Expression of interferon-inducible genes in RD-114 cells

Systematic identification of anti-interferon function on hepatitis C virus genome reveals p7 as an immune evasion protein

Hepatitis C virus (HCV) encodes mechanisms to evade the multilayered antiviral actions of the host immune system. Great progress has been made in elucidating the strategies HCV employs to down-regulate interferon (IFN) production, impede IFN signaling transduction, and impair IFN-stimulated gene (ISG) expression. However, there is a limited understanding of the mechanisms governing how viral proteins counteract the antiviral functions of downstream IFN effectors due to the lack of an efficient approach to identify such interactions systematically. To study the mechanisms by which HCV antagonizes the IFN responses, we have developed a high-throughput profiling platform that enables mapping of HCV sequences critical for anti-IFN function at high resolution. Genome-wide profiling performed with a 15-nt insertion mutant library of HCV showed that mutations in the p7 region conferred high levels of IFN sensitivity, which could be alleviated by the expression of WT p7 protein. This finding suggests that p7 protein of HCV has an immune evasion function. By screening a liver-specific ISG library, we identified that IFI6-16 significantly inhibits the replication of p7 mutant viruses without affecting WT virus replication. In contrast, knockout of IFI6-16 reversed the IFN hypersensitivity of p7 mutant virus. In addition, p7 was found to be coimmunoprecipitated with IFI6-16 and to counteract the function of IFI6-16 by depolarizing the mitochondria potential. Our data suggest that p7 is a critical immune evasion protein that suppresses the antiviral IFN function by counteracting the function of IFI6-16.

Differentially expressed genes after viral haemorrhagic septicaemia virus infection in olive flounder (Paralichthys olivaceus)

A strain of viral haemorrhagic septicaemia virus (VHSV) was isolated from cultured olive flounder (Paralichthys olivaceus) during epizootics in South Korean. This strain showed high mortality to olive flounder in in vivo challenge experiment. The complete genomic RNA sequences were determined and phylogenetic analysis of the amino acid sequences of glycoprotein revealed that this isolate was grouped into genotype IVa of genus Novirhabdovirus. Expression profile of genes in olive flounder was analyzed at day 1 and day3 after infection with this VHSV isolate by using cDNA microarray containing olive flounder 13K cDNA clones. Microarray analysis revealed 785 up-regulated genes and 641 down-regulated genes by at least two-fold in virus-infected fish compared to healthy control groups. Among 785 up-regulated genes, we identified seven immune response-associated genes, including the interferon (IFN)-induced 56-kDa protein (IFI56), suppressor of cytokine signaling 1 (SOCS1), interleukin 8 (IL-8), cluster of differentiation 83 (CD83), α-globin (HBA), VHSV-induced protein-6 (VHSV6), and cluster of differentiation antigen 9 (CD9). Our results confirm previous reports that even virulent strain of VHSV induces expression of genes involved in protective immunity against VHSV.

IFN-γR -/- mice show an enhanced liver and brain metallothionein I+II response to endotoxin but not to immobilization stress

Interferon-γ (IFN-γ) is known for its important antiviral activity and other immunomodulatory actions. In in vitro studies, this cytokine has also been involved in the control of metallothionein (MT) synthesis. MT is a low molecular weight protein comprised of several isoforms called MT-I to MT-IV; of these, MT-1+11 are widely expressed, whereas MT-III and MT-IV are rather tissue specific. In the present report, we have studied in vivo the role of IFN-γ for a normal liver and brain MT-I+II response to immobilization stress and to an inflammatory process caused by bacterial lipopolysaccharide (LPS, endotoxin), using mice carrying a null mutation in the IFN-γ receptor gene (IFN-γR-/-). Liver MT-I mRNA and MT-(I+II) protein levels during stress of IFN-γR-/- mice were similar to those of the two parental mouse strains used to generate them, namely C57BU6 and 129/Sv mice, and that of the F1 C57BU6 x 129/Sv offspring mice. In contrast, liver MT response to LPS was significantly higher in the IFN-γR-/- mice than in the other strains. MT-I+II response to LPS was also higher in IFN-γR-/- mice in medulla plus pons and tended to in hypothalamus, hippocampus, and cerebellum, but not in the remaining brain. These results suggest that a role of IFN-γ on liver and brain MT-I+II response to stress is unlikely, but that this cytokine exerts an inhibitory effect on the signaling pathways activated by LPS involved in MT-I+II regulation. In situ hybridization analysis for MT-I and MT-III mRNAs of control mice revealed significant effects of the functional IFN-γ deficiency on MT-I but not MT-III mRNA levels in the dentate gyrus and the habenula, while no effects were observed in the remaining brain areas studied.

