Regulation of the Erythroid Transcription Factor NF-E2 by Cyclic Adenosine Monophosphate–Dependent Protein Kinase

Hematopoietic transcription factors and differential cofactor binding regulate PRKACB isoform expression

Hematopoietic differentiation is controlled by key transcription factors, which regulate stem cell functions and differentiation. TAL1 is a central transcription factor for hematopoietic stem cell development in the embryo and for gene regulation during erythroid/megakaryocytic differentiation. Knowledge of the target genes controlled by a given transcription factor is important to understand its contribution to normal development and disease. To uncover direct target genes of TAL1 we used high affinity streptavidin/biotin-based chromatin precipitation (Strep-CP) followed by Strep-CP on ChIP analysis using ChIP promoter arrays. We identified 451 TAL1 target genes in K562 cells. Furthermore, we analysed the regulation of one of these genes, the catalytic subunit beta of protein kinase A (PRKACB), during megakaryopoiesis of K562 and primary human CD34+ stem cell/progenitor cells. We found that TAL1 together with hematopoietic transcription factors RUNX1 and GATA1 binds to the promoter of the isoform 3 of PRKACB (Cβ3). During megakaryocytic differentiation a coactivator complex on the Cβ3 promoter, which includes WDR5 and p300, is replaced with a corepressor complex. In this manner, activating chromatin modifications are removed and expression of the PRKACB-Cβ3 isoform during megakaryocytic differentiation is reduced. Our data uncover a role of the TAL1 complex in controlling differential isoform expression of PRKACB. These results reveal a novel function of TAL1, RUNX1 and GATA1 in the transcriptional control of protein kinase A activity, with implications for cellular signalling control during differentiation and disease.

Regulation and function of the NFE2 transcription factor in hematopoietic and non-hematopoietic cells

The NFE2 transcription factor was identified over 25 years ago. The NFE2 protein forms heterodimers with small MAF proteins, and the resulting complex binds to regulatory elements in a large number of target genes. In contrast to other CNC transcription family members including NFE2L1 (NRF1), NFE2L2 (NRF2) and NFE2L3 (NRF3), which are widely expressed, earlier studies had suggested that the major sites of NFE2 expression are hematopoietic cells. Based on cell culture studies it was proposed that this protein acts as a critical regulator of globin gene expression. However, the knockout mouse model displayed only mild erythroid abnormalities, while the major phenotype was a defect in megakaryocyte biogenesis. Indeed, absence of NFE2 led to severely impaired platelet production. A series of recent data, also summarized here, shed new light on the various functional roles of NFE2 and the regulation of its activity. NFE2 is part of a complex regulatory network, including transcription factors such as GATA1 and RUNX1, controlling megakaryocytic and/or erythroid cell function. Surprisingly, it was recently found that NFE2 also has a role in non-hematopoietic tissues, such as the trophoblast, in which it is also expressed, as well as the bone, opening the door to new research areas for this transcription factor. Additional data showed that NFE2 function is controlled by a series of posttranslational modifications. Important strides have been made with respect to the clinical significance of NFE2, linking this transcription factor to hematological disorders such as polycythemias.

Differential roles of cAMP and cGMP in megakaryocyte maturation and platelet biogenesis

The cyclic nucleotides cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) regulate the activity of protein kinase A (PKA) and protein kinase G (PKG), respectively. This process helps maintain circulating platelets in a resting state. Here we studied the role of cAMP and cGMP in the regulation of megakaryocyte (MK) differentiation and platelet formation. Cultured, platelet-producing MKs were differentiated from fetal livers harvested from 13.5 days postcoital mouse embryos. MK development was accompanied by a dramatic increase in cAMP production and expression of soluble guanylate cyclase, PKG, and PKA as well as their downstream targets vasodilator-stimulated phosphoprotein (VASP) and MENA. Stimulation of prostaglandin E(1) receptor/adenylyl cyclase or soluble guanylate cyclase/PKG in cultured MKs increased VASP phosphorylation, indicating that these components share a common signaling pathway. To dissect out the role of cyclic nucleotides in MK differentiation, cAMP/PKA and cGMP/PKG signaling were alternately blocked in cultured MKs. Down-regulation of cAMP pathway effectors decreased MK numbers and ploidy. Notably, cGMP levels increased at the beginning of MK development and returned to basal levels in parallel with MK maturation. However, inhibition of cGMP pathway effectors had no effect on MK development. In addition, platelet release from mature MKs was enhanced by cGMP and inhibited by cAMP. Our data suggest that cAMP plays an important role in MK differentiation, while cAMP and cGMP have opposite effects on platelet production. Identifying the signaling pathways that underpin MK development and proplatelet formation will provide greater insights into thrombopoiesis and may potentially yield useful therapeutic targets.

