Dibutyryl-cAMP up-regulates nur77 expression via histone modification during neurite outgrowth in PC12 cells

Clemastine fumarate alleviates endoplasmic reticulum stress through the Nur77/GFPT2/CHOP pathway after ischemia/reperfusion in rat hearts

BACKGROUND AND PURPOSE

Clemastine fumarate (CLE) is an H1 receptor (H1R) antagonist that is used clinically to treat various allergic disorders. It blocks histamine release from mast cells and inhibits H1R. Preliminary studies have shown that CLE can reduce myocardial ischemia/reperfusion (I/R) injury. In this study, we confirmed the efficacy of CLE against myocardial I/R injury using in vivo and in vitro examinations.

EXPERIMENTAL APPROACH

To test the efficacy of CLE against myocardial I/R injury, we established a rat model of myocardial hypoxia/reperfusion injury. A series of assessments were conducted to determine cardiac function, measure areas of myocardial infarction, and analyze the histopathological changes. Additionally, we developed a rat model of cardiomyocyte hypoxia/reoxygenation (H/R); in both models, we quantified the expression levels of key markers and cardiac injury-specific proteins to assess the biochemical milieu influenced by CLE treatment.

KEY RESULTS

Our findings demonstrated that CLE reduced the expression of nerve growth factor-induced gene B (Nur77), glutamine-fructose-6-phosphate transaminase 2 (GFPT2), and C/EBP homologous protein (CHOP) and decreased the area of myocardial infarction and the degree of endoplasmic reticulum stress. CLE pretreatment ameliorated abnormal fibers and myocardial edema and reduced the inflammatory cell infiltration caused by I/R injury. While Nur77 overexpression aggravated cardiac function, these effects were ameliorated by the downregulation of Nur77.

CONCLUSION AND IMPLICATIONS

We anticipate that these results validate the hypothesis that CLE mitigates apoptosis and reduces endogenous stress within myocardial cells by modulating Nur77, GFPT2, and CHOP expression. These findings elucidate the therapeutic mechanisms by which CLE alleviates myocardial I/R injury. In addition, they will serve as a new theoretical foundation for developing future treatment strategies and enhancing clinical applications in cardiac care.

OMICS Analyses Unraveling Related Gene and Protein-Driven Molecular Mechanisms Underlying PACAP 38-Induced Neurite Outgrowth in PC12 Cells

The study aimed to understand mechanism/s of neuronal outgrowth in the rat adrenal-derived pheochromocytoma cell line (PC12) under pituitary adenylate cyclase-activating polypeptide (PACAP) treatment. Neurite projection elongation was suggested to be mediated via Pac1 receptor-mediated dephosphorylation of CRMP2, where GSK-3β, CDK5, and Rho/ROCK dephosphorylated CRMP2 within 3 h after addition of PACAP, but the dephosphorylation of CRMP2 by PACAP remained unclear. Thus, we attempted to identify the early factors in PACAP-induced neurite projection elongation via omics-based transcriptomic (whole genome DNA microarray) and proteomic (TMT-labeled liquid chromatography-tandem mass spectrometry) analyses of gene and protein expression profiles from 5-120 min after PACAP addition. The results revealed a number of key regulators involved in neurite outgrowth, including known ones, called 'Initial Early Factors', e.g., genes Inhba, Fst, Nr4a1,2,3, FAT4, Axin2, and proteins Mis12, Cdk13, Bcl91, CDC42, including categories of 'serotonergic synapse, neuropeptide and neurogenesis, and axon guidance'. cAMP signaling and PI3K-Akt signaling pathways and a calcium signaling pathway might be involved in CRMP2 dephosphorylation. Cross-referencing previous research, we tried to map these molecular components onto potential pathways, and we may provide important new information on molecular mechanisms of neuronal differentiation induced by PACAP. Gene and protein expression data are publicly available at NCBI GSE223333 and ProteomeXchange, identifier PXD039992.

