GATA-1- and FOG-dependent activation of megakaryocytic alpha IIB gene expression

Megakaryocytes Are Regulators of the Tumor Microenvironment and Malignant Hematopoietic Progenitor Cells in Myelofibrosis

Megakaryocytes (MKs) are multifunctional hematopoietic cells that produce platelets, serve as components of bone marrow (BM) niches that support the development of hematopoietic stem and progenitor cell (HSPC) and provide inflammatory signals. MKs can dynamically change their activities during homeostasis and following stress, thereby regulating hematopoietic stem cell (HSC) function. Myelofibrosis (MF) is a progressive chronic myeloproliferative neoplasm (MPN) characterized by hyperactivation of JAK/STAT signaling and MK hyperplasia, which is associated with an aberrant inflammatory signature. Since JAK1/2 inhibitor alone is incapable of depleting the malignant HSC clones or reversing BM fibrosis, the identification of mechanisms that cooperate with MF JAK/STAT signaling to promote disease progression might help in developing combination therapies to modify disease outcomes. Chronic inflammation and MK hyperplasia result in an abnormal release of TGFβ1, which plays a critical role in the pathobiology of MF by contributing to the development of BM fibrosis. Dysregulated TGFβ signaling can also alter the hematopoietic microenvironment supporting the predominance of MF-HSCs and enhance the quiescence of the reservoir of wild-type HSCs. Upregulation of TGFβ1 levels is a relatively late event in MF, while during the early pre-fibrotic stage of MF the alarmin S100A8/S100A9 heterocomplex promotes pro-inflammatory responses and sustains the progression of MF-HSCs. In this review, we will discuss the recent advances in our understanding of the roles of abnormal megakaryopoiesis, and the altered microenvironment in MF progression and the development of novel combined targeted therapies to disrupt the aberrant interplay between MKs, the BM microenvironment and malignant HSCs which would potentially limit the expansion of MF-HSC clones.

The Role of GATA in Mammalian Reproduction

GATA transcription factors are emerging as critical players in mammalian reproductive development and function. GATA-4 contributes to fetal male gonadal development by regulating genes mediating Müllerian duct regression and the onset of testosterone production. GATA-2 expression appears to be sexually dimorphic being transiently expressed in the germ cell lineage of the fetal ovary but not the fetal testis. In the reproductive system, GATA-1 is exclusively expressed in Sertoli cells at specific seminiferous tubule stages. In addition, GATA-4 and GATA-6 are localized primary to ovarian and testicular somatic cells. The majority of cell transfection studies demonstrate that GATA-1 and GATA-4 can stimulate inhibin subunit gene promoter constructs. Other studies provide strong evidence that GATA-4 and GATA-6 can activate genes mediating gonadal cell steroidogenesis. GATA-2 and GATA-3 are found in pituitary and placental cells and can regulate alpha-glycoprotein subunit gene expression. Gonadal expression and activation of GATAs appear to be regulated in part by gonadotropin signaling via the cyclic AMP-protein kinase A pathway. This review will cover the current knowledge regarding GATA expression and function at all levels of the reproductive axis.

Systems biology of megakaryocytes

The molecular pathways that regulate megakaryocyte production have historically been identified through multiple candidate gene approaches. Several transcription factors critical for generating megakaryocytes were identified by promoter analysis of megakaryocyte-specific genes, and their biological roles then verified by gene knockout studies; for example, GATA-1, NF-E2, and RUNX1 were identified in this way. In contrast, other transcription factors important for megakaryopoiesis were discovered through a systems approach; for example, c-Myb was found to be critical for the erythroid versus megakaryocyte lineage decision by genome-wide loss-of-function studies. The regulation of the levels of these transcription factors is, for the most part, cell intrinsic, although that assumption has recently been challenged. Epigenetics also impacts megakaryocyte gene expression, mediated by histone acetylation and methylation. Several cytokines have been identified to regulate megakaryocyte survival, proliferation, and differentiation, most prominent of which is thrombopoietin. Upon binding to its receptor, the product of the c-Mpl proto-oncogene, thrombopoietin induces a conformational change that activates a number of secondary messengers that promote cell survival, proliferation, and differentiation, and down-modulate receptor signaling. Among the best studied are the signal transducers and activators of transcription (STAT) proteins; phosphoinositol-3-kinase; mitogen-activated protein kinases; the phosphatases PTEN, SHP1, SHP2, and SHIP1; and the suppressors of cytokine signaling (SOCS) proteins. Additional signals activated by these secondary mediators include mammalian target of rapamycin; β(beta)-catenin; the G proteins Rac1, Rho, and CDC42; several transcription factors, including hypoxia-inducible factor 1α(alpha), the homeobox-containing proteins HOXB4 and HOXA9, and a number of signaling mediators that are reduced, including glycogen synthase kinase 3α(alpha) and the FOXO3 family of forkhead proteins. More recently, systematic interrogation of several aspects of megakaryocyte formation have been conducted, employing genomics, proteomics, and chromatin immunoprecipitation (ChIP) analyses, among others, and have yielded many previously unappreciated signaling mechanisms that regulate megakaryocyte lineage determination, proliferation, and differentiation. This chapter focuses on these pathways in normal and neoplastic megakaryopoiesis, and suggests areas that are ripe for further study.

