Expression of the papillomavirus E2 protein in HeLa cells leads to apoptosis

Hepatitis B doubly spliced protein (HBDSP) promotes hepatocellular carcinoma cell apoptosis via ETS1/GATA2/YY1-mediated p53 transcription

IMPORTANCE

Hepatitis B virus (HBV) spliced variants are associated with viral persistence or pathogenicity. Hepatitis B doubly spliced protein (HBDSP), which has been previously reported as a pleiotropic transactivator protein, can potentially serve as an HBV virulence factor. However, the underlying mechanisms of HBDSP in HBV-associated liver diseases remain to be elucidated. In this study, we revealed that HBDSP promotes cellular apoptosis and induces wt-p53-dependent apoptotic signaling pathway in wt-p53 hepatocellular cells by transactivating p53 transcription, and increases the release of HBV progeny. Therefore, HBDSP may promote the HBV particles release through wt-p53-dependent hepatocellular apoptosis. Our findings suggest that blocking HBDSP-induced wt-p53-dependent apoptosis might have therapeutic values for chronic hepatitis B.

Utilization of a cell-penetrating peptide-adaptor for delivery of human papillomavirus protein E2 into cervical cancer cells to arrest cell growth and promote cell death

BACKGROUND

Human papillomavirus (HPV) is the causative agent of nearly all forms of cervical cancer, which can arise upon viral integration into the host genome and concurrent loss of viral regulatory gene E2. Gene-based delivery approaches show that E2 reintroduction reduces proliferative capacity and promotes apoptosis in vitro.

AIMS

This work explored if our calcium-dependent protein-based delivery system, TAT-CaM, could deliver functional E2 protein directly into cervical cancer cells to limit proliferative capacity and induce cell death.

MATERIALS AND RESULTS

TAT-CaM and the HPV16 E2 protein containing a CaM-binding sequence (CBS-E2) were expressed and purified from Escherichia coli. Calcium-dependent binding kinetics were verified by biolayer interferometry. Equimolar TAT-CaM:CBS-E2 constructs were delivered into the HPV16+ SiHa cell line and uptake verified by confocal microscopy. Proliferative capacity was measured by MTS assay and cell death was measured by release of lactate dehydrogenase. As a control, human microvascular cells (HMECs) were used. As expected, TAT-CaM bound CBS-E2 with high affinity in the presence of calcium and rapidly disassociated upon its removal. After introduction by TAT-CaM, fluorescently labeled CBS-E2 was detected in cellular interiors by orthogonal projections taken at the depth of the nucleus. In dividing cells, E2 relocalized to regions associated with the mitotic spindle. Cells receiving a daily dose of CBS-E2 for 4 days showed a significant reduction in metabolic activity at low doses and increased cell death at high doses compared to controls. This phenotype was retained for 7 days with no further treatments. When subcultured on day 12, treated cells regained their proliferative capacity.

CONCLUSIONS

Using the TAT-CaM platform, bioactive E2 protein was delivered into living cervical cancer cells, inducing senescence and cell death in a time- and dose-dependent manner. These results suggest that this nucleic acid and virus-free delivery method could be harnessed to develop novel, effective protein therapeutics.

The Antiproliferative Effect of the "Early" Protein E2 of Papillomavirus HPV16 on Testis Tumors of Mice Induced by the Injection of HeLa Cells

The antiproliferative effect of the "early" protein E2 of the high-risk oncogenic human papillomavirus HPV16 on mouse testis tumors, which were induced by the intramuscular injection of HeLa cells, was discovered. The regression of tumors was maximum in the first 2 days after the oral vaccination with HPV16 E2 (500 mg per mouse) and then gradually decreased to the control variant. A typical monolayer of HeLa cells on the cultivation flask bottom, of which only 18% were functionally active, appeared after seeding testis tissue cells on DMEM medium.

Carbon-ion irradiation overcomes HPV-integration/E2 gene-disruption induced radioresistance of cervical keratinocytes

To date, only few data exist on mechanisms underlying the human papillomavirus (HPV)-associated irradiation response. It has been suggested, that the viral E2 gene plays an important role in that context. The aim of the current study is to compare the effect of photon- and carbon-ion (12C)-radiation therapy (RT) on cells with different HPV and E2 gene status. We hypothesized that 12C-RT might overcome the radioresistance of E2 gene-disrupted cells. We analyzed four different cell lines that differed in HPV status or E2 gene status. Cells were irradiated with either photons or 12C. Clonogenic survival, cell cycle and expression of Rb and p53 were analyzed. Radiosensitivity seemed to be dependent on E2 gene status and type of RT. 12C-RT led to lower surviving fractions, indicating higher radiosensitivity even in cells with disrupted E2 gene. The observed relative biological effectiveness (RBE) of 12C-RT for C33a/Caski and W12/S12 was 1.3/4 and 2.7/2.5, respectively. Cell cycle regulation after both photon- and 12C-RT was dependent on HPV status and on E2 gene status. Furthermore, the effect of RT on expression of p53 and Rb seemed to be dependent on E2 gene status and type of RT. We showed that 12C-RT overcomes HPV-integration induced radioresistance. The effect of RT on cell cycle regulation as well as on expression of p53 and Rb seemed to be dependent on HPV status, E2 gene status and type of RT. Differences in Rb expression and cell cycle regulation may play a role for enhanced radiosensitivity to 12C-RT of cells with disrupted E2 gene.

Detection of human papillomavirus in cases of head and neck squamous cell carcinoma by RNA-seq and VirTect

Next-generation sequencing provides an opportunity to detect viral species from RNA-seq data of human tissues, but existing computational approaches do not perform optimally on clinical samples. We developed a bioinformatic method called VirTect for detecting viruses in neoplastic human tissues using RNA-seq data. Here, we used VirTect to analyze RNA-seq data from 363 head and neck squamous cell carcinoma (HNSCC) patients and identified 22 human papillomavirus (HPV)-induced HNSCCs. These predictions were validated by manual review of pathology reports on histopathologic specimens. VirTect showed better performance in recall and accuracy compared to the two existing prediction methods, VirusFinder and VirusSeq, in identifying viral sequences from RNA-seq data. The majority of HPV carcinogenesis studies thus far have been performed on cervical cancer and generalized to HNSCC. Our results suggest that carcinogenesis of HPV-induced HNSCC and other cases of HNSCC involve different genes, so understanding the underlying molecular mechanisms will have a significant impact on therapeutic approaches and outcomes. In summary, RNA-seq together with VirTect can be an effective solution for the detection of viruses from tumor samples and can facilitate the clinicopathologic characterization of various types of cancers with broad applications for oncology.

Short half-life of HPV16 E6 and E7 mRNAs sensitizes HPV16-positive tonsillar cancer cell line HN26 to DNA-damaging drugs

Here we show that treatment of the HPV16-positive tonsillar cancer cell line HN26 with DNA alkylating cancer drug melphalan-induced p53 and activated apoptosis. Melphalan reduced the levels of RNA polymerase II and cellular transcription factor Sp1 that were associated with HPV16 DNA. The resulting inhibition of transcription caused a rapid loss of the HPV16 early mRNAs encoding E6 and E7 as a result of their inherent instability. As a consequence of HPV16 E6 and E7 down-regulation, the DNA damage inflicted on the cells by melphalan caused induction of p53 and activation of apoptosis in the HN26 cells. The BARD1-negative phenotype of the HN26 cells may have contributed to the failure to repair DNA damage caused by melphalan, as well as to the efficient apoptosis induction. Finally, nude mice carrying the HPV16 positive tonsillar cancer cells responded better to melphalan than to cisplatin, the chemotherapeutic drug of choice for tonsillar cancer. We concluded that the short half-life of the HPV16 E6 and E7 mRNAs renders HPV16-driven tonsillar cancer cells particularly sensitive to DNA damaging agents such as melphalan since melphalan both inhibits transcription and causes DNA damage.

