International Union of Pharmacology. LXIII. Retinoid X receptors

Dynamic interplay of nuclear receptors in tumor cell plasticity and drug resistance: Shifting gears in malignant transformations and applications in cancer therapeutics

Recent advances have brought forth the complex interplay between tumor cell plasticity and its consequential impact on drug resistance and tumor recurrence, both of which are critical determinants of neoplastic progression and therapeutic efficacy. Various forms of tumor cell plasticity, instrumental in facilitating neoplastic cells to develop drug resistance, include epithelial-mesenchymal transition (EMT) alternatively termed epithelial-mesenchymal plasticity, the acquisition of cancer stem cell (CSC) attributes, and transdifferentiation into diverse cell lineages. Nuclear receptors (NRs) are a superfamily of transcription factors (TFs) that play an essential role in regulating a multitude of cellular processes, including cell proliferation, differentiation, and apoptosis. NRs have been implicated to play a critical role in modulating gene expression associated with tumor cell plasticity and drug resistance. This review aims to provide a comprehensive overview of the current understanding of how NRs regulate these key aspects of cancer biology. We discuss the diverse mechanisms through which NRs influence tumor cell plasticity, including EMT, stemness, and metastasis. Further, we explore the intricate relationship between NRs and drug resistance, highlighting the impact of NR signaling on chemotherapy, radiotherapy and targeted therapies. We also discuss the emerging therapeutic strategies targeting NRs to overcome tumor cell plasticity and drug resistance. This review also provides valuable insights into the current clinical trials that involve agonists or antagonists of NRs modulating various aspects of tumor cell plasticity, thereby delineating the potential of NRs as therapeutic targets for improved cancer treatment outcomes.

Rational design and virtual screening identify mimetics of the RXR agonist valerenic acid

The ligand-sensing transcription factor retinoid X receptor (RXR) is the universal heterodimer partner of nuclear receptors and involved in multiple physiological processes. Its pharmacological modulation holds therapeutic potential in cancer and neurodegeneration but many available RXR ligands lack specificity. The sesquiterpenoid valerenic acid has been identified as RXR agonist with unprecedented subtype and homodimer preference. Here, we identified simplified mimetics of the complex natural product by rational design and virtual screening that exhibited similar activity profiles on RXR and informed about structural elements contributing to the favorable activity.

Discovery of small molecules from natural compound databases as potent retinoid X alpha receptor agonists to treat Alzheimer's disease

Alzheimer's is characterized as a progressive neurodegenerative disease due to beta-amyloid accumulation in the brain. Some previous studies reported that RXR agonists could be effective in the treatment of Alzheimer's disease. There are currently numerous attempts being made to discover a natural RXR agonist that is more potent than 9-cis-retinoic acid (9CR). One of the most efficient resources for finding high-potential compounds is natural databases. In this study, 81215 compounds from the IB screen library as natural databases were docked against the RXR-alpha binding site. The best compounds discovered interact with the RXR-alpha binding site with a lower binding energy (-11 to -13 kcal/mol) than the binding energy of -10.94 kcal/mol for 9-cis, which means that these compounds could interact stronger with RXR-alpha than 9CR. All selected compounds could pass the blood-brain barrier. Physiochemical properties assessment indicated that all compounds passed Lipinski's rule and had the potential to be oral drug candidates. The stability of protein-ligand complexes during a timescale of 100 ns by Molecular Dynamics simulation demonstrated that all compounds could effectively interact with the RXR binding site. The molecular mechanics/Poisson-Boltzmann surface area (MM/PBSA) represented that all selected hit compounds had a better binding affinity to the alpha RXR binding site compared to 9CR, which means these hit compounds had potential drug candidates for the treatment of Alzheimer's disease. However, experimental assessment is needed to validate this result.Communicated by Ramaswamy H. Sarma.

Structure-Guided Design of a Highly Potent Partial RXR Agonist with Superior Physicochemical Properties

Retinoid X receptors (RXRs, NR2B1-3) hold therapeutic potential in oncology, neurodegeneration, and metabolic diseases, but traditional RXR agonists mimicking the natural ligand 9-cis retinoic acid exhibit poor physicochemical properties, pharmacokinetics, and safety profiles. Improved RXR ligands are needed to exploit RXR modulation as a promising therapeutic concept in various indications beyond its current role in second-line cancer treatment. Here, we report the co-crystal structure of RXR in complex with a novel pyrimidine-based ligand and the structure-informed optimization of this scaffold to highly potent and highly soluble RXR agonists. Focused structure-activity relationship elucidation and rigidization resulted in a substantially optimized partial RXR agonist with low nanomolar potency, no cytotoxic activity, and very favorable physicochemical properties highlighting this promising scaffold for the development of next-generation RXR targeting drugs.

Structural Fusion of Natural and Synthetic Ligand Features Boosts RXR Agonist Potency

The retinoid X receptors (RXRs) are ligand-activated transcription factors involved in, for example, differentiation and apoptosis regulation. Currently used reference RXR agonists suffer from insufficient specificity and poor physicochemical properties, and improved tools are needed to capture the unexplored therapeutic potential of RXR. Endogenous vitamin A-derived RXR ligands and the natural product RXR agonist valerenic acid comprise acrylic acid residues with varying substitution patterns to engage the critical ionic contact with the binding site arginine. To mimic and exploit this natural ligand motif, we probed its structural fusion with synthetic RXR modulator scaffolds, which had profound effects on agonist activity and remarkably boosted potency of an oxaprozin-derived RXR agonist chemotype. Bioisosteric replacement of the acrylic acid to overcome its pan-assay interference compounds (PAINS) character enabled the development of a highly optimized RXR agonist chemical probe.

Retinoid X Receptor agonists as selective modulators of the immune system for the treatment of cancer

Upon heterodimerizing with other nuclear receptors, retinoid X receptors (RXR) act as ligand-dependent transcription factors, regulating transcription of critical signaling pathways that impact numerous hallmarks of cancer. By controlling both inflammation and immune responses, ligands that activate RXR can modulate the tumor microenvironment. Several small molecule agonists of these essential receptors have been synthesized. Historically, RXR agonists were tested for inhibition of growth in cancer cells, but more recent drug discovery programs screen new molecules for inhibition of inflammation or activation of immune cells. Bexarotene is the first successful example of an effective therapeutic that molecularly targets RXR; this drug was approved to treat cutaneous T cell lymphoma and is still used as a standard of care treatment for this disease. No additional RXR agonists have yet achieved FDA approval, but several promising novel compounds are being developed. In this review, we provide an overview of the multiple mechanisms by which RXR signaling regulates inflammation and tumor immunity. We also discuss the potential of RXR-dependent immune cell modulation for the treatment or prevention of cancer and concomitant challenges and opportunities.

Bexarotene drives the self-renewing proliferation of adult neural stem cells, promotes neuron-glial fate shift, and regulates late neuronal differentiation

Treatment with bexarotene, a selective retinoid X receptor (RXR) agonist, significantly improves behavioral dysfunctions in various neurodegenerative animal models. Additionally, it activates neurodevelopmental and plasticity pathways in the brains of adult mice. Our objective was to investigate the impact of RXR activation by bexarotene on adult neural stem cells (aNSC) and their cell lineages. To achieve this, we treated NSCs isolated from the subventricular zone (SVZ) of adult rat brains from the proliferative stage to the differentiated status. The results showed that bexarotene-treated aNSC exhibited increased BrdU incorporation, SOX2+ dividing cell pairs, and cell migration from neurospheres, revealing that the treatment promotes self-renewing proliferation and cell motility in SVZ-aNCS. Furthermore, bexarotene induced a cell fate shift characterized by a significant increase in GFAP+/S100B+ differentiated astrocytes, which uncovers the participation of activated-RXR in astrogenesis. In the neuronal lineage, the fate shift was counteracted by bexarotene-induced enhancement of NeuN+ nuclei together with neurite network outgrowth, indicating that the RXR agonist stimulates SVZ-aNCS neuronal differentiation at later stages. These findings establish new connections between RXR activation, astro- and neurogenesis in the adult brain, and contribute to the development of therapeutic strategies targeting nuclear receptors for neural repair.

