The distribution of binding sites for human chorionic gonadotropin (hCG) in the preovulatory follicle was studied by autoradiography. An ovulatory dose (10 IU/rat) of [125I]hCG (1.4 muCi/IU) was administered intravenously, and large Graafian follicles were isolated 3 h later by microdissection. Injection of excess unlabeled hCG (500 IU/rat) prevented uptake of radioactivity by the follicle, indicating that binding of iodinated hormone was confined to specific and saturable receptor sites. The density of bound hormone molecules was highest in the theca interna and in three to four layers of mural granulosa cells adjacent to the basement membrane; labeling was chiefly associated with the cell borders. No significant binding could be detected either on the oocyte or on the cumulus cells surrounding the oocyte. We therefore suggest that the induction of ovum maturation does not require attachment of the hormone to the oocyte itself or to follicle cells in its immediate vicinity.

According to the current model of steroid hormone action oestrogen is thought to bind to its receptor in the cytoplasm of target cells and the oestrogen-receptor complex is then translocated into the nucleus. This model is based on evidence obtained in homogenized cell preparations in which free receptor is associated with the cytosol, whereas steroid-bound receptor is associated with the nuclear fraction. Some data suggest, however, that the unfilled receptor may reside in the nucleus, and that cytosolic localization represents an extraction artefact. We have now reinvestigated the subcellular distribution of unfilled oestrogen receptor using cytochalasin B-induced enucleation to obtain cytoplast and nucleoplast fractions from receptor-containing GH3 cells derived from rat pituitary tumours. We found that cytoplasts prepared from GH3 cells contain little oestrogen-binding activity and that most of the unfilled oestrogen receptors are associated with the nuclear fraction. We therefore suggest that the standard model is in error and that the unoccupied receptor is nuclear in the intact cell.

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ABSTRACr Primary cultures of rat pituitary cells respond to estradiol-17# by increased incorporation of radiolabeled precursors into prolactin but not into the bulk of other cellular proteins. The rate of increase in prolactin synthesis is dose dependent, reaching maximal levels in the physiological ran e of estradiol. At a concentration of 1 nM, estradiol, diethylsti bestrol, and estriol are stimulatory whereas androgens, progesterone, and corticosterone are without significant effect. Exposure of pituitary cells to 10 nM estradiol resuled in a 500% increase in prolactin synthesis after 7 days of culture. The results indicate that estradiol can stimulate prolactin synthesis through direct action on the pituitary. There is an increasing body of evidence implicating estradiol in the regulation of prolactin synthesis and secretion. Recent work from our laboratory demonstrated that treatment of rats with estradiol specifically increased the incorporation of precursors into prolactin (1) Worthington) and 0.25% (wt/vol) Viokase (GIBCO). Cell viability was assessed by trypan blue exclusion (9). DNA content of the cells was estimated by the method of Burton (10). Cells were plated in Falcon flasks (2-4 X 105 cells per 25 cm2, 87-95% viable) that had been treated with 2 ml of 0.001% (wt/vol) poly(L-lysine) (Sigma) to increase plating efficiency. Culture medium consisted of Dulbecco's modified Eagle's medium buffered with 25 mM Hepes and supplemented with 10 1Ag of insulin (Sigma) per ml, 15% (vol/vol) horse serum, 2.5% (vol/ vol) fetal calf serum, 100 units of penicillin G per ml, 0.5 ,ug of amphotericin B (GIBCO) per ml, and 50 MAg of gentamycin (Schering) per ml. Sera were treated with dextran-coated charcoal (11) to remove free steroids. Steroid hormones were dissolved in ethanol and diluted in the medium so that the final ethanol concentration did not exceed 0.1%. Flasks were loosely capped and placed in a water-jacketed incubator at 370C in an atmosphere of 95% air/5% CO2. Media were changed on the third day (day 3) after plating (day 0) and every 2-3 days thereafter.Incubation with Radiolabeled Precursors. Flasks were rinsed with leucine-free Earle's minimal essential medium (GIBCO) and then incubated for 60 min at 370C in either leucine-free medium to which 5 ,Ci of ['4C]leucine was added per ml or in medium made 30,uM in leucine to which 10 MCi of [3H]leucine was added per ml. For analysis on gels, cells were removed and homogenized in 0.4 ml of 1% (wt/vol) sodium dodecyl sulfate (NaDodSO4)/4 M urea/50 mM Tris-HCl, pH 7.4 (Tris/NaDodSO4/urea), stored at -200C, and analyzed on 0.8 X 12 cm NaDodSO4/polyacrylamide gels as described (1).For immunoprecipitation, the cells were removed and homogenized in 0.4 ml of ice-cold 0.15 M NaCI/10 mM sodium phosphate, pH 7.4/10 mM leucine/1% Triton X-100/0.5% sodium deoxycholate (Pi/NaCI/Triton/deoxycholate) and stored at -20'C until assayed. Aliquots of medium (0.1 ml) were analyzed on 0.6 X S cm, 7.5% acrylamide gels as described (1).Immunoprecipitation. Cell homogenates ...

The severe growth retardation and sterility characteristic of Snell and Ames dwarfs, two nonallelic, recessive mouse mutants, have been attributed to a deficiency in pituitary production of GH and PRL. We have investigated the synthesis of these hormones in normal and homozygous dwarf mice of both strains at various stage of development to determine whether the mutations prevent initial development of the pituitary capacity to produce these hormones. Synthesis of radiolabeled GH and PRL was assayed by two-dimensional electrophoresis and immunoprecipitation using goat antisera to mouse GH and PRL. Litters in which all pups were mutant were produced by mating adult dwarfs made fertile by implanting a normal pituitary under the kidney capsule. We found that GH and PRL synthesis was undetectable in Snell or Ames dwarfs at all stages of development examined, including the neonatal period when dwarf pups are indistinguishable from normal littermates. Furthermore, we detected no immunoprecipitable peptides which might represent mutant hormones in these animals. Assays of GH synthetic rates in heterozygous animals indicated that there may be a slight negative effect of the mutant gene on GH synthesis in Snell animals. It was concluded that in the homozygous condition, both types of dwarf alleles result in failure of the pituitary to initiate GH and PRL synthesis.

The last 50 years has seen an exponential rise in the published reports about estrogen action. The model to describe the early events in the mechanism of action of estrogens via the estrogen receptor is updated in this paper to incorporate some of the recent data on the subcellular localization of the receptor. New evidence suggests that the receptor is a nuclear protein, so it appears that estrogens must first diffuse into the nuclear compartment to initiate estrogen action via the receptor complex. This review traces the development of potent estrogenic compounds by the study of their structure-activity relationships. Studies of structure-activity relationships in vivo using Allen Doisy or 3-day uterine weight tests can provide much valuable information, but the assays suffer from the complex problems of pharmacokinetics and metabolic transformation. Studies in vitro using primary cultures of rat pituitary or uterine cells to assay the ability of a compound to induce prolactin synthesis or progesterone receptor synthesis, respectively, can provide essential information about the structural requirements for a compound to produce estrogenic effects. Nevertheless, it should be pointed out that studies in vivo are required to determine whether a compound is metabolically activated to an estrogen. Estrogen receptor binding models are presented to describe the changes in a molecule that will predict high affinity for the ligand and agonist, partial agonist and antagonist properties of the ligand-receptor complex. Most estrogenic pesticides and phytoestrogens comform to the predictions of the estrogen receptor binding model.

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