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T G Muldoon

Publications and source records attributed to T G Muldoon.

At least 19 recordsLinked to original sources
PubMed1992

The metabolic pathways for hormonal steroids appear to be reflected in the stereochemistry of DNA.

Computer graphics and energy calculations were employed to examine the relative fit of progesterone and its major biosynthetic precursors and inactive metabolites into partially unwound double stranded DNA. Progesterone was found to be the best fitting molecule; moreover, it was the only compound which exhibited full stereochemical complementarity by inserting completely between base pairs and forming optimal hydrogen bonds with both deoxyribose-phosphate backbones. Intermediates in each step of the biosynthetic and degradation pathways were progressively increasing and decreasing fits into DNA, respectively. When the fits of various possible stereoisomers were examined, the positions of functional groups manifest in the known biosynthetic precursors were found to provide the best possible fitting structures. Conversely, the positions of functional groups of known inactive metabolites provided the worst possible fitting structures. These findings coupled with previous reports showing that the specific biological function assigned to each class of steroid hormone correlates with the formation of a unique pattern of donor/acceptor linkages confirms that hormonal structures are indeed rare in their capacity to form "lock and key" complexes with DNA. Given that all possible linkages to DNA are not yet accounted for, the existence of other naturally occurring compounds with salient biological function is predicted.

Computer Simulation↗
PubMed1991

Effects of steroidal and non-steroidal antiandrogens on the androgen binding properties of the rat ventral prostate androgen receptor.

Steroidal (cyproterone acetate) and non-steroidal (RU23908 and hydroxyflutamide) antiandrogens are able to block testosterone-induced increases in nuclear androgen receptor (AR) in the prostate of 1-day orchidectomized rats, but when given alone, RU23908 and hydroxyflutamide increase nuclear AR (RU23908 greater than hydroxyflutamide) in the same animal model. The increases in nuclear AR induced by antiandrogen alone or with testosterone alone are blocked by cycloheximide 1 h after administration, suggesting that androgen or antiandrogens induce de novo AR synthesis. Concomitant to nuclear AR accumulation, testosterone is able to induce depletion of cytosol and microsomal AR. Blockade of testosterone-induced depletion of microsomal AR, but not of cytosol AR, occurs in the presence of antiandrogens. Cyproterone acetate has a higher relative binding affinity (RBA) for microsomal AR and cytosol AR than RU23908 or hydroxyflutamide. This phenomenon is in good agreement with the degree of inhibition by these compounds of the association rate of androgen for the microsomal AR. This correlation between RBA and inhibition of the initial rate of hormone binding to the receptor is not found for cytosol AR. The results show that antiandrogens are not 'pure' antagonists of androgen action and they are potent agonists in the absence of testosterone. Furthermore, testosterone alone or antiandrogens per se regulate AR levels acutely by protein-synthesis dependent mechanisms of action, in rat ventral prostate.

Androgen Antagonists↗
PubMed1991

Characterization of estrogen-binding sites associated with the endoplasmic reticulum of rat uterus.

Microsomes prepared from rat uterine homogenates harbor high-affinity (Ka = 10(10) M-1), low-capacity binding sites for estrogens. Previous work from our laboratory has demonstrated that these estrophiles are located on endoplasmic reticulum and are not cytosolic contaminants of the membrane preparation. Subfractionation of microsomes into granular and agranular membranes and polysomes revealed approximately equal distribution of estrogen-binding activity among each of these constituents. These binding sites were fully extractable with 0.6 M KCl. Microsomal estrophiles solubilized under conditions of low ionic strength and complexed with estradiol migrated as 8S forms on continuous sucrose gradients. In the presence of 0.4 M KCl, the solubilized binding sites exhibit a sedimentation coefficient of 4S. Extracted binding sites do not undergo heat-induced transformation from a 4S to 5S species. The monoclonal antibody JS34/32 interacted with the endoplasmic reticulum-associated estrogen-binding sites when present in 50-fold molar excess, but not at lower antibody to binding site ratios. In comparison, the rat uterine cytosolic estrogen receptor formed complexes with JS34/32 at antibody to receptor ratios as low as 2:1. These results suggest that the endoplasmic reticulum possesses estrogen-binding sites with biochemical properties that differ from those of the classically described cytosolic (loosely associated nuclear) estrogen receptor.

