Summary. Background: Vascular smooth muscle cell (VSMC) migration is a critical process in arterial remodeling. Purified plasminogen activator inhibitor-1 (PAI-1) is reported to both promote and inhibit VSMC migration on two-dimensional (D) surfaces. Objective: To determine the effects of PAI-1 and vitronectin (VN) expressed by VSMC themselves on migration through physiological collagen matrices. Methods: We studied migration of wild-type (WT), PAI-1-deficient, VN-deficient, PAI-1/VN doubly-deficient (DKO) and PAI-1-transgenic (Tg) VSMC through three-D collagen gels. Results: WT VSMC migrated significantly slower than PAI-1-and VN-deficient VSMC, but significantly faster than DKO VSMC. Experiments with recombinant PAI-1 suggested that basal VSMC PAI-1 expression inhibits migration by binding VN, which is secreted by VSMC and binds collagen. However, PAI-1-over-expressing Tg VSMC migrated faster than WT VSMC. Reconstitution experiments with recombinant PAI-1 mutants suggested that the pro-migratory effect of PAI-1 over-expression required its anti-plasminogen activator (PA) and LDL receptor-related protein (LRP) binding functions, but not VN binding. While promoting VSMC migration in the absence of PAI-1, VN inhibited the pro-migratory effect of active PAI-1. Conclusions: In isolation, VN and PAI-1 are each pro-migratory. However, via formation of a high-affinity, non-motogenic complex, PAI-1 and VN each buffers the otherÕs pro-migratory effect. The level of PAI-1 expression by VSMC and the concentration of VN in extracellular matrix are critical determinants of whether PAI-1 and VN promote or inhibit migration. These findings help to rectify previously conflicting reports and suggest that PAI-1/ VN stoichiometry plays an important role in VSMC migration and vascular remodeling.Keywords: collagen, PAI-1, vascular smooth muscle cell, vitronectin.Intimal hyperplasia is a central process in acquired vascular diseases, such as atherosclerosis and restenosis after balloon angioplasty. A key step in intimal hyperplasia is the migration of vascular smooth muscle cells (VSMC) from the media, through the extracellular matrix (ECM) composed of collagen, elastin and multiple other components, into the intima. The plasminogen activation (PA) system plays a major role in the regulation of cell migration and the development of intimal hyperplasia [1,2]. Plasminogen activator inhibitor-1 (PAI-1) is the primary physiological inhibitor of tissue-and urinary-type plasminogen activators (t-PA and u-PA, respectively) and a major regulator of fibrinolysis [3]. PAI-1 is present in plasma, platelets, endothelial cells, VSMC and the ECM. PAI-1 expression in the vascular wall is increased in several human vascular diseases characterized by neointima formation, suggesting that PAI-1 may regulate the development of intimal hyperplasia [4,5]. PAI-1 binds vitronectin (VN), an adhesive glycoprotein present in ECM that plays key roles in cell adhesion and migration [6,7]. Binding of PAI-1 inhibits VNÕs interactions with its receptors on VSMC, ...