A review of regulation of gene expression by steroid hormone receptors

Ubiquitination of nuclear receptors

Nuclear receptors (NRs) are cellular proteins, which upon ligand activation, act to exert regulatory control over transcription and subsequent expression. Organized via systemic classification into seven subfamilies, NRs partake in modulating a vast expanse of physiological functions essential for maintenance of life. NRs display particular characteristics towards ubiquitination, the process of addition of specific ubiquitin tags at appropriate locations. Orchestrated through groups of enzymes harboring a diverse array of specialized structural components, the ubiquitination process emphatically alters the fate or downstream effects of NRs. Such influence is especially prominent in transcriptional processes such as promoter clearing for optimization and degradation pathways eliminating or recycling targeted proteins. Ultimately, the ubiquitination of NRs carries significant implications in terms of generating pathological clinical manifestations. Increasing evidence from studies involving patients and disease models suggests a role for ubiquitinated NRs in virtually every organ system. This supports the broad repertoire of roles that NRs play in the body, including modulatory conductors, facilitators, responders to external agents, and critical constituents for pharmacological or biological interventions. This review aims to cover relevant background and mechanisms of NRs and ubiquitination, with a focus towards elucidating subsequent pathophysiology and therapeutics in clinical disorders encompassing such ubiquitinated NRs.

Peptide Binding Identifies an ERα Conformation That Generates Selective Activity in Multiple In Vitro Assays

Drugs such as tamoxifen, which act at the estrogen receptor (ER), have very different in vitro and in vivo effects from those of the native hormone. Previous research has established that different ligands induce distinct conformational changes in the ER, thus affecting the interactions of the receptor with cell-specific coactivating or corepressing proteins (cofactors) and estrogen response elements (EREs), thus potentially driving differing biological effects. Affinity-selected peptides have been used to probe the conformational changes that occur within the ER upon binding various ligands. In this study, the authors characterize the ability of several peptides to be recruited to liganded ER under cellular conditions. Approximating ER conformation via recruitment of this peptide to the ER is concluded to be a better predictor of the agonist nature of an ER ligand under these different cellular contexts than is a canonical cotransfection transactivation assay.

Voltage-clamp studies of the inhibition of gamma-aminobutyric acid response by glucocorticoids in bullfrog primary afferent neurons

Acute effects of glucocorticoids on the response to gamma-aminobutyric acid (GABA) were examined in primary afferent neurons in bullfrog spinal ganglia, using intracellular and voltage-clamp recording techniques. Prednisolone and hydrocortisone (5 microM to 1 mM) caused a dose-dependent decrease in the amplitude of GABA-induced depolarization, while having no effect on the membrane potential and resistance of the neuron. Prednisolone depressed the muscimol-induced depolarization. Nipecotic acid, a blocker of GABA uptake, did not influence the inhibitory action of prednisolone. Voltage-clamp analyses showed that the inward current induced by an iontophoretic application of GABA (GABA current) was suppressed by prednisolone and hydrocortisone. The depression of the GABA current is neither due to a blockage of open channels nor a facilitation of the desensitization of GABA receptors. Prednisolone shifted the dose-response curve of the GABA current downward. The double-reciprocal (Lineweaver-Burk) plot showed that the maximum GABA current was reduced by prednisolone, suggesting a non-competitive antagonism. These results suggest that glucocorticoids suppress the GABA-induced chloride current, decreasing the number of functional channels associated with GABAA receptor.

Immunological difference between ribonuclease and temperature, time and salt-induced forms of the estrogen receptor detected by a monoclonal antibody

The effect of RNAase A on the activation of the estrogen receptor from fetal guinea pig uterus was studied by DNA-cellulose binding assay and immunorecognition of the estradiol-receptor complex by the monoclonal antibody D547 raised against the human estrogen receptor. After RNAase treatment at 4 degrees C or 25 degrees C the binding of the receptor to DNA-cellulose doubled. This stimulation was partially prevented by sodium molybdate. RNAase treatment did not modify the interaction of the receptor with the monoclonal antibody D547; this antibody, as was demonstrated previously, selectively recognizes the activated form of the receptor when activation has been induced by temperature, time or high salt concentrations. In addition, RNAase had little or no effect on the transformation of the 8-9 S receptor to more slowly sedimenting forms under low salt concentrations. These observations suggest that even if RNAase induces receptor activation, which can be inferred from the increase in its binding to DNA-cellulose, the conformational modifications of the receptor molecule involved in this process are apparently different from those induced by factors such as temperature, time or high-salt concentrations.

