Harper's Illustrated Biochemistry-McGraw-Hill Medical (2003)

THE DIVERSITY OF THE ENDOCRINE SYSTEM / 435

ferent effects in one cell or in different cells. With the Table 42–2. Determinants of the target
discovery of specific cell-surface and intracellular hor-
mone receptors, the definition of a target has been ex- cell response.
panded to include any cell in which the hormone (lig-
and) binds to its receptor, whether or not a biochemical The number, relative activity, and state of occupancy of the
or physiologic response has yet been determined. specific receptors on the plasma membrane or in the
cytoplasm or nucleus.
Several factors determine the response of a target cell
to a hormone. These can be thought of in two general The metabolism (activation or inactivation) of the hormone in
ways: (1) as factors that affect the concentration of the the target cell.
hormone at the target cell (see Table 42–1) and (2) as
factors that affect the actual response of the target cell The presence of other factors within the cell that are neces-
to the hormone (see Table 42–2). sary for the hormone response.

HORMONE RECEPTORS ARE Up- or down-regulation of the receptor consequent to the
OF CENTRAL IMPORTANCE interaction with the ligand.

Receptors Discriminate Precisely Postreceptor desensitzation of the cell, including down-
regulation of the receptor.
One of the major challenges faced in making the hor-
mone-based communication system work is illustrated logically relevant: (1) binding should be specific, ie, dis-
in Figure 42–1. Hormones are present at very low con- placeable by agonist or antagonist; (2) binding should
centrations in the extracellular fluid, generally in the be saturable; and (3) binding should occur within the
range of 10–15 to 10–9 mol/L. This concentration is concentration range of the expected biologic response.
much lower than that of the many structurally similar
molecules (sterols, amino acids, peptides, proteins) and Both Recognition & Coupling
other molecules that circulate at concentrations in the Domains Occur on Receptors
10–5 to 10–3 mol/L range. Target cells, therefore, must
distinguish not only between different hormones pre- All receptors have at least two functional domains. A
sent in small amounts but also between a given hor- recognition domain binds the hormone ligand and a
mone and the 106- to 109-fold excess of other similar second region generates a signal that couples hormone
molecules. This high degree of discrimination is pro- recognition to some intracellular function. Coupling
vided by cell-associated recognition molecules called re- (signal transduction) occurs in two general ways.
ceptors. Hormones initiate their biologic effects by Polypeptide and protein hormones and the cate-
binding to specific receptors, and since any effective cholamines bind to receptors located in the plasma
control system also must provide a means of stopping a membrane and thereby generate a signal that regulates
response, hormone-induced actions generally terminate various intracellular functions, often by changing the
when the effector dissociates from the receptor. activity of an enzyme. In contrast, steroid, retinoid, and
thyroid hormones interact with intracellular receptors,
A target cell is defined by its ability to selectively and it is this ligand-receptor complex that directly pro-
bind a given hormone to its cognate receptor. Several vides the signal, generally to specific genes whose rate of
biochemical features of this interaction are important in transcription is thereby affected.
order for hormone-receptor interactions to be physio-
The domains responsible for hormone recognition
Table 42–1. Determinants of the concentration and signal generation have been identified in the pro-
tein polypeptide and catecholamine hormone receptors.
of a hormone at the target cell. Steroid, thyroid, and retinoid hormone receptors have
several functional domains: one site binds the hormone;
The rate of synthesis and secretion of the hormones. another binds to specific DNA regions; a third is in-
The proximity of the target cell to the hormone source (dilu- volved in the interaction with other coregulator pro-
teins that result in the activation (or repression) of gene
tion effect). transcription; and a fourth may specify binding to one
The dissociation constants of the hormone with specific or more other proteins that influence the intracellular
trafficking of the receptor.
plasma transport proteins (if any).
The conversion of inactive or suboptimally active forms of the The dual functions of binding and coupling ulti-
mately define a receptor, and it is the coupling of hor-
hormone into the fully active form. mone binding to signal transduction—so-called recep-
The rate of clearance from plasma by other tissues or by tor-effector coupling—that provides the first step in
amplification of the hormonal response. This dual pur-
digestion, metabolism, or excretion. pose also distinguishes the target cell receptor from the

436 / CHAPTER 42

❁N ❁ ✴❖ ◗ ECF Figure 42–1. Specificity and selectivity of
❁ ✪ content hormone receptors. Many different molecules
N✪ ❙ circulate in the extracellular fluid (ECF), but
✧ ❉ ❃ ✧ Hormone only a few are recognized by hormone recep-
❙ ❁ Receptor tors. Receptors must select these molecules
✴ ❍ from among high concentrations of the other
✧ ✪❉ Cell types molecules. This simplified drawing shows that
❃ ❖ a cell may have no hormone receptors (1),
❍ ◗ have one receptor (2+5+6), have receptors for
several hormones (3), or have a receptor but
❉ ❁ no hormone in the vicinity (4).

12 3 45 6

plasma carrier proteins that bind hormone but do not HORMONES CAN BE CLASSIFIED
generate a signal (see Table 42–6).
IN SEVERAL WAYS
Receptors Are Proteins
Hormones can be classified according to chemical com-
Several classes of peptide hormone receptors have been position, solubility properties, location of receptors,
defined. For example, the insulin receptor is a het- and the nature of the signal used to mediate hormonal
erotetramer (α2β2) linked by multiple disulfide bonds action within the cell. A classification based on the last
in which the extracellular α subunit binds insulin and two properties is illustrated in Table 42–3, and general
the membrane-spanning β subunit transduces the sig- features of each group are illustrated in Table 42–4.
nal through the tyrosine protein kinase domain located
in the cytoplasmic portion of this polypeptide. The re- The hormones in group I are lipophilic. After secre-
ceptors for insulin-like growth factor I (IGF-I) and tion, these hormones associate with plasma transport or
epidermal growth factor (EGF) are generally similar in carrier proteins, a process that circumvents the problem
structure to the insulin receptor. The growth hormone of solubility while prolonging the plasma half-life of the
and prolactin receptors also span the plasma mem- hormone. The relative percentages of bound and free
brane of target cells but do not contain intrinsic pro- hormone are determined by the binding affinity and
tein kinase activity. Ligand binding to these receptors, binding capacity of the transport protein. The free hor-
however, results in the association and activation of a mone, which is the biologically active form, readily tra-
completely different protein kinase pathway, the Jak- verses the lipophilic plasma membrane of all cells and
Stat pathway. Polypeptide hormone and catecho- encounters receptors in either the cytosol or nucleus of
lamine receptors, which transduce signals by altering target cells. The ligand-receptor complex is assumed to
the rate of production of cAMP through G-proteins, be the intracellular messenger in this group.
are characterized by the presence of seven domains that
span the plasma membrane. Protein kinase activation The second major group consists of water-soluble
and the generation of cyclic AMP, (cAMP, 3′5′- hormones that bind to the plasma membrane of the tar-
adenylic acid; see Figure 20–5) is a downstream action get cell. Hormones that bind to the surfaces of cells
of this class of receptor (see Chapter 43 for further de- communicate with intracellular metabolic processes
tails). through intermediary molecules called second messen-
gers (the hormone itself is the first messenger), which
A comparison of several different steroid receptors are generated as a consequence of the ligand-receptor
with thyroid hormone receptors revealed a remarkable interaction. The second messenger concept arose from
conservation of the amino acid sequence in certain re- an observation that epinephrine binds to the plasma
gions, particularly in the DNA-binding domains. This membrane of certain cells and increases intracellular
led to the realization that receptors of the steroid or cAMP. This was followed by a series of experiments in
thyroid type are members of a large superfamily of nu- which cAMP was found to mediate the effects of many
clear receptors. Many related members of this family hormones. Hormones that clearly employ this mecha-
have no known ligand at present and thus are called or- nism are shown in group II.A of Table 42–3. To date,
phan receptors. The nuclear receptor superfamily plays only one hormone, atrial natriuretic factor (ANF), uses
a critical role in the regulation of gene transcription by cGMP as its second messenger, but other hormones
hormones, as described in Chapter 43. will probably be added to group II.B. Several hor-
mones, many of which were previously thought to af-
fect cAMP, appear to use ionic calcium (Ca2+) or

THE DIVERSITY OF THE ENDOCRINE SYSTEM / 437

Table 42–3. Classification of hormones by Table 42–4. General features of hormone classes.

mechanism of action. Group I Group II

I. Hormones that bind to intracellular receptors Types Steroids, iodothyro- Polypeptides, proteins,
Androgens
Calcitriol (1,25[OH]2-D3) nines, calcitriol, glycoproteins, cate-
Estrogens
Glucocorticoids retinoids cholamines
Mineralocorticoids
Progestins Solubility Lipophilic Hydrophilic
Retinoic acid
Thyroid hormones (T3 and T4) Transport Yes No
proteins
II. Hormones that bind to cell surface receptors
A. The second messenger is cAMP: Plasma half- Long (hours to Short (minutes)
α2-Adrenergic catecholamines life days)
β-Adrenergic catecholamines
Adrenocorticotropic hormone Receptor Intracellular Plasma membrane
Antidiuretic hormone Mediator
Calcitonin Receptor-hormone cAMP, cGMP, Ca2+,
Chorionic gonadotropin, human complex metabolites of complex
Corticotropin-releasing hormone phosphoinositols,
Follicle-stimulating hormone kinase cascades
Glucagon
Lipotropin metabolites of complex phosphoinositides (or both) as
Luteinizing hormone the intracellular signal. These are shown in group II.C
Melanocyte-stimulating hormone of the table. The intracellular messenger for group II.D
Parathyroid hormone is a protein kinase-phosphatase cascade. Several of these
Somatostatin have been identified, and a given hormone may use
Thyroid-stimulating hormone more than one kinase cascade. A few hormones fit into
B. The second messenger is cGMP: more than one category, and assignments change as
Atrial natriuretic factor new information is brought forward.
Nitric oxide
C. The second messenger is calcium or phosphatidyl- DIVERSITY OF THE ENDOCRINE SYSTEM
inositols (or both):
Acetylcholine (muscarinic) Hormones Are Synthesized in a
α1-Adrenergic catecholamines Variety of Cellular Arrangements
Angiotensin II
Antidiuretic hormone (vasopressin) Hormones are synthesized in discrete organs designed
Cholecystokinin solely for this specific purpose, such as the thyroid (tri-
Gastrin iodothyronine), adrenal (glucocorticoids and mineralo-
Gonadotropin-releasing hormone corticoids), and the pituitary (TSH, FSH, LH, growth
Oxytocin hormone, prolactin, ACTH). Some organs are designed
Platelet-derived growth factor to perform two distinct but closely related functions.
Substance P For example, the ovaries produce mature oocytes and
Thyrotropin-releasing hormone the reproductive hormones estradiol and progesterone.
D. The second messenger is a kinase or phosphatase The testes produce mature spermatozoa and testos-
cascade: terone. Hormones are also produced in specialized cells
Chorionic somatomammotropin within other organs such as the small intestine
Epidermal growth factor (glucagon-like peptide), thyroid (calcitonin), and kid-
Erythropoietin ney (angiotensin II). Finally, the synthesis of some hor-
Fibroblast growth factor mones requires the parenchymal cells of more than one
Growth hormone organ—eg, the skin, liver, and kidney are required for
Insulin the production of 1,25(OH)2-D3 (calcitriol). Examples
Insulin-like growth factors I and II of this diversity in the approach to hormone synthesis,
Nerve growth factor each of which has evolved to fulfill a specific purpose,
Platelet-derived growth factor are discussed below.
Prolactin

438 / CHAPTER 42

Hormones Are Chemically Diverse MANY HORMONES ARE MADE

Hormones are synthesized from a wide variety of chem- FROM CHOLESTEROL

ical building blocks. A large series is derived from cho- Adrenal Steroidogenesis

lesterol. These include the glucocorticoids, mineralo- The adrenal steroid hormones are synthesized from
cholesterol. Cholesterol is mostly derived from the
corticoids, estrogens, progestins, and 1,25(OH)2-D3 plasma, but a small portion is synthesized in situ from
(see Figure 42–2). In some cases, a steroid hormone is acetyl-CoA via mevalonate and squalene. Much of the
cholesterol in the adrenal is esterified and stored in cy-
the precursor molecule for another hormone. For ex- toplasmic lipid droplets. Upon stimulation of the
adrenal by ACTH, an esterase is activated, and the free
ample, progesterone is a hormone in its own right but cholesterol formed is transported into the mitochon-
drion, where a cytochrome P450 side chain cleav-
is also a precursor in the formation of glucocorticoids, age enzyme (P450scc) converts cholesterol to preg-
nenolone. Cleavage of the side chain involves sequential
mineralocorticoids, testosterone, and estrogens. Testos- hydroxylations, first at C22 and then at C20, followed by
side chain cleavage (removal of the six-carbon fragment
terone is an obligatory intermediate in the biosynthesis isocaproaldehyde) to give the 21-carbon steroid (Figure
42–3, top). An ACTH-dependent steroidogenic acute
of estradiol and in the formation of dihydrotestosterone regulatory (StAR) protein is essential for the transport
of cholesterol to P450scc in the inner mitochondrial
(DHT). In these examples, described in detail below, membrane.

the final product is determined by the cell type and the All mammalian steroid hormones are formed from
cholesterol via pregnenolone through a series of reac-
associated set of enzymes in which the precursor exists. tions that occur in either the mitochondria or endoplas-
mic reticulum of the adrenal cell. Hydroxylases that re-
The amino acid tyrosine is the starting point in the quire molecular oxygen and NADPH are essential, and
dehydrogenases, an isomerase, and a lyase reaction are
synthesis of the catecholamines and of the thyroid hor- also necessary for certain steps. There is cellular speci-
ficity in adrenal steroidogenesis. For instance, 18-
mones tetraiodothyronine (thyroxine; T4) and triiodo- hydroxylase and 19-hydroxysteroid dehydrogenase,
tthhaytrothneinyere(qTu3)ire(Ftihgeuaredd4i2ti–o2n).oTf i3oadnindeT(a4saIr−e) unique in which are required for aldosterone synthesis, are found
for bioac- only in the zona glomerulosa cells (the outer region of
the adrenal cortex), so that the biosynthesis of this min-
tivity. Because dietary iodine is very scarce in many eralocorticoid is confined to this region. A schematic
representation of the pathways involved in the synthesis
parts of the world, an intricate mechanism for accumu- of the three major classes of adrenal steroids is pre-
lating and retaining I− has evolved. sented in Figure 42–4. The enzymes are shown in the
rectangular boxes, and the modifications at each step
Many hormones are polypeptides or glycoproteins. are shaded.

These range in size from thyrotropin-releasing hor- A. MINERALOCORTICOID SYNTHESIS

mone (TRH), a tripeptide, to single-chain polypeptides Synthesis of aldosterone follows the mineralocorticoid
pathway and occurs in the zona glomerulosa. Preg-
like adrenocorticotropic hormone (ACTH; 39 amino nenolone is converted to progesterone by the action of
two smooth endoplasmic reticulum enzymes, 3␤-
acids), parathyroid hormone (PTH; 84 amino acids), hydroxysteroid dehydrogenase (3␤-OHSD) and ⌬5,4-
isomerase. Progesterone is hydroxylated at the C21 posi-
and growth hormone (GH; 191 amino acids) (Figure tion to form 11-deoxycorticosterone (DOC), which is an
active (Na+-retaining) mineralocorticoid. The next hy-
42–2). Insulin is an AB chain heterodimer of 21 and 30 droxylation, at C11, produces corticosterone, which has
glucocorticoid activity and is a weak mineralocorticoid (it
amino acids, respectively. Follicle-stimulating hormone has less than 5% of the potency of aldosterone). In some
species (eg, rodents), it is the most potent glucocorticoid.
(FSH), luteinizing hormone (LH), thyroid-stimulating

hormone (TSH), and chorionic gonadotropin (CG) are

glycoprotein hormones of αβ heterodimeric structure.

The α chain is identical in all of these hormones, and

distinct β chains impart hormone uniqueness. These

hormones have a molecular mass in the range of 25–30

kDa depending on the degree of glycosylation and the

length of the β chain.

Hormones Are Synthesized & Modified

For Full Activity in a Variety of Ways

Some hormones are synthesized in final form and se-
creted immediately. Included in this class are the hor-
mones derived from cholesterol. Others such as the cat-
echolamines are synthesized in final form and stored in
the producing cells. Others are synthesized from pre-
cursor molecules in the producing cell, then are
processed and secreted upon a physiologic cue (insulin).
Finally, still others are converted to active forms from
precursor molecules in the periphery (T3 and DHT).
All of these examples are discussed in more detail
below.

THE DIVERSITY OF THE ENDOCRINE SYSTEM / 439

A. CHOLESTEROL DERIVATIVES CH2OH CH3
CO
OH OH CO
OH OH
CH2

HO O O HO HO OH
17ß-Estradiol Testosterone Progesterone 1,25(OH)2-D3
Cortisol

B. TYROSINE DERIVATIVES

I I HO H
OH OH
O
CH2CH COOH HO C C NH2
I NH2 HH
T3
Norepinephrine

I I CH2CH COOH HO H
OH NH2 HO O H CH3
O C C NH
I
I HH
T4 Epinephrine

C. PEPTIDES OF VARIOUS SIZES

12 3 1 2 3 4 5 6 7 8 9 10 11
12
(pyro) Glu Hls Pro NH2
Sser Tsyer Sser Mseert Gselur Hselsr Psheer Asergr Tsrepr Gselyr Lsyesr
TRH Pseror 13

Conserved region; required for full biologic activity sVearl

24 23 22 21 20 19 18 17 16 Gselyr

Lsyesr 14

25 Pro Tyr Val Lys Vsaelr Pseror Asergr Asergr Lsyesr 15
Asp

26
Ala

27 Gly Variable region; not required for biologic activity

Glu Assepr Gselnr Sser Aselar Gselur Aselar Psheer Pseror Lseeur Gselur Psheer
28

29 30 31 32 33 34 35 36 37 38 39

Structure of human ACTH.

D. GLYCOPROTEINS (TSH, FSH, LH) ACTH

αcommon subunits
βunique subunits

Figure 42–2. Chemical diversity of hormones. A. Cholesterol derivatives. B. Tyrosine derivatives.
C. Peptides of various sizes D. Glycoproteins (TSH, FSH, LH) with common α subunits and unique β
subunits.

440 / CHAPTER 42

Cholesterol side chain cleavage 21 CH3

CCCC 20 C O
CCC
C ACTH 18 17 HC
(cAMP) 12 16 CCCC
CD OC
P450scc 11 13 15
B
+
Cholesterol
19 14
9
1
A 2 10 8

HO 3 57
6
HO 4

Pregnenolone + isocaproaldehyde

Basic steroid hormone structures

CH2OH CH3
CO
OH OH CO

HO OH

HO O OO

17β—Estradiol Testosterone Cortisol Progesterone

Estrane group (C18) Androstane group (C19) Pregnane group (C21)

Figure 42–3. Cholesterol side-chain cleavage and basic steroid hormone structures. The basic sterol rings are iden-
tified by the letters A–D. The carbon atoms are numbered 1–21 starting with the A ring. Note that the estrane group
has 18 carbons (C18), etc.