Transcription factor networks in erythroid cell and megakaryocyte development

Erythroid cells and megakaryocytes are derived from a common precursor, the megakaryocyte-erythroid progenitor. Although these 2 closely related hematopoietic cell types share many transcription factors, there are several key differences in their regulatory networks that lead to differential gene expression downstream of the megakaryocyte-erythroid progenitor. With the advent of next-generation sequencing and our ability to precisely define transcription factor chromatin occupancy in vivo on a global scale, we are much closer to understanding how these 2 lineages are specified and in general how transcription factor complexes govern hematopoiesis.

Platelets generated from human embryonic stem cells are functional in vitro and in the microcirculation of living mice

Platelets play an essential role in hemostasis and atherothrombosis. Owing to their short storage time, there is constant demand for this life-saving blood component. In this study, we report that it is feasible to generate functional megakaryocytes and platelets from human embryonic stem cells (hESCs) on a large scale. Differential-interference contrast and electron microscopy analyses showed that ultrastructural and morphological features of hESC-derived platelets were indistinguishable from those of normal blood platelets. In functional assays, hESC-derived platelets responded to thrombin stimulation, formed microaggregates, and facilitated clot formation/retraction in vitro. Live cell microscopy demonstrated that hESC-platelets formed lamellipodia and filopodia in response to thrombin activation, and tethered to each other as observed in normal blood. Using real-time intravital imaging with high-speed video microscopy, we have also shown that hESC-derived platelets contribute to developing thrombi at sites of laser-induced vascular injury in mice, providing the first evidence for in vivo functionality of hESC-derived platelets. These results represent an important step toward generating an unlimited supply of platelets for transfusion. Since platelets contain no genetic material, they are ideal candidates for early clinical translation involving human pluripotent stem cells.

GATA transcription factors in the developing reproductive system

Previous work has firmly established the role for both GATA4 and FOG2 in the initial global commitment to sexual fate, but their (joint or individual) function in subsequent steps remained unknown. Hence, gonad-specific deletions of these genes in mice were required to reveal their roles in sexual development and gene regulation. The development of tissue-specific Cre lines allowed for substantial advances in the understanding of the function of GATA proteins in sex determination, gonadal differentiation and reproductive development in mice. Here we summarize the recent work that examined the requirement of GATA4 and FOG2 proteins at several critical stages in testis and ovarian differentiation. We also discuss the molecular mechanisms involved in this regulation through the control of Dmrt1 gene expression in the testis and the canonical Wnt/ß-catenin pathway in the ovary.

NuRD mediates activating and repressive functions of GATA-1 and FOG-1 during blood development

GATA transcription factors interact with FOG proteins to regulate tissue development by activating and repressing transcription. FOG-1 (ZFPM1), a co-factor for the haematopoietic factor GATA-1, binds to the NuRD co-repressor complex through a conserved N-terminal motif. Surprisingly, we detected NuRD components at both repressed and active GATA-1/FOG-1 target genes in vivo. In addition, while NuRD is required for transcriptional repression in certain contexts, we show a direct requirement of NuRD also for FOG-1-dependent transcriptional activation. Mice in which the FOG-1/NuRD interaction is disrupted display defects similar to germline mutations in the Gata1 and Fog1 genes, including anaemia and macrothrombocytopaenia. Gene expression analysis in primary mutant erythroid cells and megakaryocytes (MKs) revealed an essential function for NuRD during both the repression and activation of select GATA-1/FOG-1 target genes. These results show that NuRD is a critical co-factor for FOG-1 and underscore the versatile use of NuRD by lineage-specific transcription factors to activate and repress gene transcription in the appropriate cellular and genetic context.

