Induction of DNA damage by dimethylarsine, a metabolite of inorganic arsenics, is for the major part likely due to its peroxyl radical

Understanding arsenic carcinogenicity by the use of animal models

Although numerous epidemiological studies have indicated that human arsenic exposure is associated with increased incidences of bladder, liver, skin, and lung cancers, limited attempts have been made to understand mechanisms of carcinogenicity using animal models. Dimethylarsinic acid (DMA), an organic arsenic compound, is a major metabolite of ingested inorganic arsenics in mammals. Recent in vitro studies have proven DMA to be a potent clastogenic agent, capable of inducing DNA damage including double strand breaks and cross-link formation. In our attempts to clarify DMA carcinogenicity, we have recently shown carcinogenic effects of DMA and its related metabolites using various experimental protocols in rats and mice: (1) a multi-organ promotion bioassay in rats; (2) a two-stage promotion bioassay by DMA of rat urinary bladder and liver carcinogenesis; (3) a 2-year carcinogenicity test of DMA in rats; (4) studies on the effects of DMA on lung carcinogenesis in rats; (5) promotion of skin carcinogenesis by DMA in keratin (K6)/ornithine decarboxylase (ODC) transgenic mice; (6) carcinogenicity of DMA in p53(+/-) knockout and Mmh/8-OXOG-DNA glycolase (OGG1) mutant mice; (7) promoting effects of DMA and related organic arsenicals in rat liver; (8) promoting effects of DMA and related organic arsenicals in a rat multi-organ carcinogenesis test; and (9) 2-year carcinogenicity tests of monomethylarsonic acid (MMA) and trimethylarsine oxide (TMAO) in rats. The results revealed that the adverse effects of arsenic occurred either by promoting and initiating carcinogenesis. These data, as covered in the present review, suggest that several mechanisms may be involved in arsenic carcinogenesis.

Oxidative stress and apoptosis in metal ion-induced carcinogenesis

Epidemiological evidence suggests that exposure to certain metals causes carcinogenesis. The mechanisms of metal-induced carcinogenesis have been pursued in chemical, biochemical, cellular, and animal models. Significant evidence has accumulated that oxidative stress may be a common pathway in cellular responses to exposure to different metals. For example, in the last few years evidence in support of a correlation between the generation of reactive oxygen species, DNA damage, tumor promotion, and arsenic exposure has strengthened. This article summarizes the current literature on metal-mediated oxidative stress, apoptosis, and their relation to metal-mediated carcinogenesis, concentrating on arsenic and chromium.

Tissue distribution of arsenic species in rabbits after single and multiple parenteral administration of arsenic trioxide: tissue accumulation and the reversibility after washout are tissue-selective

Parenteral administration of arsenic trioxide has recently been recognized as an effective antineoplastic therapy, especially for the treatment of acute promyelocytic leukemia. Its efficacy and toxicity are concentration-dependent and are related to the fractions of different arsenic species and the degree of methylation. In this study, arsenic trioxide was given parenterally to rabbits as a single dose or as a daily dose (0.2, 0.6, and 1.5 mg/kg) for 30 days. The blood and organ concentrations of the arsenic species, including As(III), dimethylarsinic acid (DMA), and monomethylarsonic acid (MMA), were studied on day 1 (single-dose study), day 30 (multiple dosing study), and day 60 (reversibility study). As(III) was the major detectable arsenic species in the blood. The pharmacokinetic parameters (total clearance, area under the curve, etc.) for As(III) indicated a limit for the capacity to eliminate As(III) at the dose of 1.5 mg/kg, and were quite the same after a single dose or chronic multiple dosing. In tissues, DMA was found to be the major metabolite and the concentrations of DMA, As(III), and MMA in general increased with the dose, with the increase most significant at a dose of 1.5 mg/kg. However, normalized tissue distribution of As(III) in the kidney on day 1, but not on day 30, was nonlinear. Along with decreased levels of As(III) and increased levels of DMA, an inducible capacity for methylating As(III) to DMA after chronic dosing in kidney was suggested. The tissue concentration of DMA was highest in lung and liver, and the normalized tissue distributions in liver on day 30 were nonlinear, suggesting a limit in eliminating DMA after a chronic high load of As(III). Tissue concentrations of As(III), DMA, and MMA in bladder increased dramatically after chronic dosing. However, after washout for 30 days, As(III), DMA, and MMA were all undetectable in bladder and liver. However, As(III) in hair and low levels of DMA in lung, kidney, heart and hair were still detected. In conclusion, in rabbits we found a similar pharmacological profile after a single dose or chronic multiple dosing of parenteral arsenic trioxide, with a limiting metabolizing capacity at a dose of 1.5 mg/kg. Tissue accumulation of arsenic species, mainly DMA, and its reversibility after washout were tissue-selective. The potential for late toxicities of arsenic trioxide in organs with a significant tendency for arsenic accumulation with low reversibility should be closely monitored.

Do arsenosugars pose a risk to human health? The comparative toxicities of a trivalent and pentavalent arsenosugar

Seafood frequently contains high concentrations of arsenic (approximately 10-100 mg/kg dry weight). In marine algae (seaweed), this arsenic occurs predominantly as ribose derivatives known collectively as arsenosugars. Although it is clear that arsenosugars are not acutely toxic, there is a possibility of arsenosugars having slight chronic toxicity. In general, trivalent arsenicals are more toxic than their pentavalent counterparts, so in this work we examine the hypothesis that trivalent arsenosugars might be significantly more toxic than pentavalent arsenosugars in vitro. We compared the in vitro toxicity of (R)-2,3-dihydroxypropyl-5-deoxy-5-dimethylarsinoyl-beta-D-riboside, a pentavalent arsenosugar, to that of its trivalent counterpart, (R)-2,3-dihydroxypropyl-5-deoxy-5-dimethylarsino-beta-D-riboside. The trivalent arsenosugar nicked plasmid DNA, whereas the pentavalent arsenosugar did not. The trivalent arsenosugar was more cytotoxic (IC50 = 200 microM, 48 h exposure) than its pentavalent counterpart (IC50 > 6000 microM, 48 h exposure) in normal human epidermal keratinocytes in vitro as determined via the neutral red uptake assay. However, both the trivalent and the pentavalent arsenosugars were significantly less toxic than MMA(III), DMA(III), and arsenate. Neither the pentavalent arsenosugar nor the trivalent arsenosugar were mutagenic in Salmonella TA104. The trivalent arsenosugar was readily formed by reaction of the pentavalent arsenosugar with thiol compounds, including, cysteine, glutathione, and dithioerythritol. This work suggests that the reduction of pentavalent arsenosugars to trivalent arsenosugars in biology might have environmental consequences, especially because seaweed consumption is a significant environmental source for human exposure to arsenicals.

Biokinetics and subchronic toxic effects of oral arsenite, arsenate, monomethylarsonic acid, and dimethylarsinic acid in v-Ha-ras transgenic (Tg.AC) mice

Previous research demonstrated that 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment increased the number of skin papillomas in v-Ha-ras transgenic (Tg.AC) mice that had received sodium arsenite [(As(III)] in drinking water, indicating that this model is useful for studying the toxic effects of arsenic in vivo. Because the liver is a known target of arsenic, we examined the pathophysiologic and molecular effects of inorganic and organic arsenical exposure on Tg.AC mouse liver in this study. Tg.AC mice were provided drinking water containing As(III), sodium arsenate [As(V)], monomethylarsonic acid [(MMA(V)], and 1,000 ppm dimethylarsinic acid [DMA(V)] at dosages of 150, 200, 1,500, or 1,000 ppm as arsenic, respectively, for 17 weeks. Control mice received unaltered water. Four weeks after initiation of arsenic treatment, TPA at a dose of 1.25 microg/200 microL acetone was applied twice a week for 2 weeks to the shaved dorsal skin of all mice, including the controls not receiving arsenic. In some cases arsenic exposure reduced body weight gain and caused mortality (including moribundity). Arsenical exposure resulted in a dose-dependent accumulation of arsenic in the liver that was unexpectedly independent of chemical species and produced hepatic global DNA hypomethylation. cDNA microarray and reverse transcriptase-polymerase chain reaction analysis revealed that all arsenicals altered the expression of numerous genes associated with toxicity and cancer. However, organic arsenicals [MMA(V) and DMA(V)] induced a pattern of gene expression dissimilar to that of inorganic arsenicals. In summary, subchronic exposure of Tg.AC mice to inorganic or organic arsenicals resulted in toxic manifestations, hepatic arsenic accumulation, global DNA hypomethylation, and numerous gene expression changes. These effects may play a role in arsenic-induced hepatotoxicity and carcinogenesis and may be of particular toxicologic relevance.

