Dimethylarsinic acid effects on DNA damage and oxidative stress related biochemical parameters in B6C3F1 mice

Differences in apoptotic signaling and toxicity between dimethylmonothioarsinic acid (DMMTAV) and its active metabolite, dimethylarsinous acid (DMAIII), in HepaRG cells: Possibility of apoptosis cascade based on diversity of active metabolites of DMMTAV

Dimethylmonothioarsinical acid (DMMTA^V), a metabolite of arsenosugars (AsSug) and arsenolipids (AsLP), which are major organoarsenicals contained in seafoods, has been a focus of our attention due to its toxicity. It has been reported that the toxicity of DMMTA^V differs according to the host cell type and that dimethylarsinous acid (DMA^III), which is a higher active metabolite of inorganic and organo arsenic compounds, may be the ultimate substance. To further elucidate the details of the mechanisms of DMMTA^V, we carried out toxicological characterization by comparing DMMTA^V and DMA^III using HepaRG cells, which are terminally differentiated hepatic cells derived from a human hepatic progenitor cell line that retains many characteristics, e.g, primary human hepatocytes including the morphology and expression of key metabolic enzymes (P450 s and GSTs, etc.) and complete expression of all nuclear receptors. HepaRG cells were induced to undergo differentiation by DMSO, which result red in increased levels of metabolic enzymes such as P450 and GST, in non-differentiated cells the cellular toxicities of DMMTA^V and DMA^III were reduced and the induction of toxicity by DMMTA^V was increased by GSH but not by DMA^III. Both DMA^III and DMMTA^V induce apoptosis and increase caspase 3/7 activity. DMA^III exposure increased the activity of caspase-9. On the contrary, DMMTA^V exposure resulted in markedly elevated activity of caspase-8 as well as caspase-9. These results suggest there are differences between the signaling pathways of apoptosis in DMA^III and DMMTA^V and that between their active metabolites. Consequently, the ultimate metabolic substance of toxicity induction of DMMTA^V may not only be DMA^III, but may also be partly due to other metabolic substances produced through the activation mechanism by GSH.

Hepatic histopathological changes and dysfunction in primates following exposure to organic arsenic diphenylarsinic acid

Organic arsenic diphenylarsinic acid (DPAA[V]) accumulates at high concentrations in the liver of primates after its subchronic administration. However, no studies on the hepatic effects of organic arsenic compounds, including DPAA(V), on primates have been reported to date. To clarify the toxicokinetics of DPAA(V) in the liver of primates, hepatic tissue specimens were collected from cynomolgus monkeys (n = 32) at 5, 29, 170, and 339 days after repeated administration of DPAA(V) for 28 days. Four histopathological changes in the specimens were observed and pathologically evaluated. Atypical ductular proliferation was found in the DPAA(V)-exposed liver throughout the period. Inflammatory cell infiltration in Glisson's capsules and lipid droplets were seen at earlier periods after administration. Conversely, inflammatory cell infiltration in liver lobules was seen later after administration. In this experiment, we did not confirm the hepatic dysfunction of DPAA(V)-exposed monkeys by blood chemistry tests. To compensate for this, we further investigated the blood from a patient who exhibited several neurological symptoms after DPAA(V) exposure. Her blood chemistry test values for aspartate transaminase, alanine transaminase, and lactate dehydrogenase were elevated, suggesting that her liver may have been damaged by DPAA(V) exposure. Together, these findings suggest that the accumulation of DPAA(V) may induce differential histopathological changes in primate hepatocytes, resulting in decreased liver function. This is the first report to investigate the liver of primates pathologically after exposure to organic arsenic DPAA(V). Our findings will help expand our knowledge regarding the effect of DPAA(V) on the liver of primates.

