ACTIVATED HUMAN PROTEIN-C PREVENTS THROMBIN-INDUCED THROMBOEMBOLISM IN MICE - EVIDENCE THAT ACTIVATED PROTEIN-C REDUCES INTRAVASCULAR FIBRIN ACCUMULATION THROUGH THE INHIBITION OF ADDITIONAL THROMBIN GENERATION

Activated protein C (APC) is a potent physiologic anticoagulant with p rofibrinolytic properties, and has been shown to prevent thrombosis in different experimental models, We investigated the effect of human AP C on thrombin-induced thromboembolism in mice, a model of acute intrav ascular fibrin deposition leading to death within minutes. APC given i ntravenously (i.v.) as a bolus 2 min before thrombin challenge (1,250 U/kg) reduced mortality in a dose-dependent manner despite the lack of thrombin inhibitor activity. Significant inhibition of thrombin-induc ed death was observed at the dose of 0.05 mg/kg, and maximal protectio n was obtained with 2 mg/kg (> 85% reduction in mortality rate). Histo logy of lung tissue revealed that APC treatment (2 mg/kg) reduced sign ificantly vascular occlusion rate (from 89.2 to 46.6%, P < 0.01), The protective effect of APC was due to the inhibition of endogenous throm bin formation as indicated by the fact that (a) the injection of human thrombin caused a marked decrease in the coagulation factors of the i ntrinsic and common pathways (but not of Factor VII), suggesting the a ctivation of blood clotting via the contact system; (b) APC pretreatme nt reduced markedly prothrombin consumption; (c) the lethal effect of thrombin was almost abolished when the animals were made deficient in vitamin K-dependent factors by warfarin treatment, and could be restor ed only by doubling the dose of thrombin, indicating that the generati on of endogenous thrombin contributes significantly to death; and (d) APC failed to protect warfarin-treated animals, in which mortality is entirely due to injected thrombin, even after protein S supplementatio n. Other results suggest that APC protects from thrombin-induced throm boembolism by rendering the formed fibrin more susceptible to plasmin degradation rather than by reducing fibrin formation: in thrombin-trea ted mice, fibrinogen consumption was not inhibited by APC; and inhibit ion of endogenous fibrinolysis by epsilon-aminocaproic or tranexamic a cid resulted in a significant reduction of the protective effect of AP C, Since APC did not enhance plasma fibrinolytic activity, as assessed by the measurement of plasminogen activator (PA) or PA inhibitor (PAI ) activities, PAI-1 antigen, or I-125-fibrin degrading activity, we sp eculate that the inhibition of additional (endogenous) thrombin format ion by APC interrupts thrombin-dependent mechanisms that make fibrin c lots more resistant to lysis, so that the intravascular deposited fibr in can be removed more rapidly by the endogenous fibrinolytic system.