Effect of sodium arsenite dose administered in the drinking water on the urinary bladder epithelium of female arsenic (+3 oxidation state) methyltransferase knockout mice

Metabolomics reveals the metabolic disturbance caused by arsenic in the mouse model of inflammatory bowel disease

Arsenic exposure has long been a significant global health concern due to its association with various human diseases. The adverse health effects of arsenic can be influenced by multiple factors, resulting in considerable individual variability. Individuals with inflammatory bowel disease (IBD) are particularly vulnerable to the effects of toxin exposure, yet the specific impact of arsenic in the context of IBD remains unclear. In this study, we employed a non-targeted metabolomics approach to investigate how arsenic exposure affects metabolic homeostasis in an IBD model using Helicobacter trogontum-infected interleukin-10 deficient mice. Our results demonstrated that arsenic exposure disrupted the balance of various metabolites, including tryptophan, polyunsaturated fatty acids, purine and pyrimidine metabolites, and branched-chain amino acids, in mice with colitis but not in those without colitis. Notably, several crucial metabolites involved in anti-inflammatory responses, oxidative stress, and energy metabolism were significantly altered in mice with colitis. These results indicate that arsenic exposure in an IBD context can lead to extensive metabolic disturbances, potentially exacerbating disease severity and impacting overall health. This study underscores the necessity of evaluating arsenic toxicity in relation to IBD to better understand and mitigate associated health risks.

On the Existence of Pnictogen Bonding Interactions in As(III) S-Adenosylmethionine Methyltransferase Enzymes

As(III) S-adenosylmethionine methyltransferases, pivotal enzymes in arsenic metabolism, facilitate the methylation of arsenic up to three times. This process predominantly yields trivalent mono- and dimethylarsenite, with trimethylarsine forming in smaller amounts. While this enzyme acts as a detoxifier in microbial systems by altering As(III), in humans, it paradoxically generates more toxic and potentially carcinogenic methylated arsenic species. The strong affinity of As(III) for cysteine residues, forming As(III)-thiolate bonds, is exploited in medical treatments, notably in arsenic trioxide (Trisenox®), an FDA-approved drug for leukemia. The effectiveness of this drug is partly due to its interaction with cysteine residues, leading to the breakdown of key oncogenic fusion proteins. In this study, we extend the understanding of As(III)'s binding mechanisms, showing that, in addition to As(III)-S covalent bonds, noncovalent O⋅⋅⋅As pnictogen bonding plays a vital role. This interaction significantly contributes to the structural stability of the As(III) complexes. Our crystallographic analysis using the PDB database of As(III) S-adenosylmethionine methyltransferases, augmented by comprehensive theoretical studies including molecular electrostatic potential (MEP), quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO) analysis, emphasizes the critical role of pnictogen bonding in these systems. We also undertake a detailed evaluation of the energy characteristics of these pnictogen bonds using various theoretical models. To our knowledge, this is the first time pnictogen bonds in As(III) derivatives have been reported in biological systems, marking a significant advancement in our understanding of arsenic's molecular interactions.

Analysis of Threshold Effect of Urinary Heavy Metal Elements on the High Prevalence of Nephrolithiasis in Men

Exposure to heavy metals in the environment exerts serious effects on kidney health. However, the effects of joint exposure on the kidneys have been rarely studied, particularly in non-occupational exposure high-risk populations. This study provided a reference threshold range of heavy metals in urine and explored the effect of joint exposure on nephrolithiasis in men. The data were obtained from the China Multi-Ethnic Cohort database, and 1502 men were included in the study. A two-piece-wise regression model was used to assess the dose-response relationship between heavy metal exposure and nephrolithiasis. The least absolute shrinkage and selection operator regression model was used to calculate the score of joint exposure to heavy metals. The threshold effect analysis revealed a linear relationship between the concentration of arsenic (As) in the urine and the prevalence of nephrolithiasis, whereas a nonlinear relationship was observed with cadmium (Cd), chromium (Cr), mercury (Hg), and lead (Pb). In addition, As, Cd, Cr, Hg, and Pb may significantly affect the joint exposure effect. Moreover, the final risk of nephrolithiasis increased by 123% (P for trend < 0.001). This study found a threshold relationship between heavy metals (Cd, Cr, Hg, Pb) in male urine and the occurrence of nephrolithiasis. Joint exposure to heavy metals in urine caused a high-risk effect on nephrolithiasis. The study provided a reference threshold value of related studies and indicated that environmental pollution caused by heavy metals should be reduced.

