Latency-associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus up-regulates transcription of human telomerase reverse transcriptase promoter through interaction with transcription factor Sp1

Kaposi's sarcoma-associated herpesvirus-encoded LANA contributes to viral latent replication by activating phosphorylation of survivin

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human gammaherpesvirus casually linked to Kaposi's sarcoma (KS), multicentric Castleman's disease (MCD), and primary effusion lymphoma (PEL). Previously, we showed that LANA encoded by KSHV upregulates expression of survivin, a member of the inhibitor of apoptosis (IAP) family. This leads to an increase in the rate of cell proliferation of KSHV-infected B cells. LANA is required for tethering of the KSHV episome to the host chromosomes and efficiently segregates the viral genomes into dividing tumor cells. Here we show that LANA interacts with Aurora kinase B (AK-B) and induces phosphorylation of survivin at residue T34. Phosphorylation of survivin specifically on residue T34 enhances the activity of p300 and inhibits the activity of histone deacetylase 1 (HDAC-1), which then leads to an increase in acetylation of histone H3 on the viral genome. Phosphorylation of survivin specifically on residue T34 upregulates the activities of histone acetyltransferases and deacetylases, which then leads to an increase in viral copy number in KSHV-infected B cells. This results in a boost of KSHV replication in latently infected B-lymphoma cells. The studies showed that LANA can also function to regulate viral replication prior to mitosis of the latently infected cells, suggesting that LANA possesses a novel role in regulating KSHV replication in infected B cells.

IMPORTANCE

This work represents a report of KSHV latent protein LANA and its interactions with AK-B leading to induction of phosphorylation of the oncoprotein survivin at residue T34. Phosphorylation of survivin specifically on residue T34 upregulates the activities of histone acetyltransferases and deacetylases. This leads to an increase in viral copy number in KSHV-infected B cells. These studies support a role for LANA in regulating KSHV replication through posttranslation modification in KSHV-infected B cells.

Tumor viruses and replicative immortality--avoiding the telomere hurdle

Tumor viruses promote cell proliferation in order to gain access to an environment suitable for persistence and replication. The expression of viral products that promote growth transformation is often accompanied by the induction of multiple signs of telomere dysfunction, including telomere shortening, damage of telomeric DNA and chromosome instability. Long-term survival and progression to full malignancy require the bypassing of senescence programs that are triggered by the damaged telomeres. Here we review different strategies by which tumor viruses interfere with telomere homeostasis during cell transformation. This frequently involves the activation of telomerase, which assures both the integrity and functionality of telomeres. In addition, recent evidence suggests that oncogenic viruses may activate a recombination-based mechanism for telomere elongation known as Alternative Lengthening of Telomeres (ALT). This error-prone strategy promotes genomic instability and could play an important role in viral oncogenesis.

A structural basis for BRD2/4-mediated host chromatin interaction and oligomer assembly of Kaposi sarcoma-associated herpesvirus and murine gammaherpesvirus LANA proteins

Kaposi sarcoma-associated herpesvirus (KSHV) establishes a lifelong latent infection and causes several malignancies in humans. Murine herpesvirus 68 (MHV-68) is a related γ₂-herpesvirus frequently used as a model to study the biology of γ-herpesviruses in vivo. The KSHV latency-associated nuclear antigen (kLANA) and the MHV68 mLANA (orf73) protein are required for latent viral replication and persistence. Latent episomal KSHV genomes and kLANA form nuclear microdomains, termed ‘LANA speckles’, which also contain cellular chromatin proteins, including BRD2 and BRD4, members of the BRD/BET family of chromatin modulators. We solved the X-ray crystal structure of the C-terminal DNA binding domains (CTD) of kLANA and MHV-68 mLANA. While these structures share the overall fold with the EBNA1 protein of Epstein-Barr virus, they differ substantially in their surface characteristics. Opposite to the DNA binding site, both kLANA and mLANA CTD contain a characteristic lysine-rich positively charged surface patch, which appears to be a unique feature of γ₂-herpesviral LANA proteins. Importantly, kLANA and mLANA CTD dimers undergo higher order oligomerization. Using NMR spectroscopy we identified a specific binding site for the ET domains of BRD2/4 on kLANA. Functional studies employing multiple kLANA mutants indicate that the oligomerization of native kLANA CTD dimers, the characteristic basic patch and the ET binding site on the kLANA surface are required for the formation of kLANA ‘nuclear speckles’ and latent replication. Similarly, the basic patch on mLANA contributes to the establishment of MHV-68 latency in spleen cells in vivo. In summary, our data provide a structural basis for the formation of higher order LANA oligomers, which is required for nuclear speckle formation, latent replication and viral persistence.

Kaposi sarcoma-associated herpesvirus (KSHV) causes Kaposi Sarcoma, Primary Effusion lymphoma and the plasma cell variant of Multicentric Castleman's Disease. Its oncogenic effect is linked to its ability to persist in a latent form for the life time of infected individuals. During latency viral genomes are replicated and passed to daughter cells in synchrony with the infected cell without the formation of new virions. A key viral protein in this process is the latency-associated nuclear antigen, LANA. In latently infected cells, viral genomes and LANA form characteristic nuclear microdomains, termed ‘LANA speckles’, which also contain cellular chromatin components. We have solved the crystal structure of the c-terminal, DNA-binding, domain (CTD) of KSHV LANA (kLANA) and its homologue mLANA of a related murine γ₂-herpesvirus, which is frequently used as a model to study latent persistence in vivo. We also identified the binding site for two chromatin proteins, BRD2/4, by NMR spectroscopy. We demonstrate the functional importance of these structural features, and their contribution to latent replication and ‘LANA speckle’ formation, in cell culture and in vivo experiments. Our results provide a structural basis for the assembly of LANA-containing nuclear structures that are required for latent viral replication and persistence.

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.

Comprehensive analysis of LANA interacting proteins essential for viral genome tethering and persistence

Kaposi’s sarcoma associated herpesvirus is tightly linked to multiple human malignancies including Kaposi’s sarcoma (KS), Primary Effusion Lymphoma (PEL) and Multicentric Castleman’s Disease (MCD). KSHV like other herpesviruses establishes life-long latency in the infected host by persisting as chromatin and tethering to host chromatin through the virally encoded protein Latency Associated Nuclear Antigen (LANA). LANA, a multifunctional protein, is capable of binding to a large number of cellular proteins responsible for transcriptional regulation of various cellular and viral pathways involved in blocking cell death and promoting cell proliferation. This leads to enhanced cell division and replication of the viral genome, which segregates faithfully in the dividing tumor cells. The mechanism of genome segregation is well known and the binding of LANA to nucleosomal proteins, throughout the cell cycle, suggests that these interactions play an important role in efficient segregation. Various biochemical methods have identified a large number of LANA binding proteins, including histone H2A/H2B, histone H1, MeCP2, DEK, CENP-F, NuMA, Bub1, HP-1, and Brd4. These nucleosomal proteins may have various functions in tethering of the viral genome during specific phases of the viral life cycle. Therefore, we performed a comprehensive analysis of their interaction with LANA using a number of different assays. We show that LANA binds to core nucleosomal histones and also associates with other host chromatin proteins including histone H1 and high mobility group proteins (HMGs). We used various biochemical assays including co-immunoprecipitation and in-vivo localization by split GFP and fluorescence resonance energy transfer (FRET) to demonstrate their association.

