Differential role of ERK in cAMP-induced Nurr1 expression in N2A and C6 cells

Models of Herpes Simplex Virus Latency

Our current understanding of HSV latency is based on a variety of clinical observations, and in vivo, ex vivo, and in vitro model systems, each with unique advantages and drawbacks. The criteria for authentically modeling HSV latency include the ability to easily manipulate host genetics and biological pathways, as well as mimicking the immune response and viral pathogenesis in human infections. Although realistically modeling HSV latency is necessary when choosing a model, the cost, time requirement, ethical constraints, and reagent availability are also equally important. Presently, there remains a pressing need for in vivo models that more closely recapitulate human HSV infection. While the current in vivo, ex vivo, and in vitro models used to study HSV latency have limitations, they provide further insights that add to our understanding of latency. In vivo models have shed light on natural infection routes and the interplay between the host immune response and the virus during latency, while in vitro models have been invaluable in elucidating molecular pathways involved in latency. Below, we review the relative advantages and disadvantages of current HSV models and highlight insights gained through each.

Mechanisms of NURR1 Regulation: Consequences for Its Biological Activity and Involvement in Pathology

NURR1 (Nuclear receptor-related 1 protein or NR4A2) is a nuclear protein receptor transcription factor with an essential role in the development, regulation, and maintenance of dopaminergic neurons and mediates the response to stressful stimuli during the perinatal period in mammalian brain development. The dysregulation of NURR1 activity may play a role in various diseases, including the onset and progression of neurodegenerative diseases, and several other pathologies. NURR1 is regulated by multiple mechanisms, among which phosphorylation by kinases or SUMOylation are the best characterized. Both post-translational modifications can regulate the activity of NURR1, affecting its stability and transcriptional activity. Other non-post-translational regulatory mechanisms include changes in its subcellular distribution or interaction with other protein partners by heterodimerization, also affecting its transcription activity. Here, we summarize the currently known regulatory mechanisms of NURR1 and provide a brief overview of its participation in pathological alterations.

The Critical Role of Nurr1 as a Mediator and Therapeutic Target in Alzheimer's Disease-related Pathogenesis

Several studies have revealed that the transcription factor nuclear receptor related 1 (Nurr1) plays several roles not only in the regulation of gene expression related to dopamine synthesis, but also in alternative splicing, and miRNA targeting. Moreover, it regulates cognitive functions and protects against inflammation-induced neuronal death. In particular, the role of Nurr1 in the pathogenesis of Parkinson's disease (PD) has been well investigated; for example, it has been shown that it restores behavioral and histological impairments in PD models. Although many studies have evaluated the connection between Nurr1 and PD pathogenesis, the role of Nurr1 in Alzheimer's disease (AD) remain to be studied. There have been several studies describing Nurr1 protein expression in the AD brain. However, only a few studies have examined the role of Nurr1 in the context of AD. Therefore, in this review, we highlight the overall effects of Nurr1 under the neuropathologic conditions related to AD. Furthermore, we suggest the possibility of using Nurr1 as a therapeutic target for AD or other neurodegenerative disorders.

From cell lines to pluripotent stem cells for modelling Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder characterized by loss of dopaminergic (DAergic) neurons in the substantia nigra (SN) that contributes to the main motor symptoms of the disease. At present, even if several advancements have been done in the last decades, the molecular and cellular mechanisms involved in the pathogenesis are far to be fully understood. Accordingly, the establishment of reliable in vitro experimental models to investigate the early events of the pathogenesis represents a key issue in the field. However, to mimic and reproduce in vitro the complex neuronal circuitry involved in PD-associated degeneration of DAergic neurons still remains a highly challenging issue. Here we will review the in vitro PD models used in the last 25 years of research, ranging from cell lines, primary rat or mice neuronal cultures to the more recent use of human induced pluripotent stem cells (hiPSCs) and, finally, the development of 3D midbrain organoids.

