Sensitivity to carcinogenesis is increased and chemopreventive efficacy of enzyme inducers is lost in nrf2 transcription factor-deficient mice

The chemopreventive actions of oltipraz (4-methyl-5-[2-pyrazinyl]-1,2-dithiole-3-thione) against many structurally diverse classes of carcinogens, including polycyclic aromatic hydrocarbons such as benzo[a]pyrene (B[ a]P), have been largely associated with the induction of phase 2 detoxifying enzymes. Induction of these genes is thought to occur through transcriptional activation of the antioxidant response elements (ARE) found in their promoter regions. The transcription factor Nrf2 has been shown to bind to the ARE sequences and directly activate transcription. Several studies in nrf2-deficient mice have confirmed the importance of Nrf2 for both constitutive and inducible expression of phase 2 genes. An additional chemopreventive effect of oltipraz stems from its capacity as a competitive inhibitor of certain cytochromes P450 involved in the bioactivation of B[ a]P and other carcinogens.; The hypothesis that mice lacking the Nrf2 transcription factor would be more susceptible to B[a]P-induced neoplasia than wild-type mice, because of diminished expression of phase 2 enzymes, was addressed in this thesis. The effects of oltipraz as a phase 2 enzyme inducer or cytochrome P450 inhibitor on both B[a]P-induced neoplasia of the forestomach and B[a]P-DNA adduct levels were evaluated.; The results of the carcinogenesis studies showed that nrf2-deficient mice were substantially more susceptible to B[a]P-induced neoplasia of the forestomach. Moreover, loss of expression of Nrf2 completely abrogated the chemopreventive action of oltipraz, when administered 48 h before B[a]P (a time interval allowing maximal induction of many phase 2 enzymes). Furthermore, oltipraz had no protective effect on tumor burden in the forestomach of nrf2-deficient mice when administered 1 h before B[a]P (a time that selectively optimizes for possible inhibition of cytochromes P450). Increased number of gastric tumors in nrf2-deficient mice was presumably the result of lower constitutive expression of phase 2 enzymes involved in B[a]P detoxication pathways. This was revealed by Northern blot analyses and enzymatic activity assays. To confirm the role of nrf2 genotype on B[ a]P disposition, levels of B[a]P-DNA adducts were measured as tetrols released from DNA isolated from target (forestomach) and non-target tissues (liver and lung) of wild-type and nrf2-deficient mice treated with either vehicle or oltipraz 1 or 48 h before B[a ]P. These tetrols were measure using high-pressure liquid chromatography (HPLC) with fluorescence detection. Levels of B[a]P-DNA adducts in forestomach were significantly higher in nrf2-disrupted mice. Oltipraz pretreatment 1h or 48 h before B[a]P administration had no protective effect on forestomach tetrol levels in nrf2-deficient mice. A significant reduction was observed in wild-type mice treated with oltipraz 48 h, but not 1 h, before the carcinogen. In contrast, oltipraz administration reduced the levels of B[a]P-DNA adducts in liver and lung at both time intervals, suggesting that Nrf2-independent mechanisms ( e.g. P450 inhibition) for oltipraz could occur in vivo. Combining all treatments and genotypes, a strong correlation (R2 = 0.91) between levels of B[a]P-DNA adducts in forestomach and subsequent yield of tumors was observed. (Abstract shortened by UMI.)...