Methionine adenosyltransferase (MAT) is an essential enzyme required for S-adenosylmethionine biosynthesis. Hepatic MAT activity falls during chronic liver injury, and mice lacking Mat1a develop spontaneous hepatocellular carcinoma by 18 months. We have previously demonstrated that CD133+CD45- oval cells isolated from 16-month-old Mat1a-/- mice represent a liver cancer stem cell population. The transforming growth factor β (TGF-β) pathway constitutes a central signaling network in proliferation, apoptosis, and tumorigenesis. In this study, we tested the response of tumorigenic liver stem cells to TGF-β. CD133+CD45- oval cells were isolated from premalignant 16-month-old Mat1a-/- mice by flow cytometry and expanded as five clone lines derived from a single cell. All clone lines demonstrated expression of both hepatocyte and cholangiocyte markers and maintained a small population (0.5% to 2%) of CD133+ cells in vitro, and three of five clone lines produced tumors. Although TGF-β1 inhibited cell growth equally in CD133%- and CD133+ cells from each clone line, the CD133+ population demonstrated significant resistance to TGF-β-induced apoptosis compared with CD133- cells. Furthermore, CD133+ cells demonstrated a substantial increase in mitogenactivated protein kinase (MAPK) pathway activation, as demonstrated by phosphorylated extracellular signal-regulated kinase levels before and after TGF-β stimulation. MAPK inhibition using mitogen-activated protein kinase kinase 1 (MEK1) inhibitor PD98059 led to a significant increase in TGF-β-induced apoptosis in CD133+ cells. Conversely, a constitutively active form of MEK1 blocked the apoptotic effects of TGF-β in CD133- cells. Conclusion: CD133± liver cancer stem cells exhibit relative resistance to TGF-β-induced apoptosis. One mechanism of resistance to TGF-β-induced apoptosis in CD133+ cancer stem cells is an activated mitogenactivated protein kinase/extracellular signal-regulated kinase pathway.
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