Inhibition of pp60(c-Src) reduces Bcl-X-L expression and reverses the transformed phenotype of cells overexpressing EGF and HER-2 receptors

Tumors that overexpress HER-2/neu receptor or exhibit enhanced EGFR signali ng have been reported to possess constitutively activated Src family kinase s especially pp60(c-Srcs). High levels of pp60(c-Src) activity have also be en reported for cell lines that overexpress the EGFR or the chimeric EGFR-H ER-2 receptor. It has therefore been suggested that Src kinases may contrib ute significantly to the oncogenic phenotype of these cells and to the degr ee of malignancy of tumors that overexpress EGFR family receptors, In this study we show that the induced expression of c-SRC antisense RNA or the app lication of a selective Src kinase inhibitor induces growth arrest, program med cell death and reverses the transformed properties of cells that overex press EGFR or HER-2 receptors. We show that inhibition of Src kinase expres sion or activity results in the reduction of Stat3 tyrosine phosphorylation , decline of Bcl-X-1, expression, and induction of cell death. Using a cons truct in which the promoter of Bcl-X, which possesses putative Stat3 sites, is tethered to the luciferase reporter gene, we show that inhibition of Sr c activity or expression induces a decline in Bcl-S expression. We also sho w that the expression of activated Src induces activation of the Bcl-X prom oter. This activation is inhibited by the expression of kinase dead Src or of Stat3 beta, the dominant-negative form of Stall, Taken together, these r esults support the hypothesis that Src positively regulates the transformed phenotype of cells overexpressing EGFR family kinases, Furthermore, these results also suggest that Src positively regulates Bcl-X-L, expression via Stat3 activation and thus acts not only as a potent mitogenic signaling ele ment, but also as an anti-apoptotic signaling protein. The combination of b oth activities probably confers upon activated Src its oncogenic activity. Since Src kinase is activated in many tumors, pp60(c-Src) kinase inhibitors may prole useful as anti-cancer agents for mana types of cancer.