The HPV8 E6 protein targets the Hippo and Wnt signaling pathways as part of its arsenal to restrain keratinocyte differentiation

IMPORTANCE

Human papillomaviruses (HPVs) infect basal epithelial cells and cause a dramatic expansion of basal-like, proliferative cells. This reflects the ability of papillomaviruses to delay keratinocyte differentiation, thereby maintaining aspects of the basal cell identity of persistently infected cells. This may enable papillomaviruses to establish and maintain long-term infections in squamous epithelial tissues. Previous work has revealed that the ability of β-HPV8 E6 protein to inhibit Notch and transforming growth factor β signaling importantly contributes to this activity. Here, we present evidence that HPV8 E6 also subverts Hippo and Wnt signaling and that these activities also aid in restraining keratinocyte differentiation.

Expression of Progesterone Receptor A as an Independent Negative Prognosticator for Cervical Cancer

The role of progesterone receptor A (PRA) for the survival outcome of cervical cancer patients is ambiguous. In mouse models, it has been shown that PRA plays a rather protective role in cancer development. The aim of this study was to assess its expression by immunohistochemistry in 250 cervical cancer tissue samples and to correlate the results with clinicopathological parameters including patient survival. PRA expression was positively correlated with the International Federation of Gynecology and Obstetrics (FIGO) classification scores. PRA was significantly overexpressed in adenocarcinomas compared to squamous epithelial carcinoma subtypes. Correlation analyses revealed a trend association with the HPV virus protein E6, a negative correlation with p16 and a positive correlation with EP3. PRA expression was also associated with the expression of RIP140, a transcriptional coregulator that we previously identified as a negative prognostic factor for survival in cervical cancer patients. Univariate survival analyses revealed PRA as a negative prognosticator for survival in patients with cervical adenocarcinoma. Multivariate analyses showed that simultaneous expression of RIP140 and PRA was associated with the worst survival, whereas with negative RIP140, PRA expression alone was associated with the best survival. We can therefore assume that the effect of nuclear PRA on overall survival is dependent upon nuclear RIP140 expression.

miR-34a negatively regulates cell cycle factor Cdt2/DTL in HPV infected cervical cancer cells

MicroRNAs have emerged as an important regulator of cell cycle and various other cellular processes. Aberration in microRNAs has been linked with development of several cancers and other diseases but still very little is known about the mechanism by which they regulate these cellular events. High risk human papilloma virus (HR HPV) is the causative agent of 99% of cervical cancer cases which attenuates multiple tumor suppressors and checkpoint factors of the host cell. The viral proteins also stabilize many oncogenic factors, including an essential cell cycle regulator Cdt2/DTL which in turn promotes cell transformation and proliferation. In this study, we report that a micro-RNA, miR-34a by suppressing HPV E6 protein, destabilizes Cdt2/DTL protein level in HPV infected cervical cancer cell lines. Destabilization of Cdt2 stabilizes pro-apoptotic and onco-suppressor proteins like p21 and Set8 and suppresses cell proliferation, invasion and migration capabilities of the HPV positive cervical cancer cells. Overexpression of either HPV E6 or Cdt2 genes along with miR-34a restored back the suppressed proliferation rate. This study is the first-ever report to show that miR-34a regulates cell cycle factor Cdt2 by suppressing viral E6 protein level, thus opening up the possibility of exploring miR-34a as a specific therapy for cervical cancer treatment.

PPI Modulators of E6 as Potential Targeted Therapeutics for Cervical Cancer: Progress and Challenges in Targeting E6

Advanced cervical cancer is primarily managed using cytotoxic therapies, despite evidence of limited efficacy and known toxicity. There is a current lack of alternative therapeutics to treat the disease more effectively. As such, there have been more research endeavors to develop targeted therapies directed at oncogenic host cellular targets over the past 4 decades, but thus far, only marginal gains in survival have been realized. The E6 oncoprotein, a protein of human papillomavirus origin that functionally inactivates various cellular antitumor proteins through protein–protein interactions (PPIs), represents an alternative target and intriguing opportunity to identify novel and potentially effective therapies to treat cervical cancer. Published research has reported a number of peptide and small-molecule modulators targeting the PPIs of E6 in various cell-based models. However, the reported compounds have rarely been well characterized in animal or human subjects. This indicates that while notable progress has been made in targeting E6, more extensive research is needed to accelerate the optimization of leads. In this review, we summarize the current knowledge and understanding of specific E6 PPI inhibition, the progress and challenges being faced, and potential approaches that can be utilized to identify novel and potent PPI inhibitors for cervical cancer treatment.

