Kaposi's sarcoma-associated herpesvirus suppression of DUSP1 facilitates cellular pathogenesis following de novo infection

KSHV vIL-6 promotes SIRT3-induced deacetylation of SERBP1 to inhibit ferroptosis and enhance cellular transformation by inducing lipoyltransferase 2 mRNA degradation

Ferroptosis, a defensive strategy commonly employed by the host cells to restrict pathogenic infections, has been implicated in the development and therapeutic responses of various types of cancer. However, the role of ferroptosis in oncogenic Kaposi's sarcoma-associated herpesvirus (KSHV)-induced cancers remains elusive. While a growing number of non-histone proteins have been identified as acetylation targets, the functions of these modifications have yet to be revealed. Here, we show KSHV reprogramming of host acetylation proteomics following cellular transformation of rat primary mesenchymal precursor. Among them, SERPINE1 mRNA binding protein 1 (SERBP1) deacetylation is increased and required for KSHV-induced cellular transformation. Mechanistically, KSHV-encoded viral interleukin-6 (vIL-6) promotes SIRT3 deacetylation of SERBP1, preventing its binding to and protection of lipoyltransferase 2 (Lipt2) mRNA from mRNA degradation resulting in ferroptosis. Consequently, a SIRT3-specific inhibitor, 3-TYP, suppresses KSHV-induced cellular transformation by inducing ferroptosis. Our findings unveil novel roles of vIL-6 and SERBP1 deacetylation in regulating ferroptosis and KSHV-induced cellular transformation, and establish the vIL-6-SIRT3-SERBP1-ferroptosis pathways as a potential new therapeutic target for KSHV-associated cancers.

Antiviral immunity of grouper MAP kinase phosphatase 1 to Singapore grouper iridovirus infection

Singapore grouper iridovirus (SGIV), with various mechanisms for evading and modulating host, has inflicted heavy economic losses in the grouper aquaculture. MAP kinase phosphatase 1 (MKP-1) regulates mitogen-activated protein kinases (MAPKs) to mediate the innate immune response. Here, we cloned EcMKP-1, an MKP-1 homolog from the orange-spotted grouper Epinephelus coioides, and investigated its role in the infection of SGIV. In juvenile grouper, EcMKP-1 was highly upregulated and peaked at different times after injection with lipopolysaccharide, polyriboinosinic polyribocytidylic acid and SGIV. EcMKP-1 expression in heterologous fathead minnow cells was able to suppress SGIV infection and replication. Furthermore, EcMKP-1 was a negative regulator of c-Jun N-terminal kinase (JNK) phosphorylation early in SGIV infection. EcMKP-1 decreased the apoptotic percentage and caspase-3 activity during the late stage of SGIV replication. Our results demonstrate critical functions of EcMKP-1 in antiviral immunity, JNK dephosphorylation and anti-apoptosis during SGIV infection.

Kaposi's Sarcoma-associated Herpesvirus microRNA mutants modulate cancer hallmark phenotypic differences in human endothelial cells