Arsenic toxicokinetic modeling and risk analysis: Progress, needs and applications

Arsenic (As) poses unique challenges in PBTK model development and risk analysis applications. Arsenic metabolism is complex, adequate information to attribute specific metabolites to particular adverse effects in humans is sparse, and measurement of relevant metabolites in biological media can be difficult. Multiple As PBTK models have been published and used or adapted for use in various exposure and risk analysis applications. These applications illustrate the broad utility of PBTK models for exposure and dose-response analysis, particularly for arsenic where multi-pathway, multi-route exposures and multiple toxic effects are of concern. Arsenic PBTK models have been used together with exposure reconstruction and dose-response functions to estimate risk of specific adverse health effects due to drinking water exposure and consumption of specific foodstuffs (e.g. rice, seafood), as well as to derive safe exposure levels and develop consumption advisories. Future refinements to arsenic PBTK models can enhance the confidence in such analyses. Improved estimates for methylation biotransformation parameters based on in vitro to in vivo extrapolation (IVIVE) methods and estimation of interindividual variability in key model parameters for specific toxicologically relevant metabolites are two important areas for consideration.

Origins, fate, and actions of methylated trivalent metabolites of inorganic arsenic: progress and prospects

The toxic metalloid inorganic arsenic (iAs) is widely distributed in the environment. Chronic exposure to iAs from environmental sources has been linked to a variety of human diseases. Methylation of iAs is the primary pathway for metabolism of iAs. In humans, methylation of iAs is catalyzed by arsenic (+ 3 oxidation state) methyltransferase (AS3MT). Conversion of iAs to mono- and di-methylated species (MAs and DMAs) detoxifies iAs by increasing the rate of whole body clearance of arsenic. Interindividual differences in iAs metabolism play key roles in pathogenesis of and susceptibility to a range of disease outcomes associated with iAs exposure. These adverse health effects are in part associated with the production of methylated trivalent arsenic species, methylarsonous acid (MAsIII) and dimethylarsinous acid (DMAsIII), during AS3MT-catalyzed methylation of iAs. The formation of these metabolites activates iAs to unique forms that cause disease initiation and progression. Taken together, the current evidence suggests that methylation of iAs is a pathway for detoxification and for activation of the metalloid. Beyond this general understanding of the consequences of iAs methylation, many questions remain unanswered. Our knowledge of metabolic targets for MAsIII and DMAsIII in human cells and mechanisms for interactions between these arsenicals and targets is incomplete. Development of novel analytical methods for quantitation of MAsIII and DMAsIII in biological samples promises to address some of these gaps. Here, we summarize current knowledge of the enzymatic basis of MAsIII and DMAsIII formation, the toxic actions of these metabolites, and methods available for their detection and quantification in biomatrices. Major knowledge gaps and future research directions are also discussed.

Arsenic methylation - Lessons from three decades of research

Between 1990 and 2020, our understanding of the significance of arsenic biomethylation changed in remarkable ways. At the beginning of this period, the conversion of inorganic arsenic into mono- and di-methylated metabolites was viewed primarily as a process that altered the kinetic behavior of arsenic. By increasing the rate of clearance of arsenic, the formation of methylated metabolites reduced exposure to this toxin; that is, methylation was detoxification. By 2020, it was clear that at least some of the toxic effects associated with As exposure depended on formation of methylated metabolites containing trivalent arsenic. Because the trivalent oxidation state of arsenic is associated with increased potency as a cytotoxin and clastogen, these findings were consistent with methylation-related changes in the dynamic behavior of arsenic. That is, methylation was activation. Our current understanding of the role of methylation as a modifier of kinetic and dynamic behaviors of arsenic is the product of research at molecular, cellular, organismic, and population levels. This information provides a basis for refining our estimates of risk associated with long term exposure to inorganic arsenic in environmental media, food, and water. This report summarizes the growth of our knowledge of enzymatically catalyzed methylation of arsenic over this period and considers the prospects for new discoveries.

Diverse genetic backgrounds play a prominent role in the metabolic phenotype of CC021/Unc and CC027/GeniUNC mice exposed to inorganic arsenic

Arsenic methyltransferase (AS3MT) is the key enzyme in the pathway for the methylation of inorganic arsenic (iAs), a potent human carcinogen and diabetogen. AS3MT converts iAs to mono- and dimethylated arsenic species (MAs, DMAs) that are excreted mainly in urine. Polymorphisms in AS3MT is a key genetic factor affecting iAs metabolism and toxicity. The present study examined the role of As3mt polymorphisms in the susceptibility to the diabetogenic effects of iAs exposure using two Collaborative Cross mouse strains, CC021/Unc and CC027/GeniUnc, carrying different As3mt haplotypes. Male mice from the two strains were exposed to iAs in drinking water (0, 0.1 or 50 ppm) for 11 weeks. Blood glucose and plasma insulin levels were measured after 6-h fasting and 15 min after i.p. injection of glucose. Body composition was determined using magnetic resonance imaging. To asses iAs metabolism, the concentrations of iAs, MAs and DMAs were measured in urine. The results show that CC021 mice, both iAs-exposed and controls, had higher body fat percentage, lower fasting blood glucose, higher fasting plasma insulin, and were more insulin resistant than their CC027 counterparts. iAs exposure had a minor effect on diabetes indicators and only in CC027 mice. Blood glucose levels 15 min after glucose injection were significantly higher in CC027 mice exposed to 0.1 ppm iAs than in control mice. No significant differences were found in the concentrations or proportions of arsenic species in urine of CC021 and CC027 mice at the same exposure level. These results suggest that the differences in As3mt haplotypes did not affect the profiles of iAs or its metabolites in mouse urine. The major differences in diabetes indicators were associated with the genetic backgrounds of CC021 and CC027 mice. The effects of iAs exposure, while minor, were genotype- and dose-dependent.

