The macrophage and B cell-specific transcription factor PU.1 is related to the ets oncogene

The transcription factor Spi-1/PU.1 interacts with the potential splicing factor TLS

Spi-1/PU.1 is an Ets protein deregulated by insertional mutagenesis during the murine Friend erythroleukemia. The overexpression of the normal protein in a proerythroblastic cell prevents its terminal differentiation. In normal hematopoiesis Spi-1/PU.1 is a transcription factor that plays a key role in normal myeloid and B lymphoid differentiation. Moreover, Spi-1/PU.1 binds RNA and interferes in vitro with the splicing process. Here we report that Spi-1 interacts in vivo with TLS (translocated in liposarcoma), a RNA-binding protein involved in human tumor-specific chromosomal translocations. This interaction appears functionally relevant, since TLS is capable of reducing the abilities of Spi-1/PU.1 to bind DNA and to transactivate the expression of a reporter gene. In addition, we observe that TLS is potentially a splicing factor. It promotes the use of the distal 5' splice site during the E1A pre-mRNA splicing. This effect is counterpoised in vivo by Spi-1. These data suggest that alteration of pre-mRNA alternative splicing by Spi-1 could be involved in the transformation of an erythroblastic cell.

The macrosialin promoter directs high levels of transcriptional activity in macrophages dependent on combinatorial interactions between PU.1 and c-Jun

Macrosialin is a transmembrane glycoprotein that is highly expressed in macrophages. In the present studies, macrosialin mRNA levels are shown to be markedly up-regulated during macrophage differentiation of bone marrow progenitor cells in response to macrophage colony-stimulating factor and granulocyte-macrophage colony-stimulating factor. To investigate the mechanisms responsible for regulation of macrosialin expression, we have isolated the macrosialin gene and performed an initial analysis of its transcriptional regulatory elements. The macrosialin promoter and 7.0 kilobase pairs of 5'-flanking information direct high levels of reporter gene activity in monocyte/macrophage-like cells, but little or no expression in nonmyeloid cells. This pattern of expression is dependent on regulatory elements located between -7.0 and -2.5 kilobase pairs from the transcriptional start site that exhibit strong enhancer activity in macrophages and repressor activity in nonmyeloid cells. Analysis of the proximal macrosialin promoter indicates that combinatorial interactions between at least four classes of transcriptional activators, including PU.1/Spi-1 and members of the AP-1 family are required for basal promoter function. PU.1/Spi-1 and c-Jun act synergistically to activate the macrosialin promoter in a nonmyeloid cell line, suggesting that combinatorial interactions between these proteins are involved in regulating macrosialin expression during macrophage differentiation.

Role of PU.1 in hematopoiesis

The ETS-family transcription factor PU.1 is expressed in hematopoietic tissues, with significant levels of expression in the monocytic and B lymphocytic lineages. PU.1 is identical to the Spi-1 proto-oncogene which is associated with the generation of spleen focus-forming virus-induced erythroleukemias. An extensive body of in vitro gene regulatory studies has implicated PU.1 as an important, versatile regulator of B lymphoid- and myeloid-specific genes. The first half of the review is designed to coalesce data generated from studies examining the two PU.1 "knockout" animals, which have prompted a reevaluation of the proposed function of PU.1 during hematopoiesis. During hematopoiesis, PU.1 is required for development along the lymphoid and myeloid lineages but needs to be downregulated during erythropoiesis. These unique functional characteristics of PU.1 will be exemplified by contrasting the function of PU.1 with other transcription factors required during fetal hematopoiesis. The second half of this review will reexamine the functional characteristics of PU.1 deduced from traditional biochemical and transactivation assays in light of recent experiments examining the functional behavior of PU.1 in an embryonic stem cell in vitro differentiation system. Working models of how PU.1 regulates promoter and enhancer regions in the B cell and myeloid lineage will be presented and discussed.

A Two-Step Mechanism for Recruitment of Pip by PU.1

Transcription of the Ig κ light chain gene is controlled in part by the 3′ κ enhancer. Two of the proteins that bind to the 3′ enhancer, PU.1 and Pip, show tissue-restricted expression and may be responsible for the tissue specificity of 3′ enhancer activity. PU.1 alone can bind to DNA; however, Pip cannot bind to its 3′ enhancer site in electrophoretic mobility shift assays, unless recruited by PU.1. Previously, we showed that the PU.1 PEST domain (rich in the amino acids proline, glutamate, serine, and threonine; sequences 118–160) is necessary for Pip recruitment to DNA. Here we used detailed mutagenic analyzes of PU.1 to more precisely identify sequences required for Pip recruitment by electrophoretic mobility shift assay. We found that mutation of three segments within the PU.1 PEST domain (118–125, 133–139, and 141–147) modulated the efficiency of Pip recruitment, while mutation of sequences between residues 88–118 and 154–168 had no effect. Interestingly, we found that the PU.1 ETS domain (residues 170 to 255) is both necessary and sufficient for Pip interaction in solution and that other ETS domain proteins can physically interact with Pip as well. Our results suggest that Pip recruitment to DNA by PU.1 occurs via a two-step mechanism. First, a physical interaction that is not sufficient to recruit Pip occurs via the PU.1 ETS domain. Second, a conformational change in the PU.1 PEST domain, apparently mediated by serine phosphorylation, induces a conformational change in Pip enabling it to bind to DNA. We also show that the PU.1 PEST domain does not target PU.1 for rapid turnover.

Identification of Distinct Regions of 5′ Flanking DNA That Mediate Constitutive, IFN-γ, STAT1, and TGF-β-Regulated Expression of the Class II Transactivator Gene

Class II transactivator (CIITA) is a master regulator required for constitutive and IFN-γ-inducible expression of class II MHC genes. Although the role of CIITA is greatly appreciated, the mechanisms underlying constitutive and IFN-γ-induced expression of CIITA are not understood. The study of CIITA induction is extremely important, but has been fraught with difficulty. This study describes for the first time a large (7-kb) fragment of 5′ flanking sequences that mediates the B cell-specific, IFN-γ-induced, and TGF-β-suppressed expression of CIITA. This pattern of expression matches the authentic expression of the endogenous gene. Within the 7-kb fragment, sequences that lie between nucleotides −545 and −113 relative to the transcriptional start site are critical for constitutive promoter expression in B cells. In contrast, inducible activation of CIITA by IFN-γ requires sequences contained in an additional 4 kb of upstream DNA. This region mediates an IFN-γ response when linked to either the endogenous CIITA promoter or a heterologous promoter. A role for STAT1 in regulation of the CIITA promoter is shown by the rescue of IFN-γ induction by expression of STAT1 in STAT1-defective U3A cells. TGF-β significantly inhibits IFN-γ-mediated induction of the CIITA promoter in 2fTGH fibroblasts, which indicates that the promoter is a target for TGF-β. This inhibition is achieved by suppression of the basal promoter. This study provides a focal point for understanding the mechanism of B cell-specific, IFN-γ-induced, and TGF-β-suppressed expression of CIITA.

Developmental biology of hematopoiesis

Hematopoietic stem cells are at the top of a hierarchy that regulates the generation of a vast repertoire of blood cells during the lifetime of a vertebrate. Recent experiments, using a vast variety of embryonic systems, shed new light on the origin of stem cells and the genes that function to regulate and maintain hematopoietic differentiation programs. Two distinct populations of stem cells develop--derived initially from transient, extraembryonic source and later from a stable, intraembryonic source; it is possible that both are generated from a pro-HSC able to respond differentially to local inductions. The initial blood cells develop from ventral mesoderm. The blood-forming region develops as a result of signaling from specific, secreted, embryonic growth factors, including the bone morphogenetic proteins. Stem cells give rise to progenitors that are restricted progressively in their ability to contribute to specific lineages. This process is regulated by transcription factors, whose functions are confirmed through genetic analyses. The identification of highly conserved, embryonic signaling pathways and transcription regulatory genes illustrates the enormous utility of analyzing embryonic hematopoiesis in frog, chick, fish, and mouse systems to further our understanding of human stem cells.

