Two molecular weight forms of human 2',5'-oligoadenylate synthetase have different activation requirements

Cellular origins of dsRNA, their recognition and consequences

Double-stranded RNA (dsRNA) is associated with most viral infections - it either constitutes the viral genome (in the case of dsRNA viruses) or is generated in host cells during viral replication. Hence, nearly all organisms have the capability of recognizing dsRNA and mounting a response, the primary aim of which is to mitigate the potential infection. In vertebrates, a set of innate immune receptors for dsRNA induce a multitude of cell-intrinsic and cell-extrinsic immune responses upon dsRNA recognition. Notably, recent studies showed that vertebrate cells can accumulate self-derived dsRNAs or dsRNA-like species upon dysregulation of several cellular processes, activating the very same immune pathways as in infected cells. On the one hand, such aberrant immune activation in the absence of infection can lead to pathogenesis of immune disorders, such as Aicardi-Goutières syndrome. On the other hand, the same innate immune reaction can be induced in a controlled setting for a therapeutic benefit, as occurs in immunotherapies. In this Review, we describe mechanisms by which immunostimulatory dsRNAs are generated in mammalian cells, either by viruses or by the host cells, and how cells respond to them, with the focus on recent developments regarding the role of cellular dsRNAs in immune modulation.

Double-Stranded RNA Sensors and Modulators in Innate Immunity

Detection of double-stranded RNAs (dsRNAs) is a central mechanism of innate immune defense in many organisms. We here discuss several families of dsRNA-binding proteins involved in mammalian antiviral innate immunity. These include RIG-I-like receptors, protein kinase R, oligoadenylate synthases, adenosine deaminases acting on RNA, RNA interference systems, and other proteins containing dsRNA-binding domains and helicase domains. Studies suggest that their functions are highly interdependent and that their interdependence could offer keys to understanding the complex regulatory mechanisms for cellular dsRNA homeostasis and antiviral immunity. This review aims to highlight their interconnectivity, as well as their commonalities and differences in their dsRNA recognition mechanisms.

Synergistic Anti-Human Immunodeficiency Viral (HIV-1) Effect of the Immunomodulator Ampligen (Mismatched Double-Stranded RNA) with Inhibitors of Reverse Transcriptase and HIV-1 Regulatory Proteins

The potent antiviral effect of double stranded RNA, such as the mismatched poly(l)·poly(C12U) [Ampligen], 2′,3′-dideoxy-3′-fluorothymidine (FddThd) and antisense oligodeoxynucleotides (ODN) has been established in in vitro systems using cells infected with the human immunodeficiency virus type 1 (HIV-1). We report here that the immunomodulator poly(l)·poly(C12U) interacts synergistically with (1) the reverse transcriptase inhibitor FddThd (FIC value: 0.43), (2) the modified (5′- and 3′-end capped thioates) antisense ODN-4 directed against the splice acceptor site of the HIV-1/ tat gene (FIC value: 0.66) and (3) also with pyronin Y, a compound which prevents binding of HIV-1 Rev protein to the HIV-1 RRE element. These data suggest that combinations of poly(l)·poly(C12U), a stimulator of the natural antiviral protection system of the cells, with compounds targeting HIV1-specific processes should be considered as candidate treatments of AIDS patients.

Activation of RNase L is dependent on OAS3 expression during infection with diverse human viruses