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.

Arsenite Causes DNA Damage in Keratinocytes Via Generation of Hydroxyl Radicals

Arsenic is an environmental and occupational toxin. Dermatologic toxicities due to arsenic exposure are well-documented and include basal cell and squamous cell carcinomas. However, the mechanism of arsenic-induced skin cancer is not well-understood. Recent studies indicate that arsenic exposure results in the generation of reactive oxygen species (ROS) and oxidative stress. Here, we examined the chemical nature of the specific ROS, studied the interrelationship among these species, and identified the specific species that is responsible for the subsequent DNA damage in a spontaneously immortalized keratinocyte cell line. We detected the formation of O(2)(*)(-) and H(2)O(2) in keratinocytes incubated with arsenite [As(III)] but not with arsenate. As(III)-induced DNA damage was detected in a concentration-dependent manner and evident at low micromolar concentrations. Catalase, an H(2)O(2) scavenger, eliminated H(2)O(2) and reduced the As(III)-mediated DNA damage. Superoxide dismutase, by enhancing the production of H(2)O(2) and (*)OH, significantly increased the As(III)-mediated DNA damage. Sodium formate, a competitive scavenger for (*)OH, and deferoxamine, a metal chelator, both reduced the DNA damage. These results suggest that exposure to arsenite generates O(2)(*)(-) and H(2)O(2), and (*)OH, derived from H(2)O(2), is responsible, at least in part, for the observed DNA damage. These findings demonstrate arsenic-induced formation of specific ROS and provide the direct evidence of (*)OH-mediated DNA damage in keratinocytes, which may play an important role in the mechanism for arsenic-induced skin carcinogenicity.

The potential biological mechanisms of arsenic-induced diabetes mellitus

Although epidemiologic studies carried out in Taiwan, Bangladesh, and Sweden have demonstrated a diabetogenic effect of arsenic, the mechanisms remain unclear and require further investigation. This paper reviewed the potential biological mechanisms of arsenic-induced diabetes mellitus based on the current knowledge of the biochemical properties of arsenic. Arsenate can substitute phosphate in the formation of adenosine triphosphate (ATP) and other phosphate intermediates involved in glucose metabolism, which could theoretically slow down the normal metabolism of glucose, interrupt the production of energy, and interfere with the ATP-dependent insulin secretion. However, the concentration of arsenate required for such reaction is high and not physiologically relevant, and these effects may only happen in acute intoxication and may not be effective in subjects chronically exposed to low-dose arsenic. On the other hand, arsenite has high affinity for sulfhydryl groups and thus can form covalent bonds with the disulfide bridges in the molecules of insulin, insulin receptors, glucose transporters (GLUTs), and enzymes involved in glucose metabolism (e.g., pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase). As a result, the normal functions of these molecules can be hampered. However, a direct effect on these molecules caused by arsenite at physiologically relevant concentrations seems unlikely. Recent evidence has shown that treatment of arsenite at lower and physiologically relevant concentrations can stimulate glucose transport, in contrary to an inhibitory effect exerted by phenylarsine oxide (PAO) or by higher doses of arsenite. Induction of oxidative stress and interferences in signal transduction or gene expression by arsenic or by its methylated metabolites are the most possible causes to arsenic-induced diabetes mellitus through mechanisms of induction of insulin resistance and beta cell dysfunction. Recent studies have shown that, in subjects with chronic arsenic exposure, oxidative stress is increased and the expression of tumor necrosis factor alpha (TNFalpha) and interleukin-6 (IL-6) is upregulated. Both of these two cytokines have been well known for their effect on the induction of insulin resistance. Arsenite at physiologically relevant concentration also shows inhibitory effect on the expression of peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear hormone receptor important for activating insulin action. Oxidative stress has been suggested as a major pathogenic link to both insulin resistance and beta cell dysfunction through mechanisms involving activation of nuclear factor-kappaB (NF-kappaB), which is also activated by low levels of arsenic. Although without supportive data, superoxide production induced by arsenic exposure can theoretically impair insulin secretion by interaction with uncoupling protein 2 (UCP2), and oxidative stress can also cause amyloid formation in the pancreas, which could progressively destroy the insulin-secreting beta cells. Individual susceptibility with respect to genetics, nutritional status, health status, detoxification capability, interactions with other trace elements, and the existence of other well-recognized risk factors of diabetes mellitus can influence the toxicity of arsenic on organs involved in glucose metabolism and determine the progression of insulin resistance and impaired insulin secretion to a status of persistent hyperglycemia or diabetes mellitus. In conclusions, insulin resistance and beta cell dysfunction can be induced by chronic arsenic exposure. These defects may be responsible for arsenic-induced diabetes mellitus, but investigations are required to test this hypothesis.

Role of reactive oxygen species in skin carcinogenesis

Reactive oxygen species (ROS) are associated not only with initiation, but also with promotion and progression in the multistage carcinogenesis model. In the present review, we will focus on the involvement of ROS in skin carcinogenesis, especially that induced by ultraviolet (UV) radiation. UV-specific DNA damage has been well studied thus far. However, recent reports have revealed the previously unknown participation of oxidative stress in UV-induced skin carcinogenesis. Indeed, in addition to transition-type mutations at dipyrimidine sites, G:C to T:A transversions, which may be induced by the presence of 8-oxoguanine during DNA replication, are frequently observed in the ras oncogene and p53 tumor suppressor gene in human skin cancers of sun-exposed areas and in UV-induced mouse skin cancers. Recent studies have shown that not only UV-B, but also UV-A is involved in UV-induced carcinogenesis. A wide variety of biological phenomena other than direct influence by UV, such as inflammatory and immunological responses and oxidative modifications of DNA and proteins, appear to play roles in UV-induced skin carcinogenesis. Furthermore, it has become clear that genetic diseases such as xeroderma pigmentosum show deficient repair of oxidatively modified DNA lesions. The involvement of ROS in skin carcinogeneisis caused by arsenic and chemical carcinogens will also be discussed.

Carcinogenic and systemic health effects associated with arsenic exposure--a critical review

Arsenic and arsenic containing compounds are human carcinogens. Exposure to arsenic occurs occupationally in several industries, including mining, pesticide, pharmaceutical, glass and microelectronics, as well as environmentally from both industrial and natural sources. Inhalation is the principal route of arsenic exposure in occupational settings, while ingestion of contaminated drinking water is the predominant source of significant environmental exposure globally. Drinking water contamination by arsenic remains a major public health problem. Acute and chronic arsenic exposure via drinking water has been reported in many countries of the world, where a large proportion of drinking water is contaminated with high concentrations of arsenic. General health effects that are associated with arsenic exposure include cardiovascular and peripheral vascular disease, developmental anomalies, neurologic and neurobehavioural disorders, diabetes, hearing loss, portal fibrosis, hematologic disorders (anemia, leukopenia and eosinophilia) and multiple cancers: significantly higher standardized mortality rates and cumulative mortality rates for cancers of the skin, lung, liver, urinary bladder, kidney, and colon in many areas of arsenic pollution. Although several epidemiological studies have documented the sources of exposure and the global impact of arsenic contamination, the mechanisms by which arsenic induces health effects, including cancer, are not well characterized. Further research is needed to provide a better understanding of the pathobiology of arsenic-induced diseases and to better define the toxicologic pathology of arsenic in various organ systems. In this review, we provide and discuss the underlying pathology and nature of arsenic-induced lesions. Such information is critical for understanding the magnitude of health effects associated with arsenic exposure throughout the world.

