Fibrin induction of tissue plasminogen activator expression in corneal endothelial cells in vitro

Fibrin is deposited in the anterior chamber of the eye in response to injury and can damage corneal endothelial cells (CEC). Fibrin degradation is plasmin dependent and is regulated by the balance between 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 hypothesized that fibrin can regulate fibrinolysis in the anterior segment by modulating PA/PAI expression in CEC.

Bovine CEC (BCEC) were treated for 3 to 72 hours with in situ polymerized fibrin (2 mg/ml) +/- 35S-methionine, cycloheximide, or actinomycin D. Polymerization was thrombin catalyzed, and control BCEC were incubated with or without thrombin or polymerization by-products. PA and PAI in conditioned medium, fibrin matrix, and cell fractions were analyzed by PA-specific zymographic and enzymatic assays.

Fibrin treatment induced a dramatic (> 20-fold) accumulation of extracellular, fibrin-bound PA. This activity was identified as t-PA by its Mw (70 kD) affinity for fibrin and sensitivity to inhibition by Erythrina. Induction of t-PA was not 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), or protein synthesis in general were unaffected. Fibrin induction of t-PA was not accompanied by changes in cellular t-PA levels and was dependent on both RNA and protein synthesis.

Fibrin selectively induces t-PA expression in CEC. Induced t-PA is released extracellularly and binds exclusively to the fibrin matrix. These findings suggest a role for fibrin and CEC in the regulation of fibrinolysis in the anterior segment of the eye.