Gene Ontology-driven transcriptional analysis of CD34+ cell-initiated megakaryocytic cultures identifies new transcriptional regulators of megakaryopoiesis

Differentiation of hematopoietic stem and progenitor cells is an intricate process controlled in large part at the level of transcription. While some key megakaryocytic transcription factors have been identified, the complete network of megakaryocytic transcriptional control is poorly understood. Using global gene expression microarray analysis, Gene Ontology-based functional annotations, and a novel interlineage comparison with parallel, isogenic granulocytic cultures as a negative control, we closely examined the mRNA level of transcriptional regulators in megakaryocytes derived from human mobilized peripheral blood CD34(+) hematopoietic cells. This approach identified 199 differentially expressed transcription factors or transcriptional regulators. We identified and detailed the transcriptional kinetics of most known megakaryocytic transcription factors including GATA1, FLI1, and MAFG. Furthermore, many genes with transcription factor activity or transcription factor binding activity were identified in megakaryocytes that had not previously been associated with that lineage, including BTEB1, NR4A2, FOXO1A, MEF2C, HDAC5, VDR, and several genes associated with the tumor suppressor p53 (HIPK2, FHL2, and TADA3L). Protein expression and nuclear localization were confirmed in megakaryocytic cells for four of the novel candidate megakaryocytic transcription factors: FHL2, MXD1, E2F3, and RFX5. In light of the hypothesis that transcription factors expressed in a particular differentiation program are important contributors to such a program, these data substantially expand our understanding of transcriptional regulation in megakaryocytic differentiation of stem and progenitor cells.

Comparative, genome-scale transcriptional analysis of CHRF-288-11 and primary human megakaryocytic cell cultures provides novel insights into lineage-specific differentiation

OBJECTIVES

Little is known about the transcriptional events underlying megakaryocytic (Mk) differentiation. We sought to identify genes and pathways previously unassociated with megakaryopoiesis and to evaluate the CHRF-288-11 (CHRF) megakaryoblastic cell line as a model system for investigating megakaryopoiesis.

METHODS

Using DNA microarrays, Q-RT-PCR, and protein-level assays, we compared the dynamic gene expression pattern of phorbol ester-induced differentiation of CHRF cells to cytokine-induced Mk differentiation of human mobilized peripheral blood CD34(+) cells.

RESULTS

Transcriptional patterns of well-known Mk genes were similar between the two systems. CHRF cells constitutively express some early Mk genes including GATA-1. Expression patterns of apoptosis-related genes suggested that increased p53 activity is involved in Mk apoptosis, and this was confirmed by p53-DNA-binding activity data and flow-cytometric analysis of the p53 target gene BBC3. Certain Rho and G-protein-coupled-receptor signaling pathway components were upregulated, including genes not previously associated with Mk cells. Ontological analysis revealed upregulation of defense-response genes, including both known and candidate platelet-derived contributors to inflammation. Upregulation of interferon-responsive genes occurred in the cell line, but not in the primary cells, likely due to a known genetic mutation in the JAK2/STAT5 signaling pathway.

CONCLUSIONS

This analysis of megakaryopoiesis, which integrates dynamic gene expression data with protein abundance and activity assays, has identified a number of genes and pathways that may help govern megakaryopoiesis. Furthermore, the transcriptional data support the hypothesis that CHRF cells resemble an early Mk phenotype and, with certain limitations, exhibit genuine transcriptional features of Mk differentiation upon treatment with phorbol esters.

The GATA factor Serpent cross-regulates lozenge and u-shaped expression during Drosophila blood cell development

The Drosophila GATA factor Serpent interacts with the RUNX factor Lozenge to activate the crystal cell program, whereas SerpentNC binds the Friend of GATA protein U-shaped to limit crystal cell production. Here, we identified a lozenge minimal hematopoietic cis-regulatory module and showed that lozenge-lacZ reporter-gene expression was autoregulated by Serpent and Lozenge. We also showed that upregulation of u-shaped was delayed until after lozenge activation, consistent with our previous results that showed u-shaped expression in the crystal cell lineage is dependent on both Serpent and Lozenge. Together, these observations describe a feed forward regulatory motif, which controls the temporal expression of u-shaped. Finally, we showed that lozenge reporter-gene activity increased in a u-shaped mutant background and that forced expression of SerpentNC with U-shaped blocked lozenge- and u-shaped-lacZ reporter-gene activity. This is the first demonstration of GATA:FOG regulation of Runx and Fog gene expression. Moreover, these results identify components of a Serpent cross-regulatory sub-circuit that can modulate lozenge expression. Based on the sub-circuit design and the combinatorial control of crystal cell production, we present a model for the specification of a dynamic bi-potential regulatory state that contributes to the selection between a Lozenge-positive and Lozenge-negative state.

