DEFINITION
CCAAT-enhancer-binding proteins are a family of transcription factors, composed of six members called from C/EBP α to C/EBP ζ.
THE GENE
CEBPB
The protein encoded by this intronless gene is a bZIP transcription factor which can bind as a homodimer to certain DNA regulatory regions. It can also form heterodimers with the related proteins CEBP-alpha, CEBP-delta, and CEBP-gamma. The encoded protein is important in the regulation of genes involved in immune and inflammatory responses and has been shown to bind to the IL-1 response element in the IL-6 gene, as well as to regulatory regions of several acute-phase and cytokine genes. In addition, the encoded protein can bind the promoter and upstream element and stimulate the expression of the collagen type I gene.
CHEMICAL STRUCTURE AND IMAGES
When relevant for the function
- Primary structure
- Secondary structure
- Tertiary structure
- Quaternary structure
Protein Aminoacids Percentage of LAP and LIP
SYNTHESIS AND TURNOVER
mRNA synthesis
protein synthesis
post-translational modifications
degradation
CELLULAR FUNCTIONS
cellular localization,
biological function
C/EBPα and C/EBPβ transcripts harbor small upstream open reading frames (uORF) in their mRNAs that sense the activity of translation initiation factors and relay initiation to alternative in-frame start sites. As a result, truncated proteins that lack part of the N-terminal sequences are generated. Besides C/EBPs, many transcripts of key regulatory genes involved in growth, differentiation, and proliferation harbor uORFs, suggesting an important role of uORF-mediated translational control in mammalian development and physiology. Nevertheless, genetic models in mammals are lacking to explore the physiological relevance of uORF-regulated translation initiation (from Valérie Bégay, Jeske J. Smink, Klaus Wethmar, Berlin)
first AA LAP: methionine
first AA LIP: methionine (third AUG)
Descombes and Schibler described two rat C/EBPβ proteins which either activated the albumin promoter (liver-activating protein [LAP]) or repressed the albumin promoter (liver-enriched inhibitory protein [LIP]) ((20)). Both were transcribed from the same mRNA by using in-frame AUGs. Both proteins share the 145 COOH-terminal amino acids that contain the basic DNA-binding domain and the leucine zipper dimerization helix. LAP uses the first and second AUG start site and LIP the third, resulting in only LAP having the transcriptional activation domain ((20)). Consequently, LAP is a longer protein (∼39 kD) than LIP (∼16 kD). However, LIP has higher binding affinity for its DNA cognate sequences, resulting in its ability to attenuate the transcriptional stimulation of LAP in substoichiometric amounts. Small changes in the LIP to LAP ratio resulted in large differences in transcription, with promoter repression occurring at ratios >0.2 ((20), (21)). LIP is also induced in mouse liver by LPS ((48)). Our data suggest that Kupffer cells may contribute to the increase in LIP observed in the liver after LPS treatment.
Type I Interferon Induces Inhibitory 16-kD CCAAT/ Enhancer Binding Protein (C/EBP)β, Repressing the HIV-1 Long Terminal Repeat in Macrophages: Pulmonary Tuberculosis Alters C/EBP Expression, Enhancing HIV-1 Replication 1998
C/EBPβ proteins which either activated the albumin promoter (liver-activating protein [LAP]) or repressed the albumin promoter (liver-enriched inhibitory protein [LIP], truncated form)
- Proliferation Truncated C/EBP isoforms sustain proliferation, whereas full-length forms are inhibitors of cell division.Previously, we showed that anaplastic large cell lymphoma and Hodgkin Lymphoma express predominantly truncated C/EBPβ. Rapamycin, an antibiotic that inhibits mTOR signaling, shuts down the truncated C/EBP isoform and concomitantly inhibits growth in both types of lymphomas. Ectopic expression of truncated C/EBPβ restored proliferation, suggesting that truncated C/EBPβ represents a translationally controlled oncogene.
- Ostoporosis. Altogether, the data showed that differential regulation of MafB gene expression by C/EBPβ isoforms determines the balance of bone turnover and that the control of C/EBPβ isoform translation represents a target for osteoporosis treatment.
- Cell signaling and Ligand transport
- Structural proteins
REGULATION
10 20 30 40 50 60
M QRLVAWDPA CLPLPPPPPA FKS M EVANFY YEADCLAAAY GGKAAPAAPP AARPGPRPPA
70 80 90 100 110 120
GELGSIGDHE RAIDFSPYLE PLGAPQAPAP ATATDTFEAA PPAPAPAPAS SGQHHDFLSD
130 140 150 160 170 180
LFSDDYGGKN CKKPAEYGYV SLGRLGAAKG ALHPGCFAPL H PPPPPPPPP AELKAEPGFE
190 200 210 220 230 240
PADCKRKEEA GAPGGGAG M A AGFPYALRAY LGYQAVPSGS SGSLSTSSSS SPPGTPSPAD
250 260 270 280 290 300
AKAPPTACYA GAAPAPSQVK SKAKKTVDKH SDEYKIRRER NNIAVRKSRD KAKMRNLETQ
310 320 330 340
HKVLELTAEN ERLQKKVEQL SRELSTLRNL FKQLPEPLLA SSGHC
P9
UBR4_HUMAN
P8
Q6PKD3_HUMAN (Radixin) ERM protein family
h4. Calmodulin
Calmodulin controls liver proliferation via interactions with C/EBPbeta-LAP and C/EBPbeta-LIP. 2010
J Biol Chem. 2010 Jul 23;285(30):23444-56. Epub 2010 May 24.
