Proteomic identification of lynchpin urokinase plasminogen activator receptor protein interactions associated with epithelial cancer malignancy

Urokinase plasminogen activator (uPA) and its high affinity receptor (uPAR) play crucial proteolytic and non-proteolytic roles in cancer metastasis. In addition to promoting plasmin-mediated degradation of extracellular matrix barriers, cell surface engagement of uPA through uPAR binding results in the activation of a suite of diverse cellular signal transduction pathways. Because uPAR is bound to the plasma membrane through a glycosyl-phosphatidylinositol anchor, these signalling sequelae are thought to occur through the formation of multi-protein cell surface complexes involving uPAR. To further characterize uPAR-driven protein complexes, we co-immunoprecipitated uPAR from the human ovarian cancer cell line, OVCA 429, and employed sensitive proteomic methods to identify the uPAR-associated proteins. Using this strategy, we identified several known, as well as numerous novel, uPAR associating proteins, including the epithelial restricted integrin, αvβ6. Reverse immunoprecipitation using anti-β6 integrin subunit monoclonal antibodies confirmed the co-purification of this protein with uPAR. Inhibition of uPAR and/or β6 integrin subunit using neutralizing antibodies resulted in the inhibition of uPA-mediated ERK 1/2 phosphorylation and subsequent cell proliferation. These data suggest that the association of β6 integrin (and possibly other lynchpin cancer regulatory proteins) with uPAR may be crucial in co-transmitting uPA signals that induce cell proliferation. Our findings support the notion that uPAR behaves as a lynchpin in promoting tumorigenesis by forming functionally active multiprotein complexes.