Arsenite and its biomethylated metabolites interfere with the formation and repair of stable BPDE-induced DNA adducts in human cells and impair XPAzf and Fpg

Biomonitoring Study of Toxic Metal(loid)s: Levels in Lung Adenocarcinoma Patients

Lung cancer is a leading cause of cancer deaths worldwide. The aim of this study was to investigate heavy metal(loid)s (Cd, Pb, Hg, Cr, Mn, Mo, Ni, and As) in lung cancer patients in order to elucidate their role as lung cancer environmental risk factors. Sixty-three patients of both sexes with adenocarcinoma stage IIIB or IV were enrolled in this research. The heavy metal(loid) urine concentrations were measured using ICP-MS. Arsenic was quantified above 10 μg/L in 44.44% of the samples. Nickel urinary concentrations above the ToxGuide reference levels were found in 50.79% of the samples, while lead was quantified in 9.52% of the urine samples. The urinary chromium levels were above the mean ToxGuide levels in 41.27% of the patients and were significantly higher in men in comparison with women (p = 0.035). The chromium urinary concentrations were positively associated with the CRP serum levels (p = 0.037). Cadmium was quantified in 61.90% of the samples with levels significantly higher in females than in males (p = 0.023), which was associated with smoking habits. Mercury was measured above the limit of quantification in 63.49% of the samples and was not associated with amalgam dental fillings. However, the Hg urinary concentrations were correlated positively with the ALT (p = 0.02), AST (p < 0.001), and GGT (p < 0.001) serum levels. In 46.03% of the samples, the Mo concentrations were above 32 μg/L, the mean value for healthy adults according to the ToxGuide, and 9.52% of the patients had Mn levels higher than 8 μg/L, the reference value for healthy adults based on ToxGuide data. The obtained results are preliminary, and further studies are needed to have a deeper insight into metal(loid) exposure's association with lung cancer development, progression, and survival prediction.

Risk assessment of small organoarsenic species in food

The European Commission asked EFSA for a risk assessment on small organoarsenic species in food. For monomethylarsonic acid MMA(V), decreased body weight resulting from diarrhoea in rats was identified as the critical endpoint and a BMDL10 of 18.2 mg MMA(V)/kg body weight (bw) per day (equivalent to 9.7 mg As/kg bw per day) was calculated as a reference point (RP). For dimethylarsinic acid DMA(V), increased incidence in urinary bladder tumours in rats was identified as the critical endpoint. A BMDL10 of 1.1 mg DMA(V)/kg bw per day (equivalent to 0.6 mg As/kg bw per day) was calculated as an RP. For other small organoarsenic species, the toxicological data are insufficient to identify critical effects and RPs, and they could not be included in the risk assessment. For both MMA(V) and DMA(V), the toxicological database is incomplete and a margin of exposure (MOE) approach was applied for risk characterisation. The highest chronic dietary exposure to DMA(V) was estimated in 'Toddlers', with rice and fish meat as the main contributors across population groups. For MMA(V), the highest chronic dietary exposures were estimated for high consumers of fish meat and processed/preserved fish in 'Infants' and 'Elderly' age class, respectively. For MMA(V), an MOE of ≥ 500 was identified not to raise a health concern. For MMA(V), all MOEs were well above 500 for average and high consumers and thus do not raise a health concern. For DMA(V), an MOE of 10,000 was identified as of low health concern as it is genotoxic and carcinogenic, although the mechanisms of genotoxicity and its role in carcinogenicity of DMA(V) are not fully elucidated. For DMA(V), MOEs were below 10,000 in many cases across dietary surveys and age groups, in particular for some 95th percentile exposures. The Panel considers that this would raise a health concern.

AsIII Selectively Induces a Disorder-to-Order Transition in the Metalloid Binding Region of the AfArsR Protein

Arsenic is highly toxic and a significant threat to human health, but certain bacteria have developed defense mechanisms initiated by AsIII binding to AsIII-sensing proteins of the ArsR family. The transcriptional regulator AfArsR responds to AsIII and SbIII by coordinating the metalloids with three cysteines, located in a short sequence of the same monomer chain. Here, we characterize the binding of AsIII and HgII to a model peptide encompassing this fragment of the protein via solution equilibrium and spectroscopic/spectrometric techniques (pH potentiometry, UV, CD, NMR, PAC, EXAFS, and ESI-MS) combined with DFT calculations and MD simulations. Coordination of AsIII changes the peptide structure from a random-coil to a well-defined structure of the complex. A trigonal pyramidal AsS3 binding site is formed with almost exactly the same structure as observed in the crystal structure of the native protein, implying that the peptide possesses all of the features required to mimic the AsIII recognition and response selectivity of AfArsR. Contrary to this, binding of HgII to the peptide does not lead to a well-defined structure of the peptide, and the atoms near the metal binding site are displaced and reoriented in the HgII model. Our model study suggests that structural organization of the metal site by the inducer ion is a key element in the mechanism of the metalloid-selective recognition of this protein.

Effect of Long-Term Low-Dose Arsenic Exposure on DNA Methylation and Gene Expression in Human Liver Cells

Millions of people around the world are exposed to elevated levels of arsenic through food or drinking water. Epidemiological studies have linked chronic arsenic exposure to an increased risk of several cancers, cardiovascular disease, central nervous system neuropathies, and genotoxic as well as immunotoxic effects. In addition to the induction of oxidative stress and inhibition of DNA repair processes, epigenetic effects, including altered DNA methylation patterns resulting in aberrant gene expression, may contribute to carcinogenicity. However, the underlying mechanisms by which chronic micromolar concentrations of arsenite affect the methylation status of DNA are not fully understood. In this study, human HepG2 hepatocarcinoma cells were treated with 0.5-10 μM sodium arsenite for 24 h, 10, or 20 days. During these periods, the effects on global DNA methylation, cell cycle phase distribution, and gene expression were investigated. While no impact on DNA methylation was seen after short-term exposure, global hypomethylation was observed at both long-term exposure periods, with concomitant induction of the DNA methyltransferase genes DNMT1 and DNMT3B, while DNMT3A was slightly down-regulated. Pronounced time- and concentration-dependent effects were also seen in the case of genes involved in DNA damage response and repair, inflammation, oxidative stress response, and metal homeostasis. These results suggest that chronic low-dose arsenite exposure can lead to global hypomethylation. As an underlying mechanism, the consistent down-regulation of DNA methyltransferase genes could be excluded; alternatively, interactions at the protein level could play an important role.

Arsenite Impairs BRCA1-Dependent DNA Double-Strand Break Repair, a Mechanism Potentially Contributing to Genomic Instability

BRCA1 is a key player in maintaining genomic integrity with multiple functions in DNA damage response (DDR) mechanisms. Due to its thiol-rich zinc-complexing domain, the protein may also be a potential target for redox-active and/or thiol-reactive (semi)metal compounds. The latter includes trivalent inorganic arsenic, which is indirectly genotoxic via induction of oxidative stress and inhibition of DNA repair pathways. In the present study, we investigated the effect of NaAsO2 on the transcriptional and functional DDR. Particular attention was paid to the potential impairment of BRCA1-mediated DDR mechanisms by arsenite by comparing BRCA1-deficient and -proficient cells. At the transcriptional level, arsenite itself activated several DDR mechanisms, including a pronounced oxidative stress and DNA damage response, mostly independent of BRCA1 status. However, at the functional level, a clear BRCA1 dependency was observed in both cell cycle regulation and cell death mechanisms after arsenite exposure. Furthermore, in the absence of arsenite, the lack of functional BRCA1 impaired the largely error-free homologous recombination (HR), leading to a shift towards the error-prone non-homologous end-joining (NHEJ). Arsenic treatment also induced this shift in BRCA1-proficient cells, indicating BRCA1 inactivation. Although BRCA1 bound to DNA DSBs induced via ionizing radiation, its dissociation was impaired, similarly to the downstream proteins RAD51 and RAD54. A shift from HR to NHEJ by arsenite was further supported by corresponding reporter gene assays. Taken together, arsenite appears to negatively affect HR via functional inactivation of BRCA1, possibly by interacting with its RING finger structure, which may compromise genomic stability.