Interplay between DNA tumor viruses and the host DNA damage response

Viruses encounter many challenges within host cells in order to replicate their nucleic acid. In the case of DNA viruses, one challenge that must be overcome is recognition of viral DNA structures by the host DNA damage response (DDR) machinery. This is accomplished in elegant and unique ways by different viruses as each has specific needs and sensitivities dependent on its life cycle. In this review, we focus on three DNA tumor viruses and their interactions with the DDR. The viruses Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), and human papillomavirus (HPV) account for nearly all of the virus-associated human cancers worldwide. These viruses have also been excellent models for the study of oncogenic virus-mediated cell transformation. In this review, we will discuss how each of these viruses engage and subvert aspects of the host DDR. The first level of DDR engagement is a result of the genetic linkage between the oncogenic potential of these viruses and their ability to replicate. Namely, the promotion of cells from quiescence into the cell cycle to facilitate virus replication can be sensed through aberrant cellular DNA replication structures which activate the DDR and hinder cell transformation. DNA tumor viruses subvert this growth-suppressive DDR through changes in viral oncoprotein expression which ultimately facilitate virus replication. An additional level of DDR engagement is through direct detection of replicating viral DNA. These interactions parallel those observed in other DNA virus systems in that the need to subvert these intrinsic sensors of aberrant DNA structure in order to replicate must be in place. DNA tumor viruses are no exception. This review will cover the molecular features of DNA tumor virus interactions with the host DDR and the consequences for virus replication.

Emmprin and KSHV: new partners in viral cancer pathogenesis

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Emmprin regulates pathogenic elements relevant to virus-associated cancer, including drug resistance and cell migration.

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Kaposi's sarcoma-associated herpesvirus (KSHV) regulates emmprin expression and downstream function.

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Targeting emmprin or its interacting proteins at the cell surface suppresses KSHV-induced pathogenesis in vitro.

Emmprin (CD147; basigin) is a multifunctional glycoprotein expressed at higher levels by cancer cells and stromal cells in the tumor microenvironment. Through direct effects within tumor cells and promotion of tumor–stroma interactions, emmprin participates in induction of tumor cell invasiveness, angiogenesis, metastasis and chemoresistance. Although its contribution to cancer progression has been widely studied, the role of emmprin in viral oncogenesis still remains largely unclear, and only a small body of available literature implicates emmprin-associated mechanisms in viral pathogenesis and tumorigenesis. We summarize these data in this review, focusing on the role of emmprin in pathogenesis associated with the Kaposi sarcoma-associated herpesvirus (KSHV), a common etiology for cancers arising in the setting of immune suppression. We also discuss future directions for mechanistic studies exploring roles for emmprin in viral cancer pathogenesis.

Viral genome maintenance and latent replication of human gammaherpesviruses

During gammaherpesvirus latency, only a few genes are expressed and required for maintenance of viral latency over a long period. While the expressed latent viral proteins play functional roles in viral latent DNA replication, they do not have replication-associated enzymatic activity such as polymerase or helicase activity. Viral genomes are detected in a similar copy number per infected cell, suggesting that the viral genome is replicated and segregated using host replication machinery. Kaposi’s sarcoma-associated herpesvirus and EBV have trans-acting elements required for viral genome maintenance during latency; LANA1 and EBNA1, respectively. The proteins recruit host replication-associated proteins at their latent origins, leading to initiation of viral replication and segregation with host chromosomes once per cell cycle. In addition, viral latent origins (cis-elements) provide trans-element-binding sites as well as a sufficient space for recruitment of cellular factors. In this review, we describe the molecular mechanisms required for replication of the viral genome during latency, including interactions with cellular factors and the interplay between viral trans- and cis-elements.

Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen regulates the KSHV epigenome by association with the histone demethylase KDM3A

Kaposi's sarcoma-associated herpesvirus (KSHV) latent genomes are tethered to host histones to form a minichromosome also known as an "episome." Histones, which are core components of chromatin, are heavily modified by various histone-targeting enzymes. Posttranslational modifications of histones significantly influence accessibility of transcriptional factors and thus have profound effects on gene expression. Recent studies showed that epigenetic marks on the KSHV episome are well organized, exemplified by the absence of histone H3 lysine 9 (H3K9) methylation, a heterochromatic histone mark, from immediate early and latent gene promoters in naturally infected cells. The present study revealed a mechanistic insight into KSHV epigenome regulation via a complex consisting of LANA and the H3K9me1/2 histone demethylase JMJD1A/KDM3A. This complex was isolated from HeLa cell nuclear extracts stably expressing LANA and was verified by coimmunoprecipitation analyses and with purified proteins. LANA recruitment sites on the KSHV genome inversely correlated with H3K9me2 histone marks in naturally infected cells, and methylation of H3K9 significantly inhibited LANA binding to the histone H3 tail. Chromatin immunoprecipitation coupled with KSHV tiling arrays identified the recruitment sites of the complex, while depletion of LANA expression or overexpression of a KDM3A binding-deficient mutant decreased KDM3A recruitment to the KSHV genome. Finally, ablation of KDM3A expression from latently KSHV-infected cells significantly inhibited KSHV gene expression, leading to decreased KSHV replication during reactivation. Taken together, our results suggest that LANA may play a role in regulation of epigenetic marks on the KSHV genome, which is in part through association with the histone demethylase KDM3A.

Telomeres and viruses: common themes of genome maintenance

Genome maintenance mechanisms actively suppress genetic instability associated with cancer and aging. Some viruses provoke genetic instability by subverting the host's control of genome maintenance. Viruses have their own specialized strategies for genome maintenance, which can mimic and modify host cell processes. Here, we review some of the common features of genome maintenance utilized by viruses and host chromosomes, with a particular focus on terminal repeat (TR) elements. The TRs of cellular chromosomes, better known as telomeres, have well-established roles in cellular chromosome stability. Cellular telomeres are themselves maintained by viral-like mechanisms, including self-propagation by reverse transcription, recombination, and retrotransposition. Viral TR elements, like cellular telomeres, are essential for viral genome stability and propagation. We review the structure and function of viral repeat elements and discuss how they may share telomere-like structures and genome protection functions. We consider how viral infections modulate telomere regulatory factors for viral repurposing and can alter normal host telomere structure and chromosome stability. Understanding the common strategies of viral and cellular genome maintenance may provide new insights into viral-host interactions and the mechanisms driving genetic instability in cancer.

