Numbers of Figures : 7 Figures ; Numbers of Supporting Figures and Table : 8 Supporting Figures and 2 Supporting Tables

Hepatocellular carcinoma(HCC)is a highly aggressive liver tumor containing cancer stem cells (CSCs) that participate in tumor invasion, therapeutic resistance and tumor relapse causing poor outcome and limited therapeutic options. Histone deacetylatase SIRT1 has been shown to be upregulated in human cancers. However, its role in liver CSCs was unknown yet. In this study, we explored biological functions of SIRT1 in liver CSCs. Our data showed that SIRT1 is highly expressed in liver CSCs and is decreased during the differentiation. And high levels of SIRT1 predict an decreased probability of survival in patients with HCC. SIRT1 is responsible for maintenance of self-renewal and tumorigenicity of liver CSCs and overexpression of exogenous SIRT1 can restore self-renewal of non-CSCs. We demonstrated that SOX2 is a main downstream regulator in SIRT1-mediated self-renewal and tumorigenicity potential of liver CSCs. Mechanistically, SIRT1 regulates transcription of SOX2 gene by chromatin-based epigenetic changes, which are dependent of DNA methylation. This effect is achieved by alternation of histone modification and interaction with DNMT3A, resulting in hypermethylation of SOX2 promoter. Furthermore, we demonstrated that IGF signaling plays an important role in maintaining SIRT1 expression through the increased SIRT1 protein stability. Conclusions: Our findings highlight the importance of SIRT1 in biology of the liver CSCs. SIRT1 may serve as a molecular target for therapy of HCC.