Apoptosis is promoted by the dsRNA-activated factor (DRAF1) during viral infection independent of the action of interferon or p53

Papaverine Enhances the Oncolytic Effects of Newcastle Disease Virus on Breast Cancer In Vitro and In Vivo

Breast cancer is a lethal disease in females worldwide and needs effective treatment. Targeting cancer cells with selective and safe treatment seems like the best choice, as most chemotherapeutic drugs act unselectively. Papaverine showed promising antitumor activity with a high safety profile and increased blood flow through vasodilation. At the same time, it was widely noticed that virotherapy using the Newcastle disease virus proved to be safe and selective against a broad range of cancer cells. Furthermore, combination therapy is favorable, as it attacks cancer cells with multiple mechanisms and enhances virus entrance into the tumor mass, overcoming cancer cells' resistance to therapy. Therefore, we aimed at assessing the novel combination of the AMHA1 strain of Newcastle disease virus (NDV) and nonnarcotic opium alkaloid (papaverine) against breast cancer models in vitro and in vivo. Methods. In vitro experiments used two human breast cancer cell lines and one normal cell line and were treated with NDV, papaverine, and a combination. The study included a cell viability MTT assay, morphological analysis, and apoptosis detection. Animal experiments used the AN3 mouse mammary adenocarcinoma tumor model. Evaluation of the antitumor activity included growth inhibition measurement; the immunohistochemistry assay measured caspase protein expression. Finally, a semiquantitative microarray assay was used to screen changes in apoptotic proteins. In vitro, results showed that the combination therapy induces synergistic cytotoxicity and apoptosis against cancer cells with a negligible cytotoxic effect on normal cells. In vivo, combination treatment induced a significant antitumor effect with an obvious regression in tumor size and a remarkable and significant expression of caspase-3, caspase-8, and caspase-9 compared to monotherapies. Microarray analysis shows higher apoptosis protein levels in the combination therapy group. In conclusion, this study demonstrated the role of papaverine in enhancing the antitumor activity of NDV, suggesting a promising strategy for breast cancer therapy through nonchemotherapeutic drugs.

Characterization of the Impact of Merkel Cell Polyomavirus-Induced Interferon Signaling on Viral Infection

Merkel cell polyomavirus (MCPyV) has been associated with approximately 80% of Merkel cell carcinoma (MCC), an aggressive and increasingly incident skin cancer. The link between host innate immunity, viral load control, and carcinogenesis has been established but poorly characterized. We previously established the importance of the STING and NF-κB pathways in the host innate immune response to viral infection. In this study, we further discovered that MCPyV infection of human dermal fibroblasts (HDFs) induces the expression of type I and III interferons (IFNs), which in turn stimulate robust expression of IFN-stimulated genes (ISGs). Blocking type I IFN downstream signaling using an IFN-β antibody, JAK inhibitors, and CRISPR knockout of the receptor dramatically repressed MCPyV infection-induced ISG expression but did not significantly restore viral replication activities. These findings suggest that IFN-mediated induction of ISGs in response to MCPyV infection is not crucial to viral control. Instead, we found that type I IFN exerts a more direct effect on MCPyV infection postentry by repressing early viral transcription. We further demonstrated that growth factors normally upregulated in wounded or UV-irradiated human skin can significantly stimulate MCPyV gene expression and replication. Together, these data suggest that in healthy individuals, host antiviral responses, such as IFN production induced by viral activity, may restrict viral propagation to reduce MCPyV burden. Meanwhile, growth factors induced by skin abrasion or UV irradiation may stimulate infected dermal fibroblasts to promote MCPyV propagation. A delicate balance of these mutually antagonizing factors provides a mechanism to support persistent MCPyV infection. IMPORTANCE Merkel cell carcinoma is an aggressive skin cancer that is particularly lethal to immunocompromised individuals. Though rare, MCC incidence has increased significantly in recent years. There are no lasting and effective treatments for metastatic disease, highlighting the need for additional treatment and prevention strategies. By investigating how the host innate immune system interfaces with Merkel cell polyomavirus, the etiological agent of most of these cancers, our studies identified key factors necessary for viral control, as well as conditions that support viral propagation. These studies provide new insights for understanding how the virus balances the effects of the host immune defenses and of growth factor stimulation to achieve persistent infection. Since virus-positive MCC requires the expression of viral oncogenes to survive, our observation that type I IFN can repress viral oncogene transcription indicates that these cytokines could be explored as a viable therapeutic option for treating patients with virus-positive MCC.

The chromatin remodeling protein ATRX positively regulates IRF3-dependent type I interferon production and interferon-induced gene expression

The chromatin remodeling protein alpha thalassemia/mental retardation syndrome X-linked (ATRX) is a component of promyelocytic leukemia nuclear bodies (PML-NBs) and thereby mediates intrinsic immunity against several viruses including human cytomegalovirus (HCMV). As a consequence, viruses have evolved different mechanisms to antagonize ATRX, such as displacement from PML-NBs or degradation. Here, we show that depletion of ATRX results in an overall impaired antiviral state by decreasing transcription and subsequent secretion of type I IFNs, which is followed by reduced expression of interferon-stimulated genes (ISGs). ATRX interacts with the transcription factor interferon regulatory factor 3 (IRF3) and associates with the IFN-β promoter to facilitate transcription. Furthermore, whole transcriptome sequencing revealed that ATRX is required for efficient IFN-induced expression of a distinct set of ISGs. Mechanistically, we found that ATRX positively modulates chromatin accessibility specifically upon IFN signaling, thereby affecting promoter regions with recognition motifs for AP-1 family transcription factors. In summary, our study uncovers a novel co-activating function of the chromatin remodeling factor ATRX in innate immunity that regulates chromatin accessibility and subsequent transcription of interferons and ISGs. Consequently, ATRX antagonization by viral proteins and ATRX mutations in tumors represent important strategies to broadly compromise both intrinsic and innate immune responses.

ATRX is a member of a family of chromatin remodeling proteins required for deposition of the histone variant H3.3 at specific genomic regions. This is important to maintain silencing at these sites. Furthermore, ATRX represents a component of PML nuclear bodies (PML-NBs) which are considered as enigmatic nuclear protein accumulations exhibiting a tight link to cell-intrinsic restriction of viral infections. Previous studies demonstrated that many viruses target ATRX by either displacement or degradation. So far, it is believed that this serves to alleviate ATRX-instituted silencing of viral gene expression. Our results reveal a novel and unexpectedly broad function of ATRX as a co-activator of the innate immune response. We show that ATRX is required for both DNA and RNA sensing pathways to activate interferon (IFN) gene expression as well as for upregulation of a distinct set of interferon-stimulated genes. Assessment of chromatin accessibility detected that IFN acts as a switch to regulate the function of ATRX in heterochromatin remodeling. ATRX positively modulates chromatin accessibility specifically upon IFN signaling, thereby affecting promoter regions with recognition motifs for AP-1 family transcription factors. Loss of ATRX due to viral infection or due to tumor mutations may thus broadly compromise cellular innate immunity.

The IRF family of transcription factors: Inception, impact and implications in oncogenesis

Members of the interferon-regulatory factor (IRF) proteins family were originally identified as transcriptional regulators of the Type I interferon system. Thanks to consistent advances made in our understanding of the immunobiology of innate receptors, it is now clear that several IRFs are critical for the elicitation of innate pattern recognition receptors, and—as a consequence—for adaptive immunity. In addition, IRFs have attracted great attentions as they modulate cellular responses that are involved in tumorigenesis. The regulation of oncogenesis by IRFs has important implications for understanding the host susceptibility to several Types of cancers, their progression, as well as the potential for therapeutic interventions.