JAK-STAT and AKT pathway-coupled genes in erythroid progenitor cells through ontogeny

Background

It has been reported that the phosphatidylinositol 3-kinase (PI3K)-AKT signaling pathway regulates erythropoietin (EPO)-induced survival, proliferation, and maturation of early erythroid progenitors. Erythroid cell proliferation and survival have also been related to activation of the JAK-STAT pathway. The goal of this study was to observe the function of EPO activation of JAK-STAT and PI3K/AKT pathways in the development of erythroid progenitors from hematopoietic CD34⁺ progenitor cells, as well as to distinguish early EPO target genes in human erythroid progenitors during ontogeny.

Methods

Hematopoietic CD34⁺ progenitor cells, isolated from fetal and adult hematopoietic tissues, were differentiated into erythroid progenitor cells. We have used microarray analysis to examine JAK-STAT and PI3K/AKT related genes, as well as broad gene expression modulation in these human erythroid progenitor cells.

Results

In microarray studies, a total of 1755 genes were expressed in fetal liver, 3844 in cord blood, 1770 in adult bone marrow, and 1325 genes in peripheral blood-derived erythroid progenitor cells. The erythroid progenitor cells shared 1011 common genes. Using the Ingenuity Pathways Analysis software, we evaluated the network pathways of genes linked to hematological system development, cellular growth and proliferation. The KITLG, EPO, GATA1, PIM1 and STAT3 genes represent the major connection points in the hematological system development linked genes. Some JAK-STAT signaling pathway-linked genes were steadily upregulated throughout ontogeny (PIM1, SOCS2, MYC, PTPN11), while others were downregulated (PTPN6, PIAS, SPRED2). In addition, some JAK-STAT pathway related genes are differentially expressed only in some stages of ontogeny (STATs, GRB2, CREBB). Beside the continuously upregulated (AKT1, PPP2CA, CHUK, NFKB1) and downregulated (FOXO1, PDPK1, PIK3CG) genes in the PI3K-AKT signaling pathway, we also observed intermittently regulated gene expression (NFKBIA, YWHAH).

Conclusions

This broad overview of gene expression in erythropoiesis revealed transcription factors differentially expressed in some stages of ontogenesis. Finally, our results show that EPO-mediated proliferation and survival of erythroid progenitors occurs mainly through modulation of JAK-STAT pathway associated STATs, GRB2 and PIK3 genes, as well as AKT pathway-coupled NFKBIA and YWHAH genes.

A monopartite sequence is essential for p45 NF-E2 nuclear translocation, transcriptional activity and platelet production

BACKGROUND

p45 NF-E2 is a bZIP transcription factor crucial for thrombopoiesis, as indicated by the fact that loss of p45 NF-E2 function results in dramatic embryonic lethal thrombopoietic defects and its overexpression boosts platelet release.

OBJECTIVES

In the present study, we set out to identify the sequences responsible for p45 NF-E2 nuclear import, evaluate its transport mechanism and ascertain its functional significance.

METHODS

A series of p45 NF-E2 deletion constructs fused to green fluorescent protein (GFP) was created and their cellular localization examined in mammalian cells, with the factor responsible for nuclear import identified using an in vitro transport assay. A p45 NF-E2 derivative mutated in the nuclear targeting sequence (NLS) was generated and its biological activity compared with wild type (wt) in luciferase assays, and proplatelet and platelet production measured in murine megakaryocytes transduced with a retroviral vector.

