CCAAT/enhancer binding proteins are critical components of the transcriptional regulation of hematopoiesis (Review)

Transcriptional Regulation of the Human IL5RA Gene through Alternative Promoter Usage during Eosinophil Development

Regulation of the IL-5 receptor alpha (IL5RA) gene is complicated, with two known promoters (P1 and P2) driving transcription, and two known isoforms (transmembrane and soluble) dichotomously affecting the signaling potential of the protein products. Here, we sought to determine the patterns of P1 and P2 promoter usage and transcription factor occupancy during primary human eosinophil development from CD34⁺ hematopoietic stem cell progenitors. We found that during eosinophilopoiesis, both promoters were active but subject to distinct temporal regulation, coincident with combinatorial interactions of transcription factors, including GATA-1, PU.1, and C/EBP family members. P1 displayed a relatively constant level of activity throughout eosinophil development, while P2 activity peaked early and waned thereafter. The soluble IL-5Rα mRNA peaked early and showed the greatest magnitude fold-induction, while the signaling-competent transmembrane isoform peaked moderately. Two human eosinophilic cell lines whose relative use of P1 and P2 were similar to eosinophils differentiated in culture were used to functionally test putative transcription factor binding sites. Transcription factor occupancy was then validated in primary cultures by ChIP. We conclude that IL-5-dependent generation of eosinophils from CD34⁺ precursors involves complex and dynamic activity including both promoters, several interacting transcription factors, and both signaling and antagonistic protein products.

Landscape and Dynamics of the Transcriptional Regulatory Network During Natural Killer Cell Differentiation

Natural killer (NK) cells are essential in controlling cancer and infection. However, little is known about the dynamics of the transcriptional regulatory machinery during NK cell differentiation. In this study, we applied the assay of transposase accessible chromatin with sequencing (ATAC-seq) technique in a home-developed in vitro NK cell differentiation system. Analysis of ATAC-seq data illustrated two distinct transcription factor (TF) clusters that dynamically regulate NK cell differentiation. Moreover, two TFs from the second cluster, FOS-like 2 (FOSL2) and early growth response 2 (EGR2), were identified as novel essential TFs that control NK cell maturation and function. Knocking down either of these two TFs significantly impacted NK cell differentiation. Finally, we constructed a genome-wide transcriptional regulatory network that provides a better understanding of the regulatory dynamics during NK cell differentiation.

Major vault protein plays important roles in viral infection

Viral replication and related protein expression inside the host cells, and host antiviral immune responses can lead to the occurrence of diverse diseases. With the outbreak of viral infection, a large number of newly diagnosed and died patients infected with various viruses are still reported every year. Viral infection has already been one of the major global public health issues and lead to huge economic and social burdens. Studying of viral pathogenesis is a very important way to find methods for prevention, diagnosis, and cure of viral infection; more evidence has confirmed that major vault protein (MVP) is closely associated with viral infection and pathogenesis, and this review is intended to provide a broad relationship between viruses and MVP to stimulate the interest of related researchers.

Contributions of Eosinophils to Human Health and Disease

The human eosinophil has long been thought to favorably influence innate mucosal immunity but at times has also been incriminated in disease pathophysiology. Research into eosinophil biology has uncovered a number of interesting contributions by eosinophils to health and disease. However, it appears that not all eosinophils from all species are created equal. It remains unclear, for example, exactly how having eosinophils benefits the human host when helminth infections in the developed world have become scarce. This review focuses on our current state of knowledge as it relates to human eosinophils. When information is lacking, we discuss lessons learned from mouse studies that may or may not directly apply to human biology and disease. It is an exciting time to be an "eosinophilosopher" because the use of biologic agents that selectively target eosinophils provides an unprecedented opportunity to define the contribution of this cell to eosinophil-associated human diseases.

