Steroids and Prostogladins

Question 1

What are the 2 main types of steroid hormone?

Answer 1

1) Corticosteroids = Mineralocorticoids (aldosterone), glucocorticoids (cortisol), androgens (DHEA)
2) Gonadal steroids = Progestogens (progesterone), Androgens (testosterone), oestrogens (oestradiol)

Question 2

What is the starting building block of all steroid hormones

Answer 2

Cholesterol.

Carbon rings are sequentially removed and oxygen containing groups are added

Question 3

What are the 4 Major sources of cholesterol that are used for steroid hormone biosynthesis

Answer 3

1) De novo cholesterol biosynthesis from acetate using HMG Co-A reductase.
2) Found in plasma membranes
3) Intracellular lipid droplets (containing cholesteryl esters such as cholesteryl oleate)
4) Plasma lipoproteins (primarily low-density lipoprotein cholesterol)

Question 4

What is the rate determining step in steroid hormone synthesis from cholesterol. And what enzyme catalyses the reaction

Answer 4

The conversion of cholesterol (a 27C molecule) to Pregnenolone (21C molecule).

This reaction is catalysed by the cytochrome p450 cholesterol side chain cleavage enzyme. CYP11A1

Question 5

Why is the conversion of cholesterol to pregnenolone the rate determining step of steroid hormone synthesis

Answer 5

The CYP11A1 needed for catalysis is located on inner mitochrondrial membane.
Cholesterol needs BOTH in order to pass into mitochondria

1) Steroidogenic acute regulator StAR
2) And Mitochondrial peripheral benzodiazepine receptors

StAR acts as a ligand for the benzo receptor.

Question 6

What is the significance of StAR or Steroidogenic acute regulator and lipoid congenital adrenal hyperplasia (not this is a form of normal CAH, but a congenital form)

Answer 6

In this condition StAR does not work in foetal adrenal glands. Therefore cholesterol cannot be transported into mitochondria leading to accumulation of cholesteryl esters forming lipid droplets in the adrenal cortex.

Therefore adrenals fail to make any corticosteroids = Leading to problems inflating lungs as no glucocorticoids in utero, and salt-water crisis because no aldoesterone. Most pregnancies will not be viable.

Question 7

What are the 2 main form of enzymes used for steroid biosynthesis

Answer 7

1) CYP enzymes

2) Hydroxysteroid dehydrogenase (HSD) enzymes

Question 8

What are the main function of the CYP enzymes in steroid biosynthesis? And how does it do them

Answer 8

1) Hydroxylation mainly = This basically just works to increase the solubility of the steroid hormone in fluid like blood, ECF, and urine.
- It does this converting NADPH to NADP+H+, and taking the electron via a ferredoxin reductase and ferredoxin and finally onto haem centre of CYP enzyme

2) Some CYP enzymes including 17a hydroxylase (CYP17A) also act like lyase enzymes to cleave C-C bones

Question 9

What are the main function of the HSD enzyme in steroid synthesis and how do they work?

Answer 9

HSD enzymes are oxidoreductase enzymes that are part of the alcohol dehydrogase family. This means they work to oxidise/reduce steroid precursors

1) 3b-HSD = An oxidising enzyme, takes electron to turn OH to O double bond. Uses the electron to turn NAD+ to NADH. Example is converting weak steroid pregnenolone to more potent ones like progesterone and androstenedione
2) 17B-HSD = Reductase enzyme that does the opp. Turns O double bone into OH by taking an electron from NADPH and turning it into NADP+. Example is converting androstenedione or estrone into testosterone and estradiol. Which are also both more potent.

Question 10

How are there tissue specific patterns of steroidogenesis

Answer 10

The pathways of steroidogenesis, and what end-products are made are actually dependant on what CYP and HSD enzymes are expressed in that particular tissue.
It is different in the testis leydig cells, follicular theca cells, gollicular granulosa, and luteal cells.

Question 11

Can the placenta make steroids

Answer 11

The placenta is an ‘incomplete endocrine gland’ because it lacks the enzymes required to synthesis a full range of steroids. It does not have CYP17A needed to convert progesterons to androgens. Instead it relies on foetal adrenals and maternal liver to make these, with steroids passing between placenta, and foetal+maternal circulations.

Question 12

What can be used as a surrogate marker to assess functional status of the foetal adrenal gland?

Answer 12

16a-hydroxyandrostenedione is secreted by the foetal zone of the foetal adrenal cortex. This can be converted/aromatised to estriol in the placenta. This is a unique steroid only made in placenta. And therefore can be a surrogate marker.

Question 13

How does testosterones work in periphery to cause virilisation. Steroid hormones often need extensive metabolism in the periphery to work

Answer 13

Testosterone has be converted within genital skin fibroblasts to an androgen metabolite 5a-dihydrotestosterone (DHT), which is done with the 5a-reductase 2 enzyme.

Question 14

How do steroid hormones work to close the epiphyses of the long bones

Answer 14

Testosterone needs to be metabolised by CYP19 to oestradiol. This acts on epiphyses via the oestrogen receptor, not the androgen receptor.

