Retinoic acid-inducible gene-I is induced by interleukin-1beta in cultured human gingival fibroblasts

The Killer's Web: Interconnection between Inflammation, Epigenetics and Nutrition in Cancer

Inflammation is a key contributor to both the initiation and progression of tumors, and it can be triggered by genetic instability within tumors, as well as by lifestyle and dietary factors. The inflammatory response plays a critical role in the genetic and epigenetic reprogramming of tumor cells, as well as in the cells that comprise the tumor microenvironment. Cells in the microenvironment acquire a phenotype that promotes immune evasion, progression, and metastasis. We will review the mechanisms and pathways involved in the interaction between tumors, inflammation, and nutrition, the limitations of current therapies, and discuss potential future therapeutic approaches.

Duck LGP2 Downregulates RIG-I Signaling Pathway-Mediated Innate Immunity Against Tembusu Virus

In mammals, the retinoic acid-inducible gene I (RIG-I)-like receptors (RLR) has been demonstrated to play a critical role in activating downstream signaling in response to viral RNA. However, its role in ducks’ antiviral innate immunity is less well understood, and how gene-mediated signaling is regulated is unknown. The regulatory role of the duck laboratory of genetics and physiology 2 (duLGP2) in the duck RIG-I (duRIG-I)-mediated antiviral innate immune signaling system was investigated in this study. In duck embryo fibroblast (DEF) cells, overexpression of duLGP2 dramatically reduced duRIG-I-mediated IFN-promotor activity and cytokine expression. In contrast, the knockdown of duLGP2 led to an opposite effect on the duRIG-I-mediated signaling pathway. We demonstrated that duLGP2 suppressed the duRIG-I activation induced by duck Tembusu virus (DTMUV) infection. Intriguingly, when duRIG-I signaling was triggered, duLGP2 enhanced the production of inflammatory cytokines. We further showed that duLGP2 interacts with duRIG-I, and this interaction was intensified during DTMUV infection. In summary, our data suggest that duLGP2 downregulated duRIG-I mediated innate immunity against the Tembusu virus. The findings of this study will help researchers better understand the antiviral innate immune system’s regulatory networks in ducks.

Immune Regulator Retinoic Acid-Inducible Gene I (RIG-I) in the Pathogenesis of Cardiovascular Disease

Retinoic acid-inducible gene I (RIG-I) is a cytosolic pattern recognition receptor that contains two CARD domains, an RNA helicase domain, and a C-terminal domain. RIG-I initiates antiviral innate immunity by recognizing exogenous viral RNAs/DNAs. However, some studies have reported that RIG-I activation leads to damage in various organs and tissues in diverse circumstances. Recent studies have shown that RIG-I is involved in cancer, lupus nephritis, immunoglobulin A nephropathy, Crohn’s disease, and atherosclerosis. These reports indicate that RIG-I not only participates in antiviral signaling pathways but also exerts an influence on non-viral infectious diseases. RIG-I is widely expressed in immune and non-immune cells including smooth muscle cells, endothelial cells, and cardiomyocytes. A succinct overview of RIG-I and its signaling pathways, with respect to the cardiovascular system, will aid in the development of novel therapeutics for cardiovascular diseases. In this review, we summarize the structure, activation, signaling pathways, and role of RIG-I in cardiovascular diseases.

Innate Immune Responses to Avian Influenza Viruses in Ducks and Chickens

Mallard ducks are important natural hosts of low pathogenic avian influenza (LPAI) viruses and many strains circulate in this reservoir and cause little harm. Some strains can be transmitted to other hosts, including chickens, and cause respiratory and systemic disease. Rarely, these highly pathogenic avian influenza (HPAI) viruses cause disease in mallards, while chickens are highly susceptible. The long co-evolution of mallard ducks with influenza viruses has undoubtedly fine-tuned many immunological host–pathogen interactions to confer resistance to disease, which are poorly understood. Here, we compare innate responses to different avian influenza viruses in ducks and chickens to reveal differences that point to potential mechanisms of disease resistance. Mallard ducks are permissive to LPAI replication in their intestinal tissues without overtly compromising their fitness. In contrast, the mallard response to HPAI infection reflects an immediate and robust induction of type I interferon and antiviral interferon stimulated genes, highlighting the importance of the RIG-I pathway. Ducks also appear to limit the duration of the response, particularly of pro-inflammatory cytokine expression. Chickens lack RIG-I, and some modulators of the signaling pathway and may be compromised in initiating an early interferon response, allowing more viral replication and consequent damage. We review current knowledge about innate response mediators to influenza infection in mallard ducks compared to chickens to gain insight into protective immune responses, and open questions for future research.

