CCAAT/enhancer-binding protein (C/EBP) beta is acetylated at multiple lysines: acetylation of C/EBPbeta at lysine 39 modulates its ability to activate transcription

Multiple muscle wasting-related transcription factors are acetylated in dexamethasone-treated muscle cells

Recent studies suggest that the expression and activity of the histone acetyltransferase p300 are upregulated in catabolic muscle allowing for acetylation of cellular proteins. The function of transcription factors is influenced by posttranslational modifications, including acetylation. It is not known if transcription factors involved in the regulation of muscle mass are acetylated in atrophying muscle. We determined cellular levels of acetylated C/EBPβ, C/EBPδ, FOXO1, FOXO3a, and NF-kB/p65 in dexamethasone-treated L6 muscle cells, a commonly used in vitro model of muscle wasting. The role of p300 in dexamethasone-induced transcription factor acetylation and myotube atrophy was examined by transfecting muscle cells with p300 siRNA. Treatment of L6 myotubes with dexamethasone resulted in increased cellular levels of acetylated C/EBPβ and δ, FOXO1 and 3a, and p65. Downregulation of p300 with p300 siRNA reduced acetylation of transcription factors and decreased dexamethasone-induced myotube atrophy and expression of the ubiquitin ligase MuRF1. The results suggest that several muscle wasting-related transcription factors are acetylated supporting the concept that posttranslational modifications of proteins regulating gene transcription may be involved in the loss of muscle mass. The results also suggest that acetylation of the transcription factors is at least in part regulated by p300 and plays a role in glucocorticoid-induced muscle atrophy. Targeting molecules that regulate acetylation of transcription factors may help reduce the impact of muscle wasting.

C/EBPβ regulates dexamethasone-induced muscle cell atrophy and expression of atrogin-1 and MuRF1

Muscle wasting in catabolic patients is in part mediated by glucocorticoids and is associated with increased expression and activity of the transcription factor C/EBPβ. It is not known, however, if C/EBPβ is causally linked to glucocorticoid-induced muscle atrophy. We used dexamethasone-treated L6 myoblasts and myotubes to test the role of C/EBPβ in glucocorticoid-induced expression of the muscle-specific ubiquitin ligases atrogin-1 and MuRF1, protein degradation, and muscle atrophy by transfecting cells with C/EBPβ siRNA. In myoblasts, silencing C/EBPβ expression with siRNA inhibited dexamethasone-induced increase in protein degradation, atrogin-1 and MuRF1 expression, and muscle cell atrophy. Similar effects of C/EBPβ siRNA were seen in myotubes except that the dexamethasone-induced increase in MuRF1 expression was not affected by C/EBPβ siRNA in myotubes. In additional experiments, overexpressing C/EBPβ did not influence atrogin-1 or MuRF1 expression in myoblasts or myotubes. Taken together, our observations suggest that glucocorticoid-induced muscle wasting is at least in part regulated by C/EBPβ. Increased C/EBPβ expression alone, however, is not sufficient to upregulate atrogin-1 and MuRF1 expression.

The histone acetyltransferase p300 promotes intrinsic axonal regeneration

Axonal regeneration and related functional recovery following axonal injury in the adult central nervous system are extremely limited, due to a lack of neuronal intrinsic competence and the presence of extrinsic inhibitory signals. As opposed to what occurs during nervous system development, a weak proregenerative gene expression programme contributes to the limited intrinsic capacity of adult injured central nervous system axons to regenerate. Here we show, in an optic nerve crush model of axonal injury, that adenoviral (cytomegalovirus promoter) overexpression of the acetyltransferase p300, which is regulated during retinal ganglion cell maturation and repressed in the adult, can promote axonal regeneration of the optic nerve beyond 0.5 mm. p300 acetylates histone H3 and the proregenerative transcription factors p53 and CCAAT-enhancer binding proteins in retinal ganglia cells. In addition, it directly occupies and acetylates the promoters of the growth-associated protein-43, coronin 1 b and Sprr1a and drives the gene expression programme of several regeneration-associated genes. On the contrary, overall increase in cellular acetylation using the histone deacetylase inhibitor trichostatin A, enhances retinal ganglion cell survival but not axonal regeneration after optic nerve crush. Therefore, p300 targets both the epigenome and transcription to unlock a post-injury silent gene expression programme that would support axonal regeneration.

