Regulation of the erythroid Kruppel-like factor (EKLF) gene promoter by the erythroid transcription factor GATA-1

Roles of Nuclear Orphan Receptors TR2 and TR4 during Hematopoiesis

TR2 and TR4 (NR2C1 and NR2C2, respectively) are evolutionarily conserved nuclear orphan receptors capable of binding direct repeat sequences in a stage-specific manner. Like other nuclear receptors, TR2 and TR4 possess important roles in transcriptional activation or repression with developmental stage and tissue specificity. TR2 and TR4 bind DNA and possess the ability to complex with available cofactors mediating developmental stage-specific actions in primitive and definitive erythrocytes. In erythropoiesis, TR2 and TR4 are required for erythroid development, maturation, and key erythroid transcription factor regulation. TR2 and TR4 recruit and interact with transcriptional corepressors or coactivators to elicit developmental stage-specific gene regulation during hematopoiesis.

Association of DDX5/p68 protein with the upstream erythroid enhancer element (EHS1) of the gene encoding the KLF1 transcription factor

EKLF/KLF1 is an essential transcription factor that plays a global role in erythroid transcriptional activation. Regulation of KLF1 is of interest, as it displays a highly restricted expression pattern, limited to erythroid cells and its progenitors. Here we use biochemical affinity purification to identify the DDX5/p68 protein as an activator of KLF1 by virtue of its interaction with the erythroid-specific DNAse hypersensitive site upstream enhancer element (EHS1). We further show that this protein associates with DEK and CTCF. We postulate that the range of interactions of DDX5/p68 with these and other proteins known to interact with this element render it part of the enhanseosome complex critical for optimal expression of KLF1 and enables the formation of a proper chromatin configuration at the Klf1 locus. These individual interactions provide quantitative contributions that, in sum, establish the high-level activity of the Klf1 promoter and suggest they can be selectively manipulated for clinical benefit.

Identification of a genomic DNA sequence that quantitatively modulates KLF1 transcription factor expression in differentiating human hematopoietic cells

The onset of erythropoiesis is under strict developmental control, with direct and indirect inputs influencing its derivation from the hematopoietic stem cell. A major regulator of this transition is KLF1/EKLF, a zinc finger transcription factor that plays a global role in all aspects of erythropoiesis. Here, we have identified a short, conserved enhancer element in KLF1 intron 1 that is important for establishing optimal levels of KLF1 in mouse and human cells. Chromatin accessibility of this site exhibits cell-type specificity and is under developmental control during the differentiation of human CD34+ cells towards the erythroid lineage. This site binds GATA1, SMAD1, TAL1, and ETV6. In vivo editing of this region in cell lines and primary cells reduces KLF1 expression quantitatively. However, we find that, similar to observations seen in pedigrees of families with KLF1 mutations, downstream effects are variable, suggesting that the global architecture of the site is buffered towards keeping the KLF1 genetic region in an active state. We propose that modification of intron 1 in both alleles is not equivalent to complete loss of function of one allele.

The N6-methyladenosine methyltransferase METTL16 enables erythropoiesis through safeguarding genome integrity

During erythroid differentiation, the maintenance of genome integrity is key for the success of multiple rounds of cell division. However, molecular mechanisms coordinating the expression of DNA repair machinery in erythroid progenitors are poorly understood. Here, we discover that an RNA N⁶-methyladenosine (m⁶A) methyltransferase, METTL16, plays an essential role in proper erythropoiesis by safeguarding genome integrity via the control of DNA-repair-related genes. METTL16-deficient erythroblasts exhibit defective differentiation capacity, DNA damage and activation of the apoptotic program. Mechanistically, METTL16 controls m⁶A deposition at the structured motifs in DNA-repair-related transcripts including Brca2 and Fancm mRNAs, thereby upregulating their expression. Furthermore, a pairwise CRISPRi screen revealed that the MTR4-nuclear RNA exosome complex is involved in the regulation of METTL16 substrate mRNAs in erythroblasts. Collectively, our study uncovers that METTL16 and the MTR4-nuclear RNA exosome act as essential regulatory machinery to maintain genome integrity and erythropoiesis.

Krüppel-Like Factor 1: A Pivotal Gene Regulator in Erythropoiesis

Krüppel-like factor 1 (KLF1) plays a crucial role in erythropoiesis. In-depth studies conducted on mice and humans have highlighted its importance in erythroid lineage commitment, terminal erythropoiesis progression and the switching of globin genes from γ to β. The role of KLF1 in haemoglobin switching is exerted by the direct activation of β-globin gene and by the silencing of γ-globin through activation of BCL11A, an important γ-globin gene repressor. The link between KLF1 and γ-globin silencing identifies this transcription factor as a possible therapeutic target for β-hemoglobinopathies. Moreover, several mutations have been identified in the human genes that are responsible for various benign phenotypes and erythroid disorders. The study of the phenotype associated with each mutation has greatly contributed to the current understanding of the complex role of KLF1 in erythropoiesis. This review will focus on some of the principal functions of KLF1 on erythroid cell commitment and differentiation, spanning from primitive to definitive erythropoiesis. The fundamental role of KLF1 in haemoglobin switching will be also highlighted. Finally, an overview of the principal human mutations and relative phenotypes and disorders will be described.

Genetic Disruption of KLF1 K74 SUMOylation in Hematopoietic System Promotes Healthy Longevity in Mice

The quest for rejuvenation and prolonged lifespan through transfusion of young blood has been studied for decades with the hope of unlocking the mystery of the key substance(s) that exists in the circulating blood of juvenile organisms. However, a pivotal mediator has yet been identified. Here, atypical findings are presented that are observed in a knockin mouse model carrying a lysine to arginine substitution at residue 74 of Krüppel-like factor 1 (KLF1/EKLF), the SUMOylation-deficient Klf1^K74R/K74R mouse, that displayed significant improvement in geriatric disorders and lifespan extension. Klf1^K74R/K74R mice exhibit a marked delay in age-related physical performance decline and disease progression as evidenced by physiological and pathological examinations. Furthermore, the KLF1(K74R) knockin affects a subset of lymphoid lineage cells; the abundance of tumor infiltrating effector CD8⁺ T cells and NKT cells is increased resulting in antitumor immune enhancement in response to tumor cell administration. Significantly, infusion of hematopoietic stem cells (HSCs) from Klf1^K74R/K74R mice extends the lifespan of the wild-type mice. The Klf1^K74R/K74R mice appear to be an ideal animal model system for further understanding of the molecular/cellular basis of aging and development of new strategies for antiaging and prevention/treatment of age-related diseases thus extending the healthspan as well as lifespan.

The complex landscape of haematopoietic lineage commitments is encoded in the coarse-grained endogenous network

Haematopoietic lineage commitments are presented by a canonical roadmap in which haematopoietic stem cells or multipotent progenitors (MPPs) bifurcate into progenitors of more restricted lineages and ultimately mature to terminally differentiated cells. Although transcription factors playing significant roles in cell-fate commitments have been extensively studied, integrating such knowledge into the dynamic models to understand the underlying biological mechanism remains challenging. The hypothesis and modelling approach of the endogenous network has been developed previously and tested in various biological processes and is used in the present study of haematopoietic lineage commitments. The endogenous network is constructed based on the key transcription factors and their interactions that determine haematopoietic cell-fate decisions at each lineage branchpoint. We demonstrate that the process of haematopoietic lineage commitments can be reproduced from the landscape which orchestrates robust states of network dynamics and their transitions. Furthermore, some non-trivial characteristics are unveiled in the dynamical model. Our model also predicted previously under-represented regulatory interactions and heterogeneous MPP states by which distinct differentiation routes are intermediated. Moreover, network perturbations resulting in state transitions indicate the effects of ectopic gene expression on cellular reprogrammes. This study provides a predictive model to integrate experimental data and uncover the possible regulatory mechanism of haematopoietic lineage commitments.

