Characterization and cell cycle regulation of the major Kaposi's sarcoma-associated herpesvirus (human herpesvirus 8) latent genes and their promoter

Human Viral Oncoproteins and Ubiquitin-Proteasome System

Some human cancers worldwide may be related to human tumor viruses. Knowing, controlling, and managing the viruses that cause cancers remain a problem. Also, tumor viruses use ubiquitin-proteasome system (UPS) that can alter host cellular processes through UPS. Human tumor viruses cause persistent infections, due to their ability to infect their host cells without killing them. Tumor viruses such as Epstein-Barr virus, hepatitis C virus, hepatitis B virus, human papillomaviruses, human T cell leukemia virus, Kaposi's sarcoma-associated herpesvirus, and Merkel cell polyomavirus are associated with human malignancies. They interfere with the regulation of cell cycle and control of apoptosis, which are important for cellular functions. These viral oncoproteins bind directly or indirectly to the components of UPS, modifying cellular pathways and suppressor proteins like p53 and pRb. They can also cause progression of malignancy. In this review, we focused on how viral oncoproteins bind to the components of the UPS and how these interactions induce the degradation of cellular proteins for their survival.

Murine Gammaherpesvirus 68 ORF45 Stimulates B2 Retrotransposon and Pre-tRNA Activation in a Manner Dependent on Mitogen-Activated Protein Kinase (MAPK) Signaling

RNA polymerase III (RNAPIII) transcribes a variety of noncoding RNAs, including tRNA (tRNA) and the B2 family of short interspersed nuclear elements (SINEs). B2 SINEs are noncoding retrotransposons that possess tRNA-like promoters and are normally silenced in healthy somatic tissue. Infection with the murine gammaherpesvirus MHV68 induces transcription of both SINEs and tRNAs, in part through the activity of the viral protein kinase ORF36. Here, we identify the conserved MHV68 tegument protein ORF45 as an additional activator of these RNAPIII loci. MHV68 ORF45 and ORF36 form a complex, resulting in an additive induction RNAPIII and increased ORF45 expression. ORF45-induced RNAPIII transcription is dependent on its activation of the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) signaling pathway, which in turn increases the abundance of the RNAPIII transcription factor Brf1. Other viral and nonviral activators of MAPK/ERK signaling also increase the levels of Brf1 protein, B2 SINE RNA, and tRNA, suggesting that this is a common strategy to increase RNAPIII activity. IMPORTANCE Gammaherpesviral infection alters the gene expression landscape of a host cell, including through the induction of noncoding RNAs transcribed by RNA polymerase III (RNAPIII). Among these are a class of repetitive genes known as retrotransposons, which are normally silenced elements and can copy and spread throughout the genome, and transfer RNAs (tRNAs), which are fundamental components of protein translation machinery. How these loci are activated during infection is not well understood. Here, we identify ORF45 from the model murine gammaherpesvirus MHV68 as a novel activator of RNAPIII transcription. To do so, it engages the MAPK/ERK signaling pathway, which is a central regulator of cellular response to environmental stimuli. Activation of this pathway leads to the upregulation of a key factor required for RNAPIII activity, Brf1. These findings expand our understanding of the regulation and dysregulation of RNAPIII transcription and highlight how viral cooption of key signaling pathways can impact host gene expression.

Alternative RNA splicing in cancer: what about adult T-cell leukemia?

Eukaryotic cells employ a broad range of mechanisms to regulate gene expression. Among others, mRNA alternative splicing is a key process. It consists of introns removal from an immature mRNA (pre-mRNA) via a transesterification reaction to create a mature mRNA molecule. Large-scale genomic studies have shown that in the human genome, almost 95% of protein-encoding genes go through alternative splicing and produce transcripts with different exons combinations (and sometimes retained introns), thus increasing the proteome diversity. Considering the importance of RNA regulation in cellular proliferation, survival, and differentiation, alterations in the alternative splicing pathway have been linked to several human cancers, including adult T-cell leukemia/lymphoma (ATL). ATL is an aggressive and fatal malignancy caused by the Human T-cell leukemia virus type 1 (HTLV-1). HTLV-1 genome encodes for two oncoproteins: Tax and HBZ, both playing significant roles in the transformation of infected cells and ATL onset. Here, we review current knowledge on alternative splicing and its link to cancers and reflect on how dysregulation of this pathway could participate in HTLV-1-induced cellular transformation and adult T-cell leukemia/lymphoma development.

Kaposi Sarcoma, a Trifecta of Pathogenic Mechanisms

Kaposi's sarcoma is a rare disease with four known variants: classic, epidemic, endemic and iatrogenic (transplant-related), all caused by an oncogenic virus named Human Herpes Virus 8. The viral infection in itself, along with the oncogenic properties of HHV8 and with immune system dysfunction, forms the grounds on which Kaposi's Sarcoma may develop. Infection with HHV8 occurs through saliva via close contacts, blood, blood products, solid organ donation and, rarely, vertical transmission. Chronic inflammation and oncogenesis are promoted by a mix of viral genes that directly promote cell survival and transformation or interfere with the regular cell cycle and cell signaling (of particular note: LANA-1, v-IL6, vBCL-2, vIAP, vIRF3, vGPCR, gB, K1, K8.1, K15). The most common development sites for Kaposi's sarcoma are the skin, mucocutaneous zones, lymph nodes and visceral organs, but it can also rarely appear in the musculoskeletal system, urinary system, endocrine organs, heart or eye. Histopathologically, spindle cell proliferation with slit-like vascular spaces, plasma cell and lymphocyte infiltrate are characteristic. The clinical presentation is heterogenic depending on the variant; some patients have indolent disease and others have aggressive disease. The treatment options include highly active antiretroviral therapy, surgery, radiation therapy, chemotherapy, and immunotherapy. A literature search was carried out using the MEDLINE/PubMed, SCOPUS and Google Scholar databases with a combination of keywords with the aim to provide critical, concise, and comprehensive insights into advances in the pathogenic mechanism of Kaposi's sarcoma.

Sperm associated antigen 9 promotes oncogenic KSHV-encoded interferon regulatory factor-induced cellular transformation and angiogenesis by activating the JNK/VEGFA pathway

Kaposi's sarcoma (KS), caused by Kaposi's sarcoma-associated herpesvirus (KSHV), is a highly angioproliferative disseminated tumor of endothelial cells commonly found in AIDS patients. We have recently shown that KSHV-encoded viral interferon regulatory factor 1 (vIRF1) mediates KSHV-induced cell motility (PLoS Pathog. 2019 Jan 30;15(1):e1007578). However, the role of vIRF1 in KSHV-induced cellular transformation and angiogenesis remains unknown. Here, we show that vIRF1 promotes angiogenesis by upregulating sperm associated antigen 9 (SPAG9) using two in vivo angiogenesis models including the chick chorioallantoic membrane assay (CAM) and the matrigel plug angiogenesis assay in mice. Mechanistically, vIRF1 interacts with transcription factor Lef1 to promote SPAG9 transcription. vIRF1-induced SPAG9 promotes the interaction of mitogen-activated protein kinase kinase 4 (MKK4) with JNK1/2 to increase their phosphorylation, resulting in enhanced VEGFA expression, angiogenesis, cell proliferation and migration. Finally, genetic deletion of ORF-K9 from KSHV genome abolishes KSHV-induced cellular transformation and impairs angiogenesis. Our results reveal that vIRF1 transcriptionally activates SPAG9 expression to promote angiogenesis and tumorigenesis via activating JNK/VEGFA signaling. These novel findings define the mechanism of KSHV induction of the SPAG9/JNK/VEGFA pathway and establish the scientific basis for targeting this pathway for treating KSHV-associated cancers.

Induction of Kaposi's Sarcoma-Associated Herpesvirus-Encoded Thymidine Kinase (ORF21) by X-Box Binding Protein 1

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent for Kaposi sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman disease (MCD). Like other herpesviruses, it has latent and lytic repertoires. However, there is evidence that some lytic genes can be directly activated by certain cellular factors. Cells undergoing endoplasmic reticulum stress express spliced X-box binding protein 1 (XBP-1s). XBP-1s is also present in large amounts in germinal center B cells. XBP-1s can activate the KSHV replication and transcription activator (RTA) and lytic replication. It can also directly activate KSHV-encoded viral interleukin-6 (vIL-6) and, thus, contribute to the pathogenesis of KSHV MCD. KSHV thymidine kinase (TK), the ORF21 gene product, can enhance the production of dTTP and is important for lytic replication. It can also phosphorylate zidovudine and ganciclovir to toxic moieties, enabling treatment of KSHV-MCD with these drugs. We show here that XBP-1s can directly activate ORF21 and that this activation is mediated primarily through two XBP-response elements (XRE) on the ORF21 promoter region. Deletion or mutation of these elements eliminated XBP-1s-induced upregulation of the promoter, and chromatin immunoprecipitation studies provide evidence that XBP-1s can bind to both XREs. Exposure of PEL cells to a chemical inducer of XBP-1s can induce ORF21 within 4 hours, and ORF21 expression in the lymph nodes of patients with KSHV-MCD is predominantly found in cells with XBP-1. Thus, XBP-1s may directly upregulate KSHV ORF21 and, thus, contribute to the pathogenesis of KSHV-MCD and the activity of zidovudine and valganciclovir in this disease.IMPORTANCE Spliced X-box binding protein 1 (XBP-1s), part of the unfolded protein response and expressed in developing germinal center B cells, can induce Kaposi's sarcoma-associated herpesvirus (KSHV) lytic replication and directly activate viral interleukin-6 (vIL-6). We show here that XBP-1s can also directly activate KSHV ORF21, a lytic gene. ORF21 encodes KSHV thymidine kinase (TK), which increases the pool of dTTP for viral replication and enhances lytic replication. Direct activation of ORF21 by XBP-1s can enhance viral replication in germinal center B cells and contribute to the pathogenesis of KSHV multicentric Castleman disease (MCD). KSHV-MCD is characterized by systemic inflammation caused, in part, by lytic replication and overproduction of KSHV vIL-6 in XBP-1s-expressing lymph node plasmablasts. KSHV thymidine kinase can phosphorylate zidovudine and ganciclovir to toxic moieties, and direct activation of ORF21 by XBP-1s may also help explain the effectiveness of zidovudine and valganciclovir in the treatment of KSHV-MCD.

