Characterization of a gp91-phox promoter element that is required for interferon gamma-induced transcription

Physiological roles of NOX/NADPH oxidase, the superoxide-generating enzyme

NADPH oxidase is a superoxide (O₂^•−)-generating enzyme first identified in phagocytes, essential for their bactericidal activities. Later, in non-phagocytes, production of O₂^•− was also demonstrated in an NADPH-dependent manner. In the last decade, several non-phagocyte-type NADPH oxidases have been identified. The catalytic subunit of these oxidases, NOX, constitutes the NOX family. There are five homologs in the family, NOX1 to NOX5, and two related enzymes, DUOX1 and DUOX2. Transgenic or gene-disrupted mice of the NOX family have also been established. NOX/DUOX proteins possess distinct features in the dependency on other components for their enzymatic activities, tissue distributions, and physiological functions. This review summarized the characteristics of the NOX family proteins, especially focused on their functions clarified through studies using gene-modified mice.

GCN5 regulates the superoxide-generating system in leukocytes via controlling gp91-phox gene expression

The superoxide anion (O(2)(-))-generating system is an important mechanism of innate immune response against microbial infection in phagocytes and is involved in signal transduction mediated by various physiological and pathological signals in phagocytes and other cells, including B lymphocytes. The O(2)(-)-generating system is composed of five specific proteins: p22-phox, gp91-phox, p40-phox, p47-phox, p67-phox, and a small G protein, Rac. Little is known regarding epigenetic regulation of the genes constituting the O(2)(-)-generating system. In this study, by analyzing the GCN5 (one of most important histone acetyltransferases)-deficient DT40 cell line, we show that GCN5 deficiency causes loss of the O(2)(-)-generating activity. Interestingly, transcription of the gp91-phox gene was drastically downregulated (to ∼4%) in GCN5-deficient cells. To further study the involvement of GCN5 in transcriptional regulation of gp91-phox, we used in vitro differentiation system of U937 cells. When human monoblastic U937 cells were cultured in the presence of IFN-γ, transcription of gp91-phox was remarkably upregulated, and the cells were differentiated to macrophage-like cells that can produce O(2)(-). Chromatin immunoprecipitation assay using the U937 cells during cultivation with IFN-γ revealed not only that association of GCN5 with the gp91-phox gene promoter was significantly accelerated, but also that GCN5 preferentially elevated acetylation levels of H2BK16 and H3K9 surrounding the promoter. These results suggested that GCN5 regulates the O(2)(-)-generating system in leukocytes via controlling the gp91-phox gene expression as a supervisor. Our findings obtained in this study should be useful in understanding the molecular mechanisms involved in epigenetic regulation of the O(2)(-)-generating system in leukocytes.

NOX/NADPH oxidase, the superoxide-generating enzyme: its transcriptional regulation and physiological roles

NADPH oxidase is a superoxide (O(2)(-))-generating enzyme first identified in phagocytes that shows bactericidal activities. It has been reported that O(2)(-) is also produced in an NADPH-dependent manner in non-phagocytes. In the last decade, non-phagocyte-type NADPH oxidases have been identified, and the catalytic subunit NOX family has been found to be composed of five homologs, NOX1 to NOX5, and two related enzymes, DUOX1 and DUOX2. These NOX proteins have distinct features in dependency on other components for maximal enzymatic activity, tissue distribution, expressional regulation, and physiological functions. This review summarized the distinct characteristics of NOX family proteins, especially focusing on their functions and mechanisms of their expressional regulation.

IFNgamma mediates DUOX2 expression via a STAT-independent signaling pathway

The biological roles of the dual oxidases, DUOX1 and DUOX2, are dependent upon the tissue in which they are expressed. However, the mechanisms that control DUOX expression in these tissues are largely unexplored. Given the known role of DUOX for host defense in the gut and respiratory tract, we characterized potential mechanisms that control DUOX2 expression in response to interferon gamma (IFNgamma) in respiratory tract epithelium. We discovered that IFNgamma-mediated DUOX2 expression was regulated by a STAT-independent, JAK-independent pathway. These data provide insights into a novel IFNgamma signaling pathway with potential importance for regulation of host defense responses.

Regulation of NOX1 expression by GATA, HNF-1alpha, and Cdx transcription factors

NOX1, a member of the NOX family of NADPH oxidases, is expressed primarily in colon epithelium, where it may function in host defense and growth regulation. We investigated factors responsible for its transcriptional regulation in vitro and its expression in vivo. Analysis of promoter constructs in the CaCo2 cell line identified a complex element between -422 and -291 critical for promoter activity. This element contained four sites that bound GATA-4, -5, and -6 in vitro with varied affinities. One site also bound the caudal-related homeodomain proteins Cdx1 and Cdx2, whereas another also bound hepatocyte nuclear factor-1alpha (HNF-1alpha). GATA-6, HNF-1alpha, and Cdx2 also bound to this region in the intact chromatin of CaCo2 cells. These factors demonstrated cooperativity when transactivating the NOX1 promoter. NOX1 mRNA was detected in human colon epithelial cells along the crypt-villus axis. A gradient of NOX1 mRNA expression was seen in the colons of normal as well as germ-free mice, with significantly higher levels in distal compared with proximal segments. The expression gradients of NOX1 mRNA in the colon paralleled those of GATA-6, HNF-1alpha, and Cdx1. These data indicate that developmental, tissue-restricted transcription factors play a key role in NOX1 regulation in vivo.

Activation of SHP2 Protein-tyrosine Phosphatase Increases HoxA10-induced Repression of the Genes Encoding gp91PHOX and p67PHOX

The CYBB and NCF2 genes encode the phagocyte oxidase proteins gp91(PHOX) and p67(PHOX), respectively. These genes are transcribed after the promyelocyte stage of differentiation, and transcription continues until cell death. In undifferentiated myeloid cells, homologous cis-elements in the CYBB and NCF2 genes are repressed by the homeodomain transcription factor HoxA10. During cytokine-induced myelopoiesis, tyrosine phosphorylation of HoxA10 decreases binding affinity for the CYBB and NCF2 cis-elements. This abrogates HoxA10-induced transcriptional repression as differentiation proceeds. Therefore, mechanisms involved in differentiation stage-specific HoxA10 tyrosine phosphorylation are of interest because HoxA10 phosphorylation modulates myeloid-specific gene transcription. In this study, we found that HoxA10 is a substrate for SHP2 protein-tyrosine phosphatase in undifferentiated myeloid cells. In contrast, HoxA10 is a substrate for a constitutively active mutant form of SHP2 in both undifferentiated and differentiating myeloid cells. Expression of such SHP2 mutants results in persistent HoxA10 repression of CYBB and NCF2 transcription during myelopoiesis. Both HoxA10 overexpression and activating SHP2 mutations have been described in human myeloid malignancies. Therefore, our results suggest that these mutations could cooperate, leading to decreased myeloid-specific gene transcription and functional differentiation block in myeloid cells with both defects.