Retinoid X receptor gamma dictates the activation threshold of group 2 innate lymphoid cells and limits type 2 inflammation in the small intestine

Group 2 innate lymphoid cells (ILC2s) are crucial in promoting type 2 inflammation that contributes to both anti-parasite immunity and allergic diseases. However, the molecular checkpoints in ILC2s that determine whether to immediately launch a proinflammatory response are unknown. Here, we found that retinoid X receptor gamma (Rxrg) was highly expressed in small intestinal ILC2s and rapidly suppressed by alarmin cytokines. Genetic deletion of Rxrg did not impact ILC2 development but facilitated ILC2 responses and the tissue inflammation induced by alarmins. Mechanistically, RXRγ maintained the expression of its target genes that support intracellular cholesterol efflux, which in turn reduce ILC2 proliferation. Furthermore, RXRγ expression prevented ILC2 response to mild stimulations, including low doses of alarmin cytokine and mechanical skin injury. Together, we propose that RXRγ expression and its mediated lipid metabolic states function as a cell-intrinsic checkpoint that confers the threshold of ILC2 activation in the small intestine.

Structural Modification of the Natural Product Valerenic Acid Tunes RXR Homodimer Agonism

Retinoid X receptors (RXR) are ligand-sensing transcription factors with a unique role in nuclear receptor signaling as universal heterodimer partners. RXR modulation holds potential in cancer, neurodegeneration and metabolic diseases but adverse effects of RXR activation and lack of selective modulators prevent further exploration as therapeutic target. The natural product valerenic acid has been discovered as RXR agonist with unprecedented preference for RXR subtype and homodimer activation. To capture structural determinants of this activity profile and identify potential for optimization, we have studied effects of structural modification of the natural product on RXR modulation and identified an analogue with enhanced RXR homodimer agonism.

Evaluation of Cisplatin-Induced Acute Renal Failure Amelioration Using Fondaparinux and Alteplase

Acute renal failure (ARF) is a deleterious condition with increased mortality or healthcare costs or dialysis-dependent end-stage renal disease. The study aims to compare prophylaxis with fondaparinux (Fund) vs. treatment with alteplase (Alt) in ameliorating cisplatin (Cis)-induced ARF. Sixty male mice were equally divided randomly into six groups of control, Cis, Alt, and Cis + Alt groups receiving normal saline for 10 days. All four groups except for the control received Cis (30 mg/kg, i.p.) on day 7, and 6 h later, both the Alt groups received Alt (0.9 mg/kg, i.v.). The animal groups Fund and Fund + Cis received Fund (5 mg/kg, i.p.) for 10 days, and the Fund + Cis group on day 7 received Cis. All the animal groups were euthanized 72 h after the Cis dose. The Fund + Cis group showed significantly increased expression levels of platelet count, retinoid X receptor alpha (RXR-α) and phosphorylated Akt (p-Akt) in addition to decreased levels of urea, blood urea nitrogen (BUN), uric acid, white blood cells (WBCs), red blood cells (RBCs), relative kidney body weight, kidney injury score, glucose, prothrombin (PT), A Disintegrin And Metalloproteinases-10 (ADAM10), extracellular matrix deposition, protease-activated receptor 2 (PAR-2), and fibrinogen expression when compared to the Cis-only group. Meanwhile, the Cis + Alt group showed increased caspase-3 expression in addition to decreased levels of urea, BUN, uric acid, WBCs, RBCs, glucose, platelet count and PT expression with a marked decrease in PAR-2 protein expression compared to the Cis group. The creatinine levels for both the Fund + Cis and Cis + Alt groups were found to be comparable to those of the Cis-only group. The results demonstrate that the coagulation system's activation through the stimulation of PAR-2 and fibrinogen due to Cis-induced ADAM10 protein expression mediated the apoptotic pathway, as indicated by caspase-3 expression through the p-Akt pathway. This is normally accompanied by the loss of RXR-α distal and proximal tubules as lipid droplets. When the animals were pre-treated with the anticoagulant, Fund, the previous deleterious effect was halted while the fibrinolytic agent, Alt, most of the time failed to treat Cis-induced toxicity.

Discovery of bipyridine amide derivatives targeting pRXRα-PLK1 interaction for anticancer therapy

Retinoid X receptor alpha (RXRα) is an important therapeutic target of cancer. Recently, small molecules (e.g.,XS-060 and its derivatives), which can significantly induce RXRα-dependent mitotic arrest by inhibiting pRXRα-PLK1 interaction, have been demonstrated as excellent anticancer agents. To further obtain novel RXR-targeted antimitotic agents with excellent bioactivity and drug-like properties, we herein synthesized two new series of bipyridine amide derivatives with XS-060 as the lead compound. In the reporter gene assay, most synthesized compounds showed antagonistic activity against RXRα. The most active compound, bipyridine amide B9 (BPA-B9), showed better activity than XS-060, with excellent RXRα-binding affinity (KD = 39.29 ± 1.12 nM) and anti-proliferative activity against MDA-MB-231 (IC50 = 16 nM, SI > 3). Besides, a docking study revealed a proper fitting of BPA-B9 into the coactivator binding site of RXRα, rationalizing its potent antagonistic effect on RXRα transactivation. Further, the mechanism studies revealed that the anticancer activity of BPA-B9 was dependent on its cellular RXRα-targeted mechanism, such as inhibiting pRXRα-PLK1 interaction and inducing RXRα-dependent mitotic arrest. Besides, BPA-B9 displayed better pharmacokinetics than the lead XS-060. Further, animal assays indicated BPA-B9 had significant anticancer efficacy in vivo with no considerable side effects. Together, our study reveals a novel RXRα ligand BPA-B9 targeting the pRXRα-PLK1 interaction, with great potential as a promising anticancer drug candidate for further development.

Computational refinement identifies functional destructive single nucleotide polymorphisms associated with human retinoid X receptor gene

Alterations in the nuclear retinoid X receptor (RXRs) signalling have been implicated in neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, stroke, multiple sclerosis and glaucoma. Single nucleotide polymorphisms (SNPs) are the main cause underlying single nucleic acid variations which in turn determine heterogeneity within various populations. These genetic polymorphisms have been suggested to associate with various degenerative disorders in population-wide analysis. This bioinformatics study was designed to investigate, search, retrieve and identify deleterious SNPs which may affect the structure and function of various RXR isoforms through a computational and molecular modelling approach. Amongst the 1,813 retrieved SNPs several were found to be deleterious with rs140464195_G139R, rs368400425_R358W and rs368586400_L383F RXRα mutant variants being the most detrimental ones causing changes in the interatomic interactions and decreasing the flexibility of the mutant proteins. Molecular genetics analysis identified seven missense mutations in RXRα/β/γ isoforms. Two novel mutations SNP IDs (rs1588299621 and rs1057519958) were identified in RXRα isoform. We used several in silico prediction tools such as SIFT, PolyPhen, I-Mutant, Protein Variation Effect Analyzer (PROVEAN), PANTHER, SNP&Go, PhD-SNP and SNPeffect to predict pathogenicity and protein stability associated with RXR mutations. The structural assessment by DynaMut tool revealed that hydrogen bonds were affected along with hydrophobic and carbonyl interactions resulting in reduced flexibility at the mutated residue positions but ultimately stabilizing the molecule as a whole. Summarizing, analysis of the missense mutations in RXR isoforms showed a mix of conclusive and inconclusive genotype-phenotype correlations suggesting the use of sophisticated computational analysis tools for studying RXR variants.Communicated by Ramaswamy H. Sarma.

