A novel, erythroid cell-specific murine transcription factor that binds to the CACCC element and is related to the Krüppel family of nuclear proteins

Ethanol-responsive genes: identification of transcription factors and their role in metabolomics

Transcription factors (TFs) and their combinatorial control on cis-regulatory elements play critical role in the co-expression of genes. This affects the interaction of genes in the transcriptome and thus may affect signals that cascade through cellular pathways. Using a combination of bioinformatic approaches, we sought to identify such common combinations of TFs in a set of ethanol-responsive (ER) genes and assess the role of ethanol in affecting multiple pathways through their co-regulation. Our results show that the metallothionein genes are regulated by TF motifs cAMP responsive element binding protein (CREB) and metal-activated transcription factor 1 and primarily involved in zinc ion homeostasis. We have also identified new target genes, Synaptojanin 1 and tryptophan hydroxylase 1, potentially regulated by this module. Altered arrangement of TF-binding sites in the module may direct the action of these and other target genes in intracellular signaling cascades, cell growth and/or maintenance. In addition to CREB, other key TFs identified are ecotropic viral integration site-1 and SP1. These modulate the contribution of the target ER genes in cell cycle regulation and apoptosis or programmed cell death. Multiple lines of evidence confirm the above findings and indicate that different groups of ER genes are involved in different biological processes and their co-regulation most likely results from different sets of regulatory modules. These findings associate the role of the ER genes studied and their potential TF modules with alcohol response pathways and phenotypes.

Characterization of the heme synthesis enzyme coproporphyrinogen oxidase (CPO) in zebrafish erythrogenesis

Hemoglobin consists of heme and globin proteins and is essential for oxygen transport in all vertebrates. Although biochemical features of heme synthesis enzymes have been well characterized, the function of these enzymes in early embryogenesis is not fully understood. We found that the sixth heme synthesis enzyme, coproporphyrinogen oxidase (CPO), is predominantly expressed in the intermediate cell mass (ICM) that is a major site of zebrafish primitive hematopoiesis. Knockdown of zebrafish CPO using anti-sense morpholinos (CPO-MO) leads to a significant suppression of hemoglobin production without apparent reduction of blood cells. Injection of human CPO RNA, but not a mutant CPO RNA that is similar to a mutant responsible for a hereditary coproporphyria (HCP), restores hemoglobin production in the CPO-MO-injected embryos. Furthermore, expression of CPO in the ICM is severely suppressed in both vlad tepes/gata1 mutants and in biklf-MO-injected embryos. In contrast, over-expression of biklf and gata1 significantly induces ectopic CPO expression. The function of CPO in heme biosynthesis is apparently conserved between zebrafish and human, suggesting that CPO-MO-injected zebrafish embryos might be a useful in vivo assay system to measure the biological activity of human CPO mutations.

Thalassaemia-like carriers not linked to the beta-globin gene cluster

This study describes the largest series reported to date, of individuals belonging to unrelated families carrying a beta-thalassaemia-like phenotype in whom the beta-globin gene was found to be structurally intact by sequence analysis. This genetic determinant appears haematologically heterogeneous, displaying either a silent beta-thalassaemia-like phenotype or a typical beta-thalassaemia carrier-like phenotype in different families. Compound heterozygosity for both beta-thalassaemia-like determinant and typical beta-thalassaemia allele resulted either in thalassaemia intermedia or thalassaemia major. By linkage analysis both the silent and the typical beta-like determinants were found not to be linked to the beta-globin cluster. Sequence analysis of the hypersensitive site cores of locus control region and of the genes coding for the transcription factors erythroid Kruppel-like factor and nuclear factor (erythroid-derived 2) were normal. beta-globin mRNA levels determined by real-time polymerase chain reaction were reduced in both types of beta-like carriers. These results indicate the existence of causative genetic determinants not yet molecularly defined, but most likely, resulting from either the reduction or loss of function of a gene coding for unknown transcriptional regulator(s) of the beta-globin gene. The knowledge of these rare beta-thalassaemia-like determinants have implications for clinical and, especially, prenatal diagnosis of beta-thalassaemia.

EKLF/KLF1 is ubiquitinated in vivo and its stability is regulated by activation domain sequences through the 26S proteasome

Erythroid Krüppel-like factor (EKLF/KLF1) is an erythroid specific, C(2)H(2) zinc finger transcription factor that is essential for the proper chromatin structure and expression of the adult beta-globin gene. Herein, we determine that 26S proteasome inhibitors lead to an accumulation of EKLF protein in murine erythroleukemia (MEL) cells. In addition, EKLF half-life in both MEL cells (<3h) and fetal liver cells (between 6 and 9h) is stabilized in the presence of these inhibitors. EKLF is ubiquitinated in vivo, however its modification does not rely on a particular internal lysine. Finally, EKLF contains two PEST sequences within its N-terminus that have no effect on the ability of EKLF to be ubiquitinated but contribute to its destabilization.

Shear stress regulation of Krüppel-like factor 2 expression is flow pattern-specific

Flow patterns in blood vessels contribute to focal distribution of atherosclerosis; the underlying mechanotransduction pathways remain to be investigated. We demonstrate that different flow patterns elicit distinct responses of Krüppel-like factor-2 (KLF2) in endothelial cells (ECs) in vitro and in vivo. While pulsatile flow with a significant forward direction induced sustained expression of KLF2 in cultured ECs, oscillatory flow with little forward direction caused prolonged suppression after a transient induction. The suppressive effect of oscillatory flow was Src-dependent. Immunohistochemical studies on ECs at arterial branch points revealed that KLF2 protein levels were related to local hemodynamics. Such flow-associated expression patterns were also demonstrated in a rat aortic restenosis model. Inhibition of KLF2 with siRNA sensitized ECs to oxidized LDL-induced apoptosis, indicating a protective role of KLF2. In conclusion, differential regulation of KLF2 may mediate the distinct vascular effects induced by various patterns of shear stress.

Human KLF17 is a new member of the Sp/KLF family of transcription factors

The Sp/KLF transcription factors perform a variety of biological functions, but are related in that they bind GC-box and CACCC-box sequences in DNA via a highly conserved DNA-binding domain. A database homology search, using the zinc finger DNA-binding domain characteristic of the family, has identified human KLF17 as a new family member that is most closely related to KLFs 1-8 and 12. KLF17 appears to be the human orthologue of the previously reported mouse gene, zinc finger protein 393 (Zfp393), although it has diverged significantly. The DNA-binding domain is the most conserved region, suggesting that both the murine and the human forms recognize the same binding sites in DNA and may retain similar functions. We show that human KLF17 can bind G/C-rich sites via its zinc fingers and is able to activate transcription from CACCC-box elements. This is the first report of the DNA-binding characteristics and transactivation activity of human KLF17, which, together with the homology it displays to other KLF proteins, put it in the Sp/KLF family.