Phosphorylation of the chromatin binding domain of KSHV LANA

The Kaposi sarcoma associated herpesvirus (KSHV) latency associated nuclear antigen (LANA) is expressed in all KSHV associated malignancies and is essential for maintenance of KSHV genomes in infected cells. To identify kinases that are potentially capable of modifying LANA, in vitro phosphorylation assays were performed using an Epstein Barr virus plus LANA protein microarray and 268 human kinases purified in active form from yeast. Interestingly, of the Epstein-Barr virus proteins on the array, the EBNA1 protein had the most similar kinase profile to LANA. We focused on nuclear kinases and on the N-terminus of LANA (amino acids 1–329) that contains the LANA chromatin binding domain. Sixty-three nuclear kinases phosphorylated the LANA N-terminus. Twenty-four nuclear kinases phosphorylated a peptide covering the LANA chromatin binding domain (amino acids 3–21). Alanine mutations of serine 10 and threonine 14 abolish or severely diminish chromatin and histone binding by LANA. However, conversion of these residues to the phosphomimetic glutamic acid restored histone binding suggesting that phosphorylation of serine 10 and threonine 14 may modulate LANA function. Serine 10 and threonine 14 were validated as substrates of casein kinase 1, PIM1, GSK-3 and RSK3 kinases. Short-term treatment of transfected cells with inhibitors of these kinases found that only RSK inhibition reduced LANA interaction with endogenous histone H2B. Extended treatment of PEL cell cultures with RSK inhibitor caused a decrease in LANA protein levels associated with p21 induction and a loss of PEL cell viability. The data indicate that RSK phosphorylation affects both LANA accumulation and function.

The Kaposi sarcoma associated herpesvirus (KSHV) is associated with cancers that have an increased incidence in individuals with compromised immune systems. KSHV expresses a protein, LANA, that is needed to maintain KSHV genomes in infected cells and also promotes the growth of KSHV associated tumors. Kinases regulate protein function through phosphorylation. To identify kinases that may affect LANA function, we performed a screen in which 268 human kinases were isolated and tested for the ability to phosphorylate LANA in vitro. We focused on the region of LANA that contains the chromatin binding domain, a motif essential for tethering KSHV genomes to the cell chromatin and maintaining latent infection. We identified serine 10 and threonine 14 as amino acids within the chromatin binding domain whose phosphorylation was important for histone binding. Serine 10 and threonine 14 were targets of the CK1, PIM1, GSK-3 and RSK3 kinases. Treatment with an inhibitor of RSK kinase reduced LANA binding to histones, decreased LANA protein levels and caused a loss of KSHV infected PEL cell viability. Our experiments show that phosphorylation affects LANA function and suggest that KSHV infected cells may be particularly vulnerable to kinase inhibitors.

The ubiquitin-specific protease USP7 modulates the replication of Kaposi's sarcoma-associated herpesvirus latent episomal DNA

Kaposi's sarcoma herpesvirus (KSHV) belongs to the gamma-2 Herpesviridae and is associated with three neoplastic disorders: Kaposi's sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman's disease (MCD). The viral latency-associated nuclear antigen 1 (LANA) is expressed in all latently KSHV-infected cells and is involved in viral latent replication and maintenance of the viral genome. We show that LANA interacts with the ubiquitin-specific protease USP7 through its N-terminal TRAF (tumor necrosis factor [TNF] receptor-associated factor) domain. This interaction involves a short sequence (amino acids [aa] 971 to 986) within the C-terminal domain of LANA with strong similarities to the USP7 binding site of the Epstein-Barr virus (EBV) EBNA-1 protein. A LANA mutant with a deletion of the identified USP7 binding site showed an enhanced ability to replicate a plasmid containing the KSHV latent origin of replication but was comparable to the wild-type LANA (LANA WT) with regard to the regulation of viral and cellular promoters. Furthermore, the LANA homologues of two other gamma-2 herpesviruses, MHV68 and RRV, also recruit USP7. Our findings suggest that recruitment of USP7 to LANA could play a role in the regulation of viral latent replication. The recruitment of USP7, and its role in herpesvirus latent replication, previously described for the latent EBNA-1 protein of the gamma-1 herpesvirus (lymphocryptovirus) EBV (M. N. Holowaty et al., J. Biol. Chem. 278:29987-29994, 2003), may thereby be a conserved feature among gammaherpesvirus latent origin binding proteins.

Infection, mutation, and cancer evolution

An understanding of oncogenesis can be fostered by an integration of mechanistic studies with evolutionary considerations, which help explain why these mechanisms occur. This integration emphasizes infections and mutations as joint essential causes for many cancers. It suggests that infections may play a broader causal role in oncogenesis than has been generally appreciated. An evolutionary perspective also suggests that oncogenic viruses will tend to be transmitted by routes that provide infrequent opportunities for transmission, such as transmission by sexual and salivary contact. Such routes increase the intensity of natural selection for persistence within hosts, and molecular mechanisms for persistence often compromise critical barriers to oncogenesis, particularly cell cycle arrest, apoptosis, and a cap on the total number of divisions that a cell can undergo.

The latency-associated nuclear antigen, a multifunctional protein central to Kaposi's sarcoma-associated herpesvirus latency

Latency-associated nuclear antigen (LANA) is encoded by the Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) open reading frame 73. LANA is expressed during latent KSHV infection of cells, including tumor cells, such as primary effusion lymphoma, KS and multicentric Castleman's disease. Latently infected cells have multiple extrachromosomal copies of covalently closed circular KSHV genomes (episomes) that are stably maintained in proliferating cells. LANA's best characterized function is that of mediating episome persistence. It does so by binding terminal repeat sequences to the chromosomal matrix, thus ensuring episome replication with each cell division and efficient DNA segregation to daughter nuclei after mitosis. To achieve these functions, LANA associates with different host cell proteins, including chromatin-associated proteins and proteins involved in DNA replication. In addition to episome maintenance, LANA has transcriptional regulatory effects and affects cell growth. LANA exerts these functions through interactions with different cell proteins.

Kaposi's sarcoma herpesvirus upregulates Aurora A expression to promote p53 phosphorylation and ubiquitylation

Aberrant expression of Aurora A kinase has been frequently implicated in many cancers and contributes to chromosome instability and phosphorylation-mediated ubiquitylation and degradation of p53 for tumorigenesis. Previous studies showed that p53 is degraded by Kaposi's sarcoma herpesvirus (KSHV) encoded latency-associated nuclear antigen (LANA) through its SOCS-box (suppressor of cytokine signaling, LANA(SOCS)) motif-mediated recruitment of the EC(5)S ubiquitin complex. Here we demonstrate that Aurora A transcriptional expression is upregulated by LANA and markedly elevated in both Kaposi's sarcoma tissue and human primary cells infected with KSHV. Moreover, reintroduction of Aurora A dramatically enhances the binding affinity of p53 with LANA and LANA(SOCS)-mediated ubiquitylation of p53 which requires phosphorylation on Ser215 and Ser315. Small hairpin RNA or a dominant negative mutant of Aurora A kinase efficiently disrupts LANA-induced p53 ubiquitylation and degradation, and leads to induction of p53 transcriptional and apoptotic activities. These studies provide new insights into the mechanisms by which LANA can upregulate expression of a cellular oncogene and simultaneously destabilize the activities of the p53 tumor suppressor in KSHV-associated human cancers.