HDAC1 disrupts the tricarboxylic acid (TCA) cycle through the deacetylation of Nur77 and promotes inflammation in ischemia-reperfusion mice

Histone deacetylase enzymes (HDACs) regulate protein acetylation. HDAC1 is known to enhance ischemia/reperfusion (I/R) injury, but its underlying mechanism(s) of action have not been defined. Here, in vivo mouse models of myocardial I/R were used to investigate the role of HDAC1 during I/R myocardial injury. We show that HDAC1 enhances the inflammatory responses of I/R mice. Using a constructed macrophage H/R (hypoxia/ regeneration) injury model (Raw264.7 cells), we identified Nur77 as a HDAC1 target in macrophages. Nur77 deficient macrophages failed to downregulate IDH1 (isocitrate dehydrogenase 1) and accumulated succinic acid and other tricarboxylic acid (TCA) cycle-derived metabolites in a glutamine-independent manner. These data show that the inhibition of HDAC1 ameliorates H/R-inflammation in macrophages through the regulation of Nur77 and the TCA cycle.

Centella asiatica (L.) Urban. Attenuates Cell Damage in Hydrogen Peroxide-Induced Oxidative Stress in Transgenic Murine Embryonic Stem Cell Line-Derived Neural-Like Cells: A Preliminary Study for Potential Treatment of Alzheimer's Disease

BACKGROUND

Centella asiatica (L.) (C. asiatica) is commonly known in South East and South East Asia communities for its nutritional and medicinal benefits. Besides being traditionally used to enhance memory and accelerate wound healing, its phytochemicals have been extensively documented for their neuroprotective, neuroregenerative, and antioxidant properties.

OBJECTIVE

The present study aims to investigate the effects of a standardized raw extract of C. asiatica (RECA) on hydrogen peroxide (H2O2)-induced oxidative stress and apoptotic death in neural-like cells derived from mouse embryonic stem (ES) cell line.

METHODS

A transgenic mouse ES cell (46C) was differentiated into neural-like cells using 4-/4+ protocol with addition of all-trans retinoic acid. These cells were then exposed to H2O2 for 24 h. The effects of RECA on H2O2-induced neural-like cells were assessed through cell viability, apoptosis, and reactive oxygen species (ROS) assays, as well as neurite length measurement. The gene expression levels of neuronal-specific and antioxidant markers were assessed by RT-qPCR analysis.

RESULTS

Pre-treatment with H2O2 for 24 hours, in a dose-dependent manner, damaged neural-like cells as marked by a decrease in cell viability, substantial increase in intracellular ROS accumulation, and increase in apoptotic rate compared to untreated cells. These cells were used to treat with RECA. Treatment with RECA for 48 h remarkably restored cell survival and promoted neurite outgrowth in the H2O2- damaged neurons by increasing cell viability and decreasing ROS activity. RT-qPCR analysis revealed that RECA upregulated the level of antioxidant genes such as thioredoxin-1 (Trx-1) and heme oxygenase-1 (HO-1) of treated cells, as well as the expression level of neuronal-specific markers such as Tuj1 and MAP2 genes, suggesting their contribution in neuritogenic effect.

CONCLUSION

Our findings indicate that RECA promotes neuroregenerative effects and exhibits antioxidant properties, suggesting a valuable synergistic activity of its phytochemical constituents, thus, making the extract a promising candidate in preventing or treating oxidative stress-associated Alzheimer's disease.

Hepatic macrophages act as a central hub for relaxin-mediated alleviation of liver fibrosis

Relaxin is an antifibrotic peptide hormone previously assumed to directly reverse the activation of hepatic stellate cells for liver fibrosis resolution. Using nanoparticle-mediated delivery, here we show that, although relaxin gene therapy reduces liver fibrosis in vivo, in vitro treatment fails to induce quiescence of the activated hepatic stellate cells. We show that hepatic macrophages express the primary relaxin receptor, and that, on relaxin binding, they switch from the profibrogenic to the pro-resolution phenotype. The latter releases exosomes that promote the relaxin-mediated quiescence of activated hepatic stellate cells through miR-30a-5p. Building on these results, we developed lipid nanoparticles that preferentially target activated hepatic stellate cells in the fibrotic liver and encapsulate the relaxin gene and miR-30a-5p mimic. The combinatorial gene therapy achieves synergistic antifibrosis effects in models of mouse liver fibrosis. Collectively, our findings highlight the key role that macrophages play in the relaxin-primed alleviation of liver fibrosis and demonstrate a proof-of-concept approach to devise antifibrotic strategies through the complementary application of nanotechnology and basic science.

