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STEROID RECEPTORS & STEROID ACTIONS
Kina M.M. McDougall15 July, 2014
OBJECTIVES
Review structure and function of steroid receptors
Review molecular signaling pathway Discuss common steroid hormones & their
controlled pathways
TYPES OF HORMONES
Tyrosine derivatives Steroids Peptides Proteins
Tyrosine derivatives
Steroids Peptides(<20 amino acids)
Proteins(>20 amino acids)
Epinephrine Testosterone Oxytocin Insulin
Norepinephrine Estradiol Vasopressin Glucagon
Dopamine Progesterone Angiotensin ACTH
T3 Cortisol MSH TSH
Thyroxine Aldosterone Somatostatin Prolactin
Vitamin D TRH Motilin
Gastrin LH
CCK GnRH
Growth Hormone
secretin
CRH
GHRH
PTH
Calcitonin
HCG
STEROID HORMONES - OVERVIEW
Hydrophobic/Lipophilic (lipid soluble) Require transporter proteins in the blood Receptors located inside cells Diffuse into cells and binds to respective
receptor Receptor has a DNA binding domain & interacts
with specific response elements associated with certain genes
Lipid soluble hormones control gene expression (action that requires time)
STEROID HORMONES - OVERVIEW
Steroid hormone receptors are intracellular Steroid hormones diffuse into the cell and bind to
nuclear receptors (NRs)
2 main classes of nuclear receptors Type I: most steroid hormones Type II: Vit D, thyroid hormone
NUCLEAR RECEPTORS
Structure Large protein complexes Made up of at least 2 protein subunits
Ligand-binding subunit Heat shock protein 90 (hsp90) Other proteins:
FKBP52, Cyp40, hsp70, p23
NUCLEAR RECEPTORS
Ligand binding subunit: 6 segments (A-F) from N-terminal to C-terminal A/B domain C domain (DNA binding domain, DBD) D domain (“hinge”) E domain (ligand binding domain, LBD) F domain
NUCLEAR RECEPTORS
N-terminal A/B domain Most variable region among all members of
NR super family in terms of length and sequence
Function not well defined In some receptors, the A/B domain is
believed to contain a positive transcriptional activation factor that is ligand independent
COMPANION SITE FOR BASIC MEDICAL
ENDOCRINOLOGY, 4TH EDITION. BY DR. GOODMAN
NUCLEAR RECEPTORS
C domain Contains DNA binding domain (DBD) Includes 2 subunits known as zinc fingers
Loop containing 4 cysteines stabilized by bonds with Zn2+ ions
Amino acid sequence between 1st & 2nd fingers is responsible for establishing specific contacts with DNA
Overall structure similar throughout NR superfamily Crucial for recognizing DNA sequences regulated by
the receptor Binds hormone response element (HRE) on DNA
NUCLEAR RECEPTORS
D domain Flexible segment known as the “hinge” May be important for spatial configuration of
the protein
COMPANION SITE FOR BASIC MEDICAL
ENDOCRINOLOGY, 4TH EDITION. BY DR. GOODMAN
NUCLEAR RECEPTORS
E domain Contains the ligand binding domain (LBD) LBD consists of 12 α helical segments (H1-
H12) Carries transcription-activating function AF-2,
which is strictly dependent on hormone binding
Discriminates between respective ligand molecules
NUCLEAR RECEPTORS
F domain Weakly conserved (variable) No known function
COMPANION SITE FOR BASIC MEDICAL
ENDOCRINOLOGY, 4TH EDITION. BY DR. GOODMAN
NUCLEAR RECEPTORS
Hsp90 Is present as a dimer Attached to C (DBD), D, and E (LBD) domains Passively prevents the hormone-binding
component from binding DNA However, can actively detach hormone-binding
subunit from DNA Also has protective function
Disruption of hsp90 activity leads to loss of hormone-binding or receptor function
NUCLEAR RECEPTORS
Ligand binding Binds within a hydrophobic pocket created by
the α helical segments Major structural change induced by ligand
binding is an internal folding of the C-terminal helix H12, which forms a cap on the ligand binding pocket
NUCLEAR RECEPTORS
Receptor Activation Hormone-binding subunit is released
from remainder of “native complex”
DNA-binding Binding occurs at hormone response elements (HREs) HREs consist of two hexa- or pentanucleotides (“half-
sites”) organized as direct or inverted repeats separated by a spacer
Each half-site interacts with one molecule of the receptor
NUCLEAR RECEPTORS
Dimerization Dimerization of NRs is essential for transcriptional
activation of gene expression NR association with DNA promotes dimerization However, dimer formation can occur when NR
receptors are not bound to DNA NR dimerize as homodimers (Type I NRs) or
heterodimers (Type II NRs) Homodimers: bind half-sites organized as inverted repeats Heterodimers: bind half-sites organized as direct or
