The immediate-early gene Egr-1 regulates the activity of the thymidine kinase promoter at the G0-to-G1 transition of the cell cycle

Glial Cell AMPA Receptors in Nervous System Health, Injury and Disease

Glia form a central component of the nervous system whose varied activities sustain an environment that is optimised for healthy development and neuronal function. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA)-type glutamate receptors (AMPAR) are a central mediator of glutamatergic excitatory synaptic transmission, yet they are also expressed in a wide range of glial cells where they influence a variety of important cellular functions. AMPAR enable glial cells to sense the activity of neighbouring axons and synapses, and as such many aspects of glial cell development and function are influenced by the activity of neural circuits. However, these AMPAR also render glia sensitive to elevations of the extracellular concentration of glutamate, which are associated with a broad range of pathological conditions. Excessive activation of AMPAR under these conditions may induce excitotoxic injury in glial cells, and trigger pathophysiological responses threatening other neural cells and amplifying ongoing disease processes. The aim of this review is to gather information on AMPAR function from across the broad diversity of glial cells, identify their contribution to pathophysiological processes, and highlight new areas of research whose progress may increase our understanding of nervous system dysfunction and disease.

Molecular targets and signaling pathways regulated by nuclear translocation of syndecan-1

Background

The cell-surface heparan sulfate proteoglycan syndecan-1 is important for tumor cell proliferation, migration, and cell cycle regulation in a broad spectrum of malignancies. Syndecan-1, however, also translocates to the cell nucleus, where it might regulate various molecular functions.

Results

We used a fibrosarcoma model to dissect the functions of syndecan-1 related to the nucleus and separate them from functions related to the cell-surface. Nuclear translocation of syndecan-1 hampered the proliferation of fibrosarcoma cells compared to the mutant lacking nuclear localization signal. The growth inhibitory effect of nuclear syndecan-1 was accompanied by significant accumulation of cells in the G0/G1 phase, which indicated a possible G1/S phase arrest.

We implemented multiple, unsupervised global transcriptome and proteome profiling approaches and combined them with functional assays to disclose the molecular mechanisms that governed nuclear translocation and its related functions. We identified genes and pathways related to the nuclear compartment with network enrichment analysis of the transcriptome and proteome. The TGF-β pathway was activated by nuclear syndecan-1, and three genes were significantly altered with the deletion of nuclear localization signal: EGR-1 (early growth response 1), NEK11 (never-in-mitosis gene a-related kinase 11), and DOCK8 (dedicator of cytokinesis 8). These candidate genes were coupled to growth and cell-cycle regulation. Nuclear translocation of syndecan-1 influenced the activity of several other transcription factors, including E2F, NFκβ, and OCT-1. The transcripts and proteins affected by syndecan-1 showed a striking overlap in their corresponding biological processes. These processes were dominated by protein phosphorylation and post-translation modifications, indicative of alterations in intracellular signaling. In addition, we identified molecules involved in the known functions of syndecan-1, including extracellular matrix organization and transmembrane transport.

Conclusion

Collectively, abrogation of nuclear translocation of syndecan-1 resulted in a set of changes clustering in distinct patterns, which highlighted the functional importance of nuclear syndecan-1 in hampering cell proliferation and the cell cycle. This study emphasizes the importance of the localization of syndecan-1 when considering its effects on tumor cell fate.

Electronic supplementary material

The online version of this article (10.1186/s12860-017-0150-z) contains supplementary material, which is available to authorized users.

P2Y2 purinergic receptor activation is essential for efficient hepatocyte proliferation in response to partial hepatectomy

Extracellular nucleotides via activation of P2 purinergic receptors influence hepatocyte proliferation and liver regeneration in response to 70% partial hepatectomy (PH). Adult hepatocytes express multiple P2Y (G protein-coupled) and P2X (ligand-gated ion channels) purinergic receptor subtypes. However, the identity of key receptor subtype(s) important for efficient hepatocyte proliferation in regenerating livers remains unknown. To evaluate the impact of P2Y2 purinergic receptor-mediated signaling on hepatocyte proliferation in regenerating livers, wild-type (WT) and P2Y2 purinergic receptor knockout (P2Y2-/-) mice were subjected to 70% PH. Liver tissues were analyzed for activation of early events critical for hepatocyte priming and subsequent cell cycle progression. Our findings suggest that early activation of p42/44 ERK MAPK (5 min), early growth response-1 (Egr-1) and activator protein-1 (AP-1) DNA-binding activity (30 min), and subsequent hepatocyte proliferation (24-72 h) in response to 70% PH were impaired in P2Y2-/- mice. Interestingly, early induction of cytokines (TNF-α, IL-6) and cytokine-mediated signaling (NF-κB, STAT-3) were intact in P2Y2-/- remnant livers, uncovering the importance of cytokine-independent and nucleotide-dependent early priming events critical for subsequent hepatocyte proliferation in regenerating livers. Hepatocytes isolated from the WT and P2Y2-/- mice were treated with ATP or ATPγS for 5-120 min and 12-24 h. Extracellular ATP alone, via activation of P2Y2 purinergic receptors, was sufficient to induce ERK phosphorylation, Egr-1 protein expression, and key cyclins and cell cycle progression of hepatocytes in vitro. Collectively, these findings highlight the functional significance of P2Y2 purinergic receptor activation for efficient hepatocyte priming and proliferation in response to PH.

