PMCOL 343 Glucocorticoids

What does the HPA axis regulate

- Metabolism

- Immune response

- Autonomic nervous system

Describe pathway of the HPA axis

- Whole circuit is stimulated by circadian rhythms, emotional stress, or physical stress

- CHR and AVP (vasopressin) are released from the hypothalamus onto the anterior pituitary

- CRH binds and activates receptors in the pituitary and induces ACTH synthesis and release from corticotrophin

- AVP has weak effect on corticotrophin, it just potentiates CRH

- ACTH then acts on adrenal cortex

Aldosterone

- Main physiological mineralocorticoid

- In the zona glomerulosa of Adrenal Cortex

- Angil and K+ stimulates secretion

Cortisol

- Main physiological glucocorticoid

- In the Zona fasciculata of Adrenal cortex

- ACTH stimulates release

Androstenedione

- Weak androgen steroid hormone

- In the Zona Reticularis of the Adrenal cortex

- ACTH stimulates release

What is the base ingredient of all adrenal cortex hormones

Cholesterol

17alpha-hydroxylase

- Enzyme that converts Pregnenolone to 17-OH pregnenolone

- Found in Zona Fasciculata and reticularis

21beta-hydroxylase

- Enzyme that converts Progesterone to 11-deoxycorticosterone (In Zona fasiculata and glomerulosa)

- Also converts 17OH-pregneolone to 11-deoxycotrisol (In Zona Fasiculata)

11beta-Hydroxylase

- Converts 11-deoxycotrisol to cortisol (In Zona Fasiculata)

- 11- deoxycorticosterone to corticosterone (In Zona fasiculata and glomerulosa)

Describe Negative feedback regulation of HPA axis

- Cortisol inhibits CRH secretion from the hypothalamus by direct effects on hypothalamic CRH neurons

- Cortisol inhibits ACTH secretion from Pituitary via GR

Rapid and delayed negative feedback in the hypothalamus

Decreased CRH levels is rapid, delayed is decreased mRNA levels

Rapid and delayed negative feedback in the Pituitary

- Rapid response is inhibiting response of corticotrophs to CRH

- Delayed response is suppression of expression of ACTH precursor

Rhythmic Regulation of the HPA axis

- Cortisol is secreted pulsatile, ultraradina rhythm of around 90mins. Peaks in the middle of the night

- GCs levels are regulated in circadian manner and peaks in the morning, then steadily declines

- ACTH id circadian rhythm and peaks in the early morning

Glucocorticoid metabolism in the Liver

11Beta-HSD1 activates cortisone to cortisol

Glucocorticoid metabolism in the kidney

11Beta-HSD2 inactivates cortisol to cortisone

Cells expressing 11Beta-HSD1 vs 11Beta-HSD2

cells with 11Beta-HSD1 increase local GC activation and binding to GR while 11Beta-HSD2 rapidly inactivate GCs

11Beta-HSD2

- High affinity enzyme

- Present in kidney, sweat gland, salivary gland, colon, CNS areas involved in salt and appetite and volume regulation and autonomic control

- protects MR from inappropriate binding and activation by GCs

11Beta-HSD1

- Low affinity enzyme

- Widespread distribution

- Amplifies local concentration of GCs

Describe GC signalling

- Around ~90% cortisol is bound to CBG (inactive)

- Proteases cleave this compound and release cortisol

- Lipophilic cortisol diffuses through membrane and then binds to GC receptor

- Logan bound GR translocates to the nucleus to exert genomic effects

GRalpha

Cannonical receptor associated with transcriptional regulation by GCs

GRbeta

Does not bind to ligand but may have dominant-negative effect of GRalpha

Homodimers of GRalpha

- A dimer that is just GRalpha

- Bind to glucocorticoid response element (GRE) to regulate gene expression

Heterodimer GRa-GRb

- Function as dominant negative inhibitors, antagonizing activity of GRalpha

Direct Dimers - DNA interaction

- Ligand bound GRalpha homodimers bind to GREs and causes an increase in gene transcription

- Can also bid to DNA negative GREs (nGREs) and gene transcription is inhibited

Composite GRE binding

- Monomer signalling

- Ligand-bound monomeric GRaplha binds to GRE which recruits a co-activator which transactivates a secondary gene

- Can also recruit a co-repressor which would trasnrepress another gene

- In this model, GRE has no direct effect on genes

GC-GR signalling: tethering

- Monomeric

- Ligand-bound monomeric GRalpha physically interacts with another transcription factor without contacting DNA

