Involvement of the transcription factor PU.1/Spi-1 in myeloid cell-restricted expression of an interferon-inducible gene encoding the human high-affinity Fc gamma receptor

PU.1 and partners: regulation of haematopoietic stem cell fate in normal and malignant haematopoiesis

During normal haematopoiesis, cell development and differentiation programs are accomplished by switching ‘on’ and ‘off’ specific set of genes. Specificity of gene expression is primarily achieved by combinatorial control, i.e. through physical and functional interactions among several transcription factors that form sequence-specific multiprotein complexes on regulatory regions (gene promoters and enhancers). Such combinatorial gene switches permit flexibility of regulation and allow numerous developmental decisions to be taken with a limited number of regulators. The haematopoietic-specific Ets family transcription factor PU.1 regulates many lymphoid- and myeloid-specific gene promoters and enhancers by interacting with multiple proteins during haematopoietic development. Such protein–protein interactions regulate DNA binding, subcellular localization, target gene selection and transcriptional activity of PU.1 itself in response to diverse signals including cytokines, growth factors, antigen and cellular stresses. Specific domains of PU.1 interact with many protein motifs such as bHLH, bZipper, zinc fingers and paired domain for regulating its activity. This review focuses on important protein–protein interactions of PU.1 that play a crucial role in regulation of normal as well as malignant haematopoiesis. Precise delineation of PU.1 protein-partner interacting interface may provide an improved insight of the molecular mechanisms underlying haematopoietic stem cell fate regulation. Its interactions with some proteins could be targeted to modulate the aberrant signalling pathways for reversing the malignant phenotype and to control the generation of specific haematopoietic progeny for treatment of haematopoietic disorders.

Identification and characterization of a PU.1/Spi-B binding site in the bovine leukemia virus long terminal repeat

Bovine leukemia virus (BLV) is a B-lymphotropic oncogenic retrovirus whose transcriptional promoter is located in the viral 5' long terminal repeat (LTR). To date, no B-lymphocyte-specific cis-regulatory element has been identified in this region. Since ETS proteins are known to regulate transcription of numerous retroviruses, we searched for the presence in the BLV promoter region of binding sites for PU.1/Spi-1, a B-cell- and macrophage-specific ETS family member. In this report, nucleotide sequence analysis of the viral LTR identified a PUbox located at -95/-84 bp. We demonstrated by gel shift and supershift assays that PU.1 and the related Ets transcription factor Spi-B interacted specifically with this PUbox. A 2-bp mutation (GGAA-->CCAA) within this motif abrogated PU.1/Spi-B binding. This mutation caused a marked decrease in LTR-driven basal gene expression in transient transfection assays of B-lymphoid cell lines, but did not impair the responsiveness of the BLV promoter to the virus-encoded transactivator Tax(BLV). Moreover, ectopically expressed PU.1 and Spi-B proteins transactivated the BLV promoter in a PUbox-dependent manner. Taken together, our results provide the first demonstration of regulation of the BLV promoter by two B-cell-specific Ets transcription factors, PU.1 and Spi-B. The PU.1/Spi-B binding site identified here could play an important role in BLV replication and B-lymphoid tropism.

TIF1beta functions as a coactivator for C/EBPbeta and is required for induced differentiation in the myelomonocytic cell line U937

Representational difference analysis (RDA) cloning has identified transcriptional intermediary factor 1 beta (TIF1beta) as a gene inducibly expressed early during myeloid differentiation of the promyelocytic cell lines HL-60 and U937. To assess the role of TIF1beta, U937 cell lines were made that expressed antisense-hammerhead ribozymes targeted specifically against TIF1beta mRNA. These cells failed to differentiate into macrophages, as determined by several criteria: a nonadherent morphology, a failure to arrest cell cycle, lowered levels of macrophage-specific cell surface markers, resistance to Legionella pneumophila infection, a loss of the ability to phagocytose and chemotax, and decreased expression of chemokine mRNAs. One way TIF1beta acts in macrophage differentiation is to augment C/EBPbeta transcriptional activity. Furthermore, we show by EMSA supershifts and coimmunoprecipitation that C/EBPbeta and TIF1beta physically interact. Although TIF1beta is necessary for macrophage differentiation of U937 cells, it is not sufficient, based on the inability of ectopically expressed TIF1beta to induce or augment phorbol ester-induced macrophage differentiation. We conclude that TIF1beta plays an important role in the terminal differentiation program of macrophages, which involves the coactivation of C/EBPbeta and induction of C/EBPbeta-responsive myeloid genes.

