Role of the retinoblastoma pathway in senescence triggered by repression of the human papillomavirus E7 protein in cervical carcinoma cells

HPV-16 E7 Interacts with the Endocytic Machinery via the AP2 Adaptor μ2 Subunit

Human papillomavirus (HPV) E7 plays a major role in HPV-induced malignancy, perturbing cell cycle regulation, and driving cell proliferation. Major targets of cancer-causing HPV E7 proteins are the pRB family of tumor suppressors, which E7 targets for proteasome-mediated degradation and whose interaction is promoted through an acidic patch, downstream of the LXCXE motif in E7, that is subject to phosphorylation by casein kinase II (CKII). In this study we show that HPV-16 E7 targets the AP2-complex, which plays a critical role in cargo recognition in clathrin-mediated endocytosis. Intriguingly, HPV-16 E7 contains a specific amino acid sequence for AP2 recognition, and this overlaps the pRb LXCXE recognition sequence but involves completely different amino acid residues. HPV-16 E7 does this by binding to the AP2-μ2 adaptor protein subunit via residues 25-YEQL-28 within the LXCXE motif. Point mutations at Y25 within 22-LYCYE-26 suggest that the interaction of E7 with AP2-μ2 is independent from pRB binding. In cells, this interaction is modulated by acidic residues downstream of LXCXE, with the binding being facilitated by CKII-phosphorylation of the serines at positions 31 and 32. Finally, we also show that association of HPV-16 E7 with the AP2 adaptor complex can contribute to cellular transformation under low-nutrient conditions, which appears to be mediated, in part, through inhibition of AP2-mediated internalization of epidermal growth factor receptor (EGFR). This indicates that E7 can modulate endocytic transport pathways, with one such component, EGFR, most likely contributing toward the ability of E7 to induce cell transformation and malignancy. These studies define a new and unexpected role for HPV-16 E7 in targeting clathrin-mediated endocytosis. IMPORTANCE Despite being a very small protein, HPV-E7 has a wide range of functions within the infected cell, many of which can lead to cell transformation. High-risk HPV-E7 deregulates the function of many cellular proteins, perturbing cellular homeostasis. We show that a novel target of HPV-E7 is the clathrin-adaptor protein 2 complex (AP2) μ2 subunit, interacting via residues within E7's pRB-binding region. Mutational studies show that an AP2 recognition motif is present in the CR2 region and is conserved in >50 HPV types, suggesting a common function for this motif in HPV biology. Mutational analysis suggests that this motif is important for cellular transformation, potentially modulating endocytosis of growth factor receptors such as EGFR, and thus being a novel activity of E7 in modulating clathrin-mediated endocytosis and cargo selection. This study has important implications for the molecular basis of E7 function in modulating protein trafficking at the cell surface.

HPV16 E7 Nucleotide Variants Found in Cancer-Free Subjects Affect E7 Protein Expression and Transformation

The human papillomavirus (HPV) type 16 E7 oncogene is critical to carcinogenesis and highly conserved. Previous studies identified a preponderance of non-synonymous E7 variants amongst HPV16-positive cancer-free controls compared to those with cervical cancer. To investigate the function of E7 variants, we constructed full-length HPV16 E7 genes and tested variants at positions H9R, D21N, N29S, E33K, T56I, D62N, S63F, S63P, T64M, E80K, D81N, P92L, and P92S (found only in controls); D14E, N29H cervical intraepithelial neoplasia (CIN2), and P6L, H51N, R77S (CIN3). We determined the steady-state level of cytoplasmic and nuclear HPV16 E7 protein. All variants from controls showed a reduced level of E7 protein, with 7/13 variants having lower protein levels. In contrast, 2/3 variants from the CIN3 precancer group had near-wild type E7 levels. We assayed the activity of representative variants in stably transfected NIH3T3 cells. The H9R, E33K, P92L, and P92S variants found in control subjects had lower transforming activity than D14E and N29H variants (CIN2), and the R77S (CIN3) had activity only slightly reduced from wild-type E7. In addition, R77S and WT E7 caused increased migration of NIH3T3 cells in a wound-healing assay compared with H9R, E33K, P92L, and P92S (controls) and D14E (CIN2). These data provide evidence that the E7 variants found in HPV16-positive cancer-free women are partially defective for transformation and cell migration, further demonstrating the importance of fully active E7 in cancer development.

Novel effect of the high risk-HPV E7 CKII phospho-acceptor site on polarity protein expression

Background

Oncogenic Human Papillomaviruses (HPVs) base their transforming potential on the action of both E6 and E7 viral oncoproteins, which perform cooperative or antagonistic actions and thus interfere with a variety of relevant cellular targets. Among them, the expression of some PDZ-containing polarity proteins, as DLG1 and hScrib, is altered during the HPV life cycle and the consequent malignant transformation. Together with the well-established interference of E6 with PDZ proteins, we have recently shown that E7 viral oncoprotein is also responsible for the changes in abundance and localization of DLG1 observed in HPV-associated lesions. Given that the mechanisms involved remained only partially understood, we here thoroughly analyse the contribution of a crucial E7 post-translational modification: its CKII-dependent phosphorylation. Moreover, we extended our studies to hScrib, in order to investigate possible conserved regulatory events among diverse PDZ targets of HPV.

Methods

We have acutely analysed the expression of DLG1 and hScrib in restrictive conditions for E7 phosphorylation by CKII in epithelial culture cells by western blot and confocal fluorescence microscopy. We made use of genome-edited HPV-positive cells, specific inhibitors of CKII activity and transient expression of the viral oncoproteins, including a mutant version of E7.

Results

We here demonstrate that the functional phosphorylation of E7 oncoprotein by the CKII cellular kinase, a key regulatory event for its activities, is also crucial to counteract the E6-mediated degradation of the PDZ-polarity protein DLG1 and to promote its subcellular redistribution. Moreover, we show that the CKII-dependent phosphorylation of E7 is able to control the expression of another PDZ target of HPV: hScrib. Remarkably, we found this is a shared feature among different oncogenic HPV types, suggesting a common path towards viral pathogenesis.

Conclusions

The present study sheds light into the mechanisms behind the misexpression of PDZ-polarity proteins during HPV infections. Our findings stress the relevance of the CKII-mediated regulation of E7 activities, providing novel insights into the joint action of HPV oncoproteins and further indicating a conserved and most likely crucial mechanism during the viral life cycle and the associated transformation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-10105-5.