Regulation of novel superoxide-producing NAD(P)H oxidases

Deliberate production of reactive oxygen species (ROS) are catalyzed by enzymes that belong to the NAD(P)H oxidase (Nox) family. The human genome contains seven members of the Nox family: the superoxide-producing enzymes Nox1 through Nox5 and the dual oxidases Duox1 and Duox2 that release hydrogen peroxide but not superoxide. Among them, the classical member gp91( phox )/Nox2 functions as the phagocyte NADPH oxidase, playing a crucial role in host defense. Although Nox2, heterodimerized with its membrane-spanning partner p22( phox ), is inactive in resting cells, during phagocytosis it forms an active complex with soluble regulatory proteins such as the organizer p47( phox ), the activator p67( phox ), and the small GTPase Rac. Here the authors describe how the novel superoxide-producing Nox oxidases (Nox1, 3, 4, and 5) with different functions are regulated by p22( phox ), the Nox organizers, the Nox activators, and Rac, and how their expression is controlled at the transcriptional level.

NF-kappaB regulates phagocytic NADPH oxidase by inducing the expression of gp91phox

The superoxide-generating phagocytic NADPH oxidase is an important component of the innate immune response against microbial agents, and is involved in shaping the cellular response to a variety of physiological and pathological signals. One of the downstream targets of NADPH oxidase-derived radicals is the ubiquitous transcription factor NF-kappaB, which controls the expression of a large array of genes involved in immune function and cell survival. Here we show that NF-kappaB itself is a key factor in controlling NADPH oxidase expression and function. In monocytic and microglial cell lines, the expression of the NADPH oxidase subunit gp91(phox) was induced by lipopolysaccharide/interferon gamma treatment and was inhibited in cells constitutively expressing IkappaBalpha. Furthermore, inducible reactive oxygen species production was inhibited in IkappaBalpha overexpressing cells. gp91(phox) expression was very low in RelA(-/-) fibroblasts and could be induced by reconstituting these cells with p65/RelA. Thus, gp91(phox) expression is dependent on the presence of p65/RelA. We also found that gp91(phox) transcription is dependent on NF-kappaB and we identified two potential cis-acting elements in the murine gp91(phox) promoter that control NF-kappaB-dependent regulation. The findings raise the possibility of a positive feedback loop in which NF-kappaB activation by oxidative stress leads to further radical production via NADPH oxidase.

HoxA10 represses transcription of the gene encoding p67phox in phagocytic cells

p67(phox) and gp91(phox) are components of the phagocyte-specific respiratory burst oxidase that are encoded by the NCF2 and CYBB genes, respectively. These genes are transcribed exclusively in myeloid cells that have differentiated beyond the promyelocyte stage. In mature phagocytes, NCF2 and CYBB transcription continues until cell death and further increases in response to IFN-gamma and other inflammatory mediators. Because p67(phox) and gp91(phox) expression profiles are similar, we hypothesize that common transcription factors interact with homologous cis elements in the CYBB and NCF2 genes to coordinate transcription. Previously, we identified a negative CYBB promoter cis element that is repressed by the homeodomain transcription factor HoxA10. We found that transcriptional repression requires HoxA10-dependent recruitment of histone deacetylase activity to the CYBB cis element. In response to IFN-gamma, phosphorylation of two tyrosine residues in the HoxA10 homeodomain decreases binding to CYBB promoter, thereby abrogating HoxA10-mediated repression. In the current studies, we investigate the possibility that HoxA10 similarly represses NCF2 transcription. We identify a sequence in the NCF2 promoter that is homologous to the HoxA10-binding CYBB cis element. We find that this NCF2 promoter sequence functions as a negative cis element that is repressed by HoxA10 in a tyrosine phosphorylation and histone deacetylase-dependent manner. Our results suggest that cytokine-stimulated pathways regulate HoxA10-mediated repression of the CYBB and NCF2 genes in differentiating myeloid cells and in mature phagocytes during the inflammatory response. Because p67(phox) and gp91(phox) are rate-limiting components for respiratory burst activity, our studies may identify rational therapeutic targets to modulate free radical generation in pathological conditions.

Anaplasma phagocytophilum modulates gp91phox gene expression through altered interferon regulatory factor 1 and PU.1 levels and binding of CCAAT displacement protein

Infection of neutrophil precursors with Anaplasma phagocytophilum, the causative agent of human granulocytic ehrlichiosis, results in downregulation of the gp91(phox) gene, a key component of NADPH oxidase. We now show that repression of gp91(phox) gene transcription is associated with reduced expression of interferon regulatory factor 1 (IRF-1) and PU.1 in nuclear extracts of A. phagocytophilum-infected cells. Loss of PU.1 and IRF-1 correlated with increased binding of the repressor, CCAAT displacement protein (CDP), to the promoter of the gp91(phox) gene. Reduced protein expression of IRF-1 was observed with or without gamma interferon (IFN-gamma) stimulation, and the defect in IFN-gamma signaling was associated with diminished binding of phosphorylated Stat1 to the Stat1 binding element of the IRF-1 promoter. The diminished levels of activator proteins and enhanced binding of CDP account for the transcriptional inhibition of the gp91(phox) gene during A. phagocytophilum infection, providing evidence of the first molecular mechanism that a pathogen uses to alter the regulation of genes that contribute to an effective respiratory burst.

