Characterization of the human herpesvirus 8 (Kaposi's sarcoma-associated herpesvirus) oncogene, kaposin (ORF K12)

Unearthing the role of septins in viral infections

Septin proteins are a subfamily of closely related GTP-binding proteins conserved in all species except for higher plants and perform essential biological processes. Septins self-assemble into heptameric or octameric complexes and form higher-order structures such as filaments, rings, or gauzes by end-to-end binding. Their close association with cell membrane components makes them central in regulating critical cellular processes. Due to their organisation and properties, septins function as diffusion barriers and are integral in providing scaffolding to support the membrane's curvature and stability of its components. Septins are also involved in vesicle transport and exocytosis through the plasma membrane by co-localising with exocyst protein complexes. Recently, there have been emerging reports of several human and animal diseases linked to septins and abnormalities in their functions. Most of our understanding of the significance of septins during microbial diseases mainly pertains to their roles in bacterial infections but not viruses. This present review focuses on the known roles of septins in host-viral interactions as detailed by various studies.

Novel Functions and Virus-Host Interactions Implicated in Pathogenesis and Replication of Human Herpesvirus 8

Human herpesvirus 8 (HHV-8) is classified as a γ2-herpesvirus and is related to Epstein-Barr virus (EBV), a γ1-herpesvirus. One important aspect of the γ-herpesviruses is their association with neoplasia, either naturally or in animal model systems. HHV-8 is associated with B-cell-derived primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD), endothelial-derived Kaposi's sarcoma (KS), and KSHV inflammatory cytokine syndrome (KICS). EBV is also associated with a number of B-cell malignancies, such as Burkitt's lymphoma, Hodgkin's lymphoma, and posttransplant lymphoproliferative disease, in addition to epithelial nasopharyngeal and gastric carcinomas. Despite the similarities between these viruses and their associated malignancies, the particular protein functions and activities involved in key aspects of virus biology and neoplastic transformation appear to be quite distinct. Indeed, HHV-8 specifies a number of proteins for which counterparts had not previously been identified in EBV, other herpesviruses, or even viruses in general, and these proteins are believed to play vital functions in virus biology and to be involved centrally in viral pathogenesis. Additionally, a set of microRNAs encoded by HHV-8 appears to modulate the expression of multiple host proteins to provide conditions conductive to virus persistence within the host and possibly contributing to HHV-8-induced neoplasia. Here, we review the molecular biology underlying these novel virus-host interactions and their potential roles in both virus biology and virus-associated disease.

Altering the Anti-inflammatory Lipoxin Microenvironment: a New Insight into Kaposi's Sarcoma-Associated Herpesvirus Pathogenesis

Lipoxins are host anti-inflammatory molecules that play a vital role in restoring tissue homeostasis. The efficacy of lipoxins and their analog epilipoxins in treating inflammation and its associated diseases has been well documented. Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL) are two well-known inflammation related diseases caused by Kaposi's sarcoma-associated herpesvirus (KSHV). Controlling inflammation is one of the strategies adopted to treat KS and PEL, a primary motivation for exploring and evaluating the therapeutic potential of using lipoxins. This study documents how KSHV manipulates and downregulates the secretion of the anti-inflammatory lipoxin A4 in host cells and the viral factors involved in this process using in vitro KS and PEL cells as models. The presence of the lipoxin A4 receptor/formyl peptidyl receptor (ALX/FPR) in KS patient tissue sections and in vitro KS and PEL cell models offers a novel possibility for treating KS and PEL with lipoxins. Treating de novo KSHV-infected endothelial cells with lipoxin and epilipoxin creates an anti-inflammatory environment by decreasing the levels of NF-κB, AKT, ERK1/2, COX-2, and 5-lipoxygenase. Lipoxin treatment on CRISPR/CAS9 technology-mediated ALX/FPR gene deletion revealed the importance of the lipoxin receptor ALX for effective lipoxin signaling. A viral microRNA (miRNA) cluster was identified as the primary factor contributing to the downregulation of lipoxin A4 secretion in host cells. The KSHV miRNA cluster probably targets enzyme 15-lipoxygenase, which is involved in lipoxin A4 synthesis. This study provides a new insight into the potential treatment of KS and PEL using nature's own anti-inflammatory molecule, lipoxin.

IMPORTANCE

KSHV infection has been shown to upregulate several host proinflammatory factors, which aid in its survival and pathogenesis. The influence of KSHV infection on anti-inflammatory molecules is not well studied. Since current treatment methods for KS and PEL are fraught with unwanted side effects and low efficiency, the search for new therapeutics is therefore imperative. The use of nature's own molecule lipoxin as a drug is promising. This study opens up new domains in KSHV research focusing on how the virus modulates lipoxin secretion and warrants further investigation of the therapeutic potential of lipoxin using in vitro cell models for KS and PEL.

Lipoxins exert antiangiogenic and anti-inflammatory effects on Kaposi's sarcoma cells