Identification of the Differentially Expressed Genes Involved in the Synergistic Neurotoxicity of an HIV Protease Inhibitor and Methamphetamine

BACKGROUND

The abuse of psychostimulants such as methamphetamine (METH) is common in human immunodeficiency virus (HIV)-infected individuals. Acquired immunodeficiency syndrome (AIDS) patients taking METH and antiretroviral drugs could suffer severe neurologic damage and cognitive impairment.

OBJECTIVE

To reveal the underlying neuropathologic mechanisms of an HIV protease inhibitor (PI) combined with METH, growth-inhibition tests of dopaminergic cells and RNA sequencing were performed.

METHODS

A combination of METH and PI caused more growth inhibition of dopaminergic cells than METH alone or a PI alone. Furthermore, we identified differentially expressed gene (DEG) patterns in the METH vs. untreated cells (1161 genes), PI vs. untreated cells (16 genes), METH-PI vs. PI (3959 genes), and METH-PI vs. METH groups (14 genes).

RESULTS

The DEGs in the METH-PI co-treatment group were verified in the brains of a mouse model using quantitative polymerase chain reaction and were involved mostly in the regulatory functions of cell proliferation and inflammation.

CONCLUSION

Such identification of key regulatory genes could facilitate the study of their neuroprotective potential in the users of METH and PIs.

Molecular mechanism of nur77 gene expression and downstream target genes in the early stage of forskolin-induced differentiation in PC12 cells

Forskolin promotes neuronal differentiation of PC12 cells via the PKA-CREB-dependent signaling pathway. Activation of PKA by forskolin phosphorylates CREB, which then binds to CRE sites in numerous gene promoters. However, it is unclear which gene contains the CRE sites responsible for forskolin-induced neuronal differentiation. In this study, we investigated how an immediate early gene, nur77, which has CRE sites in the promoter region, contributes to the early stage of differentiation of forskolin-treated PC12 cells. After treatment with forskolin, expression of Nur77 was upregulated within 1 hr. In addition, knockdown of nur77 inhibited neurite outgrowth induced by forskolin. We also revealed that the specific four CRE sites near the transcriptional start site (TSS) of nur77 were strongly associated with phosphorylated CREB within 1 hr after treatment with forskolin. To analyze the roles of these four sites, reporter assays using the nur77 promoter region were performed. The results showed that nur77 expression was mediated through three of the CRE sites, -242, -222, and -78, and that -78, the nearest of the three to the TSS of nur77, was particularly important. An analysis of neuronal markers controlled by Nur77 after A-CREB-Nur77-Synapsin1 signaling pathway plays a pivotal role in differentiation of forskolin-induced PC12 cells.

Calcium-dependent Nr4a1 expression in mouse Leydig cells requires distinct AP1/CRE and MEF2 elements

The nuclear receptor NR4A1 is expressed in steroidogenic Leydig cells where it plays pivotal roles by regulating the expression of several genes involved in steroidogenesis and male sex differentiation including Star, HSD3B2, and Insl3 Activation of the cAMP and Ca(2+) signaling pathways in response to LH stimulation leads to a rapid and robust activation of Nr4a1 gene expression that requires the Ca(2+)/CAMKI pathway. However, the downstream transcription factor(s) have yet to be characterized. To identify potential Ca(2+)/CaM effectors responsible for hormone-induced Nr4a1 expression, MA-10 Leydig cells were treated with forskolin to increase endogenous cAMP levels, dantrolene to inhibit endoplasmic reticulum Ca(2+) release, and W7 to inhibit CaM activity. We identified Ca(2+)-responsive elements located in the discrete regions of the Nr4a1 promoter, which contain binding sites for several transcription factors such as AP1, CREB, and MEF2. We found that one of the three AP1/CRE sites located at -255 bp is the most responsive to the Ca(2+) signaling pathway as are the two MEF2 binding sites at -315 and -285 bp. Furthermore, we found that the hormone-induced recruitment of phospho-CREB and of the co-activator p300 to the Nr4a1 promoter requires the Ca(2+) pathway. Lastly, siRNA-mediated knockdown of CREB impaired NR4A1 expression and steroidogenesis. Together, our data indicate that the Ca(2+) signaling pathway increases Nr4a1 expression in MA-10 Leydig cells, at least in part, by enhancing the recruitment of coactivator most likely through the MEF2, AP1, and CREB transcription factors thus demonstrating an important interplay between the Ca(2+) and cAMP pathways in regulating Nr4a1 expression.

