Integrated microarray and multiplex cytokine analyses of Kaposi's Sarcoma Associated Herpesvirus viral FLICE Inhibitory Protein K13 affected genes and cytokines in human blood vascular endothelial cells

CXCL1 confers a survival advantage in Kaposi's sarcoma-associated herpesvirus-infected human endothelial cells through STAT3 phosphorylation

Many cytokines produced by Kaposi's sarcoma-associated herpesvirus (KSHV)-infected cells have been shown to participate in the pathogenesis of KSHV. Determination of the exact role of cytokines in Kaposi's sarcoma (KS) pathogenesis is limited, however, by the difficulty to manipulate the target genes in human endothelial cells. In this study, we sought to elucidate the role of cytokines in KSHV-infected human immortalized endothelial cell line (HuARLT cells) by knockout (KO) of the corresponding target genes using the CRISPR/Cas9 system. The cytokine production profile of KSHV-infected HuARLT cells was analyzed using a protein array, and several cytokines were found to be highly upregulated following KSHV infection. This study focused on CXCL1, which was investigated by knocked out in HuARLT cells. KSHV-infected CXCL1 KO cells underwent increased cell death compared to KSHV-infected wild-type (WT) cells and mock-infected CXCL1 KO cells. Lytic replication was not observed in KSHV-infected WT nor CXCL1 KO cells. Phosphorylation of STAT3 was significantly suppressed in KSHV-infected CXCL1 KO cells. Additionally, inhibitors of STAT3 and CXCL1 induced cell death in KSHV-infected endothelial cells. Our results show that CXCL1 production is required for the survival of KSHV-infected endothelial cells, and the CXCL1 to STAT3 phosphorylation signaling pathway may be a therapeutic target for KS.

Modulation of Angiogenic Processes by the Human Gammaherpesviruses, Epstein-Barr Virus and Kaposi's Sarcoma-Associated Herpesvirus

Angiogenesis is the biological process by which new blood vessels are formed from pre-existing vessels. It is considered one of the classic hallmarks of cancer, as pathological angiogenesis provides oxygen and essential nutrients to growing tumors. Two of the seven known human oncoviruses, Epstein–Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV), belong to the Gammaherpesvirinae subfamily. Both viruses are associated with several malignancies including lymphomas, nasopharyngeal carcinomas, and Kaposi’s sarcoma. The viral genomes code for a plethora of viral factors, including proteins and non-coding RNAs, some of which have been shown to deregulate angiogenic pathways and promote tumor growth. In this review, we discuss the ability of both viruses to modulate the pro-angiogenic process.

Integrated systems biology analysis of KSHV latent infection reveals viral induction and reliance on peroxisome mediated lipid metabolism

Kaposi’s Sarcoma associated Herpesvirus (KSHV), an oncogenic, human gamma-herpesvirus, is the etiological agent of Kaposi’s Sarcoma the most common tumor of AIDS patients world-wide. KSHV is predominantly latent in the main KS tumor cell, the spindle cell, a cell of endothelial origin. KSHV modulates numerous host cell-signaling pathways to activate endothelial cells including major metabolic pathways involved in lipid metabolism. To identify the underlying cellular mechanisms of KSHV alteration of host signaling and endothelial cell activation, we identified changes in the host proteome, phosphoproteome and transcriptome landscape following KSHV infection of endothelial cells. A Steiner forest algorithm was used to integrate the global data sets and, together with transcriptome based predicted transcription factor activity, cellular networks altered by latent KSHV were predicted. Several interesting pathways were identified, including peroxisome biogenesis. To validate the predictions, we showed that KSHV latent infection increases the number of peroxisomes per cell. Additionally, proteins involved in peroxisomal lipid metabolism of very long chain fatty acids, including ABCD3 and ACOX1, are required for the survival of latently infected cells. In summary, novel cellular pathways altered during herpesvirus latency that could not be predicted by a single systems biology platform, were identified by integrated proteomics and transcriptomics data analysis and when correlated with our metabolomics data revealed that peroxisome lipid metabolism is essential for KSHV latent infection of endothelial cells.

