Alternative splicing generates isoforms of human neuron-derived orphan receptor-1 (NOR-1) mRNA

Skeletal muscle Nur77 and NOR1 insulin responsiveness is blunted in obesity and type 2 diabetes but improved after exercise training

Obesity and type 2 diabetes (T2DM) are characterized by a blunted metabolic response to insulin, and strongly manifests in skeletal muscle insulin resistance. The orphan nuclear receptors, Nur77 and NOR1, regulate insulin-stimulated nutrient metabolism where Nur77 and NOR1 gene expression is increased with acute aerobic exercise and acute insulin stimulation. Whether Nur77 or NOR1 are associated with the insulin-sensitizing effects of chronic aerobic exercise training has yet to be elucidated. Fourteen lean healthy controls (LHC), 12 obese (OB), and 10 T2DM individuals (T2DM) underwent hyperinsulinemic-euglycemic clamps with skeletal muscle biopsies. Muscle was analyzed for Nur77 and NOR1 gene and protein expression at basal and insulin-stimulated conditions. Furthermore, a subcohort of 18 participants (OB, n = 12; T2DM, n = 6) underwent a 12-week aerobic exercise intervention (85% HRmax , 60 min/day, 5 days/week). In response to insulin infusion, LHC increased protein expression of Nur77 (8.7 ± 3.2-fold) and NOR1 (3.6 ± 1.1-fold), whereas OB and T2DM remained unaffected. Clamp-derived glucose disposal rates correlated with Nur77 (r2  = 0.14) and NOR1 (r2  = 0.12) protein expression responses to insulin, whereas age (Nur77: r2  = 0.22; NOR1: r2  = 0.25) and BMI (Nur77: r2  = 0.22; NOR1: r2  = 0.42) showed inverse correlations, corroborating preclinical data. In the intervention cohort, exercise improved Nur77 protein expression in response to insulin (PRE: -1.2 ± 0.3%, POST: 6.2 ± 1.5%). Also, insulin treatment of primary human skeletal muscle cells increased Nur77 and NOR1 protein. These findings highlight the multifactorial nature of insulin resistance in human obesity and T2DM. Understanding the regulation of Nur77 and NOR1 in skeletal muscle and other insulin-sensitive tissues will create opportunities to advance therapies for T2DM.

The nuclear receptor NOR-1 regulates the small muscle protein, X-linked (SMPX) and myotube differentiation

Recent works have highlighted the role of NOR-1 in both smooth and skeletal muscle, and have proposed this nuclear receptor as a nexus that coordinates muscle performance and metabolic capacity. However, no muscle specific genes regulated by NOR-1 have been identified so far. To identify NOR-1 target genes, we over-expressed NOR-1 in human vascular smooth muscle cells (VSMC). These cells subjected to sustained over-expression of supraphysiological levels of NOR-1 experienced marked phenotypic changes and up-regulated the skeletal muscle protein X-linked (SMPX), a protein typically expressed in striated muscle and associated to cell shape. By transcriptional studies and DNA-protein binding assays, we identified a non-consensus NBRE site in human SMPX promoter, critical for NOR-1 responsiveness. The expression of SMPX was higher in human skeletal muscle myoblasts (HSMM) than in human VSMC, and further increased in HSMM differentiated to myotubes. NOR-1 silencing prevented SMPX expression in HSMM, as well as their differentiation to myotubes, but the up-regulation of SMPX was dispensable for HSMM differentiation. Our results indicate that NOR-1 regulate SMPX in human muscle cells and acts as a muscle regulatory factor, but further studies are required to unravel its role in muscle differentiation and hypertrophy.

Orphan nuclear receptor Nur77 mediates fasting-induced hepatic fibroblast growth factor 21 expression

