Photomutagenicity of anhydroretinol and 5,6-epoxyretinyl palmitate in 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.

Size- and coating-dependent cytotoxicity and genotoxicity of silver nanoparticles evaluated using in vitro standard assays

The physicochemical characteristics of silver nanoparticles (AgNPs) may greatly alter their toxicological potential. To explore the effects of size and coating on the cytotoxicity and genotoxicity of AgNPs, six different types of AgNPs, having three different sizes and two different coatings, were investigated using the Ames test, mouse lymphoma assay (MLA) and in vitro micronucleus assay. The genotoxicities of silver acetate and silver nitrate were evaluated to compare the genotoxicity of nanosilver to that of ionic silver. The Ames test produced inconclusive results for all types of the silver materials due to the high toxicity of silver to the test bacteria and the lack of entry of the nanoparticles into the cells. Treatment of L5718Y cells with AgNPs and ionic silver resulted in concentration-dependent cytotoxicity, mutagenicity in the Tk gene and the induction of micronuclei from exposure to nearly every type of the silver materials. Treatment of TK6 cells with these silver materials also resulted in concentration-dependent cytotoxicity and significantly increased micronucleus frequency. With both the MLA and micronucleus assays, the smaller the AgNPs, the greater the cytotoxicity and genotoxicity. The coatings had less effect on the relative genotoxicity of AgNPs than the particle size. Loss of heterozygosity analysis of the induced Tk mutants indicated that the types of mutations induced by AgNPs were different from those of ionic silver. These results suggest that AgNPs induce cytotoxicity and genotoxicity in a size- and coating-dependent manner. Furthermore, while the MLA and in vitro micronucleus assay (in both types of cells) are useful to quantitatively measure the genotoxic potencies of AgNPs, the Ames test cannot.

Quantitative analysis of the relative mutagenicity of five chemical constituents of tobacco smoke in the mouse lymphoma assay

Quantifying health-related biological effects, like genotoxicity, could provide a way of distinguishing between tobacco products. In order to develop tools for using genotoxicty data to quantitatively evaluate the risk of tobacco products, we tested five carcinogens found in cigarette smoke, 4-aminobiphenyl (4-ABP), benzo[a]pyrene (BaP), cadmium (in the form of CdCl2), 2-amino-3,4-dimethyl-3H-imidazo[4,5-f]quinoline (MeIQ) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), in the mouse lymphoma assay (MLA). The resulting mutagenicity dose responses were analyzed by various quantitative approaches and their strengths and weaknesses for distinguishing responses in the MLA were evaluated. L5178Y/Tk (+/-) 3.7.2C mouse lymphoma cells were treated with four to seven concentrations of each chemical for 4h. Only CdCl2 produced a positive response without metabolic activation (S9); all five chemicals produced dose-dependent increases in cytotoxicity and mutagenicity with S9. The lowest dose exceeding the global evaluation factor, the benchmark dose producing a 10%, 50%, 100% or 200% increase in the background frequency (BMD10, BMD50, BMD100 and BMD200), the no observed genotoxic effect level (NOGEL), the lowest observed genotoxic effect level (LOGEL) and the mutagenic potency expressed as a mutant frequency per micromole of chemical, were calculated for all the positive responses. All the quantitative metrics had similar rank orders for the agents' ability to induce mutation, from the most to least potent as CdCl2(-S9) > BaP(+S9) > CdCl2(+S9) > MeIQ(+S9) > 4-ABP(+S9) > NNK(+S9). However, the metric values for the different chemical responses (i.e. the ratio of the greatest value to the least value) for the different chemicals ranged from 16-fold (BMD10) to 572-fold (mutagenic potency). These results suggest that data from the MLA are capable of discriminating the mutagenicity of various constituents of cigarette smoke, and that quantitative analyses are available that can be useful in distinguishing between the exposure responses.

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.

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.

Phototoxicity of herbal plants and herbal products

Plants are used by humans in daily life in many different ways, including as food, herbal medicines, and cosmetics. Unfortunately, many natural plants and their chemical constituents are photocytotoxic and photogenotoxic, and these phototoxic phytochemicals are widely present in many different plant families. To date, information concerning the phototoxicity and photogenotoxicity of many plants and their chemical constituents is limited. In this review, we discuss phototoxic plants and their major phototoxic constituents; routes of human exposure; phototoxicity of these plants and their constituents; general mechanisms of phototoxicity of plants and phototoxic components; and several representative phototoxic plants and their photoactive chemical constituents.

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.

Cytotoxicity and mutagenicity of retinol with ultraviolet A irradiation in mouse lymphoma cells

Vitamin A (all-trans-retinol; retinol) is an essential human nutrient and plays an important role in several biological functions. However, under certain circumstances, retinol treatment can cause free radical generation and induce oxidative stress. In this study, we investigated photocytotoxicity and photomutagenicity of retinol using L5178Y/Tk(+/-) mouse lymphoma cells concomitantly exposed to retinol and ultraviolet A (UVA) light. While the cells treated with retinol alone at the doses of 5 or 10microg/ml in the absence of light did not increase the mutant frequency (MF) in the Tk gene, the treatment of the cells with 1-4microg/ml retinol under UVA light (1.38mW/cm(2) for 30min) increased the MF in the Tk gene in a dose-responsive manner. To elucidate the underlying mechanism of action, we also examined the mutational types of the Tk mutants by determining their loss of heterozygosity (LOH) at four microsatellite loci spanning the entire chromosome 11 on which the Tk gene is located. The mutational spectrum for the retinol+UVA treatment was significantly different from those of the control and UVA alone. More than 93% of the mutants from retinol+UVA treatment lost heterozygosity at the Tk1 locus and the major type (58%) of mutations was LOHs extending to D11Mit42, an alternation involving approximately 6cM of the chromosome, whereas the main type of mutations in the control was non-LOH mutations. These results suggest that retinol is mutagenic when exposed to UVA in mouse lymphoma cells through a clastogenic mode-of-action.