A truncated isoform of C/EBPbeta, C/EBPbeta-LIP, is required for liver proliferation. This isoform is expressed at high levels in proliferating liver and in liver tumors. However, high levels of C/EBPbeta-LIP are also observed in non-proliferating livers during acute phase response (APR). In this paper we present mechanisms by which liver regulates activities of C/EBPbeta-LIP. We found that calmodulin (CaM) inhibits the ability of C/EBPbeta-LIP to promote liver proliferation during APR through direct interactions. This activity of CaM is under negative control of Ca(2+), which is reduced in nuclei of livers with APR, whereas it is increased in nuclei of proliferating livers. A mutant CaM, which does not interact with C/EBPbeta-LIP, also fails to inhibit the growth promotion activity of C/EBPbeta-LIP. Down-regulation of CaM in livers of LPS-treated mice causes liver proliferation via activation of C/EBPbeta-LIP. Overexpression of C/EBPbeta-LIP above levels of CaM also initiates liver proliferation in LPS-treated mice. In addition, CaM regulates transcriptional activity of another isoform of C/EBPbeta, C/EBPbeta-LAP, and might control liver biology through the regulation of both isoforms of C/EBPbeta. In searching for molecular mechanisms by which C/EBPbeta-LIP promotes cell proliferation, we found that C/EBPbeta-LIP releases E2F.Rb-dependent repression of cell cycle genes by a disruption of E2F1.Rb complexes and by a direct interaction with E2F-dependent promoters. CaM inhibits these growth promotion activities of C/EBPbeta-LIP and, therefore, supports liver quiescence. Thus, our findings discover a new pathway of the regulation of liver proliferation that involves calcium-CaM signaling.
DIAGNOSTIC USE
Papers c/ebp lip lap
Differential control of the CCAAT/enhancer-binding protein beta (C/EBPbeta) products liver-enriched transcriptional activating protein (LAP) and liver-enriched transcriptional inhibitory protein (LIP) and the regulation of gene expression during the response to endoplasmic reticulum stress., 2008
J Biol Chem. 2008 Aug 15;283(33):22443-56. Epub 2008 Jun 11.
- The accumulation of unfolded proteins in the endoplasmic reticulum (ER) triggers a stress response program that protects cells early in the response and can lead to apoptosis during prolonged stress. The basic leucine zipper transcription factor, CCAAT/enhancer-binding protein beta (C/EBPbeta), is one of the genes with increased expression during ER stress. Translation of the C/EBPbeta mRNA from different initiation codons leads to the synthesis of two transcriptional activators (LAP-1 and -2) and a transcriptional repressor (LIP). The LIP/LAP ratio is a critical factor in C/EBPbeta-mediated gene transcription. It is shown here that the LIP/LAP ratio decreased by 5-fold during the early phase of ER stress and increased by 20-fold during the late phase, mostly because of changes in LIP levels. The early decrease in LIP required degradation via the proteasome pathway and phosphorylation of the translation initiation factor, eIF2alpha. The increased LIP levels during the late phase were due to increased synthesis and increased stability of the protein. It is proposed that regulation of synthesis and degradation rates during ER stress controls the LIP/LAP ratio. The importance of C/EBPbeta in the ER-stress response program was demonstrated using C/EBPbeta-deficient mouse embryonic fibroblasts. It is shown that C/EBPbeta attenuates expression of pro-survival ATF4 target genes in late ER stress and enhances expression of cell death-associated genes downstream of CHOP. The inhibitory effect of LIP on ATF4-induced transcription was demonstrated for the cat-1 amino acid transporter gene. We conclude that regulation of LIP/LAP ratios during ER stress is a novel mechanism for modulating the cellular stress response.
