Kaposi's sarcoma-associated herpesvirus and Kaposi's sarcoma

Kaposi's sarcoma-associated herpesvirus (KSHV) is present in all epidemiologic forms of Kaposi's sarcoma (KS). The KSHV genome contains several open reading frames which are potentially implicated in the development of KS. Some are unique to KSHV; others are homologous to cellular genes. The putative role of these genes in the genesis of KS is discussed.     

Virion proteins of Kaposi's sarcoma-associated herpesvirus

The proteins that compose a herpesvirus virion are thought to contain the functional information required for de novo infection, as well as virion assembly and egress. To investigate functional roles of Kaposi's sarcoma-associated herpesvirus (KSHV) virion proteins in viral productive replication and de novo infection, we attempted to identify virion proteins from purified KSHV by a proteomic approach. Extracellular KSHV virions were purified from phorbol-12-tetradecanoate-13-acetate-induced BCBL-1 cells through double-gradient ultracentrifugation, and their component proteins were resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Thirty prominent protein bands were excised and subjected to high-performance liquid chromatography ion trap mass spectrometric analysis. This study led to the identification of 24 virion-associated proteins. These include five capsid proteins, eight envelope glycoproteins, six tegument proteins, and five proteins whose locations in the virions have not yet been defined. Putative tegument proteins encoded by open reading frame 21 (ORF21), ORF33, and ORF45 were characterized and found to be resistant to protease digestion when purified virions were treated with trypsin, confirming that they are located within the virion particles. The ORF64-encoded large tegument protein was found to be associated with capsid but sensitive to protease treatment, suggesting its unique structure and array in KSHV virions. In addition, cellular beta-actin and class II myosin heavy chain type A were found inside KSHV virions and associated with tegument-capsid structure. Identification of KSHV virion proteins makes it possible to study the functional roles of these virion proteins in KSHV replication and pathogenicity.     

Molecular virology of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV), the most recently discovered human tumour virus, is the causative agent of Kaposi's sarcoma, primary effusion lymphoma and some forms of Castleman's disease. KSHV is a rhadinovirus, and like other rhadinoviruses, it has an extensive array of regulatory genes obtained from the host cell genome. These pirated KSHV proteins include homologues to cellular CD21, three different beta-chemokines, IL-6, BCL-2, several different interferon regulatory factor homologues, Fas-ligand ICE inhibitory protein (FLIP), cyclin D and a G-protein-coupled receptor, as well as DNA synthetic enzymes including thymidylate synthase, dihydrofolate reductase, DNA polymerase, thymidine kinase and ribonucleotide reductases. Despite marked differences between KSHV and Epstein-Barr virus, both viruses target many of the same cellular pathways, but use different strategies to achieve the same effects. KSHV proteins have been identified which inhibit cell-cycle regulation checkpoints, apoptosis control mechanisms and the immune response regulatory machinery. Inhibition of these cellular regulatory networks app ears to be a defensive means of allowing the virus to escape from innate antiviral immune responses. However, due to the overlapping nature of innate immune and tumour-suppressor pathways, inhibition of these regulatory networks can lead to unregulated cell proliferation and may contribute to virus-induced tumorigenesis.     

Remodeling of endothelial adherens junctions by Kaposi's sarcoma-associated herpesvirus

Vascular endothelial cadherin (VE-cadherin) connects neighboring endothelial cells (ECs) via interendothelial junctions and regulates EC proliferation and adhesion during vasculogenesis and angiogenesis. The cytoplasmic domain of VE-cadherin recruits α- and β-catenins and γ-catenin, which interact with the actin cytoskeleton, thus modulating cell morphology. Dysregulation of the adherens junction/cytoskeletal axis is a hallmark of invasive tumors. We now demonstrate that the transmembrane ubiquitin ligase K5/MIR-2 of Kaposi's sarcoma-associated herpesvirus targets VE-cadherin for ubiquitin-mediated destruction, thus disturbing EC adhesion. In contrast, N-cadherin levels in K5-expressing cells were increased compared to those in control cells. Steady-state levels of α- and β-catenins and γ-catenin in K5-expressing ECs were drastically reduced due to proteasomal destruction. Moreover, the actin cytoskeleton was rearranged, resulting in the dysregulation of EC barrier function as measured by electric cell-substrate impedance sensing. Our data represent the first example of a viral protein targeting adherens junction proteins and suggest that K5 contributes to EC proliferation, vascular leakage, and the reprogramming of the EC proteome during Kaposi's sarcoma tumorigenesis.     

