Asbestos fibers are mutagenic after all: new signs of orthodoxy for a paradoxical group of carcinogens

Asbestos ingestion and gastrointestinal cancer: a possible underestimated hazard

The presence of asbestos fibres (AFs) in drinking water could be linked with gastrointestinal cancers. However, it is not regulated in several countries due to conflicting evidence. Areas covered: Some reports mainly associated AF ingestion with gastric and colorectal cancer. Experimental evidence suggested a role for timing and extent of exposure, and showed that ingested AFs induce toxic effects on the stomach, ileum and colon, histological alterations and negative effects at a molecular level, cross the placenta and enter foetal organs (including the liver), and seem able to act as a co-carcinogen agent. Occupational studies suggest associations between asbestos exposure and intrahepatic cholangiocarcinoma, and observations exist indicating the possibility that AFs could enter the liver and bile through enteric absorption. Expert commentary: A risk threshold (AF concentration in drinking water) for digestive cancers has not been convincingly identified so far and regulations, where adopted, have weak scientific basis and may not be adequate. With further and more definitive studies, evidence might become sufficient to justify monitoring plans, persuade countries with no current limits to set a maximum level of AFs in drinking water and might induce a revision of the existing legislations, pointing to efficient primary prevention policies.

The role of genotoxicity in asbestos-induced mesothelioma: an explanation for the differences in carcinogenic potential among fiber types

The mechanism(s) underlying asbestos toxicity associated with the pathogenesis of mesothelioma has been a challenge to unravel for more than 60 years. A significant amount of research has focused on the characteristics of different fiber types and their potential to induce mesothelioma. These mechanistic studies of fiber toxicity have proceeded along two lines: those demonstrating biochemical mechanisms by which fibers induce disease and those investigating human susceptibility. Most recent studies focused on in vitro genotoxic effects induced by asbestos as the mechanism responsible for asbestos-induced disease. Although asbestos exerts a genotoxic effect at certain concentrations in vitro, a positive response in these tests does not indicate that the chemical is likely to produce an increased risk of carcinogenesis in exposed human populations. Thus far, findings from studies on the effects of fiber type in mesothelial cells are seriously flawed by a lack of a dose response relationship. The common limitation of these in vitro experiments is the lack of attention paid to the complexities of the human anatomy, biochemistry and physiology, which make the observed effects in these experimental systems difficult to extrapolate to persons in the workplace. Mechanistic differences between carcinogenic and genotoxic processes indicate why tests for genotoxicity do not provide much insight regarding the ability to predict carcinogenic potential in workers exposed to asbestos doses in the post-Occupational Safety and Health Administration era. This review discusses the existing literature on asbestos-induced genotoxicity and explains why these studies may or may not likely help characterize the dose-response curve at low dose.

Asbestos fibres in drinking water: are they carcinogenic or not?

The paper discusses the possibility of a carcinogenic effect of consuming drinking water contaminated by asbestos fibres.

On the mechanism of cogenotoxic action between ingested amphibole asbestos fibres and benzo[a]pyrene: II. Tissue specificity studies using comet assay

Epidemiological data seem to be equivocal on the probable increase in cancer incidence in populations exposed to asbestos-fibre contaminated drinking water. Although animal experiments failed to demonstrate carcinogenicity of oral asbestos exposure, the large surface area of the fibres, however, creates the possibility of cogenotoxic action with adsorbed water-borne organics. In our animal model, rats were gavaged with untreated UICC crocidolite and anthophyllite fibres and fibres that had been allowed to adsorb benzo[a]pyrene molecules from aqueous solutions. Peritoneal macrophages and intestine, parietal peritoneum and omentum samples were obtained from the animals after 24 h. The alkaline single-cell microgel electrophoresis assay (comet assay) was performed on cells isolated from the solid tissues. Tail moment was applied as a basis of evaluation following image analysis. Our results indicate high levels of DNA strand breaks in the cells prepared from the omentum and intestine. We could also demonstrate a significant potentiating effect of the adsorbed carcinogen on the induction of DNA damage in the omentum. The parietal peritoneum and macrophages were not involved in the early genotoxic alterations under study. Our results support the molecular model of asbestos cocarcinogenesis, including both asbestos-induced deletions and mutations caused by a mutagen carried by the same fibres.

