Kaposi's sarcoma-associated herpesvirus latent protein LANA interacts with HIF-1 alpha to upregulate RTA expression during hypoxia: Latency control under low oxygen conditions

Hypoxic reactivation of Kaposi's sarcoma associated herpesvirus

Hypoxic reactivation of Kaposi's sarcoma-associated herpesvirus (KSHV) refers to the phenomenon under low oxygen where the virus goes from latent to lytic replication. Typically, healthy cells generally cease cell division and DNA replication under hypoxic conditions due to limited resources, and the presence of physiological inhibitors. This restricted replication under hypoxic conditions is considered an employed strategy of the cell to minimize energy consumption. However, cancerous cells continuously replicate and divide in hypoxic conditions by reprogramming several aspects of their cell physiology, including but not limited to metabolism, cell cycle, DNA replication, transcription, translation, and the epigenome. KSHV infection, similar to cancerous cells, is known to bypass hypoxia-induced restrictions and undergo reactivation to produce progeny viruses. In previous studies we have mapped several aspects of cell physiology that are manipulated by KSHV through its latent antigens during hypoxic conditions, which allows for a permissive environment for its replication. We discuss the major strategies utilized by KSHV to bypass hypoxia-induced repression. We also describe the KSHV-encoded antigens responsible for modulating these cellular processes important for successful viral replication and persistence in hypoxia.

Targeting hypoxia-inducible factors in malignancies caused by Kaposis sarcoma associated herpesvirus

In this editorial, we highlight the potential use of inhibitors of hypoxia-inducible factors (HIFs) for the use in Kaposi's sarcoma associated herpesvirus (KSHV) (also known as human herpesvirus-8) related malignancies. The past 20 years has accumulated detailed knowledge of the role of these factors in ensuring the maintenance of the KSHV in infected cells, in aiding the growth of the virus infected cells and aiding in the spread of virus from infected cells by inducing lytic reactivation. Today, a wide range of inhibitors for HIFs are currently being clinically evaluated for use in treating a variety of cancers. We discuss the current state of this research area as it relates to KSHV malignancies and describe pre-clinical and clinical evidence of drugs that target HIF to back up the idea that these inhibitors could be a novel way to treat KSHV related diseases.

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 cis-regulatory elements as sensors of cellular states and environmental cues

To withstand a hostile cellular environment and replicate, viruses must sense, interpret, and respond to many internal and external cues. Retroviruses and DNA viruses can intercept these cues impinging on host transcription factors via cis-regulatory elements (CREs) in viral genomes, allowing them to sense and coordinate context-specific responses to varied signals. Here, we explore the characteristics of viral CREs, the classes of signals and host transcription factors that regulate them, and how this informs outcomes of viral replication, immune evasion, and latency. We propose that viral CREs constitute central hubs for signal integration from multiple pathways and that sequence variation between viral isolates can rapidly rewire sensing mechanisms, contributing to the variability observed in patient outcomes.

An atlas of chromatin landscape in KSHV-infected cells during de novo infection and reactivation

Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic γ-herpesvirus with a double-stranded DNA capable of establishing latent infection in the host cell. During latency, only a limited number of viral genes are expressed in infected host cells, and that helps the virus to evade host immune cell response. During primary infection, the KSHV genome is chromatinized and maintained as an episome, which is tethered to the host chromosome via Latency Associated Nuclear Antigen (LANA). The KSHV episome undergoes the same chromatin modification with the host cell chromosome and, therefore, is regulated by various epigenetic modifications, such as DNA methylation, histone methylation, and histone acetylation. The KSHV genome is also organized in a spatiotemporal manner by forming genomic loops, which enable simultaneous and coordinated control of dynamic gene transcription, particularly during the lytic replication phase. The genome-wide approaches and advancing bioinformatic tools have increased the resolution of studies on the dynamic transcriptional control and our understanding of KSHV latency-lytic switch regulation. We will summarize our current understanding of the epigenetic gene regulation on the KSHV chromatin.

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

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

Hypoxia and HIF-1α promote lytic de novo KSHV infection

IMPORTANCE

The current view is that the default pathway of Kaposi's sarcoma-associated herpesvirus (KSHV) infection is the establishment of latency, which is a prerequisite for lifelong infection and viral oncogenesis. This view about KSHV infection is supported by the observations that KSHV latently infects most of the cell lines cultured in vitro in the absence of any environmental stresses that may occur in vivo. The goal of this study was to determine the effect of hypoxia, a natural stress stimulus, on primary KSHV infection. Our data indicate that hypoxia promotes euchromatin formation on the KSHV genome following infection and supports lytic de novo KSHV infection. We also discovered that hypoxia-inducible factor-1α is required and sufficient for allowing lytic KSHV infection. Based on our results, we propose that hypoxia promotes lytic de novo infection in cells that otherwise support latent infection under normoxia; that is, the environmental conditions can determine the outcome of KSHV primary infection.

Hypoxia and hypoxia-inducible factors in Kaposi sarcoma-associated herpesvirus infection and disease pathogenesis

Kaposi sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi sarcoma and several other tumors and hyperproliferative diseases seen predominantly in human immunodeficiency virus-infected and other immunocompromised persons. There is an increasing body of evidence showing that hypoxia and hypoxia-inducible factors (HIFs) play important roles in the biology of KSHV and in the pathogenesis of KSHV-induced diseases. Hypoxia and HIFs can induce lytic activation of KSHV and KSHV can in turn lead to a hypoxic-like state in infected cells. In this review, we describe the complex interactions between KSHV biology, the cellular responses to hypoxia, and the pathogenesis of KSHV-induced diseases. We also describe how interference with HIFs can lead to decreased tumor growth and/or death of infected cells and KSHV-induced tumors. Finally, we show how these observations may lead to novel strategies for the treatment of KSHV-induced diseases.

Kaposi sarcoma-associated herpesvirus latency-associated nuclear antigen: more than a key mediator of viral persistence

Kaposi sarcoma-associated herpesvirus (KSHV), or human herpesvirus-8, is an oncogenic herpesvirus. Its latency-associated nuclear antigen (LANA) is essential for the persistence of KSHV in latently infected cells. LANA mediates replication of the latent viral genome during the S phase of a dividing cell and partitions episomes to daughter cells by attaching them to mitotic chromosomes. It also mediates the establishment of latency in newly infected cells through epigenetic mechanisms and suppresses the activation of the productive replication cycle. Furthermore, LANA promotes the proliferation of infected cell by acting as a transcriptional regulator and by modulating the cellular proteome through the recruitment of several cellular ubiquitin ligases. Finally, LANA interferes with the innate and adaptive immune system to facilitate the immune escape of infected cells.

Oncogenic viruses and host lipid metabolism: a new perspective

As noncellular organisms, viruses do not have their own metabolism and rely on the metabolism of host cells to provide energy and metabolic substances for their life cycles. Increasing evidence suggests that host cells infected with oncogenic viruses have dramatically altered metabolic requirements and that oncogenic viruses produce substances used for viral replication and virion production by altering host cell metabolism. We focused on the processes by which oncogenic viruses manipulate host lipid metabolism and the lipid metabolism disorders that occur in oncogenic virus-associated diseases. A deeper understanding of viral infections that cause changes in host lipid metabolism could help with the development of new antiviral agents as well as potential new therapeutic targets.

Epigenetic Reprogramming of Kaposi's Sarcoma-Associated Herpesvirus during Hypoxic Reactivation

The biphasic life cycle (latent and lytic) of Kaposi's sarcoma-associated Herpesvirus (KSHV) is regulated by epigenetic modification of its genome and its associated histone proteins. The temporal events driving epigenetic reprogramming of the KSHV genome on initial infection to establish latency has been well studied, but the reversal of these epigenetic changes during lytic replication, especially under physiological conditions such as hypoxia, has not been explored. In this study, we investigated epigenetic reprogramming of the KSHV genome during hypoxic reactivation. Hypoxia induced extensive enrichment of both transcriptional activators and repressors on the KSHV genome through H3K4Me3, H3K9Me3, and H3K27Me3, as well as histone acetylation (H3Ac) modifications. In contrast to uniform quantitative enrichment with modified histones, a distinct pattern of RTA and LANA enrichment was observed on the KSHV genome. The enrichment of modified histone proteins was due to their overall higher expression levels, which was exclusively seen in KSHV-positive cells. Multiple KSHV-encoded factors such as LANA, RTA, and vGPCR are involved in the upregulation of these modified histones. Analysis of ChIP-sequencing for the initiator DNA polymerase (DNAPol1α) combined with single molecule analysis of replicated DNA (SMARD) demonstrated the involvement of specific KSHV genomic regions that initiate replication in hypoxia.

A viral interferon regulatory factor degrades RNA-binding protein hnRNP Q1 to enhance aerobic glycolysis via recruiting E3 ubiquitin ligase KLHL3 and decaying GDPD1 mRNA

Reprogramming of host metabolism is a common strategy of viral evasion of host cells, and is essential for successful viral infection and induction of cancer in the context cancer viruses. Kaposi's sarcoma (KS) is the most common AIDS-associated cancer caused by KS-associated herpesvirus (KSHV) infection. KSHV-encoded viral interferon regulatory factor 1 (vIRF1) regulates multiple signaling pathways and plays an important role in KSHV infection and oncogenesis. However, the role of vIRF1 in KSHV-induced metabolic reprogramming remains elusive. Here we show that vIRF1 increases glucose uptake, ATP production and lactate secretion by downregulating heterogeneous nuclear ribonuclear protein Q1 (hnRNP Q1). Mechanistically, vIRF1 upregulates and recruits E3 ubiquitin ligase Kelch-like 3 (KLHL3) to degrade hnRNP Q1 through a ubiquitin-proteasome pathway. Furthermore, hnRNP Q1 binds to and stabilizes the mRNA of glycerophosphodiester phosphodiesterase domain containing 1 (GDPD1). However, vIRF1 targets hnRNP Q1 for degradation, which destabilizes GDPD1 mRNA, resulting in induction of aerobic glycolysis. These results reveal a novel role of vIRF1 in KSHV metabolic reprogramming, and identifying a potential therapeutic target for KSHV infection and KSHV-induced cancers.