Molecular Diagnostics in Human Papillomavirus-Related Head and Neck Squamous Cell Carcinoma

The incidence of human papillomavirus (HPV)-related head and neck squamous cell carcinoma continues to increase. Accurate diagnosis of the HPV status of a tumor is vital, as HPV+ versus HPV- tumors represent two unique biological and clinical entities with different treatment strategies. High-risk HPV subtypes encode oncoproteins E6 and E7 that disrupt cellular senescence and ultimately drive tumorigenesis. Current methods for detection of HPV take advantage of this established oncogenic pathway and detect HPV at various biological stages. This review article provides an overview of the existing technologies employed for the detection of HPV and their current or potential future role in management and prognostication.

The HPV-18 E7 CKII phospho acceptor site is required for maintaining the transformed phenotype of cervical tumour-derived cells

The Human Papillomavirus E7 oncoprotein plays an essential role in the development and maintenance of malignancy, which it achieves through targeting a number of critical cell control pathways. An important element in the ability of E7 to contribute towards cell transformation is the presence of a Casein Kinase II phospho-acceptor site within the CR2 domain of the protein. Phosphorylation is believed to enhance E7 interaction with a number of different cellular target proteins, and thereby increase the ability of E7 to enhance cell proliferation and induce malignancy. However, there is little information on how important this site in E7 is, once the tumour cells have become fully transformed. In this study, we have performed genome editing of the HPV-18 E7 CKII recognition site in C4-1 cervical tumour-derived cells. We first show that mutation of HPV18 E7 S32/S34 to A32/A34 abolishes CKII phosphorylation of E7, and subsequently we have isolated C4-1 clones containing these mutations in E7. The cells continue to proliferate, but are somewhat more slow-growing than wild type cells, reach lower saturation densities, and are also more susceptible to low nutrient conditions. These cells are severely defective in matrigel invasion assays, partly due to downregulation of matrix metalloproteases (MMPs). Mechanistically, we find that phosphorylation of E7 plays a direct role in the ability of E7 to activate AKT signaling, which in turn is required for optimal levels of MMP secretion. These results demonstrate that the E7 CKII phospho-acceptor site thus continues to play an important role for E7's activity in cells derived from cervical cancers, and suggests that blocking this activity of E7 could be expected to have therapeutic potential.

Virus like particles as a platform for cancer vaccine development

Cancers have killed millions of people in human history and are still posing a serious health problem worldwide. Therefore, there is an urgent need for developing preventive and therapeutic cancer vaccines. Among various cancer vaccine development platforms, virus-like particles (VLPs) offer several advantages. VLPs are multimeric nanostructures with morphology resembling that of native viruses and are mainly composed of surface structural proteins of viruses but are devoid of viral genetic materials rendering them neither infective nor replicative. In addition, they can be engineered to display multiple, highly ordered heterologous epitopes or peptides in order to optimize the antigenicity and immunogenicity of the displayed entities. Like native viruses, specific epitopes displayed on VLPs can be taken up, processed, and presented by antigen-presenting cells to elicit potent specific humoral and cell-mediated immune responses. Several studies also indicated that VLPs could overcome the immunosuppressive state of the tumor microenvironment and break self-tolerance to elicit strong cytotoxic lymphocyte activity, which is crucial for both virus clearance and destruction of cancerous cells. Collectively, these unique characteristics of VLPs make them optimal cancer vaccine candidates. This review discusses current progress in the development of VLP-based cancer vaccines and some potential drawbacks of VLPs in cancer vaccine development. Extracellular vesicles with close resembling to viral particles are also discussed and compared with VLPs as a platform in cancer vaccine developments.

Putative biomarkers for cervical cancer: SNVs, methylation and expression profiles

Cervical cancer is primarily caused by Human papillomavirus (HPV) infection, but other factors such as smoking habits, co-infections and genetic background, can also contribute to its development. Although this cancer is avoidable, it is the fourth most frequent type of cancer in females worldwide and can only be treated with chemotherapy and radical surgery. There is a need for biomarkers that will enable early diagnosis and targeted therapy for this type of cancer. Therefore, a systems biology pipeline was applied in order to identify potential biomarkers for cervical cancer, which show significant reports in three molecular aspects: DNA sequence variants, DNA methylation pattern and alterations in mRNA/protein expression levels. CDH1, CDKN2A, RB1 and TP53 genes were selected as putative biomarkers, being involved in metastasis, cell cycle regulation and tumour suppression. Other ten genes (CDH13, FHIT, PTEN, MLH1, TP73, CDKN1A, CACNA2D2, TERT, WIF1, APC) seemed to play a role in cervical cancer, but the lack of studies prevented their inclusion as possible biomarkers. Our results highlight the importance of these genes. However, further studies should be performed to elucidate the impact of DNA sequence variants and/or epigenetic deregulation and altered expression of these genes in cervical carcinogenesis and their potential as biomarkers for cervical cancer diagnosis and prognosis.

Effects of Methylation Status of CpG Sites within the HPV16 Long Control Region on HPV16-Positive Head and Neck Cancer Cells

Objective

To map comprehensively the methylation status of the CpG sites within the HPV16 long control region (LCR) in HPV-positive cancer cells, and to explore further the effects of methylation status of HPV16 LCR on cell bioactivity and E6 and E7 expression. In addition, to analyze the methylation status of the LCR in HPV-positive oropharyngeal squamous cell carcinoma (OPSCC) patients.

Methods and Materials

Methylation patterns of HPV16 LCR in UM-SCC47, CaSki, and SiHa cells and HPV16-positiive OPSCC specimens were detected by bisulfite-sequencing PCR and TA cloning. For cells treated with 5-aza-2′-deoxycytidine and E6 and E7 knockdown, MTS and trypan blue staining, annexin-V and 7-AAD staining, and prodidium iodide were used to evaluate cell growth and cell proliferation, cell apoptosis, and cell cycle arrest, respectively. E6 and E7 mRNA and protein expression were analyzed by quantitative real-time PCR and immunocytochemistry, respectively.

Results

Hypermethylation status of the LCR in UM-SCC47 (79.8%) and CaSki cells (90.0%) and unmethylation status of the LCR in SiHa cells (0%) were observed. Upon demethylation, the cells with different methylation levels responded differently during growth, apoptosis, and cell cycle arrest, as well as in terms of their E6 and E7 expression. In HPV16-positive OPSCC patients, the methylation rates were 9.5% in the entire LCR region, 13.9% in the 5′-LCR, 6.0% in the E6 enhancer, and 9.5% in the p97 promoter, and hypermethylation of p97 promoter was found in a subset of cases (20.0%, 2/10).