Animals↗
PubMed1991

Role of microsomal receptors in steroid hormone action.

Ventral prostate was used as a system to study the nature and properties of microsomal androgen receptor. The endoplasmic reticulum from rat ventral prostate contains high-affinity, low-capacity binding sites for androgen that are intrinsic to this intracellular compartment. Microsomal androgen receptors are not due to plasma membrane or cytosol contamination and they display a fast turnover, with depletion after 1 hour and complete replenishment 6 hours after androgen stimuli. Cycloheximide, but not actinomycin D, inhibits microsomal androgen receptor replenishment, indicating that testosterone may control microsomal receptor levels acutely by posttranscriptional mechanisms. Microsomal androgen receptor is a 5S protein that has a higher stability than its cytosolic counterpart, regardless of the presence of ligand. It does not become activated after heat or salt treatment. After extraction of binding sites, microsomes are capable of accepting cytosol mibolerone-receptor complexes to a level similar to the concentration of depleted binding sites; microsomes from nontarget tissues do not manifest such capability. The results indicate the coexistence of a non-DNA-binding form of androgen receptor in the microsomal membranes with the typical DNA-binding form of androgen receptor present in the cytosol of ventral prostate homogenates. Microsomal androgen receptor may represent an additional level of regulation of androgen action in the intact target cell.

Animals↗
PubMed1991

3-phenylacetylamino-2,6-piperidinedione inhibition of rat Nb2 lymphoma cell mitogenesis.

3-Phenylacetylamino-2,6-piperidinedione (A10), an amino acid analog, has been reported to possess antineoplastic activity against certain neoplastic tissues. The antimitogenic properties of A10 were studied by determining its effect on prolactin (PRL)- and interleukin 2 (IL-2)-stimulated mitogenic responses in the rat Nb2 lymphoma cell line. The addition of A10 (1-12 mM) to PRL (0.4 ng/ml)-stimulated cells inhibited growth in a dose-dependent manner. DNA synthesis patterns studied by thymidine incorporation demonstrated that A10 was significantly inhibitory (25% at 20 hr; 50% at 40 hr, P less than 0.01). IL-2 stimulation of mitogenesis was also sensitive to A10 inhibition. The inhibition of PRL stimulated mitogenesis was reversible when A10 was removed after 24 hr of culture and A10 showed no toxicity in a chromium release assay. These data suggest that A10 effects may be cytostatic, rather than cytotoxic.

Animals↗
PubMed1990

Specific binding of androgen and androgen-receptor complex by microsomes from rat ventral prostate.

Microsomes from rat ventral prostate show the presence of a high affinity-low capacity population of androgen-binding sites with affinity for ionic exchange resin similar to that of cytosol androgen receptor (AR), as manifested by similar results obtained with hydroxylapatite. The affinity for mibolerone was similar for both forms (Ka = 0.5-2.9 x 10(10) M-1). The membrane-bound form can be extracted in hypotonic buffer, with retention of binding properties. Isotonic sucrose allowed higher degree of extractability of the microsomal AR than 10% (v/v) glycerol. The presence of hormone lends stability to the microsomal AR, while high salt or nonionic detergents have a deleterious effect on their longevity. The microsomal receptor form is not sensitive to serine-proteases as opposed to the cytosol AR. After exhaustive extraction of binding sites, microsomes are capable of accepting cytosol mibolerone-receptor complexes to a level corresponding to the concentration of depleted binding sites; microsomes from non-target tissue do not manifest such capability. Microsomal AR complexes do not bind DNA and they are not activated after heat treatment. Mixed preparations of extracted microsomal complexes with cytosol complexes showed heat-induced increased ability to bind DNA to the same level of diluted cytosol complex alone, indicating the absence of a microsomal inhibitor of DNA binding. The results indicate the co-existence of a non-DNA binding form of the AR in the microsomal membranes with the classical DNA binding form of the AR present in the cytosol of ventral prostate homogenates.

