Heavy metals are naturally occurring elements that have a high atomic weight and a density at least five times greater than that of water. Their multiple industrial, domestic, agricultural, medical, and technological applications have led to their wide distribution in the environment, raising concerns over their potential effects on human health and the environment. Their toxicity depends on several factors including the dose, route of exposure, and chemical species, as well as the age, gender, genetics, and nutritional status of exposed individuals. Because of their high degree of toxicity, arsenic, cadmium, chromium, lead, and mercury rank among the priority metals that are of public health significance. These metallic elements are considered systemic toxicants that are known to induce multiple organ damage, even at lower levels of exposure. They are also classified as human carcinogens (known or probable) according to the US Environmental Protection Agency and the International Agency for Research on Cancer. This review provides an analysis of their environmental occurrence, production and use, potential for human exposure, and molecular mechanisms of toxicity, genotoxicity, and carcinogenicity.

/pdf/heavy-metal-toxicity-and-the-environment-32mfkxp6nb.pdf

Heavy metal toxicity and the environment.

Disinfection by-products (DBPs) are formed when disinfectants (chlorine, ozone, chlorine dioxide, or chloramines) react with naturally occurring organic matter, anthropogenic contaminants, bromide, and iodide during the production of drinking water. Here we review 30 years of research on the occurrence, genotoxicity, and carcinogenicity of 85 DBPs, 11 of which are currently regulated by the U.S., and 74 of which are considered emerging DBPs due to their moderate occurrence levels and/or toxicological properties. These 74 include halonitromethanes, iodo-acids and other unregulated halo-acids, iodo-trihalomethanes (THMs), and other unregulated halomethanes, halofuranones (MX [3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone] and brominated MX DBPs), haloamides, haloacetonitriles, tribromopyrrole, aldehydes, and N-nitrosodimethylamine (NDMA) and other nitrosamines. Alternative disinfection practices result in drinking water from which extracted organic material is less mutagenic than extracts of chlorinated water. However, the levels of many emerging DBPs are increased by alternative disinfectants (primarily ozone or chloramines) compared to chlorination, and many emerging DBPs are more genotoxic than some of the regulated DBPs. Our analysis identified three categories of DBPs of particular interest. Category 1 contains eight DBPs with some or all of the toxicologic characteristics of human carcinogens: four regulated (bromodichloromethane, dichloroacetic acid, dibromoacetic acid, and bromate) and four unregulated DBPs (formaldehyde, acetaldehyde, MX, and NDMA). Categories 2 and 3 contain 43 emerging DBPs that are present at moderate levels (sub- to low-mug/L): category 2 contains 29 of these that are genotoxic (including chloral hydrate and chloroacetaldehyde, which are also a rodent carcinogens); category 3 contains the remaining 14 for which little or no toxicological data are available. In general, the brominated DBPs are both more genotoxic and carcinogenic than are chlorinated compounds, and iodinated DBPs were the most genotoxic of all but have not been tested for carcinogenicity. There were toxicological data gaps for even some of the 11 regulated DBPs, as well as for most of the 74 emerging DBPs. A systematic assessment of DBPs for genotoxicity has been performed for approximately 60 DBPs for DNA damage in mammalian cells and 16 for mutagenicity in Salmonella. A recent epidemiologic study found that much of the risk for bladder cancer associated with drinking water was associated with three factors: THM levels, showering/bathing/swimming (i.e., dermal/inhalation exposure), and genotype (having the GSTT1-1 gene). This finding, along with mechanistic studies, highlights the emerging importance of dermal/inhalation exposure to the THMs, or possibly other DBPs, and the role of genotype for risk for drinking-water-associated bladder cancer. More than 50% of the total organic halogen (TOX) formed by chlorination and more than 50% of the assimilable organic carbon (AOC) formed by ozonation has not been identified chemically. The potential interactions among the 600 identified DBPs in the complex mixture of drinking water to which we are exposed by various routes is not reflected in any of the toxicology studies of individual DBPs. The categories of DBPs described here, the identified data gaps, and the emerging role of dermal/inhalation exposure provide guidance for drinking water and public health research.

Occurrence, genotoxicity, and carcinogenicity of regulated and emerging disinfection by-products in drinking water: a review and roadmap for research.

n this article, we review recent findings related to estrogen exposure and the risk of breast cancer, the mechanisms that may be involved, and the clinical implications of these findings. The weight of evidence indicates that exposure to estrogen is an important determinant of the risk of breast cancer. The mechanisms of carcinogenesis in the breast caused by estrogen include the metabolism of estrogen to genotoxic, mutagenic metabolites and the stimulation of tissue growth. Together, these processes cause initiation, promotion, and progression of carcinogenesis. Insight into the mechanisms of the causation of cancer by estrogen will identify determinants of susceptibility to breast cancer and new targets for prevention and therapeutic intervention.

/pdf/estrogen-carcinogenesis-in-breast-cancer-1bgx0pl4j1.pdf

Estrogen carcinogenesis in breast cancer.

