Plasminogen binding to rat hepatocytes in primary culture and to thin slices of rat liver

Abstract

Human 125I-plasminogen bound readily to rat hepatocytes in primary culture at 4°C and at 37°C. Binding was inhibited by lysine and reversed by lysine, ε-aminocaproic acid, or nonradiolabeled plasminogen. The K(d) for binding of 125I-plasminogen to hepatocytes was 0.59 ± 0.16 μmol/L, as determined from the saturation isotherm by nonlinear regression (r2 = 0.99) and the Scatchard transformation by linear regression (r2 = 0.93). The number of sites per cell was 14.1 ± 1.1 x 106. Fibrinogen synthesis and secretion by hepatocytes was insufficient to account for the major fraction of plasminogen binding, as determined by enzyme-linked immunosorbent assay (ELISA). Polyacrylamide gel electrophoresis and trichloroacetic acid precipitation studies demonstrated that plasminogen is neither activated nor degraded when bound to hepatocytes at 37°C. Thin slices of whole rat liver (500 μm), isolated and prepared totally at 4°C, bound 125I-plasminogen. Binding was inhibited by lysine. 125I-albumin binding to liver slices was minimal and not inhibited by lysine. Activation of plasminogen by tissue plasminogen activator (t-PA) was enhanced by hepatocytes in primary culture. When lysine was included in the media, the enhanced rate of activation was no longer observed. After activation with t-PA, much of the plasmin remained associated with hepatocyte surfaces and was partially protected from inhibition by α2-antiplasmin. These studies suggest that hepatocyte plasminogen binding sites may provide important surface anticoagulant activity.