Repression mechanisms of the I-A beta gene of the major histocompatibility complex

The mechanisms of regulation of I-A beta gene expression in the murine major histocompatibility complex by transcriptional repression are reviewed. Active and passive repression mechanisms are presented. The transcription factor PU.1 actively inhibits the expression of I-A beta through the binding to a DNA sequence near the Y box, a cis-element in the promoter necessary for transcription. This interaction probably interferes with the preinitiation complex assembly. NF-Y is a transcription factor that binds to the Y box and has two constituents: NF-YA (that binds weakly to DNA) and NF-YB (that increases the binding of NF-YA to DNA). The dbpA protein represses the expression of I-A beta by a quenching mechanism, forming a complex with NF-YA and the dbpB protein by sequestering the NF-YB protein. A similar mechanism is observed with the glucocorticoid receptor that binds to the X-box binding proteins and inhibits their interaction with the X box. These results are examples of cross-talk between proteins, which may help us to understand the regulation of I-A beta gene expression.

Induction of interleukin-12 p40 transcript by CD40 ligation via activation of nuclear factor-kappaB

Interleukin-12 is produced in response to infection with bacteria or parasites or to bacterial constituents such as LPS in monocytes/macrophages and dendritic cells, and also generated by the interaction between activated T cells and antigen-presenting cells via CD40-CD40 ligand (CD40L). So far, transcriptional analyses of p40 have been carried out only using bacterial constituents such as LPS as stimuli. In the present study, we have characterized the transcriptional induction of p40 by CD40 ligation in a human B lymphoblastoid cell line, Daudi, and a human acute monocytic leukemia cell line, THP-1. These cells, stimulated by an agonistic monoclonal antibody against CD40 or by transfection with a CD40L expression vector, secreted p40 and showed enhanced p40 mRNA expression. Sequence analysis of the p40 promoter region identified two potential nuclear factor (NF)-kappaB binding sites conserved between mouse and human. Electrophoretic mobility shift assay revealed that the potential NF-kappaB binding sequence which is located around 120 bp upstream of the transcription initiation site in murine and human p40 genes formed an NF-kappaB complex with nuclear extract from Daudi cells stimulated by CD40 ligation. Moreover, transfection of Daudi cells with the polymerized NF-kappaB binding sequence ligated to a thymidine kinase/chloramphenicol acetyltransferase (CAT) reporter plasmid greatly induced CAT activity, but transfection with the polymerized mutated NF-kappaB binding sequence did not. These results suggest that the NF-kappaB binding site located around 120 bp upstream of the transcription initiation site in murine and human p40 promoter regions could be important for the p40 induction by CD40 ligation via activation of NF-kappaB.

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.

Murine Macrophage Mannose Receptor Promoter Is Regulated by the Transcription Factors PU.1 and SP1

The mannose receptor (MR) is a transmembrane protein that functions primarily as a phagocytic receptor for a wide range of microorganisms. Its expression appears to be restricted to tissue macrophages and Langerhans cells. To gain an understanding of the regulation of the gene, we have isolated the 5′ flanking sequence of the murine MR gene and have analyzed a 536-bp sequence upstream of the ATG start site for transcriptional activity. This sequence lacks a TATA box but contains an initiator (Inr) consensus element overlapping the single transcriptional start site. Transcription factor binding sites contained within this sequence include PU.1, Sp1, ETS, GATA, and MYB motifs. Serial 100-bp deletions of this promoter fragment fused to a luciferase reporter gene showed various patterns of activity when transfected into different cell types. In myeloid cells, sequence elements upstream of bp −300 appeared to have a silencing effect on promoter activity. Of the four potential PU.1 binding sites contained within the fragment, one site (at −164) bound the PU.1 factor most strongly, whereas the adjacent PU.1 site (at −177 bp) bound PU.1 to a lesser degree. Mutations of these sites decreased transcriptional activity but did not abolish it. However, promoter activity was abrogated when both the −164 bp PU.1 site and the adjacent −177 bp PU.1 site were mutated. In addition, mutation of the Sp1 site also significantly reduced promoter activity. Cotransfection studies in Drosophila Schneider cells indicated that PU.1 and Sp1 may function synergistically in transactivating the murine MR. This study indicates that MR gene expression is regulated in part by the interaction between the ubiquitously expressed factor Sp1 and the lymphoid/myeloid factor PU.1 and provides a basis for studying the regulation of this gene.

PU.1/Pip and Basic Helix Loop Helix Zipper Transcription Factors Interact With Binding Sites in the CD20 Promoter to Help Confer Lineage- and Stage-Specific Expression of CD20 in B Lymphocytes

CD20 is a B-lineage–specific gene expressed at the pre–B-cell stage of B-cell development that disappears on differentiation to plasma cells. As such, it serves as an excellent paradigm for the study of lineage and developmental stage-specific gene expression. Using in vivo footprinting we identified two sites in the promoter at −45 and −160 that were occupied only in CD20+ B cells. The −45 site is an E box that binds basic helix-loop-helix-zipper proteins whereas the −160 site is a composite PU.1 and Pip binding site. Transfection studies with reporter constructs and various expression vectors verified the importance of these sites. The composite PU.1 and Pip site likely accounts for both lineage and stage-specific expression of CD20 whereas the CD20 E box binding proteins enhance overall promoter activity and may link the promoter to a distant enhancer.

Complex expression pattern of the TNF region gene LST1 through differential regulation, initiation, and alternative splicing

Recently, a novel gene, LST1, was identified in the tumor necrosis factor region of the HLA complex, 4 kb centromeric of the lymphotoxin beta gene. By analyzing several full-length cDNA clones and the genomic DNA, we identified seven exons and four introns, spanning 2.7 kb. Isolation of mouse LST1 cDNA clones established the open reading frame. LST1 transcription is characterized by four alternative transcription initiation sites and extensive alternative splicing. The derived polypeptides vary with regard to the presence of the hydrophobic N-terminus and in short internal sequences. In addition, alternative splicing results in LST1 mRNAs encoding different carboxy-terminal sequences. LST1 is predominantly transcribed in monocytes, and mRNA levels increase upon stimulation with interferon-gamma, with a concomitant change in the mRNA pattern resulting in an enhanced expression of the short LST1 transcripts. These data suggest that LST1 may have a specific role in monocytes and possibly also in T cells. Moreover, we found that the recently published cDNA 1C7 is encoded just centromeric of LST1.

