Elsevier

Cytokine

Volume 54, Issue 1, April 2011, Pages 6-19
Cytokine

Review Article
The CCAAT/enhancer (C/EBP) family of basic-leucine zipper (bZIP) transcription factors is a multifaceted highly-regulated system for gene regulation

https://doi.org/10.1016/j.cyto.2010.12.019Get rights and content

Abstract

The C/EBP family of proteins represents an important group of bZIP transcription factors that are key to the regulation of essential functions such as cell cycle, hematopoiesis, skeletal development, and host immune responses. They are also intimately associated with tumorigenesis and viral disease. These proteins are regulated at multiple levels, including gene induction, alternative translational initiation, post-translational modification, and protein–protein interaction. This review attempts to integrate recent reports with more than 20 years of previous effort focused on this fascinating collection of regulators.

Introduction

Recent developments in molecular biology have resulted in a greater understanding of gene regulation by transcription factors. Despite the ubiquitous nature of transcription factor binding sites, these elements mediate accurate transcriptional responses. Many genes are transcribed in specific cell types or at distinct developmental stages [1], [2], [3]. One of the key issues throughout the process is the mechanism by which transcription factors can precisely control the diversity and specificity of the complex patterns of eukaryotic gene regulation. One explanation involves fine-tuning of tissue-specific and signal-dependent transcription factors, and the arrangement of activator/repressor binding sites on genes. Combinations of interactions among activators, repressors, and their functional modifiers further generate considerable diversity. In addition, post-translational modification provides further opportunities to regulate gene products with an almost unlimited regulatory potential. One such transcription factor family is the CCAAT/enhancer-binding protein (C/EBP) family. The general subject of physiological and pathological roles of C/EBP proteins in gene regulation has been covered by a vast number of excellent original reports and reviews. These include overviews with a specific focus on cell-cycle and cancer [4], [5], [6], [7], extracellular signaling [8], [9], [10], tissue development [11], [12], [13], [14], [15], [16], and viral pathogenesis [17]. Although some of these reviews may appear chronologically dated, they remain extremely relevant. Consequently, this survey is not intended to replicate previous accounts, except for clarification, extension and recent advances on this topic. Although not all of the examples described herein involve classic cytokine pathways, both the activation of C/EBP by cytokines and their involvement in cytokine gene expression underscore the importance of a thorough understanding of biochemical and structural nuances in evaluating relevant mechanisms directly related to cytokine function.

Section snippets

The C/EBP family of bZIP transcription factors

Most transcription factors consist of a collection of structural and functional domains. Factors that interact directly with DNA possess a DNA binding domain (DBD) that usually provides cognate recognition of specific sequence information. Some carry transcription activation domains (TAD) important for protein–protein interactions that lead directly or indirectly to the initiation of transcription. Multimerization domains provide for the formation of homomers and heteromers. Heteromeric

DNA binding and dimerization of C/EBP via the bZIP domain

The bZIP domain interaction with DNA has been structurally characterized for several members of the superfamily. However the X-ray structure for C/EBPα bZIP–DNA complex [22] reveals the nature of target site specificity for this family in which there is a common basic region providing palindromic α-helical recognition of the DNA major groove. One important characteristic of the structure is that it provides at least one explanation for the asymmetric nature of a binding site interacting with a

Binding of C/EBP to gene regulatory elements

C/EBP was initially considered to be a protein involved in liver-specific gene expression and designated as C/EBPα [18], [26]. C/EBPβ was identified as a nuclear factor that binds to an IL-1-responsive element in the IL6 gene [34]. In early studies, C/EBPβ-binding sequences were found in regulatory regions of several genes associated with the acute-phase response, inflammation, and hematopoiesis. These include those that code for: C reactive protein; α1-acid glycoprotein; α2-macroglobuin;

Intra- and inter-family C/EBP homotypic heterodimerization

In addition to homodimers, C/EBP proteins can form homotypic heterodimers, especially with members of the CREB/ATF family, via the leucine zipper dimerization domain [47] greatly expanding the repertoire of DNA-binding specificities and altering functional activities and trans-activation potential (Fig. 3).

Such unique transcriptional specificities have been reported in C/EBPζ and C/EBPγ (Fig. 4). C/EBPζ (induced in the acute-phase response, adipocytic differentiation and cellular stress) is

Translational regulation of C/EBP isoforms

Eukaryotic protein translation requires the formation of the eukaryotic initiation factor (eIF4F) translation initiation complex that consists of the cap-binding protein eIF4E, the ATP-dependent helicase eIF4A, and a large scaffolding protein eIF4G. Alternative C/EBP translation initiation from a single mRNA generates modified isoforms with distinct transcription activation potential. The mTOR signaling pathway controls the ratio of C/EBP isoform expression through eIF4E [74]. This mechanism

Phosphorylation of C/EBPβ

The signal transduction pathway mediated by cytokines is initiated by activation of multiple protein kinases. C/EBP proteins contain various phosphorylation sites (Fig. 1 and Table 4). There have been numerous reports, dealing with phosphorylation of C/EBP family members. Phosphorylation and dephosphorylation can cause conformational changes and affect DNA binding to cognate sequences or cooperative activity with cofactors, resulting in modulation of protein–protein interactions and

