Lipid mediators of inflammation

Question 1

What are eicosanoids?

Answer 1

• PGs, Thtomboxanes and leukotrienes are collectively known as eicosanoids
• There are two pathways:
• cyclic (cycloxygenase) pathway -> Prostanoids (PGs and TXs)
• Lipoxygenase pathway -> LTs and lipoxins

Question 2

Why are eicosanoids important?

Answer 2

• They have powerful inflammatory actions

- targets of major anti-inflammatory drugs

Question 3

Give 4 examples of anti-inflammatory drug types

Answer 3

• NSAIDs (aspirin)
• Glucocorticoids (methotrexate)
• Lipoxygenase inhibitors
• Leukotriene antagonists - expensive but very good for aspirin-induced asthma

Question 4

What is the process of prostanoid formation?

Answer 4

• Prostanoids are not ‘ready-to-go’ (unlike histamine)
• Prostanoids are generated from arachidonic acid (AA, poly-unsaturated fatty acid). This is rate-limiting step
• AAs are produced from phospholipids (PLs) via 1-step/2-step pathways
• These steps are triggered by many agents, e.g. thrombin on platelets and antigen-antibody reactions on mast cells

Question 5

How is AA made?

Answer 5

1 step
- PLs (via Phospholipase A2) –> AA

2 step

• PLs (Via PLC) –> DAG (via DAG lipase) –> AA
• PLs (Via PLD) –> phosphatidic acid (via PLA2) –> AA

Question 6

What sort of stimulus causes AA to come out of esterification with the membrane phospholipid?

Answer 6

• Inflammatory stimuli such as Bradykinin and adrenaline

Question 7

What are cyclooxygensases?

Answer 7

• Enzyme that converts AA to prostanoids

- Two main isoforms (COX-1/2)

Question 8

COX-1 vs COX-2 vs COX-3

Answer 8

COX-1

• Constitutively active
• Responsible for physiological roles of PGs/TXs such as regulation of TPR, renal blood flow, platelet aggregation, gastric cytoprotection

COX-2

• Needs to be stimulated (by inflammatory cytokines such as IL-1b or TNFa)
• Responsible for roles of PGs/TXs in inflammatory responses (pain and fever)

COX-3

• Variant of COX-1
• Pain perception of CNS

Question 9

What is the cyclooxygenase pathway?

DRAW IT

Answer 9

• AA (via COX) –> PGG2
• PGG2 is very unstable and so is quickly converted to PGH2
• PGH2 -> Thrombxane synthase -> TXA2
• PGH2 -> tissue-specific isomerases (mPGES-1) -> PGD2/PGE2/PGf2a
• PGH2 -> prostacyclin synthase -> PGI2

Question 10

What do the specific prostanoids do?

Answer 10

• TXA2 = platelet aggregator
• PGD2 = allergic responses, bronchoconstrictor, inhibits platelet aggregation
• PGE2 = myometrium contraction in labour
• PGF2a = bronchoconstrictor and uterine contraction
• PGI2 = inhibits platelet activation and vasodilator

Question 11

What is homeostatic balance?

Answer 11

• TXA2 is a platelet aggregator
• PGD2 and PGI2 inhibit platelet aggregation and activation
• The balance between them means that your blood isnt always clotted

Question 12

Give 2 drugs that inhibit thromboxane synthase

Answer 12

• Aspirin
• picotamide
• Ifetroban
• Terutroban

Question 13

Give 2 drugs that inhibit prostacyclin synthase

Answer 13

• Epoprostenol
• Iloprost
• Treprostinil

Question 14

What is the lipoxygenase pathway?

DRAW IT

Answer 14

• FLAP (5-lipoxygenase activator protein) acts on AA
• 5-lipoxygenase then acts on it to produce 5-HPETE
• 5-lipoxygenase then acts again to produce LTA4
• In turn, glutathiones transpeptidases and various AAs (e.g. glycine and cysteine) are all added to produce sulphidopeptide LTs (LTC4, LTD4, LTE4)
• If LTA4 hydrolase acts on LTA4 instead, then it produces LTB4
• other lipoxygenases can act on AA/LTA4 to make lipoxins

Question 15

What are some anti-inflammatory lipid mediators?

Answer 15

• Lipoxins

- Cyclopentenone PGs (CyPGs)

Question 16

How do Lipoxins and CyPGs carry out the anti-inflammatory actions?