Polyamine biosynthesis and eIF5A hypusination are modulated by the DNA tumor virus KSHV and promote KSHV viral infection

Polyamines are critical metabolites involved in various cellular processes and often dysregulated in cancers. Kaposi’s sarcoma-associated Herpesvirus (KSHV), a defined human oncogenic virus, leads to profound alterations of host metabolic landscape to favor development of KSHV-associated malignancies. In our studies, we identified that polyamine biosynthesis and eIF5A hypusination are dynamically regulated by KSHV infection through modulation of key enzymes (ODC1 and DHPS) of these pathways. During KSHV latency, ODC1 and DHPS are upregulated along with increase of hypusinated eIF5A (hyp-eIF5A), while hyp-eIF5A is further induced along with reduction of ODC1 and intracellular polyamines during KSHV lytic reactivation. In return these metabolic pathways are required for both KSHV lytic reactivation and de novo infection. Further analysis unraveled that synthesis of critical KSHV latent and lytic proteins (LANA, RTA) depends on hypusinated-eIF5A. We also demonstrated that KSHV infection can be efficiently and specifically suppressed by inhibitors targeting these pathways. Collectively, our results illustrated that the dynamic and profound interaction of a DNA tumor virus (KSHV) with host polyamine biosynthesis and eIF5A hypusination pathways promote viral propagation, thus defining new therapeutic targets to treat KSHV-associated malignancies.

Understanding virus-host interactions is crucial to develop and improve therapies. Kaposi’s sarcoma associated Herpesvirus (KSHV) is a human gamma-herpesvirus which deeply modulates the host metabolism and is associated with various cancers of endothelial and lymphoid origin. Polyamines are critical metabolites often dysregulated in cancers. In this study we demonstrated KSHV dynamically modulates polyamine metabolism to favor eIF5A hypusination and translation of critical KSHV latent and lytic proteins (LANA, RTA). Consequently, we found KSHV lytic switch from latency and de novo infection were dependent on polyamines and hypusination and pharmacological inhibition efficiently and specifically restricted KSHV infection. Our study provides new insights into KSHV alteration of the host metabolism and describe new therapeutic targets to treat KSHV-associated malignancies.

The regulation of KSHV lytic reactivation by viral and cellular factors

Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic herpesvirus that exhibits two distinct phases of infection in the host-latent and lytic. The quiescent latent phase is defined by limited expression of a subset of viral proteins and microRNAs, and an absence of virus production. KSHV periodically reactivates from latency to undergo active lytic replication, leading to production of new infectious virions. This switch from the latent to the lytic phase requires the viral protein regulator of transcription activator (RTA). RTA, along with other virally encoded proteins, is aided by host factors to facilitate this transition. Herein, we highlight the key host proteins that are involved in mediating RTA activation and KSHV lytic replication and discuss the cellular processes in which they function. We will also focus on the modulation of viral reactivation by the innate immune system, and how KSHV influences key immune signaling pathways to aid its own lifecycle.

RNA Granules in Antiviral Innate Immunity: A Kaposi's Sarcoma-Associated Herpesvirus Journey

RNA granules are cytoplasmic, non-membranous ribonucleoprotein compartments that form ubiquitously and are often referred to as foci for post-transcriptional gene regulation. Recent research on RNA processing bodies (PB) and stress granules (SG) has shown wide implications of these cytoplasmic RNA granules and their components in suppression of RNA translation as host intracellular innate immunity against infecting viruses. Many RNA viruses either counteract or co-opt these RNA granules; however, many fundamental questions about DNA viruses with respect to their interaction with these two RNA granules remain elusive. Kaposi’s sarcoma-associated herpesvirus (KSHV), a tumor-causing DNA virus, exhibits two distinct phases of infection and encodes ∼90 viral gene products during the lytic phase of infection compared to only a few (∼5) during the latent phase. Thus, productive KSHV infection relies heavily on the host cell translational machinery, which often links to the formation of PB and SG. One major question is how KSHV counteracts the hostile environment of RNA granules for its productive infection. Recent studies demonstrated that KSHV copes with the translational suppression by cellular RNA granules, PB and SG, by expressing ORF57, a viral RNA-binding protein, during KSHV lytic infection. ORF57 interacts with Ago2 and GW182, two major components of PB, and prevents the scaffolding activity of GW182 at the initial stage of PB formation in the infected cells. ORF57 also interacts with protein kinase R (PKR) and PKR-activating protein (PACT) to block PKR dimerization and kinase activation, and thus inhibits eIF2α phosphorylation and SG formation. The homologous immediate-early regulatory protein ICP27 of herpes simplex virus type 1 (HSV-1), but not the EB2 protein of Epstein-Barr virus (EBV), shares this conserved inhibitory function with KSHV ORF57 on PB and SG. Through KSHV ORF57 studies, we have learned much about how a DNA virus in the infected cells is equipped to evade host antiviral immunity for its replication and productive infection. KSHV ORF57 would be an excellent viral target for development of anti-KSHV-specific therapy.

Hypoxia and HIF-1 Trigger Marek’s Disease Virus Reactivation in Lymphoma-Derived Latently Infected T Lymphocytes

Latency is a hallmark of herpesviruses, allowing them to persist into their host without virions production. Acute exposure to hypoxia (below 3% O 2 ) was identified as a trigger of latent-to-lytic switch (reactivation) for human oncogenic gamma-herpesviruses (KSHV and EBV). Therefore, we hypothesized that hypoxia could also induce reactivation of Marek’s disease virus (MDV), sharing biological properties with EBV and KSHV (notably oncogenic properties), into lymphocytes. Acute exposure to hypoxia (1% O 2 ) of two MDV-latently infected cell lines derived from MD tumors (3867K and MSB-1) induced MDV reactivation. A bioinformatic analysis of the RB-1B MDV genome revealed 214 putative hypoxia-response element consensus sequences on 119 open reading frames. RT-qPCR analysis showed five MDV genes strongly upregulated early after hypoxia. In 3867K cells under normoxia, pharmacological agents mimicking hypoxia (MLN4924 and CoCl 2 ) increased MDV reactivation, but to a lower level than real hypoxia. Overexpression of wild-type or stabilized human hypoxia inducible factor-1α (HIF-1α) in MSB-1 cells in normoxia also promoted MDV reactivation. In such conditions, lytic cycle was detected in cells with a sustainable HIF-1α expression, but also in HIF-1α negative cells, indicating that MDV reactivation is mediated by HIF-1, in a direct and/or indirect manner. Lastly, we demonstrated by a reporter assay that HIF-1α overexpression induced the transactivation of two viral promoters, shown upregulated in hypoxia. These results suggest that hypoxia may play a crucial role in the late lytic replication phase observed in vivo in MDV-infected chickens exhibiting tumors, since a hypoxic microenvironment is a hallmark of most solid tumors.

IMPORTANCE

Latent-to-lytic switch of herpesviruses (aka reactivation) is responsible for pathology recurrences and/or viral shedding. Studying physiological triggers of reactivation is therefore important for health to limit lesions and viral transmission. Marek's disease virus (MDV) is a potent oncogenic alpha-herpesvirus establishing latency in T-lymphocytes and causing lethal T-lymphomas in chickens. In vivo , a second lytic phase is observed during tumoral stage. Hypoxia being a hallmark of tumors, we wondered whether hypoxia induces MDV reactivation in latently-infected T-lymphocytes, like previously shown for EBV and KSHV in B-lymphocytes. In this study, we demonstrated that acute hypoxia (1% O2) triggers MDV reactivation in two MDV transformed T-cell lines. We provide some molecular basis of this reactivation by showing that hypoxia inducible factor (HIF-1) overexpression induces MDV reactivation to a similar extend than hypoxia after 24 hours. Hypoxia is therefore a reactivation stimulus shared by mammalian and avian oncogenic herpesviruses of different genus.

Hesperetin inhibits KSHV reactivation and is reversed by HIF1α overexpression

Kaposi's sarcoma-associated herpesvirus (KSHV), an oncogenic virus, has two life cycle modes: the latent and lytic phases. KSHV lytic reactivation is important for both viral propagation and KSHV-induced tumorigenesis. The KSHV replication and transcription activator (RTA) protein is essential for lytic reactivation. Hesperetin, a citrus polyphenolic flavonoid, has antioxidant, anti-inflammatory, hypolipidemic, cardiovascular and anti-tumour effects. However, the effects of hesperetin on KSHV replication and KSHV-induced tumorigenesis have not yet been reported. Here, we report that hesperetin induces apoptotic cell death in BCBL-1 cells in a dose-dependent manner. Hesperetin inhibits KSHV reactivation and reduces the production of progeny virus from KSHV-harbouring cells. We also confirmed that HIF1α promotes the RTA transcriptional activities and lytic cycle-refractory state of KSHV-infected cells. Hesperetin suppresses HIF1α expression to inhibit KSHV lytic reactivation. These results suggest that hesperetin may represent a novel strategy for the treatment of KSHV infection and KSHV-associated lymphomas.

