Studies of cytogenetic effects of sodium arsenicals on mammalian cells in vitro

Arsenic methylation - Lessons from three decades of research

Between 1990 and 2020, our understanding of the significance of arsenic biomethylation changed in remarkable ways. At the beginning of this period, the conversion of inorganic arsenic into mono- and di-methylated metabolites was viewed primarily as a process that altered the kinetic behavior of arsenic. By increasing the rate of clearance of arsenic, the formation of methylated metabolites reduced exposure to this toxin; that is, methylation was detoxification. By 2020, it was clear that at least some of the toxic effects associated with As exposure depended on formation of methylated metabolites containing trivalent arsenic. Because the trivalent oxidation state of arsenic is associated with increased potency as a cytotoxin and clastogen, these findings were consistent with methylation-related changes in the dynamic behavior of arsenic. That is, methylation was activation. Our current understanding of the role of methylation as a modifier of kinetic and dynamic behaviors of arsenic is the product of research at molecular, cellular, organismic, and population levels. This information provides a basis for refining our estimates of risk associated with long term exposure to inorganic arsenic in environmental media, food, and water. This report summarizes the growth of our knowledge of enzymatically catalyzed methylation of arsenic over this period and considers the prospects for new discoveries.

Genetic toxicology of a paradoxical human carcinogen, arsenic: a review

Arsenic is widely distributed in nature in air, water and soil in the form of either metalloids or chemical compounds. It is used commercially, as pesticide, wood preservative, in the manufacture of glass, paper and semiconductors. Epidemiological and clinical studies indicate that arsenic is a paradoxical human carcinogen that does not easily induce cancer in animal models. It is one of the toxic compounds known in the environment. Intermittent incidents of arsenic contamination in ground water have been reported from several parts of the world. Arsenic containing drinking water has been associated with a variety of skin and internal organ cancers. The wide human exposure to this compound through drinking water throughout the world causes great concern for human health. In the present review, we have attempted to evaluate and update the mutagenic and genotoxic effects of arsenic and its compounds based on available literature.

Genotoxicity of arsenical compounds

With respect to global human health hazard, arsenic (As) is one of the most important environmental single substance toxicants. Currently, millions of people all over the world are exposed to the ubiquitous element in exposure levels leading to long-term toxicity, in particular cancer. Unfortunately, it has not been elucidated up to now how As mechanistically leads to the induction of neoplasia. Besides its tumorigenic potential, As has been shown to be genotoxic in a wide variety of different experimental set-ups and biological endpoints. In vitro, the element was shown to induce chromosomal mutagenicity like micronuclei, chromosome aberrations, and sister chromatid exchanges. It mainly acts clastogenic but also has an aneugenic potential. Instead, its potential to induce point mutations is very low in bacterial as well as in mammalian cell systems. However, in combined exposure with point mutagens in vitro, As was shown to enhance the frequency of chemical mutations in a synergistic manner. Additionally, As was shown to induce chromosome aberrations and micronuclei in vivo in experiments with mice. After long-term exposure to As-contaminated drinking water, the great majority of human biomonitoring studies found elevated frequencies of DNA lesions like micronuclei or chromosome aberrations. Respective occupational studies are few. Like it is the case for As carcinogenicity, it is not known through which mechanism the genotoxicity of As is mediated, although the data available indicate that As may act indirectly on DNA, i.e. via mechanisms like interference of regulation of DNA repair or integrity. Because of the indirect mode of action, it has been discussed as well that As's genotoxicity may underlie a sublinear dose-response relationship. However, various problems like non-standardized test systems and experimental variability make it impossible to prove such statement. Basically, to be able to improve risk assessment, it is of crucial importance to scientifically approach the mechanistic way of induction of As's genotoxicity and carcinogenicity.

