Regulation of the interaction between glycogen synthase kinase 3 and the Kaposi's sarcoma-associated herpesvirus latency-associated nuclear antigen

Glycogen Synthase Kinase-3 Interaction Domain Enhances Phosphorylation of SARS-CoV-2 Nucleocapsid Protein

A structural protein of SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2), nucleocapsid (N) protein is phosphorylated by glycogen synthase kinase (GSK)-3 on the serine/arginine (SR) rich motif located in disordered regions. Although phosphorylation by GSK-3β constitutes a critical event for viral replication, the molecular mechanism underlying N phosphorylation is not well understood. In this study, we found the putative alpha-helix L/FxxxL/AxxRL motif known as the GSK-3 interacting domain (GID), found in many endogenous GSK-3β binding proteins, such as Axins, FRATs, WWOX, and GSKIP. Indeed, N interacts with GSK-3β similarly to Axin, and Leu to Glu substitution of the GID abolished the interaction, with loss of N phosphorylation. The N phosphorylation is also required for its structural loading in a virus-like particle (VLP). Compared to other coronaviruses, N of Sarbecovirus lineage including bat RaTG13 harbors a CDK1-primed phosphorylation site and Gly-rich linker for enhanced phosphorylation by GSK-3β. Furthermore, we found that the S202R mutant found in Delta and R203K/G204R mutant found in the Omicron variant allow increased abundance and hyper-phosphorylation of N. Our observations suggest that GID and mutations for increased phosphorylation in N may have contributed to the evolution of variants.

A pyridinium‑type fullerene derivative suppresses primary effusion lymphoma cell viability via the downregulation of the Wnt signaling pathway through the destabilization of β‑catenin

Primary effusion lymphoma (PEL) is defined as a rare subtype of non-Hodgkin's B cell lymphoma, which is caused by Kaposi's sarcoma-associated herpesvirus (KSHV) in immunosuppressed patients. PEL is an aggressive type of lymphoma and is frequently resistant to conventional chemotherapeutics. Therefore, the discovery of novel drug candidates for the treatment of PEL is of utmost importance. In order to discover potential novel anti-tumor compounds against PEL, the authors previously developed a pyrrolidinium-type fullerene derivative, 1,1,1′,1′-tetramethyl [60]fullerenodipyrrolidinium diiodide (derivative #1), which induced the apoptosis of PEL cells via caspase-9 activation. In the present study, the growth inhibitory effects of pyrrolidinium-type (derivatives #1 and #2), pyridinium-type (derivatives #3 and #5 to #9) and anilinium-type fullerene derivatives (derivative #4) against PEL cells were evaluated. This analysis revealed a pyridinium-type derivative (derivative #5; 3- 5′-(etho-xycarbonyl)-1′,5′-dihydro-2′H-[5,6]fullereno-C₆₀-I_h-[1,9-c]pyrrol-2′-yl]-1-methylpyridinium iodide), which exhibited antitumor activity against PEL cells via the downregulation of Wnt/β-catenin signaling. Derivative #5 suppressed the viability of KSHV-infected PEL cells compared with KSHV-uninfected B-lymphoma cells. Furthermore, derivative #5 induced the destabilization of β-catenin and suppressed β-catenin-TCF4 transcriptional activity in PEL cells. It is known that the constitutive activation of Wnt/β-catenin signaling is essential for the growth of KSHV-infected cells. The Wnt/β-catenin activation in KSHV-infected cells is mediated by KSHV latency-associated nuclear antigen (LANA). The data demonstrated that derivative #5 increased β-catenin phosphorylation, which resulted in β-catenin polyubiquitination and subsequent degradation. Thus, derivative #5 overcame LANA-mediated β-catenin stabilization. Furthermore, the administration of derivative #5 suppressed the development of PEL cells in the ascites of SCID mice with tumor xenografts derived from PEL cells. On the whole, these findings provide evidence that the pyridinium-type fullerene derivative #5 exhibits antitumor activity against PEL cells in vitro and in vivo. Thus, derivative #5 may be utilized as a novel therapeutic agent for the treatment of PEL.

Epigenetics of Cutaneous Sarcoma

Epigenetic changes influence various physiological and pathological conditions in the human body. Recent advances in epigenetic studies of the skin have led to an appreciation of the importance of epigenetic modifications in skin diseases. Cutaneous sarcomas are intractable skin cancers, and there are no curative therapeutic options for the advanced forms of cutaneous sarcomas. In this review, we discuss the detailed molecular effects of epigenetic modifications on skin sarcomas, such as dermatofibrosarcoma protuberans, angiosarcoma, Kaposi's sarcoma, leiomyosarcoma, and liposarcoma. We also discuss the application of epigenetic-targeted therapy for skin sarcomas.

Viral Manipulation of the Host Epigenome as a Driver of Virus-Induced Oncogenesis

Tumorigenesis due to viral infection accounts for a high fraction of the total global cancer burden (15–20%) of all human cancers. A comprehensive understanding of the mechanisms by which viral infection leads to tumor development is extremely important. One of the main mechanisms by which viruses induce host cell proliferation programs is through controlling the host’s epigenetic machinery. In this review, we dissect the epigenetic pathways through which oncogenic viruses can integrate their genome into host cell chromosomes and lead to tumor progression. In addition, we highlight the potential use of drugs based on histone modifiers in reducing the global impact of cancer development due to viral infection.

Identification of Mubritinib (TAK 165) as an inhibitor of KSHV driven primary effusion lymphoma via disruption of mitochondrial OXPHOS metabolism

KSHV-associated cancers have poor prognoses and lack therapeutics that selectively target viral gene functions. We developed a screening campaign to identify known drugs that could be repurposed for the treatment of KSHV-associated cancers. We focused on primary effusion lymphoma (PEL), which has particularly poor treatment outcomes. We developed a luciferase reporter assay to test the ability of drugs to inhibit DNA binding of the KSHV LANA DNA binding domain (DBD). In parallel, we screened drugs for selective inhibition of a KSHV+ PEL cells. While potent hits were identified in each assay, only one hit, Mubritinib, was found to score in both assays. Mubritinib caused PEL cells to undergo cell cycle arrest with accumulation of sub-G1 population and Annexin V. Mubritinib inhibited LANA binding to KSHV terminal repeat (TR) DNA in KSHV+ PEL cells, but did not lead to KSHV lytic cycle reactivation. Mubritinib was originally identified as a receptor tyrosine kinase (RTK) inhibitor selective for HER2/ErbB2. But recent studies have revealed that Mubritinib can also inhibit the electron transport chain (ETC) complex at nanomolar concentrations. We found that other related ETC complex inhibitors (Rotenone and Deguelin) exhibited PEL cell growth inhibition while RTK inhibitors failed. Seahorse analysis demonstrated that Mubritinib selectively inhibits the maximal oxygen consumption (OCR) in PEL cells and metabolomics revealed changes in ATP/ADP and ATP/AMP ratios. These findings indicate that PEL cells are selectively sensitive to ETC complex inhibitors and provide a rationale for repurposing Mubritinib for selective treatment of PEL.

