Functional p53 signaling in Kaposi's sarcoma-associated herpesvirus lymphomas: implications for therapy

The Kaposi's sarcoma-associated herpesvirus (KSHV/HHV8) is associated with Kaposi's sarcoma (KS) as well as primary effusion lymphomas (PEL). The expression of viral proteins capable of inactivating the p53 tumor suppressor protein has been implicated in KSHV oncogenesis. However, DNA-damaging drugs such as doxorubicin are clinically efficacious against PEL and KS, suggesting that p53 signaling remains intact despite the presence of KSHV. To investigate the functionality of p53 in PEL, we examined the response of a large number of PEL cell lines to doxorubicin. Two out of seven (29%) PEL cell lines harbored a mutant p53 allele (BCBL-1 and BCP-1) which led to doxorubicin resistance. In contrast, all other PEL containing wild-type p53 showed DNA damage-induced cell cycle arrest, p53 phosphorylation, and p53 target gene activation. These data imply that p53-mediated DNA damage signaling was intact. Supporting this finding, chemical inhibition of p53 signaling in PEL led to doxorubicin resistance, and chemical activation of p53 by the Hdm2 antagonist Nutlin-3 led to unimpaired induction of p53 target genes as well as growth inhibition and apoptosis.     

Cdk1 inhibition induces mutually inhibitory apoptosis and reactivation of Kaposi's sarcoma-associated herpesvirus

Primary effusion lymphoma (PEL) cells are predominantly infected by the latent form of Kaposi's sarcoma-associated herpesvirus (KSHV), with virus reactivation occurring in a small percentage of cells. Latency enables KSHV to persist in the host cell and promotes tumorigenesis through viral gene expression, thus presenting a major barrier to the elimination of KSHV and the treatment of PEL. Therefore, it is important to identify cellular genes that are essential for PEL cell survival or the maintenance of KSHV latency. Here we report that cyclin-dependent kinase 1 (Cdk1) inhibition can induce both apoptosis and KSHV reactivation in a population of PEL cells. Caspases, but not p53, are required for PEL cell apoptosis induced by Cdk1 inhibition. p38 kinase is activated by Cdk1 inhibition and mediates KSHV reactivation. Interestingly, upon Cdk1 inhibition, KSHV is reactivated predominantly in the nonapoptotic subpopulation of PEL cells. We provide evidence that this is due to mutual inhibition between apoptosis and KSHV reactivation. In addition, we found that KSHV reactivation activates protein kinase B (AKT/PKB), which promotes cell survival and facilitates KSHV reactivation. Our study thus establishes a key role for Cdk1 in PEL cell survival and the maintenance of KSHV latency and reveals a multifaceted relationship between KSHV reactivation and PEL cell apoptosis.     

Ribosomal protein S6 interacts with the latency-associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus

The latency-associated nuclear antigen (LANA) is central to the maintenance of Kaposi's sarcoma-associated herpesvirus (KSHV) and to the survival of KSHV-carrying tumor cells. In an effort to identify interaction partners of LANA, we purified authentic high-molecular-weight complexes of LANA by conventional chromatography followed by immunoprecipitation from the BC-3 cell line. This is the first analysis of LANA-interacting partners that is not based on forced ectopic expression of LANA. Subsequent tandem mass spectrometry (MS/MS) analysis identified many of the known LANA-interacting proteins. We confirmed LANA's interactions with histones. Three classes of proteins survived our stringent four-step purification procedure (size, heparin, anion, and immunoaffinity chromatography): two heat shock proteins (Hsp70 and Hsp96 precursor), signal recognition particle 72 (SRP72), and 10 different ribosomal proteins. These proteins are likely involved in structural interactions within LANA high-molecular-weight complexes. Here, we show that ribosomal protein S6 (RPS6) interacts with LANA. This interaction is mediated by the N-terminal domain of LANA and does not require DNA or RNA. Depletion of RPS6 from primary effusion lymphoma (PEL) cells dramatically decreases the half-life of full-length LANA. The fact that RPS6 has a well-established nuclear function beyond its role in ribosome assembly suggests that RPS6 (and by extension other ribosomal proteins) contributes to the extraordinary stability of LANA.     

