3'-azido-3'-deoxythymidine induces deletions in L5178Y mouse lymphoma cells

Detection of Loss of Heterozygosity in Tk-Deficient Mutants from L5178Y Tk+/--3.7.2C Mouse Lymphoma Cells

The mouse lymphoma assay (MLA), a forward mutation assay using the Tk^+/--3.7.2C clone of the L5178Y mouse lymphoma cell line and the Thymidine kinase (Tk) gene, has been widely used as an in vitro genetic toxicity assay for more than four decades. The MLA can evaluate the ability of mutagens to induce a wide range of genetic events including both gene mutations and chromosomal mutations and has been recommended as one component of several genotoxicity test batteries. Tk-deficient mutants often exhibit chromosomal abnormalities involving the distal end of chromosome 11 where the Tk gene is located, in mice, and the type of chromosome alteration can be analyzed using a loss of heterozygosity (LOH) approach. LOH has been considered an important event in human tumorigenesis and can result from any of the following several mechanisms: large deletions, mitotic recombination, and chromosome loss. In this chapter, the authors describe the procedures for the detection of LOH in the Tk mutants from the MLA, and apply LOH analysis for understanding the types of genetic damage that is induced by individual chemicals.

In vitro investigation of the mutagenic potential of Aloe vera extracts

A 2-year cancer bioassay in rodents with a preparation of Aloe vera whole leaf extract administered in drinking water showed clear evidence of carcinogenic activity. To provide insight into the identity and mechanisms associated with mutagenic components of the Aloe vera extracts, we used the mouse lymphoma assay to evaluate the mutagenicity of the Aloe vera whole leaf extract (WLE) and Aloe vera decolorized whole leaf extract (WLD). The WLD extract was obtained by subjecting WLE to activated carbon-adsorption. HPLC analysis indicated that the decolorization process removed many components from the WLE extract, including anthraquinones. Both WLE and WLD extracts showed cytotoxic and mutagenic effects in mouse lymphoma cells but in different concentration ranges, and WLD induced about 3-fold higher levels of intracellular reactive oxygen species than WLE. Molecular analysis of mutant colonies from cells treated with WLE and WLD revealed that the primary type of damage from both treatments was largely due to chromosome mutations (deletions and/or mitotic recombination). The fact that the samples were mutagenic at different concentrations suggests that while some mutagenic components of WLE were removed by activated carbon filtration, components with pro-oxidant activity and mutagenic activity remained. The results demonstrate the utility of the mouse lymphoma assay as a tool to characterize the mutagenic activity of fractionated complex botanical mixtures to identify bioactive components.

Mechanistic evaluation of Ginkgo biloba leaf extract-induced genotoxicity in L5178Y cells

Ginkgo biloba has been used for many thousand years as a traditional herbal remedy and its extract has been consumed for many decades as a dietary supplement. Ginkgo biloba leaf extract is a complex mixture with many constituents, including flavonol glycosides and terpene lactones. The National Toxicology Program 2-year cancer bioassay found that G. biloba leaf extract targets the liver, thyroid gland, and nose of rodents; however, the mechanism of G. biloba leaf extract-associated carcinogenicity remains unclear. In the current study, the in vitro genotoxicity of G. biloba leaf extract and its eight constituents was evaluated using the mouse lymphoma assay (MLA) and Comet assay. The underlying mechanisms of G. biloba leaf extract-associated genotoxicity were explored. Ginkgo biloba leaf extract, quercetin, and kaempferol resulted in a dose-dependent increase in the mutant frequency and DNA double-strand breaks (DSBs). Western blot analysis confirmed that G. biloba leaf extract, quercetin, and kaempferol activated the DNA damage signaling pathway with increased expression of γ-H2AX and phosphorylated Chk2 and Chk1. In addition, G. biloba leaf extract produced reactive oxygen species and decreased glutathione levels in L5178Y cells. Loss of heterozygosity analysis of mutants indicated that G. biloba leaf extract, quercetin, and kaempferol treatments resulted in extensive chromosomal damage. These results indicate that G. biloba leaf extract and its two constituents, quercetin and kaempferol, are mutagenic to the mouse L5178Y cells and induce DSBs. Quercetin and kaempferol likely are major contributors to G. biloba leaf extract-induced genotoxicity.

