Crosslink formation between DNA and nuclear proteins by in vivo and in vitro exposure of cells to dimethylarsinic acid

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.

Dimethylarsinic Acid in Drinking Water Changed the Morphology of Urinary Bladder but Not the Expression of DNA Repair Genes of Bladder Transitional Epithelium in F344 Rats

Inorganic arsenic increases urinary bladder transitional cell carcinoma in humans. In F344 rats, dimethylarsinic acid (DMA[V]) increases transitional cell carcinoma. Arsenic-induced inhibition of DNA repair has been reported in cultured cell lines and in lymphocytes of arsenic-exposed humans, but it has not been studied in urinary bladder. Should inhibition of DNA damage repair in transitional epithelium occur, it may contribute to carcinogenesis or cocarcinogenesis. We investigated morphology and expression of DNA repair genes in F344 rat transitional cells following up to 100 ppm DMA(V) in drinking water for four weeks. Mitochondria were very sensitive to DMA(V), and swollen mitochondria appeared to be the main source of vacuoles in the transitional epithelium. Real-time reverse transcriptase polymerase chain reaction (Real-Time RT PCR) showed the mRNA levels of tested DNA repair genes, ataxia telangectasia mutant (ATM), X-ray repair cross-complementing group 1 (XRCC1), excision repair cross-complementing group 3/xeroderma pigmentosum B (ERCC3/XPB), and DNA polymerase β (Polβ), were not altered by DMA(V). These data suggested that either DMA(V) does not affect DNA repair in the bladder or DMA(V) affects DNA repair without affecting baseline mRNA levels of repair genes. The possibility remains that DMA(V) may lower damage-induced increases in repair gene expression or cause post-translational modification of repair enzymes.

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.

Dimethylarsinic acid: Results of chronic toxicity/oncogenicity studies in F344 rats and in B6C3F1 mice

Dimethylarsinic acid (DMA(V), cacodylic acid), a foliar herbicide, was administered in the diet to B6C3F1 mice (at dose levels of 0, 8, 40, 200, and 500 ppm) and to F344 rats (at dose levels of 0, 2, 10, 40, and 100 ppm) for 2 years, according to US EPA guidelines. In mice, there were no treatment-related tumors observed at any site. Treatment-related progressive glomerulonephropathy and nephrocalcinosis were observed in the kidneys in both sexes. The incidence of vacuolation of the epithelium in the urinary bladder was increased in both sexes, but was not associated with cytotoxicity, necrosis or hyperplasia. Based on non-neoplastic lesions found in the urinary bladder, the NOEL for mice was assessed to be 40 ppm in males and 8 ppm in females. In rats, treatment-related mortality occurred early in the study in five males in the 100 ppm group and in one male in the 40 ppm group. Papillomas and carcinomas with degeneration of the urothelium, necrosis and urothelial cell hyperplasia, were found in the urinary bladders of both sexes. In male rats, one papilloma was found in each of the 10 and 40 ppm groups; one urothelial cell carcinoma was found in the 2 ppm group and two in the 100 ppm group. Four papillomas and six urothelial cell carcinomas were found in the female 100 ppm group. Non-neoplastic treatment-related kidney lesions were confined to the 40 and 100 ppm levels and included necrosis, pyelonephritis, medullary nephrocalcinosis and tubular cystic dilation, hyperplasia of the epithelial lining of the papilla, and pelvic urothelial cell hyperplasia. All of these kidney changes appear to be related to an increase in the aging nephropathy of the rat. Dose-related increases in the height of the thyroid follicular epithelium were also noted in males and females, however, such changes reflect an adaptive response of the thyroid to decreased levels of circulating thyroid hormone, rather than an adverse effect. Based on the kidney and bladder lesions, the NOEL for non-neoplastic and neoplastic lesions was considered to be 10 ppm in males and females. Based on these studies, DMA(V) is carcinogenic only in rats and only at relatively high doses, with the urinary bladder as the target organ. Female rats appear to be more sensitive to the effects of DMA(V) than male rats. DMA(V) is not carcinogenic in mice.

Chromatin immunoprecipitation assay

Association between proteins and DNA is crucial for many vital cellular functions such as gene transcription, DNA replication and recombination, repair, segregation, chromosomal stability, cell cycle progression, and epigenetic silencing. It is important to know the genomic targets of DNA-binding proteins and the mechanisms by which they control and guide gene regulation pathways and cellular proliferation. Chromatin immunoprecipitation (ChIP) is an important technique in the study of protein-gene interactions. Using ChIP, DNA-protein interactions are studied within the context of the cell. The basic steps in this technique are fixation, sonication, immunoprecipitation, and analysis of the immunoprecipitated DNA. Although ChIP is a very versatile tool, the procedure requires the optimization of reaction conditions. Several modifications to the original ChIP technique have been published to improve the success and to enhance the utility of this procedure. This review addresses the critical parameters and the variants of ChIP as well as the different analytical tools that can be combined with ChIP to enable better understanding of DNA-protein interactions in vivo.