Proteomic Profiling Identifies Kaposi's Sarcoma-Associated Herpesvirus (KSHV)-Encoded LANASIM-Associated Proteins in Hypoxia

Hypoxia signaling is a key regulator in the development and progression of many types of human malignancies, including viral cancers. The latency-associated nuclear antigen (LANA), encoded by Kaposi's sarcoma-associated herpesvirus (KSHV) during latency, is a multifunctional protein that plays an essential role in viral episome maintenance and lytic gene silencing for inducing tumorigenesis. Although our previous studies have shown that LANA contains a SUMO-interacting motif (LANA^SIM), and hypoxia reduces SUMOylated KAP1 association with LANA^SIM, the physiological proteomic network of LANA^SIM-associated cellular proteins in response to hypoxia is still unclear. In this study, we individually established cell lines stably expressing wild-type LANA (LANA^WT) and its SIM-deleted mutant (LANA^dSIM) and treated them with or without hypoxia, followed by coimmunoprecipitation and mass spectrometry analysis to systemically identify the hypoxia-responsive profile of LANA^SIM-associated cellular proteins. We found that in hypoxia, the number of cellular proteins associated with LANA^WT instead of LANA^dSIM was dramatically increased. Functional network analysis revealed that two major pathways, which included cytoskeleton organization and DNA/RNA binding and processing pathways, were significantly enriched for 28 LANA^SIM-associated proteins in response to hypoxia. HNRNPU was one of the proteins consistently identified that interacted with LANA^SIM in different proteomic screening systems and responded to hypoxia. This study provides a proteomic profile of LANA^SIM-associated proteins in hypoxia and facilitates our understanding of the role of the collaboration between viral infection and the hypoxia response in inducing viral persistence and tumorigenesis. IMPORTANCE Kaposi's sarcoma-associated herpesvirus (KSHV) has been reported to be involved in the regulation of host proteins in response to hypoxic stress. LANA, one of the key latent proteins, contains a SUMO-interacting motif (LANA^SIM) and reduces the association with SUMOylated KAP1 upon hypoxic treatment. However, the physiological systematic network of LANA^SIM-associated cellular proteins in hypoxia is still unclear. Here, we revealed two major pathways, which included cytoskeleton organization and DNA/RNA binding and processing pathways, that were significantly enriched for 28 LANA^SIM-associated proteins in hypoxia. This discovery not only provides a proteomic profile of LANA^SIM-associated proteins in hypoxia but also facilitates our understanding of the collaboration between viral infection and hypoxic stress in inducing viral persistence and tumorigenesis.

Impact of Hypoxia over Human Viral Infections and Key Cellular Processes

Oxygen is essential for aerobic cells, and thus its sensing is critical for the optimal maintenance of vital cellular and tissue processes such as metabolism, pH homeostasis, and angiogenesis, among others. Hypoxia-inducible factors (HIFs) play central roles in oxygen sensing. Under hypoxic conditions, the α subunit of HIFs is stabilized and forms active heterodimers that translocate to the nucleus and regulate the expression of important sets of genes. This process, in turn, will induce several physiological changes intended to adapt to these new and adverse conditions. Over the last decades, numerous studies have reported a close relationship between viral infections and hypoxia. Interestingly, this relation is somewhat bidirectional, with some viruses inducing a hypoxic response to promote their replication, while others inhibit hypoxic cellular responses. Here, we review and discuss the cellular responses to hypoxia and discuss how HIFs can promote a wide range of physiological and transcriptional changes in the cell that modulate numerous human viral infections.

KSHV-encoded vCyclin can modulate HIF1α levels to promote DNA replication in hypoxia

The cellular adaptive response to hypoxia, mediated by high HIF1α levels includes metabolic reprogramming, restricted DNA replication and cell division. In contrast to healthy cells, the genome of cancer cells, and Kaposi’s sarcoma associated herpesvirus (KSHV) infected cells maintains replication in hypoxia. We show that KSHV infection, despite promoting expression of HIF1α in normoxia, can also restrict transcriptional activity, and promoted its degradation in hypoxia. KSHV-encoded vCyclin, expressed in hypoxia, mediated HIF1α cytosolic translocation, and its degradation through a non-canonical lysosomal pathway. Attenuation of HIF1α levels by vCyclin allowed cells to bypass the block to DNA replication and cell proliferation in hypoxia. These results demonstrated that KSHV utilizes a unique strategy to balance HIF1α levels to overcome replication arrest and induction of the oncogenic phenotype, which are dependent on the levels of oxygen in the microenvironment.

HIF1α-Regulated Expression of the Fatty Acid Binding Protein Family Is Important for Hypoxic Reactivation of Kaposi's Sarcoma-Associated Herpesvirus

The hypoxic microenvironment and metabolic reprogramming are two major contributors to the phenotype of oncogenic virus-infected cells. Infection by Kaposi's sarcoma-associated herpesvirus (KSHV) stabilizes hypoxia-inducible factor 1α (HIF1α) and reprograms cellular metabolism. We investigated the comparative transcriptional regulation of all major genes involved in fatty acid and amino acid metabolism in KSHV-positive and -negative cells grown under normoxic or hypoxic conditions. We show a distinct regulation of genes involved in both fatty acid and amino acid metabolism in KSHV-positive cells grown in either normoxic or hypoxic conditions, with a particular focus on genes involved in the acetyl coenzyme A (acetyl-CoA) pathway. The fatty acid binding protein (FABP) family of genes, specifically FABP1, FABP4, and FABP7, was also observed to be synergistically upregulated in hypoxia by KSHV. This pattern of FABP gene expression was also seen in naturally infected KSHV BC3 or BCBL1 cells when compared to KSHV-negative DG75 or BL41 cells. Two KSHV-encoded antigens, which positively regulate HIF1α, the viral G-protein coupled receptor (vGPCR), and the latency-associated nuclear antigen (LANA) were shown to drive upregulation of the FABP gene transcripts. Suppression of FABPs by RNA interference resulted in an adverse effect on hypoxia-dependent viral reactivation. Overall, this study provides new evidence, which supports a rationale for the inhibition of FABPs in KSHV-positive cells as potential strategies, for the development of therapeutic approaches targeting KSHV-associated malignancies.IMPORTANCE Hypoxia is a detrimental stress to eukaryotes and inhibits several cellular processes, such as DNA replication, transcription, translation, and metabolism. Interestingly, the genome of Kaposi's sarcoma-associated herpesvirus (KSHV) is known to undergo productive replication in hypoxia. We investigated the comparative transcriptional regulation of all major genes involved in fatty acid and amino acid metabolism in KSHV-positive and -negative cells grown under normoxic or hypoxic conditions. Several metabolic pathways were observed differentially regulated by KSHV in hypoxia, specifically, the fatty acid binding protein (FABP) family genes (FABP1, FABP4, and FABP7). KSHV-encoded antigens, vGPCR and LANA, were shown to drive upregulation of the FABP transcripts. Suppression of FABPs by RNA interference resulted in an adverse effect on hypoxia-dependent viral reactivation. Overall, this study provides new evidence, which supports a rationale for the inhibition of FABPs in KSHV-positive cells as potential strategies, for the development of therapeutic approaches targeting KSHV-associated malignancies.

KSHV Reprogramming of Host Energy Metabolism for Pathogenesis

Reprogramming of energy metabolism is a key for cancer development. Kaposi’s sarcoma-associated herpesvirus (KSHV), a human oncogenic herpesvirus, is tightly associated with several human malignancies by infecting B-lymphocyte or endothelial cells. Cancer cell energy metabolism is mainly dominated by three pathways of central carbon metabolism, including aerobic glycolysis, glutaminolysis, and fatty acid synthesis. Increasing evidence has shown that KSHV infection can alter central carbon metabolic pathways to produce biomass for viral replication, as well as the survival and proliferation of infected cells. In this review, we summarize recent studies exploring how KSHV manipulates host cell metabolism to promote viral pathogenesis, which provides the potential therapeutic targets and strategies for KSHV-associated cancers.

Gallic acid inhibits Kaposi's Sarcoma-associated herpesvirus lytic reactivation by suppressing RTA transcriptional activities

Kaposi's sarcoma-associated herpesvirus (KSHV), an oncogenic virus, has two life cycle modes: the latent and lytic phases. KSHV lytic reactivation is known to be important both for viral propagation and for KSHV-induced tumorigenesis. The KSHV replication and transcription activator (RTA) protein is essential for lytic reactivation. Gallic acid (GA), one of the most abundant phenolic acids in the plant kingdom, has been shown potential chemotherapeutic efficacy against microbial and cancer. However, the effects of GA on KSHV replication and KSHV-induced tumorigenesis have not yet been reported. Here, we report that GA induces apoptotic cell death in BCBL-1 cells in a dose-dependent manner. GA inhibits KSHV reactivation and reduces the production of progeny virus from KSHV-harboring cells. GA inhibits RTA transcriptional activities by suppressing its binding to target gene promoters. These results suggest that GA may represent a novel strategy for the treatment of KSHV infection and KSHV-associated lymphomas.