Kaposi's sarcoma (KS) results from the transformation of Kaposi's sarcoma-associated herpesvirus (KSHV)-infected endothelial cells. The contribution of the KSHV microRNAs (miRNAs) to the process of oncogenesis in endothelial cells has not been fully elucidated. To better understand the contributions of individual miRNAs to oncogenesis-related cellular phenotypes, we used KSHV miRNA knockout mutants, each one lacking one of the twelve miRNA genes. An additional mutant lacked all miRNAs. Since KSHV infection causes a variety of phenotypic changes in endothelial cells, we tested the mutants for their ability to effect such changes in Telomerase-Immortalized Vein Endothelial (TIVE) cells infected with each of the mutant viruses. Wild type- and mutant-infected as well as uninfected cells were evaluated for perturbations to proliferation, migration, tubule formation, and glycolysis. We found broad variation between the different viruses in these aspects. With respect to proliferation rate, ΔmiR-K12-3, ΔmiR-K12-8, and ΔmiR-K12-11 showed significant impairment. Cells infected with ΔmiR-K12-11 had reduced migration. In tubule formation, the ΔmiR-K12-5, -6, and -7 viruses were deficient. At the same time, cells infected with the ΔmiR-K12-10 virus showed dysregulated glycolysis. By combining these observations with previously published KSHV miRNA targetome lists from ribonomics data, we were able to functionally validate a number of new miRNA targets in specific pathways. As proof of concept, miR-K12-3 was shown to target Cathepsin D, a strong promoter of apoptosis. Taken together, the results demonstrate that KSHV miRNAs play different roles in inducing the phenotypic changes which are characteristic of transformed cells.Importance: Kaposi's sarcoma-associated herpesvirus (KSHV) causes Kaposi's sarcoma (KS). The contribution of KSHV microRNAs (miRNAs) to oncogenesis is not fully understood. This is particularly true for human endothelial cells, the cell type from which KS tumors are derived. Here we used a panel of KSHV miRNA knockout viruses in order to shed light on the roles of individual miRNAs in the process of transformation. Latently infected endothelial cells were studied for phenotypic changes related to cancer, including proliferation, migration, angiogenesis, glycolysis, and apoptosis. The mutant-infected cell lines displayed a wide range of phenotypes in these selected measures of oncogenesis which differed from wild type-infected cells and from each other. These results indicate that KSHV miRNAs contribute to different aspects of oncogenesis, and that each one has a unique role to play.

Dysregulation of Dual-Specificity Phosphatases by Epstein-Barr Virus LMP1 and Its Impact on Lymphoblastoid Cell Line Survival

The strongest evidence of the oncogenicity of Epstein-Barr virus (EBV) in vitro is its ability to immortalize human primary B lymphocytes into lymphoblastoid cell lines (LCLs). Yet the underlying mechanisms explaining how the virus tempers the growth program of the host cells have not been fully elucidated. The mitogen-activated protein kinases (MAPKs) are implicated in many cellular processes and are constitutively activated in LCLs. We questioned the expression and regulation of the dual-specificity phosphatases (DUSPs), the main negative regulator of MAPKs, during EBV infection and immortalization. Thirteen DUSPs, including 10 typical and 3 atypical types of DUSPs, were tested. Most of them were downregulated after EBV infection. Here, a role of viral oncogene latent membrane protein 1 (LMP1) in limiting DUSP6 and DUSP8 expression was identified. Using MAPK inhibitors, we found that LMP1 activates extracellular signal-regulated kinase (ERK) or p38 to repress the expression of DUSP6 and DUSP8, with corresponding substrate specificity. Morphologically, overexpression of DUSP6 and DUSP8 attenuates the ability of EBV-immortalized LCL cells to clump together. Mechanistically, apoptosis induced by restoring DUSP6 and DUSP8 in LCLs indicated a novel mechanism for LMP1 to provide a survival signal during EBV immortalization. Collectively, this report provides the first description of the interplay between EBV genes and DUSPs and contributes considerably to the interpretation of MAPK regulation in EBV immortalization.IMPORTANCE Infections by the ubiquitous Epstein-Barr virus (EBV) are associated with a wide spectrum of lymphomas and carcinomas. It has been well documented that activation levels of MAPKs are found in cancer cells to translate various external or intrinsic stimuli into cellular responses. Physiologically, the dual-specificity phosphates (DUSPs) exhibit great ability in regulating MAPK activities with respect to their capability of dephosphorylating MAPKs. In this study, we found that DUSPs were generally downregulated after EBV infection. EBV oncogenic latent membrane protein 1 (LMP1) suppressed DUSP6 and DUSP8 expression via MAPK pathway. In this way, LMP1-mediated MAPK activation was a continuous process. Furthermore, DUSP downregulation was found to contribute greatly to prevent apoptosis of EBV-infected cells. To sum up, this report sheds light on a novel molecular mechanism explaining how EBV maintains the unlimited proliferation status of the immortalized cells and provides a new link to understand EBV-induced B cell survival.

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.

kshv-mir-k12-1-5p promotes cell growth and metastasis by targeting SOCS6 in Kaposi's sarcoma cells

Background: Kaposi’s sarcoma (KS) is a highly disseminated angiogenic tumour of endothelial cells. Many deregulated miRNAs, including kshv-mir-k12-1-5p, have been identified in KS. kshv-mir-k12-1-5p plays important roles in KS. However, the underlying mechanism is not fully understood. The aim of this study was to investigate the exact functions of kshv-mir-k12-1-5p in KS cells.

