Mutagenicity of the oral carcinogen 4-nitroquinoline-1-oxide in cultured BigBlue rat tongue epithelial cells and fibroblasts

The Lambda Select cII Mutation Detection System

A number of transgenic animal models and mutation detection systems have been developed for mutagenicity testing of carcinogens in mammalian cells. Of these, transgenic mice and the Lambda (λ) Select cII Mutation Detection System have been employed for mutagenicity experiments by many research groups worldwide. Here, we describe a detailed protocol for the Lambda Select cII mutation assay, which can be applied to cultured cells of transgenic mice/rats or the corresponding animals treated with a chemical/physical agent of interest. The protocol consists of the following steps: (1) isolation of genomic DNA from the cells or organs/tissues of transgenic animals treated in vitro or in vivo, respectively, with a test compound; (2) recovery of the lambda shuttle vector carrying a mutational reporter gene (i.e., cII transgene) from the genomic DNA; (3) packaging of the rescued vectors into infectious bacteriophages; (4) infecting a host bacteria and culturing under selective conditions to allow propagation of the induced cII mutations; and (5) scoring the cII-mutants and DNA sequence analysis to determine the cII mutant frequency and mutation spectrum, respectively.

Mutation Analysis in Cultured Cells of Transgenic Rodents

To comply with guiding principles for the ethical use of animals for experimental research, the field of mutation research has witnessed a shift of interest from large-scale in vivo animal experiments to small-sized in vitro studies. Mutation assays in cultured cells of transgenic rodents constitute, in many ways, viable alternatives to in vivo mutagenicity experiments in the corresponding animals. A variety of transgenic rodent cell culture models and mutation detection systems have been developed for mutagenicity testing of carcinogens. Of these, transgenic Big Blue® (Stratagene Corp., La Jolla, CA, USA, acquired by Agilent Technologies Inc., Santa Clara, CA, USA, BioReliance/Sigma-Aldrich Corp., Darmstadt, Germany) mouse embryonic fibroblasts and the λ Select cII Mutation Detection System have been used by many research groups to investigate the mutagenic effects of a wide range of chemical and/or physical carcinogens. Here, we review techniques and principles involved in preparation and culturing of Big Blue® mouse embryonic fibroblasts, treatment in vitro with chemical/physical agent(s) of interest, determination of the cII mutant frequency by the λ Select cII assay and establishment of the mutation spectrum by DNA sequencing. We describe various approaches for data analysis and interpretation of the results. Furthermore, we highlight representative studies in which the Big Blue® mouse cell culture model and the λ Select cII assay have been used for mutagenicity testing of diverse carcinogens. We delineate the advantages of this approach and discuss its limitations, while underscoring auxiliary methods, where applicable.

Protective effects of sesame oil on 4-NQO-induced oxidative DNA damage and lipid peroxidation in rats

Sesame oil could be considered as a potent antioxidant and dietary supplement. It possesses antimutagenic, anti-inflammatory and anti-cardiac toxicity. In the view of available findings, the current study focused on determining the protective effects of sesame oil on 4-Nitroquinoline-1-oxide (4-NQO) -induced oxidative DNA damage and lipid peroxidation (LPO) in rats. Seven groups of Wistar albino rats each with 6 either sex were used. Groups were given vehicle control and sesame oil alone orally and 4-NQO (30 mg/kg) by intraperitoneal injection. Following the four dose levels (1, 2, 4, and 8 ml/kg orally), sesame oil plus 4-NQO were also tested. After 24 hours of 4-NQO injection, blood samples were drawn by venipuncture. DNA damage (8-hydroxy-2-deoxy guanosine; 8-OHdG) and LPO were estimated. LPO from the 4-NQO-treated group was 2.5-fold higher than that of the control LPO. Pretreatment with sesame oil reduced this by 16-61%. 8-OHdG DNA damage from 4-NQO was found to be 3-fold higher than that of controls. Pretreatment with sesame oil effectively protected against DNA damage in a dose-dependent fashion. This study indicates that the antioxidant, sesame oil, effectively protected DNA damage and LPO induced by 4-NQO.

