A review of animal models for the study of arsenic carcinogenesis

Relevance of Carcinogen-Induced Preclinical Cancer Models

Chemical agents can cause cancer in animals by damaging their DNA, mutating their genes, and modifying their epigenetic signatures. Carcinogen-induced preclinical cancer models are useful for understanding carcinogen-induced human cancers, as they can reproduce the diversity and complexity of tumor types, as well as the interactions with the host environment. However, these models also have some drawbacks that limit their applicability and validity. For instance, some chemicals may be more effective or toxic in animals than in humans, and the tumors may differ in their genetics and phenotypes. Some chemicals may also affect normal cells and tissues, such as by causing oxidative stress, inflammation, and cell death, which may alter the tumor behavior and response to therapy. Furthermore, some chemicals may have variable effects depending on the exposure conditions, such as dose, route, and duration, as well as the animal characteristics, such as genetics and hormones. Therefore, these models should be carefully chosen, validated, and standardized, and the results should be cautiously interpreted and compared with other models. This review covers the main features of chemically induced cancer models, such as genetic and epigenetic changes, tumor environment, angiogenesis, invasion and metastasis, and immune response. We also address the pros and cons of these models and the current and future challenges for their improvement. This review offers a comprehensive overview of the state of the art of carcinogen-induced cancer models and provides new perspectives for cancer research.

Effects of arsenic exposure on trace element levels in the hippocampus and cortex of rats and their gender differences

BACKGROUND

Exposure to arsenic (As) is a major public health challenge worldwide. Chronic exposure to As can cause various human health effects, including skin diseases, cardiovascular disease, neurological disorders, and cancer. Studies have shown that As exposure can lead to disturbances in the balance of trace elements in the body. Moreover, As readily crosses the blood-brain barrier and can be enriched in the hippocampus and cortex, causing neurotoxic damage. At present, there are few reports on the effect of As on trace element levels in the central nervous system (CNS). Therefore, we sought to explore As-induced neurotoxicity and the effects of As on CNS trace element levels.

METHODS

An As-induced neurological injury model in rats was established by feeding As chow for 90 days of continuous exposure, and 19 elements were detected in the hippocampus and cortex of As-exposed rats by inductively coupled plasma mass spectrometry.

RESULTS

The results showed that the As levels in the hippocampus and cortex of As-exposed rats were significantly higher than those in the control group, The As levels in the cortex were significantly higher than in the hippocampus group. The levels of Cd, Ho, and Rb were increased in the hippocampus and decreased in Au, Ba, Ce, Cs, Pd, Se, Sr, and Tl in the As-exposed group, while the levels of Cd and Rb were increased and Se and Au were decreased in the cortex. Significant gender differences in the effects of As on hippocampal Cd, Ba, Rb, and Sr, and cortical Cd and Mo.

CONCLUSION

It is suggested that elemental imbalance may be a risk factor for developing As toxicity plays a synergistic or antagonistic role in As-induced toxicity and is closely related to As-induced CNS damage.

Malignant growth of arsenic-transformed cells depends on activated Akt induced by reactive oxygen species

Arsenic is an identified carcinogen for humans.In this study, chronic exposure of human hepatocyte L-02 to low-doses of inorganic arsenic caused cell malignant proliferation. Meanwhile, compared with normal L-02 cells, arsenic-transformed malignant cells, L-02-As displayed more ROS and significantly higher Cyclin D1 expression as well as aerobic glycolysis. Moreover, Akt activation is followed by the upregulation of Cyclin D1 and HK2 expression in L-02-As cells, since inhibition of Akt activity by Ly294002 attenuated the colony formation in soft agar and decreased the levels of Cyclin D1 and HK2. In addition, scavenging of ROS by NAC resulted in a decreased expression of phospho-Akt, HK2 and Cyclin D1, and attenuates the ability of anchorage-independent growth ofL-02-As cells, suggested that ROS mediated the Akt activation in L-02-As cells. In summary, our results demonstrated that ROS contributes to the malignant phenotype of arsenic-transformed human hepatocyte L-02-As via the activation of Akt pathway.

Application of the in vivo oxidative stress reporter Hmox1 as mechanistic biomarker of arsenic toxicity

Inorganic arsenic (iAs) is a naturally occurring metalloid present in drinking water and polluted air exposing millions of people globally. Epidemiological studies have linked iAs exposure to the development of numerous diseases including cognitive impairment, cardiovascular failure and cancer. Despite intense research, an effective therapy for chronic arsenicosis has yet to be developed. Laboratory studies have been of great benefit in establishing the pathways involved in iAs toxicity and providing insights into its mechanism of action. However, the in vivo analysis of arsenic toxicity mechanisms has been difficult by the lack of reliable in vivo biomarkers of iAs's effects. To address this issue we have applied the use of our recently developed stress reporter models to study iAs toxicity. The reporter mice Hmox1 (oxidative stress/inflammation; HOTT) and p21 (DNA damage) were exposed to iAs at acute and chronic, environmentally relevant, doses. We observed induction of the oxidative stress reporters in several cell types and tissues, which was largely dependent on the activation of transcription factor NRF2. We propose that our HOTT reporter model can be used as a surrogate biomarker of iAs-induced oxidative stress, and it constitutes a first-in-class platform to develop treatments aimed to counteract the role of oxidative stress in arsenicosis. Indeed, in a proof of concept experiment, the HOTT reporter mice were able to predict the therapeutic utility of the antioxidant N-acetyl cysteine in the prevention of iAs associated toxicity.

Long-Term Combinatorial Exposure to Trichloroethylene and Inorganic Arsenic in Genetically Heterogeneous Mice Results in Renal Tubular Damage and Cancer-Associated Molecular Changes

Trichloroethylene (TCE) and inorganic arsenic (iAs) are environmental contaminants that can target the kidney. Chronic exposure to TCE is associated with increased incidence of renal cell carcinoma, while co-exposure to TCE and iAs likely occurs in exposed human populations, such as those near Superfund sites. In order to better understand the kidney health consequences of TCE and/or iAs exposure, a genetically heterogeneous mouse population derived from FVB/NJ and CAST/EiJ mouse strains and deficient for multidrug resistance genes (Abcb1atm1Bor , Abcb1btm1Bor ) was chronically exposed for 52-weeks to varying concentrations of TCE and iAs. Although no exposure group resulted in primary renal cell tumors, kidneys from exposed mice did have significant increases in histologic and biochemical evidence of renal tubular disease with each toxicant alone and with combined exposure, with males having significantly higher levels of damage. Although no added increase in tubular disease was observed with combination exposure compared to single toxicants, molecular changes in kidneys from mice that had the combined exposure were similar to those previous observed in an embryonic stem cell assay for the P81S TCE-induced renal cell carcinoma mutation in the Von Hippel-Lindau syndrome (VHL) gene. While this model more accurately reflects human exposure conditions, development of primary renal tumors observed in humans following chronic TCE exposure was not reproduced even after inclusion of genetic heterogeneity and co-carcinogenic iAs.

