Kaposi's sarcoma-associated herpesvirus viral interferon regulatory factor 4 (vIRF4/K10) is a novel interaction partner of CSL/CBF1, the major downstream effector of Notch signaling

Biologically significant interaction of human herpesvirus 8 viral interferon regulatory factor 4 with ubiquitin-specific protease 7

Human herpesvirus 8 (HHV-8), associated with Kaposi sarcoma, primary effusion lymphoma (PEL), and multicentric Castleman disease, encodes four interferon regulatory factor homologs, vIRFs 1-4, that interact with and inhibit various mediators of host-cell defense against virus infection. A cellular protein targeted by all the vIRFs is ubiquitin-specific protease 7 (USP7); while replication-modulatory and latently infected PEL-cell pro-viability phenotypes of USP7 targeting have been identified for vIRFs 1-3, the significance of the interaction of vIRF-4 with USP7 has remained undetermined. Here we show, through genetic ablation of the vIRF-4-USP7 interaction in infected cells, that vIRF-4 association with USP7 is necessary for optimal expression of vIRF-4 and normal HHV-8 replication. Findings from experiments on transfected and infected cells identified ubiquitination of vIRF-4 via K48-linkage and USP7-binding-associated suppression of vIRF-4 ubiquitination and, in infected cells, increased vIRF-4 expression. Analysis of IFN-I induction and associated signaling as a function of vIRF-4 and its interaction with USP7 identified a role of each in innate-immune suppression. Finally, activation via K63-polyubiquitination of the innate-immune signaling mediator TRAF3 was found to be suppressed by vIRF-4 in a USP7-binding-associated manner in infected cells, but not in transfected cells, likely via binding-regulated expression of vIRF-4. Together, our data identify the first examples of vIRF ubiquitination and a vIRF substrate of USP7, enhanced expression of vIRF-4 via its interaction with USP7, and TRAF3-inhibitory activity of vIRF-4. The findings address, for the first time, the biological significance of the interaction of vIRF-4 with USP7 and reveal a mechanism of vIRF-4-mediated innate-immune evasion and pro-replication activity via TRAF3 regulation.

IMPORTANCE

HHV-8 homologs of cellular interferon regulatory factors (IRFs), involved in host-cell defense against virus infection, interact in an inhibitory fashion with IRFs and other mediators of antiviral innate immunity. These interactions are of demonstrated or hypothesized importance for successful primary, productive (lytic), and latent (persistent) infection by HHV-8. While HHV-8 vIRF-4 is known to interact physically with USP7 deubiquitinase, a key regulator of various cellular proteins, the functional and biological significance of the interaction has not been addressed. The present study identifies the interaction as important for HHV-8 productive replication and, indeed, for vIRF-4 expression and reveals a new function of vIRF-4 via inhibition of the activity of TRAF3, a pivotal mediator of host-cell antiviral activity through activation of cellular IRFs and induction of type-I interferons. These findings identify potential targets for the development of novel anti-HHV-8 agents, such as those able to disrupt vIRF-4-USP7 interaction or vIRF-4-stabilizing USP7 activity.

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.

Kaposi's Sarcoma-Associated Herpesvirus Lytic Replication Is Independent of Anaphase-Promoting Complex Activity

