C/EBPepsilon -/- mice: increased rate of myeloid proliferation and apoptosis

CCAAT/Enhancer-Binding Protein ε27 Antagonism of GATA-1 Transcriptional Activity in the Eosinophil Is Mediated by a Unique N-Terminal Repression Domain, Is Independent of Sumoylation and Does Not Require DNA Binding

CCAAT/enhancer binding protein epsilon (C/EBPε) is required for eosinophil differentiation, lineage-specific gene transcription, and expression of C/EBPε³² and shorter 27kD and 14kD isoforms is developmentally regulated during this process. We previously defined the 27kD isoform (C/EBPε²⁷) as an antagonist of GATA-1 transactivation of the eosinophil's major basic protein-1 (MBP1) P2-promoter, showing C/EBPε²⁷ and GATA-1 physically interact. In the current study, we used a Tat-C/EBPε²⁷ fusion protein for cell/nuclear transduction of an eosinophil myelocyte cell line to demonstrate that C/EBPε²⁷ is a potent repressor of MBP1 transcription. We performed structure-function analyses of C/EBPε²⁷ mapping its repressor domains, comparing it to C/EBPε³² and C/EBPε¹⁴, using GATA-1 co-transactivation of the MBP1-P2 promoter. Results show C/EBPε²⁷ repression of GATA-1 is mediated by its unique 68aa N-terminus combined with previously identified RDI domain. This repressor activity does not require, but is enhanced by, DNA binding via the basic region of C/EBPε²⁷ but independent of sumoylation of the RDI core "VKEEP" sumoylation site. These findings identify the N-terminus of C/EBPε²⁷ as the minimum repressor domain required for antagonism of GATA-1 in the eosinophil. C/EBPε²⁷ repression of GATA-1 occurs via a combination of both C/EBPε²⁷-GATA-1 protein-protein interaction and C/EBPε²⁷ binding to a C/EBP site in the MBP1 promoter. The C/EBPε²⁷ isoform may serve to titrate and/or turn off eosinophil granule protein genes like MBP1 during eosinophil differentiation, as these genes are ultimately silenced in the mature cell. Understanding the functionality of C/EBPε²⁷ in eosinophil development may prove promising in developing therapeutics that reduce eosinophil proliferation in allergic diseases.

Granulopoiesis and granules of human neutrophils

Granules are essential for the ability of neutrophils to fulfill their role in innate immunity. Granule membranes contain proteins that react to environmental cues directing neutrophils to sites of infection and initiate generation of bactericidal oxygen species. Granules are densely packed with proteins that contribute to microbial killing when liberated to the phagosome or extracellularly. Granules are, however, highly heterogeneous and are traditionally subdivided into azurophil granules, specific granules, and gelatinase granules in addition to secretory vesicles. This review will address issues pertinent to formation of granules, which is a process intimately connected to maturation of neutrophils from their precursors in the bone marrow. We further discuss possible mechanisms by which decisions are made regarding sorting of proteins to constitutive secretion or storage in granules and how degranulation of granule subsets is regulated.

Chromatin-remodeling factor SMARCD2 regulates transcriptional networks controlling differentiation of neutrophil granulocytes

We identify SMARCD2 (SWI/SNF-related, matrix-associated, actin-dependent regulator of chromatin, subfamily D, member 2), also known as BAF60b (BRG1/Brahma-associated factor 60b), as a critical regulator of myeloid differentiation in humans, mice, and zebrafish. Studying patients from three unrelated pedigrees characterized by neutropenia, specific granule deficiency, myelodysplasia with excess of blast cells, and various developmental aberrations, we identified three homozygous loss-of-function mutations in SMARCD2. Using mice and zebrafish as model systems, we showed that SMARCD2 controls early steps in the differentiation of myeloid-erythroid progenitor cells. In vitro, SMARCD2 interacts with the transcription factor CEBPɛ and controls expression of neutrophil proteins stored in specific granules. Defective expression of SMARCD2 leads to transcriptional and chromatin changes in acute myeloid leukemia (AML) human promyelocytic cells. In summary, SMARCD2 is a key factor controlling myelopoiesis and is a potential tumor suppressor in leukemia.