RESULTS

Here we show that residues 271-273 are essential for nuclear import of p45 NF-E2 in COS-7 and in primary bone marrow cells. The p45 NF-E2 NLS facilitates nuclear import specifically via importin (IMP) 7. Although within the DNA-binding domain of p45 NF-E2, the NLS is not essential for DNA-binding, but is crucial for transcriptional activation and biological activity; where, in contrast to wt, a mutant derivative with a mutated NLS failed to promote proplatelet and platelet production in murine megakaryocytes.

CONCLUSIONS

The NLS is critical for p45 NF-E2 function, with the present study being the first to demonstrate the importance of NLS-dependent nuclear import of p45 NF-E2 for platelet development.

JNK-mediated turnover and stabilization of the transcription factor p45/NF-E2 during differentiation of murine erythroleukemia cells

Regulation of the homeostatic concentrations of specific sets of transcription factors is essential for correct programming of cell proliferation and differentiation. We have characterized the signal transduction pathways regulating the catabolisis of p45/NF-E2, a bZIP factor activating the erythroid and megakaryocytic gene transcription. Through use of different approaches including nano-scale proteomics, we show that activated-JNK, or Phospho-JNK (P-JNK), physically interacts with p45/NF-E2 and phosphorylates its Ser157 residue. This reaction leads to the poly-ubiquitination of p45/NF-E2 at one or more of six Lys residues, one of which being also a sumoylation site, and its degradation through the proteasome pathway. Significantly, this regulatory pathway of p45/NF-E2 by P-JNK exists only in uninduced murine erythroleukemia (MEL) cells but not in differentiated MEL cells in which JNK is inactivated on DMSO induction. Based on the above data and analysis of the chromatin-binding kinetics of p45/NF-E2 and the erythroid gene repressor Bach1 during the early phase of MEL differentiation, we suggest a model for the regulation of erythroid maturation. In the model, the posttranslational modifications and turnover of p45/NF-E2, as mediated by P-JNK, contribute to the control of its homeostatic concentration and consequently, its regulatory functions in the progression of erythroid differentiation and erythroid gene expression.

NF-E2 domination over Nrf2 promotes ROS accumulation and megakaryocytic maturation

In megakaryocytes, the maturation process and oxidative stress response appear to be closely related. It has been suggested that increased oxygen tension and reactive oxygen species (ROS) promote megakaryopoiesis and that the expression of stress-responsive genes responsible for ROS elimination declines during megakaryocytic maturation. NF-E2 p45 is an essential regulator of megakaryopoiesis, whereas Nrf2 is a key activator of stress-responsive genes. Because p45 and Nrf2 have similar DNA-binding specificities, we hypothesized that p45 competes with Nrf2 to repress stress-responsive genes and achieves favorable intracellular conditions to allow ROS to be efficiently used as signaling molecules. We conducted comprehensive gene expression profiling with wild-type and p45-null megakaryocytes and examined the functional relationship between p45 and Nrf2. We found that 2 characteristic gene clusters are defined within p45 target genes: platelet genes and cytoprotective genes. The former are unique targets activated by p45, whereas the latter are common targets of p45 and Nrf2. Further analysis suggested that, as a less efficacious activator, p45 maintains moderate expression of cytoprotective genes through competing with Nrf2 and promotes ROS accumulation. Increased ROS enhanced platelet gene expression. These results suggest that p45 dominates over Nrf2 to enhance megakaryocytic maturation by promoting ROS accumulation.

Transcriptional control of erythropoiesis: emerging mechanisms and principles

Transcriptional networks orchestrate fundamental biological processes, including hematopoiesis, in which hematopoietic stem cells progressively differentiate into specific progenitors cells, which in turn give rise to the diverse blood cell types. Whereas transcription factors recruit coregulators to chromatin, leading to targeted chromatin modification and recruitment of the transcriptional machinery, many questions remain unanswered regarding the underlying molecular mechanisms. Furthermore, how diverse cell type-specific transcription factors function cooperatively or antagonistically in distinct cellular contexts is poorly understood, especially since genes in higher eukaryotes commonly encompass broad chromosomal regions (100 kb and more) and are littered with dispersed regulatory sequences. In this article, we describe an important set of transcription factors and coregulators that control erythropoiesis and highlight emerging transcriptional mechanisms and principles. It is not our intent to comprehensively survey all factors implicated in the transcriptional control of erythropoiesis, but rather to underscore specific mechanisms, which have potential to be broadly relevant to transcriptional control in diverse systems.

