Two tissue-specific factors bind the erythroid promoter of the human porphobilinogen deaminase gene

Ski negatively regulates erythroid differentiation through its interaction with GATA1

The Ski oncoprotein dramatically affects cell growth, differentiation, and/or survival. Recently, Ski was shown to act in distinct signaling pathways including those involving nuclear receptors, transforming growth factor beta, and tumor suppressors. These divergent roles of Ski are probably dependent on Ski's capacity to bind multiple partners with disparate functions. In particular, Ski alters the growth and differentiation program of erythroid progenitor cells, leading to malignant leukemia. However, the mechanism underlying this important effect has remained elusive. Here we show that Ski interacts with GATA1, a transcription factor essential in erythropoiesis. Using a Ski mutant deficient in GATA1 binding, we show that this Ski-GATA1 interaction is critical for Ski's ability to repress GATA1-mediated transcription and block erythroid differentiation. Furthermore, the repression of GATA1-mediated transcription involves Ski's ability to block DNA binding of GATA1. This finding is in marked contrast to those in previous reports on the mechanism of repression by Ski, which have described a model involving the recruitment of corepressors into DNA-bound transcription complexes. We propose that Ski cooperates in the process of transformation in erythroid cells by interfering with GATA1 function, thereby contributing to erythroleukemia.

Sequences downstream of the erythroid promoter are required for high level expression of the human alpha-spectrin gene

Alpha-spectrin is a membrane protein critical for the flexibility and stability of the erythrocyte. We are attempting to identify and characterize the molecular mechanisms controlling the erythroid-specific expression of the alpha-spectrin gene. Previously, we demonstrated that the core promoter of the human alpha-spectrin gene directed low levels of erythroid-specific expression only in the early stages of erythroid differentiation. We have now identified a region 3' of the core promoter that contains a DNase I hypersensitive site and directs high level, erythroid-specific expression in reporter gene/transfection assays. In vitro DNase I footprinting and electrophoretic mobility shift assays identified two functional GATA-1 sites in this region. Both GATA-1 sites were required for full activity, suggesting that elements binding to each site interact in a combinatorial manner. This region did not demonstrate enhancer activity in any orientation or position relative to either the alpha-spectrin core promoter or the thymidine kinase promoter in reporter gene assays. In vivo studies using chromatin immunoprecipitation assays demonstrated hyperacetylation of this region and occupancy by GATA-1 and CBP (cAMP-response element-binding protein (CREB)-binding protein). These results demonstrate that a region 3' of the alpha-spectrin core promoter contains a GATA-1-dependent positive regulatory element that is required in its proper genomic orientation. This is an excellent candidate region for mutations associated with decreased alpha-spectrin gene expression in patients with hereditary spherocytosis and hereditary pyropoikilocytosis.

Inhibitory effect of tellimagrandin I on chemically induced differentiation of human leukemia K562 cells

Tellimagrandin I is a hydrolysable tannin compound widely present in plants. In this study, the effect of tellimagrandin I on chemically induced erythroid and megakaryocytic differentiation was investigated using K562 cells as differentiation model. It was found that tellimagrandin I not only inhibited the hemoglobin synthesis in butyric acid (BA)- and hemin-induced K562 cells with IC50 of 3 and 40microM, respectively, but also inhibited other erythroid differentiation marker including acetylcholinesterase (AChE) and glycophorin A (GPA) in BA-induced K562 cells. Tellimagrandin I also inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced expression of CD61 protein, a megakaryocytic marker. RT-PCR analysis showed that tellimagrandin I decreased the expression of erythroid genes (gamma-globin and porphobilinogen deaminase (PBGD)) and related transcription factors (GATA-1 and NF-E2) in BA-induced K562 cells, whereas tellimagrandin I induced the overexpresison of GATA-2 transcription factor that played negative regulation on erythroid differentiation. These results indicated that tellimagrandin I had inhibitory effects on erythroid and megakaryocytic differentiation, which suggested that tannins like tellimagrandin I might influence the anti-tumor efficiency of some drugs and the hematopoiesis processes.

Dynamic changes in transcription factor complexes during erythroid differentiation revealed by quantitative proteomics

During erythroid differentiation, beta-globin gene expression is regulated by the locus control region (LCR). The transcription factor NF-E2p18/MafK binds within this region and is essential for beta-globin expression in murine erythroleukemia (MEL) cells. Here we use the isotope-coded affinity tag (ICAT) technique of quantitative mass spectrometry to compare proteins interacting with NF-E2p18/MafK during differentiation. Our results define MafK as a 'dual-function' molecule that shifts from a repressive to an activating mode during erythroid differentiation. The exchange of MafK dimerization partner from Bach1 to NF-E2p45 is a key step in the switch from the repressed to the active state. This shift is associated with changes in the interaction of MafK with co-repressors and co-activators. Thus, our results suggest that in addition to its role as a cis-acting activator of beta-globin gene expression in differentiated erythroid cells, the LCR also promotes an active repression of beta-globin transcription in committed cells before terminal differentiation.

Transcriptional activities of cholecystokinin promoter haplotypes and their relevance to panic disorder susceptibility

We previously identified a polymorphic compound short tandem repeat (STR) in the 5'-regulatory region of the cholecystokinin (CCK) gene, and showed that when the STRs were classified into three groups based on length and linkage disequilibrium behavior with neighboring variants, the medium class allele was significantly associated with panic disorder. The present study examined the transcriptional activity of the CCK promoter construct containing the STR and downstream -188A/G variation. The STRs acted as transcriptional repressors with a similar potency among the three classes, but the long (L) class STR exhibited a synergistic effect on decreasing promoter activity when combined with -188G. The haplotype composed of the L class of STR and -188G was significantly less frequent in panic disorder (P = 0.0032; odds ratio, 95% confidence interval = 0.06, 0.01-0.69). These results suggest that the L-(-188G) haplotype may act as a protective factor against panic by reducing the expression of anxiogenic CCK.

Cyclin E expression during development in Caenorhabditis elegans

Our interest in the coordination of cell cycle control and differentiation has led us to investigate the Caenorhabditis elegans cye-1 gene encoding the G(1) cell cycle regulator cyclin E. We have studied the expression and function of cye-1 by using monoclonal antibodies directed against CYE-1 protein, cye-1::GFP reporter genes, and a cye-1 chromosomal deletion mutation. We show that a ubiquitous embryonic pattern of expression becomes restricted and dynamic during postembryonic development. Promoter analysis reveals a relatively small region of cis-acting sequences that are necessary for the complex pattern of expression of this gene. Our studies demonstrate that two other G(1) cell cycle genes, encoding cyclin D and CDK4/6, have similarly compact promoter requirements. This suggests that a relatively simple mechanism of regulation may underlie the dynamic developmental patterns of expression exhibited by these three G(1) cell cycle genes. Our analysis of a new cye-1 deletion allele confirms and extends previous studies of two point mutations in the gene.

Identification of distal regulatory regions in the human alpha IIb gene locus necessary for consistent, high-level megakaryocyte expression

The alphaIIb/beta3-integrin receptor is present at high levels only in megakaryocytes and platelets. Its presence on platelets is critical for hemostasis. The tissue-specific nature of this receptor's expression is secondary to the restricted expression of alphaIIb, and studies of the alphaIIb proximal promoter have served as a model of a megakaryocyte-specific promoter. We have examined the alphaIIb gene locus for distal regulatory elements. Sequence comparison between the human (h) and murine (m) alphaIIb loci revealed high levels of conservation at intergenic regions both 5' and 3' to the alphaIIb gene. Additionally, deoxyribonuclease (DNase) I sensitivity mapping defined tissue-specific hypersensitive (HS) sites that coincide, in part, with these conserved regions. Transgenic mice containing various lengths of the h(alpha)IIb gene locus, which included or excluded the various conserved/HS regions, demonstrated that the proximal promoter was sufficient for tissue specificity, but that a region 2.5 to 7.1 kb upstream of the h(alpha)IIb gene was necessary for consistent expression. Another region 2.2 to 7.4 kb downstream of the gene enhanced expression 1000-fold and led to levels of h(alpha)IIb mRNA that were about 30% of the native m(alpha)IIb mRNA level. These constructs also resulted in detectable h(alpha)IIb/m(beta)3 on the platelet surface. This work not only confirms the importance of the proximal promoter of the alphaIIb gene for tissue specificity, but also characterizes the distal organization of the alphaIIb gene locus and provides an initial localization of 2 important regulatory regions needed for the expression of the alphaIIb gene at high levels during megakaryopoiesis.