A Positive Regulatory Feedback Loop between EKLF/KLF1 and TAL1/SCL Sustaining the Erythropoiesis

The erythroid Krüppel-like factor EKLF/KLF1 is a hematopoietic transcription factor binding to the CACCC DNA motif and participating in the regulation of erythroid differentiation. With combined use of microarray-based gene expression profiling and the promoter-based ChIP-chip assay of E14.5 fetal liver cells from wild type (WT) and EKLF-knockout (Eklf^−/−) mouse embryos, we identified the pathways and direct target genes activated or repressed by EKLF. This genome-wide study together with the molecular/cellular analysis of the mouse erythroleukemic cells (MEL) indicate that among the downstream direct target genes of EKLF is Tal1/Scl. Tal1/Scl encodes another DNA-binding hematopoietic transcription factor TAL1/SCL, known to be an Eklf activator and essential for definitive erythroid differentiation. Further identification of the authentic Tal gene promoter in combination with the in vivo genomic footprinting approach and DNA reporter assay demonstrate that EKLF activates the Tal gene through binding to a specific CACCC motif located in its promoter. These data establish the existence of a previously unknow positive regulatory feedback loop between two DNA-binding hematopoietic transcription factors, which sustains mammalian erythropoiesis.

Current understanding of human megakaryocytic-erythroid progenitors and their fate determinants

PURPOSE OF REVIEW

This review focuses on our current understanding of fate decisions in bipotent megakaryocyte-erythroid progenitors (MEPs). Although extensive research has been carried out over decades, our understanding of how MEP commit to the erythroid versus megakaryocyte fate remains unclear.

RECENT FINDINGS

We discuss the isolation of primary human MEP, and focus on gene expression patterns, epigenetics, transcription factors and extrinsic factors that have been implicated in MEP fate determination. We conclude with an overview of the open debates in the field of MEP biology.

SUMMARY

Understanding MEP fate is important because defects in megakaryocyte and erythrocyte development lead to disease states such as anaemia, thrombocytopenia and leukaemia. MEP also represent a model system for studying fundamental principles underlying cell fate decisions of bipotent and pluripotent progenitors, such that discoveries in MEP are broadly applicable to stem/progenitor cell biology.

Constitutive Activation of the Canonical NF-κB Pathway Leads to Bone Marrow Failure and Induction of Erythroid Signature in Hematopoietic Stem Cells

Constitutive activation of the canonical NF-κB pathway has been associated with a variety of human pathologies. However, molecular mechanisms through which canonical NF-κB affects hematopoiesis remain elusive. Here, we demonstrate that deregulated canonical NF-κB signals in hematopoietic stem cells (HSCs) cause a complete depletion of HSC pool, pancytopenia, bone marrow failure, and premature death. Constitutive activation of IKK2 in HSCs leads to impaired quiescence and loss of function. Gene set enrichment analysis (GSEA) identified an induction of “erythroid signature” in HSCs with augmented NF-κB activity. Mechanistic studies indicated a reduction of thrombopoietin (TPO)-mediated signals and its downstream target p57 in HSCs, due to reduced c-MpI expression in a cell-intrinsic manner. Molecular studies established Klf1 as a key suppressor of c-MpI in HSPCs with increased NF-κB. In essence, these studies identified a previously unknown mechanism through which exaggerated canonical NF-κB signals affect HSCs and cause pathophysiology.

Nakagawa and Rathinam demonstrate that constitutive activation of IKK2 in HSCs causes a complete depletion of the HSC pool and impairs HSC functions due to a loss of “sternness” signature and an induction of erythroid signature.

Survey and evaluation of mutations in the human KLF1 transcription unit

Erythroid Krüppel-like Factor (EKLF/KLF1) is an erythroid-enriched transcription factor that plays a global role in all aspects of erythropoiesis, including cell cycle control and differentiation. We queried whether its mutation might play a role in red cell malignancies by genomic sequencing of the KLF1 transcription unit in cell lines, erythroid neoplasms, dysplastic disorders, and leukemia. In addition, we queried published databases from a number of varied sources. In all cases we only found changes in commonly notated SNPs. Our results suggest that if there are mutations in KLF1 associated with erythroid malignancies, they are exceedingly rare.

Ribonuclease inhibitor 1 regulates erythropoiesis by controlling GATA1 translation

Ribosomal proteins (RP) regulate specific gene expression by selectively translating subsets of mRNAs. Indeed, in Diamond-Blackfan anemia and 5q- syndrome, mutations in RP genes lead to a specific defect in erythroid gene translation and cause anemia. Little is known about the molecular mechanisms of selective mRNA translation and involvement of ribosomal-associated factors in this process. Ribonuclease inhibitor 1 (RNH1) is a ubiquitously expressed protein that binds to and inhibits pancreatic-type ribonucleases. Here, we report that RNH1 binds to ribosomes and regulates erythropoiesis by controlling translation of the erythroid transcription factor GATA1. Rnh1-deficient mice die between embryonic days E8.5 and E10 due to impaired production of mature erythroid cells from progenitor cells. In Rnh1-deficient embryos, mRNA levels of Gata1 are normal, but GATA1 protein levels are decreased. At the molecular level, we found that RNH1 binds to the 40S subunit of ribosomes and facilitates polysome formation on Gata1 mRNA to confer transcript-specific translation. Further, RNH1 knockdown in human CD34+ progenitor cells decreased erythroid differentiation without affecting myelopoiesis. Our results reveal an unsuspected role for RNH1 in the control of GATA1 mRNA translation and erythropoiesis.

Role of tissue-specific promoter DNA methylation in regulating the human EKLF gene

Erythroid Krüppel-like factor (EKLF/KLF1) is an erythroid-specific transcription factor whose activity is essential for erythropoiesis. The underlying mechanisms for EKLF specifically restricted to erythroid cells are of great interest but remain incompletely understood. To explore the epigenetic regulation of EKLF expression by promoter DNA methylation, we investigated the methylation status of the EKLF promoter and EKLF gene expression from a panel of human tissues. We observed that erythroid-specific hypomethylation of the EKLF promoter in adult erythroid cells was positively associated with EKLF expression. Demethylation of the EKLF promoter by 5-aza-2'-deoxycytidine led to elevated EKLF expression in non-erythroid cells. We further uncovered that EKLF promoter DNA methylation reduced the binding affinity for the transcription factors GATA1 and c-myb (MYB), which in turn silenced EKLF expression. These results suggest that hypomethylation of the EKLF promoter has functional significance in the establishment and maintenance of erythroid-specific gene expression.

