New insight into intrachromosomal deletions induced by chrysotile in the gpt delta transgenic mutation assay

Do Carbon Nanotubes and Asbestos Fibers Exhibit Common Toxicity Mechanisms?

During the last two decades several nanoscale materials were engineered for industrial and medical applications. Among them carbon nanotubes (CNTs) are the most exploited nanomaterials with global production of around 1000 tons/year. Besides several commercial benefits of CNTs, the fiber-like structures and their bio-persistency in lung tissues raise serious concerns about the possible adverse human health effects resembling those of asbestos fibers. In this review, we present a comparative analysis between CNTs and asbestos fibers using the following four parameters: (1) fibrous needle-like shape, (2) bio-persistent nature, (3) high surface to volume ratio and (4) capacity to adsorb toxicants/pollutants on the surface. We also compare mechanisms underlying the toxicity caused by certain diameters and lengths of CNTs and asbestos fibers using downstream pathways associated with altered gene expression data from both asbestos and CNT exposure. Our results suggest that indeed certain types of CNTs are emulating asbestos fiber as far as associated toxicity is concerned.

Malignant Pleural Mesothelioma: Genetic and Microenviromental Heterogeneity as an Unexpected Reading Frame and Therapeutic Challenge

Mesothelioma is a malignancy of serosal membranes including the peritoneum, pleura, pericardium and the tunica vaginalis of the testes. Malignant mesothelioma (MM) is a rare disease with a global incidence in countries like Italy of about 1.15 per 100,000 inhabitants. Malignant Pleural Mesothelioma (MPM) is the most common form of mesothelioma, accounting for approximately 80% of disease. Although rare in the global population, mesothelioma is linked to industrial pollutants and mineral fiber exposure, with approximately 80% of cases linked to asbestos. Due to the persistent asbestos exposure in many countries, a worldwide progressive increase in MPM incidence is expected for the current and coming years. The tumor grows in a loco-regional pattern, spreading from the parietal to the visceral pleura and invading the surrounding structures that induce the clinical picture of pleural effusion, pain and dyspnea. Distant spreading and metastasis are rarely observed, and most patients die from the burden of the primary tumor. Currently, there are no effective treatments for MPM, and the prognosis is invariably poor. Some studies average the prognosis to be roughly one-year after diagnosis. The uniquely poor mutational landscape which characterizes MPM appears to derive from a selective pressure operated by the environment; thus, inflammation and immune response emerge as key players in driving MPM progression and represent promising therapeutic targets. Here we recapitulate current knowledge on MPM with focus on the emerging network between genetic asset and inflammatory microenvironment which characterize the disease as amenable target for novel therapeutic approaches.

Modulation of N-Methyl-N-nitrosourea Mutagenesis in Mouse Embryo Fibroblasts Derived from the gpt Delta Mouse by an Inhibitor of the O6-Methylguanine Methyltransferase, MGMT

DNA methylating agents are abundant in the environment and are sometimes used in cancer chemotherapy. They react with DNA to form methyl-DNA adducts and byproduct lesions that can be both toxic and mutagenic. Foremost among the mutagenic lesions is O⁶-methylguanine (m6G), which base pairs with thymine during replication to cause GC → AT mutations. The gpt delta C57BL/6J mouse strain of Nohmi et al. (Mol. Mutagen 1996, 28, 465-70) reliably produces mutational spectra of many DNA damaging agents. In this work, mouse embryo fibroblasts (MEFs) were made from gpt delta C57BL/6J mice and evaluated as a screening tool to determine the qualitative and quantitative features of mutagenesis by N-methyl-N-nitrosourea (MNU), a direct-acting DNA alkylator that serves as a model for environmental N-nitrosamines, such as N-nitrosodimethylamine and therapeutic agents such as Temozolomide. The DNA repair protein MGMT (O⁶-methylguanine DNA methyltransferase) protects against environmental mutagenesis by DNA methylating agents and, by removing m6G, limits the therapeutic potential of Temozolomide in cancer therapy. The gpt delta MEFs were treated with MNU to establish dose-dependent toxicity. In parallel, MNU mutagenicity was determined in the presence and absence of the MGMT inhibitor AA-CW236 (4-(2-(5-(chloromethyl)-4-(4-(trifluoromethoxy)phenyl)-1H-1,2,3-triazol-1-yl)ethyl)-3,5-dimethylisoxazole). With and without the inhibitor, the principal mutagenic event of MNU was GC → AT, but more mutations were observed when the inhibitor was present. Evidence that the mutagenic lesion was m6G was based on mass spectral data collected using O⁶-methyl-d₃-guanine as an internal standard; m6G levels were higher in AA-CW236 treated MEFs by an amount proportional to the higher mutation frequency seen in the same cells. This work establishes gpt delta MEFs as a versatile tool for probing mutagenesis by environmental and therapeutic agents and as a cell culture model in which chemical genetics can be used to determine the impact of DNA repair on biological responses to DNA damaging agents.