Long-Term Health Effects and Underlying Biological Mechanisms of Developmental Exposure to Arsenic

PURPOSE OF REVIEW

Exposure to inorganic arsenic (iAs) via drinking water represents a significant global public health threat with chronic exposure associated with cancer, skin lesions, neurological impairment, and cardiovascular diseases. Particularly susceptible populations include the developing fetus and young children. This review summarizes some of the critical studies of the long-term health effects and underlying biological mechanisms related to developmental exposure to arsenic. It also highlights the complex factors, such as the sex of the exposed individual, that contribute to susceptibility to the later life health effects of iAs.

RECENT FINDINGS

Studies in animal models, as well as human population-based studies, have established that prenatal and early life iAs exposures are associated with long-term effects, and many of these effects display sexually dimorphic responses. As an underlying molecular basis, recent epidemiologic and toxicologic studies have demonstrated that changes to the epigenome may play a key mechanistic role underlying many of the iAs-associated health outcomes. Developmental exposure to iAs results in early and later life health effects. Mechanisms underlying these outcomes are likely complex, and include disrupted key biological pathways with ties to the epigenome. This highlights the importance of continued research, particularly in animal models, to elucidate the important underpinnings (e.g., timing of exposure, metabolism, dose) of these complex health outcomes and to identify the biological mechanisms underlying sexual dimorphism in iAs-associated diseases. Future research should investigate preventative strategies for the protection from the detrimental health endpoints associated with early life exposure to iAs. Such strategies could include potential interventions focused on dietary supplementation for example the adoption of a folate-rich diet.

Developmental Windows of Susceptibility to Inorganic Arsenic: A Survey of Current Toxicologic and Epidemiologic Data

Globally, millions of people are exposed to elevated levels of inorganic arsenic (iAs) via drinking water. Exposure to iAs is associated with a wide range of negative health outcomes, including cancers, skin lesions, neurological impairment, cardiovascular diseases, and an increased susceptibility to infection. Among those exposed to iAs, the developing fetus and young children represent particularly sensitive subpopulations. Specifically, it has been noted in animal models and human populations that prenatal and early life iAs exposures are associated with diseases occurring during childhood and later in life. Recent epidemiologic and toxicologic studies have also demonstrated that epigenetic alterations may play a key mechanistic role underlying many of the iAs-associated health outcomes, including the carcinogenic and immunologic effects of exposure. This review summarizes some of the key studies related to prenatal and early life iAs exposure and highlights the complexities in isolating the precise developmental windows of exposure associated with these health outcomes.

Proteomic profiling reveals candidate markers for arsenic-induced skin keratosis

Proteomics technology is an attractive biomarker candidate discovery tool that can be applied to study large sets of biological molecules. To identify novel biomarkers and molecular targets in arsenic-induced skin lesions, we have determined the protein profile of arsenic-affected human epidermal stratum corneum by shotgun proteomics. Samples of palm and foot sole from healthy subjects were analyzed, demonstrating similar protein patterns in palm and sole. Samples were collected from the palms of subjects with arsenic keratosis (lesional and adjacent non-lesional samples) and arsenic-exposed subjects without lesions (normal). Samples from non-exposed healthy individuals served as controls. We found that three proteins in arsenic-exposed lesional epidermis were consistently distinguishably expressed from the unaffected epidermis. One of these proteins, the cadherin-like transmembrane glycoprotein, desmoglein 1 (DSG1) was suppressed. Down-regulation of DSG1 may lead to reduced cell-cell adhesion, resulting in abnormal epidermal differentiation. The expression of keratin 6c (KRT6C) and fatty acid binding protein 5 (FABP5) were significantly increased. FABP5 is an intracellular lipid chaperone that plays an essential role in fatty acid metabolism in human skin. This raises a possibility that overexpression of FABP5 may affect the proliferation or differentiation of keratinocytes by altering lipid metabolism. KRT6C is a constituent of the cytoskeleton that maintains epidermal integrity and cohesion. Abnormal expression of KRT6C may affect its structural role in the epidermis. Our findings suggest an important approach for future studies of arsenic-mediated toxicity and skin cancer, where certain proteins may represent useful biomarkers of early diagnoses in high-risk populations and hopefully new treatment targets. Further studies are required to understand the biological role of these markers in skin pathogenesis from arsenic exposure.

Arsenic toxicity to cladocerans isolated and associated with iron: implications for aquatic environments

ABSTRACT Arsenic is an ametal ubiquitous in nature and known by its high toxicity. Many studies have tried to elucidate the arsenic metabolism in the cell and its impact to plants, animals and human health. In aqueous phase, inorganic arsenic is more common and its oxidation state (As III and As V) depends on physical and chemical environmental conditions. The aim of this study was to evaluate toxicity of arsenic to Daphnia similis and Ceriodaphnia silvestrii, isolated and associated with iron. The results showed differences in toxicity of As III and As V to both species. Effective concentration (EC50) mean values were 0.45 mg L-1 (As III) and 0.54 mg L-1 (As V) for D. similis, and 0.44 mg L-1 (As III) and 0.69 mg L-1 (As V) for C. silvestrii. However, As V IC25 mean value was 0.59 mg L-1, indicating that C. silvestrii has mechanisms to reduce arsenic toxicity. On the other hand, when associated with iron at 0.02 and 2.00 mg L-1, EC50 values decreased for D. similis (0.34 and 0.38 mg L-1) as well as C. silvestrii (0.37 and 0.37 mg L-1), showing synergistic effect of these substances.

Antioxidant Potential of Spirulina platensis Mitigates Oxidative Stress and Reprotoxicity Induced by Sodium Arsenite in Male Rats

The present study aimed to examine the protective role of Spirulina platensis (S. platensis) against arsenic-induced testicular oxidative damage in rats. Arsenic (in the form of NaAsO₂ at a dose of 6.3 mg/kg body weight for 8 weeks) caused a significant accumulation of arsenic in testicular tissues as well as a decrease in the levels of testicular superoxide dismutase (SOD), catalase (CAT), reduced glutathione, and zinc. Moreover, it significantly decreased plasma testosterone, luteinizing hormone (LH), triiodothyronine (T₃), and thyroxine (T₄) levels and reduced sperm motility and sperm count. Arsenic (AS) led to a significant increase in testicular malondialdehyde (MDA), tumour necrosis factor alpha (TNF-α), nitric oxide (NO), and sperm abnormalities. S. platensis at a dose of 300 mg/kg was found to attenuate As-induced oxidative stress, testicular damage, and sperm abnormalities by its potent antioxidant activity. S. platensis may represent a potential therapeutic option to protect the testicular tissue from arsenic intoxication.