Regulation of Nuclear Receptor Nur77 by miR-124

The nuclear receptor Nur77 is commonly upregulated in adult cancers and has oncogenic functions. Nur77 is an immediate-early response gene that acts as a transcription factor to promote proliferation and protect cells from apoptosis. Conversely, Nur77 can translocate to the mitochondria and induce apoptosis upon treatment with various cytotoxic agents. Because Nur77 is upregulated in cancer and may have a role in cancer progression, it is of interest to understand the mechanism controlling its expression. MicroRNAs (miRNAs) are responsible for inhibiting translation of their target genes by binding to the 3'UTR and either degrading the mRNA or preventing it from being translated into protein, thereby making these non-coding endogenous RNAs vital regulators of every cellular process. Several miRNAs have been predicted to target Nur77; however, strong evidence showing the regulation of Nur77 by any miRNA is lacking. In this study, we used a luciferase reporter assay containing the 3'UTR of Nur77 to screen 296 miRNAs and found that miR-124, which is the most abundant miRNA in the brain and has a role in promoting neuronal differentiation, caused the greatest reduction in luciferase activity. Interestingly, we discovered an inverse relationship in Daoy medulloblastoma cells and undifferentiated granule neuron precursors in which Nur77 is upregulated and miR-124 is downregulated. Exogenous expression to further elevate Nur77 levels in Daoy cells increased proliferation and viability, but knocking down Nur77 via siRNA resulted in the opposite phenotype. Importantly, exogenous expression of miR-124 reduced Nur77 expression, cell viability, proliferation, and tumor spheroid size in 3D culture. In all, we have discovered miR-124 to be downregulated in instances of medulloblastoma in which Nur77 is upregulated, resulting in a proliferative state that abets cancer progression. This study provides evidence for increasing miR-124 expression as a potential therapy for cancers with elevated levels of Nur77.

Nur77 Was Essential for Neurite Outgrowth and Involved in Schwann Cell Differentiation After Sciatic Nerve Injury

Nur77, together with Nurr1 and NOR-1, constitutes the NR4A subgroup of orphan nuclear receptors and plays critical roles in cell proliferation, differentiation, migration, and apoptosis. Among them, Nur77 is universally well known to contribute to neurite outgrowth. However, information regarding its regulation and possible function in the peripheral nervous system is still limited. In this study, we performed a sciatic nerve injury model in adult rats and detected an increased expression of Nur77 in the sciatic nerve, which was similar to the expression of Oct-6. Immunofluorescence indicated that Nur77 was located in both axons and Schwann cells. In vitro, we observed enhanced expression of Nur77 during the process of both basic fibroblast growth factor (bFGF)-induced Schwann cells differentiation and nerve growth factor (NGF)-induced PC12 cell neurite outgrowth. In vitro and in vivo experiments indicated that inhibiting the function of Nur77 by specific short hairpin RNA could depress Schwann cells myelinization and axons regeneration. Collectively, all these results suggested that upregulation of Nur77 might be involved in Schwann cells differentiation and neurite elongation following sciatic nerve crush.

Paliperidone and aripiprazole differentially affect the strength of calcium-secretion coupling in female pituitary lactotrophs

Hyperprolactinemia is a common adverse in vivo effect of antipsychotic medications that are used in the treatment of patients with schizophrenia. Here, we compared the effects of two atypical antipsychotics, paliperidone and aripiprazole, on cAMP/calcium signaling and prolactin release in female rat pituitary lactotrophs in vitro. Dopamine inhibited spontaneous cAMP/calcium signaling and prolactin release. In the presence of dopamine, paliperidone rescued cAMP/calcium signaling and prolactin release in a concentration-dependent manner, whereas aripiprazole was only partially effective. In the absence of dopamine, paliperidone stimulated cAMP/calcium signaling and prolactin release, whereas aripiprazole inhibited signaling and secretion more potently but less effectively than dopamine. Forskolin-stimulated cAMP production was facilitated by paliperidone and inhibited by aripiprazole, although the latter was not as effective as dopamine. None of the compounds affected prolactin transcript activity, intracellular prolactin accumulation, or growth hormone secretion. These data indicate that paliperidone has dual hyperprolactinemic actions in lactotrophs i) by preserving the coupling of spontaneous electrical activity and prolactin secretion in the presence of dopamine and ii) by inhibiting intrinsic dopamine receptor activity in the absence of dopamine, leading to enhanced calcium signaling and secretion. In contrast, aripiprazole acts on prolactin secretion by attenuating, but not abolishing, calcium-secretion coupling.

