An association between human papillomavirus (HPV) infection and the development of cervical cancer was initially reported over 30 years ago, and today there is overwhelming evidence that certain subtypes of HPV are the causative agents of these malignancies. The p53 and retinoblastoma proteins are well-characterized targets of the HPV E6 and E7 oncoproteins, but recent studies have shown that the alteration of additional pathways are equally important for transformation. These additional factors are crucial regulators of cell cycle progression, telomere maintenance, apoptosis and chromosomal stability. Understanding how HPV oncoproteins modify these activities provides novel insights into the basic mechanisms of oncogenesis.

During the past 20 years, several types of human papillomaviruses (HPVs) have been identified that cause specific types of cancers. The etiology of cancer of the cervix has been linked to several types of HPV, with a high preponderance of HPV16. The role of these virus infections has been established 1) by the regular presence of HPV DNA in the respective tumor biopsy specimens, 2) by the demonstration of viral oncogene expression (E6 and E7) in tumor material, 3) by the identification of transforming properties of these genes, 4) by the requirement for E6 and E7 expression for maintaining the malignant phenotype of cervical carcinoma cell lines, 5) by the interaction of viral oncoproteins with growth-regulating host-cell proteins, and 6) by epidemiologic studies pointing to these HPV infections as the major risk factor for cervical cancer development. In addition to cancer of the cervix, a major proportion of anal, perianal, vulvar, and penile cancers appears to be linked to the same HPV infections. In addition, close to 20% of oropharyngeal cancers contain DNA from the same types of HPV. Recent evidence also points to a possible role of other HPV infections in squamous cell carcinomas of the skin. This review covers recent developments in understanding molecular mechanisms of HPV carcinogenesis, mainly discussing functions of viral oncoproteins and the regulation of viral oncogenes by host-cell factors. Modifications in host-cell genes, most likely engaged in the control of HPV gene expression in proliferating cells, emerge as important events in HPV-mediated carcinogenesis.

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer with an annual incidence of approximately 400,000 worldwide. Although the principal risk factors for head and neck cancer remain tobacco and alcohol use, human papillomavirus (HPV) has recently been found to be etiologically associated with 20 to 25% of HNSCC, mostly in the oropharynx. HPV causes human cancers by expressing two viral oncoproteins, E6 and E7. These oncoproteins degrade and destabilize two major tumor suppressor proteins, p53 and pRb, through ubiquitination. Additional studies have shown that E6 and E7 can directly bind to multiple host proteins other than p53 and pRb (e.g., Bak and p21(Cip1)), further contributing to genetic instability. However, expression of E6 and E7 alone is not sufficient for cellular transformation, and the additional genetic alterations necessary for malignant progression in the setting of virus-induced genomic instability are unknown. In addition to the etiological differences, HPV-positive cancers are clinically distinct when compared with HPV-negative cancers with regard to treatment response and survival outcome, with tumor HPV-positivity being a favorable prognostic biomarker. Further understanding of carcinogenesis and clinical behavior of HPV-positive cancers will improve disease prevention, patient care, and surveillance strategies for HNSCC patients.

Human papillomaviruses (HPV) are believed to be the primary causal agents for development of pre-neoplastic and malignant lesions of the uterine cervix, and high-risk types such as type 16 and 18 are associated with more than 90% of all cervical carcinomas. The E6 and E7 genes of HPV are thought to play causative roles, since E6 promotes the degradation of p53 through its interaction with E6AP, an E3 ubiquitin ligase, whereas E7 binds to the retinoblastoma protein (pRb) and disrupts its complex formation with E2F transcription factors. Although prophylactic vaccines have become available, it is still necessary to clarify the mechanisms of HPV-induced carcinogenesis because of the widespread nature of HPV infection. Approximately 493,000 new cases of cervical cancer are diagnosed each year with approximately 274,000 mortalities due to invasive cervical cancer. In the present article, the mechanisms of HPV16 E6- and E7-induced multistep carcinogenesis and recently identified functions of these onco-proteins are reviewed.