The serine protease inhibitor elafin maintains normal growth control by opposing the mitogenic effects of neutrophil elastase

The serine protease inhibitor, elafin, is a critical component of the epithelial barrier against neutrophil elastase (NE). Elafin is downregulated in the majority of breast cancer cell lines compared to normal human mammary epithelial cells (HMECs). Here, we evaluated the role of elafin and NE on proliferation and tumorigenesis. Elafin is induced in growth factor deprived HMECs as they enter a quiescent (G0) state, suggesting that elafin is a counterbalance against the mitogenic effects of NE in G0 HMECs. Stable knockdown of elafin compromises the ability of HMECs to maintain G0-arrest during long-term growth factor deprivation; this effect can be reversed by re-expression of wild-type elafin, but not elafin-M25G lacking protease inhibitory function. These results suggest that NE, which is largely contributed by activated neutrophils in the tumor microenvironment, may be negatively regulating the ability of elafin to arrest cells in G0. In fact when purified NE was added to elafin knockdown HMECs, these cells demonstrated greater sensitivity to the growth promoting effects of purified NE. Activation of ERK signaling, downstream of toll-like receptor 4, was essential to the mitogenic effect of NE on HMECs. These findings were next translated to patient samples, and immunohistochemical analysis of normal breast tissue revealed robust elafin expression in the mammary epithelium; however, elafin expression was dramatically downregulated in a significant proportion of human breast tumor specimens. The loss of elafin expression during breast cancer progression may promote tumor growth as a consequence of increased NE-activity. To address the role of NE in mammary tumorigenesis, we next examined if deregulated NE-activity enhances mammary tumor growth. NE knockout in the C3(1)TAg mouse model of mammary tumorigenesis suppressed proliferation and reduced the kinetics of tumor growth. Overall, the imbalance between NE and its inhibitors, such as elafin, presents an important therapeutic target in breast cancer.

MicroRNA-gene association as a prognostic biomarker in cancer exposes disease mechanisms

The transcriptional networks that regulate gene expression and modifications to this network are at the core of the cancer phenotype. MicroRNAs, a well-studied species of small non-coding RNA molecules, have been shown to have a central role in regulating gene expression as part of this transcriptional network. Further, microRNA deregulation is associated with cancer development and with tumor progression. Glioblastoma Multiform (GBM) is the most common, aggressive and malignant primary tumor of the brain and is associated with one of the worst 5-year survival rates among all human cancers. To study the transcriptional network and its modifications in GBM, we utilized gene expression, microRNA sequencing, whole genome sequencing and clinical data from hundreds of patients from different datasets. Using these data and a novel microRNA-gene association approach we introduce, we have identified unique microRNAs and their associated genes. This unique behavior is composed of the ability of the quantifiable association of the microRNA and the gene expression levels, which we show stratify patients into clinical subgroups of high statistical significance. Importantly, this stratification goes unobserved by other methods and is not affiliated by other subsets or phenotypes within the data. To investigate the robustness of the introduced approach, we demonstrate, in unrelated datasets, robustness of findings. Among the set of identified microRNA-gene associations, we closely study the example of MAF and hsa-miR-330-3p, and show how their co-behavior stratifies patients into prognosis clinical groups and how whole genome sequences tells us more about a specific genomic variation as a possible basis for patient variances. We argue that these identified associations may indicate previously unexplored specific disease control mechanisms and may be used as basis for further study and for possible therapeutic intervention.

Despite major progress and improved understanding of Glioblastoma Multiforme, the disease is still associated with poor prognosis. The identification of genomic regulatory mechanisms, their affiliation with clinical outcome and the association between specific modifications in genome sequence that can explain gain and loss of such regulatory activity, combine to suggest specific disease mechanisms and possible means of intervention in the course of the disease. We report here a method and its implementation in exposing possible regulatory mechanisms in GBM. At the core of this method is the employment of associations between micro RNAs and genes as a quantifiable metric. Identification of these associations and their affiliation with clinical features, combined with the availability of whole genome sequences, brings forward specific micro RNAs and their associated genes. Affiliation of specific genomic sequences with clinical outcome thus translates personal genomics into tumor relevant decision-making.

Krox20/EGR2 deficiency accelerates cell growth and differentiation in the monocytic lineage and decreases bone mass

Krox20/EGR2, one of the 4 early growth response genes, is a highly conserved transcription factor implicated in hindbrain development, peripheral nerve myelination, tumor suppression, and monocyte/macrophage cell fate determination. Here, we established a novel role for Krox20 in postnatal skeletal metabolism. Microcomputed tomographic analysis of 4- and 8-week-old mice revealed a low bone mass phenotype (LBM) in both the distal femur and the vertebra of Krox20(+/-) mice. This was attributable to accelerated bone resorption as demonstrated in vivo by increased osteoclast number and serum C-terminal telopeptides, a marker for collagen degradation. Krox20 haploinsufficiency did not reduce bone formation in vivo, nor did it compromise osteoblast differentiation in vitro. In contrast, growth and differentiation were significantly stimulated in preosteoclast cultures derived from Krox20(+/-) splenocytes, suggesting that the LBM is attributable to Krox20 haploinsufficiency in the monocytic lineage. Furthermore, Krox20 silencing in preosteoclasts increased cFms expression and response to macrophage colony-stimulating factor, leading to a cell-autonomous stimulation of cell-cycle progression. Our data indicate that the antimitogenic role of Krox20 in preosteoclasts is the predominant mechanism underlying the LBM phenotype of Krox20-deficient mice. Stimulation of Krox20 expression in preosteoclasts may present a viable therapeutic strategy for high-turnover osteoporosis.

