PU.1 and USF are required for macrophage-specific mannose receptor promoter activity

Transcriptional regulation of the HIV-1 inhibitory factor human mannose receptor 1 by the myeloid-specific transcription factor PU.1

HIV-1 infection of human macrophages leads to the downmodulation of human mannose receptor 1 (hMRC1), a cell-surface glycoprotein that is involved in the host innate immune response. We previously reported that downmodulation of hMRC1 involves the transactivator of transcription (Tat)-dependent transcriptional silencing of the hMRC1 promoter. However, the inhibitory effect of Tat on hMRC1 transcription was indirect and involved inhibition of the transcriptional activator PU.1, which normally upregulates hMRC1 expression in macrophages and other myeloid cells. We cloned a 284-bp fragment of the hMRC1 promoter, and within it, we identified four PU.1 box elements. We assessed the relative contribution of each of the four PU.1 boxes to PU.1-dependent transcriptional regulation and, surprisingly, found that only one of the four PU.1 boxes [PU.1(b)] was critically required for PU.1-mediated upregulation of luciferase expression. Transfer of this PU.1 box to a heterologous promoter conferred PU.1 responsiveness to an otherwise PU.1 insensitive promoter. Electrophoretic mobility shift assays identified this PU.1 box as a direct binding site for PU.1 both in the context of the hMRC1 promoter and the heterologous promoter. Furthermore, mutational analysis of the PU.1 protein identified the C-terminal DNA-binding domain in PU.1 as the region responsible for interaction with the PU.1 box. Recombinant HIV-1 Tat protein did not bind to the hMRC1 promoter element but efficiently interfered with the binding of PU.1 protein to the hMRC1 promoter. Thus, Tat is likely to inhibit the formation of active PU.1 transcription complexes, presumably by binding to and depleting common transcriptional cofactors.IMPORTANCEHIV-1 infection of cells results in the modulation of cellular gene expression by virus-encoded proteins in a manner that benefits the virus. We reported that HIV-1 transactivator of transcription (Tat) dysregulates the expression of the human mannose receptor 1 (hMRC1). hMRC1 is involved in the innate immune response of macrophages to foreign pathogens. Tat does not act directly on the hMRC1 promoter but instead inhibits PU.1, a cellular transcription factor regulating hMRC1 gene expression. Here, we characterize the PU.1-dependent regulation of hMRC1 expression. We identified four potential PU.1 binding sites in the hMRC1 promoter region but found that only one, PU.1(b), functioned as a true binding site for PU.1. Transfer of the PU.1(b) box to a heterologous promoter did not activate this promoter per se but rendered it responsive to PU.1. Our results support the view that PU.1 acts as a transcriptional co-factor whose activity can be regulated by HIV-1 Tat.

Hematopoietic upstream stimulating factor 1 deficiency is associated with increased atherosclerosis susceptibility in LDL receptor knockout mice

Total body upstream stimulatory factor 1 (USF1) deficiency in mice is associated with brown adipose tissue activation and a marked protection against the development of obesity and atherosclerotic lesions. Functional expression of USF1 has also been detected in monocytes and monocyte-derived macrophages. In the current study we therefore tested whether selective hematopoietic USF1 deficiency can also beneficially impact the development of atherosclerosis. For this purpose, LDL receptor knockout mice were transplanted with bone marrow from USF1 knockout mice or their wild-type littermate controls and subsequently fed a Western-type diet for 20 weeks to stimulate atherosclerotic lesion development. Strikingly, absence of USF1 function in bone marrow-derived cells was associated with exacerbated blood leukocyte (+ 100%; P < 0.01) and peritoneal leukocyte (+ 50%; P < 0.05) lipid loading and an increased atherosclerosis susceptibility (+ 31%; P < 0.05). These effects could be attributed to aggravated hyperlipidemia, i.e. higher plasma free cholesterol (+ 33%; P < 0.001) and cholesteryl esters (+ 39%; P < 0.001), and the development of hepatosteatosis. In conclusion, we have shown that hematopoietic USF1 deficiency is associated with an increased atherosclerosis susceptibility in LDL receptor knockout mice. These findings argue against a contribution of macrophage-specific USF1 deficiency to the previously described beneficial effect of total body USF1 deficiency on atherosclerosis susceptibility in mice.

