Receptor structure: a personal assessment of the current status

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

Agonist-antagonist activity of anti-estrogens in the human breast cancer cell line MCF-7: an hypothesis for the interaction with a site distinct from the estrogen binding site

Non-steroidal anti-estrogens exhibit an extremely complex pharmacology because of their estrogenic and anti-estrogenic effects in different species. Recently, we have reported evidence for an immunochemical difference in the estrogen receptor (ER) when it is occupied with anti-estrogens as compared to estrogens (Martin et al., 1988). In this study, we have compared immunoreactivity of MCF-7 cell estrogen receptor when bound to anti-estrogen versus estrogen. We show that the occupation of ER with antiproliferative concentrations of various anti-estrogens leads to the appearance of additional antigenic determinants for the H222 monoclonal anti-estrogen receptor antibody. When performing ER immunoassay after sedimentation of estrogen receptors on sucrose gradients, we show that exposure of new epitopes induced by anti-estrogens can occur on a 4 s molecular form related to the 66 kDa monomeric estrogen receptor. Also, when ER are previously occupied by estradiol, the addition of low anti-estrogen concentrations, which are unable to displace estradiol from the estrogen receptor, leads to a significant increase of H222 epitopes. Our results led us to propose a molecular model for anti-estrogen-receptor interaction in which their dual agonist/antagonist activity may be due to the occupation of distinct binding sites on the estrogen receptor.

Oestrogen enhances the responsiveness of the MMTV-LTR to glucocorticoid in ZR-75-1 human breast cancer cells

Treatment of ZR-75-1 human breast cancer cells with oestrogen has no direct effect on the expression of a transfected MMTV-LTR but enhances its inducibility in response to glucocorticoid treatment. This effect which can be produced with both oestradiol and diethylstilbestrol is specific to induction of the MMTV-LTR, no effect of the treatment on expression driven by the RSV-LTR being observed. The effect can be observed in cells pre-treated with oestrogen prior to removal of DNA and glucocorticoid addition but not in cells where oestrogen is added after removal of the DNA. The possible mechanisms of these effects and their relationship to the induction of oestrogen-responsive genes by this hormone are discussed.

Rapid activation of glycogen phosphorylase by steroid hormones in cultured rat hepatocytes

Testosterone (40-300 microM), oestradiol (20-500 microM), progesterone (20-500 microM), dexamethasone (10 nM-1 microM) and corticosterone (1-10 microM) activate glycogen phosphorylase rapidly when added directly to hepatocytes. The activation of phosphorylase was concentration-dependent and occurred after 10 min for dexamethasone, 30 min for testosterone and 60 min for oestradiol and progesterone. This rapid effect does not appear to be dependent on a stimulation of protein synthesis, it is independent of an increase in cyclic AMP, and it is not diminished by the presence of ornithine decarboxylase inhibitors. The stimulation of phosphorylase activity is diminished by depleting the incubation medium of Ca2+ in the presence of 0.5 mM-EGTA, and therefore it may involve changes in the distribution of Ca2+ in the hepatocytes. These results may explain some of the pharmacological effects of sex steroids, and also might contribute to the physiological actions of glucocorticoids.

Levels of the 90kd heat shock protein and resistance to glucocorticoid-mediated cell killing in a range of human and murine lymphocyte cell lines

The 90kd heat shock protein is associated with the receptors for glucocorticoids and other steroids. The steroid is believed to function by releasing the receptor from hsp90 allowing the receptor to bind to DNA and activate transcription. The levels of hsp90 have been measured in a variety of lymphoid cell lines and compared with their sensitivity to steroid mediating killing. No correlation of high hsp90 levels with steroid resistance was observed. However, increased levels of hsp90 in response to steroid were observed in a number of cell lines. The significance of these findings is discussed in terms of the modulation of the cells response to steroid treatment and the mechanism of steroid resistance.

Assessing the early luteal phase in in vitro fertilization cycles: relationships between plasma steroids, endometrial receptors, and endometrial histology

The corpus luteum-endometrial unit was investigated in in vitro fertilization (IVF) cycles using endocrine, morphologic, and biochemical measurements on the day normally scheduled for embryo transfer (day 16), in 12 stimulated and 4 natural cycles. Advanced endometrial histologic maturity was recorded in 9 of the 12 stimulated cycles. No in-phase endometria were seen when the preovulatory plasma estradiol (E2) was greater than 500 pg/ml or the day 16 plasma progesterone (P) greater than 10 ng/ml in natural or stimulated cycles. Significant negative correlations were noted between both preovulatory E2 and day 16 P and the concentration of cytosolic progesterone receptor (PRc). Advanced endometrial maturity tended to be associated with low concentrations of PRc. Regardless of endometrial maturity, the natural cycle was characterized by low cytosolic E2 receptors (ERc) and high PRc, whereas the concentration of both receptors was usually greatly reduced in stimulated cycles. It is concluded that the advanced endometrial maturation observed in stimulated IVF cycles is a consequence of the production of supraphysiologic levels of sex steroids by the corpus luteum that cause profound modifications of endometrial receptor dynamics.