Regulation of mitochondrial mRNA stability by RNase L is translation-dependent and controls IFNalpha-induced apoptosis

Interferons (IFNs) inhibit the growth of many different cell types by altering the expression of specific genes. IFNs activities are partly mediated by the 2'-5' oligoadenylates-RNase L RNA decay pathway. RNase L is an endoribonuclease requiring activation by 2'-5' oligoadenylates to cleave single-stranded RNA. Here, we present evidence that degradation of mitochondrial mRNA by RNase L leads to cytochrome c release and caspase 3 activation during IFNalpha-induced apoptosis. We identify and characterize the mitochondrial translation initiation factor (IF2mt) as a new partner of RNase L. Moreover, we show that specific inhibition of mitochondrial translation with chloramphenicol inhibits the IFNalpha-induced degradation of mitochondrial mRNA by RNase L. Finally, we demonstrate that overexpression of IF2mt in human H9 cells stabilizes mitochondrial mRNA, inhibits apoptosis induced by IFNalpha and partially reverses IFNalpha-cell growth inhibition. On the basis of our results, we propose a model describing how RNase L regulates mitochondrial mRNA stability through its interaction with IF2mt.

Small ISGs coming forward

Interferons (IFNs) were first characterized as antiviral proteins. Since then, IFNs have proved to be involved in malignant, angiogenic, inflammatory, immune, and fibrous diseases and, thus, possess a broad spectrum of pathophysiologic properties. IFNs activate a cascade of intracellular signaling pathways leading to upregulation of more than 1000 IFN-stimulated genes (ISGs) within the cell. The function of some of the IFN-induced proteins is well described, whereas that of many others remain poorly characterized. This review focuses on three families of small intracellular and intrinsically nonsecreted proteins (10-20 kDa) separated into groups according to their amino acid sequence similarity: the ISG12 group (6-16, ISG12, and ISG12-S), the 1-8 group (9-27/Leu13, 1-8U, and 1-8D), and the ISG15 group (ISG15/UCRP). These IFN-induced genes are abundantly and widely expressed and mainly induced by type I IFN. ISG15 is very well described and is a member of the ubiquitin-like group of proteins. 9-27/Leu-13 associates with CD81/TAPA-1 and plays a role in B cell development. The functions of 1-8U, 1-8D, 6-16, ISG12, and ISG12-S proteins are unknown at present.

Astrocyte-targeted expression of interleukin-3 and interferon-alpha causes region-specific changes in metallothionein expression in the brain

Transgenic mice expressing IL-3 and IFN-alpha under the regulatory control of the GFAP gene promoter (GFAP-IL3 and GFAP-IFNalpha mice) exhibit a cytokine-specific, late-onset chronic-progressive neurological disorder which resemble many of the features of human diseases such as multiple sclerosis, Aicardi-Goutières syndrome, and some viral encephalopathies including HIV leukoencephalopathy. In this report we show that the metallothionein-I+II (MT-I+II) isoforms were upregulated in the brain of both GFAP-IL3 and GFAP-IFNalpha mice in accordance with the site and amount of expression of the cytokines. In the GFAP-IL3 mice, in situ hybridization analysis for MT-I RNA and radioimmunoassay results for MT-I+II protein revealed that a significant upregulation was observed in the cerebellum and medulla plus pons at the two ages studied, 1-3 and 6-10 months. Increased MT-I RNA levels occurred in the Purkinje and granular layers of the cerebellum, as well as in its white matter tracts. In contrast to the cerebellum and brain stem, MT-I+II were downregulated by IL-3 in the hippocampus and the remaining brain in the older mice. In situ hybridization for MT-III RNA revealed a modest increase in the cerebellum, which was confirmed by immunohistochemistry. MT-III immunoreactivity was present in cells that were mainly round or amoeboid monocytes/macrophages and in astrocytes. MT-I+II induction was more generalized in the GFAP-IFNalpha (GIFN12 and GIFN39 lines) mice, with significant increases in the cerebellum, thalamus, hippocampus, and cortex. In the high expressor line GIFN39, MT-III RNA levels were significantly increased in the cerebellum (Purkinje, granular, and molecular layers), thalamus, and hippocampus (CA2/CA3 and especially lacunosum molecular layers). Reactive astrocytes, activated rod-like microglia, and macrophages, but not the perivenular infiltrating cells, were identified as the cellular sources of the MT-I+II and MT-III proteins. The pattern of expression of the different MT isoforms in these transgenic mice differed substantially, demonstrating unique effects associated with the expression of each cytokine. The results indicate that the MT expression in the CNS is significantly affected by the cytokine-induced inflammatory response and support a major role of these proteins during CNS injury.

