Regulation of RANTES chemokine gene expression requires cooperativity between NF-kappa B and IFN-regulatory factor transcription factors

Expression and function of chemokines during viral infections: from molecular mechanisms to in vivo function

Recruitment and activation of leukocytes are important for elimination of microbes, including viruses, from infected areas. Chemokines constitute a group of bioactive peptides that regulate leukocyte migration and also contribute to activation of these cells. Chemokines are essential mediators of inflammation and important for control of viral infections. The profile of chemokine expression contributes to shaping the immune response during viral infection, whereas viral subversion of the chemokine system allows the virus to evade antiviral activities of the host. In this review, we discuss the role of chemokines in host-defense against virus infections, and we also look deeper into the virus-cell interactions that trigger chemokine expression as well as the cellular signaling cascades involved.

Lactobacilli and streptococci induce inflammatory chemokine production in human macrophages that stimulates Th1 cell chemotaxis

Macrophages have a central role in innate-immune responses to bacteria. In the present work, we show that infection of human macrophages with Gram-positive pathogenic Streptococcus pyogenes or nonpathogenic Lactobacillus rhamnosus GG enhances mRNA expression of inflammatory chemokine ligands CCL2/monocyte chemoattractant protein-1 (MCP-1), CCL3/macrophage-inflammatory protein-1alpha (MIP-1alpha), CCL5/regulated on activation, normal T expressed and secreted, CCL7/MCP-3, CCL19/MIP-3beta, and CCL20/MIP-3alpha and CXC chemokine ligands CXCL8/interleukin (IL)-8, CXCL9/monokine induced by interferon-gamma (IFN-gamma), and CXCL10/IFN-inducible protein 10. Bacteria-induced CCL2, CCL7, CXCL9, and CXCL10 mRNA expression was partially dependent on ongoing protein synthesis. The expression of these chemokines and of CCL19 was dependent on bacteria-induced IFN-alpha/beta production. CCL19 and CCL20 mRNA expression was up-regulated by IL-1beta or tumor necrosis factor alpha (TNF-alpha), and in addition, IFN-alpha together with TNF-alpha further enhanced CCL19 gene expression. Synergy between IFN-alpha and TNF-alpha was also seen for CXCL9 and CXCL10 mRNA expression. Bacteria-stimulated macrophage supernatants induced the migration of T helper cell type 1 (Th1) cells, suggesting that in human macrophages, these bacteria can stimulate efficient inflammatory chemokine gene expression including those that recruit Th1 cells to the site of inflammation. Furthermore, L. rhamnosus-induced Th1 chemokine production could in part explain the proposed antiallergenic properties of this bacterium.

Functional characterization of the human phosphodiesterase 7A1 promoter

In this paper, the human phosphodiesterase 7A1 (h PDE7A1 ) promoter region was identified and functionally characterized. Transient transfection experiments indicated that a 2.9 kb fragment of the h PDE7A1 5'-flanking region, to position -2907, has strong promoter activity in Jurkat T-cells. Deletion analysis showed that the proximal region, up to position -988, contains major cis -regulatory elements of the h PDE7A1 promoter. This minimal promoter region contains a regulatory CpG island which is essential for promoter activity. The CpG island contains three potential cAMP-response-element-binding protein (CREB)-binding sites that, as judged by in vivo dimethyl sulphate (DMS) footprinting, are occupied in Jurkat T-cells. Moreover, over-expression of CREB results in increased promoter activity, but, on the other hand, promoter activity decreases when a dominant-negative form of CREB (KCREB) is over-expressed. In vivo DMS footprinting strongly indicates that other transcription factors, such Ets-2, nuclear factor of activated T-cells 1 (NFAT-1) and nuclear factor kappaB (NF-kappaB), might also contribute to the regulation of h PDE7A1 promoter. Finally, h PDE7A1 promoter was found to be induced by treatment with PMA, but not by treatment with dibutyryl cAMP or forskolin. These results provide insights into the factors and mechanisms that regulate expression of the h PDE7A gene.

Sendai virus targets inflammatory responses, as well as the interferon-induced antiviral state, in a multifaceted manner

We have used cDNA arrays to compare the activation of various cellular genes in response to infection with Sendai viruses (SeV) that contain specific mutations. Three groups of cellular genes activated by mutant SeV infection, but not by wild-type SeV, were identified in this way. While some of these genes are well known interferon (IFN)-stimulated genes, others, such as those for interleukin-6 (IL-6) and IL-8, are not directly induced by IFN. The gene for beta IFN (IFN-beta), which is critical for initiating an antiviral response, was also specifically activated in mutant SeV infections. The SeV-induced activation of IFN-beta was found to depend on IFN regulatory factor 3, and the activation of all three cellular genes was independent of IFN signaling. Mutations that disrupt four distinct elements in the SeV genome (the leader RNA, two regions of the C protein, and the V protein) all lead to enhanced levels of IFN-beta mRNA, and at least three of these viral genes also appear to be involved in preventing activation of IL-8. Our results suggest that SeV targets the inflammatory and adaptive immune responses as well as the IFN-induced intracellular antiviral state by using a multifaceted approach.

Differential activation of NF-kappa B, AP-1, and C/EBP in endotoxin-tolerant rats: mechanisms for in vivo regulation of glomerular RANTES/CCL5 expression

Chemokines play a pivotal role in the regulation of inflammatory cell infiltration in glomerular immune injury. To characterize mechanisms relevant for the regulation of chemokine expression in vivo, the LPS-mediated model of renal inflammation in rats was used in which we have previously demonstrated that the chemokine RANTES/CCL5 is expressed and secreted in glomeruli. Glomerular RANTES/CCL5 expression in this model correlated with an increased glomerular binding activity of the transcription factors AP-1, C/EBP, and NF-kappaB. To gain further insight into the functional roles of these transcription factors in the regulation of glomerular RANTES/CCL5 expression, we cloned the rat RANTES/CCL5 promoter and established the model of in vivo LPS tolerance. In tolerant rats, LPS-induced glomerular RANTES/CCL5 expression and activation of the transcription factors AP-1 and C/EBP were significantly reduced using both consensus and rat RANTES/CCL5-specific oligonucleotides. Reduced glomerular NF-kappaB binding activity after LPS injection could be demonstrated in tolerant rats only when using rat RANTES/CCL5-specific oligonucleotides. Reduced binding activity to this RANTES/CCL5-specific NF-kappaB binding site in the context of broad NF-kappaB activation might be due to changes in transcription factor interactions or chromatin remodeling processes.