The Duality of Arsenic Metabolism: Impact on Human Health

Arsenic is a naturally occurring hazardous element that is environmentally ubiquitous in various chemical forms. Upon exposure, the human body initiates an elimination pathway of progressive methylation into relatively less bioreactive and more easily excretable pentavalent methylated forms. Given its association with decreasing the internal burden of arsenic with ensuing attenuation of its related toxicities, biomethylation has been applauded for decades as a pure route of arsenic detoxification. However, the emergence of detectable trivalent species with profound toxicity has opened a long-standing debate regarding whether arsenic methylation is a detoxifying or bioactivating mechanism. In this review, we approach the topic of arsenic metabolism from both perspectives to create a complete picture of its potential role in the mitigation or aggravation of various arsenic-related pathologies.

Differing effects of inorganic and organic arsenic on uptake and distribution of multi-elements in Rice grain

Arsenic (As) pollution can lead to an element imbalance in rice. A hydroponic study was carried out to examine the influence of inorganic (arsenate) and organic (dimethylarsinic acid (DMA)) arsenic compounds on the concentration and distribution of iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), nickel (Ni), carbon (C), nitrogen (N), and sulfur (S) in rice caryopsis at maturity using laser confocal microscopy and synchrotron X-ray fluorescence (SXRF). Results showed that treatments with inorganic (iAs) and organic (DMA) arsenic did not change the distribution characteristics of the above elements in rice grains. Fe, Mn, and iAs were mainly limited to the ventral ovular vascular trace, while Cu, Zn, and DMA extended into the endosperm. This implies that milling processes are likely to remove a majority of Fe, Mn, and iAs, but not Cu, Zn, and DMA. With regard to the average fluorescent intensity of the rice endosperm, iAs exposure caused significant reductions in Mn (53%), Fe (40%), Cu (27%), and Zn (74%) while DMA treatments decreased Mn (49%), Fe (37%), and Zn (21%). Compared with DMA, iAs exerted more influence on the reduction of these elements in rice caryopsis. In addition, the elemental analysis revealed a significant 12.7% increase for N and 8% reduction for S in DMA-treated rice caryopsis while a significant decrease of 24.0% for S in iAs-exposed rice caryopsis. These findings suggest that Cu, Zn, and S are more easily impacted by iAs, while N is mostly affected by DMA.

Mode of action-based risk assessment of genotoxic carcinogens

The risk assessment of chemical carcinogens is one major task in toxicology. Even though exposure has been mitigated effectively during the last decades, low levels of carcinogenic substances in food and at the workplace are still present and often not completely avoidable. The distinction between genotoxic and non-genotoxic carcinogens has traditionally been regarded as particularly relevant for risk assessment, with the assumption of the existence of no-effect concentrations (threshold levels) in case of the latter group. In contrast, genotoxic carcinogens, their metabolic precursors and DNA reactive metabolites are considered to represent risk factors at all concentrations since even one or a few DNA lesions may in principle result in mutations and, thus, increase tumour risk. Within the current document, an updated risk evaluation for genotoxic carcinogens is proposed, based on mechanistic knowledge regarding the substance (group) under investigation, and taking into account recent improvements in analytical techniques used to quantify DNA lesions and mutations as well as "omics" approaches. Furthermore, wherever possible and appropriate, special attention is given to the integration of background levels of the same or comparable DNA lesions. Within part A, fundamental considerations highlight the terms hazard and risk with respect to DNA reactivity of genotoxic agents, as compared to non-genotoxic agents. Also, current methodologies used in genetic toxicology as well as in dosimetry of exposure are described. Special focus is given on the elucidation of modes of action (MOA) and on the relation between DNA damage and cancer risk. Part B addresses specific examples of genotoxic carcinogens, including those humans are exposed to exogenously and endogenously, such as formaldehyde, acetaldehyde and the corresponding alcohols as well as some alkylating agents, ethylene oxide, and acrylamide, but also examples resulting from exogenous sources like aflatoxin B1, allylalkoxybenzenes, 2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline (MeIQx), benzo[a]pyrene and pyrrolizidine alkaloids. Additionally, special attention is given to some carcinogenic metal compounds, which are considered indirect genotoxins, by accelerating mutagenicity via interactions with the cellular response to DNA damage even at low exposure conditions. Part C finally encompasses conclusions and perspectives, suggesting a refined strategy for the assessment of the carcinogenic risk associated with an exposure to genotoxic compounds and addressing research needs.

Possible differences in the mechanism of malignant transformation of HaCaT cells by arsenite and its dimethyl metabolites, particularly dimethylthioarsenics

BACKGROUND

As a confirmed human carcinogen, arsenic can cause skin cancer, lung cancer, etc. However, its carcinogenic mechanism is still unclear. In recent years, the oxidative stress hypothesis has become widely accepted. In mammals it has been found that arsenic can be converted to dimethylarsinous acid (DMAIII) and dimethylmonothioarsinic acid (DMMTAV) through a series of methylation and redox reactions. DMAIII and DMMTAV are highly toxic.

METHODS

Human keratinocytes (HaCaT) were exposed to different concentrations of NaAsO2 (IAsIII), DMMTAV and DMAIII for 24 h. Reactive oxygen species (hydrogen peroxide and superoxide), oxidative damage markers (8-hydroxydeoxyguanosine and malondialdehyde), and antioxidant markers (glutathione and superoxide dismutase) were measured. In addition, sulfane sulfurs were measured in HaCaT cells and a cell-free system.

RESULTS

In the DMMTAV and DMAIII treatment groups, the levels of hydrogen peroxide and superoxide in HaCaT cells were higher than in the IAsIII treatment groups at the same dose. Levels of 8-OHdG and MDA in the DMMTAV and DMAIII treatment groups were also higher than those in the IAsIII treatment groups at the same dose. However, in the DMMTAV and DMAIII treatment groups, the levels of GSH and SOD activity were lower than that in the IAsIII treatment groups. In DMMTAV-treated HaCaT cells, sulfane sulfurs were produced. Further, it was found that DMMTAV could react with DMDTAV to form persulfide in the cell-free system, which may explain the mechanism of the formation of sulfane sulfurs in DMMTAV-treated HaCaT cells.

CONCLUSIONS

DMMTAV and DMAIII more readily induce reactive oxygen species (ROS) and cause oxidative damage in HaCaT cells than inorganic arsenic. Further, the persulfide formed by the reaction of DMMTAV and DMDTAV produced from the metabolism of DMMTAV may induce a stronger reductive defense mechanism than GSH against the intracellular oxidative stress of DMMTAV. However, the cells exposed to arsenite are transformed by the continuous nuclear translocation of Nrf2 due to oxidative stress, and the persulfide from dimethylthioarsenics may promote Nrf2 by the combination with thiol groups, especially redox control key protein, Keap1, eventually cause nuclear translocation of sustained Nrf2.

