Viral interferon regulatory factor 1 of Kaposi's sarcoma-associated herpesvirus (human herpesvirus 8) binds to, and inhibits transactivation of, CREB-binding protein

Novel Functions and Virus-Host Interactions Implicated in Pathogenesis and Replication of Human Herpesvirus 8

Human herpesvirus 8 (HHV-8) is classified as a γ2-herpesvirus and is related to Epstein-Barr virus (EBV), a γ1-herpesvirus. One important aspect of the γ-herpesviruses is their association with neoplasia, either naturally or in animal model systems. HHV-8 is associated with B-cell-derived primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD), endothelial-derived Kaposi's sarcoma (KS), and KSHV inflammatory cytokine syndrome (KICS). EBV is also associated with a number of B-cell malignancies, such as Burkitt's lymphoma, Hodgkin's lymphoma, and posttransplant lymphoproliferative disease, in addition to epithelial nasopharyngeal and gastric carcinomas. Despite the similarities between these viruses and their associated malignancies, the particular protein functions and activities involved in key aspects of virus biology and neoplastic transformation appear to be quite distinct. Indeed, HHV-8 specifies a number of proteins for which counterparts had not previously been identified in EBV, other herpesviruses, or even viruses in general, and these proteins are believed to play vital functions in virus biology and to be involved centrally in viral pathogenesis. Additionally, a set of microRNAs encoded by HHV-8 appears to modulate the expression of multiple host proteins to provide conditions conductive to virus persistence within the host and possibly contributing to HHV-8-induced neoplasia. Here, we review the molecular biology underlying these novel virus-host interactions and their potential roles in both virus biology and virus-associated disease.

Of Keeping and Tipping the Balance: Host Regulation and Viral Modulation of IRF3-Dependent IFNB1 Expression

The type I interferon (IFN) response is a principal component of our immune system that allows to counter a viral attack immediately upon viral entry into host cells. Upon engagement of aberrantly localised nucleic acids, germline-encoded pattern recognition receptors convey their find via a signalling cascade to prompt kinase-mediated activation of a specific set of five transcription factors. Within the nucleus, the coordinated interaction of these dimeric transcription factors with coactivators and the basal RNA transcription machinery is required to access the gene encoding the type I IFN IFNβ (IFNB1). Virus-induced release of IFNβ then induces the antiviral state of the system and mediates further mechanisms for defence. Due to its key role during the induction of the initial IFN response, the activity of the transcription factor interferon regulatory factor 3 (IRF3) is tightly regulated by the host and fiercely targeted by viral proteins at all conceivable levels. In this review, we will revisit the steps enabling the trans-activating potential of IRF3 after its activation and the subsequent assembly of the multi-protein complex at the IFNβ enhancer that controls gene expression. Further, we will inspect the regulatory mechanisms of these steps imposed by the host cell and present the manifold strategies viruses have evolved to intervene with IFNβ transcription downstream of IRF3 activation in order to secure establishment of a productive infection.

USP7-Dependent Regulation of TRAF Activation and Signaling by a Viral Interferon Regulatory Factor Homologue

Human herpesvirus 8 (HHV-8) encodes four viral interferon regulatory factors (vIRFs 1 to 4), all of which are expressed during lytic replication and inhibit a variety of antiviral signaling pathways. Viral IRFs 1, 2, and 3 are also expressed during latency in primary effusion lymphoma (PEL) cells, and vIRF-1 and vIRF-3 have been reported to promote PEL cell viability. Viral IRFs 1, 3, and 4 are known to interact with ubiquitin-specific protease 7 (USP7); interactions of vIRF-1 and vIRF-3 with USP7 promote PEL cell viability and regulate productive replication. Here, we report that vIRF-2 also targets USP7, utilizing a PSTS motif matching the USP7 N-terminal domain-binding A/PxxS consensus, but uniquely requires catalytic domain residues for intracellular interaction. In functional and mechanistic analyses, tumor necrosis factor receptor-associated factor (TRAF)-mediated signaling and associated polyubiquitination of TRAFs 3 and 6, specifically, were regulated negatively by USP7 and positively by vIRF-2-USP7 interaction, the latter competing for USP7-TRAF association. Using depletion, depletion-complementation, and targeted mutagenesis approaches, vIRF-2 was determined to promote latent PEL cell viability, likely independently of USP7 interaction, while lytic replication was inhibited by vIRF-2, in part or in whole via USP7 interaction. Together, our data identify a new molecular determinant of USP7 recognition, TRAF3/6-specific targeting by the deubiquitinase, associated activation of these TRAFs by vIRF-2, and activities of vIRF-2 and vIRF-2-USP7 interaction in HHV-8 latent and lytic biology.IMPORTANCE Human herpesvirus 8-encoded IRF homologues were the first to be identified in a virus. Through inhibitory interactions with cellular IRFs and other mediators of antiviral signaling, the vIRFs are believed to be essential for productive replication and also for latency in particular cell types. The deubiquitinase USP7 is a regulator of key cellular pathways, modulates HHV-8 latent and lytic infection, and is targeted by vIRFs 1, 3, and 4. Here, we report that vIRF-2 also interacts with USP7, via a means distinguishable from USP7 interactions with other vIRFs and other proteins, that this interaction modulates antiviral signaling via disruption of USP7 interactions with innate immune signaling proteins TRAF3 and TRAF6, and that vIRF-2 targeting of USP7 regulates HHV-8 productive replication. The presented data are the first to identify vIRF-2 targeting of USP7 and its role in HHV-8 biology, expanding our understanding of the repertoire and importance of virus-host interactions.

