Human papillomavirus 16 E5 oncogene contributes to two stages of skin carcinogenesis

Epidermal growth factor receptor-dependent stimulation of differentiation by human papillomavirus type 16 E5

Human papillomaviruses (HPVs) infect keratinocytes of stratified squamous epithelia, and persistent infection with high-risk HPV types, such as HPV16, may lead to the development of malignancies. HPV evades host immunity in part by linking its gene expression to the host differentiation program, and therefore relies on differentiation to complete its life cycle. Based on previous reports indicating that the HPV16 protein E5 is important in the late stages of the differentiation-dependent life cycle, we found that organotypic cultures harboring HPV16 genomes lacking E5 showed reduced markers of terminal differentiation compared to wild type HPV16-containing cultures. We found that epidermal growth factor receptor (EGFR) levels and activation were increased in an E5-depdendent manner in these tissues, and that EGFR promoted terminal differentiation and expression of the HPV16 L1 gene. These findings suggest a function for E5 in preserving the ability of HPV16 containing keratinocytes to differentiate, thus facilitating the production of new virus progeny.

Preclinical Models of Laryngeal Papillomavirus Infection: A Scoping Review

OBJECTIVE

Laryngeal human papillomavirus (HPV) infection causes recurrent respiratory papillomatosis (RRP) and accounts for up to 25% of laryngeal cancers. Lack of satisfactory preclinical models is one reason that treatments for these diseases are limited. We sought to assess the literature describing preclinical models of laryngeal papillomavirus infection.

DATA SOURCES

PubMed, Web of Science, and Scopus were searched from the inception of database through October 2022.

REVIEW METHODS

Studies searched were screened by two investigators. Eligible studies were peer-reviewed, published in English, presented original data, and described attempted models of laryngeal papillomavirus infection. Data examined included type of papillomavirus, infection model, and results including success rate, disease phenotype, and viral retention.

RESULTS

After screening 440 citations and 138 full-text studies, 77 studies published between 1923 and 2022 were included. Models used low-risk HPV or RRP (n = 51 studies), high-risk HPV or laryngeal cancer (n = 16), both low- and high-risk HPV (n = 1), and animal papillomaviruses (n = 9). For RRP, 2D and 3D cell culture models and xenografts retained disease phenotypes and HPV DNA in the short term. Two laryngeal cancer cell lines were consistently HPV-positive in multiple studies. Animal laryngeal infections with animal papillomaviruses resulted in disease and long-term retention of viral DNA.

CONCLUSIONS

Laryngeal papillomavirus infection models have been researched for 100 years and primarily involve low-risk HPV. Most models lose viral DNA after a short duration. Future work is needed to model persistent and recurrent diseases, consistent with RRP and HPV-positive laryngeal cancer.

LEVEL OF EVIDENCE

NA Laryngoscope, 133:3256-3268, 2023.

Molecular Mechanisms of MmuPV1 E6 and E7 and Implications for Human Disease

Human papillomaviruses (HPVs) cause a substantial amount of human disease from benign disease such as warts to malignant cancers including cervical carcinoma, head and neck cancer, and non-melanoma skin cancer. Our ability to model HPV-induced malignant disease has been impeded by species specific barriers and pre-clinical animal models have been challenging to develop. The recent discovery of a murine papillomavirus, MmuPV1, that infects laboratory mice and causes the same range of malignancies caused by HPVs provides the papillomavirus field the opportunity to test mechanistic hypotheses in a genetically manipulatable laboratory animal species in the context of natural infections. The E6 and E7 proteins encoded by high-risk HPVs, which are the HPV genotypes associated with human cancers, are multifunctional proteins that contribute to HPV-induced cancers in multiple ways. In this review, we describe the known activities of the MmuPV1-encoded E6 and E7 proteins and how those activities relate to the activities of HPV E6 and E7 oncoproteins encoded by mucosal and cutaneous high-risk HPV genotypes.

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 type 16 E6 induces cell competition

High-risk human papillomavirus (HPV) infections induce squamous epithelial tumors in which the virus replicates. Initially, the virus-infected cells are untransformed, but expand in both number and area at the expense of uninfected squamous epithelial cells. We have developed an in vitro assay in which colonies of post-confluent HPV16 expressing cells outcompete and displace confluent surrounding uninfected keratinocytes. The enhanced colony competition induced by the complete HPV16 genome is conferred by E6 expression alone, not by individual expression of E5 or E7, and requires E6 interaction with p53. E6-expressing keratinocytes undermine and displace adjacent normal keratinocytes from contact with the attachment substrate, thereby expanding the area of the E6-expressing colony at the expense of normal keratinocytes. These new results separate classic oncogenicity that is primarily conferred by HPV16 E7 from cell competition that we show is primarily conferred by E6 and provides a new biological role for E6 oncoproteins from high-risk human papillomaviruses.

Microbial infections can change the fate and behavior of normal vertebrate cells to resemble oncogenic cells. High-risk papillomaviruses induce infected squamous epithelial cells to form tumors, some of which evolve into malignancies. The present work shows that the enhanced competitiveness of HPV16-infected cells for the basal cell surface is primarily due to the expression of the E6 oncoprotein and not the E7 or E5 oncoproteins. Compared to normal keratinocytes, E6 induces a super-competitor phenotype while E5 and E7 do not. This work shows the importance of measuring oncoprotein traits not only as cell autonomous traits, but in the context of competition with uninfected cells and shows the potential of papillomavirus oncoproteins to be novel genetic probes for the analysis of cell competition.

HPV16 E6 gene polymorphisms and the functions of the mutation site in cervical cancer among Uygur ethnic and Han nationality women in Xinjiang, China

BACKGROUND

To investigate the genotype distribution of human papillomavirus (HPV) in infected Uygur and Han women in Xinjiang, China; analyze the HPV16 E6 gene polymorphism site and relationship with the development of cervical cancer.

METHODS

The HPV16 E6 sequence was analyzed using the European standard prototype to perform an evolutionary tree. HPV16 E6-T295/T350, G295/G350, and T295/G350 GV230 vectors were stably transfected into cervical cancer C33A cells to analyze the cell proliferation, migration and invasion, apoptosis by CCK8 and clonogenic assays, transwell and cell scratch assays, FACS experiments.

RESULTS

The total HPV infection rate was 26.390% (760/2879), whereas the Uygur 22.87% (196/857) and the Han was 27.89% (564/2022) (P < 0.05). Among 110 mutations, 65 cases of E6 genes were mutated at nucleotide 350 (T350G) with the leucine changing to valine (L83V). Moreover, there were 7 cases of E6 gene mutated at nucleotide 295 (T295G) with aspartic changing to glutamic (D64E). When E6 vector(s) of mutations sites were transfected into C33A cells, they were found to promote cellular proliferation, migration, invasion, and inhibit apoptosis. T295/G350-E6 was significantly stronger than G295/G350 and T295/T350, G295/G350 was significantly stronger than T295/T350 (P < 0.05). The T295/G350 had the strongest effect on C33A cells and G295/G350 was significantly stronger than T295/T350 (P < 0.05).