Vaccination Strategies for the Control and Treatment of HPV Infection and HPV-Associated Cancer

Human papillomavirus (HPV) is the most common sexually transmitted infection, currently affecting close to 80 million Americans. Importantly, HPV infection is recognized as the etiologic factor for numerous cancers, including cervical, vulval, vaginal, penile, anal, and a subset of oropharyngeal cancers. The prevalence of HPV infection and its associated diseases are a significant problem, affecting millions of individuals worldwide. Likewise, the incidence of HPV infection poses a significant burden on individuals and the broader healthcare system. Between 2011 and 2015, there were an estimated 42,700 new cases of HPV-associated cancers each year in the United States alone. Similarly, the global burden of HPV is high, with around 630,000 new cases of HPV-associated cancer occurring each year. In the last decade, a total of three preventive major capsid protein (L1) virus-like particle-based HPV vaccines have been licensed and brought to market as a means to prevent the spread of HPV infection. These prophylactic vaccines have been demonstrated to be safe and efficacious in preventing HPV infection. The most recent iteration of the preventive HPV vaccine, a nanovalent, L1-VLP vaccine, protects against a total of nine HPV types (seven high-risk and two low-risk HPV types), including the high-risk types HPV16 and HPV18, which are responsible for causing the majority of HPV-associated cancers. Although current prophylactic HPV vaccines have demonstrated huge success in preventing infection, existing barriers to vaccine acquisition have limited their widespread use, especially in low- and middle-income countries, where the burden of HPV-associated diseases is highest. Prophylactic vaccines are unable to provide protection to individuals with existing HPV infections or HPV-associated diseases. Instead, therapeutic HPV vaccines capable of generating T cell-mediated immunity against HPV infection and associated diseases are needed to ameliorate the burden of disease in individuals with existing HPV infection. To generate a cell-mediated immune response against HPV, most therapeutic vaccines target HPV oncoproteins E6 and E7. Several types of therapeutic HPV vaccine candidates have been developed including live-vector, protein, peptide, dendritic cell, and DNA-based vaccines. This chapter will review the commercially available prophylactic HPV vaccines and discuss the recent progress in the development of therapeutic HPV vaccines.

Human papillomavirus E6: Host cell receptor, GRP78, binding site prediction

Human papillomavirus (HPV) is the main cervical cancer-promoting element that is transmitted through sexual routes. Anal, head, and throat cancers are also reported to be accompanied by HPV infection. E6 is one of the HPV nonstructural proteins, which is responsible for cell differentiation by targeting tumor suppressor genes, p105Rb and p53. E6 was reported to be stabilized by two chaperone proteins; glucose-regulated protein 78 (GRP78) and heat shock protein 90. GRP78 is responsible for the unfolded protein response of the cells, and it was reported to be upregulated in many cancers, including cervical cancer. It was reported that knocking out GRP78 destabilizes E6 leading to faster degradation of E6 in vivo. The current work predicts the possible binding mode between E6 and GRP78 based on sequence and structural similarities.

EGFR overexpression increases radiotherapy response in HPV-positive head and neck cancer through inhibition of DNA damage repair and HPV E6 downregulation

High-risk Human Papillomavirus (HPV) infections have recently emerged as an independent risk factor in head and neck squamous cell carcinoma (HNSCC). There has been a marked increase in the incidence of HPV-induced HNSCC subtype, which demonstrates different genetics with better treatment outcome. Despite the favourable prognosis of HPV-HNSCC, the treatment modality, consisting of high dose radiotherapy (RT) in combination with chemotherapy (CT), remains similar to HPV-negative tumours, associated with toxic side effects. Epidermal growth factor receptor (EGFR) is overexpressed in over 80% of HNSCC and correlates with RT resistance. EGFR inhibitor Cetuximab is the only FDA approved targeted therapy for both HNSCC subtypes, however the response varies between HNSCC subtypes. In HPV-negative HNSCC, Cetuximab sensitises HNSCC to RT improving survival rates. To reduce adverse cytotoxicity of CT, Cetuximab has been approved for treatment de-escalation of HPV-positive HNSCC. The results of several recent clinical trials have concluded differing outcome to HPV-negative HNSCC. Here we investigated the role of EGFR in HPV-positive HNSCC response to RT. Remarkably, in HPV-positive HNSCC cell lines and in vivo tumour models, EGFR activation was strongly indicative of increased RT response. In response to RT, EGFR activation induced impairment of DNA damage repair and increased RT response. Furthermore, EGFR was found to downregulate HPV oncoproteinE6 expression and induced p53 activity in response to RT. Collectively, our data uncovers a novel role for EGFR in virally induced HNSCC and highlights the importance of using EGFR-targeted therapies in the context of the genetic makeup of cancer.

