Protein kinase C regulates mitochondrial targeting of Nur77 and its family member Nor-1 in thymocytes undergoing apoptosis

NOR-1 Overexpression Elevates Myoglobin Expression via PERM1 and Enhances Mitochondrial Function and Endurance in Skeletal Muscles of Aged Mice

Skeletal muscle health and function deteriorate with age, ultimately leading to impaired mobility and disability. Exercise is among the most effective interventions to mitigate muscle dysfunction in aging and reverse deficits. However, low attrition and an impaired capacity to exercise may limit its utility in improving muscle function in aged persons. Therefore, it is crucial to advance our mechanistic understanding of the molecular transducers of exercise to identify new and innovative drug targets to improve muscle health. Transcriptomic profiling of the human response to exercise has revealed that the nuclear receptor NR4A3 (NOR-1) is among the most responsive genes to acute exercise. Previously, we observed that in vitro knockdown of NOR-1 alters metabolic signaling in C2C12 myotubes. Specifically, we found that expression of PERM1, CKMT2, myoglobin, and mTORC1 signaling were perturbed during the knockdown of NOR-1. Herein, we extend these findings and observe that a NOR-1-PERM1-myoglobin axis regulates myoglobin expression in vitro. Furthermore, we found that aging is associated with reduced skeletal muscle NOR-1 expression. Although it is well known that exercise improves aged muscle function, whether overexpression of the exercise-responsive gene NOR-1 can confer benefits and improve muscle function in an aged context has not been evaluated. We found that the overexpression of NOR-1 in aged muscle results in enhanced muscle endurance, mitochondrial respiration, and elevated expression of NOR-1 responsive genes that we previously identified in loss of function studies. However, we also observed that overexpression of NOR-1 did not improve maximal muscle torque production and resulted in a small but significant loss of muscle wet weight that was concomitant with elevated autophagy signaling. Our data suggest that NOR-1 expression may reduce muscle fatigability and that NOR-1 drives myoglobin expression in a PERM1-dependent manner.

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

The nuclear receptor subfamily 4 group A1 in human disease

Nuclear receptor 4A1 (NR4A1), a member of the NR4A subfamily, acts as a gene regulator in a wide range of signaling pathways and responses to human diseases. Here, we provide a brief overview of the current functions of NR4A1 in human diseases and the factors involved in its function. A deeper understanding of these mechanisms can potentially improve drug development and disease therapy.

Comprehensive insights into the function and molecular and pharmacological regulation of neuron-derived orphan receptor 1, an orphan receptor

Neuron-derived orphan receptor 1 (NOR1), also called nuclear receptor subfamily 4 group A member 3 (NR4A3), is a nuclear receptor belonging to the NR4A family. Since no endogenous ligand has been identified to date, NOR1 is also referred to as an orphan receptor. NOR1 is expressed in a variety of cells and tissues, including neurons, vascular smooth muscle cells, T lymphocytes, dendritic cells, tumor cells, heart, liver, and pancreas. Because NOR1 was first identified in apoptotic neurons, it is functionally associated with the regulation of cell migration and the growth of neuronal synapses. In-depth studies have shown that NOR1 can be edited by the immediate early gene and functions as a transcription factor. NOR1 has been shown to be rapidly induced by a number of stimulants including growth factors, fatty acids, and neurotransmitters. Elevated NOR1 levels may be involved in a number of pathophysiological processes. These include regulation of cellular apoptosis and regeneration, neuron formation, contextual fearing memory, inflammation, vascular smooth muscle proliferation, insulin secretion, and tumor development, whereby NOR1 mediates the pathogenesis of numerous diseases such as cerebral ischemia, depression, post-traumatic stress disorder, atherosclerosis, abdominal aortic aneurysm, cardiac hypertrophy, diabetes, osteoarthritis, rheumatoid arthritis, and cancer. However, to date, comprehensive insights into the function of NOR1 are not available in sources published online. In this review, we provide a brief overview of the function and molecular and pharmacological regulation of NOR1 in various pathological or physiological conditions to advance the development of NOR1 as a novel target for disease treatment.

Nuclear receptor NOR-1 (Neuron-derived Orphan Receptor-1) in pathological vascular remodelling and vascular remodelling

Vascular cells and their interaction with inflammatory cells and the immune system play a key role in pathological vascular remodeling. A large number of genes and proteins regulated in a coordinated manner by a small number of transcription factors are involved in this process. In recent years, research on a small subfamily of transcription factors, the NR4A subfamily, has had a major impact on our understanding of vascular biology. The NR4A1 (Nur77), NR4A2 (Nurr1) and NR4A3 (NOR-1) receptors are products of early response genes whose expression is induced by multiple pathophysiological and physical stimuli. Their wide distribution in different tissues and cells places them in the control of numerous processes such as cell differentiation, proliferation, survival and apoptosis, as well as inflammation and the metabolism of lipids and carbohydrates. This review analyzes the role of these receptors, particularly NOR-1, in pathological vascular remodeling associated with atherosclerosis, abdominal aortic aneurysm and pulmonary arterial hypertension.

NR4A nuclear receptors in T and B lymphocytes: Gatekeepers of immune tolerance. Immunol Rev 2022;307:116-133. [PMID: 35174510 DOI: 10.1111/imr.13072]

Random VDJ recombination early in T and B cell development enables the adaptive immune system to recognize a vast array of evolving pathogens via antigen receptors. However, the potential of such randomly generated TCRs and BCRs to recognize and respond to self-antigens requires layers of tolerance mechanisms to mitigate the risk of life-threatening autoimmunity. Since they were originally cloned more than three decades ago, the NR4A family of nuclear hormone receptors have been implicated in many critical aspects of immune tolerance, including negative selection of thymocytes, peripheral T cell tolerance, regulatory T cells (Treg), and most recently in peripheral B cell tolerance. In this review, we discuss important insights from many laboratories as well as our own group into the function and mechanisms by which this small class of primary response genes promotes self-tolerance and immune homeostasis to balance the need for host defense against the inherent risks posed by the adaptive immune system.