A new triphenylethylene compound, Fc-1157a. I. Hormonal effects

The basic pharmacological and biochemical properties of a new antiestrogen, Fc-1157a, are described. Fc-1157a is bound specifically and with high affinity to estrogen receptors. The binding is competitive with estradiol. Fc-1157a treatment induces translocation of estrogen receptors from cytoplasm to nucleus. The turnover rate of nuclear estrogen receptors is markedly lower than with estradiol, but is more rapid than after tamoxifen. Fc-1157a is an almost pure antiestrogen in rat uterus, but has intrinsic estrogenic activity in mouse uterus. In animal experiments Fc-1157a has shown antitumor properties, which are described in the companion paper.

Immunorecognition of the active form of the oestrogen receptor by using a monoclonal antibody

In previous studies, two forms (alpha and beta) of the oestrogen receptor, with different immunological characteristics, were observed in the cytosol fraction of fetal guinea-pig uterus, by using a monoclonal antibody to the human oestrogen receptor (D547Sp gamma). Only the alpha form was recognized by the antibody, shifting its sedimentation coefficient in high-salt sucrose gradients. The present work investigated the effect of several factors (time, temperature, high salt concentrations and Na2MoO4) on the interconversion of these two forms. Only the beta form was observed when cytosol was incubated with oestradiol for only 2-3 h, but 20 h later, 40-60% of the total oestradiol-receptor complexes were found as the alpha form. The transformation from the beta to the alpha form was accelerated by temperature (25 degrees C, 15 min) and exposure to high salt concentrations (0.4 M-KCl). On the other hand, Na2MoO4 completely blocked the transformation induced by time and temperature, but had little effect on that induced by KCl. The appearance of the alpha form always correlated with an increase in receptor binding to nuclei and DNA-cellulose. Finally, it was found that the isolated beta form, recovered from the gradient, was transformed into the alpha form after overnight dialysis under reduced pressure. The present data suggest that the alpha form, which is recognized by the monoclonal antibody, is the activated form of the oestrogen receptor.

Binding of [3H]estradiol- and [3H]H1285-receptor complexes to rabbit uterine chromatin

In the present study we investigated the binding characteristics of estrogen and antiestrogen-receptor complexes to rabbit uterine chromatin. Activated or nonactivated estrogen receptors were partially purified by DEAE-cellulose chromatography using low (1 mM) or high (10 mM) concentrations of sodium molybdate. Activated [3H]estradiol-receptor complexes showed enhanced binding to chromatin acceptor sites unmasked by 1 M, 4 M and 6 M guanidine hydrochloride. We also examined the chromatin-binding characteristics of the estrogen receptors when bound by the high-affinity triphenylethylene antiestrogen, H1285. The acceptor site activity for the [3H]H1285-receptor complexes was markedly decreased at sites unmasked by 4 M and 6 M guanidine hydrochloride. Further, the nonactivated receptor complexes showed very low binding to deproteinized chromatin. The estrogen-receptor chromatin-acceptor sites were tissue specific and saturable. These chromatin acceptor sites differ in their affinity and capacity (number of binding sites per cell) for the estrogen- and antiestrogen-receptor complexes. Thus, we suggest that the differences in the physiological and physiochemical properties of estrogens and antiestrogens may be related to their differential interaction with uterine chromatin subfractions.