Induction of lacZ mutations in MutaMouse primary hepatocytes

We have developed an in vitro mutation assay using primary hepatocytes from the transgenic MutaMouse. Primary hepatocytes were isolated using a two-step perfusion method with purification by Percoll, cultured, and treated with benzo[a]pyrene (BaP), 2-amino-1-methyl-6-phenyl- imidazo[4,5-b]pyridine (PhIP), 3-nitrobenzoanthrone (3-NBA), and cigarette smoke condensate (CSC). The mean lacZ mutant frequency (MF) for the solvent control was approximately twofold greater than the spontaneous MF observed in liver tissue. A concentration-dependent increase in MF (up to 3.7-fold above control) was observed following exposure to BaP. Fourfold and twofold increases in mutant frequency were observed for 3-NBA and PhIP exposures, respectively, without the addition of any exogenous metabolic activation. A slight but statistically significant increase in lacZ MF was observed for CSC, but only at the lowest concentration. This is the first report demonstrating that mutations can be detected in cultured primary hepatocytes from MutaMouse. The preliminary results presented suggest that the MutaMouse primary hepatocyte mutagenicity assay can be used as a cost-effective tool for screening of environmental mutagens and therapeutic products.

Protective effects of solvent fractions of Mentha spicata (L.) leaves evaluated on 4-nitroquinoline-1-oxide induced chromosome damage and apoptosis in mouse bone marrow cells

Spearmint leaves (Mentha spicata L.) contain high levels of antioxidants that are known to protect against both exogenous and endogenous DNA damage. In this study, the protective effects of the hexane fraction (HF), chloroform fraction (CF) and ethyl acetate fraction (EAF) in an ethanol extract from M. spicata were evaluated against 4-nitroquinoline-1-oxide (4-NQO) induced chromosome damage and apoptosis in bone marrow cells of Swiss albino mice. Two (EAF; 80 and 160 mg/ kg body weight - bw) or three (HF and CF; 80, 160 and 320 mg/ kg bw) doses of solvent fractions or vehicle control (25% DMSO in water) were administered orally for five consecutive days. Upon the sixth day, 4-NQO was injected intraperitoneally. The animals were killed the following day. Other control groups were comprised of animals treated with either the vehicle control or the various doses of solvent fractions, but with no 4-NQO treatment. 4-NQO induced micro-nucleated polychromatic erythrocytes (MnPCEs) in all the test groups. However, pre-treatment of animals with the solvent fractions significantly reduced the 4-NQO-induced MnPCEs as well as the percentage of apoptotic cells. The reduction of both MnPCE and apoptosis was more evident following the pre-treatment of animals with 160 mg/kg bw EAF.

Genotoxicity of 3-nitrobenzanthrone and 3-aminobenzanthrone in MutaMouse and lung epithelial cells derived from MutaMouse

FE1 lung epithelial cells derived from MutaMouse are a new model system to provide in vitro mutagenicity data with the potential to predict the outcome of an in vivo MutaMouse test. 3-Nitrobenzanthrone (3-NBA) is a potent mutagen and suspected human carcinogen identified in diesel exhaust and urban air pollution. We investigated the mutagenicity and DNA binding of 3-NBA and its main metabolite 3-aminobenzanthrone (3-ABA) in vitro and in vivo in the MutaMouse assay. Mice were treated with 3-NBA or 3-ABA (0, 2 or 5 mg/kg body weight/day) by gavage for 28 days and 28 days later lacZ mutant frequency (MF) was determined in liver, lung and bone marrow. For both compounds, dose-related increases in MF were seen in liver and bone marrow, but not in lung; mutagenic activity was approximately 2-fold lower for 3-ABA than for 3-NBA. With 3-NBA, highest DNA adduct levels (measured by (32)P-post-labelling) were found in liver (approximately 230 adducts per 10(8) nucleotides) with levels 20- to 40-fold lower in bone marrow and lung. With 3-ABA, DNA adduct levels were again highest in the liver, but approximately 4-fold lower than for 3-NBA. FE1 cells were exposed to up to 10 microg/ml 3-NBA or 3-ABA for 6 h with or without exogenous activation (S9) and harvested after 3 days. For 3-NBA, there was a dose-related increase in MF both with and without S9 mix, which was >10 times higher than observed in vivo. At the highest concentration of 3-ABA (10 microg/ml), we found only around a 2-fold increase in MF relative to controls. DNA adduct formation in FE1 cells was dose-dependent for both compounds, but 10- to 20-fold higher for 3-NBA compared to 3-ABA. Collectively, our data indicate that MutaMouse FE1 cells are well suited for cost-effective testing of suspected mutagens with different metabolic activation pathways as a guide for subsequent in vivo MutaMouse testing.

