Hormone Biosynthesis,
Metabolism, and Mechanism of
Action
Adrian Quesada Rojas, MD
Definitions
Hormone: Substance produced in tissue =>
bloodstream => responsive cells
Provides means of communication
chemical regulatory and signaling
Bloodstream
Paracrine: cell to cell (contiguos)
Autocrine: same cell
Intracrine: same cell (unsecreted)
Nomenclature
Steroid hormones
Sex steroids (Cholesterol derivatives)
3 groups (number of carbon atoms)
21 carbons: pregnane nucleus
19 carbons: androstane
18 carbons: estrane
Nomenclature
Lipoproteins and Cholesterol
All steroid prod organs can synth (acetate)
LPP: transport of non polar fat in polar solvent
 Chylomicrons: chlol 10, TG 90 formed in GI
 VLDL: more dense
 IDL: removal of some TG from VLDL interior
 LDL: end prod VLDL catabolism (50% chol)
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major carrier of chol in plasma
 HDL: highest protein content
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Lipoproteins and Cholesterol
Steroidogenesis
Reactions:
Cleavage of side chain
Conv hydroxyl <=> ketones (dehydrogenase)
Addition of OH group (hydroxylation)
Creation of double bonds (remov of H)
Addition of hydrogen (saturation)
Enz are dehydrogenases or C P450 oxidases
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Steroidogenesis
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Rate limit step: transfer Chol from outer to inner
mitochondrial membrane (SSC)
Once Pregnenolone: two ways in ovary
D5 pathway (3-b hydroxysteroids) => DHEA
D4 pathway (3-ketone) => 17a hydroxyP
Conversion of pregnenolone to P:
3b hydroxysteroid dehydrogenase
D4-5isomerase reaction (3-OH group to ketone +
transfer double bond 5-6 to 4-5)
Steroidogenesis
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P hydroxylated at 17 position
17a OH P (precursor of C19 androgens)
By peroxide formation at C20, followed by
epoxidation of C17, C20 carbons, side chain is
split off forming Androstenedione
17 ketone may be reduced to 17b OH to form
testosterone (C 19)
C 19 steroids => aromatase => C18 phenolic
steroid estrogen (E1 and E2)
Steroidogenesis
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Alternative
Pregnenolone conv to D5 3b OH C19 steroid
(DHA) by 17a OH lation followed by SCC
With D4 3 ketone formation, DHA is converted into
Androstenedione (C19 steroids)
C 19 steroids undergo aromatization
(hydroxylation of angular 19 methyl group, followed by
oxydation and loss of 19 C as formaldehyde and
dehydrogenation)
Steroidogenesis
Two Cell System
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FSH receptors on granulosa
FSH receptors are induced by FSH
LH receptors on theca initially, as follicle
grows FSH induces LH receptors on
granulosa
FSH ind aromatase activity on granulosa
Actions regulated by autocrine and
paracrine factors
Two Cell System
Blood Transport of Steroids
Most E and T bound to protein carrier (SHBG)
 30% is bound to albumin
 1% free
Hyperthyroidism, pregnancy, E adm
SHBG
Corticoids, A, P, GH, Insulin
SHBG
SHBG inversely related to weight
Hyperinsulinemia (low SHBG predictor of DM)
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Body fat distrib (central=> hyperinsulinemia)
Blood Transport of Steroids
Estrogen Metabolism
Ovary => Estradiol / estrone
 Estriol =>
periph metab of estrone / E
not a secretory prod of ovary
conversion to less active form
Androgens are precursors of estrogens
Adrenal gland (source of A, Androstenedione)
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Estrogen Metabolism
Estrogen Metabolism
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E2 100-300 mg/day
Androstenedione 3 mg/day ( 1% conversion) =>
20- 30 % of E1 (produced every day)
Progesterone Metabolism
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NO peripheral conversion
Secretion from adrenal and ovaries
Preov < 1 mg /day
Post ov 20-30 mg /day
10-20% of P is excreted as Pregnanediol
Pre ov <1 mg/day, post ov 3-6 mg/d (home
ovulation test)
Androgen Metabolism
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Ovary: DHA, Androstenedione, testosterone
Adrenal cortex: Gluco, mineralo, sex steroid
 Sex steroids (less than ovary)
 DHA (½ adrenal, ¼ ovary, ¼ periph)
 Androstenedione (½ adrenal, ½ ovary)
 Testost 0.2-0.3 mg/day
 50% periph conv of androstenedione and DHA
 25% adrenal
 25% ovary
Androgen Metabolism
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Androgens Test binding capacity
Androg effects depends on unbound fraction
Hirsutism
Test => 5a reductase => DHT (princ androg)
Androstenedione > Test (women)
 DHT is derived from Androst and DHA
DHT largely metabolized intracell
 Only 1/10 of levels of Test
 Not all tissue requires DHT from Test (Wolffian)
Cellular Mechanism of Action
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Two major types of hormone action
Tropic hormones (recep at cell memb)
Steroids (recep inside the cell)
Multiple cell receptors:
Intracellular:
In the nucleus (transcription activity)
G protein:
Single polypeptide chain (cell memb)
Cellular Mechanism of Action
Ion gate channel:
Cell surface, mult units (ACh)
Intrinsic enz activ:
Trans memb recep with intracell component
with tyrosine or serine kinase activity
Mechanism of action
Steroid hormones
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Includes:
 Hormone diffusion across cell memb
 Steroid binding to receptor protein
 Interaction of Horm-Recep complex with DNA
 Synthesis of mRNA
 Transport of mRNA to ribosomes
 Protein synthesis in cytoplasm
Mechanism of action
Steroid hormones
Mechanism of action
Steroid hormones
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Biological activity
Mantained only while H-R complex attached
to nuclear site
Duration of exposure to hormone is as
important as dose
Major factor in potency differences among
various estrogens is length of time E-R
complex occupies nucleus
Mechanism of action
Steroid hormones
Cortisol and P must circulate in large
concentrations 2 to receptor complexes short
half-lives in nucleus
H-R complex after gene activation (processing)
Rapid degradation of R unbound to E
Much slower degradation of bound R
Continuous presence of E is important factor in
