Arsenic(+3) and DNA methyltransferases, and arsenic speciation in tadpole and frog life stages of western clawed frogs (Silurana tropicalis) exposed to arsenate

Gender difference in arsenic biotransformation is an important metabolic basis for arsenic toxicity

BACKGROUND

Arsenic metabolism enzymes can affect the toxic effects of arsenic. However, the effects of different genders on the metabolites and metabolic enzymes in liver arsenic metabolism is still unclear. This study analyzed the gender differences of various arsenic metabolites and metabolic enzymes and further explored the effects of gender differences on arsenic metabolism in liver tissues of rats.

METHODS

Rats were treated with high/medium/low doses of iAs³⁺ or iAs⁵⁺. Liver pathological changes were observed with electron microscopy. The monomethyl aracid (MMA) and dimethyl aracid (DMA) was determined by high performance liquid chromatography-hydride generation atomic fluorescence spectroscopy. S-adenosylmethionine (SAM), arsenate respiratory reductase (ARR), nicotinamide adenine dinucleotide (NAD), purine nucleoside phosphorylase (PNP), pyruvate kinase (PK), and myeloperoxidase (MPO) SAM, ARR, NAD, PNP, PK, and MPO were determined by enzyme-linked immunoassay. RT-qPCR was used to determine Arsenic (+ 3 oxidation state) methyltransferase (AS3MT).

RESULTS

The iAs³⁺ and iAs⁵⁺ at high doses induced pathological changes in the liver, such as increased heterochromatin and lipid droplets. Compared within the same group, MMA and DMA were statistically significant in iAs^{3 +} high, iAs^{3 +} medium and iAs⁵⁺ low dose groups (P < 0.05). MMA of male rats in iAs³⁺ high and medium groups was higher than that of female rats, and the DMA of male rats was lower than that of female rats. As3MT mRNA in the male iAs³⁺ high group was higher than that of females. Besides, compared between male and female, only in iAS³⁺ low dose, iAS³⁺ medium dose, iAS⁵⁺ low dose, and iAS⁵⁺ medium dose groups, there was significant difference in SAM level (P < 0.05). Compared within the same group, male rats had significantly higher PNP and ARR activities while lower PK activity than female rats (P < 0.05). Between the male and female groups, only the iAS³⁺ high dose and medium dose group had a statistically significant difference (P < 0.05). The NAD activity of females in iAS³⁺ high dose group was higher than that of males.

CONCLUSION

The gender differences in the arsenic metabolism enzymes may affect the biotransformation of arsenic, which may be one of the important mechanisms of arsenic toxicity of different sexes and different target organs.

Synonymous single nucleotide polymorphism in arsenic (+3) methyltransferase of the Western mosquitofish (Gambusia affinis) and its gene expression among field populations

Naturally occurring arsenic is toxic at extremely low concentrations, yet some species persist even in high arsenic environments. We wanted to test if these species show evidence of evolution associated with arsenic exposure. To do this, we compared allelic variation across 872 coding nucleotides of arsenic (+3) methyltransferase (as3mt) and whole fish as3mt gene expression from three field populations of Gambusia affinis, from water sources containing low (1.9 ppb), medium-low (3.3 ppb), and high (15.7 ppb) levels of arsenic. The high arsenic site exceeds the US EPA's Maximum Contamination Level for drinking water. Medium-low and high populations exhibited homozygosity, and no sequence variation across all animals sampled. Eleven of 24 fish examined (45.8%) in the low arsenic population harbored synonymous single nucleotide polymorphisms (SNPs) in exons 4 and/or 10. SNP presence in the low arsenic population was not associated with differences in as3mt transcript levels compared to fish from the medium-low site, where SNPs were noted; however, as3mt expression in fish from the high arsenic concentration site was significantly lower than the other two sites. Low sequence variation in fish populations from sites with medium-low and high arsenic concentrations suggests greater selective pressure on this allele, while higher variation in the low population suggests a relaxed selection. Our results suggest gene regulation associated with arsenic detoxification may play a more crucial role in influencing responses to arsenic than polymorphic gene sequence. Understanding microevolutionary processes to various contaminants require the evaluation of multiple populations across a wide range of pollution exposures.

