Binding of arsenicals to proteins in an in vitro methylation system

The relevance of arsenic speciation analysis in health & medicine

Long term exposure to arsenic through consumption of contaminated groundwater has been a global issue since the last five decades; while from an alternate standpoint, arsenic compounds have emerged as unparallel chemotherapeutic drugs. This review highlights the contribution from arsenic speciation studies that have played a pivotal role in the progression of our understanding of the biological behaviour of arsenic in humans. We also discuss the limitations of the speciation studies and their association with the interpretation of arsenic metabolism. Chromatographic separation followed by spectroscopic detection as well as the utilization of biotinylated pull-down assays, protein microarray and radiolabelled arsenic have been instrumental in identifying hundreds of metabolic arsenic conjugates, while, computational modelling has predicted thousands of them. However, these species exhibit a variegated pattern, which supports more than one hypothesis for the metabolic pathway of arsenic. Thus, the arsenic species are yet to be integrated into a coherent mechanistic pathway depicting its chemicobiological fate. Novel biorelevant arsenic species have been identified due to significant evolution in experimental methodologies. However, these methods are specific for the identification of only a group of arsenicals sharing similar physiochemical properties; and may not be applicable to other constituents of the vast spectrum of arsenic species. Consequently, the identity of arsenic binding partners in vivo and the sequence of events in arsenic metabolism are still elusive. This resonates the need for additional focus on the extraction and characterization of both low and high molecular weight arsenicals in a combinative manner.

Monomethylated arsenic was the Major methylated arsenic in Red blood cells of acute promyelocytic leukemia patients treated with arsenic trioxide

BACKGROUND

Arsenic trioxide (ATO) has been successfully applied in the treatment of acute promyelocytic leukemia (APL). Arsenic metabolites including inorganic arsenic and methylated arsenic could lead to different toxicity and curative effect. This study aims to establish a method to determine arsenic species in red blood cells (RBCs), clarify the distribution characteristics of arsenic species in RBCs.

METHODS

Steady state blood samples were collected from 97 APL patients. H₂O₂ and HClO₄ were used to release the hemoglobin bounding arsenic and precipitate protein. Arsenite (iAs^III), arsenate (iAs^V), monomethylarsonic acid (MMA^V) and dimethylarsinic acid (DMA^V) in plasma and RBCs were detected by HPLC-HG-AFS. Free and bound arsenic species in RBCs were separated by 30 kDa molecular mass cutoff filters and determined to evaluate hemoglobin binding capacity of different arsenic species.

RESULTS

The method was validated with accuracy ranged from 84.75% to 104.13%. Arsenic species in RBCs followed the trend iAs > MMA > DMA (p < 0.01), while the concentration of DMA was significantly higher than iAs and MMA in plasma (p < 0.01). The correlation between iAs concentration in plasma and corresponding RBCs arsenic level was weak. And the concentrations of DMA and MMA in plasma were moderately positive correlated with those in RBCs. Hemoglobin-binding ratios of iAs, MMA and DMA were all over 70 %.

CONCLUSIONS

In this study, we provided a reliable method to determine arsenic species in RBCs of APL patients treated with ATO by HPLC-HG-AFS. It was confirmed that the concentration of DMA is the highest in plasma, while MMA is the most predominant methylated arsenic in RBCs. High affinity of MMA with human Hb was responsible for the accumulation of arsenic in RBCs of APL patients.

Imaging vicinal dithiol of arsenic-binding proteins in the mouse brain with amplification by gold nanocluster Au22(GSH)18

A quantitative imaging strategy for the vicinal dithiol (VD) of arsenic-binding proteins in the mouse brain is reported. 2-p-Aminophenyl-1,3,2-dithiarsenolane (PAO-EDT) couples to gold nanoclusters Au₂₂(GSH)₁₈ to form conjugate Au₂₂-PAO-EDT (APE). PAO-EDT in APE selectively binds VD with 1 : 1 stoichiometry. After tagging the mouse brain with APE, VD imaging is realized by laser ablation ICP-MS. VD correlates linearly with ¹⁹⁷Au in APE offering a 22-fold amplification and a LOD of 5.43 nM. It is found that the cerebral cortex and hippocampus are most affected in an arsenic poisoned mouse brain. This study provides useful information for further understanding the mechanisms underlying the biological effects of arsenic on the living body.

Arsenic exposure: A public health problem leading to several cancers

Arsenic, a metalloid and naturally occurring element, is one of the most abundant elements in the earth's crust. Water is contaminated by arsenic through natural sources (underground water, minerals and geothermal processes) and anthropogenic sources such as mining, industrial processes, and the production and use of pesticides. Humans are exposed to arsenic mainly by drinking contaminated water, and secondarily through inhalation and skin contact. Arsenic exposure is associated with the development of vascular disease, including stroke, ischemic heart disease and peripheral vascular disease. Also, arsenic increases the risk of tumors of bladder, lungs, kidneys and liver, according to the International Agency for Research on Cancer and the Food and Drug Administration. Once ingested, an estimated 70-90% of inorganic arsenic is absorbed by the gastrointestinal tract and widely distributed through the blood to different organs, primarily to the liver, kidneys, lungs and bladder and secondarily to muscle and nerve tissue. Arsenic accumulates in the organs, especially in the liver. Its excretion mostly takes place through urination. The toxicokinetics of arsenic depends on the duration of exposure, pathway of ingestion, physicochemical characteristics of the compound, and affected biological species. The present review outlines of arsenic toxic effects focusing on different cancer types whit highest prevalence's by exposure to this metalloid and signaling pathways of carcinogenesis.

Thiolation in arsenic metabolism: a chemical perspective

In recent years, methylated thioarsenicals have been widely detected in various biological and environmental matrices, suggesting their broad involvement and biological importance in arsenic metabolism. However, very little is known about the formation mechanism of methylated thioarsenicals and the relation between arsenic methylation and thiolation processes. It is timely and necessary to summarize and synthesize the reported information on thiolated arsenicals for an improved understanding of arsenic thiolation. To this end, we examined the proposed formation pathways of methylated oxoarsenicals and thioarsenicals from a chemical perspective and proposed a novel arsenic metabolic scheme, in which arsenic thiolation is integrated with methylation (instead of being separated from methylation as currently reported). We suggest in the new scheme that protein-bound pentavalent arsenicals are critical intermediates that connect methylation and thiolation, with protein binding of pentavalent methylated thioarsenical being a key step for arsenic thiolation. This informative review on arsenic thiolation from the chemical perspective will be helpful to better understand the arsenic metabolism at the molecular level and the toxicological effects of arsenic species.