J. PB, Larsen P, Taylor RE, Zhang J. Molecular mechanisms by which splice modulator GEX1A inhibits leukaemia development and progression

INTRODUCTION

Splice modulators have been assessed clinically in treating haematologic malignancies exhibiting splice factor mutations and acute myeloid leukaemia. However, the mechanisms by which such modulators repress leukaemia remain to be elucidated.

OBJECTIVES

The primary goal of this assessment was to assess the molecular mechanism by which the natural splice modulator GEX1A kills leukaemic cells in vitro and within in vivo mouse models.

METHODS

Using human leukaemic cell lines, we assessed the overall sensitivity these cells have to GEX1A via EC₅₀ analysis. We subsequently analysed its effects using in vivo xenograft mouse models and examined whether cell sensitivities were correlated to genetic characteristics or protein expression levels. We also utilised RT-PCR and RNAseq analyses to determine splice change and RNA expression level differences between sensitive and resistant leukaemic cell lines.

RESULTS

We found that, in vitro, GEX1A induced an MCL-1 isoform shift to pro-apoptotic MCL-1S in all leukaemic cell types, though sensitivity to GEX1A-induced apoptosis was negatively associated with BCL-xL expression. In BCL-2-expressing leukaemic cells, GEX1A induced BCL-2-dependent apoptosis by converting pro-survival BCL-2 into a cell killer. Thus, GEX1A + selective BCL-xL inhibition induced synergism in killing leukaemic cells, while GEX1A + BCL-2 inhibition showed antagonism in BCL-2-expressing leukaemic cells. In addition, GEX1A sensitised FLT3-ITD⁺ leukaemic cells to apoptosis by inducing aberrant splicing and repressing the expression of FLT3-ITD. Consistently, in in vivo xenografts, GEX1A killed the bulk of leukaemic cells via apoptosis when combined with BCL-xL inhibition. Furthermore, GEX1A repressed leukaemia development by targeting leukaemia stem cells through inhibiting FASTK mitochondrial isoform expression across sensitive and non-sensitive leukaemia types.

CONCLUSION

Our study suggests that GEX1A is a potent anti-leukaemic agent in combination with BCL-xL inhibitors, which targets leukaemic blasts and leukaemia stem cells through distinct mechanisms.

NR4A3: A Key Nuclear Receptor in Vascular Biology, Cardiovascular Remodeling, and Beyond

The mechanisms committed in the activation and response of vascular and inflammatory immune cells play a major role in tissue remodeling in cardiovascular diseases (CVDs) such as atherosclerosis, pulmonary arterial hypertension, and abdominal aortic aneurysm. Cardiovascular remodeling entails interrelated cellular processes (proliferation, survival/apoptosis, inflammation, extracellular matrix (ECM) synthesis/degradation, redox homeostasis, etc.) coordinately regulated by a reduced number of transcription factors. Nuclear receptors of the subfamily 4 group A (NR4A) have recently emerged as key master genes in multiple cellular processes and vital functions of different organs, and have been involved in a variety of high-incidence human pathologies including atherosclerosis and other CVDs. This paper reviews the major findings involving NR4A3 (Neuron-derived Orphan Receptor 1, NOR-1) in the cardiovascular remodeling operating in these diseases.

Atypic SUMOylation of Nor1/NR4A3 regulates neural cell viability and redox sensitivity

Neuron-derived orphan receptor 1, NR4A3 (Nor1)/NR4A3 is an orphan nuclear receptor involved in the transcriptional control of developmental and neurological functions. Oxidative stress-induced conditions are primarily associated with neurological defects in humans, yet the impact on Nor1-mediated transcription of neuronal genes remains with unknown mechanism. Here, we demonstrate that Nor1 is a non-conventional target of SUMO2/3 conjugation at Lys-137 contained in an atypic ψKxSP motif referred to as the pSuM. Nor1 pSuM SUMOylation differs from the canonical process with the obligate phosphorylation of Ser-139 by Ras signaling to create the required negatively charged interface for SUMOylation. Additional phosphorylation at sites flanking the pSuM is also mediated by the coordinated action of protein kinase casein kinase 2 to function as a small ubiquitin-like modifier enhancer, regulating Nor1-mediated transcription and proteasomal degradation. Nor1 responsive genes involved in cell proliferation and metabolism, such as activating transcription factor 3, cyclin D1, CASP8 and FADD-like apoptosis regulator, and enolase 3 were upregulated in response to pSuM disruption in mouse HT-22 hippocampal neuronal cells and human neuroblastoma SH-SY5Y cells. We also identified critical antioxidant genes, such as catalase, superoxide dismutase 1, and microsomal glutathione S-transferase 2, as responsive targets of Nor1 under pSuM regulation. Nor1 SUMOylation impaired gene transcription through less effective Nor1 chromatin binding and reduced enrichment of histone H3K27ac marks to gene promoters. These effects resulted in decreased neuronal cell growth, increased apoptosis, and reduced survival to oxidative stress damage, underlying the role of pSuM-modified Nor1 in redox homeostasis. Our findings uncover a hierarchical post-translational mechanism that dictates Nor1 non-canonical SUMOylation, disrupting Nor1 transcriptional competence, and neuroprotective redox sensitivity.

'Nur'turing tumor T cell tolerance and exhaustion: novel function for Nuclear Receptor Nur77 in immunity

The nuclear receptor Nur77 is expressed in a multitude of tissues, regulating cell differentiation and homeostasis. Dysregulation of Nur77 signaling is associated with cancer, cardiovascular disease, and disorders of the CNS. The role of Nur77 in T cells has been studied for almost 30 years now. There is a clear appreciation that Nur77 is crucial for apoptosis of self‐reactive T cells. However, the regulation and function of Nur77 in mature T cells remains largely unclear. In an exciting development, Nur77 has been recently demonstrated to impinge on cancer immunotherapy involving chimeric antigen receptor (CAR) T cells and tumor infiltrating lymphocytes (TILs). These studies indicated that Nur77 deficiency reduced T cell tolerance and exhaustion, thus raising the effectiveness of immune therapy in mice. Based on these novel insights, it may be proposed that regulation of Nur77 activity holds promise for innovative drug development in the field of cellular immunotherapy in cancer. In this review, we therefore summarize the role of Nur77 in T cell selection and maturation; and further develop the idea of targeting its activity in these cells as a potential strategy to augment current cancer immunotherapy treatments.

