The erythroid Krüppel-like factor transactivation domain is a critical component for cell-specific inducibility of a beta-globin promoter

Erythroid differentiation in mouse erythroleukemia cells depends on Tmod3-mediated regulation of actin filament assembly into the erythroblast membrane skeleton

Erythroid differentiation (ED) is a complex cellular process entailing morphologically distinct maturation stages of erythroblasts during terminal differentiation. Studies of actin filament (F-actin) assembly and organization during terminal ED have revealed essential roles for the F-actin pointed-end capping proteins, tropomodulins (Tmod1 and Tmod3). Tmods bind tropomyosins (Tpms), which enhance Tmod capping and F-actin stabilization. Tmods can also nucleate F-actin assembly, independent of Tpms. Tmod1 is present in the red blood cell (RBC) membrane skeleton, and deletion of Tmod1 in mice leads to a mild compensated anemia due to mis-regulated F-actin lengths and membrane instability. Tmod3 is not present in RBCs, and global deletion of Tmod3 leads to embryonic lethality in mice with impaired ED. To further decipher Tmod3's function during ED, we generated a Tmod3 knockout in a mouse erythroleukemia cell line (Mel ds19). Tmod3 knockout cells appeared normal prior to ED, but showed defects during progression of ED, characterized by a marked failure to reduce cell and nuclear size, reduced viability, and increased apoptosis. Tmod3 does not assemble with Tmod1 and Tpms into the Triton X-100 insoluble membrane skeleton during ED, and loss of Tmod3 had no effect on α1,β1-spectrin and protein 4.1R assembly into the membrane skeleton. However, F-actin, Tmod1 and Tpms failed to assemble into the membrane skeleton during ED in absence of Tmod3. We propose that Tmod3 nucleation of F-actin assembly promotes incorporation of Tmod1 and Tpms into membrane skeleton F-actin, and that this is integral to morphological maturation and cell survival during erythroid terminal differentiation.

Neomorphic effects of the neonatal anemia (Nan-Eklf) mutation contribute to deficits throughout development

Transcription factor control of cell-specific downstream targets can be significantly altered when the controlling factor is mutated. We show that the semi-dominant neonatal anemia (Nan) mutation in the EKLF/KLF1 transcription factor leads to ectopic expression of proteins that are not normally expressed in the red blood cell, leading to systemic effects that exacerbate the intrinsic anemia in the adult and alter correct development in the early embryo. Even when expressed as a heterozygote, the Nan-EKLF protein accomplishes this by direct binding and aberrant activation of genes encoding secreted factors that exert a negative effect on erythropoiesis and iron use. Our data form the basis for a novel mechanism of physiological deficiency that is relevant to human dyserythropoietic anemia and likely other disease states.

Molecular identification and functional analysis of KLF2 in Plecoglossus altivelis (ayu): It's regulatory role in monocyte/macrophage activation

Monocytes/macrophages (MO/MФ) play an important role in the response to infection in Plecoglossus altivelis (ayu). However, the role of transcription factors in the function of ayu MO/MФ is poorly understood. Here, we cloned the cDNA sequence of the Kruppel-like factor 2 (PaKLF2) gene from ayu. Phylogenetic analysis indicated that PaKLF2 was closest to that of Atlantic salmon (Salmo salar). Real time quantitative PCR (RT-qPCR) revealed that the PaKLF2 mRNA level was highest in the peripheral blood mononuclear cells among all tested tissues. The mRNA expression of PaKLF2 was upregulated in the head kidney, liver, spleen, and brain after Listonella anguillarum infection. Subsequently, PaKLF2 was expressed and purified to prepare anti-PaKLF2 antibodies. After L. anguillarum challenge, the PaKLF2 mRNA and protein levels were significantly upregulated in ayu MO/MФ. Moreover, PaKLF2 knockdown in MO/MФ resulted in the enhancement of cytokine production as well as phagocytotic and bactericidal capability. Therefore, PaKLF2 may modulate the immune response in ayu by suppressing the function of MO/MФ.

Therapeutic fetal-globin inducers reduce transcriptional repression in hemoglobinopathy erythroid progenitors through distinct mechanisms

Pharmacologic augmentation of γ-globin expression sufficient to reduce anemia and clinical severity in patients with diverse hemoglobinopathies has been challenging. In studies here, representative molecules from four chemical classes, representing several distinct primary mechanisms of action, were investigated for effects on γ-globin transcriptional repressors, including components of the NuRD complex (LSD1 and HDACs 2-3), and the downstream repressor BCL11A, in erythroid progenitors from hemoglobinopathy patients. Two HDAC inhibitors (MS-275 and SB939), a short-chain fatty acid derivative (sodium dimethylbutyrate [SDMB]), and an agent identified in high-throughput screening, Benserazide, were studied. These therapeutics induced γ-globin mRNA in progenitors above same subject controls up to 20-fold, and increased F-reticulocytes up to 20%. Cellular protein levels of BCL11A, LSD-1, and KLF1 were suppressed by the compounds. Chromatin immunoprecipitation assays demonstrated a 3.6-fold reduction in LSD1 and HDAC3 occupancy in the γ-globin gene promoter with Benserazide exposure, 3-fold reduction in LSD-1 and HDAC2 occupancy in the γ-globin gene promoter with SDMB exposure, while markers of gene activation (histone H3K9 acetylation and H3K4 demethylation), were enriched 5.7-fold. These findings identify clinical-stage oral therapeutics which inhibit or displace major co-repressors of γ-globin gene transcription and may suggest a rationale for combination therapy to produce enhanced efficacy.

