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

Global gene expression associated with hepatocarcinogenesis in adult male mice induced by in utero arsenic exposure

Our previous work has shown that exposure to inorganic arsenic in utero produces hepatocellular carcinoma (HCC) in adult male mice. To explore further the molecular mechanisms of transplacental arsenic hepatocarcinogenesis, we conducted a second arsenic transplacental carcinogenesis study and used a genomewide microarray to profile arsenic-induced aberrant gene expression more extensively. Briefly, pregnant C3H mice were given drinking water containing 85 ppm arsenic as sodium arsenite or unaltered water from days 8 to 18 of gestation. The incidence of HCC in adult male offspring was increased 4-fold and tumor multiplicity 3-fold after transplacental arsenic exposure. Samples of normal liver and liver tumors were taken at autopsy for genomic analysis. Arsenic exposure in utero resulted in significant alterations (p < 0.001) in the expression of 2,010 genes in arsenic-exposed liver samples and in the expression of 2,540 genes in arsenic-induced HCC. Ingenuity Pathway Analysis revealed that significant alterations in gene expression occurred in a number of biological networks, and Myc plays a critical role in one of the primary networks. Real-time reverse transcriptase-polymerase chain reaction and Western blot analysis of selected genes/proteins showed > 90% concordance. Arsenic-altered gene expression included activation of oncogenes and HCC biomarkers, and increased expression of cell proliferation-related genes, stress proteins, and insulin-like growth factors and genes involved in cell-cell communications. Liver feminization was evidenced by increased expression of estrogen-linked genes and altered expression of genes that encode gender-related metabolic enzymes. These novel findings are in agreement with the biology and histology of arsenic-induced HCC, thereby indicating that multiple genetic events are associated with transplacental arsenic hepatocarcinogenesis.

The impact of diet and betel nut use on skin lesions associated with drinking-water arsenic in Pabna, Bangladesh

An established exposure-response relationship exists between water arsenic levels and skin lesions. Results of previous studies with limited historical exposure data, and laboratory animal studies suggest that diet may modify arsenic metabolism and toxicity. In this study, we evaluated the effect of diet on the risk of arsenic-related skin lesions in Pabna, Bangladesh. Six hundred cases and 600 controls loosely matched on age and sex were enrolled at Dhaka Community Hospital, Bangladesh, in 2001-2002. Diet, demographic data, and water samples were collected. Water samples were analyzed for arsenic using inductively coupled plasma mass spectroscopy. Betel nut use was associated with a greater risk of skin lesions in a multivariate model [odds ratio (OR) = 1.67; 95% confidence interval (CI), 1.18-2.36]. Modest decreases in risk of skin lesions were associated with fruit intake 1-3 times/month (OR = 0.68; 95%CI, 0.51-0.89) and canned goods at least 1 time/month (OR = 0.41; 95% CI, 0.20-0.86). Bean intake at least 1 time/day (OR = 1.89; 95% CI, 1.11-3.22) was associated with increased odds of skin lesions. Betel nut use appears to be associated with increased risk of developing skin lesions in Bangladesh. Increased intake of fruit and canned goods may be associated with reduced risk of lesions. Increased intake of beans may be associated with an increased risk of skin lesions. The results of this study do not provide clear support for a protective effect of vegetable and overall protein consumption against the development of skin lesions, but a modest benefit cannot be excluded.

Distinct gene expression profiles in immortalized human urothelial cells exposed to inorganic arsenite and its methylated trivalent metabolites

Inorganic arsenic is an environmental carcinogen. The generation of toxic trivalent methylated metabolites complicates the study of arsenic-mediated carcinogenesis. This study systematically evaluated the effect of chronic treatment with sodium arsenite (iAs(III)), monomethylarsonous acid (MMA(III)), and dimethylarsinous acid (DMA(III)) on immortalized human uroepithelial cells (SV-HUC-1 cells) using cDNA microarray. After exposure for 25 passages to iAs(III) (0.5 microM), MMA(III) (0.05, 0.1, or 0.2 microM), or DMA(III) (0.2 or 0.5 microM), significant compound-specific morphologic changes were observed. A set of 114 genes (5.7% of the examined genes) was differentially expressed in one or more sets of arsenical-treated cells compared with untreated controls. Expression analysis showed that exposure of cells to DMA(III) resulted in a gene profile different from that in cells exposed to iAs(III) or MMA(III), and that the iAs(III)-induced gene profile was closest to that in the tumorigenic HUC-1-derived 3-methylcholanthrene-induced tumorigenic cell line MC-SV-HUC T2, which was derived from SV-HUC-1 cells by methylcholanthrene treatment. Of the genes affected by all three arsenicals, only one, that coding for interleukin-1 receptor, type II, showed enhanced expression, a finding confirmed by the reduced increase in NF-kappaB (nuclear factor kappa B) activity seen in response to interleukin-1beta in iAs(III)-exposed cells. The expression of 11 genes was suppressed by all three arsenicals. 5-Aza-deoxycytidine partially restored the transcription of several suppressed genes, showing that epigenetic DNA methylation was probably involved in arsenical-induced gene repression. Our data demonstrate that chronic exposure to iAs(III), MMA(III), or DMA(III) has different epigenetic effects on urothelial cells and represses NF-kappaB activity.

Assessment of lymphocyte subpopulations and cytokine secretion in children exposed to arsenic

Exposure of several human populations to arsenic has been associated with a high incidence of detrimental dermatological and carcinogenic effects. To date, studies examining the immunotoxic effects of arsenic in humans, and specifically in children, are lacking. Therefore, we evaluated several parameters of immunological status in a group of children exposed to arsenic through their drinking water. Peripheral blood mononuclear cells (PBMCs) of 90 children (6 to 10 years old) were collected. Proportions of lymphocyte subpopulations, PBMC mitogenic proliferative response, and urinary arsenic levels were evaluated. Increased urine arsenic levels were associated with a reduced proliferative response to phytohemaglutinin (PHA) stimulation (P=0.005), CD4 subpopulation proportion (P=0.092), CD4/CD8 ratio (P=0.056), and IL-2 secretion levels (P=0.003). Increased arsenic exposure was also associated with an increase in GM-CSF secretion by mononucleated cells (P=0.000). We did not observe changes in CD8, B, or NK cell proportions, nor did we observe changes in the secretion of IL-4, IL-10, or IFN-gamma by PHA-activated PBMCs. These data indicate that arsenic exposure could alter the activation processes of T cells, such that an immunosuppression status that favors opportunistic infections and carcinogenesis is produced together with increased GM-CSF secretion that may be associated with chronic inflammation.

Altered iron homeostasis involvement in arsenite-mediated cell transformation

Chronic exposure to low doses of arsenite causes transformation of human osteogenic sarcoma (HOS) cells. Although oxidative stress is considered important in arsenite-induced cell transformation, the molecular and cellular mechanisms by which arsenite transforms human cells are still unknown. In the present study, we investigated whether altered iron homeostasis, known to affect cellular oxidative stress, can contribute to the arsenite-mediated cell transformation. Using arsenite-induced HOS cell transformation as a model, it was found that total iron levels are significantly higher in transformed HOS cells in comparison to parental control HOS cells. Under normal iron metabolism conditions, iron homeostasis is tightly controlled by inverse regulation of ferritin and transferrin receptor (TfR) through iron regulatory proteins (IRP). Increased iron levels in arsenite transformed cells should theoretically lead to higher ferritin and lower TfR in these cells than in controls. However, the results showed that both ferritin and TfR are decreased, apparently through two different mechanisms. A lower ferritin level in cytoplasm was due to the decreased mRNA in the arsenite-transformed HOS cells, while the decline in TfR was due to a lowered IRP-binding activity. By challenging cells with iron, it was further established that arsenite-transformed HOS cells are less responsive to iron treatment than control HOS cells, which allows accumulation of iron in the transformed cells, as exemplified by significantly lower ferritin induction. On the other hand, caffeic acid phenethyl ester (CAPE), an antioxidant previously shown to suppress As-mediated cell transformation, prevents As-mediated ferritin depletion. In conclusion, our results suggest that altered iron homeostasis contributes to arsenite-induced oxidative stress and, thus, may be involved in arsenite-mediated cell transformation.