The effect of histone deacetylase inhibitors on AHSP expression

Alpha-hemoglobin stabilizing protein (AHSP) is a molecular chaperone that can reduce the damage caused by excess free α-globin to erythroid cells in patients with impaired β-globin chain synthesis. We assessed the effect of sodium phenylbutyrate and sodium valproate, two histone deacetylase inhibitors (HDIs) that are being studied for the treatment of hemoglobinopathies, on the expression of AHSP, BCL11A (all isoforms), γ-globin genes (HBG1/2), and some related transcription factors including GATA1, NFE2, EKLF, KLF4, and STAT3. For this purpose, the K562 cell line was cultured for 2, 4, and 6 days in the presence and absence of sodium phenylbutyrate and sodium valproate. Relative real-time qRT-PCR analysis of mRNA levels was performed to determine the effects of the two compounds on gene expression. Expression of all target mRNAs increased significantly (p < 0.05), except for the expression of BCL11A, which was down-regulated (p < 0.05) in the cells treated with both compounds relative to the levels measured for untreated cells. The findings indicated that sodium valproate had a more considerable effect than sodium phenylbutyrate (p < 0.0005) on BCL11A repression and the up-regulation of other studied genes. γ-Globin and AHSP gene expression continuously increased during the culture period in the treated cells, with the highest gene expression observed for 1 mM sodium valproate after 6 days. Both compounds repressed the expression of BCL11A (-XL, -L, -S) and up-regulated GATA1, NFE2, EKLF, KLF4, STAT3, AHSP, and γ-globin genes expression. Moreover, sodium valproate showed a stronger effect on repressing BCL11A and escalating the expression of other target genes. The findings of this in vitro experiment could be considered in selecting drugs for clinical use in patients with β-hemoglobinopathies.

KLF1 directly activates expression of the novel fetal globin repressor ZBTB7A/LRF in erythroid cells

Genes encoding the human β-like hemoglobin proteins undergo a developmental switch from fetal γ-globin to adult β-globin expression around the time of birth. β-hemoglobinopathies, such as sickle-cell disease and β-thalassemia, result from mutations affecting the adult β-globin gene. The only treatment options currently available carry significant adverse effects. Analyses of heritable variations in fetal hemoglobin (HbF) levels have provided evidence that reactivation of the silenced fetal γ-globin genes in adult erythroid cells is a promising therapy. The γ-globin repressor BCL11A has become the major focus, with several studies investigating its regulation and function as a first step to inhibiting its expression or activity. However, a second repression mechanism was recently shown to be mediated by the transcription factor ZBTB7A/LRF, suggesting that understanding the regulation of ZBTB7A may also be useful. Here we show that Krüppel-like factor 1 (KLF1) directly drives expression of ZBTB7A in erythroid cells by binding to its proximal promoter. We have also uncovered an erythroid-specific regulation mechanism, leading to the upregulation of a novel ZBTB7A transcript in the erythroid compartment. The demonstration that ZBTB7A, like BCL11A, is a KLF1 target gene also fits with the observation that reduced KLF1 expression or activity is associated with HbF derepression.

Applying attractor dynamics to infer gene regulatory interactions involved in cellular differentiation

The dynamics of gene regulatory networks (GRNs) guide cellular differentiation. Determining the ways regulatory genes control expression of their targets is essential to understand and control cellular differentiation. The way a regulatory gene controls its target can be expressed as a gene regulatory function. Manual derivation of these regulatory functions is slow, error-prone and difficult to update as new information arises. Automating this process is a significant challenge and the subject of intensive effort. This work presents a novel approach to discovering biologically plausible gene regulatory interactions that control cellular differentiation. This method integrates known cell type expression data, genetic interactions, and knowledge of the effects of gene knockouts to determine likely GRN regulatory functions. We employ a genetic algorithm to search for candidate GRNs that use a set of transcription factors that control differentiation within a lineage. Nested canalyzing functions are used to constrain the search space to biologically plausible networks. The method identifies an ensemble of GRNs whose dynamics reproduce the gene expression pattern for each cell type within a particular lineage. The method's effectiveness was tested by inferring consensus GRNs for myeloid and pancreatic cell differentiation and comparing the predicted gene regulatory interactions to manually derived interactions. We identified many regulatory interactions reported in the literature and also found differences from published reports. These discrepancies suggest areas for biological studies of myeloid and pancreatic differentiation. We also performed a study that used defined synthetic networks to evaluate the accuracy of the automated search method and found that the search algorithm was able to discover the regulatory interactions in these defined networks with high accuracy. We suggest that the GRN functions derived from the methods described here can be used to fill gaps in knowledge about regulatory interactions and to offer hypotheses for experimental testing of GRNs that control differentiation and other biological processes.

Erythroid activator NF-E2, TAL1 and KLF1 play roles in forming the LCR HSs in the human adult β-globin locus

The β-like globin genes are developmental stage specifically transcribed in erythroid cells. The transcription of the β-like globin genes requires erythroid specific activators such as GATA-1, NF-E2, TAL1 and KLF1. However, the roles of these activators have not fully elucidated in transcription of the human adult β-globin gene. Here we employed hybrid MEL cells (MEL/ch11) where a human chromosome containing the β-globin locus is present and the adult β-globin gene is highly transcribed by induction. The roles of erythroid specific activators were analyzed by inhibiting the expression of NF-E2, TAL1 or KLF1 in MEL/ch11 cells. The loss of each activator decreased the transcription of human β-globin gene, locus wide histone hyperacetylation and the binding of other erythroid specific activators including GATA-1, even though not affecting the expression of other activators. Notably, sensitivity to DNase I was reduced in the locus control region (LCR) hypersensitive sites (HSs) with the depletion of activators. These results indicate that NF-E2, TAL1 and KLF1, all activators play a primary role in HSs formation in the LCR. It might contribute to the transcription of human adult β-globin gene by allowing the access of activators and cofactors. The roles of activators in the adult β-globin locus appear to be different from the roles in the early fetal locus.

Opposing regulation of megakaryopoiesis by LPA receptors 2 and 3 in K562 human erythroleukemia cells

Erythrocytes and megakaryocytes (MK) are derived from a common progenitor that undergoes lineage specification. Lysophosphatidic acid (LPA), a lipid growth factor was previously shown to be a regulator for erythropoietic process through activating LPA receptor 3 (LPA3). However, whether LPA affects megakaryopoiesis remains unclear. In this study, we used K562 leukemia cell line as a model to investigate the roles of LPA in MK differentiation. We demonstrated that K562 cells express both LPA2 and LPA3, and the expression levels of LPA2 are higher than LPA3. Treatment with phorbol 12-myristate 13-acetate, a commonly used inducer of megakaryopoiesis, reciprocally regulates the expressions of LPA2 and LPA3. By pharmacological blockers and knockdown experiments, we showed that activation of LPA2 suppresses whereas, LPA3 promotes megakaryocytic differentiation in K562. The LPA2-mediated inhibition is dependent on β-catenin translocation, whereas reactive oxygen species (ROS) generation is a downstream signal for activation of LPA3. Furthermore, the hematopoietic transcriptional factors GATA-1 and FLI-1, appear to be involved in these regulatory mechanisms. Taken together, our results suggested that LPA2 and LPA3 may function as a molecular switch and play opposing roles during megakaryopoiesis of K562 cells.

