The orphan nuclear receptor TR4 is a vitamin A-activated nuclear receptor

PGC-1α agonism induces fetal hemoglobin and exerts antisickling effects in sickle cell disease

Sickle cell disease is a growing health burden afflicting millions around the world. Clinical observation and laboratory studies have shown that the severity of sickle cell disease is ameliorated in individuals who have elevated levels of fetal hemoglobin. Additional pharmacologic agents to induce sufficient fetal hemoglobin to diminish clinical severity is an unmet medical need. We recently found that up-regulation of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) can induce fetal hemoglobin synthesis in human primary erythroblasts. Here, we report that a small molecule, SR-18292, increases PGC-1α leading to enhanced fetal hemoglobin expression in human erythroid cells, β-globin yeast artificial chromosome mice, and sickle cell disease mice. In SR-18292-treated sickle mice, sickled red blood cells are significantly reduced, and disease complications are alleviated. SR-18292, or agents in its class, could be a promising additional therapeutic for sickle cell disease.

Nuclear Receptors and the Hidden Language of the Metabolome

Nuclear hormone receptors (NHRs) are a family of ligand-regulated transcription factors that control key aspects of development and physiology. The regulation of NHRs by ligands derived from metabolism or diet makes them excellent pharmacological targets, and the mechanistic understanding of how NHRs interact with their ligands to regulate downstream gene networks, along with the identification of ligands for orphan NHRs, could enable innovative approaches for cellular engineering, disease modeling and regenerative medicine. We review recent discoveries in the identification of physiologic ligands for NHRs. We propose new models of ligand-receptor co-evolution, the emergence of hormonal function and models of regulation of NHR specificity and activity via one-ligand and two-ligand models as well as feedback loops. Lastly, we discuss limitations on the processes for the identification of physiologic NHR ligands and emerging new methodologies that could be used to identify the natural ligands for the remaining 17 orphan NHRs in the human genome.

Roles of Nuclear Orphan Receptors TR2 and TR4 during Hematopoiesis

TR2 and TR4 (NR2C1 and NR2C2, respectively) are evolutionarily conserved nuclear orphan receptors capable of binding direct repeat sequences in a stage-specific manner. Like other nuclear receptors, TR2 and TR4 possess important roles in transcriptional activation or repression with developmental stage and tissue specificity. TR2 and TR4 bind DNA and possess the ability to complex with available cofactors mediating developmental stage-specific actions in primitive and definitive erythrocytes. In erythropoiesis, TR2 and TR4 are required for erythroid development, maturation, and key erythroid transcription factor regulation. TR2 and TR4 recruit and interact with transcriptional corepressors or coactivators to elicit developmental stage-specific gene regulation during hematopoiesis.

Cellular and micro-environmental responses influencing the antitumor activity of all-trans retinoic acid in breast cancer

All-trans retinoic acid (ATRA) is the most relevant and functionally active metabolite of Vitamin-A. From a therapeutic standpoint, ATRA is the first example of pharmacological agent exerting its anti-tumor activity via a cell differentiating action. In the clinics, ATRA is used in the treatment of Acute Promyelocytic Leukemia, a rare form of myeloid leukemia with unprecedented therapeutic results. The extraordinary effectiveness of ATRA in the treatment of Acute Promyelocytic Leukemia patients has raised interest in evaluating the potential of this natural retinoid in the treatment of other types of neoplasias, with particular reference to solid tumors.The present article provides an overview of the available pre-clinical and clinical studies focussing on ATRA as a therapeutic agent in the context of breast cancer from a holistic point of view. In detail, we focus on the direct effects of ATRA in breast cancer cells as well as the underlying molecular mechanisms of action. In addition, we summarize the available information on the action exerted by ATRA on the breast cancer micro-environment, an emerging determinant of the progression and invasive behaviour of solid tumors. In particular we discuss the recent evidences of ATRA activity on the immune system. Finally, we analyse and discuss the results obtained with the few ATRA-based clinical trials conducted in the context of breast cancer.

Structures of human TR4LBD-JAZF1 and TR4DBD-DNA complexes reveal the molecular basis of transcriptional regulation

Testicular nuclear receptor 4 (TR4) modulates the transcriptional activation of genes and plays important roles in many diseases. The regulation of TR4 on target genes involves direct interactions with DNA molecules via the DNA-binding domain (DBD) and recruitment of coregulators by the ligand-binding domain (LBD). However, their regulatory mechanisms are unclear. Here, we report high-resolution crystal structures of TR4DBD, TR4DBD-DNA complexes and the TR4LBD-JAZF1 complex. For DNA recognition, multiple factors come into play, and a specific mutual selectivity between TR4 and target genes is found. The coactivators SRC-1 and CREBBP can bind at the interface of TR4 originally occupied by the TR4 activation function region 2 (AF-2); however, JAZF1 suppresses the binding through a novel mechanism. JAZF1 binds to an unidentified surface of TR4 and stabilizes an α13 helix never reported in the nuclear receptor family. Moreover, the cancer-associated mutations affect the interactions and the transcriptional activation of TR4 in vitro and in vivo, respectively. Overall, our results highlight the crucial role of DNA recognition and a novel mechanism of how JAZF1 reinforces the autorepressed conformation and influences the transcriptional activation of TR4, laying out important structural bases for drug design for a variety of diseases, including diabetes and cancers.

Testicular orphan receptor 4 induced hepatic stellate cells activation via the regulation of TGF-β receptor Ⅰ/Smad2/3 signaling pathway

INTRODUCTION AND OBJECTIVES

Liver fibrosis is a common pathological change in many chronic liver diseases. Activation of hepatic stellate cells (HSCs) is the core event in liver fibrosis. This study aimed to investigate the role of testicular orphan receptor 4 (TR4) in the activation of HSCs.

MATERIALS AND METHODS

In vivo, bile duct ligation (BDL)-induced rat liver fibrosis model was established, and the expressions of TR4 and α-smooth muscle actin (α-SMA) in liver tissues were detected. In vitro, TR4 knockdown and overexpression in JS-1 cells using lentiviral vectors were constructed, and the expressions of TR4, α-SMA, Col-I, and TGF-β1/smads and retinoid X receptor (RXR) pathway-related genes were detected.

RESULTS

TR4 was highly expressed in BDL-induced fibrotic liver, accompanied by increased expression of α-SMA. Knockdown of TR4 significantly inhibited the expressions of α-SMA, Col-I, p-TβRI, and p-Smad2/3, and up-regulated the expression of RXRα in HSCs in vitro. In contrast, TR4 overexpression significantly increased the expressions of α-SMA, Col-I, p-TβRI, and p-Smad2/3, and inhibited the expression of RXRα.