Activation of plasmacytoid dendritic cells by Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is associated with multiple human malignancies, including Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. Following primary infection, KSHV typically goes through a brief period of lytic replication prior to the establishment of latency. Plasmacytoid dendritic cells (pDCs) are the major producers of type 1 interferon (IFN), primarily in response to virus infection. Toll-like receptors (TLRs) are key components of the innate immune system, and they serve as pathogen recognition receptors that stimulate the host antiviral response. pDCs express exclusively TLR7 and TLR9, and it is through these TLRs that the type 1 interferon response is activated in pDCs. Currently, it is not known whether KSHV is recognized by pDCs and whether activation of pDCs occurs in response to KSHV infection. We now report evidence that KSHV can infect human pDCs and that pDCs are activated upon KSHV infection, as measured by upregulation of CD83 and CD86 and by IFN-α secretion. We further show that induction of IFN-α occurs through activation of TLR9 signaling and that a TLR9 inhibitor diminishes the production and secretion of IFN-α by KSHV-infected pDCs.     

Novel Kaposi's sarcoma-associated herpesvirus homolog in baboons

Kaposi's sarcoma (KS) and lymphoproliferative diseases induced by KS-associated herpesvirus (KSHV/human herpesvirus 8) cause substantial morbidity and mortality in human immunodeficiency virus-infected individuals. To understand KSHV biology it is useful to investigate closely related rhadinoviruses naturally occurring in nonhuman primates. Here we report evidence for a novel KSHV homolog in captive baboon species (Papio anubis and other). Using degenerate PCR we identified a novel rhadinovirus, PapRV2, that has substantial sequence identity to two essential KSHV genes, the viral polymerase and thymidylate synthase. A subset of animals exhibited detectable PapRV2 viral load in peripheral blood mononuclear cells. Extensive serological analysis of nearly 200 animals in the colony demonstrated that the majority carried cross-reacting antibodies that recognize KSHV or macaque rhadinovirus antigens. Seroreactivity increased with age, similar to the age-specific prevalence of KSHV in the human population. This establishes baboons as a novel resource to investigate rhadinovirus biology, which can be developed into an animal model system for KSHV-associated human diseases, vaccine development, and therapy evaluation.     

Epidemiology and pathogenesis of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma (KS) occurs in Europe and the Mediterranean countries (classic KS) and Africa (endemic KS), immunosuppressed patients (iatrogenic or post-transplant KS) and those with acquired immune deficiency syndrome (AIDS), especially among those who acquired human immunodeficiency virus sexually (AIDS-KS). KS-associated herpesvirus (KSHV or HHV-8) is unusual among herpesviruses in having a restricted geographical distribution. Like KS, which it induces in immunosuppressed or elderly people, the virus is prevalent in Africa, in Mediterranean countries, among Jews and Arabs and certain Amerindians. Distinct KSHV genotypes occur in different parts of the world, but have not been identified as having a differential pathogenesis. KSHV is aetiologically linked to three distinct neoplasms: (i) KS, (ii) primary effusion lymphoma, and (iii) plasmablastic multicentric Castleman's disease. The histogenesis, clonality and pathology of the tumours are described, together with the epidemiology and possible modes of transmission of the virus.     

