Metformin decreases Cyp26a1 to prevent hepatocarcinogenesis through down- regulating CD8+ T cells

Hepatocellular carcinoma (HCC) is a highly heterogeneous cancer, which limits the selectivity of prevention and treatment. Preclinical and clinical studies suggested that in patients with diabetes, prolonged use of metformin, the AMPK activator, was associated with a reduction of HCC incidence. This association promotes us to investigate the possible functions and mechanisms of metformin in HCC without diabetes backgrounds. Here, we found that several unique pathways that changed during chronic liver injury of Fah-/- mice, including glucose metabolic process and retinol metabolism. Further, metformin suppressed the tumor formation in chronic liver injury of Fah-/- mice. RNA sequencing, in vivo and in vitro experiments showed that metformin suppressed Cyp26a1 gene expression of hepatocyte. Moreover, the down-regulation of Cyp26a1 leads to the increased level of all-trans-retinoic acid (atRA), which could suppress the tumor formation in our model. On the other hand, flow multicolor analysis showed that the cell number and proportion of cancer promoting (pro-tumor) CD8+ T cells increased significantly during chronic liver injury in Fah-/- mice, and both metformin and atRA treatment could reduce the number and proportion of pro-tumor CD8+ T cells. We also found metformin decreased the Cyp26a1 expression through the AMPK/JNK/c-Jun pathway. In short, the association between the metformin and atRA may explain the commonness of their anti-tumor activities. Our findings highlight the importance of targeting the precancerous microenvironment for the prognosis, prevention and treatment of HCC.