Determinants of binding and internalization of tissue-type plasminogen activator by human vascular smooth muscle and endothelial cells.

Vascular injury induced by angioplasty is often followed by smooth muscle cell (SMC) proliferation, migration, and accumulation of extracellular matrix. Since plasminogen activators and their receptors may be important both in cell migration and the clearance of plasminogen activators, we studied the binding, internalization, and degradation of radiolabeled tissue-type plasminogen activator (t-PA) by explant cultures of human vascular SMC. Binding of t-PA to SMC at 4 degrees C was rapid, specific, saturable, and inhibitable by antibodies to plasminogen activator inhibitor type 1 (PAI-1). At 37 degrees C, labeled t-PA was internalized and degraded by SMC but not by human umbilical vein endothelial cells. Internalization and degradation was mediated by the low density lipoprotein receptor related protein/alpha 2-macroglobulin receptor (LRP) in that these processes were inhibited by an anti-LRP antibody, recombinant LRP-associated protein, urokinase-type plasminogen activator-PAI-1 complexes, and lactoferrin. The portion of t-PA most important for internalization after complexing with PAI-1 is likely to be in the finger and/or epidermal growth factor domains or in the carbohydrate at amino acid 117, in that the internalization of preformed t-PA.PAI-1 complexes or complexes formed on the cell surface was inhibited by an excess of active site-blocked wild type t-PA, but not by an active site blocked t-PA variant missing these domains. These studies are consistent with a model in which t-PA binds initially to SMC-associated PAI-1 with subsequent t-PA.PAI-1 internalization via LRP. SMC may play an important role in clearing t-PA-PAI-1 complexes from within the vessel wall.