Ets target genes: past, present and future

Ets is a family of transcription factors present in species ranging from sponges to human. All family members contain an approximately 85 amino acid DNA binding domain, designated the Ets domain. Ets proteins bind to specific purine-rich DNA sequences with a core motif of GGAA/T, and transcriptionally regulate a number of viral and cellular genes. Thus, Ets proteins are an important family of transcription factors that control the expression of genes that are critical for several biological processes, including cellular proliferation, differentiation, development, transformation, and apoptosis. Here, we tabulate genes that are regulated by Ets factors and describe past, present and future strategies for the identification and validation of Ets target genes. Through definition of authentic target genes, we will begin to understand the mechanisms by which Ets factors control normal and abnormal cellular processes.

IL-2 and long-term T cell activation induce physical and functional interaction between STAT5 and ETS transcription factors in human T cells

Activation of Stat5 by many cytokines implies that it cannot alone insure the specificity of the regulation of its target genes. We have evidenced a physical and functional interaction between members of two unrelated transcription factor families, Ets-1, Ets-2 and Stat5, which could contribute to the proliferative response to interleukin 2. Competition with GAS- and EBS-specific oligonucleotides and immunoassays with a set of anti-Stat and anti-Ets families revealed that the IL-2-induced Stat5-Ets complex recognizes several GAS motifs identified as target sites for activated Stat5 dimers. Coimmunoprecipitation experiments evidenced that a Stat5/Ets-1/2 complex is formed in vivo in absence of DNA. GST-pull down experiments demonstrated that the C-terminal domain of Ets-1 is sufficient for this interaction in vitro. Cotransfection experiments in Kit225 T cells resulted in cooperative transcriptional activity between both transcription factors in response to a combination of IL-2, PMA and ionomycin. A Stat5-Ets protein complex was the major inducible DNA-binding complex bound to the human IL-2rE GASd/EBSd motif in long-term proliferating normal human T cells activated by CD2 and CD28. These results suggest that the inducible Stat5-Ets protein interaction plays a role in the regulation of gene expression in response to IL-2 in human T lymphocytes.

TFEC Is a Macrophage-Restricted Member of the Microphthalmia-TFE Subfamily of Basic Helix-Loop-Helix Leucine Zipper Transcription Factors

The murine homologue of the TFEC was cloned as part of an analysis of the expression of the microphthalmia-TFE (MiT) subfamily of transcription factors in macrophages. TFEC, which most likely acts as a transcriptional repressor in heterodimers with other MiT family members, was identified in cells of the mononuclear phagocyte lineage, coexpressed with all other known MiT subfamily members (Mitf, TFE3, TFEB). Northern blot analysis of several different cell lineages indicated that the expression of murine TFEC (mTFEC) was restricted to macrophages. A 600-bp fragment of the TATA-less putative proximal promoter of TFEC shares features with many known macrophage-specific promoters and preferentially directs luciferase expression in the RAW264.7 macrophage cell line in transient transfection assays. Five of six putative Ets motifs identified in the TFEC promoter bind the macrophage-restricted transcription factor PU.1 under in vitro conditions and in transfected 3T3 fibroblasts; the minimal luciferase activity of the TFEC promoter could be induced by coexpression of PU.1 or the related transcription factor Ets-2. The functional importance of the tissue-restricted expression of TFEC and a possible role in macrophage-specific gene regulation require further investigation, but are likely to be linked to the role of the other MiT family members in this lineage.

PU.1 regulates both cytokine-dependent proliferation and differentiation of granulocyte/macrophage progenitors

