mda-5: An interferon-inducible putative RNA helicase with double-stranded RNA-dependent ATPase activity and melanoma growth-suppressive properties

TLR signaling

Mammalian Toll-like receptors (TLRs) play a critical role in detection of invading pathogens as well as triggering of subsequent inflammatory and immune responses. Each TLR recognizes distinct microbial components and activates different signaling pathways by selective utilization of adaptor molecules. The signaling via TLRs is delivered from the cell surface and/or the endosome. Recently, the intracytoplasmic detection system of microbes has been identified in mammals as well. Peptidoglycan breakdown products and double-stranded RNA are sensed by NOD family and RNA helicase domain containing proteins, respectively. Thus, mammals make use of both receptor-type and intracellular proteins as detectors of invading pathogens.

mda-7/IL-24: multifunctional cancer-specific apoptosis-inducing cytokine

"Differentiation therapy" provides a unique and potentially effective, less toxic treatment paradigm for cancer. Moreover, combining "differentiation therapy" with molecular approaches presents an unparalleled opportunity to identify and clone genes mediating cancer growth control, differentiation, senescence, and programmed cell death (apoptosis). Subtraction hybridization applied to human melanoma cells induced to terminally differentiate by treatment with fibroblast interferon (IFN-beta) plus mezerein (MEZ) permitted cloning of melanoma differentiation associated (mda) genes. Founded on its novel properties, one particular mda gene, mda-7, now classified as a member of the interleukin (IL)-10 gene family (IL-24) because of conserved structure, chromosomal location, and cytokine-like properties has become the focus of attention of multiple laboratories. When administered by transfection or adenovirus-transduction into a spectrum of tumor cell types, melanoma differentiation associated gene-7/interleukin-24 (mda-7/IL-24) induces apoptosis, whereas no toxicity is apparent in normal cells. mda-7/IL-24 displays potent "bystander antitumor" activity and also has the capacity to enhance radiation lethality, to induce immune-regulatory activities, and to inhibit tumor angiogenesis. Based on these remarkable attributes and effective antitumor therapy in animal models, this cytokine has taken the important step of entering the clinic. In a Phase I clinical trial, intratumoral injections of adenovirus-administered mda-7/IL-24 (Ad.mda-7) was safe, elicited tumor-regulatory and immune-activating processes, and provided clinically significant activity. This review highlights our current understanding of the diverse activities and properties of this novel cytokine, with potential to become a prominent gene therapy for cancer.

PRRs in pathogen recognition

The innate immune system provides the first line of host defense against invading microorganisms before the development of adaptive immune responses. Innate immune responses are initiated by germline-encoded pattern recognition receptors (PRRs), which recognize specific structures of microorganisms. Toll-like receptors (TLRs) are pattern-recognition receptors that sense a wide range of microorganisms, including bacteria, fungi, protozoa and viruses. TLRs exist either on the cell surface or in the lysosome/endosome compartment and induce innate immune responses. Recently, cytoplasmic PRRs have been identified which detect pathogens that have invaded the cytosol. This review focuses on the pathogen recognition of PRRs in innate immunity.

Type I Inteferon Gene Induction by the Interferon Regulatory Factor Family of Transcription Factors

Induction of type I interferons (IFNs) by viruses and other pathogens is crucial for innate immunity, and it is mediated by the activation of pattern-recognition receptors, such as Toll-like receptors and cytosolic receptors such as RIG-I and MDA5. The type I IFN induction is primarily controlled at the gene transcriptional level, wherein a family of transcription factors, interferon regulatory factors (IRFs), plays central roles. Here, we summarize the recent studies on IRFs, providing a paradigm of how genes are ingeniously regulated during immune responses. We also consider some evolutional aspects on the IFN-IRF system.

Role of RNA helicases in HIV-1 replication

Viruses are replication competent genomes which are relatively gene-poor. Even the largest viruses (i.e. Herpesviruses) encode only slightly >200 open reading frames (ORFs). However, because viruses replicate obligatorily inside cells, and considering that evolution may be driven by a principle of economy of scale, it is reasonable to surmise that many viruses have evolved the ability to co-opt cell-encoded proteins to provide needed surrogate functions. An in silico survey of viral sequence databases reveals that most positive-strand and double-stranded RNA viruses have ORFs for RNA helicases. On the other hand, the genomes of retroviruses are devoid of virally-encoded helicase. Here, we review in brief the notion that the human immunodeficiency virus (HIV-1) has adopted the ability to use one or more cellular RNA helicases for its replicative life cycle.

Dissociation of a MAVS/IPS-1/VISA/Cardif-IKKepsilon molecular complex from the mitochondrial outer membrane by hepatitis C virus NS3-4A proteolytic cleavage

Intracellular RNA virus infection is detected by the cytoplasmic RNA helicase RIG-I that plays an essential role in signaling to the host antiviral response. Recently, the adapter molecule that links RIG-I sensing of incoming viral RNA to downstream signaling and gene activation events was characterized by four different groups; MAVS/IPS-1-1/VISA/Cardif contains an amino-terminal CARD domain and a carboxyl-terminal mitochondrial transmembrane sequence that localizes to the mitochondrial membrane. Furthermore, the hepatitis C virus NS3-4A protease complex specifically targets MAVS/IPS-1/VISA/Cardif for cleavage as part of its immune evasion strategy. With a novel search program written in python, we also identified an uncharacterized protein, KIAA1271 (K1271), containing a single CARD-like domain at the N terminus and a Leu-Val-rich C terminus that is identical to that of MAVS/IPS-1/VISA/Cardif. Using a combination of biochemical analysis, subcellular fractionation, and confocal microscopy, we now demonstrate that NS3-4A cleavage of MAVS/IPS-1/VISA/Cardif/K1271 results in its dissociation from the mitochondrial membrane and disrupts signaling to the antiviral immune response. Furthermore, virus-induced IKKepsilon kinase, but not TBK1, colocalized strongly with MAVS at the mitochondrial membrane, and the localization of both molecules was disrupted by NS3-4A expression. Mutation of the critical cysteine 508 to alanine was sufficient to maintain mitochondrial localization of MAVS/IPS-1/VISA/Cardif and IKKepsilon in the presence of NS3-4A. These observations provide an outline of the mechanism by which hepatitis C virus evades the interferon antiviral response.

TLR signaling

The Toll-like receptor (TLR) family plays an instructive role in innate immune responses against microbial pathogens, as well as the subsequent induction of adaptive immune responses. TLRs recognize specific molecular patterns found in a broad range of microbial pathogens such as bacteria and viruses, triggering inflammatory and antiviral responses and dendritic cell maturation, which result in the eradication of invading pathogens. Individual TLRs interact with different combinations of adapter proteins and activate various transcription factors such as nuclear factor (NF)-kappaB, activating protein-1 and interferon regulatory factors, driving a specific immune response. This review outlines the recent advances in our understanding of TLR-signaling pathways and their roles in immune responses. Further, we also discuss a new concept of TLR-independent mechanisms for recognition of microbial pathogens.

