The mammalian target of rapamycin (mTOR)/S6K1 signaling pathway controls cell growth and proliferation. To assess the importance of S6K1 in the balance between death and survival in the liver, we have generated immortalized hepatocyte cell lines from wild-type and S6K1-deficient (S6K1(-/-)) mice. In S6K1(-/-) hepatocytes, caspase-8 and the pro-apoptotic protein Bid were constitutively down-regulated as compared with wild-type. Moreover, S6K1(-/-) hepatocytes failed to respond to the apoptotic trigger of death receptor activation. Neither caspase-8 activation nor FLIP(L) degradation in response to tumor necrosis factor alpha (TNF-alpha) or anti-Fas antibody (Jo2) was observed in cells lacking S6K1. Downstream events such as Bid cleavage, cytochrome C release, caspase-3 activation, DNA laddering, as well as the percentage of apoptotic cells were attenuated as compared with wild-type. In addition, the anti-apoptotic protein Bclx(L) was down-regulated in TNF-alpha-treated or Jo2-treated wild-type hepatocytes, but this response was abolished in S6K1(-/-)cells. In vivo, S6K1-deficient mice were protected against concanavalin A-induced apoptosis. The withdrawal of growth factors strongly induced apoptosis in wild-type, but not in S6K1(-/-) hepatocytes. S6K1 deficiency did not decrease Bclx(L)/Bim ratio on serum withdrawal, thereby protecting cells from cytochrome C release and DNA fragmentation. At the molecular level, the lack of S6K1-mediated negative feedback decreased insulin receptor substrate-1 (IRS-1) serine phosphorylation, resulting in activation of survival pathways mediated by phosphatidylinositol 3-kinase/Akt and extracellular signal-regulated kinase (ERK). However, S6K1(-/-) hepatocytes underwent apoptosis on serum withdrawal in combination with phosphatidylinositol 3-kinase (PI3K) or ERK inhibitors.
Conclusion: This finding might explain the mechanism of resistance to mTOR inhibitors in cancer treatments and strongly suggests that the inhibition of S6K1 could protect against acute liver failure and, in combination with inhibitors that abrogate the sustained activation of Akt and ERK, could improve the efficacy of hepatocarcinoma (HCC) treatment.