Chromatinization of the KSHV Genome During the KSHV Life Cycle

Kaposi's sarcoma-associated herpesvirus (KSHV) belongs to the gamma herpesvirus family and is the causative agent of various lymphoproliferative diseases in humans. KSHV, like other herpesviruses, establishes life-long latent infection with the expression of a limited number of viral genes. Expression of these genes is tightly regulated by both the viral and cellular factors. Recent advancements in identifying the expression profiles of viral transcripts, using tilling arrays and next generation sequencing have identified additional coding and non-coding transcripts in the KSHV genome. Determining the functions of these transcripts will provide a better understanding of the mechanisms utilized by KSHV in altering cellular pathways involved in promoting cell growth and tumorigenesis. Replication of the viral genome is critical in maintaining the existing copies of the viral episomes during both latent and lytic phases of the viral life cycle. The replication of the viral episome is facilitated by viral components responsible for recruiting chromatin modifying enzymes and replication factors for altering the chromatin complexity and replication initiation functions, respectively. Importantly, chromatin modification of the viral genome plays a crucial role in determining whether the viral genome will persist as latent episome or undergo lytic reactivation. Additionally, chromatinization of the incoming virion DNA, which lacks chromatin structure, in the target cells during primary infection, helps in establishing latent infection. Here, we discuss the recent advancements on our understating of KSHV genome chromatinization and the consequences of chromatin modifications on viral life cycle.

KSHV LANA--the master regulator of KSHV latency

Kaposi's sarcoma associated herpesvirus (KSHV), like other human herpes viruses, establishes a biphasic life cycle referred to as dormant or latent, and productive or lytic phases. The latent phase is characterized by the persistence of viral episomes in a highly ordered chromatin structure and with the expression of a limited number of viral genes. Latency Associated Nuclear Antigen (LANA) is among the most abundantly expressed proteins during latency and is required for various nuclear functions including the recruitment of cellular machineries for viral DNA replication and segregation of the replicated genomes to daughter cells. LANA achieves these functions by recruiting cellular proteins including replication factors, chromatin modifying enzymes and cellular mitotic apparatus assembly. LANA directly binds to the terminal repeat region of the viral genome and associates with nucleosomal proteins to tether to the host chromosome. Binding of LANA to TR recruits the replication machinery, thereby initiating DNA replication within the TR. However, other regions of the viral genome can also initiate replication as determined by Single Molecule Analysis of the Replicated DNA (SMARD) approach. Recent, next generation sequence analysis of the viral transcriptome shows the expression of additional genes during latent phase. Here, we discuss the newly annotated latent genes and the role of major latent proteins in KSHV biology.

Epstein-Barr virus nuclear antigen 3C interact with p73: Interplay between a viral oncoprotein and cellular tumor suppressor

The p73 protein has structural and functional homology with the tumor suppressor p53, which plays an important role in cell cycle regulation, apoptosis, and DNA repair. The p73 locus encodes both a tumor suppressor (TAp73) and a putative oncogene (ΔNp73). p73 May play a significant role in p53-deficient lymphomas infected with Epstein-Barr virus (EBV). EBV produces an asymptomatic infection in the majority of the global population, but it is associated with several human B-cell malignancies. The EBV-encoded Epstein-Barr virus nuclear antigen 3C (EBNA3C) is thought to disrupt the cell cycle checkpoint by interacting directly with p53 family proteins. Doxorubicin, a commonly used chemotherapeutic agent, induces apoptosis through p53 and p73 signaling such that the lowΔNp73 level promotes the p73-mediated intrinsic pathway of apoptosis. In this report, we investigated the mechanism by which EBV infection counters p73α-induced apoptosis through EBNA3C.

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.

Quorum sensing in the squid-Vibrio symbiosis

Quorum sensing is an intercellular form of communication that bacteria use to coordinate group behaviors such as biofilm formation and the production of antibiotics and virulence factors. The term quorum sensing was originally coined to describe the mechanism underlying the onset of luminescence production in cultures of the marine bacterium Vibrio fischeri. Luminescence and, more generally, quorum sensing are important for V. fischeri to form a mutualistic symbiosis with the Hawaiian bobtail squid, Euprymna scolopes. The symbiosis is established when V. fischeri cells migrate via flagella-based motility from the surrounding seawater into a specialized structure injuvenile squid called the light organ. The cells grow to high cell densities within the light organ where the infection persists over the lifetime of the animal. A hallmark of a successful symbiosis is the luminescence produced by V. fischeri that camouflages the squid at night by eliminating its shadow within the water column. While the regulatory networks governing quorum sensing are critical for properly regulating V. fischeri luminescence within the squid light organ, they also regulate luminescence-independent processes during symbiosis. In this review, we discuss the quorum-sensing network of V. fischeri and highlight its impact at various stages during host colonization.

