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.

Molecular basis of agonism and antagonism in the oestrogen receptor.

The RXR heterodimers and orphan receptors

I. Introduction II. Molecular Aspects A. PPAR isotypes: identity, genomic organization and chromosomal localization B. DNA binding properties C. PPAR ligand-binding properties D. Alternative pathways for PPAR activation E. PPAR-mediated transactivation properties III. Physiological Aspects A. Differential expression of PPAR mRNAs B. PPAR target genes and functions in fatty acid metabolism C. PPARs and control of inflammatory responses D. PPARs and atherosclerosis E. PPARs and the development of the fetal epidermal permeability barrier F. PPARs, carcinogenesis, and control of the cell cycle IV. Conclusions

Peroxisome proliferator-activated receptors: nuclear control of metabolism.

Retinoids play an important role in development, differentiation, and homeostasis. The discovery of retinoid receptors belonging to the superfamily of nuclear ligand-activated transcriptional regulators has revolutionized our molecular understanding as to how these structurally simple molecules exert their pleiotropic effects. Diversity in the control of gene expression by retinoid signals is generated through complexity at different levels of the signaling pathway. A major source of diversity originates from the existence of two families of retinoid acid (RA) receptors (R), the RAR isotypes (alpha, beta, and gamma) and the three RXR isotypes (alpha, beta, and gamma), and their numerous isoforms, which bind as RXR/RAR heterodimers to the polymorphic cis-acting response elements of RA target genes. The possibility of cross-modulation (cross-talk) with cell-surface receptors signaling pathways, as well as the finding that RARs and RXRs interact with multiple putative coactivators and/or corepressors, generates additional levels of complexity for the array of combinatorial effects that underlie the pleiotropic effects of retinoids. This review focuses on recent developments, particularly in the area of structure-function relationships.

A decade of molecular biology of retinoic acid receptors.

The Structural Basis of Estrogen Receptor/Coactivator Recognition and the Antagonism of This Interaction by Tamoxifen

The crystal structure of the human retinoid-X receptor RXR-alpha ligand-binding domain reveals a previously undiscovered fold of an antiparallel alpha-helical sandwich, packed as dimeric units. Two helices and one loop form the homodimerization surface, and hydrophobic heptad repeats participate in stabilizing the fold. The existence of a ligand-binding pocket is proposed that would allow 9-cis retinoic acid to interact with different functional modules, including the AF-2 activating domain. Several lines of evidence indicate that the overall structure is a prototype fold of ligand-binding domains of nuclear receptors.

Crystal structure of the ligand-binding domain of the human nuclear receptor RXR-alpha.

The ability of nuclear receptors (NRs) to activate transcription of target genes requires the binding of cognate ligands to their ligand-binding domains (LBDs). Information provided by the three-dimensional structures of the unliganded RXR alpha and the liganded RAR gamma LBDs has been incorporated into a general alignment of the LBDs of all NRs. A twenty amino-acid region constitutes a NR-specific signature and contains most of the conserved residues that stabilize the core of the canonical fold of NR LBDs. A common ligand-binding pocket, involving predominantly hydrophobic residues, is inferred by homology modelling of the human RXR alpha and glucocorticoid receptor ligand-binding sites according to the RAR gamma holo-LBD structure. Mutant studies support these models, as well as a general mechanism for ligand-induced activation deduced from the comparison of the transcriptionally active RAR gamma holo- and inactive RXR alpha apo-LBD structures.

A canonical structure for the ligand-binding domain of nuclear receptors.

Nuclear receptor ligand-binding domains: three-dimensional structures, molecular interactions and pharmacological implications

https://www.cell.com/molecular-cell/pdf/S1097-2765(00)80424-4.pdf

Crystal structure of a heterodimeric complex of RAR and RXR ligand-binding domains.

Osh/Orp proteins transport sterols between organelles and are involved in phosphoinositide metabolism. The link between these two aspects remains elusive. Using novel assays, we address the influence of membrane composition on the ability of Osh4p/Kes1p to extract, deliver, or transport dehydroergosterol (DHE). Surprisingly, phosphatidylinositol 4-phosphate (PI(4)P) specifically inhibited DHE extraction because PI(4)P was itself efficiently extracted by Osh4p. We solve the structure of the Osh4p-PI(4)P complex and reveal how Osh4p selectively substitutes PI(4)P for sterol. Last, we show that Osh4p quickly exchanges DHE for PI(4)P and, thereby, can transport these two lipids between membranes along opposite routes. These results suggest a model in which Osh4p transports sterol from the ER to late compartments pinpointed by PI(4)P and, in turn, transports PI(4)P backward. Coupled to PI(4)P metabolism, this transport cycle would create sterol gradients. Because the residues that recognize PI(4)P are conserved in Osh4p homologues, other Osh/Orp are potential sterol/phosphoinositol phosphate exchangers.

/pdf/osh4p-exchanges-sterols-for-phosphatidylinositol-4-phosphate-2nlbbkqnyd.pdf

William Bourguet

Papers

Crystal structure of the ligand-binding domain of the human nuclear receptor RXR-alpha.

A canonical structure for the ligand-binding domain of nuclear receptors.

Nuclear receptor ligand-binding domains: three-dimensional structures, molecular interactions and pharmacological implications

Crystal structure of a heterodimeric complex of RAR and RXR ligand-binding domains.

Osh4p exchanges sterols for phosphatidylinositol 4-phosphate between lipid bilayers