Cantharidin Induces Apoptosis and Promotes Differentiation of AML Cells Through Nuclear Receptor Nur77-Mediated Signaling Pathway

Background

Acute myeloid leukemia (AML) is a hematopoietic malignancy characterized by uncontrolled proliferation and accumulation of myeloblasts in the bone marrow (BM), blood, and other organs. The nuclear receptors Nur77 is a common feature in leukemic blasts and has emerged as a key therapeutic target for AML. Cantharidin (CTD), a main medicinal component of Mylabris (blister beetle), exerts an anticancer effect in multiple types of cancer cells.

Purpose

This study aims to characterize the anti-AML activity of CTD in vitro and in vivo and explore the potential role of Nur77 signaling pathway.

Study Design/Methods

The inhibition of CTD on cell viability was performed in different AML cells, and then the inhibition of CTD on proliferation and colony formation was detected in HL-60 cells. Induction of apoptosis and promotion of differentiation by CTD were further determined. Then, the potential role of Nur77 signaling pathway was assessed. Finally, anti-AML activity was evaluated in NOD/SCID mice.

Results

In our study, CTD exhibited potent inhibition on cell viability and colony formation ability of AML cells. Moreover, CTD significantly induced the apoptosis, which was partially reversed by Z-VAD-FMK. Meanwhile, CTD promoted the cleavage of caspases 8, 3 and PARP in HL-60 cells. Furthermore, CTD obviously suppressed the proliferation and induced the cell cycle arrest of HL-60 cells at G2/M phase. Meanwhile, CTD effectively promoted the differentiation of HL-60 cells. Notably, CTD transiently induced the expression of Nur77 protein. Interestingly, CTD promoted Nur77 translocation from the nucleus to the mitochondria and enhanced the interaction between Nur77 and Bcl-2, resulting in the exposure of the BH3 domain of Bcl-2, which is critical for the conversion of Bcl-2 from an antiapoptotic to a proapoptotic protein. Importantly, silencing of Nur77 attenuated CTD-induced apoptosis, reversed CTD-mediated cell cycle arrest and differentiation of HL-60 cells. Additionally, CTD also exhibited an antileukemic effect in NOD/SCID mice with the injection of HL-60 cells into the tail vein.

Conclusions

Our studies suggest that Nur77-mediated signaling pathway may play a critical role in the induction of apoptosis and promotion of differentiation by CTD on AML cells.

Disruption of Beta-Cell Mitochondrial Networks by the Orphan Nuclear Receptor Nor1/Nr4a3

Nor1, the third member of the Nr4a subfamily of nuclear receptor, is garnering increased interest in view of its role in the regulation of glucose homeostasis. Our previous study highlighted a proapoptotic role of Nor1 in pancreatic beta cells and showed that Nor1 expression was increased in islets isolated from type 2 diabetic individuals, suggesting that Nor1 could mediate the deterioration of islet function in type 2 diabetes. However, the mechanism remains incompletely understood. We herein investigated the subcellular localization of Nor1 in INS832/13 cells and dispersed human beta cells. We also examined the consequences of Nor1 overexpression on mitochondrial function and morphology. Our results show that, surprisingly, Nor1 is mostly cytoplasmic in beta cells and undergoes mitochondrial translocation upon activation by proinflammatory cytokines. Mitochondrial localization of Nor1 reduced glucose oxidation, lowered ATP production rates, and inhibited glucose-stimulated insulin secretion. Western blot and microscopy images revealed that Nor1 could provoke mitochondrial fragmentation via mitophagy. Our study unveils a new mode of action for Nor1, which affects beta-cell viability and function by disrupting mitochondrial networks.

Role of the Orphan Nuclear Receptor NR4A Family in T-Cell Biology

The nuclear orphan receptors NR4A1, NR4A2, and NR4A3 are immediate early genes that are induced by various signals. They act as transcription factors and their activity is not regulated by ligand binding and are thus regulated via their expression levels. Their expression is transiently induced in T cells by triggering of the T cell receptor following antigen recognition during both thymic differentiation and peripheral T cell responses. In this review, we will discuss how NR4A family members impact different aspects of the life of a T cell from thymic differentiation to peripheral response against infections and cancer.

The orphan nuclear receptor Nor1/Nr4a3 is a negative regulator of β-cell mass

The Nr4a subfamily of nuclear receptor comprises three members in mammalian cells: Nur77/Nr4a1, Nurr1/Nr4a2, and Nor1/Nr4a3. Nr4a proteins play key roles in the regulation of glucose homeostasis in peripheral metabolic tissues. However, their biological functions in β-cells remain relatively uncharacterized. Here we sought to investigate the potential role of Nor1 in the regulation of β-cell mass and, in particular, β-cell survival/apoptosis. We used histological analysis to examine the consequences of genetic deletion of either Nur77 and Nor1 on β-cell mass, investigated the expression patterns of Nr4as in human islets and INS cells and performed gain- and loss-of-function experiments to further characterize the role of Nor1 in β-cell apoptosis. Surprisingly, Nor1 knockout mice displayed increased β-cell mass, whereas mice with genetic deletion of Nur77 did not exhibit any significant differences compared with their WT littermates. The increase in β-cell mass in Nor1 knockout mice was accompanied by improved glucose tolerance. A gene expression study performed in both human islets and INS cells revealed that Nor1 expression is significantly increased by pro-inflammatory cytokines and, to a lesser extent, by elevated concentrations of glucose. Nor1 overexpression in both INS and human islet cells caused apoptosis, whereas siRNA-mediated Nor1 knockdown prevented cytokine-induced β-cell death. Finally, Nor1 expression was up-regulated in islets of individuals with type 2 diabetes. Altogether, our results uncover that Nor1 negatively regulates β-cell mass. Nor1 represents a promising molecular target in diabetes treatment to prevent β-cell destruction.

