Molecular characterization of the Xenopus CCAAT-enhancer binding protein beta gene promoter

Cross-talk between the transcription factor Sp1 and C/EBPβ modulates TGFβ1 production to negatively regulate the expression of chemokine RANTES

RANTES is a key chemokine for atherosclerosis, and obesity is associated with progression of atherosclerosis. Substance P (SP) increases glucose uptake and accumulation of lipids in adipocytes, and SP may upregulate RANTES expression. This study investigated the mechanism of RANTES expression by human M1 macrophages stimulated with SP.

SP upregulated RANTES protein expression, whereas aprepitant (an NK1R antagonist) blunted this response. Pretreatment of macrophages with BIRB796 (a combined p38γ/p38δ inhibitor) led to a significant decrease of RANTES expression. Next, we investigated the effect of several NK1R internalization factors on RANTES expression, including GRK2, β-arrestin 2, dynamin, ROCK, and TGFβ1. Exposure of macrophages to SP upregulated TGFβ1 expression. Silencing of β-arrestin 2 or GRK2 significantly enhanced the RANTES protein level after stimulation by SP, whereas TGFβ1/2/3 siRNA or dynasore (a dynamin inhibitor) decreased RANTES and Y-27632 (a ROCK inhibitor) had no effect. Surprisingly, silencing of transcription factor specificity protein 1 (Sp1) or inhibition of Sp1 activity by mithramycin led to significant upregulation of TGFβ1 protein and corresponding enhancement of RANTES expression (by ELISA or western blotting), whereas siRNA for C/EBPβ attenuated expression of both TGFβ1 and RANTES. Next, we investigated transcriptional cross-talk among Sp1 and C/EBPβ, TIF1β, or Fli-1 in relation to RANTES expression. Compared with TIF1β or Fli-1 siRNA, C/EBPβ siRNA showed significantly stronger inhibition of RANTES production by Sp1 siRNA-transfected macrophages after stimulation with SP.

In conclusion, transcription factor Sp1 engages in cross-talk with C/EBPβ and modulates TGFβ1 production to negatively regulate RANTES expression in macrophages stimulated with SP.

In conclusion, cross-talk between the transcription factor Sp1 and C/EBPβ modulates TGFβ1 production to negatively regulate expression of the atherogenic chemokine RANTES in SP-stimulated macrophages, while RANTES is upregulated by SP via the p38γδMAPK/C/EBPβ/TGFβ1 signaling pathway.

Cyclic AMP responsive element binding proteins are involved in 'emergency' granulopoiesis through the upregulation of CCAAT/enhancer binding protein β

In contrast to the definitive role of the transcription factor, CCAAT/Enhancer binding protein α (C/EBPα), in steady-state granulopoiesis, previous findings have suggested that granulopoiesis during emergency situations, such as infection, is dependent on C/EBPβ. In this study, a novel lentivirus-based reporter system was developed to elucidate the molecular switch required for C/EBPβ-dependency. The results demonstrated that two cyclic AMP responsive elements (CREs) in the proximal promoter region of C/EBPβ were involved in the positive regulation of C/EBPβ transcription during granulocyte-macrophage colony-stimulating factor (GM-CSF)–induced differentiation of bone marrow cells. In addition, the transcripts of CRE binding (CREB) family proteins were readily detected in hematopoietic stem/progenitor cells. CREB was upregulated, phosphorylated and bound to the CREs in response to GM-CSF stimulation. Retroviral transduction of a dominant negative CREB mutant reduced C/EBPβ mRNA levels and significantly impaired the proliferation/differentiation of granulocyte precursors, while a constitutively active form of CREB facilitated C/EBPβ transcription. These data suggest that CREB proteins are involved in the regulation of granulopoiesis via C/EBPβ upregulation.

