The human herpes virus 8-encoded viral FLICE inhibitory protein physically associates with and persistently activates the Ikappa B kinase complex

HIV-associated cancers and lymphoproliferative disorders caused by Kaposi sarcoma herpesvirus and Epstein-Barr virus

SUMMARYWithin weeks of the first report of acquired immunodeficiency syndrome (AIDS) in 1981, it was observed that these patients often had Kaposi sarcoma (KS), a hitherto rarely seen skin tumor in the USA. It soon became apparent that AIDS was also associated with an increased incidence of high-grade lymphomas caused by Epstein-Barr virus (EBV). The association of AIDS with KS remained a mystery for more than a decade until Kaposi sarcoma-associated herpesvirus (KSHV) was discovered and found to be the cause of KS. KSHV was subsequently found to cause several other diseases associated with AIDS and human immunodeficiency virus (HIV) infection. People living with HIV/AIDS continue to have an increased incidence of certain cancers, and many of these cancers are caused by EBV and/or KSHV. In this review, we discuss the epidemiology, virology, pathogenesis, clinical manifestations, and treatment of cancers caused by EBV and KSHV in persons living with HIV.

Activation of glucocorticoid receptor signaling inhibits KSHV-induced inflammation and tumorigenesis

IMPORTANCE

Kaposi's sarcoma (KS) is the most common cancer in HIV-infected patients caused by Kaposi's sarcoma-associated herpesvirus (KSHV) infection. Hyperinflammation is the hallmark of KS. In this study, we have shown that KSHV mediates hyperinflammation by inducing IL-1α and suppressing IL-1Ra. Mechanistically, KSHV miRNAs and vFLIP induce hyperinflammation by activating the NF-κB pathway. A common anti-inflammatory agent dexamethasone blocks KSHV-induced hyperinflammation and tumorigenesis by activating glucocorticoid receptor signaling to suppress IL-1α and induce IL-1Ra. This work has identified IL-1-mediated inflammation as a potential therapeutic target and dexamethasone as a potential therapeutic agent for KSHV-induced malignancies.

Activation of glucocorticoid receptor signaling inhibits KSHV-induced inflammation and tumorigenesis

Hyperinflammation is the hallmark of Kaposi's sarcoma (KS), the most common cancer in AIDS patients caused by Kaposi's sarcoma-associated herpesvirus (KSHV) infection. However, the role and mechanism of induction of inflammation in KS remain unclear. In a screening for inhibitors of KSHV-induced oncogenesis, over half of the identified candidates were anti-inflammatory agents including dexamethasone functions by activating glucocorticoid receptor (GR) signaling. Here, we examined the mechanism mediating KSHV-induced inflammation. We found that numerous inflammatory pathways were activated in KSHV-transformed cells. Particularly, interleukin-1 alpha (IL-1α) and IL-1 receptor antagonist (IL-1Ra) from the IL-1 family were the most induced and suppressed cytokines, respectively. We found that KSHV miRNAs mediated IL-1α induction while both miRNAs and vFLIP mediated IL-1Ra suppression. Furthermore, GR signaling was inhibited in KSHV-transformed cells, which was mediated by vFLIP and vCyclin. Dexamethasone treatment activated GR signaling, and inhibited cell proliferation and colony formation in soft agar of KSHV-transformed cells but had a minimal effect on matched primary cells. Consequently, dexamethasone suppressed the initiation and growth of KSHV-induced tumors in mice. Mechanistically, dexamethasone suppressed IL-1α but induced IL-1Ra expression. Treatment with recombinant IL-1α protein rescued the inhibitory effect of dexamethasone while overexpression of IL-1Ra caused a weak growth inhibition of KSHV-transformed cells. Furthermore, dexamethasone induced IκBα expression resulting in inhibition of NF-κB pathway and IL-1α expression. These results reveal an important role of IL-1 pathway in KSHV-induced inflammation and oncogenesis, which can be inhibited by dexamethasone-activated GR signaling, and identify IL-1-mediated inflammation as a potential therapeutic target for KSHV-induced malignancies.

CRISPR screens identify novel regulators of cFLIP dependency and ligand-independent, TRAIL-R1-mediated cell death

Kaposi's sarcoma-associated herpesvirus (KSHV) causes primary effusion lymphoma (PEL). PEL cell lines require expression of the cellular FLICE inhibitory protein (cFLIP) for survival, although KSHV encodes a viral homolog of this protein (vFLIP). Cellular and viral FLIP proteins have several functions, including, most importantly, the inhibition of pro-apoptotic caspase 8 and modulation of NF-κB signaling. To investigate the essential role of cFLIP and its potential redundancy with vFLIP in PEL cells, we first performed rescue experiments with human or viral FLIP proteins known to affect FLIP target pathways differently. The long and short isoforms of cFLIP and molluscum contagiosum virus MC159L, which are all strong caspase 8 inhibitors, efficiently rescued the loss of endogenous cFLIP activity in PEL cells. KSHV vFLIP was unable to fully rescue the loss of endogenous cFLIP and is therefore functionally distinct. Next, we employed genome-wide CRISPR/Cas9 synthetic rescue screens to identify loss of function perturbations that can compensate for cFLIP knockout. Results from these screens and our validation experiments implicate the canonical cFLIP target caspase 8 and TRAIL receptor 1 (TRAIL-R1 or TNFRSF10A) in promoting constitutive death signaling in PEL cells. However, this process was independent of TRAIL receptor 2 or TRAIL, the latter of which is not detectable in PEL cell cultures. The requirement for cFLIP is also overcome by inactivation of the ER/Golgi resident chondroitin sulfate proteoglycan synthesis and UFMylation pathways, Jagunal homolog 1 (JAGN1) or CXCR4. UFMylation and JAGN1, but not chondroitin sulfate proteoglycan synthesis or CXCR4, contribute to TRAIL-R1 expression. In sum, our work shows that cFLIP is required in PEL cells to inhibit ligand-independent TRAIL-R1 cell death signaling downstream of a complex set of ER/Golgi-associated processes that have not previously been implicated in cFLIP or TRAIL-R1 function.

Bcl-xL is required to protect endothelial cells latently infected with KSHV from virus induced intrinsic apoptosis

Kaposi's Sarcoma herpesvirus (KSHV) is the etiologic agent of Kaposi's Sarcoma (KS), a highly vascularized tumor common in AIDS patients and many countries in Africa. KSHV is predominantly in the latent state in the main KS tumor cell, the spindle cell, a cell expressing endothelial cell markers. To identify host genes important for KSHV latent infection of endothelial cells we previously used a global CRISPR/Cas9 screen to identify genes necessary for the survival or proliferation of latently infected cells. In this study we rescreened top hits and found that the highest scoring gene necessary for infected cell survival is the anti-apoptotic Bcl-2 family member Bcl-xL. Knockout of Bcl-xL or treatment with a Bcl-xL inhibitor leads to high levels of cell death in latently infected endothelial cells but not their mock counterparts. Cell death occurs through apoptosis as shown by increased PARP cleavage and activation of caspase-3/7. Knockout of the pro-apoptotic protein, Bax, eliminates the requirement for Bcl-xL. Interestingly, neither Bcl-2 nor Mcl-1, related and often redundant anti-apoptotic proteins of the Bcl-2 protein family, are necessary for the survival of latently infected endothelial cells, likely due to their lack of expression in all the endothelial cell types we have examined. Bcl-xL is not required for the survival of latently infected primary effusion lymphoma (PEL) cells or other cell types tested. Expression of the KSHV major latent locus alone in the absence of KSHV infection led to sensitivity to the absence of Bcl-xL, indicating that viral gene expression from the latent locus induces intrinsic apoptosis leading to the requirement for Bcl-xL in endothelial cells. The critical requirement of Bcl-xL during KSHV latency makes it an intriguing therapeutic target for KS tumors.

