Herpesvirus-encoded GPCRs rewire cellular signaling

Cardiovascular and haematological pathology in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS): A role for viruses

ME/CFS is a debilitating chronic condition that often develops after viral or bacterial infection. Insight from the study of Long COVID/Post Acute Sequelae of COVID-19 (PASC), the post-viral syndrome associated with SARS-CoV-2 infection, might prove to be useful for understanding pathophysiological mechanisms of ME/CFS. Disease presentation is similar between the two conditions, and a subset of Long COVID patients meet the diagnostic criteria for ME/CFS. Since Long COVID is characterized by significant vascular pathology - including endothelial dysfunction, coagulopathy, and vascular dysregulation - the question of whether or not the same biological abnormalities are of significance in ME/CFS arises. Cardiac abnormalities have for a while now been documented in ME/CFS cohorts, with recent studies demonstrating major deficits in cerebral blood flow, and hence vascular dysregulation. A growing body of research is demonstrating that ME/CFS is accompanied by platelet hyperactivation, anomalous clotting, a procoagulant phenotype, and endothelial dysfunction. Endothelial damage and dysregulated clotting can impair substance exchange between blood and tissues, and result in hypoperfusion, which may contribute to the manifestation of certain ME/CFS symptoms. Here we review the ME/CFS literature to summarize cardiovascular and haematological findings documented in patients with the condition, and, in this context, briefly discuss the potential role of previously-implicated pathogens. Overall, cardiac and haematological abnormalities are present within ME/CFS cohorts. While atherosclerotic heart disease is not significantly associated with ME/CFS, suboptimal cardiovascular function defined by reduced cardiac output, impaired cerebral blood flow, and vascular dysregulation are, and these abnormalities do not appear to be influenced by deconditioning. Rather, these cardiac abnormalities may result from dysfunction in the (autonomic) nervous system. Plenty of recently published studies are demonstrating significant platelet hyperactivity and endothelial dysfunction in ME/CFS, as well as anomalous clotting processes. It is of particular importance to determine to what extent these cardiovascular and haematological abnormalities contribute to symptom severity, and if these two systems can be targeted for therapeutic purposes. Viral reservoirs of herpesviruses exist in ME/CFS, and most likely contribute to cardiovascular and haematological dysfunction directly or indirectly. This review highlights the potential of studying cardiac functioning, the vasculature, and coagulation system in ME/CFS.

The mouse cytomegalovirus G protein-coupled receptor homolog, M33, coordinates key features of in vivo infection via distinct components of its signalling repertoire

Common to all cytomegalovirus (CMV) genomes analysed to date is the presence of G protein-coupled receptors (GPCR). Animal models of CMV provide insights into their role in viral fitness. The mouse cytomegalovirus (MCMV) GPCR, M33, facilitates dendritic cell (DC)-dependent viremia, the extravasation of blood-borne infected DC to the salivary gland and the frequency of reactivation events from latently-infected tissue explants. Constitutive G protein-coupled M33 signalling is required for these phenotypes, although the contribution of distinct biochemical pathways activated by M33 is unknown. M33 engages G_q/11 to constitutively activate phospholipase C β (PLCβ) and downstream cyclic AMP response-element binding protein (CREB) in vitro. Identification of a MCMV M33 mutant (M33_ΔC38) for which CREB signalling was disabled, but PLCβ activation was preserved, provided the opportunity to investigate their relevance in vivo. Following intranasal infection with MCMV M33_ΔC38, the absence of M33 CREB G_q/11-dependent signalling correlated with reduced mobilisation of lytically-infected DC to draining lymph node high endothelial venules (HEVs) and reduced viremia compared with wild type MCMV. In contrast, M33_ΔC38-infected DC within the vascular compartment extravasated to the salivary glands via a pertussis toxin-sensitive, G_i/o-dependent and CREB-independent mechanism. In the context of MCMV latency, spleen explants from M33_ΔC38-infected mice were markedly attenuated for reactivation. Taken together, these data demonstrate that key features of the MCMV lifecycle are coordinated in diverse tissues by distinct pathways of the M33 signalling repertoire. IMPORTANCE G protein-coupled receptors (GPCRs) act as cell surface molecular "switches" which regulate the cellular response to environmental stimuli. All cytomegalovirus (CMV) genomes analysed to date possess GPCR homologs with phylogenetic evidence for independent gene capture events, signifying important in vivo roles. The mouse CMV (MCMV) GPCR homolog, designated M33, is important for cell-associated virus spread and for the establishment and/or reactivation of latent MCMV infection. The signalling repertoire of M33 is distinct from cellular GPCRs and little is known of the relevance of component signalling pathways for in vivo M33 function. In this report, we show temporal and tissue-specific M33 signalling is required facilitating in vivo infection. Understanding the relevance of the viral GPCR signalling profiles for in vivo function will provide opportunities for future targeted interventions.

