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Humans are chronically exposed to mixtures of xenobiotics referred to as endocrine-disrupting chemicals (EDCs). A vast body of literature links exposure to these chemicals with increased incidences of reproductive, metabolic, or neurological disorders. Moreover, recent data demonstrate that, when used in combination, chemicals have outcomes that cannot be predicted from their individual behavior. In its heterodimeric form with the retinoid X receptor (RXR), the pregnane X receptor (PXR) plays an essential role in controlling the mammalian xenobiotic response and mediates both beneficial and detrimental effects. Our previous work shed light on a mechanism by which a binary mixture of xenobiotics activates PXR in a synergistic fashion. Structural analysis revealed that mutual stabilization of the compounds within the ligand-binding pocket of PXR accounts for the enhancement of their binding affinity. In order to identify and characterize additional active mixtures, we combined a set of cell-based, biophysical, structural, and in vivo approaches. Our study reveals features that confirm the binding promiscuity of this receptor and its ability to accommodate bipartite ligands. We reveal previously unidentified binding mechanisms involving dynamic structural transitions and covalent coupling and report four binary mixtures eliciting graded synergistic activities. Last, we demonstrate that the robust activity obtained with two synergizing PXR ligands can be enhanced further in the presence of RXR environmental ligands. Our study reveals insights as to how low-dose EDC mixtures may alter physiology through interaction with RXR–PXR and potentially several other nuclear receptor heterodimers.

Key Points Small-molecule ligands that modulate the action of retinoic acid receptors (RARs) and retinoid X receptors (RXRs) — members of the nuclear receptor superfamily — have been used as anticancer drugs. However, their use can be limited by side effects. RARs and RXRs are each expressed from three isotypic genes (α,β and γ). The receptors are composed of a modular structure with several domains and associated functions. The main domains are the DNA-binding domain (DBD) and the ligand-binding domain (LBD). Drug discovery efforts are focusing on isotype-selective modulators to reduce toxicity associated with current retinoid therapy; elicit more specific biological responses, because the tissue distribution of retinoid receptor isotypes is not uniform; better define the physiological role of each retinoid receptor isotype; and clarify the potential of RXR-targeted pharmacology. Various crystal structures of RARs and RXRs have been elucidated, allowing for the correlation of ligand structure and (allosterically altered) receptor structure with the functional properties of the ligand-bound receptor. This information will provide guidelines for the design of novel analogues, and advances in synthetic organic chemistry will also be useful in the development of isotype-selective ligands with a range of activities from agonists to inverse agonists through to powerful antagonists. Retinoic acid receptors (RARs) are important drug targets for cancer therapy and prevention, and the potential of rexinoids for the treatment of metabolic diseases is increasingly being recognized. This article reviews recent structural data for RARs and retinoid X receptors (RXRs), discusses strategies in the design of selective RXR and RAR modulators, and consider lessons that can be learned for the design of selective nuclear-receptor modulators in general. Retinoic acid receptors (RARs) and retinoid X receptors (RXRs) are members of the nuclear receptor superfamily whose effects on cell growth and survival can be modulated therapeutically by small-molecule ligands. Although compounds that target these receptors are powerful anticancer drugs, their use is limited by toxicity. An improved understanding of the structural biology of RXRs and RARs and recent advances in the chemical synthesis of modified retinoid and rexinoid ligands should enable the rational design of more selective agents that might overcome such problems. Here, we review structural data for RXRs and RARs, discuss strategies in the design of selective RXR and RAR modulators, and consider lessons that can be learned for the design of selective nuclear-receptor modulators in general.

The potential for food contact chemicals to disrupt genetic programs in development and metabolism raises concerns. Nuclear receptors (NRs) control many of these programs, and the retinoid-X receptor (RXR) is a DNA-binding partner for one-third of the NRs. RXR disruption could generate adverse outcomes in several NR pathways. We used machine learning and other in silico methods to identify RXR-interacting candidates from a list of over 57,000 chemicals. Butylphenols comprised the largest, high-probability, structural group (58 compounds); several are food contact chemicals with widespread commercial use. In vitro ToxCast data suggested that bulky, aliphatic substitution at C4 of 2,6-di-tert-butylphenol facilitated RXR activation. We tested six butylphenols with increasing bulk at C4 in vivo for their ability to disrupt thyroid hormone receptor (TR) signaling, using an integrated luciferase reporter driven by TR-RXR binding and quantifiable morphological changes in a Xenopus laevis precocious metamorphosis assay. Three tert-butylphenols potentiated TH action at nanomolar concentrations. Molecular modeling showed the three positives formed more frequent, stable interactions with RXRα, and bulkiness at C4 increased steric complementarity with the RXR ligand-binding pocket. Our findings establish a paradigm for machine learning coupled with a convenient, in vivo validation approach to identify chemicals interacting with RXR-NR-controlled genetic pathways.

