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Department of Cancer and Cell Biology, University of Cincinnati College of Medicine, 3125 Eden Avenue, Cincinnati, Ohio 45267-0521, USA1 Nuclear Receptor Biology Laboratory, Pennington Biomedical Research Center, Louisiana State University, Baton Rouge, Louisiana 70808, USA
(Correspondence should be addressed to S A Khan; Email: sohaib.khan{at}uc.edu)
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Abstract |
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Background |
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and β proteins display ubiquitous yet differential expression in many tissues, including adipose, adrenal, bone, brain, breast, colon, inner ear, liver, lung, pancreas, pituitary, sympathetic ganglia, thyroid, and urogenital (Diel 2002, Nilsson & Gustafsson 2002, Gustafsson 2003, Matthews & Gustafsson 2003, Heldring et al. 2007b). Accordingly, estrogen signaling affects the cardiovascular, central nervous, immune, musculoskeletal, and reproductive systems, and manifests in a plethora of physiological symptoms and diseases, perhaps most notable of which is breast cancer. ER and ERβ have both overlapping and unique functions (Matthews & Gustafsson 2003), and ERβ is thought to act as an inhibitor of ER activity (Hall & McDonnell 1999). Knowledge of the tissue-specific expression and the subsequent role of ER and ERβ proteins in disease will provide opportunities to develop novel treatments for colon cancer, depression, hearing disorders, infertility, leukemia, osteoporosis, prostate and endometrial cancers, and others (Gustafsson 2003, Matthews & Gustafsson 2003).
ER and ERβ are members of the nuclear receptor (NR) superfamily and have evolutionarily conserved modular structures (Mangelsdorf et al. 1995, Renaud & Moras 2000). The N-terminal activation function-1 (AF-1) region is responsible for ligand-independent transcriptional activation of NR proteins and contains multiple phosphorylation sites. The DNA-binding domain is C-terminal to the AF-1 region and functions in DNA recognition and dimerization. The next principal domain, the AF-2 region/ligand-binding domain (LBD), is responsible for ligand binding, dimerization, and coregulator recognition. The F domain, located C-terminal to the LBD, is not well conserved among NR proteins, and its function is complex and poorly understood.
The LBD is the primary target for NR drug discovery (Hall & McDonnell 2005, Moore et al. 2006). In most traditional approaches, pharmaceuticals designed to modulate NR activity compete for access to the cognate ligand-binding pocket. ER ligands that display differential activities when compared with the biological ligand estradiol-17β (E2) are referred to as selective ER modulators (SERMs; Jordan & Robinson 1987, McDonnell 1999, Jordan & O'Malley 2007). The development of SERMs has significantly impacted the means by which estrogen-related diseases are treated. An ideal SERM is the one that beneficially modulates the ER, specific to a particular cell type, tissue, or disease, without unwanted side effects in other cells or tissues. ER agonists are commonly used for hormone replacement treatment in postmenopausal women (Burkman 2003) and have been useful in the treatment of diseases such as osteoporosis (Delmas et al. 1997). ER antagonists, including the type I antiestrogen tamoxifen, are used in the treatment of ER-positive breast cancer (Vogel 2003) (the ligand classification nomenclature used herein follows the format of type I (nonsteroidal) or type II (steroidal), distinct from that presented in McDonnell et al. 1995, Hall & McDonnell 2005). Despite their usefulness, prolonged exposure to these compounds causes a number of negative side effects. For example, tamoxifen treatment of breast cancer can result in undesired agonist behavior in certain tissues, including increased incidence of endometrial cancer (Fisher et al. 1998, 2005). The mixed behavior of commonly used SERMs has spearheaded many efforts to create novel compounds that lack negative side effects. These efforts, however, have proved difficult as SERM activity exhibits differential dose-, cell-, and tissue-dependent effects, which manifest as a result of recognized (differential tissue expression of coactivators and corepressors) and poorly understood (pharmacological behavior) consequences.
For many years, type I antiestrogens were noted to exhibit unusual pharmacological behavior compared with type II ‘pure’ antiestrogens (Jensen 2001, Jensen & Khan 2004), which include ICI 182 780 and RU 58 668. Type I antiestrogens display mixed agonist/antagonist activity in a dose-dependent manner (agonist at low concentrations; antagonist at higher concentrations), whereas type II antiestrogens display strictly antagonist activity. Furthermore, ER was shown to bind nearly twice as much antiestrogen when compared with E2 (Hedden et al. 1995). These studies and others led to the proposal of a unified two-site model describing the action of antiestrogens (Jensen & Khan 2004). This model relates the agonist activity of type I antiestrogens to the occupancy of the cognate ligand-binding site, whereas antagonist activity results from an additional interaction with a secondary location. Recently, the proposal of a second ligand-binding site was corroborated with the report of a crystal structure of ERβ LBD bound to two molecules of 4-hydroxytamoxifen (HT; Wang et al. 2006). Herein, we briefly describe the history of the pharmacological behavior of type I antiestrogens and support for a second ER ligand-binding site, building up to the structural model describing the second binding site for HT within the LBD of ERβ. Furthermore, we discuss the implications of the second binding site in the inhibition of ER/NR-coactivator interactions, as well as in the definition of the agonist or antagonist state based on the position of helix 12 in the LBD.
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Biochemical history of the dual agonist/antagonist activity of type I antiestrogens |
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Type I and type II antiestrogens were shown to expose an ER epitope specific for the monoclonal antibody H222 (Martin et al. 1988, Berthois et al. 1994, Hedden et al. 1995). These studies, which were carried out in the presence of a saturating amount of E2, led to the hypothesis that exposure of the H222-specific epitope results from the interaction of the antiestrogens at a distinct site from the cognate ligand-binding site. This alleged second ligand-binding site appeared to have a lower affinity for ligand, when compared with the cognate ligand-binding site for E2. In support, sucrose sedimentation and tritiated ligand (Fig. 1A) experiments revealed that ER from MCF-7 cytosol binds approximately twice the amount of type I (HT) or type II (RU 58 668) antiestrogens when compared with E2 when studied in the low nM range (Hedden et al. 1995). Interestingly, the concentration at which the second site-binding event occurs correlates to approximately the same concentration where agonism changes to antagonism (Fig. 1B; Katzenellenbogen et al. 1987). Another study raised the possibility of a second ligand-binding site for HT, as a high degree of ‘nonspecific’ HT binding was observed (Raam et al. 1983). This study reported an HT:ER binding ratio of 1.7 ligand per receptor using a dextran-coated charcoal assay – the same assay and approximate binding ratio reported in an aforementioned study (Hedden et al. 1995). The authors state that it was not possible to determine whether HT is binding to the E2-binding site or another functionally relevant site, but that it was in fact the same protein moiety as the E2-binding site. By contrast, the type II antiestrogen RU 58 668, which has a lower affinity for ER when compared with E2 (Van de Velde et al. 1994), does not display a concentration-dependent change from agonism to antagonism, affording a sole antagonist role. This was suggested to occur via two approximately simultaneous binding events for RU 58 668 (Jensen & Khan 2004). However, binding of type II compounds to the cognate ligand-binding site is likely sufficient to induce an antagonist response. Structural studies (discussed in a later section) reveal that the steroid portion of these compounds binds in the cognate ligand-binding pocket, whereas the bulky R group component of these compounds associates in the AF-2 surface (also discussed later as the possible second ligand-binding site). In this scenario, low concentrations of ligand resulting in binding to the cognate-binding site would be sufficient to induce an antagonist response. Higher ligand concentrations may therefore result in binding to an additional second site, but because the primary binding event causes antagonism, no change in pharmacological behavior is observed. Of note, it has also been reported that E2 can expose an ER epitope specific for the H222 monoclonal antibody at seemingly high concentrations that resulted in a decrease in the stimulatory activity of E2 (Berthois et al. 1994).
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20–30 times more abundant in the liver when compared with tumor cell lines (Chailleux et al. 1994). It has been suggested that AEBS regulates the free intracellular concentrations of antiestrogens (Kudolo et al. 1984). More recently, it was reported that non-antiestrogen ligands targeting AEBS increase the anti-proliferative activity of antiestrogens (End et al. 2001, Baum & Kirschmeier 2003, de Bono et al. 2003, Dalenc et al. 2005). It has been further suggested that prolonged exposure to antiestrogens may result in the production of AEBS (Furr & Jordan 1984), implying that the inhibition of AEBS antiestrogen binding resulting in increased cellular bioavailability of antiestrogens. Tamoxifen-sensitive, but not tamoxifen-insensitive, cell lines show immunoreactivity to the H222 antibody (Naundorf et al. 2000) and, interestingly, no differences in the ER nucleotide sequences of the ER LBD in tamoxifen-sensitive and -insensitive cell lines were observed. This suggests that either sequences removed from the LBD or other biomacromolecules, such as AEBS or perhaps CtIP (Wu et al. 2007), may impart tamoxifen sensitivity directly or indirectly. This notion is further supported by the observation that ER mutations occur at a low frequency in breast cancer patients and do not account for most tamoxifen-resistant breast tumors (Karnik et al. 1994, Naundorf et al. 2000). When taken together with the possibility that AEBS displays differential and tissue-specific concentrations and its possible role in regulating intracellular antiestrogen concentrations, it is tempting to speculate that perhaps AEBS is present at different concentrations in tamoxifen-sensitive (lower) and tamoxifen-insensitive (higher) cell lines, resulting in the observed dose-, cell-, and tissue-specific mixed agonist/antagonist activity. Therefore, it may be the case that small molecular inhibitors of AEBS that prevent it from binding tamoxifen may allow tamoxifen-insensitive cell lines to respond to tamoxifen treatment. Further work is needed to determine the effects of estrogens and antiestrogens on H222 reactivity and delineate a possible role for AEBS in regulating the intracellular concentrations of these compounds with respect to ER function. Altogether, these studies support a unified two-site model for antiestrogen activity (reviewed in Jensen & Khan 2004), which associates the agonist activity of type I antiestrogens to occupancy of the E2-binding site, and the antagonist activity to an additional interaction with a secondary location. This model suggests that the mixed agonist/antagonist activity is a consequence of the weak ligand affinity for the second (antagonist) ligand-binding site for type I antagonists, when compared with the cognate (agonist) ligand-binding site. We note other studies reporting results in corroboration of the two-site antiestrogen, or more generally, antihormone model for ER/NR action. Low concentrations of E2 and bisphenol A induced a stimulatory/proliferative response in rat cerebellum (Zsarnovszky et al. 2005). However, increasing concentrations were less efficacious and resulted in the inhibition of cellular proliferation. The authors suggested a model in which ‘the rapid-acting ER complex contains a high-affinity (picomolar) stimulatory binding site and a lower affinity (nanomolar) inhibitory binding site’ (Zsarnovszky et al. 2005). In another study, a significant dose-dependent increase in H222 immunoreactivity was observed in tamoxifen-sensitive MaCa 3366 ER/progesterone receptor (PR)-positive mammary carcinoma cells (Naundorf et al. 2000). Finally, it was reported that a mutant of the androgen receptor (AR) was unable to bind the agonist ligand progesterone but able to bind the antagonist ligand RU 486, suggesting a distinct binding site on AR for RU 486 but not progesterone (Vegeto et al. 1992, Gao & McPhaul 1998).
