Soluble thrombomodulin quenches thrombin-mediated neutralization of PAI-1 activity and inhibits fibrinolysis through a TAFI independent mechanism

Objective: To evaluate the effect of thrombomodulin on thrombin-mediated neutralization of plasminogen activator inhibitor 1 (PAI-1) activity which results in the enhancement of the fibrinolytic activity. Design: We studied the effect of recombinant human soluble TM (rhs TM) on the interaction between human thrombin and PAI-1. Its subsequent effect on tissue plasminogen activator (tPA)-induced lysis of PAI-1 enriched fibrin clot was also evaluated. Results: rhsTM abolished the high molecular weight complex formation between thrombin and PAI-1 and quenched the neutralization of PAI-1 activity by thrombin in a dose-dependent manner. rhsTM also caused dose-dependent inhibition of tPA-induced lysis of PAI-1 enriched fibrin clots in a purified system, which had been shown to be accelerated by increasing concentration of thrombin by neutralizing PAI-1 activity. This inhibition was not observed when PAI-1 was not present in the fibrin clot. Euglobulin clot lysis time (ECLT), which is determined by the balance between tPA and PAI-1, was prolonged by rhsTM. This prolongation was partially abolished by anti-PAI-1 polyclonal IgG, but was unaffected by potato carboxyl peptidase inhibitor. Conclusion: The inhibition of thrombin-dependent enhancement of fibrinolysis by TM appears to involve a mechanism of quenching of thrombin-mediated neutralization of PAI-1 activity which is independent of thrombin activatable fibrinolysis inhibitor (TAFI). (C) 1999 Harcourt Publishers Ltd.