Plasmid DNA damage caused by stibine and trimethylstibine

Antimony is classified as "possibly carcinogenic to humans" and there is also sufficient evidence for antimony carcinogenicity in experimental animals. Stibine is a volatile inorganic antimony compound to which humans can be exposed in occupational settings (e.g., lead-acid battery charging). Because it is highly toxic, stibine is considered a significant health risk; however, its genotoxicity has received little attention. For the work reported here, stibine was generated by sodium borohydride reduction of potassium antimony tartrate. Trimethylstibine is a volatile organometallic antimony compound found commonly in landfill and sewage fermentation gases at concentrations ranging between 0.1 and 100 microg/m3. Trimethylstibine is generally considered to pose little environmental or health risk. In the work reported here, trimethylstibine was generated by reduction of trimethylantimony dichloride using either sodium borohydride or the thiol compounds, dithioerythritol (DTE), L-cysteine, and glutathione. Here we report the evaluation of the in vitro genotoxicities of five antimony compounds-potassium antimony tartrate, stibine, potassium hexahydroxyantimonate, trimethylantimony dichloride, and trimethylstibine-using a plasmid DNA-nicking assay. Of these five antimony compounds, only stibine and trimethylstibine were genotoxic (significant nicking to pBR 322 plasmid DNA). We found stibine and trimethylstibine to be about equipotent with trimethylarsine using this plasmid DNA-nicking assay. Reaction of trimethylantimony dichloride with either glutathione or L-cysteine to produce DNA-damaging trimethylstibine was observed with a trimethylantimony dichloride concentration as low as 50 microM and L-cysteine or glutathione concentrations as low as 500 and 200 microM, respectively, for a 24 h incubation.

The reaction of allyl and benzylarsonic acids with thiols: mechanistic aspects and implications for dioxygen activation by trivalent arsenic compounds

The reaction of allyl and benzylarsonic acids with thiophenol gives not only the expected diphenyl alkyldithioarsonites and diphenyl disulfide but also various other compounds arising from the decomposition at the arsenic(V) oxidation level (the arsonic acids) by thiophenol and at the arsenic(III) oxidation level (mainly the alkyldithioarsonites) by thiophenol and by dissolved dioxygen. The reaction of these arsonic acids with 4-nitrothiophenol, which is not oxidized by dioxygen, revealed that the arsenic(III) of these alkyldithioarsonites is the active atom towards dioxygen. However, the reaction of allyl, benzyl, and 2-picolylarsonic acids with DL-penicillamine gives the expected products with no or very small oxidative decomposition. The decomposition pathways of allyl and benzylarsonic acids were elucidated. The results are briefly discussed in the contexts of the use of arsonic acids in chemotherapy and the ability of arsenic(III) compounds to generate reactive oxygen species.

Active arsenic species produced by GSH-dependent reduction of dimethylarsinic acid cause micronuclei formation in peripheral reticulocytes of mice

Dimethylarsine and trivalent dimethylated arsenic, metabolites of inorganic arsenics, have received considerable attention in current research because of their biological activities. We attempted to determine the appearance of micronucleated reticulocytes (MNRETs) in mouse peripheral blood following intraperitoneal administration of dimethylarsinous iodide (DMI) and trimethylarsine (TMA), model compounds of trivalent dimethylated arsenic and dimethylarsine, respectively. A significant increase in the number of MNRETs was observed with TMA, but not with DMI. Furthermore, MNRETs only appeared with 10.6 mg/kg of dimethylarsinic acid (DMA) following its co-injection with reduced glutathione (GSH). These results suggest that micronucleus formation may need further metabolic reduction of trivalent dimethylated arsenic, i.e. the production of dimethylarsine, by an excess amount of GSH. Meanwhile, the increase in MNRETs by administration of arsenite at 7.6 mg/kg, an equivalent dose to DMA as As, was remarkably diminished by co-administration with GSH. These results indicate that GSH plays an important role in the genotoxic process of arsenics, particularly by dimethylated arsenic.

Dimethylarsine and trimethylarsine are potent genotoxins in vitro

The mechanism of arsenic carcinogenesis is unclear. A complicating factor receiving increasing attention is that arsenic is biomethylated to form various metabolites. Eleven different arsenicals were studied for in vitro genotoxicity to supercoiled DNA (pBR 322 and phiX174). Five arsenicals showed various degrees of positivity-monomethylarsonous acid, dimethylarsinous acid, monomethylarsine, dimethylarsine, and trimethylarsine. Supercoiled DNA, blotted on nitrocellulose filter paper, was exposed to gaseous arsines by suspending the filter paper above aqueous reaction mixtures of sodium borohydride and an appropriate arsenical. All three methylated arsines damaged DNA; inorganic arsine did not. Arsines were generated in situ in reaction mixtures containing DNA by reaction of sodium borohydride with arsenite, monomethylarsonous acid, dimethylarsinous acid, and trimethylarsine oxide, at pH 8.0. Both dimethylarsine and trimethylarsine (generated from 200 micro M dimethylarsinous acid and trimethylarsine oxide, respectively) damaged DNA in less than 30 min. Under certain conditions, the two most potent genotoxic arsines, trimethylarsine and dimethylarsine, are about 100 times more potent than dimethylarsinous acid (the most potent genotoxic arsenical previously known). There was no evidence to suggest that anything other than the arsines caused the DNA damage. Possible models for the biological production of arsines were examined. The coenzymes, NADH and NADPH, are biological hydride donors. When NADH or NADPH (5 mM) were incubated with dimethylarsinous acid (0-2 mM) for 2 h, DNA damage was increased by at least 10-fold. A possible explanation for this result is that these compounds react with dimethylarsinous acid to generate dimethylarsine. DNA was incubated with a dithiol compound, dithioerythritol (5 mM), and trimethylarsine oxide (0.5 mM) for 2 h, and the reduction of trimethylarsine oxide to trimethylarsine resulted in DNA damage.

Arsenite-induced formation of hydroxyl radical in the striatum of awake rats

Recent studies on the mechanisms of arsenite toxicity report that some of its effects have been traced to the generation of reactive oxygen species during oxidative stress. In this study we analyze the formation of hydroxyl radicals in the brain of awake, freely moving rats, in order to obtain direct evidence of arsenic-induced oxidative stress in this tissue. We examined the time-course of hydroxyl radical formation in the striatum of both female and male rats who underwent a direct infusion during 60 min of different concentrations of arsenite in that structure through a microdialysis probe. We report here that basal levels of hydroxyl radical production in female rats are significantly higher than those in male rats (91.9+/-16.1 vs. 59.2+/-18.1 pmol/ml, P<0.001) and that the treatment with arsenite induced significant increases of hydroxyl radical formation over basal levels at 50, 100, 200 and 400 microM (95, 98, 98 and 99% increases, respectively, P<0.05 in all cases). The maximal response to 100 microM arsenite is significantly higher in female than in male rats (194.6+/-50.1 female rats and 88.1+/-11.6 pmol/ml male rats, P=0.036). These results support the participation of hydroxyl radicals in arsenic-induced disturbances in the central nervous system.

Epidemiologic considerations to assess altered DNA methylation from environmental exposures in cancer

Epidemiologic studies in human populations have identified a broad spectrum of risk factors for cancer. Gene-damaging agents have been a primary focus of cancer epidemiology; however, all xenobiotics do not interact with DNA directly. Some exogenous agents induce epigenetic changes. In view of this, markers that measure changes to the epigenome must also be incorporated into molecular epidemiologic studies. We review the current understanding of the impact of exogenous agents including: micronutrients, chemotherapeutic agents, metals, and others, on DNA methylation. Two categories of genes are described: (1) genes that can alter susceptibility to aberrant DNA methylation and (2) genes that increase susceptibility to cancer when they are silenced through DNA methylation. Methods for incorporating markers of DNA methylation status into etiologic investigations of the impact of environmental exposures on disease (e.g., cancer) are discussed.

