Role of Myc in hepatocellular proliferation and hepatocarcinogenesis.

Background & Aims Myc is involved in cell growth, proliferation, apoptosis, energy metabolism, and differentiation. Whether it is essential for hepatocellular proliferation and carcinogenesis is unclear due to a lack of an efficient hepatocyte-specific Myc disruption model. This study used a novel genetic model to investigate the involvement of Myc in hepatocellular proliferation and hepatocarcinogenesis in mice. Methods Temporal hepatocyte-specific Myc disruption was achieved by use of the tamoxifen-inducible Cre-ER T2 recombinase system under control of the serum albumin promoter. Hepatocyte proliferation was assessed by administering peroxisome proliferator-activated receptor α (PPARα) agonist Wy-14,643. A diethylnitrosamine-induced liver cancer model was used to evaluate the role of Myc in hepatocarcinogenesis. Results Tamoxifen administration induced recombination of Myc specifically in hepatocytes of Myc fl/fl,ERT2-Cre mice. When treated with a known hepatocellular proliferative stimulus Wy-14,643, Myc fl/fl,ERT2-Cre mice showed a lower liver/body weight ratio and suppressed hepatocyte proliferation as compared to Myc fl/fl mice. Hepatic expression of cell cycle control genes, DNA repair genes, and Myc target gene miRNAs were upregulated in Wy-14,643-treated Myc fl/fl mouse livers, but not in Wy-14,643-treated Myc fl/fl,ERT2-Cre livers. However, no differences were observed in the lipid-lowering effect of Wy-14,643 between Myc fl/fl,ERT2-Cre and Myc fl/fl mice, consistent with no differences in the expression of several PPARα target genes involved in fatty acid β-oxidation. Moreover, when subjected to the diethylnitrosamine liver cancer bioassay, Myc fl/fl,ERT2-Cre mice exhibited a markedly lower incidence of tumor formation compared with Myc fl/fl mice. Conclusions Myc plays an essential role in hepatocellular proliferation and liver tumorigenesis.