Nuclear Estrogen Receptor Release from Antiestrogen Suppression: Amplified Induction of Progesterone Receptor in MCF-7 Human Breast Cancer Cells*

Abstract

This study describes a mechanism by which antiestrogen-suppressed MCF-7 human breast cancer cells in long term culture are rescued by estradiol. In these cells, nafoxidine (1 μM) is a potent inhibitor of cell growth, and unlike estradiol, nafoxidine fails to induce progesterone receptors at any dose tested. If in suppressed cells, nafoxidine-containing medium is replaced by medium containing estradiol (10 nM), cell growth resumes (as measured by increases in total protein and DNA), and progesterone receptor is synthesized. The average rate of progesterone receptor induction in cells treated with estradiol after nafoxidine suppression is 7-fold greater than that of cells which were given estradiol after growth in hormone-free medium. The cells rescued by estradiol from antiestrogen suppression have a 3-fold greater progesterone receptor content at the end of the study than do those cells which received estradiol without nafoxidine pretreatment. The lag period normally seen before progesterone receptor levels start to rise is markedly shortened in rescued cells. During nafoxidine treatment, estrogen receptors translocate to the nucleus and remain at high steady state levels. Estradiol rescue is characterized by direct replacement of nafoxidine by estradiol on the nuclear receptor complex, with subsequent nuclear receptor processing to low steady state levels. Estradiol doses which fail to initiate receptor processing also fail to rescue cells from antiestrogen suppression. At no time are cytoplasmic receptors restored, yet cell growth and specific protein synthesis resume. Thus, the estradiol which becomes bound in the nucleus after nafoxidine treatment is biologically effective. Actinomycin D was used to study the nature of the nuclear acceptor sites of the estrogen and antiestrogen receptor complexes. The inhibitor permits exchange of estradiol for nafoxidine on nuclear receptors but inhibits subsequent estrogen receptor processing. Thus, the prior presence of nafoxidine on nuclear chromatin fails to protect the receptor-processing mechanism from inhibition by actinomycin. (Endocrinology108: 1703, 1981)