Biotransformation of arsenic trioxide by AS3MT favors eradication of acute promyelocytic leukemia: revealing the hidden facts

Arsenic trioxide (ATO) is one of the most effective drugs for treatment of acute promyelocytic leukemia (APL). It could specifically target the PML/RARα fusion oncoprotein stability and induces APL cell differentiation as well as apoptosis. Although many studies have been conducted to document the anticancer effects and mechanism of ATO, there is little information about the association between biotransformation of ATO to active arsenic metabolites and APL therapy. Generally, ATO can be rapidly converted into trivalent methylated metabolites by arsenic (+3 oxidation state) methyltransferase (AS3MT) mostly in liver and redistributed to bloodstream of APL patients who receiving ATO treatment, thereby leading to a balance between cytotoxicity and differentiation, which is proposed to be the key event in successful treatment of APL. In this review, we comprehensively discussed possible roles of AS3MT and methylated arsenic metabolites in APL therapy, so as to reveal the association between individual differences of AS3MT expression and activity with the therapeutic efficacy of ATO in APL patients.

The Human Gut Microbiome's Influence on Arsenic Toxicity

PURPOSE OF REVIEW

Arsenic exposure is a public health concern of global proportions with a high degree of interindividual variability in pathologic outcomes. Arsenic metabolism is a key factor underlying toxicity, and the primary purpose of this review is to summarize recent discoveries concerning the influence of the human gut microbiome on the metabolism, bioavailability, and toxicity of ingested arsenic. We review and discuss the current state of knowledge along with relevant methodologies for studying these phenomena.

RECENT FINDINGS

Bacteria in the human gut can biochemically transform arsenic-containing compounds (arsenicals). Recent publications utilizing culture-based approaches combined with analytical biochemistry and molecular genetics have helped identify several arsenical transformations by bacteria that are at least possible in the human gut and are likely to mediate arsenic toxicity to the host. Other studies that directly incubate stool samples in vitro also demonstrate the gut microbiome's potential to alter arsenic speciation and bioavailability. In vivo disruption or elimination of the microbiome has been shown to influence toxicity and body burden of arsenic through altered excretion and biotransformation of arsenicals. Currently, few clinical or epidemiological studies have investigated relationships between the gut microbiome and arsenic-related health outcomes in humans, although current evidence provides strong rationale for this research in the future.

SUMMARY

The human gut microbiome can metabolize arsenic and influence arsenical oxidation state, methylation status, thiolation status, bioavailability, and excretion. We discuss the strength of current evidence and propose that the microbiome be considered in future epidemiologic and toxicologic studies of human arsenic exposure.

Gene expression and DNA methylation regulation of arsenic in mouse bladder tissues and in human urothelial cells

According to a report of the International Agency for Research on Cancer, arsenic and inorganic arsenic compounds are classified into Group 1 carcinogens with regard to human health. Epidemiological studies indicate that arsenic is one of the main risk factors for the development of bladder cancer. In the present study, arsenic-altered gene expression in mouse bladder tissues and in human urothelial cells was compared. In the mouse model, sodium arsenite-induced mouse urothelial hyperplasia and intracellular inclusions were present. Following DNA array analysis, four genes with differential expression were selected for quantitative real-time PCR assay. The genes were the following: Cystathionine β-synthase (CBS), adenosine A1 receptor (ADORA1), metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) and Wnt inhibitory factor 1 (Wif1). The results indicated a significant increase in the levels of Cbs and Adora1. The analysis of the DNA CpG methylation levels of the mouse Cbs and Adora1 genes revealed no significant change. In contrast to these observations, the four genes were further analyzed in the human normal urothelial cell line SV-HUC1. The data indicated that WIF1 gene expression was decreased by sodium arsenite, whereas this was not noted for CBS, MALAT1 and ADORA1. Sodium arsenite decreased mRNA and protein expression levels of the WIF1 gene. In addition, the methylation levels of the WIF1 gene were increased. Sodium arsenite inhibited cell proliferation and promoted cell migration as demonstrated in cell functional assays. The gene status was compared in 8 human urothelial cell lines, and WIF1 mRNA expression levels were determined to be higher, whereas DNA CpG methylation levels were lower in SV-HUC1 cells compared with those noted in the other 7 bladder cancer cell lines. In summary, the data indicated that sodium arsenite decreased WIF1 gene expression and promoted cell migration. The increased methylation levels of WIF1 DNA CpG could be a potential biomarker for bladder cancer.