Tescalcin is an essential factor in megakaryocytic differentiation associated with Ets family gene expression

We show here that the process of megakaryocytic differentiation requires the presence of the recently discovered protein tescalcin. Tescalcin is dramatically upregulated during the differentiation and maturation of primary megakaryocytes or upon PMA-induced differentiation of K562 cells. This upregulation requires sustained signaling through the ERK pathway. Overexpression of tescalcin in K562 cells initiates events of spontaneous megakaryocytic differentiation, such as expression of specific cell surface antigens, inhibition of cell proliferation, and polyploidization. Conversely, knockdown of this protein in primary CD34+ hematopoietic progenitors and cell lines by RNA interference suppresses megakaryocytic differentiation. In cells lacking tescalcin, the expression of Fli-1, Ets-1, and Ets-2 transcription factors, but not GATA-1 or MafB, is blocked. Thus, tescalcin is essential for the coupling of ERK cascade activation with the expression of Ets family genes in megakaryocytic differentiation.

Regulation of Drosophila friend of GATA gene, u-shaped, during hematopoiesis: a direct role for serpent and lozenge

Friend of GATA proteins interact with GATA factors to regulate development in a variety of tissues. We analyzed cis- and trans-regulation of the Drosophila gene, u-shaped, to better understand the transcriptional control of this important gene family during hematopoiesis. Using overlapping genomic fragments driving tissue-specific reporter-gene (lacZ) expression, we identified two minimal hematopoietic enhancers within the 7.4 kb region upstream of the transcription start site. One enhancer was active in all classes of hemocytes, whereas the other was active in hemocyte precursors and plasmatocytes only. The GATA factor, Serpent, directly regulated the activity of both enhancers. However, activity in the crystal cell lineage not only required Serpent but also the RUNX homologue, Lozenge. This is the first demonstration of GATA and RUNX direct regulation of Friend of GATA gene expression and provides additional evidence for the combinatorial control of crystal cell lineage commitment by Serpent, Lozenge, and U-shaped. In addition, we analyzed cis-regulation of ush expression in the lymph gland and identified similarities and differences between regulatory strategies used during embryonic and lymph gland hematopoiesis. The results of these studies provide information to analyze further the regulation of this conserved gene family and its role during hematopoietic lineage commitment.

Integrin alphaIIbbeta3 induces the adhesion and activation of mast cells through interaction with fibrinogen

Integrin alphaIIb, a well-known marker of megakaryocyte-platelet lineage, has been recently recognized on hemopoietic progenitors. We now demonstrate that integrin alphaIIbbeta3 is highly expressed on mouse and human mast cells including mouse bone marrow-derived mast cells, peritoneal mast cells, and human cord blood-derived mast cells, and that its binding to extracellular matrix proteins leads to enhancement of biological functions of mast cells in concert with various stimuli. With exposure to various stimuli, including cross-linking of FcepsilonRI and stem cell factor, mast cells adhered to extracellular matrix proteins such as fibrinogen and von Willebrand factor in an integrin alphaIIbbeta3-dependent manner. In addition, the binding of mast cells to fibrinogen enhanced proliferation, cytokine production, and migration and induced uptake of soluble fibrinogen in response to stem cell factor stimulation, implicating integrin alphaIIbbeta3 in a variety of mast cell functions. In conclusion, mouse and human mast cells express functional integrin alphaIIbbeta3.

Megakaryocyte biology and related disorders

Platelets, derived from megakaryocytes, have an essential role in thrombosis and hemostasis. Over the past 10 years, a great deal of new information has been obtained concerning the various aspects of hematopoiesis necessary to maintain a steady-state platelet level to support physiologic hemostasis. Here we discuss the differentiation of HSCs into megakaryocytes, with emphasis on the key cytokine signaling pathways and hematopoietic transcription factors. Recent insight into these processes elucidates the molecular bases of numerous acquired and inherited hematologic disorders. It is anticipated that the growing knowledge in these areas may be exploited for new therapeutic strategies to modulate both platelet numbers and their thrombogenicity.

GATA-1: friends, brothers, and coworkers

GATA-1 is the founding member of the GATA family of transcription factors. GATA-1 and GATA family member GATA-2 are expressed in erythroid and megakaryocytic lineages, in which they play a crucial role in cell maturation and differentiation. GATA-1 regulates the transcription of many specific and nonspecific erythroid genes by binding to DNA at the consensus sequence WGATAR, which is recognized by all of the GATA family of transcription factors. However, it was identified in eosinophilic cells and also in Sertoli cells in testis. Its activity depends on close cooperation with a functional network of cofactors, among them Friend of GATA, PU.1, and CBP/p300. The GATA-1 protein structure has been well described and includes two zinc fingers that are directly involved in the interaction with DNA and other proteins in vivo. GATA-1 mutations in the zinc fingers can cause deregulation of required interactions and lead to severe dysfunction in the hematopoietic system.