IKKepsilon and TBK1 are essential components of the IRF3 signaling pathway

The transcription factors interferon regulatory factor 3 (IRF3) and NF-kappaB are required for the expression of many genes involved in the innate immune response. Viral infection, or the binding of double-stranded RNA to Toll-like receptor 3, results in the coordinate activation of IRF3 and NF-kappaB. Activation of IRF3 requires signal-dependent phosphorylation, but little is known about the signaling pathway or kinases involved. Here we report that the noncanonical IkappaB kinase homologs, IkappaB kinase-epsilon (IKKepsilon) and TANK-binding kinase-1 (TBK1), which were previously implicated in NF-kappaB activation, are also essential components of the IRF3 signaling pathway. Thus, IKKepsilon and TBK1 have a pivotal role in coordinating the activation of IRF3 and NF-kappaB in the innate immune response.

17beta-estradiol inhibits the production of RANTES in human keratinocytes

A chemokine, regulated upon activation, normal T cell expressed and secreted (RANTES) attracts T helper-1 cells and macrophages. The production of RANTES is enhanced in keratinocytes of psoriatic skin lesions, which may contribute to the inflammatory infiltrate. It is known that estrogen regulates the natural course of psoriasis. We examined the in vitro effects of 17beta-estradiol on RANTES production by human keratinocytes. 17beta-estradiol inhibited tumor necrosis factor-alpha or interleukin-1beta-induced RANTES secretion, mRNA expression, and promoter activity in keratinocytes, and these effects of 17beta-estradiol were counteracted by estrogen receptor antagonist ICI 182 780. Two nuclear factor kappaB elements on RANTES promoter were required for tumor necrosis factor-alpha or interleukin-1beta-induced transcription and involved in the inhibition by 17beta-estradiol. 17beta-estradiol inhibited nuclear factor kappaB transcriptional activity, whereas it did not inhibit DNA binding of nuclear factor kappaB or phosphorylation or degradation of the inhibitor of nuclear factor kappaB alpha in tumor necrosis factor-alpha or interleukin-1beta-stimulated keratinocytes. 17beta-estradiol-induced inhibition of nuclear factor kappaB transcriptional activity and RANTES promoter activity was rescued by overexpression of a coactivator cyclic AMP response element-binding protein (CREB) or nuclear factor kappaB p65 but not by steroid receptor coactivator-1 or nuclear factor kappaB p50. The overexpression of CREB-binding protein rescued 17beta-estradiol-induced inhibition of transcription mediated by a chimeric protein, GAL4-p65286-551, which contained GAL4 DNA binding domain fused to C-terminal transactivating domain of p65 (amino acids 286-551). The transfection of estrogen receptor alpha or estrogen receptor beta into estrogen receptor-negative SKBR3 cells resulted in 17beta-estradiol-mediated inhibition of transcription via GAL4-p65286-551. These results suggest that 17beta-estradiol-bound estrogen receptor may inhibit nuclear factor kappaB-dependent transcription of RANTES gene by competing with p65 for limiting amounts of CREB-binding protein.

Synthesis of glucocorticoid-induced leucine zipper (GILZ) by macrophages: an anti-inflammatory and immunosuppressive mechanism shared by glucocorticoids and IL-10

Glucocorticoids and interleukin 10 (IL-10) prevent macrophage activation. In murine lymphocytes, glucocorticoids induce expression of glucocorticoid-induced leucine zipper (GILZ), which prevents the nuclear factor kappaB (NF-kappaB)-mediated activation of transcription. We investigated whether GILZ could account for the deactivation of macrophages by glucocorticoids and IL-10. We found that GILZ was constitutively produced by macrophages in nonlymphoid tissues of humans and mice. Glucocorticoids and IL-10 stimulated the production of GILZ by macrophages both in vitro and in vivo. Transfection of the macrophagelike cell line THP-1 with the GILZ gene inhibited the expression of CD80 and CD86 and the production of the proinflammatory chemokines regulated on activation normal T-cell expressed and secreted (CCL5) and macrophage inflammatory protein 1alpha (CCL3). It also prevented toll-like receptor 2 production induced by lipopolysaccharide, interferongamma, or an anti-CD40 mAb, as well as NF-kappaB function. In THP-1 cells treated with glucocorticoids or IL-10, GILZ was associated with the p65 subunit of NF-kappaB. Activated macrophages in the granulomas of patients with Crohn disease or tuberculosis do not produce GILZ. In contrast, GILZ production persists in tumor-infiltrating macrophages in Burkitt lymphomas. Therefore, GILZ appears to play a key role in the anti-inflammatory and immunosuppressive effects of glucocorticoids and IL-10. Glucocorticoid treatment stimulates GILZ production, reproducing an effect of IL-10, a natural anti-inflammatory agent. The development of delayed-type hypersensitivity reactions is associated with the down-regulation of GILZ gene expression within lesions. In contrast, the persistence of GILZ gene expression in macrophages infiltrating Burkitt lymphomas may contribute to the failure of the immune system to reject the tumor.

Fine tuning the transcriptional regulation of the CXCL1 chemokine

Constitutive activation of the transcription factor nuclear factor-κB (NF-κB) plays a major role in inflammatory diseases as well as cancer by inducing the endogenous expression of many proinflammatory proteins such as chemokines, and facilitating escape from apoptosis. The constitutive expression of chemokines such as CXCL1 has been correlated with growth, angiogenesis, and metastasis of cancers such as melanoma. The transcription of CXCL1 is regulated through interactions of NF-κB with other transcriptional regulatory molecules such as poly(ADP-ribose) polymerase-1 (PARP-1) and cAMP response element binding protein (CREB)-binding protein (CBP). It has been proposed that these two proteins interact with NF-κB and other enhancers to form an enhanceosome at the promoter region of CXCL1 and modulate CXCL1 transcription. In addition to these positive cofactors, a negative regulator, CAAT displacement protein (CDP), may also be involved in the transcriptional regulation of CXCL1. It has been postulated that the elevated expression of CXCL1 in melanomas is due to altered interaction between these molecules. CDP interaction with the promoter down-regulates transcription, whereas PARP and/or CBP interactions enhance transcription. Thus, elucidation of the interplay between components of the enhanceosome of this gene is important in finding more efficient and new therapies for conditions such as cancer as well as acute and chronic inflammatory diseases.