CONCLUSIONS

The positive HPV infection rates differed between the Uygur and Han in Xinjiang, China, and the genotype distribution of infection was different. After transfecting C33A cells with different eukaryotic expression vectors, the T295/G350 mutation site promoted the proliferation, migration, and invasion of C33A cells to a greater extent than G295/G350; however, G295/G350 had a stronger effect than T295/T350.

Human Papillomaviruses as Infectious Agents in Gynecological Cancers―Oncogenic Properties of Viral Proteins

Human papillomaviruses (HPVs), which belong to the Papillomaviridae family, constitute a group of small nonenveloped double-stranded DNA viruses. HPV has a small genome that only encodes a few proteins, and it is also responsible for 5% of all human cancers, including cervical, vaginal, vulvar, penile, anal, and oropharyngeal cancers. HPV types may be classified as high- and low-risk genotypes (HR-HPVs and LR-HPVs, respectively) according to their oncogenic potential. HR-HPV 16 and 18 are the most common types worldwide and are the primary types that are responsible for most HPV-related cancers. The activity of the viral E6 and E7 oncoproteins, which interfere with critical cell cycle points such as suppressive tumor protein p53 (p53) and retinoblastoma protein (pRB), is the major contributor to HPV-induced neoplastic initiation and progression of carcinogenesis. In addition, the E5 protein might also play a significant role in tumorigenesis. The role of HPV in the pathogenesis of gynecological cancers is still not fully understood, which indicates a wide spectrum of potential research areas. This review focuses on HPV biology, the distribution of HPVs in gynecological cancers, the properties of viral oncoproteins, and the molecular mechanisms of carcinogenesis.

The Mouse Papillomavirus Epigenetic Signature Is Characterised by DNA Hypermethylation after Lesion Regression

Papillomaviruses (PVs) are double-stranded DNA tumour viruses that can infect cutaneous and mucosal epidermis. Human papillomavirus (HPV) types have been linked to the causality of cutaneous squamous cell carcinoma (cSCC); however, HPV DNA is not always detected in the resultant tumour. DNA methylation is an epigenetic change that can contribute to carcinogenesis. We hypothesise that the DNA methylation pattern in cells is altered following PV infection. We tested if DNA methylation was altered by PV infection in the mouse papillomavirus (MmuPV1) model. Immunosuppressed mice were infected with MmuPV1 on cutaneous tail skin. Immunosuppression was withdrawn for some mice, causing lesions to spontaneously regress. Reduced representation bisulphite sequencing was carried out on DNA from the actively infected lesions, visibly regressed lesions, and mock-infected control mice. DNA methylation libraries were generated and analysed for differentially methylated regions throughout the genome. The presence of MmuPV1 sequences was also assessed. We identified 834 predominantly differentially hypermethylated fragments in regressed lesions, and no methylation differences in actively infected lesions. The promoter regions of genes associated with tumorigenicity, including the tumour suppressor protein DAPK1 and mismatch repair proteins MSH6 and PAPD7, were hypermethylated. Viral DNA was detected in active lesions and in some lesions that had regressed. This is the first description of the genome-wide DNA methylation landscape for active and regressed MmuPV1 lesions. We propose that the DNA hypermethylation in the regressed lesions that we report here may increase the susceptibility of cells to ultraviolet-induced cSCC.

The human papillomavirus oncoproteins: a review of the host pathways targeted on the road to transformation

Persistent infection with high-risk human papillomaviruses (HR-HPVs) is the causal factor in over 99 % of cervical cancer cases, and a significant proportion of oropharyngeal and anogenital cancers. The key drivers of HPV-mediated transformation are the oncoproteins E5, E6 and E7. Together, they act to prolong cell-cycle progression, delay differentiation and inhibit apoptosis in the host keratinocyte cell in order to generate an environment permissive for viral replication. The oncoproteins also have key roles in mediating evasion of the host immune response, enabling infection to persist. Moreover, prolonged infection within the cellular environment established by the HR-HPV oncoproteins can lead to the acquisition of host genetic mutations, eventually culminating in transformation to malignancy. In this review, we outline the many ways in which the HR-HPV oncoproteins manipulate the host cellular environment, focusing on how these activities can contribute to carcinogenesis.

The Merkel Cell Polyomavirus T Antigens Function as Tumor Promoters in Murine Skin

Simple Summary

Merkel cell polyomavirus, a recently discovered human virus, is linked to the development of a rare form of skin cancer called Merkel cell carcinoma. The virus does not replicate in cancer cells, yet there is continued expression of viral proteins known as T antigens. The T antigens are believed to contribute to Merkel cell carcinoma development, yet how they do so remains an active area of research. In this study, we used transgenic mice expressing the viral T antigens in their skin to determine at which stage of skin cancer development these viral proteins function. We discovered that the Merkel cell polyomavirus T antigens function as tumor promoters, rather than tumor initiators, in the skin. These findings suggest that other tumor-initiating events may cooperate with the tumor-promoting activities of the viral T antigens, thus providing important insight into how Merkel cell polyomavirus can cause cancer in human skin.

Abstract

Merkel cell polyomavirus (MCPyV) causes the majority of human Merkel cell carcinomas (MCC), a rare but highly aggressive form of skin cancer. We recently reported that constitutive expression of MCC tumor-derived MCPyV tumor (T) antigens in the skin of transgenic mice leads to hyperplasia, increased proliferation, and spontaneous epithelial tumor development. We sought to evaluate how the MCPyV T antigens contribute to tumor formation in vivo using a classical, multi-stage model for squamous cell carcinoma development. In this model, two chemical carcinogens, DMBA and TPA, contribute to two distinct phases of carcinogenesis—initiation and promotion, respectively—that are required for tumors to develop. By treating the MCPyV transgenic mice with each chemical carcinogen, we determined how the viral oncogenes contributed to carcinogenesis. We observed that the MCPyV T antigens synergized with the tumor initiator DMBA, but not with the tumor promoter TPA, cause tumors. Therefore, the MCPyV tumor antigens function primarily as tumor promoters, similar to that seen with human papillomavirus (HPV) oncoproteins. These studies provide insight into the role of MCPyV T antigen expression in tumor formation in vivo and contribute to our understanding of how MCPyV may function as a human DNA tumor virus.