Animals↗
PubMed1990

Role of 17 beta-hydroxysteroid dehydrogenase in the modulation of nuclear estradiol receptor binding by progesterone in the rat anterior pituitary gland and the uterus.

Progesterone has been shown to decrease occupied pituitary and uterine nuclear estradiol receptor (E2R) binding in mature and immature estrogen-primed rats. Progesterone has also been shown to stimulate pituitary but not uterine 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) in the rat. The conversion of estradiol to its less active metabolite estrone by 17 beta-HSD and activation of phosphatase are among mechanisms considered to be involved in the reduction of E2R. To determine if 17 beta-HSD stimulation was a mechanism by which progesterone induced nuclear E2R decrease, the synthetic estrogen ethinylestradiol, which is not oxidized by 17 beta-HSD, was used instead of estradiol to prime adult ovariectomized rats. When ethinylestradiol-primed rats received 0.8, 2.0 or 4.0 mg/kg body wt of progesterone 2 h before sacrifice, the total and occupied nuclear E2R accumulation in the anterior pituitary by a subsequent ethinylestradiol injection 1 h later did not show any decrease. This response was different from that observed previously in estradiol-primed animals in which progesterone showed a multiphasic decrease of occupied form of nuclear E2R. However, in the uterus of ethinylestradiol-primed rats, a partial decrease of total and occupied nuclear E2R accumulation was observed in the presence of the three doses of progesterone used. The decrease of uterine nuclear E2R with the three progesterone doses was different from the dose-dependent effect of progesterone observed in the uterus of estradiol-primed rats. Affinity constants of the interaction between [3H]estradiol and the nuclear E2R were similar among groups treated with ethinylestradiol, estradiol and progesterone. These results demonstrate the involvement of 17 beta-HSD in the reduction of anterior pituitary gland E2R by progesterone in the estradiol-treated animals. Furthermore, the mechanism of decrease of E2R by progesterone in the uterus appears to be different from the pituitary gland.

17-Hydroxysteroid Dehydrogenases↗
PubMed1990

Association of androgen binding sites with the endoplasmic reticulum of rat ventral prostate.

Microsomes from ventral prostate of 24-h castrated rats contain a single set of tissue-specific high-affinity, low-capacity androgen binding sites. These sites are indigenous to the endoplasmic reticulum, as shown by purification procedures associated with marker enzymes and electron microscopic analyses. When prostatic microsomal membranes are separated from plasma membranes using the nuclear or the mitochondrial pellets as the source of fractionation in sucrose gradients, the androgen binding activity is selectively associated with fractions rich in rough endoplasmic reticulum and ribosomes. Eighty-four percent of the total content of Na+/K+ adenosine triphosphatase (ATPase) and only 27% of the total binding capacity were concentrated in fractions rich in smooth-surfaced vesicular membranes, when nuclear suspensions constituted the membrane source. In contrast, the region of the same gradient when enriched in rough endoplasmic reticulum and deficient in plasma membrane content contained 73% of the androgen-binding capacity and only 14% of the ATPase. For fractions collected using mitochondrial suspensions as starting material, the ratio (total glucose-6-phosphatase/total binding capacity) was closer to 1.0 than similar ratios of ATPase/binding capacity, indicating co-sedimentation of binding sites with microsomal membranes and not with plasma membranes. Na+/K+ ATPase, but not 5' nucleotidase, is a valid plasma membrane marker for ventral prostate. Microsomal androgen receptors may constitute a new level of regulation of androgen action in target cells.

Androgens↗
PubMed1990

Inhibitory effect of 5-alpha-dihydroprogesterone on nuclear estrogen receptor binding of the anterior pituitary and uterus in the rat.