inverted repeats
TYPE I NUCLEAR RECEPTORS
TYPE II NUCLEAR RECEPTORS
Olefsky J M J. Biol. Chem. 2001;276:36863-36864
STRUCTURE OF NUCLEAR RECEPTORS
NUCLEAR RECEPTORS
Mechanisms of regulation Ligand-dependent activation
Ligand-bound receptor increases transcription of a target gene to which it is bound
Transcriptional activation itself is mediated primarily by the LBD
Ligand-independent repression of transcription
Ligand-dependent negative regulation
NUCLEAR RECEPTORS
Factors modulating receptor activity Receptor concentration Ligand concentration Ligand function Concentrations and types of coactivators and
corepressors Phosphorylation state of NR
STEROID HORMONE - PRODUCTION
Formed enzymatically through a series of modifications of their common precursor cholesterol
Minimal storage of steroid hormones in their cells of origin
Synthesis and secretion are aspects of the same process Steroid hormones diffuse across the plasma
membrane as soon as they are formed
STEROID HORMONES
Main steroids in humans: Adrenal steroids
Glucocorticoids Mineralocortictoids Androgens
Gonadal steroids Ovarian Testicular
Vitamin D
Mineralocorticoids
Glucocorticoids
Androgens
Catecholamines
ADRENAL STEROIDS
Companion site for Basic Medical Endocrinology, 4th Edition. by Dr. Goodman
Copyright © 2009 by Academic Press. All rights reserved.
GLUCOCORTICOIDS
HYPOTHALAMIC-PITUITARY-ADRENAL AXIS
CIRCADIAN RHYTHM OF ACTH & CORTISOL
Companion site for Basic Medical Endocrinology, 4th Edition. by Dr. Goodman
Copyright © 2009 by Academic Press. All rights reserved.
GLUCOCORTICOID PRODUCTION
ACTH acts by binding to a specific cell-surface receptor, the melanocortin receptor-2 (MCR-2)
ACTH up-regulates expression of these receptors, thereby increasing the steroidogenic response to further ACTH stimulation
ACTH binding to its receptors activates adenylyl cyclase, increasing cyclic AMP (cAMP) production, which in turn stimulates cAMP-dependent protein kinase (protein kinase A) and phosphorylation of a number of proteins.
STIMULATION OF STEROIDOGENESIS BY ACTH
Companion site for Basic Medical Endocrinology, 4th Edition. by Dr. Goodman Copyright © 2009 by Academic Press. All
rights reserved.
EXAMPLE OF HORMONAL IMBALANCE
Companion site for Basic Medical Endocrinology, 4th Edition. by Dr. Goodman Copyright © 2009 by Academic Press. All
rights reserved.
FIGURE 4.25
CORTICOSTEROID TRANSPORT More than 90% of cortisol is bound
Predominantly to cortisol-binding globulin (CBG) Small amount to albumin Free cortisol is biologically active (only small fraction of
total) CBG is made in liver and has high affinity for
cortisol CBG levels are increased by:
Estrogen (2-3 fold in pregnancy) Chronic hepatitis
CBG levels are reduced by: Glucocorticoids Cirrhosis, nephrosis, hyperthyroidism
CORTICOSTEROID METABOLISM
Cortisol half-life is 70-120 minutes Free cortisol is excreted through kidneys
1% of total cortisol secretion
Most of cortisol metabolized in liver and kidney to inactive metabolites
GLUCOCORTICOID METABOLISM
Thyroid hormone Hyperthyroidism increases cortisol metabolism and
clearance and hypothyroidism causes the converse Effect on hepatic HSD11B1 and 5α/5β-reductases
IGF-1 Increases cortisol clearance by inhibiting hepatic
HSD11B1 Cortisol
Increases metabolism by inducing 6β-hydroxylase Drugs
Rifampin and phenytoin induce 6β-hydroxylase
SYSTEMIC GLUCOCORTICOID EFFECTS
MINERALOCORTICOIDS
MINERALOCORTICOIDS
Aldosterone is the main mineralocorticoid in humans, with 100-150 mcg/day produced
Mineralocorticoids have a much more restricted role compared to glucocorticoids Fluid (sodium) balance Potassium balance
RENIN-ANGIOTENSIN SYSTEM
COMPANION SITE FOR BASIC MEDICAL ENDOCRINOLOGY, 4TH EDITION. BY DR. GOODMAN
MINERALOCORTICOID SYNTHESIS
Have same precursor as cortisol Produced in adrenal glomerulosa
Low concentration of CYP17 (17-hydroxylase) Only zone that has CYP11B2 (aldosterone synthase)
Production and release are stimulated by Angiotensin II Rise in serum potassium Decline in serum sodium (minor role)
ALDOSTERONE SECRETION
Angiotensin II and hyperkalemia act on the zona glomerulosa
Promote conversion of cholesterol to pregnenolone and corticosterone to aldosterone via stimulation of aldosterone synthase Chronic sodium restriction leads to a 10-fold increase in
messenger RNA for aldosterone synthase and in aldosterone synthase activity
Zona glomerulosa is extremely sensitive to change in K+
