Chronic inorganic arsenic exposure induces hepatic global and individual gene hypomethylation: implications for arsenic hepatocarcinogenesis.
Inorganic arsenic is a human carcinogen that can target the liver, but its carcinogenic mechanisms are still unknown. Global DNA hypomethylation occurs during arsenic-induced malignant transformation in rodent liver cells. DNA hypomethylation can increase gene expression, particularly when occurring in the promoter region CpG sites, and may be a non-genotoxic mechanism of carcinogenesis. Thus, in the present study liver samples of male mice exposed to 0 (control) or 45 p.p.m. arsenic (as NaAsO(2)) in the drinking water for 48 weeks were analyzed for gene expression and DNA methylation. Chronic arsenic exposure caused hepatic steatosis, a lesion also linked to consumption of methyl-deficient diets. Microarray analysis of liver samples showed arsenic induced aberrant gene expression including steroid-related genes, cytokines, apoptosis-related genes and cell cycle-related genes. In particular, the expression of the estrogen receptor-alpha (ER-alpha), and cyclin D1 genes were markedly increased. RT-PCR and immunohistochemistry confirmed arsenic-induced increases in hepatic ER-alpha and cyclin D1 transcription and translation products, respectively. Arsenic induced hepatic global DNA hypomethylation, as evidenced by 5-methylcytosine content of DNA and by the methyl acceptance assay. Arsenic also markedly reduced the methylation within the ER-alpha gene promoter region, as assessed by methylation-specific PCR, and this reduction was statistically significant in 8 of 13 CpG sites within the promoter region. Overall, in controls 28.3% of the ER-alpha promoter region CpG sites were methylated, but only 2.9% were methylated after chronic arsenic exposure. Thus, long-term exposure of mice to arsenic in the drinking water can induce aberrant gene expression, global DNA hypomethylation, and the hypomethylation of the ER-alpha gene promoter, all of which could potentially contribute to arsenic hepatocarcinogenesis.
Chen
Hua
H
Laboratory of Comparative Carcinogenesis, National Cancer Institute at the National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina, USA.
Li
ShuanFang
S
Liu
Jie
J
Diwan
Bhalchandra A
BA
Barrett
J Carl
JC
Waalkes
Michael P
MP
D000818:Animals; D018053:Arsenites / Q000633:toxicity*; D014408:Biomarkers, Tumor / Q000235:genetics / Q000378:metabolism; D002471:Cell Transformation, Neoplastic / Q000187:drug effects* / Q000235:genetics; D019938:Cyclin D1 / Q000235:genetics; D019175:DNA Methylation / Q000187:drug effects*; D047628:Estrogen Receptor alpha; D020869:Gene Expression Profiling; D007124:Immunoenzyme Techniques; D008099:Liver / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology*; D008297:Male; D051379:Mice; D020411:Oligonucleotide Array Sequence Analysis; D011401:Promoter Regions, Genetic / Q000235:genetics; D011960:Receptors, Estrogen / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017670:Sodium Compounds / Q000633:toxicity*; D013439:Sulfhydryl Reagents; D014867:Water
D018053:Arsenites; D014408:Biomarkers, Tumor; D047628:Estrogen Receptor alpha; D011960:Receptors, Estrogen; D017670:Sodium Compounds; D013439:Sulfhydryl Reagents; D014867:Water; D019938:Cyclin D1; C017947:sodium arsenite