natural anticoagulant system and fibrinolysis

Question 1

What are the different roles of thrombin?

Answer 1

1. Procoagulant
   - fibrinogen to fibrin
   - activates FV, FVIII, FXI to their FVa, FVIIIa, FXIa form
2. Anticoagulant
   - tPA release
   - Protein C activation
3. Angiogenesis
   - PAR activation
   - VGEF release
   - Metaloprotease activation
4. Inflammation
   - induction of Il-6 and Il-8
   - Chemotaxis and activation of monocytes

Question 2

List the natural anticoagulants and what they act on.

Answer 2

1. TFPI-Tissue factor inhibitor: inhibits TF:FVIIa and Xa, prevents from forming tenase complex
2. Protein C and Protein S system: Protein C binds to EPCR, is activated by thrombin/thrombomodulin complex. APC binds to endothelial cells and platelets in complex with Protein S and destroys FVIIIa and FVa
3. Antithrombin: assoc with heparin like proteoglycan, inhibits thrombin irreversibly, also FIXa, and FXa

Question 3

How does the protein C and protein S system work?

Answer 3

• Protein C is bound to endothelial protein C receptor (EPCR) and is activated by thrombin/thrombomodulin complex
• APC binds to platelets and endothelial cells in complex with protein S and degrades F VIIIa and F Va.
• inhibition is sequential, the APC/PS complex degrades F Va first then degrades F VIIIa

Question 4

How does antithrombin promote anticoagulation?

Answer 4

• active form when bound to heparin-like preoteglycan (- charge), which bind to + lysine domain on antithrombin
• anthithrombin inhibits thrombin, F IXa and FXa, but prefers thrombin
• high molecular weight heparin complexes with AT and thrombin, prefers to inhibit thrombin
• small heparin molecules activate AT so that it prefers inhibition of F Xa (therapeutic)
• Also Heparin Cofactor II: function unknown

Question 5

Describe what is needed for the activation and inhibition of plasminogen to plasmin.

Answer 5

• Tissue plasminogen activator (tPA) and Urokinase Plasminogen activator (uPA) activate plasminogen to plasmin
• plasmin is a serine protease, binds to lysine residues on fibrin and cleaves fibrin as well as other proteins
• alpha-2-plasmin inhibitor (a-2-PI) inhibit plasmin
• –in mature clot: a-2-PI linked to fibrin by F XIIIa, resistant to plasmin degradation

Question 6

What is the role of plasmin in angiogenesis?

Answer 6

• can degrade basement membrane proteins

- can activate metalloproteinases, which degrade collagen

Question 7

Describe how tPA activates plasminogen

Answer 7

• tPA actively secreted by endothelial cells after activation by thrombin +cytokines
• tPA requires either fibrin or Annexin II cofactors
• fibrin binds tPA and plasminogen to its surface via lysine residues–>allows plasmin to be formed on fibrin
• annexin II is on endothelial surface and bind plasminogen and tPA, plasmin remains on surface of endothelial cell

Question 8

How does uPA activate plasminogen to plasmin

Answer 8

• single chain uPA (sc-uPA) made and secreted by endothelial cell
• sc-uPa requires cofactor, believed to be urokinase plasminogen activator receptor (uPAR)
• uPAR lowers Kd for sc-uPa
• uPA receptors on monocytes and macrophages

Question 9

How is fibrinolysis inhibited?

Answer 9

• tPA and uPA inhibited by Plasminogen Activator Inhibitor 1 (PAI-1)
• in plasma is inactive, only activated when bound to sub endothelial matrix protein vitronectin (unless tPA actively secreted, then active PAI-1 secreted by endothelial cell or released by activated platelets
• Thrombin Activated Fibrinolysis Inhibitor (TAFI) -activated by thrombin/thrombomodulin complex - removes lysine and arginine residues from fibrin - reduces binding of plasmin and plasminogen to fibrin molecules, inhibits the making and activity of plasmin