The fasting-induced hepatic hormone, fibroblast growth factor 21 (FGF21), is a potential candidate for the treatment of metabolic syndromes. Although peroxisome proliferator-activated receptor (PPAR)α is known to play a major role in the induction of hepatic FGF21 expression, other fasting-induced transcription factors that induce FGF21 expression have not yet been fully studied. In the present study, we investigated whether the fasting-induced activation of the orphan nuclear receptor Nur77 increases hepatic FGF21 expression. We found that fasting induced hepatic Nur77 and FGF21 expression. Glucagon and forskolin increased Nur77 and FGF21 expression in vivo and in vitro, respectively, and adenovirus-mediated overexpression of Nur77 (Ad-Nur77) increased FGF21 expression in vitro and in vivo. Moreover, knockdown of endogenous Nur77 expression by siRNA-Nur77 abolished the effect of forskolin on FGF21 expression. The results of ChIP assays, EMSA, and mutagenesis analysis showed that Nur77 bound to the putative NBRE of the FGF21 promoter in cultured hepatocytes and fasting induced Nur77 binding to the FGF21 promoter in vivo. Knockdown of PPARα partially inhibited forskolin-induced FGF21 expression, suggesting PPARα involvement in glucagon-stimulated FGF21 expression. In addition, double knockdown of PPARα and Nur77 further diminished FGF21 expression in cultured hepatocytes. In conclusion, this study shows that Nur77 mediates fasting-induced hepatic FGF21 expression, and suggests an alternative mechanism via which hepatic FGF21 transcription is mediated under fasting conditions.

Transcriptional regulation of fibroblast growth factor 21 expression

Fibroblast growth factor 21 (FGF21) is an attractive target for treating metabolic disease due to its wide-ranging beneficial effects on glucose and lipid metabolism. Circulating FGF21 levels are increased in insulin-resistant states; however, endogenous FGF21 fails to improve glucose and lipid metabolism in obesity, suggesting that metabolic syndrome is an FGF21-resistant state. Therefore, transcription factors for FGF21 are potential drug targets that could increase FGF21 expression in obesity and reduce FGF21 resistance. Despite many studies on the metabolic effects of FGF21, the transcriptional regulation of FGF21 gene expression remains controversial and is not fully understood. As the FGF21 transcription factor pathway is one of the most promising targets for the treatment of metabolic syndrome, further investigation of FGF21 transcriptional regulation is required.

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.

Naturally occurring C-terminal splice variants of nuclear receptors

Alternative mRNA splicing in the region encoding the C-terminus of nuclear receptors results in receptor variants lacking the entire ligand-binding domain (LBD), or a part of it, and instead contain a sequence of splice variant-specific C-terminal amino acids. A total of thirteen such splice variants have been shown to occur in vertebrates, and at least nine occur in humans. None of these receptor variants appear to be able to bind endogenous ligands and to induce transcription on promoters containing the response element for the respective canonical receptor variant. Interestingly, ten of these C-terminal splice variants have been shown to display dominant-negative activity on the transactivational properties of their canonical equivalent. Research on most of these splice variants has been limited, and the dominant-negative effect of these receptor variants has only been demonstrated in reporter assays in vitro, using transiently transfected receptors and reporter constructs. Therefore, the in vivo function and relevance of most C-terminal splice variants remains unclear. By reviewing the literature on the human glucocorticoid receptor beta-isoform (hGRbeta), we show that the dominant-negative effect of hGRbeta is well established using more physiologically relevant readouts. The hGR beta-isoform may alter gene transcription independent from the canonical receptor and increased hGRbeta levels correlate with glucocorticoid resistance and the occurrence of several immune-related diseases. Thus, available data suggests that C-terminal splice variants of nuclear receptors act as dominant-negative inhibitors of receptor-mediated signaling in vivo, and that aberrant expression of these isoforms may be involved in the pathogenesis of a variety of diseases.

Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

International Union of Pharmacology. LXVI. Orphan nuclear receptors

Half of the members of the nuclear receptors superfamily are so-called "orphan" receptors because the identity of their ligand, if any, is unknown. Because of their important biological roles, the study of orphan receptors has attracted much attention recently and has resulted in rapid advances that have helped in the discovery of novel signaling pathways. In this review we present the main features of orphan receptors, discuss the structure of their ligand-binding domains and their biological functions. The paradoxical existence of a pharmacology of orphan receptors, a rapidly growing and innovative field, is highlighted.

The orphan nuclear receptor, NOR-1, is a target of beta-adrenergic signaling in skeletal muscle