C21 hydroxylation is necessary for both mineralocorticoid costerone or aldosterone, depending on the cell type).
and glucocorticoid activity, but most steroids with a C17 17α-Hydroxylase is a smooth endoplasmic reticulum
hydroxyl group have more glucocorticoid and less miner- enzyme that acts upon either progesterone or, more
alocorticoid action. In the zona glomerulosa, which does commonly, pregnenolone. 17α-Hydroxyprogesterone is
not have the smooth endoplasmic reticulum enzyme hydroxylated at C21 to form 11-deoxycortisol, which is
17α-hydroxylase, a mitochondrial 18-hydroxylase is pres- then hydroxylated at C11 to form cortisol, the most po-
ent. The 18-hydroxylase (aldosterone synthase) acts on tent natural glucocorticoid hormone in humans. 21-Hy-
corticosterone to form 18-hydroxycorticosterone, which droxylase is a smooth endoplasmic reticulum enzyme,
is changed to aldosterone by conversion of the 18-alcohol whereas 11β-hydroxylase is a mitochondrial enzyme.
to an aldehyde. This unique distribution of enzymes and Steroidogenesis thus involves the repeated shuttling of
the special regulation of the zona glomerulosa by K+ and substrates into and out of the mitochondria.
angiotensin II have led some investigators to suggest that,
in addition to the adrenal being two glands, the adrenal C. ANDROGEN SYNTHESIS
cortex is actually two separate organs.
The major androgen or androgen precursor produced by
B. GLUCOCORTICOID SYNTHESIS the adrenal cortex is dehydroepiandrosterone (DHEA).
Most 17-hydroxypregnenolone follows the glucocorticoid
Cortisol synthesis requires three hydroxylases located in pathway, but a small fraction is subjected to oxidative fis-
the fasciculata and reticularis zones of the adrenal cortex sion and removal of the two-carbon side chain through
that act sequentially on the C17, C21, and C11 positions. the action of 17,20-lyase. The lyase activity is actually
The first two reactions are rapid, while C11 hydroxyla- part of the same enzyme (P450c17) that catalyzes 17α-
tion is relatively slow. If the C11 position is hydroxylated hydroxylation. This is therefore a dual function protein.
first, the action of 17␣-hydroxylase is impeded and the The lyase activity is important in both the adrenals and
mineralocorticoid pathway is followed (forming corti-

THE DIVERSITY OF THE ENDOCRINE SYSTEM / 441

Cholesterol

SCC CH3 17α-HYDROXYLASE CH3

CO CO 17,20-LYASE O
— OH

HO HO HO
Pregnenolone 17-Hydroxypregnenolone Dehydroepiandrosterone

3β-HYDROXYSTEROID DEHYDROGENASE: ∆5,4 ISOMERASE

CH3 CH3 O
CO
CO
P450c17 — OH P450c17

O O O
Progesterone 17-Hydroxyprogesterone ∆4 ANDROSTENE-3,17-DION

21-HYDROXYLASE CH2OH
CH2OH
CO CO
— OH

O O
11-Deoxycorticosterone 11-Deoxycortisol

11β-HYDROXYLASE

CH2OH CH2OH
CO
HO CO
HO — OH

O O
Corticosterone
CORTISOL

18-HYDROXYLASE
18-HYDROXYDEHYDROGENASE

O CH2OH
CO

HO H C

O
ALDOSTERONE

Figure 42–4. Pathways involved in the synthesis of the three major classes of
adrenal steroids (mineralocorticoids, glucocorticoids, and androgens). Enzymes
are shown in the rectangular boxes, and the modifications at each step are
shaded. Note that the 17α-hydroxylase and 17,20-lyase activities are both part of
one enzyme, designated P450c17. (Slightly modified and reproduced, with permis-
sion, from Harding BW: In: Endocrinology, vol 2. DeGroot LJ [editor]. Grune & Stratton,
1979.)

442 / CHAPTER 42 are generally inactive or less active than the parent com-
pound. Metabolism by the second pathway, which is less
the gonads and acts exclusively on 17α-hydroxy-contain- efficient, occurs primarily in target tissues and produces
ing molecules. Adrenal androgen production increases the potent metabolite dihydrotestosterone (DHT).
markedly if glucocorticoid biosynthesis is impeded by the
lack of one of the hydroxylases (adrenogenital syn- The most significant metabolic product of testos-
drome). DHEA is really a prohormone, since the actions terone is DHT, since in many tissues, including
of 3β-OHSD and ∆5,4-isomerase convert the weak andro- prostate, external genitalia, and some areas of the skin,
gen DHEA into the more potent androstenedione. this is the active form of the hormone. The plasma con-
Small amounts of androstenedione are also formed in the tent of DHT in the adult male is about one-tenth that
adrenal by the action of the lyase on 17α-hydroxyproges- of testosterone, and approximately 400 µg of DHT is
terone. Reduction of androstenedione at the C17 position produced daily as compared with about 5 mg of testos-
results in the formation of testosterone, the most potent terone. About 50–100 µg of DHT are secreted by the
adrenal androgen. Small amounts of testosterone are pro- testes. The rest is produced peripherally from testos-
duced in the adrenal by this mechanism, but most of this terone in a reaction catalyzed by the NADPH-depen-
conversion occurs in the testes. dent 5␣-reductase (Figure 42–6). Testosterone can
thus be considered a prohormone, since it is converted
Testicular Steroidogenesis into a much more potent compound (dihydrotestos-
terone) and since most of this conversion occurs outside
Testicular androgens are synthesized in the interstitial the testes. Some estradiol is formed from the peripheral
tissue by the Leydig cells. The immediate precursor of aromatization of testosterone, particularly in males.
the gonadal steroids, as for the adrenal steroids, is cho-
lesterol. The rate-limiting step, as in the adrenal, is de- Ovarian Steroidogenesis
livery of cholesterol to the inner membrane of the mito-
chondria by the transport protein StAR. Once in the The estrogens are a family of hormones synthesized in a
proper location, cholesterol is acted upon by the side variety of tissues. 17β-Estradiol is the primary estrogen
chain cleavage enzyme P450scc. The conversion of cho- of ovarian origin. In some species, estrone, synthesized
lesterol to pregnenolone is identical in adrenal, ovary, in numerous tissues, is more abundant. In pregnancy,
and testis. In the latter two tissues, however, the reac- relatively more estriol is produced, and this comes from
tion is promoted by LH rather than ACTH. the placenta. The general pathway and the subcellular
localization of the enzymes involved in the early steps
The conversion of pregnenolone to testosterone re- of estradiol synthesis are the same as those involved in
quires the action of five enzyme activities contained in androgen biosynthesis. Features unique to the ovary are
three proteins: (1) 3β-hydroxysteroid dehydrogenase (3β- illustrated in Figure 42–7.
OHSD) and ∆5,4-isomerase; (2) 17α-hydroxylase and
17,20-lyase; and (3) 17β-hydroxysteroid dehydrogenase Estrogens are formed by the aromatization of andro-
(17β-OHSD). This sequence, referred to as the proges- gens in a complex process that involves three hydroxyla-
terone (or ⌬4) pathway, is shown on the right side of Fig- tion steps, each of which requires O2 and NADPH. The
ure 42–5. Pregnenolone can also be converted to testos- aromatase enzyme complex is thought to include a
terone by the dehydroepiandrosterone (or ⌬5) pathway, P450 monooxygenase. Estradiol is formed if the sub-
which is illustrated on the left side of Figure 42–5. The ∆5 strate of this enzyme complex is testosterone, whereas es-
route appears to be most used in human testes. trone results from the aromatization of androstenedione.

The five enzyme activities are localized in the micro- The cellular source of the various ovarian steroids has
somal fraction in rat testes, and there is a close func- been difficult to unravel, but a transfer of substrates be-
tional association between the activities of 3β-OHSD tween two cell types is involved. Theca cells are the source
and ∆5,4-isomerase and between those of a 17α-hydrox- of androstenedione and testosterone. These are converted
ylase and 17,20-lyase. These enzyme pairs, both con- by the aromatase enzyme in granulosa cells to estrone and
tained in a single protein, are shown in the general reac- estradiol, respectively. Progesterone, a precursor for all
tion sequence in Figure 42–5. steroid hormones, is produced and secreted by the corpus
luteum as an end-product hormone because these cells do
Dihydrotestosterone Is Formed From not contain the enzymes necessary to convert proges-
Testosterone in Peripheral Tissues terone to other steroid hormones (Figure 42–8).

Testosterone is metabolized by two pathways. One in- Significant amounts of estrogens are produced by
volves oxidation at the 17 position, and the other in- the peripheral aromatization of androgens. In human
volves reduction of the A ring double bond and the 3-ke- males, the peripheral aromatization of testosterone to
tone. Metabolism by the first pathway occurs in many estradiol (E2) accounts for 80% of the production of
tissues, including liver, and produces 17-ketosteroids that the latter. In females, adrenal androgens are important

THE DIVERSITY OF THE ENDOCRINE SYSTEM / 443

CH3 CH3
CO CO

HO 3β–HYDROXYSTEROID DEHYDROGENASE AND ∆5,4 ISOMERASE HO
Pregnenolone Progesterone

17α-HYDROXYLASE* 17α-HYDROXYLASE*

CH3 CH3
CO CO

OH OH

HO O
17α-Hydroxypregnenolone 17α-Hydroxyprogesterone

17,20-LYASE* 17,20-LYASE*

O O

HO O
Dehydroepiandrosterone Androstenedione

Figure 42–5. Pathways of testos- 17β-HYDROXYSTEROID 17β-HYDROXYSTEROID
terone biosynthesis. The pathway on DEHYDROGENASE DEHYDROGENASE
the left side of the figure is called the ∆5
or dehydroepiandrosterone pathway; OH OH

the pathway on the right side is called HO O
the ∆4 or progesterone pathway. The as- ∆5-Androstenediol TESTOSTERONE
terisk indicates that the 17α-hydroxy-
lase and 17,20-lyase activities reside in a

single protein, P450c17.

444 / CHAPTER 42

OH OH

5α-REDUCTASE

NADPH

OO
H

Testosterone DIHYDROTESTOSTERONE (DHT)

Figure 42–6. Dihydrotestosterone is formed from testosterone through action of the
enzyme 5α-reductase.

Cholesterol Pregnenolone 17α-Hydroxypregnenolone Dehydroepiandrosterone
Progesterone 17α-Hydroxyprogesterone
Other Androstenedione
metabolites
Testosterone

AROMATASE AROMATASE
O
OH

HO HO
ESTRONE (E1) 17β-ESTRADIOL (E2)

16α-Hydroxylase Other metabolites

OH
OH

HO

Estriol

Figure 42–7. Biosynthesis of estrogens. (Slightly modified and reproduced, with permission, from Ganong
WF: Review of Medical Physiology, 20th ed. McGraw-Hill, 2001.)

THE DIVERSITY OF THE ENDOCRINE SYSTEM / 445

Acetate CH3 most of the precursor for 1,25(OH)2-D3 synthesis is
Cholesterol CO produced in the malpighian layer of the epidermis from
7-dehydrocholesterol in an ultraviolet light-mediated,
HO nonenzymatic photolysis reaction. The extent of this
Pregnenolone conversion is related directly to the intensity of the ex-
posure and inversely to the extent of pigmentation in
CH3 the skin. There is an age-related loss of 7-dehydrocho-
CO lesterol in the epidermis that may be related to the neg-
ative calcium balance associated with old age.
O
B. LIVER
Progesterone
A specific transport protein called the vitamin D-bind-
Figure 42–8. Biosynthesis of progesterone in the ing protein binds vitamin D3 and its metabolites and
corpus luteum. moves vitamin D3 from the skin or intestine to the
liver, where it undergoes 25-hydroxylation, the first
substrates, since as much as 50% of the E2 produced obligatory reaction in the production of 1,25(OH)2-
during pregnancy comes from the aromatization of an- D3. 25-Hydroxylation occurs in the endoplasmic retic-
drogens. Finally, conversion of androstenedione to ulum in a reaction that requires magnesium, NADPH,
estrone is the major source of estrogens in post- molecular oxygen, and an uncharacterized cytoplasmic
menopausal women. Aromatase activity is present in factor. Two enzymes are involved: an NADPH-depen-
adipose cells and also in liver, skin, and other tissues. dent cytochrome P450 reductase and a cytochrome
Increased activity of this enzyme may contribute to the P450. This reaction is not regulated, and it also occurs
“estrogenization” that characterizes such diseases as cir- with low efficiency in kidney and intestine. The
rhosis of the liver, hyperthyroidism, aging, and obesity. 25(OH)2-D3 enters the circulation, where it is the
major form of vitamin D found in plasma, and is trans-
1,25(OH)2-D3 (Calcitriol) Is Synthesized ported to the kidney by the vitamin D-binding protein.
From a Cholesterol Derivative
C. KIDNEY
1,25(OH)2-D3 is produced by a complex series of enzy-
matic reactions that involve the plasma transport of pre- 25(OH)2-D3 is a weak agonist and must be modified
cursor molecules to a number of different tissues (Figure by hydroxylation at position C1 for full biologic activ-
42–9). One of these precursors is vitamin D—really not ity. This is accomplished in mitochondria of the renal
a vitamin, but this common name persists. The active proximal convoluted tubule by a three-component
molecule, 1,25(OH)2-D3, is transported to other organs monooxygenase reaction that requires NADPH, Mg2+,
where it activates biologic processes in a manner similar molecular oxygen, and at least three enzymes: (1) a
to that employed by the steroid hormones. flavoprotein, renal ferredoxin reductase; (2) an iron sul-
fur protein, renal ferredoxin; and (3) cytochrome P450.
A. SKIN This system produces 1,25(OH)2-D3, which is the most
Small amounts of the precursor for 1,25(OH)2-D3 syn- potent naturally occurring metabolite of vitamin D.
thesis are present in food (fish liver oil, egg yolk), but
CATECHOLAMINES & THYROID
HORMONES ARE MADE FROM TYROSINE

Catecholamines Are Synthesized in Final
Form & Stored in Secretion Granules

Three amines—dopamine, norepinephrine, and epi-
nephrine—are synthesized from tyrosine in the chro-
maffin cells of the adrenal medulla. The major product
of the adrenal medulla is epinephrine. This compound
constitutes about 80% of the catecholamines in the
medulla, and it is not made in extramedullary tissue. In
contrast, most of the norepinephrine present in organs
innervated by sympathetic nerves is made in situ (about
80% of the total), and most of the rest is made in other
nerve endings and reaches the target sites via the circu-

446 / CHAPTER 42

Sunlight

7-Dehydrocholesterol Previtamin D3 Vitamin D3
25-Hydroxylase
SKIN Other LIVER
metabolites
25-Hydroxycholecalciferol (25[OH]-D3)

24-Hydroxylase 1 α-Hydroxylase

24,25(OH)2-D3 KIDNEY 1,25(OH)2-D3

1,24,25(OH)3-D3 OH

24
27

25
26

CH2 CH2

HO HO HO OH
7-Dehydrocholesterol Vitamin D3 1,25(OH)2-D3

Figure 42–9. Formation and hydroxylation of vitamin D3. 25-Hydroxylation takes place in the liver, and the
other hydroxylations occur in the kidneys. 25,26(OH)2-D3 and 1,25,26(OH)3-D3 are probably formed as well. The
formulas of 7-dehydrocholesterol, vitamin D3, and 1,25(OH)2-D3 are also shown. (Modified and reproduced, with
permission, from Ganong WF: Review of Medical Physiology, 20th ed. McGraw-Hill, 2001.)

lation. Epinephrine and norepinephrine may be pro- As the rate-limiting enzyme, tyrosine hydroxylase is regu-
duced and stored in different cells in the adrenal lated in a variety of ways. The most important mecha-
medulla and other chromaffin tissues. nism involves feedback inhibition by the catecholamines,
which compete with the enzyme for the pteridine cofac-
The conversion of tyrosine to epinephrine requires tor. Catecholamines cannot cross the blood-brain barrier;
four sequential steps: (1) ring hydroxylation; (2) decar- hence, in the brain they must be synthesized locally. In
boxylation; (3) side chain hydroxylation to form norepi- certain central nervous system diseases (eg, Parkinson’s
nephrine; and (4) N-methylation to form epinephrine. disease), there is a local deficiency of dopamine synthesis.
The biosynthetic pathway and the enzymes involved are L-Dopa, the precursor of dopamine, readily crosses the
illustrated in Figure 42–10. blood-brain barrier and so is an important agent in the
treatment of Parkinson’s disease.
A. TYROSINE HYDROXYLASE IS RATE-LIMITING
B. DOPA DECARBOXYLASE IS PRESENT IN ALL TISSUES
FOR CATECHOLAMINE BIOSYNTHESIS
This soluble enzyme requires pyridoxal phosphate for
Tyrosine is the immediate precursor of catecholamines, the conversion of L-dopa to 3,4-dihydroxyphenylethyl-
and tyrosine hydroxylase is the rate-limiting enzyme in amine (dopamine). Compounds that resemble L-dopa,
catecholamine biosynthesis. Tyrosine hydroxylase is such as α-methyldopa, are competitive inhibitors of
found in both soluble and particle-bound forms only in this reaction. α-Methyldopa is effective in treating
tissues that synthesize catecholamines; it functions as an some kinds of hypertension.
oxidoreductase, with tetrahydropteridine as a cofactor, to
convert L-tyrosine to L-dihydroxyphenylalanine (L-dopa).

THE DIVERSITY OF THE ENDOCRINE SYSTEM / 447

O the conversion of dopamine to norepinephrine occurs
in this organelle.
H C OH
D. PHENYLETHANOLAMINE-N-METHYLTRANSFERASE
HO C C NH2 (PNMT) CATALYZES THE PRODUCTION
OF EPINEPHRINE
HH
Tyrosine PNMT catalyzes the N-methylation of norepinephrine
to form epinephrine in the epinephrine-forming cells
TYROSINE of the adrenal medulla. Since PNMT is soluble, it is as-
HYDROXYLASE sumed that norepinephrine-to-epinephrine conversion
occurs in the cytoplasm. The synthesis of PNMT is in-
O duced by glucocorticoid hormones that reach the
HO medulla via the intra-adrenal portal system. This special
system provides for a 100-fold steroid concentration
H C OH gradient over systemic arterial blood, and this high
intra-adrenal concentration appears to be necessary for
HO C C NH2 the induction of PNMT.

HH

Dopa

DOPA
DECARBOXYLASE

HO T3 & T4 Illustrate the Diversity
HH in Hormone Synthesis

HO C C NH2 The formation of triiodothyronine (T3) and tetra-
HH iodothyronine (thyroxine; T4) (see Figure 42–2) illus-
trates many of the principles of diversity discussed in
DOPAMINE this chapter. These hormones require a rare element
DOPAMINE (iodine) for bioactivity; they are synthesized as part of a
β-HYDROXYLASE very large precursor molecule (thyroglobulin); they are
stored in an intracellular reservoir (colloid); and there is
HO H peripheral conversion of T4 to T3, which is a much
OH more active hormone.

HO C C NH2 The thyroid hormones T3 and T4 are unique in that
HH iodine (as iodide) is an essential component of both. In
most parts of the world, iodine is a scarce component of
NOREPINEPHRINE soil, and for that reason there is little in food. A com-
plex mechanism has evolved to acquire and retain this
PNMT crucial element and to convert it into a form suitable
for incorporation into organic compounds. At the same
HO H time, the thyroid must synthesize thyronine from tyro-
HO O H CH3 sine, and this synthesis takes place in thyroglobulin
(Figure 42–11).
C C NH
Thyroglobulin is the precursor of T4 and T3. It is a
HH large iodinated, glycosylated protein with a molecular
EPINEPHRINE mass of 660 kDa. Carbohydrate accounts for 8–10% of
the weight of thyroglobulin and iodide for about
Figure 42–10. Biosynthesis of catecholamines. 0.2–1%, depending upon the iodine content in the
(PNMT, phenylethanolamine-N-methyltransferase.) diet. Thyroglobulin is composed of two large subunits.
It contains 115 tyrosine residues, each of which is a po-
C. DOPAMINE ␤-HYDROXYLASE (DBH) CATALYZES tential site of iodination. About 70% of the iodide
THE CONVERSION OF DOPAMINE TO NOREPINEPHRINE in thyroglobulin exists in the inactive precursors,
monoiodotyrosine (MIT) and diiodotyrosine (DIT),
DBH is a monooxygenase and uses ascorbate as an elec- while 30% is in the iodothyronyl residues, T4 and T3.
tron donor, copper at the active site, and fumarate as When iodine supplies are sufficient, the T4:T3 ratio is
modulator. DBH is in the particulate fraction of the about 7:1. In iodine deficiency, this ratio decreases, as
medullary cells, probably in the secretion granule; thus, does the DIT:MIT ratio. Thyroglobulin, a large mole-
cule of about 5000 amino acids, provides the confor-

448 / CHAPTER 42

FOLLICULAR SPACE WITH COLLOID

Oxidation I+ + Tgb Iodination* MIT MIT Coupling* T3 MIT
DIT DIT DIT DIT
PEROXIDASE
H2O2 DIT Tgb T4
Tgb MIT DIT
I– MIT MIT
DIT T4
DIT DIT

O2 Phagocytosis

NADPH NADP+ Tgb and
H+ pinocytosis

THYROID CELL Lysosomes
Tgb

Secondary
Tgb lysosome

Tyrosine

Deiodination* Hydrolysis

I– MIT

DEIODINASE DIT

I–

Trans- Concentration* T3, T4
porter Release
Na+-K+ ATPase

EXTRACELLULAR SPACE

I – T3, T4

Figure 42–11. Model of iodide metabolism in the thyroid follicle. A follicular cell is shown facing the follicular
lumen (top) and the extracellular space (at bottom). Iodide enters the thyroid primarily through a transporter
(bottom left). Thyroid hormone synthesis occurs in the follicular space through a series of reactions, many of
which are peroxidase-mediated. Thyroid hormones, stored in the colloid in the follicular space, are released from
thyroglobulin by hydrolysis inside the thyroid cell. (Tgb, thyroglobulin; MIT, monoiodotyrosine; DIT, diiodotyro-
sine; T3, triiodothyronine; T4, tetraiodothyronine.) Asterisks indicate steps or processes that are inherited enzyme
deficiencies which cause congenital goiter and often result in hypothyroidism.

