Eicosanoids

Question 1

What are the Eicosanoids?

Answer 1

Oxidation products of 20-carbon FA’s…..
predominantly of arachidonic acid.

Classical eicosanoids - prostanoids and leukotrienes (LT).

Non-classical include:
Endocannabinoids and Lipoxins, resolvins, isoprostanes.

Question 2

What are the Prostanoids?

Answer 2

Prostaglandins (PG), Prostacyclins (PGI) and Thromboxanes (TX)…

Question 3

How are leukotrienes synthesised?

Answer 3

Classical eicosanoids.

Arachidonic acid is converted to 5-HPETE by 5-Lipoxygenase.

5-HPETE forms LTA4 - the precursor to LTB4 or LTC4, LTD4 and LTE4….

LTC4, LTD4 and LTE4 are cysteinyl LTs which act via cysLT receptors.

LTB4 = chemotactic + neutrophil-depedent vascular permeability = via BLt1 receptor.

Question 4

What are the actions of Leukotrienes?

Answer 4

Bronchoconstriction = LTC4 and LTD4
Oedema = LTC4 and LTD4 + LTB4
Chemotaxis = LTB4
Inflammation.

Question 5

How are Leukotrienes related to bronchoconstriction and oedema?

Answer 5

LTC4 and LTD4 are very potent human bronchial SM contractor agonists
(1000 times more potent than histamine).

LTC4 and LTD4 stimulate vascular permeability INDEPENDENTLY to neutrophils.
LTB4 is released by neutrophils = neutrophil dependent vascular permeability

Question 6

How are LTs related to Chemotaxis?

Answer 6

LTB4 = neutrophil-dependent vascular permeability.
And a potent chemotactic agent for recruiting inflammatory cells by acting on BLT1 receptors….

Question 7

How are LTs implicated in RA?

Answer 7

LTs are present at high levels in synovial fluid of RA patients…

Question 8

What are the various anti-LT therapies?

Answer 8

Zileuton - Inhibits 5-lipoxygenase.
Stops the conversion of arachidonic acid to 5-HPETE.
= Stops the formation of ALL LEUKOTRIENES….

Glucocorticoids inhibits PLA2 to reduce arachidonic acid synthesis from membrane phospholipids…
= Indirect inhibition.

Interestingly, no evidence to suggest statins decrease leukotriene synthesis.

Zafirlukast and Montelukaast block LTC4/LTD4 via cysLT receptor?

Question 9

How are prostaglandins synthesised?

Answer 9

Under basal conditions the rate of synthesis of PGs is low, until inflammation.

Cyclo-oxygenase form PGH2 from arachidonic acid.

Tissue-specific isomerases then convert PGH2 to TxA2, or PGE2, PGI2 etc.

Question 10

What are the various Prostanoid receptors?

Answer 10

TxA2 acts on TP receptors.
PGI2 acts on IP receptors.
PGE2 acts on EP1 - EP4 receptors
= all GPCRs..

Question 11

What prostanoids initiate labour?

What prostanoids decrease Gastric acid production + enhance production of mucus?

Answer 11

PGF2a and PGE2 initiate labour!

PGE2.

Question 12

What are the physiological functions of PGs?

Answer 12

Initiate labour - PGF2a and PGE2.

Inhibit gastric acid secretion + increase gastric mucus production = PGE2.

Inhibit platelet aggregation and cause vasodilation = PGI2 - Gs coupled = PKA…

Platelet aggregation and vasoconstriction = TXA2…
(dense granules release = Gq coupled)

The perception of pain!!!

Question 13

What PGs are involved in vascular SM tone and platelets?

Answer 13

TxA2 acts on Gq coupled TP receptors to induce platelet aggregation and vasoconstriction.

PGI2 acts on Gs coupled IP receptors to induce vasodilation and inhibit aggregation of platelets

Question 14

What are the pro-inflammatory actions of prostaglandins?

Answer 14

EP2 receptor has both pro and anti-inflammatory actions.

EP2 in vascular SM = Gs coupled = vasodilation.
= pro

EP2 in leukocytes = Gs coupled = inhibits function.
= anti

Via the same Gs-coupled EP2 receptor!!

Question 15

How are PGs related to pain perception?

Answer 15

EP1 receptors are expressed on c fibre sensory nerves.

Ep1 receptor KO mice have reduce perception of pain!!!

