KSHV viral cyclin binds to p27KIP1 in primary effusion lymphomas

Hijacking of nucleotide biosynthesis and deamidation-mediated glycolysis by an oncogenic herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi's sarcoma (KS) and multiple types of B cell malignancies. Emerging evidence demonstrates that KSHV reprograms host-cell central carbon metabolic pathways, which contributes to viral persistence and tumorigenesis. However, the mechanisms underlying KSHV-mediated metabolic reprogramming remain poorly understood. Carbamoyl-phosphate synthetase 2, aspartate transcarbamoylase, and dihydroorotase (CAD) is a key enzyme of the de novo pyrimidine synthesis, and was recently identified to deamidate the NF-κB subunit RelA to promote aerobic glycolysis and cell proliferation. Here we report that KSHV infection exploits CAD for nucleotide synthesis and glycolysis. Mechanistically, KSHV vCyclin binds to and hijacks cyclin-dependent kinase CDK6 to phosphorylate Ser-1900 on CAD, thereby activating CAD-mediated pyrimidine synthesis and RelA-deamidation-mediated glycolytic reprogramming. Correspondingly, genetic depletion or pharmacological inhibition of CDK6 and CAD potently impeded KSHV lytic replication and thwarted tumorigenesis of primary effusion lymphoma (PEL) cells in vitro and in vivo. Altogether, our work defines a viral metabolic reprogramming mechanism underpinning KSHV oncogenesis, which may spur the development of new strategies to treat KSHV-associated malignancies and other diseases.

Demographic characteristics and prognosis of HHV8-positive diffuse large B-cell lymphoma, not otherwise specified: Insights from a population-based study with a 10-year follow-up

HHV8-positive diffuse large B-cell lymphoma, not otherwise specified (HDN) is a subtype of lymphoma that usually arises in association with HHV8-positive multicentric Castleman disease. However, the epidemiology, treatment patterns, and survival outcomes of HDN are poorly understood. A retrospective analysis was performed for 67 patients with HDN diagnosed from 2011 to 2020 using the SEER database. The demographic characteristics, treatment modalities, and survival outcomes of HDN patients were evaluated. Kaplan-Meier analysis and Cox regression analysis were employed to identify prognostic factors for overall survival (OS) and disease-specific survival (DSS). The age-adjusted incidence rate of HDN was 0.010 per 100,000 person-years. The median age at diagnosis was 51.8 years with male predominance. The primary site distribution was mainly nodal (79.1%), while the extranodal sites were rarely involved (20.9%). The majority of patients were white (65.7%). Only 3.0% of patients received radiotherapy, while 55.2% received chemotherapy. The 1-year, 3-year, and 5-year OS was 67.4%, 65.6%, 58.4%, and 56.3%, respectively, and the corresponding DSS was 73.1%, 73.1%, and 67.8%, respectively. The diagnosis year group of 2016-2020 had a significantly worse OS than the diagnosis year group of 2011-2015 (P = .040), but not for DSS (P = .074). No significant survival improvement was observed in patients underwent chemotherapy. Age and marital status were independent prognostic factors for OS, and age was an independent prognostic factor for DSS. In conclusion, HDN is a rare and aggressive disease, our study provides a comprehensive overview of the epidemiology, treatment patterns and survival outcomes of HDN patients for the first time. We revealed that older age and marital status of single were associated with worse survival of HDN, while chemotherapy was not associated with improved survival outcomes in HDN patients.

CDK4/6 inhibitors sensitize gammaherpesvirus-infected tumor cells to T-cell killing by enhancing expression of immune surface molecules

BACKGROUND

The two oncogenic human gammaherpesviruses, Kaposi sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV), both downregulate immune surface molecules, such as MHC-I, ICAM-1, and B7-2, enabling them to evade T-cell and natural killer cell immunity. Both also either encode for human cyclin homologues or promote cellular cyclin activity, and this has been shown to be important for proliferation and survival of gammaherpesvirus-induced tumors. CDK4/6 inhibitors, which are approved for certain breast cancers, have been shown to enhance expression of MHC-I in cell lines and murine models of breast cancer, and this was attributed to activation of interferons by endogenous retrovirus elements. However, it was not known if this would occur in gammaherpesvirus-induced tumors in which interferons are already activated.

