Signaling pathways regulating aromatase and cyclooxygenases in normal and malignant breast cells

Abstract

Aromatase (estrogen synthase) is the cytochrome P450 enzyme complex that converts C(19) androgens to C(18) estrogens. Aromatase activity has been demonstrated in breast tissue in vitro, and expression of aromatase is highest in or near breast tumor sites. Thus, local regulation of aromatase by both endogenous factors as well as exogenous medicinal agents will influence the levels of estrogen available for breast cancer growth. The prostaglandin PGE(2) increases intracellular cAMP levels and stimulates estrogen biosynthesis, and our recent studies have shown a strong linear association between CYP19 expression and the sum of cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) expression in breast cancer specimens. Knowledge of the signaling pathways that regulate the expression and enzyme activity of aromatase and cyclooxygenases (COXs) in stromal and epithelial breast cells will aid in understanding the interrelationships of these two enzyme systems and potentially identify novel targets for regulation. The effects of epidermal growth factor (EGF), transforming growth factor-beta (TGFbeta), and tetradecanoyl phorbol acetate (TPA) on aromatase and COXs were studied in primary cultures of normal human adipose stromal cells and in cell cultures of normal immortalized human breast epithelial cells MCF-10F, estrogen-responsive human breast cancer cells MCF-7, and estrogen-unresponsive human breast cancer cells MDA-MB-231. Levels of the constitutive COX isozyme, COX-1, were not altered by the various treatments in the cell systems studied. In breast adenocarcinoma cells, EGF and TGFbeta did not alter COX-2 levels at 24h, while TPA induced COX-2 levels by 75% in MDA-MB-231 cells. EGF and TPA in MCF-7 cells significantly increased aromatase activity while TGFbeta did not. In contrast to MCF-7 cells, TGFbeta and TPA significantly increased activity in MDA-MB-231 cells, while only a modest increase with EGF was observed. Untreated normal adipose stromal cells exhibited high basal levels of COX-1 but low to undetectable levels of COX-2. A dramatic induction of COX-2 was observed in the adipose stromal cells by EGF, TGFbeta, and TPA. Aromatase enzyme activity in normal adipose stromal cells was significantly increased by EGF, TGFbeta and TPA after 24h of treatment. In summary, the results of this investigation on the effects of several paracrine and/or autocrine signaling pathways in the regulation of expression of aromatase, COX-1, and COX-2 in breast cells has identified more complex relationships. Overall, elevated levels of these factors in the breast cancer tissue microenvironment can result in increased aromatase activity (and subsequent increased estrogen biosynthesis) via autocrine mechanisms in breast epithelial cells and via paracrine mechanisms in breast stromal cells. Furthermore, increased secretion of prostaglandins such as PGE(2) from constitutive COX-1 and inducible COX-2 isozymes present in epithelial and stromal cell compartments will result in both autocrine and paracrine actions to increase aromatase expression in the tissues.