Search

Dietary polyphenols, including flavonoids, have been implied to have cancer preventive properties. Suggested mechanisms include inhibition of carcinogen-activating cytochrome P450 (CYP) transcription and activities. These studies have focused mainly on CYP1A1. However, CYP1B1 has recently been shown to be of particular importance in smoking-induced oral and oesophageal cancer. Previous observations in our laboratory demonstrated that methoxylated flavonoids may be effective inhibitors of CYP1A1 transcription and activity as well as being orally bioavailable. In this study, an initial screening of 19 methoxylated flavones, using the ethoxyresorufin de-ethylation assay in human oral squamous cell carcinoma SCC-9 cells pretreated with 1 μM benzo[a]pyrene, identified six strongly inhibitory compounds for further studies. The effect of these flavones on CYP1B1 mRNA expression was measured with quantitative branched DNA methodology. Four of the compounds – 3′,4′-dimethoxyflavone and 5,7,4′-trimethoxyflavone and, in particular, 7,3′-dimethoxyflavone and 7,4′-dimethoxyflavone – were potent inhibitors of CYP1B1 mRNA expression. Two of the more common unmethylated polyphenols – curcumin and quercetin – were also potent inhibitors. Whereas most unmethylated polyphenols, such as curcumin and quercetin, have very poor bioavailability, the high metabolic stability of the methoxylated flavones studied here suggests that these CYP1B1 inhibitors may also be effective in-vivo. [ABSTRACT FROM AUTHOR]

The roles of CYP1A1 and 1B1 in tobacco smoke carcinogen, e.g. benzo[a]pyrene (BaP), induced DNA binding and their inhibition by the dietary polyphenol 5,7-dimethoxyflavone (DMF), compared with 3′,4′-dimethoxyflavone (3′,4′-DMF) and resveratrol, were investigated in the human oral epithelial squamous cell carcinoma (SCC)-9 cells. A low concentration of BaP (1 μM) dramatically induced BaP-DNA adduct formation (∼40-fold) in a time-dependent manner, while it only increased CYP1A1/1B1 activities, as measured by ethoxyresorufin O-deethylation, ∼3-fold. Furthermore, BaP induced both CYP1B1 and CYP1A1 mRNA and protein expression, as determined by the branched DNA assay and western blot analysis, but with considerably higher levels of CYP1B1. Combined treatment of SCC-9 cells with 1 μM BaP and 20 μM DMF inhibited BaP–DNA adduct formation. The mechanism of this effect appeared to be direct inhibition of CYP1B1 enzyme with a Ki value of 0.58 μM, a highly potent inhibition considering the high cellular uptake of DMF in the SCC-9 cells. DMF also inhibited CYP1A1, but not CYP1B1 protein, and mRNA expression in the cells. In an extension to other polyphenols, the structural analog 3′,4′-DMF, in contrast to DMF, inhibited the expression of CYP1B1 both at the mRNA and protein levels. Surprisingly, in contrast to previous studies in other cell types, resveratrol had no effect on CYP1B1 in the SCC-9 cells. Based on this study, CYP1B1 mRNA may be an early biomarker of oral cancer, being a sensitive signal for tobacco-carcinogen exposure. Methoxylated dietary flavonoids, e.g. DMF and 3′,4′-DMF, may be potent chemoprotectants by direct inhibition of CYP1B1/1A1 function and/or their protein expression. [ABSTRACT FROM AUTHOR]

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 M) 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 (220 M) 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 M. 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. [ABSTRACT FROM AUTHOR]