Liver-enriched inhibitory protein (LIP) actively inhibits preadipocyte differentiation through histone deacetylase 1 (HDAC1)

The CCAAT/enhancer-binding protein β (C/EBPβ) is expressed as three isoforms (LAP*, liver-enriched activating protein (LAP), and liver-enriched inhibitory protein (LIP)) that differentially regulate gene expression. The interplay between LAP*, LAP, and LIP in regulating cellular processes is largely unknown, and LIP has been largely regarded to repress transcription through a passive heterodimerization-dependent mechanism. Recently, we have shown that p300/GCN5 and mSin3A/HDAC1 differentially regulate the ability of C/EBPβ to stimulate preadipocyte differentiation through activation of C/ebpα transcription. Here, we have mapped requirements for binding of mSin3A/HDAC1 to LAP/LAP* and LIP to a 4-amino acid motif in the central region of LAP/LAP* (residues 153-156) and the N terminus of LIP. Reducing mSin3A/HDAC1 binding to LAP/LAP* and LIP through deletion of this motif reduced the recruitment of HDAC1 to the C/ebpα promoter and increased preadipocyte differentiation stimulated by insulin and 1-methyl-3-isobutylxanthine. Additional studies showed that the interaction of HDAC1 with LIP provides for active repression of C/ebpα transcription and is largely responsible for the ability of LIP and HDAC1 to repress preadipocyte differentiation. Thus, although mSin3A/HDAC1 interacted readily with LAP/LAP* in addition to LIP and that expression of LAP/LAP* was sufficient to recruit HDAC1 to the C/ebpα promoter, mutations in C/ebpβ that abrogated HDAC1 association to LAP/LAP* in the absence of LIP provided no additional stimulation of differentiation or transcription beyond the deletion of LIP alone. The implication of these results for the interaction between p300/GCN5 and mSin3A/HDAC1 in regulating C/EBPα transcription and preadipocyte differentiation are discussed.

C/EBPβ mediates growth hormone-regulated expression of multiple target genes

Regulation of c-Fos transcription by GH is mediated by CCAAT/enhancer binding protein β (C/EBPβ). This study examines the role of C/EBPβ in mediating GH activation of other early response genes, including Cyr61, Btg2, Socs3, Zfp36, and Socs1. C/EBPβ depletion using short hairpin RNA impaired responsiveness of these genes to GH, as seen for c-Fos. Rescue with wild-type C/EBPβ led to GH-dependent recruitment of the coactivator p300 to the c-Fos promoter. In contrast, rescue with C/EBPβ mutated at the ERK phosphorylation site at T188 failed to induce GH-dependent recruitment of p300, indicating that ERK-mediated phosphorylation of C/EBPβ at T188 is required for GH-induced recruitment of p300 to c-Fos. GH also induced the occupancy of phosphorylated C/EBPβ and p300 on Cyr61, Btg2, and Socs3 at predicted C/EBP-cAMP response element-binding protein motifs in their promoters. Consistent with a role for ERKs in GH-induced expression of these genes, treatment with U0126 to block ERK phosphorylation inhibited their GH-induced expression. In contrast, GH-dependent expression of Zfp36 and Socs1 was not inhibited by U0126. Thus, induction of multiple early response genes by GH in 3T3-F442A cells is mediated by C/EBPβ. A subset of these genes is regulated similarly to c-Fos, through a mechanism involving GH-stimulated ERK 1/2 activation, phosphorylation of C/EBPβ, and recruitment of p300. Overall, these studies suggest that C/EBPβ, like the signal transducer and activator of transcription proteins, regulates multiple genes in response to GH.

Acetylation-deacetylation of the transcription factor Nrf2 (nuclear factor erythroid 2-related factor 2) regulates its transcriptional activity and nucleocytoplasmic localization

Activation of Nrf2 by covalent modifications that release it from its inhibitor protein Keap1 has been extensively documented. In contrast, covalent modifications that may regulate its action after its release from Keap1 have received little attention. Here we show that CREB-binding protein induced acetylation of Nrf2, increased binding of Nrf2 to its cognate response element in a target gene promoter, and increased Nrf2-dependent transcription from target gene promoters. Heterologous sirtuin 1 (SIRT1) decreased acetylation of Nrf2 as well as Nrf2-dependent gene transcription, and its effects were overridden by dominant negative SIRT1 (SIRT1-H355A). The SIRT1-selective inhibitors EX-527 and nicotinamide stimulated Nrf2-dependent gene transcription, whereas resveratrol, a putative activator of SIRT1, was inhibitory, mimicking the effect of SIRT1. Mutating lysine to alanine or to arginine at Lys(588) and Lys(591) of Nrf2 resulted in decreased Nrf2-dependent gene transcription and abrogated the transcription-activating effect of CREB-binding protein. Furthermore, SIRT1 had no effect on transcription induced by these mutants, indicating that these sites are acetylation sites. Microscope imaging of GFP-Nrf2 in HepG2 cells as well as immunoblotting for Nrf2 showed that acetylation conditions resulted in increased nuclear localization of Nrf2, whereas deacetylation conditions enhanced its cytoplasmic rather than its nuclear localization. We posit that Nrf2 in the nucleus undergoes acetylation, resulting in binding, with basic-region leucine zipper protein(s), to the antioxidant response element and consequently in gene transcription, whereas deacetylation disengages it from the antioxidant response element, thereby resulting in transcriptional termination and subsequently in its nuclear export.

Sirtuins and inflammation: Friends or foes?