Regulation of RANTES promoter activation in alveolar epithelial cells after cytokine stimulation

Regulated on activation, normal T cell expressed, and presumably secreted (RANTES) is a member of the CC chemokine family of proteins implicated in a variety of diseases characterized by lung eosinophilia and inflammation, strongly produced by stimulated airway epithelial cells. Because such cytokines as tumor necrosis factor (TNF)-alpha and interferon-gamma (IFN-gamma) have been shown to enhance RANTES induction in airway epithelial cells and RANTES gene expression appears to be differentially regulated depending on the cell type and the stimulus applied, in this study we have elucidated mechanisms that operate to control RANTES induction on exposure to TNF-alpha and/or IFN-gamma. Our results indicate that TNF-alpha and IFN-gamma synergistically induce RANTES protein secretion and mRNA expression. RANTES transcription is activated only after stimulation with TNF-alpha, but not IFN-gamma, which affects RANTES mRNA stabilization. Promoter deletion and mutagenesis experiments indicate that the nuclear factor (NF)-kappaB site is the most important cis-regulatory element controlling TNF-induced RANTES transcription, although NF-interleukin-6 binding site, cAMP responsive element (CRE), and interferon-stimulated responsive element (ISRE) also play a significant role. TNF-alpha stimulation induces nuclear translocation of interferon regulatory factor (IRF)-3, which in viral infection binds the RANTES ISRE and is necessary for activation of RANTES transcription. However, TNF-induced IRF-3 translocation does not result in IRF-3 binding to the RANTES ISRE. Although viral infection can activate an ISRE-driven promoter, TNF cannot, indicating that RANTES gene enhancers are controlled in a stimulus-specific fashion. Identification of molecular mechanisms involved in RANTES gene expression is fundamental for developing strategies to modulate lung inflammatory responses.

Expression of the chemokines MCP-1/CCL2 and RANTES/CCL5 is differentially regulated by infiltrating inflammatory cells

BACKGROUND

Chemokines are involved in the regulation of the cellular renal infiltrate in glomerulonephritis; however, it is unclear to which degree resident glomerular cells or infiltrating leukocytes contribute to the formation of chemokines in glomerular inflammatory lesions. We therefore examined whether monocytes/macrophages play a role in the expression of the C-C chemokines MCP-1/CCL2 and RANTES/CCL5 in renal tissue in a lipopolysaccharide (LPS)-induced model of inflammation, where previously we have shown increased glomerular RANTES expression and glomerular infiltration of ED-1-positive cells.

METHODS

Inflammatory lesions were induced by an intraperitoneal injection of LPS. The infiltration of monocytes into the glomerulus was reduced by two experimental approaches. First, rats were depleted of monocytes by the use of specific monocyte-antisera or by cytotoxic drugs. Second, the infiltration of monocytes into the kidney was reduced by using intercellular adhesion molecule-1 (ICAM-1) knockout mice.

RESULTS

Both experimental approaches demonstrated a significant reduction in the number of infiltrating monocytes/macrophages after lipopolysaccharide injection. This reduction in the infiltration of inflammatory cells was associated with significantly reduced RANTES/CCL5 mRNA expression. However, MCP-1/CCL2 mRNA expression was not inhibited after the LPS injection by monocyte/macrophage depletion. Also, the increase in nuclear factor-kappaB (NF-kappaB) binding activity after the LPS injection was not reduced in pretreated animals. The experiments therefore demonstrate that infiltrating monocytes/macrophages contribute to increased RANTES/CCL5 mRNA expression in inflammatory renal lesions, whereas MCP-1/CCL2 mRNA expression and NF-kappaB activation were not reduced by monocyte/macrophage depletion.

CONCLUSION

MCP-1/CCL2 released from renal tissue upon stimulation plays a major role in the regulation of monocyte/macrophage infiltration, which contributes significantly to increased renal RANTES/CCL5 expression. This cross-talk between resident renal cells and monocytes/macrophages is therefore likely to boost the number of infiltrating inflammatory cells.

Down regulation of the interleukin-8 promoter by human papillomavirus type 16 E6 and E7 through effects on CREB binding protein/p300 and P/CAF

Previously, we reported that human papillomavirus (HPV) type 16 E6 binds to C/H1, C/H3, and the C-terminal domains of coactivators p300 and CBP, causing the modulation of the transcription of certain genes controlled by NF-kappaB (p65 or relA) and p53. To establish the biological significance of these observations, we have focused on the transcriptional regulation of interleukin-8 (IL-8), a potent chemoattractant for T lymphocytes and neutrophils, which is also essential for the initiation of the local immune response. The IL-8 promoter is regulated by NF-kappaB/p65 in response to tumor necrosis factor alpha and requires the cooperation of the coactivators CBP/p300 and steroid receptor coactivator 1 (SRC-1) and the p300/CBP-associated factor (P/CAF) for optimal activation. Here we report that, in the presence of HPV-16 E6, the promoter activity of IL-8 was repressed. Moreover, from the mutational analysis of the IL-8 promoter, we found that E6 down-regulates the IL-8 promoter activity through the NF-kappaB/p65 binding site. This inhibition appears to result from the ability of HPV-16 E6 to compete with NF-kappaB/p65 and SRC-1 for binding to the N terminus and C terminus of CBP, respectively. Reporter data also showed that E7 represses IL-8 promoter activity, though to a lesser extent than E6 but, like E6, the repression by E7 is through the NF-kappaB/p65 binding site. E7 was shown for the first time to bind to P/CAF, and the binding was necessary for the down regulation of the IL-8 promoter. E6 and E7 together inhibited transcription of the IL-8 promoter to a greater extent than either alone. Finally, by RNase protection assay, we showed that the synthesis of endogenous IL-8 mRNA was repressed in keratinocytes stably expressing E6 and E7. Taken together, the results provide evidence that E6 and E7 can cooperatively disrupt IL-8 transcription through disruption of transcriptional active complexes, and this may have important consequences for immune responses in infected hosts.

Nf-kappa B, chemokine gene transcription and tumour growth

The constitutive expression of angiogenic and tumorigenic chemokines by tumour cells facilitates the growth of tumours. The transcription of these angiogenic and tumorigenic chemokine genes is modulated, in part, by the nuclear factor-kappa B (NF-kappa B) family of transcription factors. In some tumours, there is constitutive activation of the kinases that modulate the activity of inhibitor of NF-kappa B (I kappa B) kinase (IKK), which leads to the constitutive activation of members of the NF-kappa B family. This activation of NF-kappa B is associated with the dysregulation of transcription of genes that encode cytokines, chemokines, adhesion factors and inhibitors of apoptosis. In this review, I discuss the factors that lie upstream of the NF-kappa B cascade that are activated during tumorigenesis and the role of the putative NF-kappa B enhanceosome in constitutive chemokine gene transcription during tumorigenesis.