State-of-the-art methodological investigation of carotenoid activity and metabolism - from organic synthesis via metabolism to biological activity - exemplified by a novel retinoid signalling pathway

Carotenoids are the most abundant lipophilic secondary plant metabolites and their dietary intake has been related to a large number of potential health benefits relevant for humans, including even reduced total mortality. An important feature is their potential to impact oxidative stress and inflammatory pathways, by interacting with transcription factors. For example, they may act as precursors of bioactive derivatives activating nuclear hormone receptor mediated signalling. These bioactive derivatives, originating e.g. from β-carotene, i.e. retinoids / vitamin A, can activate the nuclear hormone receptors RARs (retinoic acid receptors). Due to new analytical insights, various novel metabolic pathways were recently outlined to be mediated via distinct nuclear hormone receptor activating pathways that were predicted and further confirmed. In this article, we describe old and novel metabolic pathways from various carotenoids towards novel ligands of alternative nuclear hormone receptors. However, to fully elucidate these pathways, a larger array of techniques and tools, starting from organic synthesis, lipidomics, reporter models, classical in vitro and in vivo models and further omics-approaches and their statistical evaluation are needed to comprehensively and conclusively study this topic. Thus, we further describe state-of-the-art techniques from A to Ω elucidating carotenoid biological mediated activities and describe in detail required materials and methods needed - in practical protocol form - for the various steps of carotenoid investigations.

Different Treatment Modalities of Oral Lichen Planus-A Narrative Review

Oral lichen planus (OLP) is a chronic inflammatory disease of unknown etiology which affects the oral mucosa. OLP varies in its clinical features from a reticular form that is, in most cases, asymptomatic, to atrophic-erosive, and is accompanied by symptoms of burning sensation and pain followed by difficulty in eating. Given the fact that OLP is a disease of unknown etiology, the treatment is symptomatic and involves suppressing the signs and symptoms of the disease using various topical and systemic drugs. The first line of therapy for treating symptomatic OLP is topical corticosteroids, whereas systemic corticosteroids are used for treating persistent lesions that do not respond to local treatment. However, the lack of convincing evidence on the efficacy of previous therapies, including topical corticosteroids, and numerous side effects that have appeared over recent years has resulted in the emergence and development of new therapeutic options. Some of the therapies mentioned are tacrolimus, efalizumab, dapson, interferon, retinoic acid, photochemotherapy with psoralen and ultraviolet A rays (PUVA), aloe vera, antimalarials, antibiotics and others. These therapies only partially meet the properties of efficacy and safety of use, thus justifying the continuous search and testing of new treatment methods.

Rational Design of a New RXR Agonist Scaffold Enabling Single-Subtype Preference for RXRα, RXRβ, and RXRγ

The three retinoid X receptor subtypes (RXRα, RXRβ, RXRγ) exhibit critical regulatory roles in cell proliferation and differentiation, metabolism, and inflammation. Due to their importance in nuclear receptor signaling, RXRs are widely distributed and pan-RXR agonists cause adverse effects, but the three highly conserved RXR ligand binding sites render the development of subtype-selective ligands a major challenge. We have fused elements of known RXR ligands to obtain a new RXR agonist chemotype on which minor structural modifications enabled the development of tools with single-subtype preference for RXRα, RXRβ, and RXRγ. Molecular modeling indicated different binding conformations and interaction patterns with the RXR LBDs as factors of preferential binding. In a phenotypic adipocyte differentiation experiment, only the RXRα preferential tool enhanced the adipogenic effects of pioglitazone, suggesting this subtype as particularly relevant in adipogenesis and highlighting the set of subtype-preferential RXR agonist tools as suitable for functional cellular studies.

Design and in vitro characterization of RXR variants as tools to investigate the biological role of endogenous rexinoids

Retinoid X receptors (RXRα, β, and γ) are essential members of the nuclear receptor (NR) superfamily of ligand-dependent transcriptional regulators that bind DNA response elements and control the expression of large gene networks. As obligate heterodimerization partners of many NRs, RXRs are involved in a variety of pathophysiological processes. However, despite this central role in NR signaling, there is still no consensus regarding the precise biological functions of RXRs and the putative role of the endogenous ligands (rexinoids) previously proposed for these receptors. Based on available crystal structures, we introduced a series of amino acid substitutions into the ligand-binding pocket of all three RXR subtypes in order to alter their binding properties. Subsequent characterization using a battery of cell-based and in vitro assays led to the identification of a double mutation abolishing the binding of any ligand while keeping the other receptor functions intact and a triple mutation that selectively impairs interaction with natural rexinoids but not with some synthetic ligands. We also report crystal structures that help understand the specific ligand-binding capabilities of both variants. These RXR variants, either fully disabled for ligand binding or retaining the property of being activated by synthetic compounds, represent unique tools that could be used in future studies to probe the presence of active endogenous rexinoids in tissues/organs and to investigate their role in vivo. Last, we provide data suggesting a possible involvement of fatty acids in the weak interaction of RXRs with corepressors.

Acute retinol mobilization by retinol-binding protein 4 in mouse liver induces fibroblast growth factor 21 expression

Hepatocytes secrete retinol-binding protein 4 (RBP4) into circulation, thereby mobilizing vitamin A from the liver to provide retinol for extrahepatic tissues. Obesity and insulin resistance are associated with elevated RBP4 levels in the blood. However, in a previous study, we observed that chronically increased RBP4 by forced Rbp4 expression in the liver does not impair glucose homeostasis in mice. Here, we investigated the effects of an acute mobilization of hepatic vitamin A stores by hepatic overexpression of RBP4 in mice. We show that hepatic retinol mobilization decreases body fat content and enhances fat turnover. Mechanistically, we found that acute retinol mobilization increases hepatic expression and serum levels of fibroblast growth factor 21 (FGF21), which is regulated by retinol mobilization and retinoic acid in primary hepatocytes. Moreover, we provide evidence that the insulin-sensitizing effect of FGF21 is associated with organ-specific adaptations in retinoid homeostasis. Taken together, our findings identify a novel crosstalk between retinoid homeostasis and FGF21 in mice with acute RBP4-mediated retinol mobilization from the liver.

On modeling and utilizing chemical compound information with deep learning technologies: A task-oriented approach

A large number of chemical compounds are available in databases such as PubChem and ZINC. However, currently known compounds, though large, represent only a fraction of possible compounds, which is known as chemical space. Many of these compounds in the databases are annotated with properties and assay data that can be used for drug discovery efforts. For this goal, a number of machine learning algorithms have been developed and recent deep learning technologies can be effectively used to navigate chemical space, especially for unknown chemical compounds, in terms of drug-related tasks. In this article, we survey how deep learning technologies can model and utilize chemical compound information in a task-oriented way by exploiting annotated properties and assay data in the chemical compounds databases. We first compile what kind of tasks are trying to be accomplished by machine learning methods. Then, we survey deep learning technologies to show their modeling power and current applications for accomplishing drug related tasks. Next, we survey deep learning techniques to address the insufficiency issue of annotated data for more effective navigation of chemical space. Chemical compound information alone may not be powerful enough for drug related tasks, thus we survey what kind of information, such as assay and gene expression data, can be used to improve the prediction power of deep learning models. Finally, we conclude this survey with four important newly developed technologies that are yet to be fully incorporated into computational analysis of chemical information.