C/EBP-δ positively regulates MDSC expansion and endothelial VEGFR2 expression in tumor development

Vascular endothelial cells and Gr-1+CD11b+ myeloid derived suppressor cells (MDSCs) are two important components that constitute the tumor microenvironment. Targeting these cells offers the potential to halt tumor growth. In this study, we report a common mediator in C/EBP-δ that regulates both components and aids in tumor development. C/EBP-δ is elevated in tumor derived MDSCs. Interestingly, genetic deletion of C/EBP-δ in mice significantly impaired MDSC expansion in response to tumor progression, but it had no effect on Gr-1+CD11b+ cell production in normal development. It suggests a specific role of C/EBP-δ in emergency myelopoiesis under tumor conditions. Consistent with the pro tumor functions of MDSCs, loss of C/EBP-δ resulted in reduced tumor angiogenesis and tumor growth. Moreover, we found expression of C/EBP-δ in vascular endothelial cells. C/EBP-δ regulated cell motility, endothelial network formation and vascular sprouting. Notably, inactivation of C/EBP-δ in endothelial cells specifically inhibited the expression of VEGFR2 but not VEGFR1. Ectopic expression of C/EBP-δ increased and knockdown of the gene decreased VEGFR2 expression. C/EBP-δ is recruited to the promoter region of VEGFR2, indicative of transcriptional regulation. Collectively, this study has identified a positive mediator in C/EBP-δ, which regulates tumor induced MDSC expansion and VEGFR2 expression in endothelium. Considering the importance of MDSCs and endothelial cells in tumor progression, targeting C/EBP-δ may provide an interesting means for cancer therapy, killing two birds with one stone.

C/EBP β mRNA expression is upregulated and positively correlated with the expression of TNIP1/TNFAIP3 in peripheral blood mononuclear cells from patients with systemic lupus erythematosus

CCAAT/enhancer-binding protein β (C/EBP β) has important roles in numerous signaling pathways. The expression of the majority of regulators and target gene products of C/EBP β, including tumor necrosis factor α-induced protein 3 (TNFAIP3) and TNFAIP3-interacting protein 1 (TNIP1), are upregulated in patients with systemic lupus erythematosus (SLE). The aim of the present study was to investigate whether C/EBP β expression is associated with SLE pathogenesis and correlates with TNIP1 and TNFAIP3 expression. Quantitative reverse transcription-polymerase chain reaction analysis was used to assess the expression of C/EBP β, TNIP1, and TNFAIP3 mRNA in peripheral blood mononuclear cells (PBMC) from 20 patients with SLE and 20 healthy controls. Spearman's rank test was used to determine the correlation between C/EBP β expression and SLE disease activity, and that between C/EBP β expression and TNIP1/TNFAIP3 expression in PBMCs from patients with SLE. C/EBP β mRNA expression was markedly increased in patients with SLE compared with healthy controls. The expression of C/EBP β was positively correlated with the SLE disease activity index and negatively correlated with the serum level of complement components C3 and C4. In addition, C/EBP β mRNA expression was increased in PBMCs from SLE patients that were positive for antinuclear, anti-Smith and anti-nRNP antibodies, compared with the antibody negative SLE patients. Furthermore, the mRNA expression levels of C/EBP β in patients with SLE was positively correlated with TNIP1 and TNFAIP3 expression. The results of the current study suggest that the increased expression of C/EBP β in PBMCs and the interaction between C/EBP β and TNIP1/TNFAIP3 may be involved in the pathogenesis of SLE.

α1B -Adrenoceptor signalling regulates bone formation through the up-regulation of CCAAT/enhancer-binding protein δ expression in osteoblasts

BACKGROUND AND PURPOSE

The sympathetic nervous system regulates bone remodelling, in part, through ß2 -adrenoceptor signalling. However, the physiological role of α1 -adrenoceptor signalling in bone in vivo remains unclear. Therefore, to obtain a deeper understanding of bone remodelling by the sympathetic nervous system, we investigated the role of α1B -adrenoceptor signalling in bone metabolism.

