Lytic replication-associated protein (RAP) encoded by Kaposi sarcoma-associated herpesvirus causes p21CIP-1-mediated G1 cell cycle arrest through CCAAT/enhancer-binding protein-alpha

Modulation of Cell Cycle Kinases by Kaposi's Sarcoma-Associated Herpesvirus

The cell cycle is governed by kinase activity that coordinates progression through a series of regulatory checkpoints, preventing the division of damaged cells. The Kaposi's sarcoma-associated herpesvirus (KSHV) encodes multiple genes that modulate or co-opt the activity of these kinases, shaping the cellular environment to promote viral persistence. By advancing the cell cycle, KSHV facilitates latent replication and subsequent transmission of viral genomes to daughter cells, while also contributing to the establishment of multiple cancer types. Conversely, during viral lytic replication, KSHV extends the resting phase of the cell cycle to prevent cellular DNA synthesis that would otherwise compete for essential replication precursors. This review will examine the mechanisms KSHV has evolved to control the kinase activity regulating host cell cycle progression.

KSHV transactivator-derived small peptide traps coactivators to attenuate MYC and inhibits leukemia and lymphoma cell growth

In herpesvirus replicating cells, host cell gene transcription is frequently down-regulated because important transcriptional apparatuses are appropriated by viral transcription factors. Here, we show a small peptide derived from the Kaposi's sarcoma-associated herpesvirus transactivator (K-Rta) sequence, which attenuates cellular MYC expression, reduces cell proliferation, and selectively kills cancer cell lines in both tissue culture and a xenograft tumor mouse model. Mechanistically, the peptide functions as a decoy to block the recruitment of coactivator complexes consisting of Nuclear receptor coactivator 2 (NCOA2), p300, and SWI/SNF proteins to the MYC promoter in primary effusion lymphoma cells. Thiol(SH)-linked alkylation for the metabolic sequencing of RNA (SLAM seq) with target-transcriptional analyses further confirm that the viral peptide directly attenuates MYC and MYC-target gene expression. This study thus provides a unique tool to control MYC activation, which may be used as a therapeutic payload to treat MYC-dependent diseases such as cancers and autoimmune diseases.

Anticancer activities of parthenolide in primary effusion lymphoma preclinical models

The sesquiterpene lactone parthenolide is a major component of the feverfew medicinal plant, Tanacetum parthenium. Parthenolide has been extensively studied for its anti-inflammatory and anticancer properties in several tumor models. Parthenolide's antitumor activities depend on several mechanisms but it is mainly known as an inhibitor of the nuclear factor-κB (NF-κB) pathway. This pathway is constitutively activated and induces cell survival in primary effusion lymphoma (PEL), a rare aggressive AIDS-related lymphoproliferative disorder that is commonly caused by the human herpesvirus 8 (HHV-8) infection. The aim of this study is to evaluate the targeted effect of Parthenolide both in vitro and in vivo. Herein, parthenolide significantly inhibited cell growth, induced G₀ /G₁ cell cycle arrest, and induced massive apoptosis in PEL cells and ascites. In addition, parthenolide inhibited the NF-ĸB pathway suppressing IĸB phosphorylation and p65 nuclear translocation. It also reduced the expression of the DNA methylase inhibitor (DNMT1). Parthenolide induced HHV-8 lytic gene expression without inhibiting latent viral gene expression. Importantly, DMAPT, the more soluble parthenolide prodrug, promoted delay in ascites development and prolonged the survival of PEL xenograft mice. This study supports the therapeutic use of parthenolide in PEL and encourages its further clinical development.

Novel modulators of p53-signaling encoded by unknown genes of emerging viruses

The p53 transcription factor plays a key role both in cancer and in the cell-intrinsic response to infections. The ORFEOME project hypothesized that novel p53-virus interactions reside in hitherto uncharacterized, unknown, or hypothetical open reading frames (orfs) of human viruses. Hence, 172 orfs of unknown function from the emerging viruses SARS-Coronavirus, MERS-Coronavirus, influenza, Ebola, Zika (ZIKV), Chikungunya and Kaposi Sarcoma-associated herpesvirus (KSHV) were de novo synthesized, validated and tested in a functional screen of p53 signaling. This screen revealed novel mechanisms of p53 virus interactions and two viral proteins KSHV orf10 and ZIKV NS2A binding to p53. Originally identified as the target of small DNA tumor viruses, these experiments reinforce the notion that all viruses, including RNA viruses, interfere with p53 functions. These results validate this resource for analogous systems biology approaches to identify functional properties of uncharacterized viral proteins, long non-coding RNAs and micro RNAs.

New viruses are constantly emerging. The ORFEOME project was based on the hypothesis that every virus, regardless of its molecular makeup and biology should encode functions that intersect the p53 signaling network, since p53 guards the cell from genomic insults, of which depositing a foreign, viral nucleic acid is one. The result of the ORFEOME screen of proteins without any known function, of predicted open reading frames and of suspected non-coding RNAs is the identification of two viral proteins that interact with p53. The first one, orf10, is encoded by Kaposi Sarcoma-associated herpesvirus and the second one, NS2A, is encoded by the Zika virus.

Productive herpesvirus lytic replication in primary effusion lymphoma cells requires S-phase entry

Gammaherpesviruses establish lifelong latent infection in B lymphocytes and are the causative agent of several B-cell malignancies and lymphoproliferative disorders. While a quiescent latent infection allows these pathogens to evade immune detection, initiation of an alternative lifecycle stage, known as lytic replication, is an essential step in the production and dissemination of infectious progeny. Although cessation of cellular proliferation is an eventual consequence of lytic induction, exactly how gammaherpesviruses manipulate the cell cycle prior to amplification of viral DNA remains under debate. Here we show that the onset of Kaposi's sarcoma-associated herpesvirus (KSHV) lytic reactivation in B cells leads to S-phase accumulation and that exit from G1 is required for efficient viral DNA replication. We also show that lytic replication leads to an S-phase-specific activation of the DNA damage response (DDR) that is abrogated when lytic replication is restricted to G0/G1. Finally, we observe that expression of early lytic viral genes results in cellular replication stress with increased stalling of DNA replication forks. Overall, we demonstrate that S-phase entry is important for optimal KSHV replication, that G1 arresting compounds are effective inhibitors of viral propagation, and that lytic-induced cell-cycle arrest could occur through the obstruction of cellular replication forks and subsequent activation of the DDR.

Molecular Virology of KSHV in the Lymphocyte Compartment-Insights From Patient Samples and De Novo Infection Models

The incidence of Kaposi’s sarcoma-associated herpesvirus (KSHV)-associated Kaposi Sarcoma has declined precipitously in the present era of effective HIV treatment. However, KSHV-associated lymphoproliferative disorders although rare, have not seen a similar decline. Lymphoma is now a leading cause of death in people living with HIV (PLWH), indicating that the immune reconstitution provided by antiretroviral therapy is not sufficient to fully correct the lymphomagenic immune dysregulation perpetrated by HIV infection. As such, novel insights into the mechanisms of KSHV-mediated pathogenesis in the immune compartment are urgently needed in order to develop novel therapeutics aimed at prevention and treatment of KSHV-associated lymphoproliferations. In this review, we will discuss our current understanding of KSHV molecular virology in the lymphocyte compartment, concentrating on studies which explore mechanisms unique to infection in B lymphocytes.

The bZIP Proteins of Oncogenic Viruses

Basic leucine zipper (bZIP) transcription factors (TFs) govern diverse cellular processes and cell fate decisions. The hallmark of the leucine zipper domain is the heptad repeat, with leucine residues at every seventh position in the domain. These leucine residues enable homo- and heterodimerization between ZIP domain α-helices, generating coiled-coil structures that stabilize interactions between adjacent DNA-binding domains and target DNA substrates. Several cancer-causing viruses encode viral bZIP TFs, including human T-cell leukemia virus (HTLV), hepatitis C virus (HCV) and the herpesviruses Marek’s disease virus (MDV), Epstein–Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV). Here, we provide a comprehensive review of these viral bZIP TFs and their impact on viral replication, host cell responses and cell fate.