The anaphase-promoting complex, or cyclosome (APC/C), is a large E3 ubiquitin ligase composed of 14 subunits. The activity of APC/C oscillates during the cell cycle to ensure a timely transition through each phase by promoting the degradation of important cell cycle regulators. Of the human herpesviruses, cytomegalovirus (HCMV) and Epstein-Barr virus (EBV) both impair the activity of APC/C during their lytic replication cycle through virus-encoded protein kinases. Here, we addressed whether the oncogenic Kaposi's sarcoma-associated herpesvirus (KSHV) deregulates the activity of APC/C during the lytic replication cycle. To this end, we used the well-characterized iSLK.219 cell model of KSHV infection and established a new infection model of primary lymphatic endothelial cells (LECs) infected with a lytically replicating KSHV BAC16 mutant. In contrast to those of EBV and HCMV, the KSHV lytic cycle occurs while the APC/C is active. Moreover, interfering with the activity of APC/C did not lead to major changes in the production of infectious virus. We further investigated whether rereplication stress induced by the unscheduled activation of the APC/C-CDH1 complex affects the number and integrity of KSHV viral episomes. Deep sequencing of the viral episomes and host chromosomes in iSLK.219 cells revealed that, while distinct regions in the cellular chromosomes were severely affected by rereplication stress, the integrity of the viral episomes remained unaltered.IMPORTANCE DNA viruses have evolved complex strategies to gain control over the cell cycle. Several of them target APC/C, a key cellular machinery that controls the timely progression of the cell cycle, by either blocking or enhancing its activity. Here, we investigated the activity of APC/C during the lytic replication cycle of KSHV and found that, in contrast to that of KSHV's close relatives EBV and HCMV, KSHV lytic replication occurs while the APC/C is active. Perturbing APC/C activity by depleting a core protein or the adaptor proteins of the catalytic domain, and hence interfering with normal cell-cycle progression, did not affect virus replication. This suggests that KSHV has evolved to replicate independently of the activity of APC/C and in various cell cycle conditions.

Structurally conserved binding motifs of transcriptional regulators to notch nuclear effector CSL

This mini review discusses the protein complexes comprised of the universal Notch signaling transcription factor, CSL (CBF1/Su(H)/Lag-1), and its activating or repressing transcriptional coregulation partners. Many of these complex structures have been solved crystallographically as well as undergoing extensive binding studies with wild-type and mutant variants. Notch signaling is critically important in a large variety of basic biological processes: cell proliferation, differentiation, cell cycle control to name a few. Aberrant Notch thus remains a coveted target for pharmaceutical intervention. To that end, we provide a molecular-level summary of the similarities and differences in the Notch coregulator complexes that ultimately govern these processes. We highlight a conserved binding motif that multiple superficially unrelated proteins have adopted to become involved in Notch target gene regulation. As CSL-interacting small molecules begin to be characterized, this review will provide insight to potential binding sites and differential complex disruption.

Impact statement

Proper Notch signaling regulation is informed by many distinct protein complexes involving a single nuclear effector. A decade of research into these protein complexes yields multiple crystal structures and a wealth of binding data to guide drug development for Notch-related diseases – cancer, cardiovascular, development disorders.

Kaposi's sarcoma-associated herpesvirus vIRF2 protein utilizes an IFN-dependent pathway to regulate viral early gene expression

Kaposi’s sarcoma-associated herpesvirus (KSHV; human herpesvirus 8) belongs to the subfamily of Gammaherpesvirinae and is the etiological agent of Kaposi’s sarcoma as well as of two lymphoproliferative diseases: primary effusion lymphoma and multicentric Castleman disease. The KSHV life cycle is divided into a latent and a lytic phase and is highly regulated by viral immunomodulatory proteins which control the host antiviral immune response. Among them is a group of proteins with homology to cellular interferon regulatory factors, the viral interferon regulatory factors 1–4. The KSHV vIRFs are known as inhibitors of cellular interferon signaling and are involved in different oncogenic pathways. Here we characterized the role of the second vIRF protein, vIRF2, during the KSHV life cycle. We found the vIRF2 protein to be expressed in different KSHV positive cells with early lytic kinetics. Importantly, we observed that vIRF2 suppresses the expression of viral early lytic genes in both newly infected and reactivated persistently infected endothelial cells. This vIRF2-dependent regulation of the KSHV life cycle might involve the increased expression of cellular interferon-induced genes such as the IFIT proteins 1, 2 and 3, which antagonize the expression of early KSHV lytic proteins. Our findings suggest a model in which the viral protein vIRF2 allows KSHV to harness an IFN-dependent pathway to regulate KSHV early gene expression.

The life cycle of Kaposi Sarcoma herpesvirus involves both persistence in a latent form and productive replication to generate new viral particles. How the virus switches between latency and productive (‘lytic’) replication is only partially understood. Here we show that a viral homologue of interferon regulatory factors, vIRF2, antagonizes lytic protein expression in endothelial cells. It does this by inducing the expression of cellular interferon-regulated genes such as IFIT 1–3, which in turn dampens early viral gene expression. This observation suggests that vIRF2 allows KSHV to harness the interferon pathway to regulate early viral gene expression in endothelial cells.