The Myb-related protein MYPOP is a novel intrinsic host restriction factor of oncogenic human papillomaviruses

The skin represents a physical and chemical barrier against invading pathogens, which is additionally supported by restriction factors that provide intrinsic cellular immunity. These factors detect viruses to block their replication cycle. Here, we uncover the Myb-related transcription factor, partner of profilin (MYPOP) as a novel antiviral protein. It is highly expressed in the epithelium and binds to the minor capsid protein L2 and the DNA of human papillomaviruses (HPV), which are the primary causative agents of cervical cancer and other tumors. The early promoter activity and early gene expression of the oncogenic HPV types 16 and 18 is potently silenced by MYPOP. Cellular MYPOP-depletion relieves the restriction of HPV16 infection, demonstrating that MYPOP acts as a restriction factor. Interestingly, we found that MYPOP protein levels are significantly reduced in diverse HPV-transformed cell lines and in HPV-induced cervical cancer. Decades ago it became clear that the early oncoproteins E6 and E7 cooperate to immortalize keratinocytes by promoting degradation of tumor suppressor proteins. Our findings suggest that E7 stimulates MYPOP degradation. Moreover, overexpression of MYPOP blocks colony formation of HPV and non-virally transformed keratinocytes, suggesting that MYPOP exhibits tumor suppressor properties.

Therapeutic vaccines for high-risk HPV-associated diseases

Cancer is the second leading cause of death worldwide, and it is estimated that Human papillomavirus (HPV) related cancers account for 5% of all human cancers. Current HPV vaccines are extremely effective at preventing infection and neoplastic disease; however, they are prophylactic and do not clear established infections. Therapeutic vaccines which trigger cell-mediated immune responses for the treatment of established infections and malignancies are therefore required. The E6 and E7 early genes are ideal targets for vaccine therapy due to their role in disruption of the cell cycle and their constitutive expression in premalignant and malignant tissues. Several strategies have been investigated for the development of therapeutic vaccines, including live-vector, nucleic acid, peptide, protein-based and cell-based vaccines as well as combinatorial approaches, with several vaccine candidates progressing to clinical trials. With the current understanding of the HPV life cycle, molecular mechanisms of infection, carcinogenesis, tumour biology, the tumour microenvironment and immune response mechanisms, an approved HPV therapeutic vaccine seems to be a goal not far from being achieved. In this article, the status of therapeutic HPV vaccines in clinical trials are reviewed, and the potential for plant-based vaccine production platforms described.

HPV16 viral load and physical state measurement as a potential immediate triage strategy for HR-HPV-infected women: a study in 644 women with single HPV16 infections

High genome copy number (viral load) of human papillomavirus (HPV) is being discussed as a risk factor for high-grade cervical lesions. However, conflicting data about the integration status or viral load of the virus as risk factors for prevalent high-grade squamous intraepithelial lesions (HSIL) are found in the literature. To investigate whether viral load and/or integration status are indicative for prevalent ASCUS/LSIL or HSIL, we determined the HPV16 viral load and the physical state of the genome in 644 women with single HPV16 infections stratified by their cytology results from a large Danish population-based cohort consisting of 40,399 women. Cervical smear samples were tested using a multiplex quantitative real-time PCR (qPCR) with primers specific for HPV16 E2, E6 and beta actin, allowing simultaneous determination of the genome's physical state and the viral copy number per cell. The associations of viral load and physical state with cervical abnormalities were assessed using multinomial logistic regression. We found that a 10-fold increase in viral load was significantly associated with the presence of ASCUS/LSIL (OR=3.91; 95% CI, 2.49-6.13) and HSIL (OR=4.1; 95% CI, 2.45-6.68). A significant association with HSIL was observed for primarily integrated genomes (OR=6.68; 95% CI, 1.45-30.8). Among women with integrated viral genomes, we observed a trend towards increased risk of ASCUS/LSIL (OR=1.32; 95% CI -2.90-3.44) and HSIL (OR=5.10; 95% CI -0.67-38.9) per 10-fold increase in viral load, although not statistically significant. In conclusion, increasing viral load and integrated viral genomes were significantly associated with prevalent HSIL, thus indicating that viral load and physical state may potentially be useful triage markers for HPV16-positive women during cervical screening.

Human Papillomavirus Types 16 and 18 Early-expressed Proteins Differentially Modulate the Cellular Redox State and DNA Damage

Oxidative stress has been proposed as a risk factor for cervical cancer development. However, few studies have evaluated the redox state associated with human papillomavirus (HPV) infection. The aim of this work was to determine the role of the early expressed viral proteins E1, E2, E6 and E7 from HPV types 16 and 18 in the modulation of the redox state in an integral form. Therefore, generation of reactive oxygen species (ROS), concentration of reduced glutathione (GSH), levels and activity of the antioxidant enzymes catalase and superoxide dismutase (SOD) and deoxyribonucleic acid (DNA) damage, were analysed in epithelial cells ectopically expressing the viral proteins. Our research shows that E6 oncoproteins decreased GSH and catalase protein levels, as well as its enzymatic activity, which was associated with an increase in ROS production and DNA damage. In contrast, E7 oncoproteins increased GSH, as well as catalase protein levels and its activity, which correlated with a decrease in ROS without affecting DNA integrity. The co-expression of both E6 and E7 oncoproteins neutralized the effects that were independently observed for each of the viral proteins. Additionally, the combined expression of E1 and E2 proteins increased ROS levels with the subsequent increase in the marker for DNA damage phospho-histone 2AX (γH2AX). A decrease in GSH, as well as SOD2 levels and activity were also detected in the presence of E1 and E2, even though catalase activity increased. This study demonstrates that HPV early expressed proteins differentially modulate cellular redox state and DNA damage.

High-Risk Alphapapillomavirus Oncogenes Impair the Homologous Recombination Pathway

Persistent high-risk genus human Alphapapillomavirus (HPV) infections cause nearly every cervical carcinoma and a subset of tumors in the oropharyngeal tract. During the decades required for HPV-associated tumorigenesis, the cellular genome becomes significantly destabilized. Our analysis of cervical tumors from four separate data sets found a significant upregulation of the homologous-recombination (HR) pathway genes. The increased abundance of HR proteins can be replicated in primary cells by expression of the two HPV oncogenes (E6 and E7) required for HPV-associated transformation. HPV E6 and E7 also enhanced the ability of HR proteins to form repair foci, and yet both E6 and E7 reduce the ability of the HR pathway to complete double-strand break (DSB) repair by about 50%. The HPV oncogenes hinder HR by allowing the process to begin at points in the cell cycle when the lack of a sister chromatid to serve as a homologous template prevents completion of the repair. Further, HPV E6 attenuates repair by causing RAD51 to be mislocalized away from both transient and persistent DSBs, whereas HPV E7 is only capable of impairing RAD51 localization to transient lesions. Finally, we show that the inability to robustly repair DSBs causes some of these lesions to be more persistent, a phenotype that correlates with increased integration of episomal DNA. Together, these data support our hypothesis that HPV oncogenes contribute to the genomic instability observed in HPV-associated malignancies by attenuating the repair of damaged DNA.IMPORTANCE This study expands the understanding of HPV biology, establishing a direct role for both HPV E6 and E7 in the destabilization of the host genome by blocking the homologous repair of DSBs. To our knowledge, this is the first time that both viral oncogenes were shown to disrupt this DSB repair pathway. We show that HPV E6 and E7 allow HR to initiate at an inappropriate part of the cell cycle. The mislocalization of RAD51 away from DSBs in cells expressing HPV E6 and E7 hinders HR through a distinct mechanism. These observations have broad implications. The impairment of HR by HPV oncogenes may be targeted for treatment of HPV⁺ malignancies. Further, this attenuation of repair suggests HPV oncogenes may contribute to tumorigenesis by promoting the integration of the HPV genome, a common feature of HPV-transformed cells. Our data support this idea since HPV E6 stimulates the integration of episomes.

