54: Adrenal Gland

Question 1

Describe the synthesis of steroid hormones in the adrenal cortex, including regulation by ACTH, CRH, diurnal variation, and feedback mechanisms.

Answer 1

Hormone synthesis in adrenal cortex:

• Common precursor of adrenal cortical hormones is cholesterol, LDL cholesterol or de novo synthesized cholesterol.
• Except for cytosolic 3beta-hydroxysteroid dehydrogenase, the rest belong to the family of cytochrome P-450 oxidase.
• The cholesterol to pregnenolone step is rate limiting (catalyzed by 20,22 desmolase, aka P450 side-chain cleavage enzyme).

Glomerulosa layer lacks 17 alpha-hydroxylase but has aldosterone synthase. There is no storage pool of presynthesized aldosterone. Secretion of aldosterone by the adrenal is limited by the rate at which the glomerulosa cells can synthesize the hormone.

Fasciculata and reticularis layers lack aldosterone synthase but have 17-alpha-hydroxylase.

Androgen synthesis:

Dehydroepiandrosterone (DHEA) and androstenedione are two adrenal androgens. They are far less potent than testosterone or dihydrotestosterone. However, androstenedione can be converted to testerone in the peripheral tissue.

DHEA reaches peak production in ones 2’s. Decline starts in early 30’s and a 75 yo has ~20% of a 25 yo. It is considered to be important for maintenance of sex drive in females after menopause.

Cortisol:

The synthesis of cortisol in the adrenal gland is stimulated by the anterior lobe of the pituitary gland with adrenocorticotropic hormone (ACTH) production of ACTH is in turn stimulated by corticotropin-releasing hormone (CRH), released by the hypothalamus.

CRH is a 41-amino acid neuropeptide made in the paraventricular of the hypothalamus. CRH receptors are G- protein coupled receptors on the cell membrane of corticotroph cells. Hormone binding activates signaling through second messengers lead to a rise in intra celllular calcium concentration which lead exocytosis of preformed ACTH. Prolonged CRH receptor activation also lead to increased gene transcription and synthesis of the ACTH precursor.

ACTH is a 39-amino acid peptide hormone secreted by the corticotroph cells of the anterior pituitary. It is produced by post-translational processing of a large precursor protein (i.e., a preprohormone) called pro- opiomelanocortin (POMC). ACTH receptor is also a G-protein coupled receptor called melanocortin-2 receptor. The resulting increase in cAMP activates PKA, which phosphorylates a variety of proteins. A rapid effect of ACTH is to stimulate the rate-limiting step in cortisol formation, that is, the conversion of cholesterol to pregenolone. In addition, prolonged activation of the receptor increase the synthesis of several proteins involved in cortisol synthesis.

Corticotrophs synthesize ACTH by complex post-translation processing of a large precursor protein called pro-opiomelanocortin (POMC), a precursor for several peptide hormones.
In the anterior pituitary, processing of POMC gives rise to ACTH, and beta- lipotropin.
During fetal life and pregnancy, the intermediate pituitary lobe processes POMC to yield different hormones including alpha-MSH and beta-endorphin.

Circulating cortisol exerts negative feedback control on the release of both ACTH and CRH:

1) In the anterior pituitary, cortisol inhibits the expression of POMC gene (POMC = precursor) and inhibits the release of presynthesized ACTH stored in vesicles.
2) In the hypothalamus, cortisol decreases the mRNA and peptide levels of CRH and inhibits the release of presynthesized CRH.

Circadiam rhythm and cortisol secretion:

The amount of cortisol present in the serum undergoes diurnal variation, with the highest levels present in the early morning, and lower levels in the evening. Information about thelight/dark cycle is transmitted from the retina to the suprachiasmatic nucleus of the hypothalamus which controls the circadium rhythms of the body. CRH release from hypothalamus is in pulses which results pulsatile secretion of ACTH. The greatest ACTH secretory activity occurs in the morning hours and diminishes late in the afternoon and early evening. Rhythmic secretion of ACTH directly controls the diurnal variation of cortisol production.

Stress and cortisol secretion:
Physical, psychological and biochemical stresses enhance CRH secretion and thus enhanced ACTH secretion which result in increased secretion of cortisol. For example, hypoglycemia stimulates the secretion of both CRH and ACTH which leads to increased release of cortisol that tend to raise blood glucose levels.

Question 2

Explain the causes and consequences of over secretion and under secretion of steroid hormones, including Addison Disease, Cushing Disease, and enzyme deficiencies.

Answer 2

21-alpha HYDROXYLASE DEFICIENCY (most common form of congenital adrenal hyperplasia)

21-alpha HYDROXYLASE DEFICIENCY is a genetic mutation that causes decreased production of cortisol and aldosterone.

