Arsenite enhances DNA double-strand breaks and cell killing of methyl methanesulfonate-treated cells by inhibiting the excision of alkali-labile sites

Molecular Mechanisms of Arsenic-Induced Disruption of DNA Repair

Exposure to arsenic in contaminated drinking water is an emerging public health problem that impacts more than 200 million people worldwide. Accumulating lines of evidence from epidemiological studies revealed that chronic exposure to arsenic can result in various human diseases including cancer, type 2 diabetes, and neurodegenerative disorders. Arsenic is also classified as a Group I human carcinogen. In this review, we survey extensively different modes of action for arsenic-induced carcinogenesis, with focus being placed on arsenic-mediated impairment of DNA repair pathways. Inorganic arsenic can be bioactivated by methylation, and the ensuing products are highly genotoxic. Bioactivation of arsenicals also elicits the production of reactive oxygen and nitrogen species (ROS and RNS), which can directly damage DNA and modify cysteine residues in proteins. Results from recent studies suggest zinc finger proteins as crucial molecular targets for direct binding to As3+ or for modifications by arsenic-induced ROS/RNS, which may constitute a common mechanism underlying arsenic-induced perturbations of DNA repair.

The Arsenic-Binding Aptamer Cannot Bind Arsenic: Critical Evaluation of Aptamer Selection and Binding

An arsenic-binding aptamer named Ars-3 was reported in 2009, and it has been used for detection of As(III) in more than two dozen papers. In this work, we performed extensive binding assays using isothermal titration calorimetry, various DNA-staining dyes, and gold nanoparticles. By carefully comparing Ars-3 and a few random control DNA sequences, no specific binding of As(III) was observed in each case. Therefore, we conclude that Ars-3 cannot bind As(III). Possible reasons for some of the previously reported binding and detection were speculated to be related to the adsorption of As(III) onto gold surfaces, which were used in many related sensor designs, and As(III)/Au interactions were not considered before. The selection data in the original paper were then analyzed in terms of sequence alignment, secondary structure prediction, and dissociation constant measurement. These steps need rigorous testing before confirming specific binding of newly selected aptamers. This study calls for attention to the gap between aptamer selection and biosensor design, and the gap needs to be filled by careful binding assays to further the growth of the aptamer field.

Antagonistic effect of N-ethylmaleimide on arsenic-mediated oxidative stress-induced poly(ADP-ribosyl)ation and cytotoxicity

Long-term exposure to arsenic has been known to induce neoplastic initiation and progression in several organs; however, the role of arsenic (As₂ O₃ ) in oxidative stress-mediated DNA damage remains elusive. One of the immediate cellular responses to DNA damage is poly(ADP-ribosyl)ation (PARylation), which mediates DNA repair and enhances cell survival. In this study, we found that oxidative stress (H₂ O₂ )-induced PARylation was suppressed by As₂ O₃ exposure in different human cancer cells. Moreover, As₂ O₃ treatment promoted H₂ O₂ -induced DNA damage and apoptosis, leading to increased cell death. We found that N-ethylmaleimide (NEM), an organic compound derived from maleic acid, could reverse As₂ O₃ -mediated effects, thus enhancing PARylation with attenuated cell death and increased cell survival. Pharmacologic inhibition of glutathione with l-buthionine-sulfoximine blocked the antagonistic effect of NEM on As₂ O₃ , thereby continuing As₂ O₃ -mediated suppression of PARylation and causing DNA damage. Our findings identify NEM as a potential antidote against As₂ O₃ -mediated DNA damage in a glutathione-dependent manner. Copyright © 2016 John Wiley & Sons, Ltd.

Arsenic induces DNA damage via reactive oxygen species in human cells

To elucidate arsenic-induced oxidative DNA damage, the genotoxicity of arsenic in human cells was comparatively studied with single cell gel electrophoresis (SCGE) assay in combination with the observation of the protective effects of dimethyl sulfoxide (DMSO) and catalase. Arsenic, at the concentration of 2.4 μM by coincubation for 24 hours, significantly induced DNA damage in HL60, a human promyelocytic leukemia cell line. In contrast, significant DNA damage was found in human mononucleocytes at the concentration of 4.8 μM or above. The cells were incubated separately with DMSO (12 mM/l), a well-known hydroxyl radical (OH(-)) scavenger, and catalase (1,300 U/ml), a hydrogen peroxide (H(2)O(2)) scavenger, for 6 hours and then further coincubated with various concentrations of arsenic for 24 hours at 37°C and 5% CO(2). The findings showed that both DMSO and catalase significantly reduced the arsenic-induced tail moment, a parameter of total damaged DNA, in HL60 and mononucleocytes. Hence our findings indicate that arsenic, with micromolar concentrations, induces typical and various extents of DNA damage in human cells via reactive oxygen species in a dose-dependent manner.

