Article

Involvement of N-6 Adenine-Specific DNA Methyltransferase 1 (N6AMT1) in Arsenic Biomethylation and Its Role in Arsenic-Induced Toxicity

Genes and Environment Laboratory, Division of Environmental Health Sciences, School of Public Health, University of California-Berkeley, Berkeley, California 94720, USA.
Environmental Health Perspectives (Impact Factor: 7.03). 12/2010; 119(6):771-7. DOI: 10.1289/ehp.1002733
Source: PubMed

ABSTRACT In humans, inorganic arsenic (iAs) is metabolized to methylated arsenical species in a multistep process mainly mediated by arsenic (+3 oxidation state) methyltransferase (AS3MT). Among these metabolites is monomethylarsonous acid (MMAIII), the most toxic arsenic species. A recent study in As3mt-knockout mice suggests that unidentified methyltransferases could be involved in alternative iAs methylation pathways. We found that yeast deletion mutants lacking MTQ2 were highly resistant to iAs exposure. The human ortholog of the yeast MTQ2 is N-6 adenine-specific DNA methyltransferase 1 (N6AMT1), encoding a putative methyltransferase.
We investigated the potential role of N6AMT1 in arsenic-induced toxicity.
We measured and compared the cytotoxicity induced by arsenicals and their metabolic profiles using inductively coupled plasma-mass spectrometry in UROtsa human urothelial cells with enhanced N6AMT1 expression and UROtsa vector control cells treated with different concentrations of either iAsIII or MMAIII.
N6AMT1 was able to convert MMAIII to the less toxic dimethylarsonic acid (DMA) when overexpressed in UROtsa cells. The enhanced expression of N6AMT1 in UROtsa cells decreased cytotoxicity of both iAsIII and MMAIII. Moreover, N6AMT1 is expressed in many human tissues at variable levels, although at levels lower than those of AS3MT, supporting a potential participation in arsenic metabolism in vivo.
Considering that MMAIII is the most toxic arsenical, our data suggest that N6AMT1 has a significant role in determining susceptibility to arsenic toxicity and carcinogenicity because of its specific activity in methylating MMAIII to DMA and other unknown mechanisms.

Download full-text

Full-text

Available from: Martyn Smith, Sep 23, 2014
0 Followers
 · 
149 Views
  • Source
    • "Arsenic N6AMT1 (N-6 adenine-specific DNA methyltransferase 1, putative) and MYST1 (MYST histone acetyltransferase 1) are genes that play an important role in arsenic methylation and histone acetylation, respectively. Results from studies in yeast [Jo et al., 2009a,b] and validation in mammalian cells [Ren et al., 2011] suggested that N6AMT1 and MYST1 are candidate human susceptibility genes involved in arsenic toxicity. Studies are undergoing for investigating the biochemical roles of these two proteins in in vitro studies and examining the role of N6AMT1 polymorphisms in susceptibility to lung cancer associated with arsenic exposure in DNA samples from a case control study conducted in Chile. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Predictive toxicology plays an important role in the assessment of toxicity of chemicals and the drug development process. While there are several well-established in vitro and in vivo assays that are suitable for predictive toxicology, recent advances in high-throughput analytical technologies and model systems are expected to have a major impact on the field of predictive toxicology. This commentary provides an overview of the state of the current science and a brief discussion on future perspectives for the field of predictive toxicology for human toxicity. Computational models for predictive toxicology, needs for further refinement and obstacles to expand computational models to include additional classes of chemical compounds are highlighted. Functional and comparative genomics approaches in predictive toxicology are discussed with an emphasis on successful utilization of recently developed model systems for high-throughput analysis. The advantages of three-dimensional model systems and stem cells and their use in predictive toxicology testing are also described. Environ. Mol. Mutagen., 2014. © 2014 Wiley Periodicals, Inc.
    Environmental and Molecular Mutagenesis 12/2014; 55(9). DOI:10.1002/em.21885 · 2.55 Impact Factor
  • Source
    • "In contrast to correlative genomic approaches such as transcriptomics, functional genomics directly identifies the genes tied to a phenotypic outcome (such as growth or toxicity). We have previously used this approach with several toxicants and shown the capability to translate results from yeast to mammalian systems (Jo et al., 2009a,b; Zhang et al., 2010; Ren et al., 2011). We also performed temporal protein expression profiling of yeast stress response. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Benzo[a]pyrene (BaP) is a ubiquitous, potent, and complete carcinogen resulting from incomplete organic combustion. BaP can form DNA adducts but other mechanisms may play a role in toxicity. We used a functional toxicology approach in S. cerevisiae to assess the genetic requirements for cellular resistance to BaP. In addition, we examined translational activities of key genes involved in various stress response pathways. We identified multiple genes and processes involved in modulating BaP toxicity in yeast which support DNA damage as a primary mechanism of toxicity, but also identify other potential toxicity pathways. Gene ontology enrichment analysis indicated that DNA damage and repair as well as redox homeostasis and oxidative stress are key processes in cellular response to BaP suggesting a similar mode of action of BaP in yeast and mammals. Interestingly, toxicant export is also implicated as a potential novel modulator of cellular susceptibility. In particular, we identified several transporters with human orthologs (solute carrier family 22) which may play a role in mammalian systems.
    Frontiers in Genetics 02/2013; 3:316. DOI:10.3389/fgene.2012.00316
  • Source
    • "This suggests that either other methyltransferases or perhaps anaerobic intestinal microbiota can methylate orally administered arsenicals. Recently, it has been shown that N-6 adeninespecific DNA methyltransferase 1 (N6AMT1) can biotransform MMA III to DMA via the use of N6AMT1-transfected UROtsa cells, with N6AMT1 endowing resistance to the cells against iAs III and MMA III (Ren et al. 2011). iAs is converted to MMA V and DMA V non-enzymatically in the presence of methyl vitamin B 12 (methylcobalamin) and GSH (Nakamura et al. 2009; Zakharyan and Aposhian 1999). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Arsenic is a worldwide environmental pollutant and a human carcinogen. It is well recognized that the toxicity of arsenicals largely depends on the oxidoreduction states (trivalent or pentavalent) and methylation levels (monomethyl, dimethyl, and trimethyl) that are present during the process of metabolism in mammals. However, presently, the specifics of the metabolic pathway of inorganic arsenicals have yet to be confirmed. In mammals, there are two possible mechanisms that have been proposed for the metabolic pathway of inorganic arsenicals, oxidative methylation, and glutathione conjugation. Oxidative methylation, which was originally proposed in fungi, is based on findings that arsenite (iAs(III)) is sequentially converted to monomethylarsonic acid (MMA(V)) and dimethylarsinic acid (DMA(V)) in both humans and in laboratory animals such as mice and rats. However, recent in vitro observations have demonstrated that arsenic is only methylated in the presence of glutathione (GSH) or other thiol compounds, which strongly suggests that arsenic is methylated in trivalent forms. The glutathione conjugation mechanism is supported by findings that have shown that most intracellular arsenicals are trivalent and excreted from cells as GSH conjugates. Since non-conjugated trivalent arsenicals are highly reactive with thiol compounds and are easily converted to less toxic corresponding pentavalent arsenicals, the arsenic-glutathione conjugate stability may be the most important factor for determining the toxicity of arsenicals. In addition, "being a non-anionic form" also appears to be a determinant of the toxicity of oxo-arsenicals or thioarsenicals. The present review discusses both the metabolism of arsenic and the toxicity of arsenic metabolites.
    Archives of Toxicology 07/2012; 87(6). DOI:10.1007/s00204-012-0904-5 · 5.08 Impact Factor