Human DNA Virus Exploitation of the MAPK-ERK Cascade

The extracellular signal-regulated kinases (ERKs) comprise a particular branch of the mitogen-activated protein kinase cascades (MAPK) that transmits extracellular signals into the intracellular environment to trigger cellular growth responses. Similar to other MAPK cascades, the MAPK-ERK pathway signals through three core kinases—Raf, MAPK/ERK kinase (MEK), and ERK—which drive the signaling mechanisms responsible for the induction of cellular responses from extracellular stimuli including differentiation, proliferation, and cellular survival. However, pathogens like DNA viruses alter MAPK-ERK signaling in order to access DNA replication machineries, induce a proliferative state in the cell, or even prevent cell death mechanisms in response to pathogen recognition. Differential utilization of this pathway by multiple DNA viruses highlights the dynamic nature of the MAPK-ERK pathway within the cell and the importance of its function in regulating a wide variety of cellular fates that ultimately influence viral infection and, in some cases, result in tumorigenesis.

Molecular Biology of KSHV in Relation to HIV/AIDS-Associated Oncogenesis

Discovered in 1994, Kaposi's sarcoma-associated herpesvirus (KSHV) has been associated with four human malignancies including Kaposi's sarcoma, primary effusion lymphoma, a subset of multicentric Castleman's disease, and KSHV inflammatory cytokine syndrome. These malignancies mostly occur in immunocompromised patients including patients with acquired immunodeficiency syndrome and often cause significant mortality because of the lack of effective therapies. Significant progresses have been made to understand the molecular basis of KSHV infection and KSHV-induced oncogenesis in the last two decades. This chapter provides an update on the recent advancements focusing on the molecular events of KSHV primary infection, the mechanisms regulating KSHV life cycle, innate and adaptive immunity, mechanism of KSHV-induced tumorigenesis and inflammation, and metabolic reprogramming in KSHV infection and KSHV-transformed cells.

Quantitative RNAseq analysis of Ugandan KS tumors reveals KSHV gene expression dominated by transcription from the LTd downstream latency promoter

KSHV is endemic in Uganda and the HIV epidemic has dramatically increased the incidence of Kaposi sarcoma (KS). To investigate the role of KSHV in the development of KS, we obtained KS biopsies from ART-naïve, HIV-positive individuals in Uganda and analyzed the tumors using RNAseq to globally characterize the KSHV transcriptome. Phylogenetic analysis of ORF75 sequences from 23 tumors revealed 6 distinct genetic clusters with KSHV strains exhibiting M, N or P alleles. RNA reads mapping to specific unique coding sequence (UCDS) features were quantitated using a gene feature file previously developed to globally analyze and quantitate KSHV transcription in infected endothelial cells. A pattern of high level expression was detected in the KSHV latency region that was common to all KS tumors. The clear majority of transcription was derived from the downstream latency transcript promoter P3(LTd) flanking ORF72, with little evidence of transcription from the P1(LTc) latency promoter, which is constitutive in KSHV-infected lymphomas and tissue-culture cells. RNAseq data provided evidence of alternate P3(LTd) transcript editing, splicing and termination resulting in multiple gene products, with 90% of the P3(LTd) transcripts spliced to release the intronic source of the microRNAs K1-9 and 11. The spliced transcripts encode a regulatory uORF upstream of Kaposin A with alterations in intervening repeat sequences yielding novel or deleted Kaposin B/C-like sequences. Hierarchical clustering and PCA analysis of KSHV transcripts revealed three clusters of tumors with different latent and lytic gene expression profiles. Paradoxically, tumors with a latent phenotype had high levels of total KSHV transcription, while tumors with a lytic phenotype had low levels of total KSHV transcription. Morphologically distinct KS tumors from the same individual showed similar KSHV gene expression profiles suggesting that the tumor microenvironment and host response play important roles in the activation level of KSHV within the infected tumor cells.

Kaposi’s sarcoma (KS) is among the world’s most common AIDS-associated malignancies. The Kaposi sarcoma-associated herpesvirus (KSHV) was first identified in KS tumors and is now known to be the causative agent of all forms of KS, including classical, endemic, iatrogenic and HIV-associated. KSHV is endemic to sub-Saharan Africa with high infection rates in children and adults. Compounded with the high rate of HIV and AIDS in this area, pediatric and adult KS are some of the most common malignancies with the highest fatality rates. We used RNA deep sequencing to characterize KSHV expression in a large collection of KS biopsies from HIV-infected Ugandans. Using a novel approach to quantitate expression in complex genomes like KSHV, we found that RNA from a single KSHV promoter within the latency region constituted the majority of KSHV transcripts in the KS tumors. Alternate RNA processing produced different spliced and un-spliced transcripts with different coding potentials. Differential expression of other KSHV genes was detected which segregated the tumors into three different types depending on their expression of lytic or latency genes. Quantitative analysis of KSHV expression in KS tumors provides an important basis for future studies on the role of KSHV in the development of KS.

Circular DNA tumor viruses make circular RNAs

Epstein-Barr virus (EBV) and Kaposi's sarcoma herpesvirus (KSHV) cause ∼2% of all human cancers. RNase R-resistant RNA sequencing revealed that both gammaherpesviruses encode multiple, uniquely stable, circular RNAs (circRNA). EBV abundantly expressed both exon-only and exon-intron circRNAs from the BamHI A rightward transcript (BART) locus (circBARTs) formed from a spliced BART transcript and excluding the EBV miRNA region. The circBARTs were expressed in all verified EBV latency types, including EBV-positive posttransplant lymphoproliferative disease, Burkitt lymphoma, nasopharyngeal carcinoma, and AIDS-associated lymphoma tissues and cell lines. Only cells infected with the B95-8 EBV strain, with a 12-kb BART locus deletion, were negative for EBV circBARTs. Less abundant levels of EBV circRNAs originating from LMP2- and BHLF1-encoding genes were also identified. The circRNA sequencing of KSHV-infected primary effusion lymphoma cells revealed a KSHV-encoded circRNA from the vIRF4 locus (circvIRF4) that was constitutively expressed. In addition, KSHV polyadenylated nuclear (PAN) RNA locus generated a swarm (>100) of multiply backspliced, low-abundance RNase R-resistant circRNAs originating in both sense and antisense directions consistent with a novel hyperbacksplicing mechanism. In EBV and KSHV coinfected cells, exon-only EBV circBARTs were located more in the cytoplasm, whereas the intron-retaining circBARTs were found in the nuclear fraction. KSHV circvIRF4 and circPANs were detected in both nuclear and cytoplasmic fractions. Among viral circRNAs tested, none were found in polysome fractions from KSHV-EBV coinfected BC1 cells, although low-abundance protein translation from viral circRNAs could not be excluded. The circRNAs are a new class of viral transcripts expressed in gammaherpesvirus-related tumors that might contribute to viral oncogenesis.

Kaposi's Sarcoma-Associated Herpesvirus Latency-Associated Nuclear Antigen: Replicating and Shielding Viral DNA during Viral Persistence

Kaposi's sarcoma herpesvirus (KSHV) establishes lifelong latency. The viral latency-associated nuclear antigen (LANA) promotes viral persistence by tethering the viral genome to cellular chromosomes and by participating in latent DNA replication. Recently, the structure of the LANA C-terminal DNA binding domain was solved and new cytoplasmic variants of LANA were discovered. We discuss how these findings contribute to our current view of LANA structure and assembly and of its role during viral persistence.

Quantitative Analysis of the KSHV Transcriptome Following Primary Infection of Blood and Lymphatic Endothelial Cells

The transcriptome of the Kaposi’s sarcoma-associated herpesvirus (KSHV/HHV8) after primary latent infection of human blood (BEC), lymphatic (LEC) and immortalized (TIME) endothelial cells was analyzed using RNAseq, and compared to long-term latency in BCBL-1 lymphoma cells. Naturally expressed transcripts were obtained without artificial induction, and a comprehensive annotation of the KSHV genome was determined. A set of unique coding sequence (UCDS) features and a process to resolve overlapping transcripts were developed to accurately quantitate transcript levels from specific promoters. Similar patterns of KSHV expression were detected in BCBL-1 cells undergoing long-term latent infections and in primary latent infections of both BEC and LEC cultures. High expression levels of poly-adenylated nuclear (PAN) RNA and spliced and unspliced transcripts encoding the K12 Kaposin B/C complex and associated microRNA region were detected, with an elevated expression of a large set of lytic genes in all latently infected cultures. Quantitation of non-overlapping regions of transcripts across the complete KSHV genome enabled for the first time accurate evaluation of the KSHV transcriptome associated with viral latency in different cell types. Hierarchical clustering applied to a gene correlation matrix identified modules of co-regulated genes with similar correlation profiles, which corresponded with biological and functional similarities of the encoded gene products. Gene modules were differentially upregulated during latency in specific cell types indicating a role for cellular factors associated with differentiated and/or proliferative states of the host cell to influence viral gene expression.