Promoters of type I interferon genes from Atlantic salmon contain two main regulatory regions

Recognition of viral nucleic acids by vertebrate host cells results in the synthesis and secretion of type I interferons (IFN-alpha/beta), which induce an antiviral state in neighboring cells. We have cloned the genes and promoters of two type I IFNs from Atlantic salmon. Both genes have the potential to encode IFN transcripts with either a short or a long 5'-untranslated region, apparently controlled by two distinct promoter regions, PR-I and PR-II, respectively. PR-I is located within 116 nucleotides upstream of the short transcript and contains a TATA-box, two interferon regulatory factor (IRF)-binding motifs, and a putative nuclear factor kappa B (NFkappaB)-binding motif. PR-II is located 469-677 nucleotides upstream of the short transcript and contains three or four IRF-binding motifs and a putative ATF-2/c-Jun element. Complete and truncated versions of the promoters were cloned in front of a luciferase reporter gene and analyzed for promoter activity in salmonid cells. Constructs containing PR-I were highly induced after treatment with the dsRNA poly(I:C), and promoter activity appeared to be dependent on NFkappaB. In contrast, constructs containing exclusively PR-II showed poor poly(I:C)-inducible activity. PR-I is thus the main control region for IFN-alpha/beta synthesis in salmon. Two pathogenic RNA viruses, infectious pancreatic necrosis virus and infectious salmon anemia virus, were tested for their ability to stimulate the minimal PR-I, but only the latter was able to induce promoter activity. The established IFN promoter-luciferase assay will be useful in studies of host-virus interactions in Atlantic salmon, as many viruses are known to encode proteins that prevent IFN synthesis by inhibition of promoter activation.

Recruitment of an interferon molecular signaling complex to the mitochondrial membrane: disruption by hepatitis C virus NS3-4A protease

Recent advances in the understanding of the signaling pathways leading to the host antiviral response to hepatitis C virus (HCV), the mechanisms used by HCV to evade the immune response, and the development of small molecule inhibitors of HCV have generated optimism that novel therapeutic approaches to control HCV disease may soon be available. HCV infection is detected by the cytoplasmic, RNA helicase RIG-I that plays an essential role in signaling to the host antiviral response. Recently the adapter molecule that links RIG-I sensing of incoming viral RNA to downstream signaling and gene activation events was characterized by four different groups: MAVS/IPS-1-1/VISA/Cardif contains an amino-terminal CARD domain and carboxyl-terminal mitochondrial transmembrane sequence that localizes to the mitochondrial membrane. Furthermore, the hepatitis C virus NS3-4A protease complex specifically targets MAVS/IPS-1/VISA/Cardif for cleavage as part of its immune evasion strategy. Using a combination of biochemical analysis, subcellular fractionation and confocal microscopy, we demonstrate that: (1) NS3-4A cleavage of MAVS/IPS-1/VISA/Cardif causes relocation from the mitochondrial membrane to the cytosolic fraction, resulting in disruption of signaling to the antiviral immune response; (2) disruption requires a function NS3-4A protease; (3) a point mutant of MAVS/IPS-1/VISA/Cardif (Cys508Ala) is not cleaved from the mitochondria by active protease; and (4) the virus-induced IKK epsilon kinase, but not TBK1, co-localizes strongly with MAVS at the mitochondrial membrane and the localization of both molecules is disrupted by NS3-4A expression. These observations provide an outline of the mechanism by which HCV evades the IFN antiviral response.

The interferon-inducible RNA helicase, mda-5, is involved in measles virus-induced expression of antiviral cytokines

Activation of host cell antiviral responses is mediated by receptors detecting the presence of viruses. Here we have studied the role of double-stranded RNA (dsRNA) binding molecules melanoma differentiation-associated gene 5 (mda-5), retinoic acid inducible gene I (RIG-I), and Toll-like receptor 3 (TLR3) in measles virus (MV)-induced expression of antiviral cytokines and chemokines in human A549 lung epithelial cells and human umbilical vein endothelial cells (HUVECs). We show that MV infection results in the activation of mda-5, RIG-I, and TLR3 gene expression that is followed by high expression of interferon (IFN)-beta, interleukin (IL)-28 and IL-29, CCL5, and CXCL10 genes. We also demonstrate that IFN-alpha and IFN-beta upregulate mda-5, RIG-I, and TLR3 gene expression in epithelial and endothelial cell lines. Forced expression of mda-5, but not that of RIG-I or TLR3, leads to enhanced IFN-beta promoter activity in MV-infected A549 cells. Our results suggest that IFN-inducible mda-5 is involved in MV-induced expression of antiviral cytokines.

Retinoic acid inducible gene-I and mda-5 are involved in influenza A virus-induced expression of antiviral cytokines

Activation of host cell antiviral responses is mediated by pattern recognition receptors. Cytoplasmic RNA helicases, retinoic acid inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (mda-5) have been identified to function as receptors for double-stranded RNA. Here we show that interferon (IFN)-alpha pretreatment enhances influenza A virus-induced expression of IFN-alpha, IFN-beta, interleukin (IL)-28 and IL-29 genes in human dendritic cells and epithelial cell lines. Both IFN-alpha and IFN-beta strongly enhanced RIG-I and mda-5 mRNA and protein expression in these cell types. Expression of RIG-I and mda-5 gene constructs, but not that of TLR3, lead to a dramatic enhancement of IFN-beta promoter driven transcription in influenza A virus-infected epithelial cells. Furthermore, dominant negative RIG-I gene construct inhibited influenza A virus-induced IFN-beta promoter activity. In conclusion, our results show that in epithelial cells influenza A virus-induced antiviral cytokine gene expression is triggered by RIG-I and mda-5, whose expression is positively regulated by IFN-alpha.

TLRs, NLRs and RLRs: a trinity of pathogen sensors that co-operate in innate immunity

Significant advances in our understanding of innate immunity have been made following the identification of three families of pathogen sensors: Toll-like receptors (TLRs), NOD-like receptors (NLRs) and RIG-I-like receptors (RLRs). Members of the TLR family recognize bacteria, viruses, fungi and protozoa; NLRs with known functions detect bacteria, and RLRs are anti-viral. It is likely that interplay between these families ensures the efficient co-ordination of innate immune responses, through either synergistic or co-operative signalling. Important interactions occur between TLRs and certain NLRs for inducing the pro-inflammatory cytokine interleukin (IL)-1beta. TLRs induce pro-IL-1beta production and prime NLR-containing multi-protein complexes, termed "inflammasomes", to respond to bacterial products and products of damaged cells. This results in caspase-1 activation and the subsequent processing of pro-IL-1beta to its active form. In this article, we hypothesize that during the first phase of the host response to infection, an important interplay occurs between these families, providing a substantial combinatorial repertoire in innate immunity.

Toll-like receptors and RNA helicases: two parallel ways to trigger antiviral responses

The early detection by the host of invading microorganisms, including viruses, depends on a limited number of specific receptors that recognize pathogen-associated molecular patterns (PAMPs). A few of these PAMPs, including ssRNA and dsRNA, are recognized by Toll-like receptors (TLR)-7/8 and TLR3, respectively. Activation of an antiviral TLR-dependent signaling cascade leads to the activation of the key transcription factors IRF and NF-kappaB, which promote antiviral responses through induction of specific genes. Recently, a second system has been described, which relies on the cytoplasmic recognition of dsRNA by RNA helicases such as RIG-I. In this review, we discuss the mechanistic aspects of these important arms of the host innate response to dsRNA and a few viral strategies utilized to counteract them.