EXPERIMENTAL APPROACH

Prazosin, a nonspecific α1 -adrenoceptor antagonist, was administered for 2 weeks in C57BL6 mice, and efficacy was evaluated by bone microarchitecture using microcomputed tomography and determination of bone formation by fluorescent labelling of bone. We also compared the bone phenotype of α1B -adrenoceptor null mice (α1B (-/-) ) with that of wild-type littermates.

KEY RESULTS

We demonstrated that the systemic administration of prazosin decreased bone formation. In addition, α1B -adrenoceptor-deficient mice had a lower bone mass due to decreased bone formation but did not exhibit any changes in bone-resorbing activity. Furthermore, stimulation with phenylephrine, a non-specific α1 -adrenoceptor agonist, increased the expression of the transcriptional factor CCAAT/enhancer-binding protein δ (Cebpd) in MC3T3-E1 osteoblastic cells. The overexpression of Cebpd induced cellular proliferation in MC3T3-E1 cells, whereas the silencing of Cebpd suppressed it.

CONCLUSIONS AND IMPLICATIONS

Taken together, these results suggested that α1B -adrenoceptor signalling is required for bone formation and regulated cellular proliferation through a mechanism relevant to the up-regulation of Cebpd in osteoblasts and, thus, provide new evidence for the physiological importance of α1B -adrenoceptor signalling in bone homeostasis.

NLS-RARα Inhibits the Effects of All-trans Retinoic Acid on NB4 Cells by Interacting with P38α MAPK

Nuclear localization signal retinoic acid receptor alpha(NLS-RARα), which forms from the cleavage of promyelocytic leukemia-retinoic acid receptor alpha(PML-RARα) protein by neutrophil elastase(NE), possesses an important role in the occurrence and development of acute promyelocytic leukemia(APL). However, the potential mechanism underlying the effects of NLS-RARα on APL is still not entirely clear. Here, we investigated the effects of NLS-RARα on APL NB4 cells and its mechanism. We found that all-trans retinoic acid(ATRA) could promote differentiation while inhibit proliferation of APL NB4 cells via upregulating the expression of phosphorylated p38α mitogen-activated protein kinase(p-p38α MAPK). We also found that NLS-RARα could inhibit differentiation while accelerate proliferation of NB4 cells via downregulating the expression of p-p38α protein in the presence of ATRA. Furthermore, immunofluorescence and co-immunoprecipitation assays confirmed NLS-RARα interacted with p38α protein directly. Finally, application of PD169316, an inhibitor of p38α protein, suggested that recruitment p38α-combinded NLS-RARα by ATRA eventually caused activation of p38α protein. In summary, our study demonstrated that ATRA cound promote differentiation while inhibit proliferation of APL NB4 cells via activating p38α protein after recruiting p38α-combinded NLS-RARα, while NLS-RARα could inhibit the effects of ATRA in the process.

Application of quantitative trait locus mapping and transcriptomics to studies of the senescence-accelerated phenotype in rats

Background

Etiology of complex disorders, such as cataract and neurodegenerative diseases including age-related macular degeneration (AMD), remains poorly understood due to the paucity of animal models, fully replicating the human disease. Previously, two quantitative trait loci (QTLs) associated with early cataract, AMD-like retinopathy, and some behavioral aberrations in senescence-accelerated OXYS rats were uncovered on chromosome 1 in a cross between OXYS and WAG rats. To confirm the findings, we generated interval-specific congenic strains, WAG/OXYS-1.1 and WAG/OXYS-1.2, carrying OXYS-derived loci of chromosome 1 in the WAG strain. Both congenic strains displayed early cataract and retinopathy but differed clinically from OXYS rats. Here we applied a high-throughput RNA sequencing (RNA-Seq) strategy to facilitate nomination of the candidate genes and functional pathways that may be responsible for these differences and can contribute to the development of the senescence-accelerated phenotype of OXYS rats.