HGF-promoted motility in primary human melanocytes depends on CD44v6 regulated via NF-kappa B, Egr-1, and C/EBP-beta

The regulation of CD44v6, a variant of the CD44 family of glycosylated adhesion molecules, through hepatocyte growth factor (HGF) has implications for motility in primary human melanocytes. We show that exposure of primary human melanocytes to HGF results in an increase of CD44v6 expression. Immunostaining of melanocytic lesions revealed low cytoplasmic positivity of CD44v6 in some nevi but high membranous expression in primary cutaneous melanomas, and cutaneous and lymph node metastases. HGF-dependent CD44v6 regulation in melanocytes is NF-kappaB dependent because BAY 11-7082, an inhibitor of NF-kappaB activation, but not interference with the mitogen-activated protein kinase or phosphatidylinositol 3-kinase cascade, antagonized HGF-induced CD44v6 expression. NF-kappaB-mediated transcriptional regulation of CD44v6 involves the transcription factors Egr-1 and CCAAT enhancer-binding protein-beta (C/EBP-beta). In gel shift assays, the initial binding of p100/p52 NF-kappaB, C/EBP-beta, and Egr-1 to the CD44 promoter experienced reshuffling toward increased affinity of C/EBP-beta after HGF stimulation. A blocking antibody to CD44v6 decreased HGF-induced c-Met phosphorylation as well as enhanced random- and site-directed migration. Our data show that HGF-induced motility in primary human melanocytes depends on c-Met-CD44v6 interaction, and that HGF-enhanced CD44v6 expression is required for motility and transcriptional upregulation of CD44v6, presumably mediated through a complex comprising NF-kappaB/C/EBP-beta and Egr-1.

Early biomarkers and potential mediators of ventilation-induced lung injury in very preterm lambs

Background

Bronchopulmonary dysplasia (BPD) is closely associated with ventilator-induced lung injury (VILI) in very preterm infants. The greatest risk of VILI may be in the immediate period after birth, when the lungs are surfactant deficient, still partially filled with liquid and not uniformly aerated. However, there have been very few studies that have examined this immediate post-birth period and identified the initial injury-related pathways that are activated. We aimed to determine if the early response genes; connective tissue growth factor (CTGF), cysteine rich-61 (CYR61) and early growth response 1 (EGR1), were rapidly induced by VILI in preterm lambs and whether ventilation with different tidal volumes caused different inflammatory cytokine and early response gene expression.

Methods

To identify early markers of VILI, preterm lambs (132 d gestational age; GA, term ~147 d) were resuscitated with an injurious ventilation strategy (V_T 20 mL/kg for 15 min) then gently ventilated (5 mL/kg) for 15, 30, 60 or 120 min (n = 4 in each). To determine if early response genes and inflammatory cytokines were differentially regulated by different ventilation strategies, separate groups of preterm lambs (125 d GA; n = 5 in each) were ventilated from birth with a V_T of 5 (VG5) or 10 mL/kg (VG10) for 135 minutes. Lung gene expression levels were compared to levels prior to ventilation in age-matched control fetuses.

Results

CTGF, CYR61 and EGR1 lung mRNA levels were increased ~25, 50 and 120-fold respectively (p < 0.05), within 30 minutes of injurious ventilation. VG5 and VG10 caused significant increases in CTGF, CYR61, EGR1, IL1-β, IL-6 and IL-8 mRNA levels compared to control levels. CTGF, CYR61, IL-6 and IL-8 expression levels were higher in VG10 than VG5 lambs; although only the IL-6 and CYR61 mRNA levels reached significance.

Conclusion

CTGF, CYR61 and EGR1 may be novel early markers of lung injury and mechanical ventilation from birth using relatively low tidal volumes may be less injurious than using higher tidal volumes.

hEGR1 is induced by EGF, inhibited by gefitinib in bladder cell lines and related to EGF receptor levels in bladder tumours

The effect of EGF and gefitinib on two EGFR-positive human bladder cancer cell lines has been investigated using array-based gene expression profiling. The most prominent transcript, increased up to 6.7-fold by EGF compared with controls in RT112 cells, was human early growth response protein 1 (hEGR1). This induction was prevented by gefitinib. The hEGR1 mRNA in EGF-treated samples was reduced in the presence of gefitinib, as was hEGR1 protein in cell lysates. In the RT4 cells, hEGR1 expression was halved in the presence of EGF and gefitinib in combination. In bladder tumour samples, there was a significant correlation between hEGR1 mRNA detected by RT-PCR and EGFR detected by ligand binding, (P=0.042). The induction by EGF of the hEGR1 gene, mRNA and protein in RT112 cells, and its inhibition by gefitinib, together with the detection of hEGR1 mRNA in bladder tumours, suggests that hEGR1 may be important in the EGFR growth-signalling pathway in bladder cancer and should be further investigated for its prognostic significance and as a potential therapeutic target.