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.

Pattern of excretion of arsenic compounds [arsenite, arsenate, MMA(V), DMA(V)] in urine of children compared to adults from an arsenic exposed area in Bangladesh

Urinary arsenic is generally considered as the most reliable indicator of recent exposure to inorganic arsenic and is used as the main bio-marker of exposure. However, due to the different toxicity of arsenic compounds, speciation of arsenic in urine is generally considered to be more convenient for health risk assessment than measuring total arsenic concentration. Additionally, it can give valuable information about the metabolism of arsenic species within the body. In our study, for exposed group--42 urine samples were collected from Datterhat (South) village of Madaripur district, Bangladesh and an average arsenic concentration in their drinking water was 376 microg/L (range 118 to 620 microg/L). For control group, 27 urine samples were collected from a non-affected district, Badhadamil village of Medinipur district, West Bengal, India, where arsenic concentration in their drinking water is below 3 microg/L. The arsenic species in the urine were separated and quantified by using HPLC-ICP-MS. The sum of inorganic arsenic and its metabolites was also determined by FI-HG-AAS. Results indicate that average total urinary arsenic metabolites in children's urine is higher than adults and total arsenic excretion per kg body weight is also higher for children than adults. For arsenic species between adults and children, it has been observed that inorganic arsenic (In-As) in average is 2.36% and MMA is 6.55% lower for children than adults while DMA is 8.91% (average) higher in children than adults. The efficiency of the methylation process is also assessed by the ratio between urinary concentration of putative product and putative substrate of the arsenic metabolic pathway. Higher values mean higher methylation capacity. Results show the values of the MMA/In-As ratio for adults and children are 0.93 and 0.74 respectively. These results indicate that first reaction of the metabolic pathway is more active in adults than children. But a significant increase in the values of the DMA/MMA ratio in children than adults of exposed group (8.15 vs. 4.11 respectively) indicates 2nd methylation step is more active in children than adults. It has also been shown that the distribution of the values of DMA/MMA ratio to exposed group decrease with increasing age (2nd methylation process). Thus from these results we may infer that children retain less arsenic in their body than adults. This may also explain why children do not show skin lesions compared to adults when both are drinking same contaminated water.

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.

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.

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.

Genotoxicity of arsenical compounds

With respect to global human health hazard, arsenic (As) is one of the most important environmental single substance toxicants. Currently, millions of people all over the world are exposed to the ubiquitous element in exposure levels leading to long-term toxicity, in particular cancer. Unfortunately, it has not been elucidated up to now how As mechanistically leads to the induction of neoplasia. Besides its tumorigenic potential, As has been shown to be genotoxic in a wide variety of different experimental set-ups and biological endpoints. In vitro, the element was shown to induce chromosomal mutagenicity like micronuclei, chromosome aberrations, and sister chromatid exchanges. It mainly acts clastogenic but also has an aneugenic potential. Instead, its potential to induce point mutations is very low in bacterial as well as in mammalian cell systems. However, in combined exposure with point mutagens in vitro, As was shown to enhance the frequency of chemical mutations in a synergistic manner. Additionally, As was shown to induce chromosome aberrations and micronuclei in vivo in experiments with mice. After long-term exposure to As-contaminated drinking water, the great majority of human biomonitoring studies found elevated frequencies of DNA lesions like micronuclei or chromosome aberrations. Respective occupational studies are few. Like it is the case for As carcinogenicity, it is not known through which mechanism the genotoxicity of As is mediated, although the data available indicate that As may act indirectly on DNA, i.e. via mechanisms like interference of regulation of DNA repair or integrity. Because of the indirect mode of action, it has been discussed as well that As's genotoxicity may underlie a sublinear dose-response relationship. However, various problems like non-standardized test systems and experimental variability make it impossible to prove such statement. Basically, to be able to improve risk assessment, it is of crucial importance to scientifically approach the mechanistic way of induction of As's genotoxicity and carcinogenicity.

