C/EBPalpha and G-CSF receptor signals cooperate to induce the myeloperoxidase and neutrophil elastase genes

Secondary bile acids function through the vitamin D receptor in myeloid progenitors to promote myelopoiesis

Metabolic products of the microbiota can alter hematopoiesis. However, the contribution and site of action of bile acids is poorly understood. Here, we demonstrate that the secondary bile acids, deoxycholic acid (DCA) and lithocholic acid (LCA), increase bone marrow myelopoiesis. Treatment of bone marrow cells with DCA and LCA preferentially expanded immunophenotypic and functional colony-forming unit-granulocyte and macrophage (CFU-GM) granulocyte-monocyte progenitors (GMPs). DCA treatment of sorted hematopoietic stem and progenitor cells (HSPCs) increased CFU-GMs, indicating that direct exposure of HSPCs to DCA sufficed to increase GMPs. The vitamin D receptor (VDR) was required for the DCA-induced increase in CFU-GMs and GMPs. Single-cell RNA sequencing revealed that DCA significantly upregulated genes associated with myeloid differentiation and proliferation in GMPs. The action of DCA on HSPCs to expand GMPs in a VDR-dependent manner suggests microbiome-host interactions could directly affect bone marrow hematopoiesis and potentially the severity of infectious and inflammatory disease.

The clinical characteristics and prognosis of Chinese acute myeloid leukemia patients with CSF3R mutations

INTRODUCTION

The colony-stimulating factor 3 receptor (CSF3R) controls the proliferation of myeloid progenitors and differentiation into neutrophils. However, the clinical features and prognostic significance of CSF3R mutations in primary acute myeloid leukemia (AML) patients are still unclear.

METHODS

158 newly diagnosed AML patients were retrospectively evaluated in our study. Amplicon-based next-generation sequencing (NGS) and multiplex-nested reverse-transcription polymerase chain reaction (RT-PCR) were used to investigate the 34 genes and 43 fusion genes associated with leukemia. In addition, clinical features, mutation incidence, and survival outcomes were compared between patients with CSF3R mutation and patients with wild-type CSF3R.

RESULTS

In our study, CSF3R mutations were found in 7.6% (12/158) cases. The membrane-proximal amino acid substitution T618I (58.3%) was the most frequent mutation. CSF3R mutations were associated with higher WBC counts (P = .035). CEBPA mutation, TET2 mutation, and RUNX1-RUNX1T1 translocation were the most common co-mutations of CSF3R. The CSF3R gene was mutually exclusive with signal transduction genes (P = .029), while positively associated with TET2 mutations (P = .014). CSF3R mutations had no effect on CR1 (P = .935), R (P = .625) and OS (P = .1172). Patients with CSF3R mutations had a worse DFS (P = .0352) than those with wild-type CSF3R. Multivariate survival analysis showed that CSF3R mutation was an independent risk factor for DFS of primary AML patients (HR=2.048, 95%CI: 1.006-4.170, P = .048).

CONCLUSION

AML patients with CSF3R mutations had unique clinical features and gene co-mutation spectrum. CSF3R mutation was an independent risk factor for DFS and could be a potential prognostic marker and therapeutic target for Chinese primary AML patients.

Laboratory evaluation and prognostication among adults and children with CEBPA-mutant acute myeloid leukemia

CEBPA-mutant acute myeloid leukemia (AML) encompasses clinically and biologically distinct subtypes of AML in both adults and children. CEBPA-mutant AML may occur with monoallelic (moCEBPA) or biallelic (biCEBPA) mutations, which can be somatic or germline, with each entity impacting prognosis in unique ways. BiCEBPA AML is broadly associated with a favorable prognosis, but differences in the type and location of CEBPA mutations as well as the presence of additional leukemogenic mutations can lead to heterogeneity in survival. Concurrent FLT3-ITD mutations have a well-documented negative effect on survival in adult biCEBPA AML, whereas support for a negative prognostic effect of mutations in TET2, DNMT3A, WT1, CSF3R, ASXL1, and KIT is mixed. NPM1 and GATA2 mutations may have a positive prognostic impact. MoCEBPA AML has similar survival outcomes compared to AML with wild-type CEBPA, and risk stratification is determined by other cytogenetic and molecular findings. Germline CEBPA mutations may lead to familial biCEBPA AML after acquisition of second somatic CEBPA mutation, with variable penetrance and age. BiCEBPA AML in children is likely a favorable-risk diagnosis as it is in adults, but the role of a single CEBPA mutation and the impact of concurrent leukemogenic mutations are not clear in this population. Laboratory evaluation of the CEBPA gene includes PCR-based fragment-length analysis, Sanger sequencing, and next-generation sequencing. Phenotypic analysis using multiparameter flow cytometry can also provide additional data in evaluating CEBPA, helping to assess for the likelihood of mutation presence.

Prognostic impact of CSF3R mutations in favorable risk childhood acute myeloid leukemia

Truncation mutations in the granulocyte colony-stimulating factor receptor gene (CSF3R) are a rare abnormality in pediatric acute myeloid leukemia, and are usually associated either with mutations in CEBPA or with t(8;21). Through sequencing of over 2000 patients, the authors demonstrated that, although CSF3R mutations with associated t(8;21) still had an excellent response, CSF3R mutation abrogated the favorable risk of CEBPA mutation alone.

Transcriptional regulation of neutrophil differentiation and function during inflammation

Neutrophils are the most abundant leukocytes in innate immunity where they elicit powerful effector functions to eliminate invading pathogens and modulate the adaptive as well as the innate immune response. Neutrophil function must be tightly regulated during inflammation and infection to avoid additional tissue damage. Increasing evidence suggests that transcription factors (TFs) function as key regulators to modulate transcriptional output, thereby controlling cell fate decision and the inflammatory responses. However, the molecular mechanisms underlying neutrophil differentiation and function during inflammation remain largely uncharacterized. Here, we provide a comprehensive overview of TFs known to be crucial for neutrophil maturation and in the signaling pathways that control neutrophil differentiation and activation. We also outline how emerging genomic and single-cell technologies may facilitate further discovery of neutrophil transcriptional regulators.

Myeloid lineage enhancers drive oncogene synergy in CEBPA/CSF3R mutant acute myeloid leukemia

Acute Myeloid Leukemia (AML) develops due to the acquisition of mutations from multiple functional classes. Here, we demonstrate that activating mutations in the granulocyte colony stimulating factor receptor (CSF3R), cooperate with loss of function mutations in the transcription factor CEBPA to promote acute leukemia development. The interaction between these distinct classes of mutations occurs at the level of myeloid lineage enhancers where mutant CEBPA prevents activation of a subset of differentiation associated enhancers. To confirm this enhancer-dependent mechanism, we demonstrate that CEBPA mutations must occur as the initial event in AML initiation. This improved mechanistic understanding will facilitate therapeutic development targeting the intersection of oncogene cooperativity.