A review on exposure and effects of arsenic in passerine birds

Arsenic (As) is a metalloid of high concern because of its toxic effects for plants and animals. However, it is hard to find information on this metalloid in passerines. This review presents a comprehensive overview of As exposure and effects in birds, and more particularly in passerines, as a result of an extensive search of the literature available. Internal tissues are the most frequently analyzed matrices for As determination in passerines (37.5% of the reviewed studies used internal tissues), followed by feathers and eggs (32.5% each), feces (27.5%), and finally blood (15%). A clear tendency is found in recent years to the use of non-destructive samples. Most studies on As concentrations in passerines have been done in great tit (Parus major; 50%), followed by pied flycatcher (Ficedula hypoleuca; 22.5%). Some factors such as diet and migratory status are crucial on the interspecific differences in As exposure. More studies are needed to elucidate if intraspecific factors like age or gender affect As concentrations in different tissues. The literature review shows that studies on As concentrations in passerines have been done mainly in the United States (30%), followed by Belgium (22.5%), and Finland (20%), making evident the scarce or even lack of information in some countries, so we recommend further research in order to overcome the data gap, particularly in the southern hemisphere. Studies on humans, laboratory animals and birds have found a wide range of effects on different organ systems when they are exposed to different forms of As. This review shows that few field studies on As exposure and effects in passerines have been done, and all of them are correlative so far. Arsenic manipulation experiments on passerines are recommended to explore the adverse effects of As in free-living populations at similar levels to those occurring in the environment.

CAPSULE

This review summarizes the most interesting published studies on As exposure and effects in passerines.

Baseline comorbidities in a skin cancer prevention trial in Bangladesh

BACKGROUND

Epidemiologic research suggests that increased cancer risk due to chronic arsenic exposure persists for several decades even after the exposure has terminated. Observational studies suggest that antioxidants exert a protective effect on arsenical skin lesions and cancers among those chronically exposed to arsenic through drinking water. This study reports on the design, methods and baseline analyses from the Bangladesh Vitamin E and Selenium Trial (BEST), a population-based chemoprevention study conducted among adults in Bangladesh with visible arsenic toxicity.

MATERIALS AND METHODS

Bangladesh Vitamin E and Selenium Trial is a 2 × 2 full factorial, double-blind, randomized controlled trial of 7000 adults having manifest arsenical skin lesions evaluating the efficacy of 6-year supplementation with alpha-tocopherol (100 mg daily) and L-selenomethionine (200 μg daily) for the prevention of nonmelanoma skin cancer.

RESULTS

In cross-sectional analyses, we observed significant associations of skin lesion severity with male gender (female prevalence odds ratio (POR) = 0.87; 95% CI = 0.79-0.96), older age (aged 36-45 years, POR = 1.27; 95% CI = 1.13-1.42; aged 46-55 years, POR = 1.44; 95% CI = 1.27-1.64 and aged 56-65 years, POR = 1.50; 95% CI = 1.26-1.78 compared with aged 25-35 years), hypertension (POR = 1.29; 95% CI = 1.08-1.55), diabetes (POR = 2.13; 95% CI = 1.32-3.46), asthma (POR = 1.55; 95% CI = 1.03-2.32) and peptic ulcer disease (POR = 1.20; 95% CI = 1.07-1.35).

CONCLUSIONS

We report novel associations between arsenical skin lesions with several common chronic diseases. With the rapidly increasing burden of preventable cancers in developing countries, efficient and feasible chemoprevention study designs and approaches, such as employed in BEST, may prove both timely and potentially beneficial in conceiving cancer chemoprevention trials in Bangladesh and beyond.

Arsenic: toxicity, oxidative stress and human disease

Arsenic (As) is a toxic metalloid element that is present in air, water and soil. Inorganic arsenic tends to be more toxic than organic arsenic. Examples of methylated organic arsenicals include monomethylarsonic acid [MMA(V)] and dimethylarsinic acid [DMA(V)]. Reactive oxygen species (ROS)-mediated oxidative damage is a common denominator in arsenic pathogenesis. In addition, arsenic induces morphological changes in the integrity of mitochondria. Cascade mechanisms of free radical formation derived from the superoxide radical, combined with glutathione-depleting agents, increase the sensitivity of cells to arsenic toxicity. When both humans and animals are exposed to arsenic, they experience an increased formation of ROS/RNS, including peroxyl radicals (ROO•), the superoxide radical, singlet oxygen, hydroxyl radical (OH•) via the Fenton reaction, hydrogen peroxide, the dimethylarsenic radical, the dimethylarsenic peroxyl radical and/or oxidant-induced DNA damage. Arsenic induces the formation of oxidized lipids which in turn generate several bioactive molecules (ROS, peroxides and isoprostanes), of which aldehydes [malondialdehyde (MDA) and 4-hydroxy-nonenal (HNE)] are the major end products. This review discusses aspects of chronic and acute exposures of arsenic in the etiology of cancer, cardiovascular disease (hypertension and atherosclerosis), neurological disorders, gastrointestinal disturbances, liver disease and renal disease, reproductive health effects, dermal changes and other health disorders. The role of antioxidant defence systems against arsenic toxicity is also discussed. Consideration is given to the role of vitamin C (ascorbic acid), vitamin E (α-tocopherol), curcumin, glutathione and antioxidant enzymes such as superoxide dismutase, catalase and glutathione peroxidase in their protective roles against arsenic-induced oxidative stress.