A global role for EKLF in definitive and primitive erythropoiesis

Erythroid Kruppel-like factor (EKLF, KLF1) plays an important role in definitive erythropoiesis and beta-globin gene regulation but failure to rectify lethal fetal anemia upon correction of globin chain imbalance suggested additional critical EKLF target genes. We employed expression profiling of EKLF-null fetal liver and EKLF-null erythroid cell lines containing an inducible EKLF-estrogen receptor (EKLF-ER) fusion construct to search for such targets. An overlapping list of EKLF-regulated genes from the 2 systems included alpha-hemoglobin stabilizing protein (AHSP), cytoskeletal proteins, hemesynthesis enzymes, transcription factors, and blood group antigens. One EKLF target gene, dematin, which encodes an erythrocyte cytoskeletal protein (band 4.9), contains several phylogenetically conserved consensus CACC motifs predicted to bind EKLF. Chromatin immunoprecipitation demonstrated in vivo EKLF occupancy at these sites and promoter reporter assays showed that EKLF activates gene transcription through these DNA elements. Furthermore, investigation of EKLF target genes in the yolk sac led to the discovery of unexpected additional defects in the embryonic red cell membrane and cytoskeleton. In short, EKLF regulates global erythroid gene expression that is critical for the development of primitive and definitive red cells.

Developmental regulation of yolk sac hematopoiesis by Kruppel-like factor 6

Krüppel-like factor 6 (KLF6) is a member of a growing family of transcription factors that share a common 3 C2H2 zinc finger DNA binding domain and have broad activity in regulating proliferation and development. We have previously established that Klf6 is expressed in neuronal tissue, hindgut, heart, lung, kidney, and limb buds during midgestation. To explore the potential role of Klf6 in mouse development, we analyzed Klf6-/- mice and found that the homozygous mutation is embryonic lethal by embryonic day (E) 12.5 and associated with markedly reduced hematopoiesis and poorly organized yolk sac vascularization. Additionally, mRNA levels of Scl and Gata1 were reduced by approximately 80% in Klf6-/- yolk sacs. To further analyze this phenotype, we generated Klf6-/- embryonic stem (ES) cells by homologous recombination, and compared their capacity to differentiate into the hematopoietic lineage with that of either Klf6+/- or Klf6+/+ ES cells. Consistent with the phenotype in the early embryo, Klf6-/- ES cells displayed significant hematopoietic defects following differentiation into EBs. Prolongation of epiblast-like cells and delays in mesoderm induction were also observed in the Klf6-/- EBs, associated with delayed expression of Brachyury, Klf1, and Gata1. Forced expression of KLF6 using a tet-inducible system enhanced the hematopoietic potential of wild-type EBs. Collectively, these findings implicate Klf6 in ES-cell differentiation and hematopoiesis.

Isolation of TPO-dependent subclones from the multipotent 32D cell line

Using thrombopoietin (TPO), as selective pressure, several TPO-dependent clones were isolated from the murine multipotential IL-3-dependent cell line 32D. Four of them were fully characterized. They depended on TPO for survival and proliferation and, although retaining the capacity to grow in IL-3, did not respond to either EPO, G-CSF or GM-CSF. 32D TPO cells were heterogeneous in morphology and ranged from small cells, with a DNA content nearly tetraploid and a modal chromosome no. 66, to cells 50-75 microm in diameter containing multiple (up to 5-6) interconnected nuclei with a clear megakaryocyte (Mk) morphology by electron microscopy. Cell sorter isolation and single cell cloning experiments indicated that the small cells were those capable to proliferate in TPO and to generate the larger ones over time. 32D TPO cells expressed Mk-specific markers by FACS (CD41, CD61 and 2D5) and RT-PCR (acetyl cholinesterase E and platelet factor 4) and their unique profile, by gene array analysis, included expression of urokinase plasminogen activator surface receptor (CD87 or uPAR), plasminogen activator inhibitor and coagulation factor II (thrombin) receptor (Cf2r). In addition, by quantitative RT-PCR, 32D TPO clones expressed levels of Gata1 similar to those expressed by freshly isolated Mks (DeltaCt approximately 4.7 in both cases). In conclusion, the 32D TPO subclones described here are among the few pure Mk cell lines isolated so far and, for their unique properties, may prove themselves as a useful model to study Mk differentiation.

A functional screen for Krüppel-like factors that regulate the human γ-globin gene through the CACCC promoter element

Krüppel-like factors (KLFs) have been systematically screened as potential candidates to regulate human gamma-globin gene expression through its CACCC element. Initially, 21 human proteins that have close sequence similarity to EKLF/KLF1, a known regulator of the human beta-globin gene, were identified. The phylogenetic relationship of these 22 KLF/Sp1 proteins was determined. KLF2/LKLF, KLF3/BKLF, KLF4/GKLF, KLF5/IKLF, KLF8/BKLF3, KLF11/FKLF, KLF12/AP-2rep and KLF13/FKLF2 were chosen for functional screening. Semi-quantitative RT-PCR demonstrated that all eight of these candidates are present in human erythroid cell lines, and that the expression of the KLF2, 4, 5 and 12 mRNAs changed significantly upon erythroid differentiation. Each of the eight KLF mRNAs is expressed in mouse erythroid tissues, throughout development. UV cross-linking assays suggest that multiple erythroid proteins from human cell lines and chicken primary cells interact with the gamma-globin CACCC element. In co-transfection assays in K562 cells, it was demonstrated that KLF2, 5 and 13 positively regulate, and KLF8 negatively regulates, the gamma-globin gene through the CACCC promoter element. The data collectively suggest that multiple KLFs may participate in the regulation of gamma-globin gene expression and that KLF2, 5, 8 and 13 are prime candidates for further study.

Regulation of human fetal hemoglobin: new players, new complexities

The human globin genes are among the most extensively characterized in the human genome, yet the details of the molecular events regulating normal human hemoglobin switching and the potential reactivation of fetal hemoglobin in adult hematopoietic cells remain elusive. Recent discoveries demonstrate physical interactions between the beta locus control region and the downstream structural gamma- and beta-globin genes, and with transcription factors and chromatin remodeling complexes. These interactions all play roles in globin gene expression and globin switching at the human beta-globin locus. If the molecular events in hemoglobin switching were better understood and fetal hemoglobin could be more fully reactivated in adult cells, the insights obtained might lead to new approaches to the therapy of sickle cell disease and beta thalassemia by identifying specific new targets for molecular therapies.