The oncogenic pathways of papillomaviruses

Papillomaviruses are oncogenic DNA viruses and induce hyperplastic benign lesions of both cutaneous and mucosal tissues in their various hosts, including many domestic and wild animals as well as humans. There are some Papillomavirus genotypes that can infect hosts different from their own, such as BPV 1 and BPV 2 originated from cattle, which can also infect horses and are responsible for fibroblastic tumours in horses. This review article summarizes the origin and evolution of papillomaviruses as an etiological agent in the historical process. The main focus in this review is the evaluation of the interactions between high-risk papillomavirus oncoproteins and programmed cell-death pathways. It further exemplifies the role of these interactions in the malignant cell transformation process. In parallel with this, the use and importance of the bovine model system to enlighten the papillomavirus-associated cancers is discussed with an in-depth examination. Furthermore, it focuses on the epidemiological situation of BPV infections in Turkey in the cattle herds.

Cellular Functions of HPV16 E5 Oncoprotein during Oncogenic Transformation

The human papillomavirus (HPV) is recognized as the main etiologic agent associated with cervical cancer. HPVs are epitheliotropic, and the ones that infect the mucous membranes are classified into low-risk (LR) and high-risk (HR) types. LR-HPVs produce benign lesions, whereas HR-HPVs produce lesions that may progress to cancer. HR-HPV types 16 and 18 are the most frequently found in cervical cancer worldwide. E6 and E7 are the major HPV oncogenic proteins, and they have been profusely studied. Moreover, it has been shown that the HPV16 E5 (16E5) oncoprotein generates transformation, although the molecular mechanisms through which it carries out its activity have not been well defined. In contrast to E6 and E7, the E5 open reading frame is lost during the integration of the episomal HPV DNA into the cellular genome. This suggests that E5 acts at the early stages of the transformation process. In this review, we focused on the biochemical characteristics and functions of the HPV E5 oncoprotein, mainly on its association with growth factor receptors and other cellular proteins. Knowledge of the HPV E5 biology is important to understand the role of this oncoprotein in maintaining the viral cycle through the modulation of proliferation, differentiation, and apoptosis, as well as the alteration of other processes, such as survival, adhesion, migration, and invasion during early carcinogenesis. Finally, we summarized recent research that uses the E5 oncoprotein as a therapeutic target, promising a novel approach to the treatment of cervical cancer in its early stages.

The human papillomavirus 16 E5 gene potentiates MmuPV1-Dependent pathogenesis

The papillomavirus E5 gene contributes to transformation and tumorigenesis; however, its exact function in these processes and viral pathogenesis is unclear. While E5 is present in high-risk mucosotropic HPVs that cause anogenital and head and neck cancers, it is absent in cutaneous HPVs and the recently discovered mouse papillomavirus (MmuPV1), which causes papillomas and squamous cell carcinomas of the skin and mucosal epithelia in laboratory mice. We infected K14E5 transgenic mice, which express the high-risk mucosotropic HPV16 E5 gene in stratified epithelia, with MmuPV1 to investigate the effects of E5 on papillomavirus-induced pathogenesis. Skin lesions in MmuPV1-infected K14E5 mice had earlier onset, higher incidence, and reduced frequency of spontaneous regression compared to those in non-transgenic mice. K14E5 mice were also more susceptible to cervicovaginal cancers when infected with MmuPV1 and treated with estrogen compared to non-transgenic mice. Our studies support the hypothesis that E5 contributes to papillomavirus-induced pathogenesis.

The human CIB1-EVER1-EVER2 complex governs keratinocyte-intrinsic immunity to β-papillomaviruses

Patients with epidermodysplasia verruciformis (EV) and biallelic null mutations of TMC6 (encoding EVER1) or TMC8 (EVER2) are selectively prone to disseminated skin lesions due to keratinocyte-tropic human β-papillomaviruses (β-HPVs), which lack E5 and E8. We describe EV patients homozygous for null mutations of the CIB1 gene encoding calcium- and integrin-binding protein-1 (CIB1). CIB1 is strongly expressed in the skin and cultured keratinocytes of controls but not in those of patients. CIB1 forms a complex with EVER1 and EVER2, and CIB1 proteins are not expressed in EVER1- or EVER2-deficient cells. The known functions of EVER1 and EVER2 in human keratinocytes are not dependent on CIB1, and CIB1 deficiency does not impair keratinocyte adhesion or migration. In keratinocytes, the CIB1 protein interacts with the HPV E5 and E8 proteins encoded by α-HPV16 and γ-HPV4, respectively, suggesting that this protein acts as a restriction factor against HPVs. Collectively, these findings suggest that the disruption of CIB1-EVER1-EVER2-dependent keratinocyte-intrinsic immunity underlies the selective susceptibility to β-HPVs of EV patients.

Alkyl-imino sugars inhibit the pro-oncogenic ion channel function of human papillomavirus (HPV) E5

Despite the availability of prophylactic vaccines the burden of human papillomavirus (HPV) associated malignancy remains high and there is a need to develop additional therapeutic strategies to complement vaccination. We have previously shown that the poorly characterised E5 oncoprotein forms a virus-coded ion channel or viroporin that was sensitive to the amantadine derivative rimantadine. We now demonstrate that alkylated imino sugars, which have antiviral activity against a number of viruses, inhibit E5 channel activity in vitro. Using molecular modelling we predict that imino sugars intercalate between E5 protomers to prevent channel oligomerisation. We explored the ability of these viroporin inhibitors to block E5-mediated activation of mitogenic signalling in keratinocytes. Treatment with either rimantadine or imino sugars prevented ERK-MAPK phosphorylation and reduced cyclin B1 expression in cells expressing E5 from a number of high-risk HPV types. Moreover, viroporin inhibitors also reduced ERK-MAPK activation and cyclin B1 expression in differentiating primary human keratinocytes containing high-risk HPV18. These observations provide evidence of a key role for E5 viroporin function during the HPV life cycle. Viroporin inhibitors could be utilised for stratified treatment of HPV associated tumours prior to virus integration, or as true antiviral therapies to eliminate virus prior to malignant transformation.

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Imino sugars inhibit the viroporin activity of the E5 oncoprotein.

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Imino sugars likely interact at E5 protomer interfaces within a channel to prevent oligomerisation.

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Imino sugars and adamantanes block mitogenic signalling mediated by E5 from a range of high-risk HPV types.

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Viroporin inhibitors reduce mitogenic signalling in differentiating primary keratinocytes containing high-risk HPV18.

