Altered regulation of Src tyrosine kinase by transforming growth factor β1 in a human hepatoma cell line

Transforming growth factor ps (TGF-beta s) are the potent growth inhibitors for various cell types. Certain transformed cells, however, show poor response to TGF-beta induced growth inhibition, which contributes to their uncontrolled proliferation. Recently, we have reported that TGF-beta(1) induces degradation of activated Src tyrosine kinase in rat fibroblasts, To elucidate the alteration in TGF-beta(1) signaling pathway in turner cells that cannot respond to the cytokine, we compared the effects of TGF-beta(1) on Src kinase in two human hepatoma cell lints, TGF-beta(1)-insensitive Mahlavu cells and TGF-beta(1)-sensitive HepG2 cells, TGF-beta(1) decreased Src kinase activity in HepG2 cells, but increased cellular Src levels and Src kinase activity in Mahlavu cells. Go-incubation of Mahlavu cells with TGF-beta 1 and 12-O-tetradecanoyl phorbol 13-acetate (TPA) decreased Src protein levels and Src kinase activity, inducing TGF-beta(1) sensitivity. TGF-beta(1) induced tyrosine dephosphorylation of Ras guanosine triphosphatase-activating protein (Ras-GAP) and Ras inactivation in HepG2 cells, but induced Ras-GAP phosphorylation and Ras activation in Mahlavu cells. The Src kinase inhibitor abolished the increase of Src kinase activity in TGF-beta(1)-treated Mahlavu cells, and induced TGF-beta(1) sensitivity. These findings suggest that regulation of Src kinase by TGF-beta(1) is altered in Mahlavu cells. The altered regulation of Src may contribute to TGF-beta(1) insensitivity in this cell line, at least in part through activation of Ras.