Mammalian SP/KLF transcription factors: bring in the family

The advent of the genome projects has provided new avenues to explore the question of how DNA sequence information is used appropriately by mammalian cells. Regulation of transcription is not the only, but is certainly a very important, mechanism involved in this process. We can now identify all the genes encoding transcription factors belonging to a certain class and study their biological functions in unprecedented detail through the use of an array of biomolecular tools. It is important to use rigorous and uniform definitions for the classification of transcription factors, because this helps us to comprehend the functions of transcription factor families in biological networks. Here, we propose an unambiguous nomenclature for the members of the Specificity Protein/Krüppel-like Factor (SP/KLF) transcription factor family.

Distal enhancer of the mouseFGF-4 gene and its human counterpart exhibit differential activity: Critical role of a GT box

Previous studies have shown that there is a strict requirement for fibroblast growth factor-4 (FGF-4) during mammalian embryogenesis, and that FGF-4 expression in embryonic stem (ES) cells and embryonal carcinoma (EC) cells are controlled by a powerful downstream distal enhancer. More recently, mouse ES cells were shown to express significantly more FGF-4 mRNA than human ES cells. In the work reported here, we demonstrate that mouse EC cells also express far more FGF-4 mRNA than human EC cells. Using a panel of FGF-4 promoter/reporter gene constructs, we demonstrate that the enhancer of the mouse FGF-4 gene is approximately tenfold more active than its human counterpart. Moreover, we demonstrate that the critical difference between the mouse and the human FGF-4 enhancer is a 4 bp difference in the sequence of an essential GT box. Importantly, we demonstrate that changing 4 bp in the human enhancer to match the sequence of the mouse GT box elevates the activity of the human FGF-4 enhancer to the same level as that of the mouse enhancer. We extended these studies by examining the roles of Sp1 and Sp3 in FGF-4 expression. Although we demonstrate that Sp3, but not Sp1, can activate the FGF-4 promoter when artificially tethered to the FGF-4 enhancer, we show that Sp3 is not essential for expression of FGF-4 mRNA in mouse ES cells. Finally, our studies with human EC cells suggest that the factor responsible for mediating the effect of the mouse GT box is unlikely to be Sp1 or Sp3, and this factor is either not expressed in human EC cells or it is not sufficiently active in these cells.

KLF2 is essential for primitive erythropoiesis and regulates the human and murine embryonic beta-like globin genes in vivo

The Krüppel-like factors (KLFs) are a family of C2/H2 zinc finger DNA-binding proteins that are important in controlling developmental programs. Erythroid Krüppel-like factor (EKLF or KLF1) positively regulates the beta-globin gene in definitive erythroid cells. KLF2 (LKLF) is closely related to EKLF and is expressed in erythroid cells. KLF2-/- mice die between embryonic day 12.5 (E12.5) and E14.5, because of severe intraembryonic hemorrhaging. They also display growth retardation and anemia. We investigated the expression of the beta-like globin genes in KLF2 knockout mice. Our results show that KLF2-/- mice have a significant reduction of murine embryonic Ey- and beta h1-globin but not zeta-globin gene expression in the E10.5 yolk sac, compared with wild-type mice. The expression of the adult beta(maj)- and beta(min)-globin genes is unaffected in the fetal livers of E12.5 embryos. In mice carrying the entire human globin locus, KLF2 also regulates the expression of the human embryonic epsilon-globin gene but not the adult beta-globin gene, suggesting that this developmental-stage-specific role is evolutionarily conserved. KLF2 also plays a role in the maturation and/or stability of erythroid cells in the yolk sac. KLF2-/- embryos have a significantly increased number of primitive erythroid cells undergoing apoptotic cell death.

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.

β-Minor globin gene expression is preferentially reduced in EKLF Knock-Out mice

The CACCC box is duplicated in the beta-globin gene promoter of humans and other mammals. While the function of the proximal element as a binding site for EKLF has already been well established, the role of the distal element remains unclear. Mice present two adult beta-globin genes, beta-major and beta-minor, bearing a single CACCC box, the consensus sequence of which is identical to that of the proximal or distal human element, respectively. In the present study we analyzed the mRNA expression of beta-minor and beta-major in EKLF Knock-Out (KO) mice in comparison to wild-type (wt) littermates. The murine early fetal liver up to day 13/14 post coitum (pc) expresses mainly beta-minor globin chains. Nevertheless, expression of the beta-minor globin gene in EKLF KO mice has not been assessed to date. We provide evidence that expression of the beta-minor globin gene is dependent upon EKLF and is more affected by EKLF deprivation than the beta-major gene. The results obtained support a general role of EKLF in beta-globin gene activation and are in agreement with models involving an advantage of the LCR proximal respect to distal gene.

C/EBPδ and C/EBPγ bind the CCAAT-box in the human β-globin promoter and modulate the activity of the CACC-box binding protein, EKLF

Developmental- and tissue-specific expression of globin genes is mediated by a few key elements within the proximal promoter of each gene. DNA-binding assays previously identified NF-Y, GATA-1, C/EBPbeta and C/EBPgamma as candidate regulators of beta-globin transcription via the CCAAT-box, a promoter element situated between CACC- and TATA-boxes. We have identified C/EBPdelta as an additional beta-globin CCAAT-box binding protein. In reporter assays, we show that C/EBPdelta can co-operate with EKLF, a CACC-box binding protein, to activate the beta-globin promoter, whereas C/EBPgamma inhibits the transcriptional activity of EKLF in this assay.

Vascular implications of the Krüppel-like family of transcription factors

The Krüppel-like factor (KLF) family is a recently highlighted group of zinc finger transcription factors given their important biological roles which include the vasculature. KLF2, KLF4, KLF5, and KLF6 are notable factors that have been implicated in developmental as well as pathological vascular processes. In this brief review, we provide an up-to-date summary of the physiological functions and cellular effects as well as transcriptional regulatory mechanisms of the vascular KLFs. Through such, we aim to provide a working view for understanding the pathological actions of KLFs in the vasculature.