Genotoxicity profile of azidothymidine in vitro

Azidothymidine (Zidovudine, AZT) is part of the standard care of treatment for acquired immunodeficiency syndrome since many years. A great number of studies on the genotoxic potential of AZT have been published, but no comprehensive hypothesis yet explains all observations. We investigated a multitude of genotoxic endpoints, both in vitro and in vivo, with the goal to complete the picture. The mutagenic potential of AZT in bacteria was found to be restricted to strains with an "ochre" target sequence and could be abrogated both by thymidine supplementation and rat liver S9 mix. Single-strand breaks in mammalian cells were detected in the comet assay after short-term treatment (3h) with AZT, which did not induce micronuclei. The latter were mainly seen after prolonged exposure (24 and 48h) and are probably not directly related to AZT incorporation into DNA. Our data demonstrate that short-term exposure to low AZT concentrations does not induce biologically relevant micronucleation. Only treatment with high concentrations of AZT for prolonged time periods manifests in substantial micronucleus induction. Furthermore, we found that high concentrations of thymidine have no effect in the comet assay but increase micronucleus frequency in a manner very similar to AZT. These results lead us to the following hypothesis: AZT is triphosphorylated and then incorporated into DNA strands, leading to mutations and cytotoxicity. Cellular attempts to repair these DNA lesions as well as stalled replication forks due to chain termination are detectable with the comet assay. Increased micronucleus frequency is likely related to nucleotide pool imbalance.

Nitroxide TEMPO: a genotoxic and oxidative stress inducer in cultured cells

2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) is a low molecular weight nitroxide and stable free radical. In this study, we investigated the cytotoxicity and genotoxicity of TEMPO in mammalian cells using the mouse lymphoma assay (MLA) and in vitro micronucleus assay. In the absence of metabolic activation (S9), 3mM TEMPO produced significant cytotoxicity and marginal mutagenicity in the MLA; in the presence of S9, treatment of mouse lymphoma cells with 1-2mM TEMPO resulted in dose-dependent decreases of the relative total growth and increases in mutant frequency. Treatment of TK6 human lymphoblastoid cells with 0.9-2.3mM TEMPO increased the frequency of both micronuclei (a marker for clastogenicity) and hypodiploid nuclei (a marker of aneugenicity) in a dose-dependent manner; greater responses were produced in the presence of S9. Within the dose range tested, TEMPO induced reactive oxygen species and decreased glutathione levels in mouse lymphoma cells. In addition, the majority of TEMPO-induced mutants had loss of heterozygosity at the Tk locus, with allele loss of ⩽34Mbp. These results indicate that TEMPO is mutagenic in the MLA and induces micronuclei and hypodiploid nuclei in TK6 cells. Oxidative stress may account for part of the genotoxicity induced by TEMPO in both cell lines.

Silver nanoparticle-induced mutations and oxidative stress in mouse lymphoma cells

Silver nanoparticles (Ag-NPs) have increasingly been used for coatings on various textiles and certain implants, for the treatment of wounds and burns, as a water disinfectant, and in air-freshener sprays. The wide use of Ag-NPs may have potential human health impacts. In this study, the mutagenicity of 5-nm Ag-NPs was evaluated in the mouse lymphoma assay system, and modes of action were assessed using standard alkaline and enzyme-modified Comet assays and gene expression analysis. Treatments of L5178Y/Tk(+/-) mouse lymphoma cells with 5-nm uncoated Ag-NPs resulted in a significant yield of mutants at doses between 3 and 6 μg/mL; the upper range was limited by toxicity. Loss of heterozygosity analysis of the Tk mutants revealed that treatments with uncoated Ag-NPs induced mainly chromosomal alterations spanning less than 34 megabase pairs on chromosome 11. Although no significant induction of DNA damage in Ag-NP-treated mouse lymphoma cells was observed in the standard Comet assay, the Ag-NP treatments induced a dose-responsive increase in oxidative DNA damage in the enzyme-modified Comet assay in which oxidative lesion-specific endonucleases were added. Gene expression analysis using an oxidative stress and antioxidant defense polymerase chain reaction (PCR) array showed that the expressions of 17 of the 59 genes on the arrays were altered in the cells treated with Ag-NPs. These genes are involved in production of reactive oxygen species, oxidative stress response, antioxidants, oxygen transporters, and DNA repair. These results suggest that 5 nm Ag-NPs are mutagenic in mouse lymphoma cells due to induction of oxidative stress by the Ag-NPs.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Genotoxic effects of acrylamide and glycidamide in mouse lymphoma cells