Gene within gene configuration and expression of the Drosophila melanogaster genes lethal(2) neighbour of tid [l(2)not] and lethal(2) relative of tid[l(2)rot]

In this paper, we describe the structure and temporal expression pattern of the Drosophila melanogaster genes l(2)not and l(2)rot located at locus 59F5 vis à vis the tumor suppressor gene l(2)tid described previously and exhibiting a gene within gene configuration. The l(2)not protein coding region, 1530 nt, is divided into two exons by an intron, 2645 nt, harboring the genes l(2)rot, co-transcribed from the same DNA strand, and l(2)tid, co-transcribed from the opposite DNA strand, located vis à vis. To determine proteins encoded by the genes described in this study polyclonal rabbit antibodies (Ab), anti-Not and anti-Rot, were generated. Immunostaining of developmental Western blots with the anti-Not Ab resulted in the identification of a 45-kDa protein, Not45, which is smaller than the Not56 protein predicted from the sequence. Its localization in endoplasmic reticulum (ER) was established by immunoelectron microscopy of Drosophila melanogaster Schneider 2 cells. Not45 shows significant homology to yeast ALG3 protein acting as a dolichol mannosyltransferase in the asparagine-linked glycosylation. It is synthesized ubiquitously throughout embryonic life. The protein predicted from the l(2)rot sequence, Rot57, shows a homology to the NS2B protein of the yellow fever virus1 (yefv1). The results of l(2)rot RNA analysis by developmental Northern blot and by in situ RNA localization, as well as the results of the protein analysis via Western blot and immunohistochemistry suggest that l(2)rot is transcribed but not translated. Since RNAs encoded by the genes l(2)tid and l(2)rot are complementary and l(2)rot is presumably not translated we performed preliminary experiments on the function of the l(2)rot RNA as a natural antisense RNA (asRNA) regulator of l(2)tid expression, expressed in the same temporal and spatial manner as the l(2)tid- and l(2)not RNA. l(2)tid knock-out by antisense RNA yielded late embryonic lethality resulting from multiple morphogenetic defects.

Activation of the megakaryocyte-specific gene platelet basic protein (PBP) by the Ets family factor PU.1

Platelet basic protein (PBP) is a chemokine family member that is only found in platelets and their precursors megakaryocytes. The PBP gene is physically linked to the gene for another platelet-specific chemokine, platelet factor 4. While the biological basis of platelet factor 4 expression has been pursued by others, the regulatory features controlling the platelet-specific expression of PBP have not been investigated. In this article, we examined the molecular basis by which this megakaryocyte-specific gene is regulated. Transient expression studies of truncated reporter constructs containing from 4.5 to 0.1 kilobases of the functional PBP gene 5'-flanking region, demonstrated that the proximal 0.1 kilobases of the promoter was sufficient for high levels of expression in human erythroleukemia and CHRF-288 cells, two megakaryocytic cell lines. However, none of these constructs was expressed above background levels in HeLa and 293 cells, two non-megakaryocytic cell lines. Further truncation of this promoter suggested that there was an important regulatory element(s) within a pyrimidine-rich tract. Mobility shift analysis of the pyrimidine-rich tract defined a region between -85 and -64 which bound to a nuclear factor(s). This region contains sequences matching the consensus Ets-binding site from -78 to -75 base pairs. In particular, we noted that this site matched a PU.1 consensus sequence known as a PU box. Mobility shift and supershift studies with nuclear extracts as well as recombinant PU.1 protein and anti-PU.1 antibody further confirmed that PU.1 was the specific Ets family factor that bound to this site. Transient expression assays using reporter constructs which contained point mutations that abrogated PU.1 binding also significantly reduced PBP promoter activity in human erythroleukemia and CHRF cells. In addition, while all reporter gene constructs containing PBP promoters were completely inactive in HeLa cells, transactivation experiments using a PU.1 expression construct demonstrated that exogenous expression of PU.1 could increase reporter gene expression up to 8-fold in these cells. Finally, the role of PU.1 in PBP gene expression was compared between wild-type and PU.1-null embryonic stem (ES) cells that were differentiated in vitro into cells that resembled megakaryocytes both morphologically and immunologically. We found that PBP gene expression in the differentiated PU.1(-/-) null ES cells (as determined by semi-quantitative reverse transcriptase-polymerase chain reaction) was more than four times lower than that in the wild-type ES cells, while other platelet-specific genes were expressed equally or similarly in the two ES cell lines. Previous reports have shown that PU.1 is expressed in several hematopoietic lineages, including megakaryocytes. However, the functional role of PU.1 has only been previously demonstrated in the myeloid and lymphoid lineages. Therefore, our studies are the first to show the biological importance of this nuclear factor in the regulated expression of a megakaryocyte-specific gene.

Multifunctional proteins suggest connections between transcriptional and post-transcriptional processes

Recent findings indicate that substantial cross-talk may exist between transcriptional and post-transcriptional processes. Firstly, there are suggestions that specific promoters influence the post-transcriptional fate of transcripts, pointing to communication between protein complexes assembled on DNA and nascent pre-mRNA. Secondly, an increasing number of proteins appear to be multifunctional, participating in transcriptional and post-transcriptional events. The classic example is TFIIIA, required for both the transcription of 5S rRNA genes and the packaging of 5S rRNA. TFIIIA is now joined by the Y-box proteins, which bind DNA (transcription activation and repression) and RNA (mRNA packaging). Furthermore, the tumour suppressor WT1, at first thought to be a typical transcription factor, may also be involved in splicing; conversely, hnRNP K, a bona fide pre-mRNA-binding protein, appears to be a transcription factor. Other examples of multifunctional proteins are mentioned: notably PTB, Sxl, La and PU.1. It is now reasonable to assert that some proteins, which were first identified as transcription factors, could just as easily have been identified as splicing factors, hnRNP, mRNP proteins and vice versa. It is no longer appropriate to view gene expression as a series of compartmentalised processes; instead, multifunctional proteins are likely to co-ordinate different steps of gene expression.

A Novel, Myeloid Transcription Factor, C/EBPε, Is Upregulated During Granulocytic, But Not Monocytic, Differentiation

Human C/EBPε is a newly cloned CCAAT/enhancer-binding transcription factor. Initial studies indicated it may be an important regulator of human myelopoiesis. To elucidate the range of expression of C/EBPε, we used reverse transcription-polymerase chain reaction (RT-PCR) analysis and examined its expression in 28 hematopoietic and 14 nonhematopoietic cell lines, 16 fresh myeloid leukemia samples, and normal human hematopoietic stem cells and their mature progeny. Prominent expression of C/EBPε mRNA occurred in the late myeloblastic and promyelocytic cell lines (NB4, HL60, GFD8), the myelomonoblastic cell lines (U937 and THP-1), the early myeloblast cell lines (ML1, KCL22, MDS92), and the T-cell lymphoblastic leukemia cell lines CEM and HSB-2. For the acute promyelocytic leukemia cell line NB4, C/EBPε was the only C/EBP family member that was easily detected by RT-PCR. No C/EBPε mRNA was found in erythroid, megakaryocyte, basophil, B lymphoid, or nonhematopoietic cell lines. Most acute myeloid leukemia samples (11 of 12) from patients expressed C/EBPε. Northern blot and RT-PCR analyses showed that C/EBPε mRNA decreased when the HL60 and KG-1 myeloblast cell lines were induced to differentiate toward macrophages. Similarly, Western blot analysis showed that expression of C/EBPε protein was either unchanged or decreased slightly as the promyelocytic cell line NB4 differentiated down the macrophage-like pathway after treatment with a potent vitamin D3 analog (KH1060). In contrast, C/EBPε protein levels increased dramatically as NB4 cells were induced to differentiate down the granulocytic pathway after exposure to 9-cis retinoic acid. Furthermore, very early, normal hematopoietic stem cells (CD34+/CD38−), purified from humans had very weak expression of C/EBPε mRNA, but levels increased as these cells differentiated towards granulocytes. Likewise, purified granulocytes appeared to express higher levels of C/EBPε mRNA than purified macrophages. Addition of phosphothiolated antisense, but not sense oligonucleotides to C/EBPε, decreased clonal growth of HL-60 and NB4 cells by about 50% compared with control cultures. Taken together, our results indicate that expression of C/EBPε is restricted to hematopoietic tissues, especially myeloid cells as they differentiate towards granulocytes and inhibition of its expression in HL-60 and NB4 myeloblasts and promyelocytes decreased their proliferative capacity. Therefore, this transcriptional factor may play an important role in the process of normal myeloid development.

