MafB is an interaction partner and repressor of Ets-1 that inhibits erythroid differentiation

Equivalence in the genetic control of hindbrain segmentation in fish and mouse

The vertebrate hindbrain is subdivided into a series of rhombomeres whose segmental organization serves to pattern the architecture and innervation of the developing head. The zebrafish gene valentino is required cell-autonomously in the development of rhombomeres 5 and 6, and valentino mutants lack visible hindbrain segmentation caudal to the r3/4 boundary (Moens, C. B., Yan, Y.-L., Appel, B., Force, A. G., and Kimmel, C. B. (1996) Development 122, 3981–3990). Here we show that valentino is the zebrafish homologue of the mouse segmentation gene kreisler, which encodes a bZip transcription factor. The valentino gene is expressed in a manner consistent with its proposed role in subdividing rhombomeres 5 and 6 from their common precursor ‘proto-segment’ in the presumptive hindbrain, a process that we also demonstrate is reflected in the normal order of appearance of rhombomere boundaries. As well as having similar phenotypes with respect to visible hindbrain segmentation and patterns of marker gene expression, valentino and kreisler mutants have similar pharyngeal arch and inner ear defects, consistent with a conserved role for this gene in hindbrain segmentation and in patterning of the head periphery.

Selection of peptides that functionally replace a zinc finger in the Sp1 transcription factor by using a yeast combinatorial library

We have developed a strategy for the identification of peptides able to functionally replace a zinc finger domain in a transcription factor. This strategy could have important ramifications for basic research on gene regulation and for the development of therapeutic agents. In this study in yeast, we expressed chimeric proteins that included a random peptide combinatorial library in association with two zinc finger domains and a transactivating domain. The library was screened for chimeric proteins capable of activating transcription from a target sequence in the upstream regulatory regions of selectable or reporter genes. In a screen of approximately 1.5 x 10(7) transformants we identified 30 chimeric proteins that exhibited transcriptional activation, some of which were able to discriminate between wild-type and mutant DNA targets. Chimeric library proteins expressed as glutathione S-transferase fusions bound to double-stranded oligonucleotides containing the target sequence, suggesting that the chimeras bind directly to DNA. Surprisingly, none of the peptides identified resembled a zinc finger or other well-known transcription factor DNA binding domain.

Haemoglobin synthesis in erythropoietin-stimulated J2E cells does not require increased numbers of transferrin receptors

Changes in transferrin-receptor numbers and iron utilisation were monitored during erythropoietin-induced maturation of J2E erythroid cells. Uptake of transferrin and iron doubled 24 h after exposure to erythropoietin, due to a twofold rise in surface transferrin receptors. In addition, a tenfold increase in iron incorporation into haem was observed after erythropoietin stimulation, as iron taken up from transferrin was directed towards haem biosynthesis and away from storage in ferritin. The rise in iron chelation into haem correlated extremely well with haemoglobin synthesis. However, the increase in numbers of transferrin receptors was not essential for haemoglobin synthesis; rather, it was linked with a burst in proliferation stimulated by erythropoietin. We have shown previously that amiloride blocks erythropoietin-enhanced proliferation of J2E cells, but potentiates maturation [Callus, B. A., Tilbrook, P. A., Busfield, S. J. & Klinken, S. P. (1995) Exp. Cell Res. 219, 39-46]. Here we demonstrate that amiloride suppressed the hormone-induced increase in transferrin receptors, whereas the enhanced incorporation of iron into haem was not inhibited. Similarly, when sodium butyrate was used to induce differentiation of J2E cells, proliferation ceased and surface transferrin receptors remained unaltered, while haemoglobin production was accelerated. It was concluded from these experiments that the erythropoietin-stimulated rise in transferrin receptors during the final stages of J2E cell maturation is linked to cell division, and is not essential for haemoglobin synthesis.

The ETS-domain transcription factor family

During recent years, several significant discoveries have been made concerning the function of ETS-domain transcription factors. This family of transcription factors was originally defined on the basis of the conserved primary sequence of their DNA-binding domains. The ETS DNA-binding domain is also conserved at the structural level and is a divergent member of the winged helix-turn-helix superfamily of DNA binding proteins. This sequence conservation is reflected by their overlapping DNA-binding specificities based on the central GGAA/T motif. In addition to DNA-protein interactions, protein-protein interactions with partner proteins often play major roles in targeting ETS-domain proteins to specific promoters. Several such partner proteins have been identified. ETS-domain proteins function as either transcriptional activators or repressors and their activities are often regulated by signal transduction pathways, including the MAP kinase pathways. Specific links between such pathways and ETS-domain proteins have been established in several different experimental systems. ETS-domain transcription factors regulate a diverse array of biological functions including mammalian haematopoiesis and Drosophila eye development. In vertebrates, many ETS-domain proteins regulate embryonic and adult haematopoiesis. Deregulation of ETS-domain protein activity often leads to tumorigenesis. Future work will uncover further details of how these transcription factors work at the molecular level to regulate specific biological processes.