Cancers associated with human gammaherpesviruses

Epstein-Barr virus (EBV; human herpesvirus 4; HHV-4) and Kaposi sarcoma-associated herpesvirus (KSHV; human herpesvirus 8; HHV-8) are human gammaherpesviruses that have oncogenic properties. EBV is a lymphocryptovirus, whereas HHV-8/KSHV is a rhadinovirus. As lymphotropic viruses, EBV and KSHV are associated with several lymphoproliferative diseases or plasmacytic/plasmablastic neoplasms. Interestingly, these viruses can also infect epithelial cells causing carcinomas and, in the case of KSHV, endothelial cells, causing sarcoma. EBV is associated with Burkitt lymphoma, classic Hodgkin lymphoma, nasopharyngeal carcinoma, plasmablastic lymphoma, lymphomatoid granulomatosis, leiomyosarcoma, and subsets of diffuse large B-cell lymphoma, post-transplant lymphoproliferative disorder, and gastric carcinoma. KSHV is implicated in Kaposi sarcoma, primary effusion lymphoma, multicentric Castleman disease, and KSHV-positive diffuse large B-cell lymphoma. Pathogenesis by these two herpesviruses is intrinsically linked to viral proteins expressed during the lytic and latent lifecycles. This comprehensive review intends to provide an overview of the EBV and KSHV viral cycles, viral proteins that contribute to oncogenesis, and the current understanding of the pathogenesis and clinicopathology of their related neoplastic entities.

The role of viruses in HIV-associated lymphomas

Lymphomas are among the most common cancers in people with HIV (PWH). The lymphoma subtypes and pathogenesis of lymphoma in PWH are different from the immunocompetent population. It is well-known that HIV causes severe CD4+ T cell lymphopenia in the absence of antiretroviral therapy (ART); however, the risk of developing certain subtypes of lymphoma remains elevated even in people receiving ART with preserved CD4+ T cells. HIV contributes to lymphomagenesis and causes decreased immune surveillance via T cell depletion and dysregulation, B cell dysregulation, and the potential contribution of HIV-encoded proteins. The oncogenic gammaherpesviruses, Epstein-Barr virus (EBV) and Kaposi sarcoma herpesvirus (KSHV, also known as human herpesvirus 8), are the causative agents in the majority of HIV-associated lymphomas. HIV-associated T cell depletion and dysregulation allows EBV and KSHV to proliferate in infected B cells. Specific EBV- and KSHV-encoded proteins participate in B cell activation, and proliferation leading to B cell transformation. Understanding the distinct pathogenesis of HIV-associated lymphomas affords opportunities to develop therapies that specifically target these unique aspects and improve lymphoma outcomes in PWH. Agents being studied that target the specific roles of HIV, EBV, and KSHV in lymphomagenesis include immunotherapies, targeted agents, and cellular therapies.

KSHV/HHV8-mediated hematologic diseases

Kaposi sarcoma (KS) herpesvirus (KSHV), also known as human herpesvirus 8, is the causal agent of KS but is also pathogenetically related to several lymphoproliferative disorders, including primary effusion lymphoma (PEL)/extracavitary (EC) PEL, KSHV-associated multicentric Castleman disease (MCD), KSHV+ diffuse large B-cell lymphoma, and germinotropic lymphoproliferative disorder. These different KSHV-associated diseases may co-occur and may have overlapping features. KSHV, similar to Epstein-Barr virus (EBV), is a lymphotropic gammaherpesvirus that is preferentially present in abnormal lymphoid proliferations occurring in immunecompromised individuals. Notably, both KSHV and EBV can infect and transform the same B cell, which is frequently seen in KSHV+ EBV+ PEL/EC-PEL. The mechanisms by which KSHV leads to lymphoproliferative disorders is thought to be related to the expression of a few transforming viral genes that can affect cellular proliferation and survival. There are critical differences between KSHV-MCD and PEL/EC-PEL, the 2 most common KSHV-associated lymphoid proliferations, including viral associations, patterns of viral gene expression, and cellular differentiation stage reflected by the phenotype and genotype of the infected abnormal B cells. Advances in treatment have improved outcomes, but mortality rates remain high. Our deepening understanding of KSHV biology, clinical features of KSHV-associated diseases, and newer clinical interventions should lead to improved and increasingly targeted therapeutic interventions.

Interaction between the Hepatitis B Virus and Cellular FLIP Variants in Viral Replication and the Innate Immune System

During viral evolution and adaptation, many viruses have utilized host cellular factors and machinery as their partners. HBx, as a multifunctional viral protein encoded by the hepatitis B virus (HBV), promotes HBV replication and greatly contributes to the development of HBV-associated hepatocellular carcinoma (HCC). HBx interacts with several host factors in order to regulate HBV replication and evolve carcinogenesis. The cellular FADD-like IL-1β-converting enzyme (FLICE)-like inhibitory protein (c-FLIP) is a major factor that functions in a variety of cellular pathways and specifically in apoptosis. It has been shown that the interaction between HBx and c-FLIP determines HBV fate. In this review, we provide a comprehensive and detailed overview of the interplay between c-FLIP and HBV in various environmental circumstances. We describe strategies adapted by HBV to establish its chronic infection. We also summarize the conventional roles of c-FLIP and highlight the functional outcome of the interaction between c-FLIP and HBV or other viruses in viral replication and the innate immune system.

Antitumor effect of acanthoic acid against primary effusion lymphoma via inhibition of c-FLIP

Acanthoic acid (AA) is an active substance that is extracted from Croton oblongifolius Roxb., a traditional plant in Thailand. The antiinflammatory effect of AA on NF-κB pathway has been exclusively reported, however, its anticancer effect is still lacking. PEL is a B cell lymphoma that is mostly found in HIV patients. The prognosis and progression of PEL patients are terribly poor with a median survival time less than 6 months, so the new effective treatment is urgently needed. In this study, we found that AA effectively inhibited PEL cell proliferation with IC50s at 120-130 μM in well-representative cells, while the IC50s of AA in PBMC were higher (>200 μM). AA increased percentages of Annexin V/PI positive cells, whereas adding of caspase inhibitor (Q-VD-OPh) prevented AA-induced cell death. The antiapoptotic protein, c-FLIP, was downregulated by AA which leading to the activation of caspase-8 and -3. Combination of AA and TRAIL dramatically enhanced apoptotic cell death. In PEL xenograft model, AA at the dose of 250 mg/kg effectively inhibited PEL tumor growth without detectable toxicities assessed by mice weight and appearance.

