Interferon induced circRNAs escape herpesvirus host shutoff and suppress lytic infection.

To globally profile circRNAs, we employ RNA-Sequencing paired with chimeric junction analysis for alpha-, beta-, and gamma-herpesvirus infection. We find circRNAs are, as a population, resistant to host shutoff. We validate this observation using ectopic expression assays of human and murine herpesvirus endoribonucleases. During lytic infection, four circRNAs are commonly induced across all subfamilies of human herpesviruses, suggesting a shared mechanism of regulation. We test one such mechanism, namely how interferon-stimulation influences circRNA expression. 67 circRNAs are upregulated by either interferon-ß or -γ treatment, with half of these also upregulated during lytic infection. Using gain and loss of function studies we find an interferon-stimulated circRNA, circRELL1, inhibits lytic Herpes Simplex Virus-1 infection. We previously reported circRELL1 inhibits lytic Kaposi sarcoma-associated herpesvirus infection, suggesting a pan-herpesvirus antiviral activity. We propose a two-pronged model in which interferon-stimulated genes may encode both mRNA and circRNA with antiviral activity. This is critical in cases of host shutoff, such as alpha- and gamma-herpesvirus infection, where the mRNA products are degraded but circRNAs escape.

A virus-induced circular RNA maintains latent infection of Kaposi's sarcoma herpesvirus.

Non-coding RNAs (ncRNAs) play important roles in host-pathogen interactions; oncogenic viruses like Kaposi's sarcoma herpesvirus (KSHV) employ ncRNAs to establish a latent reservoir and persist for the life of the host. We previously reported that KSHV infection alters a novel class of RNA, circular RNAs (circRNAs). CircRNAs are alternative splicing isoforms and regulate gene expression, but their importance in infection is largely unknown. Here, we showed that a human circRNA, hsa_circ_0001400, is induced by various pathogenic viruses, namely KSHV, Epstein-Barr virus, and human cytomegalovirus. The induction of circRNAs including circ_0001400 by KSHV is co-transcriptionally regulated, likely at splicing. Consistently, screening for circ_0001400-interacting proteins identified a splicing factor, PNISR. Functional studies using infected primary endothelial cells revealed that circ_0001400 inhibits KSHV lytic transcription and virus production. Simultaneously, the circRNA promoted cell cycle, inhibited apoptosis, and induced immune genes. RNA-pull down assays identified transcripts interacting with circ_0001400, including TTI1, which is a component of the pro-growth mTOR complexes. We thus identified a circRNA that is pro-growth and anti-lytic replication. These results support a model in which KSHV induces circ_0001400 expression to maintain latency. Since circ_0001400 is induced by multiple viruses, this novel viral strategy may be widely employed by other viruses.

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.

Viral non-coding RNAs: Stealth strategies in the tug-of-war between humans and herpesviruses.

Oncogenic DNA viruses establish lifelong infections in humans, and they cause cancers, often in immunocompromised patients, despite anti-viral immune surveillance targeted against viral antigens. High-throughput sequencing techniques allowed the field to identify novel viral non-coding RNAs (ncRNAs). ncRNAs are ideal factors for DNA viruses to exploit; they are non-immunogenic to T cells, thus viral ncRNAs can manipulate host cells without evoking adaptive immune responses. Viral ncRNAs may still trigger the host innate immune response, but many viruses encode decoys/inhibitors to counter-act and evade recognition. In addition, ncRNAs can be secreted to the extracellular space and influence adjacent cells to create a pro-viral microenvironment. In this review, we present recent progress in understanding interactions between oncoviruses and ncRNAs including small and long ncRNAs, microRNAs, and recently identified viral circular RNAs. In addition, potential clinical applications for ncRNA will be discussed. Extracellular ncRNAs are suggested to be diagnostic and prognostic biomarkers and, with the realization of the importance of viral ncRNAs in tumorigenesis, approaches to target critical viral ncRNAs are emerging. Further understanding of viral utilization of ncRNAs will advance anti-viral therapeutics beyond conventional medication and vaccination.