GATA Transcription Factors in Hematologic Disease

GATA family transcription factors play essential roles in broad developmental settings. GATA-1, one of the hematopoietically expressed members, is required for normal erythroid and megakaryocytic differentiation. Over the past few years, mutations in the gene encoding GATA-1 have been linked to several human hematologic disorders, including X-linked dyserythropoietic anemia and thrombocytopenia, X-linked thrombocytopenia and beta-thalassemia, and Down syndrome acute megakaryoblastic leukemia. This review summarizes the role of GATA-1 during normal hematopoiesis and discusses how disease-associated mutations may affect its function.

Coregulation of GATA factors by the Friend of GATA (FOG) family of multitype zinc finger proteins

The Friend of GATA (FOG) family of proteins is an evolutionarily conserved class of large multitype zinc finger cofactors that bind to the amino zinc finger of GATA transcription factors and modulate their activity. Two FOG genes have been identified in mammals, both of which interact with each of the six known vertebrate GATA factors in vitro. Physical interaction between FOG and GATA proteins in vivo is essential for the development of a broad array of tissues, reflecting the overlapping expression patterns of these factors. In this review, we will discuss the identification and characterization of FOG proteins, their role in human disease, and recent studies that shed new light on their function and regulation.

Differentiation status dependent function of FOG-1

The molecular interactions between transcription factors and cofactors play crucial roles in various biological processes, including haematopoiesis. FOG-1 is a cognate cofactor of GATA-1, and the FOG-1/GATA-1 complex is essential for the haematopoietic differentiation of erythroid cells and megakaryocytes. In order to elucidate the biological functions of FOG-1 in the different contexts of cell differentiation, we analysed the effects of FOG-1 expression on haematopoietic cell differentiation, using a combination of in vitro differentiation of mouse embryonic stem (ES) cells and conditional gene expression. FOG-1 suppressed the proliferation of primitive and definitive erythroid cells in all stages of differentiation. However, FOG-1 inhibited and enhanced megakaryopoiesis in the early and late differentiation stages, respectively, through different molecular mechanisms. In addition, FOG-1 inhibited the proliferation of ES cells, the molecular mechanism of which differs from those of erythroid and megakaryocytic cells. These results suggest that FOG-1 functions in a cell differentiation context-dependent manner.

Context-dependent regulation of GATA-1 by friend of GATA-1

The transcription factor GATA-1 and its cofactor, friend of GATA-1 (FOG-1), are essential for normal erythroid development. FOG-1 physically interacts with GATA-1 to augment or inhibit its activity. The mechanisms by which FOG-1 regulates GATA-1 function are unknown. By using an assay that is based on the phenotypic rescue of a GATA-1-null erythroid cell line, we found that a conditional form of GATA-1 (GATA-1-ER) strongly induced histone acetylation at the beta-major globin promoter in vivo, consistent with previous results. In contrast, GATA-1 bearing a point mutation that impairs FOG-1 binding [GATA-1(V205M)-ER] failed to induce high levels of histone acetylation at this site. However, at DNase I-hypersensitive site (HS)3 of the beta-globin locus control region, GATA-1-induced histone acetylation was FOG-1-independent. Because the V205M mutation does not disrupt GATA-1 binding to DNA templates in vitro, we were surprised to find that in vivo GATA-1(V205M)-ER fails to bind the beta-globin promoter. However, at HS3, DNA binding by GATA-1 was FOG-1-independent, thus correlating histone acetylation with GATA-1 occupancy. Examination of additional GATA-1-dependent regulatory elements showed that the interaction with FOG-1 is required for GATA-1 occupancy at select sites, such as HS2, but is dispensable at others, including the FOG-1-independent GATA-1 target gene EKLF. Remarkably, at the GATA-2 gene, which is repressed by GATA-1, interaction with FOG-1 was dispensable for GATA-1 occupancy and was required for transcriptional inhibition and histone deacetylation. These results indicate that FOG-1 employs distinct mechanisms when cooperating with GATA-1 during transcriptional activation and repression.

Protein-protein interaction between Fli-1 and GATA-1 mediates synergistic expression of megakaryocyte-specific genes through cooperative DNA binding

