Nuclear localization of unoccupied receptors for glucocorticoids, estrogens, and progesterone in GH3 cells.

We have previously shown that the unoccupied receptor for estrogen is not present in enucleated cells (cytoplasts) and probably is nuclear in cells that have been enucleated using cytochalasin B and centrifugation. We now demonstrate, using enucleation without cytochalasin, that the unoccupied receptors for glucocorticoids and progesterone also appear to be nuclear, and that enucleation without using cytochalasin results in the same distribution of estrogen receptor as that seen when this drug is used. Enucleated cells (cytoplasts) contained only 5-10% of the concentration of unoccupied receptors found in the whole cells. The unoccupied receptors for all three hormones were recovered instead in the nucleus-containing cell fragments (nucleoplasts). The cytosolic marker lactate dehydrogenase was present in the cytoplasts at the same or higher concentration than in the whole cells or nucleoplasts. Cytoplasts were formed from approximately 90% of the cells. When the nucleoplasts were homogenized, the unoccupied receptors for all three hormones were extracted into the cytosol, as is usually seen in homogenized cells and tissues. These results are, therefore, consistent with the possibility that the unoccupied glucocorticoid and progesterone receptors, like the unoccupied estrogen receptor, are nuclear rather than cytoplasmic proteins. In addition, localization of these unfilled receptors does not appear to be an artifact of cytochalasin treatment.