Transcriptional landscape of Kaposi sarcoma tumors identifies unique immunologic signatures and key determinants of angiogenesis.

BACKGROUND: Kaposi sarcoma (KS) is a multicentric tumor caused by Kaposi sarcoma herpesvirus (KSHV) that leads to morbidity and mortality among people with HIV worldwide. KS commonly involves the skin but can occur in the gastrointestinal tract (GI) in severe cases. METHODS: RNA sequencing was used to compare the cellular and KSHV gene expression signatures of skin and GI KS lesions in 44 paired samples from 19 participants with KS alone or with concurrent KSHV-associated diseases. Analyses of KSHV expression from KS lesions identified transcriptionally active areas of the viral genome. RESULTS: The transcript of an essential viral lytic gene, ORF75, was detected in 91% of KS lesions. Analyses of host genes identified 370 differentially expressed genes (DEGs) unique to skin KS and 58 DEGs unique to GI KS lesions as compared to normal tissue. Interleukin (IL)-6 and IL-10 gene expression were higher in skin lesions as compared to normal skin but not in GI KS lesions. Twenty-six cellular genes were differentially expressed in both skin and GI KS tissues: these included Fms-related tyrosine kinase 4 (FLT4), encoding an angiogenic receptor, and Stanniocalcin 1 (STC1), a secreted glycoprotein. FLT4 and STC1 were further investigated in functional studies using primary lymphatic endothelial cells (LECs). In these models, KSHV infection of LECs led to increased tubule formation that was impaired upon knock-down of STC1 or FLT4. CONCLUSIONS: This study of transcriptional profiling of KS tissue provides novel insights into the characteristics and pathogenesis of this unique virus-driven neoplasm.

Monocyte chemoattractant protein-induced protein 1 directly degrades viral miRNAs with a specific motif and inhibits KSHV infection.

Kaposi's sarcoma-associated herpesvirus (KSHV) expresses miRNAs during latency. However, regulation of viral miRNAs remains largely unknown. Our prior studies demonstrated that MCPIP1 regulates KSHV miRNA biogenesis by degrading most KSHV pre-miRNAs through its RNase activity. Some viral pre-miRNAs are partially resistant to degradation by MCPIP1. Here, we further characterized MCPIP1 substrate specificity and its antiviral potential against KSHV infection. In vitro cleavage assays and binding assays showed that MCPIP1 cleavage efficiency is related to binding affinity. Motif-based sequence analysis identified that KSHV pre-miRNAs that are well degraded by MCPIP1 have a 5-base motif (M5 base motif) within their terminal loops and this motif region consists of multiple pyrimidine-purine-pyrimidine (YRY) motifs. We further demonstrated that mutation of this M5 base motif within terminal loop of pre-miRNAs inhibited MCPIP1-mediated RNA degradation. We also revealed that MCPIP1 has an antiviral effect against KSHV infection. MCPIP1 can reduce the expression of Dicer, which in turn restricts KSHV infection. Conclusively, our findings demonstrated that MCPIP1 inhibited KSHV infection and suppressed viral miRNA biogenesis by directly degrading KSHV pre-miRNAs and altering the expression of miRNA biogenesis factors.

Identifying and characterizing virus-encoded circular RNAs.

Circular forms of RNA were first discovered in plant viroids and later found in a variety of animal viruses. These circular RNAs lack free 5' and 3' ends, granting protection from exonucleases. This review is focused on the methods that are used to investigate virus-encoded circular RNAs. Using DNA viruses that are prevalent among human as examples, we begin with features of circular RNAs and the unique methods to enrich for circular RNAs. Next, we discuss the computational methods for RNA-sequencing analysis to discover new virus-encoded circular RNAs. Many strategies are similar to analyzing cellular RNAs, but some unique aspects of virus-encoded circular RNAs that are likely due to highly packed viral genomes and non-canonical use of splicing machinery, are described herein. We illustrate the various methods of validating expression of specific virus-encoded circular RNAs. Finally, we discuss novel methods to study functions of circular RNAs and the current technical challenges that remain for investigating virus-encoded circular RNAs.