IFN-gamma induces gp91phox expression in human monocytes via protein kinase C-dependent phosphorylation of PU.1

We previously reported that the stimulation of human blood monocytes with IFN-gamma induces the binding of PU.1 to the gp91(phox) promoter and the consequent expression of gp91(phox). In this study, we show that the effect of IFN-gamma is reproduced by the serine phosphatase inhibitor, okadaic acid, and this suggests that serine kinases could be involved in gp91(phox) expression. We also show that IFN-gamma induces the serine/threonine phosphorylation of PU.1 in cultured monocytes. This phosphorylation, as well as the IFN-gamma-induced PU.1 binding and gp91(phox) protein synthesis, is slightly affected by the casein kinase II inhibitor, daidzein, but is abrogated by the protein kinase C (PKC) -alpha and -beta inhibitor, Go6976, and by synthetic peptides with sequences based on the endogenous pseudosubstrate region of the classical PKC alpha and beta isoforms. In contrast, peptides reproducing the pseudosubstrate region of PKC epsilon were without effect. Moreover, we found that the treatment of monocytes with IFN-gamma induces the nuclear translocation and the activation of PKC alpha and beta I, but not of PKC beta II, and that the IFN-gamma-induced phosphorylation of PU.1 was greatly reduced by LY333531, a selective inhibitor of PKC beta isoforms. Finally, nuclear run-on assays demonstrated that while the PKC inhibitors, Go6976 and LY333531, decrease the IFN-gamma-induced gp91(phox) transcription, the serine phosphatase inhibitor, okadaic acid, enhances the gp91(phox) gene transcription. Our results indicate that in cultured monocytes, IFN-gamma induces the binding of PU.1 to the gp91(phox) promoter and the expression of gp91(phox) by phosphorylation of PU.1 via activation of PKC alpha and/or beta I.

Structure and regulation of the neutrophil respiratory burst oxidase: comparison with nonphagocyte oxidases

Neutrophils play an essential role in the body's innate defense against pathogens and are one of the primary mediators of the inflammatory response. To defend the host, neutrophils use a wide range of microbicidal products, such as oxidants, microbicidal peptides, and lytic enzymes. The generation of microbicidal oxidants by neutrophils results from the activation of a multiprotein enzyme complex known as the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which is responsible for transferring electrons from NADPH to O2, resulting in the formation of superoxide anion. During oxidase activation, cytosolic oxidase proteins translocate to the phagosome or plasma membrane, where they assemble around a central membrane-bound component known as flavocytochrome b. This process is highly regulated, involving phosphorylation, translocation, and multiple conformational changes. Originally, it was thought that the NADPH oxidase was restricted to phagocytes and used solely in host defense. However, recent studies indicate that similar NADPH oxidase systems are present in a wide variety of nonphagocytic cells. Although the nature of these nonphagocyte NADPH oxidases is still being defined, it is clear that they are functionally distinct from the phagocyte oxidases. It should be noted, however, that structural features of many nonphagocyte oxidase proteins do seem to be similar to those of their phagocyte counterparts. In this review, key structural and functional features of the neutrophil NADPH oxidase and its protein components are described, including a consideration of transcriptional and post-translational regulatory features. Furthermore, relevant details about structural and functional features of various nonphagocyte oxidase proteins will be included for comparison.

A functional ISRE is required for myeloid transcription of the p47phox gene

Expression of p47(phox), a component of the phagocytic NADPH oxidase, is both tissue-specific and developmentally regulated. We have investigated transcription from the p47(phox) gene promoter by reporter gene analysis of myeloid PLB985 cells stably transfected with a series of p47(phox) promoter constructs. Stable transfection with constructs containing up to 3100 bp of proximal promoter sequence demonstrated that as little as 144 bp of proximal promoter sequence was able to direct significant reporter gene activity in myeloid cells, but not in HeLa cells. Mutation of a previously uncharacterised interferon-stimulated response element (ISRE) consensus located at positions -104 to-116, or of an established binding site for the Ets family transcription factor, PU.1 (located at positions -39 to -44), abolished transcription in stably transfected myeloid cells. Electrophoretic mobility shift analysis (EMSA) with myeloid cell nuclear extracts demonstrated that an oligonucleotide containing the p47(phox) ISRE consensus was able to compete binding at another bona fide ISRE. Complexes formed on the p47(phox) ISRE itself were competed by other ISRE consensus sequences. We conclude that transcription of p47(phox) in myeloid cells requires a functional ISRE in addition to the previously identified PU.1 binding site.

Differentiation-dependent up-regulation of p47(phox) gene transcription is associated with changes in PU.1 phosphorylation and increased binding affinity

The p47(phox) gene encodes a cytosolic component of the phagocytic NADPH oxidase complex. Expression of p47(phox) is both tissue-specific and developmentally regulated. Stable transfection of the myeloid cell lines PLB985 and HL60, with reporter gene constructs containing as little as 58 bp of proximal promoter sequence, was capable of directing significant reporter gene activity in myeloid cells, which increased significantly on induction of myeloid differentiation. EMSA analysis of a binding site for the Ets family member, PU.1, located at positions -39 to -44 revealed that the pattern of complex formation changed significantly on induction of myeloid differentiation. All EMSA complexes were competed by a functional PU.1 binding site and could be supershifted in the presence of polyclonal anti-PU.1 antibody. Reaction of EMSA complexes with anti-phosphoserine antibody, treatment with phosphatase, or Western blotting of proteins captured on the PU.1 binding site, was used to demonstrate that the changes in PU.1 complex formation dependent on myeloid differentiation were associated with increased levels of PU.1 phosphorylation. Furthermore, the more highly phosphorylated forms of PU.1 were shown to have a greater affinity for the p47(phox) PU.1 consensus binding site. Up-regulated transcriptional activity in response to myeloid differentiation can therefore be correlated with increased levels of PU.1 phosphorylation and a greater binding affinity.

Identification of the human HEX1/hExo1 gene promoter and characterization of elements responsible for promoter activity

HEX1/hExo1 is a Class III nuclease of the RAD2 family with 5' to 3' exonuclease and flap structure-specific endonuclease activities. HEX1/hExo1 is expressed at low levels in a wide variety of tissues, but at higher levels in fetal liver and adult bone marrow, suggesting HEX1/hExo1 is important for hematopoietic stem cell development. A putative HEX1/hExo1 promoter fragment extending from -6240 to +1600bp exhibits cell-type specific activity in transient transfection assays. This fragment directs high luciferase reporter gene expression in the hematopoietic cell line K562, chronic myelogenous leukemia cells, but low luciferase expression in the non-hematopoietic cell line HeLa, human cervical carcinoma cells. Deletion studies identified a fragment spanning -688 to +1600bp that exhibits full transcriptional activity while a slightly shorter fragment from -658 to +1600bp exhibits significantly decreased promoter activity. In vitro binding assays revealed DNA-binding activities that interact with -687 to -681bp and -665 to -658bp elements. Oligonucleotide competition and antibody disruption studies determined that the transcription factor CREB-1 recognizes the -687 to -681bp element, while transcription factors Sp1 and Sp3 recognize the -665 to -658bp element. Mutation of either the CREB-1 or Sp1/Sp3 binding sites dramatically reduces HEX1/hExo1 promoter activity and elimination of both elements abolishes promoter activity.