Strategies to generate functionally normal neutrophils to reduce infection and infection-related mortality in cancer chemotherapy

Neutrophils form an essential part of innate immunity against infection. Cancer chemotherapy-induced neutropenia (CCIN) is a condition in which the number of neutrophils in a patient's bloodstream is decreased, leading to increased susceptibility to infection. Granulocyte colony-stimulating factor (GCSF) has been the only approved treatment for CCIN over two decades. To date, CCIN-related infection and mortality remain a significant concern, as neutrophils generated in response to administered GCSF are functionally immature and cannot effectively fight infection. This review summarizes the molecular regulatory mechanisms of neutrophil granulocytic differentiation and innate immunity development, dissects the biology of GCSF in myeloid expansion, highlights the shortcomings of GCSF in CCIN treatment, updates the recent advance of a selective retinoid agonist that promotes neutrophil granulocytic differentiation, and evaluates the benefits of developing GCSF biosimilars to increase access to GCSF biologics versus seeking a new mode to fundamentally advance GCSF therapy for treatment of CCIN.

Development of MTL-CEBPA: Small Activating RNA Drug for Hepatocellular Carcinoma

BACKGROUND

Oligonucleotide drug development has revolutionised the drug discovery field. Within this field, 'small' or 'short' activating RNAs (saRNA) are a more recently discovered category of short double-stranded RNA with clinical potential. saRNAs promote transcription from target loci, a phenomenon widely observed in mammals known as RNA activation (RNAa).

OBJECTIVE

The ability to target a particular gene is dependent on the sequence of the saRNA. Hence, the potential clinical application of saRNAs is to increase target gene expression in a sequence-specific manner. saRNA-based therapeutics present opportunities for expanding the "druggable genome" with particular areas of interest including transcription factor activation and cases of haploinsufficiency.

RESULTS AND CONCLUSION

In this mini-review, we describe the pre-clinical development of the first saRNA drug to enter the clinic. This saRNA, referred to as MTL-CEBPA, induces increased expression of the transcription factor CCAAT/enhancer-binding protein alpha (CEBPα), a tumour suppressor and critical regulator of hepatocyte function. MTL-CEBPA is presently in Phase I clinical trials for hepatocellular carcinoma (HCC). The clinical development of MTL-CEBPA will demonstrate "proof of concept" that saRNAs can provide the basis for drugs which enhance target gene expression and consequently improve treatment outcome in patients.

The blood-borne sialyltransferase ST6Gal-1 is a negative systemic regulator of granulopoiesis

Responding to systemic demands in producing and replenishing end-effector blood cells is predicated on the appropriate delivery and interpretation of extrinsic signals to the HSPCs. The data presented herein implicate the systemic, extracellular form of the glycosyltransferase ST6Gal-1 in the regulation of late-stage neutrophil development. ST6Gal-1 is typically a membrane-bound enzyme sequestered within the intracellular secretory apparatus, but an extracellular form is released into the blood from the liver. Both human and murine HSPCs, upon exposure to extracellular ST6Gal-1 ex vivo, exhibited decreased proliferation, diminished expression of the neutrophilic primary granule protein MPO, and decreased appearance of CD11b+ cells. HSPC suppression was preceded by decreased STAT-3 phosphorylation and diminished C/EBPα expression, without increased apoptosis, indicating attenuated G-CSF receptor signaling. A murine model to raise systemic ST6Gal-1 level was developed to examine the role of the circulatory enzyme in vivo. Our results show that systemic ST6Gal-1 modified the cell surface of the GMP subset of HSPCs and decreased marrow neutrophil reserves. Acute airway neutrophilic inflammation by LPS challenge was used to drive demand for new neutrophil production. Reduced neutrophil infiltration into the airway was observed in mice with elevated circulatory ST6Gal-1 levels. The blunted transition of GMPs into GPs in vitro is consistent with ST6Gal-1-attenuated granulopoiesis. The data confirm that circulatory ST6Gal-1 is a negative systemic regulator of granulopoiesis and moreover suggest a clinical potential to limit the number of inflammatory cells by manipulating blood ST6Gal-1 levels.

C/EBPα deregulation as a paradigm for leukemogenesis

Myeloid master regulator CCAAT enhancer-binding protein alpha (C/EBPα) is deregulated by multiple mechanisms in leukemia. Inhibition of C/EBPα function plays pivotal roles in leukemogenesis. While much is known about how C/EBPα orchestrates granulopoiesis, our understanding of molecular transformation events, the role(s) of cooperating mutations and clonal evolution during C/EBPα deregulation in leukemia remains elusive. In this review, we will summarize the latest research addressing these topics with special emphasis on CEBPA mutations. We conclude by describing emerging therapeutic strategies to restore C/EBPα function.

Shwachman-Bodian-Diamond syndrome (SBDS) protein deficiency impairs translation re-initiation from C/EBPα and C/EBPβ mRNAs

Mutations in the Shwachman–Bodian–Diamond Syndrome (SBDS) gene cause Shwachman–Diamond Syndrome (SDS), a rare congenital disease characterized by bone marrow failure with neutropenia, exocrine pancreatic dysfunction and skeletal abnormalities. The SBDS protein is important for ribosome maturation and therefore SDS belongs to the ribosomopathies. It is unknown, however, if loss of SBDS functionality affects the translation of specific mRNAs and whether this could play a role in the development of the clinical features of SDS. Here, we report that translation of the C/EBPα and -β mRNAs, that are indispensible regulators of granulocytic differentiation, is altered by SBDS mutations or knockdown. We show that SBDS function is specifically required for efficient translation re-initiation into the protein isoforms C/EBPα-p30 and C/EBPβ-LIP, which is controlled by a single cis-regulatory upstream open reading frame (uORF) in the 5′ untranslated regions (5′ UTRs) of both mRNAs. Furthermore, we show that as a consequence of the C/EBPα and -β deregulation the expression of MYC is decreased with associated reduction in proliferation, suggesting that failure of progenitor proliferation contributes to the haematological phenotype of SDS. Therefore, our study provides the first indication that disturbance of specific translation by loss of SBDS function may contribute to the development of the SDS phenotype.

