Effects of antioxidants on fiber mutagenesis

The chemical environment of iron in mineral fibres. A combined X-ray absorption and Mössbauer spectroscopic study

Although asbestos represents today one of the most harmful contaminant on Earth, in 72% of the countries worldwide only amphiboles are banned while controlled use of chrysotile is allowed. Uncertainty on the potential toxicity of chrysotile is due to the fact that the mechanisms by which mineral fibres induces cyto- and geno-toxic damage are still unclear. We have recently started a long term project aimed at the systematic investigation of the crystal-chemistry, bio-interaction and toxicity of the mineral fibres. This work presents a systematic structural investigation of iron in asbestos and erionite (considered the most relevant mineral fibres of social and/or economic-industrial importance) using synchrotron X-ray absorption and Mössbauer spectroscopy. In all investigated mineral fibres, iron in the bulk structure is found in octahedral sites and can be made available at the surface via fibre dissolution. We postulate that the amount of hydroxyl radicals released by the fibers depends, among other factors, upon their dissolution rate; in relation to this, a ranking of ability of asbestos fibres to generate hydroxyl radicals, resulting from available surface iron, is advanced: amosite > crocidolite ≈ chrysotile > anthophyllite > tremolite. Erionite, with a fairly high toxicity potential, contains only octahedrally coordinated Fe(3+). Although it needs further experimental evidence, such available surface iron may be present as oxide nanoparticles coating and can be a direct cause of generation of hydroxyl radicals when such coating dissolves.

The zeta potential of mineral fibres

For the first time, the zeta (ξ) potential of pathogenic mineral fibres (chrysotiles, amphiboles and erionite) was systematically investigated to shed light on the relationship between surface reactivity and fibre pathogenicity. A general model explaining the zeta potential of chrysotile, amphiboles and erionite has been postulated. In double distilled water, chrysotiles showed positive values while crocidolite and erionite showed negative values. In contact with organic solutions, all fibres exhibited negative values of zeta potential. The decrease of the surface potential is deemed to be a defensive chemical response of the macrophage cells to minimize hemolytic damage. Negatively charged surfaces favour the binding of collagen and redox activated Fe-rich proteins, to form the so-called asbestos bodies and prompt the formation of HO via the reaction with peroxide (H2O2+e(-)→HO+HO(-)). An additional mechanism accounting for higher carcinogenicity is possibly related to the Ca(2+) sequestration by the fibres with surface negative potential, impairing the mitochondrial apoptotic pathway. It was also found that with a negative zeta potential, the attractive forces prevailed over repulsions and favoured processes such as agglomeration responsible of a tumorigenic chronic inflammation.

Role of mutagenicity in asbestos fiber-induced carcinogenicity and other diseases

The cellular and molecular mechanisms of how asbestos fibers induce cancers and other diseases are not well understood. Both serpentine and amphibole asbestos fibers have been shown to induce oxidative stress, inflammatory responses, cellular toxicity and tissue injuries, genetic changes, and epigenetic alterations in target cells in vitro and tissues in vivo. Most of these mechanisms are believe to be shared by both fiber-induced cancers and noncancerous diseases. This article summarizes the findings from existing literature with a focus on genetic changes, specifically, mutagenicity of asbestos fibers. Thus far, experimental evidence suggesting the involvement of mutagenesis in asbestos carcinogenicity is more convincing than asbestos-induced fibrotic diseases. The potential contributions of mutagenicity to asbestos-induced diseases, with an emphasis on carcinogenicity, are reviewed from five aspects: (1) whether there is a mutagenic mode of action (MOA) in fiber-induced carcinogenesis; (2) mutagenicity/carcinogenicity at low dose; (3) biological activities that contribute to mutagenicity and impact of target tissue/cell type; (4) health endpoints with or without mutagenicity as a key event; and finally, (5) determinant factors of toxicity in mutagenicity. At the end of this review, a consensus statement of what is known, what is believed to be factual but requires confirmation, and existing data gaps, as well as future research needs and directions, is provided.