A review of arsenic exposure and lung cancer

As a well-established human carcinogen, arsenic has increased the risk of lung cancer over the past decades. Wide exposure to arsenic in the environment has attracted the attention of scientists. Its carcinogenicity at early life stages has been observed in certain animal studies already, yet current evidence is insufficient to extrapolate this to humans. Although the mechanisms of lung cancer induced by arsenic remain unclear, most of them are related to the biotransformation of arsenic, which would further provide target sites for precaution and therapy. This review comprehensively summarizes current studies associated to arsenic exposure and lung cancer and the mechanism of its carcinogenesis in lung cancer in three sections, namely, epidemiological studies, experimental studies, and mechanistic studies. In addition, prevention and treatment strategies as well as directions for future studies are discussed.

Inhibition of arsenite methylation induces synergistic genotoxicity of arsenite and benzo(a)pyrene diol epoxide in SCC-7 cells

A comprehensive analytical method was developed to investigate the synergistic genotoxicity of BPDE and As(iii) in SCC-7 cells.

Arsenic-containing hydrocarbons: effects on gene expression, epigenetics, and biotransformation in HepG2 cells

Arsenic-containing hydrocarbons (AsHCs), a subgroup of arsenolipids found in fish and algae, elicit substantial toxic effects in various human cell lines and have a considerable impact on cellular energy levels. The underlying mode of action, however, is still unknown. The present study analyzes the effects of two AsHCs (AsHC 332 and AsHC 360) on the expression of 44 genes covering DNA repair, stress response, cell death, autophagy, and epigenetics via RT-qPCR in human liver (HepG2) cells. Both AsHCs affected the gene expression, but to different extents. After treatment with AsHC 360, flap structure-specific endonuclease 1 (FEN1) as well as xeroderma pigmentosum group A complementing protein (XPA) and (cytosine-5)-methyltransferase 3A (DNMT3A) showed time- and concentration-dependent alterations in gene expression, thereby indicating an impact on genomic stability. In the subsequent analysis of epigenetic markers, within 72 h, neither AsHC 332 nor AsHC 360 showed an impact on the global DNA methylation level, whereas incubation with AsHC 360 increased the global DNA hydroxymethylation level. Analysis of cell extracts and cell media by HPLC-mass spectrometry revealed that both AsHCs were considerably biotransformed. The identified metabolites include not only the respective thioxo-analogs of the two AsHCs, but also several arsenic-containing fatty acids and fatty alcohols, contributing to our knowledge of biotransformation mechanisms of arsenolipids.

Arsenite Binds to the RING Finger Domain of FANCL E3 Ubiquitin Ligase and Inhibits DNA Interstrand Crosslink Repair

Human exposure to arsenic in drinking water is known to be associated with the development of bladder, lung, kidney, and skin cancers. The molecular mechanisms underlying the carcinogenic effects of arsenic species remain incompletely understood. DNA interstrand cross-links (ICLs) are among the most cytotoxic type of DNA lesions that block DNA replication and transcription, and these lesions can be induced by endogenous metabolism and by exposure to exogenous agents. Fanconi anemia (FA) is a congenital disorder manifested with elevated sensitivity toward DNA interstrand cross-linking agents, and monoubiquitination of FANCD2 by FANCL is a crucial step in FA-mediated DNA repair. Here, we demonstrated that As³⁺ could bind to the PHD/RING finger domain of FANCL in vitro and in cells. This binding led to compromised ubiquitination of FANCD2 in cells and diminished recruitment of FANCD2 to chromatin and DNA damage sites induced by 4,5',8-trimethylpsoralen plus UVA irradiation. Furthermore, clonogenic survival assay results showed that arsenite coexposure rendered cells more sensitive toward DNA interstrand cross-linking agents. Together, our study suggested that arsenite may compromise genomic stability via perturbation of the Fanconi anemia pathway, thereby conferring its carcinogenic effect.

Metabolism, toxicity and anticancer activities of arsenic compounds

A variety of studies indicated that inorganic arsenic and its methylated metabolites have paradoxical effects, namely, carcinogenic and anticancer effects. Epidemiological studies have shown that long term exposure to arsenic can increase the risk of cancers of lung, skin or bladder in man, which is probably associated with the arsenic metabolism. In fact, the enzymatic conversion of inorganic arsenic by Arsenic (+3 oxidation state) methyltransferase (AS3MT) to mono- and dimethylated arsenic species has long been considered as a major route for detoxification. However, several studies have also indicated that biomethylation of inorganic arsenic, particularly the production of trivalent methylated metabolites, is a process that activates arsenic as a toxin and a carcinogen. On the other hand, arsenic trioxide (As2O3) has recently been recognized as one of the most effective drugs for the treatment of APL. However, elaboration of the cytotoxic mechanisms of arsenic and its methylated metabolites in eradicating cancer is sorely lacking. To provide a deeper understanding of the toxicity and carcinogenicity along with them use of arsenic in chemotherapy, caution is required considering the poor understanding of its various mechanisms of exerting toxicity. Thereby, in this review, we have focused on arsenic metabolic pathway, the roles of the methylated arsenic metabolites in toxicity and in the therapeutic efficacy for the treatments of solid tumors, APL and/or non-APL malignancies.

Identification of low-molecular-weight vitellogenin 1 (Vg1)-like proteins as nucleotide excision repair (NER) factors in developing zebrafish (Danio rerio) using a transcription-based DNA repair assay

Nucleotide excision repair (NER) removes helix-distorting DNA lesions such as UV-induced pyrimidine dimers and cisplatin-induced strand crosslinking. Our earlier studies have identified low-molecular-weight proteins homologous to the 150-kDa vitellogenin 1 (Vg1) as UV-damaged DNA-binding factors expressed in developing zebrafish (Danio rerio). This present study explored if Vg1-like proteins also participated in NER in zebrafish. Immunoblot analysis of affinity-captured 12 h post-fertilization (hpf) zebrafish extract proteins showed a transient binding of a 30-kDa Vg1-like polypeptide to UV-damaged DNA. A transcription-based in vitro repair assay revealed a significant up-regulation of UVC or cisplatin-suppressed transcriptional activity of a marker cDNA driven by a SP6 RNA polymerase-regulated promotor after incubating the damaged plasmid with the extracts of 12 hpf embryos or 96 hpf larvae. The up-regulation of UV or cisplatin-suppressed transcription was abolished in the presence of a monoclonal anti-zebrafish Vg1 antibody. The differential sensitivity of UV-induced repair in 12 and 96 hpf zebrafish extracts to exogenous ATP suggested a development-dependent expression of Vg1-like NER factors. A T₄ endonuclease V digestion assay showed no inhibition of the anti-Vg1 antibody on the excision of UV-induced cyclobutane pyrimidine dimers. Our results identified the participation of Vg1-like factors in NER in developing zebrafish, and these factors may function at post-incison steps of NER.

