In vivo mutagenesis in the lungs of gpt-delta transgenic mice treated intratracheally with 1,6-dinitropyrene

Oxidative-stress-driven mutagenesis in the small intestine of the gpt delta mouse induced by oral administration of potassium bromate

Tumorigenesis induced by oxidative stress is thought to be initiated by mutagenesis, but via an indirect mechanism. The dose-response curves for agents that act by this route usually show a threshold, for unknown reasons. To gain insight into these phenomena, we have analyzed the dose response for mutagenesis induced by the oral administration of potassium bromate, a typical oxidative-stress-generating agent, to gpt delta mice. The agent was given orally for 90 d to either Nrf2+ or Nrf2-knockout (KO) mice and mutants induced in the small intestine were analyzed. In Nrf2+mice, the mutant frequency was significantly greater than in the vehicle controls at a dose of 0.6 g/L but not at 0.2 g/L, indicating that a practical threshold for mutagenesis lies between these doses. At 0.6 g/L, the frequencies of G-to-T transversions (landmark mutations for oxidative stress) and G-to-A transitions were significantly elevated. In Nrf2-KO mice, too, the total mutant frequency was increased only at 0.6 g/L. G-to-T transversions are likely to have driven tumorigenesis in the small intestine. A site-specific G-to-T transversion at guanine (nucleotide 406) in a 5'-TGAA-3' sequence in gpt, and our primer extension reaction showed that formation of the oxidative DNA base modification 8-oxo-deoxyguanosine (8-oxo-dG) at nucleotide 406 was significantly increased at doses of 0.6 and 2 g/L in the gpt delta mice. In the Apc oncogene, guanine residues in the same or similar sequences (TGAA or AGAA) are highly substituted by thymine (G-to-T transversions) in potassium bromate-induced tumors. We propose that formation of 8-oxo-dG in the T(A)GAA sequence is an initiating event in tumor formation in the small intestine in response to oxidative stress.

Extrapolation of in vitro structural alerts for mutagenicity to the in vivo endpoint

As part of the hazard and risk assessment of chemicals in man, it is important to assess the ability of a chemical to induce mutations in vivo. Because of the commonalities in the molecular initiating event, mutagenicity in vitro can correlate well to the in vivo endpoint for certain compound classes; however, the difficulty lies in identifying when this correlation holds true. In silico alerts for in vitro mutagenicity may therefore be used as the basis for alerts for mutagenicity in vivo where an expert assessment is carried out to establish the relevance of the correlation. Taking this into account, a data set of publicly available transgenic rodent gene mutation assay data, provided by the National Institute of Health Sciences of Japan, was processed in the expert system Derek Nexus against the in vitro mutagenicity endpoint. The resulting predictivity was expertly reviewed to assess the validity of the observed correlations in activity and mechanism of action between the two endpoints to identify suitable in vitro alerts for extension to the in vivo endpoint. In total, 20 alerts were extended to predict in vivo mutagenicity, which has significantly improved the coverage of this endpoint in Derek Nexus against the data set provided. Updating the Derek Nexus knowledge base in this way led to an increase in sensitivity for this data set against this endpoint from 9% to 66% while maintaining a good specificity of 89%.

Change over time of the mutagenicity in the lungs of gpt delta transgenic mice by extract of airborne particles collected from ambient air in the Tokyo metropolitan area

BACKGROUND

Previously we found that DNA adducts were accumulated in the lungs of the rats exposed to ambient air in the Tokyo metropolitan area. To examine chronological change in in vivo mutagenicity of airborne particles, extracts produced from samples of total suspended particulates (TSP) collected from urban air in 1980, 1990, and 2010 in the Tokyo metropolitan area were intratracheally administered into the lungs of gpt delta mice, and differences in mutation and mutant frequency were determined by using the gpt assay. In vivo mutations induced by the extracts were characterized and mutation hotspots were identified by DNA sequencing of the mutated gpt gene.

RESULTS

Administration of the 1990 extract at a dose of 0.3 mg/animal significantly elevated total mutant frequency to 3.3-times that in vehicle control, and the in vivo mutagenicity of the extract (induced mutation frequency per milligram extract) was estimated to be 2.0- and 2.4-times higher than that of the 2010 and 1980 extract, respectively. G-to-A transition was the most common base substitution in the vehicle control mice. However, administration of the 1990 extract increased the frequency of G-to-T transversion, which is a landmark base substitution induced by oxidative stress; furthermore, when the extract was administered at a dose of 0.15 mg, the mutant and mutation frequencies of G-to-T transversion were significantly increased to frequencies comparable with those of G-to-A transition. Similar increases in the mutant and mutation frequencies of G-to-T transversion were observed after administration of the 2010 extract. Hotspots (mutation foci identified in three or more mice) of G-to-A transition mutations at nucleotides 64 and 110 were induced by the 1980, 1990, and 2010 extracts; a hotspot of G-to-T transversions at nucleotide 406 was also induced by the 2010 extract. Previously, we showed that diesel exhaust particles or their extract, as well as 1,6-dinitropyrene, administered to mice induced these hotspots of G-to-A transitions.

CONCLUSIONS

The results of the present study suggested that mutagenesis induced by extracts produced from TSP collected in the Tokyo metropolitan area induced in vivo mutagenicity via the same mechanism underlying the induction of in vivo mutagenicity by components of diesel exhaust.

