Kaposi's sarcoma-associated herpesvirus lana2 protein interacts with the pocket proteins and inhibits their sumoylation

Recent progress in retinoblastoma: Pathogenesis, presentation, diagnosis and management

Retinoblastoma, the primary ocular malignancy in pediatric patients, poses a substantial threat to mortality without prompt and effective management. The prognosis for survival and preservation of visual acuity hinges upon the disease severity at the time of initial diagnosis. Notably, retinoblastoma has played a crucial role in unraveling the genetic foundations of oncogenesis. The process of tumorigenesis commonly begins with the occurrence of biallelic mutation in the RB1 tumor suppressor gene, which is then followed by a cascade of genetic and epigenetic alterations that correspond to the clinical stage and pathological features of the tumor. The RB1 gene, recognized as a tumor suppressor, encodes the retinoblastoma protein, which plays a vital role in governing cellular replication through interactions with E2F transcription factors and chromatin remodeling proteins. The diagnosis and treatment of retinoblastoma necessitate consideration of numerous factors, including disease staging, germline mutation status, family psychosocial factors, and the resources available within the institution. This review has systematically compiled and categorized the latest developments in the diagnosis and treatment of retinoblastoma which enhanced the quality of care for this pediatric malignancy.

Post-translational modifications on the retinoblastoma protein

The retinoblastoma protein (pRb) functions as a cell cycle regulator controlling G1 to S phase transition and plays critical roles in tumour suppression. It is frequently inactivated in various tumours. The functions of pRb are tightly regulated, where post-translational modifications (PTMs) play crucial roles, including phosphorylation, ubiquitination, SUMOylation, acetylation and methylation. Most PTMs on pRb are reversible and can be detected in non-cancerous cells, playing an important role in cell cycle regulation, cell survival and differentiation. Conversely, altered PTMs on pRb can give rise to anomalies in cell proliferation and tumourigenesis. In this review, we first summarize recent findings pertinent to how individual PTMs impinge on pRb functions. As many of these PTMs on pRb were published as individual articles, we also provide insights on the coordination, either collaborations and/or competitions, of the same or different types of PTMs on pRb. Having a better understanding of how pRb is post-translationally modulated should pave the way for developing novel and specific therapeutic strategies to treat various human diseases.

RSK1 SUMOylation is required for KSHV lytic replication

RSK1, a downstream kinase of the MAPK pathway, has been shown to regulate multiple cellular processes and is essential for lytic replication of a variety of viruses, including Kaposi's sarcoma-associated herpesvirus (KSHV). Besides phosphorylation, it is not known whether other post-translational modifications play an important role in regulating RSK1 function. We demonstrate that RSK1 undergoes robust SUMOylation during KSHV lytic replication at lysine residues K110, K335, and K421. SUMO modification does not alter RSK1 activation and kinase activity upon KSHV ORF45 co-expression, but affects RSK1 downstream substrate phosphorylation. Compared to wild-type RSK1, the overall phosphorylation level of RxRxxS*/T* motif is significantly declined in RSK1K110/335/421R expressing cells. Specifically, SUMOylation deficient RSK1 cannot efficiently phosphorylate eIF4B. Sequence analysis showed that eIF4B has one SUMO-interacting motif (SIM) between the amino acid position 166 and 170 (166IRVDV170), which mediates the association between eIF4B and RSK1 through SUMO-SIM interaction. These results indicate that SUMOylation regulates the phosphorylation of RSK1 downstream substrates, which is required for efficient KSHV lytic replication.

