Type 1 plasminogen activator inhibitor binds to fibrin via vitronectin

Type 1 plasminogen activator inhibitor (PAI-1), the primary inhibitor of ti ssue-type plasminogen activator (t-PA), circulates as a complex with the ab undant plasma glycoprotein, vitronectin. This interaction stabilizes the in hibitor in its active conformation In this report, the effects of vitronect in on the interactions of PAI-1 with fibrin clots were studied. Confocal mi croscopic imaging of platelet-poor plasma clots reveals that essentially al l fibrin-associated PAI-1 colocalizes with fibrin-bound vitronectin. Moreov er, formation of platelet-poor plasma clots in the presence of polyclonal a ntibodies specific for vitronectin attenuated the inhibitory effects of PAI -1 on t-PA-mediated fibrinolysis. Addition of vitronectin during clot forma tion markedly potentiates PAI-1-mediated inhibition of lysis of I-125-label ed fibrin clots by t-PA. This effect is dependent on direct binding interac tions of vitronectin with fibrin. There is no significant effect of fibrin- associated vitronectin on fibrinolysis in the absence of PAI-1. The binding of PAI-1 to fibrin clots formed in the absence of vitronectin was characte rized by a low affinity (K-d similar to 3.5 mu M) and rapid loss of PAT-1 i nhibitory activity over time. In contrast, a high affinity and stabilizatio n of PAI-1 activity characterized the cooperative binding of PAI-1 to fibri n formed in the presence of vitronectin. These findings indicate that plasm a PAI-1 vitronectin complexes can be localized to the surface of fibrin clo ts; by this localization, they may modulate fibrinolysis and clot reorganiz ation.