The retinoid X receptors (RXRs) are ligand-activated transcription factors which heterodimerize with a number of nuclear hormone receptors, thereby controlling a variety of (patho)-physiological processes. Although synthetic RXR ligands are developed for the treatment of various diseases, endogenous ligand(s) for these receptors have not been conclusively identified. We show here that mice lacking cellular retinol binding protein (Rbp1-/-) display memory deficits reflecting compromised RXR signaling. Using HPLC-MS and chemical synthesis we identified in Rbp1-/- mice reduced levels of 9-cis-13,14-dihydroretinoic acid (9CDHRA), which acts as an RXR ligand since it binds and transactivates RXR in various assays. 9CDHRA rescues the Rbp1-/- phenotype similarly to a synthetic RXR ligand and displays similar transcriptional activity in cultured human dendritic cells. High endogenous levels of 9CDHRA in mice indicate physiological relevance of these data and that 9CDHRA acts as an endogenous RXR ligand.

A comprehensive HDX analysis of the vitamin D receptor–retinoid X receptor (VDR-RXR) reveals extensive allosteric communication within the same polypeptide where DNA or ligand can induce alterations in distant domains of the receptors. The work also shows communication between subunits in the heterodimer. The vitamin D receptor (VDR) functions as an obligate heterodimer in complex with the retinoid X receptor (RXR). These nuclear receptors are multidomain proteins, and it is unclear how various domains interact with one another within the nuclear receptor heterodimer. Here, we show that binding of intact heterodimer to DNA alters the receptor dynamics in regions remote from the DNA-binding domains (DBDs), including the coactivator binding surfaces of both co-receptors, and that the sequence of the DNA response element can determine these dynamics. Furthermore, agonist binding to the heterodimer results in changes in the stability of the VDR DBD, indicating that the ligand itself may play a role in DNA recognition. These data suggest a mechanism by which nuclear receptors show promoter specificity and have differential effects on various target genes, providing insight into the function of selective nuclear receptor modulators.

Bexarotene is an FDA approved retinoid X-receptor (RXR) agonist for the treatment of cutaneous T-cell lymphoma, and its use in other cancers and Alzheimer’s disease is being investigated. The drug causes serious side effects, which might be reduced by chemical modifications of the molecule. To rationalize known agonists and to help identify sites for potential substitutions we present molecular simulations in which the RXR ligand-binding domain was flooded with a large number of drug-like molecules, and molecular dynamics simulations of a series of bexarotene-like ligands bound to the RXR ligand-binding domain. Based on the flooding simulations, two regions of interest for ligand modifications were identified: a hydrophobic area near the bridgehead and another near the fused ring. In addition, positional fluctuations of the phenyl ring were generally smaller than fluctuations of the fused ring of the ligands. Together, these observations suggest that the fused ring might be a good target for the design of higher affinity bexarotene-like ligands, while the phenyl ring is already optimized. In addition, notable differences in ligand position and interactions between the RXRα and RXRβ were observed, as well as differences in hydrogen bonding and solvation, which might be exploited in the development of subspecies-specific ligands.

A subset of nuclear receptors (NRs) function as permissive heterodimers with retinoid X receptor (RXR), defined by transcriptional activation in response to RXR agonist ligands. Permissive NR-RXR activation is generally understood to operate through a classical pharmacological mechanism in which RXR agonist binding enhances coactivator recruitment to the heterodimer. However, we previously demonstrated that transcriptional activation of permissive Nurr1-RXRα (NR4A2-NR2B1) heterodimers by an RXR ligand set, which included pharmacological RXR agonists and selective Nurr1-RXRα agonists that function as antagonists of RXRα homodimers, is explained by a non-classical activation mechanism involving ligand-binding domain (LBD) heterodimer dissociation (Yu et al., 2023). Here, we extend mechanistic ligand profiling of the same RXR ligand set to the evolutionarily related Nur77-RXRγ (NR4A1-NR2B3) heterodimer. Biochemical and NMR protein-protein interaction profiling, together with cellular transcription studies, indicate that activation of Nur77-RXRγ transcription by the RXR ligand set, which lacks selective Nur77-RXRγ agonists, is consistent with contributions from both classical pharmacological activation and LBD heterodimer dissociation. However, reanalysis of our previously published data for Nurr1-RXRα revealed that inclusion of selective Nurr1-RXRα agonists was essential for elucidating the LBD heterodimer dissociation mechanism. Together, our findings highlight the importance of using a more functionally diverse RXR ligand set to define the mechanism of Nur77-RXRγ activation and to further evaluate whether LBD heterodimer dissociation represents a shared activation mechanism among NR4A-RXR heterodimers relevant to neurodegenerative and inflammatory diseases.