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.

Insights into the mechanism of human papillomavirus E2-induced procaspase-8 activation and cell death

High-risk human papillomavirus (HR-HPV) E2 protein, the master regulator of viral life cycle, induces apoptosis of host cell that is independent of its virus-associated regulatory functions. E2 protein of HR-HPV18 has been found to be involved in novel FADD-independent activation of caspase-8, however, the molecular basis of this unique non-death-fold E2-mediated apoptosis is poorly understood. Here, with an interdisciplinary approach that involves in silico, mutational, biochemical and biophysical probes, we dissected and characterized the E2-procasapse-8 binding interface. Our data demonstrate direct non-homotypic interaction of HPV18 E2 transactivation domain (TAD) with α2/α5 helices of procaspase-8 death effector domain-B (DED-B). The observed interaction mimics the homotypic DED-DED complexes, wherein the conserved hydrophobic motif of procaspase-8 DED-B (F122/L123) occupies a groove between α2/α3 helices of E2 TAD. This interaction possibly drives DED oligomerization leading to caspase-8 activation and subsequent cell death. Furthermore, our data establish a model for E2-induced apoptosis in HR-HPV types and provide important clues for designing E2 analogs that might modulate procaspase-8 activation and hence apoptosis.

Stranglehold on the spindle assembly checkpoint: the human papillomavirus E2 protein provokes BUBR1-dependent aneuploidy

The Human Papillomavirus (HPV) E2 protein, which inhibits the E6 and E7 viral oncogenes, is believed to have anti-oncogenic properties. Here, we challenge this view and show that HPV-18 E2 over-activates the Spindle Assembly Checkpoint (SAC) and induces DNA breaks in mitosis followed by aneuploidy. This phenotype is associated with interaction of E2 with the Mitotic Checkpoint Complex (MCC) proteins Cdc20, MAD2 and BUBR1. While BUBR1 silencing rescues the mitotic phenotype induced by E2, p53 silencing or presence of E6/E7 (inactivating p53 and increasing BUBR1 levels respectively) both amplify it. This work pinpoints E2 as a key protein in the initiation of HPV-induced cervical cancer and identifies the SAC as a target for oncogenic pathogens. Moreover, our results suggest a role of p53 in regulating the mitotic process itself and highlight SAC over-activation in a p53-negative context as a highly pathogenic event.

Recent Insights into the Control of Human Papillomavirus (HPV) Genome Stability, Loss, and Degradation

Most human papillomavirus (HPV) antiviral strategies have focused upon inhibiting viral DNA replication, but it is increasingly apparent that viral DNA levels can be chemically controlled by approaches that promote its instability. HPVs and other DNA viruses have a tenuous relationship with their hosts. They must replicate and hide from the DNA damage response (DDR) and innate immune systems, which serve to protect cells from foreign or “non-self” DNA, and yet they draft these same systems to support their life cycles. DNA binding antiviral agents promoting massive viral DNA instability and elimination are reviewed. Mechanistic studies of these agents have identified genetic antiviral enhancers and repressors, antiviral sensitizers, and host cell elements that protect and stabilize HPV genomes. Viral DNA degradation appears to be an important means of controlling HPV DNA levels in some cases, but the underlying mechanisms remain poorly understood. These findings may prove useful not only for understanding viral DNA persistence but only in devising future antiviral strategies.

β-HPV 5 and 8 E6 disrupt homology dependent double strand break repair by attenuating BRCA1 and BRCA2 expression and foci formation

Recent work has explored a putative role for the E6 protein from some β-human papillomavirus genus (β-HPVs) in the development of non-melanoma skin cancers, specifically β-HPV 5 and 8 E6. Because these viruses are not required for tumor maintenance, they are hypothesized to act as co-factors that enhance the mutagenic capacity of UV-exposure by disrupting the repair of the resulting DNA damage. Supporting this proposal, we have previously demonstrated that UV damage signaling is hindered by β-HPV 5 and 8 E6 resulting in an increase in both thymine dimers and UV-induced double strand breaks (DSBs). Here we show that β-HPV 5 and 8 E6 further disrupt the repair of these DSBs and provide a mechanism for this attenuation. By binding and destabilizing a histone acetyltransferase, p300, β-HPV 5 and 8 E6 reduce the enrichment of the transcription factor at the promoter of two genes critical to the homology dependent repair of DSBs (BRCA1 and BRCA2). The resulting diminished BRCA1/2 transcription not only leads to lower protein levels but also curtails the ability of these proteins to form repair foci at DSBs. Using a GFP-based reporter, we confirm that this reduced foci formation leads to significantly diminished homology dependent repair of DSBs. By deleting the p300 binding domain of β-HPV 8 E6, we demonstrate that the loss of robust repair is dependent on viral-mediated degradation of p300 and confirm this observation using a combination of p300 mutants that are β-HPV 8 E6 destabilization resistant and p300 knock-out cells. In conclusion, this work establishes an expanded ability of β-HPV 5 and 8 E6 to attenuate UV damage repair, thus adding further support to the hypothesis that β-HPV infections play a role in skin cancer development by increasing the oncogenic potential of UV exposure.

Human Papillomaviruses are a family of viruses with over 100 different members that infect mucous membranes and skin. Infections with some of these viruses are linked to cancers of the cervix and oropharynx. In this work, we explore the question of whether other members of this virus family may also contribute to skin cancer by inhibiting the ability of cells to repair the damage caused from UV exposure. Here, we build on our previous work showing that the E6 protein from two of these viruses (β-HPV 5 and 8) reduces the cellular response to UV damage by decreasing the abundance of two cellular proteins (p300 and ATR) involved in repairing the UV-damaged DNA, leading to more double strand DNA breaks following UV exposure. Here we show that the loss of p300 has further deleterious consequences, specifically that it results in diminished expression of two proteins (BRCA1 and BRCA2) involved in the repair of double strand breaks. Our data shows that this results in fewer BRCA1 and BRCA2 repair foci forming at sites of damage and ultimately in attenuated repair of these lesions. Together, this work provides further support for a link between β-HPV infections and skin cancer.