Comparative Analysis of Gammaherpesvirus Circular RNA Repertoires: Conserved and Unique Viral Circular RNAs

Recent studies have identified circular RNAs (circRNAs) expressed from the Epstein-Barr virus (EBV) and Kaposi's sarcoma herpesvirus (KSHV) human DNA tumor viruses. To gain initial insights into the potential relevance of EBV circRNAs in virus biology and disease, we assessed the circRNAome of the interspecies homologue rhesus macaque lymphocryptovirus (rLCV) in a naturally occurring lymphoma from a simian immunodeficiency virus (SIV)-infected rhesus macaque. This analysis revealed rLCV orthologues of the latency-associated EBV circular RNAs circRPMS1_E4_E3a and circEBNA_U. Also identified in two samples displaying unusually high lytic gene expression was a novel rLCV circRNA that contains both conserved and rLCV-specific RPMS1 exons and whose backsplice junctions flank an rLCV lytic origin of replication (OriLyt). Analysis of a lytic infection model for the murid herpesvirus 68 (MHV68) rhadinovirus identified a cluster of circRNAs near an MHV68 lytic origin of replication, with the most abundant of these, circM11_ORF69, spanning the OriLyt. Lastly, analysis of KSHV latency and reactivation models revealed the latency associated circRNA originating from the vIRF4 gene as the predominant viral circRNA. Together, the results of this study broaden our appreciation for circRNA repertoires in the Lymphocryptovirus and Rhadinovirus genera of gammaherpesviruses and provide evolutionary support for viral circRNA functions in latency and viral replication.IMPORTANCE Infection with oncogenic gammaherpesviruses leads to long-term viral persistence through a dynamic interplay between the virus and the host immune system. Critical for remodeling of the host cell environment after the immune responses are viral noncoding RNAs that modulate host signaling pathways without attracting adaptive immune recognition. Despite the importance of noncoding RNAs in persistent infection, the circRNA class of noncoding RNAs has only recently been identified in gammaherpesviruses. Accordingly, their roles in virus infection and associated oncogenesis are unknown. Here we report evolutionary conservation of EBV-encoded circRNAs determined by assessing the circRNAome in rLCV-infected lymphomas from an SIV-infected rhesus macaque, and we report latent and lytic circRNAs from KSHV and MHV68. These experiments demonstrate utilization of the circular RNA class of RNAs across 4 members of the gammaherpesvirus subfamily, and they identify orthologues and potential homoplastic circRNAs, implying conserved circRNA functions in virus biology and associated malignancies.

Kaposi sarcoma herpesvirus pathogenesis

Kaposi sarcoma herpesvirus (KSHV), taxonomical name human gammaherpesvirus 8, is a phylogenetically old human virus that co-evolved with human populations, but is now only common (seroprevalence greater than 10%) in sub-Saharan Africa, around the Mediterranean Sea, parts of South America and in a few ethnic communities. KSHV causes three human malignancies, Kaposi sarcoma, primary effusion lymphoma, and many cases of the plasmablastic form of multicentric Castleman's disease (MCD) as well as occasional cases of plasmablastic lymphoma arising from MCD; it has also been linked to rare cases of bone marrow failure and hepatitis. As it has colonized humans physiologically for many thousand years, cofactors are needed to allow it to unfold its pathogenic potential. In most cases, these include immune defects of genetic, iatrogenic or infectious origin, and inflammation appears to play an important role in disease development. Our much improved understanding of its life cycle and its role in pathogenesis should now allow us to develop new therapeutic strategies directed against key viral proteins or intracellular pathways that are crucial for virus replication or persistence. Likewise, its limited (for a herpesvirus) distribution and transmission should offer an opportunity for the development and use of a vaccine to prevent transmission.

This article is part of the themed issue ‘Human oncogenic viruses’.