Metabolism and disposition of arsenic species from controlled oral dosing with sodium arsenite in adult female CD-1 mice. I. Pilot study to determine dosing, analytical measurements, and sampling strategies

Arsenic (As) is ubiquitous in the earth's crust, with typical dietary intake in developed countries <1 μg/kg bw/d, and atypical groundwater exposures in developing countries approaching 50 μg/kg bw/d. Arsenic exposures are linked with human diseases and doses of toxicological concern are similar to typical dietary intake estimates. The methylation of arsenite by arsenite-3-methyltransferase (As3MT) promotes the clearance of arsenic as pentavalent species, but also generates reactive trivalent intermediates. This study measured inorganic arsenic and its metabolites in pentavalent and trivalent states in blood, tissues, and excreta after oral administration of arsenite (50-200 μg/kg bw). While liver was a major site for clearance of arsenite and formation of methylated species, it also had extensive binding of trivalent intermediates; however, thiol exchange and oxidation reactions of trivalent arsenic were facile since dimethylarsinic acid (DMA^V) was the predominant species in blood and urine. Consistent evidence was observed for a non-linear relationship between doses above 50 μg/kg bw and levels of bound trivalent As metabolites. The abundance of protein-bound trivalent arsenic within target tissues should correlate with disruption of critical cellular processes, which rely on defined interactions of thiol functional groups, and could provide dose-response relationships from animal models for human risk assessment.

Thiolated arsenicals in arsenic metabolism: Occurrence, formation, and biological implications

Arsenic (As) is a notoriously toxic pollutant of health concern worldwide with potential risk of cancer induction, but meanwhile it is used as medicines for the treatment of different conditions including hematological cancers. Arsenic can undergo extensive metabolism in biological systems, and both toxicological and therapeutic effects of arsenic compounds are closely related to their metabolism. Recent studies have identified methylated thioarsenicals as a new class of arsenic metabolites in biological systems after exposure of inorganic and organic arsenicals, including arsenite, dimethylarsinic acid (DMA^V), dimethylarsinous glutathione (DMA^IIIGS), and arsenosugars. The increasing detection of thiolated arsenicals, including monomethylmonothioarsonic acid (MMMTA^V), dimethylmonothioarsinic acid (DMMTA^V) and its glutathione conjugate (DMMTA^VGS), and dimethyldithioarsinic acid (DMDTA^V) suggests that thioarsenicals may be important metabolites and play important roles in arsenic toxicity and therapeutic effects. Here we summarized the reported occurrence of thioarsenicals in biological systems, the possible formation pathways of thioarsenicals, and their toxicity, and discussed the biological implications of thioarsenicals on arsenic metabolism, toxicity, and therapeutic effects.

Sodium arsenate induce changes in fatty acids profiles and oxidative damage in kidney of rats

Six groups of rats (n = 10 per group) were exposed to 1 and 10 mg/l of sodium arsenate for 45 and 90 days. Kidneys from treated groups exposed to arsenic showed higher levels of trans isomers of oleic and linoleic acids as trans C181n-9, trans C18:1n-11, and trans C18:2n-6 isomers. However, a significant decrease in eicosenoic (C20:1n-9) and arachidonic (C20:4n-6) acids were observed in treated rats. Moreover, the "Δ5 desaturase index" and the saturated/polyunsaturated fatty acids ratio were increased. There was a significant increase in the level of malondialdehyde at 10 mg/l of treatment and in the amount of conjugated dienes after 90 days (p < 0.05). Significant kidney damage was observed at 10 mg/l by increase of plasma marker enzymes. Histological studies on the ultrastructure changes of kidney supported the toxic effect of arsenate exposure. Arsenate intoxication activates significantly the superoxide dismutase at 10 mg/l for 90 days, whereas the catalase activity was markedly inhibited in all treated groups (p < 0.05). In addition, glutathione peroxidase activity was significantly increased at 45 days and dramatically declined after 90 days at 10 mg/l (p < 0.05). A significant increase in the level of glutathione was marked for the groups treated for 45 and 90 days at 1 mg/l followed by a significant decrease for rats exposed to 10 mg/l for 90 days. An increase in the level of protein carbonyl was observed in all treated groups (p < 0.05). In conclusion, the present study provides evidence for a direct effect of arsenate on fatty acid (FA) metabolism which concerns the synthesis pathway of n-6 polyunsaturated fatty acids and leads to an increase in the trans FAs isomers. Therefore, FA-induced arsenate kidney damage could contribute to trigger kidney cancer.

Trivalent methylated arsenic metabolites induce apoptosis in human myeloid leukemic HL-60 cells through generation of reactive oxygen species

Trivalent arsenic metabolites mediate HL-60 cell apoptosis via ROS.

Studying arsenite-humic acid complexation using size exclusion chromatography-inductively coupled plasma mass spectrometry

Arsenic (As) can form complexes with dissolved organic matter (DOM), which affects the fate of arsenic in waste sites and natural environments. It remains a challenge to analyze DOM-bound As, in particular by using a direct chromatographic separation method. Size exclusion chromatography (SEC) hyphenated with UV spectrophotometer and inductively coupled plasma mass spectrometry (ICP-MS) was developed to characterize the complexation of arsenite (As(III)) with DOM. This SEC-UV-ICP-MS method is able to differentiate As(III)-DOM complexes from free As species and has the advantage of direct determination of both free and DOM-bound As(III) through mild separation. The suitability of this method for studying As(III)-DOM complexation was demonstrated by its application, in combination with the Scatchard plot and nonlinear regression of ligand binding model, for characterizing As(III) complexation with humic acid (HA) in the absence or presence of natural sand. The results suggest that, consistent with polyelectrolytic nature of HA, the As(III)-HA complexation should be accounted for by multiple classes of binding sites. By loosely classifying the binding sites into strong (S1) and weak (S2) sites, the apparent stability constants (Ks) of the resulting As-DOM complexes were calculated as logK(s1) = 6.5-7.1 while logK(s2) = 4.7-5.0.

Protective effect of conjugated linolenic acid isomers present in vegetable oils against arsenite-induced renal toxicity in rat model

OBJECTIVE

The aim of the present study was to investigate the protective effect of conjugated linolenic acid (CLnA), present in vegetable oils against arsenite-induced renal oxidative stress.

METHODS

Albino rats were divided into six groups. Group 1 was control and group 2 was treated with sodium arsenite (Sa; 10 mg/kg BW). Rats in groups 3 and 4 were treated with mixture of α-eleostearic acid and punicic acid (1:1) (0.5% and 1.0%, respectively), whereas rats in the groups 5 and 6 were treated with 0.5% of α-eleostearic acid and 0.5% of punicic acid, respectively, along with Sa by oral gavage once daily.