Rho-kinase signaling controls nucleocytoplasmic shuttling of class IIa histone deacetylase (HDAC7) and transcriptional activation of orphan nuclear receptor NR4A1

Rho-kinase (ROCK) has been well documented to play a key role in RhoA-induced actin remodeling. ROCK activation results in myosin light chain (MLC) phosphorylation either by direct action on MLC kinase (MLCK) or by inhibition of MLC phosphatase (MLCP), modulating actin-myosin contraction. We found that inhibition of the ROCK pathway in induced pluripotent stem cells, leads to nuclear export of HDAC7 and transcriptional activation of the orphan nuclear receptor NR4A1 while in cells with constitutive ROCK hyperactivity due to loss of function of the RhoGTPase activating protein Oligophrenin-1 (OPHN1), the orphan nuclear receptor NR4A1 is downregulated. Our study identify a new target of ROCK signaling via myosin phosphatase subunit (MYPT1) and Histone Deacetylase (HDAC7) at the nuclear level and provide new insights in the cellular functions of ROCK.

The histone deacetylase inhibitor trichostatin A induces neurite outgrowth in PC12 cells via the epigenetically regulated expression of the nur77 gene

Histone deacetylase (HDAC) inhibitors induce histone acetylation and gene expression by changing local chromatin structures. They can thereby influence various cells to proliferate or differentiate. It has been reported that trichostatin A (TSA) or valproic acid (VPA) can induce the neuronal differentiation of mouse embryonic neural stem cells and rat cerebellar granule cells. It is unclear however which gene is responsible for the neuronal differentiation induced by HDAC inhibitors. In this study, we investigated the contribution of immediate early gene (IEG) nur77 to the neuronal differentiation induced by TSA. We report that TSA induces neurite outgrowth in PC12 cells, and C646, an inhibitor of HAT (histone acetyl transferase) (p300), prevents TSA-induced neurite formation. The acetylation of the Lys14 residue of histone H3, and mRNA and protein expression of nur77 gene were found to be stimulated after treatment with TSA, but not in the presence of C646. A knock-down of nur77 inhibits the neurite outgrowth induced by TSA. Furthermore, the ectopic expression of nur77 significantly elicits neurite formation in PC12 cells. These results suggest that the expression of nur77, which is up-regulated via the TSA-induced acetylation of Lys14 on histone H3, is essential for the neuronal differentiation in TSA-induced PC12 cells.

Role of p53 in cAMP/PKA pathway mediated apoptosis

Deregulated β-adrenoceptor/cAMP-PKA pathway is implicated in a range of human diseases, such as neuronal loss during aging, cardiomyopathy and septic shock. The molecular mechanism of this process is, however, only poorly understood. We recently had demonstrated that the β-adrenoceptor/cAMP-PKA pathway triggers apoptosis through the transcriptional induction of the pro-apoptotic BH3-only Bcl-2 family member BIM in tissues, such as the thymus and the heart. Induction of BIM is driven by the transcriptional co-activator CBP (CREB Binding Protein) together with the proto-oncogene c-Myc. Association of CBP with c-Myc leads to altered histone acetylation and methylation pattern at the BIM promoter site [Lee et al., Cell Death Difference 20(7):941-952 (2013)]. However since CBP is a co-factor for multiple transcription factors, BH-3 only proteins other than Bim could also contribute to this apoptosis pathway. Here we provide evidence for the involvement of p53-CBP axis in apoptosis through Puma/Noxa induction, in response to β-adrenoceptor activation. Our findings highlight the molecular complexity of pathophysiology associated with a deregulated neuro-endocrine system and for developing novel therapeutic strategies for these diseases.

