S2: Steroids of the Adrenal Cortex

Question 1

What are the two parts the adrenal gland is divided into?

Answer 1

Adrenal Cortex

Adrenal Medulla

Question 2

What do the adrenal cortex and medulla secrete?

Answer 2

Adrenal cortex is the outer layer and secretes steriods

Adrenal medulla is the inner layers and secrete catecholamine’s (mainly adrenaline)

Question 3

What are the three classes of steroids?

Answer 3

Glucocorticoids - Mainly cortisol in mammals

Mineralocorticoids - Aldosterone

Androgens - sex steroids (can be metabolised in other tissues to become more potent)

Question 4

What disease does excess glucocorticoids and adrenal insufficiency cause?

Answer 4

Excess glucocorticoids = Cushing’s

Adrenal insufficiency: (primarily cortisol): Addisons

Question 5

Describe blood supply to adrenal glands

Answer 5

The adrenals are situated above the kidneys and the blood supply is in such a way that the arterial blood comes in at the outer cortex, drains through network of capillaries into and through the medulla, where at the venous end it drains out at the inner medulla.

Question 6

What are the 3 zones that cortical tissue is histologically divided into?

Answer 6

1. Zona glomerulosa
2. Zona fasiculata
3. Zona reticularis

Question 7

What does the cortical tissue having tissue specific enzyme expression mean?

Answer 7

There is layer/tissue specific enzyme expression, meaning the adrenal cortex zones can each act independently, called functional zonation of the cortex. This allows different hormones to be made in each layer.
Steroid synthesis in one layer can inhibit different enzymes in subsequent layers.

Question 8

What substance does steroid synthesis begin from?

Answer 8

Cholesterol

Question 9

What enzymes are used in steroid synthesis?

Answer 9

Cytochrome enzyme

Question 10

How is cholesterol converted to progesterone?

Answer 10

Running through this, cholesterol could enter into a cell in the zona glomerulosa, where it is converted into pregnenolone. Pregnenolone could stay in the cell and be converted to progesterone or it could enter back into the blood and move down to the zona fasiculata and under action of a new enzyme become a new molecule. Which could stay or move on etc.

Question 11

Where is aldosterone, cortisol and androstenedione produced?

Answer 11

This occurs due to zone specific expression of enzymes:

• Aldosterone - zona glomerulosa
• Cortisol - zona fasiculata
• Androstenedione - zona reticularis

Question 12

What is the functions of the adrenal steroids?

Answer 12

• • The glucocorticoids are involved in various aspects of metabolism and immune function. Stress increases release, but minimal levels essential are needed for normal function.
• The mineralocorticoids are involved in salt and water balance.
• Androgens, also called the ‘weak androgens’, have minimal function in humans with normal gonadal function.

Question 13

Describe how cortisol has a glucose conserving effect

Answer 13

It is essential to prevent fatal hypoglycemia

• Stimulates gluconeogenesis in the liver
• Catabolic functions - Lipolysis (release FA from stores) and Proteolysis. These released AA can be fed into the gluconeogenesis pathway.
• Cortisol antagonises the effect of insulin by counteracting the expression of GLUT-4 transporter (insulin induced receptor that allows uptake of glucose into muscle + adipose tissue). This increases insulin levels -> cortisol promotes insulin resistance.
• However, action of insulin is to promote lipogenesis, so although cortisol promote lipolysis the increase in insulin also promotes lipogenesis.

Question 14

What happens in excess cortisol?

Answer 14

In people with excess cortisol you tend to get promotion of hyperglycemia, insulin resistance (pre-diabetes) and promoting fat storage (with insulin overriding effects of cortisol here), which is why Cushing’s is associated with weight gain.

Question 15

Functions of glucocorticoids

Answer 15

1. Decreased glucose utilisation (glucose sparing - maintaining blood glucose during fasting)
   - Proteolysis
   - Gluconeogenesis (mainly from AA)
   - Lipolysis
   - Anabolic actions on liver
2. Cardiovascular Function
   - Required for vascular integrity
   - Cortisol deficiency can lead to inappropriate vasodilation (low BP)
   - Cortisol excess can cause hypertension via mineralocorticoid receptor

Anti-inflammatory, immunosuppressive function

• Effective drugs in highly profitable industry
• However, prolonged use can lead to side effect of excess cortisol

Question 16

Explain effect of glucocorticoids on inflammatory mediators derived from arachidonic acid (anti-inflammatory)

Answer 16

Arachidonic acid is like the mother of inflammation, a lipid derived substance that can be converted via phospholipase A2. 
Arachidonic acid can be converted into a large number of substances including prostaglandins.
Together the prostaglandins mediate the inflammatory response, the vasodilatation, increase in vascular permeability, attraction of leucocytes. 
Another class of signalling paracrine molecules are leukotrienes and involved in this also.

Cortisol inhibits the formation of arachidonic acid, it cuts off at the source the production of the inflammatory signalling molecules.

Question 17

What are prostaglandins?

Answer 17

Prostaglandins are lipid derived signalling molecules, their effects are paracrine (local, diffusion).