Arsenic and cardiovascular disease

Chronic arsenic exposure is a worldwide health problem. Although arsenic-induced cancer has been widely studied, comparatively little attention has been paid to arsenic-induced vascular disease. Epidemiological studies have shown that chronic arsenic exposure is associated with increased morbidity and mortality from cardiovascular disease. In addition, studies suggest that susceptibility to arsenic-induced vascular disease may be modified by nutritional factors in addition to genetic factors. Recently, animal models for arsenic-induced atherosclerosis and liver sinusoidal endothelial cell dysfunction have been developed. Initial studies in these models show that arsenic exposure accelerates and exacerbates atherosclerosis in apolipoprotein E-knockout mice. Microarray studies of liver mRNA and micro-RNA abundance in mice exposed in utero suggest that a permanent state of stress is induced by the arsenic exposure. Furthermore, the livers of the arsenic-exposed mice have activated pathways involved in immune responses suggesting a pro-hyperinflammatory state. Arsenic exposure of mice after weaning shows a clear dose-response in the extent of disease exacerbation. In addition, increased inflammation in arterial wall is evident. In response to arsenic-stimulated oxidative signaling, liver sinusoidal endothelium differentiates into a continuous endothelium that limits nutrient exchange and waste elimination. Data suggest that nicotinamide adenine dinucleotide phosphate oxidase-derived superoxide or its derivatives are essential second messengers in the signaling pathway for arsenic-stimulated vessel remodeling. The recent findings provide future directions for research into the cardiovascular effects of arsenic exposure.

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.

PGAM5, a Bcl-XL-interacting Protein, Is a Novel Substrate for the Redox-regulated Keap1-dependent Ubiquitin Ligase Complex

Keap1 is a BTB-Kelch substrate adaptor protein for a Cul3-dependent ubiquitin ligase complex that functions as a sensor for thiol-reactive chemopreventive compounds and oxidative stress. Inhibition of Keap1-dependent ubiquitination of the bZIP transcription factor Nrf2 enables Nrf2 to activate a cyto-protective transcriptional program that counters the damaging effects of oxidative stress. In this report we have identified a member of the phosphoglycerate mutase family, PGAM5, as a novel substrate for Keap1. The N terminus of the PGAM5 protein contains a conserved NXESGE motif that binds to the substrate binding pocket in the Kelch domain of Keap1, whereas the C-terminal PGAM domain binds Bcl-X(L). Keap1-dependent ubiquitination of PGAM5 results in proteasome-dependent degradation of PGAM5. Quinone-induced oxidative stress and the chemopreventive agent sulforaphane inhibit Keap1-dependent ubiquitination of PGAM5. The identification of PGAM5 as a novel substrate of Keap1 suggests that Keap1 regulates both transcriptional and post-transcriptional responses of mammalian cells to oxidative stress.

Folate and arsenic metabolism: a double-blind, placebo-controlled folic acid-supplementation trial in Bangladesh

BACKGROUND

Populations in South and East Asia and many other regions of the world are chronically exposed to arsenic-contaminated drinking water. To various degrees, ingested inorganic arsenic (InAs) is methylated to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) via folate-dependent one-carbon metabolism; impaired methylation is associated with adverse health outcomes. Consequently, folate nutritional status may influence arsenic methylation and toxicity.

OBJECTIVE

The objective of this study was to test the hypothesis that folic acid supplementation of arsenic-exposed adults would increase arsenic methylation.

DESIGN

Two hundred adults in a rural region of Bangladesh, previously found to have low plasma concentrations of folate (</=9 nmol/L) were enrolled in a randomized, double-blind, placebo-controlled folic acid-supplementation trial. Plasma concentrations of folate and homocysteine and urinary concentrations of arsenic metabolites were analyzed at baseline and after 12 wk of supplementation with folic acid at a dose of 400 microg/d or placebo.