RESULTS

Results revealed that activity of antioxidant enzymes and total reduced glutathione content, total protein content, and phospholipid content in kidney were decreased significantly in arsenite-treated group compared with control. Activity of nitric oxide synthase, peroxidation of lipid, protein oxidation, total cholesterol content, total lipid content of kidney, and plasma creatinine level were increased significantly (P < 0.05) in arsenite-treated rats compared with control. Fatty-acid composition of renal lipids showed significant decrease in monounsaturated fatty acid, polyunsaturated fatty acid (PUFA) content, and increase in saturated fatty acid content due to oxidative stress. PUFA such as γ-linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid decreased significantly with significant (P < 0.05) increase in arachidonic acid content after Sa treatment. Administration of blended product of both the isomers caused better restoration of renal fatty acids and other altered parameters.

CONCLUSION

CLnA isomers caused amelioration of renal oxidative stress and the isomers showed synergistic activity.

Differential toxicity and gene expression in Caco-2 cells exposed to arsenic species

Inorganic arsenic [As(V)+As(III)] and its metabolites, especially the trivalent forms [monomethylarsonous acid, MMA(III), and dimethylarsinous acid, DMA(III)], are considered the forms of arsenic with the highest degree of toxicity, linked to certain types of cancer and other pathologies. The gastrointestinal mucosa is exposed to these forms of arsenic, but it is not known what toxic effect these species may have on it. The aim of the present work was to evaluate the toxicity and some mechanisms of action of inorganic arsenic and its metabolites [monomethylarsonic acid, MMA(V), dimethylarsinic acid, DMA(V), MMA(III) and DMA(III)] in intestinal epithelial cells, using the Caco-2 human cell line as a model. The results show that the pentavalent forms do not produce toxic effects on the intestinal monolayer, but the trivalent species have a different degree of toxicity. As(III) induces death mainly by necrosis, whereas only apoptotic cells are detected after exposure to MMA(III), and for DMA(III) the percentages of apoptosis and necrosis are similar. The three forms produce reactive oxygen species, accompanied by a reduction in intracellular GSH and lipid peroxidation, the latter being especially notable in the dimethylated form. They also alter the enzyme activity of glutathione peroxidase and catalase and induce expression of stress proteins and metallothioneins. The results indicate that the trivalent forms of arsenic can affect cell viability of intestinal cells by mechanisms related to the induction of oxidative stress. Further studies are needed to evaluate how the effects observed in this study affect the structure and functionality of the intestinal epithelium.

Arsenic metabolism and thioarsenicals

Arsenic has received considerable attention in the world, since it can lead to a multitude of toxic effects and has been recognized as a human carcinogen causing cancers. Here, we focus on the current state of knowledge regarding the proposed mechanisms of arsenic biotransformation, with a little about cellular uptake, toxicity and clinical utilization of arsenicals. Since pentavalent methylated metabolites were found in animal urine after exposure to iAs(III), methylation was considered to be a detoxification process, but the discovery of methylated trivalent intermediates and thioarsenicals in urine has diverted the view and gained much interest regarding arsenic biotransformation. To further investigate the partially understood phenomena relating to arsenic toxicity and the uses of arsenic as a drug, it is important to elucidate the exact pathways involved in metabolism of this metalloid, as the toxicity and the clinical uses of arsenic can be best recognized in context of its biotransformation. Thereby, in this perspective, we have focused on arsenic metabolic pathways including three proposed mechanisms: a classic pathway by Challenger in 1945, followed by a new metabolic pathway proposed by Hayakawa in 2005 involving arsenic-glutathione complexes, while the third is a new reductive methylation pathway that is proposed by our group involving As-protein complexes. According to previous and present in vivo and in vitro experiments, we conclude that the methylation reaction takes place with simultaneous reductive rather than stepwise oxidative methylation. In addition, production of pentavalent methylated arsenic metabolites are suggested to be as the end product of metabolism, rather than intermediates.

In vitro study of intestinal transport of inorganic and methylated arsenic species by Caco-2/HT29-MTX cocultures

This study characterizes intestinal absorption of arsenic species using in vitro system Caco-2/HT29-MTX cocultures in various proportions (100/0 to 30/70). The species assayed were As(V), As(III), monomethylarsonic acid [MMA(V)], monomethylarsonous acid [MMA(III)], dimethylarsinic acid [DMA(V)], and dimethylarsinous acid [DMA(III)]. The results show that the apparent permeability (P(app)) values of pentavalent species increase significantly in the Caco-2/HT29-MTX cocultures in comparison with the Caco-2 monoculture, probably because of enhancement of paracellular transport. For MMA(III) and DMA(III), P(app) decreases in the Caco-2/HT29-MTX cell model, and for As(III), there is no change in P(app) between the two culture models. Transport studies of arsenic solubilized from cooked foods (rice, garlic, and seaweed) after applying an in vitro gastrointestinal digestion showed that arsenic absorption also varies with the model used, increasing with the incorporation of HT29-MTX in the culture. These results show the importance of choosing a suitable in vitro model when evaluating intestinal arsenic absorption processes.

Mechanisms underlying the inhibitory effects of arsenic compounds on protein tyrosine phosphatase (PTP)

Arsenic binding to biomolecules is considered one of the major toxic mechanisms, which may also be related to the carcinogenic risks of arsenic in humans. At the same time, arsenic is also known to activate the phosphorylation-dependent signaling pathways including the epidermal growth factor receptor, the mitogen-activated protein kinase and insulin/insulin-like growth factor-1 pathways. These signaling pathways originate at the level of receptor tyrosine kinases whose phosphorylation status is regulated by opposing protein tyrosine phosphatase (PTP) activity. Reversible tyrosine phosphorylation, which is governed by the balanced action of protein tyrosine kinases and phosphatases, regulates important signaling pathways that are involved in the control of cell proliferation, adhesion and migration. In the present study, we have focused on the interaction of cellular PTPs with toxic trivalent arsenite (iAs(III)) and its intermediate metabolites such as monomethylarsonous acid (MMA(III)) and dimethylarsinous acid (DMA(III)) in vitro, and then determined the arsenic binding site in PTP by the use of recombinant PTPs (e.g., PTP1B and CD45). Interestingly, the activities of PTP1B (cytoplasm-form) or CD45 (receptor-linked form) were observed to be strongly inhibited by both methylated metabolites (i.e., MMA(III) and DMA(III)) but not by iAs(III). Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS) has clearly confirmed that the organic intermediate, DMA(III) directly bound to the active site cysteine residue of PTP1B (e.g., Cys215), resulting in inhibition of enzyme activity. These results suggest that arsenic exposure may disturb the cellular signaling pathways through PTP inactivation.

