Small size, big impact: how studies of small DNA tumour viruses revolutionized biology

Epidermal Growth Factor Receptor Emerges as a Viable Target for Reducing Tumorigenicity of MDCK Cells

The MDCK cell line is perceived as better than the embryos of hen eggs for the production of influenza vaccines, but the tumorigenicity of these cells is concerning. Epidermal growth factor receptor (EGFR) is likely to be a crucial target that contributes to the tumorigenicity of MDCK cells. In this study, EGFR-knockdown and EGFR-overexpression cell lines were established. EGFR's influence on cell growth, migration, clonogenic ability, and flu virus susceptibility was evaluated in vitro, and its role in cell tumorigenicity was examined in nude mice. GST pull-down coupled with mass spectrometry (MS) and bioinformatics analysis identified EGFR-interacting proteins. The expression levels of these proteins, as well as those of PI3K-AKT- and MAPK-ERK-signaling-pathway-related molecules, were confirmed at both gene and protein levels. The result indicates that EGFR overexpression can enhance cell proliferation, migration, and clonal formation; EGFR knockdown could effectively curtail tumorigenesis and amplify the titers of influenza viruses in MDCK cells. An analysis of the underlying mechanism identified a total of 21 interacting proteins implicated in tumor formation, and among these, AKT1, CDK4, GNB2, and MAPK8 were confirmed at both gene and protein levels. EGFR can activate key factors of the PI3K-AKT signaling pathway, AKT and PI3K, and promote their phosphorylation levels. Consequently, we concluded that EGFR interacts with GNB2, facilitating transmembrane signal transduction, activating the PI3K-AKT signaling cascade, controlling cell cycle alterations, stimulating cell proliferation, and promoting tumorigenesis.

The cellular Notch1 protein promotes KSHV reactivation in an Rta-dependent manner

The cellular Notch signal transduction pathway is intimately associated with infections by Kaposi's sarcoma-associated herpesvirus (KSHV) and other gamma-herpesviruses. RBP-Jk, the cellular DNA binding component of the canonical Notch pathway, is the key Notch downstream effector protein in virus-infected and uninfected animal cells. Reactivation of KSHV from latency requires the viral lytic switch protein, Rta, to form complexes with RBP-Jk on numerous sites within the viral DNA. Constitutive Notch activity is essential for KSHV pathophysiology in models of Kaposi's sarcoma (KS) and Primary Effusion Lymphoma (PEL), and we demonstrate that Notch1 is also constitutively active in infected Vero cells. Although the KSHV genome contains >100 RBP-Jk DNA motifs, we show that none of the four isoforms of activated Notch can productively reactivate the virus from latency in a highly quantitative trans-complementing reporter virus system. Nevertheless, Notch contributed positively to reactivation because broad inhibition of Notch1-4 with gamma-secretase inhibitor (GSI) or expression of dominant negative mastermind-like1 (dnMAML1) coactivators severely reduced production of infectious KSHV from Vero cells. Reduction of KSHV production is associated with gene-specific reduction of viral transcription in both Vero and PEL cells. Specific inhibition of Notch1 by siRNA partially reduces the production of infectious KSHV, and NICD1 forms promoter-specific complexes with viral DNA during reactivation. We conclude that constitutive Notch activity is required for the robust production of infectious KSHV, and our results implicate activated Notch1 as a pro-viral member of a MAML1/RBP-Jk/DNA complex during viral reactivation.

IMPORTANCE

Kaposi's sarcoma-associated herpesvirus (KSHV) manipulates the host cell oncogenic Notch signaling pathway for viral reactivation from latency and cell pathogenesis. KSHV reactivation requires that the viral protein Rta functionally interacts with RBP-Jk, the DNA-binding component of the Notch pathway, and with promoter DNA to drive transcription of productive cycle genes. We show that the Notch pathway is constitutively active during KSHV reactivation and is essential for robust production of infectious virus progeny. Inhibiting Notch during reactivation reduces the expression of specific viral genes yet does not affect the growth of the host cells. Although Notch cannot reactivate KSHV alone, the requisite expression of Rta reveals a previously unappreciated role for Notch in reactivation. We propose that activated Notch cooperates with Rta in a promoter-specific manner that is partially programmed by Rta's ability to redistribute RBP-Jk DNA binding to the virus during reactivation.