Murine HPV16 E7-expressing transgenic skin effectively emulates the cellular and molecular features of human high-grade squamous intraepithelial lesions

Currently available vaccines prevent HPV infection and development of HPV-associated malignancies, but do not cure existing HPV infections and dysplastic lesions. Persistence of infection(s) in immunocompetent patients may reflect induction of local immunosuppressive mechanisms by HPV, providing a target for therapeutic intervention. We have proposed that a mouse, expressing HPV16 E7 oncoprotein under a Keratin 14 promoter (K14E7 mice), and which develops epithelial hyperplasia, may assist with understanding local immune suppression mechanisms that support persistence of HPV oncogene-induced epithelial hyperplasia. K14E7 skin grafts recruit immune cells from immunocompetent hosts, but consistently fail to be rejected. Here, we review the literature on HPV-associated local immunoregulation, and compare the findings with published observations on the K14E7 transgenic murine model, including comparison of the transcriptome of human HPV-infected pre-malignancies with that of murine K14E7 transgenic skin. We argue from the similarity of i) the literature findings and ii) the transcriptome profiles that murine K14E7 transgenic skin recapitulates the cellular and secreted protein profiles of high-grade HPV-associated lesions in human subjects. We propose that the K14E7 mouse may be an appropriate model to further study the immunoregulatory effects of HPV E7 expression, and can facilitate development and testing of therapeutic vaccines.

Inhibition of TGF-β and NOTCH Signaling by Cutaneous Papillomaviruses

Infections with cutaneous papillomaviruses have been linked to cutaneous squamous cell carcinomas that arise in patients who suffer from a rare genetic disorder, epidermodysplasia verruciformis, or those who have experienced long-term, systemic immunosuppression following organ transplantation. The E6 proteins of the prototypical cutaneous human papillomavirus (HPV) 5 and HPV8 inhibit TGF-β and NOTCH signaling. The Mus musculus papillomavirus 1, MmuPV1, infects laboratory mouse strains and causes cutaneous skin warts that can progress to squamous cell carcinomas. MmuPV1 E6 shares biological and biochemical activities with HPV8 E6 including the ability to inhibit TGF-β and NOTCH signaling by binding the SMAD2/SMAD3 and MAML1 transcription factors, respectively. Inhibition of TGF-β and NOTCH signaling is linked to delayed differentiation and sustained proliferation of differentiating keratinocytes. Furthermore, the ability of MmuPV1 E6 to bind MAML1 is necessary for wart and cancer formation in experimentally infected mice. Hence, experimental MmuPV1 infection in mice will be a robust and valuable experimental system to dissect key aspects of cutaneous HPV infection, pathogenesis, and carcinogenesis.

Transgenic Mouse Models of Tumor Virus Action

Genetically engineered mice (GEMs) have provided valuable insights into the carcinogenic properties of various human tumor viruses, which, in aggregate, are etiologically associated with over 15% of all human cancers. This review provides an overview of seminal discoveries made through the use of GEM models for human DNA tumor viruses. Emphasis is placed on the discoveries made in the study of human papillomaviruses, Merkel cell carcinoma-associated polyomavirus, Epstein-Barr virus, and Kaposi's sarcoma-associated herpesvirus, because GEMs have contributed extensively to our understanding of how these DNA tumor viruses directly contribute to human cancers.

The human papillomavirus replication cycle, and its links to cancer progression: a comprehensive review

HPVs (human papillomaviruses) infect epithelial cells and their replication cycle is intimately linked to epithelial differentiation. There are over 200 different HPV genotypes identified to date and each displays a strict tissue specificity for infection. HPV infection can result in a range of benign lesions, for example verrucas on the feet, common warts on the hands, or genital warts. HPV infects dividing basal epithelial cells where its dsDNA episomal genome enters the nuclei. Upon basal cell division, an infected daughter cell begins the process of keratinocyte differentiation that triggers a tightly orchestrated pattern of viral gene expression to accomplish a productive infection. A subset of mucosal-infective HPVs, the so-called ‘high risk’ (HR) HPVs, cause cervical disease, categorized as low or high grade. Most individuals will experience transient HR-HPV infection during their lifetime but these infections will not progress to clinically significant cervical disease or cancer because the immune system eventually recognizes and clears the virus. Cancer progression is due to persistent infection with an HR-HPV. HR-HPV infection is the cause of >99.7% cervical cancers in women, and a subset of oropharyngeal cancers, predominantly in men. HPV16 (HR-HPV genotype 16) is the most prevalent worldwide and the major cause of HPV-associated cancers. At the molecular level, cancer progression is due to increased expression of the viral oncoproteins E6 and E7, which activate the cell cycle, inhibit apoptosis, and allow accumulation of DNA damage. This review aims to describe the productive life cycle of HPV and discuss the roles of the viral proteins in HPV replication. Routes to viral persistence and cancer progression are also discussed.

Functional variants of human papillomavirus type 16 demonstrate host genome integration and transcriptional alterations corresponding to their unique cancer epidemiology

BACKGROUND

Human papillomaviruses (HPVs) are a worldwide burden as they are a widespread group of tumour viruses in humans. Having a tropism for mucosal tissues, high-risk HPVs are detected in nearly all cervical cancers. HPV16 is the most common high-risk type but not all women infected with high-risk HPV develop a malignant tumour. Likely relevant, HPV genomes are polymorphic and some HPV16 single nucleotide polymorphisms (SNPs) are under evolutionary constraint instigating variable oncogenicity and immunogenicity in the infected host.

RESULTS

To investigate the tumourigenicity of two common HPV16 variants, we used our recently developed, three-dimensional organotypic model reminiscent of the natural HPV infectious cycle and conducted various "omics" and bioinformatics approaches. Based on epidemiological studies we chose to examine the HPV16 Asian-American (AA) and HPV16 European Prototype (EP) variants. They differ by three non-synonymous SNPs in the transforming and virus-encoded E6 oncogene where AAE6 is classified as a high- and EPE6 as a low-risk variant. Remarkably, the high-risk AAE6 variant genome integrated into the host DNA, while the low-risk EPE6 variant genome remained episomal as evidenced by highly sensitive Capt-HPV sequencing. RNA-seq experiments showed that the truncated form of AAE6, integrated in chromosome 5q32, produced a local gene over-expression and a large variety of viral-human fusion transcripts, including long distance spliced transcripts. In addition, differential enrichment of host cell pathways was observed between both HPV16 E6 variant-containing epithelia. Finally, in the high-risk variant, we detected a molecular signature of host chromosomal instability, a common property of cancer cells.

CONCLUSIONS

We show how naturally occurring SNPs in the HPV16 E6 oncogene cause significant changes in the outcome of HPV infections and subsequent viral and host transcriptome alterations prone to drive carcinogenesis. Host genome instability is closely linked to viral integration into the host genome of HPV-infected cells, which is a key phenomenon for malignant cellular transformation and the reason for uncontrolled E6 oncogene expression. In particular, the finding of variant-specific integration potential represents a new paradigm in HPV variant biology.