Expression of the Antisense-to-Latency Transcript Long Noncoding RNA in Kaposi's Sarcoma-Associated Herpesvirus

The regulation of latency is central to herpesvirus biology. Recent transcriptome-wide surveys have uncovered evidence for promiscuous transcription across the entirety of the Kaposi's sarcoma-associated herpesvirus (KSHV) genome and postulated the existence of multiple viral long noncoding RNAs (lncRNAs). Next-generation sequencing studies are highly dependent on the specific experimental approach and particular algorithms of analysis and therefore benefit from independent confirmation of the results. The antisense-to-latency transcript (ALT) lncRNA was discovered by genome-tiling microarray (Chandriani et al., J Virol 86:7934-7942, 2010, https://doi.org/10.1128/JVI.00645-10). To characterize ALT in detail, we physically isolated this lncRNA by a strand-specific hybrid capture assay and then employed transcriptome sequencing and novel reverse transcription-PCR (RT-PCR) assays to distinguish all RNA species in the KSHV latency region. These methods confirm that ALT initiates at positions 120739/121012 and encodes a single splice site, which is shared with the 3'-coterminal K14-vGPCR/ORF74 mRNA, terminating at 130873 (GenBank accession number GQ994935), resulting in an ∼10,000-nucleotide transcript. No shorter ALT isoforms were identified. This study also identified a novel intron within the LANA 5' untranslated region using a splice acceptor at 127888. In summary, ALT joins PAN/nut1/T1.1 as a bona fide lncRNA of KSHV with potentially important roles in viral gene regulation and pathogenesis.

IMPORTANCE

Increasing data support the importance of noncoding RNAs (ncRNAs), including microRNAs (miRNAs) and lncRNAs, which have been shown to exert critical regulatory functions without coding for recognizable proteins. Defining the sequences of these ncRNAs is essential for future studies aiming to functionally characterize a specific ncRNA. Most lncRNA studies are highly dependent on high-throughput sequencing and bioinformatic analyses, few studies follow up on the initial predictions, and analyses are at times discordant. The manuscript characterizes one key viral lncRNA, ALT, by physically isolating ALT and by a sequencing-independent assay. It provides for a simple assay to monitor lncRNA expression in experimental and clinical samples. ALT is expressed antisense to the major viral latency transcripts encoding LANA as well as the viral miRNAs and thus has the potential to regulate this key part of the viral life cycle.

Nuclear Innate Immune DNA Sensor IFI16 Is Degraded during Lytic Reactivation of Kaposi's Sarcoma-Associated Herpesvirus (KSHV): Role of IFI16 in Maintenance of KSHV Latency

IFI16 (interferon gamma-inducible protein 16) recognizes nuclear episomal herpesvirus (Kaposi's sarcoma-associated herpesvirus [KSHV], Epstein-Barr virus [EBV], and herpes simplex virus 1 [HSV-1]) genomes and induces the inflammasome and interferon beta responses. It also acts as a lytic replication restriction factor and inhibits viral DNA replication (human cytomegalovirus [HCMV] and human papillomavirus [HPV]) and transcription (HSV-1, HCMV, and HPV) through epigenetic modifications of the viral genomes. To date, the role of IFI16 in the biology of latent viruses is not known. Here, we demonstrate that knockdown of IFI16 in the latently KSHV-infected B-lymphoma BCBL-1 and BC-3 cell lines results in lytic reactivation and increases in levels of KSHV lytic transcripts, proteins, and viral genome replication. Similar results were also observed during KSHV lytic cycle induction in TREX-BCBL-1 cells with the doxycycline-inducible lytic cycle switch replication and transcription activator (RTA) gene. Overexpression of IFI16 reduced lytic gene induction by the chemical agent 12-O-tetradecoylphorbol-13-acetate (TPA). IFI16 protein levels were significantly reduced or absent in TPA- or doxycycline-induced cells expressing lytic KSHV proteins. IFI16 is polyubiquitinated and degraded via the proteasomal pathway. The degradation of IFI16 was absent in phosphonoacetic acid-treated cells, which blocks KSHV DNA replication and, consequently, late lytic gene expression. Chromatin immunoprecipitation assays of BCBL-1 and BC-3 cells demonstrated that IFI16 binds to KSHV gene promoters. Uninfected epithelial SLK and osteosarcoma U2OS cells transfected with KSHV luciferase promoter constructs confirmed that IFI16 functions as a transcriptional repressor. These results reveal that KSHV utilizes the innate immune nuclear DNA sensor IFI16 to maintain its latency and repression of lytic transcripts, and a late lytic KSHV gene product(s) targets IFI16 for degradation during lytic reactivation.

IMPORTANCE

Like all herpesviruses, latency is an integral part of the life cycle of Kaposi's sarcoma-associated herpesvirus (KSHV), an etiological agent for many human cancers. Herpesviruses utilize viral and host factors to successfully evade the host immune system to maintain latency. Reactivation is a complex event where the latent episomal viral genome springs back to active transcription of lytic cycle genes. Our studies reveal that KSHV has evolved to utilize the innate immune sensor IFI16 to keep lytic cycle transcription in dormancy. We demonstrate that IFI16 binds to the lytic gene promoter, acts as a transcriptional repressor, and thereby helps to maintain latency. We also discovered that during the late stage of lytic replication, KSHV selectively degrades IFI16, thus relieving transcriptional repression. This is the first report to demonstrate the role of IFI16 in latency maintenance of a herpesvirus, and further understanding will lead to the development of strategies to eliminate latent infection.

Primary B Lymphocytes Infected with Kaposi's Sarcoma-Associated Herpesvirus Can Be Expanded In Vitro and Are Recognized by LANA-Specific CD4+ T Cells

Kaposi's sarcoma-associated herpesvirus (KSHV) has tropism for B lymphocytes, in which it establishes latency, and can also cause lymphoproliferative disorders of these cells manifesting as primary effusion lymphoma (PEL) and multicentric Castleman disease (MCD). T cell immunity is vital for the control of KSHV infection and disease; however, few models of B lymphocyte infection exist to study immune recognition of such cells. Here, we developed a model of B lymphocyte infection with KSHV in which infected tonsillar B lymphocytes were expanded by providing mitogenic stimuli and then challenged with KSHV-specific CD4(+)T cells. The infected cells expressed viral proteins found in PELs, namely, LANA and viral IRF3 (vIRF3), albeit at lower levels, with similar patterns of gene expression for the major latency, viral interleukin 6 (vIL-6), and vIRF3 transcripts. Despite low-level expression of open reading frame 50 (ORF50), transcripts for the immune evasion genes K3 and K5 were detected, with some downregulation of cell surface-expressed CD86 and ICAM. The vast majority of infected lymphocytes expressed IgM heavy chains with Igλ light chains, recapitulating the features seen in infected cells in MCD. We assessed the ability of the infected lymphocytes to be targeted by a panel of major histocompatibility complex (MHC) class II-matched CD4(+)T cells and found that LANA-specific T cells restricted to different epitopes recognized these infected cells. Given that at least some KSHV latent antigens are thought to be poor targets for CD8(+)T cells, we suggest that CD4(+)T cells are potentially important effectors for thein vivocontrol of KSHV-infected B lymphocytes.

IMPORTANCE

KSHV establishes a latent reservoir within B lymphocytes, but few models exist to study KSHV-infected B cells other than the transformed PEL cell lines, which have likely accrued mutations during the transformation process. We developed a model of KSHV-infected primary B lymphocytes that recapitulates features seen in PEL and MCD by gene expression and cell phenotype analysis, allowing the study of T cell recognition of these cells. Challenge of KSHV-infected B cells with CD4(+)T cells specific for LANA, a protein expressed in all KSHV-infected cells and malignanciesin vivo, showed that these effectors could efficiently recognize such targets. Given that the virus expresses immune evasion genes or uses proteins with intrinsic properties, such as LANA, that minimize epitope recognition by CD8(+)T cells, CD4(+)T cell immunity to KSHV may be important for maintaining the virus-host balance.

Induction of Kaposi's Sarcoma-Associated Herpesvirus-Encoded Viral Interleukin-6 by X-Box Binding Protein 1

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent for Kaposi sarcoma (KS), primary effusion lymphoma (PEL), and a subset of multicentric Castleman disease (MCD). The KSHV life cycle has two principal gene repertoires, latent and lytic. KSHV viral interleukin-6 (vIL-6), an analog of human IL-6, is usually lytic; production of vIL-6 by involved plasmablasts is a central feature of KSHV-MCD. vIL-6 also plays a role in PEL and KS. We show that a number of plasmablasts from lymph nodes of patients with KSHV-MCD express vIL-6 but not ORF45, a KSHV lytic gene. We further show that vIL-6 is directly induced by the spliced (active) X-box binding protein-1 (XBP-1s), a transcription factor activated by endoplasmic reticulum (ER) stress and differentiation of B cells in lymph nodes. The promoter region of vIL-6 contains several potential XBP-response elements (XREs), and two of these elements in particular mediate the effect of XBP-1s. Mutation of these elements abrogates the response to XBP-1s but not to the KSHV replication and transcription activator (RTA). Also, XBP-1s binds to the vIL-6 promoter in the region of these XREs. Exposure of PEL cells to a chemical inducer of XBP-1s can induce vIL-6. Patient-derived PEL tumor cells that produce vIL-6 frequently coexpress XBP-1, and immunofluorescence staining of involved KSHV-MCD lymph nodes reveals that most plasmablasts expressing vIL-6 also coexpress XBP-1. These results provide evidence that XBP-1s is a direct activator of KSHV vIL-6 and that this is an important step in the pathogenesis of KSHV-MCD and PEL.