Cooperation of STAT-1 and IRF-1 in interferon-gamma-induced transcription of the gp91(phox) gene

Interferon (IFN)-gamma induces the expression of the gp91(phox) gene both during myeloid differentiation and also in mature phagocytes through several cis-elements and their binding proteins. To find new cis-elements for this induction, transient expression assays were performed using a reporter gene driven by serially truncated gp91(phox) promoters in U937 cells. The results suggest that a critical cis-element for induction exists in the region from bp -115 to -96 of the promoter. Site-directed mutagenesis showed that a gamma-activated sequence (GAS) element at bp -100 (-100GAS) of the gp91(phox) promoter plays a pivotal role for the IFN-gamma-dependent activity of the bp -115 to +12 region of the gp91(phox) promoter. Electrophoretic mobility shift assays using several GAS competitors and specific antibodies indicated that phosphorylated STAT-1alpha specifically binds to the -100GAS. Site-directed mutagenesis showed that an interferon-stimulated response element (ISRE) at bp -88 (-88ISRE) mediates the induction of the gene by IFN-gamma in cooperation with -100GAS. Electrophoretic mobility shift assay showed that IRF-1 dominantly binds to -88ISRE in an IFN-gamma-dependent fashion. These results demonstrate a new mechanism for IFN-gamma-induced transcription of the gp91(phox) gene by the cooperation of STAT-1alpha and IRF-1 binding to -100GAS and -88ISRE, respectively.

The matrix attachment region-binding protein SATB1 interacts with multiple elements within the gp91phox promoter and is down-regulated during myeloid differentiation

The gp91(phox) gene encodes a component of the respiratory burst NADPH oxidase complex and is highly expressed in mature myeloid cells. The transcriptional repressor CCAAT displacement protein binds to at least five sites within the proximal gp91(phox) promoter and represses expression prior to terminal phagocyte differentiation. The DNA binding activity of CCAAT displacement protein decreases during terminal phagocyte differentiation, thus permitting the binding of transcriptional activators and induction of gp91(phox) expression. We report here that the matrix attachment region-binding protein SATB1 interacts with at least seven sites within the -1542 to +12-base pair gp91(phox) promoter. Four additional binding sites for CCAAT displacement protein were also identified. Furthermore, the most proximal SATB1-binding site within the gp91(phox) promoter binds specifically to the nuclear matrix fraction in vitro. SATB1 expression is down-regulated during terminal myeloid cell differentiation, coincident with induction of gp91(phox) expression. Transient transfection assays demonstrate that a SATB1-binding site derived from the gp91(phox) promoter represses promoter activity in cells expressing SATB1. These findings underscore the importance of transcriptional repression in the regulation of gp91(phox) expression and reveal a candidate myeloid cell target gene for SATB1, a factor previously found to be essential for T cell development.

SHP1 protein-tyrosine phosphatase inhibits gp91PHOX and p67PHOX expression by inhibiting interaction of PU.1, IRF1, interferon consensus sequence-binding protein, and CREB-binding protein with homologous Cis elements in the CYBB and NCF2 genes

The CYBB and NCF2 genes encode the phagocyte respiratory burst oxidase proteins, gp91PHOX and p67PHOX. Previously, we identified homologous CYBB and NCF2 cis elements that are necessary for lineage-specific transcription during late myeloid differentiation. We determined that these homologous cis elements are activated by PU.1, IRF1, interferon consensus sequence-binding protein (ICSBP), and the CREB-binding protein (CBP). Since expression of PU.1 and ICSBP is lineage-restricted, our investigations identified a mechanism of lineage-specific CYBB and NCF2 transcription. Since PU.1, IRF1, ICSBP, and CBP are expressed in undifferentiated myeloid cells, our investigations did not determine the mechanism of differentiation stage-specific CYBB and NCF2 transcription. In the current investigations, we determine that SHP1 protein-tyrosine phosphatase (SHP1-PTP) inhibits gp91PHOX and p67PHOX expression, in undifferentiated myeloid cell lines, by decreasing interaction of PU.1, IRF1, ICSBP, and CBP with the CYBB and NCF2 genes. We also determine that IRF1 and ICSBP are tyrosine-phosphorylated during interferon gamma differentiation of myeloid cell lines, and we identify IRF1 and ICSBP tyrosine residues that are necessary for CYBB and NCF2 transcription. Therefore, these investigations identify a novel mechanism by which SHP1-PTP antagonizes myeloid differentiation and determine that tyrosine phosphorylation of IRF1 and ICSPB mediates stage-specific transcriptional activation in differentiating myeloid cells.

Mechanisms of expression of NADPH oxidase components in human cultured monocytes: role of cytokines and transcriptional regulators involved

Human blood monocytes lose their capability to produce microbicidal oxidants during culture. We report that this process is associated with decreased gp91phox, p22phox and p47phox expression, release of PU.1 and CP-1 from gp91phox promoter, and PU.1 from p47phox promoter. However, in presence of IFN-gamma or TNF-alpha, the superoxide anion (O(2)(-)) production, the p47phox, gp91phox and p22phox expression, and the binding of PU.1 and CP-1 to DNA are maintained at the high levels observed in blood monocytes. To clarify the role of PU.1 in the expression of NADPH oxidase components, oligonucleotides competing for PU.1-DNA binding were added to cultured monocytes. These oligonucleotides abrogated the maintenance of gp91phox and p22phox expression by IFN-gamma and TNF-alpha, but did not inhibit the effect of these cytokines on p47phox expression and O(2)(-) production. Our results indicate that in monocytes the IFN-gamma- and TNF-alpha-induced expression of gp91phox and p22phox, but not p47phox, requires the binding of PU.1 to gp91phox promoter. However, the preservation of O(2)(-) production by IFN-gamma and TNF-alpha is unrelated to their effect on gp91phox and p22phox expression.