The Molecular Basis of Viral Inhibition of IRF- and STAT-Dependent Immune Responses

The antiviral innate immunity is the first line of host defense against virus infections. In mammalian cells, viral infections initiate the expression of interferons (IFNs) in the host that in turn activate an antiviral defense program to restrict viral replications by induction of IFN stimulated genes (ISGs), which are largely regulated by the IFN-regulatory factor (IRF) family and signal transducer and activator of transcription (STAT) family transcription factors. The mechanisms of action of IRFs and STATs involve several post-translational modifications, complex formation, and nuclear translocation of these transcription factors. However, many viruses, including human immunodeficiency virus (HIV), Zika virus (ZIKV), and herpes simplex virus (HSV), have evolved strategies to evade host defense, including alteration in IRF and STAT post-translational modifications, disturbing the formation and nuclear translocation of the transcription complexes as well as proteolysis/degradation of IRFs and STATs. In this review, we discuss and summarize the molecular mechanisms by which how viral components may target IRFs and STATs to antagonize the establishment of antiviral host defense. The underlying host-viral interactions determine the outcome of viral infection. Gaining mechanistic insight into these processes will be crucial in understanding how viral replication can be more effectively controlled and in developing approaches to improve virus infection outcomes.

Importance of Promyelocytic Leukema Protein (PML) for Kaposi's Sarcoma-Associated Herpesvirus Lytic Replication

Many DNA virus replication-related proteins are associated with promyelocytic leukemia protein (PML), a component of nuclear domain 10 (ND10), which has been investigated for its potential involvement in viral replication. In the case of Kaposi’s sarcoma-associated herpesvirus (KSHV) lytic gene products, K8 (K-bZIP), ORF59, and ORF75 have been shown to colocalize with PML, but its importance in KSHV lytic replication is still unclear. In this study, we analyzed the functional influence of PML on KSHV latency and lytic replication in KSHV-infected primary effusion lymphoma (PEL) cell lines. Stable PML-knockout (BC3-PML^KO) and PML-overexpressing BC3 cells (BC3^PML) were successfully generated and the latency and reactivation status were analyzed. The results demonstrated that neither KSHV latency nor the episome copy number was affected in BC3-PML^KO cells. In the reactivation phase, the expression dynamics of KSHV immediate-early or early lytic proteins such as RTA, K9 (vIRF1), K5, K3, ORF59, and K8 (K-bZIP) were comparable between wild-type, control BC3, and BC3-PML^KO cells. Interestingly, KSHV lytic replication, virion production, and expression of late genes were downregulated in BC3-PML^KO cells and upregulated in BC3^PML cells, compared to those in control or wild-type BC3 cells. Moreover, exogenous PML increased the size of the PML dots and recruited additional K8 (K-bZIP) to PML-NBs as dots. Therefore, PML would function as a positive regulator for KSHV lytic DNA replication by recruiting KSHV replication factors such as 8 (K-bZIP) or ORF59 to the PML-NBs.

Cellular and viral oncogenes: the key to unlocking unknowns of Kaposi's sarcoma-associated herpesvirus pathogenesis

Oncogenic viruses carry an extensive arsenal of oncogenes for hijacking cellular pathways. Notably, variations in oncogenes among tumor-producing viruses give rise to different mechanisms for cellular transformation. Specifically, Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic virus able to infect and transform a variety of cell types. The oncogenicity of KSHV disseminates from the virus' ability to induce and encode a wide variety of both cellular and viral oncogenes. Such an array of cellular and viral oncogenes enables KSHV to induce the malignant phenotype of a KSHV-associated cancer. Evolutionarily, KSHV has acquired many oncogenic homologues capable of inducing cell proliferation, cell differentiation, cell survival, and immune evasion. Integration between inducing and encoding oncogenes plays a vital role in KSHV pathogenicity. KSHV is alleged to harbor the highest number of potential oncogenes by which a virus promotes cellular transformation and malignancy. Many KSHV inducing/encoding oncogenes are mainly expressed during the latent phase of KSHV infection, a period required for virus establishment of malignant cellular transformation. Elucidation of the exact mechanism(s) by which oncogenes promote KSHV pathogenicity would not only give rise to potential novel therapeutic targets/drugs but would also add to our understanding of cancer biology. The scope of this review is to examine the roles of the most important cellular and viral oncogenes involved in KSHV pathogenicity.

Viral Evasion Strategies in Type I IFN Signaling - A Summary of Recent Developments

The immune system protects the organism against infections and the damage associated with them. The first line of defense against pathogens is the innate immune response. In the case of a viral infection, it induces the interferon (IFN) signaling cascade and eventually the expression of type I IFN, which then causes an antiviral state in the cells. However, many viruses have developed strategies to counteract this mechanism and prevent the production of IFN. In order to modulate or inhibit the IFN signaling cascade in their favor, viruses have found ways to interfere at every single step of the cascade, for example, by inducing protein degradation or cleavage, or by mediate protein polyubiquitination. In this article, we will review examples of viruses that modulate the IFN response and describe the mechanisms they use.

A Rhesus Rhadinovirus Viral Interferon (IFN) Regulatory Factor Is Virion Associated and Inhibits the Early IFN Antiviral Response

The interferon (IFN) response is the earliest host immune response dedicated to combating viral infection. As such, viruses have evolved strategies to subvert this potent antiviral response. Two closely related gammaherpesviruses, Kaposi's sarcoma-associated herpesvirus (KSHV) and rhesus macaque rhadinovirus (RRV), are unique in that they express viral homologues to cellular interferon regulatory factors (IRFs), termed viral IRFs (vIRFs). Cellular IRFs are a family of transcription factors that are particularly important for the transcription of type I IFNs. Here, we demonstrate a strategy employed by RRV to ensure rapid inhibition of virus-induced type I IFN induction. We found that RRV vIRF R6, when expressed ectopically, interacts with a transcriptional coactivator, CREB-binding protein (CBP), in the nucleus. As a result, phosphorylated IRF3, an important transcriptional regulator in beta interferon (IFN-β) transcription, fails to effectively bind to the IFN-β promoter, thus inhibiting the activation of IFN-β genes. In addition, we found R6 within RRV virion particles via immunoelectron microscopy and, furthermore, that virion-associated R6 is capable of inhibiting the type I IFN response by preventing efficient binding of IRF3/CBP complexes to the IFN-β promoter in the context of infection. The work shown here is the first example of a vIRF being associated with either the KSHV or RRV virion. The presence of this immunomodulatory protein in the RRV virion provides the virus with an immediate mechanism to evade the host IFN response, thus enabling the virus to effectively establish an infection within the host.