METHODS

Multiple KSHV/EBV-infected cell lines were treated with CDK4/6 inhibitors. The growth of viable cells and expression of surface markers was assessed. T cell activation stimulated by the treated cells was assayed by a T-cell activation bioassay. Both viral and host gene expression was surveyed using RT-qPCR.

RESULTS

Three CDK4/6 inhibitors, abemaciclib, palbociclib, and ribociclib, inhibited cell growth in KSHV-induced primary effusion lymphoma (PEL) and EBV positive Burkitt's lymphoma (BL) cell lines, and KSHV-infected human umbilical vein endothelial cells (HUVECs). Moreover, CDK4/6 inhibitors increased mRNA and surface expression of MHC-I in all three and prevented downregulation of MHC-I surface expression during lytic replication in KSHV-infected cells. CDK4/6 inhibitors also variably increased mRNA and surface expression of ICAM-1 and B7-2 in the tested lines. Abemaciclib also significantly enhanced T-cell activation induced by treated PEL and BL cells. Certain gammaherpesvirus genes as well as endogenous retrovirus (ERV) 3-1 genes were enhanced by CDK4/6 inhibitors in most PEL and BL lines and this enhancement was associated with expression of gamma interferon-induced genes including MHC-I.

CONCLUSIONS

These observations provide evidence that CDK4/6 inhibitors can induce expression of surface immune markers MHC-I, B7-2, and ICAM-1 in gammaherpesvirus-infected cell lines and induce virus-specific immunity. They can thus thwart virus-induced immune evasion. These effects, along with their direct effects on KSHV- or EBV-induced tumors, provide a rational for the clinical testing of these drugs in these tumors.

p27Kip1, an Intrinsically Unstructured Protein with Scaffold Properties

The Cyclin-dependent kinase (CDK) regulator p27^Kip1 is a gatekeeper of G1/S transition. It also regulates G2/M progression and cytokinesis completion, via CDK-dependent or -independent mechanisms. Recently, other important p27^Kip1 functions have been described, including the regulation of cell motility and migration, the control of cell differentiation program and the activation of apoptosis/autophagy. Several factors modulate p27^Kip1 activities, including its level, cellular localization and post-translational modifications. As a matter of fact, the protein is phosphorylated, ubiquitinated, SUMOylated, O-linked N-acetylglicosylated and acetylated on different residues. p27^Kip1 belongs to the family of the intrinsically unstructured proteins and thus it is endowed with a large flexibility and numerous interactors, only partially identified. In this review, we look at p27^Kip1 properties and ascribe part of its heterogeneous functions to the ability to act as an anchor or scaffold capable to participate in the construction of different platforms for modulating cell response to extracellular signals and allowing adaptation to environmental changes.

The Oncogenic Kaposi's Sarcoma-Associated Herpesvirus Encodes a Mimic of the Tumor-Suppressive miR-15/16 miRNA Family

Many tumor viruses encode oncogenes of cellular origin. Here, we report an oncoviral mimic of a cellular tumor suppressor. The Kaposi’s sarcoma-associated herpesvirus (KSHV) microRNA (miRNA) miR-K6-5p shares sequence similarity to the tumor-suppressive cellular miR-15/16 miRNA family. We show that miR-K6-5p inhibits cell cycle progression, a hallmark function of miR-16. miR-K6-5p regulates conserved miR-15/16 family miRNA targets, including many cell cycle regulators. Inhibition of miR-K6-5p in KSHV-transformed B cells confers a significant growth advantage. Altogether, our data show that KSHV encodes a functional mimic of miR-15/16 family miRNAs. While it is exceedingly well established that oncogenic viruses encode oncogenes of cellular origin, this is an unusual example of an oncogenic virus that encodes a viral mimic of a cellular tumor suppressor. Encoding a tumor-suppressive miRNA could help KSHV balance viral oncogene expression and thereby avoid severe pathogenesis in the healthy host.