Characterization of the role of protein-cysteine residues in the binding with sodium arsenite

To better characterize the interaction of protein-cysteines with sodium arsenite, arsenic-binding proteins were identified from the arsenic-resistant Chinese hamster ovary cell line SA7 using a p-aminophenylarsine oxide (PAO)-agarose matrix in combination with proteomic techniques. Twenty of the isolated arsenic-binding proteins were further peptide-mapped by MALDI-Q-TOF-MS. The binding capacity of PAO-agarose-retained proteins was then verified by re-applying Escherichia coli overexpressed recombinant proteins with various numbers of cysteine residues onto the PAO-agarose matrix. The results showed that recombinant heat shock protein 27 (HSP27, with one cysteine residue), reticulocalbin-3 (RCN3, with no cysteine residue), galectin-1 (GAL1, with six cysteine residues), but not peroxiredoxin 6 (Prdx6, with one cysteine residue but not retained by the PAO-agarose matrix), were bound to the PAO-agarose matrix. The six free cysteine residues in GAL1 were individually or double-mutated to alanine by means of site-directed mutagenesis and subjected to CD and ICP-MS analysis. The binding capacity of GAL1 for sodium arsenite was significantly attenuated in C16A, C88A and all double mutant clones. Taken together, our current data suggest that the cysteine residues in GAL1 may play a critical role in the binding of arsenic, but that in the case of RCN3 and Prdx6, this interaction may be mediated by other factors.

Direct analysis and stability of methylated trivalent arsenic metabolites in cells and tissues

Chronic ingestion of water containing inorganic arsenic (iAs) has been linked to a variety of adverse health effects, including cancer, hypertension and diabetes. Current evidence suggests that the toxic methylated trivalent metabolites of iAs, methylarsonous acid (MAs(III)) and dimethylarsinous acid (DMAs(III)) play a key role in the etiology of these diseases. Both MAs(III) and DMAs(III) have been detected in urine of subjects exposed to iAs. However, the rapid oxidation of DMAs(III) and, to a lesser extent, MAs(III) in oxygen-rich environments leads to difficulties in the analysis of these metabolites in samples of urine collected in population studies. Results of our previous work indicate that MAs(III) and DMAs(III) are relatively stable in a reducing cellular environment and can be quantified in cells and tissues. In the present study, we used the oxidation state-specific hydride generation-cryotrapping-atomic absorption spectroscopy (HG-CT-AAS) to examine the presence and stability of these trivalent metabolites in the liver of mice and in UROtsa/F35 cells exposed to iAs. Tri- and pentavalent metabolites of iAs were analyzed directly (without chemical extraction or digestion). Liver homogenates prepared in cold deionized water and cell culture medium and lysates were stored at either 0 °C or -80 °C for up to 22 days. Both MAs(III) and DMAs(III) were stable in homogenates stored at -80 °C. In contrast, DMAs(III) in homogenates stored at 0 °C began to oxidize to its pentavalent counterpart after 1 day; MAs(III) remained stable for at least 3 weeks under these conditions. MAs(III) and DMAs(III) generated in UROtsa/F35 cultures were stable for 3 weeks when culture media and cell lysates were stored at -80 °C. These results suggest that samples of cells and tissues represent suitable material for the quantitative, oxidation state-specific analysis of As in laboratory and population studies examining the metabolism or toxic effects of this metalloid.

Mathematical model insights into arsenic detoxification

Background

Arsenic in drinking water, a major health hazard to millions of people in South and East Asia and in other parts of the world, is ingested primarily as trivalent inorganic arsenic (iAs), which then undergoes hepatic methylation to methylarsonic acid (MMAs) and a second methylation to dimethylarsinic acid (DMAs). Although MMAs and DMAs are also known to be toxic, DMAs is more easily excreted in the urine and therefore methylation has generally been considered a detoxification pathway. A collaborative modeling project between epidemiologists, biologists, and mathematicians has the purpose of explaining existing data on methylation in human studies in Bangladesh and also testing, by mathematical modeling, effects of nutritional supplements that could increase As methylation.

Methods

We develop a whole body mathematical model of arsenic metabolism including arsenic absorption, storage, methylation, and excretion. The parameters for arsenic methylation in the liver were taken from the biochemical literature. The transport parameters between compartments are largely unknown, so we adjust them so that the model accurately predicts the urine excretion rates of time for the iAs, MMAs, and DMAs in single dose experiments on human subjects.

Results

We test the model by showing that, with no changes in parameters, it predicts accurately the time courses of urinary excretion in mutiple dose experiments conducted on human subjects. Our main purpose is to use the model to study and interpret the data on the effects of folate supplementation on arsenic methylation and excretion in clinical trials in Bangladesh. Folate supplementation of folate-deficient individuals resulted in a 14% decrease in arsenicals in the blood. This is confirmed by the model and the model predicts that arsenicals in the liver will decrease by 19% and arsenicals in other body stores by 26% in these same individuals. In addition, the model predicts that arsenic methyltransferase has been upregulated by a factor of two in this population. Finally, we also show that a modification of the model gives excellent fits to the data on arsenic metabolism in human cultured hepatocytes.

Conclusions

The analysis of the Bangladesh data using the model suggests that folate supplementation may be more effective at reducing whole body arsenic than previously expected. There is almost no data on the upregulation of arsenic methyltransferase in populations chronically exposed to arsenic. Our model predicts upregulation by a factor of two in the Bangladesh population studied. This prediction should be verified since it could have important public health consequences both for treatment strategies and for setting appropriate limits on arsenic in drinking water. Our model has compartments for the binding of arsenicals to proteins inside of cells and we show that these comparments are necessary to obtain good fits to data. Protein-binding of arsenicals should be explored in future biochemical studies.

Analysis of arsenic metabolites in HepG2 and AS3MT-transfected cells

It has been suggested that arsenic (+3 oxidation state) methyltransferase (AS3MT) plays a critical role in methylation of arsenic, and that arsenic-glutathione conjugate is a substrate for AS3MT-catalyzed methylation of arsenic. However, the mechanism of arsenic methylation in cells is not fully understood. Here, we have constructed T-REx-CHO-hAS3MTtr cells that transiently overexpress human AS3MT in response to tetracycline. The decreases in cell viability after exposure to sodium arsenite were greater in tetracycline-treated cells (tet(+) cells) than in untreated cells (tet(-) cells). Concentration of total cellular arsenic was significantly higher in tet(+) cells than in tet(-) cells. Speciation analyses of arsenic metabolites in whole cell lysates and cell culture medium were performed using both HepG2 cells and T-REx-CHO-hAS3MTtr cells. Speciation analyses of arsenic metabolites in lysates of T-REx-CHO-hAS3MTtr cells revealed that dimethylated arsenicals were the predominant arsenic metabolites in tet(+) cells, while methylated metabolites were not found in tet(-) cells. In contrast, less amount of methylated arsenic metabolites were found in the HepG2 cell lysates, and monomethylated trivalent arsenicals were the predominant methylated arsenic metabolites. Arsenate was found in the culture medium after 24 h culture with arsenite. A larger amount of arsenate was found in the culture medium of tet(+) or tet(-) cells compared to HepG2 cells. These findings indicated that AS3MT expression enhanced the cytotoxic effect of arsenite in tet(+) cells because these cells accumulated more arsenic metabolites than did the tet(-) cells, and accordingly, the tet(+) cells were more susceptible to arsenic than were the tet(-) cells. Oxidation--reduction of arsenic may be implicated in the toxic effects of arsenite.