THE DIVERSITY OF THE ENDOCRINE SYSTEM / 449

mation required for tyrosyl coupling and iodide organi- mones remain as integral parts of thyroglobulin until
fication necessary in the formation of the diaminoacid the latter is degraded, as described above.
thyroid hormones. It is synthesized in the basal portion
of the cell and moves to the lumen, where it is a storage A deiodinase removes I− from the inactive mono-
form of T3 and T4 in the colloid; several weeks’ supply and diiodothyronine molecules in the thyroid. This
of these hormones exist in the normal thyroid. Within mechanism provides a substantial amount of the I− used
minutes after stimulation of the thyroid by TSH, col- in T3 and T4 biosynthesis. A peripheral deiodinase in
loid reenters the cell and there is a marked increase of target tissues such as pituitary, kidney, and liver selec-
phagolysosome activity. Various acid proteases and tively removes I− from the 5′ position of T4 to make T3
peptidases hydrolyze the thyroglobulin into its con- (see Figure 42–2), which is a much more active mole-
stituent amino acids, including T4 and T3, which are cule. In this sense, T4 can be thought of as a prohor-
discharged from the basal portion of the cell (see Figure mone, though it does have some intrinsic activity.
42–11). Thyroglobulin is thus a very large prohor-
mone. SEVERAL HORMONES ARE MADE FROM
LARGER PEPTIDE PRECURSORS
Iodide Metabolism Involves
Formation of the critical disulfide bridges in insulin re-
Several Discrete Steps quires that this hormone be first synthesized as part of a
larger precursor molecule, proinsulin. This is conceptu-
The thyroid is able to concentrate I− against a strong ally similar to the example of the thyroid hormones,
electrochemical gradient. This is an energy-dependent which can only be formed in the context of a much
process and is linked to the Na+-K+ ATPase-dependent larger molecule. Several other hormones are synthesized
thyroidal I− transporter. The ratio of iodide in thyroid as parts of large precursor molecules, not because of
to iodide in serum (T:S ratio) is a reflection of the ac- some special structural requirement but rather as a
tivity of this transporter. This activity is primarily con- mechanism for controlling the available amount of the
trolled by TSH and ranges from 500:1 in animals active hormone. PTH and angiotensin II are examples
chronically stimulated with TSH to 5:1 or less in hy- of this type of regulation. Another interesting example
pophysectomized animals (no TSH). The T:S ratio in is the POMC protein, which can be processed into
humans on a normal iodine diet is about 25:1. many different hormones in a tissue-specific manner.
These examples are discussed in detail below.
The thyroid is the only tissue that can oxidize I− to a
higher valence state, an obligatory step in I− organifica- Insulin Is Synthesized as a Preprohormone
tion and thyroid hormone biosynthesis. This step in- & Modified Within the ␤ Cell
volves a heme-containing peroxidase and occurs at the
luminal surface of the follicular cell. Thyroperoxidase, a Insulin has an AB heterodimeric structure with one in-
tetrameric protein with a molecular mass of 60 kDa, re- trachain (A6–A11) and two interchain disulfide bridges
quires hydrogen peroxide as an oxidizing agent. The (A7–B7 and A20–B19) (Figure 42–12). The A and B
H2O2 is produced by an NADPH-dependent enzyme chains could be synthesized in the laboratory, but at-
resembling cytochrome c reductase. A number of com- tempts at a biochemical synthesis of the mature insulin
pounds inhibit I− oxidation and therefore its subse- molecule yielded very poor results. The reason for this
quent incorporation into MIT and DIT. The most im- became apparent when it was discovered that insulin is
portant of these are the thiourea drugs. They are used as synthesized as a preprohormone (molecular weight ap-
antithyroid drugs because of their ability to inhibit thy- proximately 11,500), which is the prototype for peptides
roid hormone biosynthesis at this step. Once iodination that are processed from larger precursor molecules. The
occurs, the iodine does not readily leave the thyroid. hydrophobic 23-amino-acid pre-, or leader, sequence di-
Free tyrosine can be iodinated, but it is not incorpo- rects the molecule into the cisternae of the endoplasmic
rated into proteins since no tRNA recognizes iodinated reticulum and then is removed. This results in the 9000-
tyrosine. MW proinsulin molecule, which provides the conforma-
tion necessary for the proper and efficient formation of
The coupling of two DIT molecules to form T4—or the disulfide bridges. As shown in Figure 42–12, the se-
of an MIT and DIT to form T3—occurs within the quence of proinsulin, starting from the amino terminal,
thyroglobulin molecule. A separate coupling enzyme is B chain—connecting (C) peptide—A chain. The
has not been found, and since this is an oxidative proinsulin molecule undergoes a series of site-specific
process it is assumed that the same thyroperoxidase cat- peptide cleavages that result in the formation of equimo-
alyzes this reaction by stimulating free radical forma- lar amounts of mature insulin and C peptide. These en-
tion of iodotyrosine. This hypothesis is supported by zymatic cleavages are summarized in Figure 42–12.
the observation that the same drugs which inhibit I− ox-
idation also inhibit coupling. The formed thyroid hor-

450 / CHAPTER 42

20

Pro Gln Leu Ser Gly Ala Gly Pro Gly

Ala Leu Connecting peptide Gly

Leu Gly
Leu

Glu Glu
Gly 10

Val

Ser Gln
Leu Gly
31 Val
Gln Gln

Lys

Arg

1 Gly – COOH Leu

Asp
Ile

NH 2– Glu
Val Asn 21

1 Phe Glu Cys Ala
Gln
Val S Tyr Glu 1
Asn Arg
Gln Cys S A chain Asn Arg
Mis Thr 30
Cys Ser Ile Cys Ser Leu Tyr Glu S
Thr 10 Leu
Gln

S

Leu S Lys

Cys Insulin Pro
S Thr
Gly
B chain Arg Tyr
Ser Phe
Mis Ala Leu Tyr Gly Phe
Leu
10 Val Glu Gly Glu

Leu Val Cys

20

Figure 42–12. Structure of human proinsulin. Insulin and C-peptide molecules are connected at two sites by
dipeptide links. An initial cleavage by a trypsin-like enzyme (open arrows) followed by several cleavages by a car-
boxypeptidase-like enzyme (solid arrows) results in the production of the heterodimeric (AB) insulin molecule
(light blue) and the C-peptide.

Parathyroid Hormone (PTH) Is Secreted as mRNA, and this is followed by an increased rate of
an 84-Amino-Acid Peptide PTH synthesis and secretion. However, about 80–90%
of the proPTH synthesized cannot be accounted for as
The immediate precursor of PTH is proPTH, which intact PTH in cells or in the incubation medium of ex-
differs from the native 84-amino-acid hormone by hav- perimental systems. This finding led to the conclusion
ing a highly basic hexapeptide amino terminal exten- that most of the proPTH synthesized is quickly de-
sion. The primary gene product and the immediate pre- graded. It was later discovered that this rate of degrada-
cursor for proPTH is the 115-amino-acid preproPTH. tion decreases when Ca2+ concentrations are low, and it
This differs from proPTH by having an additional 25- increases when Ca2+ concentrations are high. Very spe-
amino-acid amino terminal extension that, in common cific fragments of PTH are generated during its prote-
with the other leader or signal sequences characteristic olytic digestion (Figure 42–13). A number of prote-
of secreted proteins, is hydrophobic. The complete olytic enzymes, including cathepsins B and D, have
structure of preproPTH and the sequences of proPTH been identified in parathyroid tissue. Cathepsin B
and PTH are illustrated in Figure 42–13. PTH1–34 has cleaves PTH into two fragments: PTH1–36 and
full biologic activity, and the region 25–34 is primarily PTH37–84. PTH37–84 is not further degraded; however,
responsible for receptor binding. PTH1–36 is rapidly and progressively cleaved into di-
and tripeptides. Most of the proteolysis of PTH occurs
The biosynthesis of PTH and its subsequent secre- within the gland, but a number of studies confirm that
tion are regulated by the plasma ionized calcium (Ca2+) PTH, once secreted, is proteolytically degraded in other
concentration through a complex process. An acute de- tissues, especially the liver, by similar mechanisms.
crease of Ca2+ results in a marked increase of PTH

THE DIVERSITY OF THE ENDOCRINE SYSTEM / 451

Leader (pre) sequence

–6 –10 –20 –31

Pro Lys Gly Asp Ser Arg Ala Leu Phe Cys Ile Ala Leu Met Val Ile Met Val Lys Val Met Asp Lys Ala Ser Met Met NH2
sequence Ser

Val

Lys (2) (1)

Lys

–1 Arg (3)

1 Ala

Val

Ser 10 20
Glu
Ile Gln Phe Met His Asn Leu Gly Lys His Leu Ser Ser Met Glu Arg Val Glu Trp Leu Arg Lys
Lys

Full biologic activity sequence Leu
Gln

30
Asp

Val

His

C-fragment sequence Asn

50 40 Phe
Val
Asn Asp Glu Lys Lys Arg Pro Arg Gln Ser Ser Gly Asp Arg Tyr Ala Ile Ser Ala Gly Leu Ala
Val
60 Leu (4)

Val (5)

Glu

Ser

His

Gln

Lys 70 80 O
Ser C
Leu Gly Glu Ala Asp Lys Ala Asp Val Asp Val Leu Ile Lys Ala Lys Pro Gln

OH

Figure 42–13. Structure of bovine preproparathyroid hormone. Arrows indicate sites cleaved by pro-
cessing enzymes in the parathyroid gland (1–5) and in the liver after secretion of the hormone (4–5). The
biologically active region of the molecule is flanked by sequence not required for activity on target re-
ceptors. (Slightly modified and reproduced, with permission, from Habener JF: Recent advances in parathy-
roid hormone research. Clin Biochem 1981;14:223.)

Angiotensin II Is Also Synthesized tors that decreases fluid volume (dehydration, decreased
From a Large Precursor blood pressure, fluid or blood loss) or decreases NaCl
concentration stimulates renin release. Renal sympa-
The renin-angiotensin system is involved in the regula- thetic nerves that terminate in the juxtaglomerular cells
tion of blood pressure and electrolyte metabolism mediate the central nervous system and postural effects
(through production of aldosterone). The primary hor- on renin release independently of the baroreceptor and
mone involved in these processes is angiotensin II, an salt effects, a mechanism that involves the β-adrenergic
octapeptide made from angiotensinogen (Figure receptor. Renin acts upon the substrate angiotensino-
42–14). Angiotensinogen, a large α2-globulin made in gen to produce the decapeptide angiotensin I.
liver, is the substrate for renin, an enzyme produced in
the juxtaglomerular cells of the renal afferent arteriole. Angiotensin-converting enzyme, a glycoprotein
The position of these cells makes them particularly sen- found in lung, endothelial cells, and plasma, removes
sitive to blood pressure changes, and many of the physi- two carboxyl terminal amino acids from the decapep-
ologic regulators of renin release act through renal tide angiotensin I to form angiotensin II in a step that
baroreceptors. The juxtaglomerular cells are also sensi- is not thought to be rate-limiting. Various nonapeptide
tive to changes of Na+ and Cl− concentration in the analogs of angiotensin I and other compounds act as
renal tubular fluid; therefore, any combination of fac- competitive inhibitors of converting enzyme and are
used to treat renin-dependent hypertension. These are

452 / CHAPTER 42

Angiotensinogen Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu-Leu (~ 400 more amino acids)

RENIN
Angiotensin I Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu

CONVERTING ENZYME
ANGIOTENSIN II Asp-Arg-Val-Tyr-Ile-His-Pro-Phe

AMINOPEPTIDASE
Angiotensin III Arg-Val-Tyr-Ile-His-Pro-Phe

ANGIOTENSINASES

Degradation products

Figure 42–14. Formation and metabolism of angiotensins. Small arrows in-
dicate cleavage sites.

referred to as angiotensin-converting enzyme (ACE) Complex Processing Generates
inhibitors. Angiotensin II increases blood pressure by
causing vasoconstriction of the arteriole and is a very the Pro-opiomelanocortin (POMC)
potent vasoactive substance. It inhibits renin release
from the juxtaglomerular cells and is a potent stimula- Peptide Family
tor of aldosterone production. This results in Na+ re-
tention, volume expansion, and increased blood pres- The POMC family consists of peptides that act as hor-
sure. mones (ACTH, LPH, MSH) and others that may serve
as neurotransmitters or neuromodulators (endorphins)
In some species, angiotensin II is converted to the (see Figure 42–15). POMC is synthesized as a precur-
heptapeptide angiotensin III (Figure 42–14), an equally sor molecule of 285 amino acids and is processed differ-
potent stimulator of aldosterone production. In hu- ently in various regions of the pituitary.
mans, the plasma level of angiotensin II is four times
greater than that of angiotensin III, so most effects are The POMC gene is expressed in the anterior and in-
exerted by the octapeptide. Angiotensins II and III are termediate lobes of the pituitary. The most conserved
rapidly inactivated by angiotensinases. sequences between species are within the amino termi-
nal fragment, the ACTH region, and the β-endorphin
Angiotensin II binds to specific adrenal cortex region. POMC or related products are found in several
glomerulosa cell receptors. The hormone-receptor in- other vertebrate tissues, including the brain, placenta,
teraction does not activate adenylyl cyclase, and cAMP gastrointestinal tract, reproductive tract, lung, and lym-
does not appear to mediate the action of this hormone. phocytes.
The actions of angiotensin II, which are to stimulate
the conversion of cholesterol to pregnenolone and of The POMC protein is processed differently in the an-
corticosterone to 18-hydroxycorticosterone and aldos- terior lobe than in the intermediate lobe. The intermedi-
terone, may involve changes in the concentration of in- ate lobe of the pituitary is rudimentary in adult humans,
tracellular calcium and of phospholipid metabolites by but it is active in human fetuses and in pregnant women
mechanisms similar to those described in Chapter 43. during late gestation and is also active in many animal
species. Processing of the POMC protein in the periph-
eral tissues (gut, placenta, male reproductive tract) resem-

ACTH (1–39) THE DIVERSITY OF THE ENDOCRINE SYSTEM / 453

POMC (1–134)
β-LPH (42–134)

α-MSH CLIP γ-LPH β-Endorphin
(1–13) (18–39) (42–101) (104 –134)

β-MSH γ-Endorphin
(84 –101) (104 –118)

α-Endorphin
(104 –117)

Figure 42–15. Products of pro-opiomelanocortin (POMC) cleavage.
(MSH, melanocyte-stimulating hormone; CLIP, corticotropin-like inter-
mediate lobe peptide; LPH, lipotropin.)

bles that in the intermediate lobe. There are three basic there is no intracellular reservoir of these hormones.
peptide groups: (1) ACTH, which can give rise to The catecholamines, also synthesized in active form, are
α-MSH and corticotropin-like intermediate lobe peptide stored in granules in the chromaffin cells in the adrenal
(CLIP); (2) β-lipotropin (β-LPH), which can yield medulla. In response to appropriate neural stimulation,
γ-LPH, β-MSH, and β-endorphin (and thus α- and these granules are released from the cell through exocy-
γ-endorphins); and (3) a large amino terminal peptide, tosis, and the catecholamines are released into the circu-
which generates γ-MSH. The diversity of these products lation. A several-hour reserve supply of catecholamines
is due to the many dibasic amino acid clusters that are exists in the chromaffin cells.
potential cleavage sites for trypsin-like enzymes. Each of
the peptides mentioned is preceded by Lys-Arg, Arg-Lys, Parathyroid hormone also exists in storage vesicles.
Arg-Arg, or Lys-Lys residues. After the prehormone seg- As much as 80–90% of the proPTH synthesized is de-
ment is cleaved, the next cleavage, in both anterior and graded before it enters this final storage compartment,
intermediate lobes, is between ACTH and β-LPH, re- especially when Ca2+ levels are high in the parathyroid
sulting in an amino terminal peptide with ACTH and a cell (see above). PTH is secreted when Ca2+ is low in
β-LPH segment (Figure 42–15). ACTH1–39 is subse- the parathyroid cells, which contain a several-hour sup-
quently cleaved from the amino terminal peptide, and in ply of the hormone.
the anterior lobe essentially no further cleavages occur. In
the intermediate lobe, ACTH1–39 is cleaved into α-MSH The human pancreas secretes about 40–50 units of in-
(residues 1–13) and CLIP (18–39); β-LPH (42–134) is sulin daily, which represents about 15–20% of the hor-
converted to γ-LPH (42–101) and β-endorphin (104– mone stored in the B cells. Insulin and the C-peptide (see
134). β-MSH (84–101) is derived from γ-LPH. Figure 42–12) are normally secreted in equimolar
amounts. Stimuli such as glucose, which provokes insulin
There are extensive additional tissue-specific modifi- secretion, therefore trigger the processing of proinsulin to
cations of these peptides that affect activity. These insulin as an essential part of the secretory response.
modifications include phosphorylation, acetylation,
glycosylation, and amidation. A several-week supply of T3 and T4 exists in the thy-
roglobulin that is stored in colloid in the lumen of the
THERE IS VARIATION IN THE STORAGE thyroid follicles. These hormones can be released upon
stimulation by TSH. This is the most exaggerated ex-
& SECRETION OF HORMONES ample of a prohormone, as a molecule containing ap-
proximately 5000 amino acids must be first synthe-
As mentioned above, the steroid hormones and sized, then degraded, to supply a few molecules of the
1,25(OH)2-D3 are synthesized in their final active active hormones T4 and T3.
form. They are also secreted as they are made, and thus
The diversity in storage and secretion of hormones
is illustrated in Table 42–5.

454 / CHAPTER 42

Table 42–5. Diversity in the storage of hormones. lives. A notable exception is IGF-I, which is transported
bound to members of a family of binding proteins.
Hormone Supply Stored in Cell
Steroids and 1,25(OH)2-D3 None Thyroid Hormones Are Transported
Catecholamines and PTH Hours by Thyroid-Binding Globulin
Insulin Days
T3 and T4 Weeks Many of the principles discussed above are illustrated in
a discussion of thyroid-binding proteins. One-half to
SOME HORMONES HAVE PLASMA two-thirds of T4 and T3 in the body is in an extrathy-
roidal reservoir. Most of this circulates in bound form,
TRANSPORT PROTEINS ie, bound to a specific binding protein, thyroxine-
binding globulin (TBG). TBG, a glycoprotein with a
The class I hormones are hydrophobic in chemical na- molecular mass of 50 kDa, binds T4 and T3 and has the
ture and thus are not very soluble in plasma. These hor- capacity to bind 20 µg/dL of plasma. Under normal
mones, principally the steroids and thyroid hormones, circumstances, TBG binds—noncovalently—nearly all
have specialized plasma transport proteins that serve sev- of the T4 and T3 in plasma, and it binds T4 with greater
eral purposes. First, these proteins circumvent the solu- affinity than T3 (Table 42–7). The plasma half-life of
bility problem and thereby deliver the hormone to the T4 is correspondingly four to five times that of T3. The
target cell. They also provide a circulating reservoir of small, unbound (free) fraction is responsible for the bi-
the hormone that can be substantial, as in the case of the ologic activity. Thus, in spite of the great difference in
thyroid hormones. Hormones, when bound to the trans- total amount, the free fraction of T3 approximates that
port proteins, cannot be metabolized, thereby prolonging of T4, and given that T3 is intrinsically more active than
their plasma half-life (t1/2). The binding affinity of a T4, most biologic activity is attributed to T3. TBG does
given hormone to its transporter determines the bound not bind any other hormones.
versus free ratio of the hormone. This is important be-
cause only the free form of a hormone is biologically ac- Glucocorticoids Are Transported
tive. In general, the concentration of free hormone in by Corticosteroid-Binding Globulin
plasma is very low, in the range of 10–15 to 10–9 mol/L. It
is important to distinguish between plasma transport Hydrocortisone (cortisol) also circulates in plasma in
proteins and hormone receptors. Both bind hormones protein-bound and free forms. The main plasma bind-
but with very different characteristics (Table 42–6). ing protein is an α-globulin called transcortin, or cor-
ticosteroid-binding globulin (CBG). CBG is pro-
The hydrophilic hormones—generally class II and duced in the liver, and its synthesis, like that of TBG, is
of peptide structure—are freely soluble in plasma and increased by estrogens. CBG binds most of the hor-
do not require transport proteins. Hormones such as mone when plasma cortisol levels are within the normal
insulin, growth hormone, ACTH, and TSH circulate range; much smaller amounts of cortisol are bound to
in the free, active form and have very short plasma half- albumin. The avidity of binding helps determine the
biologic half-lives of various glucocorticoids. Cortisol
Table 42–6. Comparison of receptors with binds tightly to CBG and has a t1/2 of 1.5–2 hours,
transport proteins. while corticosterone, which binds less tightly, has a t1/2
of less than 1 hour (Table 42–8). The unbound (free)
Feature Receptors Transport Proteins cortisol constitutes about 8% of the total and represents
the biologically active fraction. Binding to CBG is not
restricted to glucocorticoids. Deoxycorticosterone and

