PU. 1 is not essential for early myeloid gene expression but is required for terminal myeloid differentiation

DNA methylation and chromatin structure regulate PU.1 expression

Knockout studies have shown that PU.1 is required for the normal development of many blood cell lineages, yet overexpression of this transcription factor in erythroid cells can lead to erythroleukemia. Thus, how the tissue-specific expression of PU.1 is regulated is important to our understanding of hematopoiesis. In this study, we showed that B and macrophage cell lines expressing PU.1 contained DNase I-hypersensitive sites in intron 1 and were hypomethylated at three MspI sites flanking exon 1. Results from studies using several T-cell lines suggested that the pattern of methylation changed as these cells matured. A pre-T cell line that expresses PU.1 contained DNase I-hypersensitive sites in intron 1 and was also hypomethylated at both MspI sites. Other immature T-cell lines had methylated at least one of the MspI sites and displayed no hypersensitive sites. Mature T-cell lines had a methylation pattern more similar to that of fibroblasts. Treatment of an immature T-cell line with 5-azacytidine resulted in the expression of PU.1 transcripts. These data suggest that the tissue-specific expression of PU.1 is controlled by chromatin structure and DNA methylation and that this may be a mechanism used to shut off PU.1 expression in specific cell lineages during hematopoiesis.

Multilineage embryonic hematopoiesis requires hypoxic ARNT activity

Although most cells undergo growth arrest during hypoxia, endothelial cells and placental cytotrophoblasts proliferate in response to low O(2). We demonstrate that proliferation of embryonic multilineage hematopoietic progenitors is also regulated by a hypoxia-mediated signaling pathway. This pathway requires HIF-1 (HIF-1alpha/ARNT heterodimers) because Arnt(-/-) embryoid bodies fail to exhibit hypoxia-mediated progenitor proliferation. Furthermore, Arnt(-/-) embryos exhibit decreased numbers of yolk sac hematopoietic progenitors. This defect is cell extrinsic, is accompanied by a decrease in ARNT-dependent VEGF expression, and is rescued by exogenous VEGF. Therefore, "physiologic hypoxia" encountered by embryos is essential for the proliferation or survival of hematopoietic precursors during development.

T Cell Development in PU.1-Deficient Mice

These studies address the role of PU.1 in T cell development through the analysis of PU.1−/− mice. We show that the majority of PU.1−/− thymocytes are blocked in differentiation prior to T cell commitment, and contain a population of thymocyte progenitors with the cell surface phenotype of CD44+, HSAbright, c-kitint, Thy-1−, CD25−, Sca-1−, CD4−, and CD8−. These cells correspond in both number and cell surface phenotype with uncommitted thymocyte progenitors found in wild-type fetal thymus. RT-PCR analysis demonstrated that PU.1 is normally expressed in this early progenitor population, but is down-regulated during T cell commitment. Rare PU.1−/− thymi, however, contained small numbers of thymocytes expressing markers of T cell commitment. Furthermore, almost 40% of PU.1−/− thymi placed in fetal thymic organ culture are capable of T cell development. Mature PU.1−/− thymocytes generated during organ culture proliferated and produced IL-2 in response to stimulation through the TCR. These data demonstrate that PU.1 is not absolutely required for T cell development, but does play a role in efficient commitment and/or early differentiation of most T progenitors.

A Critical Role for PU.1 in Homing and Long-Term Engraftment by Hematopoietic Stem Cells in the Bone Marrow

We have previously demonstrated that PU.1 is required for the production of lymphoid and myeloid, but not of erythroid progenitors in the fetal liver. In this study, competitive reconstitution assays show that E14.5 PU.1−/− hematopoietic progenitors (HPC) fail to sustain definitive/adult erythropoiesis or to contribute to the lymphoid and myeloid lineages. PU.1−/−HPC are unable to respond synergistically to erythropoietin plus stem cell factor and have reduced expression of c-kit, which may explain the erythroid defect. Fluorescently labeled,PU.1−/−, AA4.1+, fetal liver HPC were transferred into irradiated recipients, where they demonstrated a severely impaired ability to home to and colonize the bone marrow.PU.1−/− HPC were found to lack integrins 4 (VLA-4/CD49d), 5 (VLA-5/CD49e), and CD11b (M). Collectively, this study has shown that PU.1 plays an important role in controlling migration of hematopoietic progenitors to the bone marrow and the establishment of long-term multilineage hematopoiesis.

A Critical Role for PU.1 in Homing and Long-Term Engraftment by Hematopoietic Stem Cells in the Bone Marrow

We have previously demonstrated that PU.1 is required for the production of lymphoid and myeloid, but not of erythroid progenitors in the fetal liver. In this study, competitive reconstitution assays show that E14.5 PU.1−/− hematopoietic progenitors (HPC) fail to sustain definitive/adult erythropoiesis or to contribute to the lymphoid and myeloid lineages. PU.1−/−HPC are unable to respond synergistically to erythropoietin plus stem cell factor and have reduced expression of c-kit, which may explain the erythroid defect. Fluorescently labeled,PU.1−/−, AA4.1+, fetal liver HPC were transferred into irradiated recipients, where they demonstrated a severely impaired ability to home to and colonize the bone marrow.PU.1−/− HPC were found to lack integrins 4 (VLA-4/CD49d), 5 (VLA-5/CD49e), and CD11b (M). Collectively, this study has shown that PU.1 plays an important role in controlling migration of hematopoietic progenitors to the bone marrow and the establishment of long-term multilineage hematopoiesis.

