Unoccupied nuclear oestrogen receptors in the female rat hypothalamus. Increases on oestrogen administration.

A major proportion of the hypothalamic nuclear oestrogen receptors were available for complexing with radioactive oestradiol in vitro at 4 degrees C and were apparently unoccupied . At 6 h after oestradiol administration the content of unoccupied nuclear receptors had increased 2.5-fold and represented 71% of the total nuclear receptor content. These results suggest that unoccupied receptors may be active elements in the 'long-term' receptor population of the hypothalamus. Androgenized females had lower contents of these receptors.     

The relationship of the oestrogen and progestin receptors in the abnormal uterus of the adult anovulatory rat. Effects of neonatal treatment with testosterone propionate or clomiphene citrate.

The neonatal administration of testosterone propionate to Wistar rats resulted in anovulatory adults in persistent vaginal oestrus. Clomiphene citrate had a similar effect. In both groups of adults, hyperplasia of the uterine epithelium and occasional metaplasia was observed. The uterine nuclear and cytosol oestrogen and progestin receptors of these anovulatory rats were found to have affinities for their respective ligands similar to those of normal females. The nuclear oestrogen receptor comprised occupied and unoccupied components, as in normal females. The content of the nuclear oestrogen receptor was comparable with that of females in the late dioestrous or pro-oestrous phase. This content was higher in the clomiphene-treated group. Despite the relatively high nuclear oestrogen receptor content the content of progestin receptors, a putative index of the oestrogenic response, was lower in the treated rats than in normal adult females throughout the cycle. Administration of oestradiol to both treatment groups resulted in depletion of cytosol oestrogen receptor content 1 h later, which, however, was not reflected by an increase in the content of nuclear oestrogen receptors. There was no measurable increase in progesterone receptor content in treated rats after daily administration of oestrogen (5 microgram/rat) for 3 days. These changes in sex-hormone-receptor interactions involving an impairment of the normal oestrogenic response may be associated with the abnormal differentiation of the uterus in these sterile, anovulatory animals.     

A comparison of the relationships between progestin receptors and oestrogen receptors in neural and non-neural target tissues of the rat during the oestrous cycle.

Similar cyclic changes in the content of nuclear oestrogen receptor occurred in the hypothalamus, cerebral cortex, uterus and pituitary during the oestrous cycle. The relationship of the unoccupied to the total nuclear oestrogen receptor at each phase was similar in all these tissues. However, cyclic changes in the content of the cytosol progestin receptor occurred only in the uterus and pituitary (where they paralleled changes in the nuclear oestrogen receptor), but not in the hypothalamus or cerebral cortex.     

Unoccupied binding sites for oestrogen in nuclei of a breast tumour cell line (MCF-7).

1. A method to measure both occupied and unoccupied oestrogen receptors directly in the crude nuclear fraction of the MCF-7 cells was developed. The receptors had high affinity for oestradiol (Kd approx. 0.7 nM) and binding specificity characteristics of oestrogen receptors. 2. A substantial amount of the unoccupied receptors were found in the crude nuclear fraction. 3. Several experiments excluded the possibility that the unoccupied nuclear receptor might be a cytoplasmic contaminant. (a) Multiple extractions with Tris buffer released about 75% of the total receptor content, leaving the rest unextractable in the crude nuclear fraction. (b) Nuclei purified by centrifugation through 1.8M-sucrose and treatment with 0.7% Triton X-100, or by centrifugation through 50% glycerol with 0.1% Triton X-100 contained similar amounts of unoccupied receptors to that found in the crude nuclear fraction. (c) In cells cultured during 5 days after preconfluency a 3-fold increase in the amount of unoccupied cytoplasmic receptors occurred, whereas the amount of unoccupied nuclear receptors did not change significantly and conversely in cells exposed to increasing concentrations of oestradiol the unoccupied cytoplasmic receptor was continuously depleted but no considerable change in the unoccupied nuclear receptor was found.     

Intracellular relationships of the oestrogen receptor in the rat uterus and hypothalamus during the oestrous cycle.