Role of DNA polymerase beta in the genotoxicity of arsenic

Arsenic, an important hazard in the environment, is associated with human cancer and other degenerative diseases. However, the mechanisms underlying arsenic hazardous effects remain unclear. It has been reported arsenic exposure can result in increased cellular reactive oxygen species and oxidative DNA damage. This suggests DNA base excision repair (BER), the major pathway for repairing oxidative DNA damage, may be involved in combating arsenic hazardous effects. As a critical repair enzyme in BER, DNA polymerase beta (Pol β) might play an essential role in reducing arsenic toxicity. To test this hypothesis, we evaluated arsenic-induced cytotoxic and genotoxic effects under Pol β deficiency. Our results demonstrated that the viability of Pol β-deficient mouse embryonic fibroblasts was much lower than that of Pol β wild-type cells after treatment with arsenite (As(3+) ). An increased level of DNA damage and significantly delayed arsenite-induced DNA damage repair in Pol β-deficient cells indicated reduced repair of DNA lesions under Pol β deficiency. This was consistent with the increase in the frequency of micronuclei (MN), an indicator of chromosomal breakage, which was also observed in Pol β-deficient cells treated with arsenite. In contrast, cells harboring overexpressed Pol β resulted in a lower level of DNA damage and MN than Pol β wild-type cells, indicating overexpression of the enzyme can combat arsenic-induced genotoxic effects. In conclusion, our results indicate an important role for Pol β in repairing arsenite-induced DNA damage and maintaining chromosomal integrity and further suggest deficiency of BER may be involved in arsenic genotoxicity and carcinogenicity.

Induction of apoptosis-like cell death by pentamidine and doxorubicin through differential inhibition of topoisomerase II in arsenite-resistant L. donovani

The current study has been undertaken to investigate the sensitivity of the topoisomerase II (topo II) of wild type (Ld-Wt) and arsenite-resistant (Ld-As20) L. donovani to an anti-leishmanial agent pentamidine and an anti-cancer drug doxorubicin. We demonstrate that the cross resistance to pentamidine and doxorubicin in Ld-As20, was in part implicated through differential inhibition of topo II in Ld-Wt and Ld-As20. Further, the treatment of promastigotes at drug concentrations inhibiting 50% of topo II activity inflicted a regulated cell death sharing several apoptotic features like externalization of phosphatidylserine, loss of mitochondrial membrane potential, cytochrome C release into the cytosol, activation of cellular proteases and DNA fragmentation. The cytotoxic potential of pentamidine and doxorubicin in L. donovani has been shown to be mediated through topoisomerase II inhibition and results in inciting programmed cell death process.

Oxidative mechanism of arsenic toxicity and carcinogenesis

Arsenic is a known toxin and carcinogen that is present in industrial settings and in the environment. The mechanisms of disease initiation and progression are not fully understood. In the last a few years, there has been increasing evidence of the correlation between the generation of reactive oxygen species (ROS), DNA damage, tumor promotion, and arsenic exposure. This article summarizes the current literature on the arsenic mediated generation of ROS and reactive nitrogen species (RNS) in various biological systems. This article also discusses the role of ROS and RNS in arsenic-induced DNA damage and activation of oxidative sensitive gene expression.

Inhibition of DNA-double strand break repair by antimony compounds

DNA double strand breaks (DSBs), induced by gamma-irradiation in Chinese hamster ovary cells, were used to examine whether antimony compounds affect the repair of DNA damage. The cells were first incubated with antimony trichloride or antimony potassium tartrate (both Sb(III)) for 2 h, and then irradiated with gamma-rays at a dose of 40 Gy. The DNA DSB was quantified with pulsed field gel electrophoresis immediately after irradiation (non-repair group) as well as at 30 min post-irradiation (repair group). The degree of repair inhibition was determined by the differences in the amount of DNA DSB between non-repair and repair groups. Both antimony compounds inhibited repair of DNA DSB in a dose dependent manner. In trichloride, 0.2 mM antimony significantly inhibited the rejoining of DSB, while 0.4 mM was necessary in potassium antimony tartrate. The mean lethal doses, D(0), for the treatment with antimony trichloride and antimony potassium tartrate, were approximately 0.21 and 0.12 mM, respectively. This indicates that the repair inhibition by antimony trichloride occurred in the dose range near D(0), but the antimony potassium tartrate inhibited the repair at doses where most cells lost their proliferating ability. This is the first report to indicate that antimony compounds may inhibit the repair of radiation-induced DNA DSB.