Kinetic, isotherm and thermodynamic studies of adsorption behaviour of CNT/CuO nanocomposite for the removal of As(iii) and As(v) from water

A CNT/CuO nanocomposite prepared by precipitation method was characterized and utilized as a nanoadsorbent for the adsorption of As(iii)/As(v) from water. Maximum uptake capacities of 2267 μg g⁻¹for As(iii) and 2395 μg g⁻¹for As(v) were achieved.

Developmental exposure to 50 parts-per-billion arsenic influences histone modifications and associated epigenetic machinery in a region- and sex-specific manner in the adult mouse brain

Epidemiological studies report that arsenic exposure via drinking water adversely impacts cognitive development in children and, in adults, can lead to greater psychiatric disease susceptibility, among other conditions. While it is known that arsenic toxicity has a profound effect on the epigenetic landscape, very few studies have investigated its effects on chromatin architecture in the brain. We have previously demonstrated that exposure to a low level of arsenic (50ppb) during all three trimesters of fetal/neonatal development induces deficits in adult hippocampal neurogenesis in the dentate gyrus (DG), depressive-like symptoms, and alterations in gene expression in the adult mouse brain. As epigenetic processes control these outcomes, here we assess the impact of our developmental arsenic exposure (DAE) paradigm on global histone posttranslational modifications and associated chromatin-modifying proteins in the dentate gyrus and frontal cortex (FC) of adult male and female mice. DAE influenced histone 3K4 trimethylation with increased levels in the male DG and FC and decreased levels in the female DG (no change in female FC). The histone methyltransferase MLL exhibited a similar sex- and region-specific expression profile as H3K4me3 levels, while histone demethylase KDM5B expression trended in the opposite direction. DAE increased histone 3K9 acetylation levels in the male DG along with histone acetyltransferase (HAT) expression of GCN5 and decreased H3K9ac levels in the male FC along with decreased HAT expression of GCN5 and PCAF. DAE decreased expression of histone deacetylase enzymes HDAC1 and HDAC2, which were concurrent with increased H3K9ac levels but only in the female DG. Levels of H3 and H3K9me3 were not influenced by DAE in either brain region of either sex. These findings suggest that exposure to a low, environmentally relevant level of arsenic during development leads to long-lasting changes in histone methylation and acetylation in the adult brain due to aberrant expression of epigenetic machinery based on region and sex.

Comparison of speciated arsenic levels in the liver and brain of mice between arsenate and arsenite exposure at the early life

The aim of this study was to compare the risk from exposure to arsenate (iAs(V) ) or arsenite (iAs(III) ) at the early life. Mother mice were exposed to equimolar dose of iAs(V) and iAs(III) via drinking water during gestation and lactation. Their offspring continually drank the same water after weaning. Levels of speciated arsenic in both liver and brain were analyzed by hydride generation of volatile arsines and atomic absorption spectrophotometry (HG-AAS). In the liver, inorganic arsenic (iAs) levels significantly increased from postnatal day (PND) 15, and those on PND 35 were significantly higher than on PND 15 and 21 in iAs(III) exposed mice, but iAs levels did not significantly differ until PND 35 in iAs(V) exposed mice; Furthermore, all speciated arsenic levels on PND 35 and dimethylarsinic acid (DMA) levels on PND 1 were significantly higher in iAs(III) exposed mice than those in iAs(V) exposed mice. In the brain, iAs levels increased significantly on PND 21, but those declined sharply on PND 35 in either iAs(III) or iAs(V) exposed mice, however the mean difference between the two exposure groups was not significant; whereas DMA levels in iAs(III) exposed mice were significantly higher than those in iAs(V) exposed mice on both PND 1 and 35. In conclusion, findings from this study suggested that iAs(III) was preferentially accumulated into liver, and expected to result in more efficient methylation capacity than iAs(V) ; either iAs(V) or iAs(III) might be accumulated in the brain readily, when immature blood brain barrier can not limit it into brain. Hence, exposure to either iAs(V) or iAs(III) at the early life may increase the risk of iAs exposure in the brain.

Arsenic uptake by Lemna minor in hydroponic system

Arsenic is hazardous and causes several ill effects on human beings. Phytoremediation is the use of aquatic plants for the removal of toxic pollutants from external media. In the present research work, the removal efficiency as well as the arsenic uptake capacity of duckweed Lemna minor has been studied. Arsenic concentration in water samples and plant biomass were determined by AAS. The relative growth factor of Lemna minor was determined. The duckweed had potential to remove as well as uptake arsenic from the aqueous medium. Maximum removal of more than 70% arsenic was achieved atinitial concentration of 0.5 mg/1 arsenic on 15th day of experimental period of 22 days. Removal percentage was found to decrease with the increase in initial concentration. From BCF value, Lemna minor was found to be a hyperaccumulator of arsenic at initial concentration of 0.5 mg/L, such that accumulation decreased with increase in initial arsenic concentration.

Arsenic toxicosis in sheep: The first report from Iran

Arsenic contamination of groundwater has been previously reported in Ghopuz, a village located in the Northwest of Iran. Samples were taken from consuming and irrigation water and plants of the region for chemical analysis. A seven-year old ewe, which had lived in and fed a lifelong at the same place, with clinical signs such as weakness, wasting and inappropriate integument was necropsied. Grossly, buccal erosion, stomatitis, cutaneous ulcers and serous atrophy of fat deposits were observed. Rumen contents, wool and several tissue samples were obtained for toxicological and histopathological examinations. Mean arsenic concentration in the spring water, irrigation water and grass/algae were 70.11, 48.74 and 141.85 ppb (µg/kg), respectively. Arsenic levels were 486.73, 247.94, 127.92, 125.97 and 231.24 ppb in wool, skin, rumen contents, liver and kidney, respectively. Microscopic study revealed hyperemia and heavy parasitic infestation of the abomasal wall. Hyperemia and regeneration of renal tubule epithelia were observed in kidneys and hyperkeratosis, suppurative deep dermatitis and paniculitis were found in skin. Periacinar fibrosis and a poorly differentiated cholangiocarcinoma were seen in liver. In pancreas, reduced cell density of islands of Langerhans was noticeable. In the central nervous system, perineuronal and perivascular edema, ischemic changes in gray matter neurons, and microcavitation of white matter were present. Our findings confirmed chronic arsenic toxicosis in small ruminants in this region. It can be concluded that long-term consumption of arsenic contamined water and forage may be associated with chronic arsenic poisoning in domestic animals and human beings, with consequent neoplastic disease and induction of diabetes in this region.