Friend leukemia integration 1 (Fli-1) is a member of the Ets family of transcriptional activators that has been shown to be an important regulator during megakaryocytic differentiation. We undertook a two-hybrid screen of a K562 cDNA library to identify transcription factors that interacted with Fli-1 and were potential regulators of megakaryocyte development. Here we report the physical interaction of Fli-1 with GATA-1, a well-characterized, zinc finger transcription factor critical for both erythroid and megakaryocytic differentiation. We map the minimal domains required for the interaction and show that the zinc fingers of GATA-1 interact with the Ets domain of Fli-1. GATA-1 has previously been shown to interact with the Ets domain of the Fli-1-related protein PU.1, and the two proteins appear to inhibit each other's activity. In contrast, we demonstrate that GATA-1 and Fli-1 synergistically activate the megakaryocyte-specific promoters GPIX and GPIbalpha in transient transfections. Quantitative electrophoretic mobility shift assays using oligonucleotides derived from the GPIX promoter containing Ets and GATA binding motifs reveal that Fli-1 and GATA-1 exhibit cooperative DNA binding in which the binding of GATA-1 to DNA is increased approximately 26-fold in the presence of Fli-1 (from 4.2 to 0.16 nM), providing a mechanism for the observed transcriptional synergy. To test the effect on endogenous genes, we stably overexpressed Fli-1 in K562 cells, a line rich in GATA-1. Overexpression of Fli-1 induced the expression of the endogenous GPIX and GPIbalpha genes as measured by Northern blot and fluorescence-activated cell sorter analysis. This work suggests that Fli-1 and GATA-1 work together to activate the expression of genes associated with the terminal differentiation of megakaryocytes.

Cloning and analysis of the thrombopoietin-induced megakaryocyte-specific glycoprotein VI promoter and its regulation by GATA-1, Fli-1, and Sp1

The exposure of collagen fibers at sites of vascular injury results in the adherence of platelets and their subsequent activation. The platelet collagen receptor glycoprotein (GP)(1) VI plays a crucial role in platelet activation and thrombus formation and decreased levels or defective GPVI may lead to excessive bleeding. In addition, elevated levels of collagen receptors may predispose individuals to coronary heart disease or strokes. GPVI expression is restricted to platelets and their precursor cell, the megakaryocyte. In this study we investigate the regulation of GPVI expression and show that thrombopoietin induces its expression in the megakaryocytic cell line UT-7/TPO. A 5'-region flanking the transcription start point of the GPVI gene was cloned (-694 to +29) and we report that this putative GPVI promoter bestows megakaryocye-specific expression. Deletion analyses and site-directed mutagenesis identified Sp1(227), GATA(177), and Ets(48) sites as essential for GPVI expression. We show that transcription factors GATA-1, Fli-1, and Sp1 can bind to and activate this promoter. Finally, GPVI mRNA was detected only in megakaryocytic cell lines expressing both Fli-1 and GATA-1, and we show that overexpression of Fli-1 in a stable cell line (which expresses endogenous GATA-1 and Sp1) results in expression of the endogenous GPVI gene.

Furin gene (fur) regulation in differentiating human megakaryoblastic Dami cells: involvement of the proximal GATA recognition motif in the P1 promoter and impact on the maturation of furin substrates

The convertase furin is involved in the maturation of key growth/aggregation mediators synthesized by the platelet producers, megakaryocytes, but the regulation of furin in these cells remains unknown. Computer-assisted search of the furin promoter sequence revealed multiple potential binding motifs for GATA-1, suggesting that furin is expressed and regulated in these cells. Using megakaryoblastic Dami cells, we observed that fur mRNA expression increased gradually on phorbol 12-myristate 13-acetate-induced differentiation, reaching maximum levels (8.3-fold increase) at 10 days. Transient transfections with P1, P1A, or P1B fur-LUC-promoter constructs revealed that in Dami cells, the P1 promoter is the strongest and the most sensitive to forced expression of GATA-1. Coexpression of GATA-1 and its comodulator, Friend of GATA-1 (FOG-1), resulted in a cooperative increase in P1 activity. Deletion analysis indicated that important GATA-1-regulated sequences are located in the most proximal region of the P1 promoter. Further analysis revealed 2 potential GATA-binding motifs at positions -66 and +62. Point mutation of each of the 2 motifs indicated that the intactness of the first GATA site is required for full basal and GATA-1-stimulated promoter activity. Finally, the inhibition of furin activity through gene transfer of the inhibitor alpha1-AT-PDX led to a block in maturation of the furin substrates transforming growth factor-beta1 and platelet-derived growth factor. Taken together, these results indicate that the most proximal GATA element in the P1 promoter is needed for fur gene expression in megakaryoblastic cells. They also suggest that proper regulation of the fur gene in megakaryocytes has an impact on the activation of furin substrates involved in megakaryocyte maturation and platelet functions.