IL-17 selectively down-regulates TNF-alpha-induced RANTES gene expression in human colonic subepithelial myofibroblasts

IL-17 enhances the TNF-alpha-induced IL-6 and IL-8 secretion in human colonic subepithelial myofibroblasts. In this study, we investigated how IL-17 modulates RANTES secretion in these cells. TNF-alpha potently induced RANTES secretion, but IL-17 dose-dependently inhibited the TNF-alpha-induced RANTES secretion. This was also observed at the mRNA level. Even after pretreatment with TNF-alpha for 12 h, the inhibitory effect of IL-17 was detectable. IL-17 did not affect the TNF-alpha-induced stability of the RANTES gene. IL-17 significantly decreased the TNF-alpha-induced increase in RANTES promoter activity, and IL-17 actually blocked the TNF-alpha-induced RANTES gene transcription. EMSAs demonstrated that IL-17 did not modulate the TNF-alpha-induced NF-kappaB DNA-binding activity, but markedly decreased TNF-alpha-induced IFN regulatory factor-1 (IRF-1) DNA-binding activity. Because cooperation between NF-kappaB and IRF-1 is important in the TNF-alpha-induced RANTES gene expression, the major mechanism mediating the inhibitory effect of IL-17 may be achieved by the inhibition of IRF-1 DNA-binding activity.

GM-CSF and M-CSF modulate beta-chemokine and HIV-1 expression in microglia

Significant numbers of patients with acquired immunodeficiency syndrome (AIDS) develop CNS infection primarily in macrophages and microglial cells. Therefore, the regulation of human immunodeficiency virus type 1 (HIV-1) infection and activation of the brain mononuclear phagocytes subsequent to infection are important areas of investigation. In the current report, we studied the role of granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage-CSF (M-CSF) in the expression of antiviral beta-chemokines and HIV-1 p24 in cultures of primary human fetal microglia. We found that stimulation with GM-CSF or M-CSF induced macrophage inflammatory proteins (MIP-1alpha and MIP-1beta) and augmented RANTES expression, after HIV-1 infection of microglia. This was not due to the effect of GM-CSF on viral expression because GM-CSF was neither necessary nor stimulatory for viral infection, nor did GM-CSF enhance the expression of env-pseudotyped reporter viruses. Blocking GM-CSF-induced microglial proliferation by nocodazole had no effect on beta-chemokine or p24 expression. The functional significance of the GM-CSF-induced beta-chemokines was suggested by the finding that, in the presence of GM-CSF, exogenous beta-chemokines lost their anti-HIV-1 effects. We further show that although HIV-1-infected microglia produced M-CSF, they failed to produce GM-CSF. In vivo, GM-CSF expression was localized to activated astrocytes and some inflammatory cells in HIV-1 encephalitis, suggesting paracrine activation of microglia through GM-CSF. Our results demonstrate a complex interplay between CSFs, chemokines, and virus in microglial cells and may have bearing on the interpretation of data derived in vivo and in vitro.

Protein kinase R regulates double-stranded RNA induction of TNF-alpha but not IL-1 beta mRNA in human epithelial cells

Epithelial cells represent the initial site of respiratory viral entry and the first line of defense against such infections. This early antiviral response is characterized by an increase in the production of proinflammatory cytokines such as TNF-alpha and IL-1 beta. dsRNA, which is a common factor present during the life cycle of both DNA and RNA viruses, is known to induce TNF-alpha and IL-1 beta in a variety of cells. In this work we provide data showing that dsRNA treatment induces TNF-alpha and IL-1 beta in human lung epithelial cells via two different mechanisms. Our data show that dsRNA activation of dsRNA-activated protein kinase (PKR) is associated with induction of TNF-alpha but not IL-1 beta expression. An inhibitor of PKR activation blocked the dsRNA-induced elevations in TNF-alpha but not IL-1 beta mRNA in epithelial cells. Data obtained from infection of epithelial cells with a vaccinia virus lacking the PKR inhibitory polypeptide, E3L, revealed that PKR activation was essential for TNF-alpha but not for IL-1 beta expression. In this report, we provide experimental support for the differential regulation of proinflammatory cytokine expression by dsRNA and viral infections in human airway epithelial cells.

A novel interferon regulatory factor (IRF), IRF-10, has a unique role in immune defense and is induced by the v-Rel oncoprotein

The cloning and functional characterization of a novel interferon regulatory factor (IRF), IRF-10, are described. IRF-10 is most closely related to IRF-4 but differs in both its constitutive and inducible expression. The expression of IRF-10 is inducible by interferons (IFNs) and by concanavalin A. In contrast to that of other IRFs, the inducible expression of IRF-10 is characterized by delayed kinetics and requires protein synthesis, suggesting a unique role in the later stages of an antiviral defense. Accordingly, IRF-10 is involved in the upregulation of two primary IFN-gamma target genes (major histocompatibility complex [MHC] class I and guanylate-binding protein) and interferes with the induction of the type I IFN target gene for 2',5'-oligo(A) synthetase. IRF-10 binds the interferon-stimulated response element site of the MHC class I promoter. In contrast to that of IRF-1, which has some of the same functional characteristics, the expression of IRF-10 is not cytotoxic for fibroblasts or B cells. The expression of IRF-10 is induced by the oncogene v-rel, the proto-oncogene c-rel, and IRF-4 in a tissue-specific manner. Moreover, v-Rel and IRF-4 synergistically cooperate in the induction of IRF-10 in fibroblasts. The level of IRF-10 induction in lymphoid cell lines by Rel proteins correlates with Rel transformation potential. These results suggest that IRF-10 plays a role in the late stages of an immune defense by regulating the expression some of the IFN-gamma target genes in the absence of a cytotoxic effect. Furthermore, IRF-10 expression is regulated, at least in part, by members of the Rel/NF-kappa B and IRF families.

Transcriptional profiling of interferon regulatory factor 3 target genes: direct involvement in the regulation of interferon-stimulated genes

Ubiquitously expressed interferon regulatory factor 3 (IRF-3) is directly activated after virus infection and functions as a key activator of the immediate-early alpha/beta interferon (IFN) genes, as well as the RANTES chemokine gene. In the present study, a tetracycline-inducible expression system expressing a constitutively active form of IRF-3 (IRF-3 5D) was combined with DNA microarray analysis to identify target genes regulated by IRF-3. Changes in mRNA expression profiles of 8,556 genes were monitored after Tet-inducible expression of IRF-3 5D. Among the genes upregulated by IRF-3 were transcripts for several known IFN-stimulated genes (ISGs). Subsequent analysis revealed that IRF-3 directly induced the expression of ISG56 in an IFN-independent manner through the IFN-stimulated responsive elements (ISREs) of the ISG56 promoter. These results demonstrate that, in addition to its role in the formation of a functional immediate-early IFN-beta enhanceosome, IRF-3 is able to discriminate among ISRE-containing genes involved in the establishment of the antiviral state as a direct response to virus infection.