Human Immunodeficiency Virus-Associated Exosomes Promote Kaposi's Sarcoma-Associated Herpesvirus Infection via the Epidermal Growth Factor Receptor

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the causal agent for Kaposi’s sarcoma (KS), the most common malignancy in HIV/AIDS patients. Oral transmission through saliva is considered the most common route for spreading the virus among HIV/AIDS patients. However, the role of HIV-specific components in the cotransfection of KSHV is unclear. We demonstrate that exosomes purified from the saliva of HIV-positive patients and secreted by HIV-infected T-cell lines promote KSHV infectivity in immortalized and primary oral epithelial cells. HIV-associated exosomes promote KSHV infection, which depends on HIV trans-activation response element (TAR) RNA and EGFR of oral epithelial cells, which can be targeted for reducing KSHV infection. These results reveal that HIV-associated exosomes are a risk factor for KSHV infection in the HIV-infected population.

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the causal agent for Kaposi’s sarcoma (KS), the most common malignancy in people living with human immunodeficiency virus (HIV)/AIDS. The oral cavity is a major route for KSHV infection and transmission. However, how KSHV breaches the oral epithelial barrier for spreading to the body is not clear. Here, we show that exosomes purified from either the saliva of HIV-positive individuals or the culture supernatants of HIV-1-infected T-cell lines promote KSHV infectivity in immortalized and primary human oral epithelial cells. HIV-associated saliva exosomes contain the HIV trans-activation response element (TAR), Tat, and Nef RNAs but do not express Tat and Nef proteins. The TAR RNA in HIV-associated exosomes contributes to enhancing KSHV infectivity through the epidermal growth factor receptor (EGFR). An inhibitory aptamer against TAR RNA reduces KSHV infection facilitated by the synthetic TAR RNA in oral epithelial cells. Cetuximab, a monoclonal neutralizing antibody against EGFR, blocks HIV-associated exosome-enhanced KSHV infection. Our findings reveal that saliva containing HIV-associated exosomes is a risk factor for the enhancement of KSHV infection and that the inhibition of EGFR serves as a novel strategy for preventing KSHV infection and transmission in the oral cavity.

IMPORTANCE Kaposi’s sarcoma-associated herpesvirus (KSHV) is the causal agent for Kaposi’s sarcoma (KS), the most common malignancy in HIV/AIDS patients. Oral transmission through saliva is considered the most common route for spreading the virus among HIV/AIDS patients. However, the role of HIV-specific components in the cotransfection of KSHV is unclear. We demonstrate that exosomes purified from the saliva of HIV-positive patients and secreted by HIV-infected T-cell lines promote KSHV infectivity in immortalized and primary oral epithelial cells. HIV-associated exosomes promote KSHV infection, which depends on HIV trans-activation response element (TAR) RNA and EGFR of oral epithelial cells, which can be targeted for reducing KSHV infection. These results reveal that HIV-associated exosomes are a risk factor for KSHV infection in the HIV-infected population.

Kaposi's Sarcoma-Associated Herpesvirus K8 Is an RNA Binding Protein That Regulates Viral DNA Replication in Coordination with a Noncoding RNA

Kaposi's sarcoma-associated herpesvirus (KSHV) lytic replication and constant primary infection of fresh cells are crucial for viral tumorigenicity. The virus-encoded bZIP family protein K8 plays an important role in viral DNA replication in both viral reactivation and de novo infection. The mechanism underlying the functional role of K8 in the viral life cycle is elusive. Here, we report that K8 is an RNA binding protein that also associates with many other proteins, including other RNA binding proteins. Many protein-protein interactions involving K8 are mediated by RNA. Using a UV cross-linking and immunoprecipitation (CLIP) procedure combined with high-throughput sequencing, RNAs that are associated with K8 in BCBL-1 cells were identified, including both viral (PAN, T1.4, T0.7, etc.) and cellular (MALAT-1, MRP, 7SK, etc.) RNAs. An RNA binding motif in K8 was defined, and mutation of the motif abolished the ability of K8 to bind to many noncoding RNAs, as well as viral DNA replication during de novo infection, suggesting that the K8 functions in viral replication are carried out through RNA association. The functions of K8 and associated T1.4 RNA were investigated in detail, and the results showed that T1.4 mediates the binding of K8 to ori-Lyt DNA. The T1.4-K8 complex physically bound to KSHV ori-Lyt DNA and recruited other proteins and cofactors to assemble a replication complex. Depletion of T1.4 abolished DNA replication in primary infection. These findings provide mechanistic insights into the role of K8 in coordination with T1.4 RNA in regulating KSHV DNA replication during de novo infection.IMPORTANCE Genomewide analyses of the mammalian transcriptome revealed that a large proportion of sequence previously annotated as noncoding regions is actually transcribed and gives rise to stable RNAs. The emergence of a large number of noncoding RNAs suggests that functional RNA-protein complexes, e.g., ribosomes or spliceosomes, are not ancient relics of the last ribo-organism but would be well adapted to a regulatory role in biology. K8 has been puzzling because of its unique characteristics, such as multiple regulatory roles in gene expression and DNA replication without DNA binding capability. This study reveals the mechanism underlying its regulatory role by demonstrating that K8 is an RNA binding protein that binds to DNA and initiates DNA replication in coordination with a noncoding RNA. It is suggested that many K8 functions, if not all, are carried out through its associated RNAs.

Nuclear targeting of the betanodavirus B1 protein via two arginine-rich domains induces G1/S cell cycle arrest mediated by upregulation of p53/p21

The molecular functions of betanodavirus non-structural protein B and its role in host cell survival remain unclear. In the present study, we examined the roles of specific nuclear targeting domains in B1 localization as well as the effect of B1 nuclear localization on the cell cycle and host cell survival. The B1 protein of the Red spotted grouper nervous necrosis virus (RGNNV) was detected in GF-1 grouper cells as early as 24 hours post-infection (hpi). Using an EYFP-B1 fusion construct, we observed nuclear localization of the B1 protein (up to 99%) in GF-1 cells at 48 hpi. The nuclear localization of B1 was mediated by two arginine-rich nuclear targeting domains (B domain: ⁴⁶RRSRR⁵¹; C domain: ⁶³RDKRPRR⁷⁰) and domain C was more important than domain B in this process. B1 nuclear localization correlated with upregulation of p53 and p21^(wef1/cip1); downregulation of Cyclin D1, CDK4 and Mdm2; and G1/S cell cycle arrest in GF-1 cells. In conclusion, nuclear targeting of the RGNNV B1 protein via two targeting domains causes cell cycle arrest by up-regulating p53/p21 and down-regulating Mdm2, thereby regulating host cell survival.

Localization of Double-Strand Break Repair Proteins to Viral Replication Compartments following Lytic Reactivation of Kaposi's Sarcoma-Associated Herpesvirus

Double-strand breaks (DSBs) in DNA are recognized by the Ku70/80 heterodimer and the MRE11-RAD50-NBS1 (MRN) complex and result in activation of the DNA-PK and ATM kinases, which play key roles in regulating the cellular DNA damage response (DDR). DNA tumor viruses such as Kaposi's sarcoma-associated herpesvirus (KSHV) are known to interact extensively with the DDR during the course of their replicative cycles. Here we show that during lytic amplification of KSHV DNA, the Ku70/80 heterodimer and the MRN complex consistently colocalize with viral genomes in replication compartments (RCs), whereas other DSB repair proteins form foci outside RCs. Depletion of MRE11 and abrogation of its exonuclease activity negatively impact viral replication, while in contrast, knockdown of Ku80 and inhibition of the DNA-PK enzyme, which are involved in nonhomologous end joining (NHEJ) repair, enhance amplification of viral DNA. Although the recruitment of DSB-sensing proteins to KSHV RCs is a consistent occurrence across multiple cell types, activation of the ATM-CHK2 pathway during viral replication is a cell line-specific event, indicating that recognition of viral DNA by the DDR does not necessarily result in activation of downstream signaling pathways. We have also observed that newly replicated viral DNA is not associated with cellular histones. Since the presence and modification of these DNA-packaging proteins provide a scaffold for docking of multiple DNA repair factors, the absence of histone deposition may allow the virus to evade localization of DSB repair proteins that would otherwise have a detrimental effect on viral replication.IMPORTANCE Tumor viruses are known to interact with machinery responsible for detection and repair of double-strand breaks (DSBs) in DNA, although detail concerning how Kaposi's sarcoma-associated herpesvirus (KSHV) modulates these cellular pathways during its lytic replication phase was previously lacking. By undertaking a comprehensive assessment of the localization of DSB repair proteins during KSHV replication, we have determined that a DNA damage response (DDR) is directed to viral genomes but is distinct from the response to cellular DNA damage. We also demonstrate that although recruitment of the MRE11-RAD50-NBS1 (MRN) DSB-sensing complex to viral genomes and activation of the ATM kinase can promote KSHV replication, proteins involved in nonhomologous end joining (NHEJ) repair restrict amplification of viral DNA. Overall, this study extends our understanding of the virus-host interactions that occur during lytic replication of KSHV and provides a deeper insight into how the DDR is manipulated during viral infection.