Nucleolin links to arsenic-induced stabilization of GADD45alpha mRNA

The present study shows that arsenic induces GADD45alpha (growth arrest and DNA damage inducible gene 45alpha) mainly through post-transcriptional mechanism. Treatment of the human bronchial epithelial cell line, BEAS-2B, with arsenic(III) chloride (As3+) resulted in a significant increase in GADD45alpha protein and mRNA. However, As3+ only exhibited a marginal effect on the transcription of the GADD45alpha gene. The accumulation of GADD45alpha mRNA is largely achieved by the stabilization of GADD45alpha mRNA in the cellular response to As3+. As3+ is able to induce binding of mRNA stabilizing proteins, nucleolin and less potently, HuR, to the GADD45alpha mRNA. Although As3+ was unable to affect the expression of nucleolin, treatment of the cells with As3+ resulted in re-distribution of nucleolin from nucleoli to nucleoplasm. Silencing of the nucleolin mRNA by RNA interference reversed As3+-induced stabilization of the GADD45alpha mRNA and accumulation of the GADD45alpha protein. Stabilization of GADD45alpha mRNA, thus, represents a novel mechanism contributing to the production of GADD45alpha and cell cycle arrest in response to As3+.

Urinary 8-hydroxy-2′-deoxyguanosine in inhabitants chronically exposed to arsenic in groundwater in Cambodia

Arsenic concentrations in hair and urine, and urinary levels of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a marker of oxidative DNA damage, were examined for inhabitants of the Mekong Basin in Kratie Province, Cambodia. Also, the arsenic levels of tube-well water were determined. Total arsenic concentrations in tube-well water ranged from <1 to 886 microg L(-1), and 44.8% of these exceeded the WHO drinking water guideline of 10 microg L(-1). Elevated levels of arsenic were observed in the human hair and urine, and also a significant positive correlation was observed between the concentrations in hair and urine. These results suggest that the inhabitants are chronically exposed to arsenic through drinking the tube-well water. Levels of urinary 8-OHdG were higher for the subjects with higher arsenic levels in hair and urine, suggesting that induction of oxidative DNA damage was caused by chronic exposure to arsenic in tube-well water for the inhabitants in Kratie Province. To our knowledge, this is the first report on the oxidative DNA damage caused by chronic exposure to arsenic in groundwater for the inhabitants in Cambodia.

Human health effects from chronic arsenic poisoning--a review

The ill effects of human exposure to arsenic (As) have recently been reevaluated by government agencies around the world. This has lead to a lowering of As guidelines in drinking water, with Canada decreasing the maximum allowable level from 50 to 25 microg/L and the U.S. from 50 to 10 microg/L. Canada is currently contemplating a further decrease to 5 microg/L. The reason for these regulatory changes is the realization that As can cause deleterious effects at lower concentrations than was previously thought. There is a strong relationship between chronic ingestion of As and deleterious human health effects and here we provide an overview of some of the major effects documented in the scientific literature. As regulatory levels of As have been decreased, an increasing number of water supplies will now require removal of As before the water can be used for human consumption. While As exposure can occur from food, air and water, all major chronic As poisonings have stemmed from water and this is usually the predominant exposure route. Exposure to As leads to an accumulation of As in tissues such as skin, hair and nails, resulting in various clinical symptoms such as hyperpigmentation and keratosis. There is also an increased risk of skin, internal organ, and lung cancers. Cardiovascular disease and neuropathy have also been linked to As consumption. Verbal IQ and long term memory can also be affected, and As can suppress hormone regulation and hormone mediated gene transcription. Increases in fetal loss and premature delivery, and decreased birth weights of infants, can occur even at low (<10 microg/L) exposure levels. Malnourished people have been shown to be more predisposed to As-related skin lesions. A large percentage of the population (30-40%) that is using As-contaminated drinking water can have elevated As levels in urine, hair and nails, while showing no noticeable clinical symptoms, such as skin lesions. It is therefore important to carry out clinical tests of As exposure. Factors combining to increase/decrease the ill effects of As include duration and magnitude of As exposure, source of As exposure, nutrition, age and general health status. Analytical determinations of As poisoning can be made by examining As levels in urine, hair and toenails. Communities and individuals relying on groundwater sources for drinking water need to measure As levels to ensure that their supplies are safe. Communities with water As levels greater than 5 microg/L should consider a program to document As levels in the population.

Arsenic-Induced Cell Death in Liver and Brain of Experimental Rats

Arsenic is a well established human carcinogen and is ubiquitous in the environment. The present study demonstrates the effect of acute arsenic administration at three different doses in liver and brain of Wistar rats. Sodium arsenite was administered orally at doses of 6.3 mg/kg, 10.5 mg/kg and 12.6 mg/kg of body weight on the basis of a lethal dose 50% (LD50) for 24 hr. After administration of arsenites, liver and brain were analyzed for various parameters of oxidative stress, histopathological changes and caspase-3 activity. Glutathione levels were decreased significantly in the liver at all doses. In liver the following biochemical changes were observed, a significant lipid peroxidation and cytochrome-P450 induction along with significant decrease in catalase and superoxide dismutase was observed at 10.5 mg/kg and 12.6 mg/kg. The activity of glutathione peroxidase was increased significantly at all doses. In brain, no significant change was observed at 6.3 mg/kg. However, a significant increase in lipid peroxidation and glutathione peroxidase activity along with significant decrease in the activity of glutathione, catalase and superoxide dismutase was observed at 10.5 mg/kg and 12.6 mg/kg. The activity of glutathione-S-transferase was decreased significantly in both liver and brain at 10.5 and 12.6 mg/kg. No significant alteration in the activity of glucose-6-phosphate dehydrogenase and glutathione reductase was observed in either liver or brain at any dose. Dose-dependent histopathological changes, observed in both liver and brain are also described. A significant increase in caspase-3 activity was observed at all doses in liver and at 10.5 and 12.6 mg/kg in brain. Sodium arsenite caused DNA cleavage into fragments and manifested as "DNA laddering", a hallmark of apoptosis.

Heat shock protein 70 as an indicator of early lung injury caused by exposure to arsenic

Heat shock proteins (HSPs) are a family of highly conserved proteins that are induced by a number of stresses including toxic metals. Heat shock proteins expression has been reported to be an early and sensitive biomarker of cell stress. Arsenic is a naturally occurring metal that exists widely in the environment and is used in several industries. Exposure to arsenic is associated with the development of pulmonary cancers. We monitored changes in Hsp70 and markers of oxidative injury induced by arsenic in human pulmonary epithelial cells (BEAS-2B). Hsp70 protein, mRNA and reactive oxygen species (ROS) generation were measured after exposing cells to arsenic as markers of injury. Hsp70 protein expression showed significant 7.9-fold and 31.5-fold increase using Western blotting and ELISA assay, respectively, at a 50 microM As(III) with a 12 h exposure and an 12 h recovery time. Hsp70A and Hsp70B mRNA expression showed a two-fold increase and Hsp70C mRNA expression showed a six-fold increase. As(III)-induced Hsp70 protein expression was inhibited significantly by catalase and NAC, indicating mediation of ROS in Hsp70 expression. Intracellular glutathione (GSH) was significantly depleted by As(III) exposure. Lipid peroxidation by-product, 8-isoprostane, was increased six-fold at 24 h exposure to 20 microM As(III). Electron spin resonance and confocal microscope studies also showed As(III)-stimulated ROS generation. These results suggest that cellular injury by arsenic is mediated through ROS generation resulting in the expression of Hsp70. It is possible that Hsp70 may prove to be a sensitive biomarker for arsenic exposure with other markers of oxidative stress in human serum.

An approach to As(III) and As(V) bioavailability studies with Caco-2 cells

Foods and drinking water are the main sources of human exposure to inorganic arsenic [As(III) and As(V)]. After oral ingestion, the intestinal epithelium is the first barrier to absorption of these species. A human intestinal cell line (Caco-2) was used to evaluate cell retention and transport of As(III) (15.6-156.0 microM) and/or As(V) (15.4-170.6 microM). Cell monolayer integrity, cell viability, membrane damage and effects on cell metabolism were evaluated. Only the highest concentrations assayed [As(III): 156.0 microM; As(V): 170.6 microM] produced a cytotoxic effect with different cellular targets: As(III) altered the permeability of tight junctions, and As(V) caused uncoupling of the respiratory chain. Retention and transport of As(III) was more efficient than that of As(V). After 4h of exposure to As(III) or As(V), monolayer retention percentages varied between 0.87-2.28% and 0.14-0.39%, respectively. Transepithelial transport was greater for As(III) (5.82-7.71%) than for As(V) (not detectable-1.55%). The addition of As(III) and As(V) jointly produced a transport rate similar to that observed when they were added independently.