Cloning, expression, and bioinformatics analysis of a putative pigeon retinoid acid-inducible gene-I

Retinoid acid-inducible gene-I (RIG-I) is a major cytoplasmic RNA sensor, playing an essential role in detecting viral RNA and triggering antiviral innate immune responses. The objective of the present study was to characterize the structure and expression of the RIG-I gene in pigeons. The pigeon RIG-I (piRIG-I) was cloned from splenic lymphocytes of pigeons using reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends. The cDNA of piRIG-I contains a 147 bp 5′-untranslated regions (UTRs), a 2787 bp open reading frame, and 2962 bp 3′-UTRs. Based on this sequence, the encoded piRIG-I protein is predicted to consist of 928 amino acids, and it has conserved domains typical of RIG-I-like receptors (RLRs) including two tandem arranged N-terminal caspase recruitment domains, a domain with the signature of DExD/H box helicase (helicase domain), and a C-terminal repression domain similar to finch RIG-I, duck RIG-I, goose RIG-I, human RIG-I, and mouse RIG-I. The piRIG-I shows 82.1%, 78.6%, and 78.2% amino acid sequence identity with previously described finch RIG-I, duck RIG-I, and goose RIG-I, respectively, and 49.7%–53.8% sequence identity with mammalian homologs. Quantitative RT-PCR (qRT-PCR) analysis indicated that the piRIG-I mRNA is scarcely detected in healthy tissues, and it is strongly expressed in the thymus gland, kidney, spleen, and bursa of Fabricius. These findings lay the foundation for further research on the function and mechanism of avian RIG-I in innate immune response related to vaccinations and infectious diseases in the pigeon.

Oncostatin M induces RIG-I and MDA5 expression and enhances the double-stranded RNA response in fibroblasts

Interleukin (IL)-6-type cytokines have no direct antiviral activity; nevertheless, they display immune-modulatory functions. Oncostatin M (OSM), a member of the IL-6 family, has recently been shown to induce a distinct number of classical interferon stimulated genes (ISG). Most of them are involved in antigen processing and presentation. However, induction of retinoic acid-inducible gene (RIG)-I-like receptors (RLR) has not been investigated. Here we report that OSM has the capability to induce the expression of the DExD/H-Box RNA helicases RIG-I and melanoma differentiation antigen 5 (MDA5) as well as of the transcription factors interferon regulatory factor (IRF)1, IRF7 and IRF9 in primary fibroblasts. Induction of the helicases depends on tyrosine as well as serine phosphorylation of STAT1. Moreover, we could show that the OSM-induced STAT1 phosphorylation is predominantly counter-regulated by a strong STAT3-dependent SOCS3 induction, as Stat3 as well as Socs3 knock-down results in an enhanced and prolonged helicase and IRF expression. Other factors involved in regulation of STAT1 or IRF1 activity, like protein tyrosine phosphatase, non-receptor type 2 (PTPN2), promyelocytic leukaemia protein (PML) or small ubiquitin-related modifier 1 (SUMO1), play a minor role in OSM-mediated induction of RLR. Remarkably, OSM and interferon-γ (IFN-γ) synergize to mediate transcription of RLR and pre-treatment of fibroblasts with OSM fosters the type I interferon production in response to a subsequent encounter with double-stranded RNA. Together, these findings suggest that the OSM-induced JAK/STAT1 signalling is implicated in virus protection of non-professional immune cells and may cooperate with interferons to enhance RLR expression in these cells.