Induction of macrophage-inflammatory protein-3alpha gene expression by TNF-dependent NF-kappaB activation

Macrophage-inflammatory protein-3alpha (MIP-3alpha), also designated as liver and activation-regulated chemokine (LARC), Exodus, or CCL20, is a recently identified CC chemokine that is expected to play a crucial role in the initiation of immune responses. In this study, we describe that MIP-3alpha expression is under the direct control of NF-kappaB, a key transcription factor of immune and inflammatory responses. Overexpression of the p65/RelA subunit of NF-kappaB significantly increased the MIP-3alpha mRNA level. MIP-3alpha transcription was stimulated by TNF, and this stimulation was inhibited by an NF-kappaB inhibitor, I-kappaBalpha superrepressor. Analysis of the human MIP-3alpha promoter demonstrated a functional NF-kappaB site responsible for its expression. We also show that MIP-3alpha expression is induced in LPS-treated mouse livers that were primed with Propionibacterium acnes, which developed massive liver injury with infiltration of inflammatory cells. This induction was fully dependent on the TNF signaling cascade, because it was not observed in the livers of TNFR1-deficient mice. Furthermore, pretreatment with gliotoxin, an inhibitor of NF-kappaB activity, abrogated the P. acnes/LPS-induced MIP-3alpha expression of wild-type mice. These results clearly demonstrate that MIP-3alpha gene expression is dependent on NF-kappaB activity in vitro, and indicate that the TNFR1-mediated TNF signaling cascade that leads to NF-kappaB activation plays an essential role in MIP-3alpha expression in the murine liver injury model.

Interferon-beta activates multiple signaling cascades in primary human microglia

Microglia, the resident brain macrophages, are the principal cells involved in the regulation of inflammatory and antimicrobial responses in the CNS. Interferon-beta (IFNbeta) is an antiviral cytokine induced by viral infection or following non-specific inflammatory challenges of the CNS. Because of the well-known anti-inflammatory properties of IFNbeta, it is also used to treat multiple sclerosis, an inflammatory CNS disease. Despite the importance of IFNbeta signaling in CNS cells, little has been studied, particularly in microglia. In this report, we investigated the molecular mechanisms underlying IFNbeta-induced beta-chemokine expression in primary human fetal microglia. Multiple signaling cascades are activated in microglia by IFNbeta, including nuclear factor-kappaB (NF-kappaB), activator protein-1 (AP-1) and Jak/Stat. IFNbeta induced IkappaBalpha degradation and NF-kappaB (p65:p50) DNA binding. Inhibition of NF-kappaB by either adenoviral transduction of a super repressor IkappaBalpha, or an antioxidant inhibitor of NF-kappaB reduced expression of the beta-chemokines, regulated upon activation, normal T-cell expressed and secreted (RANTES) and macrophage inflammatory protein (MIP)-1beta. IFNbeta also induced phosphorylation of extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase, and the MAP kinase kinase 1 (MEK1) inhibitor PD98059 dose-dependently inhibited beta-chemokine mRNA and protein expression. PD98059 did not inhibit NF-kappaB binding, demonstrating that ERK was not responsible for NF-kappaB activation. Two downstream targets of ERK were identified in microglia: AP-1 and Stat1. IFNbeta induced AP-1 nuclear binding activity in microglia and this was suppressed by PD98059. Additionally, IFNbeta induced Stat1 phosphorylation at both tyrosine 701 (Y701) and serine 727 (S727) residues. S727 phosphorylation of Stat1, which is known to be required for maximal transcriptional activation, was inhibited by PD98059. Our results demonstrating multiple signaling cascades initiated by IFNbeta in primary human microglia are novel and have implications for inflammatory and infectious diseases of the CNS.

Inhibition of morphine-potentiated HIV-1 replication in peripheral blood mononuclear cells with the nuclease-resistant 2-5A agonist analog, 2-5A(N6B)

Opioids potentiate HIV-1 infection in vitro at least partly by suppressing immunoresponsive processes in human lymphocytes and monocytes. For example, it appears that morphine inhibits the interferon (IFN)-alpha, -beta, and -gamma-mediated natural antiviral defense pathways in human peripheral blood mononuclear cells (PBMC). In this study, we show that restoration of a key component of the antiviral pathway reverses morphine-potentiated HIV-1 infection of human PBMC. The data show that HIV-1 replication is potentiated and RNase L activity is inhibited after morphine administration. Because HIV-1 inhibits the antiviral pathway at the level of 2',5'-oligoadenylate (2-5A) synthetase and p68 kinase, antiviral enzymes that require double-stranded RNA, we overcame this blockade by the addition of the nuclease-resistant, nontoxic 2-5A agonist, 2-5A(N6B), to PBMC in culture. Addition of 2-5A(N6B), but not zidovudine or saquinavir, to morphine-treated PBMC completely reversed the morphine-induced potentiation of HIV-1 infection. Further, 2-5A(N6B) significantly enhanced expression of both IFN-alpha and IFN-gamma. Also, increased expression of IFN-gamma was associated with a significant increase in expression of RANTES and monocyte chemotactic protein (MCP)-1, chemokines that may inhibit HIV-1 infection by blocking viral attachment to CCR2 and CCR5 co-receptors. Our results suggest that reactivation of the antiviral pathway by 2-5A agonists may be useful to inhibit opioid-potentiated HIV-1 replication.

NF-kappa B2 is required for optimal CD40-induced IL-12 production but dispensable for Th1 cell Differentiation

NF-kappa B is a ubiquitously expressed transcription factor involved in the regulation of innate and adaptive immunity. As part of studies to define the role of various NF-kappa B family members in Th cell development and maintenance, we infected NF-kappa B2(-/-) and control mice with Leishmania major and followed disease progression. NF-kappa B2(-/-) mice on a normally resistant background develop chronic nonhealing lesions associated with uncontrolled parasite replication and a failure to develop an IFN-gamma response. We show that there are no intrinsic defects in Th cell differentiation in the absence of NF-kappa B2. Indeed, NF-kappa B2(-/-) T cells are able to develop a Th1 phenotype and protect recombination-activating gene(-/-) mice from progressive cutaneous leishmaniasis. We demonstrate instead that the susceptibility of NF-kappa B2(-/-) mice to L. major is the result of an IL-12 deficiency, and we provide evidence for a specific impairment in CD40-induced IL-12 production by macrophages lacking this transcription factor.