Simultaneous measurements were made of the specific oestrogen receptor in the nuclear and cytosol fractions prepared from the uterus and hypothalamus of 50--81-day-old female rats undergoing a 4-day oestrous cycle. In the uterus, the content of nuclear receptor fluctuated in concert with known cyclic changes in the secretion of oestrogen, being maximal at pro-oestrus. Over the period of 50--81 days, the nuclear content at all phases increased with age, again corresponding to known age-related increases in ovarian secretion of oestrogen. This age-related increase in nuclear content, averaged from the values of the different phases in each age group, was related to equivalent increases in uterine wet weight, an increase of 1 pmol of receptor being accompanied by an increase of 80--90 mg. The concentration of cytosol receptor was maintained constant, with respect to wet weight, throughout the cycle and with age, irrespective of changes in nuclear content. In the uterus of normal mature females, translocation of receptor into the nucleus did not lead to depletion of cytosol receptor, suggesting a process of continuous replenishment/synthesis. In the hypothalamus, the nuclear content of oestrogen receptor was also maximal at pro-oestrus. In contrast with the uterus, the content of hypothalamic cytosol receptor was minimal at this phase and reflects depletion of the cytosol receptor, possibly as a result of translocation. The extent of translocation was low compared with that in the uterus and did not alter with age during the age-period studied. This low nuclear binding of the receptor in vivo is discussed in relation to the presence of a cytosol factor, present in limiting amounts, which in vitro mediates the binding of cytosol receptor to oligo(dT)-cellulose. The difference in the physiological response of the uterus and of the hypothalamus to oestrogens may be related to the extent of nuclear binding of receptor.     

Comparative effects of progesterone, norgestrel, norethisterone and tamoxifen on the abnormal uterus of the anovulatory rat.

Progesterone therapy results in partial reversibility of histological abnormalities of the rat uterus exposed to constant oestrogen stimulation and is associated with a decrease in nuclear oestrogen receptor content, which may underlie the tissue response to hormone treatment [White, Moore, Elder & Lim (1982) Biochem. J. 202, 535-41]. The synthetic progestins norgestrel and norethisterone used in this study were as effective as progesterone in decreasing the content of nuclear oestrogen receptor. However, only norgestrel had an ameliorative effect on epithelial hyperplasia and metaplasia. The non-steroidal anti-oestrogen tamoxifen caused a significant decrease in both nuclear and cytosol oestrogen receptor content without any change in luminal epithelial hyperplasia and metaplasia. Each progestin caused an increase, whereas tamoxifen caused a decrease, in the proportion of nuclear oestrogen receptors that were unoccupied. Each compound caused a decrease in the content of cytosol progesterone receptor. The effectiveness of compounds used as oestrogen antagonists is discussed with reference to their mode of action.     

Discrete early changes in cellular subpopulations of rat uterine and anterior pituitary estrogen receptors in response to acute exposure to exogenous estradiol

Distribution of estrogen receptors among ligand-occupied and unoccupied species in cytosolic and nuclear subcellular compartments has been analyzed as an acute response to administration of 5 micrograms of estradiol in adult female rats. Patterns of anterior pituitary and uterine receptor turnover were monitored at intervals over a 5-h period, using either intact or 2-weeks ovariectomized animals. In terms of total cellular receptor content, initial levels were higher in castrate animals, but rapidly fell to intact levels within an hour following estradiol injection. Cycloheximide given shortly before estradiol had no effect on total pituitary receptor patterns, but appeared to result in an elevation in total uterine receptor content at early intervals. Unoccupied cytosol receptors were rapidly depleted and, with the exception of castrate pituitary samples, showed some replenishment within 5 h, all of which was cycloheximide-sensitive. Initially, occupied cytosol receptors were low in intact rats, but were present at levels approaching those of the unoccupied cytosol receptor forms in the ovariectomized rat tissues. Occupied cytosol receptor levels fluctuated in response to estradiol. Subpopulations of nuclear receptors, especially the unoccupied species, showed significant tissue specificity. In the uterus, unoccupied nuclear forms were initially present in high amounts, and the levels did not change in response to estradiol administration. In the pituitary, the levels of these receptors rose and subsequently fell over the 5-h interval. Cycloheximide conferred a similar biphasic response to estradiol upon the otherwise insensitive unoccupied nuclear forms of the uterus. Occupied nuclear receptors turned over completely during the 5-h study interval, with the kinetics being faster in the castrate than the intact tissues. Cycloheximide affected occupied nuclear forms of the uterus only, dramatically increasing their levels in response to estrogen and causing prolonged retention in the castrate animal model. Collectively, the cycloheximide effects on this system are consistent with early estrogen induction or stimulation of a protein which inhibits accumulation of occupied or unoccupied receptor species within the nucleus. This re-examination of all forms of cellular estrogen receptors as they fluctuate acutely in response to exogenous estrogen has revealed several heretofore undetected responses which must be incorporated into the overall scheme of early estrogen action.     