Lysine acetylation/deacetylation has been recognized as an important posttranslational modification regulating numerous cellular processes. Sirtuins represent novel players in these complex regulatory circuits. These NAD-dependent lysine-deacetylases have attracted much interest based on their role in the regulation of lifespan in lower organisms, and their capacity to interfere with cell growth, proliferation and survival in response to stress. Their absolute requirement for NAD suggests that these enzymes may represent an important molecular link between metabolism and several human disorders such as diabetes and cancer. More recently, the identification of several transcription factors known to play a role in the immune system as sirtuin substrates has suggested that this family of enzymes may also play an important role in the regulation of inflammation, a pathological situation with clear links to metabolism and aging in humans. We review herein the possible links between nuclear sirtuins and the regulation of an immune response, and discuss the possible strategies that may lead to the development of novel therapeutic approaches to treat inflammation by targeting sirtuin activity.

Preservation of hepatocellular functionality in cultures of primary rat hepatocytes upon exposure to 4-Me2N-BAVAH, a hydroxamate-based HDAC-inhibitor

Great efforts are being put in the development/optimization of reliable and highly predictive models for high-throughput screening of efficacy and toxicity of promising drug candidates. The use of primary hepatocyte cultures, however, is still limited by the occurrence of phenotypic alterations, including loss of xenobiotic biotransformation capacity. In the present study, the differentiation-stabilizing effect of a new histone deacetylase inhibitor 5-(4-dimethylaminobenzoyl)-aminovaleric acid hydroxamide (4-Me(2)N-BAVAH), a structural Trichostatin A (TSA)-analogue with a more favourable pharmaco-toxicological profile, was studied at a genome-wide scale by means of microarray analysis. Several genes coding for xenobiotic biotransformation enzymes were found to be positively regulated upon exposure to 4-Me(2)N-BAVAH. For CYP1A1/2B1/3A2, these observations were confirmed by qRT-PCR and immunoblot analysis. In addition, significantly higher 7-ethoxyresorufin-O-deethylase and 7-pentoxyresorufin-O-dealkylase activity levels were measured. These effects were accompanied by an increased expression of CCAAT/enhancer binding protein alpha and hepatic nuclear factor (HNF)4α, but not of HNF1α. Finally, 4-Me(2)N-BAVAH was found to induce histone H3 acetylation at the proximal promoter of the albumin, CYP1A1 and CYP2B1 genes, suggesting that chromatin remodelling is directly involved in the transcriptional regulation of these genes. In conclusion, histone deacetylase inhibitors prove to be efficient agents for better maintaining a differentiated hepatic phenotype in rat hepatocyte cultures.

Genetic analysis of nitric oxide synthase 1 variants in schizophrenia and bipolar disorder

Nitric oxide (NO) is a neurotransmitter that acts as a second messenger of the N-methyl-D-aspartate receptor and interacts with the dopaminergic and the serotonergic systems. NO involvement in pathological processes relevant to neuropsychiatric disorders stems from its ability to modulate certain forms of synaptic plasticity, and from its capacity to be transformed to a highly active free radical. Additionally, multiple links have been reported between the NO-producing enzyme, nitric oxide synthase (NOS) 1, and both schizophrenia and bipolar disorder (BPD). RNA and DNA isolated from dorsolateral-prefrontal cortices of schizophrenia patients, bipolar patients and controls (n = 26, 30 and 29, respectively) were donated by the Stanley Foundation Brain Collection. Gene expression was measured by Real-Time-PCR. Genetic polymorphisms were genotyped by restriction-fragment length-polymorphism analysis, and by product-size determination of PCR products amplified with a fluorescent primer.Expression analysis of pan-NOS1, as well as of 2 of its isoforms, "NOS1_1d" and "NOS1_1f", which differ in their first exons and translational strength, revealed a trend for pan-NOS1 over-expression (P = 0.075) in schizophrenia patients (1.33-fold), and significant over-expression (P < 0.05) of NOS1_1d and NOS1_1f in this group (1.54-fold and 1.61-fold, respectively). No expressional alteration was observed in BPD. Polymorphisms at the promoters of NOS1_1d and NOS1_1f, previously shown to be functional in vitro, revealed no significant allelic or genotypic differences among clinical groups and showed no effect on these transcripts' expression. In conclusion, understanding the molecular mechanisms underlying the over-expression of specific NOS1 isoforms, which is unique to schizophrenia, may assist in identifying targets for new drugs.

A novel mechanism involving coordinated regulation of nuclear levels and acetylation of NF-YA and Bcl6 activates RGS4 transcription

Neuronally enriched RGS4 plays a critical role attenuating G protein signaling in brain, although the mechanisms regulating RGS4 expression are unknown. Here we describe a novel mechanism for transcriptional activation of RGS4 in neuron-like PC6 cells, where RGS4 is markedly induced during confluence-induced growth arrest. Transcriptional activation of RGS4 in confluent PC6 cells was accompanied by impaired G(i/o)-dependent MAPK activation. In the human RGS4 gene promoter, we identified three phylogenetically conserved cis-elements: an inverted CCAAT box element (ICE), a cAMP response element, and a B-cell lymphoma 6 (Bcl6)-binding site. The ICE and the cAMP response element mediate activation, and the Bcl6 site mediates repression of RGS4 transcription. Activation of RGS4 transcription in confluent PC6 cells is accompanied by increases in NF-YA and C/EBPβ and decreases in Bcl6 levels in the nucleus. Increases in NF-YA and C/EBPβ lead to their increased binding to the RGS4 promoter in vivo, and dominant negative forms of these proteins repressed RGS4 promoter activity. Acetylation of NF-YA and Bcl6 were increased in postconfluent cells. Trichostatin A stimulation of RGS4 promoter activity, accompanied by increased binding of NF-YA and decreased binding of Bcl6 to the promoter, was abolished by mutation of the ICE and enhanced by mutation of the Bcl6 site. These findings demonstrate a dynamic and coordinated regulation of nuclear levels and acetylation status of trans-acting factors critical in determining the off/on state of the RGS4 promoter.