The N-terminal CEBPA mutant in acute myeloid leukemia impairs CXCR4 expression

CXC chemokine receptor 4 (CXCR4) is an essential regulator for homing and maintenance of hematopoietic stem cells within the bone marrow niches. Analysis of clinical implications of bone marrow CXCR4 expression in patients with acute myeloid leukemia showed not only higher CXCR4 expression was an independent poor prognostic factor, irrespective of age, white blood cell counts, cytogenetics, and mutation status of NPM1/FLT3-ITD and CEBPA, but also showed CXCR4 expression was inversely associated with mutations of CEBPA, a gene encoding transcription factor C/EBPα. Patients with wild-type CEBPA had significantly higher CXCR4 expression than those with mutated CEBPA. We hypothesized that CEBPA might influence the expression of CXCR4. To test this hypothesis, we first examined endogenous CXCR4 expression in 293T and K562 cells over-expressing wild-type C/EBPα p42 and demonstrated that CXCR4 levels were increased in these cells, whilst the expression of the N-terminal mutant, C/EBPα p30, diminished CXCR4 transcription. We further showed p42 was bound to the CXCR4 promoter by the chromatin immunoprecipitation assays. Induction of p42 in the inducible K562-C/EBPα cell lines increased the chemotactic migration. Moreover, decreased expression of C/EBPα by RNA interference decreased levels of CXCR4 protein expression in U937 cells, thereby abrogating CXCR4-mediated chemotaxis. Our results provide, for the first time, evidence that C/EBPα indeed regulates the activation of CXCR4, which is critical for the homing and engraftment of acute myeloid leukemia cells, while p30 mutant impairs CXCR4 expression.

C/EBPγ deregulation results in differentiation arrest in acute myeloid leukemia

C/EBPs are a family of transcription factors that regulate growth control and differentiation of various tissues. We found that C/EBPγ is highly upregulated in a subset of acute myeloid leukemia (AML) samples characterized by C/EBPα hypermethylation/silencing. Similarly, C/EBPγ was upregulated in murine hematopoietic stem/progenitor cells lacking C/EBPα, as C/EBPα mediates C/EBPγ suppression. Studies in myeloid cells demonstrated that CEBPG overexpression blocked neutrophilic differentiation. Further, downregulation of Cebpg in murine Cebpa-deficient stem/progenitor cells or in human CEBPA-silenced AML samples restored granulocytic differentiation. In addition, treatment of these leukemias with demethylating agents restored the C/EBPα-C/EBPγ balance and upregulated the expression of myeloid differentiation markers. Our results indicate that C/EBPγ mediates the myeloid differentiation arrest induced by C/EBPα deficiency and that targeting the C/EBPα-C/EBPγ axis rescues neutrophilic differentiation in this unique subset of AMLs.

C/EBPα and DEK coordinately regulate myeloid differentiation

The transcription factor C/EBPα is a critical mediator of myeloid differentiation and is often functionally impaired in acute myeloid leukemia. Recent studies have suggested that oncogenic FLT3 activity disrupts wild-type C/EBPα function via phosphorylation on serine 21 (S21). Despite the apparent role of pS21 as a negative regulator of C/EBPα transcription activity, the mechanism by which phosphorylation tips the balance between transcriptionally competent and inhibited forms remains unresolved. In the present study, we used immuno-affinity purification combined with quantitative mass spectrometry to delineate the proteins associated with C/EBPα on chromatin. We identified DEK, a protein with genetic links to leukemia, as a member of the C/EBPα complexes, and demonstrate that this association is disrupted by S21 phosphorylation. We confirmed that DEK is recruited specifically to chromatin with C/EBPα to enhance GCSFR3 promoter activation. In addition, we demonstrated that genetic depletion of DEK reduces the ability of C/EBPα to drive the expression of granulocytic target genes in vitro and disrupts G-CSF-mediated granulocytic differentiation of fresh human BM-derived CD34(+) cells. Our data suggest that C/EBPα and DEK coordinately activate myeloid gene expression and that S21 phosphorylation on wild-type C/EBPα mediates protein interactions that regulate the differentiation capacity of hematopoietic progenitors.

STAT3 controls myeloid progenitor growth during emergency granulopoiesis

Granulocyte colony-stimulating factor (G-CSF) mediates "emergency" granulopoiesis during infection, a process that is mimicked by clinical G-CSF use, yet we understand little about the intracellular signaling cascades that control demand-driven neutrophil production. Using a murine model with conditional deletion of signal transducer and activator of transcription 3 (STAT3) in bone marrow, we investigated the cellular and molecular mechanisms of STAT3 function in the emergency granulopoiesis response to G-CSF administration or infection with Listeria monocytogenes, a pathogen that is restrained by G-CSF signaling in vivo. Our results show that STAT3 deficiency renders hematopoietic progenitor cells and myeloid precursors refractory to the growth-promoting functions of G-CSF or L monocytogenes infection. STAT3 is necessary for accelerating granulocyte cell-cycle progression and maturation in response to G-CSF. STAT3 directly controls G-CSF-dependent expression of CCAAT-enhancer-binding protein β (C/EBPβ), a crucial factor in the emergency granulopoiesis response. Moreover, STAT3 and C/EBPβ coregulate c-Myc through interactions with the c-myc promoter that control the duration of C/EBPα occupancy during demand-driven granulopoiesis. These results place STAT3 as an essential mediator of emergency granulopoiesis by its regulation of transcription factors that direct G-CSF-responsive myeloid progenitor expansion.

HAT: hypergeometric analysis of tiling-arrays with application to promoter-GeneChip data

Background

Tiling-arrays are applicable to multiple types of biological research questions. Due to its advantages (high sensitivity, resolution, unbiased), the technology is often employed in genome-wide investigations. A major challenge in the analysis of tiling-array data is to define regions-of-interest, i.e., contiguous probes with increased signal intensity (as a result of hybridization of labeled DNA) in a region. Currently, no standard criteria are available to define these regions-of-interest as there is no single probe intensity cut-off level, different regions-of-interest can contain various numbers of probes, and can vary in genomic width. Furthermore, the chromosomal distance between neighboring probes can vary across the genome among different arrays.

Results

We have developed Hypergeometric Analysis of Tiling-arrays (HAT), and first evaluated its performance for tiling-array datasets from a Chromatin Immunoprecipitation study on chip (ChIP-on-chip) for the identification of genome-wide DNA binding profiles of transcription factor Cebpa (used for method comparison). Using this assay, we can refine the detection of regions-of-interest by illustrating that regions detected by HAT are more highly enriched for expected motifs in comparison with an alternative detection method (MAT). Subsequently, data from a retroviral insertional mutagenesis screen were used to examine the performance of HAT among different applications of tiling-array datasets. In both studies, detected regions-of-interest have been validated with (q)PCR.