Progesterone and its metabolite 5 alpha-dihydroprogesterone (5 alpha-DHP) have been shown to bring about gonadotropin release in the estrogen-primed ovariectomized rat. One of the actions of progesterone is the decrease in occupied estrogen receptors (E2Rs) in the anterior pituitary and uterus. This study attempted to determine if 5 alpha-DHP had a similar effect on pituitary and uterine E2Rs. Estrogen-primed ovariectomized mature female rats were injected with either vehicle, or 0.2, 0.8 or 2 mg/kg body weight (BW) of 5 alpha-DHP, 2 h before sacrifice and 1 h before the last estradiol injection (2 micrograms/rat). Nuclear E2Rs were determined by Scatchard analyses in the uterus and anterior pituitary. Total nuclear E2R levels of both tissues showed a 2-fold increase in the number of estradiol-binding sites after estradiol administration, as compared to control groups. In estradiol-primed rats, 5 alpha-DHP produced a significant decrease in total nuclear E2R levels in a tissue-specific manner. In the pituitary, there was a maximal and significant decrease in nuclear E2Rs with 0.2 and 2.0 mg/kg BW of 5 alpha-DHP as compared to estradiol alone; the intermediate dose of 0.8 mg/kg BW of 5 alpha-DHP induced a smaller nonsignificant change in nuclear E2Rs. In the uterus, 5 alpha-DHP showed a dose-dependent decrease in nuclear E2Rs. The 5 alpha-DHP effect in both tissues was due to a specific reduction in the occupied form of nuclear E2Rs levels. The unoccupied form of E2Rs was unaffected by 5 alpha-DHP administration. 5 alpha-DHP did not have any effect in the absence of estrogen priming.(ABSTRACT TRUNCATED AT 250 WORDS)

5-alpha-Dihydroprogesterone↗
PubMed1989

Effect of progesterone on the activity of occupied nuclear estrogen receptor in vitro.

In previous studies, we have demonstrated that progesterone administration in vivo can selectively alter estrogen receptor levels and distribution in the rat anterior pituitary. The present study represents an attempt to extend these observations to an in vitro model. Cytosolic and nuclear preparations of uterine homogenates from ovariectomized adult rats were shown to be capable of temperature-dependent estrogen-mediated receptor activation and translocation from cytosol to nuclei upon recombination. Addition of progesterone to isolated cytosol did not diminish estrogen receptor binding capacity over at least a 2 h period at 22 degrees C. Preincubation of the subcellular fractions with progesterone, followed by removal of free progesterone prior to cytosol-nuclear recombination, resulted in dramatic reduction in nuclear estrogen receptor activity. This action was equally apparent whether progesterone was introduced to the cytosolic or nuclear fraction, and was confined to the steroid-occupied subpopulation of nuclear receptors. The ability of this in vitro system to mimic the estrogen receptor-suppressive effect of progesterone provides a good model in which to analyze the biochemical basis for a direct estrogen-inhibitory effect of progesterone on estrogen action.

Animals↗
PubMed1989

Relative binding properties of microsomal and cytosolic androgen receptor species of the ventral prostate.

A comparison between a microsomal and a cytosolic source of receptor-like androgen-binding activity was made in the rat ventral prostate. Microsomal binders remain in solution at 90% saturation with (NH4)2SO4 and display equal affinity for 5 alpha-dihydrotestosterone (DHT) and mibolerone. They have a half-life of dissociation of steroid-receptor complexes of 96 +/- 11 h and a 5S sedimentation coefficient for the untransformed moiety, with the appearance of a 3.5S species after incubation at 24 C for 30 min. They do not acquire DNA-binding capability after heat- or salt-attempted activation. Cytosol binders precipitate upon 90% saturation with (NH4)SO4 and have higher affinity for DHT than mibolerone. The half-life for dissociation of complexes is 85 +/- 14 h, and the complex sediments as an 8S moiety which is able to transform to a 4.4S form after heat activation correlated with increased DNA-binding ability of these species. Unactivated cytosol steroid-receptor complexes are also able to bind to DNA in the presence of molybdate. Salt-induced activation of cytosol moieties only occurred in the absence of molybdate. Microsomal androgen receptor is more stable than cytosol androgen receptor independently of the presence of hormone or partial purification of the moieties; the inactivation rates of the two forms of complexes differ 3-fold. Results indicate that androgen-binding sites associated with the microsomal and cytosolic fractions of the prostate are distinct entities.