Control of megakaryocyte-specific gene expression by GATA-1 and FOG-1: role of Ets transcription factors

The transcription factor GATA-1 and its cofactor FOG-1 are essential for the normal development of erythroid cells and megakaryocytes. FOG-1 can stimulate or inhibit GATA-1 activity depending on cell and promoter context. How the GATA-1-FOG-1 complex controls the expression of distinct sets of gene in megakaryocytes and erythroid cells is not understood. Here, we examine the molecular basis for the megakaryocyte-restricted activation of the alphaIIb gene. FOG-1 stimulates GATA-1-dependent alphaIIb gene expression in a manner that requires their direct physical interaction. Transcriptional output by the GATA-1-FOG-1 complex is determined by the hematopoietic Ets protein Fli-1 that binds to an adjacent Ets element. Chromatin immunoprecipitation experiments show that GATA-1, FOG-1 and Fli-1 co-occupy the alphaIIb promoter in vivo. Expression of several additional megakaryocyte-specific genes that bear tandem GATA and Ets elements in their promoters also depends on the physical interaction between GATA-1 and FOG-1. Our studies define a molecular context for transcriptional activation by GATA-1 and FOG-1, and may explain the occurrence of tandem GATA and Ets elements in the promoters of numerous megakaryocyte-expressed genes.

Protein kinase C-zeta overexpression induces erythroid phenotype in the monocytic leukaemia cell line U937

Previous studies have established that protein kinase C-zeta (PKC-zeta) is critical for neuronal cell differentiation. However, the role of PKC-zeta in haematopoietic cell differentiation is less clear. In this study, we have investigated the influence of PKC-zeta overexpression on the phenotype of the human monocytic U937 leukaemic cells. In two PKC-zeta-overexpressing clones (U937 zetaJ and U937 zetaB), PKC-zeta expression levels and activity were three to fourfold higher, and the enzyme accumulated both in the cytoplasm and in the nucleus compared with U937 control cells. PKC-zeta-overexpressing U937 cells exhibited an erythroid phenotype characterized by high levels of glycophorin A, cell haemoglobinization, increased GATA-1 transcripts and protein expression, compared with controls. Immunoprecipitation studies revealed that GATA-1 protein was constitutively phosphorylated in PKC-zeta-overexpressing cells. Moreover, GATA-1 did not interact with PKC-zeta but interacted with ERK1, which was constitutively activated and accumulated in the nucleus of U937 zetaJ. However, ERK1 phosphorylation inhibition by PD098059 did not influence either GATA-1 phosphorylation or GATA-1/ERK1 interaction. Collectively, these results suggest a model in which PKC-zeta induces MEK-dependent ERK1 activation, ERK1 translocation to the nucleus, GATA-1/ERK1 interaction and ERK1-independent GATA-1 phosphorylation resulting in GATA-1 accumulation. To conclude, this study provides evidence for the role of PKC-zeta in erythroid gene regulation.

Molecular and transcriptional regulation of megakaryocyte differentiation

Megakaryocytes, among the rarest of hematopoietic cells, serve the essential function of producing numerous platelets. Genetic studies have recently provided rich insights into the molecular and transcriptional regulation of megakaryocyte differentiation and thrombopoiesis. Three transcription factors, GATA-1, FOG-1, and NF-E2, are essential regulators of distinct stages in megakaryocyte differentiation, extending from the birth of early committed progenitors to the final step of platelet release; a fourth factor, Fli-1, likely also plays an important role. The putative transcriptional targets of these regulators, including the NF-E2-dependent hematopoietic-specific beta-tubulin isoform beta1, deepen our understanding of molecular mechanisms in platelet biogenesis. The study of rare syndromes of inherited thrombocytopenia in mice and man has also refined the emerging picture of megakaryocyte maturation. Synthesis of platelet-specific organelles is mediated by a variety of regulators of intracellular vesicle membrane fusion, and platelet release is coordinated through extensive and dynamic reorganization of the actin and microtubule cytoskeletons. As in other aspects of hematopoiesis, characterization of recurrent chromosomal translocations in human leukemias provides an added dimension to the molecular underpinnings of megakaryocyte differentiation. Long regarded as a mysterious cell, the megakaryocyte is thus yielding many of its secrets, and mechanisms of thrombopoiesis are becoming clearer. Although this review focuses on transcriptional control mechanisms, it also discusses recent advances in broader consideration of the birth of platelets.

Functional conservation of hematopoietic factors in Drosophila and vertebrates

The GATA, Friend of GATA, and Runt homology domain protein families function during hematopoiesis to promote progenitor cell development and regulate lineage commitment and differentiation. The hematopoietic functions of these factors have been remarkably conserved across taxonomic groups, ranging from flies to humans. Furthermore, aspects of hematopoiesis and hemocyte function appear to be conserved. Thus, comparative studies using Drosophila and vertebrate models should enhance our understanding of blood cell development.

An optimized system for studies of EPO-dependent murine pro-erythroblast development

OBJECTIVE

Objectives were to develop new means to isolate useful numbers of primary progenitor cells and to quantitatively assay the stepwise maturation of erythroblasts.