Contribution of hypoxia inducible factor-1 during viral infections

Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that plays critical roles during the cellular response to hypoxia. Under normoxic conditions, its function is tightly regulated by the degradation of its alpha subunit (HIF-1α), which impairs the formation of an active heterodimer in the nucleus that otherwise regulates the expression of numerous genes. Importantly, HIF-1 participates in both cancer and infectious diseases unveiling new therapeutic targets for those ailments. Here, we discuss aspects related to the activation of HIF-1, the effects of this transcription factor over immune system components, as well as the involvement of HIF-1 activity in response to viral infections in humans. Although HIF-1 is currently being assessed in numerous clinical settings as a potential therapy for different diseases, up to date, there are no clinical studies evaluating the pharmacological modulation of this transcription factor as a possible new antiviral treatment. However, based on the available evidence, clinical trials targeting this molecule are likely to occur soon. In this review we discuss the role of HIF-1 in viral immunity, the modulation of HIF-1 by different types of viruses, as well as the effects of HIF-1 over their life cycle and the potential use of HIF-1 as a new target for the treatment of viral infections.

Oxygen Sensing and Viral Replication: Implications for Tropism and Pathogenesis

The ability to detect and respond to varying oxygen tension is an essential prerequisite to life. Several mechanisms regulate the cellular response to oxygen including the prolyl hydroxylase domain (PHD)/factor inhibiting HIF (FIH)-hypoxia inducible factor (HIF) pathway, cysteamine (2-aminoethanethiol) dioxygenase (ADO) system, and the lysine-specific demethylases (KDM) 5A and KDM6A. Using a systems-based approach we discuss the literature on oxygen sensing pathways in the context of virus replication in different tissues that experience variable oxygen tension. Current information supports a model where the PHD-HIF pathway enhances the replication of viruses infecting tissues under low oxygen, however, the reverse is true for viruses with a selective tropism for higher oxygen environments. Differences in oxygen tension and associated HIF signaling may play an important role in viral tropism and pathogenesis. Thus, pharmaceutical agents that modulate HIF activity could provide novel treatment options for viral infections and associated pathological conditions.

Polyploid Giant Cancer Cells, a Hallmark of Oncoviruses and a New Therapeutic Challenge

Tumors are renowned as intricate systems that harbor heterogeneous cancer cells with distinctly diverse molecular signatures, sizes and genomic contents. Among those various genomic clonal populations within the complex tumoral architecture are the polyploid giant cancer cells (PGCC). Although described for over a century, PGCC are increasingly being recognized for their prominent role in tumorigenesis, metastasis, therapy resistance and tumor repopulation after therapy. A shared characteristic among all tumors triggered by oncoviruses is the presence of polyploidy. Those include Human Papillomaviruses (HPV), Epstein Barr Virus (EBV), Hepatitis B and C viruses (HBV and HCV, respectively), Human T-cell lymphotropic virus-1 (HTLV-1), Kaposi's sarcoma herpesvirus (KSHV) and Merkel polyomavirus (MCPyV). Distinct viral proteins, for instance Tax for HTLV-1 or HBx for HBV have demonstrated their etiologic role in favoring the appearance of PGCC. Different intriguing biological mechanisms employed by oncogenic viruses, in addition to viruses with high oncogenic potential such as human cytomegalovirus, could support the generation of PGCC, including induction of endoreplication, inactivation of tumor suppressors, development of hypoxia, activation of cellular senescence and others. Interestingly, chemoresistance and radioresistance have been reported in the context of oncovirus-induced cancers, for example KSHV and EBV-associated lymphomas and high-risk HPV-related cervical cancer. This points toward a potential linkage between the previously mentioned players and highlights PGCC as keystone cancer cells in virally-induced tumors. Subsequently, although new therapeutic approaches are actively needed to fight PGCC, attention should also be drawn to reveal the relationship between PGCC and oncoviruses, with the ultimate goal of establishing effective therapeutic platforms for treatment of virus-associated cancers. This review discusses the presence of PGCCs in tumors induced by oncoviruses, biological mechanisms potentially favoring their appearance, as well as their consequent implication at the clinical and therapeutic level.

Regulation of KSHV Latency and Lytic Reactivation

Kaposi’s sarcoma-associated herpesvirus (KSHV) is associated with three malignancies— Kaposi’s sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman’s disease (MCD). Central to the pathogenesis of these diseases is the KSHV viral life cycle, which is composed of a quiescent latent phase and a replicative lytic phase. While the establishment of latency enables persistent KSHV infection and evasion of the host immune system, lytic replication is essential for the dissemination of the virus between hosts and within the host itself. The transition between these phases, known as lytic reactivation, is controlled by a complex set of environmental, host, and viral factors. The effects of these various factors converge on the regulation of two KSHV proteins whose functions facilitate each phase of the viral life cycle—latency-associated nuclear antigen (LANA) and the master switch of KSHV reactivation, replication and transcription activator (RTA). This review presents the current understanding of how the transition between the phases of the KSHV life cycle is regulated, how the various phases contribute to KSHV pathogenesis, and how the viral life cycle can be exploited as a therapeutic target.

Oncogenic human herpesvirus hijacks proline metabolism for tumorigenesis

Three-dimensional (3D) cell culture is well documented to regain intrinsic metabolic properties and to better mimic the in vivo situation than two-dimensional (2D) cell culture. Particularly, proline metabolism is critical for tumorigenesis since pyrroline-5-carboxylate (P5C) reductase (PYCR/P5CR) is highly expressed in various tumors and its enzymatic activity is essential for in vitro 3D tumor cell growth and in vivo tumorigenesis. PYCR converts the P5C intermediate to proline as a biosynthesis pathway, whereas proline dehydrogenase (PRODH) breaks down proline to P5C as a degradation pathway. Intriguingly, expressions of proline biosynthesis PYCR gene and proline degradation PRODH gene are up-regulated directly by c-Myc oncoprotein and p53 tumor suppressor, respectively, suggesting that the proline-P5C metabolic axis is a key checkpoint for tumor cell growth. Here, we report a metabolic reprogramming of 3D tumor cell growth by oncogenic Kaposi's sarcoma-associated herpesvirus (KSHV), an etiological agent of Kaposi's sarcoma and primary effusion lymphoma. Metabolomic analyses revealed that KSHV infection increased nonessential amino acid metabolites, specifically proline, in 3D culture, not in 2D culture. Strikingly, the KSHV K1 oncoprotein interacted with and activated PYCR enzyme, increasing intracellular proline concentration. Consequently, the K1-PYCR interaction promoted tumor cell growth in 3D spheroid culture and tumorigenesis in nude mice. In contrast, depletion of PYCR expression markedly abrogated K1-induced tumor cell growth in 3D culture, not in 2D culture. This study demonstrates that an increase of proline biosynthesis induced by K1-PYCR interaction is critical for KSHV-mediated transformation in in vitro 3D culture condition and in vivo tumorigenesis.

Induction of Kaposi's Sarcoma-Associated Herpesvirus-Encoded Thymidine Kinase (ORF21) by X-Box Binding Protein 1

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent for Kaposi sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman disease (MCD). Like other herpesviruses, it has latent and lytic repertoires. However, there is evidence that some lytic genes can be directly activated by certain cellular factors. Cells undergoing endoplasmic reticulum stress express spliced X-box binding protein 1 (XBP-1s). XBP-1s is also present in large amounts in germinal center B cells. XBP-1s can activate the KSHV replication and transcription activator (RTA) and lytic replication. It can also directly activate KSHV-encoded viral interleukin-6 (vIL-6) and, thus, contribute to the pathogenesis of KSHV MCD. KSHV thymidine kinase (TK), the ORF21 gene product, can enhance the production of dTTP and is important for lytic replication. It can also phosphorylate zidovudine and ganciclovir to toxic moieties, enabling treatment of KSHV-MCD with these drugs. We show here that XBP-1s can directly activate ORF21 and that this activation is mediated primarily through two XBP-response elements (XRE) on the ORF21 promoter region. Deletion or mutation of these elements eliminated XBP-1s-induced upregulation of the promoter, and chromatin immunoprecipitation studies provide evidence that XBP-1s can bind to both XREs. Exposure of PEL cells to a chemical inducer of XBP-1s can induce ORF21 within 4 hours, and ORF21 expression in the lymph nodes of patients with KSHV-MCD is predominantly found in cells with XBP-1. Thus, XBP-1s may directly upregulate KSHV ORF21 and, thus, contribute to the pathogenesis of KSHV-MCD and the activity of zidovudine and valganciclovir in this disease.IMPORTANCE Spliced X-box binding protein 1 (XBP-1s), part of the unfolded protein response and expressed in developing germinal center B cells, can induce Kaposi's sarcoma-associated herpesvirus (KSHV) lytic replication and directly activate viral interleukin-6 (vIL-6). We show here that XBP-1s can also directly activate KSHV ORF21, a lytic gene. ORF21 encodes KSHV thymidine kinase (TK), which increases the pool of dTTP for viral replication and enhances lytic replication. Direct activation of ORF21 by XBP-1s can enhance viral replication in germinal center B cells and contribute to the pathogenesis of KSHV multicentric Castleman disease (MCD). KSHV-MCD is characterized by systemic inflammation caused, in part, by lytic replication and overproduction of KSHV vIL-6 in XBP-1s-expressing lymph node plasmablasts. KSHV thymidine kinase can phosphorylate zidovudine and ganciclovir to toxic moieties, and direct activation of ORF21 by XBP-1s may also help explain the effectiveness of zidovudine and valganciclovir in the treatment of KSHV-MCD.

