Induction of aromatase (CYP19) expression in breast cancer cells through a nongenomic action of estrogen receptor α

Aromatase plays a critical role in breast cancer development by converting androgen to estrogen. In this report, results are presented to demonstrate that estrogen, the product of aromatase, can up-regulate its expression. Estrogen receptor (ER) transient transfection experiments were performed using the SK-BR-3 breast cancer cell line, which is ER negative and expresses aromatase. When SK-BR-3 cells were transfected with the expression plasmid pCI-ERalpha, but not pCI-ERbeta, aromatase activity was elevated by 17beta-estradiol (E-2) in a dose-dependent manner. The E-2 induction could be enhanced by cotransfection with the coactivator GRIP and suppressed by antiestrogens such as tamoxifen and ICI 182,780. The aromatase activity in the ERalpha-transfected SK-BR-3 cells could also be induced by environmental chemicals that were known to have an estrogen-like activity. Using aromatase gene exon Is-specific reverse transcription-PCR, the level of promoter I.1-driven transcripts was found to be elevated in E-2-treated ERalpha-transfected cells. This suggested that E-2 induced aromatase expression through the up-regulation of promoter I.1. Using DNA deletion analysis of the 5'-flanking region of promoter I.1, the section between -300 and -280 bp upstream from exon I.1 was identified to be important for mediating E-2 induction. However, a direct binding of ERalpha to this 20-bp region could not be demonstrated. It was found that E-2 induction could be suppressed by the mitogen-activated protein/extracellular signal-regulated kinase kinase inhibitor, PD98059, and the epidermal growth factor receptor tyrosine kinase inhibitor, PD153035 hydrochloride. A significant induction of aromatase expression was also detected in ER-positive MCF-7 breast cancer cells after transfection with pCI-ERalpha and E-2 treatment. Furthermore, after ERalpha transfection and E2 treatment, the aromatase activity in Her-2-overexpressing MCF-7 cells was drastically higher than that of the wild-type MCF-7 cells. In addition, aromatase induction in MCF-7 cells could also be suppressed by PD153035 hydrochloride. These results suggest that E-2 up-regulates aromatase expression by a nongenomic action of ERalpha via cross-talk with growth factor-mediated pathways.