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-ER alpha, but not pCI-ER beta, 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 GRIP1 and suppressed by antiestrogens such as tamoxifen and ICI 182,780. The aromatase activity in the ER alpha-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 ER alpha-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 ER alpha 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-ER alpha and E(2) treatment. Furthermore, after ER alpha transfection and E(2) 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 ER alpha via cross-talk with growth factor-mediated pathways.