Binding of tissue plasminogen activator to cultured human endothelial cells

Tissue plasminogen activator (t-PA) and urokinase (u-PA), the major activators of plasminogen, are synthesized and released from endothelial cells. We previously demonstrated specific and functional binding of plasminogen to cultured human umbilical vein endothelial cells (HUVEC). In the present study we found that t-PA could bind to HUVEC. Binding of t-PA to HUVEC was specific, saturable, plasminogen-independent, and did not require lysine binding sites. The t-PA bound in a rapid and reversible manner, involving binding sites of both high (Kd, 28.7 +/- 10.8 pM; Bmax, 3,700 +/- 300) and low (Kd, 18.1 +/- 3.8 nM; Bmax 815,000 +/- 146,000) affinity. t-PA binding was 70% inhibited by a 100-fold molar excess of u-PA. When t-PA was bound to HUVEC, its apparent catalytic efficiency increased by three- or fourfold as measured by plasminogen activation. HUVEC-bound t-PA was active site-protected from its rapidly acting inhibitor: plasminogen activator inhibitor. These results demonstrate that t-PA specifically binds to HUVEC and that such binding preserves catalytic efficiency with respect to plasminogen activation. Therefore, endothelial cells can modulate hemostatic and thrombotic events at the cell surface by providing specific binding sites for activation of plasminogen.