Identification, Characterization, and Functional Analysis of Sp1 Transcript Variants Expressed in Germ Cells During Mouse Spermatogenesis1

The SP family of zinc-finger transcription factors are important mediators of selective gene activation during embryonic development and cellular differentiation. SP-binding GC-box domains are common cis-regulatory elements present in the promoters of several genes expressed in a developmentally specific manner in differentiating mouse germ cells. Four Sp1 cDNAs were isolated from a mouse pachytene spermatocyte cDNA library and characterized by DNA sequence analysis. Northern blot studies revealed that these cDNAs corresponded to 3 full-length Sp1 transcripts (4.1, 3.7, and 3.2 kilobases [kb]) and an additional 1.4-kb 5'-truncated Sp1 transcript that are temporally expressed during spermatogenesis. Quantitative real-time polymerase chain reaction studies verified that the highest levels of Sp1 transcript expression of 4.1, 3.7, and 3.2 kb occur in the primary spermatocytes. The spatial and temporal expression patterns of these Sp1 transcripts and their encoded 60-kDa and 90-kDa SP1 proteins were demonstrated using in situ hybridization and immunohistochemical analyses. To assess the transcriptional properties of these SP1 transcription factors, SP-deficient Drosophila SL2 cells were stably transfected with the respective Sp1 cDNA expression vectors and cotransfected with either Ldh2, Ldh3, or Creb promoter/luciferase reporter constructs. The levels of SP-mediated luciferase expression observed depended on the structure of the glutamine-rich transactivation domains and the number of GC-box elements present in the respective promoters. The alterations observed in germ cells in the patterns of expression of the Sp1 transcripts encoding the 60-kDa and 90-kDa SP1 isoforms suggest that these SP1 factors may be involved in mediating stage-specific and cell type-specific gene expression during mouse spermatogenesis.

Lyn deficiency reduces GATA-1, EKLF and STAT5, and induces extramedullary stress erythropoiesis

In vitro studies have implicated the Lyn tyrosine kinase in erythropoietin signaling. In this study, we show that J2E erythroid cells lacking Lyn have impaired signaling and reduced levels of transcription factors STAT5a, EKLF and GATA-1. Since mice lacking STAT5, EKLF or GATA-1 have red cell abnormalities, this study also examined the erythroid compartment of Lyn(-/-) mice. Significantly, STAT5, EKLF and GATA-1 levels were appreciably lower in Lyn(-/-) erythroblasts, and the phenotype of Lyn(-/-) animals was remarkably similar to GATA-1(low) animals. Although young adult Lyn-deficient mice had normal hematocrits, older mice developed anemia. Grossly enlarged erythroblasts and florid erythrophagocytosis were detected in the bone marrow of mice lacking Lyn. Markedly elevated erythroid progenitors and precursor levels were observed in the spleens, but not bone marrow, of Lyn(-/-) animals indicating that extramedullary erythropoiesis was occurring. These data indicate that Lyn(-/-) mice display extramedullary stress erythropoiesis to compensate for intrinsic and extrinsic erythroid defects.

An emerging role for Krüppel-like factors in vascular biology

The Krüppel-like family of transcription factors play diverse roles regulating cellular differentiation and tissue development. Accumulating evidence supports an important role for these factors in vascular biology. This review examines the current knowledge of this gene family's role in key cell types that critically regulate vessel biology under physiologic and pathologic states.

An Erythroid-Specific Chromatin Opening Element Increases β-Globin Gene Expression from Integrated Retroviral Gene Transfer Vectors

Gene therapy strategies requiring long-term high-level expression from integrated genes are currently limited by inconsistent levels of expression. This may be observed as variegated, silenced or position-dependent gene expression. Each of these phenomena involve suppressive chromatin structures. We hypothesized that by actively conferring an open chromatin structure on integrated vectors would increase transgene expression. To test this idea we used a 100bp element from the β-globin locus control region (LCR) which is able to independently open local chromatin structure in erythroid tissues. This element includes binding sites for GATA-1, NF-E2, EKLF and Sp-1 and is evolutionarily conserved. We constructed a series of MSCV-based vectors containing the β-globin gene driven by a minimal β-globin promoter with combinations of the HSFE and LCR derived enhancer elements. Pools of MEL clones containing integrated vectors were analyzed for chromatin structure and β-globin gene expression. The HSFE increased the extent of nuclease sensitive chromatin over the promoters of the constructs. The most effective vector included tandem copies of the HSFE and produced a 5-fold increase in expression compared to the promoter alone. These results indicate that the HSFE is able to augment the opening of β-globin promoter chromatin structure and significantly increase gene expression in the context of an integrated retroviral vector.

A novel silent beta-thalassemia mutation in the distal CACCC box affects the binding and responsiveness to EKLF

The silent beta-thalassemia mutation, beta(+)-101C-->T, is the only mutation currently described in the distal beta-globin CACCC box. We present a novel mutation, a C-->G transversion, in the same position. Expression analysis in heterozygous subjects demonstrated that the mutation determines a 20% reduction in the output of the beta-globin gene. DNA-protein interaction and transactivation analysis correlated the decrease in the beta-globin synthesis with the reduced binding and transactivation of EKLF to the mutant promoter. These data predict that the beta-101C-->G mutation will display a silent thalassemia phenotype similar to that of the beta-101C-->T mutation.

The distalβ-globin CACCC box is required for maximal stimulation of theβ-globin gene by EKLF

The transcription factor erythroid Kruppel-like factor (EKLF) specifically activates the beta-globin gene by interacting with the proximal beta-globin CACCC box, a known hot spot for thalassaemia mutations. This study investigated whether EKLF could also bind to, and activate from, the distal CACCC, which is a rare site of thalassaemia mutations. Using band shift and transient expression analysis with wild type, single and double CACCC mutants, we established that the distal CACCC box is weakly bound by EKLF, but, when mutated, significantly impairs EKLF-dependent beta-globin stimulation. Thus, EKLF requires both CACCC boxes to maximally stimulate the beta-globin gene.

Inhibition of β protein 1 expression enhances β-globin promoter activity and β-globin mRNA levels in the human erythroleukemia (K562) cell line

OBJECTIVE

In this paper, we report new observations related to the mechanism of the negative regulation of the important adult beta-globin gene in the erythroid cells at the embryonic-fetal stage of their development. We focused on the role of the silencer II region located upstream of the beta-globin gene, which along with its cognate binding protein BP1, negatively regulates beta-globin transcription.

MATERIALS AND METHODS

We prepared plasmid constructs containing the wild-type silencer II sequence, a mutated silencer II sequence, or a mutated control sequence in the beta-globin promoter 690-bp insert, which in turn was linked to an enhanced green fluorescent protein (EGFP) reporter gene. A human erythroleukemia cell line (K562) with embryonic-fetal phenotype was transfected with these EGFP constructs.