Kaposi’s Sarcoma herpesvirus (KSHV) is the etiologic agent of Kaposi’s Sarcoma, the most common tumor of AIDS patients. KSHV modulates host cell signaling and metabolism to maintain a life-long latent infection. To unravel the underlying cellular mechanisms modulated by KSHV, we used multiple global systems biology platforms to identify and integrate changes in both cellular protein expression and transcription following KSHV infection of endothelial cells, the relevant cell type for KS tumors. The analysis identified several interesting pathways including peroxisome biogenesis. Peroxisomes are small cytoplasmic organelles involved in redox reactions and lipid metabolism. KSHV latent infection increases the number of peroxisomes per cell and proteins involved in peroxisomal lipid metabolism are required for the survival of latently infected cells. In summary, through integration of multiple global systems biology analyses we were able to identify novel pathways that could not be predicted by one platform alone and found that lipid metabolism in a small cytoplasmic organelle is necessary for the survival of latent infection with a herpesvirus.

Herpesviral G protein-coupled receptors activate NFAT to induce tumor formation via inhibiting the SERCA calcium ATPase

G protein-coupled receptors (GPCRs) constitute the largest family of proteins that transmit signal to regulate an array of fundamental biological processes. Viruses deploy diverse tactics to hijack and harness intracellular signaling events induced by GPCR. Herpesviruses encode multiple GPCR homologues that are implicated in viral pathogenesis. Cellular GPCRs are primarily regulated by their cognate ligands, while herpesviral GPCRs constitutively activate downstream signaling cascades, including the nuclear factor of activated T cells (NFAT) pathway. However, the roles of NFAT activation and mechanism thereof in viral GPCR tumorigenesis remain unknown. Here we report that GPCRs of human Kaposi’s sarcoma-associated herpesvirus (kGPCR) and cytomegalovirus (US28) shortcut NFAT activation by inhibiting the sarcoplasmic reticulum calcium ATPase (SERCA), which is necessary for viral GPCR tumorigenesis. Biochemical approaches, entailing pharmacological inhibitors and protein purification, demonstrate that viral GPCRs target SERCA2 to increase cytosolic calcium concentration. As such, NFAT activation induced by vGPCRs was exceedingly sensitive to cyclosporine A that targets calcineurin, but resistant to inhibition upstream of ER calcium release. Gene expression profiling identified a signature of NFAT activation in endothelial cells expressing viral GPCRs. The expression of NFAT-dependent genes was up-regulated in tumors derived from tva-kGPCR mouse and human KS. Employing recombinant kGPCR-deficient KSHV, we showed that kGPCR was critical for NFAT-dependent gene expression in KSHV lytic replication. Finally, cyclosporine A treatment diminished NFAT-dependent gene expression and tumor formation induced by viral GPCRs. These findings reveal essential roles of NFAT activation in viral GPCR tumorigenesis and a mechanism of “constitutive” NFAT activation by viral GPCRs.

G protein-coupled receptors (GPCRs) constitute the largest family of proteins that transmit signal across plasma membrane. Herpesviral GPCRs (vGPCRs) activate diverse signaling cascades and are implicated in viral pathogenesis (e.g., tumor development). In contrast to cellular GPCRs that are chiefly regulated via cognate ligand-association, vGPCRs are constitutively active independent of ligand-binding. vGPCRs provide useful tools to dissect signal transduction from plasma membrane receptors to nuclear transcription factors. To probe the activation of nuclear factor of T cells (NFAT), we demonstrate that vGPCRs target the ER calcium ATPase to increase cytosolic calcium concentration and activate NFAT. Inhibition of NFAT activation impairs tumor formation induced by vGPCRs, implying the antitumor therapeutic potential via disabling NFAT activation.

Systemic expression of Kaposi sarcoma herpesvirus (KSHV) Vflip in endothelial cells leads to a profound proinflammatory phenotype and myeloid lineage remodeling in vivo

KSHV is the causative agent of Kaposi sarcoma (KS), a spindle-shaped endothelial cell neoplasm accompanied by an inflammatory infiltrate. To evaluate the role of KSHV vFLIP in the pathogenesis of KS, we constructed mice with inducible expression of vFLIP in endothelial cells. Abnormal cells with endothelial marker expression and fusiform appearance were observed in several tissues reminiscent of the spindle cells found in KS. Serum cytokines displayed a profound perturbation similar to that described in KSHV inflammatory cytokine syndrome (KICS), a recently described clinical condition characterized by elevated IL6 and IL10. An increased myeloid component with suppressive immune phenotype was found, which may contribute to functional changes in the microenvironment and cellular heterogeneity as observed in KS. These mice represent the first in vivo demonstration that vFLIP is capable of inducing vascular abnormalities and changes in host microenvironment with important implications for understanding the pathogenesis and treating KSHV-associated diseases.