Viral G Protein-Coupled Receptors: Attractive Targets for Herpesvirus-Associated Diseases

Herpesviruses are ubiquitous pathogens that establish lifelong, latent infections in their host. Spontaneous reactivation of herpesviruses is often asymptomatic or clinically manageable in healthy individuals, but reactivation events in immunocompromised or immunosuppressed individuals can lead to severe morbidity and mortality. Moreover, herpesvirus infections have been associated with multiple proliferative cardiovascular and post-transplant diseases. Herpesviruses encode viral G protein-coupled receptors (vGPCRs) that alter the host cell by hijacking cellular pathways and play important roles in the viral life cycle and these different disease settings. In this review, we discuss the pharmacological and signaling properties of these vGPCRs, their role in the viral life cycle, and their contribution in different diseases. Because of their prominent role, vGPCRs have emerged as promising drug targets, and the potential of vGPCR-targeting therapeutics is being explored. Overall, these vGPCRs can be considered as attractive targets moving forward in the development of antiviral, cancer, and/or cardiovascular disease treatments. SIGNIFICANCE STATEMENT: In the last decade, herpesvirus-encoded G protein-coupled receptors (GPCRs) have emerged as interesting drug targets with the growing understanding of their critical role in the viral life cycle and in different disease settings. This review presents the pharmacological properties of these viral receptors, their role in the viral life cycle and different diseases, and the emergence of therapeutics targeting viral GPCRs.

Control of Cytokines in Latent Cytomegalovirus Infection

Human cytomegalovirus (HCMV) has evolved a number of mechanisms for long-term co-existence within its host. HCMV infects a wide range of cell types, including fibroblasts, epithelial cells, monocytes, macrophages, dendritic cells, and myeloid progenitor cells. Lytic infection, with the production of infectious progeny virions, occurs in differentiated cell types, while undifferentiated myeloid precursor cells are the primary site of latent infection. The outcome of HCMV infection depends partly on the cell type and differentiation state but is also influenced by the composition of the immune environment. In this review, we discuss the role of early interactions between HCMV and the host immune system, particularly cytokine and chemokine networks, that facilitate the establishment of lifelong latent infection. A better understanding of these cytokine signaling pathways could lead to novel therapeutic targets that might prevent latency or eradicate latently infected cells.

The constitutive activity of the viral-encoded G protein-coupled receptor US28 supports a complex signalling network contributing to cancer development

US28 is a viral G protein-coupled receptor (GPCR) encoded by the human cytomegalovirus (HCMV). This receptor, expressed both during lytic replication and viral latency, is required for latency. US28 is binding to a wide variety of chemokines but also exhibits a particularly high constitutive activity robustly modulating a wide network of cellular pathways altering the host cell environment to benefit HCMV infection. Several studies suggest that US28-mediated signalling may contribute to cancer progression. In this review, we discuss the unique structural characteristics that US28 acquired through evolution that confer a robust constitutive activity to this viral receptor. We also describe the wide downstream signalling network activated by this constitutive activation of US28 and discuss how these signalling pathways may promote and support important cellular aspects of cancer.