Progesterone therapy results in partial reversibility of uterine abnormalities of the adult anovulatory rat.

The effects of progesterone therapy (5 mg, administered subcutaneously daily for 6 days) on the abnormal uterus of adult anovulatory Wistar rats have been studied. These rats, rendered anovulatory by neonatal treatment with testosterone propionate or clomiphene citrate, displayed severe hyperplasia and metaplasia of the uterine luminal epithelium and a disproportionately high content of nuclear oestrogen receptor, as a result of constant oestrogen stimulation unrelieved by progesterone [White, Moore, Elder & Lim (1981) Biochem. J. 196, 557-565]. Progesterone therapy resulted in the virtual elimination of the hyperplasia and metaplasia and a corresponding decrease in the content of nuclear oestrogen receptor with the proportion of the unoccupied nuclear receptor being increased to values exhibited by normal cyclic females. There was also a decrease in the content of progestin receptors, a putative index of oestrogenic stimulation. Further, in the testosterone-treated group, progesterone therapy resulted in the restoration of oestrogen receptor translocational responses to oestradiol stimulation. Progesterone treatment of these anovulatory rats thus provides a model system for investigating the biochemical mechanisms underlying progestin antagonism and regulation of oestrogen-stimulated cell proliferation.     

Developmental changes in the content of oestrogen receptors in the hypothalamus of the female rat.

Hypothalamic cytosol and nuclear oestrogen receptors are present at birth. A 2-fold increase in cytoplasmic receptor content occurs by the second week, whereas the first significant and equivalent increase in nuclear receptor occurs in the fourth week. The latter reflects reported increases in oestradiol availability thought to lead to complete feminine sexual differentiation. The presence of nuclear receptors in the newborn suggests a requirement for oestrogenic stimulation in early development.     

Evidence for the origin of the unoccupied oestrogen receptor in nuclei of a human breast-cancer cell line (MCF-7)

The origin of the unoccupied nuclear oestrogen receptor (R_n) was studied. Three working hypotheses were investigated. (a) R_n is a dissociation product of the oestrogen occupied nuclear receptor (ER_n). (b) ER_n is only partially occupied, so that additional binding may occur at 0°C (the temperature at which oestradiol saturates unoccupied sites). (c) R_n is derived from the penetration of unoccupied cytoplasmic receptor (R_c) into the nucleus. The MCF-7 cell line was used as a model in the present investigation. The amount of unoccupied receptors was measured by saturation with 7.5nm-[³H]oestradiol at 0°C, whereas the occupied receptors were measured by exchange at 30°C. The cells at preconfluency were exposed to a medium fortified with 10nm-[³H]oestradiol for 1h, washed and cultured up to 5 days in fresh growth medium. The distribution of oestradiol receptors was determined before exposure and during the following 5 days. After 1h exposure only ER_n was found in the nuclear fraction. Thereafter ER_n declined continuously so that on day 5 it approached 15% of its value measured 1h after exposure. Although after 3 days about 80% of ER_n disappeared no R_n appeared, which contradicts hypotheses (a) and (b). On day 4 R_n and R_c appeared simultaneously. The appearance of R_n and R_c was not prevented by culturing the cells in an oestrogen-free medium, supporting hypothesis (c). Exposure of cells to increasing concentration of [³H]oestradiol (0.1–10nm) for 1h resulted in a parallel increase in ER_n without increasing the amount of unoccupied binding sites, which contradicts hypothesis (b). The present study supports the hypothesis (c), i.e., R_c may also penetrate the nucleus without binding to oestradiol.     

The oestrogen receptor in the rat uterus in relation to intra-uterine devices and the oestrous cycle.