Sepsis and glucocorticoids upregulate p300 and downregulate HDAC6 expression and activity in skeletal muscle

Muscle wasting during sepsis is in part regulated by glucocorticoids. In recent studies, treatment of cultured muscle cells in vitro with dexamethasone upregulated expression and activity of p300, a histone acetyl transferase (HAT), and reduced expression and activity of the histone deacetylases-3 (HDAC3) and -6, changes that favor hyperacetylation. Here, we tested the hypothesis that sepsis and glucocorticoids regulate p300 and HDAC3 and -6 in skeletal muscle in vivo. Because sepsis-induced metabolic changes are particularly pronounced in white, fast-twitch skeletal muscle, most experiments were performed in extensor digitorum longus muscles. Sepsis in rats upregulated p300 mRNA and protein levels, stimulated HAT activity, and reduced HDAC6 expression and HDAC activity. The sepsis-induced changes in p300 and HDAC expression were prevented by the glucocorticoid receptor antagonist RU38486. Treatment of rats with dexamethasone increased expression of p300 and HAT activity, reduced expression of HDAC3 and -6, and inhibited HDAC activity. Finally, treatment with the HDAC inhibitor trichostatin A resulted in increased muscle proteolysis and expression of the ubiquitin ligase atrogin-1. Taken together, our results suggest for the first time that sepsis-induced muscle wasting may be regulated by glucocorticoid-dependent hyperacetylation caused by increased p300 and reduced HDAC expression and activity. The recent development of pharmacological HDAC activators may provide a novel avenue to prevent and treat muscle wasting in sepsis and other catabolic conditions.

CCAAT/enhancer binding protein beta2 is involved in growth hormone-regulated insulin-like growth factor-II gene expression in the liver of rainbow trout (Oncorhynchus mykiss)

Previously, we showed that levels of different CCAAT/enhancer binding protein (C/EBP) mRNAs in the liver of rainbow trout were modulated by GH and suggested that C/EBPs might be involved in GH-induced IGF-II gene expression. As a step toward further investigation, we have developed monospecific polyclonal antibodies to detect rainbow trout C/EBPalpha, -beta1, -beta2, and -delta2 isoform proteins. Injection of GH into adult rainbow trout resulted in a significant increase of C/EBPbeta1, C/EBPbeta2, and C/EBPdelta2 proteins in the liver. Chromatin immunoprecipitation analysis revealed that C/EBPbeta2 binds to multiple sites at the 5' promoter/regulatory region, introns, and the 3' untranslated region of the IGF-II gene. GH treatment reduced C/EBPbeta2 binding to several of these regions at 6 h after injection. The decreased occupancy of C/EBPbeta2 coincided well with an increase of histone H4 acetylation at the proximal promoter and elevation of the IGF-II mRNA level. Immunoblotting analysis showed that C/EBPbeta2 existed predominately as a truncated form in the liver, and cotransfection analysis further showed that the truncated C/EBPbeta2 acted as a negative regulator on IGF-II proximal promoter. GH treatment caused deacetylation of C/EBPbeta2 in the liver. In addition, we observed a GH-dependent interaction of C/EBPbeta2 with a complex involving histone H1. All together, these results suggest that C/EBPbeta2 was regulated at multiple levels by GH, and C/EBPbeta2 may play a suppressive role in mediating GH-induced IGF-II expression in the liver of rainbow trout.

Protein acetylation in synaptic plasticity and memory

Posttranslational modifications of proteins regulate various processes in the cells. The seminal role of phosphorylation in synaptic plasticity and memory has been established using several different model systems. Recently, an important role for another posttranslational modification, acetylation, particularly of histones, has emerged in these processes. This review focuses on the role of activity-dependent protein acetylation in synaptic plasticity and memory.

The importance of being flexible: the case of basic region leucine zipper transcriptional regulators

Large volumes of protein sequence and structure data acquired by proteomic studies led to the development of computational bioinformatic techniques that made possible the functional annotation and structural characterization of proteins based on their primary structure. It has become evident from genome-wide analyses that many proteins in eukaryotic cells are either completely disordered or contain long unstructured regions that are crucial for their biological functions. The content of disorder increases with evolution indicating a possibly important role of disorder in the regulation of cellular systems. Transcription factors are no exception and several proteins of this class have recently been characterized as premolten/molten globules. Yet, mammalian cells rely on these proteins to control expression of their 30,000 or so genes. Basic region:leucine zipper (bZIP) DNA-binding proteins constitute a major class of eukaryotic transcriptional regulators. This review discusses how conformational flexibility "built" into the amino acid sequence allows bZIP proteins to interact with a large number of diverse molecular partners and to accomplish their manifold cellular tasks in a strictly regulated and coordinated manner.