On the mechanism of cogenotoxic action between ingested amphibole asbestos fibres and benzo[a]pyrene: I. Urinary and serum mutagenicity studies with rats

Recently, there has been concern that ingested asbestos may cause an increase in cancer incidence in populations exposed to fibre-contaminated drinking water. Although animal experiments failed to demonstrate carcinogenicity of the oral asbestos exposure, the high adsorption capacity of the fibres creates the possibility of cocarcinogenic action with adsorbed organics. In a simple in vivo model we demonstrated earlier that UICC crocidolite and anthophyllite asbestos fibres were able to adsorb carcinogen molecules from aqueous solutions. When orally administered, these fibres increased the sister chromatid exchange frequency in bone marrow cells of rats. In the present study we tried to follow the desorption and metabolization processes of carcinogenic benzo[a]pyrene molecules transported by the ingested fibres using the highly sensitive Salmonella/Ames mutagenicity assay. The bacterial test was performed on concentrated serum and urine samples of the treated animals by using the TA98 and 100 strains in the presence and absence of liver microsomal and deconjugating enzymes. All sets of urine and serum samples failed to show mutagenic activity indicating a lack of both desorption in the serum and the ability of the liver to metabolize. Considering our results, the cytogenetic impact demonstrated earlier in the bone marrow can be explained by a local action of accumulated and transported carcinogen molecules.

Chrysotile asbestos fibers mediate homologous recombination in Rat2 lambda fibroblasts: implications for carcinogenesis

Asbestos fibers are widespread environmental carcinogens whose mutagenicity is now established. Nonetheless, the molecular nature of these mutations and the mechanisms by which they accelerate carcinogenesis remain poorly understood. We have assessed the ability of asbestos fibers to promote homologous recombination, a potent mechanism for generating intrachromosomal rearrangements, such as deletions, and mitotic recombination. For this, we have developed a new assay which determines the extent to which a marker gene present in DNA introduced by asbestos can recombine with homologous genes residing in a transfected cell. We have demonstrated that Calidria chrysotile fibers are mutagenic and are able to mediate transfection of molecularly marked mutant lacI genes in a manner that results in their preferential recombination with homologous wild-type genes in the transfected cell. Asbestos induced recombination events may play a significant role in asbestos mutagenesis and carcinogenesis, and promotion of recombination may underlie the well-recognized synergy of asbestos with other carcinogens.

In vivo studies on genotoxicity and cogenotoxicity of ingested UICC anthophyllite asbestos

Early cytogenetic action of oral exposure to UICC anthophyllite, an amphibole type of asbestos, was studied in Fischer-344 rats. The animals were gavaged with a suspension of untreated fibres (50 mg/kg) and fibres which had been allowed to adsorb benzo[alpha]pyrene molecules from aqueous solutions of 0.25-2.5 micrograms/ml. HPLC measurements indicated effective adsorption of the benzo[alpha]pyrene molecules on the fibres. The authors consider this system a suitable model for the drinking of water containing asbestos fibres and organic micropollutants. The formation of micronuclei and sister chromatid exchanges was studied in bone marrow samples taken from animals 24 h after oral administration of suspensions. Whereas anthophyllite fibres failed to induce cytogenetic alterations, fibres pretreated with the polycyclic aromatic solutions caused dose-dependent increase in the sister chromatid exchange frequencies. The observed cytogenetic impact can be explained by a local action of carcinogen molecules accumulated and subsequently transported. The results support the hypothesis that epidemiological evidence of carcinogenicity of asbestos in potable water may rather be explained by cogenotoxic action of the asbestos fibres and biologically active organic micropollutants adsorbed on their surface.