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 125I-labeled plasminogen and 125I-labeled plasmin to HGEC that involved the lysine binding sites of both ligands. 125I-plasminogen and 125I-plasmin bound to the same receptors with different affinities: Kd = 1.20 +/- 0.08 and 0.30 +/- 0.05 microM, 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 (Km) = 0.80 +/- 0.54 vs. 3.15 +/- 0.78 microM, P < 0.0001; catalytic constant (kcat) = 0.39 +/- 0.17 vs. 0.50 +/- 0.29 s-1, not significant; kcat/Km = 0.57 +/- 0.35 vs. 0.16 +/- 0.11 microM-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 its inhibitor, alpha-2-antiplasmin, whereas solution-phase plasmin was not.(ABSTRACT TRUNCATED AT 250 WORDS)