Human CYP1B1 is regulated by estradiol via estrogen receptor

Human cytochrome P450 (CYP) 1B1 is a key enzyme in the metabolism of 17beta-estradiol (E2). CYP1B1 is mainly expressed in endocrine-regulated tissues, such as mammary, uterus, and ovary. Because many CYP enzymes are likely to be induced by the substrates themselves, we examined whether the human CYP1B1 expression is regulated by E2 in the present study. Real-time reverse transcription-PCR analysis revealed that treatment with 10 nM E2 for 12 h induced CYP1B1 mRNA expression in estrogen receptor (ER)-positive MCF-7 cells. Luciferase reporter assays using MCF-7 cells showed a significant transactivation up to 7-fold by E2 with a reporter plasmid containing a region from -152 to +25 of the human CYP1B1 gene. A computer-assisted homology search indicated a putative estrogen response element (ERE) between -63 and -49 in the CYP1B1 promoter region. Specific binding of ERalpha to the putative ERE was demonstrated by chromatin immunoprecipitation assays and gel shift analyses. With reporter plasmids containing the wild or mutated putative ERE on the CYP1B1 gene and the wild or mutated ERalpha expression vectors, luciferase assays using Ishikawa cells demonstrated that the putative ERE and ERalpha are essential for the transactivation by E2. Because endometrial tissue is highly regulated by estrogens, the expression pattern of CYP1B1 protein in human endometrial specimens was examined by immunohistochemistry. The staining of CYP1B1 was stronger in glandular epithelial cells during a proliferative phase than those during a secretory phase, consistent with the pattern of estrogen secretion. These findings clearly indicated that the human CYP1B1 is regulated by estrogen via ERalpha. Because 4-hydroxylation of estrogen by CYP1B1 leads to decrease of the estrogenic activity but the produced metabolite is toxicologically active, our findings suggest a clinical significance in the estrogen-regulated CYP1B1 expression for the homeostasis of estrogens as well as estrogen-dependent carcinogenesis.