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.

The efficacy of monoisoamyl ester of dimercaptosuccinic acid in chronic experimental arsenic poisoning in mice

The therapeutic efficacy of monoisoamyl meso-2,3-dimercaptosuccinic acid (MiADMSA), a new monoester of 2,3-dimercaptosuccinic acid on arsenic induced oxidative stress in liver and kidneys, alterations in hematopoietic system and depletion of arsenic burden was assessed, in mice. Three different doses of MiADMSA (25, 50 or 100 mg/kg) for five consecutive days were administered in chronically arsenic exposed mice (10 ppm in drinking water for six months). Oral administration of MiADMSA particularly at a dose of 50 mg/kg, produced relatively more pronounced beneficial effects on the inhibited blood delta-aminolevulinic acid dehydratase (ALAD), biochemical variables indicative of hepatic and renal oxidative stress and depletion of arsenic concentration in blood, liver and kidneys, compared with intraperitoneal administration of the drug. The treatment with MiADMSA although, produced essential metals imbalance which could be a restrictive factor for the possible therapeutic use of this compound in chronic arsenic poisoning and thus require further exploration.

Methylated trivalent arsenicals as candidate ultimate genotoxic forms of arsenic: induction of chromosomal mutations but not gene mutations

Arsenic is a prevalent human carcinogen whose mutagenicity has not been characterized fully. Exposure to either form of inorganic arsenic, As(III) or As(V), can result in the formation of at least four organic metabolites: monomethylarsonic acid, monomethylarsonous acid (MMA(III)), dimethylarsinic acid, and dimethylarsinous acid (DMA(III)). The methylated trivalent species, as well as some of the other species, have not been evaluated previously for the induction of chromosome aberrations, sister chromatid exchanges (SCE), or toxicity in cultured human peripheral blood lymphocytes; for mutagenicity in L5178Y/Tk(+/-) mouse lymphoma cells or in the Salmonella reversion assay; or for prophage-induction in Escherichia coli. Here we evaluated the arsenicals in these assays and found that MMA(III) and DMA(III) were the most potent clastogens of the six arsenicals in human lymphocytes and the most potent mutagens of the six arsenicals at the Tk(+/-) locus in mouse lymphoma cells. The dimethylated arsenicals were also spindle poisons, suggesting that they may be ultimate forms of arsenic that induce aneuploidy. Although the arsenicals were potent clastogens, none were potent SCE inducers, similar to clastogens that act via reactive oxygen species. None of the six arsenicals were gene mutagens in Salmonella TA98, TA100, or TA104; and neither MMA(III) nor DMA(III) induced prophage. Our results show that both methylated As(V) compounds were less cytotoxic and genotoxic than As(V), whereas both methylated As(III) compounds were more cytotoxic and genotoxic than As(III). Our data support the view that MMA(III) and DMA(III) are candidate ultimate genotoxic forms of arsenic and that they are clastogens and not gene mutagens. We suggest that the clastogenicity of the other arsenicals is due to their metabolism by cells to MMA(III) or DMA(III).

Acute arsenite-induced 8-hydroxyguanine is associated with inhibition of repair activity in cultured human cells

8-Hydroxyguanine (8-OH-Gua) is one of the major modified bases in DNA produced by oxidative damage. Human lung carcinoma cells (A549) were treated with 0.5-2mM sodium arsenite for 4h. By an immunohistochemical type procedure, 8-OH-Gua was clearly detected in A549 cells using a fluorescence microscope and an increase in the percentage of A549 cells with oxidative DNA damage was observed using flow cytometry. The formation of 8-OH-Gua in DNA was also detected by a HPLC-ECD. A dose-dependent increase in oxidative DNA damage in A549 cells with increasing arsenite concentrations was obtained. Therefore, oxidative stress is induced after arsenite treatment. Furthermore, we also found that arsenite decreased the activity of the 8-OH-Gua repair enzyme, hOGG1 (8-oxoguanine-DNA glycosylase 1) as well as its gene and protein expression. We conclude that the 8-OH-Gua level in cultured human cells increases partly by the generation of reactive oxygen species (ROS) and partly by the influence on hOGG1 expression, followed by the inhibition of the repair activity for 8-OH-Gua.

Endonuclease III, formamidopyrimidine-DNA glycosylase, and proteinase K additively enhance arsenic-induced DNA strand breaks in human cells

We report here that sequential digestion with endonuclease III, formamidopyrimidine-DNA glycosylase, and proteinase K in Tris buffer markedly increased the sensitivity for detecting DNA damage in arsenic-treated cells. These three enzymes increased DNA strand breaks in an additive manner. By using this sequential-enzyme-digestion comet assay, we demonstrated that trivalent inorganic arsenic induced more DNA damage than monomethylarsonous acid, monomethylarsonic acid, and dimethylarsinic acid in human blood cell lines. However, trivalent inorganic arsenic was far less potent than monomethylarsonous acid in inhibiting pyruvate dehydrogenase activity. Therefore, different mechanisms are involved in inhibiting pyruvate dehydrogenase activity and inducing DNA damage. Our results also indicate while trivalent inorganic arsenic induced more endonuclease III-digestible adducts, monomethylarsonous acid and monomethylarsonic acid induced more proteinase K-digestible adducts. These results suggest there is a difference in the mechanism for inducing DNA damage between inorganic and organic methylated arsenic compounds.

Carcinogenicity of dimethylarsinic acid in male F344 rats and genetic alterations in induced urinary bladder tumors

Arsenic is a well-documented human carcinogen, and contamination with this heavy metal is of global concern, presenting a major issue in environmental health. However, the mechanism by which arsenic induces cancer is unknown, in large part due to the lack of an appropriate animal model. In the present set of experiments, we focused on dimethylarsinic acid (DMA), a major metabolite of arsenic in most mammals including humans. We provide, for the first time, the full data, including detailed pathology, of the carcinogenicity of DMA in male F344 rats in a 2-year bioassay, along with the first assessment of the genetic alteration patterns in the induced rat urinary bladder tumors. Additionally, to test the hypothesis that reactive oxygen species (ROS) may play a role in DMA carcinogenesis, 8-hydroxy-2'-deoxyguanosine (8-OHdG) formation in urinary bladder was examined. In experiment 1, a total of 144 male F344 rats at 10 weeks of age were randomly divided into four groups that received DMA at concentrations of 0, 12.5, 50 and 200 p.p.m. in the drinking water, respectively, for 104 weeks. From weeks 97-104, urinary bladder tumors were observed in 8 of 31 and 12 of 31 rats in groups treated with 50 and 200 p.p.m. DMA, respectively, and the preneoplastic lesion, papillary or nodular hyperplasias (PN hyperplasia), was noted in 12 and 14 rats, respectively. DMA treatment did not cause tumors in other organs and no urinary bladder tumors or preneoplastic lesions were evident in the 0 and 12.5 p.p.m.-treated groups. Urinary levels of arsenicals increased significantly in a dose-responsive manner except for arsenobetaine (AsBe). DMA and trimethylarsine oxide (TMAO) were the major compounds detected in the urine, with small amounts of monomethylarsonic acid (MMA) and tetramethylarsonium (TeMa) also detected. Significantly increased 5-bromo-2'-deoxyuridine (BrdU) labeling indices were observed in the morphologically normal epithelium of the groups treated with 50 and 200 p.p.m. DMA. Mutation analysis showed that DMA-induced rat urinary bladder tumors had a low rate of H-ras mutations (2 of 20, 10%). No alterations of the p53, K-ras or beta-catenin genes were detected. Only one TCC (6%) demonstrated nuclear accumulation of p53 protein by immunohistochemistry. In 16 of 18 (89%) of the TTCs and 3 of 4 (75%) of the papillomas, decreased p27(kip1) expression could be demonstrated. Cyclin D1 overexpression was observed in 26 of 47 (55%) PN hyperplasias, 3 of 4 (75%) papillomas, and 10 of 18 (56%) TCCs. As a molecular marker of oxidative stress, increased COX-2 expression was noted in 17 of 18 (94%) TCCs, 4 of 4 (100%) papillomas, and 39 of 47 (83%) PN hyperplasias. In experiment 2, 8-OHdG formation in urinary bladder was significantly increased after treatment with 200 p.p.m. DMA in the drinking water for 2 weeks compared with the controls. The studies demonstrated DMA to be a carcinogen for the rat urinary bladder and suggested that DMA exposure may be relevant to the carcinogenic risk of inorganic arsenic in humans. Diverse genetic alterations observed in DMA-induced urinary bladder tumors imply that multiple genes are involved in stages of DMA-induced tumor development. Furthermore, generation of ROS is likely to play an important role in the early stages of DMA carcinogenesis.