Lipoxin A4 (LXA4) is an endogenously produced host molecule with anti-inflammatory resolution effects. Previous studies demonstrated it to be involved in anti-vascular endothelial growth factor (VEGF)-mediated angiogenesis and in a possible anticancer role via interaction with its receptor, lipoxin A 4 receptor (ALXR). Here, we examined the effects of LXA4 and its epimer 15-epi-LXA4 in inhibiting proinflammatory and angiogenic functions in a human Kaposi's sarcoma tumor-derived cell line (KS-IMM). KS-IMM cells expressed increased levels of inflammatory cyclooxygenase 2 (COX-2) and 5-lipoxygenase (5-LO) pathway enzymes when compared with human microvascular dermal endothelial cells (HMVEC-d). KS-IMM cells secreted high levels of prostaglandin E2 (PGE2) and chemotactic leukotriene B4 (LTB4). Treatment with LXA4 or 15-epi-LXA4 effectively reduced the levels of COX-2, 5-LO proteins, and secretion of PGE2 and LTB4 in KS-IMM cells. LXA4 or 15-epi-LXA4 treatment also decreased secretion of proinflammatory interleukin 6 (IL-6) and IL-8 cytokines but induced the secretion of anti-inflammatory IL-10. LXA4 treatment reduced the phosphorylation of VEGF receptor (VEGFR) and ephrin family receptor tyrosine kinases. LXA4 treatment effectively induced dephosphorylation of multiple cellular kinases such as Focal Adhesion Kinase, Protein kinase B, nuclear factor kappa-light-chain-enhancer of activated B cells, and Extracellular signal-regulated kinases (ERK)1/2, and reduced angiogenic factor VEGF-C secretion in KS cells. LX treatment drastically induced the Src-homology 2 domain-containing phosphatase tyrosine (Y542) phosphatase and reduced VEGFR-2 phosphorylation at sites Y1059, Y1175, and Y1212. Treatment of KS-IMM cells with LXA4 resulted in selective localization of VEGFR-2 in nonlipid raft (non-LR) and ALXR to LR fractions. These results demonstrated that LXA4 or 15-epi-LXA4 induce anti-inflammatory and antiangiogenic effects in KS cells and suggest that treatment with LXs is an attractive novel strategy against KS.

Chromatinization of the KSHV Genome During the KSHV Life Cycle

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

MicroRNA-mediated transformation by the Kaposi's sarcoma-associated herpesvirus Kaposin locus

The human oncogenic Kaposi's sarcoma-associated herpesvirus (KSHV) expresses a set of ∼20 viral microRNAs (miRNAs). miR-K10a stands out among these miRNAs because its entire stem-loop precursor overlaps the coding sequence for the Kaposin (Kap) A/C proteins. The ectopic expression of KapA has been reported to lead to transformation of rodent fibroblasts. However, these experiments inadvertently also introduced miR-K10a, which raises the question whether the transforming activity of the locus could in fact be due to miR-K10a expression. To answer this question, we have uncoupled miR-K10a and KapA expression. Our experiments revealed that miR-K10a alone transformed cells with an efficiency similar to that when it was coexpressed with KapA. Maintenance of the transformed phenotype was conditional upon continued miR-K10a but not KapA protein expression, consistent with its dependence on miRNA-mediated changes in gene expression. Importantly, miR-K10a taps into an evolutionarily conserved network of miR-142-3p targets, several of which are expressed in 3T3 cells and are also known inhibitors of cellular transformation. In summary, our studies of miR-K10a serve as an example of an unsuspected function of an mRNA whose precursor is embedded within a coding transcript. In addition, our identification of conserved miR-K10a targets that limit transformation will point the way to a better understanding of the role of this miRNA in KSHV-associated tumors.

IMPORTANCE

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human tumor virus. The viral Kaposin locus has known oncogenic potential, which has previously been attributed to the encoded KapA protein. Here we show that the virally encoded miR-K10a miRNA, whose precursor overlaps the KapA-coding region, may account for the oncogenic properties of this locus. Our data suggest that miR-K10a mimics the cellular miRNA miR-142-3p and thereby represses several known inhibitors of oncogenic transformation. Our work demonstrates that functional properties attributed to a coding region may in fact be carried out by an embedded noncoding element and sheds light on the functions of viral miR-K10a.

Glutamate secretion and metabotropic glutamate receptor 1 expression during Kaposi's sarcoma-associated herpesvirus infection promotes cell proliferation

Kaposi's sarcoma associated herpesvirus (KSHV) is etiologically associated with endothelial Kaposi's sarcoma (KS) and B-cell proliferative primary effusion lymphoma (PEL), common malignancies seen in immunocompromised HIV-1 infected patients. The progression of these cancers occurs by the proliferation of cells latently infected with KSHV, which is highly dependent on autocrine and paracrine factors secreted from the infected cells. Glutamate and glutamate receptors have emerged as key regulators of intracellular signaling pathways and cell proliferation. However, whether they play any role in the pathological changes associated with virus induced oncogenesis is not known. Here, we report the first systematic study of the role of glutamate and its metabotropic glutamate receptor 1 (mGluR1) in KSHV infected cell proliferation. Our studies show increased glutamate secretion and glutaminase expression during de novo KSHV infection of endothelial cells as well as in KSHV latently infected endothelial and B-cells. Increased mGluR1 expression was detected in KSHV infected KS and PEL tissue sections. Increased c-Myc and glutaminase expression in the infected cells was mediated by KSHV latency associated nuclear antigen 1 (LANA-1). In addition, mGluR1 expression regulating host RE-1 silencing transcription factor/neuron restrictive silencer factor (REST/NRSF) was retained in the cytoplasm of infected cells. KSHV latent protein Kaposin A was also involved in the over expression of mGluR1 by interacting with REST in the cytoplasm of infected cells and by regulating the phosphorylation of REST and interaction with β-TRCP for ubiquitination. Colocalization of Kaposin A with REST was also observed in KS and PEL tissue samples. KSHV infected cell proliferation was significantly inhibited by glutamate release inhibitor and mGluR1 antagonists. These studies demonstrated that elevated glutamate secretion and mGluR1 expression play a role in KSHV induced cell proliferation and suggest that targeting glutamate and mGluR1 is an attractive therapeutic strategy to effectively control the KSHV associated malignancies.

