Functional Analysis of the Cellular Receptor for Urokinase in Plasminogen Activation

Plasminogen activation catalyzed by the urokinase-type plasminogen activator (uPA) constitutes a reciprocal zymogen activation system, as plasmin can efficiently activate pro-uPA, the single-chain zymogenic form of the protease. We have previously shown that the overall efficiency of this plasminogen activation system is greatly enhanced by its assembly on the cell surface, involving binding of pro-uPA to its cellular binding site uPAR, and the concurrent cellular binding of plasminogen. We have now studied the effect of a recombinant soluble form of uPAR (residues 1–277) on the proteolytic reactions of this system. In contrast to the increased efficiencies of plasminogen activation and pro-uPA activation observed with cell-surface uPAR, soluble uPAR had an inhibitory effect on both of these individual reactions. Soluble uPAR also caused no increase in the low, but discernible, intrinsic activity of pro-uPA. Consistent with the observations on the isolated reactions, the overall activity of the pro-uPA-mediated plasminogen activation system was significantly inhibited. These observations confirm the previous interpretation of the observations made with cell-surface uPAR that the mechanism of the enhanced plasmin generation is due to the catalytically favorable interaction of uPAR-bound uPA/pro-uPA with cell-bound plasminogen/plasmin, rather than direct effects on the properties of uPA or pro-uPA on binding to uPAR.