Subcellular distribution of estrogen receptor and progesterone receptor with and without specific ligand

The estrogen receptor (ER) and progesterone receptor (PR) content of cultured human breast carcinoma cells (MCF-7) was determined by biochemical assay, immunoblot analysis, and immunohistochemical assay under varying conditions of hormonal stimulation. The ER and PR content in cytosolic and nuclear extracts varied with steroid treatment. However, both the amount and distribution of each receptor in these extracts was virtually the same when determined by steroid binding and immunoblot analyses. Two immunocytochemical parameters (staining intensity and proportion of cells stained) correlated with the quantitative analyses of ER and PR, but not with the subcellular distribution. When MCF-7 cells were grown for 4 days in charcoal-stripped serum without phenol red, 93% of total ER was found in the cytosol (10 mM KCl), whereas short-term treatment with 5 nM estradiol resulted in the appearance of 82% of total ER in the nuclear extract (400 mM KCl). With either cell treatment only nuclear staining for ER was observed. Progesterone receptor was virtually undetectable in the same cells by any method. After 4 days of treatment by 5 nM estradiol, PR was strongly induced (50-fold) in MCF-7 cells as determined by all three methods. As observed for ER, 95% of total induced PR was found in the cytosol in the absence of a progestin. Short-term treatment with 5 nM ORG 2058, a synthetic progestin, resulted in the appearance of 42% of total PR in the nuclear extract. However, only strong nuclear staining for PR was observed in either the presence or absence of a progestin. These findings are consistent with the current view of ER and PR as nuclear receptors present in at least two forms. One of these, the unoccupied form of the receptor, is easily removed from the nucleus by hypotonic buffers during the cell homogenization process and appears in the cytosolic extract. The other form of the receptor, the steroid-occupied form, is more tightly bound to nuclear components and is removed from nuclei only under more vigorous extraction conditions.