Metabolic Control by DNA Tumor Virus-Encoded Proteins

Viruses co-opt a multitude of host cell metabolic processes in order to meet the energy and substrate requirements for successful viral replication. However, due to their limited coding capacity, viruses must enact most, if not all, of these metabolic changes by influencing the function of available host cell regulatory proteins. Typically, certain viral proteins, some of which can function as viral oncoproteins, interact with these cellular regulatory proteins directly in order to effect changes in downstream metabolic pathways. This review highlights recent research into how four different DNA tumor viruses, namely human adenovirus, human papillomavirus, Epstein-Barr virus and Kaposi's associated-sarcoma herpesvirus, can influence host cell metabolism through their interactions with either MYC, p53 or the pRb/E2F complex. Interestingly, some of these host cell regulators can be activated or inhibited by the same virus, depending on which viral oncoprotein is interacting with the regulatory protein. This review highlights how MYC, p53 and pRb/E2F regulate host cell metabolism, followed by an outline of how each of these DNA tumor viruses control their activities. Understanding how DNA tumor viruses regulate metabolism through viral oncoproteins could assist in the discovery or repurposing of metabolic inhibitors for antiviral therapy or treatment of virus-dependent cancers.

Novel Functions and Virus-Host Interactions Implicated in Pathogenesis and Replication of Human Herpesvirus 8

Human herpesvirus 8 (HHV-8) is classified as a γ2-herpesvirus and is related to Epstein-Barr virus (EBV), a γ1-herpesvirus. One important aspect of the γ-herpesviruses is their association with neoplasia, either naturally or in animal model systems. HHV-8 is associated with B-cell-derived primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD), endothelial-derived Kaposi's sarcoma (KS), and KSHV inflammatory cytokine syndrome (KICS). EBV is also associated with a number of B-cell malignancies, such as Burkitt's lymphoma, Hodgkin's lymphoma, and posttransplant lymphoproliferative disease, in addition to epithelial nasopharyngeal and gastric carcinomas. Despite the similarities between these viruses and their associated malignancies, the particular protein functions and activities involved in key aspects of virus biology and neoplastic transformation appear to be quite distinct. Indeed, HHV-8 specifies a number of proteins for which counterparts had not previously been identified in EBV, other herpesviruses, or even viruses in general, and these proteins are believed to play vital functions in virus biology and to be involved centrally in viral pathogenesis. Additionally, a set of microRNAs encoded by HHV-8 appears to modulate the expression of multiple host proteins to provide conditions conductive to virus persistence within the host and possibly contributing to HHV-8-induced neoplasia. Here, we review the molecular biology underlying these novel virus-host interactions and their potential roles in both virus biology and virus-associated disease.

Mechanistic Insights into Chemoresistance Mediated by Oncogenic Viruses in Lymphomas

Viral lymphomagenesis induced by infection with oncogenic viruses, such as Kaposi’s sarcoma associated herpesvirus (KSHV), Epstein–Barr virus (EBV) and human T-cell leukemia virus (HTLV-1), represents a group of aggressive malignancies with a diverse range of pathological features. Combined chemotherapy remains the standard of care for these virus-associated lymphomas; however, frequent chemoresistance is a barrier to achieving successful long-term disease-free survival. There is increasing evidence that indicates virus-associated lymphomas display more resistance to cytotoxic chemotherapeutic agents than that observed in solid tumors. Although the tumor microenvironment and genetic changes, such as key oncogene mutations, are closely related to chemoresistance, some studies demonstrate that the components of oncogenic viruses themselves play pivotal roles in the multidrug chemoresistance of lymphoma cells. In this review, we summarize recent advances in the understanding of the mechanisms through which oncogenic viruses mediate lymphoma cell chemoresistance, with a particular focus on KSHV and EBV, two major oncogenic viruses. We also discuss the current challenges to overcome these obstacles in the treatment of virus-associated lymphomas.