Concentration Very low Very high

(thousands/cell) (billions/µL)

Binding affinity High (pmol/L to Low (µmol/L range) Table 42–7. Comparison of T4 and T3 in plasma.
nmol/L range)

Binding specificity Very high Low Total Free Hormone t 1⁄2
in Blood
Saturability Yes No Hormone Percent (days)

(µg/dL) of Total ng/dL Molarity 6.5
1.5
Reversibility Yes Yes T4 8 0.03 ~2.24 3.0 × 10−11

Signal transduction Yes No T3 0.15 0.3 ~0.4 ~0.6 × 10−11

THE DIVERSITY OF THE ENDOCRINE SYSTEM / 455

Table 42–8. Approximate affinities of steroids for binding capacity they probably buffer against sudden
serum-binding proteins. changes in the plasma level. Because the metabolic
clearance rates of these steroids are inversely related to
SHBG1 CBG1 the affinity of their binding to SHBG, estrone is cleared
more rapidly than estradiol, which in turn is cleared
Dihydrotestosterone 1 > 100 more rapidly than testosterone or DHT.
Testosterone 2 > 100
Estradiol 5 > 10 SUMMARY
Estrone > 10 > 100
Progesterone > 100 ~2 • The presence of a specific receptor defines the target
Cortisol > 100 ~3 cells for a given hormone.
Corticosterone > 100 ~5
• Receptors are proteins that bind specific hormones
1Affinity expressed as Kd (nmol/L). and generate an intracellular signal (receptor-effector
coupling).
progesterone interact with CBG with sufficient affinity
to compete for cortisol binding. Aldosterone, the most • Some hormones have intracellular receptors; others
potent natural mineralocorticoid, does not have a spe- bind to receptors on the plasma membrane.
cific plasma transport protein. Gonadal steroids bind
very weakly to CBG (Table 42–8). • Hormones are synthesized from a number of precur-
sor molecules, including cholesterol, tyrosine per se,
Gonadal Steroids Are Transported and all the constituent amino acids of peptides and
proteins.
by Sex Hormone-Binding Globulin
• A number of modification processes alter the activity
Most mammals, humans included, have a plasma β- of hormones. For example, many hormones are syn-
globulin that binds testosterone with specificity, rela- thesized from larger precursor molecules.
tively high affinity, and limited capacity (Table 42–8).
This protein, usually called sex hormone-binding • The complement of enzymes in a particular cell type
globulin (SHBG) or testosterone-estrogen-binding allows for the production of a specific class of steroid
globulin (TEBG), is produced in the liver. Its produc- hormone.
tion is increased by estrogens (women have twice the
serum concentration of SHBG as men), certain types of • Most of the lipid-soluble hormones are bound to
liver disease, and hyperthyroidism; it is decreased by rather specific plasma transport proteins.
androgens, advancing age, and hypothyroidism. Many
of these conditions also affect the production of CBG REFERENCES
and TBG. Since SHBG and albumin bind 97–99% of
circulating testosterone, only a small fraction of the Bartalina L: Thyroid hormone-binding proteins: update 1994. En-
hormone in circulation is in the free (biologically ac- docr Rev 1994;13:140.
tive) form. The primary function of SHBG may be to
restrict the free concentration of testosterone in the Beato M et al: Steroid hormone receptors: many actors in search of
serum. Testosterone binds to SHBG with higher affin- a plot. Cell 1995;83:851.
ity than does estradiol (Table 42–8). Therefore, a
change in the level of SHBG causes a greater change in Dai G, Carrasco L, Carrasco N: Cloning and characterization of
the free testosterone level than in the free estradiol level. the thyroid iodide transporter. Nature 1996;379:458.

Estrogens are bound to SHBG and progestins to DeLuca HR: The vitamin D story: a collaborative effort of basic
CBG. SHBG binds estradiol about five times less avidly science and clinical medicine. FASEB J 1988;2:224.
than it binds testosterone or DHT, while progesterone
and cortisol have little affinity for this protein (Table Douglass J, Civelli O, Herbert E: Polyprotein gene expression:
42–8). In contrast, progesterone and cortisol bind with Generation of diversity of neuroendocrine peptides. Annu
nearly equal affinity to CBG, which in turn has little Rev Biochem 1984;53:665.
avidity for estradiol and even less for testosterone,
DHT, or estrone. Miller WL: Molecular biology of steroid hormone biosynthesis.
Endocr Rev 1988;9:295.
These binding proteins also provide a circulating
reservoir of hormone, and because of the relatively large Nagatsu T: Genes for human catecholamine-synthesizing enzymes.
Neurosci Res 1991;12:315.

Russell DW, Wilson JD: Steroid 5 alpha-reductase: two genes/two
enzymes. Annu Rev Biochem 1994;63:25.

Russell J et al: Interaction between calcium and 1,25-dihydroxy-
vitamin D3 in the regulation of preproparathyroid hormone
and vitamin D receptor mRNA in avian parathyroids. En-
docrinology 1993;132:2639.

Steiner DF et al: The new enzymology of precursor processing en-
doproteases. J Biol Chem 1992;267:23435.

Hormone Action & 43
Signal Transduction

Daryl K. Granner, MD

BIOMEDICAL IMPORTANCE as described in Chapter 42, ie, based on the location of
their specific cellular receptors and the type of signals
The homeostatic adaptations an organism makes to a generated. Group I hormones interact with an intracel-
constantly changing environment are in large part ac- lular receptor and group II hormones with receptor
complished through alterations of the activity and recognition sites located on the extracellular surface of
amount of proteins. Hormones provide a major means the plasma membrane of target cells. The cytokines, in-
of facilitating these changes. A hormone-receptor inter- terleukins, and growth factors should also be considered
action results in generation of an intracellular signal in this latter category. These molecules, of critical im-
that can either regulate the activity of a select set of portance in homeostatic adaptation, are hormones in
genes, thereby altering the amount of certain proteins the sense that they are produced in specific cells, have
in the target cell, or affect the activity of specific pro- the equivalent of autocrine, paracrine, and endocrine
teins, including enzymes and transporter or channel actions, bind to cell surface receptors, and activate
proteins. The signal can influence the location of pro- many of the same signal transduction pathways em-
teins in the cell and can affect general processes such as ployed by the more traditional group II hormones.
protein synthesis, cell growth, and replication, perhaps
through effects on gene expression. Other signaling SIGNAL GENERATION
molecules—including cytokines, interleukins, growth
factors, and metabolites—use some of the same general The Ligand-Receptor Complex Is the
mechanisms and signal transduction pathways. Exces- Signal for Group I Hormones
sive, deficient, or inappropriate production and release
of hormones and of these other regulatory molecules The lipophilic group I hormones diffuse through the
are major causes of disease. Many pharmacotherapeutic plasma membrane of all cells but only encounter their
agents are aimed at correcting or otherwise influencing specific, high-affinity intracellular receptors in target
the pathways discussed in this chapter. cells. These receptors can be located in the cytoplasm or
in the nucleus of target cells. The hormone-receptor
HORMONES TRANSDUCE SIGNALS TO complex first undergoes an activation reaction. As
shown in Figure 43–2, receptor activation occurs by at
AFFECT HOMEOSTATIC MECHANISMS least two mechanisms. For example, glucocorticoids
diffuse across the plasma membrane and encounter
The general steps involved in producing a coordinated their cognate receptor in the cytoplasm of target cells.
response to a particular stimulus are illustrated in Ligand-receptor binding results in the dissociation of
Figure 43–1. The stimulus can be a challenge or a heat shock protein 90 (hsp90) from the receptor. This
threat to the organism, to an organ, or to the integrity step appears to be necessary for subsequent nuclear lo-
of a single cell within that organism. Recognition of the calization of the glucocorticoid receptor. This receptor
stimulus is the first step in the adaptive response. At the also contains nuclear localization sequences that assist
organismic level, this generally involves the nervous sys- in the translocation from cytoplasm to nucleus. The
tem and the special senses (sight, hearing, pain, smell, now activated receptor moves into the nucleus (Figure
touch). At the organismic or cellular level, recognition 43–2) and binds with high affinity to a specific DNA
involves physicochemical factors such as pH, O2 ten- sequence called the hormone response element
sion, temperature, nutrient supply, noxious metabo- (HRE). In the case illustrated, this is a glucocorticoid
lites, and osmolarity. Appropriate recognition results in response element, or GRE. Consensus sequences for
the release of one or more hormones that will govern HREs are shown in Table 43–1. The DNA-bound, lig-
generation of the necessary adaptive response. For pur- anded receptor serves as a high-affinity binding site for
poses of this discussion, the hormones are categorized

456

HORMONE ACTION & SIGNAL TRANSDUCTION / 457

STIMULUS Recognition

Group I hormones Group II hormones Hormone release

Hormone•receptor complex Many different signals Signal generation
Effects

Gene Transporters Protein Protein
transcription Channels translocation modification

COORDINATED RESPONSE TO STIMULUS

Figure 43–1. Hormonal involvement in responses to a stimulus. A challenge to the in-
tegrity of the organism elicits a response that includes the release of one or more hormones.
These hormones generate signals at or within target cells, and these signals regulate a vari-
ety of biologic processes which provide for a coordinated response to the stimulus or chal-
lenge. See Figure 43–8 for a specific example.

one or more coactivator proteins, and accelerated gene gests that these hormones exert their dominant effect
transcription typically ensues when this occurs. By con- on modulating gene transcription, but they—and many
trast, certain hormones such as the thyroid hormones of the hormones in the other classes discussed below—
and retinoids diffuse from the extracellular fluid across can act at any step of the “information pathway” illus-
the plasma membrane and go directly into the nucleus. trated in Figure 43–3. Direct actions of steroids in the
In this case, the cognate receptor is already bound to cytoplasm and on various organelles and membranes
the HRE (the thyroid hormone response element have also been described.
[TRE], in this example). However, this DNA-bound
receptor fails to activate transcription because it is com- GROUP II (PEPTIDE &
plexed with a corepressor. Indeed, this receptor- CATECHOLAMINE) HORMONES
corepressor complex serves as an active repressor of gene HAVE MEMBRANE RECEPTORS
transcription. The association of ligand with these re- & USE INTRACELLULAR MESSENGERS
ceptors results in dissociation of the corepressor. The
liganded receptor is now capable of binding one or Many hormones are water-soluble, have no transport
more coactivators with high affinity, resulting in the ac- proteins (and therefore have a short plasma half-life),
tivation of gene transcription. The relationship of hor- and initiate a response by binding to a receptor located
mone receptors to other nuclear receptors and to coreg- in the plasma membrane (see Tables 42–3 and 42–4).
ulators is discussed in more detail below. The mechanism of action of this group of hormones
can best be discussed in terms of the intracellular sig-
By selectively affecting gene transcription and the nals they generate. These signals include cAMP (cyclic
consequent production of appropriate target mRNAs, AMP; 3′,5′-adenylic acid; see Figure 18–5), a nu-
the amounts of specific proteins are changed and meta- cleotide derived from ATP through the action of
bolic processes are influenced. The influence of each of adenylyl cyclase; cGMP, a nucleotide formed by gua-
these hormones is quite specific; generally, the hor- nylyl cyclase; Ca2+; and phosphatidylinositides. Many
mone affects less than 1% of the genes, mRNA, or pro- of these second messengers affect gene transcription, as
teins in a target cell; sometimes only a few are affected. described in the previous paragraph; but they also influ-
The nuclear actions of steroid, thyroid, and retinoid
hormones are quite well defined. Most evidence sug-

458 / CHAPTER 43

−

Cytoplasm +
TRE
− +
TRE TRE

GRE GRE GRE

hsp +
hsp
+

Nucleus

Figure 43–2. Regulation of gene expression by class I hormones.
Steroid hormones readily gain access to the cytoplasmic compartment
of target cells. Glucocorticoid hormones (solid triangles) encounter
their cognate receptor in the cytoplasm, where it exists in a complex
with heat shock protein 90 (hsp). Ligand binding causes dissociation of
hsp and a conformational change of the receptor. The receptor•ligand
complex then traverses the nuclear membrane and binds to DNA with
specificity and high affinity at a glucocorticoid response element (GRE).
This event triggers the assembly of a number of transcription coregula-
tors (᭝+ ), and enhanced transcription ensues. By contrast, thyroid hor-
mones and retinoic acid (᭹) directly enter the nucleus, where their
cognate receptors are already bound to the appropriate response ele-
ments with an associated transcription repressor complex (᭺− ). This
complex, which consists of molecules such as N-CoR or SMRT (see
Table 43–6) in the absence of ligand, actively inhibits transcription. Lig-
and binding results in dissociation of the repressor complex from the
receptor, allowing an activator complex to assemble. The gene is then
actively transcribed.

ence a variety of other biologic processes, as shown in cloned from various mammalian species. A wide variety
Figure 43–1. of responses are mediated by the GPCRs.

G Protein-Coupled Receptors (GPCR) cAMP Is the Intracellular Signal
for Many Responses
Many of the group II hormones bind to receptors that
couple to effectors through a GTP-binding protein in- Cyclic AMP was the first intracellular signal identified
termediary. These receptors typically have seven hy- in mammalian cells. Several components comprise a
drophobic plasma membrane-spanning domains. This system for the generation, degradation, and action of
is illustrated by the seven interconnected cylinders ex- cAMP.
tending through the lipid bilayer in Figure 43–4. Re-
ceptors of this class, which signal through guanine nu- A. ADENYLYL CYCLASE
cleotide-bound protein intermediates, are known as Different peptide hormones can either stimulate (s) or
G protein-coupled receptors, or GPCRs. To date, inhibit (i) the production of cAMP from adenylyl cy-
over 130 G protein-linked receptor genes have been

HORMONE ACTION & SIGNAL TRANSDUCTION / 459

Table 43–1. The DNA sequences of several Gene
hormone response elements (HREs).1

TRANSCRIPTION

Hormone or Effector HRE DNA Sequence

Glucocorticoids GRE Primary transcript Degradation
Progestins MODIFICATION/PROCESSING
Mineralocorticoids PRE ←GGTACA NNN TGTTCT
Androgens MRE ←← NUCLEUS
←
ARE mRNA Degradation
←
Estrogens ERE ←AGGTCA ––– TGA/TCCT Transport

Thyroid hormone TRE mRNA Active inactive
Retinoic acid RARE AGGTCA N3,4,5, AGGTCA
Vitamin D VDRE degradation

cAMP CRE TGACGTCA CYTOPLASM TRANSLATION

1Letters indicate nucleotide; N means any one of the four can be Protein Modification
used in that position. The arrows pointing in opposite directions degradation
illustrate the slightly imperfect inverted palindromes present in
many HREs; in some cases these are called “half binding sites” be- Figure 43–3. The “information pathway.” Informa-
cause each binds one monomer of the receptor. The GRE, PRE, tion flows from the gene to the primary transcript to
MRE, and ARE consist of the same DNA sequence. Specificity may mRNA to protein. Hormones can affect any of the steps
be conferred by the intracellular concentration of the ligand or involved and can affect the rates of processing, degra-
hormone receptor, by flanking DNA sequences not included in dation, or modification of the various products.
the consensus, or by other accessory elements. A second group of
HREs includes those for thyroid hormones, estrogens, retinoic tively. In the case of αs, this modification disrupts the
acid, and vitamin D. These HREs are similar except for the orienta- intrinsic GTP-ase activity; thus, αs cannot reassociate
tion and spacing between the half palindromes. Spacing deter- with βγ and is therefore irreversibly activated. ADP-
mines the hormone specificity. VDRE (N=3), TRE (N=4), and RARE
(N=5) bind to direct repeats rather than to inverted repeats. An- ribosylation of αi-2 prevents the dissociation of αi-2
other member of the steroid receptor superfamily, the retinoid X from βγ, and free αi-2 thus cannot be formed. αs activ-
receptor (RXR), forms heterodimers with VDR, TR, and RARE, and ity in such cells is therefore unopposed.
these constitute the trans-acting factors. cAMP affects gene tran-
scription through the CRE.

There is a large family of G proteins, and these are

clase, which is encoded by at least nine different genes part of the superfamily of GTPases. The G protein

(Table 43–2). Two parallel systems, a stimulatory (s) family is classified according to sequence homology

one and an inhibitory (i) one, converge upon a single into four subfamilies, as illustrated in Table 43–3.

catalytic molecule (C). Each consists of a receptor, Rs or There are 21 α, 5 β, and 8 γ subunit genes. Various
Ri, and a regulatory complex, Gs and Gi. Gs and Gi are
each trimers composed of α, β, and γ subunits. Because combinations of these subunits provide a large number
the α subunit in Gs differs from that in Gi, the pro-
teins, which are distinct gene products, are designated of possible αβγ and cyclase complexes.
αs and αi. The α subunits bind guanine nucleotides.
The β and γ subunits are always associated (βγ) and ap- The α subunits and the βγ complex have actions in-
pear to function as a heterodimer. The binding of a hor-
dependent of those on adenylyl cyclase (see Figure
mone to Rs or Ri results in a receptor-mediated activa- of αi stimulate K+
tion of G, which entails the exchange of GDP by GTP 43–4 and Table 43–3). Some forms and some αs mole-
on α and the concomitant dissociation of βγ from α. channels and inhibit Ca2+ channels,

The αs protein has intrinsic GTPase activity. The cules have the opposite effects. Members of the Gq fam-
active form, αs•GTP, is inactivated upon hydrolysis of ily activate the phospholipase C group of enzymes. The
the GTP to GDP; the trimeric Gs complex (αβγ) is βγ complexes have been associated with K+ channel
then re-formed and is ready for another cycle of activa-
stimulation and phospholipase C activation. G proteins
tion. Cholera and pertussis toxins catalyze the ADP-
ribosylation of αs and αi-2 (see Table 43–3), respec- are involved in many important biologic processes in

addition to hormone action. Notable examples include

olfaction (αOLF) and vision (αt). Some examples are
listed in Table 43–3. GPCRs are implicated in a num-

ber of diseases and are major targets for pharmaceutical

agents.

460 / CHAPTER 43 N
H
N

EE

γ γ
β β αs
αs
GDP
GT P
C C
No hormone: inactive effector Bound hormone (H): active effector

Figure 43–4. Components of the hormone receptor–G protein effector system. Receptors that

couple to effectors through G proteins (GPCR) typically have seven membrane-spanning domains. In
the absence of hormone (left), the heterotrimeric G-protein complex (α, β, γ) is in an inactive guano-
sine diphosphate (GDP)-bound form and is probably not associated with the receptor. This complex is
anchored to the plasma membrane through prenylated groups on the βγ subunits (wavy lines) and

᭛perhaps by myristoylated groups on α subunits (not shown). On binding of hormone ( H ) to the re-

ceptor, there is a presumed conformational change of the receptor—as indicated by the tilted mem-

brane spanning domains—and activation of the G-protein complex. This results from the exchange of
GDP with guanosine triphosphate (GTP) on the α subunit, after which α and βγ dissociate. The α sub-
unit binds to and activates the effector (E). E can be adenylyl cyclase, Ca2+, Na+, or Cl− channels (αs), or
it could be a K+ channel (αi), phospholipase Cβ (αq), or cGMP phosphodiesterase (αt). The βγ subunit
can also have direct actions on E. (Modified and reproduced, with permission, from Granner DK in: Princi-
ples and Practice of Endocrinology and Metabolism, 3rd ed. Becker KL [editor]. Lippincott, 2000.)

Table 43–2. Subclassification of group II.A B. PROTEIN KINASE
hormones.
In prokaryotic cells, cAMP binds to a specific protein
Hormones That Stimulate Hormones That Inhibit called catabolite regulatory protein (CRP) that binds
Adenylyl Cyclase Adenylyl Cyclase directly to DNA and influences gene expression. In eu-
(Hs) (Hl) karyotic cells, cAMP binds to a protein kinase called
protein kinase A (PKA) that is a heterotetrameric mol-
ACTH Acetylcholine ecule consisting of two regulatory subunits (R) and two
ADH α2-Adrenergics catalytic subunits (C). cAMP binding results in the fol-
β-Adrenergics Angiotensin II lowing reaction:
Calcitonin Somatostatin
CRH 4 cAMP + R2C2 a R2 ⋅(4 cAMP) + 2C
FSH
Glucagon The R2C2 complex has no enzymatic activity, but
hCG the binding of cAMP by R dissociates R from C,
LH thereby activating the latter (Figure 43–5). The active
LPH C subunit catalyzes the transfer of the γ phosphate of
MSH ATP to a serine or threonine residue in a variety of pro-
PTH teins. The consensus phosphorylation sites are -Arg-
TSH Arg/Lys-X-Ser/Thr- and -Arg-Lys-X-X-Ser-, where X
can be any amino acid.