Transcriptional Heterogeneity and Lineage Commitment in Myeloid Progenitors

Within the bone marrow, stem cells differentiate and give rise to diverse blood cell types and functions. Currently, hematopoietic progenitors are defined using surface markers combined with functional assays that are not directly linked with in vivo differentiation potential or gene regulatory mechanisms. Here, we comprehensively map myeloid progenitor subpopulations by transcriptional sorting of single cells from the bone marrow. We describe multiple progenitor subgroups, showing unexpected transcriptional priming toward seven differentiation fates but no progenitors with a mixed state. Transcriptional differentiation is correlated with combinations of known and previously undefined transcription factors, suggesting that the process is tightly regulated. Histone maps and knockout assays are consistent with early transcriptional priming, while traditional transplantation experiments suggest that in vivo priming may still allow for plasticity given strong perturbations. These data establish a reference model and general framework for studying hematopoiesis at single-cell resolution.

C/EBP transcription factors in human squamous cell carcinoma: selective changes in expression of isoforms correlate with the neoplastic state

The CCAAT/Enhancer Binding Proteins (C/EBPs) are a family of leucine-zipper transcription factors that regulate physiological processes such as energy metabolism, inflammation, cell cycle, and the development and differentiation of several tissues including skin. Recently, a role for C/EBPs in tumor cell proliferation and differentiation has been proposed, but the incomplete characterization in the literature of multiple translational isoforms of these proteins has made interpretation of these roles difficult. Therefore, we have carefully reexamined C/EBP isoform expression in human non-melanoma skin cancers. C/EBPα, C/EBPβ, and C/EBPδ were analyzed histologically in squamous cell carcinomas (SCC). The individual isoforms of C/EBPα and C/EBPβ were examined by immunofluorescent digital imaging, western blotting and DNA binding activity (electrophoretic mobility shift analysis). Expression of all C/EBP family proteins was decreased in SCC tumors. Suppression was greatest for C/EBPα, less for C/EBPβ, and least for C/EBPδ. Western analyses confirmed that C/EBPα p42 and p30 isoforms were decreased. For C/EBPβ, only the abundant full-length isoform (C/EBPβ−1, LAP*, 55 kD) was reduced, whereas the smaller isoforms, C/EBPβ−2 (LAP, 48 kD) and C/EBPβ−3 (LIP, 20 kD), which are predominantly nuclear, were significantly increased in well- and moderately-differentiated SCC (up to 14-fold for C/EBPβ−3). These elevations correlated with increases in PCNA, a marker of proliferation. Although C/EBPβ displayed increased post-translational modifications in SCC, phosphorylation of C/EBPβ−1 (Thr 235) was not altered. C/EBP-specific DNA binding activity in nuclear and whole-cell extracts of cultured cells and tumors was predominantly attributable to C/EBPβ. In summary, two short C/EBPβ isoforms, C/EBPβ−2 and C/EBPβ−3, represent strong candidate markers for epithelial skin malignancy, due to their preferential expression in carcinoma versus normal skin, and their strong correlation with tumor proliferation.

A crucial role for the ubiquitously expressed transcription factor Sp1 at early stages of hematopoietic specification

Mammalian development is regulated by the interplay of tissue-specific and ubiquitously expressed transcription factors, such as Sp1. Sp1 knockout mice die in utero with multiple phenotypic aberrations, but the underlying molecular mechanism of this differentiation failure has been elusive. Here, we have used conditional knockout mice as well as the differentiation of mouse ES cells as a model with which to address this issue. To this end, we examined differentiation potential, global gene expression patterns and Sp1 target regions in Sp1 wild-type and Sp1-deficient cells representing different stages of hematopoiesis. Sp1(-/-) cells progress through most embryonic stages of blood cell development but cannot complete terminal differentiation. This failure to fully differentiate is not seen when Sp1 is knocked out at later developmental stages. For most Sp1 target and non-target genes, gene expression is unaffected by Sp1 inactivation. However, Cdx genes and multiple Hox genes are stage-specific targets of Sp1 and are downregulated at an early stage. As a consequence, expression of genes involved in hematopoietic specification is progressively deregulated. Our work demonstrates that the early absence of active Sp1 sets a cascade in motion that culminates in a failure of terminal hematopoietic differentiation and emphasizes the role of ubiquitously expressed transcription factors for tissue-specific gene regulation. In addition, our global side-by-side analysis of the response of the transcriptional network to perturbation sheds a new light on the regulatory hierarchy of hematopoietic specification.