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.

Locus control regions

Locus control regions (LCRs) are operationally defined by their ability to enhance the expression of linked genes to physiological levels in a tissue-specific and copy number-dependent manner at ectopic chromatin sites. Although their composition and locations relative to their cognate genes are different, LCRs have been described in a broad spectrum of mammalian gene systems, suggesting that they play an important role in the control of eukaryotic gene expression. The discovery of the LCR in the beta-globin locus and the characterization of LCRs in other loci reinforces the concept that developmental and cell lineage-specific regulation of gene expression relies not on gene-proximal elements such as promoters, enhancers, and silencers exclusively, but also on long-range interactions of various cis regulatory elements and dynamic chromatin alterations.

Molecular aspects of embryonic hemoglobin function

In order to provide the appropriate level of oxygen transport to respiring tissues, we need to produce a molecular oxygen transporting system to supplement oxygen diffusion and solubility. This supplementation is provided by hemoglobin. The role of hemoglobin in providing oxygen transport from lung to tissues in the adult is well-documented and functional characteristics of the fetal hemoglobin, which provide placental oxygen exchange, are also well understood. However the characteristics of the three embryonic hemoglobins, which provide oxygen transport during the first three months of gestation, are not well recognized. This review seeks to describe the state of our understanding of the temporal control of the expression of these proteins and the oxygen binding characteristics of the individual protein molecules. The modulation of the oxygen binding properties of these proteins, by the various allosteric effectors, is described and the structural origins of these characteristics are probed.

Integration and diversity of the regulatory network composed of Maf and CNC families of transcription factors

Recent progress in the analysis of transcriptional regulation has revealed the presence of an exquisite functional network comprising the Maf and Cap 'n' collar (CNC) families of regulatory proteins, many of which have been isolated. Among Maf factors, large Maf proteins are important in the regulation of embryonic development and cell differentiation, whereas small Maf proteins serve as obligatory heterodimeric partner molecules for members of the CNC family. Both Maf homodimers and CNC-small Maf heterodimers bind to the Maf recognition element (MARE). Since the MARE contains a consensus TRE sequence recognized by AP-1, Jun and Fos family members may act to compete or interfere with the function of CNC-small Maf heterodimers. Overall then, the quantitative balance of transcription factors interacting with the MARE determines its transcriptional activity. Many putative MARE-dependent target genes such as those induced by antioxidants and oxidative stress are under concerted regulation by the CNC family member Nrf2, as clearly proven by mouse germline mutagenesis. Since these genes represent a vital aspect of the cellular defense mechanism against oxidative stress, Nrf2-null mutant mice are highly sensitive to xenobiotic and oxidative insults. Deciphering the molecular basis of the regulatory network composed of Maf and CNC families of transcription factors will undoubtedly lead to a new paradigm for the cooperative function of transcription factors.

Regulatory heme and trichloroethylene intoxication: A possible explanation of the case of "A Civil Action"

In 1998, a amovie entitled "A Civil Action" was released. The movie described the Woburn case, begun in 1982 and concluded in 1990, one of the most famous cases of trichloroethylene pollution. In a small town near Boston, twelve children died of leukemia, which seemed attributable to trichloroethylene contamination of the drinking water. The victims, however, could not win the case, since evidence that the identified chemicals could cause leukemia and other human illnesses was rather sketchy. There have been many cases of trichloroethylene pollution in industrial nations including Japan, therefore, we reconsidered the missing link. Our conclusion is that the disease occurred not by a direct effect of the chemical hazard on biological macromolecules but by an indirect effect through the physiological system such as signal transduction and transcriptional regulation. In 1984, we reported a marked reduction in the regulatory heme pool by trichloroethylene exposure, however, the biological significance was not well understood. Recently, we found that the DNA binding activity of Bach1, a negative regulator of genes, is controlled by heme, the regulation of which seems to explain how leukemia develops. The heterodimer of Bach1 with MafK recognizes Maf recognition elements (MAREs) competing with the erythroid type positive regulator, a complex of NF-E2 with MafK. Bach1/MafK occupies MAREs under lower heme conditions, whereas MAREs are open to NF-E2/MafK along with increasing heme concentration. Since the NF-E2/MafK function is closely related to normal erythroid differentiation, of which disorders such as sideroblastic anemia are often related to neoplasia; i.e., a clonal disorder that can progress to leukemia. Thus, a marked decline in regulatory heme by trichloroethylene intoxication could be one of the pathways to leukemia.

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.

In vivo formation of a human beta-globin locus control region core element requires binding sites for multiple factors including GATA-1, NF-E2, erythroid Kruppel-like factor, and Sp1

The active elements of the beta-globin locus control region (LCR) are located within domains of unique chromatin structure. These nuclease hypersensitive sites (HSs) are characterized by high DNase I sensitivity, erythroid specificity, similar nucleosomal structure, and evolutionarily conserved clusters of cis-acting elements that are required for the formation and function of the core elements. To determine the requirements for HS core formation in the setting of nuclear chromatin, we constructed a series of artificial HS cores containing binding sites for GATA-1, NF-E2, and Sp1. In contrast to the results of previous in vitro experiments, we found that when constructs were stably integrated in mouse erythroleukemia cells the binding sites for NF-E2, GATA-1, or Sp1 alone or in any combination were unable to form core HS structures. We subsequently identified two new cis-acting elements from the LCR HS4 core that, when combined with the NF-E2, Sp1, and tandem inverted GATA elements, result in core structure formation. Both new cis-acting elements bind Sp1, and one binds erythroid Kruppel-like factor (EKLF). We conclude that in vivo beta-globin LCR HS core formation is more complex than previously thought and that several factors are required for this process to occur.

Hemin-induced activation of the thioredoxin gene by Nrf2. A differential regulation of the antioxidant responsive element by a switch of its binding factors

Thioredoxin plays an important role in various cellular processes through redox regulation. Here, we have demonstrated that thioredoxin expression is transcriptionally induced in K562 cells by hemin (ferriprotoporphyrin IX) through activation of a regulatory region positioned from -452 to -420 bp of the thioredoxin gene. Insertion of a mutation in the antioxidant responsive element (ARE)/AP-1 consensus binding sequence in this region abolished the response to hemin. With electrophoretic mobility shift and DNA affinity assays, we have shown that the NF-E2p45/small Maf complex constitutively binds to the ARE. The binding of the Nrf2/small Maf complex to ARE was induced by hemin, whereas the binding of Jun/Fos proteins to ARE was induced by phorbol 12-myristate 13-acetate, but not hemin. Hemin induced nuclear translocation of Nrf2 but did not affect nuclear expression of Jun/Fos proteins. Overexpression of Nrf2 augmented the response to hemin in a dose-dependent manner. In contrast, overexpression of the dominant negative mutant of Nrf2 suppressed hemin-induced activation through the ARE. We show here hemin-induced activation of the thioredoxin gene by Nrf2 through the ARE and propose a novel mechanism of the regulation of the ARE through a switch of its binding factors.