RESULTS

Flow cytometry and fluorescence digital imaging showed about threefold increase in the beta-globin promoter activity of the mutated silencer II construct. Introduction of a small interfering RNA (siRNA) complementary to BP1 into the cells caused a 75% decrease in BP1 expression and a simultaneous approximately 40% elevation of beta-globin promoter activity as well as an increase in beta-globin mRNA levels, as compared with controls. We detected no changes in the mRNA levels of positive regulators of hemoglobin transcription such as EKLF and GATA-1.

CONCLUSION

Our results support the involvement of BP1 in the mechanism of the negative regulation of beta-globin transcription. A better understanding of this mechanism may lay the groundwork for novel gene therapy approaches to inhibit the expression of abnormal structural variants of adult beta globin, such as sickle hemoglobin.

Site-specific acetylation of the fetal globin activator NF-E4 prevents its ubiquitination and regulates its interaction with the histone deacetylase, HDAC1

Acetylation provides one mechanism by which the functional diversity of individual transcription factors can be expanded. This is valuable in the setting of complex multigene loci that are regulated by a limited number of proteins, such as the human beta-globin locus. We have studied the role of acetylation in the regulation of the transcription factor NF-E4, a component of a protein complex that facilitates the preferential expression of the human gamma-globin genes in fetal erythroid cells. We have shown that NF-E4 interacts directly with, and serves as a substrate for, the acetyltransferase co-activator PCAF. Acetylation of NF-E4 is restricted to a single residue (Lys(43)) in the amino-terminal domain of the protein and results in two important functional consequences. Acetylation of NF-E4 prolongs the protein half-life by preventing ubiquitin-mediated degradation. This stabilization is PCAF-dependent, since enforced expression in fetal/erythroid cells of a mutant form of PCAF lacking the histone acetyltransferase domain (PCAFDeltaHAT) decreases NF-E4 stability. Acetylation of Lys(43) also reduces the interaction between NF-E4 and HDAC1, potentially maximizing the activating ability of the factor at the gamma-promoter. These results provide further demonstration that co-activators, such as PCAF, can influence individual transcription factor properties at multiple levels to alter their effects on gene expression.

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.

Enterocyte differentiation marker intestinal alkaline phosphatase is a target gene of the gut-enriched Kruppel-like factor

We have examined the role that the transcription factor gut-enriched Krüppel-like factor (KLF4 or GKLF) plays in activating the enterocyte differentiation marker gene intestinal alkaline phosphatase (IAP). A yeast one-hybrid screen was used to identify proteins interacting with a previously identified cis-element (IF-III) located within the human IAP gene promoter. DNA-protein interactions were determined by using EMSA. Northern blot analysis was used to study RNA expression in human colon cancer RKO cells engineered to overexpress KLF4. Transient transfections with IAP-luciferase reporter constructs were used to characterize the mechanisms by which KLF4 activates IAP transcription. The yeast one-hybrid screen and EMSA identified KLF4 as binding to IF-III. RKO cells induced to overexpress KLF4 demonstrated a corresponding dose-dependent increase in IAP expression, and EMSA with nuclear extract from these cells confirmed that KLF4 binds to the IF-III element. Transient transfections revealed that KLF4 transactivated the IAP gene largely via a critical segment in the IAP promoter that includes the IF-III cis-element. Mutant KLF4 constructs failed to fully activate IAP. We have identified the enterocyte differentiation marker IAP as a KLF4 target gene. IAP transactivation by KLF4 is likely mediated through a critical region located within the proximal IAP promoter region.

The Krüppel-like factor epiprofin is expressed by epithelium of developing teeth, hair follicles, and limb buds and promotes cell proliferation

We identified a cDNA clone for epiprofin, which is preferentially expressed in teeth, by differential hybridization using DNA microarrays from an embryonic day 19.5 mouse molar cDNA library. Sequence analysis revealed that this cDNA encodes a member of the Krüppel-like factor family containing three characteristic C2H2-type zinc finger motifs. The full-length cDNA was obtained by the 5' Cap capture method. Except for its 5'-terminal sequence, the epiprofin mRNA sequence is almost identical to the predicted sequence of Krüppel-like factor 14/Sp6 (specificity protein 6), which was previously identified in expressed sequence tag data bases and GenBank by an Sp1 zinc finger DNA-binding domain search (Scohy, S., Gabant, P., Van Reeth, T., Hertveldt, V., Dreze, P. L., Van Vooren, P., Riviere, M., Szpirer, J., and Szpirer, C. (2000) Genomics 70, 93-101). This sequence difference is due to differences in the assignment of the location of exon 1. In situ hybridization revealed that epiprofin mRNA is expressed by proliferating dental epithelium, differentiated odontoblast, and also hair follicle matrix epithelium. In addition, whole mount in situ hybridization showed transient expression of epiprofin mRNA in cells of the apical ectodermal ridge in developing limbs and the posterior neuropore. Transfection of an epiprofin expression vector revealed that this molecule is localized in the nucleus and promotes cell proliferation. Thus, epiprofin is a highly cell- and tissue-specific nuclear protein expressed primarily by proliferating epithelial cells of teeth, hair follicles, and limbs that may function in the development of these tissues by regulating cell growth.

Differential expression of Ran GTPase during HMBA-induced differentiation in murine erythroleukemia cells

Murine erythroleukemia (MEL) cells undergo erythroid differentiation in vitro when treated with hexamethylene bisacetamide (HMBA). To identify genes involved in the commitment of MEL cells to differentiate, we screened a cDNA library constructed from HMBA-induced cells by differential hybridization and isolated GTPase Ran as a down-regulated gene. We observed that Ran was expressed in a biphasic mode. Following a decrease in mRNA level during the initial hours of induction, Ran re-expressed at 24-48 h, and gradually declined again. To investigate the role of Ran during MEL differentiation we constructed MEL transfectants capable to express or block Ran mRNA production constitutively. No effects were observed on cell growth and proliferation. Blockage of Ran, however, interfered with MEL cell differentiation resulting in a decrease of cell survival in the committed population.