IMPORTANCE

Kaposi sarcoma herpesvirus (KSHV)-associated multicentric Castleman disease (KSHV-MCD) is characterized by severe inflammatory symptoms caused by an excess of cytokines, particularly KSHV-encoded viral interleukin-6 (vIL-6) produced by lymph node plasmablasts. vIL-6 is usually a lytic gene. We show that a number of KSHV-MCD lymph node plasmablasts express vIL-6 but do not have full lytic KSHV replication. Differentiating lymph node B cells express spliced (active) X-box binding protein-1 (XBP-1s). We show that XBP-1s binds to the promoter of vIL-6 and can directly induce production of vIL-6 through X-box protein response elements on the vIL-6 promoter region. We further show that chemical inducers of XBP-1s can upregulate production of vIL-6. Finally, we show that most vIL-6-producing plasmablasts from lymph nodes of KSHV-MCD patients coexpress XBP-1s. These results demonstrate that XBP-1s can directly induce vIL-6 and provide evidence that this is a key step in the pathogenesis of KSHV-MCD and other KSHV-induced diseases.

Mechanism of angiopoietin-1 upregulation in Kaposi's sarcoma-associated herpesvirus-infected PEL cell lines

Angiopoietin-1 (ANGPT-1) is a secreted glycoprotein that was first characterized as a ligand of the Tie2 receptor. In a previous study using microarray analysis, we found that the expression of ANGPT-1 was upregulated in Kaposi's sarcoma-associated herpesvirus (KSHV)-infected primary effusion lymphoma (PEL) cell lines compared with that in uninfected Burkitt and other leukemia cell lines. Other authors have also reported focal expression of ANGPT-1 mRNA in biopsy specimens of Kaposi's sarcoma (KS) tissue from patients with AIDS. Here, to confirm these findings, we examined the expression and secretion levels of ANGPT-1 in KSHV-infected PEL cell lines and address the mechanisms of ANGPT-1 transcriptional regulation. We also showed that ANGPT-1 was expressed and localized in the cytoplasm and secreted into the supernatant of KSHV-infected PEL cells. Deletion studies of the regulatory region revealed that the region encompassing nucleotides -143 to -125 of the ANGPT-1-regulating sequence was responsible for this upregulation. Moreover, an electrophoretic mobility shift assay and chromatin immunoprecipitation, followed by quantitative PCR, suggested that some KSHV-infected PEL cell line-specific DNA-binding factors, such as OCT-1, should be involved in the upregulation of ANGPT-1 in a sequence-dependent manner.

IMPORTANCE

We confirmed that ANGPT-1 was expressed in and secreted from KSHV-infected PEL cells and that the transcriptional activity of ANGPT-1 was upregulated. A 19-bp fragment was identified as the region responsible for ANGPT-1 upregulation through binding with OCT-1 as a core factor in PEL cells. This study suggests that ANGPT-1 is overproduced in KSHV-infected PEL cells, which could affect the pathophysiology of AIDS patients with PEL.

A viral genome landscape of RNA polyadenylation from KSHV latent to lytic infection

RNA polyadenylation (pA) is one of the major steps in regulation of gene expression at the posttranscriptional level. In this report, a genome landscape of pA sites of viral transcripts in B lymphocytes with Kaposi sarcoma-associated herpesvirus (KSHV) infection was constructed using a modified PA-seq strategy. We identified 67 unique pA sites, of which 55 could be assigned for expression of annotated ∼90 KSHV genes. Among the assigned pA sites, twenty are for expression of individual single genes and the rest for multiple genes (average 2.7 genes per pA site) in cluster-gene loci of the genome. A few novel viral pA sites that could not be assigned to any known KSHV genes are often positioned in the antisense strand to ORF8, ORF21, ORF34, K8 and ORF50, and their associated antisense mRNAs to ORF21, ORF34 and K8 could be verified by 3′RACE. The usage of each mapped pA site correlates to its peak size, the larger (broad and wide) peak size, the more usage and thus, the higher expression of the pA site-associated gene(s). Similar to mammalian transcripts, KSHV RNA polyadenylation employs two major poly(A) signals, AAUAAA and AUUAAA, and is regulated by conservation of cis-elements flanking the mapped pA sites. Moreover, we found two or more alternative pA sites downstream of ORF54, K2 (vIL6), K9 (vIRF1), K10.5 (vIRF3), K11 (vIRF2), K12 (Kaposin A), T1.5, and PAN genes and experimentally validated the alternative polyadenylation for the expression of KSHV ORF54, K11, and T1.5 transcripts. Together, our data provide not only a comprehensive pA site landscape for understanding KSHV genome structure and gene expression, but also the first evidence of alternative polyadenylation as another layer of posttranscriptional regulation in viral gene expression.

A genome-wide polyadenylation landscape in the expression of human herpesviruses has not been reported. In this study, we provide the first genome landscape of viral RNA polyadenylation sites in B cells from KSHV latent to lytic infection by using a modified PA-seq protocol and selectively validated by 3′ RACE. We found that KSHV genome contains 67 active pA sites for the expression of its ∼90 genes and a few antisense transcripts. Among the mapped pA sites, a large fraction of them are for the expression of cluster genes and the production of bicistronic or polycistronic transcripts from KSHV genome and only one-third are used for the expression of single genes. We found that the size of individual PA peaks is positively correlated with the usage of corresponding pA site, which is determined by the number of reads within the PA peak from latent to lytic KSHV infection, and the strength of cis-elements surrounding KSHV pA site determines the expression level of viral genes. Lastly, we identified and experimentally validated alternative polyadenylation of KSHV ORF54, T1.5, and K11 during viral lytic infection. To our knowledge, this is the first report on alternative polyadenylation events in KSHV infection.

A critical Sp1 element in the rhesus rhadinovirus (RRV) Rta promoter confers high-level activity that correlates with cellular permissivity for viral replication

KSHV establishes characteristic latent infections in vitro, while RRV, a related macaque rhadinovirus, establishes characteristic permissive infections with virus replication. We identified cells that are not permissive for RRV replication and recapitulate the latent KSHV infection and reactivation processes. The RRV replication and transactivator (Rta) promoter was characterized in permissive and non-permissive cells and compared to the KSHV Rta promoter. Both promoters contained a critical Sp1 element, had equivalent activities in different cell types, and were inhibited by LANA. RRV and KSHV infections were non-permissive in cells with low Rta promoter activity. While RRV infections were permissive in cells with high basal promoter activity, KSHV infections remained non-permissive. Our studies suggest that RRV lacks the Rta-inducible LANA promoter that is responsible for LANA inhibition of the KSHV Rta promoter and induction of latency during KSHV infection. Instead, the outcome of RRV infection is determined by host factors, such as Sp1.

Next-generation sequence analysis of the genome of RFHVMn, the macaque homolog of Kaposi's sarcoma (KS)-associated herpesvirus, from a KS-like tumor of a pig-tailed macaque

The complete sequence of retroperitoneal fibromatosis-associated herpesvirus Macaca nemestrina (RFHVMn), the pig-tailed macaque homolog of Kaposi's sarcoma-associated herpesvirus (KSHV), was determined by next-generation sequence analysis of a Kaposi's sarcoma (KS)-like macaque tumor. Colinearity of genes was observed with the KSHV genome, and the core herpesvirus genes had strong sequence homology to the corresponding KSHV genes. RFHVMn lacked homologs of open reading frame 11 (ORF11) and KSHV ORFs K5 and K6, which appear to have been generated by duplication of ORFs K3 and K4 after the divergence of KSHV and RFHV. RFHVMn contained positional homologs of all other unique KSHV genes, although some showed limited sequence similarity. RFHVMn contained a number of candidate microRNA genes. Although there was little sequence similarity with KSHV microRNAs, one candidate contained the same seed sequence as the positional homolog, kshv-miR-K12-10a, suggesting functional overlap. RNA transcript splicing was highly conserved between RFHVMn and KSHV, and strong sequence conservation was noted in specific promoters and putative origins of replication, predicting important functional similarities. Sequence comparisons indicated that RFHVMn and KSHV developed in long-term synchrony with the evolution of their hosts, and both viruses phylogenetically group within the RV1 lineage of Old World primate rhadinoviruses. RFHVMn is the closest homolog of KSHV to be completely sequenced and the first sequenced RV1 rhadinovirus homolog of KSHV from a nonhuman Old World primate. The strong genetic and sequence similarity between RFHVMn and KSHV, coupled with similarities in biology and pathology, demonstrate that RFHVMn infection in macaques offers an important and relevant model for the study of KSHV in humans.

The open chromatin landscape of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic gammaherpesvirus which establishes latent infection in endothelial and B cells, as well as in primary effusion lymphoma (PEL). During latency, the viral genome exists as a circular DNA minichromosome (episome) and is packaged into chromatin analogous to human chromosomes. Only a small subset of promoters, those which drive latent RNAs, are active in latent episomes. In general, nucleosome depletion ("open chromatin") is a hallmark of eukaryotic regulatory elements such as promoters and transcriptional enhancers or insulators. We applied formaldehyde-assisted isolation of regulatory elements (FAIRE) followed by next-generation sequencing to identify regulatory elements in the KSHV genome and integrated these data with previously identified locations of histone modifications, RNA polymerase II occupancy, and CTCF binding sites. We found that (i) regions of open chromatin were not restricted to the transcriptionally defined latent loci; (ii) open chromatin was adjacent to regions harboring activating histone modifications, even at transcriptionally inactive loci; and (iii) CTCF binding sites fell within regions of open chromatin with few exceptions, including the constitutive LANA promoter and the vIL6 promoter. FAIRE-identified nucleosome depletion was similar among B and endothelial cell lineages, suggesting a common viral genome architecture in all forms of latency.

