Urokinase-type plasminogen activator induces tyrosine phosphorylation of a78-kDa protein in H-157 cells

Studies from our laboratory have shown that exposure of human lung epitheli al cells to urokinase plasminogen activator (uPA) induces their proliferati on. This effect of uPA is likely to occur via activation of signal transduc tion pathways. To elucidate uPA-induced signal transduction mechanisms, we exposed H-157 cells to uPA and determined the induced tyrosine phosphorylat ion profile of proteins. We demonstrate that, in these cells, uPA prominent ly induced tyrosine phosphorylation of a 78-kDa protein. This effect was ob served as early as 30 min and was sustained for at least 24 h. Treatment of cells with agents that abrogate uPA receptor (uPAR) function, including ne utralizing anti-uPAR antibody, phosphatidylinositol-specific phospholipase C, or a selective antagonist that blocks the association of uPA with uPAR ( Angstrom 5 compound), all failed to prevent uPA-induced tyrosine phosphoryl ation. B-428, an active site inhibitor of uPA activity, prevented the uPA e ffect. Treatment of cells with hepatocyte growth factor, vascular endotheli al growth factor, or transforming growth factor-beta, all of which are know n to be activated by a uPA-dependent pathway, did not stimulate tyrosine ph osphorylation of the 78-kDa protein. uPA induced an increase in [H-3]thymid ine incorporation into DNA, and cell numbers were unaffected in the presenc e of Angstrom 5. These results demonstrate that, in H-157 cells, uPA induce s tyrosine phosphorylation of a 78-kDa protein via a proteolysis-dependent but uPAR-independent mechanism. This novel signaling pathway represents a p utative mechanism by which uPA could influence epithelial cell proliferatio n.