Estrogen Receptor in Rat Liver: Translocation to the Nucleusin Vivo*

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Abstract
Following in vivo ethinyl estradiol (EE2) administration (100 .mu.g s.c.) to adult female rats, the estradiol-specific binding sites (ESBS) of liver cytosol were markedly reduced at 30 and 60 min. The reduction at 30 min was to 1/4 of that found in rats treated with vehicle alone. Coincident with this reduction, nuclear ESBS were increased. The ESBS of partially purified cytosol and of dense sucrose-purified nuclei were determined by gel filtration after incubations with 3H-estradiol using exchange assay conditions. An elevated temperature during the exchange assay incubations was necessary to demonstrate most of the ESBS in purified nuclei of EE2-treated rats and suggested that estrogens are attached at these sites. Following administration of 5 .mu.g EE2, the decrease in cytosol ESBS and the increase in nuclear ESBS were smaller. In contrast, 5 .mu.g EE2 was as effective as 100 .mu.g EE2 in substantially increasing the ESBS observed in uterine nuclear fractions. The low level of ESBS found in whole brain purified nuclei was unchanged by 100 .mu.g EE2 administration. The steroid specificity and proteolytic enzyme sensitivity of the purified nuclear ESBS of treated rats were similar to that of the partially purified cytosol ESBS of rats treated with vehicle alone. The data are consistent with the ESBS being estrogen receptor proteins which translocate from the liver cytosol to the nucleus after estrogen administration in vivo.