Persistent Innate Immune Stimulation Results in IRF3-Mediated but Caspase-Independent Cytostasis

Persistent virus infection continuously produces non-self nucleic acids that activate cell-intrinsic immune responses. However, the antiviral defense evolved as a transient, acute phase response and the effects of persistently ongoing stimulation onto cellular homeostasis are not well understood. To study the consequences of long-term innate immune activation, we expressed the NS5B polymerase of Hepatitis C virus (HCV), which in absence of viral genomes continuously produces immune-stimulatory RNAs. Surprisingly, within 3 weeks, NS5B expression declined and the innate immune response ceased. Proteomics and functional analyses indicated a reduced proliferation of those cells most strongly stimulated, which was independent of interferon signaling but required mitochondrial antiviral signaling protein (MAVS) and interferon regulatory factor 3 (IRF3). Depletion of MAVS or IRF3, or overexpression of the MAVS-inactivating HCV NS3/4A protease not only blocked interferon responses but also restored cell growth in NS5B expressing cells. However, pan-caspase inhibition could not rescue the NS5B-induced cytostasis. Our results underline an active counter selection of cells with prolonged innate immune activation, which likely constitutes a cellular strategy to prevent persistent virus infections.

Caspase-Dependent Suppression of Type I Interferon Signaling Promotes Kaposi's Sarcoma-Associated Herpesvirus Lytic Replication

An important component of lytic infection by Kaposi's sarcoma-associated herpesvirus (KSHV) is the ability of the virus to evade the innate immune response, specifically type I interferon (IFN) responses that are triggered by recognition of viral nucleic acids. Inhibition of type I IFN responses by the virus promotes viral replication. Here, we report that KSHV uses a caspase-dependent mechanism to block type I IFN, in particular IFN-β, responses during lytic infection. Inhibition of caspases during KSHV reactivation resulted in increased TBK1/IKKε-dependent phosphorylation of IRF3 as well as elevated levels of IFN-β transcription and secretion. The increased secretion of IFN-β upon caspase inhibition reduced viral gene expression, viral DNA replication, and virus production. Blocking IFN-β production or signaling restored viral replication. Overall, our results show that caspase-mediated regulation of pathogen sensing machinery is an important mechanism exploited by KSHV to evade innate immune responses.IMPORTANCE KSHV is the causative agent of Kaposi's sarcoma (KS), an AIDS-defining tumor that is one of the most common causes of cancer death in sub-Saharan Africa. In this study, we examined the role of a set of cellular proteases, called caspases, in the regulation of immune responses during KSHV infection. We demonstrate that caspases prevent the induction and secretion of the antiviral factor IFN-β during replicative KSHV infection. The reduced IFN-β production allows for high viral gene expression and viral replication. Therefore, caspases are important for maintaining KSHV replication. Overall, our results suggest that KSHV utilizes caspases to evade innate immune responses, and that inhibiting caspases could boost the innate immune response to this pathogen and potentially be a new antiviral strategy.

Pro-apoptotic signaling induced by Retinoic acid and dsRNA is under the control of Interferon Regulatory Factor-3 in breast cancer cells

Breast cancer is one of the most lethal malignancies for women. Retinoic acid (RA) and double-stranded RNA (dsRNA) are considered signaling molecules with potential anticancer activity. RA, co-administered with the dsRNA mimic polyinosinic-polycytidylic acid (poly(I:C)), synergizes to induce a TRAIL (Tumor-Necrosis-Factor Related Apoptosis-Inducing Ligand)- dependent apoptotic program in breast cancer cells. Here, we report that RA/poly(I:C) co-treatment, synergically, induce the activation of Interferon Regulatory Factor-3 (IRF3) in breast cancer cells. IRF3 activation is mediated by a member of the pathogen recognition receptors, Toll-like receptor-3 (TLR3), since its depletion abrogates IRF3 activation by RA/poly(I:C) co-treatment. Besides induction of TRAIL, apoptosis induced by RA/poly(I:C) correlates with the increased expression of pro-apoptotic TRAIL receptors, TRAIL-R1/2, and the inhibition of the antagonistic receptors TRAIL-R3/4. IRF3 plays an important role in RA/poly(I:C)-induced apoptosis since IRF3 depletion suppresses caspase-8 and caspase-3 activation, TRAIL expression upregulation and apoptosis. Interestingly, RA/poly(I:C) combination synergizes to induce a bioactive autocrine/paracrine loop of type-I Interferons (IFNs) which is ultimately responsible for TRAIL and TRAIL-R1/2 expression upregulation, while inhibition of TRAIL-R3/4 expression is type-I IFN-independent. Our results highlight the importance of IRF3 and type-I IFNs signaling for the pro-apoptotic effects induced by RA and synthetic dsRNA in breast cancer cells.

Label-Free Quantitative Proteomic Analysis of Differentially Expressed Membrane Proteins of Pulmonary Alveolar Macrophages Infected with Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus and Its Attenuated Strain

Significant differences exist between the highly pathogenic (HP) porcine reproductive and respiratory syndrome virus (PRRSV) and its attenuated pathogenic (AP) strain in the ability to infect host cells. The mechanisms by which different virulent strains invade host cells remain relatively unknown. In this study, pulmonary alveolar macrophages (PAMs) are infected with HP‐PRRSV (HuN4) and AP‐PRRSV (HuN4‐F112) for 24 h, then harvested and subjected to label‐free quantitative MS. A total of 2849 proteins are identified, including 95 that are differentially expressed. Among them, 26 proteins are located on the membrane. The most differentially expressed proteins are involved in response to stimulus, metabolic process, and immune system process, which mainly have the function of binding and catalytic activity. Cluster of differentiation CD163, vimentin (VIM), and nmII as well as detected proteins are assessed together by string analysis, which elucidated a potentially different infection mechanism. According to the function annotations, PRRSV with different virulence may mainly differ in immunology, inflammation, immune evasion as well as cell apoptosis. This is the first attempt to explore the differential characteristics between HP‐PRRSV and its attenuated PRRSV infected PAMs focusing on membrane proteins which will be of great help to further understand the different infective mechanisms of HP‐PRRSV and AP‐PRRSV.

Fish IRF3 up-regulates the transcriptional level of IRF1, IRF2, IRF3 and IRF7 in CIK cells

Interferon Regulatory Factors (IRFs) belong to a family of transcription factor involved in transcriptional regulation of type I IFN and IFN-stimulated genes (ISG) in cells. In the present study, an IRF3 full-length cDNA (termed CiIRF3, JX999055) and its promoter sequence were cloned by homology cloning strategy and genome walking from grass carp (Ctenopharyngodon idella). The full-length cDNA sequence of CiIRF3 is comprised of a 5'UTR (195 bp), a 3'UTR (269 bp) and a largest open reading frame (ORF) of 1377 bp encoding a polypeptide of 458 amino acids. CiIRF3 has a conservative DNA-binding domain (DBD) at N-terminal and a relatively conserved interferon regulatory factors association domain (IAD). Phylogenetic tree analysis indicated that CiIRF3 gathers together with other IRF-3 from higher vertebrates in the same branch. The promoter sequence of CiIRF3 (596 bp) consists of three IRF-E, a C/EBP beta, a NF-kappa B and a TATA-BOX. CiIRF3 was constitutively expressed at low level in different grass carp tissues but was rapidly up-regulated with Poly I:C stimulation. To study the molecular mechanism of CiIRF3 regulating the transcription of IRFs, CiIRF3 was expressed in Escherichia coli BL21 and purified by affinity chromatography with the Ni-NTA His-Bind Resin. Gel mobility shift assays revealed the affinity of CiIRF3 protein with promoters of CiIRF1, CiIRF2, CiIRF3 and CiIRF7 respectively. Then, CIK cells were co-transfected with pcDNA3.1-CiIRF3, pGL3-promotor (pGL3-CiIRF1, pGL3-CiIRF2, pGL3-CiIRF3, pGL3-CiIRF7) and luciferase reporter vector respectively. The cotransfection experiment showed that CiIRF3 increased the promoter activity of CiIRF1, CiIRF2, CiIRF3 and CiIRF7. Furthermore, overexpression of CiIRF3 in CIK cells also up-regulated the expressions of CiIRF1, CiIRF2, CiIRF3 and CiIRF7. So, CiIRF3 can improve the transcriptional level of CiIRF1, CiIRF2, CiIRF3 and CiIRF7.

