Increase in mutation frequency in lung of Big Blue® rat by exposure to diesel exhaust

Abstract

Exposure to diesel exhaust (DE) is known to cause lung tumors in rats. To clarify the mutagenicity of DE, we estimated mutant frequency (MF) and determined the mutation spectra in rat lung after exposure to DE using lambda/lacI transgenic rats (Big Blue® system). Male Big Blue® rats (6 weeks old); were exposed for 4 weeks to 1 or 6 mg/m3 DE, which contains suspended particulate matter. Control rats were maintained in filtered clean air. After exposure to 6 mg/m3 DE, MF in lung was 4.8-fold higher than in control rats (P < 0.01), but no increase in MF was observed in rats exposed to 1 mg/m3 DE. Sixty-nine mutants were identified after exposure to 6 mg/m3 DE. The major mutations were A:T→G:C (18 mutations) and G:C→A:T (19 mutations) transitions. Remarkably, G→T transversion of the lad gene at site 221 was a hot-spot induced by exposure to DE, and there were complex mutations in which multiple mutations occurred in a single mutant, especially in the rats exposed to 6 mg/m3 DE. DNA adducts formed by DE were analyzed using a 32P-post-label TLC method and the amount of 8-hydroxydeoxyguanosine (8-OHdG) was measured using HPLC. Relative adduct level and amount of 8-OHdG were significantly increased in the rats exposed to 6 mg/m3 DE compared with the controls (3.0- and 2.2-fold, respectively; P < 0.01). The level of cytochrome P450 1A1 mRNA was shown by northern blot analysis to be significantly increased in the lungs of rats exposed to 6 mg/m3 DE (5.5-fold; P < 0.01). These results indicate that DE causes lesions in genomic DNA and acts as a mutagen in rat lung.