Gene expression profiling of responses to dimethylarsinic acid in female F344 rat urothelium

Gene expression profiling has been shown to be useful for identifying underlying mechanisms of toxicity, determining patterns of biological response, and elucidating candidate markers of exposure and response. Inorganic arsenic (iAs) is a human carcinogen and epidemiologic evidence implicates it in the development of urinary bladder cancer. Dimethylarsinic acid (DMA), the major excreted metabolite of iAs in humans, is a known rat bladder carcinogen. To examine the changes associated with DMA exposure, microarray analysis of the urothelium was performed in female F344 rats exposed to non-toxic and toxic doses of DMA in their drinking water for 28 days. A novel method for isolating predominantly urothelial cells was developed. Gene expression profiling of the urothelium using a custom 2-dye spotted array revealed that DMA treatment modulated the expression of transcripts of genes that regulate apoptosis, cell cycle regulation and the oxidative stress response. Expression of genes mapping to pathways involved in cancer control processes were also altered after DMA exposure. Morphological data suggested a dose dependent increase in cellular toxicity. Significant changes in differential gene expression were present after all treatments event at doses where standard toxicological responses were not detectable. The greatest perturbation in gene expression was present in rats after treatment with 40 ppm DMA. Doses which produced no histologic or ultrastructural evidence of toxicity (non-toxic) could be differentiated from toxic doses based on the expression of a subset of genes, which control cell signaling and the stress response. These reported changes in gene expression show similarities between the mechanisms of action of DMA in vivo and those previously described for iAs in vitro. These data illustrate the utility of transcriptional profiling and its potential in predicting key mechanistic pathways involved in toxicity and as a time efficient tool to inform the mode of action analysis in risk assessment.

Oxidation and methylation status determine the effects of arsenic on the mitotic apparatus

We investigated the spindle inhibitory properties of six arsenicals differing in their methylation or oxidation state. Human lymphoblasts were exposed for 6 h to either sodium arsenate (NaAs(V)), sodium arsenite (NaAs(III)), monomethylarsonic acid (MMA(V)), monomethylarsonous acid (MMA(III)), dimethylarsinic acid (DMA(V)), or dimethylarsinous acid (DMA(III)). After exposure slides were prepared, and the mitotic indices (MI) were assessed. We also exposed tubulin directly to each arsenical and spectrophotometrically measured its effect on polymerization. NaAs(V) caused a small but significant increase in MI. MMA(V) also caused only a slight increase in MI that just reached statistical significance. In contrast, DMA(V) caused a significant increase in MI, producing approximately 75% the MI of demecolcine and approximately 4 times the MI of the control. NaAs(III) had no significant effect on MI and was quite toxic. MMA(III) induced more than a twofold increase in MI compared to the control, which was about 40% that caused by demecolcine. On a micromolar basis, MMA(III) was the most potent of the arsenicals tested. DMA(III) gave inconsistent results. None of the pentavalent arsenicals had a substantial effect (either inhibition or enhancement) on GTP-induced polymerization of tubulin. In contrast, NaAs(III) inhibited polymerization at concentrations of 1 mM and above and MMA(III) and DMA(III) at 10 microM and above. Taken together, these results present a complex picture of how arsenicals may affect cells. These studies demonstrate that the metabolites of arsenic are active not only as chromosome breaking and DNA damaging agents but can also interfere with cell division via tubulin disruption.

Inorganic arsenic compounds cause oxidative damage to DNA and protein by inducing ROS and RNS generation in human keratinocytes

Arsenic is a naturally occurring element that is present in food, soil, and water. Inorganic arsenic can accumulate in human skin and is associated with increased risk of skin cancer. Oxidative stress due to arsenic exposure is proposed as one potential mode of carcinogenic action. The purpose of this study is to investigate the specific reactive oxygen and nitrogen species that are responsible for the arsenic-induced oxidative damage to DNA and protein. Our results demonstrated that exposure of human keratinocytes to trivalent arsenite caused the generation of 8-hydroxyl-2'-deoxyguanine (8-OHdG) and 3-nitrotyrosine (3-NT) in a concentration- and time-dependent manner. Pentavalent arsenate had similar effects, but to a significantly less extent. The observed oxidative damage can be suppressed by pre-treating cells with specific antioxidants. Furthermore, we found that pre-treating cells with Nomega-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase (NOS), or with 5,10,15,20-tetrakis (N-methyl-4'-pyridyl) porphinato iron (III) chloride (FeTMPyP), a decomposition catalyst of peroxynitrite, suppressed the generation of both 8-OHdG and 3-NT, which indicated that peroxynitrite, a product of the reaction of nitric oxide and superoxide, played an important role in arsenic-induced oxidative damage to both DNA and protein. These findings highlight the involvement of peroxynitrite in the molecular mechanism underlying arsenic-induced human skin carcinogenesis.

Therapeutic effects of Moringa oleifera on arsenic-induced toxicity in rats

Moringa oleifera Lamarack (English: Horseradish-tree, Drumstick-tree; Hindi: Saijan; Sanskrit: Shigru) belongs to the Moringaceae family, is generally known in the developing world as a vegetable, a medicinal plant and a source of vegetable oil. Besides, the plant is reported to have various biological activities, including hypocholesterolemic agent, regulation of thyroid hormone status, anti-diabetic agent, gastric ulcers, anti-tumor agent and hypotensive agent, used for treating various diseases such as inflammation, cardiovascular and liver diseases. Therapeutic efficacy of oral administration of seed powder of M. oleifera (500mg/kg, orally, once daily) post arsenic exposure (100ppm in drinking water for 4 months) was investigated in rats. Animals exposed to arsenic(III) showed a significant inhibition of δ-aminolevulinic acid dehydratase (ALAD) activity, decrease in reduced glutathione (GSH) level and an increase in reactive oxygen species (ROS) in blood. On the other hand, a significant decrease in hepatic ALAD, and an increase in δ-aminolevulinic acid synthetase (ALAS) activity was noted after arsenic exposure. These changes were accompanied by an increase in thiobarbiturc acid reactive substances (TBARS) level in liver and kidney. Activities of liver, kidney and brain superoxide dismutase (SOD) and catalase also showed a decrease on arsenic exposure. Administration of M. oleifera seed powder post arsenic exposure, exhibited significant recovery in blood ALAD activity while, it restored blood GSH and ROS levels. Most of the other blood biochemical variables remained unchanged on M. oleifera supplementation. A significant protection in the altered ALAD and ALAS activities of liver and TBARS level in liver and kidney was however, observed after M. oleifera administration. Interestingly, there was a marginal but significant depletion of arsenic from blood, liver and kidneys. The results, thus lead us to conclude that post arsenic exposure administration with the seed powder of M. oleifera has significant role in protecting animals from arsenic-induced oxidative stress and in the depletion of arsenic concentration. Further studies thus can be recommended for determining the effect of co-administrating seed powder of M. oleifera during chelation therapy with a thiol chelator.

Cytogenetic evaluation of arsenic trioxide toxicity in Sprague-Dawley rats

Acute exposure to arsenic trioxide has been reported to induce death and/or multiple organ damage with symptoms including nausea, vomiting, diarrhea, gastrointestinal hemorrhage, cerebral edema, tachycardia, dysrhythmias and hypovolemic shock. Its toxic effects are due to its ability to bind to sulfhydryl groups of proteins and to inhibit energy production. Although the chronic exposure to arsenic trioxide has been linked to various types of cancer, such as skin, liver, lung, bladder and kidney neoplasms, studies of its carcinogenic potential in animals have not been conclusive. In this study, we investigated the genotoxic potential of arsenic trioxide in bone-marrow cells obtained from Sprague-Dawley rats; using chromosomal aberrations (CA), mitotic index (MI) and micronuclei (MN) formation as the toxicological endpoints. Four groups of six male rats each, weighing approximately 60+/-2 g per rat, were injected intraperitoneally, once a day for 5 days with doses of 5, 10, 15 and 20 mg/kg body weight (BW) of arsenic trioxide dissolved in distilled water. A control group was also made of six animals injected with distilled water without chemical. All the animals were sacrificed at the end of the treatment period. Chromosome and micronuclei preparation was obtained from bone-marrow cells following standard protocols. Arsenic trioxide exposure significantly increased the number of structural chromosomal aberrations, the frequency of micronucleated cells and decreased the mitotic index in treated groups when compared with the control group. Our results demonstrate that arsenic trioxide has a clastogenic/genotoxic potential as measured by the bone-marrow CA and MN tests in Sprague-Dawley rats.

