THE UROKINASE-TYPE PLASMINOGEN-ACTIVATOR RECEPTOR MEDIATES TYROSINE PHOSPHORYLATION OF FOCAL ADHESION PROTEINS AND ACTIVATION OF MITOGEN-ACTIVATED PROTEIN-KINASE IN CULTURED ENDOTHELIAL-CELLS

Urokinase-type plasminogen activator (uPA) binds to cells via a specif ic glycosylphosphatidyilinositol-anchored receptor. Although occupancy of the uPA receptor (uPAR) has been shown to alter cellular function and to induce gene expression, the signaling mechanism has not been ch aracterized. Urokinase induced an increase in the tyrosine phosphoryla tion of multiple proteins in bovine aortic endothelial cells. In contr ast, low molecular weight uPA did not induce this response. Analysis b y immunoblotting demonstrated tyrosine phosphorylation of focal adhesi on kinase (FAK), the focal adhesion-associated proteins paxillin and p 130(cas), and mitogen-activated protein kinase (MAPK) following the oc cupancy of the uPAR by uPA Treatment of cells with phosphatidylinosito l-specific phospholipase C, which cleaves glycosylphosphatidylinositol -linked proteins from the cell surface, blocked the uPA-induced tyrosi ne phosphorylation of FAK, indicating the requirement of an intact uPA R on the cell surface. The uPA-induced activation of MAPK was complete ly inhibited by genistein, but not by ethylphenyl)-7-(t-butyl)pyrazolo [3,4-d]pyrimidine, a specific inhibitor of Src family kinases, Thus, t his study demonstrates a novel role for the uPAR in endothelial cell s ignal transduction that invoices the activation of FAK and MAPK, which are mediated by the receptor-binding domain of uPA. This may have imp ortant implications for the mechanism through which uPA influences cel l migration and differentiation.