Structural analysis of the Kaposi's sarcoma-associated herpesvirus K1 protein

Malignancy and viral infections in Sub-Saharan Africa: A review

The burden of malignancy related to viral infection is increasing in Sub-Saharan Africa (SSA). In 2018, approximately 2 million new cancer cases worldwide were attributable to infection. Prevention or treatment of these infections could reduce cancer cases by 23% in less developed regions and about 7% in developed regions. Contemporaneous increases in longevity and changes in lifestyle have contributed to the cancer burden in SSA. African hospitals are reporting more cases of cancer related to infection (e.g., cervical cancer in women and stomach and liver cancer in men). SSA populations also have elevated underlying prevalence of viral infections compared to other regions. Of 10 infectious agents identified as carcinogenic by the International Agency for Research on Cancer, six are viruses: hepatitis B and C viruses (HBV and HCV, respectively), Epstein-Barr virus (EBV), high-risk types of human papillomavirus (HPV), Human T-cell lymphotropic virus type 1 (HTLV-1), and Kaposi's sarcoma herpesvirus (KSHV, also known as human herpesvirus type 8, HHV-8). Human immunodeficiency virus type 1 (HIV) also facilitates oncogenesis. EBV is associated with lymphomas and nasopharyngeal carcinoma; HBV and HCV are associated with hepatocellular carcinoma; KSHV causes Kaposi's sarcoma; HTLV-1 causes T-cell leukemia and lymphoma; HPV causes carcinoma of the oropharynx and anogenital squamous cell cancer. HIV-1, for which SSA has the greatest global burden, has been linked to increasing risk of malignancy through immunologic dysregulation and clonal hematopoiesis. Public health approaches to prevent infection, such as vaccination, safer injection techniques, screening of blood products, antimicrobial treatments and safer sexual practices could reduce the burden of cancer in Africa. In SSA, inequalities in access to cancer screening and treatment are exacerbated by the perception of cancer as taboo. National level cancer registries, new screening strategies for detection of viral infection and public health messaging should be prioritized in SSA's battle against malignancy. In this review, we discuss the impact of carcinogenic viruses in SSA with a focus on regional epidemiology.

Update of the global distribution of human gammaherpesvirus 8 genotypes

Human gammaherpesvirus 8 (HHV-8) consists of six major clades (A–F) based on the genetic sequence of the open reading frame (ORF)-K1. There are a few conflicting reports regarding the global distribution of the different HHV-8 genotypes. This study aimed to determine the global distribution of the different HHV-8 genotypes based on phylogenetic analysis of the ORF-K1 coding region using sequences published in the GenBank during 1997–2020 and construct a phylogenetic tree using the maximum likelihood algorithm with the GTR + I + G nucleotide substitution model. A total of 550 sequences from 38 countries/origins were analysed in this study. Genotypes A and C had similar global distributions and were prevalent in Africa and Europe. Genotype B was prevalent in Africa. Of the rare genotypes, genotype D was reported in East Asia and Oceania and genotype E in South America, while genotype F was prevalent in Africa. The highest genotypic diversity was reported in the American continent, with Brazil housing five HHV-8 genotypes (A, B, C, E, and F). In this study, we present update of the global distribution of HHV-8 genotypes, providing a basis for future epidemiological and evolutionary studies of HHV-8.

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.

Oncogenic human herpesvirus hijacks proline metabolism for tumorigenesis

Three-dimensional (3D) cell culture is well documented to regain intrinsic metabolic properties and to better mimic the in vivo situation than two-dimensional (2D) cell culture. Particularly, proline metabolism is critical for tumorigenesis since pyrroline-5-carboxylate (P5C) reductase (PYCR/P5CR) is highly expressed in various tumors and its enzymatic activity is essential for in vitro 3D tumor cell growth and in vivo tumorigenesis. PYCR converts the P5C intermediate to proline as a biosynthesis pathway, whereas proline dehydrogenase (PRODH) breaks down proline to P5C as a degradation pathway. Intriguingly, expressions of proline biosynthesis PYCR gene and proline degradation PRODH gene are up-regulated directly by c-Myc oncoprotein and p53 tumor suppressor, respectively, suggesting that the proline-P5C metabolic axis is a key checkpoint for tumor cell growth. Here, we report a metabolic reprogramming of 3D tumor cell growth by oncogenic Kaposi's sarcoma-associated herpesvirus (KSHV), an etiological agent of Kaposi's sarcoma and primary effusion lymphoma. Metabolomic analyses revealed that KSHV infection increased nonessential amino acid metabolites, specifically proline, in 3D culture, not in 2D culture. Strikingly, the KSHV K1 oncoprotein interacted with and activated PYCR enzyme, increasing intracellular proline concentration. Consequently, the K1-PYCR interaction promoted tumor cell growth in 3D spheroid culture and tumorigenesis in nude mice. In contrast, depletion of PYCR expression markedly abrogated K1-induced tumor cell growth in 3D culture, not in 2D culture. This study demonstrates that an increase of proline biosynthesis induced by K1-PYCR interaction is critical for KSHV-mediated transformation in in vitro 3D culture condition and in vivo tumorigenesis.

