Breast tumor aromatase: functional role and transcriptional regulation

Aromatase has been shown to be expressed at a higher level in human breast cancer tissue than in normal breast tissue, by means of enzyme activity measurement, immunocytochemistry, and RT-PCR analysis. Cell culture including MCF-7 breast cancer cells, animal experiments using aromatase-transfected breast cancer cells, and transgenic mouse studies have demonstrated that estrogen production in situ plays a more important role than circulating estrogens in breast tumor promotion. In addition, tumor aromatase is believed to be able to stimulate breast cancer growth through both autocrine and paracrine pathways, as demonstrated by a three-dimensional cell culture study. RT-PCR and gene transcriptional studies have revealed that the aromatase promoter is switched from a glucocorticoid-stimulated promoter, I.4, in normal tissue to cAMP-stimulated promoters, I.3 and II, in cancerous tissue. Recently, we identified and characterized a cAMP-responsive element (CREaro) upstream from promoter I.3 by DNA deletion and mutational analyses. Our results from promoter functional analysis also demonstrated an interaction between the CREaro and the silencer element (S1) that was identified previously in our laboratory. In the presence of cAMP, the positive regulatory CREaro can overcome the action of the silencer on the function of promoter I.3. On the basis of results generated from our own and other laboratories, we propose that, in normal breast adipose stromal cells and fibroblasts, aromatase expression is driven by promoter I.4 (glucocorticoid dependent), and that the action of promoters I.3 and II is suppressed by the silencer negative regulatory element. However, in cancer cells and surrounding adipose stromal cells, the cAMP level increases, and aromatase promoters are switched to cAMP-dependent promoters - I.3 and II. Furthermore, we applied the yeast one-hybrid screening method to search for proteins interacting with the silencer element, S1. The major protein identified was ERRalpha-1; however, SF-1, which is present in the ovary, is not detected in breast cancer tissue. Using a reporter plasmid with the aromatase genomic fragment containing promoter I.3 and S1, in breast cancer SK-BR-3 cells, ERRalpha-1 was found to have a positive regulatory function. It is believed that the silencer element in the human aromatase gene may function differently in different tissues, as a result of distinct expression patterns of transcription factors.