Clinical and Therapeutic Implications of Epstein-Barr Virus in HIV-Related Lymphomas

The incidence of lymphomas is increased in people living with HIV (PLWH). Aggressive B-cell non-Hodgkin lymphomas (NHLs) are the most common and are considered an AIDS-defining cancer (ADC). Although Hodgkin lymphoma (HL) is not considered an ADC, its incidence is also increased in PLWH. Among all HIV-related lymphomas (HRL), the prevalence of Epstein-Barr virus (EBV) is high. It has been shown that EBV is involved in different lymphomagenic mechanisms mediated by some of its proteins, contributing to the development of different lymphoma subtypes. Additionally, cooperation between both HIV and EBV can lead to the proliferation of aberrant B-cells, thereby being an additional lymphomagenic mechanism in EBV-associated HRL. Despite the close relationship between EBV and HRL, the impact of EBV on clinical aspects has not been extensively studied. These lymphomas are treated with the same therapeutic regimens as the general population in combination with cART. Nevertheless, new therapeutic strategies targeting EBV are promising for these lymphomas. In this article, the different types of HRL are extensively reviewed, focusing on the influence of EBV on the epidemiology, pathogenesis, clinical presentation, and pathological characteristics of each lymphoma subtype. Moreover, novel therapies targeting EBV and future strategies to treat HRL harboring EBV are discussed.

MALT1 as a promising target to treat lymphoma and other diseases related to MALT1 anomalies

Mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) is a key adaptor protein that regulates the NF-κB pathway, in which MALT1 functions as a scaffold protein and protease to trigger downstream signals. The abnormal expression of MALT1 is closely associated with lymphomagenesis and other diseases, including solid tumors and autoimmune diseases. MALT1 is the only protease in the underlying pathogenesis of these diseases, and its proteolytic activity can be pharmacologically regulated. Therefore, MALT1 is a potential and promising target for anti-lymphoma and other MALT1-related disease treatments. Currently, the development of MALT1 inhibitors is still in its early stages. This review presents an overview of MALT1, particularly its X-ray structures and biological functions, and elaborates on the pathogenesis of diseases associated with its dysregulation. We then summarize previously reported MALT1 inhibitors, focusing on their molecular structure, biological activity, structure-activity relationship, and limitations. Finally, we propose future research directions to accelerate the discovery of novel MALT1 inhibitors with clinical applications. Overall, this review provides a comprehensive and systematic overview of MALT1-related research advances and serves as a theoretical basis for drug discovery and research.

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.

Modulation of virus-induced NF-κB signaling by NEMO coiled coil mimics

Protein-protein interactions featuring intricate binding epitopes remain challenging targets for synthetic inhibitors. Interactions of NEMO, a scaffolding protein central to NF-κB signaling, exemplify this challenge. Various regulators are known to interact with different coiled coil regions of NEMO, but the topological complexity of this protein has limited inhibitor design. We undertook a comprehensive effort to block the interaction between vFLIP, a Kaposi’s sarcoma herpesviral oncoprotein, and NEMO using small molecule screening and rational design. Our efforts reveal that a tertiary protein structure mimic of NEMO is necessary for potent inhibition. The rationally designed mimic engages vFLIP directly causing complex disruption, protein degradation and suppression of NF-κB signaling in primary effusion lymphoma (PEL). NEMO mimic treatment induces cell death and delays tumor growth in a PEL xenograft model. Our studies with this inhibitor reveal the critical nexus of signaling complex stability in the regulation of NF-κB by a viral oncoprotein.

NF-κB signalling involves the scaffold protein NEMO, which can be bound by the oncoprotein vFLIP to promote cell survival and oncogenic transformation. Here the authors rationally engineer a tertiary protein mimic of NEMO to disrupt the vFLIP-NEMO interaction to induce cell death.

Narciclasine, an isocarbostyril alkaloid, has preferential activity against primary effusion lymphoma

Primary effusion lymphoma (PEL) is a subtype of non-Hodgkin lymphoma associated with infection by Kaposi sarcoma-associated herpes virus (KSHV). PEL is an aggressive disease with extremely poor prognosis when treated with conventional chemotherapy. Narciclasine, a natural product present in Amaryllidaceae family of flowering plants including daffodils, belongs to a class of molecules termed ‘isocarbostyril alkaloid’. We have found that narciclasine displays preferential cytotoxicity towards PEL at low nanomolar concentrations and is approximately 10 and 100-fold more potent than its structural analogs lycoricidine and lycorine, respectively. Narciclasine arrested cell-cycle progression at the G₁ phase and induced apoptosis in PEL, which is accompanied by activation of caspase-3/7, cleavage of PARP and increase in the surface expression of Annexin-V. Although narciclasine treatment resulted in a marked decrease in the expression of MYC and its direct target genes,time-course experiments revealed that MYC is not a direct target of narciclasine. Narciclasine treatment neither induces the expression of KSHV-RTA/ORF50 nor the production of infectious KSHV virions in PEL. Finally, narciclasine provides dramatic survival advantages to mice in two distinct mouse xenograft models of PEL. In conclusion, our results suggest that narciclasine could be a promising agent for the treatment of PEL.

Viral FLIP blocks Caspase-8 driven apoptosis in the gut in vivo

A strict cell death control in the intestinal epithelium is indispensable to maintain barrier integrity and homeostasis. In order to achieve a balance between cell proliferation and cell death, a tight regulation of Caspase-8, which is a key player in controlling apoptosis, is required. Caspase-8 activity is regulated by cellular FLIP proteins. These proteins are expressed in different isoforms (cFLIP_long and cFLIP_short) which determine cell death and survival. Interestingly, several viruses encode FLIP proteins, homologous to cFLIP_short, which are described to regulate Caspase-8 and the host cell death machinery. In the current study a mouse model was generated to show the impact of viral FLIP (vFLIP) from Kaposi’s Sarcoma-associated Herpesvirus (KSHV)/ Human Herpesvirus-8 (HHV-8) on cell death regulation in the gut. Our results demonstrate that expression of vFlip in intestinal epithelial cells suppressed cFlip expression, but protected mice from lethality, tissue damage and excessive apoptotic cell death induced by genetic cFlip deletion. Finally, our model shows that vFlip expression decreases cFlip mediated Caspase-8 activation in intestinal epithelial cells. In conclusion, our data suggests that viral FLIP neutralizes and compensates for cellular FLIP, efficiently counteracting host cell death induction and facilitating further propagation in the host organism.

δ-Aminolevulinate induces fetal hemoglobin expression by enhancing cellular heme biosynthesis

Sickle cell disease (SCD) and β-thalassemia are inherited blood disorders caused by genetic defects in the β-globin gene on chromosome 11, producing severe disease in people worldwide. Induction of fetal hemoglobin consisting of two α-globin and two γ-globin chains ameliorates the clinical symptoms of both disorders. In the present study, we investigated the ability of δ-aminolevulinate (ALA), the heme precursor, to activate γ-globin gene expression as well as its effects on cellular functions in erythroid cell systems. We demonstrated that ALA induced γ-globin expression at both the transcriptional and protein levels in the KU812 erythroid cell line. Using inhibitors targeting two enzymes in the heme biosynthesis pathway, we showed that cellular heme biosynthesis was involved in ALA-mediated γ-globin activation. Moreover, the transcription factor NRF2 (nuclear factor [erythroid-derived 2]-like 2), a critical regulator of the cellular antioxidant response, was activated by ALA and contributed to mechanisms of γ-globin activation; ALA did not affect cell proliferation and was not toxic to cells. Subsequent studies demonstrated ALA-induced γ-globin activation in erythroid progenitors generated from normal human CD34+ stem cells. These data support future study to explore the potential of stimulating intracellular heme biosynthesis by ALA or similar compounds as a novel therapeutic strategy for treating SCD and β-thalassemia.