Surveillance of tuberculosis among HIV infected persons in three different regions of Nepal

Tuberculosis itself is a major public health problem in Nepal. The emergence of HIV has caused sharp increase in TB incidence in the community. Surveillance of TB/HIV co-infection helps in developing effective TB/HIV control strategies. The objective of this study was to measure prevalence of TB among HIV/AIDS patients in the selected regions. Between December 2006 and May 2008, a cross-sectional study was conducted in three different settings namely Tribhuvan University Teaching hospital, Kathmandu; Regional Tuberculosis Centre, Pokhara and Shree Siddhanath Science Campus, Mahendranagar. After taking informed consent pre-structured questionnaire was administered. Sputum specimen was collected from HIV/AIDS patients to investigate tuberculosis by culture and microscopy. Data analysis was done using SPSS 11.5. Of the 394 HIV infected persons, 225 (57.1%) were male and 169 (42.9%) female. The overall prevalence of TB was found to be 8.1%. Clinical signs and symptoms were significantly lower in those patients undergoing ART (chi2 value ranging from 4.19 to 9.13). However, development of tuberculosis is independent of ART status (chi2 value 1.14) and CD4 level (chi2 value 3.25). TB case detection rate by cultural technique was found to be twice as superior as direct microscopy. It can be concluded that smear negative TB constitute the significant proportion of TB cases in HIV infected persons.

The RBP-Jκ binding sites within the RTA promoter regulate KSHV latent infection and cell proliferation

Kaposi's sarcoma-associated herpesvirus (KSHV) is tightly linked to at least two lymphoproliferative disorders, primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD). However, the development of KSHV-mediated lymphoproliferative disease is not fully understood. Here, we generated two recombinant KSHV viruses deleted for the first RBP-Jκ binding site (RTA_1st) and all three RBP-Jκ binding sites (RTA_all) within the RTA promoter. Our results showed that RTA_1st and RTA_all recombinant viruses possess increased viral latency and a decreased capability for lytic replication in HEK 293 cells, enhancing colony formation and proliferation of infected cells. Furthermore, recombinant RTA_1st and RTA_all viruses showed greater infectivity in human peripheral blood mononuclear cells (PBMCs) relative to wt KSHV. Interestingly, KSHV BAC36 wt, RTA_1st and RTA_all recombinant viruses infected both T and B cells and all three viruses efficiently infected T and B cells in a time-dependent manner early after infection. Also, the capability of both RTA_1st and RTA_all recombinant viruses to infect CD19+ B cells was significantly enhanced. Surprisingly, RTA_1st and RTA_all recombinant viruses showed greater infectivity for CD3+ T cells up to 7 days. Furthermore, studies in Telomerase-immortalized human umbilical vein endothelial (TIVE) cells infected with KSHV corroborated our data that RTA_1st and RTA_all recombinant viruses have enhanced ability to persist in latently infected cells with increased proliferation. These recombinant viruses now provide a model to explore early stages of primary infection in human PBMCs and development of KSHV-associated lymphoproliferative diseases.

Kaposi's sarcoma-associated herpesvirus (KSHV) is tightly linked to at least two lymphoproliferative disorders, primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD). The life cycle of KSHV consists of latent and lytic phase. RTA is the master switch for viral lytic replication. In this study, we first show that recombinant viruses deleted for the RBP-Jκ sites within the RTA promoter have a decreased capability for lytic replication, and thus enhanced colony formation and proliferation of infected cells. Interestingly, the recombinant viruses show greater infectivity in human peripheral blood mononuclear cells (PBMCs). The recombinant viruses also infected CD19+ B cells and CD3+ T cells with increased efficiency in a time-dependent manner and now provide a model which can be used to explore the early stages of primary infection in human PBMCs, as well as the development of KSHV-associated lymphoproliferative diseases.