Results

First, the size and map position of QTL-derived congenic segments were determined by comparative analysis of coding single-nucleotide polymorphisms (SNPs), which were identified for OXYS, WAG, and congenic retinal RNAs after sequencing. The transferred locus was not what we expected in WAG/OXYS-1.1 rats. In rat retina, 15442 genes were expressed. Coherent sets of differentially expressed genes were identified when we compared RNA-Seq retinal profiles of 20-day-old WAG/OXYS-1.1, WAG/OXYS-1.2, and OXYS rats. The genes most different in the average expression level between the congenic strains included those generally associated with the Wnt, integrin, and TGF-β signaling pathways, widely involved in neurodegenerative processes. Several candidate genes (including Arhgap33, Cebpg, Gtf3c1, Snurf, Tnfaip3, Yme1l1, Cbs, Car9 and Fn1) were found to be either polymorphic in the congenic loci or differentially expressed between the strains. These genes may contribute to the development of cataract and retinopathy.

Conclusions

This study is the first RNA-Seq analysis of the rat retinal transcriptome generated with 40 mln sequencing read depth. The integration of QTL and transcriptomic analyses in our study forms the basis of future research into the relationship between the candidate genes within the congenic regions and specific changes in the retinal transcriptome as possible causal mechanisms that underlie age-associated disorders.

Rad51 supports triple negative breast cancer metastasis

In contrast to extensive studies on familial breast cancer, it is currently unclear whether defects in DNA double strand break (DSB) repair genes play a role in sporadic breast cancer development and progression. We performed analysis of immunohistochemistry in an independent cohort of 235 were sporadic breast tumours. This analysis suggested that RAD51 expression is increased during breast cancer progression and metastasis and an oncogenic role for RAD51 when deregulated. Subsequent knockdown of RAD51 repressed cancer cell migration in vitro and reduced primary tumor growth in a syngeneic mouse model in vivo. Loss of RAD51 also inhibited associated metastasis not only in syngeneic mice but human xenografts and changed the metastatic gene expression profile of cancer cells, consistent with inhibition of distant metastasis. This demonstrates for the first time a new function of RAD51 that may underlie the proclivity of patients with RAD51 overexpression to develop distant metastasis. RAD51 is a potential biomarker and attractive drug target for metastatic triple negative breast cancer, with the capability to extend the survival of patients, which is less than 6 months.

C/EBPβ mediates osteoclast recruitment by regulating endothelial progenitor cell expression of SDF-1α

Integration of tissue-engineered bone grafts with the host bone is vital for the healing of critical-size bone defects. An important aspect of this process is bone resorption, which must be carried out by osteoclasts derived from the host. However, the mechanism underlying recruitment of host osteoclast precursors to graft sites remains unclear. Endothelial progenitor cells (EPCs) mobilize from the bone marrow into the circulation and home to sites of angiogenesis such as tissue remodeling. Since EPCs express SDF-1, and C/EBPβ is known to regulate SDF-1α expression, we hypothesized that EPCs may recruit CXCR4-expressing host osteoclast precursors to the repair area and that this recruitment may be mediated through C/EBPβ signaling. Using an inflammatory EPC model we showed that EPCs upregulate protein levels of both SDF-1α and C/EBPβ. A luciferase assay confirmed that C/EBPβ acts on the SDF-1α promoter in these cells, and that binding is increased under conditions of inflammation, while silencing of C/EBPβ reduces expression of SDF-1 α and C/EBPβ. Using RAW264.7 cells as a model of osteoclastic monocyte precursors, we investigated their responses to migratory factors in EPC conditioned medium. We showed that RAW264.7 cells migrate towards conditioned medium from EPCs treated with IL-1β, an effect which could be abolished by silencing C/EBPβ in EPCs, and was almost completely blocked by silencing CXCR4 in RAW264.7 cells. These findings show that EPCs respond to inflammatory stimuli by signaling to osteoclast precursors via SDF-1, and that C/EBPβ mediates this response.

