Estradiol as an anti-aromatase agent in human breast cancer cells

Abstract

Estradiol (E 2) is an important risk factor in the development and progression of breast cancer. However, a “direct effect” of E 2 in breast cancerization has not yet been demonstrated. The estrogen receptor complex can mediate the activation of oncogens, proto-oncogens, nuclear proteins and other target genes that can be involved in the transformation of normal to cancerous cells. Breast cancer cells possess all the enzymes (sulfatase, aromatase, 17β-hydroxysteroid dehydrogenase (17β-HSD)) necessary for the local bioformation of E 2. In the last years, many studies have shown that treatment of breast cancer patients using anti-aromatase agents has beneficial therapeutic effects. The aromatase activity is very low in most breast cancer cells but was significantly increased in a hormone-dependent breast cancer cell line: the MCF-7aro, using the aromatase cDNA transfection and G-418 (neomycin) selection. In the present study, we explore the effect of E 2 on the aromatase activity of this cell line. The MCF-7aro cell line was a gift from Dr. S. Chen (Beckman Research Institute, Duarte, U.S.A.). For experiments the cells were stripped of endogenous steroids and incubated with physiological concentrations of [ 3H]-testosterone (5 × 10 −9 mol/l) alone or in the presence of E 2 (5 × 10 −5, 5 × 10 −7 and 5 × 10 −9 mol/l) for 24 h at 37 °C. The cellular radioactivity uptake was determined in the ethanolic supernatant and the DNA content in the remaining pellet. [ 3H]-E 2, [ 3H]-estrone ([ 3H]-E 1) and [ 3H]-testosterone were characterized by thin layer chromatography and quantified using the corresponding standard. It was observed that [ 3H]-testosterone is converted mainly into [ 3H]-E 2 and not to E 1, which suggests very low or absence of oxidative 17β-HSD (type 2) activity in these experimental conditions. The aromatase activity, corresponding to the conversion of [ 3H]-testosterone to [ 3H]-E 2 after 24 h, is relatively high, since the concentration of E 2 was 2.74 ± 0.11 pmol/mg DNA in the non-treated cells. E 2 inhibits this conversion by 77, 57 and 21%, respectively, at the concentrations of 5 × 10 −5, 5 × 10 −7 and 5 × 10 −9 mol. In previous studies, it was demonstrated that E 2 exerts a potent anti-sulfatase activity in the MCF-7 and T-47D breast cancer cells. The present data show that E 2 can also block the aromatase activity. The dual inhibition of the aromatase and sulfatase activities, two crucial enzymes for the biosynthesis of E 2 by E 2 itself in breast cancer add interesting and attractive information for the use of estrogen therapeutic treatments.