Arsenic trioxide inhibits nuclear receptor function via SEK1/JNK-mediated RXRalpha phosphorylation

We have previously published that 2 proven treatments for acute promyelocytic leukemia, As2O3 and retinoic acid, can be antagonistic in vitro. We now report that As2O3 inhibits ligand-induced transcription of the retinoic acid receptor, as well as other nuclear receptors that heterodimerize with the retinoid X receptor alpha (RXRalpha). As2O3 did not inhibit transactivation of the estrogen receptor or the glucocorticoid receptor, which do not heterodimerize with RXRalpha. We further show that As2O3 inhibits expression of several target genes of RXRalpha partners. Phosphorylation of RXRalpha has been reported to inhibit nuclear receptor signaling, and we show by in vivo labeling and phosphoamino acid detection that As2O3 phosphorylated RXRalpha in the N-terminal ABC region exclusively on serine residues. Consistent with our previous data implying a role for JNK in As2O3-induced apoptosis, we show that pharmacologic or genetic inhibition of JNK activation decreased As2O3-induced RXRalpha phosphorylation and blocked the effects of As2O3 on RXRalpha-mediated transcription. A mutational analysis indicated that phosphorylation of a specific serine residue, S32, was primarily responsible for inhibition of RXRalpha-mediated transcription. These data may provide some insight into the rational development of chemotherapeutic combinations involving As2O3 as well as into molecular mechanisms of arsenic-induced carcinogenesis resulting from environmental exposure.

Caffeic acid phenethyl ester (CAPE) prevents transformation of human cells by arsenite (As) and suppresses growth of As-transformed cells

Recent evidence suggests that inflammatory cytokines and growth factors contribute to arsenite (As)-induced human carcinogenesis. We investigated the expression of inflammatory cytokine mRNAs during the transformation process induced by chronic As exposure in non-tumorigenic human osteogenic sarcoma (N-HOS) cells using gene arrays, and results were confirmed by RT-PCR and protein arrays. Caffeic acid phenethyl ester (CAPE), a naturally occurring immunomodulating agent, was used to evaluate the role of inflammatory factors in the process of As-mediated N-HOS cell transformation and in As-transformed HOS (AsT-HOS) cells. We found that an 8-week continuous exposure of N-HOS to 0.3 microM arsenite resulted in HOS cell transformation. That exposure also caused substantial decreases in inflammatory cytokine mRNAs, such as interleukin (IL) IL-1alpha, IL-2, IL-8, IL-18, MCP-1, TGF-beta2, and TNF-alpha, while it increased c-jun mRNA in a time-dependent manner. Co-incubation of N-HOS with As and CAPE (0.5-2.5 microM) prevented As-mediated declines in cytokine mRNAs in the co-treated cells, as well as their transformation to anchorage independence, while it caused decreases in c-jun mRNA. CAPE (up to 10 microM) had no effect on growth of N-HOS cells. However, CAPE (1-10 microM) treatment of AsT-HOS cells inhibited cell growth, induced cell cycle G2/M arrest, and triggered apoptosis, accompanied by changes in cytokine gene expression, as well as decreases in cyclin B1 and cdc2 abundance. Resveratrol (RV) and (-)(.) epigallocatechin gallate (EGCG), preventive agents present in grapes and green tea, respectively, induced similar changes in AsT-HOS cell growth but required much higher doses than CAPE to cause 50% growth arrest (<2.5 microM CAPE versus 25 microM RV or 50 microM EGCG). Overall, our findings suggest that inflammatory cytokines play an important role in the suppressive effects of CAPE on As-induced cell transformation and in the selective cytotoxicity of CAPE to As-transformed HOS cells.

Arsenite binding to synthetic peptides based on the Zn finger region and the estrogen binding region of the human estrogen receptor-alpha

We selected the estrogen receptor protein for study because of prior results indicating that arsenite is a "potential nonsteroidal environmental estrogen". We utilized radioactive (73)As-labeled arsenite and vacuum filtration methodology to determine the binding affinity of arsenite to synthetic peptides. A zinc finger region containing four free sulfhydryls and the hormone binding region containing three free sulfhydryls based on the human estrogen receptor-alpha were studied. Peptide 15 (RYCAVCNDYASGYHYGVWSCEGCKA) bound arsenite with a K(d) of 2.2 microM and B(max) (maximal binding capacity) of 89 nmol/mg protein. Peptide 10 (LECAWQGKCVEGTEHLYSMKCKNV) had a K(d) of 1.3 microM and B(max) of 59 nmol/mg protein. In contrast, the monothiol peptide 19 (LEGAWQGKGVEGTEHLYSMKCKNV) bound arsenite with a higher K(d) of 124 microM and a B(max) of 26 nmol/mg protein. In our studies, amino acids other than cysteine (including methionine and histidine) did not bind arsenite at all. Peptides modeled on the estrogen receptor with two or more nearby free sulfhydryls (two or five intervening amino acids) had low K(d) values in the 1-4 microM range. Peptides containing single sulfhydryls or two sulfhydryls spaced 17 amino acids apart had higher K(d) values in the 100-200 microM range, demonstrating lower affinity. With the exception of peptide 24 which had an unusually high B(max) value of 234 nmol/mg, the binding capacity of the studied peptides was proportional to the number of free cysteines. Binding of trivalent arsenicals to peptides and proteins can contribute to arsenic toxicity and carcinogenicity via altered peptide/protein structure and enzyme function.

Arsenic exposure, urinary arsenic speciation, and peripheral vascular disease in blackfoot disease-hyperendemic villages in Taiwan

Long-term exposure to ingested inorganic arsenic is associated with peripheral vascular disease (PVD) in the blackfoot disease (BFD)-hyperendemic area in Taiwan. This study further examined the interaction between arsenic exposure and urinary arsenic speciation on the risk of PVD. A total of 479 (220 men and 259 women) adults residing in the BFD-hyperendemic area were studied. Doppler ultrasound was used to diagnose PVD. Arsenic exposure was estimated by an index of cumulative arsenic exposure (CAE). Urinary levels of total arsenic, inorganic arsenite (As(III)) and arsenate (As(V)), monomethylarsonic acid (MMA(V)), and dimethylarsinic acid (DMA(V)) were determined. Primary methylation index [PMI = MMA(V)/(As(III) + As(V))] and secondary methylation index (SMI = DMA(V)/MMA(V)) were calculated. The association between PVD and urinary arsenic parameters was evaluated with consideration of the interaction with CAE and the confounding effects of age, sex, body mass index, total cholesterol, triglycerides, cigarette smoking, and alcohol consumption. Results showed that aging was associated with a diminishing capacity to methylate inorganic arsenic and women possessed a more efficient arsenic methylation capacity than men did. PVD risk increased with a higher CAE and a lower capacity to methylate arsenic to DMA(V). The multivariate-adjusted odds ratios for CAE of 0, 0.1-15.4, and >15.4 mg/L x year were 1.00, 3.41 (0.74-15.78), and 4.62 (0.96-22.21), respectively (P < 0.05, trend test); and for PMI < or = 1.77 and SMI > 6.93, PMI > 1.77 and SMI > 6.93, PMI > 1.77 and SMI < or = 6.93, and PMI < or = 1.77 and SMI < or = 6.93 were 1.00, 2.93 (0.90-9.52), 2.85 (1.05-7.73), and 3.60 (1.12-11.56), respectively (P < 0.05, trend test). It was concluded that individuals with a higher arsenic exposure and a lower capacity to methylate inorganic arsenic to DMA(V) have a higher risk of developing PVD in the BFD-hyperendemic area in Taiwan.