Emergence of multidrug resistance in leukemia cells during chemotherapy: mechanisms and prevention

Multifactorial resistance to extracellular stimuli is one of the major factors of tumor progression. Cells can acquire a multidrug resistant (MDR) phenotype in response to a wide variety of stress-inducing agents including chemotherapeutic drugs. In addition to the mechanisms expressed in the tumor prior to chemotherapy (presumably these mechanisms allowed tumor cells to escape the control of growth and differentiation), a complex phenotype of pleiotropic resistance is presented in the residual or recurrent tumor. This review analyzes the molecular mechanisms of MDR acquisition with the focus on hematopoietic malignancies. In particular, the chemotherapy-induced up-regulation of P-glycoprotein, a broad-specificity transmembrane efflux pump, is considered a major event in establishment of MDR in leukemia cells that were sensitive before drug exposure. The pharmacological and genetic approaches to prevent the acquisition of Pgp-mediated MDR during chemotherapy are discussed.

Recognition of the measles virus nucleocapsid as a mechanism of IRF-3 activation

The mechanisms of cellular recognition for virus infection remain poorly understood despite the wealth of information regarding the signaling events and transcriptional responses that ensue. Host cells respond to viral infection through the activation of multiple signaling cascades, including the activation of NF-kappaB, c-Jun/ATF-2 (AP-1), and the interferon regulatory factors (IRFs). Although viral products such as double-stranded RNA (dsRNA) and the processes of viral binding and fusion have been implicated in the activation of NF-kappaB and AP-1, the mechanism(s) of IRF-1, IRF-3, and IRF-7 activation has yet to be fully elucidated. Using recombinant measles virus (MeV) constructs, we now demonstrate that phosphorylation-dependent IRF-3 activation represents a novel cellular detection system that recognizes the MeV nucleocapsid structure. At low multiplicities of infection, IRF-3 activation is dependent on viral transcription, since UV cross-linking and a deficient MeV containing a truncated polymerase L gene failed to induce IRF-3 phosphorylation. Expression of the MeV nucleocapsid (N) protein, without the requirement for any additional viral proteins or the generation of dsRNA, was sufficient for IRF-3 activation. In addition, the nucleocapsid protein was found to associate with both IRF-3 and the IRF-3 virus-activated kinase, suggesting that it may aid in the colocalization of the kinase and the substrate. Altogether, this study suggests that IRF-3 recognizes nucleocapsid structures during the course of an MeV infection and triggers the induction of interferon production.

Immunopathogenesis of vasculitis

Vasculitis is defined by the presence of leukocytes in the vessel wall with reactive damage to mural structures, leading to tissue ischemia and necrosis. The immunopathologic events that initiate the process of vascular inflammation and blood vessel damage are unclear. Damage of vascular endothelial cells and the recruitment and accumulation of the inflammatory infiltrate are determined by the endothelial cell and the bystanders, including the expression of adhesion molecules, the secretion of peptides and hormones, and the specific interaction with inflammatory cells. In addition to the endothelial cells, which provide costimulatory function, other cellular components and nonendothelial structures of the vessel wall are involved in controlling the inflammatory process, serve as antigen-presenting cells, and contribute with inflammatory mediators.

A non-classical ISRE/ISGF3 pathway mediates induction of RANTES gene transcription by type I IFNs

RANTES (regulated upon activation normal T cell expressed and secreted) is a chemoattractant cytokine important in the generation of inflammatory responses and human immunodeficiency virus resistance. In hematopoietic cells, RANTES is over-expressed by type I interferons (IFN-alpha and IFN-beta). The upstream region of the RANTES gene promoter contains a distal low affinity IFN-stimulated response element (ISRE). Specific mutagenesis in this ISRE-like motif abolished the activation of RANTES transcription by type I IFNs. Examination of the ISRE binding factors strongly suggested that signal transducer and activator of transcription (Stat)-2 and p48/IFN-stimulated gene factor 3gamma (ISGF3gamma) are not required for the induction of RANTES by type I IFNs. The specific requirement of Stat-1 was demonstrated using Stat-1-deficient U3A cells. These results revealed a non-classical ISRE/ISGF3 signal transduction pathway for the induction of RANTES by type I IFNs.

Structure and function of IRF-7

Interferon (IFN) regulatory factors (IRF) are a family of transcription factors with multiple functions. IRF-7 was initially cloned within the biologic context of Epstein-Barr virus (EBV) latency and discovered to have an intimate relation with the EBV primary oncogenic protein, latent membrane protein-1 (LMP-1). EBV regulates and uses IRF-7 as a secondary mediator for several target genes involved in latency and immune regulation. Other than its functions in EBV latency, IRF-7 has been identified as one of the major players in virally induced IFN production that is central to innate immunity. Thus, IRF-7 plays important roles in a variety of biologic systems.

Curcumin inhibits activation of Vgamma9Vdelta2 T cells by phosphoantigens and induces apoptosis involving apoptosis-inducing factor and large scale DNA fragmentation

Curcumin, in addition to its role as a spice, has been used for centuries to treat inflammatory disorders. Although the mechanism of action remains unclear, it has been shown to inhibit the activation of NF-kappaB and AP-1, transcription factors required for induction of many proinflammatory mediators. Due to its low toxicity it is currently under consideration as a broad anti-inflammatory, anti-tumor cell agent. In this study we investigated whether curcumin inhibited the response of gammadelta T cells to protease-resistant phosphorylated derivatives found in the cell wall of many pathogens. The results showed that curcumin levels > or =30 microM profoundly inhibited isopentenyl pyrophosphate-induced release of the chemokines macrophage inflammatory protein-1alpha and -1beta and RANTES. Curcumin also blocked isopentenyl pyrophosphate-induced activation of NF-kappaB and AP-1. Commencing around 16 h, treatment with curcumin lead to the induction of cell death that could not be reversed by APC, IL-15, or IL-2. This cytotoxicity was associated with increased annexin V reactivity, nuclear expression of active caspase-3, cleavage of poly(ADP-ribose) polymerase, translocation of apoptosis-inducing factor to the nucleus, and morphological evidence of nuclear disintegration. However, curcumin led to only large scale DNA chromatolysis, as determined by a combination of TUNEL staining and pulse-field and agarose gel electrophoresis, suggesting a predominantly apoptosis-inducing factor-mediated cell death process. We conclude that gammadelta T cells activated by these ubiquitous Ags are highly sensitive to curcumin, and that this effect may contribute to the anti-inflammatory properties of this compound.