Daxx is a transcriptional repressor of CCAAT/enhancer-binding protein beta

CCAAT/enhancer-binding Protein beta (C/EBPbeta) is a member of the bZIP transcription factor family that is expressed in various tissues, including cells of the hematopoietic system. C/EBPbeta is involved in tissue-specific gene expression and thereby takes part in fundamental cellular processes such as proliferation and differentiation. Here, we show that the activity of C/EBPbeta is negatively regulated by the transcriptional co-repressor Daxx. C/EBPbeta was found to directly interact with Daxx after overexpression as well as on the endogenous level. Glutathione S-transferase pulldown assays showed that Daxx binds via amino acids 190-400 to the C-terminal part of C/EBPbeta. Co-expression of C/EBPbeta changed the sub-nuclear Daxx distribution pattern from predominantly POD-localized to nucleoplasmic. Daxx suppressed basal and p300-enhanced transcriptional activity of C/EBPbeta. Furthermore, Daxx decreased the C/EBPbeta-dependent phosphorylation of p300, which in turn was associated with a diminished level of p300-mediated C/EBPbeta acetylation. Co-expression of promyelocytic leukemia protein abrogated the repressive effect of Daxx on C/EBPbeta as well as the direct interaction of Daxx and C/EBPbeta, presumably by re-recruiting Daxx to PML-oncogenic domains. In acute promyelocytic leukemia (APL) cells, C/EBPbeta activity is known to be required for all-trans-retinoic acid-induced cell differentiation and disease remission. We show that all-trans-retinoic acid as well as arsenic trioxide treatment leads to a reduced C/EBPbeta fraction associated with Daxx suggesting a relief of Daxx-dependent C/EBPbeta repression as an important molecular event leading to APL cell differentiation. Overall, our data identify Daxx as a new negative regulator of C/EBPbeta and provide first clues for a link between abrogation of Daxx-C/EBPbeta complex formation and APL remission.

Activation of SIRT1 by resveratrol represses transcription of the gene for the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) by deacetylating hepatic nuclear factor 4alpha

The SIRT1 activators isonicotinamide (IsoNAM), resveratrol, fisetin, and butein repressed transcription of the gene for the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (PEPCK-C). An evolutionarily conserved binding site for hepatic nuclear factor (HNF) 4alpha (-272/-252) was identified, which was required for transcriptional repression of the PEPCK-C gene promoter caused by these compounds. This site contains an overlapping AP-1 binding site and is adjacent to the C/EBP binding element (-248/-234); the latter is necessary for hepatic transcription of PEPCK-C. AP-1 competed with HNF4alpha for binding to this site and also decreased HNF4alpha stimulation of transcription from the PEPCK-C gene promoter. Chromatin immunoprecipitation experiments demonstrated that HNF4alpha and AP-1, but not C/EBPbeta, reciprocally bound to this site prior to and after treating HepG2 cells with IsoNAM. IsoNAM treatment resulted in deacetylation of HNF4alpha, which decreased its binding affinity to the PEPCK-C gene promoter. In HNF4alpha-null Chinese hamster ovary cells, IsoNAM and resveratrol failed to repress transcription from the PEPCK-C gene promoter; overexpression of HNF4alpha in Chinese hamster ovary cells re-established transcriptional inhibition. Exogenous SIRT1 expression repressed transcription, whereas knockdown of SIRT1 by RNA interference reversed this effect. IsoNAM decreased the level of mRNA for PEPCK-C but had no effect on mRNA for glucose-6-phosphatase in AML12 mouse hepatocytes. We conclude that SIRT1 activation inhibited transcription of the gene for PEPCK-C in part by deacetylation of HNF4alpha. However, SIRT1 deacetylation of other key regulatory proteins that control PEPCK-C gene transcription also likely contributed to the inhibitory effect.

Computational and functional analysis of growth hormone (GH)-regulated genes identifies the transcriptional repressor B-cell lymphoma 6 (Bc16) as a participant in GH-regulated transcription