Evaluation of in vivo mutagenesis for assessing the health risk of air pollutants

Various kind of chemical substances, including man-made chemical products and unintended products, are emitted to ambient air. Some of these substances have been shown to be mutagenic and therefore to act as a carcinogen in humans. National pollutant inventories (e.g., Pollutant Release and Transfer Registration in Japan) have estimated release amounts of man-made chemical products, but a major concern is the release of suspended particulate matter containing potent mutagens, for example, polycyclic aromatic hydrocarbons and related compounds generated by the combustion of fossil fuel, which are not estimated by PRTR system. In situ exposure studies have revealed that DNA adducts in the lung, and possibly mutations in germline cells are induced in rodents by inhalation of ambient air, indicating that evaluating in vivo mutations is important for assessing environmental health risks. Transgenic rodent systems (Muta, Big Blue, and gpt delta) are good tools for analyzing in vivo mutations induced by a mixture of chemical substances present in the environment. Following inhalation of diesel exhaust (used as a model mixture), mutation frequency was increased in the lung of gpt delta mice and base substitutions were induced at specific guanine residues (mutation hotspots) on the target transgenes. Mutation hotspots induced by diesel exhaust were different from those induced by benzo[a]pyrene, a typical mutagen in ambient air, but nearly identical to those induced by 1,6-dinitropyrene contained in diesel exhaust. Comparison between mutation hotspots in the TP53 (p53) gene in human lung cancer (data extracted from the IARC TP53 database) and mutations we identified in gpt delta mice showed that G to A transitions centered in CGT and CGG trinucleotides were mutation hotspots on both TP53 genes in human lung cancers and gpt genes in transgenic mice that inhaled diesel exhaust. The carcinogenic potency (TD50 value) of genotoxic carcinogen was shown to be correlated with the in vivo mutagenicity (total dose per increased mutant frequency). These results suggest that the mutations identified in transgenic rodents can help identify environmental mutagens that cause cancer.

Alterations in the mutagenicity and mutation spectrum induced by benzo[a]pyrene instilled in the lungs of gpt delta mice of various ages

INTRODUCTION

To examine whether the mutagenic potential of lung exposure to air-borne environmental mutagens is age dependent, we administered 1 mg of benzo[a]pyrene intratracheally to 11- and 24-month old (middle-aged and old, respectively) gpt delta transgenic mice that harbor gpt (guanine phosphoribosyltransferase) genes integrated in the genomic DNA as a target for mutation detection, and then analyzed the benzo[a]pyrene-induced and spontaneous in vivo mutations and mutation spectrum in the lungs.

RESULTS

The mutant frequencies in the lungs of the 11- and 24-month-old control (vehicle-treated) gpt delta mice were 1.14 ± 0.22 × 10(-5) and 1.00 ± 0.20 × 10(-5), respectively, which are significantly higher than that observed for the control 3-month-old (young) mice (0.59 ± 0.13 × 10(-5)) in our previous studies, indicating that spontaneous mutation in the lung increases with age. The mutant frequencies in 11- and 24-month-old mice treated with benzo [a] pyrene were 1.5- and 2.3-fold, respectively, that of the age-matched control mice, and 4.3-fold that of the 3-month-old mice in our previous studies. Analysis of mutation spectra showed that both G:C to A:T transitions and G:C to T:A transversions were predominant in the lungs of control mice at all ages. In benzo [a] pyrene-treated mice in our previous studies, G:C to T:A transversions were the predominant type of mutation (55 %) at 3 months. Here we found that their frequency was dramatically reduced to 18 % by 24 months, and the G:C to A:T transitions became the predominant type of mutation in 24-month-old mice (41 % [16 % at CpG sites]).

CONCLUSIONS

Our findings suggest that susceptibility to benzo[a]pyrene is highest in young mice and is elevated again in old age. The elevation of G:C to A:T transitions was observed following benzo [a] pyrene administration in the lungs of aged mice, and accelerated cytidine deamination is speculated to contribute to this elevation.

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.

Mutations in the lungs of gpt delta transgenic mice following inhalation of diesel exhaust

Diesel exhaust (DE) is a major airborne pollutant of urban areas. It contains various polycyclic aromatic hydrocarbons (PAH) and nitrated PAHs. In this study, gpt delta mice were treated with inhalation of 1 or 3 mg m(-3) DE, or a single intratracheal instillation of diesel exhaust particles (DEP) or DEP extract. In the lungs of mice treated with inhalation of 3 mg m(-3) DE for 12 weeks, the mutant frequency (MF) was 3.2-fold higher than that of the control group (1.90 x 10(-5) and 0.59 x 10(-5), respectively). An instillation of DEP and DEP extract resulted in a significant dose-dependent linear increase in MF. In mice treated with 0.5 mg DEP and 0.2 mg DEP extract, the MFs were 3.0- and 2.7-fold higher than that of the control group, respectively. The mutagenic potency (MF mg(-1)) of DEP extract (5.6 x 10(-5)) was double that of DEP (2.7 x 10(-5)), suggesting that the mutagenicity of the latter is derived primarily from compounds in the extract, which itself is responsible for ca. 50% of the weight of DEP. G:C-->A:T transitions were the predominant gpt mutation induced by all three treatments and G:C-->T:A transversions were induced by DEP and DEP extract. Guanine bases centered in nucleotide sequences such as GGA, TGA, CGG, and CGT were the major mutation targets of all three treatments. Thus, our results suggest that the mutagens contained in DEP such as PAH and nitrated PAHs induce mutations and may be responsible for carcinogenesis caused by inhalation of DE.