Malignant mesothelioma as an oxidative stress-induced cancer: An update

Malignant mesothelioma (MM) is a relatively rare cancer that occurs almost exclusively following respiratory exposure to asbestos in humans. Its pathogenesis is closely associated with iron overload and oxidative stress in mesothelial cells. On fiber exposure, mesothelial cells accumulate fibers simultaneously with iron, which either performs physical scissor function or catalyzes free radical generation, leading to oxidative DNA damage such as strand breaks and base modifications, followed by activation of intracellular signaling pathways. Chrysotile, per se without iron, causes massive hemolysis and further adsorbs hemoglobin. Exposure to indigestible foreign materials also induces chronic inflammation, involving consistent generation of free radicals and subsequent activation of NALP3 inflammasomes in macrophages. All of these contribute to mesothelial carcinogenesis. Genomic alterations most frequently involve homozygous deletion of INK4A/4B, and other pathways such as Hippo and TGF-β pathways are also affected in MM. Recently, analyses of familial MM sorted out BAP1 as a novel responsible tumor suppressor gene, whose function is not fully elucidated. Five-year survival of mesothelioma is still ~8%, and this cancer is increasing worldwide. Connective tissue growth factor, a secretory protein creating a vicious cycle mediated by β-catenin, has been recognized as a hopeful target for therapy, especially in sarcomatoid subtype. Recent research outcomes related to microRNAs and cancer stem cells also offer additional novel targets for the treatment of MM. Iron reduction as chemoprevention of mesothelioma is helpful at least in an animal preclinical study. Integrated approaches to fiber-induced oxidative stress would be necessary to overcome this currently fatal disease.

In vivo genotoxicity of 1-methylnaphthalene from comprehensive toxicity studies with B6C3F1 gpt delta mice

1-Methylnaphthalene (1-MN), a constituent of the polycyclic aromatic hydrocarbons (PAHs), is a lung carcinogen in mice. However, conventional genotoxicity tests such as the Ames test and sister chromatid exchange (SCE) test have yielded equivocal results. In the present study, the in vivo genotoxicity of 1-methylnaphthalene (1-MN) together with its toxicological profile was investigated in a 13-week repeated dose toxicity study of 1-MN using B6C3F1 gpt delta mice. In the serum biochemistry, significant increases in AST and ALP were observed in males of the 0.15% 1-MN group. From histopathological examination, the incidence of single cell necrosis in the liver was significantly increased in males of the 0.15% 1-MN group; however, no changes were observed in the lungs, the target organ of 1-MN. In an in vivo mutation assay, no changes in mutant frequencies of gpt and red/gam (Spi-) in lung DNA of 1-MN treated mice were observed at 13 weeks. In addition, there were no significant differences in the proliferating cell nuclear antigen (PCNA)-positive ratios in bronchiolar epithelial cells among the groups for either sex. These results suggest that 1-MN at a carcinogenic dose not induce overt toxicity for any organs and has no in vivo genotoxicity in the lungs.