For insight into transcriptional mechanisms mediating physiological responses to GH, data mining was performed on a profile of GH-regulated genes induced or inhibited at different times in highly responsive 3T3-F442A adipocytes. Gene set enrichment analysis indicated that GH-regulated genes are enriched in pathways including phosphoinositide and insulin signaling and suggested that suppressor of cytokine signaling 2 (SOCS2) and phosphoinositide 3' kinase regulatory subunit p85alpha (Pik3r1) are important targets. Model-based Chinese restaurant clustering identified a group of genes highly regulated by GH at times consistent with its key physiological actions. This cluster included IGF-I, phosphoinositide 3' kinase p85alpha, SOCS2, and cytokine-inducible SH2-containing protein. It also contains the most strongly repressed gene in the profile, B cell lymphoma 6 (Bcl6), a transcriptional repressor. Quantitative real-time PCR verified the strong decrease in Bcl6 mRNA after GH treatment and induction of the other genes in the cluster. Transcriptional network analysis of the genes implicated signal transducer and activator of transcription (Stat) 5 as hub regulating the most responsive genes, Igf1, Socs2, Cish, and Bcl6. Transcriptional activation analysis demonstrated that Bcl6 inhibits SOCS2-luciferase and blunts its stimulation by GH. Occupancy of endogenous Bcl6 on SOCS2 DNA decreased after GH treatment, whereas occupancy of Stat5 increased concomitantly. Thus, GH-mediated inhibition of Bcl6 expression may reverse the repression of SOCS2 and facilitate SOCS2 activation by GH. Together these analyses identify Bcl6 as a participant in GH-regulated gene expression and suggest an interplay between the repressor Bcl6 and the activator Stat5 in regulating genes, which contribute to GH responses.

C/EBPalpha expression is partially regulated by C/EBPbeta in response to DNA damage and C/EBPalpha-deficient fibroblasts display an impaired G1 checkpoint

We observed that C/EBPα is highly inducible in primary fibroblasts by DNA damaging agents that induce strand breaks, alkylate and crosslink DNA as well as those that produce bulky DNA lesions. Fibroblasts deficient in C/EBPα (C/EBPα^-/-) display an impaired G₁ checkpoint as evidenced by inappropriate entry into S-phase in response to DNA damage and these cells also display an enhanced G₁ to S transition in response to mitogens. The induction of C/EBPα by DNA damage in fibroblasts does not require p53. EMSA analysis of nuclear extracts prepared from UVB- and MNNG-treated fibroblasts revealed increased binding of C/EBPβ to a C/EBP consensus sequence and ChIP analysis revealed increased C/EBPβ binding to the C/EBPα promoter. To determine whether C/EBPβ has a role in the regulation of C/EBPα we treated C/EBPβ^-/- fibroblasts with UVB or MNNG. We observed C/EBPα induction was impaired in both UVB- and MNNG- treated C/EBPβ^-/- fibroblasts. Our study reveals a novel role for C/EBPβ in the regulation of C/EBPα in response to DNA damage and provides definitive genetic evidence that C/EBPα has a critical role in the DNA damage G₁ checkpoint.

Role of epigenetics in liver-specific gene transcription, hepatocyte differentiation and stem cell reprogrammation

Controlling both growth and differentiation of stem cells and their differentiated somatic progeny is a challenge in numerous fields, from preclinical drug development to clinical therapy. Recently, new insights into the underlying molecular mechanisms have unveiled key regulatory roles of epigenetic marks driving cellular pluripotency, differentiation and self-renewal/proliferation. Indeed, the transcription of genes, governing cell-fate decisions during development and maintenance of a cell's differentiated status in adult life, critically depends on the chromatin accessibility of transcription factors to genomic regulatory and coding regions. In this review, we discuss the epigenetic control of (liver-specific) gene-transcription and the intricate interplay between chromatin modulation, including histone (de)acetylation and DNA (de)methylation, and liver-enriched transcription factors. Special attention is paid to their role in directing hepatic differentiation of primary hepatocytes and stem cells in vitro.

Misexpression of CCAAT/enhancer binding protein beta causes osteopenia

CCAAT/enhancer binding proteins (C/EBPs) are expressed by osteoblasts and adipocytes during differentiation. C/EBP beta is critical for adipogenesis; however, its role in osteoblastogenesis is unclear, and its function in the postnatal skeleton is not known. To study C/EBP beta in osteoblasts in vivo, we created transgenic mice expressing full length C/EBP beta under the control of a 3.8 kb fragment of the human osteocalcin promoter. Two transgenic lines were established in a friend leukemia virus strain B genetic background, and compared with wild type littermate controls. Both C/EBP beta transgenic lines exhibited osteopenia, with a 30% decrease in bone volume, due to a decrease in trabecular number. The number of osteoblasts and osteoclasts per bone perimeter was not changed. Bone marrow stromal cells from C/EBP beta transgenics showed reduced mineralization, and reduced alkaline phosphatase mRNA levels. Calvarial osteoblasts from C/EBP beta transgenics displayed reduced alkaline phosphatase activity. To determine the consequences of the Cebpb deletion in vivo, the phenotype of Cebpb null mice was compared with that of wild type controls of identical genetic composition. Cebpb null mice exhibited reduced weight, body fat, and bone mineral density, and decreased bone volume, due to a decrease in trabecular number. The number of osteoblasts and osteoclasts per bone perimeter was not changed. C/EBP beta downregulation by RNA interference in calvarial osteoblasts had no effect on osteoblast differentiation/function. The phenotype of the Cebpb inactivation may be secondary to systemic indirect effects, and to direct effects of C/EBP beta in osteoblasts. In conclusion, C/EBP beta plays a role in mesenchymal cell differentiation and its misexpression in vivo causes osteopenia.

