Angel R. de Lera

International Union of Pharmacology. LXIII. Retinoid X Receptors

The physiological effects of retinoic acids (RAs) are mediated by members of two families of nuclear receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs), which are encoded by three distinct human genes, RXRα, RXRβ, and RXRγ. RARs bind both all-trans- and 9-cis-RA, whereas only the 9-cis-RA stereoisomer binds to RXRs. As RXR/RAR heterodimers, these receptors control the transcription of RA target genes through binding to RA-response elements. This review is focused on the structure, mode of action, ligands, expression, and pharmacology of RXRs. Given their role as common partners to many other members of the nuclear receptor superfamily, these receptors have been the subject of intense scrutiny. Moreover, and despite numerous studies since their initial discovery, RXRs remain enigmatic nuclear receptors, and there is still no consensus regarding their role. Indeed, multiple questions about the actual biological role of RXRs and the existence of an endogenous ligand have still to be answered.

RAR and RXR modulation in cancer and metabolic disease

Retinoic acid receptors (RARs) are ligand-controlled transcription factors that function as heterodimers with retinoid X receptors (RXRs) to regulate cell growth and survival. The success of RAR modulation in the treatment of acute promyelocytic leukaemia (APL) has stimulated considerable interest in the development of RAR and RXR modulators. This has been aided by recent advances in the understanding of the biological role of RARs and RXRs and in the design of selective receptor modulators that might overcome the limitations of current drugs. Here, we discuss the challenges and opportunities for therapeutic strategies based on RXR and RAR modulators, with a focus on cancer and metabolic diseases such as diabetes and obesity.

International Union of Pharmacology. LX. Retinoic Acid Receptors

Retinoid is a term for compounds that bind to and activate retinoic acid receptors (RARα, RARβ, and RARγ), members of the nuclear hormone receptor superfamily. The most important endogenous retinoid is all-trans-retinoic acid. Retinoids regulate a wide variety of essential biological processes, such as vertebrate embryonic morphogenesis and organogenesis, cell growth arrest, differentiation and apoptosis, and homeostasis, as well as their disorders. This review summarizes the considerable amount of knowledge generated on these receptors.

Structural basis for the high all-trans-retinaldehyde reductase activity of the tumor marker AKR1B10

AKR1B10 is a human aldo-keto reductase (AKR) found to be elevated in several cancer types and in precancerous lesions. In vitro, AKR1B10 exhibits a much higher retinaldehyde reductase activity than any other human AKR, including AKR1B1 (aldose reductase). We here demonstrate that AKR1B10 also acts as a retinaldehyde reductase in vivo. This activity may be relevant in controlling the first step of retinoic acid synthesis. Up-regulation of AKR1B10, resulting in retinoic acid depletion, may lead to cellular proliferation. Both in vitro and in vivo activities of AKR1B10 were inhibited by tolrestat, an AKR1B1 inhibitor developed for diabetes treatment. The crystal structure of the ternary complex AKR1B10–NADP+–tolrestat was determined at 1.25-Å resolution. Molecular dynamics models of AKR1B10 and AKR1B1 with retinaldehyde isomers and site-directed mutagenesis show that subtle differences at the entrance of the retinoid-binding site, especially at position 125, are determinant for the all-trans-retinaldehyde specificity of AKR1B10. Substitutions in the retinaldehyde cyclohexene ring also influence the specificity. These structural features should facilitate the design of specific inhibitors, with potential use in cancer and diabetes treatments.

Characterization of the Interaction between Retinoic Acid Receptor/Retinoid X Receptor (RAR/RXR) Heterodimers and Transcriptional Coactivators through Structural and Fluorescence Anisotropy Studies

Retinoid receptors (RARs and RXRs) are ligand-activated transcription factors that regulate the transcription of target genes by recruiting coregulator complexes at cognate promoters. To understand the effects of heterodimerization and ligand binding on coactivator recruitment, we solved the crystal structure of the complex between the RARβ/RXRα ligand-binding domain heterodimer, its 9-cis retinoic acid ligand, and an LXXLL-containing peptide (termed NR box 2) derived from the nuclear receptor interaction domain (NID) of the TRAP220 coactivator. In parallel, we measured the binding affinities of the isolated NR box 2 peptide or the full-length NID of the coactivator SRC-1 for retinoid receptors in the presence of various types of ligands. Our correlative analysis of three-dimensional structures and fluorescence data reveals that heterodimerization does not significantly alter the structure of individual subunits or their intrinsic capacity to interact with NR box 2. Similarly, we show that the ability of a protomer to recruit NR box 2 does not vary as a function of the ligand binding status of the partner receptor. In contrast, the strength of the overall association between the heterodimer and the full-length SRC-1 NID is dictated by the combinatorial action of RAR and RXR ligands, the simultaneous presence of the two receptor agonists being required for highest binding affinity. We identified an LXXLL peptide-driven mechanism by which the concerted reorientation of three phenylalanine side chains generates an “aromatic clamp” that locks the RXR activation helix H12 in the transcriptionally active conformation. Finally, we show how variations of helix H11-ligand interactions can alter the communication pathway linking helices H11, H12, and the connecting loop L11-12 to the coactivator-binding site. Together, our results reveal molecular and structural features that impact on the ligand-dependent interaction of the RAR/RXR heterodimer with nuclear receptor coactivators.