Regulation of CCAAT/enhancer-binding protein (C/EBP) α in human-cytomegalovirus-infected fibroblasts

CCAAT/enhancer-binding protein (C/EBP) α, a member of the C/EBP family of transcription factors, is known to be involved in gene expression and DNA replication of human cytomegalovirus (HCMV). This study aimed to understand the regulation of endogenous C/EBPα during HCMV infection using an in vitro infection model. The expression and localization of C/EBPα were investigated in fibroblasts infected with HCMV. The overexpression of C/EBP homologous protein (CHOP), the endogenous inhibitor of C/EBP, was also employed to test the involvement of C/EBPα during HCMV infection. Our data showed that HCMV infection increases the expression of the full-length C/EBPα isoform (p42) especially during the late stage of infection at the transcriptional and post-translational levels. The increased p42 accumulated in the viral DNA replication compartment. p42 expression was not induced in cells treated with UV-irradiated virus or in cells infected with normal virus in the presence of ganciclovir. CHOP-mediated inhibition of C/EBP activity suppressed viral gene expression and DNA replication, which lowered the level of viral production. Together, our data suggest that HCMV-mediated C/EBPα regulation might play a beneficial role in the lytic cycle of HCMV.

Activation of DNA Damage Response Pathways during Lytic Replication of KSHV

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the causative agent of several human malignancies. Human tumour viruses such as KSHV are known to interact with the DNA damage response (DDR), the molecular pathways that recognise and repair lesions in cellular DNA. Here it is demonstrated that lytic reactivation of KSHV leads to activation of the ATM and DNA-PK DDR kinases resulting in phosphorylation of multiple downstream substrates. Inhibition of ATM results in the reduction of overall levels of viral replication while inhibition of DNA-PK increases activation of ATM and leads to earlier viral release. There is no activation of the ATR-CHK1 pathway following lytic replication and CHK1 phosphorylation is inhibited at later times during the lytic cycle. Despite evidence of double-strand breaks and phosphorylation of H2AX, 53BP1 foci are not consistently observed in cells containing lytic virus although RPA32 and MRE11 localise to sites of viral DNA synthesis. Activation of the DDR following KSHV lytic reactivation does not result in a G1 cell cycle block and cells are able to proceed to S-phase during the lytic cycle. KSHV appears then to selectively activate DDR pathways, modulate cell cycle progression and recruit DDR proteins to sites of viral replication during the lytic cycle.

Modulation of DNA damage and repair pathways by human tumour viruses

With between 10% and 15% of human cancers attributable to viral infection, there is great interest, from both a scientific and clinical viewpoint, as to how these pathogens modulate host cell functions. Seven human tumour viruses have been identified as being involved in the development of specific malignancies. It has long been known that the introduction of chromosomal aberrations is a common feature of viral infections. Intensive research over the past two decades has subsequently revealed that viruses specifically interact with cellular mechanisms responsible for the recognition and repair of DNA lesions, collectively known as the DNA damage response (DDR). These interactions can involve activation and deactivation of individual DDR pathways as well as the recruitment of specific proteins to sites of viral replication. Since the DDR has evolved to protect the genome from the accumulation of deleterious mutations, deregulation is inevitably associated with an increased risk of tumour formation. This review summarises the current literature regarding the complex relationship between known human tumour viruses and the DDR and aims to shed light on how these interactions can contribute to genomic instability and ultimately the development of human cancers.

White spot syndrome virus IE1 and WSV056 modulate the G1/S transition by binding to the host retinoblastoma protein

DNA viruses often target cellular proteins to modulate host cell cycles and facilitate viral genome replication. However, whether proliferation of white spot syndrome virus (WSSV) requires regulation of the host cell cycle remains unclear. In the present study, we show that two WSSV paralogs, IE1 and WSV056, can interact with Litopenaeus vannamei retinoblastoma (Rb)-like protein (lv-RBL) through the conserved LxCxE motif. Further investigation revealed that IE1 and WSV056 could also bind to Drosophila retinoblastoma family protein 1 (RBF1) in a manner similar to how they bind to lv-RBL. Using the Drosophila RBF-E2F pathway as a model system, we demonstrated that both IE1 and WSV056 could sequester RBF1 from Drosophila E2F transcription factor 1 (E2F1) and subsequently activate E2F1 to stimulate the G1/S transition. Our findings provide the first evidence that WSSV may regulate cell cycle progression by targeting the Rb-E2F pathway.

CD30 targeting with brentuximab vedotin: a novel therapeutic approach to primary effusion lymphoma

Primary effusion lymphoma (PEL) is an aggressive subtype of non-Hodgkin lymphoma characterized by short survival with current therapies, emphasizing the urgent need to develop new therapeutic approaches. Brentuximab vedotin (SGN-35) is an anti-CD30 monoclonal antibody (cAC10) conjugated by a protease-cleavable linker to a microtubule-disrupting agent, monomethyl auristatin E. Brentuximab vedotin is an effective treatment of relapsed CD30-expressing Classical Hodgkin and systemic anaplastic large cell lymphomas. Herein, we demonstrated that PEL cell lines and primary tumors express CD30 and thus may serve as potential targets for brentuximab vedotin therapy. In vitro treatment with brentuximab vedotin decreased cell proliferation, induced cell cycle arrest, and triggered apoptosis of PEL cell lines. Furthermore, in vivo brentuximab vedotin promoted tumor regression and prolonged survival of mice bearing previously reported UM-PEL-1 tumors as well as UM-PEL-3 tumors derived from a newly established and characterized Kaposi's sarcoma-associated herpesvirus- and Epstein-Barr virus-positive PEL cell line. Overall, our results demonstrate for the first time that brentuximab vedotin may serve as an effective therapy for PEL and provide strong preclinical indications for evaluation of brentuximab vedotin in clinical studies of PEL patients.

Efficacious proteasome/HDAC inhibitor combination therapy for primary effusion lymphoma

Primary effusion lymphoma (PEL) is a rare form of aggressive B cell lymphoma caused by Kaposi's sarcoma-associated herpesvirus (KSHV). Current chemotherapy approaches result in dismal outcomes, and there is an urgent need for new PEL therapies. Previously, we established, in a direct xenograft model of PEL-bearing immune-compromised mice, that treatment with the proteasome inhibitor, bortezomib (Btz), increased survival relative to that after treatment with doxorubicin. Herein, we demonstrate that the combination of Btz with the histone deacetylase (HDAC) inhibitor suberoylanilidehydroxamic acid (SAHA, also known as vorinostat) potently reactivates KSHV lytic replication and induces PEL cell death, resulting in significantly prolonged survival of PEL-bearing mice. Importantly, Btz blocked KSHV late lytic gene expression, terminally inhibiting the full lytic cascade and production of infectious virus in vivo. Btz treatment led to caspase activation and induced DNA damage, as evidenced by the accumulation of phosphorylated γH2AX and p53. The addition of SAHA to Btz treatment was synergistic, as SAHA induced early acetylation of p53 and reduced interaction with its negative regulator MDM2, augmenting the effects of Btz. The eradication of KSHV-infected PEL cells without increased viremia in mice provides a strong rationale for using the proteasome/HDAC inhibitor combination therapy in PEL.