CCAAT/enhancer-binding protein beta: its role in breast cancer and associations with receptor tyrosine kinases

The CCAAT/enhancer-binding proteins (C/EBPs) are a family of leucine-zipper transcription factors that regulate gene expression to control cellular proliferation, differentiation, inflammation and metabolism. Encoded by an intronless gene, C/EBPbeta is expressed as several distinct protein isoforms (LAP1, LAP2, LIP) whose expression is regulated by the differential use of several in-frame translation start sites. LAP1 and LAP2 are transcriptional activators and are associated with differentiation, whereas LIP is frequently elevated in proliferative tissue and acts as a dominant-negative inhibitor of transcription. However, emerging evidence suggests that LIP can serve as a transcriptional activator in some cellular contexts, and that LAP1 and LAP2 might also have unique actions. The LIP:LAP ratio is crucial for the maintenance of normal growth and development, and increases in this ratio lead to aggressive forms of breast cancer. This review discusses the regulation of C/EBPbeta activity by post-translational modification, the individual actions of LAP1, LAP2 and LIP, and the functions and downstream targets that are unique to each isoform. The role of the C/EBPbeta isoforms in breast cancer is discussed and emphasis is placed on their interactions with receptor tyrosine kinases.

Transcriptional regulation of the human growth hormone receptor (hGHR) gene V2 promoter by transcriptional activators and repressor

The V2 transcript is the major ubiquitously expressed human GH receptor (hGHR) mRNA in all tissues examined to date. In a previous investigation, we defined the V2 promoter as TATA-less and exhibiting many characteristics of a housekeeping gene promoter. We also demonstrated that its basal activity is determined by several different cis-regulatory regions within both the promoter and the V2 exon. In the present study, we used luciferase-reporter, site-directed mutagenesis, gel shift, chromatin immunoprecipitation, and quantitative RT-PCR assays to investigate the ability of certain transcription factors to regulate hGHR V2 transcription through these regions in mammalian cells, including human adipocytes. Ets1 was found to transactivate the V2 proximal promoter through specific Ets sites. Two CCAAT/enhancer-binding protein (C/EBP) family members [C/EBP-homologous protein (CHOP) and C/EBPbeta] enhanced V2 transcription via different pathways: indirectly, by association with a V2 exon region (CHOP), and directly, using a V2 proximal promoter noncanonical binding site (C/EBPbeta). The Notch signaling mediator, Hes1, potently suppressed V2 promoter activity through interaction with two Hes sites within the V2 exon. We propose that these transcriptional factors regulate hGHR V2 expression by acting as downstream nuclear effectors, linking specific signaling cascades (e.g. MAPK and Notch) triggered by different growth factor-, development-, and nutrition- as well as stress-related stimuli. Our data also suggest that these factors are likely to be important in the differentiation-induced increase in V2 mRNA expression in adipocytes, with Ets1 and CHOP functioning at the preadipocyte stage to prepare the cells for differentiation and increasing C/EBPs and decreasing Hes1 levels contributing during adipocyte maturation.

Transcription factors Sp1 and C/EBP regulate NRAMP1 gene expression

The natural resistance-associated macrophage protein 1 (Nramp1), which belongs to a conserved family of membrane metal transporters, contributes to phagocyte-autonomous antimicrobial defense mechanisms. Genetic polymorphisms in the human NRAMP1 gene predispose to susceptibility to infectious or inflammatory diseases. To characterize the transcriptional mechanisms controlling NRAMP1 expression, we previously showed that a 263 bp region upstream of the ATG drives basal promoter activity, and that a 325 bp region further upstream confers myeloid specificity and activation during differentiation of HL-60 cells induced by vitamin D. Herein, the major transcription start site was mapped in the basal region by S1 protection assay, and two cis-acting elements essential for myeloid transactivation were characterized by in vitro DNase footprinting, electrophoretic mobility shift experiments, in vivo transfection assays using linker-mutated constructs, and chromatin immunoprecipitation assays in differentiated monocytic cells. One distal cis element binds Sp1 and is required for NRAMP1 myeloid regulation. Another site in the proximal region binds CCAAT enhancer binding proteins alpha or beta and is crucial for transcription. This study implicates Sp1 and C/EBP factors in regulating the expression of the NRAMP1 gene in myeloid cells.

G9a-mediated lysine methylation alters the function of CCAAT/enhancer-binding protein-beta

The functional capacity of the transcriptional regulatory CCAAT/enhancer-binding protein-beta (C/EBPbeta) is governed by protein interactions and post-translational protein modifications. In a proteome-wide interaction screen, the histone-lysine N-methyltransferase, H3 lysine 9-specific 3 (G9a), was found to directly interact with the C/EBPbeta transactivation domain (TAD). Binding between G9a and C/EBPbeta was confirmed by glutathione S-transferase pulldown and co-immunoprecipitation. Metabolic labeling showed that C/EBPbeta is post-translationally modified by methylation in vivo. A conserved lysine residue in the C/EBPbeta TAD served as a substrate for G9a-mediated methylation. G9a, but not a methyltransferase-defective G9a mutant, abrogated the transactivation potential of wild type C/EBPbeta. A C/EBPbeta TAD mutant that contained a lysine-to-alanine exchange was resistant to G9a-mediated inhibition. Moreover, the same mutation conferred super-activation of a chromatin-embedded, endogenous C/EBPbeta target gene. Our data identify C/EBPbeta as a direct substrate of G9a-mediated post-translational modification that alters the functional properties of C/EBPbeta during gene regulation.