Apoptosis signal-regulating kinase 1 in peptidoglycan-induced COX-2 expression in macrophages

In this study, we investigated the role of ASK1 in PGN-induced C/EBPbeta activation and COX-2 expression in RAW 264.7 macrophages. The PGN-induced COX-2 expression was attenuated by the DNs of ASK1, JNK1, JNK2, a JNK inhibitor (SP600125), and an AP-1 inhibitor (curcumin). PGN caused ASK1 dephosphorylation time-dependently at Ser967, dissociation from the ASK1-14-3-3 complex, and subsequent ASK1 activation. In addition, PGN activated PP2A and suppression of PP2A by okadaic acid markedly inhibited PGN-induced ASK1 Ser967 dephosphorylation and COX-2 expression. PGN induced the activation of the JNK-AP-1 signaling cascade downstream of ASK1. PGN-increased C/EBPbeta expression and DNA-binding activity were inhibited by the ASK1-JNK-AP-1 signaling blockade. COX-2 promoter luciferase activity induced by PGN was attenuated in cells transfected with the COX-2 reporter construct possessing the C/EBP-binding site mutation. In addition, the ASK1-JNK-AP-1-C/EBPbeta cascade was activated in human peripheral mononuclear cells exposure to PGN. The TLR2 agonist Pam(3)CSK(4) was also shown to induce ASK1 Ser967 dephosphorylation, JNK and c-jun phosphorylation, C/EBPbeta activation, and COX-2 expression in RAW 264.7 macrophages. PGN-induced COX-2 promoter luciferase activity was prevented by selective inhibition of TLR2 and c-Jun in RAW 264.7 macrophages. Our data demonstrate that PGN might activate the TLR2-mediated PP2A-ASK1-JNK-AP-1-C/EBPbeta cascade and subsequent COX-2 expression in RAW 264.7 macrophages.

Inhibition of collagen gene expression by interferon-gamma: novel role of the CCAAT/enhancer binding protein beta (C/EBPbeta)

By inhibiting collagen synthesis, interferon-gamma (IFN-gamma) plays a key role in maintaining connective tissue homeostasis, but the mechanisms are not well-understood. In addition to intracellular signaling through the canonical JAK-STAT transduction pathway, IFN-gamma was recently shown to regulate gene expression via the CCAAT/enhancer-binding protein beta (C/EBPbeta) as well. Because C/EBPbeta is a crucial mediator of immune and inflammatory responses, and has been implicated in regulation of collagen synthesis by tumor necrosis factor-alpha, we examined its role in the inhibitory effects of IFN-gamma. The results demonstrated that IFN-gamma caused increased C/EBPbeta expression in dermal fibroblasts and enhanced its binding to cognate DNA sequences in the alpha2(I) procollagen gene (COL1A2) promoter in vitro and in vivo. Disruption of C/EBP binding by deletion or site-directed mutagenesis abrogated the inhibition of collagen promoter activity in transient transfection assays, as did cotransfection with dominant negative C/EBPbeta, indicating a functional role of cellular C/EBPbeta in mediating the IFN-gamma response. Rapid phosphorylation of the ERK1/2 MAP kinases induced by IFN-gamma was accompanied by phosphorylation and nuclear translocation of cellular C/EBPbeta, and pretreatment of fibroblasts with ERK1/2 kinase inhibitor blocked C/EBPbeta phosphorylation, as well as inhibition of COL1A2 promoter activity, elicited by IFN-gamma. These results provide compelling evidence for a novel C/EBPbeta-dependent IFN-gamma signaling pathway responsible for inhibition of collagen gene transcription. Taken together with recent reports, the findings indicate that intracellular pathways mediating negative regulation of collagen synthesis in response to distinct inflammatory signals that converge on C/EBPbeta.

Upregulation of CCAAT/enhancer binding protein β in activated astrocytes and microglia

