PROTEASE NEXIN-1-UROKINASE COMPLEXES ARE INTERNALIZED AND DEGRADED THROUGH A MECHANISM THAT REQUIRES BOTH UROKINASE RECEPTOR AND ALPHA(2)-MACROGLOBULIN RECEPTOR

After binding to its receptor (uPAR), active cell-surface urokinase (u PA) is not internalized while the complex formed by uPA with plasminog en activator inhibitor type 1 (PAI-1) is internalized and degraded. In ternalization and degradation require binding to uPAR and subsequently an interaction with the alpha(2)-macroglobulin receptor (alpha(2)-MR) . To analyze the generality of this mechanism, we studied the internal ization of uPA by recombinant protease nexin-1 (rPN-1), an inhibitor o f thrombin, uPA, and plasmin. I-125-uPA.rPN-1 complexes bound specific ally to uPAR; internalization occurred efficiently, and its time cours e was essentially the same as for uPA.PAI-1. Internalization required binding to uPAR since it could be blocked by the anti-uPAR monoclonal antibodies, by the uPAR antagonist amino-terminal fragment of uPA, and by the removal of uPAR by the treatment of cells with phosphatidylino sitol-specific phospholipase C.As for uPA.PAI-1, the internalization o f uPA.rPN-1 also required alpha(2)-MR, since it could be inhibited by the 39-kDa alpha(2)-macroglobulin receptor/low density lipoprotein rec eptor-associated protein, a ligand for the alpha(2)-MR. Finally, we sh ow by ligand blot analysis that the uPA.rPN-1 complex, like uPA.PAI-1 but unlike free uPA, bound specifically to both uPAR and alpha(2)-MR.