In studying biological roles of interferon regulatory factor (IRF)-1 tumor suppressor in cervical carcinogenesis, we found that HPV E7 is functionally associated with IRF-1. Binding assays indicate a physical interaction between IRF-1 and HPV E7 in vivo and in vitro. The carboxyl-terminal transactivation domain of IRF-1 was required for the interaction. Transient co-expression of E7 significantly inhibits the IRF-1-mediated activation of IFN-beta promoter in NIH-3T3 cells. Co-transfection of E7 mutants reveals that the pRb-binding portion of E7 is necessary for the E7-mediated inactivation of IRF-1. It was next determined whether histone deacetylase (HDAC) is involved in the inactivation mechanism as recently suggested, where the carboxyl-terminal zinc finger domain of E7 associates with NURD complex containing HDAC. When trichostatin A, an inhibitor of HDAC, was treated, the repressing activity of E7 was released in a dose-dependent manner. Furthermore, the mutation of zinc finger abrogates such activity without effect on the interaction with IRF-1. These results suggest that HPV E7 interferes with the transactivation function of IRF-1 by recruiting HDAC to the promoter. The immune-promoting role of IRF-1 evokes the idea that our novel finding might be important for the elucidation of the E7-mediated immune evading mechanism that is frequently found in cervical cancer.

We report here that the E7 oncoprotein encoded by the oncogenic human papillomavirus (HPV) type 16 binds to the glycolytic enzyme type M2 pyruvate kinase (M2-PK). M2-PK occurs in a tetrameric form with a high affinity to its substrate phosphoenolpyruvate and a dimeric form with a low affinity to phosphoenolpyruvate, and the transition between both conformations regulates the glycolytic flux in tumor cells. The glycolytic intermediate fructose 1, 6-bisphosphate induces the reassociation of the dimeric to the tetrameric form of M2-PK. The expression of E7 in an experimental cell line shifts the equilibrium to the dimeric state despite a significant increase in the fructose 1,6-bisphosphate levels. Investigations of HPV-16 E7 mutants and the nononcogenic HPV-11 subtype suggest that the interaction of HPV-16 E7 with M2-PK may be linked to the transforming potential of the viral oncoprotein.

Human papillomaviruses (HPVs) are a family of small double-stranded DNA viruses that have a tropism for the epithelia of the genital and upper respiratory tracts and for the skin. Approximately 150 HPV types have been discovered so far, which are classified into several genera based on their DNA sequence. Approximately 15 high-risk mucosal HPV types are clearly associated with cervical cancer; HPV16 and HPV18 are the most carcinogenic since they are responsible for approximately 50% and 20% of all cervical cancers worldwide, respectively. It is now also clear that these viruses are linked to a subset of other genital cancers, as well as head and neck cancers. Due to their high level of carcinogenic activity, HPV16 and HPV18 are the most studied HPV types so far. Biological studies have highlighted the key roles in cellular transformation of the products of two viral early genes, E6 and E7. Many of the mechanisms of E6 and E7 in subverting the regulation of fundamental cellular events have been fully characterized, contributing not only to our knowledge of how the oncogenic viruses promote cancer development but also to our understanding of basic cell biology. Despite HPV research resulting in extraordinary achievements in the last four decades, significantly improving the screening and prophylaxis of HPV-induced lesions, additional research is necessary to characterize the biology and epidemiology of the vast number of HPV types that have been poorly investigated so far, with a final aim of clarifying their potential roles in other human diseases.

Human papillomaviruses (HPVs) are the causative agents of a number of human cancers, of which cervical cancer is the most important. This occurs following persistent infection with a limited number of viral subtypes and is characterized by continued expression of the viral E6 and E7 oncoproteins. A unique characteristic of the cancer-causing HPV types is the presence of a PDZ recognition motif on the carboxy terminus of the E6 oncoprotein. Through this motif, E6 directs the proteasome-mediated degradation of cellular proteins involved in the regulation of cell polarity and in cell proliferation control. These include components of the Scrib and Par polarity complexes, as well as a number of other PDZ domain-containing substrates. Thus, PVs are now providing novel insights into the functioning of many of these cellular proteins, and into which of these functions, in particular, are relevant for maintaining normal cellular homeostasis. In this review, we discuss the biological consequences of papillomaviral targeting of these cell polarity regulators, both with respect to the viral life cycle and, most importantly, to the development of HPV-induced malignancy.