Virion-wide protein interactions of Kaposi's sarcoma-associated herpesvirus

Herpesvirus virions are highly organized structures built through specific protein-protein interactions. Thus, revelation of the protein interactions among virion proteins will shed light on the processes and the mechanisms of virion formation. Recently, we identified 24 virion proteins of Kaposi's sarcoma-associated herpesvirus (KSHV), using a proteomic approach (F. X. Zhu et al., J. Virol. 79:800-811, 2005). In the current study, a comprehensive analysis of protein-protein interaction between KSHV virion proteins was carried out using yeast two-hybrid (Y2H) and coimmunoprecipitation (co-IP) approaches. Every pairwise combination between KSHV tegument and capsid proteins, between tegument and envelope proteins, and among tegument proteins was tested for possible binary interaction. Thirty-seven protein-protein interactions were identified by both Y2H and co-IP analyses. The results revealed interactions between tegument and capsid proteins such as that of open reading frame 64 (ORF64) with ORF25 (major capsid protein [MCP]), ORF62 (triplex-1 [TRI-1]), and ORF26 (TRI-2). Many interactions were detected among the tegument proteins. ORF64 was found to interact with several tegument proteins including ORF11, ORF21, ORF33, ORF45, ORF63, ORF75, and ORF64 itself, suggesting that ORF64 may serve as a hub protein and play a role in recruiting tegument proteins during tegumentation and virion assembly. Our investigation also revealed redundant interactions between tegument proteins and envelope glycoproteins. These interactions are believed to contribute to final envelopment in virion assembly. Overall, this study allows us to establish a virion-wide protein interaction map, which provides insight into the architecture of the KSHV virion and sets up a foundation for exploring the functions of these proteins in viral particle assembly.     

The molecular pathology of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is the eighth and most recently identified human herpesvirus (HHV-8). KSHV was discovered in 1994 by Chang et al. who used representational difference analysis to search for DNA sequences present in AIDS-associated KS but not in adjacent normal skin [1]. The virus has since been shown to be specifically associated with all forms of this disease and has fulfilled all of Hill's criteria for causation (reviewed in ). KSHV is also found in all cases of primary effusion lymphoma and in a plasmablastic variant of multicentric Castleman's disease. Over the last few years a wealth of data has been gained on the role of KSHV genes during infection. This review is an attempt to assemble this information into a more complete picture of how KSHV may cause disease.     

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.     

Control of Kaposi's sarcoma-associated herpesvirus reactivation induced by multiple signals

The ability to control cellular functions can bring about many developments in basic biological research and its applications. The presence of multiple signals, internal as well as externally imposed, introduces several challenges for controlling cellular functions. Additionally the lack of clear understanding of the cellular signaling network limits our ability to infer the responses to a number of signals. This work investigates the control of Kaposi's sarcoma-associated herpesvirus reactivation upon treatment with a combination of multiple signals. We utilize mathematical model-based as well as experiment-based approaches to achieve the desired goals of maximizing virus reactivation. The results show that appropriately selected control signals can induce virus lytic gene expression about ten folds higher than a single drug; these results were validated by comparing the results of the two approaches, and experimentally using multiple assays. Additionally, we have quantitatively analyzed potential interactions between the used combinations of drugs. Some of these interactions were consistent with existing literature, and new interactions emerged and warrant further studies. The work presents a general method that can be used to quantitatively and systematically study multi-signal induced responses. It enables optimization of combinations to achieve desired responses. It also allows identifying critical nodes mediating the multi-signal induced responses. The concept and the approach used in this work will be directly applicable to other diseases such as AIDS and cancer.     

Subversion of autophagy by Kaposi's sarcoma-associated herpesvirus impairs oncogene-induced senescence

Acute oncogenic stress can activate autophagy and facilitate permanent arrest of the cell cycle through a failsafe mechanism known as oncogene-induced senescence (OIS). Kaposi's sarcoma-associated herpesvirus (KSHV) proteins are known to subvert autophagic pathways, but the link to Kaposi's sarcoma pathogenesis is unclear. We find that oncogenic assault caused by latent KSHV infection elicits DNA damage responses (DDRs) characteristic of OIS, yet infected cells display only modest levels of autophagy and fail to senesce. These aberrant responses result from the combined activities of tandemly expressed KSHV v-cyclin and v-FLIP proteins. v-Cyclin deregulates the cell cycle, triggers DDRs, and if left unchecked can promote autophagy and senescence. However, during latency v-FLIP blocks v-cyclin-induced autophagy and senescence in a manner that requires intact v-FLIP ATG3-binding domains. Together, these data reveal a coordinated viral gene expression program that usurps autophagy, blocks senescence, and facilitates the proliferation of KSHV-infected cells.