Impact statement

Inherited mutations in the β-globin-like genes result in the most common forms of genetic blood disease including sickle cell disease (SCD) and β-thalassemia worldwide. Therefore, effective inexpensive therapies that can be distributed widely are highly desirable. Currently, drug-mediated fetal hemoglobin (HbF) induction can ameliorate clinical symptoms of SCD and β-thalassemia and is the most effective strategy for developing new therapeutic options. In the current study, we confirmed that δ-Aminolevulinate (ALA), the precursor of heme, induces γ-globin expression at both the transcriptional and translational levels in primary human erythroid progenitors. Moreover, the results indicate activation of the transcription factor NRF2 (nuclear factor (erythroid-derived 2)-like 2) by ALA to enhance HbF expression. These data support future study to explore the potential of stimulating intracellular heme biosynthesis by ALA or similar compounds as a novel therapeutic strategy for treating SCD and β-thalassemia.

Molecular Biology of KSHV in Relation to HIV/AIDS-Associated Oncogenesis

Discovered in 1994, Kaposi's sarcoma-associated herpesvirus (KSHV) has been associated with four human malignancies including Kaposi's sarcoma, primary effusion lymphoma, a subset of multicentric Castleman's disease, and KSHV inflammatory cytokine syndrome. These malignancies mostly occur in immunocompromised patients including patients with acquired immunodeficiency syndrome and often cause significant mortality because of the lack of effective therapies. Significant progresses have been made to understand the molecular basis of KSHV infection and KSHV-induced oncogenesis in the last two decades. This chapter provides an update on the recent advancements focusing on the molecular events of KSHV primary infection, the mechanisms regulating KSHV life cycle, innate and adaptive immunity, mechanism of KSHV-induced tumorigenesis and inflammation, and metabolic reprogramming in KSHV infection and KSHV-transformed cells.

Modulation of Innate Immune Signaling Pathways by Herpesviruses

Herpesviruses can be detected by pattern recognition receptors (PRRs), which then activate downstream adaptors, kinases and transcription factors (TFs) to induce the expression of interferons (IFNs) and inflammatory cytokines. IFNs further activate the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway, inducing the expression of interferon-stimulated genes (ISGs). These signaling events constitute host innate immunity to defeat herpesvirus infection and replication. A hallmark of all herpesviruses is their ability to establish persistent infection in the presence of active immune response. To achieve this, herpesviruses have evolved multiple strategies to suppress or exploit host innate immune signaling pathways to facilitate their infection. This review summarizes the key host innate immune components and their regulation by herpesviruses during infection. Also we highlight unanswered questions and research gaps for future perspectives.

Suppression of the SAP18/HDAC1 complex by targeting TRIM56 and Nanog is essential for oncogenic viral FLICE-inhibitory protein-induced acetylation of p65/RelA, NF-κB activation, and promotion of cell invasion and angiogenesis

Kaposi's sarcoma (KS), a highly invasive and angiogenic tumor of endothelial spindle-shaped cells, is the most common AIDS-associated cancer caused by KS-associated herpesvirus (KSHV) infection. KSHV-encoded viral FLICE-inhibitory protein (vFLIP) is a viral oncogenic protein, but its role in the dissemination and angiogenesis of KSHV-induced cancers remains unknown. Here, we report that vFLIP facilitates cell migration, invasion, and angiogenesis by downregulating the SAP18-HDAC1 complex. vFLIP degrades SAP18 through a ubiquitin-proteasome pathway by recruiting E3 ubiquitin ligase TRIM56. Further, vFLIP represses HDAC1, a protein partner of SAP18, by inhibiting Nanog occupancy on the HDAC1 promoter. Notably, vFLIP impairs the interaction between the SAP18/HDAC1 complex and p65 subunit, leading to enhancement of p65 acetylation and NF-κB activation. Our data suggest a novel mechanism of vFLIP activation of the NF-κB by decreasing the SAP18/HDAC1 complex to promote the acetylation of p65 subunit, which contributes to vFLIP-induced activation of the NF-κB pathway, cell invasion, and angiogenesis. These findings advance our understanding of the mechanism of KSHV-induced pathogenesis, and providing a rationale for therapeutic targeting of the vFLIP/SAP18/HDAC1 complex as a novel strategy of AIDS-KS.

The Expression of CD154 by Kaposi's Sarcoma Cells Mediates the Anti-Apoptotic and Migratory Effects of HIV-1-Tat Protein

Kaposi's sarcoma (KS) is a malignancy associated to conditions of immune system impairment such as HIV-1 infection and post-transplantation therapy. Here we report that HIV-1-Tat protein, at concentrations well below those detected in AIDS patients, up-regulates the expression of both CD40 and CD154 on KS cells. This occurred also in the presence of vincristine, that at doses shown to induce apoptosis decreased the expression of both CD40 and CD154 on KS cells. The treatment with a soluble CD40-muIg fusion protein (CD40 fp) that prevents the binding of CD154 with cell surface CD40, as well as the transfection with a vector for soluble CD40 (KS sCD40), decreased the anti-apoptotic effect of Tat. Moreover, Tat-induced motility of KS cells was inhibited by soluble CD40 fp. Tat also enhanced the expression of intracellular proteins known to transduce signals triggered by CD40 engagement, in particular TRAF-3. Tat as well as soluble CD154 (sCD154) prevented vincristine-induced reduction of TRAF-3 in KS cells transfected with a vector for neomycin resistance (KS psv-neo), but not in KS sCD40. Immunoprecipitation studies showed that Tat induced CD40 / TRAF-3 association and that this binding was abrogated upon the incubation with the soluble CD40 fp. These data suggest that Tat activates the CD40-CD154 pathway by enhancing the membrane expression of CD40 and in particular of CD154, and by activating the TRAF-3-dependent signaling pathway of CD40. These findings indicate that the CD40-CD154 pathway mediates the anti-apoptotic and migratory effects of HIV-1-Tat, suggesting the potential therapeutic benefits of blocking CD40 activation in HIV-1-associated KS.

Chronic intestinal inflammation in mice expressing viral Flip in epithelial cells

Viruses are present in the intestinal microflora and are currently discussed as a potential causative mechanism for the development of inflammatory bowel disease. A number of viruses, such as Human Herpesvirus-8, express homologs to cellular FLIPs, which are major contributors for the regulation of epithelial cell death. In this study we analyzed the consequences of constitutive expression of HHV8-viral FLIP in intestinal epithelial cells (IECs) in mice. Surprisingly, expression of vFlip disrupts tissue homeostasis and induces severe intestinal inflammation. Moreover vFlip^IEC-tg mice showed reduced Paneth cell numbers, associated with excessive necrotic cell death. On a molecular level vFlip expression altered classical and alternative NFκB activation. Blocking of alternative NFκB signaling by deletion of Ikka in vivo largely protected mice from inflammation and Paneth cell loss induced by vFLIP. Collectively, our data provide functional evidence that expression of a single viral protein in IECs can be sufficient to disrupt epithelial homeostasis and to initiate chronic intestinal inflammation.

Transactivation of human endogenous retrovirus K (HERV-K) by KSHV promotes Kaposi's sarcoma development

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of several human cancers such as Kaposi's sarcoma (KS), which represents the most common AIDS-associated malignancy that lacks effective treatment options. Despite its clear role in AIDS malignancies, the fact that only a small set of KSHV-infected patients will eventually develop these tumors implies that additional co-factors are required for the development of KSHV-related cancers. In the current study, we demonstrate for the first time that KSHV de novo infection or viral latent proteins are able to transactivate human endogenous retrovirus K (HERV-K) through a variety of cellular signaling pathways and transcriptional factors. Moreover, we found that HERV-K transactivation, particularly activation of its encoded oncogenic NP9 protein, plays an important role in KSHV pathogenesis and tumorigenesis in vitro and in vivo. Our data provide innovative insights into the mechanisms of HERV-K transactivation contributing to viral oncogenesis, which may represent a promising target for KS treatment.