Protein kinase activities were originally described as
being “cAMP-dependent” or “cAMP-independent.” This

HORMONE ACTION & SIGNAL TRANSDUCTION / 461

Table 43–3. Classes and functions of selected G proteins.1,2

Class or Type Stimulus Effector Effect
Gs
Glucagon, β-adrenergics ↑ Adenylyl cyclase Gluconeogenesis, lipolysis,
αs ↑ Cardiac Ca2+, Cl−, and Na+ channels glycogenolysis

αolf Odorant ↑ Adenylyl cyclase Olfaction

Gi Acetylcholine, ↓ Adenylyl cyclase Slowed heart rate
αi-1,2,3 ↑ Potassium channels
α2-adrenergics ↓ Calcium channels
M2 cholinergics

α0 Opioids, endorphins ↑ Potassium channels Neuronal electrical activity

αt Light ↑ cGMP phosphodiesterase Vision

Gq M1 cholinergics ↑ Phospholipase C-β1 ↑ Muscle contraction
αq α1-Adrenergics ↑ Phospholipase c-β2 and

α11 α1-Adrenergics ↑ Blood pressure

G12 ? Cl− channel ?
α12

1Modified and reproduced, with permission, from Granner DK in: Principles and Practice of Endocrinology and Metabolism, 3rd

ed. Becker KL (editor). Lippincott, 2000.
2The four major classes or families of mammalian G proteins (Gs, Gi, Gq, and G12) are based on protein sequence homology.
Representative members of each are shown, along with known stimuli, effectors, and well-defined biologic effects. Nine iso-

forms of adenylyl cyclase have been identified (isoforms I–IX). All isoforms are stimulated by αs; αi isoforms inhibit types V
and VI, and α0 inhibits types I and V. At least 16 different α subunits have been identified.

classification has changed, as protein phosphorylation is ment binding protein (CREB). CREB binds to a
now recognized as being a major regulatory mecha- cAMP responsive element (CRE) (see Table 43–1) in
nism. Several hundred protein kinases have now been its nonphosphorylated state and is a weak activator of
described. The kinases are related in sequence and transcription. When phosphorylated by PKA, CREB
structure within the catalytic domain, but each is a binds the coactivator CREB-binding protein CBP/
unique molecule with considerable variability with re- p300 (see below) and as a result is a much more potent
spect to subunit composition, molecular weight, au- transcription activator.
tophosphorylation, Km for ATP, and substrate speci-
ficity. D. PHOSPHODIESTERASES

C. PHOSPHOPROTEINS Actions caused by hormones that increase cAMP con-
centration can be terminated in a number of ways, in-
The effects of cAMP in eukaryotic cells are all thought cluding the hydrolysis of cAMP to 5′-AMP by phos-
to be mediated by protein phosphorylation-dephosphor- phodiesterases (see Figure 43–5). The presence of these
ylation, principally on serine and threonine residues. hydrolytic enzymes ensures a rapid turnover of the sig-
The control of any of the effects of cAMP, including nal (cAMP) and hence a rapid termination of the bio-
such diverse processes as steroidogenesis, secretion, ion logic process once the hormonal stimulus is removed.
transport, carbohydrate and fat metabolism, enzyme in- There are at least 11 known members of the phospho-
duction, gene regulation, synaptic transmission, and diesterase family of enzymes. These are subject to regu-
cell growth and replication, could be conferred by a lation by their substrates, cAMP and cGMP; by hor-
specific protein kinase, by a specific phosphatase, or by mones; and by intracellular messengers such as calcium,
specific substrates for phosphorylation. These substrates probably acting through calmodulin. Inhibitors of
help define a target tissue and are involved in defining phosphodiesterase, most notably methylated xanthine
the extent of a particular response within a given cell. derivatives such as caffeine, increase intracellular cAMP
For example, the effects of cAMP on gene transcription and mimic or prolong the actions of hormones through
are mediated by the protein cyclic AMP response ele- this signal.

462 / CHAPTER 43

ATP • Mg2+ R2C2
Inactive PKA
Active
adenylyl cAMP C2 + R2
cyclase Phosphodiesterase Active PKA
5′-AMP
Cell
membrane

Mg2+ • ATP

Protein Phosphoprotein

Phosphatase

Physiologic
effects

Figure 43–5. Hormonal regulation of cellular processes through
cAMP-dependent protein kinase (PKA). PKA exists in an inactive form as
an R2C2 heterotetramer consisting of two regulatory and two catalytic
subunits. The cAMP generated by the action of adenylyl cyclase (acti-
vated as shown in Figure 43–4) binds to the regulatory (R) subunit of
PKA. This results in dissociation of the regulatory and catalytic subunits
and activation of the latter. The active catalytic subunits phosphorylate
a number of target proteins on serine and threonine residues. Phos-
phatases remove phosphate from these residues and thus terminate
the physiologic response. A phosphodiesterase can also terminate the
response by converting cAMP to 5′-AMP.

E. PHOSPHOPROTEIN PHOSPHATASES heat-stable protein inhibitors regulate type I phos-
phatase activity. Inhibitor-1 is phosphorylated and acti-
Given the importance of protein phosphorylation, it is vated by cAMP-dependent protein kinases; and in-
not surprising that regulation of the protein dephos- hibitor-2, which may be a subunit of the inactive
phorylation reaction is another important control phosphatase, is also phosphorylated, possibly by glyco-
mechanism (see Figure 43–5). The phosphoprotein gen synthase kinase-3.
phosphatases are themselves subject to regulation by
phosphorylation-dephosphorylation reactions and by a cGMP Is Also an Intracellular Signal
variety of other mechanisms, such as protein-protein
interactions. In fact, the substrate specificity of the Cyclic GMP is made from GTP by the enzyme gua-
phosphoserine-phosphothreonine phosphatases may be nylyl cyclase, which exists in soluble and membrane-
dictated by distinct regulatory subunits whose binding bound forms. Each of these isozymes has unique physi-
is regulated hormonally. The best-studied role of regu- ologic properties. The atriopeptins, a family of peptides
lation by the dephosphorylation of proteins is that of produced in cardiac atrial tissues, cause natriuresis, di-
glycogen metabolism in muscle. Two major types of uresis, vasodilation, and inhibition of aldosterone secre-
phosphoserine-phosphothreonine phosphatases have tion. These peptides (eg, atrial natriuretic factor) bind
been described. Type I preferentially dephosphorylates to and activate the membrane-bound form of guanylyl
the β subunit of phosphorylase kinase, whereas type II cyclase. This results in an increase of cGMP by as much
dephosphorylates the α subunit. Type I phosphatase is as 50-fold in some cases, and this is thought to mediate
implicated in the regulation of glycogen synthase, phos- the effects mentioned above. Other evidence links
phorylase, and phosphorylase kinase. This phosphatase cGMP to vasodilation. A series of compounds, includ-
is itself regulated by phosphorylation of certain of its ing nitroprusside, nitroglycerin, nitric oxide, sodium
subunits, and these reactions are reversed by the action nitrite, and sodium azide, all cause smooth muscle re-
of one of the type II phosphatases. In addition, two

HORMONE ACTION & SIGNAL TRANSDUCTION / 463

laxation and are potent vasodilators. These agents in- B. CALMODULIN
crease cGMP by activating the soluble form of guanylyl
cyclase, and inhibitors of cGMP phosphodiesterase (the The calcium-dependent regulatory protein is calmod-
drug sildenafil [Viagra], for example) enhance and pro- ulin, a 17-kDa protein that is homologous to the mus-
long these responses. The increased cGMP activates cle protein troponin C in structure and function.
cGMP-dependent protein kinase (PKG), which in turn Calmodulin has four Ca2+ binding sites, and full occu-
phosphorylates a number of smooth muscle proteins. pancy of these sites leads to a marked conformational
Presumably, this is involved in relaxation of smooth change, which allows calmodulin to activate enzymes
muscle and vasodilation. and ion channels. The interaction of Ca2+ with calmod-
ulin (with the resultant change of activity of the latter)
Several Hormones Act Through is conceptually similar to the binding of cAMP to PKA
Calcium or Phosphatidylinositols and the subsequent activation of this molecule.
Calmodulin can be one of numerous subunits of com-
Ionized calcium is an important regulator of a variety of plex proteins and is particularly involved in regulating
cellular processes, including muscle contraction, stimu- various kinases and enzymes of cyclic nucleotide gener-
lus-secretion coupling, the blood clotting cascade, en- ation and degradation. A partial list of the enzymes reg-
zyme activity, and membrane excitability. It is also an ulated directly or indirectly by Ca2+, probably through
intracellular messenger of hormone action. calmodulin, is presented in Table 43–4.

A. CALCIUM METABOLISM In addition to its effects on enzymes and ion trans-
port, Ca2+/calmodulin regulates the activity of many
The extracellular calcium (Ca2+) concentration is about structural elements in cells. These include the actin-
5 mmol/L and is very rigidly controlled. Although sub- myosin complex of smooth muscle, which is under β-
stantial amounts of calcium are associated with intracel- adrenergic control, and various microfilament-medi-
lular organelles such as mitochondria and the endoplas- ated processes in noncontractile cells, including cell
mic reticulum, the intracellular concentration of free or motility, cell conformation changes, mitosis, granule re-
ionized calcium (Ca2+) is very low: 0.05–10 µmol/L. In lease, and endocytosis.
spite of this large concentration gradient and a favor-
able transmembrane electrical gradient, Ca2+ is re- C. CALCIUM IS A MEDIATOR OF HORMONE ACTION
strained from entering the cell. A considerable amount A role for Ca2+ in hormone action is suggested by the
of energy is expended to ensure that the intracellular observations that the effect of many hormones is (1)
Ca2+ is controlled, as a prolonged elevation of Ca2+ in blunted by Ca2+-free media or when intracellular cal-
the cell is very toxic. A Na+/Ca2+ exchange mechanism cium is depleted; (2) can be mimicked by agents that
that has a high capacity but low affinity pumps Ca2+ increase cytosolic Ca2+, such as the Ca2+ ionophore
out of cells. There also is a Ca2+/proton ATPase-depen- A23187; and (3) influences cellular calcium flux. The
dent pump that extrudes Ca2+ in exchange for H+. This regulation of glycogen metabolism in liver by vaso-
has a high affinity for Ca2+ but a low capacity and is pressin and α-adrenergic catecholamines provides a
probably responsible for fine-tuning cytosolic Ca2+. good example. This is shown schematically in Figures
Furthermore, Ca2+ ATPases pump Ca2+ from the cy- 18–6 and 18–7.
tosol to the lumen of the endoplasmic reticulum. There
are three ways of changing cytosolic Ca2+: (1) Certain Table 43–4. Enzymes and proteins regulated by
hormones (class II.C, Table 42–3) by binding to recep- calcium or calmodulin.
tors that are themselves Ca2+ channels, enhance mem-
brane permeability to Ca2+ and thereby increase Ca2+ Adenylyl cyclase
influx. (2) Hormones also indirectly promote Ca2+ in- Ca2+-dependent protein kinases
flux by modulating the membrane potential at the Ca2+-Mg2+ ATPase
plasma membrane. Membrane depolarization opens Ca2+-phospholipid-dependent protein kinase
voltage-gated Ca2+ channels and allows for Ca2+ influx. Cyclic nucleotide phosphodiesterase
(3) Ca2+ can be mobilized from the endoplasmic reticu- Some cytoskeletal proteins
lum, and possibly from mitochondrial pools. Some ion channels (eg, L-type calcium channels)
Nitric oxide synthase
An important observation linking Ca2+ to hormone Phosphorylase kinase
action involved the definition of the intracellular targets Phosphoprotein phosphatase 2B
of Ca2+ action. The discovery of a Ca2+-dependent reg- Some receptors (eg, NMDA-type glutamate receptor)
ulator of phosphodiesterase activity provided the basis
for a broad understanding of how Ca2+ and cAMP in-
teract within cells.

464 / CHAPTER 43

A number of critical metabolic enzymes are regu- face receptors such as those for acetylcholine, antidi-
lated by Ca2+, phosphorylation, or both, including
glycogen synthase, pyruvate kinase, pyruvate carboxy- uretic hormone, and α1-type catecholamines are, when
lase, glycerol-3-phosphate dehydrogenase, and pyruvate occupied by their respective ligands, potent activators
dehydrogenase.
of phospholipase C. Receptor binding and activation of
D. PHOSPHATIDYLINOSITIDE METABOLISM AFFECTS
CA2+-DEPENDENT HORMONE ACTION phospholipase C are coupled by the Gq isoforms (Table
43–3 and Figure 43–6). Phospholipase C catalyzes the
Some signal must provide communication between the
hormone receptor on the plasma membrane and the in- hydrolysis of phosphatidylinositol 4,5-bisphosphate to
tracellular Ca2+ reservoirs. This is accomplished by
products of phosphatidylinositol metabolism. Cell sur- inositol trisphosphate (IP3) and 1,2-diacylglycerol (Fig-
ure 43–7). Diacylglycerol is itself capable of activating

protein kinase C (PKC), the activity of which also de-
pends upon Ca2+. IP3, by interacting with
tracellular receptor, is an effective releaser a specific in-
of Ca2+ from

Ca2+

PIP2 Receptor Diacylglycerol
Endoplasmic reticulum G protein
Phospholipase C +
Mitochondrion Protein kinase C
Inositol–P3
(IP3) (PKC)

Ca2+
Calmodulin

Ca2+-Calmodulin

++

Specific Multifunctional

calmodulin kinase calmodulin kinase

Proteins Phosphoproteins
Physiologic responses
Other
proteins

Figure 43–6. Certain hormone-receptor interactions result in the activation of phospholipase C. This ap-

pears to involve a specific G protein, which also may activate a calcium channel. Phospholipase C results in
generation of inositol trisphosphate (IP3), which liberates stored intracellular Ca2+, and diacylglycerol (DAG),
a potent activator of protein kinase C (PKC). In this scheme, the activated PKC phosphorylates specific sub-
strates, which then alter physiologic processes. Likewise, the Ca2+-calmodulin complex can activate specific

kinases, two of which are shown here. These actions result in phosphorylation of substrates, and this leads to
altered physiologic responses. This figure also shows that Ca2+ can enter cells through voltage- or ligand-
gated Ca2+ channels. The intracellular Ca2+ is also regulated through storage and release by the mitochon-

dria and endoplasmic reticulum. (Courtesy of JH Exton.)

HORMONE ACTION & SIGNAL TRANSDUCTION / 465

R1 R2

P

OH OH P Phospholipase C R1 R2 OH

OH 1,2-Diacylglycerol
(DAG)

P
P

Phosphatidylinositol 4,5-bisphosphate OH OH P
(PIP2)
Figure 43–7. Phospholipase C cleaves PIP2
into diacylglycerol and inositol trisphosphate. R1 OH
generally is stearate, and R2 is usually arachido-
nate. IP3 can be dephosphorylated (to the inac- P
tive I-1,4-P2) or phosphorylated (to the potentially
active I-1,3,4,5-P4). Inositol 1,4,5-trisphosphate
(IP3)

intracellular storage sites in the endoplasmic reticulum. ligand-activated tyrosine kinase activity. Several recep-
Thus, the hydrolysis of phosphatidylinositol 4,5-bis- tors—generally those involved in binding ligands in-
phosphate leads to activation of PKC and promotes an volved in growth control, differentiation, and the in-
increase of cytoplasmic Ca2+. As shown in Figure 43–4, flammatory response—either have intrinsic tyrosine
the activation of G proteins can also have a direct ac- kinase activity or are associated with proteins that are
tion on Ca2+ channels. The resulting elevations of cy- tyrosine kinases. Another distinguishing feature of this
tosolic Ca2+ activate Ca2+–calmodulin-dependent kinases class of hormone action is that these kinases preferen-
and many other Ca2+–calmodulin-dependent enzymes. tially phosphorylate tyrosine residues, and tyrosine
phosphorylation is infrequent (< 0.03% of total amino
Steroidogenic agents—including ACTH and cAMP acid phosphorylation) in mammalian cells. A third dis-
in the adrenal cortex; angiotensin II, K+, serotonin, tinguishing feature is that the ligand-receptor interac-
ACTH, and cAMP in the zona glomerulosa of the tion that results in a tyrosine phosphorylation event ini-
adrenal; LH in the ovary; and LH and cAMP in the tiates a cascade that may involve several protein kinases,
Leydig cells of the testes—have been associated with in- phosphatases, and other regulatory proteins.
creased amounts of phosphatidic acid, phosphatidyl-
inositol, and polyphosphoinositides (see Chapter 14) in A. INSULIN TRANSMITS SIGNALS
the respective target tissues. Several other examples
could be cited. BY SEVERAL KINASE CASCADES

The roles that Ca2+ and polyphosphoinositide break- The insulin, epidermal growth factor (EGF), and IGF-I
down products might play in hormone action are pre- receptors have intrinsic protein tyrosine kinase activities
sented in Figure 43–6. In this scheme the activated pro- located in their cytoplasmic domains. These activities
tein kinase C can phosphorylate specific substrates, are stimulated when the receptor binds ligand. The re-
which then alter physiologic processes. Likewise, the ceptors are then autophosphorylated on tyrosine
Ca2+-calmodulin complex can activate specific kinases. residues, and this initiates a complex series of events
These then modify substrates and thereby alter physio- (summarized in simplified fashion in Figure 43–8). The
logic responses. phosphorylated insulin receptor next phosphorylates
insulin receptor substrates (there are at least four of
Some Hormones Act Through these molecules, called IRS 1–4) on tyrosine residues.
Phosphorylated IRS binds to the Src homology 2
a Protein Kinase Cascade (SH2) domains of a variety of proteins that are directly
involved in mediating different effects of insulin. One
Single protein kinases such as PKA, PKC, and Ca2+- of these proteins, PI-3 kinase, links insulin receptor ac-
calmodulin (CaM)-kinases, which result in the phos- tivation to insulin action through activation of a num-
phorylation of serine and threonine residues in target ber of molecules, including the kinase PDK1 (phospho-
proteins, play a very important role in hormone action. inositide-dependent kinase-1). This enzyme propagates
The discovery that the EGF receptor contains an intrin- the signal through several other kinases, including PKB
sic tyrosine kinase activity that is activated by the bind- (akt), SKG, and aPKC (see legend to Figure 43–8 for
ing of the ligand EGF was an important breakthrough. definitions and expanded abbreviations). An alternative
The insulin and IGF-I receptors also contain intrinsic

466 / CHAPTER 43 INSULIN

RECOGNITION
(HYPERGLYCEMIA)

P-Y Y-P YY

IRS 1-4 IRS 1-4
YY

SIGNAL P-Y Y-P GRB2/mSOS

GENERATION +

PTEN PI3 - p21Ras
Raf-1
+ kinase MEK
MAP
PKB mTOR
SGK kinase
aPKC +
? p70S6K

Protein translocation Enzyme activity Gene transcription Cell growth
DNA synthesis
EFFECTS Glucose transporter Insulin receptor PEPCK HKII Early response
Insulin receptor Protein phosphatases Glucagon Glucokinase
IGF-II receptor Phosphodiesterases* IGFBP1 > 100 others genes

Others

Figure 43–8. Insulin signaling pathways. The insulin signaling pathways provide an excellent example of the
“recognition → hormone release → signal generation → effects” paradigm outlined in Figure 43–1. Insulin is re-
leased in response to hyperglycemia. Binding of insulin to a target cell-specific plasma membrane receptor results in
a cascade of intracellular events. Stimulation of the intrinsic tyrosine kinase activity of the insulin receptor marks the
initial event, resulting in increased tyrosine (Y) phosphorylation (Y → Y-P) of the receptor and then one or more of
the insulin receptor substrate molecules (IRS 1–4). This increase in phosphotyrosine stimulates the activity of many
intracellular molecules such as GTPases, protein kinases, and lipid kinases, all of which play a role in certain meta-
bolic actions of insulin. The two best-described pathways are shown. First, phosphorylation of an IRS molecule
(probably IRS-2) results in docking and activation of the lipid kinase, PI-3 kinase, which generates novel inositol lipids
that may act as “second messenger” molecules. These, in turn, activate PDK1 and then a variety of downstream sig-
naling molecules, including protein kinase B (PKB or akt), SGK, and aPKC. An alternative pathway involves the activa-
tion of p70S6K and perhaps other as yet unidentified kinases. Second, phosphorylation of IRS (probably IRS-1) re-
sults in docking of GRB2/mSOS and activation of the small GTPase, p21RAS, which initiates a protein kinase cascade
that activates Raf-1, MEK, and the p42/p44 MAP kinase isoforms. These protein kinases are important in the regula-
tion of proliferation and differentiation of several cell types. The mTOR pathway provides an alternative way of acti-
vating p70S6K and appears to be involved in nutrient signaling as well as insulin action. Each of these cascades may
influence different physiologic processes, as shown. Each of the phosphorylation events is reversible through the
action of specific phosphatases. For example, the lipid phosphatase PTEN dephosphorylates the product of the PI-3
kinase reaction, thereby antagonizing the pathway and terminating the signal. Representative effects of major ac-
tions of insulin are shown in each of the boxes. The asterisk after phosphodiesterase indicates that insulin indirectly
affects the activity of many enzymes by activating phosphodiesterases and reducing intracellular cAMP levels.
(IGFBP, insulin-like growth factor binding protein; IRS 1–4, insulin receptor substrate isoforms 1–4); PI-3 kinase, phos-
phatidylinositol 3-kinase; PTEN, phosphatase and tensin homolog deleted on chromosome 10; PKD1, phosphoinosi-
tide-dependent kinase; PKB, protein kinase B; SGK, serum and glucocorticoid-regulated kinase; aPKC, atypical pro-
tein kinase C; p70S6K, p70 ribosomal protein S6 kinase; mTOR, mammalian target of rapamycin; GRB2, growth factor
receptor binding protein 2; mSOS, mammalian son of sevenless; MEK, MAP kinase kinase and ERK kinase; MAP
kinase, mitogen-activated protein kinase.)