In vitro assays of inorganic arsenic methylation

Inorganic arsenic is extensively metabolized to produce mono-, di-, and trimethylated products. The formation of these metabolites produces a variety of intermediates that differ from inorganic arsenic in terms of patterns of distribution and retention and in toxic effects. In order to elucidate the pathway for arsenic methylation, it was necessary to develop a reliable in vitro assay system in which the formation of methylated metabolites could be monitored. Here, in vitro assay system that uses the postmicrosomal supernate from rat liver is used as the source of the enzymatic activity that catalyzes methylation reactions. This system can be used to study the requirements for methylation reactions (e.g., identifying the donor of methyl groups) and for screening of compounds as potential activators or inhibitors of arsenic methylation.

Speciation, formation, stability and analytical challenges of human arsenic metabolites

Human arsenic metabolism produces a number of species with varying toxicities; the presence of some has been identified while the existence of others has been postulated through indirect evidence. Speciation methods for the analysis of arsenite (AsIII), monomethylarsonous acid (MMAIII), dimethylarsinous acid (DMAIII), arsenate (AsV), monomethylarsonic acid (MMAV), dimethylarsinic acid (DMAV), arsino-glutathione (As(GS)3), monomethylarsino-glutathione (MMA(GS)2) and dimethylarsino-glutathione (DMA(GS)) were developed in this study through the use of cation exchange and reverse phase chromatography in a complementary manner. Electrospray ionization mass spectrometry (ESI-MS) was used for molecular identification of the arsenicals while inductively coupled plasma mass spectrometry (ICP-MS) was employed for quantitation purposes. Validation of the developed methods against each other for the quantitation of trivalent and pentavalent arsenicals was performed. The effect of reduced glutathione (GSH) concentration on the formation of arsenic-glutathione (As-GSH) complexes was studied. In the presence of glutathione, the occurrence of chromatographic artifacts on the cation exchange column was observed. The stability of trivalent arsenicals and As-GSH complexes was studied at various pH conditions. The results shed light on the importance of sample preparation, storage and proper choice of analytical column for the accurate identification of the As species. Reinvestigation of some of the previously reported As speciation studies of glutathione-rich biological samples needs to be performed for the verification of occurrence of As-GSH complexes and DMAIII.

Comparative functional genomic analysis identifies distinct and overlapping sets of genes required for resistance to monomethylarsonous acid (MMAIII) and arsenite (AsIII) in yeast

Arsenic is a human toxin and carcinogen commonly found as a contaminant in drinking water. Arsenite (As(III)) is the most toxic inorganic form, but recent evidence indicates that the metabolite monomethylarsonous acid (MMA(III)) is even more toxic. We have used a chemical genomics approach to identify the genes that modulate the cellular toxicity of MMA(III) and As(III) in the yeast Saccharomyces cerevisiae. Functional profiling using homozygous deletion mutants provided evidence of the requirement of highly conserved biological processes in the response against both arsenicals including tubulin folding, DNA double-strand break repair, and chromatin modification. At the equitoxic doses of 150 microM MMA(III) and 300 microM As(III), genes related to glutathione metabolism were essential only for resistance to the former, suggesting a higher potency of MMA(III) to disrupt glutathione metabolism than As(III). Treatments with MMA(III) induced a significant increase in glutathione levels in the wild-type strain, which correlated to the requirement of genes from the sulfur and methionine metabolic pathways and was consistent with the induction of oxidative stress. Based on the relative sensitivity of deletion strains deficient in GSH metabolism and tubulin folding processes, oxidative stress appeared to be the primary mechanism of MMA(III) toxicity whereas secondary to tubulin disruption in the case of As(III). Many of the identified yeast genes have orthologs in humans that could potentially modulate arsenic toxicity in a similar manner as their yeast counterparts.

Effects of selenium on the structure and function of recombinant human S-adenosyl-L-methionine dependent arsenic (+3 oxidation state) methyltransferase in E. coli

The effects of Se(IV) on the structure and function of recombinant human arsenic (+3 oxidation state) methyltransferase (AS3MT) purified from the cytoplasm of Escherichia coli were studied. The coding region of human AS3MT complementary DNA was amplified from total RNA extracted from HepG2 cell by reverse transcription PCR. Soluble and active human AS3MT was expressed in the E. coli with a Trx fusion tag under a lower induction temperature of 25 degrees C. Spectra (UV-vis, circular dichroism, and fluorescence) were first used to probe the interaction of Se(IV) and recombinant human AS3MT and the structure-function relationship of the enzyme. The recombinant human AS3MT had a secondary structure of 29.0% alpha-helix, 23.9% beta-pleated sheet, 17.9% beta-turn, and 29.2% random coil. When Se(IV) was added, the content of the alpha-helix did not change, but that of the beta-pleated sheet increased remarkably in the conformation of recombinant human AS3MT. Se(IV) inhibited the enzymatic methylation of inorganic As(III) in a concentration-dependent manner. The IC(50) value for Se(IV) was 2.38 muM. Double-reciprocal (1/V vs. 1/[inorganic As(III)]) plots showed Se(IV) to be a noncompetitive inhibitor of the methylation of inorganic As(III) by recombinant human AS3MT with a K (i) value of 2.61 muM. We hypothesized that Se(IV) interacts with the sulfhydryl group of cysteine(s) in the structural residues rather than the cysteines of the active site (Cys156 and Cys206). When Se(IV) was combined with cysteine(s) in the structural residues, the conformation of recombinant human AS3MT changed and the enzymatic activity decreased. Considering the quenching of tryptophan fluorescence, Cys72 and/or Cys226 are deduced to be primary targets for Se(IV).