Differential activity of the -2.7 kb chicken lysozyme enhancer in macrophages of different ontogenic origins is regulated by C/EBP and PU.1 transcription factors

Expression of the chicken lysozyme gene is upregulated during macrophage maturation. Recently, an additional regulatory feature was discovered: the gene is differentially expressed in macrophages of embryonic/fetal and adult origin. The lysozyme gene is only weakly expressed in mature embryo-derived macrophages, whereas there is a high level of expression in macrophages derived from adult animals. This finding provided a molecular tool to investigate the heretofore ill-defined differences between embryonic/fetal- and adult-type macrophages. We showed that the low expression in the embryo is associated with reduced activity of the myeloid-specific -2.7 kb lysozyme enhancer. Our protein-binding analyses and transfection studies demonstrated that this enhancer, in order to be fully active in activated macrophages, requires the combined action of C/EBPs, PU.1, and a third, as yet unidentified, protein binding to an AP-1-like site. Of these three, PU.1 and C/EBPs display significantly reduced nuclear DNA-binding activities in embryo-derived macrophages compared with adult-type cells. These results point to different roles of C/EBPs and PU.1 in embryonic/fetal and adult myelopoiesis.

C/EBP Bypasses Granulocyte Colony-Stimulating Factor Signals to Rapidly Induce PU.1 Gene Expression, Stimulate Granulocytic Differentiation, and Limit Proliferation in 32D cl3 Myeloblasts

Within hematopoiesis, C/EBP is expressed only in myeloid cells, and PU.1 is expressed mainly in myeloid and B-lymphoid cells. C/EBP-deficient mice lack the neutrophil lineage and retain monocytes, whereas PU.1-deficient mice lack monocytes and have severely reduced neutrophils. We expressed a C/EBP-estrogen receptor ligand-binding domain fusion protein, C/EBPWT-ER, in 32D cl3 myeloblasts. 32D cl3 cells proliferate in interleukin-3 (IL-3) and differentiate to neutrophils in granulocyte colony-stimulating factor (G-CSF). In the presence of estradiol, C/EBPWT-ER induced morphologic differentiation and the expression of the myeloperoxidase, lactoferrin, and G-CSF receptor mRNAs. C/EBPWT-ER also induced a G1/S cell cycle block, with induction of p27 and Rb hypophosphorylation. bcr-ablp210 prevented 32D cl3 cell differentiation. Activation of C/EBP-ER in 32D-bcr-ablp210 or Ba/F3 B-lymphoid cells induced cell cycle arrest independent of terminal differentiation. C/EBPWT-ER induced endogenous PU.1 mRNA within 8 hours in both 32D cl3 and Ba/F3 cells, even in the presence of cycloheximide, indicating that C/EBP directly activates the PU.1 gene. However, activation of a PU.1-ER fusion protein in 32D cl3 cells induced myeloperoxidase (MPO) RNA but not terminal differentiation. Thus, C/EBP acts downstream of G-CSF and upstream of PU.1, p27, and potentially other factors to induce myeloblasts to undergo granulocytic differentiation and cell cycle arrest.

C/EBP Bypasses Granulocyte Colony-Stimulating Factor Signals to Rapidly Induce PU.1 Gene Expression, Stimulate Granulocytic Differentiation, and Limit Proliferation in 32D cl3 Myeloblasts

Within hematopoiesis, C/EBP is expressed only in myeloid cells, and PU.1 is expressed mainly in myeloid and B-lymphoid cells. C/EBP-deficient mice lack the neutrophil lineage and retain monocytes, whereas PU.1-deficient mice lack monocytes and have severely reduced neutrophils. We expressed a C/EBP-estrogen receptor ligand-binding domain fusion protein, C/EBPWT-ER, in 32D cl3 myeloblasts. 32D cl3 cells proliferate in interleukin-3 (IL-3) and differentiate to neutrophils in granulocyte colony-stimulating factor (G-CSF). In the presence of estradiol, C/EBPWT-ER induced morphologic differentiation and the expression of the myeloperoxidase, lactoferrin, and G-CSF receptor mRNAs. C/EBPWT-ER also induced a G1/S cell cycle block, with induction of p27 and Rb hypophosphorylation. bcr-ablp210 prevented 32D cl3 cell differentiation. Activation of C/EBP-ER in 32D-bcr-ablp210 or Ba/F3 B-lymphoid cells induced cell cycle arrest independent of terminal differentiation. C/EBPWT-ER induced endogenous PU.1 mRNA within 8 hours in both 32D cl3 and Ba/F3 cells, even in the presence of cycloheximide, indicating that C/EBP directly activates the PU.1 gene. However, activation of a PU.1-ER fusion protein in 32D cl3 cells induced myeloperoxidase (MPO) RNA but not terminal differentiation. Thus, C/EBP acts downstream of G-CSF and upstream of PU.1, p27, and potentially other factors to induce myeloblasts to undergo granulocytic differentiation and cell cycle arrest.

Differentiation of the Mononuclear Phagocyte System During Mouse Embryogenesis: The Role of Transcription Factor PU.1

During mouse embryogenesis, macrophage-like cells arise first in the yolk sac and are produced subsequently in the liver. The onset of liver hematopoiesis is associated with the transition from primitive to definitive erythrocyte production. This report addresses the hypothesis that a similar transition in phenotype occurs in myelopoiesis. We have used whole mount in situ hybridization to detect macrophage-specific genes expressed during mouse development. The mouse c-fms mRNA, encoding the receptor for macrophage colony-stimulating factor (CSF-1), was expressed on phagocytic cells in the yolk sac and throughout the embryo before the onset of liver hematopoiesis. Similar cells were detected using the mannose receptor, the complement receptor (CR3), or the Microphthalmia transcription factor (MITF) as mRNA markers. By contrast, other markers including the F4/80 antigen, the macrophage scavenger receptor, the S-100 proteins, S100A8 and S100A9, and the secretory product lysozyme appeared later in development and appeared restricted to only a subset of c-fms–positive cells. Two-color immunolabeling on disaggregated cells confirmed that CR3 and c-fmsproteins are expressed on the same cells. Among the genes appearing later in development was the macrophage-restricted transcription factor, PU.1, which has been shown to be required for normal adult myelopoiesis. Mice with null mutations in PU.1 had normal numbers of c-fms–positive phagocytes at 11.5dpc. PU.1(−/−) embryonic stem cells were able to give rise to macrophage-like cells after cultivation in vitro. The results support previous evidence that yolk sac–derived fetal phagocytes are functionally distinct from those arising in the liver and develop via a different pathway.