Cicadidae Periostracum, the Cast-Off Skin of Cicada, Protects Dopaminergic Neurons in a Model of Parkinson's Disease

Parkinson's disease (PD) is characterized by dopaminergic neuronal loss in the substantia nigra pars compacta (SNPC) and the striatum. Nuclear receptor-related 1 protein (Nurr1) is a nuclear hormone receptor implicated in limiting mitochondrial dysfunction, apoptosis, and inflammation in the central nervous system and protecting dopaminergic neurons and a promising therapeutic target for PD. Cicadidae Periostracum (CP), the cast-off skin of Cryptotympana pustulata Fabricius, has been used in traditional medicine for its many clinical pharmacological effects, including the treatment of psychological symptoms in PD. However, scientific evidence for the use of CP in neurodegenerative diseases, including PD, is lacking. Here, we investigated the protective effects of CP on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine- (MPTP-) induced PD in mice and explored the underlying mechanisms of action, focusing on Nurr1. CP increased the expression levels of Nurr1, tyrosine hydroxylase, DOPA decarboxylase, dopamine transporter, and vesicular monoamine transporter 2 via extracellular signal-regulated kinase phosphorylation in differentiated PC12 cells and the mouse SNPC. In MPTP-induced PD, CP promoted recovery from movement impairments. CP prevented dopamine depletion and protected against dopaminergic neuronal degradation via mitochondria-mediated apoptotic proteins such as B-cell lymphoma 2 (Bcl-2), Bcl-2-associated X, cytochrome c, and cleaved caspase-9 and caspase-3 by inhibiting MPTP-induced neuroinflammatory cytokines, inducible nitric oxide synthase, cyclooxygenase 2, and glial/microglial activation. Moreover, CP inhibited lipopolysaccharide-induced neuroinflammatory cytokines and response levels and glial/microglial activation in BV2 microglia and the mouse brain. Our findings suggest that CP might contribute to neuroprotective signaling by regulating neurotrophic factors primarily via Nurr1 signaling, neuroinflammation, and mitochondria-mediated apoptosis.

Molecular Insights into NR4A2(Nurr1): an Emerging Target for Neuroprotective Therapy Against Neuroinflammation and Neuronal Cell Death

NR4A2 is a nuclear receptor and a transcription factor, with distinctive physiological features. In the cell nuclei of the central nervous system, it is widely expressed and identified as a crucial regulator of dopaminergic (DA) neuronal differentiation, survival, and maintenance. Importantly, it has regulated different genes crucial for dopaminergic signals, and its expression has been diminished in both aged and PD post-mortem brains and reduced in PD patients. In microglia and astrocytes, the expression of NR4A2 has been found where it can be capable of inhibiting the expression of proinflammatory mediators; hence, it protected inflammation-mediated DA neuronal death. In addition, NR4A2 plays neuroprotective role via regulating different signals. However, NR4A2 has been mainly focused on Parkinson's research, but, in recent times, it has been studied in Alzheimer's disease (AD), multiple sclerosis (MS), and stroke. Altered expression of NR4A2 is connected to AD progression, and activation of its may improve cognitive function. It is downregulated in peripheral blood mononuclear cells of MS patients; nonetheless, its role in MS has not been fully clear. miR-145-5p known as a putative regulator of NR4A2 and in a middle cerebral artery occlusion/reperfusion model, anti-miR-145-5p administration promoted neurological outcomes in rat. To date, various activators and modulators of NR4A2 have been discovered and investigated as probable therapeutic drugs in neuroinflammatory and neuronal cell death models. The NR4A2 gene and cell-based therapy are described as promising drug candidates for neurodegenerative diseases. Moreover, microRNA might have a crucial role in neurodegeneration via affecting NR4A2 expression. Herein, we present the role of NR4A2 in neuroinflammation and neuronal cell death focusing on neurodegenerative conditions and display NR4A2 as a promising therapeutic target for the therapy of neuroprotection.

Rhein induces apoptosis and autophagy in human and rat glioma cells and mediates cell differentiation by ERK inhibition

In this study, we investigated the anticancer potentials of Rhein, an anthraquinone derivative of most commonly used Chinese rhubarb on the rat F98 glioma cells. The experimental studies revealed that Rhein induced cell cycle arrest, caspase mediated apoptosis. It results in the formation of intracellular acidic vesicles in cytoplasm, leading to autophagy. Differentiation of viable cells towards elongation of matured astrocytes was proved by monitoring dramatic changes in morphological characteristics as well as identified from the elevation of glial fibrillary acidic protein (GFAP) expression. Rhein treatment did not alter the phosphorylated MAPKs activation including p-38, JNK and NF-κB, transcription unit whereas rhein significantly inhibited ERK1/2 activation in F98 glioma cells. PD98059, a specific inhibitor for ERK activation imitates rhein effects on morphology and expressions of GFAP but did not help to induce any apoptosis or autophagy. Collective data exhibited that potentials of rhein in anti-cancer property in ERK-independent apoptosis and autophagy in association with downregulated ERK-dependent differentiation process of glioma cell lines.