Conclusions

Our study revealed two different methylation levels of the LCR in HPV16-positive cancer cells and OPSCC patients, which may represent different carcinogenesis mechanisms of HPV-positive cancers cells. Demethylating the meCpGs in HPV16 LCR might be a potential target for a subgroup of HPV16-positive patients with head and neck squamous cell carcinoma.

Review and Updates of Immunohistochemistry in Selected Salivary Gland and Head and Neck Tumors

Context

Immunohistochemistry is a useful tool for diagnosing salivary gland and head and neck tumors.

Objective

To review immunohistochemical markers, which can aid in the diagnosis of selected salivary gland and head and neck tumors.

Data Sources

Literature review and authors' personal practice experience.

Conclusions

Salivary gland and head and neck tumors include a large diverse group of tumors with complex and overlapping histologic features. Immunohistochemistry plays an important role in resolving the differential diagnosis of some salivary gland and head and neck tumors and can provide information for the prognosis of certain tumors.

SMCX and components of the TIP60 complex contribute to E2 regulation of the HPV E6/E7 promoter

An important step in the malignant progression of HPV-associated lesions is the dysregulation of expression of the viral E6 and E7 oncogenes. This is often achieved through the loss of expression of E2, which represses the HPV LCR promoter and E6/E7 expression. Our previous studies confirmed a role for Brd4 in mediating the E2 transcriptional repression function, and identified JARID1C/SMCX and EP400 as contributors to E2-mediated repression. Here we show that TIP60, a component of the TIP60/TRRAP histone acetyltransferase complex, also contributes to the E2 repression function, and we extend our studies on SMCX. Di- and tri-methyl marks on histone H3K4 are reduced in the presence of E2 and SMCX, suggesting a mechanism by which SMCX contributes to E2-mediated repression of the HPV LCR. Together, these findings lead us to hypothesize that E2 recruits histone-modifying cellular proteins to the HPV LCR, resulting in transcriptional repression of E6 and E7.

Host cell factor-1 recruitment to E2F-bound and cell-cycle-control genes is mediated by THAP11 and ZNF143

Host cell factor-1 (HCF-1) is a metazoan transcriptional coregulator essential for cell-cycle progression and cell proliferation. Current models suggest a mechanism whereby HCF-1 functions as a direct coregulator of E2F proteins, facilitating the expression of genes necessary for cell proliferation. In this report, we show that HCF-1 recruitment to numerous E2F-bound promoters is mediated by the concerted action of zinc finger transcription factors THAP11 and ZNF143, rather than E2F proteins directly. THAP11, ZNF143, and HCF-1 form a mutually dependent complex on chromatin, which is independent of E2F occupancy. Disruption of the THAP11/ZNF143/HCF-1 complex results in altered expression of cell-cycle control genes and leads to reduced cell proliferation, cell-cycle progression, and cell viability. These data establish a model in which a THAP11/ZNF143/HCF-1 complex is a critical component of the transcriptional regulatory network governing cell proliferation.

MiR-101 induces senescence and prevents apoptosis in the background of DNA damage in MCF7 cells

Moderately increased DNA damage due to the exogenous miR-101 (4 fold) over-expression in MCF7 cells was substantiated by an increase in the number of γ-H2AX foci, correlating with a simple-to-do Halo-assay. miR-101 induced mild/moderate DNA damage favoured senescence rather than apoptosis. An experimental support emanated from the induced mild/moderate DNA damage with 1 µM/5 µM etoposide in MCF7 cells, which resulted in an endogenous miR-101 over-expression (10/4 fold, respectively), followed by senescence. On the other hand, the severe DNA damage induced with 10 µM etoposide, resulted in a low (<1 fold) endogenous expression of miR-101 and an elevated percentage of apoptotic cells. Using bioinformatics tools along with in-vitro and in-vivo validations, miR-101 was found to target and downregulate the mRNA expression of UBE2N and SMARCA4, involved in DNA damage repair (DDR) pathways. Recovery of the expression of the two novel targets in anti-miR-101 transfection validated the results. We conclude that a threshold range of over-expressed miR-101, capable of inducing mild/moderate DNA damage, is sensed by cells to become senescent. The observation derives further support from in-silico protein-protein network analysis where the two novel targets showed their involvement in senescence pathway.

Targeted silencing of MLL5β inhibits tumor growth and promotes gamma-irradiation sensitization in HPV16/18-associated cervical cancers

We previously identified a novel MLL5 isoform, MLL5β, which was essential for E6 and E7 transcriptional activation in HPV16/18-associated cervical cancers. In this report, we investigated the potential of RNAi-mediated silencing of MLL5β through the use of MLL5β-siRNA as a novel therapeutic strategy for HPV16/18-positive cervical cancer. We observed concurrent downregulation of E6 and E7 after MLL5β silencing, leading to growth inhibition via the activation of apoptosis and senescence in the HeLa cell model. This corresponded with the enhanced antitumor effects of MLL5β-siRNA compared with E6- or E7-siRNA single treatments. Significant reduction in tumor size after MLLβ-siRNA treatment in the HeLa xenograft tumor model further emphasized the importance of MLL5β in HPV16/18-associated tumor growth and the potential of RNAi therapeutics that target MLL5β. We also identified MLL5β as a modulator of gamma-irradiation (IR) sensitization properties of cisplatin. We observed that while MLL5β silencing alone was enough to evoke cisplatin-like IR sensitization in tumor cells in vitro, overexpression of MLL5β inhibited the ability of cisplatin to sensitize HeLa cells to IR-induced cytotoxicity. MLL5β-siRNA-IR cotreatment was also observed to enhance tumor growth inhibition in vivo. Taken together, our findings highlight the potential of targeted silencing of MLL5β via the use of MLL5β-siRNA as a novel therapeutic strategy and propose that MLL5β-siRNA could be a viable alternative for cisplatin in the current cisplatin-based chemotherapeutics for HPV16/18-associated cervical cancers.

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 E7 proteins

E7 is an accessory protein that is not encoded by all papillomaviruses. The E7 amino terminus contains two regions of similarity to conserved regions 1 and 2 of the adenovirus E1A protein, which are also conserved in the simian vacuolating virus 40 large tumor antigen. The E7 carboxyl terminus consists of a zinc-binding motif, which is related to similar motifs in E6 proteins. E7 proteins play a central role in the human papillomavirus life cycle, reprogramming the cellular environment to be conducive to viral replication. E7 proteins encoded by the cancer-associated alpha human papillomaviruses have potent transforming activities, which together with E6, are necessary but not sufficient to render their host squamous epithelial cell tumorigenic. This article strives to provide a comprehensive summary of the published research studies on human papillomavirus E7 proteins.