Pharmacokinetics, metabolism, and carcinogenicity of arsenic

The carcinogenicity of arsenic in humans has been unambiguously demonstrated in a variety of epidemiological studies encompassing geographically diverse study populations and multiple exposure scenarios. Despite the abundance of human data, our knowledge of the mechanism(s) responsible for the carcinogenic effects of arsenic remains incomplete. A deeper understanding of these mechanisms is highly dependent on the development of appropriate experimental models, both in vitro and in vivo, for future mechanistic investigations. Suitable in vitro models would facilitate further investigation of the critical chemical species (arsenate/arsenite/MMA/DMA) involved in the carcinogenic process, as well as the evaluation of the generation and role of ROS. Mechanisms underlying the clastogenic effects of arsenic, its role in modulating DNA methylation, and the phenomenon of inducible tolerance could all be more completely investigated using in vitro models. The mechanisms involved in arsenic's inhibition of ubiquitin-mediated proteolysis demand further attention, particularly with respect to its effects on cell proliferation and DNA repair. Exploration of the mechanisms responsible for the protective or anticarcinogenic effects of arsenic could also enhance our understanding of the cellular and molecular interactions that influence its carcinogenicity. In addition, appropriate in vivo models must be developed that consider the action of arsenic as a promoter and/or progressor. In vivo models that allow further investigation of the comutagenic effects of arsenic are also especially necessary. Such models may employ initiation-promotion-progression bioassays or transgenic animals. Both in vitro and in vivo models have the potential to greatly enhance our current understanding of the cellular and molecular interactions of arsenic and its metabolites in target tissues. However, refinement of our knowledge of the mechanistic aspects of arsenic carcinogenicity is not alone sufficient; an understanding of the pharmacokinetics and target tissue doses of the critical chemical species is essential. Additionally, a more thorough characterization of species differences in the tissue kinetics of arsenic and its methylated metabolites would facilitate the development of more accurate and relevant PBPK models. Improved models could be used to further investigate the existence of a methylation threshold for arsenic and its relevance to arsenic carcinogenicity in humans. The significance of alterations in relative tissue concentrations of SAM and SAH deserves further attention, particularly with respect to their role in modulating methyltransferases involved in arsenic metabolism and DNA methylation. The importance of genetic polymorphisms and nutrition in influencing methyltransferase activities must not be overlooked. In vivo models are necessary to evaluate these factors; transgenic or knockout models would be particularly useful in the investigation of methylation polymorphisms. Further evaluation of methylation polymorphisms in human populations is also warranted. Other in vivo models incorporating dietary manipulation could provide valuable insight into the role of nutrition in the carcinogenicity of arsenic. With more complete knowledge of the pharmacokinetics of arsenic metabolism and the mechanisms associated with its carcinogenic effects, development of more reliable risk assessment strategies are possible. Integration of data, both pharmacokinetic and mechanistic in nature, will lead to more accurate descriptions of the interactions that occur between the active chemical species and cellular constituents which lead to the development of cancer. This knowledge, in turn, will facilitate the development of more accurate and reliable risk assessment strategies for arsenic.

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 specificity of protein-DNA crosslinking by formaldehyde: in vitro and in drosophila embryos

Formaldehyde crosslinking has been widely used to study binding of specific proteins to DNA elements in intact cells. However, previous studies have not determined if this crosslinker preserves the bona fide pattern of DNA binding. Here we show that formaldehyde crosslinking of Drosophila embryos maps an interaction of the transcription factor Zeste to a known target element in the Ultrabithorax promoter. This data agrees broadly with previous mapping of the same Zeste binding sites by in vivo UV crosslinking, though the formaldehyde method does give a low, possibly artifactual signal on other DNA fragments that is not detected by the UV method. We also demonstrate, using an in vitro assay, that formaldehyde crosslinking accurately reflects the DNA binding specificities of both Zeste and a second transcription factor, Eve. The crosslinking reagent methylene blue is shown to preserve DNA binding specificity in vitro as well. Our results suggest that crosslinking by formaldehyde, and possibly also by methylene blue, provide an accurate guide to the interaction of proteins with their high affinity target sites in cells.

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

Adult female B6C3F1 mice were given 720 mg/kg of DMA by oral gavage at one of three times (2 h, 15 h, or at both 21 and 4 h) before sacrifice. Significant (P < 0.05) decreases in liver GSH and GSSG contents (15-37%) were observed. Some evidence of DMA-induced hepatic DNA damage (at the P < 0.10 level only) was observed. Pulmonary and hepatic ODC activities were reduced (19-59%) by DMA treatment. Overall, these biochemical studies show that mice are much less responsive to DMA than rats.