A protein array screen for Kaposi's sarcoma-associated herpesvirus LANA interactors links LANA to TIP60, PP2A activity, and telomere shortening

The Kaposi's sarcoma-associated herpesvirus (KSHV) LANA protein functions in latently infected cells as an essential participant in KSHV genome replication and as a driver of dysregulated cell growth. To identify novel LANA protein-cell protein interactions that could contribute to these activities, we performed a proteomic screen in which purified, adenovirus-expressed Flag-LANA protein was incubated with an array displaying 4,192 nonredundant human proteins. Sixty-one interacting cell proteins were consistently detected. LANA interactions with high-mobility group AT-hook 1 (HMGA1), HMGB1, telomeric repeat binding factor 1 (TRF1), xeroderma pigmentosum complementation group A (XPA), pygopus homolog 2 (PYGO2), protein phosphatase 2A (PP2A)B subunit, Tat-interactive protein 60 (TIP60), replication protein A1 (RPA1), and RPA2 proteins were confirmed in coimmunoprecipitation assays. LANA-associated TIP60 retained acetyltransferase activity and, unlike human papillomavirus E6 and HIV-1 TAT proteins, LANA did not reduce TIP60 stability. The LANA-bound PP2A B subunit was associated with the PP2A A subunit but not the catalytic C subunit, suggesting a disruption of PP2A phosphatase activity. This is reminiscent of the role of simian virus 40 (SV40) small t antigen. Chromatin immunoprecipitation (ChIP) assays showed binding of RPA1 and RPA2 to the KSHV terminal repeats. Interestingly, LANA expression ablated RPA1 and RPA2 binding to the cell telomeric repeats. In U2OS cells that rely on the alternative mechanism for telomere maintenance, LANA expression had minimal effect on telomere length. However, LANA expression in telomerase immortalized endothelial cells resulted in telomere shortening. In KSHV-infected cells, telomere shortening may be one more mechanism by which LANA contributes to the development of malignancy.

Carboxyl-terminal amino acids 1052 to 1082 of the latency-associated nuclear antigen (LANA) interact with RBP-Jκ and are responsible for LANA-mediated RTA repression

Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8, is closely associated with several malignancies, including Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. KSHV can establish lifelong latency in the host, but the mechanism is not fully understood. Previous studies have proposed a feedback model in which the viral replication and transcription activator (RTA) can induce the expression of the latency-associated nuclear antigen (LANA) during early infection. LANA, in turn, represses transcription and RTA function to establish and maintain KSHV latency. The interaction between LANA and the recombination signal sequence binding protein Jκ (RBP-Jκ, also called CSL), a major transcriptional repressor of the Notch signaling pathway, is essential for RTA repression. In the present study, we show that the LANA carboxyl-terminal amino acids 1052 to 1082 are responsible for the LANA interaction with RBP-Jκ. The secondary structure of the LANA carboxyl terminus resembles the RBP-Jκ-associated module (RAM) of Notch receptor. Furthermore, deletion of the region of LANA residues 1052 to 1082 resulted in aberrant expression of RTA, leading to elevated viral lytic replication. For the first time, we dissected a conserved RBP-Jκ binding domain in LANA and demonstrated that this domain was indispensable for LANA-mediated repression of KSHV lytic genes, thus helping the virus maintain latency and control viral reactivation.

Post-Translational Modifications of Kaposi's Sarcoma-Associated Herpesvirus Regulatory Proteins - SUMO and KSHV

KSHV latency can be envisioned as an outcome that is balanced between factors that promote viral gene expression and lytic replication against those that facilitate gene silencing and establish or maintain latency. A large body of work has focused on the activities of the key viral regulatory proteins involved in KSHV latent or lytic states. Moreover, recent studies have also begun to document the importance of epigenetic landscape evolution of the KSHV viral genome during latency and reactivation. However, one area of KSHV molecular virology that remains largely unanswered is the precise role of post-translational modifications on the activities of viral factors that function during latency and reactivation. In this review, we will summarize the post-translational modifications associated with three viral factors whose activities contribute to the viral state. The viral proteins discussed are the two major KSHV encoded transcription factors, K-Rta (KSHV replication and transcriptional activator) and K-bZIP (KSHV basic leucine zipper) and the viral latency-associated nuclear antigen (LANA). A special emphasis will be placed on the role of the sumoylation pathway in the modulation of the KSHV lifecycle. Newly uncovered small ubiquitin-like modifier (SUMO)-associated properties of LANA and K-Rta will also be presented, namely LANA histone targeting SUMO E3 ligase activity and K-Rta SUMO-targeted ubiquitin ligase function.

Kaposi sarcoma-associated herpesvirus (KSHV) induces a functional tumor-associated phenotype for oral fibroblasts

The Kaposi sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi sarcoma (KS), the most common HIV/AIDS-associated tumor worldwide. Involvement of the oral cavity portends a poor prognosis for patients with KS, but mechanisms for KSHV regulation of the oral tumor microenvironment are largely unknown. Infiltrating fibroblasts are found with KS lesions, and KSHV establishes latent infection within human primary fibroblasts in vitro, but contributions for KSHV-infected fibroblasts to the KS microenvironment have not been previously characterized. Secretion of pro-migratory factors and intratumoral invasion are characteristics of tumor-associated fibroblasts (TAF) found in the microenvironment of non-viral malignancies. In the present study, we show that latent KSHV infection of primary human fibroblasts isolated from the oral cavity enhances their secretion of KS-promoting cytokines and intrinsic invasiveness through VEGF-dependent mechanisms. Moreover, we find that KSHV induces these effects through Sp1- and Egr2-dependent transcriptional activation of the Extracellular Matrix MetalloPRoteinase INducer (emmprin). These data implicate KSHV activation of emmprin in the induction of a "TAF-like" phenotype for oral fibroblasts in the KS microenvironment and support the potential utility of targeting TAFs and/or emmprin in the treatment of oral KS.

Protein arginine methyltransferase 1-directed methylation of Kaposi sarcoma-associated herpesvirus latency-associated nuclear antigen

The Kaposi sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA) is a multifunctional protein with roles in gene regulation and maintenance of viral latency. Post-translational modification of LANA is important for functional diversification. Here, we report that LANA is subject to arginine methylation by protein arginine methyltransferase 1 in vitro and in vivo. The major arginine methylation site in LANA was mapped to arginine 20. This site was mutated to either phenylalanine (bulky hydrophobic, constitutive methylated mimetic) or lysine (positively charged, non-arginine methylatable) residues. The significance of the methylation in LANA function was examined in both the isolated form and in the context of the viral genome through the generation of recombinant KSHV. In addition, authentic LANA binding sites on the KSHV episome in naturally infected cells were identified using a whole genome KSHV tiling array. Although mutation of the methylation site resulted in no significant difference in KSHV LANA subcellular localization, we found that the methylation mimetic mutation resulted in augmented histone binding in vitro and increased LANA occupancy at identified LANA target promoters in vivo. Moreover, a cell line carrying the methylation mimetic mutant KSHV showed reduced viral gene expression relative to controls both in latency and in the course of reactivation. These results suggest that residue 20 is important for modulation of a subset of LANA functions and properties of this residue, including the hydrophobic character induced by arginine methylation, may contribute to the observed effects.