Respiratory cancer and inhaled inorganic arsenic in copper smelters workers: a linear relationship with cumulative exposure that increases with concentration

BACKGROUND

Inhalation of high levels of airborne inorganic arsenic is a recognized cause of respiratory cancer. Although multiple epidemiologic studies have demonstrated this association, there have been few analyses of the mathematical relationship between cumulative arsenic exposure and risk of respiratory cancer, and no assessment as to whether and how arsenic concentration may modify this association.

OBJECTIVES

The objective is an evaluation of the shape of the relationship between respiratory cancer mortality and cumulative inhaled arsenic exposure among copper smelter workers, and the modification of that relationship by arsenic concentration.

METHODS

We used Poisson regression methods to analyze data from a cohort of arsenic-exposed copper smelter workers under a linear-exponential model for the excess relative risk.

RESULTS

Within categories of arsenic concentration, the association between respiratory cancer and cumulative arsenic exposure was consistent with linearity. The slope of the linear relationship with cumulative exposure increased with increasing arsenic concentration category.

CONCLUSIONS

Our results suggested a direct concentration effect from inhaled inorganic arsenic, whereby the excess relative risk for a fixed cumulative exposure was greater when delivered at a higher concentration and shorter duration than when delivered at a lower concentration and longer duration.

Ameliorating effect of curcumin on sodium arsenite-induced oxidative damage and lipid peroxidation in different rat organs

The present study was conducted to investigate the antioxidative effect of curcumin against sodium arsenite-induced oxidative damage in rat. Animals were divided into four groups, the first group was used as control. Groups 2, 3 and 4 were orally treated with curcumin (15 mg/kg BW), sodium arsenite (Sa, 5 mg/kg BW) and sodium arsenite plus curcumin, respectively. Rats were orally administered their respective doses daily for 30 days. Results showed that Sa increased thiobarbituric acid-reactive substances (TBARS) in plasma, liver, kidney, lung, testes and brain. While, the activities of glutathione S-transferase, superoxide dismutase and catalase and the content of sulfhydryl groups (SH-groups) were significantly decreased in plasma and tissues compared to control. Treatment with curcumin alone reduced the levels of TBARS, while induced the activities of the antioxidant enzymes, and the levels of SH-groups. The presence of curcumin with Sa reduced the induction in the levels of TBARS and induced the decrease in the activities of antioxidant enzymes and the levels of SH-groups. Results indicated that treatment with Sa decreased body weight and increased liver weight compared to control. The presence of curcumin with Sa alleviated its toxic effects. It can be concluded that curcumin has beneficial influences and could be able to antagonize Sa toxicity.

Methylated Arsenicals: The Implications of Metabolism and Carcinogenicity Studies in Rodents to Human Risk Assessment

Monomethylarsonic acid (MMA(V)) and dimethylarsinic acid (DMA(V)) are active ingredients in pesticidal products used mainly for weed control. MMA(V) and DMA(V) are also metabolites of inorganic arsenic, formed intracellularly, primarily in liver cells in a metabolic process of repeated reductions and oxidative methylations. Inorganic arsenic is a known human carcinogen, inducing tumors of the skin, urinary bladder, and lung. However, a good animal model has not yet been found. Although the metabolic process of inorganic arsenic appears to enhance the excretion of arsenic from the body, it also involves formation of methylated compounds of trivalent arsenic as intermediates. Trivalent arsenicals (whether inorganic or organic) are highly reactive compounds that can cause cytotoxicity and indirect genotoxicity in vitro. DMA(V) was found to be a bladder carcinogen only in rats and only when administered in the diet or drinking water at high doses. It was negative in a two-year bioassay in mice. MMA(V) was negative in 2-year bioassays in rats and mice. The mode of action for DMA(V)-induced bladder cancer in rats appears to not involve DNA reactivity, but rather involves cytotoxicity with consequent regenerative proliferation, ultimately leading to the formation of carcinoma. This critical review responds to the question of whether DMA(V)-induced bladder cancer in rats can be extrapolated to humans, based on detailed comparisons between inorganic and organic arsenicals, including their metabolism and disposition in various animal species. The further metabolism and disposition of MMA(V) and DMA(V) formed endogenously during the metabolism of inorganic arsenic is different from the metabolism and disposition of MMA(V) and DMA(V) from exogenous exposure. The trivalent arsenicals that are cytotoxic and indirectly genotoxic in vitro are hardly formed in an organism exposed to MMA(V) or DMA(V) because of poor cellular uptake and limited metabolism of the ingested compounds. Furthermore, the evidence strongly supports a nonlinear dose-response relationship for the biologic processes involved in the carcinogenicity of arsenicals. Based on an overall review of the evidence, using a margin-of-exposure approach for MMA(V) and DMA(V) risk assessment is appropriate. At anticipated environmental exposures to MMA(V) and DMA(V), there is not likely to be a carcinogenic risk to humans.

Specific accumulation of arsenic compounds in green turtles (Chelonia mydas) and hawksbill turtles (Eretmochelys imbricata) from Ishigaki Island, Japan

Concentrations of total arsenic (As) and individual compounds were determined in green and hawksbill turtles from Ishigaki Island, Japan. In both species, total As concentrations were highest in muscle among the tissues. Arsenobetaine was a major compound in most tissues of both turtles. High concentrations of trimethylarsine oxide were detected in hawksbill turtles. A significant negative correlation between standard carapace length (SCL), an indicator of age, and total As levels in green turtles was found. In contrast, the levels increased with SCL of hawksbill turtles. Shifts in feeding habitats with growth may account for such a growth-dependent accumulation of As. Although concentrations of As in marine sponges, the major food of hawksbill turtles are not high compared to those in algae eaten by green turtles, As concentrations in hawksbill turtles were higher than those in green turtles, indicating that hawksbill turtles may have a specific accumulation mechanism for As.

Arsenic in marine mammals, seabirds, and sea turtles

Although there have been numerous studies on arsenic in low-trophic-level marine organisms, few studies exist on arsenic in marine mammals, seabirds, and sea turtles. Studies on arsenic species and their concentrations in these animals are needed to evaluate their possible health effects and to deepen our understanding of how arsenic behaves and cycles in marine ecosystems. Most arsenic in the livers of marine mammals, seabirds, and sea turtles is AB, but this form is absent or occurs at surprisingly low levels in the dugong. Although arsenic levels were low in marine mammals, some seabirds, and some sea turtles, the black-footed albatross and hawksbill and loggerhead turtles showed high concentrations, comparable to those in marine organisms at low trophic levels. Hence, these animals may have a specific mechanism for accumulating arsenic. Osmoregulation in these animals may play a role in the high accumulation of AB. Highly toxic inorganic arsenic is found in some seabirds and sea turtles, and some evidence suggests it may act as an endocrine disruptor, requiring new and more detailed studies for confirmation. Furthermore, DMA(V) and arsenosugars, which are commonly found in marine animals and marine algae, respectively, might pose risks to highly exposed animals because of their tendency to form reactive oxygen species. In marine mammals, arsenic is thought to be mainly stored in blubber as lipid-soluble arsenicals. Because marine mammals occupy the top levels of their food chain, work to characterize the lipid-soluble arsenicals and how they cycle in marine ecosystems is needed. These lipid-soluble arsenicals have DMA precursors, the exact structures of which remain to be determined. Because many more arsenicals are assumed to be present in the marine environment, further advances in analytical capabilities can and will provide useful future information on the transformation and cycling of arsenic in the marine environment.