The role of Toll-like receptors in immune disorders

Toll-like receptors (TLRs) play an important role in innate immunity. Individual TLRs recognise microbial components that are conserved among pathogens. Such recognition initiates necessary inflammatory immune responses and induces subsequent activation of adaptive immunity. Studies in people with polymorphisms in genes encoding TLR signalling can elucidate the relationship between TLRs and human diseases, such as infectious diseases, atherosclerosis and immunodeficiency. Indeed, accumulating data in respect to TLR signalling suggest that TLRs are closely related with the pathogenesis of autoimmune diseases. This review looks at the role of TLRs in various immune disorders, and discusses the pathogenesis of diseases.

Polynucleotide phosphorylase: an evolutionary conserved gene with an expanding repertoire of functions

RNA metabolism plays a seminal role in regulating diverse physiological processes. Polynucleotide phosphorylase (PNPase) is an evolutionary conserved 3',5' exoribonuclease, which plays a central role in RNA processing in bacteria and plants. Human polynucleotide phosphorylase (hPNPase old-35) was cloned using an inventive strategy designed to identify genes regulating the fundamental physiological processes of differentiation and senescence. Although hPNPase old-35 structurally and biochemically resembles PNPase of other species, targeted overexpression and inhibition studies reveal that hPNPase old-35 has evolved to serve more specialized functions in humans. The present review provides a global perspective on the structure and function of PNPase and then focuses on hPNPase old-35 in the contexts of differentiation and senescence.

Neonatal exposure to diethylstilbestrol alters the expression of DNA methyltransferases and methylation of genomic DNA in the epididymis of mice

Fetal and neonatal exposure to diethylstilbestrol (DES) is known to cause many abnormalities, such as cancer, in the male and female reproductive tracts later in life, and epigenetic mechanisms, such as DNA methylation, may be involved in these processes. In the present study, newborn C57BL/6 male mice were exposed to 3 mug of DES from postnatal days 1 to 5. Subsequently, the expression levels of the DNA methyltransferases Dnmt1, Dnmt3a and Dnmt3b and the transcription factors Sp1 and Sp3, which have been reported to regulate the expression of Dnmts, were examined at days 5, 14 and 30. Furthermore, restriction landmark genomic scanning (RLGS), which can analyze genome-wide DNA methylation, was performed to clarify whether or not aberrant DNA methylation was present in the epididymis of the DES-treated mice at day 30. Increased expression of Dnmt3b was observed at days 5 and 14, followed by increased expression of Dnmt1 and Dnmt3a at day 30, as evaluated by real-time RT-PCR. The expression of Sp1 was also increased at day 30. The RLGS analysis revealed that 7 loci of the genomic DNA were demethylated and 1 locus was methylated in the epididymis of the DES-treated mice. Four of these loci specifically demethylated in DES-treated mice were cloned, and all were found to be located within CpG islands near genes. In conclusion, our results indicated the possibility that DES-induced abnormalities of reproductive organs are associated with altered expression levels of DNA methyltransferases and DNA methylation.

Role of the alpha/beta interferon response in the acquisition of susceptibility to poliovirus by kidney cells in culture

Replication of poliovirus (PV) is restricted to a few sites, including the brain and spinal cord. However, this neurotropism is not conserved in cultured cells. Monkey kidney cells become susceptible to PV infection after cultivation in vitro, and cell lines of monolayer cultures from almost any tissue of primates are susceptible to PV infection. These observations suggest that cellular changes during cultivation are required for acquisition of susceptibility. The molecular basis for the cellular changes during this process is not known. We investigated the relationship between PV susceptibility and interferon (IFN) response in primary cultured kidney and liver cells derived from transgenic mice expressing human PV receptor and in several primate cell lines. Both kidneys and liver in vivo showed rapid IFN response within 6 h postinfection. However, monkey and mouse kidney cells in culture and primate cell lines, which were susceptible to PV, did not show such rapid response or showed no response at all. On the other hand, primary cultured liver cells, which were partially resistant to infection, showed rapid IFN induction. The loss of IFN inducibility in kidney cells was associated with a decrease in expression of IFN-stimulated genes involved in IFN response. Mouse kidney cells pretreated with a small dose of IFN, in turn, restored IFN inducibility and resistance to PV. These results strongly suggest that the cells in culture acquire PV susceptibility during the process of cultivation by losing rapid IFN response that has been normally maintained in extraneural tissues in vivo.

Essential role of mda-5 in type I IFN responses to polyriboinosinic:polyribocytidylic acid and encephalomyocarditis picornavirus

The innate immune system recognizes viral dsRNA through two distinct pathways; the Toll-like receptor 3 (TLR3) pathway detects dsRNA phagocytosed in endosomes; the helicases retinoic acid-induced protein I (RIG-I) and melanoma differentiation-associated gene-5 (mda-5) detect cytoplasmic dsRNA generated during viral replication. Both RIG-I and mda-5 can bind polyriboinosinic:polyribocytidylic acid (polyI:C), the synthetic analog of viral dsRNA, and mediate type I IFN responses to polyI:C and multiple RNA viruses in vitro. We generated mda-5-deficient mice and showed that mda-5 is the dominant receptor mediating type I IFN secretion in response to polyI:C in vitro and in vivo. Moreover, mda-5-/- mice exhibited a selectively impaired antiviral response to encephalomyocarditis picornavirus, indicating functional specialization of mda-5 in vivo.

Differential roles of MDA5 and RIG-I helicases in the recognition of RNA viruses

The innate immune system senses viral infection by recognizing a variety of viral components (including double-stranded (ds)RNA) and triggers antiviral responses. The cytoplasmic helicase proteins RIG-I (retinoic-acid-inducible protein I, also known as Ddx58) and MDA5 (melanoma-differentiation-associated gene 5, also known as Ifih1 or Helicard) have been implicated in viral dsRNA recognition. In vitro studies suggest that both RIG-I and MDA5 detect RNA viruses and polyinosine-polycytidylic acid (poly(I:C)), a synthetic dsRNA analogue. Although a critical role for RIG-I in the recognition of several RNA viruses has been clarified, the functional role of MDA5 and the relationship between these dsRNA detectors in vivo are yet to be determined. Here we use mice deficient in MDA5 (MDA5-/-) to show that MDA5 and RIG-I recognize different types of dsRNAs: MDA5 recognizes poly(I:C), and RIG-I detects in vitro transcribed dsRNAs. RNA viruses are also differentially recognized by RIG-I and MDA5. We find that RIG-I is essential for the production of interferons in response to RNA viruses including paramyxoviruses, influenza virus and Japanese encephalitis virus, whereas MDA5 is critical for picornavirus detection. Furthermore, RIG-I-/- and MDA5-/- mice are highly susceptible to infection with these respective RNA viruses compared to control mice. Together, our data show that RIG-I and MDA5 distinguish different RNA viruses and are critical for host antiviral responses.

Activation of Ras/Raf protects cells from melanoma differentiation-associated gene-5-induced apoptosis

Melanoma differentiation-associated gene-5 (mda-5) was the first molecule identified in nature whose encoded protein embodied the unique structural combination of an N-terminal caspase recruitment domain and a C-terminal DExD/H RNA helicase domain. As suggested by its structure, cumulative evidences documented that ectopic expression of mda-5 leads to growth inhibition and/or apoptosis in various cell lines. However, the signaling pathways involved in mda-5-mediated killing have not been elucidated. In this study, we utilized either genetically modified cloned rat embryo fibroblast cells overexpressing different functionally and structurally distinct oncogenes or human pancreatic and colorectal carcinoma cells containing mutant active ras to resolve the role of the Ras/Raf signaling pathway in mda-5-mediated growth inhibition/apoptosis induction. Rodent and human tumor cells containing constitutively activated Raf/Raf/MEK/ERK pathways were resistant to mda-5-induced killing and this protection was antagonized by intervening in this signal transduction cascade either by directly inhibiting ras activity using an antisense strategy or by targeting ras-downstream factors, such as MEK1/2, with the pharmacological inhibitor PD98059. The present findings provide a further example of potential cross-talk between growth-inhibitory and growth-promoting pathways in which the ultimate balance of these factors defines cellular homeostasis, leading to survival or induction of programmed cell death.