Emerging roles of cytomegalovirus-encoded G protein-coupled receptors during lytic and latent infection

Cytomegaloviruses (CMVs) have developed multiple diverse strategies to ensure their replicative success and to evade immune recognition. Given the fact that G protein-coupled receptors (GPCRs) are key regulators of numerous cellular processes and modify a variety of signaling pathways, it is not surprising that CMVs and other herpesviruses have hijacked mammalian GPCRs during their coevolution. Human cytomegalovirus (HCMV) encodes for four viral GPCR homologues (vGPCRs), termed US27, US28, UL33, and UL78. Although HCMV-encoded GPCRs were first described in 1990, the pivotal functions of these viral receptor proteins were detected only recently. Here, we summarize seminal knowledge on the functions of herpesviral vGPCRs with a focus on novel roles of cytomegalovirus-encoded vGPCRs for viral spread and the regulation of latency.

US28, a Virally-Encoded GPCR as an Antiviral Target for Human Cytomegalovirus Infection

Viruses continue to evolve a new strategy to take advantage of every aspect of host cells in order to maximize their survival. Due to their central roles in transducing a variety of transmembrane signals, GPCRs seem to be a prime target for viruses to pirate for their own use. Incorporation of GPCR functionality into the genome of herpesviruses has been demonstrated to be essential for pathogenesis of many herpesviruses-induced diseases. Here, we introduce US28 of human cytomegalovirus (HCMV) as the best-studied example of virally-encoded GPCRs to manipulate host GPCR signaling. In this review, we wish to summarize a number of US28-related topics including its regulation of host signaling pathways, its constitutive internalization, its structural and functional analysis, its roles in HCMV biology and pathogenesis, its proliferative activities and role in oncogenesis, and pharmacological modulation of its biological activities. This review will aid in our understanding of how pathogenic viruses usurp the host GPCR signaling for successful viral infection. This kind of knowledge will enable us to build a better strategy to control viral infection by normalizing the virally-dysregulated host GPCR signaling.

Human cytomegalovirus encoded chemokine receptor US28 activates the HIF-1α/PKM2 axis in glioblastoma cells

The human cytomegalovirus (HCMV) encoded chemokine receptor US28 promotes tumorigenesis through activation of various proliferative and angiogenic signaling pathways. Upon infection, US28 displays constitutive activity and signals in a G protein-dependent manner, hijacking the host's cellular machinery. In tumor cells, the hypoxia inducible factor-1α/pyruvate kinase M2 (HIF-1α/PKM2) axis plays an important role by supporting proliferation, angiogenesis and reprogramming of energy metabolism. In this study we show that US28 signaling results in activation of the HIF-1α/PKM2 feedforward loop in fibroblasts and glioblastoma cells. The constitutive activity of US28 increases HIF-1 protein stability through a Gαq-, CaMKII- and Akt/mTOR-dependent mechanism. Furthermore, we found that VEGF and lactate secretion are increased and HIF-1 target genes, glucose transporter type 1 (GLUT1) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), involved in glucose metabolism, are upregulated in US28 expressing cells. In addition, PKM2 is phosphorylated and found to be in a tumor-associated dimeric state upon US28 expression. Also in HCMV-infected cells HIF-1 activity is enhanced, which in part is US28-dependent. Finally, increased proliferation of cells expressing US28 is abolished upon inhibition of the HIF-1α/PKM2 cascade. These data highlight the importance of HIF-1α and PKM2 in US28-induced proliferation, angiogenesis and metabolic reprogramming.

Hijacking GPCRs by viral pathogens and tumor

G protein-coupled receptors (GPCRs) constitute the largest family of molecules that transduce signals across the plasma membrane. Herpesviruses are successful pathogens that evolved diverse mechanisms to benefit their infection. Several human herpesviruses express GPCRs to exploit cellular signaling cascades during infection. These viral GPCRs demonstrate distinct biochemical and biophysical properties that result in the activation of a broad spectrum of signaling pathways. In immune-deficient individuals, human herpesvirus infection and the expression of their GPCRs are implicated in virus-associated diseases and pathologies. Emerging studies also uncover diverse mutations in components, particularly GPCRs and small G proteins, of GPCR signaling pathways that render the constitutive activation of proliferative and survival signal, which contributes to the oncogenesis of various human cancers. Hijacking GPCR-mediated signaling is a signature shared by diseases associated with constitutively active viral GPCRs and cellular mutations activating GPCR signaling, exposing key molecules that can be targeted for anti-viral and anti-tumor therapy.