- It then enhances or decreases the capacity of partnering Transcription factor

Binding of glucocorticoids to cell membrane-associated GR

- Non-genomic effect

- Membrane bound receptors have distinct properties other than cytoplasmic ones which leads to effect within seconds to minutes

Permissive effects of glucocorticoids

Glucocorticoid does not initiate cell response, it promotes expression of other enzymes and hormones to lead to cell response

Cortisol secretion in acute stress

- overrides negative feedback control

- causes marked increase in plasma concentrations of cortisol

- Acute increase in cortisol helps survival chances

- Cortisol peaks 15-20mins after stress due to large surge of ACTH

- Pulsatility is maintained despite the greatly increased cortisol'

GC secretion in chronic stress

- Change in ratio of AVP to CRH in the hypothalamus. AVP becomes dominant.

- Decreased cortisol feedback but levels remained raised as a result of reduced cortisol feedback and reduced metabolism of cortisol

Metabolic effects of glucocorticoids: Carbohydrate metabolism

Increases circulating glucose

Metabolic effects of glucocorticoids: Protein metabolism

Increases proteolysis and inhibits protein synthesis

Metabolic effects of glucocorticoids: Lipid metabolism

Promotes fat deposition in trunk but fat breakdown in limbs

Metabolic effects of glucocorticoids: Muscle

Glucose uptake and glycogen synthesis decreases. Proteolysis increases

Metabolic effects of glucocorticoids: Liver

Gluconeogenesis increases, while glycogen synthesis decreases

Metabolic effects of glucocorticoids: Adipose tissue

Lipolysis increases (fasting), adipogenesis increases (fed state)

Metabolic effects of glucocorticoids: Brain

Regulates sleep, appetite, memory, cognition, emotions (high levels of cortisol is linked to depressive disorders)

Metabolic effects of glucocorticoids: Kidneys

decreases AVP levels and effects

Metabolic effects of glucocorticoids: Cardiovascular system

Vasoconstriction and increases blood pressure

Physiological effects of cortisol

- Inflammation: short increases of cortisol can boost immunity by limiting inflammation. However, consistently high levels can lead to inflammation and compromised immune system

- Blood pressure: More cortisol can increase blood pressure

- Controls sleep wake cycle: initiates wakefulness and the circadian rhythm

Effect of ACTH on Zona Fasciculata and Reticularis

- Absence of ACTH: undergo atrophy, severely impaired production of GCs and adrenal androgens

- Elevated levels of ACTH: hypertrophy and hyperplasia of these Zonas. Concomitant overproduction of GCs and adrenal androgens

Primary Adrenal Insufficiency

- Caused by adrenal gland failure

- Canot produce sufficient GCs and MCs

- no cortisol negative feedback, leads to high levels of CRH and ACTH

Congenital Adrenal Hyperplasia

- Most common inherited Primary Adrenal Insufficiency

- Genetic defect in 21B-hydroxylase

- ACTH stimulates tissue proliferation in adrenal cortex (adrenal hyper plasia)

- Patients have increased androgens and estrogen

What causes Primary Adrenal Insufficiency

- In adults, Addisons disease and HIV

- In kids, genetic defects

Addisons disease

- Caused by autoimmune adrenaltis, infection, or cancer

- Involves autoimmune T cells attack adrenal cells expressing 21B-hydorxylase. B-cells in local lymph nodes produce autoantibodies against 21B-hydroxylase

- Characterized by hyperpigmentation, vitiligo, weight loss, weakness, and GI problems

How to treat Addisons disease

GCs and MCs

Secondary Adrenal Insufficiency

- Caused by problems in pituitary leading to insufficient ACTH production

- Caused by tumours and autoimmune reaction

- More common than PAI

- Loss of adrenal cortisol and androstenedione production

- Increased CRH

- Aldosterone production is unaffected by due to RASS system being uninvovled

Tertiary Adrenal Insufficiency

- Problem is at level of hypothalamus

- Due to chronic administration of GCs

- May also be caused by tumours, radiation, and inflammation this region

- Diminished CRH which results in reduced ACTH

- Secretion of cortisol and androstenedione is impaired

- Aldosterone secretion is preserved

Cushing's syndrome

- Excess ACTH production

- Most common cause is a pituitary tumour

- Adrenal tumour can also cause it which leads to lots o cortisol

- Ectopic secretion of Cortisol (by other tissues) and hypercortisolism

- Characterized by red cheeks, red striation, poor wound healing, thin arms and legs, osteoporosis and fat in the trunk