A Novel, Myeloid Transcription Factor, C/EBPε, Is Upregulated During Granulocytic, But Not Monocytic, Differentiation

Human C/EBPε is a newly cloned CCAAT/enhancer-binding transcription factor. Initial studies indicated it may be an important regulator of human myelopoiesis. To elucidate the range of expression of C/EBPε, we used reverse transcription-polymerase chain reaction (RT-PCR) analysis and examined its expression in 28 hematopoietic and 14 nonhematopoietic cell lines, 16 fresh myeloid leukemia samples, and normal human hematopoietic stem cells and their mature progeny. Prominent expression of C/EBPε mRNA occurred in the late myeloblastic and promyelocytic cell lines (NB4, HL60, GFD8), the myelomonoblastic cell lines (U937 and THP-1), the early myeloblast cell lines (ML1, KCL22, MDS92), and the T-cell lymphoblastic leukemia cell lines CEM and HSB-2. For the acute promyelocytic leukemia cell line NB4, C/EBPε was the only C/EBP family member that was easily detected by RT-PCR. No C/EBPε mRNA was found in erythroid, megakaryocyte, basophil, B lymphoid, or nonhematopoietic cell lines. Most acute myeloid leukemia samples (11 of 12) from patients expressed C/EBPε. Northern blot and RT-PCR analyses showed that C/EBPε mRNA decreased when the HL60 and KG-1 myeloblast cell lines were induced to differentiate toward macrophages. Similarly, Western blot analysis showed that expression of C/EBPε protein was either unchanged or decreased slightly as the promyelocytic cell line NB4 differentiated down the macrophage-like pathway after treatment with a potent vitamin D3 analog (KH1060). In contrast, C/EBPε protein levels increased dramatically as NB4 cells were induced to differentiate down the granulocytic pathway after exposure to 9-cis retinoic acid. Furthermore, very early, normal hematopoietic stem cells (CD34+/CD38−), purified from humans had very weak expression of C/EBPε mRNA, but levels increased as these cells differentiated towards granulocytes. Likewise, purified granulocytes appeared to express higher levels of C/EBPε mRNA than purified macrophages. Addition of phosphothiolated antisense, but not sense oligonucleotides to C/EBPε, decreased clonal growth of HL-60 and NB4 cells by about 50% compared with control cultures. Taken together, our results indicate that expression of C/EBPε is restricted to hematopoietic tissues, especially myeloid cells as they differentiate towards granulocytes and inhibition of its expression in HL-60 and NB4 myeloblasts and promyelocytes decreased their proliferative capacity. Therefore, this transcriptional factor may play an important role in the process of normal myeloid development.

Thrombopoietin-induced expression of the glycoprotein IIb gene involves the transcription factor PU.1/Spi-1 in UT7-Mpl cells

Thrombopoietin (TPO) is the major regulator of proliferation and differentiation of megakaryocytes and their progenitors. These actions can be reproduced in the human megakaryoblastic cell line UT7 into which the murine TPO receptor, c-Mpl, was introduced. In these cells, TPO enhanced the expression of the specific megakaryocytic marker integrin glycoprotein (GP) IIb-IIIa while decreasing the expression of erythroid genes (Porteu, F., Rouyez, M. -C., Cocault, L., Benit, L., Charon, M., Picard, F., Gisselbrecht, S. , Souyri, M., and Dusanter-Fourt, I. (1996) Mol. Cell. Biol. 16, 2473-2482). We have now analyzed the effect of TPO on the transcriptional activity of the GPIIb promoter in these cells. Using transient transfection assays of a series of human GPIIb promoter fragments, we delineated a TPO-responsive element within the previously reported enhancer region of the promoter. Although this enhancer included GATA- and Ets-binding sites (EBSs), we found that only EBS -514 was important for TPO response. We identified PU. 1/Spi-1 as the endogenous Ets transcription factor that strongly and preferentially interacted with this enhancer EBS. This factor did not interact with other proximal EBSs in the GPIIb promoter. We next showed that TPO induced a strong and selective increase of PU. 1/Spi-1 expression and DNA binding activity in UT7-Mpl cells. In contrast, TPO did not affect the expression of Ets-1/2 while weakly increasing the levels of Fli-1. Overexpression of PU.1/Spi-1 was further shown to enhance GPIIb promoter activity in the absence and presence of TPO. Overall, our data indicated that, in UT7-Mpl cells, TPO increased the transcriptional activity of a GPIIb gene in part due to an enhanced expression of an unexpected transcription factor, the Ets family PU.1/Spi-1 factor. To our knowledge, this is the first evidence of a role for the PU.1/Spi-1 factor in the regulation of megakaryocytic genes.

Cloning and characterization of the mouse vitamin D receptor promoter

The gene encoding the mouse vitamin D receptor has been cloned. A new exon 1 has been found that changes the numbering established for the human VDR gene. Exons 2 and 3 in the human VDR gene (coding for the zinc fingers 1 and 2, respectively) are named exons 3 and 4 in the mouse vitamin D receptor. The 1.5-kb 5'-flanking region of the new exon 1 was analyzed and revealed the presence of putative cis-acting elements. Despite the absence of a TATA box, this 5'-flanking region contains several characteristics of a GC-rich promoter including four Sp1 sites present in tandem and two CCAAT boxes. Interestingly, the Sp1 site that is the most proximal to the new exon 1 overlaps a perfect site for Krox-20/24. Krox-20 is a transcription factor involved in brain development, and also in bone remodeling. In luciferase reporter gene expression assays, we showed that sequences from this 5'-flanking region elicit high transactivation activity. Furthermore, in the NIH 3T3 cell line, a 3- to 5-fold increase in response to forskolin treatment (an activator of adenylate cyclase and in turn of protein kinase A pathway) was observed.

The interferon gamma (IFN-gamma) receptor: a paradigm for the multichain cytokine receptor

With the purification and cloning of the interferon gamma (IFN-gamma) receptor chains the mechanism of IFN-gamma action and the resultant signal transduction events were delineated in remarkable detail. The interferon gamma (IFN-gamma) receptor complex consists of two chains: IFN-gammaR1, the ligand-binding chain, and IFN-gammaR2, the accessory chain. Binding of IFN-gamma causes oligomerization of the two IFN-gamma receptor subunits, IFN-gammaR1 and IFN-gammaR2, which initiates the signal transduction events: activation of Jak1 and Jak2 receptor associated protein tyrosine kinases, phosphorylation of the IFN-gammaR1 intracellular domain on Tyr440 followed by phosphorylation and activation of Stat1alpha, the latent transcriptional factor. With all these steps established, the IFN-gamma receptor complex has provided the basic model for understanding the receptors for other members of the family of class II cytokine receptors.