CCAR1/CoCoA pair-mediated recruitment of the Mediator defines a novel pathway for GATA1 function

The MED1 subunit of the Mediator transcriptional coregulator complex coactivates GATA1 and induces erythropoiesis. Here, we show the dual mechanism of GATA1- and MED1-mediated transcription. MED1 expression levels in K562 erythroleukemia cells paralleled the levels of GATA1-targeted gene transcription and erythroid differentiation. An N-terminal fragment of MED1, MED1(1-602), which is incapable of interacting with GATA1, enhanced GATA1-targeted gene transcription and erythroid differentiation, and introduction of MED1(1-602) into Med1(-/-) mouse embryonic fibroblasts (MEFs) partially rescued GATA1-mediated transcription. The C-terminal zinc-finger domain of GATA1 interacts with the MED1(1-602)-interacting coactivator CCAR1, CoCoA and MED1(681-715). CCAR1 and CoCoA synergistically enhanced GATA1-mediated transcription from the γ-globin promoter in MEFs. Recombinant GATA1, CCAR1, CoCoA and MED1(1-602) formed a complex in vitro, and GATA1, CCAR1, CoCoA and MED1 were recruited to the γ-globin promoter in K562 cells during erythroid differentiation. Therefore, in addition to the direct interaction between GATA1 and MED1, CoCoA and CCAR1 appear to relay the GATA1 signal to MED1, and multiple modes of the GATA1-MED1 axis may help to fine-tune GATA1 function during GATA1-mediated homeostasis events.

Ldb1 complexes: the new master regulators of erythroid gene transcription

Elucidation of the genetic pathways that control red blood cell development has been a central goal of erythropoiesis research over the past decade. Notably, data from several recent studies have provided new insights into the regulation of erythroid gene transcription. Transcription profiling demonstrates that erythropoiesis is mainly controlled by a small group of lineage-restricted transcription factors [Gata binding protein 1 (Gata1), T cell acute lymphocytic leukemia 1 protein (Tal1), and Erythroid Kruppel-like factor (EKLF; henceforth referred to as Klf1)]. Binding-site mapping using ChIP-Seq indicates that most DNA-bound Gata1 and Tal1 proteins are contained within higher order complexes (Ldb1 complexes) that include the nuclear adapters Ldb1 and Lmo2. Ldb1 complexes regulate Klf1, and Ldb1 complex-binding sites frequently colocalize with Klf1 at erythroid genes and cis-regulatory elements, indicating strong functional synergy between Gata1, Tal1, and Klf1. Together with new data demonstrating that Ldb1 can mediate long-range promoter-enhancer interactions, these findings provide a foundation for the first comprehensive models of the global regulation of erythroid gene transcription.

Pharmacological Induction of Fetal Hemoglobin Synthesis using Histone Deacetylase Inhibitors

A number of pharmacological agents are currently available for the induction of the fetal hemoglobin (Hb F) to treat the patients with sickle cell disease and beta-thalassemia. In the present review, we summarized the investigation and development of these Hb F-inducing agents and introduced histone deacetylase inhibitors as the new strategy to induce Hb F to treat the hemoglobin disorders

Protein arginine methyltransferase 1 interacts with and activates p38α to facilitate erythroid differentiation

Protein arginine methylation is emerging as a pivotal posttranslational modification involved in regulating various cellular processes; however, its role in erythropoiesis is still elusive. Erythropoiesis generates circulating red blood cells which are vital for body activity. Deficiency in erythroid differentiation causes anemia which compromises the quality of life. Despite extensive studies, the molecular events regulating erythropoiesis are not fully understood. This study showed that the increase in protein arginine methyltransferase 1 (PRMT1) levels, via transfection or protein transduction, significantly promoted erythroid differentiation in the bipotent human K562 cell line as well as in human primary hematopoietic progenitor CD34(+) cells. PRMT1 expression enhanced the production of hemoglobin and the erythroid surface marker glycophorin A, and also up-regulated several key transcription factors, GATA1, NF-E2 and EKLF, which are critical for lineage-specific differentiation. The shRNA-mediated knockdown of PRMT1 suppressed erythroid differentiation. The methyltransferase activity-deficient PRMT1G80R mutant failed to stimulate differentiation, indicating the requirement of arginine methylation of target proteins. Our results further showed that a specific isoform of p38 MAPK, p38α, promoted erythroid differentiation, whereas p38β did not play a role. The stimulation of erythroid differentiation by PRMT1 was diminished in p38α- but not p38β-knockdown cells. PRMT1 appeared to act upstream of p38α, since expression of p38α still promoted erythroid differentiation in PRMT1-knockdown cells, and expression of PRMT1 enhanced the activation of p38 MAPK. Importantly, we showed for the first time that PRMT1 was associated with p38α in cells by co-immunoprecipitation and that PRMT1 directly methylated p38α in in vitro methylation assays. Taken together, our findings unveil a link between PRMT1 and p38α in regulating the erythroid differentiation program and provide evidence suggesting a novel regulatory mechanism for p38α through arginine methylation.

EKLF/KLF1, a tissue-restricted integrator of transcriptional control, chromatin remodeling, and lineage determination

Erythroid Krüppel-like factor (EKLF or KLF1) is a transcriptional regulator that plays a critical role in lineage-restricted control of gene expression. KLF1 expression and activity are tightly controlled in a temporal and differentiation stage-specific manner. The mechanisms by which KLF1 is regulated encompass a range of biological processes, including control of KLF1 RNA transcription, protein stability, localization, and posttranslational modifications. Intact KLF1 regulation is essential to correctly regulate erythroid function by gene transcription and to maintain hematopoietic lineage homeostasis by ensuring a proper balance of erythroid/megakaryocytic differentiation. In turn, KLF1 regulates erythroid biology by a wide variety of mechanisms, including gene activation and repression by regulation of chromatin configuration, transcriptional initiation and elongation, and localization of gene loci to transcription factories in the nucleus. An extensive series of biochemical, molecular, and genetic analyses has uncovered some of the secrets of its success, and recent studies are highlighted here. These reveal a multilayered set of control mechanisms that enable efficient and specific integration of transcriptional and epigenetic controls and that pave the way for proper lineage commitment and differentiation.

5-aza-2'-deoxycytidine activates iron uptake and heme biosynthesis by increasing c-Myc nuclear localization and binding to the E-boxes of transferrin receptor 1 (TfR1) and ferrochelatase (Fech) genes

The hypomethylating agent 5-aza-2'-deoxycytidine (5-aza-CdR) and its derivatives have been successfully used for the treatment of myelodysplastic syndromes, and they frequently improve the anemia that usually accompanies these disorders. However, the molecular mechanisms underlying this action remain poorly understood. In this study, we used two erythroid models, murine erythroid leukemia cells and erythroid burst-forming unit-derived erythroblasts, to show that 5-aza-CdR induced erythroid differentiation and increased the expression of transferrin receptor 1 (TfR1) and ferrochelatase (Fech), thereby increasing iron uptake and heme biosynthesis. We have identified new regulatory E-boxes that lie outside of CpG islands in the TfR1 and Fech promoters, and the methylation status of these sites can be altered by 5-aza-CdR treatment. This in turn altered the binding of the transcription factor c-Myc to these promoter elements. Furthermore, 5-aza-CdR promoted the nuclear translocation of c-Myc and its binding to Max to form functional complexes. The coordinated actions of 5-aza-CdR on the methylation status of the target genes and in stimulating the nuclear translocation of c-Myc provide new molecular insights into the regulation of E-boxes and explain, at least in part, the increased erythroid response to 5-aza-CdR treatment.

