Review
Mechanism of arsenic carcinogenesis: an integrated approach

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Abstract

Epidemiological evidence shows an association between inorganic arsenic in drinking water and increased risk of skin, lung and bladder cancers. The lack of animal models has hindered mechanistic studies of arsenic carcinogenesis in the past, but some promising new models for these cancers are now available. The various forms of arsenic to which humans are exposed, either directly or via metabolism of inorganic arsenic to various methylated forms, further complicate the issue of mechanism, since these compounds can have different effects, both genotoxic and non-genotoxic. This review will try to integrate all of these issues, with a strong bias toward effects that are produced by environmentally relevant arsenic concentrations.

Introduction

Arsenic-related publications have greatly increased in recent years, partly as a result of the enormous disaster in the Bengal region of India and neighboring Bangladesh where millions have been exposed to high levels of arsenic in drinking water. In West Bengal alone, nine districts encompassing an area of 38,000 km2 and with a population of about 42.7 million are affected [1]. In addition, the recent re-examination by the USEPA of arsenic levels in drinking water was a stimulus to research on the mechanism of arsenic carcinogenesis. In the absence of clear knowledge as to the mechanism of arsenic carcinogenesis, the default assumption (based on the idea that a carcinogen forms DNA adducts, and therefore even a single adduct has a finite chance of causing cancer) was used, leading to a controversial linear extrapolation from observable exposures with no threshold.

Some excellent recent reviews on various aspects of arsenic metabolism, toxicity, and carcinogenicity already exist [2], [3], [4], [5], [6], [7]. The genetic and molecular toxicology of arsenic was previously reviewed by this author [8] and more recently Basu et al. [9] has reviewed arsenic’s genetic toxicology, giving an extensive compendium of the behavior of arsenic compounds in various genetic toxicology assays. This review will focus on work related to the possible mechanisms of arsenic carcinogenicity, both genotoxic and non-genotoxic, published in peer-reviewed journals during the past 10 years. It will not address the very important issue of arsenic as a chemotherapeutic agent for acute promyelocytic leukemia and other cancers, and the related topic of arsenic-induced apoptosis. These topics have been addressed by others [7], [10], [11], [12].

Because any event which could lead to carcinogenesis must of necessity allow clonal expansion of altered cells, this review will not contain much discussion of in vitro studies using concentrations of arsenic compounds that lead to extensive apoptosis or necrotic cell death. It is becoming increasingly clear that high dose exposure to arsenic compounds differs from low dose exposure with regard to genotoxicity [13], types of reactive species formed [14], signal pathways activated [15] and gene expression [16]. Many “stress proteins” seem to be induced only at high dose [17]. For most human cells treated in clonal culture (<1000 cells/dish) the IC50 for arsenite (clonal survival) is between 0.2 and 2 μM and for rodent cells it can be an order of magnitude higher [18]. When cells are treated in subconfluent monolayer cultures, which usually contain at least 5×105 cells, IC50 values for clonal survival after replating can be 10-fold higher than for direct exposure in clonal culture. Although many studies claim to be using “non-toxic” concentrations of arsenicals, the criteria are often weak, based on assays which are not as stringent as clonal survival or even growth inhibition as a substitute for cells which do not form colonies. We have recently shown that many of these alternate assays are performed too soon after exposure to arsenite, so that dying cells are not scored (Komissarova and Rossman, manuscript in preparation).

Section snippets

Arsenic as a human carcinogen

Arsenic is released into the atmosphere from both natural and anthropogenic sources. Global natural emissions of arsenic and arsenic compounds have been estimated to be 8000 t each year, whereas anthropogenic emissions are about three times higher [19]. Chronic arsenic exposure is of concern mainly because of its carcinogenic effects. Inorganic arsenic was one of the earliest identified human carcinogens. Medical treatment of psoriasis with Fowler’s solution (1% potassium arsenite) resulted in

Metabolism

It has been known for some time that arsenite is more toxic than arsenate. This may be due in part to different rates of cellular uptake. At equimolar concentration, arsenite accumulation in many cell types is much faster compared with that of arsenate [29], [30], [31], [32], [33]. It has been suggested that since arsenite is uncharged at physiological pH, it can pass through the cell membrane faster than can arsenate which is negatively charged. However, it is now becoming clear that both