The 2',5'-oligoadenylate (2-5A) synthetase (OAS)-RNase L system is an IFN-induced antiviral pathway. RNase L activity depends on 2-5A, synthesized by OAS. Although all three enzymatically active OAS proteins in humans--OAS1, OAS2, and OAS3--synthesize 2-5A upon binding dsRNA, it is unclear which are responsible for RNase L activation during viral infection. We used clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein-9 nuclease (Cas9) technology to engineer human A549-derived cell lines in which each of the OAS genes or RNase L is knocked out. Upon transfection with poly(rI):poly(rC), a synthetic surrogate for viral dsRNA, or infection with each of four viruses from different groups (West Nile virus, Sindbis virus, influenza virus, or vaccinia virus), OAS1-KO and OAS2-KO cells synthesized amounts of 2-5A similar to those synthesized in parental wild-type cells, causing RNase L activation as assessed by rRNA degradation. In contrast, OAS3-KO cells synthesized minimal 2-5A, and rRNA remained intact, similar to infected RNase L-KO cells. All four viruses replicated to higher titers in OAS3-KO or RNase L-KO A549 cells than in parental, OAS1-KO, or OAS2-KO cells, demonstrating the antiviral effects of OAS3. OAS3 displayed a higher affinity for dsRNA in intact cells than either OAS1 or OAS2, consistent with its dominant role in RNase L activation. Finally, the requirement for OAS3 as the major OAS isoform responsible for RNase L activation was not restricted to A549 cells, because OAS3-KO cells derived from two other human cell lines also were deficient in RNase L activation.

Antiviral actions of interferons

Tremendous progress has been made in understanding the molecular basis of the antiviral actions of interferons (IFNs), as well as strategies evolved by viruses to antagonize the actions of IFNs. Furthermore, advances made while elucidating the IFN system have contributed significantly to our understanding in multiple areas of virology and molecular cell biology, ranging from pathways of signal transduction to the biochemical mechanisms of transcriptional and translational control to the molecular basis of viral pathogenesis. IFNs are approved therapeutics and have moved from the basic research laboratory to the clinic. Among the IFN-induced proteins important in the antiviral actions of IFNs are the RNA-dependent protein kinase (PKR), the 2',5'-oligoadenylate synthetase (OAS) and RNase L, and the Mx protein GTPases. Double-stranded RNA plays a central role in modulating protein phosphorylation and RNA degradation catalyzed by the IFN-inducible PKR kinase and the 2'-5'-oligoadenylate-dependent RNase L, respectively, and also in RNA editing by the IFN-inducible RNA-specific adenosine deaminase (ADAR1). IFN also induces a form of inducible nitric oxide synthase (iNOS2) and the major histocompatibility complex class I and II proteins, all of which play important roles in immune response to infections. Several additional genes whose expression profiles are altered in response to IFN treatment and virus infection have been identified by microarray analyses. The availability of cDNA and genomic clones for many of the components of the IFN system, including IFN-alpha, IFN-beta, and IFN-gamma, their receptors, Jak and Stat and IRF signal transduction components, and proteins such as PKR, 2',5'-OAS, Mx, and ADAR, whose expression is regulated by IFNs, has permitted the generation of mutant proteins, cells that overexpress different forms of the proteins, and animals in which their expression has been disrupted by targeted gene disruption. The use of these IFN system reagents, both in cell culture and in whole animals, continues to provide important contributions to our understanding of the virus-host interaction and cellular antiviral response.

Antisense RNA: function and fate of duplex RNA in cells of higher eukaryotes

There is ample evidence that cells of higher eukaryotes express double-stranded RNA molecules (dsRNAs) either naturally or as the result of viral infection or aberrant, bidirectional transcriptional readthrough. These duplex molecules can exist in either the cytoplasmic or nuclear compartments. Cells have evolved distinct ways of responding to dsRNAs, depending on the nature and location of the duplexes. Since dsRNA molecules are not thought to exist naturally within the cytoplasm, dsRNA in this compartment is most often associated with viral infections. Cells have evolved defensive strategies against such molecules, primarily involving the interferon response pathway. Nuclear dsRNA, however, does not induce interferons and may play an important posttranscriptional regulatory role. Nuclear dsRNA appears to be the substrate for enzymes which deaminate adenosine residues to inosine residues within the polynucleotide structure, resulting in partial or full unwinding. Extensively modified RNAs are either rapidly degraded or retained within the nucleus, whereas transcripts with few modifications may be transported to the cytoplasm, where they serve to produce altered proteins. This review summarizes our current knowledge about the function and fate of dsRNA in cells of higher eukaryotes and its potential manipulation as a research and therapeutic tool.