Up-regulation of ROS by mitochondria-dependent bystander signaling contributes to genotoxicity of bystander effects

Genomic instability can be observed in bystander cells. However, the underlying mechanism(s) is still relatively unclear. In a previous study, we found that irradiated cells released mitochondria-dependent intracellular factor(s) which could lead to bystander gamma-H2AX induction. In this paper, we used normal (rho(+)) and mtDNA-depleted (rho(0)) human-hamster hybrid cells to investigate mitochondrial effects on the genotoxicity in bystander effect through medium transfer experiments. Through the detection of DNA double-strand breaks with gamma-H2AX, we found that the fraction of gamma-H2AX positive cells changed with time when irradiation conditioned cell medium (ICCM) were harvested. ICCM harvested from irradiated rho(+) cells at 10 min post-irradiation (rho(+) ICCM(10 min)) caused larger increases of bystander gamma-H2AX induction comparing to rho(0) ICCM(10 min), which only caused a slight increase of bystander gamma-H2AX induction. The rho(+) ICCM(10 min) could also result in the up-regulation of ROS production (increased by 35% at 10 min), while there was no significant increase in cells treated with rho(0) ICCM(10 min). We treated cells with dimethyl sulfoxide (DMSO), the scavenger of ROS, and quenched gamma-H2AX induction by rho(+) ICCM. Furthermore, after the medium had been transferred and the cells were continuously cultured for 7 days, we found significantly increased CD59(-) gene loci mutation (increased by 45.9%) and delayed cell death in the progeny of rho(+) ICCM-treated bystander cells. In conclusion, the work presented here suggested that up-regulation of the mitochondria-dependent ROS might be very important in mediating genotoxicity of bystander effects.

Molecular and genetic changes in asbestos-related lung cancer

Asbestos-exposure is associated with an increased risk of lung cancer, one of the leading causes of cancer deaths worldwide. Asbestos is known to induce DNA and chromosomal damage as well as aberrations in signalling pathways, such as the MAPK and NF-kappaB cascades, crucial for cellular homeostasis. The alterations result from both indirect effects through e.g. reactive oxygen/nitrogen species and direct mechanical disturbances of cellular constituents. This review describes the current knowledge on genomic and pathway aberrations characterizing asbestos-related lung cancer. Specific asbestos-associated molecular signatures can assist the development of early biomarkers, molecular diagnosis, and molecular targeted treatments for asbestos-exposed lung cancer patients.

Mechanism of fiber carcinogenesis: from reactive radical species to silencing of the beta igH3 gene

Although the U.S. Environmental Protection Agency has restricted the industrial use of regulated forms of asbestos in the United States since the early 1970s, environmental exposure to asbestos remains a health concern in the United States and is a significant health issue among developing countries. Exposure to asbestos is associated with chronic pulmonary diseases and cancer of the lung, pleura, and peritoneum. The mechanism of fiber carcinogenesis is far from clear and is likely to be complex, depending on fiber dimensions, surface properties, and physical durability. The induction of reactive oxygen and nitrogen species upon phagocytosis of fibers plays an important role in fiber genotoxicity. The beta igH3, a secreted protein induced by the transforming growth factor-beta and essential for cell adhesion, is downregulated in asbestos-induced tumorigenic human bronchial epithelial cells. Ectopic expression of the beta igH3 gene abrogates the tumorigenic phenotype and suggests that the gene plays a causal role in fiber carcinogenesis. A better understanding of the carcinogenic mechanism of asbestos and other mineral fibers will provide useful information on interventional and preventive measures for asbestos-mediated diseases such as human pleural and peritoneal mesotheliomas.

Induction of 8-hydroxydeoxyguanosine by man made vitreous fibres and crocidolite asbestos administered intraperitoneally in rats

Inhaled fibres with certain physico-chemical properties are known to induce mesothelioma in humans. The induction of reactive oxygen (ROS) or nitrogen species (RNS) have been suggested as molecular mechanism of fibre induced carcinogenesis. In earlier studies we were able to demonstrate that crocidolite asbestos in vivo induces mutations in transgenic rats with a specific molecular spectrum that indicates the involvement of 8-hydroxydeoxyguanosine (8-OHdG) as pre-mutagenic adduct. 8-OHdG may be induced by primary (direct) and/or secondary (cellular mediated) mechanisms. Therefore, the induction of 8-OHdG as well as the inflammatory response of animals treated with fibre samples significantly differing in their physico-chemical characteristics was investigated. As appropriate system to study mesothelioma carcinogenesis, intraperitoneal injection in rats was used with samples of UICC crocidolite, crocidolite with reduced iron content, and a vitreous fibre (MMVF 11). Equal numbers of carcinogenic fibres from each sample revealed significant comparable increases in 8-OHdG induction. Parameters of inflammation (percentage of macrophages and TNF-alpha secretion) correlated significantly with the induction of 8-OHdG, 10 weeks after treatment.