beta-Adrenergic receptor (beta-AR) agonists induce Nur77 mRNA expression in the C2C12 skeletal muscle cell culture model and elicit skeletal muscle hypertrophy. We previously demonstrated that Nur77 (NR4A1) is involved in lipolysis and gene expression associated with the regulation of lipid homeostasis. Subsequently it was demonstrated by another group that beta-AR agonists and cold exposure-induced Nur77 expression in brown adipocytes and brown adipose tissue, respectively. Moreover, NOR-1 (NR4A3) was hyperinduced by cold exposure in the nur77(-/-) animal model. These studies underscored the importance of understanding the role of NOR-1 in skeletal muscle. In this context we observed 30-480 min of beta-AR agonist treatment significantly and transiently increased expression of the orphan nuclear receptor NOR-1 in both mouse skeletal muscle tissue (plantaris) and C2C12 skeletal muscle cells. Specific beta(2)- and beta(3)-AR agonists had similar effects as the pan-agonist and were blocked by the beta-AR antagonist propranolol. Moreover, in agreement with these observations, isoprenaline also significantly increased the activity of the NOR-1 promoter. Stable exogenous expression of a NOR-1 small interfering RNA (but not the negative control small interfering RNA) in skeletal muscle cells significantly repressed endogenous NOR-1 mRNA expression and led to changes in the expression of genes involved in the control of lipid use and muscle mass underscored by a dramatic increase in myostatin mRNA expression. Concordantly the myostatin promoter was repressed by NOR-1 expression. In conclusion, NOR-1 is highly responsive to beta-adrenergic signaling and regulates the expression of genes controlling fatty acid use and muscle mass.

The NR4A orphan nuclear receptor NOR1 is induced by platelet-derived growth factor and mediates vascular smooth muscle cell proliferation

Members of the nuclear hormone receptor superfamily function as key transcriptional regulators of inflammation and proliferation in cardiovascular diseases. In addition to the ligand-dependent peroxisome proliferator-activated receptors and liver X receptors, this family of transcription factors includes a large number of orphan receptors, and their role in vascular diseases remains to be investigated. The neuron-derived orphan receptor-1 (NOR1) belongs to the ligand-independent NR4A subfamily, which has been implicated in cell proliferation, differentiation, and apoptosis. In this study, we demonstrate NOR1 expression in vascular smooth muscle cells (SMC) of human atherosclerotic lesions. In response to mitogenic stimulation with platelet-derived growth factor (PDGF), SMC rapidly express NOR1 through an ERK-MAPK-dependent signaling pathway. 5'-deletion analysis, site-directed mutagenesis, and transactivation experiments demonstrate that PDGF-induced NOR1 expression is mediated through a cAMP-response element-binding protein (CREB)-dependent transactivation of the NOR1 promoter. Consequently, short interfering RNA-mediated depletion of CREB abolished PDGF-induced NOR1 expression in SMC. Furthermore, PDGF induced Ser-133 phosphorylation of CREB and subsequent binding to the CRE sites of the endogenous NOR1 promoter. Functional analysis demonstrated that PDGF induces NOR1 transactivation of its consensus NGFI-B-response elements (NBRE) in SMC. We finally demonstrate that SMC isolated from NOR1-deficient mice exhibit decreased cell proliferation and characterize cyclin D1 and D2 as NOR1 target genes in SMC. These experiments indicate that PDGF-induced NOR1 transcription in SMC is mediated through CREB-dependent transactivation of the NOR1 promoter and further demonstrate that NOR1 functions as a key transcriptional regulator of SMC proliferation.

Coexpression of NOR1 and SIX3 proteins in extraskeletal myxoid chondrosarcomas without detectable NR4A3 fusion genes

Extraskeletal myxoid chondrosarcoma (EMC) is a rare mesenchymal tumor cytogenetically characterized by reciprocal translocations, such as t(9;22)(q22;q12) and t(9;17)(q22;q11), which result in EWSR1/NR4A3 and TAF15/NR4A3 fusion genes (alias EWS/NOR1, TAF2N/NOR1), respectively. NOR1 is an orphan nuclear receptor and acts as a transcription factor that can bind to its putative coactivator, SIX3. Although the NOR1 fusion protein has been implicated in oncogenesis of EMC, a small fraction of EMC lacks detectable rearrangements of the NR4A3 gene or 9q22. We report a case of EMC with no detectable NR4A3 gene alterations, as assessed with various molecular techniques including reverse transcription-polymerase chain reaction (RT-PCR), Southern blotting, interphase fluorescence in situ hybridization, and PCR single-strand conformation polymorphism-but with coexpression of native NOR1 and SIX3. In our survey of another 18 EMCs, we identified one more case expressing both NOR1 and SIX3 but lacking NR4A3 fusion. Fourteen tumors with detectable NR4A3 fusion genes (EWSR1-NR4A3; TAF15-NR4A3) expressed neither native NOR1 nor SIX3. SIX3 expression is normally confined specifically to the developing eye and fetal forebrain, although the expression of NR4A3 is largely ubiquitous. Our data suggest that aberrant coexpression of NOR1 and SIX3 is a potential alternative mechanism underlying the development of EMC.

