Mitotic disturbances and micronucleus induction in Syrian hamster embryo fibroblast cells caused by asbestos fibers

Analyzing biological and molecular characteristics and genomic damage induced by exposure to asbestos

Asbestos is one of the most important occupational carcinogens. Currently, about 125 million people worldwide are exposed to asbestos in the workplace. According to global estimates, at least 107,000 people die each year from lung cancer, mesothelioma, and asbestosis as a result of occupational exposure to asbestos. The high pathogenicity of this material is currently known, being associated with the development of pulmonary diseases, of which lung cancer is the main cause of death due to exposure to this mineral. Pulmonary diseases related to asbestos are a common clinical problem and a major health concern worldwide. Extensive research has identified many important pathogenic mechanisms; however, the precise molecular mechanisms involved, and the generated genomic damage that lead to the development of these diseases, are not completely understood. The modes of action that underlie this type of disease seem to differ depending on the type of fiber, lung clearance, and genetics. This evidences the need to increase our knowledge about these effects on human health. This review focuses on the characteristics of asbestos and the cellular and genomic damage generated in humans via exposure.

Tumors that mimic asbestos-related mesothelioma: time to consider a genetics-based tumor registry?

The diagnosis of mesothelioma is not always straightforward, despite known immunohistochemical markers and other diagnostic techniques. One reason for the difficulty is that extrapleural tumors resembling mesothelioma may have several possible etiologies, especially in cases with no meaningful history of amphibole asbestos exposure. When the diagnosis of mesothelioma is based on histologic features alone, primary mesotheliomas may resemble various primary or metastatic cancers that have directly invaded the serosal membranes. Some of these metastatic malignancies, particularly carcinomas and sarcomas of the pleura, pericardium and peritoneum, may undergo desmoplastic reaction in the pleura, thereby mimicking mesothelioma, rather than the primary tumor. Encasement of the lung by direct spread or metastasis, termed pseudomesotheliomatous spread, occurs with several other primary cancer types, including certain late-stage tumors from genetic cancer syndromes exhibiting chromosomal instability. Although immunohistochemical staining patterns differentiate most carcinomas, lymphomas, and mestastatic sarcomas from mesotheliomas, specific genetic markers in tumor or somatic tissues have been recently identified that may also distinguish these tumor types from asbestos-related mesothelioma. A registry for genetic screening of mesothelioma cases would help lead to improvements in diagnostic criteria, prognostic accuracy and treatment efficacy, as well as improved estimates of primary mesothelioma incidence and of background rates of cancers unrelated to asbestos that might be otherwise mistaken for mesothelioma. This information would also help better define the dose-response relationships for mesothelioma and asbestos exposure, as well as other risk factors for mesothelioma and other mesenchymal or advanced metastatic tumors that may be indistinguishable by histology and staining characteristics.

Distinct affinity of nuclear proteins to the surface of chrysotile and crocidolite

The inhalation of asbestos is a risk factor for the development of malignant mesothelioma and lung cancer. Based on the broad surface area of asbestos fibers and their ability to enter the cytoplasm and nuclei of cells, it was hypothesized that proteins that adsorb onto the fiber surface play a role in the cytotoxicity and carcinogenesis of asbestos fibers. However, little is known about which proteins adsorb onto asbestos. Previously, we systematically identified asbestos-interacting proteins and classified them into eight sub-categories: chromatin/nucleotide/RNA-binding proteins, ribosomal proteins, cytoprotective proteins, cytoskeleton-associated proteins, histones and hemoglobin. Here, we report an adsorption profile of proteins for the three commercially used asbestos compounds: chrysotile, crocidolite and amosite. We quantified the amounts of adsorbed proteins by analyzing the silver-stained gels of sodium dodecyl sulfate-polyacrylamide gel electrophoresis with ImageJ software, using the bands for amosite as a standard. We found that histones were most adsorptive to crocidolite and that chromatin-binding proteins were most adsorptive to chrysotile. The results suggest that chrysotile and crocidolite directly interact with chromatin structure through different mechanisms. Furthermore, RNA-binding proteins preferably interacted with chrysotile, suggesting that chrysotile may interfere with transcription and translation. Our results provide novel evidence demonstrating that the specific molecular interactions leading to carcinogenesis are different between chrysotile and crocidolite.

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.

XRCC1 deficiency sensitizes human lung epithelial cells to genotoxicity by crocidolite asbestos and Libby amphibole

BACKGROUND

Asbestos induces DNA and chromosomal damage, but the DNA repair pathways protecting human cells against its genotoxicity are largely unknown. Polymorphisms in XRCC1 have been associated with altered susceptibility to asbestos-related diseases. However, it is unclear whether oxidative DNA damage repaired by XRCC1 contributes to asbestos-induced chromosomal damage.

OBJECTIVES

We sought to examine the importance of XRCC1 in protection against genotoxic effects of crocidolite and Libby amphibole asbestos.

METHODS

We developed a genetic model of XRCC1 deficiency in human lung epithelial H460 cells and evaluated genotoxic responses to carcinogenic fibers (crocidolite asbestos, Libby amphibole) and nongenotoxic materials (wollastonite, titanium dioxide).

