Protein C and fibrinolysis: a link between coagulation and fibrinolysis

The effect of purified human activated protein C (APC) on fibrinolysis was studied by using in vitro clot lysis techniques. Clots were formed from citrated blood or plasma (supplemented with 125I-labeled fibrinogen) by adding thrombin and Ca(2+)-ions; lysis of the clots was achieved by the addition of tissue-type plasminogen activator before clot formation. The gradual release of labeled fibrin degradation products from the clot into the supernatant was taken as a measure for the lysis rate. It was demonstrated that the acceleration of clot lysis by APC added before clot formation depends on the presence of Protein S, Ca(2+)-ions and phospholipids. These observations suggest a role of APC as anticoagulant in clot lysis, since the cofactors for the expression of its anticoagulant and profibrinolytic effect are very similar. Indeed, we could demonstrate that the profibrinolytic effect of APC in vitro is associated with reduction of thrombin generation through the coagulation cascade by inactivation of factor VIIIa and factor Va. For instance, APC did not accelerate the lysis of factor X deficient blood clots. More generally, thrombin generation was associated with retarded fibrinolysis in vitro. Consequently anticoagulants such as APC or Heparin are profibrinolytic, whereas pro-coagulants such as phospholipids (in cell-free plasma) inhibit fibrinolysis through the generation of thrombin. Thrombin thus plays a crucial role as a link between coagulation and fibrinolysis. As thrombin is able to inhibit the lysis of blood and plasma clots, and not of purified fibrin clots, we hypothesize that thrombin inhibits lysis through an as yet unidentified mediator in plasma.