Viral Interplay with the Host Sumoylation System

Viruses have evolved elaborate means to regulate diverse cellular pathways in order to create a cellular environment that facilitates viral survival and reproduction. This includes enhancing viral macromolecular synthesis and assembly, as well as preventing antiviral responses, including intrinsic, innate, and adaptive immunity. There are numerous mechanisms by which viruses mediate their effects on the host cell, and this includes targeting various cellular post-translational modification systems, including sumoylation. The wide-ranging impact of sumoylation on cellular processes such as transcriptional regulation, apoptosis, stress response, and cell cycle control makes it an attractive target for viral dysregulation. To date, proteins from both RNA and DNA virus families have been shown to be modified by SUMO conjugation, and this modification appears critical for viral protein function. More interestingly, members of the several viral families have been shown to modulate sumoylation, including papillomaviruses, adenoviruses, herpesviruses, orthomyxoviruses, filoviruses, and picornaviruses. This chapter will focus on mechanisms by which sumoylation both impacts human viruses and is used by viruses to promote viral infection and disease.

The Latency-Associated Nuclear Antigen of Kaposi's Sarcoma-Associated Herpesvirus Inhibits Expression of SUMO/Sentrin-Specific Peptidase 6 To Facilitate Establishment of Latency

Kaposi's sarcoma-associated herpesvirus (KSHV), which belongs to the Gammaherpesviridae, typically displays two different phases in its life cycle, the latent phase and the lytic phase. Latency-associated nuclear antigen (LANA), the primary viral product during latency, has been reported to bind to a series of cellular gene promoters to modulate gene transcription. To systemically elucidate the cellular genes regulated by LANA, we identified genome-wide LANA binding sites by chromatin immunoprecipitation coupled with sequencing (ChIP-seq). We stratified ChIP-seq data and found that LANA might be involved in the macromolecule catabolic process. Specifically, we found and verified that LANA could directly bind to the promoter of the SUMO/sentrin-specific peptidase 6 (SENP6) gene in vivo and in vitro LANA could repress SENP6 promoter activity in a dose-dependent manner in a reporter gene assay. LANA expression was sufficient to inhibit endogenous SENP6 expression at both the RNA and protein levels. Moreover, SENP6 overexpression in KSHV-infected cells reduced LANA at the protein level. Mechanistically, we found that SENP6 could interact with LANA and reduce the formation of sumoylated LANA, which relies on the desumoylation ability of SENP6. During de novo infection, SENP6 overexpression would decrease the abundance of LANA and enhance viral gene expression, which would hamper the establishment of latency. Taken together, these data suggest that KSHV-encoded LANA could inhibit SENP6 expression to regulate the abundance of itself, which may play an important role in controlling the establishment of latency.IMPORTANCE LANA, as a key latent protein produced by KSHV, is responsible for episome persistence and regulates viral reactivation. In the present study, our results demonstrated that LANA could bind to the promoter region of the SENP6 gene and inhibit SENP6 expression while the regulated SENP6 could in turn modulate the abundance of LANA through desumoylation. This delicate regulation may provide important insights to explain the abundance of LANA during KSHV latency.

Rhadinoviral interferon regulatory factor homologues

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

Modulation of global SUMOylation by Kaposi's sarcoma-associated herpesvirus and its effects on viral gene expression

Some viruses have evolved to exploit the host SUMOylation system to regulate their own replication. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes K-bZIP, a SUMO E3 ligase catalyzing the SUMOylation of viral and host proteins. KSHV also encodes replication and transcriptional activator (RTA), a SUMO-targeted ubiquitin ligase catalyzing the ubiquitination of SUMOylated proteins and targeting them for degradation. Using chronic KSHV-infected TRE × BCBL-1 RTA cells, the expression kinetics of K-bZIP and RTA, and the global SUMOylation level were detected. The endogenous K-bZIP protein increased dramatically after the induction of the RTA gene that is tetracycline responsive, but then decreased rapidly after peaking at 8 h post tetracycline treatment. Consistently, the global SUMO-conjugated proteins increased and remained at high levels until 8 h, and decreased afterward, correlating with the expression kinetics of RTA and K-bZIP. In luciferase reporter assays, transfection of 293T cells with SUMO2 expression plasmid reduced the RTA transactivations of immediate-early genes k8, orf45, and orf50, but enhanced the RTA transactivations of other viral genes including orf57, pan, k2, orf8, and orf73. These results indicated that KSHV might regulate gene expression and viral replication schedule through modulation of the global SUMOylation level, probably via RTA, and RTA-regulated K-bZIP.

