K13 blocks KSHV lytic replication and deregulates vIL6 and hIL6 expression: a model of lytic replication induced clonal selection in viral oncogenesis

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.

Narciclasine, an isocarbostyril alkaloid, has preferential activity against primary effusion lymphoma

Primary effusion lymphoma (PEL) is a subtype of non-Hodgkin lymphoma associated with infection by Kaposi sarcoma-associated herpes virus (KSHV). PEL is an aggressive disease with extremely poor prognosis when treated with conventional chemotherapy. Narciclasine, a natural product present in Amaryllidaceae family of flowering plants including daffodils, belongs to a class of molecules termed ‘isocarbostyril alkaloid’. We have found that narciclasine displays preferential cytotoxicity towards PEL at low nanomolar concentrations and is approximately 10 and 100-fold more potent than its structural analogs lycoricidine and lycorine, respectively. Narciclasine arrested cell-cycle progression at the G₁ phase and induced apoptosis in PEL, which is accompanied by activation of caspase-3/7, cleavage of PARP and increase in the surface expression of Annexin-V. Although narciclasine treatment resulted in a marked decrease in the expression of MYC and its direct target genes,time-course experiments revealed that MYC is not a direct target of narciclasine. Narciclasine treatment neither induces the expression of KSHV-RTA/ORF50 nor the production of infectious KSHV virions in PEL. Finally, narciclasine provides dramatic survival advantages to mice in two distinct mouse xenograft models of PEL. In conclusion, our results suggest that narciclasine could be a promising agent for the treatment of PEL.

Metabolic Reprogramming of the Host Cell by Human Adenovirus Infection

Viruses are obligate intracellular parasites that alter many cellular processes to create an environment optimal for viral replication. Reprogramming of cellular metabolism is an important, yet underappreciated feature of many viral infections, as this ensures that the energy and substrates required for viral replication are available in abundance. Human adenovirus (HAdV), which is the focus of this review, is a small DNA tumor virus that reprograms cellular metabolism in a variety of ways. It is well known that HAdV infection increases glucose uptake and fermentation to lactate in a manner resembling the Warburg effect observed in many cancer cells. However, HAdV infection induces many other metabolic changes. In this review, we integrate the findings from a variety of proteomic and transcriptomic studies to understand the subtleties of metabolite and metabolic pathway control during HAdV infection. We review how the E4ORF1 protein of HAdV enacts some of these changes and summarize evidence for reprogramming of cellular metabolism by the viral E1A protein. Therapies targeting altered metabolism are emerging as cancer treatments, and similar targeting of aberrant components of virally reprogrammed metabolism could have clinical antiviral applications.

Regulation of the Host Antiviral State by Intercellular Communications

Viruses usually induce a profound remodeling of host cells, including the usurpation of host machinery to support their replication and production of virions to invade new cells. Nonetheless, recognition of viruses by the host often triggers innate immune signaling, preventing viral spread and modulating the function of immune cells. It conventionally occurs through production of antiviral factors and cytokines by infected cells. Virtually all viruses have evolved mechanisms to blunt such responses. Importantly, it is becoming increasingly recognized that infected cells also transmit signals to regulate innate immunity in uninfected neighboring cells. These alternative pathways are notably mediated by vesicular secretion of various virus- and host-derived products (miRNAs, RNAs, and proteins) and non-infectious viral particles. In this review, we focus on these newly-described modes of cell-to-cell communications and their impact on neighboring cell functions. The reception of these signals can have anti- and pro-viral impacts, as well as more complex effects in the host such as oncogenesis and inflammation. Therefore, these “broadcasting” functions, which might be tuned by an arms race involving selective evolution driven by either the host or the virus, constitute novel and original regulations of viral infection, either highly localized or systemic.

Immunomodulatory drugs target IKZF1-IRF4-MYC axis in primary effusion lymphoma in a cereblon-dependent manner and display synergistic cytotoxicity with BRD4 inhibitors

Primary effusion lymphoma (PEL) is an aggressive type of non-Hodgkin lymphoma localized predominantly in body cavities. Kaposi’s sarcoma-associated herpes virus is the causative agent of PEL. PEL is an incurable malignancy and has extremely poor prognosis when treated with conventional chemotherapy. Immunomodulatory drugs (IMiDs) lenalidomide and pomalidomide are FDA approved drugs for the treatment of various ailments. IMiDs display pronounced anti-proliferative effect against majority of PEL cell lines within their clinically achievable concentrations, by arresting cells at G₀/G₁ phase of cell-cycle, and without any induction of KSHV lytic-cycle reactivation. Although microarray examination of PEL cells treated with lenalidomide revealed activation of interferon (IFN) signaling, blocking the IFN pathway did not block the anti-PEL activity of IMiDs. The anti-PEL effects of IMiDs involved cereblon-dependent suppression of IRF4 and rapid degradation of IKZF1, but not IKZF3. Small hairpin-RNA (shRNA) mediated knockdown of MYC enhanced the cytotoxicity of IMiDs. Bromodomain and extraterminal domain (BET) proteins are epigenetic readers which perform a vital role in chromatin remodeling and transcriptional regulation. BRD4, a widely expressed transcriptional coactivator, belongs to BET family of proteins, which has been shown to co-occupy the super-enhancers associated with MYC. Specific BRD4 inhibitors were developed which suppress MYC transcriptionally. Lenalidomide displayed synergistic cytotoxicity with several structurally distinct BRD4 inhibitors (JQ-1, IBET151, and PFI-1). Furthermore, combined administration of lenalidomide and BRD4 inhibitor JQ-1 significantly increased the survival of PEL bearing NOD.SCID mice in an orthotopic xenograft model as compared to either agent alone. These results provide compelling evidence for clinical testing of IMiDs alone and in combination with BRD4 inhibitors for PEL.

Inefficient Codon Usage Impairs mRNA Accumulation: the Case of the v-FLIP Gene of Kaposi's Sarcoma-Associated Herpesvirus

Latent Kaposi's sarcoma-associated herpesvirus (KSHV) genomes encode a homolog of cellular FLICE-inhibitory proteins (termed v-FLIP) that activates NF-κB and can trigger important proinflammatory and antiapoptotic changes in latently infected cells. The protein is present at very low levels in infection and has generally been difficult to efficiently express in recombinant vectors. Here we show that codon usage in the v-FLIP gene is strikingly suboptimal. Optimization of codon use in expression vectors, as expected, restores efficient protein expression. Surprisingly, however, it also dramatically increases the steady-state level of v-FLIP mRNA, at least in part by increasing mRNA stability. When codon-optimized v-FLIP sequences are reintroduced into intact KSHV genomes, the resulting virus expresses readily detectable monocistronic v-FLIP mRNAs that are undetectable in wild-type (WT) infection by blot hybridization, suggesting that such RNAs are in fact transcribed in WT infection but fail to accumulate. The overexpression of v-FLIP by codon-optimized latent genomes results in a 5- to 7-fold decrement in virus production following lytic induction, indicating that maximizing NF-κB signaling is deleterious to induction. These studies provide a clear explanation for the evolution of inefficient codon usage in this gene and point to a strong connection between translational efficiency and RNA accumulation in mammalian cells.

