Eicosanoids

Question 1

what are eicosanoids?

Answer 1

oxidation products of 20-carbon fatty acids
- Arachidonic acid
- Dihomo gamma-linoleic acid
- Eicosapentanoic acid

Question 2

2 types of eicosanoids

Answer 2

[classical]
- prostanoids (prostaglandins, prostacyclins, thromboxanes)
- leukotrienes

[non-classical]
- Lipoxins
- resolvins
- isoprostanes
- endocannabinoids

Question 3

prostaglandin synthesis

Answer 3

membrane phospholipids

[PLA2]

arachidonic acid

[cyclo-oxygenase]

PGH2

tissue-specific isomerases

TxA2, PGD2, PGE2, PGI2, PGF2-alpha

Question 4

what type of receptors do prostaglandins act on?

Answer 4

specific GPCRs on target cells

Question 5

PGE2 receptors

Answer 5

EP1 to EP4

Question 6

prostacyclin (PGI2) receptor

Answer 6

IP

Question 7

Thromboxane (Tx) receptor

Answer 7

TP

Question 8

PDG2 receptors

Answer 8

DP1 and DP2

Question 9

Physiological functions of PGs

Answer 9

1. Initiation of labour (PGF2-alpha and PGE2)
2. Inhibition of gastric secretion, increased gastric mucus production (PGE2)
3. Inhibition of platelet aggregation and vasodilation
   
   • PGI2 from endothelium
4. Platelet aggregation and vasoconstriction
   - TXA2 from platelets

Question 10

Pro and anti-inflammatory actions of PGs

Answer 10

[EP2]

pro-inflammatory receptor -> Gs/AC mediated elevation of cAMP in smooth muscle vasodilates

potentially anti-inflammatory receptor -> Gs/AC mediated elevation of cAMP in leucocytes inhibits function

Question 11

Von Frey - pain perception test

Answer 11

prostaglandin receptors are on sensory nerves (increase pain signals to brain)
-> EP1 receptor knock-out mice have DECREASED pain perception

Question 12

EP3 receptor

Answer 12

activates leucocytes and mast cells

Gi receptor -> reduces AC/cAMP signaling

enhances function and oedema formation

Question 13

prostaglandins in fever

Answer 13

elevation of hypothalamic thermostat (increased metabolism = heat production)

protective against infection

dangerous if prolonged or severe

regulated by production + action of PGE2 in HYPOTHALAMUS
-> cerebroventricular injection of PGE2 leads to fever
-> role for EP3 receptor

Question 14

what are the 2 isoforms of cyclo-oxygenase

Answer 14

[encoded by separate genes]

COX-1 and COX-2

Question 15

COX-1

Answer 15

“house-keeping” enzyme

products important in normal function of stomach, intestine, kidney and platelets

Question 16

COX-2

Answer 16

important in inflammation

Question 17

COX-3

Answer 17

splice variant of COX-1 gene

potentially expressed in CNX

previously suggested as paracetamol target

Question 18

what are NSAIDs?

Answer 18

non-steroidal anti-inflammatory drugs

Question 19

what is the most common mechanism of NSAIDs?

Answer 19

inhibition of COX enzymes

Question 20

aspirin

Answer 20

Covalent binding

Acetylates COX

Irreversible inhibition of COX

Releases salicylate - further actions?

Question 21

Ibuprofen

Answer 21

Competitive inhibitor of arachidonate binding to COX

Question 22

COX-2 selectivity

Answer 22

inhibition of COX-2 but not COX-1 across entire therapeutic dose range

Question 23

how is COX-1 measured?

Answer 23

as platelet Tx production

Question 24

how is COX-2 measured?

Answer 24

as monocyte PGE2

Question 25

how is NSAID selectivity for COX-1 and COX-2 studied?

Answer 25

POTENT RATIO

IC50 values for blocking the 2 enzymes

Question 26

side effects of aspirin-like drugs

Answer 26

gastric irritation/bleeding

renal toxicity

bleeding

=> due to blocking “house-keeping” COX-1 and reducing cytoprotective effects of PGs

Question 27

how does aspirin work?

Answer 27

irreversibly acetylates cyclo-oxygenase -> platelet TXA2 production ceases (lack of nucleus)

endothelial cells make new COX and so PGI2 still released

Question 28

when are the effects of aspirin useful?

Answer 28

Acute myocardial infarction

Coronary artery by-pass

After angioplasty and stenting

Acute thrombotic stroke

Pulmonary embolism and thrombosis

Question 29

normal action of platelets (no aspirin treatment)

Answer 29

TxA2 from platelets

PGI2 from vessel wall endothelium

Balance - controlled thrombus function

Question 30

aspirin-treated platelets

Answer 30

COX is inactivated by low-dose aspirin

Platelet COX-1 inactivated for life of the platelet - no TxA2

Blood vessel COX rapidly re-synthesised (hours)
- PGI2 levels maintained

=> decreased thrombus function

Question 31

leukotrienes - bronchoconstriction

Answer 31

LTC4 and LTD4 constrict human bronchial smooth muscle -> x1000 more potent than histamine

Increase effects of other constrictor agent

Question 32

leukotrienes - Oedema

Answer 32

LTC4 and LTD4 stimulate increased vascular permeability (neutrophil independent)

LTB4 increases vascular permeability (neutrophil dependent)

Question 33

leukotrienes - chemotaxis

Answer 33

LTB4 potent chemotactic agent for inflammatory cells

BLT1 receptor

Question 34

leukotrienes - inflammation

Answer 34

High levels of synovial fluid of Rheumatoid arthritis patients

Question 35

Leukotrienes (LTs)

Answer 35

arachidonic acid

[5-lipooxygenase]

5-HPETE

LTA4
-> LTB4 (chemotactic; BLT receptor
-> LTD4 -> LTE4 (cysLT receptor; bronchoconstriction; increased vascular permeability)

Question 36

anti-LP therapies

Answer 36

glucocorticoids - indirect inhibition of PLA2

Zileuton - targets 5-lipoxygenase

zafirlukast, montelukast - targets leukotrients

Question 37

future targets for anti-inflammatory drugs

Answer 37

PGE2 = main mediator of inflammation

Prostaglandin E synthase inhibitors - mPGES-1

Increase degradation of PGE2

Antagonist of EP receptor

Question 38

Why are eicosanoids important?

Answer 38

Potent vasodilators (PGE2, PGI2)

Increase vascular permeability (LTs)

Pain (PGE2, PGI2)

Fever (PGE2)

Synergy with other mediators
- Histamine
- Bradykinin
- Chemotaxins

Important modulators of cell function
- Direct effects
- Via effects on nuclear transcription factors?