In Vitro Nuclear Translocation of the Estrogen Receptor in the Hepatic Parenchymal Cell from Male Rats

Isolated, purified, viable parenchymal cells from adult male rat liver were characterized for the distribution of the estrogen receptor. Demonstration of the cytosol estrogen receptor required its partial purification by fractionation with ammonium sulfate. Crude cytosol from the isolated cells contained high levels of the recently described, male-specific, nonreceptor, steroid binding protein. The estrogen receptor found in the partially purified cytosol fraction of parenchymal cells was at a level near that found in similar preparations from whole liver. The properties of these partially purified cytosol estrogen receptors closely resembled those of cytosol estrogen receptors from whole liver of male or female rats, or from rat uterus. The distribution of the two classes of cytosol binding sites for estrogens was then studied after exposure of the cells to ethinyl estradiol. After in vitro incubation with ethinyl estradiol, cytosol and nuclear estrogen receptors were determined by exchange assay with [ 3 H]estradiol. Cytosol estrogen receptor levels were rapidly diminished, while receptor levels in highly purified nuclei were rapidly increased. In contrast, levels of the nonreceptor, male-specific sex steroid binder in cytosol were not diminished following exposure of the cells to ethinyl estradiol. There was a similar steroid specificity (during exchange with [ 3 H]estradiol) of cytosol estrogen receptors of cells not exposed to estrogen and of nuclear estrogen receptors of cells exposed to ethinyl estradiol for 30 min. The levels of occupied receptors calculated by exchange assay at the time of maximal occupation of the nuclear receptors compared well with the value obtained after incubating [ 3 H]ethinyl estradiol with the cells. The data are consistent with estrogen receptor translocation from the cytoplasmic to nuclear compartment in the isolated parenchymal cell from adult male rat liver. Several differences were found comparing this translocation process in hepatic parenchymal cells from male rats to results obtained previously using this cell type from female rats. In male liver cells, the onset of translocation was slower, the nuclear retention of the receptor longer, and the half maximal concentration of ethinyl estradiol required for nuclear translocation approximately two-fold higher than in female liver cells.