Human α-defensin expression is not dependent on CCAAT/enhancer binding protein-ε in a murine model

Specific granule deficiency (SGD) is a rare congenital disorder characterized by recurrent infections. The disease is caused by inactivating mutations of the CCAAT/enhancer binding protein-ε (C/EBP-ε) gene. As a consequence, specific and gelatinase granules lack most matrix proteins. Furthermore, azurophil granules contain diminished amounts of their most abundant proteins, α-defensins, also known as human neutrophil peptides (HNPs). In accordance with this, in vitro models have demonstrated induction of HNPs by C/EBP-ε. Since mice do not express myeloid defensins, they cannot per se be used to characterize the role of C/EBP-ε in controlling HNP expression in vivo. We therefore crossed a transgenic HNP-1-expressing mouse with the Cebpe^-/- mouse to study the in vivo significance of C/EBP-ε for HNP-1 transcription and expression. Surprisingly, neither expression nor processing of HNP-1 was affected by lack of C/EBP-ε in these mice. Transduction of C/EBP-ε into primary bone marrow cells from HNP-1 mice induced some HNP-1 expression, but not to levels comparable to expression human cells. Taken together, our data infer that the HNP-1 of the transgenic mouse does not show an expression pattern equivalent to endogenous secondary granule proteins. This limits the use of these transgenic mice as a model for human conditions.

miRNA-130a regulates C/EBP-ε expression during granulopoiesis

CCAAT/enhancer binding protein-ε (C/EBP-ε) is considered a master transcription factor regulating terminal neutrophil maturation. It is essential for expression of secondary granule proteins, but it also regulates proliferation, cell cycle, and maturation during granulopoiesis. Cebpe(-/-) mice have incomplete granulocytic differentiation and increased sensitivity toward bacterial infections. The amount of C/EBP-ε messenger RNA (mRNA) increases with maturation from myeloblasts with peak level in myelocytes (MC)/metamyelocytes (MM), when the cells stop proliferating followed by a decline in more mature cells. In contrast, C/EBP-ε protein is virtually detectable only in the MC/MM population, indicating that expression in more immature cells could be inhibited by microRNAs (miRNAs). We found that miRNA-130a (miR-130a) regulates C/EBP-ε protein expression in both murine and human granulocytic precursors. Overexpression of miR-130a in a murine cell line downregulated C/EBP-ε protein and lactoferrin (Ltf), cathelicidin antimicrobial protein (Camp), and lipocalin-2 (Lcn2) mRNA expression giving rise to cells with a more immature phenotype, as seen in the Cebpe(-/-) mouse. Introduction of a C/EBP-ε mRNA without target site for miR-130a restored both C/EBP-ε production, expression of Camp and Lcn2, and resulted in the cells having a more mature phenotype. We conclude that miR-130a is important for the regulation of the timed expression of C/EBP-ε during granulopoiesis.

Regulation of monocyte differentiation by specific signaling modules and associated transcription factor networks

Monocyte/macrophages are important players in orchestrating the immune response as well as connecting innate and adaptive immunity. Myelopoiesis and monopoiesis are characterized by the interplay between expansion of stem/progenitor cells and progression towards further developed (myelo)monocytic phenotypes. In response to a variety of differentiation-inducing stimuli, various prominent signaling pathways are activated. Subsequently, specific transcription factors are induced, regulating cell proliferation and maturation. This review article focuses on the integration of signaling modules and transcriptional networks involved in the determination of monocytic differentiation.