CONCLUSIONS

TR4 may promote the activation of HSCs by up-regulating TβR I/Smad2/3 signaling pathway and down-regulating RXRα signaling, thereby promoting the progression of liver fibrosis. Our findings may provide a new therapeutic target against hepatic fibrosis.

Structural overview and perspectives of the nuclear receptors, a major family as the direct targets for small-molecule drugs

The nuclear receptors (NRs) are an evolutionarily related family of transcription factors, which share certain common structural characteristics and regulate the expressions of various genes by recognizing different response elements. NRs play important roles in cell differentiation, proliferation, survival and apoptosis, rendering them indispensable in many physiological activities including growth and metabolism. As a result, dysfunctions of NRs are closely related to a variety of diseases, such as diabetes, obesity, infertility, inflammation, the Alzheimer's disease, cardiovascular diseases, prostate and breast cancers. Meanwhile, small-molecule drugs directly targeting NRs have been widely used in the treatment of above diseases. Here we summarize recent progress in the structural biology studies of NR family proteins. Compared with the dozens of structures of isolated DNA-binding domains (DBDs) and the striking more than a thousand of structures of isolated ligand-binding domains (LBDs) accumulated in the Protein Data Bank (PDB) over thirty years, by now there are only a small number of multi-domain NR complex structures, which reveal the integration of different NR domains capable of the allosteric signal transduction, or the detailed interactions between NR and various coregulator proteins. On the other hand, the structural information about several orphan NRs is still totally unavailable, hindering the further understanding of their functions. The fast development of new technologies in structural biology will certainly help us gain more comprehensive information of NR structures, inspiring the discovery of novel NR-targeting drugs with a new binding site beyond the classic LBD pockets and/or a new mechanism of action.

TLX, an Orphan Nuclear Receptor With Emerging Roles in Physiology and Disease

TLX (NR2E1), an orphan member of the nuclear receptor superfamily, is a transcription factor that has been described to be generally repressive in nature. It has been implicated in several aspects of physiology and disease. TLX is best known for its ability to regulate the proliferation of neural stem cells and retinal progenitor cells. Dysregulation, overexpression, or loss of TLX expression has been characterized in numerous studies focused on a diverse range of pathological conditions, including abnormal brain development, psychiatric disorders, retinopathies, metabolic disease, and malignant neoplasm. Despite the lack of an identified endogenous ligand, several studies have described putative synthetic and natural TLX ligands, suggesting that this receptor may serve as a therapeutic target. Therefore, this article aims to briefly review what is known about TLX structure and function in normal physiology, and provide an overview of TLX in regard to pathological conditions. Particular emphasis is placed on TLX and cancer, and the potential utility of this receptor as a therapeutic target.

Synthetic Retinoids as Potential Therapeutics in Prostate Cancer-An Update of the Last Decade of Research: A Review

Prostate cancer (PC) is the second most common tumor in males. The search for appropriate therapeutic options against advanced PC has been in process for several decades. Especially after cessation of the effectiveness of hormonal therapy (i.e., emergence of castration-resistant PC), PC management options have become scarce and the prognosis is poor. To overcome this stage of disease, an array of natural and synthetic substances underwent investigation. An interesting and promising class of compounds constitutes the derivatives of natural retinoids. Synthesized on the basis of the structure of retinoic acid, they present unique and remarkable properties that warrant their investigation as antitumor drugs. However, there is no up-to-date compilation that consecutively summarizes the current state of knowledge about synthetic retinoids with regard to PC. Therefore, in this review, we present the results of the experimental studies on synthetic retinoids conducted within the last decade. Our primary aim is to highlight the molecular targets of these compounds and to identify their potential promise in the treatment of PC.

Targeting the TR4 nuclear receptor with antagonist bexarotene can suppress the proopiomelanocortin signalling in AtT-20 cells

Drug options for the life‐threatening Cushing's disease are limited, and surgical resection or radiation therapy is not invariably effective. Testicular receptor 4 (TR4) has been identified as a novel drug target to treat Cushing's disease. We built the structure model of TR4 and searched the TR4 antagonist candidate via in silico virtual screening. Bexarotene was identified as an antagonist of TR4 that can directly interact with TR4 ligand binding domain (TR4‐LBD) and induces a conformational change in the secondary structure of TR4‐LBD. Bexarotene suppressed AtT‐20 cell growth, proopiomelanocortin (POMC) expression and adrenocorticotropin (ACTH) secretion. Mechanism dissection revealed that bexarotene could suppress TR4‐increased POMC expression via promoting the TR4 translocation from the nucleus to the cytoplasm. This TR4 translocation might then result in reducing the TR4 binding to the TR4 response element (TR4RE) on the 5’ promoter region of POMC. Results from in vivo mouse model also revealed that oral bexarotene administration markedly suppressed ACTH‐secreting tumour growth, adrenal enlargement and the secretion of ACTH and corticosterone in mice with already established tumours. Together, these results suggest that bexarotene may be developed as a potential novel therapeutic drug to better suppress Cushing's disease.

Identification of Small-Molecule Regulators of Testicular Receptor 4 via a Drug Repurposing Screening

The testicular receptor 4 (TR4) is a nuclear receptor implicated in multiple pathological processes, including cancer development, chemotherapy, and radiotherapy resistance. However, no effective TR4 small-molecule regulator is available to date. Here, we assessed a physical-interaction-based surface plasmon resonance imaging assay for discovery of TR4 regulators. We screened 1018 FDA-approved drugs and obtained 126 drugs with K _D values below 10^-6 M. The dual-luciferase-based biological assay verified four activatory compounds and two inhibitory compounds against TR4. Among them, nilotinib exhibited the most potent inhibitor, with an EC₅₀ of 1.05 μM, while genistein represented the most potent activator, with an EC₅₀ of 2.42 μM. Both drugs were predicted to bind in the ligand binding pocket of TR4. The circular dichroism spectroscopic assay revealed differed conformation changes upon nilotinib or genistein binding. These results established our combined physical and biological approaches as a highly effective way to identify and develop new TR4 regulators.

Nuclear Receptors as Regulators of Pituitary Corticotroph Pro-Opiomelanocortin Transcription

The hypothalamic–pituitary–adrenal (HPA) axis plays a critical role in adaptive stress responses and maintaining organism homeostasis. The pituitary corticotroph is the central player in the HPA axis and is regulated by a plethora of hormonal and stress related factors that synergistically interact to activate and temper pro-opiomelanocortin (POMC) transcription, to either increase or decrease adrenocorticotropic hormone (ACTH) production and secretion as needed. Nuclear receptors are a family of highly conserved transcription factors that can also be induced by various physiologic signals, and they mediate their responses via multiple targets to regulate metabolism and homeostasis. In this review, we summarize the modulatory roles of nuclear receptors on pituitary corticotroph cell POMC transcription, describe the unique and complex role these factors play in hypothalamic–pituitary–adrenal axis (HPA) regulation and discuss potential therapeutic targets in disease states.