Transcriptional regulation in megakaryocytes: the thrombopoietin receptor gene as a model

The MPL gene codes for the thrombopoietin receptor, whose ligand specifically controls megakaryocytic differentiation. In order to understand the molecular basis for the megakaryocyte-specific expression of MPL, we analyzed the regulatory elements of this gene. Two regions are hypersensitive to DNase I in nuclei of cells that express MPL: the promoter and a portion of intron 6. The latter behaves as a chromatin-dependent enhancer. A 200 bp fragment of the promoter is sufficient for high-level specific expression. This fragment can bind several transacting factors in vitro, including GATA-1 and members of the Ets family. GATA-1 binds with low affinity to a unique GATA motif at -70 in the MPL promoter, and destruction of this site yields only a modest decrease in expression level in human erythroleukemia (HEL) cells. Ets proteins also bind with low affinity to two sites. One is located at position -15 and its destruction reduces expression to 50%; the other is located immediately downstream of the GATA motif and plays a crucial role in expression of the promoter in HEL cells, as its inactivation reduces expression to 15%. This study indicates a molecular basis for the coregulation of markers of megakaryocyte differentiation. Finally, we describe other nuclear factor binding sites that may be involved in the cell-type-specific expression of MPL.

Cyclin D3 and megakaryocyte development: exploration of a transgenic phenotype

The roles of cell cycle regulatory proteins in megakaryocyte development are poorly understood. We have previously demonstrated that cyclin D3 is expressed in megakaryocytes and is induced upon treatment with Mpl ligand. Transgenic mice in which cyclin D3 is overexpressed in the megakaryocytic lineage show features similar to in vivo Mpl ligand treatment, including increased megakaryocyte number and ploidy. Terminal maturation and platelet production are not enhanced, however, and transgenic megakaryocytes show a defect in demarcation membrane development. We have examined expression of the transcription factor nuclear factor (NF)-E2, known to be involved in cytoplasmic maturation and platelet fragmentation, in these transgenic mice and controls treated with Mpl ligand. Our findings demonstrate marked induction of NF-E2 mRNA in control megakaryocytes in response to Mpl ligand, but no NF-E2 increase in transgenic cells, potentially explaining the lack of platelet increase in these transgenic mice. Transgenic megakaryocytes treated with Mpl ligand display a limited increase in NF-E2. In response to literature reports of Mpl ligand-induced transient increases in p21Cip1/WAF1 mRNA in polyploidizing megakaryocytic cell lines, we have examined p21 transcript levels in both normal and transgenic megakaryocytes. In normal mouse spleen, only a small percentage of megakaryocytes express detectable levels of p21 mRNA, with the majority of these cells expressing at high intensity. p21 levels are not affected by treatment with Mpl ligand, while the frequency of expressing cells increases transiently. Transgenic megakaryocytes exposed to Mpl ligand also show an increased frequency of p21-positive cells, and stimulation with Mpl ligand resulted in a further increase in this frequency. The nature of this effect will require further investigation.

Nuclear factors bind to a conserved DNA element that modulates transcription of Anopheles gambiae trypsin genes

The Anopheles gambiae trypsin family consists of seven genes that are transcribed in the gut of female mosquitoes in a temporal coordinated and mutually exclusive manner, suggesting the involvement of a complex transcription regulatory mechanism. We identified a highly conserved 12-nucleotide motif present in all A. gambiae and Anopheles stephensi trypsin promoters. We investigated the role of this putative trypsin regulatory element (PTRE) in controlling the transcription of the trypsin genes. Gel shift experiments demonstrated that nuclear proteins of A. gambiae cell lines formed two distinct complexes with probes encompassing the PTRE sequence. Mapping of the binding sites revealed that one of the complex has the specificity of a GATA transcription factor. Promoter constructs containing mutations in the PTRE sequence that selectively abolished the binding of either one or both complexes exerted opposite effects on the transcriptional activity of trypsin promoters in A. gambiae and Aedes aegypti cell lines. In addition, the expression of a novel GATA gene was highly enriched in A. gambiae guts. Taken together our data prove that factors binding to the PTRE region are key regulatory elements possibly involved in the blood meal-induced repression and activation of transcription in early and late trypsin genes.

p45(NFE2) is a negative regulator of erythroid proliferation which contributes to the progression of Friend virus-induced erythroleukemias

In previous studies, we identified a common site of retroviral integration designated Fli-2 in Friend murine leukemia virus (F-MuLV)-induced erythroleukemia cell lines. Insertion of F-MuLV at the Fli-2 locus, which was associated with the loss of the second allele, resulted in the inactivation of the erythroid cell- and megakaryocyte-specific gene p45(NFE2). Frequent disruption of p45(NFE2) due to proviral insertion suggests a role for this transcription factor in the progression of Friend virus-induced erythroleukemias. To assess this possibility, erythroleukemia was induced by F-MuLV in p45(NFE2) mutant mice. Since p45(NFE2) homozygous mice mostly die at birth, erythroleukemia was induced in +/- and +/+ mice. We demonstrate that +/- mice succumb to the disease moderately but significantly faster than +/+ mice. In addition, the spleens of +/- mice were significantly larger than those of +/+ mice. Of the 37 tumors generated from the +/- and +/+ mice, 10 gave rise to cell lines, all of which were derived from +/- mice. Establishment in culture was associated with the loss of the remaining wild-type p45(NFE2) allele in 9 of 10 of these cell lines. The loss of a functional p45(NFE2) in these cell lines was associated with a marked reduction in globin gene expression. Expression of wild-type p45(NFE2) in the nonproducer erythroleukemic cells resulted in reduced cell growth and restored the expression of globin genes. Similarly, the expression of p45(NFE2) in these cells also slows tumor growth in vivo. These results indicate that p45(NFE2) functions as an inhibitor of erythroid cell growth and that perturbation of its expression contributes to the progression of Friend erythroleukemia.

Ectopic expression of transcription factor NF-E2 alters the phenotype of erythroid and monoblastoid cells

In this study, regulation of transcription factor NF-E2 was examined in differentiating erythroid and myeloid cells, and the impact of raising NF-E2 concentrations within these cell types was assessed. NF-E2 was expressed in the J2E erythroid cell line, but the levels increased only marginally during erythropoietin-induced differentiation. In contrast, rare myeloid variants of J2E cells did not express NF-E2. Although NF-E2 was present in M1 monoblastoid cells, it was undetectable as these cells matured into macrophages. Compared with erythroid cells, transcription of the NF-E2 gene was reduced, and the half-life of the mRNA was significantly shorter in monocytoid cells. Ectopic expression of NF-E2 had a profound impact upon the J2E cells; morphologically mature erythroid cells spontaneously emerged in culture, but the cells failed to synthesize hemoglobin, even in the presence of erythropoietin. Although proliferation and viability increased in the NF-E2-transfected J2E cells, their responsiveness to erythropoietin was severely diminished. Strikingly, increasing the expression of NF-E2 in M1 cells produced sublines that contained erythroid or immature megakaryocytic cells. Finally, overexpression of NF-E2 in primary hemopoietic progenitors from fetal liver increased erythroid colony formation in the absence of erythropoietin. These data demonstrate that elevated NF-E2 (i) had a dominant effect on the phenotype and maturation of J2E erythroid cells, (ii) was able to reprogram the M1 monocytoid line, and (iii) promoted the development of erythroid colonies by normal progenitors.