Listeria monocytogenesInduces the Expression of Retinoic Acid-Inducible Gene-I

Retinoic acid-inducible gene-I (RIG-I) is considered to play a role in innate immunity against virus infections. We showed by immunohistochemical study that RIG-I expression is upregulated in vivo in hepatic Kupffer cells and in splenic reticular cells of mice infected with Listeria monocytogenes. Both heat-killed L. monocytogenes and live L. monocytogenes induced the expression of RIG-I in cultured RAW264.7 murine macrophage-like cells in vitro. RIG-I may also be involved in innate immunity against Listeria infection.

Passive carriage of rabies virus by dendritic cells

The rabies virus (RABV) is highly neurotropic and it uses evasive strategies to successfully evade the host immune system. Because rabies is often fatal, understanding the basic processes of the virus-host interactions, particularly in the initial events of infection, is critical for the design of new therapeutic approaches to target RABV. Here, we examined the possible role of dendritic cells (DCs) in the transmission of RABV to neural cells at peripheral site of exposure. Viral replication only occurred at a low level in the DC cell line, JAWS II, after its infection with either pathogenic RABV (CVS strain) or low-pathogenic RABV (ERA strain), and no progeny viruses were produced in the culture supernatants. However, both viral genomic RNAs were retained in the long term after infection and maintained their infectivity. The biggest difference between CVS and ERA was in their ability to induce type I interferons. Although the ERA-infected JAWS II cells exhibited cytopathic effect and were apparently killed by normal spleen cells in vitro, the CVS-infected JAWS II cells showed milder cytopathic effect and less lysis when cocultured with spleen cells. Strongly increased expression of major histocompatibility complex classes I, costimulatory molecules (CD80 and CD86), type I interferons and Toll- like receptor 3, and was observed only in the ERA-inoculated JAWS II cells and not in those inoculated with CVS. During the silencing of the cellular immune response in the DCs, the pathogenic CVS strain cryptically maintained an infectious viral genome and was capable of transmitting infectious RABV to permissive neural cells. These findings demonstrate that DCs may play a role in the passive carriage of RABV during natural rabies infections.

Expression and function of galectins in the endometrium and at the human feto-maternal interface

Galectins are classified as lectins that share structural similarities and bind β-galactosides via a conserved carbohydrate recognition domain. So far 16 out of 19 identified galectins were shown to be present in humans and numerous studies revealed galectins as pivotal modulators of cell death, differentiation and growth. Galectins were highlighted to interact with both the adaptive and innate immune response. In the field of reproductive medicine and placenta research different roles for galectins have been proposed. Several galectins, being abundantly present at the human feto-maternal interphase and endometrium, were hypothesized to significantly contribute to endometrial receptivity and pregnancy physiology. Hence, this review outlines selected aspects of galectin action within endometrial function and at the feto-maternal interphase. Further current knowledge on galectins in reproductive and pregnancy disorders like endometriosis, abortion or preeclampsia is summarized.

Sequence analysis of a putative goose RIG-I gene

Li, G., Li, J., Tian, Y., Wang, DE., Shen, J., Tao, Z., Xu, J. and Lu, L. 2012. Sequence analysis of a putative goose RIG-I gene. Can. J. Anim. Sci. 92: 143–151. Retinoid acid-inducible gene-I (RIG-I) is a critical cytoplasmic RNA sensor which plays an important role in the recognition of, and response to, influenza virus and other RNA viruses. In the present study, A 3808-bp cDNA encoding goose RIG-I (goRIG-I) was cloned from splenic lymphocytes of geese using RT-PCR and rapid amplification of cDNA ends (RACE) techniques. The encoded protein, which is predicted to consist of 933 amino acids, has a molecular weight of 106.4 kDa and includes an N-terminal caspase recruitment domain (CARD), a domain with the signature of DExD/H box helicase (helicase domain), and a C-terminal repression domain (RD) similar to duck RIG-I (duRIG-I), human RIG-I, and mouse RIG-I. The goRIG-I showed 93.8 and 78.0% amino acid sequence identity with previously described duRIG-I and finch RIG-I, respectively, and 48.9–53.0% sequence identity with mammalian homologs. Quantitative RT-PCR analysis indicated that the goRIG-I gene is strongly expressed in the liver, lung, brain, spleen, and bursa of Fabricius. These findings lay the foundation for further research on the function and mechanism of avian RIG-I in innate immunity.