FUS Negatively Regulates Kaposi's Sarcoma-Associated Herpesvirus Gene Expression

Kaposi’s sarcoma-associated herpesvirus (KSHV) is a human gammaherpesvirus and the etiological agent of Kaposi’s sarcoma. KSHV is also causally associated with the development of lymphoproliferative diseases, including primary effusion lymphoma (PEL). KSHV reactivation from latency plays an integral role in the progression to KSHV-associated disease as several lytic proteins have angiogenic and anti-apoptotic functions essential to the tumor microenvironment. Thus, restriction of KSHV reactivation represents an attractive therapeutic target. Here, we demonstrate that the cellular protein Fused-in-sarcoma (FUS) restricts KSHV lytic reactivation in PEL and in an epithelial cell-based model. Depletion of FUS significantly enhances viral mRNA and protein expression, resulting in increased viral replication and production of infectious virions. Chromatin immunoprecipitation analyses demonstrate that FUS is present at several KSHV lytic cycle genes during the latent stage of infection. We further demonstrate that FUS interacts with RNA polymerase II and negatively affects Serine-2 phosphorylation of its C-terminal domain at the KSHV RTA gene, decreasing nascent RNA synthesis. Knockdown of FUS increases transcription of RTA, thus driving enhanced expression of KSHV lytic genes. Collectively, these data reveal a novel role for FUS in regulating viral gene expression and are the first to demonstrate its role as a viral restriction factor.

Peroxisomes support human herpesvirus 8 latency by stabilizing the viral oncogenic protein vFLIP via the MAVS-TRAF complex

Human herpesvirus 8 (HHV-8) is causally related to human malignancies. HHV-8 latent viral FLICE-inhibitory protein (vFLIP) is a viral oncoprotein that is linked to pathogenesis, but how its expression is regulated is largely unknown. In an attempt to understand the role of the mitochondrial antiviral signaling (MAVS) adaptor in HHV-8 infection, we discovered that vFLIP expression was post-translationally up-regulated by the MAVS signaling complex on peroxisomes. Furthermore, we demonstrated that vFLIP could be targeted to the peroxisomes, where it was oncogenically active, in a PEX19-dependent manner. Targeted disruption of vFLIP and MAVS interaction resulted in a decrease in vFLIP expression and selectively promoted death of latently HHV-8-infected cells, providing therapeutic potential for treating HHV-8 diseases. Collectively, our experimental results suggest novel involvement of peroxisomes and MAVS in the stabilization of vFLIP and thereby in the establishment or maintenance of HHV-8 latency and associated pathogenesis.

Kaposi sarcoma herpesvirus pathogenesis

Kaposi sarcoma herpesvirus (KSHV), taxonomical name human gammaherpesvirus 8, is a phylogenetically old human virus that co-evolved with human populations, but is now only common (seroprevalence greater than 10%) in sub-Saharan Africa, around the Mediterranean Sea, parts of South America and in a few ethnic communities. KSHV causes three human malignancies, Kaposi sarcoma, primary effusion lymphoma, and many cases of the plasmablastic form of multicentric Castleman's disease (MCD) as well as occasional cases of plasmablastic lymphoma arising from MCD; it has also been linked to rare cases of bone marrow failure and hepatitis. As it has colonized humans physiologically for many thousand years, cofactors are needed to allow it to unfold its pathogenic potential. In most cases, these include immune defects of genetic, iatrogenic or infectious origin, and inflammation appears to play an important role in disease development. Our much improved understanding of its life cycle and its role in pathogenesis should now allow us to develop new therapeutic strategies directed against key viral proteins or intracellular pathways that are crucial for virus replication or persistence. Likewise, its limited (for a herpesvirus) distribution and transmission should offer an opportunity for the development and use of a vaccine to prevent transmission.

This article is part of the themed issue ‘Human oncogenic viruses’.

KSHV co-infection down-regulates HPV16 E6 and E7 from cervical cancer cells

High-risk human papillomavirus (HPV) infection is the etiological agent of some malignancies such as cervical, oral and oropharyngeal cancers. Kaposi sarcoma-associated herpesvirus (KSHV) represents a principal causative agent of several human cancers arising in those immunocompromised patients. Interestingly, KSHV DNA has been detected in the oral cavity and the female genital tract, although its detection rate in cervical samples is very low and few reports are about KSHV/HPV co-infection. Therefore, it remains unclear about the role of KSHV co-infection in the development of HPV-related neoplasias. In the current study, we report that HPV16-integrated cervical cancer cell-line SiHa is susceptible to KSHV latent infection and replication. We also have found that KSHV infection or viral latent proteins are capable of reducing HPV16 E6/E7 expression through the manipulation of cellular microRNA function. Array analysis indicates that KSHV infection induces some inflammatory cytokines/chemokines production as well as up-regulates a series of interferon-induced genes expression, which may facilitate host immune defense system attacking these co-infected cells and clearance of viruses. Together, our data have provided possible explanations for very low detection rate of KSHV shedding as well as of KSHV/HPV co-infection in cervical samples and/or cervical cancer cells.

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.

Kaposi's sarcoma herpesvirus (KSHV) microRNA K12-1 functions as an oncogene by activating NF-κB/IL-6/STAT3 signaling

The human oncogenic virus Kaposi's sarcoma herpesvirus (KSHV) is the most common cause of malignancies among AIDS patients. KSHV possesses over hundred genes, including 25 microRNAs (miRNAs). The roles of these miRNAs and many other viral genes in KSHV biology and pathogenesis remain largely unknown. Accordingly, the molecular mechanisms by which KSHV induces tumorigenesis are still poorly defined. Here, we identify KSHV miRNA K12-1 (miR-K12-1) as a novel viral oncogene by activating two important transcription factors, nuclear factor-κb (NF-κB) and signal transducer and activator of transcription 3 (STAT3). Interestingly, miR-K12-1 activates STAT3 indirectly through inducing NF-κB activation and NF-κB-dependent expression of the cytokine interleukin-6 (IL-6) by repressing the expression of the NF-κB inhibitor IκBα. Accordingly, expression of ectopic IκBα or knockdown of NF-κB RelA, IL-6 or STAT3 prevents expression of cell growth genes and suppresses the oncogenicities of both miR-K12-1 and KSHV. These data identify miR-K12-1/NF-κB/IL-6/STAT3 as a novel oncogenic signaling underlying KSHV tumorigenesis. These data also provide the first evidence showing that IL-6/STAT3 signaling acts as an essential mediator of NF-κB oncogenic actions. These findings significantly improve our understanding of KSHV pathogenesis and oncogenic interaction between NF-κB and STAT3.

Viral Oncology: Molecular Biology and Pathogenesis

Oncoviruses are implicated in approximately 12% of all human cancers. A large number of the world's population harbors at least one of these oncoviruses, but only a small proportion of these individuals go on to develop cancer. The interplay between host and viral factors is a complex process that works together to create a microenvironment conducive to oncogenesis. In this review, the molecular biology and oncogenic pathways of established human oncoviruses will be discussed. Currently, there are seven recognized human oncoviruses, which include Epstein-Barr Virus (EBV), Human Papillomavirus (HPV), Hepatitis B and C viruses (HBV and HCV), Human T-cell lymphotropic virus-1 (HTLV-1), Human Herpesvirus-8 (HHV-8), and Merkel Cell Polyomavirus (MCPyV). Available and emerging therapies for these oncoviruses will be mentioned.