Kaposi's sarcoma associated herpesvirus (KSHV), prevalent in immunosuppressed HIV infected individuals and transplant recipients, is etiologically associated with cancers such as endothelial Kaposi's sarcoma (KS) and B-cell primary effusion lymphoma (PEL). Both KS and PEL develop from the unlimited proliferation of KSHV infected cells. Increased secretion of various host cytokines and growth factors, and the activation of their corresponding receptors, are shown to be contributing to the proliferation of KSHV latently infected cells. Glutamate, a neurotransmitter, is also involved in several cellular events including cell proliferation. In the present study, we report that KSHV-infected latent cells induce the secretion of glutamate and activation of metabotropic glutamate receptor 1 (mGluR1), and KSHV latency associated LANA-1 and Kaposin A proteins are involved in glutaminase and mGluR1 expression. Our functional analysis showed that elevated secretion of glutamate and mGluR1 activation is linked to increased proliferation of KSHV infected cells and glutamate release inhibitor and glutamate receptor antagonists blocked the proliferation of KSHV infected cells. These studies show that proliferation of cancer cells latently infected with KSHV in part depends upon glutamate and glutamate receptor and therefore could potentially be used as therapeutic targets for the control and elimination of KSHV associated cancers.

Molecular biology of human herpesvirus 8: novel functions and virus-host interactions implicated in viral pathogenesis and replication

Human herpesvirus 8 (HHV-8), also known as Kaposi's sarcoma-associated herpesvirus (KSHV), is the second identified human gammaherpesvirus. Like its relative Epstein-Barr virus, HHV-8 is linked to B-cell tumors, specifically primary effusion lymphoma and multicentric Castleman's disease, in addition to endothelial-derived KS. HHV-8 is unusual in its possession of a plethora of "accessory" genes and encoded proteins in addition to the core, conserved herpesvirus and gammaherpesvirus genes that are necessary for basic biological functions of these viruses. The HHV-8 accessory proteins specify not only activities deducible from their cellular protein homologies but also novel, unsuspected activities that have revealed new mechanisms of virus-host interaction that serve virus replication or latency and may contribute to the development and progression of virus-associated neoplasia. These proteins include viral interleukin-6 (vIL-6), viral chemokines (vCCLs), viral G protein-coupled receptor (vGPCR), viral interferon regulatory factors (vIRFs), and viral antiapoptotic proteins homologous to FLICE (FADD-like IL-1β converting enzyme)-inhibitory protein (FLIP) and survivin. Other HHV-8 proteins, such as signaling membrane receptors encoded by open reading frames K1 and K15, also interact with host mechanisms in unique ways and have been implicated in viral pathogenesis. Additionally, a set of micro-RNAs encoded by HHV-8 appear to modulate expression of multiple host proteins to provide conditions conducive to virus persistence within the host and could also contribute to HHV-8-induced neoplasia. Here, we review the molecular biology underlying these novel virus-host interactions and their potential roles in both virus biology and virus-associated disease.

Role of the Crosstalk between Autophagy and Apoptosis in Cancer

Autophagy and apoptosis are catabolic pathways essential for organismal homeostasis. Autophagy is normally a cell-survival pathway involving the degradation and recycling of obsolete, damaged, or harmful macromolecular assemblies; however, excess autophagy has been implicated in type II cell death. Apoptosis is the canonical programmed cell death pathway. Autophagy and apoptosis have now been shown to be interconnected by several molecular nodes of crosstalk, enabling the coordinate regulation of degradation by these pathways. Normally, autophagy and apoptosis are both tumor suppressor pathways. Autophagy fulfils this role as it facilitates the degradation of oncogenic molecules, preventing development of cancers, while apoptosis prevents the survival of cancer cells. Consequently, defective or inadequate levels of either autophagy or apoptosis can lead to cancer. However, autophagy appears to have a dual role in cancer, as it has now been shown that autophagy also facilitates the survival of tumor cells in stress conditions such as hypoxic or low-nutrition environments. Here we review the multiple molecular mechanisms of coordination of autophagy and apoptosis and the role of the proteins involved in this crosstalk in cancer. A comprehensive understanding of the interconnectivity of autophagy and apoptosis is essential for the development of effective cancer therapeutics.

Preparation and application of polyclonal antibodiesagainst KSHV v-cyclin

We prepared rabbit polyclonal antibodies against Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded v-cyclin (ORF 72) and detected the natural viral protein using these polyclonal antibodies. Three antigenic polypeptides of v-cyclin were designed and synthesized. A fragment of the v-cyclin gene was cloned into a eukaryotic expression vector pEF-MCS-Flag-IRES/Puro to construct a recombinant vector, pEF v-cyclin. Then, pEF v-cyclin was transfected into 293T and EA.hy926 cells to obtain v-cyclin-Flag fusion proteins. Six New Zealand white rabbits were immunized with KLH-conjugated peptides to generate polyclonal antibodies against v-cyclin. The polyclonal antibodies were then characterized by ELISA and Western blotting assays. Finally, the polyclonal antibodies against v-cyclin were used to detect natural viral protein expressed in BCBL-1, BC-3, and JSC-1 cells. The results showed that using the Flag antibody, v-cyclin-Flag fusion protein was detected in 293T and EA.hy926 cells transfected with pEF-v-cyclin. Furthermore, ELISA showed that the titer of the induced polyclonal rabbit anti-v-cyclin antibodies was higher than 1:8,000. In Western blotting assays, the antibodies reacted specifically with the v-cyclin-Flag fusion protein as well as the natural viral protein. The recombinant expression vector pEF-v-cyclin was constructed successfully, and the polyclonal antibodies prepared can be used for various biological tests including ELISA and Western blotting assays.

A-to-I RNA editing: the "ADAR" side of human cancer

Carcinogenesis is a complex, multi-stage process depending on both endogenous and exogenous factors. In the past years, DNA mutations provided important clues to the comprehension of the molecular pathways involved in numerous cancers. Recently, post-transcriptional modification events, such as RNA editing, are emerging as new players in several human diseases, including tumours. A-to-I RNA editing changes the nucleotide sequence of target RNAs, introducing A-to-I/G "mutations". Since ADAR enzymes catalyse this nucleotide conversion, their expression/activity is essential and finely regulated in normal cells. This review summarizes the available knowledge on A-to-I RNA editing in the cancer field, giving a new view on how ADARs may play a role in carcinogenesis.