Galectin-12: A protein associated with lipid droplets that regulates lipid metabolism and energy balance

Galectin-12, a member of the galectin family of animal lectins, is preferentially expressed in adipocytes. We recently reported that this galectin is localized on lipid droplets, specialized organelles for fat storage. Galectin-12 regulates lipid degradation (lipolysis) by modulating lipolytic protein kinase A (PKA) signaling. Mice deficient in galectin-12 exhibit enhanced adipocyte lipolysis, increased mitochondria respiration, reduced adiposity and ameliorated insulin resistance associated with weight gain. The results suggest that galectin-12 may be a useful target for treatment of obesity-related metabolic conditions, such as insulin resistance, metabolic syndrome, and type 2 diabetes. Most previously described galectins largely reside in the cytosol, although they can also be induced to become associated with membrane-containing structures. Along with an in-depth characterization of galectin-12, this mini-review comments on this first report of a galectin normally localized specifically in an organelle that performs an important intracellular function. Further studies will help shed light on how this protein regulates cellular homeostasis, especially energy homeostasis, and provide additional insight into the intracellular functions of galectins.

Specificity and membrane partitioning of Grsp1 signaling complexes with Grp1 family Arf exchange factors

The Arf exchange factor Grp1 selectively binds phosphatidylinositol 3,4,5-triphosphate [PtdIns(3,4,5)P(3)], which is required for recruitment to the plasma membrane in stimulated cells. The mechanisms for phosphoinositide recognition by the PH domain, catalysis of nucleotide exchange by the Sec7 domain, and autoinhibition by elements proximal to the PH domain are well-characterized. The N-terminal heptad repeats in Grp1 have also been shown to mediate homodimerization in vitro as well as heteromeric interactions with heptad repeats in the FERM domain-containing protein Grsp1 both in vitro and in cells [Klarlund, J. K., et al. (2001) J. Biol. Chem. 276, 40065-40070]. Here, we have characterized the oligomeric state of Grsp1 and Grp1 family proteins (Grp1, ARNO, and Cytohesin-1) as well as the oligomeric state, stoichiometry, and specificity of Grsp1 complexes with Grp1, ARNO, and Cytohesin-1. At low micromolar concentrations, Grp1 and ARNO are homodimeric whereas Cytohesin-1 and Grsp1 are monomeric. When mixed with Grsp1, Grp1 homodimers and Cytohesin-1 monomers spontaneously re-equilibrate to form heterodimers, whereas approximately 50% of ARNO remains homodimeric under the same conditions. Fluorescence resonance energy transfer experiments suggest that the Grsp1 heterodimers with Grp1 and Cytohesin-1 adopt a largely antiparallel orientation. Finally, formation of Grsp1-Grp1 heterodimers does not substantially influence the binding of Grp1 to the headgroups of PtdIns(3,4,5)P(3) or PtdIns(4,5)P(2), nor does it influence partitioning with liposomes containing PtdIns(3,4,5)P(3), PtdIns(4,5)P(2), and/or phosphatidylserine.

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.

Identification of neuronal target genes for CCAAT/enhancer binding proteins

CCAAT/Enhancer Binding Proteins (C/EBPs) play pivotal roles in the development and plasticity of the nervous system. Identification of the physiological targets of C/EBPs (C/EBP target genes) should therefore provide insight into the underlying biology of these processes. We used unbiased genome-wide mapping to identify 115 C/EBPbeta target genes in PC12 cells that include transcription factors, neurotransmitter receptors, ion channels, protein kinases and synaptic vesicle proteins. C/EBPbeta binding sites were located primarily within introns, suggesting novel regulatory functions, and were associated with binding sites for other developmentally important transcription factors. Experiments using dominant negatives showed C/EBPbeta to repress transcription of a subset of target genes. Target genes in rat brain were subsequently found to preferentially bind C/EBPalpha, beta and delta. Analysis of the hippocampal transcriptome of C/EBPbeta knockout mice revealed dysregulation of a high percentage of transcripts identified as C/EBP target genes. These results support the hypothesis that C/EBPs play non-redundant roles in the brain.

