Changes in gene expression and assessment of DNA methylation in primary human hepatocytes and HepG2 cells exposed to the environmental contaminants—Hexabromocyclododecane and 17-β oestradiol

We evaluated the effects of two putative non-genotoxic hepatic carcinogens, hexabromocyclododecane (HBCD) and 17-β oestradiol (E2) on global and CpG promoter DNA methylation in both primary human hepatocytes and hepatocellular carcinoma (HepG2) cells. The mRNA gene expression levels of genes involved particularly in cell cycle were also evaluated and potential correlation with DNA methylation status examined. HBCD at 0.03 and 0.3 ng/mL did not produce statistically significant differences in global genomic methylation. However, E2 (0.1 ng/mL) significantly lowered global DNA methylation levels in HepG2 cells by approximately 65% (P < 0.01). In primary hepatocytes, the promoter regions of N-cym and ERα were methylated in both control and treated groups, signifying lack of promoter demethylation by both HBCD and E2. Furthermore, CpG promoter methylation of RB1 was observed in HepG2 cells but this was unaffected by treatments. The remaining genes (p16, C-myc, H-ras, THRα, histone H3, TBK1 and TNFRα) were unmethylated in their CpG promoter regions in both test systems. Quantitative RT-PCR showed that HBCD at 0.03 ng/mL up-regulated the expression of N-cym whereas E2 up-regulated the expression of ERα and THRα genes in primary hepatocytes. In HepG2 cells, the mRNA gene expression levels of p16, RB1 and N-cym were significantly down regulated by HBCD (0.03 ng/mL) and E2 (0.1 ng/mL) while HBCD at 0.3 ng/mL, significantly down regulated the expression levels of N-cym, ERα and ERβ genes. Thus, while both HBCD and E2 may alter the expression of certain genes involved in proliferation, the mechanisms appear unrelated to DNA methylation.