Cytoplasmic location of NR4A1 in aggressive lymphomas is associated with a favourable cancer specific survival

The nuclear orphan receptor NR4A1 functions as tumour suppressor in aggressive lymphomas by pro-apoptotic genomic and non-genomic effects. Here, we immunohistochemically studied the clinico-pathological relevance of NR4A1 protein expression patterns in a cohort of 60 diffuse large B cell lymphoma (DLBCL) patients and non-neoplastic lymph nodes. We observed a significant association between high cytoplasmic NR4A1 and favourable cancer-specific survival and the germinal centre B cell-like subtype, respectively. Moreover, the percentage of lymphoma cells exhibiting cytoplasmic NR4A1 significantly correlated to those showing cleaved caspase 3. Complementary, functional profiling using gene set enrichment of Reactome pathways based on publicly available microarray data was applied to determine pathways potentially implicated in cytoplasmic localization of NR4A1 and validated by means of semi quantitative real-time PCR. The pathway analysis revealed changes in the ERK1/2 pathway, and this was corroborated by the finding that high cytoplasmic NR4A1 was associated with higher expression of ERK1/2 targets in our cohort. These data indicate that high cytoplasmic NR4A1 is associated with a favourable lymphoma-specific survival and highlights the importance of NR4A1 expression patterns as potential prognostic marker for risk assessment in aggressive lymphomas.

LncRNA TDRG1 enhances tumorigenicity in endometrial carcinoma by binding and targeting VEGF-A protein

Endometrial carcinoma is one of the most frequently diagnosed cancers in females. Long non-coding RNAs (lncRNAs) have been associated with cancer; its role in endometrial carcinoma is an emerging area of research. In this article, lncRNA TDRG1 expression in human endometrial carcinoma tissues and normal endometrial tissues was quantified by qRT-PCR. LncRNA TDRG1 was overexpressed or knocked-down in neither HEC-1B nor Ishikawa endometrial carcinoma cells, respectively, to assess cellular phenotype and expression of related molecules. Our results showed that lncRNA TDRG1 was significantly overexpressed in endometrial carcinoma tissues. Overexpression of lncRNA TDRG1 promoted endometrial carcinoma cell viability, invasion and migratory ability, inhibited apoptosis, and upregulated VEGF-A, PI3K, Bcl-2, MMP2 and survivin; knockdown of lncRNA TDRG1 had the opposite effects. LncRNA TDRG1 overexpression increased tumorigenicity in vivo and was associated with the upregulation of VEGF-A. RNA binding protein immunoprecipitation (RIP) assays confirmed that lncRNA TDRG1 directly binds to VEGF-A protein. Furthermore, knockdown of VEGFA in lncRNA TDRG1-overexpressing endometrial carcinoma cells reversed the effects of lncRNA TDRG1 on cell proliferation, invasion, migration and apoptosis. In conclusion, lncRNA TDRG1 may promote endometrial carcinoma cell proliferation and invasion by positively targeting VEGF-A and modulating relative genes.

B cell lymphoma 2 (Bcl-2) residues essential for Bcl-2's apoptosis-inducing interaction with Nur77/Nor-1 orphan steroid receptors

Apoptosis is mediated through the extrinsic or intrinsic pathway. Key regulators of the intrinsic apoptotic pathway are the family of B cell lymphoma 2 (Bcl-2) proteins. The activity of the prototypical Bcl-2 protein is usually considered antiapoptotic. However, under some conditions, Bcl-2 associates with the orphan nuclear hormone receptors Nur77 and Nor-1, converting Bcl-2 into a proapoptotic molecule. Expression of Nur77 and Nor-1 is induced by a variety of signals, including those leading to apoptosis. Translocation of Nur77/Nor-1 to mitochondria results in their association with Bcl-2, exposing the Bcl-2 homology (BH) 3 domain and causing apoptosis. However, the molecular details of this interaction are incompletely understood. Here, through extensive Bcl-2 mutagenesis and functional assays, we identified residues within Bcl-2 that are essential for its interaction with Nur77/Nor-1. Although an initial report has suggested that an unstructured loop region between the Bcl-2 BH4 and BH3 domains is required for Bcl-2's interaction with Nur77/Nor-1, we found that it is dispensable for this interaction. Instead, we found important interacting residues at the BH4 domain and crucial interacting residues between the BH1 and BH2 domains. Bcl-2 alanine mutants at this region could no longer interact with Nur77/Nor-1 and could not initiate Nur77/Bcl-2-mediated cell death. However, they still retained their anti-apoptotic capability in two different death assays. These results establish crucial residues in Bcl-2 required for Nur77/Nor-1-mediated apoptosis and point to potential new strategies for manipulating Bcl-2 function.

In vivo neutralization of α4 and β7 integrins inhibits eosinophil trafficking and prevents lung injury during tropical pulmonary eosinophilia in mice

Integrins regulate leukocyte trafficking during homeostasis and inflammatory conditions. However, the role of α4 and β7 integrins in guiding eosinophil transmigration into the lungs during filarial manifestation of Tropical Pulmonary Eosinophilia (TPE) has not been explored. In this study, mice exhibiting TPE manifestations were administered with in vivo neutralizing antibodies against integrins α4 and β7 or their combination and immuno-pathological parameters were evaluated. Results show an intact lung barrier, significantly lower lung inflammation and reduced eosinophil counts in the Bronchoalveolar lavage fluid and lungs of mice receiving anti-α4⁺ β7 treatment. Reduced eosinophil peroxidase and β-hexosaminidase activity, downregulation of inflammatory genes, lower production of inflammatory lipid intermediates like prostaglandins E2 and D2, leukotriene B4 and cysteinyl leukotrienes were also noted in anti-α4⁺ β7 treated mice. Reduced accumulation of central memory, effector memory, regulatory T cells and lower production of IL-4, IL-5, and TGF-β were other cardinal features of anti-α4⁺ β7 treated mice lungs. Flow cytometry-sorted lung eosinophils from anti-α4⁺ β7 treated mice showed higher apoptotic potential, downregulated anti-apoptotic gene Bcl-2, and exhibited reduced F-actin polymerization and calcium influx as compared to IgG controls. In summary, neutralization of α4⁺ β7 integrins impairs the transmigration, activation and survival of eosinophils and reduces TPE induced pathology in mice lungs.