Regulation of globin gene transcription by heme in erythroleukemia cells: analysis of putative heme regulatory motifs in the p45 NF-E2 transcription factor

The function of the NF-E2 transcription factor, a p45/small Maf heterodimer, was analyzed in the erythroleukemia cell lines MEL and CB3. In contrast to MEL cells, CB3 cells are null for p45 and thus express only extremely low levels of adult globin transcripts upon induction by agents promoting erythroid differentiation. We investigated the response of erythroleukemia cells to hemin treatment. Hemin rapidly induces beta-globin gene transcript levels in MEL cells, but not in CB3 cells. Stable expression of the large p45 NF-E2 subunit in CB3 cells restores hemin mediated beta-globin gene transcription, suggesting that the presence of a functional NF-E2 is required for strong induction of beta-globin mRNA levels by hemin in erythroleukemia cells. We performed mutagenesis of two potential heme-regulatory motifs (HRMs) in p45 NF-E2 and found that the mutated versions are expressed and can still recognize a NF-E2 DNA binding element. In addition, we showed that p45 NF-E2 HRM mutants are able to restore beta-globin gene transcription in CB3 cells upon induction by hemin. Our results suggest that globin gene activation by heme appears to be independent of the putative HRMs in the p45 subunit of the NF-E2 transcription factor.

Sumoylation of p45/NF-E2: nuclear positioning and transcriptional activation of the mammalian beta-like globin gene locus

NF-E2 is a transcription activator for the regulation of a number of erythroid- and megakaryocytic lineage-specific genes. Here we present evidence that the large subunit of mammalian NF-E2, p45, is sumoylated in vivo in human erythroid K562 cells and in mouse fetal liver. By in vitro sumoylation reaction and DNA transfection experiments, we show that the sumoylation occurs at lysine 368 (K368) of human p45/NF-E2. Furthermore, p45 sumoylation enhances the transactivation capability of NF-E2, and this is accompanied by an increase of the NF-E2 DNA binding affinity. More interestingly, we have found that in K562 cells, the beta-globin gene loci in the euchromatin regions are predominantly colocalized with the nuclear bodies promyelocytic leukemia protein (PML) oncogenic domains that are enriched with the PML, SUMO-1, RNA polymerase II, and sumoylatable p45/NF-E2. Chromatin immunoprecipitation assays further showed that the intact sumoylation site of p45/NF-E2 is required for its binding to the DNase I-hypersensitive sites of the beta-globin locus control region. Finally, we demonstrated by stable transfection assay that only the wild-type p45, but not its mutant form p45 (K368R), could efficiently rescue beta-globin gene expression in the p45-null, erythroid cell line CB3. These data together point to a model of mammalian beta-like globin gene activation by sumoylated p45/NF-E2 in erythroid cells.

Interleukin-1beta up-regulates the expression of thrombopoietin and transcription factors c-Jun, c-Fos, GATA-1, and NF-E2 in megakaryocytic cells

The multifunctional cytokine interleukin-1beta (IL-1beta) plays a central role in the body's immune and inflammatory responses. The mechanism of IL-1beta on thrombocytosis and megakaryocytopoiesis has remained controversial. In previous reports, we have demonstrated the expression of IL-1 receptors (IL-1RI and IL-1RII) and enhancing effects of IL-1beta on primary human megakaryocytic (MK) cells. In this study, we investigated the possible direct effects of IL-1beta on the expression of thrombopoietin (TPO) and transcription factors c-Jun, c-Fos, GATA-1, and p45 nuclear factor-E2 (NF-E2) in MK cell lines CHRF and Meg-01. Our results demonstrated that IL-1beta up-regulated messenger RNA (mRNA) and protein expressions of these transcription factors in a dose- and time-dependent manner. In CHRF cells, mRNA: c-Jun [3.4-fold, peaked at 15 minutes], c-Fos [4.2-fold, 15 minutes], GATA-1 [4.0-fold, 60 minutes], NF-E2 [3.2-fold, 120 minutes] and protein expression: c-Jun [3.0-fold, 30 minutes], c-Fos [1.7-fold, 30 minutes], GATA-1 [11.5-fold, 60 minutes], NF-E2 [12.5-fold, 120 minutes] were evidently enhanced after treatment with IL-1beta. The response to IL-1beta was consistent in the total cell and nuclear extracts and was significantly reduced by pretreatment with actinomycin D or cycloheximide. An IL-1-receptor antagonist (IL-1RA) inhibited the stimulatory effects of IL-1beta on these transcription factors by as much as 78%. TPO expression was increased by more than 9.9-fold on stimulation with IL-1beta. A TPO-neutralizing antibody did not significantly reduce the effects of IL-1beta. We conclude that IL-1beta up-regulates the expression of TPO, c-Jun, c-Fos, GATA-1, and NF-E2 in MK cells. The mechanism might be mediated by IL-1beta receptors and require transcription or protein synthesis. The direct involvement of IL-1beta in the MK lineage may provide an explanation for the phenomenon of thrombocytosis during inflammatory responses.

