Kinetics of DNA adduct formation and removal in mouse hepatocytes following in vivo exposure to 5,9-dimethyldibenzo[c,g]carbazole

5,9-Dimethyldibenzo[c,g]carbazole (DMDBC), a potent mouse hepatocarcinogen, has been shown to induce a nonlinear increase in mutant frequency in the l iver of the transgenic Muta(TM)Mouse. To gain insight into the mechanisms u nderlying the mutagenicity of DMDBC in vivo, DNA damage formation and remov al were monitored in mouse hepatocytes over 4-144 h after a single skin app lication of 10 or 90 mg/kg DMDBC, DNA adducts were measured by P-32-post-la beling. DNA repair was assessed by: (i) the unscheduled DNA synthesis (UDS) assay, which measures [H-3]thymidine incorporation into hepatocyte DNA und ergoing excision repair; (ii) the Comet assay, which detects DNA strand bre aks transiently produced between the incision and rejoining steps of the ex cision repair process. A plateau of similar to 400 DNA adducts/10(8) nucleo tides was reached 24 h after treatment with 10 mg/kg and remained unchanged until 144 h, UDS activity was significantly induced at 15 and 24 h, while no DNA strand breaks were observed at any sampling time. These results sugg est that DNA repair mechanisms were efficiently induced and the formation o f a high degree of DNA damage was avoided at this dose level, Following exp osure to 90 mg/kg DMDBC, the number of DNA adducts increased sharply to a m aximum at 24 h (similar to 8000/10(8) nucleotides) and then declined to sim ilar to 500/10(8) nucleotides at 144 h, UDS activity was markedly induced f rom 15 to 72 h, Low levels of DNA strand breaks were observed at 24 and 48 h, The formation of large numbers of DNA adducts and the emergence of DNA s trand breaks despite a strong initial induction of UDS activity suggested t hat DNA repair mechanisms were saturated at this dose level. This phenomeno n could partly account for the nonlinear induction of gene mutations previo usly reported in the liver of the transgenic Muta(TM)Mouse.