Viroporins: structure, function and potential as antiviral targets

The channel-forming activity of a family of small, hydrophobic integral membrane proteins termed 'viroporins' is essential to the life cycles of an increasingly diverse range of RNA and DNA viruses, generating significant interest in targeting these proteins for antiviral development. Viroporins vary greatly in terms of their atomic structure and can perform multiple functions during the virus life cycle, including those distinct from their role as oligomeric membrane channels. Recent progress has seen an explosion in both the identification and understanding of many such proteins encoded by highly significant pathogens, yet the prototypic M2 proton channel of influenza A virus remains the only example of a viroporin with provenance as an antiviral drug target. This review attempts to summarize our current understanding of the channel-forming functions for key members of this growing family, including recent progress in structural studies and drug discovery research, as well as novel insights into the life cycles of many viruses revealed by a requirement for viroporin activity. Ultimately, given the successes of drugs targeting ion channels in other areas of medicine, unlocking the therapeutic potential of viroporins represents a valuable goal for many of the most significant viral challenges to human and animal health.

Human papillomavirus E5 oncoprotein: function and potential target for antiviral therapeutics

ABSTRACT 

Mucosal human papillomaviruses express a small, hydrophobic, protein called E5, which plays an important role in the HPV life cycle by delaying normal epithelial cell differentiation while maintaining cell cycle progression. In addition, E5 exhibits transforming abilities in a number of cell culture systems and transgenic mouse models. Lacking any described enzymatic activity, E5 is thought to function by binding to host proteins and modulating their activities. In particular, members of the growth factor receptor family are known targets for subversion. This review article summarizes our latest understanding of this enigmatic oncoprotein, including its role in the HPV life cycle, interactions with host proteins and contribution toward tumorigenesis.

Disruption of Bcl-2 and Bcl-xL by viral proteins as a possible cause of cancer

The Bcl proteins play a critical role in apoptosis, as mutations in family members interfere with normal programmed cell death. Such events can cause cell transformation, potentially leading to cancer. Recent discoveries indicate that some viral proteins interfere with Bcl proteins either directly or indirectly; however, these data have not been systematically described. Some viruses encode proteins that reprogramme host cellular signalling pathways controlling cell differentiation, proliferation, genomic integrity, cell death, and immune system recognition. This review analyses and summarises the existing data and discusses how viral proteins interfere with normal pro- and anti-apoptotic functions of Bcl-2 and Bcl-xL. Particularly, this article focuses on how viral proteins, such as Herpesviruses, HTLV-1, HPV and HCV, block apoptosis and how accumulation of such interference predisposes cancer development. Finally, we discuss possible ways to prevent and treat cancers using a combination of traditional therapies and antiviral preparations that are effective against these viruses.

Human papillomavirus E5 oncoproteins bind the A4 endoplasmic reticulum protein to regulate proliferative ability upon differentiation

Human papillomaviruses (HPV) infect stratified epithelia and link their life cycles to epithelial differentiation. The HPV E5 protein plays a role in the productive phase of the HPV life cycle but its mechanism of action is still unclear. We identify a new binding partner of E5, A4, using a membrane-associated yeast-two hybrid system. The A4 protein co-localizes with HPV 31 E5 in perinuclear regions and forms complexes with E5 and Bap31. In normal keratinocytes, A4 is found primarily in basal cells while in HPV positive cells high levels of A4 are seen in both undifferentiated and differentiated cells. Reduction of A4 expression by shRNAs, enhanced HPV genome amplification and increased cell proliferation ability following differentiation but this was not seen in cells lacking E5. Our studies suggest that the A4 protein is an important E5 binding partner that plays a role in regulating cell proliferation ability upon differentiation.

HPV16 E5 and KGFR/FGFR2b interplay in differentiating epithelial cells

The E5 oncogenic protein of the human papillomavirus type 16 (HPV16 E5) cooperates in epithelial transformation perturbing the behaviour of differentiating suprabasal cells. Among the receptor tyrosine kinases deregulated by 16E5 expression, the key paracrine mediator of epithelial homeostasis keratinocyte growth factor receptor (KGFR/FGFR2b) is altered in its signaling and endocytic traffic in undifferentiated keratinocytes expressing 16E5 and it would represent a major target of the viral protein in differentiated cells. With the aim to specifically address the possible interplay of 16E5 with KGFR/FGFR2b in cells already committed to differentiation, we took advantage of an in vitro model for forced overexpression or depletion of KGFR in E5 expressing human keratinocytes under synchronous waves of differentiation. Quantitative RT-PCR, biochemical and immunofluorescence analysis showed that KGFR down-modulation is responsible for a E5-mediated decrease of the early differentiation marker K1 and that the receptor re-expression as well as triggering of its kinase activity and signaling are able to efficiently counteract the impairment of differentiation, providing a further demonstration of the tumor-suppressive role of KGFR in the new unexplored context of HPV16 E5-mediated carcinogenesis. In addition, KGFR induced a ligand-dependent decrease of p63 through a miR-203 independent mechanism and this effect was blocked by inhibition of the PI3K/Akt signaling, which is the main pathway involved in KGFR-dependent keratinocyte differentiation, suggesting that alterations of the KGFR/p63 crosstalk are responsible for the impairment of keratinocyte differentiation induced by 16E5 and that the opposite tumor-suppressive action of KGFR and oncogenic role of E5 might both involve p63.

The role of human papillomaviruses in oncogenesis

Human papillomaviruses (HPVs) are the causative agents of cervical and other anogenital as well as oral cancers. Approximately fifty percent of virally induced cancers in the USA are associated with HPV infections. HPVs infect stratified epithelia and link productive replication with differentiation. The viral oncoproteins, E6, E7, and E5, play important roles in regulating viral functions during the viral life cycle and also contribute to the development of cancers. p53 and Rb are two major targets of the E6 and E7 oncoproteins, but additional cellular proteins also play important roles. E5 plays an auxiliary role in contributing to the development of cancers. This review will discuss the various targets of these viral proteins and what roles they play in viral pathogenesis.

The E5 proteins

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.

Human papillomavirus proteins as prospective therapeutic targets

Human papillomaviruses (HPV) are the causative agents of a subset of cervical cancers that are associated with persistent viral infection. The HPV genome is an ∼8 kb circle of double-stranded DNA that encodes eight viral proteins, among which the products of the E6 and E7 open reading frames are recognized as being the primary HPV oncogenes. E6 and E7 are expressed in pre-malignant lesions as well as in cervical cancers; hence these proteins have been extensively studied as potential targets for HPV therapies and novel vaccines. Here we review the expression and functions of E6 and E7 in the viral vegetative cycle and in oncogenesis. We also explore the expression and functions of other HPV proteins, including those with oncogenic properties, and discuss the potential of these molecules as alternative therapeutic targets.