Conclusions

We demonstrate that HAT has increased specificity for analysis of tiling-array data in comparison with the alternative method, and that it accurately detects regions-of-interest in two different applications of tiling-arrays. HAT has several advantages over previous methods: i) as there is no single cut-off level for probe-intensity, HAT can detect regions-of-interest at various thresholds, ii) it can detect regions-of-interest of any size, iii) it is independent of probe-resolution across the genome, and across tiling-array platforms and iv) it employs a single user defined parameter: the significance level. Regions-of-interest are detected by computing the hypergeometric-probability, while controlling the Family Wise Error. Furthermore, the method does not require experimental replicates, common regions-of-interest are indicated, a sequence-of-interest can be examined for every detected region-of-interest, and flanking genes can be reported.

miR-328 functions as an RNA decoy to modulate hnRNP E2 regulation of mRNA translation in leukemic blasts

MicroRNAs and heterogeneous ribonucleoproteins (hnRNPs) are posttranscriptional gene regulators that bind mRNA in a sequence-specific manner. Here, we report that loss of miR-328 occurs in blast crisis chronic myelogenous leukemia (CML-BC) in a BCR/ABL dose- and kinase-dependent manner through the MAPK-hnRNP E2 pathway. Restoration of miR-328 expression rescues differentiation and impairs survival of leukemic blasts by simultaneously interacting with the translational regulator poly(rC)-binding protein hnRNP E2 and with the mRNA encoding the survival factor PIM1, respectively. The interaction with hnRNP E2 is independent of the microRNA's seed sequence and it leads to release of CEBPA mRNA from hnRNP E2-mediated translational inhibition. Altogether, these data reveal the dual ability of a microRNA to control cell fate both through base pairing with mRNA targets and through a decoy activity that interferes with the function of regulatory proteins.

A robust and highly efficient immune cell reprogramming system

Here we describe a lineage reprogramming system consisting of a B cell line with an estradiol-inducible form of C/EBPalpha where cells can be converted into macrophage-like cells at 100% efficiency within 2 to 3 days. The reprogrammed cells are larger, contain altered organelle and cytoskeletal structures, are phagocytic, and exhibit an inflammatory response. Time-lapse experiments showed that the cells acquire a macrophage morphology and increased migratory activity as early as 10 hr. During induction, thousands of genes become up- or downregulated, including several dozen transcription and chromatin-remodeling factors. Time-limited exposure of cells to the inducer showed that the reprogrammed cells become transgene independent within 1 to 2 days. The reprogramming can be inhibited, at least partially, by perturbation experiments with B cell and macrophage transcription factors. The tightness, robustness, and speed of the system described make it a versatile tool to study biochemical and biological aspects of lineage reprogramming.

Neutrophil elastase is severely down-regulated in severe congenital neutropenia independent of ELA2 or HAX1 mutations but dependent on LEF-1

Severe congenital neutropenia (CN) is a heterogeneous disorder of myelopoiesis which follows an autosomal dominant or autosomal recessive pattern of inheritance. Genetic analyses indicate mutations in the ELA2 gene in most patients. We have identified LEF-1 as a decisive transcription factor in granulopoiesis controlling proliferation and granulocytic differentiation by direct activation of its target gene, C/EBPα. In patients with CN, the expression of LEF-1 and C/EBPα was abrogated in myeloid progenitors leading to maturation arrest of granulopoiesis. In the present study we demonstrated that ELA2 mRNA expression in myeloid progenitors and plasma protein levels of neutrophil elastase (NE) were markedly reduced in patients with CN harboring mutations in either ELA2 or HAX-1 genes. The ELA2 gene promoter is positively regulated by the direct binding of LEF-1 or C/EBPα, documenting the role of LEF1 in the diminished ELA2 expression. We found that transduction of hematopoietic cells with LEF-1 cDNA resulted in the up-regulation of ELA2/NE synthesis, whereas inhibition of LEF-1 by shRNA led to a marked reduction in the levels of ELA2/NE. LEF-1 rescue of CD34+ cells isolated from 2 patients with CN resulted in granulocytic differentiation of the cells which was in line with increased levels of functionally active ELA2/NE.

M-CSF elevates c-Fos and phospho-C/EBPalpha(S21) via ERK whereas G-CSF stimulates SHP2 phosphorylation in marrow progenitors to contribute to myeloid lineage specification

The role of hematopoietic cytokines in lineage commitment remains uncertain. To gain insight into the contribution of cytokine signaling to myeloid lineage specification, we compared granulocyte colony-stimulating factor (G-CSF) and macrophage colony-stimulating factor (M-CSF) signaling in Ba/F3 cells expressing both the G-CSF and M-CSF receptors and in lineage-negative murine marrow cells. G-CSF and M-CSF serve as prototypes for additional cytokines that also influence immature myeloid cells. G-CSF specifically activated signal transducer and activator of transcription 3 and induced Src homology region 2 domain-containing phosphatase 2 (SHP2) phosphorylation, whereas M-CSF preferentially activated phospholipase Cgamma2, and thereby extracellular signal-regulated kinase (ERK), to stabilize c-Fos and stimulate CCAAT/enhancer-binding protein (C/EBP)alpha(S21) phosphorylation. In contrast, activation of Jun kinase or c-Jun was similar in response to either cytokine. Inhibition of ERK prevented induction of c-Fos by M-CSF and reduced C/EBPalpha phosphorylation and formation of colony-forming unit-monocytes. SHP2 inhibition reduced ERK activation in G-CSF, but not M-CSF, and reduced colony-forming unit-granulocytes, underscoring divergent pathways to ERK activation. Phorbol ester mimicked the effect of M-CSF, activating ERK independent of SHP2. In summary, M-CSF activates ERK more potently than G-CSF, and thereby induces higher levels of c-Fos and phospho-C/EBPalpha(S21), which may directly interact to favor monopoiesis, whereas G-CSF activates signal transducer and activator of transcription 3 and SHP2, potentially shifting the balance to granulopoiesis via gene induction by C/EBPalpha homodimers and via effects of SHP2 on regulators besides ERK.

NF-kappa B p50 regulates C/EBP alpha expression and inflammatory cytokine-induced neutrophil production

NF-kappaB is a key transcriptional inducer of the inflammatory response in mature myeloid cells, and also stimulates cell survival, but its role in immature myeloid cell development has not been well characterized. C/EBPalpha is required for the development of monocytic and granulocytic myeloid cells from early progenitors, and NF-kappaB and C/EBPbeta cooperatively induce several inflammatory mediators. Having found that C/EBPalpha binds NF-kappaB p50 preferentially compared with NF-kappaB p65, we have now investigated myelopoiesis in nfkb1(-/-) mice lacking NF-kappaB p50. Absence of p50 leads to a significant reduction in the number of granulocytic progenitors, CFU-granulocyte, obtained with G-CSF or GM-CSF in vitro and reduces neutrophil production in vivo in response to G-CSF, with preservation of monopoiesis in vitro in response to cytokines or LPS. To gain insight into the mechanism underlying reduced granulopoiesis in the absence of NF-kappaB p50, we assessed the expression of several myeloid regulatory proteins in lineage-negative, immature myeloid cells. Although PU.1, C/EBPbeta, and STAT3 levels were unchanged, C/EBPalpha protein and RNA levels were reduced approximately 3-fold in the absence of NF-kappaB p50. In addition, NF-kappaB p50 and C/EBPalpha bound the endogenous C/EBPalpha promoter in a chromatin immunoprecipitation assay, and NF-kappaB p50 trans activated the C/EBPalpha promoter, alone or in cooperation with C/EBPalpha. Despite reduction of C/EBPalpha, G-CSFR and M-CSFR levels were maintained in total marrow and in lineage-negative cells. Together, these data indicate that acute inflammation not only activates mature myeloid cells, but also stimulates neutrophil production via NF-kappaB p50 induction of C/EBPalpha transcription.