The E5 proteins are short transmembrane proteins encoded by many animal and human papillomaviruses. These proteins display transforming activity in cultured cells and animals, and they presumably also play a role in the productive virus life cycle. The E5 proteins are thought to act by modulating the activity of cellular proteins. Here, we describe the biological activities of the best-studied E5 proteins and discuss the evidence implicating specific protein targets and pathways in mediating these activities. The primary target of the 44-amino acid BPV1 E5 protein is the PDGF β receptor, whereas the EGF receptor appears to be an important target of the 83-amino acid HPV16 E5 protein. Both E5 proteins also bind to the vacuolar ATPase and affect MHC class I expression and cell-cell communication. Continued studies of the E5 proteins will elucidate important aspects of transmembrane protein-protein interactions, cellular signal transduction, cell biology, virus replication, and tumorigenesis.

Papillomaviruses (PVs) are established agents of human and animal cancers. They infect cutaneous and mucous epithelia. High Risk (HR) Human PVs (HPVs) are consistently associated with cancer of the uterine cervix, but are also involved in the etiopathogenesis of other cancer types. The early oncoproteins of PVs: E5, E6 and E7 are known to contribute to tumour progression. While the oncogenic activities of E6 and E7 are well characterised, the role of E5 is still rather nebulous. The widespread causal association of PVs with cancer makes their study worthwhile not only in humans but also in animal model systems. The Bovine PV (BPV) system has been the most useful animal model in understanding the oncogenic potential of PVs due to the pivotal role of its E5 oncoprotein in cell transformation. This review will highlight the differences between HPV-16 E5 (16E5) and E5 from other PVs, primarily from BPV. It will discuss the targeting of E5 as a possible therapeutic agent.

Over the last two decades, since the initial discovery of human papillomavirus (HPV) type 16 and 18 DNAs in cervical cancers by Dr. Harald zur Hausen (winner of the Nobel Prize in Physiology or Medicine, 2008), the HPVs have been well characterised as causative agents for cervical cancer. Viral DNA from a specific group of HPVs can be detected in at least 90% of all cervical cancers and two viral genes, E6 and E7, are invariably expressed in HPV-positive cervical cancer cells. Their gene products are known to inactivate the major tumour suppressors, p53 and retinoblastoma protein (pRB), respectively. In addition, one function of E6 is to activate telomerase, and E6 and E7 cooperate to effectively immortalise human primary epithelial cells. Though expression of E6 and E7 is itself not sufficient for cancer development, it seems to be either directly or indirectly involved in every stage of multi-step carcinogenesis. Epidemiological and biological studies suggest the potential efficacy of prophylactic vaccines to prevent genital HPV infection as an anti-cancer strategy. However, given the widespread nature of HPV infection and unresolved issues about the duration and type specificity of the currently available HPV vaccines, it is crucial that molecular details of the natural history of HPV infection as well as the biological activities of the viral oncoproteins be elucidated in order to provide the basis for development of new therapeutic strategies against HPV-associated malignancies. This review highlights novel functions of E6 and E7 as well as the molecular mechanisms of HPV-induced carcinogenesis.

Tuberous sclerosis complex (TSC) is a genetic disorder caused by mutations in either TSC1 or TSC2 tumor suppressor gene. TSC1 and TSC2 products, Harmatin and Tuberin, form the functional complex to serve as the negative regulator for insulin-induced phosphorylation of S6 kinase and elF4E-binding protein 1. High-risk human papillomavirus (HPV) infection is the necessary cause for cervical cancer. E6 oncoprotein encoded by HPV plays a pivotal role in carcinogenesis by interference with the host intracellular protein functions. In this study, we show that HPV16 E6 interacts with tumor suppressor gene TSC2 product, Tuberin, and results in the phosphorylation of S6 kinase and S6 even in the absence of insulin. The overexpression of Tuberin overcomes the effect of E6 on S6 kinase phosphorylation. Binding with HPV16 E6 causes the proteasome-mediated degradation of Tuberin. A DILG motif and an ELVG motif located in the carboxyl-terminal of Tuberin are required for E6 binding. In addition, the Tuberin interaction region in E6 has been mapped in the amino-terminal portion of HPV16 E6, which is different from the binding domain with p53. These results provide a possible link between E6-induced oncogenesis and the insulin-stimulated cell proliferation signaling pathway.