Moderate superoxide production is an early promoter of mitochondrial biogenesis in differentiating N2a neuroblastoma cells

Reactive oxygen species (ROS) have been widely considered as harmful for cell development and as promoters of cell aging by increasing oxidative stress. However, ROS have an important role in cell signaling and they have been demonstrated to be beneficial by triggering hormetic signals, which could protect the organism from later insults. In the present study, N2a murine neuroblastoma cells were used as a paradigm of cell-specific (neural) differentiation partly mediated by ROS. Differentiation was triggered by the established treatments of serum starvation, forskolin or dibutyryl cyclic AMP. A marked differentiation, expressed as the development of neurites, was detected by fixation and staining with coomassie brilliant blue after 48 h treatment. This was accompanied by an increase in mitochondrial mass detected by mitotracker green staining, an increased expression of the peroxisome proliferator-activated receptor gamma (PPARγ) coactivator 1-alpha (PGC-1α) and succinate dehydrogenase activity as detected by MTT. In line with these results, an increase in free radicals, specifically superoxide anion, was detected in differentiating cells by flow cytometry. Superoxide scavenging by MnTBAP and MAPK inhibition by PD98059 partially reversed differentiation and mitochondrial biogenesis. In this way, we demonstrated that mitochondrial biogenesis and differentiation are mediated by superoxide and MAPK cues. Our data suggest that differentiation and mitochondrial biogenesis in N2a cells are part of a hormetic response which is triggered by a modest increase of superoxide anion concentration within the mitochondria.

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.

Luteolin downregulates IL-1β-induced MMP-9 and -13 expressions in osteoblasts via inhibition of ERK signalling pathway

The inhibitory effect of four structurally related flavonoids, apigenin, baicalein, luteolin and quercetin on the matrix metalloproteinase (MMP)-9 and -13 expressions in osteoblasts was investigated. Treatment with IL-1β induced both MMP-9 and -13 mRNA expressions as measured by quantitative real-time PCR. Luteolin and apigenin decreased IL-1β-induced MMP-9 and -13 mRNA expressions, whereas baicalein and quercetin showed little effects. Related to signalling, treatment with IL-1β induced ERK phosphorylation as measured by Western blotting. Further studies showed that transfection with a constitutively active form of the Ras protein (Ras(V12)) induced stronger ERK phosphorylation and upregulated MMP-9 and -13 mRNA expressions. However, transfection with a dominant-negative form of the Ras protein (Ras(N17)) inhibited the ERK activation and MMP-9 and -13 mRNA expressions induced by IL-1β, which supported the involvement of ERK signalling in IL-1β-induced MMP-9 and -13 expressions. Treatment with luteolin effectively inhibited the IL-1β-induced ERK activation in dose-dependent manner. Taken together, luteolin might inhibit IL-1β-induced MMP-9 and -13 expressions, in part, via inhibition of ERK signalling.

The role of NR4A transcription factors in memory formation

In various physiological contexts, Nr4a genes are transcribed in response to external stimuli as part of an immediate early response that initiates a cascade of gene expression ultimately leading to distinct physiological outcomes in each of these contexts. The signaling pathway that initiates Nr4a gene expression in most of these contexts consists of elevated intracellular cAMP activating PKA, which in turn leads to phosphorylation of CREB and new gene synthesis. This cAMP-PKA-CREB pathway is a central molecular pathway in the formation of a long-term memory. Indeed, learning induces Nr4a family gene expression, and long-term memory formation requires at least two waves of transcription after learning, suggesting that NR4A nuclear receptors may contribute to the second of these waves of gene expression. In this article, we review insights gained in other physiological contexts regarding Nr4a function and regulation, and highlight how these lessons can be applied to the study of memory formation.