Kaposi's Sarcoma-Associated Herpesvirus LANA Modulates the Stability of the E3 Ubiquitin Ligase RLIM

The Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded latency-associated nuclear antigen (LANA) protein functions in latently infected cells as an essential participant in KSHV genome replication and as a driver of dysregulated cell growth. In a previous study, we have identified LANA-interacting proteins using a protein array screen. Here, we explore the effect of LANA on the stability and activity of RLIM (RING finger LIM-domain-interacting protein, encoded by the RNF12 gene), a novel LANA-interacting protein identified in that protein screen. RLIM is an E3 ubiquitin ligase that leads to the ubiquitination and degradation of several transcription regulators, such as LMO2, LMO4, LHX2, LHX3, LDB1, and the telomeric protein TRF1. Expression of LANA leads to downregulation of RLIM protein levels. This LANA-mediated RLIM degradation is blocked in the presence of the proteasome inhibitor, MG132. Therefore, the interaction between LANA and RLIM could be detected in coimmunoprecipitation assay only in the presence of MG132 to prevent RLIM degradation. A RING finger mutant RLIM is resistant to LANA-mediated degradation, suggesting that LANA promotes RLIM autoubiquitination. Interestingly, we found that LANA enhanced the degradation of some RLIM substrates, such as LDB1 and LMO2, and prevented RLIM-mediated degradation of others, such as LHX3 and TRF1. We also show that transcription regulation by RLIM substrates is modulated by LANA. RLIM substrates are assembled into multiprotein transcription regulator complexes that regulate the expression of many cellular genes. Therefore, our study identified another way KSHV can modulate cellular gene expression.IMPORTANCE E3 ubiquitin ligases mark their substrates for degradation and therefore control the cellular abundance of their substrates. RLIM is an E3 ubiquitin ligase that leads to the ubiquitination and degradation of several transcription regulators, such as LMO2, LMO4, LHX2, LHX3, LDB1, and the telomeric protein TRF1. Here, we show that the Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded LANA protein enhances the ubiquitin ligase activity of RLIM, leading to enhanced RLIM autoubiquitination and degradation. Interestingly, LANA enhanced the degradation of some RLIM substrates, such as LDB1 and LMO2, and prevented RLIM-mediated degradation of others, such as LHX3 and TRF1. In agreement with protein stability of RLIM substrates, we found that LANA modulates transcription by LHX3-LDB1 complex and suggest additional ways LANA can modulate cellular gene expression. Our study adds another way a viral protein can regulate cellular protein stability, by enhancing the autoubiquitination and degradation of an E3 ubiquitin ligase.

Bioinformatics analysis of key biomarkers and pathways in KSHV infected endothelial cells

Kaposi sarcoma (KS) is an endothelial tumor etiologically related to Kaposi sarcoma herpesvirus (KSHV) infection. The aim of our study was to screen out candidate genes of KSHV infected endothelial cells and to elucidate the underlying molecular mechanisms by bioinformatics methods. Microarray datasets GSE16354 and GSE22522 were downloaded from Gene Expression Omnibus (GEO) database. the differentially expressed genes (DEGs) between endothelial cells and KSHV infected endothelial cells were identified. And then, functional enrichment analyses of gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis were performed. After that, Search Tool for the Retrieval of Interacting Genes (STRING) was used to investigate the potential protein-protein interaction (PPI) network between DEGs, Cytoscape software was used to visualize the interaction network of DEGs and to screen out the hub genes. A total of 113 DEGs and 11 hub genes were identified from the 2 datasets. GO enrichment analysis revealed that most of the DEGs were enrichen in regulation of cell proliferation, extracellular region part and sequence-specific DNA binding; KEGG pathway enrichments analysis displayed that DEGs were mostly enrichen in cell cycle, Jak-STAT signaling pathway, pathways in cancer, and Insulin signaling pathway. In conclusion, the present study identified a host of DEGs and hub genes in KSHV infected endothelial cells which may serve as potential key biomarkers and therapeutic targets, helping us to have a better understanding of the molecular mechanism of KS.

Towards Better Understanding of KSHV Life Cycle: from Transcription and Posttranscriptional Regulations to Pathogenesis

Kaposi’s sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus-8 (HHV-8), is etiologically linked to the development of Kaposi’s sarcoma, primary effusion lymphoma, and multicentric Castleman’s disease. These malignancies often occur in immunosuppressed individuals, making KSHV infection-associated diseases an increasing global health concern with persistence of the AIDS epidemic. KSHV exhibits biphasic life cycles between latent and lytic infection and extensive transcriptional and posttranscriptional regulation of gene expression. As a member of the herpesvirus family, KSHV has evolved many strategies to evade the host immune response, which help the virus establish a successful lifelong infection. In this review, we summarize the current research status on the biology of latent and lytic viral infection, the regulation of viral life cycles and the related pathogenesis.

Kaposi's Sarcoma-Associated Herpesvirus LANA-Adjacent Regions with Distinct Functions in Episome Segregation or Maintenance

Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA) is a 1,162-amino-acid protein that mediates episome persistence of viral genomes. LANA binds the KSHV terminal-repeat (TR) sequence through its carboxy-terminal domain to mediate DNA replication. LANA simultaneously binds mitotic chromosomes and TR DNA to segregate virus genomes to daughter cell nuclei. Amino-terminal LANA attaches to chromosomes by binding histones H2A/H2B, and carboxy-terminal LANA contributes to mitotic-chromosome binding. Although amino- and carboxy-terminal LANA are essential for episome persistence, they are not sufficient, since deletion of all internal LANA sequence renders LANA highly deficient for episome maintenance. Internal LANA sequence upstream of the internal repeat elements contributes to episome segregation and persistence. Here, we investigate this region with a panel of LANA deletion mutants. Mutants retained the ability to associate with mitotic chromosomes and bind TR DNA. In contrast to prior results, deletion of most of this sequence did not reduce LANA's ability to mediate DNA replication. Deletions of upstream sequence within the region compromised segregation of TR DNA to daughter cells, as assessed by retention of green fluorescent protein (GFP) expression from a replication-deficient TR plasmid. However, deletion of this upstream sequence did not reduce episome maintenance. In contrast, deletions that included an 80-amino-acid sequence immediately downstream resulted in highly deficient episome persistence. LANA with this downstream sequence deleted maintained the ability to replicate and segregate TR DNA, suggesting a unique role for the residues. Therefore, this work identifies adjacent LANA regions with distinct roles in episome segregation and persistence.IMPORTANCE KSHV LANA mediates episomal persistence of viral genomes. LANA binds the KSHV terminal-repeat (TR) sequence to mediate DNA replication and tethers KSHV DNA to mitotic chromosomes to segregate genomes to daughter cell nuclei. Here, we investigate LANA sequence upstream of the internal repeat elements that contributes to episome segregation and persistence. Mutants with deletions within this sequence maintained the ability to bind mitotic chromosomes or bind and replicate TR DNA. Deletion of upstream sequence within the region reduced segregation of TR DNA to daughter cells, but not episome maintenance. In contrast, mutants with deletions of 80 amino acids immediately downstream were highly deficient for episome persistence yet maintained the ability to replicate and segregate TR DNA, the two principal components of episome persistence, suggesting another role for the residues. In summary, this work identifies adjacent LANA sequence with distinct roles in episome segregation and persistence.