Microenvironmental influences on mutagenesis in mammary epithelial cells

Tumor progression may be viewed as an evolutionary process at the cellular level. Because blood supply to solid tumors is inadequate, the cancer cells face a hostile microenvironment characterized by hypoxia or anoxia, acidic extracellular pH and nutrient deficiencies. It has been proposed that these factors result in increased levels of spontaneous mutagenesis and thereby contribute to tumor progression. We have examined spontaneous mutagenesis in vitro and in vivo, using previously characterized cell lines (mammary epithelial cells [ME] and mammary fibroblast cells [MFib]) from the mammary gland of the BigBluetrade mark rat, carrying a transgene construct suitable for the detection of mutations. Cells were exposed in vitro to control conditions, low pH, or to glucose deprivation, under normoxic or hypoxic culture conditions, and were also grown as xenografted tumors in immune-deficient mice. We examined cell survival and mutant frequency/spectrum at the cII locus. Significant increases in mutant frequency were observed in ME cells exposed to hypoxia alone or in combination with no glucose; the latter condition also resulted in reduced clonogenic survival. Cells grown as xenografts and then recovered and expanded in culture also had elevated frequencies of spontaneous mutations. We observed a shift in the spontaneous mutation spectrum between the ME cells and the MET cells (cultured in vitro or isolated from mouse xenograft tumors). These results support the concept that the tumor microenvironment contributes to tumor progression by enhancing spontaneous mutagenesis, that different cell types from the same organ can respond differently to these stresses and that differences in microenvironment may influence the types of mutations that arise.

Development and characterization of a stable epithelial cell line from Muta Mouse lung

We have isolated and characterized a stable epithelial cell line from Muta Mouse lung that is a suitable complement to the in vivo assay system. The cells are contact inhibited, forming a flat monolayer, and retain several epithelial/pulmonary characteristics. The genome is stable across more than 50 generations, with a modal chromosome number of 78. Spontaneous rates of micronuclei (19.2 +/- 1.4 per 1,000), sister chromatid exchanges (0.25 +/- 0.004 per chromosome), and chromosome aberrations ( approximately 4%) are lower than, or comparable to, other transgenic cell lines currently used in mutagenicity research. Fluorescence in situ hybridization analyses showed that 80% of cells contain three lambdagt10lacZ loci. Slot-blot analyses indicated that the average cell contains approximately 17 transgene monomers. Spontaneous mutant frequency at the lacZ transgene is stable (39.8 +/- 1.1 x 10(-5)), and the direct-acting mutagens N-ethyl-N-nitrosourea and ICR-191 yielded increases in mutant frequency of 6.3- and 3.2-fold above control, respectively. Benzo[a]pyrene (BaP) exposure increased mutant frequency more than 25-fold above control and did not require an exogenous metabolic activation mixture. Inhibition of Cyp1A1 by 5 microM alpha-naphthoflavone eliminated BaP mutagenesis. Activation and mutation induction by the heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine required a low concentration (0.05% v/v) of exogenous rat liver S9. High activity of alpha, micro, and pi glutathione-S-transferase isozymes appears to confer resistance to the cytotoxic effects of xenobiotics. The cell line is a suitable complement to the in vivo Muta Mouse assay, and provides an opportunity for routine in vitro mutagenicity testing using an endpoint that is identical to that employed in vivo.