Re-expression of HPV16 E2 in SiHa (human cervical cancer) cells potentiates NF-κB activation induced by TNF-α concurrently increasing senescence and survival

Re-expression of E2 in human papillomavirus (HPV) transformed tumour cells can induce apoptosis; however, some evidences also attribute an important role to E2 in sustaining tumorigenesis. In the present paper, we studied the effects of tumour necrosis factor (TNF)-α-mediated NF-κB (nuclear factor kappa-light-chain-enhancer of activated B-cells) activation on E2-induced senescence in HPV16-integrated SiHa cells. The results show that E2 inhibits endogenous E6 gene expression and sensitizes SiHa cells to TNF-α-induced NF-κB activation. Under this condition there was an increase in the expression of senescent proteins p53, p21, p27 and p16 and senescence-associated (SA)-β-galactosidase activity indicating that TNF-α augments E2-mediated senescence. Re-expression of E2 expression with TNF-α treatment resulted in an increase in the expression of anti-apoptotic Bcl2 (B-cell lymphoma 2) protein and other pro-survival genes like cyclin D1 (cyc D1), survivin and hTERT (human telomerase reverse transcriptase). Concomitantly, E2 + TNF-α combination increased the survival of SiHa cells by positive changes in viability, proliferation and colony formation. E2-induced apoptotic tendency shifted towards senescence in presence of TNF-α by arresting the cells at both G₀/G₁ and G₂/M phases, thus enhancing cell survival. Another observation in the present study is the significant up-regulation of key senescence messaging factors regulated by NF-κB namely interleukin (IL)-6, IL-8, high-mobility group protein A (HMGA)1 and B (HMGB)1 in E2-transfected cells treated with TNF-α. Our data provide a mechanistic basis and a new insight for the role of TNF-α and E2 in linking cellular senescence, tumorigenesis and HPV re-infection.

Human papillomavirus (HPV)16 E2 potentiates NF-κB (nuclear factor kappa-light-chain-enhancer of activated B-cells) activation induced by tumour necrosis factor (TNF)-α in SiHa (human cervical cancer) cells and significantly influences cell viability, apoptosis and expression of pro-survival genes regulated by NF-κB.

Detection of rare and possibly carcinogenic human papillomavirus genotypes as single infections in invasive cervical cancer

The contribution of carcinogenic human papillomavirus (HPV) types to the burden of cervical cancer has been well established. However, the role and contribution of phylogenetically related HPV genotypes and rare variants remains uncertain. In a recent global study of 8977 HPV-positive invasive cervical carcinomas (ICCs), the genotype remained unidentified in 3.7% by the HPV SPF10 PCR-DEIA-LiPA25 (version 1) algorithm. The 331 ICC specimens with unknown genotype were analysed by a novel sequence methodology, using multiple selected short regions in L1. This demonstrated HPV genotypes that have infrequently or never been detected in ICC, ie HPV26, 30, 61, 67, 68, 69, 73 and 82, and rare variants of HPV16, 18, 26, 30, 34, 39, 56, 67, 68, 69, 82 and 91. These are not identified individually by LiPA25 and only to some extent by other HPV genotyping assays. Most identified genotypes have a close phylogenetic relationship with established carcinogenic HPVs and have been classified as possibly carcinogenic by IARC. Except for HPV85, all genotypes in α-species 5, 6, 7, 9 and 11 were encountered as single infections in ICCs. These species of established and possibly carcinogenic HPV types form an evolutionary clade. We have shown that the possibly carcinogenic types were detected only in squamous cell carcinomas, which were often keratinizing and diagnosed at a relatively higher mean age (55.3 years) than those associated with established carcinogenic types (50.9 years). The individual frequency of the possibly carcinogenic types in ICCs is low, but together they are associated with 2.25% of the 8338 included ICCs with a single HPV type. This fraction is greater than seven of the established carcinogenic types individually. This study provides evidence that possibly carcinogenic HPV types occur as single infections in invasive cervical cancer, strengthening the circumstantial evidence of a carcinogenic role.

Regression of human papillomavirus intraepithelial lesions is induced by MVA E2 therapeutic vaccine

Human papilloma viruses can induce warts, condylomas, and other intraepithelial cervical lesions that can progress to cancer. Cervical cancer is a serious problem in developing countries because early detection is difficult, and thus proper early treatment is many times missing. In this phase III clinical trial, we evaluated the potential use of MVA E2 recombinant vaccinia virus to treat intraepithelial lesions associated with papillomavirus infection. A total of 1176 female and 180 male patients with intraepithelial lesions were studied. They were injected with 10(7) MVA E2 virus particles directly into their uterus, urethra, vulva, or anus. Patients were monitored by colposcopy and cytology. Immune response was determined by measuring the antibody titer against MVA E2 virus and by analyzing the cytotoxic activity against cancer cells bearing papillomavirus DNA. Papillomavirus was determined by the Hybrid Capture method or by polymerase chain reaction analysis. By histology, 1051 (89.3%) female patients showed complete elimination of lesions after treatment with MVA E2. In 28 (2.4%) female patients, the lesion was reduced to CIN 1. Another 97 (8.3%) female patients presented isolated koilocytes after treatment. In men, all lesions were completely eliminated. All MVA E2-treated patients developed antibodies against the MVA E2 vaccine and generated a specific cytotoxic response against papilloma-transformed cells. Papillomavirus DNA was not detected after treatment in 83% of total patients treated. MVA E2 did not generate any apparent side effects. These data suggest that therapeutic vaccination with MVA E2 vaccine is an excellent candidate to stimulate the immune system and generate regression in intraepithelial lesions when applied locally.

Berberine alters epigenetic modifications, disrupts microtubule network, and modulates HPV-18 E6-E7 oncoproteins by targeting p53 in cervical cancer cell HeLa: a mechanistic study including molecular docking

Increased evidence of chemo-resistance, toxicity and carcinogenicity necessitates search for alternative approaches for determining next generation cancer therapeutics and targets. We therefore tested the efficacy of plant alkaloid berberine on human papilloma virus (HPV) -18 positive cervical cancer cell HeLa systematically-involving certain cellular, viral and epigenetic factors. We observed disruptions of microtubule network and changes in membrane topology due to berberine influx through confocal and atomic force microscopies (AFM). We examined nuclear uptake, internucleosomal DNA damages, mitochondrial membrane potential (MMP) alterations and cell migration assays to validate possible mode of cell death events. Analytical data on interactions of berberine with pBR322 through fourier transform infrared (FTIR) and gel migration assay strengthen berberine׳s biologically significant DNA binding abilities. We measured cellular uptake, DNA ploidy and DNA strand-breaks through fluorescence activated cell sorting (FACS). To elucidate epigenetic modifications, in support of DNA binding associated processes, if any, we conducted methylation-specific restriction enzyme (RE) assay, methylation specific-PCR (MSP) and expression studies of histone proteins. We also analyzed differential interactions and localization of cellular tumor suppressor p53 and viral oncoproteins HPV-18 E6-E7 through siRNA approach. We further made in-silico approaches to determine possible binding sites of berberine on histone proteins. Overall results indicated cellular uptake of berberine through cell membrane depolarization causing disruption of microtubule networks and its biological DNA binding abilities that probably contributed to epigenetic modifications. Results of modulation in p53 and viral oncoproteins HPV-18 E6-E7 by berberine further proved its potential as a promising chemotherapeutic agent in cervical cancer.

Regulation of human genome expression and RNA splicing by human papillomavirus 16 E2 protein

Human papillomavirus 16 (HPV16) is causative in human cancer. The E2 protein regulates transcription from and replication of the viral genome; the role of E2 in regulating the host genome has been less well studied. We have expressed HPV16 E2 (E2) stably in U2OS cells; these cells tolerate E2 expression well and gene expression analysis identified 74 genes showing differential expression specific to E2. Analysis of published gene expression data sets during cervical cancer progression identified 20 of the genes as being altered in a similar direction as the E2 specific genes. In addition, E2 altered the splicing of many genes implicated in cancer and cell motility. The E2 expressing cells showed no alteration in cell growth but were altered in cell motility, consistent with the E2 induced altered splicing predicted to affect this cellular function. The results present a model system for investigating E2 regulation of the host genome.

