Nuclear receptor and apoptosis initiator NGFI-B is a substrate for kinase ERK2

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.

The Ginsenoside Compound K Suppresses Stem-Cell-like Properties and Colorectal Cancer Metastasis by Targeting Hypoxia-Driven Nur77-Akt Feed-Forward Signaling

Hypoxia reprograms cancer stem cells. Nur77, an orphan nuclear receptor, highly expresses and facilitates colorectal cancer (CRC) stemness and metastasis under a hypoxic microenvironment. However, safe and effective small molecules that target Nur77 for CSC depletion remain unexplored. Here, we report our identification of the ginsenoside compound K (CK) as a new ligand of Nur77. CK strongly inhibits hypoxia-induced CRC sphere formation and CSC phenotypes in a Nur77-dependent manner. Hypoxia induces an intriguing Nur77-Akt feed-forward loop, resulting in reinforced PI3K/Akt signaling that is druggable by targeting Nur77. CK directly binds and modulates Nur77 phosphorylation to block the Nur77-Akt activation loop by disassociating Nur77 from the p63-bound Dicer promoter. The transcription of Dicer that is silenced under a hypoxia microenvironment is thus reactivated by CK. Consequently, the expression and processing capability of microRNA let-7i-5p are significantly increased, which targets PIK3CA mRNA for decay. The in vivo results showed that CK suppresses cancer stemness and metastasis without causing significant adverse effects. Given that the majority of FDA-approved and currently clinically tested PI3K/Akt inhibitors are reversible ATP-competitive kinase antagonists, targeting Nur77 for PI3K/Akt inactivation may provide an alternative strategy to overcoming concerns about drug selectivity and safety. The mechanistic target identification provides a basis for exploring CK as a promising nutraceutical against CRC.

Malayoside, a cardenolide glycoside extracted from Antiaris toxicaria Lesch, induces apoptosis in human non-small lung cancer cells via MAPK-Nur77 signaling pathway

Lung cancer is the leading cause of cancer deaths in the world. Non-small cell lung cancer (NSCLC), with poor prognosis and resistance to chemoradiotherapy, is the most common histological type of lung cancer. Therefore, it is necessary to develop new and more effective treatment strategy for NSCLC. Nur77, an orphan member of the nuclear receptor superfamily, induces apoptosis in cancer cells including NSCLC cells, by high expression and translocation to mitochondria. Small molecules trigger expression and mitochondrial localization of Nur77 may be an ideal anti-cancer drug candidate. Here, we report malayoside, a cardiac glycoside in the extract of Antiaris toxicaria Lesch., had different sensitivities to NSCLC cells. Malayoside induced apoptosis in NCI-H460 cells. Meanwhile, malayoside induced Nur77 expression and mitochondrial localization, and its induction of apoptosis was Nur77-dependent. To investigate the molecular mechanism of malayoside inducing Nur77 and apoptosis, we found that malayoside activated MAPK signaling pathway, including both ERK and p38 phosphorylation. The suppression of MAPK signaling activation inhibited the expression of Nur77 and apoptosis induced by malayoside. Our studies in nude mice showed that malayside potently inhibited the growth of tumor cells in vivo. Furthermore, the anti-cancer effect of malayosidwas in vivo was also related to the elevated expression of Nur77, p-ERK, and p-p38 proteins. Our results suggest that malayoside possesses an anti-NSCLC activity in vitro and in vivo mainly via activation of MAPK-Nur77 signaling pathway, indicating that malayoside is a promising chemotherapeutic candidate for NSCLC.

RAF-MEK-ERK pathway in cancer evolution and treatment

The RAF-MEK-ERK signaling cascade is a well-characterized MAPK pathway involved in cell proliferation and survival. The three-layered MAPK signaling cascade is initiated upon RTK and RAS activation. Three RAF isoforms ARAF, BRAF and CRAF, and their downstream MEK1/2 and ERK1/2 kinases constitute a coherently orchestrated signaling module that directs a range of physiological functions. Genetic alterations in this pathway are among the most prevalent in human cancers, which consist of numerous hot-spot mutations such as BRAF^V600E. Oncogenic mutations in this pathway often override otherwise tightly regulated checkpoints to open the door for uncontrolled cell growth and neoplasia. The crosstalk between the RAF-MEK-ERK axis and other signaling pathways further extends the proliferative potential of this pathway in human cancers. In this review, we summarize the molecular architecture and physiological functions of the RAF-MEK-ERK pathway with emphasis on its dysregulations in human cancers, as well as the efforts made to target the RAF-MEK-ERK module using small molecule inhibitors.

The nuclear receptor NR4A1 is regulated by SUMO modification to induce autophagic cell death

NR4A is a nuclear receptor protein family whose members act as sensors of cellular environment and regulate multiple processes such as metabolism, proliferation, migration, apoptosis, and autophagy. Since the ligand binding domains of these receptors have no cavity for ligand interaction, their function is most likely regulated by protein abundance and post-translational modifications. In particular, NR4A1 is regulated by protein abundance, phosphorylation, and subcellular distribution (nuclear-cytoplasmic translocation), and acts both as a transcription factor and as a regulator of other interacting proteins. SUMOylation is a post-translational modification that can affect protein stability, transcriptional activity, alter protein-protein interactions and modify intracellular localization of target proteins. In the present study we evaluated the role of SUMOylation as a posttranslational modification that can regulate the activity of NR4A1 to induce autophagy-dependent cell death. We focused on a model potentially relevant for neuronal cell death and demonstrated that NR4A1 needs to be SUMOylated to induce autophagic cell death. We observed that a triple mutant in SUMOylation sites has reduced SUMOylation, increased transcriptional activity, altered intracellular distribution, and more importantly, its ability to induce autophagic cell death is impaired.

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.

Altered regulation of Nur77 nuclear receptor gene expression in the mesocorticolimbic regions of rat brain by amphetamine sensitization

The mechanisms underlying psychostimulant drug-induced sensitization include long-term cellular and molecular adaptations in dopaminergic circuits. Nur77, a member of the Nur family of transcription factors, is expressed in brain regions receiving dopamine inputs and plays a role in activity-induced synaptic modification. Here we evaluated changes in Nur77 mRNA levels in the medial prefrontal cortex (mPFC), dorsal striatum (Str) and nucleus accumbens (NAc) of rats receiving a repeated, sensitizing regimen of amphetamine (AMPH). Results were compared to two groups of controls - animals receiving repeated injections of saline (Rp-SAL) or with no treatment (CON). Two weeks after the last injection, the effect of an acute challenge dose of AMPH on Nur77 expression was evaluated using in-situ hybridization. Repeated AMPH treatment (Rp-AMPH) increased the levels of Nur77 mRNA in the mPFC, NAc core and shell regions. However, the effects of an acute injection of AMPH in each of the three groups of animals was distinct. Whereas an acute AMPH led to a significant increase of Nur77 in all brain regions of the CON animals, it had no significant effect in Rp-SAL animals. Interestingly, in acute AMPH-injected Rp-AMPH animals, Nur77 mRNA levels in the mPFC, Str and NAc regions were significantly lower compared to CON and Rp-SAL animals treated with acute AMPH. There was a positive correlation between AMPH -induced locomotor activity and Nur77 mRNA expression in CON animals; however, this relationship was absent in Rp-SAL and Rp-AMPH animals. The data suggest that Nur77 is a part of neuroadaptive changes caused by either mild stress of repeated injections as well as AMPH-sensitization and may play a role in abnormal behaviors induced by the drug.