Arsenic co-exposure potentiates benzo[a]pyrene genotoxicity

Co-exposures to complex mixtures of arsenic and polycyclic aromatic hydrocarbons such as benzo[a]pyrene (BaP) are common in the environment. These two environmental pollutants are carcinogenic, but the nature of their molecular interactions in the induction of cancer is not well understood. Additive or synergistic interactions have been proposed to explain why arsenic, which is not a potent mutagen itself, is comutagenic with a variety of DNA-damaging agents. We have examined the genotoxicity of BaP-arsenic mixtures. We find that exposure of mouse hepatoma Hepa-1 cells to low concentrations of arsenite increases BaP-DNA adduct levels by as much as 18-fold. This effect requires the activation of BaP by cytochrome p450 1A1 (CYP1A1), although arsenite does not alter BaP-inducible CYP1A1 enzymatic activity, suggesting that arsenite acts downstream of metabolic BaP activation. Glutathione homeostasis was important in modulating the potency of arsenite. In cells depleted of reduced glutathione, arsenite increased BaP-DNA adduct formation by an even greater degree than in cells co-treated with BaP and arsenite in control medium. Although arsenic comutagenicity has been attributed to inhibition of DNA repair, arsenite treatment did not alter adduct removal kinetics in BaP-treated cells, suggesting that mechanisms upstream of DNA repair are responsible for increased adduct levels. Concentrations of arsenite and BaP that had no measurable mutagenic effect alone, increased mutation frequency at the Hprt locus by eight-fold when given in combination, demonstrating a comutagenic response between BaP and arsenite. These results provide strong support for the positive interaction between arsenic and PAH-induced cancer observed in epidemiology studies, and help to identify additional mechanistic steps likely to be involved in arsenic comutagenesis.

Arsenite pretreatment attenuates benzo[a]pyrene cytotoxicity in a human lung adenocarcinoma cell line by decreasing cyclooxygenase-2 levels

Both simultaneous and sequential exposure to arsenite and benzo[a]pyrene (BaP) potentially occur in human populations drinking arsenic-contaminated water or burning arsenic-contaminated coal. Although arsenite and BaP are both well-documented hazardous substances and human carcinogens, interactions between these two agents have not been well defined. In this study, we demonstrated that posttreatment with arsenite synergistically enhanced the cytotoxicity of BaP for a human lung adenocarcinoma cell line, CL3. In contrast, pretreatment of CL3 cells with arsenite attenuated BaP cytotoxicity. Involvement of heat-shock protein 70 and heme oxygenase-1 in this arsenite-mediated attenuation of BaP cytotoxicity was ruled out. Our data also indicated that arsenite pretreatment did not affect the BaP-mediated induction of CYP1A1, the initial enzyme involved in its metabolic activation, but did result in a significant decrease in mRNA and protein levels of cyclooxygenase-2 (COX-2), which is required to convert the BaP metabolite BaP 7,8-dihydrodiol to the ultimate epoxide. In contrast to the high susceptibility of CL3 cells to BaP, the human lung carcinoma cells, H460, and CL3R15 cells (arsenic-resistant CL3 cells) showed normal CYP1A1 inducibility by BaP, had negligible amounts of COX-2, and were highly resistant to BaP. The involvement of COX-2 in BaP activation was confirmed by transfection of H460 cells with a recombinant adenovirus, Ad-pgk-Cox2, coding for COX-2, which resulted in a significant increase in the levels of the COX-2 product prostaglandin E2 in the medium and in the susceptibility of H460 cells to BaP. The present study confirms the importance of COX-2 in BaP activation and demonstrates that the arsenite-mediated attenuation of BaP cytotoxicity is mediated by a reduction in COX-2 levels.

Influence of sodium arsenite on the genotoxicity of potassium dichromate and ethyl methanesulfonate: studies with the wing spot test in Drosophila

The wing spot test in Drosophila melanogaster was used to investigate the genotoxicity of arsenic and its effects on the action of two clearly genotoxic agents: potassium dichromate (PDC) and ethyl methanesulfonate (EMS). This assay is based on the principle that the loss of heterozygosity of the suitable recessive markers multiple wing hairs (mwh) and flare-3 (flr(3)) can lead to the formation of mutant clones of larval cells, which are then expressed as spots on the wings of adult flies. These spots can be attributed to different genotoxic events: either mitotic recombination or mutation (deletion, point mutation, and specific types of translocation). Pretreatments and chronic cotreatments were comparatively used for combined treatments. From the results obtained it is evident that sodium arsenite (SA) does not increase the frequency of any of the three categories of spots recorded (small, large, and twin spots) at the concentrations tested. The effects of SA in combination with PDC, in both cotreatments and pretreatments, indicate that SA almost suppressed the clones induced by PDC. Nevertheless, no effects of arsenic were observed with respect to the pre- and cotreatments with EMS. Thus, SA does not modify the frequencies of mutant clones induced by EMS.

