Mats Carlquist

Molecular basis of agonism and antagonism in the oestrogen receptor.

Oestrogens are involved in the growth, development and homeostasis of a number of tissues. The physiological effects of these steroids are mediated by a ligand-inducible nuclear transcription factor, the oestrogen receptor (ER). Hormone binding to the ligand-binding domain (LBD) of the ER initiates a series of molecular events culminating in the activation or repression of target genes. Transcriptional regulation arises from the direct interaction of the ER with components of the cellular transcription machinery,. Here we report the crystal structures of the LBD of ER in complex with the endogenous oestrogen, 17β-oestradiol, and the selective antagonist raloxifene, at resolutions of 3.1 and 2.6 Å, respectively. The structures provide a molecular basis for the distinctive pharmacophore of the ER and its catholic binding properties. Agonist and antagonist bind at the same site within the core of the LBD but demonstrate different binding modes. In addition, each class of ligand induces a distinct conformation in the transactivation domain of the LBD, providing structural evidence of the mechanism of antagonism.

Structure of the Ligand-Binding Domain of Oestrogen Receptor Beta in the Presence of a Partial Agonist and a Full Antagonist

Oestrogens exert their physiological effects through two receptor subtypes. Here we report the three‐dimensional structure of the oestrogen receptor beta isoform (ERβ) ligand‐binding domain (LBD) in the presence of the phyto‐oestrogen genistein and the antagonist raloxifene. The overall structure of ERβ‐LBD is very similar to that previously reported for ERα. Each ligand interacts with a unique set of residues within the hormone‐binding cavity and induces a distinct orientation in the AF‐2 helix (H12). The bulky side chain of raloxifene protrudes from the cavity and physically prevents the alignment of H12 over the bound ligand. In contrast, genistein is completely buried within the hydrophobic core of the protein and binds in a manner similar to that observed for ERs endogenous hormone, 17β‐oestradiol. However, in the ERβ–genistein complex, H12 does not adopt the distinctive ’agonist‘ position but, instead, lies in a similar orientation to that induced by ER antagonists. Such a sub‐optimal alignment of the transactivation helix is consistent with genisteins partial agonist character in ERβ and demonstrates how ERs transcriptional response to certain bound ligands is attenuated.

Structural Insights into the Mode of Action of a Pure Antiestrogen

BACKGROUND Estrogens exert their effects on target tissues by binding to a nuclear transcription factor termed the estrogen receptor (ER). Previous structural studies have demonstrated that each class of ER ligand (agonist, partial agonist, and SERM antagonist) induces distinctive orientations in the receptors carboxy-terminal transactivation helix. The conformation of this portion of the receptor determines whether ER can recruit and interact with the components of the transcriptional machinery, thereby facilitating target gene expression. RESULTS We have determined the structure of rat ERbeta ligand binding domain (LBD) in complex with the pure antiestrogen ICI 164,384 at 2.3 A resolution. The binding of this compound to the receptor completely abolishes the association between the transactivation helix (H12) and the rest of the LBD. The structure reveals that the terminal portion of ICIs bulky side chain substituent protrudes from the hormone binding pocket, binds along the coactivator recruitment site, and physically prevents H12 from adopting either its characteristic agonist or AF2 antagonist orientation. CONCLUSIONS The binding mode adopted by the pure antiestrogen is similar to that seen for other ER antagonists. However, the size and resultant positioning of the ligands side chain substituent produces a receptor conformation that is distinct from that adopted in the presence of other classes of ER ligands. The novel observation that binding of ICI results in the complete destabilization of H12 provides some indications as to a possible mechanism for pure receptor antagonism.

The three-dimensional structure of the liver X receptor beta reveals a flexible ligand-binding pocket that can accommodate fundamentally different ligands.

The structures of the liver X receptor LXRβ (NR1H2) have been determined in complexes with two synthetic ligands, T0901317 and GW3965, to 2.1 and 2.4 Å, respectively. Together with its isoform LXRα (NR1H3) it regulates target genes involved in metabolism and transport of cholesterol and fatty acids. The two LXRβ structures reveal a flexible ligand-binding pocket that can adjust to accommodate fundamentally different ligands. The ligand-binding pocket is hydrophobic but with polar or charged residues at the two ends of the cavity. T0901317 takes advantage of this by binding to His-435 close to H12 while GW3965 orients itself with its charged group in the opposite direction. Both ligands induce a fixed “agonist conformation” of helix H12 (also called the AF-2 domain), resulting in a transcriptionally active receptor.

Characterization of bacterially expressed rat estrogen receptor β ligand binding domain by mass spectrometry: Structural comparison with estrogen receptor α

Abstract Functional rat estrogen receptor β ligand binding domain (rERβ LBD, aa 210–485) and human estrogen receptor α ligand binding domain (hERα LBD, aa 301–553) were expressed in Escherichia coli. Hormone binding assays revealed that both ERβ and ERα LBDs bound the natural ligand estradiol (E2) with similar affinity (Kd ∼ 100 pM). Competitive binding experiments were carried out with ICI 164384, 4-hydroxytamoxifen, 16α-bromo-estradiol, and genistein employing [3H]E2 as a tracer. No significant differences in responses of ERα and ERβ LBDs to ICI 164384 and 4-hydroxytamoxifen were observed. 16α-Bromo-estradiol and genistein discriminated between the ER subtypes and acted as ERα and ERβ selective ligands, respectively. Final purification of recombinant proteins was achieved on an E2 affinity column, where they were subjected to in situ carboxymethylation. The partially carboxymethylated proteins actively bound E2. The carboxymethylated rERβ LBD had a molecular mass of 32251.6 Da, equivalent to the calculated mass with the addition of three carboxymethyl groups. No other proteins (of lower or higher molecular mass) were detected, so the LBD was considered structurally authentic and pure. By using a combination of intact protein mass spectrometric fragmentation and trypsin proteolysis (98% sequence coverage), it was established that rERβ cysteine-289 and -354 were not carboxymethylated on the affinity column, suggesting that they were shielded from alkylation in the E2-bound conformation state. Concurrent analysis of hERα LBD showed that under the same experimental conditions, the two equivalent ERα cysteines were not alkylated (αC381 and αC447). These data support close structural relationship between the E2-bound ERα LBD and ERβ LBD proteins.