Cyclooxygenase-2-prostaglandin E2-eicosanoid receptor inflammatory axis: a key player in Kaposi's sarcoma-associated herpes virus associated malignancies

The role of cyclooxygenase-2 (COX-2), its lipid metabolite prostaglandin E2 (PGE2), and Eicosanoid (EP) receptors (EP; 1-4) underlying the proinflammatory mechanistic aspects of Burkitt's lymphoma, nasopharyngeal carcinoma, cervical cancer, prostate cancer, colon cancer, and Kaposi's sarcoma (KS) is an active area of investigation. The tumorigenic potential of COX-2 and PGE2 through EP receptors forms the mechanistic context underlying the chemotherapeutic potential of nonsteroidal anti-inflammatory drugs (NSAIDs). Although role of the COX-2 is described in several viral associated malignancies, the biological significance of the COX-2/PGE2/EP receptor inflammatory axis is extensively studied only in Kaposi's sarcoma-associated herpes virus (KSHV/HHV-8) associated malignancies such as KS, a multifocal endothelial cell tumor and primary effusion lymphoma (PEL), a B cell-proliferative disorder. The purpose of this review is to summarize the salient findings delineating the molecular mechanisms downstream of COX-2 involving PGE2 secretion and its autocrine and paracrine interactions with EP receptors (EP1-4), COX-2/PGE2/EP receptor signaling regulating KSHV pathogenesis and latency. KSHV infection induces COX-2, PGE2 secretion, and EP receptor activation. The resulting signal cascades modulate the expression of KSHV latency genes (latency associated nuclear antigen-1 [LANA-1] and viral-Fas (TNFRSF6)-associated via death domain like interferon converting enzyme-like- inhibitory protein [vFLIP]). vFLIP was also shown to be crucial for the maintenance of COX-2 activation. The mutually interdependent interactions between viral proteins (LANA-1/vFLIP) and COX-2/PGE2/EP receptors was shown to play key roles in the biological mechanisms involved in KS and PEL pathogenesis such as blockage of apoptosis, cell cycle regulation, transformation, proliferation, angiogenesis, adhesion, invasion, and immune-suppression. Understanding the COX-2/PGE2/EP axis is very important to develop new safer and specific therapeutic modalities for KS and PEL. In addition to COX-2 being a therapeutic target, EP receptors represent ideal targets for pharmacologic agents as PGE2 analogues and their blockers/antagonists possess antineoplastic activity, without the reported gastrointestinal and cardiovascular toxicity observed with few a NSAIDs.

Kaposi's sarcoma: a computational approach through protein-protein interaction and gene regulatory networks analysis

Interactomic data for Kaposi's Sarcoma Associated Herpes virus (KSHV)-the causative agent of vascular origin tumor called Kaposi's sarcoma-is relatively modest to date. The objective of this study was to assign functions to the previously uncharacterized ORFs in the virus using computational approaches and subsequently fit them to the host interactome landscape on protein, gene, and cellular level. On the basis of expression data, predicted RNA interference data, reported experimental data, and sequence based functional annotation we also tried to hypothesize the ORFs role in lytic and latent cycle during viral infection. We studied 17 previously uncharacterized ORFs in KSHV and the host-virus interplay seems to work in three major functional pathways-cell division, transport, metabolic and enzymatic in general. Studying the host-virus crosstalk for lytic phase predicts ORF 10 and ORF 11 as a predicted virus hub whereas PCNA is predicted as a host hub. On the other hand, ORF31 has been predicted as a latent phase inducible protein. KSHV invests a lion's share of its coding potential to suppress host immune response; various inflammatory mediators such as IFN-γ, TNF, IL-6, and IL-8 are negatively regulated by the ORFs while Il-10 secretion is stimulated in contrast. Although, like any other computational prediction, the study requires further validation, keeping into account the reproducibility and vast sample size of the systems biology approach the study allows us to propose an integrated network for host-virus interaction with good confidence. We hope that the study, in the long run, would help us identify effective dug against potential molecular targets.

CTCF regulates Kaposi's sarcoma-associated herpesvirus latency transcription by nucleosome displacement and RNA polymerase programming

CCCTC-binding factor (CTCF) has been implicated in various aspects of viral and host chromatin organization and transcriptional control. We showed previously that CTCF binds to a cluster of three sites in the first intron of the Kaposi's sarcoma-associated herpesvirus (KSHV) multicistronic latency-associated transcript that encodes latency-associated nuclear antigen (LANA), viral cyclin (vCyclin), vFLIP, viral microRNAs, and kaposin. We show here that these CTCF binding sites regulate mRNA production, RNA polymerase II (RNAPII) programming, and nucleosome organization of the KSHV latency transcript control region. We also show that KSHV bacmids lacking these CTCF binding sites have elevated and altered ratios of spliced latency transcripts. CTCF binding site mutations altered RNAPII and RNAPII-accessory factor interactions with the latency control region. CTCF binding sites were required for the in vitro recruitment of RNAPII to the latency control region, suggesting that direct interactions between CTCF and RNAPII contribute to transcription regulation. Histone modifications in the latency control region were also altered by mutations in the CTCF binding sites. Finally, we show that CTCF binding alters the regular phasing of nucleosomes in the latency gene transcript and intron, suggesting that nucleosome positioning can be an underlying biochemical mechanism of CTCF function. We propose that RNAPII interactions and nucleosome displacement serve as a biochemical basis for programming RNAPII in the KSHV transcriptional control region.

Unbiased mutagenesis of MHV68 LANA reveals a DNA-binding domain required for LANA function in vitro and in vivo

The Latency-Associated Nuclear Antigen (LANA), encoded by ORF73, is a conserved gene among the γ2-herpesviruses (rhadinoviruses). The Kaposi's Sarcoma-Associated Herpesvirus (KSHV) LANA is consistently expressed in KSHV-associated malignancies. In the case of the rodent γ2-herpesvirus, murine gammaherpesvirus 68 (MHV68), the LANA homolog (mLANA) is required for efficient virus replication, reactivation from latency and immortalization of murine fetal liver-derived B cells. To gain insights into mLANA function(s), knowing that KSHV LANA binds DNA and can modulate transcription of a variety of promoters, we sought out and identified a mLANA-responsive promoter which maps to the terminal repeat (TR) of MHV68. Notably, mLANA strongly repressed activity from this promoter. We extended these analyses to demonstrate direct, sequence-specific binding of recombinant mLANA to TR DNA by DNase I footprinting. To assess whether the DNA-binding and/or transcription modulating function is important in the known mLANA phenotypes, we generated an unbiased library of mLANA point mutants using error-prone PCR, and screened a large panel of mutants for repression of the mLANA-responsive promoter to identify loss of function mutants. Notably, among the mutant mLANA proteins recovered, many of the mutations are in a predicted EBNA-1-like DNA-binding domain. Consistent with this prediction, those tested displayed loss of DNA binding activity. We engineered six of these mLANA mutants into the MHV68 genome and tested the resulting mutant viruses for: (i) replication fitness; (ii) efficiency of latency establishment; and (iii) reactivation from latency. Interestingly, each of these mLANA-mutant viruses exhibited phenotypes similar to the mLANA-null mutant virus, indicating that DNA-binding is critical for mLANA function.

The human gammaherpesviruses Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) are tightly associated with a number of different cancers. Unfortunately, due to their very narrow host tropism, characterizing the pathogenesis of these viruses has been difficult. Infection of laboratory mice with the rodent gammaherpesvirus, murine gammaherpesvirus 68 (MHV68), has proven to be an excellent approach for understanding how these viruses cause disease. One of the MHV68 encoded proteins, which is also found in KSHV, is called LANA and in the case of KSHV-associated diseases LANA expression is consistently detected in infected cells. Here we show that the MHV68 LANA shares a key function with the KSHV homolog—namely, modulating gene expression. Using a random mutagenesis protocol, we identified mLANA mutants that had lost transcriptional regulatory activity. We engineered these mutations back into the virus, used the viruses to infect mice, and find that this function is critical to LANA function in vivo and in vitro. This method, combined with the knowledge gained here, sets the stage for future studies to identify mutant forms of LANA that could be used to block wild type LANA function or, alternatively, to design drugs that target LANA function.

An alternative Kaposi's sarcoma-associated herpesvirus replication program triggered by host cell apoptosis

Kaposi's sarcoma-associated herpesvirus (KSHV) is linked to several neoplastic diseases: Kaposi's sarcoma, primary effusion lymphoma (PEL), and multicentric Castleman's disease (MCD). KSHV replicates actively, via a controlled gene expression program, but can also remain latent. It had been thought that the transition from latent to lytic replication was controlled exclusively by the replication and transcription activator protein RTA (open reading frame 50 [ORF50] gene product). A dominant-negative (DN) ORF50 mutant, ORF50ΔSTAD, blocks gene expression and replication. We produced a PEL cell line derivative containing both latent KSHV genomes and an inducible ORF50ΔSTAD. We unexpectedly found that induction of apoptosis triggered high-level viral replication, even when DN ORF50ΔSTAD was present, suggesting that apoptosis triggers KSHV replication through a distinct RTA-independent pathway. We verified that apoptosis triggers KSHV replication independent of RTA using ORF50 small interfering RNA (siRNA) and also showed that caspase activity is required to trigger KSHV replication. We showed that when apoptosis triggers KSHV replication, the kinetics of late gene expression is accelerated by 12 to 24 h and that virus produced following apoptosis has reduced infectivity. KSHV therefore appears to replicate via two distinct pathways, a conventional pathway requiring RTA, with slower replication kinetics, producing virus with higher infectivity, and an alternative apoptosis-triggered pathway that does not require RTA, has faster replication kinetics, and produces virus with lower infectivity. The existence of a distinct apoptosis-triggered, accelerated replication pathway may have evolutionary advantages for the virus and clinical significance for the treatment of KSHV-associated neoplasms. It also provides further evidence that KSHV can sense and react to its environment.

