Co-occupancy of the interferon regulatory element of the class II transactivator (CIITA) type IV promoter by interferon regulatory factors 1 and 2

Interferon Regulatory Factors 1 and 2 Play Different Roles in MHC II Expression Mediated by CIITA in Grass Carp, Ctenopharyngodon idella

Expression of major histocompatibility complex class II (MHC II) molecules, which determines both the immune repertoire during development and subsequent triggering of immune responses, is always under the control of a unique (MHC class II) transactivator, CIITA. The IFN-γ-inducible MHC II expression has been extensively and thoroughly studied in humans, but not in bony fish. In this study, the characterization of CIITA was identified and its functional domains were analyzed in grass carp. The absence of GAS and E-box in the promoter region of grass carp CIITA, might imply that the cooperative interaction between STAT1 and USF1 to active the CIITA expression, found in mammals, is not present in bony fish. After the transfection of IFN-γ or IFN-γ rel, only IFN-γ could induce MHC II expression mediated by CIITA. Moreover, interferon regulatory factor (IRF) 2, which cooperates with IRF1 to active the CIITA promoter IV expression in mammals, played an antagonistic role to IRF1 in the activation of grass carp CIITA. These data suggested that grass carp, compared with mammals, has both conservative and unique mechanisms in the regulation of MHC II expression.

MHC class II associated stomach cancer mutations correlate with lack of subsequent tumor development

The role of tumor cell expression of major histocompatibility class II (MHCII) has been controversial, with evidence indicating that tumor cell expression of MHCII may lead to an anti-tumor immune response and to tumor cell apoptosis and that MHCII has pro-tumorigenic functions. The cancer genome atlas (TCGA) indicates numerous deleterious mutations for the highly specific, MHCII transcriptional activation proteins, RFX5, RFXAP, RFXANK and CIITA. Also, mutations in the non-polymorphic, human leukocyte antigen (HLA)-DRA gene, which encodes the heavy chain for the most prominent human MHCII molecule, HLA-DR, are common. For many, if not most TCGA cancer datasets, the MHCII specific mutations do not associate with clinical outcomes. However, stomach carcinoma represents an exception, where the data indicate that MHCII-specific mutations are associated with a more favorable outcome. These data raise the question of whether stomach cancer mutations represent effective haploinsufficiency or whether mutations that are associated with a favorable outcome occur with other stomach cancer molecular features that limit the function of the two alleles that represent these MHCII-related proteins.

Upregulation of microRNA-450 inhibits the progression of lung cancer in vitro and in vivo by targeting interferon regulatory factor 2

MicroRNAs (miRNAs) are a class of non‑coding RNAs that play pivotal roles in human lung cancer development. The majority of studies have focused on either non-small cell lung cancer (NSCLC) or small cell lung cancer (SCLC). In the present study, we investigated a plausible mechanism of action of miR‑450 in these types of lung cancer. We found that the level of miR‑450 was decreased in lung cancer cell lines, as well as in solid tumors. As exemplified in the H510A (SCLC) and H2291 (NSCLC) cells, transfection with lentivirus carrying miR‑450 upregulated miR‑450 expression and significantly attenuated lung cancer cell proliferation and invasion, as well as the growth of implantated tumors. Interferon regulatory factor 2 (IRF2) was also verified to be a direct target of miR‑450 in lung cancer cells. The overexpression of IRF2 in the H510A and H2291 cells abrogated the inhibitory effects of miR‑450 on lung cancer cell proliferation and invasion. Taken together, in this study, we identified a novel role of miR‑450 in lung cancer. miR-450 targets IRF2 and thus supresses lung cancer cell proliferation and invasion.

Polycomb Repressive Complex 2 Confers BRG1 Dependency on the CIITA Locus

CIITA (or MHC2TA) coordinates constitutive and IFN-γ-induced expression of MHC class II genes. IFN-γ responsiveness of CIITA requires BRG1 (SMARCA4), the ATPase engine of the chromatin remodeling SWI/SNF complex (also called BAF). SWI/SNF is defective in many human cancers, providing a mechanism to explain IFN-γ resistance. BRG1 dependency is mediated through remote elements. Short CIITA reporters lacking these elements respond to IFN-γ, even in BRG1-deficient cells, suggesting that BRG1 counters a remote repressive influence. The nature of this distal repressor is unknown, but it would represent a valuable therapeutic target to reactivate IFN-γ responsiveness in cancer. In this article, we show that the polycomb repressive complex 2 (PRC2) components EZH2 and SUZ12, as well as the associated histone mark H3K27me3, are codetected at interenhancer regions across the CIITA locus. IFN-γ caused a BRG1-dependent reduction in H3K27me3, associated with nucleosome displacement. SUZ12 knockdown restored IFN-γ responsiveness in BRG1-null cells, and it mimicked the ability of BRG1 to induce active histone modifications (H3K27ac, H3K4me) at the -50-kb enhancer. Thus, PRC2 confers BRG1 dependency on the CIITA locus. Our data suggest that, in addition to its known roles in promoting stemness and proliferation, PRC2 may inhibit immune surveillance, and it could be targeted to reactivate CIITA expression in SWI/SNF deficient cancers.

IRF-2 is over-expressed in pancreatic cancer and promotes the growth of pancreatic cancer cells

Pancreatic cancer is one of the most malignant diseases in the world. Interferon regulator factor 2 (IRF-2), an interferon regulatory factor, has been known to act as an oncogene in distinct types of cancer. In this study, we found that the expression of IRF-2 was up-regulated in primary pancreatic cancer samples and associated with tumor size, differentiation, tumor-node-metastasis stage, and survival of the patients. In pancreatic cancer cells, knockdown on the expression of IRF-2 inhibited cell growth in the liquid culture and on the soft agar. Mechanistically, IRF-2 modulated the growth of pancreatic cancer cells through regulating proliferation and apoptosis effectors, such as cyclin D1 and BAX. Collectively, these results suggest that IRF-2 plays an important role in the tumorigenesis of pancreatic cancer and down-regulation of IRF-2 would be a new treatment target for pancreatic cancer.