Granulocytic Differentiation of Normal Hematopoietic Precursor Cells Induced by Transcription Factor PU.1 Correlates With Negative Regulation of the c-myb Promoter

Numerous transcription factors allow hematopoietic cells to respond to lineage- and stage-specific cytokines and/or to act as their effectors. The transcription factors PU.1 and c-Myb are essential for hematopoiesis, most likely acting at distinct stages of differentiation, but sharing a common set of target genes. To determine whether PU.1 and c-Myb are functionally interrelated, murine bone marrow (BM) cells and 32Dcl3 murine myeloid precursor cells were infected with a retrovirus carrying a PU.1 cDNA and assessed for myeloid colony formation and for granulocytic differentiation, respectively. Compared with noninfected normal BM cells or to cells infected with an empty virus, hematopoietic precursor cells expressing PU.1 formed an increased number of interleukin-3 (IL-3) and granulocyte colony-stimulating factor (G-CSF )–stimulated colonies. Moreover, granulocytic differentiation of 32Dcl3 cells constitutively expressing PU.1 was accelerated, as indicated by morphology and by expression of differentiation markers. Downregulation of c-Myb protein levels by expression of an antisense c-myb construct was also associated with a faster kinetics of 32Dcl3 granulocytic differentiation. Sequence analysis of the 5′ flanking region of the c-myb gene revealed a consensus PU box at position +16 to +21 able to specifically interact in electrophoretic mobility shift assays with either bacterially synthesized PU.1 protein or whole cell extracts from differentiated 32Dcl3 cells. Transient expression of PU.1 in cotransfection assays in different cell lines resulted in inhibition of chloramphenicol acetyl transferase activity driven by different segments of the c-myb promoter. Moreover, such an effect was dependent on an intact PU box. Thus, the ability of PU.1 to potentiate terminal myeloid differentiation appears to involve downregulation of c-myb expression, an essential step during differentiation of hematopoietic precursor cells.

Human alpha-galactosidase A: high plasma activity expressed by the -30G-->A allele

Human alpha-galactosidase A (EC 3.2.1.22; alpha-Gal A) is the lysosomal exoglycosidase responsible for the hydrolysis of terminal alpha-galactosyl residues from glycoconjugates and is the defective enzyme causing Fabry disease (McKusick 301500). An unusally elevated level of plasma alpha-Gal A activity (> 2.5 times the normal mean) was detected in two unrelated normal males and the elevated activities were inherited as X-linked traits in their families. Sequencing of the alpha-Gal A coding region, intron/exon boundaries and 5'-flanking region from the proband identified a single mutation, a G-->A transition 30 nt upstream from the initiation of translation codon in exon 1. The -30G-->A mutation occurred in a putative NF kappa B/Ets consensus binding site that was recently shown to inhibit protein binding to the 5'-untranslated region of the gene, providing a possible explanation for its high activity. To further characterize the mutation, the mRNA and protein expressed by this variant allele were studied. Purified plasma and lymphoblast alpha-Gal A activity from individuals with the -30G-->A mutation had normal physical and kinetic properties. In vitro translation of mRNAs from the cloned normal and high plasma activity alleles resulted in similar levels of alpha-Gal A protein, indicating that this mutation did not enhance translation. These findings suggest that the -30G-->A mutation in the 5'-untranslated region of the alpha-Gal A gene enhances transcription, presumably by interfering with the binding of negatively-acting transcription factors which normally decrease alpha-Gal A expression in various cells. Preliminary studies of the frequency of the -30G-->A mutation in 395 unrelated normal males of mixed ancestry revealed two additional unrelated individuals who had high plasma enzymatic activity and the mutation, confirming the effect of this mutation on enzyme expression and suggesting that about 0.5% of normal individuals have high plasma alpha-Gal A activity due to this variant allele.

SA-1, a nuclear protein encoded by one member of a novel gene family: molecular cloning and detection in hemopoietic organs

We report the molecular cloning of a novel gene family. The first member of this family was cloned from a mouse lambda gt11 cDNA library using the B92 monoclonal antibody (mAb) raised against stromal cell extracts. This was followed by RACE-PCR using mRNA from the stromal cell line. A 4 kb cDNA was obtained encoding a unique protein sequence of 1258 aa, that we designate stromal antigen (SA)-1. The human SA-1 gene was cloned by homology from a thymus cDNA library and the sequence of the predicted protein was found to be highly homologous to the murine SA-1 (>98.9%). Another cDNA was cloned and the deduced protein (SA-2) was 71% homologous to SA-1. Northern blot and PCR analysis indicated that on the mRNA level the SA-1 gene is expressed in all tissues analyzed and probably encodes a single transcript. The identification of SA-1 protein in tissues and cells required combined immunoprecipitation and Western blotting using a polyclonal antiserum raised against a predicted peptide of SA-1 and the B92 mAb. Using this assay we identified a protein of about 120 kDa in hemopoietic organs. Subcellular fractionation indicated that SA-1 is a nuclear protein. Thus, despite the ubiquitous expression on the mRNA level, the protein was predominantly detected in hemopoietic organs and may therefore be controlled on a post-transcriptional level. The SA-1 gene described in this study is highly conserved between mouse and man. This implies a crucial function for this protein.

Transcriptional regulation during myelopoiesis

The coordinated production of all blood cells from a common stem cell is a highly regulated process involving successive stages of commitment and differentiation. From analyses of mice deficient in transcription factor genes and from the characterizations of chromosome breakpoints in human leukemias, it has become evident that transcription factors are important regulators of hematopoiesis. During myelopoiesis, which includes the development of granulocytic and monocytic lineages, transcription factors from several families are active, including AML1/CBF beta, C/EBP, Ets, c-Myb, HOX, and MZF-1. Few of these factors are expressed exclusively in myeloid cells; instead it appears that they cooperatively regulate transcription of myeloid-specific genes. Here we discuss recent advances in transcriptional regulation during myelopoiesis.

Isolation and characterization of a novel epithelium-specific transcription factor, ESE-1, a member of the ets family

We report here the isolation of a novel, highly tissue-restricted member of the ets transcription factor/oncogene family, ESE-1 (for epithelium-specific Ets), which has features distinct from those of any other ets-related factor. ESE-1 contains two putative DNA binding domains: an ETS domain, which is unique in that the 5' half shows relatively weak homology to known ets factors, and an A/T hook domain, found in HMG proteins and various other nuclear factors. In contrast to any known ets factors, ESE-1 is expressed exclusively in epithelial cells. ESE-1 expression is induced during terminal differentiation of the epidermis and in a primary human keratinocyte differentiation system. The keratinocyte terminal differentiation marker gene, SPRR2A, is a putative target for ESE-1, since SPRR2A expression during keratinocyte differentiation correlates with induction of ESE-1 expression, and ESE-1 binds with high affinity to and transactivates the ets binding site in the SPRR2A promoter. ESE-1 also binds to and transactivates the enhancer of the Endo A gene, a potential target for ESE-1 in simple epithelia. Due to the important role that other ets factors play in cellular differentiation, ESE-1 is expected to be a critical regulator of epithelial cell differentiation.

PU.1 negatively regulates the CD11c integrin gene promoter through recognition of the major transcriptional start site

CD11c integrin expression is restricted to myeloid cells and activated B lymphocytes, mainly through the collaborative action of Sp1 and members of the AP-1 and C/EBP transcription factor families on the proximal region of the CD11c gene promoter. While analyzing the role of an initiator-like sequence at the major transcriptional start site, an inverted consensus GGAA Ets binding site was identified as a negative regulatory element whose disruption increases the activity of the CD11c promoter. The GGAA element was specifically recognized by PU.1 in THP-1 monocytic cells and by PU.1 and GABP-related proteins in U937 promonocytic cells. Mutational analysis indicated that PU.1 recognition depends not only on the GGAA consensus element but also on flanking sequences. The functional relevance of PU.1 binding was assayed in transactivation experiments in HeLa cells, where PU.1 co-expression led to a significant decrease in the activity of the CD11c promoter, demonstrating that PU.1 inhibits the activity of the CD11c promoter through a PU.1 binding site located at the major transcriptional start site (PU1-5). The inhibitory action of PU.1 on CD11c is in contrast with its positive regulatory effect on the CD11b and CD18 integrin gene promoters, which might contribute to the differentially regulated expression of CD11b/CD18 and CD11c/CD18 during monocyte extravasation and terminal maturation. In addition, since PU.1 transcriptional activity correlates with macrophage proliferation, PU.1 might modulate CD11c gene transcription according to the proliferative state of the cell.