IMPORTANCE

This study reports that inefficient codon usage in a herpesviral gene is strikingly correlated with the inability of its mRNA to accumulate in cells; correction of efficient translatability restores RNA abundance. A similar correlation has been reported in yeast species, but the mechanisms operating in mammalian cells appear substantially different.

Kaposi's sarcoma-associated herpesvirus induces Nrf2 activation in latently infected endothelial cells through SQSTM1 phosphorylation and interaction with polyubiquitinated Keap1

Nuclear factor erythroid 2-related factor 2 (Nrf2), the cellular master regulator of the antioxidant response, dissociates from its inhibitor Keap1 when activated by stress signals and participates in the pathogenesis of viral infections and tumorigenesis. Early during de novo infection of endothelial cells, KSHV induces Nrf2 through an intricate mechanism involving reactive oxygen species (ROS) and prostaglandin E2 (PGE2). When we investigated the Nrf2 activity during latent KSHV infection, we observed increased nuclear serine-40-phosphorylated Nrf2 in human KS lesions compared to that in healthy tissues. Using KSHV long-term-infected endothelial cells (LTC) as a cellular model for KS, we demonstrated that KSHV infection induces Nrf2 constitutively by extending its half-life, increasing its phosphorylation by protein kinase Cζ (PKCζ) via the infection-induced cyclooxygenase-2 (COX-2)/PGE2 axis and inducing its nuclear localization. Nrf2 knockdown in LTC decreased expression of antioxidant genes and genes involved in KS pathogenesis such as the NAD(P)H quinone oxidase 1 (NQO1), gamma glutamylcysteine synthase heavy unit (γGCSH), the cysteine transporter (xCT), interleukin 6 (IL-6), and vascular endothelial growth factor A (VEGF-A) genes. Nrf2 activation was independent of oxidative stress but dependent on the autophagic protein sequestosome-1 (SQSTM1; p62). SQSTM1 levels were elevated in LTC, a consequence of protein accumulation due to decreased autophagy and Nrf2-mediated transcriptional activation. SQSTM1 was phosphorylated on serine-351 and -403, while Keap1 was polyubiquitinated with lysine-63-ubiquitin chains, modifications known to increase their mutual affinity and interaction, leading to Keap1 degradation and Nrf2 activation. The latent KSHV protein Fas-associated death domain-like interleukin-1β-converting enzyme-inhibitory protein (vFLIP) increased SQSTM1 expression and activated Nrf2. Collectively, these results demonstrate that KSHV induces SQSTM1 to constitutively activate Nrf2, which is involved in the regulation of genes participating in KSHV oncogenesis.

IMPORTANCE

The transcription factor Nrf2 is activated by stress signals, including viral infection, and responds by activating the transcription of cytoprotective genes. Recently, Nrf2 has been implicated in oncogenesis and was shown to be activated during de novo KSHV infection of endothelial cells through ROS-dependent pathways. The present study was undertaken to determine the mechanism of Nrf2 activation during prolonged latent infection of endothelial cells, using an endothelial cell line latently infected with KSHV. We show that Nrf2 activation was elevated in KSHV latently infected endothelial cells independently of oxidative stress but dependent on the autophagic protein sequestosome-1 (SQSTM1), which was involved in the degradation of the Nrf2 inhibitor Keap1. Furthermore, our results indicated that the KSHV latent protein vFLIP participates in Nrf2 activation. This study suggests that KSHV hijacks the host's autophagic protein SQSTM1 to induce Nrf2 activation, thereby manipulating the infected host gene regulation to promote KS pathogenesis.

Targeting Myc in KSHV-associated primary effusion lymphoma with BET bromodomain inhibitors

Primary effusion lymphoma (PEL) is an aggressive form of non-Hodgkin's B-cell lymphoma associated with infection by Kaposi's sarcoma-associated herpes virus (KSHV). (+)-JQ1 and I-BET151 are two recently described novel small-molecule inhibitors of BET bromodomain chromatin-associated proteins that have shown impressive preclinical activity in cancers in which MYC is overexpressed at the transcriptional level due to chromosomal translocations that bring the MYC gene under the control of a super-enhancer. PEL cells, in contrast, lack structural alterations in the MYC gene, but have deregulated Myc protein due to the activity of KSHV-encoded latent proteins. We report that PEL cell lines are highly sensitive to bromodomain and extra-terminal (BET) bromodomain inhibitors-induced growth inhibition and undergo G0/G1 cell-cycle arrest, apoptosis and cellular senescence, but without the induction of lytic reactivation, upon treatment with these drugs. Treatment of PEL cell lines with BET inhibitors suppressed the expression of MYC and resulted in a genome-wide perturbation of MYC-dependent genes. Silencing of BRD4 and MYC expression blocked cell proliferation and cell-cycle progression, while ectopic expression of MYC from a retroviral promoter rescued cells from (+)-JQ1-induced growth arrest. In a xenograft model of PEL, (+)-JQ1 significantly reduced tumor growth and improved survival. Taken collectively, our results demonstrate that the utility of BET inhibitors may not be limited to cancers in which genomic alterations result in extremely high expression of MYC and they may have equal or perhaps greater activity against cancers in which the MYC genomic locus is structurally intact and c-Myc protein is deregulated at the post-translational level and is only modestly overexpressed.

High prevalence of Human Herpesvirus 8 in schizophrenic patients

Many studies have reported an association between Herpes family viruses and an increased risk of schizophrenia, but the role of Human Herpesvirus 8 (HHV8) has never been investigated. This study aimed to assess HHV8 prevalence in schizophrenic patients as well as the possible association between HHV8 infection and schizophrenia clinical features. We consecutively enrolled 108 patients meeting fourth edition of Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) criteria of schizophrenia and 108 age and sex matched controls. Data about a number of demographic characteristics and potential HHV8 risk factors of infection were collected. Standardized psychopathology measures, disease severity and functioning level were obtained using Positive and Negative Syndrome Scale (PANSS), Brief Psychiatric Rating Scale (BPRS), Scale for the Assessment of Negative Symptoms (SANS), Scale for the Assessment of Positive Symptoms (SAPS), Clinical Global Impressions (CGI) and Global Assessment of functioning (GAF). The presence of anti-HHV8 antibodies was analyzed using an indirect immunofluorescence assay. A higher prevalence of HHV8 infection in schizophrenic patients than in controls was found. Marital status, having children, sexual behavior and risk factors of blood transmission were not associated with HHV8 prevalence. However, among schizophrenic patients, HHV8 prevalence was statically associated with positive symptoms. To our knowledge, this would be the first report of a possible role of HHV8 in the pathogenesis of schizophrenia. To prove this hypothesis, further investigation of HHV8 in schizophrenia with larger samples is needed.