Biochemical and functional interactions of human papillomavirus proteins with polycomb group proteins

The role of enzymes involved in polycomb repression of gene transcription has been studied extensively in human cancer. Polycomb repressive complexes mediate oncogene-induced senescence, a principal innate cell-intrinsic tumor suppressor pathway that thwarts expansion of cells that have suffered oncogenic hits. Infections with human cancer viruses including human papillomaviruses (HPVs) and Epstein-Barr virus can trigger oncogene-induced senescence, and the viruses have evolved strategies to abrogate this response in order to establish an infection and reprogram their host cells to establish a long-term persistent infection. As a consequence of inhibiting polycomb repression and evading oncogene induced-senescence, HPV infected cells have an altered epigenetic program as evidenced by aberrant homeobox gene expression. Similar alterations are frequently observed in non-virus associated human cancers and may be harnessed for diagnosis and therapy.

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.

Emerging roles of RB family: new defense mechanisms against tumor progression

The retinoblastoma (RB) family of proteins, including RB1/p105, retinoblastoma-like 1 (RBL1/p107), and retinoblastoma-like 2 (RBL2/p130), is principally known for its central role on cell cycle regulation. The inactivation of RB proteins confers a growth advantage and underlies multiple types of tumors. Recently, it has been shown that RB proteins have other important roles, such as preservation of chromosomal stability, induction and maintenance of senescence and regulation of apoptosis, cellular differentiation, and angiogenesis. RB proteins are involved in many cellular pathways and act as transcriptional regulators able to bind several transcription factors, thus antagonizing or potentiating their functions. Furthermore, RB proteins might control the expression of specific target genes by recruiting chromatin remodeling enzymes. Although many efforts have been made to dissect the different functions of RB proteins, it remains still unclear which are necessary for cancer suppression and the role they play at distinct steps of carcinogenesis. Moreover, RB proteins can behave differently in various cell types or cell states. Elucidating the intricate RB protein network in regulating cell fate might provide the knowledge necessary to explain their potent tumor suppressor activity and to design novel therapeutic strategies.

Human papillomavirus type 16 E7 oncoprotein engages but does not abrogate the mitotic spindle assembly checkpoint

The mitotic spindle assembly checkpoint (SAC) ensures faithful chromosome segregation during mitosis by censoring kinetochore-microtubule interactions. It is frequently rendered dysfunctional during carcinogenesis causing chromosome missegregation and genomic instability. There are conflicting reports whether the HPV16 E7 oncoprotein drives chromosomal instability by abolishing the SAC. Here we report that degradation of mitotic cyclins is impaired in cells with HPV16 E7 expression. RNAi-mediated depletion of Mad2 or BubR1 indicated the involvement of the SAC, suggesting that HPV16 E7 expression causes sustained SAC engagement. Mutational analyses revealed that HPV16 E7 sequences that are necessary for retinoblastoma tumor suppressor protein binding as well as sequences previously implicated in binding the nuclear and mitotic apparatus (NuMA) protein and in delocalizing dynein from the mitotic spindle contribute to SAC engagement. Importantly, however, HPV16 E7 does not markedly compromise the SAC response to microtubule poisons.

Cervical cancers require the continuous expression of the human papillomavirus type 16 E7 oncoprotein even in the presence of the viral E6 oncoprotein

High-risk human papillomaviruses (HPV), such as HPV-16, are etiologic agents of a variety of anogenital and oral malignancies, including nearly all cases of cervical cancer. Cervical cancers arising in transgenic mice that express HPV-16 E7 in an inducible manner require the continuous expression of E7 for their maintenance. However, in HPV-associated cancers in vivo, E6 and E7 invariably are coexpressed. In this study, we investigated whether cervical cancers rely on the continuous expression of E7 in the context of constitutively expressed E6. We placed the inducible HPV-16 E7 transgene onto a background in which HPV-16 E6 was constitutively expressed. In transgenic mice with high-grade cervical dysplastic lesions and cervical cancer, repressing the expression of E7 led to the regression of all cancers and the vast majority of high-grade dysplastic lesions. In addition, cervical cancers were occasionally observed in transgenic mice in which E7 was repressed and then reexpressed. Our findings indicate that even in the presence of constitutively expressed E6, the continuous expression of E7 is required for the maintenance of cervical cancers and most precancerous lesions. These data have important implications for the potential clinical use of drugs designed to inhibit the expression and/or function of E7 to treat HPV-associated cancers.

The switch from pRb/p105 to Rb2/p130 in DNA damage and cellular senescence

Cellular senescence is a response to genotoxic stress that results in an irreversible cell cycle arrest. Activation of this pathway relies on the activity of the retinoblastoma proteins and proteins of the DNA damage response cascade. Here, we discuss the functional relevance of the switch from pRb/p105 to Rb2/p130 that becomes apparent when cells enter senescent arrest.

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

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

Concurrent loss of the PTEN and RB1 tumor suppressors attenuates RAF dependence in melanomas harboring (V600E)BRAF

Identifying the spectrum of genetic alterations that cooperate with critical oncogenes to promote transformation provides a foundation for understanding the diversity of clinical phenotypes observed in human cancers. Here, we performed integrated analyses to identify genomic alterations that co-occur with oncogenic BRAF in melanoma and abrogate cellular dependence upon this oncogene. We identified concurrent mutational inactivation of the PTEN and RB1 tumor suppressors as a mechanism for loss of BRAF/MEK dependence in melanomas harboring (V600E)BRAF mutations. RB1 alterations were mutually exclusive with loss of p16(INK4A), suggesting that whereas p16(INK4A) and RB1 may have overlapping roles in preventing tumor formation, tumors with loss of RB1 exhibit diminished dependence upon BRAF signaling for cell proliferation. These findings provide a genetic basis for the heterogeneity of clinical outcomes in patients treated with targeted inhibitors of the mitogen-activated protein kinase pathway. Our results also suggest a need for comprehensive screening for RB1 and PTEN inactivation in patients treated with RAF and MEK-selective inhibitors to determine whether these alterations are associated with diminished clinical benefit in patients whose cancers harbor mutant BRAF.

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.