A cluster of transcripts encoded by KSHV ORF30-33 gene locus

Kaposi's sarcoma-associated herpesvirus ORF30-33 locus encodes four genes with unknown functions. We performed transcriptional mapping of these genes. Northern-hybridization, 5'- and 3'-rapid amplification of cDNA ends, and DNA sequencing identified four transcripts of 3.7, 3.6, 2.7, and 1.4 kb, none of which has alternative splicing. While all transcripts have the same termination site, their start sites vary. All transcripts are not expressed or only weakly expressed in latent cells but can be chemically induced. The 3.7 and 3.6 kb transcripts contain all four genes and are sensitive to cycloheximide (CH) but resistant to phosphonoacetic acid (PAA), indicating that they are early lytic transcripts. The 2.7 kb transcript contains ORF32 and ORF33 genes while the 1.4 kb transcript contains the ORF33 gene. Both transcripts are sensitive to CH and PAA, indicating that they are late lytic transcripts. Furthermore, we identified four promoters with functional TATA boxes, none of which is directly transactivated by RTA. Examination of the 5' untranslated region of ORF31 failed to identify any functional internal ribosome entry sites. These results define the transcriptional patterns of the ORF30-33 locus, which should help the delineation of its function.

Kaposi's sarcoma-associated herpesvirus latency-associated nuclear antigen induction by hypoxia and hypoxia-inducible factors

Hypoxia and hypoxia-inducible factors (HIFs) play an important role in the Kaposi's sarcoma-associated herpesvirus (KSHV) life cycle. In particular, hypoxia can activate lytic replication of KSHV and specific lytic genes, including the replication and transcription activator (RTA), while KSHV infection in turn can increase the levels and activity of HIFs. In the present study, we show that hypoxia increases the levels of mRNAs encoding KSHV latency-associated nuclear antigen (LANA) in primary effusion lymphoma (PEL) cell lines and also increases the levels of LANA protein. Luciferase reporter assays in Hep3B cells revealed a moderate activation of the LANA promoter region by hypoxia as well as by cotransfection with degradation-resistant HIF-1α or HIF-2α expression plasmids. Computer analysis of a 1.2-kb sequence upstream of the LANA translational start site identified six potential hypoxia-responsive elements (HRE). Sequential deletion studies revealed that much of this activity was mediated by one of these HREs (HRE 4R) oriented in the 3' to 5' direction and located between the constitutive (LTc) and RTA-inducible (LTi) mRNA start sites. Site-directed mutation of this HRE substantially reduced the response to both HIF-1α and HIF-2α in a luciferase reporter assay. Electrophoretic mobility shift assays (EMSA) and chromatin immunoprecipitation (ChIP) assays demonstrated binding of both HIF-1α and HIF-2α to this region. Also, HIF-1α was found to associate with RTA, and HIFs enhanced the activation of LTi by RTA. These results provide evidence that hypoxia and HIFs upregulate both latent and lytic KSHV replication and play a central role in the life cycle of this virus.

Mechanism of glycyrrhizic acid inhibition of Kaposi's sarcoma-associated herpesvirus: disruption of CTCF-cohesin-mediated RNA polymerase II pausing and sister chromatid cohesion

Glycyrrhizic acid (GA), a derivative of licorice, selectively inhibits the growth of lymphocytes latently infected with Kaposi's sarcoma-associated herpesvirus. The mechanism involves the deregulation of the multicistronic latency transcript, including the failure to generate the mature forms of viral mRNA encoding LANA. We show here that GA disrupts an RNA polymerase II (RNAPII) complex that accumulates at the CTCF-cohesin binding site within the first intron of the latency transcript. GA altered the enrichment of the RNAPII pausing complex, along with pausing factors SPT5 and NELF-A, at the intragenic CTCF-cohesin binding sites. GA blocked the interaction of cohesin subunit SMC3 with another cohesin subunit, RAD21, and reduced SPT5 interaction with RNAPII. Covalent coupling of GA to a solid support revealed that GA interacts with several cellular proteins, including SMC3 and SPT5, but not their respective interaction partners RAD21 and RNAPII. GA treatment also inhibited the transcription of some cellular genes, like c-myc, which contain a similar CTCF-cohesin binding site within the first intron. We also found that GA leads to a more general loss of sister chromatid cohesion for cellular chromosomes. These findings suggest that RNAPII pauses at intragenic CTCF-cohesin binding sites and that abrogation of this pausing by GA leads to loss of proper mRNA production and defects in sister chromatid cohesion, a process important for both viral and cellular chromosome stability.

Kaposi's sarcoma-associated herpesvirus-induced angiogenin plays roles in latency via the phospholipase C gamma pathway: blocking angiogenin inhibits latent gene expression and induces the lytic cycle

During de novo infection of human dermal microvascular endothelial cells (HMVEC-d), Kaposi's sarcoma-associated herpesvirus (KSHV) induced the multifunctional angiogenin (ANG) protein, which entered the nuclei and nucleoli of infected cells and stimulated 45S rRNA gene transcription, proliferation, and tube formation, which were inhibited by blocking ANG nuclear translocation with the antibiotic neomycin (S. Sadagopan et al., J. Virol. 83:3342-3364, 2009). ANG was induced by KSHV latency protein LANA-1 (open reading frame 73 [ORF73]). Here we examined the presence and functions of ANG in KSHV-positive (KSHV(+)) primary effusion lymphoma (PEL/BCBL) cells. Significant ANG gene expression and secretion were observed in KSHV(+) (BCBL-1 and BC-3) and KSHV(+) and Epstein-Barr virus-positive (KSHV(+) EBV(+)) (JSC-1) PEL cells and in BJAB-KSHV cells but not in EBV(-) KSHV(-) lymphoma cells (Akata, Loukes, Ramos, and BJAB), EBV(+) lymphoma cells (Akata-EBV and Raji), and cells from an EBV(+) lymphoblastoid cell line, thus suggesting a specific association of ANG in KSHV biology. Inhibition of nuclear translocation of ANG resulted in reduced BCBL-1 and TIVE-LTC (latently infected endothelial) cell survival and proliferation, while EBV(-) and EBV(+) Akata cells were unaffected. Blocking nuclear transport of ANG inhibited latent ORF73 gene expression and increased lytic switch ORF50 gene expression, both during de novo infection and in latently infected cells. A greater quantity of infectious KSHV was detected in the supernatants of neomycin-treated BCBL-1 cells than 12-O-tetradecanoylphorbol-13-acetate (TPA)-treated cells. Neomycin treatment and ANG silencing inhibited phospholipase Cγ (PLC-γ) and AKT phosphorylation, and in contrast, ANG induced ORF73 expression and PLC-γ and AKT phosphorylation. Further studies provided evidence that blockage of PLC-γ activation by neomycin appears to be mediating the inhibition of latent gene expression, since treatment with the conventional PLC-γ inhibitor U73122 also showed similar results. Silencing of ANG also resulted in reduced cell survival, reduced ORF73 gene expression, and lytic gene activation in BCBL-1 and TIVE-LTC cells and during de novo infection. Taken together, these studies suggest that KSHV has evolved to exploit ANG for its advantage via a so-far-unexplored PLC-γ pathway for maintaining its latency.

NF-κB as a target for oncogenic viruses

NF-κB is a pivotal transcription factor that controls cell survival and proliferation in diverse physiological processes. The activity of NF-κB is tightly controlled through its cytoplasmic sequestration by specific inhibitors, IκBs. Various cellular stimuli induce the activation of an IκB kinase, which phosphorylates IκBs and triggers their proteasomal degradation, causing nuclear translocation of activated NF-κB. Under normal conditions, the activation of NF-κB occurs transiently, thus ensuring rapid but temporary induction of target genes. Deregulated NF-κB activation contributes to the development of various diseases, including cancers and immunological disorders. Accumulated studies demonstrate that the NF-κB signaling pathway is a target of several human oncogenic viruses, including the human T cell leukemia virus type 1, the Kaposi sarcoma-associated herpesvirus, and the Epstein-Bar virus. These viruses encode specific oncoproteins that target different signaling components of the NF-κB pathway, leading to persistent activation of NF-κB. This chapter will discuss the molecular mechanisms by which NF-κB is activated by the viral oncoproteins.

Why do viruses cause cancer? Highlights of the first century of human tumour virology

The year 2011 marks the centenary of Francis Peyton Rous's landmark experiments on an avian cancer virus. Since then, seven human viruses have been found to cause 10-15% of human cancers worldwide. Viruses have been central to modern cancer research and provide profound insights into both infectious and non-infectious cancer causes. This diverse group of viruses reveals unexpected connections between innate immunity, immune sensors and tumour suppressor signalling that control both viral infection and cancer. This Timeline article describes common features of human tumour viruses and discusses how new technologies can be used to identify infectious causes of cancer.

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.