Involvement of neuron-derived orphan receptor-1 (NOR-1) in LDL-induced mitogenic stimulus in vascular smooth muscle cells: role of CREB

OBJECTIVE

Low density lipoproteins (LDLs) modulate the expression of key genes involved in atherogenesis. Recently, we have shown that the transcription factor neuron-derived orphan receptor-1 (NOR-1) is involved in vascular smooth muscle cell (VSMC) proliferation. Our aim was to analyze whether NOR-1 is involved in LDL-induced mitogenic effects in VSMC.

METHODS AND RESULTS

LDL induced NOR-1 expression in a time- and dose-dependent manner. Antisense oligonucleotides against NOR-1 inhibit DNA synthesis induced by LDL in VSMCs as efficiently as antisense against the protooncogene c-fos. The upregulation of NOR-1 mRNA levels by LDL involves pertusis-sensitive G protein-coupled receptors, Ca2+ mobilization, protein kinases A (PKA) and C (PKC) activation, and mitogen-activated protein kinase pathways (MAPK) (p44/p42 and p38). LDL promotes cAMP response element binding protein (CREB) activation (phosphorylation in Ser133). In transfection assays a dominant-negative of CREB inhibits NOR-1 promoter activity, while mutation of specific (cAMP response element) CRE sites in the NOR-1 promoter abolishes LDL-induced NOR-1 promoter activity.

CONCLUSIONS

In VSMCs, LDL-induced mitogenesis involves NOR-1 upregulation through a CREB-dependent mechanism. CREB could play a role in the modulation by LDL of key genes (containing CRE sites) involved in atherogenesis.

Deacetylase activity is required for cAMP activation of a subset of CREB target genes

Many hormones activate transcription by raising the level of cAMP within cells. In one well studied pathway, cAMP induces protein kinase A to phosphorylate the transcription factor CREB, which binds to a consensus sequence, the cAMP-regulated enhancer, found in many target genes. A generally accepted model suggests that phosphorylated CREB recruits the histone acetyltransferase CBP to activate transcription. In contrast, histone deacetylases have been linked to the cessation of CREB-dependent transcription. Here we tested this model in the regulation of endogenous CREB target genes. We used a constitutively active CREB mutant and microarray analysis to identify target genes in PC12 cells. We then tested the role of histone deacetylase activity in cAMP activation of four of these genes (c-FOS, ICER, NOR-1, and NUR77) by treating cells with the histone deacetylase inhibitor trichostatin A. Consistent with the generally accepted model, trichostatin A enhanced activation of c-FOS and NUR77 by cAMP. Surprisingly, trichostatin A blocked activation of ICER and NOR-1. The block of ICER and NOR-1 activation persisted in the presence of cycloheximide, indicating that the trichostatin A effect did not depend on new protein synthesis. This unexpected role of histone deacetylases in transcriptional activation of certain endogenous CREB target genes was not apparent in transfected reporter genes. Chromatin immunoprecipitation analysis indicated that the differential roles of histone deacetylases in activating or repressing CREB target genes was manifested at the level of preinitiation complex recruitment. These data indicate that histone deacetylases differentially regulate CREB target genes by contributing to either activation or cessation of transcription.

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.

Parathyroid hormone induces the nuclear orphan receptor NOR-1 in osteoblasts

Parathyroid hormone (PTH) significantly affects osteoblast function by altering gene expression. We have identified neuron-derived orphan receptor-1 (NOR-1) as a PTH-induced primary gene in osteoblastic cells. NOR-1, Nurr1, and Nur77 comprise the NGFI-B nuclear orphan receptor family and Nurr1 and Nur77 are PTH-induced primary osteoblastic genes. Ten nM PTH maximally induced NOR-1 mRNA at 2h in primary mouse osteoblasts and at 1h in mouse calvariae. Cycloheximide pretreatment did not inhibit PTH-induced NOR-1 mRNA. PTH activates cAMP-protein kinase A (PKA), protein kinase C (PKC), and calcium signaling. Forskolin (PKA activator) and PMA (PKC activator) mimicked PTH-induced NOR-1 mRNA. Ionomycin (calcium ionophore) and PTH(3-34), which do not activate PKA, failed to induce NOR-1 mRNA. PKA inhibition with H89 blocked PTH- and FSK-induced NOR-1 mRNA. PMA pretreatment to deplete PKC inhibited PMA-induced, but not PTH-induced, NOR-1 mRNA. We conclude that NOR-1 is a PTH-regulated primary osteoblastic gene that is induced mainly through cAMP-PKA signaling.