The gut microbiome is required for full protection against acute arsenic toxicity in mouse models

Arsenic poisons an estimated 200 million people worldwide through contaminated food and drinking water. Confusingly, the gut microbiome has been suggested to both mitigate and exacerbate arsenic toxicity. Here, we show that the microbiome protects mice from arsenic-induced mortality. Both antibiotic-treated and germ-free mice excrete less arsenic in stool and accumulate more arsenic in organs compared to control mice. Mice lacking the primary arsenic detoxification enzyme (As3mt) are hypersensitive to arsenic after antibiotic treatment or when derived germ-free, compared to wild-type and/or conventional counterparts. Human microbiome (stool) transplants protect germ-free As3mt-KO mice from arsenic-induced mortality, but protection depends on microbiome stability and the presence of specific bacteria, including Faecalibacterium. Our results demonstrate that both a functional As3mt and specific microbiome members are required for protection against acute arsenic toxicity in mouse models. We anticipate that the gut microbiome will become an important explanatory factor of disease (arsenicosis) penetrance in humans, and a novel target for prevention and treatment strategies.

It is unclear whether the gut microbiome can mitigate or exacerbate arsenic toxicity. Here, Coryell et al. show that the human gut microbiome protects mice from arsenic-induced mortality, with protection levels correlating with the relative abundance of the human commensal Faecalibacterium.

Multidrug Resistance Protein 1 (MRP1/ABCC1)-Mediated Cellular Protection and Transport of Methylated Arsenic Metabolites Differs between Human Cell Lines

The ATP-binding cassette (ABC) transporter multidrug resistance protein 1 (MRP1/ABCC1) protects cells from arsenic (a proven human carcinogen) through the cellular efflux of arsenic triglutathione [As(GS)3] and the diglutathione conjugate of monomethylarsonous acid [MMA(GS)2]. Previously, differences in MRP1 phosphorylation (at Y920/S921) and N-glycosylation (at N19/N23) were associated with marked differences in As(GS)3 transport kinetics between HEK293 and HeLa cell lines. In the current study, cell line differences in MRP1-mediated cellular protection and transport of other arsenic metabolites were explored. MRP1 expressed in HEK293 cells reduced the toxicity of the major urinary arsenic metabolite dimethylarsinic acid (DMAV), and HEK-WT-MRP1-enriched vesicles transported DMAV with high apparent affinity and capacity (Km 0.19 µM, Vmax 342 pmol⋅mg-1protein⋅min-1). This is the first report that MRP1 is capable of exporting DMAV, critical for preventing highly toxic dimethylarsinous acid formation. In contrast, DMAV transport was not detected using HeLa-WT-MRP1 membrane vesicles. MMA(GS)2 transport by HeLa-WT-MRP1 vesicles had a greater than threefold higher Vmax compared with HEK-WT-MRP1 vesicles. Cell line differences in DMAV and MMA(GS)2 transport were not explained by differences in phosphorylation at Y920/S921. DMAV did not inhibit, whereas MMA(GS)2 was an uncompetitive inhibitor of As(GS)3 transport, suggesting that DMAV and MMA(GS)2 have nonidentical binding sites to As(GS)3 on MRP1. Efflux of different arsenic metabolites by MRP1 is likely influenced by multiple factors, including cell and tissue type. This could have implications for the impact of MRP1 on both tissue-specific susceptibility to arsenic-induced disease and tumor sensitivity to arsenic-based therapeutics.

The Structure of an As(III) S-Adenosylmethionine Methyltransferase with 3-Coordinately Bound As(III) Depicts the First Step in Catalysis

Arsenic is a ubiquitous environmental toxic substance and a Class 1 human carcinogen. Arsenic methylation by the enzyme As(III) S-adenosylmethionine (SAM) methyltransferase (ArsM in microbes or AS3MT in animals) detoxifies As(III) in microbes but transforms it into more toxic and potentially more carcinogenic methylated species in humans. We previously proposed a reaction pathway for ArsM/AS3MT that involves initial 3-coordinate binding of As(III). To date, reported structures have had only 2-coordinately bound trivalent arsenicals. Here we report a crystal structure of CmArsM from Cyanidioschyzon sp.5508 in which As(III) is 3-coordinately bound to three conserved cysteine residues with a molecule of the product S-adenosyl-l-homocysteine bound in the SAM binding site. We propose that this structure represents the first step in the catalytic cycle. In a previously reported SAM-bound structure, a disulfide bond is formed between two conserved cysteine residues. Comparison of these two structures indicates that there is a conformational change in the N-terminal domain of CmArsM that moves a loop to allow formation of the 3-coordinate As(III) binding site. We propose that this conformational change is an initial step in the As(III) SAM methyltransferase catalytic cycle.

Toxicodynamics of Lead, Cadmium, Mercury and Arsenic- induced kidney toxicity and treatment strategy: A mini review

Environmental pollution has become a concerning matter to human beings. Flint water crisis in the USA pointed out that pollution by heavy metal is getting worse day by day, predominantly by Lead, Cadmium, Mercury and Arsenic. Despite of not having any biological role in flora and fauna, they exhibit detrimental effect following exposure (acute or chronic). Even at low dose, they affect brain, kidney and heart. Oxidative stress has been termed as cause and effect in heavy metal-induced kidney toxicity. In treatment strategy, different chelating agent, vitamins and minerals are included, though chelating agents has been showed different fatal drawbacks. Interestingly, plants and plants derived compounds had shown possible effectiveness against heavy metals induced kidney toxicity. This review will provide detail information on toxicodynamics of Pb, Cd, Hg and As, treatment strategy along with the possible beneficiary role of plant derived compound to protect kidney.