Hematopoietic-specific β1 tubulin participates in a pathway of platelet biogenesis dependent on the transcription factor NF-E2

The cellular and molecular bases of platelet release by terminally differentiated megakaryocytes represent important questions in cell biology and hematopoiesis. Mice lacking the transcription factor NF-E2 show profound thrombocytopenia, and their megakaryocytes fail to produce proplatelets, the microtubule-based precursors of blood platelets. Using mRNA subtraction between normal and NF-E2–deficient megakaryocytes, cDNA was isolated encoding β1 tubulin, the most divergent β tubulin isoform. In NF-E2–deficient megakaryocytes, β1 tubulin mRNA and protein are virtually absent. The expression of β1 tubulin is exquisitely restricted to platelets and megakaryocytes, where it appears late in differentiation and localizes to microtubule shafts and coils within proplatelets. Restoring NF-E2 activity in a megakaryoblastic cell line or in NF-E2–deficient primary megakaryocytes rescues the expression of β1 tubulin. Re-expressing β1 tubulin in isolation does not, however, restore proplatelet formation in the defective megakaryocytes, indicating that other critical factors are required; indeed, other genes identified by mRNA subtraction also encode structural and regulatory components of the cytoskeleton. These findings provide critical mechanistic links between NF-E2, platelet formation, and selected microtubule proteins, and they also provide novel molecular insights into thrombopoiesis.

Hematopoietic-specific β1 tubulin participates in a pathway of platelet biogenesis dependent on the transcription factor NF-E2

The cellular and molecular bases of platelet release by terminally differentiated megakaryocytes represent important questions in cell biology and hematopoiesis. Mice lacking the transcription factor NF-E2 show profound thrombocytopenia, and their megakaryocytes fail to produce proplatelets, the microtubule-based precursors of blood platelets. Using mRNA subtraction between normal and NF-E2–deficient megakaryocytes, cDNA was isolated encoding β1 tubulin, the most divergent β tubulin isoform. In NF-E2–deficient megakaryocytes, β1 tubulin mRNA and protein are virtually absent. The expression of β1 tubulin is exquisitely restricted to platelets and megakaryocytes, where it appears late in differentiation and localizes to microtubule shafts and coils within proplatelets. Restoring NF-E2 activity in a megakaryoblastic cell line or in NF-E2–deficient primary megakaryocytes rescues the expression of β1 tubulin. Re-expressing β1 tubulin in isolation does not, however, restore proplatelet formation in the defective megakaryocytes, indicating that other critical factors are required; indeed, other genes identified by mRNA subtraction also encode structural and regulatory components of the cytoskeleton. These findings provide critical mechanistic links between NF-E2, platelet formation, and selected microtubule proteins, and they also provide novel molecular insights into thrombopoiesis.

The human ankyrin-1 gene is selectively transcribed in erythroid cell lines despite the presence of a housekeeping-like promoter

To begin to study the sequence variations identified in the 5′ flanking genomic DNA of the ankyrin gene in ankyrin-deficient hereditary spherocytosis patients and to provide additional insight into our understanding of the regulation of genes encoding erythrocyte membrane proteins, we have identified and characterized the erythroid promoter of the human ankyrin-1 gene. This compact promoter has characteristics of a housekeeping gene promoter, including very high G+C content and enzyme restriction sites characteristic of an HTF-island, no TATA, InR, or CCAAT consensus sequences, and multiple transcription initiation sites. In vitro DNAseI footprinting analyses revealed binding sites for GATA-1, CACCC-binding, and CGCCC-binding proteins. Transfection of ankyrin promoter/reporter plasmids into tissue culture cell lines yielded expression in erythroid, but not muscle, neural, or HeLa cells. Electrophoretic mobility shift assays, including competition and antibody supershift experiments, demonstrated binding of GATA-1, BKLF, and Sp1 to core ankyrin promoter sequences. In transfection assays, mutation of the Sp1 site had no effect on reporter gene expression, mutation of the CACCC site decreased expression by half, and mutation of the GATA-1 site completely abolished activity. The ankyrin gene erythroid promoter was transactivated in heterologous cells by forced expression of GATA-1 and to a lesser degree BKLF.

The human ankyrin-1 gene is selectively transcribed in erythroid cell lines despite the presence of a housekeeping-like promoter

To begin to study the sequence variations identified in the 5′ flanking genomic DNA of the ankyrin gene in ankyrin-deficient hereditary spherocytosis patients and to provide additional insight into our understanding of the regulation of genes encoding erythrocyte membrane proteins, we have identified and characterized the erythroid promoter of the human ankyrin-1 gene. This compact promoter has characteristics of a housekeeping gene promoter, including very high G+C content and enzyme restriction sites characteristic of an HTF-island, no TATA, InR, or CCAAT consensus sequences, and multiple transcription initiation sites. In vitro DNAseI footprinting analyses revealed binding sites for GATA-1, CACCC-binding, and CGCCC-binding proteins. Transfection of ankyrin promoter/reporter plasmids into tissue culture cell lines yielded expression in erythroid, but not muscle, neural, or HeLa cells. Electrophoretic mobility shift assays, including competition and antibody supershift experiments, demonstrated binding of GATA-1, BKLF, and Sp1 to core ankyrin promoter sequences. In transfection assays, mutation of the Sp1 site had no effect on reporter gene expression, mutation of the CACCC site decreased expression by half, and mutation of the GATA-1 site completely abolished activity. The ankyrin gene erythroid promoter was transactivated in heterologous cells by forced expression of GATA-1 and to a lesser degree BKLF.

Direct interaction of NF-E2 with hypersensitive site 2 of the β-globin locus control region in living cells

The human β-globin locus control region (LCR) confers high-level, tissue-specific expression to the β-globin genes. Tandem Maf recognition elements (MAREs) within the hypersensitive site 2 (HS2) subregion of the LCR are important for the strong enhancer activity of the LCR. Multiple proteins are capable of interacting with these sites in vitro, including the erythroid cell- and megakaryocyte-specific transcription factor, NF-E2. The importance of NF-E2 for β-globin gene expression is evident in murine erythroleukemia cells lacking the p45 subunit of NF-E2. These CB3 cells have a severe defect in - and β-globin gene transcription, which can be restored by expression of NF-E2. However, mice nullizygous for p45 express nearly normal levels of β-globin. Thus, either a redundant factor(s) exists in mice that can functionally replace NF-E2, or NF-E2 does not function through the LCR to regulate β-globin gene expression. To address this issue, we asked whether NF-E2 binds directly to the tandem MAREs of HS2 in intact cells. Using a chromatin immunoprecipitation assay, we provide evidence for NF-E2 binding directly and specifically to HS2 in living erythroleukemia cells and in mouse fetal liver. The specific immunoisolation of HS2 sequences was dependent on the presence of p45 and on intact MAREs within HS2. These results support a direct role for NF-E2 in the regulation of β-globin gene expression through activation of the LCR.

Direct interaction of NF-E2 with hypersensitive site 2 of the β-globin locus control region in living cells

The human β-globin locus control region (LCR) confers high-level, tissue-specific expression to the β-globin genes. Tandem Maf recognition elements (MAREs) within the hypersensitive site 2 (HS2) subregion of the LCR are important for the strong enhancer activity of the LCR. Multiple proteins are capable of interacting with these sites in vitro, including the erythroid cell- and megakaryocyte-specific transcription factor, NF-E2. The importance of NF-E2 for β-globin gene expression is evident in murine erythroleukemia cells lacking the p45 subunit of NF-E2. These CB3 cells have a severe defect in - and β-globin gene transcription, which can be restored by expression of NF-E2. However, mice nullizygous for p45 express nearly normal levels of β-globin. Thus, either a redundant factor(s) exists in mice that can functionally replace NF-E2, or NF-E2 does not function through the LCR to regulate β-globin gene expression. To address this issue, we asked whether NF-E2 binds directly to the tandem MAREs of HS2 in intact cells. Using a chromatin immunoprecipitation assay, we provide evidence for NF-E2 binding directly and specifically to HS2 in living erythroleukemia cells and in mouse fetal liver. The specific immunoisolation of HS2 sequences was dependent on the presence of p45 and on intact MAREs within HS2. These results support a direct role for NF-E2 in the regulation of β-globin gene expression through activation of the LCR.