Recent animal models for inorganic arsenic carcinogenesis

Until recently, arsenic compounds were the only compounds that IARC considered to have sufficient evidence for human carcinogenicity, but inadequate evidence for animal carcinogenicity [53]. Early tumorigenesis experiments in four species of animals given inorganic arsenic compounds by different routes of exposure either failed or had serious flaws [27]. There are several reports of lung carcinogenesis using intratracheal instillation of rats and hamsters with inorganic arsenic compounds alone

Genotoxicity by low level inorganic arsenic

When studying the genetic toxicology of arsenite in cultured cells, it must be kept in mind that some cell lines are able to methylate arsenite. Although hepatocytes are not often used for assessing genotoxicity, rat hepatocytes have a fast rate of methylation compared with human hepatocytes, while the methylation capacities of human keratinocytes and bronchial cells were less than 1% that of human hepatocytes [32]. The methylation rate of normal human keratinocytes did not exceed 0.25 pmol

Oxidant production and oxidative DNA damage induced by arsenite

Although arsenite does not react directly with DNA, cells treated with arsenite show evidence of oxidative DNA damage. The concept that arsenite increases oxidant levels is supported by studies demonstrating protection against arsenite genotoxicity by GSH elevation and antioxidants Vitamin E, catalase, superoxide dismutase (SOD), and squalene [81], [89], [109], [110], [111], [112], [113]. H2O2-resistant CHO cells are cross-resistant to arsenite [114]. Mutagenicity of AL cells by arsenite was

In vitro transformation by arsenicals

In contrast to its weak mutagenicity, arsenite induces cell transformation of various cells to a more malignant phenotype. Some early studies found a lack of mutagenesis in the same cells that were transformed by arsenite suggesting a non-mutagenic mode of transformation. For example, no mutagenicity was seen at two loci in Syrian hamster embryo (SHE) cells, where arsenite caused cell transformation and cytogenetic damage [157]. These cells did, however, undergo gene amplification at the dhfr

DNA methylation, gene amplification, and genomic instability induced by arsenite

It has become increasingly apparent that aberrant promoter methylation at CpG sites alters gene function, which may give a selective advantage to neoplastic cells in the same way that mutations do [164]. The first report of arsenite inducing methylation changes was the increased cytosine methylation in the p53 promoter in human adenocarcinoma A549 cells [165]. Later it was found that there was both hypo- and hypermethylation (of different genes) in human kidney UOK cells treated with arsenite

Enhancing effects of arsenite

One of the most serious threats to genome stability is replication of DNA with unrepaired or badly repaired damage. Low concentrations of arsenite which are not mutagenic nevertheless can effect the mutagenicity of other carcinogens, probably by interfering with DNA repair. Arsenite enhances the mutagenicity of UV in E. coli [186] and the mutagenicity and/or clastogenicity of UV, N-methyl-N-nitrosourea (MNU), diepoxybutane, X-rays, and methylmethane sulfonate in mammalian cells [75], [78], [104]

Effects of arsenite on DNA damage response and cell cycle control

Many different DNA lesions can trigger common signaling pathways that collectively are referred to as the DNA damage response [212]. An important feature of this pathway is the slowing or arrest of the cell cycle which is thought to be necessary to allow efficient DNA repair to take place prior to DNA replication. If damaged DNA is replicated, it may be mutated or lost due to chromosome breaks. Cell cycle checkpoints are signal transduction pathways that prevent late events from being initiated

Carcinogenicity of methylated arsenic species

DMAV (cacodylic acid) is widely used in herbicides. Human exposure occurs during production and use of these herbicides as well as from food contaminated with them and through ingestion of some seaweed in which it occurs naturally [19]. As discussed above, DMAV is the main urinary metabolite of inorganic arsenic. It is also the major metabolite of the arsenosugars that occur naturally in seaweed [230]. When rodents and human ingest DMAV, most of it is rapidly excreted unchanged, but

Genotoxicity of methylated metabolites of arsenic

Various pathways of metabolism have been proposed to explain the effects of DMAV. These include: (1) reduction of DMAV to DMAIII, which is more toxic and genotoxic (discussed below); (2) formation of TMAO; (3) reduction of DMAV to yield dimethyl arsine, which forms the peroxyradical (see Fig. 1), hydroxyl radical, and superoxide [245], [246]. To determine whether DMAIII mediated the bladder hyperplasia in rats, 2,3-dimercaptopropane-1-sulfonic acid (DMPS), a chelator of trivalent arsenicals,

