Sensitive detection of air-born mutagens and their evaluation using gene knockout mice

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A transcription factor, Nrf2, is essential for expression of phase II enzymes and antioxidant enzymes, and contributes to a protection system against xenobiotics. We intended to reveal how mutant frequency is increased under Nrf2-deficient condition. We used gpt delta mice as a model animal for determining characteristics of in vivo mutagenesis caused by environmental mutagens. Intratracheal instillation of mutagens such as benzo[a]pyrene (B[a]P) and 1,6-dinitropyrene(1,6-DNP) caused to increase the mutant frequency linearly depending of the dose, and in vivo mutagenic potency of 1,6-DNA was shown to be higher than that of B[a]P. A predominant base substitution caused by B[a]P and 1,6-DNP was G>T and G>A, respectively, and positions of mutation hotspots on gpt gene was different between these compounds. Exposure of diesel exhaust for 12 weeks at the concentration of 3 mg/m^3 resulted to elevate the mutant frequency depending on duration of exposure. As a result of intratracheal instillation, mutagenicity of diesel exhaust particles (DEP) was shown to be derived from the extract of DEP. A predominant base substitution induced by exposure of diesel exhaust, as well as administration of DEP, was G>A, and the positions of mutation hotspots were identical to those of 1,6-DNP, suggesting that major mutagens in diesel exhaust are nitropyrenes and related compounds. We produced Nrf2-deficient gpt delta mice, and B[a]P was administered to lungs of the Nrf2(+/-) and Nrf2(-/-) mice. The mutant frequency in B[a]P-treated Nrf2(-/-) mice became twice compared to Nrf2(+/-) mice.