Retinoic acid inducible gene-I, more than a virus sensor

Retinoic acid inducible gene-I (RIG-I) is a caspase recruitment domain (CARD) containing protein that acts as an intracellular RNA receptor and senses virus infection. After binding to double stranded RNA (dsRNA) or 5'-triphosphate single stranded RNA (ssRNA), RIG-I transforms into an open conformation, translocates onto mitochondria, and interacts with the downstream adaptor mitochondrial antiviral signaling (MAVS) to induce the production of type I interferon and inflammatory factors via IRF3/7 and NF-κB pathways, respectively. Recently, accumulating evidence suggests that RIG-I could function in non-viral systems and participate in a series of biological events, such as inflammation and inflammation related diseases, cell proliferation, apoptosis and even senescence. Here we review recent advances in antiviral study of RIG-I as well as the functions of RIG-I in other fields.

Function and regulation of retinoic acid-inducible gene-I

Antiviral innate immunity is triggered by sensing viral nucleic acids. RIG-I (retinoic acid-inducible gene-I) is an intracellular molecule that responds to viral nucleic acids and activates downstream signaling, resulting in the induction of members of the type I interferon (IFN) family, which are regarded among the most important effectors of the innate immune system. Although RIG-I is expressed ubiquitously in the cytoplasm, its levels are subject to transcriptional and post-transcriptional regulation. RIG-I belongs to the IFN-stimulated gene (ISG) family, but certain cells regulate its expression through IFN-independent mechanisms. Several lines of evidence indicate that deregulated RIG-I signaling is associated with autoimmune disorders. Further studies suggest that RIG-I has functions in addition to those directly related to its role in RNA sensing and host defense. We have much to learn and discover regarding this interesting cytoplasmic sensor so that we can capitalize on its properties for the treatment of viral infections, immune disorders, cancer, and perhaps other conditions.

Pregnancy and interferon tau regulate DDX58 and PLSCR1 in the ovine uterus during the peri-implantation period

Interferon τ (IFNT), the pregnancy recognition signal in ruminants, abrogates the luteolytic mechanism for maintenance of the corpus luteum for production of progesterone (P(4)). This study examined the expression of DEAD (Asp-Glu-Ala-Asp) box polypeptide 58 (DDX58) and phospholipid scramblase 1 (PLSCR1) mRNAs in the ovine uterus as these genes were increased most in 2fTGH (STAT1 positive) cells by IFNT. The results of this study indicated that IFNT regulates expression of DDX58 and PLSCR1 mRNAs in the ovine uterus, which confirmed the results of the in vitro transcriptional profiling experiment with the 2fTGH (parental STAT1 positive) and U3A (STAT1 null) cell lines. Steady-state levels of DDX58 and PLSCR1 mRNAs increased in cells of the ovine uterus between days 12 and 20 of pregnancy, but not between days 10 and 16 of the estrous cycle. The expression of DDX58 and PLSCR1 mRNAs was greatest in endometrial stromal cells, but there was transient expression in uterine luminal and superficial glandular epithelial cells. P(4) alone did not induce expression of DDX58 and PLSCR1 mRNAs; however, intrauterine injections of IFNT did induce expression of DDX58 and PLSCR1 mRNAs in the endometria of nonpregnant ewes independent of effects of P(4). These results indicate that IFNT induces expression of DDX58 and PLSCR1 in ovine endometrial cells via the classical STAT1-mediated cell signaling pathway. Based on their known biological effects, DDX58 and PLSCR1 are IFN-stimulated genes, which may increase the antiviral status of cells of the pregnant uterus to protect against viral infection and/or enhance secretion of type I IFNs that inhibit viral replication.

