Genotoxic Mechanisms of Asbestos Fibers:  Role of Extranuclear Targets

Abstract

Asbestos fibers are carcinogenic to both humans and experimental animals. The continued discoveries of exposure routes whereby the general public is exposed to asbestos suggest a long-term, low-dose exposure for a large number of people. However, the mechanisms by which asbestos induces malignancy are not entirely understood. In previous studies, we have shown that asbestos is an effective gene and chromosomal mutagen when assayed using the highly sensitive AL mutation assay and that the mutagenicity is mediated by reactive oxygen species. The objective of the present study is to determine the origin of these radical species, particularly reactive nitrogen species, in fiber mutagenesis. Using the radical probe 5',6'-chloromethyl-2',7'-dihydroxyphenoxazine diacetate to trap reactive radical species, we showed that crocidolite increased the levels of oxyradicals in cytoplasts, in the absence of the nucleus, in a dose-dependent manner, which was reduced significantly by cotreatment with the radical scavenger dimethyl sulfoxide. Treatment of enucleated cells with crocidolite asbestos followed by rescue fusion using karyoplasts from control cells resulted in significant mutant induction, indicating that the nuclear-cytoplasmic interaction is necessary for fiber mutagenesis. Using the fluorescent probe 2,3-diaminonaphthotriazole, crocidolite fibers were shown to induce a dose-dependent increase of nitric oxide production, which was suppressed significantly by concurrent treatment with the nitric oxide synthase inhibitor, NG-methyl-L-arginine (L-NMMA). Similarly, there was a dose-dependent decrease in the mutation yield induced by crocidolites in the presence of graded doses of L-NMMA. These data showed that extranuclear targets play an essential role in the initiation of oxidative damage that mediates fiber mutagenesis in mammalian cells.

Emerging roles of T helper 17 and regulatory T cells in lung cancer progression and metastasis

“…A well-documented example is asbestos, which refers to six unique silicate mineral fibers: chrysotile, amosite, crocidolite, tremolite, anthophyllite and actinolite. Inhaled asbestos fibers larger than 20 μm are not efficiently phagocytosed and remain in lung tissue, where they induce fibrosis, inflammation and eventually, carcinogenesis [15, 25]. Most of the inflammation driven effects of asbestos exposure are a consequence of increased ROS production [26].…”

Molecular Pathways: Targeting Mechanisms of Asbestos and Erionite Carcinogenesis in Mesothelioma

“…DNA damage may be directly caused through mechanical interference of asbestos fibers with chromosome segregation during mitosis (16), or, more likely, indirectly through asbestos-related induction of mesothelial cells and macrophages to generate mutagenic reactive oxygen (ROS) and nitrogen (iNOS) species, both of which are mutagenic in vitro (17, 18). The generation of oxidants by macrophages as they attempt to digest asbestos fibers may trigger the activation of several signaling pathways.…”

Hypoxia/Reoxygenation-Induced Mutations in Mammalian Cells Detected by the Flow Cytometry Mutation Assay and Characterized by Mutant Spectrum

“…Many studies have demonstrated the ability of DMSO to reduce oxygen radical-induced mutagenesis (32, 33). DMSO increased the survival of hypoxia-treated cells by ~30% and decreased the mutant yield in the flow cytometry mutation assay significantly.…”