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, 17beta-oestradiol, and the selective antagonist raloxifene, at resolutions of 3.1 and 2.6 A, 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.

Oestrogens exert their physiological effects through two receptor subtypes. Here we report the three-dimensional structure of the oestrogen receptor beta isoform (ERbeta) ligand-binding domain (LBD) in the presence of the phyto-oestrogen genistein and the antagonist raloxifene. The overall structure of ERbeta-LBD is very similar to that previously reported for ERalpha. 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 ER's endogenous hormone, 17beta-oestradiol. However, in the ERbeta-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 genistein's partial agonist character in ERbeta and demonstrates how ER's transcriptional response to certain bound ligands is attenuated.

The structures of the liver X receptor LXR␤ (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 LXR␣ (NR1H3) it regulates target genes involved in metabolism and transport of cholesterol and fatty acids. The two LXR␤ 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.Liver X receptors (LXR) 1 are members of the superfamily of nuclear receptors. These transcription factors regulate target genes through a dynamic series of interactions with specific DNA response elements as well as transcriptional coregulators. The binding of ligand has profound effects on these interactions and has the potential to trigger both gene activation and, in some cases, gene silencing. There are 48 sequence-related nuclear receptors in humans and the family comprises receptors that recognize hormones, both steroidal and non-steroidal, but also receptors responding to metabolic intermediates and to xenobiotics. There are also a number of so-called orphan receptors where the natural ligand is unknown. Some of the receptors show a very specific and high affinity ligand binding, like the thyroid hormone receptors, whereas others have a substantially lower affinity for their ligands and are less discriminating in their ligand selectivity. Like many of the other nonsteroid hormone receptors, LXR functions as a heterodimer with the retinoid X receptor (RXR) to regulate gene expression (1, 2). Together with peroxisome proliferator-activated receptor (PPAR) and farnesoid X receptor (FXR), LXRs represent a subclass of so-called permissive RXR heterodimers. In this subclass, the RXR heterodimers can be activated independently by either the RXR ligand, the partner's ligand, or synergistically by both (3).LXRs consist of two closely related receptor isoforms encoded by separate genes, LXR␣ (NR1H3) and LXR␤ (NR1H2). LXR␣ shows tissue-restricted expression with the highest mRNA levels in the liver and somewhat lower levels in the kidney, small intestine, spleen, and adrenal gland (4, 5). In contrast, LXR␤ is ubiquitously expressed (6, 7). Both LXR isoforms can be activated by specific oxysterols that are formed in vivo (2,8,9). In view of the high degree of homology between the LXR isoforms (75% identity in the ligand-binding domain (LBD), 54% identity overall), it is perhaps not surprising that few subtypespecific biological responses have been described and that information on subtype selective ligands is limited. LXRs have been shown to regulate several genes involved in cholesterol and lipid homeos...

A series of substituted 2-benzyl-3-aryl-7-trifluoromethylindazoles were prepared as LXR modulators. These compounds were partial agonists in transactivation assays when compared to 1 (T0901317) and were slightly weaker with respect to potency and efficacy on LXRalpha than on LXRbeta. Lead compounds in this series 12 (WAY-252623) and 13 (WAY-214950) showed less lipid accumulation in HepG2 cells than potent full agonists 1 and 3 (WAY-254011) but were comparable in efficacy to 1 and 3 with respect to cholesterol efflux in THP-1 foam cells, albeit weaker in potency. Compound 13 reduced aortic lesion area in LDLR knockout mice equivalently to 3 or positive control 2 (GW3965). In a 7-day hamster model, compound 13 showed a lesser propensity for plasma TG elevation than 3, when the compounds were compared at doses in which they elevated ABCA1 and ABCG1 gene expression in duodenum and liver at equal levels. In contrast to results previously published for 2, the lack of TG effect of 13 correlated with its inability to increase liver fatty acid synthase (FAS) gene expression, which was up-regulated 4-fold by 3. These results suggest indazoles such as 13 may have an improved profile for potential use as a therapeutic agent.

A structure-based approach was used to optimize our new class of quinoline LXR modulators leading to phenyl acetic acid substituted quinolines 15 and 16. Both compounds displayed good binding affinity for LXRbeta and LXRalpha and were potent activators in LBD transactivation assays. The compounds also increased expression of ABCA1 and stimulated cholesterol efflux in THP-1 cells. Quinoline 16 showed good oral bioavailability and in vivo efficacy in a LDLr knockout mouse model for lesions.