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.

Arsenic toxicity and potential mechanisms of action

Exposure to the metalloid arsenic is a daily occurrence because of its environmental pervasiveness. Arsenic, which is found in several different chemical forms and oxidation states, causes acute and chronic adverse health effects, including cancer. The metabolism of arsenic has an important role in its toxicity. The metabolism involves reduction to a trivalent state and oxidative methylation to a pentavalent state. The trivalent arsenicals, including those methylated, have more potent toxic properties than the pentavalent arsenicals. The exact mechanism of the action of arsenic is not known, but several hypotheses have been proposed. At a biochemical level, inorganic arsenic in the pentavalent state may replace phosphate in several reactions. In the trivalent state, inorganic and organic (methylated) arsenic may react with critical thiols in proteins and inhibit their activity. Regarding cancer, potential mechanisms include genotoxicity, altered DNA methylation, oxidative stress, altered cell proliferation, co-carcinogenesis, and tumor promotion. A better understanding of the mechanism(s) of action of arsenic will make a more confident determination of the risks associated with exposure to this chemical.

Arsenic inhibits the repair of DNA damage induced by benzo(a)pyrene

In order to study the effect of arsenic on DNA damage, Sprague-Dawley rats were dosed with sodium arsenite (10 mg/kg) with or without 800 microg of benzo(a)pyrene (BP) by intramammilary injection. The animals were sacrificed on day 1, 3, 5, 10 and 27 and the mammary gland tissues were collected for DNA adduct measurement using a (32)P post-labeling assay. Animals dosed with arsenic alone did not show any DNA adducts. DNA adduct levels in rats dosed with BP alone reached a maximum level by day 5, reducing to 13% of this level by day 27. Adduct levels in rats dosed with arsenic and BP also reached a maximum by day 5 but only 80% of the level observed in the BP group. However, 84% of this amount still remained by day 27. The First Nucleotide Change (FNC) technique was used for the screening of 115 samples of various tissues from mice that had been chronically exposed to sodium arsenate for over 2 years revealed that inorganic arsenic did not attack the two putative hotspots (codons 131 and 154) of the hOGG1 gene. These results support the hypothesis that arsenic exerts its biological activity through DNA repair inhibition.

In vivo genotoxicity evaluation of dimethylarsinic acid in MutaMouse

Dimethylarsinic acid (DMA) induces DNA damage in the lung by formation of various peroxyl radical species. The present study was conducted to evaluate whether arsenite or its metabolite, DMA, could initiate carcinogenesis via mutagenic DNA lesions in vivo that can be attributed to oxidative damage. A transgenic mouse model, MutaMouse, was used in this study and mutations in the lacZ transgene and in the endogenous cII gene were assessed. When DMA was intraperitoneally injected into MutaMice at a dose of 10.6 mg/kg per day for 5 consecutive days, it caused only a weak increase in the mutant frequency (MF) of the lacZ gene in the lung, which was at most 1.3-fold higher than in the untreated control animals. DMA did not appreciably raise the MF in the bladder or bone marrow. Further analysis of the cII gene in the lung, the organ in which DMA induced the DNA damage, revealed only a marginal increase in the MF. Following DMA administration, no change in the cII mutation spectra was observed, except for a slight increase in the G:C to T:A transversion. Administration of arsenic trioxide (arsenite) at a dose of 7.6 mg/kg per day did not result in any increase in the MF of the lacZ gene in the lung, kidney, bone marrow, or bladder. Micronucleus formation was also evaluated in peripheral blood reticulocytes (RETs). The assay for micronuclei gave marginally positive results with arsenite, but not with DMA. These results suggest that the mutagenicity of DMA and arsenite might be too low to be detected in the MutaMouse.

Induction of genotoxic damage is not correlated with the ability to methylate arsenite in vitro in the leukocytes of four mammalian species

Arsenic is a natural drinking water contaminant that impacts the health of large populations of people throughout the world; however, the mode or mechanism by which arsenic induces cancer is unclear. In a series of in vitro studies, we exposed leukocytes from humans, mice, rats, and guinea pigs to a range of sodium arsenite concentrations to determine whether the lymphocytes from these species showed differential sensitivity to the induction of micronuclei (MN) assessed in cytochalasin B-induced binucleate cells. We also determined the capacity of the leukocytes to methylate arsenic by measuring the production of MMA [monomethylarsinic acid (MMA(V)) and monomethylarsonous acid (MMA(III))] and DMA [dimethylarsinic acid (DMA(V)) and dimethylarsonous acid (DMA(III))]. The results indicate that cells treated for 2 hr at the G(0) stage of the cell cycle with sodium arsenite showed only very small to negligible increases in MN after mitogenic stimulation. Treatment of actively cycling cells produced induction of MN with increasing arsenite concentration, with the human, rat, and mouse lymphocytes being much more sensitive to MN induction than those of the guinea pig. These data gave an excellent fit to a linear model. The leukocytes of all four species, including the guinea pig (a species previously thought not to methylate arsenic), were able to methylate arsenic, but there was no clear correlation between the ability to methylate arsenic and the induction of MN.

The cellular metabolism and systemic toxicity of arsenic

Although it has been known for decades that humans and many other species convert inorganic arsenic to mono- and dimethylated metabolites, relatively little attention has been given to the biological effects of these methylated products. It has been widely held that inorganic arsenicals were the species that accounted for the toxic and carcinogenic effects of this metalloid and that methylation was properly regarded as a mechanism for detoxification of arsenic. Elucidation of the metabolic pathway for arsenic has changed our understanding of the significance of methylation. Both methylated and dimethylated arsenicals that contain arsenic in the trivalent oxidation state have been identified as intermediates in the metabolic pathway. These compounds have been detected in human cells cultured in the presence of inorganic arsenic and in urine of individuals who were chronically exposed to inorganic arsenic. Methylated and dimethylated arsenicals that contain arsenic in the trivalent oxidation state are more cytotoxic, more genotoxic, and more potent inhibitors of the activities of some enzymes than are inorganic arsenicals that contain arsenic in the trivalent oxidation state. Hence, it is reasonable to describe the methylation of arsenic as a pathway for its activation, not as a mode of detoxification. This review summarizes the current knowledge of the processes that control the formation and fate of the methylated metabolites of arsenic and of the biological effects of these compounds. Given the considerable interest in the dose-response relationships for arsenic as a toxin and a carcinogen, understanding the metabolism of arsenic may be critical to assessing the risk associated with chronic exposure to this element.

Oral exposure of dimethylarsinic acid, a main metabolite of inorganic arsenics, in mice leads to an increase in 8-Oxo-2'-deoxyguanosine level, specifically in the target organs for arsenic carcinogenesis

We have proposed that oral administration of dimethylarsinic acid (DMA), a metabolite of inorganic arsenics in mammals, rather than inorganic arsenics themselves, promotes lung and skin tumors by way of the metabolic production of free radicals such as dimethylarsenic peroxy radical [(CH(3))(2)AsOO*]. The purpose of the present study was to examine if dimethylarsenic has the ability to induce oxidative damage. 8-oxo-2'-deoxyguanosine (8-oxodG) was used as a biomarker of DNA oxidation. The oral administration of DMA enhanced significantly the amounts of 8-oxodG specifically in the target organs (skin, lung, liver, and urinary bladder) of arsenic carcinogenesis and also in urine, whereas arsenite did not. The dimethylarsenics thus may play an important role in arsenic carcinogenesis through the induction of oxidative damage, particularly of base oxidation.