Genetic, biochemical, and clinical features of chronic granulomatous disease

The reduced nicotinamide dinucleotide phosphate (NADPH) oxidase complex allows phagocytes to rapidly convert O2 to superoxide anion which then generates other antimicrobial reactive oxygen intermediates, such as H2O2, hydroxyl anion, and peroxynitrite anion. Chronic granulomatous disease (CGD) results from a defect in any of the 4 subunits of the NADPH oxidase and is characterized by recurrent life-threatening bacterial and fungal infections and abnormal tissue granuloma formation. Activation of the NADPH oxidase requires translocation of the cytosolic subunits p47phox (phagocyte oxidase), p67phox, and the low molecular weight GT-Pase Rac, to the membrane-bound flavocytochrome, a heterodimer composed of the heavy chain gp91phox and the light chain p22phox. This complex transfers electrons from NADPH on the cytoplasmic side to O2 on the vacuolar or extracellular side, thereby generating superoxide anion. Activation of the NADPH oxidase requires complex rearrangements between the protein subunits, which are in part mediated by noncovalent binding between src-homology 3 domains (SH3 domains) and proline-rich motifs. Outpatient management of CGD patients relies on the use of prophylactic antibiotics and interferon-gamma. When infection is suspected, aggressive effort to obtain culture material is required. Treatment of infections involves prolonged use of systemic antibiotics, surgical debridement when feasible, and, in severe infections, use of granulocyte transfusions. Mouse knockout models of CGD have been created in which to examine aspects of pathophysiology and therapy. Gene therapy and bone marrow transplantation trials in CGD patients are ongoing and show great promise.

Cloning of a mammalian transcriptional activator that binds unmethylated CpG motifs and shares a CXXC domain with DNA methyltransferase, human trithorax, and methyl-CpG binding domain protein 1

Ligand screening was utilized to isolate a human cDNA that encodes a novel CpG binding protein, human CpG binding protein (hCGBP). This factor contains three cysteine-rich domains, two of which exhibit homology to the plant homeodomain finger domain. A third cysteine-rich domain conforms to the CXXC motif identified in DNA methyltransferase, human trithorax, and methyl-CpG binding domain protein 1. A fragment of hCGBP that contains the CXXC domain binds to an oligonucleotide probe containing a single CpG site, and this complex is disrupted by distinct oligonucleotide competitors that also contain a CpG motif(s). However, hCGBP fails to bind oligonucleotides in which the CpG motif is either mutated or methylated, and it does not bind to single-stranded DNA or RNA probes. Furthermore, the introduction of a CpG dinucleotide into an unrelated oligonucleotide sequence is sufficient to produce a binding site for hCGBP. Native hCGBP is detected as an 88-kDa protein by Western analysis and is ubiquitously expressed. The DNA-binding activity of native hCGBP is apparent in electrophoretic mobility shift assays, and hCGBP trans-activates promoters that contain CpG motifs but not promoters in which the CpG is ablated. These data indicate that hCGBP is a transcriptional activator that recognizes unmethylated CpG dinucleotides, suggesting a role in modulating the expression of genes located within CpG islands.

Recruitment of CREB-Binding Protein by PU.1, IFN-Regulatory Factor-1, and the IFN Consensus Sequence-Binding Protein Is Necessary for IFN-γ-Induced p67phox and gp91phox Expression

Activation of the phagocyte respiratory burst oxidase requires interaction between the oxidase components p47phox, p67phox, p22phox, and gp91phox. IFN-γ induces transcription of the genes encoding p67phox (the NCF2 gene) and gp91phox (the CYBB gene) during monocyte differentiation, and also in mature monocytes. In these studies, we identify an NCF2 cis element, necessary for IFN-γ-induced p67phox expression, and determine that this element is activated by cooperation between the transcription factors PU.1, IFN regulatory factor 1 (IRF1), and the IFN consensus-binding protein (ICSBP). Previously, we identified a CYBB cis element, necessary for IFN-γ-induced gp91phox expression, and also activated by this transcription factor combination. In these investigations, we determine that recruitment of a coactivator protein, CBP (the CREBbinding protein), to the CYBB or NCF2 promoter is the molecular mechanism of transcriptional activation by PU.1, IRF1, and ICSBP. Also, we determine that the multiprotein interaction of CBP with PU.1, IRF1, and ICSBP requires either the CYBB- or NCF2--binding site. Because IFN-γ induces simultaneous expression of p67phox and gp91phox, these investigations identify a molecular event that coordinates oxidase gene transcription during the inflammatory response. Also, these investigations identify CBP recruitment by cooperation between PU.1, IRF1, and ICSBP as a novel molecular mechanism for IFN-γ-induced activation of myeloid genes that are involved in the system of host defense.

Critical flanking sequences of PU.1 binding sites in myeloid-specific promoters

The myeloid-specific transcription factor PU.1 is essential for expression of p47(phox), a component of the superoxide-forming phagocyte NADPH oxidase. The consensus PU.1 binding sequence (GAGGAA) is located on the non-coding strand from position -40 to -45 relative to the transcriptional start site of the p47phox promoter. A promoter construct extending to -46 was sufficient to drive tissue-specific expression of the luciferase reporter gene, but extension of the promoter from -46 to -48 resulted in a significant increase in reporter expression. Mutations of the nucleotides G at -46 and/or T at -47 reduced both reporter expression and PU.1 binding, whereas mutations at -48 had no effect. The PU.1 binding avidity of these sequences correlated closely with their capacity to dictate reporter gene transcription. In parallel studies on the functional PU.1 site in the promoter of CD18, mutations of nucleotides G and T at positions -76 and -77 (corresponding to -46 and -47, respectively, of the p47phox promoter) reduced PU.1 binding and nearly abolished the contribution of this element to promoter activity. We conclude that the immediate flanking nucleotides of the PU.1 consensus motif have significant effects on PU.1 binding avidity and activity and that this region is the dominant cis element regulating p47phox expression.

Overexpression of CCAAT Displacement Protein Represses the Promiscuously Active Proximal gp91phox Promoter

CCAAT displacement protein (CDP) is a transcriptional repressor that restricts expression of the gp91phox gene to mature myeloid cells. CDP interacts with multiple sites within the −450 to +12 bp human gp91phox promoter, and down-regulation of CDP DNA-binding activity is required for induction of gp91phox transcription in mature phagocytes. Truncation of the gp91phox promoter to −102 to +12 bp removes 4 CDP-binding sites and reveals a promiscuous promoter activity that is active in some nonphagocytic cells. A cis-element at −90 bp is required for derepressed transcription and serves as a binding site for multiple transcriptional activators. We now report that this element also serves as a binding site for CDP. The affinity of CDP for this element is relatively weak compared with upstream CDP-binding sites within the promoter, consistent with the promiscuous transcriptional activity exhibited by the −102 to +12 bp gp91phox promoter fragment. Further analysis of the proximal promoter reveals an additional weak-affinity CDP-binding site centered at approximately −20 bp. Overexpression of cloned CDP represses the −102 to +12 bp gp91phox promoter, indicating that these proximal CDP-binding sites are functionally significant. The constellation of transcriptional activators and a repressor that interacts with the −90 bp cis-element is identical to that observed for a promoter element at −220 bp, reflecting the highly modular organization of the gp91phoxpromoter. These studies illustrate the complex interplay between transcriptional activators and a repressor that contribute to the myeloid-restricted expression of the gp91phox gene.