The transcription factor CCAAT/enhancer binding protein beta (C/EBPbeta) regulates the expression of key genes in inflammation but little is known about the involvement of C/EBPbeta in glial activation. In this report, we have studied the patterns of astroglial and microglial C/EBPbeta expression in primary mouse cortical cultures. We show that both astrocytes and microglia express C/EBPbeta in untreated mixed glial cultures. C/EBPbeta is upregulated when glial activation is induced by lipopolysaccharide (LPS). The LPS-induced upregulation of glial C/EBPbeta is rapid (2 h at mRNA level, 4 h at protein level). It is elicited by low concentrations of LPS (almost maximal effect at 1 ng/mL) and it is reversed by the protein synthesis inhibitor cycloheximide. C/EBPbeta nuclear levels increase both in astrocytes and microglia after LPS treatment, and the response is more marked in microglia. The LPS-induced increase in microglial C/EBPbeta is prevented by coadministration of the MAP kinase inhibitors SB203580 (p38 inhibitor) + SP600125 (JNK inhibitor) or SB203580 + U0126 (ERK inhibitor). Systemic injection of LPS also increases brain nuclear levels of C/EBPbeta as shown by Western blot, and this increase is localized in microglial cells as shown by double immunofluorescence, in the first report to our knowledge of C/EBPbeta expression in activated glial cells in vivo. These findings support a role for C/EBPbeta in the activation of astrocytes and, particularly, microglia. Given the nature of the C/EBPbeta-regulated genes, we hypothesize that this factor participates in neurotoxic effects associated with glial activation. (c) 2006 Wiley-Liss, Inc.

Short upstream region drives dynamic expression of hypoxia-inducible factor 1alpha during Xenopus development

Hypoxia-inducible factor 1alpha (HIF-1alpha) plays a central role in regulating oxygen-dependent gene expression and is involved in a range of pathways implicated in cellular survival, proliferation, and development. While the posttranslational regulation of HIF-1alpha is well characterized, the relative importance of its control at the transcriptional level during development remains less clear. Although the mouse and human promoter regions have been analyzed in vitro, to date, there has been no in vivo analysis of any vertebrate HIF-1alpha promoter. To investigate the transcriptional regulation of HIF-1alpha during development of the amphibian Xenopus laevis, we have described the gene's expression pattern and isolated the xHIF-1alpha upstream regulatory regions. We show xHIF-1alpha mRNA to be constitutively expressed at low levels throughout embryogenesis, but with significant up-regulation during gastrula stages, and subsequently, in specific regions of the central nervous system and axial tissues. Our functional analysis using a series of truncated xHIF-1alpha promoter constructs demonstrates that a 173-bp region of the proximal promoter, which is 100% conserved among five allelic variants, is sufficient to drive correct expression in transgenic embryos. Although these results are corroborated by a parallel set of in vitro transfection experiments in a Xenopus cell line, some key differences suggest the importance of using transgenic methods in conjunction with in vitro assays.

Activity-dependent hepatocyte growth factor expression and its role in organogenesis and cancer growth suppression

Studies by Murphy et al. have shown that neuronal stimulation can activate immediate early genes that code for transcription factors. Recent data suggest that Ca(2+) elevation in both neuronal cytoplasmic and nuclear compartments is responsible for the coupling of synaptic excitation to gene expression. Deisseroth et al. suggest that Ca(2+) influx through L-type voltage-sensitive Ca(2+) channels (VSCCs) activates cytoplasmic Ca(2+) targets such as calmodulin (CaM). The Ca(2+)-CaM complex then translocates to the nucleus leading to Ca(2+) and cAMP response element-binding protein (CREB) phosphorylation and gene expression. Reports have shown that L-type VSCCs are found on the vagus nerve. Other studies have suggested that activation of L-type VSCCs leads to a Ca(2+) store-dependent elevation of nuclear [Ca(2+)] that triggers gene expression by more direct activation of nuclear Ca(2+)/CaM-dependent protein kinase (CaMK). Moreover, nuclear transcription factors such as DREAM are themselves Ca(2+)-dependent, further supporting the importance of both nuclear and cytoplasmic Ca(2+) elevation in regulating gene expression. Our simulation studies suggest that intense synaptic stimulation in combination with amplification by release from intracellular Ca(2+) stores can produce elevations in nuclear Ca(2+) concentration and CaMK phosphorylation leading to CREB phosphorylation and gene expression. One of the downstream events would be the production of hepatocyte growth factor (HGF). HGF has trophic, repair, therapeutic or mitotic effect on kidney, pancreas, spleen, liver, lung, heart and spinal cord. These organs and systems' regeneration can be achieved by either upregulation of HGF release from the vagus nerve or upregulation of HGF production within the system (spinal cord). Conversely, inhibition of HGF release from the vagus nerve can inhibit cancer growth. Vagus nerve seems to be the nerve that nature intends to regulate organ growth and regeneration, it is very possible that other than HGF and injurin, other growth factors could be found in the vagus nerve. Electrical depolarization and hyperpolarization of the vagus nerve would be the most natural and effective way to induce organ regeneration and suppress cancer growth, respectively. A similar pathway seems to exist for different organs as HGF has trophic, repair, therapeutic or mitotic effect on different vagally innervated organs.

