Regulation of the urokinase-type plasminogen activator receptor on vascular smooth muscle cells is under the control of thrombin and other mitogens

The urokinase-type plasminogen activator receptor (u-PAR) was demonstrated on cultured smooth muscle cells (SMCs) of bovine aorta. Binding of ¹²⁵I-urokinase-type plasminogen activator (u-PA) was concentration dependent and saturable within 45-60 minutes. A similar concentration and time dependence was found in functional plasminogen activation studies. Human two-chain high-molecular-weight u-PA and its proenzyme (pro-u-PA) bound specifically with identical affinity (K_d). Activation of pro-u-PA was strongly accelerated on binding to SMCs and occurred only in the presence of plasminogen on the cell surface. A 100-fold molar excess of unlabeled high-molecular-weight u-PA effectively blocked binding of the radiolabeled ligands; tissue-type plasminogen activator, plasminogen, low-molecular-weight u-PA, and unrelated proteins did not. ^12SI-u-PA binding was abolished by a monoclonal antibody against the specific u-PA sequence responsible for u-PAR binding. Binding of u-PA sharply decreased on SMC exposure to phosphatidylinositol-specific phospholipase C, confirming the glycan phospholipid cell anchorage of u-PAR. Bovine and human α-thrombin (240 nM) increased the binding of ¹²⁵I-u-PA fivefold, translating into an increase in the number of sites per cell from about 10⁵ to 5 ×10⁵ without significant change in the K_d (1.29±0.39 nM). Active site blockade of thrombin by D-Phe-Pro-Arg-chloromethyl ketone resulted in the total loss of stimulatory activity, as did the use of the inactive active site thrombin mutant, S205A. Hirugen (100 μM), which blocks the anion-binding exosite of thrombin, blocked u-PAR stimulating activity. Thus, both the catalytic activity and integrity of the exosite are important for thrombin's stimulatory activity. Other SMC mitogens (epidermal growth factor, transforming growth factor-α1, basic fibroblast growth factor, platelet-derived growth factor, and phorbol 12-myristate 13-acetate) increased u-PAR expression on SMCs six- to 20-fold while concomitantly increasing K_d four-to 10-fold. In all cases the induction of u-PAR was dependent on de novo protein synthesis. These observations assign a possible role for thrombin and other mitogens in u-PAR regulation, thereby influencing the pericellular proteolysis that is important in SMC migration and atheromatous plaque development.