An intact retinoblastoma protein-binding site in Merkel cell polyomavirus large T antigen is required for promoting growth of Merkel cell carcinoma cells

Merkel cell carcinoma (MCC) is a highly aggressive skin cancer that frequently harbours Merkel cell polyomavirus (MCV) DNA integrated in the genome of the tumor cells. In our study, we elaborate our recent finding that MCV-positive MCC cell lines require the expression of the viral T antigens (TA). Indeed, in a xeno-transplantation model, we prove that TA expression is essential also in an in vivo situation, as knock down of TA leads to tumor regression. Moreover, rescuing TA short hairpin RNA (shRNA)-treated MCV-positive MCC cells by ectopic expression of shRNA-insensitive TAs clearly demonstrates that the observed effect is caused by TA knockdown. Notably, introduction of a mutation in the LTA protein interfering with LTA binding to the retinoblastoma protein (RB) ablated this rescue. The importance of this interaction was further confirmed as LTA-specific knockdown leads to explicit cell growth inhibition. In summary, the presented data demonstrate that established MCV-positive MCC tumors critically depend on TA expression, in particular the LTA and RB interaction, for sustained tumor growth. Consequently, interference with LTA/RB interaction appears as promising strategy to treat MCC.

Epstein-Barr virus (EBV) Rta-mediated EBV and Kaposi's sarcoma-associated herpesvirus lytic reactivations in 293 cells

Epstein–Barr virus (EBV) Rta belongs to a lytic switch gene family that is evolutionarily conserved in all gamma-herpesviruses. Emerging evidence indicates that cell cycle arrest is a common means by which herpesviral immediate-early protein hijacks the host cell to advance the virus's lytic cycle progression. To examine the role of Rta in cell cycle regulation, we recently established a doxycycline (Dox)-inducible Rta system in 293 cells. In this cell background, inducible Rta modulated the levels of signature G1 arrest proteins, followed by induction of the cellular senescence marker, SA-β-Gal. To delineate the relationship between Rta-induced cell growth arrest and EBV reactivation, recombinant viral genomes were transferred into Rta-inducible 293 cells. Somewhat unexpectedly, we found that Dox-inducible Rta reactivated both EBV and Kaposi's sarcoma-associated herpesvirus (KSHV), to similar efficacy. As a consequence, the Rta-mediated EBV and KSHV lytic replication systems, designated as EREV8 and ERKV, respectively, were homogenous, robust, and concurrent with cell death likely due to permissive lytic replication. In addition, the expression kinetics of EBV lytic genes in Dox-treated EREV8 cells was similar to that of their KSHV counterparts in Dox-induced ERKV cells, suggesting that a common pathway is used to disrupt viral latency in both cell systems. When the time course was compared, cell cycle arrest was achieved between 6 and 48 h, EBV or KSHV reactivation was initiated abruptly at 48 h, and the cellular senescence marker was not detected until 120 h after Dox treatment. These results lead us to hypothesize that in 293 cells, Rta-induced G1 cell cycle arrest could provide (1) an ideal environment for virus reactivation if EBV or KSHV coexists and (2) a preparatory milieu for cell senescence if no viral genome is available. The latter is hypothetical in a transient-lytic situation.

miR-29 and miR-30 regulate B-Myb expression during cellular senescence

Cellular senescence is a form of irreversible growth arrest and a major tumor suppressor mechanism. We show here that the miR-29 and miR-30 microRNA families are up-regulated during induced and replicative senescence and that up-regulation requires activation of the Rb pathway. Expression of a reporter construct containing the 3'UTR of the B-Myb oncogene is repressed during senescence, and repression is blocked by mutations in conserved miR-29 and miR-30 binding sites in the B-Myb 3'UTR. In proliferating cells, transfection of miR-29 and miR-30 represses a reporter construct containing the wild-type but not the mutant B-Myb 3'UTR, and repression of the mutant 3'UTR is reinstituted by compensatory mutations in miR-29 and miR-30 that restore binding to the mutant sites. miR-29 and miR-30 introduction also represses expression of endogenous B-Myb and inhibits cellular DNA synthesis. Finally, interference with miR-29 and miR-30 expression inhibits senescence. These findings demonstrate that miR-29 and miR-30 regulate B-Myb expression by binding to its 3'UTR and suggest that these microRNAs play an important role in Rb-driven cellular senescence.

Interkeratin peptide-protein interactions that promote HPV16 E7 gene expression

Human papillomavirus (HPV) 16 E7 gene product encodes the major transforming activity of the virus so as to induce neoplastic transformation. Continued expression of HPV16 E7 protein is required for both the establishment and maintenance of the transformed cellular phenotype. Therefore, understanding of the molecular and biochemical factors leading to the expression of E7 protein is important in relation to HPV-associated diseases. Previously, we identified a rare codon usage and a specific interaction between cytokeratin (CK) 7 and HPV16 E7 mRNA as factors modulating HPV16 E7 expression. In the present study we report that CK19, a biochemical marker of squamous oral and cervical cancer carcinogenesis, promotes the expression of HPV16 E7 oncoprotein by binding to the CK792-97SEQIKA peptide. These findings shed light on the dynamic functionality of the intermediate filament cytoskeleton, open new perspectives for investigating the role of CKs in controlling HPV16 E7 expression, and suggest new therapeutic avenues for HPV-associated carcinomas.

Use of tissue microarray to facilitate oncology research

HPV-positive oropharyngeal squamous cell carcinomas (OSCC) represent a distinct disease entity from traditional OSCC. We hypothesized that for HPV DNA-positive cases, p16 expression status differentiates the biologically relevant ones.We determined HPV16DNA viral load in a cohort of 79 oropharyngeal squamous cell cancers by real-time polymerase chain reaction (PCR). We used cervical cancer as a disease model for HPV-initiated epithelial cancer. In cervical cancer, p53 and Rb expression is reduced, while p16 expression is increased. We used TMA technology to facilitate interrogation of this cohort for p53, Rb, and p16 protein expression using a quantitative, in situ method of protein analysis (AQUA analysis). Our results indeed delineate three biologically and clinically distinct types of oropharyngeal squamous cell cancers based on HPV-DNA determination and p16 expression status: one class of HPV-negative/p16-nonexpressing (HPV-negative), one class of HPV-positive/p16-nonexpressing (HPV-inactive), and one class of HPV positive/p16-expressing (HPV-active) oropharyngeal tumors. We demonstrated that only the HPV-active tumors share a similar molecular phenotype to cervical cancers, and are the ones associated with favorable prognosis.