Substrate stiffness influences phenotype and function of human antigen-presenting dendritic cells

Dendritic cells (DCs) are specialized immune cells that scan peripheral tissues for foreign material or aberrant cells and, upon recognition of such danger signals, travel to lymph nodes to activate T cells and evoke an immune response. For this, DCs travel large distances through the body, encountering a variety of microenvironments with different mechanical properties such as tissue stiffness. While immune-related pathological conditions such as fibrosis or cancer are associated with tissue stiffening, the role of tissue stiffness in regulating key functions of DCs has not been studied yet. Here, we investigated the effect of substrate stiffness on the phenotype and function of DCs by conditioning DCs on polyacrylamide substrates of 2, 12 and 50 kPa. Interestingly, we found that C-type lectin expression on immature DCs (iDCs) is regulated by substrate stiffness, resulting in differential antigen internalization. Furthermore, we show that substrate stiffness affects β2 integrin expression and podosome formation by iDCs. Finally, we demonstrate that substrate stiffness influences CD83 and CCR7 expression on mature DCs, the latter leading to altered chemokine-directed migration. Together, our results indicate that DC phenotype and function are affected by substrate stiffness, suggesting that tissue stiffness is an important determinant for modulating immune responses.

PU.1 is involved in the immune response to Aspergillus fumigatus through upregulating Dectin-1 expression

Background

Invasive aspergillosis is a life-threatening disease, and its incidence has increased in the recent past. Dectin-1 recognizes β-glucans and mediates innate immune responses to Aspergillus fumigatus. Transcription factor PU.1 has been the focus of recent research due to its role in inflammation and infection. However, its role in Dectin-1 regulation during A. fumigatus infection remains to be elucidated.

Methods

THP-1 cells were stimulated with A. fumigatus conidia. We then used real-time RT-PCR, Western blot, and immunofluorescence assays to analyze the mRNA and protein levels and cellular distribution, respectively, of Dectin-1 and PU.1 in stimulated THP-1 cells. Additionally, we used the luciferase reporter assays, chromatin immunoprecipitation (ChIP) assays, electrophoretic mobility shift assays (EMSA), and RNA interference experiments to investigate the role of PU.1 in Dectin-1 regulation.

Results

Our results revealed that Dectin-1 mRNA and protein levels as well as the PU.1 protein level were increased in THP-1 cells stimulated with A. fumigatus conidia, while the mRNA expression level did not significantly change between the stimulated and control groups. We also observed that PU.1 translocated into the nucleus in stimulated THP-1 cells. The results of the luciferase reporter assay showed that PU.1 promoted human Dectin-1 (hDectin-1) gene activity. ChIP and EMSA indicated that PU.1 could bind with hDectin-1 gene promoter at three potential transcription factor-binding sites (TFBSs). In addition, knockdown of PU.1 significantly decreased Dectin-1 expression.

Conclusions

This study demonstrated the novel role of PU.1 in the immune response to A. fumigatus through upregulation of Dectin-1 expression and its translocation to the nucleus in A. fumigatus-stimulated THP-1 cells.

Cell-Type Specific Determinants of NRAMP1 Expression in Professional Phagocytes

The Natural resistance-associated macrophage protein 1 (Nramp1 or Solute carrier 11 member 1, Slc11a1) transports divalent metals across the membrane of late endosomes and lysosomes in professional phagocytes. Nramp1 represents an ancient eukaryotic cell-autonomous defense whereas the gene duplication that yielded Nramp1 and Nramp2 predated the origin of Sarcopterygians (lobe-finned fishes and tetrapods). SLC11A1 genetic polymorphisms associated with human resistance to tuberculosis consist of potential regulatory variants. Herein, current knowledge of the regulation of SLC11A1 gene expression is reviewed and comprehensive analysis of ENCODE data available for hematopoietic cell-types suggests a hypothesis for the regulation of SLC11A1 expression during myeloid development and phagocyte functional polarization. SLC11A1 is part of a 34.6 kb CTCF-insulated locus scattered with predicted regulatory elements: a 3' enhancer, a large 5' enhancer domain and four elements spread around the transcription start site (TSS), including several C/EBP and PU.1 sites. SLC11A1 locus ends appear mobilized by ETS-related factors early during myelopoiesis; activation of both 5' and 3' enhancers in myelo-monocytic cells correlate with transcription factor binding at the TSS. Characterizing the corresponding cis/trans determinants functionally will establish the mechanisms involved and possibly reveal genetic variation that impacts susceptibility to infectious or immune diseases.