Sodium arsenite reduces proliferation of human activated T-cells by inhibition of the secretion of interleukin-2

Arsenic (As) is a common metalloid which contaminates drinking water in several regions of the world and chronic exposure is associated with skin, lung, bladder, and kidney cancer. Previous studies suggest that arsenic exposure leads to a diminution of phytohemaglutinin (PHA) stimulated T cell proliferation in humans. In order to understand the mechanism of this suppression, the effect of As was evaluated on the expression of CD25, and IL-2 secretion in human peripheral blood mononuclear cells (PBMC). Inhibition of proliferation was observed in all donors studied. Most of the donors did not show any change in the expression of CD25, but IL-2 secretion was inhibited in 6 of the 7 donors tested. Proliferative inhibition was due to a suboptimal levels of IL-2 secreted by lymphocytes, since the addition of recombinant IL-2 to the cultures reversed in a dose-dependent fashion the inhibitory effect of As. The determination of the mRNA of IL-2 and the intracellular IL-2 levels demonstrated that the inhibition is not at the transcriptional level. Electron microscopy studies revealed that cellular ultrastructure in Golgi apparatus, mitochondria, cytoskeleton, and perinuclear membrane were altered. These alterations suggest that due to sodium arsenite effects on cytoskeleton, the intracellular secretion of proteins is affected, including the one of IL-2, leading to an impaired proliferation of the T cells when stimulated with PHA.

Spontaneous and induced sister chromatid exchanges and delayed cell proliferation in peripheral lymphocytes of Bowen's disease patients and matched controls of arseniasis-hyperendemic villages in Taiwan

A total of 15 newly-developed Bowen's disease patients and 34 age-sex-residence-matched controls were recruited from three arseniasis-hyperendemic villages in Taiwan to compare spontaneous and arsenic-induced sister chromatid exchanges (SCEs), proportion of cells with high frequencies of SCEs (HFCs), and replication index (RI) in their peripheral lymphocytes. Arsenic-induced Bowen's disease patients were found to have significantly higher spontaneous SCEs and HFCs and a lower spontaneous RI than in matched controls without or with adjustment for age, gender, cigarette smoking, alcohol drinking, tea drinking, status of major diseases, HBsAg carrier status and arsenic exposure indices through multivariate analysis. Sodium arsenite was found to increase SCEs and HFCs and to decrease RI in a dose-response pattern for both cases and controls. The arsenic-induced decrease in RI was significantly greater in arsenic-induced Bowen's disease patients than in matched controls. The arsenic-induced increases in SCEs and HFCs were also consistently, but not statistically significantly, higher in arsenic-induced Bowen's disease patients than in matched controls at all arsenite treatment levels of 0.5, 1.0 and 2.0 microM. The arsenic-induced increase in cytogenetic damages and decrease in cell proliferation among arsenic-induced Bowen's disease patients compared with matched controls may result from their long-term exposure to inorganic arsenic through consumption of high-arsenic artesian well water, elevated individual genetic and acquired susceptibility to arsenic-induced damage, or both.

The nature and significance of public exposure to arsenic: a review of its relevance to South West England