Activation of Wnt signaling pathway by human papillomavirus E6 and E7 oncogenes in HPV16-positive oropharyngeal squamous carcinoma cells

We sought to determine the role of human papillomavirus (HPV) E6 and E7 oncogenes in nuclear beta-catenin accumulation, a hallmark of activated canonical Wnt signaling pathway. We used HPV16-positive oropharyngeal cancer cell lines 147T and 090, HPV-negative cell line 040T, and cervical cell lines SiHa (bearing integrated HPV16) and HeLa (bearing integrated HPV18) to measure the cytoplasmic and nuclear beta-catenin levels and the beta-catenin/Tcf transcriptional activity before and after E6/E7 gene silencing. Repression of HPV E6 and E7 genes induced a substantial reduction in nuclear beta-catenin levels. Luciferase assay showed that transcriptional activation of Tcf promoter by beta-catenin was lower after silencing. The protein levels of beta-catenin are tightly regulated by the ubiquitin/proteasome system. We therefore performed expression analysis of regulators of beta-catenin degradation and nuclear transport and showed that seven in absentia homologue (Siah-1) mRNA and protein levels were substantially upregulated after E6/E7 repression. Siah-1 protein promotes the degradation of beta-catenin through the ubiquitin/proteasome system. To determine whether Siah-1 is important for the proteasomal degradation of beta-catenin in HPV16-positive oropharyngeal cancer cells, we introduced a Siah-1 expression vector into 147T and 090 cells and found substantial reduction of endogenous beta-catenin in these cells. Thus, E6 and E7 are involved in beta-catenin nuclear accumulation and activation of Wnt signaling in HPV-induced cancers. In addition, we show the significance of the endogenous Siah-1-dependent ubiquitin/proteasome pathway for beta-catenin degradation and its regulation by E6/E7 viral oncoproteins in HPV16-positive oropharyngeal cancer cells.

Genome-wide siRNA screen identifies SMCX, EP400, and Brd4 as E2-dependent regulators of human papillomavirus oncogene expression

An essential step in the pathogenesis of human papillomavirus (HPV)-associated cancers is the dysregulated expression of the viral oncogenes. The papillomavirus E2 protein can silence the long control region (LCR) promoter that controls viral E6 and E7 oncogene expression. The mechanisms by which E2 represses oncogene expression and the cellular factors through which E2 mediates this silencing are largely unknown. We conducted an unbiased, genome-wide siRNA screen and series of secondary screens that identified 96 cellular genes that contribute to the repression of the HPV LCR. In addition to confirming a role for the E2-binding bromodomain protein Brd4 in E2-mediated silencing, we identified a number of genes that have not previously been implicated in E2 repression, including the demethylase JARID1C/SMCX as well as EP400, a component of the NuA4/TIP60 histone acetyltransferase complex. Each of these genes contributes independently and additively to E2-mediated silencing, indicating that E2 functions through several distinct cellular complexes to repress E6 and E7 expression.

Cellular players in the herpes simplex virus dependent apoptosis balancing act

Apoptosis is triggered as an intrinsic defense against numerous viral infections. Almost every virus encodes apoptotic modulators, and the herpes simplex viruses (HSV) are no exception. During HSV infection, there is an intricate balance between pro- and anti-apoptotic factors that delays apoptotic death until the virus has replicated. Perturbations in the apoptotic balance can cause premature cell death and have the potential to dramatically alter the outcome of infection. Recently, certain cellular genes have been shown to regulate sensitivity to HSV-dependent apoptosis. This review summarizes current knowledge of the cellular genes that impact the apoptotic balance during HSV infection.

High-throughput cell-based screen for chemicals that inhibit infection by simian virus 40 and human polyomaviruses

We developed a high-throughput, cell-based screen to identify chemicals that inhibit infection by the primate polyomaviruses. The screen is based on the detection of compounds that inhibit the ability of a replication-defective simian virus 40 (SV40)-based viral vector to cause growth arrest in HeLa cells by repressing the expression of the endogenous human papillomavirus E7 oncogene in these cells. We identified two compounds, ellagic acid and spiperone, that suppressed the ability of the SV40 recombinant virus to inhibit cellular DNA synthesis. These compounds caused a marked reduction of the ability of wild-type SV40 to productively infect permissive monkey cells, even when the compounds were added several hours after infection. The fraction of cells expressing SV40 large T antigen and the levels of T antigen mRNA were reduced in infected human and monkey cells treated with ellagic acid and spiperone, suggesting that these compounds block a step in the virus life cycle prior to SV40 early gene expression. Ellagic acid and spiperone also inhibited large T antigen expression by BK virus and JC virus, two important, pathogenic human polyomaviruses.

E6 and e7 gene silencing and transformed phenotype of human papillomavirus 16-positive oropharyngeal cancer cells

BACKGROUND

The E6 and E7 genes of human papillomavirus type 16 (HPV16) encode oncoproteins that bind and degrade p53 and retinoblastoma (pRb) tumor suppressors, respectively. We examined the effects of repressing E6 and E7 oncogene expression on the transformed phenotype of HPV16-positive oropharyngeal cancer cell lines.

METHODS

Human oropharyngeal squamous cell cancer 147T and 090 (harboring integrated HPV16 DNA) and 040T (HPV DNA-negative) cells were infected with retroviruses that expressed a short hairpin RNA (shRNA) targeting the HPV16 E6 and E7 genes or a scrambled-sequence control shRNA. Flow cytometry, terminal deoxynucleotidyltransferase-mediated UTP end-labeling assay, and immunoblotting for annexin V were used to assess apoptosis in shRNA-infected cell lines. Biochemical analysis involved quantitative real-time polymerase chain reaction analysis of p53- and pRb-target gene expression and immunoblotting for p53 and pRb protein expression.

RESULTS

In 147T and 090 cells, shRNA-mediated inhibition of HPV16 E6 and E7 expression reduced the E6 and E7 mRNA levels by more than 85% compared with control cells that expressed a scrambled-sequence shRNA. E6 and E7 repression resulted in restoration of p53 and pRB protein expression, increased expression of p53-target genes (p21 and FAS), decreased expression of genes whose expression is increased in the absence of functional pRb (DEK and B-MYB), and induced substantial apoptosis in 147T and 090 cells compared with the control shRNA-infected cells (from 13.4% in uninfected to 84.3% in infected 147T cells and from 3.3% in uninfected to 71.2% in infected 090 cells).

CONCLUSION

Repression of E6 and E7 oncogenes results in restoration of p53 and pRb suppressor pathways and induced apoptosis in HPV16-positive oropharyngeal squamous cell cancer cell lines.