The ASK family kinases differentially mediate induction of type I interferon and apoptosis during the antiviral response

Viral infection activates host defense mechanisms, including the production of type I interferon (IFN) and the apoptosis of infected cells. We investigated whether these two antiviral responses were differentially regulated in infected cells. We showed that the mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK) apoptosis signal-regulating kinase 1 (ASK1) was activated in cells by the synthetic double-stranded RNA analog polyinosinic:polycytidylic acid [poly(I:C)] and by RNA viruses, and that ASK1 played an essential role in both the induction of the gene encoding IFN-β (IFNB) and apoptotic cell death. In contrast, we found that the MAPKKK ASK2, a modulator of ASK1 signaling, was essential for ASK1-dependent apoptosis, but not for inducing IFNB expression. Furthermore, genetic deletion of either ASK1 or ASK2 in mice promoted the replication of influenza A virus in the lung. These results indicated that ASK1 and ASK2 are components of the antiviral defense mechanism and suggested that ASK2 acts as a key modulator that promotes apoptosis rather than the type I IFN response. Because ASK2 is selectively present in epithelium-rich tissues, such as the lung, ASK2-dependent apoptosis may contribute to an antiviral defense in tissues with a rapid repair rate in which cells could be readily replaced.

Interferon regulatory factor 3 protects against adverse neo-intima formation

AIMS

Vascular smooth muscle cell (VSMC) proliferation is central to the pathophysiology of neo-intima formation. Interferon regulatory factor 3 (IRF3) inhibits the growth of cancer cells and fibroblasts. However, the role of IRF3 in vascular neo-intima formation is unknown. We evaluated the protective role of IRF3 against neo-intima formation in mice and the underlying mechanisms.

METHODS AND RESULTS

IRF3 expression was down-regulated in VSMCs after carotid wire injury in vivo, and in SMCs after platelet-derived growth factor (PDGF)-BB challenge in vitro. Global knockout of IRF3 (IRF3-KO) led to accelerated neo-intima formation and proliferation of VSMCs, whereas the opposite was seen in SMC-specific IRF3 transgenic mice. Mechanistically, we identified IRF3 as a novel regulator of peroxisome proliferator-activated receptor γ (PPARγ), a negative regulator of SMC proliferation after vascular injury. Binding of IRF3 to the AB domain of PPARγ in the nucleus of SMCs facilitated PPARγ transactivation, resulting in decreased proliferation cell nuclear antigen expression and suppressed proliferation. Overexpression of wild-type, but not truncated, IRF3 with a mutated IRF association domain (IAD) retained the ability to exert anti-proliferative effect.

CONCLUSIONS

IRF3 inhibits VSMC proliferation and neo-intima formation after vascular injury through PPARγ activation.

A death-promoting role for ISG54/IFIT2

The cellular responses to infection are many, and include programmed cell death to inhibit microbial dissemination and the production and secretion of interferons (IFNs), which confer resistance to uninfected cells. In addition to the antimicrobial effects of IFNs, these cytokines have been used clinically for the treatment of various neoplasias to inhibit proliferation and stimulate apoptosis. However, the precise mechanisms of action of IFNs remain to be completely understood. One of the primary response genes induced after an infection or treatment with type I or III IFN is known as IFN-stimulated gene 54 (ISG54) or IFN-induced gene with tetratricopeptide repeats 2 (IFIT2). ISG54/IFIT2 is a member of a family of IFN-induced genes related in the sequence and structure. Expression of this protein has been found to promote cellular apoptosis by a mitochondrial pathway dependent on the action of Bcl2 proteins. ISG54/IFIT2 does not function as a monomer, and it forms complexes with itself and with the related ISG56/IFIT1 and ISG60/IFIT3 proteins to elicit complex cellular responses. The apoptotic response to ISG54/IFIT2 may contribute to other functions that have been reported, including translational regulation, inhibition of tumor colonization, and protection against a lethal viral infection.

The DNA damage response induces IFN

This study reveals a new complexity in the cellular response to DNA damage: activation of IFN signaling. The DNA damage response involves the rapid recruitment of repair enzymes and the activation of signal transducers that regulate cell-cycle checkpoints and cell survival. To understand the link between DNA damage and the innate cellular defense that occurs in response to many viral infections, we evaluated the effects of agents such as etoposide that promote dsDNA breaks. Treatment of human cells with etoposide led to the induction of IFN-stimulated genes and the IFN-α and IFN-λ genes. NF-κB, known to be activated in response to DNA damage, was shown to be a key regulator of this IFN gene induction. Expression of an NF-κB subunit, p65/RelA, was sufficient for induction of the human IFN-λ1 gene. In addition, NF-κB was required for the induction of IFN regulatory factor-1 and -7 that are able to stimulate expression of the IFN-α and IFN-λ genes. Cells that lack the NF-κB essential modulator lack the ability to induce the IFN genes following DNA damage. Breaks in DNA are generated during normal physiological processes of replication, transcription, and recombination, as well as by external genotoxic agents or infectious agents. The significant finding of IFN production as a stress response to DNA damage provides a new perspective on the role of IFN signaling.

Interferon regulatory transcription factor 3 protects mice from uterine horn pathology during Chlamydia muridarum genital infection

Mice with the type I interferon (IFN) receptor gene knocked out (IFNAR KO mice) or deficient for alpha/beta IFN (IFN-α/β) signaling clear chlamydial infection earlier than control mice and develop less oviduct pathology. Initiation of host IFN-β transcription during an in vitro chlamydial infection requires interferon regulatory transcription factor 3 (IRF3). The goal of the present study was to characterize the influence of IRF3 on chlamydial genital infection and its relationship to IFN-β expression in the mouse model. IRF3 KO mice were able to resolve infection as well as control mice, overcoming increased chlamydial colonization and tissue burden early during infection. As previously observed for IFNAR KO mice, IRF3 KO mice generated a potent antigen-specific T cell response. However, in contrast to IFNAR KO mice, IRF3 KO mice exhibited unusually severe dilatation and pathology in the uterine horns but normal oviduct pathology after infection. Although IFN-β expression in vivo was dependent on the presence of IRF3 early in infection (before day 4), the IFN-independent function of IRF3 was likely driving this phenotype. Specifically, early during infection, the number of apoptotic cells and the number of inflammatory cells were significantly less in uterine horns from IRF3 KO mice than in those from control mice, despite an increased chlamydial burden. To delineate the effects of IFN-β versus IRF3, neutralizing IFN-β antibody was administered to wild-type (WT) mice during chlamydial infection. IFN-β depletion in WT mice mimicked that in IFNΑR KO mice but not that in IRF3 KO mice with respect to both chlamydial clearance and reduced oviduct pathology. These data suggest that IRF3 has a role in protection from uterine horn pathology that is independent of its function in IFN-β expression.

The interferon stimulated gene 54 promotes apoptosis

The ability of interferons (IFNs) to inhibit viral replication and cellular proliferation is well established, but the specific contribution of each IFN-stimulated gene (ISG) to these biological responses remains to be completely understood. In this report we demonstrate that ISG54, also known as IFN-induced protein with tetratricopeptide repeats 2 (IFIT2), is a mediator of apoptosis. Expression of ISG54, independent of IFN stimulation, elicits apoptotic cell death. Cell death and apoptosis were quantified by propidium iodide uptake and annexin-V staining, respectively. The activation of caspase-3, a key mediator of the execution phase of apoptosis, was clearly apparent in cells expressing ISG54. The anti-apoptotic B cell lymphoma-xl (Bcl-xl) protein inhibited the apoptotic effects of ISG54 as did the anti-apoptotic adenoviral E1B-19K protein. In addition, ISG54 was not able to promote cell death in the absence of pro-apoptotic Bcl family members, Bax and Bak. Analyses of binding partners of ISG54 revealed association with two homologous proteins, ISG56/IFIT1 and ISG60/IFIT3. In addition, ISG60 binding negatively regulates the apoptotic effects of ISG54. The results reveal a previously unidentified role of ISG54 in the induction of apoptosis via a mitochondrial pathway and shed new light on the mechanism by which IFN elicits anti-viral and anti-cancer effects.