Modulators of the structural dynamics of the retinoid X receptor to reveal receptor function.

Retinoid X receptors (RXRα, -β, and -γ) occupy a central position in the nuclear receptor superfamily, because they form heterodimers with many other family members and hence are involved in the control of a variety of (patho)physiologic processes. Selective RXR ligands, referred to as rexinoids, are already used or are being developed for cancer therapy and have promise for the treatment of metabolic diseases. However, important side effects remain associated with existing rexinoids. Here we describe the rational design and functional characterization of a spectrum of RXR modulators ranging from partial to pure antagonists and demonstrate their utility as tools to probe the implication of RXRs in cell biological phenomena. One of these ligands renders RXR activity particularly sensitive to coactivator levels and has the potential to act as a cell-specific RXR modulator. A combination of crystallographic and fluorescence anisotropy studies reveals the molecular details accounting for the agonist-to-antagonist transition and provides direct experimental evidence for a correlation between the pharmacological activity of a ligand and its impact on the structural dynamics of the activation helix H12. Using RXR and its cognate ligands as a model system, our correlative analysis of 3D structures and dynamic data provides an original view on ligand actions and enables the establishment of mechanistic concepts, which will aid in the development of selective nuclear receptor modulators.

A unique secondary-structure switch controls constitutive gene repression by retinoic acid receptor

In the absence of ligand, some nuclear receptors, including retinoic acid receptor (RAR), act as transcriptional repressors by recruiting corepressor complexes to target genes. This constitutive repression is crucial in metazoan reproduction, development and homeostasis. However, its specific molecular determinants had remained obscure. Using structural, biochemical and cell-based assays, we show that the basal repressive activity of RAR is conferred by an extended β-strand that forms an antiparallel β-sheet with specific corepressor residues. Agonist binding induces a β-strand–to–α-helix transition that allows for helix H11 formation, which in turn provokes corepressor release, repositioning of helix H12 and coactivator recruitment. Several lines of evidence suggest that this structural switch could be implicated in the intrinsic repressor function of other nuclear receptors. Finally, we report on the molecular mechanism by which inverse agonists strengthen corepressor interaction and enhance gene silencing by RAR.

Functions, therapeutic applications, and synthesis of retinoids and carotenoids.

Carotenoids Rosana Álvarez,† Beleń Vaz,† Hinrich Gronemeyer,‡ and Ángel R. de Lera*,† †Departamento de Química Orgańica, Centro de Investigacioń Biomed́ica (CINBIO), and Instituto de Investigacioń Biomed́ica de Vigo (IBIV), Universidade de Vigo, 36310 Vigo, Spain ‡Department of Functional Genomics and Cancer, Institut de Geńet́ique et de Biologie Molećulaire et Cellulaire (IGBMC)/CNRS/INSERM/ULP, BP 10142, 67404 Illkirch Cedex, C. U. de Strasbourg, France

Rational design of RAR-selective ligands revealed by RARβ crystal stucture

The crystal structure of the ligand‐binding domain of RARβ, a suspect tumour suppressor, reveals important features that distinguish it from the two other RAR isotypes. The most striking difference is an extra cavity allowing RARβ to bind more bulky agonists. Accordingly, we identified a ligand that shows RARβ selectivity with a 100‐fold higher affinity to RARβ than to α or γ isotypes. The structural differences between the three RAR ligand‐binding pockets revealed a rationale explaining how a single retinoid can be at the same time an RARα, γ antagonist and an RARβ agonist. In addition, we demonstrate how to generate an RARβ antagonist by gradually modifying the bulkiness of a single substitution. Together, our results provide structural guidelines for the synthesis of RARβ‐selective agonists and antagonists, allowing for the first time to address pharmacologically the tumour suppressor role of RARβ in vitro and in animal models.

A General Synthesis of Alkenyl-Substituted Benzofurans, Indoles, and Isoquinolones by Cascade Palladium-Catalyzed Heterocyclization/Oxidative Heck Coupling

Structurally diverse C3-alkenylbenzofurans, C3-alkenylindoles, and C4-alkenylisoquinolones are efficiently prepared by using consecutive Sonogashira and cascade Pd-catalyzed heterocyclization/oxidative Heck couplings from readily available ortho-iodosubstituted phenol, aniline, and benzamide substrates, alkynes, and functionalized olefins. The cyclization of O- and N-heteronucleophiles follows regioselective 5-endo-dig- or 6-endo-dig-cyclization modes, whereas the subsequent Heck-type coupling with both mono- and disubstituted olefins takes place stereoselectively with exclusive formation of the E isomers in most cases.