Mouse interferon regulatory factor-2: expression, purification and DNA binding activity

Interferon regulatory factor-2 (IRF-2) is a mammalian transcription factor for Interferon and Interferon inducible genes; biologically, plays an important role in cell growth regulation and has been shown to be a potential oncogene. We have expressed, purified recombinant Murine IRF-2 (349 amino acid) as a GST (Glutathione-S-Transferase)-IRF-2 soluble fusion protein in E. coli XL-1 blue cells. Recombinant GST-IRF-2 was biologically active in terms of its DNA binding activity with IRF-E oligo (GAAAGT)4. GST-alone expressed from the vector did not bind to it. We observed five different molecular mass complexes of GST-IRF-2/DNA (1-5) with IRF-E, which were competed out by 100×-fold molar excess of IRF-E, suggesting that the complexes were specific for IRF-2. Such GAAANN (N=any nucleotide) hexa nucleotides occur in the promoters of many virus and interferon-inducible mammalian genes. Multimeric GAAAGT/C sequences are inducible by virus, IFN, dsRNA and IRF-1/2. Multiple GST-IRF-2/DNA complexes may be helpful to understand the mechanism of DNA binding activity of IRF-2.

Marek's disease virus influences the expression of genes associated with IFN-gamma-inducible MHC class II expression

Chickens infected with Marek's disease virus (MDV) become lifelong carriers regardless of their susceptibility to clinical disease. Therefore various viral immune-evasive mechanisms must play a role in MDV-host interactions. MDV has previously been shown to influence the expression of major histocompatibility complex (MHC) class II molecules. However, little is known about the underlying mechanisms of this phenomenon. In the present study, we studied the effect of MDV infection on the expression of several genes associated with IFN-gamma-inducible MHC class II expression at 4, 7, 14, and 21 days post-infection (dpi). There was a significant (p Collapse

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MESH Headings

• Animals
• Chickens/virology
• Gene Expression Profiling
• Genes, MHC Class II
• Immunologic Surveillance/genetics
• Interferon Regulatory Factor-1/genetics
• Interferon-gamma/immunology
• Mardivirus/immunology
• Marek Disease/genetics
• Marek Disease/immunology
• Nuclear Proteins/genetics
• Poultry Diseases/genetics
• Poultry Diseases/immunology
• Receptors, Interferon/genetics
• STAT1 Transcription Factor/metabolism
• Signal Transduction
• Time Factors
• Trans-Activators/genetics
• Interferon gamma Receptor

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Grants Collapse

Affiliation(s)

Niroshan Thanthrige-Don Department of Pathobiology, University of Guelph, Guelph, Ontario, Canada
Leah R Read
Mohamed Faizal Abdul-Careem
Hakimeh Mohammadi
Amirul I Mallick
Shayan Sharif

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Substantially reduced expression of PIAS1 is associated with colon cancer development

PURPOSE

Protein inhibitors of activated STATs (PIAS) regulate the interferon-gamma (IFN-gamma) signaling pathway, which has numerous effects on tumor development and tumor cell biology. PIAS's also regulate STAT family members not directly involved in IFN-gamma signaling. This project was designed to assess PIAS1 expression in colon cancer.

METHODS

To determine whether PIAS1, one of the PIAS family members, or IFN-gamma signaling pathway components could be used to stratify colon tumors, we stained tissue microarrays for PIAS1, interferon regulatory factor-1 (IRF-1) and STAT1alpha.

RESULTS

PIAS1 staining of the colon cancer tissue microarrays indicated a strong correlation of normal colon cells, and adenomas, with high expression of both PIAS1 and IRF-1.

CONCLUSION

The PIAS1 results in particular may represent a basis for new approaches for efficiently distinguishing adenomas from colon cancer.

MHC class II transactivator (CIITA) expression is upregulated in multiple myeloma cells by IFN-gamma

The MHC class II transactivator (CIITA) acts in the cell nucleus as the master regulator of MHC class II (MHC II) gene expression. It is important to study CIITA regulation in multiple myeloma since MHC expression is central to ability of myeloma cells to present antigen and to the ability of the immune system to recognize and destroy this malignancy. Regulation of CIITA by IFN-gamma in B lymphocytes occurs through the CIITA type IV promoter (pIV), one of the four potential promoters (pI-pIV) of this gene. To investigate regulation of CIITA by IFN-gamma in multiple myeloma cells, first the ability of these cells to respond to IFN-gamma was examined. RT-PCR analyses show that IFN-gammaR1, the IFN-gamma-binding chain of the IFN-gamma receptor, is expressed in myeloma cells and IRF-1 expression increases in response to IFN-gamma treatment. Western blotting demonstrates that STAT1 is activated by phosphorylation in response to IFN-gamma. RT-PCR and functional promoter analyses show that IFN-gamma upregulates the activity of CIITA pIV, as does ectopic expression of IRF-1 or IRF-2. In vivo protein/DNA binding studies demonstrate protein binding at the GAS, E box and IRF-E sites. In vitro studies confirm the binding of IRF-1 and IRF-2 to CIITA pIV. Although multiple myeloma cells express PRDI-BF1/Blimp-1, a factor that represses both the CIITA type III and IV promoters, they retain the capability to upregulate CIITA pIV and MHC II expression in response to IFN-gamma treatment. These findings are the first to demonstrate that although PRDI-BF1/Blimp-1 diminishes the constitutive ability of these cells to present antigen by limiting CIITA and MHC II expression, it is possible to enhance this expression through the use of cytokines, like IFN-gamma.