Introduction of human gammaherpesvirus 8 genotypes A, B, and C into Brazil from multiple geographic regions

Variations in the open reading frame (ORF) K1 gene sequence of human gammaherpesvirus 8 (HHV-8) has led to the identification of 6 major genotypic clades (A, B, C, D, E, and F) in specimens isolated from around the world. These clades exhibit clear clustering among individuals in different ethnic groups and from different geographic regions. The human population of Brazil varies greatly in ethnicity because of multiple immigration events from Africa, Europe, Asia, and indigenous communities. However, there is scant information about the HHV-8 genotypes currently circulating in Brazil. Here, we describe HHV-8 genotypic diversity in isolates from Brazilian HIV-infected patients living with Kaposi's sarcoma (KS) by analysis of the complete ORF-K1 region. We also identified the most likely geographic origins of these different Brazilian genotypes. We extracted HHV-8 DNA (24 positive samples) from individuals with HIV/KS from the states of São Paulo and Rio de Janeiro, amplified the ORF-K1 gene using nested PCR (about 870 base pairs), performed sequencing and phylogenetic analysis, and then calculated the mean genetic distances of Brazilian sequences from sequences in other regions of the world (523 sequences analyzed). Phylogenetic analysis showed that genotypes C, A, and B were present in 45.8 %, 29.2 % and 25 % of the isolates from Brazil, respectively. These isolates grouped into separate clades, rather than a single monophyletic cluster. Mean genetic distance analyses suggested that these genotypes were introduced into the Brazil multiple times from different geographical regions. HHV-8/A isolates appear to be from Ukraine, Russia, and the Tartar ethnic group; HHV-8/B isolates appear to be from Congo and Democratic Republic of the Congo; and HHV-8/C isolates appear to be from Australia, Algeria, England, and French Guiana. These results contribute to a better understanding of the genetic diversity and origins of HHV-8 strains circulating in Brazil, and will provide a foundation for further epidemiological and evolutionary studies of HHV-8.

The Kaposi's sarcoma-associated herpesvirus (KSHV) non-structural membrane protein K15 is required for viral lytic replication and may represent a therapeutic target

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the infectious cause of the highly vascularized tumor Kaposi’s sarcoma (KS), which is characterized by proliferating spindle cells of endothelial origin, extensive neo-angiogenesis and inflammatory infiltrates. The KSHV K15 protein contributes to the angiogenic and invasive properties of KSHV-infected endothelial cells. Here, we asked whether K15 could also play a role in KSHV lytic replication. Deletion of the K15 gene from the viral genome or its depletion by siRNA lead to reduced virus reactivation, as evidenced by the decreased expression levels of KSHV lytic proteins RTA, K-bZIP, ORF 45 and K8.1 as well as reduced release of infectious virus. Similar results were found for a K1 deletion virus. Deleting either K15 or K1 from the viral genome also compromised the ability of KSHV to activate PLCγ1, Erk1/2 and Akt1. In infected primary lymphatic endothelial (LEC-rKSHV) cells, which have previously been shown to spontaneously display a viral lytic transcription pattern, transfection of siRNA against K15, but not K1, abolished viral lytic replication as well as KSHV-induced spindle cell formation. Using a newly generated monoclonal antibody to K15, we found an abundant K15 protein expression in KS tumor biopsies obtained from HIV positive patients, emphasizing the physiological relevance of our findings. Finally, we used a dominant negative inhibitor of the K15-PLCγ1 interaction to establish proof of principle that pharmacological intervention with K15-dependent pathways may represent a novel approach to block KSHV reactivation and thereby its pathogenesis.

Both the latent and lytic replication phases of the KSHV life cycle are thought to contribute to its persistence and pathogenesis. The non-structural signaling membrane protein K15 is involved in the angiogenic and invasive properties of KSHV-infected endothelial cells. Here we show that the K15 protein is required for virus replication, early viral gene expression and virus production through its activation of the cellular signaling pathways PLCγ1 and Erk 1/2. K15 is abundantly expressed in KSHV-infected lymphatic endothelial cells (LECs) and contributes to KSHV-induced endothelial spindle cell formation. The abundant K15 protein expression observed in LECs is also observed in KS tumors. We also show that it may be possible to target K15 in order to intervene therapeutically with KSHV lytic replication and pathogenesis.

Diversity of human herpesvirus 8 genotypes in patients with AIDS and non-AIDS associated Kaposi's sarcoma, Castleman's disease and primary effusion lymphoma in Argentina

HHV-8 genotypes are distributed heterogeneously worldwide. The variable K1 gene and the conserved ORF26E region serve to genotype. The aim of the study was to characterize HHV-8 isolates from patients with AIDS, classical, and iatrogenic KS, primary effusion lymphoma and Castleman's disease and one organ donor from Argentina by analysis of ORFK1 and ORF26E regions. DNA was extracted from fresh or paraffin embedded biopsies, blood, and saliva samples and submitted to HHV-8 PCR. Phylogenetic analyses of ORFK1 showed that subtypes C (C1, C2, and C3), B1 and A (A1, A2, and A3) were present in 70.8%, 16.7%, and 12.5% of cases, respectively. Analyses of ORF26E fragment revealed that most strains (45.8%) were subtype A/C while the remaining fall into K, J, B2, R, and D subtypes. Linkage between ORFK1-ORF26E subtypes corresponded to reported relationships, except for one strain that clustered with B1 (K1 African) and D (ORF26E Asian-Pacific) subtypes. This research reveals predominance of subtype C, a broad spectrum of HHV-8 genotypes and reports the first isolation of the African B genotype in Argentina.