The Kaposi's sarcoma-associated herpesvirus (KSHV) non-structural membrane protein K15 is required for viral lytic replication and may represent a therapeutic target

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the infectious cause of the highly vascularized tumor Kaposi’s sarcoma (KS), which is characterized by proliferating spindle cells of endothelial origin, extensive neo-angiogenesis and inflammatory infiltrates. The KSHV K15 protein contributes to the angiogenic and invasive properties of KSHV-infected endothelial cells. Here, we asked whether K15 could also play a role in KSHV lytic replication. Deletion of the K15 gene from the viral genome or its depletion by siRNA lead to reduced virus reactivation, as evidenced by the decreased expression levels of KSHV lytic proteins RTA, K-bZIP, ORF 45 and K8.1 as well as reduced release of infectious virus. Similar results were found for a K1 deletion virus. Deleting either K15 or K1 from the viral genome also compromised the ability of KSHV to activate PLCγ1, Erk1/2 and Akt1. In infected primary lymphatic endothelial (LEC-rKSHV) cells, which have previously been shown to spontaneously display a viral lytic transcription pattern, transfection of siRNA against K15, but not K1, abolished viral lytic replication as well as KSHV-induced spindle cell formation. Using a newly generated monoclonal antibody to K15, we found an abundant K15 protein expression in KS tumor biopsies obtained from HIV positive patients, emphasizing the physiological relevance of our findings. Finally, we used a dominant negative inhibitor of the K15-PLCγ1 interaction to establish proof of principle that pharmacological intervention with K15-dependent pathways may represent a novel approach to block KSHV reactivation and thereby its pathogenesis.

Both the latent and lytic replication phases of the KSHV life cycle are thought to contribute to its persistence and pathogenesis. The non-structural signaling membrane protein K15 is involved in the angiogenic and invasive properties of KSHV-infected endothelial cells. Here we show that the K15 protein is required for virus replication, early viral gene expression and virus production through its activation of the cellular signaling pathways PLCγ1 and Erk 1/2. K15 is abundantly expressed in KSHV-infected lymphatic endothelial cells (LECs) and contributes to KSHV-induced endothelial spindle cell formation. The abundant K15 protein expression observed in LECs is also observed in KS tumors. We also show that it may be possible to target K15 in order to intervene therapeutically with KSHV lytic replication and pathogenesis.

Oncolytic Reactivation of KSHV as a Therapeutic Approach for Primary Effusion Lymphoma

Primary effusion lymphoma (PEL) is an aggressive subtype of non-Hodgkin lymphoma caused by Kaposi's sarcoma-associated herpesvirus (KSHV) infection. Currently, treatment options for patients with PEL are limited. Oncolytic viruses have been engineered as anticancer agents and have recently shown increased therapeutic promise. Similarly, lytic activation of endogenous viruses from latently infected tumor cells can also be applied as a cancer therapy. In theory, such a therapeutic strategy would induce oncolysis by viral replication, while simultaneously stimulating an immune response to viral lytic cycle antigens. We examined the combination of the FDA-approved drug ingenol-3-angelate (PEP005) with epigenetic drugs as a rational therapeutic approach for KSHV-mediated malignancies. JQ1, a bromodomain and extra terminal (BET) protein inhibitor, in combination with PEP005, not only robustly induced KSHV lytic replication, but also inhibited IL6 production from PEL cells. Using the dosages of these agents that were found to be effective in reactivating HIV (as a means to clear latent virus with highly active antiretroviral therapy), we were able to inhibit PEL growth in vitro and delay tumor growth in a PEL xenograft tumor model. KSHV reactivation was mediated by activation of the NF-κB pathway by PEP005, which led to increased occupancy of RNA polymerase II onto the KSHV genome. RNA-sequencing analysis further revealed cellular targets of PEP005, JQ1, and the synergistic effects of both. Thus, combination of PEP005 with a BET inhibitor may be considered as a rational therapeutic approach for the treatment of PEL. Mol Cancer Ther; 16(11); 2627-38. ©2017 AACR.

Epigenetic manipulation of host chromatin by Kaposi sarcoma-associated herpesvirus: a tumor-promoting factor?

Molecular and epidemiological evidence links Kaposi sarcoma-associated herpesvirus (KSHV) to a number of malignancies of endothelial or B cell origin. As for most virus-associated cancers, however, the tumor initiating and promoting events remain poorly understood. Given the emerging role of epigenetic alterations as drivers of human cancers, an interesting (and as of yet under-explored) hypothesis is that viral manipulation of host cell chromatin may contribute to the pathogenesis of KSHV-associated tumors. We here review the current knowledge regarding the interplay between KSHV-encoded factors and host chromatin and discuss how epigenetic alterations may contribute to the pathogenesis of KSHV-associated tumors.

Rhadinoviral interferon regulatory factor homologues

Kaposi's sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8 (HHV8) is a gammaherpesvirus and the etiological agent of Kaposi's sarcoma, primary effusion lymphoma and multicentric Castleman disease. The KSHV genome contains genes for a unique group of proteins with homology to cellular interferon regulatory factors, termed viral interferon regulatory factors (vIRFs). This review will give an overview over the oncogenic, antiapoptotic and immunomodulatory characteristics of KSHV and related vIRFs.

Kaposi's Sarcoma-Associated Herpesvirus Latency-Associated Nuclear Antigen: Replicating and Shielding Viral DNA during Viral Persistence

Kaposi's sarcoma herpesvirus (KSHV) establishes lifelong latency. The viral latency-associated nuclear antigen (LANA) promotes viral persistence by tethering the viral genome to cellular chromosomes and by participating in latent DNA replication. Recently, the structure of the LANA C-terminal DNA binding domain was solved and new cytoplasmic variants of LANA were discovered. We discuss how these findings contribute to our current view of LANA structure and assembly and of its role during viral persistence.

Identification of a nucleoside analog active against adenosine kinase-expressing plasma cell malignancies

Primary effusion lymphoma (PEL) is a largely incurable malignancy of B cell origin with plasmacytic differentiation. Here, we report the identification of a highly effective inhibitor of PEL. This compound, 6-ethylthioinosine (6-ETI), is a nucleoside analog with toxicity to PEL in vitro and in vivo, but not to other lymphoma cell lines tested. We developed and performed resistome analysis, an unbiased approach based on RNA sequencing of resistant subclones, to discover the molecular mechanisms of sensitivity. We found different adenosine kinase-inactivating (ADK-inactivating) alterations in all resistant clones and determined that ADK is required to phosphorylate and activate 6-ETI. Further, we observed that 6-ETI induces ATP depletion and cell death accompanied by S phase arrest and DNA damage only in ADK-expressing cells. Immunohistochemistry for ADK served as a biomarker approach to identify 6-ETI-sensitive tumors, which we documented for other lymphoid malignancies with plasmacytic features. Notably, multiple myeloma (MM) expresses high levels of ADK, and 6-ETI was toxic to MM cell lines and primary specimens and had a robust antitumor effect in a disseminated MM mouse model. Several nucleoside analogs are effective in treating leukemias and T cell lymphomas, and 6-ETI may fill this niche for the treatment of PEL, plasmablastic lymphoma, MM, and other ADK-expressing cancers.