Kaposi's Sarcoma-Associated Herpesvirus LANA Modulates the Stability of the E3 Ubiquitin Ligase RLIM

The Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded latency-associated nuclear antigen (LANA) protein functions in latently infected cells as an essential participant in KSHV genome replication and as a driver of dysregulated cell growth. In a previous study, we have identified LANA-interacting proteins using a protein array screen. Here, we explore the effect of LANA on the stability and activity of RLIM (RING finger LIM-domain-interacting protein, encoded by the RNF12 gene), a novel LANA-interacting protein identified in that protein screen. RLIM is an E3 ubiquitin ligase that leads to the ubiquitination and degradation of several transcription regulators, such as LMO2, LMO4, LHX2, LHX3, LDB1, and the telomeric protein TRF1. Expression of LANA leads to downregulation of RLIM protein levels. This LANA-mediated RLIM degradation is blocked in the presence of the proteasome inhibitor, MG132. Therefore, the interaction between LANA and RLIM could be detected in coimmunoprecipitation assay only in the presence of MG132 to prevent RLIM degradation. A RING finger mutant RLIM is resistant to LANA-mediated degradation, suggesting that LANA promotes RLIM autoubiquitination. Interestingly, we found that LANA enhanced the degradation of some RLIM substrates, such as LDB1 and LMO2, and prevented RLIM-mediated degradation of others, such as LHX3 and TRF1. We also show that transcription regulation by RLIM substrates is modulated by LANA. RLIM substrates are assembled into multiprotein transcription regulator complexes that regulate the expression of many cellular genes. Therefore, our study identified another way KSHV can modulate cellular gene expression.IMPORTANCE E3 ubiquitin ligases mark their substrates for degradation and therefore control the cellular abundance of their substrates. RLIM is an E3 ubiquitin ligase that leads to the ubiquitination and degradation of several transcription regulators, such as LMO2, LMO4, LHX2, LHX3, LDB1, and the telomeric protein TRF1. Here, we show that the Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded LANA protein enhances the ubiquitin ligase activity of RLIM, leading to enhanced RLIM autoubiquitination and degradation. Interestingly, LANA enhanced the degradation of some RLIM substrates, such as LDB1 and LMO2, and prevented RLIM-mediated degradation of others, such as LHX3 and TRF1. In agreement with protein stability of RLIM substrates, we found that LANA modulates transcription by LHX3-LDB1 complex and suggest additional ways LANA can modulate cellular gene expression. Our study adds another way a viral protein can regulate cellular protein stability, by enhancing the autoubiquitination and degradation of an E3 ubiquitin ligase.

Targeting Kaposi's Sarcoma-Associated Herpesvirus ORF21 Tyrosine Kinase and Viral Lytic Reactivation by Tyrosine Kinase Inhibitors Approved for Clinical Use

Kaposi's sarcoma-associated herpesvirus (KSHV) is the cause of three human malignancies: Kaposi's sarcoma, primary effusion lymphoma, and the plasma cell variant of multicentric Castleman disease. Previous research has shown that several cellular tyrosine kinases play crucial roles during several steps in the virus replication cycle. Two KSHV proteins also have protein kinase function: open reading frame (ORF) 36 encodes a serine-threonine kinase, while ORF21 encodes a thymidine kinase (TK), which has recently been found to be an efficient tyrosine kinase. In this study, we explore the role of the ORF21 tyrosine kinase function in KSHV lytic replication. By generating a recombinant KSHV mutant with an enzymatically inactive ORF21 protein, we show that the tyrosine kinase function of ORF21/TK is not required for the progression of the lytic replication in tissue culture but that it is essential for the phosphorylation and activation to toxic moieties of the antiviral drugs zidovudine and brivudine. In addition, we identify several tyrosine kinase inhibitors, already in clinical use against human malignancies, which potently inhibit not only ORF21 TK kinase function but also viral lytic reactivation and the development of KSHV-infected endothelial tumors in mice. Since they target both cellular tyrosine kinases and a viral kinase, some of these compounds might find a use in the treatment of KSHV-associated malignancies.IMPORTANCE Our findings address the role of KSHV ORF21 as a tyrosine kinase during lytic replication and the activation of prodrugs in KSHV-infected cells. We also show the potential of selected clinically approved tyrosine kinase inhibitors to inhibit KSHV TK, KSHV lytic replication, infectious virion release, and the development of an endothelial tumor. Since they target both cellular tyrosine kinases supporting productive viral replication and a viral kinase, these drugs, which are already approved for clinical use, may be suitable for repurposing for the treatment of KSHV-related tumors in AIDS patients or transplant recipients.

Kaposi's sarcoma-associated herpesvirus is cell-intrinsically controlled in latency in microgravity

In the next several decades, humans will explore deep space, including Mars. During long-term space flight, astronauts will be exposed to various physical stressors. Among these stressors, microgravity may compromise the immune system. Consistently, the reactivation of several latent herpesviruses has been reported in astronauts. Although herpesvirus infection status is determined by both cell-intrinsic and -extrinsic factors, it remains unclear which factors play major roles in the virus reactivation in microgravity. Here, using Kaposi's sarcoma-associated herpesvirus (KSHV)-infected cells, we found that KSHV is cell-intrinsically controlled in latency in microgravity. Innate immunity appeared to be unaffected in microgravity, while the expression of some restriction factors against KSHV, such as CTCF and AMPK, was upregulated. Collectively, the infected cells in microgravity can control KSHV in latency, possibly by unimpaired innate immunity and upregulated KSHV restriction factors. This is the first pilot study of the conflicts between cell-intrinsic defense systems and viruses in microgravity and provides fundamental information regarding host-virus interactions in microgravity.

Herpesviruses and the Unfolded Protein Response

Herpesviruses usurp cellular stress responses to promote viral replication and avoid immune surveillance. The unfolded protein response (UPR) is a conserved stress response that is activated when the protein load in the ER exceeds folding capacity and misfolded proteins accumulate. The UPR aims to restore protein homeostasis through translational and transcriptional reprogramming; if homeostasis cannot be restored, the UPR switches from "helper" to "executioner", triggering apoptosis. It is thought that the burst of herpesvirus glycoprotein synthesis during lytic replication causes ER stress, and that these viruses may have evolved mechanisms to manage UPR signaling to create an optimal niche for replication. The past decade has seen considerable progress in understanding how herpesviruses reprogram the UPR. Here we provide an overview of the molecular events of UPR activation, signaling and transcriptional outputs, and highlight key evidence that herpesviruses hijack the UPR to aid infection.

Identification of viral SIM-SUMO2-interaction inhibitors for treating primary effusion lymphoma

Primary effusion lymphoma (PEL) is an aggressive B-cell malignancy without effective treatment, and caused by the infection of Kaposi’s sarcoma-associated herpesvirus (KSHV), predominantly in its latent form. Previously we showed that the SUMO2-interacting motif within the viral latency-associated nuclear antigen (LANA^SIM) is essential for establishment and maintenance of KSHV latency. Here, we developed a luciferase based live-cell reporter system to screen inhibitors selectively targeting the interaction between LANA^SIM and SUMO2. Cambogin, a bioactive natural product isolated from the Garcinia genus (a traditional herbal medicine used for cancer treatment), was obtained from the reporter system screening to efficiently inhibit the association of SUMO2 with LANA^SIM, in turn reducing the viral episome DNA copy number for establishment and maintenance of KSHV latent infection at a low concentration (nM). Importantly, Cambogin treatments not only specifically inhibited proliferation of KSHV-latently infected cells in vitro, but also induced regression of PEL tumors in a xenograft mouse model. This study has identified Cambogin as a novel therapeutic agent for treating PEL as well as eliminating persistent infection of oncogenic herpesvirus.

Primary effusion lymphoma is a common AIDS-associated malignancy caused by infection with Kaposi’s sarcoma-associated herpesvirus (KSHV), and is currently absence of efficient and specific treatment. Natural product from herbal medicines is a major source of drug discovery for the treatment of a variety of diseases. In this study, the authors demonstrated that Cambogin, a polycyclic polyprenylated acylphloroglucinols (PPAPs) isolated from the branches of Garcinia esculenta (a tropical evergreen tree and traditional cancer treatment across Southern Asia), is a potent and effective inhibitor of KSHV-latently infected cells at a low concentration (nM) in vitro and in vivo, through targeting viral LANA^SIM-SUMO2 interaction.

A role of hypoxia-inducible factor 1 alpha in Murine Gammaherpesvirus 68 (MHV68) lytic replication and reactivation from latency

The hypoxia-inducible factor 1 alpha (HIF1α) protein and the hypoxic microenvironment are critical for infection and pathogenesis by the oncogenic gammaherpesviruses (γHV), Kaposi sarcoma herpes virus (KSHV) and Epstein-Barr virus (EBV). However, understanding the role of HIF1α during the virus life cycle and its biological relevance in the context of host has been challenging due to the lack of animal models for human γHV. To study the role of HIF1α, we employed the murine gammaherpesvirus 68 (MHV68), a rodent pathogen that readily infects laboratory mice. We show that MHV68 infection induces HIF1α protein and HIF1α-responsive gene expression in permissive cells. siRNA silencing or drug-inhibition of HIF1α reduce virus production due to a global downregulation of viral gene expression. Most notable was the marked decrease in many viral genes bearing hypoxia-responsive elements (HREs) such as the viral G-Protein Coupled Receptor (vGPCR), which is known to activate HIF1α transcriptional activity during KSHV infection. We found that the promoter of MHV68 ORF74 is responsive to HIF1α and MHV-68 RTA. Moreover, Intranasal infection of HIF1αLoxP/LoxP mice with MHV68 expressing Cre- recombinase impaired virus expansion during early acute infection and affected lytic reactivation in the splenocytes explanted from mice. Low oxygen concentrations accelerated lytic reactivation and enhanced virus production in MHV68 infected splenocytes. Thus, we conclude that HIF1α plays a critical role in promoting virus replication and reactivation from latency by impacting viral gene expression. Our results highlight the importance of the mutual interactions of the oxygen-sensing machinery and gammaherpesviruses in viral replication and pathogenesis.

