Nicotinamide N-Methyltransferase Suppression Participates in Nickel-Induced Histone H3 Lysine9 Dimethylation in BEAS-2B Cells

Background: Nickel compounds are well-established human carcinogens with weak mutagenic activity. Histone methylation has been proposed to play an important role in nickel-induced carcinogenesis. Nicotinamide N-methyltransferase (NNMT) decreases histone methylation in several cancer cells by altering the cellular ratio of S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH). However, the role of NNMT in nickel-induced histone methylation remains unclear. Methods: BEAS-2B cells were exposed to different concentrations of nickel chloride (NiCl2) for 72 h or 200 μM NiCl2 for different time periods. Histone H3 on lysine 9 (H3K9) mono-, di-, and trimethylation and NNMT protein levels were measured by western blot analysis. Expressions of NNMT mRNA and the H3k9me2-associated genes, mitogen-activated protein kinase 3 (MAP2K3) and dickkopf1 (DKK1), were determined by qPCR analysis. The cellular ratio of nicotinamide adenine dinucleotide (NAD+) to reduced NAD (NADH) and SAM/SAH ratio were determined. Results: Exposure of BEAS-2B cells to nickel increased H3K9 dimethylation (H3K9me2), suppressed the expressions of H3K9me2-associated genes (MAP2K3 and DKK1), and induced NNMT repression at both the protein and mRNA levels. Furthermore, over-expression of NNMT inhibited nickel-induced H3K9me2 and altered the cellular SAM/SAH ratio. Additionally, the NADH oxidant phenazine methosulfate (PMS) not only reversed the nickel-induced reduction in NAD+/NADH but also inhibited the increase in H3K9me2. Conclusions: These findings indicate that the repression of NNMT may underlie nickel-induced H3K9 dimethylation by altering the cellular SAM/SAH ratio.