The following scenario develops:
• decreased production of cortisol and aldosterone
• increased ACTH production (negative feedback gone)
• adrenal hyperplasia
• increased androgen production
• ambiguous genitalia in females

Diagnosis:
Elevation of 17-hydroxprogesterone before and after adrenocorticotropin (ACTH) stimulation test. Diagnosis should be confirmed by molecular genetic analysis of CYP21 gene (encoding 21-alpha hydroxylase).

Hormone replacement therapy:
Glucocorticoids such as prednisolone and dexamethasone, provide a reliable substitute for cortisol, thereby reducing ACTH levels. Reducing ACTH also reduces the stimulus for continued hyperplasia and overproduction of androgens. In other words, glucocorticoid replacement is the primary method of reducing the excessive adrenal androgen production in both sexes.

Mineralocorticoids are replaced in all infants with salt-wasting and in most patients with elevated renin levels. Fludrocortisone is the only pharmaceutically available mineralocorticoid.

Consequences of 21-a-hydroxylase deficiency:

1) decreased aldosterone synthesis = loss of salt, hypotension, dehydration
2) decreased cortisol synthesis = hypoglycemia, increased size of adrenal gland (hyperplasia)
3) increased androgen synthesis = virilizing effect in males (virilization refers to the biological development of sex differences, changes that make a male body different from a female body.) & ambiguous he/she genitalia in females

17-a-hydroxylase deficiency:

Biochemical consequences:

1) Reduced cortisol and androgen synthesis.
2) Increased cortisterone (a weak glucococorticoid, which mitigates the adrenal insufficiency) and aldosterone, which causes hypertension and hypokalemia.
3) Reduced estrogen synthesis (in the ovary).

Clinical symptoms:
Hypertension

Hypokalemia

Sexual infantilism in genetic females

Pseudohermaphroditism and sexual infantilism in genetic males

Hormone replacement therapy:

Hypertension and mineralocorticoid excess is treated with glucocorticoid replacement.

Genetic females need estrogen treatment. Genetic males may need surgery or testosterone treatment.

Consequences of 17-a-hydroxylase deficiency:

1) Increased aldosterone synthesis = hypertension & hypokalemia (low K+)
2) Decreased cortisol but increased coticosterone = increased size of adrenal gland (hyperplasia)
3) Decreased androgen synthesis = Sexual infantilism in genetic females & Pseudohermaphroditism and sexual infantilism in genetic males

Cushing’s syndrome:
Occurs when the body’s tissues are exposed to excessive levels of cortisol for long periods of time. It is often caused by prolonged use of immunosuppression drugs such as prednisone, adrenal tumors and tumors that increase ACTH such as pituitary adenomas, ectopic ACTH syndromes.

Cushing’s syndrome refers to excess cortisol of any etiology.

Cushing’s disease refers only to hypercortisolism secondary to excess production of adrenocorticotropin (ACTH) from a pituitary*** gland adenoma.

Cushing’s symptoms:
• Moon face, florid complexion (red face), upper body obesity, rounded face, increased fat around the neck, and thinning arms and legs.

• change in body fat distribution

• skin thinning and fragility

• osteopenia (low bone mineral density)

• increased frequency and severity of infections

• muscle wasting and weakness (excess cortisol)

• glucose intolerance

• hypokalemia and hypertension

Diagnosis:

Dexamethasone suppression test (administration of dexamethasone and frequent determination of cortisol and ACTH levels). Dexamethasone is a potent synthetic member of the glucocorticoid.

Take urinary or salivary cortisol measurement.

Other pituitary hormones may need to be determined if pituitary adenoma is suspected.

If adrenal tumor: cotisol high & ACTH low

If ACTH producing tumor: cortisol high & ACTH high

Treatment of Cushing’s disease:

Surgery to remove adrenal adenoma or ACTH producing adenoma. If both adrenals are removed replacement with hydrocortisone or prednisolone is imperative. An alternative therapy is to inhibit cortisol synthesis by drugs such asketoconazole and metyrapone. These drugs have limited efficacy.

Addison’s disease = HYPOadrenal function:

• autoimmune disease (TB infection)
• lack of aldosterone=hypotension & hyperalkalemia (high K+)
• lack of cortisol = hypoglycemia, weakness, weight loss, poor tolerance to stress
• primary adrenal disease & increased ACTH causes hyper pigmentation of skin & mucous membranes. Increased pigmentation of skin = high levels of ACTH.

The symptoms of Addison’s disease are caused by the failure of the adrenal glands to produce enough cortisol.

In primary adrenal insufficiency (classic Addison’s disease) the hormone aldosterone is also deficient.

Many of the symptoms of Addison’s disease arise due to the hyposecretion of aldosterone leading to hyperkalemia + hypotension. Often the production of adrenaline is also diminished or eliminated.