Promotion of NCI-Black-Reiter male rat bladder carcinogenesis by dimethylarsinic acid an organic arsenic compound

Dimethylarsinic acid (DMAA) is a major metabolite of inorganic arsenicals in mammals. In the present study, we investigated its promoting effects on urinary bladder carcinogenesis in NCI-Black-Reiter (NBR) rats, which lack alpha2u-globulin synthesizing ability. Male 9-14-week-old NBR rats were treated sequentially with 0.05% N-butyl-N-(4-hydroxybutyl)-nitrosamine (BBN) for 4 weeks and then given 100 ppm DMAA in their drinking water (group 1) for 32 weeks. Induction of preneoplastic lesions (papillary or nodular hyperplasia) in this DMAA-treated group was significantly increased as compared to the carcinogen alone control group (P < 0.01). The development of carcinomas was also enhanced and a significant increase in the 5-bromo-2'-deoxyuridine (BrdU) labeling index of the urinary bladder epithelial cells was observed for the DMAA treatment group. These results indicate that DMAA has promoting effects on urinary bladder carcinogenesis even in NBR rats, so its effects are not dependent on the presence of alpha2u-globulin.

Physiologically based models of metal kinetics

The issues confronting the modeler of metals kinetics are somewhat different from those with which the modeler of organic chemical behavior is faced. Particularly important features of metals kinetics include metal-protein binding and metal-metal interactions. Reduction, and for some metals oxidation, is frequently an intrinsic part of metal metabolism. Alkylation/dealkylation reactions may or may not render the metal less active, and the behavior of alkylated or dealkylated metabolites must often be included in a complete kinetic model. Despite these complexities, the kinetics of metals are as amenable to the techniques of physiologically based modeling as are the kinetics of organic chemicals. Like all models, those for metals kinetics have the potential to organize a variety of observations, sometimes including apparently inconsistent observations, into a coherent framework of behavior, to identify needs for more complete experimental information, and to assist the risk assessor in making judgments concerning dose-response relationships. Development of physiologically based models of the kinetic behavior of metals is in its very early stages. The kinetics of only four metals, arsenic, chromium, mercury, and lead, have been modeled with any degree of completeness. Of these, the lead model is the most fully realized at the present time. The chromium and mercury models are still in the process of development, and experimental data are being gathered to support further development and refinement of the arsenic model. We may expect to see continued progress made on these models and their practical applications, as well as the development of new models for other toxicologically significant metals such as cadmium, manganese, nickel, and aluminum.

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.

Arsenic and antimony: comparative approach on mechanistic toxicology

A chemico-toxicological similarity between arsenic and antimony exists and their toxicology is often seen. Indeed, both elements possess several common properties, e.g. they are clastogenic but not mutagenic in the trivalent state and they have a carcinogenic potential: trivalent arsenicals are known to be human carcinogens and antimony(III) oxide (by inhalation) has been shown to cause lung cancer in female rats. For years, arsenic has been known to be environmentally toxic. Elevated human exposure to this element, mostly caused by the intake of contaminated tap water, is associated with increased incidences of cancer at various sites. It is still not clear how arsenic compounds exert their genotoxic effect. It may be connected with an inhibition of DNA repair or the induction of oxidative stress. Little work has been done on the toxicology of antimony as it is less widely present in the environment. There is evidence that in mammals antimony, unlike arsenic, is not detoxified via methylation but it still remains unclear what mechanism is responsible for antimony's genotoxicity. In general, there is little information known about this element to accurately determine its impact on human health. Thus, the aim of this paper is to review current knowledge for future risk assessment and further scientific work.

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.

Effect of hepatic methyl donor status on urinary excretion and DNA damage in B6C3F1 mice treated with sodium arsenite

This study evaluated the effect of hepatic methyl donor status on the ability of sodium arsenite (2.5, 5.0 and 10.0 mg/kg) administered by gavage once or on four consecutive days to induce DNA damage in male B6C3F1 mice. Maintenance on a choline-deficient (CD) diet prior to treatment resulted in mice with hepatic methyl donor deficiency (HMDD) and altered arsenical metabolism, as demonstrated by a decreased total urinary excretion of inorganic and organic arsenicals. The alkaline (pH > 13) Single Cell Gel (SCG) assay was used to evaluate for the induction of DNA damage (single strand breaks, alkali labile sites, DNA crosslinking) in blood leukocytes, liver parenchymal cells, and cells sampled from bladder, lung, and skin, while the bone marrow erythrocyte micronucleus (MN) assay was used to assess for the induction of chromosomal damage in bone marrow cells. Treatment with sodium arsenite once or four times induced a significant decrease in DNA migration (indicative of DNA crosslinking) in bladder and liver parenchymal cells of hepatic methyl donor sufficient (HMDS) mice, but in skin cells of HMDD mice. Both HMDD and HMDS mice exhibited a significant increase in the frequency of micronucleated polychromatic erythrocytes (MN-PCE) in bone marrow following four, but not following one, treatments. However, the positive response occurred at a lower dose for HMDS mice and, in these mice, bone marrow toxicity, as demonstrated by a significant reduction in the percentage of PCE, was present also. These results indicate that hepatic methyl donors deficiency significantly decreases the total urinary excretion of orally administered sodium arsenite and markedly modulates target organ arsenic-induced DNA damage, with an apparent shift from liver and bladder to skin.