Metallothionein synthesis induced by interferon alpha/beta in mice of various zinc status

We studied the ability of interferon alpha/beta (IFN) to induce metallothionein (MT) synthesis in mice. Male mice were injected intraperitoneally with mouse IFN (5 x 10(5) IU/mouse). Plasma Zn levels were reduced at 4 hr after injection, reached a minimum value at 6 hr, and then returned to the control level at 8 hr. Hepatic MT concentrations began to increase at 4 hr and reached maximum values at 6 hr. Induction of MT gene expression and protein synthesis was confirmed by Northern blot analysis and radioimmunoassay, respectively. The induction of MT synthesis in the liver by IFN was dose-dependent. The data suggest that induction of MT-mRNA and the protein in the liver by IFN occurs rapidly but is rather transient. Furthermore, MT synthesis was not induced by IFN in the liver of mice given a Zn-deficient diet, whereas IFN induced increases in the activity of 2',5'-oligoadenylate synthetase in the spleen were unaffected by Zn status. Thus, induction of hepatic MT synthesis by IFN is influenced by Zn status.

Metallothionein induction in cultured rat hepatocytes by arthritic rat serum, activated macrophages, interleukin-6, interleukin-11 and leukaemia inhibitory factor

Potential mediators of hepatic metallothionein (MT) synthesis in adjuvant-induced arthritis were investigated in cultured rat hepatocytes. Sera from arthritic rats (14 d post-adjuvant treatment) in the presence of Zn (50 mumol/L)+dexamethasone (Dex; 1 mumol/L) increased metallothionein (MT) accumulation by 34% above that obtained with control rat serum with Zn+Dex. Endogenous IL-6 activity in serum from arthritic rats was 93 +/- 49 U/mL and was undetectable in control rat serum. The activities of TNF, IL-1 and corticosterone concentrations were the same in control and arthritic rats. The accumulation of MT in hepatocytes in the presence of Zn (10 mumol/L)+Dex (1 mumol/L) was enhanced 29% and 49% by media from lipopolysaccharide (LPS)-stimulated peritoneal macrophage (PMM) and Kupffer cell cultures (KCM), respectively. The response with PMM and KCM was quantitatively the same as that with interleukin-6 (IL-6). Analysis of PMM and KCM showed activities of 1,000-10,000 U/mL for IL-6, 100-1000 U/mL for TNF and < 10,000 U/mL for IL-1, the latter detected only in PMM. LPS alone enhanced the accumulation of MT above Zn+Dex in a dose dependent manner. A significant LPS response was obtained at 5 mg/L with a maximal stimulation above Zn+Dex of 38% at 10 mg/L. This direct stimulation of MT by LPS was not part of the response observed with PMM and KCM where the final LPS concentration in culture was only 0.1 mg/L. Other cytokines capable of synergy with Zn+Dex on MT synthesis were investigated. Interleukin-11 (IL-11) increased the Zn+Dex induction in a dose dependent manner with maximal stimulation at 100 U/mL of 40%. A small stimulation of 12% above Zn+Dex was obtained with leukaemia inhibitory factor (LIF) at concentrations greater than 100 U/mL. No enhancement of the Zn+Dex response was obtained with interleukin-3 (1000 U/mL), interleukin-4 (10 micrograms/L), platelet activating factor (5 nmol/L) or granulocyte-colony stimulating factor (5 micrograms/L). Neither IL-11 nor LIF enhanced the response obtained with Zn+Dex+IL-6. The results demonstrate that mediators present in arthritic rat serum and in LPS-stimulated PMM and KCM cause a quantitatively similar response on MT accumulation as IL-6. IL-11 and to a lesser extent LIF, are also potential mediators of MT synthesis in inflammation.