Trapping of transcription factors with symmetrical DNA using thiol-disulfide exchange chemistry

Green fluorescent protein (GFP) fused to the C-terminal 100 amino acids of CAAT enhancer binding protein (C/EBP) also containing an N-terminal (His)(6) tag (GFP-C/EBP) was used as a transcription factor model to test whether thiol-disulfide exchange reactions could be used to successfully purify transcription factors. A symmetrical dithiol oligonucleotide with dual CAAT elements was constructed with 5' and 3' thiols. Upon reduction, circular dichroism confirms it spontaneously anneals with its internally complementary sequence to form the hairpin structure: 5'-HS-GCAGATTGCGCAATCTGC 3'-HS-CGTCTAACGCGTTAGACG The specific GFP-C/EBP protein-DNA complex, formed in solution at nM concentrations, could then be recovered (trapped) via thiol-disulfide exchange with a disulfide thiopropyl-Sepharose and eluted with dithiothreitol. GFP-C/EBP was isolated from crude bacterial extract treated with iodoacetamide; DNA binding by GFP-C/EBP was unaltered by carboxyamidomethylation. Eluted GFP-C/EBP was of high purity by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The protein, after in-gel digestion with trypsin, was also characterized by capillary reversed-phase liquid chromatography-nano-electrospray ionization-tandem mass spectrometry and the results analyzed using MASCOT software searching of the non-redundant protein database. A score of 1874 with a sequence coverage of 51% encompassing both termini and internal sequences for the match with GFP-C/EBP confirms its identity and sequence. The method has high potential for the identification and characterization of transcription factors and other DNA-binding proteins.

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.

Mechanisms of eosinophilia in the pathogenesis of hypereosinophilic disorders

The increased numbers of activated eosinophils in the blood and tissues that typically accompany hypereosinophilic disorders result from a variety of mechanisms. Exciting advances in translating discoveries achieved from mouse models and molecular strategies to the clinic have led to a flurry of new therapeutics specifically designed to target eosinophil-associated diseases. So far, this form of hypothesis testing in humans in vivo through pharmacology generally has supported the paradigms generated in vitro and in animal models, raising hopes that a spectrum of novel therapies soon may become available to help those who have eosinophil-associated diseases.

CCAAT/enhancer-binding protein phosphorylation biases cortical precursors to generate neurons rather than astrocytes in vivo

The intracellular mechanisms that bias mammalian neural precursors to generate neurons versus glial cells are not well understood. We demonstrated previously that the growth factor-regulated mitogen-activated protein kinase kinase (MEK) and its downstream target, the CCAAT/enhancer-binding protein (C/EBP) family of transcription factors, are essential for neurogenesis in cultured cortical precursor cells (Ménard et al., 2002). Here, we examined a role for this pathway during cortical cell fate determination in vivo using in utero electroporation of the embryonic cortex. These studies demonstrate that inhibition of the activity of either MEK or the C/EBPs inhibits the genesis of neurons in vivo. Moreover, the MEK pathway mediates phosphorylation of C/EBPbeta in cortical precursors, and expression of a C/EBPbeta construct in which the MEK pathway phosphorylation sites are mutated inhibits neurogenesis. Conversely, expression of a C/EBPbeta construct, in which the same sites are mutated to glutamate and therefore are "constitutively" phosphorylated, enhances neurogenesis in the early embryonic cortex. A subpopulation of precursors in which C/EBP activity is inhibited are maintained as cycling precursors in the ventricular/subventricular zone of the cortex until early in postnatal life, when they have an enhanced propensity to generate astrocytes, presumably in response to gliogenic signals in the neonatal environment. Thus, activation of an MEK-C/EBP pathway in cortical precursors in vivo biases them to become neurons and against becoming astrocytes, thereby acting as a growth factor-regulated switch.