Arsenite induces oxidative DNA adducts and DNA-protein cross-links in mammalian cells

Arsenic is generally recognized as a nonmutagenic carcinogen because sodium arsenite induces DNA damage only at very high concentrations. In this study we demonstrate that arsenite concentrations above 0.25 microM induce DNA strand breaks in both human leukemia cells and Chinese hamster ovary cells. Therefore, DNA damage may be involved in arsenic-induced carcinogenesis. Formamidopyrimidine-DNA glycosylase and proteinase K greatly increased DNA strand breaks in arsenite-treated cells, providing evidence that a large portion of arsenite-induced DNA strand breaks come from excision of oxidative DNA adducts and DNA-protein cross-links. Because DNA strand breaks appear only temporarily during excision repair, the level of detectable DNA strand breaks will be low at any given time point. For this reason many previous studies have only detected low levels of DNA strand breaks. We also show that catalase, and inhibitors of calcium, nitric oxide synthase, superoxide dismutase, and myeloperoxidase, could modulate arsenite-induced DNA damage. We conclude that arsenite induces DNA adducts through calcium-mediated production of peroxynitrite, hypochlorous acid, and hydroxyl radicals.

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.

Inhibition of repair of radiation-induced DNA double-strand breaks by nickel and arsenite

The effect of arsenite or nickel on the repair of DNA double-strand breaks (DSBs) was studied in gamma-irradiated Chinese hamster ovary cells using pulsed-field gel electrophoresis. After treatment with nickel chloride or arsenite for 2 h, cells were irradiated with gamma rays at a dose of 40 Gy, and the numbers of DNA DSBs were measured immediately after irradiation as well as at 30 min postirradiation. Both arsenite and nickel(II) inhibited repair of DNA DSBs in a concentration-dependent manner; 0.08 mM arsenite significantly inhibited the rejoining of DSBs, while 76 mM nickel was necessary to observe a clear inhibition. The mean lethal concentrations for the arsenite and nickel(II) treatments were approximately 0.12 and 13 mM, respectively. This indicates that the inhibition of repair by arsenite occurred at a concentration at which appreciable cell survival occurred, but that nickel(II) inhibited repair only at cytotoxic concentrations at which the cells lost their proliferative ability. These novel observations provide insight into the mechanisms underlying the effects of combined exposure to arsenite and ionizing radiation in our environment.

Are metals dietary carcinogens?

Humans have been in contact with metals almost since the beginning of our existence. In fact, one cannot even think on human evolution without considering the great role played by metals in mankind's development. Metals are common moieties of molecules involved in a wide variety of biological processes, and hence are found in virtually all living organisms. Some metals are essential for human nutrition; others are found as contaminants in foodstuffs. One feature of the normal human diet which is frequently found is the simultaneous presence of both essential and toxic metals. Other factors important in the risk-evaluation analysis of metals are their pharmacokinetics, interactions among them and with other major components of the diet, and, especially, the great differences in the dietary habits of different populations and in the regional distribution of metals. In attempting to understand the role which dietary metals could play in human carcinogenesis, we found that the many factors involved and the lack of specific information made it difficult to reach firm conclusions on the hazards of dietary metals. We hope that this paper will raise the interest of genetic toxicologists in the subject and will consequently facilitate a risk analysis of the carcinogenic potential of dietary metals.

Mutational spectrum of p53 gene in arsenic-related skin cancers from the blackfoot disease endemic area of Taiwan

To understand the role of p53 tumour suppressor gene in the carcinogenesis of arsenic-related skin cancers from the blackfoot disease endemic area of Taiwan, we collected tumour samples from 23 patients with Bowen's disease, seven patients with basal cell carcinomas (BCC) and nine patients with squamous cell carcinomas (SCC). The result showed that p53 gene mutations were found in 39% of cases with Bowen's disease (9/23), 28.6% of cases with BCC (2/7) and 55.6% of cases with SCC (5/9). Most of the mutation sites were located on exon 5 and exon 8. Moreover, the results from direct sequencing indicated that missense mutations were found at codon 149 (C-->T) in one case, codon 175 (G-->A) in three cases, codon 273 (G-->C) in three cases, codon 292 (T-->A) in one case, codon 283 (G-->T) in one case, codon 172 (T-->C) in one case and codon 284 (C-->A) in one case. In addition, silent mutations were also found in four cases. These mutations were located at codons 174, 253, 289 and 298 respectively. In immunohistochemistry analysis, p53 overexpression was found in 43.5% (10/23) of cases with Bowen's disease, 14% (1/7) of cases with BCC and 44% (4/9) of cases with SSC. These findings showed that p53 gene mutation rate in arsenic-related skin cancers from the blackfoot disease endemic area of Taiwan is high and that the mutation types are different from those in UV-induced skin cancers.