To the best of our knowledge, no additional, conclusive biochemical studies of ER antiestrogen stoichiometry have been reported in support of, or opposition to, the two-site model. However, we note a number of studies reported with either unique or somewhat conflicting observations. For example, a set of studies revealed that one molecule of HT, but not E2, out of two total molecules, dissociates from the ER dimer after binding to the estrogen response element (ERE; Klinge et al. 1996a,b, 1998), perhaps via interaction with an accessory protein that interacts with E2-bound, but not HT-bound, ER (Anolik et al. 1996). In the aforementioned study in support of the two-site antiestrogen model (Raam et al. 1983), other assays performed report a HT:ER ratio of 1. However, the authors suggest that this value may be due to more extensive dissociation of ligand during washes. The data suggest that not all but some population of ER proteins had two bound HT ligands. It is tempting to speculate that this may be a result of a weaker affinity, second binding site. In other studies, the dependence of HT or E2 on the binding of ER to nuclei extracted from cellular tissues was analyzed without mention of a specific ligand:protein stoichiometry measurement (Cushing et al. 1985, Klinge et al. 1987, 1989). Additionally, visual inspection of data that report a 1:1 ratio for HT-bound ER via Scatchard analysis (Klinge et al. 1989) suggests a more complex binding curve. For cases like these where Scatchard analysis is used, which often results in errors when determining the number of binding sites (Klotz 1982, Faguet 1986), computer software utilizing a nonlinear regression analysis would be more appropriate for fitting equations and extracting rate parameters. Furthermore, in the aforementioned studies, as well as those not mentioned here, it is sometimes difficult to note whether the use of ‘nonspecific binding’, which implies low-affinity binding sites, is the same as ‘background signal’, as the two are not the same. In any case, additional studies aimed at probing the binding stoichiometry of agonists and type I/II antiestrogens and the resulting pharmacological phenotype are warranted. However, crystallographic structural studies described below provide additional support for a second, antagonistic ligand-binding site for the type II compound tamoxifen.
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Structural validation of a second NR ligand-binding site |
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-helical sandwich. One HT molecule is located in the cognate ligand-binding pocket, whereas the second HT molecule is located in the hydrophobic groove of the coactivator-binding (AF-2) surface, comprising helices 3, 4, 5, and 12. This surface provides a binding site for coactivator proteins containing the LXXLL recognition motif (Savkur & Burris 2004). The binding of the second HT molecule in the AF-2 surface primarily involves hydrophobic and van der Waals interactions. The unsubstituted phenyl group of the second HT molecule is buried deep into the hydrophobic cavity of the coactivator surface, which was noted to likely provide the main contribution to binding in the AF-2 surface. Consistent with the aforementioned biological data suggesting a second low-affinity binding site, the second HT molecule has higher crystallographic temperature factors and is not fully buried when compared with the HT molecule in the cognate ligand-binding site. The charge clamp residues Lys314 and Glu493, which are important for recognition of the LXXLL motif, are not involved in the interaction with the second HT molecule. We note that ERβ residues involved in binding the second HT molecule are highly conserved in ER and ERβ proteins in other species (Fig. 3). Again we note that although, for example, tamoxifen is a pure agonist in mouse under certain circumstances and tissues (Furr & Jordan 1984, Jordan 1984), the tamoxifen-binding complex AEBS displays differential tissue-specific concentration that may not allow the intracellular concentration of tamoxifen to saturate the lower affinity second binding site in some tissues if present at sufficient concentrations.
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90° from the agonist position, allowing it to occupy the AF-2 coactivator binding surface (Fig. 4D). A partial agonist is thought to induce an intermediate conformation (Fig. 4E), termed a quasi-antagonist position (Pike et al. 2000), whereby a suitable coactivator interaction can dynamically switch helix 12 between partial and full agonist positions.
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(PPARA) LBD bound to the antagonist ligand GW6471 and the NCOR peptide (Fig. 4G; Xu et al. 2002). The significance of the position of helix 12 in the HT-ERβ LBD structure remains elusive. However, the degree to which it is displaced and the secondary helix 12 interaction with the second HT molecule leads to the speculation that its novel placement could provide a means of coregulator binding through the specific recognition of the second HT molecule in the AF-2 surface.
We note that the crystal structure of ER LBD bound to HT does not show a second HT molecule bound to the AF-2 surface (Shiau et al. 1998). The ERβ LBD structure bound to two HT molecules (Wang et al. 2006) was determined from protein that had ten times the molar concentration of the protein in the crystallization buffer. By contrast, ligand-bound ER LBD (Shiau et al. 1998) was obtained by eluting the protein from an E2-affinity column with the ligand of interest (Greene et al. 1980) – a commonly used method to purify ER LBD protein for crystallography (Goldstein et al. 2001). The authors do not specify if ligand was included during the crystallization process; therefore, it is possible that the second binding site in this study was unoccupied due to the unavailability of additional ligand at saturating conditions to bind to the AF-2 surface. This is supported by the notion that the binding of HT to the AF-2 surface is of lower affinity compared with the cognate ligand-binding pocket (Wang et al. 2006). Recently, it was reported that the Y537S ER LBD mutation allows for the conformational stabilization during protein purification and subsequently allows for the addition of ligands to purified apoprotein (Nettles et al. 2008).
More recently, structural studies of AR (Estébanez-Perpiñá et al. 2007a) and thyroid hormone receptor β (THRB; Estébanez-Perpiñá et al. 2007b) LBDs were reported, which corroborate the concept of a non-cognate ligand-binding site in LBD of NR proteins. The structure of AR LBD was determined bound to natural ligand (dihydrotestosterone; DHT) and small molecules found to inhibit AR/coregulator interactions – [4-(4-hydroxy-3-iodo-phenoxy)-3,5-diiodo-phenyl]-acetic acid, 4HY; 3,5,3'triiodothyronine, T3. The small molecules bind in the AF-2 surface (referred to as BF-2) and a nearby surface (BF-3), which comprises helix 1, the helix 3–5 loop, and helix 9 (Fig. 4H and I). The BF-3 surface is nearly the same size as the AF-2 region and is a known target for mutations in prostate cancer and androgen insensitivity syndrome. The small molecules bind to the AR LBD with IC50
50 µM and weaken the binding of coactivator peptides, suggesting that small molecules binding to sites other than the cognate ligand-binding site can modulate NR-coactivator interactions. In the case of THRB (Estébanez-Perpiñá et al. 2007b), a high-throughput screen identified the aromatic β-enone 1-(4-hexylphenyl)-prop-2-en-1-one (HPPE) molecule as an inhibitor of steroid receptor coactivator-2 (SRC-2) interaction, and crystallographic studies revealed that HPPE binds to the AF-2 surface of THRB. Indeed, the authors confirm that molecules targeted to the AF-2 regions have a great potential in modulating NR coregulator recruitment.
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Additional studies describing an antagonist-specific NR ligand-binding site |
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and ERβ, suggesting a second E2-independent binding site (Tyulmenkov & Klinge 2000). Fluorescence experiments monitoring the binding of THCK, which is an agonist for ER and an antagonist for ERβ, suggested the molecule induced different ER and ERβ conformations, providing a possible origin for the modes of agonist and antagonist action. A molecular modeling study was conducted to determine the location of THCK binding (van Hoorn 2002). Structures of the LBD of ER and ERβ were analyzed for surface cavities and grooves, and three sites of interest were identified (Fig. 5): 1) the conserved, cognate ligand-binding site, 2) the AF-2 coactivator-binding site, and 3) a site proximal to the steroid-binding site comprising the helix 2/β-sheet region. The results of this study, however, were inconclusive for providing a basis by which THCK binding to the helix 2/β-sheet region could result in agonist properties for ER and antagonist properties for ERβ.
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,25(OH)2-vitamin D3 (1,25D) analogs do not compete well with [3H]1,25D, but did induce agonist/antagonist non-genomic properties, suggested the existence of an alternative vitamin D receptor (VDR) ligand-binding site (Mizwicki et al. 2004). Molecular modeling studies identified a possible alternative ligand-binding site, termed the A-pocket (alternative ligand-binding pocket). The A-pocket comprises a surface analogous to the aforementioned ER helix 2/β-sheet region (van Hoorn 2002) and was postulated to accept ligands of different shape compared with the G-pocket (genomic pocket), which is analogous to cognate ER ligand-binding site (Rochel et al. 2000, Tocchini-Valentini et al. 2001, Norman et al. 2004). The authors suggest the A-pocket affords a rapid, non-genomic response, distinct from the genomic response afforded by the G-pocket.
Although these studies indeed support the concept of an additional ligand-binding site for ER and VDR proteins, the proposed site (helix 2/β-sheet region; Fig. 5) differs from the second ligand-binding site in the AF-2 surface reported for AR (Estébanez-Perpiñá et al. 2007a), ERβ (Wang et al. 2006), and THRB (Estébanez-Perpiñá et al. 2007b). It is possible that this site represents an additional ligand-binding site, but this remains to be experimentally verified. Recently, the helix 2/β-sheet surface in ER was shown to be a previously unidentified ligand-independent coregulator interaction site (Kong et al. 2005). The authors propose that this site is highly unlikely to bind natural small molecule ligands. In support of this study, targeted mutations to this site in ER, which is evolutionarily conserved among other members of the NR superfamily, resulted in reduced ligand-binding affinity (cognate site) and reduced SRC-1 interaction (Raviscioni et al. 2006). Furthermore, studies of PPARG (Bruning et al. 2007), retinoid X receptor (RXRA; Yan et al. 2004, 2006, Nahoum et al. 2007), and ER (Dai et al. 2008) reveal that the binding of partial agonists in the cognate-ligand binding site results in a change in dynamics in the helix 2/β-sheet region. Changes in conformational dynamics have been shown to correlate positively to regions important for allosteric control of protein function (generally discussed in Kern & Zuiderweg 2003, Busenlehner & Armstrong 2005, Swain & Gierasch 2006, Henzler-Wildman & Kern 2007), further supporting the concept that the helix 2/β-sheet region is a control surface in NR proteins. Notwithstanding these results, the possibility exists for the development of small molecules to modulate the function of this surface.