Gene structure and transcription of IRF-1 and IRF-7 in the mandarin fish Siniperca chuatsi

The genes of IRF-1 and IRF-7 have been cloned from the mandarin fish (Siniperca chuatsi). The IRF-1 gene has 4919 nucleotides (nt) and contains 10 exons and 9 introns, with an open reading frame (ORF) of 903nt encoding 301aa. The IRF-7 gene has 6057nt and also contains 10 exons and 9 introns, with an ORF of 1308nt encoding 436aa. The IRF-1 and IRF-7 genes have only one copy each in the genome. The transcription of IRF-1 and IRF-7 in different organs was analyzed by real-time PCR, and both molecules were constitutively expressed. The IRF-1 and IRF-7 mRNAs were abundant in gill, spleen, kidney and pronephros. The temporal transcriptional changes for IRF-1, IRF-7 and Mx were investigated within 48h after poly I: C stimulation in liver, gill, spleen and pronephros. An increased transcription was detected for IRF-1 and IRF-7 12h post-stimulation, being earlier than the transcription of Mx protein; however, IRF-1 and IRF-7 transcription decreased while the Mx protein was stable at 48h post-stimulation.

Repression of IFN-γ Induction of Class II Transactivator: A Role for PRDM1/Blimp-1 in Regulation of Cytokine Signaling

MHC class II is expressed in restricted lineages and is modulated in response to pathogens and inflammatory stimuli. This expression is controlled by MHC CIITA, which is transcribed from multiple promoters. Although factors required for induction of CIITA are well characterized, less is known about the mechanisms leading to repression of this gene. During plasma cell differentiation, B lymphocyte-induced maturation protein-1 (PRDM1/Blimp-1) represses promoter (p)III of CIITA, responsible for constitutive expression in B cells. pIV is inducible by IFN-gamma in epithelia, macrophages and B cells. An IFN regulatory factor-element (IRF-E) in CIITA-pIV, which is bound by IRF-1 and IRF-2, is necessary for this response. This site matches the PRDM1/Blimp-1 consensus binding site, and PRDM1/Blimp-1 is expressed in cell lineages in which this promoter is operative. We, therefore, investigated whether PRDM1 regulates CIITA-pIV and found that PRDM1 bound to CIITA-pIV in vivo and the IRF-E in vitro. PRDM1 repressed IFN-gamma-mediated induction of a CIITA-pIV luciferase reporter in a fashion dependent on an intact consensus sequence and competes with IRF-1/IRF-2 for binding to the IRF-E and promoter activation. In human myeloma cell lines that express IRFs, PRDM1 occupancy of CIITA-pIV was associated with resistance to IFN-gamma stimulation, while short interfering RNA knockdown of PRDM1 led to up-regulation of CIITA. Our data indicate that PRDM1 is a repressor of CIITA-pIV, identifying a target of particular relevance to macrophages and epithelia. These findings support a model in which PRDM1/Blimp-1 can modulate the cellular response to IFN-gamma by competing with IRF-1/IRF-2 dependent activation of target promoters.

Positive regulatory domain I-binding factor 1 mediates repression of the MHC class II transactivator (CIITA) type IV promoter

MHC class II transactivator (CIITA), a co-activator that controls MHC class II (MHC II) transcription, functions as the master regulator of MHC II expression. Persistent activity of the CIITA type III promoter (pIII), one of the four potential promoters of this gene, is responsible for constitutive expression of MHC II by B lymphocytes. In addition, IFN-gamma induces expression of CIITA in these cells through the type IV promoter (pIV). Positive regulatory domain 1-binding factor 1 (PRDI-BF1), called B lymphocyte-induced maturation protein 1 (Blimp-1) in mice, represses the expression of CIITA pIII in plasma and multiple myeloma cells. To investigate regulation of CIITA pIV expression by PRDI-BF1 in the B lymphocyte lineage, protein/DNA-binding studies, and functional promoter analyses were performed. PRDI-BF1 bound to the IFN regulatory factor-element (IRF-E) site in CIITA pIV. Ectopic expression of either PRDI-BF1 or Blimp-1 repressed this promoter in B lymphocytes. In vitro binding and functional analyses of CIITA pIV demonstrated that the IRF-E is the target of this repression. In vivo genomic footprint analysis demonstrated protein binding at the IRF-E site of CIITA pIV in U266 myeloma cells, which express PRDI-BF1. PRDI-BF1beta, a truncated form of PRDI-BF1 that is co-expressed in myeloma cells, also bound to the IRF-E site and repressed CIITA pIV. These findings demonstrate for the first time that, in addition to silencing expression of CIITA pIII in B lymphocytes, PRDI-BF1 is capable of binding and suppressing CIITA pIV.

Regulation of MHC class II gene expression by the class II transactivator

MHC class II molecules are pivotal for the adaptive immune system, because they guide the development and activation of CD4+ T helper cells. Fulfilling these functions requires that the genes encoding MHC class II molecules are transcribed according to a strict cell-type-specific and quantitatively modulated pattern. This complex gene-expression profile is controlled almost exclusively by a single master regulatory factor, which is known as the class II transactivator. As we discuss here, differential activation of the three independent promoters that drive expression of the gene encoding the class II transactivator ultimately determines the exquisitely regulated pattern of MHC class II gene expression.