Predicted AS3MT Proteins Methylate Arsenic and Support Two Major Phylogenetic AS3MT Groups

Inorganic arsenic is one of the most toxic and carcinogenic substances in the environment, but many organisms, including humans, methylate inorganic arsenic to mono-, di-, and trimethylated arsenic metabolites, which the organism can excrete. In humans and other eukaryotic organisms, the arsenite methyltransferase (AS3MT) protein methylates arsenite. AS3MT sequences from eukaryotic organisms group phylogenetically with predicted eubacterial AS3MT sequences, which has led to the suggestion that AS3MT was acquired from eubacteria by multiple events of horizontal gene transfer. In this study, we evaluated whether 55 (out of which 47 were predicted based on protein sequence similarity) sequences encoding putative AS3MT orthologues in 47 species from different kingdoms can indeed methylate arsenic. Fifty-three of the proteins showed arsenic methylating capacity. For example, the predicted AS3MT of the human gut bacterium Faecalibacterium prausnitzii methylated arsenic efficiently. We performed a kinetic analysis of 14 AS3MT proteins representing two phylogenetically distinct clades (Group 1 and 2) that each contain both eubacterial and eukaryotic sequences. We found that animal and bacterial AS3MTs in Group 1 rarely produce trimethylated arsenic, whereas Hydra vulgaris and the bacterium Rhodopseudomonas palustris in Group 2 produce trimethylated arsenic metabolites. These findings suggest that animals during evolution have acquired different arsenic methylating phenotypes from different bacteria. Further, it shows that humans carry two bacterial systems for arsenic methylation: one bacterium-derived AS3MT from Group 1 incorporated in the human genome and one from Group 2 in F. prausnitzii present in the gut microbiome.

Extreme Arsenic and Antimony Uptake and Tolerance in Toad Tadpoles during Development in Highly Contaminated Wetlands

The effects of extreme concentrations of toxic metalloids, such as arsenic (As) and antimony (Sb), on larval amphibians are not well-understood. We sampled Western Toad tadpoles (Anaxyrus boreas) living in As- and Sb-contaminated wetlands throughout their development. Although the tadpoles completed metamorphosis, they accumulated among the highest concentrations of As and Sb ever reported for a living vertebrate (3866.9 mg/kg; 315.0 mg/kg (dry weight), respectively). Ingestion of contaminated sediment had a more important role in metalloid accumulation than aqueous exposure alone. Metalloids were initially concentrated in the gut; however, by metamorphosis, the majority were found in other tissues. These concentrations subsequently decreased with the onset of metamorphosis, yet remained quite elevated. Sublethal effects, including delayed development and reduced size at metamorphosis, were associated with elevated metalloid exposure. The presence of organic arsenicals in tadpole tissues suggests they have the ability to biomethylate inorganic As compounds. The arsenical trimethyl arsine oxide accounted for the majority of extractable organic As, with lesser amounts of monomethylarsonic acid and dimethylarsinic acid. Our findings demonstrate remarkable tolerance of toad tadpoles to extreme metalloid exposure and implicate physiological processes mediating that tolerance.

AS3MT-mediated tolerance to arsenic evolved by multiple independent horizontal gene transfers from bacteria to eukaryotes

Organisms have evolved the ability to tolerate toxic substances in their environments, often by producing metabolic enzymes that efficiently detoxify the toxicant. Inorganic arsenic is one of the most toxic and carcinogenic substances in the environment, but many organisms, including humans, metabolise inorganic arsenic to less toxic metabolites. This multistep process produces mono-, di-, and trimethylated arsenic metabolites, which the organism excretes. In humans, arsenite methyltransferase (AS3MT) appears to be the main metabolic enzyme that methylates arsenic. In this study, we examined the evolutionary origin of AS3MT and assessed the ability of different genotypes to produce methylated arsenic metabolites. Phylogenetic analysis suggests that multiple, independent horizontal gene transfers between different bacteria, and from bacteria to eukaryotes, increased tolerance to environmental arsenic during evolution. These findings are supported by the observation that genetic variation in AS3MT correlates with the capacity to methylate arsenic. Adaptation to arsenic thus serves as a model for how organisms evolve to survive under toxic conditions.