Kaposi’s sarcoma (KS) is the most common cancer in men infected with HIV, and also among the most frequent malignancies in Sub-Equatorial Africa. KS is a tumor of endothelial cell origin that is caused by infection with a gamma-herpesvirus, called KS herpesvirus (KSHV) or human herpesvirus 8 (HHV-8). KSHV vFLIP is a viral oncoprotein expressed during latent infection. We report here the generation and characterization of mice expressing KSHV vFLIP in an inducible manner in endothelial cells. Transgenic mice showed: 1) systemic endothelial abnormalities, with the presence of fusiform cells reminiscent of the spindle cells found in KS, 2) development of a profound perturbation in serum cytokines, reminiscent of the cytokine storm characteristic of KSHV-associated cytokine syndrome (KICS), and 3) remodeling of myeloid differentiation with expansion of myeloid cells displaying a suppressive immunophenotype that potentially favors host immune evasion, angiogenesis and tumor progression. This is the first example of significant changes in myeloid differentiation, vascular abnormalities and cytokine perturbation entirely initiated by ectopic expression of a single viral gene, making this mouse model a useful system to dissect the mechanisms viruses use to manipulate the host microenvironment culminating in sabotage of immunity and development of vascular lesions.

NEMO is essential for Kaposi's sarcoma-associated herpesvirus-encoded vFLIP K13-induced gene expression and protection against death receptor-induced cell death, and its N-terminal 251 residues are sufficient for this process

Kaposi's sarcoma-associated herpesvirus-encoded viral FLICE inhibitory protein (vFLIP) K13 was originally believed to protect virally infected cells against death receptor-induced apoptosis by interfering with caspase 8/FLICE activation. Subsequent studies revealed that K13 also activates the NF-κB pathway by binding to the NEMO/inhibitor of NF-κB (IκB) kinase gamma (IKKγ) subunit of an IKK complex and uses this pathway to modulate the expression of genes involved in cellular survival, proliferation, and the inflammatory response. However, it is not clear if K13 can also induce gene expression independently of NEMO/IKKγ. The minimum region of NEMO that is sufficient for supporting K13-induced NF-κB has not been delineated. Furthermore, the contribution of NEMO and NF-κB to the protective effect of K13 against death receptor-induced apoptosis remains to be determined. In this study, we used microarray analysis on K13-expressing wild-type and NEMO-deficient cells to demonstrate that NEMO is required for modulation of K13-induced genes. Reconstitution of NEMO-null cells revealed that the N-terminal 251 amino acid residues of NEMO are sufficient for supporting K13-induced NF-κB but fail to support tumor necrosis factor alpha (TNF-α)-induced NF-κB. K13 failed to protect NEMO-null cells against TNF-α-induced cell death but protected those reconstituted with the NEMO mutant truncated to include only the N-terminal 251 amino acid residues [the NEMO(1-251) mutant]. Taken collectively, our results demonstrate that NEMO is required for modulation of K13-induced genes and the N-terminal 251 amino acids of NEMO are sufficient for supporting K13-induced NF-κB. Finally, the ability of K13 to protect against TNF-α-induced cell death is critically dependent on its ability to interact with NEMO and activate NF-κB.

IMPORTANCE

Kaposi's sarcoma-associated herpesvirus-encoded vFLIP K13 is believed to protect virally infected cells against death receptor-induced apoptosis and to activate the NF-κB pathway by binding to adaptor protein NEMO/IKKγ. However, whether K13 can also induce gene expression independently of NEMO and the minimum region of NEMO that is sufficient for supporting K13-induced NF-κB remain to be delineated. Furthermore, the contribution of NEMO and NF-κB to the protective effect of K13 against death receptor-induced apoptosis is not clear. We demonstrate that NEMO is required for modulation of K13-induced genes and its N-terminal 251 amino acids are sufficient for supporting K13-induced NF-κB. The ability of K13 to protect against TNF-α-induced cell death is critically dependent on its ability to interact with NEMO and activate NF-κB. Our results suggest that K13-based gene therapy approaches may have utility for the treatment of patients with NEMO mutations and immunodeficiency.