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Inconsistencies between the two-site antagonist model and ER LBD structures highlight the importance of the F domain |
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and ERβ LBD bound to two HT molecules (Shiau et al. 1998, Wang et al. 2006). The two-site model associates the agonist activity of type I antiestrogens, such as HT, to occupancy of the E2-binding site, and the antagonist activity results from an additional interaction with a secondary location. Again, this is supported by studies that reveal that ER can bind nearly twice as much antiestrogen compared with E2, and that the second binding event of the type I antiestrogen HT correlates to a switch from agonism to antagonism (Fig. 1).
The discrepancy between the two-site antiestrogen model and the agonist/antagonist position of helix 12 in ER structures may originate from the fact that the ER LBD structures were determined without the C-terminal F domain. By contrast, the biochemical data supporting the two-site model were collected using full-length ER, which includes the F domain. Only a few NR structures have been solved with sequences C-terminal to helix 12, including glucocorticoid receptor (GR; Bledsoe et al. 2002, Kauppi et al. 2003), PR (Williams & Sigler 1998), and PPARA (Xu et al. 2002). However, these NRs contain small C-terminal sequences (10–12 amino acids or less) and are not generally considered as F domains. On the other hand, the F domains of ER and ERβ are relatively large (41 and 30 amino acids respectively) and believed to possess conformational flexibility, as well as differences in secondary structure (Skafar & Koide 2006).
Although no structural data (X ray crystallographic or NMR) currently exist for the ER F domain, biochemical and theoretical studies have shed some light on its function (reviewed in Skafar & Zhao 2008). The most important fact related to the inconsistencies between biological full-length ER and structural ER LBD-only data is that deletion or specific mutations of the F domain eliminates the ability of tamoxifen to act as an agonist (Montano et al. 1995, Schwartz et al. 2002, Koide et al. 2007b). This key observation suggests that the structures of ligand-bound ER LBD, determined without the F domain, may not depict the precise role of helix 12 in determining the active state of full-length ER. Again, this is significant because the structural studies prior to the HT-ERβ LBD structure (Wang et al. 2006) suggest that one molecule of tamoxifen bound in the cognate ligand-binding pocket is sufficient to induce an antagonist AF-2 conformation, which is in direct conflict with biochemical studies (reviewed in Jensen & Khan 2004). Furthermore, mutation or truncation of the F domain has been shown to: alter antagonist activity, making tamoxifen a more effective antagonist (Montano et al. 1995, Schwartz et al. 2002); increase the affinity for E2 (Schwartz et al. 2002, Skafar & Koide 2006); remove a partial inhibition of dimerization (Peters & Khan 1999); alter the ability of E2 to overcome the antagonist activity of tamoxifen (Skafar & Koide 2006); alter the activity of an ERE-driven promoter in response to E2 or HT (Koide et al. 2007b); increase the degree to which ER binds coactivators, including nuclear receptor-interacting protein (RIP)-140 (Peters & Khan 1999) and SRC-1 (Koide et al. 2002); eliminate the E2-dependent activation of ER/SP1-mediated transcription (Kim et al. 2003); and remove the protection of ligand-unbound ER from proteolysis (Tateishi et al. 2006). Interestingly, helix 12, but not the F domain, was found to be essential for ER interactions with cytokeratins 8 and 18 (Long & Nephew 2006), suggesting an F domain-independent action that adds further complexity. Similar observations detailing the role of the F domain on the function of other NR proteins have been noted (Modarress et al. 1997, Sladek et al. 1999, Suaud et al. 1999, Ruse et al. 2002, Bertin et al. 2004, Farboud & Privalsky 2004, Petrescu et al. 2005, Takayama et al. 2007). In particular, in a study of the hepatocyte nuclear factor-4 (HNF4A), the authors assert that the characteristics of F domain-truncated mutants ‘may not reflect the structure, ligand-binding, ligand-conformational responsiveness and cooperativity of full-length HNF4A’ (Petrescu et al. 2005).
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Evidence supporting a change from the strict use of helix 12 positioning in determining the NR agonist/antagonist state |
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(Heldring et al. 2007a), removal of helix 12 from a LBD-only protein construct significantly enhanced the binding of corepressor peptides that specifically recognize HT-bound ER (Paige et al. 1999, Huang et al. 2002, Heldring et al. 2004). The corepressor peptides recognize a novel HT-induced binding surface, distinct from the agonist-induced AF-2 surface (Heldring et al. 2004). Furthermore, it was necessary to remove helix 12 from the protein construct in order to yield crystals of ER in complex with the corepressor peptides. It is tempting to speculate that the structural position of helix 12 was not favorable for corepressor interaction, perhaps because it was not in a conformation similar to the HT-ERβ LBD structure with a HT molecule bound in the AF-2 coactivator surface (Fig. 4F) or the GW6471–PPARA–NCOR complex (Fig. 4G). Interestingly, a small molecule (p-chloromercuribenzene sulfonic acid) used to reduce crystal lattice motion in the study of ICI 164 384-ERβ (Pike et al. 2001) was found to bind in a similar region to the displaced helix 12 in HT-ERβ (Wang et al. 2006). It is further tempting to speculate about functional relevance of this surface with respect to helix 12, the F domain, or domain/protein interactions in general. Structures of AR LBD with small molecules bound in the AF-2 coactivator surface reveal that the sequence C-terminal to helix 12 makes β-sheet contacts to the loop region between helices 9 and 10 (Fig. 4H and I; dotted oval regions), whereas helix 12 adopts an agonist-like conformation (Estébanez-Perpiñá et al. 2007a). GR (Bledsoe et al. 2002) and PR (Williams & Sigler 1998) LBD proteins bound to agonist also make β-sheet contacts between the C-terminal helix 12 extension and the helix 9-helix 10 loop. Unfortunately, it has not been possible to obtain crystallographic structural data for the ER F domain (region C-terminal to helix 12) or the loop between helices 9 and 10. If this type of β-sheet interaction observed in the AR LBD structures is conserved in ER and other NR proteins, it suggests that helix 12 might be tethered into an agonist-like position, capping the cognate ligand-binding site, regardless of the type of ligand bound in the cognate ligand-binding site, further raising the question whether helix 12 blocks the AF-2 surface in the antagonist functional state.
A recent study of PPARG LBD (Bruning et al. 2007) tested the generally accepted model that asserts the level of NR agonism is associated with the degree to which the AF-2 coactivator surface is stabilized, namely helix 12 dynamics and positioning. Crystal structures and hydrogen exchange data obtained for six ligand-bound PPARG LBD complexes, varying from full agonists to intermediate/partial agonists, reveal that individual ligands induce unique conformational dynamics. In particular, full agonists induce a change in helix 12 dynamics, compared with ligand-free PPARG, whereas intermediate and partial agonists do not alter the dynamics of helix 12 at all. Furthermore, intermediate and partial agonists differentially alter the dynamics of the helix 2/β-sheet region. The conclusion from this study, namely that activation of PPARG is not solely determined by helix 12 positioning and dynamics, led the authors to conclude that the ‘understanding of allosteric signaling must be extended beyond the idea of a dynamic helix 12 acting as a molecular switch’ (Bruning et al. 2007). Similar observations were noted for the LBD of RXRA (Yan et al. 2004, 2006, Nahoum et al. 2007) and, more recently, ER (Dai et al. 2008). In the case of ER, different groups of hydrogen exchange signatures are observed for ER agonists and SERMs, suggesting that hydrogen exchange signatures can provide a method for predicting tissue-specific ligand response. Notably, the ligands display differential stabilization in helix 2, helix 2/helix 3 loop, helix 3, helix 6, the β-sheet 1/2 region, and helix 11. Moreover, two distinct groups of SERMs are noted, based on the degree of protection in the β-sheet region. Raloxifene-like molecules give rise to an intermediate degree of protection in the β-sheet region, whereas HT-like molecules afford a greater degree of protection. Of note, helix 12 displays no observable protection to hydrogen exchange upon binding any of the ligands studied, adding further uncertainty to the role of helix 12 conformational dynamics in determining the ER/NR agonist or antagonist state.
The need for an extended model of NR agonist/antagonist action is further supported by the evidence revealing the importance of a number of complex factors in determining the response to a particular ligand and the subsequent active state of the NR protein. These factors include, but are not limited to, interactions between different domains within NR proteins (Métivier et al. 2002, Glaros et al. 2006, Li et al. 2006), the notion that agonist ligands can recruit coactivator and corepressor proteins (Folkertsma et al. 2007, Gurevich et al. 2007), the observation that coregulator/transcriptional machinery interactions can influence whether tamoxifen acts as an agonist or antagonist in a particular cell environment (Smith et al. 1997, Shang & Brown 2002, Smith & O'Malley 2004, Kressler et al. 2007), NR mobility, dynamics, and degradation within the cell (Hager et al. 2004, Smith & O'Malley 2004), as well as the nature of the receptor binding to DNA (direct or indirect), including ligand dissociation after binding DNA (Anolik et al. 1996, Klinge et al. 1996a), different NR isoforms, and post-translational modifications of the receptor complex (discussed further in Wehling 1997, Chen et al. 1999, Beato & Klug 2000, Lee & Lee Kraus 2001, Nilsson et al. 2001, Steinmetz et al. 2001, Cavaillès 2002, Heinlein & Chang 2002, Stallcup et al. 2003, Perissi & Rosenfeld 2005, Duma et al. 2006, Faus & Haendler 2006, Li & Shang 2007) – all of which are not included in the functional interpretation of the LBD structural data. A recent AR LBD structural report revealed that the binding modes of coactivator and corepressor peptides are structurally similar (Jouravel et al. 2007), leading the authors to suggest that ‘the AF-2 region is utilized as a ‘communicating’ binding site for both repression and activation of AR.’ Thus, NR activation is not simply described using an ‘on/off’ switch model, where helix 12 positioning determines the degree of agonism, but rather a complicated combination of multiple factors resulting in the final functional outcome.