In vivo deficiency of both C/EBPβ and C/EBPε results in highly defective myeloid differentiation and lack of cytokine response

The CCAAT/enhancer binding proteins (C/EBPs) are transcription factors involved in hematopoietic cell development and induction of several inflammatory mediators. Here, we generated C/EBPβ and C/EBPε double-knockout (bbee) mice and compared their phenotypes to those of single deficient (bbEE and BBee) and wild-type (BBEE) mice. The bbee mice were highly susceptible to fatal infections and died within 2-3 months. Morphologically, their neutrophils were blocked at the myelocytes/metamyelocytes stage, and clonogenic assays of bone marrow cells indicated a significant decrease in the number of myeloid colonies of the bbee mice. In addition, the proportion of hematopoietic progenitor cells [Lin(-)Sca1(+)c-Kit(+)] in the bone marrow of the bbee mice was significantly increased, reflecting the defective differentiation of the myeloid compartment. Furthermore, microarray expression analysis of LPS- and IFNγ-activated bone marrow-derived macrophages from bbee compared to single knockout mice revealed decreased expression of essential immune response-related genes and networks, including some direct C/EBP-targets such as Marco and Clec4e. Overall, the phenotype of the bbee mice is distinct from either the bbEE or BBee mice, demonstrating that both transcription factors are crucial for the maturation of neutrophils and macrophages, as well as the innate immune system, and can at least in part compensate for each other in the single knockout mice.

Human C/EBP-epsilon activator and repressor isoforms differentially reprogram myeloid lineage commitment and differentiation

CCAAT enhancer-binding protein-epsilon (C/EBP-epsilon) is required for the terminal differentiation of neutrophils and eosinophils. Human C/EBP-epsilon is expressed as 4 isoforms (32, 30, 27, and 14 kDa) through differential RNA splicing, and alternative promoters and translational start sites. The C/EBP-epsilon(32/30) isoforms are transcriptional activators, whereas C/EBP-epsilon(27) interacts with and represses GATA-1 transactivation of eosinophil promoters. C/EBP-epsilon(14) contains only DNA-binding and -dimerization domains and may function as a dominant-negative regulator. To define functional activities for these C/EBP-epsilon isoforms in myelopoiesis, human CD34(+) progenitors were transduced with internal ribosomal entry site-enhanced green fluorescent protein retroviral vectors encoding the 32/30, 27, and 14-kDa isoforms, purified by fluorescence-activated cell sorter, and analyzed in colony-forming assays and suspension cultures. Progenitors transduced with C/EBP-epsilon(32/30) default exclusively to eosinophil differentiation and gene expression, independent of interleukin-5, and regardless of inclusion of cytokines to induce other lineages. In contrast, the putative repressor C/EBP-epsilon(27) isoform strongly inhibits eosinophil differentiation and gene expression, including GATA-1, promoting granulocyte (neutrophil)-macrophage differen-tiation. The C/EBP-epsilon(14) repressor isoform strongly inhibits eosinophil development and gene expression, promoting erythroid differentiation, an effect enhanced by erythropoietin. Thus, C/EBP-epsilon isoforms can reprogram myeloid lineage commitment and differentiation consistent with their predicted activities based on activator and repressor domains and in vitro functional activities.

The lamin B receptor under transcriptional control of C/EBPepsilon is required for morphological but not functional maturation of neutrophils

The lamin B receptor (LBR) is an integral nuclear envelope protein that interacts with chromatin and has homology to sterol reductases. Mutations in LBR result in Pelger-Huët anomaly and HEM-Greenberg skeletal dysplasia, whereas in mice Lbr mutations result in ichthyosis. To further understand the function of the LBR and its role in disease, we derived a novel mouse model with a gene-trap insertion into the Lbr locus (Lbr(GT/GT)). Phenotypically, the Lbr(GT/GT) mice are similar to ichthyosis mice. The Lbr(GT/GT) granulocytes lack a mature segmented nucleus and have a block in late maturation. Despite these changes in nuclear morphology, the innate granulocyte immune function in the killing of Staphylococcus aureus bacteria appears to be intact. Granulocyte differentiation requires the transcription factor C/EBPepsilon. We identified C/EBPepsilon binding sites within the Lbr promoter and used EMSAs and luciferase assays to show that Lbr is transcriptionally regulated by C/EBPepsilon. Our findings indicate that the Lbr(GT/GT) mice are a model for Pelger-Huët anomaly and that Lbr, under transcriptional regulation of C/EBPepsilon, is necessary for morphological but not necessarily functional granulocyte maturation.