Disrupting galectin-1 interactions with N-glycans suppresses hypoxia-driven angiogenesis and tumorigenesis in Kaposi's sarcoma

Disrupting Gal-1 interactions with N-glycans prevents hypoxia-driven angiogenesis to suppress tumorigenesis of Kaposi’s sarcoma

Kaposi’s sarcoma (KS), a multifocal vascular neoplasm linked to human herpesvirus-8 (HHV-8/KS-associated herpesvirus [KSHV]) infection, is the most common AIDS-associated malignancy. Clinical management of KS has proven to be challenging because of its prevalence in immunosuppressed patients and its unique vascular and inflammatory nature that is sustained by viral and host-derived paracrine-acting factors primarily released under hypoxic conditions. We show that interactions between the regulatory lectin galectin-1 (Gal-1) and specific target N-glycans link tumor hypoxia to neovascularization as part of the pathogenesis of KS. Expression of Gal-1 is found to be a hallmark of human KS but not other vascular pathologies and is directly induced by both KSHV and hypoxia. Interestingly, hypoxia induced Gal-1 through mechanisms that are independent of hypoxia-inducible factor (HIF) 1α and HIF-2α but involved reactive oxygen species–dependent activation of the transcription factor nuclear factor κB. Targeted disruption of Gal-1–N-glycan interactions eliminated hypoxia-driven angiogenesis and suppressed tumorigenesis in vivo. Therapeutic administration of a Gal-1–specific neutralizing mAb attenuated abnormal angiogenesis and promoted tumor regression in mice bearing established KS tumors. Given the active search for HIF-independent mechanisms that serve to couple tumor hypoxia to pathological angiogenesis, our findings provide novel opportunities not only for treating KS patients but also for understanding and managing a variety of solid tumors.

Leucine zipper domain is required for Kaposi sarcoma-associated herpesvirus (KSHV) K-bZIP protein to interact with histone deacetylase and is important for KSHV replication

The Kaposi sarcoma-associated herpesvirus (KSHV; or human herpesvirus-8)-encoded protein called K-bZIP (also named K8) was found to be multifunctional. In this study, we discovered that K-bZIP interacts with histone deacetylase (HDAC) 1/2 in 12-O-tetradecanoylphorbol-13-acetate-stimulated BCBL-1 lymphocyte cells. K-bZIP appears to repress HDAC activity through this interaction, which we determined to be independent of K-bZIP SUMOylation. We dissected the domains of K-bZIP and found that the leucine zipper (LZ) domain is essential for the interaction of K-bZIP and HDAC. In addition, we constructed a KSHV bacterial artificial chromosome (BAC) with LZ domain-deleted K-bZIP (KSHVdLZ) and transfected this mutated KSHV BAC DNA into HEK 293T cells. As a result, it was consistently found that K-bZIP without its LZ domain failed to interact with HDAC2. We also showed that the interaction between K-bZIP and HDAC is necessary for the inhibition of the lytic gene promoters (ORF50 and OriLyt) of KSHV by K-bZIP. Furthermore, we found that the LZ domain is also important for the interaction of K-bZIP with the promoters of ORF50 and OriLyt. Most interestingly, although it was found to have suppressive effects on the promoters of ORF50 and OriLyt, KSHVdLZ replicates at a significantly lower level than its BAC-derived revertant (KSHVdLZRev) or KSHVWT (BAC36) in HEK 293T cells. The defectiveness of KSHVdLZ replication can be partially rescued by siRNA against HDAC2. Our results suggest that the function of K-bZIP interaction with HDAC is two-layered. 1) K-bZIP inhibits HDAC activity generally so that KSHVdLZ replicates at a lower level than does KSHVWT. 2) K-bZIP can recruit HDAC to the promoters of OriLyt and ORF50 through interaction with HDAC for K-bZIP to have a temporary repressive effect on the two promoters.

Epstein-Barr virus (EBV) Rta-mediated EBV and Kaposi's sarcoma-associated herpesvirus lytic reactivations in 293 cells

Epstein–Barr virus (EBV) Rta belongs to a lytic switch gene family that is evolutionarily conserved in all gamma-herpesviruses. Emerging evidence indicates that cell cycle arrest is a common means by which herpesviral immediate-early protein hijacks the host cell to advance the virus's lytic cycle progression. To examine the role of Rta in cell cycle regulation, we recently established a doxycycline (Dox)-inducible Rta system in 293 cells. In this cell background, inducible Rta modulated the levels of signature G1 arrest proteins, followed by induction of the cellular senescence marker, SA-β-Gal. To delineate the relationship between Rta-induced cell growth arrest and EBV reactivation, recombinant viral genomes were transferred into Rta-inducible 293 cells. Somewhat unexpectedly, we found that Dox-inducible Rta reactivated both EBV and Kaposi's sarcoma-associated herpesvirus (KSHV), to similar efficacy. As a consequence, the Rta-mediated EBV and KSHV lytic replication systems, designated as EREV8 and ERKV, respectively, were homogenous, robust, and concurrent with cell death likely due to permissive lytic replication. In addition, the expression kinetics of EBV lytic genes in Dox-treated EREV8 cells was similar to that of their KSHV counterparts in Dox-induced ERKV cells, suggesting that a common pathway is used to disrupt viral latency in both cell systems. When the time course was compared, cell cycle arrest was achieved between 6 and 48 h, EBV or KSHV reactivation was initiated abruptly at 48 h, and the cellular senescence marker was not detected until 120 h after Dox treatment. These results lead us to hypothesize that in 293 cells, Rta-induced G1 cell cycle arrest could provide (1) an ideal environment for virus reactivation if EBV or KSHV coexists and (2) a preparatory milieu for cell senescence if no viral genome is available. The latter is hypothetical in a transient-lytic situation.

Getting the message direct manipulation of host mRNA accumulation during gammaherpesvirus lytic infection

The Gammaherpesvirinae subfamily of herpesviruses comprises lymphotropic viruses, including the oncogenic human pathogens Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus. During lytic infection, gammaherpesviruses manipulate host gene expression to optimize the cellular environment for viral replication and to evade the immune response. Additionally, although a lytically infected cell will itself be killed in the process of viral replication, lytic infection can contribute to pathogenesis by inducing the secretion of paracrine factors with functions in cell survival and proliferation, and angiogenesis. The mechanisms by which these viruses manipulate host gene expression are varied and target the accumulation of cellular mRNAs and their translation, signaling pathways, and protein stability. Here, we discuss how gammaherpesviral proteins directly influence host mRNA biogenesis and stability, either selectively or globally, in order to fine-tune the cellular environment to the advantage of the virus. Appreciation of the mechanisms by which these viruses interface with and adapt normal cellular processes continues to inform our understanding of gammaherpesviral biology and the regulation of mRNA accumulation and turnover in our own cells.

Histone demethylase JMJD2A regulates Kaposi's sarcoma-associated herpesvirus replication and is targeted by a viral transcriptional factor

The switch between the latency and lytic cycles of Kaposi's sarcoma-associated herpesvirus (KSHV) is accompanied by specific alterations of histone codes. Recently, comprehensive analysis of histone modifications of KSHV showed the deposition of H3K27me3 across the KSHV genome with two specific regions occupied by the heterochromatin marker H3K9me3. Here, we show that knockdown of JMJD2A, an H3K9me3 demethylase, attenuates viral titers, whereas its overexpression increases KSHV reactivation. JMJD2A is localized in regions of latent viral chromosomes that are deficient in the H3K9me3 mark, indicating that JMJD2A may be responsible for the low level of this mark on viral chromatin. The presence of JMJD2A on the latent genome maintains H3K9 in unmethylated form and signals the readiness of specific sets of viral genes to be reactivated. The demethylase activity of JMJD2A is important for KSHV reactivation, because a demethylase-deficient mutant cannot restore the JMJD2A knockdown phenotype. Interestingly, we found that the KSHV encoded K-bZIP associated with JMJD2A, resulting in the inhibition of demethylase activity of JMJD2A both in vivo and in vitro. Inhibition of JMJD2A by K-bZIP is likely due to a physical interaction which blocks substrate accessibility. A consequence of such an inhibition is increasing global levels of H3K9me3 and gene silencing. Consistently, K-bZIP overexpression resulted in a repression of ∼80% of the ≥2-fold differentially regulated genes compared to results for the uninduced control cells. The consequences of K-bZIP targeting JMJD2A during viral replication will be discussed. To our knowledge, this is the first description of a viral product shown to be a potent inhibitor of a host cellular histone demethylase.