Reversal of interferon-gamma-resistant phenotype by poly(I:C): possible involvement of ISGF2 (IRF1) in interferon-gamma-mediated induction of the IDO gene

Indoleamine 2,3-dioxygenase (IDO) is induced in many cell lines by interferon-gamma (IFN-gamma) treatment. IDO mRNA increases rapidly from 4 h after IFN-gamma treatment to at least 24 h after treatment in ME180 cells. The IFN-gamma-resistant mutant of ME180, IR3B6B, expresses only one-sixth the amount of IDO message after IFN-gamma treatment and very low levels of IDO. However, pretreatment of these mutants with poly(I:C) restores normal levels of IDO mRNAs and IDO activity. Since IRF1 mRNA induction is also low in IR3B6B cells after IFN-gamma treatment, we examined whether there was any relationship between IRF1 induction and IDO induction by IFN-gamma. The steady-state level of IRF1 mRNA was elevated by treating IR3B6B cells with poly(I:C) and IFN-gamma. Poly(I:C)-mediated reversal of IFN-gamma-resistant phenotype and induction of IDO and IRF1 messages are inhibited by 2-aminopurine. Transient transfection of IRF1 cDNA in ME180 cells resulted in activation of IDO transcription. Nuclear extracts prepared from IFN-gamma-treated ME180 and IR3B6B cells affected differently the mobility of a 80-bp DNA fragment of the 5' regulatory region of IDO gene. Pretreatment of IR3B6B cells with poly(I:C) and addition of IFN-gamma resulted in increased DNA binding of nuclear proteins to the DNA. Pre- and post-treatment of nuclear extract of IFN-gamma-treated ME180 cells with anti-IRF1 antibody resulted in a super shift in mobility of the probe with the abolishment of normal gel-shift pattern.(ABSTRACT TRUNCATED AT 250 WORDS)

A collection of mRNA species that are inducible in the RAW 264.7 mouse macrophage cell line by gamma interferon and other agents

To identify genes induced during macrophage activation, a cDNA library was prepared from cultures of the RAW 264.7 mouse macrophage cell line that had been treated with conditioned medium from mitogen-stimulated spleen cells, and the cDNA library was screened by differential plaque hybridization. Eleven cDNA clones, designated CRG-1 through CRG-11, corresponding to mRNA species inducible in RAW 264.7 cells by the spleen cell conditioned medium, were isolated. Inductions were not blocked by cycloheximide. All of the mRNAs were inducible by gamma interferon, and some were also inducible by alpha and beta interferons, by lipopolysaccharide, by phorbol 12-myristate 13-acetate, and by the calcium ionophore A23187. Sequencing of the cDNAs revealed that CRG-1, CRG-3, and CRG-5 are cDNAs of recently identified transcription factors IRF-1, zif/268, and LRF-1 respectively. As previously reported, CRG-2 and CRG-10 (MIG) encode new members of the platelet factor 4 family of cytokines. CRG-6 corresponds to a new member of a family of interferon-inducible genes clustered on mouse chromosome 1, CRG-9 corresponds to a prostaglandin synthase homolog, CRG-8 corresponds to beta 2-microglobulin, and CRG-4 corresponds to metallothionein II. CRG-11 contains sequences of a truncated L1Md repetitive element as well as nonrepetitive sequences. The nonrepetitive sequence of CRG-11 as well as the sequences of CRG-7 are not closely related to published sequences. The CRG genes and proteins are of interest because of their involvement in macrophage activation, because of their roles as mediators of the effects of gamma interferon and other pleiotropic agents, and because of their usefulness as tools for studying the signal pathways through which gamma interferon and other inducers exert their effects on gene and protein expression.