TEL gene rearrangements in myeloid malignancy

The TEL gene is a recently described, "promiscuous" gene with a role in both myeloid and lymphoid malignancy. It is unusual since there may be more than one mechanism by which its rearrangement through chromosomal translocation is leukemogenic. This article discusses the four potential mechanisms of TEL-mediated transformation. It is conceivable that the TEL gene is the common target for various translocations precisely because of this pleiotropy of pathogenic mechanisms by which TEL gene rearrangements can lead to cell transformation.

Expression of two myeloid cell-specific genes requires the novel transcription factor, c-fes expression factor

The protein product of the c-fes proto-oncogene has been implicated in the normal development of myeloid cells (macrophages and granulocytes). We have previously shown that 151 base pairs of c-fes 5'-flanking sequences are sufficient for myeloid cell-specific expression and include functional binding sites for Sp1, PU.1, and a novel nuclear factor (Heydemann, A., Juang, G., Hennessy, K., Parmacek, M. S., and Simon, M. C. (1996) Mol. Cell. Biol. 16, 1676-1686). This novel hematopoietic transcription factor, termed FEF (c-fes expression factor), binds to a cis-acting element that is located at nucleotides -9 to -4 of the c-fes promoter between two Ets binding sites (at -19 to -15 and -4 to +1) which bind PU.1. We now show that a FEF binding site exists in the myeloid cell-specific regulatory region of a second gene, the -2.7-kilobase pair enhancer of chicken lysozyme. The lysozyme FEF site is immediately 5' to a PU. 1 site, analogous to their arrangement in the c-fes promoter, and allows the formation of a preliminary FEF consensus site, 5'-GAAT(C/G)A-3'. This consensus site does not match any sites for known transcription factors. Importantly, although PU.1 binds immediately 3' of the FEF site in both the c-fes promoter and the chicken lysozyme enhancer (CLE), we show that they bind independently. The FEF sites are required for high levels of transcription by both the CLE and the c-fes promoter in transient transfection experiments. Importantly, elimination of the CLE FEF site abolishes all transcriptional activity of this enhancer element. Mutation of the adjacent PU.1 site in either the c-fes promoter or the CLE, reduces activity by approximately 50%. Therefore, transcription of both lysozyme and fes in myeloid cells requires FEF and PU.1. UV cross-linking experiments show that the FEF binding activity consists of a single 70-kDa protein in both human and murine cell lines. FEF binding activity is not affected by antibodies that specifically recognize a number of cloned transcription factors. Collectively, these data indicate that we have identified a novel transcription factor that is functionally important for the expression of at least two myeloid cell-specific genes.

The expression of a novel, epithelium-specific ets transcription factor is restricted to the most differentiated layers in the epidermis

Ets proteins have been implicated in the regulation of gene expression during a variety of biological processes, including growth control, differentiation, development and transformation. More than 35 related proteins containing the 'ets domain' have now been found which specifically interact with DNA sequences encompassing the core tetranucleotide GGAA. Although ets responsive genes have been identified in the epidermis, little is known about their distribution and function in this tissue. We have now demonstrated that epidermis and cultured epidermal keratinocytes synthesize numerous ets proteins. The expression of some of these proteins is regulated as a function of differentiation. Among these is a novel ets transcription factor with a dual DNA-binding specificity, which we have called jen. The expression of jen is not only epithelial specific, but it is the only ets protein so far described, and one of the very few transcription factors whose expression is restricted to the most differentiated epidermal layers. We show that two epidermal marker genes whose expression coincides with that of jen are transregulated by this protein in a complex mode which involves interactions with other transcriptional regulators such as Sp1 and AP1.

The Maf transcription factors: regulators of differentiation

Since the identification of the v-maf oncogene in an avian tumor virus, the Maf protein family has grown rapidly, forming a unique subclass of basic-leucine zipper transcription (bZIP) factors. Maf family members appear to play important roles in the regulation of differentiation.

