After nuclear translocation of estrogen receptors in MCF-7 human breast cancer cells, a processing takes place resulting in a 30-70% decline in the number of estradiol-binding sites measured in nuclear extracts. We have investigated the mechanism of estrogen receptor processing and obtained evidence that multiple events are involved. We confirm, as others have shown previously, that processing involves a decrease in the amount of estradiol binding in MCF-7 cells. In addition, evidence is provided for the generation of a rapidly dissociating population of estradiol-binding sites as an early event in processing. There is a single, slowly dissociating population of estrogen binding sites when MCF-7 cells are exposed to estradiol in the presence of actinomycin D, an inhibitor of receptor processing. One hour after the addition of sufficient estradiol to induce receptor processing, an additional, more rapidly dissociating population of estrogen binding sites is detected. When cells are exposed to estradiol and ethidium bromide, a drug which shares many actions with actinomycin D, but does not inhibit receptor processing, the rapidly dissociating population of estradiol-binding sites is again observed. Significantly, the loss of estradiol-binding sites from MCF-7 cells associated with processing between 1 and 6 h of estradiol exposure, occurs exclusively from the rapidly dissociating population of sites. Whole cell equilibrium-binding assays were performed with MCF-7 cells after 30 min or 5 h of estradiol exposure to determine whether the detected changes in estradiol dissociation reflected affinity changes in a subpopulation of estrogen-binding sites. Although the number of sites detected per cell varied with the assay method employed, binding to a single saturable class of higher affinity sites is always observed. High affinity estradiol-binding sites were reduced by 45% after a 5-h incubation with estradiol in both assay methods. The loss of estradiol binding during processing may therefore be explained by the conversion of certain high affinity estrogen receptors to a rapidly dissociating form which then fails to rebind hormone, or undergoes subsequent reactions that destroy hormone binding activity. Additionally, after 6 h of exposure to estradiol, the remaining receptor-bound estradiol dissociates from intact cells with a rate increased by 50% over that seen from the slow dissociating receptors present at 1 h. Therefore, during receptor processing the estradiol-receptor complexes which remain, or components interacting with these receptors, have also been altered. We conclude that processing of estrogen receptors in MCF-7 cells is more complex than previously believed, and involves both kinetic changes in receptor properties and loss of binding capacity.
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