Effects of exposure protocols on induction of kinetochore-plus and -minus micronuclei by arsenite in diploid human fibroblasts

Arsenic, widely distributed in the environment, is a potent human carcinogen. Arsenite genotoxicity has been observed in a variety of cells and animal systems. However, the underlying mechanism is not completely clear. In this study, human fibroblasts (HFW) were treated with 1.25-10 microM arsenite for 24 h (low dose and long exposure) and 5-80 microM for 4 h (high dose and short exposure), and the arsenite accumulation, cytotoxicity, and micronucleus (MN) induction were examined. By these two different protocols, HFW cells showed equivalent levels of arsenite accumulation, but exhibited different kinetics of cell killing and different types of MN generation. Arsenite induced mainly kinetochore-positive MN (K+-MN) in HFW cells by low dose exposure whereas mainly kinetochore-negative MN (K--MN) was induced by high dose exposure. Catalase reduced both K+- and K--MN induced by these two exposure protocols. Except for the case of K+-MN induction by the high dose exposure protocol, N-acetyl-cysteine (NAC) in both low and high dose protocols was also shown to effectively reduce arsenite-induced MN. The present results imply that oxidative stress is involved in arsenite-induced MN in diploid human fibroblasts. However, different protocols for arsenite exposure may result in different cellular damage.

Calcium-dependent nitric oxide production is involved in arsenite-induced micronuclei

Arsenic, a human carcinogen is known to induce sister-chromatid exchanges, chromosome aberrations and micronuclei (MN), but its mechanisms remain unknown. Recently, independent studies have suggested that intracellular calcium and reactive oxygen species are involved in arsenite-induced MN, and nitric oxide (NO) is involved in arsenite-induced poly(ADP-ribosylation). The aim of this research is to investigate the involvement of these molecules in arsenite-induced MN. The intracellular oxidant level and calcium level were monitored with a flow cytometer by using dichlorofluorescein diacetate and fluo3-AM, respectively. The NO production was estimated from the nitrite in cell culture medium with a spectrophotometer by using diaminonaphthalene. The results show that a 4-h treatment with arsenite above 5 microM, caused a dose-dependent increase of oxidant, NO, as well as intracellular calcium level. The arsenite-increased intracellular oxidant level was inhibited by NO synthase inhibitors, S-methyl-l-thiocitrulline and Nomega-nitro-l-arginine methyl ester and calcium chelators, ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, and 2-[(2-bis-[carboxymethyl]-amino-5-methylphenoxy)-methyl]-6-methoxy-8- bis[carboxy-methyl]aminoquinoline, but not by catalase inhibitor, 3-aminotriazole. The arsenite-increased NO could also be suppressed by NO synthase inhibitors and calcium chelator. However, the arsenite-increased intracellular calcium level was inhibited by calcium chelators, but not by NO synthase inhibitors. A 4-h treatment with arsenite above 10 microM, also induced MN dose-dependently. The arsenite-increased MN could be reduced by NO synthase inhibitors, calcium chelators, as well as superoxide dismutase and uric acid. These results suggest the involvement of peroxynitrite in arsenite-induced MN. We surmise that the disturbance of NO production may cause cardio/peripheral vascular disorders, and the peroxynitrite-mediated DNA damages may cause genetic instability and, hence, cancers in arsenic-exposed humans.

Association between the clastogenic effect in peripheral lymphocytes and human exposure to arsenic through drinking water

We describe the association between structural chromosome aberrations (CAs) and parameters of exposure to arsenic among 42 individuals exposed to arsenic through well waters in Finland. The median concentration of arsenic in the wells was 410 microg/l, the total arsenic concentrations in urine (As-tot) was 180 microg/l, and in hair 1.3 microg/g, for current users (n = 32) of contaminated wells. Urinary arsenic species and CAs were also analyzed in eight control individuals from the same village who consumed water which contained arsenic <1.0 microg/l (detection limit). Increased arsenic exposure, indicated best by increased concentrations of arsenic species (inorganic arsenic, methylarsonic acid (MMA), dimethylarsinic acid (DMA)) in urine, was associated with increased frequency of CAs. The increased urinary ratio of MMA/As-tot and the decreased ratio of DMA/As-tot were associated with increased CAs when all aberration types, including gaps, were considered. Associations between CAs and arsenic exposure indicators were stronger among current users than among persons who had stopped using the contaminated well water for 2-4 months before sampling (ex-users, n = 10). Furthermore, there was a positive but not statistically significant association between CAs and arsenic in hair among the current users, but not among the ex-users, who still had relatively high arsenic concentrations in hair. The results suggest that the effect observed in the present study reflects relatively recent arsenic exposure.