Discovery of Kaposi's sarcoma herpesvirus-encoded circular RNAs and a human antiviral circular RNA.

Noncoding RNAs have substantial effects in host-virus interactions. Circular RNAs (circRNAs) are novel single-stranded noncoding RNAs which can decoy other RNAs or RNA-binding proteins to inhibit their functions. The role of circRNAs is largely unknown in the context of Kaposi's sarcoma herpesvirus (KSHV). We hypothesized that circRNAs influence viral infection by inhibiting host and/or viral factors. Transcriptome analysis of KSHV-infected primary endothelial cells and a B cell line identified human circRNAs that are differentially regulated upon infection. We confirmed the expression changes with divergent PCR primers and RNase R treatment of specific circRNAs. Ectopic expression of hsa_circ_0001400, a circRNA induced by infection, suppressed expression of key viral latent gene LANA and lytic gene RTA in KSHV de novo infections. Since human herpesviruses express noncoding RNAs like microRNAs, we searched for viral circRNAs encoded in the KSHV genome. We performed circRNA-Seq analysis with RNase R-treated, circRNA-enriched RNA from KSHV-infected cells. We identified multiple circRNAs encoded by the KSHV genome that are expressed in KSHV-infected endothelial cells and primary effusion lymphoma (PEL) cells. The KSHV circRNAs are located within ORFs of viral lytic genes, are up-regulated upon the induction of the lytic cycle, and alter cell growth. Viral circRNAs were also detected in lymph nodes from patients of KSHV-driven diseases such as PEL, Kaposi's sarcoma, and multicentric Castleman's disease. We revealed new host-virus interactions of circRNAs: human antiviral circRNAs are activated in response to KSHV infection, and viral circRNA expression is induced in the lytic phase of infection.

RNA Sequencing Reveals that Kaposi Sarcoma-Associated Herpesvirus Infection Mimics Hypoxia Gene Expression Signature.

Kaposi sarcoma-associated herpesvirus (KSHV) causes several tumors and hyperproliferative disorders. Hypoxia and hypoxia-inducible factors (HIFs) activate latent and lytic KSHV genes, and several KSHV proteins increase the cellular levels of HIF. Here, we used RNA sequencing, qRT-PCR, Taqman assays, and pathway analysis to explore the miRNA and mRNA response of uninfected and KSHV-infected cells to hypoxia, to compare this with the genetic changes seen in chronic latent KSHV infection, and to explore the degree to which hypoxia and KSHV infection interact in modulating mRNA and miRNA expression. We found that the gene expression signatures for KSHV infection and hypoxia have a 34% overlap. Moreover, there were considerable similarities between the genes up-regulated by hypoxia in uninfected (SLK) and in KSHV-infected (SLKK) cells. hsa-miR-210, a HIF-target known to have pro-angiogenic and anti-apoptotic properties, was significantly up-regulated by both KSHV infection and hypoxia using Taqman assays. Interestingly, expression of KSHV-encoded miRNAs was not affected by hypoxia. These results demonstrate that KSHV harnesses a part of the hypoxic cellular response and that a substantial portion of hypoxia-induced changes in cellular gene expression are induced by KSHV infection. Therefore, targeting hypoxic pathways may be a useful way to develop therapeutic strategies for KSHV-related diseases.

Virus-Mediated Alterations in miRNA Factors and Degradation of Viral miRNAs by MCPIP1.