Genomic structure and 5'-flanking sequences of rat N-acetylglucosaminyltransferase I gene and regulatory role of its transcriptional diversity

It has been known that mouse, rat, and human N-acetylglucosaminyltransferase I (GnT-I) genes produce at least two transcripts, which differ in their 5'-untranslated region (5'-UTR) length, and the longer transcript is preferentially expressed in brains. However, the physiological meaning of this brain-specific expression pattern was unknown. We cloned the rat GnT-I gene and analyzed its structure. It consisted of five exons, and four of them coded only 5'-UTRs. A putative NF-kappaB binding site was found in the 5'-flanking sequence for the transcript that was previously shown to be induced by inflammation. The unusually long 5'-UTR of the major GnT-I transcript in rat brain was shown to inhibit protein production from the following coding sequence in COS7 cells. Comparison of the GnT-I protein/mRNA ratio in rat brain and liver showed that GnT-I mRNA in the brain was translated 3.8-times less efficiently than in the liver. Implications are discussed of these results in regulation of GnT-I expression in rat brain.

Erythroid gene suppression by NF-kappa B

NF-kappa B/Rel transcription factors play essential roles to mediate the immune response and apoptosis, and they have also been implicated in cellular differentiation such as erythropoiesis. To elucidate the possible role(s) of NF-kappa B in erythroid gene regulation and erythropoiesis, we have carried out transient transfection studies of the human embryonic/fetal erythroid cell line K562 and mouse adult erythroid MEL cells. It is shown that tumor necrosis factor-alpha represses the transcription activity directed by either alpha or zeta globin promoter in a dose-dependent manner. Furthermore, different NF-kappa B family members could effectively repress the transfected alpha-like globin promoters in K562 as well as in MEL cells. The involvement of NF-kappa B pathway is supported by the ability of a NF-kappa B-specific, dominant negative mutant to block the tumor necrosis factor-alpha or p65-mediated suppression of the alpha-like globin promoter activities. The suppression appears to be mediated through cis-linked HS-40 enhancer. Finally, stably transfected K562 cells overexpressing p65 contain reduced amounts of the p45/NF-E2 RNA and functional NF-E2 proteins. Our studies have identified a new set of targets of NF-kappa B. We suggest that the relatively high activity of the NF-kappa B pathway in early erythroid progenitors is involved in the suppression of erythroid-specific genes. Later in differentiation, together with other changes, the decline of the amounts of the NF-kappa B family of factors leads to derepression and consequent increase of NF-E2, which in turn would activate a subset of erythroid-specific genes.

Differential regulation of SOCS genes in normal and transformed erythroid cells

The SOCS family of genes are negative regulators of cytokine signalling with SOCS-1 displaying tumor suppressor activity. SOCS-1, CIS and SOCS-3 have been implicated in the regulation of red blood cell production. In this study, a detailed examination was conducted on the expression patterns of these three SOCS family members in normal erythroid progenitors and a panel of erythroleukemic cell lines. Unexpectedly, differences in SOCS gene expression were observed during maturation of normal red cell progenitors, viz changes to CIS were inversely related to the alterations of SOCS-1 and SOCS-3. Similarly, these SOCS genes were differentially expressed in transformed erythoid cells - erythroleukemic cells immortalized at an immature stage of differentiation expressed SOCS-1 and SOCS-3 mRNA constitutively, whereas in more mature cell lines SOCS-1 and CIS were induced only after exposure to erythropoietin (Epo). Significantly, when ectopic expression of the tyrosine kinase Lyn was used to promote differentiation of immature cell lines, constitutive expression of SOCS-1 and SOCS-3 was completely suppressed. Modulation of intracellular signalling via mutated Epo receptors in mature erythroleukemic lines also highlighted different responses by the three SOCS family members. Close scrutiny of SOCS-1 revealed that, despite large increases in mRNA levels, the activity of the promoter did not alter after erythropoietin stimulation; in addition, erythroid cells from SOCS-1-/- mice displayed increased sensitivity to Epo. These observations indicate complex, stage-specific regulation of SOCS genes during normal erythroid maturation and in erythroleukemic cells.

5-azacytidine reactivates the erythroid differentiation potential of the myeloid-restricted murine cell line 32D Ro

32D cells grown for 1 year in interleukin-3 (IL-3) and granulocyte colony-stimulating factor (G-CSF) generated the 32D Ro cell line which retained the parental mast cell phenotype but lost ability to generate erythroid cells in response to erythropoietin (EPO). In order to clarify the mechanisms underlying such restriction, we compared 32D and 32D Ro cells for their capacity to express erythroid-specific transcription factors (Gata1, Gata2, Scl, Nef2, Eklf, and Id) and the capacity of short exposure to 5-azacytidine (5-AzaC) to reactivate erythroid differentiation potential in 32D Ro cells. By Northern analysis, the two cell lines expressed similar levels of all these genes. However, after being treated with 5-AzaC, 32D Ro cells acquired the ability to generate EPO-dependent clones (1 clone/10(4) cells) which gave rise to EPO-dependent cell lines. All the 10 EPO-responsive cell lines independently isolated from 5-AzaC-treated 32D Ro cells had erythroid morphology and expressed high levels of alpha- and beta-globin. In contrast, none of the IL-3-dependent and granulocyte/macrophage colony-stimulating factor-dependent clones concurrently isolated, as a control, showed erythroid properties. Therefore, 5-AzaC treatment reactivates the potential of the myeloid-restricted 32D Ro cells to generate EPO-responsive erythroid clones suggesting that gene methylation played an important role in the G-CSF-mediated restriction/activation of the differentiation potential of these cells.

Histone acetyltransferases and deacetylases in the control of cell proliferation and differentiation

Histone acetylation and deacetylation are chromatin-modifying processes that have fundamental importance for transcriptional regulation. Transcriptionally active chromatin regions show a high degree of histone acetylation, whereas deacetylation events are generally linked to transcriptional silencing. Many of the acetylating and deacetylating enzymes were originally identified as transcriptional coactivators or repressors. Their histone-modifying enzymatic activity was discovered more recently, opening up a whole new area of research. Histone acetyltransferases such as CREB-binding protein (CBP) and PCAF are involved in processes as diverse as promoting cell cycle progression and regulating differentiation. A controlled balance between histone acetylation and deacetylation seems to be essential for normal cell growth. Both histone acetyltransferases and deacetylases are involved in the development of diseases, including neurodegenerative disorders and cancer. Treatments that target these enzymes are already under clinical investigation.