Current In Vitro and In Vivo Models to Study MCPyV-Associated MCC

Merkel cell polyomavirus (MCPyV) is the only human polyomavirus currently known to cause human cancer. MCPyV is believed to be an etiological factor in at least 80% of cases of the rare but aggressive skin malignancy Merkel cell carcinoma (MCC). In these MCPyV+ MCC tumors, clonal integration of the viral genome results in the continued expression of two viral proteins: the viral small T antigen (ST) and a truncated form of the viral large T antigen. The oncogenic potential of MCPyV and the functional properties of the viral T antigens that contribute to neoplasia are becoming increasingly well-characterized with the recent development of model systems that recapitulate the biology of MCPyV+ MCC. In this review, we summarize our understanding of MCPyV and its role in MCC, followed by the current state of both in vitro and in vivo model systems used to study MCPyV and its contribution to carcinogenesis. We also highlight the remaining challenges within the field and the major considerations related to the ongoing development of in vitro and in vivo models of MCPyV+ MCC.

Small DNA tumor viruses and human cancer: Preclinical models of virus infection and disease

Human tumor viruses cause various human cancers that account for at least 15% of the global cancer burden. Among the currently identified human tumor viruses, two are small DNA tumor viruses: human papillomaviruses (HPVs) and Merkel cell polyomavirus (MCPyV). The study of small DNA tumor viruses (adenoviruses, polyomaviruses, and papillomaviruses) has facilitated several significant biological discoveries and established some of the first animal models of virus-associated cancers. The development and use of preclinical in vivo models to study HPVs and MCPyV and their role in human cancer is the focus of this review. Important considerations in the design of animal models of small DNA tumor virus infection and disease, including host range, cell tropism, choice of virus isolates, and the ability to recapitulate human disease, are presented. The types of infection-based and transgenic model strategies that are used to study HPVs and MCPyV, including their strengths and limitations, are also discussed. An overview of the current models that exist to study HPV and MCPyV infection and neoplastic disease are highlighted. These comparative models provide valuable platforms to study various aspects of virus-associated human disease and will continue to expand knowledge of human tumor viruses and their relationship with their hosts.

Human Papillomavirus L2 Capsid Protein Stabilizes γ-Secretase during Viral Infection

Intracellular trafficking of human papillomavirus (HPV) during virus entry requires γ-secretase, a cellular protease consisting of a complex of four cellular transmembrane (TM) proteins. γ-secretase typically cleaves substrate proteins but it plays a non-canonical role during HPV entry. γ-secretase binds to the HPV minor capsid protein L2 and facilitates its insertion into the endosomal membrane. After insertion, L2 protrudes into the cytoplasm, which allows HPV to bind other cellular factors required for proper virus trafficking into the retrograde transport pathway. Here, we further characterize the interaction between γ-secretase and HPV L2. We show that γ-secretase is required for cytoplasmic protrusion of L2 and that L2 associates strongly with the PS1 catalytic subunit of γ-secretase and stabilizes the γ-secretase complex. Mutational studies revealed that a putative TM domain in HPV16 L2 cannot be replaced by a foreign TM domain, that infectivity of HPV TM mutants is tightly correlated with γ-secretase binding and stabilization, and that the L2 TM domain is required for protrusion of the L2 protein into the cytoplasm. These results provide new insight into the interaction between γ-secretase and L2 and highlight the importance of the native HPV L2 TM domain for proper virus trafficking during entry.

The long and winding road: human papillomavirus entry and subcellular trafficking

Human papillomaviruses (HPVs) infect and replicate in differentiating mucosal and cutaneous epithelium. Most HPV infections are asymptomatic or cause transient benign neoplasia. However, persistent infections by oncogenic HPV types can progress to cancer. During infectious entry into host keratinocytes, HPV particles interact with many host proteins, beginning with major capsid protein L1 binding to cellular heparan sulfate and a series of enzymatic capsid modifications that promote infectious cellular entry. After utilizing the endosomal pathway to uncoat the viral genome (vDNA), the minor capsid protein L2/vDNA complex is retrograde trafficked to the Golgi, and thereafter, to the nucleus where viral transcription initiates. Post-Golgi trafficking is dependent on mitosis, with L2-dependent tethering of vDNA to mitotic chromosomes before accumulation at nuclear substructures in G1. This review summarizes the current knowledge of the HPV entry pathway, the role of cellular proteins in this process, and notes many gaps in our understanding.