NEMO is essential for Kaposi's sarcoma-associated herpesvirus-encoded vFLIP K13-induced gene expression and protection against death receptor-induced cell death, and its N-terminal 251 residues are sufficient for this process

Kaposi's sarcoma-associated herpesvirus-encoded viral FLICE inhibitory protein (vFLIP) K13 was originally believed to protect virally infected cells against death receptor-induced apoptosis by interfering with caspase 8/FLICE activation. Subsequent studies revealed that K13 also activates the NF-κB pathway by binding to the NEMO/inhibitor of NF-κB (IκB) kinase gamma (IKKγ) subunit of an IKK complex and uses this pathway to modulate the expression of genes involved in cellular survival, proliferation, and the inflammatory response. However, it is not clear if K13 can also induce gene expression independently of NEMO/IKKγ. The minimum region of NEMO that is sufficient for supporting K13-induced NF-κB has not been delineated. Furthermore, the contribution of NEMO and NF-κB to the protective effect of K13 against death receptor-induced apoptosis remains to be determined. In this study, we used microarray analysis on K13-expressing wild-type and NEMO-deficient cells to demonstrate that NEMO is required for modulation of K13-induced genes. Reconstitution of NEMO-null cells revealed that the N-terminal 251 amino acid residues of NEMO are sufficient for supporting K13-induced NF-κB but fail to support tumor necrosis factor alpha (TNF-α)-induced NF-κB. K13 failed to protect NEMO-null cells against TNF-α-induced cell death but protected those reconstituted with the NEMO mutant truncated to include only the N-terminal 251 amino acid residues [the NEMO(1-251) mutant]. Taken collectively, our results demonstrate that NEMO is required for modulation of K13-induced genes and the N-terminal 251 amino acids of NEMO are sufficient for supporting K13-induced NF-κB. Finally, the ability of K13 to protect against TNF-α-induced cell death is critically dependent on its ability to interact with NEMO and activate NF-κB.

IMPORTANCE

Kaposi's sarcoma-associated herpesvirus-encoded vFLIP K13 is believed to protect virally infected cells against death receptor-induced apoptosis and to activate the NF-κB pathway by binding to adaptor protein NEMO/IKKγ. However, whether K13 can also induce gene expression independently of NEMO and the minimum region of NEMO that is sufficient for supporting K13-induced NF-κB remain to be delineated. Furthermore, the contribution of NEMO and NF-κB to the protective effect of K13 against death receptor-induced apoptosis is not clear. We demonstrate that NEMO is required for modulation of K13-induced genes and its N-terminal 251 amino acids are sufficient for supporting K13-induced NF-κB. The ability of K13 to protect against TNF-α-induced cell death is critically dependent on its ability to interact with NEMO and activate NF-κB. Our results suggest that K13-based gene therapy approaches may have utility for the treatment of patients with NEMO mutations and immunodeficiency.

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.

A purine scaffold HSP90 inhibitor BIIB021 has selective activity against KSHV-associated primary effusion lymphoma and blocks vFLIP K13-induced NF-κB

PURPOSE

Kaposi sarcoma-associated herpes virus (KSHV)-associated primary effusion lymphomas (PEL) have extremely poor prognosis when treated with conventional chemotherapy. KSHV-encoded viral FLICE-inhibitory protein (vFLIP) K13 binds to the IkappaB kinase (IKK) complex to constitutively activate the NF-κB pathway, which has been shown to be essential for the survival and proliferation of PEL cells. The molecular chaperone HSP90 is a component of the IKK complex and is required for its activity.

EXPERIMENTAL DESIGN

We have analyzed the effect of HSP90 inhibitors on the survival and proliferation of PEL cells and on the activity of the NF-κB pathway.

RESULTS

We show that BIIB021, a purine scaffold-based orally administrable HSP90 inhibitor, shows preferential cytotoxicity toward PEL cells as compared with non-PEL cells. The cytotoxic effect of BIIB021 against PEL was associated with induction of cell-cycle arrest and apoptosis. BIIB021 blocked the expression of a number of cellular proteins involved in the regulation of cell cycle and apoptosis. BIIB021 also blocked constitutive NF-κB activity present in PEL cells, in part, by blocking the interaction of vFLIP K13 with the IKK complex subunits. In a xenograft model of PEL, BIIB021 significantly reduced tumor growth.

CONCLUSION

BIIB021 blocks constitutive NF-κB activity in PEL and shows preferential antitumor activity against PEL in vitro and in vivo. BIIB021 may be a promising agent for treatment of PEL.

KSHV ORF67 encoded lytic protein localizes on the nuclear membrane and alters emerin distribution

p29, a newly identified Kaposi's sarcoma-associated herpesvirus (KSHV) protein, is the product of ORF67, the positional homolog of the conserved herpesvirus protein UL34. Like its homologues in other herpesviruses, p29 is expressed early during viral lytic cycle, and is localized on the nuclear rim. Upon chemical induction of viral replication in primary effusion lymphoma cells, p29 interacts with p33, encoded by ORF69, the positional homolog of the conserved herpesvirus protein UL31, and both proteins colocalize on the nuclear membrane. IFA and biochemical analysis of infected or transfected cells showed that p29 expression resulted in delocalization and hyperphosphorylation of emerin, whereas other nuclear lamin associated proteins, such as LUMA, LB1 and LBR were not affected. Mislocalization of emerin was robustly increased upon combined expression of p29 and p33, suggesting that emerin destabilization might represent the first step in nuclear lamina disassembling, a process necessary for nucleocapsid maturation.

Kaposi's sarcoma-associated herpesvirus oncoprotein K13 protects against B cell receptor-induced growth arrest and apoptosis through NF-κB activation

Kaposi's sarcoma-associated herpesvirus (KSHV) has been linked to the development of Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease (MCD). We have characterized the role of KSHV-encoded viral FLICE inhibitory protein (vFLIP) K13 in the modulation of anti-IgM-induced growth arrest and apoptosis in B cells. We demonstrate that K13 protects WEHI 231, an immature B-cell line, against anti-IgM-induced growth arrest and apoptosis. The protective effect of K13 was associated with the activation of the NF-κB pathway and was deficient in a mutant K13 with three alanine substitutions at positions 58 to 60 (K13-58AAA) and a structural homolog, vFLIP E8, both of which lack NF-κB activity. K13 upregulated the expression of NF-κB subunit RelB and blocked the anti-IgM-induced decline in c-Myc and rise in p27(Kip1) that have been associated with growth arrest and apoptosis. K13 also upregulated the expression of Mcl-1, an antiapoptotic member of the Bcl2 family. Finally, K13 protected the mature B-cell line Ramos against anti-IgM-induced apoptosis through NF-κB activation. Inhibition of anti-IgM-induced apoptosis by K13 may contribute to the development of KSHV-associated lymphoproliferative disorders.

Negative elongation factor-mediated suppression of RNA polymerase II elongation of Kaposi's sarcoma-associated herpesvirus lytic gene expression

Genome-wide chromatin immunoprecipitation assays indicate that the promoter-proximal pausing of RNA polymerase II (RNAPII) is an important postinitiation step for gene regulation. During latent infection, the majority of Kaposi's sarcoma-associated herpesvirus (KSHV) genes is silenced via repressive histone marks on their promoters. Despite the absence of their expression during latency, however, several lytic promoters are enriched with activating histone marks, suggesting that mechanisms other than heterochromatin-mediated suppression contribute to preventing lytic gene expression. Here, we show that the RNAPII-mediated transcription of the KSHV OriLytL, K5, K6, and K7 (OriLytL-K7) lytic genes is paused at the elongation step during latency. Specifically, the RNAPII-mediated transcription is stalled by the host's negative elongation factor (NELF) at the promoter regions of OriLytL-K7 lytic genes during latency, leading to the hyperphosphorylation of the serine 5 residue and the hypophosphorylation of the serine 2 of the C-terminal domain of the RNAPII large subunit, a hallmark of stalled RNAPII. Consequently, depletion of NELF expression induced transition of stalled RNAPII into a productive transcription elongation at the promoter-proximal regions of OriLytL-K7 lytic genes, leading to their RTA-independent expression. Using an RTA-deficient recombinant KSHV, we also showed that expression of the K5, K6, and K7 lytic genes was highly inducible upon external stimuli compared to other lytic genes that lack RNAPII on their promoters during latency. These results indicate that the transcription elongation of KSHV OriLytL-K7 lytic genes is inhibited by NELF during latency, but can also be promptly reactivated in an RTA-independent manner upon external stimuli.