Signal Transduction Pathways Associated with KSHV-Related Tumors

Signal transduction pathways play a key role in the regulation of cell growth, cell differentiation, cell survival, apoptosis, and immune responses. Bacterial and viral pathogens utilize the cell signal pathways by encoding their own proteins or noncoding RNAs to serve their survival and replication in infected cells. Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8 (HHV-8), is classified as a rhadinovirus in the γ-herpesvirus subfamily and was the eighth human herpesvirus to be discovered from Kaposi's sarcoma specimens. KSHV is closely associated with an endothelial cell malignancy, Kaposi's sarcoma, and B-cell malignancies, primary effusion lymphoma, and multicentric Castleman's disease. Recent studies have revealed that KSHV manipulates the cellular signaling pathways to achieve persistent infection, viral replication, cell proliferation, anti-apoptosis, and evasion of immune surveillance in infected cells. This chapter summarizes recent developments in our understanding of the molecular mechanisms used by KSHV to interact with the cell signaling machinery.

KSHV and the Role of Notch Receptor Dysregulation in Disease Progression

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of two human cancers, Kaposi's Sarcoma (KS) and primary effusion lymphoma (PEL), and a lymphoproliferation, Multicentric Castleman's Disease (MCD). Progression to tumor development in KS is dependent upon the reactivation of the virus from its latent state. We, and others, have shown that the Replication and transcriptional activator (Rta) protein is the only viral gene product that is necessary and sufficient for viral reactivation. To induce the reactivation and transcription of viral genes, Rta forms a complex with the cellular DNA binding component of the canonical Notch signaling pathway, recombination signal binding protein for Jk (RBP-Jk). Formation of this Rta:RBP-Jk complex is necessary for viral reactivation to occur. Expression of activated Notch has been shown to be dysregulated in KSHV infected cells and to be necessary for cell growth and disease progression. Studies into the involvement of activated Notch in viral reactivation have yielded varied results. In this paper, we review the current literature regarding Notch dysregulation by KSHV and its role in viral infection and cellular pathogenesis.

Rhadinoviral interferon regulatory factor homologues

Kaposi's sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8 (HHV8) is a gammaherpesvirus and the etiological agent of Kaposi's sarcoma, primary effusion lymphoma and multicentric Castleman disease. The KSHV genome contains genes for a unique group of proteins with homology to cellular interferon regulatory factors, termed viral interferon regulatory factors (vIRFs). This review will give an overview over the oncogenic, antiapoptotic and immunomodulatory characteristics of KSHV and related vIRFs.

Kaposi Sarcoma-associated Herpesvirus: mechanisms of oncogenesis

Kaposi Sarcoma-associated Herpesvirus (KSHV, HHV8) causes three human malignancies, Kaposi Sarcoma (KS), an endothelial tumor, as well as Primary Effusion Lymphoma (PEL) and the plasma cell variant of Multicentric Castleman's Disease (MCD), two B-cell lymphoproliferative diseases. All three cancers occur primarily in the context of immune deficiency and/or HIV infection, but their pathogenesis differs. KS most likely results from the combined effects of an endotheliotropic virus with angiogenic properties and inflammatory stimuli and thus represents an interesting example of a cancer that arises in an inflammatory context. Viral and cellular angiogenic and inflammatory factors also play an important role in the pathogenesis of MCD. In contrast, PEL represents an autonomously growing malignancy that is, however, still dependent on the continuous presence of KSHV and the action of several KSHV proteins.

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.

Kaposi's sarcoma-associated herpesvirus viral interferon regulatory factor 4 (vIRF4) targets expression of cellular IRF4 and the Myc gene to facilitate lytic replication

Besides an essential transcriptional factor for B cell development and function, cellular interferon regulatory factor 4 (c-IRF4) directly regulates expression of the c-Myc gene, which is not only associated with various B cell lymphomas but also required for herpesvirus latency and pathogenesis. Kaposi's sarcoma-associated herpesvirus (KSHV), the etiological agent of Kaposi's sarcoma and primary effusion lymphoma, has developed a unique mechanism to deregulate host antiviral innate immunity and growth control by incorporating four viral homologs (vIRF1 to -4) of cellular IRFs into its genome. Previous studies have shown that several KSHV latent proteins, including vIRF3, vFLIP, and LANA, target the expression, function, and stability of c-Myc to establish and maintain viral latency. Here we report that the KSHV vIRF4 lytic protein robustly suppresses expression of c-IRF4 and c-Myc, reshaping host gene expression profiles to facilitate viral lytic replication. Genomewide gene expression analysis revealed that KSHV vIRF4 grossly affects host gene expression by upregulating and downregulating 118 genes and 166 genes, respectively, by at least 2-fold. Remarkably, vIRF4 suppressed c-Myc expression by 11-fold, which was directed primarily by the deregulation of c-IRF4 expression. Real-time quantitative PCR (RT-qPCR), single-molecule in situ hybridization, and chromatin immunoprecipitation assays showed that vIRF4 not only reduces c-IRF4 expression but also competes with c-IRF4 for binding to the specific promoter region of the c-Myc gene, resulting in drastic suppression of c-Myc expression. Consequently, the loss of vIRF4 function in the suppression of c-IRF4 and c-Myc expression ultimately led to a reduction of KSHV lytic replication capacity. These results indicate that the KSHV vIRF4 lytic protein comprehensively targets the expression and function of c-IRF4 to downregulate c-Myc expression, generating a favorable environment for viral lytic replication. Finally, this study further reinforces the important role of the c-Myc gene in KSHV lytic replication and latency.