Study of arsenic mutagenesis using the plasmid shuttle vector pZ189 propagated in DNA repair proficient human cells

Arsenic is considered a human carcinogen and although it is non-mutagenic in bacterial or human cells, arsenic interacts synergistically with genotoxic agents in the production of mutations. To gain insight into the possible mechanisms of action of arsenic in mutagenesis we studied the effects of sodium arsenite exposure on UV mutagenesis using the pZ189 shuttle vector system in DNA repair proficient GM 637 human fibroblasts. The purpose of the study was to determine whether arsenic alone induces mutations in the supF gene and whether the combination of arsenic and UV irradiation leads to a yield of mutants greater than the sum of the arsenic or UV treatments alone. Treatment of fibroblasts for 72 h with 5.0 microM of sodium arsenite alone produced significant increases in the pZ189 mutant frequency; 1 and 2.5 microM arsenite were not mutagenic. UV irradiation (320 J/m2) increased the yield of mutants 3.5-fold above the background rate. When UV-irradiated plasmid was allowed to replicate in fibroblasts treated with 1, 2.5, or 5.0 microM arsenite, the yields of mutations were significantly greater (p < 0.01) than the yield expected if the effects of each treatment were simply additive. The greatest potentiation of UV-induced mutations (4.9-fold) was observed at 1 microM arsenite, a concentration that was neither mutagenic itself nor cytotoxic. Restriction digest and DNA sequencing analyses indicated that arsenite alone produces both large-scale rearrangements, frameshifts and base substitutions. Hotspots for deletions were observed to be associated with a previously reported deletion hotspot involving 5'-CpC and runs of cytosines. Base substitutions observed involved A:T-->T:A transversions. The results indicate that arsenite alone is mutagenic in human cells using the supF reporter gene. The pZ189 shuttle vector may provide a model to study the molecular nature of co-mutagenesis of arsenic and other environmental agents. Further characterization of arsenic's effects on DNA repair and mutational spectra may be useful in the development of molecular markers in studies of arsenic carcinogenesis in human populations.

Inhibition of poly(ADP-ribose) polymerase by arsenite

Inorganic arsenic is considered a human carcinogen based principally on epidemiological evidence. Unlike most initiating chemicals, arsenic is inactive or extremely weak in its ability to directly induce gene mutations. Arsenite has been shown, however, to enhance mutagenicity when present with other agents such as UV radiation. Synergistic potentiation of chromosomal damage has been shown with co-treatment with DNA-crosslinking agents. Arsenite at low concentrations is known to be highly selective in reacting with closely spaced (vicinal) dithiol groups in proteins. Poly(ADP-ribose) polymerase (PARP) is known to contain such vicinal dithiol groups. Stimulation of PARP is an immediate response of eukaryotic cells to DNA strand breaks and has been implicated in DNA repair. The effect of treatment with sodium arsenite on PARP activity was assessed as follows: Molt-3 cells (a human T-cell lymphoma-derived cell line) in culture were treated for 24 h with concentrations of sodium arsenite ranging from 2.5 up to 25 microM. Speciation of inorganic arsenic and cell viability were determined. Cell cycle kinetics were measured by flow cytometry. Poly(ADP-ribose) synthesis was assayed using a palindromic decameric deoxynucleotide to stimulate enzyme activity. Results show that arsenite decreases PARP activity in a dose-dependent manner with an approximately 50% decrease in enzyme activity at 10 microM arsenite and 80% viability. The percent of cells in S-phase increases with increasing concentration of arsenite. These results provide further indication that arsenite may potentiate genetic damage through reaction with dithiols in DNA repair proteins such as PARP, perhaps resulting in interference with normal repair function.

Disruption of microtubule assembly and spindle formation as a mechanism for the induction of aneuploid cells by sodium arsenite and vanadium pentoxide

Arsenic and vanadium are important environmental and industrial pollutants. Due to their widespread occurrence and potential genotoxicity, we studied the aneuploidy-inducing effects of these elements in cultured human lymphocytes using a variety of techniques including fluorescence in situ hybridization (FISH) with DNA probes for chromosomes 1 and 7, immunostaining of the lymphocyte spindle apparatus, and an in vitro assay measuring the polymerization and depolymerization of tubulin. Dose-related increases in hyperdiploidy were seen in lymphocyte cultures treated with sodium arsenite (NaAsO2) or vanadium pentoxide (V2O5) over concentrations ranging from 0.001 to 0.1 microM. NaAsO2-treated cells from different donors exhibited similar hyperdiploid frequencies, whereas substantial inter-individual variability was seen in the V2O5-treated cells. Examination of the spindle apparatus using an anti-beta-tubulin antibody indicated that these compounds might disrupt spindle formation by interacting with microtubules. Additional in vitro assays using purified tubulin indicated that both compounds inhibited microtubule assembly and induced tubulin depolymerization. These results indicate that in vitro exposure to both NaAsO2 and V2O5 can induce aneuploidy in human lymphocytes, and that this effect may occur through a disruption of microtubule function.

