Introduction to Endocrinology

Question 1

What are the three groups of hormone?

Answer 1

Protein/peptide
Steroid
Miscellaneous

Question 2

What are peptide hormones initially produced as? Give an example.

Answer 2

Inactive prohormones

E.g. ACTH is produced as pro-opiomelanocortin (POMC)

Question 3

Describe the synthesis of protein/peptide hormones.

Answer 3

Amino acids are delivered to the cell. These are used to synthesise the peptide hormones in the same way that all proteins are synthesised. Genome –> mRNA –> protein

Question 4

What are all steroid hormones derived from?

Answer 4

Cholesterol

Question 5

How is cholesterol stored in cells and what do they look like in the cell?

Answer 5

Cholesterol is delivered in LDLs and then it is stored as fatty acid esters, which appear as fatty droplets in the cell. Steroid producing cells have a lot of fat droplets.

Question 6

Where are most steroid hormones produced?

Answer 6

Adrenals and gonads

Question 7

What enzyme breaks down fatty acid esters?

Answer 7

Esterase

Question 8

Where does steroid hormone synthesis take place within the cell?

Answer 8

Mitochondrion

Question 9

How does cholesterol get into the mitochondrion?

Answer 9

Via StAR proteins (steroidogenic acute regulatory proteins)

Question 10

As all steroid hormones are derived from cholesterol, what determines the final hormone produced?

Answer 10

The combination of enzymes that are found within the cell

Question 11

In terms of secretion, how are steroid hormones and peptide hormones different?

Answer 11

Steroid hormones are secreted immediately into the blood (as they are produced). Peptide hormones are packaged into vesicles and remain by the cell surface awaiting a signal before they are exocytosed.

Question 12

State some key features of protein/polypeptide hormones and state how they are stored.

Answer 12

Protein/polypeptide hormones are stored in the cells that produce them. They have a short half life.

Question 13

How are steroid hormones transported/stored?

Answer 13

They are transported in the blood - bound to carrier proteins such as albumin or other specific carriers such as cortisol and corticosteroid binding globulin.

Question 14

State two ways in which the steroid hormone equilibrium in the blood can be shifted.

Answer 14

Uptake of steroid hormones by tissue - leads to the release of some of the protein bound hormone to re-establish previous free steroid hormone levels.
Rise in plasma protein levels - more plasma proteins means that you are more likely that the plasma proteins will bind to the hormone so more protein bound hormone is formed.

Question 15

Give two examples of how the steroid hormone equilibrium can be shifted.

Answer 15

Cortisol is a stress hormone and in times of stress, more of the free cortisol enters the tissues so the free cortisol levels fall.
In pregnancy more plasma proteins are synthesised.

Question 16

Describe the mechanism of action of ACTH in stimulating cortisol production.

Answer 16

ACTH receptors are usually G-protein coupled receptors.
Binding of ACTH to the receptor activates adenylate cyclase, which converts ATP to cAMP. The increase in cAMP activates Protein Kinase A. Esterase is phosphorylated, and so cholesterol is liberated. Phosphorylation of the StAR protein activates the StAR protein allowing cholesterol to move into the mitochondrion. This then stimulates steroid hormone production.

Question 17

What type of receptor do steroid hormones bind to?

Answer 17

Intracellular receptors - they act as transcription factors

Question 18

Describe the feedback loop of ACTH and cortisol.

Answer 18

The anterior pituitary is stimulated by stress and ACTH is released. ACTH acts on the adrenal gland to increase cortisol production. Cortisol returns to the anterior pituitary and switches off cortisol production.