Constitutive activation of T cells by γ2-herpesviral GPCR through the interaction with cellular CXCR4

Members of the herpesviral family use multiple strategies to hijack infected host cells and exploit cellular signaling for their pathogenesis and latent infection. Among the most intriguing weapons in the arsenal of pathogenic herpesviruses are the constitutively active virally-encoded G protein-coupled receptors (vGPCRs). Even though vGPCRs contribute to viral pathogenesis such as immune evasion and proliferative disorders, the molecular details of how vGPCRs continuously activate cellular signaling are largely unknown. Here, we report that the vGPCR of Herpesvirus saimiri (HVS), an oncogenic γ2-herpesvirus, constitutively activates T cells via a heteromeric interaction with cellular CXCR4. Constitutive T cell activation also occurs with expression of the vGPCR of Kaposi's sarcoma-associated herpesvirus (KSHV), but not the vGPCR of Epstein-Barr virus. Expression of HVS vGPCR down-regulated the surface expression of CXCR4 but did not induce the degradation of the chemokine receptor, suggesting that vGPCR/CXCR4 signaling continues in cytosolic compartments. The physical association of vGPCR with CXCR4 was demonstrated by proximity ligation assay as well as immunoprecipitation. Interestingly, the constitutive activation of T cells by HVS vGPCR is independent of proximal T cell receptor (TCR) signaling molecules, such as TCRβ, Lck, and ZAP70, whereas CXCR4 silencing by shRNA abolished T cell activation by vGPCRs of HVS and KSHV. Furthermore, previously identified inactive vGPCR mutants failed to interact with CXCR4. These findings on the positive cooperativity of vGPCR with cellular CXCR4 in T cell activation extend our current understanding of the molecular mechanisms of vGPCR function and highlight the importance of heteromerization for GPCR activity.

The K1 Protein of Kaposi's Sarcoma-Associated Herpesvirus Augments Viral Lytic Replication

The K1 gene product of Kaposi's sarcoma-associated herpesvirus (KSHV) is encoded by the first open reading frame (ORF) of the viral genome. To investigate the role of the K1 gene during the KSHV life cycle, we constructed a set of recombinant viruses that contained either wild-type (WT) K1, a deleted K1 ORF (KSHVΔK1), stop codons within the K1 ORF (KSHV-K15×STOP), or a revertant K1 virus (KSHV-K1REV). We report that the recombinant viruses KSHVΔK1 and KSHV-K15×STOP displayed significantly reduced lytic replication compared to WT KSHV and KSHV-K1REV upon reactivation from latency. Additionally, cells infected with the recombinant viruses KSHVΔK1 and KSHV-K15×STOP also yielded smaller amounts of infectious progeny upon reactivation than did WT KSHV- and KSHV-K1REV-infected cells. Upon reactivation from latency, WT KSHV- and KSHV-K1REV-infected cells displayed activated Akt kinase, as evidenced by its phosphorylation, while cells infected with viruses deleted for K1 showed reduced phosphorylation and activation of Akt kinase. Overall, our results suggest that K1 plays an important role during the KSHV life cycle.

IMPORTANCE

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of three human malignancies, and KSHV K1 is a signaling protein that has been shown to be involved in cellular transformation and to activate the phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR pathway. In order to investigate the role of the K1 protein in the life cycle of KSHV, we constructed recombinant viruses that were deficient for K1. We found that K1 deletion viruses displayed reduced lytic replication compared to the WT virus and also yielded smaller numbers of infectious progeny. We report that K1 plays an important role in the life cycle of KSHV.

The Modulation of Apoptotic Pathways by Gammaherpesviruses

Apoptosis or programmed cell death is a tightly regulated process fundamental for cellular development and elimination of damaged or infected cells during the maintenance of cellular homeostasis. It is also an important cellular defense mechanism against viral invasion. In many instances, abnormal regulation of apoptosis has been associated with a number of diseases, including cancer development. Following infection of host cells, persistent and oncogenic viruses such as the members of the Gammaherpesvirus family employ a number of different mechanisms to avoid the host cell’s “burglar” alarm and to alter the extrinsic and intrinsic apoptotic pathways by either deregulating the expressions of cellular signaling genes or by encoding the viral homologs of cellular genes. In this review, we summarize the recent findings on how gammaherpesviruses inhibit cellular apoptosis via virus-encoded proteins by mediating modification of numerous signal transduction pathways. We also list the key viral anti-apoptotic proteins that could be exploited as effective targets for novel antiviral therapies in order to stimulate apoptosis in different types of cancer cells.

MiRNA-891a-5p mediates HIV-1 Tat and KSHV Orf-K1 synergistic induction of angiogenesis by activating NF-κB signaling

Co-infection with HIV-1 and Kaposi's sarcoma-associated herpesvirus (KSHV) is the cause of aggressive AIDS-related Kaposi's sarcoma (AIDS-KS) characterized by abnormal angiogenesis. The impact of HIV-1 and KSHV interaction on the pathogenesis and extensive angiogenesis of AIDS-KS remains unclear. Here, we explored the synergistic effect of HIV-1 Tat and KSHV oncogene Orf-K1 on angiogenesis. Our results showed that soluble Tat or ectopic expression of Tat enhanced K1-induced cell proliferation, microtubule formation and angiogenesis in chorioallantoic membrane and nude mice models. Mechanistic studies revealed that Tat promoted K1-induced angiogenesis by enhancing NF-κB signaling. Mechanistically, we showed that Tat synergized with K1 to induce the expression of miR-891a-5p, which directly targeted IκBα 3′ untranslated region, leading to NF-κB activation. Consequently, inhibition of miR-891a-5p increased IκBα level, prevented nuclear translocation of NF-κB p65 and ultimately suppressed the synergistic effect of Tat- and K1-induced angiogenesis. Our results illustrate that, by targeting IκBα to activate the NF-κB pathway, miR-891a-5p mediates Tat and K1 synergistic induction of angiogenesis. Therefore, the miR-891a-5p/NF-κB pathway is important in the pathogenesis of AIDS-KS, which could be an attractive therapeutic target for AIDS-KS.