Overexpression of CCAAT Displacement Protein Represses the Promiscuously Active Proximal gp91phox Promoter

CCAAT displacement protein (CDP) is a transcriptional repressor that restricts expression of the gp91phox gene to mature myeloid cells. CDP interacts with multiple sites within the −450 to +12 bp human gp91phox promoter, and down-regulation of CDP DNA-binding activity is required for induction of gp91phox transcription in mature phagocytes. Truncation of the gp91phox promoter to −102 to +12 bp removes 4 CDP-binding sites and reveals a promiscuous promoter activity that is active in some nonphagocytic cells. A cis-element at −90 bp is required for derepressed transcription and serves as a binding site for multiple transcriptional activators. We now report that this element also serves as a binding site for CDP. The affinity of CDP for this element is relatively weak compared with upstream CDP-binding sites within the promoter, consistent with the promiscuous transcriptional activity exhibited by the −102 to +12 bp gp91phox promoter fragment. Further analysis of the proximal promoter reveals an additional weak-affinity CDP-binding site centered at approximately −20 bp. Overexpression of cloned CDP represses the −102 to +12 bp gp91phox promoter, indicating that these proximal CDP-binding sites are functionally significant. The constellation of transcriptional activators and a repressor that interacts with the −90 bp cis-element is identical to that observed for a promoter element at −220 bp, reflecting the highly modular organization of the gp91phoxpromoter. These studies illustrate the complex interplay between transcriptional activators and a repressor that contribute to the myeloid-restricted expression of the gp91phox gene.

YY1 binds five cis-elements and trans-activates the myeloid cell-restricted gp91(phox) promoter

Four transcriptional activating cis-elements within the gp91(phox) promoter bind a protein complex of similar mobility and binding specificity, denoted BID (binding increased during differentiation). The intensity of BID complexes increases upon myeloid cell differentiation, coincident with induction of gp91(phox) expression, and BID competes with the transcriptional repressor CDP for binding to each of these promoter elements. To determine the identity of BID, an expression library was ligand screened with the BID-binding site that surrounds the -145-base pair (bp) region of the gp91(phox) promoter. One recovered factor that exhibits the expected binding specificity is YY1, a ubiquitous multifunctional transcription factor. BID complexes that form with the four binding sites within the gp91(phox) promoter are disrupted by YY1 antiserum, and a fifth YY1-binding site was detected in the -412-bp promoter region. Overexpression of YY1 in transient co-transfection assays trans-activates a minimal promoter containing two copies of the -145-bp binding site from the gp91(phox) promoter. Neither the level of YY1 protein nor DNA binding activity increases during myeloid cell differentiation. These studies identify a target gene of YY1 function in mature myeloid cells, and demonstrate that YY1 function can be controlled during myeloid development by the modulation of a competing DNA-binding factor.

Modulation of the gene network connected to interferon-gamma in liver regeneration from oval cells

Suppression subtractive hybridization was used to clone genes associated with proliferation of oval cells in rat liver regenerating after a 70% partial hepatectomy combined with the feeding of 2-acetylaminofluorene. A subset of the identified genes comprised interferon-gamma receptor alpha subunit (IFN-gammaRalpha), gp91phox, interleukin-1beta (IL-1beta), lymphocyte function-associated molecule-1alpha (LFA-1), eukaryotic initiation factor-2-associated 67-kd protein (eIF-2-associated 67-kd protein), and alpha-fetoprotein, which constitute part of the cellular program modulated by IFN-gamma. Therefore, expression analysis performed by Northern blotting and immunohistochemistry were extended to include IFN-gamma, the IFN-gamma receptor beta subunit (IFN-gammaRbeta), three secondary response genes induced by interaction of IFN-gamma with IFN-gamma receptor complexes, ie, IL-1beta-converting enzyme (ICE), intercellular adhesion molecule-1 (ICAM-1), and urokinase-type plasminogen activator receptor (uPAR), and a cytokine inducing IFN-gamma expression, ie, interleukin-18 (IL-18). The Northern blot analysis showed that all examined genes were modulated when progenitor-like oval cells were activated and recruited for liver regeneration. Immunohistochemistry localized the subunits of the IFN-gamma receptor complex, IFN-gammaRalpha and IFN-gammaRbeta, the secondary response genes uPAR and ICAM-1, the IFN-gamma-inducing factor IL-18, and ICE to the ductular structures of oval cells. In contrast, during liver regeneration after a 70% partial hepatectomy, only modulation of IL-1beta and ICE was observed. Our results, therefore, indicate that IFN-gamma-mediated events may be particularly important when cells in the bile ductules must respond to liver damage by production of ductular oval cells.

Elf-1 and PU.1 Induce Expression of gp91phox Via a Promoter Element Mutated in a Subset of Chronic Granulomatous Disease Patients

The cytochrome b heavy chain (gp91phox) is the redox center of the NADPH-oxidase and is highly expressed in mature myeloid cells. Point mutations at −57, −55, −53, and −52 bp of the gp91phox promoter have been detected in patients with chronic granulomatous disease (CGD; Newburger et al,J Clin Invest 94:1205, 1994; and Suzuki et al, Proc Natl Acad Sci USA 95:6085, 1998). We report that Elf-1 and PU.1,ets family members highly expressed in myeloid cells, bind to this promoter element. Either factor trans-activates the −102 to +12 bp gp91phox promoter when overexpressed in nonhematopoietic HeLa cells or the PLB985 myeloid cell line. However, no synergy of gp91phox promoter activation occurs when both Elf-1 and PU.1 are overexpressed. Introduction of the −57 bp or −55 bp CGD mutations into the gp91phoxpromoter significantly reduces the binding affinity of Elf-1 and PU.1 and also reduces the ability of these factors to trans-activate the promoter. These results indicate that Elf-1 and PU.1 contribute to directing the lineage-restricted expression of the gp91phox gene in phagocytes and that failure of these factors to effectively interact with this promoter results in CGD.