Inorganic arsenic inhibits the nucleotide excision repair pathway and reduces the expression of XPC

Chronic exposure to arsenic, most often through contaminated drinking water, has been linked to several types of cancer in humans, including skin and lung cancer. However, the mechanisms underlying its role in causing cancer are not well understood. There is evidence that exposure to arsenic can enhance the carcinogenicity of UV light in inducing skin cancers and may enhance the carcinogenicity of tobacco smoke in inducing lung cancers. The nucleotide excision repair (NER) pathway removes different types of DNA damage including those produced by UV light and components of tobacco smoke. The aim of the present study was to investigate the effect of sodium arsenite on the NER pathway in human lung fibroblasts (IMR-90 cells) and primary mouse keratinocytes. To measure NER, we employed a slot-blot assay to quantify the introduction and removal of UV light-induced 6-4 photoproducts (6-4 PP) and cyclobutane pyrimidine dimers (CPDs). We find a concentration-dependent inhibition of the removal of 6-4 PPs and CPDs in both cell types treated with arsenite. Treatment of both cell types with arsenite resulted in a significant reduction in the abundance of XPC, a protein that is critical for DNA damage recognition in NER. The abundance of RNA expressed from several key NER genes was also significantly reduced by treatment of IMR-90 cells with arsenite. Finally, treatment of IMR-90 cells with MG-132 abrogated the reduction in XPC protein, suggesting an involvement of the proteasome in the reduction of XPC protein produced by treatment of cells with arsenic. The inhibition of NER by arsenic may reflect one mechanism underlying the role of arsenic exposure in enhancing cigarette smoke-induced lung carcinogenesis and UV light-induced skin cancer, and it may provide some insights into the emergence of arsenic trioxide as a chemotherapeutic agent.

Kinetics and thermodynamics of zinc(II) and arsenic(III) binding to XPA and PARP-1 zinc finger peptides

Inhibition of DNA repair is an established mechanism of arsenic co-carcinogenesis, and may be perpetuated by the binding of As(III) to key zinc finger (zf) DNA repair proteins. Validated molecular targets of As(III) include the first zinc finger domain of Poly (ADP-Ribose) Polymerase 1 (PARP-1), and the zinc finger domain of Xeroderma Pigmentosum Complementation Group A (XPA). In order to gain an understanding of the thermodynamic and kinetic parameters of the interaction of As(III) with these two zinc finger motifs, a fluorescence based approach was used to investigate Zn(II) and As(III) binding to synthetic model peptides corresponding to the zf motif of XPA and first zf motif of PARP-1, referred to in this paper as XPAzf and PARP-1zf-1, respectively. While XPAzf and PARP-1zf-1 display similar relative affinities for As(III), PARP-1zf-1 shows a potential kinetic advantage over XPAzf for As(III) binding, with a rate constant for the fast phase of formation of As(III)-PARP-1zf-1 approximately 4-fold higher than for As(III)-XPAzf. However, the binding of Zn(II) with either peptide proceeds at a faster rate than As(III). Notably, XPAzf demonstrates comparable affinities for binding both metals, while PARP-1zf-1 shows a slightly higher affinity for Zn(II), suggesting that the relative concentrations of Zn(II) and As(III) in a system may significantly influence which species predominates in zinc finger occupancy. These results provide insight into the mechanisms underlying interactions between zinc finger structures and As(III), and highlight the potential utility of zinc supplementation in mitigating adverse effects of As(III) on zinc finger functions in vivo.

Arsenite Targets the Zinc Finger Domains of Tet Proteins and Inhibits Tet-Mediated Oxidation of 5-Methylcytosine

Arsenic toxicity is a serious public health problem worldwide that brings more than 100 million people into the risk of arsenic exposure from groundwater and food contamination. Although there is accumulating evidence linking arsenic exposure with aberrant cytosine methylation in the global genome or at specific genomic loci, very few have investigated the impact of arsenic on the oxidation of 5-methylcytosine (5-mC) mediated by the Ten-eleven translocation (Tet) family of proteins. Owing to the high binding affinity of As(III) toward cysteine residues, we reasoned that the highly conserved C3H-type zinc fingers situated in Tet proteins may constitute potential targets for arsenic binding. Herein, we found that arsenite could bind directly to the zinc fingers of Tet proteins in vitro and in cells, and this interaction substantially impaired the catalytic efficiency of Tet proteins in oxidizing 5-mC to 5-hydroxymethylcytosine (5-hmC), 5-formylcytosine (5-foC), and 5-carboxylcytosine (5-caC). Treatments with arsenite also led to a dose-dependent decrease in the level of 5-hmC, but not 5-mC, in DNA isolated from HEK293T cells overexpressing the catalytic domain of any of the three Tet proteins and from mouse embryonic stem cells. Together, our study unveiled, for the first time, that arsenite could alter epigenetic signaling by targeting the zinc fingers of Tet proteins and perturbing the Tet-mediated oxidation of 5-mC in vitro and in cells. Our results offer important mechanistic understanding of arsenic epigenotoxicity and carcinogenesis in mammalian systems and may lead to novel approaches for the chemoprevention of arsenic toxicity.

Arsenic Disruption of DNA Damage Responses-Potential Role in Carcinogenesis and Chemotherapy

Arsenic is a Class I human carcinogen and is widespread in the environment. Chronic arsenic exposure causes cancer in skin, lung and bladder, as well as in other organs. Paradoxically, arsenic also is a potent chemotherapeutic against acute promyelocytic leukemia and can potentiate the cytotoxic effects of DNA damaging chemotherapeutics, such as cisplatin, in vitro. Arsenic has long been implicated in DNA repair inhibition, cell cycle disruption, and ubiquitination dysregulation, all negatively impacting the DNA damage response and potentially contributing to both the carcinogenic and chemotherapeutic potential of arsenic. Recent studies have provided mechanistic insights into how arsenic interferes with these processes including disruption of zinc fingers and suppression of gene expression. This review discusses these effects of arsenic with a view toward understanding the impact on the DNA damage response.

DNA damage in Mexican children living in high-risk contaminated scenarios

The aim of this study was to evaluate the deoxyribonucleic acid (DNA) damage (as a biomarker of biological effects) in children living in areas at high risk of contamination in Mexico using the comet assay. The alkaline comet assay was performed in order to assess DNA damage levels in blood cells of 276 children living in eleven communities in four states of Mexico. Moreover, levels of arsenic and 1-hydroxypyrene (1-OHP) in urine and lead and total DDT [sum of 1,1-dichloro-2,2-bis(p-chlorophenyl) ethylene (DDE) and 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT)] in blood were quantified. We found urinary 1-OHP levels between <LOD and 14.5 μmol/mol creatinine; for arsenic, the urinary levels were 3.5-180 μg/g creatinine (range). Lead levels in blood ranged from 0.5 to 24 μg/dL and finally, the levels of total DDT (DDE and DDT) ranged from <LOD to 32,000 ng/g lipid. Regarding DNA damage (comet assay), the most important finding in our study was that children exposed to a chemical mixture [high levels of exposure to polycyclic aromatic hydrocarbons (PAHs) and DDT were found] had the significant highest DNA damage level (p<0.05) in their blood cells (olive tail moment=7.5±3.5), when compared with DNA damage levels in children living in the other scenarios assessed in this work. Finally, significant correlations were observed between urinary arsenic levels (r=0.32, p<0.05); urinary 1-OHP levels (r=0.65, p<0.01); total DDT in blood levels (r=0.59, p<0.01) and DNA damage. In conclusion, the data indicates that children living in areas which are at high risk of contamination showed high levels of biomarkers of exposure in urine or blood. Moreover, the exposure levels contribute to DNA damage and suggest an increased health risk in studied sites at risk of great pollution.

