Estrogen Receptor in Rat Liver: Translocation to the Nucleus in Isolated Parenchymal Cells*

Isolated, purified, viable parenchymal cells from adult female rat liver were characterized for their estrogen receptor distribution. The isolated cells contained cytosol-binding sites specific for [3H]estradiol ([3H]E2; determined after partial purification) at a level near that found in cytosol prepared from whole liver. The properties of these binding sites closely resembled those of putative estrogen receptors found in cytosol prepared from liver or uterus in adult female rats. The estrogen receptor distribution was then studied after exposure of the cells to ethinyl estradiol (EE2; the commonly used estrogen in oral contraceptives). After in vitro incubation with EE2, cytosol and nuclear estrogen receptors were determined by exchange assay with [([3H]E2. Cytosol receptor levels were rapidly depleted, while receptor levels in highly purified nuclei were rapidly increased. The cytosol. of cells not exposed to EE2 and nuclei of cells exposed to EE2 for 15 min contained binding sites for [([3H]E2 with similar steroid specificities during exchange with [([3H]E2. Both cytosol and nuclear binding sites could be precipitated with protamine sulfate or hydroxylapatite. The half-maximal doses for cytosol receptor depletion and nuclear receptor accumulation were 1 × 10-7 and 5 × 10-7 M EE2, respectively. The levels of occupied receptor calculated by exchange assay at the time of maximal nuclear receptor occupation compared well to the values obtained after incubating [([3H]E2 with the hepatocytes. The data are consistent with the binding sites for [3H]estrogen being estrogen receptors which translocate from cytoplasm to nucleus in isolated liver parenchymal cells. An estrogen of oral contraceptives, EE2 is capable of attaching to and promoting translocation of the liver parenchymal cell estrogen receptor.