The Dynamics of Intracellular Estrogen Receptor Regulation as Influenced by 17β-Estradiol1

Estrogen receptor turnover within the cells of responsive tissue has been investigated as a function of the circulating level of 17β-estradiol. In the anterior pituitary, hypothalamus and uterus of the ovariectomized rat, depletion of cytoplasmic receptors by translocation to the nucleus occurs within 1 h of injection of 10 μg of 17β-estradiol and subsequent replenishment of these receptors can be blocked completely by cycloheximide administration concomitant with the estrogen. The cycloheximide block of early replenishment is partial if the drug is given 30 min after 17β3-estradiol and is completely absent if given 1 h after estrogen. Actinomycin D treatment 1 h prior to 17β-estradiol inhibits replenishment in the anterior pituitary and uterus, but not in the hypothalamus. Analysis of tissue distribution of [3H] Actinomycmn D indicates that the drug has very limited accessibility to the hypothalamus. Although cytoplasmic estrogen receptor number/ mg of cytosol protein is relatively constant in animals of different endocrine states, the magnitude of the cytosol receptor replenishment response varies directly with the endogenous hormonal levels in both male and female rats. Thus, intact animals replenish more than castrate animals, which in turn replenish more than do castrate-adrenalectomized rats, although significant amounts of replenishment are still demonstrable even in the latter group. Cycloheximide inhibition of estrogen-induced replenishment in intact animals is less pronounced than in castrates and replacement of estrogen to the castrate animals decreases the cycloheximide sensitivity to a level similar to that of the intact animals. Estrogen-induced cytoplasmic receptor replenishment can be delayed by a second 17β-estradiol injection, but a replenishment phase does ultimately occur. Subcellular distribution of radiolabeled estrogen in uterine cells, as a function of time following injection of 17β-estradiol, differs between ovariectomized and estrogen-primed ovariectomized animals. In the castrate rats, binding of estrogen traces the following route: cytoplasm → nucleus → microsomes → nucleus. In the primed castrate animals, the path of movement is: cytoplasm → nucleus → (microsomes) → cytoplasm. Microsomes extracted with 0.4M KC1 3 h after exposure to [3H]17β-estradiol show binding in the 4S region of low-salt sucrose gradients, with little or no radioactivity in the 7S region. Microsomes extracted 5 H after estrogen have saturable binding components in both the 4S and 7S regions. A model is presented in which possible interpretations of the role of estrogen in regulation of receptor dynamics are summarized, in an attempt to clarify the contributions of synthesis, regeneration and reutilization to the overall process of receptor depletion and replenishment.