Estrogen metabolism is suggested to play an important role in estrogen-induced breast carcinogenesis. Epidemiologic studies suggest that diets rich in phytoestrogens are associated with a reduced risk of breast cancer. Phytoestrogens are biologically active plant compounds that structurally mimic 17beta-estradiol (E(2)). We hypothesize that phytoestrogens, may provide protection against breast carcinogenesis by altering the expression of estrogen-metabolizing enzymes cytochrome P450 1A1 (Cyp1A1) and 1B1 (Cyp1B1). Cyp1A1 and Cyp1B1 are responsible for the metabolism of E(2) to generate 2-hydroxyestradiol (2-OHE(2)) and 4-hydroxyestradiol (4-OHE(2)), respectively. Studies suggest that 2-OHE(2) and 2-methoxyestradiol may protect against breast carcinogenesis, while 4-OHE(2) is carcinogenic in rodent models. Thus, agents that increase the metabolism of E(2) by Cyp1A1 to produce 2-OHE(2) may have chemoprotective properties. The human immortalized non-neoplastic breast cell line MCF10F was treated with quercetin at 10 and 50muM concentrations for time points ranging from 3 to 48h. Total RNA and protein were isolated. Real-time PCR was used to measure the expression of Cyp1A1 and Cyp1B1 mRNA. Quercetin treatment produced differential regulation of Cyp1A1 and Cyp1B1 mRNA expression in a time- and dose-dependent manner. Treatment with 10 and 50 microM doses of quercetin produced 6- and 11-times greater inductions of Cyp1A1 mRNA over Cyp1B1 mRNA, respectively. Furthermore, quercetin dramatically increased Cyp1A1 protein levels and only slightly increased Cyp1B1 protein levels in MCF10F cells. Thus, our data suggest that phytoestrogens may provide protection against breast cancer by modulating expression of estrogen-metabolizing genes such that production of the highly carcinogenic estrogen metabolite 4-OHE(2) by Cyp1B1 is reduced and the production of the less genotoxic 2-OHE(2) by Cyp1A1 is increased.