Expression of the MHC class II transactivator (CIITA) type IV promoter in B lymphocytes and regulation by IFN-gamma

The MHC class II transactivator (CIITA), the master regulator of MHC class II (MHC II) expression, is a co-activator that controls MHC II transcription. Human B lymphocytes express MHC II constitutively due to persistent activity of CIITA promoter III (pIII), one of the four potential promoters (pI-pIV) of this gene. Although increases in MHC II expression in B cells in response to cytokines have been observed and induction of MHC II and CIITA by IFN-gamma has been studied in a number of different cell types, the specific effects of IFN-gamma on CIITA expression in B cells have not been studied. To investigate the regulation of CIITA expression by IFN-gamma in B cells, RT-PCR, in vivo and in vitro protein/DNA binding studies, and functional promoter analyses were performed. Both MHC II and CIITA type IV-specific RNAs increased in human B lymphocytes in response to IFN-gamma treatment. CIITA promoter analysis confirmed that pIV is IFN-gamma inducible in B cells and that the GAS and IRF-E sites are necessary for full induction. DNA binding of IRF-1 and IRF-2, members of the IFN regulatory factor family, was up-regulated in B cells in response to IFN-gamma and increased the activity of CIITA pIV. In vivo genomic footprint analysis demonstrated proteins binding at the GAS, IRF-E and E box sites of CIITA pIV. Although CIITA pIII is considered to be the hematopoietic-specific promoter of CIITA, these findings demonstrate that pIV is active in B lymphocytes and potentially contributes to the expression of CIITA and MHC II in these cells.

IFN regulatory factor-2 cooperates with STAT1 to regulate transporter associated with antigen processing-1 promoter activity

Class I MHC complexes (MHC(I)) are essential in mediating immune response. The transport of antigenic peptides (TAP) to MHC(I) and the stable expression of MHC(I) on the cell surface require the presence of a dedicated TAP. In this study we report that IFN-gamma and thrombopoietin (TPO) strongly increase TAP1 protein expression in megakaryocytes, followed by an enhanced expression of MHC(I) on the cell surface. This expression parallels the enhanced TAP1 promoter activity and TAP1 mRNA expression, which are independent of protein synthesis. We also show that this cytokine-dependent expression of TAP1 transcripts depends on STAT1 and IFN regulatory factor-2 (IRF-2), but not on IRF-1, and provide evidence that IRF-2 constitutively binds to the TAP1 gene promoter and enhances TAP1 promoter activity. We show that IRF-2 forms a complex with STAT1 and the cytokine-responsive region of the TAP1 promoter in any TPO or IFN-gamma target cells tested. Interaction of IRF-2 and STAT1 on the promoter depends on the DNA-binding domain of IRF-2. Overall, our data indicate that TPO and IFN-gamma activate the expression of TAP1 via a new mechanism that involves functional cooperation between STAT1 and IRF-2 on the TAP1 promoter.

Oct-1 DNA binding activity unresponsive to retinoblastoma protein expression prevents MHC class II induction in a non-small cell lung carcinoma cell line

Numerous human tumor lines fail to induce major histocompatibility (MHC) class II expression following interferon-gamma (IFN-gamma) treatment, a response that is considered to be a normal function for almost all human parenchymal and connective tissue cell-types. The effect of MHC class II non-inducibility on solid tumor growth is controversial, but an extensive body of literature indicates that tumor cell MHC class II expression can lead to an antitumor response or tumor tolerance, depending on a number of variables. Thus, understanding the molecular basis for MHC class II induction failures in solid tumor cells will likely lead to ideas for manipulating the antitumor immune response. To date, a handful of tumor associated molecular anomalies have accounted for all the known failures of MHC class II inducibility. In particular, lack of the retinoblastoma tumor suppressor protein (Rb) has been shown in both human and mouse cells to be strongly associated with failure to induce MHC class II. The basis for this relationship is traceable to, among other things, high level Oct-1 DNA binding activity in Rb-defective cells, which represses the prototypical human MHC class II gene, HLA-DRA. Ordinarily, re-establishment of Rb expression leads to elimination of, or substantially reduced Oct-1 DNA binding activity and to rescue of HLA-DRA inducibility. However, in the case of one non-small cell lung carcinoma line (NSCLC), Rb re-expression failed to rescue HLA-DRA inducibility despite successful re-establishment of Rb-function. We now report that this failure is traceable to the failure of Rb to rescue normal Oct-1 function. Furthermore, histone deacetylase inhibitor treatment allows a bypass of the Rb requirement and facilitates the MHC class II induction in this NSCLC line.

Regulation of the human leukocyte-derived arginine aminopeptidase endoplasmic reticulum-aminopeptidase 2 gene by interferon-?

The leukocyte-derived arginine aminopeptidase (L-RAP) is the second aminopeptidase localized in the endoplasmic reticulum (ER) processing antigenic peptides presented to major histocompatibility complex (MHC) class I molecules. In this study, the genomic organization of the gene encoding human L-RAP was determined and the regulatory mechanism of its expression was elucidated. The entire genomic structure of the L-RAP gene is similar to both placental leucine aminopeptidase (P-LAP) and adipocyte-derived leucine aminopeptidase (A-LAP) genes, confirming the close relationship of these three enzymes. Interferon (IFN)-gamma up-regulates the expression of the L-RAP gene. Deletion and site-directed mutagenic analyses of the 5'-flanking region of the L-RAP gene and electrophoretic mobility shift assay indicated that while interferon regulatory factor (IRF)-2 is important in the basal condition, IRF-1 is the primary regulator of IFN-gamma-mediated augmentation of the gene expression. In addition, PU.1, a member of the E26 transformation-specific family of transcription factors, also plays a role in the regulation of gene expression. The maximum expression of the gene was achieved by coexpression of IRF-1 and PU.1 in HEK293 cells and IRF-2 suppressed the IRF-1-mediated enhancement of gene expression, suggesting that IFN-gamma-induced L-RAP gene expression is cooperatively regulated by IRFs and PU.1 transcription factors.