HPV16 E2-mediated potentiation of NF-κB activation induced by TNF-α involves parallel activation of STAT3 with a reduction in E2-induced apoptosis

Human papilloma virus is associated with cervical and other tumors, and several cellular conditions also play an important role in carcinogenesis. Human papilloma virus (HPV)-infected cells exhibit activation of NF-κB and STAT3 (mediators of inflammation), but little is known about their regulation by HPV. This study attempts to understand the role of HPV16 E2, an important early protein of HPV16, in the regulation of NF-κB and STAT3 by reporter assays, quantitative reverse transcriptase-polymerase chain reaction, and immunoblotting. We demonstrate that E2 enhances NF-κB activation induced by TNF-α, a proinflammatory cytokine, in both non-tumor- and tumor-derived epithelial cell lines besides potentiating STAT3 transcriptional activity induced by TNF-α in HEK293 cells. E2 increases the expression of RelA and its transcriptional activation, and retention of E2 was observed in the nucleus with significant interaction with RelA (immunoprecipitation) upon TNF-α treatment. Transfection with shRNA-RelA or pretreatment with a STAT3 inhibitor had a negative effect on the ability of E2 to enhance TNF-α-induced NF-κB activation. Experiments with co-expression of a mutant of STAT3 with E2 also suggested that the activation of STAT3 is indispensible for TNF-α-induced NF-κB activation. Inhibition of STAT3 activation enhanced E2-induced apoptosis, whereas parallel activation of NF-κB and STAT3 by the combined action of E2 and TNF-α increased the expression of their common targets, cyclin-D1, c-Myc, survivin, and Bcl-2, leading to a decrease in E2-induced apoptosis (viability and cell cycle). Our results reveal novel mechanisms by which E2 may regulate NF-κB and STAT3 activation in the presence of TNF-α with implications on the survival of HPV-infected cells.

E2 proteins of high risk human papillomaviruses down-modulate STING and IFN-κ transcription in keratinocytes

In the early stages of human papillomavirus (HPV) infection, the viral proteins elicit specific immune responses that can participate to regression of ano-genital lesions. HPV E6 protein for instance can reduce type I interferon (IFN) including IFN-κ that is involved in immune evasion and HPV persistence. To evaluate the role of E2 protein in innate immunity in HPV16-associated cervical lesions, genome-wide expression profiling of human primary keratinocytes (HPK) transduced by HPV16 E2 was investigated using microarrays and innate immunity associated genes were specifically analyzed. The analyses showed that the expression of 779 genes was modulated by HPV16E2 and 92 of them were genes associated with innate immunity. Notably IFN-κ and STING were suppressed in HPK expressing the E2 proteins of HPV16 or HPV18 and the trans-activation amino-terminal domain of E2 was involved in the suppressive effect. The relationship between STING, IFN-κ and interferon stimulated genes (ISGs) in HPK was confirmed by gene silencing and real time PCR. The expression of STING and IFN-κ were further determined in clinical specimens by real time PCR. STING and IFN-κ were down-modulated in HPV positive low grade squamous intraepithelial lesions compared with HPV negative controls. This study demonstrates that E2 proteins of high risk HPV reduce STING and IFN-κ transcription and its downstream target genes that might be an immune evasion mechanism involved in HPV persistence and cervical cancer development.

Manipulation of cellular DNA damage repair machinery facilitates propagation of human papillomaviruses

In general, the interplay among viruses and DNA damage repair (DDR) pathways can be divided based on whether the interaction promotes or inhibits the viral lifecycle. The propagation of human papillomaviruses is both promoted and inhibited by DDR proteins. As a result, HPV proteins both activate repair pathways, such as the ATM and ATR pathways, and inhibit other pathways, most notably the p53 signaling pathway. Indeed, the role of HPV proteins, with regard to the DDR pathways, can be divided into two broad categories. The first set of viral proteins, HPV E1 and E2 activate a DNA damage response and recruit repair proteins to viral replication centers, where these proteins are likely usurped to replicate the viral genome. Because the activation of the DDR response typically elicits a cell cycle arrest that would impeded the viral lifecycle, the second set of HPV proteins, HPV E6 and E7, prevents the DDR response from pausing cell cycle progression or inducing apoptosis. This review provides a detailed account of the interactions among HPV proteins and DDR proteins that facilitate HPV propagation.

A functional interaction of E7 with B-Myb-MuvB complex promotes acute cooperative transcriptional activation of both S- and M-phase genes. (129 c)

High-risk human papillomaviruses are causative agents of cervical cancer. Viral protein E7 is required to establish and maintain the pro-oncogenic phenotype in infected cells, but the molecular mechanisms by which E7 promotes carcinogenesis are only partially understood. Our transcriptome analyses in primary human fibroblasts transduced with the viral protein revealed that E7 activates a group of mitotic genes via the activator B-Myb-MuvB complex. We show that E7 interacts with the B-Myb, FoxM1 and LIN9 components of this activator complex, leading to cooperative transcriptional activation of mitotic genes in primary cells and E7 recruitment to the corresponding promoters. E7 interaction with LIN9 and FoxM1 depended on the LXCXE motif, which is also required for pocket protein interaction and degradation. Using E7 mutants for the degradation of pocket proteins but intact for the LXCXE motif, we demonstrate that E7 functional interaction with the B-Myb-MuvB complex and pocket protein degradation are two discrete functions of the viral protein that cooperate to promote acute transcriptional activation of mitotic genes. Transcriptional level of E7 in patient's cervical lesions at different stages of progression was shown to correlate with those of B-Myb and FoxM1 as well as other mitotic gene transcripts, thereby linking E7 with cellular proliferation and progression in cervical cancer in vivo. E7 thus can directly activate the transcriptional levels of cell cycle genes independently of pocket protein stability.

The papillomavirus E2 proteins

The papillomavirus E2 proteins are pivotal to the viral life cycle and have well characterized functions in transcriptional regulation, initiation of DNA replication and partitioning the viral genome. The E2 proteins also function in vegetative DNA replication, post-transcriptional processes and possibly packaging. This review describes structural and functional aspects of the E2 proteins and their binding sites on the viral genome. It is intended to be a reference guide to this viral protein.

Localization of HPV-18 E2 at mitochondrial membranes induces ROS release and modulates host cell metabolism

Papillomavirus E2 proteins are predominantly retained in the nuclei of infected cells, but oncogenic (high-risk) HPV-18 and 16 E2 can shuttle between the host nucleus and cytoplasm. We show here that cytoplasmic HPV-18 E2 localizes to mitochondrial membranes, and independent mass spectrometry analyses of the E2 interactome revealed association to the inner mitochondrial membrane including components of the respiratory chain. Mitochondrial E2 association modifies the cristae morphology when analyzed by electron microscopy and increases production of mitochondrial ROS. This ROS release does not induce apoptosis, but instead correlates with stabilization of HIF-1α and increased glycolysis. These mitochondrial functions are not shared by the non-oncogenic (low-risk) HPV-6 E2 protein, suggesting that modification of cellular metabolism by high-risk HPV E2 proteins could play a role in carcinogenesis by inducing the Warburg effect.

The modulation of apoptosis by oncogenic viruses

Transforming viruses can change a normal cell into a cancer cell during their normal life cycle. Persistent infections with these viruses have been recognized to cause some types of cancer. These viruses have been implicated in the modulation of various biological processes, such as proliferation, differentiation and apoptosis. The study of infections caused by oncogenic viruses had helped in our understanding of several mechanisms that regulate cell growth, as well as the molecular alterations leading to cancer. Therefore, transforming viruses provide models of study that have enabled the advances in cancer research. Viruses with transforming abilities, include different members of the Human Papillomavirus (HPV) family, Hepatitis C virus (HCV), Human T-cell Leukemia virus (HTLV-1), Epstein Barr virus (EBV) and Kaposi’s Sarcoma Herpesvirus (KSHV).