In addition to occupational exposures to acrylamide (AA), concerns about AA health risks for the general population have been recently raised due to the finding of AA in food. In this study, we evaluated the genotoxicity of AA and its metabolite glycidamide (GA) in L5178Y/Tk(+/-) mouse lymphoma cells. The cells were treated with 2-18 mM of AA or 0.125-4 mM of GA for 4 h without metabolic activation. The DNA adducts, mutant frequencies and the types of mutations for the treated cells were examined. Within the dose range tested, GA induced DNA adducts of adenine and guanine [N3-(2-carbamoyl-2-hydroxyethyl)-adenine and N7-(2-carbamoyl-2-hydroxyethyl)-guanine] in a linear dose-dependent manner. The levels of guanine adducts were consistently about 60-fold higher across the dose range than those of adenine. In contrast, no GA-derived DNA adducts were found in the cells treated with any concentrations of AA, consistent with a lack of metabolic conversion of AA to GA. However, the mutant frequency was significantly increased by AA at concentrations of 12 mM and higher. GA was mutagenic starting with the 2mM dose, suggesting that GA is much more mutagenic than AA. The mutant frequencies were increased with increasing concentrations of AA and GA, mainly due to an increase of proportion of small colony mutants. To elucidate the underlying mutagenic mechanism, we examined the loss of heterozygosity (LOH) at four microsatellite loci spanning the entire chromosome 11 for mutants induced by AA or GA. Compared to GA induced mutations, AA induced more mutants whose LOH extended to D11Mit22 and D11Mit74, an alteration of DNA larger than half of the chromosome. Statistical analysis of the mutational spectra revealed a significant difference between the types of mutations induced by AA and GA treatments (P=0.018). These results suggest that although both AA and GA generate mutations through a clastogenic mode of action in mouse lymphoma cells, GA induces mutations via a DNA adduct mechanism whereas AA induces mutations by a mechanism not involving the formation of GA adducts.

Transplacental carcinogenicity of 3'-azido-3'-deoxythymidine in B6C3F1 mice and F344 rats

The prophylactic use of zidovudine (3'-azido-3'-deoxythymidine, AZT) during pregnancy greatly reduces transmission of HIV-1 from infected mothers to their infants; however, the affinity of host cell DNA polymerases for AZT also allows for its incorporation into host cell DNA, predisposing to cancer development. To expand upon previous transplacental carcinogenesis assays performed in CD-1 mice, the transplacental carcinogenicity of AZT was evaluated in a second mouse strain and a second rodent species. Date-mated female mice and rats were gavaged daily with 0, 80, 240, or 480 mg AZT/kg bw during the last 7 days of gestation. At 2 years postpartum, male and female B6C3F1 mouse and F344 rat offspring (n = 44-46 of each sex and species/treatment group) were necropsied for gross and microscopic tissue examinations. Under the conditions of these two-year studies, there was clear evidence of carcinogenic activity based upon significant dose-related trends and increases in the incidences of hemangiosarcoma in male mice and mononuclear cell leukemia in female rats. There was some evidence of carcinogenic activity in the livers of male mice based upon a positive trend and an increased incidence of hepatic carcinoma in the high-dose AZT group. The incidence of gliomas in female rats exceeded the historical background rates for gliomas in F344 rats. P53 overexpression was detected in some AZT-treated mouse neoplasms. These and other cancer-related findings confirm and extend those of previous transplacental carcinogenicity studies of AZT in mice, support the need for long-term follow-up of nucleoside reverse transcriptase inhibitor (NRTI)-exposed children, and indicate the necessity for effective protective strategies against NRTI-induced side effects.

