EXPRESSION OF PLASMINOGEN/PLASMIN RECEPTORS ON HUMAN GLOMERULAR EPITHELIAL-CELLS

The aim of the present study was to display plasminogen/plasmin receptors on human glomerular epithelial cell (HGEC) membranes and to determine the properties of receptor-bound plasminogen at the cell surface. Using an immortalized human glomerular epithelial cell line (E71-A1), we found a specific, saturable, and reversible binding of I-125-labeled plasminogen and I-125-labeled plasmin to HGEC that involved the lysine binding sites of both ligands. I-125-plasminogen and I-125-plasmin bound to the same receptors with different affinities: K-d = 1.20 +/- 0.08 and 0.30 +/- 0.05 mu M, respectively (P < 0.001). The number of binding sites per cell was 6.00 +/- 0.56 x 10(6) for plasminogen and 2.00 +/- 0.47 x 10(6) for plasmin (P < 0.05). Similar receptor affinities were found on isolated glomeruli, on immortalized and nonimmortalized HGEC, and on purified HGEC membranes. The apparent kinetic constants of plasminogen activation by receptor-bound urokinase-type plasminogen activator (u-PA) compared with solution-phase u-PA [Michaelis constant (K-m) = 0.80 +/- 0.54 vs. 3.15 +/- 0.78 mu M, P < 0.0001; catalytic constant (k(cat)) = 0.39 +/- 0.17 vs. 0.50 +/- 0.29 s(-1), not significant; h(cat)/K-m = 0.57 +/- 0.35 vs. 0.16 +/- 0.11 mu M(-1).s(-1), P < 0.05, respectively] showed a higher efficiency of plasminogen activation at the cell surface. When measured by a chromogenic assay using S22-51, receptor-bound plasmin activity on HGEC was partly protected from ifs inhibitor, alpha-2-antiplasmin, whereas solution-phase plasmin was not. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography, solution-phase I-125-plasmin was complexed with alpha-2-antiplasmin, whereas receptor-bound I-125-plasmin was not. The number of plasminogen/plasmin receptors was significantly decreased by a 24-h incubation with 50 nM dexamethasone or 10 nM thrombin without change in receptor affinity. These results show that HGEC express plasminogen/plasmin receptors, which, combined with the previously described autocrine saturation of u-PA receptors, constitute a complete membrane-bound plasminogen-activating system. Plasminogen/plasmin receptors focalize plasmin activity at the cell surface and could play a role,in the regulation of glomerular proteolytic activity.