Lipocalin 2: novel component of proinflammatory signaling in Alzheimer's disease

Alzheimer's disease (AD) is associated with an altered immune response, resulting in chronic increased inflammatory cytokine production with a prominent role of TNF-α. TNF-α signals are mediated by two receptors: TNF receptor 1 (TNFR1) and TNF receptor 2 (TNFR2). Signaling through TNFR2 is associated with neuroprotection, whereas signaling through TNFR1 is generally proinflammatory and proapoptotic. Here, we have identified a TNF-α-induced proinflammatory agent, lipocalin 2 (Lcn2) via gene array in murine primary cortical neurons. Further investigation showed that Lcn2 protein production and secretion were activated solely upon TNFR1 stimulation when primary murine neurons, astrocytes, and microglia were treated with TNFR1 and TNFR2 agonistic antibodies. Lcn2 was found to be significantly decreased in CSF of human patients with mild cognitive impairment and AD and increased in brain regions associated with AD pathology in human postmortem brain tissue. Mechanistic studies in cultures of primary cortical neurons showed that Lcn2 sensitizes nerve cells to β-amyloid toxicity. Moreover, Lcn2 silences a TNFR2-mediated protective neuronal signaling cascade in neurons, pivotal for TNF-α-mediated neuroprotection. The present study introduces Lcn2 as a molecular actor in neuroinflammation in early clinical stages of AD.

Macrophage receptors for influenza A virus: role of the macrophage galactose-type lectin and mannose receptor in viral entry

Although sialic acid has long been recognized as the primary receptor determinant for attachment of influenza virus to host cells, the specific receptor molecules that mediate viral entry are not known for any cell type. For the infection of murine macrophages by influenza virus, our earlier study indicated involvement of a C-type lectin, the macrophage mannose receptor (MMR), in this process. Here, we have used direct binding techniques to confirm and characterize the interaction of influenza virus with the MMR and to seek additional macrophage surface molecules that may have potential as receptors for viral entry. We identified the macrophage galactose-type lectin (MGL) as a second macrophage membrane C-type lectin that binds influenza virus and is known to be endocytic. Binding of influenza virus to MMR and MGL occurred independently of sialic acid through Ca(2+)-dependent recognition of viral glycans by the carbohydrate recognition domains of the two lectins; influenza virus also bound to the sialic acid on the MMR. Multivalent ligands of the MMR and MGL inhibited influenza virus infection of macrophages in a manner that correlated with expression of these receptors on different macrophage populations. Influenza virus strain A/PR/8/34, which is poorly glycosylated and infects macrophages poorly, was not recognized by the C-type lectin activity of either the MMR or the MGL. We conclude that lectin-mediated interactions of influenza virus with the MMR or the MGL are required for the endocytic uptake of the virus into macrophages, and these lectins can thus be considered secondary or coreceptors with sialic acid for infection of this cell type.

Lipopolysaccharide-dependent interaction between PU.1 and c-Jun determines production of lipocalin-type prostaglandin D synthase and prostaglandin D2 in macrophages

Previously, we reported that expression of lipocalin-prostaglandin D synthase (L-PGDS) is inducible in macrophages and protects from Pseudomonas pneumonia. Here, we investigated the mechanism by which L-PGDS gene expression is induced in macrophages. A promoter analysis of the murine L-PGDS promoter located a binding site of PU.1, a transcription factor essential for macrophage development and inflammatory gene expression. A chromatin immunoprecipitation assay showed that PU.1 bound to the cognate site in the endogenous L-PGDS promoter in response to LPS. Overexpression of PU.1, but not of PU.1(S148A), a mutant inert to casein kinase II (CKII) or NF-kappaB-inducing kinase (NIK), induced L-PGDS in RAW 264.7 cells. Conversely, siRNA silencing of PU.1 expression blunted productions of L-PGDS and prostaglandin D2 (PGD(2)). LPS treatment induced formation of the complex of PU.1 and cJun on the PU.1 site, but inactivation of cJun by treatment with JNK or p38 kinase inhibitor abolished the complex, and suppressed PU.1 transcriptional activity for L-PGDS gene expression. Together, these results show that PU.1, activated by CKII or NIK, cooperates with MAPK-activated cJun to maximally induce L-PGDS expression in macrophages following LPS treatment, and suggest that PU.1 participates in innate immunity through the production of L-PGDS and PGD(2).

