17α-Estradiol Is a Biologically Active Estrogen in Human Breast Cancer Cells in Tissue Culture*

Abstract

The biological effects of 17α-estradiol (17α-E) and its interaction with estrogen receptors were studied in the MCF-7 human breast cancer cell line. Competition for [³H]17β- estradiol ([³H]17β-E) binding shows that 17α-E binds to receptor with high affinity and has a dissociation constant (K(i) estimated to be 0.7 nM. Upon binding with 17α-E, the cytosol receptor is translocated to the nucleus and is then rapidly depleted or processed in the same manner as the 17β-E-receptor complex. The nuclear 17α-E-receptor complex was determined to be biologically active by its ability to stimulate an increase in the progesterone receptor content and to reverse antiestrogen inhibition of cellular proliferation and DNA polymerase activity. The estrogenic potency of 17α-E, estimated from the dose-response curves as the median effective dose in stimulating progesterone receptor content and in reversing antiestrogen inhibition, is about one tenth the potency of 17β-E. Competition curves show that 17α-E binds to the cytosol estrogen receptor with about one third the affinity of 17β-E, so the correlation between relative binding affinity and biological potency is not perfect. The correlation, however, is reasonably good compared with that in animal studies, in which 17α-E displays negligible biological activity. Gas chromatography and mass spectrometry rule out the possibility that the observed estrogenic activity of 17α-E was due to contamination of the preparation with the more active 17β-E. The enhanced estrogenic potency of 17a-E in MCF-7 cells raises the question of whether the stereospecificity and, thus, the sensitivity of the estrogen receptors in breast cancer cells may be different than those in normal target tissues. Enhanced estrogenic activity may also be due simply to the nature of the tissue culture system, which allows continuous exposure of the cells to hormone; a condition which may not be achieved in some animal studies.