Dioxin increases C/EBPbeta transcription by activating cAMP/protein kinase A

The environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD = dioxin) has been shown to increase the expression of C/EBPbeta. The modulated expression of C/EBPbeta has been suggested to be associated with toxic responses of TCDD such as wasting syndrome, diabetes, and inhibition of adipocyte differentiation. This study focused on the regulatory mechanism of TCDD-mediated transcriptional activation of C/EBPbeta. Elevated C/EBPbeta mRNA and protein levels in mouse embryonic fibroblasts (C3H10T(1/2)) and in mouse hepatoma cells (Hepa1c1c7) were correlated with increased binding affinity of the C/EBPbeta protein. Transfection studies with different deletion constructs of the CCAAT/enhancer-binding protein promoter indicated that a small region located 60-120 bp upstream of the start site of transcription is required for activation of the C/EBPbeta gene by TCDD in both cell lines tested. Further analysis using mutation constructs of the C/EBPbeta promoter demonstrated that activation of the C/EBPbeta promoter is mediated through incomplete cAMP-response element-binding protein (CREB) sites located close to the TATA box of the C/EBPbeta gene. The protein kinase A (PKA) inhibitor H89 completely blocks the TCDD-dependent effect on C/EBPbeta promoter activity, indicating that TCDD activates CREB binding via a cAMP/PKA pathway, which is supported by the increased cAMP level and PKA activity observed after TCDD treatment. Gel shift analyses demonstrated that CREB itself binds to the putative CREB motif that mediates the TCDD-dependent effect on C/EBPbeta gene transcription. Cotransfection experiments with CREB and PKA expression plasmids further supported our conclusions that the TCDD-dependent effect on C/EBPbeta transcription is mediated via PKA-dependent CREB activation.

Interleukin-6 represses the transcription of the CCAAT/enhancer binding protein-alpha gene in hepatoma cells by inhibiting its ability to autoactivate the proximal promoter region

The cytokine interleukin-6 (IL-6) plays key roles in the immune and inflammatory responses, acute-phase reaction and hematopoiesis. Such biological actions of IL-6 are characterised by both the activation and the inhibition of gene transcription. Unfortunately, in contrast to gene activation, the mechanism by which IL-6 suppresses transcription remains largely unclear. We have, therefore, investigated this aspect using the Xenopus laevis CCAAT/enhancer binding protein-alpha (C/EBPalpha) gene promoter as a model. We show by transient transfection assays of various promoter-luciferase DNA constructs into hepatoma cells that a C/EBP recognition sequence in the proximal promoter region is essential for the IL-6-mediated repression. Electrophoretic mobility shift assays showed that C/EBPalpha was the major protein that bound to this site and, consistent with its expression pattern, the binding was reduced when the cells were exposed to IL-6. Co-transfection assays revealed for the first time that the ability of C/EBPalpha, but not C/EBPbeta or Sp1, to transactivate the promoter was decreased dramatically when the cells were incubated with IL-6. These studies, therefore, identify a novel mechanism for IL-6-mediated repression of gene transcription that involves a reduction in C/EBPalpha-mediated activation.