Predinsolone

Short acting synthetic derivative glucocorticoid drug

Methylpredinsolone

Short acting synthetic derivative glucocorticoid drug

Tramicnolone

Intermediate acting synthetic derivative glucocorticoid drug

Dexamethasone

Long acting synthetic derivative glucocorticoid drug

Betamethasone

Long acting synthetic derivative glucocorticoid drug

Differences between synthetic and endogenous GCs

- potency (Synthetic GCs are much better activators)

- specificity (Synthetic GCs bind with higher affinity to GCs and lower affinity to MRs compared to endogenous)

- bioavailability

Structural modifications of synthetic GCs optimize:

- Pharmacokinetics

- Bioavailability

- Cross-reactivity with MRs

2 therapeutic uses of GCs

- Replacement therapy in acute and chronic insufficiency

- Non-endocrine uses like anti-inflammation, immunosuppressive, and anti proliferative (cancer)

Hydrocortisone

- Current drug of choice for replacement therapy

- Aims to replicate the circadian rhythm of cortisol in secretion

- Continuous subcutaneous hydrocortisone infusion (CSHI), replicates closely to cortisol physiology

GCs as anti-inflammatory agents

- Considered the best ones

- Mediated via transrepression of pro-inflammatory proteins and transactivation of key anti-inflammatory proteins like Annexin 1 and GILZ

Anti-inflammatory MOA via Co-repressors

1) GCs enter cell

2) combines with receptor

3) translocates to nucleus

4) Monomeric action with NF-kB which represses COX-2 and Pro-inflammatory Prostaglandins

5) AND/OR GC/GR complex can form a dimeric action with nGRE which reduces transcription of interleukins, interferon, and Tumor necrosis factor

Anti-inflammatory MOA via Co-activators

1) GCs enter cell

2) combines with receptor

3) translocates to nucleus

4) Forms Dimer with GRE

5) Increases transcription of anti-inflammatory genes like Ik-Beta-alpha, GILZ, and Annexin A1

6) These factors inhibit phospholipase A2 and Leukocyte infiltration

describe inhibition of COX 2

GCS inhibits NFkB which inhibits COX2Stops the formation of Prostanoids like prostaglandins, prostacyclin, and Thromboxane

Describe action of Annexin A1

- belongs to superfamily of calcium dependent phospholipid binding proteins

- inhabits action of phospholipase A2

- This blocks the release of arachidonic acid and in prevents synthesis of eicosanoids (whole inflammation process)

Describe basic inflammatory process

- Bacteria enter body

- infection recognized by tissue resident immune cells. initial contact of phagocytes with pathogens initiate antimicrobial mechanisms

- Inflammatory mediators act on smooth muscle cells to induce vasodilation. Nitric oxide and other mediators increase vascular permeability

- Phagocytes are recruited to site of pathogen entrance

- Phagocytosis of apoptotic neutrophils by macrophages leads to the transduction of anti-inflammatory signals

What do GCs and Annexin 1 increase/decrease in immune response

- Decrease transmigration of neutrophil

- Increase neutrophil detachment

- Increase macrophage expression, phagocytosis, and apoptotic neutrophil

Drawbacks of GC therapy

- Adverse effects in long periods/high doses (hypertension, central obesity, hyperglycaemia, immunosuppression)

- Patients can be GC resistant (inherited via mutations in GR gene or acquired)

Selective GR Modulators (SGRMs)

- GCs side effects are mostly mediated by dimeric GR transactivation

- Anti-inflammatory effects of GCs are largely due to GR transrepression

- SGRMS are novel steroid or non-steroids that favour trans depression actions of the GR over trasnactivating

GCs Nanoformulations

- Encapsulations of GCs in nanostrucutres of diverse chemical nature

- Selectively target individuals cell types, tissues, and organs

- Reduce systemic side effects like: Surface-modifed liposomes, Inorganic-organic hybrid nanoparticles, and different drug conjugates

Ketoconazole

- Inhibitor of GC synthesis

- Inhibits 17alpha-hydroxylase (stops conversion of pregenolone to 17-OH Pregeneolone in zone fasciculata and reticularis

- anti-fungal

- Used to treat Cushing's syndrome

- However it can increase progesterone and aldosterone

Metyrapone

- Inhibitor of GC synthesis

- Inhbits 11beta-hydroxylase (stops conversion of 11-deoxycorticosterone to corticosterone and stops conversion of 11deoxycortisol to cortisol)

- Treatment of Cushing's syndrome

- Chronic use may lead to hirsutism and hypertension

Mifepristone

- GC antagonist

- Synthetic steroid

- Bids to GR and Progesterone receptors

- Inhibits activation of GR

- has higher binding affinity to GC than dexamethasone

- Used in operable patients with ectopic ACTH secretion or adrenal carcinoma