SB203580 enhances interleukin-1 receptor antagonist gene expression in IFN-γ-stimulated BV2 microglial cells through a composite nuclear factor-κB/PU.1 binding site

Interleukin-1 receptor antagonist (IL-1ra) is a naturally occurring antagonist of IL-1alpha and IL-1beta binding to the IL-1 receptors and alleviates various inflammatory reactions. Therefore, the upregulation of IL-1ra expression is important for preventing and/or treating inflammatory diseases including many neurodegenerative diseases. This study found that SB203580, which is generally known as a p38 MAP kinase inhibitor and an anti-inflammatory agent, increased the level of IL-1ra expression in IFN-gamma-stimulated BV2 microglial cells. This effect is believed to occur through the inhibition of protein kinase B (PKB), independently of the p38 MAP kinase pathways. Further mechanistic studies using an IL-1ra promoter revealed that a composite NF-kappaB/PU.1 binding site plays an important role in this SB203580-mediated upregulation of IL-1ra. Considering that IFN-gamma is a major stimulator of the innate and adaptive immune responses, the upregulation of anti-inflammatory IL-1ra expression by SB203580 in the IFN-gamma-stimulated microglia might provide a new therapeutic modality for various inflammatory diseases of the central nervous system.

HIV-1 Tat interaction with cyclin T1 represses mannose receptor and the bone morphogenetic protein receptor-2 transcription

Macrophage transcription is significantly altered by HIV-1 infection. HIV Tat, an immediate-early product of the viral lifecycle, interacts with host transcription factors to alter host gene expression. We have previously shown that Tat represses transcription from the mannose receptor (MR) and the bone morphogenetic protein receptor-2 (BMPR2) promoters. The current study shows that transcriptional repression of these receptors involves Tat interaction with cyclin T1. Assays using U937 human monocytic cells transiently expressing MR or BMPR2 promoter-luciferase constructs demonstrated equal repression by one- and two-exon Tat gene products. A mutant Tat expression vector encoding Tat protein lacking the cyclin T1 binding domain failed to inhibit MR and BMPR2 promoter activities. Over-expression of cyclin T1 in the presence of wild-type Tat resulted in recovered activity from both promoters. Finally, two inhibitors of cyclin-dependent kinase 9 (a dominant negative CDK9 and flavopiridol) repressed activity from the MR and BMPR2 promoters.

PU.1 regulates the tissue-specific expression of dendritic cell-specific intercellular adhesion molecule (ICAM)-3-grabbing nonintegrin

Dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) is a cell surface C-type lectin expressed on myeloid dendritic cells and certain tissue macrophages, which mediates antigen capture for processing and presentation and participates in intercellular interactions with naive T lymphocytes or endothelial cells. In their strategy to evade immunosurveillance, numerous pathogenic microorganisms, including human immunodeficiency virus and Mycobacterium, bind to DC-SIGN in order to gain access to dendritic cells. We present evidence that PU.1 dictates the basal and cell-specific activity of DC-SIGN gene-regulatory region through in vivo occupancy of two functional Ets elements, whose integrity is required for PU.1 responsiveness and for the cooperative actions of PU.1 and other transcription factors (Myb, RUNX) on the DC-SIGN gene proximal regulatory region. In addition, protein analysis and gene profiling experiments indicate that DC-SIGN and PU.1 are coordinately expressed upon classical and alternative macrophage activation and during dendritic cell maturation. Moreover, small interfering RNA-mediated reduction of PU.1 expression results in diminished DC-SIGN cellular levels. Altogether, these results indicate that PU.1 is involved in the myeloid-specific expression of DC-SIGN in myeloid cells, a contribution that can be framed within the role that PU.1 has on the acquisition of the antigen uptake molecular repertoire by dendritic cells and macrophages.

Correlation between differentiation plasticity and mRNA expression profiling of CD34+-derived CD14− and CD14+ human normal myeloid precursors

In spite of their apparently restricted differentiation potentiality, hematopoietic precursors are plastic cells able to trans-differentiate from a maturation lineage to another. To better characterize this differentiation plasticity, we purified CD14- and CD14+ myeloid precursors generated by 'in vitro' culture of human CD34+ hematopoietic progenitors. Morphological analysis of the investigated cell populations indicated that, as expected, they consisted of granulocyte and monocyte precursors, respectively. Treatment with differentiation inducers revealed that CD14- cells were bipotent granulo-monocyte precursors, while CD14+ cells appeared univocally committed to a terminal macrophage maturation. Flow cytometry analysis demonstrated that the conversion of granulocyte precursors to the mono-macrophage maturation lineage occurs through a differentiation transition in which the granulocyte-related myeloperoxidase enzyme and the monocyte-specific CD14 antigen are co-expressed. Expression profiling evidenced that the observed trans-differentiation process was accompanied by a remarkable upregulation of the monocyte-related MafB transcription factor.