Requirement of novel amino acid fragments of orphan nuclear receptor TR3/Nur77 for its functions in angiogenesis

Pathological angiogenesis is a hallmark of many diseases. We demonstrated that TR3/Nur77 is an excellent target for pro-angiogenesis and anti-angiogenesis therapies. Here, we report that TR3 transcriptionally regulates endothelial cell migration, permeability and the formation of actin stress fibers that is independent of RhoA GTPase. 1) Amino acid residues 344-GRR-346 and de-phosphorylation of amino acid residue serine 351 in the DNA binding domain, and 2) phosphorylation of amino acid residues in the 41-61 amino acid fragment of the transactivation domain, of TR3 are required for its induction of the formation of actin stress fibers, cell proliferation, migration and permeability. The 41-61 amino acid fragment contains one of the three potential protein interaction motifs in the transactivation domain of TR3, predicted by computational modeling and analysis. These studies further our understanding of the molecular mechanism, by which TR3 regulates angiogenesis, identify novel therapeutic targeted sites of TR3, and set the foundation for the development of high-throughput screening assays to identify compounds targeting TR3/Nur77 for pro-angiogenesis and anti-angiogenesis therapies.

The interplay of NR4A receptors and the oncogene-tumor suppressor networks in cancer

Nuclear receptor (NR) subfamily 4 group A (NR4A) is a family of three highly homologous orphan nuclear receptors that have multiple physiological and pathological roles, including some in cancer. These NRs are reportedly dysregulated in multiple cancer types, with many studies demonstrating pro-oncogenic roles for NR4A1 (Nur77) and NR4A2 (Nurr1). Additionally, NR4A1 and NR4A3 (Nor-1) are described as tumor suppressors in leukemia. The dysregulation and functions of the NR4A members are due to many factors, including transcriptional regulation, protein-protein interactions, and post-translational modifications. These various levels of intracellular regulation result from the signaling cross-talk of the NR4A members with various signaling pathways, many of which are relevant to cancer and likely explain the family members' functions in oncogenesis and tumor suppression. In this review, we discuss the multiple functions of the NR4A receptors in cancer and summarize a growing body of scientific literature that describes the interconnectedness of the NR4A receptors with various oncogene and tumor suppressor pathways.

Differential regulation of orphan nuclear receptor TR3 transcript variants by novel vascular growth factor signaling pathways

Angiogenesis is a hallmark of many diseases, including cancer, ischemic heart disease, inflammation, and others. It is well known that vascular endothelial growth factor (VEGF) is the most important angiogenic factor. Recently, we demonstrated that orphan nuclear receptor TR3 (mouse Nur77 and rat NGFI-B) plays critical roles in tumor growth and angiogenesis induced by VEGF-A in vitro and in vivo. However, the signaling pathways that mediate the expression of TR3 induced by VEGF are still not completely understood. Here we reported that 3 TR3 transcript variants (TR3-TVs) are expressed at differential levels, and regulated differentially in endothelial cells. While the expression of TR3-TV1 is relatively low, the expression of TR3-TV2 is up-regulated markedly, and the expression of TR3-TV3 is up-regulated moderately in endothelial cells induced by VEGF-A. The kinetics of the induction of these TR3-TVs is different. We also found that several signaling pathways, including calcium-PLC-PKC-PKD1 pathway, NF-κB pathway, and MAP kinase (ERK, p38, and JNK) pathways are important for VEGF-A-induced TR3-TV2 and TR3-TV3 mRNA induction. More important, we found that VEGF-A or VEGF-E, but not VEGF-B, nor placenta growth factor (PlGF), induces the phosphorylation of insulin-like growth factor-1 receptor (IGF-1R) and the interaction of VEGF receptor 2/kinase insert domain receptor (VEGFR2/KDR) with IGF-1R, which mediates the expression of TR3-TV2, but not TR3-TV3. Taking together, we demonstrate that TR3-TVs are differentially regulated by VEGF-A and identify a novel signaling pathway by which VEGF-A and VEGF-E, but neither VEGF-B, nor PlGF, induce the interaction of VEGFR2/KDR with IGF-1R, resulting in IGF-1R transactivation to induce the high level expression of TR3-TV2. Our data not only elucidate the signaling pathways by which TR3-TVs are regulated, but extend the molecular mechanism, by which VEGF-A-induced angiogenesis. These studies should permit the development of screening assays for compounds that inhibit VEGF signaling.

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.

Nur transcription factors in stress and addiction

The Nur transcription factors Nur77 (NGFI-B, NR4A1), Nurr1 (NR4A2), and Nor-1 (NR4A3) are a sub-family of orphan members of the nuclear receptor superfamily. These transcription factors are products of immediate early genes, whose expression is rapidly and transiently induced in the central nervous system by several types of stimuli. Nur factors are present throughout the hypothalamus-pituitary-adrenal (HPA) axis where are prominently induced in response to stress. Drugs of abuse and stress also induce the expression of Nur factors in nuclei of the motivation/reward circuit of the brain, indicating their participation in the process of drug addiction and in non-hypothalamic responses to stress. Repeated use of addictive drugs and chronic stress induce long-lasting dysregulation of the brain motivation/reward circuit due to reprogramming of gene expression and enduring alterations in neuronal function. Here, we review the data supporting that Nur transcription factors are key players in the molecular basis of the dysregulation of neuronal circuits involved in chronic stress and addiction.

The nuclear receptor NR4A1 induces a form of cell death dependent on autophagy in mammalian cells

The control of cell death is a biological process essential for proper development, and for preventing devastating pathologies like cancer and neurodegeneration. On the other hand, autophagy regulation is essential for protein and organelle degradation, and its dysfunction is associated with overlapping pathologies like cancer and neurodegeneration, but also for microbial infection and aging. In the present report we show that two evolutionarily unrelated receptors--Neurokinin 1 Receptor (NK(1)R,) a G-protein coupled receptor, and Insulin-like Growth Factor 1 Receptor (IGF1R), a tyrosine kinase receptor--both induce non-apoptotic cell death with autophagic features and requiring the activity of the autophagic core machinery proteins PI3K-III, Beclin-1 and Atg7. Remarkably, this form of cell death occurs in apoptosis-competent cells. The signal transduction pathways engaged by these receptors both converged on the activation of the nuclear receptor NR4A1, which has previously been shown to play a critical role in some paradigms of apoptosis and in NK(1)R-induced cell death. The activity of NR4A1 was necessary for IGF1R-induced cell death, as well as for a canonical model of cell death by autophagy induced by the presence of a pan-caspase inhibitor, suggesting that NR4A1 is a general modulator of this kind of cell death. During cell death by autophagy, NR4A1 was transcriptionally competent, even though a fraction of it was present in the cytoplasm. Interestingly, NR4A1 interacts with the tumor suppressor p53 but not with Beclin-1 complex. Therefore the mechanism to promote cell death by autophagy might involve regulation of gene expression, as well as protein interactions. Understanding the molecular basis of autophagy and cell death mediation by NR4A1, should provide novel insights and targets for therapeutic intervention.