Modulation of interferon regulatory factor 5 activities by the Kaposi sarcoma-associated herpesvirus-encoded viral interferon regulatory factor 3 contributes to immune evasion and lytic induction

Multiple Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded proteins with potential roles in KSHV-associated neoplasms have been identified. KSHV encodes 4 genes with homology to transcription factors of the interferon (IFN) regulatory factor (IRF) family. Viral IRF3 (vIRF3) is expressed in latently KSHV-infected primary effusion lymphoma (PEL) cells and was recently shown to be essential for the survival of PEL cells. The focus of this study was to determine the mechanism(s) of vIRF3 oncogenic activity contributing to KSHV-associated lymphoma. We report that vIRF3 interacts with the amino-terminal DNA binding domain of human IRF5, leading to a complex manipulation of IRF5 function. vIRF3 associated with both exogenous and endogenous IRF5, thereby inhibiting IRF5-mediated IFN promoter activation and the synthesis of biologically active type I IFNs by blocking its binding to endogenous IFNA promoters. The function of this interaction was not limited to the IFN system as IRF5-mediated cell growth regulation was significantly altered by overexpression of vIRF3 in B cells. vIRF3 prevented IRF5-mediated growth inhibition and G2/M cell cycle arrest. Important, IRF5 was upregulated by the protein kinase C agonist 12-O-tetradecanoyl-phorbol-13-acetate in BCBL1 PEL cells and interaction with vIRF3 was observed at the endogenous p21 promoter in response to 12-O-tetradecanoyl-phorbol-13-acetate, suggesting that these 2 proteins cooperate in the regulation of lytic cycle-induced G1 arrest, which is an important early step for the reactivation of KSHV. In conclusion, cellular IRF5 and vIRF3 interact, leading to the functional modulation of IRF5-mediated type I IFN expression and cell cycle regulation. These findings support an important role for vIRF3 in immune evasion and cell proliferation that likely contribute to the survival of PEL cells.

MicroRNAs encoded by Kaposi's sarcoma-associated herpesvirus regulate viral life cycle

Kaposi's sarcoma-associated herpesvirus (KSHV) is linked with Kaposi's sarcoma and lymphomas. The pathogenesis of KSHV depends on the balance between two phases of the viral cycle: latency and lytic replication. In this study, we report that KSHV-encoded microRNAs (miRNAs) function as regulators by maintaining viral latency and inhibiting viral lytic replication. MiRNAs are short, noncoding, small RNAs that post-transcriptionally regulate the expression of messenger RNAs. Of the 12 viral miRNAs expressed in latent KSHV-infected cells, we observed that expression of miR-K3 can suppress both viral lytic replication and gene expression. Further experiments indicate that miR-K3 can regulate viral latency by targeting nuclear factor I/B. Nuclear factor I/B can activate the promoter of the viral immediate-early transactivator replication and transcription activator (RTA), and depletion of nuclear factor I/B by short hairpin RNAs had similar effects on the viral life cycle to those of miR-K3. Our results suggest a role for KSHV miRNAs in regulating the viral life cycle.

Pivotal advance: Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded microRNA specifically induce IL-6 and IL-10 secretion by macrophages and monocytes

Macrophages are an important source of inflammatory cytokines generated during the innate immune response,but in the microenvironment of certain tumors,macrophages promote tumor progression through their preferential secretion of cytokines that support tumor cell growth and suppress antitumoral immune responses. KSHV is the causative agent of KS and lymphomas preferentially arising in immuno compromised patients, and specific cytokines, including IL-6 and IL-10, have been implicated in KSHV-associated cancer pathogenesis. However, the contribution of KSHV-infected macrophages to the cytokine milieu within KSHV-related tumors is unclear. We found that individual KSHV-encoded miRNA induce IL-6 and IL-10 secretion independently and additively by murine macrophages and human myelomonocytic cells. Bioinformatics analysis identified KSHV miRNA binding sites formiR-K12-3 and miR-K12-7 within the 3'UTR of the basic region/leucine zipper motif transcription factor C/EBPbeta, a known regulator of IL-6 and IL-10 transcriptional activation.Subsequent immunoblot analyses revealed that miR-K12-3 and miR-K12-7 preferentially reduce expression of C/EBPbeta p20 (LIP), an isoform of C/EBPbeta known to function as a negative transcription regulator. In addition,RNA interference specifically targeting LIP induced basal secretion of IL-6 and IL-10 by macrophages.Taken together, these data support a role for KSHV miRNA in the programming of macrophage cytokine responses in favor of KSHV-related tumor progression.

Initiation of lytic DNA replication in Epstein-Barr virus: search for a common family mechanism

Herpesviruses are a complex family of dsDNA viruses that are a major cause of human disease. All family members share highly related viral replication proteins, such as DNA polymerase, ssDNA-binding proteins and processivity factors. Consequently, it is generally thought that lytic replication occurs through a common and conserved mechanism. However, considerable evidence indicates that proteins controlling initiation of DNA replication vary greatly among the herepesvirus subfamilies. In this article, we focus on some of the known mechanisms that regulate Epstein-Barr virus lytic-cycle replication, and compare this to other herpesvirus family members. Our reading of the literature leads us to conclude that diverse viral mechanisms generate a common nucleoprotein prereplication structure that can be recognized by a highly conserved family of viral replication enzymes.

Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a SUMO E3 ligase that is SIM-dependent and SUMO-2/3-specific

Sumoylation has emerged as a major post-translational modification of cellular proteins, affecting a variety of cellular processes. Viruses have exploited the sumoylation pathway to advance their own replication by evolving several ways to perturb the host sumoylation apparatus. However, there has been no report of virally encoded enzymes directly involved in catalyzing the sumoylation reaction. Here, we report that the K-bZIP protein encoded by Kaposi's sarcoma-associated herpesvirus (KSHV) is a SUMO E3 ligase with specificity toward SUMO2/3. K-bZIP is a nuclear factor that functions to modulate viral gene expression and to prolong the G1 phase, allowing viral transcription and translation to proceed at the early stage of infection. In addition to functioning as a transcriptional factor, we show that K-bZIP carries a SIM (SUMO-interacting motif), which specifically binds to SUMO-2/3 but not SUMO-1. K-bZIP catalyzes its own SUMO modification as well as that of its interacting partners such as the cellular tumor suppressor proteins p53 and Rb, both in vitro and in vivo. This reaction depends on an intact SIM. Sumoylation of p53 leads to its activation and K-bZIP is recruited to several p53 target chromatin sites in a SIM-dependent manner. In addition to the identification of a viral SUMO-2/3 E3 ligase, our results provide additional insights into the mechanisms whereby K-bZIP induces cell cycle arrest.

Regulation of the retinoblastoma proteins by the human herpesviruses

Viruses are obligate intracellular parasites that alter the environment of infected cells in order to replicate more efficiently. One way viruses achieve this is by modulating cell cycle progression. The main regulators of progression out of G0, through G1, and into S phase are the members of the retinoblastoma (Rb) family of tumor suppressors. Rb proteins repress the transcription of genes controlled by the E2F transcription factors. Because the expression of E2F-responsive genes is required for cell cycle progression into the S phase, Rb arrests the cell cycle in G0/G1. A number of viral proteins directly target Rb family members for inactivation, presumably to create an environment more hospitable for viral replication. Such viral proteins include the extensively studied oncoproteins E7 (from human papillomavirus), E1A (from adenovirus), and the large T (tumor) antigen (from simian virus 40). Elucidating how these three viral proteins target and inactivate Rb has proven to be an invaluable approach to augment our understanding of both normal cell cycle progression and carcinogenesis. In addition to these proteins, a number of other virally-encoded inactivators of the Rb family have subsequently been identified including a surprising number encoded by human herpesviruses. Here we review how the human herpesviruses modulate Rb function during infection, introduce the individual viral proteins that directly or indirectly target Rb, and speculate about what roles Rb modulation by these proteins may play in viral replication, pathogenesis, and oncogenesis.

Mutual inhibition between Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus lytic replication initiators in dually-infected primary effusion lymphoma

Background

Both Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV) are members of the human gamma herpesvirus family: each is associated with various human cancers. The majority of AIDS-associated primary effusion lymphoma (PEL) are co-infected with both KSHV and EBV. Dually-infected PELs selectively switch from latency to lytic replication of either KSHV or EBV in response to chemical stimuli. KSHV replication and transcription activator (K-RTA) is necessary and sufficient for the switch from KSHV latency to lytic replication, while EBV BZLF1 gene product (EBV-Z) is a critical initiator for induction of EBV lytic replication.