Human papillomaviruses (HPVs) infect epithelia and can lead to the development of lesions, some of which have malignant potential. HPV type 16 (HPV16) is the most oncogenic genotype and causes various types of cancer, including cervical, anal, and head and neck cancers. However, despite significant research, our understanding of the mechanism by which HPV16 binds to and enters host cells remains fragmented. Over several decades, many HPV receptors and entry pathways have been described. This review puts those studies into context and offers a model of HPV16 binding and entry as a framework for future research. Our model suggests that HPV16 binds to heparin sulfate proteoglycans (HSPGs) on either the epithelial cell surface or basement membrane through interactions with the L1 major capsid protein. Growth factor receptors may also become activated through HSPG/growth factor/HPV16 complexes that initiate signaling cascades during early virion-host cell interactions. After binding to HSPGs, the virion undergoes conformational changes, leading to isomerization by cyclophilin B and proprotein convertase-mediated L2 minor capsid protein cleavage that increases L2 N terminus exposure. Along with binding to HSPGs, HPV16 binds to α6 integrins, which initiate further intracellular signaling events. Following these primary binding events, HPV16 binds to a newly identified L2-specific receptor, the annexin A2 heterotetramer. Subsequently, clathrin-, caveolin-, lipid raft-, flotillin-, cholesterol-, and dynamin-independent endocytosis of HPV16 occurs.

The papillomavirus E5 proteins are short, hydrophobic transforming proteins. The transmembrane E5 protein encoded by bovine papillomavirus transforms cells by activating the platelet-derived growth factor beta receptor tyrosine kinase in a ligand-independent fashion. The bovine papillomavirus E5 protein forms a stable complex with the receptor, thereby inducing receptor dimerization and activation, trans-phosphorylation, and recruitment of cellular signaling proteins to the receptor. The E5 proteins of the human papillomaviruses also appear to affect the activity of growth factor receptors and their signaling pathways. The interaction of papillomavirus E5 proteins with a subunit of the vacuolar ATPase may also contribute to transformation. Further analysis of these unique mechanisms of viral transformation will yield new insight into the regulation of growth factor receptor activity and cellular signal transduction pathways.

Approximately 20% of all cancers are associated with infectious agents. Among them, human papillomaviruses (HPVs) are very common and are now recognized as the etiological agent of cervical cancer, the second most common cancer in women worldwide, and they are increasingly linked with other forms of dysplasia. Carcinogenesis is a complex and multistep process requiring the acquisition of several genetic and/or epigenetic alterations. HPV-induced neoplasia, however, is in part mediated by the intrinsic functions of the viral proteins. In order to replicate its genome, HPV modulates the cell cycle, while deploying mechanisms to escape the host immune response, cellular senescence and apoptosis. As such, HPV infection leads directly and indirectly to genomic instability, further favouring transforming genetic events and progression to malignancy. This review aims to summarize our current understanding of the molecular mechanisms exploited by HPV to induce neoplasia, with an emphasis on the role of the 2 viral oncoproteins E6 and E7. Greater understanding of the role of HPV proteins in these processes will ultimately aid in the development of antiviral therapies, as well as unravel general mechanisms of oncogenesis.

Infection with Human papillomaviruses (HPVs) leads to the development of a wide-range of cancers, accounting for 5% of all human cancers. A prominent example is cervical cancer, one of the leading causes of cancer death in women worldwide. It has been well established that tumor development and progression induced by HPV infection is driven by the sustained expression of two oncogenes E6 and E7. The expression of E6 and E7 not only inhibits the tumor suppressors p53 and Rb, but also alters additional signalling pathways that may be equally important for transformation. Among these pathways, the phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signalling cascade plays a very important role in HPV-induced carcinogenesis by acting through multiple cellular and molecular events. In this review, we summarize the frequent amplification of PI3K/Akt/mTOR signals in HPV-induced cancers and discuss how HPV oncogenes E6/E7/E5 activate the PI3K/Akt/mTOR signalling pathway to modulate tumor initiation and progression and affect patient outcome. Improvement of our understanding of the mechanism by which the PI3K/Akt/mTOR signalling pathway contributes to the immortalization and carcinogenesis of HPV-transduced cells will assist in devising novel strategies for preventing and treating HPV-induced cancers.

