CAH Flashcards
What are steroid hormone receptors?
- found within the cell, typically in cytoplasm or nucleus
- ligands are small lipophilic molecules
- receptor encoded by a single gene
- able to bind to DNA
- function as transcription factors
How do steroid hormones work?
- most hydrophobic steroid are bound to plasma protein carriers as they cannot travel in water, only unbound hormones can diffuse into target cell
- bind to receptor that use secondary messengers
- receptor-hormone complex binds to DNA and activates or represses one or more genes
- activated genes create new mRNA –> protein synthesis
What are type 1 nuclear receptors and how do they work?
- work as homo-dimers
- normally found in cytoplasm
- e.g. glucocorticoids, mineralocorticoids, androgens
- when hormone binds, move from cytoplasm to uncles and bind to sequence of DNA called hormone response elements (HRE’s)
- nuclear receptor/DNA complex then recruits other proteins which transcribe DNA downstream from HRE to mRNA and eventually protein causing a change in cell function
What are type 2 nuclear receptors and how do they work?
- work at hetero-dimers
- all bind as heterosexual-dimers with RXR (retinoid X receptor)
- e.g. VDR (vitamin D), RAR and TR
- found in nucleus normally bound to response element already
- these receptors have proteins on them (co-repressors) which stop transcription
- activated by removing co-repressor when ligand binds
Describe the structure of the steroid hormone receptors
- 3 domains: transactivation, DNA binding and hormone binding
- within those there are 5 further domains: A,B,C,D,E and sometimes F
A/B: AF-1 = always on, when hormone binds to receptor, AF-2 works with AF-1 to drastically up regulate gene transcription (synergisation)
C/D: DNA binding areas, NLS = nuclear locational signal (allows entry into nucleus to bind to DNA). Hinge that allows dimerisation is here
E: within domain there is AF-2- which is a transcriptional activation domain, becomes active when hormone bound
What is a zinc finger?
- within the C domain
- enables proteins to bind to DNA
- second zinc finger domain involved in dimerisation
What are HRE’s?
Hormone response elements
- within receptor there is a set sequence recognition of DNA
- allow for specificity
- dimeric and 2 halves for 2 receptors as its dimeric
- vary in structure
How does a hormone alter gene transcription?
- binding via receptor to target sequences of DNA
- HRE
- located in regulatory regions of target gene
- usually 5’, close to core promoter
- 6bp hexamer - core recognition motif
- usually 2 half-sites, intervening base pairs
How does a receptor recognise a specific HRE?
P-box contains:
- zinc fingers which enable DNA docking
- HRE which allows for specificity
What is ACTH and how is it synthesised?
Adrenocorticotrophic hormone - aka adrenocorticotrophin
- polypeptide of 39 amino acids
Synthesis:
- large precursor Pro-opiomelanocortin (POMC) is the source of several important biologically active substances
- ACTH = cut from POMC
How is cortisol released in response to ACTH?
- stress = hypothalamus releases CRH (corticotrophin releasing hormone)
- interacts with receptor to cause release of ACTH
- interacts with receptor to cause cortisol release from adrenal cortex
What are the effects of ACTH?
- ACTH stimulates G-protein receptor coupled to cAMP
- this stimulates the enzyme that converts cholesterol to cortisol or sex steroid precursors
- ACTH rises with stress - this is used clinically to test corticotroph function following insulin challenge
What is the basic structure of the adrenal glands?
- sit on top of kidney
- split into the adrenal cortex and medulla
What are the adrenal cortex hormones?
- corticosteroids: glucocorticoids, mineralocorticoids
- -> synthesised with P450 cytochrome enzymes
- (weak) androgens
- -> derived from cholesterol (synthesised from acetate)
Describe the histological zonation of the adrenal glands
Cortex split into 3 zones:
1. Zona glomerulosa
2. Zona fasciculata
3. Zona reticularis
(All 3 responsible for corticosteroid synthesis)
Medulla responsible for catecholamine synthesis
How is cholesterol imported into mitochondria?
- by Steroidogenic acute regulatory (StAR) protein
- transported from mitochondrial membrane into inner mitochondrial membrane
- this is the RATE LIMITING STEP
Describe the structure and regulation of StAR protein
- contains a cholesterol transfer domain
- produced in response to stimulation usually through a cAMP secondary messenger system
- promoted by ACTH and LH
- suppressed by alcohol
How is pregnenolone formed?
cholesterol –> pregnenolone
- using cytochrome P450scc
(scc = side chain cleavage)
- occurs within inner membrane of mitochondria
- two hyrdoxylase reactions produce 20,22-dihydroxycholesterol
- final stage = cleavage of bond between carbon 20 and 22 to produce pregnenolone
How is P450scc regulated?
- requires electrons for function
- works in a complex with 2 other proteins: adrenodoxin reductive and adrenodoxin (these provide initial electrons)
- always active but dependent on supply of cholesterol
- ACTH induces expression of all the genes in the complex
What are glucocorticoids?
- C21 steroids
Examples: - cortisol (aka hydrocortisone)
- corticosterone
*cortisol binds, cortisone does not, cortisone is converted by the body to cortisol
Describe the biosynthesis of glucocorticoids
CYP11B1
- metabolises precursors to make cortisol
- requires different layers of enzymes
How is cortisol transported in the blood?
- 90%+ bound to plasma protein
- -> transcortin (CBG) - 80+%
- -> albumin - 10%
Why is cortisol transported bound to plasma protein?
- lipophilic = doesn’t like water therefore transporting cortisol in blood is difficult
- transcortin = carrier protein for cortisol
- in circulation, cortisol exists in 2 forms, bound and unbound
How does bound cortisol become unbound?
Random equilibrium means carrier proteins randomly drop off molecules
What can impact cortisol levels in the body?
- emotion via limbic system
- biochemical stressors
- drive for diurnal rhythm
How is ACTH generated?
- produced from single gene = POMC
- ACTH produced when POMC gets cleaved by enzymes
- ACTH acts on adrenal but is also a prohormone
- can be cleaved into a-MSH (regulates melanocytes) and CLIP (no known function)
What are the effects of ACTH?
- interacts with cell surface receptor (melanocortin 2 receptor) and uses secondary messengers as peptide receptor
- cAMP activates secondary messenger
Immediate = increased cholesterol into mitochondria Subsequent = increased gene transcription of hydroxylases, increased LDL receptors Long-term = increased size and functional complexity of organelles, increase size and number of cells => HYPERPLASIA
What effects do glucocorticoids have on metabolism?
CARBOHYDRATES
- increased plasma glucose
- increased hepatic gluconeogenesis
- inhibits glucose entry into tissues
PROTEIN muscles - increase breakdown of protein --> amino acids --> wasting/ growth retardation liver - increase uptake of amino acids --> protein synthesis --> gluconeogenesis
FAT
- increased metabolism of fatty acids from adipose tissue –> gluconeogenesis
- redistribution to extremities
What are the effects of glucocorticoids on bones?
- decreased absorption of Ca2+
- increased excretion
- inhibition of osteoblasts –> osteoporosis
What are the effects of glucocorticoids on the CNS and the immune system?
CNS: affects mood and cognition
Immune system:
- decreased lymphocytes, eosinophils (disease fighting white blood cells)
- increased neutrophils, RBCs and platelets
What are the therapeutic actions of glucocorticoids?
- Anti-inflammatory: potentially decrease leukocyte action - rheumatoid arthritis: potential proteolysis at site of damage
- Anti-allergic: decreased histamine synthesis and release
- Immunosuppression