Preventive mechanism of cellular glutathione in monomethylarsonic acid-induced cytolethality

Human pentavalent arsenic metabolic intermediate, monomethylarsonic acid (MMAs(V)), is a major arsenic type found in the blood in chronic arsenic poisoning patients, but little information is available on its toxicity potential or mechanisms of action. In this study, we investigated the molecular mechanisms of in vitro cytolethality of MMAs(V) using rat liver TRL 1215 cells. Cellular arsenic concentrations reached the nanomolar range in TRL 1215 cells when cells were exposed to millimolar levels of MMAs(V), and most of the MMAs(V) was not metabolized during the 48-h incubation. Under these conditions, MMAs(V) showed significant cytolethality when cellular reserves of reduced glutathione (GSH) were depleted. Morphological and biochemical evidence confirmed that MMAs(V) induced both necrosis and apoptosis in the cellular GSH-depleted cells. MMAs(V) significantly enhanced cellular caspase 3 activity in the cellular GSH-depleted cells, and a caspase 3 inhibitor blocked MMAs(V)-induced apoptosis. MMAs(V) also enhanced the production of cellular reactive oxygen species (ROS) in the cellular GSH-depleted cells, and addition of a membrane-permeable radical trapping reagent completely prevented both MMAs(V)-induced cellular caspase 3 activation and cytolethality in these cells. These observations suggest that MMAs(V) typically generates harmful ROS in cells, and cellular GSH prevents cytolethality by scavenging these toxic ROS. However, when cellular GSH levels are decreased, MMAs(V) induces oxidative stress in the cells, and this leads to apoptosis and/or necrosis depending on the cellular ROS/GSH ratio.

Butylhydroquinone protects cells genetically deficient in glutathione biosynthesis from arsenite-induced apoptosis without significantly changing their prooxidant status

Arsenic, first among the top environmentally hazardous substances, is associated with skin, lung, liver, kidney, prostate, and bladder cancer. Arsenic is also a cardiovascular and a central nervous system toxicant, and it has genotoxic and immunotoxic effects. Paradoxically, arsenic trioxide is used successfully in the treatment of acute promyelocytic leukemia and multiple myeloma. Arsenic induces oxidative stress, and its toxicity is decreased by free thiols and increased by glutathione depletion. To further characterize the role of glutathione and oxidative stress in the toxicity of arsenic, we have used fetal fibroblasts from Gclm(-/-) mice, which lack the modifier subunit of glutamate-cysteine ligase, the rate-limiting enzyme in glutathione biosynthesis. Gclm(-/-) mouse embryo fibroblasts (MEFs) are eight times more sensitive to arsenite-induced apoptotic death. Because of a dramatic decrease in glutathione levels, Gclm(-/-) MEFs have a high prooxidant status that is not significantly relieved by treatment with the phenolic antioxidant tBHQ; however, tBHQ blocks arsenite-induced apoptosis in both Gclm(+/+) and Gclm(-/-) cells, although it raises a significant antioxidant response only in Gclm(+/+) cells. Global gene expression profiles indicate that tBHQ is significantly effective in reversing arsenite-induced gene deregulation in Gclm(+/+) but not in Gclm(-/-) MEFs. This effect of tBHQ is evident in the expression of metalloproteases and chaperones, and in the expression of genes involved in DNA damage and repair, protein biosynthesis, cell growth and maintenance, apoptosis, and cell cycle regulation. These results suggest that regulation of glutathione levels by GCLM determines the sensitivity to arsenic-induced apoptosis by setting the overall ability of the cells to mount an effective antioxidant response.

Methyl-parathion and organophosphorous pesticide metabolites modify the activation status and interleukin-2 secretion of human peripheral blood mononuclear cells

Organophosphorous (OP) compounds are the most commonly used pesticides. There are reports on susceptibility to the toxic effects of OP pesticides, but no information exists regarding the toxicity of their metabolites. To determine the metabolites' contribution to the OP pesticide immunotoxic effects, human peripheral blood mononuclear cells (PBMCs) were treated with the parent compound methyl-parathion (MP) and the following OP pesticide alkyl-phosphorous metabolites: diethylphosphate (DEP), diethylthiophosphate (DETP), diethyldithiophosphate (DEDTP), dimethylphosphate (DMP), and dimethyldithiophosphate (DMDTP). Activation and function of the PBMCs were examined by assessment of phytohemagglutinin (PHA)-induced proliferative response, interleukin-2 (IL-2) secretion, and CD25 and CD69 expression. Treatments with DMP, DEP, DETP and DEDTP for 48h produced significant toxicity in human PBMCs, but did not affect their proliferative response to PHA. Only MP reduced cell proliferation by 30%. DEDTP decreased the proportion of PBMCs expressing CD25. This effect was associated with a reduction of IL-2 secretion, which was also reduced by MP and DMP treatments. In contrast, DETP and DEDTP treatments increased the expression of CD69. DMP, DETP and DEDTP were more consistently involved in modulating the PBMC response to PHA.

Intracellular calcium disturbances induced by arsenic and its methylated derivatives in relation to genomic damage and apoptosis induction

Arsenic and its methylated derivatives are contaminants of air, water, and food and are known as toxicants and carcinogens. Arsenic compounds are also being used as cancer chemotherapeutic agents. In humans, inorganic arsenic is metabolically methylated to mono-, di-, and trimethylated forms. Recent findings suggest that the methylation reactions represent a toxification rather than a detoxification pathway. In recent years, the correlation between arsenic exposure, cytotoxicity and genotoxicity, mutagenicity, and tumor promotion has been established, as well as the association of arsenic exposure with perturbation of physiologic processes, generation of reactive oxygen species, DNA damage, and apoptosis induction. Trivalent forms of arsenic have been found to induce apoptosis in several cellular systems with involvement of membrane-bound cell death receptors, activation of caspases, release of calcium stores, and changes of the intracellular glutathione level. It is well known that calcium ion deregulation plays a critical role in apoptotic cell death. A calcium increase in the nuclei might lead to toxic effects in the cell. In this review, we highlight the relationship between induced disturbances of calcium homeostasis, genomic damage, and apoptotic cell death caused by arsenic and its organic derivatives.

The role of trivalent dimethylated arsenic in dimethylarsinic acid-promoted skin and lung tumorigenesis in mice: tumor-promoting action through the induction of oxidative stress

We investigated the relationship between lung- and skin-tumor promotion and oxidative stress caused by administration of dimethylarsinic acid (DMA(V)) in mice. The incidence of lung tumors induced by lung tumor initiator (4NQO) and DMA(V) were, as well as 8-oxo-2'-deoxyguanosine (8-oxodG), suppressed by cotreatment with (-)epigallocatechin gallate (EGCG). When mice were topically treated with trivalent dimethylated arsenic (DMA(III)), a further reductive metabolite of DMA(V), not only an increase in skin tumors but also an elevation of 8-oxodG in epidermis were observed. These results suggest that tumor promotion due to DMA(V) administration is mediated by DMA(III) through the induction of oxidative stress.

DNA–protein cross-links and sister chromatid exchanges induced by dimethylarsinic acid in human fibroblasts cells

Biotransformation of inorganic arsenic to form both methylarsinic acid (MA) and dimethylarsinic acid (DMA) has traditionally been considered as a mechanism to facilitate the detoxification and excretion of arsenic. However, the methylation of inorganic arsenic as a detoxification mechanism has been questioned due to recent studies revealing an important role of organic arsenic in the induction of genetic damage. In a previous report a reduction of DNA migration after treatment of cells with DMA was described. In order to further evaluate the possible induction of protein-DNA adducts, an experiment was performed taking into account other parameters and modifications of the standard alkaline comet assay. In addition, the results obtained with the comet assay were compared with those obtained by analyzing the induction of sister chromatid exchanges (SCEs). SCE frequencies were significantly increased in treated cells in relation to controls (p<0.001). Furthermore, in the standard alkaline comet assay, as well as in the control assay for proteinase K treatment, a significant dose-dependent reduction in tail moment was observed. Nevertheless, the post-treatment with proteinase K induced the release of proteins joined to the DNA and consequently, a dose-dependent increment in DNA migration was observed (p<0.001). These results suggest that DNA-protein cross-links may be an important genotoxic effect induced by dimethylarsinic acid in human MRC-5 cells.

Chronic arsenic exposure and urinary 8-hydroxy-2'-deoxyguanosine in an arsenic-affected area in Inner Mongolia, China

OBJECTIVE

Recent studies have shown that generation of reactive oxidants during arsenic metabolism can play an important role in arsenic-induced injury. The purpose of this study was to examine the relationship between arsenic in drinking water and oxidative stress in humans by measuring 8-Hydroxy-2'-deoxyguanosine (8-OHdG).