Signal Transduction Pathways Associated with KSHV-Related Tumors

Signal transduction pathways play a key role in the regulation of cell growth, cell differentiation, cell survival, apoptosis, and immune responses. Bacterial and viral pathogens utilize the cell signal pathways by encoding their own proteins or noncoding RNAs to serve their survival and replication in infected cells. Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8 (HHV-8), is classified as a rhadinovirus in the γ-herpesvirus subfamily and was the eighth human herpesvirus to be discovered from Kaposi's sarcoma specimens. KSHV is closely associated with an endothelial cell malignancy, Kaposi's sarcoma, and B-cell malignancies, primary effusion lymphoma, and multicentric Castleman's disease. Recent studies have revealed that KSHV manipulates the cellular signaling pathways to achieve persistent infection, viral replication, cell proliferation, anti-apoptosis, and evasion of immune surveillance in infected cells. This chapter summarizes recent developments in our understanding of the molecular mechanisms used by KSHV to interact with the cell signaling machinery.

The Latency-Associated Nuclear Antigen of Kaposi's Sarcoma-Associated Herpesvirus Inhibits Expression of SUMO/Sentrin-Specific Peptidase 6 To Facilitate Establishment of Latency

Kaposi's sarcoma-associated herpesvirus (KSHV), which belongs to the Gammaherpesviridae, typically displays two different phases in its life cycle, the latent phase and the lytic phase. Latency-associated nuclear antigen (LANA), the primary viral product during latency, has been reported to bind to a series of cellular gene promoters to modulate gene transcription. To systemically elucidate the cellular genes regulated by LANA, we identified genome-wide LANA binding sites by chromatin immunoprecipitation coupled with sequencing (ChIP-seq). We stratified ChIP-seq data and found that LANA might be involved in the macromolecule catabolic process. Specifically, we found and verified that LANA could directly bind to the promoter of the SUMO/sentrin-specific peptidase 6 (SENP6) gene in vivo and in vitro LANA could repress SENP6 promoter activity in a dose-dependent manner in a reporter gene assay. LANA expression was sufficient to inhibit endogenous SENP6 expression at both the RNA and protein levels. Moreover, SENP6 overexpression in KSHV-infected cells reduced LANA at the protein level. Mechanistically, we found that SENP6 could interact with LANA and reduce the formation of sumoylated LANA, which relies on the desumoylation ability of SENP6. During de novo infection, SENP6 overexpression would decrease the abundance of LANA and enhance viral gene expression, which would hamper the establishment of latency. Taken together, these data suggest that KSHV-encoded LANA could inhibit SENP6 expression to regulate the abundance of itself, which may play an important role in controlling the establishment of latency.IMPORTANCE LANA, as a key latent protein produced by KSHV, is responsible for episome persistence and regulates viral reactivation. In the present study, our results demonstrated that LANA could bind to the promoter region of the SENP6 gene and inhibit SENP6 expression while the regulated SENP6 could in turn modulate the abundance of LANA through desumoylation. This delicate regulation may provide important insights to explain the abundance of LANA during KSHV latency.

Kaposi's sarcoma-associated herpesvirus ORF34 is essential for late gene expression and virus production

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi’s sarcoma, primary effusion lymphoma, and multicentric Castleman’s disease. KSHV establishes a life-long infection in its host and alternates between a latent and lytic infection state. During lytic infection, lytic-related genes are expressed in a temporal manner and categorized as immediate early, early, and late gene transcripts. ORF34 is an early-late gene that interacts with several viral transcription-associated factors, however its physiological importance remains poorly understood. Here, we investigated the role of ORF34 during KSHV infection by generating ORF34-deficient KSHV, using a bacterial artificial chromosome system. Our results reveal that ORF34-deficient KSHV exhibited significantly attenuated late gene expression and viral production but did not affect viral DNA replication. ORF34 interacted with transcription factors ORF18, ORF24, ORF31, and ORF66, and a novel ORF34-interaction partner, ORF23. The C-terminal region of ORF34 was important for interaction with ORF24 and viral production. Our data support a model, in which ORF34 serves as a hub for recruiting a viral transcription complex to ORF24 to promote late viral gene expression.

Methylseleninic acid and sodium selenite induce severe ER stress and subsequent apoptosis through UPR activation in PEL cells

Selenium compounds such as methylseleninic acid (MSA) and sodium selenite (SS) have been widely evaluated as potential anti-cancer agents in the clinical setting. Primary effusion lymphoma (PEL) is a non-Hodgkin's B-cell lymphoma, associated with immunosuppressed individuals, such as post-transplant or AIDS patients. Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of PEL and Kaposi's sarcoma. Here, we found that MSA and SS markedly inhibited the growth of PEL cells compared with KSHV-uninfected B cells. MSA and SS caused ER stress, inducing the unfolded protein response (UPR) pathway in PEL cells that resulted in pro-apoptotic UPR, and finally apoptosis. The expression of UPR-related molecules (GRP78 and GADD34) and pro-apoptotic UPR molecules (CHOP, Bim, or Puma) were augmented in PEL cells treated with MSA or SS. In addition, these compounds induced the activation of caspase-4, an ER stress specific caspase, as well as caspase-3,-7, and -9 in PEL cells. We confirmed that thapsigargin which is an inducer of ER stress, dramatically decreased the viability of PEL cells, compared with KSHV-uninfected Ramos cells. We also investigated whether MSA or SS caused oxidization of cellular proteins in PEL cells. MSA and SS increased the levels of oxidative proteins in PEL cells, and the anti-oxidant agent (N-acetyl-l-cysteine) restored cell viability and suppressed caspase-7 activation in PEL cells treated with MSA or SS. Finally, we confirmed that MSA and SS induced neither lytic replication nor viral production in PEL cells. Taken together, MSA and SS could serve as lead compounds for the development of novel and effective drugs against PEL without the risk of de novo KSHV production.

