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.