Enhanced cell proliferation by arsenicals

Cell proliferation that results from mitogenic stimuli or from regeneration after cytotoxicity can enhance carcinogenesis [260]. Proliferation of mammalian cells is regulated by a number of mitogens including growth factors, mitogenic lipids, inflammatory cytokines and hormones, modified by integrin-mediated adhesion [261]. A considerable amount of evidence suggests that arsenite (and perhaps some of its metabolites) act as co-carcinogens in part by activating signal transduction pathways which

Additional considerations

Tumorigenesis is often accompanied by mechanisms that override cellular programs for terminal differentiation. Arsenite prevents terminal differentiation of pre-adipocytes stimulated by insulin+dexamethasone [288]. In this system, arsenite blocked the upregulation of c/EBPα and p21 that normally accompanies differentiation [289]. It also suppresses differentiation of a human keratinocyte line [290], an affect mediated by AP-1 [291]. Perhaps the arsenite-induced activation of c-jun blocks the

Conclusion

Both arsenite and its metabolites can have a variety of genotoxic effects, which may be mediated by oxidants or free radical species. All of these species also have effects on signaling pathways leading to proliferative responses. There are interesting differences in the activities of inorganic and organic species both in terms of target organ carcinogenicity and genotoxic and toxic mechanisms. Animal experiments show that following chronic exposure, arsenite accumulates in the skin and hair

Acknowledgements

We thank Eleanor Cordisco for her expert help in document preparation. We also thank Dr. Jerry Solomon for his help with Fig. 1 and Dr. Barry Rosen for his advice on arsenic metabolism. This work was supported by United States Public Health Service Grants CA73610, ES09252, and P42 ES10344, and is part of NYU’s Nelson Institute of Environmental Medicine Center programs supported by Grants ES00260 from NIEHS and CA16087 from NCI.

References (299)

  • L.M. Del Razo et al.

    Stress proteins induced by arsenic

    Toxicol. Appl. Pharmacol.

    (2001)
  • S.A. Lerman et al.

    Arsenic uptake and metabolism by liver cells is dependent on arsenic oxidation state

    Chem.-Biol. Interact.

    (1983)
  • M. Delnomdedieu et al.

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

    Chem. Biol. Interact.

    (1995)
  • L. Vega et al.

    Differential effects of trivalent and pentavalent arsenicals on cell proliferation and cytokine secretion in normal human epidermal keratinocytes

    Toxicol. Appl. Pharmacol.

    (2001)
  • R.N. Huang et al.

    Cellular uptake of trivalent arsenite and pentavalent arsenate in KB cells cultured in phosphate-free medium

    Toxicol. Appl. Pharmacol.

    (1996)
  • D.J. Thomas et al.

    The cellular metabolism and systemic toxicity of arsenic

    Toxicol. Appl. Pharmacol.

    (2001)
  • S. Lin et al.

    A novel S-adenosyl-l-methionine:arsenic(III) methyltransferase from rat liver cytosol

    J. Biol. Chem.

    (2002)
  • J.S. Petrick et al.

    Monomethylarsonous acid (MMIII) is more toxic than arsenite in Chang human hepatocytes

    Toxicol. Appl. Pharmacol.

    (2000)
  • N. Ishinishi et al.

    Tumorigenicity of arsenic trioxide to the lung in Syrian golden hamsters by intermittent instillations

    Cancer Lett.

    (1983)
  • G. Pershagen et al.

    Carcinomas of the respiratory tract in hamsters given arsenic trioxide and/or benzo(a)pyrene by the pulmonary route

    Environ. Res.

    (1984)
  • D.R. Germolec et al.

    Arsenic can mediate skin neoplasia by chronic stimulation of keratinocyte-derived growth factors

    Mutat. Res.

    (1997)
  • M.I. Luster et al.

    Role of keratinocyte-derived cytokines in chemical toxicity

    Toxicol. Lett.

    (1995)
  • Y. Chen et al.

    K6/ODC transgenic mice as a sensitive model for carcinogen identification

    Toxicol. Lett.

    (2000)
  • F.R. de Gruijl

    Skin cancer and solar UV radiation

    Eur. J Cancer

    (1999)
  • T.G. Rossman et al.

