ERS18 Adrenal Steroidogenesis Flashcards
Adrenal Steroidogenesis significance
- Steroid hormone concentrations are regulated at level of SYNTHESIS rather than secretion
- steroidogenic cells do ***NOT store preformed hormones / steroids
- once formed can freely diffuse across cell membrane (∵ lipophilic)
- need to be bound to plasma proteins in serum for transport - Enzymatic defects in adrenal steroidogenesis lead to ***Congenital Adrenal Hyperplasia (CAH) —> a common group of endocrine disorders
Steroid hormones vs Peptide hormones
- Storage of preformed hormone
- Steroid: Very little
- Peptide: 1 day - Mechanism of secretion
- Steroid: Diffusion through plasma membrane
- Peptide: Exocytosis of storage vesicles - ***Binding to plasma proteins
- Steroid: Yes (serum albumins + other specific carriers)
- Peptide: Rare - Lifetime in blood plasma
- Steroid: Hours
- Peptide: Mins - Time course of action
- Steroid: Hours - Days
- Peptide: Mins - Hours - Receptors
- Steroid: Cytosolic / Nuclear
- Peptide: Plasma membrane -
**MOA
- Steroid: Receptor-hormone complex controls **transcription and stability of mRNA (+ non-genomic pathways)
- Peptide: Hormone binding triggers synthesis of cytosolic ***2nd messengers / protein kinase activity
Steroid hormones
Steroidogenesis: ALL derived from Cholesterol (C27)
—> progressive **loss of carbons + **hydroxylation (oxidation)
Key features:
- Small hydrophobic (∵ benzene ring)
- Subtle structural differences between them
C18 steroid / Estrane
- Estrogen —> Estradiol
C19 steroid / Androstane
- Androgens —> Testosterone
C21 steroid / Pregnane
- Progestogens —> Progesterone
- Glucocorticoids —> Cortisol
- Mineralocorticoids —> Aldosterone
Adrenalcortical Steroid Biosynthesis
6 enzymes:
- CYP11A
- 3β-HSD (only one not CYP450 enzyme)
- CYP17
- CYP21
- CYP11B1
- CYP11B2
***Properties of CYP450 family of Hydroxylases
- Cyto: present in **mitochondria, **ER (microsomes)
- Chrome P450: Heme-containing —> absorbs at 450 nm when complexed with CO
- Heme with Fe (Fe2+ Fe3+) (electron rich planar ring) —> allow accept / donate electrons to facilitate electron transfer
RH + O2 + NADPH + H —> ***ROH + H2O + NADP
- Mixed-function oxygenases (∵ both RH and NADPH + H are oxidised: 2 oxidation processes)
- Monooxygenase / Hydroxylase (∵ only 1 atom of O2 is incorporated into RH substrate)
- Multiple substrates: drugs / carcinogens / endogenous compounds (e.g. steroid hormones, bile acids, prostaglandins etc.)
- About 100 different isoenzymes: Different but overlapping specificity found in human (57 genes identified)
- Multiprotein complexes consisted of 2 functional components:
—> Electron-donating system + CYP450 (heme group bind substrates and carry out reactions) - Within adrenocortical cells, mitochondrial and microsomal enzymes have distinct electron-donating systems (將electrons由NADPH + H帶到去heme group to carry out reaction —> oxidation / hydroxylation)
—> Mitochondrial electron carrying component: **Adrenodoxin reductase, Adrenodoxin
—> Microsomal electron carrying component: **Flavoprotein
***Biosynthesis of Adrenalcortical steroids from Cholesterol
Matrix of reactions (sideway + downward parallel reactions)
***Differential expression of CYP in appropriate corticol zones
—> accounting of differences in steroid hormone biosynthesis
—> outer zone: CYP11B2 —> Aldosterone production
—> inner zone: CYP17, CYP11B1 —> Cortisol production
- CYP11A
- **Rate-limiting step
- 3 reactions:
—> 20α-hydroxylation
—> 22-hydroxylation
—> Cleavage of cholesterol side chain at C20-22 —> lose 6 carbons (unstable bond at di-ol)
- Mitochondrial
- expressed in **ALL primary steroidogenic tissues (adrenal cortex, ovary, testicular Leydig cells, placenta)
- stimulated by ACTH, steroid hormones -ve feedback to ACTH - 3β-Hydroxysteroid dehydrogenase (3β-HSD) (non-cytochrome step)
- 2 reactions:
—> Oxidation of 3β-OH group to 3-ketone
—> Isomerisation of C5,6 double to C4,5 position
- Microsomal
- also primary steroidogenic tissues + breast, prostate, liver, skin - CYP17 (**sideway)
- 2 reactions:
—> 17α-hydroxylation
—> 17, 20-lyase reaction
- formation of **Glucocorticoid + **Androgen (but NOT Aldosterone)
—> Glucocorticoid: Cortisol (eventually)
—> Androgen: DHEA, DHEA-S (highest), Androstenedione
- expressed in many **Steroidogenic tissues
- expressed in inner zones (Zona Fasciculata + Zona Reticularis) but NOT outer zone of Adrenal cortex
—> Glucocorticoid (Corticol) synthesis
—> NOT synthesise Aldosterone
- Microsomal - CYP21
- 2 reactions