C/EBPβ has been associated with regulation of the asparagine synthase (AS) gene (23)
CEBPB e H2O2
C/EBP hydrogen peroxide
CEBPB and infections
Type I Interferon Induces Inhibitory 16-kD CCAAT/ Enhancer Binding Protein (C/EBP)β, Repressing the HIV-1 Long Terminal Repeat in Macrophages: Pulmonary Tuberculosis Alters C/EBP Expression, Enhancing HIV-1 Replication 1998
Liver
All-trans retinoic acid down-regulates human albumin gene expression through the induction of C/EBPβ-LIP, 2006
Stable conditional expression and effect of C/ebpβ-LIP in adipocytes using the pSLIK system. 2013
Murine 3T3-L1 adipocytes are widely used as a cellular model of obesity. However, whereas transfection of 3T3-L1 preadipocytes is straightforward, ectopic gene expression in mature 3T3-L1 adipocytes has proved challenging. Here, we used the pSLIK vector system to generate stable doxycycline-inducible expression of the liver-enriched inhibitor protein isoform of CCAAT/enhancer binding protein β (C/ebpβ (Cebpb)) (C/EBPβ-LIP) in fully differentiated 3T3-L1 adipocytes. Because overexpression of C/ebpβ-LIP impairs adipocyte differentiation, the C/ebpβ-LIP construct was first integrated in 3T3-L1 preadipocytes but expression was induced only when adipocytes were fully differentiated. Increased C/EBPβ-LIP in mature adipocytes down-regulated C/ebpβ target genes including 11β-hydroxysteroid dehydrogenase type 1, phosphoenolpyruvate carboxykinase and fatty acid binding protein 4 but had no effect on asparagine synthetase, demonstrating that transcriptional down-regulation by C/ebpβ-LIP in 3T3-L1 adipocytes is not a general effect. Importantly, these genes were modulated in a similar manner in adipose tissue of mice with genetically increased C/ebpβ-LIP levels. The use of the pSLIK system to conditionally express transgenes in 3T3-L1 cells could be a valuable tool to dissect adipocyte physiology.
KEYWORDS:
Transcriptional induction of the human asparagine synthetase gene during the unfolded protein response does not require the ATF6 and IRE1/XBP1 arms of the pathway. 2009
The UPR (unfolded protein response) pathway comprises three signalling cascades mediated by the ER (endoplasmic reticulum) stress-sensor proteins PERK [PKR (double-stranded RNA-activated protein kinase)-like ER kinase], IRE1 (inositol-requiring kinase 1) and ATF6 (activating transcription factor 6). The present study shows that ASNS (asparagine synthetase) transcription activity was up-regulated in HepG2 cells treated with the UPR activators thapsigargin and tunicamycin. ChIP (chromatin immunoprecipitation) analysis demonstrated that during ER stress, ATF4, ATF3 and C/EBPbeta (CCAAT/enhancer-binding protein beta) bind to the ASNS proximal promoter region that includes the genomic sequences NSRE (nutrient-sensing response element)-1 and NSRE-2, previously implicated by mutagenesis in UPR activation. Consistent with increased ASNS transcription, ChIP analysis also demonstrated that UPR signalling resulted in enhanced recruitment of general transcription factors, including RNA Pol II (polymerase II), to the ASNS promoter. The ASNS gene is also activated by the AAR (amino acid response) pathway following amino acid deprivation of tissue or cells. Immunoblot analysis of HepG2 cells demonstrated that simultaneous activation of the AAR and UPR pathways did not further increase the ASNS or ATF4 protein abundance when compared with triggering either pathway alone. In addition, siRNA (small interfering RNA)-mediated knockdown of XBP1 (X-box-binding protein 1), ATF6alpha or ATF6beta expression did not affect ASNS transcription, whereas siRNA against ATF4 suppressed ASNS transcription during UPR activation. Collectively, these results indicate that the PERK/p-eIF2alpha (phosphorylated eukaryotic initiation factor 2alpha)/ATF4 signalling cascade is the only arm of the UPR that is responsible for ASNS transcriptional induction during ER stress. Consequently, ASNS NSRE-1 and NSRE-2, in addition to ERSE (ER stress response element)-I, ERSE-II and the mUPRE (mammalian UPR element), function as mammalian ER-stress-responsive sequences.
Amino-acid limitation induces transcription from the human C/EBPbeta gene via an enhancer activity located downstream of the protein coding sequence. 2005
Abstract
For animals, dietary protein is critical for the nutrition of the organism and, at the cellular level, protein nutrition translates into amino acid availability. Amino acid deprivation triggers the AAR (amino acid response) pathway, which causes enhanced transcription from specific target genes. The present results show that C/EBPbeta (CCAAT/enhancer-binding protein beta) mRNA and protein content were increased following the deprivation of HepG2 human hepatoma cells of a single amino acid. Although there was a modest increase in mRNA half-life following histidine limitation, the primary mechanism for the elevated steady-state mRNA was increased transcription. Transient transfection documented that C/EBPbeta genomic fragments containing the 8451 bp 5' upstream of the transcription start site did not contain amino-acid-responsive elements. However, deletion analysis of the genomic region located 3' downstream of the protein coding sequence revealed that a 93 bp fragment contained an amino-acid-responsive activity that functioned as an enhancer. Exogenous expression of ATF4 (activating transcription factor 4), known to activate other genes through amino acid response elements, caused increased transcription from reporter constructs containing the C/EBPbeta enhancer in cells maintained in complete amino acid medium. Chromatin immunoprecipitation demonstrated that RNA polymerase II is bound at the C/EBPbeta promoter and at the 93 bp regulatory region in vivo, whereas ATF4 binds to the enhancer region only. Immediately following amino acid removal, the kinetics of binding for ATF4, ATF3, and C/EBPbeta itself to the 93 bp regulatory region were similar to those observed for the amino-acid-responsive asparagine synthetase gene. Collectively the findings show that expression of C/EBPbeta, which contributes to the regulation of amino-acid-responsive genes, is itself controlled by amino acid availability through transcription.