Methodology/Principal Findings

We show K-RTA and EBV-Z are co-localized and physically interact with each other in dually-infected PELs. K-RTA inhibits the EBV lytic replication by nullifying EBV-Z-mediated EBV lytic gene activation. EBV-Z inhibits KSHV lytic gene expression by blocking K-RTA-mediated transactivations. The physical interaction between K-RTA and EBV-Z are required for the mutual inhibition of the two molecules. The leucine heptapeptide repeat (LR) region in K-RTA and leucine zipper region in EBV-Z are involved in the physical interactions of the two molecules. Finally, initiation of KSHV lytic gene expression is correlated with the reduction of EBV lytic gene expression in the same PEL cells.

Conclusions/Significance

In this report, how the two viruses interact with each other in dually infected PELs is addressed. Our data may provide a possible mechanism for maintaining viral latency and for selective lytic replication in dually infected PELs, i.e., through mutual inhibition of two critical lytic replication initiators. Our data about putative interactions between EBV and KSHV would be applicable to the majority of AIDS-associated PELs and may be relevant to the pathogenesis of PELs.

Kaposi's sarcoma-associated herpesvirus ORF57 functions as a viral splicing factor and promotes expression of intron-containing viral lytic genes in spliceosome-mediated RNA splicing

Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57 facilitates the expression of both intronless viral ORF59 genes and intron-containing viral K8 and K8.1 genes (V. Majerciak, N. Pripuzova, J. P. McCoy, S. J. Gao, and Z. M. Zheng, J. Virol. 81:1062-1071, 2007). In this study, we showed that disruption of ORF57 in a KSHV genome led to increased accumulation of ORF50 and K8 pre-mRNAs and reduced expression of ORF50 and K-bZIP proteins but had no effect on latency-associated nuclear antigen (LANA). Cotransfection of ORF57 and K8beta cDNA, which retains a suboptimal intron of K8 pre-mRNA due to alternative splicing, promoted RNA splicing of K8beta and production of K8alpha (K-bZIP). Although Epstein-Barr virus EB2, a closely related homolog of ORF57, had a similar activity in the cotransfection assays, herpes simplex virus type 1 ICP27 was inactive. This enhancement of RNA splicing by ORF57 correlates with the intact N-terminal nuclear localization signal motifs of ORF57 and takes place in the absence of other viral proteins. In activated KSHV-infected B cells, KSHV ORF57 partially colocalizes with splicing factors in nuclear speckles and assembles into spliceosomal complexes in association with low-abundance viral ORF50 and K8 pre-mRNAs and essential splicing components. The association of ORF57 with snRNAs occurs by ORF57-Sm protein interaction. We also found that ORF57 binds K8beta pre-mRNAs in vitro in the presence of nuclear extracts. Collectively our data indicate that KSHV ORF57 functions as a novel splicing factor in the spliceosome-mediated splicing of viral RNA transcripts.

Binding of Kaposi's sarcoma-associated herpesvirus K-bZIP to interferon-responsive factor 3 elements modulates antiviral gene expression

Kaposi's sarcoma-associated herpesvirus encodes numerous regulatory proteins capable of modulating viral and cellular gene expression and affecting host cell functions. K-bZIP, a leucine zipper-containing transcription factor encoded by ORFK8, is one such protein. During infection, transcription of the ORFK8 early gene is turned on by the immediate-early replication and transcription factor activator (RTA). One described function of the K-bZIP nuclear protein is to interact with and repress RTA-mediated transactivation of viral promoters, including that of the K8 gene. In the present work, we provide evidence that the expression of K-bZIP results in the activation of the ifn-beta gene. Of interest, ifn-beta gene activation by K-bZIP is independent of interferon (IFN)-responsive factor 3 (IRF-3) and nuclear factor kappaB (NF-kappaB) activation. Using a DNA binding affinity assay and electromobility shift assay, we report that K-bZIP binds efficiently to the PRDIII-I region of the beta IFN (IFN-beta) promoter, and, in doing so, it prevents the attachment of activated IRF-3 but not that of NF-kappaB or ATF2/c-Jun to the IFN-beta promoter sequence. As a consequence, ifn-beta gene activation in response to IFN inducers such as Sendai virus infection or expression of retinoic acid-inducible gene I, mitochondrial antiviral signaling protein, or TANK-binding kinase 1 (TBK-1) is severely impaired (>90%) by the presence of K-bZIP. K-bZIP also prevents the activation of RANTES and CXCL11, whose promoters are also regulated by IRF-3. Lysine 158 (target for SUMO conjugation), threonine 111, and serine 167 (targets for phosphorylation) mutants of K-bZIP were equally effective as wild-type K-bZIP in mediating the repression of TBK-1-activated ifn-beta gene expression. Lastly, the overexpression of CREB binding protein could not reverse the K-bZIP repression of TBK-1-activated ifn-beta gene expression. In all, our results indicate that K-bZIP binds directly to the PRDIII-I region of the IFN-beta promoter and, as a consequence, causes a low level of ifn-beta gene transcription. In doing so, K-bZIP prevents IRF-3 from binding to the IFN-beta promoter and precludes the formation of the enhanceosome, which is required for maximal ifn-beta gene transcription. A new role for K-bZIP as a protein involved in immune evasion is therefore uncovered.

[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.

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.

A dynamic model for induced reactivation of latent virus

We develop a deterministic mathematical model to describe reactivation of latent virus by chemical inducers. This model is applied to the reactivation of latent KSHV in BCBL-1 cell cultures with butyrate as the inducing agent. Parameters for the model are first estimated from known properties of the exponentially growing, uninduced cell cultures. Additional parameters that are necessary to describe induction are determined from fits to experimental data from the literature. Our initial model provides good agreement with two independent sets of experimental data, but also points to the need for a new class of experiments which are required for further understanding of the underlying mechanisms.

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.

Kaposi's sarcoma-associated herpesvirus-encoded protein kinase and its interaction with K-bZIP

The oncogenic herpesvirus, Kaposi's sarcoma-associated herpesvirus, also identified as human herpesvirus 8, contains genes producing proteins that control transcription and influence cell signaling. Open reading frame 36 (ORF36) of this virus encodes a serine/threonine protein kinase, which is designated the viral protein kinase (vPK). Our recent efforts to elucidate the role of vPK in the viral life cycle have focused on identifying viral protein substrates and determining the effects of vPK-mediated phosphorylation on specific steps in viral replication. The vPK gene was transcribed into 4.2-kb and 3.6-kb mRNAs during the early and late phases of viral reactivation. vPK is colocalized with viral DNA replication/transcription compartments as marked by a polymerase processivity factor, and K-bZIP, a protein known to bind the viral DNA replication origin (Ori-Lyt) and to regulate viral transcription. The vPK physically associated with and strongly phosphorylated K-bZIP at threonine 111, a site also recognized by the cyclin-dependent kinase Cdk2. Both K-bZIP and vPK were corecruited to viral promoters targeted by K-bZIP as well as to the Ori-Lyt region. Phosphorylation of K-bZIP by vPK had a negative impact on K-bZIP transcription repression activity. The extent of posttranslational modification of K-bZIP by sumoylation, a process that influences its repression function, was decreased by vPK phosphorylation at threonine 111. Our data thus identify a new role of vPK as a modulator of viral transcription.

Kaposi sarcoma-associated herpesvirus and other viruses in human lymphomagenesis

Kaposi sarcoma-associated herpesvirus (KSHV), also called human herpesvirus 8 (HHV-8), is associated with a specific subset of lymphoproliferative disorders. These include two main categories. The first is primary effusion lymphomas and related solid variants. The second is multicentric Castleman disease, from which KSHV-positive plasmablastic lymphomas can arise. KSHV contributes to lymphomagenesis by subverting the host cell molecular signaling machinery to deregulate cell growth and survival. KSHV expresses a selected set of genes in the lymphoma cells, encoding viral proteins that play important roles in KSHV lymphomagenesis. Deregulation of the NF-kappaB pathway is an important strategy used by KSHV to promote lymphoma cell survival, and the viral protein vFLIP is essential for this process. Two other viruses that are well documented to be causally associated with lymphoid neoplasia in humans are Epstein-Barr virus (EBV/HHV-4) and human T-cell lymphotropic virus (HTLV-1). Both of these are similar to KSHV in their use of viral proteins to promote cell survival by deregulating the NF-kappaB pathway. Here we review the basic information and recent developments that have contributed to our knowledge of lymphomas caused by KSHV and other viruses. The understanding of the mechanisms of viral lymphomagenesis should lead to the identification of novel therapeutic targets and to the development of rationally designed therapies.