Characterization of retinoic acid-inducible gene-I expression in primary murine glia following exposure to vesicular stomatitis virus

Vesicular stomatitis virus (VSV) is a negative-sense single-stranded RNA virus that closely resembles its deadly cousin, rabies virus. In mice, VSV elicits a rapid and severe T cell-independent encephalitis, indicating that resident glial cells play an important role in the initiation of central nervous system (CNS) inflammation. Recently, retinoic acid-inducible gene I (RIG-I)-like helicases have been shown to function as intracellular pattern recognition receptors for replicative viral RNA motifs. In the present study, we demonstrate that the expression of two members of this RIG-I-like receptor family (RLR), RIG-I and melanoma differentiation-associated antigen 5 (MDA5), are elevated in mouse brain tissue following intranasal administration of VSV. Using isolated cultures of primary murine glial cells, we demonstrate that microglia and astrocytes constitutively express both RIG-I and MDA5 transcripts and protein. Importantly, we show that such expression is elevated following challenge with VSV or another negative-sense RNA virus, Sendai virus. The authors provide evidence that such induction is indirect and secondary to the production of soluble mediators by infected cells. Circumstantial evidence for the functional nature of RLR expression in glial cells comes from the observation that microglia express the RLR downstream effector molecule, interferon promoter stimulator-1, and demonstrate diminished levels of the negative RLR regulator, laboratory of genetics and physiology 2, following viral challenge. These findings raise the exciting possibility that RLR molecules play important roles in the detection of viral CNS pathogens and the initiation of protective immune responses or, alternatively, the progression of damaging inflammation within the brain.

An essential role for RIG-I in toll-like receptor-stimulated phagocytosis

Retinoic acid-inducible gene-I (RIG-I) plays an important role in antiviral response by recognizing double-stranded RNA. Here we demonstrate an unanticipated role of RIG-I in Toll-like receptor (TLR)-stimulated phagocytosis. Stimulation with lipopolysaccharide (LPS), a ligand of TLR4, induced the expression of RIG-I in macrophages. Depletion of RIG-I by RNAi or gene targeting inhibited the LPS-induced phagocytosis of bacteria. Cellular processes involved in phagocytosis, such as small GTPase Cdc42/Rac1 activation, actin polymerization, and actin-regulator Arp2/3 recruitment, were also impaired in RIG-I-deficient macrophages activated by LPS. Moreover, RIG-I(-/-) mice were found to be more susceptible to infection with Escherichia coli as compared to wild-type mice. Thus, the regulatory functions of RIG-I are strikingly broad, including a role not only in antiviral responses but in antibacterial responses as well.

IFN-epsilon mediates TNF-alpha-induced STAT1 phosphorylation and induction of retinoic acid-inducible gene-I in human cervical cancer cells

Retinoic acid inducible gene-I (RIG-I) plays important roles during innate immune responses to viral infections and as a transducer of cytokine signaling. The mechanisms of RIG-I up-regulation after cytokine stimulation are incompletely characterized. It was previously reported that IFN-gamma induces the expression of RIG-I in endothelial cells. In this study, we characterized the mechanism of type I IFN-mediated up-regulation of RIG-I in HeLa cells and found that, in addition to type I IFN, TNF-alpha, a cytokine that regulates innate immune responses, induced expression of RIG-I. To investigate whether TNF-alpha- and type I IFN-mediated up-regulations of RIG-I were causally related, we studied the kinetics of these responses. Our results were consistent with a model in which TNF-alpha functioned upstream of type I IFNs. The ability of TNF-alpha to up-regulate RIG-I required protein synthesis, expression of functional type I IFNRs, and STAT1 signaling. We also found that IFN-epsilon was the only IFN isoform expressed constitutively in HeLa cells and that its expression was up-regulated in response to stimulation with TNF-alpha. The mechanism of up-regulation involved stabilization of IFN-epsilon mRNA in the absence of transcriptional activation. Silencing the expression of IFN-epsilon attenuated STAT1 expression and phosphorylation and inhibited RIG-I expression, providing additional support for the participation of IFN-epsilon upstream of STAT1. Our findings support a sequential mechanism whereby TNF-alpha leads to stabilization of IFN-epsilon mRNA, increased IFN-epsilon synthesis, engagement of type I IFNRs, increased STAT1 expression and phosphorylation, and up-regulation of RIG-I expression. These findings have implications for our understanding of the immune responses that follow cytokine stimulation.