Single molecule analysis of replicated DNA reveals the usage of multiple KSHV genome regions for latent replication

Kaposi's sarcoma associated herpesvirus (KSHV), an etiologic agent of Kaposi's sarcoma, Body Cavity Based Lymphoma and Multicentric Castleman's Disease, establishes lifelong latency in infected cells. The KSHV genome tethers to the host chromosome with the help of a latency associated nuclear antigen (LANA). Additionally, LANA supports replication of the latent origins within the terminal repeats by recruiting cellular factors. Our previous studies identified and characterized another latent origin, which supported the replication of plasmids ex-vivo without LANA expression in trans. Therefore identification of an additional origin site prompted us to analyze the entire KSHV genome for replication initiation sites using single molecule analysis of replicated DNA (SMARD). Our results showed that replication of DNA can initiate throughout the KSHV genome and the usage of these regions is not conserved in two different KSHV strains investigated. SMARD also showed that the utilization of multiple replication initiation sites occurs across large regions of the genome rather than a specified sequence. The replication origin of the terminal repeats showed only a slight preference for their usage indicating that LANA dependent origin at the terminal repeats (TR) plays only a limited role in genome duplication. Furthermore, we performed chromatin immunoprecipitation for ORC2 and MCM3, which are part of the pre-replication initiation complex to determine the genomic sites where these proteins accumulate, to provide further characterization of potential replication initiation sites on the KSHV genome. The ChIP data confirmed accumulation of these pre-RC proteins at multiple genomic sites in a cell cycle dependent manner. Our data also show that both the frequency and the sites of replication initiation vary within the two KSHV genomes studied here, suggesting that initiation of replication is likely to be affected by the genomic context rather than the DNA sequences.

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

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

At a crossroads: human DNA tumor viruses and the host DNA damage response

Human DNA tumor viruses induce host cell proliferation in order to establish the necessary cellular milieu to replicate viral DNA. The consequence of such viral-programmed induction of proliferation coupled with the introduction of foreign replicating DNA structures makes these viruses particularly sensitive to the host DNA damage response machinery. In fact, sensors of DNA damage are often activated and modulated by DNA tumor viruses in both latent and lytic infection. This article focuses on the role of the DNA damage response during the life cycle of human DNA tumor viruses, with a particular emphasis on recent advances in our understanding of the role of the DNA damage response in EBV, Kaposi's sarcoma-associated herpesvirus and human papillomavirus infection.

The manipulation of miRNA-gene regulatory networks by KSHV induces endothelial cell motility

miRNAs have emerged as master regulators of cancer-related events. miRNA dysregulation also occurs in Kaposi sarcoma (KS). Exploring the roles of KS-associated miRNAs should help to identify novel angiogenesis and lymphangiogenesis pathways. In the present study, we show that Kaposi sarcoma-associated herpesvirus (KSHV), the etiological agent of KS, induces global miRNA changes in lymphatic endothelial cells (LECs). Specifically, the miR-221/miR-222 cluster is down-regulated, whereas miR-31 is up-regulated. Both latent nuclear antigen (LANA) and Kaposin B repress the expression of the miR-221/miR-222 cluster, which results in an increase of endothelial cell (EC) migration. In contrast, miR-31 stimulates EC migration, so depletion of miR-31 in KSHV-transformed ECs reduces cell motility. Analysis of the putative miRNA targets among KSHV-affected genes showed that ETS2 and ETS1 are the downstream targets of miR-221 and miR-222, respectively. FAT4 is one of the direct targets of miR-31. Overexpression of ETS1 or ETS2 alone is sufficient to induce EC migration, whereas a reduction in FAT4 enhances EC motility. Our results show that KSHV regulates multiple miRNA-mRNA networks to enhance EC motility, which eventually contributes to KS progression by promoting the spread of malignant KS progenitor cells. Targeting KSHV-regulated miRNAs or genes might allow the development of novel therapeutic strategies that induce angiogenesis or allow the treatment of pathogenic (lymph)angiogenesis.

Gammaherpesvirus and lymphoproliferative disorders in immunocompromised patients

Two lymphotropic human gamma herpesviruses can cause lymphoproliferative disorders: Epstein Barr virus (EBV, formally designated as human herpesvirus 4) and Kaposi sarcoma herpesvirus (KSHV, also called human herpesvirus 8). Individuals with inherited or acquired immunodeficiency have a greatly increased risk of developing a malignancy caused by one of these two viruses. Specific types of lymphoproliferations, including malignant lymphomas, occur in individuals with HIV infection, transplant recipients and children with primary immunodeficiency. Some of these diseases, such as Hodgkin's and non-Hodgkin lymphoma resemble those occurring in immunocompetent patients, but the proportion of tumors in which EBV is present is increased. Others, like primary effusion lymphoma and polymorphic post-transplant lymphoproliferative disorder are rarely seen outside the context of a specific immunodeficient state. Understanding the specific viral associations in selected lymphoproliferative disorders, and the insights into the molecular mechanisms of viral oncogenesis, will lead to better treatments for these frequently devastating diseases.

Kaposi's sarcoma-associated herpesvirus-encoded latency-associated nuclear antigen reduces interleukin-8 expression in endothelial cells and impairs neutrophil chemotaxis by degrading nuclear p65

Latency-associated nuclear antigen 1 (LANA-1) of Kaposi's sarcoma-associated herpesvirus (KSHV) is the major viral latent protein and functions as a multifaceted protein. Here, we report that LANA-1 attenuates the endothelial response to tumor necrosis factor alpha (TNF-α) stimulation and inhibits consequent neutrophil chemotaxis. Reporter assays showed that LANA-1 constantly repressed nuclear factor (NF)-κB transactivity upon TNF-α stimulation. We also found that LANA-1 decreased nuclear p65 protein levels through enhancement of polyubiquitylation-mediated p65 degradation and that an elongin B/elongin C-cullin 5-LANA-1-p65 complex assembled by LANA-1 was responsible for this enhanced p65 degradation. In telomerase-immortalized human umbilical vein endothelial cells, LANA-1 was demonstrated to repress interleukin-8 expression, which was involved in neutrophil recruitment to the inflammatory site. Through an in vitro transmigration assay, we determined a suppressive effect of LANA-1 on neutrophil chemotaxis. Our work suggests that KSHV LANA-1 is a negative modulator of acute inflammation and sheds light on a new mechanism by which KSHV during the latent life cycle evades the host innate immune response.

Cooperative roles for emmprin and LYVE-1 in the regulation of chemoresistance for primary effusion lymphoma

The Kaposi's sarcoma-associated herpesvirus is the causative agent of primary effusion lymphoma (PEL), for which cytotoxic chemotherapy represents the standard of care. The high mortality associated with PEL may be explained in part by resistance of these tumors to chemotherapy. The membrane-bound glycoprotein emmprin (CD147) enhances chemoresistance in tumors through effects on transporter expression, trafficking and interactions. Interactions between hyaluronan and hyaluronan receptors on the cell surface also facilitate emmprin-mediated chemoresistance. Whether emmprin or hyaluronan-receptor interactions regulate chemotherapeutic resistance for virus-associated malignancies is unknown. Using human PEL tumor cells, we found that PEL sensitivity to chemotherapy is directly proportional to expression of emmprin, the lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1) and a drug transporter known as the breast cancer resistance protein/ABCG2 (BCRP), and that emmprin, LYVE-1 and BCRP interact with each other and colocalize on the PEL cell surface. In addition, we found that emmprin induces chemoresistance in PEL cells through upregulation of BCRP expression, and RNA interference targeting of emmprin, LYVE-1 or BCRP enhances PEL cell apoptosis induced by chemotherapy. Finally, disruption of hyaluronan-receptor interactions using small hyaluronan oligosaccharides reduces expression of emmprin and BCRP while sensitizing PEL cells to chemotherapy. Collectively, these data support interdependent roles for emmprin, LYVE-1 and BCRP in chemotherapeutic resistance for PEL.