RESULTS

XRCC1 knockdown sensitized cells to the clastogenic and cytotoxic effects of oxidants [hydrogen peroxide (H₂O₂), bleomycin] but not to the nonoxidant paclitaxel. XRCC1 knockdown strongly enhanced genotoxicity of amphibole fibers as evidenced by elevated formation of clastogenic micronuclei. Crocidolite induced primarily clastogenic micronuclei, whereas Libby amphibole induced both clastogenic and aneugenic micronuclei. Crocidolite and bleomycin were potent inducers of nuclear buds, which were enhanced by XRCC1 deficiency. Libby amphibole and H₂O₂ did not induce nuclear buds, irrespective of XRCC1 status. Crocidolite and Libby amphibole similarly activated the p53 pathway.

CONCLUSIONS

Oxidative DNA damage repaired by XRCC1 (oxidized bases, single-strand breaks) is a major cause of chromosomal breaks induced by crocidolite and Libby amphibole. Nuclear buds are a novel biomarker of genetic damage induced by exposure to crocidolite asbestos, which we suggest are associated with clustered DNA damage. These results provide mechanistic evidence for the epidemiological association between XRCC1 polymorphisms and susceptibility to asbestos-related disease.

Chromosomal damage in two species of aquatic turtles (Emys orbicularis and Mauremys caspica) inhabiting contaminated sites in Azerbaijan

The Caspian region and specifically the Apsheron peninsula of Azerbaijan are known to be polluted with a variety of environmental contaminants. These complex mixtures of contaminants make risk assessment difficult. We used the flow cytometry method (FCM) and the micronucleus assay (MN) to assess chromosomal damage in aquatic turtles (Emys orbicularis, the European pond turtle; and Mauremys caspica, the Caspian turtle) inhabiting contaminated wetlands in Azerbaijan. Evidence of genetic damage was found for two sites, Neftchala and Sumgayit, relative to a reference site, Ali Bairamly. Sediment samples from each site were analyzed for PAHs and mercury to evaluate potential contaminant associations with genetic damage. A significant positive correlation was documented between three-ring PAH sediment concentrations and FCM estimates of chromosomal damage in E. orbicularis. These data combine to show that the contaminated wetlands in Sumgayit and Neftchala are genotoxic and that three-ring PAHs are likely a significant influence on observed genotoxicity.

A Molecular Epidemiology Case Control Study on Pleural Malignant Mesothelioma

Pleural malignant mesothelioma is an uncommon neoplasm usually associated with asbestos exposure. The increasing incidence of malignant mesothelioma cases involving individuals with low levels of asbestos exposure suggests a complex carcinogenetic process with the involvement of other cofactors. Cytogenetic studies revealed the complexity of the genetic changes involved in this neoplasm reflecting the accumulation of genomic damage. One of the most used methodologies for assessing genomic damage is the cytokinesis-blocked micronucleus test applied in peripheral blood lymphocytes (PBL). This approach allows the detection of chromosomal alterations expressed in binucleated cells after nuclear division in vitro. This marker could provide a tool for assessing genetically determined constitutional differences in chromosomal instability. A biomonitoring study was carried out to evaluate the micronuclei frequency in PBLs of patients with pleural malignant mesothelioma with respect to lung cancer, healthy, and risk controls as a marker of cancer susceptibility in correlation with the presence of SV40. A significant increased micronuclei frequency was observed in patients with malignant mesothelioma in comparison with all the other groups, the mean micronuclei frequency was double in patients with malignant mesothelioma compared with healthy controls, risk controls, and patients with lung adenocarcinoma (median 11.4 binucleated cells with micronuclei/1,000 binucleated cells versus 6.2, 6.1, and 5.1, respectively). Our data indicate that human T lymphocyte samples carry DNA sequences coding for SV40 large T antigen at low prevalence, both in cancer cases and controls. Evidence of cytogenetic damage revealed as micronuclei frequency in mesothelioma cancer patients could be related to exogenous and endogenous cofactors besides asbestos exposure.

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.

Oncogenes and tumor-suppressor genes in mesothelioma--a synopsis

Invariably mesothelioma is diagnosed late in the development of the disease when treatment is no longer effective. Therefore, a key to reducing the mortality rate of this neoplasm is knowledge of the general sequence of genetic events between initiation of mesothelial cells and the emergence of the metastatic tumor cells. Unfortunately, relatively little is known about the early changes in the genesis of this disease. Of the known changes, the most frequent are in the tumor-suppressor genes p16INK4a and NF2 and possibly the SV40 virus large T-antigen oncogene. The molecular nature of the changes in these genes as well as other alterations are addressed in this overview.

Mechanisms of fiber-induced genotoxicity

The mechanisms of particle-induced genotoxicity have been investigated mainly with asbestos fibers. The results are summarized and discussed in this paper. DNA damage can be produced by oxidoreduction processes generated by fibers. The extent of damage yield depends on experimental conditions: if iron is present, either on fibers or in the medium, damage is increased. However, iron reactivity does not explain all the results obtained in cell-free systems, as breakage of plasmid DNA was not directly associated with the amount of iron released by the fibers. The proximity of DNA to the site of generation of reactive oxygen species (ROS) is important because these species have an extremely short half-life. Damage to cellular DNA can be produced by oxidoreduction processes that originate from cells during phagocytosis. Secondary molecules that are more stable than ROS are probably involved in DNA damage. Oxidoreduction reactions originating from cells can induce mutations. Genotoxicity is also demonstrated by chromosomal damage associated with impaired mitosis, as evidenced by chromosome missegregation, spindle changes, alteration of cell cycle progression, formation of aneuploid and polyploid cells, and nuclear disruption. In some of these processes, the particle state and fiber dimensions are considered important parameters in the generation of genotoxic effects.