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

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

Regulation of delta-aminolevulinic acid dehydratase by krüppel-like factor 1

Krüppel-like factor 1(KLF1) is a hematopoietic-specific zinc finger transcription factor essential for erythroid gene expression. In concert with the transacting factor GATA1, KLF1 modulates the coordinate expression of the genes encoding the multi-enzyme heme biosynthetic pathway during erythroid differentiation. To explore the mechanisms underpinning KLF1 action at the gene loci regulating the first 3 steps in this process, we have exploited the K1-ERp erythroid cell line, in which KLF1 translocates rapidly to the nucleus in response to treatment with 4-OH-Tamoxifen (4-OHT). KLF1 acts as a differentiation-independent transcriptional co-regulator of delta-aminolevulinic acid dehydratase (Alad), but not 5-aminolevulinate synthase gene (Alas2) or porphobilinogen deaminase (Pbgd). Similar to its role at the β-globin promoter, KLF1 induces factor recruitment and chromatin changes at the Alad1b promoter in a temporally-specific manner. In contrast to these changes, we observed a distinct mechanism of histone eviction at the Alad1b promoter. Furthermore, KLF1-dependent events were not modulated by GATA1 factor promoter co-occupancy alone. These results not only enhance our understanding of erythroid-specific modulation of heme biosynthetic regulation by KLF1, but provide a model that will facilitate the elucidation of novel KLF1-dependent events at erythroid gene loci that are independent of GATA1 activity.

ATP-dependent mitochondrial porphyrin importer ABCB6 protects against phenylhydrazine toxicity

Abcb6 is a mammalian mitochondrial ATP-binding cassette (ABC) transporter that regulates de novo porphyrin synthesis. In previous studies, haploinsufficient (Abcb6(+/-)) embryonic stem cells showed impaired porphyrin synthesis. Unexpectedly, Abcb6(-/-) mice derived from these stem cells appeared phenotypically normal. We hypothesized that other ATP-dependent and/or -independent mechanisms conserve porphyrins. Here, we demonstrate that Abcb6(-/-) mice lack mitochondrial ATP-driven import of coproporphyrin III. Gene expression analysis revealed that loss of Abcb6 results in up-regulation of compensatory porphyrin and iron pathways, associated with elevated protoporphyrin IX (PPIX). Phenylhydrazine-induced stress caused higher mortality in Abcb6(-/-) mice, possibly because of sustained elevation of PPIX and an inability to convert PPIX to heme despite elevated ferrochelatase levels. Therefore, Abcb6 is the sole ATP-dependent porphyrin importer, and loss of Abcb6 produces up-regulation of heme and iron pathways necessary for normal development. However, under extreme demand for porphyrins (e.g. phenylhydrazine stress), these adaptations appear inadequate, which suggests that under these conditions Abcb6 is important for optimal survival.

Recombinant erythroid Kruppel-like factor fused to GATA1 up-regulates delta- and gamma-globin expression in erythroid cells

The β-hemoglobinopathies sickle cell disease and β-thalassemia are among the most common human genetic disorders worldwide. Hemoglobin A2 (HbA2, α₂δ₂) and fetal hemoglobin (HbF, α₂γ₂) both inhibit the polymerization of hemoglobin S, which results in erythrocyte sickling. Expression of erythroid Kruppel-like factor (EKLF) and GATA1 is critical for transitioning hemoglobin from HbF to hemoglobin A (HbA, α₂β₂) and HbA2. The lower levels of δ-globin expression compared with β-globin expression seen in adulthood are likely due to the absence of an EKLF-binding motif in the δ-globin proximal promoter. In an effort to up-regulate δ-globin to increase HbA2 expression, we created a series of EKLF-GATA1 fusion constructs composed of the transactivation domain of EKLF and the DNA-binding domain of GATA1, and then tested their effects on hemoglobin expression. EKLF-GATA1 fusion proteins activated δ-, γ-, and β-globin promoters in K562 cells, and significantly up-regulated δ- and γ-globin RNA transcript and protein expression in K562 and/or CD34(+) cells. The binding of EKLF-GATA1 fusion proteins at the GATA1 consensus site in the δ-globin promoter was confirmed by chromatin immunoprecipitation assay. Our studies demonstrate that EKLF-GATA1 fusion proteins can enhance δ-globin expression through interaction with the δ-globin promoter, and may represent a new genetic therapeutic approach to β-hemoglobinopathies.

Identification of fetal hemoglobin-inducing agents using the human leukemia KU812 cell line

Fetal hemoglobin (HbF) ameliorates the clinical severity of sickle cell disease; therefore continued research to identify efficacious HbF-inducing agents is desirable. In this study, we investigated KU812 leukemia cells that express the fetal γ-globin and adult β-globin genes, as a system for screening and discovery of novel HbF inducers. KU812 cells were analyzed in the presence or absence of fetal bovine serum and then expression levels of the globin genes, cell surface markers and transcription factors were quantified by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). For comparison, primary erythroid cells were grown in a two-phase liquid culture system. After drug inductions for 48-72 h, globin mRNA and HbF levels were quantified by RT-qPCR and enzyme-linked immunosorbent assay, respectively. Erythroid markers and transcription factors expression levels in KU812 cells were comparable to days 7-14 erythroid cells. We also tested several drugs including butyrate, trichostatin A, scriptaid, suberoylanilide hydroxamic acid and hydroxyurea, which induced γ-globin in KU812 cells; however, some agents also induced β-globin. A novel agent STI-571 was studied in the system, which non-selectively induced the globin genes. Additional studies showed comparable globin gene response patterns in KU812 and primary erythroid cells after treatments with the various drug inducers. Mechanisms of drug-mediated γ-globin induction in KU812 cells require signaling through the p38 mitogen-activated protein kinase pathway similar to that previously demonstrated in primary erythroid cells. These data suggest that KU812 cells serve as a good screening system to identify potential HbF inducers for the treatment of β-hemoglobinopathies.

Mammalian Krüppel-like factors in health and diseases

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

Role of Kruppel-like factors in leukocyte development, function, and disease

The Krüppel-like transcription factor (KLF) family participates in diverse aspects of cellular growth, development, differentiation, and activation. Recently, several groups have identified new connections between the function of these factors and leukocyte responses in health and disease. Gene targeting of individual KLFs in mice has uncovered novel and unexpected physiologic roles among myeloid and lymphocyte cell lineage maturation, particularly in the bone marrow niche and blood. In addition, several KLF family members are downstream targets of stimuli and signaling pathways critical to T-cell trafficking, T regulatory cell differentiation or suppressor function, monocyte/macrophage activation or renewal, and B memory cell maturation or activation. Indeed, KLFs have been implicated in subtypes of leukemia, lymphoma, autoimmunity, and in acute and chronic inflammatory disease states, such as atherosclerosis, diabetes, and airway inflammation, raising the possibility that KLFs and their upstream signals are of therapeutic interest. This review focuses on the relevant literature of Krüppel-like factors in leukocyte biology and their implications in clinical settings.