SIRT1 and AMPK pathways are essential for the proliferation and survival of primary effusion lymphoma cells

Primary effusion lymphoma (PEL) is a rare and aggressive B-cell lymphoma with a dismal prognosis caused by infection of Kaposi's sarcoma-associated herpesvirus. Despite the findings that numerous viral genes and cellular pathways are essential for the proliferation and survival of PEL cells, there is currently no effective therapeutic treatment for PEL. Here, we report that the metabolic sensor SIRT1 is functionally required for sustaining the proliferation and survival of PEL cells. Knockdown of SIRT1 with specific shRNAs or inhibition of SIRT1 with an inhibitor (tenovin-6) induced cell cycle arrest and apoptosis in PEL cells. We detected high levels of AMPK activation in PEL cells, reflected in AMPKα1 phosphorylation at T174. Knockdown or inhibition of SIRT1 reduced AMPK activation, indicating that SIRT1 was required for AMPK activation. Interestingly, knockdown of AMPK with specific shRNAs or inhibition of AMPK with the inhibitor compound C recapitulated the phenotype of SIRT1, and induced cell cycle arrest and apoptosis, whereas overexpression of a constitutively active AMPK construct rescued the cytotoxic effect of SIRT1 knockdown. Remarkably, treatment with tenovin-6 effectively inhibited the initiation and progression of PEL, and significantly extended the survival of mice in a murine PEL model. Taken together, these results illustrate that the SIRT1-AMPK axis is essential for maintaining the proliferation and survival of PEL and identify SIRT1 and AMPK as potential therapeutic targets, and tenovin-6 as a candidate therapeutic agent for PEL patients. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Kaposi Sarcoma Herpesvirus (KSHV) Latency-Associated Nuclear Antigen (LANA) recruits components of the MRN (Mre11-Rad50-NBS1) repair complex to modulate an innate immune signaling pathway and viral latency

Kaposi Sarcoma Herpesvirus (KSHV), a γ2-herpesvirus and class 1 carcinogen, is responsible for at least three human malignancies: Kaposi Sarcoma (KS), Primary Effusion Lymphoma (PEL) and Multicentric Castleman's Disease (MCD). Its major nuclear latency protein, LANA, is indispensable for the maintenance and replication of latent viral DNA in infected cells. Although LANA is mainly a nuclear protein, cytoplasmic isoforms of LANA exist and can act as antagonists of the cytoplasmic DNA sensor, cGAS. Here, we show that cytosolic LANA also recruits members of the MRN (Mre11-Rad50-NBS1) repair complex in the cytosol and thereby inhibits their recently reported role in the sensing of cytoplasmic DNA and activation of the NF-κB pathway. Inhibition of NF-κB activation by cytoplasmic LANA is accompanied by increased lytic replication in KSHV-infected cells, suggesting that MRN-dependent NF-κB activation contributes to KSHV latency. Cytoplasmic LANA may therefore support the activation of KSHV lytic replication in part by counteracting the activation of NF-κB in response to cytoplasmic DNA. This would complement the recently described role of cytoplasmic LANA in blocking an interferon response triggered by cGAS and thereby promoting lytic reactivation. Our findings highlight a second point at which cytoplasmic LANA interferes with the innate immune response, as well as the importance of the recently discovered role of cytoplasmic MRN complex members as innate sensors of cytoplasmic DNA for the control of KSHV replication.

The Itch ubiquitin ligase is required for KSHV RTA induced vFLIP degradation

Expression of Kaposi's sarcoma herpesvirus vFLIP, a potent activator of NFkB signaling, promotes latency. Inhibition of NFkB signaling promotes lytic reactivation. We previously reported that lytic inducer, RTA, inhibits vFLIP induced NFkB signaling by inducing the degradation of vFLIP via the proteasome. Here we report that the cellular ubiquitin ligase, Itch, is required for RTA induced degradation of vFLIP. Expression of either Itch targeting shRNA or a dominant negative mutant of the ubiquitin ligase both increased the stability of vFLIP in the presence of RTA. Itch potently ubiquitinated vFLIP in vivo and in vitro. We provide evidence for interaction between RTA, vFLIP and Itch and we identified an RTA resistant mutant of vFLIP that is unable to interact with Itch. These observations contribute to our understanding of how RTA counteracts the activities of vFLIP.

Azidothymidine Sensitizes Primary Effusion Lymphoma Cells to Kaposi Sarcoma-Associated Herpesvirus-Specific CD4+ T Cell Control and Inhibits vIRF3 Function

Kaposi sarcoma-associated herpesvirus (KSHV) is linked with the development of Kaposi sarcoma and the B lymphocyte disorders primary effusion lymphoma (PEL) and multi-centric Castleman disease. T cell immunity limits KSHV infection and disease, however the virus employs multiple mechanisms to inhibit efficient control by these effectors. Thus KSHV-specific CD4+ T cells poorly recognize most PEL cells and even where they can, they are unable to kill them. To make KSHV-infected cells more sensitive to T cell control we treated PEL cells with the thymidine analogue azidothymidine (AZT), which sensitizes PEL lines to Fas-ligand and TRAIL challenge; effector mechanisms which T cells use. PELs co-cultured with KSHV-specific CD4+ T cells in the absence of AZT showed no control of PEL outgrowth. However in the presence of AZT PEL outgrowth was controlled in an MHC-restricted manner. To investigate how AZT sensitizes PELs to immune control we first examined BJAB cells transduced with individual KSHV-latent genes for their ability to resist apoptosis mediated by stimuli delivered through Fas and TRAIL receptors. This showed that in addition to the previously described vFLIP protein, expression of vIRF3 also inhibited apoptosis delivered by these stimuli. Importantly vIRF3 mediated protection from these apoptotic stimuli was inhibited in the presence of AZT as was a second vIRF3 associated phenotype, the downregulation of surface MHC class II. Although both vFLIP and vIRF3 are expressed in PELs, we propose that inhibiting vIRF3 function with AZT may be sufficient to restore T cell control of these tumor cells.

Kaposi sarcoma-associated herpesvirus (KSHV) can cause disease in humans in the form of B lymphocyte disorders such as primary effusion lymphoma (PEL) and multicentric Castleman disease. Where tested, these are highly resistant to immune control by KSHV-specific T cells. To investigate how such KSHV-infected cells can be made more sensitive to T cell control we treated PEL lines with azidothymidine (AZT), which has been shown to induce sensitivity in such lines to the mechanisms which T cells use to kill targets. We found this allowed the T cells to control in vitro lymphoma growth. The ability of the T cells to control PEL cell growth was found to correlate with AZT mediated inhibition of function of the KSHV protein vIRF3 which we show has the ability to protect cells from killing by immune effector mechanisms. These studies suggest that the therapeutic drug AZT may be of use to tip the virus host balance away from the virus by interfering with this immune evasion and pro-survival protein, potentially allowing better control by the host.

Multicentric Castleman disease: Where are we now?

Multicentric Castleman disease (MCD) encompasses a spectrum of conditions that give rise to overlapping clinicopathological manifestations. The fundamental pathogenetic mechanism involves dysregulated cytokine activity that causes systemic inflammatory symptoms as well as lymphadenopathy. The histological changes in lymph nodes resemble in part the findings originally described in the unicentric forms Castleman disease, both hyaline vascular and plasma cell variants. In MCD caused by Kaposi sarcoma-associated herpesvirus/human herpesvirus-8 (KSHV/HHV8), the cytokine over activity is caused by viral products, which can also lead to atypical lymphoproliferations and potential progression to lymphoma. In cases negative for KSHV/HHV8, so-called idiopathic MCD, the hypercytokinemia can result from various mechanisms, which ultimately lead to different constellations of clinical presentations and varied pathology in lymphoid tissues. In this article, we review the evolving concepts and definitions of the various conditions under the eponym of Castleman disease, and summarize current knowledge regarding the histopathology and pathogenesis of lesions within the MCD spectrum.