IMPORTANCE

Kaposi's sarcoma-associated herpesvirus (KSHV) and the closely related rhesus macaque rhadinovirus (RRV) are the only viruses known to encode viral homologues to cellular interferon regulatory factors (IRFs), known as vIRFs. In KSHV, these proteins have been shown to play major roles in a variety of cellular processes and are particularly important in the evasion of the host type I interferon (IFN) response. In this study, we delineate the immunomodulatory mechanism of an RRV vIRF and its ability to assist the virus in rapid immune evasion by being prepackaged within the virion, thus providing evidence, for the first time, of a virion-associated vIRF. This work further contributes to our understanding of the mechanisms behind immunomodulation by the RRV vIRFs during infection.

Molecular biology of human herpesvirus 8: novel functions and virus-host interactions implicated in viral pathogenesis and replication

Human herpesvirus 8 (HHV-8), also known as Kaposi's sarcoma-associated herpesvirus (KSHV), is the second identified human gammaherpesvirus. Like its relative Epstein-Barr virus, HHV-8 is linked to B-cell tumors, specifically primary effusion lymphoma and multicentric Castleman's disease, in addition to endothelial-derived KS. HHV-8 is unusual in its possession of a plethora of "accessory" genes and encoded proteins in addition to the core, conserved herpesvirus and gammaherpesvirus genes that are necessary for basic biological functions of these viruses. The HHV-8 accessory proteins specify not only activities deducible from their cellular protein homologies but also novel, unsuspected activities that have revealed new mechanisms of virus-host interaction that serve virus replication or latency and may contribute to the development and progression of virus-associated neoplasia. These proteins include viral interleukin-6 (vIL-6), viral chemokines (vCCLs), viral G protein-coupled receptor (vGPCR), viral interferon regulatory factors (vIRFs), and viral antiapoptotic proteins homologous to FLICE (FADD-like IL-1β converting enzyme)-inhibitory protein (FLIP) and survivin. Other HHV-8 proteins, such as signaling membrane receptors encoded by open reading frames K1 and K15, also interact with host mechanisms in unique ways and have been implicated in viral pathogenesis. Additionally, a set of micro-RNAs encoded by HHV-8 appear to modulate expression of multiple host proteins to provide conditions conducive to virus persistence within the host and could also contribute to HHV-8-induced neoplasia. Here, we review the molecular biology underlying these novel virus-host interactions and their potential roles in both virus biology and virus-associated disease.

The crystal structure of the DNA-binding domain of vIRF-1 from the oncogenic KSHV reveals a conserved fold for DNA binding and reinforces its role as a transcription factor

Kaposi’s sarcoma-associated herpesvirus encodes four viral homologues to cellular interferon regulatory factors (IRFs), where the most studied is vIRF-1. Even though vIRF-1 shows sequence homology to the N-terminal DNA-binding domain (DBD) of human IRFs, a specific role for this domain in vIRF-1’s function has remained uncertain. To provide insights into the function of the vIRF-1 DBD, we have determined the crystal structure of it in complex with DNA and in its apo-form. Using a thermal stability shift assay (TSSA), we show that the vIRF-1 DBD binds DNA, whereas full-length vIRF-1 does not, suggesting a cis-acting regulatory mechanism in similarity to human IRFs. The complex structure of vIRF-1 DBD reveals interactions with the DNA backbone and the positioning of two arginines for specific recognition in the major grove. A superimposition with human IRF-3 reveals a similar positioning of the two specificity-determining arginines, and additional TSSAs indicate binding of vIRF-1 to an IRF-3 operator consensus sequence. The results from this study, therefore, provide support that vIRF-1 has evolved to bind DNA and plays a role in DNA binding in the context of transcriptional regulation and might act on some of the many operator sequences controlled by human IRF-3.

The viral interferon regulatory factors of kaposi's sarcoma-associated herpesvirus differ in their inhibition of interferon activation mediated by toll-like receptor 3

Kaposi's sarcoma-associated herpesvirus (KSHV) infection is correlated with three human malignancies and can establish lifelong latent infection in multiple cell types within its human host. In order to establish and maintain infection, KSHV utilizes multiple mechanisms to evade the host immune response. One such mechanism is the expression of a family of genes with homology to cellular interferon (IFN) regulatory factors (IRFs), known as viral IRFs (vIRFs). We demonstrate here that KSHV vIRF1, -2, and -3 have a differential ability to block type I interferon signaling mediated by Toll-like receptor 3 (TLR3), a receptor we have previously shown to be activated upon KSHV infection. vIRF1, -2, and -3 inhibited TLR3-driven activation of IFN transcription reporters. However, only vIRF1 and vIRF2 inhibited increases in both IFN-β message and protein levels following TLR3 activation. The expression of vIRF1 and vIRF2 also allowed for increased replication of a virus known to activate TLR3 signaling. Furthermore, vIRF1 and vIRF2 may block TLR3-mediated signaling via different mechanisms. Altogether, this report indicates that vIRFs are able to block IFN mediated by TLRs but that each vIRF has a unique function and mechanism for blocking antiviral IFN responses.

Viral interferon regulatory factors decrease the induction of type I and type II interferon during rhesus macaque rhadinovirus infection

Kaposi's sarcoma-associated herpesvirus and rhesus macaque rhadinovirus (RRV), two closely related gammaherpesviruses, are unique in their expression of viral homologs of cellular interferon regulatory factors (IRFs), termed viral IRFs (vIRFs). To assess the role of vIRFs during de novo infection, we have utilized the bacterial artificial chromosome clone of wild-type RRV(17577) (WT(BAC) RRV) to generate a recombinant virus with all 8 of the vIRFs deleted (vIRF-ko RRV). The infection of primary rhesus fibroblasts and peripheral blood mononuclear cells (PBMCs) with vIRF-ko RRV resulted in earlier and increased induction of type I interferon (IFN) (IFN-α/β) and type II IFN (IFN-γ). Additionally, plasmacytoid dendritic cells maintained higher levels of IFN-α production in PBMC cultures infected with vIRF-ko RRV than in cultures infected with WT(BAC) RRV. Moreover, the nuclear accumulation of phosphorylated IRF-3, which is necessary for the induction of type I IFN, was also inhibited following WT(BAC) RRV infection. These findings demonstrate that during de novo RRV infection, vIRFs are inhibiting the induction of IFN at the transcriptional level, and one potential mechanism for this is the disruption of the activation and localization of IRF-3.