PU.1 is a unique regulatory protein required for the generation of both the innate and the adaptive immune system. It functions exclusively in a cell-intrinsic manner to control the development of granulocytes, macrophages, and B and T lymphocytes. We demonstrate that mutation of the PU.1 gene causes a severe reduction in myeloid (granulocyte/macrophage) progenitors. PU.1 -/- myeloid progenitors can proliferate in vitro in response to the multilineage cytokines interleukin-3 (IL-3), IL-6 and stem cell factor but are unresponsive to the myeloid-specific cytokines granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF and M-CSF. The failure of PU.1 -/- progenitors to respond to G-CSF is bypassed by transient signaling with IL-3. In the presence of IL-3 and G-CSF, PU.1 -/- progenitors can differentiate into granulocytic precursors containing myeloperoxidase-positive granules. Thus PU.1 is not essential for specification of granulocytic precursors, but is required for their further differentiation. The failure of PU.1 -/- progenitors to respond to M-CSF is due to lack of c-fms gene transcription. Transduction of c-fms into PU.1 -/- myeloid progenitors bypasses the block to M-CSF-dependent proliferation but does not induce detectable macrophage differentiation. Therefore, PU. 1 appears to be essential for specification of monocytic precursors. Importantly, retroviral transduction of PU.1 into mutant progenitors restores responsiveness to myeloid-specific cytokines and development of mature granulocytes and macrophages. Thus PU.1 controls myelopoiesis by regulating both proliferation and differentiation pathways.

IFN-γ Induction of the Human Monocyte Chemoattractant Protein (hMCP)-1 Gene in Astrocytoma Cells: Functional Interaction Between an IFN-γ-Activated Site and a GC-Rich Element

We characterized regulation of the human monocyte chemoattractant protein-1 (hMCP-1) gene by IFN-γ in astrocytoma cells, because astroglial cells express chemokines in several central nervous system inflammatory states. It was found that IFN-γ-induced hMCP-1 transcription was rapid, transient, and mediated by a 213-bp promoter-proximal regulatory region of the gene. Our studies on both in vitro and in vivo states of the hMCP-1 regulatory region established requirement of an IFN-γ-activated site (GAS) and the presence of IFN-γ-inducible GAS-binding activity involving at least STAT-1α for IFN-γ-induced hMCP-1 expression. Unexpectedly, in vivo genomic footprinting of the proximal regulatory region of the IFN-γ-induced gene revealed protection of a GC-rich sequence (GC box) with the same temporal pattern as that seen at the GAS; in vitro, this GC-rich element is associated with nuclear factor Sp1. These observations suggested a cooperative interaction between the GAS and the GC box element. Interestingly, site-specific mutations that abolished GC-box or GAS-element function produced clearly disparate results. Disruption of the GC box did not affect fold induction by IFN-γ but reduced promoter-reporter expression by half. Conversely, GAS mutation abrogated induction but did not affect the magnitude of expression. These results establish the importance of the GAS element for induction of hMCP-1 and further our understanding of IFN-γ-mediated transcriptional induction by providing the first evidence in vivo for inducible signaling to the GC box by this cytokine.

PU.1/Spi-1 Is Essential for the B Cell-Specific Activity of the Mouse CD72 Promoter

CD72 is a 45-kDa glycoprotein that is predominantly expressed on cells of the B lineage, except for plasma cells. Its expression pattern is representative of many B cell-specific proteins, which are essential for B cell development and activation but are down-regulated after B cells become terminally differentiated plasma cells. We have examined the promoter region of the mouse CD72 gene to identify sequences responsible for this regulatory pattern. The CD72 gene does not have an obvious TATAA box. Primer extension assays identified multiple transcription initiation sites. Deletion analyses have identified the 255-bp minimal promoter required for tissue-specific and developmental stage-specific expression. DNase I footprinting analysis of the CD72 minimal promoter revealed three protected elements: FP I, FP II, and FP III. Sequences corresponding to FP I or III gave increased reporter gene activity specifically in B cells, but not in T cells or NIH-3T3 cells. Sequences corresponding to FP II gave increased reporter gene activity in mature B cells, but not in plasma cells or non-B cells. Electrophoretic mobility shift assays and DNase I protection analyses revealed that FP I was bound by the transcription factor PU.1/Spi-1. Transient reporter analyses with plasmid bearing the mutated PU.1 binding site showed that binding of PU.1 is necessary for the increase in CD72 promoter activity in B cells. These results suggest that the 255-bp CD72 promoter confers both tissue specificity and developmental stage specificity, and that the B cell and macrophage-specific transcription factor PU.1 is essential for regulating the tissue specificity of the mouse CD72 promoter.