Kaposi's sarcoma-associated herpesvirus (KSHV), the causative agent of Kaposi's sarcoma, encodes 25 mature viral miRNAs. MCP-1-induced protein-1 (MCPIP1), a critical regulator of immune homeostasis, has been shown to suppress miRNA biosynthesis via cleavage of precursor miRNAs through its RNase domain. We demonstrate that MCPIP1 can directly cleave KSHV and EBV precursor miRNAs and that MCPIP1 expression is repressed following de novo KSHV infection. In addition, repression with siRNAs to MCPIP1 in KSHV-infected cells increased IL-6 and KSHV miRNA expression, supporting a role for MCPIP1 in IL-6 and KSHV miRNA regulation. We also provide evidence that KSHV miRNAs repress MCPIP1 expression by targeting the 3'UTR of MCPIP1. Conversely, expression of essential miRNA biogenesis components Dicer and TRBP is increased following latent KSHV infection. We propose that KSHV infection inhibits a negative regulator of miRNA biogenesis (MCPIP1) and up-regulates critical miRNA processing components to evade host mechanisms that inhibit expression of viral miRNAs. KSHV-mediated alterations in miRNA biogenesis represent a novel mechanism by which KSHV interacts with its host and a new mechanism for the regulation of viral miRNA expression.

Kaposi's Sarcoma-Associated Herpesvirus MicroRNAs Target GADD45B To Protect Infected Cells from Cell Cycle Arrest and Apoptosis.

Kaposi's sarcoma is one of the most common malignancies in HIV-infected individuals. The responsible agent, Kaposi's sarcoma-associated herpesvirus (KSHV; HHV8), expresses multiple microRNAs (miRNAs), but the targets and functions of these miRNAs are not completely understood. After infection in primary endothelial cells with KSHV, growth arrest DNA damage-inducible gene 45 beta (GADD45B) is one of the most repressed genes using genomic expression profiling. GADD45B was also repressed in mRNA expression profiling experiments when KSHV miRNAs were introduced to uninfected cells. We hypothesized that KSHV miRNAs target human GADD45B to protect cells from consequences of DNA damage, which can be triggered by viral infection. Expression of GADD45B protein is induced by the p53 activator, Nutlin-3, and KSHV miRNA-K9 inhibits this induction. In addition, Nutlin-3 increased apoptosis and cell cycle arrest based on flow cytometry assays. KSHV miR-K9 protected primary endothelial cells from apoptosis and cell cycle arrest following Nutlin-3 treatment. Similar protective phenotypes were seen for targeting GADD45B with short interfering RNAs (siRNAs), as with miR-K9. KSHV miR-K9 also decreased the protein levels of cleaved caspase-3, cleaved caspase-7, and cleaved poly(ADP-ribose) polymerase (PARP). In B lymphocytes latently infected with KSHV, specific inhibitors of KSHV miR-K9 led to increased GADD45B expression and apoptosis, indicating that miR-K9 is important for reducing apoptosis in infected cells. Furthermore, ectopic expression of GADD45B in KSHV-infected cells promoted apoptosis. Together, these results identify a new miRNA target and demonstrate that KSHV miRNAs are important for protecting infected cells from DNA damage responses. IMPORTANCE: Kaposi's sarcoma-associated herpesvirus is a leading cause of cancers in individuals with AIDS. Promoting survival of infected cells is essential for maintaining viral infections. A virus needs to combat various cellular defense mechanisms designed to eradicate the viral infection. One such response can include DNA damage response factors, which can promote an arrest in cell growth and trigger cell death. We used a new approach to search for human genes repressed by small nucleic acids (microRNAs) expressed by a gammaherpesvirus (KSHV), which identified a gene called GADD45B as a target of microRNAs. Repression of GADD45B, which is expressed in response to DNA damage, benefited survival of infected cells in response to a DNA damage response. This information could be used to design new treatments for herpesvirus infections.

On the Oxidation State of Manganese Ions in Li-Ion Battery Electrolyte Solutions.