The Krüppel-associated box (KRAB)-zinc finger protein Kid-1 and the Wilms' tumor protein WT1, two transcriptional repressor proteins, bind to heteroduplex DNA

Zinc finger proteins of the Cys2His2 class represent a large group of DNA-binding proteins. A major subfamily of those proteins, the Krüppel-associated box (KRAB) domain-containing Cys2His2-zinc finger proteins, have been described as potent transcriptional repressors. So far, however, no DNA-binding sites for KRAB domain-containing zinc finger proteins have been isolated. Using a polymerase chain reaction-based selection strategy with double- and single-stranded DNA, we failed to reveal a binding site for Kid-1, one member of KRAB-zinc finger proteins. Binding of Kid-1 both to single- and homoduplex double-stranded DNA was negligible. We now present evidence that Kid-1 binds to heteroduplex DNA. Similar to Kid-1, the non-KRAB-zinc finger protein WT1 also bound avidly to heteroduplex DNA (both the -KTS and +KTS splice variant of WT1), whereas the POU domain protein Oct-6, the ets domain protein Ets-1 and the RING finger of BRCA-1 did not bind to heteroduplex DNA. Binding of WT1 to heteroduplex DNA was markedly reduced in naturally occurring mutants. The recognition of certain DNA structures by transcriptional repressor proteins may therefore represent a more common phenomenon than previously thought.

The Ets transcription factors interact with each other and with the c-Fos/c-Jun complex via distinct protein domains in a DNA-dependent and -independent manner

The transcription factors Fos, Jun, and Ets regulate the expression of human stromelysin-1 and collagenase-1 genes. Recently, we found that ERG, an Ets family member, activates collagenase-1 gene but not stromelysin-1 by physically interacting with c-Fos/c-Jun. Interestingly, ERG binds to stromelysin-1 promoter and represses its activation by ETS2. Here, to investigate the molecular mechanism of this regulation, we have used an in vitro protein-protein interaction assay and studied the transcription factor interactions of ETS2. We found that ETS2 could weakly associate with in vitro synthesized ETS1, c-Fos, and c-Jun and strongly with c-Fos/c-Jun complex and ERG via several distinct ETS2 domains including the C-terminal region that contains the DNA-binding domain. Strikingly, these interactions were stabilized in vitro by DNA as they were inhibited by ethidium bromide. Both the N-terminal region, comprising the transactivation domain, and the C-terminal region of ETS2 associated with ERG and, interestingly, the interaction of ERG through the transactivation domain of ETS2 was DNA-independent. The DNA-dependent interaction of ETS2 with c-Fos/c-Jun was enhanced by specific DNA fragments requiring two Ets-binding sites of the stromelysin-1 promoter. Using the two hybrid system, we also demonstrated that ETS2 interacts with c-Jun or ERG in vivo.

The lineage commitment of haemopoietic progenitor cells

Multipotent haemopoietic progenitor cells appear to be 'primed' for commitment by co-expression of a multiplicity of genes characteristic of different lineages. Lineage commitment proceeds as the consolidation of a distinct pattern of gene expression out of this milieu.

Complex regulation of transferrin receptors during erythropoietin-induced differentiation of J2E erythroid cells--elevated transcription and mRNA stabilisation produce only a modest rise in protein content

The regulation of transferrin-receptor synthesis was studied in J2E erythroid cells induced to differentiate with erythropoietin. Nuclear run-on assays demonstrated that transcription of the transferrin-receptor gene rose markedly after erythropoietin treatment. In addition, transferrin-receptor mRNA was stabilised and this was associated with an increase in the activity of the RNA-binding protein IRP (iron regulatory protein). As a result of increased transcription and mRNA stabilisation, steady-state RNA levels increased 10-20-fold. However, despite these large increases in mRNA, translation only doubled; consequently, modest increases in total protein and surface transferrin receptors were observed. Moreover, this rise in transferrin receptors was transient, and correlated with a burst of proliferation shortly after erythropoietin treatment. The expected inverse relationship between transferrin receptors and ferritin did not occur during J2E maturation as translation of both ferritin subunits increased when transferrin-receptor mRNA levels rose. Analysis of mutant J2E clones incapable of synthesising haemoglobin revealed that surface transferrin-receptor levels were only 15-25% that of the parental erythroid line. We propose that the surface expression of transferrin receptors in J2E cells is governed by three factors: basal levels essential for normal growth in culture; elevated levels needed for haemoglobin synthesis; and a transient erythropoietin-induced increase that is required for the final burst of proliferation. It was concluded that the regulation of transferrin-receptor production in erythropoietin-stimulated J2E cells is complex and that there are several sites of control.

A role for the ETS domain transcription factor PEA3 in myogenic differentiation

Activation of adult myoblasts called satellite cells during muscle degeneration is an important aspect of muscle regeneration. Satellite cells are believed to be the only myogenic stem cells in adult skeletal muscle and the source of regenerating muscle fibers. Upon activation, satellite cells proliferate, migrate to the site of degeneration, and become competent to fuse and differentiate. We show here that the transcription factor polyomavirus enhancer activator 3 (PEA3) is expressed in adult myoblasts in vitro when they are proliferative and during the early stages of differentiation. Overexpression of PEA3 accelerates differentiation, whereas blocking of PEA3 function delays myoblast fusion. PEA3 activates gene expression following binding to the ets motif most efficiently in conjunction with the transcription factor myocyte enhancer factor 2 (MEF2). In vivo, PEA3 is expressed in satellite cells only after muscle degeneration. Taken together, these results suggest that PEA3 is an important regulator of activated satellite cell function.