Biology and oncogenicity of the Kaposi sarcoma herpesvirus K1 protein

The Kaposi sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8, is a gammaherpesvirus etiologically linked to the development of Kaposi sarcoma, primary effusion lymphomas, and multicentric Castleman disease in humans. KSHV is unique among other human herpesviruses because of the elevated number of viral products that mimic human cellular proteins, such as a viral cyclin, a viral G protein-coupled receptor, anti-apoptotic proteins (e.g., v-bcl2 and v-FLIP), viral interferon regulatory factors, and CC chemokine viral homologues. Several KSHV products have oncogenic properties, including the transmembrane K1 glycoprotein. KSHV K1 is encoded in the viral ORFK1, which is the most variable portion of the viral genome, commonly used to discriminate among viral genotypes. The extracellular region of K1 has homology with the light chain of lambda immunoglobulin, and its cytoplasmic region contains an immunoreceptor tyrosine-based activation motif (ITAM). KSHV K1 ITAM activates several intracellular signaling pathways, notably PI3K/AKT. Consequently, K1 expression inhibits proapoptotic proteins and increases the life-span of KSHV-infected cells. Another remarkable effect of K1 activity is the production of inflammatory cytokines and proangiogenic factors, such as vascular endothelial growth factor. KSHV K1 immortalizes primary human endothelial cells and transforms rodent fibroblasts in vitro; moreover, K1 induces tumors in vivo in transgenic mice expressing this viral protein. This review aims to consolidate and discuss the current knowledge on this intriguing KSHV protein, focusing on activities of K1 that can contribute to the pathogenesis of KSHV-associated human cancers.

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.

HIV-1 Nef and KSHV oncogene K1 synergistically promote angiogenesis by inducing cellular miR-718 to regulate the PTEN/AKT/mTOR signaling pathway

Kaposi's sarcoma (KS) is an AIDS-defining cancer with aberrant neovascularization caused by KS-associated herpesvirus (KSHV). Although the interaction between HIV-1 and KSHV plays a pivotal role in promoting the aggressive manifestations of KS, the pathogenesis underlying AIDS-KS remains largely unknown. Here we examined HIV-1 Nef protein promotion of KSHV oncoprotein K1-induced angiogenesis. We showed that both internalized and ectopic expression of Nef in endothelial cells synergized with K1 to facilitate vascular tube formation and cell proliferation, and enhance angiogenesis in a chicken CAM model. In vivo experiments further indicated that Nef accelerated K1-induced angiogenesis and tumorigenesis in athymic nu/nu mice. Mechanistic studies revealed that Nef and K1 synergistically activated PI3K/AKT/mTOR signaling by downregulating PTEN. Furthermore, Nef and K1 induced cellular miR-718, which inhibited PTEN expression by directly targeting a seed sequence in the 3' UTR of its mRNA. Inhibition of miR-718 expression increased PTEN synthesis and suppressed the synergistic effect of Nef- and K1-induced angiogenesis and tumorigenesis. These results indicate that, by targeting PTEN, miR-718 mediates Nef- and K1-induced angiogenesis via activation of AKT/mTOR signaling. Our results demonstrate an essential role of miR-718/AKT/mTOR axis in AIDS-KS and thus may represent an attractive therapeutic target.

KSHV: pathways to tumorigenesis and persistent infection

Kaposi's sarcoma-associated herpesvirus (KSHV; also known as human herpesvirus 8) is the etiologic agent of Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. These cancers often occur in the context of immunosuppression, which has made KSHV-associated malignancies an increasing global health concern with the persistence of the AIDS epidemic. KSHV has also been linked to several acute inflammatory diseases. KSHV exists between a lytic and latent lifecycle, which allows the virus to transition between active replication and quiescent infection. KSHV encodes a number of proteins and small RNAs that are thought to inadvertently transform host cells while performing their functions of helping the virus persist in the infected host. KSHV also has an arsenal of components that aid the virus in evading the host immune response, which help the virus establish a successful lifelong infection. In this comprehensive chapter, we will discuss the diseases associated with KSHV infection, the biology of latent and lytic infection, and individual proteins and microRNAs that are known to contribute to host cell transformation and immune evasion.

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.

Review of latent and lytic phase biomarkers in Kaposi's sarcoma

INTRODUCTION

Kaposi's sarcoma (KS) is a vascular neoplasm with distinct clinical-epidemiological subtypes and varied clinical presentations. While the association of KS with human herpesvirus-8 (HHV8, KSHV) infection is well known, additional factors are needed for tumorigenesis. The precise sequence of events involved in KS development, progression and regression continues to be investigated. The discovery of KSHV biomarkers is helpful for diagnostic purposes, for understanding KS pathogenesis and for identifying potential druggable targets.

AREAS COVERED

This article reviews a number of key biomarkers relevant for the diagnosis of KS and HHV8-related pathogenesis. New developments in KS, potential therapeutic targets and the challenges involved in their discovery are highlighted.

EXPERT OPINION

Although there is currently no cure for KS, continued research devoted to uncovering biomarkers and understanding their pathogenic roles remains encouraging. The hope is that sometime soon one of these candidate targets will provide a curative therapy for this enigmatic sarcoma.