Modulation of the Nur77-Bcl-2 apoptotic pathway by p38α MAPK

Orphan nuclear receptor Nur77 promotes apoptosis by targeting mitochondria through interaction with Bcl-2, an event that converts Bcl-2 from a survival to killer. However, how the Nur77-Bcl-2 apoptotic pathway is regulated remains largely unknown. In this study, we examined the regulation of the Nur77-Bcl-2 pathway by CCE9, a xanthone compound. Our results demonstrated that the apoptotic effect of CCE9 depended on its induction of Nur77 expression, cytoplasmic localization, and mitochondrial targeting. The activation of the Nur77-Bcl-2 pathway by CCE9 was associated with its activation of p38α MAPK. Inhibition of p38α MAPK activation by knocking down or knocking out p38α MAPK impaired the effect of CCE9 on inducing apoptosis and the expression and cytoplasmic localization of Nur77. In addition, CCE9 activation of p38α MAPK resulted in Bcl-2 phosphorylation and Bcl-2 interaction with Nur77, whereas inhibition of p38α MAPK activation or expression suppressed the interaction. Moreover, mutating Ser87 and Thr56 in the loop of Bcl-2, which are known to be phosphorylated by p38α MAPK, impaired the ability Bcl-2 to interact with Nur77. Together, our results reveal a profound role of p38α MAPK in regulating the Nur77-Bcl-2 apoptotic pathway through its modulation of Nur77 expression, Bcl-2 phosphorylation, and their interaction.

Endogenous Nur77 Is a Specific Indicator of Antigen Receptor Signaling in Human T and B Cells

Distinguishing true Ag-stimulated lymphocytes from bystanders activated by the inflammatory milieu has been difficult. Nur77 is an immediate early gene whose expression is rapidly upregulated by TCR signaling in murine T cells and human thymocytes. Nur77-GFP transgenes serve as specific TCR and BCR signaling reporters in murine transgenic models. In this study, we demonstrate that endogenous Nur77 protein expression can serve as a reporter of TCR and BCR specific signaling in human PBMCs. Nur77 protein amounts were assessed by immunofluorescence and flow cytometry in T and B cells isolated from human PBMCs obtained from healthy donors that had been stimulated by their respective Ag receptors. We demonstrate that endogenous Nur77 is a more specific reporter of Ag-specific signaling events than the commonly used CD69 activation marker in both human T and B cells. This is reflective of the disparity in signaling pathways that regulate the expression of Nur77 and CD69. Assessing endogenous Nur77 protein expression has great potential to identify Ag-activated lymphocytes in human disease.

In Silico Adoption of an Orphan Nuclear Receptor NR4A1

A 4.1μs molecular dynamics simulation of the NR4A1 (hNur77) apo-protein has been undertaken and a previously undetected druggable pocket has become apparent that is located remotely from the ‘traditional’ nuclear receptor ligand-binding site. A NR4A1/bis-indole ligand complex at this novel site has been found to be stable over 1 μs of simulation and to result in an interesting conformational transmission to a remote loop that has the capacity to communicate with a NBRE within a RXR-α/NR4A1 heterodimer. Several features of the simulations undertaken indicate how NR4A1 can be affected by alternate-site modulators.

Orphan Nuclear Receptor Nur77 Inhibits Cardiac Hypertrophic Response to Beta-Adrenergic Stimulation

The orphan nuclear receptor Nur77 plays critical roles in cardiovascular diseases, and its expression is markedly induced in the heart after beta-adrenergic receptor (β-AR) activation. However, the functional significance of Nur77 in β-AR signaling in the heart remains unclear. By using Northern blot, Western blot, and immunofluorescent staining assays, we showed that Nur77 expression was markedly upregulated in cardiomyocytes in response to multiple hypertrophic stimuli, including isoproterenol (ISO), phenylephrine (PE), and endothelin-1 (ET-1). In a time- and dose-dependent manner, ISO increases Nur77 expression in the nuclei of cardiomyocytes. Overexpression of Nur77 markedly inhibited ISO-induced cardiac hypertrophy by inducing nuclear translocation of Nur77 in cardiomyocytes. Furthermore, cardiac overexpression of Nur77 by intramyocardial injection of Ad-Nur77 substantially inhibited cardiac hypertrophy and ameliorated cardiac dysfunction after chronic infusion of ISO in mice. Mechanistically, we demonstrated that Nur77 functionally interacts with NFATc3 and GATA4 and inhibits their transcriptional activities, which are critical for the development of cardiac hypertrophy. These results demonstrate for the first time that Nur77 is a novel negative regulator for the β-AR-induced cardiac hypertrophy through inhibiting the NFATc3 and GATA4 transcriptional pathways. Targeting Nur77 may represent a potentially novel therapeutic strategy for preventing cardiac hypertrophy and heart failure.