Double-stranded RNAs containing multiple IU pairs are sufficient to suppress interferon induction and apoptosis

Adenosine deaminases acting on RNA (ADARs) catalyze hyperediting of long double-stranded RNAs (dsRNAs), whereby up to 50% of adenosines are converted to inosine (I). Although hyperedited dsRNAs (IU-dsRNAs) have been implicated in various cellular functions, we now provide evidence for another role. We show that IU-dsRNA suppresses the induction of interferon-stimulated genes (ISGs) and apoptosis by poly(IC). Moreover, we show that IU-dsRNA inhibits the activation of interferon regulatory factor 3 (IRF3), which is essential for the induction of ISGs and apoptosis. Finally, we speculate that the inhibition of IRF3 results from specific binding of IU-dsRNA to MDA-5 or RIG-I, both of which are cytosolic sensors for poly(IC). Although our data are consistent with a previous study in which ADAR1 deletion resulted in increased expression of ISGs and apoptosis, we show that IU-dsRNA per se suppresses ISGs and apoptosis. We therefore propose that any IU-dsRNA generated by ADAR1 can inhibit both pathways.

Regulation of immunity and oncogenesis by the IRF transcription factor family

Nine interferon regulatory factors (IRFs) compose a family of transcription factors in mammals. Although this family was originally identified in the context of the type I interferon system, subsequent studies have revealed much broader functions performed by IRF members in host defense. In this review, we provide an update on the current knowledge of their roles in immune responses, immune cell development, and regulation of oncogenesis.

Viral apoptosis is induced by IRF-3-mediated activation of Bax

Upon infection with many RNA viruses, the cytoplasmic retinoic acid inducible gene-I (RIG-I) pathway activates the latent transcription factor IRF-3, causing its nuclear translocation and the induction of many antiviral genes, including those encoding interferons. Here, we report a novel and distinct activity of IRF-3, in virus-infected cells, that induces apoptosis. Using genetically defective mouse and human cell lines, we demonstrated that, although both pathways required the presence of RIG-I, IPS1, TRAF3 and TBK1, only the apoptotic pathway required the presence of TRAF2 and TRAF6 in addition. More importantly, transcriptionally inactive IRF-3 mutants, such as the one missing its DNA-binding domain, could efficiently mediate apoptosis. Apoptosis was triggered by the direct interaction of IRF-3, through a newly identified BH3 domain, with the pro-apoptotic protein Bax, their co-translocation to the mitochondria and the resulting activation of the mitochondrial apoptotic pathway. Thus, IRF-3 is a dual-action cytoplasmic protein that, upon activation, translocates to the nucleus or to the mitochondrion and triggers two complementary antiviral responses of the infected cell.

Cell type-dependent proapoptotic role of Bcl2L12 revealed by a mutation concomitant with the disruption of the juxtaposed Irf3 gene

The generation of mice lacking the expression of the IRF3 transcription factor (Irf3(-/-) mice) has revealed its crucial role in the activation of the type I IFN response. The Bcl2l12 gene, encoding Bcl2L12 protein structurally related to the Bcl-2 family, was found to almost overlap with the Irf3 gene, and the null mutation previously introduced into the Irf3 allele resulted in the functional inactivation of the Bcl2l12 gene; therefore, the mice are correctly termed Irf3(-/-)Bcl2l12(-/-) mice. Embryonic fibroblasts from Irf3(-/-)Bcl2l12(-/-) mice (Irf3(-/-)Bcl2l12(-/-) MEFs) showed resistance to DNA damage-induced apoptosis, accompanied by impaired caspase cleavage. This apoptotic defect in Irf3(-/-)Bcl2l12(-/-) MEFs was rescued by the ectopic expression of Bcl2L12, but not IRF3. The Bcl2L12-mediated apoptotic response depended on the cell type and extracellular stimulus. In contrast, the previously reported defect in the induction of type I IFN genes by nucleic acids in Irf3(-/-)Bcl2l12(-/-) MEFs was rescued by expressing IRF3, but not Bcl2L12. Thus, our present study revealed, on the one hand, a cell type-dependent proapoptotic function of Bcl2L12 and, on the other hand, confirmed the essential role of IRF3 in type I IFN response.

Transcriptional re-programming of primary macrophages reveals distinct apoptotic and anti-tumoral functions of IRF-3 and IRF-7

The immunoregulatory transcriptional modulators - IFN-regulatory factor (IRF)-3 and IRF-7 - possess similar structural features but distinct gene-regulatory potentials. For example, adenovirus-mediated transduction of the constitutively active form of IRF-3 triggered cell death in primary human MPhi, whereas expression of active IRF-7 induced a strong anti-tumoral activity in vitro. To further characterize target genes involved in these distinct cellular responses, transcriptional profiles of active IRF-3- or IRF-7-transduced primary human MPhi were compared and used to direct further mechanistic studies. The pro-apoptotic BH3-only protein Noxa was identified as a primary IRF-3 target gene and an essential regulator of IRF-3, dsRNA and vesicular stomatitis virus-induced cell death. The critical role of IRF-7 and type I IFN production in increasing the immunostimulatory capacity of MPhi was also evaluated; IRF-7 increased the expression of a broad range of IFN-stimulated genes including immunomodulatory cytokines and genes involved in antigen processing and presentation. Furthermore, active IRF-7 augmented the cross-presentation capacity and tumoricidal activity of MPhi and led to an anti-tumor response against the B16 melanoma model in vivo. Altogether, these data further highlight the respective functions of IRF-3 and IRF-7 to program apoptotic, immune and anti-tumor responses.

The IKK-related kinases: from innate immunity to oncogenesis

Over the past four years, the field of the innate immune response has been highly influenced by the discovery of the IkappaB kinase (IKK)-related kinases, TANK Binding Kinase 1 (TBK1) and IKKi, which regulate the activity of interferon regulatory factor (IRF)-3/IRF-7 and NF-kappaB transcription factors. More recently, additional essential components of the signaling pathways that activate these IKK homologues have been discovered. These include the RNA helicases RIGi and MDA5, and the downstream mitochondrial effector known as CARDIF/MAVS/VISA/IPS-1. In addition to their essential functions in controlling the innate immune response, recent studies have highlighted a role of these kinases in cell proliferation and oncogenesis. The canonical IKKs are well recognized to be a bridge linking chronic inflammation to cancer. New findings now suggest that the IKK-related kinases TBK1 and IKKi also participate in signaling pathways that impact on cell transformation and tumor progression. This review will therefore summarize and discuss the role of TBK1 and IKKi in cellular transformation and oncogenesis by focusing on their regulation and substrate specificity.

Toll-like receptor 3 triggers apoptosis of human prostate cancer cells through a PKC-alpha-dependent mechanism

Toll-like receptors (TLRs) are known to play a key role in the innate immune system particularly in inflammatory response against invading pathogens. Recent reports strongly indicate that they play important roles in cancer cells. Prostate cancer represents one of the most common cancer for which no cure is available once metastatic and androgen refractory. Since TLR3 has been recently suggested as a possible therapeutic target in some cancer cell lines, we studied TLR3 expression and functionality in two human prostate cancer cell lines, LNCaP and PC3. We report that both cell lines express TLR3 and that the TLR3 agonist poly (I:C) activates mitogen-activated protein kinases and induces inhibition of proliferation as well as caspase-dependent apoptosis. By using pharmacological and genetic approaches, we demonstrate the involvement of TLR3 in poly (I:C)-induced effects. We also show that a novel interferon-independent pathway involving protein kinase C (PKC)-alpha activation, upstream of p38 and c-jun N-terminal kinase, is responsible for poly (I:C) pro-apoptotic effects on LNCaP cells. To our knowledge, this is the first report describing a role of PKC-alpha in poly (I:C)-mediated apoptosis. The comprehension of the mechanisms underlying TLR3-mediated apoptosis can contribute tools to develop new agonists useful for the treatment of prostate cancer.

The IRF family transcription factors in immunity and oncogenesis

The interferon regulatory factor (IRF) family, consisting of nine members in mammals, was identified in the late 1980s in the context of research into the type I interferon system. Subsequent studies over the past two decades have revealed the versatile and critical functions performed by this transcription factor family. Indeed, many IRF members play central roles in the cellular differentiation of hematopoietic cells and in the regulation of gene expression in response to pathogen-derived danger signals. In particular, the advances made in understanding the immunobiology of Toll-like and other pattern-recognition receptors have recently generated new momentum for the study of IRFs. Moreover, the role of several IRF family members in the regulation of the cell cycle and apoptosis has important implications for understanding susceptibility to and progression of several cancers.