A computational profiling of changes in gene expression and transcription factors induced by vFLIP K13 in primary effusion lymphoma

Infection with Kaposi's sarcoma associated herpesvirus (KSHV) has been linked to the development of primary effusion lymphoma (PEL), a rare lymphoproliferative disorder that is characterized by loss of expression of most B cell markers and effusions in the body cavities. This unique clinical presentation of PEL has been attributed to their distinctive plasmablastic gene expression profile that shows overexpression of genes involved in inflammation, adhesion and invasion. KSHV-encoded latent protein vFLIP K13 has been previously shown to promote the survival and proliferation of PEL cells. In this study, we employed gene array analysis to characterize the effect of K13 on global gene expression in PEL-derived BCBL1 cells, which express negligible K13 endogenously. We demonstrate that K13 upregulates the expression of a number of NF-κB responsive genes involved in cytokine signaling, cell death, adhesion, inflammation and immune response, including two NF-κB subunits involved in the alternate NF-κB pathway, RELB and NFKB2. In contrast, CD19, a B cell marker, was one of the genes downregulated by K13. A comparison with K13-induced genes in human vascular endothelial cells revealed that although there was a considerable overlap among the genes induced by K13 in the two cell types, chemokines genes were preferentially induced in HUVEC with few exceptions, such as RANTES/CCL5, which was induced in both cell types. Functional studies confirmed that K13 activated the RANTES/CCL5 promoter through the NF-κB pathway. Taken collectively, our results suggest that K13 may contribute to the unique gene expression profile, immunophenotype and clinical presentation that are characteristics of KSHV-associated PEL.

Kaposi's sarcoma associated herpesvirus encoded viral FLICE inhibitory protein K13 activates NF-κB pathway independent of TRAF6, TAK1 and LUBAC

BACKGROUND

Kaposi's sarcoma associated herpesvirus encoded viral FLICE inhibitory protein (vFLIP) K13 activates the NF-κB pathway by binding to the NEMO/IKKγ subunit of the IκB kinase (IKK) complex. However, it has remained enigmatic how K13-NEMO interaction results in the activation of the IKK complex. Recent studies have implicated TRAF6, TAK1 and linear ubiquitin chains assembled by a linear ubiquitin chain assembly complex (LUBAC) consisting of HOIL-1, HOIP and SHARPIN in IKK activation by proinflammatory cytokines.

METHODOLOGY/PRINCIPAL FINDINGS

Here we demonstrate that K13-induced NF-κB DNA binding and transcriptional activities are not impaired in cells derived from mice with targeted disruption of TRAF6, TAK1 and HOIL-1 genes and in cells derived from mice with chronic proliferative dermatitis (cpdm), which have mutation in the Sharpin gene (Sharpin(cpdm/cpdm)). Furthermore, reconstitution of NEMO-deficient murine embryonic fibroblast cells with NEMO mutants that are incapable of binding to linear ubiquitin chains supported K13-induced NF-κB activity. K13-induced NF-κB activity was not blocked by CYLD, a deubiquitylating enzyme that can cleave linear and Lys63-linked ubiquitin chains. On the other hand, NEMO was required for interaction of K13 with IKK1/IKKα and IKK2/IKKβ, which resulted in their activation by "T Loop" phosphorylation.

CONCLUSIONS/SIGNIFICANCE

Our results demonstrate that K13 activates the NF-κB pathway by binding to NEMO which results in the recruitment of IKK1/IKKα and IKK2/IKKβ and their subsequent activation by phosphorylation. Thus, K13 activates NF-κB via a mechanism distinct from that utilized by inflammatory cytokines. These results have important implications for the development of therapeutic agents targeting K13-induced NF-κB for the treatment of KSHV-associated malignancies.

Modulation of Immune System by Kaposi's Sarcoma-Associated Herpesvirus: Lessons from Viral Evasion Strategies

Kaposi's sarcoma-associated herpesvirus (KSHV), a member of the herpesvirus family, has evolved to establish a long-term, latent infection of cells such that while they carry the viral genome gene expression is highly restricted. Latency is a state of cryptic viral infection associated with genomic persistence in their host and this hallmark of KSHV infection leads to several clinical-epidemiological diseases such as KS, a plasmablastic variant of multicentric Castleman's disease, and primary effusion lymphoma upon immune suppression of infected hosts. In order to sustain efficient life-long persistency as well as their life cycle, KSHV dedicates a large portion of its genome to encode immunomodulatory proteins that antagonize its host's immune system. In this review, we will describe our current knowledge of the immune evasion strategies employed by KSHV at distinct stages of its viral life cycle to control the host's immune system.

Kaposi's Sarcoma-Associated Herpesvirus Genome Replication, Partitioning, and Maintenance in Latency

Kaposi's sarcoma-associated herpesvirus (KSHV) is thought to be an oncogenic member of the γ-herpesvirus subfamily. The virus usually establishes latency upon infection as a default infection pattern. The viral genome replicates according to the host cell cycle by recruiting the host cellular replication machinery. Among the latently expressing viral factors, LANA plays pivotal roles in viral genome replication, partitioning, and maintenance. LANA binds with two LANA-binding sites (LBS1/2) within a terminal repeat (TR) sequence and is indispensable for viral genome replication in latency. The nuclear matrix region seems to be important as a replication site, since LANA as well as cellular replication factors accumulate there and recruit the viral replication origin in latency (ori-P) by its binding activity to LBS. KSHV ori-P consists of LBS followed by a 32-bp GC-rich segment (32GC). Although it has been reported that LANA recruits cellular pre-replication complexes (pre-RC) such as origin recognition complexes (ORCs) to the ori-P through its interaction with ORCs, this mechanism does not account completely for the requirement of the 32GC. On the other hand, there are few reports about the partitioning and maintenance of the viral genome. LANA interacts with many kinds of chromosomal proteins, including Brd2/RING3, core histones, such as H2A/H2B and histone H1, and so on. The detailed molecular mechanisms by which LANA enables KSHV genome partitioning and maintenance still remain obscure. By integrating the findings reported thus far on KSHV genome replication, partitioning, and maintenance in latency, we will summarize what we know now, discuss what questions remain to be answered, and determine what needs to be done next to understand the mechanisms underlying viral replication, partitioning, and maintenance strategy.