Definition of functionally and structurally distinct repressive states in the nuclear receptor PPARγ

The repressive states of nuclear receptors (i.e., apo or bound to antagonists or inverse agonists) are poorly defined, despite the fact that nuclear receptors are a major drug target. Most ligand bound structures of nuclear receptors, including peroxisome proliferator-activated receptor γ (PPARγ), are similar to the apo structure. Here we use NMR, accelerated molecular dynamics and hydrogen-deuterium exchange mass spectrometry to define the PPARγ structural ensemble. We find that the helix 3 charge clamp positioning varies widely in apo and is stabilized by efficacious ligand binding. We also reveal a previously undescribed mechanism for inverse agonism involving an omega loop to helix switch which induces disruption of a tripartite salt-bridge network. We demonstrate that ligand binding can induce multiple structurally distinct repressive states. One state recruits peptides from two different corepressors, while another recruits just one, providing structural evidence of ligand bias in a nuclear receptor.

The repressive states of peroxisome proliferator-activated receptor γ (PPARγ) are ill-defined, despite nuclear receptors being a major drug target. Here authors demonstrate multiple structurally distinct repressive states, providing a structural rationale for ligand bias in a nuclear receptor.

Strategies for developing pregnane X receptor antagonists: Implications from metabolism to cancer

Pregnane X receptor (PXR) is a ligand-activated nuclear receptor (NR) that was originally identified as a master regulator of xenobiotic detoxification. It regulates the expression of drug-metabolizing enzymes and transporters to control the degradation and excretion of endobiotics and xenobiotics, including therapeutic agents. The metabolism and disposition of drugs might compromise their efficacy and possibly cause drug toxicity and/or drug resistance. Because many drugs can promiscuously bind and activate PXR, PXR antagonists might have therapeutic value in preventing and overcoming drug-induced PXR-mediated drug toxicity and drug resistance. Furthermore, PXR is now known to have broader cellular functions, including the regulation of cell proliferation, and glucose and lipid metabolism. Thus, PXR might be involved in human diseases such as cancer and metabolic diseases. The importance of PXR antagonists is discussed in the context of the role of PXR in xenobiotic sensing and other disease-related pathways. This review focuses on the development of PXR antagonists, which has been hampered by the promiscuity of PXR ligand binding. However, substantial progress has been made in recent years, suggesting that it is feasible to develop selective PXR antagonists. We discuss the current status, challenges, and strategies in developing selective PXR antagonists. The strategies are based on the molecular mechanisms of antagonism in related NRs that can be applied to the design of PXR antagonists, primarily driven by structural information.

Targeting TR4 nuclear receptor with antagonist bexarotene increases docetaxel sensitivity to better suppress the metastatic castration-resistant prostate cancer progression

Prostate cancer (PCa) is the second leading cause of cancer death in men in America, and there are no curative options for metastatic castration-resistant prostate cancer (mCRPC). Docetaxel (DTX) has been used as a standard chemotherapy for the mCRPC. However, resistance to DTX is a significant clinical problem as half of patients fail to respond to therapy. The TR4 nuclear receptor has been reported to play an important role in PCa progression, however, its linkage to the DTX resistance remains unclear. Here we found that TR4 was upregulated after DTX chemotherapy in the mCRPC cells and patients, and TR4 expression is correlated with DTX sensitivity with a higher level conferring chemo-resistance. Targeting TR4 with an antagonist bexarotene (Bex, a derivative of retinoid) suppressed the TR4 transactivation with increased DTX chemo-sensitivity. Mechanism dissection studies revealed that TR4 might alter the DTX chemo-sensitivity via modulating the TR4/lincRNA-p21/HIF-1α/VEGF-A signaling. Together, these results suggest that targeting this newly identified TR4/lincRNA-p21/HIF-1α/VEGF-A signaling with Bex, an FDA-approved drug, may increase the DTX chemo-sensitivity to better suppress the mCRPC progression.

Modulation of retinoid signaling: therapeutic opportunities in organ fibrosis and repair

The vitamin A metabolite, retinoic acid, is an important signaling molecule during embryonic development serving critical roles in morphogenesis, organ patterning and skeletal and neural development. Retinoic acid is also important in postnatal life in the maintenance of tissue homeostasis, while retinoid-based therapies have long been used in the treatment of a variety of cancers and skin disorders. As the number of people living with chronic disorders continues to increase, there is great interest in extending the use of retinoid therapies in promoting the maintenance and repair of adult tissues. However, there are still many conflicting results as we struggle to understand the role of retinoic acid in the multitude of processes that contribute to tissue injury and repair. This review will assess our current knowledge of the role retinoic acid signaling in the development of fibroblasts, and their transformation to myofibroblasts, and of the potential use of retinoid therapies in the treatment of organ fibrosis.

Targeting the TR4 nuclear receptor-mediated lncTASR/AXL signaling with tretinoin increases the sunitinib sensitivity to better suppress the RCC progression

Renal cell carcinoma (RCC) is one of the most lethal urological tumors. Using sunitinib to improve the survival has become the first-line therapy for metastatic RCC patients. However, the occurrence of sunitinib resistance in the clinical application has curtailed its efficacy. Here we found TR4 nuclear receptor might alter the sunitinib resistance to RCC via altering the TR4/lncTASR/AXL signaling. Mechanism dissection revealed that TR4 could modulate lncTASR (ENST00000600671.1) expression via transcriptional regulation, which might then increase AXL protein expression via enhancing the stability of AXL mRNA to increase the sunitinib resistance in RCC. Human clinical surveys also linked the expression of TR4, lncTASR, and AXL to the RCC survival, and results from multiple RCC cell lines revealed that targeting this newly identified TR4-mediated signaling with small molecules, including tretinoin, metformin, or TR4-shRNAs, all led to increase the sunitinib sensitivity to better suppress the RCC progression, and our preclinical study using the in vivo mouse model further proved tretinoin had a better synergistic effect to increase sunitinib sensitivity to suppress RCC progression. Future successful clinical trials may help in the development of a novel therapy to better suppress the RCC progression.