Selective oncolytic effect of an attenuated Newcastle disease virus (NDV-HUJ) in lung tumors

Newcastle disease virus (NDV), an avian paramyxovirus, has a potential oncolytic effect that may be of significance in the treatment of a variety of cancer diseases. An attenuated lentogenic isolate of NDV (HUJ) demonstrated a selective cytopathic effect upon a panel of human and mouse lung tumor cells, as compared to human nontumorigenic lung cells. The virus-selective oncolytic effect is apoptosis dependent, and related to higher levels of viral transcription, translation and progeny virus formation. Furthermore, NDV-HUJ oncolytic activity is directed in-cis and not through induction of cytokines, that may act in-trans on neighboring cells. Development of primary lung tumors and of the consequent metastasis in mice inoculated with mouse lung tumor cells 3LL-D122 was decreased following treatment with NDV-HUJ. The preferential killing of the tumor cells is not due to a deficiency in the interferon (IFN) system, as expression of the IFN-beta gene, in the infected cells, is properly induced. Moreover, pretreatment with IFN effectively protected the tumor cells from the virus oncolytic effect. We conclude therefore, that NDV-HUJ should have a significant benefit in the treatment of lung cancer as well as other malignancies.

Phosphorylation of IRF-3 on Ser 339 generates a hyperactive form of IRF-3 through regulation of dimerization and CBP association

The IkappaB kinase-related kinases, TBK1 and IKKi, were recently shown to be responsible for the C-terminal phosphorylation of IRF-3. However, the identity of the phosphoacceptor site(s) targeted by these two kinases remains unclear. Using a biological assay based on the IRF-3-mediated production of antiviral cytokines, we demonstrate here that all Ser/Thr clusters of IRF-3 are required for its optimal transactivation capacity. In vitro kinase assays using full-length His-IRF-3 as a substrate combined with mass spectrometry analysis revealed that serine 402 and serine 396 are directly targeted by TBK1. Analysis of Ser/Thr-to-Ala mutants revealed that the S396A mutation, located in cluster II, abolished IRF-3 homodimerization, CBP association, and nuclear accumulation. However, production of antiviral cytokines was still present in IRF-3 S396A-expressing cells. Interestingly, mutation of serine 339, which is involved in IRF-3 stability, also abrogated CBP association and dimerization without affecting gene transactivation as long as serine 396 remained available for phosphorylation. Complementation of IRF-3-knockout mouse embryonic fibroblasts also revealed a compensatory mechanism of serine 339 and serine 396 in the ability of IRF-3 to induce expression of the interferon-stimulated genes ISG56 and ISG54. These data lead us to reconsider the current model of IRF-3 activation. We propose that conventional biochemical assays used to measure IRF-3 activation are not sensitive enough to detect the small fraction of IRF-3 needed to elicit a biological response. Importantly, our study establishes a molecular link between the role of serine 339 in IRF-3 homodimerization, CBP association, and its destabilization.

A cell-type-specific requirement for IFN regulatory factor 5 (IRF5) in Fas-induced apoptosis

Apoptosis is a highly regulated process of cell suicide that occurs during development, host defense, and pathophysiology. The transcription factor IFN regulatory factor 5 (IRF5), known to be involved in the activation of innate immune responses, recently has been shown to be critical for DNA damage-induced apoptosis and tumor suppression. Here, we report on a cell-type-specific role of IRF5 in promoting apoptosis upon signaling through the death receptor Fas (CD95/APO-1/TNFRSF6). In particular, we show that mice deficient in the Irf5 gene are resistant to hepatic apoptosis and lethality in response to the in vivo administration of a Fas-activating monoclonal antibody, and that IRF5 is involved in a stage of Fas signaling that precedes the activation of caspase 8 and c-Jun N-terminal kinase (JNK). In addition to hepatocytes, IRF5 is also required for apoptosis in dendritic cells activated by hypomethylated CpG but not in thymocytes and embryonic fibroblasts in vitro. Thus, these findings reveal a cell-type-specific function for IRF5 in the complex regulatory mechanism of death-receptor-induced apoptosis.

IRF-3 activation by Sendai virus infection is required for cellular apoptosis and avoidance of persistence

Here, we report that specific manipulations of the cellular response to virus infection can cause prevention of apoptosis and consequent establishment of persistent infection. Infection of several human cell lines with Sendai virus (SeV) or human parainfluenza virus 3, two prototypic paramyxoviruses, caused slow apoptosis, which was markedly accelerated upon blocking the action of phosphatidylinositol 3-kinases (PI3 kinases) in the infected cells. The observed apoptosis required viral gene expression and the action of the caspase 8 pathway. Although virus infection activated PI3 kinase, as indicated by AKT activation, its blockage did not inhibit JNK activation or IRF-3 activation. The action of neither the Jak-STAT pathway nor the NF-kappaB pathway was required for apoptosis. In contrast, IRF-3 activation was essential, although induction of the proapototic protein TRAIL by IRF-3 was not required. When IRF-3 was absent or its activation by the RIG-I pathway was blocked, SeV established persistent infection, as documented by viral protein production and infectious virus production. Introduction of IRF-3 in the persistently infected cells restored the cells' ability to undergo apoptosis. These results demonstrated that in our model system, IRF-3 controlled the fate of the SeV-infected cells by promoting apoptosis and preventing persistence.

IRF-3-dependent and augmented target genes during viral infection

Activation of the transcription factor interferon regulatory factor-3 (IRF-3) is an essential event in the innate immune response to viral infection. To understand the contribution of IRF-3 to host defense, we used a systems biology approach to analyze global gene expression dependent on IRF-3. Comparison of expression profiles in cells from IRF-3 knockout animals or wild-type siblings following viral infection revealed three sets of induced genes, those that are strictly dependent on IRF-3, augmented with IRF-3, or not responsive to IRF-3. Products of identified IRF-3 target genes are involved in innate or acquired immunity, or in the regulation of cell cycle, apoptosis and proliferation. These results reveal the global effects of one transcription factor in the immune response and provide information to evaluate the integrated response to viral infection.

Hepatitis C virus triggers apoptosis of a newly developed hepatoma cell line through antiviral defense system

BACKGROUND & AIMS

Hepatitis C virus (HCV) has a tendency to cause chronic viral infection. Viral evasion of host immune systems plays a key role in the pathogenesis of HCV. However, the interaction between HCV and hepatocyte innate antiviral defense systems is not understood. The aim of this study was to examine how human hepatocytes respond to HCV infection.

METHODS

We have established a novel human hepatoma cell line, LH86, from a well-differentiated hepatocellular carcinoma tissue. An infectious HCV isolate, JFH-1, was used to infect LH86 cells. HCV replication and apoptosis after viral infection were examined. Mechanisms of HCV-induced apoptosis were determined. Type I interferon induction and the relevant signaling molecules were examined.

RESULTS

LH86 cells permitted JFH-1 HCV infection. The viral infection caused massive apoptosis. The apoptosis was related to viral replication, because blocking viral entry with anti-CD81 or suppressing viral replication with interferon protected cells from HCV-induced apoptosis. The HCV-induced apoptosis appeared to be triggered by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and its receptors, death receptor 4 and death receptor 5, which were up-regulated in HCV infection. HCV also activated interferon response factor 3, which induced expression of interferon and TRAIL in LH86 cells.

CONCLUSIONS

Our study showed that a specific HCV isolate, JFH-1, is cytopathic in this new hepatoma cell line. LH86 cells mount an intact innate antiviral defense through induction of interferon and triggering apoptosis of infected cells. This study reveals a novel mechanism by which host hepatocytes respond to acute HCV infection.

The response of mammalian cells to double-stranded RNA

Double-stranded RNA (dsRNA) has long been recognized as a central component of the interferon (IFN) system. It was originally characterized as a key mediator of IFN induction in response to virus infection. Subsequently, it was identified as a prime activator of the antiviral response. In recent years the discovery of the RNA interference (RNAi) pathway in mammals has renewed interest in dsRNA-mediated cellular responses. This has coincided with the identification of key components of the IFN induction pathway. Here, we present an overview of the current knowledge of dsRNA-mediated pathways in mammalian cells and introduce a link between these pathways and application of RNAi.