HORMONE ACTION & SIGNAL TRANSDUCTION / 467

pathway downstream from PKD1 involves p70S6K and balancing actions of phosphatases. Two mechanisms
perhaps other as yet unidentified kinases. A second major are employed to initiate this cascade. Some hormones,
pathway involves mTOR. This enzyme is directly regu- such as growth hormone, prolactin, erythropoietin, and
lated by amino acids and insulin and is essential for the cytokines, initiate their action by activating a tyro-
p70S6K activity. This pathway provides a distinction sine kinase, but this activity is not an integral part of
between the PKB and p70S6K branches downstream the hormone receptor. The hormone-receptor interac-
from PKD1. These pathways are involved in protein tion promotes binding and activation of cytoplasmic
translocation, enzyme activity, and the regulation, by protein tyrosine kinases, such as Tyk-2, Jak1, or
insulin, of genes involved in metabolism (Figure 43–8). Jak2. These kinases phosphorylate one or more cyto-
Another SH2 domain-containing protein is GRB2, plasmic proteins, which then associate with other dock-
which binds to IRS-1 and links tyrosine phosphoryla- ing proteins through binding to SH2 domains. One
tion to several proteins, the result of which is activation such interaction results in the activation of a family of
of a cascade of threonine and serine kinases. A pathway cytosolic proteins called signal transducers and activa-
showing how this insulin-receptor interaction activates tors of transcription (STATs). The phosphorylated
the mitogen-activated protein (MAP) kinase pathway STAT protein dimerizes and translocates into the nu-
and the anabolic effects of insulin is illustrated in Fig- cleus, binds to a specific DNA element such as the in-
ure 43–8. The exact roles of many of these docking terferon response element, and activates transcription.
proteins, kinases, and phosphatases remain to be estab- This is illustrated in Figure 43–9. Other SH2 docking
lished. events may result in the activation of PI 3-kinase, the
MAP kinase pathway (through SHC or GRB2), or G
B. THE JAK/STAT PATHWAY IS USED protein-mediated activation of phospholipase C (PLCγ)
with the attendant production of diacylglycerol and ac-
BY HORMONES AND CYTOKINES tivation of protein kinase C. It is apparent that there is
a potential for “cross-talk” when different hormones ac-
Tyrosine kinase activation can also initiate a phosphor- tivate these various signal transduction pathways.
ylation and dephosphorylation cascade that involves the
action of several other protein kinases and the counter-

Ligand

RR RR RR

JAK JAK P JAK JAK P P JAK JAK P

PP P P STAT P
PP P
P
X SH2 P

x = SHC Dimerization
GRB2 and
PLCγ
PI-3K nuclear
GAP translocation

Figure 43–9. Initiation of signal transduction by receptors linked to Jak ki-
nases. The receptors (R) that bind prolactin, growth hormone, interferons, and cy-
tokines lack endogenous tyrosine kinase. Upon ligand binding, these receptors
dimerize and an associated protein (Jak1, Jak2, or TYK) is phosphorylated. Jak-P,
an active kinase, phosphorylates the receptor on tyrosine residues. The STAT pro-
teins associate with the phosphorylated receptor and then are themselves phos-
phorylated by Jak-P. STAT᭺P dimerizes, translocates to the nucleus, binds to spe-
cific DNA elements, and regulates transcription. The phosphotyrosine residues of
the receptor also bind to several SH2 domain-containing proteins. This results in
activation of the MAP kinase pathway (through SHC or GRB2), PLCγ, or PI-3 kinase.

468 / CHAPTER 43

C. THE NF-␬B PATHWAY IS Glucocorticoid hormones are therapeutically useful
agents for the treatment of a variety of inflammatory and
REGULATED BY GLUCOCORTICOIDS immune diseases. Their anti-inflammatory and im-
munomodulatory actions are explained in part by the in-
The transcription factor NF-κB is a heterodimeric hibition of NF-κB and its subsequent actions. Evidence
complex typically composed of two subunits termed for three mechanisms for the inhibition of NF-κB by
p50 and p65 (Figure 43–10). Normally, NF-κB is kept glucocorticoids has been presented: (1) Glucocorticoids
sequestered in the cytoplasm in a transcriptionally inac- increase IκB mRNA, which leads to an increase of IκB
tive form by members of the inhibitor of NF-κB (IκB) protein and more efficient sequestration of NF-κB in the
family. Extracellular stimuli such as proinflammatory cytoplasm. (2) The glucocorticoid receptor competes
cytokines, reactive oxygen species, and mitogens lead to with NF-κB for binding to coactivators. (3) The gluco-
activation of the IκB kinase complex, IKK, which is a corticoid receptor directly binds to the p65 subunit of
heterohexameric structure consisting of α, β, and γ sub- NF-κB and inhibits its activation (Figure 43–10).
units. IKK phosphorylates IκB on two serine residues,
and this targets IκB for ubiquitination and subsequent HORMONES CAN INFLUENCE
degradation by the proteasome. Following IκB degra- SPECIFIC BIOLOGIC EFFECTS BY
dation, free NF-κB can now translocate to the nucleus, MODULATING TRANSCRIPTION
where it binds to a number of gene promoters and acti-
vates transcription, particularly of genes involved in the The signals generated as described above have to be
inflammatory response. Transcriptional regulation by translated into an action that allows the cell to effec-
NF-κB is mediated by a variety of coactivators such as tively adapt to a challenge (Figure 43–1). Much of this
CREB binding protein (CBP), as described below (Fig-
ure 43–13).

NF-κB Activators

Proinflammatory cytokines

Bacterial and viral infection

Reactive oxygen species

Mitogens IKK

complex

γγ PP
IκB
αββα Proteasome
1 Ubiquitin

IκB p50 p65 Cytoplasm
p50 p65 Nucleus
Coactivators
p50 p65 2

3

Target gene

Figure 43–10. Regulation of the NF-κB pathway. NF-κB consists of two sub-
units, p50 and p65, which regulate transcription of many genes when in the
nucleus. NF-κB is restricted from entering the nucleus by IκB, an inhibitor of
NF-κB. IκB binds to—and masks—the nuclear localization signal of NF-κB.
This cytoplasmic protein is phosphorylated by an IKK complex which is acti-

vated by cytokines, reactive oxygen species, and mitogens. Phosphorylated
IκB can be ubiquitinylated and degraded, thus releasing its hold on NF-κB. Glu-
cocorticoids affect many steps in this process, as described in the text.

HORMONE ACTION & SIGNAL TRANSDUCTION / 469

Figure 43–11. The hormone response transcrip- AFE codGinegneregion
tion unit. The hormone response transcription unit HRE
is an assembly of DNA elements and bound pro- AFE
teins that interact, through protein-protein interac-
tions, with a number of coactivator or corepressor AF
molecules. An essential component is the hormone R
response element which binds the ligand ( )- R
bound receptor (R). Also important are the acces- AF p160
sory factor elements (AFEs) with bound transcrip-
tion factors. More than two dozen of these p300
accessory factors (AFs), which are often members of
the nuclear receptor superfamily, have been linked BTC
to hormone effects on transcription. The AFs can in-
teract with each other, with the liganded nuclear
receptors, or with coregulators. These components
communicate with the basal transcription complex
through a coregulator complex that can consist of
one or more members of the p160, corepressor,
mediator-related, or CBP/p300 families (see
Table 43–6).

adaptation is accomplished through alterations in the tion initiation site, but they may be located within the
rates of transcription of specific genes. Many different coding region of the gene, in introns. HREs were de-
observations have led to the current view of how hor- fined by the strategy illustrated in Figure 39–11. The
mones affect transcription. Some of these are as follows: consensus sequences illustrated in Table 43–1 were ar-
(1) Actively transcribed genes are in regions of “open” rived at through analysis of several genes regulated by a
chromatin (defined by a susceptibility to the enzyme given hormone using simple, heterologous reporter sys-
DNase I), which allows for the access of transcription tems (see Figure 39–10). Although these simple HREs
factors to DNA. (2) Genes have regulatory regions, and bind the hormone-receptor complex more avidly than
transcription factors bind to these to modulate the fre- surrounding DNA—or DNA from an unrelated
quency of transcription initiation. (3) The hormone- source—and confer hormone responsiveness to a re-
receptor complex can be one of these transcription fac- porter gene, it soon became apparent that the regula-
tors. The DNA sequence to which this binds is called a tory circuitry of natural genes must be much more
hormone response element (HRE; see Table 43–1 for complicated. Glucocorticoids, progestins, mineralocor-
examples). (4) Alternatively, other hormone-generated ticoids, and androgens have vastly different physiologic
signals can modify the location, amount, or activity of actions. How could the specificity required for these ef-
transcription factors and thereby influence binding to fects be achieved through regulation of gene expression
the regulatory or response element. (5) Members of a by the same HRE (Table 43–1)? Questions like this
large superfamily of nuclear receptors act with—or in a have led to experiments which have allowed for elabora-
manner analogous to—hormone receptors. (6) These tion of a very complex model of transcription regula-
nuclear receptors interact with another large group of tion. For example, the HRE must associate with other
coregulatory molecules to effect changes in the tran- DNA elements (and associated binding proteins) to
scription of specific genes. function optimally. The extensive sequence similarity
noted between steroid hormone receptors, particularly
Several Hormone Response Elements in their DNA-binding domains, led to discovery of the
(HREs) Have Been Defined nuclear receptor superfamily of proteins. These—and
a large number of coregulator proteins—allow for a
Hormone response elements resemble enhancer ele- wide variety of DNA-protein and protein-protein inter-
ments in that they are not strictly dependent on posi- actions and the specificity necessary for highly regulated
tion and location. They generally are found within a physiologic control. A schematic of such an assembly is
few hundred nucleotides upstream (5′) of the transcrip- illustrated in Figure 43–11.

470 / CHAPTER 43

A/B C D E F

N AF-1 DBD Hinge LBD AF-2 C

××

GR, MR, PR TR, RAR, VDR COUP-TF, TR2, GEN8
AR, ER PPARα, β, γ HNF-4, TLX

FXR, CAR, LXR,

PXR/SXR

Receptors: Steroid class RXR partnered Orphans
Binding: Homodimers Homodimers
Ligand: Steroids Heterodimers ?
DNA element: Inverted 9-Cis RA + (x) Direct repeats
repeat Direct repeats

Figure 43–12. The nuclear receptor superfamily. Members of this family are
divided into six structural domains (A–F). Domain A/B is also called AF-1, or the
modulator region, because it is involved in activating transcription. The C do-
main consists of the DNA-binding domain (DBD). The D region contains the
hinge, which provides flexibility between the DBD and the ligand-binding do-
main (LBD, region E). The amino (N) terminal part of region E contains AF-2, an-
other domain important for transactivation. The F region is poorly defined. The
functions of these domains are discussed in more detail in the text. Receptors
with known ligands, such as the steroid hormones, bind as homodimers on in-
verted repeat half-sites. Other receptors form heterodimers with the partner
RXR on direct repeat elements. There can be nucleotide spacers of one to five
bases between these direct repeats (DR1–5). Another class of receptors for
which ligands have not been determined (orphan receptors) bind as homo-
dimers to direct repeats and occasionally as monomers to a single half-site.

There Is a Large Family of Nuclear receptor [RXR] partner), or as monomers. The target
response element consists of one or two half-site con-
Receptor Proteins sensus sequences arranged as an inverted or direct re-
peat. The spacing between the latter helps determine
The nuclear receptor superfamily consists of a diverse set binding specificity. Thus, a direct repeat with three,
of transcription factors that were discovered because of a four, or five nucleotide spacer regions specifies the
sequence similarity in their DNA-binding domains. This binding of the vitamin D, thyroid, and retinoic acid re-
family, now with more than 50 members, includes the ceptors, respectively, to the same consensus response
nuclear hormone receptors discussed above, a number of element (Table 43–1). A multifunctional ligand-
other receptors whose ligands were discovered after the binding domain (LBD) is located in the carboxyl ter-
receptors were identified, and many putative or orphan minal half of the receptor. The LBD binds hormones
receptors for which a ligand has yet to be discovered. or metabolites with selectivity and thus specifies a par-
ticular biologic response. The LBD also contains do-
These nuclear receptors have several common struc- mains that mediate the binding of heat shock proteins,
tural features (Figure 43–12). All have a centrally lo- dimerization, nuclear localization, and transactivation.
cated DNA-binding domain (DBD) that allows the The latter function is facilitated by the carboxyl termi-
receptor to bind with high affinity to a response ele- nal transcription activation function (AF-2 domain),
ment. The DBD contains two zinc finger binding mo- which forms a surface required for the interaction with
tifs (see Figure 39–14) that direct binding either as ho-
modimers, as heterodimers (usually with a retinoid X

HORMONE ACTION & SIGNAL TRANSDUCTION / 471

Group I GH, Prl,

Insulin, GPCR Hormones Cytokines, etc
EGF, etc
TNF, etc

cAMP Retinoic acid, Jak

RAS IRS estrogen, Plasma
membrane
MEK vitamin D,
MAPK NFκB•IκB
glucocorticoids, NFκB

PKA etc STATs

Nuclear STATs NFκB Nuclear
CREB receptors membrane
AP-1

CBP
p300

Figure 43–13. Several signal transduction pathways converge on
CBP/p300. Ligands that associate with membrane or nuclear receptors even-
tually converge on CBP/p300. Several different signal transduction pathways
are employed. EGF, epidermal growth factor; GH, growth hormone; Prl, pro-
lactin; TNF, tumor necrosis factor; other abbreviations are expanded in the
text.

coactivators. A highly variable hinge region separates Another group of orphan receptors that as yet have no
the DBD from the LBD. This region provides flexibil- known ligand bind as homodimers or monomers to di-
ity to the receptor, so it can assume different DNA- rect repeat sequences.
binding conformations. Finally, there is a highly vari-
able amino terminal region that contains another trans- As illustrated in Table 43–5, the discovery of the nu-
activation domain referred to as AF-1. Less well de- clear receptor superfamily has led to an important un-
fined, the AF-1 domain may provide for distinct derstanding of how a variety of metabolites and xenobi-
physiologic functions through the binding of different otics regulate gene expression and thus the metabolism,
coregulator proteins. This region of the receptor, detoxification, and elimination of normal body prod-
through the use of different promoters, alternative ucts and exogenous agents such as pharmaceuticals.
splice sites, and multiple translation initiation sites, Not surprisingly, this area is a fertile field for investiga-
provides for receptor isoforms that share DBD and tion of new therapeutic interventions.
LBD identity but exert different physiologic responses
because of the association of various coregulators with A Large Number of Nuclear Receptor
this variable amino terminal AF-1 domain. Coregulators Also Participate
in Regulating Transcription
It is possible to sort this large number of receptors
into groups in a variety of ways. Here they are discussed Chromatin remodeling, transcription factor modification
according to the way they bind to their respective DNA by various enzyme activities, and the communication
elements (Figure 43–12). Classic hormone receptors for between the nuclear receptors and the basal transcrip-
glucocorticoids (GR), mineralocorticoids (MR), estro- tion apparatus are accomplished by protein-protein in-
gens (ER), androgens (AR), and progestins (PR) bind teractions with one or more of a class of coregulator
as homodimers to inverted repeat sequences. Other molecules. The number of these coregulator molecules
hormone receptors such as thyroid (TR), retinoic acid now exceeds 100, not counting species variations and
(RAR), and vitamin D (VDR) and receptors that bind splice variants. The first of these to be described was the
various metabolite ligands such as PPAR α β, and γ, CREB-binding protein, CBP. CBP, through an
FXR, LXR, PXR/SXR, and CAR bind as heterodimers, amino terminal domain, binds to phosphorylated serine
with retinoid X receptor (RXR) as a partner, to direct 137 of CREB and mediates transactivation in response
repeat sequences (see Figure 43–12 and Table 43–5). to cAMP. It thus is described as a coactivator. CBP and

472 / CHAPTER 43

Table 43–5. Nuclear receptors with special ligands.1

Receptor Partner Ligand Process Affected

Peroxisome PPARα RXR (DR1) Fatty acids Peroxisome proliferation

Proliferator- PPARβ Fatty acids

activated PPARγ Fatty acids Lipid and carbohydrate metabolism

Eicosanoids,

thiazolidinediones

Farnesoid X FXR RXR (DR4) Farnesol, bile acids Bile acid metabolism

Liver X LXR RXR (DR4) Oxysterols Cholesterol metabolism

Xenobiotic X CAR RXR (DR5) Androstanes

Phenobarbital Protection against certain drugs, toxic

Xenobiotics metabolites, and xenobiotics

PXR RXR (DR3) Pregnanes

Xenobiotics

1Many members of the nuclear receptor superfamily were discovered by cloning, and the corresponding
ligands were then identified. These ligands are not hormones in the classic sense, but they do have a
similar function in that they activate specific members of the nuclear receptor superfamily. The recep-
tors described here form heterodimers with RXR and have variable nucleotide sequences separating the
direct repeat binding elements (DR1–5). These receptors regulate a variety of genes encoding cy-
tochrome p450s (CYP), cytosolic binding proteins, and ATP-binding cassette (ABC) transporters to influ-
ence metabolism and protect cells against drugs and noxious agents.

its close relative, p300, interact directly or indirectly Table 43–6. Some mammalian coregulator
with a number of signaling molecules, including activa- proteins.
tor protein-1 (AP-1), signal transducers and activators
of transcription (STATs), nuclear receptors, and CREB I. 300-kDa family of coactivators
(Figure 39–11). CBP/p300 also binds to the p160
family of coactivators described below and to a number CBP CREB-binding protein
of other proteins, including viral transcription factor
Ela, the p90rsk protein kinase, and RNA helicase A. It is p300 Protein of 300 kDa
important to note that CBP/p300 also has intrinsic
histone acetyltransferase (HAT) activity. The impor- II. 160-kDa family of coactivators
tance of this is described below. Some of the many ac-
tions of CBP/p300, which appear to depend on intrin- A. SRC-1 Steroid receptor coactivator 1
sic enzyme activities and its ability to serve as a scaffold
for the binding of other proteins, are illustrated in Fig- NCoA-1 Nuclear receptor coactivator 1
ure 43–11. Other coregulators may serve similar func-
tions. B. TIF2 Transcriptional intermediary factor 2

Several other families of coactivator molecules have GRIP1 Glucocorticoid receptor-interacting protein
been described. Members of the p160 family of coac-
tivators, all of about 160 kDa, include (1) SRC-1 and NCoA-2 Nuclear receptor coactivator 2
NCoA-1; (2) GRIP 1, TIF2, and NCoA-2; and (3)
p/CIP, ACTR, AIB1, RAC3, and TRAM-1 (Table C. p/CIP p300/CBP cointegrator-associated protein 1
43–6). The different names for members within a sub-
family often represent species variations or minor splice ACTR Activator of the thyroid and retinoic acid
variants. There is about 35% amino acid identity be-
tween members of the different subfamilies. The p160 receptors
coactivators share several properties. They (1) bind
nuclear receptors in an agonist and AF-2 transactiva- AIB Amplified in breast cancer
tion domain-dependent manner; (2) have a conserved
amino terminal basic helix-loop-helix (bHLH) motif RAC3 Receptor-associated coactivator 3
(see Chapter 39); (3) have a weak carboxyl terminal
transactivation domain and a stronger amino terminal TRAM-1 TR activator molecule 1

III. Corepressors

NCoR Nuclear receptor corepressor

SMRT Silencing mediator for RXR and TR

IV. Mediator-related proteins

TRAPs Thyroid hormone receptor-associated

proteins

DRIPs Vitamin D receptor-interacting proteins

ARC Activator-recruited cofactor

HORMONE ACTION & SIGNAL TRANSDUCTION / 473

transactivation domain in a region that is required for • Many hormone responses are accomplished through
the CBP/p16O interaction; (4) contain at least three of alterations in the rate of transcription of specific
the LXXLL motifs required for protein-protein inter- genes.
action with other coactivators; and (5) often have HAT
activity. The role of HAT is particularly interesting, as • The nuclear receptor superfamily of proteins plays a
mutations of the HAT domain disable many of these central role in the regulation of gene transcription.
transcription factors. Current thinking holds that these
HAT activities acetylate histones and result in remodel- • These receptors, which may have hormones, metabo-
ing of chromatin into a transcription-efficient environ- lites, or drugs as ligands, bind to specific DNA ele-
ment; however, other protein substrates for HAT- ments as homodimers or as heterodimers with RXR.
mediated acetylation have been reported. Histone Some—orphan receptors—have no known ligand
acetylation/deacetylation is proposed to play a critical but bind DNA and influence transcription.
role in gene expression.
• Another large family of coregulator proteins remodel
A small number of proteins, including NCoR and chromatin, modify other transcription factors, and
SMRT, comprise the corepressor family. They func- bridge the nuclear receptors to the basal transcription
tion, at least in part, as described in Figure 43–2. An- apparatus.
other family includes the TRAPs, DRIPs, and ARC
(Table 43–6). These so-called mediator-related pro- REFERENCES
teins range in size from 80 kDa to 240 kDa and are
thought to be involved in linking the nuclear receptor- Arvanitakis L et al: Constitutively signaling G-protein-coupled re-
coactivator complex to RNA polymerase II and the ceptors and human disease. Trends Endocrinol Metab
other components of the basal transcription apparatus. 1998;9:27.