Molecular regulation of UV-induced DNA repair

Ultraviolet (UV) radiation from sunlight is a major etiologic factor for skin cancer, the most prevalent cancer in the United States, as well as premature skin aging. In particular, UVB radiation causes formation of specific DNA damage photoproducts between pyrimidine bases. These DNA damage photoproducts are repaired by a process called nucleotide excision repair, also known as UV-induced DNA repair. When left unrepaired, UVB-induced DNA damage leads to accumulation of mutations, predisposing people to carcinogenesis as well as to premature aging. Genetic loss of nucleotide excision repair leads to severe disorders, namely, xeroderma pigmentosum (XP), trichothiodystrophy (TTD) and Cockayne syndrome (CS), which are associated with predisposition to skin carcinogenesis at a young age as well as developmental and neurological conditions. Regulation of nucleotide excision repair is an attractive avenue to preventing or reversing these detrimental consequences of impaired nucleotide excision repair. Here, we review recent studies on molecular mechanisms regulating nucleotide excision repair by extracellular cues and intracellular signaling pathways, with a special focus on the molecular regulation of individual repair factors.

Arsenic-induced promoter hypomethylation and over-expression of ERCC2 reduces DNA repair capacity in humans by non-disjunction of the ERCC2-Cdk7 complex

Arsenic in drinking water is of critical concern in West Bengal, India, as it results in several physiological symptoms including dermatological lesions and cancers. Impairment of the DNA repair mechanism has been associated with arsenic-induced genetic damage as well as with several cancers. ERCC2 (Excision Repair Cross-Complementing rodent repair, complementation group 2), mediates DNA-repair by interacting with Cdk-activating kinase (CAK) complex, which helps in DNA proof-reading during transcription. Arsenic metabolism alters epigenetic regulation; we tried to elucidate the regulation of ERCC2 in arsenic-exposed humans. Water, urine, nails, hair and blood samples from one hundred and fifty seven exposed and eighty eight unexposed individuals were collected. Dose dependent validation was done in vitro using HepG2 and HEK-293. Arsenic content in the biological samples was higher in the exposed individuals compared with the content in unexposed individuals (p < 0.001). Bisulfite-modified methylation specific PCR showed a significant (p < 0.0001) hypomethylation of the ERCC2 promoter in the arsenic-exposed individuals. Densitometric analysis of immunoblots showed a nearly two-fold increase in expression of ERCC2 in exposed individuals, but there was an enhanced genotoxic insult as measured by micronuclei frequency. Immuno-precipitation and western blotting revealed an increased (p < 0.001) association of Cdk7 with ERCC2 in highly arsenic exposed individuals. The decrease in CAK activity was determined by observing the intensity of Ser(392) phosphorylation in p53, in vitro, which decreased with an increase in arsenic dose. Thus we infer that arsenic biotransformation leads to promoter hypomethylation of ERCC2, which in turn inhibits the normal functioning of the CAK-complex, thus affecting DNA-repair; this effect was highest among the arsenic exposed individuals with dermatological lesions.

The broccoli-born isothiocyanate sulforaphane impairs nucleotide excision repair: XPA as one potential target

The isothiocyanate sulforaphane (SFN), the major hydrolysis product of glucosinolates present in broccoli, has frequently been proposed to exert anticarcinogenic properties, mainly due to the induction of the nrf2/Keap1/ARE-signaling pathway. As potential underlying mechanism, a SFN-dependent zinc release from Keap1, the negative regulator of nrf2, has been described. This raises the question whether SFN is able to interfere with other zinc binding structures as well, for example those essential for DNA repair. Within this study, a SFN-induced deliberation of zinc from a synthesized peptide resembling the zinc binding domain of the xeroderma pigmentosum A (XPA) protein was observed starting at 50 μM SFN. Since XPA is absolutely required for nucleotide excision repair, the impact of SFN on the repair of (+)-anti-benzo[a]pyrene 7,8-diol-9,10-epoxide ((+)-anti-BPDE)-induced DNA adducts in HCT 116 cells was investigated. While preincubation with SFN did not affect initial lesion levels, a dose-dependent repair inhibition of (+)-anti-BPDE-induced DNA damage during the first 12 h after lesion induction was observed, starting at 1 μM SFN. In contrast, the later phase of DNA repair was not impaired by SFN. In support of an inactivation of XPA also in cells, SFN increased the (+)-anti-BPDE-induced cytotoxicity XPA dependently in XP12RO cells. Comparison of p53-proficient and p53-deficient cells revealed no difference in SFN-induced DNA repair inhibition, indicating that p53 is no cellular target of SFN. Since DNA repair processes are required to maintain DNA integrity, the presented data suggest a potential impairment of genomic stability by SFN.

Arsenite-induced ROS/RNS generation causes zinc loss and inhibits the activity of poly(ADP-ribose) polymerase-1

Arsenic enhances the genotoxicity of other carcinogenic agents such as ultraviolet radiation and benzo[a]pyrene. Recent reports suggest that inhibition of DNA repair is an important aspect of arsenic cocarcinogenesis, and DNA repair proteins such as poly(ADP ribose) polymerase (PARP)-1 are direct molecular targets of arsenic. Although arsenic has been shown to generate reactive oxygen/nitrogen species (ROS/RNS), little is known about the role of arsenic-induced ROS/RNS in the mechanism underlying arsenic inhibition of DNA repair. We report herein that arsenite-generated ROS/RNS inhibits PARP-1 activity in cells. Cellular exposure to arsenite, as well as hydrogen peroxide and NONOate (nitric oxide donor), decreased PARP-1 zinc content, enzymatic activity, and PARP-1 DNA binding. Furthermore, the effects of arsenite on PARP-1 activity, DNA binding, and zinc content were partially reversed by the antioxidant ascorbic acid, catalase, and the NOS inhibitor, aminoguanidine. Most importantly, arsenite incubation with purified PARP-1 protein in vitro did not alter PARP-1 activity or DNA-binding ability, whereas hydrogen peroxide or NONOate retained PARP-1 inhibitory activity. These results strongly suggest that cellular generation of ROS/RNS plays an important role in arsenite inhibition of PARP-1 activity, leading to the loss of PARP-1 DNA-binding ability and enzymatic activity.

In vitro toxicological characterization of two arsenosugars and their metabolites

SCOPE

In their recently published Scientific Opinion on Arsenic in Food, the European Food Safety Authority concluded that a risk assessment for arsenosugars is currently not possible, largely because of the lack of relevant toxicological data. To address this issue, we carried out a toxicological in vitro characterization of two arsenosugars and six arsenosugar metabolites.

METHODS AND RESULTS

The highly pure synthesized arsenosugars, DMA(V) -sugar-glycerol and DMA(V) -sugar-sulfate, investigated in this study, as well as four metabolites, oxo-dimethylarsenoacetic acid (oxo-DMAA(V) ), oxo-dimethylarsenoethanol (oxo-DMAE(V) ), thio-DMAA(V) and thio-DMAE(V) , exerted neither cytotoxicity nor genotoxicity up to 500 μM exposure in cultured human bladder cells. However, two arsenosugar metabolites, namely dimethyl-arsinic acid (DMA(V) ) and thio-dimethylarsinic acid (thio-DMA(V) ), were toxic to the cells; thio-DMA(V) was even slightly more cytotoxic than arsenite. Additionally, intestinal bioavailability of the arsenosugars was assessed applying the Caco-2 intestinal barrier model. The observed low, but significant transfer rates of the arsenosugars across the barrier model provide further evidence that arsenosugars are intestinally bioavailable.