Human papillomavirus type 16 E7 oncoprotein associates with E2F6

The papillomavirus life cycle is intimately coupled to the differentiation state of the infected epithelium. Since papillomaviruses lack most of the rate-limiting enzymes required for genome synthesis, they need to uncouple keratinocyte differentiation from cell cycle arrest and maintain or reestablish a replication-competent state within terminally differentiated keratinocytes. The human papillomavirus (HPV) E7 protein appears to be a major determinant for this activity and induces aberrant S-phase entry through the inactivation of the retinoblastoma tumor suppressor and related pocket proteins. In addition, E7 can abrogate p21 and p27. Together, this leads to the activation of E2F1 to E2F5, enhanced expression of E2F-responsive genes, and increased cdk2 activity. E2F6 is a pRB-independent, noncanonical member of the E2F transcription factor family that acts as a transcriptional repressor. E2F6 expression is activated in S phase through an E2F-dependent mechanism and thus may provide a negative-feedback mechanism that slows down S-phase progression and/or exit in response to the activation of the other E2F transcription factors. Here, we show that low- and high-risk HPV E7 proteins, as well as simian virus 40 T antigen and adenovirus E1A, can associate with and inactivate the transcriptional repression activity of E2F6, thereby subverting a critical cellular defense mechanism. This may result in the extended S-phase competence of HPV-infected cells. E2F6 is a component of polycomb group complexes, which bind to silenced chromatin and are critical for the maintenance of cell fate. We show that E7-expressing cells show decreased staining for E2F6/polycomb complexes and that this is at least in part dependent on the association with E2F6.

Gene expression reveals two distinct groups of anal carcinomas with clinical implications

Human papillomavirus (HPV) is a major aetiological agent in anal carcinomas. We here present a study of global gene expression using microarray hybridisation in a collection of anal carcinoma biopsies. Quantitative PCR was used to verify expression of selected genes. All biopsies contained integrated DNA of human papillomavirus subtype 16 (HPV16) and expressed HPV16 E7 mRNA. No other subspecies of HPV were detected in these 13 biopsies as assessed by PCR amplification and DNA sequencing. Unsupervised cluster analysis, based on global mRNA expression, divided the tumour biopsies into two distinct groups. Cluster analysis based on a number of high-risk HPV and/or E2F-regulated genes reproduced this biopsy grouping, suggesting that integrated HPV16 substantially influenced global gene expression in approximately half the biopsies studied. The levels of HPV16 E7 mRNA were significantly different between the two groups, but with considerable overlap. Thus, influence on global gene expression could not be absolutely ascribed to the expression level of HPV16. To investigate whether this distinction in gene expression had prognostic impact, we studied protein expression in an independent cohort of 55 anal carcinomas not included in the microarray study of two differentially expressed candidate genes, minichromosome maintenance complex component 7 (MCM7) and cyclin-dependent kinase inhibitor 2A (CDKN2A or p16). HPV status was assessed by in situ hybridisation. There was a significant association between in situ staining for HPV E7 mRNA and immunostaining for CDKN2A (p16) and MCM7 protein. CDKN2A (p16) mRNA was found significantly differentially expressed between the two tumour groups. However, cluster analysis on genes directly regulated by CDKN2A (p16) could not reproduce this split of biopsies into two groups, suggesting that the transcriptional regulatory activity of CDKN2A in these biopsies is inhibited. Furthermore, protein expression of CDKN2A (p16) could not be associated with survival. MCM7 is directly regulated by E2F and induced by HPV, and its mRNA was found differentially expressed between the two tumour groups. High level of MCM7 protein was found to be associated with both improved relapse-free survival (RFS, P=0.02) and cancer-specific survival (CSS, P=0.03) in anal cancer patients treated with radiation with or without additional chemotherapy.

Single-dose, therapeutic vaccination of mice with vesicular stomatitis virus expressing human papillomavirus type 16 E7 protein

We are developing recombinant attenuated vesicular stomatitis virus (VSV) as a vaccine vector to generate humoral and cell-mediated immune responses. Here, we explore the use of VSV vaccines for cancer immunotherapy. Immunotherapy targeting high-risk human papillomavirus (HPV) lesions has the potential to benefit HPV-infected individuals and cervical cancer patients by generating cytotoxic T cells that kill tumor cells that express viral antigens. A single dose of VSV expressing the HPV type 16 (HPV16) E7 oncogene was used for therapeutic vaccination of mice bearing TC-1 syngeneic tumors, which express HPV16 E7. HPV16 E7-specific T cells were generated and displayed cytotoxic activity against the tumor cells. By 14 days postvaccination, average tumor volumes were 10-fold less in the vaccinated group than in mice that received the empty-vector VSV, and regression of preexisting tumors occurred in some cases. This antitumor effect was CD8 T-cell dependent. Our results demonstrate antitumor responses to HPV16 E7 and suggest that recombinant-VSV-based vaccination should be explored as a therapeutic strategy for cervical carcinoma and other HPV-associated cancers.

The E8 repression domain can replace the E2 transactivation domain for growth inhibition of HeLa cells by papillomavirus E2 proteins

Continuous expression of the human papillomavirus (HPV) oncoproteins E6 and E7 is required for the growth of cervical cancer cell lines. So far, only the overexpression of the wild type papillomavirus E2 protein has been shown to induce growth arrest in HPV18-positive HeLa cells by repressing E6/E7 transcription. Growth arrest by E2 requires the aminoterminal transcription activation domain in addition to the carboxyterminal DNA-binding domain. Several papillomaviruses such as the carcinogenic HPV31 express in addition to E2 an E8(wedge)E2C fusion protein in which the E8 domain, which is required for repression of replication and transcription, replaces the E2 activation domain. In this report, we demonstrate that the HPV31 E8(wedge)E2C protein is able to inhibit the growth of HeLa cells but not of HPV-negative C33A cervical cancer cells. Growth repression by E8(wedge)E2C correlates with repression of the endogenous HPV18 E6/E7 promoter and the reappearance of E6- and E7-regulated p53, pRb and p21 proteins, suggesting that E8(wedge)E2C inhibits growth by reactivating dormant tumor suppressor pathways. Growth inhibition requires an intact E8 repression domain in addition to the carboxyterminal E2C DNA binding domain. Chromatin immunoprecipitation experiments suggest that the E8 repression domain enhances binding to the HPV18 promoter sequence in vivo. In summary, our results demonstrate that the small E8 repression domain can functionally replace the large E2 transactivation domain for growth inhibition of HeLa cervical cancer cells.

p53 and hTERT determine sensitivity to viral apoptosis

Apoptosis is a potent host defense against microbes. Most viruses have adapted strategies to counteract this response. Herpes simplex virus (HSV) produces a balance between pro- and antiapoptotic processes during infection. When antiapoptotic signals become limiting, infected cells die through HSV-dependent apoptosis (HDAP). Oncogenic pathways were previously implicated in HDAP susceptibility. Here, we exploited our ability to selectively express all, one, or no oncogenes in the well-defined HeLa cell system to dissect the requirements for HDAP. Human papillomavirus E6 and E7 oncogene expression was inhibited by the E2 viral repressor. Sole expression of E6 mediated HDAP sensitization. Next, two known cellular targets of E6 were independently modulated. This demonstrated that E6 sensitizes HeLa cells to HDAP through hTERT and p53. Given the universality of the apoptotic antiviral response, p53 and telomerase regulation will likely be important for counteracting host defenses in many other viral infections.