Apoptosis, or programmed cell death, is a tightly regulated process that plays an important role in development and homeostasis. Additionally, it functions as an antiviral defense mechanism. The deregulation of apoptosis has been implicated in the etiology of diverse diseases, including cancer. Oncogenic viruses employ different mechanisms to inhibit the apoptotic process, allowing the propagation of infected and damaged cells. During this process, some viral proteins are able to evade the immune system, while others can directly interact with the caspases involved in apoptotic signaling. In some instances, viral proteins can also promote apoptosis, which may be necessary for an accurate regulation of the initial stages of infection.

Current understanding of the role of the Brd4 protein in the papillomavirus lifecycle

The Brd4 protein is an epigenetic reader that is central to regulation of cellular transcription and mitotic bookmarking. The transcription and replication proteins of many viruses interact with Brd4. We describe the multiple roles of Brd4 in the papillomavirus lifecycle.

Structural insights into a wildtype domain of the oncoprotein E6 and its interaction with a PDZ domain

The high-risk human papilloma virus (HPV) oncoproteins E6 and E7 interact with key cellular regulators and are etiological agents for tumorigenesis and tumor maintenance in cervical cancer and other malignant conditions. E6 induces degradation of the tumor suppressor p53, activates telomerase and deregulates cell polarity. Analysis of E6 derived from a number of high risk HPV finally yielded the first structure of a wild-type HPV E6 domain (PDB 2M3L) representing the second zinc-binding domain of HPV 51 E6 (termed 51Z2) determined by NMR spectroscopy. The 51Z2 structure provides clues about HPV-type specific structural differences between E6 proteins. The observed temperature sensitivity of the well-folded wild-type E6 domain implies a significant malleability of the oncoprotein in vivo. Hence, the structural differences between individual E6 and their malleability appear, together with HPV type-specific surface exposed side-chains, to provide the structural basis for the different interaction networks reported for individual E6 proteins. Furthermore, the interaction of 51Z2 with a PDZ domain of hDlg was analyzed. Human Dlg constitutes a prototypic representative of the large family of PDZ proteins regulating cell polarity, which are common targets of high-risk HPV E6. Nine C-terminal residues of 51Z2 interact with the second PDZ domain of hDlg2. Surface plasmon resonance in conjunction with the NMR spectroscopy derived complex structure (PDB 2M3M) indicate that E6 residues N-terminal to the canonical PDZ-BM of E6 significantly contribute to this interaction and increase affinity. The structure of the complex reveals how residues outside of the classical PDZ-BM enhance the affinity of E6 towards PDZ domains. Such mechanism facilitates successful competition of E6 with cellular PDZ-binding proteins and may apply to PDZ-binding proteins of other viruses as well.

Liberation of functional p53 by proteasome inhibition in human papilloma virus-positive head and neck squamous cell carcinoma cells promotes apoptosis and cell cycle arrest

Human papilloma virus (HPV) infection represents an emerging risk factor in head and neck squamous cell carcinoma (HNSCC). In contrast to HPV-negative HNSCC, most cases of HPV-positive HNSCC encode wild-type p53, although the p53 protein in these cells is rapidly degraded via HPV E6-mediated ubiquitination and subsequent proteasomal degradation. This unique feature of HPV-positive HNSCC has raised hope that liberation of wild-type p53 from the E6 protein may have therapeutic benefit in this disease. Indeed, suppression of E6 expression promotes apoptosis in HPV-positive HNSCC cell lines. However, the role of p53 in mediating this cell death has not been determined. Here, we demonstrate that siRNAs targeting the E6/E7 RNA, or treatment with the proteasome inhibitor bortezomib, resulted in upregulation of functional p53 and p53 gene targets in three HPV-positive HNSCC cell lines, but not in HPV-negative HNSCC cells. Apoptosis induced by E6/E7 siRNA in HPV-positive cells was found to be dependent on p53, while bortezomib-induced cell death was modestly p53-dependent. Treatment with subtoxic doses of bortezomib led to cell cycle arrest in HPV-positive, but not HPV-negative HNSCC cells. Moreover, this cell cycle arrest was mediated by p53 and the cell cycle inhibitor p21, the product of a p53 target gene. Collectively, these findings establish that wild-type p53 encoded by HPV-positive HNSCC cells, once liberated from HPV E6, can play important roles in promoting apoptosis and cell cycle arrest.

The transcription factors TBX2 and TBX3 interact with human papillomavirus 16 (HPV16) L2 and repress the long control region of HPVs

The minor capsid protein L2 of human papillomaviruses (HPVs) has multiple functions during the viral life cycle. Although L2 is required for effective invasion and morphogenesis, only a few cellular interaction partners are known so far. Using yeast two-hybrid screening, we identified the transcription factor TBX2 as a novel interaction partner of HPV type 16 (HPV16) L2. Coimmunoprecipitations and immunofluorescence analyses confirmed the L2-TBX2 interaction and revealed that L2 also interacts with TBX3, another member of the T-box family. Transcription of the early genes during HPV infection is under the control of an upstream enhancer and early promoter region, the long control region (LCR). In promoter-reporter gene assays, we observed that TBX2 and TBX3 repress transcription from the LCR and that this effect is enhanced by L2. Repression of the HPV LCR by TBX2/3 seems to be a conserved mechanism, as it was also observed with the LCRs of different HPV types. Finally, interaction of TBX2 with the LCR was detected by chromatin immunoprecipitation, and we found a strong colocalization of L2 and TBX2 in HPV16-positive cervical intraepithelial neoplasia (CIN) I-II tissue sections. These results suggest that TBX2/3 might play a role in the regulation of HPV gene expression during the viral life cycle.

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.

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.

The transcriptional regulator gene E2 of the Human Papillomavirus (HPV) 16 influences the radiosensitivity of cervical keratinocytes

Background

Clinical studies have demonstrated that HPV induced tumors constitute a specific subclass of cancer with a better response to radiation treatment. The purpose of this study was to investigate meaning of viral E2-gene for radiosensitivity.

Methods

W12 cells contain episomal HPV 16 genomes, whereas S12 cells, which derive from the W12 line, contain HPV DNA as integrated copies. Clonogenic survival was analyzed using 96-well in vitro test. Using flow cytometry cell cycle analyses were performed. Expression of pRb and p53 were analyzed using intracellular staining.

Results

W12 cells (intact E2 gene) showed a lower survival fraction than S12 cells. W12 cells developed a G2/M block 24 h after irradiation with 2 Gy whereas S12 showed no G2/M bloc. After irradiation S12 cells developed polyploidy and pRb-positive cells decreased. W12 cells showed no change of pRb-positive cells.

Conclusions

Depending on E2 gene status differences in cell cycle regulation might cause radioresistance. The E2/E7/pRb pathway seems to influence HPV-induced radiosensitivity. Our experiments demonstrated an effect of HPV on radiosensitivity of cervical keratinocytes via viral transcription regulator E2 pathway.

Oncolytic viruses & their specific targeting to tumour cells

Cancer is one of the major causes of death worldwide. In spite of achieving significant successes in medical sciences in the past few decades, the number of deaths due to cancer remains unchecked. The conventional chemotherapy and radiotherapy have limited therapeutic index and a plethora of treatment related side effects. This situation has provided an impetus for search of novel therapeutic strategies that can selectively destroy the tumour cells, leaving the normal cells unharmed. Viral oncotherapy is such a promising treatment modality that offers unique opportunity for tumour targeting. Numerous viruses with inherent anti-cancer activity have been identified and are in different phases of clinical trials. In the era of modern biotechnology and with better understanding of cancer biology and virology, it has become feasible to engineer the oncolytic viruses (OVs) to increase their tumour selectivity and enhance their oncolytic activity. In this review, the mechanisms by which oncolytic viruses kill the tumour cells have been discussed as also the development made in virotherapy for cancer treatment with emphasis on their tumour specific targeting.

