Vascular endothelial growth factor induction of the angiogenic phenotype requires Ras activation

We investigated the role of Ras in vascular endothelial growth factor (VEGF)-mediated signal transduction and the promotion of angiogenic changes primary endothelial cells. We find that VEGF potently induces Ras activation and that this step is essential for the stimulation by VEGF of several cellular changes associated with angiogenesis, including proliferation, migration, and branching morphogenesis in three-dimensional culture. Inhibition of Ras signaling induced subtle changes in the actin architecture but had no effect on the phosphatidylinositol 3-kinase (PI3K) or p38 signaling pathways. In contrast, activation of ERK was largely dependent on Ras. Although inhibiting ERK activity completely suppressed cell proliferation and partially blocked in vitro differentiation, neither ERK nor PI3K activity was required for VEGF-induced migration. These data provide the first direct demonstration that inhibition of Ras signal transduction is anti-angiogenic. Interestingly, VEGF signal transduction bifurcates both upstream and downstream of Ras, with different Ras-dependent signals controlling endothelial cell proliferation and migration, essential components of the angiogenic response.