Modulation of genotoxic effects in asbestos-exposed primary human mesothelial cells by radical scavengers, metal chelators and a glutathione precursor

The genotoxicity of asbestos fibers is generally mediated by reactive oxygen species (ROS) and by insufficient antioxidant protection. To further elucidate which radicals are involved in asbestos-mediated genotoxicity and to which extent, we have carried out experiments with the metal chelators deferoxamine (DEF) and phytic acid (PA), and with the radical scavengers superoxide dismutase (SOD), dimethylthiourea (DMTU) and the glutathione precursor Nacystelyn trade mark (NAL). We investigated the influence of these compounds on the potency of crocidolite, an amphibole asbestos fiber with a high iron content (27%), and chrysotile, a serpentine asbestos fiber with a low iron content (2%), to induce micronuclei (MN) in human mesothelial cells (HMC) after an exposure time of 24-72 h. Our results show that the number of crocidolite-induced MN is significantly reduced after pretreatment of fibers with PA and DEF. This effect was not observed with chrysotile. In contrast, simultaneous treatment of cells with asbestos and the OH*scavenging DMTU or the O2- -scavenging SOD significantly decreased the number of MN induced by chrysotile and crocidolite. In particular, DMTU almost completely suppressed micronucleus induction by both fiber types. A similar effect was observed in the presence of the H(2)O(2)-scavenging NAL after chrysotile treatment of HMC. By means of kinetochore analysis, it could be shown that the number of clastogenic events is decreased after PA and DEF pretreatment of fibers as well as after application of the above-mentioned scavengers. Our results show that chrysotile asbestos induces an increased release of H(2)O(2) in contrast to crocidolite. Also, the iron content of the fiber plays an important role in radical formation, but nevertheless, chrysotile produces oxy radicals to a similar extent as crocidolite, probably by phagocytosis-mediated oxidative bursting.

Oxidative Stress,hogg1Expression and NF‐κB Activity in Cells Exposed to Low Level Chromium

Chromium compounds are carcinogenic to the human lung, although the detailed biochemical mechanism is still unclear. To understand the carcinogenic mechanism in cells exposed to low level hexavalent chromium, we measured the generation of reactive oxygen species (ROS) and 8-hydroxydeoxyguanosine (8-OH-dG), the transcription of hogg1, which encodes an 8-OH-dG repair protein, and NF-kappaB activation levels in the A549 human lung epithelial cell line after exposure to Cr (VI) at concentrations of 12.5 to 800 microM. In A549 cells, ROS levels and DNA binding by NF-kappaB increased in proportion to the concentration of Cr (VI). These increases were diminished by pretreatment with catalase, superoxide dismutase, or D-mannitol, but the levels of 8-OH-dG and expression of hogg1 did not change significantly with Cr (VI) exposure. These results suggest that the induction of ROS and the activation of NF-kappaB are important in the carcinogenic mechanism of Cr (VI), but it is unlikely that Cr (VI) concentrations below 800 microM increase 8-OH-dG levels or the expression of hogg1.

The effect of iron on the biological activities of erionite and mordenite

Epidemiological data has demonstrated that environmental and/or occupational exposure to mineral particulates may result in the development of pulmonary fibrosis, bronchogenic carcinoma and malignant mesothelioma many years following exposure. It has been suggested that the genotoxic effects of fibrous particulates, such as asbestos, is due in part to the generation of reactive oxygen species (ROS) from iron associated with the particulates. However, the molecular mechanisms by which mineral particulates induce ROS that results in genotoxic damage remains unclear. The naturally occurring zeolites, erionite and mordenite share several physiochemical properties but they elicit very different biological responses, with erionite, a fibrous particulate, being highly toxic, and mordenite, a nonfibrous particulate, being relatively benign. We are using these natural zeolites as a model system to determine what physicochemical properties of these zeolites are responsible for their biological response(s) and to evaluate the parameters that influence these responses. The purpose of the present study was to determine the mutagenic potential of erionite and mordenite and to determine whether this mutagenic potential was modulated by iron. The results of this study using the Chinese hamster ovary cell line AS52 demonstrated that erionite was more cytotoxic than mordenite. However, the cytotoxicity of both zeolites was increased in the presence of physiological concentrations of ferrous chloride. Ferrous ions (5-20 microM) significantly (p<0.001) increased the cytotoxicity of mordenite, but only at the highest concentration (16 microg/cm(2)) of mordenite tested. Conversely, only the highest concentration (20 microM) of ferrous ion significantly (p<0.001) increased the cytotoxicity of erionite, but this enhanced cytotoxicity occurred over a wider concentration range (6-16 microg/cm(2)) of erionite. Mordenite was not mutagenic at any of the concentrations tested, and the mutagenic potential of mordenite was not enhanced by the addition of ferrous ion. Conversely, erionite was mutagenic in a dose-response manner at concentrations greater than 6 microg/cm(2) and the mutagenic potential of erionite was significantly enhanced by the addition of ferrous ions. These results suggest that while the cytotoxicity of mordenite and erionite may be related to the ability of these fibers to transport iron into a cell, the different coordination state of iron associated with the two fiber surfaces is critical for inducing genotoxic damage.