We demonstrate herein that Mn3+ and not Mn2+, as commonly accepted, is the dominant dissolved manganese cation in LiPF6-based electrolyte solutions of Li-ion batteries with lithium manganate spinel positive and graphite negative electrodes chemistry. The Mn3+ fractions in solution, derived from a combined analysis of electron paramagnetic resonance and inductively coupled plasma spectroscopy data, are ∼80% for either fully discharged (3.0 V hold) or fully charged (4.2 V hold) cells, and ∼60% for galvanostatically cycled cells. These findings agree with the average oxidation state of dissolved Mn ions determined from X-ray absorption near-edge spectroscopy data, as verified through a speciation diagram analysis. We also show that the fractions of Mn3+ in the aprotic nonaqueous electrolyte solution are constant over the duration of our experiments and that disproportionation of Mn3+ occurs at a very slow rate.

Kaposi's sarcoma-associated herpesvirus microRNAs repress breakpoint cluster region protein expression, enhance Rac1 activity, and increase in vitro angiogenesis.

UNLABELLED: MicroRNAs (miRNAs) are small, ∼ 22-nucleotide-long RNAs that regulate gene expression posttranscriptionally. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes 12 pre-miRNAs during latency, and the functional significance of these microRNAs during KSHV infection and their cellular targets have been emerging recently. Using a previously reported microarray profiling analysis, we identified breakpoint cluster region mRNA (Bcr) as a cellular target of the KSHV miRNA miR-K12-6-5p (miR-K6-5). Bcr protein levels were repressed in human umbilical vein endothelial cells (HUVECs) upon transfection with miR-K6-5 and during KSHV infection. Luciferase assays wherein the Bcr 3' untranslated region (UTR) was cloned downstream of a luciferase reporter showed repression in the presence of miR-K6-5, and mutation of one of the two predicted miR-K6-5 binding sites relieved this repression. Furthermore, inhibition or deletion of miR-K6-5 in KSHV-infected cells showed increased Bcr protein levels. Together, these results show that Bcr is a direct target of the KSHV miRNA miR-K6-5. To understand the functional significance of Bcr knockdown in the context of KSHV-associated disease, we hypothesized that the knockdown of Bcr, a negative regulator of Rac1, might enhance Rac1-mediated angiogenesis. We found that HUVECs transfected with miR-K6-5 had increased Rac1-GTP levels and tube formation compared to HUVECs transfected with control miRNAs. Knockdown of Bcr in latently KSHV-infected BCBL-1 cells increased the levels of viral RTA, suggesting that Bcr repression by KSHV might aid lytic reactivation. Together, our results reveal a new function for both KSHV miRNAs and Bcr in KSHV infection and suggest that KSHV miRNAs, in part, promote angiogenesis and lytic reactivation. IMPORTANCE: Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) infection is linked to multiple human cancers and lymphomas. KSHV encodes small nucleic acids (microRNAs) that can repress the expression of specific human genes, the biological functions of which are still emerging. This report uses a variety of approaches to show that a KSHV microRNA represses the expression of the human gene called breakpoint cluster region (Bcr). Repression of Bcr correlated with the activation of a protein previously shown to cause KS-like lesions in mice (Rac1), an increase in KS-associated phenotypes (tube formation in endothelial cells and vascular endothelial growth factor [VEGF] synthesis), and modification of the life cycle of the virus (lytic replication). Our results suggest that KSHV microRNAs suppress host proteins and contribute to KS-associated pathogenesis.

Proteomic screening of human targets of viral microRNAs reveals functions associated with immune evasion and angiogenesis.

