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Blood, Vol. 93 No. 11 (June 1), 1999:
pp. 3839-3846
Characterization of Cell-Associated Plasminogen Activation
Catalyzed by Urokinase-Type Plasminogen Activator, but
Independent of Urokinase Receptor (uPAR, CD87)
Colin Longstaff,
R. Elizabeth Merton,
Pere Fabregas, and
Jordi Felez
From The National Institute for Biological Standards and Control,
South Mimms, Hertfordshire, UK, and the Institut de Recerca Oncologica
(IRO), Hospital Duran i Reynals, Barcelona, Spain.
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 106
to 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.

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