Interaction of NF-E2 in the human beta-globin locus control region before chromatin remodeling

When transcription is initiated under repressive conditions, such as when chromatin are packed together, binding followed by the functioning of key components in the transcriptional apparatus should be appropriately facilitated in the chromatin architecture. We provide evidence that the erythroid-specific enhancer- binding protein NF-E2 interacts with the cognate motif at DNase I-hypersensitive site 2 of the human beta-globin locus control region in a repressive state. The nucleosome containing the NF-E2-binding site showed characteristic rotational and translational phases in vitro. The binding site had less affinity to the histone octamers than nearby regions while showing greater accessibility to DNase I and micrococcal nuclease. Furthermore, the motif was recognized by the exogenous NF-E2 protein expressed in HeLa cells, which have a repressive state of chromatin at the beta-globin locus, as shown by ligation-mediated PCR and chromatin immunoprecipitation assay. These lines of evidence indicate that NF-E2 interacts with the cognate motif on the nucleosome before chromatin is remodeled.

Identification and characterization of a consensus DNA binding element for the zinc finger transcription factor TIEG/EGRalpha

TGFbeta-Inducible Early Gene (TIEG) and the alternatively-transcribed Early Growth Response Gene alpha (EGRalpha) share a Cys(2)His(2) three-zinc finger region with high homology to Sp1 within its zinc finger region. Three-zinc finger transcription factors bind to GC-rich sequences, with small variations in consensus sequence between subfamilies. In this work, a consensus sequence was identified for TIEG/EGRalpha by expressing and purifying the zinc finger region of the protein, and using this to select a binding site from a random oligonucleotide library by iterative cycles of nitrocellulose filter binding and PCR. A fusion of the TIEG/EGRalpha with the VP16 activation domain supported transcription from this site when cloned in front of a heterologous promoter. Mutational analysis of the binding site identified a GT-rich core (5'-GGTGTG-3') that was necessary for binding, with mutations outside of this region causing only a small to moderate decrease in binding.

Transcriptional profiling of Krüppel-like factor 4 reveals a function in cell cycle regulation and epithelial differentiation

Krüppel-like factor 4 (KLF4) is an epithelially enriched, zinc finger-containing transcription factor, the expression of which is associated with growth arrest. Constitutive expression of KLF4 inhibits G1/S transition of the cell cycle but the manner by which it accomplishes this effect is unclear. To better understand the biochemical function of KLF4, we identified its target genes using cDNA microarray analysis in an established human cell line containing inducible KLF4. RNA extracted from induced and control cells were hybridized differentially to microarray chips containing 9600 human cDNAs. In all, 84 genes with significantly increased expression and 107 genes with significantly reduced expression due to KLF4 induction were identified. The affected genes are sorted to several clusters on the basis of functional relatedness. A major cluster belongs to genes involved in cell-cycle control. Within this cluster, many up-regulated genes are inhibitors of the cell cycle and down-regulated genes are promoters of the cell cycle. Another up-regulated gene cluster includes nine keratin genes, of which seven are located in a specific region on chromosome 12. The results indicate that KLF4 is involved in the control of cell proliferation and does so by eliciting changes in expression of numerous cell-cycle regulatory genes in a concerted manner. Furthermore, KLF4 regulates expression of a group of epithelial-specific keratin genes in a manner consistent with a potential locus control region function.

Gene structure and promoter analysis of the rat BTEB2 gene

Rat BTEB2 protein is a transcription factor with three zinc fingers that binds to GC box, and is expressed in the placenta, intestine, and testis. To understand mechanisms of gene expression of BTEB2, we have cloned the rat BTEB2 gene from a rat liver genomic library and determined the gene structure. The BTEB2 gene contained 4 exons. In the process of cloning of the BTEB2 gene, we cloned two pseudogenes for BTEB2, one of which was a processed gene. The upstream region of the bona fide gene was fused to a luciferase reporter gene, and the generated BTEB2-luciferase chimeric plasmid was transiently transfected into HeLa cells that expressed endogenous BTEB2 mRNA. Significant expression of luciferase activity was observed. Deletion analysis of the promoter region of the BTEB2 gene revealed that at least three regions are important for the activity. Upon investigation of cis-acting elements in the regions, the GC box, CCAAT box and NF-1 binding site were found. As binding factors, Sp1, CBFa, and NF-1 were identified to the DNA elements by gel mobility shift assays using specific antibodies.

Analysis of the 5' promoters for human IL-3 and GM-CSF receptor alpha genes

The receptors for human interleukin-3 (hIL-3R) and granulocyte-macrophage colony-stimulating factor (hGM-CSFR) consist of an alpha subunit, specific for each cytokine, and a beta subunit, common to IL-3, GM-CSF, and IL-5. We cloned genomic DNA covering 1.5 kb of the 5' flanking region of the hIL-3R alpha gene and identified multiple transcription start sites by 5(')-RACE and primer extension analyses. By use of transient transfection experiments, two regions (nt -363 to -331 and -106 to -92) of the hIL-3R alpha promoter appeared to have significant transcription-enhancing activities. Electrophoresis mobility shift assays revealed the binding of Sp1 and unidentified proteins to these regions. Deletion of a putative PU.1 binding site did not affect the promoter activity. We then analyzed 2.5 kb of the hGM-CSFR alpha gene and found the proximal PU.1 binding site to be important for transcription-enhancing activity, as previously reported. These results suggest that different transcriptional activation mechanisms are employed for the transcriptional regulation of hIL-3 and hGM-CSF receptor alpha genes.

The Kruppel-like factor Zf9 and proteins in the Sp1 family regulate the expression of HSP47, a collagen-specific molecular chaperone

In several cells and tissues the synthesis of HSP47, a collagen-specific molecular chaperone in the endoplasmic reticulum, is closely correlated with the synthesis of collagen. We previously reported that the Sp1 binding site at -210 bp in the promoter region and the first and second introns are required for the tissue-specific expression of HSP47 in transgenic mice (Hirata, H., Yamamura, I., Yasuda, K., Kobayashi, A., Tada, N., Suzuki, M., Hirayoshi, K., Hosokawa, N., and Nagata, K. (1999) J. Biol. Chem. 274, 35703-35710). Here, we analyze how these introns influence the transcriptional regulation of the hsp47 gene in BALB/c 3T3 cells, which produce high levels of HSP47. In vitro promoter analysis using a luciferase reporter and gel mobility shift analysis revealed that two cis-acting elements in the first and second introns, BS5-B and EP7-D, respectively, are required for the activation of hsp47 in BALB/c 3T3 cells. Several members of the Kruppel-like factor (KLF) family of proteins were identified as BS5-B-binding proteins by yeast one-hybrid analysis using these elements as baits. One of these proteins, KLF-6/Zf9, binds to the BS5-B element and activates expression of the reporter construct when transfected into cells. Chromatin immunoprecipitation assay analysis revealed that the endogenous KLF-6/Zf9 binds the BS5-B elements that contain the CACCC motif, which is a consensus recognition sequence for other proteins in the KLF family. We also showed that BS5-B and EP7-D are bound by two members of the Sp1 family, Sp2 and Sp3. These results suggest that at least three sequences are required for the constitutive expression of hsp47 in BALB/c 3T3 cells: the -210 bp Sp1 binding site, the BS5-B element in the first intron, and the EP7-D element in the second intron. We suggest that KLF proteins regulate the transcription of hsp47 by binding the BS5-B element in cooperation with Sp2 and/or Sp3.