cAMP/PKA-mediated regulation of erythropoiesis

The role of cyclic AMP (cAMP) as second messenger in erythropoiesis has been suggested in the early 1980s. However, careful analysis showed that cAMP is not generated in direct response to the main erythropoiesis-controlling cytokines such as erythropoietin (Epo). As a result, cAMP disappeared from the central stage in research of erythropoiesis. Instead, other signal transduction pathways, including the Ras/extracellular regulated kinase (ERK)-pathway, the phosphatidylinositol 3-kinase (P13K) and the signal transducer and activator of transcription (STAT5)-pathways, have been found and explored. In concert, these signaling pathways control the transcriptional machinery of erythroid cells. Although cAMP is not directly generated in response to Epo stimulation, it has recently been demonstrated that increased cAMP-levels and in particular the cAMP-dependent protein kinase A (PKA) can modulate erythroid signal transduction pathways. In some cases, like the ERK-signaling pathway, PKA affects signal transduction by regulating the balance between specific phosphatases and kinases. In other cases, such as the STAT5 pathway, PKA enhances Epo signaling by inducing recruitment of additional co-regulators of transcription. In addition to STAT5, PKA also activates other transcription factors that are required for erythroid gene expression. This review discusses the impact of cAMP/PKA on Epo-mediated signaling pathways and summarizes the role of cAMP in malignant erythropoiesis.

Oxidized Phospholipids Induce Expression of Human Heme Oxygenase-1 Involving Activation of cAMP-responsive Element-binding Protein

Heme oxygenase-1 (HO-1) catalyzes the rate-limiting step in heme degradation, protects against oxidative stress, and shows potent anti-inflammatory effects. Oxidized phospholipids, which are generated during inflammation and apoptosis, modulate the inflammatory response by inducing the expression of several genes including HO-1. Here we investigated the signaling pathways and transcriptional events involved in the induction of HO-1 gene expression by oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (OxPAPC) in human umbilical vein endothelial cells. OxPAPC up-regulated HO-1 mRNA and protein in a time- and concentration-dependent manner, whereas pro-inflammatory agents like TNF-alpha and lipopolysaccharide did not significantly induce HO-1 expression in human umbilical vein endothelial cells. Signaling pathways involved in the OxPAPC-mediated HO-1 induction included protein kinases A and C, as well as the mitogen-activated protein kinases p38 and ERK. The cAMP-responsive element-binding protein (CREB) was phosphorylated via these pathways in response to OxPAPC treatment and expression of a dominant-negative mutant of CREB inhibited OxPAPC-induced activity of a human heme oxygenase-1 promoter-driven luciferase reporter construct. We identified a cAMP-responsive element and a Maf recognition element to be involved in the transcriptional activation of the HO-1 promoter by OxPAPC. In gel shift assays we observed binding of CREB to the cAMP-responsive element after OxPAPC treatment. Induction of HO-1 expression by lipid oxidation products via CREB may represent a feedback mechanism to limit inflammation and associated tissue damage.