Disruption of NF-kappa B signaling and chemokine gene activation by retroviral mediated expression of IKK gamma/NEMO mutants

Phosphorylation of I kappa Bs--the cytoplasmic inhibitors of the NF-kappa B transcription factors--is the key event which triggers activation of the NF-kappa B cascade. Signal-mediated phosphorylation of I kappa B alpha is mediated by a multiprotein complex, the I kappa B kinase (IKK) complex, which is composed of at least three identified subunits. Two of these polypeptides, IKK alpha and IKK beta, also known as IKK1 and IKK2, are the catalytic subunits of the kinase complex and phosphorylate I kappa B alpha and I kappa B beta. The third component, NEMO/IKK gamma, does not exhibit kinase activity, but rather constitutes a regulatory subunit. In the present study, C-terminal truncated forms of IKK gamma--Delta C-IKK gamma 306 and Delta C-IKK gamma 261--were stably expressed in the myeloid cell line U937 by retroviral-mediated gene transfer. Overexpression of Delta C-IKK gamma resulted in a reduction in IKK kinase activity in vitro, a subsequent decrease in NF-kappa B DNA binding activity, and inhibition of chemokine gene induction in response to TNFalpha stimulation or paramyxovirus infection. This study demonstrates the efficacy of Delta C-IKK gamma as a repressor of IKK signaling and NF-kappa B activation and suggests a potential gene therapy approach to limit chronic inflammation due to chemokine hyperactivation.

Multiple cis regulatory elements control RANTES promoter activity in alveolar epithelial cells infected with respiratory syncytial virus

Respiratory syncytial virus (RSV) produces intense pulmonary inflammation, in part through its ability to induce chemokine synthesis in infected airway epithelial cells. RANTES (regulated upon activation, normally T-cell expressed and presumably secreted) is a CC chemokine which recruits and activates monocytes, lymphocytes, and eosinophils, all cell types present in the lung inflammatory infiltrate induced by RSV infection. In this study, we analyzed the mechanism of RSV-induced RANTES promoter activation in human type II alveolar epithelial cells (A549 cells). Promoter deletion and mutagenesis experiments indicate that RSV requires the presence of five different cis regulatory elements, located in the promoter fragment spanning from -220 to +55 nucleotides, corresponding to NF-kappaB, C/EBP, Jun/CREB/ATF, and interferon regulatory factor (IRF) binding sites. Although site mutations of the NF-kappaB, C/EBP, and CREB/AP-1 like sites reduce RSV-induced RANTES gene transcription to 50% or less, only mutations affecting IRF binding completely abolish RANTES inducibility. Supershift and microaffinity isolation assays were used to identify the different transcription factor family members whose DNA binding activity was RSV inducible. Expression of dominant negative mutants of these transcription factors further established their central role in virus-induced RANTES promoter activation. Our finding that the presence of multiple cis regulatory elements is required for full activation of the RANTES promoter in RSV-infected alveolar epithelial cells supports the enhanceosome model for RANTES gene transcription, which is absolutely dependent on binding of IRF transcription factors. The identification of regulatory mechanisms of RANTES gene expression is fundamental for rational design of inhibitors of RSV-induced lung inflammation.

Hepatitis C virus nonstructural 5A protein induces interleukin-8, leading to partial inhibition of the interferon-induced antiviral response

Hepatitis C virus (HCV), a major cause of liver disease worldwide, is frequently resistant to the antiviral alpha interferon (IFN). The HCV nonstructural 5A (NS5A) protein has been implicated in HCV antiviral resistance in many studies. NS5A antagonizes the IFN antiviral response in vitro, and one mechanism is via inhibition of a key IFN-induced enzyme, the double-stranded-RNA-activated protein kinase (PKR). In the present study we determined if NS5A uses other strategies to subvert the IFN system. Expression of full-length NS5A proteins from patients who exhibited a complete response (FL-NS5A-CR) or were nonresponsive (FL-NS5A-NR) to IFN therapy in HeLa cells had no effect on IFN induction of IFN-stimulated gene factor 3 (ISGF-3). Expression of mutant NS5A proteins lacking 110 (NS5A-DeltaN110), 222 (NS5A-DeltaN222), and 334 amino-terminal amino acids and mutants lacking 117 and 230 carboxy-terminal amino acids also had no effect on ISGF-3 induction by IFN. Expression of FL-NS5A-CR and FL-NS5A-NR did not affect IFN-induced STAT-1 tyrosine phosphorylation or upregulation of PKR and major histocompatibility complex class I antigens. However, NS5A expression in human cells induced interleukin 8 (IL-8) mRNA and protein, and this effect correlated with inhibition of the antiviral effects of IFN in an in vitro bioassay. NS5A induced transcription of a reporter gene driven by the IL-8 promoter, and the first 133 bp of the IL-8 promoter made up the minimal domain required for NS5A transactivation. NS5A-DeltaN110 and NS5A-DeltaN222 stimulated the IL-8 promoter to higher levels than did the full-length NS5A protein, and this correlated with increased nuclear localization of the proteins. Additional mutagenesis of the IL-8 promoter suggested that NF-kappaB and AP-1 were important in NS5A-DeltaN222 transactivation in the presence of tumor necrosis factor alpha and that NF-IL-6 was inhibitory to this process. This study suggests that NS5A inhibits the antiviral actions of IFN by at least two mechanisms and provides the first evidence for a biological effect of the transcriptional activity of the NS5A protein. During HCV infection, viral proteins may induce chemokines that contribute to HCV antiviral resistance and pathogenesis.

IL-1 induced chemokine production through the association of Syk with TNF receptor-associated factor-6 in nasal fibroblast lines

The fibroblasts stimulated by cytokines released the chemokine and recruited the infiltrating cells, including eosinophils, that play a key role in the pathogenesis of airway disease. We established the human fibroblast lines showing high Syk expression and the lines showing low Syk expression from pieces of nasal polyp. IL-1 induces the interaction of TNFR-associated factor (TRAF) 6 with IL-1R-associated kinase, which is rapidly recruited to the IL-1R after IL-1 induction, whereas TRAF2 participates in TNF-alpha-signaling. In the present study, we found that Syk played a different role in IL-1- and TNF-alpha-induced chemokine production through a signaling complex involving Syk and TRAF6. Overexpression of wild-type Syk by gene transfer enhanced RANTES production from nasal fibroblasts stimulated with IL-1. The decrease of Syk expression by the administration of Syk antisense inhibited RANTES production in response to IL-1. However, the change of Syk expression did not affect RANTES production by TNF-alpha stimulation. We concluded that Syk is required for the IL-1-induced chemokine production through the association with TRAF-6 in fibroblasts of nasal polyps.