Prevalence of JC polyomavirus genomic sequences from the large T-antigen and non-coding control regions among Bulgarian patients with primary brain tumors

A total of 111 fresh brain biopsies from patients with primary brain tumors were examined for JC polyomavirus sequences from the Large T antigen encoding region (LT) and the viral non-coding control region (NCCR). SYBR Green and TaqMan real-time polymerase chain reaction assays were used. In the glioblastoma group of 39 patients 48.7% were positive for LT sequences. Among the astrocytoma group (19 patients) and the oligodendroglioma group (12 patients) 31.6% and 33.3% were also positive. The prevalence of LT genomic sequences among the other groups was as follows: in 2 out of 3 oligoastrocytomas; in 3 out 5 gangliogliomas; in 2 out of 5 meduloblastomas; in 1 out 3 pineocytomas; and in none of the tested 5 ependimomas. All positive samples had a late threshold cycle that varied from 36 to 49, indicative of very low starting viral number. Only 21 of all the 111 samples were positive for NCCR. Low copy number in range of 10-1,000 was present. Notably, only 8 of all NCCR positive specimens were also LT positive. It might be suggested that the disproportion between the results for LT and NCCR is either due to clonally integrated LT fragments, with loss of genetic material, or changes in the NCCR. The latter would alter the productive course of the infection and may establish a premise for continuous interaction of viral regulatory proteins with cell molecules that are responsible for the control of the cell cycle. This may lead subsequently to malignant transformation.

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.

Deletion of Kaposi's sarcoma-associated herpesvirus FLICE inhibitory protein, vFLIP, from the viral genome compromises the activation of STAT1-responsive cellular genes and spindle cell formation in endothelial cells

Kaposi's sarcoma herpesvirus (KSHV) Fas-associated death domain (FADD)-like interleukin-1 beta-converting enzyme (FLICE)-inhibitory protein, vFLIP, has antiapoptotic properties, is a potent activator of the NF-κB pathway, and induces the formation of endothelial spindle cells, the hallmark of Kaposi's sarcoma, when overexpressed in primary endothelial cells. We used a reverse genetics approach to study several functions of KSHV vFLIP in the context of the whole viral genome. Deletion of the gene encoding vFLIP from a KSHV genome cloned in a bacterial artificial chromosome (BAC) reduced the ability of the virus to persist and induce spindle cell formation in primary human umbilical vein endothelial cells (HUVECs). Only a few, mainly interferon (IFN)-responsive, genes were expressed in wild-type KSHV (KSHV-wt)-infected endothelial cells at levels higher than those in KSHV-ΔFLIP-infected endothelial cells, in contrast to the plethora of cellular genes induced by overexpressed vFLIP. In keeping with this observation, vFLIP induces the phosphorylation of STAT1 and STAT2 in an NF-κB-dependent manner in endothelial cells. vFLIP-dependent phosphorylation of STAT1 and STAT2 could be demonstrated after endothelial cells were infected with KSHV-wt, KSHV-ΔFLIP, and a KSHV-vFLIP revertant virus. These findings document the impact of KSHV vFLIP on the transcriptome of primary endothelial cells during viral persistence and highlight the role of vFLIP in the activation of STAT1/STAT2 and STAT-responsive cellular genes by KSHV.

The single RBP-Jkappa site within the LANA promoter is crucial for establishing Kaposi's sarcoma-associated herpesvirus latency during primary infection

Kaposi's sarcoma-associated herpesvirus (KSHV; or human herpesvirus 8 [HHV8]) is implicated in the pathogenesis of many human malignancies including Kaposi's sarcoma (KS), multicentric Castleman's disease (MCD), and primary effusion lymphoma (PEL). KSHV infection displays two alternative life cycles, referred to as the latent and lytic or productive cycle. Previously, we have reported that the replication and transcription activator (RTA), a major lytic cycle transactivator, contributes to the development of KSHV latency by inducing latency-associated nuclear antigen (LANA) expression during early stages of infection by targeting RBP-Jκ, the master regulator of the Notch signaling pathway. Here, we generated a bacterial artificial chromosome (BAC) KSHV recombinant virus with a deletion of the RBP-Jκ site within the LANA promoter to evaluate the function of the RBP-Jκ cognate site in establishing primary latent infection. The results showed that genetic disruption of the RBP-Jκ binding site within the KSHV LANA promoter led to enhanced expression of the KSHV-encoded immediate early RTA, resulting in an increase in lytic replication during primary infection of human peripheral blood mononuclear cells (PBMCs). This system provides a powerful tool for use in indentifying additional cellular and viral molecules involved in LANA-mediated latency maintenance during the early stages of KSHV infection.

Constitutive NF-kappaB activation confers interleukin 6 (IL6) independence and resistance to dexamethasone and Janus kinase inhibitor INCB018424 in murine plasmacytoma cells

Myeloma cells are dependent on IL6 for their survival and proliferation during the early stages of disease, and independence from IL6 is associated with disease progression. The role of the NF-κB pathway in the IL6-independent growth of myeloma cells has not been studied. Because human herpesvirus 8-encoded K13 selectively activates the NF-κB pathway, we have used it as a molecular tool to examine the ability of the NF-κB pathway to confer IL6 independence on murine plasmacytomas. We demonstrated that ectopic expression of K13, but not its NF-κB-defective mutant or a structural homolog, protected plasmacytomas against IL6 withdrawal-induced apoptosis and resulted in emergence of IL6-independent clones that could proliferate long-term in vitro in the absence of IL6 and form abdominal plasmacytomas with visceral involvement when injected intraperitoneally into syngeneic mice. These IL6-independent clones were dependent on NF-κB activity for their survival and proliferation but were resistant to dexamethasone and INCB018424, a selective Janus kinase 1/2 inhibitor. Ectopic expression of human T cell leukemia virus 1-encoded Tax protein, which resembles K13 in inducing constitutive NF-κB activation, similarly protected plasmacytoma cells against IL6 withdrawal-induced apoptosis. Although K13 is known to up-regulate IL6 gene expression, its protective effect was not due to induction of endogenous IL6 production but instead was associated with sustained expression of several antiapoptotic members of the Bcl2 family upon IL6 withdrawal. Collectively, these results demonstrate that NF-κB activation cannot only promote the emergence of IL6 independence during myeloma progression but can also confer resistance to dexamethasone and INCB018424.

Kaposi's sarcoma-associated herpesvirus infection of endothelial cells inhibits neutrophil recruitment through an interleukin-6-dependent mechanism: a new paradigm for viral immune evasion

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of Kaposi's sarcoma (KS), an endothelial cell (EC) neoplasm characterized by dysregulated angiogenesis and inflammation. KSHV infection of EC causes production of proinflammatory mediators, regarded as possible initiators of the substantial mononuclear leukocyte recruitment seen in KS. Conversely, KSHV immune evasion strategies exist, such as degradation of EC leukocyte adhesion receptors by viral proteins. Here, we report the effects of KSHV infection of primary EC on recruitment of flowing leukocytes. Infection did not initiate adhesion of any leukocyte subset per se. However, on cytokine-stimulated EC, KSHV specifically inhibited neutrophil, but not PBL or monocyte, transmigration, an observation consistent with the inflammatory cell profile found in KS lesions in vivo. This inhibition could be recapitulated on uninfected EC using supernatant from infected cultures. These supernatants contained elevated levels of human interleukin 6 (hIL-6), and both the KSHV- and the supernatant-induced inhibitions of neutrophil transmigration were abrogated in the presence of a hIL-6 neutralizing antibody. Furthermore, preconditioning of EC with hIL-6 mimicked the effect of KSHV. Using RNA interference (RNAi), we show that upregulation of suppressor of cytokine signaling 3 (SOCS3) was necessary for this effect of hIL-6. These studies reveal a novel paracrine mode of KSHV immune evasion, resulting in reduced recruitment of neutrophils, a cell type whose antiviral and antitumor roles are becoming increasingly appreciated. Moreover, the findings have implications for our understanding of the contribution of hIL-6 to the pathogenesis of other inflammatory disorders and tumors in which this cytokine is abundant.