Expression of retinoic acid-inducible gene-I (RIG-I) in macrophages: possible involvement of RIG-I in atherosclerosis

AIM

Retinoic acid-inducible gene-I (RIG-I) is one of the genes induced by interferon (IFN)-gamma which plays an important role in atherosclerosis. The aim of this study is to examine if RIG-I is involved in atherosclerosis.

METHODS

The expression of RIG-I in atherosclerotic lesions in human aorta was examined by immunohistochemical analysis. The expression of RIG-I in THP-1 monocytic cell line or human monocyte-derived macrophages was studied by western blot and RT-PCR analyses.

RESULTS

Intense immunoreactivity for RIG-I was detected in intimal macrophages in atherosclerotic lesions. IFN-gamma slightly enhanced the RIG-I expression in THP-1 cells. Treatment of the cells with phorbol 12-myristate 13-acetate, which induces the differentiation of the cells into macrophage-like cells, significantly enhanced the IFN-gamma -induced RIG-I expression. IFN-gamma also stimulated the expression of RIG-I in monocyte-derived macrophages.

CONCLUSION

These results suggest that RIG-I may be involved in differentiation and activation of macrophages, playing a role in atherosclerosis.

Interleukin-1 induces the expression of vascular endothelial growth factor in human pericardial mesothelial cells

Vascular endothelial growth factor (VEGF) is a mitogen for endothelial cells. We have studied the production of VEGF by human pericardial mesothelial cells. Mesothelial cells were separated by scraping the pericardial surface during cardiac surgery and cultured. When stimulated with interleukin (IL)-1alpha, pericardial mesothelial cells expressed VEGF mRNA and protein in concentration- and time-dependent manners. Hypoxia was also found to enhance mesothelial VEGF mRNA expression. The cells expressed mRNA for Flt-1 (VEGF receptor 1) and Flk-1 (VEGF receptor 2), and exogenous VEGF was found to have migration-promoting activity on cultured cells. We conclude that pericardial mesothelial cells express VEGF, which may serve as an autocrine growth-regulatory mechanism.

Retinoic acid-inducible gene-I mediates RANTES/CCL5 expression in U373MG human astrocytoma cells stimulated with double-stranded RNA

Retinoic acid-inducible gene-I (RIG-I) mediates part of the cell signaling in response to viral infection. Polyinosinic-polycytidilic acid (poly IC) is a synthetic double-stranded RNA (dsRNA) and mimics viral infection when applied to cell cultures. The CC chemokine, RANTES (regulated on activation, normal T-cell expressed and secreted), is a potent attractant for inflammatory cells such as memory T-lymphocytes, monocytes and eosinophils. In the present study, we demonstrated that poly IC enhances the expression of RIG-I in U373MG human astrocytoma cells. The RNA interference of RIG-I resulted in the suppression of the poly IC-induced RANTES expression. Pretreatment of the cells with SB203580, an inhibitor of p38 mitogen-activated protein kinase, and dexamethasone inhibited the poly IC-induced expression of RIG-I. Furthermore, poly IC upregulated RIG-I in normal human astrocytes in culture and the in vivo injection of poly IC into the striatum of the mouse brain induced the expression of RIG-I in astrocytes. We conclude that RIG-I may be involved in immune reactions against viral infection, at least in part, through the regulation of RANTES expression in astrocytes.

Cytokine modulation of retinoic acid-inducible gene-I (RIG-I) expression in human epidermal keratinocytes

BACKGROUND

Retinoic acid-inducible gene-I (RIG-I) is a member of the DExH/D box family proteins and designated as a putative RNA helicase, which plays various roles in gene expression and cellular functions in response to a variety of RNA viruses.

OBJECTIVE

The present study was designed to investigate the effects of interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha on RIG-I expression in human keratinocytes, and the expression of RIG-I in skin lesions of psoriasis vulgaris, in which IFN-gamma and TNF-alpha are considered to be involved in its pathogenesis.