Mechanisms of kidney toxicity for chromium- and arsenic-based preservatives: potential involvement of a pro-oxidative pathway

Metals have been extensively used for the preservation of wood. Among metallic conservatives, mixtures of chromated copper arsenate (CCA) were thoroughly used. However, the release and consequent mobilization of such compounds by biota, may culminate in the exertion of toxic chemical effects. The present study intended to show the toxicological effects caused by arsenic (7.2 mg/kg body weight), chromium (10.2 mg/kg Cr body weight) and the commercial mixture CCA (7.2 mg/kg As body weight and 10.2 mg/kg Cr body weight) in mice, namely the oxidative stress response (catalase - CAT, glutathione peroxidase - GPx, and glutathione-S-transferases - GSTs), in kidney tissues. The determination of the tested parameters was performed after exposure; organisms were exposed, and then sacrificed at two distinct periods, namely 14 and 96 h after the administration of toxicants. Exposure to chromium and arsenic induced significant modifications in the redox state of the test organisms, evidenced by significant alterations in GSTs and GPx activities. No alterations were found concerning the activity of catalase. These findings showed that the chemical mixture used as household product may exert significant toxicological outcomes in exposed animals, such as rodents, conditioning their redox homeostasis and antioxidant response.

Sodium arsenite and arsenic trioxide differently affect the oxidative stress, genotoxicity and apoptosis in A549 cells: an implication for the paradoxical mechanism

Although arsenic toxicity greatly depends on its chemical forms, few studies have taken into account the paradoxical phenomenon which is manifested by that sodium arsenite (NaAsO₂) acts as a potent carcinogen but arsenic trioxide (As₂O₃) serves as an effective therapeutic agent. In this study, we compared the in vitro effects of NaAsO₂ and As₂O₃ on cell viability, colony formation, cell cycle progression, apoptosis, genotoxicity and oxidative stress in human lung adenocarcinoma A549 cells. Our results demonstrated that both NaAsO₂ and As₂O₃ caused oxidative stress, genotoxicity, cytotoxicity, cell cycle arrest as well as apoptosis, while As₂O₃ induced higher production of reactive oxygen species (ROS) with a more remarkable decrease in superoxide dismutase (SOD) activities and intracellular levels of glutathione (GSH) than NaAsO₂. Moreover, the degree of DNA damage, chromosomal breakage, cell cycle arrest and apoptosis in As₂O₃-treated cells were more severe than those in NaAsO₂-treated cells. These findings suggest that differential effects and mechanisms of NaAsO₂ and As₂O₃ may responsible for the paradoxical effects of arsenic on the carcinogenesis and anticancer function.

Differential methylation of the arsenic (III) methyltransferase promoter according to arsenic exposure

Inorganic arsenic is methylated in the body by arsenic (III) methyltransferase (AS3MT). Arsenic methylation is thought to play a role in arsenic-related epigenetic phenomena, including aberrant DNA and histone methylation. However, it is unclear whether the promoter of the AS3MT gene, which codes for AS3MT, is differentially methylated as a function of arsenic exposure. In this study, we evaluated AS3MT promoter methylation according to exposure, assessed by urinary arsenic excretion in a stratified random sample of 48 participants from the Strong Heart Study who had urine arsenic measured at baseline and DNA available from 1989 to 1991 and 1998-1999. For this study, all data are from the 1989-1991 visit. We measured AS3MT promoter methylation at its 48 CpG loci by bisulphite sequencing. We compared mean  % methylation at each CpG locus by arsenic exposure group using linear regression adjusted for study centre, age and sex. A hypomethylated region in the AS3MT promoter was associated with higher arsenic exposure. In vitro, arsenic induced AS3MT promoter hypomethylation, and it increased AS3MT expression in human peripheral blood mononuclear cells. These findings may suggest that arsenic exposure influences the epigenetic regulation of a major arsenic metabolism gene.

What is the best biomarker to assess arsenic exposure via drinking water?

Arsenic (As) is a ubiquitous element. The current WHO guideline for As in drinking water is 10 μg/L. Furthermore, about 130 million people have only access to drinking water containing more than 10 g As/L. Although numerous studies have shown the related adverse effects of As, sensitive appropriate biomarkers are still required for studies of environmental epidemiology. A review of the literature has shown that various biomarkers are used for such research. Their limits and advantages are highlighted in this paper: (i) the detection of As or its derivatives in the blood is an indication of the dose ingested but it is not evidence of chronic intoxication. (ii) The detection of As in urine is an indispensible procedure because it is a good marker for internal dose. It has been demonstrated to correlate well for a number of chronic effects related to As levels in drinking water. However confounding factors must be taken into account to avoid misinterpretation and this may require As speciation. (iii) As in the hair and nails reflects the level of long term exposure but it is difficult to relate the level with the dose ingested. (iv) Some studies showed a correlation between urinary As and urinary and blood porphyrins. However, it is difficult to use only porphyrins as a biomarker in a population survey carried out without doing further studies. (v) Genotoxic effects are based on the characterization of these potential effects. Most studies have detected increases in DNA damage, sister chromatid exchange, micronuclei or chromosomal aberrations in populations exposed to As in drinking water. Micronuclei assay is the technique of choice to follow these populations, because it is sensitive and easy to use. To conclude, whatever epidemiological studies are, the urinary and toenail biomarkers are useful to provide indications of internal dose. Moreover, micronuclei assay can be complementary use as biomarker of early effects.

Protective effects of N-acetylcysteine against arsenic-induced oxidative stress and reprotoxicity in male mice

Arsenic is a well-known environmental toxic metalloid element and carcinogen that affects multiple organ systems including tissue lipid peroxidation and reproduction. The present study was aimed to investigate the protective role of N-acetylcysteine (NAC) on arsenic-induced testicular oxidative damage and antioxidant and steroidogeneic enzymes and sperm parameters in mice. Arsenic was administered through drinking water to mice at a concentration of 4.0 ppm sodium arsenite (actual concentration 2.3 ppm arsenic) for 35 days. The body weight of treated mice did not show significant change as compared with the control mice. In arsenic exposed mice there was a significant decrease in the weight of the testis, epididymis and prostate gland as compared with the control animals. Significant reduction was observed in epididymal sperm count, motile sperms and viable sperms in mice exposed to arsenic indicate decreased spermatogenesis and poor sperm quality. The activity levels of testicular 3β- and 17β-hydroxysteroid dehydrogenases and circulatory levels of testosterone were also decreased in arsenic treated mice indicating reduced steroidogenesis. A significant increase in the activities of lipid peroxidation and a significant decrease in the activities of antioxidant enzymes were observed in the testis of mice exposed to arsenic. In addition, significant increase in the testicular arsenic levels was observed during arsenic intoxication. No significant changes in the oxidation status and selected reproductive variables were observed in the N-acetylcysteine alone treated mice. Whereas, intra-peritoneal injection of NAC to arsenic exposed mice showed a significant increase in the weights of reproductive organs, reduction in arsenic-induced oxidative stress in the tissues and improvement in steroidogenesis over arsenic-exposed mice indicating the beneficial role of N-acetylcysteine to counteract arsenic-induced oxidative stress and to restore the suppressed reproduction in male mice.