Precise developmental regulation of Ets family transcription factors during specification and commitment to the T cell lineage

Ets family transcription factors control the expression of a large number of genes in hematopoietic cells. Here we show strikingly precise differential expression of a subset of these genes marking critical, early stages of mouse lymphocyte cell-type specification. Initially, the Ets family member factor Erg was identified during an arrayed cDNA library screen for genes encoding transcription factors expressed specifically during T cell lineage commitment. Multiparameter fluorescence-activated cell sorting for over a dozen cell surface markers was used to isolate 18 distinct primary-cell populations representing discrete T cell and B cell developmental stages, pluripotent lymphoid precursors, immature NK-like cells and myeloid hematopoietic cells. These populations were monitored for mRNA expression of the Erg, Ets-1, Ets-2, Fli-1, Tel, Elf-1, GABPalpha, PU.1 and Spi-B genes. The earliest stages in T cell differentiation show particularly dynamic Ets family gene regulation, with sharp transitions in expression correlating with specification and commitment events. Ets, Spi-B and PU.1 are expressed in these stages but not by later T-lineage cells. Erg is induced during T-lineage specification and then silenced permanently, after commitment, at the beta-selection checkpoint. Spi-B is transiently upregulated during commitment and then silenced at the same stage as Erg. T-lineage commitment itself is marked by repression of PU.1, a factor that regulates B-cell and myeloid genes. These results show that the set of Ets factors mobilized during T-lineage specification and commitment is different from the set that maintains T cell gene expression during thymocyte repertoire selection and in all classes of mature T cells.

A novel stromal cell-dependent hematopoietic cell line established from temperature-sensitive SV40 T-antigen transgenic mice

A novel primitive hematopoietic cell line, THS119, was established from lineage marker negative (Lin-)/Sca-1+ cells from bone marrow of temperature-sensitive (ts) SV40 T-antigen transgenic mice after lengthy passaging by coculture with TBR59 bone marrow stromal cells. THS119 cells exhibited immature primitive hematopoietic cells such as forming cobblestones underneath the stromal cell layers. They retained properties of hematopoietic stem cells as shown by expression of c-Kit, Sca-1 and CD34low, but lacked hematopoietic lineage surface markers of differentiated hematopoietic cells (Gr-1, TER119, Mac-1, CD3, B220). RT-PCR analysis showed that THS119 cells exhibited multiple expression of both earlier developmental markers of myeloid, lymphoid and the hematopoietic cell specific transcription factors. THS119 cells showed temperature-dependent growth reflecting ts T-antigen, and their maintenance was TBR59 stromal cell-dependent. The requirement of stromal cells could not be replaced by cytokines, however, an IL-3 or IL-7 dependent cell line was generated after prolonged culture of THS119 cells on the stromal cells in the presence of these cytokines, and these cytokine-dependent cell lines exhibited phenotypes similar to the parental cells in their gene expression. SCF/c-Kit interaction is one factor required for their maintenance, but involvement of other factor(s) in the conditioned medium of TBR59 stromal cells was suggested. A novel immature hematopoietic cell line, THS119, may provide an appropriate experimental system to resolve how hematopoietic cells are kept in a primitive phase within a hematopoietic microenvironment.

Commitment to the Monocytic Lineage Occurs in the Absence of the Transcription Factor PU.1

Mice homozygous for the disruption of the PU.1 (Spi-1) gene do not produce mature macrophages. In determining the role of PU.1 in macrophage differentiation, the present study investigated whether or not there was commitment to the monocytic lineage in the absence of PU.1. Early PU.1−/− myeloid colonies were generated from neonate liver under conditions that promote primarily macrophage and granulocyte/macrophage colonies. These PU.1−/− colonies were found to contain cells with monocytic characteristics as determined by nonspecific esterase stain and the use of monoclonal antibodies that recognize early monocyte precursors, including Moma-2, ER-MP12, ER-MP20, and ER-MP58. In addition, early myeloid cells could be grown from PU.1−/− fetal liver cultures in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF). Similar to the PU.1 null colonies, the GM-CSF–dependent cells also possessed early monocytic characteristics, including the ability to phagocytize latex beads. The ability of PU.1−/− progenitors to commit to the monocytic lineage was also verified in vivo by flow cytometry and cytochemical analysis of primary neonate liver cells. The combined data shows that PU.1 is absolutely required for macrophage development after commitment to this lineage.

Commitment to the Monocytic Lineage Occurs in the Absence of the Transcription Factor PU.1

Mice homozygous for the disruption of the PU.1 (Spi-1) gene do not produce mature macrophages. In determining the role of PU.1 in macrophage differentiation, the present study investigated whether or not there was commitment to the monocytic lineage in the absence of PU.1. Early PU.1−/− myeloid colonies were generated from neonate liver under conditions that promote primarily macrophage and granulocyte/macrophage colonies. These PU.1−/− colonies were found to contain cells with monocytic characteristics as determined by nonspecific esterase stain and the use of monoclonal antibodies that recognize early monocyte precursors, including Moma-2, ER-MP12, ER-MP20, and ER-MP58. In addition, early myeloid cells could be grown from PU.1−/− fetal liver cultures in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF). Similar to the PU.1 null colonies, the GM-CSF–dependent cells also possessed early monocytic characteristics, including the ability to phagocytize latex beads. The ability of PU.1−/− progenitors to commit to the monocytic lineage was also verified in vivo by flow cytometry and cytochemical analysis of primary neonate liver cells. The combined data shows that PU.1 is absolutely required for macrophage development after commitment to this lineage.

The Ets transcription factor ERM is Th1-specific and induced by IL-12 through a Stat4-dependent pathway

Interleukin 12 (IL-12)-induced T helper 1 (Th1) development requires Stat4 activation. However, antigen-activated Th1 cells can produce interferon gamma (IFN-gamma) independently of IL-12 and Stat4 activation. Thus, in differentiated Th1 cells, factors regulated by IL-12 and Stat4 may be involved in IFN-gamma production. Using subtractive cloning, we identified ERM, an Ets transcription factor, to be a Th1-specific, IL-12-induced gene. IL-12-induction of ERM occurred in wild-type and Stat1-deficient, but not Stat4-deficient, T cells, suggesting ERM is Stat4-inducible. Retroviral expression of ERM did not restore IFN-gamma production in Stat4-deficient T cells, but augmented IFN-gamma expression in Stat4-heterozygous T cells. Ets factors frequently regulate transcription via cooperative interactions with other transcription factors, and ERM has been reported to cooperate with c-Jun. However, in the absence of other transcription factors, ERM augmented expression of an IFN-gamma reporter by only 2-fold. Thus, determining the requirement for ERM in Th1 development likely will require gene targeting.