Morrison et al. report that the tumor virus KSHV encodes a mimic of a cellular tumor suppressor. KSHV miR-K6-5p phenocopies miR-16-induced cell cycle inhibition, shares mRNA targets and binding sites with miR-16, and negatively regulates proliferation in KSHV-infected cells.

Epigenetic control in Kaposi sarcoma-associated herpesvirus infection and associated disease

Kaposi sarcoma-associated herpesvirus (KSHV) is the etiologic agent of several malignancies of endothelial and B-cell origin. The fact that latently infected tumor cells in these malignancies do not express classical viral oncogenes suggests that pathogenesis of KSHV-associated disease results from multistep processes that, in addition to constitutive viral gene expression, may require accumulation of cellular alterations. Heritable changes of the epigenome have emerged as an important co-factor that contributes to the pathogenesis of many non-viral cancers. Since KSHV encodes a number of factors that directly or indirectly manipulate host cell chromatin, it is an intriguing possibility that epigenetic reprogramming also contributes to the pathogenesis of KSHV-associated tumors. The fact that heritable histone modifications have also been shown to regulate viral gene expression programs in KSHV-infected tumor cells underlines the importance of epigenetic control during latency and tumorigenesis. We here review what is presently known about the role of epigenetic regulation of viral and host chromatin in KSHV infection and discuss how viral manipulation of these processes may contribute to the development of KSHV-associated disease.

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

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

Signal Transduction Pathways Associated with KSHV-Related Tumors

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

B-cell Lymphoproliferative Disorders Associated with Primary and Acquired Immunodeficiency

The diagnosis of lymphoproliferative disorders associated with immunodeficiency can be challenging because many of these conditions have overlapping clinical and pathologic features and share similarities with their counterparts in the immunocompetent setting. There are subtle but important differences between these conditions that are important to recognize for prognostic and therapeutic purposes. This article provides a clinicopathologic update on how understanding of these B-cell lymphoproliferations in immunodeficiency has evolved over the past decade.

The Modulation of Apoptotic Pathways by Gammaherpesviruses

Apoptosis or programmed cell death is a tightly regulated process fundamental for cellular development and elimination of damaged or infected cells during the maintenance of cellular homeostasis. It is also an important cellular defense mechanism against viral invasion. In many instances, abnormal regulation of apoptosis has been associated with a number of diseases, including cancer development. Following infection of host cells, persistent and oncogenic viruses such as the members of the Gammaherpesvirus family employ a number of different mechanisms to avoid the host cell’s “burglar” alarm and to alter the extrinsic and intrinsic apoptotic pathways by either deregulating the expressions of cellular signaling genes or by encoding the viral homologs of cellular genes. In this review, we summarize the recent findings on how gammaherpesviruses inhibit cellular apoptosis via virus-encoded proteins by mediating modification of numerous signal transduction pathways. We also list the key viral anti-apoptotic proteins that could be exploited as effective targets for novel antiviral therapies in order to stimulate apoptosis in different types of cancer cells.

Triptolide decreases expression of latency-associated nuclear antigen 1 and reduces viral titers in Kaposi's sarcoma-associated and herpesvirus-related primary effusion lymphoma cells

Kaposi's sarcoma-associated herpesvirus (KSHV) can establish a life-long persistence in the host after primary infection and is associated with certain malignancies, which are resistant to conventional chemotherapeutic agents with a poor prognosis. Latency-associated nuclear antigen 1 (LANA1) encoded by KSHV is essential for segregation, replication and maintenance of viral genome. In addition, LANA1 upregulates the transcriptional activity of signal transducer and activator of transcription 3 (STAT3), which plays an important role in promoting survival of KSHV-associated primary effusion lymphoma (PEL) cells. Furthermore, LANA1 mediates transcriptional modulation of KSHV and host genome in host cells. In the present study, the antitumor effect of triptolide was assessed. CCK-8 assays were performed to demonstrate that the proliferations of PEL cells were efficiently inhibited by triptolide in a dose- and time-dependent manner. Flow cytometric results indicated that triptolide induced cell cycle arrest and apoptosis. Western blot results suggested that triptolide downregulated LANA1 expression and reduced half-life of LANA1 in the KSHV-infected malignant cells. Viral titer experiments indicated that triptolide treatment impaired the number of viral DNA copies and the production of virions in BCBL-1 cells. Triptolide also suppressed STAT3 activity and inhibited secretion of IL-6 in PEL cells. In a mouse xenograft model of primary effusion lymphoma by BCBL-1 cells, triptolide treatment significantly inhibited ascites formation and diffused organ infiltration. These results indicate that triptolide impairs the expression of LANA1 and shows antitumor activity against PEL in vitro and in vivo. Triptolide may be a potential agent for treatment of PEL.