The constitutive and inducible expression of Nurr1, a key regulator of dopaminergic neuronal differentiation, in human neural and non-neural cell lines

Nur-related factor 1 (Nurr1), nerve growth factor-induced gene B (NGFI-B) and neuron-derived orphan receptor-1 (NOR-1) constitute the orphan nuclear receptor subfamily of transcription factors. Previous studies showed that midbrain dopaminergic neuronal precursor cells failed to differentiate in Nurr1-deficient mice. To investigate a role of Nurr1 in human neuronal function, Nurr1 mRNA expression was studied in human neural cell lines by RT-PCR and northern blot analysis. Nurr1, NGFI-B and NOR-1 mRNA were coexpressed in all human neural and nonneural cell lines under the serum-containing culture condition, except for SK-N-SH neuroblastoma, in which Nurr1 mRNA was undetectable. The levels of Nurr1, NGFI-B and NOR-1 mRNA were elevated markedly in NTera2 teratocarcinoma-derived neurons (NTera2-N), a model of differentiated human neurons, following a 1.5 or 3 h-exposure to 1 mM dibutyryl cyclic AMP or 100 nm phorbol 12-myristate 13-acetate. NGFI-B mRNA levels were also elevated in NTera2-N cells by exposure to 100 ng/mL brain-derived neurotrophic factor (BDNF). To identify Nurr1-target genes, the mRNA expression of 27 genes potentially involved in dopaminergic neuronal differentiation and survival, including BDNF, glia-derived neurotrophic factor, their receptors, tyrosine hydroxylase and alpha-synuclein, were studied in HEK293 cells following overexpression of Nurr1. None of these genes examined, however, showed significant changes. These results indicate that Nurr1, NGFI-B and NOR-1 mRNA are expressed constitutively in various human neural and non-neural cell lines under the serum-containing culture condition, and their levels are up-regulated in human neurons by activation of protein kinase A or protein kinase C pathway, although putative coactivators expressed in dopaminergic neuronal precursor cells might be required for efficient transcriptional activation of Nurr1-target genes.

Molecular genetic characterization of the EWS/CHN and RBP56/CHN fusion genes in extraskeletal myxoid chondrosarcoma

Extraskeletal myxoid chondrosarcoma (EMC) is a soft-tissue neoplasm cytogenetically characterized by the translocations t(9;22)(q22;q11-12) or t(9;17)(q22;q11), generating EWS/CHN or RBP56/CHN fusion genes, respectively. In the present study, 18 EMCs were studied both cytogenetically and at the molecular level. Chromosomal aberrations were detected in 16 samples: 13 with involvement of 9q22 and 22q11-12, and three with rearrangements of 9q22 and 17q11. Fifteen cases had an EWS/CHN fusion transcript and three had an RBP56/CHN transcript. The most frequent EWS/CHN transcript (type 1; 10 tumors), involved fusion of EWS exon 12 with CHN exon 3, and the second most common (type 5; two cases) was fusion of EWS exon 13 with CHN exon 3. In all tumors with RBP56/CHN fusion, exon 6 of RBP56 was fused to exon 3 of CHN. By genomic XL PCR and sequence analyses, the breakpoints from 14 cases were mapped in the EWS, RBP56, and CHN genes. In CHN, 12 breakpoints were found in intron 2 and only two in intron 1. In EWS, the breaks occurred in introns 7 (one break), 12 (eight breaks), and 13 (one break), and in RBP56 in intron 6. Repetitive elements such as Alu and LINE sequences seem to have limited, if any, importance in the genesis of EWS/CHN and RBP56/CHN chimeras. Furthermore, there were no chi, chi-like, topoisomerase II, or translin consensus sequences in the introns harboring the translocation breakpoints, nor could the number of topo I sites in EWS, RBP56, and CHN introns explain the uneven distribution of the breakpoints among EWS or CHN introns. Additional genetic events, such as nucleotide insertions, homologies at the junction, deletions, duplications, and inversions, were found to accompany the translocations, indicating that the chromosomal translocations do not require sequence-specific recombinases or extensive homology between the recombined sequences.

