Quercetin inhibits benzo[a]pyrene-induced DNA adducts in human Hep G2 cells by altering cytochrome P-450 1A1 gene expression

Quercetin is one of the most abundant of the naturally occurring flavonoids. If has been estimated that about 25-50 mg of quercetin are consumed from the daily diet. The chemopreventive effect of quercetin on dietary carcinogen has been intensely studied in animal models; however, knowledge regarding the molecular mechanism is still limited. In this study, the human hepatoma Hep G2 cell line was used to investigate how quercetin prevents benzo[a]pyrene (B[a]P)-induced DNA adducts. The Hep G2 cells were treated with 10 mu M B[a]P for 18 hours in the presence or absence of quercetin. The DNA adduct levels, evaluated by P-32 postlabeling, decreased in a dose-dependent manner after treatment with quercetin. Cytochrome P-450 1A1 (CYP1A1) and glutathione S-transferase involvement have been well demonstrated in the modulation of B[a]P-induced DNA damage. From the assays of both enzyme activities, quercetin inhibits CYP1A1-linked ethoxyresorufin O-dealkylase activity more effectively than glutathione S-transferase activity. To elucidate the molecular mechanisms, reverse transcriptase-polymerase chain reaction and Western blot were used to evaluate whether the decrease in CYP1A1 enzyme activity by quercetin is mediated because of alterations of CYP1A1 transcription or mRNA stability. The results indicated that quercetin significantly inhibits B[a]P-induced CYP1A1 mRNA and protein expression. From these findings, we conclude that quercetin suppresses B[a]P-induced DNA damage in human Hep G2 cells by altering CYP1A1 gene expression. Thus we suggest that dietary quercetin may have a long-term preventive effect on chemical carcinogenesis, especially in people who eat a diet rich in fruits and vegetables.