Epigenetic Regulation of Cancer Stem Cell Marker CD133 by Transforming Growth Factor-β

Hepatocellular carcinoma (HCC) is the third leading cause of cancer mortality worldwide. CD 133, a transmembrane glycoprotein, is an important cell surface marker for both stem cells and cancer stem cells in various tissues including liver. CD 133 expression has been recently linked to poor prognosis in HCC patients. CD133+ liver cancer cells are characterized by resistance to chemotherapy, self-renewal, multilineage potential, increased colony formation, and in vivo cancer initiation at limited dilution. Recent studies demonstrate that CD 133 expression is regulated by DNA methylation. In this study, we explored the role of transforming growth factor beta (TGF beta), a multifunctional cytokine that plays a critical role in chronic liver injury, in the regulation of CD133 expression. TGF beta 1 is capable of up-regulating CD 133 expression specifically within the Huh7 HCC cell line in a time- and dose-dependent manner. Most important, TGF beta 1-induced CD133+ Huh7 cells demonstrate increased tumor initiation in vivo. Forced expression of inhibitory Smads, including Smad6 and Smad7, attenuated TGF beta 1-induced CD133 expression. Within CD 133 Huh7 cells, TGF beta 1 stimulation inhibited the expression of DNA methyl-transferases (DNMT) 1 and DNMT3 beta, which are critical in the maintenance of regional DNA methylation, and global DNMT activity in CD133 Huh7 cells was inhibited by TGF beta 1. DNMT3 beta inhibition by TGF beta 1 was partially rescued with overexpression of inhibitory Smads. Lastly, TGF beta 1 treatment led to significant demethylation in CD133 promoter-1 in CD133 Huh7 cells. Conclusion: TGF beta 1 is able to regulate CD133 expression through inhibition of DNMT1 and DNMT3 beta expression and subsequent demethylation of promoter-1. TGF beta 1-induced CD 133+ Huh7 cells are tumorigenic. The mechanism by which TGF beta induces CD 133 expression is partially dependent on the Smads pathway. (HEPATOLOGY 2010;51:1635-1644)