Recent insights on the role and regulation of retinoic acid signaling during epicardial development

The vitamin A metabolite, retinoic acid, carries out essential and conserved roles in vertebrate heart development. Retinoic acid signals via retinoic acid receptors (RAR)/retinoid X receptors (RXRs) heterodimers to induce the expression of genes that control cell fate specification, proliferation, and differentiation. Alterations in retinoic acid levels are often associated with congenital heart defects. Therefore, embryonic levels of retinoic acid need to be carefully regulated through the activity of enzymes, binding proteins and transporters involved in vitamin A metabolism. Here, we review evidence of the complex mechanisms that control the fetal uptake and synthesis of retinoic acid from vitamin A precursors. Next, we highlight recent evidence of the role of retinoic acid in orchestrating myocardial compact zone growth and coronary vascular development.

Vitamin A signaling and homeostasis in obesity, diabetes, and metabolic disorders

Much evidence has accumulated in the literature over the last fifteen years that indicates vitamin A has a role in metabolic disease prevention and causation. This literature proposes that vitamin A can affect obesity development and the development of obesity-related diseases including insulin resistance, type 2 diabetes, hepatic steatosis and steatohepatitis, and cardiovascular disease. Retinoic acid, the transcriptionally active form of vitamin A, accounts for many of the reported associations. However, a number of proteins involved in vitamin A metabolism, including retinol-binding protein 4 (RBP4) and aldehyde dehydrogenase 1A1 (ALDH1A1, alternatively known as retinaldehyde dehydrogenase 1 or RALDH1), have also been identified as being associated with metabolic disease. Some of the reported effects of these vitamin A-related proteins are proposed to be independent of their roles in assuring normal retinoic acid homeostasis. This review will consider both human observational data as well as published data from molecular studies undertaken in rodent models and in cells in culture. The primary focus of the review will be on the effects that vitamin A per se and proteins involved in vitamin A metabolism have on adipocytes, adipose tissue biology, and adipose-related disease, as well as on early stage liver disease, including non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH).

Design, Synthesis, and Actions of an Innovative Bispecific Designer Peptide

Despite optimal current therapies, cardiovascular disease remains the leading cause for death worldwide. Importantly, advances in peptide engineering have accelerated the development of innovative therapeutics for diverse human disease states. Additionally, the advancement of bispecific therapeutics targeting >1 signaling pathway represents a highly innovative strategy for the treatment of cardiovascular disease. We, therefore, engineered a novel, designer peptide, which simultaneously targets the pGC-A (particulate guanylyl cyclase A) receptor and the MasR (Mas receptor), potentially representing an attractive cardiorenoprotective therapeutic for cardiovascular disease. We engineered a novel, bispecific receptor activator, NPA7, that represents the fusion of a 22-amino acid sequence of BNP (B-type natriuretic peptide; an endogenous ligand of pGC-A) with Ang 1-7 (angiotensin 1-7)-the 7-amino acid endogenous activator of MasR. We assessed NPA7's dual receptor activating actions in vitro (second messenger production and receptor interaction). Further, we performed an intravenous peptide infusion comparison study in normal canines to study its biological actions in vivo, including in the presence of an MasR antagonist. Our in vivo and in vitro studies demonstrate the successful synthesis of NPA7 as a bispecific receptor activator targeting pGC-A and MasR. In normal canines, NPA7 possesses enhanced natriuretic, diuretic, systemic, and renal vasorelaxing and cardiac unloading properties. Importantly, NPA7's actions are superior to that of the individual native pGC-A or MasR ligands. These studies advance NPA7 as a novel, bispecific designer peptide with potential cardiorenal therapeutic benefit for the treatment of cardiovascular disease, such as hypertension and heart failure.

BAP1 regulation of the key adaptor protein NCoR1 is critical for γ-globin gene repression

Human globin gene production transcriptionally "switches" from fetal to adult synthesis shortly after birth and is controlled by macromolecular complexes that enhance or suppress transcription by cis elements scattered throughout the locus. The DRED (direct repeat erythroid-definitive) repressor is recruited to the ε-globin and γ-globin promoters by the orphan nuclear receptors TR2 (NR2C1) and TR4 (NR2C2) to engender their silencing in adult erythroid cells. Here we found that nuclear receptor corepressor-1 (NCoR1) is a critical component of DRED that acts as a scaffold to unite the DNA-binding and epigenetic enzyme components (e.g., DNA methyltransferase 1 [DNMT1] and lysine-specific demethylase 1 [LSD1]) that elicit DRED function. We also describe a potent new regulator of γ-globin repression: The deubiquitinase BRCA1-associated protein-1 (BAP1) is a component of the repressor complex whose activity maintains NCoR1 at sites in the β-globin locus, and BAP1 inhibition in erythroid cells massively induces γ-globin synthesis. These data provide new mechanistic insights through the discovery of novel epigenetic enzymes that mediate γ-globin gene repression.

The emerging roles of orphan nuclear receptors in prostate cancer

Orphan nuclear receptors are members of the nuclear receptor (NR) superfamily and are so named because their endogenous physiological ligands are either unknown or may not exist. Because of their important regulatory roles in many key physiological processes, dysregulation of signalings controlled by these receptors is associated with many diseases including cancer. Over years, studies of orphan NRs have become an area of great interest because their specific physiological and pathological roles have not been well-defined, and some of them are promising drug targets for diseases. The recently identified synthetic small molecule ligands, acting as agonists or antagonists, to these orphan NRs not only help to understand better their functional roles but also highlight that the signalings mediated by these ligand-independent NRs in diseases could be therapeutically intervened. This review is a summary of the recent advances in elucidating the emerging functional roles of orphan NRs in cancers, especially prostate cancer. In particular, some orphan NRs, RORγ, TR2, TR4, COUP-IFII, ERRα, DAX1 and SHP, exhibit crosstalk or interference with androgen receptor (AR) signaling in either normal or malignant prostatic cells, highlighting their involvement in prostate cancer progression as androgen and AR signaling pathway play critical roles in this process. We also propose that a better understanding of the mechanism of actions of these orphan NRs in prostate gland or prostate cancer could help to evaluate their potential value as therapeutic targets for prostate cancer.

Structures and regulation of non-X orphan nuclear receptors: A retinoid hypothesis

Nuclear receptors are defined as a family of ligand regulated transcription factors [1-6]. While this definition reflects that ligand binding is a key property of nuclear receptors, it is still a heated subject of debate if all the nuclear receptors (48 human members) can bind ligands (ligands referred here to both physiological and synthetic ligands). Recent studies in nuclear receptor structure biology and pharmacology have undoubtedly increased our knowledge of nuclear receptor functions and their regulation. As a result, they point to new avenues for the discovery and development of nuclear receptor regulators, including nuclear receptor ligands. Here we review the recent literature on orphan nuclear receptor structural analysis and ligand identification, particularly on the orphan nuclear receptors that do not heterodimerize with retinoid X receptors, which we term as non-X orphan receptors. We also propose a speculative "retinoid hypothesis" for a subset of non-X orphan nuclear receptors, which we hope to help shed light on orphan nuclear receptor biology and drug discovery. This article is part of a Special Issue entitled 'Orphan Nuclear Receptors'.