Enhanced expression of the mannose receptor by endothelial cells of the liver and spleen microvascular beds in the macrophage-deficient PU.1 null mouse

Mice null for the haematopoietic lineage-specific transcription factor PU.1 lack mature Mphi and are compromised in their ability to clear cellular debris from the blood circulation. We investigated the possibility that non-professional phagocytes may partially compensate for the lack of Mphi in clearance functions. In the absence of Kupffer cells (resident liver Mphi) in the PU.1 null mice, electron microscopy revealed ingested debris in sinusoidal endothelial cells and hepatocytes although debris was also seen free in blood vessels. To investigate whether an increased clearance function of non-professional phagocytes might be linked to expression of Mphi-associated phagocytic and pinocytic receptors by other cells in PU.1 null mouse, we examined expression of several candidate proteins by immunocytochemistry and Western blotting. We found mannose receptor (MR) comparably expressed in PU.1 null and PU.1+ mice liver and spleen whereas class A scavenger receptor was substantially reduced and complement receptor 3 was absent in PU.1 null animals. By morphometric analysis, liver and spleen sinusoidal endothelial cells were seen to express significantly more MR in the PU.1 null mouse. This study provides the first evidence of apparently compensatory alterations in the microvasculature of the Mphi-deficient PU.1 null mouse.

Pea3 Transcription Factor Cooperates with USF-1 in Regulation of the Murine bax Transcription without Binding to an Ets-binding Site

The Pea3 transcription factor (which belongs to the PEA3 group) from the Ets family has been shown to be involved in mammary embryogenesis and oncogenesis. However, except for proteinases, only few of its target genes have been reported. In the present report, we identified bax as a Pea3 up-regulated gene. We provide evidence of this regulation by using Pea3 overexpression and Pea3 silencing in a mammary cell line. Both Pea3 and Erm, another member of the PEA3 group, are able to transactivate bax promoter fragments. Although the minimal Pea3-regulated bax promoter does not contain an Ets-binding site, two functional upstream stimulatory factor-regulated E boxes are present. We further demonstrate the ability of Pea3 and USF-1 to cooperate for the transactivation of the bax promoter, mutation of the E boxes dramatically reducing the Pea3 transactivation potential. Although Pea3 did not directly bind to the minimal bax promoter, we provide evidence that USF-1 could form a ternary complex with Pea3 and DNA. Taken together, our results suggest that Pea3 may regulate bax transcription via the interaction with USF-1 but without binding to DNA.

IL-4 modulates transcriptional control of the mannose receptor in mouse FSDC dendritic cells

The mannose receptor is a 175 kDa protein found on the surface of macrophages and dendritic cells whose functions include clearance of extracellular hydrolases, internalization of pathogens, and antigen capture. Receptor expression is closely linked to the functional state of these cells and is regulated by cytokines. Previous work has shown that treatment of macrophages and dendritic cells with interleukin-4 leads to increased mannose receptor expression. We have examined the mechanism of this IL-4-mediated up-regulation in the murine dendritic cell line FSDC. IL-4 increased mannose receptor activity, protein, and mRNA. The mannose receptor promoter was functional in FSDCs using transient transfection assays, and IL-4 treatment increased promoter activity 2.6-fold. The responsive region was localized to the proximal 228 bp. Electrophoretic mobility shift assays detected an IL-4-inducible protein that bound to the mannose receptor promoter at a site spanning the region between -147 and -108 bp. The sequence TTAC(N)4CACC (-135 and -124 bp) is similar to the IL-4 response region in the Fc receptor II. Mutation of the flanking TT and CC in this motif blocked IL-4 responsiveness and binding of the IL-4-induced mannose receptor binding protein. This protein does not appear to be STAT6 since neither an anti-STAT6 antibody nor a STAT6 consensus oligonucleotide altered factor binding.