Analysis of the modulation of transcriptional activity in myelopoiesis and leukemogenesis

Acute myeloid leukemia (AML) is still associated with a mortality of 60 to 80%. AML is characterized by a block in myeloid differentiation. The transcription factors PU.1 and C/EBPalpha are responsible for normal myeloid differentiation from stem cells to monocytes or granulocytes. In particular, PU.1 induces expression of the macrophage colony-stimulating factor (M-CSF) receptor and the development of monocytes, whereas C/EBPalpha increases the expression of the granulocyte colony-stimulating factor (G-CSF) receptor and leads to mature granulocytes. In AML, chromosomal aberrations result in oncoproteins such as AML1/ETO, PML/RARalpha, or activated Ras, which can deregulate genes important for normal myelopoiesis. Thus, AML1/ETO can bind to the transcription factor C/EBPalpha, inhibit C/EBPalpha-dependent transcription, and block granulocytic differentiation. However, AML1/ETO can also synergize with the transcription factor AML1 to enhance the activity of the M-CSF receptor promoter. On the other hand, the PML/RARalpha fusion protein causes transcriptional repression by recruiting the nuclear corepressor (N-CoR) histone deacetylase complex to the DNA, which results in decreased histone acetylation and a repressive chromatin organization. Here we describe methods to investigate whether and how signaling agonists induce myeloid differentiation and how oncoproteins might cause AML by modulating the activity of transcription factors that are pivotal for normal myeloid development.

Distinct promoters control transmembrane and cytosolic protein tyrosine phosphatase epsilon expression during macrophage differentiation

We have recently isolated two cDNAs encoding two forms of transmembrane and cytosolic protein tyrosine phosphatase epsilon (PTPepsilon). In this study, the 5' end of the rat PTPepsilon gene was isolated and characterized. Transmembrane PTPepsilon (PTPepsilonM) and cytosolic PTPepsilon (PTPepsilonC) were encoded by a single gene. 5' RACE analysis and RNase protection assay showed that the mRNA of each PTPepsilon isoform was transcribed from different promoters. The putative promoter regions of two alternative first exons lacked a TATA box, but contained potential recognition sites for several transcription factors. Reverse transcription PCR analysis revealed that PTPepsilonC mRNA was up-regulated during interleukin 6-induced differentiation of murine leukemia M1 cells, whereas PTPepsilonM mRNA was down-regulated. With the use of luciferase as a reporter gene, the promoter activities of the 5'-flanking regions were examined during phorbol myristate acetate-induced differentiation of HL-60 cells. In the differentiated HL-60 cells, the activity of the PTPepsilonC promoter, but not that of PTPepsilonM, was dramatically elevated. Furthermore, we found that PTPepsilonC mRNA is highly expressed in mouse peritoneal macrophages and enhanced during activation by lipopolysaccharide. These results suggest that the different promoters control expression of PTPepsilon isoforms during the differentiation and/or activation of macrophages.

Transcriptional regulation of the stem cell leukemia gene by PU.1 and Elf-1

The SCL gene, also known as tal-1, encodes a basic helix-loop-helix transcription factor that is pivotal for the normal development of all hematopoietic lineages. SCL is expressed in committed erythroid, mast, and megakaryocytic cells as well as in hematopoietic stem cells. Nothing is known about the regulation of SCL transcription in mast cells, and in other lineages GATA-1 is the only tissue-specific transcription factor recognized to regulate the SCL gene. We have therefore analyzed the molecular mechanisms underlying SCL expression in mast cells. In this paper, we demonstrate that SCL promoter 1a was regulated by GATA-1 together with Sp1 and Sp3 in a manner similar to the situation in erythroid cells. However, SCL promoter 1b was strongly active in mast cells, in marked contrast to the situation in erythroid cells. Full activity of promoter 1b was dependent on ETS and Sp1/3 motifs. Transcription factors PU.1, Elf-1, Sp1, and Sp3 were all present in mast cell extracts, bound to promoter 1b and transactivated promoter 1b reporter constructs. These data provide the first evidence that the SCL gene is a direct target for PU.1, Elf-1, and Sp3.

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.

Normal myeloid development requires both the glutamine-rich transactivation domain and the PEST region of transcription factor PU.1 but not the potent acidic transactivation domain

Gene targeting of transcription factor PU.1 results in an early block to fetal hematopoiesis, with no detectable lymphoid or myeloid cells produced in mouse embryos. Furthermore, PU.1(-/-) embryonic stem (ES) cells fail to differentiate into Mac-1(+) and F4/80(+) macrophages in vitro. We have previously shown that a PU.1 transgene under the control of its own promoter restores the ability of PU. 1(-/-) ES cells to differentiate into macrophages. In this study, we take advantage of our PU.1(-/-) ES cell rescue system to genetically test which previously identified PU.1 functional domains are necessary for the development of mature macrophages. PU.1 functional domains include multiple N-terminal acidic and glutamine-rich transactivation domains, a PEST domain, several serine phosphorylation sites, and a C-terminal Ets DNA binding domain, all delineated and characterized by using standard biochemical and transactivational assays. By using the production of mature macrophages as a functional readout in our assay system, we have established that the glutamine-rich transactivation domain, a portion of the PEST domain, and the DNA binding domain are required for myelopoiesis. Deletion of three acidic domains, which exhibit potent transactivation potential in vitro, had no effect on the ability of PU.1 to promote macrophage development. Furthermore, mutagenesis of four independent sites of serine phosphorylation also had no effect on myelopoiesis. Collectively, our results indicate that PU.1 interacts with important regulatory proteins during macrophage development via the glutamine-rich and PEST domains. The PU.1(-/-) ES cell rescue system represents a powerful, in vitro strategy to functionally map domains of PU.1 essential for normal hematopoiesis and the generation of mature macrophages.

