Cigarette smoking is a major risk factor in lung
carcinogenesis via
carcinogens such as
polycyclic aromatic hydrocarbons (PAHs) and
nitrosamines. In this study, we used
benzo[a]pyrene (BaP) as the classic PAH compound and BEAS-2B cells, a model of normal human bronchial epithelial cells, to investigate whether
5,7-dimethoxyflavone (5,7-DMF) and
3',4'-DMF compared with
resveratrol (RV) have chemopreventive properties in this
cancer. Exposure of BEAS-2B cells to [(3)H]BaP (1 microM) showed increasing binding to
DNA up to 72 h of exposure, about 20-fold higher than that at 0.5 h exposure. BaP exposure also increased both
CYP1A1/1B1 and
microsomal epoxide hydrolase (mEH)
enzyme activities with a maximum 10-fold increase at 48 h. BaP induced
CYP1A1 protein and
mRNA levels maximally after 48 h. In contrast, although CYP1B1
mRNA was rapidly induced, its
protein expression showed a very poor response. Simultaneous treatment with BaP and 5,7-DMF,
3',4'-DMF or RV for 48 h inhibited BaP-
DNA binding by > or =75%, with
3',4'-DMF being the most effective. 5,7-DMF affected
CYP1A1 mRNA levels only modestly, whereas
3',4'-DMF was a potent inhibitor. The catalytic activity of
CYP1A1/1B1 was reduced over 95% after exposure to 5,7-DMF,
3',4'-DMF or RV, most effectively by
3',4'-DMF. BaP-induced mEH activity was not affected by treatment with 5,7-DMF, but was significantly inhibited by
3',4'-DMF. In contrast, mEH activity was notably increased by RV. Most importantly, western blotting showed all three
polyphenols dramatically reducing BaP-induced
CYP1A1 protein expression. Both 5,7-DMF and
3',4'-DMF demonstrated very high, about 40-fold, accumulation in BEAS-2B cells. In summary, BaP exposure results in a high level of
DNA binding in BEAS-2B cells, which is mainly mediated by induction of
CYP1A1 protein, just as in the human lung. Two methoxylated dietary
flavonoids with highly specific effects on BaP bioactivation block this
DNA binding and
CYP1A1 protein expression as effectively as RV, thus making them potential chemopreventive agents for BaP-induced lung
carcinogenesis.