Interferon-gamma stimulates the expression of the inducible cAMP early repressor in macrophages through the activation of casein kinase 2. A potentially novel pathway for interferon-gamma-mediated inhibition of gene transcription

Interferon-gamma (IFN-gamma) is a pleiotropic cytokine that modulates the immune function, cell proliferation, apoptosis, macrophage activation, and numerous other cellular responses. These biological actions of IFN-gamma are characterized by both the activation and the inhibition of gene transcription. Unfortunately, in contrast to gene activation, the mechanisms through which the cytokine suppresses gene transcription remain largely unclear. We show here for the first time that exposure of macrophages to IFN-gamma leads to a dramatic induction in the expression of the inducible cAMP early repressor (ICER), a potent inhibitor of gene transcription. In addition, a synergistic action of IFN-gamma and calcium in the activation of ICER expression was identified. The IFN-gamma-mediated activation of ICER expression was not blocked by H89, bisindoylmaleimide, SB202190, PD98059, W7, and AG490, which inhibit protein kinase A, protein kinase C, p38 mitogen-activated protein kinase, extracellular signal-regulated kinase, calcium-calmodulin-dependent protein kinase, and Janus kinase-2, respectively. In contrast, apigenin, a selective casein kinase 2 (CK2) inhibitor, was found to inhibit response. Consistent with this finding, IFN-gamma stimulated CK2 activity and the level of phosphorylated cAMP response element-binding protein, which is known to induce ICER gene transcription, and this response was inhibited in the presence of apigenin. These studies, therefore, identify a previously uncharacterized pathway, involving the IFN-gamma-mediated stimulation of CK2 activity, activation of cAMP response element-binding protein, and increased production of ICER, which may then play an important role in the inhibition of macrophage gene transcription by this cytokine.

The role of C/EBPbeta in mammary gland development and breast cancer

The CCAAT/enhancer binding protein (C/EBP) family of bZIP transcription factors control the proliferation and differentiation of a variety of tissues. While C/EBPalpha and -delta are also expressed in the mammary gland, the multiple protein isoforms of C/EBPbeta appear to play a critical role in mammary gland development and breast cancer. Targeted deletion of all the C/EBPbeta isoforms results in a severe inhibition of lobuloalveolar development and a block to functional differentiation, as well as more subtle changes in ductal morphogenesis. The altered expression of a number of molecular markers, including the progesterone, estrogen, and prolactin receptors, the transporter proteins (NKCC1 and aquaporin 5), and several markers of skin differentiation (Sprr2A and keratin 6), suggests that germline deletion of C/EBPbeta results in an altered cell fate. Thus, C/EBPbeta appears to play a role in the specification of progenitor cell fate not only in the mammary gland, but also in a number of other tissues.

Potentiation of lipopolysaccharide-inducible cyclooxygenase 2 expression by C2-ceramide via c-Jun N-terminal kinase-mediated activation of CCAAT/enhancer binding protein beta in macrophages

Ceramide, formed by sphingomyelinase, is involved in the expression of cyclooxygenase-2 (COX-2). This study examines the effect of C2-ceramide (C2), a cell-permeable ceramide analog, on the lipopolysaccharide (LPS)-inducible COX-2 expression and signaling pathways. C2 did not induce COX-2 but potentiated LPS-inducible COX-2 expression in Raw264.7 cells, whereas dihydro-C2 was inactive. Treatment of cells with C2 notably increased LPS-inducible CCAAT/enhancer binding protein (C/EBP) DNA binding. Antibody supershift experiments revealed that LPS-induced C/EBP DNA binding activity depended on C/EBP beta and C/EBP delta but not C/EBP alpha, C/EBP epsilon or CBP/p300. C/EBP beta contributed to C2-enhanced DNA binding activity. 4-(4-Fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl) 1H-imidazole (SB203580), a p38 kinase inhibitor, completely inhibited LPS-inducible and C2-potentiated LPS-inducible COX-2 expression. Enhancement of LPS-inducible COX-2 expression and C/EBP DNA binding by C2 was abrogated in dominant-negative mutant of JNK1 [JNK1(-)] cells. 2'-Amino-3'-methoxyflavone (PD98059) or stable transfection with dominant-negative mutant of MKK1 decreased COX-2 induction by LPS but failed to inhibit C2-enhanced LPS induction of COX-2. Transfection with dominant-negative mutant of C/EBP inhibited the ability of C2 to potentiate the induction of COX-2 by LPS. In LPS-treated cells, C2 enhanced both the nuclear translocation and the expression of LPS-inducible C/EBP beta with an increase in AP-1 DNA binding activity. These enhancements were abolished by JNK1(-) transfection. AP-1 decoy oligonucleotide suppressed C2-potentiated C/EBP beta expression, indicating that AP-1 was responsible for C2-mediated C/EBP beta expression. These results demonstrate that C2 increases C/EBP beta-mediated COX-2 induction by LPS and that the pathway of JNK1 but not ERK1/2 is responsible for C/EBP beta activation involving activator protein-1-mediated enhanced C/EBP beta expression.