KSHV viral cyclin interferes with T-cell development and induces lymphoma through Cdk6 and Notch activation in vivo

Kaposi's sarcoma herpesvirus (KSHV)-encoded v-cyclin, a homolog of cellular cyclin D2, activates cellular CDK6, promotes G1-S transition of the cell cycle, induces DNA damage, apoptosis, autophagy and is reported to have oncogenic potential. Here we show that in vivo expression of v-cyclin in the B- and T-cell lymphocyte compartments results in a markedly low survival due to high penetrance of early-onset T-cell lymphoma and pancarditis. The v-cyclin transgenic mice have smaller pre-tumorigenic lymphoid organs, showing decreased cellularity, and increased proliferation and apoptosis. Furthermore, v-cyclin expression resulted in decreased amounts of CD3-expressing mature T-cells in the secondary lymphoid organs concurrent with alterations in the T-cell subpopulations of the thymus. This suggests that v-cyclin interferes with normal T-cell development. As the Notch pathway is recognized for its role in both T-cell development and lymphoma initiation, we addressed the role of Notch in the v-cyclin-induced alterations. Fittingly, we demonstrate induction of Notch3 and Hes1 in the pre-tumorigenic thymi and lymphomas of v-cyclin expressing mice, and show that lymphoma growth and viability are dependent on activated Notch signaling. Notch3 transcription and growth of the lymphomas was dependent on CDK6, as determined by silencing of CDK6 expression or chemical inhibition, respectively. Our work here reveals a viral cyclin-CDK6 complex as an upstream regulator of Notch receptor, suggesting that cyclins can play a role in the initiation of Notch-dependent lymphomagenesis.

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.

KSHV vCyclin counters the senescence/G1 arrest response triggered by NF-κB hyperactivation

Many oncogenic viruses activate NF-κB as a part of their replicative cycles. We have shown recently that persistent and potentially oncogenic activation of NF-κB by the human T-lymphotropic virus 1 (HTLV-1) oncoprotein Tax immediately triggers a host senescence response mediated by cyclin-dependent kinase inhibitors: p21^CIP1/WAF1 (p21) and p27^Kip1 (p27) Here we demonstrate that RelA/NF-κB activation by Kaposi sarcoma herpesvirus (KSHV) latency protein vFLIP also leads to p21/p27 up-regulation and G1 cell cycle arrest. Remarkably, KSHV vCyclin, another latency protein co-expressed with vFLIP from a bicistronic latency-specific mRNA, was found to prevent the senescence and G1 arrest induced by HTLV-1 Tax and vFLIP respectively. This is due to the known ability of vCyclin/CDK6 complex to resist p21 and p27 inhibition and cause p27 degradation²³. In KSHV-transformed BCBL-1 cells, sustained vFLIP expression with shRNA-mediated vCyclin depletion resulted in G1 arrest. The functional interdependence of vFLIP and vCyclin explains why they are co-translated from the same viral mRNA. Importantly, deregulation of the G1 cyclin-dependent kinase can facilitate chronic IKK/NF-κB activation.

