The interaction between human papillomavirus type 16 and FADD is mediated by a novel E6 binding domain

Protein-DNA Interactions Regulate Human Papillomavirus DNA Replication, Transcription, and Oncogenesis

Human papillomavirus (HPV) is a group of alpha papillomaviruses that cause various illnesses, including cancer. There are more than 160 types of HPV, with many being "high-risk" types that have been clinically linked to cervical and other types of cancer. "Low-risk" types of HPV cause less severe conditions, such as genital warts. Over the past few decades, numerous studies have shed light on how HPV induces carcinogenesis. The HPV genome is a circular double-stranded DNA molecule that is approximately 8 kilobases in size. Replication of this genome is strictly regulated and requires two virus-encoded proteins, E1 and E2. E1 is a DNA helicase that is necessary for replisome assembly and replication of the HPV genome. On the other hand, E2 is responsible for initiating DNA replication and regulating the transcription of HPV-encoded genes, most importantly the E6 and E7 oncogenes. This article explores the genetic characteristics of high-risk HPV types, the roles of HPV-encoded proteins in HPV DNA replication, the regulation of transcription of E6 and E7 oncogenes, and the development of oncogenesis.

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.

A high-content AlphaScreen™ identifies E6-specific small molecule inhibitors as potential therapeutics for HPV+ head and neck squamous cell carcinomas

The incidence of human papillomavirus-positive head and neck squamous cell carcinoma (HPV⁺-HNSCC) has increased dramatically over the past decades due to an increase in infection of the oral mucosa by HPV. The etiology of HPV⁺-HNSCC is linked to expression of the HPV oncoprotein, E6, which influences tumor formation, growth and survival. E6 effects this oncogenic phenotype in part through inhibitory protein-protein interactions (PPIs) and accelerated degradation of proteins with tumor suppressor properties, such as p53 and caspase 8. Interfering with the binding between E6 and its cellular partners may therefore represent a reasonable pharmacological intervention in HPV⁺ tumors. In this study, we probed a small-molecule library using AlphaScreen™ technology to discover novel E6 inhibitors. Following a cascade of screens we identified and prioritized one hit compound. Structure activity relationship (SAR) studies of this lead uncovered an analog, 30-hydroxygambogic acid (GA-OH), that displayed improved activity. Further testing of this analog in a panel of HPV⁺ and HPV^- cell lines showed good potency and a large window of selectivity as demonstrated by apoptosis induction and significant inhibition of cell growth, cell survival in HPV⁺ cells. In summary, GA-OH may serve as a starting point for the development of potent E6-specific inhibitors.

Effects of HPV16 E6 protein on Daxx-induced apoptosis in C33A cells

Aims

Daxx is a highly conserved nuclear protein with an important role in transcription, apoptosis and other cell processes. We investigated the role of HPV16 E6 in Daxx-induced apoptosis through their interactions in C33A cells.

Methods

The binding of HPV16 E6 and Daxx was confirmed in C33A cells using co-immunoprecipitation and indirect immunofluorescence assays. Quantitative PCR and western blotting were performed to determine the RNA and protein expressions of Daxx, respectively. Automatic cell count and MTT assays were performed to investigate the proliferation of C33A cells. The apoptosis rate of C33A cells was determined via flow cytometry using Annexin V-FITC/PI staining. The relative activity of caspase-8 was tested using ELISA.

Results

HPV16 E6 can bind with Daxx and cause its translocation in C33A cells. The transfected HPV16 E6 can cause a decrease in relative quantification for Daxx in Daxx-overexpressing cells. After Daxx transfection, cell proliferation was found to decrease sharply and cell apoptosis to increase sharply. However, when HPV16 E6 was co-transfected with Daxx, this decrease and increase both became gentle. Similarly, HPV16 E6 made the Daxx-induced increase in caspase-8 activity milder.

Conclusions

HPV16 E6 is involved in inhibiting apoptosis through deregulation of Daxx-induced caspase-8 activities.