Chlamydia pneumoniae infection promotes a proliferative phenotype in the vasculature through Egr-1 activation in vitro and in vivo

Atherosclerosis is characterized by inflammation and proliferation of vascular cells. The intracellular bacterium Chlamydia (Chlamydophila) pneumoniae uses blood monocytes [peripheral blood mononuclear cells (PBMCs)] for dissemination, has been found to persist in atherosclerotic lesions, and has been implicated in atherogenesis by small GTPase activation and T lymphocyte recruitment. Infection of human coronary artery smooth muscle cells with C. pneumoniae significantly induced mRNA and protein for the angiogenic transcription factor Egr-1, resulting in enhanced coronary artery smooth muscle cell proliferation, which was reduced by transfection with small interfering RNA duplexes targeted at Egr-1 mRNA. These effects required viable chlamydiae and depended on p44/42 mitogen-activated protein kinase activity but not on the p38 mitogen-activated protein kinase pathway. Postinfectious Egr-1 mRNA up-regulation in arterial vessels was confirmed ex vivo in a rat aortic ring model of focal vascular chlamydial infection. An in vivo model based on the injection of C. pneumoniae-infected PBMCs into mice confirmed Egr-1 mRNA up-regulation within 24 h of endovascular infection. Arterial injury from repeated direct chlamydial infections and cell-to-cell contact with C. pneumoniae-infected PBMCs might represent a chronic focus of proliferative activity linked to the media proliferation seen in advanced atherosclerosis. Overall, chlamydial infection induces a proliferative phenotype in vascular cells via transcription factor Egr-1 activation in vitro, ex vivo, and in vivo.

The transcription factor early growth response factor-1 (EGR-1) promotes apoptosis of neuroblastoma cells

Early growth response factor-1 (EGR-1) is an immediate early gene, which is rapidly activated in quiescent cells by mitogens or in postmitotic neurons after depolarization. EGR-1 has been involved in diverse biological functions such as cell growth, differentiation and apoptosis. Here we report that enforced expression of the EGR-1 gene induces apoptosis, as determined by flow cytometry and terminal deoxynucleotidyl transferase-mediated dUTP-fluorescein nick-end labelling (TUNEL) analysis, in murine Neuro2A cells. In accordance with this role of EGR-1 in cell death, antisense oligonucleotides increase cell viability in cells cultured in the absence of serum. This apoptotic activity of the EGR-1 appears to be mediated by p73, a member of the p53 family of proteins, since an increase in the amount of p73 is observed in clones stably expressing the EGR-1 protein. We also observed an increase in the transcriptional activity of the mdm2 promoter in cells overexpressing EGR-1, which is paralleled by a marked decrease in the levels of p53 protein, therefore excluding a role of this protein in mediating EGR-1-induced apoptosis. Our results suggest that EGR-1 is an important factor involved in neuronal apoptosis.

Extracellular ATP stimulates the early growth response protein 1 (Egr-1) via a protein kinase C-dependent pathway in the human osteoblastic HOBIT cell line

Extracellular nucleotides exert an important role in controlling cell physiology by activating intracellular signalling cascades. Osteoblast HOBIT cells express P2Y(1) and P2Y(2) G-protein-coupled receptors, and respond to extracellular ATP by increasing cytosolic calcium concentrations. Early growth response protein 1 (Egr-1) is a C(2)H(2)-zinc-finger-containing transcriptional regulator responsible for the activation of several genes involved in the control of cell proliferation and apoptosis, and is thought to have a central role in osteoblast biology. We show that ATP treatment of HOBIT cells increases Egr-1 protein levels and binding activity via a mechanism involving a Ca(2+)-independent protein kinase C isoform. Moreover, hypotonic stress and increased medium turbulence, by inducing ATP release, result in a similar effect on Egr-1. Increased levels of Egr-1 protein expression and activity are achieved at very early times after stimulation (5 min), possibly accounting for a rapid way for changing the osteoblast gene-expression profile. A target gene for Egr-1 that is fundamental in osteoblast physiology, COL1A2, is up-regulated by ATP stimulation of HOBIT cells in a timescale that is compatible with that of Egr-1 activation.

High-level expression of Egr-1 and Egr-1-inducible genes in mouse and human atherosclerosis

To understand the mRNA transcript profile in the human atherosclerotic lesion, RNA was prepared from the fibrous cap versus adjacent media of 13 patients undergoing carotid endarterectomy. cDNA expression arrays bearing 588 known genes indicated that lesions express unexpectedly high levels of the early growth response gene, Egr-1 (NGFI-A), a zinc-finger transcription factor that modulates a cluster of stress-responsive genes including PDGF and TGF-beta. Expression of Egr-1 was an average of 5-fold higher in the lesion than in the adjacent media, a result confirmed by RT-PCR, and many Egr-1-inducible genes were also strongly elevated in the lesion. Time-course analyses revealed that Egr-1 was not induced ex vivo. Immunocytochemistry indicated that Egr-1 was expressed prominently in the smooth muscle-actin positive cells, particularly in areas of macrophage infiltration, and in other cell types, including endothelial cells. Induction of atherosclerosis in LDL receptor-null mice by feeding them a high-fat diet resulted in a progressive increase in Egr-1 expression in the aorta. Thus, induction of Egr-1 by atherogenic factors may be a key step in coordinating the cellular events that result in vascular lesions.

Analysis of prolactin-modulated gene expression profiles during the Nb2 cell cycle using differential screening techniques

BACKGROUND

Rat Nb2-11C lymphoma cells are dependent on prolactin for proliferation and are widely used to study prolactin signaling pathways. To investigate the role of this hormone in the transcriptional mechanisms that underlie prolactin-stimulated mitogenesis, five different techniques were used to isolate differentially expressed transcripts: mRNA differential display, representational difference analysis (RDA), subtractive suppressive hybridization (SSH), analysis of weakly expressed candidate genes, and differential screening of an organized library.

RESULTS

About 70 transcripts were found to be modulated in Nb2 cells following prolactin treatment. Of these, approximately 20 represent unknown genes. All cDNAs were characterized by northern blot analysis and categorized on the basis of their expression profiles and the functions of the known genes. We compared our data with other cell-cycle-regulated transcripts and found several new potential signaling molecules that may be involved in Nb2 cell growth. In addition, abnormalities in the expression patterns of several transcripts were detected in Nb2 cells, including the constitutive expression of the immediate-early gene EGR-1. Finally, we compared the differential screening techniques in terms of sensitivity, efficiency and occurrence of false positives.

