Preferential nuclear binding of estrogen in the formalin-fixed rat uterus

Characterization of rat uterine estrogen receptors in vivo

In vivo binding of [3H]estradiol ([3H]E2) in the rat uterus was performed by an intraluminal perfusion of the ligand for different time periods. In this way the binding takes place in the intact organ before processing the tissue. In 10 min, with 10 nM [3H]E2 apparent saturation or steady state incorporation of the [3H]E2 was achieved with a similar distribution of the label between cytosol and nuclear fractions. In vitro, the subcellular localization of the estrogen receptor (ER) is influenced by the extent of tissue damage. With the intact organ the ER subcellular distribution approaches that of the in vivo perfusion. With increasing [3H]E2 in the perfusate it was possible to obtain a "saturation" curve and to derive the kinetic parameters. For cytosol: Kd 16 nM; Bmax 235 fmol/mg prot. For nucleus: Kd 2.7 nM; Bmax 103 fmol/mg prot. To follow the time course of the ER movement in vivo, "pulse and wait" experiments were designed. Both uterine horns were perfused for 1 min. One of the horns was immediately processed (0 time) and the other was left in place after the perfusion for different periods. At 0 time 90% of the bound label appeared in the cytosol. At 5, 15 and 30 min, the label in the cytosol decreased and that of the nucleus increased approx. to 50%. Thus, translocation of the bound label from cytosol to nucleus was apparent. The role of the cytoplasm-nucleus ER traffic in the regulation of gene transcription by estrogens is discussed.