Steroid receptor recycling and its possible role in the modulation of steroid hormone action

Urban Renewal in the Nucleus: Is Protein Turnover by Proteasomes Absolutely Required for Nuclear Receptor-Regulated Transcription?

The importance of the ubiquitin proteasome pathway in higher eukaryotes has been well established in cell cycle regulation, signal transduction, and cell differentiation, but has only recently been linked to nuclear hormone receptor-regulated gene transcription. Characterization of a number of ubiquitin proteasome pathway enzymes as coactivators and observations that several nuclear receptors are ubiquitinated and degraded in the course of their nuclear activities provide evidence that ubiquitin proteasome-mediated protein degradation plays an integral role in eukaryotic transcription. In addition to receptors, studies have revealed that coactivators are ubiquitinated and degraded via the proteasome. The notion that the ubiquitin proteasome pathway is involved in gene transcription is further strengthened by the fact that ubiquitin proteasome pathway enzymes are recruited to the promoters of target genes and that proteasome-dependent degradation of nuclear receptors is required for efficient transcriptional activity. These findings suggest that protein degradation is coupled with nuclear receptor coactivation activity. It is possible that the ubiquitin proteasome pathway modulates transcription by promoting remodeling and turnover of the nuclear receptor-transcription complex. In this review, we discus the possible role of the ubiquitin proteasome pathway in nuclear hormone receptor-regulated gene transcription.

Steroid hormones and temperature induce changes of binding parameters of their receptors in intact cells

When MCF-7 cells were treated with 17 beta-estradiol, dexamethasone, or promegestone at 37 degrees C, the KD of receptors for their cognate ligands was found to decrease as compared to that measured at 2 degrees C. Cell incubation with hormone at 37 degrees C did not affect the Bmax of glucocorticoid and progesterone receptors, but caused a 40% increase of that of estrogen receptor. This increase required the presence of ligand, was insensitive to cycloheximide, and was completed within 10 min of cell incubation at physiological temperature. We conclude that an early step of estrogen action is the recruitment of pre-existing receptor molecules through activation of their binding capacity.

Receptor-mediated methylprednisolone pharmacodynamics in rats: Steroid-induced receptor down-regulation

Several approaches to receptor down-regulation were examined to extend previous receptor/gene-mediated pharmacokinetic/dynamic models of corticosteroids. Down-regulation of the glucocorticoid receptor was considered as an instantaneous event or as a gradual steroid-receptor-mediated process. Concentrations of plasma methylprednisolone, free hepatic cytosolic receptors, and the activity of hepatic tyrosine aminotransferase (TAT) enzyme were measured for 16 hr following administration of 0, 10, and 50 mg/kg methylprednisolone sodium succinate to 93 adrenalectomized rats. Receptor down-regulation was best described by a fractional decrement in the rate of return of free cytosolic glucocorticoid receptor. Predicted values for free receptor, bound receptor, nuclear bound receptor, and transfer compartments were in accord with the expected rank order values based on the high and low steroid doses. Model parameter estimates were independent of dose and described the rapid depletion of free cytosolic receptor, late-phase return of cytosolic receptor to a new baseline level that was 20-40% lower than control, and the TAT induction/dissipation pattern following steroid dosing. The microscopic association and dissociation constants describing the steroid-receptor interaction were 0.23 L/nmole per hr (k(on)) and 4.74 hr-1 (k(off)) for methylprednisolone compared to previously obtained values of 0.20 L/nmole per hr and 15.7 hr-1 for the related steroid prednisolone. The time course of TAT induction was similar to that observed previously for prednisolone. Efficiency of TAT induction was more closely related to steroid receptor occupancy than plasma methylprednisolone concentrations due to receptor saturability and receptor recycling.