Piracy of prostaglandin E2/EP receptor-mediated signaling by Kaposi's sarcoma-associated herpes virus (HHV-8) for latency gene expression: strategy of a successful pathogen

Kaposi's sarcoma-associated herpes virus (KSHV) is implicated in the pathogenesis of KS, a chronic inflammation-associated malignancy. Cyclooxygenase-2 (COX-2) and its metabolite prostaglandin E2 (PGE2), two pivotal proinflammatory/oncogeneic molecules, are proposed to play roles in the expression of major KSHV latency-associated nuclear antigen-1 (LANA-1). Microsomal PGE2 synthase, PGE2, and its receptors (EP1, EP2, EP3, and EP4) were detected in KS lesions with the distinct staining of EP2/EP4 in KS lesions. In latently infected endothelial TIVE-LTC cells, EP receptor antagonists downregulated LANA-1 expression as well as Ca(2+), p-Src, p-PI3K, p-PKCzeta/lambda, and p-NF-kappaB, which are also some of the signal molecules proposed to be important in KS pathogenesis. Exogenous PGE2 and EP receptor agonists induced the LANA-1 promoter in 293 cells, and YY1, Sp1, Oct-1, Oct-6, C/EBP, and c-Jun transcription factors seem to be involved in this induction. PGE2/EP receptor-induced LANA-1 promoter activity was downregulated significantly by the inhibition of Ca(2+), p-Src, p-PI3K, p-PKCzeta/lambda, and p-NF-kappaB. These findings implicate the inflammatory PGE2/EP receptors and the associated signal molecules in herpes virus latency and uncover a novel paradigm that shows the evolution of KSHV genome plasticity to use inflammatory response for its survival advantage of maintaining latent gene expression. These data also suggest that potential use of anti-COX-2 and anti-EP receptor therapy may not only ameliorate the chronic inflammation associated with KS but could also lead to elimination of the KSHV latent infection and the associated KS lesions.

KSHV and the pathogenesis of Kaposi sarcoma: listening to human biology and medicine

The linkage of Kaposi sarcoma (KS) to infection by a novel human herpesvirus (Kaposi sarcoma-associated herpesvirus [KSHV]) is one of the great successes of contemporary biomedical research and was achieved by using advanced genomic technologies in a manner informed by a nuanced understanding of epidemiology and clinical investigation. Ongoing efforts to understand the molecular mechanisms by which KSHV infection predisposes to KS continue to be powerfully influenced by insights emanating from the clinic. Here, recent developments in KS pathogenesis are reviewed, with particular emphasis on clinical, pathologic, and molecular observations that highlight the many differences between this process and tumorigenesis by other oncogenic viruses.

Array-based transcript profiling and limiting-dilution reverse transcription-PCR analysis identify additional latent genes in Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is a B-lymphotropic herpesvirus strongly linked to both lymphoproliferative diseases and Kaposi's sarcoma. The viral latency program of KSHV is central to persistent infection and plays important roles in the pathogenesis of KSHV-related tumors. Up to six polypeptides and 18 microRNAs are known to be expressed in latency, but it is unclear if all major latency genes have been identified. Here, we have employed array-based transcript profiling and limiting-dilution reverse transcription-PCR (RT-PCR) methodologies to explore this issue in several KSHV-infected cell lines. Our results show that RNAs encoding the K1 protein are found at low levels in most latently infected cell lines. The gene encoding v-IL-6 is also expressed as a latent transcript in some contexts. Both genes encode powerful signaling molecules with particular relevance to B cell biology: K1 mimics signaling through the B cell receptor, and v-IL-6 promotes B cell survival. These data resolve earlier controversies about K1 and v-IL-6 expression and indicate that, in addition to core latency genes, some transcripts can be expressed in KSHV latency in a context-dependent manner.

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

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

Involvement of SSRP1 in latent replication of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (also named human herpesvirus 8) is a gamma-herpesvirus that undergoes both lytic and latent infection. During latent infection, two viral elements are required: latency-associated nuclear antigen (LANA), which functions as an origin binding protein, and the latent origin, which resides within the terminal repeats (TRs) of the viral genome. Previously, we identified two cis-elements within the TRs which are required for latent DNA replication: two LANA binding sites (LBS1 and LBS2 [LBS1/2]) and a GC-rich replication element (RE) upstream of LBS1/2. To further characterize the RE, we constructed a 71-bp minimal replicon (MR) and performed a detailed mutational analysis. Our data indicate that the first 8 nucleotides within the RE are critical for replication. Moreover, both the position and the distance between the RE and LBS1/2 can affect origin replication activity, suggesting that the RE may function as a loading pad for cellular proteins involved in replication. Using biotinylated DNA fragments of wild-type or mutant MRs as probes, we identified 30 proteins that preferentially bind to the origin. Among these proteins, structure-specific recognition protein 1 (SSRP1), a subunit of the FACT complex, and telomeric repeat binding factor 2 (TRF2) formed complexes with LANA at the MR region. Furthermore, the small interfering RNA-based knockdown of SSRP1, but not the dominant-negative-based knockdown of TRF2, significantly decreased the efficiency of LANA-dependent DNA replication. These results indicate that SSRP1 is a novel cellular protein involved in LANA-dependent DNA replication.

Kaposi sarcoma-associated herpes virus (KSHV) G protein-coupled receptor (vGPCR) activates the ORF50 lytic switch promoter: a potential positive feedback loop for sustained ORF50 gene expression

KSHV vGPCR, a lytic cycle associated protein, induces several signaling pathways leading to the activation of various transcription factors and consequently the expression of cellular and viral genes. Though the role of vGPCR in KSHV tumorigenicity has been well studied, its function related to the viral life cycle is poorly understood. Reduction in vGPCR by RNA interference also resulted in the reduction in KSHV lytic switch ORF50 gene and protein expression. Induction of vGPCR by doxycycline in BC3.14 cells also resulted in more KSHV production. When this was explored, induction of the ORF50 promoter by vGPCR expression was observed. Further examination of the molecular mechanisms by which vGPCR regulates the ORF50 promoter, using various ORF50 promoter constructs, revealed that induction of ORF50 promoter by vGPCR did not involve AP1 but was dependent on Sp1 and Sp3 transcription factors. vGPCR signaling led to an increase in Sp1 and Sp3 DNA binding activity and a decrease in histone deacetylase (HDAC) activity. These activities were pertussis toxin independent, did not involve Rho and Rac-GTPases and involved the heterotrimeric G protein subunits Galpha12 and Galphaq. Studies using pharmacologic inhibitors and dominant-negative proteins identified phospholipase C, the novel protein kinase C (novel PKC) family and protein kinase D (PKD) as part of the signaling initiated by vGPCR leading to ORF50 promoter activation. Taken together, this study suggests a role for vGPCR in the sustained expression of ORF50 which could lead to a continued activation of lytic cycle genes and ultimately to successful viral progeny formation.

Disruption of LANA in rhesus rhadinovirus generates a highly lytic recombinant virus

Rhesus monkey rhadinovirus (RRV) is a gammaherpesvirus that is closely related to human Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8). RRV is the closest relative to KSHV that has a fully sequenced genome and serves as an in vitro and an in vivo model system for KSHV. The latency-associated nuclear antigen (LANA) protein of both KSHV and RRV plays key roles in the establishment and maintenance of these herpesviruses. We have constructed a RRV recombinant virus (RRVDeltaLANA/GFP) in which the RRV LANA open reading frame has been disrupted with a green fluorescent protein (GFP) expression cassette generated by homologous recombination. The integrity of the recombinant virus was confirmed by diagnostic PCR, restriction digestion, Southern blot analysis, and whole-genome sequencing. We compared the single-step and multistep replication kinetics of RRVDeltaLANA/GFP, RRV-GFP, wild-type (WT) RRV H26-95, and a revertant virus using traditional plaque assays, as well as real-time quantitative PCR-based genome quantification assays. The RRVDeltaLANA/GFP recombinant virus exhibited significantly higher lytic replicative properties compared to RRV-GFP, WT RRV, or the revertant virus. This was observed upon de novo infection and in the absence of chemical inducers such as phorbol esters. In addition, by using a quantitative real-time PCR-based viral array, we are the first to report differences in global viral gene expression between WT and recombinant viruses. The RRVDeltaLANA/GFP virus displayed increased lytic gene transcription at all time points postinfection compared to RRV-GFP. Moreover, we also examined several cellular genes that are known to be repressed by KSHV LANA and report that these genes are derepressed during de novo lytic infection with the RRVDeltaLANA/GFP virus compared to RRV-GFP. Finally, we also demonstrate that the RRVDeltaLANA/GFP virus fails to establish latency in B cells, as measured by the loss of GFP-positive cells and intracellular viral genomes.

Cell cycle control of Kaposi's sarcoma-associated herpesvirus latency transcription by CTCF-cohesin interactions

Kaposi's sarcoma-associated herpesvirus (KSHV) latency is characterized by the highly regulated transcription of a few viral genes essential for genome maintenance and host cell survival. A major latency control region has been identified upstream of the divergent promoters for the multicistronic transcripts encoding LANA (ORF73), vCyclin (ORF72), and vFLIP (ORF71) and for the complementary strand transcript encoding K14 and vGPCR (ORF74). Previous studies have shown that this major latency control region is occupied by the cellular chromatin boundary factor CTCF and chromosome structural maintenance proteins SMC1, SMC3, and RAD21, which comprise the cohesin complex. Deletion of the CTCF-cohesin binding site caused an inhibition of cell growth and viral genome instability. We now show that the KSHV genes regulated by CTCF-cohesin are under cell cycle control and that mutation of the CTCF binding sites abolished cell cycle-regulated transcription. Cohesin subunits assembled at the CTCF binding sites and bound CTCF proteins in a cell cycle-dependent manner. Subcellular distribution of CTCF and colocalization with cohesins also varied across the cell cycle. Ectopic expression of Rad21 repressed CTCF-regulated transcription of KSHV lytic genes, and a Rad21-CTCF chimeric protein converted CTCF into an efficient transcriptional repressor of KSHV genes normally activated in the G(2) phase. We conclude that cohesins interact with CTCF in mid-S phase and repress CTCF-regulated genes in a cell cycle-dependent manner. We propose that the CTCF-cohesin complex plays a critical role in regulating the cell cycle control of viral gene expression during latency and that failure to maintain cell cycle control of latent transcripts inhibits host cell proliferation and survival.