Minireview: Nuclear hormone receptor 4A signaling: implications for metabolic disease

Numerous members of the nuclear hormone receptor (NR) superfamily have been demonstrated to regulate metabolic function in a cell- and tissue-specific manner. This review brings together recent studies that have associated members of the NR superfamily, the orphan NR4A subgroup, with the regulation of metabolic function and disease. The orphan NR4A subgroup includes Nur77 (NR4A1), Nurr1 (NR4A2), and Nor-1 (NR4A3). Expression of these receptors is induced in multiple tissues by a diverse range of stimuli, including stimuli associated with metabolic function, such as: β-adrenoceptor agonists, cold, fatty acids, glucose, insulin, cholesterol, and thiazolidinediones. In vitro and in vivo gain- and loss-of-function studies in major metabolic tissues (including skeletal muscle, adipose, and liver cells and tissues) have associated the NR4A subgroup with specific aspects of lipid, carbohydrate, and energy homeostasis. Most excitingly, although these orphan receptors do not have known endogenous ligands, several small molecule agonists have recently been identified. The preliminary studies reviewed in this manuscript suggest that therapeutic exploitation of the NR4A subgroup may show utility against dyslipidemia, obesity, diabetes, and cardiovascular disease.

Differentiation of mouse Neuro 2A cells into dopamine neurons

Neuro 2A (N2a) is a mouse neural crest-derived cell line that has been extensively used to study neuronal differentiation, axonal growth and signaling pathways. A convenient characteristic of these cells is their ability to differentiate into neurons within a few days. However, most differentiation methods reported for N2a cells do not provide information about the neuronal types obtained after each treatment. In this study, we evaluated the generation of N2a dopamine neurons following treatment with a number of factors known to induce neuronal differentiation. Our results showed that N2a cells express Nurr-related factor 1 (Nurr1) and produce low levels of tyrosine hydroxylase (TH) and dopamine. Both TH and dopamine levels were significantly enhanced in the presence of dibutyryl cyclic adenosine monophosphate (dbcAMP), as evidenced by Western blot, immunocytochemistry and high performance liquid chromatography (HPLC). In contrast to dbcAMP, other factors such as transforming growth factor beta1 (TGF beta 1), bone morphogenetic protein 4 (BMP4), glial cell-derived neurotrophic factor (GDNF) and retinoic acid (RA) did not increase TH expression. Further investigation confirmed that the effect of dbcAMP on production of TH-positive neurons was mediated through cyclic AMP (cAMP) responsive element binding protein (CREB) and it was antagonized by RA. Thus, although various treatments can be used to generate N2a neurons, only dbcAMP significantly enhanced the formation of dopamine neurons. Taken together, this study provided a simple and reliable method to generate dopamine neurons for rapid and efficient physiological and pharmacological assays.

A tense situation: forcing tumour progression

Cells within tissues are continuously exposed to physical forces including hydrostatic pressure, shear stress, and compression and tension forces. Cells dynamically adapt to force by modifying their behaviour and remodelling their microenvironment. They also sense these forces through mechanoreceptors and respond by exerting reciprocal actomyosin- and cytoskeletal-dependent cell-generated force by a process termed 'mechanoreciprocity'. Loss of mechanoreciprocity has been shown to promote the progression of disease, including cancer. Moreover, the mechanical properties of a tissue contribute to disease progression, compromise treatment and might also alter cancer risk. Thus, the changing force that cells experience needs to be considered when trying to understand the complex nature of tumorigenesis.

mRNA expression of the Nurr1 and NGFI-B nuclear receptor families following acute and chronic administration of methamphetamine