AKTivation of PI3K/AKT/mTOR signaling pathway by KSHV

As an obligate intracellular parasite, Kaposi sarcoma-associated herpesvirus (KSHV) relies on the host cell machinery to meet its needs for survival, viral replication, production, and dissemination of progeny virions. KSHV is a gammaherpesvirus that is associated with three different malignancies: Kaposi sarcoma (KS), and two B cell lymphoproliferative disorders, primary effusion lymphoma (PEL) and multicentric Castleman’s disease. KSHV viral proteins modulate the cellular phosphatidylinositol-3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling pathway, which is a ubiquitous pathway that also controls B lymphocyte proliferation and development. We review the mechanisms by which KSHV manipulates the PI3K/AKT/mTOR pathway, with a specific focus on B cells.

Modulation of autophagy-like processes by tumor viruses

Autophagy is an intracellular degradation pathway for long-lived proteins and organelles. This process is activated above basal levels upon cell intrinsic or environmental stress and dysregulation of autophagy has been linked to various human diseases, including those caused by viral infection. Many viruses have evolved strategies to directly interfere with autophagy, presumably to facilitate their replication or to escape immune detection. However, in some cases, modulation of autophagy appears to be a consequence of the virus disturbing the cell’s metabolic signaling networks. Here, we summarize recent advances in research at the interface of autophagy and viral infection, paying special attention to strategies that human tumor viruses have evolved.

Hsp90 and Hsp40/Erdj3 are required for the expression and anti-apoptotic function of KSHV K1

Kaposi sarcoma-associated herpesvirus (KSHV) is a member of the gammaherpesvirus family. It is the etiological agent of three different human cancers, Kaposi sarcoma (KS), primary effusion lymphoma (PEL) and multicentric Castleman disease. The far left end of the KSHV genome encodes a unique transmembrane glycoprotein called K1. K1 possesses the ability to transform rodent fibroblasts and block apoptosis. K1 has also been shown to activate the PI3K/Akt/mTOR pathway in different cells. Using tandem affinity purification, we identified heat shock protein 90beta (Hsp90beta) and endoplasmic reticulum-associated Hsp40 (Erdj3/DnaJB11), as cellular binding partners of K1. Interactions of K1 with Hsp90beta and Hsp40 were confirmed by co-immunoprecipitation in both directions. Furthermore, K1 also interacted with the Hsp90alpha isoform. We report that small-interfering RNAs directed against Hsp90 and Hsp40/Erdj3, as well as pharmacological inhibitors of Hsp90, dramatically reduced K1 expression, suggesting that K1 is a client protein of these chaperones. In addition, both Hsp90 and Hsp40/Erdj3 were essential for K1's anti-apoptotic function. Finally, we report that the Hsp90 inhibitors, 17-AAG and 17-DMAG, can suppress the proliferation of KSHV-positive PEL cell lines and exhibited IC(50) values of 50 nM and below.

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.

Kaposi's sarcoma-associated herpesvirus viral interferon regulatory factor 4 targets MDM2 to deregulate the p53 tumor suppressor pathway

Cells infected by viruses utilize interferon (IFN)-mediated and p53-mediated irreversible cell cycle arrest and apoptosis as part of the overall host surveillance mechanism to ultimately block viral replication and dissemination. Viruses, in turn, have evolved elaborate mechanisms to subvert IFN- and p53-mediated host innate immune responses. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes several viral IFN regulatory factors (vIRF1 to vIRF4) within a cluster of loci, their functions being primarily to inhibit host IFN-mediated innate immunity and deregulate p53-mediated cell growth control. Despite its significant homology and similar genomic location to other vIRFs, vIRF4 is distinctive, as it does not target and antagonize host IFN-mediated signal transduction. Here, we show that KSHV vIRF4 interacts with the murine double minute 2 (MDM2) E3 ubiquitin ligase, leading to the reduction of p53, a tumor suppressor, via proteasome-mediated degradation. The central region of vIRF4 is required for its interaction with MDM2, which led to the suppression of MDM2 autoubiquitination and, thereby, a dramatic increase in MDM2 stability. Consequently, vIRF4 expression markedly enhanced p53 ubiquitination and degradation, effectively suppressing p53-mediated apoptosis. These results indicate that KSHV vIRF4 targets and stabilizes the MDM2 E3 ubiquitin ligase to facilitate the proteasome-mediated degradation of p53, perhaps to circumvent host growth surveillance and facilitate viral replication in infected cells. Taken together, the indications are that the downregulation of p53-mediated cell growth control is a common characteristic of the four KSHV vIRFs and that p53 is indeed a key factor in the host's immune surveillance program against viral infections.

Mechanism of Fas signaling regulation by human herpesvirus 8 K1 oncoprotein

BACKGROUND

Herpesvirus 8 (HHV-8) oncoprotein K1 is linked to lymphoproliferation and suppression of apoptosis mediated by the Fas death receptor. Expression of K1 in transgenic mice induces accumulation of lymphoid tissue cells and lymphoma.

METHODS

To examine how K1 and Fas interact to suppress apoptosis, K1-Fas binding was studied in human embryonic kidney (HEK) and lymphoma (BJAB) cells that expressed wild-type K1 or a K1 Ig domain deletion mutant and were treated with Fas ligand (FasL) or an agonistic Fas antibody, using immunoprecipitation and Western blot analysis. Cleavage of caspase-3 and apoptosis was compared in liver samples from mice that were transfected with empty vector vs with plasmids expressing wild-type K1 or a K1 Ig deletion mutant and treated with agonistic Fas antibody for 7 hours. These studies used immunohistochemical staining and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay. All statistical tests were two-sided.