Mitochondrial translocation of Nur77 induced by ROS contributed to cardiomyocyte apoptosis in metabolic syndrome

Metabolic syndrome is a major risk factor for cardiovascular diseases, and increased cardiomyocyte apoptosis which contributes to cardiac dysfunction after myocardial ischemia/reperfusion (MI/R) injury. Nur77, a nuclear orphan receptor, is involved in such various cellular events as apoptosis, proliferation, and glucose and lipid metabolism in several cell types. Apoptosis is positively correlated with mitochondrial translocation of Nur77 in the cancer cells. However, the roles of Nur77 on cardiac myocytes in patients with metabolic syndrome remain unclear. The objective of this study was to determine whether Nur77 may contribute to cardiac apoptosis in patients with metabolic syndrome after I/R injury, and, if so, to identify the underlying molecular mechanisms responsible. We used leptin-deficient (ob/ob) mice to make metabolic syndrome models. In this report, we observed that, accompanied by the substantial decline in apoptosis inducer Nur77, MI/R induced cardiac dysfunction was manifested as cardiomyopathy and increased ROS. Using the neonatal rat cardiac myocytes cultured in a high-glucose and high-fat medium, we found that excessive H2O2 led to the significant alteration in mitochondrial membrane potential and translocation of Nur77 from the nucleus to the mitochondria. However, inhibition of the relocation of Nur77 to mitochondria via Cyclosporin A reversed the changes in membrane potential mediated by H2O2 and reduced myocardial cell injury. Therefore, these data provide a potential underlying mechanism for cardiac dysfunction in metabolic syndrome and the suppression of Nur77 translocation may provide an effective approach to reduce cardiac injury in the process.

17β-estradiol delays 6-OHDA-induced apoptosis by acting on Nur77 translocation from the nucleus to the cytoplasm

Nuclear receptors (Nurs) represent a large family of gene expression regulating proteins. Gathering evidence indicates an important role for Nurs as transcription factors in dopamine neurotransmission. Nur77, a member of the Nur superfamily, plays a role in mediating the effects of antiparkinsonian and neuroleptic drugs. Besides, Nur77 survival and apoptotic roles depend largely on its subcellular localization. Estrogens are known for their neuroprotective properties, as demonstrated in animal and clinical studies. However, their action on Nur77 translocation pertaining to neuroprotection has not been investigated yet. The aim of our study was to perform a kinetic study on the effect of neurotoxic 6-hydroxydopamine (6-OHDA) and 17β-estradiol (E2) on the subcellular localization of Nur77 with reference to the modulation of apoptosis in PC12 cells. Our results demonstrate that E2 administration alone does not affect Nur77 cytoplasmic/nuclear ratio, mRNA levels, or apoptosis in PC12 cells. The neurotoxin 6-OHDA significantly enhances cytoplasmic localization of Nur77 after merely 3 h, while precipitating apoptosis. 6-OHDA also increases Nur77 transcription, which could partly explain the rise in cytoplasmic localization of the protein. Finally, treatment with both E2 and 6-OHDA delays Nur77 accumulation in the cytoplasm and delays cell death for a few hours in our cellular paradigm. Pre-treatment with E2 does not alter the increase in levels of Nur77 mRNA produced by 6-OHDA, suggesting that a raise in nuclear translocation is likely responsible for the stabilization of the cytoplasmic/nuclear ratio until 6 h. These results suggest an intriguing cooperation between E2 and Nur77 toward cellular fate guidance.

NGF-induced cell differentiation and gene activation is mediated by integrative nuclear FGFR1 signaling (INFS)

Nerve growth factor (NGF) is the founding member of the polypeptide neurotrophin family responsible for neuronal differentiation. To determine whether the effects of NGF rely upon novel Integrative Nuclear FGF Receptor-1 (FGFR1) Signaling (INFS) we utilized the PC12 clonal cell line, a long-standing benchmark model of sympathetic neuronal differentiation. We demonstrate that NGF increases expression of the fgfr1 gene and promotes trafficking of FGFR1 protein from cytoplasm to nucleus by inhibiting FGFR1 nuclear export. Nuclear-targeted dominant negative FGFR1 antagonizes NGF-induced neurite outgrowth, doublecortin (dcx) expression and activation of the tyrosine hydroxylase (th) gene promoter, while active constitutive nuclear FGFR1 mimics the effects of NGF. NGF increases the expression of dcx, th, βIII tubulin, nurr1 and nur77, fgfr1and fibroblast growth factor-2 (fgf-2) genes, while enhancing binding of FGFR1and Nur77/Nurr1 to those genes. NGF activates transcription from isolated NurRE and NBRE motifs. Nuclear FGFR1 transduces NGF activation of the Nur dimer and raises basal activity of the Nur monomer. Cooperation of nuclear FGFR1 with Nur77/Nurr1 in NGF signaling expands the integrative functions of INFS to include NGF, the first discovered pluripotent neurotrophic factor.