KSHV RTA abolishes NFκB responsive gene expression during lytic reactivation by targeting vFLIP for degradation via the proteasome

Kaposi's sarcoma herpesvirus (KSHV) is a gamma-2 herpesvirus present in all cases of Kaposi's sarcoma, primary effusion lymphoma (PEL), and some cases of multicentric Castleman's disease. Viral FLICE inhibitory protein (vFLIP) is a latently expressed gene that has been shown to be essential for survival of latently infected PEL cells by activating the NFκB pathway. Inhibitors of either vFLIP expression or the NFĸB pathway result in enhanced lytic reactivation and apoptosis. We have observed a decrease in vFLIP protein levels and of NFκB activation in the presence of the KSHV lytic switch protein RTA. Whereas vFLIP alone induced expression of the NFĸB responsive genes ICAM1 and TNFα, inclusion of RTA decreased vFLIP induced ICAM1 and TNFα expression in both co-transfected 293T cells and in doxycycline induced TREx BCBL1 cells. RTA expression resulted in proteasome dependent destabilization of vFLIP. Neither RTA ubiquitin E3 ligase domain mutants nor a dominant-negative RAUL mutant abrogated this effect, while RTA truncation mutants did, suggesting that RTA recruits a novel cellular ubiquitin E3 ligase to target vFLIP for proteasomal degradation, allowing for inhibition of NFĸB responsive gene expression early during lytic reactivation.

Cyclooxygenase-2-prostaglandin E2-eicosanoid receptor inflammatory axis: a key player in Kaposi's sarcoma-associated herpes virus associated malignancies

The role of cyclooxygenase-2 (COX-2), its lipid metabolite prostaglandin E2 (PGE2), and Eicosanoid (EP) receptors (EP; 1-4) underlying the proinflammatory mechanistic aspects of Burkitt's lymphoma, nasopharyngeal carcinoma, cervical cancer, prostate cancer, colon cancer, and Kaposi's sarcoma (KS) is an active area of investigation. The tumorigenic potential of COX-2 and PGE2 through EP receptors forms the mechanistic context underlying the chemotherapeutic potential of nonsteroidal anti-inflammatory drugs (NSAIDs). Although role of the COX-2 is described in several viral associated malignancies, the biological significance of the COX-2/PGE2/EP receptor inflammatory axis is extensively studied only in Kaposi's sarcoma-associated herpes virus (KSHV/HHV-8) associated malignancies such as KS, a multifocal endothelial cell tumor and primary effusion lymphoma (PEL), a B cell-proliferative disorder. The purpose of this review is to summarize the salient findings delineating the molecular mechanisms downstream of COX-2 involving PGE2 secretion and its autocrine and paracrine interactions with EP receptors (EP1-4), COX-2/PGE2/EP receptor signaling regulating KSHV pathogenesis and latency. KSHV infection induces COX-2, PGE2 secretion, and EP receptor activation. The resulting signal cascades modulate the expression of KSHV latency genes (latency associated nuclear antigen-1 [LANA-1] and viral-Fas (TNFRSF6)-associated via death domain like interferon converting enzyme-like- inhibitory protein [vFLIP]). vFLIP was also shown to be crucial for the maintenance of COX-2 activation. The mutually interdependent interactions between viral proteins (LANA-1/vFLIP) and COX-2/PGE2/EP receptors was shown to play key roles in the biological mechanisms involved in KS and PEL pathogenesis such as blockage of apoptosis, cell cycle regulation, transformation, proliferation, angiogenesis, adhesion, invasion, and immune-suppression. Understanding the COX-2/PGE2/EP axis is very important to develop new safer and specific therapeutic modalities for KS and PEL. In addition to COX-2 being a therapeutic target, EP receptors represent ideal targets for pharmacologic agents as PGE2 analogues and their blockers/antagonists possess antineoplastic activity, without the reported gastrointestinal and cardiovascular toxicity observed with few a NSAIDs.