One enhancer mediates mafK transcriptional activation in both hematopoietic and cardiac muscle cells

Members of the small Maf family of transcription factors play important roles in hematopoiesis. Using transgenic assays, we discovered a tissue-specific enhancer 3' to the mafK gene. This enhancer directs mafK transcription in hematopoietic as well as in developing cardiac muscle cells, and was thus designated the hematopoietic and cardiac enhancer of mafK (HCEK). Only two of four GATA consensus motifs identified within HCEK contributed to enhancer activity, and both of these sites were required for both cardiac and hematopoietic transcriptional activation. The expression profile of MafK significantly overlapped that of GATA-1 in hematopoietic cells and of GATA-4/-6 in cardiac tissues. Each of these GATA factors bound with high specificity to both of the critical GATA sites in HCEK. Hence, the mafK gene is regulated by different GATA proteins in the hematopoietic and cardiac compartments through the same two GATA-binding sites in HCEK. These data provide the first in vivo demonstration that distinct members of a related transcription factor family activate the tissue-specific expression of a single target gene using the same cis-regulatory element.

GATA-1 is a potential repressor of anti-Müllerian hormone expression during the establishment of puberty in the mouse

Anti-Müllerian hormone (AMH), also known as Müllerian inhibiting substance (MIS), is one of the earliest and best-known markers of Sertoli cell differentiation and is expressed until around puberty. The present study is aimed at the better understanding of the molecular pathways involved in testicular development and establishment of adult functions with regards to AMH regulation. We found, within the mouse AMH promoter, putative GATA motifs (A/T)GATA(A/G), known to be specifically bound by members of the GATA transcription factor family. We then carried out RNase protection assays and immunohistochemical techniques aimed at comparing precisely the chronological expression patterns of AMH and GATA-1, this latter being expressed in the testis after birth. Using both approaches we found an inverse and close relationship between AMH and GATA-1 mRNA and protein expression during the pre-pubertal period. These results allowed us to define a transitory 4-5-day period, starting from 3 dpp when both proteins are heterogeneously expressed in Sertoli cells and showed that the appearance of GATA-1 is associated with the decrease of AMH expression in these cells. Furthermore DNA-protein interaction in in vitro studies showed first that GATA-1 binds with various affinities on sites found in the AMH promoter and second that the proximity of the two strongest affinity sites leads to a synergistic binding effect. Altogether, the present study suggests that GATA-1 participates in AMH gene repression during the pre-pubertal period.

Analysis of ferrochelatase expression during hematopoietic development of embryonic stem cells

Ferrochelatase, the last enzyme in the heme pathway, chelates protoporphyrin IX and iron to form heme and is mutated in protoporphyria. The ferrochelatase gene is expressed in all tissues at low levels to provide heme for essential heme-containing proteins and is up-regulated during erythropoiesis for the synthesis of hemoglobin. The human ferrochelatase promoter contains 2 Sp1 cis-elements and GATA and NF–E2 sites, all of which bind their cognatetrans-acting factors in vitro. To investigate the role of these elements during erythropoiesis, we introduced expression of the green fluorescent protein (EGFP) transgenes driven by various ferrochelatase promoter fragments into a single locus in mouse embryonic stem cells. EGFP expression was monitored during hematopoietic differentiation in vitro using flow cytometry. We show that a promoter fragment containing the Sp1 sites, the NF–E2 and GATA elements, was sufficient to confer developmental-specific expression of the EGFP transgene, with an expression profile identical to that of the endogenous gene. In this system the −0.275 kb NF–E2 cis-element is required for erythroid-enhanced expression, the GATA cis-element functions as a stage-specific repressor and enhancer, and elements located between −0.375kb and −1.1kb are necessary for optimal levels of expression. Ferrochelatase mRNA increased before the primitive erythroid-cell stage without a concomitant increase in ferrochelatase protein, suggesting the presence of a translational control mechanism. Because of the sensitivity of this system, we were able to assess the effect of an A-to-G polymorphism identified in the promoters of patients with protoporphyria. There was no effect of the G haplotype on transcriptional activity of the −1.1 kb transgene.

Analysis of ferrochelatase expression during hematopoietic development of embryonic stem cells

Ferrochelatase, the last enzyme in the heme pathway, chelates protoporphyrin IX and iron to form heme and is mutated in protoporphyria. The ferrochelatase gene is expressed in all tissues at low levels to provide heme for essential heme-containing proteins and is up-regulated during erythropoiesis for the synthesis of hemoglobin. The human ferrochelatase promoter contains 2 Sp1 cis-elements and GATA and NF–E2 sites, all of which bind their cognatetrans-acting factors in vitro. To investigate the role of these elements during erythropoiesis, we introduced expression of the green fluorescent protein (EGFP) transgenes driven by various ferrochelatase promoter fragments into a single locus in mouse embryonic stem cells. EGFP expression was monitored during hematopoietic differentiation in vitro using flow cytometry. We show that a promoter fragment containing the Sp1 sites, the NF–E2 and GATA elements, was sufficient to confer developmental-specific expression of the EGFP transgene, with an expression profile identical to that of the endogenous gene. In this system the −0.275 kb NF–E2 cis-element is required for erythroid-enhanced expression, the GATA cis-element functions as a stage-specific repressor and enhancer, and elements located between −0.375kb and −1.1kb are necessary for optimal levels of expression. Ferrochelatase mRNA increased before the primitive erythroid-cell stage without a concomitant increase in ferrochelatase protein, suggesting the presence of a translational control mechanism. Because of the sensitivity of this system, we were able to assess the effect of an A-to-G polymorphism identified in the promoters of patients with protoporphyria. There was no effect of the G haplotype on transcriptional activity of the −1.1 kb transgene.

Regulation of mouse p45 NF-E2 transcription by an erythroid-specific GATA-dependent intronic alternative promoter

The erythroid-enriched transcription factor NF-E2 is composed of two subunits, p45 and p18, the former of which is mainly expressed in the hematopoietic system. We have isolated and characterized the mouse p45 NF-E2 gene; we show here that, similar to the human gene, the mouse gene has two alternative promoters, which are differentially active during development and in different hematopoietic cells. Transcripts from the distal promoter are present in both erythroid and myeloid cells; however, transcripts from an alternative proximal 1b promoter, lying in the first intron, are abundant in erythroid cells, but barely detectable in myeloid cells. During development, both transcripts are detectable in yolk sac, fetal liver, and bone marrow. Transfection experiments show that proximal promoter 1b has a strong activity in erythroid cells, which is completely dependent on the integrity of a palindromic GATA-1 binding site. In contrast, the distal promoter 1a is not active in this assay. When the promoter 1b is placed 3' to the promoter 1a and reporter gene, in an arrangement that resembles the natural one, it acts as an enhancer to stimulate the activity of the upstream promoter la.

Regulatory mechanisms of cellular response to oxidative stress

An antioxidant responsive element (ARE) or electrophile responsive element (EpRE) mediates the transcriptional activation of genes encoding phase II drug metabolizing enzymes. The ARE consensus sequence shows high similarity to an erythroid gene regulatory element, and based on the observation, we have recently found that transcription factor Nrf2 is essential for the coordinate induction of phase II detoxifying enzymes. The expression of anti-oxidative stress enzyme genes is also regulated by Nrf2. Detailed analysis of the regulatory mechanisms of Nrf2 activity has ultimately led us to the identification of a new protein, which we have named Keap1, that suppresses Nrf2 activity by specific binding to its evolutionarily-conserved N-terminal Neh2 regulatory domain.