Genetic ecotoxicology of asbestos pollution in the house mouse Mus musculus domesticus

PURPOSE

We tested the genetic diversity in wild mice (Mus musculus domesticus) inhabiting the asbestos-polluted area as a model for the long-term mutagenic effect of asbestos. Hazardous effects of deposited asbestos persist in the environment because of low rate of fiber disintegration. The upper layers of the soil in the vicinity of a former asbestos factory are nearly "saturated" with asbestos fibers and dust. Natural populations of mice dwell in this area and are constantly exposed to asbestos fibers.

METHODS

We measured the microsatellites genetic diversity of wild mice (Mus musculus domesticus) inhabiting the asbestos-polluted area as a model for the long-term mutagenic effect of this environmental toxin.

RESULTS

The six tested microsatellites were highly polymorphic, revealing 111 different alleles for the two sampled populations. Effective number of alleles was slightly higher in the polluted population relative to the control population, while observed heterozygosity was lower. The chromatographic profile of the polluted population exhibited a significantly higher number of bands, probably resulting from somatic mutations, in addition to the ordinary microsatellite band profiles.

CONCLUSIONS

Long-term exposure to asbestos fibers significantly elevates the level of somatic mutations. It also leads to a relatively high level of observed homozygosity, a phenomenon that may be associated with loss of heterozygosity. Based on the mice population, our data suggest elevated health risks for humans living in an asbestos-polluted area.

Mutation spectrum in FE1-MUTA(TM) Mouse lung epithelial cells exposed to nanoparticulate carbon black

It has been shown previously that carbon black (CB), Printex 90 exposure induces cII and lacZ mutants in the FE1-Muta(TM) Mouse lung epithelial cell line and causes oxidatively damaged DNA and the production of reactive oxygen species (ROS). The purpose of this study was to determine the mutation spectrum in the cII gene of Printex 90 exposed cells. Cells exposed to CB have a substantially different mutation spectrum in the cII gene compared with vehicle exposed controls. The mutation spectra differ both in the positions (P < 0.0001) and types of the mutations (P < 0.0001). Exposure to Printex 90 increased the number of single base deletions by 2.3-fold and larger deletions by 1.9-fold. Most single base deletions were within two repetitive sequences in cII, but the large deletions were not. The mechanism behind the large deletions is not yet known. The largest increases in base substitutions were observed in G:C→T:A, G:C→C:G, and A:T→T:A transversion mutations; this is in keeping with a genetic finger print of ROS and is further substantiated by the observations that Printex 90 generates ROS and oxidatively damaged DNA.

The fate of chrysotile-induced multipolar mitosis and aneuploid population in cultured lung cancer cells

Chrysotile is one of the six types of asbestos, and it is the only one that can still be commercialized in many countries. Exposure to other types of asbestos has been associated with serious diseases, such as lung carcinomas and pleural mesotheliomas. The association of chrysotile exposure with disease is controversial. However, in vitro studies show the mutagenic potential of chrysotile, which can induce DNA and cell damage. The present work aimed to analyze alterations in lung small cell carcinoma cultures after 48 h of chrysotile exposure, followed by 2, 4 and 8 days of recovery in fiber-free culture medium. Some alterations, such as aneuploid cell formation, increased number of cells in G2/M phase and cells in multipolar mitosis were observed even after 8 days of recovery. The presence of chrysotile fibers in the cell cultures was detected and cell morphology was observed by laser scanning confocal microscopy. After 4 and 8 days of recovery, only a few chrysotile fragments were present in some cells, and the cellular morphology was similar to that of control cells. Cells transfected with the GFP-tagged α-tubulin plasmid were treated with chrysotile for 24 or 48 h and cells in multipolar mitosis were observed by time-lapse microscopy. Fates of these cells were established: retention in metaphase, cell death, progression through M phase generating more than two daughter cells or cell fusion during telophase or cytokinesis. Some of them were related to the formation of aneuploid cells and cells with abnormal number of centrosomes.

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.

