The cation independent mannose-6-phosphate receptor/ insulin-like growth factor ii receptor (cimpr) modulates the subcellular distribution of the urokinase receptor (upar)

We have previously shown that the endocytic LDL receptor-related protein (LRP) can modulate cell surface expression of uPAR by internalizing the ternary uPA:PAl-l:uPAR complex. This uPA:serpin-induced uptake of uPAR is blocked by the 39-40 kDa LRP ligand RAP. Subsequent experiments showed that cells cultured in the presence of RAP exhibited significant lysosomal staining for uPAR, demonstrating a sorting mechanism independent of LRP. By affinity chromatography using recombinant soluble uPAR (r-uPAR), we purified a 225 kDa protein which by aa microsequencing was identified as the ubiquitous lysosomal sorting receptor CIMPR, also present at the cell surface. Binding of IGFII and p-galactosidase, and recognition by specific antibodies confirmed the identity. Both r-uPAR and purified wild-type uPAR bound to a CIMPR affinity column in a mannose-6-phophate (M6P) sensitive manner. Whereas the binding site in r-uPAR reside in N-linked M6P residues present in domains 2+3, the PI-glycan of the GPI-anchor accounts for binding to CIMPR in wild-type uPAR. Cells transfected with bovine CIMPR, but not receptor negative controls obtained from knock-out mice, targeted labeled ruPAR to lysosomes. Moreover, the ratio between intracellularly located and surfacebound endogenous uPAR was significantly higher in the CiMPR transfectants as compared to control cells. Finally, metabolically labeled uPAR in primary cultures of human trophoblasts could be cross-linked to the endogenous CIMPR, demonstrating a direct contact between the two receptors. In conclusion, we have shown that the subcellular distribution of uPAR is modulated by the CIMPR, and we suggest that CIMPR may modulate the invasive phenotype of e.g. cancer cells, by regulating surface expression of uPAR.