In South West England, more than two hundred years of intensive exploitation of metalliferous ore deposits, combined with the natural processes of pedogenesis from mineral-rich parent rocks, has resulted in the creation of a aignificant area of arsenic-contaminated wastes and soils. The scale of arsenic dispersion by natural and anthropogenic processes is such that 722 km(2) of land contains concentrations of arsenic in excess of 110 μg g(-1), more than twice the maximum that might be expected in a normal soil.The general rationale for the clean-up of derelict and contaminated mining sites often includes aesthetic factors and the desirability of preventing the dispersion of contaminants beyond the site boundaries. Only in extreme cases is public health directly invoked as justification for remediation. In South West England, if arsenic constitutes a genuine threat to the public, an increased rate of site remediation would be justified. The primary purpose of this review is to establish whether or not widespread arsenic contamination (principally of soils) has any measurable effects on public health in South West England, and how this might affect current contaminated site remediation policy. The review is based on data from previous research in the region, and other relevant international studies of mining and smelting communities, and other populations exposed to elevated arsenic concentrations. The literature reviewed also includes the determination of the extent and sources of contamination, and pathways between source and man.While the contamination of potable waters in some countries has led to measurable health effects, this scenario has not yet been identified in South West England, and there is little reason to believe that significantly contaminated potable water supplies would escape detection for extended periods of time under the current monitoring regime.In relative terms (based on both globaland local data), one of the most significant links between contaminated soils and humans appears to be contaminated food stuffs. In absolute terms, such exposure is low due to the natural constraints on arsenic uptake by herbage, cereal crops and vegetables, and the food chain does not appear to have been significantly compromised in South West England. Chronic health effects are unlikely as excessive arsenic concentrations in locally grown food crops remain rare.With the problems of confounding medical and social factors, it is not surprising that studies in South West England have failed to identify chronic exposure to arsenic at very low concentrations as a significant health risk. Those studies that indicate otherwise do not stand up to close scrutiny. It appears that the number of additional deaths arising from the widespread arsenic contamination in South West England is small. The relative benefits of a costly statistical study to actually determine the number of additional deaths might be considered minimal, but one major area could benefit from further studies: the sensitivity of certain population sub-groups to environmental arsenic exposure. Of particular interest are children, for whom significant exposure to arsenic via soil ingestion may be occurring.Based on available information, there appears to be no justification for a large programme of site remediation. Resources should, however, be expended on enlightening the general public, and private and governmental organisations as regards the gap between the perceived and actual significance of arsenic contamination in South West England.

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

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

Rationale for selecting exfoliated bladder cell micronuclei as potential biomarkers for arsenic genotoxicity

Biomarkers of effect have important potential in epidemiology, since they may enable ascertainment of exposure-effect associations in relatively inexpensive cross-sectional studies, with confirmation by short follow-up after cessation of exposure. Arsenic is known to cause human skin and lung cancer, and may also cause various internal cancers including bladder, kidney, and liver cancer. The strongest epidemiological association between arsenic ingestion and an internal cancer is that with bladder cancer. Epidemiological studies of a Taiwanese population exposed to high levels of arsenic from drinking water reported relative risks for bladder cancer well above any other known environmental carcinogen. Populations at increased risk for bladder cancer from other exposures, such as smoking and schistosomiasis infection, have elevated frequencies of micronuclei in exfoliated bladder cells. We have therefore proposed that the bladder cell micronucleus assay could be an appropriate biological marker of genotoxic effect of arsenic exposure. In this paper, we present the rationale for choosing the bladder cell micronucleus assay as a potential biomarker of effect for arsenic. We also briefly describe the studies we are conducting using this biomarker in currently exposed populations.

Inorganic arsenic compounds: are they carcinogenic, mutagenic, teratogenic?

This review examines and evaluates the literature on the ability of inorganic arsenic compounds to cause cancer in humans and laboratory animals. The epidemiological data that supports the position that inorganic arsenical derivatives are carcinogenic in humans is convincing and difficult to deny because of their consistency. These data are from studies of different occupational exposures such as smelter and pesticide workers, as well as from studies of drinking water, wines and medicinal tonics that contained or were contaminated with inorganic compounds of arsenic. Indeed, positive dose-response relationships between cancer incidence or mortality with many inorganic arsenical substances have been shown. Despite the presence of data which confuse the interpretation and evaluation of epidemiological data, associated neoplasms of the lungs, skin and gastrointestinal systems have been observed as a result of exposure to inorganic arsenic compounds.The mechanism of carcinogenicity of inorganic arsenical substances in humans is unknown. Inorganic arsenic compounds are not carcinogenic in laboratory animals by most routes of administration. However, further studies (subchronic, chronic, carcinogenic) using intratracheal and other conventional routes in other animal species would appear to be warranted. Moreso, especially since there is no evidence that organic arsenic compounds are carcinogenic in numerous mammalian species. Inorganic derivatives of arsenic are not mutagenic but may be teraiogenic. This latter conclusion is dependent on the method of administration and size of the dose, as well as on the species of animal used for the study.

