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Altered regulation of Src tyrosine kinase by transforming growth factor beta1 in a human hepatoma cell line
1Second Department of Internal Medicine, Osaka University Medical School, Suita, Japan.
Abstract:
Transforming growth factor betas (TGF-betas) 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-beta1 induces degradation of activated Src tyrosine kinase in rat fibroblasts. To elucidate the alteration in TGF-beta signaling pathway in tumor cells that cannot respond to the cytokine, we compared the effects of TGF-beta1 on Src kinase in two human hepatoma cell lines, TGF-beta1-insensitive Mahlavu cells and TGF-beta1-sensitive HepG2 cells. TGF-beta1 decreased Src kinase activity in HepG2 cells, but increased cellular Src levels and Src kinase activity in Mahlavu cells. Co-incubation of Mahlavu cells with TGF-beta1 and 12-O-tetradecanoyl phorbol 13-acetate (TPA) decreased Src protein levels and Src kinase activity, inducing TGF-beta1 sensitivity. TGF-beta1 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-beta1-treated Mahlavu cells, and induced TGF-beta1 sensitivity. These findings suggest that regulation of Src kinase by TGF-beta1 is altered in Mahlavu cells. The altered regulation of Src may contribute to TGF-beta1 insensitivity in this cell line, at least in part through activation of Ras.
Insights
Transforming growth factor-beta1 (TGF-beta1) alters Src kinase regulation in hepatoma cells. Ineffective Src regulation by TGF-beta1 contributes to tumor cell insensitivity and uncontrolled proliferation.
Area of Science:
- Cell Biology
- Molecular Oncology
- Signal Transduction
Background:
- Transforming growth factor betas (TGF-betas) are potent inhibitors of cell growth.
- Tumor cells often exhibit resistance to TGF-beta-induced growth inhibition, promoting uncontrolled proliferation.
- TGF-beta1 has been shown to induce degradation of activated Src tyrosine kinase in fibroblasts.
Purpose of the Study:
- To investigate the alterations in the TGF-beta signaling pathway in tumor cells resistant to TGF-beta.
- To compare the effects of TGF-beta1 on Src kinase in TGF-beta1-insensitive (Mahlavu) and TGF-beta1-sensitive (HepG2) human hepatoma cell lines.
Main Methods:
- Comparative analysis of TGF-beta1 effects on Src kinase activity and levels in Mahlavu and HepG2 cells.
- Assessment of Src kinase inhibitor effects on Mahlavu cell sensitivity to TGF-beta1.
- Investigation of TGF-beta1-induced changes in Ras guanosine triphosphatase-activating protein (Ras-GAP) phosphorylation and Ras activation.
Main Results:
- TGF-beta1 decreased Src kinase activity in HepG2 cells but increased Src levels and activity in Mahlavu cells.
- Co-treatment of Mahlavu cells with TGF-beta1 and TPA restored TGF-beta1 sensitivity by decreasing Src levels and activity.
- TGF-beta1 induced Ras-GAP dephosphorylation and Ras inactivation in HepG2 cells, contrasting with Ras-GAP phosphorylation and Ras activation in Mahlavu cells.
- A Src kinase inhibitor abolished the increased Src activity in TGF-beta1-treated Mahlavu cells, restoring TGF-beta1 sensitivity.
Conclusions:
- Regulation of Src kinase by TGF-beta1 is significantly altered in Mahlavu hepatoma cells.
- Altered Src kinase regulation, potentially through Ras activation, contributes to TGF-beta1 insensitivity in Mahlavu cells.
- Targeting Src kinase may offer a strategy to overcome TGF-beta1 resistance in certain cancers.