Integration of in vivo genotoxicity and short-term carcinogenicity assays using F344 gpt delta transgenic rats: in vivo mutagenicity of 2,4-diaminotoluene and 2,6-diaminotoluene structural isomers

An important trend in current toxicology is the replacement, reduction, and refinement of the use of experimental animals (the 3R principle). We propose a model in which in vivo genotoxicity and short-term carcinogenicity assays are integrated with F344 gpt delta transgenic rats. Using this model, the genotoxicity of chemicals can be identified in target organs using a shuttle vector λ EG10 that carries reporter genes for mutations; short-term carcinogenicity is determined by the formation of glutathione S-transferase placenta form (GST-P) foci in the liver. To begin validating this system, we examined the genotoxicity and hepatotoxicity of structural isomers of 2,4-diaminotoluene (2,4-DAT) and 2,6-diaminotoluene (2,6-DAT). Although both compounds are genotoxic in the Ames/Salmonella assay, only 2,4-DAT induces tumors in rat livers. Male F344 gpt delta rats were fed diet containing 2,4-DAT at doses of 125, 250, or 500 ppm for 13 weeks or 2,6-DAT at a dose of 500 ppm for the same period. The mutation frequencies of base substitutions, mainly at G:C base pairs, were significantly increased in the livers of 2,4-DAT–treated rats at all three doses. In contrast, virtually no induction of genotoxicity was identified in the kidneys of 2,4-DAT–treated rats or in the livers of 2,6-DAT–treated rats. GST-P–positive foci were detected in the livers of rats treated with 2,4-DAT at a dose of 500 ppm but not in those treated with 2,6-DAT. Integrated genotoxicity and short-term carcinogenicity assays may be useful for early identifying genotoxic and nongenotoxic carcinogens in a reduced number of experimental animals.

DNA double-strand breaks by asbestos, silica, and titanium dioxide: possible biomarker of carcinogenic potential?

DNA double-strand breaks (DSBs) can result in cell death or genetic alterations when cells are subjected to radiation, exposure to toxins, or other environmental stresses. A complex DNA-damage-response pathway is activated to repair the damage, and the inability to repair these breaks can lead to carcinogenesis. One of the earliest responses to DNA DSBs is the phosphorylation of a histone, H2AX, at serine 139 (gamma-H2AX), which can be detected by a fluorescent antibody. A study was undertaken to compare the induction of DNA DSBs in normal (small airway epithelial) cells and cancer cells (A549) after exposure to asbestos (crocidolite), a proven carcinogen, silica, a suspected carcinogen, and titanium dioxide (TiO(2)), an inert particle recently reported to be carcinogenic in animals. The results indicate that crocidolite induced greater DNA DSBs than silica and TiO(2), regardless of cell type. DNA DSBs caused by crocidolite were higher in normal cells than in cancer cells. Silica and TiO(2) induced higher DNA DSBs in cancer cells than in normal cells. The production of reactive oxygen species was found to be highest in cells exposed to crocidolite, followed, in potency, by silica and TiO(2). The generation of reactive oxygen species was higher in normal cells than in cancer cells. Cell viability assay indicated that crocidolite caused the greatest cytotoxicity in both cell types. Apoptosis, measured by caspase 3/7 and poly (ADP-Ribose) polymerase activation, was highest in crocidolite-exposed cells, followed by TiO(2) and silica. The results of this study indicate that crocidolite has a greater carcinogenic potential than silica and TiO(2), judged by its ability to cause sustained genomic instability in normal lung cells.

Genotoxic responses to titanium dioxide nanoparticles and fullerene in gpt delta transgenic MEF cells

BACKGROUND

Titanium dioxide (TiO2) nanoparticles and fullerene (C60) are two attractive manufactured nanoparticles with great promise in industrial and medical applications. However, little is known about the genotoxic response of TiO2 nanoparticles and C60 in mammalian cells. In the present study, we determined the mutation fractions induced by either TiO2 nanoparticles or C60 in gpt delta transgenic mouse primary embryo fibroblasts (MEF) and identified peroxynitrite anions (ONOO-) as an essential mediator involved in such process.

