Metformin targets liver tumor-initiating cells through the PI3K/Akt/mTOR survival pathway

Liver tumor-initiating cells(T-ICs) are thought to be inherently resistant to the cytotoxic effects of chemotherapy, and can self-renewal and maintain tumor-initiating potential. Therefore, effective anticancer research strategies should target the unique properties of T-ICs. In this study, we found that metformin, a first-line drug of choice for the treatment of type 2 diabetes, inhibited liver T-ICs both in vivo and in vitro. Metformin inhibited the formation of hepatospheres and epithelial-specific antigen-positive(ESA,CD133?) cell colonies by hepatocellular carcinoma(HCC)cell lines. Metformin also downregulated the expression of several T-IC-related genes which are involved in the signaling pathways, governing the self-renewal, proliferation and differentiation of T-ICs. Furthermore, the targeting of liver T-ICs by metformin was PI-3-kinase-Akt-mTOR(PI3K/Akt/mTOR)-pathway dependent. The PI3K/Akt/mTOR inhibitor LY294002 and rapamycin abolished the inhibitory effect of metformin on CD133?cells, and the PI3K/Akt/mTOR stimulator EGF promoted the inhibitory effect of metformin on CD133?cells. Metformin also dramatically decreased the tumor volume and number of CD133 expressing tumor cells in a xenograft mouse model. Metformin exerted a synergistic effect with cisplatin to target both T-ICs and non-T-ICs, and resulted in the smallest tumor volume and lowest number of CD133 expressing tumor cells. This study indicates that the antidiabetic drug metformin could potentially be used in combination therapy with chemotherapeutic agents to improve the treatment of liver cancer.