Essential role of RBP-Jkappa in activation of the K8 delayed-early promoter of Kaposi's sarcoma-associated herpesvirus by ORF50/RTA

KSHV K8 gene is activated by virally encoded transactivator RTA in delayed-early stage of viral reactivation. Three RTA-responsive elements (RREs) were identified in the promoter. Among them, RRE-II was found to be the most critical cis-acting element for RTA transactivation. In this report, the mechanism underlying RTA-mediated activation of the K8 delayed-early promoter was investigated. A DNA affinity purification study demonstrated that RRE-II was bound by cellular protein RBP-Jkappa, a sequence-specific DNA binding protein and a primary target of the Notch signaling pathway. Inspection of the RRE-II sequence revealed a potential recognition sequence for RBP-Jkappa (GTGAGAA) between the nucleotides -102 and -108 relative to the transcription initial site. Removal or mutation of the motif abolished RBP-Jkappa binding to the K8 promoter and as a consequence, RTA failed to bind to and activate the promoter. An essential role of RBP-Jkappa in the transcription of the K8 promoter was demonstrated by diminishment of the promoter activity in RBP-Jkappa-null murine embryonic fibroblasts. Taken together, RTA activates the K8 promoter through an indirect binding mechanism, i.e. being recruited to the K8 promoter through interaction with RBP-Jkappa bound to an RBP-Jkappa motif in the promoter.

The porcine lymphotropic herpesvirus 1 encodes functional regulators of gene expression

The porcine lymphotropic herpesviruses (PLHV) are discussed as possible risk factors in xenotransplantation because of the high prevalence of PLHV-1, PLHV-2 and PLHV-3 in pig populations world-wide and the fact that PLHV-1 has been found to be associated with porcine post-transplant lymphoproliferative disease. To provide structural and functional knowledge on the PLHV immediate-early (IE) transactivator genes, the central regions of the PLHV genomes were characterized by genome walking, sequence and splicing analysis. Three spliced genes were identified (ORF50, ORFA6/BZLF1(h), ORF57) encoding putative IE transactivators, homologous to (i) ORF50 and BRLF1/Rta, (ii) K8/K-bZIP and BZLF1/Zta and (iii) ORF57 and BMLF1 of HHV-8 and EBV, respectively. Expressed as myc-tag or HA-tag fusion proteins, they were located to the cellular nucleus. In reporter gene assays, several PLHV-promoters were mainly activated by PLHV-1 ORF50, to a lower level by PLHV-1 ORFA6/BZLF1(h) and not by PLHV-1 ORF57. However, the ORF57-encoded protein acted synergistically on ORF50-mediated activation.

Notch and Wnt signaling: mimicry and manipulation by gamma herpesviruses

A small number of fundamental cell signaling pathways are key to the regulation of proliferation and differentiation responses during normal development. Two of these pathways, the Notch and Wnt pathways, have proven to be attractive targets for virus interaction and manipulation. In general, viral gene expression and replication are intimately linked to the differentiation state of the infected cell and, in the case of the gamma herpesviruses, establishment of a lifelong persistent infection in the host is also dependent on the proliferative expansion of an infected B cell population. This review examines the ways in which the gamma herpesviruses Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) have exploited the Notch and Wnt pathways to advance their own life cycles. The virus-pathway interactions are compared with the mechanisms and outcome of cellular Notch and Wnt signaling.

Kaposi's sarcoma-associated herpesvirus K8beta is derived from a spliced intermediate of K8 pre-mRNA and antagonizes K8alpha (K-bZIP) to induce p21 and p53 and blocks K8alpha-CDK2 interaction

Kaposi's sarcoma-associated herpesvirus (KSHV) is a lymphotropic DNA tumor virus that induces Kaposi's sarcoma and AIDS-related primary effusion lymphoma. KSHV open reading frame 50 and K8 genes in early viral lytic infection express, respectively, a tricistronic and a bicistronic pre-mRNA, which undergo alternative splicing to create two major spliced mRNA isoforms, alpha and beta, by inclusion (beta) or exclusion (alpha) of an intron at nucleotides 75563 to 75645. This intron contains some suboptimal features, which cause the intron 5' splice site (ss) to interact weakly with U1 snRNA and the 3' ss to bind a U2 auxiliary factor, U2AF, with low affinity. Optimization of this intron in K8 (K8 intron 2) promoted the interaction of the 5' ss with U1 and the 3' ss with U2AF, resulting in a substantial increase in intron splicing. Splicing of K8 intron 2 has also been shown to be stimulated by the splicing of a downstream intron. This was confirmed by the insertion of a human beta-globin intron into the K8beta exon 3-exon 4 splice junction, which promoted splicing of K8beta intron 2 and conversion of the K8beta mRNA to the K8alpha mRNA that encodes a K-bZIP protein. Intron 2 contains a premature termination codon, yet the K8beta mRNA is insensitive to nonsense-mediated mRNA decay, suggesting that the truncated K8beta protein may have a biological function. Indeed, although the truncated K8beta protein is missing only a C-terminal leucine zipper domain from the K-bZIP, its expression antagonizes the ability of the K-bZIP to induce p53 and p21 and blocks K-bZIP-CDK2 interaction through interfering K8alpha mRNA production.

KSHV viral cyclin inactivates p27KIP1 through Ser10 and Thr187 phosphorylation in proliferating primary effusion lymphomas

Kaposi sarcoma herpesvirus (KSHV) infection is consistently associated with primary effusion lymphomas (PELs) that are non-Hodgkin lymphomas of B-cell origin. All PEL cells are latently infected with KSHV and express latent viral proteins such as the viral cyclin (v-cyclin), which has previously been implicated in down-regulation of cell-cycle inhibitor p27KIP1 levels via phosphorylation on Thr187. PEL cells retain high levels of p27KIP1 but yet proliferate actively, which has left the biologic significance of this p27KIP1 destabilization somewhat elusive. We have recently demonstrated that v-cyclin and p27KIP1 stably associate in PEL cells. Here we demonstrate that v-cyclin together with its kinase partner CDK6 phosphorylates the associated p27KIP1 in PEL cells, which represent a biologically relevant model system for KSHV pathobiology. During latent viral replication p27KIP1 was phosphorylated by v-cyclin-CDK6 predominantly on Ser10, which enhances its cytoplasmic localization. Interestingly, upon reactivation of KSHV lytic cycle, v-cyclin-CDK6 phosphorylated p27KIP1 on Thr187, which resulted in down-regulation of p27KIP1 protein levels. These findings indicate that v-cyclin modulates the cell-cycle inhibitory function of p27KIP1 by phosphorylation in PELs, and also suggest a novel role for v-cyclin in the lytic reactivation of KSHV. (Blood. 2006;107:725-732)

Kaposi's sarcoma-associated herpesvirus K-bZIP represses gene transcription via SUMO modification

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human gammaherpesvirus implicated in AIDS-related neoplasms. Previously, we demonstrated that the early lytic gene product K-bZIP is a transcriptional repressor that affects a subset of viral gene transcriptions mediated by the viral transactivator K-Rta (Y. Izumiya et al. J. Virol. 77:1441-1451, 2003). Sumoylation has emerged as an important posttranslational modification that affects the location and function of cellular and viral proteins and also plays a significant role in transcriptional repression along with Ubc9, the E2 SUMO conjugation enzyme. Here, we provide evidence that K-bZIP is sumoylated at the lysine 158 residue and associates with Ubc9 both in a cell-free system and in virus-infected BCBL-1 cells. Reporter assays showed that the expression of SUMO-specific protease 1 attenuated the transcriptional repression activity of K-bZIP. The expression of a K-bZIPK158R mutant, which was no longer sumoylated, exhibited the reduced transcriptional repression activity. This indicates that sumoylation plays an important part in the transcriptional repression activity of K-bZIP. Finally, chromatin immunoprecipitation experiments demonstrated that K-bZIP interacts with and recruits Ubc9 to specific KSHV promoters. Thus, our data indicate that K-bZIP is a SUMO adaptor, which recruits Ubc9 to specific viral target promoters, thereby exerting its transcriptional repression activity.