Materials and methods: The biological functions of kshv-mir-k12-1-5p were studied using CCK-8, apoptosis, migration and invasion assays. Bioinformatics software was used to identify the target gene (SOCS6) of kshv-mir-k12-1-5p. A dual luciferase assay, Western blot (WB) and quantitative real-time polymerase chain reaction (q-PCR) were performed to further verify the target gene. The underlying molecular mechanisms of kshv-mir-k12-1-5p in KS cells were also explored.

Results: kshv-mir-k12-1-5p can promote the proliferation, migration and invasion of KS cells and inhibit cell apoptosis. Suppressor of cytokine signalling 6 (SOCS6) was identified as a direct target of kshv-mir-k12-1-5p, and kshv-mir-k12-1-5p can downregulate SOCS6 expression. In addition, knockdown of SOCS6 rescued the effects of kshv-mir-k12-1-5p inhibitor. Hence, a direct relationship between kshv-mir-k12-1-5p and SOCS6 was confirmed.

Conclusions: kshv-mir-k12-1-5p promotes the malignant phenotype of KS cells by targeting SOCS6, suggesting that kshv-mir-k12-1-5p could be a potential therapeutic target for KS.

Memantine ameliorates pulmonary inflammation in a mice model of COPD induced by cigarette smoke combined with LPS

An enhanced chronic inflammatory response in the airways has been regarded as a critical characteristic of chronic obstructive pulmonary disease (COPD). Memantine, an N-methyl-d-aspartate (NMDA) receptors antagonist, has been reported to alleviate lung inflammation. In this study, we investigated the effect and mechanism of memantine on the COPD model induced by cigarette smoke (CS) combined with LPS. Mice and RAW264.7 cells were treated with LPS in the presence or absence of CS. We performed H&E staining to analysis the lung histopathological characteristics. Cytokines (IL-6, TNF-α, and IFN-γ) levels in bronchoalveolar lavage fluid (BALF), lung tissue homogenates and RAW264.7 cell culture medium were determined. Glutamate levels in plasma and culture medium of RAW264.7 were determined. The intracellular Ca2+ flux in RAW264.7 cells was measured by fluo-3 AM staining. The protein levels of NR-1, xCT, ERK1/2, and AKT signaling in the lung tissue and cells were investigated. The result showed that CS and LPS stimulation caused inflammation response, a significant increase in the release of cytokines, including TNF-α, IL-6, and IFN-γ, the elevated release of glutamate and protein levels of NR-1 and xCT, increased Ca2+ influx, and the activation of the ERK1/2 pathway in vitro and in vivo. The above effects of CS and LPS stimulation could be significantly attenuated by memantine treatment. In conclusion, memantine can effectively ameliorate pulmonary inflammation in CS + LPS-induced COPD in mice via reducing NR-1 and xCT expression, glutamate release, Ca2+ influx, and the phosphorylation of Erk1/2. We provided a possible mechanism by which memantine ameliorates COPD in mice.

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.

Effects of silencing the DUSP1 gene using lentiviral vector-mediated siRNA on the release of proinflammatory cytokines through regulation of the MAPK signaling pathway in mice with acute pancreatitis