The Association and Significance of p53 in Gynecologic Cancers: The Potential of Targeted Therapy

Dysfunction of p53 is observed in the many malignant tumors. In cervical cancer, p53 is inactivated by degradation through the complex with human papilloma virus (HPV) oncoprotein E6 and E6-associated protein (E6AP), an E3 ubiquitin protein ligase. In endometrial cancer, overexpression of p53 in immunohistochemistry is a significant prognostic factor. A discrepancy between p53 overexpression and TP53 mutations is observed in endometrioid endometrial cancer, indicating that the accumulation of p53 protein can be explained by not only gene mutations but also dysregulation of the factors such as ERβ and MDM2. Furthermore, the double-positive expression of immunoreactive estrogen receptor (ER) β and p53 proteins is closely associated with the incidence of metastasis and/or recurrence. High-grade serous ovarian carcinoma (HGSC) arises from secretary cells in the fallopian tube. The secretary cell outgrowth (SCOUT) with TP53 mutations progresses to HGSC via the p53 signature, serous intraepithelial lesion (STIL), and serous intraepithelial carcinoma (STIC), indicating that TP53 mutation is associated with carcinogenesis of HGSC. Clinical application targeting p53 has been approved for some malignant tumors. Gene therapy by the adenovirus-mediated p53 gene transfer system is performed for head and neck cancer. A clinical phase III trial using MDM2/X inhibitors, idasanutlin (RG7388) combined with cytarabine, is being performed involving relapse/refractory acute myeloid leukemia patients. The use of adenoviruses as live vectors which encode wild-type p53 has given promising results in cervical cancer patients.

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.

[Analysis of the current status of research on human papillomavirus-associated head and neck cancers based on recent Chinese literature]

OBJECTIVE

To analyze the current status of research on human papillomavirus (HPV)-associated head and neck cancers in China using national and international references.

METHODS

We searched for bibliographies in the electronic databases of Sichuan University, including Chinese Biological and Medical Database (CBM), China Science and Technology Journal Database (VIP), and Wanfang Journal Database. Then, data was selected and extracted to analyze the basic characteristics of HPV-related head and neck cancers, the general prevalence of HPV, and the effects of various clinic pathological indicators on HPV patients.

RESULTS

The general prevalence of HPV was 34.05% among civil patients with head and neck cancers. Our analysis of different clinical and pathological indices that affect HPV-associated oral squamous cell carcinomas revealed that non-smoking and non-drinking groups had higher HPV infection rates compared with smoking (P=0.002) and drinking (P<
0.005) groups. In addition, N2-N3 stage patients had higher detectable rates than N0-N1 stage patients (P=0.027).

CONCLUSIONS

The majority of civil research studies on HPV-associated head and neck cancers are concentrated on clinical studies. Thus, the molecular neuropathology and signaling pathway of HPV should be further explored in the future to advance prognostic methods. Moreover, the perspective view of therapeutic vaccines and molecular-targeting treatments should receive close attention. 
.

FADD regulates NF-κB activation and promotes ubiquitination of cFLIPL to induce apoptosis

Tumor Necrosis Factor-α canonically induces the activation of NF-κB and associated gene product cellular FLICE-like inhibitory protein (cFLIPL) to promote cell survival. Previously, we demonstrated that ectopic expression of the Fas associated death domain (FADD) diminishes the expression of cFLIPL and transduces caspases-8 mediated apoptosis, independent of FasL stimulation in HEK 293T cells. However, the underlying molecular mechanism of FADD mediated ablation of cFLIP and NF-κB signaling to determining the fate of cell death or survival remains elusive. Here, we explored a novel molecular mechanism of FADD mediated apoptotic cell death that was directed by ubiquitination of cFLIPL and inhibition of NF-κB activation, independent of TNF-α stimulation. We found that induced expression of FADD firmly interacts with procaspase-8 and precludes cFLIPL to from the death inducing signaling complex (DISC). In addition, FADD negatively regulates cellular inhibitor of apoptosis protein 2 (cIAP2) and Bcl-2. Furthermore, FADD restrains cIAP2 expression and interacts with RIP1 and procaspase-8 to accomplish apoptotic cell death signaling. Interestingly, FADD was also found to promote JNK1 mediated activation of E3 ubiquitin ligase ITCH to degrade cFLIPL that may lead to commencement of apoptosis. Thus, FADD is an important regulator for determining the fate of cell death or survival.