Animals↗
PubMed1988

Microsomal receptor for steroid hormones: functional implications for nuclear activity.

Target tissues for steroid hormones are responsive by virtue of and to the extent of their content of functional intracellular receptors. Recent years have seen a shift in considerations of the cellular dynamics and distribution of these receptors, with current views favoring predominant intranuclear localization in the intact cell. This paper summarizes our analyses of the microsomal estrogen and androgen binding capability of rat uterine and ventral prostate tissue, respectively; these studies have revealed a set of high affinity sites that may act as a conduit for estrogen traversing the cell en route to the nucleus. These sites have many properties in common with cytosolic receptors, with the salient difference of a failure to activate to a more avid DNA-binding form under conditions which permit such activation of cytosolic receptors. The microsomal estrogen-binding proteins also have appreciable affinity for progesterone, another distinction from other known cellular estrogen receptor species. Various experimental approaches were employed to demonstrate that the microsomal receptors were not simply cytosol contaminants; the most convincing evidence is the recent successful separation of the cytosolic and microsomal forms by differential ammonium sulfate precipitation. Discrete subfractionation of subcellular components on successive sucrose gradients, with simultaneous assessments of binding capability and marker enzyme concentrations, indicates that the major portion of the binding is localized within the vesicles of the endoplasmic reticulum free of significant plasma membrane contamination. The microsomal receptors are readily solubilized by extraction with high- or low-salt-containing buffers or with steroid. The residual microsomes following such extraction have the characteristics of saturable acceptor sites for cytosolic estrogen-receptor complexes. The extent to which these sites will accept the cytosolic complexes is equal to the concentration of microsomal binding sites extracted. These observations suggest three possible roles for the microsomal receptor-like proteins: (a) modulation of estrogen access to nuclear binding sites; (b) formation of functional complexes which diffuse to other extranuclear sites to alter non-genomic cellular processes; (c) regulation of nuclear concentration of estrogen-receptor complexes by virtue of producing microsomal acceptor sites for uptake of free or loosely associated nuclear complexes, previously thought to exist in the cytoplasm.

Animals↗
PubMed1988

Actions of an endogenous antitumorigenic agent on mammary tumor development and modeling analysis of its capacity for interacting with DNA.

Antineoplaston A10 (3-phenylacetylamino-2,6-piperidinedione) is an agent derived from human urine which is remarkable for its antineoplastic activity and lack of toxicity. This study deals with the discovery of a hormonal component to the action of A10 on rodent tumor formation and approaches to the delineation of its mechanism of action. Oral administration of A10 dramatically delays onset of spontaneous mammary tumor incidence in female and orchidectomized male C3H+ mice. The action in the male qualitatively mimics that of androgens. In the rat, A10 ingestion is very effective in preventing carcinogen-induced mammary tumors, but does not cause regression of pre-established tumors. It selectively blocks the occurrence of the estrogen-sensitive subpopulation of tumors, acting in a fashion completely analogous to that of tamoxifen; in contrast to tamoxifen, however, A10 has no measurable affinity for the estrogen receptor. Modeling studies demonstrate a stereoselective capacity for interaction between A10 and specific deoxyribonucleotide base pair sequences. The potential affinity is lower than that for classical intercalating antitumor agents, and the stereospecificity differs from that which we have previously established for estrogens. We offer an interpretation of the data in terms of direct effect of A10 at the genomic level to alter the cellular responsiveness to steroid hormones.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1988

Hormones and their receptors.

Hormone action at the cellular level begins with binding of the hormone to highly specific protein receptor molecules. Detailed studies of this complex interaction have placed mechanistic aspects of hormone action on a firm biochemical basis and permitted development of a number of receptor-based diagnostic and prognostic tests that are now in routine clinical use. This overview summarizes present understanding of this rapidly developing area and relates it to specific disease processes.