Targetable water-soluble polymer-drug conjugates for the treatment of visceral leishmaniasis

The present work describes the synthesis, characterization, and biological evaluation of targetable N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-anti-leishmanial drug conjugates for the treatment of visceral leishmaniasis (VL). Conjugates of HPMA copolymer with NPC1161, an 8-aminoquinoline analog with anti-leishmanial activity, containing N-acetylmannosamine (ManN) in the side chains were synthesized and characterized. In vitro anti-leishmanial efficacy of the conjugates was determined in mouse peritoneal macrophages infected with Leishmania donovani amastigotes. The conjugates were tested against mammalian KB cells for cytotoxicity. The effect of ManN content on uptake was evaluated in RAW 264 murine macrophages. In vivo anti-leishmanial efficacy was evaluated at 1 mg/kg intravenous dose in BALB/c mice. HPMA copolymer-NPC1161 conjugates with 5 mole% or higher ManN content were significantly (p<0.0001) more active (ED50<15 microg/ml) than nontargeted conjugates (ED50>30 microg/ml). All conjugates were relatively nontoxic towards the mammalian cells. Significantly (p<0.003) higher uptake was observed for targeted conjugates compared to nontargeted conjugates. The targeted conjugates were significantly more effective in vivo (67-80% inhibition, p<0.0001) than nontargeted conjugate (47% inhibition). HPMA copolymers containing ManN in the side chains can potentially reduce the toxicity and increase efficacy of anti-leishmanial drugs for the treatment of VL.

Pulmonary surfactant protein A up-regulates activity of the mannose receptor, a pattern recognition receptor expressed on human macrophages

Inhaled particulates and microbes are continually cleared by a complex array of lung innate immune determinants, including alveolar macrophages (AMs). AMs are unique cells with an enhanced capacity for phagocytosis that is due, in part, to increased activity of the macrophage mannose receptor (MR), a pattern recognition receptor for various microorganisms. The local factors that "shape" AM function are not well understood. Surfactant protein A (SP-A), a major component of lung surfactant, participates in the innate immune response and can enhance phagocytosis. Here we show that SP-A selectively enhances MR expression on human monocyte-derived macrophages, a process involving both the attached sugars and collagen-like domain of SP-A. The newly expressed MR is functional. Monocyte-derived macrophages on an SP-A substrate demonstrated enhanced pinocytosis of mannose BSA and phagocytosis of Mycobacterium tuberculosis lipoarabinomannan-coated microspheres. The newly expressed MR likely came from intracellular pools because: 1) up-regulation of the MR by SP-A occurred by 1 h, 2) new protein synthesis was not necessary for MR up-regulation, and 3) pinocytosis of mannose BSA via MR recycling was increased. AMs from SP-A(-/-) mice have reduced MR expression relative to SP-A(+/+). SP-A up-regulation of MR activity provides a mechanism for enhanced phagocytosis of microbes by AMs, thereby enhancing lung host defense against extracellular pathogens or, paradoxically, enhancing the potential for intracellular pathogens to enter their intracellular niche. SP-A contributes to the alternative activation state of the AM in the lung.

Toll-like receptor 2 and 4 (TLR2 and TLR4) agonists differentially regulate secretory interleukin-1 receptor antagonist gene expression in macrophages

Treatment of macrophages with lipopolysaccharide (LPS) from Gram-negative bacteria or peptidoglycan (PGN) from Gram-positive bacteria activates multiple intracellular signaling pathways and a large, diverse group of nuclear transcription factors. The signaling receptors for PGN and LPS are now known to be the Toll-like receptors 2 and 4 (TLR2 and -4, respectively). While a large body of literature indicates that the members of the TLR family activate nearly identical cytoplasmic signaling programs, several recent reports have suggested that the functional outcomes of signaling via TLR2 or TLR4 are not equivalent. In the current studies, we compared the responses of the secretory IL-1 receptor antagonist (sIL-1Ra) gene to both LPS and PGN. Both LPS and PGN induced IL-1Ra gene expression; however, the combination of both stimuli synergistically increased sIL-1Ra mRNA expression and promoter activity, suggesting that the signals induced by PGN and LPS are not equivalent. While both LPS and PGN utilized the PU.1-binding sites in the proximal sIL-1Ra promoter region to generate a full response, additional distinct promoter elements were utilized by LPS or PGN. Activation of p38 stress-activated protein kinase was required for LPS- or PGN-induced IL-1Ra gene expression, but the p38-responsive promoter elements localized to distinct regions of the sIL-1Ra gene. Additionally, while the LPS-induced, p38-dependent response was dependent upon PU.1 binding, the PGN-induced, p38 response was not. Collectively, these data indicated that while some of the intracellular signaling events by TLR2 and TLR4 agonists are similar, there are clearly distinct differences in the responses elicited by these two bacterial products.