Flavonol and imidazole derivatives block HPV16 E6 activities and reactivate apoptotic pathways in HPV⁺ cells

High-risk human papillomaviruses (HR-HPVs) cause nearly all cases of cervical cancer, as well as approximately 30% of head and neck cancers. HPV 16 E6, one of two major viral oncogenes, protects cells from apoptosis by binding to and accelerating the degradation of several proteins important in apoptotic signaling, including caspase 8 and p53. We proposed that blocking the interactions between HPV E6 and its partners using small molecules had the potential to re-sensitize HPV(+) cells to apoptosis. To test this idea, we screened libraries of small molecules for candidates that could block E6/caspase 8 binding and identified several candidates from different chemical classes. We tested hits for dose-dependency and specificity in vitro and for toxicity in a cell-based assay and then used this information to select the two best candidates for further testing: myricetin, a flavonol, and spinacine, an imidazole amino-acid derivative of histidine. Both compounds clearly inhibited the ability of E6 to bind in vitro to both caspase 8 and E6AP, the protein that mediates p53 degradation. In addition, both compounds were able to increase the level of caspase 8 and p53 in SiHa cervical cancer cells, resulting in an increase of caspase 3/7 activity. Finally, both myricetin and spinacine sensitized HPV(+) cervical and oral cancer cells, but not HPV(-) cervical and oral cancer cells, to apoptosis induced by the cancer-specific ligand TRAIL, as well as the chemotherapeutic agents doxorubicin and cisplatin. New therapies based on this work may improve treatment for HPV(+) cancer patients.

Recent Progress in Therapeutic Treatments and Screening Strategies for the Prevention and Treatment of HPV-Associated Head and Neck Cancer

The rise in human papillomavirus (HPV)-associated head and neck squamous cell carcinoma (HNSCC) has elicited significant interest in the role of high-risk HPV in tumorigenesis. Because patients with HPV-positive HNSCC have better prognoses than do their HPV-negative counterparts, current therapeutic strategies for HPV⁺ HNSCC are increasingly considered to be overly aggressive, highlighting a need for customized treatment guidelines for this cohort. Additional issues include the unmet need for a reliable screening strategy for HNSCC, as well as the ongoing assessment of the efficacy of prophylactic vaccines for the prevention of HPV infections in the head and neck regions. This review also outlines a number of emerging prospects for therapeutic vaccines, as well as for targeted, molecular-based therapies for HPV-associated head and neck cancers. Overall, the future for developing novel and effective therapeutic agents for HPV-associated head and neck tumors is promising; continued progress is critical in order to meet the challenges posed by the growing epidemic.

Novel Functions of the Human Papillomavirus E6 Oncoproteins

Human papillomaviruses (HPVs) infect the epidermis as well as mucous membranes of humans. They are the causative agents of anogenital tract and some oropharyngeal cancers. Infections begin in the basal epithelia, where the viral genome replicates slowly along with its host cell. As infected cells begin to differentiate and progress toward the periphery, the virus drives proliferation in cells that would otherwise be quiescent. To uncouple differentiation from continued cellular propagation, HPVs express two oncoproteins, HPV E6 and E7. This review focuses on high-risk α-HPV E6, which in addition to supporting viral replication has transforming properties. HPV E6 promotes p53 degradation and activates telomerase, but the multifaceted oncoprotein has numerous other functions that are highlighted here.

Redox control of viral carcinogenesis: The human papillomavirus paradigm

BACKGROUND

Cervical cancer is the second most common neoplastic disease among women worldwide. The initiating event of such cancer is the infection with certain types of human papillomavirus (HPV), a very common condition in the general population. However, the majority of HPV infections is subclinical and transitory and is resolved spontaneously. Intriguingly, viral oncogene expression, although necessary, is not per se sufficient to promote cervical cancer and other factors are involved in the progression of infected cells to the full neoplastic phenotype. In this perspective it has been suggested that the redox balance and the oxidative stress (OS) may represent interesting and under-explored candidates as promoting factors in HPV-initiated carcinogenesis.

