Akita, Hirofumi2; Marquardt, Jens U3; Durkin, Marian E4; Kitade, Mitsuteru3; Seo, Daekwan3; Conner, Elizabeth A4; Andersen, Jesper Bøje6; Factor, Valentina M5; Thorgeirsson, Snorri S4
1 Andersen Group, BRIC Research Groups, BRIC, Københavns Universitet2 Department of Surgery, Osaka University, Graduate School of Medicine.3 Laboratory of Experimental Carcinogenesis, Center for Cancer Research, National Cancer Institute.4 National Cancer Institute5 Lab. of Experimental Carcinogenesis, National Cancer Institute/NIH.6 Andersen Group, BRIC Research Groups, BRIC, Københavns Universitet
Activation of c-MYC is an oncogenic hallmark of many cancers including liver cancer, and is associated with a variety of adverse prognostic characteristics. Despite a causative role during malignant transformation and progression in hepatocarcinogenesis, consequences of c-MYC activation for the biology of hepatic cancer stem cells (CSCs) are undefined. Here, distinct levels of c-MYC over-expression were established by using two dose-dependent tetracycline inducible systems in 4 hepatoma cell lines with different p53 mutational status. The CSCs were evaluated using side-population approach as well as standard in vitro and in vivo assays. Functional repression of p53 was achieved by lentiviral shRNA transduction. The results show that c-MYC expression levels have a differential impact on liver CSC characteristics. At low levels, c-MYC activation led to increased proliferation and enhanced CSC properties including activation of reprogramming transcription factors and CSC marker expression (e.g. NANOG, OCT4 and EpCAM), expansion of side population and acceleration of tumor growth upon subcutaneous transplantation into immunocompromised mice. However, when exceeding a threshold level, c-MYC induced a pro-apoptotic program and loss of CSC potential both in vitro and in vivo. Mechanistically, c-MYC induced self-renewal capacity of liver cancer cells was exerted in a p53 dependent manner. Low c-MYC activation increased spheroid formation in p53-deficient tumor cells, whereas p53-dependent effects were blunted in the absence of MYC overexpression. Together, our results confirm the role of c-MYC as a master regulator during hepatocarcinogenesis and establish a new gatekeeper role for p53 in repressing c-MYC induced CSC phenotype in liver cancer cells.