Retromer stabilizes transient membrane insertion of L2 capsid protein during retrograde entry of human papillomavirus

Retromer, a cellular protein trafficking complex, sorts human papillomaviruses (HPVs) into the retrograde pathway for transport of HPV to the nucleus during virus entry. Here, we conducted a protein modulation screen to isolate four artificial transmembrane proteins called traptamers that inhibit different steps of HPV entry. By analyzing cells expressing pairs of traptamers, we ordered the trafficking steps during entry into a coherent pathway. One traptamer stimulates ubiquitination of the L2 capsid protein or associated proteins and diverts incoming virus to the lysosome, whereas the others act downstream by preventing sequential passage of the virus through retrograde compartments. Complex genetic interactions between traptamers revealed that a cell-penetrating peptide (CPP) on L2 mediates transient insertion of L2 into the endosome membrane, which is stabilized by retromer-L2 binding. These results define the retrograde entry route taken by HPV and show that retromer can play a role in CPP-mediated membrane insertion.

A Comprehensive Proteomics Analysis of the JC Virus (JCV) Large and Small Tumor Antigen Interacting Proteins: Large T Primarily Targets the Host Protein Complexes with V-ATPase and Ubiquitin Ligase Activities While Small t Mostly Associates with Those Having Phosphatase and Chromatin-Remodeling Functions

The oncogenic potential of both the polyomavirus large (LT-Ag) and small (Sm t-Ag) tumor antigens has been previously demonstrated in both tissue culture and animal models. Even the contribution of the MCPyV tumor antigens to the development of an aggressive human skin cancer, Merkel cell carcinoma, has been recently established. To date, the known primary targets of these tumor antigens include several tumor suppressors such as pRb, p53, and PP2A. However, a comprehensive list of the host proteins targeted by these proteins remains largely unknown. Here, we report the first interactome of JCV LT-Ag and Sm t-Ag by employing two independent “affinity purification/mass spectroscopy” (AP/MS) assays. The proteomics data identified novel targets for both tumor antigens while confirming some of the previously reported interactions. LT-Ag was found to primarily target the protein complexes with ATPase (v-ATPase and Smc5/6 complex), phosphatase (PP4 and PP1), and ligase (E3-ubiquitin) activities. In contrast, the major targets of Sm t-Ag were identified as Smarca1/6, AIFM1, SdhA/B, PP2A, and p53. The interactions between “LT-Ag and SdhB”, “Sm t-Ag and Smarca5”, and “Sm t-Ag and SDH” were further validated by biochemical assays. Interestingly, perturbations in some of the LT-Ag and Sm t-Ag targets identified in this study were previously shown to be associated with oncogenesis, suggesting new roles for both tumor antigens in novel oncogenic pathways. This comprehensive data establishes new foundations to further unravel the new roles for JCV tumor antigens in oncogenesis and the viral life cycle.

Towards a multi-level and a multi-disciplinary approach to DNA oncovirus virulence

One out of 10 cancers is estimated to arise from infections by a handful of oncogenic viruses. These infectious cancers constitute an opportunity for primary prevention through immunization against the viral infection, for early screening through molecular detection of the infectious agent, and potentially for specific treatments, by targeting the virus as a marker of cancer cells. Accomplishing these objectives will require a detailed understanding of the natural history of infections, the mechanisms by which the viruses contribute to disease, the mutual adaptation of viruses and hosts, and the possible viral evolution in the absence and in the presence of the public health interventions conceived to target them. This issue showcases the current developments in experimental tissue-like and animal systems, mathematical models and evolutionary approaches to understand DNA oncoviruses. Our global aim is to provide proximate explanations to the present-day interface and interactions between virus and host, as well as ultimate explanations about the adaptive value of these interactions and about the evolutionary pathways that have led to the current malignant phenotype of oncoviral infections. This article is part of the theme issue 'Silent cancer agents: multi-disciplinary modelling of human DNA oncoviruses'.

DNA Tumor Viruses and Their Contributions to Molecular Biology

This summer marks the 51st anniversary of the DNA tumor virus meetings. Scientists from around the world will gather in Trieste, Italy, to report their latest results and to agree or disagree on the current concepts that define our understanding of this diverse class of viruses. This article offers a brief history of the impact the study of these viruses has had on molecular and cancer biology and discusses obstacles and opportunities for future progress.