CREB-binding protein (CBP) regulates β-adrenoceptor (β-AR)-mediated apoptosis

Catecholamines regulate the β-adrenoceptor/cyclic AMP-regulated protein kinase A (cAMP/PKA) pathway. Deregulation of this pathway can cause apoptotic cell death and is implicated in a range of human diseases, such as neuronal loss during aging, cardiomyopathy and septic shock. The molecular mechanism of this process is, however, only poorly understood. Here we demonstrate that the β-adrenoceptor/cAMP/PKA pathway triggers apoptosis through the transcriptional induction of the pro-apoptotic BH3-only Bcl-2 family member Bim in tissues such as the thymus and the heart. In these cell types, the catecholamine-mediated apoptosis is abrogated by loss of Bim. Induction of Bim is driven by the transcriptional co-activator CBP (CREB-binding protein) together with the proto-oncogene c-Myc. Association of CBP with c-Myc leads to altered histone acetylation and methylation pattern at the Bim promoter site. Our findings have implications for understanding pathophysiology associated with a deregulated neuroendocrine system and for developing novel therapeutic strategies for these diseases.

Small-molecule-mediated axonal branching in Caenorhabditis elegans

An in vivo system for monitoring small-molecule-mediated neuronal branching has been developed by using C. elegans. Growth-promoting compounds can be detected by visual inspection of GFPlabeled cholinergic neurons, as axonal branching occurs following treatment with neurotrophic agents. Investigation of the structure-activity relationship of the neurotrophic natural product clovanemagnolol (1) led us to a comparable chemically edited derivative.

Expression of the murine Nr4a1 gene is controlled by three distinct genomic loci

The transcription factor NR4A1 belongs to the class of orphan nuclear receptors without known ligand which might control its activity. Here we examined its transcriptional regulation in response to elevated cAMP levels in HL-1 cardiac myocytes and in the heart in vivo. We report, that murine Nr4a1 is expressed from distinct promoters located upto -11.5 kb from the first coding exon. Whereas in HL-1 cells only two of the three distinct transcripts were induced by cAMP, in the heart all transcripts were regulated in response to β-adrenergic stimulation.

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.

Role of mitochondrial activation in PACAP dependent neurite outgrowth

Pituitary adenylate cyclase-activating polypeptide (PACAP) increases neurite outgrowth, although signaling via its receptor PACAP-specific receptor (PAC1R) has not been fully characterized. Because mitochondria also play an important role in neurite outgrowth, we examined whether mitochondria contribute to PACAP-mediated neurite outgrowth. When mouse primary hippocampal neurons and Neuro2a cells were exposed to PACAP, neurite outgrowth and the mitochondrial membrane potential increased in both cell types. These results were reproduced using the PAC1R-specific agonist maxadilan and the adenylate cyclase activator forskolin, whereas the protein kinase A inhibitor H89 and mitochondrial uncoupling agent carbonyl cyanide m-chlorophenyl hydrazone (CCCP) inhibited these effects. Expression levels of peroxisome proliferator-activated receptor γ coactivator 1α (Pgc1α), a master regulator of mitochondrial activation, and its downstream effectors, such as cytochrome C and cytochrome C oxidase subunit 4, increased in response to PACAP. Knocking down Pgc1α expression using small interfering RNA or treatment with CCCP significantly attenuated neurite outgrowth and reduced the mitochondrial membrane potential in PACAP-treated cells. These data suggest that mitochondrial activation plays a key role in PACAP-induced neurite outgrowth via a signaling pathway that includes PAC1R, PKA, and Pgc1α.