METHODS

We performed a cross-sectional study in an arsenic-affected village in Hetao Plain, Inner Mongolia, China. A total of 134 of the 143 inhabitants (93.7%) of the village participated in the study. The levels of 8-OHdG, arsenic and its metabolites were measured in urine collected from the participants. Regression analyses were performed to investigate the relationship between arsenic species and 8-OHdG levels in urine.

RESULTS

In the polluted village, monomethylarsenic was significantly higher in subjects with arsenic dermatosis than those without dermatosis despite no difference in mean levels of arsenic in well water between both types of subject. For subjects with arsenic dermatosis, arsenic species and metabolites in urine are significantly associated with 8-OHdG, while there was no statistically significant relationship for subjects without arsenic dermatosis. For all residents of the polluted village, the levels of dimethylarsenic and 8-OHdG were significantly higher for those who had been exposed to well water for more than 12 years.

CONCLUSIONS

These results provide evidence of a link between exposure to arsenic from drinking water and oxidative stress, which may play an important role in arsenic-involved injuries.

Micronuclei assessment in buccal cells of people environmentally exposed to arsenic in northern Chile

To determine the genotoxic risk associated to environmental arsenic exposure, the frequency of micronuclei in buccal cells (BCMN) of people drinking arsenic-contaminated water has been evaluated. A group of 105 individuals from the Antofagasta region (north Chile), and 102 individuals from the area of Concepcion, used as reference group, were included in the study. Arsenic concentration in drinking water was high (0.75 mg/L) in the Antofagasta area, 75-fold the maximum recommended level by WHO (0.01 mg/L), while the values obtained in Concepcion were significantly lower (0.002 mg/L). Individual measures of arsenic exposure were also determined in fingernails, which clearly confirm the existence of chronic exposure in the sampled populations from the Antofagasta region (10.15 microg/g versus 3.57 microg/g). The cytogenetic results indicate that, although the BCMN frequency is higher in exposed than in controls, this increase does not attain statistical significance. When the exposure biomarkers were related with the cytogenetic values, no correlations were observed between BCMN and arsenic content in water or in fingernails. In addition, the genotoxicity values do not seem to be related to the ethnic origin from people belonging to the exposed group. As a conclusion it appears that, in the studied population, the chronic ingestion of arsenic-contaminated water does not induce cytogenetic damage, measured as micronuclei, in the cells of the oral mucous in a significant extent.

MECHANISMS OF ARSENITE-MEDIATED DECREASES IN BENZO[K]FLUORANTHENE-INDUCED HUMAN CYTOCHROME P4501A1 LEVELS IN HEPG2 CELLS

Polycyclic aromatic hydrocarbons (PAHs) and heavy metals are often environmental cocontaminants that could interact to alter PAH carcinogenicity. The heavy metal, arsenite, and the PAH, benzo[k]fluoranthene, were used as prototypes to investigate, in human HepG2 cells, mechanisms whereby the bioactivation of benzo[k]fluoranthene by human CYP1A1 could be diminished by arsenite-mediated decreases in CYP1A1 induction by benzo[k]fluoranthene. To determine whether arsenite down-regulates CYP1A1 transcription, quantitative real-time reverse transcriptase-polymerase chain reaction assays and luciferase reporter gene expression assays were used with HepG2 cells treated with benzo[k]fluoranthene and arsenite, separately and as a mixture. Benzo[k]fluoranthene (0.5 microM) and arsenite (5 microM) markedly decreased benzo[k]fluoranthene-mediated induction of CYP1A1 mRNA by 45%. Plasmids containing the CYP1A1 promoter region (pHu-1A1-FL) were induced 7.4-fold over vehicle by benzo[k]fluoranthene (0.5 microM), whereas arsenite (1, 2.5, or 5 microM) decreased reporter gene expression by 46%, 45%, and 61%, respectively. The plasmid, pHu-1A1-Delta100-FL, lacked xenobiotic response element (XRE) sites at -1061 and -981 and showed greater responsiveness relative to pHu-1A1-FL, by 1.7-fold. Benzo[k]fluoranthene (0.5 microM) and arsenite (1, 2.5, or 5 microM) decreased reporter gene expression by 0%, 27%, and 39%, respectively, relative to expression levels produced by benzo[k]fluoranthene alone. Arsenite is stable for at least 48 h in the HepG2 cell medium with respect to its ability to diminish CYP1A1 benzo[k]fluoranthene induction. Arsenite did not affect benzo[k]fluoranthene induction directly through XRE sites, nor did it affect the stability of CYP1A1 mRNA. Thus, arsenite affects the transcriptional regulation of the benzo[k]fluoranthene-mediated induction of CYP1A1 and could diminish PAH carcinogenicity by decreasing bioactivation by CYP1A1.

Association of chromosomal alterations with arsenite-induced tumorigenicity of human HaCaT keratinocytes in nude mice

Inorganic arsenic is a well-documented human carcinogen. Chronic low-dose exposure to inorganic arsenic is associated with an increased incidence of a variety of cancers, including skin, lung, bladder, and liver cancer. Because genetic alterations often occur during cancer development, the objective of this study was to explore what types of genetic alterations were induced by chronic exposure of human HaCaT cells to arsenic. After 20 passages in the presence of inorganic trivalent arsenite at concentrations of 0.5 or 1 microM, HaCaT cells had higher intracellular levels of glutathione, became more resistance to arsenite, and showed an increased frequency of micronuclei. Furthermore, the previously nontumorigenic HaCaT cells became tumorigenic, as shown by subcutaneous injection into Balb/c nude mice. Cell lines derived from the tumors formed by injection of arsenite-exposed HaCaT cells into nude mice expressed higher levels of keratin 6, a proliferation marker of keratinocytes, than did parental HaCaT cells, whereas the expression of keratins 5, 8, and 10 was significantly decreased. Comparative genomic hybridization demonstrated chromosomal alterations in the 11 cell lines derived from these tumors; all 11 showed significant loss of chromosome 9q, and seven showed significant gain of chromosome 4q. The present results show that long-term exposure to low doses of arsenite transformed nontumorigenic human keratinocytes to cells that were tumorigenic in nude mice and that chromosomal alterations were observed in all cell lines established from the tumors.

Integrating phenotypic and expression profiles to map arsenic-response networks

By integrating phenotypic and transcriptional profiling and mapping the data onto metabolic and regulatory networks, it was shown that arsenic probably channels sulfur into glutathione for detoxification, leads to indirect oxidative stress by depleting glutathione pools, and alters protein turnover via arsenation of sulfhydryl groups on proteins.

Background

Arsenic is a nonmutagenic carcinogen affecting millions of people. The cellular impact of this metalloid in Saccharomyces cerevisiae was determined by profiling global gene expression and sensitivity phenotypes. These data were then mapped to a metabolic network composed of all known biochemical reactions in yeast, as well as the yeast network of 20,985 protein-protein/protein-DNA interactions.

Results

While the expression data unveiled no significant nodes in the metabolic network, the regulatory network revealed several important nodes as centers of arsenic-induced activity. The highest-scoring proteins included Fhl1, Msn2, Msn4, Yap1, Cad1 (Yap2), Pre1, Hsf1 and Met31. Contrary to the gene-expression analyses, the phenotypic-profiling data mapped to the metabolic network. The two significant metabolic networks unveiled were shikimate, and serine, threonine and glutamate biosynthesis. We also carried out transcriptional profiling of specific deletion strains, confirming that the transcription factors Yap1, Arr1 (Yap8), and Rpn4 strongly mediate the cell's adaptation to arsenic-induced stress but that Cad1 has negligible impact.

Conclusions

By integrating phenotypic and transcriptional profiling and mapping the data onto the metabolic and regulatory networks, we have shown that arsenic is likely to channel sulfur into glutathione for detoxification, leads to indirect oxidative stress by depleting glutathione pools, and alters protein turnover via arsenation of sulfhydryl groups on proteins. Furthermore, we show that phenotypically sensitive pathways are upstream of differentially expressed ones, indicating that transcriptional and phenotypic profiling implicate distinct, but related, pathways.