Background: Nuclear receptors are transcription factors of central importance in human biology and associated diseases. Much of the knowledge related to their major functions, such as ligand and DNA binding or dimerization, derives from functional studies undertaken in classical model animals. It has become evident, however, that a deeper understanding of these molecular functions requires uncovering how these characteristics originated and diversified during evolution, by looking at more species. In particular, the comprehension of how dimerization evolved from ancestral homodimers to a more sophisticated state of heterodimers has been missing, due to a too narrow phylogenetic sampling. Here, we experimentally and phylogenetically define the evolutionary trajectory of nuclear receptor dimerization by analyzing a novel NR7 subgroup, present in various metazoan groups, including cnidarians, annelids, mollusks, sea urchins, and amphioxus, but lost in vertebrates, arthropods, and nematodes.Results: We focused on NR7 of the cephalochordate amphioxus B. lanceolatum. We present a complementary set of functional, structural, and evolutionary analyses that establish that NR7 lies at a pivotal point in the evolutionary trajectory from homodimerizing to heterodimerizing nuclear receptors. The crystal structure of the NR7 ligand-binding domain suggests that the isolated domain is not capable of dimerizing with the ubiquitous dimerization partner RXR. In contrast, the full-length NR7 dimerizes with RXR in a DNA-dependent manner and acts as a constitutively active receptor. The phylogenetic and sequence analyses position NR7 at a pivotal point, just between the basal class I nuclear receptors that form monomers or homodimers on DNA and the derived class II nuclear receptors that exhibit the classical DNA-independent RXR heterodimers.Conclusions: Our data suggest that NR7 represents the \"missing link\" in the transition between class I and class II nuclear receptors and that the DNA independency of heterodimer formation is a feature that was acquired during evolution. Our studies define a novel paradigm of nuclear receptor dimerization that evolved from DNA-dependent to DNA-independent requirements. This new concept emphasizes the importance of DNA in the dimerization of nuclear receptors, such as the glucocorticoid receptor and other members of this pharmacologically important oxosteroid receptor subfamily. Our studies further underline the importance of studying emerging model organisms for supporting cutting-edge research.

Retinoid X receptors (RXRα, β and γ) are recently known to be cancer chemotherapies targets. The ligand binding domains of RXRs have been crystallized, but the information of RXRγ ligand binding site is not yet available due to the lack of liganded complex. A thorough understanding of the ligand binding sites is essential to study RXRs and may result in cancer therapeutic breakthrough. Thus we aimed to study the RXRγ ligand binding site and find out the differences between the three subtypes. Alignment and molecular simulation were carried out for identifying the RXRγ ligand binding site, characterizing the RXRγ ligand binding mode and comparing the three RXRs. The result has indicated that the RXRγ ligand binding site is defined by helices H5, H10, β-sheet s1 and the end loop. Besides hydrophobic interactions, the ligand 9-cis retinoic acid interacts with RXRγ through a hydrogen bond with Ala106, a salt bridge with Arg95 and the π-π interactions with Phe217 and Phe218. The binding modes exhibit some similarities among RXRs, such as the interactions with Arg95 and Ala106. Nonetheless, owing to the absence of Ile47, Cys48, Ala50, Ala51 and residues 225∼237 in the active site, the binding pocket in RXRγ is two times larger than those of RXRα and RXRβ. Meanwhile, spatial effects of Trp84, Arg95, Ala106, Phe217 and Phe218 help to create a differently shaped binding pocket as compared to those of RXRα and RXRβ. Consequently, the ligand in RXRγ undergoes a “standing” posing which is distinct from the other two RXRs.

The principal abnormalities of the metabolic syndrome are abdominal obesity, atherogenic dyslipidemia, hypertension, insulin resistance, inflammation, and prothrombotic states. This review focuses on the retinoid X receptor (RXR) and its partners in the metabolic syndrome. RXR and its partners are nuclear receptors that function as ligand-dependent transcription factors. The ligands are lipids, and the system functions as a cellular lipid sensor. Agonists and inhibitors of these receptors are promising treatments for this widespread syndrome. This review focuses on the retinoid X receptor (RXR) and its partners in the metabolic syndrome. Agonists and inhibitors of these receptors are promising treatments for this widespread syndrome. The metabolic syndrome, also known as syndrome X, is characterized by abdominal obesity, atherogenic dyslipidemia, hypertension, insulin resistance, inflammation, and prothrombotic states. 1 Diagnostic of the metabolic syndrome are abnormalities in three or more of the clinical criteria of the Adult Treatment Panel III of the National Cholesterol Education Program, which include the following: a waist circumference of more than 102 cm in men and more than 88 cm in women; a triglyceride level of 150 mg per deciliter or more; a level of high-density lipoprotein (HDL) cholesterol of less than 40 mg per deciliter in men and less than 50 . . .