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

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

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

Adenovirus Early Proteins and Host Sumoylation

The human adenovirus genome is transported into the nucleus, where viral gene transcription, viral DNA replication, and virion assembly take place. Posttranslational modifications by small ubiquitin-like modifiers (SUMOs) are implicated in the regulation of diverse cellular processes, particularly nuclear events. It is not surprising, therefore, that adenovirus modulates and utilizes the host sumoylation system. Adenovirus early proteins play an important role in establishing optimal host environments for virus replication within infected cells by stimulating the cell cycle and counteracting host antiviral defenses. Here, we review findings on the mechanisms and functional consequences of the interplay between human adenovirus early proteins and the host sumoylation system.

Human Oncogenic Herpesvirus and Post-translational Modifications - Phosphorylation and SUMOylation

Pathogens, especially viruses, evolve abilities to utilize cellular machineries to facilitate their survival and propagation. Post-translational modifications (PTMs), especially phosphorylation and SUMOylation, that reversibly modulate the function and interactions of target proteins are among the most important features in cell signaling pathways. PTM-dependent events also serve as one of the favorite targets for viruses. Among the seven unambiguous human oncogenic viruses, hepatitis B virus (HBV), hepatitis C virus (HCV), Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), human papillomavirus (HPV), Human T lymphotrophic virus-1 (HTLV-1), and Merkel cell polyomavirus (MCPyV), two are herpesviruses. The life cycle of herpesviruses consists of latent and lytic phases and the rapid switch between these states includes global remodeling of the viral genome from heterochromatin-to-euchromatin. The balance between lytic replication and latency is essential for herpesvirus to maintain a persistent infection through a combination of viral propagation and evasion of the host immune response, which consequently may contribute to tumorigenesis. It is no surprise that the swift reversibility of PTMs, especially SUMOylation, a modification that epigenetically regulates chromatin structure, is a major hijack target of the host for oncogenic herpesviruses. In this brief review, we summarize the varied ways in which herpesviruses engage the host immune components through PTMs, focusing on phosphorylation and SUMOylation.

KSHV latent protein LANA2 inhibits sumo2 modification of p53

Tumor suppressor p53 plays a crucial antiviral role and targeting of p53 by viral proteins is a common mechanism involved in virus oncogenesis. The activity of p53 is tightly regulated at the post-translational levels through a myriad of modifications. Among them, modification of p53 by SUMO has been associated with the onset of cellular senescence. Kaposi´s sarcoma-associated herpesvirus (KSHV) expresses several proteins targeting p53, including the latent protein LANA2 that regulates polyubiquitylation and phosphorylation of p53. Here we show that LANA2 also inhibits the modification of p53 by SUMO2. Furthermore, we show that the reduction of p53-SUMO2 conjugation by LANA2, as well as the p53-LANA2 interaction, both require the SUMOylation of the viral protein and its interaction with SUMO or SUMOylated proteins in a non-covalent manner. Finally, we show that the control of p53-SUMO2 conjugation by LANA2 correlates with its ability to inhibit SUMO2- and type I interferon-induced senescence. These results highlight the importance of p53 SUMOylation in the control of virus infection and suggest that viral oncoproteins could contribute to viral infection and cell transformation by abrogating p53 SUMOylation.