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.

Apoptosis inducer NGFI-B is degraded by the proteasome and stabilized by treatment with EGF

NGFI-B is a nuclear receptor and immediate early gene that is upregulated in many different tumour cell lines. As it is involved in cell death and survival, it has been suggested as a target for anti-cancer drugs. The protein level of NGFI-B is important for its functions and may be regulated through induction or stabilization. NGFI-B protein stability was studied using the protein synthesis inhibitor cycloheximide in CV1 cells transiently transfected with NGFI-B. Inhibiting the proteasome with MG132 stabilized NGFI-B, indicating that the proteasome is responsible for break-down of NGFI-B, as it is for many nuclear receptors. In order to determine regions responsible for the break-down of NGFI-B two N-terminal regions with high PEST-scores were deleted. Deletion of amino acids 122-195 containing a PEST-sequence which includes an ERK2 phosphorylation target, gave a more stable protein. In addition, treatment of the cells with the ERK2 activator EGF increased the stability of wild type NGFI-B. We then tested whether a mutation at threonine 142 influenced the stability of NGFI-B. We found that the phosphorylation-mimicking mutant NGFI-B T142E had an increased stability, while the non-phosphorylable mutant (T142A) showed similar stability to the wild type. Thus, EGF-stimulation of cells may be a mechanism for priming the cells for effects of NGFI-B by increasing its stability.

Prolyl isomerase Pin1 stabilizes and activates orphan nuclear receptor TR3 to promote mitogenesis

Pin1 regulates a subset of phosphoproteins by isomerizing phospho-Ser/Thr-Pro motifs via a 'post-phosphorylation' mechanism. Here, we characterize TR3 as a novel Pin1 substrate, and the mitogenic function of TR3 depends on Pin1-induced isomerization. There are at least three phospho-Ser-Pro motifs on TR3 that bind to Pin1. The Ser95-Pro motif of TR3 is the key site through which Pin1 enhances TR3 stability by retarding its degradation. Pin1 can also catalyze TR3 through phospho-Ser431-Pro motif, which is phosphorylated by extracellular signal-regulated kinase 2 (ERK2), resulting in enhanced TR3 transactivation. Furthermore, Pin1 not only facilitates TR3 targeting to the promoter of cyclin D2, a novel downstream target of TR3, but also promotes TR3 to recruit p300, thereby inducing cell proliferation. Importantly, we found that Pin1 is indispensable for TR3 to promote tumor growth both in vitro and in vivo. Our study thus suggests that Pin1 has an important role in cell proliferation by isomerizing TR3.

p38 MAPKs regulate the expression of genes in the dopamine synthesis pathway through phosphorylation of NR4A nuclear receptors

In Drosophila, the melanization reaction is an important defense mechanism against injury and invasion of microorganisms. Drosophila tyrosine hydroxylase (TH, also known as Pale) and dopa decarboxylase (Ddc), key enzymes in the dopamine synthesis pathway, underlie the melanin synthesis by providing the melanin precursors dopa and dopamine, respectively. It has been shown that expression of Drosophila TH and Ddc is induced in various physiological and pathological conditions, including bacterial challenge; however, the mechanism involved has not been fully elucidated. Here, we show that ectopic activation of p38 MAPK induces TH and Ddc expression, leading to upregulation of melanization in the Drosophila cuticle. This p38-dependent melanization was attenuated by knockdown of TH and Ddc, as well as by that of Drosophila HR38, a member of the NR4A family of nuclear receptors. In mammalian cells, p38 phosphorylated mammalian NR4As and Drosophila HR38 and potentiated these NR4As to transactivate a promoter containing NR4A-binding elements, with this transactivation being, at least in part, dependent on the phosphorylation. This suggests an evolutionarily conserved role for p38 MAPKs in the regulation of NR4As. Thus, p38-regulated gene induction through NR4As appears to function in the dopamine synthesis pathway and may be involved in immune and stress responses.

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.

Different changes in protein and phosphoprotein levels result from serum starvation of high-grade glioma and adenocarcinoma cell lines

Tumor cells undergoing serum starvation in vitro partially mimic metabolically stressed cells trying to adjust to a changed environment in vivo by inducing signal transduction and gene expression so that the tumor continues to grow. Our hypothesis is that the changes in protein and phosphoprotein levels after serum starvation may reflect the adapted phenotype of the tumor, which could be targeted for therapy. We used reverse-phase protein microarrays to interrogate five high-grade glioma cell lines and seven adenocarcinoma cell lines for differences in the level of 81 proteins and 25 phosphoproteins. All cell lines were studied in the well-fed condition of growth with 10% FBS and the starved condition of 0.5% FBS. Protein expression levels were normalized to beta-actin and trichotomized as increased (+1, upper 75th quartile), decreased (-1, lowest 25th quartile), or unchanged (0, others) to focus on the patterns of the biggest (and hopefully most robust) changes in protein and phosphoprotein levels. We examined these trichotomized values to better understand Starved-Fed differences among the cell lines and thereby gain better/clearer insight into the effects of serum starvation on potential cellular responses. In general, the expression of proteins and phosphoproteins 24 h after FBS starvation increased more often in glioma lines than in adenocarcinoma lines, which appeared to have fewer increased protein scores and more decreased scores. Many of the proteins increased in gliomas were downstream targets of the PTEN-PI-3 kinase-AKT, EGFR-MAPK-Stat, and transcription activator-polyamine signaling pathways. In adenocarcinomas, the expression of proteins and phosphoproteins generally increased in apoptosis pathways, while there were minor fluctuations in the other pathways above. Contrawise, gliomas become resistant to apoptosis after 24 h of serum starvation and upregulate transcription activators and polyamines more so than adenocarciomas.

Endosomal endothelin-converting enzyme-1: a regulator of beta-arrestin-dependent ERK signaling

Neuropeptide signaling at the cell surface is regulated by metalloendopeptidases, which degrade peptides in the extracellular fluid, and beta-arrestins, which interact with G protein-coupled receptors (GPCRs) to mediate desensitization. beta-Arrestins also recruit GPCRs and mitogen-activated protein kinases to endosomes to allow internalized receptors to continue signaling, but the mechanisms regulating endosomal signaling are unknown. We report that endothelin-converting enzyme-1 (ECE-1) degrades substance P (SP) in early endosomes of epithelial cells and neurons to destabilize the endosomal mitogen-activated protein kinase signalosome and terminate signaling. ECE-1 inhibition caused endosomal retention of the SP neurokinin 1 receptor, beta-arrestins, and Src, resulting in markedly sustained ERK2 activation in the cytosol and nucleus, whereas ECE-1 overexpression attenuated ERK2 activation. ECE-1 inhibition also enhanced SP-induced expression and phosphorylation of the nuclear death receptor Nur77, resulting in cell death. Thus, endosomal ECE-1 attenuates ERK2-mediated SP signaling in the nucleus to prevent cell death. We propose that agonist availability in endosomes, here regulated by ECE-1, controls beta-arrestin-dependent signaling of endocytosed GPCRs.

