Platelet-endothelial interaction

Question 1

Pro-inflammatory cytokines and their effects

Answer 1

• ILs (particularly 1 and 6), TNFa, INFg, VEGF
• Cause: increased TF, fibrinogen, vEF, adhesion molecules on endothelium
• They also cause: decreased thrombomodulin (TM)
• Overall: increases chance of thrombus formation
• Also increases thrombus formation chance: bad genes, bad food, obesity

Question 2

Formation of atherosclerosis: fatty streak

Answer 2

• Accumulation of cholesterol-rich lipids in sub endothelial (intima) space (usually at site of flow disturbances like bifurcation)
• The lipids become modified and oxidized, then invoke an inflammatory response by causing endothelium (EC) and smooth muscle cells (SMC) to release pro-inflammatory cytokines
• This stimulates the migration of macrophages to the lipids
• The macs ingest the oxidized lipids and become foam cells
• Accumulation of foam cells is the fatty streak

Question 3

Formation of atherosclerosis: fibrous plaque 1

Answer 3

• Foam cells (activated macs) express high levels of TF
• The foam cells will cause thrombosis if they are exposed to blood, but this is limited by the endothelial cells
• If the endothelium is denuded microthrombi and platelets form over the fatty streak, the beginning of the fibrous plaque

Question 4

Formation of atherosclerosis: fibrous plaque 2

Answer 4

• The platelets release PDGF and TGF-b and, combined w/ mitogenic effects of thrombin, stimulate proliferation of SMCs and fibroblasts and increase synthesis of interstitial collagen
• Platelets also release PAI1, leading to a local inhibition of fibrinolysis
• The decreased generation of plasmin also leads to depressed local activation of collagen-degrading metalloproteinases (MMPs)
• Overall effect is an uncontrolled accumulation of collagen in the lipid-rich plaque

Question 5

Rupture of the fibrous plaque

Answer 5

• Continual growth of the plaque results in an atheroma (large plaque)
• Remodeling of the fibrous plaque leads to a cap over the surface of the plaque, with an underlying matrix of lipid and TF rich foam cells (and other proteins like collagen)
• Rupturing of the cap over the atheroma causes the majority of arterial thrombi and MI
• The exposed collagen can activate platelets, which can be amplified by the excessive TF leading to massive thrombin production
• Rupture of fibrous plaque could be in part due to excess expression of MMPs, leading to destabilization of the fibrous cap (?)

Question 6

Hemostasis in the venous system

Answer 6

• Dependent on the expression of procoagulant factors, resulting in thrombin stimulation and platelet aggregation
• In the low shear venous system platelet aggregation is mediated by binding of platelets to fibrinogen (GPIIb/IIIa)
• The activated platelets provide a thrombogenic surface, further enhancing thrombin generation
• Prophylaxis and Rx for venous thrombi involved anticoagulants that inhibit formation of thrombin

Question 7

Hemostasis in the arterial system

Answer 7

• Expression of endothelial procoagulants alone is insufficient to generate amount of thrombin needed for hemostatic plug
• Exposure of subendothelium (rich in collagen and vWF) allows for rapid adhesion (GPIb/IX to vWF and GPVI/GPIa/IIa to collagen) and activation of platelets
• Platelet aggregation is supported by the binding of vWF to GPIIb/IIIa (not fibrinogen)
• Since arterial clots are predominantly composed of platelets, Rx and prophylaxis inhibit platelet activation

Question 8

Activation of platelets in hemostasis

Answer 8

• Activated platelets will express P-selectin on their membranes
• This will bing to mono/mac-derived micro vesicles that are rich in TF (via P-selectin binding to PSGL-1)
• Upon the binding of these two, their membranes will fuse and platelets will now express TF on their surface, greatly increasing their coagulability

Question 9

Anti-platelet Rx

Answer 9

• Mostly for prevention and Rx of arterial thrombosis
• Targets: inhibition of primary agonist signaling (thrombin inhibition w/ Hirudin and Bivalrudin)
• Inhibition of secondary (amplifying) signaling (aspirin and thienopyridines)
• Inhibition of GPIIb/IIIa receptor aggregation (Abciximab, eptifibatide, tirofiban)

Question 10

Aspirin (ASA)

Answer 10

• Irreversibly inhibits platelet COX1 to prevent the formation of TxA2
• Prevents aggregation and activation
• ASA is inactivated quickly in liver, so the only the platelets in the portal circulation are inactivated
• This is to our advantage, since all of our platelets pass through the portal circulation within a short period of time and thus many are inactivated
• This spares the rest of our cells (such as systemic endothelium) from having their COX irreversibly inhibited

Question 11

Thienopyridines (ticlopidine,clopidogrel)

Answer 11

• Irreversible inhibitors of the ADP receptor (P2Y12)
• Active inhibitory agent is produced metabolically in the liver (gets activated in liver)
• Repeated doses are required before effective platelet ADP receptor blockade is active

Question 12

Platelet GPIIb/IIIa receptor inhibitors

Answer 12

• Primary receptor for platelet aggregation in low and high shear blood flow
• Congenital deficiency (glanzman’s thrombasthenia) results in severe life-long bleeding
• Both reversible (eptifibatide and tirofiban) and irreversible (abciximab) antagonists for the receptor exist
• These are usually only used in the short term due to their tendency to cause bleeding diatheses