Studies on the mechanism of glucocorticoid hormone induced alterations in rat thymic transcription--I. Evidence from reconstituted cross-over transcription assays that sequential increases and decreases in transcription are due to changes in the activity of RNA polymerase II rather than in the activity of chromatin template

In experiments to determine the mechanism of glucocorticoid induced decreases in thymic transcription, adrenalectomized rats were injected with hydrocortisone (50 mg/kg) or vehicle. Thymic nuclei were used to prepare chromatins and soluble nuclear extracts containing RNA polymerase II for cross-over experiments. With calf thymus DNA or rat thymic chromatins as templates limiting RNA polymerase II from rats treated with hydrocortisone 3 h previously had 130% of the [3H]UMP incorporating activity of RNA polymerase II from control vehicle treated rats. In contrast, limiting RNA polymerase II from rats treated with hydrocortisone 12 h previously had 40-50% of the [3H]UMP incorporating activity of RNA polymerase II from controls. When limiting calf thymus DNA or rat thymic chromatins were used in 12 h cross-over experiments. Individual RNA polymerases II produced equal [3H]UMP incorporations, but RNA polymerase II activity from hydrocortisone treated rats was again only 50% of control values. Thus with template saturation, RNA polymerase II from hydrocortisone treated rats could not transcribe rat thymic chromatin templates to the level achieved by RNA polymerase II from control rats. This suggests that the activity, rather than the amount, of RNA polymerase II from hydrocortisone treated rats is reduced. Double reciprocal plots of [3H]UMP incorporation on rat chromatins with increasing concentrations of RNA polymerases II were made at 12 h. The apparent Km for RNA polymerase II from animals treated with hydrocortisone was identical to that of RNA polymerase II from controls, but the Vmax of RNA polymerase II from hydrocortisone treated animals was reduced. These data suggest the presence of an inhibitor of transcription or an RNA polymerase II defective in its capacity to initiate and/or elongate RNA transcripts. Further experiments demonstrated that these effects were not due to steroid induced changes in ribonuclease or protease activities.

Characterization and assay of progesterone-binding components in DMBA-induced rat mammary carcinoma tissue after progesterone administration

Nuclear and cytoplasmic progesterone-binding components were characterized and measured in DMBA-induced rat mammary carcinoma tissue, before and after progesterone administration. Rats, bearing growing tumors, were ovariectomized and then primed for two days with estradiol. Biopsy specimens were taken prior to or following administration of progesterone. Nuclear binding was assayed in the 0.4 M KCl extract of the nuclear fraction using [3H]R5020 as ligand. The receptor character of the binding was demonstrated by: (1) high affinity (Kd approximately 2 nM); (2) specificity: competition by R5020 and progesterone, minimal competition by 17 alpha-hydroxyprogesterone, corticosterone, testosterone and estradiol; (3) sedimentation constant at about 3S in a sucrose density gradient. Similar characteristics displayed the cytoplasmic receptor before and after progesterone administration. Progesterone receptor distribution in the nuclear extract and cytosol were determined in 36 tumors. The levels of total receptors (cytoplasmic plus nuclear) before and after progesterone administration varied widely, however the average values found after progesterone administration were significantly lower, 1.59 +/- 0.20 pmol/mg DNA compared to 2.58 +/- 0.32 pmol/mg DNA. Before progesterone administration only cytoplasmic receptors were found. One hour after progesterone administration a variable amount of the receptor (0-40%) was found in the nucleus of the tumorous tissue. In uteri of the same rats a uniform distribution of receptors (about 40% in the nucleus) was found after progesterone administration. A defect in the translocation process might be considered in the tumors with low receptors level, which suggests that DMBA-tumors may not respond uniformly to progesterone administration.

Initial characterisation of oestrogen receptors in canine mammary tumour lines maintained in nude mice

Proteins binding oestradiol with high affinity and low capacity have been characterised and quantitated in 4 newly established canine mammary tumour lines, MS306, V5500, F5010 and PD6014, maintained in nude mice. These receptor proteins, specific for oestrogens, were found in all the tumour lines, both in initial implants and all subsequent passages. Receptors had equilibrium dissociation constants for oestradiol in the range of 33-210 pM, and sedimentation coefficients of 4 S and 8 S.

