OBJECTIVE: We investigated the role of matrix metalloproteinase-8 (MMP8) in neointima formation and in vascular smooth muscle cell (VSMC) migration and proliferation. APPROACH AND RESULTS: After carotid artery wire injuring, MMP8(-/-)/apoE(-/-) mice had fewer proliferating cells in neointimal lesions and smaller lesion sizes. Ex vivo assays comparing VSMCs isolated from MMP8 knockout and wild-type mice showed that MMP8 knockout decreased proliferation and migration. Proteomics analysis revealed that a disintegrin and metalloproteinase domain-containing protein 10 (ADAM10) had lower concentrations in MMP8 knockout VSMC culture media than in MMP8 wild-type VSMC culture media. Western blot, flow cytometric, and immunocytochemical analyses showed that MMP8 knockout VSMCs contained more pro-ADAM10 but less mature ADAM10, more N-cadherin, and β-catenin in the plasma membrane but less β-catenin in the nucleus and less cyclin D1. Treatment of MMP8 wild-type VSMCs with an ADAM10 inhibitor, GI254023X, or siRNA knockdown of ADAM10 in MMP8 wild-type VSMCs inhibited proliferation and migration, increased N-cadherin and β-catenin in the plasma membrane, reduced β-catenin in the nucleus, and decreased cyclin D1 expression. Incubation of MMP8 knockout VSMCs with a recombinant ADAM10 rescued the proliferative and migratory ability of MMP8 knockout VSMCs and increased cyclin D1 expression. Furthermore, immunohistochemical analyses showed colocalization of ADAM10 with VSMCs and N-cadherin, and nuclear accumulation of β-catenin in the neointima in apoE(-/-)/MMP8(+/+) mice. CONCLUSIONS: MMP8 enhances VSMC proliferation via an ADAM10, N-cadherin, and β-catenin-mediated pathway and plays an important role in neointima formation.