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Relevance to targeted ER drug development |
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and ERβ in complex with these ligands reveal that two residues in particular, Met336 and Met421 in ER or Leu384 and Ile373 in ERβ, are likely responsible for the ERβ-type selectivity. Modified versions of the ERβ-specific phytoestrogen genistein (Kuiper et al. 1998) were used to develop additional ERβ-specific ligands (Mewshaw et al. 2005), which were found to be active in two inflammation models, suggesting its use in treating chronic inflammatory diseases, including inflammatory bowel disease or rheumatoid arthritis, and later noted to be effective for treating severe sepsis (Cristofaro et al. 2006). A derivative of the phytoestrogen, 8-prenylnaringenin, was found to be a full agonist for ER and an antagonist for ERβ (Roelens et al. 2006), suggesting that a molecule can be developed to directly influence the opposing roles of ER and ERβ (Hall & McDonnell 1999, Matthews & Gustafsson 2003).
Alternative methods for developing antagonist compounds to directly modulate ER-coactivator interactions have also been reported. Instead of taking the traditional approach of designing a molecule to compete for the cognate ligand-binding site, these methods aim to block NR signaling by means of direct inhibition of NR-coactivator binding. One method involves the use of peptidomimetics, which are small peptides based on the LXXLL NR recognition motif designed to bind in the AF-2 surface and inhibit ER–protein interactions (Geistlinger & Guy 2003a,b, Leduc et al. 2003, Geistlinger et al. 2004). This approach supports a wide variety of natural (amino acid-like) and non-natural side-chain scaffolds that bind to ER with relatively high affinity (nM-µM). However, the large size of these molecules precludes their use in a clinical setting. Another approach involves the development of small molecules designed to inhibit protein–protein interactions (broadly reviewed in Peczuh & Hamilton 2000, Toogood 2002, Pagliaro et al. 2004, Zhao & Chmielewski 2005). The first proof-of-principle ER-coactivator inhibitors were pyrimidine-based molecules that mimic the three leucine residues of the coactivator LXXLL peptide helix and bind to the AF-2 surface (Rodriguez et al. 2004). However, these molecules could not be studied in vivo due to their low binding affinity. More recently, it was reported that bicyclo[2.2.2]octane compounds are effective structural mimics of two key leucine residues within the LXXLL motif that bind to the hydrophobic groove of the AF-2 surface (Zhou et al. 2007). Additionally, cell- and computer-based screening methods have been used to identify novel ER antagonist compounds that block the interaction between ER and the coactivator SRC-3 and inhibit endogenous ER function in MCF-7 cells (Shao et al. 2004). These compounds did not displace E2 from the ligand-binding site, but were found to bind directly to the LBD of ER, implying that they function through the inhibition of ER-coactivator interactions. The HT-ERβ (Wang et al. 2006), PPARA (Estébanez-Perpiñá et al. 2007a), and THRB (Estébanez-Perpiñá et al. 2007b) LBD structural studies, which reveal that the small molecules indeed bind to the AF-2 surface, support the concept of small molecule modulators of NR activity. Additionally, the observation that helix 12 from a neighboring ERβ LBD molecule in the crystal lattice associates with the second HT molecule bound in the AF-2 surface suggests that it is possible to target a SERM to the AF-2 surface with molecular properties that could recognize or fine-tune ER interaction with specific coregulator proteins.
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Concluding remarks |
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/mouse RXRA heterodimer (Bourguet et al. 2000) NR LBDs resulting in an antagonist helix 12 conformation were developed using truncated F domain mutants. In the case of the mouse RXRA (Bourguet et al. 2000), an antagonist helix 12 conformation was observed only with a constitutively active LBD mutant (F318A). The authors note that the conformation of helix 12 in the F318A RXRA structure does not correlate with biological studies that reveal both native and F318A RXRA proteins bound to the coactivator TIF2. Interestingly, the authors also note that deletion of helix 12 and, therefore, its presence in the AF-2 surface is not mandatory for antagonism (Voegel et al. 1996). Finally, a study of GR bound to the antagonist RU 486 (Kauppi et al. 2003) described three antagonist-bound structures (GR1, GR2, and GR3). In the GR2 structure, helix 12 was absent due to enterokinase cleavage. In the GR3 structure, helix 12 from a domain-swapped molecule binds to the AF-2 surface of another GR molecule between crystallographically identical subunits in the GR dimer. The structural refinement of GR1 was not finalized due to poor data, nor was it submitted to the Protein Data Bank; however, the GR1 structure was used as an example for the antagonist conformation of helix 12 in GR. One possible example of a native antagonist helix 12 conformation is found in the structure of PPARA bound to the antagonist molecule GW6471 and the NCOR peptide (Xu et al. 2002). The conformation of helix 12 in the PPARA structure is similar to the secondary interaction involving helix 12 from a neighboring molecule in the crystal lattice of the ERβ LBD structure bound to two HT molecules (Fig. 4F and G; Wang et al. 2006). However, the sequence C-terminal to helix 12 in PPARA is short (5 amino acids) and may allow for more flexibility in the positioning of its helix 12 compared with other NR proteins with large F domains. For many other NR proteins, no wild-type structures bound to an antagonist molecule have been reported to the best of our knowledge. Most, if not all, discussions concerning the structural basis of antagonism in these proteins, as well as efforts for structure-based development of NR antagonists, are related to antagonist ER structures as a model system. These observations, as well as increasing experimental evidence supporting a role for changes in ligand-induced conformational dynamics rather than the structural conformation of helix 12, suggest that a new model for the NR antagonist state is needed. An accurate understanding of the NR antagonist state will undoubtedly lead to improved intelligent design of NR pharmaceutical modulators. Nature has precisely tuned the NR cognate ligand-binding site to have high affinity for its natural ligand. The studies described reveal potential for rational drug design approaches to target small molecules to functionally important surfaces, such as the AF-2 surface, which circumvents competition with the cognate ligand-binding site. Further studies are needed to address a number of issues pertaining to the relationship between the two-site antiestrogen model and the active state (agonist/antagonist activity) of ER proteins. In particular, what is the biological role of the second tamoxifen, or more generally, antiestrogen molecule in determining ER activity? What are the precise roles of helix 12, the helix 2/β-strand surface, the F domain, other domains N-terminal to the LBD, and differences in the SERM-induced conformational dynamics in regulating coregulator binding? Biochemical data suggest that antagonism induced by type I compounds arises as a result of a second, concentration-dependent ligand-binding event – is SERM agonism or antagonism dependent on the tissue-specific concentrations of these molecules? The action of type II compounds is more uncertain, as biochemical data suggest that ER can bind twice as many of these compounds as well. However, as noted above, because the extended R group portions of these molecules associate in the AF-2 surface, the binding of a single type II compound should be sufficient to induce an antagonist response.
As no structural data exist for agonist or antagonist molecules bound to a NR protein with an intact F domain, nor has the possibility for a second ligand-binding site been extensively explored, it is unclear whether the commonly accepted agonist or antagonist position of helix 12 in LBD-only protein constructs is functionally relevant. Additional structural (crystallographic or NMR spectroscopy) and functional studies are warranted to explore the role of the F domain in response to agonist and antagonist ligands, as well as its possible role in two-site tamoxifen binding to the AF-2 surface of ER. In addition to pure structural studies, the use of monobodies may provide a useful tool to study different conformations of NR proteins (Batori et al. 2002, Huang et al. 2006, Koide et al. 2007a,b). This concept is realized in a recent E2-ER structure bound to the E2#23 FN3 monobody, where a helix structure within a loop of the monobody domain binds to the ER AF-2 surface (Fig. 4J). Until future studies reveal the long-awaited, three-dimensional structure of a full-length NR protein, or the LBD and F domains together, caution should be taken in the functional interpretation of LBD-only biological and structural studies.
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Note added in proof |
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Chandra V, Huang P, Hamuro Y, Raghuram S, Wang Y, Burris TP & Rastinejad F 2008 Structure of the intact PPAR-
–RXR- nuclear receptor complex on DNA. Nature [in press].
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Declaration of interest |
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Funding |
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Acknowledgements |
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References |
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Batori V, Koide A & Koide S 2002 Exploring the potential of the monobody scaffold: effects of loop elongation on the stability of a fibronectin type III domain. Protein Engineering 15 1015–1020.
Baum C & Kirschmeier P 2003 Preclinical and clinical evaluation of farnesyltransferase inhibitors. Current Oncology Reports 5 99–107.[Medline]
Beato M & Klug J 2000 Steroid hormone receptors: an update. Human Reproduction Update 6 225–236.
Berthois Y, Pons M, Dussert C, Crastes de Paulet A & Martin PM 1994 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. Molecular and Cellular Endocrinology 99 259–268.[Medline]
Bertin B, Sasorith S, Caby S, Oger F, Cornette J, Wurtz J-M & Pierce RJ 2004 Unique functional properties of a member of the Fushi Tarazu-Factor 1 family from Schistosoma mansoni. Biochemical Journal 382 337–351.[Medline]
Billas IML, Moulinier L, Rochel N & Moras D 2001 Crystal structure of the ligand-binding domain of the ultraspiracle protein USP, the ortholog of retinoid X receptors in insects. Journal of Biological Chemistry 276 7465–7474.
Black LJ, Jones CD & Falcone JF 1983 Antagonism of estrogen action with a new benzothiophene derived antiestrogen. Life Sciences 32 1031–1036.[CrossRef][Web of Science][Medline]
Bledsoe RK, Montana VG, Stanley TB, Delves CJ, Apolito CJ, McKee DD, Consler TG, Parks DJ, Stewart EL, Willson TM et al. 2002 Crystal structure of the glucocorticoid receptor ligand binding domain reveals a novel mode of receptor dimerization and coactivator recognition. Cell 110 93–105.[CrossRef][Web of Science][Medline]
de Bono JS, Tolcher AW & Rowinsky EK 2003 Farnesyltransferase inhibitors and their potential in the treatment of breast carcinoma. Seminars in Oncology 30 79–92.[Web of Science][Medline]
Bourguet W, Vivat V, Wurtz J-M, Chambon P, Gronemeyer H & Moras D 2000 Crystal structure of a heterodimeric complex of RAR and RXR ligand-binding domains. Molecular Cell 5 289–298.[CrossRef][Web of Science][Medline]
Bruning JB, Chalmers MJ, Prasad S, Busby SA, Kamenecka TM, He Y, Nettles KW & Griffin PR 2007 Partial agonists activate PPARgamma using a helix 12 independent mechanism. Structure 15 1258–1271.[Medline]
Brzozowski AM, Pike ACW, Dauter Z, Hubbard RE, Bonn T, Engström O, Öhman L, Greene GL, Gustafsson J-Å & Carlquist M 1997 Molecular basis of agonism and antagonism in the oestrogen receptor. Nature 389 753–758.[CrossRef][Medline]
Burkman RT 2003 Hormone replacement therapy. Current controversies. Minerva Ginecologica 55 107–116.[Medline]
Busenlehner LS & Armstrong RN 2005 Insights into enzyme structure and dynamics elucidated by amide H/D exchange mass spectrometry. Archives of Biochemistry and Biophysics 433 34–46.[CrossRef][Web of Science][Medline]
Cavaillès VEstrogens and receptors: an evolving conceptClimacteric 5 Suppl_2 2002 20–26.