Inflammatory CXCL12-CXCR4/CXCR7 axis mediates G-protein signaling pathway to influence the invasion and migration of nasopharyngeal carcinoma cells

This study explored whether the migration, invasion, and apoptosis of nasopharyngeal carcinoma (NPC) cells were affected by the CXCR4/CXCR7-CXCL12 axis and if this mechanism was related to G-protein signaling pathway. A total of 72 NPC patients admitted in our hospital between April 2013 and February 2015 were incorporated in this study. Immunohistochemistry was performed to compare the expression levels of CXCR4, CXCR7, and CXCL12 between NPC tissues and adjacent normal tissues. Then, the correlation analysis was implemented to assess the association among CXCR4, CXCR7, and CXCL12 expressions. Jellyfish glow protein experiment was carried out after the cultivation of CNE-2Z cell lines in order to observe the intracellular calcium mobilization resulted from G-protein activation contributed by CXCR4/CXCR7-CXCL12 axis. The impact of CXCR4/CXCR7-CXCL12 axis on the migration and invasion of NPC cells was explored using transwell experiments. Finally, the anti-apoptosis effects of CXCR4/CXCR7-CXCL12 axis on NPC cells were investigated by the splicing of poly ADP-ribose polymerase (PARP). Compared to NPC patients with low-grade (stage I-II) tumor node metastasis (TNM) and those without lymph node metastasis, the expression of CXCR4, CXCR7, and CXCL12 were significantly higher in NPC patients with high-grade (stage III-IV) TNM and those with lymph node metastasis (P < 0.05). Moreover, there was significant positive correlation between the expression level of CXCL12 and CXCR7 (r s = 0.484, P < 0.001) as well as the expression level of CXCL12 and CXCR4 (r s = 0.414, P < 0.001). As suggested by cellular experiments using CNE-2Z, the calcium mobilization degree induced by CXCR4-CXCL12 axis in activating G proteins seemed to be slightly more effective than that induced by CXCR4/CXCR7-CXCL12 axis, while the CXCR7-CXCL12 axis could hardly activate calcium mobilization. Furthermore, the transwell experiment showed that CXCR4/CXCR7-CXCL12 axis could exacerbate the migration and invasion of NPC cells (P < 0.05). The transwell experiment also suggested that the CXCR4/CXCR7-CXCL12 axis was associated with the expression of matrix metallo proteinase 9 (MMP9) which is a substance in the downstream of G-protein pathways (P < 0.05). Results from PARP shear zone also indicated that the CXCR4/CXCR7-CXCL12 axis could suppress NPC cell apoptosis (P < 0.05). The expressional levels of CXCR4, CXCR7, and CXCL12 significantly varied with clinical stages and status of lymph node metastasis of NPC patients. This revealed potential indicators which can be used for NPC prognosis. Additionally, the CXCR4/CXCR7-CXCL12 axis may regulate the expression of downstream proteins (e.g., MMP-9) through the activation of G-protein signaling pathways. These conclusions may provide key evidence for NPC aetiology which can be further investigated to develop novel molecular targets for NPC treatments.

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

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

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

Modulation of cellular signaling by herpesvirus-encoded G protein-coupled receptors

Human herpesviruses (HHVs) are widespread infectious pathogens that have been associated with proliferative and inflammatory diseases. During viral evolution, HHVs have pirated genes encoding viral G protein-coupled receptors (vGPCRs), which are expressed on infected host cells. These vGPCRs show highest homology to human chemokine receptors, which play a key role in the immune system. Importantly, vGPCRs have acquired unique properties such as constitutive activity and the ability to bind a broad range of human chemokines. This allows vGPCRs to hijack human proteins and modulate cellular signaling for the benefit of the virus, ultimately resulting in immune evasion and viral dissemination to establish a widespread and lifelong infection. Knowledge on the mechanisms by which herpesviruses reprogram cellular signaling might provide insight in the contribution of vGPCRs to viral survival and herpesvirus-associated pathologies.