The world according to Maf

Maf family proteins are so named because of their structural similarity to the founding member, the oncoprotein v-Maf. The small Maf proteins (MafF, MafG and MafK), as do all family members, include a characteristic basic region linked to a leucine zipper (b-Zip) domain which mediate DNA binding and subunit dimerization respectively. The small Maf proteins form homodimers or heterodimers with other b-Zip proteins present in the cell and bind to Maf recognition elements (MARE) in DNA. Since they lack known transcriptional activation domains, the small Maf proteins function either as obligatory heterodimeric partner molecules with numerous large subunits, discussed below, or alternatively as homo- or heterodimeric transcriptional repressors. The three small Maf proteins are expressed in a number of overlapping tissues, but their expression profiles nonetheless appear to be under meticulous tissue- and developmental stage-specific control. The MARE bears a striking resemblance to the NF-E2 binding sequence. NF-E2 binding sites in the human beta-globin locus control region have been directly implicated as integral components in the circuitry required for eliciting changes in chromatin structure that precede globin gene activation. While the NF-E2 DNA sequence has been shown to be important for erythroid-specific gene regulation, a growing list of other genes may also be regulated through the same, or very similar, cis elements in non-erythroid cells. Taken together, these observations argue that comprehensive analysis of the activities of the small Maf proteins may provide a unique perspective for expanding our understanding of transcriptional regulation that can be elicited through interacting transcription factor networks.

The expression pattern of the mafB/kr gene in birds and mice reveals that the kreisler phenotype does not represent a null mutant

The recessive mouse mutation kreisler affects hindbrain segmentation and inner ear development in homozygous mice. The mouse gene affected by the mutation was found to encode a basic domain leucine-zipper (bZIP)-type transcription factor of the Maf-family named kr (Cordes, S.P. and Barsh, G.S. (1994) Cell 79, 1025-1034). The avian bZIP transcription factor mafB, which shows high homology to kr, has been identified as an interaction partner of c-Ets 1 (Sieweke, M.H., Tekotte, M.H., Frampton, J. and Graf, T. (1996) Cell 85, 49-60). Here we demonstrate by Southern blot analysis that mafB is the avian homologue of kr, and present a detailed pattern of its expression during avian and murine embryonic development. Consistent with the kreisler phenotype, mafB is expressed in avians in the tissues which are affected by the mouse mutation: rhombomeres 5 and 6 (r5 and r6) and the neural crest derived from these rhombomeres. However, our analysis reveals a variety of additional expression sites: mafB/kr expression persists in vestibular and acoustic nuclei and is also observed in differentiating neurons of the spinal cord and brain stem. Restricted expression sites are found in the mesonephros, the perichondrium, and in the hemopoietic system. Since these expression sites are conserved between mouse and chicken we reexamined homozygous kreisler mice for unrevealed phenotypes in the hemopoietic system. However, peritoneal macrophages from homozygous kreisler mice were found to be functionally normal and still expressed mafB/kr. Other adult tissues examined from homozygous kreisler mice had also not lost mafB/kr expression. Our results thus indicate that the kreisler mutation involves a tissue specific gene inactivation and suggest additional roles for mafB/kr in later developmental and differentiation processes that are not revealed by the mutation.

GABP Cooperates With c-Myb and C/EBP to Activate the Neutrophil Elastase Promoter

Neutrophil elastase (NE) is a serine protease that is transcriptionally regulated during early myeloid differentiation. The murine NE (mNE) promoter contains functionally important c-Myb, C/EBP, and ets binding sites. Deletion of the ets site reduced promoter activity by 90%. Although the ets transcription factor, PU.1, bound to this ets site, it only modestly activated the mNE promoter. Here, we show that a second transcription factor from myeloid cells — GABP — binds to the mNE ets site but strongly activates the mNE promoter. GABP is a heteromeric transcription factor complex that consists of GABPα, an ets factor, and GABPβ, a Notch-related protein. GABPα bound to the mNE ets site and, in turn, recruited GABPβ to form a transcriptionally active complex. GABPα and PU.1 competed with each other for binding to the mNE ets site. GABP increased the activity of the mNE promoter sevenfold in U937 myeloid cells. GABP cooperated with c-Myb and C/EBPα to activate the mNE promoter more than 85-fold in otherwise nonpermissive, nonhematopoietic NIH 3T3 cells. Thus, GABP binds to the crucial mNE promoter ets site and powerfully activates its expression alone and in cooperation with the transcription factors c-Myb and C/EBP.

