Polymerase chain reaction-based deletion analysis of spontaneous and arsenite-enhanced gpt mutants in CHO-AS52 cells

Arsenic trioxide mutational spectrum analysis in the mouse lymphoma assay

It has been well documented that long-term exposure to inorganic arsenic induces cancers and vascular diseases in a dose-response relationship. Nevertheless, arsenic has also demonstrated to have anticancer activity; thus, arsenic trioxide (ATO, As2O3) is an inorganic trivalent arsenic form, currently used in the treatment against acute promyelocytic leukaemia (APL). The open discussion about how arsenic compounds induce genotoxic damage has moved us to evaluate the mutational spectrum induced by ATO in mouse lymphoma cells. Thus, 49 Tk-/- mutant colonies obtained in the mouse lymphoma assay (MLA), after treatments lasting for 4h with 10microM ATO, and 49 spontaneous mutant colonies from independent untreated cultures, were used to analyse and to characterise the mutational spectrum induced by this arsenic compound, to understand its mechanism of action. RT-PCR analysis of Tk cDNA and PCR amplifications of eight selected microsatellite sequences, located on chromosome 11, were used to carry out this screening. Our results show that, in mouse lymphoma cells, ATO is a strong clastogenic compound inducing large deletions, at chromosomal level, covering the Tk gene, as well as other regions of chromosome 11.

Further evidence against a direct genotoxic mode of action for arsenic-induced cancer

Arsenic in drinking water, a mixture of arsenite and arsenate, is associated with increased skin and other cancers in Asia and Latin America, but not the United States. Arsenite alone in drinking water does not cause skin cancers in experimental animals; therefore, it is not a complete carcinogen in skin. We recently showed that low concentrations of arsenite enhanced the tumorigenicity of solar UV irradiation in hairless mice, suggesting arsenic cocarcinogenesis with sunlight in skin cancer and perhaps with different carcinogenic partners for lung and bladder tumors. Cocarcinogenic mechanisms could include blocking DNA repair, stimulating angiogenesis, altering DNA methylation patterns, dysregulating cell cycle control, induction of aneuploidy and blocking apoptosis. Arsenicals are documented clastogens but not strong mutagens, with weak mutagenic activity reported at highly toxic concentrations of inorganic arsenic. Previously, we showed that arsenite, but not monomethylarsonous acid (MMA[III]), induced delayed mutagenesis in HOS cells. Here, we report new data on the mutagenicity of the trivalent methylated arsenic metabolites MMA(III) and dimethylarsinous acid [DMA(III)] at the gpt locus in Chinese hamster G12 cells. Both methylated arsenicals seemed mutagenic with apparent sublinear dose responses. However, significant mutagenesis occurred only at highly toxic concentrations of MMA(III). Most mutants induced by MMA(III) and DMA(III) exhibited transgene deletions. Some non-deletion mutants exhibited altered DNA methylation. A critical discussion of cell survival leads us to conclude that clastogenesis occurs primarily at highly cytotoxic arsenic concentrations, casting further doubt as to whether a genotoxic mode of action (MOA) for arsenicals is supportable.

Role of oxidative damage in the genotoxicity of arsenic

Arsenic is a well-established human carcinogen and is ubiquitous in the environment. For decades, arsenic has been considered to be a nongenotoxic carcinogen because it is only weakly active or, more often, completely inactive in bacterial and mammalian cell mutation assays. In this review, evidence is presented that when assayed using model systems in which both intragenic and multilocus mutations can readily be detected, arsenic is, indeed, found to be a strong, dose-dependent mutagen which induces mostly multilocus deletions. Furthermore, the roles of reactive oxygen and reactive nitrogen species in mediating the genotoxic response are presented in a systematic and logical fashion in support of a working model. The data suggest that antioxidants may be a useful interventional treatment in reducing the deleterious effects of arsenic.

Arsenite induces delayed mutagenesis and transformation in human osteosarcoma cells at extremely low concentrations

Arsenite is a human multisite carcinogen, but its mechanism of action is not known. We recently found that extremely low concentrations (</=0.1 microM) of arsenite transform human osteosarcoma TE85 (HOS) cells to anchorage-independence. In contrast to other carcinogens which transform these cells within days of exposure, almost 8 weeks of arsenite exposure are required for transformation. We decided to reexamine the question of arsenite mutagenicity using chronic exposure in a spontaneous mutagenesis assay we previously developed. Arsenite was able to cause a delayed increase in mutagenesis at extremely low concentrations (</=0.1 microM) in a dose-dependent manner. The increase in mutant frequency occurred after almost 20 generations of growth in arsenite. Transformation required more than 30 generations of continuous exposure. We also found that arsenite induced gene amplification of the dihydrofolate reductase (DHFR) gene in a dose-dependent manner. Since HOS cells are able to methylate arsenite at a very low rate, it was possible that active metabolites such as monomethylarsonous acid (MMA(III)) contributed to the delayed mutagenesis and transformation in these cells. However, when the assay was repeated with MMA(III), we found no significant increase in mutagenesis or transformation, suggesting that arsenite-induced delayed mutagenesis and transformation are not caused by arsenite's metabolites, but by arsenite itself. Our results suggest that long-term exposure to low concentrations of arsenite may affect signaling pathways that result in a progressive genomic instability.