Structure and function of the CSL-KyoT2 corepressor complex: a negative regulator of Notch signaling

Notch refers to a highly conserved cell-to-cell signaling pathway with essential roles in embryonic development and tissue maintenance. Dysfunctional signaling causes human disease, highlighting the importance of pathway regulation. Notch signaling ultimately results in the activation of target genes, which is regulated by the nuclear effector CSL (CBF-1/RBP-J, Su(H), Lag-1). CSL dually functions as an activator and a repressor of transcription through differential interactions with coactivator or corepressor proteins, respectively. Although the structures of CSL-coactivator complexes have been determined, the structures of CSL-corepressor complexes are unknown. Here, using a combination of structural, biophysical, and cellular approaches, we characterize the structure and function of CSL in complex with the corepressor KyoT2. Collectively, our studies provide molecular insights into how KyoT2 binds CSL with high affinity and competes with coactivators, such as Notch, for binding CSL. These studies are important for understanding how CSL functions as both an activator and a repressor of transcription of Notch target genes.

Distinct roles of Kaposi's sarcoma-associated herpesvirus-encoded viral interferon regulatory factors in inflammatory response and cancer

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent associated with Kaposi's sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman disease (MCD). Similar to other herpesviruses, KSHV has two life cycles, latency and lytic replication. In latency, the KSHV genome persists as a circular episome in the nucleus of the host cell and only a few viral genes are expressed. In this review, we focus on oncogenic, antiapoptotic, and immunomodulating properties of KSHV-encoded homologues of cellular interferon regulatory factors (IRFs)--viral IRF1 (vIRF1) to vIRF4--and their possible role in the KSHV-mediated antiviral response, apoptosis, and oncogenicity.

Evasion of adaptive and innate immune response mechanisms by γ-herpesviruses

γ-Herpesviral immune evasion mechanisms are optimized to support the acute, lytic and the longterm, latent phase of infection. During acute infection, specific immune modulatory proteins limit, but also exploit, the antiviral activities of cell intrinsic innate immune responses as well as those of innate and adaptive immune cells. During latent infection, a restricted gene expression program limits immune targeting and cis-acting mechanisms to reduce the antigen presentation as well as antigenicity of latency-associated proteins. Here, we will review recent progress in our understanding of γ-herpesviral immune evasion strategies.

Abortive lytic reactivation of KSHV in CBF1/CSL deficient human B cell lines

Since Kaposi's sarcoma associated herpesvirus (KSHV) establishes a persistent infection in human B cells, B cells are a critical compartment for viral pathogenesis. RTA, the replication and transcription activator of KSHV, can either directly bind to DNA or use cellular DNA binding factors including CBF1/CSL as DNA adaptors. In addition, the viral factors LANA1 and vIRF4 are known to bind to CBF1/CSL and modulate RTA activity. To analyze the contribution of CBF1/CSL to reactivation in human B cells, we have successfully infected DG75 and DG75 CBF1/CSL knock-out cell lines with recombinant KSHV.219 and selected for viral maintenance by selective medium. Both lines maintained the virus irrespective of their CBF1/CSL status. Viral reactivation could be initiated in both B cell lines but viral genome replication was attenuated in CBF1/CSL deficient lines, which also failed to produce detectable levels of infectious virus. Induction of immediate early, early and late viral genes was impaired in CBF1/CSL deficient cells at multiple stages of the reactivation process but could be restored to wild-type levels by reintroduction of CBF1/CSL. To identify additional viral RTA target genes, which are directly controlled by CBF1/CSL, we analyzed promoters of a selected subset of viral genes. We show that the induction of the late viral genes ORF29a and ORF65 by RTA is strongly enhanced by CBF1/CSL. Orthologs of ORF29a in other herpesviruses are part of the terminase complex required for viral packaging. ORF65 encodes the small capsid protein essential for capsid shell assembly. Our study demonstrates for the first time that in human B cells viral replication can be initiated in the absence of CBF1/CSL but the reactivation process is severely attenuated at all stages and does not lead to virion production. Thus, CBF1/CSL acts as a global hub which is used by the virus to coordinate the lytic cascade.