A20/TNFAIP3 inhibits NF-κB activation induced by the Kaposi's sarcoma-associated herpesvirus vFLIP oncoprotein

Kaposi's sarcoma-associated herpesvirus (KSHV) K13/vFLIP (viral Flice-inhibitory protein) induces transcription of numerous genes through NF-κB activation, including pro-inflammatory cytokines, which contribute to the pathogenesis of Kaposi's sarcoma (KS). In this study, we report that KSHV vFLIP induces the expression of the NF-κB regulatory proteins A20, ABIN-1 and ABIN-3 (A20-binding NF-κB inhibitors) in primary human endothelial cells, and that KS spindle cells express A20 in KS tissue. In reporter assays, A20 strongly impaired vFLIP-induced NF-κB activation in 293T cells, but ABIN-1 and ABIN-3 did not. Mutational analysis established that the C-terminal domain (residues 427-790) is critical for A20 modulation of NF-κB, but the ubiquitin-editing OTU (ovarian tumor) domain is not. In functional assays, A20 inhibited vFLIP-induced expression of the chemokine IP-10, reduced vFLIP-induced cell proliferation and increased IKK1 protein levels. Thus, we demonstrate that A20 negatively regulates NF-κB activation directly induced by KSHV vFLIP. By attenuating excessive and prolonged vFLIP-induced NF-κB activation that could be harmful to KSHV-infected cells, A20 likely has an important role in the pathogenesis of KSHV-associated diseases, in which vFLIP is expressed.

Recent advances in Kaposi sarcoma herpesvirus-associated multicentric Castleman disease

PURPOSE OF REVIEW

The discovery of Kaposi sarcoma herpesvirus (KSHV) led to recognition of KSHV-associated multicentric Castleman disease (MCD) as a distinct lymphoproliferative disorder. The pathogenesis of KSHV-MCD is attributed to proliferation of KSHV-infected B cells, production of KSHV-encoded viral interleukin 6 by these cells, and dysregulation of human interleukin 6 and interleukin 10. This article reviews advances in the field of disease pathogenesis and targeted therapies.

RECENT FINDINGS

Our understanding of the pathogenesis of KSHV-MCD has increased in recent years and improved therapies have been developed. Recent studies demonstrate that the anti-CD20 monoclonal antibody, rituximab, as well as virus-activated cytotoxic therapy using high-dose zidovudine and valganciclovir, can control symptoms and decrease adenopathy. With treatment, 1-year survival now exceeds 85%. Interestingly, even in the absence of pathologic findings of MCD, KSHV-infected patients may have inflammatory symptoms, excess cytokine production, and elevated KSHV viral load similar to KSHV-associated MCD. The term KSHV-associated inflammatory cytokine syndrome has been proposed to describe such patients.

SUMMARY

Recent advances in targeted therapy have improved outcomes in KSHV-MCD, and decreased need for cytotoxic chemotherapy. Improved understanding of the pathogenesis of KSHV-MCD and KSHV-associated inflammatory cytokine syndrome is needed, and will likely lead to additional advances in therapy for these disorders.

A computational profiling of changes in gene expression and transcription factors induced by vFLIP K13 in primary effusion lymphoma

Infection with Kaposi's sarcoma associated herpesvirus (KSHV) has been linked to the development of primary effusion lymphoma (PEL), a rare lymphoproliferative disorder that is characterized by loss of expression of most B cell markers and effusions in the body cavities. This unique clinical presentation of PEL has been attributed to their distinctive plasmablastic gene expression profile that shows overexpression of genes involved in inflammation, adhesion and invasion. KSHV-encoded latent protein vFLIP K13 has been previously shown to promote the survival and proliferation of PEL cells. In this study, we employed gene array analysis to characterize the effect of K13 on global gene expression in PEL-derived BCBL1 cells, which express negligible K13 endogenously. We demonstrate that K13 upregulates the expression of a number of NF-κB responsive genes involved in cytokine signaling, cell death, adhesion, inflammation and immune response, including two NF-κB subunits involved in the alternate NF-κB pathway, RELB and NFKB2. In contrast, CD19, a B cell marker, was one of the genes downregulated by K13. A comparison with K13-induced genes in human vascular endothelial cells revealed that although there was a considerable overlap among the genes induced by K13 in the two cell types, chemokines genes were preferentially induced in HUVEC with few exceptions, such as RANTES/CCL5, which was induced in both cell types. Functional studies confirmed that K13 activated the RANTES/CCL5 promoter through the NF-κB pathway. Taken collectively, our results suggest that K13 may contribute to the unique gene expression profile, immunophenotype and clinical presentation that are characteristics of KSHV-associated PEL.

