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 and may be involved in regulating cell-associated plasminogen activation and pericellular proteolysis. While investigating the relationship between uPAR levels and plasmin generation, we found that uPA-catalyzed plasminogen activation is stimulated by cells which do not express uPAR. This uPAR-independent mechanism appears to be at least as effective in vitro as uPAR-dependent stimulation, such that stimulation on the order of 30-fold was observed, resulting from improvements in both apparent kcat and apparent Km. The mechanism depends on simultaneous binding of both uPA and plasminogen to the cell and requires the presence of the amino-terminal fragment (ATF), available 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 106to 107 cells/mL and may be more general. A mechanism is proposed whereby uPA can associate with binding sites on the cell surface of lower affinity, but higher capacity than uPAR, but these are sufficient to stimulate plasmin generation even at subphysiologic uPA concentrations. This mechanism is likely to operate under conditions commonly used for in vitro studies and may have some significance in vivo.