Most Addison’s disease are due to autoantibodies directed against adrenal cells containing 21-hydroxylase, an enzyme involved in the production of cortisol and aldosterone.

Diagnosis

1. Determine whether levels of cortisol are insufficient.
2. ACTH Stimulation Test: Blood cortisol, urine cortisol, or both are measured before and after a synthetic form of ACTH is given by injection. The normal response after an injection of ACTH is a rise in blood and urine cortisol levels. Adrenal insufficiency results in poor response or none at all.
3. CRH Stimulation Test: Synthetic CRH is injected intravenously. Cortisol and ACTH in the blood are measured before and after injection:

Patients with primary adrenal insufficiency have high ACTHs but do not produce cortisol.

Patients with secondary adrenal insufficiency have no/ delayed ACTH response & have deficient cortisol responses.

Treatment
Treatment for Addison’s disease involves replacing the missing cortisol and, if necessary (for primary Addison’s), fludrocortisone as replacement for the missing aldosterone.

The enzyme 11β-hydroxysteroid dehydrogenase type 2 is present in renal tubular cells and converts cortisol to cortisone, which does not readily bind to mineralocorticoid receptors. When this enzyme is deficient, cortisol, which is present in the plasma in considerably higher concentrations than aldosterone, binds the mineralocorticoids receptor and exerts mineralocorticoid effects. Hyperkalemia and decreased blood pressure reflect mineralocorticoid deficiency.

Question 3

Describe the actions and mechanisms of adrenal cortical hormones in target tissues.

Answer 3

Effects of aldosterone:

• increase Na(Cl) reabsorption from distal nephron segments
• increase potassium secretion by distal nephron segments

There is no storage pool of presynthesized aldosterone. Secretion of aldosterone by the adrenal is limited by the rate at which the glomerulosa cells can synthesize the hormone.

Once secreted, about 37% remains free in the plasma, the rest weakly binds to CBG and albumin.

The major action of aldosterone is to stimulate the kidney to reabsorb sodium and water and enhance potassium secretion. It has similar actions in the colon, salivary glands, and sweat glands.

In the target cells of the renal tubule, aldosterone increases the activity of several key proteins involved in sodium transport. It increases transcription of sodium-potassium pump and expression of apical sodium channels. The net effect of these actions is to increase sodium reabsorption and potassium secretion.

Cortisol mechanism of action:

a. Free cortisol enters target cell by diffusion
b. binds to cytoplasmic receptor

c. migrates to the nucleus

d. modulates gene transcription

In feedback inhibition loop, cortisol inhibits the expression of CRH (Corticotropin-releasing hormone) receptor and ACTH in the corticotrophs of the anterior pituitary. Cortisol inhibits transcription of POMC gene.

Cortisol metabolic effects:

• stimulates gluconeogenesis in the liver

• enhances protein breakdown in muscle cells to provide amino acid substrate for gluconeogenesis (breakdown muscle)
• stimulates lipolysis in adipose tissue in the peripheral as alternative fuel to glucose. Curiously, some fat is deposited centrally for unknown reasons.
• decreases osteoblastic activity in trabecular bone and interferes with Ca++ absorption from gut.

Cortisol anti-inflammatory effects:

• inhibits production of cytokines

• inhibits production of chemo-attractant molecules

• stabilize lysosomal enzymes

• contributes to vasoconstriction and decreased capillary permeability

Cortisol Immunosuppressive effects:

• decreases lymphocyte proliferation

• inhibits hypersensitivity reactions (especially cell-mediated)


Cortisol Other effects:

• emotional instability (receptors are expressed in the brain).

Question 4

Non-objective important stuff:

Answer 4

The adrenal cortex:

• derived from the mesoderm
• secretes glucocorticoids, mineralocorticoids, and androgens. Note that Mineralocorticoid is secreted by zona glomerulosa while glucocorticoids and androgens were secreted by zona fasciculata and zona reticularis.
• essential for life (total loss of function is fatal in 4-14 days)

The adrenal medulla:

• derived from neural crest
• epinephrine, norepinephrine, dopa, and dopamine
• modified post-ganglionic sympathetic nerve cells
• total loss is not life threatening

Cortisol transport in plasma:

• 90% bound to cortisol binding protein (CBP or transcortin)
• 7% bound to albumin

• 3% circulates “free”

Steroids like cortisone with potent glucocorticoid activity tend to increase plasma glucose concentration (like cortisol). As a result, insulin secretion is stimulated. Increased glucocorticoid activity also diminishes muscle protein. Because of feedback, cortisone administration leads to a decrease in adrenocorticotropic hormone secretion and, therefore, a decrease in plasma cortisol concentration.