CONCLUSION

In a cellular system that metabolizes arsenosugars, cellular toxicity likely arises. Thus, in strong contrast to arsenobetaine, arsenosugars cannot be categorized as nontoxic for humans and a risk to human health cannot be excluded.

Reduction of arsenite-enhanced ultraviolet radiation-induced DNA damage by supplemental zinc

Arsenic is a recognized human carcinogen and there is evidence that arsenic augments the carcinogenicity of DNA damaging agents such as ultraviolet radiation (UVR) thereby acting as a co-carcinogen. Inhibition of DNA repair is one proposed mechanism to account for the co-carcinogenic actions of arsenic. We and others find that arsenite interferes with the function of certain zinc finger DNA repair proteins. Furthermore, we reported that zinc reverses the effects of arsenite in cultured cells and a DNA repair target protein, poly (ADP-ribose) polymerase-1. In order to determine whether zinc ameliorates the effects of arsenite on UVR-induced DNA damage in human keratinocytes and in an in vivo model, normal human epidermal keratinocytes and SKH-1 hairless mice were exposed to arsenite, zinc or both before solar-simulated (ss) UVR exposure. Poly (ADP-ribose) polymerase activity, DNA damage and mutation frequencies at the Hprt locus were measured in each treatment group in normal human keratinocytes. DNA damage was assessed in vivo by immunohistochemical staining of skin sections isolated from SKH-1 hairless mice. Cell-based findings demonstrate that ssUVR-induced DNA damage and mutagenesis are enhanced by arsenite, and supplemental zinc partially reverses the arsenite effect. In vivo studies confirm that zinc supplementation decreases arsenite-enhanced DNA damage in response to ssUVR exposure. From these data we can conclude that zinc offsets the impact of arsenic on ssUVR-stimulated DNA damage in cells and in vivo suggesting that zinc supplementation may provide a strategy to improve DNA repair capacity in arsenic exposed human populations.

Metal interaction with redox regulation: an integrating concept in metal carcinogenesis?

The carcinogenicity of cadmium, arsenic, and chromium(VI) compounds has been recognized for some decades. However, the underlying molecular mechanisms seem to be complex and are not completely understood at present. Although, with the exception of chromium(VI), direct DNA damage seems to be of minor importance, interactions with DNA repair processes, tumor suppressor functions, and signal transduction pathways have been described in diverse biological systems. In addition to the induction of damage to cellular macromolecules by reactive oxygen species, the interference with cellular redox regulation by reaction with redox-sensitive protein domains or amino acids may provide one plausible mechanism involved in metal carcinogenicity. Consequences are the distortion of zinc-binding structures and the activation or inactivation of redox-regulated signal transduction pathways, provoking metal-induced genomic instability. Nevertheless, the relevance of the respective mechanisms depends on the actual metal or metal species under consideration and more research is needed to further strengthen this hypothesis.

Selectivity of arsenite interaction with zinc finger proteins

Arsenic is a carcinogenic element also used for the treatment of acute promyelocytic leukemia. The reactivity of proteins to arsenic must be associated with the various biological functions of As. Here, we investigated the selectivity of arsenite to zinc finger proteins (ZFPs) with different zinc binding motifs (C2H2, C3H, and C4). Single ZFP domain proteins were used for the direct comparison of the reactivity of different ZFPs. The binding constants and the reaction rates have been studied quantitatively. Results show that both the binding affinity and reaction rates of single-domain ZFPs follow the trend of C4 > C3H ≫ C2H2. Compared with the C2H2 motif ZFPs, the binding affinities of C3H and C4 motif ZFPs are nearly two orders of magnitude higher and the reaction rates are approximately two-fold higher. The formation of multi-domain ZFPs significantly enhances the reactivity of C2H2 type ZFPs, but has negligible effects on C3H and C4 ZFPs. Consequently, the reactivities of the three types of multi-domain ZFPs are rather similar. The 2D NMR spectra indicate that the As(III)-bound ZFPs are also unfolded, suggesting that arsenic binding interferes with the function of ZFPs.

Response to arsenate treatment in Schizosaccharomyces pombe and the role of its arsenate reductase activity

Arsenic toxicity has been studied for a long time due to its effects in humans. Although epidemiological studies have demonstrated multiple effects in human physiology, there are many open questions about the cellular targets and the mechanisms of response to arsenic. Using the fission yeast Schizosaccharomyces pombe as model system, we have been able to demonstrate a strong activation of the MAPK Spc1/Sty1 in response to arsenate. This activation is dependent on Wis1 activation and Pyp2 phosphatase inactivation. Using arsenic speciation analysis we have also demonstrated the previously unknown capacity of S. pombe cells to reduce As (V) to As (III). Genetic analysis of several fission yeast mutants point towards the cell cycle phosphatase Cdc25 as a possible candidate to carry out this arsenate reductase activity. We propose that arsenate reduction and intracellular accumulation of arsenite are the key mechanisms of arsenate tolerance in fission yeast.

Arsenic toxicity: the effects on plant metabolism

The two forms of inorganic arsenic, arsenate (AsV) and arsenite (AsIII), are easily taken up by the cells of the plant root. Once in the cell, AsV can be readily converted to AsIII, the more toxic of the two forms. AsV and AsIII both disrupt plant metabolism, but through distinct mechanisms. AsV is a chemical analog of phosphate that can disrupt at least some phosphate-dependent aspects of metabolism. AsV can be translocated across cellular membranes by phosphate transport proteins, leading to imbalances in phosphate supply. It can compete with phosphate during phosphorylation reactions, leading to the formation of AsV adducts that are often unstable and short-lived. As an example, the formation and rapid autohydrolysis of AsV-ADP sets in place a futile cycle that uncouples photophosphorylation and oxidative phosphorylation, decreasing the ability of cells to produce ATP and carry out normal metabolism. AsIII is a dithiol reactive compound that binds to and potentially inactivates enzymes containing closely spaced cysteine residues or dithiol co-factors. Arsenic exposure generally induces the production of reactive oxygen species that can lead to the production of antioxidant metabolites and numerous enzymes involved in antioxidant defense. Oxidative carbon metabolism, amino acid and protein relationships, and nitrogen and sulfur assimilation pathways are also impacted by As exposure. Readjustment of several metabolic pathways, such as glutathione production, has been shown to lead to increased arsenic tolerance in plants. Species- and cultivar-dependent variation in arsenic sensitivity and the remodeling of metabolite pools that occurs in response to As exposure gives hope that additional metabolic pathways associated with As tolerance will be identified.