The molecular epidemiology of asbestos and tobacco in lung cancer

Asbestos is a well-known toxin and lung carcinogen. Epidemiologic studies have established tobacco smoke and asbestos exposures synergistically interact to enhance lung cancer risk. The biologic mechanism responsible for this interaction has been the subject of considerable debate. Studies have suggested that asbestos may act as a carcinogen by generating free radical and reactive oxygen species, by inducing tissue injury and subsequent cellular growth, via large-scale chromosome loss and by enhancing delivery of tobacco carcinogens to the respiratory epithelium. Recent molecular epidemiologic approaches further suggest that asbestos enhances the mutagenicity of tobacco carcinogens and that it acts, at least in part, independent of the tissue damage responsible for fibrosis.

Chrysotile-mediated imbalance in the glutathione redox system in the development of pulmonary injury

A significant depletion in the content of glutathione (GSH) and alteration in GSH redox system enzymes were observed in the lung of chrysotile-exposed animals (5 mg) during different developmental stages of asbestosis. In the alveolar macrophages (AM) of exposed animals, the depletion in GSH started from day 1 and reached a maximum at day 16, whereas in lung tissue the maximum depletion was observed when fibrosis has matured. It appears that cellular GSH depletion triggers oxidative stress in the system as observed from increased thiobarbituric acid reactive substance (TBARS) production and alteration in the activities of glutathione peroxidase (GPx), glutathione reductase (GR), glucose 6-phosphate dehydrogenase (G6PD) and glutathione S-transferase (GST), the enzymes regulating oxidative tone. The depletion in GSH was also observed in red blood cells (RBC) of the exposed animals reaching a maximum when fibrosis matured. Thus the observed depletion in GSH, ascorbic acid and alteration in GSH redox system enzymes may be involved in fibrosis and carcinogenesis induced by chrysotile.

Biological effects of naturally occurring and man-made fibres: in vitro cytotoxicity and mutagenesis in mammalian cells

Cytotoxicity and mutagenicity of tremolite, erionite and the man-made ceramic (RCF-1) fibre were studied using the human-hamster hybrid A(L) cells. Results from these fibres were compared with those of UICC Rhodesian chrysotile fibres. The A(L) cell mutation assay, based on the S1 gene marker located on human chromosome 11, the only human chromosome contained in the hybrid cell, has been shown to be more sensitive than conventional assays in detecting deletion mutations. Tremolite, erionite and RCF-1 fibres were significantly less cytotoxic to A(L) cells than chrysotile. Mutagenesis studies at the HPRT locus revealed no significant mutant yield with any of these fibres. In contrast, both erionite and tremolite induced dose-dependent S1- mutations in fibre-exposed cells, with the former inducing a significantly higher mutant yield than the latter fibre type. On the other hand, RCF-1 fibres were largely non-mutagenic. At equitoxic doses (cell survival at approximately 0.7), erionite was found to be the most potent mutagen among the three fibres tested and at a level comparable to that of chrysotile fibres. These results indicate that RCF-1 fibres are non-genotoxic under the conditions used in the studies and suggest that the high mesothelioma incidence previously observed in hamster may either be a result of selective sensitivity of hamster pleura to fibre-induced chronic irritation or as a result of prolonged fibre treatment. Furthermore, the relatively high mutagenic potential for erionite is consistent with its documented carcinogenicity.