Innate immune recognition of viral infection

Induction of the antiviral innate immune response depends on recognition of viral components by host pattern-recognition receptors. Members of the Toll-like receptor family have emerged as key sensors that recognize viral components such as nucleic acids. Toll-like receptor signaling results in the production of type I interferon and inflammatory cytokines and leads to dendritic cell maturation and establishment of antiviral immunity. Cells also express cytoplasmic RNA helicases that function as alternative pattern-recognition receptors through recognition of double-stranded RNA produced during virus replication. These two classes of pattern-recognition receptor molecules are expressed in different intracellular compartments and induce type I interferon responses via distinct signaling pathways.

Pathogen recognition and innate immunity

Microorganisms that invade a vertebrate host are initially recognized by the innate immune system through germline-encoded pattern-recognition receptors (PRRs). Several classes of PRRs, including Toll-like receptors and cytoplasmic receptors, recognize distinct microbial components and directly activate immune cells. Exposure of immune cells to the ligands of these receptors activates intracellular signaling cascades that rapidly induce the expression of a variety of overlapping and unique genes involved in the inflammatory and immune responses. New insights into innate immunity are changing the way we think about pathogenesis and the treatment of infectious diseases, allergy, and autoimmunity.

mda-9/Syntenin: a positive regulator of melanoma metastasis

Metastasis is a significant event in cancer progression and continues to pose the greatest challenge for a cancer cure. Defining genes that control metastasis in vivo may provide new targets for intervening in this process with profound therapeutic implications. Melanoma differentiation associated gene-9 (mda-9) was initially identified by subtraction hybridization as a novel gene displaying biphasic expression during terminal differentiation in human melanoma cells. Mda-9, also known as syntenin, is a PDZ-domain protein overexpressed in many types of human cancers, where it is believed to function in tumor progression. However, a functional role of mda-9/syntenin in tumor growth and metastasis and the signaling pathways involved in mediating these biological activities remain to be defined. Evidence is now provided, using weakly and highly metastatic isogenic melanoma variants, that mda-9/syntenin regulates metastasis. Expression of mda-9/syntenin correlates with advanced stages of melanoma progression. Regulating mda-9/syntenin expression using a replication-incompetent adenovirus expressing either sense or antisense mda-9/syntenin modifies the transformed phenotype and alters metastatic ability in immortal human melanocytes and metastatic melanoma cells in vitro and in vivo in newborn rats. A direct relationship is observed between mda-9/syntenin expression and increased phosphorylation of focal adhesion kinase, c-Jun-NH2-kinase, and p38. This study provides the first direct link between mda-9/syntenin expression and tumor cell dissemination in vivo and indicates that mda-9/syntenin expression activates specific signal transduction pathways, which may regulate melanoma tumor progression. Based on its ability to directly alter metastasis, mda-9/syntenin provides a promising new focus for melanoma cancer research with potential therapeutic applications for metastatic diseases.

Is mda-7/IL-24 a "magic bullet" for cancer?

The "holy grail" of cancer therapy is to identify and exploit genetic elements and signal transduction pathways capable of selectively destroying tumor cells without eliciting harmful effects in normal cells or tissues. To achieve this objective, subtraction hybridization was combined with a "differentiation therapy" model of cancer in which human melanoma cells were induced to revert to a more "normal" state, growth arrest irreversibly, and terminally differentiate by treatment with fibroblast IFN and mezerein. This strategy permitted the cloning of a variety of genes involved in regulating important physiologic processes, including cell cycle, response to cytokines and viruses, tumorigenesis and metastasis, cancer growth control, apoptosis, and senescence. A specific gene, melanoma differentiation-associated gene-7/interleukin-24 (mda-7/IL-24), displaying cancer-specific apoptosis-inducing properties isolated using this scheme has now come into the limelight as a new gene therapy for divergent cancers. Although the mechanism of cancer cell selectivity of mda-7/IL-24 remains to be delineated, numerous attributes enable this gene as an effective therapy for cancer, including an ability to discriminate between normal and cancer cells, induce apoptosis in diverse tumor cells, promote "bystander" antitumor effects, inhibit tumor growth and angiogenesis in animal models, synergize with radiation, and modulate immune responses. These unique features combined with successful transition into the clinic instill confidence that mda-7/IL-24, as a single or more likely as part of a combinatorial approach, may provide profound therapeutic benefit for cancer patients.

CARD games between virus and host get a new player

A growing family of cellular proteins encoding the caspase activation and recruitment domain (CARD) has a crucial role in immunity by sensing virus infection and signaling antiviral immune defenses. Four independent studies have identified a novel CARD-containing protein, variously called IPS-1, MAVS, VISA and Cardif, which is an essential signaling adaptor of the host defense mediating CARD-CARD interactions with retinoic acid inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDAS), sensors of virus infection. Disruption of this novel signaling pathway by hepatitis C virus (HCV) might provide a foundation for viral persistence.

“Endogenous adjuvant” activity of the RNA components of lupus autoantigens Sm/RNP and Ro 60

OBJECTIVE

Most lupus patients produce autoantibodies against small ribonucleoproteins such as Sm/RNP and Ro 60 (containing U1 and Y1-Y5 RNAs, respectively). We undertook this study to investigate whether the RNA components of these antigens, which contain extensive tracts of single- and double-stranded RNA, signatures of viral infection, activate innate immunity.

METHODS

U1 and Y RNAs were affinity purified from K562 cells. Murine bone marrow-derived dendritic cells (DCs), human HEK 293 cells, and murine RAW264.7 cells were stimulated with U1 RNA and other known Toll-like receptor (TLR) ligands. Expression of the interferon (IFN)-inducible gene Mx1 and other genes was quantified using real-time polymerase chain reaction, and cytokine production was measured by enzyme-linked immunosorbent assay. DC maturation was assessed using flow cytometry.

RESULTS

Purified U1 and Y1-Y5 RNAs and synthetic stem-loop II of U1 RNA stimulated type I IFN (IFN-I) production by cell lines and murine bone marrow-derived DCs and promoted DC maturation (CD86 expression). U1 RNA-stimulated, but not TLR-3 ligand-stimulated, IFN-I was blocked by bafilomycin A1, indicating that immunostimulation by U1 RNA requires endosomal acidification. Myeloid differentiation factor 88-deficient cells responded poorly to U1 RNA, suggesting that an endosomal TLR, probably TLR-7, mediates the stimulatory effects of U1 RNA. U1 RNA-induced IFN-I and interleukin-6 production also were protein kinase R (PKR) dependent (abrogated by 2-aminopurine and greatly reduced in PKR-/- cells).

CONCLUSION

We conclude that the RNA components of the Ro 60 (Y1-Y5 RNA) and Sm/RNP (U1 RNA) small ribonucleoproteins act as endogenous adjuvants that could play a role in the pathogenesis of autoimmunity by stimulating DC maturation and IFN-I production.