Molecular processes in cellular arsenic metabolism

Elucidating molecular processes that underlie accumulation, metabolism and binding of iAs and its methylated metabolites provides a basis for understanding the modes of action by which iAs acts as a toxin and a carcinogen. One approach to this problem is to construct a conceptual model that incorporates available information on molecular processes involved in the influx, metabolism, binding and efflux of arsenicals in cells. This conceptual model is initially conceived as a non-quantitative representation of critical molecular processes that can be used as a framework for experimental design and prediction. However, with refinement and incorporation of additional data, the conceptual model can be expressed in mathematical terms and should be useful for quantitative estimates of the kinetic and dynamic behavior of iAs and its methylated metabolites in cells. Development of a quantitative model will be facilitated by the availability of tools and techniques to manipulate molecular processes underlying transport of arsenicals across cell membranes or expression and activity of enzymes involved in methylation of arsenicals. This model of cellular metabolism might be integrated into more complex pharmacokinetic models for systemic metabolism of iAs and its methylated metabolites. It may also be useful in development of biologically based dose-response models describing the toxic and carcinogenic actions of arsenicals.

A case-control study of GST polymorphisms and arsenic related skin lesions

BACKGROUND

Polymorphisms in GSTT1, GSTM1 and GSTP1 impact detoxification of carcinogens by GSTs and have been reported to increase susceptibility to environmentally related health outcomes. Individual factors in arsenic biotransformation may influence disease susceptibility. GST activity is involved in the metabolism of endogenous and exogenous compounds, including catalyzing the formation of arsenic-GSH conjugates.

METHODS

We investigated whether polymorphisms in GSTT1, GSTP1 and GSTM1 were associated with risk of skin lesions and whether these polymorphisms modify the relationship between drinking water arsenic exposure and skin lesions in a case control study of 1200 subjects frequency matched on age and gender in community clinics in Pabna, Bangladesh in 2001-2002.

RESULTS AND DISCUSSION

GSTT1 homozygous wildtype status was associated with increased odds of skin lesions compared to the null status (OR1.56 95% CI 1.10-2.19). The GSTP1 GG polymorphism was associated with greater odds of skin lesions compared to GSTP1 AA, (OR 1.86 (95%CI 1.15-3.00). No evidence of effect modification by GSTT1, GSTM1 or GSTP1 polymorphisms on the association between arsenic exposure and skin lesions was detected.

CONCLUSION

GSTT1 wildtype and GSTP1 GG are associated with increased risk of skin lesions.

Expression and activity of arsenic methyltransferase Cyt19 in rat tissues

Arsenic has been reportedly metabolized by the repetitive reduction and methylation, and is excreted mainly in urine as methylated arsenicals such as monomethylarsonic acid (MMA(V)) and dimethylarsinic acid (DMA(V)). Although it has been demonstrated that the methylation of arsenic is catalyzed by arsenic methyltransferase, Cyt19, very little is known about the characteristics of this enzyme. We investigated mRNA and protein levels of Cyt19 and the enzyme activity of Cyt19 in rat tissues by Northern and Western blottings and high-performance liquid chromatography-inductively coupled argon plasma mass spectrometry (HPLC-ICP MS). Both mRNA and protein levels of the liver were higher than those of other tissues. An intermediate expression of Cyt19 mRNA was observed in the heart and testis. Methylated arsenicals were found to be produced by the liver cytosol using the current HPLC-ICP MS method.

Study of interactions between arsenicals and thioredoxins (human and E. coli) using mass spectrometry

Thioredoxin (Trx) plays an important role in achieving redox balances in cells and protecting the cells from oxidative damage. However, little is known about how arsenic affects Trx chemically. It is conceivable that trivalent arsenicals may bind to Trx, which has a highly conserved -CysGlyProCys- sequence. The objective of this study is to characterize the binding of seven arsenic species with Trx from E. coli and humans, using two mass spectrometry techniques. The arsenic-Trx complexes and the free arsenicals were well separated by size-exclusion liquid chromatography (LC) and detected with inductively coupled plasma mass spectrometry (ICPMS). The LC/ICPMS analyses showed that the trivalent arsenic species were able to form complexes with both human and E. coli Trx. Determination of binding constants indicated that affinity to Trx was higher for monomethylarsonous acid (MMA(III)) and phenylarsine oxide (PhAs(III)) than inorganic arsenite (iAs(III)) and dimethylarsinous acid (DMA(III)), probably because MMA(III) and PhAs(III) were able to form stable complexes by binding to two vicinal cysteines in the -CysGlyProCys- region of the Trx. The complexes of arsenicals with both human and E. coli Trx were further characterized by nano-electrospray tandem mass spectrometry. Binding stoichiometries for different arsenic species were consistent with the available cysteine residues in the Trx. Mass spectral evidence also suggests that the pentavalent arsenicals could be reduced by Trx. This study provides the first detailed chemical characterization of the interactions between Trx and arsenic species.

Chronic arsenic exposure increases TGFalpha concentration in bladder urothelial cells of Mexican populations environmentally exposed to inorganic arsenic

Inorganic arsenic (iAs) is a well-established carcinogen and human exposure has been associated with a variety of cancers including those of skin, lung, and bladder. High expression of transforming growth factor alpha (TGF-alpha) has associated with local relapses in early stages of urinary bladder cancer. iAs exposures are at least in part determined by the rate of formation and composition of iAs metabolites (MAs(III), MAs(V), DMAs(III), DMAs(V)). This study examines the relationship between TGF-alpha concentration in exfoliated bladder urothelial cells (BUC) separated from urine and urinary arsenic species in 72 resident women (18-51 years old) from areas exposed to different concentrations of iAs in drinking water (2-378 ppb) in central Mexico. Urinary arsenic species, including trivalent methylated metabolites were measured by hydride generation atomic absorption spectrometry method. The concentration of TGF-alpha in BUC was measured using an ELISA assay. Results show a statistically significant positive correlation between TGF-alpha concentration in BUC and each of the six arsenic species present in urine. The multivariate linear regression analyses show that the increment of TGF-alpha levels in BUC was importantly associated with the presence of arsenic species after adjusting by age, and presence of urinary infection. People from areas with high arsenic exposure had a significantly higher TGF-alpha concentration in BUC than people from areas of low arsenic exposure (128.8 vs. 64.4 pg/mg protein; p<0.05). Notably, exfoliated cells isolated from individuals with skin lesions contained significantly greater amount of TGF-alpha than cells from individuals without skin lesions: 157.7 vs. 64.9 pg/mg protein (p=0.003). These results suggest that TGF-alpha in exfoliated BUC may serve as a susceptibility marker of adverse health effects on epithelial tissue in arsenic-endemic areas.