Physical state and viral load as predictive biomarkersfor persistence and progression of HPV16-positive cervical lesions: results from a population based long-term prospective cohort study

Persistent infection with a high risk (hr) human papillomavirus (HPV) has been established as the main cause of cervical cancer and high-grade cervical intraepithelial neoplasia (CIN3). Because most infections are transient, testing for hrHPV lacks specificity and has a low positive predictive value. It has been suggested that additional parameters like viral load and physical status of the viral genome could improve the effectiveness of HPV-based screening. We investigated the association between HPV16 viral load and physical state with viral persistence or risk of incident CIN3 or worse in a population-based prospective cohort study comprising 8656 women (20-29 years). All participants had two gynecological examinations two years apart and were followed through the nationwide Danish Pathology Data Bank (median follow-up: 12.9 yrs). Seventynine cervical swabs from women with a persistent HPV16 infection were available for analysis. For comparison we selected a random age-matched sample of transiently HPV16 infected women (N=91). Persistently infected women with incident CIN3 or cancer (CIN3+; N=31) were compared to women with normal cytology during follow up (non-progressors; N=39). Quantitative real-time PCR for HPV16E6, E2 and IFNb1 was done to determine the HPV16 viral load and the E2/E6 ratio was used as a surrogate marker for integration. Women with normal cytology who became persistently HPV16 infected had a significantly lower HPV16 load at baseline than women who cleared the infection (median 4.72 copies/cell versus median 20.0 copies/cell, respectively; p=0.0003). There was no difference in viral load at enrollment between women who progressed to CIN3+ and women who stayed cytologically normal (p=0.85). At the second examination viral load tended to be higher in women who progressed, but the difference was not statistically significant (p=0.39). The E2/E6 ratio was shown to be lower in the persistently infected group (p<0.0001) already at the first examination, but no difference between non-progressors and CIN3+ cases was observed at any of the two examinations (p=0.61 and 0.86). Lower viral load and integration of the viral genome are predictive for the persistence of HPV16 DNA, but not for the progression of a persistent HPV16 infection to CIN3+ in women with normal cytology.

Molecular Mechanism and Potential Targets for Blocking HPV-Induced Lesion Development

Persistent infection with high-risk HPV is the etiologic agent associated with the development of cervical cancer (CC) development. However, environmental, social, epidemiological, genetic, and host factors may have a joint influence on the risk of disease progression. Cervical lesions caused by HPV infection can be removed naturally by the host immune response and only a small percentage may progress to cancer; thus, the immune response is essential for the control of precursor lesions and CC. We present a review of recent research on the molecular mechanisms that allow HPV-infected cells to evade immune surveillance and potential targets of molecular therapy to inhibit tumor immune escape.

Direct identification of an HPV-16 tumor antigen from cervical cancer biopsy specimens

Persistent infection with high-risk human papilloma viruses (HPV) is the worldwide cause of many cancers, including cervical, anal, vulval, vaginal, penile, and oropharyngeal. Since T cells naturally eliminate the majority of chronic HPV infections by recognizing epitopes displayed on virally altered epithelium, we exploited Poisson detection mass spectrometry (MS³) to identify those epitopes and inform future T cell-based vaccine design. Nine cervical cancer biopsies from HPV-16 positive HLA-A^*02 patients were obtained, histopathology determined, and E7 oncogene PCR-amplified from tumor DNA and sequenced. Conservation of E7 oncogene coding segments was found in all tumors. MS³ analysis of HLA-A^*02 immunoprecipitates detected E7_11–19 peptide (YMLDLQPET) in seven of the nine tumor biopsies. The remaining two samples were E7_11–19 negative and lacked the HLA-A^*02 binding GILT thioreductase peptide despite possessing binding-competent HLA-A^*02 alleles. Thus, the conserved E7_11–19 peptide is a dominant HLA-A^*02 binding tumor antigen in HPV-16 transformed cervical squamous and adenocarcinomas. Findings that a minority of HLA-A^*02:01 tumors lack expression of both E7_11–19 and a peptide from a thioreductase important in processing of cysteine-rich proteins like E7 underscore the value of physical detection, define a potential additional tumor escape mechanism and have implications for therapeutic cancer vaccine development.

Primary human cervical carcinoma cells require human papillomavirus E6 and E7 expression for ongoing proliferation

Repression of human papillomavirus (HPV) E6 and E7 oncogenes in established cervical carcinoma cell lines causes senescence due to reactivation of cellular tumor suppressor pathways. Here, we determined whether ongoing expression of HPV16 or HPV18 oncogenes is required for the proliferation of primary human cervical carcinoma cells in serum-free conditions at low passage number after isolation from patients. We used an SV40 viral vector expressing the bovine papillomavirus E2 protein to repress E6 and E7 in these cells. To enable efficient SV40 infection and E2 gene delivery, we first incubated the primary cervical cancer cells with the ganglioside GM1, a cell-surface receptor for SV40 that is limiting in these cells. Repression of HPV in primary cervical carcinoma cells caused them to undergo senescence, but the E2 protein had little effect on HPV-negative primary cells. These data suggest that E6 and E7 dependence is an inherent property of human cervical cancer cells.

Targeting the human papillomavirus E6 and E7 oncogenes through expression of the bovine papillomavirus type 1 E2 protein stimulates cellular motility

Expression of the high-risk human papillomavirus (HPV) E6 and E7 oncogenes is essential for the initiation and maintenance of cervical cancer. The repression of both was previously shown to result in activation of their respective tumor suppressor targets, p53 and pRb, and subsequent senescence induction in cervical cancer cells. Consequently, viral oncogene suppression is a promising approach for the treatment of HPV-positive tumors. One well-established method of E6/E7 repression involves the reexpression of the viral E2 protein which is usually deleted in HPV-positive cancer cells. Here, we show that, surprisingly, bovine papillomavirus type 1 (BPV1) E2 but not RNA interference-mediated E6/E7 repression in HPV-positive cervical cancer cells stimulates cellular motility and invasion. Migration correlated with the dynamic formation of cellular protrusions and was dependent upon cell-to-cell contact. While E2-expressing migratory cells were senescent, migration was not a general feature of cellular senescence or cell cycle arrest and was specifically observed in HPV-positive cervical cancer cells. Interestingly, E2-expressing cells not only were themselves motile but also conferred increased motility to admixed HeLa cervical cancer cells. Together, our data suggest that repression of the viral oncogenes by E2 stimulates the motility of E6/E7-targeted cells as well as adjacent nontargeted cancer cells, thus raising the possibility that E2 expression may unfavorably increase the local invasiveness of HPV-positive tumors.

HPV E6 proteins target Ubc9, the SUMO conjugating enzyme

The human papillomavirus oncogenic protein, E6, interacts with a number of cellular proteins, and for some targets, E6 directs their degradation through the ubiquitin-proteasome pathway. Post-translational modification with ubiquitin-like modifiers, such as SUMO, also influences protein activities, protein-protein interactions, and protein stability. We report that the high risk HPVE6 proteins reduce the intracellular quantity of the sole SUMO conjugation enzyme, Ubc9, concomitant with decreased host sumoylation. E6 did not significantly influence transcription of Ubc9, indicating that the effects were likely at the protein level. Consistent with typical E6-mediated proteasomal degradation, E6 bound to Ubc9 in vitro, and required E6AP for reduction of Ubc9 levels. Under stable E6 expression conditions in differentiating keratinocytes there was a decrease in Ubc9 and a loss of numerous sumoylated targets indicating a significant perturbation of the normal sumoylation profile. While E6 is known to inhibit PIASy, a SUMO ligase, our results suggest that HPV E6 also targets the Ubc9 protein to modulate host cell sumoylation, suggesting that the sumoylation system may be an important target during viral reproduction and possibly the subsequent development of cervical cancer.