RESULTS

Immunoprecipitation and Western blot analysis of transfected HEK and BJAB cells revealed that wild-type K1 but not Ig-deleted K1 binds to Fas and prevents Fas activation by FasL or by an agonistic Fas antibody. More mice that were transfected with wild-type K1 (7 of 10) than mice transfected with empty vector (3 of 13) or the K1 Ig deletion mutant (0 of 6) survived treatment with the agonistic Fas antibody. Compared with vector-transfected mice, livers of wild-type K1-transfected mice contained fewer cells in which caspase-3 was cleaved (87.6% vs 58.0%, difference = 29.6%, 95% confidence interval [CI] = 19.2% to 40.0%; P = .003) and fewer apoptotic cells (83.7% vs 34.2%, difference = 49.5%, 95% CI = 39.8% to 59.2%; P = .003).

CONCLUSIONS

K1 blocks Fas signaling by directly binding to Fas through the Ig-like domain of K1 and preventing binding of FasL. The relative resistance of cancer cells to Fas-mediated apoptosis may be due to the inhibition of Fas by Ig domain-containing proteins.

Critical role for endocytosis in the regulation of signaling by the Kaposi's sarcoma-associated herpesvirus K1 protein

Kaposi's sarcoma-associated herpesvirus (KSHV) is a member of the gammaherpesvirus family. KSHV is the etiologic agent of Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. The first open reading frame of the KSHV genome encodes a type 1 transmembrane glycoprotein named K1. K1 is structurally similar to the B-cell receptor (BCR), and its cytoplasmic tail contains an immunoreceptor tyrosine-based activation motif that can activate Syk kinase and the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. Recent evidence suggests that receptor signaling occurs not only at the cell membrane, but from intracellular compartments as well. We have found that K1 is internalized in a clathrin-dependent manner, and efficient internalization is coupled to its signaling function. Once internalized, K1 traffics from the early endosome to the recycling endosome. Interestingly, blocking K1's activation of Syk and PI3K prevents K1 from internalizing. We have also found that blocking clathrin-mediated endocytosis prevents downstream signaling by K1. These results strongly suggest that internalization of K1 is intimately associated with normal signaling. When K1 internalization was examined in B lymphocytes, we found that K1 cointernalized with the BCR. Altogether, these results suggest that K1's signaling function is tightly coupled to its internalization.

Increased tumorigenicity of cells carrying recombinant human herpesvirus 8

Human herpesvirus 8 (HHV-8) infection is associated with the development of Kaposi's sarcoma and primary effusion lymphoma. The cloning of the HHV-8 genome into a bacterial artificial chromosome (BAC) allows researchers to mutate and identify the relative importance of HHV-8 genes essential for growth and replication in tissue culture systems. However, in vivo models to study the impact of such mutations are very limited. Consequently, the objective of this study was to determine whether cells carrying the HHV-8 BAC would form tumors when injected into mice, enabling the use of this model to assess the influence of viral gene mutation on tumorigenesis. To do so, 293T and 293T-E1 cells carrying recombinant HHV-8 were injected into SCID mice and tumor growth was analyzed. Our results clearly show that mice injected with 293T-E1 cells had a significantly higher tumor incidence level as well as increased tumor volumes and weights compared to mice injected with 293T control cells. Cells carrying the HHV-8 genome grew faster and more aggressively in SCID mice than control 293T cells, highlighting the oncogenic properties of HHV-8. The model presented could therefore be used for the identification of HHV-8 genes contributing to tumorigenesis in the context of the entire viral genome.

K1 protein of human herpesvirus 8 suppresses lymphoma cell Fas-mediated apoptosis

Expression of the K1 gene of human herpesvirus 8 activates nuclear factor-kappaB and induces lymph node hyperplasia and lymphomas in transgenic mice. To further delineate its role in cell survival, we determined whether K1 altered apoptosis of lymphoma cells. K1 protein is expressed in Kaposi sarcoma and primary effusion lymphoma. We retrovirally transfected BJAB lymphoma, THP-1, U937, and Kaposi sarcoma SLK cells to express K1 and a K1 mutant with the deleted immunoreceptor tyrosine-based activation motif (K1m). We challenged cells with an agonistic anti-Fas antibody, Fas ligand, irradiation, and tumor necrosis factor-related apoptosis-inducing ligand. K1 transfectants but not K1m transfectants exhibited reduced levels of apoptosis induced by the anti-Fas antibody but not apoptosis induced by the tumor necrosis factor-related apoptosis-inducing ligand or irradiation. K1 expression resulted in reduced apoptosis rates as shown in several assays. K1 induced a modest reduction in levels of Fas-associated death domain protein, and procaspase 8 recruited to the death-inducing signaling complex. Finally, K1 transfectants cleaved procaspase 8 at significantly lower rates than did K1m transfectants. K1-transfected mice, compared with vector-transfected mice, showed lower death rates after challenge with anti-Fas antibody. K1 may contribute to lymphoma development by stimulating cell survival by selectively blocking Fas-mediated apoptosis.

Modern evolutionary history of the human KSHV genome

The genomes of several human herpesviruses, including Kaposi sarcoma (KS) herpesvirus (KSHV), display surprisingly high levels of both genetic diversity and clustered subtyping at certain loci. We have been interested in understanding this phenomenon with the hope that it might be a useful diagnostic tool for viral epidemiology, and that it might provide some insights about how these large viral genomes evolve over a relatively short timescale. To do so, we have carried out extensive PCR DNA sequence analysis across the genomes of 200 distinct KSHV samples collected from KS patients around the world. Here we review and summarize current understanding of the origins of KSHV variability, the spread of KSHV and its human hosts out of Africa, the existence of chimeric genomes, and the concept that different segments of the genome have had different evolutionary histories.