Molecular biology of Kaposi's sarcoma-associated herpesvirus and related oncogenesis

Kaposi's Sarcoma-associated Herpesvirus (KSHV), also known as human herpesvirus 8 (HHV-8), is the most recently identified human tumor virus,and is associated with the pathogenesis of Kaposi's sarcoma and two lymphoproliferative disorders known to occur frequently in AIDS patients-primary effusion lymphoma and multicentric Castleman disease. In the 15 years since its discovery, intense studies have demonstrated an etiologic role for KSHV in the development of these malignancies. Here, we review the recent advances linked to understanding KSHV latent and lytic life cycle and the molecular mechanisms of KSHV-mediated oncogenesis in terms of transformation, cell signaling, cell growth and survival, angiogenesis, immune invasion and response to microenvironmental stress, and highlight the potential therapeutic targets for blocking KSHV tumorigenesis.

Molecular mechanisms of viral immune evasion proteins to inhibit MHC class I antigen processing and presentation

Viral products inhibit MHC class I antigen processing and presentation via three major pathways: inhibition of major histocompatibility complex (MHC) class I expression on cells, blockade of peptide trafficking and loading on MHC class I molecules, and inhibition of peptide generation in host cells. Viral products also interfere with IFN-gamma -mediated JAK/STAT signal transduction in cells. These results imply that viral proteins probably inhibit the function of IFN-gamma in MHC class I antigen presentation via inactivation of JAK/STAT signal transduction in host cells. Mechanisms of viral products to inhibit IFN-gamma -mediated MHC class I antigen presentation were summarized in this literature review.

Epigenetic regulation of Kaposi's sarcoma-associated herpesvirus replication

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi's sarcoma and B-lymphocyte disorders, primary effusion lymphoma (PEL) and Multicentric Castleman's Disease (MCD). KSHV usually exists in a latent form in which the viral genome is circularized into an extrachormosomal episome. However, induction of lytic replication by environmental stimuli or chemical agents is important for the spread of KSHV. The switch between latency and lytic replication is regulated by epigenetic factors. Hypomethylation of the promoter of replication and transcription activator (RTA), which is essential for the lytic switch, leads to KSHV reactivation. Histone acetylation induces KSHV replication by influencing protein-protein-associations and transcription factor binding. Histone modifications also determine chromatin structure and nucleosome positioning, which are important for KSHV DNA replication during latency. The association of KSHV proteins with chromatin remodeling complexes promotes the open chromatin structure needed for transcription factor binding and DNA replication. Additionally, post-translational modification of KSHV proteins is important for the regulation of RTA activity and KSHV replication. KSHV may also cause epigenetic modification of the host genome, contributing to promoter hypermethylation of tumor suppressor genes in KSHV-associated neoplasias.

Identification of the DNA sequence interacting with Kaposi's sarcoma-associated herpesvirus viral interferon regulatory factor 1

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of Kaposi's sarcoma. The open reading frame (K9) of KSHV encodes viral interferon regulatory factor 1 (vIRF1), which functions as a repressor of interferon-mediated signal transduction. The amino-terminal region of vIRF1 displays significant homology to the DNA-binding domain of cellular interferon regulatory factors, supporting the theory that the protein interacts with specific DNA sequences. Here, we identify the consensus sequence of vIRF1-binding sites from a pool of random oligonucleotides. Moreover, our data show that vIRF1 interacts with the K3:viral dihydrofolate reductase:viral interleukin 6 promoter region in the KSHV genome.

Molecular biology of KSHV in relation to AIDS-associated oncogenesis

KSHV has been established as the causative agent of KS, PEL, and MCD, malignancies occurring more frequently in AIDS patients. The aggressive nature of KSHV in the context of HIV infection suggests that interactions between the two viruses enhance pathogenesis. KSHV latent infection and lytic reactivation are characterized by distinct gene expression profiles, and both latency and lytic reactivation seem to be required for malignant progression. As a sophisticated oncogenic virus, KSHV has evolved to possess a formidable repertoire of potent mechanisms that enable it to target and manipulate host cell pathways, leading to increased cell proliferation, increased cell survival, dysregulated angiogenesis, evasion of immunity, and malignant progression in the immunocompromised host. Worldwide, approximately 40.3 million people are currently living with HIV infection. Of these, a significant number are coinfected with KSHV. The complex interplay between the two viruses dramatically elevates the risk for development of KSHV-induced malignancies, KS, PEL, and MCD. Although HAART significantly reduces HIV viral load, the entire T-cell repertoire and immune function may not be completely restored. In fact, clinically significant immune deficiency is not necessary for the induction of KSHV-related malignancy. Because of variables such as lack of access to therapy noncompliance with prescribed treatment, failure to respond to treatment and the development of drug-resistant strains of HIV, KSHV-induced malignancies will continue to present as major health concerns.

Kaposi sarcoma herpesvirus-encoded interferon regulator factors

The Kaposi sarcoma herpesvirus (KSHV) encodes multiple proteins that disrupt host antiviral responses, including four viral proteins that have homology to the interferon regulatory factor (IRF) family of transcription factors. At least three of the KSHV vIRFs (vIRFs 1-3) alter responses to cellular IRFs and to interferons (IFNs), whereas functional changes resulting from the fourth vIRF (vIRF-4) have not been reported. The vIRFs also affect other important regulatory proteins in the cell, including responses to transforming growth factor beta (TGF-beta) and the tumor suppressor protein p53. This review examines the expression of the vIRFs during the life cycle of KSHV and the functional consequences of their expression.