COX-2/PGE2: molecular ambassadors of Kaposi's sarcoma-associated herpes virus oncoprotein-v-FLIP

Kaposi's sarcoma herpesvirus (KSHV) latent oncoprotein viral FLICE (FADD-like interferon converting enzyme)-like inhibitory protein (v-FLIP) or K13, a potent activator of NF-κB, has well-established roles in KSHV latency and oncogenesis. KSHV-induced COX-2 represents a novel strategy employed by KSHV to promote latency and inflammation/angiogenesis/invasion. Here, we demonstrate that v-FLIP/K13 promotes tumorigenic effects via the induction of host protein COX-2 and its inflammatory metabolite PGE2 in an NF-κB-dependent manner. In addition to our previous studies demonstrating COX-2/PGE2's role in transcriptional regulation of KSHV latency promoter and latent gene expression, the current study adds to the complexity that though LANA-1 (latency associated nuclear antigen) is utilizing COX-2/PGE2 as critical factors for its transcriptional regulation, it is the v-FLIP/K13 gene in the KSHV latency cluster that maintains continuous COX-2/PGE2 levels in the infected cells. We demonstrate that COX-2 inhibition, via its chemical inhibitors (NS-398 or celecoxib), reduced v-FLIP/K13-mediated NF-κB induction, and extracellular matrix (ECM) interaction-mediated signaling, mitochondrial antioxidant enzyme manganese superoxide dismutase (MnSOD) levels, and subsequently downregulated detachment-induced apoptosis (anoikis) resistance. vFLIP expression mediated the secretion of cytokines, and spindle cell differentiation activated the phosphorylation of p38, RSK, FAK, Src, Akt and Rac1-GTPase. The COX-2 inhibition in v-FLIP/K13-HMVECs reduced inflammation and invasion/metastasis-related genes, along with reduced anchorage-independent colony formation via modulating ‘extrinsic' as well as ‘intrinsic' cell death pathways. COX-2 blockade in v-FLIP/K13-HMVEC cells drastically augmented cell death induced by removal of essential growth/survival factors secreted in the microenvironment. Transformed cells obtained from anchorage-independent colonies of COX-2 inhibitor-treated v-FLIP/K13-HMVEC cells expressed lower levels of endothelial–mesenchymal transition genes such as slug, snail and twist, and higher expression of the tumor-suppressor gene, E-cadherin. Taken together, our study provides strong evidences that FDA-approved COX-2 inhibitors have great potential in blocking tumorigenic events linked to KSHV's oncogenic protein v-FLIP/K13.

Deletion of Kaposi's sarcoma-associated herpesvirus FLICE inhibitory protein, vFLIP, from the viral genome compromises the activation of STAT1-responsive cellular genes and spindle cell formation in endothelial cells

Kaposi's sarcoma herpesvirus (KSHV) Fas-associated death domain (FADD)-like interleukin-1 beta-converting enzyme (FLICE)-inhibitory protein, vFLIP, has antiapoptotic properties, is a potent activator of the NF-κB pathway, and induces the formation of endothelial spindle cells, the hallmark of Kaposi's sarcoma, when overexpressed in primary endothelial cells. We used a reverse genetics approach to study several functions of KSHV vFLIP in the context of the whole viral genome. Deletion of the gene encoding vFLIP from a KSHV genome cloned in a bacterial artificial chromosome (BAC) reduced the ability of the virus to persist and induce spindle cell formation in primary human umbilical vein endothelial cells (HUVECs). Only a few, mainly interferon (IFN)-responsive, genes were expressed in wild-type KSHV (KSHV-wt)-infected endothelial cells at levels higher than those in KSHV-ΔFLIP-infected endothelial cells, in contrast to the plethora of cellular genes induced by overexpressed vFLIP. In keeping with this observation, vFLIP induces the phosphorylation of STAT1 and STAT2 in an NF-κB-dependent manner in endothelial cells. vFLIP-dependent phosphorylation of STAT1 and STAT2 could be demonstrated after endothelial cells were infected with KSHV-wt, KSHV-ΔFLIP, and a KSHV-vFLIP revertant virus. These findings document the impact of KSHV vFLIP on the transcriptome of primary endothelial cells during viral persistence and highlight the role of vFLIP in the activation of STAT1/STAT2 and STAT-responsive cellular genes by KSHV.

cIAPs block Ripoptosome formation, a RIP1/caspase-8 containing intracellular cell death complex differentially regulated by cFLIP isoforms