SCOPE OF THE REVIEW

The current review discusses the possible interplay between the viral mechanisms modulating cell homeostasis and redox sensitive mechanisms. Experimental data and indirect evidences are presented on the activity of viral dependent functions on i) the regulation of enzymes and compounds involved in OS; ii) the protection from oxidation of detoxifying/antiapoptotic enzymes and redox-sensitive transcription factors; iii) the suppression of apoptosis; and iv) the modulation of host microRNAs regulating genes associated with antioxidant defense.

MAJOR CONCLUSIONS

The resulting tangled scenario suggests that viral hosting cells adapt their metabolisms in order to support their growth and survival in the increasingly oxidant micro-environment associated with HPV tumor initiation and progression.

GENERAL SIGNIFICANCE

HPV can modulate the host cell redox homeostasis in order to favor infection and possibly tumor transformation. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

The small splice variant of HPV16 E6, E6, reduces tumor formation in cervical carcinoma xenografts

High-risk types of human papillomavirus (HPV) cause nearly all cases of cervical cancer. The E6 oncoprotein is produced as a full-length variant (E6) as well as several shorter isoforms (E6). E6 inhibits certain oncogenic activities of E6, suggesting that it might play an anti-oncogenic role in vivo. To test this, we created E6-expressing SiHa (HPV(+)) and C33A (HPV(-)) cells, then examined the ability of both the parental and E6-expressing cells to form tumors in nude mice. We found that over-expression of E6 indeed decreased the growth of tumors derived from both SiHa and C33A cells, with the reduction greatest in tumors derived from E6-expressing SiHa cells. These findings point to multiple anti-oncogenic characteristics of E6, some of which likely involve down-regulation of the full-length isoform, and others that are independent of HPV. These data represent the first demonstration of biologically-relevant E6 activities distinct from those of the full-length isoform in vivo.

hpvPDB: An Online Proteome Reserve for Human Papillomavirus

Human papillomavirus (HPV) infection is the leading cause of cancer mortality among women worldwide. The molecular understanding of HPV proteins has significant connotation for understanding their intrusion in the host and designing novel protein vaccines and anti-viral agents, etc. Genomic, proteomic, structural, and disease-related information on HPV is available on the web; yet, with trivial annotations and more so, it is not well customized for data analysis, host-pathogen interaction, strain-disease association, drug designing, and sequence analysis, etc. We attempted to design an online reserve with comprehensive information on HPV for the end users desiring the same. The Human Papillomavirus Proteome Database (hpvPDB) domiciles proteomic and genomic information on 150 HPV strains sequenced to date. Simultaneous easy expandability and retrieval of the strain-specific data, with a provision for sequence analysis and exploration potential of predicted structures, and easy access for curation and annotation through a range of search options at one platform are a few of its important features. Affluent information in this reserve could be of help for researchers involved in structural virology, cancer research, drug discovery, and vaccine design.

HPV-16 exposed mouse embryos: a potential model for pregnancy wastage

PURPOSE

Placentas from spontaneous abortions and preterm deliveries have a higher prevalence of Human papillomavirus (HPV) compared to placentas from elective abortions and term births. The objective was to determine the effects of HPV-16 on the adhesion and implantation properties of early embryo trophoblasts.

METHODS

Two-cell mouse embryos were cultured (medium G2, 5 % CO2, 37 °C) for 72-96 h and exposed to either HPV-16 rich SiHa cell lysates which were refrigerated after mechanical lysis, thawed lysates which had been frozen for freeze/thaw lysis method, or control medium, incubated (4-5 days) and evaluated by microscopy (N = 96 embryos, 3 repeated experiments). Trophoblasts were stained and images were digitized. Adhesion and dimension data were analyzed by Chi-square and t test, respectively.