Ubiquitination of the HPV Oncoprotein E6 Is Critical for E6/E6AP-Mediated p53 Degradation

High-risk Human papillomaviruses (HPVs) types are associated with more than 90% of premalignant and malignant squamous lesions of the uterine cervix. The E6 oncoprotein of high-risk HPVs is a key determinant in cell transformation because it induces the degradation of the host pro-apoptotic tumor suppressor p53. E6 recruits the intracellular ubiquitin ligase E6AP and subsequently induces proteasome-dependent p53 degradation. Neither E6 nor E6AP alone interact with p53; however, the precise mechanism of the functional regulation of the E6/E6AP/p53 complex is unclear. Here, we showed that the high-risk HPV E6 proteins are ubiquitinated during E6/E6AP/p53 complex assembly and degraded by the proteasome system. Increasing p53 expression enhanced E6/E6AP/p53 assembly and facilitated E6 ubiquitination and degradation. The dominant negative mutant of p53 R175H, which does not efficiently bind E6, decreased E6 ubiquitination and increased stability. Furthermore, we showed that the ubiquitin ligase E6AP is essential for E6 ubiquitination, and downregulation of E6AP expression increased E6 stability. We also showed that p53 R175H inhibited E6-mediated p53 degradation. Consistently, the host deubiquitinating enzyme USP15 removed ubiquitin chains from E6 proteins and inhibited E6-mediated p53 degradation. Crucially, ectopic expression of either p53 R175H or USP15 promoted p53-triggered apoptosis in human cervical cancer cells. Considering the importance of ubiquitinated E6 on p53 degradation, the disruption of E6 ubiquitination represents an attractive pharmacological intervention against HPV-positive human cervical cancer.

iASPP mediates p53 selectivity through a modular mechanism fine-tuning DNA recognition

The most frequently mutated protein in human cancer is p53, a transcription factor (TF) that regulates myriad genes instrumental in diverse cellular outcomes including growth arrest and cell death. Cell context-dependent p53 modulation is critical for this life-or-death balance, yet remains incompletely understood. Here we identify sequence signatures enriched in genomic p53-binding sites modulated by the transcription cofactor iASPP. Moreover, our p53-iASPP crystal structure reveals that iASPP displaces the p53 L1 loop-which mediates sequence-specific interactions with the signature-corresponding base-without perturbing other DNA-recognizing modules of the p53 DNA-binding domain. A TF commonly uses multiple structural modules to recognize its cognate DNA, and thus this mechanism of a cofactor fine-tuning TF-DNA interactions through targeting a particular module is likely widespread. Previously, all tumor suppressors and oncoproteins that associate with the p53 DNA-binding domain-except the oncogenic E6 from human papillomaviruses (HPVs)-structurally cluster at the DNA-binding site of p53, complicating drug design. By contrast, iASPP inhibits p53 through a distinct surface overlapping the E6 footprint, opening prospects for p53-targeting precision medicine to improve cancer therapy.

Identification of potential binding pocket on viral oncoprotein HPV16 E6: a promising anti-cancer target for small molecule drug discovery

BACKGROUND

Several human cancers, especially cervical cancer are caused by the infection of high risk strains of human papillomaviruses (HPV), notably HPV16. It is implicated that the oncoprotein E6 expressed from HPV, is inhibiting the apoptotic pathway by binding to adaptor molecule FADD (Fas-associated death domain). Inhibiting E6 interactions with FADD could provide a promising treatment for cervical cancer. There are few small molecules reported to inhibit such interactions. However, the FADD binding site information on the HPV E6 is not currently available. This binding site information may provide an opportunity to design new small molecule inhibitors to treat E6 mediated cancers. In this study we report the possible binding pocket on HPV16 E6 oncoprotein by using activity data of reported inhibitors through a stepwise molecular modeling approach.

RESULTS

Blind docking and removing duplicates followed by visual inspection to determine ligand-receptor interactions provided 68 possible binding sites on the E6 protein. Individual docking of all known inhibitors lead to the identification of 28 pockets having some kind of correlation with their activity data. It was also observed that several of these pockets overlapped with each other, having some amino acids in common. Amino acids Leu50 and Cys51 were identified as key E6 residues for high affinity ligand binding which are seen in most of these pockets. In most cases, ligands demonstrated a hydrogen bond interaction with Cys51. Ala61, Arg131 and Gln107 were also frequently observed showing interactions among these pockets. A few amino acids unique to each ligand were also identified representing additional interactions at the receptor site.

CONCLUSIONS

After determining receptor-ligand interactions between E6 oncoprotein and the six known inhibitors, the amino acids Cys51, Leu50, Arg102, Arg131, Leu67, Val62, and Gln107 were identified to have importance in E6 inhibition. It was generally observed that Leu50 and Cys51 are necessary for high binding affinity with Cys51 being essential for hydrogen bonding. This study identified a potential binding pocket for the E6 inhibitors. Identification of the ligand binding pocket helps to design novel inhibitors of HPV16 E6 oncoprotein as a promising treatment for cervical cancer.