Low-potency glucocorticoid hydrocortisone has similar neurotoxic effects as high-potency glucocorticoid dexamethasone on neurons in the immature chicken cerebellum

High-potency glucocorticoids (GC) are used in the prophylaxis and treatment of neonatal bronchopulmonal dysplasia, but there is concern about side effects on the developing brain. Recently, the low-potency GC hydrocortisone (HC) has been suggested as an alternative to high-potency GC. We compared the neurotoxic effects of HC with the high-potency GC dexamethasone (DEX) in chicken cerebellum. A single dose of GC was injected into the egg at embryonic day 16 and the death of granule neurons in histologic sections of the cerebellar cortex was examined 24 h later. DEX and HC showed a similar dose-dependent induction of morphological apoptosis and caspase-3 activation in the internal granular layer. A doubling of the apoptosis rate compared to the basal rate was seen for the highest dose of DEX (5 mg/kg) and medium dose of HC (1 mg/kg). In cultures of embryonic chicken cerebellar granule cells, DEX and HC increased cell death and induced rapid caspase-3 activation in a similar dose-dependent manner. Transfection of granule cells with a luciferase reporter gene revealed that the dose needed for the activation of gene transcription (classical signalling pathway) with DEX was much lower than for HC. In conclusion, HC does not present itself as a safer drug than DEX in this model. In addition, it appears that DEX and HC induce apoptosis in immature granule neurons via a non-genomic (non-classical) mechanism.

A Nurr1 point mutant, implicated in Parkinson's disease, uncouples ERK1/2-dependent regulation of tyrosine hydroxylase transcription

The orphan nuclear receptor NURR1 is critical for the development of mesencephalic dopamine neurons and directly regulates tyrosine hydroxylase (TH) via specific NGFI-B response elements (NBRE). We identified a Parkinson's disease patient with a NURR1 mutation, resulting in a p.Ser125Cys change, immediately adjacent to the putative ERK1/2 phosphorylation site. Here we show, in dopaminergic SK-N-AS human neuroblastoma cells, that this substitution markedly attenuated NURR1-induced transcriptional activation through a human TH promoter NBRE. Furthermore, in SK-N-AS cells co-transfected with the dopamine-D2S receptor and NURR1, the dopamine-D2 agonist quinpirole stimulated ERK1/2 phosphorylation and enhanced transcriptional activation by wild-type NURR1 but not the p.Ser125Cys NURR1 mutant, and these actions were blocked by the specific MEK1/2 inhibitor PD98059. These results indicate that Ser125 is critical for basal and ERK1/2-induced NURR1 activity and suggest a role for this and other NURR1 mutations in the regulation of dopamine synthesis and predisposition to Parkinson's disease.

Targeting Nur77 translocation

The ultimate growth of a tumour depends on not only the rate of tumour cell proliferation, but also the rate of tumour cell death (apoptosis). Nur77 (also known as TR3 or NGFI-B), an orphan member of the nuclear receptor superfamily, controls both survival and death of cancer cells. A wealth of recent experimental data demonstrates that the Nur77 activities are regulated through its subcellular localisation. In the nucleus, Nur77 functions as an oncogenic survival factor, promoting cancer cell growth. In contrast, it is a potent killer when migrating to mitochondria, where it binds to Bcl-2 and converts its survival phenotype, triggering cytochrome c release and apoptosis. Agents, such as 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (AHPN/CD437), which induce Nur77 migration from the nucleus to mitochondria, effectively induce apoptosis of cancer cells. Moreover, Nur77 translocation is highly controlled by retinoid X receptor (RXR), suggesting a role of RXR ligands in regulating the process. Thus, translocation of Nur77 from the nucleus to mitochondria represents a new paradigm in cancer cell apoptosis, and targeting the Nur77 translocation by AHPN/CD437 or RXR ligands promises to effectively restrict cancer cell growth by simultaneously promoting cancer cell death and suppressing cancer cell proliferation.

Immature CD4+CD8+ thymocytes and mature T cells regulate Nur77 distinctly in response to TCR stimulation

The orphan steroid receptor, Nur77, is thought to be a central participant in events leading to TCR-mediated clonal deletion of immature thymocytes. Interestingly, although both immature and mature murine T cell populations rapidly up-regulate Nur77 after TCR stimulation, immature CD4+CD8+ thymocytes respond by undergoing apoptosis, whereas their mature descendants respond by dividing. To understand these developmental differences in susceptibility to the proapoptotic potential of Nur77, we compared its regulation and compartmentalization and show that mature, but not immature, T cells hyperphosphorylate Nur77 in response to TCR signals. Nur77 resides in the nucleus of immature CD4+CD8+ thymocytes throughout the course of its expression and is not found in either the organellar or cytoplasmic fractions. However, hyperphosphorylation of Nur77 in mature T cells, which is mediated by both the MAPK and PI3K/Akt pathways, shifts its localization from the nucleus to the cytoplasm. The failure of immature CD4+CD8+ thymocytes to hyperphosphorylate Nur77 in response to TCR stimulation may be due in part to decreased Akt activity at this developmental stage.

6-Mercaptopurine, an activator of Nur77, enhances transcriptional activity of HIF-1alpha resulting in new vessel formation

Hypoxia-inducible factor-1alpha (HIF-1alpha) plays a central role in oxygen homeostasis. Previously, we reported that the orphan nuclear receptor Nur77 functions in stabilizing HIF-1alpha. Here, we demonstrate that 6-mercaptopurine (6-MP), an activator of the NR4A family members, enhances transcriptional activity of HIF-1. 6-MP enhanced the protein-level of HIF-1alpha as well as vascular endothelial growth factor (VEGF) in a dose- and time-dependent manner. The induction of HIF-1alpha was abolished by the transfection of either a dominant-negative Nur77 mutant or si-Nur77, indicating a critical role of Nur77 in the 6-MP action. The HIF-1alpha protein level remained up to 60 min in the presence of 6-MP when de novo protein synthesis was blocked by cycloheximide, suggesting that 6-MP induces stabilization of the HIF-1alpha protein. The fact that 6-MP decreased the association of HIF-1alpha with von Hippel-Lindau protein and the acetylation of HIF-1alpha, may explain how 6-MP induced stability of HIF-1alpha. Further, 6-MP induced the transactivation function of HIF-1alpha by recruiting co-activator cyclic-AMP-response-element-binding protein. Finally, 6-MP enhanced the expression of HIF-1alpha and VEGF, and the formation of capillary tubes in human umbilical vascular endothelial cells. Together, our results provide a new insight for 6-MP action in the stabilization of HIF-1alpha and imply a potential application of 6-MP in hypoxia-associated human vascular diseases.

Post-translational control of Nur77

Nur77 is a nuclear orphan receptor that has been implicated in both cell survival and apoptosis. With the exception of T-cells, translocation of Nur77 to the cytoplasm promotes cell death, while its retention in the nucleus promotes survival and proliferation. Nur77 appears to be a true orphan receptor, indicating that its activity must be controlled by ligand-independent mechanisms. Here, we discuss the role of phosphorylation in the regulation of Nur77.