METHODS

Approaches involved dosing mice with thiamphenicol (TAP) to yield staged cohorts of pro-erythroid cells; optimizing conditions for their EPO-dependent in vitro growth and survival; developing assays for CFU-E maturation; analyzing stage-specific transcript expression; and expressing a heterologous, erythroid-specific tag (EE372) in transgenic mice.

RESULTS

Per TAP-treated mouse, 3 x 10(7) highly EPO-responsive erythroid progenitor cells were generated that represented up to 30% of total splenocytes and showed strict dependence on EPO for survival, growth, and immediate response gene expression. In this developing cohort, a tightly programmed sequence of gene expression was observed, and maximal expression of c-kit, EPO receptor, and beta-globin transcripts occurred at 72, 96, and 120 hours post-TAP withdrawal, respectively. Also, the newly discovered erythroid-specific dual-specificity kinase, DYRK3, was revealed to be expressed at a late CFU-E stage. In vitro, these progenitor cells matured stepwise from high FALS Ter119- cells (24-hour culture) to high FALS Ter119+ cells (24-36 hours) to low FALS Ter119+ maturing erythroblasts (40-48 hours) and sharp differences in their morphologies were observed. Finally, a MACS-based procedure for the purification of erythroid progenitor cells from TAP-treated EE372 transgenic mice also was developed.

CONCLUSIONS

A comprehensive new system for isolating large numbers of primary murine erythroid progenitor cells and quantitatively monitoring their development is established that should serve well in investigations of endogenous and pharmacological regulators of red blood cell development.

Transcriptional regulation of hemopoiesis

The regulation of blood cell formation, or hemopoiesis, is central to the replenishment of mature effector cells of innate and acquired immune responses. These cells fulfil specific roles in the host defense against invading pathogens, and in the maintenance of homeostasis. The development of hemopoietic cells is under stringent control from extracellular and intracellular stimuli that result in the activation of specific downstream signaling cascades. Ultimately, all signal transduction pathways converge at the level of gene expression where positive and negative modulators of transcription interact to delineate the pattern of gene expression and the overall cellular hemopoietic response. Transcription factors, therefore, represent a nodal point of hemopoietic control through the integration of the various signaling pathways and subsequent modulation of the transcriptional machinery. Transcription factors can act both positively and negatively to regulate the expression of a wide range of hemopoiesis-relevant genes including growth factors and their receptors, other transcription factors, as well as various molecules important for the function of developing cells. The expression of these genes is dependent on the complex interactions between transcription factors, co-regulatory molecules, and specific binding sequences on the DNA. Recent advances in various vertebrate and invertebrate systems emphasize the importance of transcription factors for hemopoiesis control and the evolutionary conservation of several of such mechanisms. In this review we outline some of the key issues frequently identified in studies of the transcriptional regulation of hemopoietic gene expression. In teleosts, we expect that the characterization of several of these transcription factors and their regulatory mechanisms will complement recent advances in a number of fish systems where identification of cytokine and other hemopoiesis-relevant factors are currently under investigation.

mDYRK3 kinase is expressed selectively in late erythroid progenitor cells and attenuates colony-forming unit-erythroid development

DYRKs are a new subfamily of dual-specificity kinases that was originally discovered on the basis of homology to Yak1, an inhibitor of cell cycle progression in yeast. At present, mDYRK-3 and mDYRK-2 have been cloned, and mDYRK-3 has been characterized with respect to kinase activity, expression among tissues and hematopoietic cells, and possible function during erythropoiesis. In sequence, mDYRK-3 diverges markedly in noncatalytic domains from mDYRK-2 and mDYRK-1a, but is 91.3% identical overall to hDYRK-3. Catalytically, mDYRK-3 readily phosphorylated myelin basic protein (but not histone 2B) and also appeared to autophosphorylate in vitro. Expression of mDYRK-1a, mDYRK-2, and mDYRK-3 was high in testes, but unlike mDYRK1a and mDYRK 2, mDYRK-3 was not expressed at appreciable levels in other tissues examined. Among hematopoietic cells, however, mDYRK-3 expression was selectively elevated in erythroid cell lines and primary pro-erythroid cells. In developmentally synchronized erythroid progenitor cells, expression peaked sharply following exposure to erythropoietin plus stem cell factor (SCF) (but not SCF alone), and in situ hybridizations of sectioned embryos revealed selective expression of mDYRK-3 in fetal liver. Interestingly, antisense oligonucleotides to mDYRK-3 were shown to significantly and specifically enhance colony-forming unit-erythroid colony formation. Thus, it is proposed that mDYRK-3 kinase functions as a lineage-restricted, stage-specific suppressor of red cell development. (Blood. 2001;97:901-910)