METHODS

The mRNA and protein expression of RIG-I was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting. Immunohistochemical staining of RIG-I was examined on psoriatic skin section.

RESULTS

The levels of RIG-I mRNA and protein were upregulated in HaCaT keratinocytes in the presence of IFN-gamma or TNF-alpha. Immunohistochemically, RIG-I was detected in the cytoplasm of the spinous and basal layers of psoriatic skin.

CONCLUSION

Our results suggest that RIG-I might operate not only as a RNA helicase but also as a mediator of the cytokine network in the inflammatory skin diseases, such as psoriasis vulgaris.

Retinoic acid-inducible gene-I is induced in gingival fibroblasts by lipopolysaccharide or poly IC: possible roles in interleukin-1beta, -6 and -8 expression

Retinoic acid-inducible gene-I (RIG-I) is a member of the DExH family of proteins, and little is known of its biological function in the oral region. We previously reported that interleukin 1beta (IL-1beta) induced RIG-I expression in gingival fibroblasts. In this study, we studied the mechanism of RIG-I expression induced by lipopolysaccharide (LPS) or double-stranded RNA (dsRNA) in gingival fibroblasts. We also addressed the role of RIG-I in the expression of IL-1beta, IL-6 and IL-8 in gingival fibroblasts stimulated with LPS or dsRNA. We stimulated cultured human gingival fibroblasts with LPS or dsRNA, and examined the expression of RIG-I mRNA and protein. The effect of cycloheximide, a protein synthesis inhibitor, on RIG-I induction by these stimuli was examined. The expression of IL-1beta, IL-6 and IL-8 in gingival fibroblasts transfected with RIG-I cDNA stimulated with LPS or dsRNA was examined. LPS or dsRNA induced the expression of mRNA and protein for RIG-I in concentration- and time-dependent manners. We also examined the localization of RIG-I, and found that it was expressed in cytoplasm. Cycloheximide did not suppress the LPS or dsRNA-induced RIG-I expression. Introduction of RIG-I cDNA into gingival fibroblasts resulted in enhanced expression of IL-1beta, IL-6 and IL-8; moreover, overexpression of RIG-I stimulated with LPS or dsRNA synergistically increased expression of IL-1beta, IL-6 and IL-8. RIG-I may have important roles in the innate immune response in the regulation of IL-1beta, IL-6 and IL-8 expression in gingival fibroblasts in response to LPS and dsRNA.

The RNA helicase Lgp2 inhibits TLR-independent sensing of viral replication by retinoic acid-inducible gene-I

The paramyxovirus Sendai (SV), is a well-established inducer of IFN-alphabeta gene expression. In this study we show that SV induces IFN-alphabeta gene expression normally in cells from mice with targeted deletions of the Toll-IL-1 resistance domain containing adapters MyD88, Mal, Toll/IL-1R domain-containing adaptor inducing IFN-beta (TRIF), and TRIF-related adaptor molecule TLR3, or the E3 ubiquitin ligase, TNFR-associated factor 6. This TLR-independent induction of IFN-alphabeta after SV infection is replication dependent and mediated by the RNA helicase, retinoic acid-inducible gene-I (RIG-I) and not the related family member, melanoma differentiation-associated gene 5. Furthermore, we characterize a RIG-I-like RNA helicase, Lgp2. In contrast to RIG-I or melanoma differentiation-associated gene 5, Lgp2 lacks signaling caspase recruitment and activation domains. Overexpression of Lgp2 inhibits SV and Newcastle disease virus signaling to IFN-stimulated regulatory element- and NF-kappaB-dependent pathways. Importantly, Lgp2 does not prevent TLR3 signaling. Like RIG-I, Lgp2 binds double-stranded, but not single-stranded, RNA. Quantitative PCR analysis demonstrates that Lgp2 is present in unstimulated cells at a lower level than RIG-I, although both helicases are induced to similar levels after virus infection. We propose that Lgp2 acts as a negative feedback regulator of antiviral signaling by sequestering dsRNA from RIG-I.