Urokinase-type plasminogen activator induces tyrosine phosphorylation of a 78-kDa protein in H-157 cells

Studies from our laboratory have shown that exposure of human lung epithelial cells to urokinase plasminogen activator (uPA) induces their proliferation. This effect of uPA is likely to occur via activation of signal transduction pathways. To elucidate uPA-induced signal transduction mechanisms, we exposed H-157 cells to uPA and determined the induced tyrosine phosphorylation profile of proteins. We demonstrate that, in these cells, uPA prominently induced tyrosine phosphorylation of a 78-kDa protein. This effect was observed 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 neutralizing anti-uPAR antibody, phosphatidylinositol-specific phospholipase C, or a selective antagonist that blocks the association of uPA with uPAR (A5 compound), all failed to prevent uPA-induced tyrosine phosphorylation. B-428, an active site inhibitor of uPA activity, prevented the uPA effect. Treatment of cells with hepatocyte growth factor, vascular endothelial growth factor, or transforming growth factor-beta, all of which are known to be activated by a uPA-dependent pathway, did not stimulate tyrosine phosphorylation of the 78-kDa protein. uPA induced an increase in [(3)H]thymidine incorporation into DNA, and cell numbers were unaffected in the presence of A5. These results demonstrate that, in H-157 cells, uPA induces tyrosine phosphorylation of a 78-kDa protein via a proteolysis-dependent but uPAR-independent mechanism. This novel signaling pathway represents a putative mechanism by which uPA could influence epithelial cell proliferation.