Stress-induced changes of glucocorticoid receptor in rat liver

The effect of corticosterone injection and of acute and repeated stress on rat liver cytosol glucocorticoid receptor was studied to ascertain whether corticosterone-induced glucocorticoid receptor (GR) regulation also takes place in intact animals as it does in adrenalectomized ones. Adult male rats were exposed to six different stressors (swimming, 10 mg/kg histamine i.p., 500 mU/kg vasopressin s.c., heat, immobilization and cold) acutely or three times daily for 18 days (repeated stress). Each of the stressors applied acutely provoked a pronounced increase of plasma corticosterone with subsequent induction of hepatic tyrosine aminotransferase activity. Depletion of cytosol receptor was however only noticed after swimming and histamine injection. On the other hand, sustained hypersecretion of corticosterone evoked by repeated stress significantly reduced the number of GR in rat liver cytosol without any change in Kd. It is concluded that in the presence of intact adrenal glands cytosol receptors are more resistant to corticosterone-induced depletion than in their absence. Further, repeated stress causes down-regulation of GR in the liver, most probably by sustained corticosterone secretion, yet the effect of other stress factors cannot be excluded.

Rapid homologous up-regulation of binding capacity of androgen receptors in intact cells

Incubation of MCF 7 cells with 5 alpha-dihydrotestosterone (DHT) at 37 degrees C led to a 70% increase in the Bmax of androgen receptor, as compared to the values measured at 2 degrees C, without detectable changes in equilibrium dissociation constants. When MCF 7 cells were incubated with hormone at 2 degrees C, to reach steady-state levels of androgen-receptor complex, a subsequent temperature shift to 37 degrees C induced a rapid (t 1/2 = 3 min) cycloheximide-insensitive increase in DHT binding to androgen receptor. MCF 7 cell treatments at 37 degrees C either before or after incubation with DHT at 2 degrees C showed that up-regulation of binding capacity of androgen receptor could be observed only if hormone is present during incubation at physiological temperature.

Expression of estrogen receptor and its messenger ribonucleic acid in the MCF-7 cell line: multiparametric analysis of its processing and regulation by estrogen

Experimentally, a portion of the detectable cellular estrogen receptor (ER) is seen to disappear in human breast cancer cells submitted to estradiol treatment. In this study, we have applied several detection methods to analyze the loss (processing) then the replenishment of ER in the MCF-7 cell line. Radioligand exchange assay and enzyme immunoassay revealed an accumulation of ER in the nuclei with a concomitant depletion in cytosol shortly after the addition of estradiol in cell culture. Then, a time-dependent decrease of ER level in the nuclear compartment without rescue in the cytosol was observed. When an immunocytochemical assay was performed on whole cells treated with estradiol, a similar decrease of ER number was shown, indicating that a decrease in the extractability of estradiol-filled ER was not involved in the processing. Analysis of ER mRNA also indicated that the estrogen treatment induces a time-dependent decrease of its expression. Measurement of [35S]methionine-labeled ER following the arrest of the hormone treatment suggested that ER replenishment was due to newly synthesized receptors. Sucrose gradient experiments confirmed the generation of small molecular forms of ER, following its binding with estradiol. All these data are indicative of estrogen-receptor complex degradation. We also confirm that estrogen regulates ER level through the decrease of its mRNA expression.

Stabilization of glucocorticoid-receptor interactions in vitro by removal of RNA bound to receptor complexes in vivo

The dissociation of the steroid from glucocorticoid-receptor-RNA complexes at 5 degrees C was evaluated in cytosolic and nuclear extracts prepared from Hela cells crosslinked in vivo with glutaraldehyde. Sample treatment with catalytically active RNase A prevented the dissociation of the steroid which was induced by sample dilution with buffer. Dilution of the extracts with boiled cytosol, instead, stabilized steroid-receptor interactions. We conclude that some heat-stable factor should be also associated with glucocorticoid-receptor-RNA complexes from crosslinked cells, stabilizing steroid-receptor interactions, and we propose that it could counteract the labilizing effect of RNA.

Regulation of epidermal growth factor-receptor by estrogen and antiestrogen in the human breast cancer cell line MCF-7

Regulation of breast tumor proliferation depends in a large part on a variety of hormones and growth factors. In this report we show that estrogen and antiestrogen modulate epidermal growth factor-receptor (EGF-R) level in the human breast cancer MCF-7 cells with opposite mechanisms. Although a short-term treatment (24h to 48h) with estradiol leads to a decrease in EGF-R number, the addition of hormone in cell culture for 5 days increases EGF-R level with a maximal effect observed at 10(-10) M estradiol. In contrast, when cells are treated with the antiestrogen hydroxytamoxifen, a dose-dependent decrease in EGF-R level occurs. We also report that EGF is able to induce estrogen receptors and, to a lesser extent, progesterone receptors when added to MCF-7 cell cultures. These results demonstrate an interaction between both estrogen receptor and EGF receptor growth promoting systems in target cells. The implications of such an interaction in the understanding of human breast cancer hormone responsiveness and, in the development of therapies, are discussed.

