Regulation of Fibrinolysis in Plasma by TAFI and Protein C Is Dependent on the Concentration of Thrombomodulin

Summary Thrombin activatable fibrinolysis inhibitor (TAFI) is a carboxypeptidase B-like proenzyme, that after activation down regulates fibrinolysis. TAFI is activated by thrombin in the presence of the cofactor thrombomodulin (TM). By stimulation of TAFI activation TM down regulates fibrinolysis, however TM is also a cofactor in the activation of protein C. Activated protein C (APC) can up regulate fibrinolysis by limiting the activation of TAFI via the attenuation of thrombin production. We studied these counteracting fibrinolytic properties of TM in plasma by measuring the activation of TAFI during tissue factor induced coagulation. TAFI activation was stimulated at low concentrations of TM but decreased at higher concentrations of TM. Similarly, the clot lysis times increased at low concentrations of TM but decreased at higher concentrations of TM. The reduction of TAFI activation at high TM concentrations was found to be dependent on a functional protein C pathway. The concentration of TM is therefore an important factor in the regulation of TAFI activation and in the regulation of fibrinolysis. High concentrations of TM result in up regulation of fibrinolysis, whereas low concentrations of TM have a down regulatory effect on fibrinolysis. These results suggest that fibrinolysis might be differentially regulated by TM in different parts of the body depending on the local TM concentration in the vasculature. Abbreviations: TAFI, thrombin activatable fibrinolysis inhibitor; TAFIa, activated TAFI; CPI, carboxypeptidase inhibitor from potato tubers; tPA, tissue-type plasminogen activator; TM, thrombo-modulin; TM4-6, Thrombomodulin fragment containing EGF domains 4 to 6; HMEC, human microvascular endothelial cell.