Cytochrome P450 enzymes that metabolize estrogens are expressed in the mammary gland, uterus, brain and other target tissues for estrogen action, and this results in the formation of hydroxylated estrogens in these tissues. Estradiol metabolites formed in target tissues at or near estrogen receptors may either be inactive or have important biological effects, and changes in the activities of estrogen-metabolizing enzymes in target tissues may profoundly influence estrogen action. Although some active estrogen metabolites exert hormonal effects in target tissues by interaction with the classical estrogen receptor, other metabolites appear to elicit unique biological responses that are not associated with activation of this receptor. Therefore, some of the many actions of estradiol may not be caused by estradiol per se, but may result from the formation of active estrogen metabolite(s) which function as local mediators or may activate their own unique receptors or effectors. This is an important area in need of more research. The present paper represents a review of the literature and perspectives by the authors on the functional role of estrogen metabolism in target tissues.

https://www.hormonebalance.org/images/documents/Zhu%2098estrogen%20metabolism%20in%20target%20cells%20Carc.pdf

Functional role of estrogen metabolism in target cells: review and perspectives.

Unlimited cellular proliferation depends on counteracting the telomere attrition that accompanies DNA replication. In human cancers this usually occurs through upregulation of telomerase activity, but in 10-15% of cancers - including some with particularly poor outcome - it is achieved through a mechanism known as alternative lengthening of telomeres (ALT). ALT, which is dependent on homologous recombination, is therefore an important target for cancer therapy. Although dissection of the mechanism or mechanisms of ALT has been challenging, recent advances have led to the identification of several genes that are required for ALT and the elucidation of the biological significance of some phenotypic markers of ALT. This has enabled development of a rapid assay of ALT activity levels and the construction of molecular models of ALT.

Alternative lengthening of telomeres: models, mechanisms and implications

Diethylstilbestrol (DES) has been demonstrated previously to induce morphological and neoplastic transformation of Syrian hamster embryo cells in the absence of any measurable induction of gene mutation. To determine if DES induces cell transformation by a genetic mechanism at the chromosomal level, the effect of DES on structural aberrations and numerical chromosome changes was examined in asynchronous and synchronized cells. Over the concentration range which is optimal for cell transformation, DES failed to induce any increase in chromosome aberrations in the cells. In contrast, significant numerical chromosome changes were observed in DES-treated cultures. The percentage of metaphases with a near diploid chromosome number increased to 19% at 48 hr after treatment. By comparison, cells from control cultures contained only 1 to 2% aneuploid metaphases with a near diploid chromosome number. No significant increase in the number of metaphases with a near tetraploid number (>70) of chromosomes was observed in the DES-treated cultures. DES induced both chromosome loss and gain, and no significant difference was detected between the number of hyperdiploid and hypodiploid cells. Chromosome loss or gain was observed for chromosomes in each karyotype group. These findings suggest that DES induces chromosome nondisjunction. Synchronized cell cultures were obtained by first growing the cells in 1% serum and then in 10% serum with hydroxyurea which blocked the cells at the G1-S border. Upon release of the hydroxyurea block, the cells entered into S phase in a very synchronous manner. The cells were treated for 3 hr during one of four time periods after hydroxyurea release. During the first period, the cells were primarily in early S phase, while the second period included cells in late S phase. During the third period most of the cells were undergoing mitosis, while in the fourth period most of the cells were in G1 phase, although some mitotic cells were observed. Treatment of the synchronized cells with DES during early or late S phase resulted in little morphological transformation. However, treatment during the third period, when the majority of the cells were in mitosis, resulted in a peak of transformation which was 15 times the level observed in cultures treated in early or late S phase. Treatment during the fourth time period resulted in a reduced level of cell transformation. Treatment of synchronized cultures with DES resulted also in a cell cycle-dependent induction of aneuploid cells which paralleled the induction of cell transformation, with the greatest level observed following treatment during mitosis. No increase in the percentage of polyploid metaphases or chromosome aberrations was observed in the DES-treated synchronized cells. Parallel dose-response curves for cell transformation and aneuploidy induction by DES were observed when the synchronized cultures were treated during the mitotic phase of the cell cycle. Possible mechanisms for DES-induced aneuploidy and the evidence supporting a role for nonrandom numerical chromosome changes in neoplastic development, as well as significance of aneuploidy in cancer, are discussed.

https://cancerres.aacrjournals.org/content/canres/43/8/3814.full.pdf

Aneuploidy induction and cell transformation by diethylstilbestrol: a possible chromosomal mechanism in carcinogenesis.