The pathogenic NY-1 hantavirus G1 cytoplasmic tail inhibits RIG-I- and TBK-1-directed interferon responses

Hantaviruses cause two diseases with prominent vascular permeability defects, hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome. All hantaviruses infect human endothelial cells, although it is unclear what differentiates pathogenic from nonpathogenic hantaviruses. We observed dramatic differences in interferon-specific transcriptional responses between pathogenic and nonpathogenic hantaviruses at 1 day postinfection, suggesting that hantavirus pathogenesis may in part be determined by viral regulation of cellular interferon responses. In contrast to pathogenic NY-1 virus (NY-1V) and Hantaan virus (HTNV), nonpathogenic Prospect Hill virus (PHV) elicits early interferon responses following infection of human endothelial cells. We determined that PHV replication is blocked in human endothelial cells and that RNA and protein synthesis by PHV, but not NY-1V or HTNV, is inhibited at 2 to 4 days postinfection. The addition of antibodies to beta interferon (IFN-beta) blocked interferon-directed MxA induction by >90% and demonstrated that hantavirus infection induces the secretion of IFN-beta from endothelial cells. Coinfecting endothelial cells with NY-1V and PHV resulted in a 60% decrease in the induction of interferon-responsive MxA transcripts by PHV and further suggested the potential for NY-1V to regulate early IFN responses. Expression of the NY-1V G1 cytoplasmic tail inhibited by >90% RIG-I- and downstream TBK-1-directed transcription from interferon-stimulated response elements or beta-interferon promoters in a dose-dependent manner. In contrast, expression of the NY-1V nucleocapsid or PHV G1 tail had no effect on RIG-I- or TBK-1-directed transcriptional responses. Further, neither the NY-1V nor PHV G1 tails inhibited transcriptional responses directed by a constitutively active form of interferon regulatory factor 3 (IRF-3 5D), and IRF-3 is a direct target of TBK-1 phosphorylation. These findings indicate that the pathogenic NY-1V G1 protein regulates cellular IFN responses upstream of IRF-3 phosphorylation at the level of the TBK-1 complex. These findings further suggest that the G1 cytoplasmic tail contains a virulence element which determines the ability of hantaviruses to bypass innate cellular immune responses and delineates a mechanism for pathogenic hantaviruses to successfully replicate within human endothelial cells.

Involvement of the IκB Kinase (IKK)-Related Kinases Tank-Binding Kinase 1/IKKi and Cullin-Based Ubiquitin Ligases in IFN Regulatory Factor-3 Degradation

Activation of the innate arm of the immune system following pathogen infection relies on the recruitment of latent transcription factors involved in the induction of a subset of genes responsible for viral clearance. One of these transcription factors, IFN regulatory factor 3 (IRF-3), is targeted for proteosomal degradation following virus infection. However, the molecular mechanisms involved in this process are still unknown. In this study, we show that polyubiquitination of IRF-3 increases in response to Sendai virus infection. Using an E1 temperature-sensitive cell line, we demonstrate that polyubiquitination is required for the observed degradation of IRF-3. Inactivation of NEDD8-activating E1 enzyme also results in stabilization of IRF-3 suggesting the NEDDylation also plays a role in IRF-3 degradation following Sendai virus infection. In agreement with this observation, IRF-3 is recruited to Cullin1 following virus infection and overexpression of a dominant-negative mutant of Cullin1 significantly inhibits the degradation of IRF-3 observed in infected cells. We also asked whether the C-terminal cluster of phosphoacceptor sites of IRF-3 could serve as a destabilization signal and we therefore measured the half-life of C-terminal phosphomimetic IRF-3 mutants. Interestingly, we found them to be short-lived in contrast to wild-type IRF-3. In addition, no degradation of IRF-3 was observed in TBK1(-/-) mouse embryonic fibroblasts. All together, these data demonstrate that virus infection stimulates a host cell signaling pathway that modulates the expression level of IRF-3 through its C-terminal phosphorylation by the IkappaB kinase-related kinases followed by its polyubiquitination, which is mediated in part by a Cullin-based ubiquitin ligase.

Newcastle disease virus exerts oncolysis by both intrinsic and extrinsic caspase-dependent pathways of cell death

Newcastle disease virus (NDV), an avian paramyxovirus, is tumor selective and intrinsically oncolytic. Here, we present evidence that genetically modified, recombinant NDV strains are cytotoxic to human tumor cell lines of ecto-, endo-, and mesodermal origin. We show that cytotoxicity against tumor cells is due to multiple caspase-dependent pathways of apoptosis independent of interferon signaling competence. The signaling pathways of NDV-induced, cancer cell-selective apoptosis are not well understood. We demonstrate that NDV triggers apoptosis by activating the mitochondrial/intrinsic pathway and that it acts independently of the death receptor/extrinsic pathway. Caspase-8-methylated SH-SY5Y neuroblastoma cells are as sensitive to NDV as other caspase-8-competent cells. This demonstrates that NDV is likely to act primarily through the mitochondrial death pathway. NDV infection results in the loss of mitochondrial membrane potential and the subsequent release of the mitochondrial protein cytochrome c, but the second mitochondrion-derived activator of caspase (Smac/DIABLO) is not released. In addition, we describe early activation of caspase-9 and caspase-3. In contrast, cleavage of caspase-8, which is predominantly activated by the death receptor pathway, is a TNF-related, apoptosis-inducing ligand (TRAIL)-induced late event in NDV-mediated apoptosis of tumor cells. Our data, therefore, indicate that the death signal(s) generated by NDV in tumor cells ultimately converges at the mitochondria and that it acts independently of the death receptor pathway. Our cytotoxicity studies demonstrate that recombinant NDV could be developed as a cancer virotherapy agent, either alone or in combination with therapeutic transgenes. We have also shown that trackable oncolytic NDV could be developed without any reduction in oncolytic efficacy.

Differential activation of IFN regulatory factor (IRF)-3 and IRF-5 transcription factors during viral infection

Members of the IFN regulatory factor (IRF) family regulate gene expression critical to immune response, hemopoiesis, and proliferation. Although related by homology at their N-terminal DNA-binding domain, they display individual functional properties. The distinct properties result from differences in regulated expression, response to activating signals, and interaction with DNA regulatory elements. IRF-3 is expressed ubiquitously and is activated by serine phosphorylation in response to viral infection or TLR signaling. Evidence indicates that the kinases TANK-binding kinase 1 and inhibitor of NF-kappaB kinase-epsilon specifically phosphorylate and thereby activate IRF-3. We evaluated the contribution of another member of the IRF family, IRF-5, during viral infection since prior studies provided varied results. Analysis of phosphorylation, nuclear translocation, dimerization, binding to CREB-binding protein, recognition of DNA, and induction of gene expression were used comparatively with IRF-3 as a measure of IRF-5 activation. IRF-5 was not activated by viral infection; however, expression of TANK-binding kinase 1 or inhibitor of NF-kappaB kinase-epsilon did provide clear activation of IRF-5. IRF-5 is therefore distinct in its activation profile from IRF-3. However, similar to the biological effects of IRF-3 activation, a constitutively active mutation of IRF-5 promoted apoptosis. The apoptosis was inhibited by expression of Bcl-x(L) but not a dominant-negative mutation of the Fas-associated death domain. These studies support the distinct activation profiles of IRF-3 in comparison to IRF-5, but reveal a potential shared biological effect.

Single-stranded RNA viruses inactivate the transcriptional activity of p53 but induce NOXA-dependent apoptosis via post-translational modifications of IRF-1, IRF-3 and CREB

To characterize the mechanisms underlying apoptosis induced by viral infection, transcriptional activation of genes encoding members of the 'BH3-only' family of proteins was analysed during the course of virus infection. Among these genes, only NOXA is transcriptionally activated by vesicular stomatitis virus (VSV), sendai virus (SV), measles virus, herpes simplex virus, or dsRNA and required for efficient apoptosis of cells. Transcriptional activation of NOXA by VSV or SV is independent of p53, but requires the presence of interferon regulatory factor 1 (IRF-1), IRF-3 and cAMP-responsive element binding protein (CREB). Binding to and transactivation of the NOXA promoter by each of these transcription factors is governed by post-translational modification involving different pathways for each factor. Thus, SV infection activates IRF-3 and CREB by phosphorylation triggered by Toll like receptor 3 signalling, and a pathway involving calcium-independent phopholipase A2, respectively. In addition transactivation induced by IRF-1 during viral infection correlates with a 10 kDa increase in its molecular weight, suggesting a covalent linkage with a previously unknown regulatory polypeptide.