CCAAT/enhancer-binding proteins: structure, function and regulation

CCAAT/enhancer binding proteins (C/EBPs) are a family of transcription factors that all contain a highly conserved, basic-leucine zipper domain at the C-terminus that is involved in dimerization and DNA binding. At least six members of the family have been isolated and characterized to date (C/EBP alpha[bond]C/EBP zeta), with further diversity produced by the generation of different sized polypeptides, predominantly by differential use of translation initiation sites, and extensive protein-protein interactions both within the family and with other transcription factors. The function of the C/EBPs has recently been investigated by a number of approaches, including studies on mice that lack specific members, and has identified pivotal roles of the family in the control of cellular proliferation and differentiation, metabolism, inflammation and numerous other responses, particularly in hepatocytes, adipocytes and haematopoietic cells. The expression of the C/EBPs is regulated at multiple levels during several physiological and pathophysiological conditions through the action of a range of factors, including hormones, mitogens, cytokines, nutrients and certain toxins. The mechanisms through which the C/EBP members are regulated during such conditions have also been the focus of several recent studies and have revealed an immense complexity with the potential existence of cell/tissue- and species-specific differences. This review deals with the structure, biological function and the regulation of the C/EBP family.

A novel role of Sp1 and Sp3 in the interferon-gamma -mediated suppression of macrophage lipoprotein lipase gene transcription

The regulation of macrophage lipoprotein lipase by cytokines is of potentially crucial importance in the pathogenesis of atherosclerosis. We have shown previously that macrophage lipoprotein lipase expression is suppressed by interferon-gamma (IFN-gamma) at the transcriptional level. We investigated the regulatory sequence elements and the transcription factors that are involved in this response. We demonstrated that the -31/+187 sequence contains the minimal IFN-gamma-responsive elements. Electrophoretic mobility shift assays showed that the binding of proteins to two regions in the -31/+187 sequence was reduced dramatically when the cells were exposed to IFN-gamma. Both competition electrophoretic mobility shift assays and antibody supershift assays showed that the interacting proteins were composed of Sp1 and Sp3. Mutations of the Sp1/Sp3-binding sites in the minimal IFN-gamma-responsive elements abolished the IFN-gamma-mediated suppression of promoter activity, whereas multimers of the sequence were able to impart the response to a heterologous promoter. Western blot analysis showed that IFN-gamma reduced the steady state levels of Sp3 protein. In contrast, the cytokine decreased the DNA binding activity of Sp1 without affecting the protein levels. These studies therefore reveal a novel mechanism for IFN-gamma-mediated regulation of macrophage gene transcription.

Involvement of CCAAT/enhancer-binding protein in regulation of the rat serine:pyruvate/alanine:glyoxylate aminotransferase gene expression

In the rat liver, transcription of the serine:pyruvate/alanine:glyoxylate aminotransferase (SPT/AGT) gene occurs from two sites, +1 and +66, in exon 1, resulting in the formation of two mRNAs, one for a precursor of mitochondrial SPT/AGT and the other for peroxisomal SPT/AGT, respectively. In this study, we attempted to characterize the downstream promoter responsible for generation of peroxisomal SPT/AGT. The minimal downstream promoter was confined to the +21-+90 region. We demonstrated that C/EBPalpha and C/EBPbeta bound around the downstream start site (+66) contribute to the promoter activity. The downstream promoter activity is also regulated positively by a short inverted repeat, located 20-30 bp upstream of the downstream start site, through a protein factor(s) bound to this region. On the other hand, the sequence just downstream of the start site may negatively regulate the promoter activity.