CONCLUSIONS

Using these techniques to determine which genes are differentially expressed in Nb2 lymphoma cells, we have obtained valuable insight into the potential functions of some of these genes in the cell cycle. Although this information is preliminary, comparison with other eukaryotic models of cell-cycle progression enables identification of expression abnormalities and proteins potentially involved in signal transduction, which could indicate new directions for research.

Estrogen activates raf-1 kinase and induces expression of Egr-1 in MCF-7 breast cancer cells

We have investigated whether the raf-1 kinase, a downstream mediator of both receptor tyrosine kinase and protein kinase C signalling, is activated by estrogen (E2) in an estrogen receptor positive human breast cancer cell line. Autophosphorylation of raf-1 kinase was studied after treatment of MCF-7 cells with E2. E2-deprived cells contained low levels of raf-1 kinase activity. Treatment of cells for 1 min with E2 resulted in raf-1 autophosphorylation which was maximal within 5 min. Western blot analysis showed that raf-1 undergoes an electrophoretic mobility shift following E2 treatment. Egr-1 is a zinc finger-containing transcription factor which is expressed in association with raf-1 activation. Untreated MCF-7 cells expressed low levels of Egr-1 while E2 treatment resulted in an induction of egr-1 mRNA expression. These kinetics followed closely behind the E2 induction of c-myc mRNA. Egr-1 protein was similarly low in E2-deprived MCF-7 cells and was transiently increased following E2 treatment. Several studies have suggested that kinase activity may play a role in estrogen receptor (ER) activation. While activated v-raf failed to augment ER activation of transcription in transient transfection assays, a dominant negative mutant of raf-1 inhibited E2-induced transcription by 50% primarily as a result of increased baseline levels of E2 independent transcription. The results show that E2 can induce raf-1 kinase activity in MCF-7 breast cancer cells associated with the expression of an early growth response gene and modulation of ER signalling.

Human thymidine kinase can functionally replace herpes simplex virus type 1 thymidine kinase for viral replication in mouse sensory ganglia and reactivation from latency upon explant

Herpes simplex virus type 1 thymidine kinase exhibits a strikingly broad substrate specificity. It is capable of phosphorylating deoxythymidine and deoxyuridine as does human thymidine kinase, deoxycytidine as does human deoxycytidine kinase, the cytosolic kinase whose amino acid sequence it most closely resembles, and thymidylate as does human thymidylate kinase. Following peripheral inoculation of mice, viral thymidine kinase is ordinarily required for viral replication in ganglia and for reactivation from latency following ganglionic explant. To determine which activity of the viral kinase is important for replication and reactivation in mouse ganglia, recombinant viruses lacking viral thymidine kinase but expressing individual human kinases were constructed. Each recombinant virus expressed the appropriate kinase activity with early kinetics following infection of cultured cells. The virus expressing human thymidine kinase exhibited thymidine phosphorylation activity equivalent to approximately 5% of that of wild-type virus in a quantitative plaque autoradiography assay. Nevertheless, it was competent for ganglionic replication and reactivation following corneal inoculation of mice. The virus expressing human thymidylate kinase was partially competent for these activities despite failing to express detectable thymidine kinase activity. The virus expressing human deoxycytidine kinase failed to replicate acutely in neurons or to reactivate from latency. Therefore, it appears that low levels of thymidine phosphorylation suffice to fulfill the role of the viral enzyme in ganglia and that this role can be partially fulfilled by thymidylate kinase activity alone.

Cyclosporin A-sensitive transcription factor Egr-3 regulates Fas ligand expression

Activation-induced transcriptional upregulation of the ligand for Fas (FasL) and the resulting apoptosis of Fas-bearing cells constitute essential steps in a host of normal and pathological processes. Here we describe an activation-inducible cis-acting regulatory element in the fasL promoter that is required for gene expression. Oligonucleotide competition and antibody supershift analyses identified two activation-induced DNA-binding species: Egr-1 (NGFI-A, krox-24, zif268, TIS-8), a transcription factor that has been implicated in growth, differentiation, and apoptosis; and Egr-3 (PILOT), a transcription factor of no previously known function. Activation-induced expression of Egr-3, like that of FasL, was inhibited by cyclosporin A, whereas expression of Egr-1 was unaffected. Transient expression of Egr-3 alone increased fasL promoter activity in a cyclosporin A-insensitive manner, whereas expression of Egr-1 had little effect. Moreover, endogenous fasL mRNA was induced in nonlymphoid cells by forced expression of Egr-3 in the absence of any other stimulus. These studies identify a critical Egr family-binding site in the fasL promoter and demonstrate that activation-induced Egr-3, but not Egr-1, directly upregulates fasL transcription in response to activating stimuli.

[Egr-1 transcription factor regulates the growth of glomerular mesangium cells]

BACKGROUND

The transcriptional regulator Early growth response gene-1 (Egr-1) is rapidly and transiently induced by various mitogens in cultured rat mesangial cells (MCs).