Retinoids induce Nur77-dependent apoptosis in mouse thymocytes

Nur77 is a transcription factor, which plays a determinant role in mediating T cell receptor-induced cell death of thymocytes. In addition to regulation of transcription, Nur77 contributes to apoptosis induction by targeting mitochondria, where it can convert Bcl-2, an anti-apoptotic protein into a proapoptotic molecule. Previous studies have demonstrated that retinoids are actively produced in the mouse thymus and can induce a transcription-dependent apoptosis in mouse thymocytes. Here we show that retinoic acids induce the expression of Nur77, and retinoid-induced apoptosis is completely dependent on Nur77, as retinoids were unable to induce apoptosis in Nur77 null thymocytes. In wild-type thymocytes retinoids induced enhanced expression of the apoptosis-related genes FasL, TRAIL, NDG-1, Gpr65 and Bid, all of them in a Nur77-dependent manner. The combined action of these proteins led to Caspase 8-dependent Bid cleavage in the mitochondria. In addition, we could demonstrate the Nur77-dependent induction of STAT1 leading to enhanced Bim expression, and the mitochondrial translocation of Nur77 leading to the exposure of the Bcl-2/BH3 domain. The retinoid-induced apoptosis was dependent on both Caspase 8 and STAT1. Our data together indicate that retinoids induce a Nur77-dependent cell death program in thymocytes activating the mitochondrial pathway of apoptosis.

Orphan nuclear receptor Nur77 Inhibits Oxidized LDL-induced differentiation of RAW264.7 murine macrophage cell line into dendritic like cells

Background

Nur77 is an orphan nuclear receptor expressed in human atheroma. In vascular cells in vitro, Nur77 expression is induced by pro-inflammatory factors, such as oxidized LDL (oxLDL).

Methods

We analyze the role of Nur77 in the oxLDL-induced differentiation of macrophages into dendritic cells (DC). The murine RAW264.7 macrophage cell line was stably transfected with expression plasmids encoding either GFP or GFP fusions with either full-length Nur77 (GFP-Nur77), Nur77 lacking the DNA binding domain (GFP-Nur77-ΔDBD) or Nur77 lacking the transactivation domain (GFP-Nur77-ΔTAD).

Results

GFP-Nur77 overexpression significantly suppressed the effect of oxLDL treatment on DC morphologic changes, expression of DC maturation markers, endocytic activity, allogeneic activation of T cell proliferation, and the activity and secretion of pro-inflammatory cytokines. Analysis of GFP-Nur77-ΔTAD and GFP-Nur77-ΔDBD indicated that the Nur77 DNA binding and transactivation domains were both required for this effect. GFP-Nur77-ΔDBD consistently had the opposite effect to GFP-Nur77, increasing DC-type differentiation in all assays. Interestingly, GFP-Nur77-ΔDBD protein was cytosolic, whereas GFP-Nur77 and GFP-Nur77-ΔTAD were both nuclear.

Conclusions

These data show that GFP-Nur77 inhibited differentiation of oxLDL-treated macrophages into DC. The effects of Nur77 on the macrophage phenotype may involve changes in its subcellular distribution.

Electronic supplementary material

The online version of this article (doi:10.1186/s12865-014-0054-z) contains supplementary material, which is available to authorized users.

Induction of Nur77 by hyperoside inhibits vascular smooth muscle cell proliferation and neointimal formation

Nur77 is an orphan nuclear receptor that belongs to the nuclear receptor 4A (NR4A) subfamily, which has been implicated in a variety of biological events, such as cell apoptosis, proliferation, inflammation, and metabolism. Activation of Nur77 has recently been shown to be beneficial for the treatment of cardiovascular and metabolic diseases. The purpose of this study is to identify novel natural Nur77 activators and investigate their roles in preventing vascular diseases. By measuring Nur77 expression using quantitative RT-PCR, we screened active ingredients extracted from Chinese herb medicines with beneficial cardiovascular effects. Hyperoside (quercetin 3-D-galactoside) was identified as one of the potent activators for inducing Nur77 expression and activating its transcriptional activity in vascular smooth muscle cells (VSMCs). We demonstrated that hyperoside, in a time and dose dependent manner, markedly increased the expression of Nur77 in rat VSMCs, with an EC50 of ∼0.83 μM. Mechanistically, we found that hyperoside significantly increased the phosphorylation of ERK1/2 MAP kinase and its downstream target cAMP response element-binding protein (CREB), both of which contributed to the hyperoside-induced Nur77 expression in rat VSMCs. Moreover, through activation of Nur77 receptor, hyperoside markedly inhibited both vascular smooth muscle cell proliferation in vitro and the carotid artery ligation-induced neointimal formation in vivo. These findings demonstrate that hyperoside is a potent natural activator of Nur77 receptor, which can be potentially used for prevention and treatment of occlusive vascular diseases.

Induction of Nur77-dependent apoptotic pathway by a coumarin derivative through activation of JNK and p38 MAPK

Coumarins are plant-derived natural products with a broad range of known pharmacological activities including anticancer effects. However, the molecular mechanisms by which this class of promising compounds exerts their anticancer effects remain largely unknown. We report here that a furanocoumarin named apaensin could effectively induce apoptosis of cancer cells through its activation of Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). Apoptosis induction by apaensin in cancer cells was suppressed by chemical inhibitors of JNK and p38 MAPK. Inhibition of the expression of orphan nuclear receptor Nur77 by small interfering RNA (siRNA) approach also abrogated the death effect of apaensin. Molecular analysis demonstrated that JNK activation was required for the nuclear export of Nur77, a known apoptotic event in cancer cells. Although p38 MAPK activation was not involved in Nur77 nuclear export, it was essential for Nur77 mitochondrial targeting through induction of Nur77 interaction with Bcl-2, which is also known to convert Bcl-2 from an antiapoptotic to a proapoptotic molecule. Together, our results identify a new natural product that targets orphan nuclear receptor Nur77 through its unique activation of JNK and p38 MAPK and provide insight into the complex regulation of the Nur77-Bcl-2 apoptotic pathway.