A role for MALT1 activity in Kaposi's sarcoma-associated herpes virus latency and growth of primary effusion lymphoma

Primary effusion lymphoma (PEL) is an incurable malignancy that develops in immunodeficient patients as a consequence of latent infection of B-cells with Kaposi's sarcoma-associated herpes virus (KSHV). Malignant growth of KSHV-infected B cells requires the activity of the transcription factor nuclear factor (NF)-κB, which controls maintenance of viral latency and suppression of the viral lytic program. Here we show that the KSHV proteins K13 and K15 promote NF-κB activation via the protease mucosa-associated lymphoid tissue lymphoma translocation protein-1 (MALT1), a key driver of NF-κB activation in lymphocytes. Inhibition of the MALT1 protease activity induced a switch from the latent to the lytic stage of viral infection, and led to reduced growth and survival of PEL cell lines in vitro and in a xenograft model. These results demonstrate a key role for the proteolytic activity of MALT1 in PEL, and provide a rationale for the pharmacological targeting of MALT1 in PEL therapy.

Interplay of Murine Gammaherpesvirus 68 with NF-kappaB Signaling of the Host

Herpesviruses establish a chronic infection in the host characterized by intervals of lytic replication, quiescent latency, and reactivation from latency. Murine gammaherpesvirus 68 (MHV68) naturally infects small rodents and has genetic and biologic parallels with the human gammaherpesviruses (gHVs), Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus. The murine gammaherpesvirus model pathogen system provides a platform to apply cutting-edge approaches to dissect the interplay of gammaherpesvirus and host determinants that enable colonization of the host, and that shape the latent or lytic fate of an infected cell. This knowledge is critical for the development of novel therapeutic interventions against the oncogenic gHVs. The nuclear factor kappa B (NF-κB) signaling pathway is well-known for its role in the promotion of inflammation and many aspects of B cell biology. Here, we review key aspects of the virus lifecycle in the host, with an emphasis on the route that the virus takes to gain access to the B cell latency reservoir. We highlight how the murine gammaherpesvirus requires components of the NF-κB signaling pathway to promote replication, latency establishment, and maintenance of latency. These studies emphasize the complexity of gammaherpesvirus interactions with NF-κB signaling components that direct innate and adaptive immune responses of the host. Importantly, multiple facets of NF-κB signaling have been identified that might be targeted to reduce the burden of gammaherpesvirus-associated diseases.

cFLIPL Interrupts IRF3-CBP-DNA Interactions To Inhibit IRF3-Driven Transcription

Type I IFN induction is critical for antiviral and anticancer defenses. Proper downregulation of type I IFN is equally important to avoid deleterious imbalances in the immune response. The cellular FLIP long isoform protein (cFLIPL) controls type I IFN production, but opposing publications show it as either an inhibitor or inducer of type I IFN synthesis. Regardless, the mechanistic basis for cFLIPL regulation is unknown. Because cFLIPL is important in immune cell development and proliferation, and is a target for cancer therapies, it is important to identify how cFLIPL regulates type I IFN production. Data in this study show that cFLIPL inhibits IFN regulatory factor 3 (IRF3), a transcription factor central for IFN-β and IFN-stimulated gene expression. This inhibition occurs during virus infection, cellular exposure to polyinosinic-polycytidylic acid, or TBK1 overexpression. This inhibition is independent of capase-8 activity. cFLIPL binds to IRF3 and disrupts IRF3 interaction with its IFN-β promoter and its coactivator protein (CREB-binding protein). Mutational analyses reveal that cFLIPL nuclear localization is necessary and sufficient for inhibitory function. This suggests that nuclear cFLIPL prevents IRF3 enhanceosome formation. Unlike other cellular IRF3 inhibitors, cFLIPL did not degrade or dephosphorylate IRF3. Thus, cFLIPL represents a different cellular strategy to inhibit type I IFN production. This new cFLIPL function must be considered to accurately understand how cFLIPL affects immune system development and regulation.

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.

Distinct Activation Mechanisms of NF-κB Regulator Inhibitor of NF-κB Kinase (IKK) by Isoforms of the Cell Death Regulator Cellular FLICE-like Inhibitory Protein (cFLIP)

The viral FLICE-like inhibitory protein (FLIP) protein from Kaposi sarcoma-associated herpesvirus activates the NF-κB pathway by forming a stable complex with a central region (amino acids 150-272) of the inhibitor of NF-κB kinase (IKK) γ subunits, thereby activating IKK. Cellular FLIP (cFLIP) forms are also known to activate the NF-κB pathway via IKK activation. Here we demonstrate that cFLIPL, cFLIPS, and their proteolytic product p22-FLIP all require the C-terminal region of NEMO/IKKγ (amino acids 272-419) and its ubiquitin binding function for activation of the IKK kinase (or kinase complex), but none form a stable complex with IKKγ. Our results further reveal that cFLIPLrequires the linear ubiquitin chain assembly complex and the kinase TAK1 for activation of the IKK kinase. Similarly, cFLIPSand p22-FLIP also require TAK1 but do not require LUBAC. In contrast, these isoforms are both components of complexes that incorporate Fas-associated death domain and RIP1, which appear essential for kinase activation. This conservation of IKK activation among the cFLIP family using different mechanisms suggests that the mechanism plays a critical role in their function.

The Role of Gammaherpesviruses in Cancer Pathogenesis

Worldwide, one fifth of cancers in the population are associated with viral infections. Among them, gammaherpesvirus, specifically HHV4 (EBV) and HHV8 (KSHV), are two oncogenic viral agents associated with a large number of human malignancies. In this review, we summarize the current understanding of the molecular mechanisms related to EBV and KSHV infection and their ability to induce cellular transformation. We describe their strategies for manipulating major cellular systems through the utilization of cell cycle, apoptosis, immune modulation, epigenetic modification, and altered signal transduction pathways, including NF-kB, Notch, Wnt, MAPK, TLR, etc. We also discuss the important EBV latent antigens, namely EBNA1, EBNA2, EBNA3’s and LMP’s, which are important for targeting these major cellular pathways. KSHV infection progresses through the engagement of the activities of the major latent proteins LANA, v-FLIP and v-Cyclin, and the lytic replication and transcription activator (RTA). This review is a current, comprehensive approach that describes an in-depth understanding of gammaherpes viral encoded gene manipulation of the host system through targeting important biological processes in viral-associated cancers.

Epigenetic Pathways of Oncogenic Viruses: Therapeutic Promises

Cancerous transformation comprises different events that are both genetic and epigenetic. The ultimate goal for such events is to maintain cell survival and proliferation. This transformation occurs as a consequence of different features such as environmental and genetic factors, as well as some types of infection. Many viral infections are considered to be causative agents of a number of different malignancies. To convert normal cells into cancerous cells, oncogenic viruses must function at the epigenetic level to communicate with their host cells. Oncogenic viruses encode certain epigenetic factors that lead to the immortality and proliferation of infected cells. The epigenetic effectors produced by oncogenic viruses constitute appealing targets to prevent and treat malignant diseases caused by these viruses. In this review, we highlight the importance of epigenetic reprogramming for virus-induced oncogenesis, with special emphasis on viral epigenetic oncoproteins as therapeutic targets. The discovery of molecular components that target epigenetic pathways, especially viral factors, is also discussed.