Erythroid expression of the human alpha-spectrin gene promoter is mediated by GATA-1- and NF-E2-binding proteins

alpha-Spectrin is a highly expressed membrane protein critical for the flexibility and stability of the erythrocyte. Qualitative and quantitative defects of alpha-spectrin are present in the erythrocytes of many patients with abnormalities of red blood cell shape including hereditary spherocytosis and elliptocytosis. We wished to determine the regulatory elements that determine the erythroid-specific expression of the alpha-spectrin gene. We mapped the 5' end of the alpha-spectrin erythroid cDNA and cloned the 5' flanking genomic DNA containing the putative alpha-spectrin gene promoter. Using transfection of promoter/reporter plasmids in human tissue culture cell lines, in vitro DNase I footprinting analyses, and gel mobility shift assays, an alpha-spectrin gene erythroid promoter with binding sites for GATA-1- and NF-E2-related proteins was identified. Both binding sites were required for full promoter activity. In transgenic mice, a reporter gene directed by the alpha-spectrin promoter was expressed in yolk sac, fetal liver, and erythroid cells of bone marrow but not adult reticulocytes. No expression of the reporter gene was detected in nonerythroid tissues. We conclude that this alpha-spectrin gene promoter contains the sequences necessary for low level expression in erythroid progenitor cells.

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.

Identification of Zfp393, a germ cell-specific gene encoding a novel zinc finger protein

Using the digital differential display program of the National Center for Biotechnology Information, we identified a contig of expression sequence tags (ESTs) which were unique to ovary, testis, and egg libraries. The full-length cDNA of this transcript was deduced and further confirmed by reverse transcriptase polymerase chain reaction (RT-PCR). The cDNA encodes a novel protein of 341 amino acids with a nuclear localization signal. The carboxyl-terminus of the protein contains three C2H2 zinc fingers, and the NH(2)-terminus is proline and serine-rich. Based on the conserved zinc finger motifs, we have termed this novel protein as zinc finger protein 393 (ZFP393). Northern blot and RT-PCR analyses revealed that Zfp393 mRNA was exclusively expressed in testis and ovary. The expression sites were further localized by in situ hybridization to step 3-8 spermatids in testis and growing oocytes in ovary. The Zfp393 gene consists of three exons spanning approximately 8 kb on the distal part of mouse chromosome 4. The carboxyl-terminal zinc finger region is highly homologous to several zinc finger-containing proteins, but no proteins were found to share sequence similarity with the NH(2)-terminal region of ZFP393. Genomic database mining and Southern blot analysis indicate that Zfp393 is a single copy gene. We hypothesize that ZFP393 functions as a germ cell-specific transcription factor that plays important roles in spermatid differentiation and oocyte development.

Krüppel-like zinc fingers bind to nuclear import proteins and are required for efficient nuclear localization of erythroid Krüppel-like factor

Erythroid Krüppel-like Factor (EKLF/KLF-1) is an erythroid-specific transcription factor that contains three C(2)H(2) zinc fingers and is required for correct chromatin structure and expression of the beta-globin locus. However, regions within the EKLF protein that serve as signals for its nuclear localization and the proteins that may enable it to become localized are unknown. Two approaches were used to address these issues. First, green fluorescent protein or pyruvate kinase was fused to EKLF domains, and localization was monitored and quantitated by confocal microscopy. Two necessary and sufficient nuclear localization signals (NLSs) were identified: one (NLS1) adjacent to the zinc finger DNA binding domain within a highly basic stretch of amino acids (275-296), and another more efficient signal (NLS2) within the zinc finger domain itself (amino acids 293-376). Interestingly, each zinc finger contributes to the overall effectiveness of NLS2 and requires an intact finger structure. Second, each NLS was tested in vitro for binding to importin proteins. Surprisingly, both EKLF NLSs, but principally the zinc finger domain, bind importin alpha and importin beta. These findings demonstrate that two nuclear localization signals target EKLF to the nucleus and suggest this transport relies primarily on a novel zinc finger/importin protein interaction.

Sp/Krüppel-like transcription factors are essential for the expression of mitochondrial glycerol phosphate dehydrogenase promoter B

The human mitochondrial glycerol phosphate dehydrogenase (hmGPD) is abundant in the normal pancreatic insulin cell and its level is lowered to 50% by high glucose and diabetes. Using Drosophila and INS-1 cells, we have analysed the hmGPD gene promoter B to characterize cis-elements and trans-acting factors that affect its regulation. We identified ten efficient Sp/Krüppel-like transcription-factor-binding sites in the promoter sequence. These sites include four GC-boxes (CCCGCCC at -227, -68, -46 and GGGCGAG at -382), three GT-boxes (CCCCACCC at -350, CCCACACCC at -257, and CACCCGCCC at -48), and three CT/GA-boxes (TCCCTCCC at -262, GGGAGGGAG at -129, and GGGAGGAGGA at -107). Transfection of Drosophila SL2 cells, which lack Sp/Krüppel-like factors, with constructs encoding either Sp1, Sp3, Sp4 or erythroid Krüppel-like factor (EKLF) specifically activates the hmGPD promoter B up to 50-fold. Promoter activation requires the Sp1 activation and the DNA binding domains. Co-transfected EKLF was synergistic with either Sp1 or Sp3. On the other hand, the basic Krüppel-like factor (BKLF) inhibited Sp1-and EKLF-mediated promoter activation. Similarly, constructs encoding either GA-binding protein (GABP) or PU.1 inhibited Sp1-mediated promoter activation. Oligonucleotide 'decoys' with potential transcription factor binding sites decreased promoter activity in both INS-1 and Drosophila cells. Significant loss of Sp- and EKLF-mediated activation was observed with some internal as well as sequential 5' deletions of the promoter DNA. The level of Sp1 protein was reduced by 50% in INS-1 cells chronically exposed to a high concentration of glucose. The results demonstrate that Sp/Krüppel-like factors are essential for mGPD gene expression.