We investigated the binding characteristics, content and intracellular distribution of nuclear and cytosolic oestrogen receptors in the uteri of rats bearing a unilateral intrauterine device, fitted 14--18 days earlier, at four phases of a 5-day oestrous cycle. The patterns of changes in wet weight and content of cytosolic and nuclear receptor that normally occur during the oestrous cycle were not altered by the presence of the device. At all stages of the cycle the intra-uterine-device-containing horn had a greater wet weight and a correspondingly higher content of cytosolic receptor than its contralateral control horn, the cellular concentration of cytosolic receptor being apparently maintained. However, the intra-uterine-device-containing horn had significantly lower cellular concentrations (i.e. per mg of DNA) of nuclear receptor, particularly at late dioestrus and pro-oestrus. Thus the treated horn showed a decreased translocation of receptor in response to increases in circulating oestrogens. Both horns contained equivalent amounts of an activating factor implicated in translocation and measured in vitro by binding of cytosol receptor to oligo(dT)-cellulose. The presence of an intra-uterine device neither altered the dissociation constants (Kd) of the nuclear and cytosolic oestrogen-receptor complexes nor the stability of the nuclear receptor complex in vitro. The decreased translocation cannot thus be directly attributed to changes in the physical properties of the receptor. This decrease may be responsible for the anti-fertility effect of the intra-uterine device (which affects only the treated horn of the bicornuate rat uterus), since implantation of the blastocyst requires correct concentrations of nuclear oestrogen receptor.     

On the mechanism of opioid-oestradiol interactions

Characteristics of opioid binding and possible relationships between oestradiol and opioid binding sites were studied in rat oestrogen sensitive tissues(uterus, preoptic area-anterior hypothalamus, median eminence-basal hypothalamus). Naloxone (Nal) and oestradiol (Oe) bindings were assessed by in vitro saturation analyses. In 800 g supernatants of both uterine and hypothalamic tissues homogenates high affinity (Kd: 2-4 X 10(-9) M) and low capacity [3H]Nal binding sites were found. These binding sites were sedimented from 800 g supernatant by further centrifugation at 10(5) g for 1 h. In competition studies [3H]Nal binding was completely prevented by morphine, while met-enkephalin and leu-enkephalin caused only a partial inhibition. [3H]Nal binding was increased by ovariectomy and decreased by Oe treatment (10 micrograms/100 g b.wt) in both tissues. The cytoplasmic [3H]Oe binding in the studied tissues seems to be affected by the naloxone binding system. After in vitro saturation of naloxone binding sites by naloxone the [3H]Oe binding to low affinity sites (type II) in hypothalamus as well as in uterus has been increased by 8- and 2-fold, respectively. These results indicate the presence of specific [3H]Nal binding in rat uterus with similar properties to those found in the hypothalamus. Furthermore an interaction between opioid and oestradiol receptor systems could be also suggested.     

Oestradiol uptake and retention, and high-affinity binding sites in cultured rabbit uterus

Oestradiol uptake and turnover was examined in rabbit uterus maintained in organ culture for up to 3 days. Serum decreased the uptake of [(3)H]oestradiol, whereas insulin had no significant effect. During the first 24h of culture unoccupied high-affinity receptors for oestradiol were markedly depleted in the cytosol. Nuclear binding sites remained high during the first day of culture, and were still present after 3 days. The stability of nuclear-bound oestradiol was confirmed by examining the turnover of radioactivity during culture of uteri of rabbits injected with [(3)H]oestradiol 6h before death. Over half of the radioactivity was retained for as long as 3 days in tissue cultured in the absence of oestrogen. In tissue cultured for 24h with unlabelled oestrogen, there was a progressive increase in the displacement of [(3)H]oestradiol as the concentration of unlabelled hormone in the medium was increased from 0.1 to 5nm. Higher concentrations of oestradiol had little additional effect. The oestradiol involved in this displacement reaction was associated with macromolecules, characterized by Sephadex G-25 chromatography and sucrose-density-gradient ultracentrifugation of the 0.4m-KCl extract of the nuclear pellet.     

The effects of oestrogens and progesterone on oestrogen receptors in female rat liver.