Erythroid Kruppel-like factor (EKLF) is recruited to the gamma-globin gene promoter as a co-activator and is required for gamma-globin gene induction by short-chain fatty acid derivatives

OBJECTIVES

The erythroid Kruppel-like factor (EKLF) is an essential transcription factor for beta-type globin gene switching, and specifically activates transcription of the adult beta-globin gene promoter. We sought to determine if EKLF is also required for activation of the gamma-globin gene by short-chain fatty acid (SCFA) derivatives, which are now entering clinical trials.

METHODS

The functional and physical interaction of EKLF and co-regulatory molecules with the endogenous human globin gene promoters was studied in primary human erythroid progenitors and cell lines, using chromatin immunoprecipitation (ChIP) assays and genetic manipulation of the levels of EKLF and co-regulators.

RESULTS AND CONCLUSIONS

Knockdown of EKLF prevents SCFA-induced expression of the gamma-globin promoter in a stably expressed microLCRbeta(pr)R(luc) (A)gamma(pr)F(luc) cassette, and prevents induction of the endogenous gamma-globin gene in primary human erythroid progenitors. EKLF is actively recruited to endogenous gamma-globin gene promoters after exposure of primary human erythroid progenitors, and murine hematopoietic cell lines, to SCFA derivatives. The core ATPase BRG1 subunit of the human SWI/WNF complex, a ubiquitous multimeric complex that regulates gene expression by remodeling nucleosomal structure, is also required for gamma-globin gene induction by SCFA derivatives. BRG1 is actively recruited to the endogenous gamma-globin promoter of primary human erythroid progenitors by exposure to SCFA derivatives, and this recruitment is dependent upon the presence of EKLF. These findings demonstrate that EKLF, and the co-activator BRG1, previously demonstrated to be required for definitive or adult erythropoietic patterns of globin gene expression, are co-opted by SCFA derivatives to activate the fetal globin genes.

Failure of terminal erythroid differentiation in EKLF-deficient mice is associated with cell cycle perturbation and reduced expression of E2F2

Erythroid Krüppel-like factor (EKLF) is a Krüppel-like transcription factor identified as a transcriptional activator and chromatin modifier in erythroid cells. EKLF-deficient (Eklf(-/-)) mice die at day 14.5 of gestation from severe anemia. In this study, we demonstrate that early progenitor cells fail to undergo terminal erythroid differentiation in Eklf(-/-) embryos. To discover potential EKLF target genes responsible for the failure of erythropoiesis, transcriptional profiling was performed with RNA from wild-type and Eklf(-/-) early erythroid progenitor cells. These analyses identified significant perturbation of a network of genes involved in cell cycle regulation, with the critical regulator of the cell cycle, E2f2, at a hub. E2f2 mRNA and protein levels were markedly decreased in Eklf(-/-) early erythroid progenitor cells, which showed a delay in the G(1)-to-S-phase transition. Chromatin immunoprecipitation analysis demonstrated EKLF occupancy at the proximal E2f2 promoter in vivo. Consistent with the role of EKLF as a chromatin modifier, EKLF binding sites in the E2f2 promoter were located in a region of EKLF-dependent DNase I sensitivity in early erythroid progenitor cells. We propose a model in which EKLF-dependent activation and modification of the E2f2 locus is required for cell cycle progression preceding terminal erythroid differentiation.

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

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

Sumoylation of EKLF promotes transcriptional repression and is involved in inhibition of megakaryopoiesis

Erythroid Krüppel-like factor (EKLF [KLF1]) is a transcriptional regulator that plays a critical role within a specific subset of hematopoietic cells, particularly in the erythroid lineage and its immediate precursor, the megakaryocyte-erythroid progenitor (MEP). We find that EKLF is posttranslationally modified by sumoylation at a single site near its amino terminus and that PIAS1 plays a critical role in this process. Mutation of this site has little effect on EKLF's ability to function as a transcriptional activator; however, it has a dramatic effect on its repressive abilities. The mechanism of repression likely involves a novel small ubiquitin-related modifier (SUMO)-dependent EKLF interaction with the Mi-2beta component of the NuRD repression complex. Mutated EKLF is attenuated in its ability to repress megakaryocyte differentiation, implicating EKLF sumoylation status in differentiative decisions emanating from the MEP. These studies demonstrate a novel mechanism by which transcription factor sumoylation can alter protein-protein interactions and bipotential lineage decisions.

A global role for zebrafish klf4 in embryonic erythropoiesis

There are two waves of erythropoiesis, known as primitive and definitive waves in mammals and lower vertebrates including zebrafish. The founding member of the Kruppel-like factor (KLF) family of CACCC-box binding proteins, EKLF/Klf1, is essential for definitive erythropoiesis in mammals but only plays a minor role in primitive erythropoiesis. Morpholino knockdown experiments have shown a role for zebrafish klf4 in primitive erythropoiesis and hatching gland formation. In order to generate a global understanding of how klf4 might influence gene expression and differentiation, we have performed expression profiling of klf4 morphants, and then performed validation of many putative target genes by qRT-PCR and whole mount in situ hybridization. We found a critical role for klf4 in embryonic globin, heme synthesis and hatching gland gene expression. In contrast, there was an increase in expression of definitive hematopoietic specific genes such as larval globin genes, runx1 and c-myb from 24 hpf, suggesting a selective role for klf4 in primitive rather than definitive erythropoiesis. In addition, we show klf4 preferentially binds CACCC box elements in the primitive zebrafish beta-like globin gene promoters. These results have global implications for primitive erythroid gene regulation by KLF-CACCC box interactions.