Horizontal gene transfers with or without cell fusions in all categories of the living matter

This article reviews the history of widespread exchanges of genetic segments initiated over 3 billion years ago, to be part of their life style, by sphero-protoplastic cells, the ancestors of archaea, prokaryota, and eukaryota. These primordial cells shared a hostile anaerobic and overheated environment and competed for survival. "Coexist with, or subdue and conquer, expropriate its most useful possessions, or symbiose with it, your competitor" remain cellular life's basic rules. This author emphasizes the role of viruses, both in mediating cell fusions, such as the formation of the first eukaryotic cell(s) from a united crenarchaeon and prokaryota, and the transfer of host cell genes integrated into viral (phages) genomes. After rising above the Darwinian threshold, rigid rules of speciation and vertical inheritance in the three domains of life were established, but horizontal gene transfers with or without cell fusions were never abolished. The author proves with extensive, yet highly selective documentation, that not only unicellular microorganisms, but the most complex multicellular entities of the highest ranks resort to, and practice, cell fusions, and donate and accept horizontally (laterally) transferred genes. Cell fusions and horizontally exchanged genetic materials remain the fundamental attributes and inherent characteristics of the living matter, whether occurring accidentally or sought after intentionally. These events occur to cells stagnating for some 3 milliard years at a lower yet amazingly sophisticated level of evolution, and to cells achieving the highest degree of differentiation, and thus functioning in dependence on the support of a most advanced multicellular host, like those of the human brain. No living cell is completely exempt from gene drains or gene insertions.

Human immunodeficiency virus type 1 Tat accelerates Kaposi sarcoma-associated herpesvirus Kaposin A-mediated tumorigenesis of transformed fibroblasts in vitro as well as in nude and immunocompetent mice

Kaposi sarcoma-associated herpesvirus (KSHV) is necessary but not sufficient to cause Kaposi sarcoma (KS). Coinfection with human immunodeficiency virus type 1 (HIV-1), in the absence of antiretroviral suppressive therapy, drastically increases the risk of KS. Previously, we identified that HIV-1 transactivative transcription protein (Tat) was an important cofactor that activated lytic cycle replication of KSHV. Here, we further investigated the potential of Tat to influence tumorigenesis induced by KSHV Kaposin A, a product of KSHV that was encoded by the open reading frame K12 (a KSHV-transforming gene). By using colony formation in soft agar, (3)H-TdR incorporation, cell cycle, and microarray gene expression analyses, we demonstrated that Tat enhanced proliferation as well as mitogen-activated protein kinase, signal transducer and activator of transcription 3, and phosphatidylinositol 3-kinase/protein kinase B signaling induced by Kaposin A in NIH3T3 cells. Animal experiments further demonstrated that Tat accelerated tumorigenesis by Kaposin A in athymic nu/nu mice. Cells obtained from primary tumors of nude mice succeeded inducing tumors in immunocompetent mice. These data suggest that Tat can accelerate tumorigenesis induced by Kaposin A. Our data present the first line of evidence that Tat may participate in KS pathogenesis by collaborating with Kaposin A in acquired immunodeficiency syndrome (AIDS)-related KS (AIDS-KS) patients. Our data also suggest that the model for Kaposin and Tat-mediated oncogenesis will contribute to our understanding of the pathogenesis of AIDS-KS at the molecular level and may even be important in exploring a novel therapeutic method for AIDS-KS.

Kaposi sarcoma: a continuing conundrum

Kaposi sarcoma (KS) remains a challenge. Its classic or Mediterranean form tends to be benign. In transplant recipients it may be less so. As part of the AIDS pandemic, of which it was an original defining component, it may be life-threatening. It is due to human herpesvirus-8, which is necessary but not sufficient to produce the disease. KS has a low prevalence in the general population of the United States and United Kingdom, with an intermediate rate in Italy and Greece, and a high one in parts of Africa. In Italy, hot spots include its southern regions, the Po River Valley, and Sardinia, possibly related to a high density of blood-sucking insects. An important challenge is to treat KS patients without immunocompromising them. The potential of effective anti-herpes virus therapy and the use of sirolimus in transplantation recipients have added new opportunities for KS prevention.

LEARNING OBJECTIVES

At the conclusion of this learning activity, participants should be able to provide the most recent information about Kaposi sarcoma in the context in which it occurs. Its classic or Mediterranean form, its pattern in transplant recipients and others iatrogenically immunosuppressed, and its occurrence as a potentially life-threatening part of the AIDS pandemic will be stressed. Its etiology and transmission will be discussed in detail to facilitate understanding of Kaposi sarcoma and of human herpesvirus-8 infection in the general population of the United States and United Kingdom, in Italy and Greece, and in certain parts of Africa. Its therapy, including the concept of doing it without immunocompromising the patient, will be stressed. New opportunities for Kaposi sarcoma prevention will also be discussed.

RNA editing of the human herpesvirus 8 kaposin transcript eliminates its transforming activity and is induced during lytic replication

Human herpesvirus 8 is the etiologic agent associated with Kaposi's sarcoma and primary effusion lymphoma (PEL). The K12 RNA, which produces as many as three variants of the kaposin protein, as well as a microRNA, is the most abundant transcript expressed in latent Kaposi's sarcoma-associated herpesvirus infection, and yet it is also induced during lytic replication. The portion of the transcript that includes the microRNA and the kaposin A sequence has been shown to have tumorigenic potential. Genome coordinate 117990, which is within this transcript, has been found to be heterogeneous, primarily in RNAs but also among viral DNA sequences. This sequence heterogeneity affects an amino acid in kaposins A and C and the microRNA. The functional effects of this sequence heterogeneity have not been studied, and its origin has not been definitively settled; both RNA editing and heterogeneity at the level of the viral genome have been proposed. Here, we show that transcripts containing A at position 117990 are tumorigenic, while those with G at this position are not. Using a highly sensitive quantitative assay, we observed that, in PEL cells under conditions where more than 60% of cDNAs derived from K12 RNA transcripts have G at coordinate 117990, there is no detectable G in the viral DNA sequence at this position, only A. This result is consistent with RNA editing by one of the host RNA adenosine deaminases (ADARs). Indeed, we observed that purified human ADAR1 efficiently edits K12 RNA in vitro. Remarkably, the amount of editing correlated with the replicative state of the virus; editing levels were nearly 10-fold higher in cells treated to induce lytic viral replication. These results suggest that RNA editing controls the function of one segment of the kaposin transcript, such that it has transforming activity during latent replication and possibly another, as-yet-undetermined, function during lytic replication.