Up-regulation of the human gamma-glutamylcysteine synthetase regulatory subunit gene involves binding of Nrf-2 to an electrophile responsive element

The rate-limiting step in the de novo synthesis of the cellular protectant glutathione is catalyzed by gamma-glutamylcysteine synthetase (GCS; also known as glutamine-L-cysteine ligase, GLCL), a heterodimer consisting of catalytic (GCS(h)) and regulatory (GCS(l)) subunits. Regulation of expression of the human gamma-glutamylcysteine synthetase regulatory subunit gene in response to beta-NF is mediated by an Electrophile Responsive Element (EpRE) [Moinova, H., and Mulcahy, R. T. (1998) J. Biol. Chem. 273, 14683-14689]. Oligonucleotide probes corresponding to wild-type and mutant EpRE sequences were used in gel-shift and super-shift analyses to identify proteins binding. Four protein:DNA complexes (a-d) with distinct mobilities were detected when the wild-type EpRE probe was incubated with nuclear extracts from control or beta-NF-treated HepG2 cells. Following beta-NF treatment, there was an increase in the intensity of a single band, band b. This band was eliminated in gel shifts employing mutant EpRE probes which abolish beta-NF inducibility, demonstrating a correlation between band b and transactivation. Super-shift analysis identified JunD, Nrf1, and Nrf2 in the EpRE-binding complexes. Antibodies to Nrf2 completely super-shifted the band b protein:DNA complex. These studies demonstrate that Nrf2 proteins recognize and bind the GCS(l) EpRE sequence to affect transactivation of the gene.

Induction of erythrocyte protein 4.2 gene expression during differentiation of murine erythroleukemia cells

Protein 4.2 (P4.2) is an important component in the erythrocyte membrane skeletal network that regulates the stability and flexibility of erythrocytes. Recently, we provided the evidence for specific P4.2 expression in erythroid cells during development (L. Zhu et al., 1998, Blood 91, 695-705). Using dimethyl sulfoxide (DMSO)-induced differentiation of murine erythroleukemia (MEL) cells as a model, transcription of the P4.2 gene was found to be induced during erythroid differentiation. To examine the mechanism for this induction, we isolated the mouse P4.2 genomic DNA containing the 5' flanking sequence and defined the location of the P4.2 promoter. Transcription of the mouse P4.2 gene initiates at multiple sites, with the major initiation site mapped at 174 nucleotides upstream of the ATG start codon. The mouse P4.2 promoter is TATA-less and contains multiple potential binding sites for erythroid transcription factors GATA-1, NF-E2, EKLF, and tal-1/SCL. Transient transfection experiments demonstrated that a 1.7-kb mouse P4.2 promoter fused with the luciferase coding regions was induced in DMSO-treated MEL cells. Deletion analysis showed that a 259-bp P4.2 promoter DNA (nucleotide position -88 to +171 relative to the major transcription initiation site designated +1), containing a GATA-binding site at position -29 to -24, could still respond to the induction in differentiated MEL cells. Importantly, mutations in the -29/-24 GATA motif rendered the promoter unresponsive to DMSO induction. Electrophoretic mobility shift assay revealed that GATA-1 could bind to the -29/-24 GATA motif and this was confirmed by the observation that the nuclear protein bound to the motif was supershifted by an anti-GATA-1 monoclonal antibody. Taken together, these results suggest that the erythroid transcription factor GATA-1 plays an important role in the induction of P4.2 gene expression during erythroid cell differentiation.

Guanine-adenine ligation-mediated polymerase chain reaction in vivo footprinting

The analysis of functional DNA regulatory sequences involved in transcriptional control is critical to establishing which proteins mediate cell-specific gene expression. The organization of erythroid LCRs is complex, consisting of multiple, interdigested cis elements. As in situ binding to these sites is determined by the accessibility of these regulatory regions in native chromatin and the availability of relevent cell-specific and ubiquitous factors, in vivo footprinting was used to define protein DNA interactions in human globin LCRs. To further enhance the detection of protein contacts with this technique, we have modified the dimethyl sulfate-based ligation-mediated PCR in vivo footprinting procedure to permit the assessment of protein binding at guanine and adenine resides, rather than exclusively at guanines. This modification, termed GA-LMPCR in vivo footprinting, was essential for the analysis of GATA-1 motifs in the alpha-LCR and HS-3 of the beta-LCR. Moreover, GA-LMPCR in vivo footprinting provided high-resolution analysis of AP-1/NF-E2 elements and revealed protein contacts at sequences that are not coincident with previously described regulatory motifs. A comprehensive discussion of this modification and sample illustrations from our studies have been presented to demonstrate the enhanced detection and resolution obtained with this procedure.

A human beta-spectrin gene promoter directs high level expression in erythroid but not muscle or neural cells

beta-Spectrin is an erythrocyte membrane protein that is defective in many patients with abnormalities of red blood cell shape including hereditary spherocytosis and elliptocytosis. It is expressed not only in erythroid tissues but also in muscle and brain. We wished to determine the regulatory elements that determine the tissue-specific expression of the beta-spectrin gene. We mapped the 5'-end of the beta-spectrin erythroid cDNA and cloned the 5'-flanking genomic DNA containing the putative beta-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, a beta-spectrin gene erythroid promoter with two binding sites for GATA-1 and one site for CACCC-related proteins was identified. All three binding sites were required for full promoter activity; one of the GATA-1 motifs and the CACCC-binding motif were essential for activity. The beta-spectrin gene promoter was able to be transactivated in heterologous cells by forced expression of GATA-1. In transgenic mice, a reporter gene directed by the beta-spectrin promoter was expressed in erythroid tissues at all stages of development. Only weak expression of the reporter gene was detected in muscle and brain tissue, suggesting that additional regulatory elements are required for high level expression of the beta-spectrin gene in these tissues.

The Megakaryocyte/Platelet-Specific Enhancer of the 2β1 Integrin Gene: Two Tandem AP1 Sites and the Mitogen-Activated Protein Kinase Signaling Cascade

The 2β1 integrin, a collagen receptor on platelets and megakaryocytes, is required for normal platelet function. Transcriptional regulation of the 2 integrin gene in cells undergoing megakaryocytic differentiation requires a core promoter between bp −30 and −92, a silencer between bp −92 and −351, and megakaryocytic enhancers in the distal 5′ flank. We have now identified a 229-bp region of the distal 5′ flank of the 2 integrin gene required for high-level enhancer activity in cells with megakaryocytic features. Two tandem AP1 binding sites with dyad symmetry are required for enhancer activity and for DNA-protein complex formation with members of the c-fos/c-jun family. The requirement for AP1 activation suggested a role for the mitogen-activated protein kinase (MAPK) signaling pathway in regulating 2 integrin gene expression. Inhibition of the MAP kinase cascade with PD98059, a specific inhibitor of MAPK kinase 1, prevented the expression of the 2 integrin subunit in cells induced to become megakaryocytic. We provide a model of megakaryocytic differentiation in which expression of the 2 integrin gene requires signaling via the MAP kinase pathway to activate two tandem AP1 binding sites in the 2 integrin enhancer.

The Megakaryocyte/Platelet-Specific Enhancer of the 2β1 Integrin Gene: Two Tandem AP1 Sites and the Mitogen-Activated Protein Kinase Signaling Cascade

The 2β1 integrin, a collagen receptor on platelets and megakaryocytes, is required for normal platelet function. Transcriptional regulation of the 2 integrin gene in cells undergoing megakaryocytic differentiation requires a core promoter between bp −30 and −92, a silencer between bp −92 and −351, and megakaryocytic enhancers in the distal 5′ flank. We have now identified a 229-bp region of the distal 5′ flank of the 2 integrin gene required for high-level enhancer activity in cells with megakaryocytic features. Two tandem AP1 binding sites with dyad symmetry are required for enhancer activity and for DNA-protein complex formation with members of the c-fos/c-jun family. The requirement for AP1 activation suggested a role for the mitogen-activated protein kinase (MAPK) signaling pathway in regulating 2 integrin gene expression. Inhibition of the MAP kinase cascade with PD98059, a specific inhibitor of MAPK kinase 1, prevented the expression of the 2 integrin subunit in cells induced to become megakaryocytic. We provide a model of megakaryocytic differentiation in which expression of the 2 integrin gene requires signaling via the MAP kinase pathway to activate two tandem AP1 binding sites in the 2 integrin enhancer.