The present study investigated the effects of dual specificity phosphatase 1 (DUSP1) gene silencing using lentiviral vector-mediated small interfering (si)RNA on the release of proinflammatory cytokines through the regulation of the mitogen‑activated protein kinase (MAPK) signaling pathway in mice with acute pancreatitis (AP). Two siRNA‑DUSP1 sequences and one scramble siRNA sequence were designed, and the expression of DUSP1 was detected using western blot analysis to screen for the one with a higher interference rate. An AP mouse model was established, and KM mice were assigned to either a control, siRNA, AP, AP+PD98059, AP+scramble, AP+siRNA or AP+PD98059+siRNA group. The expression of proinflammatory cytokines, including tumor necrosis factor (TNF)‑α, interleukin (IL)‑1β and IL‑6, high mobility group box 1 (HMGB1), and S100A12 in serum samples were detected using an enzyme‑linked immunosorbent assay at 12, 24 and 48 h post‑modeling. The serum amylase levels were also detected. The expression levels of DUSP1, TNF‑α, IL‑1β, IL‑6, HMGB1, S100A12, phosphorylated (p‑) extracellular signal‑regulated kinase (ERK), p‑c‑Jun N‑terminal kinase (JNK), p‑p38, ERK, JNK and p38 in pancreatic, liver, kidney and lung tissues were detected using reverse transcription‑quantitative polymerase chain reaction and western blot analysis. Compared with the control group, the siRNA group demonstrated marginally upregulated serum amylase, lipase, urinary trypsinogen‑2, and proinflammatory cytokines, HMGB1 and S100A12 in serum and tissues, with no statistically significant difference, elevated expression levels of p‑ERK, p‑JNK and p‑p38, and decreased expression of DUSP1. The other five groups demonstrated increased expression levels of TNF‑α, IL‑1β, IL‑6, HMGB1, S100A12, amylase, lipase and urinary trypsinogen‑2 in serum, and increased expression levels of DUSP1, TNF‑α, IL‑1β, IL‑6, HMGB1, S100A12, p‑ERK, p‑JNK and p‑p38 in tissues. Compared with the AP group, the AP+PD98059+siRNA group had decreased expression of DUSP1 in tissues, whereas the AP+PD98059 group had decreased serum expression levels of TNF‑α, IL‑1β, IL‑6, HMGB1, S100A12 and amylase, lipase and urinary trypsinogen‑2. The expression levels of TNF‑α, IL‑1β, IL‑6, HMGB1, S100A12, p‑ERK, p‑JNK, p‑p38 in tissues, and edema of pancreatic tissue were alleviated, whereas the opposite results were observed in the AP+siRNA group with the decreased expression of DUSP1. The results suggested that DUSP1 gene silencing promoted the release of proinflammatory cytokines through activation of the MAPK signaling pathway in mice with AP.

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.

Modulation of oncogenic signaling networks by Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of three human malignancies: Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. To persist and replicate within host cells, KSHV encodes proteins that modulate different signaling pathways. Manipulation of cell survival and proliferative networks by KSHV can promote the development of KSHV-associated malignancies. In this review, we discuss recent updates on KSHV pathogenesis and the viral life cycle. We focus on proteins encoded by KSHV that modulate the phosphatidylinositol-4,5-bisphosphate 3 kinase and extracellular signal-regulated kinases 1/2 pathways to create an environment favorable for viral replication and the development of KSHV malignancies.

Role of heme oxygenase-1 in the pathogenesis and tumorigenicity of Kaposi's sarcoma-associated herpesvirus

Kaposi's Sarcoma-associated Herpesvirus (KSHV) is the etiologic agent of several malignancies, including Kaposi's Sarcoma (KS), which preferentially arise in immunocompromised patients such as HIV+ subpopulation and lack effective therapeutic options. Heme oxygenase-1 (HO-1) has been reported as an important regulator of endothelial cell cycle control, proliferation and angiogenesis. HO-1 has also been found to be highly expressed in KSHV-infected endothelial cells and oral AIDS-KS lesions. We previously demonstrate that the multifunctional glycoprotein CD147 is required for KSHV/LANA-induced endothelial cell invasiveness. During the identification of CD147 controlled downstream genes by microarray analysis, we found that the expression of HO-1 is significantly elevated in both CD147-overexpressing and KSHV-infected HUVEC cells when compared to control cells. In the current study, we further identify the regulation of HO-1 expression and mediated cellular functions by both CD147 and KSHV-encoded LANA proteins. Targeting HO-1 by either RNAi or the chemical inhibitor, SnPP, effectively induces cell death of KSHV-infected endothelial cells (the major cellular components of KS) through DNA damage and necrosis process. By using a KS-like nude mouse model, we found that SnPP treatment significantly suppressed KSHV-induced tumorigenesis in vivo. Taken together, our data demonstrate the important role of HO-1 in the pathogenesis and tumorigenesis of KSHV-infected endothelial cells, the underlying regulatory mechanisms for HO-1 expression and targeting HO-1 may represent a promising therapeutic strategy against KSHV-related malignancies.