Kaposi's sarcoma associated herpesvirus (KSHV) establishes a life-long persistent infection in B cells, which constitute the viral reservoir for reactivation and production of progeny virus. Viral reactivation is associated with multiple AIDS related malignancies including Kaposi's sarcoma, an endothelial tumor, and two B cell lymphoproliferative malignancies, the primary effusion lymphoma and the multicentric Castleman's disease. CBF1/CSL is a cellular DNA binding protein that can recruit transactivators or repressors to regulatory sites in the viral and cellular genome. The replication and transcription activator (RTA) plays an essential role in the switch between latency and lytic reactivation. RTA can either bind to DNA directly or is recruited to DNA via anchor proteins like CBF1/CSL and activates transcription. In this study we used a novel cell culture model to analyze the contribution of the CBF1/CSL protein to the process of viral reactivation in human B cells. Two isogenic CBF1/CSL proficient or deficient B cell lines were latently infected with recombinant KSHV. Lytic viral gene expression, viral replication and virus production were compared. Our results suggest that viral lytic gene expression is severely attenuated but not abolished at multiple stages before and after the onset of lytic replication while virus production is below detection levels in CBF1/CSL deficient B cells.

Noncanonical NF-κB activation by the oncoprotein Tio occurs through a nonconserved TRAF3-binding motif

Members of the nuclear factor κB (NF-κB) family of transcription factors regulate many cellular functions. Activation of NF-κB signaling is commonly classified as occurring through canonical or noncanonical pathways. Most NF-κB-inducing stimuli, including the viral oncoprotein Tio, lead to a concerted activation of both NF-κB pathways; however, extensive crosstalk at multiple levels between these signaling cascades restricts the ability to discriminate between the canonical and the noncanonical effects. We showed that noncanonical NF-κB activation by Tio depends on a distinct sequence motif that directly recruits tumor necrosis factor receptor-associated factor 3 (TRAF3). Through its TRAF3-binding motif, Tio triggered a ubiquitin-independent depletion of TRAF3 from the cytosol, which prevented TRAF3 from inhibiting signaling through the noncanonical NF-κB cascade. Furthermore, the Tio-TRAF3 interaction did not affect components of the canonical NF-κB signaling pathway or the expression of target genes; thus, Tio induced noncanonical NF-κB independently of crosstalk with the canonical pathway. Together, these data identify a distinct molecular mechanism of noncanonical NF-κB activation that should enable studies into the particular functions of this pathway.

O-GlcNAc transferase inhibits KSHV propagation and modifies replication relevant viral proteins as detected by systematic O-GlcNAcylation analysis

O-GlcNAcylation is an inducible, highly dynamic and reversible post-translational modification, mediated by a unique enzyme named O-linked N-acetyl-d-glucosamine (O-GlcNAc) transferase (OGT). In response to nutrients, O-GlcNAc levels are differentially regulated on many cellular proteins involved in gene expression, translation, immune reactions, protein degradation, protein-protein interaction, apoptosis and signal transduction. In contrast to eukaryotic cells, little is known about the role of O-GlcNAcylation in the viral life cycle. Here, we show that the overexpression of the OGT reduces the replication efficiency of Kaposi's sarcoma-associated herpesvirus (KSHV) in a dose-dependent manner. In order to investigate the global impact of O-GlcNAcylation in the KSHV life cycle, we systematically analyzed the 85 annotated KSHV-encoded open reading frames for O-GlcNAc modification. For this purpose, an immunoprecipitation (IP) strategy with three different approaches was carried out and the O-GlcNAc signal of the identified proteins was properly controlled for specificity. Out of the 85 KSHV-encoded proteins, 18 proteins were found to be direct targets for O-GlcNAcylation. Selected proteins were further confirmed by mass spectrometry for O-GlcNAc modification. Correlation of the functional annotation and the O-GlcNAc status of KSHV proteins showed that the predominant targets were proteins involved in viral DNA synthesis and replication. These results indicate that O-GlcNAcylation plays a major role in the regulation of KSHV propagation.