DNA damage and decreased DNA repair in peripheral blood mononuclear cells in individuals exposed to arsenic and lead in a mining site

The aim of this study was to evaluate DNA damage and the capacity for DNA repair in children exposed to arsenic and lead. During 2006, we studied a total of 85 healthy children (aged 4-11 years) who were residents of Villa de la Paz (community A), Matehuala (community B), and Soledad de Graciano Sanchez (community C) in San Luis Potosi, Mexico. The quantification of arsenic in urine (AsU) and lead in blood (PbB) was performed by atomic absorption spectrophotometry. The alkaline comet assay was used to evaluate DNA damage and DNA repair. The highest levels of AsU and PbB in children were found in community A (44.5 μg/g creatinine for arsenic and 11.4 μg/dL for lead), followed by community B (16.8 μg/g creatinine for arsenic and 7.3 μg/dL for lead) and finally by children living in community C (12.8 μg/g creatinine for arsenic and 5.3 μg/dL for lead). When DNA damage was assessed, children living in community A had the highest DNA damage. Analysis of these same cells 1 h after a challenge with H(2)O(2) 10 μM showed a dramatic increase in DNA damage in the cells of children living in community B and community C, but not in the cells of children living in community A. Moreover, significantly higher levels of DNA damage were observed 3 h after the challenge ended (repair period) in cells from individuals living in community A. Our results show that children exposed to metals might be more susceptible to DNA alterations.

Effects of manganese and arsenic species on the level of energy related nucleotides in human cells

Cellular adenine and pyridine nucleotides play important roles in the cellular energy and redox state. An imbalance in the cellular levels of these tightly regulated energy related nucleotides can lead to oxidative stress and thus is discussed to contribute to neurotoxic and carcinogenic processes. Here we established a reliable ion-pair reversed phase HPLC based method for the parallel quantification of six energy related nucleotides (ATP, ADP, ADP-ribose, AMP, NAD(+), NADH) in cells and subsequently applied it to determine effects of manganese and arsenic species in cultured human cells. In human lung cells, MnCl(2) (≥50 μM) decreased the levels of ATP, NAD(+) and NADH as well as the NAD(+)/NADH ratio. This reflects a decline in the cellular energy metabolism, most likely resulting from a disturbance of the mitochondrial function. In contrast, cultured astrocytes were more resistant towards manganese. Regarding the arsenicals, a disturbance of the cellular energy related nucleotides was detected in lung cells for arsenite (≥50 μM), monomethylarsonous (≥1 μM), dimethylarsinous (≥1 μM) and dimethylarsinic acid (≥100 μM). Thereby, the single arsenicals seem to disturb the cellular energy and redox state by different mechanisms. Taken together, this study provides further evidence that cellular energy related nucleotides serve as sensitive indicators for toxic species exposure. When searching for a molecular mechanism of toxic compounds, the data illustrate the necessity of quantifying several energy related nucleotides in parallel, especially since ATP depletion, redox state alterations and oxidative stress are known to potentiate each other.

Advances in carcinogenic metal toxicity and potential molecular markers

Metal compounds such as arsenic, cadmium, chromium, cobalt, lead, mercury, and nickel are classified as carcinogens affecting human health through occupational and environmental exposure. However, the underlying mechanisms involved in tumor formation are not well clarified. Interference of metal homeostasis may result in oxidative stress which represents an imbalance between production of free radicals and the system's ability to readily detoxify reactive intermediates. This event consequently causes DNA damage, lipid peroxidation, protein modification, and possibly symptomatic effects for various diseases including cancer. This review discusses predominant modes of action and numerous molecular markers. Attention is paid to metal-induced generation of free radicals, the phenomenon of oxidative stress, damage to DNA, lipid, and proteins, responsive signal transduction pathways with major roles in cell growth and development, and roles of antioxidant enzymatic and DNA repair systems. Interaction of non-enzymatic antioxidants (carotenoids, flavonoids, glutathione, selenium, vitamin C, vitamin E, and others) with cellular oxidative stress markers (catalase, glutathione peroxidase, and superoxide dismutase) as well as certain regulatory factors, including AP-1, NF-κB, Ref-1, and p53 is also reviewed. Dysregulation of protective pathways, including cellular antioxidant network against free radicals as well as DNA repair deficiency is related to oncogenic stimulation. These observations provide evidence that emerging oxidative stress-responsive regulatory factors and DNA repair proteins are putative predictive factors for tumor initiation and progression.

Arsenic biotransformation as a cancer promoting factor by inducing DNA damage and disruption of repair mechanisms

Chronic exposure to arsenic in drinking water poses a major global health concern. Populations exposed to high concentrations of arsenic-contaminated drinking water suffer serious health consequences, including alarming cancer incidence and death rates. Arsenic is biotransformed through sequential addition of methyl groups, acquired from s-adenosylmethionine (SAM). Metabolism of arsenic generates a variety of genotoxic and cytotoxic species, damaging DNA directly and indirectly, through the generation of reactive oxidative species and induction of DNA adducts, strand breaks and cross links, and inhibition of the DNA repair process itself. Since SAM is the methyl group donor used by DNA methyltransferases to maintain normal epigenetic patterns in all human cells, arsenic is also postulated to affect maintenance of normal DNA methylation patterns, chromatin structure, and genomic stability. The biological processes underlying the cancer promoting factors of arsenic metabolism, related to DNA damage and repair, will be discussed here.

Toxicological Characterization of the Inorganic and Organic Arsenic Metabolite Thio-DMA in Cultured Human Lung Cells

We synthesised and toxicologically characterised the arsenic metabolite thiodimethylarsinic acid (thio-DMA^V). Successful synthesis of highly pure thio-DMA^V was confirmed by state-of-the-art analytical techniques including ¹H-NMR, HPLC-FTMS, and HPLC-ICPMS. Toxicological characterization was carried out in comparison to arsenite and its well-known trivalent and pentavalent methylated metabolites. It comprised cellular bioavailability as well as different cytotoxicity and genotoxicity end points in cultured human A549 lung cells. Of all arsenicals investigated, thio-DMA^V exerted the strongest cytotoxicity. Moreover, thio-DMA^V did not induce DNA strand breaks and an increased induction of both micronuclei and multinucleated cells occurred only at beginning cytotoxic concentrations, indicating that thio-DMA^V does not act via a genotoxic mode of action. Finally, to assess potential implications of thio-DMA^V for human health, further mechanistic studies are urgently necessary to identify the toxic mode of action of this highly toxic, unusual pentavalent organic arsenical.

Arsenicals affect base excision repair by several mechanisms

Inorganic arsenic is a strong, widespread human carcinogen. How exactly inorganic arsenic exerts carcinogenicity in humans is as yet unclear, but it is thought to be closely related to its metabolism. At exposure-relevant concentrations arsenic is neither directly DNA reactive nor mutagenic. Thus, more likely epigenetic and indirect genotoxic effects, among others a modulation of the cellular DNA damage response and DNA repair, are important molecular mechanisms contributing to its carcinogenicity. In the present study, we investigated the impact of arsenic on several base excision repair (BER) key players in cultured human lung cells. For the first time gene expression, protein level and in case of human 8-oxoguanine DNA glycosylase 1 (hOGG1) protein function was examined in one study, comparing inorganic arsenite and its trivalent and pentavalent mono- and dimethylated metabolites, also taking into account their cellular bioavailability. Our data clearly show that arsenite and its metabolites can affect several cellular endpoints related to DNA repair. Thus, cellular OGG activity was most sensitively affected by dimethylarsinic acid (DMA(V)), DNA ligase IIIα (LIGIIIα) protein level by arsenite and X-ray cross complementing protein 1 (XRCC1 protein) content by monomethylarsonic acid (MMA(V)), with significant effects starting at ≥3.2μM cellular arsenic. With respect to MMA(V), to our knowledge these effects are the most sensitive endpoints, related to DNA damage response, that have been identified so far. In contrast to earlier nucleotide excision repair related studies, the trivalent methylated metabolites exerted strong effects on the investigated BER key players only at cytotoxic concentrations. In summary, our data point out that after mixed arsenic species exposure, a realistic scenario after oral inorganic arsenic intake in humans, DNA repair might be affected by different mechanisms and therefore very effectively, which might facilitate the carcinogenic process of inorganic arsenic.