Urinary arsenic speciation and porphyrins in C57Bl/6J mice chronically exposed to low doses of sodium arsenate

Arsenic has been classified as a human carcinogen based on epidemiological data however the mechanism of its carcinogenicity is still unclear. Urinary biomarkers for chronic arsenic exposure would be valuable as an early warning indicator for timely interventions. In this study, young female C57Bl/6J mice were given drinking water containing 0, 100, 250 and 500 microg Asv/L as sodium arsenate ad libitum for 12 months. Urine was collected bimonthly for urinary arsenic methylation assay and porphyrin analysis. All detectable arsenic species showed strong linear correlation with administered dosage and the arsenic methylation patterns were similar in all three treatment groups. No significant changes of methylation patterns were observed over time for either the control or test groups. Urinary coproporphyrin III was significantly increased in the 8th month in 250 and 500 microg/L groups and remained significantly dose-related after 10 and 12 months. Coproporphyrin I also showed a significant dose-response relationship after 12 months. Our results confirm that urinary arsenic is a useful biomarker for internal dose. The alteration of porphyrin profile suggests that arsenic can affect the heme metabolism and this may occur prior to the onset of arsenic induced carcinogenesis.

Speciation of dimethylarsinous acid and trimethylarsine oxide in urine from rats fed with dimethylarsinic acid and dimercaptopropane sulfonate

Speciation of arsenic in urine from rats treated with dimethylarsinic acid (DMA(V)) alone or in combination with dimercaptopropane sulfonate (DMPS) were studied. Methods were developed for the determination of the methylarsenic metabolites, especially trace levels of dimethylarsinous acid (DMA(III)) and trimethylarsine oxide (TMAO), in the presence of a large excess of DMA(V). Success was achieved by using improved ion-exchange chromatographic separation combined with hydride generation atomic fluorescence detection. Micromolar concentrations of DMA(III) were detected in urine of rats fed with a diet supplemented with either 100 microg/g of DMA(V) or a mixture of 100 microg/g of DMA(V) and 5600 microg/g of DMPS. No significant difference in the DMA(III) concentration was observed between the two groups; however, there was a significant difference in TMAO concentrations. Urine from rats fed with the diet supplemented with DMA(V) alone contained 73 +/- 30 microM TMAO, whereas urine from rats fed with the diet supplemented with both DMA(V) and DMPS contained only 2.8 +/- 1.4 microM TMAO. Solutions containing mixtures of 100 microg/L DMA(V) or TMAO and 5600 microg/L DMPS did not show reduction of DMA(V) and TMAO. The significant decrease (p < 0.001) of the TMAO concentration in rats administered with both DMA(V) and DMPS suggests that DMPS inhibits the biomethylation of arsenic.

Arsenite induces HIF-1alpha and VEGF through PI3K, Akt and reactive oxygen species in DU145 human prostate carcinoma cells

Arsenite is widely distributed environmental toxicant in water, food and air. It is a known human carcinogen, which is strongly associated with human cancers originated from liver, nasal cavity, lung, skin, bladder, kidney, and prostate. In this study, we investigated whether arsenite induces expression of hypoxia-inducible factor 1 (HIF-1). HIF-1 is a heterodimeric basic helix-loop-helix transcription factor, composed of HIF-1alpha and HIF-1beta/ARNT subunits; and is involved in tumor growth and angiogenesis. Here we demonstrate that arsenite induces the expression of HIF-1alpha but not HIF-1beta subunit in DU145 human prostate carcinoma cells. Arsenite also increases the expression of VEGF through the induction of HIF-1. We also found that arsenite activates PI3K and Akt that are required for arsenite-induced expression of HIF-1alpha and VEGF. The induction of HIF-1 and VEGF by arsenite can not be inhibited by MAP kinase inhibitors. Arsenite causes production of reactive oxygen species (ROS). The major species of ROS required for the induction of HIF-1 and VEGF is H2O2. These data indicate that the arsenite-induced activation of PI3K/Akt signaling and the expression of HIF-1 and VEGF through the generation of ROS could be an important mechanism in the arsenite-induced carcinogenesis.

Molecular mechanisms of arsenic carcinogenesis

Arsenic is a metalloid compound that is widely distributed in the environment. Human exposure of this compound has been associated with increased cancer incidence. Although the exact mechanisms remain to be investigated, numerous carcinogenic pathways have been proposed. Potential carcinogenic actions for arsenic include oxidative stress, genotoxic damage, DNA repair inhibition, epigenetic events, and activation of certain signal transduction pathways leading to abberrant gene expression. In this article, we summarize current knowledge on the molecular mechanisms of arsenic carcinogenesis with an emphasis on ROS and signal transduction pathways.

Arsenic toxicity, mutagenesis, and carcinogenesis--a health risk assessment and management approach

A comprehensive analysis of published data indicates that arsenic exposure induces cardiovascular diseases, developmental abnormalities, neurologic and neurobehavioral disorders, diabetes, hearing loss, hematologic disorders, and various types of cancer. Although exposure may occur via the dermal, and parenteral routes, the main pathways of exposure include ingestion, and inhalation. The severity of adverse health effects is related to the chemical form of arsenic, and is also time- and dose-dependent. Recent reports have pointed out that arsenic poisoning appears to be one of the major public health problems of pandemic nature. Acute and chronic exposure to arsenic has been reported in several countries of the world where a large proportion of drinking water (groundwater) is contaminated with high concentrations of arsenic. Research has also pointed significantly higher standardized mortality rates for cancers of the bladder, kidney, skin, liver, and colon in many areas of arsenic pollution. There is therefore a great need for developing a comprehensive health risk assessment (RA) concept that should be used by public health officials and environmental managers for an effective management of the health effects associated with arsenic exposure. With a special emphasis on arsenic toxicity, mutagenesis, and carcinogenesis, this paper is aimed at using the National Academy of Science's RA framework as a guide, for developing a RA paradigm for arsenic based on a comprehensive analysis of the currently available scientific information on its physical and chemical properties, production and use, fate and transport, toxicokinetics, systemic and carcinogenic health effects, regulatory and health guidelines, analytical guidelines and treatment technologies.

Understanding arsenic carcinogenicity by the use of animal models

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

A proteome analysis of the arsenite response in cultured lung cells: evidence for in vitro oxidative stress-induced apoptosis

Arsenite is well documented as a chemotherapeutic agent capable of inducing cell death. However, the cellular response at the molecular level has not been studied extensively. In the present study, we provide for the first time a proteomic analysis of rat LECs (lung epithelial cells) treated with arsenite, with the aim of identifying defence proteins, probably expressed to protect the cells during the course of arsenic-induced apoptosis. Comparative proteome analysis was conducted on LECs and LECs treated with 40 microM arsenite to identify global changes in their protein expression profiles. Over 1000 protein spots were separated by two-dimensional electrophoresis and visualized by silver staining. Seven proteins changed expression levels significantly and were identified by matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry and database searching. The proteins up-regulated were mostly HSPs (heat-shock proteins) and antioxidative stress proteins, including HSP70, aldose reductase, haem oxygenase-1, HSP27, ferritin light chain and alphaB-crystallin. The glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase was down-regulated. Pretreatment with the thiol antioxidants glutathione or N-acetylcysteine before arsenite insult effectively abrogated the induction of these defence proteins and sustained cell viability, whereas antioxidants were protective only at earlier time points if they were added to cells after arsenite. Taken together, our results demonstrate that high levels of arsenite cause oxidative stress-induced apoptosis.

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

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

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

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

Environmental distribution, analysis, and toxicity of organometal(loid) compounds

The biochemical modification of the metals and metalloids mercury, tin, arsenic, antimony, bismuth, selenium, and tellurium via formation of volatile metal hydrides and alkylated species (volatile and involatile) performs a fundamental role in determining the environmental processing of these elements. In most instances, the formation of such species increases the environmental mobility of the element, and can result in bioaccumulation in lipophilic environments. While inorganic forms of most of these compounds are well characterized (e.g., arsenic, mercury) and some of them exhibit low toxicity (e.g., tin, bismuth), the more lipid-soluble organometals can be highly toxic. Methylmercury poisoning (e.g., Minamata disease) and tumor development in rats after exposure to dimethylarsinic acid or tributyltin oxide are just some examples. Data on the genotoxicity (and the neurotoxicity) as well as the mechanisms of cellular action of organometal(loid) compounds are, however, scarce. Many studies have shown that the production of such organometal(loid) species is possible and likely whenever anaerobic conditions (at least on a microscale) are combined with available metal(loid)s and methyl donors in the presence of suitable organisms. Such anaerobic conditions can exist within natural environments (e.g., wetlands, pond sediments) as well as within anthropogenic environmental systems (e.g., waste disposal sites and sewage treatments plants). Some methylation can also take place under aerobic conditions. This article gives an overview about the environmental distribution of organometal(loid) compounds and the potential hazardous effects on animal and human health. Genotoxic effects in vivo and in vitro in particular are discussed.