Viral cyclin promotes KSHV-induced cellular transformation and tumorigenesis by overriding contact inhibition

Kaposi sarcoma-associated herpesvirus (KSHV) is a tumor virus encoding several proto-oncogenes. However, the roles of these viral genes in KSHV-induced tumorigenesis have not been defined. In this study, we used a recently developed model of KSHV-induced cellular transformation and tumorigenesis combining with a reverse genetic system to examine the role of a KSHV latent gene vCyclin (ORF72), a cellular Cyclin D2 homolog, in KSHV-induced oncogenesis. Deletion of vCyclin did not affect cell proliferation and cell cycle progression at a low-density condition, when cells were at an active proliferation state. However, vCyclin mutant cells were contact-inhibited and arrested at G 1 phase at a high-density condition. As a result, vCyclin mutant cells formed less and smaller colonies in soft agar assay. Nude mice inoculated with vCyclin mutant cells had reduced tumor incidence and extended tumor latency and survival compared with mice inoculated with wild-type (WT) virus-infected cells. WT but not mutant virus effectively induced Cyclin-dependent kinase inhibitor p27/Kip1 Ser10 phosphorylation and cytoplasmic relocalization. shRNA knockdown of p27 released the blockage of the mutant cells from cell cycle arrest at G 1 phase at a high-density condition. Together, these results indicate that vCyclin primarily functions to enhance cellular transformation and tumorigenesis by promoting cell cycle progression and cell proliferation at a contact-inhibited condition.

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.

The Ubiquitin System and Kaposi's Sarcoma-Associated Herpesvirus

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

Nucleophosmin phosphorylation by v-cyclin-CDK6 controls KSHV latency

Nucleophosmin (NPM) is a multifunctional nuclear phosphoprotein and a histone chaperone implicated in chromatin organization and transcription control. Oncogenic Kaposi's sarcoma herpesvirus (KSHV) is the etiological agent of Kaposi's sarcoma, primary effusion lymphoma (PEL) and multicentric Castleman disease (MCD). In the infected host cell KSHV displays two modes of infection, the latency and productive viral replication phases, involving extensive viral DNA replication and gene expression. A sustained balance between latency and reactivation to the productive infection state is essential for viral persistence and KSHV pathogenesis. Our study demonstrates that the KSHV v-cyclin and cellular CDK6 kinase phosphorylate NPM on threonine 199 (Thr199) in de novo and naturally KSHV-infected cells and that NPM is phosphorylated to the same site in primary KS tumors. Furthermore, v-cyclin-mediated phosphorylation of NPM engages the interaction between NPM and the latency-associated nuclear antigen LANA, a KSHV-encoded repressor of viral lytic replication. Strikingly, depletion of NPM in PEL cells leads to viral reactivation, and production of new infectious virus particles. Moreover, the phosphorylation of NPM negatively correlates with the level of spontaneous viral reactivation in PEL cells. This work demonstrates that NPM is a critical regulator of KSHV latency via functional interactions with v-cyclin and LANA.

Latency is the predominant mode of viral persistence in KS and PEL tumors, and has a fundamental impact on KSHV tumorigenesis. Establishment and maintenance of latency involves a number of viral and cellular factors. This study provides a novel functional link between LANA and v-cyclin by showing that phosphorylation of nucleophosmin (NPM) by the v-cyclin-CDK6 kinase complex supports its interaction with LANA, and thus enables the transcriptional silencing of KSHV lytic genes needed for latency. These findings indicate that KSHV has evolved mechanisms to utilize host proteins for maintaining the latency, and underscores the role of NPM as a regulator of not only mammalian transcription but also of viral transcription. Taken together, our data suggests that a cellular protein, NPM, is a critical factor for the latency of this oncogenic human virus, and may thus represent an attractive novel target for intervention.