Hierarchical differentiation of myeloid progenitors is encoded in the transcription factor network

Hematopoiesis is an ideal model system for stem cell biology with advanced experimental access. A systems view on the interactions of core transcription factors is important for understanding differentiation mechanisms and dynamics. In this manuscript, we construct a Boolean network to model myeloid differentiation, specifically from common myeloid progenitors to megakaryocytes, erythrocytes, granulocytes and monocytes. By interpreting the hematopoietic literature and translating experimental evidence into Boolean rules, we implement binary dynamics on the resulting 11-factor regulatory network. Our network contains interesting functional modules and a concatenation of mutual antagonistic pairs. The state space of our model is a hierarchical, acyclic graph, typifying the principles of myeloid differentiation. We observe excellent agreement between the steady states of our model and microarray expression profiles of two different studies. Moreover, perturbations of the network topology correctly reproduce reported knockout phenotypes in silico. We predict previously uncharacterized regulatory interactions and alterations of the differentiation process, and line out reprogramming strategies.

Protein-protein interactions: analysis of a false positive GST pulldown result

One of the most common ways to demonstrate a direct protein-protein interaction in vitro is the glutathione-S-transferse (GST)-pulldown. Here we report the detailed characterization of a putative interaction between two transcription factor proteins, GATA-1 and Krüppel-like factor 3 (KLF3/BKLF) that show robust interactions in GST-pulldown experiments. Attempts to map the interaction interface of GATA-1 on KLF3 using a mutagenic screening approach did not yield a contiguous binding face on KLF3, suggesting that the interaction might be non-specific. NMR experiments showed that the proteins do not interact at protein concentrations of 50-100 μM. Rather, the GST tag can cause part of KLF3 to misfold. In addition to misfolding, the fact that both proteins are DNA-binding domains appears to introduce binding artifacts (possibly nucleic acid bridging) that cannot be resolved by the addition of nucleases or ethidium bromide (EtBr). This study emphasizes the need for caution in relying on GST-pulldown results and related methods, without convincing confirmation from different approaches.

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.

Transcriptional regulation by GATA1 and GATA2 during erythropoiesis

The transcription factor GATA1 regulates multiple genes in erythroid lineage cells. However, the means by which GATA1 regulates the expression of target genes during erythropoiesis remains to be elucidated. Three mechanisms have been postulated for the regulation of genes by GATA1. First, individual target genes may have multiple discrete thresholds for cellular GATA1. GATA1 has a dynamic expression profile during erythropoiesis, thus the expression of a set of GATA1 target genes may be triggered at a given stage of differentiation by cellular GATA1. Second, the expression of GATA1 target genes may be modified, at least in part, by GATA2 occupying the GATA-binding motifs. GATA2 is expressed earlier in erythropoiesis than GATA1, and prior GATA2 binding may afford GATA1 access to GATA motifs through epigenetic remodeling and thus facilitate target gene expression. Third, other regulatory molecules specific to each target gene may function cooperatively with GATA1. If GATA1 is required for the expression of such cofactors, a regulatory network will be formed and relevant gene expression will be delayed. We propose that the stage-specific regulation of erythroid genes by GATA1 is tightly controlled through a combination of these mechanisms in vivo.

Garlic accelerates red blood cell turnover and splenic erythropoietic gene expression in mice: evidence for erythropoietin-independent erythropoiesis

Garlic (Allium sativum) has been valued in many cultures both for its health effects and as a culinary flavor enhancer. Garlic's chemical complexity is widely thought to be the source of its many health benefits, which include, but are not limited to, anti-platelet, procirculatory, anti-inflammatory, anti-apoptotic, neuro-protective, and anti-cancer effects. While a growing body of scientific evidence strongly upholds the herb's broad and potent capacity to influence health, the common mechanisms underlying these diverse effects remain disjointed and relatively poorly understood. We adopted a phenotype-driven approach to investigate the effects of garlic in a mouse model. We examined RBC indices and morphologies, spleen histochemistry, RBC half-lives and gene expression profiles, followed up by qPCR and immunoblot validation. The RBCs of garlic-fed mice register shorter half-lives than the control. But they have normal blood chemistry and RBC indices. Their spleens manifest increased heme oxygenase 1, higher levels of iron and bilirubin, and presumably higher CO, a pleiotropic gasotransmitter. Heat shock genes and those critical for erythropoiesis are elevated in spleens but not in bone marrow. The garlic-fed mice have lower plasma erythropoietin than the controls, however. Chronic exposure to CO of mice on garlic-free diet was sufficient to cause increased RBC indices but again with a lower plasma erythropoietin level than air-treated controls. Furthermore, dietary garlic supplementation and CO treatment showed additive effects on reducing plasma erythropoietin levels in mice. Thus, garlic consumption not only causes increased energy demand from the faster RBC turnover but also increases the production of CO, which in turn stimulates splenic erythropoiesis by an erythropoietin-independent mechanism, thus completing the sequence of feedback regulation for RBC metabolism. Being a pleiotropic gasotransmitter, CO may be a second messenger for garlic's other physiological effects.

Mammalian Krüppel-like factors in health and diseases

The Krüppel-like factor (KLF) family of transcription factors regulates diverse biological processes that include proliferation, differentiation, growth, development, survival, and responses to external stress. Seventeen mammalian KLFs have been identified, and numerous studies have been published that describe their basic biology and contribution to human diseases. KLF proteins have received much attention because of their involvement in the development and homeostasis of numerous organ systems. KLFs are critical regulators of physiological systems that include the cardiovascular, digestive, respiratory, hematological, and immune systems and are involved in disorders such as obesity, cardiovascular disease, cancer, and inflammatory conditions. Furthermore, KLFs play an important role in reprogramming somatic cells into induced pluripotent stem (iPS) cells and maintaining the pluripotent state of embryonic stem cells. As research on KLF proteins progresses, additional KLF functions and associations with disease are likely to be discovered. Here, we review the current knowledge of KLF proteins and describe common attributes of their biochemical and physiological functions and their pathophysiological roles.

Unique biological properties and application potentials of CD34+ CD38- stem cells from various sources

OBJECTIVE

Somatic CD34+ CD38- stem cells can differentiate into cells of hematopoietic and endothelial lineages and have been clinically used to treat diseases. These stem cells can be obtained from cord blood (CB), bone marrow or granulocyte-macrophage colony-stimulating factor-mobilized peripheral blood. Unmasking genes differentially expressed in hematopoietic stem cells (HSCs) from different anatomic locations can improve our understanding of their basic biological features and help in clinical decision making when applying different HSCs.

MATERIALS AND METHODS

We performed microarray analysis on human CD34+ CD38- HSCs isolated from CB, bone marrow and peripheral blood. Systems biology and advanced bioinformatics tools were used to better understand the biological modules and genetic networks accompanying each HSC subtype.