—> Hydroxylation of Progesterone to 11-Deoxycorticosterone (DOC) (position C21) (—> **Aldosterone)
—> Hydroxylation of 17-OH Progesterone to 11-Deoxycortisol (position C21) (—> **Cortisol)
- Microsomal
- ONLY in Adrenal cortex (less severe consequence if deficiency)
- **CYP21 deficiency —> most common inherited disorder of steroid hormone biosynthesis (*90% of CAH disorder) - CYP11B (2 isoenzymes: CYP11B1, CYP11B2)
- Inner zone: **B1 isoform:
—> 11-Deoxycortisol to **Cortisol (11β-hydroxylation)
- Outer zone: **B2 isoform:
—> DOC to **Aldosterone (11β-hydroxylation + 18-hydroxylation + 18-oxidation)
- Mitochondrial
ACTH + Angiotensin II: Regulation of biosynthesis of Adrenalcortical steroids
Diurnal rhythm (+ve), Stress (+ve), Glucocorticoid (-ve feedback)
—> Hypothalamus
—> CRH
—> Anterior pituitary
—> ACTH
—> ACTH receptor (GPCR —> cAMP —> PKA)
—> stimulates 1. **CYP11A activity (secs - mins) + 2. **CYP11A transcription (hrs - days)
—>
1. Inner zone (majority) —> Cortisol (+ Corticosterone)
2. Outer zone (minor effect) —> Aldosterone (Major inducer: RAAS system —> Angiotensin II + serum K)
**Chronic increased in ACTH —> ↑ protein enzymes synthesis —> **Hyperplasia of Adrenocorticol tissue
Angiotensin II:
- intrinsic ***vasoconstrictive properties
- also bind to membrane receptor of cells of Zona glomerulosa —> stimulate ***Aldosterone synthesis
Synthesis of Adrenal androgens
CYP17 (***sideway)
- expressed in many ***steroidogenic tissues
- expressed in ***inner zones (Zona Fasciculata + Zona Reticularis)
Adrenal Androgens:
- DHEA
- DHEA-S (highest)
- Androstenedione
- low level in adrenal glands
- unclear biological effects
- metabolised to more **potent sex steroids in **extra-adrenal tissues e.g. liver, gonads, skin, adipose tissue
—> Testosterone —> Dihydrotestosterone
—> Estradiol
Congenital Adrenal Hyperplasia
Mostly (90%) due to CYP21 (21-hydroxylase) deficiency
- early diagnosis essential to effective steroid treatment
- higher mutation rate due to ***gene duplication (CYP21P pseudogene + CYP21 functional gene)
- high carrier rate
Pathogenesis: ↓ Glucocorticoid —> ↑ ACTH (∵ less -ve feedback) —> ***Enlargement of adrenal gland —> ↑ Pregnenolone —> ↑ Progesterone + 17-OH Progesterone —> Next step blocked (∵ CYP21 deficiency) —> Go to sideway reaction —> ***Accumulation of Adrenal androgens —> conversion to potent sex steroid in extra-adrenal tissues —> ***Virilisation
Complication: ↓ Aldosterone —> persistent loss of Na in urine —> ***Dehydration + ***Hypotension —> shock, sudden death
3 forms with different severity of deficiency:
- Salt-losing (~67% of Classical form)
- ***Acute adrenal crisis early in life
- Severely ↓ / Total lack of Glucocorticoid + Aldosterone production - Simple Virilising (~33% of Classical form) —> Accumulation of Androgens
- Musculinisation of external genitalia in females (Virilism)
- Early sex development, accelerated growth, early bone maturation leading to ***short statue in males - Late-onset (Non-classic)
- Manifested only in females
Result of defect in enzymes
- CYP11A —> Lipoid CAH
- 3β-HSD —> Salt-losing CAH
- CYP17 —> Hypertensive CAH (∵ can only produce Aldosterone)
- CYP21 —> Virilising CAH
- CYP11B (B1/B2) —> Virilising CAH
CYP21 and CYP21P
CYP21 / CYP21B:
- functional gene
CYP21P / CYP21A:
- accumulate a lot of mutations (most CAH mutations found in CYP21P)
- pseudogene (usually not expressed, ∵ do not have functional promoters)
- but have functional promoters —> but non-functional products (∵ many mutations)
- 98% homologous to CYP21
- mutations e.g. 8bp deletion in exon 3 —> premature termination codons —> non-functional CYP21 production
- close proximity between 21A and 21B
—> mistakes copied from 21A to 21B via multiple mechanisms e.g. Unequal crossing over, Gene conversion etc.
—> leads to ↑ CYP21B mutation
—> ***Non-functional CYP21 gene
Unequal crossing over
Tandomly arranged CYP21P and CYP21
—> ↑ chance of **Meiotic mispairing (during 1st meiotic division chromatin exchange) + **Unequal crossing over
—> Gene deletion (loss of functional CYP21)
- **Meiotic mispairing:
- CYP21P with CYP21 —> but high enough homology —> stable enough
**Unequal crossing over:
- chromatin exchange between mispaired chromosomes
—> depend on level of unequal crossing over
—> original CYP21 can be made up mainly of 5’ part of CYP21P, **including deleterious mutations characteristics of CYP21P
—> ***Non-functional CYP21 gene
Milder versions:
- only C4B complement gene affected (becomes C4A)
- CYP21 not affected