RESULTS

HPV-16 exposed embryos exhibited less adhesion through reduced implantation compared with the control (combined lysates 53.8 vs. 85.7 %, P < 0.05). Refrigerated and thawed lysate groups had similar reduced implantations (58.3 vs. 50.0 %). Of the embryos with implantation, 100 % in the refrigerated lysates were noted to have loose or abnormal adhesion. This was measured when embryos were noted to be lost after washes with HTF. There was no difference in trophoblast viability among the groups. Total trophoblast area was greater in the HPV-16 exposed frozen lysate group (1,881.8 ± 605.3 vs. control 848.8 ± 298.0 square units, mean ± SEM).

CONCLUSIONS

HPV-16 inhibited trophoblasts adhesion needed for normal implantation, but not embryo development. Total trophoblast spread was increased after HPV-16 exposure suggesting that HPV-16 altered trophoblast migration. These results suggest that HPV-16 may induce abnormal placental growth resulting in pregnancy wastage.

Modulation of apoptotic pathways by human papillomaviruses (HPV): mechanisms and implications for therapy

The ability of the host to trigger apoptosis in infected cells is perhaps the most powerful tool by which viruses can be cleared from the host organism. To avoid elimination by this mechanism, human papillomaviruses (HPV) have developed several mechanisms that enable the cells they infect to elude both extrinsic and intrinsic apoptosis. In this manuscript, we review the current literature regarding how HPV-infected cells avoid apoptosis and the molecular mechanisms involved in these events. In particular, we will discuss the modifications in intrinsic and extrinsic apoptotic pathways caused by proteins encoded by HPV early genes. Many of the current efforts regarding anti-cancer drug development are focused on directing tumor cells to undergo apoptosis. However, the ability of HPV-infected cells to resist apoptotic signals renders such therapies ineffective. Possible mechanisms for overcoming the resistance of HPV-infected tumor cells to anticancer drugs will be discussed.

Small molecule inhibitors of the HPV16-E6 interaction with caspase 8

High-risk strains of human papillomaviruses (HPVs) cause nearly all cases of cervical cancer as well as a growing number of head and neck cancers. The oncogenicity of these viruses can be attributed to the activities of their two primary oncoproteins, E6 and E7. The E6 protein has among its functions the ability to prevent apoptosis of infected cells through its binding to FADD and caspase 8. A small molecule library was screened for candidates that could inhibit E6 binding to FADD and caspase 8. Flavonols were found to possess this activity with the rank order of myricetin>morin>quercetin>kaempferol=galangin≫(apigenin, 7-hydroxyflavonol, rhamnetin, isorhamnetin, geraldol, datiscetin, fisetin, 6-hydroxyflavonol). Counter screening, where the ability of these chosen flavonols to inhibit caspase 8 binding to itself was assessed, demonstrated that myricetin, morin and quercetin inhibited GST-E6 and His-caspase 8 binding in a specific manner. The structure-activity relationships suggested by these data are unique and do not match prior reports on flavonols in the literature for a variety of anticancer assays.

The SARS coronavirus E protein interacts with PALS1 and alters tight junction formation and epithelial morphogenesis

Intercellular tight junctions define epithelial apicobasal polarity and form a physical fence which protects underlying tissues from pathogen invasions. PALS1, a tight junction-associated protein, is a member of the CRUMBS3-PALS1-PATJ polarity complex, which is crucial for the establishment and maintenance of epithelial polarity in mammals. Here we report that the carboxy-terminal domain of the SARS-CoV E small envelope protein (E) binds to human PALS1. Using coimmunoprecipitation and pull-down assays, we show that E interacts with PALS1 in mammalian cells and further demonstrate that the last four carboxy-terminal amino acids of E form a novel PDZ-binding motif that binds to PALS1 PDZ domain. PALS1 redistributes to the ERGIC/Golgi region, where E accumulates, in SARS-CoV-infected Vero E6 cells. Ectopic expression of E in MDCKII epithelial cells significantly alters cyst morphogenesis and, furthermore, delays formation of tight junctions, affects polarity, and modifies the subcellular distribution of PALS1, in a PDZ-binding motif-dependent manner. We speculate that hijacking of PALS1 by SARS-CoV E plays a determinant role in the disruption of the lung epithelium in SARS patients.