RESULTS

We identified HSC genes differentially expressed in various HSCs and found them to be involved in critical biological processes such as cell cycle regulation, cell motility, and endogenous antigen presentation. Among these three HSC types, HSCs from CB expressed the fewest rejection and immune response-associated genes, thereby showing the best potential as a transplantation source. Analysis of HSC-enriched genes using systems biology tools revealed a complex genetic network functioning in different CD34+ CD38- cells, in which several genes act as hubs, such as MYC in CB HSCs and hepatic growth factor in bone marrow HSCs, to maintain the stability or connectivity of the whole network.

CONCLUSION

This study provides the foundation for a more detailed understanding of CD34+ CD38- HSCs from different sources, and reveals the potentials of different HSCs for different clinical applications.

LRF is an essential downstream target of GATA1 in erythroid development and regulates BIM-dependent apoptosis

GATA-1-dependent transcription is essential for erythroid differentiation and maturation. Suppression of programmed cell death is also thought to be critical for this process; however, the link between these two features of erythropoiesis has remained elusive. Here, we show that the POZ-Krüppel family transcription factor, LRF (also known as Zbtb7a/Pokemon), is a direct target of GATA1 and plays an essential antiapoptotic role during terminal erythroid differentiation. We find that loss of Lrf leads to lethal anemia in embryos, due to increased apoptosis of late-stage erythroblasts. This programmed cell death is Arf and p53 independent and is instead mediated by upregulation of the proapoptotic factor Bim. We identify Lrf as a direct repressor of Bim transcription. In strong support of this mechanism, genetic Bim loss delays the lethality of Lrf-deficient embryos and rescues their anemia phenotype. Thus, our data define a key transcriptional cascade for effective erythropoiesis, whereby GATA-1 suppresses BIM-mediated apoptosis via LRF.

EKLF/KLF1 controls cell cycle entry via direct regulation of E2f2

Differentiation of erythroid cells requires precise control over the cell cycle to regulate the balance between cell proliferation and differentiation. The zinc finger transcription factor, erythroid Krüppel-like factor (EKLF/KLF1), is essential for proper erythroid cell differentiation and regulates many erythroid genes. Here we show that loss of EKLF leads to aberrant entry into S-phase of the cell cycle during both primitive and definitive erythropoiesis. This cell cycle defect was associated with a significant reduction in the expression levels of E2f2 and E2f4, key factors necessary for the induction of S-phase gene expression and erythropoiesis. We found and validated novel intronic enhancers in both the E2f2 and E2f4 genes, which contain conserved CACC, GATA, and E-BOX elements. The E2f2 enhancer was occupied by EKLF in vivo. Furthermore, we were able to partially restore cell cycle dynamics in EKLF(-/-) fetal liver upon additional genetic depletion of Rb, establishing a genetic causal link between reduced E2f2 and the EKLF cell cycle defect. Finally, we propose direct regulation of the E2f2 enhancer is a generic mechanism by which many KLFs regulate proliferation and differentiation.

Mutations in EKLF/KLF1 form the molecular basis of the rare blood group In(Lu) phenotype

Comparison of normal erythroblasts and erythroblasts from persons with the rare In(Lu) type of Lu(a-b-) blood group phenotype showed increased transcription levels for 314 genes and reduced levels for 354 genes in In(Lu) cells. Many erythroid-specific genes (including ALAS2, SLC4A1) had reduced transcript levels, suggesting the phenotype resulted from a transcription factor abnormality. A search for mutations in erythroid transcription factors showed mutations in the promoter or coding sequence of EKLF in 21 of 24 persons with the In(Lu) phenotype. In all cases the mutant EKLF allele occurred in the presence of a normal EKLF allele. Nine different loss-of-function mutations were identified. One mutation abolished a GATA1 binding site in the EKLF promoter (-124T>C). Two mutations (Leu127X; Lys292X) resulted in premature termination codons, 2 (Pro190LeufsX47; Arg319GlufsX34) in frameshifts, and 4 in amino acid substitution of conserved residues in zinc finger domain 1 (His299Tyr) or domain 2 (Arg328Leu; Arg328His; Arg331Gly). Persons with the In(Lu) phenotype have no reported pathology, indicating that one functional EKLF allele is sufficient to sustain human erythropoiesis. These data provide the first description of inactivating mutations in human EKLF and the first demonstration of a blood group phenotype resulting from mutations in a transcription factor.

Activation of Eklf expression during hematopoiesis by Gata2 and Smad5 prior to erythroid commitment

The hierarchical progression of stem and progenitor cells to their more-committed progeny is mediated through cell-to-cell signaling pathways and intracellular transcription factor activity. However, the mechanisms that govern the genetic networks underlying lineage fate decisions and differentiation programs remain poorly understood. Here we show how integration of Bmp4 signaling and Gata factor activity controls the progression of hematopoiesis, as exemplified by the regulation of Eklf during establishment of the erythroid lineage. Utilizing transgenic reporter assays in differentiating mouse embryonic stem cells as well as in the murine fetal liver, we demonstrate that Eklf expression is initiated prior to erythroid commitment during hematopoiesis. Applying phylogenetic footprinting and in vivo binding studies in combination with newly developed loss-of-function technology in embryoid bodies, we find that Gata2 and Smad5 cooperate to induce Eklf in a progenitor population, followed by a switch to Gata1-controlled regulation of Eklf transcription upon erythroid commitment. This stage- and lineage-dependent control of Eklf expression defines a novel role for Eklf as a regulator of lineage fate decisions during hematopoiesis.

MLLT3 regulates early human erythroid and megakaryocytic cell fate

Regulatory mechanisms of human hematopoiesis remain largely uncharacterized. Through expression profiling of prospectively isolated stem and primitive progenitor cells as well as committed progenitors from cord blood (CB), we identified MLLT3 as a candidate regulator of erythroid/megakaryocytic (E/Meg) lineage decisions. Through the analysis of the hematopoietic potential of primitive cord blood cells in which MLLT3 expression has been knocked down, we identify a requirement for MLLT3 in the elaboration of the erythroid and megakaryocytic lineages. Conversely, forced expression of MLLT3 promotes the output of erythroid and megakaryocytic progenitors, and analysis of MLLT3 mutants suggests that this capacity of MLLT3 depends on its transcriptional regulatory activity. Gene expression and cis-regulatory element analyses reveal crossregulatory interactions between MLLT3 and E/Meg-affiliated transcription factor GATA-1. Taken together, the data identify MLLT3 as a regulator of early erythroid and megakaryocytic cell fate in the human system.

Erythroid Kruppel-like Factor Regulates the G1 Cyclin Dependent Kinase Inhibitor p18INK4c

Erythroid Kruppel-like factor (EKLF, KLF1) is an essential erythroid cell specific C(2)H(2) zinc finger transcription factor that binds CACC box elements in promoters and distant regulatory elements to activate transcription. Forced expression of EKLF arrests cell division. The cyclin dependent kinase (Cdk) inhibitor p18(INK4c) was identified as a potential novel EKLF target gene from an expression profiling study. The p18(INK4c) protein functions as an inhibitor of Cdk4 and Cdk6 activity during early G1 phase of the cell cycle, thus acting as a physiological brake on cell division. We confirmed p18(INK4c) is downregulated in EKLF null mice by real-time PCR and Western blotting, and identified three closely associated and highly conserved EKLF binding sites (CCNCNCCCN) approximately 1 kb upstream of the p18(INK4c) transcriptional start site. We showed that EKLF binds to one of these elements by gel shift assay and demonstrated this site is capable of driving EKLF dependent transcription. We also determined by chromatin immunoprecipitation (ChIP) that this region of the p18(INK4c) promoter is bound by EKLF in erythroid cells. Thus, EKLF is a direct regulator of p18(INK4c) gene expression, and much of EKLF's role in driving erythroid cell differentiation may occur via p18(INK4c).