Structure of the murine CD156 gene, characterization of its promoter, and chromosomal location

The murine cell surface antigen mCD156 is a glycoprotein that is expressed in monocytic cell lines and consists of a metalloprotease domain, a disintegrin domain, a cysteine-rich domain, and an epidermal growth factor-like domain in the extracellular region. The mCD156 gene is composed of 24 exons and 23 introns and spans approximately 14 kilobases. The first exon encodes most of the signal peptide sequence, and the transmembrane region is encoded by a single exon (19). In contrast, the other regions are composed of multiple exons. Of these, exons 7-12 and 12-15 encode a metalloprotease domain and a disintegrin domain, respectively. Sequence analysis of the 5'-flanking DNA revealed many potential regulatory motifs. Chloramphenicol acetyltransferase analysis demonstrated that nucleotides at positions -183, -334, and -623 contained cis-acting enhancing elements in a mouse monocytic cell line, aHINS-B3. Nucleotides at positions -183 and -390 contained elements responsible for lipopolysaccharide (LPS) inducibility, although several other 5'-flanking regions were also involved in LPS responsiveness. Regions -202, -507, and -659 play a role in interferon-gamma inducibility. Some of the potential regulatory motifs and other unknown cis elements may be involved in the constitutive expression, and LPS and interferon-gamma inducibilities. The mCD156 gene was mapped to chromosome 7, region F3-F4.

The conserved 5'-untranslated leader of Spi-1 (PU.1) mRNA is highly structured and potently inhibits translation in vitro but not in vivo

The transcription factor Spi-1 (PU.1) has a central role in regulating myeloid gene expression during hematopoietic development and its overexpression has been implicated in erythroleukemic transformation. Thus regulation of Spi-1 expression has broad significance for hematopoietic development. A comparison of human and murine cDNA sequences demonstrates that the 5'-untranslated region (5'-UTR) of Spi-1 mRNA is as highly conserved as the coding region (87% identical), suggesting that this sequence may be involved in regulating expression of this protein. The experiments presented in this manuscript provide evidence that the 5'-UTR of Spi-1 contains extensive secondary structure, including three stem-loops that precede the AUG codon. Analysis of the in vitro transcribed Spi-1 5'-UTR by partial nuclease digestion sensitivity is consistent with the existence of two of these stem-loops. The 5'-UTR decreased translation of Spi-1 transcripts in reticuloctye lysates 8- to 10-fold. A series of partial deletions of the 5'-UTR identified the sequence corresponding to the stem-loop most proximal to the initiating AUG codon as sufficient for inhibition of translation. However, the effect of the 5'-UTR on translation in vivo was negligible and resulted in only a slight reduction in the number of ribosomes that became associated with the mRNA. Further, this sequence had no affect on expression of luciferase. The disparity between in vivo and in vitro effects, coupled with the observation that endogenous Spi-1 mRNA is wholly associated with polysomes in MEL cells, suggests that additional cellular mechanisms contribute to regulation of Spi-1 expression in these cells or that conservation of these sequences serves a function that is independent of translation.

PU.1 is essential for p47(phox) promoter activity in myeloid cells

Expression of the phagocyte cytosolic protein p47(phox), a component of NADPH oxidase, is restricted mainly to myeloid cells. To study the cis-elements and trans-acting factors responsible for its gene expression, we have cloned and characterized the p47(phox) promoter. A predominant transcriptional start site was identified 21 nucleotides upstream of the translation initiation codon. To identify the gene promoter sequences, transient transfections of HL-60 human myeloid cells were performed with a series of 5'-deletion p47(phox)-luciferase reporter constructs that extended as far upstream as -3050 bp relative to the transcriptional start site. The -224 and -86 constructs had the strongest p47(phox) promoter activity, whereas the -46 construct showed a major reduction in activity and the -36 construct a complete loss of activity. DNase I footprint analysis identified a protected region from -37 to -53. This region containing a consensus PU.1 site bound specifically both PU.1 present in nuclear extracts from myeloid cells and PU.1 synthesized in vitro. Mutations of this site eliminated PU.1 binding and abolished the ability of the p47(phox) promoter to direct expression of the reporter gene. The p47(phox) promoter was active in all myeloid cell lines tested (HL-60, THP-1, U937, PLB-985), but not in non-myeloid cells (HeLa, HEK293). Finally, PU.1 trans-activated the p47(phox)-luciferase constructs in HeLa cells. We conclude that, similar to certain other myeloid-specific genes, p47(phox) promoter activity in myeloid cells requires PU.1.

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.

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.

Diverse transcription factors are involved in the quantitative regulation of transcriptional activation of kappa promoters

Immunoglobulin kappa promoters show sequence divergence but conserved function between different subgroups. Here we show that three separate 5' elements are required for synergistic stimulation of transcription with the decamer in a kappa promoter. These sites are a 5' E-box, a 3' AT-rich region in the pentadecamer (pd) element, and the kappa-Y element. Elf-1 is a novel kappa-Y element ligand induced upon mitogenic stimulation of resting B lymphocytes. Furthermore, the 5' E2A-like E-box in the pd element could be substituted by an upstream stimulatory factor motif with conservation of function. Thus, the synergistic activation requirements of kappa transcription is strictly dependent on the quantitative presence of transcription factor-binding motifs 5' of the decamer, but these differ qualitatively in that they may bind an array of proteins with conserved function.

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.

MNDA dimerizes through a complex motif involving an N-terminal basic region

Human myeloid cell nuclear differentiation antigen (MNDA) is a myelomonocytic lineage-specific protein that influences gene expression through interactions with other nuclear proteins and transcription factors. MNDA also self-associates and chemical cross-linking was used to demonstrate that MNDA forms a dimer. C-terminal and internal deletion mutants were used to identify two regions in the N-terminal half of MNDA essential for self-association. One region contains an imperfect leucine zipper and the second is highly enriched in basic residues. The sequences that are essential for dimerization are separated by a highly basic amphipathic alpha-helical region which was not required for dimerization.

Transcriptional activity and constitutive nuclear localization of the ETS protein Elf-1

Elf-1 is a lymphoid-specific transcription factor that belongs to the ETS protein family. It can bind to DNA target sequences within a variety of cytokine genes. We demonstrate that Elf-1 is constitutively localized in the nucleus which is dependent on the presence of amino acids 86-265. Analysis of Gal4-Elf-1 fusion proteins revealed that the N-terminal 86 amino acids of Elf-1 contain a transcriptional activation domain, the activity of which is attenuated by an internal repression domain. Furthermore, Elf-1 interacts specifically with the E74 target sequence and can stimulate transcription driven by the E74 site independent of mitogenic signaling. Thus, Elf-1 is able to stimulate gene transcription which may be required for the development and activity of lymphocytes.