Primary Mediastinal Nodal and Extranodal Non-Hodgkin Lymphomas: Current Concepts, Historical Evolution, and Useful Diagnostic Approach: Part 1

Primary mediastinal non-Hodgkin lymphomas (PM-NHLs) represent ~5% of all NHLs and comprise lymphomas of B-cell and T-cell origin. PM-NHLs are defined as involvement of mediastinal lymph nodes, thymus, and/or mediastinal organs (heart, lung, pleura, pericardium) by NHL without evidence of systemic disease at presentation. The clinical scenario is variable and depends on the lymphoma subtype. The radiologic presentation is also variable ranging from a mediastinal mass with or without superior vena cava syndrome, a pleural or a cardiac mass associated with an effusion, or as an effusion only. The diagnosis of PM-NHLs can only be established by microscopic evaluation, and therefore, general pathologists should be aware of these tumors and familiar with their diagnostic approach. The most common anterior mediastinal NHLs (90% to 95%) are primary mediastinal large B-cell lymphoma and T lymphoblastic lymphoma. Thymic marginal zone lymphoma and mediastinal gray zone lymphoma are very rare. The remainder PM-NHLs involving middle or posterior mediastinum include diffuse large B-cell lymphoma (DLBCL) and rare cases of T-cell lymphoma, including anaplastic large cell lymphoma and breast implant-associated anaplastic large cell lymphoma extending to the anterior mediastinum. Primary pleural and cardiac NHLs are mostly DLBCLs. Other rare subtypes of PM-NHLs include DLBCL associated with chronic inflammation/pyothorax-associated lymphoma, fibrin-associated DLBCL (both EBV), and pleural and/or pericardial primary effusion lymphoma (HHV-8/EBV). We review the historical aspects, epidemiology, clinico-radiologic features, histopathology, immunohistochemistry, differential diagnosis, and relevant cytogenetic and molecular features of PM (thymic) LBCL, PM "nonthymic" DLBCL, BCL, unclassifiable, with features intermediate between DLBCL and classic Hodgkin lymphoma (mediastinal gray zone lymphoma), DLBCL associated with chronic inflammation (pyothorax-associated lymphoma), fibrin-associated DLBCL, and primary effusion lymphoma. This review represents the first part of 2 manuscripts covering PM-NHLs.

KSHV-encoded LANA protects the cellular replication machinery from hypoxia induced degradation

Kaposi’s sarcoma associated herpesvirus (KSHV), like all herpesviruses maintains lifelong persistence with its host genome in latently infected cells with only a small fraction of cells showing signatures of productive lytic replication. Modulation of cellular signaling pathways by KSHV-encoded latent antigens, and microRNAs, as well as some level of spontaneous reactivation are important requirements for establishment of viral-associated diseases. Hypoxia, a prominent characteristic of the microenvironment of cancers, can exert specific effects on cell cycle control, and DNA replication through HIF1α-dependent pathways. Furthermore, hypoxia can induce lytic replication of KSHV. The mechanism by which KSHV-encoded RNAs and antigens regulate cellular and viral replication in the hypoxic microenvironment has yet to be fully elucidated. We investigated replication-associated events in the isogenic background of KSHV positive and negative cells grown under normoxic or hypoxic conditions and discovered an indispensable role of KSHV for sustained cellular and viral replication, through protection of critical components of the replication machinery from degradation at different stages of the process. These include proteins involved in origin recognition, pre-initiation, initiation and elongation of replicating genomes. Our results demonstrate that KSHV-encoded LANA inhibits hypoxia-mediated degradation of these proteins to sustain continued replication of both host and KSHV DNA. The present study provides a new dimension to our understanding of the role of KSHV in survival and growth of viral infected cells growing under hypoxic conditions and suggests potential new strategies for targeted treatment of KSHV-associated cancer.

Hypoxia induces cell cycle arrest and DNA replication to minimize energy and macromolecular demands on the ATP stores of cells in this microenvironment. A select set of proteins functions as transcriptional activators in hypoxia. However, transcriptional and translational pathways are negatively regulated in response to hypoxia. This preserves ATP until the cell encounters more favorable conditions. In contrast, the genome of cancer cells replicates spontaneously under hypoxic conditions, and KSHV undergoes enhanced lytic replication. This unique feature by which KSHV genome is reactivated to induce lytic replication is important to elucidate the molecular mechanism by which cells can bypass hypoxia-mediated arrest of DNA replication in cancer cells. Here we provide data which shows that KSHV can manipulate the DNA replication machinery to support replication in hypoxia. We observed that KSHV can stabilize proteins involved in the pre-initiation, initiation and elongation steps of DNA replication. Specifically, KSHV-encoded LANA was responsible for this stabilization, and maintenance of endogenous HIF1α levels was required for stabilization of these proteins in hypoxia. Expression of LANA in KSHV negative cells confers protection of these replication proteins from hypoxia-dependent degradation, and knock-down of LANA or HIF1α showed a dramatic reduction in KSHV-dependent stabilization of replication-associated proteins in hypoxia. These data suggest a role for KSHV-encoded LANA in replication of infected cells, and provides a mechanism for sustained replication of both cellular and viral DNA in hypoxia.

The Kaposi's Sarcoma-Associated Herpesvirus ORF34 Protein Interacts and Stabilizes HIF-2α via Binding to the HIF-2α bHLH and PAS Domains

Hypoxia and hypoxia inducible factors (HIFs) play important roles in the Kaposi's sarcoma-associated herpesvirus (KSHV) life cycle. KSHV is the causative agent of Kaposi's sarcoma (KS) and other AIDS-related malignancies. Kaposi's sarcoma is a highly vascular tumor, which preferentially develops in the lower extremities of the body where blood vessels are often poorly oxygenated. The main cellular responses to hypoxia are mediated mainly by two isoforms of HIF, HIF-1α and HIF-2α. HIF-1α and HIF-2α have common as well as distinct functions, although they are similar in structure and function. Previously, we showed that the KSHV ORF34 protein binds HIF-1α and facilitates its degradation through the ubiquitin-proteasome pathway causing negative regulation of HIF-1α-dependent genes (Haque and Kousoulas, J Virol 87:2164-2173, 2013, https://www.doi.org/10.1128/JVI.02460-12). Herein, we show that the ORF34 gene is involved in the regulation of KSHV lytic gene expression, since deletion of ORF34 resulted in reduced immediate early and early lytic gene expression and blocked late gene expression. Coimmunoprecipitation experiments revealed that the ORF34 protein physically interacted with HIF-2α in transfected as well as in KSHV-infected cells. Utilization of ORF34 truncations revealed that three distinct domains bind HIF-2α and that both bHLH and PAS domains of HIF-2α interacted with ORF34. Unlike HIF-1α, dose-dependent coexpression of ORF34 stabilized the HIF-2α protein, ensuring HIF-2α-dependent transcriptional activity. The ORF34 protein enhanced HIF-2α ubiquitination at the bHLH and PAS domains. The results show that the KSHV ORF34 protein is involved in the KSHV life cycle by regulating the expression of HIF-1α and HIF-2α proteins.IMPORTANCE Hypoxia inducible factor 1α (HIF-1α) and HIF-2α are transcription factors which play important roles in the Kaposi's sarcoma-associated herpesvirus (KSHV) latent and lytic gene replication. Herein, we show that the ORF34 gene is involved in the regulation of KSHV lytic gene expression, since deletion of ORF34 resulted in reduced immediate early and early lytic gene expression and blocked late gene expression. In addition, we demonstrate that the KSHV ORF34 protein binds and stabilizes HIF-2α, in contrast to its role in binding HIF-1α and causing its degradation via the proteasome pathway. Thus, the KSHV ORF34 protein plays a regulatory role in the KSHV life cycle by regulating HIF-1α and HIF-2α expression.

Viral hijacking of cellular metabolism

This review discusses the current state of the viral metabolism field and gaps in knowledge that will be important for future studies to investigate. We discuss metabolic rewiring caused by viruses, the influence of oncogenic viruses on host cell metabolism, and the use of viruses as guides to identify critical metabolic nodes for cancer anabolism. We also discuss the need for more mechanistic studies identifying viral proteins responsible for metabolic hijacking and for in vivo studies of viral-induced metabolic rewiring. Improved technologies for detailed metabolic measurements and genetic manipulation will lead to important discoveries over the next decade.

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.

Systemically Administered Reovirus-Induced Downregulation of Hypoxia Inducible Factor-1α in Subcutaneous Tumors

Reovirus, which possesses a 10-segmented double-stranded RNA genome, mediates superior antitumor effects via not only virus replication in a tumor cell-specific manner but also other mechanisms distinct from virus replication. Several groups, including ours, reported the reovirus-mediated downregulation of hypoxia inducible factor-1α (HIF-1α) following infection in cultured tumor cells; however, it remained to be clarified whether reovirus downregulates the expression of HIF-1α and its target genes in tumor-bearing hosts. We found that reovirus induced significant downregulation of protein levels of HIF-1α and its target genes in the subcutaneous tumors at 120 h post-systemic administration. Expression of reovirus capsid protein σ3 was found in the pimonidazole-positive hypoxic area in the tumor. Significant levels of tumor cell apoptosis were not found in the tumors of reovirus-treated mice at this time point, suggesting that reovirus-mediated tumor cell killing did not largely contribute to the downregulation of HIF-1α protein levels in the tumors. UV-inactivated reovirus did not induce downregulation of HIF-1α expression in the tumors, indicating that virus replication was indispensable for downregulation of HIF-1α expression in the subcutaneous tumors. This study provides important information for the development of reovirus-mediated virotherapy against various types of tumors.