Preferential solubilization of Triton X-100 resistant nuclear glucocorticoid receptors by deoxyribonuclease I

Nuclei, prepared by hypo-osmotic lysis from rat thymocytes that had been incubated at 37 degrees C with 20 nM [1,2,4-3H] triamcinolone acetonide, contained 2600-2800 stably and specifically bound steroid molecules per nucleus, 70-80% of which were retained after treatment with Triton X-100. DNAase I was more effective than DNAase II in solubilizing detergent-resistant glucocorticoid-receptor complexes from nuclei at both 0 degrees C and 37 degrees C; at 0 degrees C, DNAase I digestion released 50-55% of the bound steroid with only 10% digestion of DNA. These results imply that the glucocorticoid-receptor complex is preferentially associated with transcriptionally active chromatin.

Age-dependent changes in rat liver steroid hormone receptor proteins

The formation of the steroid hormone-receptor protein complex in rat liver cytosol was investigated as a function of rat age. As reported earlier there was no age-related change in the concentration of receptor proteins in rat liver cytosol. The affinity of the specific receptor protein for both dexamethasone and corticosterone, however, decreased with age. An age-related decrease was observed in the concentration of steroid hormones that were associated with the specific receptor protein responsible for translocation of the nucleus. A reciprocal age-dependent increase in non-specific protein binding of the hormone occurred.

The binding of 3H-labelled oestradiol- and progesterone-receptor complexes to hypothalamic chromatin of male and female sheep

Chromatin isolated from hypothalamic nuclei of sexually mature entire male and female sheep was linked to cellulose in u.v. light. The saturation binding of 3H-labelled oestrogen- and progesterone-receptor complexes, prepared by (NH4)2SO4 precipitation from the 105000g supernatant of hypothalamic cytosol, was then measured in vitro in 0.15m-KCl. Saturation binding was also measured after extraction of histones and masking acidic proteins. Salt + urea was observed to be more effective than guanidine hydrochloride in unmasking receptor acceptor sites, and the binding of labelled receptor complexes to dehistonized unmasked chromatin was shown to be largely resistant to 0.4m-KCl extraction. Whereas extents of receptor-complex binding were similar to published values for comparable preparations of hen oviduct chromatin, no sex-related difference was observed. However, binding of progesterone-receptor to chromatin was greater than that of oestradiol-receptor. Binding also increased more after removal of histones and masking acidic proteins, suggesting the presence of a greater number of progesterone-receptor acceptor sites in hypothalamic chromatin than of estradiol-receptor acceptor sites. The failure to demonstrate a sex-related difference in oestradiol-receptor binding to hypothalamic chromatin in vitro is discussed.

Inhibition of vitamin A action in rat bone cultures by inhibitors of RNA and protein synthesis

Using explanted humeri of late fetal rats, retinoic acid induced a dose- and time-dependent regression of cartilage. The retinoic acid-induced release of proteoglycan into the medium was blocked by inhibitors of RNA and protein synthesis, suggesting its dependence upon continuous RNA and protein synthesis.

Progesterone-regulated changes in transcriptional events in rabbit uterus

The role of steroid receptors in the early events of progesterone action was elucidated by examining the temporal relationship between the nuclear accumulation of progestin receptor and changes in activities of RNA polymerases I and II, as well as that of chromatin template in rabbit uterus. Following a 5-day estrogen pretreatment, the animals received an intravenous injection of progesterone (10 mg), after which they were killed at timed intervals. Nuclear progestin receptor level, as measured by an exchange assay, reached the peak value 30 min after hormone administration (11 600 to 46 600 sites/nucleus) and declined to the control levels by 4 h. Changes in the activities of RNA polymerase I and II did not follow identical time courses: polymerase I rose at 30 min and remained elevated for 2 h and then declined to about 75% of the pretreatment activity, whereas RNA polymerase II activity increased more rapidly (at 15 min), and was followed by a sharp decrease to about 50% of the initial value. Thereafter, the latter enzyme activity rose slowly and reached the pretreatment level within 12 h of progesterone administration. Early changes in chromatin template activity were similar to those in RNA polymerase I with a second rise by 8--10 h. The early inhibition of transcriptional events by progesterone may result from antiestrogenic properties of this steroid. Accumulation of nuclear progestin receptor occurs at a similar time to early changes in the transcriptional events suggesting a regulatory role for the hormone receptor complexes.