METHOD AND RESULTS

Here we show Egr-1 induction in an in vivo model of mesangioproliferative glomerulonephritis (GN). A 14.9-fold increase in Egr-1 mRNA was observed 6 days after disease induction. A concomitant increase in Egr-1 protein was demonstrated by immunocytochemistry. Egr-1 was mainly localized to the nuclei of cells in mesangial localization. To test whether Egr-1 directly regulated MC proliferation, we preincubated cultured MCs with antisense oligonucleotides directed against Egr-1. The platelet-derived growth factor (PDGF)-induced increase in Egr-1 mRNA and protein levels was inhibited by 75% and 74%, respectively. At the same time Egr-1 antisense oligonucleotides dose-dependently inhibited MC-proliferation as determined by thymidine-uptake by up to 75%. Control oligonucleotides were without effects on Egr-1 mRNA, protein or MC growth.

CONCLUSION

We conclude that Egr-1 induction is a necessary step in the mitogenic signaling cascade in glomerular MCs.

Activation of mitogen-activated protein kinase couples neurotensin receptor stimulation to induction of the primary response gene Krox-24

Neurotensin (NT) is a neuropeptide that is important in a variety of biological processes such as signal transduction and cell growth. NT effects are mediated by a single class of cell-surface receptors, known as neurotensin receptors (NTRs), which exhibit structural features of the G-protein-coupled receptors superfamily. We investigated NTR signalling properties with Chinese hamster ovary (CHO) cells stably transformed with human NTR (hNTR). First, we showed that NTR stimulation by NT induced the activation of the mitogen-activated protein kinases (MAPKs) in time- and dose-dependent manners. Both p42 and p44 MAPK isoforms were retarded in gel-shift assays, which was consistent with their activation by phosphorylation. In addition we showed that NT caused a prolonged activation of MAPK as measured by in-gel kinase assay. Secondly, we demonstrated that NT induced the expression of the growth-related gene Krox-24 at the protein level, as assessed by Western-blot analysis, and at the transcriptional level, as demonstrated in CHO cells transfected with hNTR and a reporter gene for Krox-24. Activation of MAPK and induction of Krox-24 were both prevented by the NTR antagonist SR 48692, confirming the specific action on NTR. Furthermore we observed coupling of NTR to a mitogenic pathway and Krox-24 induction in the human adenocarcinoma cell line HT29, which naturally expresses NTRs. Considering coupling pathways between NTR stimulation and MAPK activation, we observed a partial inhibition by pertussis toxin (PTX) and a complete blockade by the protein kinase C (PKC) inhibitor GF 109203X. Taken together, these results suggest that (1) stimulation of NTR activates the MAPK pathway by mechanisms involving dual coupling to both PTX-sensitive and PTX-insensitive G-proteins as well as PKC activation, and (2) these effects are associated with the induction of Krox-24, which might be a target of MAPK effector.

NAB2, a corepressor of NGFI-A (Egr-1) and Krox20, is induced by proliferative and differentiative stimuli

Previous work had identified a corepressor, NAB1, which represses transcriptional activation mediated by NGFI-A (also known as Egr-1, zif268, and Krox24) and Krox20. These zinc finger transcription factors are encoded by immediate-early genes and have been implicated in a wide variety of proliferative and differentiative processes. We have isolated and characterized another corepressor, NAB2, which is highly related to NAB1 within two discrete domains. The first conserved domain of NAB2 mediates an interaction with the R1 domain of NGFI-A. NAB2 represses the activity of both NGFI-A and Krox20, and its expression is regulated by some of the same stimuli that induce NGFI-A expression, including serum stimulation of fibroblasts and nerve growth factor stimulation of PC12 cells. The human NAB2 gene has been localized to chromosome 12ql3.3-14.1, a region that is rearranged in several solid tumors, lipomas, uterine leiomyomata, and liposarcomas. Sequencing of the Caenorhabditis elegans genome has identified a gene that bears high homology to both NAB1 and NAB2, suggesting that NAB molecules fulfill an evolutionarily conserved role.

Sp1 binds two sites in the CD11c promoter in vivo specifically in myeloid cells and cooperates with AP1 to activate transcription

The leukocyte integrin gene, CD11c, is transcriptionally regulated and is expressed predominantly on differentiated cells of the myelomonocytic lineage. In this study we have demonstrated that the regions -72 to -63 and -132 to -104 of the CD11c promoter contain elements responsible for phorbol ester-induced differentiation of the myeloid cell line HL60. DNase I footprinting analysis revealed that these regions can bind purified Sp1, and supershift analysis with Sp1 antibody confirmed that Sp1 in HL60 nuclear extracts could bind these regions. Transfection analysis of CD11c promoter-chloramphenicol acetyltransferase constructs containing deletions of these Sp1-binding sites revealed that these sites are essential for expression of the CD11c gene in HL60 cells but not in the T-cell line Molt4 or the cervical carcinoma cell line HeLa. Moreover, cotransfection of pPacSp1 along with these CD11c promoter-chloramphenicol acetyltransferase constructs into Sp1-deficient Drosophila Schneider 2 cells verified that these sites are essential for Sp1-dependent expression of the CD11c promoter. In vivo genomic footprinting revealed that Sp1 contacts the CD11c promoter within the regions -69 to -63 and -116 to -105 in phorbol 12-myristate 13-acetate-differentiated HL60 cells but not in undifferentiated HL60 cells or in Molt4 or HeLa cells. Cotransfection assays in HL60 cells revealed that Sp1 acts synergistically with Ap1 to activate CD11c. Further, both Sp1 sites are capable of cooperating with AP1. In vitro DNase I footprinting analysis with purified Sp1 and c-jun proteins showed that Sp1 binding could facilitate binding of c-jun. We propose that myeloid-specific expression of the CD11c promoter and is facilitated by cooperative interaction between the Sp1- and Ap1-binding sites.