RESULTS

Both TiO2 nanoparticles and C60 dramatically increased the mutation yield, which could be abrogated by concurrent treatment with the endocytosis inhibitor, Nystatin. Under confocal scanning microscopy together with the radical probe dihydrorhodamine 123 (DHR 123), we found that there was a dose-dependent formation of ONOO- in live MEF cells exposed to either TiO2 nanoparticles or C60, and the protective effects of antioxidants were demonstrated by the nitric oxide synthase (NOS) inhibitor, NG-methyl-L-arginine (L-NMMA). Furthermore, suppression of cyclooxygenase-2 (COX-2) activity by using the chemical inhibitor NS-398 significantly reduced mutation frequency of both TiO2 nanoparticles and C60.

CONCLUSION

Our results provided novel information that both TiO2 nanoparticles and C60 were taken up by cells and induced kilo-base pair deletion mutations in a transgenic mouse mutation system. The induction of ONOO- may be a critical signaling event for nanoparticle genotoxicity.

Environmental mutagens induced transversions but not transitions in regulatory region of mitochondrial DNA

One of the long-term objectives of the research in our laboratory was to determine whether mitochondrial DNA (mtDNA) mutations were generated in cell lines exposed to a variety of known mutagens. Many of these mutagens are known to increase oxidative stress in the cell, and one potential outcome of this would be an increased incidence of point mutations in mtDNA. Recently, there has been some controversy regarding the validity of point mutations in the regulatory region of mtDNA as a predictive or causative marker for carcinogenesis. Studies were undertaken to assess whether nuclear mutagens such as arsenic (As), asbestos, and ultraviolet (UV) and gamma-radiation, induced both heteroplasmic and homoplasmic point mutations in mtDNA. A direct sequencing approach was used to reduce the occurrence of experimental errors and cross-checked all base changes with databases of known polymorphisms. Our results showed that, while base changes did occur, there was no marked difference between the number of changes in treated and untreated cells. Furthermore, in human lymphocyte samples from subjects exposed to As, most of these base changes were previously reported. Interestingly, there was an increase in the number of transversions (purine ( pyrimidine) in smokers from a human population study, but as with the findings in cell culture samples, there was no difference in the total number of base changes. Data suggest that only a change in the number of rare transversions would be indicative of an increase in point mutations in mtDNA after exposure to mutagens.

Chrysotile effects on human lung cell carcinoma in culture: 3-D reconstruction and DNA quantification by image analysis

Background

Chrysotile is considered less harmful to human health than other types of asbestos fibers. Its clearance from the lung is faster and, in comparison to amphibole forms of asbestos, chrysotile asbestos fail to accumulate in the lung tissue due to a mechanism involving fibers fragmentation in short pieces. Short exposure to chrysotile has not been associated with any histopathological alteration of lung tissue.

Methods

The present work focuses on the association of small chrysotile fibers with interphasic and mitotic human lung cancer cells in culture, using for analyses confocal laser scanning microscopy and 3D reconstructions. The main goal was to perform the analysis of abnormalities in mitosis of fibers-containing cells as well as to quantify nuclear DNA content of treated cells during their recovery in fiber-free culture medium.

Results

HK2 cells treated with chrysotile for 48 h and recovered in additional periods of 24, 48 and 72 h in normal medium showed increased frequency of multinucleated and apoptotic cells. DNA ploidy of the cells submitted to the same chrysotile treatment schedules showed enhanced aneuploidy values. The results were consistent with the high frequency of multipolar spindles observed and with the presence of fibers in the intercellular bridge during cytokinesis.

Conclusion

The present data show that 48 h chrysotile exposure can cause centrosome amplification, apoptosis and aneuploid cell formation even when long periods of recovery were provided. Internalized fibers seem to interact with the chromatin during mitosis, and they could also interfere in cytokinesis, leading to cytokinesis failure which forms aneuploid or multinucleated cells with centrosome amplification.

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.