A20 is induced by Kaposi sarcoma-associated herpesvirus-encoded viral FLICE inhibitory protein (vFLIP) K13 and blocks K13-induced nuclear factor-kappaB in a negative feedback manner

Expression of A20, a negative regulator of the NF-κB pathway, is frequently lost in several subtypes of Hodgkin and non-Hodgkin lymphoma. We report that A20 is expressed in Kaposi sarcoma-associated herpesvirus (KSHV)-infected primary effusion lymphoma cell lines, and its expression correlates closely with the expression of KSHV-encoded viral FLICE inhibitory protein K13. Ectopic expression of K13 induced A20 expression through NF-κB-mediated activation of A20 promoter. In turn, A20 blocked K13-induced NF-κB activity and up-regulation of proinflammatory cytokines CCL20 and IL-8 in a negative feedback fashion. Both the N-terminal deubiquitinating domain and the C-terminal zinc finger domain of A20 were involved in the inhibition of K13-induced NF-κB activity. Overexpression of A20 blocked K13-induced IκBα phosphorylation, NF-κB nuclear translocation, and cellular transformation. Consistent with the above, K13-induced IκBα phosphorylation and NF-κB transcriptional activation were enhanced in A20-deficient cells. Finally, A20 was found to interact physically with K13. Taken collectively, these results demonstrate that K13 is a key determinant of A20 expression in KSHV-infected cells, and A20 is a key negative regulator of K13-induced NF-κB activity. A20 might serve to control the inflammatory response to KSHV infection and protect KSHV-infected cells from apoptosis.

Immune evasion by Kaposi's sarcoma-associated herpesvirus

Persistent viral infections are often associated with serious diseases, primarily by altering functions of the host immune system. The hallmark of Kaposi's sarcoma-associated herpesvirus (KSHV) infection is the establishment of a life-long persistent infection, which leads to several clinical, epidemiological and infectious diseases, such as Kaposi's sarcoma, a plasmablastic variant of multicentric Castleman's disease, and primary effusion lymphoma. To sustain an efficient life-long persistency, KSHV dedicates a large portion of its genome to encoding immunomodulatory proteins that antagonize the immune system of its host. In this article, we highlight the strategies KSHV uses to evade, escape and survive its battle against the host's immune system.

Kaposi's sarcoma-associated herpesvirus ORF57 promotes escape of viral and human interleukin-6 from microRNA-mediated suppression

Kaposi's sarcoma-associated herpesvirus (KSHV) lytic infection increases the expression of viral and human interleukin-6 (vIL-6 and hIL-6, respectively), an important factor for cell growth and pathogenesis. Here, we report genome-wide analysis of viral RNA targets of KSHV ORF57 by a novel UV-cross-linking and immunoprecipitation (CLIP) assay. We identified 11 viral transcripts as putative ORF57 targets and demonstrate that vIL-6 mRNA is an authentic target of ORF57. Disrupting the ORF57 gene in the KSHV genome leads to inefficient expression of vIL-6. With transient transfection, the expression of vIL-6 could be enhanced greatly in the presence of ORF57 in a dose-dependent manner. We found that the open reading frame (ORF) region of vIL-6 RNA contains an MRE (MTA [ORF57]-responsive element) composed of two motifs, MRE-A and MRE-B, and binding of ORF57 to these two motifs stabilizes vIL-6 RNA and promotes vIL-6 translation. We demonstrate that vIL-6 MRE-B bears an miR-1293 binding site and that, mechanistically, ORF57 competes with miR-1293 for the same binding site to interact with vIL-6 RNA, thereby preventing vIL-6 RNA from association with the miR-1293-specified RNA-induced silencing complex (RISC). Consistent with this, ORF57 also interacts with an miR-608 binding site in the hIL-6 ORF and prevents miR-608 repression of hIL-6. Collectively, our results identify a novel function of ORF57 in being responsible for stabilization of viral and human IL-6 RNAs and the corresponding enhancement of RNA translation. In addition, our data provide the first evidence that a tumor virus may use a viral protein to interfere with microRNA (miRNA)-mediated repression of an miRNA target to induce cell proliferation and tumorigenesis during virus infection.

Mechanisms of Kaposi's Sarcoma-Associated Herpesvirus Latency and Reactivation

The life cycle of Kaposi's sarcoma-associated herpesvirus (KSHV) consists of latent and lytic replication phases. During latent infection, only a limited number of KSHV genes are expressed. However, this phase of replication is essential for persistent infection, evasion of host immune response, and induction of KSHV-related malignancies. KSHV reactivation from latency produces a wide range of viral products and infectious virions. The resulting de novo infection and viral lytic products modulate diverse cellular pathways and stromal microenvironment, which promote the development of Kaposi's sarcoma (KS). The mechanisms controlling KSHV latency and reactivation are complex, involving both viral and host factors, and are modulated by diverse environmental factors. Here, we review the cellular and molecular basis of KSHV latency and reactivation with a focus on the most recent advancements in the field.

Kaposi sarcoma-associated herpesvirus-encoded viral FLICE inhibitory protein (vFLIP) K13 cooperates with Myc to promote lymphoma in mice

Primary effusion lymphoma (PEL) is an aggressive form of lymphoma that is associated with infection by Kaposi's sarcoma-associated herpesvirus (KSHV). One of the KSHV genes expressed in PEL cells is K13, a potent activator of the NF-κB pathway. K13 transgenic mice develop lymphomas, but after a long period of latency. A possible candidate that could cooperate with K13 in the development of PEL is c-Myc, whose expression is frequently dysregulated in PEL cells. To study the cooperative interaction between K13 and c-Myc in the pathogenesis of PEL, we crossed the K13 transgenic mice to iMyc(Eμ) transgenic mice that overexpress Myc. We report that lymphomas in the K13/iMyc(Eμ) double transgenic mice developed with shorter latency and were histologically distinct from those observed in the iMyc(Eμ) mice. Lymphomas in the K13/iMyc(Eμ) mice also lacked the expression of B- and T-cell markers, thus resembling the immunophenotype of PEL. The accelerated development of lymphoma in the K13/iMyc(Eμ) mice was associated with increased expression of K13, elevated NF-κB activity and decrease in apoptosis. Taken collectively, our results demonstrate a cooperative interaction between the NF-κB and Myc pathways in lymphomagenesis.

