Estrogen receptor. Unoccupied sites in nuclei of a breast tumor cell line.

Abstract

An estrogen-responsive human breast cancer cell line (MCF-7) derived from a pleural effusion was used. This cell line contains a large amount of nuclear receptor not occupied with estrogen. In these experiments it was shown that unoccupied nuclear receptor (Rn) has most of the characteristics attributed to estrogen receptor that it can bind estradiol directly to become estrogen-occupied nuclear receptor and that it is not an artifact of tissue preparation. MCF-7 cells were grown in Falcon plastic flasks or in glass roller bottles in 5% carbon dioxide in air at 37 degrees C. Growth medium was Earles-basal minimal essential medium with supplements. Prior to homogenization cells were 90-95% viable. Cytosol and nuclear extracts were prepared. Details of methods used are described. In the MCF-7 cells a nuclear estrogen receptor with high affinity and limited capacity binding for estradiol was found. Rn comprised about 75% of the cells total population of estrogen-binding sites. Data suggested that (tritiated)-estradiol was bound directly to unoccupied nuclear sites. Studies have shown that MCF-7 cells were not dependent on estrogen for growth but were stimulated in its presence. Rn could be inducing cell growth and division even in the absence of estrogens. The increased stimulation with estradiol would then reflect the increment of additional unoccupied cytoplasmic receptor translocated to the nuclei from the cytoplasm.