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

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Abstract
5,9-Dimethyldibenzo[c,g]carbazole (DMDBC) is a synthetic derivative of the environmental pollutant 7H-dibenzo-[c,g]carbazole. DMDBC is a potent genotoxic carcinogen specific for mouse liver. Using the Muta™Mouse lacZ transgenic mouse model and a positive selection assay, we measured lacZ mutant frequency (MF) in the liver 28 days after a single s.c. administration of DMDBC at 3, 10, 30, 90 or 180 mg/kg. MF remained low at 3 and 10 mg/kg, but increased markedly from 30 mg/kg onwards. To investigate the reason for this non-linear response, we examined mechanisms potentially involved in mutation induction in the liver. Genotoxic effects such as DNA adduct formation were detected in 32P-post-labelling studies. Liver sections were examined for microscopic changes and cell proliferation. These parameters, and MF, were studied 2, 4, 7, 14, 21 and 28 days after a single s.c. administration of 10 or 90 mg/kg DMDBC. At 10 mg/kg, a dose found to double the MF on day 28, DNA adducts reached a level of 200–600 adducts per 108 nucleotides from day 4 to day 28. No changes in histology or cell proliferation were detected 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 108 nucleotides on day 2, then stabilized at ~400 adducts per 108 nucleotides on day 4. An early cytotoxic effect was detected microscopically in centrilobular hepatocytes, and was followed by liver cell proliferation. These data suggest that the marked increase in MF in Muta™Mouse 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, allowing the fixation of persistent DNA adducts into mutations.

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