Induction of cytochrome P4501A1 by 2,3,7,8-tetrachlorodibenzo-p-dioxin or indolo(3,2-b)carbazole is associated with oxidative DNA damage

Induction of cytochrome P4501A1 (CYP1A1) in the hepatoma Hepa1c1c7 cell line results in an elevation in the excretion rate of 8-oxoguanine (oxo(8)Gua), a biomarker of oxidative DNA damage and the major repair product of 8-oxo-2'-deoxyguanosine (oxo(8)dG) residues in DNA. Treatment of this cell line with 2,3,7,8-tetrachloro-p-dibenzodioxin (TCDD), a nonmetabolized environmental contaminant, and indolo(3,2-b)carbazole (ICZ), a metabolite of a natural pesticide found in cruciferous vegetables, is shown to both induce CYP1A1 activity and elevate the excretion rate of oxo(8)Gua; 7,8-benzoflavone (7,8-BF or alpha-naphthoflavone), an inhibitor of CUP1A1 activity and an antagonist of the aryl hydrocarbon (Ah) receptor, reduced the excretion rate of oxo(8)Gua. The essential role of Ah-receptor, which mediates the induction of CYP1A1, is shown by the inability of TCDD to induce CYP1A1 and to increase excretion of oxo(8)Gua in Ah receptor-defective c4 mutant cells. While there was a significant 7.0 fold increase over 2 days in the excretion rate of oxo(8)Gua into the growth medium of TCDD-treated Hepa1c1c7 cells compared to control, no significant increase was detected in the steady-state level of oxo(8)dG in the DNA presumably due to efficient DNA repair. Thus, the induction of CYP1A1 appears to lead to a leak of oxygen radicals and consequent oxidative DNA damage that could lead to mutation and cancer.