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.

VSV-tumor selective replication and protein translation

The emergence of vesicular stomatatis virus (VSV) as a potent antitumor agent has made a dissection of the molecular determinants of host-cell permissiveness to this virus an important objective. Such insight would not only enable the intelligent design of future generations of recombinant VSV vectors to combat disease, but may also resolve general features of cellular transformation that may be exploited by this virus, and perhaps other oncolytic viruses. The defective pathways underlining the oncolytic activity of VSV remain to be fully determined but recent data indicates that flaws in innate immune responses, involving the interferon (IFN) system, may commonly occur in tumor cells and thus play a large role in facilitating oncolysis. Aside from the IFN system, however, it is almost certain that other key cellular pathways may be similarly defective and therefore cooperatively contribute towards mediating rapid oncolytic virus activity. Recent data have indicated that defects in cancer cell translational regulation could be one area that may be exploited by VSV. Certainly, all viruses require cellular protein synthesis pathways to facilitate their replication and many have devised numerous mechanisms to ensure that viral mRNAs become translated at the expense of the host. Using VSV as a model, this review will discuss some of the recent developments in the fields of innate immunity and translational regulation that may help explain mechanisms of viral oncolysis.

Role of double-stranded RNA and Npro of classical swine fever virus in the activation of monocyte-derived dendritic cells

Classical swine fever virus (CSFV) is a noncytopathogenic (ncp) positive-sense RNA virus that replicates in myeloid cells including macrophages and dendritic cells (DC). The virus does not induce type I interferon (IFN-alpha/beta), which in macrophages has been related to the presence of the viral Npro gene. In the present work, the role of viral double-stranded (ds)RNA and Npro in the virus-host cell interaction has been analyzed. Higher levels of detectable dsRNA were produced by a genetically engineered cytopathogenic (cp) CSFV compared with ncp CSFV, and cp CSFV induced IFN-alpha/beta in PK-15 cells. With DC, there was only a small difference in the levels of dsRNA between the cp and ncp viruses, and no IFN-alpha/beta was produced. However, the cp virus induced a higher degree of DC maturation, in terms of CD80/86 and MHC II expression. Npro deletion mutants induced an increase in DC maturation and IFN-alpha/beta production-for both ncp and cp viruses-despite reduced replication efficiency in the DC. Deletion of Npro did not influence dsRNA levels, indicating that the interference was downstream of dsRNA turnover regulation. In conclusion, the capacity of CSFV to replicate in myeloid DC, and prevent IFN-alpha/beta induction and DC maturation, requires both regulated dsRNA levels and the presence of viral Npro.

N(pro) of classical swine fever virus is an antagonist of double-stranded RNA-mediated apoptosis and IFN-alpha/beta induction

Classical swine fever virus (CSFV) protects cells from double-stranded (ds) RNA-mediated apoptosis and IFN-alpha/beta induction. This phenotype is lost when CSFV lacks N(pro) (DeltaN(pro) CSFV). In the present study, we demonstrate that N(pro) counteracts dsRNA-mediated apoptosis and IFN-alpha/beta induction independently of other CSFV elements. For this purpose, we generated porcine SK-6 and PK-15 cell lines constitutively expressing N(pro) fused to the enhanced green fluorescent protein (EGFP). The survival of the SK6-EGFP-N(pro) cell line after polyinosinic polycytidylic acid [poly(IC)] treatment was comparable to that of CSFV-infected SK-6 cells and was significantly higher than the survival of the parent cell line. In PK-15 cells, the presence of EGFP-N(pro) prevented the DeltaN(pro) CSFV- and poly(IC)-mediated IFN-alpha/beta production. Importantly, N(pro) also inhibited IFN-alpha and IFN-beta promoter-driven luciferase expression in human cells and blocked IFN-alpha/beta induction mediated by Newcastle disease virus. This establishes a novel function for N(pro) in counteraction of the IFN-alpha/beta induction pathway.

Identification and characterization of MAVS, a mitochondrial antiviral signaling protein that activates NF-kappaB and IRF 3

Viral infection triggers host innate immune responses through activation of the transcription factors NF-kappaB and IRF 3, which coordinately regulate the expression of type-I interferons such as interferon-beta (IFN-beta). Herein, we report the identification of a novel protein termed MAVS (mitochondrial antiviral signaling), which mediates the activation of NF-kappaB and IRF 3 in response to viral infection. Silencing of MAVS expression through RNA interference abolishes the activation of NF-kappaB and IRF 3 by viruses, thereby permitting viral replication. Conversely, overexpression of MAVS induces the expression of IFN-beta through activation of NF-kappaB and IRF 3, thus boosting antiviral immunity. Epistasis experiments show that MAVS is required for the phosphorylation of IRF 3 and IkappaB and functions downstream of RIG-I, an intracellular receptor for viral RNA. MAVS contains an N-terminal CARD-like domain and a C-terminal transmembrane domain, both of which are essential for MAVS signaling. The transmembrane domain targets MAVS to the mitochondria, implicating a new role of mitochondria in innate immunity.

Cardif is an adaptor protein in the RIG-I antiviral pathway and is targeted by hepatitis C virus

Antiviral immunity against a pathogen is mounted upon recognition by the host of virally associated structures. One of these viral 'signatures', double-stranded (ds) RNA, is a replication product of most viruses within infected cells and is sensed by Toll-like receptor 3 (TLR3) and the recently identified cytosolic RNA helicases RIG-I (retinoic acid inducible gene I, also known as Ddx58) and Mda5 (melanoma differentiation-associated gene 5, also known as Ifih1 or Helicard). Both helicases detect dsRNA, and through their protein-interacting CARD domains, relay an undefined signal resulting in the activation of the transcription factors interferon regulatory factor 3 (IRF3) and NF-kappaB. Here we describe Cardif, a new CARD-containing adaptor protein that interacts with RIG-I and recruits IKKalpha, IKKbeta and IKKvarepsilon kinases by means of its C-terminal region, leading to the activation of NF-kappaB and IRF3. Overexpression of Cardif results in interferon-beta and NF-kappaB promoter activation, and knockdown of Cardif by short interfering RNA inhibits RIG-I-dependent antiviral responses. Cardif is targeted and inactivated by NS3-4A, a serine protease from hepatitis C virus known to block interferon-beta production. Cardif thus functions as an adaptor, linking the cytoplasmic dsRNA receptor RIG-I to the initiation of antiviral programmes.

Regulation of polymeric immunoglobulin receptor expression by reovirus

Polymeric immunoglobulin receptor (pIgR) transcytoses dimeric IgA and IgA-coated immune complexes from the lamina propria across epithelia and into secretions. The effect of reovirus infection on regulation of pIgR expression in the human intestinal epithelial cell line HT-29 was characterized in this report. Both replication-competent and UV-inactivated reovirus at m.o.i. equivalents of 1-100 p.f.u. per cell upregulated pIgR mRNA by 24 h post-infection and intracellular pIgR protein was increased at 48 h following exposure to UV-inactivated virus. Binding of virus to HT-29 cells was required, as pre-incubating virus with specific antiserum, but not non-immune serum, inhibited reovirus-mediated pIgR upregulation. Endosomal acidification leading to uncoating of virus is a required step for pIgR upregulation, as ammonium chloride or bafilomycin A1 pre-treatment inhibited virus-induced pIgR upregulation. Inhibition experiments using the calpain inhibitor N-acetyl-leucyl-leucyl-norleucinal suggested that calpains are involved in reovirus-mediated pIgR upregulation. Upregulation of pIgR following virus infection appears to be an innate immune response against invading pathogens that could help the host clear infection effectively. Signalling induced by microbes and their products may serve to augment pIgR-mediated transcytosis of IgA, linking the innate and acquired immune responses to viruses.