Interplay between the p53 tumor suppressor protein family and Cdk5

Cyclin-dependent kinases (Cdks) play a key role in orchestrating the coordination of cell cycle progression in proliferating cells. The escape from the proper, control of the cell cycle by the upregulation of cyclins or aberrant activation of Cdks leads to malignant transformation. In quiescent cells and/or terminally differentiated cells, the expression pattern and activity of Cdks is altered. In postmitotic neurons, expression of mitotic kinases is downregulated, whereas Cdk5 expression becomes upregulated. Similarly to other Cdks, free Cdk5 displays no enzymatic activity and requires complex formation with a specific regulatory subunit. Two activators of Cdk5 have been identified. p35 and its isoform p39 bind to, and thereby activate, Cdk5. Unlike mitotic kinases, Cdk5 does not require activating phosphorylation within the T-loop. Because p35 is a short-lived protein, the p35/Cdk5 complexes are unstable. The stability of the p35 protein is regulated by its Cdk5-mediated phosphorylation of p35. Activated p35/Cdk5 kinase phosphorylates numerous physiological targets. The proper phosphorylation of the most important substrates, such as τ protein and neurofilament H, is essential for the correct regulation of the cytoskeletal organization, thereby regulating cell adhesion, motility, and synaptic plasticity. Moreover, Cdk5 regulates the activity of the p53 tumor suppressor via phosphorylation. p53 is upregulated in multiple neuronal death paradigms, including hypoxia, ischemia, and excitotoxicity, and plays a key role in the induction of apoptosis. On the other hand, an abnormally high expression and elevated activity of Cdk5 was observed in neurodegenerative diseases, suggesting the application of Cdk inhibitors for their therapy. Considering the action of some Cdk inhibitors on the expression and activity of the p53 protein, their therapeutic efficacy must be carefully evaluated.     

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.     

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.     

Kaposi's sarcoma-associated herpesvirus latency-associated nuclear antigen interacts with multifunctional angiogenin to utilize its antiapoptotic functions

Kaposi's sarcoma-associated herpesvirus (KSHV) is etiologically associated with the angioproliferative Kaposi's sarcoma (KS). KSHV infection and the expression of latency-associated nuclear antigen (LANA-1) upregulates the angiogenic multifunctional 123-amino-acid, 14-kDa protein angiogenin (ANG), which is detected in KS lesions and in KSHV-associated primary effusion lymphoma (PEL) cells. ANG knockdown or the inhibition of ANG's nuclear translocation resulted in decreased LANA-1 gene expression and reduced KSHV-infected endothelial and PEL cell survival (Sadagopan et al., J. Virol. 83:3342-3364, 2009). Further studies here demonstrate that LANA-1 and ANG colocalize and coimmunoprecipitate in de novo infected endothelial cells and in latently infected PEL (BCBL-1 and BC-3) cells. LANA-1 and ANG interaction occurred in the absence of the KSHV genome and other viral proteins. In gel filtration chromatography analyses of BC-3 cell lysates, ANG coeluted with LANA-1, p53, and Mdm2 in high-molecular-weight fractions, and LANA-1, p53, and Mdm2 also coimmunoprecipitated with ANG. LANA-1, ANG, and p53 colocalized in KSHV-infected cells, and colocalization between ANG and p53 was also observed in LANA-1-negative cells. The deletion constructs of ANG suggested that the C-terminal region of amino acids 104 to 123 is involved in LANA-1 and p53 interactions. Silencing ANG or inhibiting its nuclear translocation resulted in decreased nuclear LANA-1 and ANG levels, decreased interactions between ANG-LANA-1, ANG-p53, and LANA-1-p53, the induction of p53, p21, and Bax proteins, the increased cytoplasmic localization of p53, the downregulation of Bcl-2, the increased cleavage of caspase-3, and the apoptosis of cells. No such effects were observed in KSHV-negative BJAB cells. The phosphorylation of p53 at serine 15, which is essential for p53 stabilization and for p53's apoptotic and cell cycle regulation functions, was increased in BCBL-1 cells transduced with short hairpin RNA targeting ANG. Together, these studies suggest that the antiapoptosis observed in KSHV-infected cells and the suppression of p53 functions are mediated in part by ANG, and KSHV has probably evolved to utilize angiogenin's multiple functions for the maintenance of its latency and cell survival. Thus, targeting ANG to induce the apoptosis of cells latently infected with KSHV is an attractive therapeutic strategy against KSHV infection and associated malignancies.     

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.     

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.     

XPC promotes MDM2-mediated degradation of the p53 tumor suppressor

Although ubiquitin receptor Rad23 has been implicated in bringing ubiquitylated p53 to the proteasome, how Rad23 recognizes p53 remains unclear. We demonstrate that XPC, a Rad23-binding protein, regulates p53 turnover. p53 protein in XPC-deficient cells remains ubiquitylated, but its association with the proteasome is drastically reduced, indicating that XPC regulates a postubiquitylation event. Furthermore, we found that XPC participates in the MDM2-mediated p53 degradation pathway via direct interaction with MDM2. XPC W690S pathogenic mutant is specifically defective for MDM2 binding and p53 degradation. p53 is known to become stabilized following UV irradiation but can be rendered unstable by XPC overexpression, underscoring a critical role of XPC in p53 regulation. Elucidation of the proteolytic role of XPC in cancer cells will help to unravel the detailed mechanisms underlying the coordination of DNA repair and proteolysis.