Over 250 PDZ (PSD95/Dlg/ZO-1) domain-containing proteins have been described in the human proteome. As many of these possess multiple PDZ domains, the potential combinations of associations with proteins that possess PBMs (PDZ-binding motifs) are vast. However, PDZ domain recognition is a highly specific process, and much less promiscuous than originally thought. Furthermore, a large number of PDZ domain-containing proteins have been linked directly to the control of processes whose loss, or inappropriate activation, contribute to the development of human malignancies. These regulate processes as diverse as cytoskeletal organization, cell polarity, cell proliferation and many signal transduction pathways. In the present review, we discuss how PBM-PDZ recognition and imbalances therein can perturb cellular homoeostasis and ultimately contribute to malignant progression.

Human papillomaviruses (HPVs) are small double-stranded circular DNA viruses with 8 kb genomes. So far, more than 150 HPVs have been identified, and 12 types of HPVs have been conclusively linked to cancer by the International Agency for Research on Cancer/World Health Organization. Expression of HPV E5, E6 and E7 oncoproteins can alter multiple signaling pathways to cause cancer. In this review, the signaling pathways activated by these oncoproteins are summarized, and targeted therapy against key signaling molecules is described. E6 can inactivate tumor protein 53 and PDZ (post synaptic density protein-drosophila disk large tumor suppressor-zonula occludens-1 proteins) while stimulating phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt), Wnt and Notch pathways. E7 can inhibit retinoblastoma protein and stimulate the PI3K/Akt pathway. Both E6 and E7 can deregulate cellular microRNA expression, which can alter cellular signaling pathways. E5 can sensitize epidermal growth factor receptor to epidermal growth factor to increase activation of PI3K/Akt and mitogen-activated protein kinase pathways. E5 can also inhibit the extrinsic apoptotic pathway. These altered signaling pathways could be critical for the initiation and maintenance of HPV-associated cancers. Therefore, targeted therapy against the key signaling molecules has therapeutic implications. Among these, the possibilities of targeting PI3K/Akt, mammalian target of rapamycin, epidermal growth factor receptor and vascular endothelial growth factor have been extensively studied in many cancers. Some inhibitors have been studied in cervical cancer in both animal models and clinical trials. Although the results are promising, further investigation is warranted.

Transcription of the catalytic subunit of telomerase (hTERT) in keratinocytes can be induced by human papillomavirus type 16 (HPV16) E6/E6AP ubiquitin ligase through degradation of the repressor, NFX1-91. Here, we demonstrate that NFX1-91 interacts with the corepressor complex mSin3A/histone deacetylase (HDAC) at the hTERT promoter. By degrading NFX1-91, E6/E6AP changes the chromatin structure at the hTERT promoter as indicated by enhanced acetylation of histones H3 and H4 as well as dimethylation of H3K4. Knockdown of NFX1-91 by short hairpin RNA (shRNA) mimics the effect of E6 and leads to acetylation of histones H3 and H4. Conversely, knockdown of E6AP by shRNA suppresses histone acetylation at the hTERT promoter. These data demonstrate that targeted degradation of NFX1-91 by E6/E6AP dissociates the mSin3A/HDAC complex from the hTERT promoter and induces hTERT transcription.

Cancer-causing HPV E6 oncoproteins are characterized by the presence of a PDZ binding motif (PBM) at their extreme carboxy terminus. It was long thought that this region of E6 had a sole function to confer interaction with a defined set of cellular substrates. However, more recent studies have shown that the E6 PBM has a complex pattern of regulation, whereby phosphorylation within the PBM can regulate interaction with two classes of cellular proteins: those containing PDZ domains and the members of the 14-3-3 family of proteins. In this review, we explore the roles that the PBM and its ligands play in the virus life cycle, and subsequently how these can inadvertently contribute towards the development of malignancy. We also explore how subtle alterations in cellular signal transduction pathways might result in aberrant E6 phosphorylation, which in turn might contribute towards disease progression.