Altered regulation of beta-like globin genes by a redesigned erythroid transcription factor

OBJECTIVE

Targeted regulation of beta-like globin genes was studied using designer zinc finger transcription factors containing the DNA binding domain of the red cell specific transcription factor erythroid Kruppel-like factor (EKLF) fused to repression domains.

METHODS

Globin gene expression was analyzed after introduction of the modified transcription factors into cell lines, embryonic stem cells and transgenic mice.

RESULTS

As would be predicted, when introduced transiently into cells these transcription factors were effective in repressing the adult beta-globin promoter CACCC element, which is the natural target for EKLF. In murine erythroleukemia cells repression of the adult beta-globin gene was accompanied by a reactivation of the endogenous embryonic betaH1-globin gene. Studies in differentiated embryonic stem cells and transgenic mice confirmed the reactivation of embryonic gene expression during development.

CONCLUSION

Our studies support a competition model for beta-globin gene expression and underscore the importance of EKLF in the embryonic/fetal-to-adult globin switch. They also demonstrate the feasibility of designer zinc finger transcription factors in the study of transcriptional control mechanisms at the beta-globin locus and as potential gene therapy agents for sickle cell disease and related hemoglobinopathies.

Chromatin-binding in vivo of the erythroid kruppel-like factor, EKLF, in the murine globin loci

EKLF is an erythroid-specific, zinc finger-containing transcription factor essential for the activation of the mammalian beta globin gene in erythroid cells of definitive lineage. We have prepared a polyclonal anti-mouse EKLF antibody suitable for Western blotting and immunoprecipitation (IP) qualities, and used it to define the expression patterns of the EKLF protein during mouse erythroid development. We have also used this antibody for the chromatin-immunoprecipitation (ChIP) assay. EKLF was found to bind in vivo at both the mouse beta-major-globin promoter and the HS2 site of beta-LCR in the mouse erythroleukemia cells (MEL) in a DMSO-inducible manner. The DMSO-induced bindings of EKLF as well as three other proteins, namely, RNA polymerase II, acetylated histone H3, and methylated histone H3, were not abolished but significantly lowered in CB3, a MEL-derived cell line with null-expression of p45/NF-E2, an erythroid-enriched factor needed for activation of the mammalian globin loci. Interestingly, binding of EKLF in vivo was also detected in the mouse alpha-like globin locus, at the adult alpha globin promoter and its far upstream regulatory element alpha-MRE (HS26). This study provides direct evidence for EKLF-binding in vivo at the major regulatory elements of the mouse beta-like globin gene clusters the data also have interesting implications with respect to the role of EKLF-chromatin interaction in mammalian globin gene regulation.

Alterations in expression and chromatin configuration of the alpha hemoglobin-stabilizing protein gene in erythroid Kruppel-like factor-deficient mice

Erythroid Krüppel-like factor (EKLF) is an erythroid zinc finger protein identified by its interaction with a CACCC sequence in the beta-globin promoter, where it establishes local chromatin structure permitting beta-globin gene transcription. We sought to identify other EKLF target genes and determine the chromatin status of these genes in the presence and absence of EKLF. We identified alpha hemoglobin-stabilizing protein (AHSP) by subtractive hybridization and demonstrated a 95 to 99.9% reduction in AHSP mRNA and the absence of AHSP in EKLF-deficient cells. Chromatin at the AHSP promoter from EKLF-deficient cells lacked a DNase I hypersensitive site and exhibited histone hypoacetylation across the locus compared to hyperacetylation of wild-type chromatin. Wild-type chromatin demonstrated a peak of EKLF binding over a promoter region CACCC box that differs from the EKLF consensus by a nucleotide. In mobility shift assays, the AHSP promoter CACCC site bound EKLF in a manner comparable to the beta-globin promoter CACCC site, indicating a broader recognition sequence for the EKLF consensus binding site. The AHSP promoter was transactivated by EKLF in K562 cells, which lack EKLF. These results support the hypothesis that EKLF acts as a transcription factor and a chromatin modulator for the AHSP and beta-globin genes and indicate that EKLF may play similar roles for other erythroid genes.

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.

The family feud: turning off Sp1 by Sp1-like KLF proteins

Sp1 is one of the best characterized transcriptional activators. The biological importance of Sp1 is underscored by the fact that several hundreds of genes are thought to be regulated by this protein. However, during the last 5 years, a more extended family of Sp1-like transcription factors has been identified and characterized by the presence of a conserved DNA-binding domain comprising three Krüppel-like zinc fingers. Each distinct family member differs in its ability to regulate transcription, and, as a consequence, to influence cellular processes. Specific activation and repression domains located within the N-terminal regions of these proteins are responsible for these differences by facilitating interactions with various co-activators and co-repressors. The present review primarily focuses on discussing the structural, biochemical and biological functions of the repressor members of this family of transcription factors. The existence of these transcriptional repressors provides a tightly regulated mechanism for silencing a large number of genes that are already known to be activated by Sp1.

Stage-specific repression by the EKLF transcriptional activator

Dynamic changes in transcription factor function can be mediated by switching its interaction with coactivators and corepressors. Erythroid Kruppel-like factor (EKLF) is an erythroid cell-specific transcription factor that plays a critical role in beta-globin gene activation via its interactions with CBP/p300 and SWI/SNF proteins. Unexpectedly, it also interacts with Sin3A and histone deacetylase 1 (HDAC1) corepressors via its zinc finger domain. We now find that selected point mutants can uncouple activation and repression and that an intact finger structure is not required for interactions with Sin3A/HDAC1 or for transrepression. Most intriguingly, EKLF repression exhibits stage specificity, with reversible EKLF-Sin3A interactions playing a key role in this process. Finally, we have located a key lysine residue that is both a substrate for CBP acetylation and required for Sin3A interaction. These data suggest a model whereby the stage of the erythroid cell alters the acetylation status of EKLF and plays a critical role in directing its coactivator-corepressor interactions and downstream transcriptional effects.