Question 15

How are steroid hormones excreted from the body

Answer 15

They need to be made more hydrophilic so that they can pass out through urine or faeces. This is a 2 step process that are catalysed primarily in the liver. Involves sequential steps of:

1) CYP or HSD enzymes = Creating polar hydroxyl (alcohol) groups
2) Conjugating enzymes = These are typically glucuronyltransferases or sulphotransferases. These add polar or charged chemical groups to the polar hydroxyl/alcohol groups.

Question 16

What are the 4 domains of the steroid hormone receptor

Answer 16

There are 5 different types of steroid hormone receptor, that each bind with a different affinity to each of the different types of steroid.

1) Ligang binding domain, which binds steroid in a hydrophobic binding pocket
2) DNA binding domain that has a pair of zinc fingers to bind DNA to make the changes
3) Dimerisation domain which facilitates interactions between pairs of activated steroid receptors
4) Transactivating factor domains, which interact with proteins in the preinitiation transcription complex

Question 17

How do steroid receptors work

Answer 17

Vacant steroid receptors are either in the cytoplasm or nucleus. Remember steroids are hydrophobic so can come through the plasma membrane.
After steroid ligand binds = Activated receptor translocates to nuclues to bind DNA and modulate target gene transcription.

Question 18

How does the steroid receptor bind to DNA/ and where

Answer 18

Dimeric receptor binds to the DNA at a specific sequence (steroid response elements) that are usually at the promoter region of the target gene

Question 19

How do steroid receptors reduce gene transcription

Answer 19

Causes activation of co-repressor proteins once binded to DNA. These recruit histone deacetylase enzymes which remove acetyl groups from adjacent histone proteins. Causes DNA to remain in a condensed state and reduces transcription

Question 20

How do steroid receptors increased gene transcription

Answer 20

Binding to DNA causes Co-activator proteins to recruit histone acetyltransferase enzymes. These cause acetylation of the histones which opens the DNA to open up and therefore increase gene transcription

Question 21

Where do we get most of our arachidonic acid? What is the name of the enzyme that liberates it, and what molecule inhibits this enzyme

Answer 21

Remember all prostagladins are prodiced from arachidonic acid.
Most of the AA is found in plasma membrane. So phospholipase A2 removes it from the membrane.

Phospholipase A2 is inhibited by Lipocortin which is upregulated by anti-inflammatory glucocorticoids lik cortisol and dexamethasone (negative feedback)

Question 22

What is the start of the prostagladin production pathway? What are the enzymes involved and what are the intermediates

Answer 22

Arachidonic acid converted into Prostogladin G2 by COX 1 enzyme. But this is then very quickly converted in prostogladin H2. This is the building block to then be worked on various prostogladin synthase enzymes to produce various types.

Question 23

List some of the prostogladin synthase enzymes and their products

Answer 23

Prostogladin H2 is the starting block, and can be acted on by PG D, PG E, PG F, or PG I synthases = And this respectively produces PG D2, PG E2, PG F2a, PGI2 (prostacyclin)

Question 24

How are prostagladins metabolised

Answer 24

The biologicla activities of PGs is based on alcohol group on C15. This can be oxidised by 15-hydroxyprostaglandin dehydrogenase causing its deactivation.
Most of this enzyme is in the lungs, and deactivates the majority of PGs.
Meaning PGs can only really act in an autocrine and paracrine effect on neighbouring cells.

Question 25

How to PG bind to their receptor

Answer 25

Even tho PGs (like steroids) are lipophilic, they actually bind instead to GCPR on cell surface. There are various PG receptors depending on the PG type, and they all have different secondary message cascades including cAMP and Ca.

Question 26

What regulates the contractility of the myometrium?

Answer 26

There is a careful balance of contractants and relaxant hormones that control the contraction.

Question 27

What are the hormones that promote myometrial contraction, and how do they act

Answer 27

Oxyotcin (post lobe of pit gland) and Prostogladins (specifically PGF2a) = Both these work by activating phospholipase C via the oxytocin receptor and the F prostanoid receptor.

Secondary messenger in contractants after receptor binding = Causes increase in Ca. This causes activation of myosin light chain kinase AND Ca dependant protein kinase to cause contractions.

Positive feedback = Oxytocin also stimulates PG synthesis and PGF2a upregulates oxytocin synthesis.

Question 28

How and which additional hormones can also promote contractility of the myometrium via oxytocin and PGF2a

Answer 28

Estradiol and cortisol steroid hormones increase expression of oxytocin and PGF receptors, and the enzymes required to synthesis PGs.

Question 29

What are the hormones that promote myometrial relaxation, and how do they act

Answer 29

Major relaant hormone = Progesterone. Works by impairing the synthesis and action of both oxytocin and PGF2a. It also increases cAMP and decreases operation of gap junctions in myometrium, so less messengers can go between to cause contraction and myometrium doesnt act like a functional syncytium

There are other relaxant hormones:
NO = Relaxes myometrium strangely independnat of its ability to raise cGMP
Relaxin = Increases cAMP
Prostacyclin = Increases cAMP

Secondary messenger in relaxants (progesterone, relaxain, prostacyclin) = is cAMP and cGMP. These then activate cAMP and cGMP dependant protein kinases that dephosphrylate myosin light chain and therefore limit contractions

Question 30

How do steroid hormone changing levels near labour change myometrial contractility

Answer 30

Near labour the plancenta produces less progesteron, meaning the progesterone:oestradiol ratio is less. Therefore meaning this promotes contracility more (from a steroid hormone only perspective)