Animals↗
PubMed1987

Receptor-weighted mechanistic approach to analysis of the actions of estrogen and progesterone on gonadotropin secretion.

This presentation has touched upon three experimental designs which we have used to examine the single premise that analysis of steroid hormone receptor activity is a viable approach to an increased understanding of steroid-gonadotropin feedback mechanisms and reproductive function. In the study of LH suppressibility in the castrate animal, we observed strikingly similar patterns of receptor turnover and LH suppression-recovery. By discriminate choice of stimulating and blocking agents which had differential access to hypothalamic sites of action, we accumulated data suggesting that the inhibitory effects of estradiol on LH secretion are transient at the pituitary level and of longer duration in the hypothalamus. This was based on the different responses to alterations in nuclear receptor retention time in the two tissues. This initial analysis of the negative segment of the feedback will need to be extended to an animal model in which positive feedback comes into play before a full picture of the receptor correlations can be drawn. The ability of LHRH to selectively stimulate estrogen receptor activity in the nucleus using isolated pituitary cells provides a mechanistic explanation for the self-priming action of LHRH. By enhancing receptor activity, LHRH increases the responsiveness of the pituitary to estrogen. This in turn intensifies the sensitivity to LHRH, and a secondary LHRH stimulus will elicit a magnified response. The ability to demonstrate selective augmentation of the functional matrix-associated receptor population, and our recent results showing that gonadotropes are indeed the responsive cells (Singh P, Muldoon TG, unpublished observations) speak to the specificity and relevance of these findings. The equal ability of LHRH to reduce nuclear estrogen receptor binding in whole cells and in isolated nuclei indicates that internalized LHRH may be capable of recognizing secondary intracellular receptors, and the system is amenable to exploration of this phenomenon. Using a combination of estrogen receptor and progesterone receptor measurements, we have begun to unravel the interrelationships between these two hormones at this level apropos of their interplay in gonadotropin regulation. The kinetics of progesterone receptor induction by estrogen are consistent with an inability of the system to respond acutely to progesterone in the absence of strong estrogen backing, but with the capability of inhibiting long-term responses to estrogen. The acute capability of progesterone, acting through its receptors, to depress nuclear estrogen receptor activity, correlates with its acute antiestrogenic stimulation of gonadotropin secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Integration of the effects of estradiol and progesterone in the modulation of gonadotropin secretion.

Estradiol secreted by the maturing follicle is the primary trigger for the surge of gonadotropins leading to ovulation. Progesterone has stimulatory or inhibitory actions on this estrogen-induced gonadotropin surge depending upon the time and dose of administration. The administration of progesterone to immature ovariectomized rats primed with a low dose of estradiol induced a well-defined LH surge and prolonged FSH release, a pattern similar to the proestrus surge of gonadotropins. A physiological role of progesterone is indicated in the normal ovulatory process because a single injection of the progesterone antagonist RU 486 on the day of proestrus in the adult cycling rat and on the day of the gonadotropin surge in the pregnant mare's serum gonadotropin stimulated immature rat resulted in an attenuated gonadotropin surge and reduced the number of ova per ovulating rat. Progesterone administration brought about a rapid LHRH release and an decrease in nuclear accumulation of estrogen receptors in the anterior pituitary but not the hypothalamus. The progesterone effect was demonstrated in vitro in the uterus and anterior pituitary and appears to be confined to occupied estradiol nuclear receptors. In in vivo experiments the progesterone effect on estradiol nuclear receptors appeared to be of approximately 2-h duration, which coincided with the time period of progesterone nuclear receptor accumulation after a single injection of progesterone. During the period of progesterone effects on nuclear estrogen receptors, the ability of estrogens to induce progesterone receptors was impaired. Based on the above results, a model is proposed for the stimulatory and inhibitory effects of progesterone on gonadotropin secretion.

Animals↗
PubMed1987

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.

Animals↗