Cellular localisation and nuclear export of the human bZIP transcription factor TCF11

TCF11 is a ubiquitous transcription factor of the CNC-bZIP family. The activity of this vital protein is strictly regulated and we have previously published that the two major translated protein forms show a clearly different transactivation ability in transient transfections. Only the full-length form is active in a variety of mammalian cells [J. Biol. Chem. 276 (2001) 17641]. Here we further investigate the complex regulation of TCF11, studying the cellular localisation of some of the different protein isoforms. The full-length form is located both in the cytoplasm and the nucleus, while the internally initiated shorter protein form is restricted to nuclear localisation. A nuclear export signal (NES) localised in the N-terminus of TCF11 is responsible for the active nuclear export of the protein. This export is highly sensitive to leptomycin B (LMB) and is largely blocked by mutating three of the leucine residues in the signal region. These results indicate that export occurs through the Crm1-mediated pathway. Due to alternative splicing within the tcf11 gene, different isoforms of the longer protein form are produced. Some of these isoforms, one identical to Nrf1, lack the NES and are thereby restricted to nuclear localisation.

DYRK3 activation, engagement of protein kinase A/cAMP response element-binding protein, and modulation of progenitor cell survival

DYRKs are a new family of dual-specificity tyrosine-regulated kinases with emerging roles in cell growth and development. Recently, we discovered that DYRK3 is expressed primarily in erythroid progenitor cells and modulates late erythropoiesis. We now describe 1) roles for the DYRK3 YTY signature motif in kinase activation, 2) the coupling of DYRK3 to cAMP response element (CRE)-binding protein (CREB), and 3) effects of DYRK3 on hematopoietic progenitor cell survival. Regarding the DYRK3 kinase domain, intactness of Tyr(333) (but not Tyr(331)) within subdomain loop VII-VIII was critical for activation. Tyr(331) plus Tyr(333) acidification (Tyr mutated to Glu) was constitutively activating, but kinase activity was not affected substantially by unique N- or C-terminal domains. In transfected 293 and HeLa cells, DYRK3 was discovered to efficiently stimulate CRE-luciferase expression, to activate a CREB-Gal4 fusion protein, and to promote CREB phosphorylation at Ser(133). Interestingly, this CREB/CRE response was also supported (50% of wild-type activity) by a kinase-inactive DYRK3 mutant as well as a DYRK3 C-terminal region and was blocked by protein kinase A inhibitors, suggesting functional interactions between protein kinase A and DYRK3. Finally, DYRK3 expression in cytokine-dependent hematopoietic FDCW2 cells was observed to inhibit programmed cell death. Thus, primary new insight into DYRK3 kinase signaling routes, subdomain activities, and possible biofunctions is provided.

Chromatin structure and control of beta-like globin gene switching

The human beta-globin locus is a complex genetic system widely used for analysis of eukaryotic gene expression. The locus consists of five functional beta-like globin genes, epsilon, (G)gamma, (A)gamma, delta, and beta, arrayed on the chromosome in the order that they are expressed during ontogeny. Globin gene expression is regulated, in part, by the locus control region, which physically consists of five DNaseI-hypersensitive sites located 6-22 Kb upstream of the epsilon -globin gene. During ontogeny two switches occur in beta-globin gene expression that reflect the changing oxygen requirements of the fetus. The first switch from embryonic epsilon - to fetal gamma-globin occurs at six weeks of gestation. The second switch from gamma- to adult delta- and beta-globin occurs shortly after birth. Throughout the locus, cis-acting elements exist that are dynamically bound by trans-acting proteins, including transcription factors, co-activators, repressors, and chromatin modifiers. Discovery of novel erythroid-specific transcription factors and a role for chromatin structure in gene expression have enhanced our understanding of the mechanism of globin gene switching. However, the hierarchy of events regulating gene expression during development, from extracellular signaling to transcriptional activation or repression, is complex. In this review we attempt to unify the current knowledge regarding the interplay of cis-acting elements, transcription factors, and chromatin modifiers into a comprehensive overview of globin gene switching.