Knockout of arsenic (+3 oxidation state) methyltransferase is associated with adverse metabolic phenotype in mice: the role of sex and arsenic exposure

Susceptibility to toxic effects of inorganic arsenic (iAs) depends, in part, on efficiency of iAs methylation by arsenic (+3 oxidation state) methyltransferase (AS3MT). As3mt-knockout (KO) mice that cannot efficiently methylate iAs represent an ideal model to study the association between iAs metabolism and adverse effects of iAs exposure, including effects on metabolic phenotype. The present study compared measures of glucose metabolism, insulin resistance and obesity in male and female wild-type (WT) and As3mt-KO mice during a 24-week exposure to iAs in drinking water (0.1 or 1 mg As/L) and in control WT and As3mt-KO mice drinking deionized water. Results show that effects of iAs exposure on fasting blood glucose (FBG) and glucose tolerance in either WT or KO mice were relatively minor and varied during the exposure. The major effects were associated with As3mt KO. Both male and female control KO mice had higher body mass with higher percentage of fat than their respective WT controls. However, only male KO mice were insulin resistant as indicated by high FBG, and high plasma insulin at fasting state and 15 min after glucose challenge. Exposure to iAs increased fat mass and insulin resistance in both male and female KO mice, but had no significant effects on body composition or insulin resistance in WT mice. These data suggest that As3mt KO is associated with an adverse metabolic phenotype that is characterized by obesity and insulin resistance, and that the extent of the impairment depends on sex and exposure to iAs, including exposure to iAs from mouse diet.

Inorganic arsenic: A non-genotoxic carcinogen

Inorganic arsenic induces a variety of toxicities including cancer. The mode of action for cancer and non-cancer effects involves the metabolic generation of trivalent arsenicals and their reaction with sulfhydryl groups within critical proteins in various cell types which leads to the biological response. In epithelial cells, the response is cell death with consequent regenerative proliferation. If this continues for a long period of time, it can result in an increased risk of cancer. Arsenicals do not react with DNA. There is evidence for indirect genotoxicity in various in vitro and in vivo systems, but these involve exposures at cytotoxic concentrations and are not the basis for cancer development. The resulting markers of genotoxicity could readily be due to the cytotoxicity rather than an effect on the DNA itself. Evidence for genotoxicity in humans has involved detection of chromosomal aberrations, sister chromatid exchanges in lymphocytes and micronucleus formation in lymphocytes, buccal mucosal cells, and exfoliated urothelial cells in the urine. Numerous difficulties have been identified in the interpretation of such results, including inadequate assessment of exposure to arsenic, measurement of micronuclei, and potential confounding factors such as tobacco exposure, folate deficiency, and others. Overall, the data strongly supports a non-linear dose response for the effects of inorganic arsenic. In various in vitro and in vivo models and in human epidemiology studies there appears to be a threshold for biological responses, including cancer.

Cellular arsenic transport pathways in mammals

Natural contamination of drinking water with arsenic results in the exposure of millions of people world-wide to unacceptable levels of this metalloid. This is a serious global health problem because arsenic is a Group 1 (proven) human carcinogen and chronic exposure is known to cause skin, lung, and bladder tumors. Furthermore, arsenic exposure can result in a myriad of other adverse health effects including diseases of the cardiovascular, respiratory, neurological, reproductive, and endocrine systems. In addition to chronic environmental exposure to arsenic, arsenic trioxide is approved for the clinical treatment of acute promyelocytic leukemia, and is in clinical trials for other hematological malignancies as well as solid tumors. Considerable inter-individual variability in susceptibility to arsenic-induced disease and toxicity exists, and the reasons for such differences are incompletely understood. Transport pathways that influence the cellular uptake and export of arsenic contribute to regulating its cellular, tissue, and ultimately body levels. In the current review, membrane proteins (including phosphate transporters, aquaglyceroporin channels, solute carrier proteins, and ATP-binding cassette transporters) shown experimentally to contribute to the passage of inorganic, methylated, and/or glutathionylated arsenic species across cellular membranes are discussed. Furthermore, what is known about arsenic transporters in organs involved in absorption, distribution, and metabolism and how transport pathways contribute to arsenic elimination are described.