Chemical crosslinking: a useful tool for evaluations of steroid receptor structures and their functional states in intact cells

A procedure of chemical crosslinking of intact cells with glutaraldehyde was employed to contribute to the understanding of glucocorticoid receptor structures and their functional states in vivo. Under optimal experimental conditions, glucocorticoid binding sites were found almost equally distributed between cytosolic and nuclear fractions of crosslinked cells. Sedimentation properties of crosslinked receptor complexes in cytosolic and nuclear extracts revealed that these entities were oligomers, which heterogeneously sedimented between 11 and 4S in the presence of 0.3 M NaCl. By anion exchange chromatography, we could establish that these receptor complex oligomers behaved as untransformed forms.

Oestrogen receptor activation and molecular forms in human breast cancer

The oestrogen receptor (ER) present in human breast cancer cytosol was resolved by high performance size exclusion chromatography into a high molecular weight form (300 kDa) either alone or together with a lower molecular weight species (60 kDa). In tumours that contained the 60 kDa species there was a significant increase both in ER activation, determined by binding to oligo(dT)-cellulose, and the concentration of progesterone receptor (PR). Despite cytosolic ER being a potential homogenisation artefact, qualitative differences observed in vitro may reflect aspects of tumour biology in vivo.

Activated type II receptors in brain cannot rebind glucocorticoids: relationship to progesterone's antiglucocorticoid actions

Exchange assays have often been used to quantitate steroid receptors when endogenous ligands are present; however, there are no reports of their successful application to activated glucocorticoid-Type II receptor complexes. In addition to investigating the reasons for this failure, the present study also examined the effects of progesterone on glucocorticoid dissociation from, and reassociation with unactivated and activated Type II receptors. Molybdate-stabilized brain cytosol from adrenal-ovariectomized mice was incubated with [3H]dexamethasone ( +/- [1H]DEX) for 40 h at 0 degree C. Afterwards free steroid was removed on Sephadex G-25 columns in the presence (unactivated receptors) or absence (activated receptors) of molybdate. Activation, as measured by DNA-cellulose binding, was achieved by incubating molybdate-free cytosol at 22 degrees C for 20 min followed by G-25 filtration in the presence of molybdate. The rates of dissociation and reassociation were then measured by incubating cytosol with [1H]triamcinolone acetonide (TA) or [3H]TA ( +/- [1H]TA) at 12 degrees C. An exchange assay was also employed in which cytosol was incubated first with [1H]DEX for 40 h at 0 degree C followed by bound-free steroid separations and 12 degrees C incubations with [3H]TA ( +/- [1H]TA). Both approaches revealed that even though activation reduced the rate of DEX dissociation from Type II receptors by 40%, it eliminated the ability of the newly unoccupied receptors to rebind glucocorticoid. Adding [1H]progesterone to occupied receptor preparations increased dissociation rate constants by nearly 3-fold, for both unactivated and activated Type II receptors. Since [1H]TA failed to prevent this effect, progesterone appears to act at an allosteric site(s) which cannot be occupied by glucocorticoids. Exchange assays revealed that progesterone-facilitated dissociation increased the rate of glucocorticoid rebinding to unactivated, but not activated Type II receptors. These results suggest that spontaneous and progesterone-facilitated termination of glucocorticoid genomic actions could be mediated by steroid dissociation since unoccupied activated Type II receptors do not rebind agonist steroid.

The A and B forms of the chicken progesterone receptor arise by alternate initiation of translation of a unique mRNA

In order to establish the origin of the A and B proteins of the chicken progesterone receptor we have expressed its cDNA in vivo in heterologous cells and in vitro in reticulocyte cell lysates. The A and B proteins were expressed from a single cDNA both in heterologous receptor negative cells and in a cell-free system. Both proteins bind progesterone and are indistinguishable from chick oviduct authentic A and B proteins in terms of size, immunoreactivity and hormone binding properties. Truncated mRNA's which lack the receptor B protein translation signal are capable of generating the receptor A protein by initiation of translation at a second internal start site. We conclude from these data that the chicken progesterone receptor A and B proteins arise most likely by alternate initiation of translation from a single mRNA transcript.