Epstein-Barr virus-encoded latent membrane protein 1 (LMP1) and LMP2A function cooperatively to promote carcinoma development in a mouse carcinogenesis model

The Epstein-Barr virus (EBV) proteins latent membrane proteins 1 and 2 (LMP1 and LMP2) are frequently expressed in EBV-associated lymphoid and epithelial cancers and have complex effects on cell signaling and growth. The effects of these proteins on epithelial cell growth were assessed in vivo using transgenic mice driven by the keratin 14 promoter (K14). The development of papillomas and carcinomas was determined in the tumor initiator and promoter model using dimethyl benzanthracene (DMBA), followed by repeated treatments of 12-O-tetradecanoyl phorbol 13-acetate (TPA). In these assays, LMP1 functioned as a weak tumor promoter and increased papilloma formation. In contrast, mice expressing LMP2A did not induce or promote papilloma formation. Transgenic LMP1 mice had slightly increased development of squamous cell carcinoma; however, the development of carcinoma was significantly increased in the doubly transgenic mice expressing both LMP1 and LMP2A. DMBA treatment induces an activating mutation in the Harvey-ras (H-ras(61)) oncogene, and this mutation was identified in most papillomas and carcinomas although several papillomas and carcinomas in K14-LMP1 and K14-LMP1/LMP2A mice lacked the mutation. Analysis of signaling pathways that are known to be activated by LMP1 and/or LMP2 indicated that all genotypes had high levels of activated extracellular signal-regulated kinase (ERK) and Stat3 in carcinomas with significantly higher activation in the doubly transgenic carcinomas. These findings suggest that, in combination, LMP1 and LMP2 contribute to carcinoma progression and that this may reflect the combined effects of the proteins on activation of multiple signaling pathways. This study is the first to characterize the effects of LMP2 on tumor initiation and promotion and to identify an effect of the combined expression of LMP1 and LMP2 on the increase of carcinoma development.

High-risk human papillomavirus E5 oncoprotein displays channel-forming activity sensitive to small-molecule inhibitors

High-risk human papillomavirus type 16 (HPV16) is the primary causative agent of cervical cancer and therefore is responsible for significant morbidity and mortality worldwide. Cellular transformation is mediated directly by the expression of viral oncogenes, the least characterized of which, E5, subverts cellular proliferation and immune recognition processes. Despite a growing catalogue of E5-specific host interactions, little is understood regarding the molecular basis of its function. Here we describe a novel function for HPV16 E5 as an oligomeric channel-forming protein, placing it within the virus-encoded "viroporin" family. The development of a novel recombinant E5 expression system showed that E5 formed oligomeric assemblies of a defined luminal diameter and stoichiometry in membranous environments and that such channels mediated fluorescent dye release from liposomes. Hexameric E5 channel stoichiometry was suggested by native PAGE studies. In lieu of high-resolution structural information, established de novo molecular modeling and design methods permitted the development of the first specific small-molecule E5 inhibitor, capable of both abrogating channel activity in vitro and reducing E5-mediated effects on cell signaling pathways. The identification of channel activity should enhance the future understanding of the physiological function of E5 and could represent an important target for antiviral intervention.

Cellular transformation by human papillomaviruses: lessons learned by comparing high- and low-risk viruses

The oncogenic potential of papillomaviruses (PVs) has been appreciated since the 1930s yet the mechanisms of virally-mediated cellular transformation are still being revealed. Reasons for this include: a) the oncoproteins are multifunctional, b) there is an ever-growing list of cellular interacting proteins, c) more than one cellular protein may bind to a given region of the oncoprotein, and d) there is only limited information on the proteins encoded by the corresponding non-oncogenic PVs. The perspective of this review will be to contrast the activities of the viral E6 and E7 proteins encoded by the oncogenic human PVs (termed high-risk HPVs) to those encoded by their non-oncogenic counterparts (termed low-risk HPVs) in an attempt to sort out viral life cycle-related functions from oncogenic functions. The review will emphasize lessons learned from the cell culture studies of the HPVs causing mucosal/genital tract cancers.

Human papillomavirus 16 E5 modulates the expression of host microRNAs

Human papillomavirus (HPV) infection is a prerequisite of developing cervical cancer, approximately half of which are associated with HPV type 16. HPV 16 encodes three oncogenes, E5, E6, and E7, of which E5 is the least studied so far. Its roles in regulating replication and pathogenesis of HPV are not fully understood. Here we utilize high-throughput screening to coordinately investigate the effect of E5 on the expression of host protein-coding and microRNA genes. MicroRNAs form a class of 22nt long noncoding RNAs with regulatory activity. Among the altered cellular microRNAs we focus on the alteration in the expression of miR-146a, miR-203 and miR-324-5p and their target genes in a time interval of 96 hours of E5 induction. Our results indicate that HPV infection and subsequent transformation take place through complex regulatory patterns of gene expression in the host cells, part of which are regulated by the E5 protein.

HPV16 E5 affects the KGFR/FGFR2b-mediated epithelial growth through alteration of the receptor expression, signaling and endocytic traffic

The E5 oncoprotein of the human papillomavirus type 16 (HPV16 E5) cooperates in cervical carcinogenesis and in epithelial transformation deregulating cell growth, survival and differentiation through the modulation of growth factor receptors. Among the epithelial receptor tyrosine kinases, the keratinocyte growth factor receptor/fibroblast growth factor receptor 2b (KGFR/FGFR2b) is a major paracrine mediator of epithelial homeostasis and appears to have an unique and unusual role in epithelial tissues, exerting a tumor-suppressive function in vitro and in vivo. With the aim to better elucidate the molecular events involved in the pathological activity of 16E5, we investigated if the viral protein would be able to affect the KGFR expression, signaling and turnover by interference with its degradative and recycling endocytic pathways. Quantitative reverse transcriptase-PCR and biochemical approaches on human keratinocytes transfected with 16E5-HA showed that E5 protein is able to induce KGFR down-modulation at both transcript and protein levels. Immunofluorescence microscopy in double-transfected cells expressing both E5 and KGFR revealed that the viral protein alters the receptor endocytic trafficking and triggers its endosomal sorting to the indirect juxtanuclear recycling pathway. The shift from lysosomal degradation to recycling at the plasma membrane correlates with a reduced phosphorylation of the fibroblast growth factor receptor substrate-2α tyrosine 196, the major docking site for Grb2-Cbl complexes responsible for receptor ubiquitination and degradation. 5'-Bromo-deoxyuridine incorporation assay demonstrated that expression of 16E5 induces a decrease in the growth response to the receptor ligands as a consequence of KGFR down-modulation, suggesting that 16E5 might have a role on HPV infection in perturbing the KGFR-mediated physiological behavior of confluent keratinocytes committed to differentiation.