Transcriptional regulation of nuclear orphan receptor, NOR-1, by Ca(2+)/calmodulin-dependent protein kinase cascade

We found that transcription of NOR-1 (neuron-derived orphan receptor-1) is induced by the Ca(2+)/calmodulin-dependent protein kinase IV (CaM-KIV) cascade by differential hybridization screening of a cDNA array using probes prepared from SH-SY5Y cells infected with recombinant adenoviruses expressing Ca(2+)/CaM-independent mutants of CaM-KIV and CaM-K kinase (CaM-KK). Induction of NOR-1 mRNA expression by the CaM-KIV cascade was confirmed by RT-PCR. Promoter analyses indicate that CaM-KIV cascade response element is located between -162 bp and -42 bp in the 1.7 kb NOR-1 promoter containing triple cAMP response elements. Disruption of each element significantly reduced the promoter activity, indicating the direct transcriptional regulation of NOR-1 by CaM-KIV cascade.

Early induction of nerve growth factor-induced genes after liver resection-reperfusion injury

BACKGROUND/AIMS

Nur-related factor 1 (Nurr1) has been implicated in liver generation after hepatectomy. We hypothesized that the genes in the nerve growth factor-induced gene B (NGFI-B) family was induced in liver ischemia-reperfusion injury.

METHODS

Expression of the NGFI-B family genes was examined by the reverse transcription-polymerase chain reaction in rat and human livers. In situ hybridization was performed to check the localization of the NGFI-B gene in rat liver. Expression of phospho-Ser-133-specific cyclic adenosine-3' :5'-monophosphate response element binding (pCREB) protein was examined by Western blot analysis and gel shift assay, since the promoter region of the NGFI-B family genes contains CRE.

RESULTS

The expression of the NGFI-B family genes were recognized within 30 min after ischemia-reperfusion in rat liver, which was augmented by cycloheximide injection. In human specimens, the NGFI-B family genes expression was stronger than that before ischemic insult. pCREB protein was detected in the rat liver sampled 15 min after reperfusion. Gel shift assay suggested that CREB bound to neuron-derived orphan receptor gene in rat liver cells.

CONCLUSIONS

We recognized the early induction of the NGFI-B family genes after ischemia-reperfusion injury in rat and human livers. A pathway via CREB may be responsible for the induction.

Structure and expression of the mouse gene encoding the orphan nuclear receptor TEC

Translocated in extraskeletal chondrosarcoma (TEC) is an orphan nuclear receptor involved in the control of cell proliferation and apoptosis and is expressed mainly in the mammalian central nervous system. To help understand the regulation of its expression, we have characterized the mouse genomic locus encoding TEC and analyzed its expression pattern in various tissues. The gene spans approximately 40 kb and contains 8 exons, of which the first two are noncoding. The promoter region does not contain any identifiable TATA box or CCAAT box elements; however, several binding sites for the transcription factors cyclic AMP-responsive element binding (CREB) protein and Spl are present. Two types of transcripts generated by alternative splicing were characterized by RT-PCR: one encodes the full-length receptor of 627 amino acids; the other encodes a truncated receptor of 429 amino acids lacking the entire carboxyl-terminal domain. Northern blots and RT-PCR analyses showed that mRNAs encoding both isoforms are expressed in all mouse tissues examined, with the highest levels being found in the brain. This expression pattern suggests that TEC may perform some basic housekeeping cellular function in addition to its role in cell proliferation.

An isoform of Nurr1 functions as a negative inhibitor of the NGFI-B family signaling

NGFI-B, Nurr1 and NOR-1 constitute a distinct subfamily within the nuclear receptor superfamily. To clarify the transcriptional regulation by the NGFI-B family, we searched for other components that can bind to the NBRE response element, a known target sequence for these transcription factors. By low stringency hybridization using the DNA binding domain of NOR-1 as a probe, a C-terminal truncated Nurr1 isoform, named Nurr2, was isolated from a mouse MC3T3-E1 cell cDNA library. Nurr2 had a novel cryptic exon located upstream in the Nurr1 promoter region, and was generated by alternative splicing at exons 1, 2 and 6. The C-terminal region was encoded by frame-shifted exon 6, and so Nurr2 lacked the C-terminal sequences corresponding to the putative ligand binding domain or dimerization domain. Quantitative reverse transcriptase-PCR experiments confirmed the presence of the Nurr2 isoform in mouse, rat and human. It was, like Nurr1, highly expressed in the pituitary and the cerebral cortex. Nurr2 and Nurr1 were also concomitantly induced by forskolin in NIH3T3 cells. Functional analysis using a reporter gene, containing NBRE response elements, indicated that while the isoform was inactive by itself, it could inhibit transactivation by the members of the NGFI-B family. These results indicate that the C-terminal truncated isoform, Nurr2, may act as a negative regulator of the NGFI-B family signaling.