Determination of trivalent methylated arsenicals in biological matrices

The enzymatically catalyzed oxidative methylation of As yields methylated arsenicals that contain pentavalent As (As(V)). Because trivalent As (As(III)) is the favored substrate for this methyltransferase, methylated arsenicals containing As(V) are reduced to trivalency in cells. Methylated arsenicals that contain As(III) are extremely potent inhibitors of NADPH-dependent flavoprotein oxidoreductases and potent cytotoxins in many cell types. Therefore, the formation of methylated arsenicals that contain As(III) may be properly regarded as an activation step, rather than a means of detoxification. Recognition of the role of methylated arsenicals that contain As(III) in the toxicity and metabolism of As emphasizes the need for analytical methods to detect and quantify these species in biological samples. Hence, a method was developed to exploit pH-dependent differences in the generation of arsines from inorganic and methylated arsenicals that contain either As(V) or As(III). Reduction with borohydride at pH 6 generated arsines from inorganic As(III), methyl As(III), and dimethyl As(III), but not from inorganic As(V), methyl As(V), and dimethyl As(V). Reduction with borohydride at pH 2 or lower generated arsines from arsenicals that contained either As(V) or As(III). Arsines are trapped in a liquid nitrogen-cooled gas chromatographic trap, which is subsequently warmed to allow separation of the hydrides by their boiling points. Atomic absorption spectrophotometry is used to detect and quantify the arsines. The detection limits (ng As ml(-1)) for inorganic As(III), methyl As(III), and dimethyl As(III) are 1.1, 1.2, and 6.5, respectively. This method has been applied to the analysis of arsenicals in water, human urine, and cultured cells. Both methyl As(III) and dimethyl As(III) are detected in urine samples from individuals who chronically consumed inorganic As-contaminated water and in human cells exposed in vitro to inorganic As(III). The reliable quantitation of inorganic and methylated arsenicals that contain As(III) in biological samples will aid the study of the toxicity of these species and may provide a new biomarker of the effects of chronic exposure to As.

Genetic toxicology of a paradoxical human carcinogen, arsenic: a review

Arsenic is widely distributed in nature in air, water and soil in the form of either metalloids or chemical compounds. It is used commercially, as pesticide, wood preservative, in the manufacture of glass, paper and semiconductors. Epidemiological and clinical studies indicate that arsenic is a paradoxical human carcinogen that does not easily induce cancer in animal models. It is one of the toxic compounds known in the environment. Intermittent incidents of arsenic contamination in ground water have been reported from several parts of the world. Arsenic containing drinking water has been associated with a variety of skin and internal organ cancers. The wide human exposure to this compound through drinking water throughout the world causes great concern for human health. In the present review, we have attempted to evaluate and update the mutagenic and genotoxic effects of arsenic and its compounds based on available literature.

Recent advances in arsenic carcinogenesis: modes of action, animal model systems, and methylated arsenic metabolites

Recent advances in our knowledge of arsenic carcinogenesis include the development of rat or mouse models for all human organs in which inorganic arsenic is known to cause cancer-skin, lung, urinary bladder, liver, and kidney. Tumors can be produced from either promotion of carcinogenesis protocols (mouse skin and lungs, rat bladder, kidney, liver, and thyroid) or from complete carcinogenesis protocols (rat bladder and mouse lung). Experiments with p53(+/-) and K6/ODC transgenic mice administered dimethylarsinic acid or arsenite have shown some degree of carcinogenic, cocarcinogenic, or promotional activity in skin or bladder. At present, with the possible exception of skin, the arsenic carcinogenesis models in wild-type animals are more highly developed than in transgenic mice. Recent advances in arsenic metabolism have suggested that methylation of inorganic arsenic may be a toxification, rather than a detoxification, pathway and that trivalent methylated arsenic metabolites, particularly monomethylarsonous acid and dimethylarsinous acid, have a great deal of biological activity. Accumulating evidence 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. In risk assessment of environmental arsenic, it is important to know and to utilize both the mode of carcinogenic action and the shape of the dose-response curve at low environmental arsenic concentrations. Although much progress has been recently made in the area of arsenic's possible mode(s) of carcinogenic action, a scientific concensus has not yet been reached. In this review, nine different possible modes of action of arsenic carcinogenesis are presented and discussed-induced chromosomal abnormalities, oxidative stress, altered DNA repair, altered DNA methylation patterns, altered growth factors, enhanced cell proliferation, promotion/progression, gene amplification, and suppression of p53.

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.

Dimethylarsinic acid induces 8-hydroxy-2'-deoxyguanosine formation in the kidney of NCI-Black-Reiter rats

Dirnethylarsenic peroxyl radical [(CH(3))(2)AsOO] has been postulated to be responsible for DNA damage induced by dimethylarsinic acid (DMA). In an effort to elucidate the possible mechanism of tumor-inducing potential of DMA, an experiment was designed to investigate the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a specific marker of oxidative base damage in the kidney tissues of NCI-Black Reiter (NBR) rats. Animals were divided into four groups and administered the vehicle - saline, 5, 10 and 20 mg/kg body weight respectively of DMA by gavage, once a day, 5 days a week, for a period of 4 weeks. DMA induced increase of 8-OHdG levels in the kidney of the rats treated, with the highest level at the dose of 10 mg/kg body weight. Analysis of the kidney for cell proliferation employing PCNA-positive index showed greater proliferation in the tissues of treated rats. However, DMA did not have any influence on apoptosis in this regimen. Histopathological examination of the kidney selections revealed the presence of vacuolated degeneration and dilation of the proximal tubule cells in two groups (10 and 20 mg/kg body weight). This study provides evidence to substantiate the role of DMA in inducing oxidative DNA damage in the kidney.

Methylated trivalent arsenic species are genotoxic

The reactivities of methyloxoarsine (MAs(III)) and iododimethylarsine (DMAs(III)), two methylated trivalent arsenicals, toward supercoiled phiX174 RFI DNA were assessed using a DNA nicking assay. The induction of DNA damage by these compounds in vitro in human peripheral lymphocytes was assessed using a single-cell gel (SCG, "comet") assay. Both methylated trivalent arsenicals were able to nick and/or completely degrade phiX174 DNA in vitro in 2 h incubations at 37 degrees C (pH 7.4) depending on concentration. MAs(III) was effective at nicking phiX174 DNA at 30 mM; however, at 150 microM DMAs(III), nicking could be observed. Exposure of phiX174 DNA to sodium arsenite (iAs(III); from 1 nM up to 300 mM), sodium arsenate (from 1 microM to 1 M), and the pentavalent arsenicals, monomethylarsonic acid (from 1 microM to 3 M) and dimethylarsinic acid (from 0.1 to 300 mM), did not nick or degrade phiX174 DNA under these conditions. In the SCG assay in human lymphocytes, methylated trivalent arsenicals were much more potent than any other arsenicals that were tested. On the basis of the slopes of the concentration-response curve for the tail moment in the SCG assay, MAs(III) and DMAs(III) were 77 and 386 times more potent than iAs(III), respectively. Because methylated trivalent arsenicals were the only arsenic compounds that were observed to damage naked DNA and required no exogenously added enzymatic or chemical activation systems, they are considered here to be direct-acting forms of arsenic that are genotoxic, though they are not, necessarily, the only genotoxic species of arsenic that could exist.