Acquisition of a phospho-acceptor site enhances HPV E6 PDZ-binding motif functional promiscuity

All cancer-causing human papillomavirus (HPV) E6 oncoproteins have a C-terminal PDZ-binding motif (PBM), which correlates with oncogenic potential. Nonetheless, several HPVs with little or no oncogenic potential also have an E6 PBM, with minor sequence differences affecting PDZ protein selectivity. Furthermore, certain HPV types have a phospho-acceptor site embedded within the PBM. We therefore compared HPV-18, HPV-66 and HPV-40 E6 proteins to examine the possible link between the ability to target multiple PDZ proteins and the acquisition of a phospho-acceptor site. The mutation of essential residues in HPV-18E6 reduces its phosphorylation, and fewer PDZ substrates are bound. In contrast, the generation of consensus phospho-acceptor sites in HPV-66 and HPV-40 E6 PBMs increases the PDZ proteins recognized. Thus, although phosphorylation of the E6 PBM and PDZ protein recognition are mutually exclusive, they are closely linked, with the acquisition of a phospho-acceptor site also contributing to an expansion in the number of PDZ proteins bound.

Cervical cancer cell lines are sensitive to sub-erythemal UV exposure

High risk human papillomavirus (HPV) infections are the causative agent in virtually every cervical cancer as well as a host of other anogenital and oropharyngeal malignancies. These viruses must activate DNA repair pathways to facilitate their replication, while avoiding the cell cycle arrest and apoptosis that can accompany DNA damage. HPV oncoproteins facilitate each of these goals, but also reduce genome stability. Our data dissect the cytotoxic and cytoprotective characteristics of HPV oncogenes in cervical cancer cells. These data show that while the transformation of keratinocytes by HPV oncogene leaves these cells more sensitive to UV, the oncogenes also protect against UV-induced apoptosis. Cisplatin and UV resistant cervical cancer cell lines were generated and probed for their sensitivity to genotoxic agents. Cervical cancer cells can acquire resistance to one DNA crosslinking agent (UV or cisplatin) without gaining broad tolerance of crosslinked DNA. Further, cisplatin resistance may or may not result in sensitivity to PARP1 inhibition.

A Cu2+ complex induces the aggregation of human papillomavirus oncoprotein E6 and stabilizes p53

Papillomavirus oncoprotein E6 is a critical factor in the modulation of cervical cancer in humans. At the molecular level, formation of the E6-E6AP-p53 ternary complex, which directs p53's degradation, is the key instigator of cancer transforming properties. Herein, a Cu²⁺ anthracenyl-terpyridine complex is described which specifically induces the aggregation of E6 in vitro and in cultured cells. For a hijacking mechanism, both E6 and E6AP are required for p53 ubiquitination and degradation. The Cu²⁺ complex interacts with E6 at the E6AP and p53 binding sites. We show that E6 function is suppressed by aggregation, rendering it incapable of hijacking p53 and thus increasing its cellular level. Therapeutic treatments of cervical cancer are currently unavailable to infected individuals. We anticipate that this Cu²⁺ complex might open up a new therapeutic avenue for the design and development of new chemical entities for the diagnosis and treatment of HPV-induced cancers.

Therapeutic DNA Vaccines for Human Papillomavirus and Associated Diseases

Human papillomavirus (HPV) has long been recognized as the causative agent of cervical cancer. High-risk HPV types 16 and 18 alone are responsible for over 70% of all cases of cervical cancers. More recently, HPV has been identified as an etiological factor for several other forms of cancers, including oropharyngeal, anogenital, and skin. Thus, the association of HPV with these malignancies creates an opportunity to control these HPV lesions and HPV-associated malignancies through immunization. Strategies to prevent or to therapeutically treat HPV infections have been developed and are still pushing innovative boundaries. Currently, commercial prophylactic HPV vaccines are widely available, but they are not able to control established infections or lesions. As a result, there is an urgent need for the development of therapeutic HPV vaccines, to treat existing infections, and to prevent the development of HPV-associated cancers. In particular, DNA vaccination has emerged as a promising form of therapeutic HPV vaccine. DNA vaccines have great potential for the treatment of HPV infections and HPV-associated cancers due to their safety, stability, simplicity of manufacturability, and ability to induce antigen-specific immunity. This review focuses on the current state of therapeutic HPV DNA vaccines, including results from recent and ongoing clinical trials, and outlines different strategies that have been employed to improve their potencies. The continued progress and improvements made in therapeutic HPV DNA vaccine development holds great potential for innovative ways to effectively treat HPV infections and HPV-associated diseases.