Regulation of major histocompatibility complex class II gene expression in trophoblast cells

Trophoblast cells are unique because they are one of the few mammalian cell types that do not express major histocompatibility complex (MHC) class II antigens, either constitutively or after exposure to IFN-gamma. The absence of MHC class II antigen expression on trophoblast cells has been postulated to be one of the essential mechanisms by which the semi-allogeneic fetus evades immune rejection reactions by the maternal immune system. Consistent with this hypothesis, trophoblast cells from the placentas of women suffering from chronic inflammation of unknown etiology and spontaneous recurrent miscarriages have been reported to aberrantly express MHC class II antigens. The lack of MHC class II antigen expression on trophoblast cells is due to silencing of expression of the class II transactivator (CIITA), a transacting factor that is essential for constitutive and IFN-gamma-inducible MHC class II gene transcription. Transfection of trophoblast cells with CIITA expression vectors activates both MHC class II and class Ia antigen expression, which confers on trophoblast cells both the ability to activate helper T cells, and sensitivity to lysis by cytotoxic T lymphocytes. Collectively, these studies strongly suggest that stringent silencing of CIITA (and therefore MHC class II) gene expression in trophoblast cells is critical for the prevention of immune rejection responses against the fetus by the maternal immune system. The focus of this review is to summarize studies examining the novel mechanisms by which CIITA is silenced in trophoblast cells. The elucidation of the silencing of CIITA in trophoblast cells may shed light on how the semi-allogeneic fetus evades immune rejection by the maternal immune system during pregnancy.

K-562 cells lack MHC class II expression due to an alternatively spliced CIITA transcript with a truncated coding region

The focus of this study was to determine the functional capacity of class II transactivator (CIITA), a regulatory factor of major histocompatibility complex (MHC) class II genes, in K-562 cells. We show that CIITA mRNA is present in K-562 cells and the interferon-gamma (IFN-gamma)-inducible CIITA promoter-IV exhibits low levels of basal activity, which is greatly enhanced upon treatment with IFN-gamma. Further study revealed that the CIITA cDNA contains an insertion of genomic sequence, which introduces a stop codon. The truncated coding region of the CIITA transcript in K-562 cells provides a possible explanation for the absence of MHC class II molecules.

Differential Usage of Class II Transactivator Promoters PI and PIV during Inflammation and Injury in Kidney

ABSTRACT. Expression of class II transactivator (CIITA), the transcriptional regulator that controls all class II expression, is controlled in cell linesin vitroby three promoters: the dendritic cell promoter PI, the B cell promoter PIII, and the interferon-γ (IFN-γ)–inducible promoter, PIV. The authors examined the promoter usagein vivoin mouse kidney in the basal state and in response to IFN-γ, endotoxin, allostimulation, and renal injury. Genetically modified mice were used to examine the dependency of each promoter on IFN-γ and on the transcription factor interferon regulatory factor 1 (IRF-1). Usage of distinct CIITA promoters was monitored by real-time reverse transcriptase polymerase chain reaction (RT-PCR) using the unique sequences in the 5′ end of the transcript from each promoter. Kidneys in both control mice and IFN-γ knockouts expressed chiefly PI- and PIV–related products. Administration of recombinant IFN-γ activated only promoter PIV. Endotoxin or allogeneic stimulation elevated the PIV-related mRNA, dependent on IFN-γ and on IRF-1. Ischemic renal injury, however, increased the PI- and PIV–driven mRNA expression in wild-type but also in IFN-γ–deficient mice. Thus thein vivocontrol of CIITA promoters in kidney is similar to that observedin vitro(i.e., basal-state usage of PI and IFN-γ–dependent usage of PIV during inflammation), but it also shows additional levels of control: IFN-γ–independent basal activity of PIV and IFN-γ–independent induction of PIV during tissue injury. E-mail: phil.halloran@ualberta.ca

Identification of novel co-repressor molecules for Interferon Regulatory Factor-2

We have identified two novel proteins that interact specifically with the C-terminal repression domain of Interferon Regulatory Factor-2 (IRF-2). These proteins, which we term IRF-2 binding proteins 1 and 2 (IRF-2BP1 and IRF-2BP2, the latter having two splicing isoforms, A and B), are nuclear proteins, and have the properties of IRF-2-dependent transcriptional co-repressors that can inhibit both enhancer-activated and basal transcription in a manner that is not dependent upon histone deacetylation. IRF-2BP1 and IRF-2BP2A/B contain an N-terminal zinc finger and a C-terminal RING finger domain of the C3HC4 subclass, but show no homology to other known transcriptional regulators; they therefore define a new family of co- repressor proteins. An alternatively spliced form of IRF-2 that lacks two amino acids (valines 177 and 178) in the central portion of the protein (IRF-2[S]) cannot bind to these co-repressors and cannot mediate repression despite having the same C- terminal repression domain as IRF-2, suggesting that the relative conformation of the DNA binding domain and the C-terminal region of IRF-2 is crucial for transcriptional repression.