continuing response
Estrogen Receptors
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ER-a (rapid turnover)
ER-b 96% homologous in DNA binding
domain and 53% homologous in the hormone
binding domain (when compared to ER-a)
Respond in comparable manner to same
hormones
Estrogen Receptors
Estrogen Receptors
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E receptors => nucleocytoplasmic shuttling
Recep can diffuse out of nucleus and be
transported back in or undergo metabolism
If shuttling impaired Receptors are degraded
rapidly
E antagonists prevent nuclear translocation and
thus increase cytoplasmic degradation
Estrogen Receptors
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Prior to binding R is inactive complex that
includes a variety of heat shock proteins
Activation => dissociation of HSP
When H binds R => conformational change
Conformational shape determines exact
message transmitted to the gene
E2, raloxifene, tamoxifen induces a different
conformational shape
Estrogen Receptors
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1.
2.
Once activated H receptor activates
transcription in partnership with several
groups of polypeptides
Transcription factors (polymerase enzyme
and DNA)
Coactivators and corepressors (intracel prot
called adaptor proteins and activate or
suppress the TAF)
Estrogen Receptors
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Phosphorylation
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Also stimulated by ligand binding
Occur in specific receptor sites
Increases potency of molecule to regulate
transcription
cAMP and prot kinase pathways increase
transcriptional activity of E-R
Estrogen Receptors
Differences ER-a and ER-b
ER-b prevalent in brain, CV system, granulosa cells
Breast expresses ER-a and ER-b
ER-b acts as natural suppressor of ER-a activity on
breast tissue
Colon contains only ER-b, reduction of colon CA in
postmenopausal on HRT may reflect
antiproliferative effect of beta receptor
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Progesterone Receptor
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Induced by E, decreased by Progestins
Two major forms (A and B)
A and B are expressed differently in Breast
and Endometrial tissues
In most cells B positive regulator of Presponsive genes, A inhibits B activity
A and B have different molecular functions,
affecting different genes
Progesterone Receptor
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Mice lacking of P receptors (A,B) are unable to
ovulate (failure to expel a mature oocyte in a fully
developed follicle)
PR-A protects against uterine and mammary gland
hyperplasia induced by PR-B
Androgen Receptor
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1.
2.
3.
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Androgens only in one of three ways:
T => DHT (Intracrine)
T itself (Endocrine)
Intracel conversion of T to E (Intracrine)
Wolffian duct derivatives (T)
Hair follicles, urogenital sinus derivatives
(req T => DHT)
Androgen Receptor
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T and DHT binds to same high affinity A-receptor
(DHT has greater affinity)
Anti-A also bind to same R (20% affinity of T)
A-R: two forms B (full length) and A (shorter)
Functional differences yet to be determined
A and P can cross react for their receptors
(pharmacologic concentrations)
Progestin can act as an anti-E and anti-A
Androgen Receptor
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Androgen Insensitivity Syndrome
Congenital abnormality in A intracel R
A-R gene is on X chromosome (Xq11-12)
X-linked disorder
Deletion of amino acids from steroid binding
domain 2 to nucleotide alterations in gene
Androgen Receptor
Agonists and Antagonists
Agonist => stimulates response
Antagonist => inhibits actions of an agonist
blockage of receptor message
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Examples:
Tamoxifen
Mifepristone
Histamine receptor antag
Agonists and Antagonists
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Physiologic Antagonists
P is not an E antagonist
Modifies E action by depleting E receptors
P can induce conversion of E2 to E1
Androgens block E actions (unclear mechan)
Agonists and Antagonists
Two groups of anti-E:
Pure anti-E
Mixed agonists antagonists
 Tamoxifen:
Similar to clomiphene
Competitively inhibits E binding to receptor
E affinity for receptor is 100-1000x better than
tamoxifen
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Agonists and Antagonists
Agonists and Antagonists
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Tamoxifen–E receptor binds to DNA
Agonistic – Antagonistic actions depends on
coactivators present in specific cell types
Estrogenic actions:
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Lowers FSH levels
Decreases Cholesterol, LDL
Stimulates P receptor synthesis
Maintenance of bone, vagina, endometrium
Agonists and Antagonists
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Causes endometrial hyperplasia, polyps and 4x
increase in endometrial cancer
Mechanism of action:
 TAF-1 and TAF-2 can both activate transcription
 Tamoxifen agonistic ability is due to activation of
TAF-1
 Antagonistic activity is due to competitive
inhibition of E-dependent activation of TAF-2
Estrogen Receptors
Agonists and Antagonists
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Response to E and anti-E depends on:
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Nature of E receptor
E response elements and promoters
Cell content of protein coactivators
Properties of ligand
Modulation by growth factors and agents
that affect phosphorylation and protein
kinases
Agonists and Antagonists
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Tamoxifen reduces risk of recurrent disease in E-R
positive and negative breast cancer
Besides binding to E-R (competitive inh),
Inhibits protein kinase C activity (phosphor)
Inhibits calmodulin-dependent cyclic AMP
phosphodiesterase (binds to calmodulin)
Stim secretion of TGF-b (inh growth CA cell)
Agonists and Antagonists
Tamoxifen: treatment should be for only 5 years
 Emergence of tamoxifen-resistant tumors, how?