Virus-associated lymphomagenesis

At least 2 billion people are affected by viral infections worldwide. The infections induce a lot of various human diseases and are one of the main causes of human mortality. In particular, they can lead to development of various human cancers. Up to 15-20% of human cancer incidence can be attributed to viruses. Although viral infections are very common in the general population, only few of them result in clinically relevant lesions. Certain associations between virus infections and malignancy are strong and irrefutable, the others are still speculative. The criteria most often used for determining the causality are the consistence of the association, either epidemiologic or at the molecular level, and oncogenicity of viruses or particular viral genes in animal models or cell cultures. Due to some ambiguity of such a determination, it is instructive to consider by specific cases what evidence is generally accepted as sufficient to establish a causal relation between virus and cancer. Lymphomas are one of the best studied cancer types closely associated with a small but definite range of viruses. Numerous data show a close interrelation between lymphomagenesis and infection by such viruses as Kaposi's sarcoma herpesvirus (KSHV), Epstein-Barr virus (EBV), hepatitis C virus (HCV), human T-cell leukemia virus (HTLV), and human immunodeficiency virus (HIV). For instance, experiments on monkeys artificially infected with viruses and data on anti-cancer effect of specific antiviral preparations strongly suggest the involvement of viruses in lymphoma development. The present review is devoted to the association of different viruses with human lymphomas and to viral genes potentially involved in the neoplastic process. The recognition of virus involvement in lymphomagenesis may facilitate new strategies for cancer therapy, diagnosis and screening and can lead to a reduction in the number of individuals at risk of disease.

Regulation of intracellular signalling by the terminal membrane proteins of members of the Gammaherpesvirinae

The human gamma(1)-herpesvirus Epstein-Barr virus (EBV) and the gamma(2)-herpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV), rhesus rhadinovirus (RRV), herpesvirus saimiri (HVS) and herpesvirus ateles (HVA) all contain genes located adjacent to the terminal-repeat region of their genomes, encoding membrane proteins involved in signal transduction. Designated 'terminal membrane proteins' (TMPs) because of their localization in the viral genome, they interact with a variety of cellular signalling molecules, such as non-receptor protein tyrosine kinases, tumour-necrosis factor receptor-associated factors, Ras and Janus kinase (JAK), thereby initiating further downstream signalling cascades, such as the MAPK, PI3K/Akt, NF-kappaB and JAK/STAT pathways. In the case of TMPs expressed during latent persistence of EBV and HVS (LMP1, LMP2A, Stp and Tip), their modulation of intracellular signalling pathways has been linked to the provision of survival signals to latently infected cells and, hence, a contribution to occasional cellular transformation. In contrast, activation of similar pathways by TMPs of KSHV (K1 and K15) and RRV (R1), expressed during lytic replication, may extend the lifespan of virus-producing cells, alter their migration and/or modulate antiviral immune responses. Whether R1 and K1 contribute to the oncogenic properties of KSHV and RRV has not been established satisfactorily, despite their transforming qualities in experimental settings.

Characterization of Kaposi's sarcoma-associated herpesvirus (KSHV) K1 promoter activation by Rta

The K1 gene of Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a 46-kDa transmembrane glycoprotein that possesses transforming properties, initiates signaling pathways in B cells, and prevents apoptosis. Here, we demonstrate a mechanism for activation of the K1 promoter by the Rta transactivator. Electrophoretic mobility shift assay (EMSA) analysis of the K1 promoter demonstrated that purified Rta protein bound to the K1 promoter at three locations, independent of other DNA-binding factors. Transcriptional assays revealed that only two of these Rta DNA-binding sites are functionally significant, and that they could impart Rta responsiveness to a heterologous E4 TATA box promoter. In addition, TATA-binding protein (TBP) bound to a TATA box element located 25 bp upstream of the K1 transcription start site and was also shown to associate with Rta by coimmunoprecipitation analysis. Rta transactivation may therefore be mediated in part through recruitment of TBP to target promoters.

Immortalization of Primary Endothelial Cells by the K1 Protein of Kaposi's Sarcoma–Associated Herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is linked to three different human cancers: Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. The Kaposi's sarcoma lesion expresses high levels of angiogenic factors and is comprised of a mixed cell population, including endothelial cells that are infected with KSHV. We find that the KSHV K1 protein is expressed in Kaposi's sarcoma lesions and can immortalize and extend the life span of primary human umbilical vein endothelial cells in culture. Vascular endothelial growth factor (VEGF) is critical for the survival of endothelial cells, and we show that expression of K1 in endothelial cells resulted in increased levels of secreted VEGF and the activation of key signaling pathways, including the VEGF/VEGF receptor and the phosphatidylinositol-3'-OH-kinase (PI3K) pathway. The SH2 binding motifs present in the cytoplasmic tail of K1 were critical for K1's ability to activate these pathways. Activation of PI3K by K1 results in activation of Akt kinase and mammalian target of rapamycin and inactivation of the proapoptotic proteins FKHR, glycogen synthase kinase-3, and Bad, which are events indicative of cell survival. Because activation of the PI3K pathway is critical for transformation of many human cells, we suggest that PI3K activation by K1 is involved in endothelial cell immortalization and contributes to KSHV-associated tumorigenesis. We also report that K1 enhances angiogenesis in vivo and increases tumor vasculature and tumor size.