Regulation of Wnt/β-catenin signaling by herpesviruses

The Wnt/β-catenin signaling pathway is instrumental in successful differentiation and proliferation of mammalian cells. It is therefore not surprising that the herpesvirus family has developed mechanisms to interact with and manipulate this pathway. Successful coexistence with the host requires that herpesviruses establish a lifelong infection that includes periods of latency and reactivation or persistence. Many herpesviruses establish latency in progenitor cells and viral reactivation is linked to host-cell proliferation and differentiation status. Importantly, Wnt/β-catenin is tightly connected to stem/progenitor cell maintenance and differentiation. Numerous studies have linked Wnt/β-catenin signaling to a variety of cancers, emphasizing the importance of Wnt/β-catenin pathways in development, tissue homeostasis and disease. This review details how the alpha-, beta-, and gammaherpesviruses interact and manipulate the Wnt/β-catenin pathway to promote a virus-centric agenda.

Kaposi's Sarcoma Herpesvirus Genome Persistence

Kaposi’s sarcoma-associated herpesvirus (KSHV) has an etiologic role in Kaposi’s sarcoma, primary effusion lymphoma, and multicentric Castleman’s disease. These diseases are most common in immunocompromised individuals, especially those with AIDS. Similar to all herpesviruses, KSHV infection is lifelong. KSHV infection in tumor cells is primarily latent, with only a small subset of cells undergoing lytic infection. During latency, the KSHV genome persists as a multiple copy, extrachromosomal episome in the nucleus. In order to persist in proliferating tumor cells, the viral genome replicates once per cell cycle and then segregates to daughter cell nuclei. KSHV only expresses several genes during latent infection. Prominent among these genes, is the latency-associated nuclear antigen (LANA). LANA is responsible for KSHV genome persistence and also exerts transcriptional regulatory effects. LANA mediates KSHV DNA replication and in addition, is responsible for segregation of replicated genomes to daughter nuclei. LANA serves as a molecular tether, bridging the viral genome to mitotic chromosomes to ensure that KSHV DNA reaches progeny nuclei. N-terminal LANA attaches to mitotic chromosomes by binding histones H2A/H2B at the surface of the nucleosome. C-terminal LANA binds specific KSHV DNA sequence and also has a role in chromosome attachment. In addition to the essential roles of N- and C-terminal LANA in genome persistence, internal LANA sequence is also critical for efficient episome maintenance. LANA’s role as an essential mediator of virus persistence makes it an attractive target for inhibition in order to prevent or treat KSHV infection and disease.

A Genome-Wide RNA Interference Screen Identifies a Role for Wnt/β-Catenin Signaling during Rift Valley Fever Virus Infection

Rift Valley fever virus (RVFV) is an arbovirus within the Bunyaviridae family capable of causing serious morbidity and mortality in humans and livestock. To identify host factors involved in bunyavirus replication, we employed genome-wide RNA interference (RNAi) screening and identified 381 genes whose knockdown reduced infection. The Wnt pathway was the most represented pathway when gene hits were functionally clustered. With further investigation, we found that RVFV infection activated Wnt signaling, was enhanced when Wnt signaling was preactivated, was reduced with knockdown of β-catenin, and was blocked using Wnt signaling inhibitors. Similar results were found using distantly related bunyaviruses La Crosse virus and California encephalitis virus, suggesting a conserved role for Wnt signaling in bunyaviral infection. We propose a model where bunyaviruses activate Wnt-responsive genes to regulate optimal cell cycle conditions needed to promote efficient viral replication. The findings in this study should aid in the design of efficacious host-directed antiviral therapeutics.

IMPORTANCE RVFV is a mosquito-borne bunyavirus that is endemic to Africa but has demonstrated a capacity for emergence in new territories (e.g., the Arabian Peninsula). As a zoonotic pathogen that primarily affects livestock, RVFV can also cause lethal hemorrhagic fever and encephalitis in humans. Currently, there are no treatments or fully licensed vaccines for this virus. Using high-throughput RNAi screening, we identified canonical Wnt signaling as an important host pathway regulating RVFV infection. The beneficial role of Wnt signaling was observed for RVFV, along with other disparate bunyaviruses, indicating a conserved bunyaviral replication mechanism involving Wnt signaling. These studies supplement our knowledge of the fundamental mechanisms of bunyavirus infection and provide new avenues for countermeasure development against pathogenic bunyaviruses.

Activation of DNA Damage Response Induced by the Kaposi's Sarcoma-Associated Herpes Virus

The human herpes virus 8 (HHV-8), also known as Kaposi sarcoma-associated herpes virus (KSHV), can infect endothelial cells often leading to cell transformation and to the development of tumors, namely Kaposi’s sarcoma (KS), primary effusion lymphoma (PEL), and the plasmablastic variant of multicentric Castleman’s disease. KSHV is prevalent in areas such as sub-Saharan Africa and the Mediterranean region presenting distinct genotypes, which appear to be associated with differences in disease manifestation, according to geographical areas. In infected cells, KSHV persists in a latent episomal form. However, in a limited number of cells, it undergoes spontaneous lytic reactivation to ensure the production of new virions. During both the latent and the lytic cycle, KSHV is programmed to express genes which selectively modulate the DNA damage response (DDR) through the activation of the ataxia telangiectasia mutated (ATM) pathway and by phosphorylating factors associated with the DDR, including the major tumor suppressor protein p53 tumor suppressor p53. This review will focus on the interplay between the KSHV and the DDR response pathway throughout the viral lifecycle, exploring the putative molecular mechanism/s that may contribute to malignant transformation of host cells.

Diallyl trisulfide induces apoptosis by suppressing NF-κB signaling through destabilization of TRAF6 in primary effusion lymphoma

The allyl sulfides, including diallyl sulfide (DAS), diallyl disulfide (DAD), and diallyl trisulfide (DAT), contained in garlic and members of the Allium family, have a variety of pharmacological activities. Therefore, allyl sulfides have been evaluated as potential novel chemotherapeutic agents. Here, we found that DAT inhibited nuclear factor-κB (NF-κB) signaling and induced apoptosis in primary effusion lymphoma (PEL), a subtype of non-Hodgkin's B-cell lymphoma caused by Kaposi's sarcoma-associated herpesvirus (KSHV). We examined the cytotoxic effects of DAS, DAD and DAT on PEL cells. DAT significantly reduced the viability of PEL cells compared with uninfected B-lymphoma cells, and induced the apoptosis of PEL cells by activating caspase-9. DAT induced stabilization of IκBα, and suppressed NF-κB transcriptional activity in PEL cells. We examined the mechanism underlying DAT-mediated IκBα stabilization. The results indicated that DAT stabilized IκBα by inhibiting the phosphorylation of IκBα by the IκB kinase (IKK) complex. Furthermore, DAT induced proteasomal degradation of TRAF6, and DAT suppressed IKKβ-phosphorylation through downregulation of TRAF6. It is known that activation of NF-κB is essential for survival of PEL cells. In fact, the NF-κB inhibitor BAY11-7082 induced apoptosis in PEL cells. In addition, DAT suppressed the production of progeny virus from PEL cells. The administration of DAT suppressed the development of PEL cells and ascites in SCID mice xenografted with PEL cells. These findings provide evidence that DAT has antitumor activity against PEL cells in vitro and in vivo, suggesting it to be a novel therapeutic agent for the treatment of PEL.