Frequency of Hprt mutant lymphocytes and micronucleated erythrocytes in p53-haplodeficient mice treated perinatally with AZT and AZT in combination with 3TC

Azidothymidine (AZT) is a nucleoside reverse transcriptase inhibitor (NRTI) that is used for reducing mother-to-child transmission of human immunodeficiency virus I. Combinations of AZT and 3'-thiacytidine (3TC) are even more effective than AZT alone. AZT, however, is a mutagen and carcinogen in rodent models and 3TC can increase the genotoxicity of AZT. Since p53 plays a key role in human and mouse tumorigenesis, p53-haplodeficient mice are currently being evaluated as a model for assessing the carcinogenicity of perinatal exposure to NRTIs. In the present study, male C57BL/6 p53(+/+) and p53(-/-) mice were mated with C3H p53(+/+) females; the pregnant females were treated on gestation day 12 through parturition with 40, 80, and 160 mg/kg of AZT or a combination of 160 mg/kg AZT and 100 mg/kg 3TC (AZT-3TC); the p53(+/+) and p53(+/-) offspring were treated daily after birth through postnatal day (PND) 28. The frequencies of micronucleated reticulocytes (MN-RETs) and micronucleated normochromatic erythrocytes (MN-NCEs) were determined on PND1, PND10, and PND28; the frequency of Hprt mutant lymphocytes was measured on PND28. The frequencies of MN-RETs and MN-NCEs were increased in treated animals at all time points; there were no differences in the responses of p53(+/+) and p53(+/-) animals treated with identical doses of NRTIs. After correction for clonal expansion, both AZT and AZT-3TC treatments induced small but significant increases in the frequency of Hprt mutant lymphocytes in p53(+/-) mice, but not in p53(+/+) mice. The data indicate that p53 haplodeficiency affects the genotoxicity of NRTIs; thus, p53(+/-) mice may be a sensitive model for evaluating the carcinogenicity of perinatal exposure to NRTIs.

A method to distinguish between the de novo induction of thymidine kinase mutants and the selection of pre-existing thymidine kinase mutants in the mouse lymphoma assay

The mouse lymphoma assay (MLA) is the most widely used in vitro mammalian gene mutation assay. It detects various mutation events involving the thymidine kinase (Tk) gene in L5178Y/Tk+/- -3.7.2C mouse lymphoma cells. Mutants are detected using a thymidine analogue that arrests the growth of cells containing a functional Tk gene. However, there are a number of potential test chemicals that are thymidine analogues, and there is a problem when using the MLA to evaluate the mutagenicity of these chemicals. Thymidine analogues are activated by Tk before eliciting their toxicity. Therefore, any pre-existing Tk-/- mutants may avoid the toxicity of the test chemical and obtain a growth advantage over the Tk+/- cells, increasing the Tk mutant frequency (MF) in the culture via a selection mechanism. This potential mutant selection effect needs to be distinguished from de novo mutant induction in order to properly evaluate the mutagenicity of these chemicals. Here we describe a simple MLA study design that can differentiate between the selection of pre-existing mutants and de novo mutant induction. Trifluorothymidine (TFT), a thymidine analogue and the selection agent normally used in the MLA, and 4-nitroquinoline-1-oxide (4-NQO), a potent mutagen, were used to treat cells from two different Tk+/- mouse lymphoma cell cultures with different background MFs (approximately 112 and 305x10(-6)). Both agents significantly increased the Tk MFs in both the normal and high background cultures (p<0.01). In 4-NQO-treated cultures, the induced MFs (MF of treated culture-MF of control) for the cultures with different background MFs were about the same (p>0.1), while in TFT-treated cultures, they were significantly different (p<0.01). In TFT-treated cultures, the fold-increases of MF (MF of treated culture/MF of control) for the cultures with different background MFs were about the same (p>0.1), while in 4-NQO-treated cultures, they were significantly different (p<0.01). This study confirms that, when de novo mutations are induced, the induced MF is the same for cultures with normal and artificially high background MFs. In situations where the increase in MF is due solely to selection of pre-existing mutants, the "induced" MF will be a multiple of the background MF and the magnitude of the increase of the induced MF will depend upon the magnitude of the background MF. Our results demonstrate that it is possible, using this experimental design, to distinguish between chemicals acting primarily via the selection of pre-existing Tk mutants and those inducing de novo mutants in the MLA.