Functional activation of the egr-1 (early growth response-1) gene by hydrogen peroxide

The redox-based regulation of gene expression is one of the fundamental mechanisms of cellular functions, and hydrogen peroxide seems to act as an intracellular second messenger of signal transduction of cytokines. Hydrogen peroxide at non-toxic doses induced the accumulation of mRNA for the early growth response-1 (egr-1) gene in mouse osteoblastic cells. The Egr-1 protein is a transcription factor that binds the GCGGGGGCG sequence and contains a zinc-finger structure that is essential for DNA binding. Egr-1 protein is sensitive to oxidative stress and loses specific DNA-binding activity when exposed to high levels of oxidative stress. Incubating cells with hydrogen peroxide at about 50 microM, however, increased the accumulation of Egr-1 protein, and the Egr-1 product seemed to be functional, judging by its binding activity to the GCGGGGGCG sequence and its ability to activate the chloramphenicol acetyltransferase reporter gene under the control of the human thymidine kinase enhancer containing the Egr-1 binding sequence. It was reported that the activity of Egr-1 protein as a transcription factor was negatively regulated by active oxygens. However, with appropriate concentrations of active oxygen, its capacity to bind a specific DNA sequence and to enhance the transcriptional activity of target genes is thought to be elevated.

Role of EGR1 in regulation of stimulus-dependent CD44 transcription in B lymphocytes

The immediate-early gene egr-1 encodes a transcription factor (EGR1) that links B-cell antigen receptor (BCR) signals to downstream activation events through the regulation of previously unidentified target genes. Here we identify the gene encoding the lymphocyte homing and migration protein CD44 as a target of EGR1 regulation in B cells. BCR-induced increases in CD44 mRNA expression and transcription levels are shown to occur in EGR1-expressing but not in nonexpressing subclones of the B-cell line WEHI-231. Kinetics of egr-1 transcription and the appearance of nuclear EGR1 protein precede CD44 induction and occur within 30 min after stimulation in the EGR1-expressing subclone. A single EGR1 binding motif is demonstrated at bp -301 of the human CD44 promoter. Cotransfection of a CD44 promoter-chloramphenicol acetyltransferase reporter construct with an egr-1 expression vector resulted in a 6.5- to 8.5-fold induction of transcriptional activity relative to an empty expression vector. The EGR1 binding motif was shown to be necessary for stimulus-induced expression of a CD44 promoter-chloramphenicol acetyltransferase reporter construct in nontransformed B lymphocytes and was required for transactivation by an EGR1 expression vector in a B-cell line. These studies identify EGR1 as an intermediary linking BCR-derived signals to the induction of CD44. The relevance of these molecular events to BCR signal transduction and antigen-stimulated B-cell-mediated immune responses is discussed.

Transcriptional regulation of the Icam-1 gene in antigen receptor- and phorbol ester-stimulated B lymphocytes: role for transcription factor EGR1

Intercellular adhesion molecule (ICAM) 1/CD54 plays an important role in T cell dependent B cell activation and for function of B lymphocytes as antigen-presenting cells. ICAM-1 expression is upregulated as a consequence of B lymphocyte antigen receptor (BCR) signaling, thereby serving to render antigen-stimulated B cells more receptive to T cell-mediated costimulatory signals. We have investigated BCR-induced expression of the Icam-1 gene in primary B cells and B cell lines and have found it to be dependent on BCR-induced expression of the transcription factor EGR1. Icam-1 transcription, induced by BCR cross-linking or bypassing the BCR with phorbol ester, is absent in a B cell line in which the EGR1-encoding gene (egr-1) is methylated and not expressed. A potential EGR1-binding site was located at -701 bp upstream of the murine Icam-1 gene transcription start site and shown by electrophoretic mobility shift assay to bind to murine EGR1. Mutation of this site in the context of 1.1 kb of the Icam-1 promoter significantly abrogated transcriptional induction by phorbol ester and anti-mu stimulation in primary B cells. A direct effect of EGR1 on the Icam-1 promoter is suggested by the ability of EGR1 expressed from an SV40-driven expression vector transactivate the wild-type Icam-1 promoter, whereas mutation of the EGR1 mutation of the EGR1 binding motif at -701 bp markedly compromises this induction. These data identify EGR1 as a signaling intermediate in BCR-stimulated B cell functional responses, specifically linking BCR signal transduction to induction of the Icam-1 gene. Furthermore, similar findings for BCR-induced CD44 gene induction (Maltzman, J.S., J.A. Carman, and J.G. Monroe. 1996. Role of EGR1 in regulation of stimulus-dependent CD44 transcription in B lymphocytes. Mol. Cell. Biol. In press) suggest that EGR1 may be an important signaling molecule for regulating levels of migration and adhesion molecules during humoral immune responses.

A Tat-induced auto-up-regulatory loop for superactivation of the human immunodeficiency virus type 1 promoter

The virus-encoded Tat protein strongly activates transcription of human immunodeficiency virus (HIV). A well-recognized mechanism involves interaction of Tat with the nascent RNA transcript of the viral tar gene; mutation of tar greatly decreases activation by Tat. However, Tat still provides a low level of activation, demonstrating that it also has a tar-independent mode of action. We propose that this tar-independent mode of Tat action is through activation of gene transcription to produce tumor necrosis factor alpha. This cytokine and other compounds that activate NF-kappa B up-regulate the HIV promoter at a low level, similarly to the second Tat action. Through this mechanism, they also activate promoters of tumor necrosis factor alpha and other cytokines and thereby establish an auto-up-regulatory loop. Activated NF-kappa B motifs in the HIV promoter synergize with Tat/tar. Mutations of these motifs decrease activation by Tat to a few percent of the wild-type value. In cooperation, the two modes of activation by Tat (tar dependent and cytokine based) set up positive up-regulatory loops which greatly superactivate transcription of HIV. Agents that block these synergistic pathways at three different steps and are more inhibitory in combination than is any one alone have been found. Thereby, multidrug modalities for transcription of HIV are proposed for virus suppression.