Benzene is a human carcinogen present naturally in petroleum and gasoline. For the simultaneous assessment of benzene-induced carcinogenicity and mutagenicity, benzene and its principal metabolites, phenol, catechol and hydroquinone were examined for their ability to induce cell transformation and genotoxic effects using the same mammalian cells in culture. Each of the four compounds induced morphological transformation of Syrian hamster embryo (SHE) cells. Catechol was the most potent, inducing transformation at concentrations of 1-30 microM, followed by hydroquinone (3-30 microM), phenol (10-100 microM) and benzene (only at 100 microM). Gene mutations at two loci in SHE cells were induced by all four compounds, with catechol being the most potent; both ouabain-resistant and 6-thioguanine-resistant mutant frequencies were increased. Chromosomal aberrations in SHE cells were especially induced by catechol, lesser by hydroquinone, and to a marginal extent by phenol at only the 100 microM concentration, whereas sister chromatid exchanges in SHE cells occurred with hydroquinone (1-30 microM), catechol (10-30 microM) and phenol (1000-3000 microM). Aneuploidy in the near diploid range of SHE cells was significantly induced by benzene and catechol. All three metabolites induced unscheduled DNA synthesis in SHE cells, whereas benzene did not. This is the first report that the cell transforming activity and mutagenicity of benzene and its metabolites were assessed with the same mammalian cells in culture. The results provide evidence that benzene and several of its metabolites are cell transforming and genotoxic to cultured mammalian cells.

Benzene-, catechol-, hydroquinone- and phenol-induced cell transformation, gene mutations, chromosome aberrations, aneuploidy, sister chromatid exchanges and unscheduled DNA synthesis in Syrian hamster embryo cells.

The effects of exposure of cultured human diploid fibroblasts (JHU-1 cells) to sodium fluoride have been studied with respect to cytotoxicity and induction of chromosome aberrations and unscheduled DNA synthesis (UDS) Cytotoxicity of NaF on JHU-1 cells, as determined by a decrease in colony-forming ability, linearly increased with increasing dose of NaF (50-150 micrograms/ml) or exposure time (1-24 h). Treatment of the cells with 50 micrograms/ml NaF for 24 h resulted in a lethality (approximately 70%) similar to that obtained with 100 micrograms/ml for 12 h. A linear increase in cytotoxicity was observed as a fraction of the product of NaF treatment time and dose. JHU-1 cells treated with 20-50 micrograms/ml NaF for 12 or 24 h were analyzed for chromosome aberrations. A significant increase in the frequency of chromosome aberrations at the chromatid level was observed in treated cells in a dose-dependent manner. For detection of UDS, confluent JHU-1 cells were cultured with medium containing low serum and then exposed to NaF in the presence of 10 mM hydroxyurea. Treatment with 100-400 micrograms NaF/ml for 4-24 h reproducibly elicited UDS in a dose-related fashion as determined by direct scintillation counting of [3H]thymidine incorporated into DNA during repair synthesis. These results suggest that NaF causes DNA damage in human diploid fibroblasts in culture.

Cytotoxicity, chromosome aberrations and unscheduled DNA synthesis in cultured human diploid fibroblasts induced by sodium fluoride.

The ability of 17 beta-estradiol to induce morphological transformation of Syrian hamster embryo cells was examined and dose-dependent increases were observed over the concentration range of 1-10 micrograms/ml. However, treatment of the cells with 17 beta-estradiol failed to induce any detectable increases in gene mutations, chromosome aberrations, sister chromatid exchanges or unscheduled DNA synthesis. In contrast, over the dose range that was effective in inducing cell transformation, 17 beta-estradiol induced numerical chromosome changes (both chromosome gains and losses). These findings are similar to the reported observations with the synthetic estrogen, diethylstilbestrol, and support the hypothesis that aneuploidy induction is important in cell transformation and possibly carcinogenesis induced by estrogens.

17β-Estradiol-induced cell transformation and aneuploidy of Syrian hamster embryo cells in culture

Treatment of Syrian hamster embryo cells in culture with 001-01 micrograms/ml of colcemid for 48 h resulted in morphological and neoplastic transformation of the cells Cell transformation was observed with doses which were non-cytotoxic and did not cause mitotic inhibition of the cells Higher dose of colcemid (greater than 01 microgram/ml) resulted in mitotic inhibition of the cells and a significant loss of colony forming ability, but no increase in the frequency of morphological transformation Treatment of the cells with transforming doses of colcemid did not result in any measurable induction of gene mutations or structural chromosome aberrations; however, numerical chromosome changes in the treated cells were observed A 14-fold increase in the number of aneuploid cells with a near diploid chromosome complement was found in cultures treated with 01 micrograms/ml colcemid and both chromosome loss and gain were induced The dose response curves for colcemid induced morphological transformation and aneuploidy were similar These results are consistent with a role of carcinogen-induced chromosome non-disjunction in carcinogenesis

Takeki Tsutsui

Papers

Aneuploidy induction and cell transformation by diethylstilbestrol: a possible chromosomal mechanism in carcinogenesis.

Benzene-, catechol-, hydroquinone- and phenol-induced cell transformation, gene mutations, chromosome aberrations, aneuploidy, sister chromatid exchanges and unscheduled DNA synthesis in Syrian hamster embryo cells.

Cytotoxicity, chromosome aberrations and unscheduled DNA synthesis in cultured human diploid fibroblasts induced by sodium fluoride.

17β-Estradiol-induced cell transformation and aneuploidy of Syrian hamster embryo cells in culture

Colcemid-induced neoplastic transformation and aneuploidy in Syrian hamster embryo cells