Increased expression of Dyrk1a in HPV16 immortalized keratinocytes enable evasion of apoptosis

Immortalization is a critical event in virus-related oncogenesis. No enough information, however, is currently available to elucidate the changes that occur in cellular molecules during immortalization. To identify potential cellular markers or regulators involving in immortalization, a paired-cell model of primary foreskin keratinocytes (FK) and HPV16 immortalized foreskin keratinocytes were established. Using mRNA differential display, RT-PCR and Northern blot methods, we have identified and confirmed that Dyrk1a (dual-specificity tyrosine-phosphorylated and regulated kinase 1A) is present and increased in HPV16 immortalized cells, but is absent in primary keratinocytes. Moreover, transfection of E7 siRNA oligo into immortalized cells leads to a diminishing E7 expression and the eventual disappearance of Dyrk1a. Similar results of Dyrk1a expressional differences could also be seen when tissue specimens were compared using LCM/real-time PCR and immunohistochemistry analysis; malignant cervical lesions contain significantly more DYRK1A than normal tissue. It was also demonstrated that raised DYRK1A could rearrange the cellular localization of FKHR (forkhead in rhabdomyosarcoma), an apoptosis activator, and suppress BAD. Importantly, this phenomenon can be reversed when endogenous Dyrk1a was knocked down in immortalized cells by RNA interference. These results suggest that the raised Dyrk1a in HPV16 immortalized keratinocytes and cervical lesions may serve as a candidate antiapoptotic factor in the FKHR regulated pathway and initiate immortalization and tumorigenesis gradually.

Human papillomavirus E7 repression in cervical carcinoma cells initiates a transcriptional cascade driven by the retinoblastoma family, resulting in senescence

This work demonstrates a central role for the retinoblastoma (Rb) family in driving the transcriptional program of induced and replicative senescence. HeLa cervical carcinoma cells rapidly undergo senescence when the human papillomavirus (HPV) type 18 E7 gene in these cells is repressed by the bovine papillomavirus (BPV) E2 protein. This senescence response requires the endogenous Rb pathway but not the p53 pathway. Microarray analysis 6 days after BPV E2 introduction into HeLa cells identified 224 cellular genes induced by E7 repression and 354 repressed genes. Many repressed genes were involved in cell cycle progression, and numerous induced genes encoded lysosomal proteins. These gene expression changes were blocked by constitutive expression of the wild-type HPV16 E7 or adenovirus E1A gene, but not by E7 or E1A mutants defective for Rb binding. Short hairpin RNAs targeting the Rb family also inhibited these gene expression changes and blocked senescence. Therefore, surprisingly, the transcriptional response to BPV E2 expression was entirely dependent on E7 repression and activation of the Rb family, and the BPV E2 protein did not directly affect the expression of cellular genes. Activation of the Rb family repressed E2F-responsive genes and stimulated transcriptional activators, thereby mobilizing multiple signals, such as repression of B-MYB and DEK, that were independently sufficient to induce senescence. There was extensive overlap between the transcriptional profiles of senescent, late-passage primary human fibroblasts and senescent cervical carcinoma cells, suggesting that this Rb family-mediated transcriptional cascade also plays a central role in replicative senescence.

Regulation of cellular senescence by Rb2/p130

Growth regulatory functions of Rb2/p130, which aim at a sustained arrest such as in quiescent or differentiated cells, qualify the protein also to act as a central regulator of growth arrest in cellular senescence. In this respect, Rb2/p130 functions are connected to signaling pathways induced by p53, which is a master regulator in cellular senescence. Here, we summarize the pathways, which specify pRb2/p130 to control this arrest program and distinguish its functions from those of pRb/p105.

Human papillomaviruses and cervical cancer

Carcinoma of the uterine cervix, a leading cause of cancer death in women worldwide, is initiated by infection with high-risk types of human papillomaviruses (HPVs). This review summarizes laboratory studies over the past 20 years that have elucidated the major features of the HPV life cycle, identified the functions of the viral proteins, and clarified the consequences of HPV infection for their host cells. This information has allowed the development of various strategies to prevent or treat infections, including prophylactic vaccination with virus-like particles, therapeutic vaccination against viral proteins expressed in cancer cells, and antiviral approaches to inhibit virus replication, spread, or pathogenesis. These strategies have the potential to cause a dramatic reduction in the incidence of cervical carcinoma and serve as the prototype for comprehensive efforts to combat virus-induced tumors.

RBP1 family proteins exhibit SUMOylation-dependent transcriptional repression and induce cell growth inhibition reminiscent of senescence

The retinoblastoma binding protein 1 (RBP1) appears to be an important factor in the repression of E2F-dependent transcription by the retinoblastoma protein (pRB) family. The recent identification of the breast carcinoma associated antigen (BCAA) as an RBP1-like protein led us to investigate its biological properties and compare them to RBP1. Like RBP1, BCAA contains a carboxy-terminal R2 domain that elicits histone deacetylase (HDAC)-dependent transcriptional repression via interactions with the SAP30 subunit of the Sin3/HDAC complex. Each RBP1 family member also contains two HDAC-independent repression activities within a region termed R1, which can be subdivided into a SUMOylated moiety (R1sigma) and a predicted alpha-helical region (R1alpha). R1alpha is embedded within the ARID region and represses basal transcription only, whereas R1sigma represses both basal and activated transcription and depends on SUMOylation. Overexpression of either RBP1 or BCAA, but not the truncated BCAAMCF-7 isoform that is overexpressed in breast cancer cells, caused a profound inhibition of cell proliferation and induced expression of a senescence marker. In each case the presence of both R1 and R2 was necessary for suppression of cell growth, suggesting that both R1 and R2 transcriptional repression activities play a role in RBP1 family protein-mediated regulation of cellular proliferation.