PU.1 regulates the expression of the human neutrophil elastase gene

PU.1 is a transcription factor present in B-cells and macrophages. Here, we report our studies on the role of PU.1 in myelopoiesis using human neutrophil elastase (HNE) as a model. HNE, a component of the primary granules of mature granulocytes, is a serine protease which is transcriptionally restricted to the late promyelocytic stage of granulocytic maturation. The first 200 bp of the HNE promoter directs myeloid specific expression of a reporter gene and a 30-bp element within this region was been identified as the major determinant of myeloid specific expression [S. Srikanth, T. Rado, A 30-bp element is responsible for the myeloid specific activity of the human neutrophil elastase promoter, J. Biol. Chem. 269 (1994) 32626-32632.]. We now show that the B-cell and macrophage specific transcription factor, PU.1, binds to the PU.1 consensus site within the 30-bp element to activate transcription. Substitution mutations within this recognition sequence results in the loss of PU.1 binding and in a 90% decrease in promoter activity in myeloid cells. Cotransfection of PU.1 and a reporter gene controlled by the HNE promoter into non-myeloid HeLa cells resulted in activation of reporter gene transcription.

Use of RDA analysis of knockout mice to identify myeloid genes regulated in vivo by PU.1 and C/EBPalpha

PU.1 and C/EBPalpha are transcription factors essential for normal myeloid development. Loss-of-function mutation of PU.1 leads to an absolute block in monocyte/macrophage development and abnormal granulocytic development while that of C/EBPalpha causes a selective block in neutrophilic differentiation. In order to understand these phenotypes, we studied the role of PU.1 and C/EBPalpha in the regulation of myeloid target genes in vivo . Northern blot analysis revealed that mRNAs encoding receptors for M-CSF, G-CSF and GM-CSF, were expressed at low levels in PU.1(-/-) fetal liver compared with wild type. To identify additional myeloid genes regulated by PU.1 and C/EBPalpha, we performed representational difference analysis (RDA), a PCR-based subtractive hybridization using fetal livers from wild type and PU.1 or C/EBPalpha knockout mice. By introducing a new modification of RDA, that of tissue-specific gene suppression, we could selectively identify a set of differentially expressed genes specific to myeloid cells. Differentially expressed genes included both primary and secondary granule protein genes. In addition, eight novel genes were identified that were upregulated in expression during myeloid differentiation. These methods provide a general strategy for elucidating the genes affected in murine knockout models.

Cross-lineage expression of Ig-beta (B29) in thymocytes: positive and negative gene regulation to establish T cell identity

Developmental commitment involves activation of lineage-specific genes, stabilization of a lineage-specific gene expression program, and permanent inhibition of inappropriate characteristics. To determine how these processes are coordinated in early T cell development, the expression of T and B lineage-specific genes was assessed in staged subsets of immature thymocytes. T lineage characteristics are acquired sequentially, with germ-line T cell antigen receptor-beta transcripts detected very early, followed by CD3epsilon and terminal deoxynucleotidyl transferase, then pTalpha, and finally RAG1. Only RAG1 expression coincides with commitment. Thus, much T lineage gene expression precedes commitment and does not depend on it. Early in the course of commitment to the T lineage, thymocytes lose the ability to develop into B cells. To understand how this occurs, we also examined expression of well defined B lineage-specific genes. Although lambda5 and Ig-alpha are not expressed, the mu 0 and I mu transcripts from the unrearranged IgH locus are expressed early, in distinct patterns, then repressed just before RAG1 expression. By contrast, RNA encoding the B cell receptor component Ig-beta was found to be transcribed in all immature thymocyte subpopulations and throughout most thymocyte differentiation. Ig-beta expression is down-regulated only during positive selection of CD4(+)CD8(-) cells. Thus several key participants in the B cell developmental program are expressed in non-B lineage-committed cells, and one is maintained even through commitment to an alternative lineage, and repressed only after extensive T lineage differentiation. The results show that transcriptional activation of "lymphocyte-specific" genes can occur in uncommitted precursors, and that T lineage commitment is a composite of distinct positive and negative regulatory events.

Development of the definitive hematopoietic hierarchy in the mouse

Recent research on the ontogeny of the hematopoietic system in mammals has shown that a simple textbook steady-state hematopoietic hierarchy can not be strictly applied to the hematopoietic cells found within the embryo. During embryonic development, hematopoietic cells originate, migrate and differentiate in a number of distinct anatomical sites such as the yolk sac AGM region and liver and thus represent various classes of cells within diverse microenvironments. In this manuscript we review both cellular and molecular aspects of developmental hematopoiesis and present our current views on the numerous complex mechanisms underlying the establishment of definitive hematopoiesis.

Differentiation-stimulated activity binds an ETS-like, essential regulatory element in the human promyelocytic defensin-1 promoter

The human HNP-defensin-1 gene encodes a peptide antibiotic found exclusively in neutrophils and is key to elimination of microbes. Expression is a marker for the granulocytic lineage and for certain stages of differentiation and is not known to be inducible in mature cells under physiological conditions. Low level of transcription also occurs in HL-60 promyelocytic leukemia cells and is greatly activated upon drug-induced granulocytic maturation and by low doses of retinoic acid, in a strictly cell-specific manner (Herwig, S., Su, Q., Ma, Y., and Tempst, P. (1996) Blood 87, 350-364). We have analyzed a 10-kilobase pair region, upstream of the defensin-1 cap site, for the presence of control elements, and we describe a minimal promoter (position -83 to +82) required to drive transcription in HL-60 cells in a quasi cell-specific manner. Our data also suggest the presence of negative regulatory elements in the -416/-191 region that may further contribute to cell specificity in a chromosomal context. The basal promoter contains two functionally essential, ETS-like (GGAA core sequence) elements. The proximal site (-22/-19) constitutively binds the PU.1 transcription factor in vitro and could function, together perhaps with an adjacent TA-rich sequence (-32/-25), in assembly of a myeloid-restricted, basal transcription factor complex. The distal site (-62/-59) interacts in vitro with an unidentified activity, distinct from PU.1, ETS-1, PEA3, and ELK-1 (factors with definite binding site similarities), and is greatly stimulated by phosphorylation during granulocytic differentiation of HL-60 cells. Identification of this protein will be important to resolve the molecular mechanisms controlling temporal, granulocytic restricted gene expression.