Effect of arsenic and cadmium on the persistence of mutagen-induced DNA lesions in human cells

The alkaline single cell gel electrophoresis (SCG test or comet assay) was used to characterize the influence of sodium arsenite (NaAsO2) and cadmium sulphate (CdSO4) on the persistence of mutagen-induced DNA lesions. Human blood and SV4O-transformed fibroblasts (MRC5CV1) were treated for 2 hr with methyl methanesulphonate (MMS) or benzo(a)pyrene (BaP). MMS induced concentration-related DNA damage in white Blood cells (WBC) and fibroblasts in similar concentrations. For the induction of DNA damage in white blood cells (WBC) and fibroblasts in similar concentrations. For the induction of DNA damage by BaP, higher concentrations had to be applied to WBC than to the fibroblast cell line. To study the influence of metal ions on the persistence of DNA lesions, treated cells were further incubated for 2 hr in the absence (postincubation) or presence (posttreatment) of NaAsO2 or CdSO4. After postincubation, MMS and BaP-induced DNA effects were reduced in both cell types, indicating that repair of DNA lesions had taken place. When the cells were posttreated with NaAsO2 or CdSO4, BaP- and MMS-induced DNA lesions persisted in both cell types, indicating an inhibition of DNA repair by these metals. The results suggest a strong interaction of arsenic and cadmium with BaP- and MMS-induced DNA repair processes.

Quantification of DNA damage and repair in amino acid auxotrophs and UV-sensitive mutants of Aspergillus nidulans using an ELISA

An ELISA used to investigate DNA repair in mammalian cells has been adapted to investigate mutagen-induced DNA damage and repair in protoplasts of Aspergillus nidulans. The assay shows a reduced rate of repair of DNA damage in methionine and arginine auxotrophs (methG and argB), which were shown previously to be hypersensitive to UV radiation and chemical mutagens. The assay also showed a considerably reduced ability to repair mutagen-induced damage in the uv-sensitive mutants uvsB and uvsH. The increased sensitivity of amino acids auxotrophs to mutagens is, therefore, correlated with a reduced capacity to repair mutagen-induced DNA damage.

Genotoxicity of two arsenic compounds in germ cells and somatic cells of Drosophila melanogaster

Two arsenic compounds, sodium arsenite (NaAsO2) and sodium arsenate (Na2HAsO4), were tested for their possible genotoxicity in germinal and somatic cells of Drosophila melanogaster. For germinal cells, the sex-linked recessive lethal test (SLRLT) and the sex chromosome loss test (SCLT) were used. In both tests, a brood scheme of 2-3-3 days was employed. Two routes of administration were used for the SLRLT: adult male injection (0.38, 0.77 mM for sodium arsenite; and 0.54, 1.08 mM for sodium arsenate) and larval feeding (0.008, 0.01, 0.02 mM for sodium arsenite; and 0.01, 0.02 mM for sodium arsenate). For the SCLT the compounds were injected into males. Controls were treated with a solution of 5% sucrose which was employed as solvent. The somatic mutation and recombination test (SMART) was run in the w+/w eye assay as well as in the mwh +/+ flr3 wing test, employing the standard and insecticide-resistant strains. In both tests, third instar larvae were treated for 6 hr with sodium arsenite (0.38, 0.77, 1.15 mM), and sodium arsenate (0.54, 1.34, 2.69 mM). In the SLRLT, both compounds were positive, but they were negative in the SCLT. The genotoxicity of both compounds was localized mainly in somatic cells, in agreement with reports on the carcinogenic potential of arsenical compounds. Sodium arsenite was an order of magnitude more toxic and mutagenic than sodium arsenate. This study confirms the reliability of the Drosophila in vivo system to test the genotoxicity of environmental compounds.