A dynamic model of glucocorticoid receptor phosphorylation and cycling in intact cells

Glucocorticoid receptors have been proposed to undergo an ATP-dependent recycling process in intact cells, and a functional role for receptor phosphorylation has been suggested. To further investigate this possibility we have examined the phosphate content of the steroid-binding protein of all glucocorticoid receptor forms which have been isolated from WEHI-7 mouse thymoma cells. By labeling of intact cells with 32Pi for 18-20 h in the absence of hormone, covalent binding of [3H]dexamethasone 21-mesylate, immunopurification and SDS-PAGE analysis, the steroid binding protein was found to contain, on average, 2-3 phosphates as phosphoserine. One third of the phosphates were associated with proteolytic fragments encompassing the C-terminal steroid-binding domain. The central DNA-binding domain was not phosphorylated, leaving the other two thirds of the phosphates localized in the N-terminal domain. The phosphate content of various receptor forms from cells incubated with 32Pi and [35S]methionine was compared using 35S to normalize for quantity of protein. In ATP-depleted cells a non-steroid-binding form of the receptor (the "null" receptor) is found tightly bound to the nucleus, even without steroid. The phosphate content of null receptors was two thirds that of cytosolic receptors from normal cells, suggesting phosphorylation-dependent cycling in the absence of hormone. Addition of glucocorticoid agonists, but not antagonist, to 32P- and 35S-labeled cells increased the phosphate content of the cytosolic steroid-binding protein up to 170%, indicating an average increase in the phosphates from about 3 to 5. After 30 min of hormone treatment the phosphate content of the steroid-binding protein of cytosolic activated (DNA-binding) and nonactivated receptors, and that of nuclear receptors extractable with high salt concentrations and/or DNase I digestion, was the same. No change in the phosphate content of the 90-kDa heat shock protein associated with unliganded and nonactivated receptors was detected following association of the free protein with the receptor and following hormone binding of the receptor. Analysis of the unextractable nuclear receptors indicated that they contained less phosphate (60% of that of cytosolic receptors), similarly to null receptors, indicating that dephosphorylation is associated with the unextractable nuclear fraction. The rate of hormone-dependent phosphorylation appeared to be much faster than the rate of dephosphorylation in the presence of hormone, the latter determined by a chase of the 32P label with unlabeled phosphate. Our results show that phosphorylation and dephosphorylation are involved in the mechanism of action of glucocorticoid receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Glucocorticoid receptors lacking hormone-binding activity are bound in nuclei of ATP-depleted cells

The glucocorticoid receptor binding capacity of rat thymus cells disappears when the cells are depleted of ATP by anaerobiosis, and rapidly reappears when ATP levels are restored. Loss and recovery of binding capacity occurs even when protein synthesis is suppressed with cycloheximide. In view of this and similar work in other cell systems, we proposed that in cells deprived of ATP the receptor is present in a form--the 'null receptor' form, as we shall call it--that cannot bind hormone. Although many subsequent observations support this idea, no direct evidence has appeared for the existence of the null receptor. We have attempted to detect the null receptor in WEHI-7 mouse thymoma cells with a monoclonal antibody to the glucocorticoid receptor. Here we report that the null receptor is bound in the nuclei of ATP-depleted cells, and is present in amounts comparable to those of receptors in normal cells.

RNA-containing nuclear binding sites for glucocorticoid-receptor complexes

RNase A treatment of HeLa cell nuclei causes a time- and concentration-dependent release of dexamethasone-receptor complexes. If nuclei are incubated in the absence of enzyme, only 60% of RNase-releasable complexes can be detected. Sucrose density gradient analysis of nuclear extracts shows that receptor complexes released by RNase treatment sediment at 3.6 S, whereas complexes obtained from untreated nuclei sediment between 7 and 3.6 S. Our results show that a fraction of dexamethasone-receptor complexes retained by HeLa cell nuclei is located in binding sites involving RNA.