Shared and unique functions of the DExD/H-box helicases RIG-I, MDA5, and LGP2 in antiviral innate immunity

The cellular protein retinoic acid-inducible gene I (RIG-I) senses intracellular viral infection and triggers a signal for innate antiviral responses including the production of type I IFN. RIG-I contains a domain that belongs to a DExD/H-box helicase family and exhibits an N-terminal caspase recruitment domain (CARD) homology. There are three genes encoding RIG-I-related proteins in human and mouse genomes. Melanoma differentiation associated gene 5 (MDA5), which consists of CARD and a helicase domain, functions as a positive regulator, similarly to RIG-I. Both proteins sense viral RNA with a helicase domain and transmit a signal downstream by CARD; thus, these proteins share overlapping functions. Another protein, LGP2, lacks the CARD homology and functions as a negative regulator by interfering with the recognition of viral RNA by RIG-I and MDA5. The nonstructural protein 3/4A protein of hepatitis C virus blocks the signaling by RIG-I and MDA5; however, the V protein of the Sendai virus selectively abrogates the MDA5 function. These results highlight ingenious mechanisms for initiating antiviral innate immune responses and the action of virus-encoded inhibitors.

Complete open reading frame (C-ORF) technology: simple and efficient technique for cloning full-length protein-coding sequences

Technical difficulties in full-length cDNA cloning hinder successful characterization of many unknown and potentially novel expressed sequence tags (ESTs). We presently describe improved methods for cDNA cloning. This scheme is based on the polymerase chain reaction and utilizes a degenerate stem-loop annealing primer (dSLAP), consisting of a stem-loop structure followed by 12 random nucleotides, and is called the C-ORF (complete open reading frame) technique. The dSLAP is designed to anneal to first-strand cDNA, while suppressing second-strand synthesis from internal sites because of its bulky stem-loop structure. The C-ORF technique consists of three steps: reverse transcription, dSLAP annealing plus the second-strand synthesis, and PCR amplification. Applications of dSLAP to both known and previously unknown cDNA targets resulted in cloning of their complete open reading frames, in most cases after a single application of the C-ORF method. The currently described protocol is simple and does not require unusual molecular biology reagents, except for reverse transcriptase, Taq polymerase and a DNA primer, which makes it readily amenable for cloning purposes in individual laboratories. Moreover, this approach has wide applicability and in principle can be used to identify the protein-coding region of virtually any gene in which limited or incomplete sequence information is available.

The alpha/beta interferon response controls tissue tropism and pathogenicity of poliovirus

Poliovirus selectively replicates in neurons in the spinal cord and brainstem, although poliovirus receptor (PVR) expression is observed in both the target and nontarget tissues in humans and transgenic mice expressing human PVR (PVR-transgenic mice). We assessed the role of alpha/beta interferon (IFN) in determining tissue tropism by comparing the pathogenesis of the virulent Mahoney strain in PVR-transgenic mice and PVR-transgenic mice deficient in the alpha/beta IFN receptor gene (PVR-transgenic/Ifnar knockout mice). PVR-transgenic/Ifnar knockout mice showed increased susceptibility to poliovirus. After intravenous inoculation, severe lesions positive for the poliovirus antigen were detected in the liver, spleen, and pancreas in addition to the central nervous system. These results suggest that the alpha/beta IFN system plays an important role in determining tissue tropism by protecting nontarget tissues that are potentially susceptible to infection. We subsequently examined the expression of IFN and IFN-stimulated genes (ISGs) in the PVR-transgenic mice. In the nontarget tissues, ISGs were expressed even in the noninfected state, and the expression level increased soon after poliovirus infection. On the contrary, in the target tissues, ISG expression was low in the noninfected state and sufficient response after poliovirus infection was not observed. The results suggest that the unequal IFN response is one of the important determinants for the differential susceptibility of tissues to poliovirus. We consider that poliovirus replication was observed in the nontarget tissues of PVR-transgenic/Ifnar knockout mice because the IFN response was null in all tissues.

The V proteins of paramyxoviruses bind the IFN-inducible RNA helicase, mda-5, and inhibit its activation of the IFN-beta promoter

Most paramyxoviruses circumvent the IFN response by blocking IFN signaling and limiting the production of IFN by virus-infected cells. Here we report that the highly conserved cysteine-rich C-terminal domain of the V proteins of a wide variety of paramyxoviruses binds melanoma differentiation-associated gene 5 (mda-5) product. mda-5 is an IFN-inducible host cell DExD/H box helicase that contains a caspase recruitment domain at its N terminus. Overexpression of mda-5 stimulated the basal activity of the IFN-beta promoter in reporter gene assays and significantly enhanced the activation of the IFN-beta promoter by intracellular dsRNA. Both these activities were repressed by coexpression of the V proteins of simian virus 5, human parainfluenza virus 2, mumps virus, Sendai virus, and Hendra virus. Similar results to the reporter assays were obtained by measuring IFN production. Inhibition of mda-5 by RNA interference or by dominant interfering forms of mda-5 significantly inhibited the activation of the IFN-beta promoter by dsRNA. It thus appears that mda-5 plays a central role in an intracellular signal transduction pathway that can lead to the activation of the IFN-beta promoter, and that the V proteins of paramyxoviruses interact with mda-5 to block its activity.

The origin of polynucleotide phosphorylase domains

In this report, we document the presence of polynucleotide phosphorylase (PNPase) in the animal eukaryotes. These proteins contain several domains, including 2 RNase PH domains (PNPase 1 and PNPase 2) which are closely related functionally and in sequence similarity to ribonuclease PH (RPH) protein. Phylogenetic analysis of the gene genealogy of these three domains suggests that PNPase was formed via a duplication event that also produced the RNase PH protein. Given the current distribution of these domains in the tree of life, these duplication events most likely occurred in the common ancestor of the three organismal superkingdoms, Archaea, Eukarya, and Bacteria. In particular, PNPase 2 and RPH are more closely related to each other than either one is to PNPase 1, suggesting a deeper differentiation of PNPase 1 in the common organismal ancestor. In addition, while PNPase 1 and PNPase 2 appear to have the same evolutionary signal as determined by the incongruence length difference (ILD) test, RPH appears to have an incongruent signal with both of the PNPase domains. This result suggests that RPH experienced different evolutionary divergence patterns than the PNPase domains, consistent with the linked nature of the two PNPase domains.

Upregulation of retinoic acid-inducible gene-I in T24 urinary bladder carcinoma cells stimulated with interferon-gamma

Urinary bladder epithelial cells play an important role in the host defense against urinary tract infections. Interferon-gamma (IFN-gamma) is a potent cytokine that regulates immune responses by inducing multiple genes in many types of cells including urinary bladder epithelial cells. Retinoic acid-inducible gene-I (RIG-I) is a member of the DExH-box family, which is involved in various reactions related to RNA metabolism, and is induced in leukemic cells by retinoic acid or in endothelial cells by lipopolysaccharide. We have studied the expression of RIG-I in T24 cells, a cell line derived from human urinary bladder epithelial carcinoma cells. IFN-gamma stimulated T24 cells to express RIG-I mRNA and protein in concentration- and time-dependent manners. Immunohistochemical analysis revealed the expression of RIG-I in the urinary bladder epithelium from a patient with chronic urinary tract infection and in a bladder epithelial carcinoma. We conclude that RIG-I may play some role in inflammatory reactions in the urinary tract epithelium.

