Genotoxicity of 3-methylcholanthrene in liver of transgenic Big Blue (R) mice

Transgenic mice provide a unique tool for studying the tissue specificity a nd mutagenic potential of chemicals. Because 3-methylcholanthrene (3MC) was found mutagenic in bacteria, clastogenic in bone marrow, and induces DNA a dducts in animals, we were interested to determinine whether this xenobioti c provokes (1) cell proliferation, (2) transcriptional activity changes, (3 ) DNA adducts, and (4) hepatic mutations in transgenic Big Blue(R) mice car rying the lambda LIZ phage shuttle vector. Big Blue C57/BI male mice were t reated with a single intraperitoneal dose of 80 mg/kg 3MC for 1, 3, 6, 14, or 30 days. Cell proliferation was checked by 5-bromo-2-deoxyuridine labeli ng and immunohistochemical detection. The maximal increase of the mitotic i ndex was evidenced after 3 days (2.9 times the control value; P < 0.01). Th e relative nucleus area, reflecting the transcriptional activity, was also the highest in the treated group after 3 days: 1.86 times the control value , on average (P < 0.01). Four major DNA adducts, determined according to th e [P-32]-postlabeling method, were evidenced in liver DNA of treated mice, 6 days after the treatment: the spot intensities increased in a time-depend ent manner. The mutant frequency of liver DNA was the highest after 14 days : 20.3 +/- 2.9 x 10(-5) in the treated vs. 7.6 +/- 2.7 x 10(-5) in the cont rol mice (P < 0.01). Sequencing of the <lambda> lad mutant plaques showed m ainly G:C --> T:A and C:G --> A:T transversions. In conclusion, 3MC at firs t induced nuclear enlargement and a slight increase of cell proliferation i n liver, Followed by parallel Formation of DNA adducts and mutations. This study shows how transgenic models allow in vivo evaluation of mechanistical ly simultaneous endpoints. Environ. Mel. Mutagen. 36:266-273, 2000. (C) 200 0 WileyLiss, Inc.