Characterization of a novel protein kinase C response element in the glucagon gene

To maintain glucose levels in blood within narrow limits, the synthesis and secretion of pancreatic islet hormones are controlled by a variety of neural, hormonal, and metabolic messengers that act through multiple signal transduction pathways. Glucagon gene transcription is stimulated by cyclic AMP and depolarization-induced calcium influx. In this study, the effect of protein kinase C on glucagon gene transcription was investigated. After transient transfection of a glucagon-reporter fusion gene into the glucagon-producing islet cell line alphaTC2, activation of protein kinase C by 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulated glucagon gene transcription. By 5' deletions, 3' deletions, internal deletion, and oligonucleotide cassette insertion, the TPA-responsive element was mapped to the G2 element (from -165 to -200). Like TPA, overexpression of oncogenic Ras (V-12 Ras) stimulated G2-mediated transcription whereas overexpression of a dominant negative Ras mutant (N-17 Ras) blocked the effect of TPA. A mutational analysis of G2 function and nuclear protein binding indicated that protein kinase C and Ras responsiveness is conferred to the glucagon gene by HNF-3beta functionally interacting with a protein that binds to a closely associated site with sequence similarity to binding sites of Ets family proteins. HNF-3beta belongs to the winged-helix family of transcription factors and has been implicated in the control of cell-specific and developmental gene expression. The results of the present study show that the cell lineage-specific transcription factor HNF-3beta is an essential component of a novel protein kinase C response element in the glucagon gene.

The promoter activity of the phospholipase C-gamma2 gene is regulated by a cell-type-specific control element

We have cloned and characterized a genomic DNA spanning the 5'-flanking region, the first and second exons, and the first intron of the human PLC-gamma2 gene. The proximal upstream region is highly GC-rich and lacks a TATA box, whereas the distal region contains several AT-rich tracts. Multiple transcription initiation sites were identified by primer extension analysis. Based on the transient transfection assays, the major transcriptional activation element was identified between -183 and +43 (G2SE) and a transcriptional repressive element was found between -303 and -184 (G2RE). The expression of PLC-gamma2 in various cell lines was examined using monoclonal anti-PLC-gamma2 antibody. PLC-gamma2 was highly expressed in B-cell lines such as Daudi, SP2, and Ramos cells, whereas it existed at very low levels in Jurkat, 3T3-L1, NBL-7, and C6Bu-1 cells. Moderate levels of PLC-gamma2 were also detected in C2C12, P19, U937, HL60, A431, and PC12 cells. The 4-kb genomic fragment upstream of -1,654 was able to activate transcription from the PLC-gamma2 promoter in Daudi and C2C12 cells, but not in Jurkat cells, which is consistent with the PLC-gamma2 protein expression levels in those cell lines. These results suggest that the cell-type-specific expression of PLC-gamma2 might be attributed to the transcriptional regulation by the upstream cis-element.

Osteopetrosis in mice lacking haematopoietic transcription factor PU.1

Osteoclasts are multinucleated cells and the principal resorptive cells of bone. Although osteoclasts are of myeloid origin, the role of haematopoietic transcription factors in osteoclastogenesis has not been explored. Here we show that messenger RNA for the myeloid- and B-cell-specific transcription factor PU.1 progressively increases as marrow macrophages assume the osteoclast phenotype in vitro. The association between PU.1 and osteoclast differentiation was confirmed by demonstrating that PU.1 expression increased with the induction of osteoclastogenesis by either 1,25-dihydroxyvitamin D3 or dexamethasone. Consistent with the participation of PU.1 in osteoclastogenesis, we found that the development of both osteoclasts and macrophages is arrested in PU.1-deficient mice. Reflecting the absence of osteoclasts, PU.1-/- mice exhibit the classic hallmarks of osteopetrosis, a family of sclerotic bone diseases. These animals were rescued by marrow transplantation, with complete restoration of osteoclast and macrophage differentiation, verifying that the PU.1 lesion is intrinsic to haematopoietic cells. The absence of both osteoclasts and macrophages in PU.1-mutant animals suggests that the transcription factor regulates the initial stages of myeloid differentiation, and that its absence represents the earliest developmental osteopetrotic mutant yet described.

I-A beta gene expression regulation in macrophages derived from mice susceptible or resistant to infection with M. bovis BCG

The innate capacity of mice to control mycobacterial multiplication early after infection is controlled by the resistant allele of the Nramp-1/Bcg gene. The Bcg gene seems to be involved in a pathway leading to macrophage activation. It differentially affects the ability of BCG-resistant and -susceptible strains of mice to express important macrophage genes including Major Histocompatibility Complex (MHC) class II genes. An inhibition of Nramp1 gene by Nramp1-ribozyme transfection in macrophages resulted in the impairment of MHC class II gene induction by IFN gamma. In this study, we have investigated the molecular mechanisms involved in IFN-gamma-induced MHC class II expression using macrophages derived from mice resistant or susceptible to mycobacterial infections (B10R and B10S, respectively). We have found that the difference in the IFN gamma-induced Ia surface protein expression between B10R and B10S macrophages correlate with a higher rate of I-A beta gene transcription. We have also studied the binding of proteins prepared from nuclear extracts of non-stimulated and IFN-gamma-stimulated B10R and B10S macrophages to the S, X and Y cis-acting elements of the I-A beta promoter. Differences observed in protein binding to the X box may explain the difference in transcription activation of the I-A beta gene. We have also found that I-A alpha and I-A beta mRNA half-lives measured in IFN gamma-stimulated cells are significantly longer in B10R, compared to B10S macrophages. Overall, our data suggest that both transcriptional and posttranscriptional regulatory mechanisms are responsible for the more efficient expression of I-A beta gene in macrophages carrying a resistant allele of Nramp1 gene.

Cloning of the novel human myeloid-cell-specific C/EBP-epsilon transcription factor

Chicken NF-M transcription factor, in cooperation with either c-Myb or v-Myb, is active in the combinatorial activation of myeloid-cell-specific genes in heterologous cell types, such as embryonic fibroblasts. In humans, similar effects were observed with homologous members of the CCAAT/enhancer-binding protein (C/EBP) family of transcriptional regulators, especially the human homolog of chicken NF-M, C/EBP-beta (NF-IL6). However, the NF-IL6 gene is expressed in a variety of nonmyeloid cell types and is strongly inducible in response to inflammatory stimuli, making it an unlikely candidate to have an exclusive role as a combinatorial differentiation switch during myelopoiesis in human cells. By using a reverse transcription-PCR-based approach and a set of primers specific for the DNA-binding domains of highly homologous members of the C/EBP family of transcriptional regulators, we have cloned a novel human gene encoding a member of the C/EBP gene family, identified as the human homolog of CRP1, C/EBP-epsilon. A 1.2-kb cDNA encoding full-length human C/EBP-epsilon was cloned from a promyelocyte-late myeloblast-derived lambda gt11 library. Molecular analysis of the cDNA and genomic clones indicated the presence of two exons encoding a protein with an apparent molecular mass of 32 kDa and a pI of 9.5. Primer extension analysis of C/EBP-epsilon mRNA detected a single major transcription start site approximately 200 bp upstream of the start codon. The putative promoter area is similar to those of several other myeloid-cell-specific genes in that it contains no TATAAA box but has a number of purine-rich stretches with multiple sites for the factors of the Ets family of transcriptional regulators. Northern blot analyses indicated a highly restricted mRNA expression pattern, with the strongest expression occurring in promyelocyte and late-myeloblast-like cell lines. Western blot and immunoprecipitation studies using rabbit anti-C/EBP-epsilon antibodies raised against the N-terminal portion of C/EBP-epsilon (amino acids 1 to 115) showed that C/EBP-epsilon is a 32-kDa nuclear phosphoprotein. The human C/EBP-epsilon protein exhibited strong and specific binding to double-stranded DNA containing consensus C/EBP sites. Cotransfection of the C/EBP-epsilon sense and antisense expression constructs together with chloramphenicol acetyltransferase reporter vectors containing myeloid-cell-specific c-mim and human myeloperoxidase promoters suggested a role for C/EBP-epsilon transcription factor in the regulation of a subset of myeloid-cell-specific genes. Transient tranfection of a promyelocyte cell line (NB4) with a C/EBP-epsilon expression plasmid increased cell growth by sevenfold, while antisense C/EBP-epsilon caused a fivefold decrease in clonal growth of these cells.

