The Thrombospondin Receptor CD47 (IAP) Modulates and Associates with a2b1 Integrin in Vascular Smooth Muscle Cells

The carboxyl-terminal domain of thrombospondin-1 enhances the migration and proliferation of smooth muscle cells. Integrin-associated protein (IAP or CD47) is a receptor for the thrombospondin-1 carboxyl-terminal cell-binding domain and binds the agonist peptide 4N1K (kRFYVVMWKk) from this domain. 4N1K peptide stimulates chemotaxis of both human and rat aortic smooth muscle cells on gelatin-coated filters. The migration on gelatin is specifically blocked by monoclonal antibodies against IAP and a b1 integrin, rather than avb3 as found previously for 4N1K-stimulated chemotaxis of endothelial cells on gelatin. Both human and rat smooth muscle cells displayed a weak migratory response to soluble type I collagen; however, the presence of 4N1K peptide or intact thrombospondin-1 provoked a synergistic chemotactic response that was partially blocked by antibodies to a2 and b1 integrin subunits and to IAP. A combination of antia2 and IAP monoclonal antibodies completely blocked chemotaxis. RGD peptide and antiavb3 mAb were without effect. 4N1K and thrombospondin-1 did not augment the chemotactic response of smooth muscle cells to fibronectin, vitronectin, or collagenasedigested type I collagen. Complex formation between a2b1 and IAP was detected by the coimmunoprecipitation of both a2 and b1 integrin subunits with IAP. These data suggest that IAP can associate with a2b1 integrin and modulate its function.