The internal Kaposi's sarcoma-associated herpesvirus LANA regions exert a critical role on episome persistence

Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA) is a 1,162-amino-acid protein that acts on viral terminal repeat (TR) DNA to mediate KSHV episome persistence. The two essential components of episome persistence are DNA replication prior to cell division and episome segregation to daughter nuclei. These functions are located within N- and C-terminal regions of LANA. N- and C-terminal regions of LANA are sufficient for TR DNA replication. In addition, N- and C-terminal regions of LANA tether episomes to mitotic chromosomes to segregate episomes to progeny cell nuclei. To generate a tethering mechanism, N-terminal LANA binds histones H2A/H2B to attach to mitotic chromosomes, and C-terminal LANA binds TR DNA and also associates with mitotic chromosomes. Here, we test the importance of the internal LANA sequence for episome persistence. We generated LANA mutants that contain N- and C-terminal regions of LANA but have most of the internal sequence deleted. As expected, the LANA mutants bound mitotic chromosomes in a wild-type pattern and also bound TR DNA as assayed by electrophoretic mobility shift assays (EMSA). The mutants mediated TR DNA replication, although with reduced efficiency compared with LANA. Despite the ability to replicate DNA and exert the chromosome and DNA binding functions necessary for segregating episomes to daughter nuclei, the mutants were highly deficient for the ability to mediate both short- and long-term episome persistence. These data indicate that internal LANA sequence exerts a critical effect on its ability to maintain episomes, possibly through effects on TR DNA replication.

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.

Kaposi's sarcoma and its associated herpesvirus

Kaposi's sarcoma (KS) is the most common cancer in HIV-infected untreated individuals. Kaposi's sarcoma-associated herpesvirus (KSHV; also known as human herpesvirus 8 (HHV8)) is the infectious cause of this neoplasm. In this Review we describe the epidemiology of KS and KSHV, and the insights into the remarkable mechanisms through which KSHV can induce KS that have been gained in the past 16 years. KSHV latent transcripts, such as latency-associated nuclear antigen (LANA), viral cyclin, viral FLIP and viral-encoded microRNAs, drive cell proliferation and prevent apoptosis, whereas KSHV lytic proteins, such as viral G protein-coupled receptor, K1 and virally encoded cytokines (viral interleukin-6 and viral chemokines) further contribute to the unique angioproliferative and inflammatory KS lesions through a mechanism called paracrine neoplasia.

Kaposi's sarcoma-associated herpesvirus inhibits interleukin-4-mediated STAT6 phosphorylation to regulate apoptosis and maintain latency

Cytokine-mediated JAK/STAT signaling controls numerous important biologic responses like immune function, cellular growth, and differentiation. Inappropriate activation of this signaling pathway is associated with a range of malignancies. Kaposi's sarcoma-associated herpesvirus (KSHV) is the infectious viral agent associated with Kaposi's sarcoma and may also contribute to B-cell disorders, which include primary effusion lymphoma (PEL) and multicentric Castleman's disease. However, regulation of cytokine-mediated lymphocytic immune response by KSHV is not fully understood. In this report, we demonstrate that KSHV suppresses the interleukin-4 (IL-4)-stimulated immune response of B-lymphocyte activation and cell proliferation. Moreover, we show that the latency-associated nuclear antigen (LANA) encoded by KSHV is essential for viral blocking of IL-4-induced signaling. LANA reduces phosphorylation of the signal transducers and activators of transcription 6 (STAT6) on Y-641 and concomitantly its DNA binding ability. Importantly, knockdown of endogenous STAT6 dramatically increases the sensitivity of PEL cells to low-serum stress or chemical-mediated cellular apoptosis and reactivation of KSHV from latent replication. Thus, these findings suggest that the IL-4/STAT6 signaling network is precisely controlled by KSHV for survival, maintenance of latency, and suppression of the host cytokine immune response of the virus-infected cells.

Bub1 and CENP-F can contribute to Kaposi's sarcoma-associated herpesvirus genome persistence by targeting LANA to kinetochores

The latency-associated nuclear antigen (LANA) encoded by Kaposi's sarcoma-associated herpesvirus (KSHV) is critical for segregation of viral episomes to progeny nuclei and allows for maintenance of the viral genome in newly divided daughter cells. LANA binds to KSHV terminal repeat (TR) DNA and simultaneously associates with chromatin-bound cellular proteins. This process tethers the viral episomes to host chromosomes. However, the mechanism of tethering is complex and involves multiple protein-protein interactions. Our previous proteomics studies which showed the association of LANA with centromeric protein F (CENP-F) prompted us to further study whether LANA targets centromeric proteins for persistence of KSHV episomes during cell division. Here we show that LANA colocalized with CENP-F as speckles, some of which are paired at centromeric regions of a subset of chromosomes in KSHV-infected JSC-1 cells. We also confirm that both the amino and carboxy termini of LANA can bind to CENP-F. Moreover, LANA associated with another kinetochore protein, Bub1 (budding uninhibited by benzimidazole 1), which is known to form a complex with CENP-F. Importantly, we demonstrated the dynamic association of LANA and Bub1/CENP-F and the colocalization between Bub1, LANA, and the KSHV episome tethered to the host chromosome using fluorescence in situ hybridization (FISH). Knockdown of Bub1 expression by lentivirus-delivered short hairpin RNA (shRNA) dramatically reduced the number of KSHV genome copies, whereas no dramatic effect was seen with CENP-F knockdown. Therefore, the interaction between LANA and the kinetochore proteins CENP-F and Bub1 is important for KSHV genome tethering and its segregation to new daughter cells, with Bub1 potentially playing a more critical role in the long-term persistence of the viral genome in the infected cell.

Telomere/telomerase interplay in virus-driven and virus-independent lymphomagenesis: pathogenic and clinical implications

Telomerase is a ribonucleoprotein complex critically involved in extending and maintaining telomeres. Unlike the majority of somatic cells, in which hTERT and telomerase activity are generally silent, normal lymphocytes show transient physiological hTERT expression and telomerase activity according to their differentiation/activation status. During lymphomagenesis, induction of persistent telomerase expression and activity may occur before or after telomere shortening, as a consequence of the different mechanisms through which transforming factors/agents may activate telomerase. Available data indicate that the timing of telomerase activation may allow the distinction of two different lymphomagenetic models: (i) an early activation of telomerase via exogenous regulators of hTERT, along with an increased lymphocyte growth and a subsequent selection of cells with increased transforming potential may characterize several virus-related lymphoid malignancies; (ii) a progressive shortening of telomeres, leading to genetic instability which favors a subsequent activation of telomerase via endogenous regulators may occur in most virus-unrelated lymphoid tumors. These models may have clinically relevant implications, particularly for the tailoring of therapeutic strategies targeting telomerase.