Multiple signaling pathways regulate the transcriptional activity of the orphan nuclear receptor NURR1

The orphan nuclear receptor nurr1 (NR4A2) is an essential transcription factor for the acquisition and maintenance of the phenotype of dopamine (DA)-synthesizing neurons in the mesencephalon. Although structurally related to ligand-regulated nuclear receptors, nurr1 is functionally atypical due to its inability to bind a cognate ligand and to activate transcription following canonical nuclear receptor (NR) rules. Importantly, the physiological stimuli that activate this NR and the signaling proteins that regulate its transcriptional activity in mesencephalic neurons are unknown. We used an affinity chromatography approach and CSM14.1 cells of mesencephalic origin to isolate and identify several proteins that interact directly with nurr1 and regulate its transcriptional activity. Notably, we demonstrate that the mitogen-activated protein kinases, ERK2 and ERK5, elevate, whereas LIM Kinase 1 inhibits nurr1 transcriptional activity. Furthermore, nurr1 recruits ERK5 to a NBRE-containing promoter and is a potential substrate for this kinase. We have identified amino acids in the A/B domain of nurr1 important for mediating the ERK5 activating effects on nurr1 transcriptional activity. Our results suggest that nurr1 acts as a point of convergence for multiple signaling pathways that likely play a critical role in differentiation and phenotypic expression of dopaminergic (DAergic) neurons.

p53 and Nur77/TR3 - transcription factors that directly target mitochondria for cell death induction

The complex apoptotic functions of the p53 tumor suppressor are central to its antineoplastic activity in vivo. Conversely, p53 function is altered or attenuated in one way or another in the majority of human cancers. Besides its well-understood action as a transcriptional regulator of multiple apoptotic genes, p53 also exerts a direct pro-apoptotic role at the mitochondria by engaging in protein-protein interactions with anti- and pro-apoptotic Bcl2 family members, thereby executing the shortest known circuitry of p53 death signaling. Nur77, also known as TR3 or NGFI-B, is a unique transcription factor belonging to the orphan nuclear receptor superfamily. Even more extreme than p53, Nur77 can exert opposing biological activities of survival and death. Its activities are regulated by subcellular distribution, expression levels, protein modification and heterodimerization with retinoid X receptor. In cancer cells, Nur77 functions in the nucleus as an oncogenic survival factor, but becomes a potent killer when certain death stimuli induce its migration to mitochondria, where it binds to Bcl2 and conformationally converts it to a killer that triggers cytochrome c release and apoptosis. This review focuses on their unexpected transcription-independent pro-death programs at mitochondria and highlights the remarkable mechanistic similarities between them. Moreover, an accumulating body of evidence provides ample rationale to further investigate how these mitochondrial p53 and Nur77 pathways could become exploitable targets for new cancer therapeutics.

Differential gene expression in the rat cochlea after exposure to impulse noise

Understanding the molecular biology of noise trauma is vital to developing effective and timely interventions. In a model of explosion-mediated impulse noise injury, differential gene expression was studied in whole rat cochlea preparations at 3 and 24 h following the exposure. We developed a technique using mRNA from a single cochlea on each oligonucleotide microarray to avoid pooling of mRNA samples. Application of a conservative statistical analysis approach resulted in the identification of 61 differentially expressed genes. Within 3 h after the exposure, there was an up-regulation of immediate early genes, mainly transcription factors and genes involved in the tissue's response to oxidative stress. No genes were found to be significantly down-regulated. At 24 h following the exposure, up-regulated genes included members of inflammatory and antioxidant pathways and one gene involved in glutathione metabolism was down-regulated. A subset of genes was confirmed by real-time reverse transcriptase-polymerase chain reaction (RT-PCR). The present study demonstrates the power of the microarray technique in providing a global view of the gene regulation following noise exposure, and in identifying genes that may be mechanistically important in hearing loss, and thereby serve as a basis for the development of therapeutic interventions.

Nur77 is phosphorylated in cells by RSK in response to mitogenic stimulation

Nur77 is a nuclear orphan receptor that is able to activate transcription independently of exogenous ligand, and has also been shown to promote apoptosis on its localization to mitochondria. Phosphorylation of Nur77 on Ser354 has been suggested to reduce ability of Nur77 to bind DNA; however, the kinase responsible for this phosphorylation in cells has not been clearly established. In the present study, we show that Nur77 is phosphorylated on this site by RSK (ribosomal S6 kinase) and MSK (mitogen- and stress-activated kinase), but not by PKB (protein kinase B) or PKA (protein kinase A), in vitro. In cells, phosphorylation of Nur77 in vivo is catalysed by RSK, which is activated downstream of the classical MAPK (mitogen-activated protein kinase) cascade. Phosphorylation of Nur77 by RSK is able to promote the binding of Nur77 to 14-3-3 proteins in vitro, however, no evidence could be seen for this interaction in cells. We have established that two related proteins, Nurr1 and Nor1, are also phosphorylated on the equivalent site by RSK in cells in response to mitogenic stimulation.

The extracellular signal-regulated kinase: multiple substrates regulate diverse cellular functions

The extracellular signal-regulated kinase (ERK) cascade is a central pathway that transmits signals from many extracellular agents to regulate cellular processes such as proliferation, differentiation and cell cycle progression. The signaling via the ERK cascade is mediated by sequential phosphorylation and activation of protein kinases in the different tiers of the cascade. Although the main core phosphorylation chain of the cascade includes Raf kinases, MEK1/2, ERK1/2 (ERKs) and RSKs, other alternatively spliced forms and distinct components exist in the different tiers, and participate in ERK signaling under specific conditions. These components enhance the complexity of the ERK cascade and thereby, enable the wide variety of functions that are regulated by it. Another factor that is important for the dissemination of ERKs' signals is the multiplicity of the cascade's substrates, which include transcription factors, protein kinases and phosphatases, cytoskeletal elements, regulators of apoptosis, and a variety of other signaling-related molecules. About 160 substrates have already been discovered for ERKs, and the list of these substrates, as well as the function and mechanism of activation of representative substrates, are described in the current review. Many of these substrates are localized in the nucleus, and seem to participate in the regulation of transcription upon stimulation. However, other substrates are found in the cytosol as well as in other cellular organelles, and those are responsible for processes such as translation, mitosis and apoptosis. Understanding of these processes may provide a full picture of the distinct, and even opposing cellular processes that are regulated by the ERK cascade.

Nuclear receptors and their coregulators in kidney

Nuclear receptors are transcription factors that are essential in embryonic development, maintenance of differentiated cellular phenotypes, metabolism, and apoptosis. Dysfunction of nuclear receptor signaling leads to a wide spectra of proliferative, reproductive, and metabolic diseases, including cancers, infertility, obesity, and diabetes. In addition, many proteins have been identified as coregulators which can be recruited by DNA-binding nuclear receptors to affect transcriptional regulation. The cellular level of coregulators is crucial for nuclear receptor-mediated transcription and many coregulators have been shown to be targets for diverse intracellular signaling pathways and posttranslational modifications. This review provides a general overview of the roles and mechanism of action of nuclear receptors and their coregulators. Since progression of renal diseases is almost always associated with inflammatory processes and/or involve metabolic disorders of lipid and glucose, cell proliferation, hypertrophy, apoptosis, and hypertension, the importance of nuclear receptors and their coregulators in these contexts will be addressed.