Arsenic-induced oxidative stress and its reversibility

This review summarizes the literature describing the molecular mechanisms of arsenic-induced oxidative stress, its relevant biomarkers, and its relation to various diseases, including preventive and therapeutic strategies. Arsenic alters multiple cellular pathways including expression of growth factors, suppression of cell cycle checkpoint proteins, promotion of and resistance to apoptosis, inhibition of DNA repair, alterations in DNA methylation, decreased immunosurveillance, and increased oxidative stress, by disturbing the pro/antioxidant balance. These alterations play prominent roles in disease manifestation, such as carcinogenicity, genotoxicity, diabetes, cardiovascular and nervous systems disorders. The exact molecular and cellular mechanisms involved in arsenic toxicity are rather unrevealed. Arsenic alters cellular glutathione levels either by utilizing this electron donor for the conversion of pentavalent to trivalent arsenicals or directly binding with it or by oxidizing glutathione via arsenic-induced free radical generation. Arsenic forms oxygen-based radicals (OH(•), O(2)(•-)) under physiological conditions by directly binding with critical thiols. As a carcinogen, it acts through epigenetic mechanisms rather than as a classical mutagen. The carcinogenic potential of arsenic may be attributed to activation of redox-sensitive transcription factors and other signaling pathways involving nuclear factor κB, activator protein-1, and p53. Modulation of cellular thiols for protection against reactive oxygen species has been used as a therapeutic strategy against arsenic. N-acetylcysteine, α-lipoic acid, vitamin E, quercetin, and a few herbal extracts show prophylactic activity against the majority of arsenic-mediated injuries in both in vitro and in vivo models. This review also updates the reader on recent advances in chelation therapy and newer therapeutic strategies suggested to treat arsenic-induced oxidative damage.

From an old remedy to a magic bullet: molecular mechanisms underlying the therapeutic effects of arsenic in fighting leukemia

Arsenic had been used in treating malignancies from the 18th to mid-20th century. In the past 3 decades, arsenic was revived and shown to be able to induce complete remission and to achieve, when combined with all-trans retinoic acid and chemotherapy, a 5-year overall survival of 90% in patients with acute promyelocytic leukemia driven by the t(15;17) translocation-generated promyelocytic leukemia-retinoic acid receptor α (PML-RARα) fusion. Molecularly, arsenic binds thiol residues and induces the formation of reactive oxygen species, thus affecting numerous signaling pathways. Interestingly, arsenic directly binds the C3HC4 zinc finger motif in the RBCC domain of PML and PML-RARα, induces their homodimerization and multimerization, and enhances their interaction with the SUMO E2 conjugase Ubc9, facilitating subsequent sumoylation/ubiquitination and proteasomal degradation. Arsenic-caused intermolecular disulfide formation in PML also contributes to PML-multimerization. All-trans retinoic acid, which targets PML-RARα for degradation through its RARα moiety, synergizes with arsenic in eliminating leukemia-initiating cells. Arsenic perturbs a number of proteins involved in other hematologic malignancies, including chronic myeloid leukemia and adult T-cell leukemia/lymphoma, whereby it may bring new therapeutic benefits. The successful revival of arsenic in acute promyelocytic leukemia, together with modern mechanistic studies, has thus allowed a new paradigm to emerge in translational medicine.

Gene expression of normal human epidermal keratinocytes modulated by trivalent arsenicals

Chronic exposure to inorganic arsenic (iAs) is associated with the development of benign and malignant human skin lesions including nonmelanoma skin cancers. The precise arsenical form(s) responsible for this carcinogenic effect are unknown, although trivalent inorganic arsenic (iAs(III)) and two of its toxic metabolites, monomethylarsonous acid (MMA(III)) and methylarsinous acid (DMA(III)), are attractive candidates. In an effort to better understand and compare their toxic effects in the skin, we compared the global gene expression profiles of normal human epidermal keratinocytes (NHEKs) exposed to varying noncytotoxic/slightly cytotoxic concentrations of iAs(III), MMA(III), and DMA(III) for 24 h. Exposure to each arsenical treatment group exhibited a dose effect in the number of altered genes and the magnitude of expression change in NHEKs. The most significant gene expression changes associated with iAs(III) and MMA(III) exposure were consistent with several key events believed to be important to As-driven skin carcinogenesis, namely induction of oxidative stress, increased transcript levels of keratinocyte growth factors, and modulation of MAPK and NF-κB pathways. At both comparable arsenical concentrations and comparable NHEK toxicity, greater potential carcinogenic effects were observed in MMA(III)-exposed NHEKs than those exposed to iAs(III), including involvement of more proinflammatory signals and increased transcript levels of more growth factor genes. In contrast, none of these above-mentioned transcriptional trends were among the most significantly altered functions in the DMA(III) treatment group. This study suggests the relative capacity of each of the tested arsenicals to drive suspected key events in As-mediated skin carcinogenesis is MMA(III) > iAs(III) with little contribution from DMA(III).

Effects of MEK and DNMT inhibitors on arsenic-treated human uroepithelial cells in relation to Cyclin-D1 and p16

Arsenic compounds are well-known toxic and carcinogenic agents, and they are widely distributed throughout the earth's crust. These compounds are associated with various human malignancies. It has been reported that there is an elevated risk of bladder cancer in an area highly contaminated with arsenic on the southwest coast of Taiwan. However, the underlying mechanisms of arsenic-associated carcinogenesis are still unclear. The cell cycle regulatory proteins are important indicators in control of cell cycle progression. Moreover, the high expression of Cyclin-D1 and loss of p16 has been associated with a worse prognosis in a variety of human cancers. Therefore, we investigated the effect of arsenic on Cyclin-D1 and p16 expression and evaluated the role of the ERK signaling pathway and DNA methylation in arsenic carcinogenesis. Our study results showed that Cyclin-D1 high expression was found in 56.3% (9/16) of urothelial carcinomas (UC) from a blackfoot disease (BFD) area and 6.3% (1/16) of UC from a non-BFD area (p=0.002). The p16 low expression in 81.2% (13/16) of UC from BFD areas was significantly lower than in non-BFD areas (25.0%; 4/16) (p=0.001). In addition, the Cyclin-D1 increased expression but decreased p16 expression in arsenite-treated SV-HUC-1 cells. However, when cells were pretreated with inhibitors (5-aza-CdR or U0126), the effects of arsenite on Cyclin-D1 and p16 expression were suppressed. Finally, these results indicated that Cyclin-D1 and p16 both might play important roles in carcinogenesis as a result of arsenic.