Identification of a novel septin 4 protein binding to human herpesvirus 8 kaposin A protein using a phage display cDNA library

Human herpesvirus 8 (HHV-8) is associated with the development of Kaposi's sarcoma and several other human malignancies. Kaposin A protein of HHV-8 has been demonstrated as inducing tumorigenic transformation, being responsible for nuclear receptor coactivators in the transforming activity. In this study, a kaposin A-interacting septin 4 variant that contained the unique GDR at the N-terminus and AAALE at the C-terminus was identified using affinity selection of a phage display library. Co-immunoprecipitation and confocal imaging revealed in vitro binding specificity and in vivo co-localization of HHV-8 kaposin A protein to the septin 4 variant. The kaposin A-interacting septin 4 variant induced cell rounding up, activated caspase-3, and up-regulated transcriptional factor NF-kappaB. By contrast, kaposin A protein showed an antagonistic effect on the biological functions of the septin 4 variant. Therefore, the interaction of kaposin A protein and the septin 4 variant was suggested as playing a possible role in the development of HHV-8-associated malignancies. This study provides insights into the mechanism of the kaposin A protein pathology, in which the interactions of kaposin A protein with cellular proteins might allow alteration of fundamental cellular processes.

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.

Endothelial cell- and lymphocyte-based in vitro systems for understanding KSHV biology

Kaposi sarcoma (KS), the most common AIDS-associated malignancy, is a multifocal tumor characterized by deregulated angiogenesis, proliferation of spindle cells, and extravasation of inflammatory cells and erythrocytes. Kaposi sarcoma-associated herpesvirus (KSHV; also human herpesvirus-8) is implicated in all clinical forms of KS. Endothelial cells (EC) harbor the KSHV genome in vivo, are permissive for virus infection in vitro, and are thought to be the precursors of KS spindle cells. Spindle cells are rare in early patch-stage KS lesions but become the predominant cell type in later plaque- and nodular-stage lesions. Alterations in endothelial/spindle cell physiology that promote proliferation and survival are thus thought to be important in disease progression and may represent potential therapeutic targets. KSHV encodes genes that stimulate cellular proliferation and migration, prevent apoptosis, and counter the host immune response. The combined effect of these genes is thought to drive the proliferation and survival of infected spindle cells and influence the lesional microenvironment. Large-scale gene expression analyses have revealed that KSHV infection also induces dramatic reprogramming of the EC transcriptome. These changes in cellular gene expression likely contribute to the development of the KS lesion. In addition to KS, KSHV is also present in B cell neoplasias including primary effusion lymphoma and multicentric Castleman disease. A combination of virus and virus-induced host factors are similarly thought to contribute to establishment and progression of these malignancies. A number of lymphocyte- and EC-based systems have been developed that afford some insight into the means by which KSHV contributes to malignant transformation of host cells. Whereas KSHV is well maintained in PEL cells cultured in vitro, explanted spindle cells rapidly lose the viral episome. Thus, endothelial cell-based systems for studying KSHV gene expression and function, as well as the effect of infection on host cell physiology, have required in vitro infection of primary or life-extended EC. This chapter includes a review of these in vitro cell culture systems, acknowledging their strengths and weaknesses and putting into perspective how each has contributed to our understanding of the complex KS lesional environment. In addition, we present a model of KS lesion progression based on findings culled from these models as well as recent clinical advances in KS chemotherapy. Thus this unifying model describes our current understanding of KS pathogenesis by drawing together multiple theories of KS progression that by themselves cannot account for the complexities of tumor development.

Regulation of KSHV lytic gene expression

The life cycle of KSHV, latency versus lytic replication, is mainly determined at the transcriptional regulation level. A viral immediate-early gene product, replication and transcription activator (RTA), has been identified as the molecular switch for initiation of the lytic gene expression program from latency. Here we review progress on two key questions: how RTA gene expression is controlled by viral proteins and cellular signals and how RTA regulates the expression of downstream viral genes. We summarize the interactions of RTA with cellular and other viral proteins. We also discuss critical issues that must be addressed in the near future.

Transcripts encoding K12, v-FLIP, v-cyclin, and the microRNA cluster of Kaposi's sarcoma-associated herpesvirus originate from a common promoter

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of three malignancies associated with AIDS and immunosuppression. Tumor cells harbor latent virus and express kaposin (open reading frame [ORF] K12), v-FLIP (ORF 71), v-Cyclin (ORF 72), and latency-associated nuclear antigen (LANA; ORF 73). ORFs 71 to 73 are transcribed as multicistronic RNAs initiating from adjacent constitutive and inducible promoters upstream of ORF 73. Here we characterize a third promoter embedded within the ORF 71-to-73 transcription unit specifying transcripts that encode ORF 71/72 or K12. These transcripts may also be the source of 11 microRNAs arranged as a cluster between K12 and ORF 71. Our studies reveal a complex arrangement of interlaced transcription units, incorporating four important protein-encoding genes required for latency and pathogenesis and the entire KSHV microRNA repertoire.