How cells respond to interferons

Interferons play key roles in mediating antiviral and antigrowth responses and in modulating immune response. The main signaling pathways are rapid and direct. They involve tyrosine phosphorylation and activation of signal transducers and activators of transcription factors by Janus tyrosine kinases at the cell membrane, followed by release of signal transducers and activators of transcription and their migration to the nucleus, where they induce the expression of the many gene products that determine the responses. Ancillary pathways are also activated by the interferons, but their effects on cell physiology are less clear. The Janus kinases and signal transducers and activators of transcription, and many of the interferon-induced proteins, play important alternative roles in cells, raising interesting questions as to how the responses to the interferons intersect with more general aspects of cellular physiology and how the specificity of cytokine responses is maintained.

The 2-5A system: modulation of viral and cellular processes through acceleration of RNA degradation

The 2-5A system is an RNA degradation pathway that can be induced by the interferons (IFNs). Treatment of cells with IFN activates genes encoding several double-stranded RNA (dsRNA)-dependent synthetases. These enzymes generate 5'-triphosphorylated, 2',5'-phosphodiester-linked oligoadenylates (2-5A) from ATP. The effects of 2-5A in cells are transient since 2-5A is unstable in cells due to the activities of phosphodiesterase and phosphatase. 2-5A activates the endoribonuclease 2-5A-dependent RNase L, causing degradation of single-stranded RNA with moderate specificity. The human 2-5A-dependent RNase is an 83.5 kDa polypeptide that has little, if any, RNase activity, unless 2-5A is present. 2-5A binding to RNase L switches the enzyme from its off-state to its on-state. At least three 2',5'-linked oligoadenylates and a single 5'-phosphoryl group are required for maximal activation of the RNase. Even though the constitutive presence of 2-5A-dependent RNase is observed in nearly all mammalian cell types, cellular amounts of 2-5A-dependent mRNA and activity can increase after IFN treatment. One well-established role of the 2-5A system is as a host defense against some types of viruses. Since virus infection of cells results in the production and secretion of IFNs, and since dsRNA is both a frequent product of virus infection and an activator of 2-5A synthesis, the replication of encephalomyocarditis virus, which produces dsRNA during its life cycle, is greatly suppressed in IFN-treated cells as a direct result of RNA decay by the activated 2-5A-dependent RNase. This review covers the organic chemistry, enzymology, and molecular biology of 2-5A and its associated enzymes. Additional possible biological roles of the 2-5A system, such as in cell growth and differentiation, human immunodeficiency virus replication, heat shock, atherosclerotic plaque, pathogenesis of Type I diabetes, and apoptosis, are presented.

A sensitive assay for the IFN-regulated 2-5A synthetase enzyme

2-5A synthetase is the central enzyme of the 2-5A system, an important mediator of interferon action. An assay capable of detecting low, yet biologically important levels of 2-5A synthetase enzyme activity is described. The purification of enzyme reaction products on SepPak C-18 cartridges resulted in a significant reduction in background, when a high specific activity substrate was used to label the 2-5A. Quantitation of labeled 2-5A by chromatography and scintillation counting provided a means of detecting femptomolar amounts of 2-5A. The combination of these procedures accounts for a 3-4 log increase in sensitivity over existing assays. This degree of sensitivity should permit a more accurate determination of the 2-5A synthetase activity in vivo leading to a better understanding of the role of the 2-5A system in virus infection and other cellular processes.

Mode of action of the anti-AIDS compound poly(I).poly(C12U) (Ampligen): activator of 2',5'-oligoadenylate synthetase and double-stranded RNA-dependent kinase