Structural basis for corepressor assembly by the orphan nuclear receptor TLX

The orphan nuclear receptor TLX regulates neural stem cell self-renewal in the adult brain and functions primarily as a transcription repressor through recruitment of Atrophin corepressors, which bind to TLX via a conserved peptide motif termed the Atro box. Zhi et al. report crystal structures of the human and insect TLX ligand-binding domain in complex with Atro box peptides. Mutations that weaken the TLX–Atrophin interaction compromise the repressive activity of TLX. In addition, mutations of corresponding residues in other members of this class of receptors diminish their repressor activities.

The orphan nuclear receptor TLX regulates neural stem cell self-renewal in the adult brain and functions primarily as a transcription repressor through recruitment of Atrophin corepressors, which bind to TLX via a conserved peptide motif termed the Atro box. Here we report crystal structures of the human and insect TLX ligand-binding domain in complex with Atro box peptides. In these structures, TLX adopts an autorepressed conformation in which its helix H12 occupies the coactivator-binding groove. Unexpectedly, H12 in this autorepressed conformation forms a novel binding pocket with residues from helix H3 that accommodates a short helix formed by the conserved ALXXLXXY motif of the Atro box. Mutations that weaken the TLX–Atrophin interaction compromise the repressive activity of TLX, demonstrating that this interaction is required for Atrophin to confer repressor activity to TLX. Moreover, the autorepressed conformation is conserved in the repressor class of orphan nuclear receptors, and mutations of corresponding residues in other members of this class of receptors diminish their repressor activities. Together, our results establish the functional conservation of the autorepressed conformation and define a key sequence motif in the Atro box that is essential for TLX-mediated repression.

Compound loss of function of nuclear receptors Tr2 and Tr4 leads to induction of murine embryonic β-type globin genes

The orphan nuclear receptors TR2 and TR4 have been shown to play key roles in repressing the embryonic and fetal globin genes in erythroid cells. However, combined germline inactivation of Tr2 and Tr4 leads to periimplantation lethal demise in inbred mice. Hence, we have previously been unable to examine the consequences of their dual loss of function in adult definitive erythroid cells. To circumvent this issue, we generated conditional null mutants in both genes and performed gene inactivation in vitro in adult bone marrow cells. Compound Tr2/Tr4 loss of function led to induced expression of the embryonic εy and βh1 globins (murine counterparts of the human ε- and γ-globin genes). Additionally, TR2/TR4 function is required for terminal erythroid cell maturation. Loss of TR2/TR4 abolished their occupancy on the εy and βh1 gene promoters, and concurrently impaired co-occupancy by interacting corepressors. These data strongly support the hypothesis that the TR2/TR4 core complex is an adult stage-specific, gene-selective repressor of the embryonic globin genes. Detailed mechanistic understanding of the roles of TR2/TR4 and their cofactors in embryonic and fetal globin gene repression may ultimately enhance the discovery of novel therapeutic agents that can effectively inhibit their transcriptional activity and be safely applied to the treatment of β-globinopathies.

ROR nuclear receptors: structures, related diseases, and drug discovery

Nuclear receptors (NRs) are ligand-regulated transcription factors that regulate metabolism, development and immunity. The NR superfamily is one of the major classes of drug targets for human diseases. Retinoic acid receptor-related orphan receptor (ROR) α, β and γ belong to the NR superfamily, and these receptors are still considered as 'orphan' receptors because the identification of their endogenous ligands has been controversial. Recent studies have demonstrated that these receptors are regulated by synthetic ligands, thus emerge as important drug targets for the treatment of multiple sclerosis, rheumatoid arthritis, psoriasis, etc. Studying the structural basis and ligand development of RORs will pave the way for a better understanding of the roles of these receptors in human diseases. Here, we review the structural basis, disease relevance, strategies for ligand identification, and current status of development of therapeutic ligands for RORs.

Insights into Orphan Nuclear Receptors as Prognostic Markers and Novel Therapeutic Targets for Breast Cancer

There is emerging evidence asserting the importance of orphan nuclear receptors (ONRs) in cancer initiation and progression. In breast cancer, there is a lot unknown about ONRs in terms of their expression profile and their transcriptional targets in the various stages of tumor progression. With the classification of breast tumors into distinct molecular subtypes, we assess ONR expression in the different breast cancer subtypes and with patient outcomes. Complementing this, we review evidence implicating ONR-dependent molecular pathways in breast cancer progression to identify candidate ONRs as potential prognostic markers and/or as therapeutic targets.

Advances in our structural understanding of orphan nuclear receptors

Nuclear receptors (NRs) are key players in the regulation of gene expression, coordinating protein assemblies upon their surfaces. NRs are regulated by ligand binding, which remodels the interaction surfaces and subsequently influences macromolecular complex formation. Structural biology has been instrumental in the discovery of some of these ligands, but there are still orphan NRs (ONRs) whose bona fide ligands have yet to be identified. Over the past decade, fundamental structural and functional breakthroughs have led to a deeper understanding of ONR actions and their multidomain organization. Here, we summarize the structural advances in ONRs with implications for the therapeutic treatment of diseases such as metabolic syndrome and cancer.

The Concise Guide to PHARMACOLOGY 2013/14: nuclear hormone receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full.

Nuclear hormone receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets.

It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Fetal globin gene repressors as drug targets for molecular therapies to treat the β-globinopathies

The human β-globin locus is comprised of embryonic, fetal, and adult globin genes that are expressed in a developmental stage-specific manner. Mutations in the globin locus give rise to the β-globinopathies, β-thalassemia and sickle cell disease, which begin to manifest symptoms around the time of birth. Although the fetal globin genes are autonomously silenced in adult-stage erythroid cells, mutations lying both within and outside the locus lead to natural variations in the level of fetal globin gene expression, and some of these significantly ameliorate the clinical symptoms of the β-globinopathies. Multiple reports have now identified several transcription factors that are involved in fetal globin gene repression in definitive (adult)-stage erythroid cells (the TR2/TR4 heterodimer, MYB, KLFs, BCL11A, and SOX6). To carry out their repression functions, chromatin-modifying enzymes (such as DNA methyltransferase, histone deacetylases, and lysine-specific histone demethylase 1) are additionally involved as a consequence of forming large macromolecular complexes with the DNA-binding subunits of these cellular machines. This review focuses on the molecular mechanisms underlying fetal globin gene silencing and possible near-future molecularly targeted therapies for treating the β-globinopathies.