Characterization of a silencer element and purification of a silencer protein that negatively regulates the human adenine nucleotide translocator 2 promoter

Expression of adenine nucleotide translocator isoform 2 (ANT2) is growth regulated. In the present study, we report the presence of a silencer region in the human ANT2 promoter and the purification of a two-component factor that recognizes a specific hexanucleotide element, GTCCTG, of the silencer. Transfection of deletion constructs shows that ANT2 silencer activity extends over a region of at least 310 nts. However, mutating the GTCCTG element completely relieves silencing activity in the context of the human ANT2 promoter. The data suggest that the GTCCTG element might be required for maintaining silencer activity of the extended silencer region. The ANT2 silencer region cloned in front of the herpes simplex virus thymidine kinase promoter confers nearly complete inhibition to the heterologous promoter. However, unlike the ANT2 promoter, mutating the GTCCTG element restores only partial activity to the herpes simplex virus thymidine kinase promoter. A protein complex consisting of two major polypeptides of 37 and 49 kDa was isolated from HeLa nuclear extracts by affinity chromatography using the GTCCTG element as the affinity resin. Cross-linking studies and Southwestern analysis indicate that p37 binds DNA. p49 appears to be loosely associated with the p37/DNA complex but is necessary for strong binding of p37. Our data implicating the GTCCTG element directly in silencing of the ANT2 promoter, together with data from the literature reporting the presence of this element within the silencer region of several additional promoters, suggest a general role of the GTCCTG element in transcriptional silencing.

The identification of a cis-element and a trans-acting factor involved in the response to polyamines and polyamine analogues in the regulation of the human spermidine/spermine N1-acetyltransferase gene transcription

The superinduction of spermidine/spermine N1-acetyltransferase (SSAT) gene has been associated with a cytotoxic response to a new class of antineoplastic polyamine analogues. The initial mechanism of SSAT superinduction is an increase in transcription in response to analogue exposure. This increased transcription appears to be modulated through the association between a nuclear protein factor and a cis-element described here as the polyamine-responsive element (PRE). The PRE was identified as a 9-base pair sequence, 5'-TATGACTAA-3', in the context of a 31-base pair stretch from -1522 to -1492 base pairs with respect to the SSAT transcriptional start site. This element binds a nuclear factor from polyamine analogue-responsive cells, but not from polyamine analogue-insensitive cells. The labeled PRE was used to clone and identify the transcription factor, Nrf-2, that binds constitutively to the PRE sequence. Although the PRE sequence shares homology to the originally identified Nrf-2 recognition sequence, the two sequences are not identical. The Nrf-2 transcription factor appears only to be present in cell types that are capable of expressing high amounts of SSAT. The results of these studies suggest that Nrf-2, bound to the PRE, plays an important regulatory role of expression of the human SSAT gene.

Identification and characterization of the human adipocyte apM-1 promoter

The human adipocyte-specific apM-1 gene encodes a secretory protein of the adipose tissue and seems to play a role in the pathogenesis of obesity. A 1.3 kb amount of the proximal promoter region has been cloned and analyzed for the presence of putative transcription factor binding sites. Several binding sites known to be involved in adipogenesis and regulation of adipocyte-specific genes (C/EBP, SREBP) are present. No TATA box, but a classical CCAAT box could be identified. To confirm functionality and cell specificity of the 1.3 kb promoter, a series of 5'-deleted fragments were ligated in front of the luciferase gene and the constructs were transfected into 3T3-L1 adipocytes. The reporter gene was effectively transcribed, as demonstrated by the expression of enzyme activity. The 5'-end of the human cDNA was completed by 5'-RACE-PCR. Several alternative transcription start sites were detected by RNase protection assay and primer extension analysis. In addition, an exon/intron boundary was mapped at the extreme 5'-end of the cDNA sequence. Genomic Southern blotting suggests that the human apM-1 gene is a single copy gene.

Sequence and structure of the rat housekeeping PBG-D isoform

Porphobilinogen deaminase (PBG-D), a key enzyme in the tetrapyrrole biosynthetic pathway, is encoded by a single gene containing two different promoters. The upstream promoter, found in all cell types, initiates the transcription of the housekeeping PBG-D isoform, whereas the downstream one is erythroid-specific. In this study, we provide the first full sequence of a 1086bp cDNA covering the coding region for the rat ubiquitous PBG-D and its primary amino acid sequence. The cDNA encodes a 39,361 Da protein composed of 361 amino acids. Nucleotide sequence comparison between both isoforms from rat shows similarities of 99.5%, with four changes (C/G) in exon 8 and only one (C/A) in exon 12. Secondary structure prediction reveals that 76.5% of the amino acids from exon 1 are located in a loop. Potential phosphorylation, glycosylation, and myristoylation sites were revealed through motif searches. Housekeeping PBG-D contains coiled-coil segments known to be involved in dynamic rearrangements in the active site.

An electrophile responsive element (EpRE) regulates beta-naphthoflavone induction of the human gamma-glutamylcysteine synthetase regulatory subunit gene. Constitutive expression is mediated by an adjacent AP-1 site

Exposure of HepG2 cells to beta-naphthoflavone (beta-NF) results in time- and dose-dependent increase in the steady-state mRNA levels for both the catalytic (GCSh) and regulatory (GCS1) subunits of gamma-glutamylcysteine synthetase (GCS) which catalyzes the rate-limiting step in the de novo synthesis of the cellular antioxidant glutathione (GSH) (Mulcahy, R. T., Wartman, M. A., Bailey, H. B., and Gipp, J. J. (1997) J. Biol. Chem. 272, 7445-7454). Cloning and sequencing of the GCS1 promoter region is reported. Regulatory sequences mediating basal and beta-NF induced expression of the GCSl gene were identified using a series of promoter/reporter fusion genes transfected into HepG2 cells. Sequences directing basal and beta-NF induced expression were localized between nucleotides -344 and -242 (numbered relative to the translation start site). Mutational analyses indicate that basal expression of the GCSl gene is directed by a consensus AP-1-binding site located 33 base pairs upstream of a consensus electrophile responsive element (EpRE) sequence; both cis-elements are capable of supporting beta-NF inducibility. Elimination of the inducible response requires simultaneous mutation of both sequences, however, in the presence of an intact EpRE the upstream AP-1 site is irrelevant to induction. Regulation of expression of both human GCS subunit genes in response to beta-NF is therefore mediated by cis-elements satisfying the consensus core EpRE motif.

Overlapping antioxidant response element and PMA response element sequences mediate basal and beta-naphthoflavone-induced expression of the human gamma-glutamylcysteine synthetase catalytic subunit gene

gamma-Glutamylcysteine synthetase (GCS), the rate-limiting enzyme in the de novo synthesis of GSH, is a heterodimer, consisting of a catalytic (GCSh) and a regulatory subunit (GCSl). We previously demonstrated that the constitutive and beta-naphthoflavone (beta-NF)-induced expression of the GCSh gene is mediated by a distal antioxidant response element (ARE), ARE4, located 3.1 kb upstream of the transcriptional start site [Mulcahy, Wartman, Bailey and Gipp (1997) J. Biol. Chem. 272, 7445-7454]. ARE4 consists of a consensus ARE sequence (5'-GTGACTCAGCG-3') containing an embedded PMA-responsive element (TRE, underlined). The relative significance of the two overlapping response elements to constitutive and beta-NF-induced expression of the GCSh gene was determined by mutational analyses. The internal activator protein-1 (AP-1)-binding sequence mediated constitutive expression of promoter/reporter transgenes, but was not required for beta-NF responsiveness. In gel-shift experiments, the TRE was necessary for binding of proteins from nuclear extracts prepared from untreated HepG2 cells. In contrast, induction by beta-NF was dependent on an intact ARE sequence, particularly the terminal GC box of ARE4. The GC box of ARE4 was shown to be essential for both basal and beta-NF-induced expression of reporter constructs. This element also influenced binding of nuclear proteins to ARE4, specifically in extracts isolated from beta-NF-treated HepG2 cells. Because previous studies indicated that ARE4 may co-operate with a separate putative ARE, the role of the neighbouring sequence (ARE3), located 34 bases downstream of ARE4, was also evaluated. Mutation of this element within a GCSh promoter/reporter did not modify the basal or beta-NF-induced expression of the transgene, demonstrating that ARE3 does not influence the constitutive or beta-NF-induced expression of the GCSh gene.