Genetic disruption of KSHV major latent nuclear antigen LANA enhances viral lytic transcriptional program

Following primary infection, KSHV establishes a lifelong persistent latent infection in the host. The mechanism of KSHV latency is not fully understood. The latent nuclear antigen (LANA or LNA) encoded by ORF73 is one of a few viral genes expressed during KSHV latency, and is consistently detected in all KSHV-related malignancies. LANA is essential for KSHV episome persistence, and regulates the expression of viral lytic genes through epigenetic silencing, and inhibition of the expression and transactivation function of the key KSHV lytic replication initiator RTA (ORF50). In this study, we used a genetic approach to examine the role of LANA in regulating KSHV lytic replication program. Deletion of LANA did not affect the expression of its adjacent genes vCyclin (ORF72) and vFLIP (ORF71). In contrast, the expression levels of viral lytic genes including immediate-early gene RTA, early genes MTA (ORF57), vIL-6 (ORF-K2) and ORF59, and late gene ORF-K8.1 were increased before and after viral lytic induction with 12-O-tetradecanoyl-phorbol-13-acetate and sodium butyrate. This enhanced expression of viral lytic genes was also observed following overexpression of RTA with or without simultaneous chemical induction. Consistent with these results, the LANA mutant cells produced more infectious virions than the wild-type virus cells did. Furthermore, genetic repair of the mutant virus reverted the phenotypes to those of wild-type virus. Together, these results have demonstrated that, in the context of viral genome, LANA contributes to KSHV latency by regulating the expression of RTA and its downstream genes.

Human herpesvirus-8 (HHV-8) sero-detection and HIV association in Kaposi's sarcoma (KS), non-KS tumors and non-neoplastic conditions

Background

The association of the human herpesvirus-8/Kaposi's sarcoma (KS)-associated herpesvirus (HHV-8/KSHV) serology with various malignancies in Tanzania is not currently well established while previous studies were based on either PCR or immunofluorescence assays [IFA] but not with a sensitive enzyme-linked immunosorbent assay (ELISA). Selected archival diagnostic biopsies (n = 184) and sera from indigenous patients with KS (n = 120), non-KS tumors (n = 24) and non-neoplastic lesions (n = 40) at Muhimbili National Hospital (MNH), Tanzania, were evaluated by diagnostic histopathology, immunohistology [anti-HHV-8 latency-associated nuclear antigen (LANA)] and serology for HIV (ELISA) and HHV-8 (IFA and ELISA).

Results

About 66.3% (n = 122) cases including AIDS-associated Kaposi's sarcoma (AKS) (n = 93), reactive conditions (n = 28) and only one non-KS tumour were HIV positive. Endemic KS (EKS) patients were mostly males (96.3%, 26/27) who were less (69.9%, 65/93) predominant in AIDS-associated (AKS). A high (89%) percentage of patients with anti-HHV-8 antibodies was found in the cohort including the HIV positive (92%) cases, males (81.2%), KS patients (93%), non-KS tumors (92%), and reactive conditions (75%). All HHV-8 seronegative KS cases were nodular stage whereas both sera and corresponding biopsies from early stage KS were HHV-8+. Assay sensitivity, positive predictive value (PPV) and specificity were 98.6%, 93.5% and 16.7% for IFA and 93.5%, 98.6% and 50.0% for ELISA respectively.

Conclusion

HHV-8 seroprevalence at MNH appears high as expected among AKS cases and males but also in non-KS patients. ELISA showed a combination of high HHV-8 sensitivity as well as higher PPV and specificity than IFA which however, showed higher sensitivity. The apparent stage-dependent, inverted serum HHV-8 immunoreactivity supports a notion of viral immune-segregation during KS development. Routine HHV-8 screening should be considered particularly in patients at risk of KS and for selection of blood/organ donations.

Cohesins localize with CTCF at the KSHV latency control region and at cellular c-myc and H19/Igf2 insulators

Cohesins, which mediate sister chromatin cohesion, and CTCF, which functions at chromatin boundaries, play key roles in the structural and functional organization of chromosomes. We examined the binding of these two factors on the Kaposi's sarcoma-associated herpesvirus (KSHV) episome during latent infection and found a striking colocalization within the control region of the major latency transcript responsible for expressing LANA (ORF73), vCyclin (ORF72), vFLIP (ORF71), and vmiRNAs. Deletion of the CTCF-binding site from the viral genome disrupted cohesin binding, and crippled colony formation in 293 cells. Clonal instability correlated with elevated expression of lytic cycle gene products, notably the neighbouring promoter for K14 and vGPCR (ORF74). siRNA depletion of RAD21 from latently infected cells caused an increase in K14 and ORF74, and lytic inducers caused a rapid dissociation of RAD21 from the viral genome. RAD21 and SMC1 also associate with the cellular CTCF sites at mammalian c-myc promoter and H19/Igf2 imprinting control region. We conclude that cohesin subunits associate with viral and cellular CTCF sites involved in complex gene regulation and chromatin organization.

Interaction of Adenovirus E1A with the HHV8 Promoter of Latent Genes: E1A Proteins are Able to Activate the HHV-8 LANAp in MV3 Reporter Cells

Human herpesvirus 8 (HHV-8) is associated with Kaposi's sarcoma, body cavity-based lymphoma, and Castleman's disease. Adenoviral (Ad) E1A proteins regulate the activity of cellular and viral promoters/enhancers and transcription factors and can suppress tumorigenicity of human cancers. As (i) HHV-8 and Ad may co-exist in immunocompromised patients and (ii) E1A might be considered as therapeutic transgene for HHV-8-associated neoplasms we investigated whether the promoter of the latency-associated nuclear antigen (LANAp) controlling expression of vCyclin, vFLIP, and LANA proteins required for latent type infection is regulated by E1A. Transfection experiments in MV3 melanoma cells revealed activation of the LANAp by Ad5 E1A constructs containing an intact N terminus (aa 1-119). In particular, an Ad12 E1A mutant, Spm2, lacking six consecutive alanine residues in the "spacer" region activated the HHV-8 promoter about 15-fold compared to vector controls. In summary, we report the activation of the LANAp by E1A as a novel interaction of E1A with a viral promoter. These data may have relevance for the management of viral infections in immunocompromised patients. A role for E1A as a therapeutic in this context remains to be defined.

KSHV infection and the pathogenesis of Kaposi's sarcoma

Kaposi's sarcoma (KS) has long been suspected of having an infectious etiology on the basis of its unusual epidemiology, histopathology, and natural history. Nearly a decade ago, a novel herpesviral genome was discovered in KS biopsies, and since that time strong epidemiologic evidence has accumulated correlating infection with this KS-associated herpesvirus (KSHV, also known as human herpesvirus 8) with the development of the disease. Here we review the evidence linking KSHV infection to KS risk and discuss current notions of how KSHV gene expression promotes the development of this remarkable neoplasm. These studies show that both latent and lytic viral replicative cycles contribute significantly-but differently-to KS development. The studies also highlight mechanistic differences between oncogenesis caused by KSHV and that caused by its distant relative Epstein-Barr virus.

Kaposi's sarcoma-associated herpesvirus induces sustained levels of vascular endothelial growth factors A and C early during in vitro infection of human microvascular dermal endothelial cells: biological implications

Kaposi's sarcoma (KS), a vascular tumor associated with human immunodeficiency virus type 1 infection, is characterized by spindle-shaped endothelial cells, inflammatory cells, cytokines, growth and angiogenic factors, and angiogenesis. KS spindle cells are believed to be of the lymphatic endothelial cell (LEC) type. Kaposi's sarcoma-associated herpesvirus (KSHV, or human herpesvirus 8) is etiologically linked to KS, and in vitro KSHV infection of primary human dermal microvascular endothelial cells (HMVEC-d) is characterized by the induction of preexisting host signal cascades, sustained expression of latency-associated genes, transient expression of a limited number of lytic genes, sustained induction of NF-kappaB and several cytokines, and growth and angiogenic factors. KSHV induced robust vascular endothelial growth factor A (VEGF-A) and VEGF-C gene expression as early as 30 min postinfection (p.i.) in serum-starved HMVEC-d, which was sustained throughout the observation period of 72 h p.i. Significant amounts of VEGF-A and -C were also detected in the culture supernatant of infected cells. VEGF-A and -C were also induced by UV-inactivated KSHV and envelope glycoprotein gpK8.1A, thus suggesting a role for virus entry stages in the early induction of VEGF and requirement of KSHV viral gene expression for sustained induction. Exogenous addition of VEGF-A and -C increased KSHV DNA entry into target cells and moderately increased latent ORF73 and lytic ORF50 promoter activation and gene expression. KSHV infection also induced the expression of lymphatic markers Prox-1 and podoplanin as early as 8 h p.i., and a paracrine effect was seen in the neighboring uninfected cells. Similar observations were also made in the pure blood endothelial cell (BEC)-TIME cells, thus suggesting that commitment to the LEC phenotype is induced early during KSHV infection of blood endothelial cells. Treatment with VEGF-C alone also induced Prox-1 expression in the BEC-TIME cells. Collectively, these studies show that the in vitro microenvironments of KSHV-infected endothelial cells are enriched, with VEGF-A and -C molecules playing key roles in KSHV biology, such as increased infection and gene expression, as well as in angiogenesis and lymphangiogenesis, thus recapitulating the microenvironment of early KS lesions.