The IRF-2 DNA binding domain facilitates the activation of the class II transactivator (CIITA) type IV promoter by IRF-1

IFN-gamma induced transcription of class II transactivator (CIITA), a major regulator of MHC class II gene expression, is directed by the CIITA type IV promoter. The IFN-gamma activation of the CIITA type IV promoter is mediated by STAT1 and IRF-1, which bind to the GAS and IRF-E of the promoter, respectively. We and others have determined that IRF-2, another member of the IRF family, also activates the CIITA type IV promoter, by binding to the IRF-E. Also, IRF-2 cooperates with IRF-1 to activate the promoter. DNA binding analyses determined that IRF-1 and IRF-2 can co-occupy the IRF-E of the CIITA type IV promoter. To further understand the mechanism of IRF-1 and IRF-2 cooperativity in the activation of CIITA type IV promoter, we characterized the binding of IRF-1 and IRF-2 to the CIITA IRF-E and mapped the domains of IRF-2 required for the cooperative transactivation. Off-rate experiments revealed that the IRF-2/IRF-E complex was more stable than the IRF-1/IRF-E complex and that the affinity of IRF-1 for the IRF-E was increased when IRF-1 co-occupied the IRF-E with IRF-2. Deletion analysis of functional domains of IRF-2 revealed that a previously described latent activation domain of IRF-2 was essential for IRF-2 transactivation and participated in cooperative activation of the CIITA promoter by IRF-1 and IRF-2. However, the DNA binding domain of IRF-2 was sufficient for cooperativity with IRF-1 in the activation of the CIITA type IV promoter. DNA binding assay demonstrated that, like the full-length IRF-2, the IRF-2 DNA binding domain could co-occupy the CIITA IRF-E with IRF-1.

Induction of the early growth response gene 1 promoter by TCR agonists and partial agonists: ligand potency is related to sustained phosphorylation of extracellular signal-related kinase substrates

Responses to partial agonist TCR signals include positive selection of thymocytes, survival of naive T cells, and homeostatic proliferation. As part of an effort to understand the molecular basis of these processes, we have determined how agonist and partial agonist ligands act differently to induce a change in gene expression. We have found that the early growth response gene 1 (Egr1) promoter is activated by agonist and partial agonist ligands, but the partial agonist induces 10-fold lower promoter activity. Both agonist and partial agonist ligands require all six serum response elements in the Egr1 promoter to reach maximum induction. Although slightly fewer cells respond to the partial agonist, all of the responding cells have reduced activity compared with the cells responding to agonist. The factors binding to the serum response elements of the Egr1 promoter form a ternary complex (TC) consisting of serum response factor and either Elk-1 or serum response factor accessory protein-1a. Formation of a stable TC and inducible promoter activity are both dependent on extracellular signal-related kinase activation. Examination of TC formation over time reveals that this complex is induced well by partial agonist ligands, but it is not sustained, whereas agonist stimulation induces longer lived TCs. Therefore, the data suggest that both agonist and partial agonist ligands can induce formation of multiple TC on the Egr1 promoter, but the ability of the agonist ligand to maintain these complexes for an extended time results in the increased potency of the agonist.

Transcription factor IRF-1 in kidney transplants mediates resistance to graft necrosis during rejection

In many circumstances kidney transplants remain viable despite extensive inflammation, permitting rejection episodes to be reversed. The mechanisms by which the kidney resists host effector mechanisms are not known. In mouse kidney transplants, resistance requires interferon-gamma (IFN-gamma), which acts on the graft to protect the graft from necrosis during the first days of rejection as well as inducing major histocompatibility complex (MHC) expression. Because some effects of IFN-gamma are mediated by transcription factor IRF-1, the role of IRF-1 in the donor tissue early phases of rejection of mouse kidney allografts was studied. H-2(b) kidneys were transplanted from mice with wild-type IRF-1 genes (WT) or mice with disrupted IRF-1 genes (IRF-1KO) into CBA (H-2(k)) recipients. At day 5 and day 7, IRF-1KO and WT kidneys were functioning despite typical rejection pathology: interstitial infiltration and tubulitis. However, function deteriorated rapidly in rejecting IRF-1KO allografts, associated with widespread epithelial necrosis, peritubular capillary congestion, glomerulitis, and fibrin thrombi in small veins by day 7. At day 21, WT kidneys were viable despite severe tubulitis and arteritis, whereas IRF-1KO kidneys showed massive necrosis of the epithelium despite patent large vessels. Compared with WT kidneys, rejecting IRF-1KO kidneys showed less induction of donor MHC yet had similar mRNA levels of perforin, granzyme B, and Fas ligand and evoked host alloantibody responses. Thus in rejecting kidney transplants, IRF-1 in the graft mediates MHC induction, but it also mediates resistance to necrosis, an effect that could be crucial to permit success in interventions against rejection.

Lipopolysaccharide activates the expression of ISG15-specific protease UBP43 via interferon regulatory factor 3

UBP43 is a protease that specifically removes a ubiquitin-like protein, ISG15, from its targets. Highest levels of UBP43 expression are detected in macrophages and in cell lines of monocytic lineage. Macrophages are important in host defense against bacterial and viral infections. The lipopolysaccharide (LPS) of the bacterial cell wall can mimic bacteria and activate monocytes/macrophages to provoke inflammatory responses. Here, we report that LPS strongly activates UBP43 expression in macrophages, which is paralleled by changes in UBP43 protein levels. Two interferon regulatory factor (IRF) binding sites in the UBP43 promoter are responsible for the induction of UBP43 expression by LPS, as well as for basal UBP43 promoter activity. We have identified two members of the IRF family (IRF-2 and IRF-3) that specifically bind to these sites. IRF-3 plays a primary role in the LPS-inducible activation of the UBP43 gene and IRF-2 confers a basal transcriptional activity to the UBP43 promoter. Furthermore, we demonstrate that LPS treatment increases the amount of ISG15-conjugates in macrophages. Coordinated induction of ISG15 and UBP43 suggests that ISG15 conjugation is a dynamic process and that a critical balance of ISG15-modification should be maintained during innate immune response.