Loss of E receptors
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Loss of cellular control / loss of E-R expression
Variant and mutant E-R
Changes in coactivators
Differential metabolism
Resistance occur when E-R is NOT dominant in
growth of CA cells
Agonists and Antagonists
Selective E Agonist / Antagonist
Raloxifene
Induce conformational shape change that
results in modified action influenced by
cellular regulating proteins
Anti-E in uterus and breast
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Agonist in certain tissues (bone, lipids)
Antiprogestins
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Mifepristone
Binding => different conformational change =>
prevents full gene activation
Has also some agonistic activity
Final response is determined by coact, corepr
Affinity to P-R is 5x greater than P
In absence of P can produce weak agonistic effect
Also works as weak anti-androgens
Tropic Hormones
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Receptor in cell membrane
R functions as ion channel or as enzime
Can also couple an active agent (messenger)
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Cyclic AMP
IP3
1,2 DG
Ca
Cyclic GMP
Tropic Hormones
Cyclic AMP
FSH, LH, hCG, TSH and ACTH
Ligand => R activates adenylate cyclase
Conversion of ATP into cAMP
cAMP binds cytoplasm receptor protein and
activates protein kinase
Catalyze phosphorilation of serine in cell
proteins => physiologic events
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Tropic Hormones
Tropic Hormones
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cAMP is degrated by PDE into 5’-AMP
PG’s stim adenylate cyclase and cAMP accum
Other tropic hormones
 Oxytocin
 Insulin
 GH
do not use Adenylate cyclase
 PRL
 hPL
Tropic Hormones
Calcium messenger system
Intracel Ca regulates cAMP, cGMP levels
Ca mess system is linked to H-R function by specific
enzyme Phospholipase C
Catalyzes hydrolysis of phosphatidylinositols (cell
membrane phospholipids)
PLC activ generates two intracell messengers
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Tropic Hormones
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IP3 and DAG
Both initiate functions of 2 parts of Ca system
First part => Ca activ Prot kinase
Sustained cellular responses
Second part
=>
Calmodulin (regulator)
Acute cell responses
Tropic Hormones
Tropic Hormones
Kinase receptors
Cell membrane receptors of
insulin, IGF, EGF, platelet-derived GF and fibroblast
GF are tyrosine kinases
Extracell, transmembrane, cytoplasmic domain
Cytoplasmic (autophospholylation)
Insulin / IGF (2a, 2b subunits)
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Tropic Hormones
Regulation of tropic hormones
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Four major components
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Autocrine and paracrine reg factors
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GF are produced by local gene expression
Modulate activ in cells (nearby or producing cells)
Involved in reproductive physiology (activin,
inhibin, IGF-I, IGF-II, TGF-b, FGF, EGF)
Also cytokines (modulate ovarian steroidogenesis)
Heterogeneity of tropic hormones (FSH, hCG)
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Glycoproteins are not single proteins (isoforms)
Regulation of tropic hormones
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Up and down-regulation of receptors
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Positive or negative modulation of receptors by
homologous hormones
Little is known regarding up regulation (PRL, GnRH)
Regulation of adenylate cyclase
Regulation of tropic hormones
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Regulation of Adenylate cyclase
G protein system:
Adenylate cyclase (composition):
 Receptor
 Guanyl nucleotide regulatory unit (GTP regul)
 Catalytic unit (converts ATP to cAMP)
Regulation of tropic hormones
Regulation of tropic hormones
Regulation of tropic hormones
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Ligand => receptor => nucleotide regulatory
unit (GTP uptake) => catalytic enzyme
converting ATP to cAMP
Coupling of regulatory unit with catalytic unit
Enzyme activity is terminated by hydrolysis of
GTP to GDP
Regulation of tropic hormones
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G protein system
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Stimulatory nucleotide regulatory G protein
Inhibitory nucleotide regulatory G protein
Not limited to cAMP signal, can activate
other messenger-generating enzymes
Regulation of tropic hormones