Stable interaction between α5β1 integrin and Tie2 tyrosine kinase receptor regulates endothelial cell response to Ang-1

During angiogenic remodeling, Ang-1, the ligand of Tie2 tyrosine kinase, is involved in vessel sprouting and stabilization through unclear effects on nascent capillaries and mural cells. In our study, we hypothesized that the Ang-1/Tie2 system could cross-talk with integrins, and be influenced by the dynamic interactions between extracellular matrix and endothelial cells (ECs). Here, we show that α5β1 specifically sensitizes and modulates Tie2 receptor activation and signaling, allowing EC survival at low concentrations of Ang-1 and inducing persistent EC motility. Tie2 and α5β1 interact constitutively; α5β1 binding to fibronectin increases this association, whereas Ang-1 stimulation recruits p85 and FAK to this complex. Furthermore, we demonstrate that Ang-1 is able to mediate selectively α5β1 outside-in FAK phosphorylation. Thus, Ang-1 triggers signaling pathways through Tie2 and α5β1 receptors that could cross-talk when Tie2/α5β1 interaction occurs in ECs plated on fibronectin. By using blocking antibodies, we consistently found that α5β1, but not αvβ3 activation, is essential to Ang-1–dependent angiogenesis in vivo.