Expression of retinoic acid-inducible gene-I in vascular smooth muscle cells stimulated with interferon-gamma

Vascular smooth muscle cells (SMC) play an important role in atherogenesis and vasospasm. Interferon-gamma (IFN-gamma) is a potent cytokine that regulates immune and inflammatory responses by inducing multiple genes in many types of cells including SMC. Retinoic acid-inducible gene-I (RIG-I) is a putative RNA helicase, but its physiological function is not known. RIG-I is induced in leukemic cells by retinoic acid or in endothelial cells by lipopolysaccharide. We have studied the expression of RIG-I in cultured SMC from human umbilical artery. IFN-gamma stimulated SMC to express RIG-I mRNA and protein in concentration- and time-dependent manners. Immunohistochemical analysis revealed the expression of RIG-I in SMC in vivo. We conclude that RIG-I may play some pathophysiological role in immune and inflammatory reactions in SMC.

The RNA helicase RIG-I has an essential function in double-stranded RNA-induced innate antiviral responses

Intracellular double-stranded RNA (dsRNA) is a chief sign of replication for many viruses. Host mechanisms detect the dsRNA and initiate antiviral responses. In this report, we identify retinoic acid inducible gene I (RIG-I), which encodes a DExD/H box RNA helicase that contains a caspase recruitment domain, as an essential regulator for dsRNA-induced signaling, as assessed by functional screening and assays. A helicase domain with intact ATPase activity was responsible for the dsRNA-mediated signaling. The caspase recruitment domain transmitted 'downstream' signals, resulting in the activation of transcription factors NF-kappaB and IRF-3. Subsequent gene activation by these factors induced antiviral functions, including type I interferon production. Thus, RIG-I is key in the detection and subsequent eradication of the replicating viral genomes.

Retinoic acid-inducible gene-I is induced by interferon-gamma and regulates the expression of interferon-gamma stimulated gene 15 in MCF-7 cells

Retinoic acid-inducible gene-I (RIG-I) is a member of the DExH box family proteins, which have diverse roles in regulation of gene expression and cellular functions. We found RIG-I mRNA and protein were expressed in MCF-7 human breast cancer cells stimulated with interferon-gamma (IFN-gamma). This effect of IFN-gamma was observed in concentration- and time-dependent manners, and IFN-gamma also induced promoter activity of RIG-I. Transfection of GFP-RIG-I cDNA into MCF-7 cells resulted in the expression of RIG-I protein in cytoplasm. Overexpression of RIG-I induced the upregulation of IFN-gamma stimulated gene 15, which has the potential to amplify the immunomodulatory effects. We conclude that IFN-gamma induces the expression of RIG-I, which may play a role in the immunological effects of IFN-gamma.

Interferon-gamma induces retinoic acid-inducible gene-I in endothelial cells

Interferon-gamma (IFN-gamma) induces expression of multiple genes in endothelial cells. Retinoic acid-inducible gene-I (RIG-I) encodes a protein belonging to the DExH-box family, but details of its physiological function are not clear. RIG-I is induced in leukemia cells by retinoic acid and in endothelial cells by lipopolysaccharide. In the present study, the authors found that IFN-gamma also induces the expression of RIG-I in human umbilical vein endothelial cells. Induction of RIG-I mRNA by IFN-gamma was not altered by the treatment with cycloheximide or interleukin-4. Fluorescent immunostaining and Western blot analysis revealed cytoplasmic distribution of RIG-I. The in situ endothelium in a normal lung tissue was also found to express RIG-I protein. Although the physiological function of RIG-I is still unknown, induction of RIG-I by IFN-gamma may play an important role in inflammatory or immunological reactions in endothelial cells.

Facilitation of mRNA deadenylation and decay by the exosome-bound, DExH protein RHAU

The AU-rich element (ARE) in the 3' untranslated region of unstable mRNAs mediate their rapid degradation. ARE binding proteins (AUBPs) have been described that either stabilize or otherwise degrade ARE-mRNAs by recruiting the exosome, a complex of 3'-to-5' exoribonucleases. We have identified RHAU, a putative DExH RNA helicase that was isolated in association with the ARE of urokinase plasminogen activator mRNA (ARE(uPA)). RHAU physically interacts with the deadenylase PARN and the human exosome and enhances the deadenylation and decay of ARE(uPA)-mRNAs. An alternatively spliced isoform of RHAU that localized to the cytoplasm had a more pronounced effect on ARE(uPA)-mRNA destabilization than full-length RHAU. Furthermore, the ATPase activity of RHAU is essential for its mRNA-destabilizing function. ARE(uPA)-mRNA recognition by RHAU may be mediated through its RNA-dependent interaction with the AUBPs HuR and NFAR1. A model is presented to describe the action of RHAU in ARE(uPA)-directed mRNA turnover.

African swine fever virus multigene family 360 and 530 genes affect host interferon response

African swine fever virus (ASFV) multigene family 360 and 530 (MGF360/530) genes affect viral growth in macrophage cell cultures and virulence in pigs (L. Zsak, Z. Lu, T. G. Burrage, J. G. Neilan, G. F. Kutish, D. M. Moore, and D. L. Rock, J. Virol. 75:3066-3076, 2001). The mechanism by which these novel genes affect virus-host interactions is unknown. To define MGF360/530 gene function, we compared macrophage transcriptional responses following infection with parental ASFV (Pr4) and an MGF360/530 deletion mutant (Pr4 Delta 35). A swine cDNA microarray containing 7,712 macrophage cDNA clones was used to compare the transcriptional profiles of swine macrophages infected with Pr4 and Pr4 Delta 35 at 3 and 6 h postinfection (hpi). While at 3 hpi most (7,564) of the genes had similar expression levels in cells infected with either virus, 38 genes had significantly increased (>2.0-fold, P < 0.05) mRNA levels in Pr4 Delta 35-infected macrophages. Similar up-regulation of these genes was observed at 6 hpi. Viral infection was required for this induced transcriptional response. Most Pr Delta 35 up-regulated genes were part of a type I interferon (IFN) response or were genes that are normally induced by double-stranded RNA and/or viral infection. These included monocyte chemoattractant protein, transmembrane protein 3, tetratricopeptide repeat protein 1, a ubiquitin-like 17-kDa protein, ubiquitin-specific protease ISG43, an RNA helicase DEAD box protein, GTP-binding MX protein, the cytokine IP-10, and the PKR activator PACT. Differential expression of IFN early-response genes in Pr4 Delta 35 relative to Pr4 was confirmed by Northern blot analysis and real-time PCR. Analysis of IFN-alpha mRNA and secreted IFN-alpha levels at 3, 8, and 24 hpi revealed undetectable IFN-alpha in mock- and Pr4-infected macrophages but significant IFN-alpha levels at 24 hpi in Pr4 Delta 35-infected macrophages. The absence of IFN-alpha in Pr4-infected macrophages suggests that MGF360/530 genes either directly or indirectly suppress a type I IFN response. An inability to suppress host type I IFN responses may account for the growth defect of Pr4 Delta 35 in macrophages and its attenuation in swine.

