Direct chemotactic action of angiopoietin-1 on mesenchymal cells in the presence of VEGF.

Angiopoietin-1 (Ang1) and its receptor, Tie2, play an important role in angiogenesis and vessel maturation. We previously reported that overexpression of Ang1 in MCF7 xenograft tumors facilitated vessel stabilization by mural cells, and that cultured SMC express Tie2. Here, we investigated whether Ang1 directly acts as a chemoattractant on mural cells or their precursors. In a Matrigel plug assay, neither Ang1 nor VEGF alone induced angiogenesis but together stimulated infiltration of non-endothelial cells that were CD31-negative, vimentin-positive and also positive for VEGFR-1 and Tie2. While negative for smooth muscle actin, reactivity for desmin suggests that the cells are mural cell precursors. VEGF treatment of cultured smooth muscle cells (SMC) upregulated Tie2 and allowed for Ang1-mediated phosphorylation of Tie2 and the AKT serine-threonine kinase. The combination of Ang1 and VEGF stimulated SMC migration in a Boyden chamber-type assay. In the presence of VEGF, Tie2 is upregulated on mural cells, allowing for a migratory response to Ang1. These findings support the view that Ang1, in concert with VEGF, can act directly on mural cells or their precursors to facilitate their recruitment to new blood vessels. This action may play an important role in vascular stabilization.