Regulation of asymmetrical cytokinesis by cAMP during meiosis I in mouse oocytes

Mammalian oocytes undergo an asymmetrical first meiotic division, extruding half of their chromosomes in a small polar body to preserve maternal resources for embryonic development. To divide asymmetrically, mammalian oocytes relocate chromosomes from the center of the cell to the cortex, but little is known about the underlying mechanisms. Here, we show that upon the elevation of intracellular cAMP level, mouse oocytes produced two daughter cells with similar sizes. This symmetrical cell division could be rescued by the inhibition of PKA, a cAMP-dependent protein kinase. Live cell imaging revealed that a symmetrically localized cleavage furrow resulted in symmetrical cell division. Detailed analyses demonstrated that symmetrically localized cleavage furrows were caused by the inappropriate central positioning of chromosome clusters at anaphase onset, indicating that chromosome cluster migration was impaired. Notably, high intracellular cAMP reduced myosin II activity, and the microinjection of phospho-myosin II antibody into the oocytes impeded chromosome migration and promoted symmetrical cell division. Our results support the hypothesis that cAMP plays a role in regulating asymmetrical cell division by modulating myosin II activity during mouse oocyte meiosis I, providing a novel insight into the regulation of female gamete formation in mammals.

Low-molecular-weight compounds having neurotrophic activity in cultured PC12 cells and neurons

Recent reports have indicated that some low-molecular-weight compounds mimic neurotrophic factors inducing neurite outgrowth and neuroprotection. Carnosic acid (CA) promotes neurite outgrowth through the activation of Nrf2 in PC12 cells. CA also protects neurons via the keap/Nrf2 transcriptional pathway from oxidative stress. Forskolin-induced neurite outgrowth is mediated by activation of the PKA signalling pathway and this PKA-mediated neurite outgrowth is achieved by the expression of nur77 in PC12 cells. In addition, forskolin at its low concentration is closely related to the cAMP-induced protective function against L-DOPA-induced cytotoxicity in PC12 cells. A HDAC inhibitor trichostatin A (TSA) increases neurite length via p53 acetylation in rat cultured cerebellar granule neurons and in cerebral cortical neurons, and also protects neurons against glutathione depletion-induced oxidative stress. Recently, it was revealed that Nrf2 and p53 bind to CBP/p300 directly, and Nur77 is acetylated in vivo and in vitro by CBP/p300. Acetylation of Nrf2, p53 and Nur77 by CBP/p300 may constitute a novel similar regulatory mechanism for low-molecular-weight compounds with neurotrophic activities.

Cyclic AMP signalling through PKA but not Epac is essential for neurturin-induced biphasic ERK1/2 activation and neurite outgrowths through GFRα2 isoforms

Cyclic AMP (cAMP) and neurotrophic factors are known to interact closely to promote neurite outgrowth and neuronal regeneration. Glial cell line-derived neurotrophic factor (GDNF) and its family member neurturin (NTN) transduce signal through a multi-component receptor complex consisting of GDNF family receptor alpha 2 (GFRα2) and Ret receptor tyrosine kinase. Neurons from GFRα2-deficient mice do not promote axonal initiation when stimulated by NTN, consistent with the role of GFRα2 in neuronal outgrowth. Multiple alternatively spliced isoforms of GFRα2 are known to be expressed in the nervous system. GFRα2a and GFRα2c but not GFRα2b promoted neurite outgrowth. It is currently unknown if cAMP signalling is differentially regulated by these isoforms. In this study, NTN activation of GFRα2a and GFRα2c but not GFRα2b induced biphasic ERK1/2 activation and phosphorylation of the major cAMP target CREB. Interestingly, inhibition of cAMP signalling significantly impaired GFRα2a and GFRα2c-mediated neurite outgrowth while cAMP agonists cooperated with GFRα2b to induce neurite outgrowth. Importantly, the specific cAMP effector PKA but not Epac was essential for NTN-induced neurite outgrowth, through transcription and translation-dependent activation of late phase ERK1/2. Taken together, these results not only demonstrated the essential role of cAMP-PKA signalling in NTN-induced biphasic ERK1/2 activation and neurite outgrowth, but also suggested cAMP-PKA signalling as a hitherto unrecognized underlying mechanism contributing to the differential neuritogenic activities of GFRα2 isoforms.