Herpesviruses and the innate immune response

Herpesvirus infection leads to the rapid induction of an innate immune response. A central aspect of this host response is the production and secretion of type I interferon. The current model of virus-mediated interferon production includes three stages: sensitization, induction, and amplification. A key mediator of all three stages is the cellular transcription factor interferon regulatory factor 3 (IRF3). Although the precise details of IRF3 activation and interferon production in response to herpesvirus infection are still being elucidated, viral proteins that block components of the interferon pathway, particularly IRF3, have been identified and characterized. In vivo studies have shown that in addition to type I interferon, interleukin-15 (IL-15) and natural killer (NK) cells also play an important role in mediating resistance to herpesvirus infection. Recent investigations have demonstrated a strong association between IRF3, interferon, IL-15, and NK cells. This review will focus on herpesvirus-mediated induction of innate immunity, the central role of the type I interferon response and mechanisms used by herpesviruses to block host antiviral immunity.

Inhibition of nuclear factor kappaB activity by viral interferon regulatory factor 3 of Kaposi's sarcoma-associated herpesvirus

Nuclear factor-kappaB (NF-kappaB) is a transcription factor that plays an important role in the immune system and cell death. Many viral proteins modulate NF-kappaB to escape host immune surveillance, promote cell survival, and enhance viral replication. In the present study, we show that NF-kappaB activity is downmodulated by viral interferon regulatory factor 3 (vIRF3), which is encoded by Kaposi's sarcoma-associated herpesvirus open-reading frame K10.5. vIRF3 repressed NF-kappaB-dependent transcription in a dose-dependent manner and inhibited the activation of NF-kappaB induced by tumor necrosis factor (TNF)-alpha. In vivo studies showed vIRF3 inhibited IkappaB kinase beta (IKKbeta) activity, but not IKKalpha activity, resulting in reduced IkappaB phosphorylation. Immunofluorescence assays showed that vIRF3 interfered with nuclear translocation of NF-kappaB. In addition, consistent with the inhibition of NF-kappaB activity, vIRF3 sensitized cells to TNF-alpha-induced apoptosis. While vIRF3 interacts with IKKbeta in vitro and in 293T cells, we were unable to demonstrate vIRF3-IKKbeta interaction in BCBL-1 cells. Our results indicate that vIRF3 can regulate the host immune system and apoptosis via inhibition of NF-kappaB activity.

Kaposi's Sarcoma–Associated Herpesvirus Viral IFN Regulatory Factor 1 Inhibits Transforming Growth Factor-β Signaling

Kaposi's sarcoma-associated herpesvirus, also called human herpesvirus 8, has been implicated in the pathogenesis of Kaposi's sarcoma, body cavity-based primary effusion lymphoma, and some forms of multicentric Castleman's disease. The Kaposi's sarcoma-associated herpesvirus open reading frame K9 encodes viral IFN regulatory factor 1 (vIRF1), which functions as a repressor of IFN-mediated signal transduction. vIRF1 expression in NIH 3T3 cells leads to transformation and consequently induces malignant fibrosarcoma in nude mice, suggesting that vIRF1 is a strong oncoprotein. Here, we show that vIRF1 inhibited transforming growth factor-beta (TGF-beta) signaling via its targeting of Smad proteins. vIRF1 suppressed TGF-beta-mediated transcription and growth arrest. vIRF1 directly interacted with both Smad3 and Smad4, resulting in inhibition of their transactivation activity. Studies using vIRF1 deletion mutants showed that the central region of vIRF1 was required for vIRF1 association with Smad3 and Smad4 and that this region was also important for inhibition of TGF-beta signaling. In addition, we found that vIRF1 interfered with Smad3-Smad4 complex formation and inhibited Smad3/Smad4 complexes from binding to DNA. These results indicate that vIRF1 inhibits TGF-beta signaling via interaction with Smads. In addition, the data indicate the TGF-beta pathway is an important target for viral oncoproteins.

Regulation of growth signalling and cell cycle by Kaposi's sarcoma-associated herpesvirus genes

Kaposi's sarcoma-associated herpesvirus (KSHV) is the primary aetiological agent of at least three malignancies associated with HIV infection and immunosuppression: Kaposi's sarcoma, primary effusion lymphoma and multicentric Castleman's disease. KSHV encodes proteins that deregulate key checkpoints in the signalling pathways governing cell proliferation, which may ultimately contribute to the virus' oncogenic potential. To alter cellular signalling associated with proliferation, these viral proteins function like growth factor ligands/receptors, signal transduction proteins, transcription factors and cell cycle regulators. This review focuses on the mechanisms by which some KSHV-encoded proteins activate signalling pathways and cell proliferation and their role in the pathogenesis of KSHV-driven mechanisms.

Overlapping CRE and E box motifs in the enhancer sequences of the bovine leukemia virus 5' long terminal repeat are critical for basal and acetylation-dependent transcriptional activity of the viral promoter: implications for viral latency

Bovine leukemia virus (BLV) infection is characterized by viral latency in a large proportion of cells containing an integrated provirus. In this study, we postulated that mechanisms directing the recruitment of deacetylases to the BLV 5' long terminal repeat (LTR) could explain the transcriptional repression of viral expression in vivo. Accordingly, we showed that BLV promoter activity was induced by several deacetylase inhibitors (such as trichostatin A [TSA]) in the context of episomal LTR constructs and in the context of an integrated BLV provirus. Moreover, treatment of BLV-infected cells with TSA increased H4 acetylation at the viral promoter, showing a close correlation between the level of histone acetylation and transcriptional activation of the BLV LTR. Among the known cis-regulatory DNA elements located in the 5' LTR, three E box motifs overlapping cyclic AMP responsive elements (CREs) in U3 were shown to be involved in transcriptional repression of BLV basal gene expression. Importantly, the combined mutations of these three E box motifs markedly reduced the inducibility of the BLV promoter by TSA. E boxes are susceptible to recognition by transcriptional repressors such as Max-Mad-mSin3 complexes that repress transcription by recruiting deacetylases. However, our in vitro binding studies failed to reveal the presence of Mad-Max proteins in the BLV LTR E box-specific complexes. Remarkably, TSA increased the occupancy of the CREs by CREB/ATF. Therefore, we postulated that the E box-specific complexes exerted their negative cooperative effect on BLV transcription by steric hindrance with the activators CREB/ATF and/or their transcriptional coactivators possessing acetyltransferase activities. Our results thus suggest that the overlapping CRE and E box elements in the BLV LTR were selected during evolution as a novel strategy for BLV to allow better silencing of viral transcription and to escape from the host immune response.