Structural and functional characterization of the human perlecan gene promoter. Transcriptional activation by transforming growth factor-beta via a nuclear factor 1-binding element

Perlecan, a modular heparan sulfate proteoglycan of basement membranes and cell surfaces, plays a crucial role in regulating the assembly of extracellular matrices and the binding of nutrients and growth factors to target cells. To achieve a molecular understanding of perlecan gene regulation, we isolated the 5'-flanking region and investigated its functional promoter activity and its response to cytokines. Transient cell transfection assays, using plasmid constructs harboring the perlecan promoter linked to the chloramphenicol acetyltransferase reporter gene, demonstrated that the largest approximately 2.5-kilobase construct contained maximal promoter activity. This promoter region was functionally active in a variety of cells of diverse histogenetic origin, thus corroborating the widespread expression of this gene product. Stepwise 5' deletion analyses demonstrated that the -461-base pair (bp) proximal promoter retained approximately 90% of the total activity, and internal deletions confirmed that the most proximal sequence was essential for proper promoter activity. Nanomolar amounts of transforming growth factor-beta induced 2-3-fold perlecan mRNA and protein core levels in normal human skin fibroblasts, and this induction was transcriptionally regulated; in contrast, tumor necrosis factor-alpha had no effect and was incapable of counteracting the effects of TGF-beta. Using additional 5' deletions and DNase footprinting analyses, we mapped the TGF-beta responsive region to a sequence of 177 bp contained between -461 and -285. This region harbored a 14-bp element similar to a TGF-beta-responsive element present in the promoters of collagen alpha1(I), alpha2(I), elastin, and growth hormone. Electrophoretic mobility shift assays and mutational analyses demonstrated that the perlecan TGF-beta-responsive element bound specifically to TGF-beta-inducible nuclear proteins with high affinity for NF-1 member(s) of transcription factors.

Overexpression of PU.1 Induces Growth and Differentiation Inhibition and Apoptotic Cell Death in Murine Erythroleukemia Cells

PU.1 is a member of the ets family of transcription factors and is expressed in Friend virus-induced murine erythroleukemia (MEL) cells as a consequence of proviral integration into the PU.1/Spi-1 locus. After induction of MEL cell differentiation by treatment with dimethylsulfoxide (DMSO), expression of the PU.1/Spi-1 gene decreased before induction of β-globin gene expression. Overexpression of PU.1 by using a zinc-inducible expression plasmid in MEL cells resulted in unexpected growth inhibition of the transfectants. When PU.1-overexpressing transfectants were treated with DMSO, growth inhibition became much pronounced and apoptosis was induced. Expression of the β-globin gene was not induced under this condition. Neither growth inhibition nor apoptosis was induced in MEL cells after expression of mutant PU.1 proteins with a deletion of the activation domain or the DNA-binding Ets domain irrespective of the presence of DMSO. Interestingly, β-globin gene expression was not induced in the transfectants expressing the former mutant, whereas it was induced in those expressing the latter one in the presence of DMSO. These results indicate that overexpression of PU.1 in MEL cells results in growth and differentiation inhibition and, in conjunction with DMSO treatment, apoptotic cell death. These results also suggest that the activation domain and the Ets domain of PU.1 contribute differently to induction of these effects.

Expression of NRAMP1 molecule in human peripheral blood leukocytes

Natural resistance or susceptibility of host to infection with several intracellular pathogens, such as Mycobacterium, Salmonella and Leishmania, is controlled in mice by the expression of a single dominant gene locus designated Lsh/Ity/Bcg. Natural resistance-associated macrophage protein gene 1 (Nramp1) was isolated as a candidate gene. Nramp1 gene encodes a 60 kDa polypeptide with 10-12 potential transmembrane domains and an evolutionary conserved consensus transport motif. The present study shows that the human NRAMP1 gene is expressed in all established hematopoietic cell lines examined, including monocytes/macrophages and B- and T-lymphocytes. In contrast, cell type-specific expressions are observed in human peripheral blood leukocytes. NRAMP1 expression is very low level in granulocytes. B- and T-lymphocytes are equivalent in the level of NRAMP1 expression. Notable expression of NRAMP1 gene can be detected in the monocyte population. These results have important implications for the host defence mechanisms and the pathogenesis of intracellular pathogens which are recognized and ingested by the mononuclear phagocyte system including monocytes/macrophages.

The nfkb1 promoter is controlled by proteins of the Ets family

The gene encoding NFKB1 is autoregulated, responding to NF-kappa B/Rel activation through NF-kappa B binding sites in its promoter, which also contains putative sites for Ets proteins. One of the Ets sites, which we refer to as EBS4, is located next to an NF-kappa B/Rel binding site, kB3, which is absolutely required for activity of the promoter in Jurkat T cells in response to activation by phorbol 12-myristate 13-acetate (PMA), PMA/ionomycin, or the Tax protein from human T cell leukemia virus type I. We show that EBS4 is, required for the full response of the nfkb1 promoter to PMA or PMA/ionomycin in Jurkat cells. EBS4 is bound by Ets-1, Elf-1, and other species. Overexpression of Ets-1 augments the response to PMA/ionomycin and this is reduced by mutation of EBS4. Elf-1 has less effect in conjunction with PMA/ionomycin, but by itself activates the promoter 12-fold. This activation is only partly affected by mutation of EBS4, and a mutant promoter that binds Ets-1, but not Elf-1, at the EBS4 site responds to PMA/ionomycin as efficiently as the wild-type. Ets proteins may be responsible for fine-tuning the activity of the nfkb1 gene in a cell-type-specific manner.

PU.1 can participate in an active enhancer complex without its transcriptional activation domain

The transcription factor PU.1 is necessary for the development of multiple hematopoietic lineages and contributes to the activity of the immunoglobulin kappa 3' enhancer. A variety of proteins bind to the 3' enhancer (PU.1, PIP, ATF1, CREM, c-Fos, c-Jun, and E2A), but the mechanism of 3'-enhancer activity and the proteins necessary for its activity are presently unclear. We show here that PU.1 participates with other transcription factors in forming a higher-order complex with 3'-enhancer DNA sequences. Each protein is necessary for formation of this complex. Individually, transcription factors that bind to the 3' enhancer do not appreciably stimulate transcription in a cell type in which the 3' enhancer is normally silent (NIH 3T3). However, mixture of multiple transcription factors (PU.1, PIP, c-Fos, and c-Jun) can greatly activate the enhancer. PU.1 is necessary for maximal enhancer activity, but mutants of PU.1 that lack the transcriptional activation domain are nearly as efficient at stimulating enhancer activity as the wild-type PU.1 protein. PU.1 apparently can activate transcription by playing an architectural role in interactions with other transcription factors.

Structural organization and characterization of the promoter region of a human carboxylesterase gene

A gene encoding a human liver carboxylesterase has been isolated and characterized. Analysis of three overlapping genomic lambda clones revealed that the gene spans about 30 kb and is made of 14 exons being 39 to 379 bp in length. The encoded protein is 550 amino acids long and is highly homologous to carboxylesterases of various mammalian species. The transcription start site was determined by 5'-RACE PCR. An additional 900 bp of DNA from the 5' flanking region of the gene was cloned and sequenced in order to elucidate the structure of the promoter. In this sequence several possible binding sites for transcription factors have been identified, but no TATA-box was present. When different parts of the putative promoter region were ligated in front of the luciferase gene and the constructs were transfected into CHO cells, the reporter gene was effectively transcribed, as demonstrated by the expression of enzyme activity.