CD147 and downstream ADAMTSs promote the tumorigenicity of Kaposi's sarcoma-associated herpesvirus infected endothelial cells

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent of several human cancers, including Kaposi's sarcoma (KS), which preferentially arise in immunocompromised patients and lack effective therapeutic options. We have previously shown that KSHV or viral protein LANA up-regulates the glycoprotein CD147, thereby inducing primary endothelial cell invasiveness. In the current study, we identify the global network controlled by CD147 in KSHV-infected endothelial cells using Illumina microarray analysis. Among downstream genes, two specific metalloproteases, ADAMTS1 and 9, are strongly expressed in AIDS-KS tissues and contribute to KSHV-infected endothelial cell invasiveness through up-regulation of IL-6 and VEGF. By using a KS-like nude mouse model, we found that targeting CD147 and downstream ADAMTSs significantly suppressed KSHV-induced tumorigenesis in vivo. Taken together, targeting CD147 and associated proteins may represent a promising therapeutic strategy against these KSHV-related malignancies.

Genomic analysis of xCT-mediated regulatory network: Identification of novel targets against AIDS-associated lymphoma

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of primary effusion lymphoma (PEL), a rapidly progressing malignancy mostly arising in HIV-infected patients. Even under conventional chemotherapy, PEL continues to portend nearly 100% mortality within several months, which urgently requires novel therapeutic strategies. We have previously demonstrated that targeting xCT, an amino acid transporter for cystine/glutamate exchange, induces significant PEL cell apoptosis through regulation of multiple host and viral factors. More importantly, one of xCT selective inhibitors, Sulfasalazine (SASP), effectively prevents PEL tumor progression in an immune-deficient xenograft model. In the current study, we use Illumina microarray to explore the profile of genes altered by SASP treatment within 3 KSHV⁺ PEL cell-lines, and discover that many genes involved in oxidative stress/antioxidant defense system, apoptosis/anti-apoptosis/cell death, and cellular response to unfolded proteins/topologically incorrect proteins are potentially regulated by xCT. We further validate 2 downstream candidates, OSGIN1 (oxidative stress-induced growth inhibitor 1) and XRCC5 (X-ray repair cross-complementing protein 5), and evaluate their functional relationship with PEL cell survival/proliferation and chemoresistance, respectively. Together, our data indicate that targeting these novel xCT-regulated downstream genes may represent a promising new therapeutic strategy against PEL and/or other AIDS-related lymphoma.

Induction of hyaluronan production by oncogenic KSHV and the contribution to viral pathogenesis in AIDS patients

Kaposi sarcoma-associated herpesvirus (KSHV) is the etiologic agent for Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL), malignancies arising primarily in immunocompromised patients particularly AIDS-patients, which still lack effective therapy. Hyaluronan (HA) is a large glucuronic acid and has been found closely related to multiple functions in cancer cells, although its role in viral oncogenesis remains largely unknown. Here we provide first evidence that KSHV de novo infection induces HA production from primary endothelial cells through upregulation of HA synthase gene 1 (Has1) and a multifunctional glycoprotein, CD147. Further data demonstrate that KSHV-induced HA production requires viral latent protein, LANA (in particular functional domain A) and MAPK/ERK signaling activities. In functions, HA production is necessary for KSHV/LANA-induced primary endothelial cell invasion, a hallmark feature for KS development. For clinical relevance, our data indicate that the KSHV+ group has higher levels of HA and Has1 activities in its plasma than the KSHV- group of cohort HIV-infected patients. Together, our findings provide innovative insights into the mechanisms of oncogenic virus activation of HA production and its role in virus-associated malignancy pathogenesis, which may help to develop novel therapeutic strategies by targeting HA and related signaling.

xCT, not just an amino-acid transporter: a multi-functional regulator of microbial infection and associated diseases

Expression of xCT, a component of the x_c^– amino-acid transporter, is essential for the uptake of cystine required for intracellular glutathione (GSH) synthesis and maintenance of the intracellular redox balance. Therefore, xCT plays an important role not only in the survival of somatic and immune cells, but also in other aspects of tumorigenesis, including the growth and malignant progression of cancer cells, resistance to anticancer drugs, and protection of normal cells against oxidative damage induced by carcinogens. xCT also functions as a factor required for infection by Kaposi’s sarcoma-associated herpesvirus (KSHV), the causative agent of Kaposi’s sarcoma (KS) and other lymphoproliferative diseases associated with HIV/AIDS. In spite of these advances, our understanding of the role of xCT in the pathogenesis of infectious diseases is still limited. Therefore, this review will summarize recent findings about the functions of xCT in diseases associated with microbial (bacterial or viral) infections, in particular KSHV-associated malignancies. We will also discuss the remaining questions, future directions, as well as evidence that supports the potential benefits of exploring system x_c^– as a target for prevention and clinical management of microbial diseases and cancer.