Interferon-gamma-induced chromatin remodeling at the CIITA locus is BRG1 dependent

SWI/SNF regulates growth control, differentiation and tumor suppression, yet few direct targets of this chromatin-remodeling complex have been identified in mammalian cells. We report that SWI/SNF is required for interferon (IFN)-gamma induction of CIITA, the master regulator of major histocompatibility complex class II expression. Despite the presence of functional STAT1, IRF-1 and USF-1, activators implicated in CIITA expression, IFN-gamma did not induce CIITA in cells lacking BRG1 and hBRM, the ATPase subunits of SWI/SNF. Reconstitution with BRG1, but not an ATPase-deficient version of this protein (K798R), rescued CIITA induction, and enhanced the rate of induction of the IFN-gamma-responsive GBP-1 gene. Not ably, BRG1 inhibited the CIITA promoter in transient transfection assays, underscoring the importance of an appropriate chromosomal environment. Chromatin immunoprecipitation revealed that BRG1 interacts directly with the endogenous CIITA promoter in an IFN-gamma-inducible fashion, while in vivo DNase I footprinting and restriction enzyme accessibility assays showed that chromatin remodeling at this locus requires functional BRG1. These data provide the first link between a cytokine pathway and SWI/SNF, and suggest a novel role for this chromatin-remodeling complex in immune surveillance.

Genetic control of MHC class II expression

The presentation of peptides to T cells by MHC class II molecules is of critical importance in specific recognition by the immune system. Expression of class II molecules is exquisitely controlled at the transcriptional level. A large set of proteins interact with the promoters of class II genes. The most important of these is CIITA, a master controller that orchestrates expression but does not bind directly to the promoter. The transcriptosome complex formed at class II promoters is a model for induction of gene expression.

IRF and tuberculosis

Tuberculosis is the most prevalent infectious disease and causes more deaths than any other, yet only 5%-10% of people infected by the causative agent, Mycobacterium tuberculosis, will develop the disease. Thus, natural resistance among humans is the norm. Fundamental immune responses to M. tuberculosis are being elucidated, including induction of interferon regulatory factor-1 (IRF-1). Moreover, IRF-1 has been found necessary for normal resistance to infection by mycobacteria in mice. Roles for IRF-1 in a plethora of immune system functions have been described. This review considers molecular responses to infection by M. tuberculosis that might account for induction of IRF-1 and highlights putative connections between immunomodulatory functions of IRF-1 and immune responses relevant to infection by M. tuberculosis. However, the complexity inherent in pleiotropy and redundancy limits the ability to draw firm conclusions. In many cases, it remains to be demonstrated that a particular function of IRF-1 is the basis for a known response to infection. For example, although IRF-1 is required for a Th1 cell-mediated, adaptive immune response in some circumstances, it is not known if the Th1 response to infection by M. tuberculosis requires IRF-1. Conversely, some known contributions by IRF-1 to fundamental aspects of the immune system are not yet proven relevant in the host response to infection. For example, it is not known if control of T cell subset development by IRF-1 is significant for host defense against M. tuberculosis. Functions of other IRF that overlap with or are distinct from the functions of IRF-1 also could be important for the immune response to M. tuberculosis.

Impaired class II transactivator expression in mice lacking interferon regulatory factor-2

Class II transactivator (CIITA) is required for both constitutive and inducible expression of MHC class II genes. IFN-gamma induced expression of CIITA in various cell types is directed by CIITA type IV promoter. The two transactivators, STAT1 and IRF-1, mediate the IFN-gamma activation of the type IV promoter by binding to the GAS and IRF-E of the promoter, respectively. In addition to IRF-1, IRF-2, another member of the IRF family, also activates the human CIITA type IV promoter, and IRF-2 cooperates with IRF-1 to activate the promoter in transient transfection assays. IRF-1 and IRF-2 can co-occupy the IRF-E of the human CIITA type IV promoter. To understand the effect of loss of IRF-2 on the endogenous CIITA expression, we assayed for CIITA expression in IRF-2 knock-out mice. Both basal and IFN-gamma induced CIITA expression were reduced in IRF-2 knock-out mice. At least half of the amount of inducible CIITA mRNA depends on IRF-2. The reduction of IFN-gamma induced CIITA mRNA in IRF-2 knock-out mice was due to the reduction of the type IV CIITA mRNA induction. The reduction of basal CIITA mRNA was apparently due to the reduction of CIITA mRNA originating from other promoters. These data indicate that IRF-2, like IRF-1, plays a critical role in the regulation of the endogenous CIITA gene. The implications in understanding the previously described phenotypes of IRF-2 defective mice are discussed.

Coactivator p300 acetylates the interferon regulatory factor-2 in U937 cells following phorbol ester treatment

Interferon regulatory factor-2 (IRF-2) is a transcription factor of the IRF family that represses interferon-mediated gene expression. In the present study, we show that human monocytic U937 cells express truncated forms of IRF-2 containing the DNA binding domain but lacking much of the C-terminal regulatory domain. U937 cells are shown to respond to phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) to induce expression of histone acetylases p300 and p300/CBP-associated factor (PCAF). In addition, TPA treatment led to the appearance of full-length IRF-2, along with a reduction of the truncated protein. Interestingly, full-length IRF-2 in TPA-treated U937 cells occurred as a complex with p300 as well as PCAF and was itself acetylated. Consistent with these results, recombinant IRF-2 was acetylated by p300 and to a lesser degree by PCAF in vitro. Another IRF member, IRF-1, an activator of interferon-mediated transcription, was also acetylated in vitro by these acetylases. Finally, we demonstrate that the addition of IRF-2 but not IRF-1 inhibits core histone acetylation by p300 in vitro. The addition of IRF-2 also inhibited acetylation of nucleosomal histones in TPA-treated U937 cells. Acetylated IRF-2 may affect local chromatin structure in vivo by inhibiting core histone acetylation and may serve as a mechanism by which IRF-2 negatively regulates interferon-inducible transcription.

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.