Chemokine receptor oligomerization and allostery

Oligomerization of chemokine receptors has been reported to influence many aspects of receptor function through allosteric communication between receptor protomers. Allosteric interactions within chemokine receptor hetero-oligomers have been shown to cause negative cooperativity in the binding of chemokines and to inhibit receptor activation in the case of some receptor pairs. Other receptor pairs can cause enhanced signaling and even activate entirely new, hetero-oligomer-specific signaling complexes and responses downstream of receptor activation. Many mechanisms contribute to these effects including direct allosteric coupling between the receptors, G protein-mediated allostery, G protein stealing, ligand sequestration, and recruitment of new intracellular proteins by exposing unique binding interfaces on the oligomerized receptors. These effects present both challenges as well as exciting opportunities for drug discovery. One of the most difficult challenges will involve determining if and when hetero-oligomers versus homomeric receptors are involved in specific disease states.

A genome-wide RNAi screen reveals a Trio-regulated Rho GTPase circuitry transducing mitogenic signals initiated by G protein-coupled receptors

Activating mutations in GNAQ and GNA11, encoding members of the Gα(q) family of G protein α subunits, are the driver oncogenes in uveal melanoma, and mutations in Gq-linked G protein-coupled receptors have been identified recently in numerous human malignancies. How Gα(q) and its coupled receptors transduce mitogenic signals is still unclear because of the complexity of signaling events perturbed upon Gq activation. Using a synthetic-biology approach and a genome-wide RNAi screen, we found that a highly conserved guanine nucleotide exchange factor, Trio, is essential for activating Rho- and Rac-regulated signaling pathways acting on JNK and p38, and thereby transducing proliferative signals from Gα(q) to the nucleus independently of phospholipase C-β. Indeed, whereas many biological responses elicited by Gq depend on the transient activation of second-messenger systems, Gq utilizes a hard-wired protein-protein-interaction-based signaling circuitry to achieve the sustained stimulation of proliferative pathways, thereby controlling normal and aberrant cell growth.

Evolutionary Analysis of Functional Divergence among Chemokine Receptors, Decoy Receptors, and Viral Receptors

Chemokine receptors (CKRs) function in the inflammatory response and in vertebrate homeostasis. Decoy and viral receptors are two types of CKR homologs with modified functions from those of the typical CKRs. The decoy receptors are able to bind ligands without signaling. On the other hand, the viral receptors show constitutive signaling without ligands. We examined the sites related to the functional difference. At first, the decoy and viral receptors were each classified into five groups, based on the molecular phylogenetic analysis. A multiple amino acid sequence alignment between each group and the CKRs was then constructed. The difference in the amino acid composition between the group and the CKRs was evaluated as the Kullback-Leibler (KL) information value at each alignment site. The KL information value is considered to reflect the difference in the functional constraints at the site. The sites with the top 5% of KL information values were selected and mapped on the structure of a CKR. The comparisons with decoy receptor groups revealed that the detected sites were biased on the intracellular side. In contrast, the sites detected from the comparisons with viral receptor groups were found on both the extracellular and intracellular sides. More sites were found in the ligand binding pocket in the analyses of the viral receptor groups, as compared to the decoy receptor groups. Some of the detected sites were located in the GPCR motifs. For example, the DRY motif of the decoy receptors was often degraded, although the motif of the viral receptors was basically conserved. The observations for the viral receptor groups suggested that the constraints in the pocket region are loose and that the sites on the intracellular side are different from those for the decoy receptors, which may be related to the constitutive signaling activity of the viral receptors.