Modulation of Rxrα Expression in Mononuclear Phagocytes Impacts on Cardiac Remodeling after Ischemia-Reperfusion Injury

Retinoid X receptors (RXRs), as members of the steroid/thyroid hormone superfamily of nuclear receptors, are crucial regulators of immune response during health and disease. RXR subtype expression is dependent on tissue and cell type, RXRα being the relevant isoform in monocytes and macrophages. Previous studies have assessed different functions of RXRs and positive implications of RXR agonists on outcomes after ischemic injuries have been described. However, the impact of a reduced Rxrα expression in mononuclear phagocytes on cardiac remodeling after myocardial infarction (MI) has not been investigated to date. Here, we use a temporally controlled deletion of Rxrα in monocytes and macrophages to determine its role in ischemia-reperfusion injury. We show that reduced expression of Rxrα in mononuclear phagocytes leads to a decreased phagocytic activity and an accumulation of apoptotic cells in the myocardium, reduces angiogenesis and cardiac macrophage proliferation in the infarct border zone/infarct area, and has an impact on monocyte/macrophage subset composition. These changes are associated with a greater myocardial defect 30 days after ischemia/reperfusion injury. Overall, the reduction of Rxrα levels in monocytes and macrophages negatively impacts cardiac remodeling after myocardial infarction. Thus, RXRα might represent a therapeutic target to regulate the immune response after MI in order to improve cardiac remodeling.

Targeting Nuclear Receptors in Lung Cancer—Novel Therapeutic Prospects

Lung cancer, the second most commonly diagnosed cancer, is the major cause of fatalities worldwide for both men and women, with an estimated 2.2 million new incidences and 1.8 million deaths, according to GLOBOCAN 2020. Although various risk factors for lung cancer pathogenesis have been reported, controlling smoking alone has a significant value as a preventive measure. In spite of decades of extensive research, mechanistic cues and targets need to be profoundly explored to develop potential diagnostics, treatments, and reliable therapies for this disease. Nuclear receptors (NRs) function as transcription factors that control diverse biological processes such as cell growth, differentiation, development, and metabolism. The aberrant expression of NRs has been involved in a variety of disorders, including cancer. Deregulation of distinct NRs in lung cancer has been associated with numerous events, including mutations, epigenetic modifications, and different signaling cascades. Substantial efforts have been made to develop several small molecules as agonists or antagonists directed to target specific NRs for inhibiting tumor cell growth, migration, and invasion and inducing apoptosis in lung cancer, which makes NRs promising candidates for reliable lung cancer therapeutics. The current work focuses on the importance of various NRs in the development and progression of lung cancer and highlights the different small molecules (e.g., agonist or antagonist) that influence NR expression, with the goal of establishing them as viable therapeutics to combat lung cancer.

Retinol Binding Protein 7 Promotes Adipogenesis in vitro and Regulates Expression of Genes Involved in Retinol Metabolism

Retinol is an essential nutrient in animals. Its metabolites, specifically retinoic acid (RA), are crucial for cell differentiation, including adipogenesis. Retinol binding protein 7 (Rbp7) is under the control of PPARγ, the master regulator of adipogenesis. However, the role of RBP7 in adipogenesis is unclear. Our study showed that Rbp7 was abundantly expressed in white and brown mouse adipose tissues and had a higher expression in adipocytes than in stromal vascular fraction. Rbp7 overexpression promoted 3T3-L1 preadipocyte differentiation with increased triglyceride accumulation and up-regulation of Pparγ, Fabp4, C/ebpα, and AdipoQ. Rbp7 deficient adipocytes had opposite effects of the overexpression, which were rescued by RA supplementation. Indirect assessment of relative nuclear RA levels using RAR response element (RARE)-Luc reporter assay demonstrated that Rbp7 overexpression significantly increased RARE-Luc reporter activity. Rbp7 overexpression significantly increased expression of Raldh1, responsible for RA production, and up-regulation of Lrat and Cyp26a1, involved in retinol storage and RA catabolism, respectively, in 3T3-L1 adipocytes. Rbp7 deficient adipocytes had opposite effects of the overexpression of those genes involved in retinol metabolism. These data suggest that RBP7 increases transcriptional activity of RARE that may induce negative feedback responses via regulation of the gene expression for retinol homeostasis. Our data indicate critical RBP7 functions in adipocytes: regulation of transcriptional activity of RARE and adipocytes differentiation, potentially providing a new target for obesity therapy.

Breaking the barriers to remyelination in multiple sclerosis

Chronically demyelinated axons are rendered susceptible to degeneration through loss of trophic support from oligodendrocytes and myelin, and this process underlies disability progression in multiple sclerosis. Promoting remyelination is a promising neuroprotective therapeutic strategy, but to date, has not been achieved through simply promoting oligodendrocyte precursor cell differentiation, and it is clear that a detailed understanding of the molecular mechanisms underlying failed remyelination is required to guide future therapeutic approaches. In multiple sclerosis, remyelination is impaired by extrinsic inhibitory cues in the lesion microenvironment including secreted effector molecules released from compartmentalized immune cells and reactive glia, as well as by intrinsic defects in oligodendrocyte lineage cells, most notably increased metabolic demands causing oxidative stress and accelerated cellular senescence. Promising advances in our understanding of the cellular and molecular mechanisms underlying these processes offers hope for strategically designed interventions to facilitate remyelination thereby resulting in robust clinical benefits.

Altered RBP1 Gene Expression Impacts Epithelial Cell Retinoic Acid, Proliferation, and Microenvironment

Vitamin A is an essential diet-derived nutrient that has biological activity affected through an active metabolite, all-trans retinoic acid (atRA). Retinol-binding protein type 1 (RBP1) is an intracellular chaperone that binds retinol and retinal with high affinity, protects retinoids from non-specific oxidation, and delivers retinoids to specific enzymes to facilitate biosynthesis of RA. RBP1 expression is reduced in many of the most prevalent cancers, including breast cancer. Here, we sought to understand the relationship between RBP1 expression and atRA biosynthesis in mammary epithelial cells, as well as RBP1 expression and atRA levels in human mammary tissue. We additionally aimed to investigate the impact of RBP1 expression and atRA on the microenvironment as well as the potential for therapeutic restoration of RBP1 expression and endogenous atRA production. Using human mammary ductal carcinoma samples and a series of mammary epithelial cell lines representing different stages of tumorigenesis, we investigated the relationship between RBP1 expression as determined by QPCR and atRA via direct liquid chromatography-multistage-tandem mass spectrometry-based quantification. The functional effect of RBP1 expression and atRA in epithelial cells was investigated via the expression of direct atRA targets using QPCR, proliferation using Ki-67 staining, and collagen deposition via picrosirius red staining. We also investigated the atRA content of stromal cells co-cultured with normal and tumorigenic epithelial cells. Results show that RBP1 and atRA are reduced in mammary tumor tissue and tumorigenic epithelial cell lines. Knock down of RBP1 expression using shRNA or overexpression of RBP1 supported a direct relationship between RBP1 expression with atRA. Increases in cellular atRA were able to activate atRA direct targets, inhibit proliferation and inhibit collagen deposition in epithelial cell lines. Conditions encountered in tumor microenvironments, including low glucose and hypoxia, were able to reduce RBP1 expression and atRA. Treatment with either RARα agonist AM580 or demethylating agent Decitabine were able to increase RBP1 expression and atRA. Cellular content of neighboring fibroblasts correlated with the RA producing capacity of epithelial cells in co-culture. This work establishes a direct relationship between RBP1 expression and atRA, which is maintained when RBP1 expression is restored therapeutically. The results demonstrate diseases with reduced RBP1 could potentially benefit from therapeutics that restore RBP1 expression and endogenous atRA.