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.

Glycogen synthase kinase-3 (GSK3): regulation, actions, and diseases

Glycogen synthase kinase-3 (GSK3) may be the busiest kinase in most cells, with over 100 known substrates to deal with. How does GSK3 maintain control to selectively phosphorylate each substrate, and why was it evolutionarily favorable for GSK3 to assume such a large responsibility? GSK3 must be particularly adaptable for incorporating new substrates into its repertoire, and we discuss the distinct properties of GSK3 that may contribute to its capacity to fulfill its roles in multiple signaling pathways. The mechanisms regulating GSK3 (predominantly post-translational modifications, substrate priming, cellular trafficking, protein complexes) have been reviewed previously, so here we focus on newly identified complexities in these mechanisms, how each of these regulatory mechanism contributes to the ability of GSK3 to select which substrates to phosphorylate, and how these mechanisms may have contributed to its adaptability as new substrates evolved. The current understanding of the mechanisms regulating GSK3 is reviewed, as are emerging topics in the actions of GSK3, particularly its interactions with receptors and receptor-coupled signal transduction events, and differential actions and regulation of the two GSK3 isoforms, GSK3α and GSK3β. Another remarkable characteristic of GSK3 is its involvement in many prevalent disorders, including psychiatric and neurological diseases, inflammatory diseases, cancer, and others. We address the feasibility of targeting GSK3 therapeutically, and provide an update of its involvement in the etiology and treatment of several disorders.

Kaposi's sarcoma-associated herpesvirus induces Nrf2 during de novo infection of endothelial cells to create a microenvironment conducive to infection

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of Kaposi's sarcoma (KS) and primary effusion B-cell lymphoma. KSHV induces reactive oxygen species (ROS) early during infection of human dermal microvascular endothelial (HMVEC-d) cells that are critical for virus entry. One of the downstream targets of ROS is nuclear factor E2-related factor 2 (Nrf2), a transcription factor with important anti-oxidative functions. Here, we show that KS skin lesions have high Nrf2 activity compared to healthy skin tissue. Within 30 minutes of de novo KSHV infection of HMVEC-d cells, we observed Nrf2 activation through ROS-mediated dissociation from its inhibitor Keap1, Ser-40 phosphorylation, and subsequent nuclear translocation. KSHV binding and consequent signaling through Src, PI3-K and PKC-ζ were also important for Nrf2 stability, phosphorylation and transcriptional activity. Although Nrf2 was dispensable for ROS homeostasis, it was essential for the induction of COX-2, VEGF-A, VEGF-D, Bcl-2, NQO1, GCS, HO1, TKT, TALDO and G6PD gene expression in KSHV-infected HMVEC-d cells. The COX-2 product PGE2 induced Nrf2 activity through paracrine and autocrine signaling, creating a feed-forward loop between COX-2 and Nrf2. vFLIP, a product of KSHV latent gene ORF71, induced Nrf2 and its target genes NQO1 and HO1. Activated Nrf2 colocalized with the KSHV genome as well as with the latency protein LANA-1. Nrf2 knockdown enhanced ORF73 expression while reducing ORF50 and other lytic gene expression without affecting KSHV entry or genome nuclear delivery. Collectively, these studies for the first time demonstrate that during de novo infection, KSHV induces Nrf2 through intricate mechanisms involving multiple signal molecules, which is important for its ability to manipulate host and viral genes, creating a microenvironment conducive to KSHV infection. Thus, Nrf2 is a potential attractive target to intervene in KSHV infection and the associated maladies.

KSHV infection of endothelial cells in vivo causes Kaposi's sarcoma and understanding the steps involved in de novo KSHV infection of these cells and the consequences is important to develop therapies to counter KSHV pathogenesis. Infection of endothelial cells in vitro is preceded by the induction of a network of host signaling agents that are necessary for virus entry, gene expression and establishment of latency. Our previous studies have implicated reactive oxygen species (ROS) as part of this network. In the current study, we show that ROS activate Nrf2, a master transcriptional regulator of genes involved in ROS homeostasis, apoptosis, glucose metabolism and angiogenesis. Besides ROS, KSHV utilizes additional aspects of host signaling to induce Nrf2 activity. We also observed that infection of endothelial cells deficient in Nrf2 resulted in downregulation of multiple genes important in KSHV pathogenesis, such as COX-2 and VEGF, and affected proper expression of two hallmark KSHV genes, lytic ORF50 and latent ORF73. Taken together, this study is the first to demonstrate the importance of Nrf2 during de novo KSHV infection of endothelial cells, and establishes Nrf2 as an attractive therapeutic target to control KSHV infection, establishment of latency and the associated cancers.

Helicobacter pylori CagA promotes Snail-mediated epithelial-mesenchymal transition by reducing GSK-3 activity

Cytotoxin-associated gene A (CagA) is an oncoprotein and a major virulence factor of H. pylori. CagA is delivered into gastric epithelial cells via a type IV secretion system and causes cellular transformation. The loss of epithelial adhesion that accompanies the epithelial-mesenchymal transition (EMT) is a hallmark of gastric cancer. Although CagA is a causal factor in gastric cancer, the link between CagA and the associated EMT has not been elucidated. Here, we show that CagA induces the EMT by stabilizing Snail, a transcriptional repressor of E-cadherin expression. Mechanistically we show that CagA binds GSK-3 in a manner similar to Axin and causes it to shift to an insoluble fraction, resulting in reduced GSK-3 activity. We also find that the level of Snail protein is increased in H. pylori infected epithelium in clinical samples. These results suggest that H. pylori CagA acts as a pathogenic scaffold protein that induces a Snail-mediated EMT via the depletion of GSK-3.

Sangivamycin induces apoptosis by suppressing Erk signaling in primary effusion lymphoma cells

Sangivamycin, a structural analog of adenosine and antibiotic exhibiting antitumor and antivirus activities, inhibits protein kinase C and the synthesis of both DNA and RNA. Primary effusion lymphoma (PEL) is an aggressive neoplasm caused by Kaposi's sarcoma-associated herpesvirus (KSHV) in immunosuppressed patients and HIV-infected homosexual males. PEL cells are derived from post-germinal center B cells, and are infected with KSHV. Herein, we asked if sangivamycin might be useful to treat PEL. We found that sangivamycin killed PEL cells, and we explored the underlying mechanism. Sangivamycin treatment drastically decreased the viability of PEL cell lines compared to KSHV-uninfected B lymphoma cell lines. Sangivamycin induced the apoptosis of PEL cells by activating caspase-7 and -9. Further, sangivamycin suppressed the phosphorylation of Erk1/2 and Akt, thus inhibiting activation of the proteins. Inhibitors of Akt and MEK suppressed the proliferation of PEL cells compared to KSHV-uninfected cells. It is known that activation of Erk and Akt signaling inhibits apoptosis and promotes proliferation in PEL cells. Our data therefore suggest that sangivamycin induces apoptosis by inhibiting Erk and Akt signaling in such cells. We next investigated whether sangivamycin, in combination with an HSP90 inhibitor geldanamycin (GA) or valproate (valproic acid), potentiated the cytotoxic effects of the latter drugs on PEL cells. Compared to treatment with GA or valproate alone, the addition of sangivamycin enhanced cytotoxic activity. Our data thus indicate that sangivamycin may find clinical utility as a novel anti-cancer agent targeting PEL.