Defective B cell receptor-mediated responses in mice lacking the Ets protein, Spi-B

Spi-B is a hematopoietic-specific Ets family transcription factor closely related to PU.1. Previous gene targeting experiments have shown that PU.1 is essential for the production of both lymphocytes and monocytes. We have now generated mice with a null mutation at the Spi-B locus. Unlike PU.1 mutant mice, Spi-B-/- mice are viable, fertile and possess mature B and T lymphocytes. However, Spi-B-/- mice exhibit severe abnormalities in B cell function and selective T cell-dependent humoral immune responses. First, although Spi-B-/- splenic B cells respond normally to lipopolysaccharide stimulation in vitro, these B cells proliferate poorly and die in response to B cell receptor (surface IgM) cross-linking. Secondly, Spi-B-/- mice display abnormal T-dependent antigenic responses in vivo and produce low levels of antigen-specific IgG1, IgG2a and IgG2b after immunization. Finally, Spi-B-/- mice show a dramatic defect in germinal center formation and maintenance. In contrast to wild-type animals, germinal centers in Spi-B-/- mice are smaller and short-lived with significantly increased numbers of apoptotic B cells. Taken together, these results demonstrate that Spi-B is essential for antigen-dependent expansion of B cells, T-dependent immune responses and maturation of normal germinal centers in vivo.

Specific DNA binding of Stat5, but not of glucocorticoid receptor, is required for their functional cooperation in the regulation of gene transcription

Prolactin and glucocorticoid hormone are signals which regulate the transcription of milk protein genes in mammary epithelial cells. We have investigated the molecular mechanisms by which these hormones cooperate in the induction of transcription. Both hormones activate latent transcription factors in the cytoplasm of mammary epithelial cells. Prolactin exerts its effect through binding to the extracellular domain of the prolactin receptor and through receptor dimerization. This leads to the activation of a protein tyrosine kinase (Jak2), which is noncovalently associated with the cytoplasmic domain of the prolactin receptor. Jak2 phosphorylates the signal transducer and transcription activator (Stat5) which causes its dimerization and nuclear translocation where Stat5 specifically binds to sequence elements in the promoter regions of milk protein genes. In comparison, the glucocorticoid receptor is activated by a lipophilic steroid ligand in the cytoplasm which causes allosteric changes in the molecule, dimerization, and nuclear localization. It has been demonstrated that Stat5 and the glucocorticoid receptor form a molecular complex which cooperates in the induction of transcription of the beta-casein gene. We have defined the DNA sequence requirements for this cooperative mechanism and have delimited the functional domains in Stat5 and the glucocorticoid receptor that are necessary for the functional interaction. We find that the Stat5 response element (Stat5RE) within the beta-casein gene promoter is sufficient to elicit the cooperative action of Stat5 and the glucocorticoid receptor on transcription. Activation of Stat5 through phosphorylation of tyrosine 694 is an absolute prerequisite for transcription. Deletion of the transactivation domain of Stat5 results in a molecule which cannot mediate transactivation by itself but can still cooperate with the glucocorticoid receptor. Mutated variants of the glucocorticoid receptor with a nonfunctional DNA binding domain or a DNA binding domain contributed by the estrogen receptor are still able to cooperate with Stat5 in transcriptional induction. Deletion of the ligand binding domain of the glucocorticoid receptor does not impede cooperation with Stat5, whereas deletion of the AF-1 transactivation domain does prevent cooperation. Our results indicate that the glucocorticoid receptor acts as a ligand-dependent coactivator of Stat5 independently of its DNA binding function.

Combinatorial determinants of tissue-specific transcription in B cells and macrophages

A tripartite domain of the immunoglobulin mu heavy-chain gene enhancer that activates transcription in B cells contains binding sites for PU.1, Ets-1, and a leucine zipper-containing basic helix-loop-helix factor. Because PU.1 is expressed only in B cells and macrophages, we tested the activity of a minimal mu enhancer fragment in macrophages by transient transfections. The minimal mu enhancer activated transcription in macrophages, and the activity was dependent on all three sites. Analysis of mutated enhancers, in which spacing and orientation of the ETS protein binding sites had been changed, suggested that the mechanisms of enhancer activation were different in B cells and macrophages. Thus, ETS protein binding sites may be combined in different ways to generate tissue-specific transcription activators. Despite the activity of the minimal enhancer in macrophages, a larger mu enhancer fragment was inactive in these cells. We propose that formation of the nucleoprotein complex that is formed on the minimal enhancer in macrophages cannot be helped by the neighboring muE elements that are essential for activity of the monomeric enhancer.