Prevalence of pulmonary tuberculosis among HIV infected persons in Pokhara, Nepal

BACKGROUND

Tuberculosis (TB) is a major public health problem in Nepal. HIV co-infection has further added a serious challenge to control TB. An early diagnosis and treatment of TB in HIV infected individuals can help reduce not only the morbidity and mortality associated in this high risk group but also to control the TB burden in Nepal.

METHODS

Clinical and bacteriological examination for pulmonary TB was carried out in 184 HIV positive persons from the community in Pokhara. Questionnaire was used to screen for symptoms and collect participants' demographic information and potential risk factors for TB.

RESULTS

Eighty (43.5%) sero-positive participants were young adult (21-30 years), followed by 71 (38.6%) middle aged (31-40 years) persons. Eleven participants were found to have Mycobacterium tuberculosis in their sputum, giving an overall prevalence of tuberculosis of 5.97%. All 11 cases were detected by Acid Fast Bacilli (AFB) culture, of which only three cases were detected by direct microscopy of AFB stained sputum smear. The majority of TB/HIV patients were asymptomatic with low AFB smear positivity. No significant association was found between TB and risk factors like ARV status, common clinical features of tuberculosis and smoking habit. However, significant association was found between the alcohol consumption and TB (X2=4.11, p<0.05).

CONCLUSIONS

High prevalence of tuberculosis (5.97%) was found among HIV infected persons. Alcohol consumption was associated with the development of tuberculosis. Majority of the studied subjects were asymptomatic for common signs and symptoms of TB. Weight loss was observed in 43% of the studied subjects followed by chest pain (39.2%), loss of appetite (34.8%), fever (34.8%) and night sweat (26.1%). Only 27% of the TB cases were found to be smear-positive.

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.

Epstein-Barr Virus nuclear antigen 1 (EBNA1) confers resistance to apoptosis in EBV-positive B-lymphoma cells through up-regulation of survivin

Resistance to apoptosis is an important component of the overall mechanism which drives the tumorigenic process. EBV is a ubiquitous human gamma-herpesvirus which preferentially establishes latent infection in viral infected B-lymphocytes. EBNA1 is typically expressed in most forms of EBV-positive malignancies and is important for replication of the latent episome in concert with replication of the host cells. Here, we investigate the effects of EBNA1 on survivin up-regulation in EBV-infected human B-lymphoma cells. We present evidence which demonstrates that EBNA1 forms a complex with Sp1 or Sp1-like proteins bound to their cis-element at the survivin promoter. This enhances the activity of the complex and up-regulates survivin. Knockdown of survivin and EBNA1 showed enhanced apoptosis in infected cells and thus supports a role for EBNA1 in suppressing apoptosis in EBV-infected cells. Here, we suggest that EBV encoded EBNA1 can contribute to the oncogenic process by up-regulating the apoptosis suppressor protein, survivin in EBV-associated B-lymphoma cells.

Nm23-H1 can induce cell cycle arrest and apoptosis in B cells

Nm23-H1 is a well-known tumor metastasis suppressor, which functions as a nucleoside-diphosphate kinase converting nucleoside diphosphates to nucleoside triphosphates with an expense of ATP. It regulates a variety of cellular activities, including proliferation, development, migration and differentiation known to be modulated by a series of complex signaling pathway. Few studies have addressed the mechanistic action of Nm23-H1 in the context of these cellular processes. To determine the downstream pathways modulated by Nm23-H1, we expressed Nm23-H1 in a Burkitt lymphoma derived B-cell line BJAB and performed pathway specific microarray analysis. The genes with significant changes in expression patterns were clustered in groups which are responsible for regulating cell cycle, p53 activities and apoptosis. We found a general reduction of cell cycle regulatory proteins including cyclins and cyclin dependent kinase inhibitors (anti proliferation), and upregulation of apoptotic genes which included caspase 3, 9 and Bcl-x. Nm23-H1 was also found to upregulate p53 and downregulate p21 expression. A number of these genes were validated by real time PCR and results from promoter assays indicated that Nm23-H1 expression downregulated cyclin D1 in a dose responsive manner. Further, we show that Nm23-H1 forms a complex with the cellular transcription factor AP1 to modulate cyclin D1 expression levels. BJAB cells expressing Nm23-H1 showed reduced proliferation rate and were susceptible to increased apoptosis which may in part be due to a direct interaction between Nm23-H1 and p53. These results suggest that Nm23-H1 may have a role in the regulation of cell cycle and apoptosis in human B-cells.