Short duration of elevated vIRF-1 expression during lytic replication of human herpesvirus 8 limits its ability to block antiviral responses induced by alpha interferon in BCBL-1 cells

Human herpesvirus 8 (HHV-8) encodes multiple proteins that disrupt the host antiviral response, including viral interferon (IFN) regulatory factor 1 (vIRF-1). The product of the vIRF-1 gene blocks responses to IFN when overexpressed by transfection, but the functional consequence of vIRF-1 that is expressed during infection with HHV-8 is not known. These studies demonstrate that BCBL-1 cells that were latently infected with HHV-8 expressed low levels of vIRF-1 that were associated with PML bodies, whereas much higher levels of vIRF-1 were transiently expressed during the lytic phase of HHV-8 replication. The low levels of vIRF-1 that were associated with PML bodies were insufficient to block alpha IFN (IFN-alpha)-induced alterations in gene expression, whereas cells that expressed high levels of vIRF-1 were resistant to some changes induced by IFN-alpha, including the expression of the double-stranded-RNA-activated protein kinase. High levels of vIRF-1 were expressed for only a short period during the lytic cascade, so many cells with HHV-8 in the lytic phase responded to IFN-alpha with increased expression of antiviral genes and enhanced apoptosis. Furthermore, the production of infectious virus was severely compromised when IFN-alpha was present early during the lytic cascade. These studies indicate that the transient expression of high levels of vIRF-1 is inadequate to subvert many of the antiviral effects of IFN-alpha so that IFN-alpha can effectively induce apoptosis and block production of infectious virus when present early in the lytic cascade of HHV-8.

Human herpesvirus 8: biology and role in the pathogenesis of Kaposi's sarcoma and other AIDS-related malignancies

Human herpesvirus type 8, or Kaposi's sarcoma-associated herpesvirus (KSHV), is the only known human g(2) herpesvirus (rhadinovirus) and the most recently discovered tumor virus. KSHV is associated with Kaposi's sarcoma and two other lymphoproliferative disorders in the AIDS setting: primary effusion lymphoma and the plasma cell variant of multicentric Castleman's disease. This review offers an update on the epidemiology and the role of KSHV genes in the development of disease.

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.

Transcription mapping of human herpesvirus 8 genes encoding viral interferon regulatory factors

The human herpesvirus 8 (HHV-8) genome contains four tandemly arranged genes encoding viral interferon regulatory factors (vIRF-1 to 4) located between genes 57 and 58. Transcript mapping techniques were employed to determine the sizes, ends and splicing patterns of mRNAs specified by these genes in HHV-8-infected cell lines untreated or chemically induced into the lytic growth cycle. Depending on the cell line used, vIRF-3 transcription was minimally or not induced (i.e. expressed with latent kinetics), whereas the other vIRFs were inducible (i.e. expressed with lytic kinetics). Each gene possessed its own promoter (or promoters) and polyadenylation sites, and all but vIRF-1 were spliced from two exons. vIRF-1 was transcribed in uninduced and induced cells from a single initiation site preceded by a TATA box, with the possible use of an additional TATA box and initiation site in uninduced cells. In induced cells, vIRF-2 was transcribed from a single major initiation site preceded by a TATA box, and vIRF-4 was expressed from two sites each preceded by a TATA box. Transcripts for these genes were insufficiently abundant in uninduced cells to map the 5'-ends. vIRF-3 lacks an obvious TATA box and exhibited heterogeneous 5'-ends in uninduced and induced cells. These data clarify and extend our understanding of the structure and transcription of the HHV-8 vIRF genes.

Human Herpesvirus 8: Biology and Role in the Pathogenesis of Kaposi's Sarcoma and Other AIDS-related Malignancies

Human herpesvirus type 8, or Kaposi's sarcoma-associated herpesvirus (KSHV), is the only known human g(2) herpesvirus (rhadinovirus) and the most recently discovered tumor virus. KSHV is associated with Kaposi's sarcoma and two other lymphoproliferative disorders in the AIDS setting: primary effusion lymphoma and the plasma cell variant of multicentric Castleman's disease. This review offers an update on the epidemiology and the role of KSHV genes in the development of disease.

Auto-activation of the transforming viral interferon regulatory factor encoded by Kaposi's sarcoma-associated herpesvirus (human herpesvirus-8)

Kaposi's sarcoma-associated herpesvirus (KSHV)/human herpesvirus-8-encoded viral interferon regulatory factor (vIRF) transforms NIH3T3 cells, represses interferon signal transduction and regulates the expression of other KSHV genes. Here, we have shown that vIRF is a transcriptional activator and auto-activates its own expression. Ectopic expression of vIRF activated the vIRF promoter in KSHV-negative 293, COS7, HeLa and BJAB cell lines in a dose-dependent fashion in a reporter assay and the expression of vIRF transcripts from endogenous viral genomes in BCBL-1 and BC-1 cells latently infected with KSHV. Deletion analysis identified two cis elements, named Vac1 and Vac2, in the vIRF promoter that were responsive to vIRF activation. vIRF auto-activation via Vac1 but not Vac2 was repressed by Tis, a transcriptional silencer in the vIRF promoter. Neither Vac1 nor Vac2 contain any interferon-stimulated response element (ISRE)-like sequences and are unresponsive to induction with interferon-beta and -gamma. These results indicate that KSHV uses the mechanism of auto-activation to regulate the expression of a viral transforming protein to efficiently evade host tumour suppressor pathways.