The regulation of host-mediated apoptosis by the E6 and E7 oncoproteins has garnered attention because it is believed to be an important strategy employed by high-risk (HR)-human papillomaviruses (HPVs) to evade immune surveillance. Additionally, the revelation that E5 can protect cells from tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis suggests that it may also play a role in undermining host defense mechanisms. Cellular transformation is an unintended consequence of persistent infection by HR-HPVs, and it is therefore likely that the primary function of E5, E6 and E7 is to regulate cell survival throughout the normal viral life cycle in order to ensure viral replication and promote the spread of progeny. The purpose of this article is to review the literature on the regulation of host-mediated apoptosis by E5, E6 and E7 that describes the mechanisms employed by HR-HPVs to persist in the host and create the conditions necessary for cellular transformation.

Extensive experimental work has conclusively demonstrated that infection with certain types of human papillomaviruses, the so-called high-risk human papillomavirus (HR-HPV), represent a most powerful human carcinogen. However, neoplastic growth is a rare and inappropriate outcome in the natural history of HPV, and a number of other events have to concur in order to induce the viral infection into the (very rare) neoplastic transformation. From this perspective, a number of putative viral, host, and environmental co-factors have been proposed as potential candidates. Among them oxidative stress (OS) is an interesting candidate, yet comparatively underexplored. OS is a constant threat to aerobic organisms being generated during mitochondrial oxidative phosphorylation, as well as during inflammation, infections, ionizing irradiation, UV exposure, mechanical and chemical stresses. Epithelial tissues, the elective target for HPV infection, are heavily exposed to all named sources of OS. Two different types of cooperative mechanisms are presumed to occur between OS and HPV: I) The OS genotoxic activity and the HPV-induced genomic instability concur independently to the generation of the molecular damage necessary for the emergence of neoplastic clones. This first mode is merely a particular form of co-carcinogenesis; and II) OS specifically interacts with one or more molecular stages of neoplastic initiation and/or progression induced by the HPV infection. This manuscript was designed to summarize available data on this latter hypothesis. Experimental data and indirect evidences on promoting the activity of OS in viral infection and viral integration will be reviewed. The anti-apoptotic and pro-angiogenetic role of NO (nitric oxide) and iNOS (inducible nitric oxide synthase) will be discussed together with the OS/HPV cooperation in inducing cancer metabolism adaptation. Unexplored/underexplored aspects of the OS interplay with the HPV-driven carcinogenesis will be highlighted. The aim of this paper is to stimulate new areas of study and innovative approaches.

Research exploring the E6-p53 and E7-pRb model has resulted in the identity of the viral gene's actions on numerous cellular proteins and processes normally involved in cell growth and proliferation. Specially, several findings have established the various ways by which the HPV-infected cell may escape controls governing cell growth and proliferation, including the fidelity of the host cell's genome and apoptosis. A large body of knowledge already generated in this area supports the view that high-risk HPV types have the ability to transform cells into a malignant phenotype. Such ability, however, is not sufficient to actually and inevitably produce cervical carcinoma, as indicated by the frequent spontaneous clearance of HPV infection and the long delay between the onset of persistent infection and emergence of the malignancy. Delay in the participation of cofactors has been suggested as explanation in this regard. However, it remains unclear how and when cofactors or factors that are innate in the HPV-infected cells launch the host cells into an irreversible progression to carcinoma.

Cell signaling pathways are the mechanisms by which cells transduce external stimuli, which control the transcription of genes, to regulate diverse biological effects. In cancer, distinct signaling pathways, such as the Wnt/β-catenin pathway, have been implicated in the deregulation of critical molecular processes that affect cell proliferation and differentiation. For example, changes in β-catenin localization have been identified in Human Papillomavirus (HPV)-related cancers as the lesion progresses. Specifically, β-catenin relocates from the membrane/cytoplasm to the nucleus, suggesting that this transcription regulator participates in cervical carcinogenesis. The E6 and E7 oncoproteins are responsible for the transforming activity of HPV, and some studies have implicated these viral oncoproteins in the regulation of the Wnt/β-catenin pathway. Nevertheless, new interactions of HPV oncoproteins with cellular proteins are emerging, and the study of the biological effects of such interactions will help to understand HPV-related carcinogenesis. Viruses 2015, 7 4735 This review addresses the accumulated evidence of the involvement of the HPV E6 and E7 oncoproteins in the activation of the Wnt/β-catenin pathway.