Early events associated with infection of Epstein-Barr virus infection of primary B-cells

Epstein Barr virus (EBV) is closely associated with the development of a vast number of human cancers. To develop a system for monitoring early cellular and viral events associated with EBV infection a self-recombining BAC containing 172-kb of the Epstein Barr virus genome BAC-EBV designated as MD1 BAC (Chen et al., 2005, J.Virology) was used to introduce an expression cassette of green fluorescent protein (GFP) by homologous recombination, and the resultant BAC clone, BAC-GFP-EBV was transfected into the HEK 293T epithelial cell line. The resulting recombinant GFP EBV was induced to produce progeny virus by chemical inducer from the stable HEK 293T BAC GFP EBV cell line and the virus was used to immortalize human primary B-cell as monitored by green fluorescence and outgrowth of the primary B cells. The infection, B-cell activation and cell proliferation due to GFP EBV was monitored by the expression of the B-cell surface antigens CD5, CD10, CD19, CD23, CD39, CD40 , CD44 and the intercellular proliferation marker Ki-67 using Flow cytometry. The results show a dramatic increase in Ki-67 which continues to increase by 6–7 days post-infection. Likewise, CD40 signals showed a gradual increase, whereas CD23 signals were increased by 6–12 hours, maximally by 3 days and then decreased. Monitoring the viral gene expression pattern showed an early burst of lytic gene expression. This up-regulation of lytic gene expression prior to latent genes during early infection strongly suggests that EBV infects primary B-cell with an initial burst of lytic gene expression and the resulting progeny virus is competent for infecting new primary B-cells. This process may be critical for establishment of latency prior to cellular transformation. The newly infected primary B-cells can be further analyzed for investigating B cell activation due to EBV infection.

Epstein-Barr Virus (EBV): infection, propagation, quantitation, and storage

Epstein-Barr virus (EBV) was first reported as the etiological agent of Burkitt's lymphoma in 1964. Since then, EBV has also been associated with nasopharyngeal carcinoma, which is highly prevalent in Southeast Asia, as well as infectious mononucleosis, complications of AIDS, and transplant-related B cell lymphomas. This virus has further been linked with T cell lymphomas and Hodgkin's disease, establishing the concept of a wide spectrum of EBV-associated malignant disorders. So far, there are a number of EBV-infected cell lines established that can be induced for production of infectious viral progeny and that facilitate the study of the mechanism of EBV-related infection, transformation, and oncogenesis. This unit describes procedures for the preparation of EBV virion particles and in vitro infection of cells with EBV. In addition, procedures for quantitation and storage of the virus are provided.

Nm23-H1 modulates the activity of the guanine exchange factor Dbl-1

Cytoskeleton rearrangement is necessary for tumor invasion and metastasis. Cellular molecules whose role is to regulate components of the cytoskeletal structure can dictate changes in cellular morphology. One of these molecules is the suppressor of tumor metastasis Nm23-H1. The level of Nm23-H1 expression has been linked to the invasiveness and metastatic potential of human cancers including melanoma and breast cancer. In this report, we demonstrate an interaction between the suppressor of tumor metastasis Nm23-H1, and Dbl-1, an oncoprotein which is associated with guanine exchange and belongs to a family of Guanine Exchange Factors (GEF). Nm23-H1 also was shown to bind pDbl which is the proto-oncoprotein of Dbl. Interestingly, the interaction between Nm23-H1 and Dbl-1 rescues the suppression of the cell motility activity Nm23-H1. Moreover, this interaction results in loss of the ability of the Dbl-1 oncoprotein to function as a GEF for the critical Rho-GTPase family member Cdc42. The loss of GTP loading onto Cdc42 resulted in a dramatic reduction in adhesion stimulated ruffles and suggests that Nm23-H1 can negatively regulate cell migration and tumor metastasis by modulating the activity of Cdc42 through direct interaction with Dbl-1.