12-O-tetradecanoylphorbol-13-acetate activation of the MDR1 promoter is mediated by EGR1

P-glycoprotein, the product of the MDR1 gene (multidrug resistance gene 1), is an energy-dependent efflux pump associated with treatment failure in some hematopoietic malignancies. Its expression is regulated during normal hematopoietic differentiation, although its function in normal hematopoietic cells is unknown. To identify cellular factors that regulate the expression of MDR1 in hematopoietic cells, we characterized the cis- and trans-acting factors mediating 12-O-tetradecanoylphorbol-13-acetate (TPA) activation of the MDR1 promoter in K562 cells. Transient-transfection assays demonstrated that an MDR1 promoter construct containing nucleotides -69 to +20 conferred a TPA response equal to that of a construct containing nucleotides -434 to +105. TPA induced EGR1 binding to the -69/+20 promoter sequences over a time course which correlated with increased MDR1 promoter activity and increased steady-state MDR1 RNA levels. The -69/+20 promoter region contains an overlapping SP1/EGR site. The TPA-responsive element was localized to the overlapping SP1/EGR site by using a synthetic reporter construct. A mutation in this site that inhibited EGR protein binding blocked the -69/+20 MDR1 promoter response to TPA. The expression of a dominant negative EGR protein also blocked the TPA response of the -69/+20 promoter construct. Finally, the expression of EGR1 was sufficient to activate a construct containing tandem MDR1 promoter SP1/EGR sites. These data suggest a role for EGR1 in modulating MDR1 promoter activity in hematopoietic cells.

Role of EGR-1 in thapsigargin-inducible apoptosis in the melanoma cell line A375-C6

Induction of apoptosis by diverse exogenous signals is dependent on elevation of intracellular Ca2+. This process of cell death can be blocked by actinomycin D, indicating that it requires gene transcription events. To identify genes that are required for apoptosis, we used thapsigargin (TG), which inhibits endoplasmic reticulum-dependent Ca(2+)-ATPase and thereby increases cytosolic Ca2+. Exposure to TG led to induction of the zinc finger transcription factor, EGR-1, and apoptosis in human melanoma cells, A375-C6. To determine the functional relevance of EGR-1 expression in TG-inducible apoptosis, we employed a dominant negative mutant which functionally competes with EGR-1 in these cells. Interestingly, the dominant negative mutant inhibited TG-inducible apoptosis. Consistent with this observation, an antisense oligomer directed against Egr-1 also led to a diminution of the number of cells that undergo TG-inducible apoptosis. These results suggest a novel regulatory role for EGR-1 in mediating apoptosis that is induced by intracellular Ca2+ elevation. We have previously shown that in these melanoma cells, EGR-1 acts to inhibit the growth arresting action of interleukin-1. Together, these results imply that EGR-1 plays inducer-specific roles in growth control.

Quantification of DNA-protein interaction by UV crosslinking

Measurement of the affinity of a protein for a promoter sequence is critical when assessing its potential to regulate transcription. Here we report that the DNA protein crosslinking (DPC) assay can be used to measure affinity, amount and molecular weight of DNA binding proteins to specific and non-specific DNA sequences. By applying a theoretical analysis to evaluate the binding data, it was shown that the affinity constants of two proteins (named DPC80 and DPC107) to the MT3 region of the mouse thymidine kinase promoter were 2 x 10(-9) M, which is 10(4) times higher than to non-specific DNA. Similar affinity constants were found when the purified proteins corresponding to DPC80 and DPC107 instead of nuclear extracts were used to assess the reliability of the DPC assay. A value for crosslinking efficiency was determined as 0.07, however, it is not needed for computation of the DNA-protein affinity, but with it the abundance of a binding protein can be estimated. In summary, the DPC assay is useful for quantifying DNA binding proteins and thereby judging their influence on transcription.

Identification of NAB1, a repressor of NGFI-A- and Krox20-mediated transcription

NGFI-A (also called Egr1, Zif268, or Krox24) and the closely related proteins Krox20, NGFI-C, and Egr3 are zinc-finger transcription factors encoded by immediate-early genes which are induced by a wide variety of extracellular stimuli. NGFI-A has been implicated in cell proliferation, macrophage differentiation, synaptic activation, and long-term potentiation, whereas Krox20 is critical for proper hindbrain segmentation and peripheral nerve myelination. In previous work, a structure/function analysis of NGFI-A revealed a 34-aa inhibitory domain that was hypothesized to be the target of a cellular factor that represses NGFI-A transcriptional activity. Using the yeast two-hybrid system, we have isolated a cDNA clone which encodes a protein that interacts with this inhibitory domain and inhibits the ability of NGFI-A to activate transcription. This NGFI-A-binding protein, NAB1, is a 570-aa nuclear protein that bears no obvious sequence homology to known proteins. NAB1 also represses Krox20 activity, but it does not influence Egr3 or NGFI-G, thus providing a mechanism for the differential regulation of this family of immediate-early transcription factors.