The administration of oestradiol-17 beta or ethynyloestradiol as well as the synthetic progestogen norethisterone acetate resulted in translocation of the oestrogen receptor. Progesterone and the synthetic progestogen (+)-norgestrel were ineffective. The increases in nuclear oestrogen receptor content 1 h after injection of each steroid were similar but different subsequently. The increase with oestradiol-17 beta extended for 3--6 h and for at least 9 h with ethynyloestradiol. With norethisterone acetate, nuclear content was still increased after 24 h. Oestrogen injection resulted in cytosol receptor depletion and a 'deficit' in receptor content extending for 6 h, whereas norethisterone acetate-induced translocation was quantitative. With injections of norethisterone acetate + ethynyloestradiol the increase at 1 h and retention of the nuclear receptors were similar to that with norethisterone acetate alone. In contrast, the depletion of cytosol receptor and its restoration were similar to that seen with ethynyloestradiol alone, suggesting that norethisterone acetate did not interfere with the oestrogen receptor replenishment. Specific binding in vitro of [3H]oestradiol-17 beta in liver cytosols was inhibited by (+)-norgestrel and norethisterone acetate, but not progesterone, at concentrations of 10--100 microM. Nuclear receptors present after norethisterone acetate injection bound oestrogen with high affinity (Kd = 1.52 nM), similar to receptors of oestrogen-injected animals. In the uterus, differential retention of nuclear receptors in response to oestrogens is associated with different cellular responses. The differences in the response of the receptor system in liver to the various steroids suggests that the corresponding tissue responses may also be dissimilar. These results are discussed in relation to the problems of liver dysfunction in oral-contraceptive users.     

Effects of fasting on the distribution of cytoplasmic and nuclear oestrogen receptors in rat liver, uterus, pituitary and hypothalamus before and after exogenous oestrogen administration

The accumulation of oestrogen receptors in the liver cell nuclei of intact female rats 45 min after administration of 100 micrograms 17 alpha-ethynyloestradiol-17 beta i.p., decreased progressively during a 72-h fast from 2550 +/- 860 to 257 +/- 67 fmol/mg DNA, a level not significantly different from that in uninjected animals. Cytoplasmic oestrogen receptor concentrations also decreased, but only to about 60% of the original level (from 84.1 +/- 27.5 to 50.3 +/- 2.09 fmol/mg protein during the fast). Similar differences were found when these parameters were examined in normally fed and 72-h-fasted ovariectomized rats. On the other hand these parameters were unaffected in uterus, pituitary and hypothalamus. Uterine cytoplasmic receptor concentrations remained at about 500 fmol/mg protein during the fasting period, those in the pituitary and hypothalamus at about 230 and 30 fmol/mg protein, respectively. Nor was in vivo translocation in these organs affected by fasting. Regardless of nutritional status, the nuclear oestrogen receptor concentrations in uterus rose from about 500 to 2000 fmol/mg DNA after ethynyloestradiol administration, those in the pituitary and hypothalamus from approximately 250 to 2000 and from 250 to 500 fmol/mg DNA respectively.     

Effect of neonatal exposure to the antioestrogens nafoxidine and CI-628 upon the development of the uterus in the prepubertal rat

Neonatal Sprague-Dawley rats were injected with the antioestrogens nafoxidine or CI-628 on Day 3 of life alone or in combination with oestradiol benzoate 24 h later. Oestrogen-stimulated glucose oxidation and cytoplasmic oestrogen binding sites of the uteri were assessed at 21-23 days of age. Neither antioestrogen antagonized the prepubertal uterine impairments produced by neonatal oestradiol treatment. Both antioestrogens administered alone produced deficits which mimicked those produced by neonatal oestrogenization. However, the agonist property of each antioestrogen was differentially expressed: treatment with CI-628 reduced prepubertal oestrogen binding sites in the uterus, but nafoxidine exposure decreased the sensitivity of the uterus to oestradiol stimulation of glucose oxidation. It is postulated that CI-628 directly affects the uterus to reduce production of oestrogen receptor protein, while nafoxidine affects the development of the uterine phosphogluconate oxidative pathway indirectly through impaired ovarian function. However, antioestrogens blocked the neonatal oestradiol-induced reduction in the oestrogen-stimulated production of actomyosin in the adult uterus. Therefore, while both CI-628 and nafoxidine are clearly agonists in the neonatal rat, each appears to exhibit cell-specific agonist and antagonist properties.     