Identification of MoKA, a novel F-box protein that modulates Krüppel-like transcription factor 7 activity

KLF7, a member of the Krüppel-like transcription factor family, is believed to regulate neurogenesis and cell cycle progression. Here, a yeast two-hybrid screen for KLF7 cofactors in the developing nervous system identified a novel 140-kDa protein named MoKA, for modulator of KLF7 activity. Interaction between MoKA and KLF7 was confirmed by the in vitro glutathione S-transferase pull-down assay and by coimmunoprecipitation of the proteins overexpressed in mammalian cells. Functional assays documented that MoKA is a KLF7 coactivator, and in situ hybridizations identified the developing nervous system and the adult testes as two sites of MoKA and Klf7 coexpression. Chromatin immunoprecipitation experiments demonstrated KLF7 binding to the p21(WAF1/Cip1) gene while transient transfection assays documented KLF7 stimulation of the p21(WAF1/Cip1) proximal promoter. Additional tests revealed that distinct structural motifs of MoKA direct interaction with KLF7 and shuttling between the nucleus and cytoplasm of asynchronously cycling cells. Altogether, our results strongly suggest that MoKA and KLF7 interact functionally to regulate gene expression during cell differentiation and identify the cell cycle regulator p21(WAF1/Cip1) as one of the targeted genes.

Haploinsufficiency of Runx1 results in the acceleration of mesodermal development and hemangioblast specification upon in vitro differentiation of ES cells

The AML1 gene (recently renamed Runx1), which encodes the DNA-binding subunit of a transcription factor of the core binding factor (CBF) family, is required for the establishment of definitive hematopoiesis. We have previously demonstrated that Runx1 is expressed in yolk sac mesodermal cells prior to the establishment of the blood islands and in the embryoid body (EB)–derived blast-colony–forming cells (BL-CFCs), the in vitro equivalent of the hemangioblast. Analysis of Runx1-deficient embryonic stem (ES) cells demonstrated that this gene is essential for the generation of normal numbers of blast colonies, the progeny of the BL-CFCs. In the present study, we analyzed the potential of Runx1+/– ES cells to determine if heterozygosity at the Runx1 locus impacts early developmental events leading to the commitment of the BL-CFCs. Our results indicate that Runx1 heterozygosity leads to an acceleration of mesodermal commitment and specification to the BL-CFCs and to the hematopoietic lineages in EBs.

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.

Effect of fetal hemoglobin-stimulating medicines on the interaction of DNA and protein of important erythroid regulatory elements

β-Thalassemia is the most common single gene disorder in the world, which is caused by the imbalance between α-globin chain and β-globin chain synthesis. Several medicines, such as 5-azacytidine, hydroxyurea, cytarabine, vinblatine, butyrate, and myleran, have been shown to be able to reactivate γ-globin chain synthesis during the adult stage, and some of them (5-azacytidine, hydroxyurea, myleran, and butyrate) have been used clinically to treat thalas semia and sickle cell disease. Much research efforts are focusing on the determination of the underlying mechanisms of medicine action. In this experiment, as an effort to probe the underlying mechanism of medicine action, we used ligation-mediated polymerase chain reaction and in vivo footprinting methods to study the DNA-protein interaction at critical erythroid regulatory elements after hydroxyurea or myleran administration to mice. Our results showed that the patterns of in vivo footprints at both the hypersensitive site 2 of the locus control region and the β-globin gene promoter were changed after medicine treatment. We proposed based on these results that the medicines' administration might result in a change in the interaction between trans-acting factors and cis-acting elements at these regions. These changes might influence the assembly of the transcription complex and, lastly, influence the expression of the β-globin gene.Key words: hydroxyurea, in vivo footprinting, ligation-mediated PCR, LCR, β-globin.

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.

Functional interplay between CBP and PCAF in acetylation and regulation of transcription factor KLF13 activity

The transcriptional co-activators CBP/p300 and PCAF participate in transcriptional activation by many factors. We have shown that both CBP/p300 and PCAF stimulate the transcriptional activation by KLF13, a member of the KLF/Sp1 family, either individually or cooperatively. Here we further investigated how CBP and PCAF acetylation regulate KLF13 activity, and how these two co-activators functionally interplay in the regulation of KLF13 activity. We found that CBP and PCAF acetylated KLF13 at specific lysine residues in the zinc finger domain of KLF13. The acetylation by CBP, however, resulted in disruption of KLF13 DNA binding. Although the acetyltransferase activity of CBP is not required for stimulating the DNA binding activity of all of the transcription factors that we have examined, the disruption of factor DNA binding by CBP acetylation is factor-specific. We further showed that PCAF and CBP act synergistically and antagonistically to regulate KLF13 DNA binding depending on the status of acetylation. PCAF blocked CBP acetylation and disruption of KLF13 DNA binding. Conversely, acetylation of KLF13 by CBP prevented PCAF stimulation of KLF13 DNA binding. PCAF blocked CBP disruption of KLF13 DNA binding by preventing CBP acetylation of KLF13. These results demonstrate that acetylation by CBP has distinct effects on transcription factor DNA binding, and that CBP and PCAF regulate each other functionally in their regulation of transcription factor DNA binding.

mSharp-1/DEC2, a basic helix-loop-helix protein functions as a transcriptional repressor of E box activity and Stra13 expression

Transcription factors belonging to the basic helix-loop-helix (bHLH) family play critical roles in the regulation of cellular differentiation of distinct cell types. In this study, we have characterized the DNA-binding and transcriptional properties of the bHLH factor mSharp-1/DEC2. mSharp-1 belongs to the Hairy/Enhancer of Split subfamily of bHLH factors and exhibits the highest structural and sequence identity with Stra13. We show that mSharp-1 specifically binds to the E box motif (CANNTG) as a homodimer and acts as a potent transcriptional repressor of MyoD- and E12-induced E box activity and differentiation. The inhibitory activity of mSharp-1 occurs through several mechanisms including occupancy of E box sites by mSharp-1 homodimers and by direct physical interaction with MyoD and E proteins. Furthermore, by using gel mobility shift assays and chromatin immunoprecipitation experiments, we have identified Stra13 as a target for mSharp-1-mediated repression. We demonstrate that transcriptional repression of Stra13 depends, in part, on binding of mSharp-1 to three conserved E box motifs in the Stra13 proximal promoter. Moreover, mSharp-1 directly interacts with the transcriptional activator Sp1 and impairs Sp1 induction of Stra13 promoter. Our results suggest that mSharp-1 functions as a transcriptional repressor by DNA binding dependent and independent mechanisms.