Kaposi's sarcoma (KS) is caused by infection with Kaposi's sarcoma-associated herpesvirus (KSHV). The virus expresses unique microRNAs (miRNAs), but the targets and functions of these miRNAs are not completely understood. In order to identify human targets of viral miRNAs, we measured protein expression changes caused by multiple KSHV miRNAs using pulsed stable labeling with amino acids in cell culture (pSILAC) in primary endothelial cells. This led to the identification of multiple human genes that are repressed at the protein level, but not at the miRNA level. Further analysis also identified that KSHV miRNAs can modulate activity or expression of upstream regulatory factors, resulting in suppressed activation of a protein involved in leukocyte recruitment (ICAM1) following lysophosphatidic acid treatment, as well as up-regulation of a pro-angiogenic protein (HIF1α), and up-regulation of a protein involved in stimulating angiogenesis (HMOX1). This study aids in our understanding of miRNA mechanisms of repression and miRNA contributions to viral pathogenesis.

The Role of OOH Binding Site and Pt Surface Structure on ORR Activities.

We present experimentally observed molecular adsorbate coverages (e.g., O(H), OOH and HOOH) on real operating dealloyed bimetallic PtMx (M = Ni or Co) catalysts under oxygen reduction reaction (ORR) conditions obtained using X-ray absorption near edge spectroscopy (XANES). The results reveal a complex Sabatier catalysis behavior and indicate the active ORR mechanism changes with Pt-O bond weakening from the O2 dissociative mechanism, to the peroxyl mechanism, and finally to the hydrogen peroxide mechanism. An important rearrangement of the OOH binding site, an intermediate in the ORR, enables facile H addition to OOH and faster O-O bond breaking on 111 faces at optimal Pt-O bonding strength, such as that occurring in dealloyed PtM core-shell nanoparticles. This rearrangement is identified by previous DFT calculations and confirmed from in situ measured OOH adsorption coverages during the ORR. The importance of surface structural effects and 111 ordered faces is confirmed by the higher specific ORR rates on solid core vs porous multi-core nanoparticles.

Kaposi's sarcoma-associated herpesvirus microRNAs target IRAK1 and MYD88, two components of the toll-like receptor/interleukin-1R signaling cascade, to reduce inflammatory-cytokine expression.

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is the causative agent of KS, an important AIDS-associated malignancy. KSHV expresses at least 18 different mature microRNAs (miRNAs). We identified interleukin-1 receptor (IL-1R)-associated kinase 1 (IRAK1) as a potential target of miR-K12-9 (miR-K9) in an array data set examining changes in cellular gene expression levels in the presence of KSHV miRNAs. Using 3'-untranslated region (3'UTR) luciferase reporter assays, we confirmed that miR-K9 and other miRNAs inhibit IRAK1 expression. In addition, IRAK1 expression is downregulated in cells transfected with miR-K9 and during de novo KSHV infection. IRAK1 is an important component of the Toll-like receptor (TLR)/IL-1R signaling cascade. The downregulation of IRAK1 by miR-K9 resulted in the decreased stimulation of NF-κB activity in endothelial cells treated with IL-1α and in B cells treated with a TLR7/8 agonist. Interestingly, miR-K9 had a greater effect on NF-κB activity than did a small interfering RNA (siRNA) targeting IRAK1 despite the more efficient downregulation of IRAK1 expression with the siRNA. We hypothesized that KSHV miRNAs may also be regulating a second component of the TLR/IL-1R signaling cascade, resulting in a stronger phenotype. Reanalysis of the array data set identified myeloid differentiation primary response protein 88 (MYD88) as an additional potential target. 3'UTR luciferase reporter assays and Western blot analysis confirmed the targeting of MYD88 by miR-K5. The presence of miR-K9 and miR-K5 inhibited the production of IL-6 and IL-8 upon the IL-1α stimulation of endothelial cells. These results demonstrate KSHV-encoded miRNAs regulating the TLR/IL-1R signaling cascade at two distinct points and suggest the importance of these pathways during viral infection.

Interferon induced circRNAs escape herpesvirus host shutoff and suppress lytic infection.