The mismatched double-stranded RNA (dsRNA), poly(I).poly(C12U), also termed Ampligen, exhibits a strong antiviral and cytoprotective effect on cells (human T-lymphoblastoid CEM cells and human T-cell line H9) infected with the human immunodeficiency virus type 1 (HIV-1). Untreated H9 cells infected with HIV-1 start to release the virus 3 days post-infection, while in the presence of 40 micrograms/ml (80 micrograms/ml) of poly(I).poly(C12U) the onset of virus production and release is retarded and does not occur before day 5 (day 6). We demonstrate that poly(I).poly(C12U) markedly extends the duration of the transient increase of 2',5'-oligoadenylate (2-5A) synthetase mRNA level and activity preceding virus production after infection of cells with HIV-1. Treatment of HeLa cells with poly(I).poly(C12U) was found to cause a significant increase in total (activated plus latent) 2-5A synthetase activity; no evidence was obtained that the level of latent (nonactivated) 2-5A synthetase is changed in cells treated with dsRNA plus interferon (IFN). Poly(I).poly(C12U) is able to bind and to activate 2-5A synthetase(s) from HeLa cell extracts. Addition of poly(I).poly(C12U) to HeLa cell extracts results in production of longer 2-5A oligomers (> or = 3 adenylate residues), which are better activators of RNase L. Both free and immobilized poly(I).poly(C12U) also bind to the dsRNA-dependent protein kinase (p68 kinase), resulting in autophosphorylation of the enzyme. Activation of the kinase by the free RNA occurs within a limited concentration range (10(-7) to 10(-6) grams/ml). Addition of HIV-1 Tat protein does not affect binding and activation of p68 kinase to poly(I).poly(C12U)-cellulose but strongly reduces the binding of the kinase to immobilized TAR RNA of HIV-1. We conclude that poly(I).poly(C12U) may antagonize Tat-mediated down-regulation of dsRNA-dependent enzymes.

Isoforms p69 and p100 of 2',5'-oligoadenylate synthetase induced differentially by interferons in vivo and in vitro

Four isoforms of 2',5'-oligoadenylate (2-5A) synthetase have been identified (40, 44, 69, and 100 kD). The 40- and 44-kD enzymes are encoded by the same gene, probably different from the genes encoding the larger isoforms. In this study, induction of the 100- and 69-KD (p100, p69) isoforms in different individuals and in different cell types was investigated after treatment with recombinant human interferons (IFN): IFN-alpha 2, IFN-beta ser, or IFN-gamma. The p69 and p100 isoforms were quantitated in cell extracts on Western blots using specific polyclonal antibodies, or their activity was measured after purification of cell extracts on immunoaffinity columns. The p69 and p100 isoforms were differentially induced in Daudi, fibroblast, and colon adenocarcinoma cells treated with IFNs. Considerable individual variations in both basal and induced levels of p69 and p100 were observed in peripheral blood mononuclear cells from normal donors cultured with IFNs in vitro, and from cancer patients treated with IFN-alpha 2 or with IFN-beta ser. These results demonstrate that the p69 and p100 isoforms are present in vivo in peripheral blood mononuclear cells and their level is increased following IFN administration. Furthermore, both the in vivo and in vitro observations indicate that the expression of these enzymes is specific to each cell type and varies among individuals.

Lymphocytic 2',5'-oligoadenylate synthetase is insensitive to dsRNA and interferon stimulation in autoimmune BB rats

The interferon (IFN)-dependent 2',5'-oligoadenylate synthetase (2-5A synthetase), which produces 2',5'-oligoadenylates from ATP, was analyzed in homogenates of isolated peripheral blood lymphocytes (PBL) from BB and Sprague-Dawley rats, man, sheep, and beagle dog. In all the examined species, the 2-5A synthetase was expressed constitutively and showed sensitivity differences to poly(I:C) (synthetic dsRNA). The 2-5A synthetase activity in the absence of poly(I:C) was high in the BB and Sprague-Dawley rat where only 2-5A dimers were synthesized. With the notable exception of PBL homogenates from BB rats, increasing poly(I:C) concentrations resulted in an increased 2-5A synthetase activity leading to the production of higher 2-5A oligomers, predominantly the octamer. Diabetes-resistant, diabetes-prone, and diabetic BB rats were indistinguishable in that their 2-5A synthetase was insensitive to poly(I:C). Preincubation of PBL from BB and Sprague-Dawley rats with up to 1,000 U/ml rat IFN elicited a moderate increase of 60% in the activity level of 2-5A synthetase. In contrast, preincubation of human PBL with human IFN-alpha led as expected to a 300% increase in 2-5A activity. Thus, the BB rat was markedly different from the other species in producing only the biologically inactive 2-5A dimers and in having a high basal 2-5A synthetase activity, that was unaffected by poly(I:C). We believe that these factors per se or together may render the BB rat more susceptible to virus attacks and/or may create a background that will facilitate the development of autoimmune processes.