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.

Beta-globin locus control region HS2 and HS3 interact structurally and functionally

The overall structure of the DNase I hypersensitive sites (HSs) that comprise the beta-globin locus control region (LCR) is highly conserved among mammals, implying that the HSs have conserved functions. However, it is not well understood how the LCR HSs, either individually or collectively, activate transcription. We analyzed the interactions of HS2, HS3 and HS4 with the human epsilon- and beta-globin genes in chromatinized episomes in fetal/embryonic K562 cells. Only HS2 activates transcription of the epsilon-globin gene, while all three HSs activate the beta-globin gene. HS3 stimulates the beta-globin gene constitutively, but HS2 and HS4 transactivation requires expression of the transcription factor EKLF, which is not present in K562 cells but is required for beta-globin expression in vivo. To begin addressing how the individual HSs may interact with one another in a complex, we linked the beta-globin gene to both the HS2 and HS3. HS2 and HS3 together resulted in synergistic stimulation of beta-globin transcription. Unexpectedly, mutated, inactive forms of HS2 impeded the activation of the beta-globin gene by HS3. Thus, there appear to be distinct interactions among the HSs and between the HSs and the globin genes. These preferential, non-exclusive interactions may underlie an important structural and functional cooperativity among the regulatory sequences of the beta-globin locus in vivo.

Functional cross-antagonism between transcription factors FLI-1 and EKLF

FLI-1 is an ETS family transcription factor which is overexpressed in Friend erythroleukemia and contributes to the blockage of differentiation of erythroleukemic cells. We show here that FLI-1 represses the transcriptional activity of the beta-globin gene promoter in MEL cells and interacts with two of its critical transactivators, GATA-1 and EKLF. Unexpectedly, FLI-1 enhances the stimulating activity of GATA-1 on a GATA-1-responsive promoter but represses that of EKLF on beta-globin and an EKLF-responsive artificial promoters. This repressive effect of FLI-1 requires the ETS DNA binding domain and its association with either the N- or C-terminal domain, which themselves interact with EKLF but not with GATA-1. Furthermore, the FLI-1 ETS domain alone behaves as an autonomous repression domain when linked to the Gal4 DNA binding domain. Taken together, these data indicate that FLI-1 represses EKLF-dependent transcription due to the repression activity of its ETS domain and its indirect recruitment to erythroid promoters by protein-protein interaction with EKLF. Reciprocally, we also show that EKLF itself represses the FLI-1-dependent megakaryocytic GPIX gene promoter, thus further suggesting that functional cross-antagonism between FLI-1 and EKLF might be involved in the control of the erythrocytic versus megakaryocytic differentiation of bipotential progenitors.

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.

Functional domains and DNA-binding sequences of RFLAT-1/KLF13, a Krüppel-like transcription factor of activated T lymphocytes

RFLAT-1/KLF13, a member of the Krüppel-like family of transcription factors, was identified as a transcription factor expressed 3-5 days after T lymphocyte activation. It binds to the promoter of the chemokine gene RANTES (regulated on activation normal T cell expressed and secreted) and regulates its "late" expression in activated T-cells. In this study, a series of experiments to define the functional domains of RFLAT-1/KLF13 were undertaken to further advance the understanding of the molecular mechanisms underlying transcriptional regulation by this factor. Using the GAL4 fusion system, distinct transcriptional activation and repression domains were identified. The RFLAT-1 minimum activation domain is localized to amino acids 1-35, whereas the repression domain resides in amino acids 67-168. Deletion analysis on the RFLAT-1 protein further supports these domain functions. The RFLAT-1 activation domain is similar to that of its closest family member, basic transcription element-binding protein 1. This domain is highly hydrophobic, and site-directed mutagenesis demonstrated that both negatively charged and hydrophobic residues are important for transactivation. The nuclear localization signal of RFLAT-1 was also identified using the RFLAT-1/green fluorescence protein fusion approach. RFLAT-1 contains two potent, independent nuclear localization signals; one is immediately upstream of the zinc finger DNA-binding domain, and the other is within the zinc fingers. Using mutational analysis, we also determined that the critical binding sequence of RFLAT-1 is CTCCC. The intact CTCCC box on the RANTES promoter is necessary for RFLAT-1-mediated RANTES transcription and is also required for the synergy between RFLAT-1 and NF-kappaB proteins.

Inhibition of CBP-mediated protein acetylation by the Ets family oncoprotein PU.1

Aberrant expression of PU.1 inhibits erythroid cell differentiation and contributes to the formation of murine erythroleukemias (MEL). The molecular mechanism by which this occurs is poorly understood. Here we show that PU.1 specifically and efficiently inhibits CBP-mediated acetylation of several nuclear proteins, including the hematopoietic transcription factors GATA-1, NF-E2, and erythroid Krüppel-like factor. In addition, PU.1 blocks acetylation of histones and interferes with acetylation-dependent transcriptional events. CBP acetyltransferase activity increases during MEL cell differentiation as PU.1 levels decline and is inhibited by sustained PU.1 expression. Finally, PU.1 inhibits the differentiation-associated increase in histone acetylation at an erythroid-specific gene locus in vivo. Together, these findings suggest that aberrant expression of PU.1 and possibly other members of the Ets family of oncoproteins subverts normal cellular differentiation in part by inhibiting the acetylation of critical nuclear factors involved in balancing cellular proliferation and maturation.