Resveratrol and Pterostilbene Exhibit Anticancer Properties Involving the Downregulation of HPV Oncoprotein E6 in Cervical Cancer Cells

Cervical cancer is one of the most common cancers in women living in developing countries. Due to a lack of affordable effective therapy, research into alternative anticancer compounds with low toxicity such as dietary polyphenols has continued. Our aim is to determine whether two structurally similar plant polyphenols, resveratrol and pterostilbene, exhibit anticancer and anti-HPV (Human papillomavirus) activity against cervical cancer cells. To determine anticancer activity, extensive in vitro analyses were performed. Anti-HPV activity, through measuring E6 protein levels, subsequent downstream p53 effects, and caspase-3 activation, were studied to understand a possible mechanism of action. Both polyphenols are effective agents in targeting cervical cancer cells, having low IC50 values in the µM range. They decrease clonogenic survival, reduce cell migration, arrest cells at the S-phase, and reduce the number of mitotic cells. These findings were significant, with pterostilbene often being more effective than resveratrol. Resveratrol and to a greater extent pterostilbene downregulates the HPV oncoprotein E6, induces caspase-3 activation, and upregulates p53 protein levels. Results point to a mechanism that may involve the downregulation of the HPV E6 oncoprotein, activation of apoptotic pathways, and re-establishment of functional p53 protein, with pterostilbene showing greater efficacy than resveratrol.

Human Papillomavirus 16 (HPV-16), HPV-18, and HPV-31 E6 Override the Normal Phosphoregulation of E6AP Enzymatic Activity

The human papillomavirus (HPV) E6 oncoproteins recruit the cellular ubiquitin ligase E6AP/UBE3A to target cellular substrates for proteasome-mediated degradation, and one consequence of this activity is the E6 stimulation of E6AP autoubiquitination and degradation. Recent studies identified an autism-linked mutation within E6AP at T485, which was identified as a protein kinase A phosphoacceptor site and which could directly regulate E6AP ubiquitin ligase activity. In this study, we have analyzed how T485-mediated regulation of E6AP might affect E6 targeting of some of its known substrates. We show that modulation of T485 has no effect on the ability of E6 to direct either p53 or Dlg for degradation. Furthermore, T485 regulation has no effect on HPV-16 or HPV-31 E6-induced autodegradation of E6AP but does affect HPV-18 E6-induced autodegradation of E6AP. In cells derived from cervical cancers, we find low levels of both phosphorylated and nonphosphorylated E6AP in the nucleus. However, ablation of E6 results in a dramatic accumulation of phospho-E6AP in the cytoplasm, whereas nonphosphorylated E6AP accumulates primarily in the nucleus. Interestingly, E6AP phosphorylation at T485 confers association with 14-3-3 proteins, and this interaction seems to be important, in part, for the ability of E6 to recruit phospho-E6AP into the nucleus. These results demonstrate that HPV E6 overrides the normal phosphoregulation of E6AP, both in terms of its enzymatic activity and its subcellular distribution.IMPORTANCE Recent reports demonstrate the importance of phosphoregulation of E6AP for its normal enzymatic activity. Here, we show that HPV E6 is capable of overriding this regulation and can promote degradation of p53 and Dlg regardless of the phosphorylation status of E6AP. Furthermore, E6 interaction with E6AP also significantly alters how E6AP is subject to autodegradation and suggests that this is not a simple stimulation of an already-existing activity but rather a redirection of E6AP activity toward itself. Furthermore, E6-mediated regulation of the subcellular distribution of phospho-E6AP appears to be dependent, in part, upon the 14-3-3 family of proteins.

Telomerase Induction in HPV Infection and Oncogenesis

Telomerase extends the repetitive DNA at the ends of linear chromosomes, and it is normally active in stem cells. When expressed in somatic diploid cells, it can lead to cellular immortalization. Human papillomaviruses (HPVs) are associated with and high-risk for cancer activate telomerase through the catalytic subunit of telomerase, human telomerase reverse transcriptase (hTERT). The expression of hTERT is affected by both high-risk HPVs, E6 and E7. Seminal studies over the last two decades have identified the transcriptional, epigenetic, and post-transcriptional roles high-risk E6 and E7 have in telomerase induction. This review will summarize these findings during infection and highlight the importance of telomerase activation as an oncogenic pathway in HPV-associated cancer development and progression.

Activation of telomerase by HPVs

Telomerase extends the ends of linear chromosomes, and its expression leads to cellular immortalization. In HPV-associated cancers, telomerase is universally detected, and this occurs by activation of the catalytic subunit of telomerase, hTERT. The expression of hTERT is affected by both high-risk HPV E6 and E7. Seminal studies over the last two decades have identified the transcriptional, epigenetic, and post-transcriptional roles high-risk E6 and E7 have in telomerase regulation. This review will summarize these findings and highlight the importance of telomerase activation as an oncogenic pathway in HPV-associated cancer development and progression.

The current state of therapeutic and T cell-based vaccines against human papillomaviruses

Human papillomavirus (HPV) is known to be a necessary factor for many gynecologic malignancies and is also associated with a subset of head and neck malignancies. This knowledge has created the opportunity to control these HPV-associated cancers through vaccination. However, despite the availability of prophylactic HPV vaccines, HPV infections remain extremely common worldwide. In addition, while prophylactic HPV vaccines have been effective in preventing infection, they are ineffective at clearing pre-existing HPV infections. Thus, there is an urgent need for therapeutic and T cell-based vaccines to treat existing HPV infections and HPV-associated lesions and cancers. Unlike prophylactic vaccines, which generate neutralizing antibodies, therapeutic, and T cell-based vaccines enhance cell-mediated immunity against HPV antigens. Our review will cover various therapeutic and T cell-based vaccines in development for the treatment of HPV-associated diseases. Furthermore, we review the strategies to enhance the efficacy of therapeutic vaccines and the latest clinical trials on therapeutic and T cell-based HPV vaccines.