Von frey pain perception test.

Question 16

What is the role of the EP3 Receptor?

Answer 16

EP3 receptors are Gi linked = inhibit AC, reduce cAMP.

EP3 receptors are expressed on leukocytes and Mast cells.
- enhance function of leukocytes and Mast cells.

EP3 also enhances oedema formation.

EP3 involved in fever…

Question 17

How are PGs related to fever?

Answer 17

Elevated hypothalamic thermostat - elevated metabolism + heat production to kill infection.

Prolonged/severe fever can be dangerous.

Hypothalamus regulates and produces PGE2.

EP3 particularly involved, Gi coupled.

Cereboventricular injections of PGE2 lead to fever.

Question 18

What are the isoforms of Cyclo-oxygenase?

Answer 18

COX-1 is constitutively active - Housekeeping enzyme.

COX-1 converts Arachidonic acid to PGH2 - to form prostanoids.
COX-1 in normal function of GI tract + platelets.

COX-2 is induced - especially during inflammation.

COX-3 relevance in humans is contested.

Question 19

How is COX-1 involved in housekeeping activity?

Answer 19

COX-1 is contstitutively active and involved in numerous homeostatic functions.

Gi tract, renal tract, platelet function + macrophage differentiation.

Question 20

How can COX-2 be induced?

Answer 20

IL-1 and TNF, as well as Growth Factors, can +ve effect on COX-2.

Glucocorticoids have -ve effect on COX-2….
(depletion of arachidonic acid?

Question 21

How does aspirin inhibit COX?

Answer 21

COX forms a hterodimer for catalytic activity.

Ser529 residues in catalytic site are irreversible acetylated by aspirin.

= obstruction of access of arachidonic acid.

= COX cannot synthesise PGH2.
= new enzyme needs to be synthesised..

Salicylate remains - which itself may have additional actions.

Question 22

How does ibuprofen work?

Answer 22

A competitive inhibitor for arachidonate binding to COX.

Question 23

What is COX-2 specificity?

Answer 23

Inhibiting COX-2 - not COX-1 - across the entire therapeutic dose range!

Diclofenac and Celecoxib.

Question 24

What are the mixed NSAIDS?

Answer 24

Ibuprofen, aspirin and Naproxen

= Ibuprofen can competitively inhibit both COX-1 and COX-2

Question 25

What are COX-1 selective NSAIDS?

Answer 25

LOW DOSE ASPIRIN and Flurbiprofen!

Question 26

What are the side effects of aspirin-like drugs?

Answer 26

Blocks the housekeeping effects of COX-1 on homeostatic functions.

Reduces cytoprotective effects of PGE2 in its ability to stimulate GI mucus and reduce Gastric acid secretion!

Renal toxicity, GI irriation and bleeding..

Question 27

What is the current theory behind Rofeoxib badness?

Answer 27

Much higher incidence of thrombo embolic events.

Endothelial PGI2 is most derived from COX-2, not COX-1.
Whilst TxA2 is most COX-1, not COX-2 derived…

Since Rofecoxib was COX-2 selective - it had suppressive effect on vasodilatory and anti-platelet effect of PGI2.

Disrupting the balance of TXA2 and PGI2 production!

Question 28

What are the adverse effects linked with COX-selectivity?

Answer 28

COX-2 = thrombosis and myocardial infarction = CV risks.

COX-1 = GI bleeding, ulcerations = GI effects…

VIGOR study - compared Rofecoxib vs Naproxen =
Rofecoxib had significantly less GI irritation, but higher risk of CV events!!!

Question 29

How is aspirin anti-thrombotic?

Answer 29

Platelets have no nucleus, so cannot re-synthesise COX.

Endothelial cells can re-synthesise COX.

Aspirin irreversibly acetylates SER529 residues in catalytic site to inactivate enzyme.
= in platelets, no longer able to synthesise PGs/TXA2 etc.

But in endothelial cells, still able to produce PGI2.
= PGI2 still released by endothelial cells, but platelets cannot produce TXA2.

= PGI2 levels maintained, with reduced TXA2
= Dereased thrombosis

Low dose aspirin - with its greater COX-1 selectivity:
has been shown to protective effect against colon and rectal cancer + delaying onset of Alzheimer’s…

And used in myocardial infarction, in angioplasty + stenting, thrombotic stroke