Viral latency and its regulation: lessons from the gamma-herpesviruses

Latency is a state of cryptic viral infection associated with genomic persistence and highly restricted gene expression. Its hallmark is reversibility: under appropriate circumstances, expression of the entire viral genome can be induced, resulting in the production of infectious progeny. Among the small number of virus families capable of authentic latency, the herpesviruses stand out for their ability to produce such infections in every infected individual and for being completely dependent upon latency as a mode of persistence. Here, we review the molecular basis of latency, with special attention to the gamma-herpesviruses, in which the understanding of this process is most advanced.

Kaposi's sarcoma-associated herpesvirus-encoded viral FLICE inhibitory protein (vFLIP) K13 suppresses CXCR4 expression by upregulating miR-146a

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV)-encoded viral FLICE inhibitory protein (vFLIP) K13 is a potent activator of the nuclear factor-kappaB (NF-kappaB) pathway. In this study, we show that infection with KHSV and ectopic expression of K13, but not its NF-kappaB-defective mutant, suppressed the expression of CXCR4. Suppression of CXCR4 by KSHV and K13 was associated with upregulated expression of miR-146a, a microRNA that is known to bind to the 3'-untranslated region of CXCR4 mRNA. Reporter studies identified two NF-kappaB sites in the promoter of miR-146a that were essential for its activation by K13. Accordingly, ectopic expression of K13, but not its NF-kappaB-defective mutant or other vFLIPs, strongly stimulated the miR-146a promoter activity, which could be blocked by specific genetic and pharmacological inhibitors of the NF-kappaB pathway. Finally, expression of CXCR4 was downregulated in clinical samples of KS and this was accompanied by an increased expression of miR-146a. Our results show that K13-induced NF-kappaB activity suppresses CXCR4 through upregulation of miR-146a. Downregulation of CXCR4 expression by K13 may contribute to KS development by promoting premature release of KSHV-infected endothelial progenitors into the circulation.

Integrated microarray and multiplex cytokine analyses of Kaposi's Sarcoma Associated Herpesvirus viral FLICE Inhibitory Protein K13 affected genes and cytokines in human blood vascular endothelial cells

Background

Kaposi's sarcoma (KS) associated herpesvirus (KSHV) is the etiological agent of KS, a neoplasm characterized by proliferating spindle cells, extensive neoangiogenesis and a prominent inflammatory infiltrate. Infection of blood vascular endothelial cells with KSHV in vitro results in their spindle cell transformation, which is accompanied by increased expression of inflammatory chemokines and cytokines, and acquisition of lymphatic endothelial markers. Mimicking the effect of viral infection, ectopic expression of KSHV-encoded latent protein vFLIP K13 is sufficient to induce spindle transformation of vascular endothelial cells. However, the effect of K13 expression on global gene expression and induction of lymphatic endothelial markers in vascular endothelial cells has not been studied.

Methods

We used gene array analysis to determine change in global gene expression induced by K13 in human vascular endothelial cells (HUVECs). Results of microarray analysis were validated by quantitative RT-PCR, immunoblotting and a multiplex cytokine array.

Results

K13 affected the expression of several genes whose expression is known to be modulated by KSHV infection, including genes involved in immune and inflammatory responses, anti-apoptosis, stress response, and angiogenesis. The NF-κB pathway was the major signaling pathway affected by K13 expression, and genetic and pharmacological inhibitors of this pathway effectively blocked K13-induced transcriptional activation of the promoter of CXCL10, one of the chemokines whose expression was highly upregulated by K13. However, K13, failed to induce expression of lymphatic markers in blood vascular endothelial cells.

Conclusion

While K13 may account for change in the expression of a majority of genes observed following KSHV infection, it is not sufficient for inducing lymphatic reprogramming of blood vascular endothelial cells.

Kaposi sarcoma-associated herpesvirus (KSHV): molecular biology and oncogenesis

Kaposi sarcoma-associated herpesvirus (KSHV) is a double-stranded DNA herpesvirus belonging to the gamma-herpesvirinae subfamily. KSHV has been associated with the development of three neoplastic diseases: Kaposi sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman disease (MCD). In this review, we discuss the three KSHV-associated malignancies, KSHV genome, latent and lytic aspects of the viral lifecycle, putative viral oncogenes, as well as therapeutic regimens used for the treatment of KS, PEL, and MCD.

Induction of CCL20 production by Kaposi sarcoma-associated herpesvirus: role of viral FLICE inhibitory protein K13-induced NF-kappaB activation

Kaposi sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8, is the etiologic agent of Kaposi sarcoma (KS), an angioproliferative lesion characterized by dramatic angiogenesis and inflammatory infiltration. In this study, we report that expression of chemokine CCL20, a potent chemoattractant of dendritic cells and lymphocytes, is strongly induced in cultured cells either by KSHV infection or on ectopic expression of viral FLICE inhibitory protein K13. This induction is caused by transcriptional activation of CCL20 gene, which is mediated by binding of the p65, p50, and c-Rel subunits of the transcription factor nuclear factor-kappaB (NF-kappaB) to an atypical NF-kappaB-binding site present in the CCL20 gene promoter. The CCL20 gene induction is defective in K13 mutants that lack NF-kappaB activity, and can be blocked by specific genetic and pharmacologic inhibitors of the NF-kappaB pathway. CCR6, the specific receptor for CCL20, is also induced in cultured cells either by KSHV infection or on K13 expression. Finally, expression of CCL20 and CCR6 is increased in clinical samples of KS. These results suggest that KSHV and K13-mediated induction of CCL20 and CCR6 may contribute to the recruitment of dendritic cells and lymphocytes into the KS lesions, and to tumor growth and metastases.

Activation of host translational control pathways by a viral developmental switch

In response to numerous signals, latent herpesvirus genomes abruptly switch their developmental program, aborting stable host–cell colonization in favor of productive viral replication that ultimately destroys the cell. To achieve a rapid gene expression transition, newly minted capped, polyadenylated viral mRNAs must engage and reprogram the cellular translational apparatus. While transcriptional responses of viral genomes undergoing lytic reactivation have been amply documented, roles for cellular translational control pathways in enabling the latent-lytic switch have not been described. Using PEL-derived B-cells naturally infected with KSHV as a model, we define efficient reactivation conditions and demonstrate that reactivation substantially changes the protein synthesis profile. New polypeptide synthesis correlates with 4E-BP1 translational repressor inactivation, nuclear PABP accumulation, eIF4F assembly, and phosphorylation of the cap-binding protein eIF4E by Mnk1. Significantly, inhibiting Mnk1 reduces accumulation of the critical viral transactivator RTA through a post-transcriptional mechanism, limiting downstream lytic protein production, and impairs reactivation efficiency. Thus, herpesvirus reactivation from latency activates the host cap-dependent translation machinery, illustrating the importance of translational regulation in implementing new developmental instructions that drastically alter cell fate.

Kaposi's sarcoma-associated herpesvirus (KSHV) is an important human pathogen and, like all herpesviruses, establishes a state of permanent residency in the infected host called latency. Major sites of KSHV latency are cells of the immune system and cells lining blood vessels. In individuals with weakened immunity, inappropriate growth of these cells driven by the resident virus can give rise to primary effusion lymphoma and Kaposi's sarcoma, respectively. These life-threatening cancers are most common in patients with HIV/AIDS and have become a major source of mortality in parts of sub-Saharan Africa. Under appropriate stimuli, herpesviruses change their relationship with the host cell and begin to manufacture proteins required to assemble new infectious virus particles that can be released and spread. To achieve this, the virus hijacks key processes within the cell and conscripts them into producing viral proteins. In this study, we describe for the first time how KSHV carefully manipulates the host protein synthesis machinery during the switch from latency to this specialized infectious virus production mode. Our results show that although overall protein synthesis is diminished, key components of the host's protein manufacturing machinery are actually stimulated, presumably to accelerate viral protein production.