The intracellular regulation of cell death pathways by cIAPs has been enigmatic. Here we show that loss of cIAPs promotes the spontaneous formation of an intracellular platform that activates either apoptosis or necroptosis. This 2 MDa intracellular complex that we designate “Ripoptosome” is necessary but not sufficient for cell death. It contains RIP1, FADD, caspase-8, caspase-10, and caspase inhibitor cFLIP isoforms. cFLIP_L prevents Ripoptosome formation, whereas, intriguingly, cFLIP_S promotes Ripoptosome assembly. When cIAPs are absent, caspase activity is the “rheostat” that is controlled by cFLIP isoforms in the Ripoptosome and decides if cell death occurs by RIP3-dependent necroptosis or caspase-dependent apoptosis. RIP1 is the core component of the complex. As exemplified by our studies for TLR3 activation, our data argue that the Ripoptosome critically influences the outcome of membrane-bound receptor triggering. The differential quality of cell death mediated by the Ripoptosome may cause important pathophysiological consequences during inflammatory responses.

Constitutive NF-kappaB activation confers interleukin 6 (IL6) independence and resistance to dexamethasone and Janus kinase inhibitor INCB018424 in murine plasmacytoma cells

Myeloma cells are dependent on IL6 for their survival and proliferation during the early stages of disease, and independence from IL6 is associated with disease progression. The role of the NF-κB pathway in the IL6-independent growth of myeloma cells has not been studied. Because human herpesvirus 8-encoded K13 selectively activates the NF-κB pathway, we have used it as a molecular tool to examine the ability of the NF-κB pathway to confer IL6 independence on murine plasmacytomas. We demonstrated that ectopic expression of K13, but not its NF-κB-defective mutant or a structural homolog, protected plasmacytomas against IL6 withdrawal-induced apoptosis and resulted in emergence of IL6-independent clones that could proliferate long-term in vitro in the absence of IL6 and form abdominal plasmacytomas with visceral involvement when injected intraperitoneally into syngeneic mice. These IL6-independent clones were dependent on NF-κB activity for their survival and proliferation but were resistant to dexamethasone and INCB018424, a selective Janus kinase 1/2 inhibitor. Ectopic expression of human T cell leukemia virus 1-encoded Tax protein, which resembles K13 in inducing constitutive NF-κB activation, similarly protected plasmacytoma cells against IL6 withdrawal-induced apoptosis. Although K13 is known to up-regulate IL6 gene expression, its protective effect was not due to induction of endogenous IL6 production but instead was associated with sustained expression of several antiapoptotic members of the Bcl2 family upon IL6 withdrawal. Collectively, these results demonstrate that NF-κB activation cannot only promote the emergence of IL6 independence during myeloma progression but can also confer resistance to dexamethasone and INCB018424.

A20 is induced by Kaposi sarcoma-associated herpesvirus-encoded viral FLICE inhibitory protein (vFLIP) K13 and blocks K13-induced nuclear factor-kappaB in a negative feedback manner

Expression of A20, a negative regulator of the NF-κB pathway, is frequently lost in several subtypes of Hodgkin and non-Hodgkin lymphoma. We report that A20 is expressed in Kaposi sarcoma-associated herpesvirus (KSHV)-infected primary effusion lymphoma cell lines, and its expression correlates closely with the expression of KSHV-encoded viral FLICE inhibitory protein K13. Ectopic expression of K13 induced A20 expression through NF-κB-mediated activation of A20 promoter. In turn, A20 blocked K13-induced NF-κB activity and up-regulation of proinflammatory cytokines CCL20 and IL-8 in a negative feedback fashion. Both the N-terminal deubiquitinating domain and the C-terminal zinc finger domain of A20 were involved in the inhibition of K13-induced NF-κB activity. Overexpression of A20 blocked K13-induced IκBα phosphorylation, NF-κB nuclear translocation, and cellular transformation. Consistent with the above, K13-induced IκBα phosphorylation and NF-κB transcriptional activation were enhanced in A20-deficient cells. Finally, A20 was found to interact physically with K13. Taken collectively, these results demonstrate that K13 is a key determinant of A20 expression in KSHV-infected cells, and A20 is a key negative regulator of K13-induced NF-κB activity. A20 might serve to control the inflammatory response to KSHV infection and protect KSHV-infected cells from apoptosis.