The exact role of these coactivators is presently Berridge M: Inositol triphosphate and calcium signalling. Nature
under intensive investigation. Many of these proteins 1993;361:315.
have intrinsic enzymatic activities. This is particularly
interesting in view of the fact that acetylation, phos- Chawla A et al: Nuclear receptors and lipid physiology: opening the
phorylation, methylation, and ubiquitination—as well X files. Science 2001;294:1866.
as proteolysis and cellular translocation—have been
proposed to alter the activity of some of these coregula- Darnell JE Jr, Kerr IM, Stark GR: Jak-STAT pathways and trans-
tors and their targets. criptional activation in response to IFNs and other extracellu-
lar signaling proteins. Science 1994;264:1415.
It appears that certain combinations of coregula-
tors—and thus different combinations of activators and Fantl WJ, Johnson DE, Williams LT: Signalling by receptor tyro-
inhibitors—are responsible for specific ligand-induced sine kinases. Annu Rev Biochem 1993;62:453.
actions through various receptors.
Giguère V: Orphan nuclear receptors: from gene to function. En-
SUMMARY docr Rev 1999;20:689.

• Hormones, cytokines, interleukins, and growth fac- Grunstein M: Histone acetylation in chromatin structure and trans-
tors use a variety of signaling mechanisms to facilitate cription. Nature 1997;389:349.
cellular adaptive responses.
Hanoune J, Defer N: Regulation and role of adenylyl cyclase iso-
• The ligand-receptor complex serves as the initial sig- forms. Annu Rev Pharmacol Toxicol 2001;41:145.
nal for members of the nuclear receptor family.
Hermanson O, Glass CK, Rosenfeld MG: Nuclear receptor coregu-
• Class II hormones, which bind to cell surface recep- lators: multiple modes of receptor modification. Trends En-
tors, generate a variety of intracellular signals. These docrinol Metab 2002;13:55.
include cAMP, cGMP, Ca2+, phosphatidylinositides,
and protein kinase cascades. Jaken S: Protein kinase C isozymes and substrates. Curr Opin Cell
Biol 1996;8:168.

Lucas P, Granner D: Hormone response domains in gene transcrip-
tion. Annu Rev Biochem 1992;61:1131.

Montminy M: Transcriptional regulation by cyclic AMP. Annu
Rev Biochem 1997;66:807

Morris AJ, Malbon CC: Physiological regulation of G protein-
linked signaling. Physiol Rev 1999;79:1373.

Walton KM, Dixon JE: Protein tyrosine phosphatases. Annu Rev
Biochem 1993;62:101.

SECTION VI

Special Topics

Nutrition, Digestion, & Absorption 44

David A. Bender, PhD, & Peter A. Mayes, PhD, DSc

BIOMEDICAL IMPORTANCE and steatorrhea. Lactose intolerance is due to lactase
deficiency leading to diarrhea and intestinal discomfort.
Besides water, the diet must provide metabolic fuels Absorption of intact peptides that stimulate antibody
(mainly carbohydrates and lipids), protein (for growth responses causes allergic reactions, and celiac disease
and turnover of tissue proteins), fiber (for roughage), is an allergic reaction to wheat gluten.
minerals (elements with specific metabolic functions),
and vitamins and essential fatty acids (organic com- DIGESTION & ABSORPTION
pounds needed in small amounts for essential metabolic OF CARBOHYDRATES
and physiologic functions). The polysaccharides, tri-
acylglycerols, and proteins that make up the bulk of the The digestion of complex carbohydrates is by hydroly-
diet must be hydrolyzed to their constituent monosac- sis to liberate oligosaccharides, then free mono- and di-
charides, fatty acids, and amino acids, respectively, be- saccharides. The increase in blood glucose after a test
fore absorption and utilization. Minerals and vitamins dose of a carbohydrate compared with that after an
must be released from the complex matrix of food be- equivalent amount of glucose is known as the glycemic
fore they can be absorbed and utilized. index. Glucose and galactose have an index of 1, as do
lactose, maltose, isomaltose, and trehalose, which give
Globally, undernutrition is widespread, leading to rise to these monosaccharides on hydrolysis. Fructose
impaired growth, defective immune systems, and re- and the sugar alcohols are absorbed less rapidly and
duced work capacity. By contrast, in developed coun- have a lower glycemic index, as does sucrose. The
tries, there is often excessive food consumption (espe- glycemic index of starch varies between near 1 to near
cially of fat), leading to obesity and to the development zero due to variable rates of hydrolysis, and that of non-
of cardiovascular disease and some forms of cancer. De- starch polysaccharides is zero. Foods that have a low
ficiencies of vitamin A, iron, and iodine pose major glycemic index are considered to be more beneficial
health concerns in many countries, and deficiencies of since they cause less fluctuation in insulin secretion.
other vitamins and minerals are a major cause of ill
health. In developed countries, nutrient deficiency is Amylases Catalyze
rare, though there are vulnerable sections of the popula- the Hydrolysis of Starch
tion at risk. Intakes of minerals and vitamins that are
adequate to prevent deficiency may be inadequate to The hydrolysis of starch by salivary and pancreatic
promote optimum health and longevity. amylases catalyze random hydrolysis of α(1→4) glyco-
side bonds, yielding dextrins, then a mixture of glucose,
Excessive secretion of gastric acid, associated with maltose, and isomaltose (from the branch points in
Helicobacter pylori infection, can result in the develop- amylopectin).
ment of gastric and duodenal ulcers; small changes in
the composition of bile can result in crystallization of
cholesterol as gallstones; failure of exocrine pancreatic
secretion (as in cystic fibrosis) leads to undernutrition

474

NUTRITION, DIGESTION, & ABSORPTION / 475

Disaccharidases Are Brush Glucose SGLT 1
Border Enzymes Na+ transporter
protein
The disaccharidases—maltase, sucrase-isomaltase (a
bifunctional enzyme catalyzing hydrolysis of sucrose and Glucose
isomaltose), lactase, and trehalase—are located on the Galactose Glucose
brush border of the intestinal mucosal cells where the re- Fructose Galactose
sultant monosaccharides and others arising from the diet
are absorbed. In most people, apart from those of north- GLUT 5
ern European genetic origin, lactase is gradually lost
through adolescence, leading to lactose intolerance. Brush
Lactose remains in the intestinal lumen, where it is a border
substrate for bacterial fermentation to lactate, resulting
in discomfort and diarrhea. Na+ -K+ Intestinal
pump epithelium
There Are Two Separate Mechanisms
for the Absorption of Monosaccharides Na+ ATP
in the Small Intestine 3Na+

Glucose and galactose are absorbed by a sodium-depen- Glucose 2K+ 2K+
dent process. They are carried by the same transport Fructose
protein (SGLT 1) and compete with each other for in- Galactose ADP
testinal absorption (Figure 44–1). Other monosaccha- + Pi
rides are absorbed by carrier-mediated diffusion. Be-
cause they are not actively transported, fructose and To capillaries GLUT 2
sugar alcohols are only absorbed down their concentra-
tion gradient, and after a moderately high intake some Figure 44–1. Transport of glucose, fructose, and
may remain in the intestinal lumen, acting as a sub-
strate for bacterial fermentation. galactose across the intestinal epithelium. The SGLT 1
transporter is coupled to the Na+-K+ pump, allowing
DIGESTION & ABSORPTION OF LIPIDS
glucose and galactose to be transported against their
The major lipids in the diet are triacylglycerols and, to a concentration gradients. The GLUT 5 Na+-independent
lesser extent, phospholipids. These are hydrophobic
molecules and must be hydrolyzed and emulsified to facilitative transporter allows fructose as well as glu-
very small droplets (micelles) before they can be ab-
sorbed. The fat-soluble vitamins—A, D, E, and K— cose and galactose to be transported with their con-
and a variety of other lipids (including cholesterol) are
absorbed dissolved in the lipid micelles. Absorption of centration gradients. Exit from the cell for all the sugars
the fat-soluble vitamins is impaired on a very low fat
diet. is via the GLUT 2 facilitative transporter.

Hydrolysis of triacylglycerols is initiated by lingual of the products of lipid digestion into micelles and lipo-
and gastric lipases that attack the sn-3 ester bond, form- somes together with phospholipids and cholesterol
ing 1,2-diacylglycerols and free fatty acids, aiding emul- from the bile. Because the micelles are soluble, they
sification. Pancreatic lipase is secreted into the small allow the products of digestion, including the fat-
intestine and requires a further pancreatic protein, coli- soluble vitamins, to be transported through the aqueous
pase, for activity. It is specific for the primary ester environment of the intestinal lumen and permit close
links—ie, positions 1 and 3 in triacylglycerols—result- contact with the brush border of the mucosal cells, al-
ing in 2-monoacylglycerols and free fatty acids as the lowing uptake into the epithelium, mainly of the je-
major end-products of luminal triacylglycerol digestion. junum. The bile salts pass on to the ileum, where
Monoacylglycerols are hydrolyzed with difficulty to most are absorbed into the enterohepatic circula-
glycerol and free fatty acids, so that less than 25% of in- tion (Chapter 26). Within the intestinal epithelium,
gested triacylglycerol is completely hydrolyzed to glyc- 1-monoacylglycerols are hydrolyzed to fatty acids and
erol and fatty acids (Figure 44–2). Bile salts, formed in glycerol and 2-monoacylglycerols are re-acylated to tri-
the liver and secreted in the bile, enable emulsification acylglycerols via the monoacylglycerol pathway. Glyc-
erol released in the intestinal lumen is not reutilized but
passes into the portal vein; glycerol released within the

INTESTINAL
LUMEN

1 Acyl PANCREATIC Acyl
2 Acyl LIPASE Acyl
OH
3 FA
Acyl

Triacylglycerol, 100% 1,2-Diacylglycerol OH
Acyl
PANCREATIC OH
LIPASE

FA 72%

Absorption OH
from Acyl

bile salt OH
micelle 2-Mono-
acylglycerol

476 ISOMERASE ACYL-CoA
SYNTHETASE
FA Acyl-
Acyl ATP, CoA CoA
OH
ACYL-CoA ATP

OH SYNTHETASE CoA

1-Mono- 6% FA
acylglycerol
Acyl
PANCREATIC OH AT
LIPASE OH
INTESTIN
FA LIPASE

OH
OH

OH
Glycerol

22%

Figure 44–2. Digestion and absorption of triacylglycer

widely but indicate the relative importance of the three ro

INTESTINAL LYMPHATIC ch44.qxd 3/16/04 11:07 AM Page 476
EPITHELIUM VESSELS

(LACTEALS)

Monoacylglycerol pathway Acyl
Acyl
Acyl

Triacylglycerol Acyl
Acyl

Acyl
Chylomicrons

Phosphatidic acid pathway Acyl
Acyl
P Acyl
A

OH OH
TP ATP

OH OH

NAL OH GLYCEROL P
E KINASE

Glycerol Glycerol
3-phosphate

Glycolysis PORTAL VEIN
Glycerol

rols. The values given for percentage uptake may vary
outes shown.

NUTRITION, DIGESTION, & ABSORPTION / 477

epithelium is reutilized for triacylglycerol synthesis via several different amino acid transporters, with specificity
the normal phosphatidic acid pathway (Chapter 24). for the nature of the amino acid side chain (large or
All long-chain fatty acids absorbed are converted to tri- small; neutral, acidic, or basic). The various amino acids
acylglycerol in the mucosal cells and, together with the carried by any one transporter compete with each other
other products of lipid digestion, secreted as chylomi- for absorption and tissue uptake. Dipeptides and tripep-
crons into the lymphatics, entering the blood stream via tides enter the brush border of the intestinal mucosal
the thoracic duct (Chapter 25). cells, where they are hydrolyzed to free amino acids,
which are then transported into the hepatic portal vein.
DIGESTION & ABSORPTION OF PROTEINS Relatively large peptides may be absorbed intact, either
by uptake into mucosal epithelial cells (transcellular) or
Few peptide bonds that are hydrolyzed by proteolytic by passing between epithelial cells (paracellular). Many
enzymes are accessible without prior denaturation of di- such peptides are large enough to stimulate antibody for-
etary proteins (by heat in cooking and by the action of mation—this is the basis of allergic reactions to foods.
gastric acid).
DIGESTION & ABSORPTION
Several Groups of Enzymes Catalyze
the Digestion of Proteins OF VITAMINS & MINERALS

There are two main classes of proteolytic digestive en- Vitamins and minerals are released from food during
zymes (proteases), with different specificities for the digestion—though this is not complete—and the avail-
amino acids forming the peptide bond to be hydrolyzed. ability of vitamins and minerals depends on the type of
Endopeptidases hydrolyze peptide bonds between spe- food and, especially for minerals, the presence of chelat-
cific amino acids throughout the molecule. They are the ing compounds. The fat-soluble vitamins are absorbed
first enzymes to act, yielding a larger number of smaller in the lipid micelles that result from fat digestion;
fragments, eg, pepsin in the gastric juice and trypsin, water-soluble vitamins and most mineral salts are
chymotrypsin, and elastase secreted into the small in- absorbed from the small intestine either by active trans-
testine by the pancreas. Exopeptidases catalyze the hy- port or by carrier-mediated diffusion followed by bind-
drolysis of peptide bonds, one at a time, from the ends ing to intracellular binding proteins to achieve concen-
of polypeptides. Carboxypeptidases, secreted in the tration upon uptake. Vitamin B12 absorption requires a
pancreatic juice, release amino acids from the free car- specific transport protein, intrinsic factor; calcium ab-
boxyl terminal, and aminopeptidases, secreted by the sorption is dependent on vitamin D; zinc absorption
intestinal mucosal cells, release amino acids from the probably requires a zinc-binding ligand secreted by the
amino terminal. Dipeptides, which are not substrates for exocrine pancreas; and the absorption of iron is limited.
exopeptidases, are hydrolyzed in the brush border of in-
testinal mucosal cells by dipeptidases. Calcium Absorption Is Dependent
on Vitamin D
The proteases are secreted as inactive zymogens; the
active site of the enzyme is masked by a small region of In addition to its role in regulating calcium homeosta-
its peptide chain, which is removed by hydrolysis of a sis, vitamin D is required for the intestinal absorption
specific peptide bond. Pepsinogen is activated to pepsin of calcium. Synthesis of the intracellular calcium-
by gastric acid and by activated pepsin (autocatalysis). In binding protein, calbindin, required for calcium ab-
the small intestine, trypsinogen, the precursor of sorption, is induced by vitamin D, which also affects
trypsin, is activated by enteropeptidase, which is se- the permeability of the mucosal cells to calcium, an ef-
creted by the duodenal epithelial cells; trypsin can then fect that is rapid and independent of protein synthesis.
activate chymotrypsinogen to chymotrypsin, proelas-
tase to elastase, procarboxypeptidase to carboxypepti- Phytic acid (inositol hexaphosphate) in cereals binds
dase, and proaminopeptidase to aminopeptidase. calcium in the intestinal lumen, preventing its absorp-
tion. Other minerals, including zinc, are also chelated
Free Amino Acids & Small Peptides Are by phytate. This is mainly a problem among people
Absorbed by Different Mechanisms who consume large amounts of unleavened whole
wheat products; yeast contains an enzyme, phytase,
The end product of the action of endopeptidases and which dephosphorylates phytate, so rendering it inac-
exopeptidases is a mixture of free amino acids, di- and tive. High concentrations of fatty acids in the intestinal
tripeptides, and oligopeptides, all of which are absorbed. lumen—as a result of impaired fat absorption—can
Free amino acids are absorbed across the intestinal mu- also reduce calcium absorption by forming insoluble
cosa by sodium-dependent active transport. There are calcium salts; a high intake of oxalate can sometimes
cause deficiency, since calcium oxalate is insoluble.

478 / CHAPTER 44

Iron Absorption Is Limited lized is carbohydrate, fat, or protein. Measurement of
& Strictly Controlled but Is
Enhanced by Vitamin C & Ethanol the ratio of the volume of carbon dioxide produced to

Although iron deficiency is a common problem, about volume of oxygen consumed (respiratory quotient; RQ)
10% of the population are genetically at risk of iron
overload (hemochromatosis), and elemental iron can is an indication of the mixture of metabolic fuels being
lead to nonenzymic generation of free radicals. Absorp-
tion of iron is strictly regulated. Inorganic iron is accu- oxidized (Table 27–1). A more recent technique per-
mulated in intestinal mucosal cells bound to an intra-
cellular protein, ferritin. Once the ferritin in the cell is mits estimation of total energy expenditure over a pe-
saturated with iron, no more can enter. Iron can only
leave the mucosal cell if there is transferrin in plasma riod of 1–2 weeks using dual isotopically labeled water,
to bind to. Once transferrin is saturated with iron, any 21H8O21i8sOlo. s2tHinisbolothst
that has accumulated in the mucosal cells will be lost from the body only in water, while
when the cells are shed. As a result of this mucosal bar- water and carbon dioxide; the differ-
rier, only about 10% of dietary iron is normally ab-
sorbed and only 1–5% from many plant foods. ence in the rate of loss of the two labels permits estima-

Inorganic iron is absorbed only in the Fe2+ (reduced) tion of total carbon dioxide production and thus oxy-
state, and for that reason the presence of reducing agents
will enhance absorption. The most effective compound gen consumption and energy expenditure.
is vitamin C, and while intakes of 40–60 mg of vitamin
C per day are more than adequate to meet requirements, Basal metabolic rate (BMR) is the energy expendi-
an intake of 25–50 mg per meal will enhance iron ab-
sorption, especially when iron salts are used to treat iron ture by the body when at rest—but not asleep—under
deficiency anemia. Ethanol and fructose also enhance
iron absorption. Heme iron from meat is absorbed sepa- controlled conditions of thermal neutrality, measured
rately and is considerably more available than inorganic
iron. However, the absorption of both inorganic and at about 12 hours after the last meal, and depends on
heme iron is impaired by calcium—a glass of milk with
a meal significantly reduces availability. weight, age, and gender. Total energy expenditure de-

ENERGY BALANCE: pends on the basal metabolic rate, the energy required

OVER- & UNDERNUTRITION for physical activity, and the energy cost of synthesizing

After the provision of water, the body’s first requirement reserves in the fed state. It is therefore possible to calcu-
is for metabolic fuels—fats, carbohydrates, and amino
acids from proteins (and ethanol) (Table 27–1). Food in- late an individual’s energy requirement from body
take in excess of energy expenditure leads to obesity,
while intake less than expenditure leads to emaciation weight, age, gender, and level of physical activity. Body
and wasting, as in marasmus and kwashiorkor. Both
obesity and severe undernutrition are associated with in- weight affects BMR because there is a greater amount
creased mortality. The body mass index, defined as
weight in kilograms divided by height in meters squared, of active tissue in a larger body. The decrease in BMR
is commonly used as a way of expressing relative obesity
to height. A desirable range is between 20 and 25. with increasing age, even when body weight remains

Energy Requirements Are Estimated by constant, is due to muscle tissue replacement by adi-
Measurement of Energy Expenditure
pose tissue, which is metabolically much less active.
Energy expenditure can be determined directly by mea-
suring heat output from the body but is normally esti- Similarly, women have a significantly lower BMR than
mated indirectly from the consumption of oxygen.
There is an energy expenditure of 20 kJ/L of oxygen do men of the same body weight because women’s bod-
consumed regardless of whether the fuel being metabo-
ies have proportionately more adipose tissue than men.