Differential roles of PPARγ vs TR4 in prostate cancer and metabolic diseases

Peroxisome proliferator-activated receptor γ (PPARγ, NR1C3) and testicular receptor 4 nuclear receptor (TR4, NR2C2) are two members of the nuclear receptor (NR) superfamily that can be activated by several similar ligands/activators including polyunsaturated fatty acid metabolites, such as 13-hydroxyoctadecadienoic acid and 15-hydroxyeicosatetraenoic acid, as well as some anti-diabetic drugs such as thiazolidinediones (TZDs). However, the consequences of the transactivation of these ligands/activators via these two NRs are different, with at least three distinct phenotypes. First, activation of PPARγ increases insulin sensitivity yet activation of TR4 decreases insulin sensitivity. Second, PPARγ attenuates atherosclerosis but TR4 might increase the risk of atherosclerosis. Third, PPARγ suppresses prostate cancer (PCa) development and TR4 suppresses prostate carcinogenesis yet promotes PCa metastasis. Importantly, the deregulation of either PPARγ or TR4 in PCa alone might then alter the other receptor's influences on PCa progression. Knocking out PPARγ altered the ability of TR4 to promote prostate carcinogenesis and knocking down TR4 also resulted in TZD treatment promoting PCa development, indicating that both PPARγ and TR4 might coordinate with each other to regulate PCa initiation, and the loss of either one of them might switch the other one from a tumor suppressor to a tumor promoter. These results indicate that further and detailed studies of both receptors at the same time in the same cells/organs may help us to better dissect their distinct physiological roles and develop better drug(s) with fewer side effects to battle PPARγ- and TR4-related diseases including tumor and cardiovascular diseases as well as metabolic disorders.

Biased, non-equivalent gene-proximal and -distal binding motifs of orphan nuclear receptor TR4 in primary human erythroid cells

We previously reported that TR2 and TR4 orphan nuclear receptors bind to direct repeat (DR) elements in the ε- and γ-globin promoters, and act as molecular anchors for the recruitment of epigenetic corepressors of the multifaceted DRED complex, thereby leading to ε- and γ-globin transcriptional repression during definitive erythropoiesis. Other than the ε- and γ-globin and the GATA1 genes, TR4-regulated target genes in human erythroid cells remain unknown. Here, we identified TR4 binding sites genome-wide using chromatin immunoprecipitation followed by massively parallel sequencing (ChIP-seq) as human primary CD34⁺ hematopoietic progenitors differentiated progressively to late erythroid precursors. We also performed whole transcriptome analyses by RNA-seq to identify TR4 downstream targets after lentiviral-mediated TR4 shRNA knockdown in erythroid cells. Analyses from combined ChIP-seq and RNA-seq datasets indicate that DR1 motifs are more prevalent in the proximal promoters of TR4 direct target genes, which are involved in basic biological functions (e.g., mRNA processing, ribosomal assembly, RNA splicing and primary metabolic processes). In contrast, other non-DR1 repeat motifs (DR4, ER6 and IR1) are more prevalent at gene-distal TR4 binding sites. Of these, approximately 50% are also marked with epigenetic chromatin signatures (such as P300, H3K27ac, H3K4me1 and H3K27me3) associated with enhancer function. Thus, we hypothesize that TR4 regulates gene transcription via gene-proximal DR1 sites as TR4/TR2 heterodimers, while it can associate with novel nuclear receptor partners (such as RXR) to bind to distant non-DR1 consensus sites. In summary, this study reveals that the TR4 regulatory network is far more complex than previously appreciated and that TR4 regulates basic, essential biological processes during the terminal differentiation of human erythroid cells.

Sequential genome-wide binding studies investigated by deep sequencing (ChIP-seq) represent a powerful tool for investigating the temporal sequence of gene activation and repression events that take place as cells differentiate. Here, we report the binding of an “orphan” nuclear receptor (one for which no ligand has been identified) to its cognate genomic regulatory sites and perform the functional analysis to validate its downstream targets as precursor cells differentiate from very early human hematopoietic progenitors into red blood cells. We discovered that when this receptor is bound at gene proximal promoters, it recognizes a different DNA sequence than when it binds to more distant regulatory sites (enhancers and silencers). Since this receptor can either activate or repress specific target genes, the data suggest the intriguing possibility that the two different modes of DNA recognition may reflect association of the receptor with different partner molecules when regulating gene expression from proximal or distal sequences.

PGC-1 coactivator activity is required for murine erythropoiesis

Peroxisome proliferator-activated receptor gamma (PPARγ) coactivator 1α (PGC-1α) and PGC-1β have been shown to be intimately involved in the transcriptional regulation of cellular energy metabolism as well as other biological processes, but both coactivator proteins are expressed in many other tissues and organs in which their function is, in essence, unexplored. Here, we found that both PGC-1 proteins are abundantly expressed in maturing erythroid cells. PGC-1α and PGC-1β compound null mutant (Pgc-1(c)) animals express less β-like globin mRNAs throughout development; consequently, neonatal Pgc-1(c) mice exhibit growth retardation and profound anemia. Flow cytometry shows that the number of mature erythrocytes is markedly reduced in neonatal Pgc-1(c) pups, indicating that erythropoiesis is severely compromised. Furthermore, hematoxylin and eosin staining revealed necrotic cell death and cell loss in Pgc-1(c) livers and spleen. Chromatin immunoprecipitation studies revealed that both PGC-1α and -1β, as well as two nuclear receptors, TR2 and TR4, coordinately bind to the various globin gene promoters. In addition, PGC-1α and -1β can interact with TR4 to potentiate transcriptional activation. These data provide new insights into our understanding of globin gene regulation and raise the interesting possibility that the PGC-1 coactivators can interact with TR4 to elicit differential stage-specific effects on globin gene transcription.

Discovery of 2-oxo-1,2-dihydrobenzo[cd]indole-6-sulfonamide derivatives as new RORγ inhibitors using virtual screening, synthesis and biological evaluation

Retinoic acid receptor-related orphan receptor γ (RORγ), a member of the nuclear hormone receptor superfamily, is a promising therapeutic target for treating Th17-mediated autoimmune diseases. We performed structure-based virtual screening targeting the RORγ ligand-binding domain. Among the tested compounds, s4 demonstrated RORγ antagonistic activities with micromolar IC50 values in both an AlphaScreen assay (20.27 μM) and a cell-based reporter gene assay (11.84 μM). Optimization of the s4 compound led to the identification of compounds 7j, 8c, 8k, and 8p, all of which displayed significantly enhanced RORγ inhibition with IC50 values of 40-140 nM. These results represent a promising starting point for developing potent small molecule RORγ inhibitors.