Retinoid X Receptor: Cellular and Biochemical Roles of Nuclear Receptor with a Focus on Neuropathological Involvement

Retinoid X receptors (RXRs) present a subgroup of the nuclear receptor superfamily with particularly high evolutionary conservation of ligand binding domain. The receptor exists in α, β, and γ isotypes that form homo-/heterodimeric complexes with other permissive and non-permissive receptors. While research has identified the biochemical roles of several nuclear receptor family members, the roles of RXRs in various neurological disorders remain relatively under-investigated. RXR acts as ligand-regulated transcription factor, modulating the expression of genes that plays a critical role in mediating several developmental, metabolic, and biochemical processes. Cumulative evidence indicates that abnormal RXR signalling affects neuronal stress and neuroinflammatory networks in several neuropathological conditions. Protective effects of targeting RXRs through pharmacological ligands have been established in various cell and animal models of neuronal injury including Alzheimer disease, Parkinson disease, glaucoma, multiple sclerosis, and stroke. This review summarises the existing knowledge about the roles of RXR, its interacting partners, and ligands in CNS disorders. Future research will determine the importance of structural and functional heterogeneity amongst various RXR isotypes as well as elucidate functional links between RXR homo- or heterodimers and specific physiological conditions to increase drug targeting efficiency in pathological conditions.

Modulation of retinoid-X-receptors differentially regulates expression of apolipoprotein genes apoc1 and apoeb by zebrafish microglia

Transcriptome analyses performed in both human and zebrafish indicate strong expression of Apoe and Apoc1 by microglia. Apoe expression by microglia is well appreciated, but Apoc1 expression has not been well-examined. PPAR/RXR and LXR/RXR receptors appear to regulate expression of the apolipoprotein gene cluster in macrophages, but a similar role in microglia in vivo has not been studied. Here, we characterized microglial expression of apoc1 in the zebrafish central nervous system (CNS) in situ and demonstrate that in the CNS, apoc1 expression is unique to microglia. We then examined the effects of PPAR/RXR and LXR/RXR modulation on microglial expression of apoc1 and apoeb during early CNS development using a pharmacological approach. Changes in apoc1 and apoeb transcripts in response to pharmacological modulation were quantified by RT-qPCR in whole heads, and in individual microglia using hybridization chain reaction (HCR) in situ hybridization. We found that expression of apoc1 and apoeb by microglia were differentially regulated by LXR/RXR and PPAR/RXR modulating compounds, respectively, during development. Our results also suggest RXR receptors could be involved in endogenous induction of apoc1 expression by microglia. Collectively, our work supports the use of zebrafish to better understand regulation and function of these apolipoproteins in the CNS.

Summary: Here we investigate expression of two apolipoprotein genes by microglia in the zebrafish model during normal development, and in contexts of pharmacological manipulations that target candidate regulatory receptors.

RXR – centralny regulator wielu ścieżek sygnałowych w organizmie

Receptory jądrowe (NRs) tworzą największą nadrodzinę czynników transkrypcyjnych, które odgrywają ważną rolę w regulacji wielu procesów biologicznych. Receptor kwasu 9-cis-retinowego (RXR) wydaje się odgrywać szczególną rolę wśród tej grupy białek, a to ma związek z jego zdolnością do tworzenia dimerów z innymi NRs. Ze względu na kontrolę ekspresji wielu genów, RXR stanowi bardzo dobry cel licznych terapii. Nieprawidłowości w szlakach modulowanych przez RXR są powiązane m.in. z chorobami neurodegeneracyjnymi, otyłością, cukrzycą, a także nowotworami. Istnieje wiele związków mogących regulować aktywność transkrypcyjną RXR. Jednak obecnie dopuszczonych do użytku klinicznego jest tylko kilka z nich. Retinoidy normalizują wzrost i różnicowanie komórek skóry i błon śluzowych, ponadto działają immunomodulująco oraz przeciwzapalnie. Stąd są stosowane przede wszystkim w chorobach skóry i w terapii niektórych chorób nowotworowych. W artykule przedstawiono ogólne wiadomości na temat RXR, jego budowy, ligandów i mechanizmu działania oraz potencjalnej roli w terapii nowotworów i zespołu metabolicznego.

The phosphorylated retinoid X receptor-α promotes diethylnitrosamine-induced hepatocarcinogenesis in mice through the activation of β-catenin signaling pathway

Previous studies have shown that phosphorylation of the retinoid X receptor-α (RXRα) is associated with the development of hepatocellular carcinoma (HCC). However, these findings were revealed using HCC cell lines that express phosphorylated-RXRα (p-RXRα) proteins; therefore, it remains unclear whether p-RXRα affects hepatocarcinogenesis in vivo. Therefore, to investigate the biological function of p-RXRα in vivo, we developed a doxycycline-inducible ES cell line and transgenic mouse, both of which overexpress the phosphomimetic mutant form of RXRα, T82D/S260D, in a doxycycline-dependent manner. We found that the development of liver tumors, especially high-grade adenoma and HCC, was enhanced in diethylnitrosamine (DEN)-induced T82D/S260D-inducible mice. Moreover, the increased incidence of liver tumors in the transgenic mice was attributable to the promotion of cell cycle progression. Interestingly, the expression of β-catenin protein and its target gene cyclin D1 was elevated in the liver tumors of DEN-treated T82D/S260D-inducible mice, concurrent with increased cytoplasmic and nuclear β-catenin protein expression, indicating its stabilization and transcriptional activation. These results indicate that p-RXRα promotes DEN-induced hepatocarcinogenesis in mice through the activation of the β-catenin signaling pathway, suggesting that p-RXRα may serve as a possible therapeutic target for HCC.

Effect of Radiation on the Essential Nutrient Homeostasis and Signaling of Retinoids in a Non-human Primate Model with Minimal Bone Marrow Sparing

ABSTRACT

High-dose radiation exposure results in hematopoietic (H) and gastrointestinal (GI) acute radiation syndromes (ARS) followed by delayed effects of acute radiation exposure (DEARE), which include damage to lung, heart, and GI. Whereas DEARE includes inflammation and fibrosis in multiple tissues, the molecular mechanisms contributing to inflammation and to the development of fibrosis remain incompletely understood. Reports that radiation dysregulates retinoids and proteins within the retinoid pathway indicate that radiation disrupts essential nutrient homeostasis. An active metabolite of vitamin A, retinoic acid (RA), is a master regulator of cell proliferation, differentiation, and apoptosis roles in inflammatory signaling and the development of fibrosis. As facets of inflammation and fibrosis are regulated by RA, we surveyed radiation-induced changes in retinoids as well as proteins related to and targets of the retinoid pathway in the non-human primate after high dose radiation with minimal bone marrow sparing (12 Gy PBI/BM2.5). Retinoic acid was decreased in plasma as well as in lung, heart, and jejunum over time, indicating a global disruption of RA homeostasis after IR. A number of proteins associated with fibrosis and with RA were significantly altered after radiation. Together these data indicate that a local deficiency of endogenous RA presents a permissive environment for fibrotic transformation.

Multi-omic Analysis of Non-human Primate Heart after Partial-body Radiation with Minimal Bone Marrow Sparing

ABSTRACT

High-dose radiation exposure results in hematopoietic and gastrointestinal acute radiation syndromes followed by delayed effects of acute radiation exposure, which encompasses multiple organs, including heart, kidney, and lung. Here we sought to further characterize the natural history of radiation-induced heart injury via determination of differential protein and metabolite expression in the heart. We quantitatively profiled the proteome and metabolome of left and right ventricle from non-human primates following 12 Gy partial body irradiation with 2.5% bone marrow sparing over a time period of 3 wk. Global proteome profiling identified more than 2,200 unique proteins, with 220 and 286 in the left and right ventricles, respectively, showing significant responses across at least three time points compared to baseline levels. High-throughput targeted metabolomics analyzed a total of 229 metabolites and metabolite combinations, with 18 and 22 in the left and right ventricles, respectively, showing significant responses compared to baseline levels. Bioinformatic analysis performed on metabolomic and proteomic data revealed pathways related to inflammation, energy metabolism, and myocardial remodeling were dysregulated. Additionally, we observed dysregulation of the retinoid homeostasis pathway, including significant post-radiation decreases in retinoic acid, an active metabolite of vitamin A. Significant differences between left and right ventricles in the pathology of radiation-induced injury were identified. This multi-omic study characterizes the natural history and molecular mechanisms of radiation-induced heart injury in NHP exposed to PBI with minimal bone marrow sparing.