p27 suppresses arsenite-induced Hsp27/Hsp70 expression through inhibiting JNK2/c-Jun- and HSF-1-dependent pathways

p27 is an atypical tumor suppressor that can regulate the activity of cyclin-dependent kinases and G(0)-to-S phase transitions. More recent studies reveal that p27 may also exhibit its tumor-suppressive function through regulating many other essential cellular events. However, the molecular mechanisms underlying these anticancer effects of p27 are largely unknown. In this study, we found that depletion of p27 expression by either gene knock-out or knockdown approaches resulted in up-regulation of both Hsp27 and Hsp70 expression at mRNA- and promoter-derived transcription as well as protein levels upon arsenite exposure, indicating that p27 provides a negative signal for regulating the expression of Hsp27 and Hsp70. Consistently, arsenite-induced activation of JNK2/c-Jun and HSF-1 pathways was also markedly elevated in p27 knock-out (p27(-/-)) and knockdown (p27 shRNA) cells. Moreover, interference with the expression or function of JNK2, c-Jun, and HSF-1, but not JNK1, led to dramatic inhibition of arsenite-induced Hsp27 and Hsp70 expression. Collectively, our results demonstrate that p27 suppresses Hsp27 and Hsp70 expression at the transcriptional level specifically through JNK2/c-Jun- and HSF-1-dependent pathways upon arsenite exposure, which provides additional important molecular mechanisms for the tumor-suppressive function of p27.

Arsenic detoxification potential of aox genes in arsenite-oxidizing bacteria isolated from natural and constructed wetlands in the Republic of Korea

Arsenic is subject to microbial interactions, which support a wide range of biogeochemical transformations of elements in natural environments such as wetlands. The arsenic detoxification potential of the bacterial strains was investigated with the arsenite oxidation gene, aox genotype, which were isolated from the natural and constructed wetlands. The isolates were able to grow in the presence of 10 mM of sodium arsenite (As(III) as NaAsO(2)) and 1 mM of D: +glucose. Phylogenetic analysis based on 16S rRNA gene sequencing indicated that these isolated strains resembled members of the genus that have arsenic-resistant systems (Acinetobacter sp., Aeromonas sp., Agrobacterium sp., Comamonas sp., Enterobacter sp., Pantoea sp., and Pseudomonas sp.) with sequence similarities of 81-98%. One bacterial isolate identified as Pseudomonas stutzeri strain GIST-BDan2 (EF429003) showed the activity of arsenite oxidation and existence of aoxB and aoxR gene, which could play an important role in arsenite oxidation to arsenate. This reaction may be considered as arsenic detoxification process. The results of a batch test showed that P. stutzeri GIST-BDan2 (EF429003) completely oxidized in 1 mM of As(III) to As(V) within 25-30 h. In this study, microbial activity was evaluated to provide a better understanding of arsenic biogeochemical cycle in both natural and constructed wetlands, where ecological niches for microorganisms could be different, with a specific focus on arsenic oxidation/reduction and detoxification.

Pentavalent methylated arsenicals are substrates of human AQP9

Liver aquaglyceroporin AQP9 facilitates movement of trivalent inorganic arsenite (As(III)) and organic monomethylarsonous acid (MAs(III)). However, the transport pathway for the two major pentavalent arsenic cellular metabolites, MAs(V) and DMAs(V), remains unknown in mammals. These products of arsenic metabolism, in particular DMAs(V), are the major arsenicals excreted in the urine of mammals. In this study, we examined the uptake of the two pentavalent organic arsenicals by human AQP9 in Xenopus laevis oocytes. Xenopus laevis oocytes microinjected with AQP9 cRNA exhibited uptake of both MAs(V) and DMAs(V) in a pH-dependent manner. The rate of transport was much higher at acidic pH (pH5.5) than at neutral pH. Hg(II), an aquaporin inhibitor, inhibited transport of As(III), MAs(III), MAs(V) and DMAs(V) via AQP9. However, phloretin, which inhibits water and glycerol permeation via AQP9, can only inhibit transport of pentavalent MAs(V) and DMAs(V) but not trivalent As(III) and MAs(III), indicating the translocation mechanisms of these arsenic species are not exactly the same. Reagents such as FCCP, valinomycin and nigericin that dissipate transmembrane proton potential or change the transmemebrane pH gradient did not significantly inhibit all arsenic transport via AQP9, suggesting the transport of pentavalent arsenic is not proton coupled. The results suggest that in addition to the initial uptake of trivalent inorganic As(III) inside cells, AQP9 plays a dual role in the detoxification of arsenic metabolites by facilitating efflux from cells.

Roles of vertebrate aquaglyceroporins in arsenic transport and detoxification

Aquaporins are important channel proteins that are responsible for the balance of cellular osmolarity and nutrient transport in vertebrates. Recently, new functions of these ancient channels have been found in the conduction of metalloid arsenic (As). Chronic As exposure through contaminated water and food sources is associated with multiple human diseases and endangers millions of people's health worldwide. Therefore, identification of the As transport pathways is necessary to elucidate the mechanisms of As carcinogenesis. Arsenic detoxification systems have been studied in multiple vertebrates such as mammalian mouse, rat, humans and nonmammalian vertebrates. Multiple transporters and enzymes have been shown to be involved in As translocation and cellular transformation. In these vertebrates, members ofaquaglyceroporins, which include AQP7 in kidney and AQP9 in liver, catalyze uptake of inorganic trivalent arsenite [As(III)]. AQP9, the major liver aquaglyceroporin, conducts both inorganic As(III) and organic monomethylarsonous acid [MMA(III)], an intermediate that is generated during the cellular methylation. As a channel that facilitates a downhill movement of substances dependent on the concentration gradient, AQP9 may play an important role in the simultaneous influx of inorganic As(III) from blood to liver and efflux of As metabolite MMA(III) from liver to blood. In this chapter, we will discuss the function ofaquaglyceroporins ofvertebrates in uptake and detoxification of the metalloid As.