Lipid oxidative damage and distribution of inorganic arsenic and its metabolites in the rat nervous system after arsenite exposure: Influence of alpha tocopherol supplementation

Inorganic arsenic (iAs) exposure causes peripheral neuropathy. Oxidative effects caused by iAs exposure in peripheral nerves have been incompletely characterized. This study analyzed arsenic and lipid oxidative damage in the brain, spinal cord, and sciatic and sensory sural nerves following arsenite exposure. This study also explored whether alpha tocopherol (alpha-TOC) administration mitigates arsenite-induced oxidative damage. Thiobarbituric acid-reactive substance (TBARS) levels and distributions of iAs and its metabolites were evaluated in male Wistar rats following 30d of sodium arsenite exposure (10mg/kg bodyweight (bw)/d, by gavage). A second group also received alpha-TOC (125mg/kg bw/d, by gavage) during the final 20d of arsenite administration. Arsenite exposure caused increased TBARS levels within each region of the nervous system; oxidative stress was most pronounced in the sural and sciatic nerves. In addition there was a positive quadratic relationship between TBARS levels and the concentration of arsenicals found in the nervous system (r(2)=0.878, p<0.001). Dimethylarsenic was the predominant metabolite of iAs found. Animals alpha-TOC-treated had a 1.7-5.2-fold reduction in TBARS levels when compared with rats that received iAs alone. These results suggest that oxidative damage may be the main mechanism of toxicity induced by exposure of the peripheral nervous system to arsenite and that such damage could be attenuated by alpha-TOC-supplementation.

Trivalent arsenicals are bound to proteins during reductive methylation

Inorganic arsenic is converted to methylated metabolites, and most is excreted in urine as dimethylarsinic acid in humans and animals. The present study was conducted to investigate the metabolism of arsenic and identify hepatic and renal metabolites of arsenic after an intravenous injection of arsenite (0.5 mg As/kg body weight) in rats. Similar levels of arsenic were found in the soluble (SUP) and nonsoluble sediment (SED) fractions of both organs after 1 h. More than 80% of the SUP arsenic was bound to high molecular weight (HMW) proteins in both organs. Arsenic bound to the HMW and SED proteins were oxidized with H(2)O(2) and released in the pentavalent forms (arsenate, monomethylarsonic, and dimethylarsinic acids). The relative ratios of the three arsenicals changed depending on organ, fraction (HMW and SED), and time. Since the arsenic metabolites/intermediates were liberated from proteins by oxidation with H(2)O(2) and recovered in the pentavalent forms, and only tri- but not pentavalent arsenicals were bound to proteins in vitro, it was deduced that arsenic metabolites bound to proteins during the successive methylation pathway are in the trivalent forms; that is, successive methylation reaction takes place with simultaneous reductive rather than stepwise oxidative methylation. Thus, on the basis of the present observations, it was proposed that inorganic arsenic was successively methylated reductively in the presence of glutathione, rather than a stepwise oxidative methylation, and pentavalent arsenicals (MMA(V) and DMA(V)) were present as end products of metabolism, rather than intermediates. We also discussed the in vitro formation of dimethylthioarsenicals after incubating dimethylarsinous acid with liver homogenate.

Arsenic in the aetiology of cancer

Arsenic, one of the most significant hazards in the environment affecting millions of people around the world, is associated with several diseases including cancers of skin, lung, urinary bladder, kidney and liver. Groundwater contamination by arsenic is the main route of exposure. Inhalation of airborne arsenic or arsenic-contaminated dust is a common health problem in many ore mines. This review deals with the questions raised in the epidemiological studies such as the dose-response relationship, putative confounders and synergistic effects, and methods evaluating arsenic exposure. Furthermore, it describes the metabolic pathways of arsenic, and its biological modes of action. The role of arsenic in the development of cancer is elucidated in the context of combined epidemiological and biological studies. However, further analyses by means of molecular epidemiology are needed to improve the understanding of cancer aetiology induced by arsenic.

Metabolism and toxicity of arsenic in human urothelial cells expressing rat arsenic (+3 oxidation state)-methyltransferase

The enzymatic methylation of inorganic As (iAs) is catalyzed by As(+3 oxidation state)-methyltransferase (AS3MT). AS3MT is expressed in rat liver and in human hepatocytes. However, AS3MT is not expressed in UROtsa, human urothelial cells that do not methylate iAs. Thus, UROtsa cells are an ideal null background in which the role of iAs methylation in modulation of toxic and cancer-promoting effects of this metalloid can be examined. A retroviral gene delivery system was used in this study to create a clonal UROtsa cell line (UROtsa/F35) that expresses rat AS3MT. Here, we characterize the metabolism and cytotoxicity of arsenite (iAs(III)) and methylated trivalent arsenicals in parental cells and clonal cells expressing AS3MT. In contrast to parental cells, UROtsa/F35 cells effectively methylated iAs(III), yielding methylarsenic (MAs) and dimethylarsenic (DMAs) containing either As(III) or As(V). When exposed to MAs(III), UROtsa/F35 cells produced DMAs(III) and DMAs(V). MAs(III) and DMAs(III) were more cytotoxic than iAs(III) in UROtsa and UROtsa/F35 cells. The greater cytotoxicity of MAs(III) or DMAs(III) than of iAs(III) was associated with greater cellular uptake and retention of each methylated trivalent arsenical. Notably, UROtsa/F35 cells were more sensitive than parental cells to the cytotoxic effects of iAs(III) but were more resistant to cytotoxicity of MAs(III). The increased sensitivity of UROtsa/F35 cells to iAs(III) was associated with inhibition of DMAs production and intracellular accumulation of MAs. The resistance of UROtsa/F35 cells to moderate concentrations of MAs(III) was linked to its rapid conversion to DMAs and efflux of DMAs. However, concentrations of MAs(III) that inhibited DMAs production by UROtsa/F35 cells were equally toxic for parental and clonal cell lines. Thus, the production and accumulation of MAs(III) is a key factor contributing to the toxicity of acute iAs exposures in methylating cells.