Kaposi's Sarcoma-Associated Herpesvirus Viral Interferon Regulatory Factor 1 Interacts with a Member of the Interferon-Stimulated Gene 15 Pathway

Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus known to establish lifelong latency in the human host. We and others have previously shown that three KSHV homologs of cellular interferon regulatory factors (IRFs), known as viral IRFs (vIRFs), participate in evasion of the host interferon (IFN) response. We report that vIRF1 interacts with the cellular interferon-stimulated gene 15 (ISG15) E3 ligase, HERC5, in the context of Toll-like receptor 3 (TLR3) activation and IFN induction. The ISG15 protein is covalently conjugated to target proteins upon activation of the interferon response. Interaction between vIRF1 and HERC5 was confirmed by immunoprecipitation, and the region between amino acids 224 and 349 of vIRF1 was required for interaction with HERC5. We further report that expression of vIRF1 in the context of TLR3 activation results in decreased ISG15 conjugation of proteins. Specifically, TLR3-induced ISG15 conjugation and protein levels of cellular IRF3, a known ISG15 target, were decreased in the presence of vIRF1 compared to the control. vIRF1 itself was also identified as a target of ISG15 conjugation. KSHV-infected cells exhibited increased ISG15 conjugation upon reactivation from latency in coordination with increased IFN. Furthermore, knockdown of ISG15 in latently infected cells resulted in a higher level of KSHV reactivation and an increase in infectious virus. These data suggest that the KSHV vIRF1 protein affects ISG15 conjugation and interferon responses and may contribute to effective KSHV replication.

IMPORTANCE

The KSHV vIRF1 protein can inhibit interferon activation in response to viral infection. We identified a cellular protein named HERC5, which is the major ligase for ISG15, as a vIRF1 binding partner. vIRF1 association with HERC5 altered ISG15 modification of cellular proteins, and knockdown of ISG15 augmented reactivation of KSHV from latency.

SUMO and KSHV Replication

Small Ubiquitin-related MOdifier (SUMO) modification was initially identified as a reversible post-translational modification that affects the regulation of diverse cellular processes, including signal transduction, protein trafficking, chromosome segregation, and DNA repair. Increasing evidence suggests that the SUMO system also plays an important role in regulating chromatin organization and transcription. It is thus not surprising that double-stranded DNA viruses, such as Kaposi's sarcoma-associated herpesvirus (KSHV), have exploited SUMO modification as a means of modulating viral chromatin remodeling during the latent-lytic switch. In addition, SUMO regulation allows the disassembly and assembly of promyelocytic leukemia protein-nuclear bodies (PML-NBs), an intrinsic antiviral host defense, during the viral replication cycle. Overcoming PML-NB-mediated cellular intrinsic immunity is essential to allow the initial transcription and replication of the herpesvirus genome after de novo infection. As a consequence, KSHV has evolved a way as to produce multiple SUMO regulatory viral proteins to modulate the cellular SUMO environment in a dynamic way during its life cycle. Remarkably, KSHV encodes one gene product (K-bZIP) with SUMO-ligase activities and one gene product (K-Rta) that exhibits SUMO-targeting ubiquitin ligase (STUbL) activity. In addition, at least two viral products are sumoylated that have functional importance. Furthermore, sumoylation can be modulated by other viral gene products, such as the viral protein kinase Orf36. Interference with the sumoylation of specific viral targets represents a potential therapeutic strategy when treating KSHV, as well as other oncogenic herpesviruses. Here, we summarize the different ways KSHV exploits and manipulates the cellular SUMO system and explore the multi-faceted functions of SUMO during KSHV's life cycle and pathogenesis.

Distinct roles of Kaposi's sarcoma-associated herpesvirus-encoded viral interferon regulatory factors in inflammatory response and cancer

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent associated with Kaposi's sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman disease (MCD). Similar to other herpesviruses, KSHV has two life cycles, latency and lytic replication. In latency, the KSHV genome persists as a circular episome in the nucleus of the host cell and only a few viral genes are expressed. In this review, we focus on oncogenic, antiapoptotic, and immunomodulating properties of KSHV-encoded homologues of cellular interferon regulatory factors (IRFs)--viral IRF1 (vIRF1) to vIRF4--and their possible role in the KSHV-mediated antiviral response, apoptosis, and oncogenicity.