Nuclear relocation of a transactivator subunit precedes target gene activation

Murine erythroleukemia (MEL) cells are a model system to study reorganization of the eukaryotic nucleus during terminal differentiation. Upon chemical induction, MEL cells undergo erythroid differentiation, leading to activation of globin gene expression. Both processes strongly depend on the transcriptional activator NF-E2. Before induction of differentiation, both subunits of the NF-E2 heterodimer are present, but little DNA-binding activity is detectable. Using immunofluorescence microscopy, we show that the two NF-E2 subunits occupy distinct nuclear compartments in uninduced MEL cells; the smaller subunit NF-E2p18 is found primarily in the centromeric heterochromatin compartment, whereas the larger subunit NF-E2p45 occupies the euchromatin compartment. Concomitant with the commitment period of differentiation that precedes globin gene activation, NF-E2p18, along with other transcriptional repressors, relocates to the euchromatin compartment. Thus, relocation of NF-E2 p18 may be a rate-limiting step in formation of an active NF-E2 complex. To understand the mechanisms of NF-E2 localization, we show that centromeric targeting of NF-E2p18 requires dimerization, but not with an erythroid-specific partner, and that the transactivation domain of NF-E2p45 may be necessary and sufficient to prevent its localization in centromeric heterochromatin. Finally, using fluorescence in situ hybridization, we show that, upon differentiation, the beta-globin gene loci relocate away from heterochromatin compartments to euchromatin. This relocation correlates with both transcriptional activation of the globin locus and relocation of NF-E2p18 away from heterochromatin, suggesting that these processes are linked.

Transcriptional regulation of the murine erythroid-specific 5-aminolevulinate synthase gene

5-Aminolevulinate synthase (ALAS) catalyzes the first step of the heme biosynthetic pathway in mammalian cells. Separate genes encode the two isoforms: ubiquitously expressed ALAS (ALAS1) and erythroid-specific ALAS (ALAS2). Transcription of the ALAS2 gene is only activated during erythroid cell differentiation. This stimulation allows for the formation of hemoglobin-specific heme. The 5'-flanking region of the mouse ALAS2 gene was studied in order to define its erythroid-specific function in transcriptional activation. Putative binding sites for the erythroid-specific nuclear factors GATA-1, NF-E2, and EKLF were identified within the first 300bp region of the mouse ALAS2 5'-flanking region. However, this 300bp region alone did not efficiently activate transient expression in erythroid MEL and K562 cell lines. Additional DNA regulatory sequences found within 300-718bp upstream of the transcription start site were required for maximal transcriptional activation, even though these regions stimulated similar expression in the non-erythroid HeLa and NIH/3T3 cells. This suggests that cis-acting elements present in the 5'-flanking region are not responsible for maintenance of transcriptional silencing in non-erythroid cell lines and that tissue-specific regulation of ALAS2 depends on other regions of the gene or on chromatin remodeling. A putative hypoxia inducible factor 1 (HIF-1) response element was identified within the 300-718bp upstream region. Significantly, two proximal GATA-1-binding sites (-118/-113 and -98/-93) and a region located within -518 to -315bp of the mouse ALAS2 promoter were essential for transcriptional activation during chemically induced differentiation of MEL cells, implying their importance in conferring erythroid specificity to the ALAS2 transcriptional activation. This is the first study to delimit the cis-acting region responsible for activation of the ALAS2 promoter upon dimethyl-sulfoxide induction in MEL cells.

Requirement of GATA-1 and p45 NF-E2 expression in butyric acid-induced erythroid differentiation

Butyric acid (BA) is known to induce overexpression of fetal hemoglobin and then erythroid differentiation. Therefore, BA is currently under clinical investigation as a potential therapy for the treatment of sickle cell disease and cancer. Nevertheless, the molecular mechanisms involved in BA-induced differentiation remain largely unknown. Previous reports have shown that BA-induced overexpression of erythroid genes occurred at the transcriptional level, suggesting the involvement of erythroid transcription factors. Here, we intend to demonstrate the requirement of GATA-1 and NF-E2 transcription factors in the BA-induced erythroid differentiation of human leukemic K562 cells. Time-course experiments showed that nuclear levels of GATA-1 and p45 NF-E2 proteins increased during BA treatment. Moreover, antisense oligodeoxynucleotides targeting either GATA-1 or p45 NF-E2 proteins inhibited both protein expression and BA-induced differentiation. In contrast, BA-induced cell growth inhibition was not affected. These results provide the first direct evidence for the requirement of GATA-1 and NF-E2 in BA-induced differentiation process.