NF-kappaB p50 plays distinct roles in the establishment and control of murine gammaherpesvirus 68 latency

NF-kappaB signaling is critical to the survival and transformation of cells infected by the human gammaherpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus. Here we have examined how elimination of the NF-kappaB transcription factor p50 from mice affects the life cycle of murine gammaherpesvirus 68 (MHV68). Notably, mice lacking p50 in every cell type were unable to establish a sufficiently robust immune response to control MHV68 infection, leading to high levels of latently infected B cells detected in the spleen and persistent virus replication in the lungs. The latter correlated with very low levels of virus-specific immunoglobulin G (IgG) in the infected p50(-/-) mice at day 48 postinfection. Because the confounding impact of the loss of p50 on the host response to MHV68 infection prevented a direct analysis of the role of this NF-kappaB family member on MHV68 latency in B cells, we generated and infected mixed p50(+/+)/p50(-/-) bone marrow chimeric mice. We show that the chimeric mice were able to control acute virus replication and exhibited normal levels of virus-specific IgG at 3 months postinfection, indicating the induction of a normal host immune response to MHV68 infection. However, in p50(+/+)/p50(-/-) chimeric mice the p50(-/-) B cells exhibited a significant defect compared to p50(+/+) B cells in supporting MHV68 latency. In addition to identifying a role for p50 in the establishment of latency, we determined that the absence of p50 in a subset of the hematopoietic compartment led to persistent virus replication in the lungs of the chimeric mice, providing evidence that p50 is required for controlling virus reactivation. Taken together, these data demonstrate that p50 is required for immune control by the host and has distinct tissue-dependent roles in the regulation of murine gammaherpesvirus latency during chronic infection.

A systems biology approach to identify the combination effects of human herpesvirus 8 genes on NF-kappaB activation

Human herpesvirus 8 (HHV-8) is the etiologic agent of Kaposi's sarcoma and primary effusion lymphoma. Activation of the cellular transcription factor nuclear factor-kappa B (NF-kappaB) is essential for latent persistence of HHV-8, survival of HHV-8-infected cells, and disease progression. We used reverse-transfected cell microarrays (RTCM) as an unbiased systems biology approach to systematically analyze the effects of HHV-8 genes on the NF-kappaB signaling pathway. All HHV-8 genes individually (n = 86) and, additionally, all K and latent genes in pairwise combinations (n = 231) were investigated. Statistical analyses of more than 14,000 transfections identified ORF75 as a novel and confirmed K13 as a known HHV-8 activator of NF-kappaB. K13 and ORF75 showed cooperative NF-kappaB activation. Small interfering RNA-mediated knockdown of ORF75 expression demonstrated that this gene contributes significantly to NF-kappaB activation in HHV-8-infected cells. Furthermore, our approach confirmed K10.5 as an NF-kappaB inhibitor and newly identified K1 as an inhibitor of both K13- and ORF75-mediated NF-kappaB activation. All results obtained with RTCM were confirmed with classical transfection experiments. Our work describes the first successful application of RTCM for the systematic analysis of pathofunctions of genes of an infectious agent. With this approach, ORF75 and K1 were identified as novel HHV-8 regulatory molecules on the NF-kappaB signal transduction pathway. The genes identified may be involved in fine-tuning of the balance between latency and lytic replication, since this depends critically on the state of NF-kappaB activity.

The cytoplasmic terminus of Kaposi's sarcoma-associated herpesvirus glycoprotein B is not essential for virion egress and infectivity

Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded glycoprotein B (gB) is an important determinant of viral infectivity and virion egress. A small interfering RNA (siRNA)-based strategy was devised to inhibit KSHV gB gene expression. Transient cotransfection of plasmids constitutively expressing gB and anti-gB siRNAs in 293 cells substantially inhibited gB mRNA levels and protein production. Similarly, transient expression of siRNAs into the primary effusion lymphoma cell line BCBL-1 caused a substantial reduction of gB transcripts and protein synthesis. TaqMan real-time PCR assays against the lytic KSHV gene ORF59 and infectivity assays on 293 cells were employed to assess the effect of inhibiting gB synthesis on virion egress from BCBL-1 cells and infectivity on 293 cells, respectively. These experiments showed that gB was essential for virion egress and infectivity. Transfection of a codon-optimized gB gene with the first 540 nucleotides altered, and therefore not recognized by anti-gB siRNAs that target the native but not the codon-optimized sequence, efficiently rescued virion egress and infectivity in BCBL-1 cells in the presence of siRNAs inhibiting wild-type gB expression. To assess the role of the cytoplasmic domain of gB in virion egress, mutant gB genes were generated specifying carboxyl terminal truncations of 25 and 58 amino acids disrupting two prominent predicted alpha-helical domains associated with virus-induced cell fusion. A third truncation removed the entire predicted cytoplasmic terminus of 84 amino acids, while a fourth truncation removed 110 amino acids, including the terminal most hydrophobic, intramembrane anchoring sequence. Virion egress experiments revealed that all truncated gBs facilitated virion egress from BCBL-1 cells, with the exception of the largest 110-amino-acid truncation, which removed the gB anchoring sequence. Importantly, the gB truncation that removed the entire predicted cytoplasmic domain increased virion egress, suggesting the presence of a egress regulation domain located proximal to the intramembrane sequence within the cytoplasmic domain of gB. All supernatant virions were infectious on 293 cells, indicating that the carboxyl terminus of gB is not essential for either virion egress or virus infectivity.

A nuclear role for Kaposi's sarcoma-associated herpesvirus-encoded K13 protein in gene regulation

Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded viral FLICE inhibitory protein K13 interacts with a cytosolic IkappaB kinase (IKK) complex to activate nuclear factor-kappaB (NF-kappaB). We recently reported that K13 antagonizes KSHV lytic regulator RTA (replication and transcription activator) and blocks lytic replication, but spares RTA-induced viral interleukin-6 (vIL6). Here we report that K13 is also present in the nuclear compartment, a property not shared by its structural homologs. K13 interacts with and activates the nuclear IKK complex, and binds to the IkappaBalpha promoter. K13 mutants that are retained in the cytosol lack NF-kappaB activity. However, neither the IKKs nor NF-kappaB activation is required for nuclear localization of K13. Instead, this ability is dependent on a nuclear localization signal located in its N-terminal 40 amino acids. Finally, K13, along with p65/RelA, binds to the promoters of a number of KSHV lytic genes, including RTA, ORF57 and vGPCR, but not to the promoter of the vIL6 gene. Thus, K13 has an unexpected nuclear role in viral and cellular gene regulation and its differential binding to the promoters of lytic genes may not only contribute to the inhibition of KSHV lytic replication, but may also account for the escape of vIL6 from K13-induced transcriptional suppression.