Ugandan Kaposi's sarcoma-associated herpesvirus phylogeny: evidence for cross-ethnic transmission of viral subtypes

OBJECTIVE

The aim of this study was to test the relationship between Kaposi's sarcoma-associated herpesvirus (KSHV) phylogeny and host ethnicity at the within-country scale.

METHODS

KSHV genomic DNA samples were isolated from 31 patients across eleven Ugandan ethnic groups. Amino acid sequences of the ORF-K1 gene were used to construct a neighbor-joining phylogenetic tree.

RESULTS

A5 and B1 variants predominated with no evidence of distinct ethnic or geographic distribution. A new K1 subtype (F) was identified in a member of the Bantu Gisu tribe and a new subtype B variant (B3) among members of the Bantu Ganda tribe.

CONCLUSIONS

The phylogeny may yet be structured by host ethnicity if members of Ugandan groups have convoluted biological origins, even as they identify with single tribes. An alternative possibility is that KSHV subtype evolution may have preceded major diversification of sub-Saharan Africans into ethnicities as we know them today, with ethnic groups beginning their histories already hosting multiple subtypes. A third alternative is that horizontal transmission of multiple KSHV subtypes may have broken up vertical lineages of the virus passed down within Ugandan populations.

Characterization of the Kaposi's sarcoma-associated herpesvirus K1 signalosome

Kaposi's sarcoma (KS) is a multifocal angiogenic tumor and appears to be a hyperplastic disorder caused, in part, by local production of inflammatory cytokines. The K1 lymphocyte receptor-like protein of KS-associated herpesvirus (KSHV) efficiently transduces extracellular signals to elicit cellular activation events through its cytoplasmic immunoreceptor tyrosine-based activation motif (ITAM). To further delineate K1-mediated signal transduction, we purified K1 signaling complexes and identified its cellular components. Upon stimulation, the K1 ITAM was efficiently tyrosine phosphorylated and subsequently interacted with cellular Src homology 2 (SH2)-containing signaling proteins Lyn, Syk, p85, PLCgamma2, RasGAP, Vav, SH2 domain-containing protein tyrosine phosphatase 1/2, and Grab2 through its phosphorylated tyrosine residues. Mutational analysis demonstrated that each tyrosine residue of K1 ITAM contributed to the interactions with cellular signaling proteins in distinctive ways. Consequently, these interactions led to the marked augmentation of cellular signal transduction activity, evidenced by the increase of cellular tyrosine phosphorylation and intracellular calcium mobilization, the activation of NF-AT and AP-1 transcription factor activities, and the production of inflammatory cytokines. These results demonstrate that KSHV K1 effectively recruits a set of cellular SH2-containing signaling molecules to form the K1 signalosome, which elicits downstream signal transduction and induces inflammatory cytokine production.

Linking KSHV to human cancer

Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8 (HHV-8), has been linked to several malignancies in humans. KSHV is the etiologic agent associated with the development of Kaposi's sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman's disease (MCD). KSHV is a double-stranded DNA virus that has been classified as a gammaherpesvirus. Here, we review the association of KSHV with human cancer, viral genes that may potentially be involved in the neoplastic process, and current therapies used to treat KS, PEL, and MCD.

Molecular epidemiology of Kaposi's sarcoma herpesvirus (KSHV) in Cuban and German patients with Kaposi's sarcoma (KS) and asymptomatic sexual contacts

Kaposi's sarcoma (KS) shows a distinct geographical and ethnic distribution. The variable K1 gene serves to differentiate the KSHV subtypes A-E, M, N, and Q. Phylogenetic characterization of 19 classical and epidemic German KS specimens revealed the Eurasian KSHV subtypes C (n = 13, including 6 classical KS) and A (n = 6), while 27 Cuban specimens showed a variety of different subtypes (A: n = 16, 4 being A5; C: n = 8; B: n = 2; and the new subtype E: n = 1). Three pairs of isolates from KS patients and peripheral blood mononuclear cells (PBMC) of their sexual partners without KS were studied for the first time and found identical, strongly arguing for sexual transmission of KSHV in this unique cohort. The unique ethnic background of the Cuban population may explain the variety of different KSHV strains.

Activation of Src kinase Lyn by the Kaposi sarcoma-associated herpesvirus K1 protein: implications for lymphomagenesis

The K1 gene of Kaposi sarcoma-associated herpesvirus (KSHV) encodes a transmembrane glycoprotein bearing a functional immunoreceptor tyrosine-based activation motif (ITAM). Previously, we reported that the K1 protein induced plasmablastic lymphomas in K1 transgenic mice, and that these lymphomas showed enhanced Lyn kinase activity. Here, we report that systemic administration of the nuclear factor kappa B (NF-kappaB) inhibitor Bay 11-7085 or an anti-vascular endothelial growth factor (VEGF) antibody significantly reduced K1 lymphoma growth in nude mice. Furthermore, in KVL-1 cells, a cell line derived from a K1 lymphoma, inhibition of Lyn kinase activity by the Src kinase inhibitor PP2 decreased VEGF induction, NF-kappaB activity, and the cell proliferation index by 50% to 75%. In contrast, human B-cell lymphoma BJAB cells expressing K1, but not the ITAM sequence-deleted mutant K1, showed a marked increase in Lyn kinase activity with concomitant VEGF induction and NF-kappaB activation, indicating that ITAM sequences were required for the Lyn kinase-mediated activation of these factors. Our results suggested that K1-mediated constitutive Lyn kinase activation in K1 lymphoma cells is crucial for the production of VEGF and NF-kappaB activation, both strongly implicated in the development of KSHV-induced lymphoproliferative disorders.