FIBRINOGEN IS A LIGAND FOR INTEGRIN ALPHA(5)BETA(1) ON ENDOTHELIAL-CELLS

Previous studies have shown that fibrinogen can associate with endothe lial cells via an Arg-Gly-Asp (RGD) recognition specificity. In the pr esent study, we have characterized the specificity of fibrinogen bindi ng to endothelial cells under different cation conditions. Fibrinogen binding to suspended endothelial cells was selectively supported by Mn 2+ and was suppressed by Ca2+. The Mn2+-supported interaction was comp letely inhibited by RGD peptides but not by alpha(v) beta(3) blocking monoclonal antibodies. In contrast, the interaction was completely blo cked by two alpha(5) beta(1) monoclonal antibodies. This interaction w as not mediated by fibronectin bound to the integrin; could be demonst rated with purified alpha(5) beta(1); and also was observed with a sec ond alpha(5) beta(1)-bearing cell type, platelets. The binding of fibr inogen to alpha(5) beta(1) On endothelial cells in the presence of Mn2 + was time-dependent, specific, saturable, and of high affinity (K-d = 65 nM). By employing anti-peptide monoclonal antibodies, the carboxyl -terminal RGD sequence at A alpha 572-574 was implicated in fibrinogen recognition by alpha(5) beta(1) Two circumstances were identified in which alpha(5) beta(1) interacted with fibrinogen in the presence of C a2+: when the receptor was activated with monoclonal antibody (8A2) or when the fibrinogen was presented as an immobilized substratum. These results identify fibrinogen as a ligand for alpha(5) beta(1) on endot helial and other cells, an interaction which may have broad biological implications.