Identification and functional characterization of the murine Rac2 gene promoter

Rac2, a member of the Rho family of GTPases, is highly expressed in myeloid cells and is a regulator of the NADPH-oxidase complex. A murine genomic clone was isolated that contains the 5' end and putative promoter region of the Rac2 gene. Ribonuclease protection experiments detected 13 transcription initiation sites scattered 50 to 130 bp upstream of the translation initiation site. Transient transfection studies revealed that -7 kb to +31 bp (relative to the strongest transcription initiation site) of the Rac2 gene 5'-flanking region exhibited strong promoter activity in both RAW 264.7 macrophage cells that express the endogenous Rac2 gene and NIH-3T3 fibroblast cells that do not express the endogenous gene. Truncated Rac2 promoter fragments containing as little as the -74 to +31 bp sequence produced full transcriptional activity. However, a -57 to +31 promoter fragment directed significantly less transcription, and a -39 to +31 promoter fragment was transcriptionally inactive. In vitro binding assays revealed sequence-specific and widely expressed DNA-binding activities that interacted within the -74 to -58 Rac2 promoter cis element. Oligonucleotide competition and antibody disruption studies indicated that these complexes contained the transcription factors Spl and Sp3. Specific ablation of the Sp1/Sp3 binding site significantly decreased Rac2 promoter activity in both RAW 264.7 and NIH-3T3 cells. Additional cis elements may be required to restrict Rac2 promoter activity to hematopoietic cells expressing the endogenous gene.

X-Linked chronic granulomatous disease: mutations in the CYBB gene encoding the gp91-phox component of respiratory-burst oxidase

Chronic granulomatous disease (CGD) is a hereditary disorder of host defense due to absent or decreased activity of phagocyte NADPH oxidase. The X-linked form of the disease derives from defects in the CYBB gene, which encodes the 91-kD glycoprotein component (termed "gp91-phox") of the oxidase. We have identified the mutations in the CYBB gene responsible for X-linked CGD in 131 consecutive independent kindreds. Screening by SSCP analysis identified mutations in 124 of the kindreds, and sequencing of all exons and intron boundary regions revealed the other seven mutations. We detected 103 different specific mutations; no single mutation appeared in more than seven independent kindreds. The types of mutations included large and small deletions (11%), frameshifts (24%), nonsense mutations (23%), missense mutations (23%), splice-region mutations (17%), and regulatory-region mutations (2%). The distribution of mutations within the CYBB gene exhibited great heterogeneity, with no apparent mutational hot spots. Evaluation of 87 available mothers revealed X-linked carrier status in all but 10. The heterogeneity of mutations and the lack of any predominant genotype indicate that the disease represents many different mutational events, without a founder effect, as is expected for a disorder with a previously lethal phenotype.

PU.1, interferon regulatory factor 1, and interferon consensus sequence-binding protein cooperate to increase gp91(phox) expression

gp91(phox) is a subunit of the phagocyte respiratory burst oxidase catalytic unit. Transcription of CYBB, the gene encoding gp91(phox), is restricted to terminally differentiated phagocytic cells. An element in the proximal CYBB promoter binds a protein complex, referred to as hematopoiesis-associated factor (HAF1), that is necessary for interferon-gamma (IFNgamma)-induced gp91(phox) expression. In these investigations, we determined that HAF1 was a multiprotein complex, cross-immunoreactive with the transcription factors PU.1, interferon regulatory factor 1 (IRF-1), and interferon consensus sequence-binding protein (ICSBP). In electrophoretic mobility shift assay, the HAF1 complex was reconstituted by either in vitro translated PU.1 with IRF-1 or PU.1 with ICSBP, but not by IRF-1 with ICSBP. HAF1a, a slower mobility complex with the same binding site specificity as HAF1, was also investigated. Similar to the HAF1 complex, the HAF1a complex was cross-immunoreactive with PU. 1, IRF-1, and ICSBP. Unlike the HAF1 complex, reconstitution of the HAF1a complex required in vitro translated PU.1 with both IRF-1 and ICSBP. An artificial promoter construct containing the HAF1/HAF1a binding site was modestly activated in the myelomonocytic cell line U937 by co-transfection either with PU.1 and IRF-1 or with PU.1 and ICSBP, but it was strongly activated by co-transfection with PU.1, IRF-1, and ICSBP. This activation required serine 148-phosphorylated PU.1. These studies describe a novel mechanism for PU.1 transcriptional activation via interaction with both IRF-1 and ICSBP, a target gene for the interaction of IRF-1 with ICSBP, and a novel activation function for ICSBP as a component of a multiprotein complex.

PU.1 as an essential activator for the expression of gp91(phox) gene in human peripheral neutrophils, monocytes, and B lymphocytes

We have reported a deficiency of a 91-kDa glycoprotein component of the phagocyte NADPH oxidase (gp91(phox)) in neutrophils, monocytes, and B lymphocytes of a patient with X chromosome-linked chronic granulomatous disease. Sequence analysis of his gp91(phox) gene revealed a single-base mutation (C --> T) at position -53. Electrophoresis mobility-shift assays showed that both PU.1 and hematopoietic-associated factor 1 (HAF-1) bound to the inverted PU.1 consensus sequence centered at position -53 of the gp91(phox) promoter, and the mutation at position -53 strongly inhibited the binding of both factors. It was also indicated that a mutation at position -50 strongly inhibited PU.1 binding but hardly inhibited HAF-1 binding, and a mutation at position -56 had an opposite binding specificity for these factors. In transient expression assay using HEL cells, which express PU.1 and HAF-1, the mutations at positions -53 and -50 significantly reduced the gp91(phox) promoter activity; however, the mutation at position -56 did not affect the promoter activity. In transient cotransfection study, PU.1 dramatically activated the gp91(phox) promoter in Jurkat T cells, which originally contained HAF-1 but not PU.1. In addition, the single-base mutation (C --> T) at position -52 that was identified in a patient with chronic granulomatous disease inhibited the binding of PU.1 to the promoter. We therefore conclude that PU.1 is an essential activator for the expression of gp91(phox) gene in human neutrophils, monocytes, and B lymphocytes.