Crosstalk between ERK2 and RXR regulates nuclear import of transcription factor NGFI-B

Transcription factor NGFI-B initiates apoptosis when allowed to translocate to mitochondria. Retinoid-X receptor (RXR), another member of the nuclear receptor family, regulates NGFI-B signaling through heterodimerization and nuclear export. Growth factor EGF activates ERK2, which phosphorylates NGFI-B and determines if NGFI-B is allowed to translocate to mitochondria. In the present study, EGF treatment resulted in an increased nuclear import of NGFI-B. Likewise, active ERK2 resulted in a preferential nuclear localization of NGFI-B. When coexpressed with RXR the nuclear import and nuclear localization induced by active ERK2 were strongly reduced. In the presence of its ligand 9-cis-retinoic acid, RXR no longer inhibited ERK2-induced nuclear import. Thus, RXR serves a permissive role for ERK2-mediated nuclear accumulation of NGFI-B. This finding represents a novel crosstalk between ERK2 and RXR signaling pathways, and explains how two independent inhibitors of apoptosis (EGF and 9-cis-retinoic acid) may cooperate to regulate nuclear targeting of apoptosis inducer NGFI-B.

Structural basis for the cell-specific activities of the NGFI-B and the Nurr1 ligand-binding domain

NGFI-B is a ligand-independent orphan nuclear receptor of the NR4A subfamily that displays important functional differences with its homolog Nurr1. In particular, the NGFI-B ligand-binding domain (LBD) exhibits only modest activity in cell lines in which the Nurr1 LBD strongly activates transcription. To gain insight into the structural basis for the distinct activation potentials, we determined the crystal structure of the NGFI-B LBD at 2.4-angstroms resolution. Superimposition with the Nurr1 LBD revealed a significant shift of the position of helix 12, potentially caused by conservative amino acids exchanges in helix 3 or helix 12. Replacement of the helix 11-12 region of Nurr1 with that of NGFI-B dramatically reduces the transcriptional activity of the Nurr1 LBD. Similarly, mutation of Met414 in helix 3 to leucine or of Leu591 in helix 12 to isoleucine (the corresponding residues found in NGFI-B) significantly affects Nurr1 transactivation. In comparison, swapping the helix 11-12 region of Nurr1 into NGFI-B results in a modest increase of activity. These observations reveal a high sensitivity of LBD activity to changes that influence helix 12 positioning. Furthermore, mutation of hydrophobic surface residues in the helix 11-12 region (outside the canonical co-activator surface constituted by helices 3, 4, and 12) severely affects Nurr1 transactivation. Together, our data suggest that a novel co-regulator surface that includes helix 11 and a specifically positioned helix 12 determine the cell type-dependent activities of the NGFI-B and the Nurr1 LBD.

Negative cross-talk between Nur77 and small heterodimer partner and its role in apoptotic cell death of hepatoma cells

Nur77, an orphan nuclear receptor, has been implicated in apoptosis of a variety of cell types, including hepatocytes. The small heterodimer partner (SHP) binds and inhibits the function of many nuclear receptors. Here, we investigated cross-talk between Nur77 and SHP during anti-Fas antibody (CH11)-mediated apoptosis of hepatic cells. Expression of SHP decreased, whereas antisense SHP enhanced, the transcriptional activity of Nur77 in HepG2 cells. SHP and Nur77 were physically associated in vivo and colocalized in the nucleus. SHP decreased the transactivation function of the N-terminal domain of Nur77 that recruits coactivators. Nur77 and SHP competitively bound to cAMP response element-binding protein-binding protein and the expression of coactivators, such as cAMP response element-binding protein-binding protein and activating signal cointegrator-2, recovered the decreased function of Nur77 caused by SHP. Finally, SHP was differentially expressed in hepatoma cell lines in that it was not detected in the interferon-gamma (IFNgamma)/CH11-sensitive SNU354, whereas it was significantly expressed in the IFNgamma/CH11-resistant HepG2. Interestingly, a stable SNU354 cell line that expressed SHP became resistant to the IFNgamma/CH11-induced apoptosis. Together, our results suggest that SHP plays a key role in the regulation of Nur77 activation and thereby in Nur77-mediated apoptosis in the liver.

Novel Function of Orphan Nuclear Receptor Nur77 in Stabilizing Hypoxia-inducible Factor-1α

Hypoxia-inducible factor-1alpha (HIF-1alpha) plays a central role in oxygen homeostasis by inducing the expression of a broad range of genes in a hypoxia-dependent manner. Here, we show that the orphan nuclear receptor Nur77 is an important regulator of HIF-1alpha. Under hypoxic conditions, Nur77 protein and transcripts were induced in a time-dependent manner. When Nur77 was exogenously introduced, it enhanced the transcriptional activity of HIF-1, whereas the dominant negative Nur77 mutant abolished the function of HIF-1. The HIF-1alpha protein was greatly increased and completely localized in the nucleus when coexpressed with Nur77. The N-terminal transactivation domain of Nur77 was required and sufficient for the activation of HIF-1alpha. The association of HIF-1alpha with von Hippel-Lindau protein was not affected, whereas that with mouse double minute 2 (MDM2) was greatly reduced in the presence of Nur77. Further we found that the expression of MDM2 was repressed at transcription level in the presence of Nur77 as well as under hypoxic conditions. Finally, PD98059 decreased Nur77-induced HIF-1alpha stability and recovered MDM2 expression, indicating that the extracellular signal-regulated kinase pathway is critical in the Nur77-induced activation of HIF-1alpha. Together, our results demonstrate a novel function for Nur77 in the stabilization of HIF-1alpha and suggest a potential role for Nur77 in tumor progression and metastasis.

ERK2 Prohibits Apoptosis-induced Subcellular Translocation of Orphan Nuclear Receptor NGFI-B/TR3

Transcription factor NGFI-B (neuronal growth factor-induced clone B), also called Nur77 or TR3, is an immediate early gene and an orphan member of the nuclear receptor family. The NGFI-B protein also has a function distinct from that of a transcription factor; it translocates to mitochondria to initiate apoptosis. Recently, it was demonstrated that NGFI-B interacts with Bcl-2 by inducing a conformational change in Bcl-2, converting it from protector to a killer. After exposing rat cerebellar granule neurons to glutamate (100 mum, 15 min), NGFI-B translocated to the mitochondria. Growth factors such as the epidermal growth factor activate the MAP kinase ERK, the activity of which may determine whether a cell survives or undergoes apoptosis. In the present study we found that the epidermal growth factor activated ERK2 in cerebellar granule neurons and that this activation prohibited glutamate-induced subcellular translocation of NGFI-B. Likewise, overexpressed active ERK2 resulted in a predominant nuclear localization of green fluorescent protein-tagged NGFI-B. Thus, activation of ERK2 may overcome apoptosis-induced subcellular translocation of NGFI-B. This finding represents a novel and rapid growth factor survival pathway that is independent of gene regulation.