A concise review of the toxicity and carcinogenicity of dimethylarsinic acid

Dimethylarsinic acid (DMA) has been used as a herbicide (cacodylic acid) and is the major metabolite formed after exposure to tri- (arsenite) or pentavalent (arsenate) inorganic arsenic (iAs) via ingestion or inhalation in both humans and rodents. Once viewed simply as a detoxification product of iAs, evidence has accumulated in recent years indicating that DMA itself has unique toxic properties. DMA induces an organ-specific lesion--single strand breaks in DNA--in the lungs of both mice and rats and in human lung cells in vitro. Mechanistic studies have suggested that this damage is due mainly to the peroxyl radical of DMA and production of active oxygen species by pulmonary tissues. Multi-organ initiation-promotion studies have demonstrated that DMA acts as a promotor of urinary bladder, kidney, liver and thyroid gland cancers in rats and as a promotor of lung tumors in mice. Lifetime exposure to DMA in diet or drinking water also causes a dose-dependent increase in urinary bladder tumors in rats, indicating that DMA is a complete carcinogen. These data collectively suggest that DMA plays a role in the carcinogenesis of inorganic arsenic.

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.

Hepatic damage caused by chronic arsenic toxicity in experimental animals

OBJECTIVE

Noncirrhotic fibrosis of the liver is common in subjects chronically consuming ground water geologically contaminated with arsenic, but the mechanism of the hepatic fibrosis is not known. Because lipid peroxidation has been implicated in the development of several other forms of hepatic fibrosis, including iron and copper overload, we have explored the roles of oxidative stress and lipid peroxidation in the causation of hepatic fibrosis in a murine model of chronic arsenic toxicity.

METHODS

Male BALB/c mice were given drinking water contaminated with arsenic (3.2 mg/L) or arsenic-free (<0.01 mg/L, control) ad libitum. Mice were sacrificed at 3, 6, 9, 12, and 15 months for examination of hepatic histology and assays of hepatic reduced glutathione content, lipid peroxidation, enzymes of the antioxidant defense system, and membrane-bound sodium/potassium ATPase (Na+/K+ ATPase).

RESULTS

After 12 months of arsenic feeding, the liver weights increased significantly as did serum aspartate aminotransferase and alanine aminotransferase. After 6 months of arsenic feeding, hepatic glutathione and the enzymes glucose-6-phosphate dehydrogenase and glutathione peroxidase were significantly lower than those of the control group. Hepatic catalase activity was significantly reduced at 9 months in the arsenic-fed group, while glutathione-S-transferase and glutathione reductase activities were also significantly reduced at 12 and 15 months. Plasma membrane Na+/K+ ATPase activity was reduced after 6 months while lipid peroxidation increased significantly after 6 months of arsenic feeding. Liver histology remained normal for the first 9 months, but showed fatty infiltration after 12 months of arsenic feeding. Histologic evidence of fibrosis was observed after 15 months.

CONCLUSION

We have demonstrated hepatic fibrosis due to long-term arsenic toxicity in an animal model. Initial biochemical evidence of hepatic membrane damage, probably due to reduction of glutathione and antioxidant enzymes, may be seen by 6 months. Continued arsenic feeding resulted in fatty liver with serum aminotransferase and alanine aminotransferase elevated at 12 months and hepatic fibrosis at 15 months. The murine model is proposed as relevant to epidemic human toxicity in areas of arsenic contamination.

Differential response of cellular antioxidant mechanism of liver and kidney to arsenic exposure and its relation to dietary protein deficiency

The effect on antioxidant defense system of liver and kidney of sub-acute i.p. exposure to sodium arsenite (3.33 mg/kg b.w. per day) for 14 days was studied in male Wistar rats fed on an adequate (18%) or a low (6%) protein diet. Following arsenic treatment, liver showed significantly enhanced concentration of glutathione and increased activities of glutathione reductase and glutathione-S-transferase on either of the dietary protein levels. Liver glutathione peroxidase and glucose-6-phosphate dehydrogenase activities increased significantly on an adequate protein diet while glutathione peroxidase activity decreased significantly on a low-protein diet. Lipid peroxidation and superoxide dismutase activity of liver remained unaltered on either of the dietary protein levels. On the other hand, kidney of arsenic-treated rats receiving either of the dietary protein levels showed significantly increased lipid peroxidation and decreased superoxide dismutase and catalase activities. Kidney glutathione content and glutathione reductase activity remained unaltered while glutathione peroxidase activity increased and glutathione-S-transferase activity decreased significantly on a low-protein diet following exposure to arsenic. On an adequate protein diet glucose-6-phosphate dehydrogenase activity in kidney, however, became significantly elevated following arsenic treatment. In Wistar rats, after 14 days of treatment with 3.33 mg As/kg b.w. i.p. the kidney seemed to be more sensitive to arsenic, and liver appears to be protected more by some of the antioxidant components, such as, glutathione, glutathione-S-transferase and glucose-6-phosphate dehydrogenase. It appears that low-protein diet influences the response of some of the cellular protective components against arsenic insult but does not lead to unique findings.

The role of orally administered dimethylarsinic acid, a main metabolite of inorganic arsenics, in the promotion and progression of UVB-induced skin tumorigenesis in hairless mice

The effect of dimethylarsinic acid (DMA) on skin tumorigenesis by UVB irradiation was examined. Hairless mice (Hos: HR-1) irradiated with UVB at a dose of 2 kJ/m(2) twice weekly, were fed with drinking water containing 1000 ppm DMA, a main metabolite of inorganic arsenics, produced more skin tumors than DMA-untreated mice. Histopathological examination revealed that the mouse malignant tumors with severe atypism appeared only in the treatment group of UVB plus 1000 ppm DMA. These positive results point out the importance of dimethylated metabolites of inorganic arsenic in the process of skin carcinogenesis.

Dose-dependent effects on tissue distribution and metabolism of dimethylarsinic acid in the mouse after intravenous administration

Most mammals methylate inorganic arsenic to dimethylarsinic acid (DMA). This organic arsenical causes organ-specific toxicity and is a multi-organ tumor promoter. The objective of this study was to examine whether dose could affect the distribution and metabolism of DMA. Female B6C3F1 mice (3-4/time point) were administered 1.11 or 111 mg/kg of DMA (1 microCi of [14C] or unlabeled) intravenously and killed serially (5-480 min). Blood was separated into plasma and red blood cell fractions and liver, kidney and lung were removed, weighed and homogenized. Tissue samples were oxidized and analyzed for DMA-derived radioactivity. Blood and several organs of the non-radioactive DMA-treated animals were digested in acid and analyzed by hydride generation atomic absorption spectrophotometry for DMA and metabolites. Concentration-time profiles showed a biexponential decrease of DMA-derived radioactivity in all tissues examined. Kidney had the highest concentration (1-20% dose/gm) of radioactivity of all tissues up to 60 min post-administration. Concentration of radioactivity was greater in plasma than red blood cells at 5 and 15 min and then was similar for the remaining time points. A dose-dependent effect on the concentration of radioactivity was observed in the lung. The retention of radioactivity in the lung was altered compared with liver and kidney, with a much longer t1/2beta and a disproportionate increase in area under the curve with increased dose. No methylated or demethylated products of DMA were detected in blood or any organ up to 8 h post-exposure. The dose-dependent distribution of DMA in the lung may have a role in the toxic effects DMA elicits in this organ.

The role of oxidative DNA damage in human arsenic carcinogenesis: detection of 8-hydroxy-2'-deoxyguanosine in arsenic-related Bowen's disease

Arsenic is widely distributed in nature in the form of either metalloids or chemical compounds, which cause a variety of pathologic conditions including cutaneous and visceral malignancies. Recently, reactive oxygen species have been hypothesized to be one of the causes of arsenic-induced carcinogenesis. 8-Hydroxy-2'-deoxyguanosine is one of the major reactive oxygen species-induced DNA base-modified products that is widely accepted as a sensitive marker of oxidative DNA damage. We studied the presence of 8-hydroxy-2'-deoxyguanosine by immunohistochemistry using N45.1 monoclonal antibody in 28 cases of arsenic-related skin neoplasms and arsenic keratosis as well as in 11 cases of arsenic-unrelated Bowen's diseases. The frequency of 8-hydroxy-2'-deoxyguanosine positive cases was significantly higher in arsenic-related skin neoplasms (22 of 28; 78%) than in arsenic-unrelated Bowen's disease (one of 11; 9%) (p < 0.001 by chi2 test). 8-Hydroxy-2'-deoxyguanosine was also detected in normal tissue adjacent to the arsenic-related Bowen's disease lesions. Furthermore, arsenic was detected by neutron activation analysis in the deparaffined skin tumor samples of arsenic-related disease (four of five; 80%), whereas arsenic was not detected in control samples. Our results strongly suggest the involvement of reactive oxygen species in arsenic-induced human skin cancer. Key word: neutron activation analysis.