Acetylation and deacetylation regulate CCAAT/enhancer binding protein beta at K39 in mediating gene transcription

The transcription factor CCAAT/enhancer binding protein beta (C/EBPbeta) contains multiple acetylation sites, including lysine (K) 39. Mutation of C/EBPbeta at K39, an acetylation site in the transcriptional activation domain, impairs transcription of C/EBPbeta target genes in a dominant-negative fashion. Further, K39 of C/EBPbeta can be deacetylated by HDAC1, and HDAC1 may decrease C/EBPbeta-mediated transcription, suggesting that acetylation of C/EBPbeta at K39 is dynamically regulated in mediating gene transcription. Acetylation of endogenous C/EBPbeta at K39 is detected in adipose tissue, and also occurs in 3T3-L1 cells undergoing adipocyte conversion. In addition, mutation of K39 in C/EBPbeta impairs activation of its target genes encoding C/EBPalpha and PPARgamma, essential mediators of adipogenesis, as well as adipocyte genes for leptin and Glut4. These findings suggest that acetylation of C/EBPbeta at K39 is an important and dynamic regulatory event that contributes to its ability to transactivate target genes, including those associated with adipogenesis and adipocyte function.

Transcriptional and post-transcriptional mechanisms for lysophosphatidic acid-induced cyclooxygenase-2 expression in ovarian cancer cells

Emerging evidence suggests that lysophosphatidic acid (LPA) is a physiological regulator of cyclooxygenase-2 (Cox-2) expression. Herein we used ovarian cancer cells as a model to investigate the molecular mechanisms that link the LPA G protein-coupled receptors (GPCRs) to Cox-2 expression. LPA stimulated Cox-2 expression and release of prostaglandins though the LPA(1), LPA(2), and LPA(5) receptors. The effect of LPA involves both transcriptional activation and post-transcriptional enhancement of Cox-2 mRNA stability. The consensus sites for C/EBP in the Cox-2 promoter were essential for transcriptional activation of Cox-2 by LPA. The NF-kappaB and AP-1 transcription factors commonly involved in inducible Cox-2 expression were dispensable. Dominant-negative C/EPBbeta inhibited LPA activation of the Cox-2 promoter and expression. Furthermore, LPA stimulated C/EBPbeta phosphorylation and activity through a novel mechanism integrating GPCR signals and a permissive activity from a receptor tyrosine kinase (RTK). This role of RTK was not consistent with LPA activation of C/EBP through transactivation of RTK, as full activation of RTKs with their own agonists only weakly stimulated C/EBP. In addition to the transcriptional activation, the RNA stabilization protein HuR bound to and protected Cox-2 mRNA in LPA-stimulated cells, indicating an active role for HuR in sustaining Cox-2 induction during physiological responses.

Ubiquitination, phosphorylation, and acetylation--triple threat in muscle wasting

Loss of muscle mass is commonly seen in patients with critical illness and is associated with increased expression of multiple genes controlling protein breakdown. Transcription factors that are activated during muscle wasting include NF-kB and members of the FOXO and C/EBP transcription factor families. The activity of these transcription factors is regulated by multiple posttranslational modifications, including ubiquitination, phosphorylation, and acetylation, providing for a complex and integrated network of regulatory mechanisms in muscle wasting. Targeting posttranslational modifications of transcription factors may prove important in the prevention and treatment of the debilitating consequences of muscle wasting.

Transgenic mice expressing an inhibitory truncated form of p300 exhibit long-term memory deficits

The formation of many forms of long-term memory requires several molecular mechanisms including regulation of gene expression. The mechanisms directing transcription require not only activation of individual transcription factors but also recruitment of transcriptional coactivators. CBP and p300 are transcriptional coactivators that interact with a large number of transcription factors and regulate transcription through multiple mechanisms, including an intrinsic histone acetyltransferase (HAT) activity. HAT activity mediates acetylation of lysine residues on the amino-terminal tails of histone proteins, thereby increasing DNA accessibility for transcription factors to activate gene expression. CBP has been shown to play an important role in long-term memory formation. We have investigated whether p300 is also required for certain forms of memory. p300 shares a high degree of homology with CBP and has been shown to interact with transcription factors known to be critical for long-term memory formation. Here we demonstrate that conditional transgenic mice expressing an inhibitory truncated form of p300 (p300Delta1), which lacks the carboxy-terminal HAT and activation domains, have impaired long-term recognition memory and contextual fear memory. Thus, our study demonstrates that p300 is required for certain forms of memory and that the HAT and carboxy-terminal domains play a critical role.