Multivalent DNA binding complex generated by small Maf and Bach1 as a possible biochemical basis for beta-globin locus control region complex

The human beta-globin locus control region (LCR) is required to properly regulate chromatin domain opening, replication timing, and globin gene activation. The LCR contains multiple NF-E2 sites (Maf recognition elements, MAREs) that allow the binding of various basic leucine zipper (bZip) proteins like p45 NF-E2, Nrf1, Nrf2, Bach1, and Bach2, in some cases as obligate heterodimers with a small Maf protein. In addition to the bZip domain, the Bach proteins bear a BTB/POZ domain, which has been implicated in the regulation of chromatin structure. We show here that Bach1 is highly expressed in hematopoietic cells and constitutes one of the two MARE-binding activities in murine erythroleukemic (MEL) cells. We further demonstrate that Bach1/MafK heterodimers interact with each other through the BTB domain, generating a multimeric and multivalent DNA binding complex. These results strongly implicate Bach1/MafK heterodimer as an architectural transcription factor that mediates interactions among multiple MAREs. Such a factor could then provide a model for assembly of the theoretical beta-globin LCR "holocomplex. " Other BTB domain proteins have already been demonstrated to be involved in remodeling chromatin, and thus this class of proteins likely promote the formation of nucleoprotein complexes required to establish the architecture of regulatory domains.

The NF-E2 transcription factor

NF-E2 belongs to the basic-leucine zipper family of dimeric transcription factors. It consists of a widely expressed 18 kDa subunit, related to chicken Maf proteins, and a tissue-restricted 45 kDa subunit, which contains a cnc domain. It is found almost exclusively in hematopoietic progenitors, and cells of the erythroid/mega/mast cell trilineage. NF-E2 is involved in regulation of globin gene transcription, acting through locus control regions (LCRs) upstream of the alpha and beta globin gene clusters. In addition, it is essential for normal platelet production. Targeted disruption of the gene encoding the 45 kDa subunit leads to severe thrombocytopenia but little if any defect in erythropoiesis, indicating that other molecules can substitute for p45 in red cell maturation in developing mice. However, retroviral integration within the p45 gene has been shown to disrupt erythroid differentiation in erythroleukemia cells; this suggests that p45 could, conceivably, be a target for pharmacologic interventions in patients with excess red cell production due to polycythemia vera.

Regulation of NF-E2 activity in erythroleukemia cell differentiation

The erythroid transcription factor NF-E2 is an obligate heterodimer composed of two different subunits (p45 and p18), each containing a basic region-leucine zipper DNA binding domain, and it plays a critical role in erythroid differentiation as an enhancer-binding protein for expression of the beta-globin gene. We show here that dimethyl sulfoxide treatment of wild-type murine erythroleukemia cells, but not a mutant clone of dimethyl sulfoxide-resistant cells, increases NF-E2 activity significantly, which involves both up-regulation of DNA binding and transactivation activities. Both activities were reduced markedly by treatment of cells with 2-aminopurine but not by genistein. Activation of the Ras-Raf-MAP kinase signaling cascade increased NF-E2 activity significantly, but this was suppressed when MafK was overexpressed. Domain analysis revealed an activation domain in the NH2-terminal region of p45 and a suppression domain in the basic region-leucine zipper of MafK. These findings indicate that induction of NF-E2 activity is essential for erythroid differentiation of murine erythroleukemia cells, and serine/threonine phosphorylation may be involved in this process. In addition, they also suggest that a MafK homodimer can suppress transcription, not only by competition for the DNA binding site, but also by direct inhibition of transcription. Hence, MafK may function as an active transcription repressor.

Genetics of erythropoiesis: induced mutations in mice and zebrafish

Production of red blood cells (erythropoiesis) in the vertebrate embryo is critical to its survival and subsequent development. As red cells are the first blood cells to appear in embryogenesis, their origin reflects commitment of mesoderm to an hematopoietic fate and provides an avenue by which to examine the development of the hematopoietic system, including the hematopoietic stem cell (HSC). We discuss the genetics of erythropoiesis as studied in two systems: the mouse and zebrafish (Danio rerio). In the mouse, targeted disruption has established several genes as essential at different stages of hematopoiesis or erythroid precursor cell maturation. In the zebrafish, numerous mutants displaying a wide range of phenotypes have been isolated, although the affected genes are unknown. In comparing mouse knockout and zebrafish mutant phenotypes, we propose a pathway for erythropoiesis that emphasizes the apparent similarity of the mutants and the complementary nature of investigation in the two species. We speculate that further genetic studies in mouse and zebrafish will identify the majority of essential genes and define a regulatory network for hematopoiesis in vertebrates.

Interaction of the CNC-bZIP factor TCF11/LCR-F1/Nrf1 with MafG: binding-site selection and regulation of transcription

We have previously shown that the widely expressed human transcription factor TCF11/LCR-F1/Nrf1 interacts with small Maf proteins and binds to a subclass of AP1-sites. Such sites are required for beta-globin 5' DNase I hypersensitive site 2 enhancer activity, erythroid porphobilinogen deaminase inducibility, hemin responsiveness by heme-oxygenase 1 and expression of the gene NAD(P)H:quinone oxidoreductase1. Here we report the optimal DNA-binding sequences for TCF11/LCR-F1/Nrf1 alone and as a heterodimer with MafG, identified by using binding-site selection. The heterodimer recognises a 5'-TGCTgaGTCAT-3' binding-site that is identical to the established NF-E2-site, the antioxidant response element and the heme-responsive element while the binding specificity of the homomer is less stringent. To investigate the activity of TCF11 through this selected site, both alone and in the presence of MafG, we have used a transient transfection assay. TCF11 alone activates transcription while MafG alone acts as a repressor. When co-expressed, MafG interferes with TCF11 transactivation in a dose dependent manner. This indicates that MafG protein, which heterodimerises efficiently with TCF11 in vitro (the heterodimer having a higher affinity for DNA than TCF11 alone), does not co-operate with TCF11 in transactivating transcription. We propose that since both these factors are widely expressed, they may act together to contribute to the negative regulation of this specific target site. Efficient positive regulation by TCF11 may require alternative partners with perhaps more restricted expression patterns.

Transcriptional activity of pannier is regulated negatively by heterodimerization of the GATA DNA-binding domain with a cofactor encoded by the u-shaped gene of Drosophila

The genes pannier (pnr) and u-shaped (ush) are required for the regulation of achaete-scute during establishment of the bristle pattern in Drosophila. pnr encodes a protein belonging to the GATA family of transcription factors, whereas ush encodes a novel zinc finger protein. Genetic interactions between dominant pnr mutants bearing lesions situated in the amino-terminal zinc finger of the GATA domain and ush mutants have been described. We show here that both wild-type Pannier and the dominant mutant form activate transcription from the heterologous alpha globin promoter when transfected into chicken embryonic fibroblasts. Furthermore, Pnr and Ush are found to heterodimerize through the amino-terminal zinc finger of Pnr and when associated with Ush, the transcriptional activity of Pnr is lost. In contrast, the mutant pnr protein with lesions in this finger associates only poorly with Ush and activates transcription even when cotransfected with Ush. These interactions have been investigated in vivo by overexpression of the mutant and wild-type proteins. The results suggest an antagonistic effect of Ush on Pnr function and reveal a new mode of regulation of GATA factors during development.