Cellular response to double-stranded RNA

The study of double-stranded RNA (dsRNA) encompasses a variety of fields. Basic research in this area has contributed to a greater mechanistic understanding of gene induction, tumor cell growth arrest, the establishment of antiviral states, and immunomodulation. Because of the possible clinical value of these molecules, physicians are now exploring the use of synthetic dsRNA to treat patients with cancer, HIV-1 disease, and immune dysfunction. Continued studies of the mechanisms of action of dsRNA are likely to suggest an even wider scope of clinical applications.

The murine 2-5A synthetase locus: three distinct transcripts from two linked genes

The cloning of cDNAs encoding murine 2-5A synthetase has identified three related transcripts, represented by a previously described cDNA clone, L3 and two novel cDNAs, L1 and L2. L1 contains an open reading frame coding for a protein that shows 70% conservation at the amino acid level when compared to the protein predicted to be encoded by L3. L2 recognizes an IFN-induced transcript 600-bp larger than the L3 transcript. These three cDNAs map to a cosmid, cII, containing two murine 2-5A synthetase genes, ME12 and ME5/ME8, situated in a head-to-tail orientation separated by approximately 8 kb. Southern analyses of ME12 and ME5/ME8 using L3, L1-specific and L2-specific probes indicate that these genes have a similar organization. cII was transiently and stably transfected into CV-1 cells. When treated with interferon, the transfected cells produced mature, murine 2-5A synthetase transcripts identified using L3 and L2-specific probes. Thus all transcripts present in IFN-treated mouse cells which are recognized by the available murine 2-5A synthetase cDNA probes map to an approximately 40 kb region of the mouse genome containing two 2-5A synthetase genes.

Regulation of 2',5'-oligoadenylate synthetase gene expression by interferons and platelet-derived growth factor

In murine BALB/c 3T3 cell cultures, either beta interferon or platelet-derived growth factor (PDGF) enhanced expression of the 2',5'-oligoadenylate synthetase mRNA and protein. The time course of induction in response to beta interferon was similar to that in response to PDGF. Of several growth factors known to be present in clotted blood serum (i.e., epidermal growth factor, transforming growth factor beta, and PDGF), only PDGF enhanced expression of 2',5'-oligoadenylate synthetase. The linkage of an interferon response element-containing segment from the 5'-flanking region of a human or murine 2',-5'-oligoadenylate synthetase gene made a heterologous gene responsive to interferon. The expression of such a gene construct in transfected cells was also induced by PDGF. Induction by PDGF was inhibited by mono- or polyclonal antibodies to murine interferon, which suggested that induction by PDGF requires interferon. Both PDGF and interferon induced nuclear factors that bound to this interferon response element-containing segment in vitro.

Molecular mechanism for the inhibitory action of interferon on hematopoiesis

Spleen cell suspensions obtained from adult mice were separated by Ficoll/Hypaque and Percoll density gradient centrifugation. The enriched erythroblast populations were maintained in liquid culture medium for 8 hours with 10,000 units of murine interferon (IFN) alpha and beta. Exposure of these cell cultures to murine IFN alpha and beta resulted in a 48% to 70% increase in 2-5adenylate synthetase (2-5AS) activity. In parallel studies, adult mice were injected daily for 1 or 2 weeks with recombinant human IFN alpha A/D (rHuIFN alpha A/D) at a dose of 10(6) or 10(7) units/kg body weight. This treatment did not significantly affect body weight but did produce a mean 70% increase in spleen wet weight and a mean 46% increase in number of nucleated cells per spleen. This increase in number of splenic hematopoietic cells did not result in a corresponding increase in number of circulating cells. In fact, during this 1 to 2 week period the hematocrit dropped from 45% to 38% in mice injected with high dose rHuIFN alpha A/D. From these findings we propose that IFN induces an early 2-5AS activity in erythroblasts and megakaryocytes. This 2-5AS activity, which is known to inhibit protein synthesis in other cell systems, is thought to be responsible for the block or prolongation in blood cell maturation observed in the present studies.