• They mediate inflammatory responses by increases vasodilation and attracting leucocytes.

Question 18

How is Arachindonic acid formed?

Answer 18

It is converted from phospholipid with the enzyme phospholipase A2

Arachindonic acid is the mother substance for inflammation

Question 19

Describe structure of glucocorticoid receptor

Answer 19

• Nuclear receptor family
• One gene for it but alternate splicing on 9th exon can lead to 2 major isoforms ( alpha and beta)
• Characteristic 3 domain structure

Question 20

Describe the mechanism of glucocorticoid receptors

Answer 20

• The receptor will bind its ligand (in this case cortisol), via the sequence in the ligand binding domain
• The receptor dimerises
• Bound receptor can bind to DNA by the DNA binding domain to the hormone response element of target gene

This is why nuclear receptors are controllers of transcription, they can turn on or turn off the transcription of numerous target genes by binding to the beginning of the gene at promoter region (in this case glucocorticoid response element)

Following binding of the receptor to the glucocorticoid response element, there can either be up-regulation or down-regulation of the transcription of that gene. What happens will depend on the gene and the combination of co-factors that are present, either repressor co-factors or activating ones.

Transactivation – Where glucocorticoid receptor enhances transcription of the target gene -> (so increased )
Transrepression – Where glucocorticoid receptor represses transcription of target gene

Question 21

What type of action are glucocorticoids though to have on genes?

Answer 21

Many anti-inflammatory effects of GCs thought to be due to transrepression of genes, which is why it is major therapeutic target (could develop more specific drugs).

Question 22

Funtion of mineralocortioid aldosterone

Answer 22

The mineralocorticoid aldosterone has a function of actively retaining salt via the kidney and hence retaining water passively.

Question 23

What does the mineralocorticoid receptor do?

Answer 23

The mineralocorticoid receptor (a nuclear receptor), induces expression (transactivation) of genes for the Na+/K+ ATPase and a particular type of K+ channel (active secretion of potassium).
It also has a role in volume regulation via RAAS.

By increasing activity of Na+/K+ you increase the Na+ gradient between the cell and the tubular fluid, so more Na+ is going to move into the cell and hence the circulation and be retained. Importantly, aldosterone is not under the control of the hypothalamo-pituitary axis. It is instead controlled as part of the renin-angiotensin network.

Any drop in perfusion or a drop in Na+ conc. (in macula densa) or sympathetic activity it will activate the renin-angiotensin pathway, through the juxtaglomerular apparatus in the nephron.

Question 24

Describe how aldosterone is secreted

Answer 24

There is first the conversion of angiotensinogen to angiotensin I by the enzyme renin. Circulating angiotensin I is converted to angiotensin II by ACE (angiotensin converting enzyme).
Ang II stimulates aldosterone secretion, which will be released according to need for sodium/water retention

Question 25

What are the affinities of cortisol and aldosterone to the mineralocorticoid receptor?

Answer 25

Cortisol and aldosterone have similar affinities for the mineralocorticoid receptor, i.e. cortisol can bind to the mineralocorticoid receptor with about the same affinity as the intended ligand, aldosterone.

Question 26

Why doesn’t cortisol bind to the mineralocorticoid receptor and stimulate salt and water retention if it has affinity for it and circulating cortisol concentrations are higher than aldosterone?

Answer 26

This would obviously be bad and can happen when cortisol levels are too high.

In general though it doesn’t happen. This is due to the enzyme 11beta-hydroxysteroid dehydrogenase type 2, which is present in the kidney and rapidly metabolises cortisol to inactive cortisone. This ensures only aldosterone activates the receptor.

11B-HSD1 converts cortisone back into cortisol.

Question 27

What can lead to apparent mineralocorticoid excess (AME)?

Answer 27

A very rare mutation can lead to inactivation of the 11B-HSD1, which leads to syndrome of apparent mineralocorticoid excess (AME). However note that anything that interferes with the functioning of this enzyme can lead to something such as this.

Question 28

Describe the metabolism of cortisol

Answer 28

• Normally, in mineralocorticoid responsive tissue, the enzyme 11B-HSD2 inactivates cortisol to tetrahydrocortisone (THE).
• In other tissues, the inactivation of cortisol involves conversion to tetrahydrocortisol (THF) end products.

Normally if you measure THE and THF in the urine they should be of equal amounts, but anything interfering with this enzyme 11B-HSD2, you will end up with less THE relative to the THF end products. This suggests an apparent mineralocorticoid excess (indicative of cortisol activation of mineralocorticoid receptor).

Question 29

How is glucocorticoid secretion controlled?

Answer 29

• Hypothalamic pituitary axis

From the hypothalamus we have released corticotrophin-releasing hormone (CRH) and then this would stimulate from anterior pituitary the release of adrenocorticotropic hormone/corticotropin (ACTH). This stimulates the adrenal cortex to release cortisol androgens.

The ACTH receptor is GPCR and via cAMP stimulates cholesterol uptake and steroid synthesis.
There is negative feedback on the pituitary and hypothalamus.

• The set point can be influenced by direct effects on the hypothalamus.