Role of host microRNAs in Kaposi's sarcoma-associated herpesvirus pathogenesis

MicroRNAs (miRNAs) are small non-coding RNA species that can bind to both untranslated and coding regions of target mRNAs, causing their degradation or post-transcriptional modification. Currently, over 2500 miRNAs have been identified in the human genome. Burgeoning evidence suggests that dysregulation of human miRNAs can play a role in the pathogenesis of a variety of diseases, including cancer. In contrast, only a small subset of human miRNAs has been functionally validated in the pathogenesis of oncogenic viruses, in particular, Kaposi’s sarcoma-associated herpesvirus (KSHV). KSHV is the etiologic agent of several human cancers, such as primary effusion lymphoma (PEL) and Kaposi’s sarcoma (KS), which are mostly seen in acquired immune deficiency syndrome (AIDS) patients or other immuno-suppressed subpopulation. This review summarizes recent literature outlining mechanisms for KSHV/viral proteins regulation of cellular miRNAs contributing to viral pathogenesis, as well as recent findings about the unique signature of miRNAs induced by KSHV infection or KSHV-related malignancies.

KSHV: pathways to tumorigenesis and persistent infection

Kaposi's sarcoma-associated herpesvirus (KSHV; also known as human herpesvirus 8) is the etiologic agent of Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. These cancers often occur in the context of immunosuppression, which has made KSHV-associated malignancies an increasing global health concern with the persistence of the AIDS epidemic. KSHV has also been linked to several acute inflammatory diseases. KSHV exists between a lytic and latent lifecycle, which allows the virus to transition between active replication and quiescent infection. KSHV encodes a number of proteins and small RNAs that are thought to inadvertently transform host cells while performing their functions of helping the virus persist in the infected host. KSHV also has an arsenal of components that aid the virus in evading the host immune response, which help the virus establish a successful lifelong infection. In this comprehensive chapter, we will discuss the diseases associated with KSHV infection, the biology of latent and lytic infection, and individual proteins and microRNAs that are known to contribute to host cell transformation and immune evasion.

Sphingosine kinase-2 maintains viral latency and survival for KSHV-infected endothelial cells

Phosphorylation of sphingosine by sphingosine kinases (SphK1 and SphK2) generates sphingosine-1-phosphate (S1P), a bioactive sphingolipid which promotes cancer cell survival and tumor progression in vivo. We have recently reported that targeting SphK2 induces apoptosis for human primary effusion lymphoma (PEL) cell lines infected by the Kaposi’s sarcoma-associated herpesvirus (KSHV), and this occurs in part through inhibition of canonical NF-κB activation. In contrast, pharmacologic inhibition of SphK2 has minimal impact for uninfected B-cell lines or circulating human B cells from healthy donors. Therefore, we designed additional studies employing primary human endothelial cells to explore mechanisms responsible for the selective death observed for KSHV-infected cells during SphK2 targeting. Using RNA interference and a clinically relevant pharmacologic approach, we have found that targeting SphK2 induces apoptosis selectively for KSHV-infected endothelial cells through induction of viral lytic gene expression. Moreover, this effect occurs through repression of KSHV-microRNAs regulating viral latency and signal transduction, including miR-K12-1 which targets IκBα to facilitate activation of NF-κB, and ectopic expression of miR-K12-1 restores NF-κB activation and viability for KSHV-infected endothelial cells during SphK2 inhibition. These data illuminate a novel survival mechanism and potential therapeutic target for KSHV-infected endothelial cells: SphK2-associated maintenance of viral latency.