Chailleux C, Poirot M, Mesange F, Bayard F & Faye J-C 1994 Characterization of the membranous antiestrogen binding protein: I. Partial purification of the protein in its active state. Journal of Receptor Research 14 23–35.[Medline]
Chen D, Ma H, Hong H, Koh SS, Huang S-M, Schurter BT, Aswad DW & Stallcup MR 1999 Regulation of transcription by a protein methyltransferase. Science 284 2174–2177.
Cheskis BJ, Greger JG, Nagpal S & Freedman LP 2007 Signaling by estrogens. Journal of Cellular Physiology 213 610–617.[CrossRef][Web of Science][Medline]
Cristofaro PA, Opal SM, Palardy JE, Parejo NA, Jhung J, Keith JC Jr & Harris HA 2006 WAY-202196, a selective estrogen receptor-beta agonist, protects against death in experimental septic shock. Critical Care Medicine 34 2188–2193.[CrossRef][Web of Science][Medline]
Cushing CL, Bambara RA & Hilf R 1985 Interactions of estrogen-receptor and antiestrogen-receptor complexes with nuclei in vitro. Endocrinology 116 2419–2429.
Dai SY, Chalmers MJ, Bruning J, Bramlett KS, Osborne HE, Montrose-Rafizadeh C, Barr RJ, Wang Y, Wang M, Burris TP et al. 2008 Prediction of the tissue-specificity of selective estrogen receptor modulators by using a single biochemical method. PNAS 105 7171–7176.
Dalenc F, Giamarchi C, Petit M, Poirot M, Favre G & Faye J-C 2005 Farnesyl-transferase inhibitor R115,777 enhances tamoxifen inhibition of MCF-7 cell growth through estrogen receptor dependent and independent pathways. Breast Cancer Research 7 R1159–R1167.[CrossRef][Medline]
Delmas PD, Bjarnason NH, Mitlak BH, Ravoux A-C, Shah AS, Huster WJ, Draper M & Christiansen C 1997 Effects of raloxifene on bone mineral density, serum cholesterol concentrations, and uterine endometrium in postmenopausal women. New England Journal of Medicine 337 1641–1647.
Diel P 2002 Tissue-specific estrogenic response and molecular mechanisms. Toxicology Letters 127 217–224.[CrossRef][Web of Science][Medline]
Duma D, Jewell CM & Cidlowski JA 2006 Multiple glucocorticoid receptor isoforms and mechanisms of post-translational modification. Journal of Steroid Biochemistry and Molecular Biology 102 11–21.[CrossRef][Web of Science][Medline]
End DW, Smets G, Todd AV, Applegate TL, Fuery CJ, Angibaud P, Venet M, Sanz G, Poignet H, Skrzat S et al. 2001 Characterization of the antitumor effects of the selective farnesyl protein transferase inhibitor R115777 in vivo and in vitro. Cancer Research 61 131–137.
Estébanez-Perpiñá E, Arnold AA, Nguyen P, Rodrigues ED, Mar E, Bateman R, Pallai P, Shokat KM, Baxter JD, Guy RK et al. 2007a A surface on the androgen receptor that allosterically regulates coactivator binding. PNAS 104 16074–16079.
Estébanez-Perpiñá E, Arnold LA, Jouravel N, Togashi M, Blethrow J, Mar E, Nguyen P, Phillips KJ, Baxter JD, Webb P et al. 2007b Structural insight into the mode of action of a direct inhibitor of coregulator binding to the thyroid hormone receptor. Molecular Endocrinology 21 2919–2928.
Faguet GB 1986 Number of receptor sites from Scatchard and Klotz graphs: complementary approaches. Journal of Cellular Biochemistry 31 243–250.[Medline]
Farboud B & Privalsky ML 2004 Retinoic acid receptor-alpha is stabilized in a repressive state by its C-terminal, isotype-specific F domain. Molecular Endocrinology 18 2839–2853.
Faus H & Haendler B 2006 Post-translational modifications of steroid receptors. Biomedicine and Pharmacotherapy 60 520–528.
Faye J-C, Lasserre B & Bayard F 1980 Antiestrogen specific, high affinity saturable binding sites in rat uterine cytosol. Biochemical and Biophysical Research Communications 93 1225–1231.[CrossRef][Medline]
Fisher B, Costantino JP, Wickerham DL, Redmond CK, Kavanah M, Cronin WM, Vogel V, Robidoux A, Dimitrov N, Atkins J et al. 1998 Tamoxifen for prevention of breast cancer: report of the National Surgical Adjuvant Breast and Bowel Project P-1 Study. Journal of the National Cancer Institute 90 1371–1388.
Fisher B, Costantino JP, Wickerham DL, Cecchini RS, Cronin WM, Robidoux A, Bevers TB, Kavanah MT, Atkins JN, Margolese RG et al. 2005 Tamoxifen for the prevention of breast cancer: current status of the National Surgical Adjuvant Breast and Bowel Project P-1 study. Journal of the National Cancer Institute 97 1652–1662.
Folkertsma S, van Noort PI, de Heer A, Carati P, Brandt R, Visser A, Vriend G & de Vlieg J 2007 The use of in vitro peptide binding profiles and in silico ligand–receptor interaction profiles to describe ligand-induced conformations of the retinoid X receptor alpha ligand-binding domain. Molecular Endocrinology 21 30–48.
Furr BJA & Jordan VC 1984 The pharmacology and clinical uses of tamoxifen. Pharmacology and Therapeutics 25 127–205.[CrossRef]
Gangloff M, Ruff M, Eiler S, Duclaud S, Wurtz JM & Moras D 2001 Crystal structure of a mutant hER ligand-binding domain reveals key structural features for the mechanism of partial agonism. Journal of Biological Chemistry 276 15059–15065.
Gao T & McPhaul MJ 1998 Functional activities of the A and B forms of the human androgen receptor in response to androgen receptor agonists and antagonists. Molecular Endocrinology 12 654–663.
Geistlinger TR & Guy RK 2003a Novel selective inhibitors of the interaction of individual nuclear hormone receptors with a mutually shared steroid receptor coactivator 2. Journal of the American Chemical Society 125 6852–6853.[CrossRef][Web of Science][Medline]
Geistlinger TR & Guy RK 2003b Steroid receptor coactivator peptidomimetics. Methods in Enzymology 364 223–246.[Medline]
Geistlinger TR, McReynolds AC & Guy RK 2004 Ligand-selective inhibition of the interaction of steroid receptor coactivators and estrogen receptor isoforms. Chemistry & Biology 11 273–281.[CrossRef][Medline]
Glaros S, Atanaskova N, Zhao C, Skafar DF & Reddy KB 2006 Activation function-1 domain of estrogen receptor regulates the agonistic and antagonistic actions of tamoxifen. Molecular Endocrinology 20 996–1008.
Goldstein SW, Bordner J, Hoth LR & Geoghegan KF 2001 Chemical and biochemical issues related to X-ray crystallography of the ligand-binding domain of estrogen receptor alpha. Bioconjugate Chemistry 12 406–413.[CrossRef][Medline]
Gottardis MM, Robinson SP, Satyaswaroop PG & Jordan VC 1988 Contrasting actions of tamoxifen on endometrial and breast tumor growth in the athymic mouse. Cancer Research 48 812–815.
Gouet P, Robert X & Courcelle E 2003 ESPript/ENDscript: extracting and rendering sequence and 3D information from atomic structures of proteins. Nucleic Acids Research 31 3320–3323.
Greene GL, Nolan C, Engler JP & Jensen EV 1980 Monoclonal antibodies to human estrogen receptor. PNAS 77 5115–5119.
Gross C, Yu M, Van Herle AJ, Giuliano AE & Juillard GJF 1993 Presence of a specific antiestrogen binding site on human follicular thyroid carcinoma cell line (UCLA RO 82 W-1): inhibition by an endogenous ligand present in human serum. Journal of Clinical Endocrinology and Metabolism 77 1361–1366.[Medline]
Gulino A & Pasqualini JR 1980 Specific binding and biological response of antiestrogens in the fetal uterus of the guinea pig. Cancer Research 40 3821–3826.
Gurevich I, Flores AM & Aneskievich BJ 2007 Corepressors of agonist-bound nuclear receptors. Toxicology and Applied Pharmacology 223 288–298.[CrossRef][Medline]
Gustafsson J-Å 2003 What pharmacologists can learn from recent advances in estrogen signalling. Trends in Pharmacological Sciences 24 479–485.[CrossRef][Medline]
Hager GL, Nagaich AK, Johnson TA, Walker DA & John S 2004 Dynamics of nuclear receptor movement and transcription. Biochimica et Biophysica Acta 1677 46–51.[Medline]
Hall JM & McDonnell DP 1999 The estrogen receptor beta-isoform (ERβ) of the human estrogen receptor modulates ER transcriptional activity and is a key regulator of the cellular response to estrogens and antiestrogens. Endocrinology 140 5566–5578.
Hall JM & McDonnell DP 2005 Coregulators in nuclear estrogen receptor action: from concept to therapeutic targeting. Molecular Interventions 5 343–357.
Hedden A, Müller V & Jensen EV 1995 A new interpretation of antiestrogen action. Annals of the New York Academy of Sciences 761 109–120.[Web of Science][Medline]
Heinlein CA & Chang C 2002 Androgen receptor (AR) coregulators: an overview. Endocrine Reviews 23 175–200.
Heldring N, Nilsson M, Buehrer B, Treuter E & Gustafsson J-Å 2004 Identification of tamoxifen-induced coregulator interaction surfaces within the ligand-binding domain of estrogen receptors. Molecular and Cellular Biology 24 3445–3459.