A collection of mRNA species that are inducible in the RAW 264.7 mouse macrophage cell line by gamma interferon and other agents

To identify genes induced during macrophage activation, a cDNA library was prepared from cultures of the RAW 264.7 mouse macrophage cell line that had been treated with conditioned medium from mitogen-stimulated spleen cells, and the cDNA library was screened by differential plaque hybridization. Eleven cDNA clones, designated CRG-1 through CRG-11, corresponding to mRNA species inducible in RAW 264.7 cells by the spleen cell conditioned medium, were isolated. Inductions were not blocked by cycloheximide. All of the mRNAs were inducible by gamma interferon, and some were also inducible by alpha and beta interferons, by lipopolysaccharide, by phorbol 12-myristate 13-acetate, and by the calcium ionophore A23187. Sequencing of the cDNAs revealed that CRG-1, CRG-3, and CRG-5 are cDNAs of recently identified transcription factors IRF-1, zif/268, and LRF-1 respectively. As previously reported, CRG-2 and CRG-10 (MIG) encode new members of the platelet factor 4 family of cytokines. CRG-6 corresponds to a new member of a family of interferon-inducible genes clustered on mouse chromosome 1, CRG-9 corresponds to a prostaglandin synthase homolog, CRG-8 corresponds to beta 2-microglobulin, and CRG-4 corresponds to metallothionein II. CRG-11 contains sequences of a truncated L1Md repetitive element as well as nonrepetitive sequences. The nonrepetitive sequence of CRG-11 as well as the sequences of CRG-7 are not closely related to published sequences. The CRG genes and proteins are of interest because of their involvement in macrophage activation, because of their roles as mediators of the effects of gamma interferon and other pleiotropic agents, and because of their usefulness as tools for studying the signal pathways through which gamma interferon and other inducers exert their effects on gene and protein expression.

Inhibition of interferon-inducible gene expression by adenovirus E1A proteins: block in transcriptional complex formation

Infection with wild-type adenovirus 5, but not with a mutant lacking the E1A gene, prevented the induction by interferon (IFN) alpha of chloramphenicol acetyltransferase (CAT) activity in HeLaM cell lines that had been permanently transfected with chimeric CAT reporter genes driven by the transcriptional regulatory regions of the IFN-responsive genes 561 and 6-16. Similar inhibition of IFN-inducible CAT activity was observed in cells that were cotransfected with the same reporter genes and plasmids expressing either the E1A 289- or 243-amino acid protein. These proteins also prevented the induction of CAT activity by IFN-gamma from a cotransfected HLA-DR alpha-CAT gene. Experiments with E1A mutants mapped the inhibitory activity to amino acid residues 38-65 of these proteins. In a HeLa cell line permanently expressing the E1A 289-amino acid protein, the replication of vesicular stomatitis virus and encephalomyocarditis virus was not inhibited by IFN-alpha, suggesting a global blockade of IFN responses. In accord with this theory, induction of 561, 1-8, and (2'-5')oligoadenylate synthetase mRNAs by IFN was blocked in these cells at the transcriptional level. The observed transcriptional inhibition could be attributed to the lack of formation of the crucial IFN-stimulated gene factor 3 (ISGF3) transcriptional complex. As shown by mobility shift assays, this complex was not formed in the nuclear extracts of IFN-treated adenovirus-infected cells or IFN-treated E1A-producing cells. These nuclear extracts were deficient in both ISGF3 alpha and ISGF3 gamma subunits. However, they did not block the formation of ISGF3 complex from exogenously added components.

Posttranscriptional regulation of beta interferon expression in erythroid Friend cells treated with gamma interferon

Treatment of Friend erythroleukemia cells (FLC) with gamma interferon (IFN-gamma) in the presence of anti-IFN-beta antibodies reduces the effectiveness of the antiviral state and the induction of 2'-5'-oligoadenylate synthetase activity, indicating that the antiviral activity of IFN-gamma in FLC is in part mediated by the production of IFN-beta. Accordingly, IFN-gamma induces a less pronounced antiviral state in FLC resistant to IFN-alpha/beta than in wild-type cells. Moreover, while results of run-on assays indicate that both IFN-alpha and -beta genes are constitutively transcribed in these cells, FLC treatment with IFN-gamma induces only IFN-beta mRNA accumulation. These results indicate that posttranscriptional mechanisms are involved in the regulation of IFN-beta and -alpha expression by IFN-gamma. The low amounts of the induced IFN-beta synergize with IFN-gamma in mounting the potent antiviral effect.