The full-length isoform of human papillomavirus 16 E6 and its splice variant E6* bind to different sites on the procaspase 8 death effector domain

Human papillomavirus 16 is a causative agent of most cases of cervical cancer and has also been implicated in the development of some head and neck cancers. The early viral E6 gene codes for two alternatively spliced isoforms, E6(large) and E6*. We have previously demonstrated the differential effects of E6(large) and E6* binding on the expression and stability of procaspase 8, a key mediator of the apoptotic pathway. Additionally, we have reported that E6 binds to the FADD death effector domain (DED) at a novel E6 binding domain. Sequence similarities between the FADD and procaspase 8 DEDs suggested a specific region for E6(large)/procaspase 8 binding, which was subsequently confirmed by mutational analysis as well as by the ability of peptides capable of blocking E6/FADD binding to also block E6(large)/caspase 8 binding. However, the binding of the smaller isoform, E6*, to procaspase 8 occurs at a different region, as deletion and point mutations that disrupt E6(large)/caspase 8 DED binding do not disrupt E6*/caspase 8 DED binding. In addition, peptide inhibitors that can block E6(large)/procaspase 8 binding do not affect the binding of E6* to procaspase 8. These results demonstrate that the residues that mediate E6*/procaspase 8 DED binding localize to a different region on the protein and employ a separate binding motif. This provides a molecular explanation for our initial findings that the two E6 isoforms affect procaspase 8 stability in an opposing manner.

The biological properties of E6 and E7 oncoproteins from human papillomaviruses

More than 100 different human papillomavirus (HPV) types have been isolated so far, and they can be sub-grouped in cutaneous or mucosal according to their ability to infect the skin or the mucosa of the genital or upper-respiratory tracts. A sub-group of human mucosal HPVs, referred to as high-risk HPV types, is responsible for approximately 5% of all human cancers, which represents one-third of all the tumours induced by viruses. Epidemiological and biological studies have shown that HPV16 is the most oncogenic type within the high-risk group. Emerging lines of evidence suggest that, in addition to the high-risk mucosal HPV types, certain cutaneous HPVs are involved in skin cancer. HPV-associated cancers are intimately linked to HPV persistence and the accumulation of chromosomal rearrangements. The products of the early genes, E6 and E7, of the high-risk mucosal HPV types play a key role in both events. Indeed, these proteins have developed a number of strategies to evade host immuno-surveillance allowing viral persistence, and to alter cell cycle and apoptosis control, facilitating the accumulation of DNA damage/mutations. Often, the two oncoproteins target the same cellular pathways with different mechanisms, showing a strong synergism in promoting cellular transformation and neutralizing the immune response. Here, we review most of the findings on the biological properties and molecular mechanisms of the oncoproteins E6 and E7 from mucosal and cutaneous HPV types.

[Molecular basis of cervical carcinogenesis by high-risk human papillomaviruses]

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

Papillomavirus E6 proteins

The papillomaviruses are small DNA viruses that encode approximately eight genes, and require the host cell DNA replication machinery for their viral DNA replication. Thus papillomaviruses have evolved strategies to induce host cell DNA synthesis balanced with strategies to protect the cell from unscheduled replication. While the papillomavirus E1 and E2 genes are directly involved in viral replication by binding to and unwinding the origin of replication, the E6 and E7 proteins have auxillary functions that promote proliferation. As a consequence of disrupting the normal checkpoints that regulate cell cycle entry and progression, the E6 and E7 proteins play a key role in the oncogenic properties of human papillomaviruses with a high risk of causing anogenital cancers (HR HPVs). As a consequence, E6 and E7 of HR HPVs are invariably expressed in cervical cancers. This article will focus on the E6 protein and its numerous activities including inactivating p53, blocking apoptosis, activating telomerase, disrupting cell adhesion, polarity and epithelial differentiation, altering transcription and reducing immune recognition.