Endothelial alpha3beta1-integrin represses pathological angiogenesis and sustains endothelial-VEGF.

Integrin alpha3beta1 is a major receptor for laminin. The expression levels of laminins-8 and -10 in the basement membrane surrounding blood vessels are known to change during tumor angiogenesis. Although some studies have suggested that certain ligands of alpha3beta1 can affect angiogenesis either positively or negatively, either a direct in vivo role for alpha3beta1 in this process or its mechanism of action in endothelial cells during angiogenesis is still unknown. Because the global genetic ablation of alpha3-integrin results in an early lethal phenotype, we have generated conditional-knockout mice where alpha3 is deleted specifically in endothelial cells (ec-alpha3-/-). Here we show that ec-alpha3-/- mice are viable, fertile, and display enhanced tumor growth, elevated tumor angiogenesis, augmented hypoxia-induced retinal angiogenesis, and increased vascular endothelial growth factor (VEGF)-mediated neovascularization ex vivo and in vivo. Furthermore, our data provide a novel method by which an integrin may regulate angiogenesis. We show that alpha3beta1 is a positive regulator of endothelial-VEGF and that, surprisingly, the VEGF produced by endothelial cells can actually repress VEGF-receptor 2 (Flk-1) expression. These data, therefore, identify directly that endothelial alpha3beta1 negatively regulates pathological angiogenesis and implicate an unexpected role for low levels of endothelial-VEGF as an activator of neovascularization.

Alphav beta3 integrin limits the contribution of neuropilin-1 to vascular endothelial growth factor-induced angiogenesis.

Both vascular endothelial growth factor receptors (VEGFR) and integrins are major regulators of VEGF-induced angiogenesis. Previous work has shown that beta3 integrin can regulate negatively VEGFR2 expression. Here we show that beta3 integrin can regulate negatively VEGF-mediated angiogenesis by limiting the interaction of the co-receptor NRP1 (neuropilin-1) with VEGFR2. In the presence of alphav beta3 integrin, NRP1 contributed minimally to VEGF-induced angiogenic processes in vivo, ex vivo, and in vitro. Conversely, when beta3 integrin expression is absent or low or its function is blocked with RGD-mimetic inhibitors, VEGF-mediated responses became NRP1-dependent. Indeed, combined inhibition of beta3 integrin and NRP1 decreased VEGF-mediated angiogenic responses further than individual inhibition of these receptors. We also show that alphav beta3 integrin can associate with NRP1 in a VEGF-dependent fashion. Our data suggest that beta3 integrin may, in part, negatively regulate VEGF signaling by sequestering NRP1 and preventing it from interacting with VEGFR2.