Cadmium down-regulates apolipoprotein E (ApoE) expression during malignant transformation of rat liver cells: direct evidence for DNA hypermethylation in the promoter region of ApoE.

There is adequate evidence for the carcinogenicity of cadmium (Cd). However, a significant unaddressed question remains as to how this metal actually causes malignant transformation (tumor initiation). Since it has been shown that Cd only has the weak direct interaction potential with DNA, the metal is recognized as an indirect genotoxicant and mutagen. Currently, Cd-mediated "epigenetic" modifications, such as changes in DNA methylation resulting in alteration in target gene expression, coupled with cancer progression, are the focus of mechanistic research. We have reported that the apolipoprotein E (ApoE) gene, a suppressor of cell invasion, is an early Cd target, and is involved in the malignant transformation of TRL 1215 rodent liver cells. Cd exposure suppresses ApoE expression which can be re-activated with 5-aza-2'-deoxycytidine, a DNA demethylating agent. In the present study, we sought direct evidence of Cd-induced DNA hypermethylation of the ApoE promoter region by performing bisulfite sequencing and real-time quantitative methylation-specific PCR. Our data clearly suggest that Cd can down-regulate the expression of ApoE via introduction of excess DNA methylation in the promoter region of ApoE during malignant transformation of TRL 1215 cells.