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 mus cle cell (SMC) proliferation, migration, and accumulation of extracell ular matrix. Since plasminogen activators and their receptors may be i mportant both in cell migration and the clearance of plasminogen activ ators, we studied the binding, internalization, and degradation of rad iolabeled tissue-type plasminogen activator (t-PA) by explant cultures of human vascular SMC. Binding of t-PA to SMC at 4-degrees-C was rapi d, specific, saturable, and inhibitable by antibodies to plasminogen a ctivator inhibitor type 1 (PAI-1). At 37-degrees-C, labeled t-PA was i nternalized and degraded by SMC but not by human umbilical vein endoth elial cells. Internalization and degradation was mediated by the low d ensity lipoprotein receptor related protein/alpha2-macroglobulin recep tor (LRP) in that these processes were inhibited by an anti-LRP antibo dy, recombinant LRP-associated protein, urokinase-type plasminogen act ivator-PAI-1 complexes, and lactoferrin. The portion of t-PA most impo rtant for internalization after complexing with PAI-1 is likely to be in the finger and/or epidermal growth factor domains or in the carbohy drate 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 act ive site blocked t-PA variant missing these domains. These studies are consistent with a model in which t-PA binds initially to SMC-associat ed PAI-1 with subsequent t-PA.PAI-1 internalization via LRP. SMC may p lay an important role in clearing t-PA.PAI-1 complexes from within the vessel wall.