Energy Requirements Increase
With Activity

The most useful way of expressing the energy cost of
physical activities is as a multiple of BMR. Sedentary
activities use only about 1.1–1.2 × BMR. By contrast,
vigorous exertion, such as climbing stairs, cross-country
skiing, walking uphill, etc, may use 6–8 × BMR.

Ten Percent of the Energy Yield of a Meal
May Be Expended in Forming Reserves

There is a considerable increase in metabolic rate after a
meal, a phenomenon known as diet-induced thermo-
genesis. A small part of this is the energy cost of secret-
ing digestive enzymes and of active transport of the
products of digestion; the major part is due to synthe-
sizing reserves of glycogen, triacylglycerol, and protein.

There Are Two Extreme Forms
of Undernutrition

Marasmus can occur in both adults and children and
occurs in vulnerable groups of all populations. Kwash-

iorkor only affects children and has only been reported NUTRITION, DIGESTION, & ABSORPTION / 479
in developing countries. The distinguishing feature of
kwashiorkor is that there is fluid retention, leading to the liver due to accumulation of fat. It was formerly be-
edema. Marasmus is a state of extreme emaciation; it is lieved that the cause of kwashiorkor was a lack of pro-
the outcome of prolonged negative energy balance. Not tein, with a more or less adequate energy intake; how-
only have the body’s fat reserves been exhausted, but ever, analysis of the diets of affected children shows that
there is wastage of muscle as well, and as the condition this is not so. Children with kwashiorkor are less
progresses there is loss of protein from the heart, liver, stunted than those with marasmus, and the edema be-
and kidneys. The amino acids released by the catabo- gins to improve early in treatment, when the child is
lism of tissue proteins are used as a source of metabolic still receiving a low-protein diet. Very commonly, an
fuel and as substrates for gluconeogenesis to maintain a infection precipitates kwashiorkor. Superimposed on
supply of glucose for the brain and red blood cells. As a general food deficiency, there is probably a deficiency
result of the reduced synthesis of proteins, there is im- of the antioxidant nutrients such as zinc, copper,
paired immune response and more risk from infections. carotene, and vitamins C and E. The respiratory burst
Impairment of cell proliferation in the intestinal mu- in response to infection leads to the production of oxy-
cosa occurs, resulting in reduction in surface area of the gen and halogen free radicals as part of the cytotoxic
intestinal mucosa and reduction in absorption of such action of stimulated macrophages. This added oxidant
nutrients as are available. stress may well trigger the development of kwashiorkor.

Patients With Advanced Cancer PROTEIN & AMINO ACID REQUIREMENTS
& AIDS Are Malnourished
Protein Requirements Can Be Determined
Patients with advanced cancer, HIV infection and by Measuring Nitrogen Balance
AIDS, and a number of other chronic diseases are fre-
quently undernourished—the condition is called The state of protein nutrition can be determined by
cachexia. Physically, they show all the signs of maras- measuring the dietary intake and output of nitrogenous
mus, but there is considerably more loss of body pro- compounds from the body. Although nucleic acids also
tein than occurs in starvation. The secretion of cy- contain nitrogen, protein is the major dietary source of
tokines in response to infection and cancer increases the nitrogen and measurement of total nitrogen intake
catabolism of tissue protein. This differs from maras- gives a good estimate of protein intake (mg N × 6.25 =
mus, in which protein synthesis is reduced but catabo- mg protein, as nitrogen is 16% of most proteins). The
lism in unaffected. Patients are hypermetabolic, ie, output of nitrogen from the body is mainly in urea and
there is a considerable increase in basal metabolic rate. smaller quantities of other compounds in urine and
Many tumors metabolize glucose anaerobically to re- undigested protein in feces, and significant amounts
lease lactate. This is used for gluconeogenesis in the may also be lost in sweat and shed skin.
liver, which is energy-consuming with a net cost of six
ATP for each mole of glucose cycled (Chapter 19). The difference between intake and output of nitroge-
There is increased stimulation of uncoupling proteins nous compounds is known as nitrogen balance. Three
by cytokines, leading to thermogenesis and increased states can be defined: In a healthy adult, nitrogen bal-
oxidation of metabolic fuels. Futile cycling of lipids oc- ance is in equilibrium when intake equals output, and
curs because hormone-sensitive lipase is activated by a there is no change in the total body content of protein.
proteoglycan secreted by tumors, resulting in liberation In a growing child, a pregnant woman, or in recovery
of fatty acids from adipose tissue and ATP-expensive from protein loss, the excretion of nitrogenous com-
reesterification in the liver to triacylglycerols, which are pounds is less than the dietary intake and there is net re-
exported in VLDL. tention of nitrogen in the body as protein, ie, positive
nitrogen balance. In response to trauma or infection—
Kwashiorkor Affects or if the intake of protein is inadequate to meet require-
Undernourished Children ments—there is net loss of protein nitrogen from the
body, ie, negative nitrogen balance. The continual ca-
In addition to the wasting of muscle tissue, loss of in- tabolism of tissue proteins creates the requirement for
testinal mucosa, and impaired immune responses seen dietary protein even in an adult who is not growing,
in marasmus, children with kwashiorkor show a num- though some of the amino acids released can be reuti-
ber of characteristic features. The defining characteristic lized. Nitrogen balance studies show that the average
is edema, associated with a decreased concentration of daily requirement is 0.6 g of protein per kilogram of
plasma proteins. In addition, there is enlargement of body weight (the factor 0.75 should be used to allow for
individual variation), or approximately 50 g/d. Average
intakes of protein in developed countries are about
80–100 g/d, ie, 14–15% of energy intake. Because

480 / CHAPTER 44 aloacetate, and α-ketoglutarate, respectively). The re-
maining amino acids are considered as nonessential, but
growing children are increasing the protein in the body, under some circumstances the requirement for them
they have a proportionately greater requirement than may outstrip the organism’s capacity for synthesis.
adults and should be in positive nitrogen balance. Even
so, the need is relatively small compared with the re- SUMMARY
quirement for protein turnover. In some countries, pro-
tein intake may be inadequate to meet these require- • Digestion involves hydrolyzing food molecules into
ments, resulting in stunting of growth. smaller molecules for absorption through the
gastrointestinal epithelium. Polysaccharides are
There Is a Loss of Body Protein in absorbed as monosaccharides; triacylglycerols as
Response to Trauma & Infection 2-monoacylglycerols, fatty acids, and glycerol; and
proteins as amino acids.
One of the metabolic reactions to major trauma, such
as a burn, a broken limb, or surgery, is an increase in • Digestive disorders arise as a result of (1) enzyme de-
the net catabolism of tissue proteins. As much as 6–7% ficiency, eg, lactase and sucrase; (2) malabsorption,
of the total body protein may be lost over 10 days. Pro- eg, of glucose and galactose due to defects in the
longed bed rest results in considerable loss of protein Na+-glucose cotransporter (SGLT 1); (3) absorption
because of atrophy of muscles. Protein is catabolized as of unhydrolyzed polypeptides, leading to immuno-
normal, but without the stimulus of exercise it is not logic responses, eg, as in celiac disease; and (4) pre-
completely replaced. Lost protein is replaced during cipitation of cholesterol from bile as gallstones.
convalescence, when there is positive nitrogen balance.
A normal diet is adequate to permit this replacement. • Besides water, the diet must provide metabolic fuels
(carbohydrate and fat) for bodily growth and activity;
The Requirement Is Not for Protein Itself protein for synthesis of tissue proteins; fiber for
but for Specific Amino Acids roughage; minerals for specific metabolic functions;
certain polyunsaturated fatty acids of the n-3 and n-6
Not all proteins are nutritionally equivalent. More of families for eicosanoid synthesis and other functions;
some than of others is needed to maintain nitrogen and vitamins, organic compounds needed in small
balance because different proteins contain different amounts for many varied essential functions.
amounts of the various amino acids. The body’s re-
quirement is for specific amino acids in the correct • Twenty different amino acids are required for pro-
proportions to replace the body proteins. The amino tein synthesis, of which nine are essential in the
acids can be divided into two groups: essential and human diet. The quantity of protein required is af-
nonessential. There are nine essential or indispensable fected by protein quality, energy intake, and physical
amino acids, which cannot be synthesized in the body: activity.
histidine, isoleucine, leucine, lysine, methionine, phen-
ylalanine, threonine, tryptophan, and valine. If one of • Undernutrition occurs in two extreme forms: maras-
these is lacking or inadequate, then—regardless of the mus in adults and children and kwashiorkor in chil-
total intake of protein—it will not be possible to main- dren. Overnutrition from excess energy intake is as-
tain nitrogen balance since there will not be enough of sociated with diseases such as obesity, type 2 diabetes
that amino acid for protein synthesis. mellitus, atherosclerosis, cancer, and hypertension.

Two amino acids—cysteine and tyrosine—can be REFERENCES
synthesized in the body, but only from essential amino
acid precursors (cysteine from methionine and tyrosine Bender DA, Bender AE: Nutrition: A Reference Handbook. Oxford
from phenylalanine). The dietary intakes of cysteine Univ Press, 1997.
and tyrosine thus affect the requirements for methio-
nine and phenylalanine. The remaining 11 amino acids Büller HA, Grand RJ: Lactose intolerance. Annu Rev Med
in proteins are considered to be nonessential or dispens- 1990;41:141.
able, since they can be synthesized as long as there is
enough total protein in the diet—ie, if one of these Fuller MF, Garlick PJ: Human amino acid requirements. Annu
amino acids is omitted from the diet, nitrogen balance Rev Nutr 1994;14:217.
can still be maintained. However, only three amino
acids—alanine, aspartate, and glutamate—can be con- Garrow JS, James WPT, Ralph A: Human Nutrition and Dietetics,
sidered to be truly dispensable; they are synthesized 10th ed. Churchill-Livingstone, 2000.
from common metabolic intermediates (pyruvate, ox-
National Academy of Sciences report on diet and health. Nutr Rev
1989;47:142.

Nielsen FH: Nutritional significance of the ultratrace elements.
Nutr Rev 1988;46:337.

Vitamins & Minerals 45

David A. Bender, PhD, & Peter A. Mayes, PhD, DSc

BIOMEDICAL IMPORTANCE mia (iron), cretinism and goiter (iodine). If present in
excess as with selenium, toxicity symptoms may occur.
Vitamins are a group of organic nutrients required in
small quantities for a variety of biochemical functions THE DETERMINATION OF NUTRIENT
and which, generally, cannot be synthesized by the REQUIREMENTS DEPENDS ON THE
body and must therefore be supplied in the diet. CRITERIA OF ADEQUACY CHOSEN

The lipid-soluble vitamins are apolar hydrophobic For any nutrient, particularly minerals and vitamins,
compounds that can only be absorbed efficiently when there is a range of intakes between that which is clearly
there is normal fat absorption. They are transported in inadequate, leading to clinical deficiency disease, and
the blood, like any other apolar lipid, in lipoproteins or that which is so much in excess of the body’s metabolic
attached to specific binding proteins. They have diverse capacity that there may be signs of toxicity. Between
functions, eg, vitamin A, vision; vitamin D, calcium these two extremes is a level of intake that is adequate
and phosphate metabolism; vitamin E, antioxidant; vi- for normal health and the maintenance of metabolic in-
tamin K, blood clotting. As well as dietary inadequacy, tegrity. Individuals do not all have the same require-
conditions affecting the digestion and absorption of the ment for nutrients even when calculated on the basis of
lipid-soluble vitamins—such as steatorrhea and disor- body size or energy expenditure. There is a range of in-
ders of the biliary system—can all lead to deficiency dividual requirements of up to 25% around the mean.
syndromes, including: night blindness and xeroph- Therefore, in order to assess the adequacy of diets, it is
thalmia (vitamin A); rickets in young children and os- necessary to set a reference level of intake high enough
teomalacia in adults (vitamin D); neurologic disorders to ensure that no one will either suffer from deficiency
and anemia of the newborn (vitamin E); and hemor- or be at risk of toxicity. If it is assumed that individual
rhage of the newborn (vitamin K). Toxicity can result requirements are distributed in a statistically normal
from excessive intake of vitamins A and D. Vitamin A fashion around the observed mean requirement, then a
and β-carotene (provitamin A), as well as vitamin E, are range of +/− 2 × the standard deviation (SD) around
antioxidants and have possible roles in atherosclerosis the mean will include the requirements of 95% of the
and cancer prevention. population.

The water-soluble vitamins comprise the B complex THE VITAMINS ARE A DISPARATE GROUP
and vitamin C and function as enzyme cofactors. Folic OF COMPOUNDS WITH A VARIETY
acid acts as a carrier of one-carbon units. Deficiency of OF METABOLIC FUNCTIONS
a single vitamin of the B complex is rare, since poor
diets are most often associated with multiple deficiency A vitamin is defined as an organic compound that is re-
states. Nevertheless, specific syndromes are characteris- quired in the diet in small amounts for the maintenance
tic of deficiencies of individual vitamins, eg, beriberi of normal metabolic integrity. Deficiency causes a spe-
(thiamin); cheilosis, glossitis, seborrhea (riboflavin); cific disease, which is cured or prevented only by restor-
pellagra (niacin); peripheral neuritis (pyridoxine); ing the vitamin to the diet (Table 45–1). However, vit-
megaloblastic anemia, methylmalonic aciduria, and amin D, which can be made in the skin after exposure
pernicious anemia (vitamin B12); and megaloblastic to sunlight, and niacin, which can be formed from the
anemia (folic acid). Vitamin C deficiency leads to essential amino acid tryptophan, do not strictly con-
scurvy. form to this definition.

Inorganic mineral elements that have a function in
the body must be provided in the diet. When the intake
is insufficient, deficiency symptoms may arise, eg, ane-

481

482 / CHAPTER 45

Table 45–1. The vitamins.

Vitamin Functions Deficiency Disease
Night blindness, xerophthalmia;
A Retinol, β-carotene Visual pigments in the retina; regulation of keratinization of skin
gene expression and cell differentiation;
β-carotene is an antioxidant Rickets = poor mineralization of bone;
osteomalacia = bone demineralization
D Calciferol Maintenance of calcium balance; enhances
intestinal absorption of Ca2+ and mobilizes Extremely rare—serious neurologic
bone mineral dysfunction
Impaired blood clotting, hemor-
E Tocopherols, tocotrienols Antioxidant, especially in cell membranes rhagic disease
Peripheral nerve damage (beriberi) or
K Phylloquinone, Coenzyme in formation of γ -carboxyglutamate central nervous system lesions
menaquinones in enzymes of blood clotting and bone matrix (Wernicke-Korsakoff syndrome)
Lesions of corner of mouth, lips, and
B1 Thiamin Coenzyme in pyruvate and α−ketoglutarate, tongue; seborrheic dermatitis
dehydrogenases, and transketolase; poorly Pellagra—photosensitive dermatitis,
B2 Riboflavin defined function in nerve conduction depressive psychosis
Disorders of amino acid metabolism,
Niacin Nicotinic acid, Coenzyme in oxidation and reduction reactions; convulsions
nicotinamide prosthetic group of flavoproteins
Megaloblastic anemia
B6 Pyridoxine, pyridoxal, Coenzyme in oxidation and reduction reactions, Pernicious anemia = megaloblastic
pyridoxamine functional part of NAD and NADP anemia with degeneration of the
spinal cord
Folic acid Coenzyme in transamination and decarboxy-
B12 Cobalamin lation of amino acids and glycogen Impaired fat and carbohydrate metab-
phosphorylase; role in steroid hormone action olism, dermatitis
Scurvy—impaired wound healing,
Coenzyme in transfer of one-carbon fragments loss of dental cement, subcutaneous
hemorrhage
Coenzyme in transfer of one-carbon fragments
and metabolism of folic acid

Pantothenic acid Functional part of CoA and acyl carrier protein:
H Biotin fatty acid synthesis and metabolism
C Ascorbic acid
Coenzyme in carboxylation reactions in gluco-
neogenesis and fatty acid synthesis

Coenzyme in hydroxylation of proline and
lysine in collagen synthesis; antioxidant;
enhances absorption of iron

LIPID-SOLUBLE VITAMINS provitamin A, as they can be cleaved to yield retinalde-
hyde and thence retinol and retinoic acid. The α-, β-,
RETINOIDS & CAROTENOIDS and γ-carotenes and cryptoxanthin are quantitatively
HAVE VITAMIN A ACTIVITY the most important provitamin A carotenoids. Al-
(Figure 45–1) though it would appear that one molecule of β-
carotene should yield two of retinol, this is not so in
Retinoids comprise retinol, retinaldehyde, and practice; 6 µg of β-carotene is equivalent to 1 µg of
retinoic acid (preformed vitamin A, found only in preformed retinol. The total amount of vitamin A in
foods of animal origin); carotenoids, found in plants,
comprise carotenes and related compounds, known as foods is therefore expressed as micrograms of retinol

equivalents. Beta-carotene and other provitamin A

carotenoids are cleaved in the intestinal mucosa by

carotene dioxygenase, yielding retinaldehyde, which is

reduced to retinol, esterified, and secreted in chylomi-

VITAMINS & MINERALS / 483

H3C CH3 CH3 CH3 H3C
CH3
CH3 CH3 H3C CH3
H3C CH3 CH3
CH3 β-Carotene

H3C CH3 CH3 CH3 CH2OH H3C CH3 CH3 CH3
CHO

Retinol CH3 Retinaldehyde

CH3 H3C CH3 CH3
COOH

Figure 45–1. β-Carotene and the major vita- CH3 CH3
min A vitamers. * Shows the site of cleavage of All-trans-retinoic acid H3C
β-carotene into two molecules of retinaldehyde
by carotene dioxygenase. COOH

9-cis-retinoic acid

crons together with esters formed from dietary retinol. ciency, both the time taken to adapt to darkness and the
The intestinal activity of carotene dioxygenase is low, ability to see in poor light are impaired.
so that a relatively large proportion of ingested β-
carotene may appear in the circulation unchanged. Retinoic Acid Has a Role
While the principal site of carotene dioxygenase attack in the Regulation of Gene
is the central bond of β-carotene, asymmetric cleavage Expression & Tissue Differentiation
may also occur, leading to the formation of 8′-, 10′-,
and 12′-apo-carotenals, which are oxidized to retinoic A most important function of vitamin A is in the con-
acid but cannot be used as sources of retinol or retin- trol of cell differentiation and turnover. All-trans-
aldehyde. retinoic acid and 9-cis-retinoic acid (Figure 45–1) regu-
late growth, development, and tissue differentiation;
Vitamin A Has a Function in Vision they have different actions in different tissues. Like the
steroid hormones and vitamin D, retinoic acid binds to
In the retina, retinaldehyde functions as the prosthetic nuclear receptors that bind to response elements of
group of the light-sensitive opsin proteins, forming DNA and regulate the transcription of specific genes.
rhodopsin (in rods) and iodopsin (in cones). Any one There are two families of nuclear retinoid receptors: the
cone cell contains only one type of opsin and is sensitive retinoic acid receptors (RARs) bind all-trans-retinoic
to only one color. In the pigment epithelium of the acid or 9-cis-retinoic acid, and the retinoid X receptors
retina, all-trans-retinol is isomerized to 11-cis-retinol (RXRs) bind 9-cis-retinoic acid.
and oxidized to 11-cis-retinaldehyde. This reacts with a
lysine residue in opsin, forming the holoprotein Vitamin A Deficiency Is a Major Public
rhodopsin. As shown in Figure 45–2, the absorption of Health Problem Worldwide
light by rhodopsin causes isomerization of the retinalde-
hyde from 11-cis to all-trans, and a conformational Vitamin A deficiency is the most important preventable
change in opsin. This results in the release of retinalde- cause of blindness. The earliest sign of deficiency is a
hyde from the protein and the initiation of a nerve im- loss of sensitivity to green light, followed by impair-
pulse. The formation of the initial excited form of ment of adaptation to dim light, followed by night
rhodopsin, bathorhodopsin, occurs within picoseconds blindness. More prolonged deficiency leads to xeroph-
of illumination. There is then a series of conformational thalmia: keratinization of the cornea and skin and
changes leading to the formation of metarhodopsin II, blindness. Vitamin A also has an important role in dif-
which initiates a guanine nucleotide amplification cas- ferentiation of immune system cells, and mild defi-
cade and then a nerve impulse. The final step is hydroly- ciency leads to increased susceptibility to infectious dis-
sis to release all-trans-retinaldehyde and opsin. The key eases. Furthermore, the synthesis of retinol-binding
to initiation of the visual cycle is the availability of protein in response to infection is reduced (it is a nega-
11-cis-retinaldehyde, and hence vitamin A. In defi- tive acute phase protein), decreasing the circulating vi-