Kinetics of induction of DNA adducts, cell proliferation and gene mutations in the liver of Muta (TM) Mice treated with 5,9-dimethyldibenzo[c,g]carbazole

5,9-Dimethyldibenzo[c,g]carbazole (DMDBC) is a synthetic derivative of the environmental pollutant 7H-dibenzo[c,g]carbazole. DMDBC is a potent genotox ic carcinogen specific for mouse liver. Using the Muta(TM)Mouse lacZ transg enic mouse model and a positive selection assay, we measured lacZ mutant fr equency (MF) in the liver 28 days after a single s.c. administration of DMD BC at 3, 10, 30, 90 or 180 mg/kg, RIF remained low at 3 and 10 mg/kg, but i ncreased markedly from 30 mg/kg onwards. To investigate the reason for this non-linear response, we examined mechanisms potentially involved in mutati on induction in the liver. Genotoxic effects such as DNA adduct formation w ere detected in P-32-post-labelling studies. Liver sections were examined f or microscopic changes and cell proliferation. These parameters, and MF, we re studied 2, 4, 7, 14, 21 and 28 days after a single s,c, administration o f 10 or 90 mg/kg DMDBC. At 10 mg/kg, a dose found to double the MF on day 2 8, DNA adducts reached a level of 200-600 adducts per 10(8) nucleotides fro m day 4 to day 28. No changes in histology or cell proliferation were detec ted at this low dose. At 90 mg/kg, MF increased gradually from day 7 to day 28 (maximum 44-fold). The DNA adduct level ranged from 400 to 4500 adducts per 10(8) nucleotides on day 2, then stabilized at similar to 400 adducts per 108 nucleotides on day 4, An early cytotoxic effect was detected micros copically in centrilobular hepatocytes, and was followed by liver cell prol iferation. These data suggest that the marked increase in MF in Muta(TM)Mou se liver after treatment in vivo with DMDBC at 90 mg/kg may be explained by the induction of replicative DNA synthesis due to a cytotoxic effect, allo wing the fixation of persistent DNA adducts into mutations.