Plasminogen activator inhibitor 1 and vitronectin protect against stenosis in a murine carotid artery ligation model

Objective-We previously reported that plasminogen activator inhibitor I (PAI-1), in the presence of vitronectin (VN), inhibits thrombin activity in vitro. Furthermore, we demonstrated in human atherosclerotic plaques the colocalization of thrombin, PAI-1, and VN, as well as activity of thrombin and PAI-1. Here, we show that PAI-1 is a local thrombin inhibitor in vivo. Methods and Results-We used the murine carotid artery ligation model to assess the role of PAI-I and VN in stenosis by using PAI-1-deficient (PAI-1(-/-)) and VN-/- mice. Ligation resulted in a smooth muscle cell (SMC)-rich intima without infiltrating cells. We show that PAI-1(-/-) and VN-/- mice generate a larger intima than wild-type mice as the result of more extensive SMC proliferation, as evidenced by cell counting and staining for proliferating cell-nuclear antigen. Conclusions-In PAI-1(-/-) mice, excessive intima formation is prevented by the thrombin-specific inhibitor hirudin. Finally, immunohistochemical analysis revealed PAI-I, VN, and (pro)thrombin antigen in intimal lesions. Our observations are compatible with inhibition of thrombin-mediated SMC proliferation by PAI-1/VN complexes.