Heldring N, Pawson T, McDonnell D, Treuter E, Gustafsson J-Å & Pike ACW 2007a Structural insights into corepressor recognition by antagonist-bound estrogen receptors. Journal of Biological Chemistry 282 10449–10455.
Heldring N, Pike A, Andersson S, Matthews J, Cheng G, Hartman J, Tujague M, Ström A, Treuter E, Warner M et al. 2007b Estrogen receptors: how do they signal and what are their targets. Physiological Reviews 87 905–931.
Henzler-Wildman K & Kern D 2007 Dynamic personalities of proteins. Nature 450 964–972.[CrossRef][Web of Science][Medline]
van Hoorn WP 2002 Identification of a second binding site in the estrogen receptor. Journal of Medicinal Chemistry 45 584–589.[CrossRef][Web of Science][Medline]
Hsieh RW, Rajan SS, Sharma SK, Guo Y, DeSombre ER, Mrksich M & Greene GL 2006 Identification of ligands with bicyclic scaffolds provides insights into mechanisms of estrogen receptor subtype selectivity. Journal of Biological Chemistry 281 17909–17919.
Huang H-J, Norris JD & McDonnell DP 2002 Identification of a negative regulatory surface within estrogen receptor alpha provides evidence in support of a role for corepressors in regulating cellular responses to agonists and antagonists. Molecular Endocrinology 16 1778–1792.
Huang J, Koide A, Nettle KW, Greene GL & Koide S 2006 Conformation-specific affinity purification of proteins using engineered binding proteins: application to the estrogen receptor. Protein Expression and Purification 47 348–354.[CrossRef][Web of Science][Medline]
Jensen EVThe curious pharmacology of the antiestrogensJJ Li, SA Li & JR Daling Hormonal Carcinogenesis III: Proceedings of the Third International Symposium. 2001SpringerNew York:139–143.
Jensen EV & Khan SA 2004 A two-site model for antiestrogen action. Mechanisms of Ageing and Development 125 679–682.[CrossRef][Medline]
Jordan VC 1984 Biochemical pharmacology of antiestrogen action. Pharmacological Reviews 36 245–276.[Web of Science][Medline]
Jordan VC & O'Malley BW 2007 Selective estrogen-receptor modulators and antihormonal resistance in breast cancer. Journal of Clinical Oncology 25 5815–5824.
Jordan VC & Robinson SP 1987 Species-specific pharmacology of antiestrogens: role of metabolism. Federation Proceedings 46 1870–1874.[Web of Science][Medline]
Jordan VC, Fischer AH & Rose DP 1981 Binding of [3H] monohydroxytamoxifen in human breast carcinoma cytosols. European Journal of Cancer 17 121–122.[Medline]
Jouravel N, Sablin E, Arnold LA, Guy RK & Fletterick RJ 2007 Interaction between the androgen receptor and a segment of its corepressor SHP. Acta Crystallographica. Section D. Biological Crystallography 63 1198–1200.[Medline]
Karnik PS, Kulkarni S, Liu XP, Budd GT & Bukowski RM 1994 Estrogen receptor mutations in tamoxifen-resistant breast cancer. Cancer Research 54 349–353.
Katzenellenbogen BS, Kendra KL, Norman MJ & Berthois Y 1987 Proliferation, hormonal responsiveness, and estrogen receptor content of MCF-7 human breast cancer cells grown in the short-term and long-term absence of estrogens. Cancer Research 47 4355–4360.
Kauppi B, Jakob C, Färnegårdh M, Yang J, Ahola H, Alarcon M, Calles K, Engström O, Harlan J, Muchmore S et al. 2003 The three-dimensional structures of antagonistic and agonistic forms of the glucocorticoid receptor ligand-binding domain: RU-486 induces a transconformation that leads to active antagonism. Journal of Biological Chemistry 278 22748–22754.
Kedjouar B, de Médina P, Oulad-Abdelghani M, Payré B, Silvente-Poirot S, Favre G, Faye J-C & Poirot M 2004 Molecular characterization of the microsomal tamoxifen binding site. Journal of Biological Chemistry 279 34048–34061.
Kern D & Zuiderweg ER 2003 The role of dynamics in allosteric regulation. Current Opinion in Structural Biology 13 748–757.[CrossRef][Web of Science][Medline]
Kian Tee M, Rogatsky I, Tzagarakis-Foster C, Cvoro A, An J, Christy RJ, Yamamoto KR & Leitman DC 2004 Estradiol and selective estrogen receptor modulators differentially regulate target genes with estrogen receptors alpha and beta. Molecular Biology of the Cell 15 1262–1272.
Kim K, Thu N, Saville B & Safe S 2003 Domains of estrogen receptor alpha (ER) required for ER/Sp1-mediated activation of GC-rich promoters by estrogens and antiestrogens in breast cancer cells. Molecular Endocrinology 17 804–817.
Klinge CM, Bambara RA, Zain S & Hilf R 1987 Estrogen receptor binding to nuclei from normal and neoplastic rat mammary tissues in vitro. Cancer Research 47 2852–2859.
Klinge CM, Knox DT, Bambara RA, Zain S & Hilf R 1989 Antiestrogen(4-hydroxytamoxifen)-charged estrogen receptor binding to nuclei from normal and neoplastic rat mammary tissues is not affected by host hormonal status. Journal of Steroid Biochemistry 33 335–340.[CrossRef][Medline]
Klinge CM, Traish AM, Bambara RA & Hilf R 1996a Dissociation of 4-hydroxytamoxifen, but not estradiol or tamoxifen aziridine, from the estrogen receptor as the receptor binds estrogen response element DNA. Journal of Steroid Biochemistry and Molecular Biology 57 51–66.[CrossRef][Web of Science][Medline]
Klinge CM, Traish AM, Driscoll MD, Hilf R & Bambara RA 1996b Site-directed estrogen receptor antibodies stabilize 4-hydroxytamoxifen ligand, but not estradiol, and indicate ligand-specific differences in the recognition of estrogen response element DNA in vitro. Steroids 61 278–289.[CrossRef][Web of Science][Medline]
Klinge CM, Studinski-Jones AL, Kulakosky PC, Bambara RA & Hilf R 1998 Comparison of tamoxifen ligands on estrogen receptor interaction with estrogen response elements. Molecular and Cellular Endocrinology 143 79–90.[CrossRef][Web of Science][Medline]
Klotz IM 1982 Numbers of receptor sites from Scatchard graphs: facts and fantasies. Science 217 1247–1249.
Knox AJ, Meegan MJ & Lloyd DG 2006 Estrogen receptors: molecular interactions, virtual screening and future prospects. Current Topics in Medicinal Chemistry 6 217–243.[CrossRef][Medline]
van den Koedijk CDMA, Vis van Heemst C, Elsendoorn GM, Thijssen JHH & Blankenstein MA 1992 Comparative affinity of steroidal and non-steroidal antioestrogens, cholesterol derivatives and compounds with a dialkylamino side chain for the rat liver antioestrogen binding site. Biochemical Pharmacology 43 2511–2518.[CrossRef][Medline]
Koide A, Abbatiello S, Rothgery L & Koide S 2002 Probing protein conformational changes in living cells by using designer binding proteins: application to the estrogen receptor. PNAS 99 1253–1258.
Koide A, Gilbreth RN, Esaki K, Tereshko V & Koide S 2007a High-affinity single-domain binding proteins with a binary-code interface. PNAS 104 6632–6637.
Koide A, Zhao C, Naganuma M, Abrams J, Deighton-Collins S, Skafar DF & Koide S 2007b Identification of regions within the F domain of the human estrogen receptor alpha that are important for modulating transactivation and protein–protein interactions. Molecular Endocrinology 21 829–842.
Kong EH, Heldring N, Gustafsson J-Å, Treuter E, Hubbard RE & Pike ACW 2005 Delineation of a unique protein–protein interaction site on the surface of the estrogen receptor. PNAS 102 3593–3598.
Kressler D, Hock MB & Kralli A 2007 Coactivators PGC-1β and SRC-1 interact functionally to promote the agonist activity of the selective estrogen receptor modulator tamoxifen. Journal of Biological Chemistry 282 26897–26907.
Kudolo GB, Elder MG & Myatt L 1984 A novel oestrogen-binding species in rat granulosa cells. Journal of Endocrinology 102 83–91.
Kuiper GGJM, Lemmen JG, Carlsson B, Corton JC, Safe SH, van der Saag PT, van der Burg B & Gustafsson J-Å 1998 Interaction of estrogenic chemicals and phytoestrogens with estrogen receptor beta. Endocrinology 139 4252–4263.
Larkin MA, Blackshields G, Brown NP, Chenna R, McGettigan PA, McWilliam H, Valentin F, Wallace IM, Wilm A, Lopez R et al. 2007 ClustalW and ClustalX version 2.0. Bioinformatics 23 2947–2948.
Leduc A-M, Trent JO, Wittliff JL, Bramlett KS, Briggs SL, Chirgadze NY, Wang Y, Burris TP & Spatola AF 2003 Helix-stabilized cyclic peptides as selective inhibitors of steroid receptor–coactivator interactions. PNAS 100 11273–11278.
Lee KC & Lee Kraus W 2001 Nuclear receptors, coactivators and chromatin: new approaches, new insights. Trends in Endocrinology and Metabolism 12 191–197.[CrossRef][Web of Science][Medline]
Lerea CC, Klinge CM, Bambara RA, Zain S & Hilf R 1987 Characterization of a cytosolic inhibitor of calf estrogen receptor binding to nuclei. Endocrinology 121 1146–1154.
Li S & Shang Y 2007 Regulation of SRC family coactivators by post-translational modifications. Cellular Signalling 19 1101–1112.[CrossRef][Medline]
Li J, Fu J, Toumazou C, Yoon H-G & Wong J 2006 A role of the amino-terminal (N) and carboxyl-terminal (C) interaction in binding of androgen receptor to chromatin. Molecular Endocrinology 20 776–785.
Long X & Nephew KP 2006 Fulvestrant (ICI 182,780)-dependent interacting proteins mediate immobilization and degradation of estrogen receptor-alpha. Journal of Biological Chemistry 281 9607–9615.