NR4A nuclear receptors are orphans but not lonesome

The NR4A subfamily of nuclear receptors consists of three mammalian members: Nur77, Nurr1, and NOR-1. The NR4A receptors are involved in essential physiological processes such as adaptive and innate immune cell differentiation, metabolism and brain function. They act as transcription factors that directly modulate gene expression, but can also form trans-repressive complexes with other transcription factors. In contrast to steroid hormone nuclear receptors such as the estrogen receptor or the glucocorticoid receptor, no ligands have been described for the NR4A receptors. This lack of known ligands might be explained by the structure of the ligand-binding domain of NR4A receptors, which shows an active conformation and a ligand-binding pocket that is filled with bulky amino acid side-chains. Other mechanisms, such as transcriptional control, post-translational modifications and protein-protein interactions therefore seem to be more important in regulating the activity of the NR4A receptors. For Nur77, over 80 interacting proteins (the interactome) have been identified so far, and roughly half of these interactions has been studied in more detail. Although the NR4As show some overlap in interacting proteins, less information is available on the interactome of Nurr1 and NOR-1. Therefore, the present review will describe the current knowledge on the interactomes of all three NR4A nuclear receptors with emphasis on Nur77.

Elevation of NR4A3 expression and its possible role in modulating insulin expression in the pancreatic beta cell

Background

NR4A3/NOR-1 is a member of the NR4A orphan nuclear receptor subfamily, which contains early response genes that sense and respond to a variety of stimuli in the cellular environment. The role of NR4A3 in insulin expression in pancreatic beta cells remains unknown.

Methods

Dynamic changes in NR4A3 were examined in a pancreatic beta-cell line, MIN6, treated with thapsigargin (TG), palmitate (PA), tunicamycin (TM), and dithiothreitol (DTT), chemicals that produce cell stress and even apoptosis. We exploited virus infection techniques to induce expression of NR4A3 or three deletion mutants, and determined expression of insulin and insulin regulatory genes in MIN6 cells.

Results

TG and PA, two endoplasmic reticulum (ER) stress inducers, were able to induce unfolded protein response (UPR) activation and elevation of NR4A3 expression in MIN6 cells, whereas TM and DTT, two other ER stress inducers, were able to induce UPR activation but not NR4A3 elevation. MIN6 cells over-expressing NR4A3 protein after adenoviral infection exhibited reduced transcription of the insulin genes Ins1 and Ins2, and reduced insulin protein secretion, which were negatively correlated with NR4A3 expression levels. Functional analysis of different deletion mutants of NR4A3 showed that deleting the activation domain AF1 or the DNA-binding domain abolished the down-regulation of insulin transcription by NR4A3 in MIN6 cells, indicating that this down-regulative role was closely related to the NR4A3 trans-activation activity. Over-expression of NR4A3 in MIN6 cells resulted in reduced mRNA transcription of the insulin positive-regulation genes, Pdx1 and NeuroD1.

Conclusion

Some ER stress inducers, such as TG or PA, are able to elevate NR4A3 expression in MIN6 cells, while others, such as TM or DTT, are not. Over-expression of NR4A3 in MIN6 cells results in down-regulation of insulin gene transcription and insulin secretion. NR4A3 reduces insulin gene expression by modulating the expression of Pdx1 and NeuroD1.

Nuclear receptor 4a3 (nr4a3) regulates murine mast cell responses and granule content

Nuclear receptor 4a3 (Nr4a3) is a transcription factor implicated in various settings such as vascular biology and inflammation. We have recently shown that mast cells dramatically upregulate Nuclear receptor 4a3 upon activation, and here we investigated the functional impact of Nuclear receptor 4a3 on mast cell responses. We show that Nuclear receptor 4a3 is involved in the regulation of cytokine/chemokine secretion in mast cells following activation via the high affinity IgE receptor. Moreover, Nuclear receptor 4a3 negatively affects the transcript and protein levels of mast cell tryptase as well as the mast cell's responsiveness to allergen. Together, these findings identify Nuclear receptor 4a3 as a novel regulator of mast cell function.

The MOC31PE immunotoxin reduces cell migration and induces gene expression and cell death in ovarian cancer cells

Background

The standard treatment of ovarian cancer with chemotherapy often leads to drug resistance and relapse of the disease, and the need for development of novel therapy alternatives is obvious. The MOC31PE immunotoxin binds to the cell surface antigen EpCAM, which is expressed by the majority of epithelial cancers including ovarian carcinomas, and we studied the cytotoxic effects of MOC31PE in ovarian cancer cells.

Methods

Investigation of the effects of MOC31PE treatment on protein synthesis, cell viability, proliferation and gene expression of the ovarian cancer cell lines B76 and HOC7.

Results

MOC31PE treatment for 24 h caused a dose-dependent reduction of protein synthesis with ID₅₀ values of less than 10 ng/ml, followed by reduced cell viability. In a gene expression array monitoring the expression of 84 key genes in cancer pathways, 13 of the genes were differentially expressed by MOC31PE treatment in comparison to untreated cells. By combining MOC31PE and the immune suppressor cyclosporin A (CsA) the MOC31PE effect on protein synthesis inhibition and cell viability increased tenfold. Cell migration was also reduced, both in the individual MOC31PE and CsA treatment, but even more when combining MOC31PE and CsA. In tumor metastasis PCR arrays, 23 of 84 genes were differentially expressed comparing CsA versus MOC31PE + CsA treatment. Increased expression of the tumor suppressor KISS1 and the nuclear receptor NR4A3 was observed, and the differential candidate gene expression was confirmed in complementary qPCR analyses. For NR4A3 this was not accompanied by increased protein expression. However, a subcellular fractionation assay revealed increased mitochondrial NR4A3 in MOC31PE treated cells, suggesting a role for this protein in MOC31PE-induced apoptotic cell death.

Conclusion

The present study demonstrates that MOC31PE may become a new targeted therapy for ovarian cancer and that the MOC31PE anti-cancer effect is potentiated by CsA.

Nur77: a potential therapeutic target in cancer

INTRODUCTION

The orphan nuclear receptor Nur77 (also known as NR4A1, NGFIB, TR3, TIS1, NAK-1, or N10) is a unique transcription factor encoded by an immediate early gene. Nur77 signaling is deregulated in many cancers and constitutes an important molecule for drug targeting.