Paramyxovirus strategies for evading the interferon response

Two genera, the Respirovirus (Sendai virus (SeV) and human parainfluenza virus (hPIV3) and the Rubulavirus (simian virus (SV) 5, SV41, mumps virus and hPIV2), of the three in the subfamily Paramyxovirinae inhibit interferon (IFN) signalling to circumvent the IFN response. The viral protein responsible for the inhibition is the C protein for respirovirus SeV and the V protein for the rubulaviruses, both of which are multifunctional accessory proteins expressed from the P gene. SeV suppresses IFN-stimulated tyrosine phosphorylation of signal transducers and activators of transcription (STATs) at an early phase of infection and further inhibits the downstream signalling without degrading any of the signalling components in most cell lines. On the contrary, the Rubulavirus V protein targets Stat1 or Stat2 for degradation. Proteasome-mediated degradation appears to be involved in most cases. Studies on the molecular mechanisms by which paramyxoviruses evade the IFN response will offer important information for modulating the JAK-STAT pathway, designing novel antiviral drugs and recombinant live vaccines, and improving paramyxovirus expression vectors for gene therapy.

Viral interferon regulatory factor 1 of Kaposi's sarcoma-associated herpesvirus interacts with a cell death regulator, GRIM19, and inhibits interferon/retinoic acid-induced cell death

Kaposi's sarcoma-associated herpesvirus (KSHV) plays a significant role in the development of Kaposi's sarcoma, primary effusion lymphoma, and some forms of multicentric Castleman's disease. The KSHV open reading frame K9 encodes the viral interferon (IFN) factor 1 (vIRF1), which downregulates IFN- and IRF-mediated transcriptional activation, and leads to cellular transformation in rodent fibroblasts and induction of tumors in nude mice. Using the yeast two-hybrid assay, we identified genes associated with retinoid-IFN-induced mortality-19 (GRIM19), which interacts directly with vIRF1, both in vivo and in vitro. The N-terminal region of vIRF1 is required for binding GRIM19. Colocalization of vIRF1 and GRIM19 was observed in 293T cells. The vIRF1 protein deregulates GRIM19-induced apoptosis in the presence of IFN/all-trans-retinoic acid (RA) and inhibits IFN/RA-induced cell death. Another DNA tumor viral protein, human papillomavirus type 16 E6, also binds GRIM19, suggesting that this is a general target of viral proteins. Our results collectively indicate that vIRF1 modulates IFN/RA-cell death signals via interactions with GRIM19.

Transcriptional regulation of Kaposi's sarcoma-associated herpesvirus-encoded oncogene viral interferon regulatory factor by a novel transcriptional silencer, Tis

Viral interferon regulatory factor (vIRF) encoded by Kaposi's sarcoma-associated herpesvirus (KSHV) has been shown to transform NIH3T3 and Rat-1 cells, inhibit interferon signal transduction, and regulate the expression of KSHV genes. We had previously characterized the vIRF core promoter and defined a 12-O-tetradecanoylphorbol-13-acetate (TPA)-responsive region in the upstream regulatory sequence of vIRF gene. Here, we have further identified a novel transcriptional silencer, named Tis in this region. Tis represses the promoter activities of vIRF and heterologous herpes simplex virus thymidine kinase genes in both position- and orientation-independent manners. Deletion analysis has identified a cis-element of 23 nucleotides that is essential for the negative regulation. Two Tis-binding protein complexes, named vR1 and vR2, were observed by electrophoretic mobility shift assays using nuclear extracts from both KSHV-negative and -positive cell lines. A sequence fragment GAGTTAATAGGTAGAG in the cis-element was shown to be required for the DNA-protein interactions as well as the repression of vIRF promoter activity. Point-mutation analysis identified TTAAT and GTTAATAG as the core sequence motifs for the binding of vR1 and vR2, respectively. These results define the function of a novel transcriptional silencer in the regulation of vIRF gene expression.

The molecular pathology of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is the eighth and most recently identified human herpesvirus (HHV-8). KSHV was discovered in 1994 by Chang et al. who used representational difference analysis to search for DNA sequences present in AIDS-associated KS but not in adjacent normal skin [1]. The virus has since been shown to be specifically associated with all forms of this disease and has fulfilled all of Hill's criteria for causation (reviewed in ). KSHV is also found in all cases of primary effusion lymphoma and in a plasmablastic variant of multicentric Castleman's disease. Over the last few years a wealth of data has been gained on the role of KSHV genes during infection. This review is an attempt to assemble this information into a more complete picture of how KSHV may cause disease.

Human herpesvirus-8 and Kaposi's sarcoma: relationship with the multistep concept of tumorigenesis

Kaposi's sarcoma (KS) develops through discrete inflammatory-angiogenic stages of polyclonal nature (early-stage lesions) to monomorphic nodules of spindle-shaped cells that can be clonal (late-stage lesions) and resemble true sarcomas. Molecular and epidemiological studies indicate that development of KS is tightly associated with infection by the human herpesvirus-8 (HHV-8). However, only individuals with specific conditions of immunodysregulation develop KS. In these individuals the systemic and tissue increase of Th-1-type cytokines (IC) reactivate HHV-8 infection, leading to increased viral load, antibody titers, and an expanded cell tropism that precedes the clinical appearance of KS. Recruitment of the virus into tissues by infected monocytes and other cell types is facilitated by the endothelial cell activation due to IC. In clinical lesions, HHV-8 infection increases with lesion stage and in late-stage lesions most of the spindle cells are latently infected, whereas only few lyrically infected cells are present, suggesting that latent genes may have a role in the transformation of the early inflammatory-hyperplastic lesion into a real sarcoma. The development of tumors, however, is regulated through a multistep process based on the acquisition by cells of several different capabilities leading to malignant growth. Here we review the available data on the expression of HHV-8-encoded genes in primary KS lesions and, in view of their biological activity, analyze their potential function in different steps of tumorigenesis. By this pragmatic approach interesting insights into potential key functions of HHV-8-encoded genes are found and steps of potential cooperativity with other viral factors (HIV-1-Tat) in the pathogenesis of KS are identified.