Metabolic reprogramming of Kaposi's sarcoma associated herpes virus infected B-cells in hypoxia

Kaposi’s sarcoma associated herpesvirus (KSHV) infection stabilizes hypoxia inducible factors (HIFs). The interaction between KSHV encoded factors and HIFs plays a critical role in KSHV latency, reactivation and associated disease phenotypes. Besides modulation of large-scale signaling, KSHV infection also reprograms the metabolic activity of infected cells. However, the mechanism and cellular pathways modulated during these changes are poorly understood. We performed comparative RNA sequencing analysis on cells with stabilized hypoxia inducible factor 1 alpha (HIF1α) of KSHV negative or positive background to identify changes in global and metabolic gene expression. Our results show that hypoxia induces glucose dependency of KSHV positive cells with high glucose uptake and high lactate release. We identified the KSHV-encoded vGPCR, as a novel target of HIF1α and one of the main viral antigens of this metabolic reprogramming. Bioinformatics analysis of vGPCR promoter identified 9 distinct hypoxia responsive elements which were activated by HIF1α in-vitro. Expression of vGPCR alone was sufficient for induction of changes in the metabolic phenotype similar to those induced by KSHV under hypoxic conditions. Silencing of HIF1α rescued the hypoxia associated phenotype of KSHV positive cells. Analysis of the host transcriptome identified several common targets of hypoxia as well as KSHV encoded factors and other synergistically activated genes belonging to cellular pathways. These include those involved in carbohydrate, lipid and amino acids metabolism. Further DNA methyltranferases, DNMT3A and DNMT3B were found to be regulated by either KSHV, hypoxia, or both synergistically at the transcript and protein levels. This study showed distinct and common, as well as synergistic effects of HIF1α and KSHV-encoded proteins on metabolic reprogramming of KSHV-infected cells in the hypoxia.

Hypoxia inducible factors (HIFs) play a critical role in survival and growth of cancerous cells, in addition to modulating cellular metabolism. Kaposi’s sarcoma associated herpesvirus (KSHV) infection stabilizes HIFs. Several factors encoded by KSHV are known to interact with up or downstream targets of HIFs. However, the process by which KSHV infection leads to stabilized HIF1α and modulation of the cellular metabolism is not understood. Comparative RNA sequencing analysis on cells with stabilized hypoxia inducible factor 1 alpha (HIF1α), of KSHV negative or positive cells led to identification of changes in global and metabolic gene expression. Our results show that hypoxia induces glucose dependency of KSHV positive cells with high glucose uptake and high lactate release. KSHV-encoded vGPCR was identified as a novel target of HIF1α regulation and a major viral antigen involved in metabolic reprogramming. Silencing of HIF1α rescued the hypoxia associated phenotype of KSHV positive cells. Analysis of the host transcriptome identified several common targets of hypoxia and KSHV-encoded factors, as well as other synergistically activated genes belonging to cellular metabolic pathways. This study showed unique, common and the synergistic effects of both HIF1α and KSHV-encoded proteins on metabolic reprogramming of KSHV-infected cells in hypoxia.

Regulation of lactate dehydrogenase in response to WSSV infection in the shrimp Litopenaeus vannamei

Lactate dehydrogenase (LDH) is key for anaerobic glycolysis. LDH is induced by the hypoxia inducible factor -1 (HIF-1). HIF-1 induces genes involved in glucose metabolism and regulates cellular oxygen homeostasis. HIF-1 is formed by a regulatory α-subunit (HIF-1α) and a constitutive β-subunit (HIF-1β). The white spot syndrome virus (WSSV) induces anaerobic glycolysis in shrimp hemocytes, associated with lactate accumulation. Although infection and lactate production are associated, the LDH role in WSSV-infected shrimp has not been examined. In this work, the effects of HIF-1 silencing on the expression of two LDH subunits (LDHvan-1 and LDHvan-2) in shrimp infected with the WSSV were studied. HIF-1α transcripts increased in gills, hepatopancreas, and muscle after WSSV infection, while HIF-1β remained constitutively expressed. The expression for both LDH subunits increased in each tissue evaluated during the WSSV infection, translating into increased enzyme activity. Glucose concentration increased in each tissue evaluated, while lactate increased in gills and hepatopancreas, but not in muscle. Silencing of HIF-1α blocked the increase of LDH expression and enzyme activity, along with glucose (all tissues) and lactate (gills and hepatopancreas) concentrations produced by WSSV infection. These results demonstrate that HIF-1 up regulates the expression of LDH subunits during WSSV infection, and that this induction contributes to substrate metabolism in energetically active tissues of infected shrimp.

KSHV LANA upregulates the expression of epidermal growth factor like domain 7 to promote angiogenesis

Kaposi’s sarcoma (KS) is a highly-vascularized tumor characterized by inflammation and extensive neo-angiogenesis. The KS tumor microenvironment is rich in inflammatory and pro-angiogenic cytokines. Here, we report that the expression of Epidermal growth factor-like domain 7 (EGFL7) is upregulated in Kaposi’s sarcoma-associated herpes virus (KSHV) infected cells. EGFL7 is a secreted pro-angiogenic cytokine that has been implicated in angiogenesis and the proliferation of endothelial cells during many pathological conditions. Our data show that KS tumors as well as primary effusion lymphoma cells have increased levels of EGFL7 compared to the uninfected cells. We determined that the expression of a KSHV latent protein, LANA (latency-associated nuclear antigen), is the main viral factor responsible for this upregulation. The modulation of EGFL7 expression by LANA involves sequestration of death domain-associated protein 6 (Daxx) from the EGFL7 promoter. Daxx acts as a suppressor of promoter activity by binding to the avian erythroblastosis virus E26 oncogene homolog 1 (Ets-1), which is the core transcription factor required for the expression of EGFL7. We additionally show that the upregulation of EGFL7 by LANA contributes to the promotion of angiogenesis since siRNA-mediated knockdown of EGFL7 reduced in vitro tubulogenesis in LANA-expressing HUVEC cells. EGFL7 promotes angiogenesis through autocrine as well as paracrine mechanisms as the supernatant from LANA expressing cells depleted of EGFL7 showed reduced tubulogenesis. This study for the first time demonstrates EGFL7 to be an important angiogenic molecule secreted during KSHV infection that could be exploited for blocking KSHV associated malignancies in conjugation with other anti-angiogenic therapies.

Hypoxia-inducible factor-1 alpha as a therapeutic target for primary effusion lymphoma

Primary effusion lymphoma (PEL) is an aggressive B-cell lymphoma with poor prognosis caused by Kaposi’s sarcoma-associated herpesvirus (KSHV). Previous studies have revealed that HIF-1α, which mediates much of the cellular response to hypoxia, plays an important role in life cycle of KSHV. KSHV infection promotes HIF-1α activity, and several KSHV genes are in turn activated by HIF-1α. In this study, we investigated the effects of knocking down HIF-1α in PELs. We observed that HIF-1α knockdown in each of two PEL lines leads to a reduction in both aerobic and anaerobic glycolysis as well as lipid biogenesis, indicating that HIF-1α is necessary for maintaining a metabolic state optimal for growth of PEL. We also found that HIF-1α suppression leads to a substantial reduction in activation of lytic KSHV genes, not only in hypoxia but also in normoxia. Moreover, HIF-1α knockdown led to a decrease in the expression of various KSHV latent genes, including LANA, vCyclin, kaposin, and miRNAs, under both normoxic and hypoxic conditions. These observations provide evidence that HIF-1α plays an important role in PEL even in normoxia. Consistent with these findings, we observed a significant inhibition of growth of PEL in normoxia upon HIF-1α suppression achieved by either HIF-1α knockdown or treatment with PX-478, a small molecule inhibitor of HIF-1α. These results offer further evidence that HIF-1α plays a critical role in the pathogenesis of PEL, and that inhibition of HIF-1α can be a potential therapeutic strategy in this disease.

Kaposi’s sarcoma-associated herpesvirus (KSHV) is an oncogenic herpesvirus that causes several malignancies including primary effusion lymphoma (PEL). PEL is an aggressive B-cell lymphoma that usually develops in a hypoxic environment. There is no standard treatment for PEL and it carries a poor prognosis. Previous studies have revealed that certain KSHV-encoded genes are activated by hypoxia-inducible factor 1 (HIF-1), an intracellular factor that mediates much of the cellular response to hypoxia. KSHV in turn can upregulate HIF-1, suggesting HIF-1 might play a substantial role in PEL oncogenesis. Here, we report for the first time the effects of suppressing HIF-1α, an oxygen-sensitive subunit of HIF-1, in PEL tumor cells. We demonstrate that suppressing HIF-1α can dramatically affect the oncogenic metabolic signature of PELs, replication of KSHV, expression of KSHV-encoded oncogenes, and the growth of PEL cells. Findings presented here not only provide new insights into the role of HIF-1α in KSHV-induced tumors but also provide a rationale for using anti-HIF-1α agents as a therapeutic strategy for PEL and potentially other KSHV-associated malignancies.