EVI1 Impairs myelopoiesis by deregulation of PU.1 function

EVI1 is an oncogene inappropriately expressed in the bone marrow (BM) of approximately 10% of myelodysplastic syndrome (MDS) patients. This disease is characterized by severe anemia and multilineage myeloid dysplasia that are thought to be a major cause of mortality in MDS patients. We earlier reported on a mouse model that constitutive expression of EVI1 in the BM led to fatal anemia and myeloid dysplasia, as observed in MDS patients, and we subsequently showed that EVI1 interaction with GATA1 blocks proper erythropoiesis. Whereas this interaction could provide the basis for the erythroid defects in EVI1-positive MDS, it does not explain the alteration of myeloid differentiation. Here, we have examined the expression of several genes activated during terminal myelopoiesis in BM cells and identified a group of them that are altered by EVI1. A common feature of these genes is their regulation by the transcription factor PU.1. We report here that EVI1 interacts with PU.1 and represses the PU.1-dependent activation of a myeloid promoter. EVI1 does not seem to inhibit PU.1 binding to DNA, but rather to block its association with the coactivator c-Jun. After mapping the PU.1-EVI1 interaction sites, we show that an EVI1 point mutant, unable to bind PU.1, restores the activation of PU.1-regulated genes and allows a normal differentiation of BM progenitors in vitro.

Essential role of C/EBPalpha in G-CSF-induced transcriptional activation and chromatin modification of myeloid-specific genes

Granulocyte colony-stimulating factor (G-CSF) regulates the proliferation and differentiation of neutrophilic progenitor cells. Here, we investigated the roles of CCAAT/enhancer-binding protein (C/EBP)alpha in the G-CSF-induced transcriptional activation and chromatin modification of the CCR2 and myeloperoxidase (MPO) genes in IL-3-dependent myeloid FDN1.1 cells. Chromatin immunoprecipitation (ChIP) and electrophoretic mobility shift assays revealed that G-CSF activates C/EBPalpha to bind target promoters. ChIP mapping experiments across the CCR2 and MPO genes showed that G-CSF induces histone H3 modifications: the acetylation of Lys9, trimethylation of Lys4 and trimethylation of Lys9. The distribution profile of the trimethylated Lys9 was distinct from that of the two other modifications. All the G-CSF-induced C/EBPalpha recruitment, transcriptional activation and histone modifications were reversed by re-stimulation with IL-3, and were abolished by short hairpin RNA (shRNA)-mediated knockdown of C/EBPalpha. These results indicate that C/EBPalpha is activated by G-CSF to bind target promoters, and plays critical roles in the transcriptional activation and dynamic chromatin modification of target genes during neutrophil differentiation.

LRG-accelerated differentiation defines unique G-CSFR signaling pathways downstream of PU.1 and C/EBPepsilon that modulate neutrophil activation

Expression of leucine-rich alpha2 glycoprotein (LRG), a member of the leucine-rich repeat family of proteins, was recently shown to be up-regulated during neutrophil differentiation. Its precise role in granulopoiesis, however, remains unknown. In this paper, we show that the transcription factors PU.1 and C/EBPepsilon that regulate the expression of multiple myeloid-specific genes also bind to the LRG promoter. We also demonstrate that LRG localizes to the same cytoplasmic compartment as myeloperoxidase and that G-CSF treatment of the 32Dcl3 myeloid cell line induces nuclear translocation of LRG. Stable transfection of LRG into 32Dcl3 cells resulted in accelerated, G-CSF-mediated neutrophil differentiation and induction of CD11b expression. In contrast, constitutive expression of LRG in 32Dwt18 cells, expressing a chimeric erythropoietin (Epo)/G-CSFR consisting of the EpoR extracellular domain fused to the G-CSFR transmembrane and cytoplasmic domains, failed to induce accelerated neutrophil differentiation and CD11b expression in response to Epo stimulation. LRG-mediated accelerated differentiation and CD11b expression were found to correlate with an increased level of phospho-Stat3 but not with PU.1 or p27(kip1) levels. Hence, similar to other genes involved in neutrophil differentiation, the expression of LRG also appears to be regulated by PU.1 and C/EBPepsilon. Collectively, these findings suggest a role for LRG in modulating neutrophil differentiation and expression of CD11b via nonredundant G-CSFR signals.

C/EBPalphap30 plays transcriptional regulatory roles distinct from C/EBPalphap42

CCAAT/enhancer binding protein alpha (C/EBPalpha) is a transcriptional regulatory factor that inhibits cell proliferation, and alternative translational initiation produces two polypeptides, C/EBPalphap30 and C/EBPalphap42. By expression profiling, it was revealed that C/EBPalphap30 specifically inhibited a unique set of genes, including MPP11, p84N5 and SMYD2, which were not affected by C/EBPalphap42 in both QSG-7701 hepatocyte cell line and QGY-7703 hepatoma cells. Semi-quantitative RT-PCR analysis independently confirmed these results. Chromatin immunoprecipitation assay showed that C/EBPalphap30 bound to the promoters of these genes more strongly than C/EBPalphap42. In clinical hepatoma samples in which C/EBPalpha was downregulated, all three genes specifically inhibited by C/EBPalphap30 were upregulated. However, repression of MPP11, p84N5 and SMYD2 genes might not be directly involved in C/EBPalphap30-mediated growth inhibition. Our data suggest that C/EBPalphap30 regulates a unique set of target genes and is more than a dominant-negative regulator of C/EBPalphap42.

LEF-1 is a decisive transcription factor in neutrophil granulopoiesis

We found that lymphoid enhancer-binding factor 1 (LEF-1) is a decisive transcription factor in granulopoiesis controlling proliferation, proper lineage commitment, and granulocytic differentiation via regulation of its target genes C/EBP-alpha, cyclin D1, c-myc, and survivin. Myeloid progenitor cells of patients with severe congenital neutropenia (CN) showed a severe downregulation of LEF-1 and its target genes expression. Expression of neutrophil elastase (NE) is also severely reduced in CN myeloid progenitors. Intriguingly, ELA2 gene promoter is positively regulated by direct binding of LEF-1 or LEF-1 target gene C/EBP-alpha. In summary we demonstrated that LEF-1 is not only crucial in lymphopoiesis, but also in myelopoiesis, documenting new functions of LEF-1.