Nur-related 1 (Nurr1) and nerve growth factor inducible-B (NGFI-B) constitute closely related subgroups of the nuclear receptor superfamily. One to three hours after 4 mg/kg acute methamphetamine (METH) administration, the levels of Nurr1 mRNA were significantly higher in the prelimbic (PrL), primary motor (M1) and primary somatosensory (S1) cortices and ventral tegmental area (VTA), as compared with the basal level. Pretreatment with 0.5 mg/kg of SCH23390 prevented the acute METH-induced increase in Nurr1 mRNA levels in these brain regions. One to three hours after 4-mg/kg acute METH administration, the levels of NGFI-B mRNA increased significantly in the PrL, M1, S1, striatum, and nucleus accumbens core (AcbC). Pretreatment with either 0.5 mg/kg of MK-801 or 0.5 mg/kg of SCH23390 prevented the acute METH-induced increase in NGFI-B mRNA levels in these brain regions. The levels of mRNAs were determined 3 h after a challenge injection of either saline or 4 mg/kg METH at the three-week withdrawal point in rats which had previously been exposed to either saline or METH (4 mg/kg/day) for 2 weeks. After the saline challenge, the group chronically exposed to METH displayed significantly higher levels of Nurr1 mRNA in the PrL, S1 and VTA, and of NGFI-B mRNA in the PrL, M1, S1, striatum and AcbC than did the group chronically treated with saline. The groups chronically exposed to METH failed to increase Nurr1 mRNA in the VTA, and NGFI-B mRNA in the AcbC, when challenged with 4 mg/kg METH. These results suggest that Nurr1 and NGFI-B mRNA play differential roles upon exposure to METH.

Enhancement of dopaminergic properties and protection mediated by neuronal activation of Ras in mouse ventral mesencephalic neurones

The poor differentiation and survival of dopaminergic neurones are practical constraints in their therapeutic applications. Here we explored the role of neuronally activated Ras in ventral mesencephalon-derived neurospheres generated from synRas mouse embryos. The expression of Val12 Ha-Ras transgene and enhanced Ras activity was evident after differentiation of the neurospheres with a corresponding activating phosphorylation of mitogen-activated protein kinase. Phosphorylation of Akt/PKB, the target kinase of phosphoinositide 3-kinase, along with phosphorylation of Bad and CREB were enhanced in synRas-derived differentiated neurosphere cultures. Furthermore, increased Nurr1 expression was associated with elevated numbers of dopaminergic neurones in synRas-derived cultures compared with the wild-type. Correspondingly, tyrosine hydroxylase promoter assays revealed enhanced transcriptional activation of the promoter in synRas-derived cultures. synRas-derived dopaminergic neurones were greatly resistant to degeneration induced by various noxious stimuli. Consistently, the transgenic expression of activated Ras attenuated the adverse 6-hydroxydopamine effects on dopaminergic neurones. Dopaminergic neurones derived from both wild-type and synRas cultures expressed voltage-gated potassium and sodium currents, fired action potentials and exhibited electrical network activity. Thus, expression of the transgene promotes survival and enhances differentiation towards a dopaminergic cell fate without altering their basic electrical properties. Our results suggest that intracellular cell therapy mimicking trophic signalling may offer potential benefit in models of human disease associated with dopamine neurone dysfunction.

Growth stimulation of human pulmonary adenocarcinoma cells and small airway epithelial cells by beta-carotene via activation of cAMP, PKA, CREB and ERK1/2

An alpha-tocopherol, beta-carotene supplementation trial (ATBC) and a chemoprevention trial with beta-carotene and retinoids (CARET trial) were conducted in the 1990s in populations at risk for the development of lung cancer. Both trials had to be discontinued due to significant increases in lung cancer and cardiovascular mortality. Clinical trials to test the cancer preventive effects of beta-carotene are still ongoing, and high concentrations of this provitamin are contained in numerous dietary supplements. Using a cell line derived from a human pulmonary adenocarcinoma (PAC) of Clara cell lineage and immortalized human small airway epithelial cells, our data show that low concentrations of beta-carotene that can be realistically expected in human tissues after oral administration caused a significant increase in intracellular cAMP and activated PKA, as well as in phosphorylation of ERK1/2 and CREB. Furthermore, the proliferation of cells was significantly stimulated by identical concentrations of beta-carotene as monitored by MTT assays. Control experiments with retinol also showed stimulation of cell proliferation and activation of PKA in both cell lines. In light of the fact that PAC is the leading type of lung cancer, these findings suggest that the growth promoting effects of beta-carotene on this cancer type observed in our experiments may have contributed to the unfortunate outcome of the ATBC and CARET trials. This interpretation is supported by the fact that elevated levels of cAMP in the cardiovascular system play a major role in the genesis of cardiovascular disease, which was also greatly promoted in the CARET trial. Our data challenge the widely accepted view that beta-carotene may be useful as a cancer preventive agent.