Isolation of an AP-1 repressor by a novel method for detecting protein-protein interactions

Transcription factor AP-1 transduces environmental signals to the transcriptional machinery. To ensure a quick response yet maintain tight control over AP-1 target genes, AP-1 activity is likely to be negatively regulated in nonstimulated cells. To identify proteins that interact with the Jun subunits of AP-1 and repress its activity, we developed a novel screen for detecting protein-protein interactions that is not based on a transcriptional readout. In this system, the mammalian guanyl nucleotide exchange factor (GEF) Sos is recruited to the Saccharomyces cerevisiae plasma membrane harboring a temperature-sensitive Ras GEF, Cdc25-2, allowing growth at the nonpermissive temperature. Using the Sos recruitment system, we identified new c-Jun-interacting proteins. One of these, JDP2, heterodimerizes with c-Jun in nonstimulated cells and represses AP-1-mediated activation.

Human MafG Is a Functional Partner for p45 NF-E2 in Activating Globin Gene Expression

Mammalian globin gene expression is activated through NF-E2 elements recognized by basic-leucine zipper proteins of the AP-1 superfamily. The specificity of NF-E2 DNA binding is determined by several nucleotides adjacent to a core AP-1 motif, comprising a recognition site for transcription factors of the Maf subfamily. Earlier work proposed that p18(MafK) forms a heterodimer with hematopoietic-specific protein p45 NF-E2 to activate transcription through NF-E2 sites. However, there was no direct evidence that p18(MafK) serves this function in vivo; in fact, mice lacking p18(MafK) have no phenotype. Here we describe a novel cDNA clone that encodes the human homolog of chicken MafG. Human MafG heterodimerizes with p45 NF-E2 and binds DNA with specificity identical to that of purified NF-E2 DNA binding activity. A tethered heterodimer of p45 and MafG is fully functional in supporting expression of α- and β-globin, and in promoting erythroid differentiation in CB3, a p45-deficient mouse erythroleukemia cell line. These results indicate that human MafG can serve as a functional partner for p45 NF-E2, and suggest that the p45/MafG heterodimer plays a role in the regulation of erythropoiesis.

Genomic structure and chromosomal localization of the novel ETS factor, PE-2 (ERF)

The members of the ETS family of transcription factors are grouped because they share a highly conserved DNA binding domain. These factors are involved in growth factor pathways and regulate both proliferation and differentiation. To identify ETS factors that may be involved in early hematopoietic progenitor regulation, we isolated a novel member of the ETS family by reverse transcriptase-PCR of the conserved DNA binding domain using degenerate oligonucleotides. This gene directs the synthesis of a 2704-nucleotide transcript whose largest open reading frame encodes a 548-amino-acid protein. Northern blot analysis reveals ubiquitous expression in all human tissues and cell lines tested, with highest levels in the testis, ovary, pancreas, and heart. Comparison of this gene with the available databases reveals very significant homology to the ETS factor PE-1 and probable near-identity with the recently cloned factor ERF. The PE-2 gene is composed of four exons spanning over 9 kb of genomic DNA. Sequence analysis of the promoter region reveals a GC-rich sequence without a TATA motif and with putative binding motifs for CREB, c-myb, and AP-1 factors. Using mouse-human somatic hybrids and FISH analysis, the PE-2 gene is localized to human chromosome 19q13.2, a region involved in translocations and deletions in leukemias and several solid tumors, suggesting that this novel ETS factor may play a role in carcinogenesis.

Physical interactions between Ets and NF-kappaB/NFAT proteins play an important role in their cooperative activation of the human immunodeficiency virus enhancer in T cells

The transcriptional regulatory elements of many inducible T-cell genes contain adjacent or overlapping binding sites for the Ets and NF-kappaB/NFAT families of transcription factors. Similar arrays of functionally important NF-kappaB/NFAT and Ets binding sites are present in the transcriptional enhancers of human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2), suggesting that this pattern of nuclear protein binding sites reflects an evolutionarily conserved mechanism for regulating inducible T-cell gene expression that has been co-opted during HIV evolution. Despite these findings, the molecular mechanisms by which Ets and NF-kappaB/NFAT proteins cooperatively regulate inducible T-cell gene expression remained unknown. In the studies described in this report, we demonstrated a physical interaction between multiple Ets and NF-kappaB/NFAT proteins both in vitro and in activated normal human T cells. This interaction is mediated by the Ets domain of Ets proteins and the C-terminal region of the Rel homology domains of NF-kappaB/NFAT proteins. In addition, the Ets-NF-kappaB/NFAT interaction requires the presence of DNA binding sites for both proteins, as it is abolished by the DNA intercalating agents propidium iodide and ethidium bromide and enhanced by the presence of synthetic oligonucleotides containing binding sites for Ets and NF-kappaB proteins. A dominant-negative mutant of NF-kappaB p50 that binds DNA but fails to interact with Ets proteins inhibits the synergistic activation of the HIV-1 and HIV-2 enhancers by NF-kappaB (p50 + p65) and Ets-1, suggesting that physical interaction between Ets and NF-kappaB proteins is required for the transcriptional activity of the HIV-1 and HIV-2 enhancers. Taken together, these findings suggest that evolutionarily conserved physical interactions between Ets and NF-kappaB/NFAT proteins are important in regulating the inducible expression of T-cell genes and viruses. These interactions represent a potential target for the development of novel immunosuppressive and antiviral therapies.