Distinct p53, p53:LANA, and LANA complexes in Kaposi's Sarcoma--associated Herpesvirus Lymphomas

The role of p53 in primary effusion lymphoma (PEL) is complicated. The latency-associated nuclear antigen (LANA) of Kaposi's sarcoma-associated herpesvirus (KSHV) binds p53. Despite this interaction, we had found that p53 was functional in PEL, i.e., able to induce apoptosis in response to DNA damage (C. E. Petre, S. H. Sin, and D. P. Dittmer, J. Virol. 81:1912-1922, 2007), and that hdm2 was overexpressed. To further elucidate the relationship between LANA, p53, and hdm2, we purified LANA complexes from PEL by column chromatography. This confirmed that LANA bound p53. However, the LANA:p53 complexes were a minority compared to hdm2:p53 and p53:p53 complexes. The half-life of p53 was not extended, which is in contrast to the half-life of simian virus 40 T antigen-transformed cells. p53:p53, LANA:p53, and LANA:LANA complexes coexisted in PEL, and each protein was able to bind to its cognate DNA element. These data suggest that under normal conditions, p53 is inactive in PEL, thus allowing for exponential growth, but that this inactivation is driven by the relative stoichiometries of LANA, hdm2, and p53. If p53 is activated by DNA damage or nutlin-3a, the complex falls apart easily, and p53 exercises its role as guardian of the genome.

Hepatitis B virus X protein upregulates transcriptional activation of human telomerase reverse transcriptase

It is well known that telomerase activation and virus infection are strongly associated with human hepatocellular carcinoma (HCC). Hepatitis B virus X protein (HBx) plays an important role in HCC pathogenesis. However, the mechanisms of HBx on telomerase activity are not well understood. To determine the potential roles of HBx in telomerase activity, both transfection and antisense assay were designed to examine the effects of HBx on telomerase in this report. Results showed that HBX gene increased telomerase activity and human telomerase reverse transcriptase (hTERT) expression in HBx-transfected cells and HBx-positive HCC samples. Co-transfection and luciferase reporter assay showed that HBx could stimulate hTERT promoter in a dose-dependent manner in different cells. Truncated and mutant reporter assays revealed that Sp1 binding sites mapped at -132 to +5 nt in hTERT promote were important for HBx-mediated upregulation of hTERT. Western blot did not show any change of Sp1 expression in HBx-transfected cells, but EMSA showed evidence of that HBx increased binding of Sp1 to its target DNA. These results provide new insights into the role of HBx in liver carcinogenesis.

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.

The latency-associated nuclear antigen interacts with MeCP2 and nucleosomes through separate domains

Kaposi's sarcoma-associated herpesvirus (KSHV)-infected cells express the latency-associated nuclear antigen (LANA) involved in the regulation of host and viral gene expression and maintenance of the KSHV latent episome. Performance of these diverse functions involves a 7-amino-acid chromatin-binding motif (CBM) situated at the amino terminus of LANA that is capable of binding directly to nucleosomes. LANA interacts with additional chromatin components, including methyl-CpG-binding protein 2 (MeCP2). Here, we show that the carboxy-terminal DNA-binding/dimerization domain of LANA provides the principal interaction with MeCP2 but that this association is modulated by the CBM. Both domains are required for LANA to colocalize with MeCP2 at chromocenters, regions of extensive pericentric heterochromatin that can be imaged by fluorescence microscopy. Within MeCP2, the methyl-CpG-binding domain (MBD) is the primary determinant for chromatin localization and acts together with the adjacent repression domains (the transcription repression domain [TRD] and the corepressor-interacting domain [CRID]) to redirect LANA to chromocenters. MeCP2 facilitates repression by LANA bound to the KSHV terminal repeats, a function that requires the MeCP2 C terminus in addition to the MBD and CRID/TRD. LANA and MeCP2 can also cooperate to stimulate transcription of the human E2F1 promoter, which lacks a LANA DNA-binding sequence, but this function requires both the N and C termini of LANA. The ability of LANA to establish multivalent interactions with histones and chromatin-binding proteins such as MeCP2 would enable LANA to direct regulatory complexes to specific chromosomal sites and thereby achieve stable reprogramming of cellular gene expression in latently infected cells.

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.

Latency-associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus (KSHV) upregulates survivin expression in KSHV-Associated B-lymphoma cells and contributes to their proliferation

Survivin is a master regulator of cell proliferation and cell viability and is highly expressed in most human tumors. The molecular network linked to survivin expression in tumors has not been completely elucidated. In this study, we show that latency-associated nuclear antigen (LANA), a multifunctional protein of Kaposi's sarcoma-associated herpesvirus (KSHV) that is found in Kaposi's sarcoma tumors, upregulates survivin expression and increases the proliferation of KSHV-infected B cells. Analysis of pathway-specific gene arrays showed that survivin expression was highly upregulated in BJAB cells expressing LANA. The mRNA levels of survivin were also upregulated in HEK 293 and BJAB cells expressing LANA. Similarly, protein levels of survivin were significantly higher in LANA-expressing, as well as KSHV-infected, cells. Survivin promoter activity assays identified GC/Sp1 and p53 cis-acting elements within the core promoter region as being important for LANA activity. Gel mobility shift assays revealed that LANA forms a complex with Sp1 or Sp1-like proteins bound to the GC/Sp1 box of the survivin promoter. In addition, a LANA/p53 complex bound to the p53 cis-acting element within the survivin promoter, indicating that upregulation of survivin expression can also occur through suppression of p53 function. Furthermore, immunohistochemistry analyses revealed that survivin expression was upregulated in KSHV-associated Kaposi's sarcoma tissue, suggesting that LANA plays an important role in the upregulation of survivin expression in KSHV-infected endothelial cells. Knockdown of survivin expression by lentivirus-delivered small hairpin RNA resulted in loss of cell proliferation in KSHV-infected cells. Therefore, upregulation of survivin expression in KSHV-associated human cells contributes to their proliferation.

The regulation of telomerase in oncogenesis

The influence that the expression of the human (glial-derived neurotrophic factor (GDNF)) neurotrophic factor has on the morphology and proliferative activity of embryonic stem cells (SC) of a mouse with R1 lineage, as well as their ability to form embroid bodies (EB), has been studied. Before that, using a PCR (polymerase chain reaction) coupled with reverse transcription, it was shown that, in this very lineage of the embryonic SC, the expression of the receptors' genes is being fulfilled for the neurotropfic RET and GFRα1 glia factor. The mouse's embryonic SC lineage has been obtained, transfected by the human GDNF gene, and has been fused with the "green" fluorescent protein (GFP) gene. The presence of the expression of the human GDNF gene in the cells was shown by northern hybridization and the synthesis of its albuminous product by immunocitochemical coloration with the use of specific antibodies. The reliable slowing-down of the embriod-body formation by the embryonic SC transfected by the GDNF gene has been shown. No significant influence of the expression of the GDNF gene on the morphology and the proliferative activity of the transfected embryonic SCs has been found when compared with the control ones.