Other post-translational modifications of C/EBP

Members of the C/EBP family often reveal post-translational modifications that can affect activity and intracellular localization. In addition to phosphorylation, acetylation, and sumoylation have also been implicated in such regulation and are nicely described in two reviews [8], [9]. In general, phosphorylation plays mostly a positive role in activating C/EBPα and C/EBPβ [8], [9], although there may be circumstances where it is inhibitory for C/EBPα [88], [105]. In contrast, sumoylation

Formation of multi-protein complexes on DNA

Activation of eukaryotic genes in response to extracellular signals requires assembly of stereospecific multi-protein complexes on DNA. Eukaryotic transcription factors bind to enhancers and promoters as multi-protein complexes. The assembly of architectural proteins mediated by DNA looping allows multi-protein–DNA interactions normally retarded by the high energetic cost of DNA bending and twisting. The resulting bend and twist of DNA provides additional specificity for the interaction. In

C/EBPβ/α and Spi-1 play crucial roles in hematopoietic cell differentiation

C/EBPα as well as Spi-1 are up-regulated during myelomonocytic development, while down-regulated in megakaryo-erythroid differentiation. Suh et al. [119] showed increased erythropoiesis in C/EBPα-/- mice and that over-expression of C/EBPα in hematopoietic progenitors from 5-FU-treated mice inhibits erythroid development, while promoting myeloid development. Murine erythroleukemia (MEL) cells expressing C/EBPα exhibit inhibited erythroid differentiation and induction of myeloid-specific genes

Unique compensatory roles of C/EBPβ and C/EBPδ

MyD88 is an important signaling pathway adaptor protein downstream of TLR. MyD88-deficient macrophages exhibit severely impaired cytokine production such as IL-12 p40, IL-6 and TNFα, but can activate both p65 and p50 NF-κB subunits in response to stimuli. However, in MyD88-deficient macrophages, C/EBPδ expression was undetectable, and LPS-induction of C/EBPβ was also markedly suppressed. In addition, the fact that C/EBPβδ double knockout in mouse embryonic fibroblasts (MEF) showed impaired

Summary and perspectives

The C/EBP family, which includes C/EBPα, the founding member of the bZIP superfamily [129], has been a consistently studied collection of transcription factors for more than two decades. During that time, investigations have led to the demonstration of the numerous regulatory mechanisms presented in this review. Recent reports have provided detailed insight into the crucial functional and physiological roles of family members in molecular processes during cellular responses to external signals.

Acknowledgments

Dr. Deborah L. Galson at The University of Pittsburgh is acknowledged for discussions and assistance in the preparation of this manuscript. P.E.A. was supported by funds from the Hunkele Dreaded Disease Fund and the Bayer School of Natural and Environmental Sciences at Duquesne University.

References (137)

  • J.A. Listman et al.

    Conserved ETS domain arginines mediate DNA binding, nuclear localization, and a novel mode of bZIP interaction

    J Biol Chem

    (2005)
  • S. Natsuka et al.

    Macrophage differentiation-specific expression of NF-IL6, a transcription factor for interleukin-6

    Blood

    (1992)
  • R. Morosetti et al.

    A novel, myeloid transcription factor, C/EBPε, is upregulated during granulocytic, but not monocytic, differentiation

    Blood

    (1997)
  • T.H. Tahirov et al.

    Structural analyses of DNA recognition by the AML1/Runx-1 Runt domain and its allosteric control by CBFβ

    Cell

    (2001)
  • S.M. Dunn et al.

    Requirement for nuclear factor (NF)-kappa B p65 and NF-interleukin-6 binding elements in the tumor necrosis factor response region of the granulocyte colony-stimulating factor promoter

    Blood

    (1994)
  • T. Alam et al.

    Trans-activation of the alpha 1-acid glycoprotein gene acute phase responsive element by multiple isoforms of C/EBP and glucocorticoid receptor

    J Biol Chem

    (1993)
  • S. Osada et al.

    DNA binding specificity of the CCAAT/enhancer-binding protein transcription factor family

    J Biol Chem

    (1996)
  • J.C. Craig et al.

    Consensus and variant cAMP-regulated enhancers have distinct CREB-binding properties

    J Biol Chem

    (2001)
  • T. Hattori et al.

    C/EBP homologous protein (CHOP) up-regulates IL-6 transcription by trapping negative regulating NF-IL6 isoform

    FEBS Lett

    (2003)
  • S. Caristi et al.

    Prostaglandin E2 induces interleukin-8 gene transcription by activating C/EBP homologous protein in human T lymphocytes

    J Biol Chem

    (2005)
  • M. Cucinotta et al.

    Regulation of interleukin-8 gene at a distinct site of its promoter by CCAAT enhancer-binding protein homologous protein in prostaglandin E2-treated human T cells

    J Biol Chem

    (2008)
  • H.M. Hu et al.

