Occupational exposure to arsenic (As) and nickel (Ni) containing compounds has been associated with lung cancer and other adverse health effects. In the present study, the alterations of histone methylation by arsenic and nickel exposure were examined. In human lung carcinoma A549 cells, exposure to inorganic trivalent As (arsenite) increased H3K9 dimethylation and decreased H3K27 trimethylation, both of which represent gene silencing marks, while increasing the global levels of the H3K4 trimethylation, a gene-activating mark. The increase in H3K9me2 by arsenic was accompanied by an increase in the histone methyltransferase G9a protein and messenger RNA levels. It was also observed that very low-dose (0.1 μM) arsenite can cause significant alteration of histone modifications. Thus, this study provides further evidence that mechanism(s) of carcinogenicity of arsenic may involve alterations of various histone tail modifications, which may in turn affect the expression of genes that may cause transformation.

Nickel exposure (1 mM) significantly increased trimethyl H3K4 in human lung carcinoma A549 cells. Hypoxia (1% oxygen) increased H3K4 trimethylation in both normal human bronchial epithelial Beas-2B cells and A549 cells. The increase of H3K4 trimethylation by hypoxia was likely caused by inhibition of H3K4 demethylating activity, as hypoxia still increased H3K4 trimethylation in methionine-deficient medium. Furthermore, an in vitro histone demethylation assay demonstrated that hypoxia decreased H3K4 demethylase activity in Beas-2B nuclear extracts since ambient oxygen tensions were required for oxidative demethylation reaction to proceed. Hypoxia only minimally increased H3K4 trimethylation in the BEAS-2B cells after knockdown of JARID1A, which is the major histone H3K4 demethylase in this cell line. However, the mRNA and protein levels of JARID1A were not affected by hypoxia. GeneChip and pathway analysis in the JARID1A-knockdown Beas-2B cells revealed that JARID1A regulates the expression of hundreds of genes involved in different cellular functions, including tumorigenesis. Knocking down of JARID1A increased H3K4 trimethylation at the promoters of HMOX1 and DAF genes. Thus, these results indicate that hypoxia may target JARID1A activity, which in turn increases H3K4 trimethylation at both global and gene specific levels, leading to an altered program of gene expression and, eventually, tumor progression.