Interlaboratory validation of a new assay for DNA-protein crosslinks

In 1992, a simple and sensitive assay for detecting DNA-protein crosslinks was developed [1]. In an effort to facilitate the greater use of the assay, a number of studies were conducted to evaluate its reliability and reproducibility. During this work, the assay was used to assess whether various metals and other compounds could induce crosslinks in cultured human lymphocytes (Epstein-Barr virus-transformed Burkitt's Lymphoma cell line). Potassium permanganate, mercury chloride, lead nitrate, magnesium perchlorate, aluminum chloride, and cadmium chloride did not induce DNA-protein crosslinks at either cytotoxic or non-cytotoxic levels. Copper sulfate, arsenic trioxide, and potassium chromate induced DNA-protein crosslinks only at cytotoxic concentrations. Acute lethality of the cells was assessed immediately after exposure to metals by trypan blue exclusion while long-term lethality was assessed by cell proliferation and trypan blue exclusion following an incubation period of 5 days after exposure to the metal compound. All metals exhibited more toxicity in the long-term lethality assay compared to the short-term assay. The cultured human lymphocytes treated with various doses of lead acetate, cadmium chloride, arsenic trioxide and copper sulfate, as well as cis-platinum and chromate, were sent to four different laboratories to compare the reliability and reproducibility of the DNA-protein crosslink assay. Depending on the chemical studied, there were quantitative differences in the results observed among the various laboratories using the assay. However, all laboratories generally showed that cis-platinum, chromate, arsenic trioxide and copper sulfate induced DNA-protein crosslinks at levels that produced acute cytotoxicity, whereas cadmium chloride and lead acetate did not.

Time dependence of accumulation and binding of inorganic and organic arsenic species in rabbit erythrocytes

The uptake by rabbit erythrocytes of 0.4 mM arsenate, As(V), monomethylarsinate, MMA(V) and dimethylarsonate, DMA(V) were compared over 24 h. In membrane-free hemolysate, the distribution of As between proteins (10 kDa) and ultrafiltrate was determined by ultrafiltration and arsenic species in the ultrafiltrate were identified by thin layer chromatography methods. 1H spin-echo Fourier transform NMR was used to follow the binding of these arsenic species to glutathione (GSH). 31P-NMR was used to observe their effects on high-energy adenine nucleotide levels (ATP, ADP). These results demonstrate that As(III) readily accumulates in cells, reaches a quasi-plateau at 78% of the total As in the incubation after 1 h and 88% of the total As after 24 h. On average, 20% of the total erythrocyte As(III) burden is associated with the protein fraction, particularly with hemoglobin (Hb). About 68% of the erythrocyte As(III) burden is bound to GSH. As(III) has no effect on ATP levels during a 5-h incubation. By comparison, As(V) enters erythrocytes more slowly (53% of the total As after 5 h). Erythrocytes take up 81% of the As(V) in the reaction system after a 24 h incubation. Of the total As burden in As(V)-exposed erythrocytes, 22% was associated with the proteins (10 kDa) and possibly reduced to As(III) and 59% was in the ultrafiltrate (8% as As(III) and 51% as As(V)). This finding indicates that, over a 24 h incubation period, the reduction of As(V) to As(III) may account for 30% of the total As in rabbit erythrocytes. As(V) present in the erythrocytes enters the phosphate pool and depletes ATP. In comparison, about 65% of the total MMA(V) or about 44% of the total DMA(V) in the incubation system is taken up by rabbit erythrocytes during a 24 h incubation. Neither organoAs species perturbed the Hb signals observed by spin-echo Fourier transform NMR and the binding to GSH was minimal. Unlike As(V), MMA(V) and DMA(V) do not perturb phosphate metabolism, showing that, despite their pentavalent oxidation state, these arsenic species are not analogs for phosphate.