Murine gamma-herpesvirus 68 latency protein M2 binds to Vav signaling proteins and inhibits B-cell receptor-induced cell cycle arrest and apoptosis in WEHI-231 B cells

The MHV-68 latent protein, M2, does not have homology to any known viral or cellular proteins, and its function is unclear. To define the role played by M2 during MHV-68 latency as well as the molecular mechanism involved, we used M2 as bait to screen a yeast two-hybrid mouse B-cell cDNA library. Vav1 was identified as an M2-interacting protein in two independent screenings. Subsequent yeast two-hybrid interaction studies showed that M2 also binds to Vav2, but not Vav3, and that three "PXXP" motifs located at the C terminus of M2 are important for this interaction. The interactions between M2 and Vav proteins were also confirmed in vivo in 293T and WEHI-231 B-cells by co-immunoprecipitation assays. Rac1/GST-PAK "pull-down" experiments and Western blot analysis using a phospho-Vav antibody demonstrated that expression of M2 in WEHI-231 cells enhances Vav activity. We further showed in WEHI-231 cells that M2 expression promotes proliferation and survival and is associated with enhanced cyclin D2 and repressed p27(Kip1), p130, and Bim expression. Taken together, these experiments suggest that M2 might have an important role in disseminating the latent virus during the establishment and maintenance of latency by modulating B-cell receptor-mediated signaling events through Vav to promote B-cell activation, proliferation, and survival.

Targeting human herpesvirus-8 for treatment of Kaposi??s sarcoma and primary effusion lymphoma

PURPOSE OF REVIEW

Human herpesvirus-8, also called the Kaposi's sarcoma herpesvirus, is present in all cases of Kaposi's sarcoma and primary effusion lymphoma and in some cases of multicentric Castleman's disease. This review discusses mechanisms by which human herpesvirus-8 contributes to tumorigenesis and how this knowledge can be used to target the virus for the treatment of these tumors.

RECENT FINDINGS

Most primary effusion lymphomas and Kaposi's sarcoma tumor cells are latently infected with human herpesvirus-8 and hence resistant to antiherpesvirus drugs that are dependent on lytic replication. In contrast, many of the cells infected with human herpesvirus-8 in multicentric Castleman's disease support lytic replication, so that clinical improvement frequently occurs in response to treatment with antiherpesvirus drugs. The resistance of latently-infected tumor cells to antiherpesvirus drugs can be overcome by inducing human herpesvirus-8 to reenter the lytic cascade in the presence of antiherpesvirus drugs. This leads to apoptosis of virally infected cells without increasing production of infectious virus. Alternatively, the replication and maintenance of the human herpesvirus-8 episome during latency can be disrupted by glycyrrhizic acid or hydroxyurea so that the virus no longer contributes to tumorigenesis. Both the innate and acquired immune systems can also be augmented to help prevent or treat human herpesvirus-8-associated tumors.

SUMMARY

Novel strategies targeting human herpesvirus-8, which is present in all cases of Kaposi's sarcoma and primary effusion lymphoma, provide opportunities for selectively killing tumor cells.

Kaposi's Sarcoma-associated Herpesvirus Activation of Vascular Endothelial Growth Factor Receptor 3 Alters Endothelial Function and Enhances Infection

Kaposi's sarcoma-associated herpesvirus (KSHV; also known as human herpesvirus 8) is the etiologic agent of Kaposi's sarcoma, an endothelial neoplasm. This gamma-herpesvirus encodes for several unique proteins that alter target cell function, including the virion envelope-associated glycoprotein B (gB). Glycoprotein B has an RGD (Arg-Gly-Asp) motif at the extracellular amino terminus region and binds to the alpha3beta1 surface integrin, which enhances virus entry. We now report that gB can activate the vascular endothelial growth factor receptor 3 (VEGFR-3) on the surface of microvascular endothelial cells and trigger receptor signaling, which can modulate endothelial migration and proliferation. Furthermore, we observed that VEGFR-3 expression and activation enhance KSHV infection and participate in KSHV-mediated transformation. These functional changes in the endothelium may contribute to the pathogenesis of Kaposi's sarcoma and suggest that interventions that inhibit gB activation of VEGFR-3 could be useful in the treatment of this neoplasm.

Mutational analysis reveals an essential role for the LXXLL motif in the transformation function of the human herpesvirus-8 oncoprotein, kaposin

Human herpesvirus-8 (HHV-8) is causally linked to Kaposi's sarcoma (KS). Sequence analysis of the genome and subsequent studies revealed several genes including kaposin, with transformation properties in cell culture. In this study, we have analyzed the requirement of Kaposin A for cellular transformation in an effort to understand its contribution towards KS pathogenesis. Comparative analysis of Kaposin with other proteins identified the LXXLL motif spanning from residues 31-35 (LVCLL). The observation that the LXXLL motif is present in nuclear receptor coactivators that mediate the interaction of coactivators with nuclear receptors has prompted us to investigate the relevance of this motif for Kaposin's function(s). Kaposin A coding sequences were cloned into a eukaryotic expression plasmid with the Flag (FL) epitope fused in-frame at the C-terminus (Kap-FL). To evaluate the role of leucine residues in the motif, site-directed mutagenesis was utilized, whereby alanine was substituted for the leucine residues (Kap-AXXAA-FL). Both Kap-FL and Kap- AXXAA-FL exhibited similar levels of expression in cells. Interestingly, the Kap-AXXAA-FL mutant failed to show transforming activity by two independent assays: anchorage-independent growth, and focus formation. Immunofluorescence (IFA) and FACS analysis indicated that Kap-FL was localized around the nucleus and at the cell surface, respectively. However, Kap-AXXAA-FL exhibited diffuse cytoplasmic staining as measured by IFA yet was still detectable on the cell surface by FACS. Ironically, both Kap-FL and Kap-AXXAAFL were able to activate the AP-1 promoter. These results support an important role for the LXXLL motif in the ability of Kaposin to induce transformation.