A first line of defense during infection is expression of interferon (IFN)-stimulated gene products which suppress viral lytic infection. To combat this, herpesviruses express endoribonucleases to deplete host RNAs. Here we demonstrate that IFN-induced circular RNAs (circRNAs) can escape viral-mediated degradation. We performed comparative circRNA expression profiling for representative alpha- (Herpes simplex virus-1, HSV-1), beta- (human cytomegalovirus, HCMV), and gamma-herpesviruses (Kaposi sarcoma herpesvirus, KSHV; murine gamma-herpesvirus 68, MHV68). Strikingly, we found that circRNAs are, as a population, resistant to host shutoff. This observation was confirmed by ectopic expression assays of human and murine herpesvirus endoribonucleases. During primary lytic infection, ten circRNAs were commonly regulated across all subfamilies of human herpesviruses, suggesting a common mechanism of regulation. We tested one such mechanism, namely how interferon-stimulation influences circRNA expression. 67 circRNAs were upregulated by either IFN-ß or -γ treatment, with half of these also upregulated during lytic infection. Using gain and loss of function studies we found an interferon-stimulated circRNA, circRELL1, inhibited lytic HSV-1 infection. We have previously reported circRELL1 inhibits lytic KSHV infection, suggesting a pan-herpesvirus antiviral activity. We propose a two-pronged model in which interferon-stimulated genes may encode both mRNA and circRNA with antiviral activity. This is critical in cases of host shutoff, such as alpha- and gamma-herpesvirus infection, where the mRNA products are degraded but circRNAs escape.

Functions of Kaposi's sarcoma-associated herpesvirus microRNAs.

MicroRNAs can modulate gene expression post-transcriptionally by altering mRNA stability and protein translation. Multiple DNA viruses express viral microRNAs and have been shown to modulate expression of host and viral genes. Through various methods of microRNA target identification, we are beginning to understand the various roles of viral miRNAs including viral replication, immune evasion and apoptosis. This review is focused on the roles of microRNAs expressed by Kaposi's sarcoma-associated herpesvirus or human herpesvirus 8. This article is part of a Special Issue entitled: "MicroRNAs in viral gene regulation".

Regulation of tumor necrosis factor-like weak inducer of apoptosis receptor protein (TWEAKR) expression by Kaposi's sarcoma-associated herpesvirus microRNA prevents TWEAK-induced apoptosis and inflammatory cytokine expression.

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is the causative agent of KS, the second most common AIDS-associated malignancy. KSHV expresses at least 18 different mature microRNAs (miRNAs) during latency. To identify cellular targets of KSHV miRNAs, we have analyzed a previously reported series of microarrays examining changes in cellular gene expression in the presence of KSHV miRNAs. Tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) receptor (TWEAKR) was among the most consistently and robustly downregulated genes in the presence of KSHV miR-K12-10a (miR-K10a). Results from luciferase assays with reporter plasmids containing the 3' untranslated region (UTR) of TWEAKR suggest a targeting of TWEAKR by miR-K10a. The mutation of two predicted miR-K10a recognition sites within the 3' UTR of TWEAKR completely disrupts inhibition by miR-K10a. The expression of TWEAKR was downregulated in cells transfected with miR-K10a as well as during de novo KSHV infection. In a KS tumor-derived endothelial cell line, the downregulation of TWEAKR by miR-K10a resulted in reduced levels of TWEAK-induced caspase activation. In addition, cells transfected with miR-K10a showed less induction of apoptosis by annexin V staining and terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) assays. Finally, the downregulation of TWEAKR by miR-K10a in primary human endothelial cells resulted in a decrease in levels of expression of the proinflammatory cytokines interleukin-8 (IL-8) and monocyte chemoattractant protein 1 (MCP-1) in response to TWEAK. These results identify and validate an important cellular target of KSHV miRNAs. Furthermore, we demonstrate that a viral miRNA protects cells from apoptosis and suppresses a proinflammatory response, which may have significant implications in the complex context of KS lesions.