Transcriptional regulation in myelopoiesis: Hematopoietic fate choice, myeloid differentiation, and leukemogenesis

Myeloid cells (granulocytes and monocytes) are derived from multipotent hematopoietic stem cells. Gene transcription plays a critical role in hematopoietic differentiation. However, there is no single transcription factor that is expressed exclusively by myeloid cells and that, alone, acts as a "master" regulator of myeloid fate choice. Rather, myeloid gene expression is controlled by the combinatorial effects of several key transcription factors. Hematopoiesis has traditionally been viewed as linear and hierarchical, but there is increasing evidence of plasticity during blood cell development. Transcription factors strongly influence cellular lineage during hematopoiesis and expression of some transcription factors can alter the fate of developing hematopoietic progenitor cells. PU.1 and CCAAT/enhancer-binding protein alpha (C/EBPalpha) regulate expression of numerous myeloid genes, and gene disruption studies have shown that they play essential, nonredundant roles in myeloid cell development. They function in cooperation with other transcription factors, co-activators, and co-repressors to regulate genes in the context of chromatin. Because of their essential roles in regulating myeloid genes and in myeloid cell development, it has been hypothesized that abnormal expression of PU.1 and C/EBPalpha would contribute to aberrant myeloid differentiation, i.e. acute leukemia. Such a direct link has been elusive until recently. However, there is now persuasive evidence that mutations in both PU.1 and C/EBPalpha contribute directly to development of acute myelogenous leukemia. Thus, normal myeloid development and acute leukemia are now understood to represent opposite sides of the same hematopoietic coin.

Microarray and serial analysis of gene expression analyses identify known and novel transcripts overexpressed in hematopoietic stem cells

The human CD34(+)/CD38(-)/Lin(-) cell subset, comprising approximately 1-10% of the CD34(+) cell population, contains few of the less primitive hematopoietic (lineage-committed) progenitor cells (HPCs) but most of the primitive in vivo engrafting (lympho-)hematopoietic stem cells (HSCs). We analyzed gene expression in CD34(+)/CD38(-)/Lin(-) cell populations isolated from normal human adult donor bone marrow, neonatal placental/umbilical cord blood, and mobilized adult donor peripheral blood stem-progenitor cells. As measured by Affymetrix microarrays, 4746 genes were expressed in CD34(+)/CD38(-)/Lin(-) cells from all three tissues. We also determined the transcriptomes of the stem cell-depleted, HPC-enriched CD34(+)/[CD38/Lin](++) cell population from each tissue. Comparison of CD34(+)/CD38(-)/Lin(-) (HSC-enriched) versus CD34(+)/[CD38/Lin](++) (HPC-enriched, HSC-depleted) cells from each tissue yielded 81 genes overrepresented and 90 genes underrepresented, common to all three of the CD34(+)/CD38(-)/Lin(-) cell populations. These transcripts, which are selectively expressed in HSCs from all three tissues, include a number of known genes (e.g., transcription factors, receptors, and signaling molecules) that might play roles in key functions (e.g., survival, self-renewal, differentiation, and/or migration/adhesion) of human HSCs. Many genes/transcripts of unknown function were also detected by microarray analysis. Serial analysis of gene expression of the bone marrow HSC and HPC populations confirmed expression of most of the overrepresented transcripts for which reliable serial analysis of gene expression tags were detected and additionally suggested that current microarrays do not detect as many as 30% of the transcripts expressed in HSCs, including a number of previously unknown transcripts. This work is a step toward full definition of the transcriptome of normal human HSCs and may identify new genes involved in leukemogenesis and cancer stem cells.

C/EBPalphap30, a myeloid leukemia oncoprotein, limits G-CSF receptor expression but not terminal granulopoiesis via site-selective inhibition of C/EBP DNA binding

Heterozygous mutations of the CEBPA gene are present in 5% of acute myeloid leukemia (AML) cases and often lead to the expression of an N-terminally truncated, 30 kDa isoform, C/EBPalphap30, from an internal translation start site. We have assessed the effect of C/EBPalphap30 on granulopoiesis utilizing C/EBPalphap30-ER, containing the estradiol receptor ligand-binding domain. In contrast to C/EBPalpha-ER, C/EBPalphap30-ER did not induce 32Dcl3 myeloid cell differentiation in IL-3. However, both isoforms, when expressed at high levels, were capable of inhibiting E2F activity in 32Dcl3 cells and of slowing their G1 to S progression. C/EBPalphap30 repressed expression of the endogenous G-CSF receptor several-fold. To facilitate investigation of the effect of C/EBPalphap30-ER on granulopoiesis downstream of G-CSF signalling, we coexpressed exogenous G-CSF receptor. C/EBPalphap30-ER/GR cells expressed several granulocytic markers in G-CSF and demonstrated nuclear maturation. Rat C/EBPalpha-ER and C/EBPalphap30-ER, expressed in 293T cells, bound the C/EBP site from the NE gene with similar affinity, as did human C/EBPalpha and C/EBPalphap30. In contrast, C/EBPalphap30 bound the C/EBP sites in the PU.1 or GR gene with 3-6-fold reduced affinity. Thus, the selective inhibition of GR expression by C/EBPalphap30-ER is due in part to its variable affinity for C/EBP sites. Variation in affinity for selected cis elements among isoforms may affect the biology of basic region-leucine zipper (bZIP) proteins.

Regulation of granulocyte and monocyte differentiation by CCAAT/enhancer binding protein α

CCAAT/enhancer binding protein alpha (C/EBPalpha)-ER induces 32Dcl3 neutrophilic differentiation and inhibits 32DPKCdelta maturation to macrophages in response to phorbol ester. In 32Dcl3 cells, C/EBPalpha-ER rapidly induces the PU.1 and C/EBPalpha RNAs even in the presence of cycloheximide, suggesting that these are direct C/EBPalpha genetic targets. C/EBPalpha strongly binds and modestly activates the murine PU.1 promoter via an evolutionarily conserved binding site. C/EBPalpha-ER variants incapable of binding DNA still slow G1 progression but do not induce differentiation. N-terminally truncated C/EBPalpha variants, including the p30 isoform expressed in a subset of AMLs, also retain the ability to slow 32D cl3 proliferation, whereas the C/EBPalpha(BRM2)-ER variant does not slow G1 progression, has a reduced capacity to induce early granulocytic markers, and does not induce terminal maturation. In 32DPKCdelta cells, C/EBPalpha-ER strongly inhibits endogenous or exogenous JunB induction, dependent upon the outer surface of the C/EBPalpha basic region, but does not inhibit c-Jun, PU.1, or C/EBPbeta expression. Exogenous JunB restores AP-1 DNA binding but does not overcome inhibition of monopoiesis by C/EBPalpha-ER. In summary, we propose that while C/EBPalpha is required for development of immature granulocyte-monocyte progenitors, C/EBPalpha subsequently inhibits monopoiesis, via inhibition of JunB express and via additional activities, and induces granulopoiesis, via induction of PU.1, C/EBPepsilon, and cell cycle arrest.