Perspectives for therapeutic HPV vaccine development

Background

Human papillomavirus (HPV) infections and associated diseases remain a serious burden worldwide. It is now clear that HPV serves as the etiological factor and biologic carcinogen for HPV-associated lesions and cancers. Although preventative HPV vaccines are available, these vaccines do not induce strong therapeutic effects against established HPV infections and lesions. These concerns create a critical need for the development of therapeutic strategies, such as vaccines, to treat these existing infections and diseases.

Main Body

Unlike preventative vaccines, therapeutic vaccines aim to generate cell-mediated immunity. HPV oncoproteins E6 and E7 are responsible for the malignant progression of HPV-associated diseases and are consistently expressed in HPV-associated diseases and cancer lesions; therefore, they serve as ideal targets for the development of therapeutic HPV vaccines. In this review we revisit therapeutic HPV vaccines that utilize this knowledge to treat HPV-associated lesions and cancers, with a focus on the findings of recent therapeutic HPV vaccine clinical trials.

Conclusion

Great progress has been made to develop and improve novel therapeutic HPV vaccines to treat existing HPV infections and diseases; however, there is still much work to be done. We believe that therapeutic HPV vaccines have the potential to become a widely available and successful therapy to treat HPV and HPV-associated diseases in the near future.

O-linked GlcNAcylation elevated by HPV E6 mediates viral oncogenesis

High-risk human papillomaviruses (HPVs) are causative agents of anogenital cancers and a fraction of head and neck cancers. The mechanisms involved in the progression of HPV neoplasias to cancers remain largely unknown. Here, we report that O-linked GlcNAcylation (O-GlcNAc) and O-GlcNAc transferase (OGT) were markedly increased in HPV-caused cervical neoplasms relative to normal cervix, whereas O-GlcNAcase (OGA) levels were not altered. Transduction of HPV16 oncogene E6 or E6/E7 into mouse embryonic fibroblasts (MEFs) up-regulated OGT mRNA and protein, elevated the level of O-GlcNAc, and promoted cell proliferation while reducing cellular senescence. Conversely, in HPV-18-transformed HeLa cervical carcinoma cells, inhibition of O-GlcNAc with a low concentration of a chemical inhibitor impaired the transformed phenotypes in vitro. We showed that E6 elevated c-MYC via increased protein stability attributable to O-GlcNAcylation on Thr58. Reduction of HPV-mediated cell viability by a high concentration of O-GlcNAc inhibitor was partially rescued by elevated c-MYC. Finally, knockdown of OGT or O-GlcNAc inhibition in HeLa cells or in TC-1 cells, a mouse cell line transformed by HPV16 E6/E7 and activated K-RAS, reduced c-MYC and suppressed tumorigenesis and metastasis. Thus, we have uncovered a mechanism for HPV oncoprotein-mediated transformation. These findings may eventually aid in the development of effective therapeutics for HPV-associated malignancies by targeting aberrant O-GlcNAc.

The high-risk HPV E6 target scribble (hScrib) is required for HPV E6 expression in cervical tumour-derived cell lines

The ability of high-risk HPV E6 oncoproteins to target cellular proteins which harbor PDZ domains is believed to play an important role in the virus life cycle and to influence the ability of these viruses to bring about malignant transformation. Whilst many of these PDZ proteins are potential tumour suppressors, involved in the control of cell polarity and cell-contact, recent studies suggest that mislocalisation or overexpression might result in the emergence of oncogenic functions. This has been shown most clearly for two E6 targets, hDlg and hScrib. In this study we show that hScrib plays such a role in HeLa cells, where its expression is required for maintaining high levels of HPV-18 E6 protein. Loss of hScrib has no effect on E6 stability but results in lower levels of E6 transcription and a reduced rate of E6 translation. We further show that, in the context of cervical tumour-derived cell lines, both hScrib and E6 cooperate in the activation of the S6 kinase signaling pathway, and thereby contribute towards maintaining high rates of protein translation. These results indicate that the residual hScrib that is present within HPV transformed cells is pro-oncogenic, and highlights the dual functions of E6 cell polarity targets.