Aldosterone secretion rises linearly in response to increasing plasma K+ concentration above 3.5 meq/L
ALDOSTERONE ACTION
Aldosterone diffuses into cells and binds the nuclear receptor, resulting in RNA transcription
Similarly to glucocoticoids, aldosterone binds to mineralecorticoid receptors which dissipates the heat shock protein receptor complex allowing dimerization
Translocation of the hormone into the nucleus results in enhanced transcription of the epithelial sodium channel (ENaC)
ALDOSTERONE ACTION
Na+/Cl- reabsorption and K+ excretion Increases open Na+ channels (ENaC) in
luminal membrane Inserting new channels Opening previously silent channels
Enhances basolateral membrane Na+/K+-ATPase activity
ALDOSTERONE ACTION
H+ excretion H+ is mainly excreted in intercalated cells of
cortex and tubular cells of outer medulla Occurs via H+-ATPase
Acid/base status does not affect aldosterone release
ALDOSTERONE ACTION
Extra-renal effects Reduces concentration of Na+ and raises that
of K+ in colonic and salivary secretions and in sweat
Generally of limited physiologic importance Colonic secretion/elimination of K+ can become
important in patients with end-stage renal disease
MINERALOCORTICOID EFFECT OF CORTISOL
The MR and GR share considerable homology 57% in the steroid binding domain 94% in the DNA binding domain
There is dual binding/overlap – with aldosterone binding to the GR and cortisol binding to the MR
Cortisol binds to the aldosterone receptor with equal affinity
MINERALOCORTICOID EFFECT OF CORTISOL
Cortisol circulates in much higher concentrations than aldosterone
However, cortisol does not act as a major mineralocorticoid Target tissues (eg aldosterone-sensitive cells in the
collecting tubules and salivary glands) possess enzymes, such as 11-beta-hydroxysteroid dehydrogenase (11β-HSD)
11β-HSD converts cortisol to cortisone and other inactive metabolites
MINERALOCORTICOID EFFECT OF CORTISOL
Inactivation or saturation of 11β-HSD leads to manifestations of hyperalodsteronism
FOR EXAMPLE???
THE CORTISOL-CORTISONE SHUTTLE
Companion site for Basic Medical Endocrinology, 4th Edition. by Dr. Goodman
Copyright © 2009 by Academic Press. All rights reserved. 51
ADRENAL ANDROGENS
ADRENAL ANDROGENS
Androgens are 19-carbon steroids
Most abundant adrenal steroid (>20 mg/day)
Produced mainly in the zona reticularis in the adrenal gland
Zona reticularis develops shortly before puberty
Companion site for Basic Medical Endocrinology, 4th Edition. by Dr. Goodman
Copyright © 2009 by Academic Press. All rights reserved.
ADRENAL ANDROGEN PRODUCTION
DHEA & DHEA-S produced from 17-hydroxypregnelonone
Zona reticularis does not have 3β-HSD Minimal testosterone production in adrenal glands
Androstenedione is converted to testosterone by
17-ketosteroid reductase in peripheral tissues
ADRENAL ANDROGEN SECRETION
DHEA demonstrates circadian rhythm Androgen secretion appears to be dissociated
from glucocorticoid secretion Chronic high-dose dexamethasone suppresses cortisol,
but decreases DHEA by 20% Aging reduces DHEA production but not cortisol Anorexia nervosa leads to decrease in DHEA but not
cortisol
ANDRENAL ANDROGEN SECRETION Prolactin
Abundant receptors on adrenocortical cells In women with prolactinomas, serum DHEAS concentrations
are weakly correlated with serum prolactin concentrations
T-lymphocytes Lymphocytes are present in zona reticularis Lymphocytes increase DHEAS four-fold in vitro Adrenal androgen secretion is reduced in patients receiving
T-lymphocyte suppressive drugs
ACTH Increase in ACTH can result in excessive production of
androgen precursors leading to excess androgen production
GONADAL STEROIDS
Androgens Estrogens Progestogens
TESTICULAR ANDROGENS
Testosterone is the major testicular steroid, comprising more than 99% of testicular steroid hormone production
Testosterone production is mainly regulated by pituitary luteinizing hormone (LH)
TESTOSTERONE METABOLISM Testosterone is converted to dihydrotestosterone
(DHT) by 5α-reductase Testosterone:DHT ratio in plasma 10-15:1
Testosterone is aromatized to estradiol by aromatase 85% of estradiol is synthesized in peripheral tissues, mostly in
adipose tissue Estradiol production increases with weight gain and age
Additionally, through the action of a series of 5α- and 5β-reductases, 17β-HSDs & hydroxylases, testosterone is converted into a number of inactive metabolites that are conjugated and excreted in the urine.
