THE THROMBOSPONDIN RECEPTOR CD47 (IAP) MODULATES AND ASSOCIATES WITH ALPHA-2-BETA-1 INTEGRIN IN VASCULAR SMOOTH-MUSCLE CELLS

The carboxyl-terminal domain of thrombospondin-1 enhances the migratio n and proliferation of smooth muscle cells. Integrin-associated protei n (IAP or CD47) is a receptor for the thrombospondin-l carboxyl-termin al cell-binding domain and binds the agonist peptide 4N1K (kRFYVVMWKk) from this domain. 4N1K peptide stimulates chemotaxis of both human an d rat aortic smooth muscle cells on gelatin-coated filters. The migrat ion on gelatin is specifically blocked by monoclonal antibodies agains t IAP and a pi integrin, rather than alpha v beta 3 as found previousl y 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 thrombosyondin-1 provoked a synergistic chemotactic response th at was partially blocked by antibodies to alpha 2 and beta 1 integrin subunits and to IAP. A combination of anti alpha 2 and IAP monoclonal antibodies completely blocked chemotaxis. RGD peptide and anti alpha v beta 3 mAb were without effect. 4N1K and thrombospondin-l did not aug ment the chemotactic response of smooth muscle cells to fibronectin, v itronectin, or collagenase-digested type I collagen. Complex formation between alpha 2 beta 1 and IAP was detected by the coimmunoprecipitat ion of both alpha 2 and beta 1 integrin subunits with IAP. These data suggest that IAP can associate with alpha 2 beta 1 integrin and modula te its function.