N-terminal region of CCAAT/enhancer-binding protein epsilon is critical for cell cycle arrest, apoptosis, and functional maturation during myeloid differentiation

CCAAT/enhancer-binding protein epsilon (C/EBPepsilon) plays a critical role in terminal myeloid differentiation. Differentiation is an integrated process of cell cycle arrest, morphological change, functional maturation, and apoptosis. However, the molecular networks underlying these events in C/EBPepsilon-induced differentiation remain poorly understood. To reveal these mechanisms, we performed a detailed molecular analysis of C/EBPepsilon-induced differentiation using an inducible form of C/EBPepsilon. The activation of C/EBPepsilon induced growth arrest, morphological differentiation, the expression of CD11b and secondary granule proteins, and apoptosis in myeloid cell lines. Unlike C/EBPalpha, C/EBPepsilon dramatically up-regulated p27 with a concomitant down-regulation of cdk4/6 and cyclin D2/A/E. Moreover, the anti-apoptotic proteins Bcl-2 and Bcl-x were down-regulated, whereas pro-apoptotic protein Bax remained unchanged. Using a variety of mutants, we revealed that these events were all regulated by the N-terminal activation domain of C/EBPepsilon. Interestingly, some of the differentiation processes such as the induction of secondary granule protein genes were clearly inhibited by c-Myc; however, inhibition of apoptosis by Bcl-x did not affect the entire differentiation processes. These data indicate the N terminus of C/EBPepsilon to be solely responsible for most aspects of myeloid differentiation, and these events were differentially affected by c-Myc.

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

The CCAAT/enhancer-binding protein (C/EBP) family of transcription factors plays an important role in controlling cell proliferation and differentiation. C/EBPalpha is a particularly potent regulator of cell-cycle exit and is induced in terminally differentiating adipocytes and myeloid cells, where it also activates differentiation-specific genes. The growth-inhibiting activity of C/EBPalpha suppresses tumorigenesis in myeloid cells and possibly other tissues. In addition, recent work has identified C/EBPalpha as a component of the p53-regulated growth arrest response elicited by DNA damage in epidermal keratinocytes. Several studies have explored the mechanism by which C/EBPalpha blocks cell-cycle progression at the G1-S boundary, and several models have been proposed but no universally accepted mechanism has emerged. Controversial issues include whether C/EBPalpha acts through an 'off-DNA' mechanism to inhibit cyclin-dependent kinases, and whether and how it functions with the RB-E2F system to repress transcription of S-phase genes. Other C/EBP-family members have also been implicated in positive and negative control of cell proliferation, and the mechanisms underlying their growth-regulatory activities are beginning to be elucidated.

Decreased expression of CCAAT/enhancer binding protein zeta (C/EBPzeta) in patients with different myeloid diseases

CCAAT/enhancer binding proteins (C/EBPs) are a family of transcription factors that have been implicated in diverse cellular functions such as cellular differentiation and proliferation, and inflammatory processes. C/EBPzeta, also known as GADD153, CHOP10, and DDIT3 has been found associated with the development of myxoid liposarcoma and the progression of melanoma. To investigate the correlation of C/EBPzeta transcript levels with the development of leukemia, samples from 187 patients with myelodysplastic syndrome (MDS), acute myeloid leukemia (AML), and chronic myeloid leukemia (CML) were examined for C/EBPzeta mRNA using real-time quantitative PCR (RQ-PCR). RQ-PCR analysis demonstrated the median levels of C/EBPzeta were significantly decreased in MDS, AML, and CML patients compared with normal controls (1.40, 0.96, 2.60 versus 14.69, P<0.0001). Significant differences were also observed between patients with CML and with AML or MDS. These results suggest that the insufficient dosage of C/EBPzeta might be involved in the development of leukemia.