    Arsenic is a cocarcinogaen with solar ultraviolet radiation for mouse skin: an animal model for arsenic carcinogenesis

    Toxicol. Appl. Pharmacol.

    (2001)
  • M.P. Waalkes et al.

    Transplacental carcinogenicity of inorganic arsenic in the drinking water: induction of hepatic, ovarian, pulmonary, and adrenal tumors in mice

    Toxicol. Appl. Pharmacol.

    (2003)
  • Z. Wang et al.

    Efflux mediated resistance to arsenicals in arsenic-resistant and -hypersensitive Chinese hamster cells

    Toxicol. Appl. Pharmacol.

    (1996)
  • T.W. Gebel

    Genotoxicity of arsenical compounds

    Int. J. Hyg. Environ. Health

    (2001)
  • M.M. Moore et al.

    Relative genotoxic potency of arsenic and its methylated metabolites

    Mutat. Res.

    (1997)
  • J.-R. Gurr et al.

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

    Mutat. Res.

    (1993)
  • T.S. Kochhar et al.

    Effect of trivalent and pentavalent arsenic in causing chromosome alterations in cultured CHO cells

    Toxicol. Lett.

    (1996)
  • D.A. Eastmond et al.

    Kinetochore localization in micronucleated cytokinesis-blocked Chinese hamster ovary cells a new and rapid assay for identifying aneuploidy-inducing agents

    Mutat. Res.

    (1989)
  • Z. Wang et al.

    Stable and inducible arsenite resistance in Chinese hamster cells

    Toxicol. Appl. Pharmacol.

    (1993)
  • E.M. Brambila et al.

    Chronic arsenic-exposed human prostate epithelial cells exhibit stable arsenic tolerance: mechanistic implications of altered cellular glutathione and glutathione S-transferase

    Toxicol. Appl. Pharmacol.

    (2002)
  • L. Vega et al.

    Aneugenic effect of sodium arsenite on human lymphocytes in vitro: an individual susceptibility effect detected

    Mutat. Res.

    (1995)
  • P. Ramirez et al.

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

    Mutat. Res.

    (1997)
  • A. Basu et al.

    Enhanced frequency of micronuclei in individuals exposed to arsenic through drinking water in West Bengal, India

    Mutat. Res.

    (2002)
  • F.N. Dulout et al.

    Chromosomal aberrations in peripheral blood lymphocytes from native Andean women and children from northwestern Argentina exposed to arsenic in drinking water

    Mutat. Res.

    (1996)
  • M.E. Gonsebatt et al.

    Cytogenetic effects in human exposure to arsenic

    Mutat. Res.

    (1997)
  • T.R. Chowdhury et al.

    Arsenic poisoning in the Ganges delta

    Nature

    (1999)
  • P.P. Simeonova et al.

    Mechanisms of arsenic carcinogenicity: genetic or epigenetic mechanisms

    J. Envrion. Pathol. Toxicol. Oncol.

    (2000)
  • M. Styblo et al.

    The role of biomethylation in toxicity and carcinogenicity of arsenic: a research update

    Environ. Health Perspect.

    (2002)
  • W.H. Miller et al.

    Mechanisms of action of arsenic trioxide

    Cancer Res.

    (2002)
  • J. Yi et al.

    The inherent cellular level of reactive oxygen species: one of the mechanisms determining apoptotic susceptibility of leukemia cells to arsenic trioxide

    Apoptosis

    (2002)
  • M.D. Pulido, A.R. Parrish, Metal-induced apoptosis: mechanisms, Mutat. Res. 233 (2003)...
  • T.G. Rossman, Molecular and genetic toxicology of arsenic, in: J. Rose (Ed.), Environmental Toxicology: Current...
  • National Research Council, Arsenic in Drinking Water, National Academy Press, Washington, DC,...
  • W.P. Tseng et al.

    Prevalence of skin cancer in an endemic area of chronic arsenicism in Taiwan

    J. Natl. Cancer Inst.

    (1968)
  • W.P. Tseng

    Effects and dose response relationships of skin cancer and blackfoot disease with arsenic

    Environ. Health Perspect.

    (1977)
  • J.M. Borgono et al.

    Arsenic in the drinking water of the city of Antofagasta: epidemiological and clinical study before and after the installation of a treatment plant

    Environ. Health Perspect.

    (1977)
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