Epigenetic regulation of the hepcidin gene expression in hepatoma cells

Hepcidin negatively regulates systemic iron levels. In response to iron overload, the bone morphogenetic protein (BMP) pathway is activated, leading to stimulation of hepcidin transcription in hepatocytes. Here, we evaluated epigenetic regulation of hepcidin expression in low-hepcidin-expressing hepatoma cell lines. The CpGs were hypomethylated at the proximal and distal regions of hepcidin promoters in the liver. In contrast, the methylation of the CpGs in both regions was high in Hepa 1-6 hepatoma cells, and the CpGs in the distal region were hypermethylated in Hep-G2 hepatoma cells. Reporter assays revealed that methylation of CpGs located on the proximal BMP-response element (RE) 1 and the distal BMP-RE2, which are critical elements responsible for BMP-mediated transcription, inhibited not only basal hepcidin transcription but also transactivation of hepcidin by the BMP pathway. DNA demethylation increased hepcidin mRNA levels in a BMP pathway-dependent manner. The heterochromatinization at BMP-RE1 and BMP-RE2 on the hepcidin promoter was higher in Hep-G2 cells than in primary hepatocytes and was negatively associated with hepcidin expression. BMP6 did not modulate abundance of H3K9me2, a repressive histone mark, on the BMP-REs in Hep-G2 cells. The methylation of CpG on the BMP-RE2 was high in some human carcinomas. The present study reveals that DNA methylation and BMP-independent heterochromatinization at the BMP-RE2 are involved in the repression of hepcidin transcription in low-hepcidin-expressing hepatoma cells. In view of the high risk of hepatocarcinoma by iron overload, the recovery of hepcidin expression through epigenetic control may be a target against the onset of hepatocarcinoma.