AREAS COVERED

Nur77 as a versatile transcription factor that displays distinct dual roles in cell proliferation and apoptosis. In addition, several recent insights into Nur77's non-genomic signaling through its physical interactions with various signaling proteins and its phosphorylation-dependent regulation will be highlighted. The possible mechanisms by which Nur77 supports carcinogenesis and specific examples in different human cancers will be summarized. Different approaches to target Nur77 using mimetics, natural products, and synthetic compounds are also described.

EXPERT OPINION

These latest findings shed light on the novel roles of Nur77 as an exploitable target for new cancer therapeutics. Further work which focuses on a more complete understanding of the Nur77 interactome as well as how the different networks of Nur77 functional interactions are orchestrated in a stimulus or context-specific way will aid the development of more selective, non-toxic approaches for targeting Nur77 in future.

Differences in NGFI-B, Nurr1, and NOR-1 expression and nucleocytoplasmic translocation in glutamate-treated neurons

NGFI-B (NR4A1, Nur77 or TR3) together with Nurr1 (NR4A2) and NOR-1 (NR4A3) constitute the NR4A subgroup of orphan nuclear receptors. They play critical roles in proliferation, differentiation, survival and apoptosis in different cell types, including neurons, immature T-cells, and different cancer cells. As ligand-independent and constitutively active receptors, the diverse biological activities of NGFI-B, Nurr1 and NOR-1 depend on their levels of expression, post-translational modifications and subcellular localization. Nuclear localization of the NR4A proteins leads to transcriptional activity, whereas NGFI-B and recently also NOR-1 have been shown to induce apoptosis by a more direct mechanism when localized at mitochondria. In the present study we investigated mRNA expression and subcellular translocation of the NR4A proteins during glutamate excitotoxicity in rat cerebellar granule neurons. NGFI-B and Nurr1 mRNA, but not NOR-1 mRNA, were induced by treatments associated with calcium influx, although their regulation seemed to be different. NR4A(gfp) fusion proteins showed a predominant nuclear localization in untreated cells. After glutamate treatment NGFI-B(gfp) translocated to cytosol and mitochondria within a few hours, whereas Nurr1(gfp) translocation was delayed, and NOR-1(gfp) mainly stayed in the nucleus. Subcellular targeting of NGFI-B seems to be tightly regulated, as a single mutation of threonine 142 altered NGFI-B(gfp) localization. Differences in expression and subcellular translocation of NGFI-B, Nurr1, and NOR-1 may reflect different functions in neurons in glutamate excitotoxicity.

IL-15 regulates homeostasis and terminal maturation of NKT cells

Semi-invariant NKT cells are thymus-derived innate-like lymphocytes that modulate microbial and tumor immunity as well as autoimmune diseases. These immunoregulatory properties of NKT cells are acquired during their development. Much has been learned regarding the molecular and cellular cues that promote NKT cell development, yet how these cells are maintained in the thymus and the periphery and how they acquire functional competence are incompletely understood. We found that IL-15 induced several Bcl-2 family survival factors in thymic and splenic NKT cells in vitro. Yet, IL-15-mediated thymic and peripheral NKT cell survival critically depended on Bcl-x(L) expression. Additionally, IL-15 regulated thymic developmental stage 2 to stage 3 lineage progression and terminal NKT cell differentiation. Global gene expression analyses and validation revealed that IL-15 regulated Tbx21 (T-bet) expression in thymic NKT cells. The loss of IL-15 also resulted in poor expression of key effector molecules such as IFN-γ, granzyme A and C, as well as several NK cell receptors, which are also regulated by T-bet in NKT cells. Taken together, our findings reveal a critical role for IL-15 in NKT cell survival, which is mediated by Bcl-x(L), and effector differentiation, which is consistent with a role of T-bet in regulating terminal maturation.

Life and death in the thymus--cell death signaling during T cell development

The thymus is an organ vital to proper T cell development, and the regulation of cell survival and death contributes significantly to its efficient function. Vital to many of the developmental processes that occur in the thymus, control over cell survival and death is orchestrated by several signaling processes. In this review, we focus on the regulation of death in early thymocytes known as CD4/CD8 double negative cells, including the roles of interleukin-7 and Bcl-2 family members in this developmental stage. We next consider the survival and death of later thymocytes that express both CD4 and CD8, the 'double-positive' thymocytes. These findings are discussed within the context of recent studies demonstrating the existence of caspase-independent cell death pathways.

Mitochondrial translocation of Nur77 mediates cardiomyocyte apoptosis

AIMS

The cascade of events leading to compromised mitochondrial integrity in response to stress is mediated by various combinatorial interactions of pro- and anti-apoptotic molecules. Nur77, an immediate early gene that encodes a nuclear orphan receptor, translocates from the nucleus to mitochondria to induce cytochrome c release and apoptosis in cancer cells in response to various pro-apoptotic treatments. However, the role of Nur77 in the cardiac setting is still unclear. The objective of this study is to determine the physiological relevance and pathophysiological importance of Nur77 in cardiomyocytes.

METHODS AND RESULTS

Myocardial Nur77 is upregulated following cardiomyopathic injury and, while expressed in the postnatal myocardium, declines in level within weeks after birth. Nur77 is localized predominantly in cardiomyocyte nuclei under normal conditions where it is not apoptotic, but translocates to mitochondria in response to oxidative stress both in vitro and in vivo. Mitochondrial localization of Nur77 induces cytochrome c release and typical morphological features of apoptosis, including chromatin condensation and DNA fragmentation. Knockdown of Nur77 rescued hydrogen peroxide-induced cardiomyocyte apoptosis.

CONCLUSION

Translocation of Nur77 from the nucleus to the mitochondria in cardiomyocytes results in the loss of mitochondrial integrity and subsequent apoptosis in response to ischaemia/reperfusion injury. Our findings identify Nur77 as a novel mediator of cardiomyocyte apoptosis and warrants further investigation of mitochondrial Nur77 translocation as a mechanism to control cell death in the treatment of ischaemic heart diseases.