CCAAT/enhancer binding protein epsilon: changes in function upon phosphorylation by p38 MAP kinase

C/EBPepsilon, a member of the CCAAT/enhancer binding protein family, is a transcription factor important in neutrophil differentiation. We have determined that it is phosphorylated on multiple serine and threonine residues and can be a target for phosphorylation by a number of kinases. We identified a threonine at amino acid 75, part of a consensus mitogen-activated protein (MAP) kinase site within the transactivation domain of C/EBPepsilon, as being phosphorylated only by p38 MAP kinase. Phosphorylation of this residue resulted in enhanced transcriptional activity on a myeloid-specific promoter in in vitro transient transfection reporter assays. We also determined that phosphorylation at Thr75 yielded a protein that was more effective at binding its cognate DNA sequence compared with the wild-type nonphosphorylated C/EBPepsilon. Stable expression of C/EBPepsilonT75A in interleukin 3 (IL-3)-dependent 32Dcl3 did not result in the up-regulation of expression of secondary granule genes compared with wild-type C/EBPepsilon or C/EBPepsilonT75D. Therefore we suggest that C/EBPepsilon is a target for p38 MAP kinase activity.

PRAM-1 is required for optimal integrin-dependent neutrophil function

PML-retinoic acid receptor alpha (RARalpha) regulated adaptor molecule 1 (PRAM-1) is an intracellular adaptor molecule that is upregulated during the induced granulocytic differentiation of promyelocytic leukemic cells and during normal human myelopoiesis. This report describes the generation of PRAM-1-deficient mice and an analysis of the function of this adaptor in neutrophil differentiation and mature neutrophil function. We demonstrate here that neutrophil differentiation is not impaired in PRAM-1-deficient mice and that PRAM-1-deficient neutrophils function normally following engagement of Fcgamma receptors. In contrast, mature PRAM-1-null neutrophils exhibit significant defects in adhesion-dependent reactive oxygen intermediate production and degranulation. Surprisingly, other integrin-dependent responses, such as cell spreading and activation of several signaling pathways, are normal. Together, these findings demonstrate the uncoupling of key integrin-dependent responses in the absence of PRAM-1 and show this adaptor to be critical for select integrin functions in neutrophils.

Dual-regulated expression of C/EBP-alpha and BMP-2 enables differential differentiation of C2C12 cells into adipocytes and osteoblasts

CCAAT/enhancer-binding proteins (C/EBPs) as well as bone morphogenic proteins (BMPs) play essential roles in mammalian cell differentiation in shaping adipogenic and osteoblastic lineages in particular. Recent evidence suggested that adipocytes and osteoblasts share a common mesenchymal precursor cell phenotype. Yet, the molecular details underlying the decision of adipocyte versus osteoblast differentiation as well as the involvement of C/EBPs and BMPs remains elusive. We have engineered C2C12 cells for dual-regulated expression of human C/EBP-alpha and BMP-2 to enable independent transcription control of both differentiation factors using clinically licensed antibiotics of the streptogramin (pristinamycin) and tetracycline (tetracycline) classes. Differential as well as coordinated expression of C/EBP-alpha and BMP-2 revealed that (i) C/EBP-alpha may differentiate C2C12 myoblasts into adipocytes as well as osteoblasts, (ii) BMP-2 prevents myotube differentiation, (iii) is incompetent in differentiating C2C12 into osteoblasts and (iv) even decreases C/EBP-alpha's osteoblast-specific differentiation potential but (v) cooperates with C/EBP-alpha on adipocyte differentiation, (vi) osteoblast formation occurs at low C/EBP-alpha levels while adipocyte-specific differentiation requires maximum C/EBP-alpha expression and that (vii) BMP-2 may bias the C/EBP-alpha-mediated adipocyte versus osteoblast differentiation switch towards fat cell formation. Dual-regulated expression technology enabled precise insight into combinatorial effects of two key differentiation factors involved in adipocyte/osteoblast lineage control which could be implemented in rational reprogramming of multipotent cells into desired cell phenotypes tailored for gene therapy and tissue engineering.