Delay of the excision of UV light-induced DNA adducts is involved in the coclastogenicity of UV light plus arsenite

Chromatid exchanges and chromatid breaks were synergistically increased by a 2-h post-treatment with arsenite (VS treatment) but not with arabinofuranosyl cytosine (VA treatment) of UV-irradiated late-G1 Chinese hamster ovary cells. In order to determine the mechanism of this UV-arsenite coclastogenicity, we have compared the effects of arsenite and arabinofuranosyl cytosine on the generation of DNA strand breaks in UV-irradiated cells by alkaline elution and alkaline sucrose sedimentation. Only very small numbers of DNA breaks were detected immediately after VS treatment, however the breaks in parental strands increased as the cells reached mitosis in drug-free medium, whereas a large number of breaks were detected immediately after VA treatment but the breaks decreased thereafter. By labelling the newly synthesized DNA, we have also shown that the VS-treated cells had more breaks in daughter strands than the VA-treated cells at the time of reaching mitosis. The effect of a 2-h post-treatment with arsenite on the excision of UV-induced DNA adducts was further investigated by using the exponentially growing cells. The results confirmed that very low amount of breaks was detectable immediately after VS treatment, however the amount of breaks increased upon the removal of arsenite. Therefore, the breaks in the daughter strands of VS-treated cells may come from DNA replication using templates containing unexcised adducts, or using broken templates. It is conceivable that gaps in the overlapping regions of parental and daughter strands may result in chromatid breaks and that misreplication, because of unexcised adducts or gaps in the parental strands, may result in chromatid exchanges.

Differential effects of luminol, nickel, and arsenite on the rejoining of ultraviolet light and alkylation-induced DNA breaks

When Chinese hamster ovary cells were treated with ultraviolet (UV) light or methyl methanesulfonate (MMS), a large number of DNA strand breaks could be detected by alkaline elution. These strand breaks gradually disappeared if the treated cells were allowed to recover in a drug-free medium. The presence of nickel or arsenite during the recovery incubation retarded the disappearance of UV-induced strand breaks, whereas the disappearance of MMS-induced strand breaks was retarded by the presence of arsenite or of luminol, a new inhibitor for poly(ADP-ribose) synthetase. Luminol, however, had no apparent effect on the repair of UV-induced DNA strand breaks, and nickel had no effect on the repair of MMS-induced DNA strand breaks. When UV- or MMS-treated cells were incubated in cytosine arabinofuranoside (AraC) plus hydroxyurea (HU), a large amount of low molecular weight DNA was detected by alkaline sucrose sedimentation. The molecular weight of these DNAs increased if the cells were further incubated in a drug-free medium. This rejoining of breaks in cells pretreated with UV plus AraC and HU was inhibited by nickel and by arsenite, but not by luminol. The rejoining of breaks in cells pretreated with MMS plus AraC and HU was inhibited by luminol and by arsenite, but not by nickel. These results suggest that different enzymes may be used in DNA resynthesis and/or ligation during the repairing of UV- and MMS-induced DNA strand breaks, and that nickel, luminol, and arsenite may have differential inhibitory effects on these enzymes.

Comparative investigations of the genotoxic effects of metals in the single cells gel (SCG) assay and the sister chromatid exchange (SCE) test

Sodium arsenite (NaAsO2) and cadmium sulphate (CdSO4) were tested for their ability to induce genotoxic effects in the single cell gel (SCG) assay and the sister chromatid exchange (SCE) test in human blood cultures in vitro. Both metals induced DNA damage in white blood cells that was expressed and detected as DNA migration in the SCG assay. Dose dependent effects were seen for cadmium in concentrations from 5 x 10(-4)-5 x 10(-3) M and for arsenic in concentrations from 2 x 10(-4)-1.5 x 10(-3) M. The distribution of DNA migration among cells, a function of dose, revealed that the majority of exposed cells expressed more DNA damage than cells from control cultures and that with increasing length of DNA migration the variability in migration among cells increased as well. Treatment of cells for 2 hr or 24 hr beginning 48 hr after the start of the blood cultures did not increase the SCE frequency in the case of cadmium but caused a small but significant SCE induction with arsenic at the highest concentration. The metal concentrations which could be investigated in the SCE test were much lower due to a strong toxic effect. Metal concentrations which were toxic in the SCE test were without visible effect in the SCG assay. Thus the two endpoints for the determination of genotoxic effects in vitro differed markedly with respect to the detection of genotoxicity induced by metals. These differences and the biological significance of the findings are discussed.

Induction of chromatid breaks and tetraploidy in Chinese hamster ovary cells by treatment with sodium arsenite during the G2 phase

Treatment of Chinese hamster ovary (CHO) cells with sodium arsenite during the G2 phase induced poorly condensed chromosomes and chromatid breaks. The induction of chromatid breaks was confirmed by the appearance of micronucleated cells after arsenite-treated G2 cells were allowed to re-enter interphase. When the duration of the G2 phase was artificially divided into 4 periods, more chromatid breaks were induced by treatment with arsenite during the very early G2 phase (or G2/S boundary). In addition to the induction of chromatid breaks, arsenite treatment also remarkably retarded the re-entry of mitotic cells into interphase. By replating and incubating arsenite-treated G2 cells in drug-free medium, we subsequently observed the appearance of a population of cells whose DNA content was between 4C and 8C, and metaphase cells with near-tetraploid chromosome numbers in the next mitotic division.