5,7-Dimethoxyflavone downregulates CYP1A1 expression and benzo[a]pyrene-induced DNA binding in Hep G2 cells

The objective of this study was to examine the ability of dietary polyphenols to inhibit cytochrome P450 (CYP) 1A1 expression and activity and benzo[a]pyrene (BaP) DNA binding, with the main emphasis on prevention of chemical-induced hepatic carcinogenesis. For this purpose we used Hep G2 cells, a good model of the normal human hepatocyte for CYP1A1 cell signaling. First, when these cells were exposed to a low concentration (1 microM) of BaP, DNA binding occurred, which dramatically increased after 6 h of treatment. BaP also dramatically induced CYP1A1 activity, protein expression and mRNA levels, the likely reason for the marked increase in DNA binding. Second, we screened 25 polyphenols with highly varying chemical structures for maximum ability to inhibit CYP1A1 activity in the Hep G2 cells. Highly varying responses were obtained, ranging from a 10-fold induction by some polyphenols to almost complete inhibition, in particular by 5,7-dimethoxyflavone (DMF), a flavonoid found in some tropical plants. Third, we examined the ability of DMF to inhibit DNA binding of BaP and the mechanisms involved. DMF (2-20 microM) inhibited BaP-induced DNA binding. DMF also inhibited BaP-induced CYP1A1 activity, CYP1A1 protein expression and mRNA levels. Moreover, DMF directly inhibited the catalytic activity of recombinant CYP1A1 with an IC50 of 0.8 microM. In conclusion, DMF was a highly potent inhibitor of BaP-induced DNA binding and CYP1A1 protein expression and activity in the Hep G2 cells. These properties may make DMF an effective chemoprotectant in chemical-induced liver cancer.