Genomic organisation and regulation of murine alpha haemoglobin stabilising protein by erythroid Kruppel-like factor

Alpha haemoglobin stabilising protein (AHSP) binds free alpha-globin chains and plays an important role in the protection of red cells, particularly during beta-thalassaemia. Murine ASHP was discovered as a GATA-1 target gene and human AHSP is directly regulated by GATA-1. More recently, AHSP was rediscovered as a highly erythroid Kruppel-like factor (EKLF) -dependent transcript. We have determined the organisation of the murine AHSP gene and compared it to orthologs. There are two CACC box elements in the proximal promoter. The proximal element is absolutely conserved, but does not bind EKLF as it is not a canonical binding site. In rodents, the distal element contains a 3 bp insertion that disrupts the typical EKLF binding consensus region. Nevertheless, EKLF binds this atypical site by gel mobility shift assay, specifically occupies the AHSP promoter in vivo in a chromatin immunoprecipitation assay, and transactivates AHSP through this CACC site in promoter-reporter assays. Our results suggest EKLF can occupy CACC elements in vivo that are not predictable from the consensus binding site inferred from structural studies. We also propose that absence of AHSP in EKLF-null red cells exacerbates the toxicity of free alpha-globin chains, which exist because of the defect in beta-globin gene activation.

Antiviral activity of CHO-SS cell-derived human omega interferon and other human interferons against HCV RNA replicons and related viruses

The fully glycosylated human omega interferon produced from CHO-SS cells (glycosylated IFN-omega) has been shown to be well-tolerated in man and to induce a sustained virologic response in patients infected with hepatitis C virus (HCV). We examined the antiviral activity of glycosylated IFN-omega and various human IFNs (IFN-alpha, -beta, -gamma and non-glycosylated bacterial (Escherichia coli) recombinant IFN-omega (non-glycosylated IFN-omega)) against HCV RNA replicons and several viruses related to HCV. Since none of the IFNs displayed cytotoxicity we compared their activities based on the effective concentration of the IFN that inhibited virus growth by 50% (EC50). Glycosylated IFN-omega was found to be the most potent antiviral agent of all the IFNs tested against bovine viral diarrhea virus (BVDV), yellow fever virus and West Nile virus. With HCV RNA replicons, non-glycosylated IFN-omega was comparable in activity to IFN-alpha while glycosylated IFN-omega was markedly more potent, indicating that glycosylation has an important effect on its activity. Drug combination analysis of glycosylated IFN-omega+ribavirin (RBV) in BVDV showed a synergy of antiviral effects similar to IFN-alpha+RBV, as well as a unique antagonism of RBV cytotoxic effects by glycosylated IFN-omega. Transcription factor (TF) profiling indicated that IFN-alpha or glycosylated IFN-omega treatment upregulated the same 17 TFs. IFN-alpha and glycosylated IFN-omega also upregulated 9 and 40 additional unique TFs, respectively. The differences in the expression of these TFs were modest, but statistically significantly different for eight of the TFs that were upregulated exclusively by glycosylated IFN-omega. The activation of these additional TFs by glycosylated IFN-omega might contribute to its high potency.

A DNA-binding protein, tfp1, involved in juvenile hormone-regulated gene expression in Locusta migratoria

A partially palindromic 15-nt. sequence upstream from a juvenile hormone-regulated gene (jhp21) was previously identified in the African migratory locust, Locusta migratoria. This sequence was proposed as a juvenile hormone (JH) response element (JHRE), and a protein that bound to it, as a transcription factor (TF). A yeast strain was constructed containing four tandem copies of the JHRE and after transfection with a cDNA library made to fat bodies from vitellogenic females, yeast one-hybrid experiments yielded sequences for four putative binding proteins. One of these sequences, corresponding to a transcript that was present in fat body irrespective of JH stimulation, encodes a 35kDa protein. This was designated tfp1 and appears to have a leucine zipper motif and a lipid-binding motif. Recombinant tfp1 bound to JHRE in electrophoretic mobility shift experiments and addition of tfp1 antibody in the binding reaction resulted in the disappearance or shift of TF. We suggest that JH induces the association of pre-existing proteins, including tfp1, to form an active complex, which binds to the JHRE upstream from jhp21 and regulates its transcription.

Genetic regulatory networks programming hematopoietic stem cells and erythroid lineage specification

Erythroid cell production results from passage through cellular hierarchies dependent on differential gene expression under the control of transcription factors responsive to changing niches. We have constructed Genetic Regulatory Networks (GRNs) describing this process, based predominantly on mouse data. Regulatory network motifs identified in E. coli and yeast GRNs are found in combination in these GRNs. Feed-forward motifs with autoregulation generate forward momentum and also control its rate, which is at its lowest in hematopoietic stem cells (HSCs). The simultaneous requirement for multiple regulators in multi-input motifs (MIMs) provides tight control over expression of target genes. Combinations of MIMs, exemplified by the SCL/LMO2 complexes, which have variable content and binding sites, explain how individual regulators can have different targets in HSCs and erythroid cells and possibly also how HSCs maintain stem cell functions while expressing lineage-affiliated genes at low level, so-called multi-lineage priming. MIMs combined with cross-antagonism describe the relationship between PU.1 and GATA-1 and between two of their target genes, Fli-1 and EKLF, with victory for GATA-1 and EKLF leading to erythroid lineage specification. These GRNs are useful repositories for current regulatory information, are accessible in interactive form via the internet, enable the consequences of perturbation to be predicted, and can act as seed networks to organize the rapidly accumulating microarray data.

The GATA site-dependent hemogen promoter is transcriptionally regulated by GATA1 in hematopoietic and leukemia cells

Hemgn (a gene symbol for hemogen in mouse, EDAG in human and RP59 in rat) encodes a nuclear protein that is highly expressed in hematopoietic tissues and acute leukemia. To characterize its regulatory mechanisms, we examined the activities of a Hemgn promoter containing 2975 bp of 5' flanking sequence and 196 bp of 5' untranslated region (5' UTR) sequence both in vitro and in vivo: this promoter is preferentially activated in a hematopoietic cell line, not in nonhematopoietic cell lines, and is sufficient to drive the transcription of a lacZ transgene in hematopoietic tissues in transgenic mice. Mutagenesis analyses showed that the 5' UTR including two highly conserved GATA boxes is critical for the promoter activity. GATA1, not GATA2, binds to the GATA binding sites and transactivates the Hemgn promoter in a dose-dependent manner. Furthermore, the expression of human hemogen (EDAG) transcripts were closely correlated with levels of GATA1 transcripts in primary acute myeloid leukemia specimens. This study suggests that the Hemgn promoter contains critical regulatory elements for its transcription in hematopoietic tissues and Hemgn is a direct target of GATA1 in leukemia cells.

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.