Synergism of type I and type II interferons in stimulating the activity of the same DNA enhancer

Type I and type II interferons (IFNs) can act synergistically to activate the transcription of the 2-5A synthetase gene. We used in vivo functional assays of sequences from the gene promoter region to determine which DNA segment mediates the gene induction by IFN gamma and the synergistic effect. We found that the type I IFN-inducible enhancer (or IRS) of the 2-5A synthetase gene also confers inducibility by type II IFN to a reporter CAT gene, though the time course and dose response of the induction by the two IFNs are quite different. A clear synergism of the two IFN in stimulating the IRS is observed at low doses of the two IFNs.

Beta and gamma interferons act synergistically to produce an antiviral state in cells resistant to both interferons individually

We showed previously that the mouse fibroblastoid cell line Ltk-aprt- is resistant to the antiviral effects of beta interferon. This lack of response reflects a partial sensitivity to the interferon that is accompanied by a failure to activate expression of several interferon-regulated genes, although certain other genes respond in a normal manner. We show here that Ltk-aprt- cells were also unable to establish an antiviral state and to activate expression of 2,5-oligo(A) synthetase when treated with gamma interferon. Strikingly, however, treatment with a combination of beta interferon and gamma interferon provided complete protection against viral replication. Although the cells were completely insensitive to up to 250 U of the interferons per ml added singly, essentially complete protection from viral cytopathic effects was achieved when as little as 10 U of each of the interferons per ml were combined. Expression of 2,5-oligo(A) synthetase was also sensitive to this synergistic effect. Activation of an antiviral state could also be achieved by sequential treatment, first with gamma interferon and then with beta interferon. Partial protection against viral replication could be achieved by pretreatment with gamma interferon for as little as 1 h before incubation with beta interferon and could be blocked by the addition of specific antibodies or by cycloheximide, indicating that gamma interferon induces the synthesis of a protein which can act synergistically with a signal produced by the beta-interferon receptor. We suggest that Ltk-aprt- cells suffer from defects in one or more components of the gene activation pathways for both type I and type II interferons. Nonetheless, gamma interferon is able to activate the expression of a gene encoding a protein required for signal transduction. This protein acts synergistically with a transient signal produced in response to beta interferon, thereby activating the expression of a further group of genes.

Interferon-induced expression of different genes is mediated by distinct regulatory pathways

Interferons (IFNs) regulate the expression of multiple genes and the regulatory sequences associated with several of these genes have been identified. How cell-surface IFN receptors communicate with such regulatory elements is as yet unknown. We have characterized the IFN responses of a mutant murine cell line (Ltk-aprt-) which is unable to mount an antiviral response when treated with IFN-beta. In contrast to its inability to activate antiviral pathways in these cells, IFN-beta inhibits their growth to the same level as observed in their parental L-929 line which is fully responsive to IFN, suggesting that distinct pathways account for antiviral and cytostatic responses. In agreement with this, Northern blot and nuclear run-on analyses show that the induction of transcription of three distinct genes (2,5(A) synthetase, BS-I-150, and BS-II-4) is blocked in Ltk-aprt- cells whereas another gene (I-8) is activated normally. Our results show that the defective responses observed in this cell line are not due to a nonfunctional IFN-beta cell-surface receptor and suggest that multiple pathways exist between the receptor and upstream gene regulatory elements.

Gene induction by interferons and double-stranded RNA: selective inhibition by 2-aminopurine

Transcription of several interferon-inducible human genes is also induced by double-stranded RNA. The nature and the mechanism of action of signals generated by interferons and by double-stranded RNA which mediate the induction of these genes are under investigation. Here we report that 2-aminopurine, a known inhibitor of protein kinases, could selectively block this induction process. Induction of mRNAs 561 and 6-16 in HeLaM cells by double-stranded RNA was completely inhibited by 10 mM 2-aminopurine, whereas cellular protein and RNA syntheses as well as the induction of metallothionein mRNA by CdCl2 were unaffected by this inhibitor. In addition, 2-aminopurine blocked the induction of the same two mRNAs and of mRNAs 2-5(A) synthetase, 2A, and 1-8 by alpha interferon and of mRNAs 2A and 1-8 by gamma interferon in HeLaM cells. The observed inhibition was at the level of transcription, and for establishing efficient inhibition, the 2-aminopurine treatment had to begin at early stages of interferon treatment. In GM2767 cells, 2-aminopurine inhibited induction of mRNAs 561 and 6-16 by double-stranded RNA but not by alpha interferon. These results suggest that double-stranded RNA-induced signal 2 is distinct from the interferon-alpha-induced signal 2 (R. K. Tiwari, J. Kusari, and G. C. Sen, EMBO J. 6:3373-3378, 1987) and that 2-aminopurine can block the former but not the latter. Moreover, it appeared that 2-aminopurine could block the production of signal 1 by interferons. This was confirmed by experiments in which we separately tested the effects of 2-aminopurine on signal 1 and signal 2 production by interferons in HeLaM cells. Although no direct experimental evidence is available as yet, our results are consistent with the hypothesis that the functioning of a protein kinase activity may be necessary for transcriptional induction of genes by double-stranded RNA and for gene induction by interferons in those cells in which signal 1 production is needed.