DAX-1 inhibits SF-1-mediated transactivation via a carboxy-terminal domain that is deleted in adrenal hypoplasia congenita

X-linked adrenal hypoplasia congenita (AHC) with hypogonadotropic hypogonadism was recently shown to be caused by mutations in a gene referred to as DAX-1, which encodes a novel member of the orphan nuclear receptor family. DAX-1 is homologous to other nuclear receptors in its carboxy-terminal region, but it lacks the characteristic zinc finger DNA-binding domain. The tissue distribution of DAX-1 (adrenal cortex, gonads, hypothalamus, and pituitary) is the same as that of another orphan nuclear receptor, steroidogenic factor 1 (SF-1), that is required for development of the adrenal glands and gonads. We examined whether DAX-1 and SF-1 might interact in the regulation of SF-1-responsive target genes. Coexpression of DAX-1 and SF-1 inhibited SF-1-mediated transactivation. DAX-1 was shown to interact directly with SF-1 in in vitro protein binding studies; however, it did not interfere with SF-1 binding to DNA in gel mobility shift assays. Transactivation by GAL4-SF-1 constructs was inhibited by DAX-1, indicating that neither the SF-1 DNA-binding domain nor the SF-1 binding sites are required for inhibition by DAX-1. A series of DAX-1 deletion mutants localized the inhibitory domain to the carboxy-terminal region of the protein. Deletion of this domain also reduced basal transcriptional silencing by GAL4-DAX-1. This inhibitory domain has been deleted in all naturally occurring AHC deletion mutants described to date. In addition, two naturally occurring point mutations in DAX-1 exhibited impaired inhibition of SF-1. We conclude that DAX-1 can inhibit SF-1 transcriptional activity and suggest that the loss of this inhibitory property in DAX-1 may account in part for the phenotype of AHC.

Interaction of human T-cell lymphotropic virus type I Tax, Ets1, and Sp1 in transactivation of the PTHrP P2 promoter

We have previously shown that the parathyroid hormone-related protein (PTHrP) promoter contains binding sites for transcription factors Ets1 and Sp1 and that human T-cell lymphotropic virus type I (HTLV-I) Tax cooperates with Ets1 to transactivate the PTHrP P2 promoter. Using the yeast two-hybrid interaction system, we now provide evidence that Tax interacts with Ets1. Moreover, a double mutation (D22A,C23S) in the Tax protein that abrogated the Tax/Ets1 interaction also inhibited the Tax/Ets1 cooperative effect, suggesting that the interaction between Tax and Ets1 is important for transactivation of the PTHrP promoter. In coimmunoprecipitation assays, we find that Tax facilitates the interaction between Ets1 and Sp1, forming a ternary complex. When the Sp1 site in the PTHrP promoter was mutated, the Tax/Ets1 cooperative effect was dramatically decreased. This suggests that Sp1 plays an important role in the Ets1-dependent Tax transactivation of the PTHrP P2 promoter. Finally, we demonstrate that Gal4-Tax is a strong activator of the Gal PTHrP promoter, implying that Tax contributes directly to the transcriptional activation of the promoter. We propose a model in which the Tax/Ets1 cooperative effect on the PTHrP P2 promoter is based on the ability of Tax, Ets1, and Sp1 to form a ternary complex on the template DNA. Tax facilitates the interaction of Ets1/Sp1 and participates directly in the transcription initiation process.

USF2/FIP associates with the b-Zip transcription factor, c-Maf, via its bHLH domain and inhibits c-Maf DNA binding activity

In screening for proteins that interact with the basic zipper (bZip) transcription factor, c-Maf, we isolated USF2/FIP. USF2 is a member of the bHLH-Zip protein family, possessing a basic (b) DNA binding region, a helix-loop-helix (HLH) motif, and a leucine zipper (Zip) structure. Mutants of USF2 that lacked a Zip formed heterodimers with c-Maf, but did not homodimerize. Deletion of the USF2 basic region or mutation of its helices abrogated its binding to c-Maf, but had no effect on homodimerization. A functional c-Maf bZip motif was necessary for both homodimerization and heterodimerization with USF2. These data suggest a tetrameric configuration for Maf-USF2 complexes. In the presence of USF2, the DNA binding activity of c-Maf was markedly reduced. Therefore, USF2 and c-Maf may interact to regulate gene expression.

