Transforming growth factor-beta1-induced Smad signaling, cell-cycle arrest and apoptosis in hepatoma cells.

Transforming growth factor-beta1 (TGFbeta) is involved in the regulation of liver cell proliferation and apoptosis, and escape of hepatoma cells from the growth restraining signals of TGFbeta has been suggested to contribute to tumor development. TGFbeta modulates gene transcription by receptor-mediated activation of Smad proteins which act as transcription factors. TGFbeta-mediated primary signaling responses as well as effects on the cell cycle and apoptosis were investigated in the human hepatoblastoma line HepG2, the rat hepatoma line FTO-2B and the mouse hepatoma line 55.1c. Activation of a Smad (Sma and Mad homolog) response-element-driven luciferase reporter by TGFbeta was very similar in all three cell lines, indicating functionality of the primary TGFbeta signaling pathway. Moreover, TGFbeta-inducible early gene was transiently activated by TGFbeta in all cell lines as shown by RT-PCR. HepG2 cells, however, were completely resistant to TGFbeta-induced growth arrest and apoptosis and 55.1c cells were only slightly susceptible to TGFbeta-induced apoptosis. By contrast, treatment of FTO-2B cells with TGFbeta led to a partial G0/G1 arrest and a strong induction of apoptosis. TGFbeta-induced apoptosis of FTO-2B cells was inhibited by dexamethasone, insulin, phenobarbital and dieldrin. Of these agents, only insulin led to a significant reduction of TGFbeta-stimulated Smad-reporter activity, suggesting that the other compounds interfere with TGFbeta-induced apoptosis downstream of Smad-mediated primary transcriptional responses at a level that may be constitutively altered in apoptosis-resistant hepatoma cell lines.