Receptor-mediated internalization and degradation of urokinase is caused by its specific inhibitor PAI-1

The receptor for urokinase plasminogen activator (uPA) has been previously shown not to internalize its ligand, but rather to focalize its activity at the cell surface, allowing a regulated cell surface plasmin dependent proteolysis. The receptor in fact binds the proenzyme pro-uPA and allows its very efficient conversion to the active two chains form. Receptor bound active uPA can also interact with its specific type 1 inhibiror (PAI-1) which is therefore able to inhibit the cell surface plasmin formation. In this paper we show that the uPA-PAI-1 complex bound to the uPA receptor is internalized and degraded. U937 cells were incubated at 4 degrees C with labeled uPA-PAI-1 (and other ligands), the temperature then raised to 37 degrees C and the fate of the ligand followed for 3 h thereafter. The uPA-PAI-1 complex was internalized into the cells (i.e. could not be dissociated by acid treatment) and thereafter degraded (i.e. appeared in the supernatant in a non TCA-precipitable form). Other ligands (free uPA, ATF and DFP-treated uPA) were not internalized nor degraded. The degradation of the uPA-PAI-1 complex is preceded by internalization and is inhibited by chloroquine, an inhibitor of lysosomal protein degradation. These data suggest the existence of a cellular cycle of uPA. After synthesis pro-uPA is secreted, bound to the receptor and activated to two chain uPA. On the surface, uPA can activate surface bound plasminogen to produce surface bound plasmin. In the presence of PAI-1 uPA activity is inhibited and plasmin production interrupted, while the uPA-PAI-1 complex is internalized and degraded.