The Activation State of αvβ3Regulates Platelet and Lymphocyte Adhesion to Intact and Thrombin-cleaved Osteopontin

Cleavage of osteopontin by thrombin has been reported to enhance cell adhesion. We asked whether thrombin could regulate the alpha(v)beta(3)-mediated adhesion of platelets and B lymphocytes to this substrate. Although there was no difference in the extent or the avidity of thrombin- and ADP-stimulated platelet adhesion to intact or thrombin-cleaved human osteopontin, both the extent and avidity of phorbol ester-stimulated B cell adhesion to thrombin-cleaved osteopontin was significantly increased. Thus, these data suggest that the ability of alpha(v)beta(3) to recognize osteopontin can be differentially regulated in a cell-specific manner. To localize the alpha(v)beta(3) binding site on osteopontin, we measured cell adhesion to the two thrombin cleavage products of osteopontin and to a series of nested RGD-containing osteopontin peptides cross-linked to albumin. Whereas ADP-stimulated platelets adhered to the amino-terminal but not the carboxyl-terminal osteopontin fragment and to the osteopontin peptide RGDSVVYGLR, phorbol ester-stimulated B cells did not adhere to this peptide, although they did so in the presence of 1 mm Mn(2+). Thus, our data confirm that thrombin cleavage enhances the accessibility of the binding motif for alpha(v)beta(3) on osteopontin, but this enhancement is also a function of the activation state of alpha(v)beta(3). Moreover, they indicate that the sequence RGDSVVYGLR contains sufficient information to specify activation-dependent alpha(v)beta(3)-mediated platelet and lymphocyte adhesion.