Answer 16

• Lipoxins recruit monocytes to clear the inflamed site of necrotic/apoptotic neutrophils - regulate the activation of neutrophils and increase their phagocytosis
• Acting with CyPG, they promote phagocytic clearance of apoptotic cells by macrophages
• CyPGs inhibit macrophage activation, which decreases uncontrolled tissue damage, and decreases NF-kB activation
• This helps to decrease activation of inflammatory genes and dampen down inflammation

Question 17

What cells make the particular eicosanoids?

Answer 17

• Mast cells = PGD2, LTs
• Platelets = TXA2
• Endothelial cells = PGI2, PGE2
• Neutrophils/ Basophils/ Eosinophils = LTs

Question 18

What receptors do the eicosanoids act on?

Answer 18

• PGs (lungs, vascular, gut, CNS, kidney, uterus) = DP, FP, IP, EP1,2,3
• TXs (vascular/platelets) = TP
• LTs (general inflammation, lungs/vascular in acute allergy) = BLT (1&2) and CysLT (1&2)

Question 19

What do DP receptors do?

Answer 19

Vasodilation, decreased platelet aggregation, bronchoconstriction

Question 20

What do FP receptors do?

Answer 20

Contraction of myometrial smooth muscle, bronchoconstriction

Question 21

What do EP receptors do?

Answer 21

EP1 = bronchioconstriction, GIT smooth muscle constriction
EP2 = Bronchodilation, vasodilation, relaxation of GITSM and increase intestinal fluid
EP3 = Contraction of GITSM, increase gastric mucus secretion, decrease gastric acid, pyrexia

Question 22

What do TP receptors do?

Answer 22

• Vasoconstriciton and platelet aggregation

Question 23

What do BLT receptors do?

Answer 23

Chemotaxis and proliferation of immune cells, increased adhesion
- LTB4 is a very potent chemotactic for neutrophils

Question 24

What do CysLT receptors do?

Answer 24

Bronchoconstriction, vasodilation, increased vascular permeability

Question 25

Give 4 examples of LT receptor antagonists

Answer 25

Zafirlukast Montelukast Pranlukast Zileuton Block receptor for Cysteinyl LTs (C,D,E)

Question 26

What do CysLTs do in general?

Answer 26

Cause airway oedema, secretion of thick mucus and smooth muscle contraction

Question 27

Why are LT antagonists useful?

Answer 27

- Prevent mild to moderate asthma - Early to late bronchoconstrictor effects of allergens - Exercise induced asthma and NSAID induced astham

Question 28

How can NSAIDs lead to asthma?

Answer 28

- Aspirin blocks COX, however the lipoxygenase pathway is still active - Therefore we still produce LTs - This give thick sticky mucus, bronchoconstriction, cellular infiltration of eosinophils + neutrophils

Question 29

What are some side effects of LT antagonists?

Answer 29

GI upset Irritability Dry mouth, thirst Rashes, oedema

Question 30

Give the order of potency of bronchoconstriction (least to highest)

Answer 30

Histamine LTE4 LTC4 LTD4

Question 31

What type of receptors are EP receptors?

Answer 31

GPCRs - EP1 = Gq - EP2 = Gs - EP3 = Gi

Question 32

How does PGI2 carry out its actions?

Answer 32

- Inhibits platelet aggregation | - Also acts on IP receptors on SM in response to shear stress -> Increase cAMP -> relaxation

Question 33

How does PGE2 protect our stomach?

Answer 33

- Acts on EP3 receptors on parietal cells to reduce gastric acid secretion - EP3 receptors on mucus secreting cells to increase the mucus secretion along with bicarbonate

Question 34

Why are fish oils beneficial for our health?

Answer 34

- Provide substrates for the generation of alternative eicosanoids - Increasing PUFA uptake means that more ecosapentoic acid (EPA) and docosahexaenoic acid (DHA) is produced, essentially replacing AA - Less AA available to less PGE2,TXB2, LTB4, 5-HETE, LTE4 produced - EPA and DHA are used as substrates for alternative eicosanoids - Increases production of LTB5, LTE4, 5-HPETE (with different strcture to ones produced from AA) - LTB5 is 10-100x less chemotactic to neutrophils

Question 35

What anti-inflammatory metabolites can be made by COX activity EPA and DPA?

Answer 35

- E-series resolvins (resolvin D1-4, EPA-derived mediators) have anti-inflammatory actions - Docosatrienes and neuroprotectins (D series resolvins, DHA-derived mediators) also have anti-inflammatory effects