Regulation of estrogen receptor-alpha gene expression by epidermal growth factor

The role of epidermal growth factor (EGF) in the regulation of estrogen rec eptor-alpha (ER-alpha) gene expression in the human breast cancer cell line MCF-7 was investigated. Treatment of cells with 0.4 ng/ml EGF resulted in an approximately 60% decrease in ER-alpha protein concentration by 6 h and the amount of receptor remained suppressed for 24 h. Ligand binding assays demonstrated that the decrease in ER-alpha protein corresponded to a simila r decrease (approximately 50%) in estradiol binding sites. Although EGF tre atment resulted in a decrease in the number of binding sites, it had no eff ect on the binding affinity of ER-alpha. The dissociation constant of the e stradiol-ER-alpha complex in the presence or absence of EGF was the same (K -d = 2.3 x 10(-10) M in control cells versus K-d = 1.98 x 10(-10) M in EGF- treated cells). The decrease in ER-alpha protein concentration paralleled a decrease in the steady-state amount of ER-alpha mRNA. By 9 h there was an approximately 60% decrease in ER-alpha mRNA. The amount of ER-alpha mRNA re mained suppressed for 48 h. Transcription run-on experiments demonstrated t hat there was a decrease of approximately 70% in ER-alpha gene transcriptio n upon EGF treatment, suggesting that the mechanism by which EGF regulates ER-alpha gene expression is transcriptional. In addition to regulating the amount of ER-alpha, EGF affected the activity of the receptor. At high conc entrations, EGF induced progesterone receptor. Estradiol and high concentra tions of EGF had an additive effect on progesterone receptor. In contrast t o high concentrations, low concentrations of EGF had no effect on progester one receptor and blocked estradiol induction. The effects of EGF on ER-alph a expression were inhibited by tyrphostins and wortmannin, suggesting that the effects of the growth factor are mediated by the EGF receptor and prote in kinase B. When the cells were placed in serum-free medium and then treat ed with EGF, there was no effect on ER-alpha protein concentration or activ ity. However, increasing concentrations of serum restored the effects of EG F on ER-alpha, suggesting that an additional serum factor was required for the EGF-mediated effect on the decrease in ER-alpha protein concentration.