Oxidation state specific analysis of arsenic species in tissues of wild-type and arsenic (+3 oxidation state) methyltransferase-knockout mice

Arsenic methyltransferase (As3mt) catalyzes the conversion of inorganic arsenic (iAs) to its methylated metabolites, including toxic methylarsonite (MAsIII) and dimethylarsinite (DMAsIII). Knockout (KO) of As3mt was shown to reduce the capacity to methylate iAs in mice. However, no data are available on the oxidation states of As species in tissues of these mice. Here, we compare the oxidation states of As species in tissues of male C57BL/6 As3mt-KO and wild-type (WT) mice exposed to arsenite (iAsIII) in drinking water. WT mice were exposed to 50mg/L As and As3mt-KO mice that cannot tolerate 50mg/L As were exposed to 0, 15, 20, 25 or 30mg/L As. iAsIII accounted for 53% to 74% of total As in liver, pancreas, adipose, lung, heart, and kidney of As3mt-KO mice; tri- and pentavalent methylated arsenicals did not exceed 10% of total As. Tissues of WT mice retained iAs and methylated arsenicals: iAsIII, MAsIII and DMAsIII represented 55%-68% of the total As in the liver, pancreas, and brain. High levels of methylated species, particularly MAsIII, were found in the intestine of WT, but not As3mt-KO mice, suggesting that intestinal bacteria are not a major source of methylated As. Blood of WT mice contained significantly higher levels of As than blood of As3mt-KO mice. This study is the first to determine oxidation states of As species in tissues of As3mt-KO mice. Results will help to design studies using WT and As3mt-KO mice to examine the role of iAs methylation in adverse effects of iAs exposure.

Mechanisms of action for arsenic in cardiovascular toxicity and implications for risk assessment

The possibility of an association between inorganic arsenic (iAs) exposure and cardiovascular outcomes has received increasing attention in the literature over the past decade. The United States Environmental Protection Agency (US EPA) is currently revising its Integrated Risk Assessment System (IRIS) review of iAs, and one of the non-cancer endpoints of interest is cardiovascular disease (CVD). Despite the increased interest in this area, substantial gaps remain in the available information, particularly regarding the mechanism of action (MOA) by which iAs could cause or exacerbate CVD. Few studies specifically address the plausibility of an association between iAs and CVD at the low exposure levels which are typical in the United States (i.e., below 100 μg As/L in drinking water). We have conducted a review and evaluation of the animal, mechanistic, and human data relevant to the potential MOAs of iAs and CVD. Specifically, we evaluated the most common proposed MOAs, which include disturbance of endothelial function and hepatic dysfunction. Our analysis of the available evidence indicates that there is not a well-established MOA for iAs in the development or progression of CVD. Few human studies of the potential MOAs have addressed plausibility at low doses and the applicability of extrapolation from animal studies to humans is questionable. However, the available evidence indicates that regardless of the specific MOA, the effects of iAs on physiological processes at the cellular level appear to operate via a threshold mechanism. This finding is consistent with the lack of association of CVD with iAs exposure in humans at levels below 100 μg/L, particularly when considering important exposure and risk modifiers such as nutrition and genetics. Based on this analysis, we conclude that there are no data supporting a linear dose-response relationship between iAs and CVD, indicating this relationship has a threshold.

Importance of being thiomethylated: formation, fate, and effects of methylated thioarsenicals

Although inorganic arsenic has long been recognized as a potent toxicant and carcinogen in humans, recent evidence shows that at least some of its effects are mediated by methylated metabolites. Elucidating the conversion of inorganic arsenic to mono-, di-, and trimethylated species has provided insights into the enzymology of this pathway and identified genetic and environmental factors that influence the susceptibility of individuals to this metalloid's adverse health effects. Notably, almost all work on the formation, fate, and effects of methylated arsenicals has focused on oxoarsenicals in which arsenic is bound to one or more oxygen atoms. However, thioarsenicals are a class of arsenicals in which a sulfur atom has replaced one or more oxygens that are bound to arsenic. Thioarsenicals have been identified as urinary metabolites in humans and other animals following exposure to inorganic arsenic. Studies find that methylated thioarsenicals exhibit kinetic behavior and toxicological properties that distinguish them from methylated oxoarsenicals. This perspective considers that formation, fate, and effects of methylated thioarsenicals with an emphasis on examining the linkages between the molecular processes that underlie both methylation and thiolation reactions. Integrating this information will provide a more comprehensive view of the relationship between the metabolism of arsenic and the risk posed by chronic exposure to this environmental contaminant.

AS3MT, GSTO, and PNP polymorphisms: impact on arsenic methylation and implications for disease susceptibility

BACKGROUND

Oral exposure to inorganic arsenic (iAs) is associated with adverse health effects. Epidemiological studies suggest differences in susceptibility to these health effects, possibly due to genotypic variation. Genetic polymorphisms in iAs metabolism could lead to increased susceptibility by altering urinary iAs metabolite concentrations.

OBJECTIVE

To examine the impact of genotypic polymorphisms on iAs metabolism.

METHODS

We screened 360 publications from PubMed and Web of Science for data on urinary mono- and dimethylated arsenic (MMA and DMA) percentages and polymorphic genes encoding proteins that are hypothesized to play roles in arsenic metabolism. The genes we examined were arsenic (+3) methyltransferase (AS3MT), glutathione-s-transferase omega (GSTO), and purine nucleoside phosphorylase (PNP). Relevant data were pooled to determine which polymorphisms are associated across studies with changes in urinary metabolite concentration.