Role of chemokines in angiogenesis: CXCL12/SDF-1 and CXCR4 interaction, a key regulator of endothelial cell responses

Chemokines are small proteins that act as cell attractants via the activation of G protein-coupled receptors. Chemokines play an important role in several pathophysiological processes such as inflammation and immunity. Many proinflammatory chemokines also support the development of vascular blood supply at the site of inflammation. Similarly, tumor-generated chemokines can contribute to tumor growth by promoting angiogenesis. Recently, significant advances have been made in understanding the contribution of chemokines to the angiogenesis process. This review will discuss first the evidence supporting the direct contribution of different chemokine subfamily members, including CC, CXC, and CX3C chemokines, as positive or negative regulators of the angiogenesis process based on the expression of their cognate receptors on endothelial cells. Additionally, the relationship between classic angiogenic factors and chemokine receptor expression on endothelial cells, and the implications of chemokine production by cancer cells will be analyzed with particular emphasis on the CXCL12/stromal-cell derived factor-1 interaction with CXCR4.

AML1 interconnected pathways of leukemogenesis

The AML1 transcription factor, identified by the cloning of the translocation t(8;21) breakpoint, is one of the most frequent targets for chromosomal translocations in leukemia. Furthermore, polysomies and point mutations can also alter AML1 function. AML1, also called CBF alpha 2, PEBP alpha 2 or RUNX1, is thus implicated in a great number of acute leukemias via a variety of pathogenic mechanisms and seems to act either as an oncogene or a tumor suppressor gene. Characterization of AML1 knockout mice has shown that AML1 is necessary for normal development of all hematopoietic lineages and alterations in the overal functional level of AML1 can have a profound effect on hematopoiesis. Numerous studies have shown that AML1 plays a vital role in the regulation of expression of many genes involved in hematopoietic cell development, and the impairment of AML1 function disregulates the pathways leading to cellular proliferation and differentiation. However, heterozygous AML1 mutations alone may not be sufficient for the development of leukemia. A cumulative process of mutagenesis involving additional genetic events in functionally related molecules, may be necessary for the development of leukemia and may determine the leukemic phenotype. We review the known AML1 target genes, AML1 interacting proteins, AML1 gene alterations and their effects on AML1 function, and mutations in AML1-related genes associated with leukemia. We discuss the interconnections between all these genes in cell signaling pathways and their importance for future therapeutic developments.

Regulation of neutrophil and eosinophil secondary granule gene expression by transcription factors C/EBP epsilon and PU.1

In the bone marrow of C/EBP epsilon(-/-) mice, expression of neutrophil secondary and tertiary granule mRNAs is absent for lactoferrin (LF), neutrophil gelatinase (NG), murine cathelin-like protein (MCLP), and the cathelin B9; it is severely reduced for neutrophil collagenase (NC) and neutrophil gelatinase-associated lipocalin (NGAL). In addition, the expression of eosinophil granule genes, major basic protein (MBP), and eosinophil peroxidase (EPX) is absent. These mice express C/EBP alpha, C/EBP beta, and C/EBP delta in the bone marrow at levels similar to those of their wild-type counterparts, suggesting a lack of functional redundancy among the family in vivo. Stable inducible expression of C/EBP epsilon and C/EBP alpha in the murine fibroblast cell line NIH 3T3 activated expression of mRNAs for B9, MCLP, NC, and NGAL but not for LF. In transient transfections of C/EBP epsilon and C/EBP alpha, B9 was strongly induced with weaker induction of the other genes. C/EBP beta and C/EBP delta proteins weakly induced B9 expression, but C/EBP delta induced NC expression more efficiently than the other C/EBPs. The expression of MBP was inefficiently induced by C/EBP epsilon alone and weakly induced with C/EBP epsilon and GATA-1, but the addition of PU.1 resulted in a striking cooperative induction of MBP in NIH 3T3 cells. Mutation of a predicted PU.1 site in the human MBP promoter-luciferase reporter construct abrogated the response to PU.1. Gel-shift analysis demonstrated binding of PU.1 to this site. MBP and EPX mRNAs were absent in a PU.1-null myeloid cell line established from the embryonic liver of PU.1(-/-) mice. Restitution of PU.1 protein expression restored MBP and EPX protein expression. This study demonstrates that C/EBP epsilon is essential and sufficient for the expression of a particular subset of neutrophil secondary granule genes. Furthermore, it indicates the importance of PU.1 in the cooperative activation of eosinophil granule genes.

The peripheral cannabinoid receptor Cb2, a novel oncoprotein, induces a reversible block in neutrophilic differentiation

We previously identified a novel common virus integration site, Evi11, by means of retroviral insertional mutagenesis. We demonstrated that the gene encoding the peripheral cannabinoid receptor (Cb2) is the potential target, suggesting that Cb2 is a proto-oncogene. To elucidate a role for this G protein-coupled receptor (GPCR) in leukemic transformation we generated a Cb2-EGFP cDNA construct that was introduced into 32D/G-CSF-R cells. These cells require interleukin 3 (IL-3) to proliferate in vitro, whereas in the presence of granulocyte-colony-stimulating factor (G-CSF) they differentiate toward mature neutrophils. We demonstrate that 32D/G-CSF-R/Cb2-EGFP cells migrate in a transwell assay in reponse to the Cb2 ligand 2-arachidonoylglycerol (2-AG), indicating that the fusion protein was functional. When cultured in the presence of G-CSF neutrophilic differentiation of Cb2-EGFP-expressing 32D/G-CSF-R cells was completely blocked. Moreover, a Cb2-specific antagonist fully recovered the G-CSF-induced neutrophilic differentiation of 32D/G-CSF-R/Cb2-EGFP cells. To investigate which signal transduction pathway(s) may be involved in the block of neutrophilic maturation, differentiation experiments were carried out using specific inhibitors of signaling routes. Interestingly, full rescue of G-CSF-induced neutrophilic differentiation was observed when cells were cultured with the mitogen-induced extracellular kinase (MEK) inhibitors, PD98059 or U0126, and partial recovery was detected with the phosphoinositide 3-kinase (PI3-K) inhibitor LY-294002. These studies demonstrate that the Cb2 receptor is an oncoprotein that blocks neutrophilic differentiation when overexpressed in myeloid precursor cells. Cb2 appears to mediate its activity through MEK/extracellular signal-related kinase (ERK) and PI3-K pathways.