Role of Kaposi's sarcoma-associated herpesvirus C-terminal LANA chromosome binding in episome persistence

Kaposi's sarcoma-associated herpesvirus (KSHV) LANA is an 1,162-amino-acid protein that tethers terminal repeat (TR) DNA to mitotic chromosomes to mediate episome persistence in dividing cells. C-terminal LANA self-associates to bind TR DNA. LANA contains independent N- and C-terminal chromosome binding regions. N-terminal LANA binds histones H2A/H2B to attach to chromosomes, and this binding is essential for episome persistence. We now investigate the role of C-terminal chromosome binding in LANA function. Alanine substitutions for LANA residues (1068)LKK(1070) and (1125)SHP(1127) severely impaired chromosome binding but did not reduce the other C-terminal LANA functions of self-association or DNA binding. The (1068)LKK(1070) and (1125)SHP(1127) substitutions did not reduce LANA's inhibition of RB1-induced growth arrest, transactivation of the CDK2 promoter, or C-terminal LANA's inhibition of p53 activation of the BAX promoter. When N-terminal LANA was wild type, the (1068)LKK(1070) and (1125)SHP(1127) substitutions also did not reduce LANA chromosome association or episome persistence. However, when N-terminal LANA binding to chromosomes was modestly diminished, the substitutions in (1068)LKK(1070) and (1125)SHP(1127) dramatically reduced both LANA chromosome association and episome persistence. These data suggest a model in which N- and C-terminal LANA cooperatively associates with chromosomes to mediate full-length LANA chromosome binding and viral persistence.

Latent membrane protein 1 of Epstein-Barr virus activates the hTERT promoter and enhances telomerase activity in B lymphocytes

Transformation of primary B lymphocytes by Epstein-Barr virus requires the establishment of a strictly latent infection, the expression of several latent viral proteins, and sustained telomerase activity. Our previous findings indicated that induction of hTERT, the rate-limiting catalytic unit of the telomerase complex, was associated with the expression of the viral latent membrane protein 1 (LMP1). In the present study, we demonstrate that ectopic expression of LMP1 in BJAB and Ramos B cells resulted in an increase of hTERT transcripts, thus suggesting that LMP1 acts at the transcriptional level. This was confirmed by transient expression of a luciferase reporter plasmid containing the hTERT promoter cotransfected with an LMP1-expressing vector or transfected into B cells in which LMP1 expression was inducible. Consistently, silencing of LMP1 by small interfering RNA resulted in a reduction of hTERT transcripts. We also provide evidence indicating that LMP1-induced hTERT activation is independently mediated by NF-kappaB and by mitogen-activated protein kinase and extracellular signal-regulated kinase 1/2 pathways, whereas CD40, Akt, and mTOR signaling has no involvement. Moreover, our results do not support a role for c-Myc in mediating these effects on hTERT, since ectopic expression of LMP1 did not upregulate c-Myc and silencing of this oncogene or E box mutagenesis failed to inhibit LMP1-induced hTERT activation. These findings indicate that LMP1 simultaneously modulates multiple signal transduction pathways in B cells to transactivate the hTERT promoter and enhance telomerase activity, thus confirming the pleiotropic nature of this viral oncoprotein.

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.

Regulation of telomerase and telomeres: human tumor viruses take control

Human tumor viruses are responsible for one-fifth of all cancers worldwide. These viruses have evolved multiple strategies to evade immune defenses and to persist in the host by establishing a latent infection. Proliferation is necessary for pretumor cells to accumulate genetic alterations and to acquire a transformed phenotype. However, each cell division is associated with a progressive shortening of the telomeres, which can suppress tumor development by initiating senescence and irreversible cell cycle arrest. Therefore, the ability of virus-infected cells to circumvent the senescence program is essential for the long-term survival and proliferation of infected cells and the likelihood of transformation. We review the multiple strategies used by human DNA and RNA tumor viruses to subvert telomerase functions during cellular transformation and carcinogenesis. Epstein-Barr virus, Kaposi sarcoma-associated herpesvirus, human papillomavirus, hepatitis B virus, hepatitis C virus, and human T-cell leukemia virus-1 each can increase transcription of the telomerase reverse transcriptase. Several viruses appear to mediate cis-activation or enhance epigenetic activation of telomerase transcription. Epstein-Barr virus and human papillomavirus have each developed posttranscriptional mechanisms to regulate the telomerase protein. Finally, some tumor virus proteins can also negatively regulate telomerase transcription or activity. It is likely that, as future studies further expose the strategies used by viruses to deregulate telomerase activity and control of telomere length, novel mechanisms will emerge and underscore the importance of increased telomerase activity in sustaining virus-infected cells and its potential in therapeutic targeting.

HTLV-1 HBZ cooperates with JunD to enhance transcription of the human telomerase reverse transcriptase gene (hTERT)

BACKGROUND

Activation of telomerase is a critical and late event in tumor progression. Thus, in patients with adult-T cell leukaemia (ATL), an HTLV-1 (Human T cell Leukaemia virus type 1)-associated disease, leukemic cells display a high telomerase activity, mainly through transcriptional up-regulation of the human telomerase catalytic subunit (hTERT). The HBZ (HTLV-1 bZIP) protein coded by the minus strand of HTLV-1 genome and expressed in ATL cells has been shown to increase the transcriptional activity of JunD, an AP-1 protein. The presence of several AP-1 binding sites in the hTERT promoter led us to investigate whether HBZ regulates hTERT gene transcription.

RESULTS

Here, we demonstrate using co-transfection assays that HBZ in association with JunD activates the hTERT promoter. Interestingly, the -378/+1 proximal region, which does not contain any AP-1 site was found to be responsible for this activation. Furthermore, an increase of hTERT transcripts was observed in cells co-expressing HBZ and JunD. Chromatin immunoprecipitation (ChIP) assays revealed that HBZ, and JunD coexist in the same DNA-protein complex at the proximal region of hTERT promoter. Finally, we provide evidence that HBZ/JunD heterodimers interact with Sp1 transcription factors and that activation of hTERT transcription by these heterodimers is mediated through GC-rich binding sites for Sp1 present in the proximal sequences of the hTERT promoter.

CONCLUSION

These observations establish for the first time that HBZ by intervening in the re-activation of telomerase, may contribute to the development and maintenance of the leukemic process.

microRNAs in viral oncogenesis

MicroRNAs are a recently discovered class of small noncoding functional RNAs. These molecules mediate post-transcriptional regulation of gene expression in a sequence specific manner. MicroRNAs are now known to be key players in a variety of biological processes and have been shown to be deregulated in a number of cancers. The discovery of viral encoded microRNAs, especially from a family of oncogenic viruses, has attracted immense attention towards the possibility of microRNAs as critical modulators of viral oncogenesis. The host-virus crosstalk mediated by microRNAs, messenger RNAs and proteins, is complex and involves the different cellular regulatory layers. In this commentary, we describe models of microRNA mediated viral oncogenesis.