RESULTS

In our review, AS3MT polymorphisms rs3740390, rs11191439, and rs11191453 were associated with statistically significant changes in percent urinary MMA. Studies of GSTO polymorphisms did not indicate statistically significant associations with methylation, and there are insufficient data on PNP polymorphisms to evaluate their impact on metabolism.

DISCUSSION

Collectively, these data support the hypothesis that AS3MT polymorphisms alter in vivo metabolite concentrations. Preliminary evidence suggests that AS3MT genetic polymorphisms may impact disease susceptibility. GSTO polymorphisms were not associated with iAs-associated health outcomes. Additional data are needed to evaluate the association between PNP polymorphisms and iAs-associated health outcomes. Delineation of these relationships may inform iAs mode(s) of action and the approach for evaluating low-dose health effects for iAs.

CONCLUSIONS

Genotype impacts urinary iAs metabolite concentrations and may be a potential mechanism for iAs-related disease susceptibility.

Characterization of intracellular inclusions in the urothelium of mice exposed to inorganic arsenic

Inorganic arsenic (iAs) is a known human carcinogen at high exposures, increasing the incidences of urinary bladder, skin, and lung cancers. In most mammalian species, ingested iAs is excreted mainly through urine primarily as dimethylarsinic acid (DMA(V)). In wild-type (WT) mice, iAs, DMA(V), and dimethylarsinous acid (DMA(III)) exposures induce formation of intramitochondrial urothelial inclusions. Arsenite (iAs(III)) also induced intranuclear inclusions in arsenic (+3 oxidation state) methyltransferase knockout (As3mt KO) mice. The arsenic-induced formation of inclusions in the mouse urothelium was dose and time dependent. The inclusions do not occur in iAs-treated rats and do not appear to be related to arsenic-induced urothelial cytotoxicity. Similar inclusions in exfoliated urothelial cells from humans exposed to iAs have been incorrectly identified as micronuclei. We have characterized the urothelial inclusions using transmission electron microscopy (TEM), DNA-specific 4',6-diamidino-2-phenylindole (DAPI), and non-DNA-specific Giemsa staining and determined the arsenical content. The mouse inclusions stained with Giemsa but not with the DAPI stain. Analysis of urothelial mitochondrial- and nuclear-enriched fractions isolated from WT (C57BL/6) and As3mt KO mice exposed to arsenate (iAs(V)) for 4 weeks showed higher levels of iAs(V) in the treated groups. iAs(III) was the major arsenical present in the enriched nuclear fraction from iAs(V)-treated As3mt KO mice. In conclusion, the urothelial cell inclusions induced by arsenicals appear to serve as a detoxifying sequestration mechanism similar to other metals, and they do not represent micronuclei.

Cytotoxicity and gene expression changes induced by inorganic and organic trivalent arsenicals in human cells

Inorganic arsenic (iAs) is a human urinary bladder, skin and lung carcinogen. iAs is metabolized to methylated arsenicals, with trivalent arsenicals more cytotoxic than pentavalent forms in vitro. In this study, cytotoxicity and gene expression changes for arsenite (iAs(III)), monomethylarsonous acid (MMA(III)) and dimethylarsinous acid (DMA(III)) were evaluated in three human cell types, urothelial (1T1), keratinocyte (HEK001) and bronchial epithelial (HBE) cells, corresponding to target organs for iAs-induced cancer. Cells were exposed to arsenicals to determine cytotoxicity and to study gene expression changes. Affymetrix chips were used to determine differentially expressed genes (DEGs) by statistical analysis. Lethal concentrations (LC50) for trivalent arsenicals in all cells ranged from 1.6 to 10μM. MMA(III) and DMA(III) had 4-12-fold greater potency compared to iAs. Increasing concentrations of iAs(III) induced more genes and additional signaling pathways in HBE cells. At equivalent cytotoxic concentrations, greater numbers of DEGs were induced in 1T1 cells compared to the other cells. Each arsenical altered slightly different signaling pathways within and between cell types, but when altered pathways from all three arsenicals were combined, they were similar between cell types. The major signaling pathways altered included NRF2-mediated stress response, interferon, p53, cell cycle regulation and lipid peroxidation. These results show a similar process qualitatively and quantitatively for all three cell types, and support a mode of action involving cytotoxicity and regenerative proliferation.

Time course of urothelial changes in rats and mice orally administered arsenite

Inorganic arsenic (arsenite and arsenate) at high exposures is a known human carcinogen, inducing tumors of the urinary bladder, skin, and lungs. In two experiments, we examined the urothelial proliferative effects of treatment with 173 ppm sodium arsenite (100 ppm arsenic) in the drinking water for 6 and 24 hr, and 3, 7, and 14 days in female F344 rats and 43.3 ppm sodium arsenite (25 ppm arsenic) in female C57BL/6 wild-type and arsenic (+3 oxidation state) methyltransferase knockout (As3mt KO) mice that are unable to methylate arsenicals. In the rat and both mouse genotypes, scanning electron microscopy showed cytotoxic urothelial changes as early as 6 hr after the start of arsenic exposure. The severity of As(III)-induced cytotoxic urothelial changes increased over time in the rat and in the As3mt KO mouse. Light microscopy showed an increase in urothelial hyperplasia in the rat. No significant increases in bromodeoxyuridine-labeling index were observed. The data support the hypothesis that the sequence of events in the mode of action for urothelial effects of orally administered inorganic arsenic in the rat and mouse involves superficial cytotoxicity with consequent regenerative increased cell proliferation similar to the findings associated with the administration of dimethylarsinic acid (DMA(V)) in rats.

