Endogenous Vegf-a Is Responsible for the Mitogenic Effect of Monocyte Chemotactic Protein-1 on Vascular Smooth Muscle Cells

Vessel wall remodelling is a complex phenomenon in which the loss of differentiation of vascular smooth muscle cells (VSMCs) occurs. We investigated the role of rat MCP-1 on rat VSMC proliferation and migration in order to identify the mechanism(s) involved in this kind of activity. Exposure to very low concentrations (1-100 pg/ml) of rat MCP-1 induced a significant proliferation of cultured rat VSMCs, assessed as cell duplication by counting total cells after exposure to test substances. MCP-1 stimulated VSMC proliferation and migration in a two-dimensional lateral sheet migration of adherent cells in culture. Endogenous VEGF-A was responsible for the mitogenic activity of MCP-1, since neutralizing anti-VEGF-A antibody inhibited cell proliferation in response to MCP-1. On the contrary, neutralizing anti-FGF-2 and anti-PDGFbb antibodies did not affect VSMC proliferation induced by MCP-1. RT-PCR and Western blot analyses showed an increased expression of either mRNA or VEGF-A protein following MCP-1 activation (10-100 pg/ml), while no Flt-1 receptor upregulation was observed. Since we have previously demonstrated that hypoxia (3%O2) is able to enhance VSMC proliferation induced by VEGF-A through Flt-1 receptor up-regulation, the effect of hypoxia on the response of VSMCs to MCP-1 was investigated. A severe hypoxia (3%O2) potentiated the growth promoting effect of MCP-1, which was able to significantly induce cell proliferation even at a concentration as low as 0.1 pg/ml. These findings demonstrate that low concentrations of rat MCP-1 are able to directly promote rat VSMC proliferation and migration through the autocrine production of VEGF-A.