Love RR, Mazess RB, Barden HS, Epstein S, Newcomb PA, Jordan VC, Carbone PP & DeMets DL 1992 Effects of tamoxifen on bone mineral density in postmenopausal women with breast cancer. New England Journal of Medicine 326 852–856.[Abstract]
Mangelsdorf DJ, Thummel C, Beato M, Herrlich P, Schütz G, Umesono K, Blumberg B, Kastner P, Mark M, Chambon P et al. 1995 The nuclear receptor superfamily: the second decade. Cell 83 835–839.[CrossRef][Web of Science][Medline]
Martin PM, Berthois Y & Jensen EV 1988 Binding of antiestrogens exposes an occult antigenic determinant in the human estrogen receptor. PNAS 85 2533–2537.
Matthews J & Gustafsson J-Å 2003 Estrogen signaling: a subtle balance between ER and ER β. Molecular Interventions 3 281–292.
McDonnell DP 1999 The molecular pharmacology of SERMs. Trends in Endocrinology and Metabolism 10 301–311.[CrossRef][Web of Science][Medline]
McDonnell DP, Clemm DL, Hermann T, Goldman ME & Pike JW 1995 Analysis of estrogen receptor function in vitro reveals three distinct classes of antiestrogens. Molecular Endocrinology 9 659–669.
Mésange F, Sebbar M, Capdevielle J, Guillemot J-C, Ferrara P, Bayard F, Poirot M & Faye J-C 2002 Identification of two tamoxifen target proteins by photolabeling with 4-(2-morpholinoethoxy)benzophenone. Bioconjugate Chemistry 13 766–772.[CrossRef][Medline]
Métivier R, Stark A, Flouriot G, Hübner MR, Brand H, Penot G, Manu D, Denger S, Reid G, Kos M et al. 2002 A dynamic structural model for estrogen receptor-alpha activation by ligands, emphasizing the role of interactions between distant A and E domains. Molecular Cell 10 1019–1032.[CrossRef][Web of Science][Medline]
Mewshaw RE, Edsall RJ Jr, Yang C, Manas ES, Xu ZB, Henderson RA, Keith JC Jr & Harris HA 2005 ERβ ligands. 3. Exploiting two binding orientations of the 2-phenylnaphthalene scaffold to achieve ERβ selectivity. Journal of Medicinal Chemistry 48 3953–3979.[CrossRef][Web of Science][Medline]
Mizwicki MT, Keidel D, Bula CM, Bishop JE, Zanello LP, Wurtz J-M, Moras D & Norman AW 2004 Identification of an alternative ligand-binding pocket in the nuclear vitamin D receptor and its functional importance in 1,25(OH)2-vitamin D3 signaling. PNAS 101 12876–12881.
Modarress KJ, Opoku J, Xu M, Sarlis NJ & Simons SS Jr 1997 Steroid-induced conformational changes at ends of the hormone-binding domain in the rat glucocorticoid receptor are independent of agonist versus antagonist activity. Journal of Biological Chemistry 272 23986–23994.
Montano MM, Müller V, Trobaugh A & Katzenellenbogen BS 1995 The carboxy-terminal F domain of the human estrogen receptor: role in the transcriptional activity of the receptor and the effectiveness of antiestrogens as estrogen antagonists. Molecular Endocrinology 9 814–825.
Moore JT, Collins JL & Pearce KH 2006 The nuclear receptor superfamily and drug discovery. ChemMedChem 1 504–523.
Murphy LC & Sutherland RL 1981a A high affinity intracellular binding site for non-steroidal oestrogen antagonists. Reviews on Endocrine-Related Cancer 9 177–184.
Murphy LC & Sutherland RL 1981b A high-affinity binding site for the antioestrogens, tamoxifen and CI 628, in immature rat uterine cytosol which is distinct from the oestrogen receptor. Journal of Endocrinology 91 155–161.
Nahoum V, Pérez E, Germain P, Rodriguez-Barrios F, Manzo F, Kammerer S, Lemaire G, Hirsch O, Royer CA, Gronemeyer H et al. 2007 Modulators of the structural dynamics of the retinoid X receptor to reveal receptor function. PNAS 104 17323–17328.
Naundorf H, Jost-Reuhl B, Becker M, Reuhl T, Neumann C & Fichtner I 2000 Differences in immunoreactivity of estrogen receptor (ER) in tamoxifen-sensitive and -resistant breast carcinomas: preclinical and first clinical investigations. Breast Cancer Research and Treatment 60 81–92.[CrossRef][Web of Science][Medline]
Nettles KW, Bruning JB, Gil G, Nowak J, Sharma SK, Hahm JB, Kulp K, Hochberg RB, Zhou H, Katzenellenbogen JA et al. 2008 NFkappaB selectivity of estrogen receptor ligands revealed by comparative crystallographic analyses. Nature Chemical Biology 4 241–247.
Nilsson S & Gustafsson J-Å 2002 Biological role of estrogen and estrogen receptors. Critical Reviews in Biochemistry and Molecular Biology 37 1–28.[CrossRef][Web of Science][Medline]
Nilsson S, Mäkelä S, Treuter E, Tujague M, Thomsen J, Andersson G, Enmark E, Pettersson K, Warner M & Gustafsson J-Å 2001 Mechanisms of estrogen action. Physiological Reviews 81 1535–1565.
Norman AW, Mizwicki MT & Norman DPG 2004 Steroid-hormone rapid actions, membrane receptors and a conformational ensemble model. Nature Reviews. Drug Discovery 3 27–41.[CrossRef][Web of Science][Medline]
Pagliaro L, Felding J, Audouze K, Nielsen SJ, Terry RB, Krog-Jensen C & Butcher S 2004 Emerging classes of protein–protein interaction inhibitors and new tools for their development. Current Opinion in Chemical Biology 8 442–449.[CrossRef][Web of Science][Medline]
Paige LA, Christensen DJ, Grøn H, Norris JD, Gottlin EB, Padilla KM, Chang C-Y, Ballas LM, Hamilton PT, McDonnell DP et al. 1999 Estrogen receptor (ER) modulators each induce distinct conformational changes in ER and ER β. PNAS 96 3999–4004.
Pearce KH, Iannone MA, Simmons CA & Gray JG 2004 Discovery of novel nuclear receptor modulating ligands: an integral role for peptide interaction profiling. Drug Discovery Today 9 741–751.[CrossRef][Web of Science][Medline]
Peczuh MW & Hamilton AD 2000 Peptide and protein recognition by designed molecules. Chemical Reviews 100 2479–2494.[CrossRef][Medline]
Perissi V & Rosenfeld MG 2005 Controlling nuclear receptors: the circular logic of cofactor cycles. Nature Reviews. Molecular Cell Biology 6 542–554.[CrossRef][Web of Science][Medline]
Peters GA & Khan SA 1999 Estrogen receptor domains E and F: role in dimerization and interaction with coactivator RIP-140. Molecular Endocrinology 13 286–296.
Petrescu AD, Hertz R, Bar-Tana J, Schroeder F & Kier AB 2005 Role of regulatory F-domain in hepatocyte nuclear factor-4alpha ligand specificity. Journal of Biological Chemistry 280 16714–16727.
Pike ACW 2006 Lessons learnt from structural studies of the oestrogen receptor. Best Practice and Research. Clinical Endocrinology and Metabolism 20 1–14.[CrossRef]
Pike ACW, Brzozowski AM, Hubbard RE, Bonn T, Thorsell AG, Engström O, Ljunggren J, Gustafsson J-Å & Carlquist M 1999 Structure of the ligand-binding domain of oestrogen receptor beta in the presence of a partial agonist and a full antagonist. EMBO Journal 18 4608–4618.[CrossRef][Web of Science][Medline]
Pike ACW, Brzozowski AM, Walton J, Hubbard RE, Bonn T & Gustafsson J-Å 2000 Structural aspects of agonism and antagonism in the oestrogen receptor. Biochemical Society Transactions 28 396–400.[Web of Science][Medline]
Pike ACW, Brzozowski AM, Walton J, Hubbard RE, Thorsell AG, Li Y-L, Gustafsson J-Å & Carlquist M 2001 Structural insights into the mode of action of a pure antiestrogen. Structure 9 145–153.[Medline]
Poulin R, Merand Y, Poirier D, Levesque C, Dufour J-M & Labrie F 1989 Antiestrogenic properties of keoxifene, trans-4-hydroxytamoxifen, and ICI 164384, a new steroidal antiestrogen, in ZR-75-1 human breast cancer cells. Breast Cancer Research and Treatment 14 65–76.[CrossRef][Web of Science][Medline]
Raam S, D'Agincourt PG & Jordan VC 1983 A comparative study of electrophoretic mobilities of [3H]-estradiol and monohydroxytamoxifen binding components in the cytosols of human breast carcinomas and sera of healthy adult females. European Journal of Cancer and Clinical Oncology 19 1457–1465.
Raviscioni M, He Q, Salicru EM, Smith CL & Lichtarge O 2006 Evolutionary identification of a subtype specific functional site in the ligand binding domain of steroid receptors. Proteins 64 1046–1057.[CrossRef][Web of Science][Medline]
Renaud JP & Moras D 2000 Structural studies on nuclear receptors. Cellular and Molecular Life Sciences 57 1748–1769.[CrossRef][Web of Science][Medline]
Rochel N, Wurtz JM, Mitschler A, Klaholz B & Moras D 2000 The crystal structure of the nuclear receptor for vitamin D bound to its natural ligand. Molecular Cell 5 173–179.[CrossRef][Web of Science][Medline]
Rodriguez AL, Tamrazi A, Collins ML & Katzenellenbogen JA 2004 Design, synthesis, and in vitro biological evaluation of small molecule inhibitors of estrogen receptor alpha coactivator binding. Journal of Medicinal Chemistry 47 600–611.[CrossRef][Web of Science][Medline]
Roelens F, Heldring N, Dhooge W, Bengtsson M, Comhaire F, Gustafsson J-Å, Treuter E & De Keukeleire D 2006 Subtle side-chain modifications of the hop phytoestrogen 8-prenylnaringenin result in distinct agonist/antagonist activity profiles for estrogen receptors alpha and beta. Journal of Medicinal Chemistry 49 7357–7365.[CrossRef][Medline]
Ruse MD Jr, Privalsky ML & Sladek FM 2002 Competitive cofactor recruitment by orphan receptor hepatocyte nuclear factor 41: modulation by the F domain. Molecular and Cellular Biology 22 1626–1638.