Transcriptional regulation of FKLF-2 (KLF13) gene in erythroid cells

FKLF-2 (KLF13) was cloned from fetal globin-expressing tissues and has been shown to be abundantly expressed in erythroid cells. In this study we examined the transcriptional regulation of the KLF13 gene. A 5.5 kb 5' flanking region cloned from mouse erythroleukemia (MEL) cell genomic DNA showed that major cis regulatory activities exist in the 550 bp sequence to the unique transcription start site, and that the promoter is more active in K562 cells than in COS-7 cells. The promoter was trans-activated by co-expressed GATA-1 through the sequence containing two CCAAT motifs, suggesting that GATA-1 is involved in the abundant expression of KLF13 mRNA in the erythroid tissue. Dual action, i.e. activating effect in COS-7 and repressive effect in K562 cell, was observed on its own promoter, suggesting a feedback mechanism for the transcriptional control of the KLF13 gene in the erythroid environment. These findings provide an insight on the mechanism of inducible mRNA expression of the KLF13 gene in erythroid cells.

Chromatin structure at the flanking regions of the human beta-globin locus control region DNase I hypersensitive site-2: proposed nucleosome positioning by DNA-binding proteins including GATA-1

The human beta-globin locus control region DNase I hypersensitive site-2 (LCR HS-2) is erythroid-specific and is located 10.9 kb upstream of the epsilon-globin gene. Most studies have only examined the core region of HS-2. However, previous studies in this laboratory indicate that positioned nucleosomes are present at the 5'- and 3'-flanking regions of HS-2. In addition, footprints were observed that indicated the involvement of DNA-binding proteins in positioning the nucleosome cores. A consensus GATA-1 site exists in the region of the 3'-footprint. In this study, using an electrophoretic mobility shift assay (EMSA) and DNase I footprinting, we confirmed that GATA-1 binds in vitro at the 3'-end of HS-2. An additional GATA-1 site was found to bind GATA-1 in vitro at a site positioned 40 bp upstream. At the 5'-end of HS-2, DNase I footprinting revealed a series of footprints showing a marked correlation with the in vivo footprints. EMSA indicated the presence of several erythroid-specific complexes in this region including GATA-1 binding. Sequence alignment for 12 mammalian species in HS-2 confirmed that the highest conservation to be in the HS-2 core. However, a second level of conservation extends from the core to the sites of the proposed positioning proteins at the HS-2 flanking regions, before declining rapidly. This indicates the importance of the HS-2 flanking regions and supports the proposal of nucleosome positioning proteins in these regions.

Regulatory elements of the EKLF gene that direct erythroid cell-specific expression during mammalian development

Erythroid Krüppel-like factor (EKLF) plays an essential role in enabling β-globin expression during erythroid ontogeny. It is first expressed in the extraembryonic mesoderm of the yolk sac within the morphologically unique cells that give rise to the blood islands, and then later within the hepatic primordia. The BMP4/Smad pathway plays a critical role in the induction of EKLF, and transient transfection analyses demonstrate that sequences located within less than 1 kb of its transcription initiation site are sufficient for high-level erythroid-specific transcription. We have used transgenic analyses to verify that 950 bp located adjacent to the EKLF start site of transcription is sufficient to generate lacZ expression within the blood islands as well as the fetal liver during embryonic development. Of particular importance are 3 regions, 2 of which overlap endogenous erythroid-specific DNase hypersensitive sites, and 1 of which includes the proximal promoter region. The onset of transgene expression mimics that of endogenous EKLF as it begins by day 7.5 (d7.5) to d8.0. In addition, it exhibits a strict hematopoietic specificity, localized only to these cells and not to the adjacent vasculature at all stages examined. Finally, expression is heterocellular, implying that although these elements are sufficient for tissue-specific expression, they do not shield against the position effects of adjacent chromatin. These analyses demonstrate that a surprisingly small DNA segment contains all the information needed to target a linked gene to the hematopoietic compartment at both early and later stages of development, and may be a useful cassette for this purpose.

Neurokinin-B transcription in erythroid cells: direct activation by the hematopoietic transcription factor GATA-1

The GATA family of transcription factors establishes genetic networks that control developmental processes including hematopoiesis, vasculogenesis, and cardiogenesis. We found that GATA-1 strongly activates transcription of the Tac-2 gene, which encodes proneurokinin-B, a precursor of neurokinin-B (NK-B). Neurokinins function through G protein-coupled transmembrane receptors to mediate diverse physiological responses including pain perception and the control of vascular tone. Whereas an elevated level of NK-B was implicated in pregnancy-associated pre-eclampsia (Page, N. M., Woods, R. J., Gardiner, S. M., Lomthaisong, K., Gladwell, R. T., Butlin, D. J., Manyonda, I. T., and Lowry, P. J. (2000) Nature 405, 797-800), the regulation of NK-B synthesis and function are poorly understood. Tac-2 was expressed in normal murine erythroid cells and was induced upon ex vivo erythropoiesis. An estrogen receptor fusion to GATA-1 (ER-GATA-1) and endogenous GATA-1 both occupied a region of Tac-2 intron-7, which contains two conserved GATA motifs. Genetic complementation analysis in GATA-1-null G1E cells revealed that endogenous GATA-2 occupied the same region of intron-7, and expression of ER-GATA-1 displaced GATA-2 and activated Tac-2 transcription. Erythroid cells did not express neurokinin receptors, whereas aortic and yolk sac endothelial cells differentially expressed neurokinin receptor subtypes. Since NK-B induced cAMP accumulation in yolk sac endothelial cells, these results suggest a new mode of vascular regulation in which GATA-1 controls NK-B synthesis in erythroid cells.

Activation and repression of interleukin-12 p40 transcription by erythroid Kruppel-like factor in macrophages

Transcription of interleukin (IL)-12 p40 in myeloid cells is attributed to the recruitment of multiple activated transcription factors such as nuclear factor kappaB (NFkappaB), CCAAT enhancer-binding protein beta, ets-2, PU.1, and so forth. We now provide the first description of the human erythroid Kruppel-like factor (EKLF) in human primary macrophages and identify the role of EKLF in IL-12 p40 expression. EKLF-specific binding to the CACCC element (-224 to -220) on the human IL-12 p40 promoter was observed in resting human primary macrophages. Functional analysis of the CACCC element revealed a dependent role for EKLF binding in activating IL-12 p40 transcription in resting RAW264.7 cells, whereas EKLF overexpression in the presence or absence of this element repressed IL-12 p40 transcription in interferon gamma/lipopolysaccharide-stimulated RAW264.7 cells. Murine endogenous IL-12 p40 mRNA was consistently induced by overexpressed EKLF in resting RAW264.7 cells, whereas EKLF suppressed IL-12 p40 expression in activated RAW264.7 cells. Modulation of nuclear binding activities at the IL-12 p40 NFkappaB half-site was induced by EKLF for down-regulation of IL-12 p40 transcription in activated RAW264.7 cells, but no effect of EKLF on NFkappaB activity was observed in resting RAW264.7 cells. Taken together, we identify EKLF as a transcription factor in macrophages able to regulate IL-12 p40 transcription depending on the cellular activation status. The bifunctional control of IL-12 p40 by EKLF and its modulation of NFkappaB support a potential function for this factor in orchestrating IL-12 p40 production in macrophages.

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.