Dimethylarsinic acid, a main metabolite of inorganic arsenics, has tumorigenicity and progression effects in the pulmonary tumors of A/J mice

The pulmonary tumorigenicity of dimethylarsinic acid (DMAA), a main metabolite of inorganic arsenics, was examined in A/J mice fed with drinking water containing DMAA for 25 and 50 weeks. Mice fed with 400 ppm DMAA for 50 weeks produced more pulmonary tumors than untreated mice (mean number per animal 1.36 versus 0.50; P < 0.05). Histological examination revealed that the number of mice which bore adenocarcinomas or papillary adenomas correlated with the concentration of DMAA given (untreated versus 400 ppm; P = 0.002), suggesting that DMAA could promote tumorigenic processes. These results are consistent with the epidemiological studies on the pulmonary carcinogenesis of arsenics and suggest that DMAA alone can act as a carcinogen in mice.

Promotion of rat hepatocarcinogenesis by dimethylarsinic acid: association with elevated ornithine decarboxylase activity and formation of 8-hydroxydeoxyguanosine in the liver

Arsenicals are epidemiologically significant chemicals in relation to induction of liver cancer in man. In the present study, we investigated the dose-dependent promotion potential of dimethylarsinic acid (DMAA), a major metabolite of inorganic arsenicals in mammals, in a rat liver carcinogenesis model. In experiment 1, glutathione-S-transferase placental form (GST-P)-positive foci, putative preneoplastic lesions, were employed as endpoints of a liver medium-term bioassay for carcinogens (Ito test). Starting 2 weeks after initiation with diethylnitrosamine, male F344 rats were treated with 0, 25, 50 or 100 ppm of DMAA in the drinking water for 6 weeks. All animals underwent two-thirds partial hepatectomy at week 3 after initiation. Examination of liver sections after termination at 8 weeks revealed dose-dependent increases in the numbers and areas of GST-P-positive foci in DMAA-treated rats as compared with controls. In experiment 2, ornithine decarboxylase activity, which is a biomarker of cell proliferation, was found to be significantly increased in the livers of rats treated with DMAA. In experiment 3, formation of 8-hydroxydeoxyguanosine, which is a marker of oxygen radical-mediated DNA damage, was significantly increased after administration of DMAA. These results indicate that DMAA has the potential to promote rat liver carcinogenesis, possibly via a mechanism involving stimulation of cell proliferation and DNA damage caused by oxygen radicals.

Metabolic methylation is a possible genotoxicity-enhancing process of inorganic arsenics

To elucidate if the metabolic methylation participates in the induction of inorganic arsenic-responsible genetic damage, arsenite (ARS) and its methylated metabolites, methanearsonic acid (MMAA) and dimethylarsinic acid (DMAA), were comparatively assayed for the induction of DNA damage by determining DNA repair synthesis using polymerization inhibitors such as aphidicolin (aph) and hydroxyurea (HU). When human alveolar epithelial type II (L-132) cells in culture were exposed to either one of these three arsenic compounds, DNA single-strand breaks resulting from the inhibition of repair polymerization were remarkably produced by exposure to DMAA at 5 to 100 microM, while not by that to ARS and MMAA even at 100 microM. Furthermore, a bromodeoxyuridine (BrdrU)-photolysis assay indicated that the induction of DNA repair synthesis was observed only in the case of exposure to DMAA. When L-132 cells were exposed to 100 microM MMAA in the presence of 10 mM S-adenosyl-L-methionine (SAM), which is a well-known methyl-group donor in metabolic methylation of arsenics, DNA repair synthesis was induced along with an increase in the amount of dimethylarsenic in the cells. These results indicate that metabolic methylation of inorganic arsenics to dimethylarsenics is predominantly involved in the induction of DNA damage.

Enzymatic methylation of arsenic compounds: IV. In vitro and in vivo deficiency of the methylation of arsenite and monomethylarsonic acid in the guinea pig

Using an in vitro assay which measures the transfer of a radiolabeled methyl moiety of S-[methyl-3H]adenosylmethionine ([3H]SAM) to arsenite or monomethylarsonate (MMA) to yield [methyl-3H]MMA or [methyl-3H]dimethylarsinate (DMA) respectively, guinea pig liver cytosol was found to be deficient in the enzyme activities which methylate these substrates. Moreover, when guinea pigs were given a single intraperitoneal dose of [73As]arsenate (400 micrograms/kg body weight, 25 microCi/kg body weight), very little or no methylated arsenic species were detected in the urine after cation exchange chromatography. The urine collected 0-12 h after arsenate injection contained 98% inorganic arsenic and less than 1% DMA. No MMA was detected in the 0-12 h urine. Urine collected 12-24 h after injection contained approximately 93% inorganic arsenic, 2% MMA and 3% DMA in five of the six animals studied. However, in the 12-24 h urine of one guinea pig, 17% of the radioactivity was DMA, 80% was inorganic arsenic and 3% was MMA. The guinea pig, like the marmoset and tamarin monkeys and unlike most other animals studied thus far, appears to be deficient as far as the enzyme activities that methylate inorganic arsenite. The results of these experiments suggest that there may be a genetic polymorphism associated with the enzymes that methylate inorganic arsenite.

Peroxy radical oxidation of thymidine

The peroxy radical (ROO) is unique among reactive oxygen species implicated in the production of DNA damage in that it possesses an extremely long half-life (order of seconds) and is predicted to have a relatively greater chemical selectivity in its reactions relative to other radical intermediates. Yet no product studies of the reactions of ROO with bases, nucleosides, or DNA have appeared, and thus no meaningful predictions can be made regarding its potential involvement in the production of DNA base damage and the mutagenic process. We report here on the reaction products formed by peroxy radical with thymidine, major target of oxidative base damage. ROO reacts with thymine to yield predominantly 5-Me oxidation products. The highly mutagenic 5-(hydroperoxymethyl)-2'-deoxyuridine, 5-formyl-2'-deoxyuridine, and 5-(hydroxymethyl)-2'-deoxyuridine are produced by peroxy radical oxidation. In contrast, 5Me oxidation products are minor products of thymidine oxidation by OH, which yields predominantly saturated derivatives via addition to the 5,6 double bound. A plausible mechanistic scheme for the formation of the base oxidation products of thymidine by peroxy radicals is presented. Attach at the deoxyribose moiety resulting in oxidative depyrimidination is also found to occur, as indicated by free base release. Phosphodiester backbone cleavage resulting in single and double strand breaks is also catalyzed by peroxy radical, as demonstrated using a plasmid nicking assay.

Enzymatic methylation of arsenic species and other new approaches to arsenic toxicity

Arsenic metabolism has typically been studied by administering arsenate or arsenite into animals and humans and then studying the metabolites excreted in the urine. Although such studies have yielded information about the beginning and the end of the metabolic pathways for the metabolism of inorganic arsenic compounds, any statements as to the molecular mechanisms of these reactions have had to be highly speculative. Now that the rabbit and the rhesus monkey liver enzymes that transfer methyl groups from S-adenosylmethionine to arsenite and monomethlyarsonic acid have been purified and the reactions characterized, meaningful investigations of species diversity and polymorphism of these enzymes have become possible. New World animals studied thus far appear to be deficient in or totally lacking these enzymes. Old World animals, with the exception of the chimpanzee, have ample amounts of arsenite and monomethylarsonic acid methyltransferases. A hypothesis that the lack of arsenite methyltransferases may have had an evolutionary advantage for certain species is proposed.