Identification and characterization of TF1(phox), a DNA-binding protein that increases expression of gp91(phox) in PLB985 myeloid leukemia cells

The CYBB gene encodes gp91(phox), the heavy chain of the phagocyte-specific NADPH oxidase. CYBB is transcriptionally inactive until the promyelocyte stage of myelopoiesis, and in mature phagocytes, expression of gp91(phox) is further increased by interferon-gamma (IFN-gamma) and other inflammatory mediators. The CYBB promoter region contains several lineage-specific cis-elements involved in the IFN-gamma response. We screened a leukocyte cDNA expression library for proteins able to bind to one of these cis-elements (-214 to -262 base pairs) and identified TF1(phox), a protein with sequence-specific binding to the CYBB promoter. Electrophoretic mobility shift assay with nuclear proteins from a variety of cell lines demonstrated binding of a protein to the CYBB promoter that was cross-immunoreactive with TF1(phox). DNA binding of this protein was increased by IFN-gamma treatment in the myeloid cell line PLB985, but not in the non-myeloid cell line HeLa. Overexpression of recombinant TF1(phox) in PLB985 cells increased endogenous gp91(phox) message abundance, but did not lead to cellular differentiation. Overexpression of TF1(phox) in myeloid leukemia cell lines increased reporter gene expression from artificial promoter constructs containing CYBB promoter sequence. These data suggested that TF1(phox) increased expression of gp91(phox).

CCAAT displacement protein, a regulator of differentiation-specific gene expression, binds a negative regulatory element within the 5' end of the human papillomavirus type 6 long control region

We have reported previously that a 636-bp fragment spanning the 5' two-thirds of the human papillomavirus type 6 (HPV6)-W50 long control region (LCR) functions as a transcriptional silencer (A. Farr, S. Pattison, B.-S. Youn, and A. Roman, J. Gen. Virol. 76:827-835, 1995). We have utilized nested deletion analyses to implicate a 66-bp sequence which appears to be critical for this activity. A comparison of the transcriptional regulatory activities of the LCRs of HPV6-W50 and HPV6b (which has a 94-bp deletion, resulting in the elimination of the 66-bp sequence) indicates that sequences within the 94-bp region negatively regulate the activity of the intact HPV6 LCR. Two sequence-specific DNA-protein interactions were visualized via electrophoretic mobility shift assays. One of the binding events is mediated by the transcriptional repressor CCAAT displacement protein (CDP), a factor which is active in undifferentiated cells but inactive in terminally differentiated cells. This conclusion is based on the following three lines of evidence: (i) a consensus CDP binding site oligonucleotide serves as a competitor in band shift assays, (ii) the band shift complex is not seen when a CDP-negative nuclear extract is used, and (iii) anti-CDP antiserum specifically inhibits the binding. These studies identify a DNA-protein interaction occurring within the 5' end of the LCR which may be important in maintaining the tight link between keratinocyte differentiation and HPV gene expression.

Cellular responses to interferon-gamma

Interferons are cytokines that play a complex and central role in the resistance of mammalian hosts to pathogens. Type I interferon (IFN-alpha and IFN-beta) is secreted by virus-infected cells. Immune, type II, or gamma-interferon (IFN-gamma) is secreted by thymus-derived (T) cells under certain conditions of activation and by natural killer (NK) cells. Although originally defined as an agent with direct antiviral activity, the properties of IFN-gamma include regulation of several aspects of the immune response, stimulation of bactericidal activity of phagocytes, stimulation of antigen presentation through class I and class II major histocompatibility complex (MHC) molecules, orchestration of leukocyte-endothelium interactions, effects on cell proliferation and apoptosis, as well as the stimulation and repression of a variety of genes whose functional significance remains obscure. The implementation of such a variety of effects by a single cytokine is achieved by complex patterns of cell-specific gene regulation: Several IFN-gamma-regulated genes are themselves components of transcription factors. The IFN-gamma response is itself regulated by interaction with responses to other cytokines including IFN-alpha/beta, TNF-alpha, and IL-4. Over 200 genes are now known to be regulated by IFN-gamma and they are listed in a World Wide Web document that accompanies this review. However, much of the cellular response to IFN-gamma can be described in terms of a set of integrated molecular programs underlying well-defined physiological systems, for example the induction of efficient antigen processing for MHC-mediated antigen presentation, which play clearly defined roles in pathogen resistance. A promising approach to the complexity of the IFN-gamma response is to extend the analysis of the less understood IFN-gamma-regulated genes in terms of molecular programs functional in pathogen resistance.

Interferon regulatory factor-2 directs transcription from the gp91phox promoter

Repressor elements in the gp91(phox) promoter are necessary to restrict tissue-specific transcription to mature phagocytes. Deletion of these elements leads to significant promoter activity in cell lines such as HEL and K562 that do not normally express gp91(phox). The -100 to +12 base pair gp91(phox) promoter region is sufficient to direct maximal de-repressed transcription in these cells. However, promoter activity is dramatically decreased following a 16-base pair truncation that deletes an interferon-stimulated response element. This element interacts with IRF-1 and IRF-2, members of the interferon regulatory factor family of transcription factors. In addition, this promoter region is bound by a factor with properties similar to BID, a DNA-binding protein that also interacts with three upstream sites within the gp91(phox) promoter. Transient transfection studies using mutated promoters indicate that both the IRF and BID binding sites are required for maximal gp91(phox) promoter activity. Overexpression of IRF-1 or IRF-2 in K562 cells leads to transactivation of gp91(phox) promoter constructs, which is dependent on the presence of an intact IRF binding site. IRF-2 predominates in macrophages that express the gp91(phox) gene as well as in HEL and K562 cells. We conclude that IRF-2 and BID activate gp91(phox) promoter activity in the absence of transcriptional repression.

CCAAT displacement protein competes with multiple transcriptional activators for binding to four sites in the proximal gp91phox promoter

CCAAT displacement protein (CDP) competes with transcriptional activating proteins for binding to each of four elements within the myeloid-specific gp91(phox) promoter. CDP exhibits the strongest affinity for a site centered at -110 base pairs (bp) of the promoter and progressively weaker affinities for three more distal binding sites. CDP binding to each site is down-regulated during terminal phagocytic differentiation, coincident with induction of gp91(phox) expression. Deletion of the high affinity CDP-binding site at -110 bp leads to inappropriate gp91(phox) promoter activity in HeLa, K562, and HEL cells. An overlapping binding site for the CCAAT box-binding factor CP1 is required for derepressed promoter activity in HeLa and K562 cells, but is dispensable in HEL cells, indicating that different cell types require distinct cis-elements for gp91(phox) promoter activity. Derepressed gp91(phox) promoter activity is further increased upon removal of a second CDP-binding site centered at -150 bp, revealing that CDP represses gp91(phox) expression via multiple cis-elements. We present a model in which restriction of gp91(phox) expression to mature myeloid cells involves competition between transcriptional activators and repressors for binding to multiple sites within the promoter.