Instigation of Notch signaling in the pathogenesis of Kaposi's sarcoma-associated herpesvirus and other human tumor viruses

The Notch pathway is a highly conserved signaling circuit with a critical role in cell-fate determination and tumor initiation. Notch is reported to regulate various key events in tumor progression, such as angiogenesis, maintenance of cancer stem cells, resistance to therapeutic agents and metastasis. This review describes the intimate interplay of human tumor viruses with the Notch signaling pathway. Special attention is paid to Kaposi's sarcoma-associated herpesvirus, the etiological agent of Kaposi's sarcoma and rare lymphoproliferative disorders. The past decade of active research has led to significant advances in understanding how Kaposi's sarcoma-associated herpesvirus exploits the Notch pathway to regulate its replication phase and to modulate the host cellular microenvironment to make it more favorable for viral persistence and spreading.

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.

Cooperation between viral interferon regulatory factor 4 and RTA to activate a subset of Kaposi's sarcoma-associated herpesvirus lytic promoters

The four Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded interferon (IFN) regulatory factor homologues (vIRF1 to vIRF4) are used to counter innate immune defenses and suppress p53. The vIRF genes are arranged in tandem but differ in function and expression. In KSHV-infected effusion lymphoma lines, K10.5/vIRF3 and K11/vIRF2 mRNAs are readily detected during latency, whereas K9/vIRF1 and K10/vIRF4 mRNAs are upregulated during reactivation. Here we show that the K10/vIRF4 promoter responds to the lytic switch protein RTA in KSHV-infected cells but is essentially unresponsive in uninfected cells. Coexpression of RTA with vIRF4 is sufficient to restore regulation, a property not shared by other vIRFs. The K9/vIRF1 promoter behaves similarly, and production of infectious virus is enhanced by the presence of vIRF4. Synergy requires the DNA-binding domain (DBD) and C-terminal IRF homology regions of vIRF4. Mutations of arginine residues within the putative DNA recognition helix of vIRF4 or the invariant cysteines of the adjacent CxxC motif abolish cooperation with RTA, in the latter case by preventing self-association. The oligomerization and transactivation functions of RTA are also essential for synergy. The K10/vIRF4 promoter contains two transcription start sites (TSSs), and a 105-bp fragment containing the proximal promoter is responsive to vIRF4/RTA. Binding of a cellular factor(s) to this fragment is altered when both viral proteins are present, suggesting a possible mechanism for transcriptional synergy. Reliance on coregulators encoded by either the host or viral genome provides an elegant strategy for expanding the regulatory potential of a master regulator, such as RTA.

The viral interferon regulatory factors of KSHV: immunosuppressors or oncogenes?

Kaposi’s sarcoma-associated herpesvirus (KSHV) is a large double-stranded DNA gammaherpesvirus, and the etiological agent for three human malignancies: Kaposi’s sarcoma, primary effusion lymphoma, and multicentric Castleman’s disease. To establish and maintain infection, KSHV has evolved unique mechanisms to evade the host immune response. Cellular interferon regulatory factors (IRFs) are a critical part of the host anti-viral immune response. KSHV encodes four homologs of IRFs, vIRF1–4, which inhibit the activity of their cellular counterparts. vIRF1, 2, and 3 have been shown to interact directly with cellular IRFs. Additionally, the vIRFs have other functions such as modulation of Myc, p53, Notch, transforming growth factor-β, and NF-κB signaling. These activities of vIRFs may contribute to KSHV tumorigenesis. KSHV vIRF1 and vIRF3 have been implicated as oncogenes, making the understanding of KSHV vIRF function vital to understanding KSHV pathogenesis.