Retinoic acid receptor subtype-specific transcriptotypes in the early zebrafish embryo

Retinoic acid (RA) controls many aspects of embryonic development by binding to specific receptors (retinoic acid receptors [RARs]) that regulate complex transcriptional networks. Three different RAR subtypes are present in vertebrates and play both common and specific roles in transducing RA signaling. Specific activities of each receptor subtype can be correlated with its exclusive expression pattern, whereas shared activities between different subtypes are generally assimilated to functional redundancy. However, the question remains whether some subtype-specific activity still exists in regions or organs coexpressing multiple RAR subtypes. We tackled this issue at the transcriptional level using early zebrafish embryo as a model. Using morpholino knockdown, we specifically invalidated the zebrafish endogenous RAR subtypes in an in vivo context. After building up a list of RA-responsive genes in the zebrafish gastrula through a whole-transcriptome analysis, we compared this panel of genes with those that still respond to RA in embryos lacking one or another RAR subtype. Our work reveals that RAR subtypes do not have fully redundant functions at the transcriptional level but can transduce RA signal in a subtype-specific fashion. As a result, we define RAR subtype-specific transcriptotypes that correspond to repertoires of genes activated by different RAR subtypes. Finally, we found genes of the RA pathway (cyp26a1, raraa) the regulation of which by RA is highly robust and can even resist the knockdown of all RARs. This suggests that RA-responsive genes are differentially sensitive to alterations in the RA pathway and, in particular, cyp26a1 and raraa are under a high pressure to maintain signaling integrity.

Minireview: role of orphan nuclear receptors in cancer and potential as drug targets

The nuclear orphan receptors for which endogenous ligands have not been identified include nuclear receptor (NR)0B1 (adrenal hypoplasia congenita critical region on chromosome X gene), NR0B2 (small heterodimer partner), NR1D1/2 (Rev-Erbα/β), NR2C1 (testicular receptor 2), NR2C2 (testicular receptor 4), NR2E1 (tailless), NR2E3 (photoreceptor-specific NR [PNR]), NR2F1 chicken ovalbumin upstream promoter transcription factor 1 (COUP-TFI), NR2F2 (COUP-TFII), NR2F6 (v-erbA-related protein), NR4A1 (Nur77), NR4A2 (Nurr1), NR4A3 (Nor1), and NR6A1 (GCNF). These receptors play essential roles in development, cellular homeostasis, and disease including cancer where over- or underexpression of some receptors has prognostic significance for patient survival. Results of receptor knockdown or overexpression in vivo and in cancer cell lines demonstrate that orphan receptors exhibit tumor-specific pro-oncogenic or tumor suppressor-like activity. For example, COUP-TFII expression is both a positive (ovarian) and negative (prostate and breast) prognostic factor for cancer patients; in contrast, the prognostic activity of adrenal hypoplasia congenita critical region on chromosome X gene for the same tumors is the inverse of COUP-TFII. Functional studies show that Nur77 is tumor suppressor like in acute leukemia, whereas silencing Nur77 in pancreatic, colon, lung, lymphoma, melanoma, cervical, ovarian, gastric, and some breast cancer cell lines induces one or more of several responses including growth inhibition and decreased survival, migration, and invasion. Although endogenous ligands for the orphan receptors have not been identified, there is increasing evidence that different structural classes of compounds activate, inactivate, and directly bind several orphan receptors. Thus, the screening and development of selective orphan receptor modulators will have important clinical applications as novel mechanism-based agents for treating cancer patients overexpressing one or more orphan receptors and also for combined drug therapies.

Nuclear and extra-nuclear effects of retinoid acid receptors: how they are interconnected

The nuclear retinoic acid receptors (RAR α, β and γ) and their isoforms are ligand-dependent regulators of transcription Transcription , which mediate the effects of all-trans retinoic acid (RA), the active endogenous metabolite of Vitamin A. They heterodimerize with Retinoid X Receptors (RXRs α, β and γ), and regulate the expression of a battery of target genes Target genes involved in cell growth and differentiation Differentiation . During the two last decades, the description of the crystallographic structures of RARs, the characterization of the polymorphic response elements of their target genes Target genes , and the identification of the multiprotein complexes involved in their transcriptional activity have provided a wealth of information on their pleiotropic effects. However, the regulatory scenario became even more complicated once it was discovered that RARs are phosphoproteins and that RA can activate kinase signaling cascades via a pool of RARs present in membrane lipid rafts. Now it is known that these RA-activated kinases Kinases translocate to the nucleus where they phosphorylate RARs and other retinoid signaling factors. The phosphorylation Phosphorylation state of the RARs dictates whether the transcriptional programs which are known to be induced by RA are facilitated and/or switched on. Thus, kinase signaling pathways appear to be crucial for fine-tuning the appropriate physiological activity of RARs.

Structural insights into gene repression by the orphan nuclear receptor SHP

Small heterodimer partner (SHP) is an orphan nuclear receptor that functions as a transcriptional repressor to regulate bile acid and cholesterol homeostasis. Although the precise mechanism whereby SHP represses transcription is not known, E1A-like inhibitor of differentiation (EID1) was isolated as a SHP-interacting protein and implicated in SHP repression. Here we present the crystal structure of SHP in complex with EID1, which reveals an unexpected EID1-binding site on SHP. Unlike the classical cofactor-binding site near the C-terminal helix H12, the EID1-binding site is located at the N terminus of the receptor, where EID1 mimics helix H1 of the nuclear receptor ligand-binding domain. The residues composing the SHP-EID1 interface are highly conserved. Their mutation diminishes SHP-EID1 interactions and affects SHP repressor activity. Together, these results provide important structural insights into SHP cofactor recruitment and repressor function and reveal a conserved protein interface that is likely to have broad implications for transcriptional repression by orphan nuclear receptors.

Local motifs involved in the canonical structure of the ligand-binding domain in the nuclear receptor superfamily

Structural and sequence alignment analyses have revealed the existence of class-dependent and -independent local motifs involved in the overall fold of the ligand-binding domain (LBD) in the nuclear receptor (NR) superfamily. Of these local motifs, three local motifs, i.e., AF-2 fixed motifs, were involved in the agonist conformation of the activation function-2 (AF-2) region of the LBD. Receptor-agonist interactions increased the stability of these AF-2 fixed motifs in the agonist conformation. In contrast, perturbation of the AF-2 fixed motifs by a ligand or another protein molecule led the AF-2 architecture to adopt an antagonist conformation. Knowledge of this process should provide us with novel insights into the 'agonism' and 'antagonism' of NRs.