Regulation of growth signalling and cell cycle by Kaposi's sarcoma-associated herpesvirus genes

Kaposi's sarcoma-associated herpesvirus (KSHV) is the primary aetiological agent of at least three malignancies associated with HIV infection and immunosuppression: Kaposi's sarcoma, primary effusion lymphoma and multicentric Castleman's disease. KSHV encodes proteins that deregulate key checkpoints in the signalling pathways governing cell proliferation, which may ultimately contribute to the virus' oncogenic potential. To alter cellular signalling associated with proliferation, these viral proteins function like growth factor ligands/receptors, signal transduction proteins, transcription factors and cell cycle regulators. This review focuses on the mechanisms by which some KSHV-encoded proteins activate signalling pathways and cell proliferation and their role in the pathogenesis of KSHV-driven mechanisms.

Ocular and neuronal cell apoptosis during HSV-1 infection: a review

HSV-1 may activate or suppress the apoptotic pathway in various cells. This review will discuss this apparent dichotomy and place particular emphasis on the different strategies HSV-1 uses to block or suppress the apoptotic pathway in various cell lines and tissues.

Human herpesvirus-8-encoded signalling ligands and receptors

Analysis of the genome of human herpesvirus 8 (HHV-8) led to the discovery of several novel genes, unique among the characterized gammaherpesviruses. These include cytokines (interleukin-6 and chemokine homologues), two putative signal-transducing transmembrane proteins encoded by genes K1 and K15 at the genome termini, and an OX-2 (CD200) receptor homologue that had not previously been identified in a gammaherpesvirus. HHV-8 also specifies a diverged version of the gammaherpesvirus-conserved G protein-coupled chemokine receptor (vGCR) and a latently expressed protein unique to HHV-8 specified by open reading frame (ORF) K12. These cytokine and receptor homologues mediate signal transduction or modulate the activities of other endogenous cytokines and receptors to enhance viral productive replication, regulate latent-lytic switching, evade host attack, or mediate cell survival. The viral signalling ligands and receptors are also potential contributors to virus-associated diseases, Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease, and so represent potentially important targets for therapeutic and antiviral drugs. Understanding these proteins' modes of action and functions in viral biology and disease is therefore of considerable importance, and the subject of this review.

Kaposi's sarcoma - still an enigma

Kaposi's sarcoma (KS) is an unusual neoplasm that has proved to be an enigma in many ways since its original description in 1872. KS, a vascular tumour that is otherwise rare, is at present the most common neoplasm in patients with AIDS. The lesions contain spindle cells that share features with endothelial cells and smooth muscle cells and are in all likelihood primitive mesenchymal cells that can form vascular channels. These cells are monoclonal in origin indicating therefore that KS is a neoplasm. The presence of a novel type of human herpes virus, KS herpesvirus (KSHV) also called human herpesvirus type 8 (HHV8) in KS lesions support a viral ethiology. KS may be mistaken in the skin for an inflammatory or other lesion, thus skin biopsy is important for correct diagnosis, with the use of immunohistochemistry or molecular biology if needed. Radiation or interferon alpha dominate in the therapeutic approaches.

Molecular genetics of Kaposi's sarcoma-associated herpesvirus (human herpesvirus-8) epidemiology and pathogenesis

Kaposi's sarcoma had been recognized as unique human cancer for a century before it manifested as an AIDS-defining illness with a suspected infectious etiology. The discovery of Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus-8, in 1994 by using representational difference analysis, a subtractive method previously employed for cloning differences in human genomic DNA, was a fitting harbinger for the powerful bioinformatic approaches since employed to understand its pathogenesis in KS. Indeed, the discovery of KSHV was rapidly followed by publication of its complete sequence, which revealed that the virus had coopted a wide armamentarium of human genes; in the short time since then, the functions of many of these viral gene variants in cell growth control, signaling apoptosis, angiogenesis, and immunomodulation have been characterized. This critical literature review explores the pathogenic potential of these genes within the framework of current knowledge of the basic herpesvirology of KSHV, including the relationships between viral genotypic variation and the four clinicoepidemiologic forms of Kaposi's sarcoma, current viral detection methods and their utility, primary infection by KSHV, tissue culture and animal models of latent- and lytic-cycle gene expression and pathogenesis, and viral reactivation from latency. Recent advances in models of de novo endothelial infection, microarray analyses of the host response to infection, receptor identification, and cloning of full-length, infectious KSHV genomic DNA promise to reveal key molecular mechanisms of the candidate pathogeneic genes when expressed in the context of viral infection.

The Kaposi's sarcoma-associated herpesvirus K12 transcript from a primary effusion lymphoma contains complex repeat elements, is spliced, and initiates from a novel promoter

Kaposi's sarcoma-associated herpesvirus (KSHV) latently infects KS tumors, primary effusion lymphomas (PELs), and PEL cell lines. K12 (T0.7) is the most abundant transcript expressed in latent KSHV infection. We characterize here the K12 transcript from a PEL tumor prior to passage in cell culture. The PEL tumor KSHV K12 transcript contained additional complex nucleotide repeat elements compared to the previously described K12 message of the BCBL-1 PEL cell line. The PEL tumor lacked kaposin B, the predominant BCBL-1 K12 protein product, but encoded kaposins A and C. The K12 transcript was spliced and the splicing event occurred in all KSHV isolates tested. The 5' end of the K12 transcript was mapped by 5' RACE (rapid amplification of cDNA ends) and S1 nuclease protection assays and was at the site of an active promoter. This work demonstrates that the K12 transcript contains variable, complex repeat elements, is spliced and is expressed from a novel KSHV promoter.