Life cycle exposure of the frog Silurana tropicalis to arsenate: Steroid- and thyroid hormone-related genes are differently altered throughout development

Arsenic contaminates water surface and groundwater worldwide. Several studies have suggested that arsenic acts as an endocrine disruptor in mammalian and non-mammalian species, although its chronic effect during development remains largely unknown. To address this question, life cycle exposures to 0, 0.3 and 0.8ppm of arsenate (pentavalent arsenic; As(V)) were performed in the Western clawed frog (Silurana tropicalis) from the gastrulae stage (developmental stage Nieuwkoop-Faber; NF12) until metamorphosis (NF66). Tissue samples were collected at the beginning of feeding (NF46; whole body), sexual development (NF56; liver), and at metamorphosis completion (NF66; liver and gonadal mesonephros complex). Real-time RT-PCR analysis quantified decreases in mRNA levels of genes related to estrogen- (estrogen receptor alpha and aromatase), androgen- (androgen receptor and steroid 5-alpha-reductase type 2), and cholesterol metabolism- (steroidogenic acute regulatory protein) at stage NF46. Similarly, arsenate decreased steroid 5-alpha-reductase type 2 expression in stage NF56 livers, but transcript increases were observed for both estrogen receptor alpha and steroidogenic acute regulatory protein at this stage. Given the changes observed in the expression of genes essential for proper sexual development, gonadal histological analysis was carried out in stage NF66 animals. Arsenate treatments did not alter sex ratio or produce testicular oocytes. On the other hand, arsenate interfered with thyroid hormone-related transcripts at NF66. Specifically, thyroid hormone receptor beta and deiodinase type 2 mRNA levels were significantly reduced after arsenate treatment in the gonadal mesonephros complex. This reduction in thyroid hormone-related gene expression, however, was not accompanied by any morphological changes measured. In summary, environmentally relevant concentrations of As(V) altered steroidogenesis-, sex steroid signaling- and thyroid hormone-related gene expression, although transcriptional changes varied among tissues and developmental stages.

Sex-Dependent effects of developmental arsenic exposure on methylation capacity and methylation regulation of the glucocorticoid receptor system in the embryonic mouse brain

Previously we have shown that prenatal moderate arsenic exposure (50 ppb) disrupts glucocorticoid receptor (GR) programming and that these changes continue into adolescence in males. However, it was not clear what the molecular mechanisms were promoting these GR programming changes or if these changes occurred in arsenic-exposed females. In the present studies, we assessed the effects of arsenic on protein and mRNA of the glucocorticoid receptor (GR) and 11β-hydroxysteroid dehydrogenase (Hsd) isozymes and compared the levels of methylation within the promoters of the Nr3c1 and Hsd11b1 genes in female fetal brain at embryonic days (E) 14 and 18. Prenatal arsenate exposure produced sex specific effects on the glucocorticoid system. Compared to males, females were resistant to arsenic induced changes in GR, 11β-Hsd-1 and 11β-Hsd-2 protein levels despite observed elevations in Nr3c1 and Hsd11b2 mRNA. This sex-specific effect was not due to differences in the methylation of the GR promoter as methylation of the Nr3c1 gene was either unchanged (region containing the egr-1 binding site) or similarly reduced (region containing the SP-1 transcription factor binding site) in both males and females exposed to arsenic. Arsenic did produce sex and age-specific changes in the methylation of Hsd11b1 gene, producing increased methylation in females at E14 and decreased methylation at E18.These changes were not attributed to changes in DNMT levels. Since arsenate metabolism could interfere with the generation of methyl donor groups, we assessed glutathione (GSH), S-adenosylmethionine (SAM) and As 3 methyltransferase (As3MT). Exposed males and females had similar levels of As3MT and SAM; however, females had higher levels of GSH/GSSH. It is possible that this greater anti-oxidative capacity within the females provides protection against low to moderate arsenate. Our data suggest that the GR signaling system in female offspring was not as affected by prenatal arsenic and predicts that female arsenic-exposed mice should have normal GR feedback regulation.