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

Abstract

Retinoid X receptors (RXR␣, -␤, and -␥) 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.crystal structure ͉ ligand design ͉ nuclear receptor ͉ agonist ͉ antagonist N uclear Receptor (NR)-controlled gene expression relies on a mechanism in which NRs recruit coregulators that are part of multiprotein complexes. These complexes correspond to chromatin-modifying and transcription-initiating machineries that act at target gene promoters in a precisely timed and sequential fashion (1). The binding of a ligand to the ligandbinding domain (LBD) of NRs constitutes the initial step of this regulatory process. In this context, the C-terminal helix H12 of LBDs plays a key role, because its position, which depends on the bound ligand, determines the type of coregulator recruited by the receptor (2). Structural studies have shown that in agonistbound NR LBDs, H12 adopts the so-called ''active'' or ''holo'' conformation and provides a binding surface for short NR interaction motifs of coactivators (3). In contrast, antagonists prevent H12 from adopting the holo position (4).Therapeutically, retinoid X receptor (RXR)-selective ligands, referred to as rexinoids, are used in cancer therapy, and previously uncharacterized rexinoid-based therapeutic paradigms are currently being explored. In addition, rexinoids have promise for use in the therapy of metabolic diseases (5, 6), but important side effects associated with existing compounds limit their use. Improved understanding of the biological role and the structural biology of RXR (7, 8) will allow the synthesis of selective modulators that might overcome the limitations of current drugs. Here,...

Implications of the binding of tamoxifen to the coactivator recognition site of the estrogen receptor

“…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, ERa (Dai et al 2008). In the case of ERa, 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.…”

Retinoid X Receptor-selective Signaling in the Regulation of Akt/Protein Kinase B Isoform-specific Expression

“…Role of RXR in Bexarotene-enhanced Myoblast Differentiation-To assess whether bexarotene-enhanced myoblast differentiation is mediated through RXR activation, we first employed a potent RXR antagonist UVI3003 (34). C2C12 myoblasts were differentiated with bexarotene in the presence of high concentrations of UVI3003, about 30-and 150-fold of K d values.…”

Retinoic acid represses CYP7A1 expression in human hepatocytes and HepG2 cells by FXR/RXR-dependent and independent mechanisms

“…DNA oligos and sequencing were provided by the Keck Biotechnology Center at Yale University. UVI3001 and UVI3003 are gifts from Prof. Angel R. de Lera (Universidade de Vigo, Spain) ( 41 ).…”