Transformation of glucocorticoid and progesterone receptors to the DNA-binding state

This brief review explores some recent observations relating to the structure of untransformed glucocorticoid and progesterone receptors and the mechanism by which the receptors are transformed to the DNA-binding state. In their molybdatestabilized, untransformed state, progesterone and glucocorticoid receptors exist as a heteromeric 8-9S complex containing one unit of steroid binding phosphoprotein and one or two units of the 90 kD heat shock protein hsp90. When the receptors are transformed, the steroid-binding protein dissociates from hsp90. In cytosol preparations, temperature-mediated dissociation proceeds much more rapidly in the presence of hormone. The dissociated receptor binds to DNA with high affinity, regardless of whether it is in the hormone-bound or the hormone-free state. These observations raise the possibility that the primary, and perhaps the only, role for the hormone is to promote dissociation of the receptor-hsp90 complex. Molybdate, vanadate, and tungstate inhibit receptor transformation to the DNA-binding form, an effect that appears to reflect the ability of these transition metal oxyanions to stabilize the complex between the steroid receptor and hsp90. By promoting the formation of disulfide bonds, hydrogen peroxide also stabilizes the glucocorticoid receptor-hsp90 complex and prevents receptor transformation. A small, heat-stable factor present in all cytosol preparations inhibits receptor transformation, and, when the factor is removed, glucocorticoid receptors are rapidly transformed. This ubiquitous factor has the physical properties of a metal anion, and it is proposed that molybdate and vanadate affect steroid receptor complexes by interacting with a metal anion-binding site that is normally occupied by this endogenous receptor-stabilizing factor.

Characterization of chromatin binding sites for different forms of uterine progesterone receptor

We tested hamster uterine progesterone receptor (Rp) forms for binding to different chromatin preparations. Similar forms of chick oviduct Rp were used for comparison. Hamster Rp elutes from DEAE-Sephacel in the two peaks, peak I at 115 mM KCl and peak II at 205 mM KCl. Chick Rp peaks I and II elute at 125 mM and 300 mM KCl, respectively. Both chick and hamster peak I displayed a higher level of binding to SDS-stripped chromatin (DNA) than to crude chromatin or 4 M guanidine hydrochloride (GuHCl)-extracted (nucleoacidic protein, NAP) chromatin while peak II bound 50% better to the NAP chromatin than to crude chromatin or DNA. 10 mM molybdate was used to stabilize Rp and to increase Rp recovery. Molybdate-stabilized hamster Rp elutes from DEAE at the peak II position and like peak II, binds poorly to DNA. Since molybdate prevents receptor activation, DNA-Rp interactions require activated Rp. Because molybdate did not prevent Rp binding to NAP chromatin, we conclude that both activated and unactivated Rp bind well to that matrix. Activated hamster Rp could be extracted from crude chromatin, NAP chromatin and DNA with 200 mM KCl. Unactivated Rp was extracted from NAP only with 6 M GuHCl or NaSCN, whereas KCl, glycerol or pyridoxal 5'-phosphate were not able to remove unactivated Rp from NAP. Various Rp forms did not compete with [3H]ORG 2058-Rp for binding to NAP but BSA did compete. Thus a large portion of Rp binding to NAP may represent nonspecific binding rather than binding to a finite number of Rp acceptor sites. These results suggest that the binding of activated Rp to crude chromatin may represent the actual acceptor sites in target cell nuclei. Since the high level of Rp binding sites in NAP chromatin may be an extraction artifact, the involvement of proposed masking proteins in regulating the availability of acceptor sites should be reconsidered. As an alternative to acceptor site regulation, changes in the Rp molecule itself may be important. Rp isolated from hamster uteri on days 1-4 of the estrous cycle was incubated with crude chromatin, NAP chromatin and DNA. The apparent level of Rp binding to chromatin and NAP chromatin increased 2.5-fold from day 1 to day 4, but Rp binding to DNA remained constant. This suggests that ovarian cycle-dependent changes occur in the unactivated Rp which affect its interactions with chromatin, and these changes disappear when receptor is activated.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization and biological relevance of a 29-kDa, oestrogen receptor-related protein

The properties of a monoclonal antibody (D5) that can immunoprecipitate human oestradiol receptor (ER) under some but not all conditions are described. The antibody recognises a 29-kDa serine phosphoprotein that is qualitatively and quantitatively related to ER but not other steroid receptors or binding proteins. p29 will not complex with untreated cytosol ER but, after ammonium sulphate, KCl, heat or phosphatase treatments, interaction occurs that can be detected by immunoprecipitation with D5; molybdate and GTP inhibit complex formation. In human endometrium, p29 is increased by oestrogen and decreased by progestins. IRMA and histochemical assays for p29 have been developed and applied to a large series of human breast tumours. Most, but not all ER+ tumours are p29+, whilst ER-tumours are rarely p29+ unless they are also PR+. p29 predicts for clinical response to hormone therapy. ER+ p29+ tumours have a higher response rate than the ER+ p29-tumours. We do not know if p29 is a previously undetected component of the oestradiol receptor machinery or whether it is a product of oestrogen action.