A mouse model for human anal cancer

Human anal cancers are associated with high-risk human papillomaviruses (HPV) that cause other anogenital cancers and head and neck cancers. As with other cancers, HPV16 is the most common high-risk HPV in anal cancers. We describe the generation and characterization of a mouse model for human anal cancer. This model makes use of K14E6 and K14E7 transgenic mice in which the HPV16 E6 and E7 genes are directed in their expression to stratified squamous epithelia. HPV16 E6 and E7 possess oncogenic properties including, but not limited to, their capacity to inactivate the cellular tumor suppressors p53 and pRb, respectively. Both E6 and E7 were found to be functionally expressed in the anal epithelia of K14E6/K14E7 transgenic mice. To assess the susceptibility of these mice to anal cancer, mice were treated topically with dimethylbenz[a]anthracene (DMBA), a chemical carcinogen that is known to induce squamous cell carcinomas in other sites. Nearly 50% of DMBA-treated HPV16 E6/E7 transgenic mice showed overt signs of tumors, whereas none of the like-treated nontransgenic mice showed tumors. Histopathologic analyses confirmed that the HPV16 transgenic mice were increased in their susceptibility to anal cancers and precancerous lesions. Biomarker analyses demonstrated that these mouse anal cancers exhibit properties that are similar to those observed in HPV-positive precursors to human anal cancer. This is the first mouse model for investigating the contributions of viral and cellular factors in anal carcinogenesis, and should provide a platform for assessing new therapeutic modalities for treating and/or preventing this type of cancer.

Human papillomavirus oncoproteins: pathways to transformation

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.

Human papillomavirus type 16 E5 oncoprotein as a new target for cervical cancer treatment

Human papillomavirus (HPV) infection is considered to be the necessary cause of cervical cancer. E6 and E7 oncoproteins of HPV have been known to play major roles in malignant transformation of cervical cells, inhibiting the tumor suppressors p53 and Rb. However, the role of E5 oncoprotein has been relatively less defined. HPV 16 E5 is a hydrophobic membrane-bound protein which associates with the Golgi apparatus, endoplasmic reticulum and perinuclear membrane. Accumulating evidences have suggested that E5 oncoprotein may also contribute to cervical carcinogenesis through modulating cellular signaling pathways in addition to augmenting the immortalization potential of E6 and E7. Multiple mechanisms, including activation of EGFR or inflammatory cell signaling pathway, have been implicated in malignant transformation by HPV 16 E5. Therefore, targeting E5 may be a rational approach for chemoprevention and treatment of cervical cancer, and understanding its oncogenic processes may help us to design novel therapeutic strategies. In this review, we discussed the roles of HPV 16 E5 in cervical carcinogenesis, altering several cellular signaling pathways involved in cell proliferation, angiogenesis and apoptosis.

A role for HPV16 E5 in cervical carcinogenesis

A subset of the mucosotropic human papillomaviruses (HPV), including HPV16, are etiologic agents for the vast majority of cervical cancers, other anogenital cancers, and a subset of head and neck squamous cell carcinomas. HPV16 encodes three oncogenes: E5, E6, and E7. Although E6 and E7 have been well-studied and clearly shown to be important contributors to these cancers, less is known about E5. In this study, we used E5 transgenic mice to investigate the role of E5 in cervical cancer. When treated for 6 months with estrogen, a cofactor for cervical carcinogenesis, E5 transgenic mice developed more severe neoplastic cervical disease than similarly treated nontransgenic mice, although no frank cancers were detected. In addition, E5 when combined with either E6 or E7 induced more severe neoplastic disease than seen in mice expressing only one viral oncogene. Prolonged treatment of E5 transgenic mice with exogenous estrogen uncovered an ability of E5 to cause frank cancer. These data indicate that E5 acts as an oncogene in the reproductive tracts of female mice.

The human papillomavirus type 16 E5 oncoprotein synergizes with EGF-receptor signaling to enhance cell cycle progression and the down-regulation of p27(Kip1)

E5 oncoprotein activity from high risk human papillomaviruses (HPVs) is associated with growth factor receptor signaling, but the function of this protein is not well understood. In this study, we investigated the role of HPV-16 E5 on the cell cycle progression during EGF-stimulation. Wild-type and NIH 3T3 cells over-expressing human EGF-receptor were transfected with HPV-16 E5 gene and the cell cycle progression was characterized. This analysis showed that the E5-expressing cells increased DNA synthesis (S-phase) by around 40%. Cell cycle protein analysis of E5-expressing cells showed a reduction in the half-life of p27(Kip1) protein as compared to control cells (18.4 vs. 12.7 h), an effect that was enhanced in EGF-stimulated cells (12.8 vs. 3.6 h). Blockage of EGF-receptor activity abrogated E5 signals as well as p27(Kip1) down-regulation. These results suggest that E5 and the EGF-receptor cooperate to enhance cell cycle entry and progression through regulating p27(Kip1) expression at protein level.

Human papillomavirus type 16 E5 protein inhibits hydrogen-peroxide-induced apoptosis by stimulating ubiquitin-proteasome-mediated degradation of Bax in human cervical cancer cells

To investigate the mechanism by which the human papillomavirus (HPV) E5 protein contributes to the carcinogenesis of uterine cervical cancer, we studied the effect of HPV E5 on apoptosis of cervical cancer cells and its underlying mechanism. Expression of HPV16 E5 protein inhibited hydrogen peroxide-induced apoptosis in C-33A cervical cancer cells. E5 decreased the expression of Bax protein, and exogenous expression of Bax abolished the anti-apoptotic effect of E5. Knockdown of E5 by small interfering RNA sensitized CaSki cervical cancer cells to hydrogen peroxide-induced apoptosis with concurrent increase in Bax expression. Transient expression of E5 significantly increased the degradation rate of Bax protein by inducing the ubiquitination. The E5-induced decrease in Bax expression was inhibited by a cyclooxygenase-2 (COX-2) inhibitor, prostaglandin E2 (PGE(2)) receptor antagonists and cyclic adenosine monophosphate-dependent protein kinase (PKA) inhibitor. Treatment with PGE(2) decreased the expression of Bax and inhibited hydrogen peroxide-induced apoptosis of C-33A cells. We concluded that HPV16 E5 protein inhibits hydrogen peroxide-induced apoptosis of cervical cancer cells by stimulating the ubiquitin-proteasome-mediated degradation of Bax protein, and the pathway involves COX-2, PGE(2) and PKA. This finding suggests the possibility that HPV 16 E5 protein contributes to cervical carcinogenesis by inhibiting apoptosis of transformed cervical epithelial cells.

Effects of human papillomavirus type 16 E5 deletion mutants on epithelial morphology: functional characterization of each transmembrane domain

Human papillomavirus type 16 (HPV-16) is the cause of cervical cancer. The HPV genome encodes three transforming proteins, E5, E6 and E7. E6 and E7 are the main transforming proteins of HPV, while the role of E5 is still poorly understood. Using three dimensional organotypic raft cultures we show that HaCaT human keratinocytes expressing HPV-16 E5 form a very perturbed epithelium, with simultaneous hyperkeratinization of some cells and defective differentiation of other cells. The basal layer is disturbed and many cells invade the collagen matrix. Many cells among the differentiated layers show characteristics of basal cells: progression through the cell cycle, expression of cytokeratin 14, lack of cytokeratin 1 and production of matrix metalloproteases (MMP). Using deletion mutants which encompass the three hydrophobic domains of E5, we have assigned the ability to promote invasion of the matrix to the first hydrophobic domain, and the capacity to induce MMP9 to the C-terminal four amino acids. We also show that invasion and production of MMP9 can be dissociated, as mutants that are still capable of invasion do not produce MMP9 and vice versa.