Neonatal ventral hippocampus lesion leads to reductions in nerve growth factor inducible-B mRNA in the prefrontal cortex and increased amphetamine response in the nucleus accumbens and dorsal striatum

Converging evidence in schizophrenia suggests prefrontal cortical neuronal deficits that correlate with exaggerated subcortical dopamine (DA) functions: Excitotoxic lesion of the ventral hippocampus (VH) in neonatal rats is widely considered a putative animal model of schizophrenia as they lead to characteristic post-pubertal emergence of behavioral and cognitive abnormalities suggesting a developmental change in the neural circuits comprising the prefrontal cortex (PFC) and subcortical DA. Nerve growth factor inducible-B (NGFI-B, also known as Nur77), an orphan nuclear receptor and transcriptional regulator, is constitutively expressed in the target structures of DA pathways. It acts as an immediate early gene with rapid modulation of its mRNA expression by stress, DA and antipsychotic drugs. The present study assessed the effects of neonatal VH (nVH) lesion and amphetamine treatment on the expression of NGFI-B mRNA in pre- and post-pubertal rats. Sprague-Dawley rat pups received bilateral injection of ibotenic acid or phosphate buffered saline in VH at postnatal (PD) 7. At PD35 and PD56, groups of sham and lesioned animals were administered with D-amphetamine (1.5 mg/kg) or saline and killed 20 min later. In situ hybridization analyses showed that the basal level of NGFI-B mRNA in saline-treated lesioned rats was significantly reduced in the medial PFC (mPFC) and cingulate cortex (CC) only at post-pubertal (PD56) age. No significant difference in NGFI-B mRNA levels was seen in the dorsal striatum or nucleus accumbens (NAcc). Amphetamine treatment increased the expression of NGFI-B mRNA in the mPFC, CC, striatum and NAcc in both control and lesioned animals of both ages. Interestingly, however, striatal and NAcc regions of lesioned rats showed a significantly greater effect of amphetamine at PD56. The data suggest that nVH lesions lead to delayed changes in PFC gene expression along with functional DAergic hyperactivity in subcortical regions.

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Alternative, nonapoptotic programmed cell death: mediation by arrestin 2, ERK2, and Nur77

Programmed cell death (pcd) may take the form of apoptosis or of nonapoptotic pcd. Whereas cysteine aspartyl-specific proteases (caspases) mediate apoptosis, the mediators of nonapoptotic cell death programs are much less well characterized. Here we report that alternative, nonapoptotic pcd induced by the neurokinin-1 receptor (NK(1)R) activated by its ligand Substance P, is mediated by a MAPK phosphorylation cascade recruited by the scaffold protein arrestin 2. The activation of the protein kinases Raf-1, MEK2, and ERK2 is essential for this form of nonapoptotic pcd, leading to the phosphorylation of the orphan nuclear receptor Nur77. NK(1)R-mediated cell death was inhibited by a dominant negative form of arrestin 2, Raf-1, or Nur77, by MEK1/2-specific inhibitors, and by RNA interference directed against ERK2 or MEK2 but not ERK1 or MEK1 and against Nur77. The MAPK pathway is also activated in neurons in primary culture undergoing NK(1)R-mediated death, since the MEK inhibitor PD98059 inhibited Substance P-induced death in primary striatal neurons. These results suggest that Nur77, which is regulated by a MAPK pathway activated via arrestin 2, modulates NK(1)R-mediated nonapoptotic pcd.

The transcription factor NGFI-B (Nur77) and retinoids play a critical role in acute neuroleptic-induced extrapyramidal effect and striatal neuropeptide gene expression

Despite extensive investigation, the cellular mechanisms responsible for neuroleptic actions remain elusive. We have previously shown that neuroleptics modulated the expression of some members of the ligand-activated transcription factors (nuclear receptors) including the nerve-growth factor inducible gene B (NGFI-B or Nur77) and retinoid X receptor (RXR) isoforms. Using genetic and pharmacological approaches, we investigated the role of NGFI-B and retinoids in acute behavioral and biochemical responses to dopamine antagonists. NGFI-B knockout (KO) mice display a profound alteration of haloperidol-induced catalepsy and striatal neuropeptide gene expression. Haloperidol-induced increase of striatal enkephalin mRNA is totally abolished in NGFI-B KO mice whereas the increase of neurotensin mRNA expression is reduced by 50%. Interestingly, catalepsy induced by raclopride, a specific dopamine D(2)/D(3) antagonist is completely abolished in NGFI-B-deficient mice whereas the cataleptic response to SCH 23390, a dopamine D(1) agonist, is preserved. Accordingly, the effects of haloperidol on striatal c-fos, Nor-1, and dynorphin mRNA expression are also preserved in NGFI-B-deficient mice. The cataleptic response and the increase of enkephalin mRNA expression induced by haloperidol can also be suppressed by administration of retinoid ligands 9-cis retinoic acid and docosahexaenoic acid. In addition, we demonstrate that haloperidol enhances colocalization of NGFI-B and RXRgamma1 isoform mRNAs, suggesting that both NGFI-B and a RXR isoform are highly coexpressed after haloperidol administration. Our data demonstrate, for the first time, that NGFI-B and retinoids are actively involved in the molecular cascade induced by neuroleptic drugs.

Early changes in lung gene expression due to high tidal volume

The purpose of this study was to use gene expression profiling to understand how adult rat lung responds to high tidal volume (HV) ventilation in vivo. HV ventilation for 30 minutes did not cause discernable lung injury (in terms of altered mechanics or histology) but caused obvious injury when continued for 90 minutes. However, at 30-minute ventilation, HV caused significant upregulation of 10 genes and suppression of 12 genes. Among the upregulated genes were transcription factors, stress proteins, and inflammatory mediators; the downregulated genes were exemplified by metabolic regulatory genes. On the basis of cluster analysis, we studied Egr-1, c-Jun, heat shock protein 70, and interleukin (IL)-1beta in further detail. Temporal studies demonstrated that Egr-1 and c-Jun were increased early and before heat shock protein 70 and IL-1beta. Spatial studies using in situ hybridization and laser capture microscopy revealed that all four genes were upregulated primarily in the bronchiolar airway epithelium. Furthermore, at 90 minutes of HV ventilation, a significant increase in intracellular IL-1beta protein was observed. Although there are limitations to gene array methodology, the current data suggest a global hypothesis that (1). the effects of HV are cumulative; (2). specific patterns of gene activation and suppression precede lung injury; and (3). alteration of gene expression after mechanical stretch is pathogenic.