An autonomous replicating element within the KSHV genome

Members of the herpesviridae family including Kaposi's sarcoma-associated herpesvirus (KSHV) persist latently in their hosts and harbor their genomes as closed circular episomes. Propagation of the KSHV genome into new daughter cells requires replication of the episome once every cell division and is considered critically dependent on expression of the virus encoded latency-associated nuclear antigen (LANA). This study demonstrates a LANA-independent mechanism of KSHV latent DNA replication. A cis-acting DNA element within a discreet KSHV genomic region termed the long unique region (LUR) can initiate and support replication of plasmids lacking LANA-binding sequences or a eukaryotic replication origin. The human cellular replication machinery proteins ORC2 and MCM3 associated with the LUR element and depletion of cellular ORC2 abolished replication of the plasmids indicating that recruitment of the host cellular replication machinery is important for LUR-dependent replication. Thus, KSHV can initiate replication of its genome independent of any trans-acting viral factors.

Kaposi's sarcoma herpesvirus-encoded latency-associated nuclear antigen stabilizes intracellular activated Notch by targeting the Sel10 protein

Deregulation of the evolutionarily conserved Notch signaling is highly correlated with oncogenesis. Intracellular activated Notch (ICN) is a protooncogene linked to the transcription activation of a number of cellular genes involved in cell cycle regulation, differentiation, and proliferation. Stability of ICN is tightly regulated by the Sel10-mediated ubiquitin-proteasome pathway. Sel10 can function as a negative regulator of Notch and exhibits activities of a tumor-suppressor protein. This article shows that the Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA) directly interacts with Sel10 and forms a complex in KSHV-infected cells. This results in suppression of ICN ubiquitination and degradation. The carboxyl terminus of LANA interacts with the F-box and WD40 domains of Sel10 and competes with ICN for binding to Sel10. This elevated level of ICN is also critical for maintaining the enhanced proliferation of KSHV-infected tumor cells. These findings describe a mechanism by which the KSHV-encoded LANA protein regulates ubiquitination of ICN mediated by the F-box component of the E3 ligase Sel10, leading to proliferation of the virus-infected cells.

ATRA promotes alpha tocopherol succinate-induced apoptosis in freshly isolated leukemic cells from chronic myeloid leukemic patients

We investigated the in vitro efficacy of all-trans retinoic acid (ATRA) and alpha-tocopherol succinate (alpha-TS) alone and in combination on the induction of cell death in freshly isolated leukemic cells obtained from chronic myeloid leukemia (CML) patients. In vitro cytotoxicity and induction of lipid peroxidation by ATRA (10 microM) and alpha-TS (25 or 50 microM) were evaluated in primary leukemic cells by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay and malondialdehyde formation respectively. Treatment of leukemic cells with alpha-TS alone or in combination with ATRA significantly (P < 0.05) decreased the cell viability in a concentration and time dependent manner as compared to peripheral blood mononuclear cells obtained from normal healthy controls. Lipid peroxidation was enhanced by 98% (P < 0.05) on combined treatment of cells with ATRA (10 microM) and alpha-TS (50 microM). ATRA alone did not enhance the externalization of phosphatidyl serine as studied by annexin-V binding using fluorescence activated cell sorter analysis, whereas in combination with alpha-TS it increased to 400% at 12 h. The treatment of leukemic cells to combination of ATRA with alpha-TS significantly decreased (P < 0.05) mitochondrial membrane potential and enhanced lysosomal destabilization. The combination of these drugs also increased mitochondrial and cytosolic reactive oxygen species (ROS) production, nitric oxide levels, and caspase-3 activity significantly and caused DNA fragmentation at 24 h in a concentration dependent manner in the leukemic cells. Our data suggest that ATRA in combination with alpha-TS efficiently induces apoptosis in leukemic cells, which may be a useful therapeutic modality in CML patients.