Current state in the development of candidate therapeutic HPV vaccines

The identification of human papillomavirus (HPV) as an etiological factor for HPV-associated malignancies creates the opportunity to control these cancers through vaccination. Currently, available preventive HPV vaccines have not yet demonstrated strong evidences for therapeutic effects against established HPV infections and lesions. Furthermore, HPV infections remain extremely common. Thus, there is urgent need for therapeutic vaccines to treat existing HPV infections and HPV-associated diseases. Therapeutic vaccines differ from preventive vaccines in that they are aimed at generating cell-mediated immunity rather than neutralizing antibodies. The HPV-encoded early proteins, especially oncoproteins E6 and E7, form ideal targets for therapeutic HPV vaccines since they are consistently expressed in HPV-associated malignancies and precancerous lesions, playing crucial roles in the generation and maintenance of HPV-associated disease. Our review will cover various therapeutic vaccines in development for the treatment of HPV-associated lesions and cancers. Furthermore, we review strategies to enhance vaccine efficacy and the latest clinical trials on therapeutic HPV vaccines.

Viral Interactions with PDZ Domain-Containing Proteins-An Oncogenic Trait?

Many of the human viruses with oncogenic capabilities, either in their natural host or in experimental systems (hepatitis B and C, human T cell leukaemia virus type 1, Kaposi sarcoma herpesvirus, human immunodeficiency virus, high-risk human papillomaviruses and adenovirus type 9), encode in their limited genome the ability to target cellular proteins containing PSD95/ DLG/ZO-1 (PDZ) interaction modules. In many cases (but not always), the viruses have evolved to bind the PDZ domains using the same short linear peptide motifs found in host protein-PDZ interactions, and in some cases regulate the interactions in a similar fashion by phosphorylation. What is striking is that the diverse viruses target a common subset of PDZ proteins that are intimately involved in controlling cell polarity and the structure and function of intercellular junctions, including tight junctions. Cell polarity is fundamental to the control of cell proliferation and cell survival and disruption of polarity and the signal transduction pathways involved is a key event in tumourigenesis. This review focuses on the oncogenic viruses and the role of targeting PDZ proteins in the virus life cycle and the contribution of virus-PDZ protein interactions to virus-mediated oncogenesis. We highlight how many of the viral associations with PDZ proteins lead to deregulation of PI3K/AKT signalling, benefitting virus replication but as a consequence also contributing to oncogenesis.

APOBEC3A possesses anticancer and antiviral effects by differential inhibition of HPV E6 and E7 expression on cervical cancer

Cervical cancer is the second most common cancer among women worldwide and is the leading cause of deaths in developing countries. Persistent infections with a subset of HPVs, called "high-risk HPVs", including HPV16 and HPV18, are the primary cause of cervical cancer. The apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like (APOBEC) family of proteins is a group of cellular enzymes that catalyze the deamination of cytidine (C) to uracil (U) in single-stranded DNA/RNA, and functions as antiviral factors in the innate immune system of the host. Recent studies have shown that APOBEC3A could restrict certain DNA viruses, including HPVs. In this study, we confirmed that the expression of APOBEC3A was decreased in cervical cancer tissues. Furthermore, APOBEC3A inhibited the cervical cells proliferation, migration as well as invasion, and promoted apoptosis depend on cytidine deaminase. In addition, APOBEC3A decreased HPV16-E6, HPV16-E7 and HPV18-E6 depend on cytidine deaminase, but no effect on HPV18-E7. Therefore, we believe that, in cervical cancer cells, the expression of APOBEC3A possesses anticancer and antiviral effects by differential inhibition of HPV E6 and E7 expression depend on cytidine deaminase.

Effect of β-sitosterol on the expression of HPV E6 and p53 in cervical carcinoma cells

AIM OF THE STUDY

In the absence of effective therapeutic strategies, cervical carcinoma continues to be second on the list of mortality rates of malignant tumours found in women. We investigated the effects of β -sitosterol, a natural product isolated from traditional Chinese herbs, on Caski and HeLa cervical carcinoma cells.

MATERIAL AND METHODS

Morphological changes were examined by light microscopy. Ultrastructures of Caski and HeLa cells treated with 20 µmol/l β-sitosterol were documented by scanning electron microscopy and transmission electron microscopy. Changes in mRNA and protein expression were quantified respectively using Real-Time qPCR and western blot methods.

RESULTS

Treatment of Caski and HeLa cells with β-sitosterol resulted in reduced expression of PCNA, indicative of an inhibitory effect on cell proliferation. This was associated with increased p53 mRNA levels and decreased amounts of HPV E6 transcripts. Expression of p53 and HPV E6 proteins followed a similar trend as that observed for the corresponding transcripts. Caski and HeLa cells treated with β-sitosterol exhibited loss of cell surface microvilli, increased electron density of cell membrane, and decreased organelles.

CONCLUSIONS

In conclusion, treatment of Caski and HeLa cells with β-sitosterol significantly suppressed the expression of genes involved in cell proliferation and oncogenic transformation, while augmenting the expression of genes involved in apoptosis and tumour suppression. Ultrastuctural characterisation of Caski and HeLa cells treated with β-sitosterol further confirmed the anti-proliferative and anti-cancer activity of this natural product isolated from traditional Chinese herbs.