A domain of TEL conserved in a subset of ETS proteins defines a specific oligomerization interface essential to the mitogenic properties of the TEL-PDGFR beta oncoprotein

TEL is a novel member of the ETS family of transcriptional regulators which is frequently involved in human leukemias as the result of specific chromosomal translocations. We show here by co-immunoprecipitation and GST chromatography analyses that TEL and TEL-derived fusion proteins form homotypic oligomers in vitro and in vivo. Deletion mutagenesis identifies the TEL oligomerization domain as a 65 amino acid region which is conserved in a subset of the ETS proteins including ETS-1, ETS-2, FLI-1, ERG-2 and GABP alpha in vertebrates and PNTP2, YAN and ELG in Drosophila. TEL-induced oligomerization is shown to be essential for the constitutive activation of the protein kinase activity and mitogenic properties of TEL-platelet derived growth factor receptor beta (PDGFR beta), a fusion oncoprotein characteristic of the leukemic cells of chronic myelomonocytic leukemia harboring a t(5;12) chromosomal translocation. Swapping experiments in which the TEL oligomerization domain was exchanged by the homologous domains of representative vertebrate ETS proteins including ETS-1, ERG-2 and GABP alpha show that oligomerization is a specific property of the TEL amino-terminal conserved domain. These results indicate that the amino-terminal domain conserved in a subset of the ETS proteins has evolved to generate a specialized protein-protein interaction interface which is likely to be an important determinant of their specificity as transcriptional regulators.

Constitutive c-ets2 expression in M1D+ myeloblast leukemic cells induces their differentiation to macrophages

The expression of c-ets2 is rapidly induced in a variety of myelomonocytic cell lines as they differentiate into macrophages. We find that constitutive expression of c-ets2 in the M1D+ myeloblast leukemic cell line (M1ets2) is sufficient to push these cells to a more differentiated state. The expression of several differentiation-specific genes is upregulated in M1ets2 cells, including those encoding macrophage-specific lysozyme M and tumor necrosis factor alpha, which are involved in bacteriolytic and inflammatory processes, respectively. Transcription factors c-jun and junB, previously shown to induce partial macrophage differentiation when overexpressed in myelomonocytic leukemia cell lines, are also upregulated in M1ets2 cells. The upregulation of junB is the result of a direct interaction of Ets2 with ets binding sites of the junB promoter, since transient or constitutive Ets2 expression in M1D+ cells activates junB transcription via ets binding sites. In addition, transfection of a dominant negative mutant of Ets2, devoid of its transcriptional activation domain, greatly reduces transcriptional activities of the junB promoter in M1ets2 cells. Finally, unlike their parental M1D+ counterparts, M1ets2 cells secrete the macrophage colony-stimulating factor, CSF-1, and are able to phagocytize. Taken together, these results show that when the immature myeloid M1D+ cell line constitutively expresses c-ets2, these cells acquire different functions of mature macrophages.

v-Myb of E26 leukemia virus up-regulates bcl-2 and suppresses apoptosis in myeloid cells

Many oncogenes have been shown to be deregulated transcription factors, yet direct target genes mediating cell transformation remain elusive. Here we describe such a target for v-Myb by exploiting a temperature-sensitive mutant of the E26 avian leukemia virus encoding Myb-Ets. Myeloblasts transformed by the mutant differentiate into macrophages or die by apoptosis when shifted to the nonpermissive temperature as a result of inactivation of v-Myb. During this process mRNA of the antiapoptotic oncoprotein Bcl-2 is down-regulated with kinetics similar to those of Mim-1, a differentiation-related protein whose expression is directly regulated by Myb. Forced expression of bcl-2 rescues the cells from apoptosis, without preventing either their withdrawal from the cell cycle or their differentiation. v-Myb appears to act directly on the bcl-2 gene, because a bcl-2 promoter-driven reporter is activated by Myb-Ets and v-Myb-VP16 and requires intact Myb binding sites within the promoter. Surprisingly, inactivation of v-Myb in multipotent progenitors transformed by E26 virus does not induce apoptosis, indicating that bcl-2 regulation by the oncoprotein is required for the transformation of some cell types but not others.