Gene induction by interferons and double-stranded RNA: selective inhibition by 2-aminopurine

Transcription of several interferon-inducible human genes is also induced by double-stranded RNA. The nature and the mechanism of action of signals generated by interferons and by double-stranded RNA which mediate the induction of these genes are under investigation. Here we report that 2-aminopurine, a known inhibitor of protein kinases, could selectively block this induction process. Induction of mRNAs 561 and 6-16 in HeLaM cells by double-stranded RNA was completely inhibited by 10 mM 2-aminopurine, whereas cellular protein and RNA syntheses as well as the induction of metallothionein mRNA by CdCl2 were unaffected by this inhibitor. In addition, 2-aminopurine blocked the induction of the same two mRNAs and of mRNAs 2-5(A) synthetase, 2A, and 1-8 by alpha interferon and of mRNAs 2A and 1-8 by gamma interferon in HeLaM cells. The observed inhibition was at the level of transcription, and for establishing efficient inhibition, the 2-aminopurine treatment had to begin at early stages of interferon treatment. In GM2767 cells, 2-aminopurine inhibited induction of mRNAs 561 and 6-16 by double-stranded RNA but not by alpha interferon. These results suggest that double-stranded RNA-induced signal 2 is distinct from the interferon-alpha-induced signal 2 (R. K. Tiwari, J. Kusari, and G. C. Sen, EMBO J. 6:3373-3378, 1987) and that 2-aminopurine can block the former but not the latter. Moreover, it appeared that 2-aminopurine could block the production of signal 1 by interferons. This was confirmed by experiments in which we separately tested the effects of 2-aminopurine on signal 1 and signal 2 production by interferons in HeLaM cells. Although no direct experimental evidence is available as yet, our results are consistent with the hypothesis that the functioning of a protein kinase activity may be necessary for transcriptional induction of genes by double-stranded RNA and for gene induction by interferons in those cells in which signal 1 production is needed.

Studies on the role of the 2'-5'-oligoadenylate synthetase-RNase L pathway in beta interferon-mediated inhibition of encephalomyocarditis virus replication

Interferons inhibit the replication of vesicular stomatitis virus (VSV), but not of encephalomyocarditis virus (EMCV), in mouse JLSV-11 cells. We report the isolation of clonal derivatives from this cell line in which the replication of both viruses is impaired by interferons. These clones were selected from the parental line by virtue of their rescue by interferon treatment from the cytopathic effects of EMCV infection. In one such clone, RK8, the replication of VSV and EMCV and the production of resident murine leukemia virus were inhibited by interferon. On the other hand, in clone RK6, which was isolated without any selection, the replication of VSV, but not of EMCV, was impaired by interferons. The levels of 2'-5'-oligoadenylate synthetase mRNA and enzyme activity were similarly elevated upon interferon treatment in the two clones. However, the level of RNase L, as determined by binding and cross-linking of a radiolabeled 2'-5'-oligoadenylate derivative, was much lower in RK6 cells than in RK8 cells. In accord with this observation, the introduction of 2'-5'-oligoadenylates into cells inhibited protein synthesis much less strongly in RK6 cells than in RK8 cells. These results are consistent with the notion that the 2'-5'-oligoadenylate-dependent RNase L may be a mediator of the inhibition of EMCV replication by interferons.