Preparation and characterization of polyclonal antibodies specific for the 69 and 100 k-dalton forms of human 2-5A synthetase

Recently, the existence of 40-, 46-, 69- and 100- kDa forms of 2',5'-oligoadenylate (2-5A) synthetase have been established in interferon-treated human cells. Using monoclonal antibodies specific for 69- and 100- kDa forms of 2-5A synthetase, we purified these proteins by immunoaffinity chromatography and raised murine polyclonal antibodies. All immunized mice developed antibodies (anti-69 or anti-100 kDa form) which were characterized by their capacity to immunoprecipitate [35S] cysteine labeled proteins from interferon-treated cells or identify these proteins by electrophoretic transfer immunoblot analysis of extracts from control and interferon-treated cells. The 69 and 100 kDa 2-5A synthetases were induced in different types of human cells, such as Daudi, BJAB, HeLa and differentiated HL-60 cells. These enzymes were not detectable nor induced in MRC5 and undifferentiated HL-60 cells.

Regulation of 2',5'-oligoadenylate synthetase gene expression by interferons and platelet-derived growth factor

In murine BALB/c 3T3 cell cultures, either beta interferon or platelet-derived growth factor (PDGF) enhanced expression of the 2',5'-oligoadenylate synthetase mRNA and protein. The time course of induction in response to beta interferon was similar to that in response to PDGF. Of several growth factors known to be present in clotted blood serum (i.e., epidermal growth factor, transforming growth factor beta, and PDGF), only PDGF enhanced expression of 2',5'-oligoadenylate synthetase. The linkage of an interferon response element-containing segment from the 5'-flanking region of a human or murine 2',-5'-oligoadenylate synthetase gene made a heterologous gene responsive to interferon. The expression of such a gene construct in transfected cells was also induced by PDGF. Induction by PDGF was inhibited by mono- or polyclonal antibodies to murine interferon, which suggested that induction by PDGF requires interferon. Both PDGF and interferon induced nuclear factors that bound to this interferon response element-containing segment in vitro.

Intracellular metabolism of the interferon mediator, 2-5A, using permeabilized cells

2-5A synthetase and 2'-phosphodiesterase, the enzymatic activities which respectively synthesize and degrade the interferon mediator 2-5A (ppp(A2'p)nA), were studied in digitonin-permeabilized cells. 2-5A synthetase was higher in permeabilized than in lysed Daudi cells. Mouse L cells appeared to contain two different 2-5A synthetase activities, one of which could be separated from 2'-phosphodiesterase activity, which was only cytosolic. Permeabilization techniques offer opportunities to investigate (2',5')-oligoadenylate intracellular metabolism, which remains incompletely known.

Purification of 2',5'-oligoadenylate synthetase from rabbit reticulocytes

The 2',5'-oligoadenylate synthetase (2-5A synthetase) from rabbit reticulocytes has been purified to apparent homogeneity. The purification procedure consists of (NH4)2SO4 fractionation (30-50% cut), specific binding of the 2-5A synthetase to and elution from the affinity matrix of polyinosinic-polycytidylic-cellulose, another (NH4)2SO4 precipitation step, and finally chromatography on DEAE-cellulose. Upon electrophoresis in sodium dodecyl sulfate polyacrylamide gel (10%), the purified enzyme migrates as a single polypeptide with an apparent molecular weight of 110,000 daltons. A sedimentation coefficient of 5.8S is obtained by glycerol density gradient centrifugation. The synthesis of 2',5'-oligoadenylate by the purified enzyme is dependent on the presence of double-stranded (ds) RNA, in the absence of which the enzyme is highly unstable. Biochemical characteristics of the purified enzyme have been defined.