Arsenate and dimethylarsinic acid in drinking water did not affect DNA damage repair in urinary bladder transitional cells or micronuclei in bone marrow

Arsenic is a human skin, lung, and urinary bladder carcinogen, and may act as a cocarcinogen in the skin and urinary bladder. Possible modes of action of arsenic carcinogenesis/cocarcinogenesis include oxidative stress induction and inhibition of DNA damage repair. We investigated the effects of arsenic in drinking water on DNA damage repair in urinary bladder transitional cells and on micronucleus formation in bone marrow. F344 rats were given 100 ppm arsenate [As(V)] or dimethylarsinic acid [DMA(V)] in drinking water for 1 week. The in vivo repair of cyclophosphamide (CP)-induced DNA damage resulting from a single oral gavage of CP, and the in vitro repair of hydrogen peroxide (H(2)O(2))- or formaldehyde-induced DNA damage, resulting from adding H(2)O(2) or formaldehyde into cell medium, were measured by the Comet assay. DMA(V) effects were not observed on either CP-induced DNA damage induction or on DNA repair. Neither DMA(V) nor As(V) increased the H(2)O(2)- or formaldehyde-induced DNA damage, and neither inhibited the repair of H(2)O(2)-induced DNA damage. Neither DMA(V) nor As(V) increased the micronucleus frequency, nor did they elevate micronucleus frequency resulting from CP treatment above the level observed by the treatment with CP alone. These results suggest that arsenic carcinogenesis/cocarcinogenesis in the urinary bladder may not be via DNA damage repair inhibition. To our knowledge this is the first report of arsenic effects on DNA damage repair in the urinary bladder.

Arsenite and its mono- and dimethylated trivalent metabolites enhance the formation of benzo[a]pyrene diol epoxide-DNA adducts in Xeroderma pigmentosum complementation group A cells

Recently, inorganic arsenite (iAs(III)) and its mono- and dimethylated metabolites have been examined for their interference with the formation and repair of benzo[a]pyrene diol epoxide (BPDE)-induced DNA adducts in human cells (Schwerdtle, ., Walter, I., and Hartwig, A. (2003) DNA Repair 2, 1449 - 1463). iAs(III) and monomethylarsonous acid (MMA(III)) were found to be able to enhance the formation of BPDE-DNA adducts, whereas dimethylarsinous acid (DMA(III)) had no enhancing effect at all. The anomaly manifested by DMA(III) prompted us to further investigate the effects of the three trivalent arsenic species on the formation of BPDE-DNA adducts. Use of a nucleotide excision repair (NER)-deficient Xeroderma pigmentosum complementation group A cell line (GM04312C) allowed us to dissect DNA damage induction from DNA repair and to examine the effects of arsenic on the formation of BPDE-DNA adducts only. At concentrations comparable to those used in the study by Schwerdtle et al., we found that each of the three trivalent arsenic species was able to enhance the formation of BPDE-DNA adducts with the potency in a descending order of MMA(III) > DMA(III) > iAs(III), which correlates well with their cytotoxicities. Similar to iAs(III), DMA(III) modulation of reduced glutathione (GSH) or total glutathione S-transferase (GST) activity could not account for its enhancing effect on DNA adduct formation. Additionally, the enhancing effects elicited by the trivalent arsenic species were demonstrated to be highly time-dependent. Thus, although our study made use of short-term assays with relatively high doses, our data may have meaningful implications for carcinogenesis induced by chronic exposure to arsenic at low doses encountered environmentally.

Effect of arsenic on p53 mutation and occurrence of teratogenic salamanders: their potential as ecological indicators for arsenic contamination

The p53 mutation in salamanders can be used as an indicator of arsenic contamination. The influence of arsenic exposure was studied on mutation of tumor suppressor gene in salamanders collected from several As-contaminated mine areas in Korea. Salamander eggs and larvae were exposed to arsenic in a toxicity test, and teratogenic salamanders found in heavy metal- and As-contaminated water from As-Bi mines were evaluated using PCR-SSCP to determine if they would be useful as an ecological indicator species. Changes in amino acids were shown to have occurred as a result of an arsenic-accumulating event that occurred after the DNA damage. In addition, both of the Hynobius leechii exposed groups were primarily affected by forms of skin damage, changes in the lateral tail/dorsal flexure and/or abnormality teratogenesis. Single-base sense mutation in codons 346 (AAG: Lys to ATG: Met), 224 (TTT: Phe to TTA: Leu), 211 (ATG: Met to AAG: Lys), 244 (TTT: Phe to TTTG: insertion), 245 (Glu GAG to Gln CAG) and 249 (TGT Cys to TGA stop) of the p53 gene were simultaneously found in mutated salamanders. Based on the results of our data illustrating the effect of arsenic exposure on the p53 mutation of salamanders in arsenic-contaminated mine areas, these mutated salamanders can be used as potential ecological indicators in the arsenic-contaminated ecosystems.

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

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

c-Jun/AP-1 pathway-mediated cyclin D1 expression participates in low dose arsenite-induced transformation in mouse epidermal JB6 Cl41 cells

Arsenic is a well-documented human carcinogen associated with skin carcinogenesis. Our previous work reveals that arsenite exposure is able to induce cell transformation in mouse epidermal cell JB6 Cl41 through the activation of ERK, rather than JNK pathway. Our current studies further evaluate downstream pathway in low dose arsenite-induced cell transformation in JB6 Cl41 cells. Our results showed that treatment of cells with low dose arsenite induced activation of c-Jun/AP-1 pathway, and ectopic expression of dominant negative mutant of c-Jun (TAM67) blocked arsenite-induced transformation. Furthermore, our data indicated that cyclin D1 was an important downstream molecule involved in c-Jun/AP-1-mediated cell transformation upon low dose arsenite exposure, because inhibition of cyclin D1 expression by its specific siRNA in the JB6 Cl41 cells resulted in impairment of anchorage-independent growth of cells induced by low dose arsenite. Collectively, our results demonstrate that c-Jun/AP-1-mediated cyclin D1 expression is at least one of the key events implicated in cell transformation upon low dose arsenite exposure.

