Accessibility of receptor-bound urokinase to type-1 plasminogen activator inhibitor.
Urokinase plasminogen activator (uPA) interacts with a surface receptor and with specific inhibitors, such as plasminogen activator inhibitor type 1 (PAI-1). These interactions are mediated by two functionally independent domains of the molecule: the catalytic domain (at the carboxyl terminus) and the growth factor domain (at the amino terminus). We have now investigated whether PAI-1 can bind and inhibit receptor-bound uPA. Binding of 125I-labeled ATF (amino-terminal fragment of uPA) to human U937 monocyte-like cells can be competed for by uPA-PAI-1 complexes, but not by PAI-1 alone. Performed 125I-labeled uPA-PAI-1 complexes can bind to uPA receptor with the same binding specificity as uPA. PAI-1 also binds to, and inhibits the activity of, receptor-bound uPA in U937 cells, as shown in U937 cells by a caseinolytic plaque assay. Plasminogen activator activity of these cells is dependent on exogenous uPA, is competed for by receptor-binding diisopropyl fluorophosphate-treated uPA, and is inhibited by the addition of PAI-1. In conclusion, in U937 cells the binding to the receptor does not shield uPA from the action of PAI-1. The possibility that in adherent cells a different localization of PAI-1 and uPA leads to protection of uPA from PAI-1 is to be considered.
Cubellis
M V
MV
Institute of Microbiology, University of Copenhagen, Denmark.
Andreasen
P
P
Ragno
P
P
Mayer
M
M
Danø
K
K
Blasi
F
F
D001667:Binding, Competitive; D002460:Cell Line; D004792:Enzyme Precursors / Q000378:metabolism*; D005341:Fibrinolysin; D006023:Glycoproteins / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008970:Molecular Weight; D010960:Plasminogen Activators / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D015849:Plasminogen Inactivators; D011956:Receptors, Cell Surface / Q000378:metabolism*; D014568:Urokinase-Type Plasminogen Activator / Q000378:metabolism*
D004792:Enzyme Precursors; D006023:Glycoproteins; D015849:Plasminogen Inactivators; D011956:Receptors, Cell Surface; D010960:Plasminogen Activators; D005341:Fibrinolysin; D014568:Urokinase-Type Plasminogen Activator