Development of the hematopoietic system

Significant advances have been made in the past year in understanding hematopoietic development. Recent studies have clarified the origin and migration of stem cells in early embryos, established potential roles for homeodomain proteins in controlling the proliferation of progenitor cells and in patterning ventral mesoderm, and demonstrated the effects of nuclear proteins on lineage programming and apoptosis.

Pax-5 (BSAP) recruits Ets proto-oncogene family proteins to form functional ternary complexes on a B-cell-specific promoter

The paired box transcription factor Pax-5 (B-cell-specific activator protein) is a key regulator of lineage-specific gene expression and differentiation in B-lymphocytes. We show that Pax-5 functions as a cell type-specific docking protein that facilitates binding of the early B-cell-specific mb-1 promoter by proteins of the Ets proto-oncogene family. Transcriptional activity of the mb-1 promoter in pre-B-cells is critically dependent on binding sites for Pax-5:Ets complexes. Ternary complex assembly requires only the Pax-5 paired box and ETS DNA-binding domains. Mutation of a single base pair in the ternary complex binding site allows for independent binding by Ets proteins but, conversely, inhibits the binding of Pax-5 by itself. Strikingly, the mutation reverses the pattern of complex assembly: Ets proteins recruit Pax-5 to bind the mutated sequence. Recruitment of Net and Elk-1, but not SAP1a, by Pax-5 defines a functional difference between closely related Ets proteins. Replacement of a valine (V68) in the ETS domain of SAP1a by aspartic acid (as found in c-Ets-1, Elk-1, and Net) enhanced ternary complex formation by more than 60-fold. Together, these observations define novel transcription factor interactions that regulate gene expression in B cells.

Excision of Ets by an inducible site-specific recombinase causes differentiation of Myb-Ets-transformed hematopoietic progenitors

BACKGROUND

The Myb-Ets protein encoded by the E26 acute avian leukemia virus is a paradigm for the function of fused transcriptional activator oncoproteins. Myb-Ets transforms hematopoietic progenitor cells (myb-Ets progenitors, MEPs) that can be induced to differentiate into eosinophilic and myeloid cells by the activation of pathways involving Ras and/or protein kinase C. The Ets portion of the fusion protein seems to be required to maintain the multipotency of MEPs: MEPs transformed with a temperature-sensitive E26 mutant with a lesion in Ets (ts 1.1) and shifted to the non-permissive temperature predominantly form erythroid cells, but also form eosinophilic and myeloid cells. This interpretation is complicated, however, by the observation that ts 1.1-transformed MEPs differ from MEPs transformed with wild-type E26 in that they expressed erythroid and eosinophil markers even at the permissive temperature.

RESULTS

In order to alleviate the problems associated with the use of temperature-sensitive mutants we have designed a vector that allows the inducible deletion of the Ets domain. To this end, we introduced FLP recombinase target sites into the E26 virus on the 5' and 3' sides of Ets and included within the same retroviral vector sequences encoding and estrogen-dependent FLP recombinase. This construct, termed FRV-3, is capable of transforming cells to produce a phenotype indistinguishable from that of MEPs obtained with wild-type virus. Hormone treatment of MEPs transformed with FRV-3 induced erythroid differentiation in a subpopulation of the cells; this subpopulation was found to have completely excised ets. However, in contrast to previous results obtained with ts 1.1-transformed MEPs, no differentiation along the eosinophilic and myeloid lineages was seen in hormone-treated FRV-3-transformed MEPs.

CONCLUSIONS

Our results demonstrate the feasibility of using a site-specific recombinase to excise a fused oncoprotein domain encoded by a retrovirus. More specifically, they show that the Ets portion of the Myb-Ets protein selectively inhibits differentiation of MEPs along the erythroid lineage, and suggests that Ets is also required for their differentiation along the eosinophil and, possibly, myeloid lineages.

The proto-oncogene c-maf is responsible for tissue-specific expression of interleukin-4

The molecular basis for the distinctive cytokine expression of CD4+ T helper 1 (Th1) and T helper 2 (Th2) subsets remains elusive. Here, we report that the proto-oncogene c-maf, a basic region/leucine zipper transcription factor, controls tissue-specific expression of IL-4. c-Maf is expressed in Th2 but not Th1 clones and is induced during normal precursor cell differentiation along a Th2 but not Th1 lineage. c-Maf binds to a c-Maf response element (MARE) in the proximal IL-4 promoter adjacent to a site footprinted by extracts from Th2 but not Th1 clones. Ectopic expression of c-Maf transactivates the IL-4 promoter in Th1 cells, B cells, and nonlymphoid cells, a function that maps to the MARE and Th2-specific footprint. Furthermore, c-Maf acts in synergy with the nuclear factor of activated T cells (NF-ATp) to initiate endogeneous IL-4 production by B cells. Manipulation of c-Maf may alter Th subset ratios in human disease.