Interaction of α9β1 integrin with thrombospondin-1 promotes angiogenesis

Thrombospondin-1 is a multifunctional protein interacting with several cell surface receptors including integrins. We found that it is a ligand for alpha 9 beta 1 integrin, and has an integrin binding site within its N-terminal domain (NoC1). Interaction of thrombospondin- 1 and its recombinant NoC1 domain with alpha 9 beta 1 integrin was confirmed in ELISA and cell adhesion assays. Binding of NoC1 to cells expressing alpha 9 beta 1 integrin activated signaling proteins such as Erk1/2 and paxillin. Blocking of this integrin by monoclonal antibody and the met-leu-asp-disintegrin inhibited dermal human microvascular endothelial cell proliferation and NoC1-induced migration of these cells. Immunohistochemical studies revealed that alpha 9 beta 1 is expressed on microvascular endothelium in several organs including skin, lung, heart and brain. NoC1 induced neovascularization in an experimental quail chorioallantoic membrane system and Matrigel plug formation assay in mice. This proangiogenic activity of NoC1 in vivo was inhibited by alpha 9 beta 1 inhibitors. In summary, our results revealed that alpha 9 beta 1 integrin expressed on microvascular endothelial cells interacts with thrombospondin-1, and this interaction is involved in modulation of angiogenesis.