The role of C/EBPbeta in mammary gland development and breast cancer

The CCAAT/enhancer binding protein (C/EBP) family of bZIP transcription factors control the proliferation and differentiation of a variety of tissues. While C/EBPalpha and -delta are also expressed in the mammary gland, the multiple protein isoforms of C/EBPbeta appear to play a critical role in mammary gland development and breast cancer. Targeted deletion of all the C/EBPbeta isoforms results in a severe inhibition of lobuloalveolar development and a block to functional differentiation, as well as more subtle changes in ductal morphogenesis. The altered expression of a number of molecular markers, including the progesterone, estrogen, and prolactin receptors, the transporter proteins (NKCC1 and aquaporin 5), and several markers of skin differentiation (Sprr2A and keratin 6), suggests that germline deletion of C/EBPbeta results in an altered cell fate. Thus, C/EBPbeta appears to play a role in the specification of progenitor cell fate not only in the mammary gland, but also in a number of other tissues.

Recruitment of p300 by C/EBPbeta triggers phosphorylation of p300 and modulates coactivator activity

Transcriptional coactivators such as p300 act as crucial elements in the eukaryotic gene regulation network. These proteins bind to various transcription factors which recruit them to specific gene regions whose chromatin structure subsequently is remodeled. Previously, we have shown that C/EBPbeta recruits p300 by interacting with the E1A-binding site of the coactivator. We now show that C/EBPbeta not only binds to p300 but also triggers massive phosphorylation of p300. This novel activity of C/EBPbeta is dependent on the E1A-binding region of p300 as well as on several subdomains of C/EBPbeta, all of which are involved in the p300-C/EBPbeta interaction. We have identified several sites of C/EBPbeta-inducible phosphorylation within the C-terminal domain of p300. Mutation of these sites substantially impairs the activity of p300 as a coactivator of C/EBPbeta. Interestingly, phosphorylation of p300 is also triggered by other C/EBP family members as well as by various other transcription factors that interact with the E1A-binding domain of p300, suggesting that this novel phosphorylation mechanism may be of general relevance.

Analysis of the Xenopus laevis CCAAT-enhancer binding protein alpha gene promoter demonstrates species-specific differences in the mechanisms for both auto-activation and regulation by Sp1

Transcription factors belonging to the CCAAT-enhancer binding protein (C/EBP) family have been implicated in the regulation of gene expression during differentiation, development and disease. Autoregulation is relatively common in the modulation of C/EBP gene expression and the murine and human C/EBPalpha genes have been shown to be auto-activated by different mechanisms. In the light of this finding, it is essential that autoregulation of C/EBPalpha genes from a wider range of different species be investigated in order to gauge the degree of commonality, or otherwise, that may exist. We report here studies that investigate the regulation of the Xenopus laevis C/EBPalpha gene (xC/EBPalpha). The -1131/+41 promoter region was capable of directing high levels of expression in both the human hepatoma Hep3B and the Xenopus kidney epithelial A6 cell lines, and was auto-activated by expression vectors specifying for xC/EBPalpha or xC/EBPss. Deletion analysis showed that the -321/+41 sequence was sufficient for both the constitutive promoter activity and auto-activation and electrophoretic mobility shift assays identified the interaction of C/EBPs and Sp1 to this region. Although deletion of either the C/EBP or the Sp1 site drastically reduced the xC/EBPalpha promoter activity, multimers of only the C/EBP site could confer autoregulation to a heterologous SV40 promoter. These results indicate that, in contrast to the human promoter and in common with the murine gene, the xC/EBPalpha promoter was subject to direct autoregulation. In addition, we demonstrate a novel species-specific action of Sp1 in the regulation of C/EBPalpha expression, with the factor able to repress the murine promoter but activate the Xenopus gene.