Protein complexes associated with the Kaposi's sarcoma-associated herpesvirus-encoded LANA

Kaposi's sarcoma-associated herpesvirus (KSHV) is the major biological cofactor contributing to development of Kaposi's sarcoma. KSHV establishes a latent infection in human B cells expressing the latency-associated nuclear antigen (LANA), a critical factor in the regulation of viral latency. LANA is known to modulate viral and cellular gene expression. We report here on some initial proteomic studies to identify cellular proteins associated with the amino and carboxy-terminal domains of LANA. The results of these studies show an association of known cellular proteins which support LANA functions and have identified additional LANA-associated proteins. These results provide new evidence for complexes involving LANA with a number of previously unreported functional classes of proteins including DNA polymerase, RNA helicase and cell cycle control proteins. The results also indicate that the amino terminus of LANA can interact with its carboxy-terminal domain. This interaction is potentially important for facilitating associations with other cell cycle regulatory proteins which include CENP-F identified in association with both the amino and carboxy-termini. These novel associations add to the diversity of LANA functions in relation to the maintenance of latency and subsequent transformation of KSHV infected cells.

The ATM/ATR signaling effector Chk2 is targeted by Epstein-Barr virus nuclear antigen 3C to release the G2/M cell cycle block

Epstein-Barr virus (EBV) infects most of the human population and persists in B lymphocytes for the lifetime of the host. The establishment of latent infection by EBV requires the expression of a unique repertoire of genes. The product of one of these viral genes, the EBV nuclear antigen 3C (EBNA3C), is essential for the growth transformation of primary B lymphocytes in vitro and can regulate the transcription of a number of viral and cellular genes important for the immortalization process. This study demonstrates an associated function of EBNA3C which involves the disruption of the G2/M cell cycle checkpoint. We show that EBNA3C-expressing lymphoblastoid cell lines treated with the drug nocodazole, which is known to block cells at the G2/M transition, did not show a G2/M-specific checkpoint arrest. Analyses of the cell cycles of cells expressing EBNA3C demonstrated that the expression of this essential EBV nuclear antigen is capable of releasing the G2/M checkpoint arrest induced by nocodazole. This G2/M arrest in response to nocodazole was also abolished by caffeine, suggesting an involvement of the ATM/ATR signaling pathway in the regulation of this cell cycle checkpoint. Importantly, we show that the direct interaction of EBNA3C with Chk2, the ATM/ATR signaling effector, is responsible for the release of this nocodazole-induced G2/M arrest and that this interaction leads to the serine 216 phosphorylation of Cdc25c, which is sequestered in the cytoplasm by 14-3-3. Overall, our data suggest that EBNA3C can directly regulate the G2/M component of the host cell cycle machinery, allowing for the release of the checkpoint block.

The minimal replicator element of the Kaposi's sarcoma-associated herpesvirus terminal repeat supports replication in a semiconservative and cell-cycle-dependent manner

Kaposi's sarcoma-associated herpesvirus (KSHV) persists as episomes in infected cells by circularizing at the terminal repeats (TRs). The KSHV episome carries multiple reiterated copies of the terminal repeat, and each copy is capable of supporting replication. Expression of the latency-associated nuclear antigen (LANA) is critical for the replication of TR-containing plasmids. A 32-bp sequence upstream of LANA binding site 1 (LBS1), referred to as RE (replication element), along with LANA binding sites 1 and 2 (RE-LBS1/2), is sufficient to support replication (J. Hu and R. Renne, J. Virol. 79:2637-2642, 2005). In this report we demonstrate that the minimal replicator element (RE-LBS1/2) replicates in synchrony with the host cellular DNA, and only once, in a cell-cycle-dependent manner. Overexpression of the mammalian replication inhibitor geminin blocked replication of the plasmid containing the minimal replicator element, confirming the involvement of the host cellular replication control mechanism, and prevented rereplication of the plasmid in the same cell cycle. Overexpression of Cdt1 also rescued the replicative ability of the RE-LBS1/2-containing plasmids. A chromatin immunoprecipitation assay performed using anti-origin recognition complex 2 (alpha-ORC2) and alpha-LANA antibodies from cells transfected with RE-LBS1/2, RE-LBS1, LBS1, or RE showed the association of ORC2 with the RE region. Expression of LANA increased the number of copies of chromatin-bound DNA of replication elements, suggesting that LANA is important for the recruitment of ORCs and may contribute to the stabilization of the replication protein complexes at the RE site.