Effects of the NEDD8-Activating Enzyme Inhibitor MLN4924 on Lytic Reactivation of Kaposi's Sarcoma-Associated Herpesvirus

The switch of Kaposi's sarcoma-associated herpesvirus (KSHV) from latency to lytic replication is a key event for viral dissemination and pathogenesis. MLN4924, a novel neddylation inhibitor, reportedly causes the onset of KSHV reactivation but impairs later phases of the viral lytic program in infected cells. Thus far, the molecular mechanism involved in the modulation of the KSHV lytic cycle by MLN4924 is not yet fully understood. Here, we confirmed that treatment of different KSHV-infected primary effusion lymphoma (PEL) cell lines with MLN4924 substantially induces viral lytic protein expression. Due to the key role of the virally encoded ORF50 protein in the latent-to-lytic switch, we investigated its transcriptional regulation by MLN4924. We found that MLN4924 activates the ORF50 promoter (ORF50p) in KSHV-positive cells (but not in KSHV-negative cells), and the RBP-Jκ-binding elements within the promoter are critically required for MLN4924 responsiveness. In KSHV-negative cells, reactivation of the ORF50 promoter by MLN4924 requires the presence of the latency-associated nuclear antigen (LANA). Under such a condition, LANA acts as a repressor to block the ORF50p activity, whereas MLN4924 treatment relieves LANA-mediated repression. Importantly, we showed that LANA is a neddylated protein and can be deneddylated by MLN4924. On the other hand, we revealed that MLN4924 exhibits concentration-dependent biphasic effects on 12-O-tetradecanoylphorbol-13-acetate (TPA)- or sodium butyrate (SB)-induced viral reactivation in PEL cell lines. In other words, low concentrations of MLN4924 promote activation of TPA- or SB-mediated viral reactivation, whereas high concentrations of MLN4924, conversely, inhibit the progression of TPA- or SB-mediated viral lytic program.IMPORTANCE MLN4924 is a neddylation (NEDD8 modification) inhibitor, which currently acts as an anti-cancer drug in clinical trials. Although MLN4924 has been reported to trigger KSHV reactivation, many aspects regarding the action of MLN4924 in regulating the KSHV lytic cycle are not fully understood. Since the KSHV ORF50 protein is the key regulator of viral lytic reactivation, we focus on its transcriptional regulation by MLN4924. We here show that activation of the ORF50 gene by MLN4924 involves the relief of LANA-mediated transcriptional repression. Importantly, we find that LANA is a neddylated protein. To our knowledge, this is the first report showing that neddylation occurs in viral proteins. Additionally, we provide evidence that different concentrations of MLN4924 have opposite effects on TPA-mediated or SB-mediated KSHV lytic cycle activation. Therefore, in clinical application, we propose that MLN4924 needs to be used with caution in combination therapy to treat KSHV-positive subjects.

HIV-associated Kaposi sarcoma and related diseases

: The search for the etiologic agent for Kaposi sarcoma led to the discovery of Kaposi sarcoma-associated herpesvirus (KSHV) in 1994. KSHV, also called human herpesvirus-8, has since been shown to be the etiologic agent for several other tumors and diseases, including primary effusion lymphoma (PEL), an extracavitary variant of PEL, KSHV-associated diffuse large B-cell lymphoma, a form of multicentric Castleman disease, and KSHV inflammatory cytokine syndrome. KSHV encodes several genes that interfere with innate and specific immunity, thwart apoptosis, enhance cell proliferation and cytokine production, and promote angiogenesis, and these play important roles in disease pathogenesis. HIV is an important cofactor in Kaposi sarcoma pathogenesis, and widespread use of antiretroviral therapy has reduced Kaposi sarcoma incidence. However, Kaposi sarcoma remains the second most frequent tumor arising in HIV-infected patients in the United States and is particularly common in sub-Saharan Africa. KSHV prevalence varies substantially in different populations. KSHV is secreted in saliva, and public health measures to reduce its spread may help reduce the incidence of KSHV-associated diseases. Although there have been advances in the treatment of Kaposi sarcoma, KSHV-multicentric Castleman disease, and PEL, improved therapies are needed, especially those that are appropriate for Kaposi sarcoma in resource-poor regions.

Hypoxia-inducible factor-1α plays roles in Epstein-Barr virus's natural life cycle and tumorigenesis by inducing lytic infection through direct binding to the immediate-early BZLF1 gene promoter

When confronted with poor oxygenation, cells adapt by activating survival signaling pathways, including the oxygen-sensitive transcriptional regulators called hypoxia-inducible factor alphas (HIF-αs). We report here that HIF-1α also regulates the life cycle of Epstein-Barr virus (EBV). Incubation of EBV-positive gastric carcinoma AGS-Akata and SNU-719 and Burkitt lymphoma Sal and KemIII cell lines with a prolyl hydroxylase inhibitor, L-mimosine or deferoxamine, or the NEDDylation inhibitor MLN4924 promoted rapid and sustained accumulation of both HIF-1α and lytic EBV antigens. ShRNA knockdown of HIF-1α significantly reduced deferoxamine-mediated lytic reactivation. HIF-1α directly bound the promoter of the EBV primary latent-lytic switch BZLF1 gene, Zp, activating transcription via a consensus hypoxia-response element (HRE) located at nt -83 through -76 relative to the transcription initiation site. HIF-1α did not activate transcription from the other EBV immediate-early gene, BRLF1. Importantly, expression of HIF-1α induced EBV lytic-gene expression in cells harboring wild-type EBV, but not in cells infected with variants containing base-pair substitution mutations within this HRE. Human oral keratinocyte (NOK) and gingival epithelial (hGET) cells induced to differentiate by incubation with either methyl cellulose or growth in organotypic culture accumulated both HIF-1α and Blimp-1α, another cellular factor implicated in lytic reactivation. HIF-1α activity also accumulated along with Blimp-1α during B-cell differentiation into plasma cells. Furthermore, most BZLF1-expressing cells observed in lymphomas induced by EBV in NSG mice with a humanized immune system were located distal to blood vessels in hypoxic regions of the tumors. Thus, we conclude that HIF-1α plays central roles in both EBV’s natural life cycle and EBV-associated tumorigenesis. We propose that drugs that induce HIF-1α protein accumulation are good candidates for development of a lytic-induction therapy for treating some EBV-associated malignancies.

Most adults throughout the world are infected with Epstein-Barr virus (EBV), a human herpesvirus frequently associated in a latent state with some cancers of epithelial and B-cell origin such as nasopharyngeal carcinoma and Burkitt lymphoma, respectively. To develop an oncolytic therapy for treating patients with EBV-associated cancers, we need a method to efficiently induce synthesis of lytic EBV proteins. The EBV protein encoded by its immediate-early BZLF1 gene usually mediates the switch into lytic viral infection. We show here that HIF-1α, a cellular transcription factor that accumulates in cells when deprived of normal levels of oxygen, can induce lytic EBV infection. HIF-1α mediates this switch by directly binding to a specific sequence located within the BZLF1 gene promoter, activating its expression. Importantly, we also show that deferoxamine, an FDA-approved drug that inhibits degradation of HIF-1α, can induce synthesis of lytic EBV proteins in some EBV-positive epithelial and lymphocytic cell lines. These findings indicate that HIF-1α-stabilizing drugs, administered in combination with nucleoside analogues such as ganciclovir, may be helpful as part of a lytic-induction therapy for treating some patients with EBV-positive malignancies.

The role of metabolic reprogramming in γ-herpesvirus-associated oncogenesis

The γ-herpesviruses, EBV and KSHV, are closely associated with a number of human cancers. While the signal transduction pathways exploited by γ-herpesviruses to promote cell growth, survival and transformation have been reported, recent studies have uncovered the impact of γ-herpesvirus infection on host cell metabolism. Here, we review the mechanisms used by γ-herpesviruses to induce metabolic reprogramming in host cells, focusing on their ability to modulate the activity of metabolic regulators and manipulate metabolic pathways. While γ-herpesviruses alter metabolic phenotypes as a means to support viral infection and long-term persistence, this modulation can inadvertently contribute to cancer development. Strategies that target deregulated metabolic phenotypes induced by γ-herpesviruses provide new opportunities for therapeutic intervention.

Reactivation and Lytic Replication of Kaposi's Sarcoma-Associated Herpesvirus: An Update

The life cycle of Kaposi’s sarcoma-associated herpesvirus (KSHV) consists of two phases, latent and lytic. The virus establishes latency as a strategy for avoiding host immune surveillance and fusing symbiotically with the host for lifetime persistent infection. However, latency can be disrupted and KSHV is reactivated for entry into the lytic replication. Viral lytic replication is crucial for efficient dissemination from its long-term reservoir to the sites of disease and for the spread of the virus to new hosts. The balance of these two phases in the KSHV life cycle is important for both the virus and the host and control of the switch between these two phases is extremely complex. Various environmental factors such as oxidative stress, hypoxia, and certain chemicals have been shown to switch KSHV from latency to lytic reactivation. Immunosuppression, unbalanced inflammatory cytokines, and other viral co-infections also lead to the reactivation of KSHV. This review article summarizes the current understanding of the initiation and regulation of KSHV reactivation and the mechanisms underlying the process of viral lytic replication. In particular, the central role of an immediate-early gene product RTA in KSHV reactivation has been extensively investigated. These studies revealed multiple layers of regulation in activation of RTA as well as the multifunctional roles of RTA in the lytic replication cascade. Epigenetic regulation is known as a critical layer of control for the switch of KSHV between latency and lytic replication. The viral non-coding RNA, PAN, was demonstrated to play a central role in the epigenetic regulation by serving as a guide RNA that brought chromatin remodeling enzymes to the promoters of RTA and other lytic genes. In addition, a novel dimension of regulation by microPeptides emerged and has been shown to regulate RTA expression at the protein level. Overall, extensive investigation of KSHV reactivation and lytic replication has revealed a sophisticated regulation network that controls the important events in KSHV life cycle.