Stabilization of Myc through heterotypic poly-ubiquitination by mLANA is critical for γ-herpesvirus lymphoproliferation

Host colonization by lymphotropic γ-herpesviruses depends critically on expansion of viral genomes in germinal center (GC) B-cells. Myc is essential for the formation and maintenance of GCs. Yet, the role of Myc in the pathogenesis of γ-herpesviruses is still largely unknown. In this study, Myc was shown to be essential for the lymphotropic γ-herpesvirus MuHV-4 biology as infected cells exhibited increased expression of Myc signature genes and the virus was unable to expand in Myc defficient GC B-cells. We describe a novel strategy of a viral protein activating Myc through increased protein stability resulting in increased progression through the cell cycle. This is acomplished by modulating a physiological post-translational regulatory pathway of Myc. The molecular mechanism involves Myc heterotypic poly-ubiquitination mediated via the viral E3 ubiquitin-ligase mLANA protein. EC5S(mLANA) modulates cellular control of Myc turnover by antagonizing SCF(Fbw7) mediated proteasomal degradation of Myc, mimicking SCF(β-TrCP). The findings here reported reveal that modulation of Myc is essential for γ-herpesvirus persistent infection, establishing a link between virus induced lymphoproliferation and disease.

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 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.

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.

The Ubiquitin System and Kaposi's Sarcoma-Associated Herpesvirus

Ubiquitination is a post-translational modification in which one or more ubiquitin molecules are covalently linked to lysine residues of target proteins. The ubiquitin system plays a key role in the regulation of protein degradation, which contributes to cell signaling, vesicular trafficking, apoptosis, and immune regulation. Bacterial and viral pathogens exploit the cellular ubiquitin system by encoding their own proteins to serve their survival and replication in infected cells. Recent studies have revealed that Kaposi’s sarcoma-associated herpesvirus (KSHV) manipulates the ubiquitin system of infected cells to facilitate cell proliferation, anti-apoptosis, and evasion from immunity. This review summarizes recent developments in our understanding of the molecular mechanisms used by KSHV to interact with the cellular ubiquitin machinery.

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.

Proteasome inhibitors induce apoptosis and reduce viral replication in primary effusion lymphoma cells

Primary effusion lymphoma (PEL) is an aggressive neoplasm caused by Kaposi's sarcoma-associated herpesvirus (KSHV). This study provides evidence that proteasomal activity is required for both survival of PEL cells stably harboring the KSHV genome and viral replication of KSHV. We evaluated the cytotoxic effects of proteasome inhibitors on PEL cells. The proteasome inhibitors MG132, lactacystin, and proteasome inhibitor I dramatically inhibited cell proliferation and induced apoptosis of PEL cells through the accumulation of p21 and p27. Furthermore, proteasome inhibitors induced the stabilization of NF-κB inhibitory molecule (IκBα) and suppressed the transcriptional activity of NF-κB in PEL cells. The NF-κB specific inhibitor BAY11-7082 also induced apoptosis in PEL cells. The constitutive activation of NF-κB signaling is essential for the survival and growth of B cell lymphoma cells, including PEL cells. NF-κB signaling is upregulated by proteasome-dependent degradation of IκBα. The suppression of NF-κB signaling by proteasome inhibitors may contribute to the induction of apoptosis in PEL cells. In addition, proteasome activity is required for KSHV replication in KSHV latently infected PEL cells. MG132 reduced the production of progeny virus from PEL cells at low concentrations, which do not affect PEL cell growth. These findings suggest that proteasome inhibitors may represent a novel strategy for the treatment of KSHV infection and KSHV-associated lymphomas.

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.

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.

Conservation of complex nuclear localization signals utilizing classical and non-classical nuclear import pathways in LANA homologs of KSHV and RFHV

ORF73 latency-associated nuclear antigen (LANA) of the Kaposi's sarcoma-associated herpesvirus (KSHV) is targeted to the nucleus of infected cells where it binds to chromatin and mediates viral episome persistence, interacts with cellular proteins and plays a role in latency and tumorigenesis. A structurally related LANA homolog has been identified in the retroperitoneal fibromatosis herpesvirus (RFHV), the macaque homolog of KSHV. Here, we report the evolutionary and functional conservation of a novel bi-functional nuclear localization signal (NLS) in KSHV and RFHV LANA. N-terminal peptides from both proteins were fused to EGFP or double EGFP fusions to examine their ability to induce nuclear transport of a heterologous protein. In addition, GST-pull down experiments were used to analyze the ability of LANA peptides to interact with members of the karyopherin family of nuclear transport receptors. Our studies revealed that both LANA proteins contain an N-terminal arginine/glycine (RG)-rich domain spanning a conserved chromatin-binding motif, which binds directly to importin β1 in a RanGTP-sensitive manner and serves as an NLS in the importin β1-mediated non-classical nuclear import pathway. Embedded within this domain is a conserved lysine/arginine-(KR)-rich bipartite motif that binds directly to multiple members of the importin α family of nuclear import adaptors in a RanGTP-insensitive manner and serves as an NLS in the classical importin α/β-mediated nuclear import pathway. The positioning of a classical bipartite kr-NLS embedded within a non-classical rg-NLS is a unique arrangement in these viral proteins, whose nuclear localization is critical to their functionality and to the virus life cycle. The ability to interact with multiple import receptors provides alternate pathways for nuclear localization of LANA. Since different import receptors can import cargo to distinct subnuclear compartments, a multifunctional NLS may provide LANA with an increased ability to interact with different nuclear components in its multifunctional role to maintain viral latency.

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.

Characterization of the interaction between latency-associated nuclear antigen and glycogen synthase kinase 3beta

The latency-associated nuclear antigen (LANA) of Karposi's sarcoma-associated herpesvirus has been reported to interact with glycogen synthase kinase 3beta (GSK-3beta) and regulate its activity, leading to inhibition of GSK-3-dependent beta-catenin degradation. In this study, the interaction between LANA and GSK-3beta was characterized further. LANA was found to interact with GSK-3beta in vitro as well as in intact cells. However, LANA did not regulate GSK-3beta kinase activity and LANA-induced upregulation of beta-catenin was GSK-3beta independent. LANA did not regulate the stability of beta-catenin or of its reported interaction partners p53 and von Hippel-Lindau protein. Additional targets of LANA are likely to mediate its malignancy-promoting function.