The Ets protein Spi-B is expressed exclusively in B cells and T cells during development

Spi-B and PU.1 are hematopoietic-specific transcription factors that constitute a subfamily of the Ets family of DNA-binding proteins. Here we show that contrary to previous reports, PU.1 and Spi-B have very different expression patterns. PU.1 is expressed at high levels in B cells, mast cells, megakaryocytes, macrophages, neutrophils, and immature erythroid cells and at lower levels in mature erythrocytes. PU.1 is completely absent from peripheral T cells and most T cell lines based on sensitive RT-PCR assays. In contrast, Spi-B is expressed exclusively in lymphoid cells and can be detected in early fetal thymus and spleen. In situ hybridizations of adult murine tissues demonstrate Spi-B mRNA in the medulla of the thymus, the white pulp of the spleen, and the germinal centers of lymph nodes. Spi-B expression is very abundant in B cells and both Spi-B mRNA and protein are detected in some T cells. In situ hybridization and Northern blot analysis suggest that Spi-B gene expression increases during B cell maturation and decreases during T cell maturation. Gel-retardation experiments show that Spi-B can bind to all putative PU.1 binding sites, but do not reveal any preferred Spi-B binding site. Finally, both PU.1 and Spi-B function as transcriptional activators of the immunoglobulin light-chain enhancer E lambda 2.4 when coexpressed with Pip (PU.1-interaction partner) in NIH-3T3 cells. Taken together, these data suggest that differences in patterns of expression between Spi-B and PU.1 distinguish the function of each protein during development of the immune system.

Sp1 binds two sites in the CD11c promoter in vivo specifically in myeloid cells and cooperates with AP1 to activate transcription

The leukocyte integrin gene, CD11c, is transcriptionally regulated and is expressed predominantly on differentiated cells of the myelomonocytic lineage. In this study we have demonstrated that the regions -72 to -63 and -132 to -104 of the CD11c promoter contain elements responsible for phorbol ester-induced differentiation of the myeloid cell line HL60. DNase I footprinting analysis revealed that these regions can bind purified Sp1, and supershift analysis with Sp1 antibody confirmed that Sp1 in HL60 nuclear extracts could bind these regions. Transfection analysis of CD11c promoter-chloramphenicol acetyltransferase constructs containing deletions of these Sp1-binding sites revealed that these sites are essential for expression of the CD11c gene in HL60 cells but not in the T-cell line Molt4 or the cervical carcinoma cell line HeLa. Moreover, cotransfection of pPacSp1 along with these CD11c promoter-chloramphenicol acetyltransferase constructs into Sp1-deficient Drosophila Schneider 2 cells verified that these sites are essential for Sp1-dependent expression of the CD11c promoter. In vivo genomic footprinting revealed that Sp1 contacts the CD11c promoter within the regions -69 to -63 and -116 to -105 in phorbol 12-myristate 13-acetate-differentiated HL60 cells but not in undifferentiated HL60 cells or in Molt4 or HeLa cells. Cotransfection assays in HL60 cells revealed that Sp1 acts synergistically with Ap1 to activate CD11c. Further, both Sp1 sites are capable of cooperating with AP1. In vitro DNase I footprinting analysis with purified Sp1 and c-jun proteins showed that Sp1 binding could facilitate binding of c-jun. We propose that myeloid-specific expression of the CD11c promoter and is facilitated by cooperative interaction between the Sp1- and Ap1-binding sites.

Spi-1/PU.1 transgenic mice develop multistep erythroleukemias

Insertional mutagenesis of the spi-1 gene is associated with the emergence of malignant proerythroblasts during Friend virus-induced acute erythroleukemia. To determine the role of spi-1/PU.1 in the genesis of leukemia, we generated spi-1 transgenic mice. In one founder line the transgene was overexpressed as an unexpected-size transcript in various mouse tissues. Homozygous transgenic animals gave rise to live-born offspring, but 50% of the animals developed a multistep erythroleukemia within 1.5 to 6 months of birth whereas the remainder survived without evidence of disease. At the onset of the disease, mice became severely anemic. Their hematopoietic tissues were massively invaded with nontumorigenic proerythroblasts that express a high level of Spi-1 protein. These transgenic proerythroblasts are partially blocked in differentiation and strictly dependent on erythropoietin for their proliferation both in vivo and in vitro. A complete but transient regression of the disease was observed after erythrocyte transfusion, suggesting that the constitutive expression of spi-1 is related to the block of the differentiation of erythroid precursors. At relapse, erythropoietin-independent malignant proerythroblasts arose. Growth factor autonomy could be partially explained by the autocrine secretion of erythropoietin; however, other genetic events appear to be necessary to confer the full malignant phenotype. These results reveal that overexpression of spi-1 is essential for malignant erythropoiesis and does not alter other hematopoietic lineages.

