FIBRIN INDUCTION OF TISSUE-PLASMINOGEN ACTIVATOR EXPRESSION IN CORNEAL ENDOTHELIAL-CELLS IN-VITRO

Purpose. Fibrin is deposited in the anterior chamber of the eye in res ponse to injury and can damage corneal endothelial cells (CEC). Fibrin degradation is plasmin dependent and is regulated by the balance betw een plasminogen activators (PA), tissue-PA (t-PA), urokinase-PA (u-PA) , and their inhibitors (PAI). Although several factors can modulate PA /PAI expression in cells, the effect of fibrin is inconclusive. We hyp othesized that fibrin can regulate fibrinolysis in the anterior segmen t by modulating PA/PAI expression in CEC. Methods. Bovine CEC (BCEC) w ere treated for 3 to 72 hours with in situ polymerized fibrin (2 mg/ml ) +/- S-35-methionine, cycloheximide, or actinomycin D. Polymerization was thrombin catalyzed, and control BCEC were incubated with or witho ut thrombin or polymerization by-products. PA and PAI in conditioned m edium, fibrin matrix, and cell fractions were analyzed by PA-specific zymographic and enzymatic assays. Results. Fibrin treatment induced a dramatic (>20-fold) accumulation of extracellular, fibrin-bound PA. Th is activity was identified as t-PA by its Mw (70 kD) affinity for fibr in and sensitivity to inhibition by Erythrina. Induction of t-PA was n ot observed in control BCEC under any condition. Fibrin induction of t -PA was selective because the levels of u-PA (45 kD), PAI-1 (50 kD), o r protein synthesis in general were unaffected. Fibrin induction of t- PA was not accompanied by changes in cellular t-PA levels and was depe ndent on both RNA and protein synthesis. Conclusions. Fibrin selective ly induces t-PA expression in CEC. Induced t-PA is released extracellu larly and binds exclusively to the fibrin matrix. These findings sugge st a role for fibrin and CEC in the regulation of fibrinolysis in the anterior segment of the eye.