Regulation of two interferon-inducible human genes by interferon, poly(rI).poly(rC) and viruses

The IFI-56K and IFI-54K genes are transcriptionally stimulated when cells are treated by interferon. We have previously shown that the IFI-56K gene is in addition directly induced by poly(rI).poly(rC), and inducer of interferon-beta. Since the regulation of the IFI-56K and IFI-54K genes by interferon are very much alike, we tested whether the IFI-54K gene is also directly regulated by poly(rI).poly(rC). Treatment of various cell lines with poly(rI).poly(rC) leads to a clear accumulation of the IFI-54K mRNA to a level which sometimes even exceeds that obtained with high doses of interferon. Several interferon-resistant cell lines were investigated for the inducibility of both the IFI-56K and IFI-54K genes by interferons, poly(rI).poly(rC) and viruses (which are the natural inducers of interferon-alpha and -beta). Both genes appear to be coordinately regulated by these inducers. It was thus interesting to search for common regulatory element(s) in the control region of these two genes. The IFI-54K gene promoter region was isolated, from which a 520-base-pair segment was sequenced and compared with the promoter region of the IFI-56K gene that we had previously sequenced. The only homology was found is a well conserved 19-bp segment located just upstream of the TATA box of these genes; interestingly, this sequence is also homologous to the minimal region needed for the inducibility by poly(rI).poly(rC) of the interferon-beta gene. This conserved sequence might be responsible for the coordinate induction of the IFI-56K and IFI-54K genes by interferon, poly(rI).poly(rC) and viruses.

Functional equivalents of interferon-mediated signals needed for induction of an mRNA can be generated by double-stranded RNA and growth factors

In our earlier studies we demonstrated that in HeLaM cells, interferon-alpha produces two functionally distinguishable signals, both of which are needed for induced transcription of mRNA 561 and other inducible mRNAs. Interferon-gamma cannot induce mRNA 561 because it produces only signal 1. Here we report that platelet-derived growth factor or epidermal growth factor could also produce signal 1. On the other hand, signal 2, which can be produced by interferon-alpha but not by interferon-gamma, could be elicited also by double-stranded RNA. Several lines of evidence suggest that the production of signal 2 by double-stranded RNA was not mediated through interferon. Interferon-induced transcription of mRNA 561 in HeLaM cells or in human fibroblast GM2767 cells was transient. However, in interferon-alpha-treated GM2767 cells, which had ceased to synthesize mRNA 561, transcription of this mRNA could be induced effectively by double-stranded RNA suggesting that this induction process could bypass the interferon-mediated down-regulation of induced transcription. Unlike HeLaM and GM2767 cells, in Daudi cells, induction of mRNA 561 by interferon-alpha was not transient. Transcription of this and two other induced mRNAs continued at a high rate even after 18 h of interferon-alpha treatment of these cells. The lack of down-regulation of interferon-induced gene expression may be responsible for interferon's acute antigrowth effects on these cells.

Clonal derivatives of the RD-114 cell line differ in their antiviral and gene-inducing responses to interferons

The human rhabdomyosarcoma cell line RD-114 is partially responsive to interferons (IFNs). In these cells, alpha interferon (IFN-alpha) or gamma interferon (IFN-gamma) inhibits the replication of some viruses but not of others. Similarly, some of the IFN-inducible mRNAs are induced poorly, whereas others are induced well. Here we report the isolation of clonal derivatives of this line which display different spectra of responses to IFNs. Among the eight extensively characterized clonal lines, one, C10, did not respond to IFN-alpha or IFN-gamma at all. Retrovirus production by each of the seven other lines was inhibited by both IFN-alpha and IFN-gamma. Replication of vesicular stomatitis virus was inhibited strongly by IFN-alpha in clone B1 but not in others, whereas it was not appreciably affected by IFN-gamma in any clone. Replication of encephalomyocarditis virus was inhibited strongly by IFN-gamma in clones A1, A2, A3, B3, and B8 and by IFN-alpha in clone A2. Neither IFN inhibited the multiplication of these clones greatly, although their doubling times were slightly increased. Five mRNAs were induced by IFNs to varying degrees in the seven clones. mRNA 2A was most strongly induced by IFN-gamma in clone A3. mRNA 1-8 was strongly induced by IFN-alpha in clone A1 and by either IFN in clones A2 and A3. The highest concentrations of 2',5'-oligoadenylate synthetase mRNA, mRNA 561, and mRNA 6-16 were in IFN-alpha-treated clones A1 and A2. These results demonstrated the existence of clonal heterogeneity in IFN responses in a cell line and strengthened the view that IFN treatment of cells generates multiple signals leading to a variety of IFN-induced phenotypes.