The normal breast epithelium of women with breast cancer displays an aberrant response to estradiol.

Breast epithelial response to estradiol may play an important role in breast cancer etiology. We have examined the relationship between serum estradiol and progesterone levels and normal breast epithelial expression of estrogen receptor (ER) alpha, progesterone receptor (PgR), and epithelial proliferation (as reflected by the Ki-67 labeling index) in 121 women (50 newly diagnosed breast cancer cases and 71 benign breast disease controls). Simultaneous samples of grossly normal breast tissue and venous blood were obtained from women undergoing breast surgery. Serum estradiol and progesterone levels were measured by radioimmunoassay; breast epithelial ER, PgR, and Ki-67 expression was measured by immunohistochemistry. Linear regression, controlled for patient age and ductal and lobular composition of the tissue, showed that the breast epithelium of control women displayed an inverse correlation between serum estradiol and ER-alpha, which was not seen in case women (P for the difference in regression slopes = 0.001). PgR expression displayed a significant positive correlation with serum estradiol in cases, but not in controls. Epithelial proliferation had no relationship to either estradiol or progesterone in both cases and controls but showed an inverse relationship with ER in controls and a direct relationship in cases (P for the difference in regression slopes = 0.066). These results suggest a dysregulation of hormonal response in the normal breast epithelium of high-risk women, with lack of regulation of ER by estradiol, increased estrogen responsiveness as reflected by PgR expression, and a dissociation of ER expression and proliferative response.