Clastogenicity evaluation of seven chemicals commonly found at hazardous industrial waste sites

7 chemicals commonly found at the industrial waste sites were tested with the Tradescantia-Micronucleus (Trad-MCN) assay to evaluate their clastogenic potential. Chemicals selected from the US EPA Superfund Priority 1 list were: aldrin, arsenic trioxide, 1,2-benz[a, h]anthracene, dieldrin, heptachlor, lead tetraacetate and tetrachloroethylene. Results of repeated tests for clastogenicity yielded the minimum effective dose (MED) for clastogenicity of 0.44 ppm for lead tetraacetate, 1.88 ppm for heptaclor, 3.81 ppm for dieldrin and arsenic trioxide and 1,2-benz[a, h]anthracene yielded positive responses at the MED of 3.96 ppm and 12.50 ppm respectively. Aldrin and tetrachloroethylene were considered to be immiscible with water, and the tests yielded negative responses. Tetrachloroethylene in gaseous state was also used to treat the flower buds. Results of tetrachloroethylene vapor phase treatment yielded a positive response at the MED of 30 ppm/min after a 2-h exposure. 5 chemicals determined to be clastogens by this test were ranked according to their MED in the descending order of potency as follows: lead tetraacetate, heptachlor, dieldrin, arsenic trioxide and 1,2-benz[a, h]anthracene. Results of this study indicate that the Trad-MCN bioassay could be effectively utilized for assessing the potential clastogenicity of the chemicals commonly found at the industrial hazardous waste sites.

Mechanisms of arsenic-induced cell transformation

Arsenic is a well-established carcinogen in humans, but there is little evidence for its carcinogenicity in animals and it is inactive as an initiator or tumor promoter in two-stage models of carcinogenicity in mice. Studies with cells in culture have provided some possible mechanisms by which arsenic and arsenical compounds may exert a carcinogenic activity. Sodium arsenite and sodium arsenate were observed to induce morphological transformation of Syrian hamster embryo cells in a dose-dependent manner. The trivalent sodium arsenite was greater than tenfold more potent than the pentavalent sodium arsenate. The compounds also exhibited toxicity; however, transformation was observed at nontoxic as well as toxic doses. At low doses, enhanced colony forming efficiency of the cells was observed. To understand the mechanism of arsenic-induced transformation, the genetic effects of the two arsenicals were examined over the same doses that induced transformation. No arsenic-induced gene mutations were detected at two genetic loci. However, cell transformation and cytogenetic effects, including endoreduplication, chromosome aberrations, and sister chromatid exchanges, were induced by the arsenicals with similar dose responses. These results support a possible role for chromosomal changes in arsenic-induced transformation. The two arsenic salts also induced another form of mutation-gene amplification. Both sodium arsenite and sodium arsenate induced a high frequency of methotrexate-resistant 3T6 cells, which were shown to have amplified copies of the dihydrofolate reductase gene.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparative analysis of data on the clastogenicity of 951 chemical substances tested in mammalian cell cultures

A literature review was conducted using original papers published during 1964-1985 on the in vitro clastogenicity of chemical substances. Results of tests on 951 chemical substances were abstracted from over 240 reports to form the database. The evaluation of these data relied on each author's original conclusion on a positive or negative outcome. Of these 951 substances, 447 (47%) were consistently positive either with or without activation; 417 (44%) were negative in the direct test but not tested with metabolic activation systems; 4 were negative but tested only with activation; and 30 (3%) were clearly negative both with and without activation. The remaining 53 substances gave variable results when tested under different experimental protocols or in different cell types, but were positive in at least one test. Although discrepant results were found associated with some cell types, the addition of metabolic activation systems tended to eliminate such variability. No one cell appeared to be superior in response to all clastogens. For screening purposes, the choice of cell may thus depend more on the general usefulness and reliability of a cell type than on a strong response to a particular chemical. However, the use of a suitable metabolic activation system does appear to be of critical importance. The concentration at which clastogenic effects were detected varied extensively for different test substances, ranging from a minimum of 4.3 X 10(-8) to 6.9 X 10(2) mM. Possible mechanisms of action for substances active at only high levels are discussed, but no satisfactory explanation is available at this time. The relevance of tests conducted at concentrations high enough to alter significantly the osmolarity and other culture conditions is considered, and caution urged in the interpretation of test results obtained under physiologically stressful conditions. The clastogenic potential was compared quantitatively using an index of effective concentration (D20) and one which estimates the number of cells with exchange aberrations expected per mg/ml (TR) for data obtained by using a uniform protocol and cultures of Chinese hamster lung (CHL) cells. Both values were distributed over a wide range, demonstrating the variety of genotoxic potential in chemicals. In general, a substance which was active at only high concentrations produced fewer exchange-type aberrations. In vivo activity, as measured by tumourigenic effect and formation of micronuclei in bone marrow, tended to be greater for substances with a D20 below 10(-2) mg/ml and a TR value over 10(3).(ABSTRACT TRUNCATED AT 400 WORDS)