Human papillomavirus 16E6 and NFX1-123 potentiate Notch signaling and differentiation without activating cellular arrest

High-risk human papillomavirus (HR HPV) oncoproteins bind host cell proteins to dysregulate and uncouple apoptosis, senescence, differentiation, and growth. These pathways are important for both the viral life cycle and cancer development. HR HPV16 E6 (16E6) interacts with the cellular protein NFX1-123, and they collaboratively increase the growth and differentiation master regulator, Notch1. In 16E6 expressing keratinocytes (16E6 HFKs), the Notch canonical pathway genes Hes1 and Hes5 were increased with overexpression of NFX1-123, and their expression was directly linked to the activation or blockade of the Notch1 receptor. Keratinocyte differentiation genes Keratin 1 and Keratin 10 were also increased, but in contrast their upregulation was only indirectly associated with Notch1 receptor stimulation and was fully unlinked to growth arrest, increased p21(Waf1/CIP1), or decreased proliferative factor Ki67. This leads to a model of 16E6, NFX1-123, and Notch1 differently regulating canonical and differentiation pathways and entirely uncoupling cellular arrest from increased differentiation.

HPV16 E6 and E6AP differentially cooperate to stimulate or augment Wnt signaling

The present study investigated the roles of E6 and E6AP in the Wnt pathway. We showed that E6 levels are markedly reduced in cells in which Wnt signaling is activated. Coexpression of wild-type or mutant E6AP (C820A) in Wnt-activated cells stabilized E6 and enhanced Wnt/β-catenin/TCF transcription. Expression of E6AP alone in nonstimulated cells elevated β-catenin level, promoted its nuclear accumulation, and activated β-catenin/TCF transcription. A knockdown of E6AP lowered β-catenin levels. Coexpression with E6 intensified the activities of E6AP. Further experiments proved that E6AP/E6 stabilize β-catenin by protecting it from proteasomal degradation. This function was dependent on the catalytic activity of E6AP, the kinase activity of GSK3β and the susceptibility of β-catenin to GSK3β phosphorylation. Thus, this study identified E6AP as a novel regulator of the Wnt signaling pathway, capable of cooperating with E6 in stimulating or augmenting Wnt/β-catenin signaling, thereby possibly contributing to HPV carcinogenesis.

Activation of interleukin-6/signal transducer and activator of transcription 3 by human papillomavirus early proteins 6 induces fibroblast senescence to promote cervical tumourigenesis through autocrine and paracrine pathways in tumour microenvironment

Although it is reported that interleukin (IL)-6/signal transducer and activator of transcription 3 (STAT3) is activated by human papillomavirus (HPV) infection in cervical cancer cells, little is known about the role of IL-6/STAT3 in tumour microenvironment during development of the disease. In this study, we found that cancer-associated fibroblasts (CAF) but not normal fibroblasts (NF) secrete high level of IL-6 with activated STAT3 and appear senescent at early passages in culture or in cervical cancer tissues infected with high-risk HPV, and that treatment of NF with recombinant IL-6 or CAF conditioned medium (CM) induces activation of STAT3 and cellular senescence. IL-6 and STAT3 are either upregulated or activated in Siha and Hela cells infected with HPV 16 or 18, but not in C33A and ME180 cells without HPV 16 or 18 infection. Overexpression of HPV early proteins 6 (E6) activates STAT3, increases IL-6 expression and tumour burden in C33A and ME180 cells, while silencing of HPV E6 by specific shRNA reduces STAT3 activation, IL-6 expression, and tumour formation in Siha and HeLa cells, so does silencing of STAT3 by specific shRNA in HeLa and C33A/E6 cells. The tumour growth of cervical cancer cells reconstituted with CAF or NF is largely affected by inhibition of fibroblast senescence with STAT3 inhibitor or with IL-6 antibody treatment. Thus, we have uncovered a mechanism that fibroblast senescence promotes cervical cancer development through high-risk HPV E6-activated IL-6/STAT3 signalling in tumour microenvironment.

An unbiased in vivo screen reveals multiple transcription factors that control HPV E6-regulated hTERT in keratinocytes

Activation of telomerase by human papillomavirus 16 (HPV16) E6 is a critical step for cell immortalization and transformation in human foreskin keratinocytes (HFKs). Multiple transcription factors have been identified as being involved in E6-induced hTERT expression. Here, we adapted an unbiased in vivo screen using a LacO-LacI system in human cells to discover hTERT promoter-interacting regulators. This approach allowed us to identify a novel hTERT repressor, Maz, which bound the hTERT promoter. E6 expression reduced Maz binding and correspondingly increased Sp1 binding at the hTERT promoter. Knockdown of Maz further increased histone acetylation, as well as hTERT expression in the presence of E6. Overall, these data indicate the utility of a novel screen for promoter-interacting and transcription-regulating proteins. These data also highlight multiple factors that normally regulate hTERT repression in HFKs, and therefore are targeted by E6 for hTERT expression.