Low-level arsenic exposure, AS3MT gene polymorphism and cardiovascular diseases in rural Texas counties

Most Americans living in rural areas use groundwater for drinking. Exposure to low-level (around the current U.S. standard 10 μg/L) arsenic in drinking water is associated with increased mortality of cardiovascular diseases. The current study was to determine if coronary heart disease, hypertension, and hyperlipidemia were associated with low-level arsenic exposure and AS3MT gene single nucleotide polymorphism (SNP) A35991G (rs10748835) in rural Texas. Subjects (156 men, 343 women, 40-96 years of age with a mean of 61) were residents from rural counties Cochran, Palmer, and Bailey, Texas. Groundwater arsenic concentration at each subject's home was estimated with ArcGIS inverse distance weighted interpolation based on the residential location's distances to surrounding wells with known water arsenic concentrations. The estimated groundwater arsenic concentration ranged from 2.2 to 15.3 (mean 6.2) μg/L in this cohort. Logistic regression analysis showed that coronary heart disease was associated with higher arsenic exposure (p<0.05) and with AS3MT genotype GG vs. AA (p<0.05) after adjustments for age, ethnicity, gender, education, smoking status, alcoholism, and anti-hyperlipidemia medication. Hypertension was associated with higher arsenic exposure, while hyperlipidemia was associated with genotype AG vs. AA of the AS3MT gene (p<0.05). Thus, coronary heart disease and its main risk factors were associated with low-level arsenic exposure, AS3MT polymorphism or both.

Arsenic-glutathione conjugate transport by the human multidrug resistance proteins (MRPs/ABCCs)

Millions of people world-wide are chronically exposed to inorganic forms of the environmental toxicant arsenic in drinking water. This has led to a public health crisis because arsenic is a human carcinogen, and causes a myriad of other adverse health effects. In order to prevent and treat arsenic-induced toxicity it is critical to understand the cellular handling of this metalloid. A large body of literature describes the importance of the cellular tripeptide glutathione (γ-Glu-Cys-Gly,GSH/GS) in the excretion of arsenic. The triglutathione conjugate of arsenite [As(III)(GS)(3)] and the diglutathione conjugate of monomethylarsonous acid [MMA(III)(GS)(2)] have been isolated from rat bile and mouse urine, and account for the majority of excreted arsenic, suggesting these are important transportable forms. The ATP-binding cassette (ABC) transporter proteins, multidrug resistance protein 1 (MRP1/ABCC1) and the related protein MRP2 (ABCC2), are thought to play an important role in arsenic detoxification through the cellular efflux of arsenic-GSH conjugates. Current knowledge on the cellular handling of arsenic with a special emphasis on the transport pathways of the arsenic-GSH conjugates As(III)(GS)(3), MMA(III)(GS)(2), and dimethylarsenic glutathione DMA(III)(GS), as well as, the seleno-bis(S-glutathionyl) arsinium ion [(GS)(2)AsSe](-) are reviewed.

Mouse arsenic (+3 oxidation state) methyltransferase genotype affects metabolism and tissue dosimetry of arsenicals after arsenite administration in drinking water

Arsenic (+3 oxidation state) methyltransferase (As3mt) catalyzes methylation of inorganic arsenic (iAs) producing a number of methylated arsenic metabolites. Although methylation has been commonly considered a pathway for detoxification of arsenic, some highly reactive methylated arsenicals may contribute to toxicity associated with exposure to inorganic arsenic. Here, adult female wild-type (WT) C57BL/6 mice and female As3mt knockout (KO) mice received drinking water that contained 1, 10, or 25 ppm (mg/l) of arsenite for 33 days and blood, liver, kidney, and lung were taken for arsenic speciation. Genotype markedly affected concentrations of arsenicals in tissues. Summed concentrations of arsenicals in plasma were higher in WT than in KO mice; in red blood cells, summed concentrations of arsenicals were higher in KO than in WT mice. In liver, kidney, and lung, summed concentrations of arsenicals were greater in KO than in WT mice. Although capacity for arsenic methylation is much reduced in KO mice, some mono-, di-, and tri-methylated arsenicals were found in tissues of KO mice, likely reflecting the activity of other tissue methyltransferases or preabsorptive metabolism by the microbiota of the gastrointestinal tract. These results show that the genotype for arsenic methylation determines the phenotypes of arsenic retention and distribution and affects the dose- and organ-dependent toxicity associated with exposure to inorganic arsenic.