Characterization of cell-associated plasminogen activation catalyzed by urokinase-type plasminogen activator, but independent of urokinase receptor (uPAR, CD87)

The 55-kD urokinase (uPA) receptor (UPAR, CD87) is capable of binding uPA a nd may be involved in regulating cell-associated plasminogen activation and pericellular proteolysis. While investigating the relationship between uPA R levels and plasmin generation, we found that uPA-catalyzed plasminogen ac tivation is stimulated by cells which do not express uPAR. This uPAR-indepe ndent mechanism appears to be at least as effective in vitro as uPAR-depend ent stimulation, such that stimulation on the order of 30-fold was observed , resulting from improvements in both apparent k(cat) and apparent K-m. The mechanism depends on simultaneous binding of both uPA and plasminogen to t he cell and requires the presence of the amino-terminal fragment (ATF), ava ilable in single chain and two chain high-molecular-weight uPA, but not low -molecular-weight uPA, Stimulation was observed in all leukemic cell lines investigated at similar optimum concentrations of 10(6) to 10(7) cells/mL a nd may be more general. A mechanism is proposed whereby uPA can associate w ith binding sites on the cell surface of lower affinity, but higher capacit y than uPAR, but these are sufficient to stimulate plasmin generation even at subphysiologic uPA concentrations. This mechanism is likely to operate u nder conditions commonly used for in vitro studies and may have some signif icance in vivo. (C) 1999 by The American Society of Hematology.