RESULTS

The increase in the proportion of total urinary arsenic excreted as DMA in the folic acid group (72% before and 79% after supplementation) was significantly (P < 0.0001) greater than that in the placebo group, as was the reduction in the proportions of total urinary arsenic excreted as MMA (13% and 10%, respectively; P < 0.0001) and as InAs (15% and 11%, respectively; P < 0.001).

CONCLUSIONS

These data indicate that folic acid supplementation to participants with low plasma folate enhances arsenic methylation. Because persons whose urine contains low proportions of DMA and high proportions of MMA and InAs have been reported to be at greater risk of skin and bladder cancers and peripheral vascular disease, these results suggest that folic acid supplementation may reduce the risk of arsenic-related health outcomes.

Folate, homocysteine, and arsenic metabolism in arsenic-exposed individuals in Bangladesh

Chronic exposure to arsenic is occurring throughout South and East Asia due to groundwater contamination of well water. Variability in susceptibility to arsenic toxicity may be related to nutritional status. Arsenic is methylated to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) via one-carbon metabolism, a biochemical pathway that is dependent on folate. The majority of one-carbon metabolism methylation reactions are devoted to biosynthesis of creatine, the precursor of creatinine. Our objectives of this cross-sectional study were to characterize the relationships among folate, cobalamin, homocysteine, and arsenic metabolism in Bangladeshi adults. Water arsenic, urinary arsenic, urinary creatinine, plasma folate, cobalamin, and homocysteine were assessed in 1,650 adults; urinary arsenic metabolites were analyzed for a subset of 300 individuals. The percentage of DMA in urine was positively associated with plasma folate (r = 0.14, p = 0.02) and negatively associated with total homocysteine (tHcys; r = -0.14, p = 0.01). Conversely, percent MMA was negatively associated with folate (r = -0.12, p = 0.04) and positively associated with tHcys (r = 0.21, p = 0.0002); percent inorganic arsenic (InAs) was negatively associated with folate (r = -0.12, p = 0.03). Urinary creatinine was positively correlated with percent DMA (r = 0.40 for males, p < 0.0001; 0.25 for females, p = 0.001), and with percent InAs (r = -0.45 for males, p < 0.0001; -0.20 for females, p = 0.01). Collectively, these data suggest that folate, tHcys, and other factors involved in one-carbon metabolism influence arsenic methylation. This may be particularly relevant in Bangladesh, where the prevalence of hyperhomocysteinemia is extremely high.

The role of active arsenic species produced by metabolic reduction of dimethylarsinic acid in genotoxicity and tumorigenesis

In recent research of arsenic carcinogenesis, many researchers have directed their attention to methylated metabolites of inorganic arsenics. Because of its high cytotoxicity and genotoxicity, trivalent dimethylated arsenic, which can be produced by the metabolic reduction of dimethylarsinic acid (DMA), has attracted considerable attention from the standpoint of arsenic carcinogenesis. In the present paper, we examined trivalent dimethylated arsenic and its further metabolites for their chemical properties and biological behavior such as genotoxicity and tumorigenicity. Our in vitro and in vivo experiments suggested that the formation of cis-thymine glycol in DNA was induced via the production of dimethylated arsenic peroxide by the reaction of trivalent dimethylated arsenic with molecular oxygen, but not via the production of common reactive oxygen species (ROS; superoxide, hydrogen peroxide, hydroxyl radical, etc.). Thus, dimethylated arsenic peroxide may be the main species responsible for the tumor promotion in skin tumorigenesis induced by exposure to DMA. Free radical species, such as dimethylarsenic radical [(CH(3))(2)As.] and dimethylarsenic peroxy radical [(CH(3))(2)AsOO.], that are produced by the reaction of molecular oxygen and dimethylarsine [(CH(3))(2)AsH], which is probably a further reductive metabolite of trivalent dimethylated arsenic, may be main agents for initiation in mouse lung tumorigenesis.