Hantavirus infection induces the expression of RANTES and IP-10 without causing increased permeability in human lung microvascular endothelial cells

Sin Nombre virus (SNV) and Hantaan virus (HTN) infect endothelial cells and are associated with different patterns of increased vascular permeability during human disease. It is thought that such patterns of increased vascular permeability are a consequence of endothelial activation and subsequent dysfunction mediated by differential immune responses to hantavirus infection. In this study, the ability of hantavirus to directly induce activation of human lung microvascular endothelial cells (HMVEC-Ls) was examined. No virus-specific modulation in the constitutive or cytokine-induced expression of cellular adhesion molecules (CD40, CD54, CD61, CD62E, CD62P, CD106, and major histocompatibility complex classes I and II) or in cytokines and chemokines (eotaxin, tumor necrosis factor alpha, interleukin 1beta [IL-1beta], IL-6, IL-8, MCP-1, MIP-1alpha, and MIP-1beta) was detected at either the protein or message level in hantavirus-infected HMVEC-Ls. Furthermore, no virus-specific enhancement of paracellular or transcellular permeability or changes in the organization and distribution of endothelial intercellular junctional proteins was observed. However, infection with either HTN or SNV resulted in detectable levels of the chemokines RANTES and IP-10 (the 10-kDa interferon-inducible protein) in HMVEC-Ls within 72 h and was associated with nuclear translocation of interferon regulatory factor 3 (IRF-3) and IRF-7. Gamma interferon (IFN-gamma)-induced expression of RANTES and IP-10 could also be detected in uninfected HMVEC-Ls and was associated with nuclear translocation of IRF-1 and IRF-3. Treatment of hantavirus-infected HMVEC-Ls with IFN-gamma for 24 h resulted in a synergistic enhancement in the expression of both RANTES and IP-10 and was associated with nuclear translocation of IRF-1, IRF-3, IRF-7, and NF-kappaB p65. These results reveal a possible mechanism by which hantavirus infection and a TH1 immune response can cooperate to synergistically enhance chemokine expression by HMVEC-Ls and trigger immune-mediated increases in vascular permeability.

HHV-8 encoded vIRF-1 represses the interferon antiviral response by blocking IRF-3 recruitment of the CBP/p300 coactivators

Human herpes virus 8 (HHV-8) has developed unique mechanisms for altering cellular proliferative and apoptotic control pathways by incorporating viral homologs to several cellular regulatory genes into its genome. One of the important pirated genes encoded by the ORF K9 reading frame is a viral homolog of the interferon regulatory factors (IRF), a family of cellular transcription proteins that regulates expression of genes involved in pathogen response, immune modulation and cell proliferation. vIRF-1 has been shown to downregulate the interferon- and IRF-mediated transcriptional activation of ISG and murine IFNA4 gene promoters. In this study we demonstrate that vIRF-1 efficiently inhibited virus-induced expression of endogenous interferon B, CC chemokine RANTES and CXC chemokine IP-10 genes. Co-expression analysis revealed that vIRF-1 selectively blocked IRF-3 but not IRF-7-mediated transactivation. vIRF-1 was able to bind to both IRF-3 and IRF-7 in vivo as detected by coimmunoprecipitation analysis, but did not affect IRF-3 dimerization, nuclear translocation and DNA binding activity. Rather, vIRF-1 interacted with the CBP/p300 coactivators and efficiently inhibited the formation of transcriptionally competent IRF-3-CBP/p300 complexes. These results illustrate that vIRF-1 is able to block the early stages of the IFN response to virus infection by interfering with the activation of IRF-3 responsive, immediate early IFN genes.

Multiple regulatory domains control IRF-7 activity in response to virus infection

Recent studies implicate the interferon regulatory factors (IRF), IRF-3 and IRF-7, as key activators of Type 1 interferon genes, as well as the RANTES (regulated on activation normal T cell expressed) chemokine gene. Both IRF-3 and IRF-7 are regulated in part by virus-induced C-terminal phosphorylation, leading to nuclear translocation, stimulation of DNA binding, and transcriptional activities. Structure-function studies with IRF-7 suggested a complex organization of the C-terminal region, with a constitutive activation domain located between amino acids 150-246, an accessory inducibility region at the very end of IRF-7 between amino acids 467 and 503, and an inhibitory region (amino acids 341-467) adjacent to the C-terminal end that interferes with transactivation. Furthermore, an element that increases basal and virus-inducible activity is located between amino acids 278 and 305. A transcriptionally active form of IRF-7 was also generated by substitution of Ser-477 and Ser-479 residues with the phosphomimetic Asp. IRF-7, particularly IRF-7(S477D/S479D), was a strong transactivator of type I interferon and RANTES chemokine gene expression. Unlike wild type IRF-3, IRF-7 overexpression was able to stimulate inteferon gene expression in the absence of virus infection. Using tagged versions of IRF-7 and IRF-3, the formation of homo- and heterodimers was detected by co-immunoprecipitation. These results demonstrate that IRF-3 and IRF-7 transcription factors possess distinct structural characteristics that impart complementary rather than redundant functional roles in cytokine gene activation.

Regulation of the promoter activity of interferon regulatory factor-7 gene. Activation by interferon snd silencing by hypermethylation

The molecular mechanism by which virus induces expression of the early inflammatory genes has not yet been completely elucidated. Previous studies indicated that the virus-mediated transcription of type I interferon (IFN) genes required activation of two members of IFN regulatory factor (IRF) family, IRF-3 and IRF-7, where the expression of IRF-7 was found to be indispensable for the induction of IFNA genes. To determine the factors that regulate expression of IRF-7 gene, as well as its inducibility by type I IFNs, we have isolated and characterized the promoter and first intron of the human IRF-7 gene. This region shows a presence of two potential interferon-sensitive response elements (ISRE/IRF-E). However, only the ISRE present in the first intron was functional and conferred interferon inducibility in a transient transfection assay. Using a pull-down assay with an oligodeoxynucleotide corresponding to this ISRE immobilized to magnetic beads, we have demonstrated that this ISRE binds ISGF3 complex and IRF-1 from the extract of IFN-treated cells but not from the untreated cells. We have further shown that the previously observed lack of expression of IRF-7 in 2fTGH fibrosarcoma cell line, correlated with hypermethylation of the CpG island in the human IRF-7 promoter. The repression of the promoter activity was relieved by treatment with DNA methyltransferase inhibitor 5-aza-deoxycytidine. In vitro methylation of IRF-7 promoter silenced IRF-7 directed expression of luciferase gene in HeLa cells that express endogenous IRF-7 gene. Whether silencing of IRF-7 by methylation is instrumental for the process of tumorigenesis remains to be determined.