Human papillomavirus E7 oncoprotein overrides the tumor suppressor activity of p21Cip1 in cervical carcinogenesis

The E7 oncoprotein of the high-risk human papillomaviruses (HPV) is thought to contribute to cervical carcinogenesis at least in part by abrogating cell cycle regulation. E7 can dysregulate the cell cycle through its interaction with several cellular proteins including the retinoblastoma suppressor protein pRb, as well as the cyclin-dependent kinase inhibitor p21(Cip1). Inactivation of pRb in cervical epithelia is not sufficient to explain the ability of E7 to cause cervical cancers in transgenic mice. In the current study, we focused on the role of p21(Cip1) in cervical cancer. Cervical disease was significantly increased in p21(-/-) mice compared with p21(+/+) mice, showing that p21(Cip1) can function as a tumor suppressor in this tissue. Importantly, the ability of E7 to induce cervical cancers was not significantly enhanced on the p21-null background, consistent with the hypothesis that the ability of E7 to inhibit p21(Cip1) contributes to its carcinogenic properties. Further supportive of this hypothesis, cervical carcinogenesis in mice expressing a mutant form of HPV-16 E7, E7(CVQ), which fails to inactivate p21(Cip1), was significantly reduced compared with that in K14E7(WT) mice expressing wild-type HPV-16 E7. However, K14E7(CVQ) mice still displayed heightened levels of cervical carcinogenesis compared with that in nontransgenic mice, indicating that activities of E7 besides its capacity to inactivate p21(Cip1) also contribute to cervical carcinogenesis. Taken together, we conclude that p21(Cip1) functions as a tumor suppressor in cervical carcinogenesis and that p21(Cip1) inactivation by HPV-16 E7 partially contributes to the contribution of E7 to cervical carcinogenesis.

Oncogenic activities of human papillomaviruses

Infectious etiologies for certain human cancers have long been suggested by epidemiological studies and studies with experimental animals. Important support for this concept came from the discovery by Harald zur Hausen's group that human cervical carcinoma almost universally contains certain "high-risk" human papillomavirus (HPV) types. Over the years, much has been learned about the carcinogenic activities of high-risk HPVs. These studies have revealed that two viral proteins, E6 and E7, that are consistently expressed in HPV-associated carcinomas, are necessary for induction and maintenance of the transformed phenotype. Hence, HPV-associated tumors are unique amongst human solid tumors in that they are universally caused by exposure to the same, molecularly defined oncogenic agents, and the molecular signal transduction pathways subverted by these viral transforming agents are frequently disrupted in other, non-virus-associated human cancers.

Human papillomavirus E5 protein induces expression of the EP4 subtype of prostaglandin E2 receptor in cyclic AMP response element-dependent pathways in cervical cancer cells

Human papillomavirus (HPV) is the major cause of uterine cervical cancer, but the role of the HPV E5 in carcinogenesis is not clearly understood. Prostaglandins are known to contribute to carcinogenesis of cervical cancer, and we therefore investigated the effect of HPV16 E5 on the expression of prostaglandin E2 (PGE2) receptors and underlying mechanisms. Stable expression of the E5 induced expression of the EP4 subtype of PGE2 receptors in C33A cervical cancer cells, and transfection of E5 small interfering RNA (siRNA) decreased it. EP4 protein expression was increased in human cervical cancer tissues, and EP4 mediated E5-induced increase in anchorage-independent colony formation and vascular endothelial growth factor expression. E5 induced cyclooxygenase-2 (COX-2) expression, and COX-2 increased PGE2 secretion and EP4 expression. The induction of EP4 by PGE2 and E5 was inhibited by an EP4 antagonist, inhibitors of cyclic adenosine monophosphate-dependent protein kinase or phosphatidylinositol 3-kinase, and a cyclic adenosine monophosphate response element (CRE) decoy. E5 increased the luciferase expression controlled by a variant CRE of the EP4 promoter, and it also increased the binding of cyclic adenosine monophosphate response element binding protein (CREB) to oligonucleotides containing this CRE. We conclude that the HPV16 E5 protein induces EP4 receptor protein in cervical cancer cells and that this induction involves epidermal growth factor receptor, COX-2, PGE2, EP2 and EP4, protein kinase A, CREB and CRE.

The first hydrophobic region of the HPV16 E5 protein determines protein cellular location and facilitates anchorage-independent growth

The human papillomavirus type 16 E5 protein (HPV16 E5) is 83 amino acids in length and contains three well-defined hydrophobic regions. The protein is expressed at very limited amounts in transfected cells and the absence of specific antibodies has strongly hampered functional analyses. To investigate the relationship between structure and function we have synthesized a codon-adapted version of the gene (hE5) and prepared a series of N-terminal and C-terminal deletions. Immunofluorescence analyses show colocaliation of the protein with calnexin, an ER marker, EEA-1, an early endosomes marker, and Lamp-2, a lysosomal marker. No major colocalization was found between hE5 and the Golgi marker 58 K. Whereas deletions at the C-terminal end of the protein do not greatly alter the localisation pattern, deletion of the first hydrophobic region results in loss of colocalisation with the ER, early endosomes and lysosomes. Further, we show that while the complete E5 protein confers to HaCaT cells the property to grow in an anchorage-independent manner, deletion of the first hydrophobic region results in loss of growth in soft agar. We conclude that the first hydrophobic region of the E5 protein largely determines the biological properties of the viral protein.

HUMAN PAPILLOMAVIRUS ASSOCIATION WITH HEAD AND NECK CANCERS: UNDERSTANDING VIRUS BIOLOGY AND USING IT IN THE DEVELOPMENT OF CANCER DIAGNOSTICS

The link between human papillomaviruses and human cervical cancers has long been established. However, human papillomaviruses (HPVs) are now being detected in another type of cancer, not previously associated with this virus, head and neck squamous cell carcinoma (HNSCC). This review will focus on experimental data supporting the view that HPVs contribute to the etiology of a subset of HNSCC. We further put forth the argument that HPV-associated HNSCC deserves to be recognized as a distinct disease in the clinic and as such needs to be appropriately diagnosed. We offer an overview of studies that have helped dissect the role of HPVs in HNSCC and that may be helpful in the development of new diagnostic tools for discriminating this type of HNSCC.