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.

Hexachlorophene suppresses beta-catenin expression by up-regulation of Siah-1 in EBV-infected B lymphoma cells

Many studies have shown that the activation of β-catenin signaling can promote oncogenesis, and it is therefore of interest to find agents that modulate this pathway. Recent work has shown using B lymphoma cells that infection by Epstein–Barr virus (EBV) and expression of its latent membrane protein (LMP)-1, cause increases in the expression of β-catenin and cellular transformation. Conversely, results from cell-based small molecule screening studies have shown that the antibiotic hexachlorophene can down-regulate β-catenin in colon cancer cells. Here we report that hexachlorophene also counteracts the elevated β-catenin levels in EBV-infected B lymphomas. This is associated with restoration in levels of Siah-1 (an E3 ubiquitin ligase that is active in β-catenin regulation) which had been diminished by LMP-1. Our results suggest that Siah-1 is targeted by both LMP-1 and hexachlorophene with opposite effects. The hexachlorophene modulation of Siah-1 and β-catenin is independent of p53 and results in reduced expression of cyclin-D1 and c-Myc (target genes of β-catenin), leading to the growth arrest of B lymphoma cells. From these results we propose that hexachlorophene may provide a novel therapeutic strategy for EBV-infected B lymphoma cells by reducing β-catenin levels via the restoration of Siah-1.

Gamma-herpesviruses and cellular signaling in AIDS-associated malignancies

gamma-Herpesviruses, Epstein-Barr virus (EBV/HHV-4) and Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8), are involved in human carcinogenesis, particularly in immunocompromised patients. Virus-associated malignancies are becoming of significant concern for the mortality of long-lived immunocompromised patients, and therefore, research of advanced strategies for AIDS-related malignancies is an important field in cancer chemotherapy. Detailed understanding of the EBV and KSHV lifecycle and related cancers at the molecular level is required for novel strategies of molecular-targeted cancer chemotherapy. The present review gives a simple outline of the functional interactions between KSHV- and EBV-viral gene products and host cell deregulated signaling pathways as possible targets of chemotherapy against AIDS-related malignancies.

The Kaposi's sarcoma-associated herpesvirus LANA protein stabilizes and activates c-Myc

The Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA) protein is functionally pleiotropic. LANA contributes to KSHV-associated pathogenesis, in part, by increasing entry of cells into S phase through a process that is driven by LANA interaction with the serine-threonine kinase glycogen synthase kinase 3 (GSK-3) and stabilization of beta-catenin. We now show that LANA affects the activity of another protein involved in cell cycle regulation, c-Myc. Sequencing of c-Myc coding sequences revealed that c-Myc in KSHV-positive primary effusion lymphoma (PEL) cell lines is wild type in the N-terminal region that regulates c-Myc protein stability. Despite this, c-Myc in PEL cells is stabilized. In LANA-expressing cells, inactivation of nuclear GSK-3 reduced phosphorylation of c-Myc at Thr58 and contributed to c-Myc stabilization by decreasing c-Myc ubiquitination. Phosphorylation of c-Myc on Ser62 also affects c-Myc stability and function. We now show that LANA increases the level of phosphorylated extracellular signal-regulated kinase 1 (ERK1) and increases ERK phosphorylation of c-Myc on Ser62. LANA also interacted with c-Myc, and c-Myc amino acids 147 to 220 were required for this interaction. LANA (L1006P) retained the ability to bind to c-Myc and activate ERK1, indicating that these events did not require LANA interaction with GSK-3. Thus, LANA stabilizes c-Myc; prevents the phosphorylation of c-Myc at Thr58, an event that promotes Myc-induced apoptosis; and independently stimulates phosphorylation of c-Myc at Ser62, an event that transcriptionally activates c-Myc. LANA-mediated manipulation of c-Myc function is likely to contribute to KSHV-associated tumorigenesis through the induction of c-Myc regulated cellular genes, as well as by the stimulation of cell cycle progression.

Kaposi's sarcoma-associated herpesvirus latency-associated nuclear antigen 1 mimics Epstein-Barr virus EBNA1 immune evasion through central repeat domain effects on protein processing

Kaposi's sarcoma-associated herpesvirus (KSHV/human herpesvirus 8 [HHV8]) and Epstein-Barr virus (EBV/HHV4) are distantly related gammaherpesviruses causing tumors in humans. KSHV latency-associated nuclear antigen 1 (LANA1) is functionally similar to the EBV nuclear antigen-1 (EBNA1) protein expressed during viral latency, although they have no amino acid similarities. EBNA1 escapes cytotoxic lymphocyte (CTL) antigen processing by inhibiting its own proteosomal degradation and retarding its own synthesis to reduce defective ribosomal product processing. We show here that the LANA1 QED-rich central repeat (CR) region, particularly the CR2CR3 subdomain, also retards LANA1 synthesis and markedly enhances LANA1 stability in vitro and in vivo. LANA1 isoforms have half-lives greater than 24 h, and fusion of the LANA1 CR2CR3 domain to a destabilized heterologous protein markedly decreases protein turnover. Unlike EBNA1, the LANA1 CR2CR3 subdomain retards translation regardless of whether it is fused to the 5' or 3' end of a heterologous gene construct. Manipulation of sequence order, orientation, and composition of the CR2 and CR3 subdomains suggests that specific peptide sequences rather than RNA structures are responsible for synthesis retardation. Although mechanistic differences exist between LANA1 and EBNA1, the primary structures of both proteins have evolved to minimize provoking CTL immune responses. Simple strategies to eliminate these viral inhibitory regions may markedly improve vaccine effectiveness by maximizing CTL responses.

[Kaposi's sarcoma-associated herpesvirus and mechanisms of oncogenesis]

Kaposi's sarcoma-associated herpesvirus (KSHV, also known as human herpesvirus 8), is well known to be responsible for Kaposi's sarcoma, the most common AIDS-related cancer. KSHV is also associated with the B cell malignancies primary effusion lymphoma and multicentric Castleman's disease. Cellular signaling pathways regulate the proliferation and differentiation during normal development and a small number of signaling pathways are involved in tumors. KSHV utilize those pathways, such as pRb-E2F, Wnt and Notch pathways, to promote driving of cell cycle and to regulate their own life-cycles (i.e., latency and lytic cycle). This review focuses on signaling pathways which KSHV gene products manipulate and discusses their contributions to tomorigenesis and regulation of viral life-cycles.

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.

