Urokinase receptor (upar/cd87) is a genuine ligand for integrins and mediates cell-cell interaction

The receptor for urokinase-type plasminogen activator (uPAR/CD87) is a 35-65 K Mr glycoprotein composed of 283 amino acid residues. It is anchored to the plasma membrane by a glycosyl phosphatidylinositol (GPI)-linkage. Binding of urokinase-type plasminogen activator (uPA) to its receptor, uPAR, regulates cellular adhesion, migration, and tumor cell invasion. It has been proposed that uPAR forms cis-interactions with integrins as an associated protein, and transduces proliferative or migratory signals to cells upon binding of uPA. However, it is still unclear how GPI-anchored uPAR, which lacks a transmembrane structure, mediates signal transduction. We found that recombinant soluble human uPAR (suPAR) synthesized in CHO cells specifically binds to recombinant human integrins a4βl, <x6βl, a9βl and (Xvβ3 on CHO cells in a dose- and cation-dependent manner. Function-blocking anti-integrin antibodies effectively blocked binding of suPAR to these integrins. Binding of suPAR to o4βl and avβ3 is blocked by known soluble ligands (e.g., soluble VCAM-1 or ROD peptide, respectively) and by the known mutations that block ligand binding in the a or βsubunit. These results suggest that uPAR is a genuine integrin ligand instead of an integrin associated protein, and competes with known ligands for binding to integrins. In addition, we demonstrate that recombinant GPI-anchored uPAR on the cell surface specifically binds to recombinant integrins o4βl, <x9βl and avβ3 on the apposing cells, suggesting that uPAR-integrin interaction may mediate cellcell interaction (trans-interaction). The present results do not rule out uPAR-integrin cisinteraction. However, the present results predict that, if uPAR interacts with integrins on the same cell-surface (cis-interaction), uPAR may bind as a ligand rather than as an associated protein. The previously unrecognized uPAR-integrin interaction allows uPAR to transduce signals through the engaged integrin without the hypothetical transmembrane adapter, and may provide a potential therapeutic target for inflammation and cancer.