[Epigenetic Regulation of Gene Expression and Hepatocyte Proliferation by Nuclear Receptor PPARA]

Chronic activation of the nuclear receptor, peroxisome proliferator-activated receptor alpha (PPARA), causes hepatocellular proliferation and increases the incidence of hepatocellular carcinoma in rodents. However, the molecular mechanisms underlying hepatocyte proliferation by activated PPARA remain ambiguous. This review focuses on the genes repressed by PPARA and describes the mechanism by which it promotes hepatocyte proliferation in mice. PPARA undergoes autoinduction, leading to its overexpression by an agonist. PPARA subsequently activates the E2F transcription factor 8 (E2f8), which then activates the ubiquitin-like protein containing the PHD and RING finger domains 1 (Uhrf1). UHRF1, in complex with histone deacetylase 1 and DNA methyltransferase 1, stimulates DNA methylation and recruitment of histone H3 containing trimethylated lysine 9 to the promoters of specific target genes, including E-cadherin/cadherin 1 (Cdh1), resulting in their downregulation. Decreased expression of CDH1 stimulates Wnt signaling, upregulation of oncogenes, including Myc and the cell cycle control genes, cyclin D1 and Jun, and enhances hepatocyte hyperproliferation. Therefore, the PPARA-E2F8-UHRF1-CDH1-Wnt signaling axis is involved in the epigenetic regulation of hepatocyte proliferation. This review provides insights into the mechanisms underlying hepatocarcinogenesis induced by non-genotoxic substances.