The zebrafish klf gene family

The Krüppel-like factor (KLF) family of genes encodes transcriptional regulatory proteins that play roles in differentiation of a diverse set of cells in mammals. For instance, the founding member KLF1 (also known as EKLF) is required for normal globin production in mammals. Five new KLF genes have been isolated from the zebrafish, Danio rerio, and the structure of their products, their genetic map positions, and their expression during development of the zebrafish have been characterized. Three genes closely related to mammalian KLF2 and KLF4 were found, as was an ortholog of mammalian KLF12. A fifth gene, apparently missing from the genome of mammals and closely related to KLF1 and KLF2, was also identified. Analysis demonstrated the existence of novel conserved domains in the N-termini of these proteins. Developmental expression patterns suggest potential roles for these zebrafish genes in diverse processes, including hematopoiesis, blood vessel function, and fin and epidermal development. The studies imply a high degree of functional conservation of the zebrafish genes with their mammalian homologs. These findings further the understanding of the KLF genes in vertebrate development and indicate an ancient role in hematopoiesis for the Krüppel-like factor gene family.

Unanticipated repression function linked to erythroid Krüppel-like factor

The erythroid cell-specific transcription factor erythroid Krüppel-like factor (EKLF) is an important activator of beta-globin gene expression. It achieves this by binding to the CACCC element at the beta-globin promoter via its zinc finger domain. The coactivators CBP and P300 interact with, acetylate, and enhance its activity, helping to explain its role as a transcription activator. Here we show that EKLF can also interact with the corepressors mSin3A and HDAC1 (histone deacetylase 1) through its zinc finger domain. When linked to a GAL4 DNA binding domain, full-length EKLF or its zinc finger domain alone can repress transcription in vivo. This repressive activity can be relieved by the HDAC inhibitor trichostatin A. Although recruitment of EKLF to a promoter is required to show repression, its zinc finger domain cannot bind directly to DNA and repress transcription simultaneously. In addition, the target promoter configuration is important for enabling EKLF to exhibit any repressive activity. These results suggest that EKLF may function in vivo as a transcription repressor and play a previously unsuspected additional role in regulating erythroid gene expression and differentiation.

Cooperative activation by GATA-4 and YY1 of the cardiac B-type natriuretic peptide promoter

YY1, a multifunctional protein essential for embryonic development, is a known repressor or activator of transcription. In cardiac and skeletal myocytes, YY1 has been described essentially as a negative regulator of muscle-specific genes. In this study, we report that YY1 is a transcriptional activator of the B-type natriuretic peptide (BNP) gene, which encodes one of the heart major secretory products. YY1 binds an element within the proximal cardiac BNP promoter, in close proximity to the high affinity binding sites for the zinc finger GATA proteins. We show that YY1 cooperates with GATA-4 to synergistically activate BNP transcription. Structure-function analysis revealed that the DNA binding domain of YY1 is sufficient for cooperative interaction with GATA-4, likely through corecruitment of the CREB-binding protein coactivator. The results suggest that YY1 and GATA factors are components of transcriptionally active complexes present in cardiac and other GATA-containing cells.

Novel transactivation domain in erythroid Kruppel-like factor (EKLF)

Erythroid Kruppel-like Factor (EKLF) is an erythroid-specific transcription factor that plays a critical role in gamma- to beta-globin gene switching during development. To identify essential domains required for EKLF transactivation function, we cotransfected a human erythroleukemia cell line (K562) with a locus control region gamma/Luc-beta/Cat reporter and an EKLF expression vector. In this assay EKLF mediates a 500-fold induction of beta/CAT expression compared with controls. To map essential transactivation domains, progressive NH(2)-terminal and internal deletion mutants of EKLF were constructed. All EKLF mutants were expressed at wild-type levels, localized to the nucleus, and bound DNA. When mutant EKLF proteins were tested for beta/CAT activation, a novel transactivation domain was identified. This novel domain, encompassing amino acids (aa) 140-358, is sufficient for maximal beta/CAT activation. An 85-amino acid subdomain within this region (aa 140-225) is essential for its activity. Interestingly, this central transactivation subdomain is functionally redundant with the amino-terminal domain (aa 1-139). Thus, EKLF possesses at least two potent transactivation domains that appear to function in a redundant manner.

Context-dependent EKLF responsiveness defines the developmental specificity of the human epsilon-globin gene in erythroid cells of YAC transgenic mice

We explored the mechanism of definitive-stage epsilon-globin transcriptional inactivity within a human beta-globin YAC expressed in transgenic mice. We focused on the globin CAC and CAAT promoter motifs, as previous laboratory and clinical studies indicated a pivotal role for these elements in globin gene activation. A high-affinity CAC-binding site for the erythroid krüppel-like factor (EKLF) was placed in the epsilon-globin promoter at a position corresponding to that in the adult beta-globin promoter, thereby simultaneously ablating a direct repeat (DR) element. This mutation led to EKLF-independent epsilon-globin transcription during definitive erythropoiesis. A second 4-bp substitution in the epsilon-globin CAAT sequence, which simultaneously disrupts a second DR element, further enhanced ectopic definitive erythroid activation of epsilon-globin transcription, which surprisingly became EKLF dependent. We finally examined factors in nuclear extracts prepared from embryonic or adult erythroid cells that bound these elements in vitro, and we identified a novel DR-binding protein (DRED) whose properties are consistent with those expected for a definitive-stage epsilon-globin repressor. We conclude that the suppression of epsilon-globin transcription during definitive erythropoiesis is mediated by the binding of a repressor that prevents EKLF from activating the epsilon-globin gene.

