Stimulation of thrombin- and plasmin-mediated activation of thrombin-activatable fibrinolysis inhibitor by anionic molecules

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a proenzyme that, once activated, attenuates fibrinolysis by removing C-terminal lysine residues from partially degraded fibrin. TAFI can be activated by thrombin or plasmin via a cleavage at Arg92 that removes the activation peptide from the enzyme, TAFIa. Thrombomodulin enhances thrombin-mediated TAFI activation and glycosaminoglycans enhance plasmin-mediated TAFI activation. The aim of this study was to investigate whether there are other anionic molecules that function as a cofactor for thrombin- or plasmin-mediated TAFI activation. TAFI activation by thrombin or plasmin was studied in the presence of physiological anionic molecules (polyphosphate, heparin, hyaluronan, DNA and dermatan sulfate) and the non-physiological sodium dodecyl sulfate (SDS). Additionally, the effect of these molecules on TAFIa stability and on thrombin-mediated protein C activation was determined. Unfractioned heparin, calcium-saturated polyphosphate with an average chain length of 100 monomers (Ca-PolyP100) and SDS significantly enhanced TAFI activation by thrombin and plasmin. Dermatan sulfate and polyphosphates with sodium as counter ion (Na-PolyP700, Na-PolyP100 and Na-PolyP70) enhanced plasmin-mediated but not thrombin-mediated TAFI activation. Additionally, unfractioned heparin, Ca-PolyP100 and SDS enhanced thrombin-mediated protein C activation. The different nature of anionic molecules capable of enhancing TAFI and protein C activation suggests a general mechanism. Several anionic molecules function as (potent) cofactors for thrombin- and plasmin-mediated TAFI activation and thrombin-mediated protein C activation. This may imply that thrombin and plasmin activity is regulated in the vasculature by more cofactors than currently appreciated