Transcriptional downregulation of ORF50/Rta by methotrexate inhibits the switch of Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 from latency to lytic replication

Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a cellular dihydrofolate reductase (DHFR) homologue. Methotrexate (MTX), a potent anti-inflammatory agent, inhibits cellular DHFR activity. We investigated the effect of noncytotoxic doses of MTX on latency and lytic KSHV replication in two KSHV-infected primary effusion lymphoma cell lines (BC-3 and BC-1) and in MTX-resistant BC-3 cells (MTX-R-BC-3 cells). Treatment with MTX completely prevented tetradecanoyl phorbol acetate-induced viral DNA replication and strongly decreased viral lytic transcript levels, even in MTX-resistant cells. However, the same treatment had no effect on transcription of cellular genes and KSHV latent genes. One of the lytic transcripts inhibited by MTX, ORF50/Rta (open reading frame), is an immediate-early gene encoding a replication-transcription activator required for expression of other viral lytic genes. Therefore, transcription of genes downstream of ORF50/Rta was inhibited, including those encoding the viral G-protein-coupled receptor (GPCR), viral interleukin-6, and K12/kaposin, which have been shown to be transforming in vitro and oncogenic in mice. Resistance to MTX has been documented in cultured cells and also in patients treated with this drug. However, MTX showed an inhibitory activity even in MTX-R-BC-3 cells. Two currently available antiherpesvirus drugs, cidofovir and foscarnet, had no effect on the transcription of these viral oncogenes and ORF50/Rta. MTX is the first example of a compound shown to downregulate the expression of ORF50/Rta and therefore prevent viral transforming gene transcription. Given that the expression of these genes may be important for tumor development, MTX could play a role in the future management of KSHV-associated malignancies.

Solution structure of a Bcl-2 homolog from Kaposi sarcoma virus

Kaposi sarcoma-associated herpes virus (KSHV) contains a gene that has functional and sequence homology to the apoptotic Bcl-2 family of proteins [Sarid, R., Sato, T., Bohenzky, R. A., Russo, J. J. & Chang, Y. (1997) Nat. Med. 3, 293-298]. The viral Bcl-2 protein promotes survival of infected cells and may contribute to the development of Kaposi sarcoma tumors [Boshoff, C. & Chang, Y. (2001) Annu. Rev. Med. 52, 453-470]. Here we describe the solution structure of the viral Bcl-2 homolog from KSHV. Comparison of the KSHV Bcl-2 structure to that of Bcl-2 and Bcl-x(L) shows that although the overall fold is the same, there are key differences in the lengths of the helices and loops. Binding studies on peptides derived from the Bcl-2 homology region 3 of proapoptotic family members indicate that the specificity of the viral protein is very different from what was previously observed for Bcl-x(L) and Bcl-2, suggesting that the viral protein has evolved to have a different mechanism of action than the host proteins.

Human herpesvirus-8 encoded Kaposin: subcellular localization using immunofluorescence and biochemical approaches

Human herpesvirus-8 (HHV-8) has been causally linked to the development of Kaposi's sarcoma (KS). DNA sequence analysis of the viral genome revealed a total of 81 open reading frames (ORF). Interestingly, only a small subset of these ORFs has been shown to be transcribed in cells latently infected with HHV-8 and in cells of the KS lesions. Among the genes active during latency, kaposin, is noted for its abundance and ability to transform cells in culture, thus implicating a potential role in KS pathogenesis. This has prompted us to undertake an investigation on elucidating the mechanism(s) by which Kaposin brings about transformation of cells. Towards this goal, we have generated an eukaryotic expression plasmid encoding Kaposin (Kap). As Kaposin is predicted to be a type II membrane protein, several strategies were utilized to address this, including the generation of Kaposin with the Flag (FL) epitope (DYKDDDDK) at the C-terminus of the protein (Kap-C-FL). Antibodies specific for Kaposin (kap-2), recognized both Kaposin and Kaposin-Flag, while antibodies against the Flag epitope recognized only Kaposin-Flag. Transfection of Kap and Kap-C-FL expression plasmid DNA into NIH3T3 cells resulted in cellular clones that exhibited a phenotypic property of transformation by forming large, multiclustered cells, when grown on soft agar. Because there is controversial data regarding the localization of Kaposin in cells, we examined the subcellular localization of Kaposin using confocal microscopy. We observed that Kaposin and Kaposin-Flag showed an intense staining surrounding the nucleus. Although there was no staining at the cell membrane of transfected cells, FACS analysis using kap-2 or Flag antibodies, under nonpermeable conditions, showed positivity. Cell fractionation studies further showed that the majority of Kaposin was detected in the nuclear fraction by Western blot analysis. The cytoplasmic and detergent soluble membrane fractions did not show Kaposin protein; however, a small amount was detected in the detergent insoluble membrane fraction. Taken together, these results suggest that Kaposin exhibits multicompartmental localization in cells.

Skin diseases associated with human herpesvirus 6, 7, and 8 infection

Relatively recently, the discovery and analysis of three new human herpesviruses, human herpesvirus (HHV)-6, HHV-7, and Kaposi's sarcoma-associated herpesvirus (KSHV), also known as HHV-8, has contributed greatly to our understanding of the pathogenesis of several common dermatoses. HHV-6 and HHV-7 are closely related beta-herpesviruses that have been linked with roseola (mostly HHV-6), severe drug eruptions (HHV-6), and pityriasis rosea (mostly HHV-7). KSHV is a gamma-herpesvirus that is now believed to be the long sought after etiologic agent of Kaposi's sarcoma. The evidence for these skin disease associations and key findings from recent basic science investigations on viral pathogenesis are discussed in this review. In addition, possible therapeutic implications of these research studies are explored.

Molecular piracy of Kaposi's sarcoma associated herpesvirus

Kaposi's Sarcoma associated Herpesvirus (KSHV) is the most recently discovered human tumor virus and is associated with the pathogenesis of Kaposi's sarcoma, primary effusion lymphoma, and Multicentric Casttleman's disease. KSHV contains numerous open reading frames with striking homology to cellular genes. These viral gene products play a variety of roles in KSHV-associated pathogenesis by disrupting cellular signal transduction pathways, which include interferon-mediated anti-viral responses, cytokine-regulated cell growth, apoptosis, and cell cycle control. In this review, we will attempt to cover our understanding of how viral proteins deregulate cellular signaling pathways, which ultimately contribute to the conversion of normal cells to cancerous cells.