Transcriptional regulation of B-cell differentiation

Transcription factors influence B cell differentiation by regulating the expression of numerous lineage-specific genes. Recent studies have identified factors that regulate differentiation of hematopoietic stem cells into B cell progenitors (PU.1 and lkaros), and further differentiation of these progenitors into mature B cells (NF kappa B, E2A, early B cell factor [EBF] and B cell specific activator protein [BSAP]). In addition, these studies demonstrate that complex interactions and redundancies among transcription factors safeguard the precise patterns of gene expression required for normal B cell differentiation.

Interaction between PU.1 and another Ets family transcription factor promotes macrophage-specific Basal transcription initiation

Numerous macrophage-restricted promoters lack TATA boxes or other conventional initiation motifs but contain high affinity binding sites (PU boxes) for the macrophage-restricted Ets family transcription factor PU.1. In RAW264 murine macrophages, multimerized PU boxes were not active as enhancers when placed upstream of a minimal promoter. To model their role in basal promoters, we inserted PU boxes into a promoterless luciferase reporter plasmid. Two sites, regardless of orientation, were necessary and sufficient to direct reporter gene expression in transient transfections of the RAW264 macrophage-like cell line. This activity was absent in transfected 3T3 fibroblasts but could be induced by PU.1 coexpression. Both the model promoter and the macrophage-specific mouse and human c-fms promoters were activated in RAW264 cells by other Ets family transcription factors, Ets-2 and Elf-1. In fibroblasts, the effects of PU.1 and Ets-2 were multiplicative, whereas overexpression of PU.1 in RAW264 cells reduced activation of c-fms or model promoters by the other Ets factors. The PU.1 and Ets-2 binding sites of the mouse c-fms promoter have been located by DNase footprinting. A conserved Ets-like motif at the transcription site, CAGGAAC, that bound only weakly to PU.1, was identified as an additional critical basal c-fms promoter element. Comparison of studies on the model promoter, c-fms and other myeloid promoters provides evidence for a conserved mechanism that involves three separate and functionally distinct Ets-like motifs.

The Role of Transcription Factor PU.I in the Activity of the Intronic Enhancer of the Eosinophil-Derived Neurotoxin (RNS2) Gene

Eosinophil-derived neurotoxin (EDN) found in the granules of human eosinophils is a cationic ribonuclease toxin. Expression of the EDN gene (RNS2) in eosinophils is dependent on proximal promoter sequences in combination with an enhancer located in the first intron. We further define here the active region of the intron using transfections in differentiated eosinophilic HL60 cells. We show that a region containing a tandem PU.I binding site is important for intronic enhancer activity. This region binds multiple forms of transcription factor PU.I as judged by gel-shift analysis and DNA affinity precipitation. Importantly, introducing point mutations in the PU.I site drastically reduces the intronic enhancer activity, showing the importance of PU.I for expression of EDN in cells of the eosinophilic lineage.

The Role of Transcription Factor PU.I in the Activity of the Intronic Enhancer of the Eosinophil-Derived Neurotoxin (RNS2) Gene

Eosinophil-derived neurotoxin (EDN) found in the granules of human eosinophils is a cationic ribonuclease toxin. Expression of the EDN gene (RNS2) in eosinophils is dependent on proximal promoter sequences in combination with an enhancer located in the first intron. We further define here the active region of the intron using transfections in differentiated eosinophilic HL60 cells. We show that a region containing a tandem PU.I binding site is important for intronic enhancer activity. This region binds multiple forms of transcription factor PU.I as judged by gel-shift analysis and DNA affinity precipitation. Importantly, introducing point mutations in the PU.I site drastically reduces the intronic enhancer activity, showing the importance of PU.I for expression of EDN in cells of the eosinophilic lineage.

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.

Yolk sac-derived murine macrophage cell line has a counterpart during ES cell differentiation

Macrophages are phagocytic hematopoietic cells involved in several immune processes, but they are also present early in mammalian development and may participate in embryonic tissue remodeling. We have isolated and characterized a cell line, Py-YSA, from the mouse yolk sac. Py-YSA cells have several functional properties of macrophages, including uptake of acetylated low density lipoprotein and phagocytic capability. They express the murine macrophage markers F4/80 and Mac-1, and they express RNA for the c-fms receptor. Their expansion in culture requires fibroblast conditioned medium or exogenous monocyte-colony stimulating factor. Murine ES (embryonic stem) cell cultures that undergo in vitro differentiation recapitulate yolk sac development, and during this process cells arise that express both Mac-1 and F4/80 and morphologically resemble the Py-YSA cells. The kinetics and distribution pattern of the Mac-1+ cells during a time course of ES cell differentiation suggest that they originate in the blood islands, and that they subsequently leave the blood islands and disperse to tissue sites. Both F4/80 and Mac-1 are first expressed in primary cultures from day 9.5 yolk sacs. The Py-YSA cultured cells thus resemble embryonic tissue macrophages by several criteria, and they share a marker profile with a cell type found in yolk sacs and differentiating ES cells. Py-YSA cells will be a useful reagent for further understanding the role of embryonic tissue macrophages in development.

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.