Post-transplant lymphoproliferative disorders: role of viral infection, genetic lesions and antigen stimulation in the pathogenesis of the disease

Post-transplant lymphoproliferative disorders (PTLD) are a life-threatening complication of solid organ transplantation or, more rarely, hematopoietic stem cell transplantation. The majority of PTLD is of B-cell origin and associated with Epstein–Barr virus (EBV) infection. PTLD generally display involvement of extranodal sites, aggressive histology and aggressive clinical behavior. The molecular pathogenesis of PTLD involves infection by oncogenic viruses, namely EBV, as well as genetic or epigenetic alterations of several cellular genes. At variance with lymphoma arising in immunocompetent hosts, whose genome is relatively stable, a fraction of PTLD are characterized by microsatellite instability as a consequence of defects in the DNA mismatch repair mechanism. Apart from microsatellite instability, molecular alterations of cellular genes recognized in PTLD include alterations of cMYC, BCL6, TP53, DNA hypermethylation, and aberrant somatic hypermutation of protooncogenes. The occurrence of IGV mutations in the overwhelming majority of PTLD documents that malignant transformation targets germinal centre (GC) B-cells and their descendants both in EBV–positive and EBV–negative cases. Analysis of phenotypic markers of B-cell histogenesis, namely BCL6, MUM1 and CD138, allows further distinction of PTLD histogenetic categories. PTLD expressing the BCL6+/MUM1+/-/CD138− profile reflect B-cells actively experiencing the GC reaction, and comprise diffuse large B-cell lymphoma (DLBCL) centroblastic and Burkitt lymphoma. PTLD expressing the BCL6−/MUM1+/CD138− phenotype putatively derive from B-cells that have concluded the GC reaction, and comprise the majority of polymorphic PTLD and a fraction of DLBCL immunoblastic. A third group of PTLD is reminiscent of post-GC and preterminally differentiated B-cells that show the BCL6−/MUM1+/CD138+ phenotype, and are morphologically represented by either polymorphic PTLD or DLBCL immunoblastic.

Viral oncogene-induced DNA damage response is activated in Kaposi sarcoma tumorigenesis

Kaposi sarcoma is a tumor consisting of Kaposi sarcoma herpesvirus (KSHV)–infected tumor cells that express endothelial cell (EC) markers and viral genes like v-cyclin, vFLIP, and LANA. Despite a strong link between KSHV infection and certain neoplasms, de novo virus infection of human primary cells does not readily lead to cellular transformation. We have studied the consequences of expression of v-cyclin in primary and immortalized human dermal microvascular ECs. We show that v-cyclin, which is a homolog of cellular D-type cyclins, induces replicative stress in ECs, which leads to senescence and activation of the DNA damage response. We find that antiproliferative checkpoints are activated upon KSHV infection of ECs, and in early-stage but not late-stage lesions of clinical Kaposi sarcoma specimens. These are some of the first results suggesting that DNA damage checkpoint response also functions as an anticancer barrier in virally induced cancers.

Recent findings have indicated that DNA hyper-replication triggered by oncogenes can induce cellular senescence, which together with the oncogene-induced DNA damage checkpoint confers a barrier to tumorigenesis. Kaposi sarcoma herpesvirus (KSHV) can infect human dermal microvascular endothelial cells (ECs) in vitro, but KSHV infection does not seem to provide growth advantage to the cells, but rather leads to retarded growth. Moreover, the proliferative index has long been known to be low in KSHV-infected spindle cells in Kaposi sarcoma (KS) tumors. Our results provide an explanation for these observations by showing that activation of the DNA damage response, exerted by KSHV and a latent viral protein v-cyclin, functions as a barrier against transformation of KSHV-infected cells. Interestingly, the antiproliferative checkpoints are activated during the initial stages of KSHV infection and KS tumorigenesis. During the course of infection, the infected cells are imposed to overcome the checkpoint, and oncogenic stress elicited by the expression of v-cyclin may further contribute to the induction of genomic instability and malignant transformation.