Understanding nuclear receptor form and function using structural biology

Nuclear receptors (NRs) are a major transcription factor family whose members selectively bind small-molecule lipophilic ligands and transduce those signals into specific changes in gene programs. For over two decades, structural biology efforts were focused exclusively on the individual ligand-binding domains (LBDs) or DNA-binding domains of NRs. These analyses revealed the basis for both ligand and DNA binding and also revealed receptor conformations representing both the activated and repressed states. Additionally, crystallographic studies explained how NR LBD surfaces recognize discrete portions of transcriptional coregulators. The many structural snapshots of LBDs have also guided the development of synthetic ligands with therapeutic potential. Yet, the exclusive structural focus on isolated NR domains has made it difficult to conceptualize how all the NR polypeptide segments are coordinated physically and functionally in the context of receptor quaternary architectures. Newly emerged crystal structures of the peroxisome proliferator-activated receptor-γ-retinoid X receptor α (PPARγ-RXRα) heterodimer and hepatocyte nuclear factor (HNF)-4α homodimer have recently revealed the higher order organizations of these receptor complexes on DNA, as well as the complexity and uniqueness of their domain-domain interfaces. These emerging structural advances promise to better explain how signals in one domain can be allosterically transmitted to distal receptor domains, also providing much better frameworks for guiding future drug discovery efforts.

The orphan nuclear receptors at their 25-year reunion

The nuclear receptor superfamily includes many receptors, identified based on their similarity to steroid hormone receptors but without a known ligand. The study of how these receptors are diversely regulated to interact with genomic regions to control a plethora of biological processes has provided critical insight into development, physiology, and the molecular pathology of disease. Here we provide a compendium of these so-called orphan receptors and focus on what has been learned about their modes of action, physiological functions, and therapeutic promise.

The roles of endogenous retinoid signaling in organ and appendage regeneration

The ability to regenerate injured or lost body parts has been an age-old ambition of medical science. In contrast to humans, teleost fish and urodele amphibians can regrow almost any part of the body with seeming effortlessness. Retinoic acid is a molecule that has long been associated with these impressive regenerative capacities. The discovery 30 years ago that addition of retinoic acid to regenerating amphibian limbs causes "super-regeneration" initiated investigations into the presumptive roles of retinoic acid in regeneration of appendages and other organs. However, the evidence favoring or dismissing a role for endogenous retinoids in regeneration processes remained sparse and ambiguous. Now, the availability of genetic tools to manipulate and visualize the retinoic acid signaling pathway has opened up new routes to dissect its roles in regeneration. Here, we review the current understanding on endogenous functions of retinoic acid in regeneration and discuss key questions to be addressed in future research.

Retinoic acid has different effects on UCP1 expression in mouse and human adipocytes

Background

Increased adipose thermogenesis is being considered as a strategy aimed at preventing or reversing obesity. Thus, regulation of the uncoupling protein 1 (UCP1) gene in human adipocytes is of significant interest. Retinoic acid (RA), the carboxylic acid form of vitamin A, displays agonist activity toward several nuclear hormone receptors, including RA receptors (RARs) and peroxisome proliferator-activated receptor δ (PPARδ). Moreover, RA is a potent positive regulator of UCP1 expression in mouse adipocytes.

Results

The effects of all-trans RA (ATRA) on UCP1 gene expression in models of mouse and human adipocyte differentiation were investigated. ATRA induced UCP1 expression in all mouse white and brown adipocytes, but inhibited or had no effect on UCP1 expression in human adipocyte cell lines and primary human white adipocytes. Experiments with various RAR agonists and a RAR antagonist in mouse cells demonstrated that the stimulatory effect of ATRA on UCP1 gene expression was indeed mediated by RARs. Consistently, a PPARδ agonist was without effect. Moreover, the ATRA-mediated induction of UCP1 expression in mouse adipocytes was independent of PPARγ coactivator-1α.

Conclusions

UCP1 expression is differently affected by ATRA in mouse and human adipocytes. ATRA induces UCP1 expression in mouse adipocytes through activation of RARs, whereas expression of UCP1 in human adipocytes is not increased by exposure to ATRA.

The crystal structure of the orphan nuclear receptor NR2E3/PNR ligand binding domain reveals a dimeric auto-repressed conformation

Photoreceptor-specific nuclear receptor (PNR, NR2E3) is a key transcriptional regulator of human photoreceptor differentiation and maintenance. Mutations in the NR2E3-encoding gene cause various retinal degenerations, including Enhanced S-cone syndrome, retinitis pigmentosa, and Goldman-Favre disease. Although physiological ligands have not been identified, it is believed that binding of small molecule agonists, receptor desumoylation, and receptor heterodimerization may switch NR2E3 from a transcriptional repressor to an activator. While these features make NR2E3 a potential therapeutic target for the treatment of retinal diseases, there has been a clear lack of structural information for the receptor. Here, we report the crystal structure of the apo NR2E3 ligand binding domain (LBD) at 2.8 Å resolution. Apo NR2E3 functions as transcriptional repressor in cells and the structure of its LBD is in a dimeric auto-repressed conformation. In this conformation, the putative ligand binding pocket is filled with bulky hydrophobic residues and the activation-function-2 (AF2) helix occupies the canonical cofactor binding site. Mutations designed to disrupt either the AF2/cofactor-binding site interface or the dimer interface compromised the transcriptional repressor activity of this receptor. Together, these results reveal several conserved structural features shared by related orphan nuclear receptors, suggest that most disease-causing mutations affect the receptor's structural integrity, and allowed us to model a putative active conformation that can accommodate small ligands in its pocket.

Recent advances in the study of testicular nuclear receptor 4

Testicular nuclear receptor 4 (TR4), also known as NR2C2 (nuclear receptor subfamily 2, group C, member 2), is a transcriptional factor and a member of the nuclear receptor family. TR4 was initially cloned from human and rat hypothalamus, prostate, and testes libraries. For almost two decades, its specific tissue distribution, genomic organization, and chromosomal assignment have been well investigated in humans and animals. However, it has been very difficult to study TR4's physiological functions due to a lack of specific ligands. Gene knock-out animal techniques provide an alternative approach for defining the biological functions of TR4. In vivo studies of TR4 gene knockout mice (TR4(-/-)) found that they display severe spinal curvature, subfertility, premature aging, and prostate prostatic intraepithelial neoplasia (PIN) development. Upstream modulators, downstream target gene regulation, feedback mechanisms, and differential modulation mediated by the recruitment of other nuclear receptors and coregulators have been identified in studies using the TR4(-/-) phenotype. With the establishment of a tissue-specific TR4(-/-) mouse model, research on TR4 will be more convenient in the future.