Calcitriol and Retinoic acid antagonize each other to suppress the production of IL-9 by Th9 cells

Distinct T helper cells, including Th9 cells help maintain homeostasis in the immune system. Vitamins play pivotal role in the immune system through many mechanisms, including regulating the differentiation of T helper cells. Calcitriol (1,25-dihydroxyvitamin D3) and retinoic acid possess hormone-like properties and are the bioactive metabolites of vitamin D and A, respectively, that signal through heterodimers containing the common retinoid X receptor. In contrast to individual treatment with the vitamins that significantly attenuates IL-9 production from Th9 cells, Th9 cells treated with both vitamins demonstrated IL-9 production similar to untreated Th9 cells. This is associated with reciprocal expression of PU.1 and Foxp3. While the recruitment of PU.1 was significantly impaired to the Il9 gene in the presence of calcitriol or retinoic acid in Th9 cells, addition of both vitamins together increased the recruitment of PU.1 to the Il9 gene. Calcitriol and retinoic acid together impaired the recruitment of HDAC1 to the Il9 gene without impacting Gcn5 recruitment. Importantly, retinoic acid negated the effect of calcitriol and impaired the binding of VDR on the Il9 gene by dampened VDR-RXR formation. Collectively, our data show that calcitriol and retinoic acid antagonize each other to regulate the differentiation of Th9 cells.

Acute Proteomic Changes in Lung after Radiation: Toward Identifying Initiating Events of Delayed Effects of Acute Radiation Exposure in Non-human Primate after Partial Body Irradiation with Minimal Bone Marrow Sparing

ABSTRACT

Radiation-induced lung injury is a delayed effect of acute radiation exposure resulting in pulmonary pneumonitis and fibrosis. Molecular mechanisms that lead to radiation-induced lung injury remain incompletely understood. Using a non-human primate model of partial body irradiation with minimal bone marrow sparing, lung was analyzed from animals irradiated with 12 Gy at timepoints every 4 d up to 21 d after irradiation and compared to non-irradiated (sham) controls. Tryptic digests of lung tissues were analyzed by liquid chromatography-tandem mass spectrometry followed by pathway analysis. Out of the 3,101 unique proteins that were identified, we found that 252 proteins showed significant and consistent responses across at least three time points post-irradiation, of which 215 proteins showed strong up-regulation while 37 proteins showed down-regulation. Canonical pathways affected by irradiation, changes in proteins that serve as upstream regulators, and proteins involved in key processes including inflammation, fibrosis, and retinoic acid signaling were identified. The proteomic profiling of lung conducted here represents an untargeted systems biology approach to identify acute molecular events in the non-human primate lung that could potentially be initiating events for radiation-induced lung injury.

Retinoids as Chemo-Preventive and Molecular-Targeted Anti-Cancer Therapies

Retinoic acid (RA) agents possess anti-tumor activity through their ability to induce cellular differentiation. However, retinoids have not yet been translated into effective systemic treatments for most solid tumors. RA signaling is mediated by the following two nuclear retinoic receptor subtypes: the retinoic acid receptor (RAR) and the retinoic X receptor (RXR), and their isoforms. The identification of mutations in retinoid receptors and other RA signaling pathway genes in human cancers offers opportunities for target discovery, drug design, and personalized medicine for distinct molecular retinoid subtypes. For example, chromosomal translocation involving RARA occurs in acute promyelocytic leukemia (APL), and all-trans retinoic acid (ATRA) is a highly effective and even curative therapeutic for APL patients. Thus, retinoid-based target discovery presents an important line of attack toward designing new, more effective strategies for treating other cancer types. Here, we review retinoid signaling, provide an update on retinoid agents and the current clinical research on retinoids in cancer, and discuss how the retinoid pathway genotype affects the ability of retinoid agents to inhibit the growth of colorectal cancer (CRC) cells. We also deliberate on why retinoid agents have not shown clinical efficacy against solid tumors and discuss alternative strategies that could overcome the lack of efficacy.

Targeting Nuclear Receptors in Neurodegeneration and Neuroinflammation

Nuclear receptors, also known as ligand-activated transcription factors, regulate gene expression upon ligand signals and present as attractive therapeutic targets especially in chronic diseases. Despite the therapeutic relevance of some nuclear receptors in various pathologies, their potential in neurodegeneration and neuroinflammation is insufficiently established. This perspective gathers preclinical and clinical data for a potential role of individual nuclear receptors as future targets in Alzheimer's disease, Parkinson's disease, and multiple sclerosis, and concomitantly evaluates the level of medicinal chemistry targeting these proteins. Considerable evidence suggests the high promise of ligand-activated transcription factors to counteract neurodegenerative diseases with a particularly high potential of several orphan nuclear receptors. However, potent tools are lacking for orphan receptors, and limited central nervous system exposure or insufficient selectivity also compromises the suitability of well-studied nuclear receptor ligands for functional studies. Medicinal chemistry efforts are needed to develop dedicated high-quality tool compounds for the therapeutic validation of nuclear receptors in neurodegenerative pathologies.

Retinoids promote penis development in sequentially hermaphroditic snails

Sexual systems are surprisingly diverse, considering the ubiquity of sexual reproduction. Sequential hermaphroditism, the ability of an individual to change sex, has emerged multiple times independently across the animal kingdom. In molluscs, repeated shifts between ancestrally separate sexes and hermaphroditism are generally found at the level of family and above, suggesting recruitment of deeply conserved mechanisms. Despite this, molecular mechanisms of sexual development are poorly known. In molluscs with separate sexes, endocrine disrupting toxins bind the retinoid X receptor (RXR), activating ectopic male development in females, suggesting the retinoid pathway as a candidate controlling sexual transitions in sequential hermaphrodites. We therefore tested the role of retinoic acid signaling in sequentially hermaphroditic Crepidula snails, which develop first into males, then change sex, maturing into females. We show that retinoid agonists induce precocious penis growth in juveniles and superimposition of male development in females. Combining RXR antagonists with retinoid agonists significantly reduces penis length in induced juveniles, while similar treatments using retinoic acid receptor (RAR) antagonists increase penis length. Transcripts of both receptors are expressed in the induced penis. Our findings therefore show that retinoid signaling can initiate molluscan male genital development, and regulate penis length. Further, we show that retinoids induce ectopic male development in multiple Crepidula species. Species-specific influence of conspecific induction of sexual transitions correlates with responsiveness to retinoids. We propose that retinoid signaling plays a conserved role in molluscan male development, and that shifts in the timing of retinoid signaling may have been important for the origins of sequential hermaphroditism within molluscs.

Creation of Fluorescent RXR Antagonists Based on CBTF-EE and Application to a Fluorescence Polarization Binding Assay

Retinoid X receptor (RXR) ligands often bind in modes in which the carboxy group forms a hydrogen bond inside the ligand-binding pocket (LBP). However, our previously reported RXR antagonist, CBTF-EE (4a), binds with its carboxy group directed outside the LBP and its alkoxy side chain located inside the LBP. Here, we examined the binding modes of 4b and 4c bearing a nitrobenzoxadiazole (NBD) or boron-dipyrromethene (BODIPY) fluorophore, respectively, at the end of the alkoxy chain of 4a. Both compounds function as RXR antagonists. 4c, but not 4b, was available for a fluorescence polarization binding assay, indicating that rotation of BODIPY, but not NBD, is restricted in the bound state. The fluorescence findings, supported by docking simulations, suggest the fluorophores are located outside the LBP, so that the binding mode of 4b and 4c is different from that of 4a. The assay results were highly correlated with those of a [³H]9-cis-retinoic acid assay.