The LKLF transcription factor is required for normal tunica media formation and blood vessel stabilization during murine embryogenesis

The transcriptional programs that regulate blood vessel formation are largely unknown. In this paper, we examine the role of the zinc finger transcription factor LKLF in murine blood vessel morphogenesis and homeostasis. By in situ hybridization and immunohistochemistry, we show that LKLF is expressed as early as embryonic day 9.5 (E9.5) in vascular endothelial cells throughout the developing mouse embryo. To better understand the function of LKLF, we used homologous recombination in embryonic stem (ES) cells to generate LKLF-deficient (LKLF-/-) mice. Both angiogenesis and vasculogenesis were normal in the LKLF-/- mice. However, LKLF-/- embryos died between E12.5 and E14.5 from severe intra-embryonic and intra-amniotic hemorrhaging. This bleeding disorder was associated with specific defects in blood vessel morphology. Umbilical veins and arteries in the LKLF-/- embryos displayed an abnormally thin tunica media and aneurysmal dilatation before rupturing into the amniotic cavity. Similarly, vascular smooth muscle cells in the aortae from the LKLF-/- animals displayed a cuboidal morphology and failed to organize into a compact tunica media. Consistent with these findings, electron microscopic analyses demonstrated endothelial cell necrosis, significant reductions in the number of vessel-wall pericytes and differentiating smooth muscle cells, and decreased deposition of extracellular matrix in the LKLF-/- vessels. Despite these defects, in situ hybridization demonstrated normal expression of platelet-derived growth factor B, Tie1, Tie2, transforming growth factor beta, and heparin-binding epidermal growth factor in the vasculature of the LKLF-/- embryos. Therefore, LKLF defines a novel transcriptional pathway in which endothelial cells regulate the assembly of the vascular tunica media and concomitant vessel wall stabilization during mammalian embryogenesis.

Interferon (IFN) consensus sequence-binding protein, a transcription factor of the IFN regulatory factor family, regulates immune responses in vivo through control of interleukin 12 expression

Mice with a null mutation of the gene encoding interferon consensus sequence-binding protein (ICSBP) develop a chronic myelogenous leukemia-like syndrome and mount impaired responses to certain viral and bacterial infections. To gain a mechanistic understanding of the contributions of ICSBP to humoral and cellular immunity, we characterized the responses of control and ICSBP-/- mice to infection with influenza A (flu) and Leishmania major (L. major). Mice of both genotypes survived infections with flu, but differed markedly in the isotype distribution of antiflu antibodies. In sera of normal mice, immunoglobulin (Ig)G2a antibodies were dominant over IgG1 antibodies, a pattern indicative of a T helper cell type 1 (Th1)-driven response. In sera of ICSBP-/- mice, however, IgG1 antibodies dominated over IgG2a antibodies, a pattern indicative of a Th2-driven response. The dominance of IgG1 and IgE over IgG2a was detected in the sera of uninfected mice as well. A seeming Th2 bias of ICSBP-deficient mice was also uncovered in their inability to control infection with L. major, where resistance is known to be dependent on IL-12 and IFN-gamma as components of a Th1 response. Infected ICSBP-deficient mice developed fulminant, disseminated leishmaniasis as a result of failure to mount a Th1-mediated curative response, although T cells remained capable of secreting IFN-gamma and macrophages of producing nitric oxide. Compromised Th1 differentiation in ICSBP-/- mice could not be attributed to hyporesponsiveness of CD4(+) T cells to interleukin (IL)-12; however, the ability of uninfected and infected ICSBP-deficient mice to produce IL-12 was markedly impaired. This indicates that ICSBP is a deciding factor in Th responses governing humoral and cellular immunity through its role in regulating IL-12 expression.

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.

Defects in Yolk Sac Hematopoiesis in Mll-Null Embryos

Translocations involving the mixed lineage leukemia gene (MLL ), the human homolog of the Drosophila gene trithorax, are one of the most common genetic alterations in human acute leukemias. Each translocation involving MLL results in loss of one functional copy of MLL and the generation of a chimeric fusion protein with potential dominant negative or neomorphic activity. Mll is a positive regulator of Hox genes, which have been implicated in both axial skeleton patterning and hematopoietic development. Previous studies indicated that Hox gene expression is altered in Mll heterozygous (+/−) and homozygous (−/−) deficient mice. To study the role of Mll in hematopoiesis and to obtain insights into leukemogenesis, we have examined the effects of haplo-insufficiency or absence of Mll by in vitro differentiation of Mll +/+, +/−, and −/− yolk sac progenitor cells. Mll −/− colonies were fewer in number, took longer to develop, and contained fewer cells than their wild-type and heterozygous counterparts. Formation of colony-forming unit-granulocyte, erythroid, macrophage, megakaryocyte (CFU-GEMM), colony-forming unit-macrophage (CFU-M), and burst-forming unit-erythroid (BFU-E) was markedly decreased in Mll −/− cultures, while numbers of colony-forming unit-erythroid (CFU-E), colony-forming unit-granulocyte (CFU-G), and colony-forming unit-granulocyte macrophage (CFU-GM) were essentially unaffected. Despite the decreased numbers of colonies present, Mll −/− cultures showed all cell types without morphologic evidence of maturation arrest. These studies indicate that Mll is required for normal numbers of hematopoietic progenitors and their proper differentiation, especially along the myeloid and macrophage pathways.

Use of embryonal stem cells in studies of molecular haemopoiesis

Although the cell biology of haemopoietic stem cells (HSC) is relatively well understood, their molecular control is less well defined. Due to the rarity of this cell type, their incompletely defined phenotype and difficulty in generating null alleles by somatic transgenesis of HSC, alternative approaches to their study have been sought. Embryonal stem (ES) cells are toti-potential, can transmit transgenes through the germ line and have recently been shown to produce HSC in vitro. This chapter reviews the utility of gene knock-outs in ES cells in the study of molecular haemopoiesis, indicates how ES cells can be used in vitro as a strategy both for the identification of genes controlling early haemopoietic events and the analysis of their function, and outlines how emerging techniques that exploit the biology of ES cells might prove to be powerful tools in the genetic dissection of the mechanisms controlling haemopoiesis.