Human cytomegalovirus-encoded UL33 and UL78 heteromerize with host CCR5 and CXCR4 impairing their HIV coreceptor activity

Human cytomegalovirus (HCMV) encodes four 7-transmembrane-spanning (7TM) proteins, US28, US27, UL33, and UL78, which present important sequence homology with human chemokine receptors. Whereas US28 binds a large range of chemokines and disturbs host cell signaling at different levels, the others are orphans with largely unknown functions. Assembly of 2 different 7TM proteins into hetero-oligomeric complexes may profoundly change their respective functional properties. We show that HCMV-encoded UL33 and UL78 form heteromers with CCR5 and CXCR4 chemokine receptors in transfected human embryonic kidney 293T cells and monocytic THP-1 cells. Expression of UL33 and UL78 had pleiotropic, predominantly negative, effects on CCR5 and CXCR4 cell surface expression, ligand-induced internalization, signal transduction, and migration without modifying the chemokine binding properties of CCR5 and CXCR4. Importantly, the coreceptor activity of CCR5 and CXCR4 for HIV was largely impaired in the presence of UL33 and UL78 without affecting expression of the primary HIV entry receptor CD4 and its interaction with CCR5 and CXCR4. Collectively, we identified the first molecular function for the HCMV-encoded orphan UL33 and UL78 7TM proteins, namely the regulation of cellular chemokine receptors through receptor heteromerization.

Human cytomegalovirus and mucoepidermoid carcinoma of salivary glands: cell-specific localization of active viral and oncogenic signaling proteins is confirmatory of a causal relationship

Human cytomegalovirus (hCMV) infection is common. Although still controversial, there is growing evidence that active hCMV infection is associated with a variety of malignancies, including brain, breast, lung, colon, and prostate. Given that hCMV is frequently resident in salivary gland (SG) ductal epithelium, we hypothesized that hCMV would be important to the pathogenesis of SG mucoepidermoid carcinoma (MEC). This was initially supported by our finding that purified CMV induces malignant transformation in SG cells in an in vitro mouse model, and utilizes a pathogenic pathway previously reported for human MEC. Here we present the histologic and molecular characterizations of 39 human SG MECs selected randomly from a repository of cases spanning 2004-2011. Serial sections were obtained from formalin-fixed, paraffin embedded, tissue blocks from previous incisional or excisional biopsies. Immunohistochemical assays were performed for active hCMV proteins (IE1 and pp65) and the activated COX/AREG/EGFR/ERK signaling pathway. All four prospective causal criteria for viruses and cancer are fully satisfied: (1) protein markers for active hCMV are present in 97% of MECs; (2) markers of active hCMV are absent in non-neoplastic SG tissues; (3) hCMV-specific proteins (IE1, pp65) are in specific cell types and expression is positively correlated with severity; (4) hCMV correlates and colocalizes with an upregulation and activation of an established oncogenic signaling pathway (COX/AREG/EGFR/ERK). Thus, the evidential support reported here and previously in a mouse model is strongly confirmatory of a causal relationship between hCMV and SG mucoepidermoid carcinoma. To our knowledge, this is the first demonstration of hCMV's role in human oncogenesis that fully responds to all of Koch's Postulates as revised for viruses and cancer. In the absence of any contrary evidence, hCMV can reasonably be designated an "oncovirus."

Ubiquitin-independent proteasomal degradation during oncogenic viral infections

Most eukaryotic proteins destined for imminent destruction are first tagged with a chain of ubiquitin molecules and are subsequently dismantled by the proteasome. Ubiquitin-independent degradation of substrates by the proteasome, however, also occurs. The number of documented proteasome-dependent, ubiquitin-independent degradation events remains relatively small but continues to grow. Proteins involved in oncogenesis and tumor suppression make up the majority of the known cases for this type of protein destruction. Provocatively, viruses with confirmed or suspected oncogenic properties are also prominent participants in the pantheon of ubiquitin-independent proteasomal degradation events. In this review, we identify and describe examples of proteasome-dependent, ubiquitin-independent protein degradation that occur during tumor virus infections, speculate why this type of protein destruction may be preferred during oncogenesis, and argue that this uncommon type of protein turnover represents a prime target for antiviral and anticancer therapeutics.