Thyroid hormone and thyroid hormone nuclear receptors: History and present state of art

The present review traces the road leading to discovery of L-thyroxine, thyroid hormone (3,5,3´-triiodo-L-thyronine, T₃) and its cognate nuclear receptors. Thyroid hormone is a pleio-tropic regulator of growth, differentiation, and tissue homeostasis in higher organisms. The major site of the thyroid hormone action is predominantly a cell nucleus. T₃ specific binding sites in the cell nuclei have opened a new era in the field of the thyroid hormone receptors (TRs) discovery. T₃ actions are mediated by high affinity nuclear TRs, TRalpha and TRbeta, which function as T₃-activated transcription factors playing an essential role as transcription-modulating proteins affecting the transcriptional responses in target genes. Discovery and characterization of nuclear retinoid X receptors (RXRs), which form with TRs a heterodimer RXR/TR, positioned RXRs at the epicenter of molecular endocrinology. Transcriptional control via nuclear RXR/TR heterodimer represents a direct action of thyroid hormone. T₃ plays a crucial role in the development of brain, it exerts significant effects on the cardiovascular system, skeletal muscle contractile function, bone development and growth, both female and male reproductive systems, and skin. It plays an important role in maintaining the hepatic, kidney and intestine homeostasis and in pancreas, it stimulates the beta-cell proliferation and survival. The TRs cross-talk with other signaling pathways intensifies the T₃ action at cellular level. The role of thyroid hormone in human cancers, acting via its cognate nuclear receptors, has not been fully elucidated yet. This review is aimed to describe the history of T₃ receptors, starting from discovery of T3 binding sites in the cell nuclei to revelation of T₃ receptors as T₃-inducible transcription factors in relation to T₃ action at cellular level. It also focuses on milestones of investigation, comprising RXR/TR dimerization, cross-talk between T₃ receptors, and other regulatory pathways within the cell and mainly on genomic action of T₃. This review also focuses on novel directions of investigation on relationships between T₃ receptors and cancer. Based on the update of available literature and the author's experimental experience, it is devoted to clinicians and medical students.

Vitamin A Update: Forms, Sources, Kinetics, Detection, Function, Deficiency, Therapeutic Use and Toxicity

Vitamin A is a group of vital micronutrients widely present in the human diet. Animal-based products are a rich source of the retinyl ester form of the vitamin, while vegetables and fruits contain carotenoids, most of which are provitamin A. Vitamin A plays a key role in the correct functioning of multiple physiological functions. The human organism can metabolize natural forms of vitamin A and provitamin A into biologically active forms (retinol, retinal, retinoic acid), which interact with multiple molecular targets, including nuclear receptors, opsin in the retina and, according to the latest research, also some enzymes. In this review, we aim to provide a complex view on the present knowledge about vitamin A ranging from its sources through its physiological functions to consequences of its deficiency and metabolic fate up to possible pharmacological administration and potential toxicity. Current analytical methods used for its detection in real samples are included as well.

Single-nucleotide polymorphisms responsible for pseudo-albinism and hypermelanosis in Japanese flounder (Paralichthys olivaceus) and reveal two genes related to malpigmentation

Paralichthys olivaceus is the kind of cold-water benthic marine fish. In the early stages of development, the symmetrical juveniles transform into an asymmetrical body shape through metamorphosis for adapting benthic life. After that, one side of the fish body is attached to the ground, and the eyes turn to the opposite side which is called ocular side. The body color also appears asymmetry. The skin on the ocular side is dark brown, and the skin on the blind side is white without pigmentation. Pseudo-albinism and hypermelanosis have been considered distinct body color disorders in flatfish. Pseudo-albinism and hypermelanosis in Paralichthys olivaceus are due to abnormal or uneven pigment distribution, due to the interaction of hereditary and environmental factors, rather than a single-nucleotide mutation of a specific gene. Here, we report three single-nucleotide polymorphisms (SNPs) responsible for both pseudo-albinism and hypermelanosis, which are located on two body color-related genes involved in melanogenesis-related pathways. c.2440C>A (P. V605I) and c.2271-96T>C are located on the Inositol 1,4,5-trisphosphate receptor type 2-like (ITPR2) (Gene ID: 109624047), they are located in exon 16 and the non-coding region, respectively, and c.2406C>A (P.H798N) is located in exon 13 of the adenylate cyclase type 6-like (AC6) gene(Gene ID: 109630770). ITPR2 and AC6 expression, which both participate in the thyroid hormone synthesis pathway associated with pseudo-albinism and hypermelanosis in P. olivaceus, were also investigated using qRT-PCR. In hypermelanotic fish, there were relatively higher levels of expression in ITPR2 and AC6 mRNA of hyper-pigmented skin of blind side than that of non-pigmented skin on the blind side and pigmented skin on the ocular side, while in pseudo-albino fish, expression level of ITPR2 and AC6 mRNA in pigmented skin of ocular side was significantly higher than that in non-pigmented skin both ocular and blind side. The results indicated that the expression of the two genes in abnormal parts of body color is positively correlated with pigmentation, suggesting that the influence of abnormal expression of two genes on the pigmentation in abnormal parts of body color deserves further study.

Oxaprozin Analogues as Selective RXR Agonists with Superior Properties and Pharmacokinetics

The retinoid X receptors (RXR) are ligand-activated transcription factors involved in multiple regulatory networks as universal heterodimer partners for nuclear receptors. Despite their high therapeutic potential in many pathologies, targeting of RXR has only been exploited in cancer treatment as the currently available RXR agonists suffer from exceptional lipophilicity, poor pharmacokinetics (PK), and adverse effects. Aiming to overcome the limitations and to provide improved RXR ligands, we developed a new potent RXR ligand chemotype based on the nonsteroidal anti-inflammatory drug oxaprozin. Systematic structure-activity relationship analysis enabled structural optimization toward low nanomolar potency similar to the well-established rexinoids. Cocrystal structures of the most active derivatives demonstrated orthosteric binding, and in vivo profiling revealed superior PK properties compared to current RXR agonists. The optimized compounds were highly selective for RXR activation and induced RXR-regulated gene expression in native cellular and in vivo settings suggesting them as excellent chemical tools to further explore the therapeutic potential of RXR.

Glossary and tutorial of xenobiotic metabolism terms used during small molecule drug discovery and development (IUPAC Technical Report)

This project originated more than 15 years ago with the intent to produce a glossary of drug metabolism terms having definitions especially applicable for use by practicing medicinal chemists. A first-draft version underwent extensive beta-testing that, fortuitously, engaged international audiences in a wide range of disciplines involved in drug discovery and development. It became clear that the inclusion of information to enhance discussions among this mix of participants would be even more valuable. The present version retains a chemical structure theme while expanding tutorial comments that aim to bridge the various perspectives that may arise during interdisciplinary communications about a given term. This glossary is intended to be educational for early stage researchers, as well as useful for investigators at various levels who participate on today’s highly multidisciplinary, collaborative small molecule drug discovery teams.

Multi-Target Approaches in Metabolic Syndrome

Metabolic syndrome (MetS) is a highly prevalent disease cluster worldwide. It requires polypharmacological treatment of the single conditions including type II diabetes, hypertension, and dyslipidemia, as well as the associated comorbidities. The complex treatment regimens with various drugs lead to drug-drug interactions and inadequate patient adherence, resulting in poor management of the disease. Multi-target approaches aim at reducing the polypharmacology and improving the efficacy. This review summarizes the medicinal chemistry efforts to develop multi-target ligands for MetS. Different combinations of pharmacological targets in context of in vivo efficacy and future perspective for multi-target drugs in MetS are discussed.