On the role of IRF in host defense

Transcription factors of the interferon (IFN) regulatory factor (IRF) family have been shown to play an essential role in the regulated expression of type I IFN genes, IFN-stimulated genes (ISG), and other cytokines and chemokines. Three members of the IRF family, IRF-3, IRF-5, and IRF-7, have been identified as acting as direct transducers of virus-mediated signaling. In infected cells, these factors are activated by phosphorylation on the serine residues, transported to the nucleus, where they bind to the promoters of IFNA and IFNB genes and tether histone transacetylases to the transcription complex enhanceosome. IFNB and IFNA subtypes are expressed at different levels in infected cells. The ratio between the relative levels of IRF-3 and IRF-7 was shown to play an essential role in the inducible expression of type I IFN genes, whereas IRF-3 alone is sufficient for expression of the IFNB gene. IRF-5 was identified recently as another inducer of IFNA genes, which has two unique properties: (1) its activation is virus specific, and (2) the profile of IFNA genes induced by IRF-5 is distinct from that induced by IRF-7. Several viruses target functions of IRF to eliminate the early inflammatory response. Kaposi's sarcoma herpesvirus (KSHV) encodes a cluster of four genes with homology to cellular IRF. Three of these vIRF were shown to inhibit induction of IFN genes and ISG in infected cells and function as dominant negative mutants of cellular IRF. The unique properties of previously uncharacterized vIRF-2 and vIRF-3 are discussed.

The transcriptional activity of cAMP response element-binding protein-binding protein is modulated by the latency associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus

A multifunctional transcription co-activator, cAMP response element-binding protein-binding protein (CBP)interacts with a number of cellular factors and participates in cell growth, transformation, and development. It is also targeted by many viral proteins for their transcriptional activity or for the regulation of cellular processes. Here, we report that the C/H3 region of CBP is targeted by the latency associated nuclear antigen (LANA) of Kaposi's sarcoma-associated herpesvirus (KSHV). LANA interferes with the interaction between CBP and c-Fos, a representative C/H3 region binding, cellular transcription factor, in vivo and in vitro. In addition, we found that LANA inhibits the transcriptional activity and the in vitro histone acetyltransferase activity of CBP, suggesting that LANA modulates the global transcriptional activities of infected cells through the interaction with CBP. These results indicate that KSHV follows one of the conserved strategies, which other viruses utilize for influencing the cellular processes.

Viral interferon regulatory factor 1 of Kaposi's sarcoma-associated herpesvirus binds to p53 and represses p53-dependent transcription and apoptosis

Kaposi's sarcoma-associated herpesvirus (KSHV) is related to the development of Kaposi's sarcoma. Open reading frame K9 of KSHV encodes viral interferon regulatory factor 1 (vIRF1), which functions as a repressor of interferon- and IRF1-mediated signal transduction. In addition, vIRF1 acts as an oncogene to induce cellular transformation. Here we show that vIRF1 directly associates with the tumor suppressor p53 and represses its functions. The vIRF1 interaction domains of p53 are the DNA binding domain (amino acids [aa] 100 to 300) and the tetramerization domain (aa 300 to 393). p53 interacts with the central region (aa 152 to 360) of vIRF1. vIRF1 suppresses p53-dependent transcription and deregulates its apoptotic activity. These results suggest that vIRF1 may regulate cellular function by inhibiting p53.

KSHV/HHV8-associated lymphoproliferations in the AIDS setting

Kaposi's sarcoma-associated herpesvirus (KSHV) or human herpesvirus 8 (HHV8) is associated with two lymphoproliferative disorders in the AIDS setting, primary effusion lymphoma (PEL) and the plasma cell variant of multicentric Castleman's disease (MCD). In PEL, KSHV persists in a latent form in most lymphoma cells, although viral production has been seen infrequently. In MCD, the viral gene expression pattern is less restrictive, virus production appears to occur and to correlate with the severity of this disease. Several viral genes may contribute to the particular features of these two disorders: among them a viral homologue of interleukin 6 (vIL6) has attracted much attention and been shown to promote the growth of plasma cells. It is thought that its activity is important in the pathogenesis of both PEL and MCD. Other viral genes, in particular a D-type cyclin homologue, the latent nuclear antigen LANA, and one or more of the viral homologues of interferon regulatory factors (vIRFs) may also contribute. Although it is conceivable that viral infection per se could explain much, if not all, of the features of MCD, it is likely that additional genetic alterations play a role in the pathogenesis of PEL.

Suppression of oncogenic viral interferon regulatory factor (vIRF) of Kaposi's sarcoma-associated herpesvirus by ribozyme-mediated cleavage

Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 (KSHV/HHV8) has been etiologically associated with several malignancies including Kaposi's sarcoma and primary effusion lymphoma. Oncogenic viral interferon regulatory factor (vIRF) encoded by KSHV ORF-K9 is a homologue of cellular interferon regulatory factor (IRF), and has been demonstrated to inhibit type I/II interferon signal transduction and transform NIH3T3 cells through the interactions with IRF-1, IRF-3, and CBP/p300 proteins. To counteract vIRF's pathogenic role, we have developed five ribozymes targeting ORF-K9 mRNA to suppress vIRF expression. The vIRF RNA substrates were cleaved up to 80% in a substrate-specific manner in transcript cleavage assays in vitro. In a transient transfection assay, two of the ribozymes efficiently suppressed the expression of vIRF protein measured by dual-color immunofluorescence assay that simultaneously detects the expression of both vIRF protein and ribozyme. Flow cytometry analysis showed that these ribozymes reduced vIRF expression up to 76%. A mutant ribozyme had no cleavage activity in vitro, but exhibited antisense effect in vivo. These results suggest that the ribozymes may provide a new approach for functional knockout of vIRF gene, and are potential candidates of antiviral therapy for KSHV-related malignancies.