Carboxymethylation of the human estrogen receptor ligand-binding domain-estradiol complex: HPLC/ESMS peptide mapping shows that cysteine 447 does not react with iodoacetic acid

Experiments were carried out to determine the degree of solvent and reagent accessibility of the cysteines in the ligand-binding domain of the human estrogen receptor (hER LBD). The cysteine residues were alkylated when human ER LBD was present in its ligand (estradiol)-bound conformation. Direct electrospray ionization mass spectrometry (ESMS) as well as liquid chromatography coupled with ESMS, and matrix-assisted laser ionization desorption time-of-flight mass spectrometry were used to determine the location and the yield of the derivatized residues after proteolysis with trypsin. We observed that the cysteine 447 was protected against alkylation under these conditions, whereas cysteines 381, 417, and 530 were fully derivatized.

An advanced biosensor for the prediction of estrogenic effects of endocrine-disrupting chemicals on the estrogen receptor alpha.

A label-free and time-resolved biosensor based on reflectometric interference spectroscopy (RIfS) has been developed to evaluate the agonistic or antagonistic effects of potential ligands with unknown behavior. The biosensor utilizes the specific interaction between the estrogen receptor α (ERα) and short specific peptides. The unique feature of these peptides allows the investigation of the behavior of ligands and the discrimination between the agonistic and antagonistic effects caused by conformational changes of the receptor. Thus, this developed biosensor allows not only the differentiation between ligands and nonligands of a receptor, but also the potential of these ligands to influence conformational changes in the receptor, leading to activation or inhibition of the receptor-dependent pathways. Owing to the robustness of the direct optical detection principle used, the biosensor is applicable to complex biological matrices, even crude cell extracts. Moreover, the reliability of the biosensor, including regeneration steps when performing subsequent measurements, has been verified.

Structural insights into the mechanisms of agonism and antagonism in oestrogen receptor isoforms

Here we summarise the results that have emerged from our structural studies on the oestrogen receptor (ER) ligand-binding domain. We have investigated the conformational effects of a variety of ligands on the structures of both ER isoforms. Each class of ligand (agonists, partial agonists and selective oestrogen receptor modulators) induces a unique conformation in the receptors ligand-dependent transcriptional activation function. Together these studies have broadened our understanding of ER function by providing a unique insight into ERs ligand specificity and the structural changes that underlie receptor agonism and antagonism.

Intact noncovalent dimer of estrogen receptor ligand-binding domain can be detected by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESMS) of the estrogen receptor ligand binding domain (ER LBD) in its estradiol-binding form was performed. A dimeric ER LBD was observed, with a greatly reduced capacity for protonation (major charge state for dimer +16 vs. +23 for a monomer). Peak broadening (probably due to heterogeneity resulting from salt and water adduct formation) adversely affected our ability to distinguish between multiple discreet dimeric species and thus prevented us from establishing an accurate average mass for the dimerized domain. A mixture of species with molecular masses between 57,240 Da and 57,900 Da was observed, which would compare to 57,274 Da, 57,546 Da, and 57,818 Da for the calculated masses of the dimer without estradiol, or with one or two bound ligand molecules, respectively. Hence, nonliganded ER LBD dimer appeared to constitute the major species. The presence of low levels of a singly liganded ER LBD dimer cannot be ruled out, but the data argue against the possibility of the ER LBD dimer carrying two molecules of estradiol. Allowing for current limitations in the technology, our data demonstrate that ESMS on a quadrupole mass spectrometer of limited mass range (4000 Da for singly charged ions) has potential utility for studying ligand-binding proteins. In particular, in future it might be possible to compare spectra obtained from agonist- and antagonist-bound receptors and determine from subtle changes in protonation state possible differences in the higher order structure of those noncovalent protein complexes.

Structural analysis of the thyroid hormone receptor ligand binding domain: studies using a quadrupole time-of-flight tandem mass spectrometer

The overall architecture of the ligand binding domain (LBD) of members of the nuclear receptor superfamily are similar. There are now standard procedures to express and purify these proteins. A rapid and sensitive method for the structural analysis of these proteins is nano-electrospray tandem mass spectrometry. In the present study we have analysed the LBD of the human thyroid hormone receptor-beta-1 (TR-beta) by quadrupole time-of-flight tandem mass spectrometry. The intact protein was analysed in a carboxymethylated form in an attempt to identify which cysteine residues are located on the surface. The protein molecular weight (31 652.5 Da) was determined with an accuracy of +/-1 Da, while masses of tryptic fragments were determined with an accuracy of at least 75 ppm. The sequence coverage of the tryptic peptide mass map was 93.2 %. Tryptic peptides were subjected to collision-induced dissociation (CID) and the resulting product ions were mass measured with an accuracy of about 100 ppm. When accurate mass measurements were made with internal calibration, mass accuracies were improved to +/-2 ppm in mass spectra, and +/-20 ppm in CID spectra. From these data it was possible to determine the presence of post-translational modifications, locate the sites of carboxymethylation and, in addition, confirm the amino acid sequence of the expressed protein. To the best of our knowledge, this is the first characterisation of the TR-LBD-beta at the protein level.