Normal sister chromatid exchange in lymphocytes from patients with multiple epidermal cancer?

Exposure to toxins in the environment and due to personal habits, e.g., tobacco smoking, may increase the rate of spontaneous sister chromatid exchange (SCE). The SCE in lymphocytes from a group of 31 patients with multiple epidermal cancer, who in the past had been exposed to various skin carcinogens, as a whole exceeded that of a control group--matched by sex, age, and smoking habits--but the difference was not statistically significant (p = 0.08). The individual SCE in these patients was also statistically independent of the nature of the carcinogenic exposure. We were unable to detect correlations between the SCE and UVC-radiation induced DNA synthesis, UVC-radiation tolerance, or rate of X-ray damage repair. This suggests that the molecular mechanisms involved in SCE induction and in repair of radiation damage are basically independent.

Effects of metals on chromosomes of higher organisms

An analysis of the available data on the clastogenic effects of metals and their compounds on higher organisms indicates some general trends. Following chronic exposure to subtoxic doses, a decrease in mitotic frequency and an increase in the number of chromosomal abnormalities are observed. These effects are usually directly proportional to the dose applied and the duration of treatment within the threshold limits. Recovery after acute treatment is inversely related to the dosage. The ultimate expression of the effects depends on certain factors, including the mode and vehicle of administration; the form administered; the test system used; the rate of detoxification, distribution, and retention in the different tissues; and interaction with foreign and endogenous substances as well as the mode of action with the biological macromolecules. In mammals, the clastogenic activity of the metals within each vertical group of the periodic table is directly proportional to the increase in atomic weight, electropositivity, and solubility of the metallic cations in water and lipids, except for Li and Ba. This pattern of inherent cytotoxicity increases with successive periods in the horizontal level. It is enhanced by the formation of covalent and coordinate covalent complexes by heavy metals with the biological macromolecules. In plants, the solubility of the metals in water is of much greater importance. The degree of dissociation of metallic salts and the rate of absorption affect significantly the frequency of chromosomal aberrations. In assessing the effects of environmental metal pollution, the presence of other metals and toxic chemicals and the level of nutrition should be taken into account, since in nature, metals occur in combination and these factors modify the cytotoxic effects to a significant extent.

Inhibition of human excision DNA repair by inorganic arsenic and the co-mutagenic effect in V79 Chinese hamster cells