A nonclassic CCAAT enhancer element binding protein binding site contributes to alpha-methylacyl-CoA racemase expression in prostate cancer

Alpha-methylacyl-CoA racemase (AMACR), an enzyme involved in branched-chain fatty acid beta-oxidation that is normally expressed at high levels in human liver, is specifically and consistently overexpressed at both mRNA and protein levels in human prostate cancer and potentially other cancer types. To characterize the mechanisms underlying transcriptional regulation of AMACR at the genetic and epigenetic levels, we performed a series of methylation and reporter assays in prostate cancer tissues and cell lines. The results ruled out altered methylation patterns as the cause of overexpression in prostate cancer cells. However, promoter deletion analysis identified an 8-bp nonclassic CCAAT enhancer element located approximately 80 bp upstream of the transcriptional initiation site that is responsible for AMACR expression in both prostate cancer cell lines and cell lines of liver origin. Deletion or mutation of this element completely abolished AMACR promoter activity. Ectopic expression of CCAAT/enhancer binding protein beta increased luciferase activity driven by a wild-type AMACR promoter sequence but not by the sequence in which the putative CCAAT/enhancer binding protein binding element had been mutated. These results implicate a nonclassic CCAAT enhancer element in the AMACR gene promoter as playing a critical role in the regulation of AMACR gene expression.

Investigation of the multimerization region of the Kaposi's sarcoma-associated herpesvirus (human herpesvirus 8) protein K-bZIP: the proposed leucine zipper region encodes a multimerization domain with an unusual structure

The K8 gene of Kaposi's sarcoma-associated herpesvirus (human herpesvirus 8) shares many functional similarities with the BZLF1 gene of Epstein-Barr virus. The protein products of K8 and BZLF1, K-bZIP (RAP, K8) and Zta (BZLF1, ZEBRA, Z) have both been proposed to be members of the bZIP family of transcription factors, forming multimers via a coiled-coil motif termed a leucine zipper. Substantial evidence supporting this model for Zta is published. Here, we demonstrate that the proposed leucine zipper region of K-bZIP (amino acids 182 to 218) is required for multimer formation but that it does not fold as a coiled coil.

The targeting of primary effusion lymphoma cells for apoptosis by inducing lytic replication of human herpesvirus 8 while blocking virus production

Primary effusion lymphoma (PEL) is a B-cell lymphoma in which human herpesvirus-8 (HHV-8) is found within all tumor cells and represents a target for selectively destroying tumor cells. HHV-8 is latent in most PEL cells and, hence, resistant to antiviral agents that inhibit lytic replication. We demonstrate that PEL cell lines containing HHV-8 without and with coinfection with Epstein-Barr virus responded to the antiseizure medication valproate with entry into the lytic cascade and production of infectious virus. Minimal cell death occurred when noninfected BL-41 cells were incubated with valproate, whereas apoptosis occurred in response to valproate in PELs that supported lytic replication of HHV-8. The anti-viral agents ganciclovir and phosphonoformic acid (PFA) blocked valproate-induced production of infectious virus without blocking entry into the lytic cascade, and apoptosis occurred at levels that were as high as when virus production was not blocked. Ganciclovir and PFA also prevented most valproate-induced expression of the late lytic viral transcript open reading frame 26 (ORF-26), but they did not block the induction of either viral interleukin-6 (vIL-6) or viral G protein-coupled receptor (vGPCR). These studies provide evidence that incubation of PELs with valproate in the presence of ganciclovir or PFA can selectively target tumor cells for apoptosis without increasing viral load.

Inhibition of infectious human herpesvirus 8 production by gamma interferon and alpha interferon in BCBL-1 cells

Human herpesvirus-8 (HHV-8) is aetiologically linked to Kaposi's sarcoma and primary effusion lymphoma. Although interferon-alpha (IFN-alpha) and interferon-gamma (IFN-gamma) are both antiviral cytokines, IFN-alpha blocks entry of HHV-8 into the lytic phase, whereas IFN-gamma induces an increase in the percentage of cells undergoing lytic replication. Multiple events in the lytic cascade must be completed to produce infectious virus. The ability of both types of IFN to affect the production of infectious virus was explored. Both IFN-alpha and IFN-gamma induced expression of the antiviral proteins double-stranded RNA-activated protein kinase (PKR) and 2'5'-oligoadenylate synthetase (2'5'-OAS) in HHV-8-infected BCBL-1 cells. Higher levels resulted from incubation with IFN-alpha than with IFN-gamma, whereas IFN-gamma induced higher levels of IRF-1 than did IFN-alpha. IFN-gamma induced a minor increase in lytic viral gene expression, which was not accompanied by a detectable increase in infectious virus. When lytic replication of HHV-8 was induced using TPA, high levels of infectious virus appeared in the conditioned medium. When IFN-gamma was present during TPA stimulation, the production of infectious virus was reduced by at least a 60 %, and IFN-alpha fully blocked TPA-induced production of infectious virus. The greater reduction of viral production that occurred with IFN-alpha is consistent with the higher levels of the antiviral proteins PKR and 2'5'-OAS induced by IFN-alpha than by IFN-gamma. These studies indicate that the augmentation of cellular antiviral defences by IFN-gamma was sufficient to prevent production of infectious virus despite IFN-gamma-induced entry of some cells into the lytic phase of HHV-8 replication.

Viral interactions with the Notch pathway

The Notch signaling pathway influences cell fate decisions, proliferation versus differentiation and cell survival. Viruses both utilize and manipulate the differentiation state of infected cells, promote or block cell cycling and employ a variety of mechanisms to evade innate cellular anti-viral responses and promote cell survival. In light of these commonalities, it is perhaps not surprising that several viruses have tapped into the Notch pathway to advance their own life cycles. This first became apparent from studies showing targeting of Epstein-Barr virus proteins to the nuclear effector of Notch signaling CSL (CBF1/RBPJk). More recently the Kaposi's sarcoma-associated herpesvirus RTA protein has been found to bind CSL. Notch pathway interactions have also been described for adenovirus SV40 and human papilloma virus. This review focuses on the herpesvirus protein interactions with the Notch pathway and the insights that these interactions have provided.

Role of protein kinase C delta in reactivation of Kaposi's sarcoma-associated herpesvirus

TPA (12-O-tetradecanoylphorbol-13-acetate), a well-known activator of protein kinase C (PKC), can experimentally induce reactivation of Kaposi's sarcoma-associated herpesvirus (KSHV) in certain latently infected cells. We selectively blocked the activity of PKC isoforms by using GF 109203X or rottlerin and demonstrated that this inhibition largely decreased lytic KSHV reactivation by TPA. Translocation of the PKCdelta isoform was evident shortly after TPA stimulation. Overexpression of the dominant-negative PKCdelta mutant supported an essential role for the PKCdelta isoform in virus reactivation, yet overexpression of PKCdelta alone was not sufficient to induce lytic reactivation of KSHV, suggesting that additional signaling molecules participate in this pathway.

Differential regulation of K8 gene expression in immediate-early and delayed-early stages of Kaposi's sarcoma-associated herpesvirus

KSHV-encoded bZip protein, namely K8, is a regulatory protein with multiple functions in immediate-early and delayed-early stages of viral life cycle. Here we report that K8 gene is expressed in both immediate-early and delayed-early phases and the transcription in different phases is controlled by distinct promoters, yielding two transcripts, an immediate-early mRNA of 1.5 kb and a delayed-early mRNA of 1.3 kb. The transcription from the immediate-early promoter is inducible by sodium butyrate or 12-O-tetradecanoylphorbol-13-acetate (TPA), but not responsive to ORF50 (Rta). The delayed-early promoter of K8 shows little response to sodium butyrate and TPA, but can be fully induced by ORF50 (Rta). In the immediate-early promoter, a 20-bp region containing two consensus Sp-1 binding sites was found to be crucial for the basal transcription and sodium butyrate induction of the promoter. In addition, mutagenesis analyses identified three ORF50 (Rta) responsive elements (RREs) in the delayed-early promoter and their roles in ORF50-dependent transactivation were investigated. The differential regulation of K8 gene expression may represent a strategy in that the virus fine-tunes the levels of K8 protein in different stages or for distinct functions. The elucidation of K8 gene expression regulation helps in understanding roles of K8 in viral replication and pathogenicity.