Expression analysis and genomic characterization of human melanoma differentiation associated gene-5, mda-5: a novel type I interferon-responsive apoptosis-inducing gene

Melanoma differentiation associated gene-5 (mda-5) was identified by subtraction hybridization as a novel upregulated gene in HO-1 human melanoma cells induced to terminally differentiate by treatment with IFN-beta+MEZ. Considering its unique structure, consisting of a caspase recruitment domain (CARD) and an RNA helicase domain, it was hypothesized that mda-5 contributes to apoptosis occurring during terminal differentiation. We have currently examined the expression pattern of mda-5 in normal tissues, during induction of terminal differentiation and after treatment with type I IFNs. In addition, we have defined its genomic structure and chromosomal location. IFN-beta, a type I IFN, induces mda-5 expression in a biphasic and dose-dependent manner. Based on its temporal kinetics of induction and lack of requirement for prior protein synthesis mda-5 is an early type I IFN-responsive gene. The level of mda-5 mRNA is in low abundance in normal tissues, whereas expression is induced in a spectrum of normal and cancer cells by IFN-beta. Expression of mda-5 by means of a replication incompetent adenovirus, Ad.mda-5, induces apoptosis in HO-1 cells as confirmed by morphologic, biochemical and molecular assays. Additionally, the combination of Ad.mda-5+MEZ further augments apoptosis as observed in Ad.null or uninfected HO-1 cells induced to terminally differentiate by treatment with IFN-beta+MEZ. The mda-5 gene is located on human chromosome 2q24 and consists of 16 exons, without pseudogenes, and is conserved in the mouse genome. Present data documents that mda-5 is a novel type I IFN-inducible gene, which may contribute to apoptosis induction during terminal differentiation and during IFN treatment. The conserved genomic and protein structure of mda-5 in human and mouse will permit analysis of the evolution and developmental aspects of this gene.

A novel cellular RNA helicase, RH116, differentially regulates cell growth, programmed cell death and human immunodeficiency virus type 1 replication

In an effort to define novel cellular factors regulating human immunodeficiency virus type 1 (HIV-1) replication, a differential display analysis has been performed on endogenously infected cells stimulated with the HIV-suppressive immunomodulator Murabutide. In this study, the cloning and identification of a Murabutide-downregulated gene, named RH116, bearing classical motifs that are characteristic of the DExH family of RNA helicases, are reported. The 116 kDa encoded protein shares 99·9 % similarity with MDA-5, an inducible RNA helicase described recently. Ectopic expression of RH116 in HeLa-CD4 cells inhibited cell growth and cell proliferation but had no measurable effect on programmed cell death. RH116 presented steady state cytoplasmic localization and could translocate to the nucleus following HIV-1 infection. Moreover, the endogenous expression of RH116, at both the transcript and protein levels, was found to be considerably upregulated after infection. Overexpression of RH116 in HIV-1-infected HeLa-CD4 cells also resulted in a dramatic increase in the level of secreted viral p24 protein. This enhancement in virus replication did not stem from upregulated proviral DNA levels but correlated with increased unspliced and singly spliced viral mRNA transcripts. These findings implicate RH116 in the regulation of HIV-1 replication and point to an apoptosis-independent role for this novel helicase in inducing cell growth arrest.

Down-regulation of Myc as a potential target for growth arrest induced by human polynucleotide phosphorylase (hPNPaseold-35) in human melanoma cells

Terminal differentiation and senescence share several common properties, including irreversible cessation of growth and changes in gene expression profiles. To identify molecules that converge in both processes, an overlapping pathway screening was employed that identified old-35, which is human polynucleotide phosphorylase (hPNPaseold-35), a 3',5'-exoribonuclease. We previously demonstrated that hPNPaseold-35 is a type I interferon-inducible gene that is also induced in senescent fibroblasts. In vitro RNA degradation assays confirmed its exoribonuclease properties, and overexpression of hPNPaseold-35 resulted in growth suppression in HO-1 human melanoma cells. The present study examined the molecular mechanism of the growth-arresting property of hPNPaseold-35. When overexpressed by means of a replication-incompetent adenoviral vector (Ad.hPNPaseold-35), hPNPaseold-35 inhibited cell growth in all cell lines tested. Analysis of cell cycle revealed that infection of HO-1 cells with Ad.hPNPaseold-35 resulted in arrest in the G1 phase and eventually apoptosis accompanied by marked reduction in the S phase. Infection with Ad.hPNPaseold-35 resulted in reduction in expression of the c-myc mRNA and Myc protein and modulated the expression of proteins regulating G1 checkpoint and apoptosis. In vitro mRNA degradation assays revealed that hPNPaseOLD-35 degraded c-myc mRNA. Overexpression of Myc partially but significantly protected HO-1 cells from Ad.hPNPaseold-35-induced growth arrest, indicating that Myc down-regulation might directly mediate the growth-inhibitory properties of Ad.hPNPaseold-35. Inhibition of hPNPaseold-35 by an antisense approach provided partial but significant protection against interferon-beta-mediated growth inhibition, thus demonstrating the biological significance of hPNPaseold-35 in interferon action.

Interferons and apoptosis

The interferons (IFNs), in addition to their well-known antiviral activities, have important roles in the control of cell proliferation and are effective agents for the treatment of a limited number of malignant diseases. IFNs not only regulate cell growth and division but also influence cell survival through their effects on apoptosis. This review describes the current state of knowledge about the mechanisms of action of these cytokines on the apoptotic machinery, with particular emphasis on the synergism that exists between the IFNs and other proapoptotic agents, such as members of the tumor necrosis factor (TNF) family. The review also discusses the physiologic and clinical implications of the effects of the IFNs on apoptosis for regulation of viral infection and tumor growth.

Molecular and functional analysis of an interferon gene from the zebrafish, Danio rerio

The interferon (IFN) family consisting of alpha IFN (IFN-alpha), IFN-beta, IFN-omega, IFN-delta, IFN-kappa, and IFN-tau is a large group of cytokines involved in the innate immune response against various microorganisms. Genes for IFN have been cloned from a variety of mammalian and avian species; however, IFN genes from lower-order vertebrates have not been forthcoming. Here, we report the cloning and characterization of the IFN gene from the zebrafish, Danio rerio. Zebrafish IFN (zfIFN) is 185 amino acids in length, with the first 22 amino acids representing a putative signal peptide. Treatment with the known IFN inducer polyinosinic acid-polycytidylic acid (poly[I]-poly[C]) resulted in an increase in zfIFN mRNA transcripts. zfIFN was also able to activate the IFN-inducible Mx promoter when cotransfected with a plasmid containing the zebrafish Mx promoter upstream of a luciferase reporter gene. To demonstrate antiviral activity, zebrafish cells were transfected with zfIFN and challenged with a fish rhabdovirus. A 36% reduction in plaque number was seen in zfIFN-transfected cells, compared to cells transfected with a control vector. Phylogenetic analysis has shown zfIFN to be approximately equally divergent from avian and mammalian IFN, consistent with its origin from an IFN present in the most recent common ancestor of these divergent lineages. A putative IFN from puffer, Fugu rubripes, was also found when zfIFN was used to search the fugu genome database, demonstrating that zfIFN can be used to find additional fish IFN genes. These results demonstrate that zebrafish can be used as an effective model for studying innate immunity and immune response to infectious disease.