Methylated Arsenicals: The Implications of Metabolism and Carcinogenicity Studies in Rodents to Human Risk Assessment

Monomethylarsonic acid (MMA(V)) and dimethylarsinic acid (DMA(V)) are active ingredients in pesticidal products used mainly for weed control. MMA(V) and DMA(V) are also metabolites of inorganic arsenic, formed intracellularly, primarily in liver cells in a metabolic process of repeated reductions and oxidative methylations. Inorganic arsenic is a known human carcinogen, inducing tumors of the skin, urinary bladder, and lung. However, a good animal model has not yet been found. Although the metabolic process of inorganic arsenic appears to enhance the excretion of arsenic from the body, it also involves formation of methylated compounds of trivalent arsenic as intermediates. Trivalent arsenicals (whether inorganic or organic) are highly reactive compounds that can cause cytotoxicity and indirect genotoxicity in vitro. DMA(V) was found to be a bladder carcinogen only in rats and only when administered in the diet or drinking water at high doses. It was negative in a two-year bioassay in mice. MMA(V) was negative in 2-year bioassays in rats and mice. The mode of action for DMA(V)-induced bladder cancer in rats appears to not involve DNA reactivity, but rather involves cytotoxicity with consequent regenerative proliferation, ultimately leading to the formation of carcinoma. This critical review responds to the question of whether DMA(V)-induced bladder cancer in rats can be extrapolated to humans, based on detailed comparisons between inorganic and organic arsenicals, including their metabolism and disposition in various animal species. The further metabolism and disposition of MMA(V) and DMA(V) formed endogenously during the metabolism of inorganic arsenic is different from the metabolism and disposition of MMA(V) and DMA(V) from exogenous exposure. The trivalent arsenicals that are cytotoxic and indirectly genotoxic in vitro are hardly formed in an organism exposed to MMA(V) or DMA(V) because of poor cellular uptake and limited metabolism of the ingested compounds. Furthermore, the evidence strongly supports a nonlinear dose-response relationship for the biologic processes involved in the carcinogenicity of arsenicals. Based on an overall review of the evidence, using a margin-of-exposure approach for MMA(V) and DMA(V) risk assessment is appropriate. At anticipated environmental exposures to MMA(V) and DMA(V), there is not likely to be a carcinogenic risk to humans.

Modulation of Murine Peritoneal Macrophage Function by Chronic Exposure to Arsenate in Drinking Water

Exposure of humans to arsenic is associated with various adverse health effects including immunotoxicity and elevated risk of cancer development. Specific mechanisms of these effects are not well understood. In the present study we investigated some functional parameters of peritoneal macrophages isolated from mice exposed for 12 weeks to sodium arsenate in drinking water at 0.5, 5, and 50 mgAs/l. The experimental conditions were matched with the environmental conditions of arsenic exposure in humans. To characterize function of the macrophages, we assessed their ability to release nitric oxide (NO), reactive oxygen species (ROS), and tumor necrosis factor-alpha (TNF-alpha) in response to common stimulants. To this end the isolated cells were stimulated with lipopolysaccharide (1 microg/ml) to assess NO and TNF-alpha production (the WEHI-164 bioassay) or with phorbol myristate acetate (5 microg/ml) to assess superoxide production (NBT reduction test). As a result, in mice exposed to 0.5, 5, and 50 mgAs/l we observed decreased production of NO (9 +/- 2, 8 +/- 2, 11 +/- 5 microM NO2-, respectively, versus 27 +/- 7 microM in control) and superoxide (41.3 +/- 18.2%, 52.8 +/- 15.1% and 55.9 +/- 12.9%, respectively, less than in control). Despite reduced NO production, expression of iNOS mRNA in RT-PCR, showed similar levels in exposed and control animals. We did not see any significant influence of the exposure on TNF-alpha release and mRNA expression. The potential consequences of decreased production of NO and superoxide by peritoneal macrophages as observed in exposed mice may suggest impaired response of the cells against infection or developing tumor cells.

Gene-mutation induction by arsenic compounds in the mouse lymphoma assay

Arsenic compounds are generally considered as poor inducers of gene mutations. To investigate the mutagenicity of several arsenic compounds at the thymidine kinase (Tk) gene, a reporter gene for mutation induction, we used the mouse lymphoma assay (MLA). This test is widely applied and detects a broad spectrum of mutational events, from point mutations to chromosome alterations. The selected arsenic compounds were two inorganic (sodium arsenite and arsenic trioxide) and four organic compounds (monomethylarsonic acid, dimethylarsinic acid, tetraphenylarsenium and arsenobetaine). The results show that sodium arsenite, arsenic trioxide, monomethylarsonic acid and dimethylarsinic acid are mutagenic, showing a clear dose-response pattern. On the other hand, tetraphenylarsenium and arsenobetaine are not mutagenic. Inorganic arsenic compounds are the more potent agents producing significant effects in the micromolar range, while the mutagenic organic arsenic compounds induce similar effects but in the millimolar range.

Expression of STAT3 and Bcl-6 oncoprotein in sodium arsenite-treated SV-40 immortalized human uroepithelial cells

Arsenic is widely distributed in the environment, and it is a proven toxic and carcinogenic agent. On the southwest coast of Taiwan, an endemic occurrence of chronic arsenical poisoning due to a high concentration of arsenic in artesian-well water has been reported. However, the mechanisms of its carcinogenic action are still unclear. The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway is an essential cascade for mediating normal functions of different cytokines in the development of the hematopoietic and immune systems. In this study, the substantial morphological changes observed in SV-40 immortalized human uroepithelial cells (SV-HUC-1) after treatment of various concentrations of arsenite were examined, and the expression of Bcl-6, Jak-2 and p-STAT3 (Tyr 705) were evaluated by immunocytochemistry and Western blotting. Our results showed that the expression of Bcl-6 increased dose-dependently in arsenite-treated urothelial cells. Sodium arsenite treatment reduced Jak-2 protein expression in a dose-dependent manner. However, treatment of SV-HUC-1 cells with arsenite at concentration ranges from 2 and 4microM for 48h dramatically increased p-STAT3 (Tyr 705), but the levels decreased at 8-40microM of arsenite. Our data suggest that arsenic-mediated inactivation of the JAK-STAT signaling pathway might be caused by Bcl-6 interaction with JAK tyrosine kinase or STAT. In conclusion, our findings indicate that arsenic inhibits JAK tyrosine kinase protein expression and suggest the interference in the JAK-STAT pathway might be through Bcl-6 playing an important role in arsenic-associated carcinogenesis.

Arsenic salts induced autophagic cell death and hypermethylation of DAPK promoter in SV-40 immortalized human uroepithelial cells

Arsenic is a well-known toxic and carcinogenic agent, and associated with various human malignancies, including skin, lung and bladder cancers. Paradoxically, arsenic trioxide has been used successfully in the treatment of patients with acute promyelocytic leukemia. In addition, arsenic could induce cell apoptosis or autophagy in malignant cells. However, the underlying mechanism of arsenic-induced carcinogenesis is still unclear. In this study, we demonstrated an increase of autophagosomes was produced in arsenic-treated SV-HUC-1 cells by using electron microscopy. In addition, increase of Beclin-1, an important regulator for the formation of autophagosome, protein expression in a dose-dependent manner was also found. By using methylation specific PCR, we revealed hypermethylation of CpG sites in the promoter region with decreased DAPK protein expression in arsenic-treated SV-HUC-1 cells. As epigenetic silencing of tumor suppressor genes by promoter hypermethylation has been found in a variety of malignancies including bladder cancer, our results provide new insights for the understanding of the mechanism of arsenic-induced carcinogenesis in urothelial cells.