Ras signaling enhances the activity of C/EBP alpha to induce granulocytic differentiation by phosphorylation of serine 248

The transcription factor C/EBP alpha regulates early steps of normal granulocyte differentiation since mice with a disruption of the C/EBP alpha gene do not express detectable levels of the granulocyte colony-stimulating factor receptor and produce no neutrophils. We have recently shown that C/EBP alpha function is also impaired in acute myeloid leukemias. However, how the transcriptional activity of C/EBP alpha is regulated both in myelopoiesis and leukemogenesis is not fully understood. The current study demonstrates that activated Ras enhances the ability of C/EBP alpha to transactivate the granulocyte colony-stimulating factor receptor promoter and a minimal promoter containing only C/EBP DNA binding sites. Ras signaling activates C/EBP alpha via the transactivation domain because it enhances the transactivation function of a fusion protein containing a Gal4 DNA binding domain and the C/EBP alpha transactivation domain and does not change C/EBP alpha DNA binding. Ras acts on serine 248 of the C/EBP alpha transactivation domain, because it does not enhance the transactivation function of a C/EBP alpha serine 248 to alanine point mutant. Interestingly, serine 248 of C/EBP alpha is a protein kinase C (PKC) consensus site, and a PKC inhibitor blocks the activation of C/EB alpha by Ras. Ras signaling leads to phosphorylation of C/EBP alpha in vivo. Finally, mutation of serine 248 to alanine obviates the ability of C/EBP alpha to induce granulocytic differentiation. These data suggest a model where Ras signaling enhances the activity of C/EBP alpha to induce granulocytic differentiation by phosphorylation of serine 248.

Transcriptional regulation of myelopoiesis

A common myeloid progenitor gives rise to both granulocytes and monocytes. The early stages of granulopoiesis are mediated by the C/EBPalpha, PU.1, RAR, CBF, and c-Myb transcription factors, and the later stages require C/EBPepsilon, PU.1, and CDP. Monocyte development requires PU.1 and interferon consensus sequence binding protein and can be induced by Maf-B, c-Jun, or Egr-1. Cytokine receptor signals modulate transcription factor activities but do not determine cell fates. Several mechanisms orchestrate the myeloid developmental program, including cooperative gene regulation, protein:protein interactions, regulation of factor levels, and induction of cell cycle arrest.

Transcriptional regulation of granulocyte and monocyte development

Granulocytes and monocytes develop from a common myeloid progenitor. Early granulopoiesis requires the C/EBPalpha, PU.1, RAR, CBF, and c-Myb transcription factors, and terminal neutrophil differentiation is dependent upon C/EBPepsilon, PU.1, Sp1, CDP, and HoxA10. Monopoiesis can be induced by Maf-B, c-Jun, or Egr-1 and is dependent upon PU.1, Sp1, and ICSBP. Signals eminating from cytokine receptors modulate factor activities but do not determine cell fates. Orchestration of the myeloid developmental program is achieved via cooperative gene regulation, via synergistic and inhibitory protein-protein interactions, via promoter auto-regulation and cross-regulation, via regulation of factor levels, and via induction of cell cycle arrest: For example, c-Myb and C/EBPalpha cooperate to activate the mim-1 and NE promoters, PU.1, C/EBPalpha, and CBF, regulate the NE, MPO, and M-CSF Receptor genes. PU.1:GATA-1 interaction and C/EBP suppression of FOG transcription inhibits erythroid and megakaryocyte gene expression. c-Jun:PU.1, ICSBP:PU.1, and perhaps Maf:Jun complexes induce monocytic genes. PU.1 and C/EBPalpha activate their own promoters, C/EBPalpha rapidly induces PU.1 and C/EBPepsilon RNA expression, and RARalpha activates the C/EBPepsilon promoter. Higher levels of PU.1 are required for monopoiesis than for B-lymphopoiesis, and higher C/EBP levels may favor granulopoiesis over monopoiesis. CBF and c-Myb stimulate proliferation whereas C/EBPalpha induces a G1/S arrest; cell cycle arrest is required for terminal myelopoiesis, perhaps due to expression of p53 or hypo-phosphorylated Rb.

CCAAT/enhancer-binding proteins are required for granulopoiesis independent of their induction of the granulocyte colony-stimulating factor receptor

Potential redundancy among members of the CCAAT/enhancer-binding protein (C/EBP) family in myeloid cells is indicated by the ability of C/EBPbeta to replace C/EBPalpha in vivo, by the expression of granulocyte colony-stimulating factor receptor (G-CSFR) on C/EBPalpha(-/-) cell lines, and by our finding that as with C/EBPalpha-estrogen receptor (C/EBPalpha-ER), either C/EBPbeta-ER or C/EBPdelta-ER can induce terminal granulopoiesis in 32D cl3 cells. To assess the consequences of globally inhibiting C/EBPs, we employed KalphaER, containing a Kruppel-associated box (KRAB) transrepression domain, the C/EBPalpha DNA-binding domain, and an ER ligand-binding domain. C/EBPs have a common DNA-binding consensus, and activation of KalphaER repressed transactivation by endogenous C/EBPs 50-fold and reduced endogenous G-CSFR expression. In 32D cl3 cells coexpressing exogenous G-CSFR, activation of KalphaER prevented and even reversed myeloperoxidase, lysozyme, lactoferrin, and C/EBPepsilon RNA induction by G-CSF. In contrast, induction of PU.1 and CD11b, a gene regulated by PU.1 but not by C/EBPs, was unaffected. A KalphaER variant incapable of binding DNA owing to an altered leucine zipper did not affect 32D cl3 differentiation. Transduction of KalphaER into murine hematopoietic progenitor cells suppressed the formation of granulocyte colony-forming units, even in cytokines that enable C/EBPalpha(-/-) progenitors to differentiate into neutrophils. The formation of macrophage and of granulocyte-macrophage colony-forming units were also inhibited, but erythroid burst-forming units grew normally. Thus, in 32D cl3 cells and perhaps normal progenitors, C/EBPs are required for granulopoiesis beyond their ability to induce receptors for G-CSF and other cytokines. One requisite activity may be activation of the C/EBPepsilon gene by C/EBPalpha, as either C/EBPalpha-ER or C/EBPbeta-ER rapidly elevated C/EBPepsilon RNA in 32D cl3 cells in the presence of cycloheximide but not actinomycin D.