Lipoteichoic acid (LTA) and lipopolysaccharides (LPS) from periodontal pathogenic bacteria facilitate oncogenic herpesvirus infection within primary oral cells

Kaposi's sarcoma (KS) remains the most common tumor arising in patients with HIV/AIDS, and involvement of the oral cavity represents one of the most common clinical manifestations of this tumor. HIV infection incurs an increased risk for periodontal diseases and oral carriage of a variety of bacteria. Whether interactions involving pathogenic bacteria and oncogenic viruses in the local environment facilitate replication or maintenance of these viruses in the oral cavity remains unknown. In the current study, our data indicate that pretreatment of primary human oral fibroblasts with two prototypical pathogen-associated molecular patterns (PAMPs) produced by oral pathogenic bacteria-lipoteichoic acid (LTA) and lipopolysaccharide (LPS), increase KSHV entry and subsequent viral latent gene expression during de novo infection. Further experiments demonstrate that the underlying mechanisms induced by LTA and/or LPS include upregulation of cellular receptor, increasing production of reactive oxygen species (ROS), and activating intracellular signaling pathways such as MAPK and NF-κB, and all of which are closely associated with KSHV entry or gene expression within oral cells. Based on these findings, we hope to provide the framework of developing novel targeted approaches for treatment and prevention of oral KSHV infection and KS development in high-risk HIV-positive patients.

Targeting xCT, a cystine-glutamate transporter induces apoptosis and tumor regression for KSHV/HIV-associated lymphoma

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the etiological agent of primary effusion lymphoma (PEL), which represents a rapidly progressing malignancy arising in HIV-infected patients. Conventional chemotherapy for PEL treatment induces unwanted toxicity and is ineffective — PEL continues to portend nearly 100% mortality within a period of months, which requires novel therapeutic strategies. The amino acid transporter, xCT, is essential for the uptake of cystine required for intracellular glutathione (GSH) synthesis and for maintaining the intracellular redox balance. Inhibition of xCT induces growth arrest in a variety of cancer cells, although its role in virus-associated malignancies including PEL remains unclear. In the current study, we identify that xCT is expressed on the surface of patient-derived KSHV⁺ PEL cells, and targeting xCT induces caspase-dependent cell apoptosis. Further experiments demonstrate the underlying mechanisms including host and viral factors: reducing intracellular GSH while increasing reactive oxygen species (ROS), repressing cell-proliferation-related signaling, and inducing viral lytic genes. Using an immune-deficient xenograft model, we demonstrate that an xCT selective inhibitor, Sulfasalazine (SASP), prevents PEL tumor progression in vivo. Together, our data provide innovative and mechanistic insights into the role of xCT in PEL pathogenesis, and the framework for xCT-focused therapies for AIDS-related lymphoma in future.

Kaposi's sarcoma-associated herpesvirus encodes a mimic of cellular miR-23

Kaposi's sarcoma-associated herpesvirus (KSHV) expresses ∼20 viral microRNAs (miRNAs) in latently infected cells. We have previously shown that two of these miRNAs function as mimics of the cellular miRNAs miR-155 and miR-142-3p. Two additional KSHV miRNAs, miR-K3+1 and miR-K3, share perfect and offset 5' homology with cellular miR-23, respectively. Here, we report a single nucleotide polymorphism that causes miR-K3+1 expression in a subset of KSHV-infected primary effusion lymphoma cell lines as a consequence of altered processing of the primary transcript by the Microprocessor complex. We confirm that miR-K3+1 regulates miR-23 targets, which is expected because these miRNAs share the entire seed region (nucleotides 2 to 8). Surprisingly, we found that miR-K3 also regulates miR-23 targets, despite offset seed sequences. In addition, the offset homology of miR-K3 to miR-23 likely allows this viral miRNA to expand its target repertoire beyond the targets of miR-23. Because miR-23 is highly expressed in endothelial cells but expressed at only low levels in B cells, we hypothesize that miR-K3 may function to introduce miR-23-like activities into KSHV-infected B cells. Together, our data demonstrate that KSHV has evolved at least three distinct viral miRNAs to tap into evolutionarily conserved cellular miRNA-regulatory networks. Furthermore, our data allow fundamental insights into the generation and functional impact of miRNA 5' end variation.