Expression of the human acute myeloid leukemia gene AML1 is regulated by two promoter regions

The human chromosome 21 AML1 gene is expressed predominantly in the hematopoietic system. In several leukemia-associated translocations AML1 is fused to other genes and transcription of the fused regions is mediated by upstream sequences that normally regulate the expression of AML1. The 5' genomic region of AML1 was cloned and sequenced. The two 5' untranslated regions (UTRs) previously identified in AML1 cDNAs were located in this region and the distance between them was established. The distal 5' UTR maps over 7 kb upstream of the proximal one. Using primer extension with mRNA, transcription start sites were identified at two distinct sites above these 5' uTRs. Sequence analysis revealed the absence of a TATA motif and the presence of Sp1, PU.1, Oct, CRE, Myb, Ets, and Ets-like binding sites in both upstream regions. Several initiator elements (Inr) that overlap the transcription start sites were also identified. These proximal and distal upstream regions and their deletion mutants were cloned in front of a luciferase reporter gene and used in transfection assays. We demonstrate that both upstream regions function as promoters in hematopoietic (Jurkat) and nonhematopoietic (HEK) cell lines. The activity of both promoters was orientation dependent and was enhanced, in a cell-type specific manner, by a heterologous enhancer sequence. These results indicate that additional control elements, either negative or positive, regulate the tissue-specific expression of AML1.

PU.1 (Spi-1) and C/EBP alpha regulate expression of the granulocyte-macrophage colony-stimulating factor receptor alpha gene

Growth factor receptors play an important role in hematopoiesis. In order to further understand the mechanisms directing the expression of these key regulators of hematopoiesis, we initiated a study investigating the transcription factors activating the expression of the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor alpha gene. Here, we demonstrate that the human GM-CSF receptor alpha promoter directs reporter gene activity in a tissue-specific fashion in myelomonocytic cells, which correlates with its expression pattern as analyzed by reverse transcription PCR. The GM-CSF receptor alpha promoter contains an important functional site between positions -53 and -41 as identified by deletion analysis of reporter constructs. We show that the myeloid and B cell transcription factor PU.1 binds specifically to this site. Furthermore, we demonstrate that a CCAAT site located upstream of the PU.1 site between positions -70 and -54 is involved in positive-negative regulation of the GM-CSF receptor alpha promoter activity. C/EBP alpha is the major CCAAT/enhancer-binding protein (C/EBP) form binding to this site in nuclear extracts of U937 cells. Point mutations of either the PU.1 site or the C/EBP site that abolish the binding of the respective factors result in a significant decrease of GM-CSF receptor alpha promoter activity in myelomonocytic cells only. Furthermore, we demonstrate that in myeloid and B cell extracts, PU.1 forms a novel, specific, more slowly migrating complex (PU-SF) when binding the GM-CSF receptor alpha promoter PU.1 site. This is the first demonstration of a specific interaction with PU.1 on a myeloid PU.1 binding site. The novel complex is distinct from that described previously as binding to B cell enhancer sites and can be formed by addition of PU.1 to extracts from certain nonmyeloid cell types which do not express PU.1, including T cells and epithelial cells, but not from erythroid cells. Furthermore, we demonstrate that the PU-SF complex binds to PU.1 sites found on a number of myeloid promoters, and its formation requires an intact PU.1 site adjacent to a single-stranded region. Expression of PU.1 in nonmyeloid cells can activate the GM-CSF receptor alpha promoter. Deletion of the amino-terminal region of PU.1 results in a failure to form the PU-SF complex and in a concomitant loss of transactivation, suggesting that formation of the PU-SF complex is of functional importance for the activity of the GM-CSF receptor alpha promoter. Finally, we demonstrate that C/EBP alpha can also active the GM-CSF receptor alpha promoter in nonmyeloid cells. These results suggest that PU.1 and C/EBP alpha direct the cell-type-specific expression of GM-CSF receptor alpha, further establish the role of PU.1 as a key regulator of hematopoiesis, and point to C/EBP alpha as an additional important factor in this process.