Double-stranded RNA-dependent protein kinase phosphorylation of the alpha-subunit of eukaryotic translation initiation factor 2 mediates apoptosis

As the molecular processes of complex cell stress signaling pathways are defined, the subsequent challenge is to elucidate how each individual event influences the final biological outcome. Phosphorylation of the translation initiation factor 2 (eIF2alpha)atSer(51) is a molecular signal that inhibits translation in response to activation of any of four diverse eIF2alpha stress kinases. We used gene targeting to replace the wild-type Ser(51) allele with an Ala in the eIF2alpha gene to test the hypothesis that translational control through eIF2alpha phosphorylation is a central death stimulus in eukaryotic cells. Homozygous eIF2alpha mutant mouse embryo fibroblasts were resistant to the apoptotic effects of dsRNA, tumor necrosis factor-alpha, and serum deprivation. TNFalpha treatment induced eIF2alpha phosphorylation and activation of caspase 3 primarily through the dsRNA-activated eIF2alpha kinase PKR. In addition, expression of a phospho-mimetic Ser(51) to Asp mutant eIF2alpha-activated caspase 3, indicating that eIF2alpha phosphorylation is sufficient to induce apoptosis. The proapoptotic effects of PKR-mediated eIF2alpha phosphorylation contrast with the anti-apoptotic response upon activation of the PKR-related endoplasmic reticulum eIF2alpha kinase, PERK. Therefore, divergent fates of death and survival can be mediated through phosphorylation at the same site within eIF2alpha. We propose that eIF2alpha phosphorylation is fundamentally a death signal, yet it may promote either death or survival, depending upon coincident signaling events.

Convergence of the NF-κB and Interferon Signaling Pathways in the Regulation of Antiviral Defense and Apoptosis

The ubiquitously expressed interferon regulatory factor 3 (IRF-3) is directly activated following virus infection and functions as a key activator of the immediate-early Type 1 interferon (IFN) genes. Using DNA microarray analysis (8,556 genes) in Jurkat T cells inducibly expressing constitutively active IRF-3, several target genes directly regulated by IRF-3 were identified. Among the genes upregulated by IRF-3 were transcripts for a subset of known IFN-stimulated genes (ISGs), including ISG56, which functions as an inhibitor of translation initiation. Phosphorylation of C-terminal Ser/Thr residues--(382)GGASSLENTVDLHISNSHPLSLTSDQY(408)-is required for IRF-3 activation. Using C-terminal point mutations and a novel phosphospecific antibody, Ser396 was characterized as the minimal phosphoacceptor site required in vivo for IRF-3 activation following Sendai virus (SeV) infection, expression of viral nucleocapsid, or double-stranded RNA (dsRNA) treatment. The identity of the virus-activated kinase (VAK) activity that targets and activates IRF-3 and IRF-7 has remained a critical missing link in the understanding of interferon signaling. We report that the IKK-related kinases-IKKepsilon/TBK-1-are components of VAK that mediate IRF-3 and IRF-7 phosphorylation and thus functionally link the NF-kappaB and IRF pathways in the development of the antiviral response.

Interferon regulatory factor 3 is involved in Toll-like receptor 4 (TLR4)- and TLR3-induced IL-12p35 gene activation

Interleukin-12 (IL-12) is a heterodimeric cytokine produced by dendritic cells (DCs) in response to Toll-like receptor (TLR) ligation. While the mechanisms regulating IL-12p40 chain gene expression are well characterized, molecular events involved in IL-12p35 chain gene activation remain to be clarified. Since IL-12p35 mRNA was induced in human DCs activated through TLR3 or TLR4 but not TLR2, we investigated the potential role of interferon regulatory factor 3 (IRF-3) in IL-12p35 gene transactivation. First, a binding site for IRF-3 named interferon-stimulated response element-1 (ISRE-1) was identified in the human IL-12p35 promoter region between nucleotides -251 and -240. The ISRE-1 site was required for IL-12p35 gene activation in RAW 264.7 cells stimulated by lipopolysaccharide (LPS) or PolyI:C. Ectopic expression of IRF-3 was found to up-regulate IL-12p35 gene activation in the same system. Furthermore, chromatin immunoprecipitation (ChIP) studies demonstrated that IRF-3 is recruited to ISRE-1 site in TLR4- or TLR3-stimulated human DCs. Finally, experiments on DCs from IRF-3-deficient mice established that TLR4-induced IL-12p35 mRNA and IL-12p70 synthesis are impaired in absence of IRF-3. We conclude that IRF-3 binds to a critical cis-acting element in the IL-12p35 gene promoter and thereby represents a key factor for the induction of IL-12p70 synthesis in DCs.

Down-regulation of p53 by double-stranded RNA modulates the antiviral response

p53 has been well characterized as a tumor suppressor gene, but its role in antiviral defense remains unclear. A recent report has demonstrated that p53 can be induced by interferons and is activated after vesicular stomatitis virus (VSV) infection. We observed that different nononcogenic viruses, including encephalomyocarditis virus (EMCV) and human parainfluenza virus type 3 (HPIV3), induced down-regulation of p53 in infected cells. Double-stranded RNA (dsRNA) and a mutant vaccinia virus lacking the dsRNA binding protein E3L can also induce this effect, indicating that dsRNA formed during viral infection is likely the trigger for down-regulation of p53. The mechanism of down-regulation of p53 by dsRNA relies on translation inhibition mediated by the PKR and RNase L pathways. In the absence of p53, the replication of both EMCV and HPIV3 was retarded, whereas, conversely, VSV replication was enhanced. Cell cycle analysis indicated that wild-type (WT) but not p53 knockout (KO) fibroblasts undergo an early-G(1) arrest following dsRNA treatment. Moreover, in WT cells the onset of dsRNA-induced apoptosis begins after p53 levels are down-regulated, whereas p53 KO cells, which lack the early-G(1) arrest, rapidly undergo apoptosis. Hence, our data suggest that the down-regulation of p53 facilitates apoptosis, thereby limiting viral replication.

Hepatitis A virus suppresses RIG-I-mediated IRF-3 activation to block induction of beta interferon

Hepatitis A virus (HAV) antagonizes the innate immune response by inhibition of double-stranded RNA (dsRNA)-induced beta interferon (IFN-beta) gene expression. In this report, we show that this is due to an interaction of HAV with the intracellular dsRNA-induced retinoic acid-inducible gene I (RIG-I)-mediated signaling pathway upstream of the kinases responsible for interferon regulatory factor 3 (IRF-3) phosphorylation (TBK1 and IKKepsilon). In consequence, IRF-3 is not activated for nuclear translocation and gene induction. In addition, we found that HAV reduces TRIF (TIR domain-containing adaptor inducing IFN-beta)-mediated IRF-3 activation, which is part of the Toll-like receptor 3 signaling pathway. As IRF-3 is necessary for IFN-beta transcription, inhibition of this factor results in efficient suppression of IFN-beta synthesis. This ability of HAV seems to be of considerable importance for HAV replication, as HAV is not resistant to IFN-beta, and it may allow the virus to establish infection and preserve the sites of virus production in later stages of the infection.

Identification of TRAIL as an interferon regulatory factor 3 transcriptional target

Interferon production and apoptosis in virus-infected cells are necessary to prevent progeny virus production and to eliminate infected cells. Paramyxovirus infection induces apoptosis through interferon regulatory factor 3 (IRF-3), but the exact mechanism of how IRF-3 functions is unknown. We show that IRF-3 is involved in the transcriptional induction of TRAIL, a key player in the apoptosis pathway. IRF-3 upregulates TRAIL transcription following viral infection and binds an interferon-stimulated response element in the TRAIL promoter. The mRNA for TRAIL and its receptor, DR5, are induced following viral infection. These studies identify TRAIL as a novel IRF-3 transcriptional target.