Disruption of differentiation in human cancer: AML shows the way

Although much is understood about the ways in which transcription factors regulate various differentiation systems, and one of the hallmarks of many human cancers is a lack of cellular differentiation, relatively few reports have linked these two processes. Recent studies of acute myeloid leukaemia (AML), however, have indicated how disruption of transcription-factor function can disrupt normal cellular differentiation and lead to cancer. This model involves lineage-specific transcription factors, which are involved in normal haematopoietic differentiation. These factors are often targeted in AML--either by direct mutation or by interference from translocation proteins. Uncovering these underlying pathways will improve the diagnosis and treatment of AML, and provide a working model for other types of human cancer, including solid tumours.

The effects of Bcr-Abl on C/EBP transcription-factor regulation and neutrophilic differentiation are reversed by the Abl kinase inhibitor imatinib mesylate

The clinical progression of chronic myeloid leukemia (CML) from chronic phase to blast crisis is characterized by the increasing failure of myeloid precursors to differentiate into mature granulocytes. This study was undertaken to investigate the influence of Bcr-Abl and of the small molecule Abl tyrosine-kinase inhibitor imatinib mesylate on granulocyte colony-stimulating factor (G-CSF)-induced neutrophilic differentiation. We show that differentiation of 32Dcl3 cells into mature granulocytes is accompanied by the increased expression of the antigens macrophage adhesion molecule-1 (Mac-1) and Gr-1, of the G-CSF receptor (G-CSFR), of myeloid transcription factors (CCAAT/enhancer-binding protein-alpha [C/EBPalpha], C/EBPepsilon, and PU.1), and of the cyclin-dependent kinase inhibitor p27(Kip1). In 32Dcl3 cells transfected with the bcr-abl gene (32D(Bcr-Abl)), G-CSF did not trigger either granulocytic differentiation or the up-regulation of C/EBPalpha, C/EBPepsilon, and the G-CSFR. This could be correlated to a defect in c-Myc down-regulation. In contrast, the up-regulation of PU.1 and p27(Kip1) by G-CSF was not affected by Bcr-Abl. Importantly, incubation of 32D(Bcr-Ablwt) cells with the kinase inhibitor imatinib mesylate prior to G-CSF stimulation completely neutralized the effects of Bcr-Abl on granulocytic differentiation and on C/EBPalpha and C/EBPepsilon expression. Taken together, the results suggest that the Bcr-Abl kinase induces a reversible block of the granulocytic differentiation program in myeloid cells by disturbing regulation of hematopoietic transcription factors such as C/EBPalpha and C/EBPepsilon.

Transcriptional mechanisms regulating myeloid-specific genes

Myeloid blood cells comprise an important component of the immune system. Proper control of both lineage- and stage-specific gene expression is required for normal myeloid cell development and function. In recent years, a relatively small number of critical transcriptional regulators have been identified that serve important roles both in myeloid cell development and regulation of lineage-restricted gene expression in mature myeloid cells. This review summarizes our current understanding of the regulation of lineage- and stage-restricted transcription during myeloid cell differentiation, how critical transcriptional regulators control myeloid cell development, and how perturbations in transcription factor function results in the development of leukemia.

Orchestration of multiple arrays of signal cross-talk and combinatorial interactions for maturation and cell death: another vision of t(15;17) preleukemic blast and APL-cell maturation

Despite intensive molecular biology investigations over the past 10 years, and an important breakthrough on how PML-RARalpha, the fusion protein resulting from t(15;17), can alter RARalpha and PML functions, no definitive views on how leukemia is generated and by what mechanism(s) the normal phenotype is restored, are yet available. 'Resistances' to pharmacological levels of all-trans-retinoic acid (ATRA) have been observed in experimental in vivo and in vitro models. In this review, we emphasize the key role played by signal cross-talk for both normal and neoplastic hemopoiesis. After an overview of reported experimental data on APL-cell maturation and apoptosis, we apply our current knowledge on signaling pathways to underline those which might generate signal cross-talks. The design of biological models suitable to decipher the integration of signal cross-talks at the transcriptional level should be our first priority today, to generate some realistic therapeutic approaches After 'Ten Years of Molecular APL', we still know very little about how the disease develops and how effective medicines work.