Structure and function of latency-associated nuclear antigen

Latency-associated nuclear antigen (LANA) encoded by open reading frame 73 (ORF73) is the major latent protein expressed in all forms of KSHV-associated malignancies. LANA is a large (222-234 kDa) nuclear protein that interacts with various cellular as well as viral proteins. LANA has been classified as an oncogenic protein as it dysregulates various cellular pathways including tumor suppressor pathways associated with pRb and p53 and can transform primary rat embryo fibroblasts in cooperation with the cellular oncogene Hras. It associates with GSK-3beta, an important modulator of Wnt signaling pathway leading to the accumulation of cytoplasmic beta-catenin, which upregulates Tcf/Lef regulated genes after entering into the nucleus. LANA also blocks the expression of RTA, the reactivation transcriptional activator, which is critical for the latency to lytic switch, and thus helps in maintaining viral latency. LANA tethers the viral episomal DNA to the host chromosomes by directly binding to its cognate binding sequence within the TR region of the genome through its C terminus and to the nucleosomes through the N terminus of the molecule. Tethering to the host chromosomes helps in efficient partitioning of the viral episomes in the dividing cells. Disruptions of LANA expression led to reduction in the episomal copies of the viral DNA, supporting its role in persistence of the viral DNA. The functions known so far suggest that LANA is a key player in KSHV-mediated pathogenesis.

Proteomic analysis of the Kaposi's sarcoma-associated herpesvirus terminal repeat element binding proteins

Terminal repeat (TR) elements of Kaposi's sarcoma-associated herpesvirus (KSHV), the potential origin sites of KSHV replication, have been demonstrated to play important roles in viral replication and transcription and are most likely also critical for the segregation of the KSHV genome to daughter cells. To search for the cellular proteins potentially involved in KSHV genome maintenance, we performed affinity chromatography analysis, using KSHV TR DNA as the affinity ligand. Proteomic analysis was then carried out to identify the TR-interacting proteins. We identified a total of 123 proteins from both KSHV-positive and -negative cells, among which most were identified exclusively from KSHV-positive cells. These proteins were categorized as proliferation/cell cycle regulatory proteins, proteins involved in spliceosome components, such as heterogeneous nuclear ribonuclear proteins, the DEAD/H family, the switch/sucrose nonfermenting protein family, splicing factors, RNA binding proteins, transcription regulation proteins, replication factors, modifying enzymes, and a number of proteins that could not be broadly categorized. To support the proteomic results, the presence of four candidate proteins, ATR, BRG1, NPM1 and PARP-1, in the elutions was further characterized in this study. The binding and colocalization of these proteins with the TR were verified using chromatin immunoprecipitation and immunofluorescence in situ hybridization analysis. These newly identified TR binding proteins provide a number of clues and potential links to understanding the mechanisms regulating the replication, transcription, and genome maintenance of KSHV. This study will facilitate the generation and testing of new hypotheses to further our understanding of the mechanisms involved in KSHV persistence and its associated pathogenesis.

Kaposi's sarcoma-associated herpesvirus latent protein LANA interacts with HIF-1 alpha to upregulate RTA expression during hypoxia: Latency control under low oxygen conditions

Hypoxia can induce lytic replication of Kaposi's sarcoma-associated herpesvirus (KSHV) in primary effusion lymphoma (PEL) cells. However, the molecular mechanism of lytic reactivation of KSHV by hypoxia remains unclear. Here we show that the latency-associated nuclear antigen (LANA), which plays a crucial role in modulating viral and cellular gene expression, directly associated with a low oxygen responder, hypoxia-inducible factor-1 alpha (HIF-1 alpha). LANA enhanced not only the transcriptional activities of HIF-1 alpha but also its mRNA level. Coimmunoprecipitation and immunofluorescence studies documented a physical interaction between LANA and HIF-1 alpha in transiently transfected 293T cells as well as in PEL cell lines during hypoxia. Through sequence analysis, several putative hypoxia response elements (HRE-1 to -6) were identified in the essential lytic gene Rta promoter. Reporter assays showed that HRE-2 (-1130 to -1123) and HRE-5 and HRE-6 (+234 to +241 and +812 to +820, respectively, within the intron sequence) were necessary and sufficient for the LANA-mediated HIF-1 alpha response. Electrophoretic mobility shift assays showed HIF-1 alpha-dependent binding of a LANA protein complex specifically to the HRE-2, -5, and -6 motifs within the promoter regulatory sequences. This study demonstrates that hypoxia-induced KSHV lytic replication is mediated at least in part through cooperation of HIF-1 alpha with LANA bound to the HRE motifs of the Rta promoter.