Kaposi sarcoma herpesvirus (KSHV) vFLIP oncoprotein induces B cell transdifferentiation and tumorigenesis in mice

Kaposi sarcoma herpesvirus (KSHV) is specifically associated with Kaposi sarcoma (KS) and 2 B cell lymphoproliferative diseases, namely primary effusion lymphoma (PEL) and multicentric Castleman disease (MCD). KS, PEL, and MCD are largely incurable and poorly understood diseases most common in HIV-infected individuals. Here, we have revealed the role of viral FLICE-inhibitory protein (vFLIP) in the initiation of PEL and MCD by specifically expressing vFLIP at different stages of B cell differentiation in vivo. Mice showed MCD-like abnormalities and immunological defects including lack of germinal centers (GCs), impaired Ig class switching, and affinity maturation. In addition, they showed increased numbers of cells expressing cytoplasmic IgM-λ, a thus far enigmatic feature of the KSHV-infected cells in MCD. B cell-derived tumors arose at high incidence and displayed Ig gene rearrangement with downregulated expression of B cell-associated antigens, which are features of PEL. Interestingly, these tumors exhibited characteristics of transdifferentiation and acquired expression of histiocytic/dendritic cell markers. These results define immunological functions for vFLIP in vivo and reveal what we believe to be a novel viral-mediated tumorigenic mechanism involving B cell reprogramming. Additionally, the robust recapitulation of KSHV-associated diseases in mice provides a model to test inhibitors of vFLIP as potential anticancer agents.

X-linked ectodermal dysplasia receptor is downregulated in breast cancer via promoter methylation

PURPOSE

The X-linked ectodermal dysplasia receptor (XEDAR) is a novel receptor of the tumor necrosis factor receptor family that binds to ectodysplasin-A2 (EDA-A2) and induces cell death. The purpose of this study was to determine the tumor-suppressive potential of XEDAR in the development of breast cancer.

EXPERIMENTAL DESIGN

We analyzed the expression of XEDAR in breast cancer cell lines and tumor samples using quantitative real-time PCR analysis and immunoblotting. We analyzed the human XEDAR gene promoter for the presence of any CpG island and examined its methylation status using methylation-specific real-time PCR. We examined the effect of 5-aza-2'-deoxycytidine on the expression of XEDAR and sensitivity to EDA-A2-induced apoptosis in breast cancer cell lines.

RESULTS

Expression of XEDAR, but not EDA-A2, was downregulated in most tumorigenic breast cancer cell lines and tumor samples. Loss of XEDAR expression correlated with the hypermethylation of its promoter. Ectopic expression of XEDAR in MDA-MB-231 cells resulted in significant induction of apoptosis and reduction in colony formation. Treatment with 5-aza-2'-deoxycytidine restored XEDAR expression in breast cancer cell lines with methylated XEDAR promoter and sensitized them to EDA-A2-induced cell death.

CONCLUSIONS

Our results suggest that XEDAR expression is downregulated in most breast cancers via promoter methylation, which may contribute to accelerated tumor development by blocking EDA-A2-induced cell death. XEDAR may represent a novel breast tumor suppressor gene, and restoration of its expression by treatment with DNA demethylating agents may represent an attractive approach for the treatment of breast cancer.

Kaposi's sarcoma-associated herpesvirus-encoded viral FLICE inhibitory protein (vFLIP) K13 suppresses CXCR4 expression by upregulating miR-146a

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV)-encoded viral FLICE inhibitory protein (vFLIP) K13 is a potent activator of the nuclear factor-kappaB (NF-kappaB) pathway. In this study, we show that infection with KHSV and ectopic expression of K13, but not its NF-kappaB-defective mutant, suppressed the expression of CXCR4. Suppression of CXCR4 by KSHV and K13 was associated with upregulated expression of miR-146a, a microRNA that is known to bind to the 3'-untranslated region of CXCR4 mRNA. Reporter studies identified two NF-kappaB sites in the promoter of miR-146a that were essential for its activation by K13. Accordingly, ectopic expression of K13, but not its NF-kappaB-defective mutant or other vFLIPs, strongly stimulated the miR-146a promoter activity, which could be blocked by specific genetic and pharmacological inhibitors of the NF-kappaB pathway. Finally, expression of CXCR4 was downregulated in clinical samples of KS and this was accompanied by an increased expression of miR-146a. Our results show that K13-induced NF-kappaB activity suppresses CXCR4 through upregulation of miR-146a. Downregulation of CXCR4 expression by K13 may contribute to KS development by promoting premature release of KSHV-infected endothelial progenitors into the circulation.