    The C/EBP bZIP domain can mediate lipopolysaccharide induction of the proinflammatory cytokines interleukin-6 and monocyte chemoattractant protein-1

    J Biol Chem

    (2000)
  • H. Gao et al.

    C/EBP γ has a stimulatory role on the IL-6 and IL-8 promoters

    J Biol Chem

    (2002)
  • S.E. Parkin et al.

    Regulation of CCAAT/enhancer-binding protein (C/EBP) activator proteins by heterodimerization with C/EBP (Ig/EBP)

    J Biol Chem

    (2002)
  • L.M. Podust et al.

    Crystal structure of the CCAAT box/enhancer-binding protein β activating transcription factor-4 basic leucine zipper heterodimer in the absence of DNA

    J Biol Chem

    (2001)
  • N. Su et al.

    C/EBP homology protein (CHOP) interacts with activating transcription factor 4 (ATF4) and negatively regulates the stress-dependent induction of the asparagine synthetase gene

    J Biol Chem

    (2008)
  • A. Gjymishka et al.

    Despite increased ATF4 binding at the C/EBP–ATF composite site following activation of the unfolded protein response, system A transporter 2 (SNAT2) transcription activity is repressed in HepG2 cells

    J Biol Chem

    (2008)
  • P. Descombes et al.

    A liver-enriched transcriptional activator protein, LAP, and a transcriptional inhibitory protein, LIP, are translated from the same mRNA

    Cell

    (1991)
  • A.J. Lincoln et al.

    Inhibition of CCAAT/enhancer-binding protein alpha and beta translation by upstream open reading frames

    J Biol Chem

    (1998)
  • C.W. Mahoney et al.

    Phosphorylation of CCAAT-enhancer binding protein by protein kinase C attenuates site-selective DNA binding

    J Biol Chem

    (1992)
  • A.M. Chumakov et al.

    Modulation of DNA binding properties of CCAAT/enhancer binding protein ε by heterodimer formation and interactions with NFκB pathway

    Blood

    (2007)
  • M. Hanlon et al.

    ERK2- and p90(Rsk2)-dependent pathways regulate the CCAAT/enhancer-binding protein-beta interaction with serum response factor

    J Biol Chem

    (2001)
  • G. Borland et al.

    Activation of protein kinase Cα by EPAC1 is required for the ERK- and CCAAT/enhancer-binding protein β-dependent induction of the SOCS-3 gene by cyclic AMP in COS1 cells

    J Biol Chem

    (2009)
  • M. Buck et al.

    Phosphorylation of rat serine 105 or mouse threonine 217 in C/EBP beta is required for hepatocyte proliferation induced by TGF alpha

    Mol Cell

    (1999)
  • M. Ptashne

    How eukaryotic transcriptional activators work

    Nature

    (1988)
  • D.P. Ramji et al.

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

    Biochem J

    (2002)
  • P.F. Johnson

    Molecular stop signs: regulation of cell-cycle arrest by C/EBP transcription factors

    J Cell Sci

    (2005)
  • S. Koschmieder et al.

    Dysregulation of the C/EBPalpha differentiation pathway in human cancer

    J Clin Oncol

    (2009)
  • H. Li et al.

    The interferon signaling network and transcription factor C/EBP-beta

    Cell Mol Immunol

    (2007)
  • J.J. Smink et al.

    Rapamycin and the transcription factor C/EBPβ as a switch in osteoclast differentiation: implications for lytic bone diseases

    J Mol Med

    (2009)
  • A.D. Friedman

    Transcriptional control of granulocyte and monocyte development

    Oncogene

    (2007)
  • T.N. Cassel et al.

    C/EBP transcription factors in the lung epithelium

    Am J Physiol Lung Cell Mol Physiol

    (2003)
  • M. Takiguchi

    The C/EBP family of transcription factors in the liver and other organs

    Int J Exp Pathol

    (1998)
  • Y. Liu et al.

    CCAAT/enhancer-binding proteins and the pathogenesis of retrovirus infection

    Future Microbiol

    (2009)
  • W.H. Landschulz et al.

    The DNA binding domain of the rat liver nuclear protein C/EBP is bipartite

    Science

    (1989)
  • E.C. LaCasse et al.

    Nuclear localization signals overlap DNA- or RNA-binding domains in nucleic acid-binding proteins

    Nucleic Acids Res

    (1995)
  • S.C. Williams et al.

    C/EBP proteins contain nuclear localization signals imbedded in their basic regions

    Gene Expr

    (1997)
  • S. Katz et al.

    The NF-M transcription factor is related to C/EBP beta and plays a role in signal transduction, differentiation and leukemogenesis of avian myelomonocytic cells

    EMBO J

    (1993)
  • Z. Cao et al.

    Regulated expression of three C/EBP isoforms during adipose conversion of 3T3–L1 cells

    Genes Dev

    (1991)
  • E. Kowenz-Leutz et al.

    Novel mechanism of C/EBP beta (NF-M) transcriptional control: activation through derepression

    Genes Dev

    (1994)
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