Structural Insights into the Interaction of the Intrinsically Disordered Co-activator TIF2 with Retinoic Acid Receptor Heterodimer (RXR/RAR)

Retinoic acid receptors (RARs) and retinoid X receptors (RXRs) form heterodimers that activate target gene transcription by recruiting co-activator complexes in response to ligand binding. The nuclear receptor (NR) co-activator TIF2 mediates this recruitment by interacting with the ligand-binding domain (LBD) of NRs trough the nuclear receptor interaction domain (TIF2_NRID) containing three highly conserved α-helical LxxLL motifs (NR-boxes). The precise binding mode of this domain to RXR/RAR is not clear due to the disordered nature of TIF2. Here we present the structural characterization of TIF2_NRID by integrating several experimental (NMR, SAXS, Far-UV CD, SEC-MALS) and computational data. Collectively, the data are in agreement with a largely disordered protein with partially structured regions, including the NR-boxes and their flanking regions, which are evolutionary conserved. NMR and X-ray crystallographic data on TIF2_NRID in complex with RXR/RAR reveal a multisite binding of the three NR-boxes as well as an active role of their flanking regions in the interaction.

Nuclear Receptors and Development of Marine Invertebrates

Nuclear Receptors (NRs) are a superfamily of transcription factors specific to metazoans that have the unique ability to directly translate the message of a signaling molecule into a transcriptional response. In vertebrates, NRs are pivotal players in countless processes of both embryonic and adult physiology, with embryonic development being one of the most dynamic periods of NR activity. Accumulating evidence suggests that NR signaling is also a major regulator of development in marine invertebrates, although ligands and transactivation dynamics are not necessarily conserved with respect to vertebrates. The explosion of genome sequencing projects and the interpretation of the resulting data in a phylogenetic context allowed significant progress toward an understanding of NR superfamily evolution, both in terms of molecular activities and developmental functions. In this context, marine invertebrates have been crucial for characterizing the ancestral states of NR-ligand interactions, further strengthening the importance of these organisms in the field of evolutionary developmental biology.

Convenient Retinoid X Receptor Binding Assay Based on Fluorescence Change of the Antagonist NEt-C343

Retinoid X receptor (RXR) modulators (rexinoids) are considered to have therapeutic potential for multiple diseases, such as Alzheimer's disease and Parkinson's disease. To overcome various disadvantages of prior screening methods, we previously developed an RXR binding assay using a fluorescent RXR ligand, CU-6PMN (4). However, this ligand binds not only at the ligand-binding domain (LBD) but also at the dimer-dimer interface of hRXRα. Here, we present a new fluorescent RXR antagonist 6-[N-ethyl-N-(5-isobutoxy-4-isopropyl-2-(11-oxo-2,3,6,7-tetrahydro-1H,5H,11H-pyrano[2,3-f]pyrido[3,2,1-ij]quinoline-10-carboxamido)phenyl)amino]nicotinic acid (NEt-C343, 7), which emits strong fluorescence only when bound to the RXR-LBD. It allows us to perform a rapid, simple, and nonhazardous binding assay that does not require bound/free separation and uses a standard plate reader. The obtained Ki values of known compounds were correlated with the Ki values obtained using the standard [³H]9cis-retinoic acid assay. This assay should be useful for drug discovery as well as for research on endocrine disruptors, functional foods, and natural products.

Centrosomal Localization of RXRα Promotes PLK1 Activation and Mitotic Progression and Constitutes a Tumor Vulnerability

Retinoid X receptor alpha (RXRα), a nuclear receptor of transcription factor, controls various physiological and pathological pathways including cellular growth, proliferation, differentiation, and apoptosis. Here, we report that RXRα is phosphorylated at its N-terminal A/B domain by cyclin-dependent kinase 1 (Cdk1) at the onset of mitosis, triggering its translocation to the centrosome, where phosphorylated-RXRα (p-RXRα) interacts with polo-like kinase 1 (PLK1) through its N-terminal A/B domain by a unique mechanism. The interaction promotes PLK1 activation, centrosome maturation, and mitotic progression. Levels of p-RXRα are abnormally elevated in cancer cell lines, during carcinogenesis in animals, and in clinical tumor tissues. An RXRα ligand XS060, which specifically inhibits p-RXRα/PLK1 interaction but not RXRα heterodimerization, promotes mitotic arrest and catastrophe in a tumor-specific manner. These findings unravel a transcription-independent action of RXRα at the centrosome during mitosis and identify p-RXRα as a tumor-specific vulnerability for developing mitotic drugs with improved therapeutic index.

Genetic variant of RXR involved in the vitamin D metabolic pathway was linked to HCV infection outcomes among a high-risk Chinese population

Genetic variation of related genes in Vitamin D (VD) metabolic pathway played an important role in antiviral immune response and chronic hepatitis C virus (HCV) infection. Retinoid X receptor (RXR) is one of the key genes in the metabolism pathway of VD. This study aims to investigate the effect of single nucleotide polymorphisms (SNPs) in RXR on the outcomes of HCV infection. Three SNPs (RXRɑ-rs4842194, rs1045570 and RXRβ-rs2076310) were genotyped using Sequenom MassARRAY platform in 515 spontaneous clearance subjects, 830 persistent infection subjects, and 1062 uninfected subjects. Multivariate stepwise regression analyss was used to identify the prediction factors for HCV infection outcomes. The USCS Brower and RNAfold web serves were performed to further explore the potential biological functions of positive SNPs. The results of logistic regression analysis after adjusting for age, gender and types of high-risk population showed that subjects with RXRβ rs2076310-T (recessive model: adjusted OR = 1.598, 95%CI = 1.126-2.267, P = 0.009; additive model: adjusted OR = 1.196, 95%CI = 1.011-1.416, P = 0.037) had a significantly increased possibility of HCV infection chronicity. Rs2076310, age, types of high-risk population and aspartate aminotransferase were independent predictors of chronic HCV infection (P < 0.05). And the area under the receiver operating characteristic curve of combined effects of these factors was 0.679. Bioinformatics analysis indicated that rs2076310 could affect the gene expression level by affecting the transcriptional regulatory activity of the corresponding gene region. These findings indicated that genetic variation of RXRβ was associated with the risk of HCV infection chronicity among a high-risk Chinese population.

Comprehensive Set of Tertiary Complex Structures and Palmitic Acid Binding Provide Molecular Insights into Ligand Design for RXR Isoforms

The retinoid X receptor (RXR) is a ligand-sensing transcription factor acting mainly as a universal heterodimer partner for other nuclear receptors. Despite presenting as a potential therapeutic target for cancer and neurodegeneration, adverse effects typically observed for RXR agonists, likely due to the lack of isoform selectivity, limit chemotherapeutic application of currently available RXR ligands. The three human RXR isoforms exhibit different expression patterns; however, they share high sequence similarity, presenting a major obstacle toward the development of subtype-selective ligands. Here, we report the discovery of the saturated fatty acid, palmitic acid, as an RXR ligand and disclose a uniform set of crystal structures of all three RXR isoforms in an active conformation induced by palmitic acid. A structural comparison revealed subtle differences among the RXR subtypes. We also observed an ability of palmitic acid as well as myristic acid and stearic acid to induce recruitment of steroid receptor co-activator 1 to the RXR ligand-binding domain with low micromolar potencies. With the high, millimolar endogenous concentrations of these highly abundant lipids, our results suggest their potential involvement in RXR signaling.