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 (uPA) is not internalized while the complex formed by uPA with plasminogen activator inhibitor type 1 (PAI-1) is internalized and degraded. Internalization 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 internalization of uPA by recombinant protease nexin-1 (rPN-1), an inhibitor of thrombin, uPA, and plasmin. 125I-uPA.rPN-1 complexes bound specifically to uPAR; internalization occurred efficiently, and its time course 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 phosphatidylinositol-specific phospholipase C. As for uPA.PAI-1, the internalization of uPA.rPN-1 also required alpha 2-MR, since it could be inhibited by the 39-kDa alpha 2-macroglobulin receptor/low density lipoprotein receptor-associated protein, a ligand for the alpha 2-MR. Finally, we show 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.