Tumor suppressor or oncogene? A critical role of the human papillomavirus (HPV) E2 protein in cervical cancer progression

The papillomavirus (PV) E2 proteins have been shown to exert many functions in the viral cycle including pivotal roles in transcriptional regulation and in viral DNA replication. Besides these historical roles, which rely on their aptitude to bind to specific DNA sequences, E2 has also been shown to modulate the host cells through direct protein interactions mainly through its amino terminal transactivation domain. We will describe here some of these new functions of E2 and their potential implication in the HPV-induced carcinogenesis. More particularly we will focus on E2-mediated modulation of the host cell cycle and consequences to cell transformation. In all, the HPV E2 proteins exhibit complex functions independent of transcription that can modulate the host cells in concert with the viral vegetative cycle and which could be involved in early carcinogenesis.

Growth inhibition of HeLa cells is a conserved feature of high-risk human papillomavirus E8^E2C proteins and can also be achieved by an artificial repressor protein

Infections with certain human papillomaviruses (HPV), such as type 16 (HPV16), 18, or 31, are a necessary risk factor for the development of cervical cancer. Transcript analyses of several HPV revealed that the viral E2 gene encodes both the E2 regulator protein and the E8∧E2C protein, which differ in their amino termini. Up to now, functional studies have focused on HPV31 E8∧E2C and demonstrated that it is a potent repressor of viral transcription and replication. However, recent analyses of HPV16 genomes have suggested that E8∧E2C proteins may differ in their activities. Therefore, we performed a comparative analysis of E8∧E2C proteins of HPV16, -18, and -31. All E8∧E2C proteins potently inhibited HPV E6/E7 oncogene promoters, and also displayed long-distance transcriptional-repression activities. Furthermore, the expression of all E8∧E2C proteins inhibited the growth of HeLa cells. Expression of E8∧E2C proteins rapidly increased the protein levels of the E6 and E7 targets p53 and p21, consistent with the repression of the endogenous HPV18 E6/E7 promoter. All E8∧E2C proteins induced G(1) arrest more efficiently than E2 proteins and activated senescence markers. Furthermore, we demonstrate that the 31E8 domain can be functionally replaced by the KRAB repression domain derived from KOX1. The KRAB-E2C fusion protein possesses long-distance transcriptional-repression activity and inhibits the growth of HeLa cells comparably to E8∧E2C. Taken together, our results suggest that the E8∧E2C proteins of HPV16, -18, and -31 are highly conserved transcriptional repressors that inhibit the growth of HeLa cells by repression of E6/E7 transcription but do not have proapoptotic activities.

Tumor viruses and cancer biology: Modulating signaling pathways for therapeutic intervention

Tumor viruses have provided relatively simple genetic systems, which can be manipulated for understanding the molecular mechanisms of the cellular transformation process. A growing body of information in the tumor virology field provides several prospects for rationally targeted therapies. However, further research is needed to better understand the multiple mechanisms utilized by these viruses in cancer progression in order to develop therapeutic strategies. Initially viruses were believed to be associated with cancers as causative agents only in animals. It was almost half a century before the first human tumor virus, Epstein-Barr virus (EBV), was identified in 1964. Subsequently, several human tumor viruses have been identified including Kaposi sarcoma associated herpesvirus (KSHV), human Papillomaviruses (HPV), Hepatitis B virus (HBV), Hepatitis C virus (HCV), Human T lymphotropic virus (HTLV-1) and recently identified Merkel cell Polyomavirus (MCPyV). Tumor viruses are sub-categorized as either DNA viruses, which include EBV, KSHV, HPV, HBV, and MCPyV, or RNA viruses such as HCV and HTLV-1. Tumor-viruses induce oncogenesis through manipulating an array of different cellular pathways. These viruses initiate a series of cellular events, which lead to immortalization and proliferation of the infected cells by disrupting the mitotic checkpoint upon infection of the host cell. This is often accomplished by functional inhibition or proteasomal degradation of many tumor suppressor proteins by virally encoded gene products. The virally infected cells can either be eliminated via cell-mediated apoptosis or persist in a state of chronic infection. Importantly, the chronic persistence of infection by tumor viruses can lead to oncogenesis. This review discusses the major human tumor associated viruses and their ability to modulate numerous cell signaling pathways, which can be targeted for potential therapeutic approaches.

Apoptosis of hepatitis B virus-infected hepatocytes prevents release of infectious virus

Apoptosis of infected cells is critically involved in antiviral defense. Apoptosis, however, may also support the release and spread of viruses. Although the elimination of infected hepatocytes is required to combat hepatitis B virus (HBV) infection, it is still unknown which consequences hepatocyte apoptosis has for the virus and whether or not it is advantageous to the virus. To study this, we designed a cell culture model consisting of both HBV-producing cell lines and primary human hepatocytes serving as an infection model. We showed that the release of mature, enveloped virions was 80% to 90% reduced 24 h after the induction of apoptosis in HBV-replicating hepatoma cells or HBV-infected hepatocytes. Importantly, HBV particles released from apoptotic hepatocytes were immature and nonenveloped and proved not to be infectious. We found an inverse correlation between the strength of an apoptotic stimulus and the infectivity of the virus particles released: the more potent the apoptotic stimulus, the higher the ratio of nonenveloped capsids to virions and the lower their infectivity. Furthermore, we demonstrated that HBV replication and, particularly, the expression of the HBx protein transcribed from the viral genome during replication do not sensitize cells to apoptosis. Our data clearly reject the hypothesis that the apoptosis of infected hepatocytes facilitates the propagation of HBV. Rather, these data indicate that HBV needs to prevent the apoptosis of its host hepatocyte to ensure the release of infectious progeny and, thus, virus spread in the liver.

HPV16 E2 is an immediate early marker of viral infection, preceding E7 expression in precursor structures of cervical carcinoma

The viral E2 gene product plays a crucial role in the human papillomavirus (HPV) vegetative cycle by regulating both transcription and replication of the viral genome. E2 is a transcriptional repressor of the E6 and E7 viral oncogenes for HPV types 16 and 18, which are involved in cervical cancers. Using new polyclonal antibodies against the HPV16 E2 protein, we showed that E2 is expressed at various precursor stages of cervical carcinoma by immunohistochemistry on paraffin-embedded clinical samples. E2 was found to be highly expressed in the nuclei and cytoplasm of cells forming the intermediate and upper layers of cervical intraepithelial neoplasia (CIN). We could show that the expressions of E2 and p16(INK4a) (surrogate marker for oncogenic E7 expression) were exclusive in most of the cases, thus implying that E2 is not expressed together with high levels of E7. Moreover, we found that E2 is expressed in a subset of columnar cells adjacent to the CIN. We could show that expression of E2 is topologically distinct from the proliferation markers p63 and Ki67, whereas it coincides with the expression of cytokeratin K13, a marker of squamous cell differentiation. Expression of E2 also topologically coincides with episomal amplification of viral genomes in the upper layers of CIN1. These in vivo data thus validate previous assumptions of the crucial role of E2 in the early steps of HPV infection and of its negative link with expression of the viral E6 and E7 oncogenes.