How to make a mesodiencephalic dopaminergic neuron

Dopaminergic neurons located in the ventral mesodiencephalon are essential for the control of voluntary movement and the regulation of emotion, and are severely affected in neurodegenerative diseases such as Parkinson's disease. Recent advances in molecular biology and mouse genetics have helped to unravel the mechanisms involved in the development of mesodiencephalic dopaminergic (mdDA) neurons, including their specification, migration and differentiation, as well as the processes that govern axonal pathfinding and their specific patterns of connectivity and maintenance. Here, we follow the developmental path of these neurons with the goal of generating a molecular code that could be exploited in cell-replacement strategies to treat diseases such as Parkinson's disease.

Regulation of osteoblast differentiation by Nurr1 in MC3T3-E1 cell line and mouse calvarial osteoblasts

The orphan nuclear receptor Nurr1 is primarily expressed in the central nervous system. It has been shown that Nurr1 is necessary for terminal differentiation of dopaminergic (DA) neurons in ventral midbrain. The receptor, however, is also expressed in other organs including bone, even though the role of Nurr1 is not yet understood. Therefore, we investigated the role of Nurr1 in osteoblast differentiation in MC3T3-E1 cells and calvarial osteoblasts derived from Nurr1 null newborn pups. Our results revealed that reduced Nurr1 expression, using Nurr1 siRNA in MC3T3-E1 cells, affected the expression of osteoblast differentiation marker genes, osteocalcin (OCN) and collagen type I alpha 1 (COL1A1), as measured by quantitative real-time PCR. The activity of alkaline phosphatase (ALP), another osteoblast differentiation marker gene, was also decreased in Nurr1 siRNA-treated MC3T3-E1 cells. In addition, Nurr1 overexpression increased OCN and COL1A1 expression. Furthermore, consistent with these results, during osteoblast differentiation, the expression of osteoblast marker genes was decreased in primary cultured mouse calvarial osteoblasts derived from Nurr1 null mice. Collectively, our results suggest that Nurr1 is important for osteoblast differentiation.

Nuclear orphan receptor Nurr1 directly transactivates the osteocalcin gene in osteoblasts

Nurr1, an NGFI-B nuclear orphan receptor, which transactivates promoters through an NGFI-B response element (NBRE), is strongly induced by parathyroid hormone through the cAMP-protein kinase A signaling pathway in osteoblasts. Here, we demonstrate that multiple agents activating diverse signaling pathways in osteoblasts induce Nurr1. The strongest Nurr1 inducers were activators of cAMP-protein kinase A-coupled signaling, followed by protein kinase C- and calcium-coupled signaling activators. Receptor tyrosine kinase activators had minimal effect, whereas serine/threonine kinase activators had no effect on basal Nurr1 mRNA levels. Computer analysis of osteoblastic promoters indicated two potential NBREs in the rat osteocalcin (Ocn) promoter. Intriguingly, the proximal site maps to the cAMP-responsive cis-element. We tested whether Nurr1 induces Ocn expression through the NBRE-like site. Recombinant and endogenous Nurr1 protein from primary mouse osteoblasts bound to a consensus NBRE in EMSAs. Nurr1 induced a consensus 3 x NBRE-luciferase reporter construct in mouse osteoblasts. Recombinant and endogenous Nurr1 protein bound to the proximal NBRE-like site in the Ocn promoter in EMSAs. Endogenous Nurr1 protein bound to this site as a monomer, because neither retinoid X receptor alpha nor retinoid X receptor beta antibody supershifted the protein-DNA complex. Ocn promoter-luciferase constructs lacking or containing a mutated proximal NBRE-like site had markedly blunted responses to Nurr1 overexpression. Finally, adenovirally expressed Nurr1 protein bound to the proximal NBRE-like site in chromatin immunoprecipitation assays and induced Ocn mRNA in primary rat osteoblasts. We conclude that Ocn is a Nurr1 target gene, which positions Nurr1 in the core of transcriptional factors regulating osteoblastic gene expression.