Molecular biology of KSHV in relation to AIDS-associated oncogenesis

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

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

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

Phosphorylation of the cyclin-dependent kinase inhibitor p21Cip1 on serine 130 is essential for viral cyclin-mediated bypass of a p21Cip1-imposed G1 arrest

K cyclin encoded by Kaposi's sarcoma-associated herpesvirus confers resistance to the cyclin-dependent kinase (cdk) inhibitors p16Ink4A, p21Cip1, and p27Kip1 on the associated cdk6. We have previously shown that K cyclin expression enforces S-phase entry on cells overexpressing p27Kip1 by promoting phosphorylation of p27Kip1 on threonine 187, triggering p27Kip1 down-regulation. Since p21Cip1 acts in a manner similar to that of p27Kip1, we have investigated the subversion of a p21Cip1-induced G1 arrest by K cyclin. Here, we show that p21Cip1 is associated with K cyclin both in overexpression models and in primary effusion lymphoma cells and is a substrate of the K cyclin/cdk6 complex, resulting in phosphorylation of p21Cip1 on serine 130. This phosphoform of p21Cip1 appeared unable to associate with cdk2 in vivo. We further demonstrate that phosphorylation on serine 130 is essential for K cyclin-mediated release of a p21Cip1-imposed G1 arrest. Moreover, we show that under physiological conditions of cell cycle arrest due to elevated levels of p21Cip1 resulting from oxidative stress, K cyclin expression enabled S-phase entry and was associated with p21Cip1 phosphorylation and partial restoration of cdk2 kinase activity. Thus, expression of the viral cyclin enables cells to subvert the cell cycle inhibitory function of p21Cip1 by promoting cdk6-dependent phosphorylation of this antiproliferative protein.

Cell signaling pathways engaged by KSHV

Kaposi's sarcoma herpesvirus (KSHV) is the eighth human herpesvirus discovered in 1994 from Kaposi's sarcoma lesion of an AIDS patient. The strong molecular and epidemiological links associating KSHV with Kaposi's sarcoma and certain lymphoproliferative disorders indicate that KSHV is required for the development of these malignancies. Although KSHV is equipped to manipulate and deregulate several cellular signaling pathways, it is not yet understood how this leads to cell transformation. Profound understanding of the interplay of viral and cellular factors in KSHV-infected cells will provide valuable information on the mechanisms of viral tumorigenesis and enable development of efficient targeted therapies for virus-induced cancers. This review focuses on the cellular signaling pathways that KSHV gene products impinge on and discusses their putative contribution to tumorigenesis.

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)

Characterization of murine gammaherpesvirus 68 v-cyclin interactions with cellular cdks

All known gamma2-herpesviruses encode a cyclin homolog with significant homology to mammalian D-type cyclins. The murine gammaherpesvirus 68 (gammaHV68) viral cyclin (v-cyclin) has been shown to be oncogenic when expression is targeted to thymocytes in transgenic mice and to be critical for virus reactivation from latency. Here, we investigate the interaction of the gammaHV68 v-cyclin with cellular cyclin-dependent kinases (cdks). We show that, in contrast to the Kaposi's sarcoma-associated herpesvirus (KSHV) v-cyclin, the gammaHV68 v-cyclin preferentially interacts with cdk2 and cdc2 but does not interact with either cdk4 or cdk6. Mutation of conserved residues, predicted to be involved in cdk binding based on the gammaHV68 v-cyclin:cdk2 crystal structure, resulted in the loss of both cdk binding and the ability to mediate phosphorylation of substrates. Like the KSHV v-cyclin, the gammaHV68 v-cyclin appears to confer expanded substrate specificity to the cellular cdk binding partners. As expected, the gammaHV68 v-cyclin:cdk complexes are able to target phosphorylation of histone H1, the retinoblastoma protein (pRb), and p27(Kip1) as assessed using in vitro kinase assays. Notably, hyperphosphorylation of pRb was observed during wt gammaHV68 replication in serum-starved murine fibroblasts, but not in cells that were either mock-infected or infected with a v-cyclin null gammaHV68. In addition, infection of serum-starved murine fibroblasts also results in a v-cyclin-dependent increase in cdk2-associated kinase activity and a concomitant decrease in the levels of p27(Kip1). Taken together, the latter studies served to validate the results of the in vitro kinase assays. Finally, in vitro kinase assays revealed that the gammaHV68 v-cyclin:cdk complexes can also phosphorylate p21(Cip1), Bcl-2, and p53. The latter suggests that, at least in vitro, the gammaHV68 v-cyclin exhibits functional characteristics of both cyclin E and cyclin A.