We investigated the lethal, UV killing-potentiating and repair-inhibiting effects of trivalent arsenic trioxide (As2O3) and pentavalent sodium arsenate (Na2HAsO4) in normal human and xeroderma pigmentosum (XP) fibroblasts. The presence of As2O3 for 24 h after UV irradiation inhibited the thymine dimer excision from the DNA of normal and XP variant cells and thus the subsequent unscheduled DNA synthesis (UDS): excision inhibitions were partial, 30-40%, at a physiological dose of 1 microgram/ml and 100% at a supralethal dose of 5 micrograms/ml. Correspondingly, As2O3 also potentiated the lethal effect of UV on excision-proficient normal and XP variant cells in a concentration-dependent manner, but not on excision-defective XP group A cells. Na2HAsO4 (As5+) was approximately an order of magnitude less effective in preventing all the above repair events than As2O3 (As3+) which is highly affinic to SH-containing proteins. The above results provide the first evidence that arsenic inhibits the excision of pyrimidine dimers. Partially repair-suppressing small doses of As2O3 (0.5 microgram/ml) and Na2HAsO4 (5 micrograms/ml) enhanced co-mutagenically the UV induction of 6-thioguanine-resistant mutations of V79 Chinese hamster cells. Thus, such a repair inhibition may be one of the basic mechanisms for the co-mutagenicity and presumably co-carcinogenicity of arsenic. XP group A and variant strains showed a unique higher sensitivity to As2O3 and Na2HAsO4 killing by a yet unidentified mechanism.

Sodium arsenite potentiates the clastogenicity and mutagenicity of DNA crosslinking agents

To see if sodium arsenite enhances the clastogenicity and the mutagenicity of DNA crosslinking agents, Chinese hamster ovary (CHO) cells and human skin fibroblasts were exposed to cis-diamminedichloroplatinum (II) (cis-Pt(II)) or 8-methoxypsoralen (8-MOP) plus long-wave ultraviolet light (UVA) and then to sodium arsenite. The results indicate that the clastogenicity of cis-Pt(II) and 8-MOP plus UVA are enhanced by the post-treatment with sodium arsenite. Chromatid breaks and exchanges are predominantly increased in doubly treated cells. Furthermore, the mutagenicity of cis-Pt(II) at the hypoxanthine-guanine phosphoribosyl transferase locus is also potentiated by sodium arsenite in CHO cells.

Sodium arsenite enhances the cytotoxicity, clastogenicity, and 6-thioguanine-resistant mutagenicity of ultraviolet light in Chinese hamster ovary cells

Cytotoxicity, chromosome aberrations, and mutations to 6-thioguanine resistance were synergistically increased by incubating the ultraviolet light (UV)-irradiated Chinese hamster ovary (CHO) cells in medium containing sodium arsenite. However, the frequencies of sister-chromatid exchanges and mutations to ouabain resistance induced by UV were not synergistically increased by sodium arsenite. The synergistic effect of sodium arsenite on UV-induced chromosome aberrations varied with cell-harvesting time and decreased with increasing time intervals between UV and sodium arsenite treatments.

Embryotoxicity of arsenite and arsenate: distribution in pregnant mice and monkeys and effects on embryonic cells in vitro

The distribution of 74As-labelled arsenate and arsenite in pregnant mice and a monkey has been studied by autoradiography and gamma counting of isolated tissues, and their in vitro toxicity to a chondrogenic system has been investigated. With both arsenic forms, given as single intravenous injections to the mother, the 74As-arsenic appeared to pass the mouse placenta relatively free and approximately to the same extent. The retention time in maternal tissues including the placenta was, however, around three times longer with arsenite than with arsenate. In early gestation, high activity was registered in the embryonic neuroepithelium, which correlates well with reported CNS malformations in rodents. In late gestation, the distribution pattern was more like that in the adults. Accumulation in skin and squamous epithelia of the upper gastrointestinal tract (oral cavity, oesophagus and oesophageal region of stomach) dominated the distribution picture, especially at a long survival interval. Arsenate, but not arsenite, showed affinity for the calcified areas of the skeleton. A marmoset monkey in late gestation receiving arsenite showed a somewhat lower rate of placental transfer than the mice. Skin and liver had the highest concentrations (at 8 hrs), both in mother and foetuses. This species is known not to methylate arsenic, resulting in stronger binding and longer retention times of arsenic as compared with other species. The stronger binding in maternal tissues may possibly explain the lower rate of placental transfer. Arsenite was shown to inhibit cartilage formation in a chick limb bud mesenchymal spot culture system (ED50 approximately 5-10 microM), while arsenate seemed to be without effect at concentrations up to 200 microM (highest tested). Arsenate, however, showed a potentiation of the arsenite toxicity.