The potential biological mechanisms of arsenic-induced diabetes mellitus

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

Helper T cell subpopulations from women are more susceptible to the toxic effect of sodium arsenite in vitro

Arsenic is known to produce inhibition as well as induction of proliferative responses in animal and human cells depending on the doses. Despite the amount of information on the immunotoxic effects of arsenic exposure in different animal models, little is known in humans. Arsenic susceptibility of lymphocyte subpopulations (T helper (Th), CD4+; T cytotoxic (Tc), CD8+) and whether arsenic effects are gender related are still to be determined. This work evaluated the in vitro toxicity of sodium arsenite on human T lymphocyte subpopulations from men and women. Peripheral blood mononuclear cells (PBMC) obtained from healthy young men and women were treated with sodium arsenite (0.01, 0.1, and 1 microM). We assessed cell viability, cell proliferation, and the proportion of Th and Tc cells after 48 or 72h of arsenic exposure in resting and phytohemagglutinin M (PHA)-activated PBMC. We observed that sodium arsenite at 1 microM was more toxic for Th than for Tc cells in PBMC from women. Besides, T lymphocytes from women were more affected by the cell proliferation inhibition induced by arsenic, suggesting that women could be more susceptible to the toxic and immunotoxic effects caused by arsenic exposure.

Mechanisms underlying arsenic carcinogenesis: hypersensitivity of mice exposed to inorganic arsenic during gestation

Inorganic arsenic is an important human carcinogen of unknown etiology. Defining carcinogenic mechanisms is critical to assessing the human health hazard of arsenic exposure but requires appropriate model systems. It has proven difficult to induced tumors in animals with inorganic arsenic alone. Several groups have studied the carcinogenic potential of inorganic arsenic in rodents, finding it to act as co-promoter or co-carcinogen, but not as a complete carcinogen. As gestation is a time of high sensitivity to chemical carcinogenesis, we performed two in utero exposure studies with inorganic arsenic. In the first study, pregnant mice received drinking water containing sodium arsenite at 0 (control), 42.5 and 85 ppm arsenic from gestation day 8 to 18, and the offspring were observed for up to 90 weeks. As adults, male offspring developed hepatocellular carcinoma (HCC) and adrenal tumors after in utero arsenite exposure. Although liver tumors were not induced by arsenic in female offspring, they did develop lung carcinoma, ovarian tumors, and uterine and oviduct preneoplasia. In a second study, the same doses of arsenic were used and the skin tumor promoting phorbol ester, TPA, was applied to the skin after birth in an effort to promote skin tumors potentially initiated by arsenic in utero. TPA did not promote dermal tumors after in utero arsenite exposure. Otherwise, results from the second chronic study largely duplicated the first and, irrespective of additional TPA exposure, arsenic exposure in utero induced HCC and adrenal tumors in males and ovarian tumors in females. In addition, combined arsenic and TPA induced a significant increase in hepatocellular tumors in female offspring, although arsenic alone was not effective. Thus, in utero inorganic arsenic exposure can act as a complete carcinogen in mice, with brief exposures consistently inducing tumors at several sites. In addition, it appears gestational arsenic can act as a tumor initiator in the female mouse liver, inducing liver lesions that can be promoted by TPA.

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

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

The metabolism of inorganic arsenic oxides, gallium arsenide, and arsine: a toxicochemical review

The aim of this review is to compare the metabolism, chemistry, and biological effects to determine if either of the industrial arsenicals (arsine and gallium arsenide) act like the environmental arsenic oxides (arsenite and arsenate). The metabolism of the arsenic oxides has been extensively investigated in the past 4 years and the differences between the arsenic metabolites in the oxidation states +III versus +V and with one or two methyl groups added have shown increased importance. The arsenic oxide metabolism has been compared with arsine (oxidation state -III) and arsenide (oxidation state between 0 to -III). The different metabolites appear to have different strengths of reaction for binding arsenic (III) to thiol groups, their oxidation-reduction reactions and their forming an arsenic-carbon bond. It is unclear if the differences in parameters such as the presence or absence of methyl metabolites, the rates of AsV reduction compared to the rates of AsIII oxidation, or the competition of phosphate and arsenate for cellular uptake are large enough to change biological effects. The arsine rate of decomposition, products of metabolism, target organ of toxic action, and protein binding appeared to support an oxidized arsenic metabolite. This arsine metabolite was very different from anything made by the arsenic oxides. The gallium arsenide had a lower solubility than any other arsenic compound and it had a disproportionate intensity of lung damage to suggest that the GaAs had a site of contact interaction and that oxidation reactions were important in its toxicity. The urinary metabolites after GaAs exposure were the same as excreted by arsenic oxides but the chemical compounds responsible for the toxic effects of GaAs are different from the arsenic oxides. The review concludes that there is insufficient evidence to equate the different arsenic compounds. There are several differences in the toxicity of the arsenic compounds that will require substantial research.

Physiologically based pharmacokinetic modeling of arsenic in the mouse

A remarkable feature of the carcinogenicity of inorganic arsenic is that while human exposures to high concentrations of inorganic arsenic in drinking water are associated with increases in skin, lung, and bladder cancer, inorganic arsenic has not typically caused tumors in standard laboratory animal test protocols. Inorganic arsenic administered for periods of up to 2 yr to various strains of laboratory mice, including the Swiss CD-1, Swiss CR:NIH(S), C57Bl/6p53(+/-), and C57Bl/6p53(+/+), has not resulted in significant increases in tumor incidence. However, Ng et al. (1999) have reported a 40% tumor incidence in C57Bl/6J mice exposed to arsenic in their drinking water throughout their lifetime, with no tumors reported in controls. In order to investigate the potential role of tissue dosimetry in differential susceptibility to arsenic carcinogenicity, a physiologically based pharmacokinetic (PBPK) model for inorganic arsenic in the rat, hamster, monkey, and human (Mann et al., 1996a, 1996b) was extended to describe the kinetics in the mouse. The PBPK model was parameterized in the mouse using published data from acute exposures of B6C3F1 mice to arsenate, arsenite, monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA) and validated using data from acute exposures of C57Black mice. Predictions of the acute model were then compared with data from chronic exposures. There was no evidence of changes in the apparent volume of distribution or in the tissue-plasma concentration ratios between acute and chronic exposure that might support the possibility of inducible arsenite efflux. The PBPK model was also used to project tissue dosimetry in the C57Bl/6J study, in comparison with tissue levels in studies having shorter duration but higher arsenic treatment concentrations. The model evaluation indicates that pharmacokinetic factors do not provide an explanation for the difference in outcomes across the various mouse bioassays. Other possible explanations may relate to strain-specific differences, or to the different durations of dosing in each of the mouse studies, given the evidence that inorganic arsenic is likely to be active in the later stages of the carcinogenic process.

Susceptibility to arsenic-induced hyperkeratosis and oxidative stress genes myeloperoxidase and catalase

Chronic exposure to inorganic arsenic is known to cause non-melanocytic skin and internal cancers in humans. We examined whether genetic susceptibility, as determined by single nucleotide polymorphisms -463G-->A and -262C-->T in the oxidative stress genes myeloperoxidase (MPO) and catalase (CAT), respectively, are associated with the risk of arsenic-induced hyperkeratotic skin lesions-precursors of skin cancer-in a case-control study in Bangladesh. Carriers of the susceptible MPO and CAT genotypes were at elevated risk (OR 2.1 and 95% CI 0.7-6.2 for MPO; OR 1.9 and 95% CI 0.8-4.7 for CAT) of hyperkeratosis after adjustment for arsenic exposure and other covariates. Subjects carrying the high-risk MPO genotype and with high arsenic exposure were at almost six times (OR 5.8; 95% CI 1.1-30.1) elevated risk of developing hyperkeratosis as compared to those carrying the low-risk genotype and with low arsenic exposure. Similarly, highly exposed subjects carrying the high-risk CAT genotype were at more than four times (OR 4.6; 95% CI 1.4-15.6) elevated risk of developing hyperkeratosis as compared to those carrying the low-risk genotype and with low arsenic exposure. Our findings, although based on small numbers, suggest that the oxidative stress genes MPO and CAT may influence the risk of arsenic-induced premalignant hyperkeratotic skin lesions.