Regulation of arginase II by interferon regulatory factor 3 and the involvement of polyamines in the antiviral response

The innate antiviral response requires the induction of genes and proteins with activities that limit virus replication. Among these, the well-characterized interferon beta (IFNB) gene is regulated through the cooperation of AP-1, NF-kappaB and interferon regulatory factor 3 (IRF-3) transcription factors. Using a constitutively active form of IRF-3, IRF-3 5D, we showed previously that IRF-3 also regulates an IFN-independent antiviral response through the direct induction of IFN-stimulated genes. In this study, we report that the arginase II gene (ArgII) as well as ArgII protein concentrations and enzymatic activity are induced in IRF-3 5D-expressing and Sendai virus-infected Jurkat cells in an IFN-independent manner. ArgII is a critical enzyme in the polyamine-biosynthetic pathway. Of the natural polyamines, spermine possesses antiviral activity and mediates apoptosis at physiological concentrations. Measurement of intracellular polyamine content revealed that expression of IRF-3 5D induces polyamine production, but that Sendai virus and vesicular stomatitis virus infections do not. These results show for the first time that the ArgII gene is an early IRF-3-regulated gene, which participates in the IFN-independent antiviral response through polyamine production and induction of apoptosis.

Involvement of Noxa in Cellular Apoptotic Responses to Interferon, Double-stranded RNA, and Virus Infection

Double-stranded RNA (dsRNA) accumulates in virally infected cells, leading to induction of genes encoding proteins involved in signaling, apoptosis, protein synthesis/processing, and cell metabolism. Noxa is a BH3-containing mitochondrial protein that contributes to apoptosis by disrupting mitochondrial outer membrane integrity. Here we demonstrate potent induction of Noxa expression by exposure of cells to dsRNA, interferon (IFN), and virus. Noxa induction was confirmed by using reverse transcriptase-PCR and immunoblot analyses in multiple human tumor cell lines. Importantly, Noxa regulation by IFN and dsRNA was independent of p53, thereby identifying a novel mechanism of Noxa induction. Ectopic expression of Noxa in HT1080 fibrosarcoma cells enhanced cellular sensitivity to viral or dsRNA/actinomycin D-induced apoptosis, typified by enhanced cytochrome c release from the mitochondrial to the cytosolic fraction and increased cleavage of caspases 3 and 9. Point and deletion mutations of Noxa confirmed that both the BH3 domain and the mitochondrial-targeting domain were necessary for enhanced cellular apoptotic responses to dsRNA, IFN, or virus. Treatment of cells with dsRNA or virus, but not etoposide, induced interaction between Noxa and Bax that required an intact Noxa BH3 domain. Interestingly, the Noxa mitochondrial-targeting domain deletion mutant interacted with Bax in a dsRNA-dependent manner and redirected Bax away from the mitochondria, thus acting as a dominant-negative protein. Together, these data suggest that Noxa is an important component of the innate immune response of cells to viral infection, leading to enhanced cellular apoptosis that may play a role in limiting viral dissemination.

Activation of TBK1 and IKKvarepsilon kinases by vesicular stomatitis virus infection and the role of viral ribonucleoprotein in the development of interferon antiviral immunity

Mounting an immune response to a viral pathogen involves the initial recognition of viral antigens through Toll-like receptor-dependent and -independent pathways and the subsequent triggering of signal transduction cascades. Among the many cellular kinases stimulated in response to virus infection, the noncanonical IKK-related kinases TBK1 and IKKepsilon have been shown to phosphorylate and activate interferon regulatory factor 3 (IRF-3) and IRF-7, leading to the production of alpha/beta interferons and the development of a cellular antiviral state. In the present study, we examine the activation of TBK1 and IKKepsilon kinases by vesicular stomatitis virus (VSV) infection in human lung epithelial A549 cells. We demonstrate that replication-competent VSV is required to induce activation of the IKK-related kinases and provide evidence that ribonucleoprotein (RNP) complex of VSV generated intracellularly during virus replication can activate TBK1 and IKKepsilon activity. In TBK1-deficient cells, IRF-3 and IRF-7 activation is significantly reduced, although transcriptional upregulation of IKKepsilon following treatment with VSV, double-stranded RNA, or RNP partially compensates for the loss of TBK1. Biochemical analyses with purified TBK1 and IKKepsilon kinases in vitro demonstrate that the two kinases exhibit similar specificities with respect to IRF-3 and IRF-7 substrates and both kinases target serine residues that are important for full transcriptional activation of IRF-3 and IRF-7. These data suggest that intracellular RNP formation contributes to the early recognition of VSV infection, activates the catalytic activity of TBK1, and induces transcriptional upregulation of IKKepsilon in epithelial cells. Induction of IKKepsilon potentially functions as a component of the amplification mechanism involved in the establishment of the antiviral state.

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.

The Ebola virus VP35 protein inhibits activation of interferon regulatory factor 3

The Ebola virus VP35 protein was previously found to act as an interferon (IFN) antagonist which could complement growth of influenza delNS1 virus, a mutant influenza virus lacking the influenza virus IFN antagonist protein, NS1. The Ebola virus VP35 could also prevent the virus- or double-stranded RNA-mediated transcriptional activation of both the beta IFN (IFN-beta) promoter and the IFN-stimulated ISG54 promoter (C. Basler et al., Proc. Natl. Acad. Sci. USA 97:12289-12294, 2000). We now show that VP35 inhibits virus infection-induced transcriptional activation of IFN regulatory factor 3 (IRF-3)-responsive mammalian promoters and that VP35 does not block signaling from the IFN-alpha/beta receptor. The ability of VP35 to inhibit this virus-induced transcription correlates with its ability to block activation of IRF-3, a cellular transcription factor of central importance in initiating the host cell IFN response. We demonstrate that VP35 blocks the Sendai virus-induced activation of two promoters which can be directly activated by IRF-3, namely, the ISG54 promoter and the ISG56 promoter. Further, expression of VP35 prevents the IRF-3-dependent activation of the IFN-alpha4 promoter in response to viral infection. The inhibition of IRF-3 appears to occur through an inhibition of IRF-3 phosphorylation. VP35 blocks virus-induced IRF-3 phosphorylation and subsequent IRF-3 dimerization and nuclear translocation. Consistent with these observations, Ebola virus infection of Vero cells activated neither transcription from the ISG54 promoter nor nuclear accumulation of IRF-3. These data suggest that in Ebola virus-infected cells, VP35 inhibits the induction of antiviral genes, including the IFN-beta gene, by blocking IRF-3 activation.

Bunyamwera virus nonstructural protein NSs counteracts interferon regulatory factor 3-mediated induction of early cell death

The genome of Bunyamwera virus (BUN; family Bunyaviridae, genus Orthobunyavirus) consists of three segments of negative-sense RNA. The smallest segment, S, encodes two proteins, the nonstructural protein NSs, which is nonessential for viral replication and transcription, and the nucleocapsid protein N. Although a precise role in the replication cycle has yet to be attributed to NSs, it has been shown that NSs inhibits the induction of alpha/beta interferon, suggesting that it plays a part in counteracting the host antiviral defense. A defense mechanism to limit viral spread is programmed cell death by apoptosis. Here we show that a recombinant BUN that does not express NSs (BUNdelNSs) induces apoptotic cell death more rapidly than wild-type virus. Screening for apoptosis pathways revealed that the proapoptotic transcription factor interferon regulatory factor 3 (IRF-3) was activated by both wild-type BUN and BUNdelNSs infection, but only wild-type BUN was able to suppress signaling downstream of IRF-3. Studies with a BUN minireplicon system showed that active replication induced an IRF-3-dependent promoter, which was suppressed by the NSs protein. In a cell line (P2.1) defective in double-stranded RNA signaling due to low levels of IRF-3, induction of apoptosis was similar for wild-type BUN and BUNdelNSs. These data suggest that the BUN NSs protein can delay cell death in the early stages of BUN infection by inhibiting IRF-3-mediated apoptosis.

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.