Oxidative stress as a possible mode of action for arsenic carcinogenesis

Many modes of action for arsenic carcinogenesis have been proposed, but few theories have a substantial mass of supporting data. Three stronger theories of arsenic carcinogenesis are production of chromosomal abnormalities, promotion of carcinogenesis and oxidative stress. This article presents the oxidative stress theory along with some supporting experimental data. In the area of which arsenic species is causually active, recent data have suggested that trivalent methylated arsenic metabolites, particularly monomethylarsonous acid (MMA(III)) and dimethylarsinous acid (DMA(III)), have a great deal of biological activity. Some evidence now indicates that these trivalent, methylated, and relatively less ionizable arsenic metabolites may be unusually capable of interacting with cellular targets such as proteins and even DNA. Thus for inorganic arsenic, oxidative methylation followed by reduction to trivalency may be a activation, rather than a detoxification pathway. This would be particularly true for arsenate. In forming toxic and carcinogenic arsenic species, reduction from the pentavalent state to the trivalent state may be as or more important than methylation of arsenic.

Plasmid DNA damage caused by methylated arsenicals, ascorbic acid and human liver ferritin

Both dimethylarsinic acid (DMA(V)) and dimethylarsinous acid (DMA(III)) release iron from human liver ferritin (HLF) with or without the presence of ascorbic acid. With ascorbic acid the rate of iron release from HLF by DMA(V) was intermediate (3.37 nM/min, P<0.05) and by DMA(III) was much higher (16.3 nM/min, P<0.001). No pBR322 plasmid DNA damage was observed from in vitro exposure to arsenate (iAs(V)), arsenite (iAs(III)), monomethylarsonic acid (MMA(V)), monomethylarsonous acid (MMA(III)) or DMA(V) alone. DNA damage was observed following DMA(III) exposure; coexposure to DMA(III) and HLF caused more DNA damage; considerably higher amounts of DNA damage was caused by coexposure of DMA(III), HLF and ascorbic acid. Diethylenetriaminepentaacetic acid (an iron chelator), significantly inhibited DNA damage. Addition of catalase (which can increase Fe(2+) concentrations) further increased the plasmid DNA damage. Iron-dependent DNA damage could be a mechanism of action of human arsenic carcinogenesis.

Oral administration of dimethylarsinic acid, a main metabolite of inorganic arsenic, in mice promotes skin tumorigenesis initiated by dimethylbenz(a)anthracene with or without ultraviolet B as a promoter

Concerning arsenic-induced tumorigenesis, an animal model must be developed for understanding the mechanism of human carcinogenesis by arsenics. To determine whether orally administered dimethylarsinic acid (DMA) promotes and causes the progression of skin tumorigenesis, an animal protocol by topical application of dimethylbenz(a)anthracene (DMBA) with or without UVB, a tumor promoter, in hairless mice was used. The administration of DMA by the oral route promoted not only the formation of papillomas induced by DMBA alone but also the formation of malignant tumors induced by way of the formation of atypical keratoses by treatment with DMBA and UVB. A phenomenon, the progression of keratoses-->atypical keratoses-->squamous cell carcinomas (SCCs), observed in the present study may resemble the development of tumors in arsenic-exposed humans. We also discussed the involvement of a reactive oxygen species (ROS), e.g., the dimethylarsenic peroxy radical [(CH3)2AsOO.], produced during the metabolic processing of DMA, in skin and in multi-organ tumorigenesis.

Arsenic species that cause release of iron from ferritin and generation of activated oxygen

The in vitro effects of four different species of arsenic (arsenate, arsenite, monomethylarsonic acid, and dimethylarsinic acid) in mobilizing iron from horse spleen ferritin under aerobic and anaerobic conditions were investigated. Dimethylarsinic acid (DMA(V)) and dimethylarsinous acid (DMA(III)) significantly released iron from horse spleen ferritin either with or without the presence of ascorbic acid, a strong synergistic agent. Ascorbic acid-mediated iron release was time-dependent as well as both DMA(III) and ferritin concentration-dependent. Iron release from ferritin by DMA(III)) alone or with ascorbic acid was not significantly inhibited by superoxide dismutase (150 or 300 units/ml). However, the iron release was greater under anaerobic conditions (nitrogen gas), which indicates direct chemical reduction of iron from ferritin by DMA(III), with or without ascorbic acid. Both DMA(V) and DMA(III)) released iron from both horse spleen and human liver ferritin. Further, the release of ferritin iron by DMA(III)) with ascorbic acid catalyzed bleomycin-dependent degradation of calf thymus DNA. These results indicate that exogenous methylated arsenic species and endogenous ascorbic acid can cause (a) the release of iron from ferritin, (b) the iron-dependent formation of reactive oxygen species, and (c) DNA damage. This reactive oxygen species pathway could be a mechanism of action of arsenic carcinogenesis in man.