Savkur RS & Burris TP 2004 The coactivator LXXLL nuclear receptor recognition motif. Journal of Peptide Research 63 207–212.[CrossRef][Web of Science][Medline]
Schwartz JA, Zhong L, Deighton-Collins S, Zhao C & Skafar DF 2002 Mutations targeted to a predicted helix in the extreme carboxyl-terminal region of the human estrogen receptor-alpha alter its response to estradiol and 4-hydroxytamoxifen. Journal of Biological Chemistry 277 13202–13209.
Shang Y & Brown M 2002 Molecular determinants for the tissue specificity of SERMs. Science 295 2465–2468.
Shao W & Brown M 2004 Advances in estrogen receptor biology: prospects for improvements in targeted breast cancer therapy. Breast Cancer Research 6 39–52.[CrossRef][Web of Science][Medline]
Shao D, Berrodin TJ, Manas E, Hauze D, Powers R, Bapat A, Gonder D, Winneker RC & Frail DE 2004 Identification of novel estrogen receptor alpha antagonists. Journal of Steroid Biochemistry and Molecular Biology 88 351–360.[CrossRef][Web of Science][Medline]
Shiau AK, Barstad D, Loria PM, Cheng L, Kushner PJ, Agard DA & Greene GL 1998 The structural basis of estrogen receptor/coactivator recognition and the antagonism of this interaction by tamoxifen. Cell 95 927–937.[CrossRef][Web of Science][Medline]
Skafar DF & Koide S 2006 Understanding the human estrogen receptor-alpha using targeted mutagenesis. Molecular and Cellular Endocrinology 246 83–90.[CrossRef][Medline]
Skafar DF & Zhao C 2008 The multifunctional estrogen receptor-alpha F domain. Endocrine 33 1–8.[Medline]
Sladek FM, Ruse MD Jr, Nepomuceno L, Huang S-M & Stallcup MR 1999 Modulation of transcriptional activation and coactivator interaction by a splicing variation in the F domain of nuclear receptor hepatocyte nuclear factor 41. Molecular and Cellular Biology 19 6509–6522.
Smith CL & O'Malley BW 2004 Coregulator function: a key to understanding tissue specificity of selective receptor modulators. Endocrine Reviews 25 45–71.
Smith CL, Nawaz Z & O'Malley BW 1997 Coactivator and corepressor regulation of the agonist/antagonist activity of the mixed antiestrogen, 4-hydroxytamoxifen. Molecular Endocrinology 11 657–666.
Stallcup MR, Kim JH, Teyssier C, Lee Y-H, Ma H & Chen D 2003 The roles of protein–protein interactions and protein methylation in transcriptional activation by nuclear receptors and their coactivators. Journal of Steroid Biochemistry and Molecular Biology 85 139–145.[CrossRef][Web of Science][Medline]
Steinmetz AC, Renaud J-P & Moras D 2001 Binding of ligands and activation of transcription by nuclear receptors. Annual Review of Biophysics and Biomolecular Structure 30 329–359.[CrossRef][Web of Science][Medline]
Suaud L, Formstecher P & Laine B 1999 The activity of the activation function 2 of the human hepatocyte nuclear factor 4 (HNF-4) is differently modulated by F domains from various origins. Biochemical Journal 340 161–169.[CrossRef][Web of Science][Medline]
Sutherland RL & Foo MS 1979 Differential binding of antiestrogens by rat uterine and chick oviduct cytosol. Biochemical and Biophysical Research Communications 91 183–191.[CrossRef][Web of Science][Medline]
Sutherland RL, Murphy LC, San Foo M, Green MD, Whybourne AM & Krozowski ZS 1980 High-affinity anti-oestrogen binding site distinct from the oestrogen receptor. Nature 288 273–275.[CrossRef][Medline]
Swain JF & Gierasch LM 2006 The changing landscape of protein allostery. Current Opinion in Structural Biology 16 102–108.[CrossRef][Web of Science][Medline]
Takayama S, Hostick U, Haendel M, Eisen J & Darimont B 2007 An F-domain introduced by alternative splicing regulates activity of the zebrafish thyroid hormone receptor alpha. General and Comparative Endocrinology 155 176–189.
Tanenbaum DM, Wang Y, Williams SP & Sigler PB 1998 Crystallographic comparison of the estrogen and progesterone receptor's ligand binding domains. PNAS 95 5998–6003.
Tateishi Y, Sonoo R, Sekiya Y-I, Sunahara N, Kawano M, Wayama M, Hirota R, Kawabe Y-I, Murayama A, Kato S et al. 2006 Turning off estrogen receptor beta-mediated transcription requires estrogen-dependent receptor proteolysis. Molecular and Cellular Biology 26 7966–7976.
Thomas T, Gallo MA & Thomas TJ 2004 Estrogen receptors as targets for drug development for breast cancer, osteoporosis and cardiovascular diseases. Current Cancer Drug Targets 4 483–499.[CrossRef][Web of Science][Medline]
Tocchini-Valentini G, Rochel N, Wurtz JM, Mitschler A & Moras D 2001 Crystal structures of the vitamin D receptor complexed to superagonist 20-epi ligands. PNAS 98 5491–5496.
Toogood PL 2002 Inhibition of protein-protein association by small molecules: approaches and progress. Journal of Medicinal Chemistry 45 1543–1558.[CrossRef][Web of Science][Medline]
Tyulmenkov VV & Klinge CM 2000 Interaction of tetrahydrocrysene ketone with estrogen receptors alpha and beta indicates conformational differences in the receptor subtypes. Archives of Biochemistry and Biophysics 381 135–142.[CrossRef][Web of Science][Medline]
Vegeto E, Allan GF, Schrader WT, Tsai MJ, McDonnell DP & O'Malley BW 1992 The mechanism of RU486 antagonism is dependent on the conformation of the carboxy-terminal tail of the human progesterone receptor. Cell 69 703–713.[CrossRef][Web of Science][Medline]
Van de Velde P, Nique F, Bouchoux F, Brémaud J, Hameau M-C, Lucas D, Moratille C, Viet S, Philibert D & Teutsch G 1994 RU 58,668, a new pure antiestrogen inducing a regression of human mammary carcinoma implanted in nude mice. Journal of Steroid Biochemistry and Molecular Biology 48 187–196.[CrossRef][Web of Science][Medline]
Voegel JJ, Heine MJS, Zechel C, Chambon P & Gronemeyer H 1996 TIF2, a 160 kDa transcriptional mediator for the ligand-dependent activation function AF-2 of nuclear receptors. EMBO Journal 15 3667–3675.[Web of Science][Medline]
Vogel CL 2003 Update on the current use of hormonals as therapy in advanced breast cancer. Anticancer Drugs 14 265–273.[Medline]
Wang Y, Chirgadze NY, Briggs SL, Khan S, Jensen EV & Burris TP 2006 A second binding site for hydroxytamoxifen within the coactivator-binding groove of estrogen receptor beta. PNAS 103 9908–9911.
Watts CK & Sutherland RL 1984 High affinity specific antiestrogen binding sites are concentrated in rough microsomal membranes of rat liver. Biochemical and Biophysical Research Communications 120 109–115.[CrossRef][Web of Science][Medline]
Watts CK & Sutherland RL 1986 Microsomal binding sites for antioestrogens in rat liver. Properties and detergent solubilization. Biochemical Journal 236 903–911.[Medline]
Watts CK, Murphy LC & Sutherland RL 1984 Microsomal binding sites for nonsteroidal anti-estrogens in MCF 7 human mammary carcinoma cells. Demonstration of high affinity and narrow specificity for basic ether derivatives of triphenylethylene. Journal of Biological Chemistry 259 4223–4229.
Wehling M 1997 Specific, nongenomic actions of steroid hormones. Annual Review of Physiology 59 365–393.[CrossRef][Web of Science][Medline]
Williams SP & Sigler PB 1998 Atomic structure of progesterone complexed with its receptor. Nature 393 392–396.[CrossRef][Medline]
Winneker RC & Clark JH 1983 Estrogenic stimulation of the antiestrogen specific binding site in rat uterus and liver. Endocrinology 112 1910–1915.
Winneker RC, Guthrie SC & Clark JH 1983 Characterization of a triphenylethylene-antiestrogen-binding site on rat serum low density lipoprotein. Endocrinology 112 1823–1827.
Wu Y-L, Yang X, Ren Z, McDonnell DP, Norris JD, Willson TM & Greene GL 2005 Structural basis for an unexpected mode of SERM-mediated ER antagonism. Molecular Cell 18 413–424.[CrossRef][Web of Science][Medline]
Wu M, Soler DR, Abba MC, Nunez MI, Baer R, Hatzis C, Llombart-Cussac A, Llombart-Bosch A & Aldaz CM 2007 CtIP silencing as a novel mechanism of tamoxifen resistance in breast cancer. Molecular Cancer Research 5 1285–1295.
Xu HE, Stanley TB, Montana VG, Lambert MH, Shearer BG, Cobb JE, McKee DD, Galardi CM, Plunket KD, Nolte RT et al. 2002 Structural basis for antagonist-mediated recruitment of nuclear co-repressors by PPARalpha. Nature 415 813–817.[Medline]
Yan X, Broderick D, Leid ME, Schimerlik MI & Deinzer ML 2004 Dynamics and ligand-induced solvent accessibility changes in human retinoid X receptor homodimer determined by hydrogen deuterium exchange and mass spectrometry. Biochemistry 43 909–917.[CrossRef][Web of Science][Medline]
Yan X, Deinzer ML, Schimerlik MI, Broderick D, Leid ME & Dawson MI 2006 Investigation of ligand interactions with human RXRalpha by hydrogen/deuterium exchange and mass spectrometry. Journal of the American Society for Mass Spectrometry 17 1510–1517.[CrossRef][Web of Science][Medline]
Zhao L & Chmielewski J 2005 Inhibiting protein–protein interactions using designed molecules. Current Opinion in Structural Biology 15 31–34.[Medline]
Zhou H-B, Collins ML, Gunther JR, Comninos JS & Katzenellenbogen JA 2007 Bicyclo[2.2.2]octanes: close structural mimics of the nuclear receptor-binding motif of steroid receptor coactivators. Bioorganic and Medicinal Chemistry Letters 17 4118–4122.
Zsarnovszky A, Le HH, Wang H-S & Belcher SM 2005 Ontogeny of rapid estrogen-mediated extracellular signal-regulated kinase signaling in the rat cerebellar cortex: potent nongenomic agonist and endocrine disrupting activity of the xenoestrogen bisphenol A. Endocrinology 146 5388–5396.
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