The role of IRF-4 in transcriptional regulation

Gene expression is a tightly regulated process involving multiple levels of control spanning histone acetylation to protein turnover. One of the first events in this cascade is transcription, which itself is a multistep process involving protein-protein interaction and macromolecular assembly. Here we review the role of the interferon (IFN) regulatory factor (IRF) transcription factor family member IRF-4 in transcriptional regulation. IRF-4 was initially characterized in lymphocytes and was shown to function as both a transcriptional repressor and activator. More recently, IRF-4 expression and function have been reported in macrophages. The ability of IRF-4 to serve as both a transcriptional activator and repressor is determined, in part, by binding to distinct DNA-binding motifs and through interaction with various additional transcription factors, most notably with the Ets family member PU.1. The details governing these functional differences are the focus of this review. Importantly, the role of posttranslational modification and nuclear translocation of IRF-4 in transcriptional regulation are addressed. Several possible paradigms of transcriptional regulation by IRF-4 are proposed, where these paradigms may describe regulatory mechanisms common to many distinct transcription factor families.

PU.1 and Multiple IFN Regulatory Factor Proteins Synergize to Mediate Transcriptional Activation of the Human IL-1β Gene

Both lymphoid and myeloid cells express two related members of the IFN regulatory factor (IRF) family of transcription factors, specifically IRF-4 and IFN consensus binding protein (ICSBP or IRF-8). We previously reported that macrophages express IRF-4 and in combination with the ETS-like protein PU.1 can synergistically activate a human IL-1beta reporter gene. Here we report that this synergy is mediated by a composite PU.1/IRF element located within an upstream enhancer known to confer cytokine- and LPS-inducible expression. In macrophages, synergistic activation of IL-1beta reporter gene expression was preferentially mediated by IRF-4, whereas IRF-4 and ICSBP were equally capable of synergizing with PU.1 when coexpressed in fibroblasts. Furthermore, coexpression of IRF-1 and IRF-2 dramatically increased the capacity of both PU.1/IRF-4 and PU.1/ICSBP to induce IL-1beta reporter gene expression in fibroblasts. The additional synergy observed with IRF-1 and IRF-2 coexpression is mediated by a region of DNA distinct from either the IL-1beta enhancer or promoter. We also assessed the capacity of these transcription factors to activate endogenous IL-1beta gene when overexpressed in human embryonic kidney 293 cells. Although ectopic expression of PU.1 alone was sufficient to activate modest levels of IL-1beta transcripts, endogenous IL-1beta expression was markedly increased following coexpression of additional IRF proteins. Thus, maximal expression of both a human IL-1beta reporter gene and the endogenous IL-1beta gene was observed in cells that coexpressed PU.1, IRF-4 (or ICSBP), IRF1, and IRF2. Together, our observations suggest that these factors may function together as an enhanceosome.

Ets target genes: past, present and future

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

HIV-1 Tat represses transcription from the mannose receptor promoter

The mannose receptor is expressed on mature macrophages and immature dendritic cells, and functions to mediate phagocytosis of pathogens and capture of Ags for delivery to MHC class II-containing intracellular compartments. It has been previously reported that HIV-1-infected macrophages have reduced functions associated with the mannose receptor, including impaired Pneumocystis carinii phagocytosis and mannosylated albumin uptake. Several HIV-1-derived proteins including the Tat protein have been shown to transcriptionally repress host cell genes. The present study was undertaken to define the role of the HIV-1-derived protein Tat in HIV-mediated mannose receptor down-regulation. Cotransfection of the human macrophage cell line U937 with a Tat expression vector and a mannose receptor promoter-luciferase reporter construct resulted in down-regulation of mannose receptor promoter activity. This repression was targeted to the basal promoter. Expression of either one- or two-exon Tat resulted in decreased promoter activity. The addition of the transactivation response element (TAR) sequence enhanced the Tat-mediated repression. Down-regulation was also seen when transfected cells were treated with exogenously added Tat protein. These results are consistent with a mechanism whereby Tat reduces mannose receptor promoter activity by interfering with the host transcriptional initiation machinery, potentially resulting in decreased levels of surface mannose receptor available for Ag or pathogen capture.

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.