Endorepellin, the C-terminal domain of perlecan, is a powerful angiogenesis inhibitor. To dissect the mechanism of endorepellin-mediated endothelial silencing, we used an antibody array against multiple tyrosine kinase receptors. Endorepellin caused a widespread reduction in phosphorylation of key receptors involved in angiogenesis and a concurrent increase in phosphatase activity in endothelial cells and tumor xenografts. These effects were efficiently hampered by function-blocking antibodies against integrin ␣21, the functional endorepellin receptor. The Src homology-2 protein phosphatase-1 (SHP-1) coprecipitated with integrin ␣2 and was phosphorylated in a dynamic fashion after endorepellin stimulation. Genetic evidence was provided by lack of an endorepellin-evoked phosphatase response in microvascular endothelial cells derived from integrin ␣21 ؊/؊ mice and by response to endorepellin in cells genetically engineered to express the ␣21 integrin, but not in cells either lacking this receptor or expressing a chimera harboring the integrin ␣2 ectodomain fused to the ␣1 intracellular domain. siRNA-mediated knockdown of integrin ␣2 caused a dose-dependent reduction of SHP-1. Finally, the levels of SHP-1 and its enzymatic activity were substantially reduced in multiple organs from ␣21 ؊/؊ mice. Our results show that SHP-1 is an essential mediator of endorepellin activity and discover a novel functional interaction between the integrin ␣2 subunit and SHP-1. (Blood. 2009; 114:4897-4906)
IntroductionThe development and maintenance of an efficient vascular system is vital for organ function and health in all higher organisms. Angiogenesis is abundant during fetal development but in the adult is a rare process mainly restricted to the female reproductive cycle and wound healing. However, deregulated angiogenesis is a prominent factor in many major diseases such as cancer, diabetes, and ischemia occurring after stroke. Controlled angiogenesis is achieved by interplay of proangiogenic and antiangiogenic factors. Proangiogenic growth factors, such as vascular endothelial growth factor (VEGF), signal by their respective receptor tyrosine kinase (RTK) to stimulate endothelial cell migration and proliferation, whereas angiogenic inhibitors counteract these actions. 1 The list of reported negative regulators of angiogenesis is long, ever growing, and truly versatile. Nevertheless, a common theme for many of them is that they are derived from limited proteolysis of extracellular matrix components and that they interact with integrin receptors. 2 The angiostatic protein endorepellin, located within the C-terminus of the heparan sulfate proteoglycan perlecan, consists of 3 laminin globular (LG1-3) domains separated by 4 epidermal growth factor-like repeats. 3 Endorepellin exerts its activity by a noncanonical cation-independent binding of the LG3 domain to the ␣2 I domain of the integrin ␣21, 4-7 a key receptor regulating angiogenesis. [8][9][10] This triggers a signaling cascade that leads to endothelial cell immobilizatio...