Oxidative modifications of proteins by sodium arsenite in human umbilical vein endothelial cells

Epidemiologic studies have demonstrated that chronic arsenic exposure is associated with the incidence of chronic diseases. This association is partly related to the increase in reactive oxygen species (ROS) overload and protein oxidation that result from arsenic exposure. In this study, we intended to identify proteins susceptible to oxidative carbonylation by sodium arsenite and the impact of carbonylation on the function of these proteins in human umbilical vein endothelial cells (HUVECs). The 2,4-dinitrophenylhydrazine (DNPH) dot-blot assay revealed that arsenite (0-50 μM) dose-dependently increased protein carbonylation. Consistent with these findings, the cellular ROS level as measured by 2',7'-dichlorofluorescein diacetate (DCHF-DA) assay was increased in cells exposed to arsenite. By two-dimensional gel electrophoresis and matrix assist laser desorption ionization time of flight mass spectrometry (MALDI-TOF/MS), one glycolytic enzyme, enolase-α, two cytoskeleton proteins, fascin (F-actin associated protein) and vimentin, and two protein quality control proteins, HSC70 (heat-shock cognate protein 70), and PDIA3 (protein disulfide isomerase family A, member 3) were identified to be arsenic-sensitive carbonlyated proteins. Accompanied by carbonylation, enolase-α activity was dose-dependently decreased and the F-actin filament network was disturbed. Taken together, our results suggest that arsenite exposure results in the generation of carbonylated proteins, and the resultant changes in energy metabolism and in the cytoskeletal network may partly lead to cell damage.

Arsenic biotransformation as a cancer promoting factor by inducing DNA damage and disruption of repair mechanisms

Chronic exposure to arsenic in drinking water poses a major global health concern. Populations exposed to high concentrations of arsenic-contaminated drinking water suffer serious health consequences, including alarming cancer incidence and death rates. Arsenic is biotransformed through sequential addition of methyl groups, acquired from s-adenosylmethionine (SAM). Metabolism of arsenic generates a variety of genotoxic and cytotoxic species, damaging DNA directly and indirectly, through the generation of reactive oxidative species and induction of DNA adducts, strand breaks and cross links, and inhibition of the DNA repair process itself. Since SAM is the methyl group donor used by DNA methyltransferases to maintain normal epigenetic patterns in all human cells, arsenic is also postulated to affect maintenance of normal DNA methylation patterns, chromatin structure, and genomic stability. The biological processes underlying the cancer promoting factors of arsenic metabolism, related to DNA damage and repair, will be discussed here.

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.

Arsenite interacts selectively with zinc finger proteins containing C3H1 or C4 motifs

Arsenic inhibits DNA repair and enhances the genotoxicity of DNA-damaging agents such as benzo[a]pyrene and ultraviolet radiation. Arsenic interaction with DNA repair proteins containing functional zinc finger motifs is one proposed mechanism to account for these observations. Here, we report that arsenite binds to both CCHC DNA-binding zinc fingers of the DNA repair protein PARP-1 (poly(ADP-ribose) polymerase-1). Furthermore, trivalent arsenite coordinated with all three cysteine residues as demonstrated by MS/MS. MALDI-TOF-MS analysis of peptides harboring site-directed substitutions of cysteine with histidine residues within the PARP-1 zinc finger revealed that arsenite bound to peptides containing three or four cysteine residues, but not to peptides with two cysteines, demonstrating arsenite binding selectivity. This finding was not unique to PARP-1; arsenite did not bind to a peptide representing the CCHH zinc finger of the DNA repair protein aprataxin, but did bind to an aprataxin peptide mutated to a CCHC zinc finger. To investigate the impact of arsenite on PARP-1 zinc finger function, we measured the zinc content and DNA-binding capacity of PARP-1 immunoprecipitated from arsenite-exposed cells. PARP-1 zinc content and DNA binding were decreased by 76 and 80%, respectively, compared with protein isolated from untreated cells. We observed comparable decreases in zinc content for XPA (xeroderma pigmentosum group A) protein (CCCC zinc finger), but not SP-1 (specificity protein-1) or aprataxin (CCHH zinc finger). These findings demonstrate that PARP-1 is a direct molecular target of arsenite and that arsenite interacts selectively with zinc finger motifs containing three or more cysteine residues.

A transgenic Drosophila model for arsenic methylation suggests a metabolic rationale for differential dose-dependent toxicity endpoints

The mechanisms by which exposure to arsenic induces its myriad pathological effects are undoubtedly complex, while individual susceptibility to their type and severity is likely to be strongly influenced by genetic factors. Human metabolism of arsenic into methylated derivatives, once presumed to result in detoxification, may actually produce species with significantly greater pathological potential. We introduce a transgenic Drosophila model of arsenic methylation, allowing its consequences to be studied in a higher eukaryote exhibiting conservation of many genes and pathways with those of human cells while providing an important opportunity to uncover mechanistic details via the sophisticated genetic analysis for which the system is particularly well suited. The gene for the human enzyme, arsenic (+3 oxidation state) methyltransferase, was introduced into nonmethylating Drosophila under inducible control. Transgenic flies were characterized for enzyme inducibility, production of methylated arsenic species, and the dose-dependent consequences for chromosomal integrity and organismal longevity. Upon enzyme induction, transgenic flies processed arsenite into mono and dimethylated derivatives identical to those found in human urine. When induced flies were exposed to 9 ppm arsenite, chromosomal stability was clearly reduced, whereas at much higher doses, adult life span was significantly increased, a seemingly paradoxical pair of outcomes. Measurement of arsenic body burden in the presence or absence of methylation suggested that enhanced clearance of methylated species might explain this greater longevity under acutely toxic conditions. Our study clearly demonstrates both the hazards and the benefits of arsenic methylation in vivo and suggests a resolution based on evolutionary grounds.

Formation and repair of tobacco carcinogen-derived bulky DNA adducts

DNA adducts play a central role in chemical carcinogenesis. The analysis of formation and repair of smoking-related DNA adducts remains particularly challenging as both smokers and nonsmokers exposed to smoke are repetitively under attack from complex mixtures of carcinogens such as polycyclic aromatic hydrocarbons and N-nitrosamines. The bulky DNA adducts, which usually have complex structure, are particularly important because of their biological relevance. Several known cellular DNA repair pathways have been known to operate in human cells on specific types of bulky DNA adducts, for example, nucleotide excision repair, base excision repair, and direct reversal involving O⁶-alkylguanine DNA alkyltransferase or AlkB homologs. Understanding the mechanisms of adduct formation and repair processes is critical for the assessment of cancer risk resulting from exposure to cigarette smoke, and ultimately for developing strategies of cancer prevention. This paper highlights the recent progress made in the areas concerning formation and repair of bulky DNA adducts in the context of tobacco carcinogen-associated genotoxic and carcinogenic effects.