The AF-1 domain of the orphan nuclear receptor NOR-1 mediates trans-activation, coactivator recruitment, and activation by the purine anti-metabolite 6-mercaptopurine

NOR-1/NR4A3 is an "orphan member" of the nuclear hormone receptor superfamily. NOR-1 and its close relatives Nurr1 and Nur77 are members of the NR4A subgroup of nuclear receptors. Members of the NR4A subgroup are induced through multiple signal transduction pathways. They have been implicated in cell proliferation, differentiation, T-cell apoptosis, chondrosarcomas, neurological disorders, inflammation, and atherogenesis. However, the mechanism of transcriptional activation, coactivator recruitment, and agonist-mediated activation remain obscure. Hence, we examined the molecular basis of NOR-1-mediated activation. We observed that NOR-1 trans-activates gene expression in a cell- and target-specific manner; moreover, it operates in an activation function (AF)-1-dependent manner. The N-terminal AF-1 domain delimited to between amino acids 1 and 112, preferentially recruits the steroid receptor coactivator (SRC). Furthermore, SRC-2 modulates the activity of the AF-1 domain but not the C-terminal ligand binding domain (LBD). Homology modeling indicated that the NOR-1 LBD was substantially different from that of hRORbeta, a closely related AF-2-dependent receptor. In particular, the hydrophobic cleft characteristic of nuclear receptors was replaced with a very hydrophilic surface with a distinct topology. This observation may account for the inability of this nuclear receptor LBD to efficiently mediate cofactor recruitment and transcriptional activation. In contrast, the N-terminal AF-1 is necessary for cofactor recruitment and can independently conscript coactivators. Finally, we demonstrate that the purine anti-metabolite 6-mercaptopurine, a widely used antineoplastic and anti-inflammatory drug, activates NOR-1 in an AF-1-dependent manner. Additional 6-mercaptopurine analogs all efficiently activated NOR-1, suggesting that the signaling pathways that modulate proliferation via inhibition of de novo purine and/or nucleic acid biosynthesis are involved in the regulation NR4A activity. We hypothesize that the NR4A subgroup mediates the genotoxic stress response and suggest that this subgroup may function as sensors that respond to genotoxicity.

Pancreatic gene expression during the initiation of acute pancreatitis: identification of EGR-1 as a key regulator

We hypothesized that genes expressed in pancreatic acinar cells during the initiation of acute pancreatitis determine the severity of the disease. Therefore, we utilized microarrays to identify those genes commonly induced in rat pancreatic acinar cells within 1-4 h in two in vivo models, caerulein and taurocholate administration. This strategy yielded 51 known genes representing a complex array of molecules, including those that are likely to either reduce or increase the severity of the disease. Novel genes identified in the current study included ATF3, BRF1, C/EBPbeta, CGRP, EGR-1, ephrinA1, villin2, ferredoxin, latexin, lipocalin, MKP-1, NGFI-B, RhoA, tissue factor (TF), and syndecan. To validate these microarray results, the role of EGR-1 was further investigated using quantitative RT-PCR, Western blotting, and immunocytochemistry. EGR-1 expression occurred within acinar cells and correlated with the development of caerulein-induced acute pancreatitis in rats. Furthermore, the levels of the inflammation-related genes MCP-1, PAI, TF, IL-6, and ICAM-1 and the extent of lung inflammation were reduced during the initiation of caerulein-induced acute pancreatitis in EGR-1-deficient mice. Thus this study identified EGR-1 and several other novel genes likely to be important in the development and severity of acute pancreatitis.

Orphan nuclear receptor Nur77 is involved in caspase-independent macrophage cell death

Activation-induced cell death in macrophages has been observed, but the mechanism remains largely unknown. Activation-induced cell death in macrophages can be independent from caspases, and the death of activated macrophages can even be triggered by the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (zVAD). Here, we show that this type of macrophage death can occur in the septic mouse model and that toll-like receptor (TLR)-2 or TLR4 signaling is required in this process. We conclude that Nur77 is involved in the macrophage death because Nur77 expression correlates with cell death, and cell death is reduced significantly in Nur77-deficient macrophages. The extracellular signal-regulated kinase pathway, which is downstream of TLR2 or TLR4, and myocyte-specific enhancer binding factor 2 (MEF2) transcription factor activity, which is up-regulated by zVAD, are required for Nur77 induction and macrophage death. Reporter gene analysis suggests that Nap, Ets, Rce, and Sp1 sites in the Nur77 promoter are regulated by TLR4 signaling and that MEF2 sites in the Nur77 promoter are regulated by zVAD treatment. MEF2 transcription factors are constitutively expressed and degraded in macrophages, and zVAD increases MEF2 transcription factor activity by preventing the proteolytic cleavage and degradation of MEF2 proteins. This paper delineates the dual signaling pathways that are required for Nur77 induction in macrophages and demonstrates a role of Nur77 in caspase-independent cell death.

Protein kinases and their involvement in the cellular responses to genotoxic stress

Cells are constantly subjected to genotoxic stress, and much has been learned regarding their response to this type of stress during the past year. In general, the cellular genotoxic response can be thought to occur in three stages: (1) damage sensing; (2) activation of signal transduction pathways; (3) biological consequences and attenuation of the response. The biological consequences, in particular, include cell cycle arrest and cell death. Although our understanding of the molecular mechanisms underlying cellular genotoxic stress responses remains incomplete, many cellular components have been identified over the years, including a group of protein kinases that appears to play a major role. Various DNA-damaging agents can activate these protein kinases, triggering a protein phosphorylation cascade that leads to the activation of transcription factors, and altering gene expression. In this review, the involvement of protein kinases, particularly the mitogen-activated protein kinases (MAPKs), at different stages of the genotoxic response is discussed.

The activation function-1 domain of Nur77/NR4A1 mediates trans-activation, cell specificity, and coactivator recruitment

Nur77/NR4A1 is an "orphan member" of the nuclear hormone receptor superfamily. Nur77 and its close relatives Nurr1 and NOR-1 bind as monomers to a consensus binding site, the nerve growth factor induced protein I-B (NGFI-B)-binding response element (NBRE). The Nur77/NURR1/NOR1 nuclear receptors are classified as immediate early response genes which are induced through multiple signal transduction pathways. They have been implicated in cell proliferation, differentiation, and apoptosis. However, the mechanism of coactivation and ligand independent trans-activation remains unclear. Hence we examined the molecular basis of Nur77-mediated cofactor recruitment and activation. We observed that Nur77 trans-activates gene expression in a cell-specific manner, and operates in an activation function-1 (AF-1)-dependent manner. The AB region encodes an uncommonly potent N-terminal AF-1 domain delimited to between amino acids 50 and 160 and is essential for the ligand-independent activation of gene expression. Steroid receptor coactivator-2 (SRC-2) modulates the activity of the N-terminal AF-1 domain. Moreover, SRC-2 dramatically potentiates the retinoid induced RXR-dependent activation of the Nur77 ligand binding domain (LBD). Interestingly, the N-terminal AB region (not the LBD) facilitates coactivator recruitment and directly interacts with SRC, p300, PCAF, and DRIP-205. Consistent with this, homology modeling indicated that the Nur77 LBD coactivator binding cleft was substantially different from that of retinoic acid receptor gamma, a closely related AF-2-dependent receptor. In particular, the hydrophobic cleft characteristic of nuclear receptors was replaced with a much more hydrophilic surface with a distinct topology. This observation accounts for the inability of this nuclear receptor LBD to directly mediate cofactor recruitment. Furthermore, the AF-1 domain physically associates with the Nur77 C-terminal LBD and synergizes with the retinoid X receptor LBD. Thus, the AF-1 domain plays a major role in Nur77-mediated transcriptional activation, cofactor recruitment, and intra- and intermolecular interactions.