Companion site for Basic Medical Endocrinology, 4th Edition. by Dr. Goodman Copyright © 2009 by Academic Press. All rights reserved.
GONADAL STEROID TRANSPORT Transported in plasma largely bound to albumin and sex
hormone-binding globulin (SHBG) Free testosterone - 2% SHBG - 44% (1000 times more affinity for testosterone than
albumin) Albumin or other proteins – 54% (much higher concentration
than SHBG) Nearly all of the albumin-bound testosterone is
available for tissue uptake bioavailable testosterone in plasma is sum of free plus albumin-bound hormone
Concentration of SHBG in men is about one-third to one-half that in women
Prepubertal boys and hypogonadal men have higher SHBG levels than normal men
GONADAL STEROID TRANSPORTSHBG concentration is decreased by:
Androgen administration Hypothyroidism
SHBG concentration is increased by: Estrogen administration Hyperthyroidism
Alterations in the SHBG concentration do not affect androgen physiology in the steady state HPA responds to acute changes in concentrations of bioavailable
testosterone Testosterone synthesis reestablishes a normal serum level of
bioavailable testosterone
ANDROGEN ACTION
Testosterone and DHT In androgen target tissues, both testosterone and DHT
bind to the androgen receptor (AR) and regulate gene expression
Testosterone binds to the androgen receptor with half the affinity of DHT, although the maximal binding capacity is similar for both androgens Main role of DHT is to amplify the androgen signal
ANDROGEN ACTION
Androgens promote nitrogen retention and stimulate muscle protein synthesis and hence increase muscle mass
Testosterone inhibits the differentiation of preadipocytes into adipocytes
ANDROGEN SIGNALING
OVARIAN STEROIDS
OVARIAN STEROIDS
The ovary secretes a variety of C19 steroids, including DHEA, androstenedione, and testosterone
They are produced by the thecal cells and to a lesser degree by the ovarian stroma
Androstenedione can be converted to estrogen or testosterone in the ovary and in extraglandular tissues.
OVARIAN STEROIDS
The principal estrogens (C18 steroids) are: Estradiol (most potent) (E2) Estrone (E1)
Secreted by the ovary Another important source of estrone is extraglandular
conversion of androstenedione in peripheral tissues Estriol (16-hydroxyestradiol) (E3)
Most abundant estrogen in urine and is produced by the metabolism of estrone and estradiol in extraovarian tissues
Aromatase: key enzyme for production of estrogens in ovary
ESTROGEN ACTION
Estradiol Regulates gonadotropin secretion Promotes development of the secondary sexual
characteristics of women Promotes uterine growth, thickening of vaginal
mucosa, thinning of cervical mucus Linear growth of ductal system of breast
OVARIAN PROGESTOGENS
The principal progestogens (C21 steroids) are: Pregnenolone
Progesterone
17-hydroxyprogesterone
Pregnenolone is of primary importance in the ovary because of its key position as precursor of all steroid hormones
Progesterone is the principal secretory product of the corpus luteum and is responsible for the progestational effects (i.e., cell differentiation and induction of secretory activity in the endometrium of the estrogen-primed uterus)
PROGESTOGEN ACTION
Progesterone is required for implantation of the fertilized ovum and maintenance of pregnancy
It also induces decidualization of the endometrium, inhibits uterine contractions, increases the viscosity of cervical mucus, promotes lateral (alveolar) development of the breast glands, and increases basal body temperature
17-hydroxyprogesterone, also secreted by the corpus luteum, has little, if any, biologic activity
SUMMARY
Steroid hormones include adrenal (glucocorticoids, mineralorticoids, androgens), gonadal (androgens, estrogens, progestogens) and vitamin D
All steroids are derived from cholesterol Steroids are hydrophobic and need transport
proteins Steroids bind nuclear receptors and modulate
gene expression
REFERENCES
Williams Textbook of Endocrinology Endocrinology, 4th Edition. by DeGroot and Jameson Companion site for Basic Medical Endocrinology, 4th
Edition. by Dr. Goodman UpToDate