Kaposi's sarcoma-associated herpesvirus LANA protein downregulates nuclear glycogen synthase kinase 3 activity and consequently blocks differentiation

The Kaposi's sarcoma-associated herpesvirus latency-associated nuclear antigen (LANA) protein interacts with glycogen synthase kinase 3 (GSK-3) and relocalizes GSK-3 in a manner that leads to stabilization of beta-catenin and upregulation of beta-catenin-responsive cell genes. The LANA-GSK-3 interaction was further examined to determine whether there were additional downstream consequences. In the present study, the nuclear GSK-3 bound to LANA in transfected cells and in BCBL1 primary effusion lymphoma cells was found to be enriched for the inactive serine 9-phosphorylated form of GSK-3. The mechanism of inactivation of nuclear GSK-3 involved LANA recruitment of the extracellular signal-regulated kinases 1 and 2 (ERK1/2) and the ribosomal S6 kinase 1 (RSK1). ERK1/2 and RSK1 coprecipitated with LANA, and LANA was a substrate for ERK1 in vitro. A model is proposed for the overall inactivation of nuclear GSK-3 that incorporates the previously described GSK-3 phosphorylation of LANA itself. Functional inactivation of nuclear GSK-3 was demonstrated by the ability of LANA to limit phosphorylation of the known GSK-3 substrates C/EBPbeta and C/EBPalpha. The effect of LANA-mediated ablation of C/EBP phosphorylation on differentiation was modeled in the well-characterized 3T3L1 adipogenesis system. LANA-expressing 3T3L1 cells were impaired in their ability to undergo differentiation and adipogenesis. C/EBPbeta induction followed the same time course as that seen in vector-transduced cells, but there was delayed and reduced induction of C/EBPbeta transcriptional targets in LANA-expressing cells. We conclude that LANA inactivates nuclear GSK-3 and modifies the function of proteins that are GSK-3 substrates. In the case of C/EBPs, this translates into LANA-mediated inhibition of differentiation.

Determination of Kaposi's sarcoma-associated herpesvirus C-terminal latency-associated nuclear antigen residues mediating chromosome association and DNA binding

Kaposi's sarcoma-associated herpesvirus latency-associated nuclear antigen (LANA) tethers viral terminal repeat (TR) DNA to mitotic chromosomes to mediate episome persistence. The 1,162-amino-acid LANA protein contains both N- and C-terminal chromosome attachment regions. The LANA C-terminal domain self-associates to specifically bind TR DNA and mitotic chromosomes. Here, we used alanine scanning substitutions spanning residues 1023 to 1145 to investigate LANA self-association, DNA binding, and C-terminal chromosome association. No residues were essential for LANA oligomerization, as assayed by coimmunoprecipitation experiments, consistent with redundant roles for amino acids in self-association. Different subsets of amino acids were important for DNA binding, as assayed by electrophoretic mobility shift assay, and mitotic chromosome association, indicating that distinct C-terminal LANA subdomains effect DNA and chromosome binding. The DNA binding domains of LANA and EBNA1 are predicted to be structurally homologous; certain LANA residues important for DNA binding correspond to those with roles in EBNA1 DNA binding, providing genetic support for at least partial structural homology. In contrast to the essential role of N-terminal LANA chromosome targeting residues in DNA replication, deficient C-terminal chromosome association did not reduce LANA-mediated DNA replication.

Molecular biology of KSHV in relation to AIDS-associated oncogenesis

KSHV has been established as the causative agent of KS, PEL, and MCD, malignancies occurring more frequently in AIDS patients. The aggressive nature of KSHV in the context of HIV infection suggests that interactions between the two viruses enhance pathogenesis. KSHV latent infection and lytic reactivation are characterized by distinct gene expression profiles, and both latency and lytic reactivation seem to be required for malignant progression. As a sophisticated oncogenic virus, KSHV has evolved to possess a formidable repertoire of potent mechanisms that enable it to target and manipulate host cell pathways, leading to increased cell proliferation, increased cell survival, dysregulated angiogenesis, evasion of immunity, and malignant progression in the immunocompromised host. Worldwide, approximately 40.3 million people are currently living with HIV infection. Of these, a significant number are coinfected with KSHV. The complex interplay between the two viruses dramatically elevates the risk for development of KSHV-induced malignancies, KS, PEL, and MCD. Although HAART significantly reduces HIV viral load, the entire T-cell repertoire and immune function may not be completely restored. In fact, clinically significant immune deficiency is not necessary for the induction of KSHV-related malignancy. Because of variables such as lack of access to therapy noncompliance with prescribed treatment, failure to respond to treatment and the development of drug-resistant strains of HIV, KSHV-induced malignancies will continue to present as major health concerns.

Molecular piracy: manipulation of the ubiquitin system by Kaposi's sarcoma-associated herpesvirus

Ubiquitination, one of several post-translational protein modifications, plays a key role in the regulation of cellular events, including protein degradation, signal transduction, endocytosis, protein trafficking, apoptosis and immune responses. Ubiquitin attachment at the lysine residue of cellular factors acts as a signal for endocytosis and rapid degradation by the 26S proteasome. It has recently been observed that viruses, especially oncogenic herpesviruses, utilise molecular piracy by encoding their own proteins to interfere with regulation of cell signalling. Kaposi's sarcoma- associated herpesvirus (KSHV) manipulates the ubiquitin system to facilitate cell proliferation, anti-apoptosis and evasion from immunity. In this review, we will describe the strategies used by KSHV at distinct stages of the viral life-cycle to control the ubiquitin system and promote oncogenesis and viral persistence.

KSHV encoded LANA upregulates Pim-1 and is a substrate for its kinase activity

Pim kinases are proto-oncogenes that are upregulated in a number of B cell cancers, including Epstein-Barr Virus (EBV) associated Burkitt's lymphoma. They have also been shown to be upregulated in Kaposi sarcoma-associated herpes virus (KSHV) infected primary B cells. Most cells in KSHV-associated tumors are latently infected and express only a small subset of viral genes, with KSHV latency associated nuclear antigen (LANA) being constitutively expressed. LANA regulates the transcription of a large number of cellular and viral genes. Here, we show that LANA upregulates transcription from the Pim-1 promoter (pPim-1) and map this activation to a region in the promoter located within the sequence (-681 to +37). We show that LANA expressing cells can proliferate faster and are better protected from drug induced apoptosis. Since transition through cell cycle check points and anti-apoptosis are functions associated with Pim-1, it is likely that higher Pim-1 expression in cells expressing LANA is responsible, at least in part, for this effect. A Pim-1 phosphorylation site was also identified within the amino-terminal domain of LANA. Using in vitro kinase assays, we confirmed that LANA was indeed a Pim-1 substrate, and the failure of Pim-1 to phosphorylate LANA mutated at SS205/6RR identified this site as the specific serine residues phosphorylated by Pim-1. This report provides valuable insight into yet another cellular signaling pathway subverted by KSHV LANA and suggests a contribution to KSHV related oncogenesis.