The cell cycle control element of histone H4 gene transcription is maximally responsive to interferon regulatory factor pairs IRF-1/IRF-3 and IRF-1/IRF-7

Interferon regulatory factors (IRFs) are transcriptional mediators of interferon-responsive signaling pathways that are involved in antiviral defense, immune response, and cell growth regulation. To investigate the role of IRF proteins in the regulation of histone H4 gene transcription, we compared the transcriptional contributions of IRF-1, IRF-2, IRF-3, and IRF-7 using transient transfection assays with H4 promoter/luciferase (Luc) reporter genes. These IRF proteins up-regulate reporter gene expression but IRF-1, IRF-3, and IRF-7 are more potent activators of the H4 promoter than IRF-2. Forced expression of different IRF combinations reveals that IRF-2 reduces IRF-1 or IRF-3 dependent activation, but does not affect IRF-7 function. Thus, IRF-2 may have a dual function in histone H4 gene transcription by acting as a weak activator at low dosage and a competitive inhibitor of other strongly activating IRFs at high levels. IRF-1/IRF-3 and IRF-1/IRF-7 pairs each mediate the highest levels of site II-dependent promoter activity and can up-regulate transcription by 120-150-fold. We also find that interferon gamma up-regulates IRF-1 and site II-dependent promoter activity. This up-regulation is not observed when the IRF site is mutated or if cells are preloaded with IRF-1. Our results indicate that IRF-1, IRF-2, IRF-3, and IRF-7 can all regulate histone H4 gene expression. The pairwise utilization of distinct IRF factors provides a flexible transcriptional mechanism for integration of diverse growth-related signaling pathways.

IFN-gamma regulation of class II transactivator promoter IV in macrophages and microglia: involvement of the suppressors of cytokine signaling-1 protein

The discovery of the class II transactivator (CIITA) transcription factor, and its IFN-gamma-activated promoter (promoter IV), have provided new opportunities to understand the molecular mechanisms of IFN-gamma-induced class II MHC expression. Here, we investigated the molecular regulation of IFN-gamma-induced murine CIITA promoter IV activity in microglia/macrophages. In the macrophage cell line RAW264.7, IFN-gamma inducibility of CIITA promoter IV is dependent on an IFN-gamma activation sequence (GAS) element and adjacent E-Box, and an IFN response factor (IRF) element, all within 196 bp of the transcription start site. In both RAW cells and the microglia cell line EOC20, two IFN-gamma-activated transcription factors, STAT-1alpha and IRF-1, bind the GAS and IRF elements, respectively. The E-Box binds upstream stimulating factor-1 (USF-1), a constitutively expressed transcription factor. Functionally, the GAS, E-Box, and IRF elements are each essential for IFN-gamma-induced CIITA promoter IV activity. The effects of the suppressors of cytokine signaling-1 (SOCS-1) protein on IFN-gamma-induced CIITA and class II MHC expression were examined. Ectopic expression of SOCS-1 inhibits IFN-gamma-induced activation of CIITA promoter IV and subsequent class II MHC protein expression. Interestingly, SOCS-1 inhibits the constitutive expression of STAT-1alpha and its IFN-gamma-induced tyrosine phosphorylation and binding to the GAS element in CIITA promoter IV. As well, IFN-gamma-induced expression of IRF-1 and its binding to the IRF element is inhibited. These results indicate that SOCS-1 may be responsible for attenuating IFN-gamma-induced CIITA and class II MHC expression in macrophages.

High level class II trans-activator induction does not occur with transient activation of the IFN-gamma signaling pathway

Gene activation in early development is highly dependent on precise concentrations of trans-acting factors for the activation of different genes at differing points in the embryo. Thus, not only is the presence or absence of a particular trans-activator or repressor relevant in determining gene activation, but also the concentration of the regulatory protein must be above or below a certain threshold for proper gene regulation. Signaling pathways in somatic cells are thought to represent cascades of on/off switches, mediated most commonly by phosphorylation. Here we demonstrate a quantitative mechanism for regulating the level of a component of the IFN-gamma signaling pathway that in effect represents the differential sensitivities of STAT1, IFN-regulatory factor-1, and class II trans-activator (CIITA) to IFN-gamma. Unlike developmental gene regulation, in which specificity of gene activation is a function of regulatory protein concentrations, specificity of gene activation in the IFN-gamma signaling pathway is regulated by the duration of the activation of the primary IFN-gamma-regulatory protein, STAT1. This result most likely explains previously reported data indicating that a minimum amount of IFN-gamma is required for MHC class II gene activation despite the fact that the level of the IFN-gamma-inducible factor directly required for MHC class II induction, CIITA, directly correlates with the level of MHC class II expression. The induction of a high level of CIITA is dependent on sustained IFN-gamma signaling. The possible implications of this result for tumorigenesis are discussed.

Cloning of an interferon regulatory factor 2 isoform with different regulatory ability

Interferons (IFNs) are a family of multifunctional proteins involved in immune activation, regulation of cell growth and antiviral response. They exert their functions by induction of several IFN-stimulated genes, including IFN regulatory factors (IRFs), a family of transcriptional regulators. One of these factors, IRF-2, was initially cloned as an antagonistic counterpart to IRF-1 with oncogenic potential. Here we describe a second isoform of IRF-2, termed IRF-2s, cloned from human and murine cells. This isoform lacks two amino acids located C-terminal of the DNA-binding domain, which is conserved in all IRF family members, leading to a change in the predicted secondary structure. Both isoforms have similar binding affinities to known target sequences in electrophoretic mobility shift assays. Using reporter gene constructs with the type IV promoter region of the MHC class II transactivator (CIITA), which is the essential factor for IFN-gamma-induced MHC class II expression, we show that the short isoform IRF-2s exhibits a weaker activation ability compared to IRF-2. Thus, our data present the first evidence of two IRF-2 isoforms with different regulatory ability.