Functional selectivity of recombinant mammalian SWI/SNF subunits

The SWI/SNF family of chromatin-remodeling complexes plays a key role in facilitating the binding of specific transcription factors to nucleosomal DNA in diverse organisms from yeast to man. Yet the process by which SWI/SNF and other chromatin-remodeling complexes activate specific subsets of genes is poorly understood. We show that mammalian SWI/SNF regulates transcription from chromatin-assembled genes in a factor-specific manner in vitro. The DNA-binding domains (DBDs) of several zinc finger proteins, including EKLF, interact directly with SWI/SNF to generate DNase I hypersensitivity within the chromatin-assembled beta-globin promoter. Interestingly, we find that two SWI/SNF subunits (BRG1 and BAF155) are necessary and sufficient for targeted chromatin remodeling and transcriptional activation by EKLF in vitro. Remodeling is achieved with only the BRG1-BAF155 minimal complex and the EKLF zinc finger DBD, whereas transcription requires, in addition, an activation domain. In contrast, the BRG1-BAF155 complex does not interact or function with two unrelated transcription factors, TFE3 and NF-kappaB. We conclude that specific domains of certain transcription factors differentially target SWI/SNF complexes to chromatin in a gene-selective manner and that individual SWI/SNF subunits play unique roles in transcription factor-directed nucleosome remodeling.

Role of gut-enriched Krüppel-like factor in colonic cell growth and differentiation

Cancer cells differ from normal cells in many aspects, including hyperproliferation and loss of differentiation. Recent research has focused on the role of transcription factors in regulating abnormal cell growth. Gut-enriched Krüppel-like factor (GKLF) is a newly identified eukaryotic zinc finger protein expressed extensively in the gastrointestinal tract. In the current study, we demonstrated that GKLF mRNA levels were significantly decreased in the dysplastic epithelium of the colon, including adenomatous polyp and cancer. GKLF immunostains in the normal colon were higher at the surface epithelium and gradually decreased toward the crypt, but this gradient was not present in the adenomatous and cancerous mucosa. Constitutive overexpression of GKLF DNA in a human colonic adenocarcinoma cell line (HT-29) decreased [(3)H]thymidine incorporation, whereas suppression of GKLF gene increased DNA synthesis, indicating that downregulation of the GKLF gene might contribute to cellular hyperproliferation. Cyclin D1 (CD1) protein level and CD1-associated kinase activity were decreased in HT-29 cell overexpressed GKLF cDNA, and CD1 promoter activity was profoundly suppressed by GKLF. When HT-29 cells were cultured in the presence of sodium butyrate, GKLF mRNA levels increased as cells acquired more differentiated phenotypes. These results suggest that GKLF plays an important role in regulating cell growth and differentiation in the colonic epithelium and that downregulation of GKLF expression may cause colonic cells to become hyperproliferative. Furthermore, GKLF appears to be a transcriptional repressor of the CD1 gene.

Erythroid Kruppel-like factor is recruited to the CACCC box in the beta-globin promoter but not to the CACCC box in the gamma-globin promoter: the role of the neighboring promoter elements

The programmed expression of the five beta-like globin genes (epsilon, (A)gamma, (G)gamma, delta, and beta) is characterized by a series of switches that are developmentally regulated. The (A)gamma- and (G)gamma- (fetus) to beta-globin (adult) switch depends on transcription factor erythroid Krüppel-like factor (EKLF), which, like Sp1, binds to CACCC boxes. EKLF is essential for the expression of the beta-globin but not the gamma-globin gene. Because both gamma-globin and beta-globin promoters contain the CACCC box, and their promoter elements are similar, it is not known why the two promoters behave so differently. In this report, we searched for the functional differences between the two promoters by studying their ability to recruit EKLF. We used the in vivo PIN*POINT assay to show that EKLF is recruited to the beta-globin promoter but not to the gamma-globin promoter. We show that this selectivity is a result of differences in surrounding promoter elements and not CACCC box alone. One of the differences between the two promoters with a functional consequence is the CCTTG repeat that is present in the gamma-globin promoter but not in the beta-globin promoter. The repeat, when inserted in the beta-globin promoter, decreases EKLF recruitment to and activity of the beta-globin promoter, suggesting that the repeat functions as a suppressor element. The CCTTG repeat can also suppress the SV40 promoter in cis, and the suppressor factor binding to the repeat can be squelched with a plasmid containing a high copy number of the repeat. These findings may have implications in designing drug targets for treatment of beta-globin disorders.

Transactivation and growth suppression by the gut-enriched Krüppel-like factor (Krüppel-like factor 4) are dependent on acidic amino acid residues and protein-protein interaction

Gut-enriched Krüppel-like factor (GKLF or KLF4) is a pleiotropic (activating and repressive) transcription factor. This study characterizes the mechanisms of transactivation by GKLF. Using a GAL4 fusion assay, the activating domain of murine GKLF was localized to the 109 amino acid residues in the N-terminus. Site-directed mutagenesis showed that two adjacent clusters of acidic residues within this region are responsible for the activating effect. Transactivation by GKLF involves intermolecular interactions as demonstrated by the ability of wild-type, but not mutated, GKLF to compete with the N-terminal activation domain. In addition, wild-type adenovirus E1A, but not a mutated E1A that failed to bind p300/CBP, inhibited transactivation by the N-terminal 109 amino acids of GKLF, suggesting that p300/CBP are GKLF's interacting partners. A physical interaction between GKLF and CBP was demonstrated by glutathione- S -transferase pull-down and by in vivo co-immuno-precipitation experiments. We also showed that the two acidic amino acid clusters are essential for this interaction, since GKLF with mutations in these residues failed to co-immunoprecipitate with CBP. Importantly, the same mutations abrogated the ability of GKLF to suppress cell growth as determined by a colony suppression assay. These studies therefore provide plausible evidence for a structural and functional correlation between the transactivating and growth-suppressing effects of GKLF.

The GATA-E box-GATA motif in the EKLF promoter is required for in vivo expression

The erythroid Krüppel-like factor (EKLF) is a key regulatory protein in globin gene expression. This zinc finger transcription factor is required for expression of the adult β globin gene, and it has been suggested that it plays an important role in the developmental switch from fetal γ to adult β globin gene expression. We have previously described a sequence element in the distal promoter region of the mouse EKLF gene that is critical for the expression of this transcription factor. The element consists of an E box motif flanked by 2 GATA-1 binding sites. Here we demonstrate that mutation of the E box or the GATA-1 consensus sequences eliminates expression from the EKLF promoter in transgenic mice. These results confirm the importance of this activator element for in vivo expression of the EKLF gene.