雌激素在兔胚泡着床中的作用——子宫雌激素受体的研究

为了验证雌激素作用存在于完整的和去卵巢兔胚泡着床过程中,本文用~3H—雌二醇交换法测定了早期妊娠 D_3、D_5、D_7子宫组织的雌二醇受体浓度,并与去卵巢后仅补充孕酮的 D_7子宫作比较。结果显示:在 D_3、D_5、D_7的子宫细胞浆和细胞核中都存在雌二醇受体。妊娠7天去卵巢兔的子宫细胞核中雌二醇受体量与妊娠7天卵巢完整的兔作比较,二者无明显差异。这说明去卵巢后子宫细胞核中雌激素受体浓度并未受影响,因此雌激素作用依然存在。关于胚泡是否有雌激素受体的问题,本工作在胚龄5、6、7天胚泡中未测得雌二醇受体。据此,胚泡着床时,雌激素的作用可能主要通过母体子宫组织而发挥其生理效应。     

New Concepts on the Action of Oestrogens in the Uterus and the Role of the Eosinophil Receptor System

Two receptor systems for oestrogens have been demonstrated in the uterus: the cytosol-nuclear receptor system and the eosinophil receptor system. It has been proposed that the cytosol-nuclear receptor system mediates the genomic response to oestrogens in the uterus, while the eosinophil receptor system is thought to mediate the uterine edema and other early oestrogenic responses in the uterus. Cortisol is known to decrease drastically the number of eosinophils in the blood and therefore to limit their availability for migration to the uterus. The present results show that cortisol also drastically reduces both the oestrogen-induced uterine eosinophilia and the uterine wet weight responses, but does not interfere with the oestrogen-induced uterine RNA and protein increases. Oestradiol-17 beta has a higher affinity than oestriol for the cytosol-nuclear receptors and is now found to be the more potent oestrogen in inducing the genomic activation in the uterus. Estriol has a higher affinity than oestradiol-17 beta for the eosinophil receptors, and therefore, oestriol is the stronger oestrogen in inducing those oestrogenic effects which are mediated by the eosinophil receptor system. We conclude that the eosinophil receptor system for oestrogens is a new system, independent of Jensen's cytosol-nuclear receptor system, and this eosinophil receptor system is involved in the mechanism of oestrogen action in the uterus.     

Rapid and sensitive detection of oestrogen receptors in cells and tissue sections by autoradiography with 125I-oestradiol

Summary The presence of receptors for steroid hormones in individual cells and tissue sections was assessed within 4–24 h using dry mount autoradiography with radio-iodinated oestradiol. Low affinity and nonspecific binding of steroids were significantly reduced by washing the cells or sections with diluted antiserum to oestradiol.For cells of the MCF-7 cell line variations in grain density were observed, indicating that cells of the MCF-7 cell line are heterogenous with respect to their cellular receptor concentrations of oestrogen receptors. Receptor-negative cells, such as peritoneal macrophages, did not retain oestradiol label.In tissue sections of rat and calf uterus, predominant labelling was observed on the endometrial gland cells and stroma.Oestradiol receptor binding in the uterus cytosol for both radio-iodinated and tritiated oestradiol showed the same qualitative characteristics as determined by sucrose gradient sedimentation profiles and a comparable amount of binding sites was found for both labels. The relative binding affinity of¹²⁵I-oestradiol compared to [³H]oestradiol is about 70–80%.The dry mount autoradiographic technique as presented can be used for rapid screening of heterogeneiety in oestrogen receptor distribution in cells and tissue sections, since this technique reveals differences in receptor concentrations on the single cell level.     

Regulation of uterine epidermal growth factor (EGF) receptors by estrogen in the mature rat and during the estrous cycle

Previous work has shown that the immature rat uterus contains epidermal growth factor (EGF) receptors and that tissue levels of this receptor are increased by the administration of exogenous estrogens. This study was undertaken to determine if estrogen administration also elevated EGF receptor levels in the mature animal and if the growth factor receptor levels varied in concert with endogenous estrogens throughout the estrous cycle. In the mature, castrate rat administration of estradiol, but not non-estrogenic steroids, causes a 2-3-fold elevation of uterine EGF receptors as judged by ligand binding. This increase is maximum in 18 h and is due to an increase in the number of binding sites. In cycling animals EGF receptor levels are low at metestrus, rise at diestrus, reach a maximum (approximately twice metestrus values) at proestrus, and then return at estrus to metestrus levels. These changes in EGF receptor levels parallel changes in plasma estrogens and occupied nuclear estrogen receptor reported by other workers. These results indicate that uterine EGF receptors are increased by exogenous estrogens in both mature and immature animals, and support a physiological role for estrogens in the regulation of this growth factor receptor.