Core binding factor cannot synergistically activate the myeloperoxidase proximal enhancer in immature myeloid cells without c-Myb

The myeloperoxidase (MPO) gene is transcribed specifically in immature myeloid cells and is regulated in part by a 414-bp proximal enhancer. Mutation of a core binding factor (CBF)-binding site at -288 decreased enhancer activity 30-fold in 32D cl3 myeloid cells cultured in granulocyte colony-stimulating factor (G-CSF). A novel functional analysis, linking the CBF-binding site to an enhancer deletion series, located at -147 an evolutionarily conserved c-Myb-binding site which was required for optimal enhancer activity and synergy with CBF in 32D cells. These sites cooperated in isolation and independent of a precise spacing. Deletional analysis carried out in the absence of the c-Myb-binding site at -147 located at -301 a second c-Myb-binding site which also synergized with CBF to activate the enhancer. A GA-rich region at -162 contributed to cooperation with CBF when the adjacent c-Myb-binding site was intact. Mutation of both c-Myb-binding sites in the context of the entire enhancer greatly impaired activation by endogenous CBF in 32D cells. Similarly, activation by c-Myb was impaired in constructs lacking the CBF-binding site. CBF and c-Myb were required for induction of MPO proximal enhancer activity when 32D cells differentiated in response to G-CSF. A fusion protein containing the Gal4 DNA-binding domain and the AML-1B activation domain, amino acids 216 to 480, activated transcription alone and cooperatively with c-Myb in nonmyeloid CV-1 cells. Determining how CBF and c-Myb synergize in myeloid cells might contribute to our understanding of leukemogenesis by the AML1-ETO, AML1-MDS1, CBFbeta-SMMHC, and v-Myb oncoproteins.

Hematopoietic-Specific Genes Are Not Induced During In Vitro Differentiation of scl-Null Embryonic Stem Cells

The helix-loop-helix transcription factor, scl, plays an essential role in hematopoietic development. Embryos in which the gene has been disrupted fail to develop yolk sac erythropoiesis, and scl-null embryonic stem cells do not contribute to hematopoiesis in chimeric mice. To analyze the molecular consequences of scl deficiency, we compared the gene expression profiles of control (wild-type and scl-heterozygous) and scl-null embryonic stem cells differentiated in vitro for up to 12 days. In control and scl-null embryoid bodies the temporal expression pattern of genes associated with the formation of ventral mesoderm, such as Brachyury, bone morphogenetic protein-4, and flk-1, was identical. Similarly, GATA-2, CD34, and c-kit, which are coexpressed in endothelial and hematopoietic lineages, were expressed normally in scl-null embryonic stem cell lines. However, hematopoietic-restricted genes, including the transcription factors GATA-1, EKLF, and PU.1 as well as globin genes and myeloperoxidase, were only expressed in wild-type and scl-heterozygous embryonic stem cells. Indirect immunofluorescence was used to confirm the observations that GATA-1 and globins were only present in control embryoid bodies but that CD34 was found on both control and scl-null embryoid bodies. These data extend the previous gene ablation studies and support a model whereby scl is absolutely required for commitment of a putative hemangioblast to the hematopoietic lineage but that it is dispensable for endothelial differentiation.

Hematopoietic-Specific Genes Are Not Induced During In Vitro Differentiation of scl-Null Embryonic Stem Cells

The helix-loop-helix transcription factor, scl, plays an essential role in hematopoietic development. Embryos in which the gene has been disrupted fail to develop yolk sac erythropoiesis, and scl-null embryonic stem cells do not contribute to hematopoiesis in chimeric mice. To analyze the molecular consequences of scl deficiency, we compared the gene expression profiles of control (wild-type and scl-heterozygous) and scl-null embryonic stem cells differentiated in vitro for up to 12 days. In control and scl-null embryoid bodies the temporal expression pattern of genes associated with the formation of ventral mesoderm, such as Brachyury, bone morphogenetic protein-4, and flk-1, was identical. Similarly, GATA-2, CD34, and c-kit, which are coexpressed in endothelial and hematopoietic lineages, were expressed normally in scl-null embryonic stem cell lines. However, hematopoietic-restricted genes, including the transcription factors GATA-1, EKLF, and PU.1 as well as globin genes and myeloperoxidase, were only expressed in wild-type and scl-heterozygous embryonic stem cells. Indirect immunofluorescence was used to confirm the observations that GATA-1 and globins were only present in control embryoid bodies but that CD34 was found on both control and scl-null embryoid bodies. These data extend the previous gene ablation studies and support a model whereby scl is absolutely required for commitment of a putative hemangioblast to the hematopoietic lineage but that it is dispensable for endothelial differentiation.

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.

Nuclear receptor steroidogenic factor 1 directs embryonic stem cells toward the steroidogenic lineage

The orphan nuclear receptor steroidogenic factor 1 (SF-1) is expressed in the adrenal gland and gonads and is an important regulator of the expression of cytochrome P-450 steroidogenic enzymes in cultured cells. Targeted disruption of the SF-1 gene in mice shows that it is a critical participant in the genetic program that promotes the development of urogenital mesoderm into the adrenal gland and gonads. To assess the ability of SF-1 to regulate this differentiation pathway, we ectopically expressed SF-1 in murine embryonic stem (ES) cells. We found that stable expression of SF-1 is sufficient to alter ES cell morphology, permit cyclic AMP (cAMP) and retinoic acid-induced expression of the endogenous side chain cleavage enzyme gene, and consequently, promote steroidogenesis. While steroid production is dependent upon SF-1, cAMP induction of steroidogenesis does not enhance the responsiveness of an SF-1-specific reporter. Furthermore, the activity of a P450SCC promoter/luciferase reporter construct, which is induced by cAMP in steroidogenic cells and ES cells converted by stable expression of SF-1, is not induced by cAMP in wild-type ES cells transiently transfected with SF-1, suggesting that the induction of downstream gene products is required before steroidogenesis can occur. We demonstrate that mutants which disrupt the DNA binding domain or the AF2 transcriptional activation domain of SF-1 do not confer the steroidogenic phenotype to ES cells. Interestingly, however, AF2 mutants fused to the VP16 activation domain do confer the steroidogenic phenotype to ES cells, but only in the presence of a portion of the ligand binding domain. These studies extend the role of SF-1 in steroidogenic tissues to that of a dominant regulator of the steroidogenic cell phenotype.

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.

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

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