Question 30

Explain the circadian rhythm control of cortisol

Answer 30

Cortisol seems to always peak early in the morning.
This is important in diagnostic testing as you need to take into account this rhythm (test in morning, normal range will be higher than if tested in eve).

Question 31

How will lack of cortisol cause excess ADH release?

Answer 31

Cortisol appears to directly suppress ADH

Question 32

How is ACTH synthesised?

Answer 32

ACTH is a peptide hormone that is synthesised from the pro-opiomelanocortin prohormone gene (POMC). It is secreted from the anterior pituitary.

Question 33

How can POMC be spliced?

Answer 33

The POMC prohormone can be spliced in various ways to give different signalling molecules, including ACTH and alpha-MSH (melanocyte stimulating hormone), present in skin promoting skin pigmentation by stimulating melanocytes.

Question 34

How can high ACTH cause skin pigmentation?

Answer 34

ACTH receptor is a member of the melanocortin group of receptors, so ACTH can also bind to other melanocortin receptors. If ACTH is high then it will bind to melanocortin receptor for alpha-MSH and stimulate melanocytes resulting in skin pigmentation.

Question 35

What is Addison’s disease?

Answer 35

Addison’s disease is a primary adrenal insufficiency, meaning the adrenal cortex is not functioning efficiently to produce sufficient amount of its hormones, especially cortisol and aldosterone.

Question 36

What happens in Addison’s disease + the cause?

Answer 36

Low levels of circulating adrenal steroids (cortisol) will via negative feedback result in increased ACTH.
Along with the effects of low cortisol (such as on CVS, low BP), there will also be side-effects of high ACTH such as skin pigmentation due to ATCH on melanocortin receptors.

A loss of aldosterone may result in hypotension, due to loss of water which usually helps maintain blood volume and pressure.

The cause is typically autoimmune destruction of the cells. ACTH acting on the adrenal cortex will have no/little effect because it is the lack of function of the adrenal cortex that is the problem.

Question 37

Symptoms of Addison’s

Answer 37

• Weakness
• Weight loss
• Pigmentation
• Postural hypotension
• Anorexia

Question 38

What is Cushing’s Syndrome and Cushing’s Disease?

Answer 38

Cushing’s syndrome are the effects of excess glucocorticoids for any reason whatsoever, the most common cause is exogenous (external), as a side effect of glucocorticoid drug treatment.

Cushing’s disease is endogenous due to increased ACTH secretion (so a secondary adrenal disorder), typically due to a pituitary adenoma. An ACTH secreting pituitary tumour which is obviously endogenous, so there is very high levels of ACTH meaning that cortisol will be turned up and despite negative feedback still working we are working at a much higher level.

Question 39

Causes of Cushing’s syndrome

Answer 39

• ACTH dependent (pituitary adenoma-secondary or ectopic ACTH secreting tumour)
• ACTH independent (adrenal adenoma or carcinoma - primary and latrogenic; effect of GC therapy)

The normal –ve feedback still works, but as there is so much more ATCH, the levels of cortisol are much greater, so –ve feedback working at a much higher level.

Question 40

Treatment for Cushing’s syndrome/disease?

Answer 40

The glucocorticoids given are not cortisol but cortisol analogues so they can activate the negative feedback system leading to low ACTH. Levels of cortisol would be low because of low stimulation from ACTH

Question 41

Difference primary and secondary cushing’s

Answer 41

Primary – high steroids, low ACTH

Secondary – would be Cushing’s disease, if it was due to a pituitary tumour, high glucocorticoids and androgens, high ACTH.

Question 42

Symptoms of Cushing’s

Answer 42

Central obesity, slim arms and legs, bruising, hirsuitism (acne, greasy skin, male pattern baldness)

Question 43

Steps to diagnose cushings

Answer 43

First step: confirm hypersecretion of cortisol
• 24-hour urinary cortisol
• Cortisol at nadir of secretion (around midnight)

Next, determine the cause;
• Plasma ACTH
Dexamethosone suppression test

Question 44

What is dexamethasone?

Answer 44

Dexamethasone is an exogenous steroid and can be used to diagnose Cushing’s disease -> that ACTH coming from pituitary. This is because there are situations where there can be ectopic sources of ACTH (e.g. lung tissue starts releasing ACTH), this would be Cushing’s syndrome but not Cushing’s disease.

Question 45

Results of dexamethasone supression test (On ACTH and cortisol)

Answer 45

ACTH~

Low doses will normally suppress ACTH secretion via negative feedback, if a low dose fails to suppress ACTH secretion then there is Cushing’s.

Higher doses will suppress ACTH secretion in Cushing’s Disease.

If there is no suppression with low or high dosage, this suggest an ectopic source of ACTH (e.g. a tumour elsewhere) or adrenal tumour.

CORTISOL ~

When you give the dexamethasone you would expect the next day cortisol to be depressed because it activates the negative feedback system.

In Cushing’s disease, low level dexamethasone would not suppress cortisol but high doses will suppress it (because the –ve feedback loop is still working).
However if ectopic source of ACTH, any dose will not be effective because the tumour is not under the feedback control.