Human papillomavirus type 16 E7 oncoprotein associates with the cullin 2 ubiquitin ligase complex, which contributes to degradation of the retinoblastoma tumor suppressor

A Novel Retinoblastoma Protein (RB) E3 Ubiquitin Ligase (NRBE3) Promotes RB Degradation and Is Transcriptionally Regulated by E2F1 Transcription Factor

Retinoblastoma protein (RB) plays critical roles in tumor suppression and is degraded through the proteasomal pathway. However, E3 ubiquitin ligases responsible for proteasome-mediated degradation of RB are largely unknown. Here we characterize a novel RB E3 ubiquitin ligase (NRBE3) that binds RB and promotes RB degradation. NRBE3 contains an LXCXE motif and bound RB in vitro. NRBE3 interacted with RB in cells when proteasome activity was inhibited. NRBE3 promoted RB ubiquitination and degradation via the ubiquitin-proteasome pathway. Importantly, purified NRBE3 ubiquitinated recombinant RB in vitro, and a U-box was identified as essential for its E3 activity. Surprisingly, NRBE3 was transcriptionally activated by E2F1/DP1. Consequently, NRBE3 affected the cell cycle by promoting G1/S transition. Moreover, NRBE3 was up-regulated in breast cancer tissues. Taken together, we identified NRBE3 as a novel ubiquitin E3 ligase for RB that might play a role as a potential oncoprotein in human cancers.

Interplay between the virus and the ubiquitin-proteasome system: molecular mechanism of viral pathogenesis

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Many viruses have evolved to utilize the host UPS for their own benefit.

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Viruses subvert the UPS to maintain optimal level/function of viral proteins.

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Viruses exploit the UPS to degrade host proteins which impede viral growth.

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The UPS serves as an important host anti-viral defense mechanism.

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The UPS is inhibited by some viruses to prevent viral clearance.

The ubiquitin–proteasome system (UPS) plays a central role in a wide range of fundamental cellular functions by ensuring protein quality control and through maintaining a critical level of important regulatory proteins. Viruses subvert or manipulate this cellular machinery to favor viral propagation and to evade host immune response. The UPS serves as a double-edged sword in viral pathogenesis: on the one hand, the UPS is utilized by many viruses to maintain proper function and level of viral proteins; while on the other hand, the UPS constitutes a host defense mechanism to eliminate viral components. To combat this host anti-viral machinery, viruses have evolved to employ the UPS to degrade or inactivate cellular proteins that limit viral growth. This review will highlight our current knowledge pertaining to the different roles for the UPS in viral pathogenesis.

Human papillomaviruses: shared and distinct pathways for pathogenesis

Over 200 types of human papillomaviruses (HPV) have been identified that infect epithelial cells at different anatomic locations. HPVs are grouped into five genera with the alpha and beta viruses being the most commonly studied. Members of the alpha HPV genus infect genital epithelia and are the causative agents of many anogenital cancers. Beta HPVs infect cutaneous epithelia and have been suggested as co-factors in the development of non-melanoma skin cancers. Recent studies have shown that activation of DNA damage pathways is important for the productive life cycle of the alpha HPVs while the beta viruses suppress their activation. These differences likely contribute to the varying types of lesions and malignancies that are associated with these viruses.

Viral Mimicry to Usurp Ubiquitin and SUMO Host Pathways

Posttranslational modifications (PTMs) of proteins include enzymatic changes by covalent addition of cellular regulatory determinants such as ubiquitin (Ub) and small ubiquitin-like modifier (SUMO) moieties. These modifications are widely used by eukaryotic cells to control the functional repertoire of proteins. Over the last decade, it became apparent that the repertoire of ubiquitiylation and SUMOylation regulating various biological functions is not restricted to eukaryotic cells, but is also a feature of human virus families, used to extensively exploit complex host-cell networks and homeostasis. Intriguingly, besides binding to host SUMO/Ub control proteins and interfering with the respective enzymatic cascade, many viral proteins mimic key regulatory factors to usurp this host machinery and promote efficient viral outcomes. Advanced detection methods and functional studies of ubiquitiylation and SUMOylation during virus-host interplay have revealed that human viruses have evolved a large arsenal of strategies to exploit these specific PTM processes. In this review, we highlight the known viral analogs orchestrating ubiquitin and SUMO conjugation events to subvert and utilize basic enzymatic pathways.

Regulation of the Wnt/β-Catenin Signaling Pathway by Human Papillomavirus E6 and E7 Oncoproteins

Cell signaling pathways are the mechanisms by which cells transduce external stimuli, which control the transcription of genes, to regulate diverse biological effects. In cancer, distinct signaling pathways, such as the Wnt/β-catenin pathway, have been implicated in the deregulation of critical molecular processes that affect cell proliferation and differentiation. For example, changes in β-catenin localization have been identified in Human Papillomavirus (HPV)-related cancers as the lesion progresses. Specifically, β-catenin relocates from the membrane/cytoplasm to the nucleus, suggesting that this transcription regulator participates in cervical carcinogenesis. The E6 and E7 oncoproteins are responsible for the transforming activity of HPV, and some studies have implicated these viral oncoproteins in the regulation of the Wnt/β-catenin pathway. Nevertheless, new interactions of HPV oncoproteins with cellular proteins are emerging, and the study of the biological effects of such interactions will help to understand HPV-related carcinogenesis. This review addresses the accumulated evidence of the involvement of the HPV E6 and E7 oncoproteins in the activation of the Wnt/β-catenin pathway.

HPV18 E7 induces the over-transcription of eIF4E gene in cervical cancer

OBJECTIVES

Eukaryotic translation initiation factor 4E (eIF4E) is overexpressed in cervical cancer (CC). However, the molecular mechanisms are unclear. This study aimed to investigate the molecular mechanism of eIF4E gene overexpression in CC.

MATERIALS AND METHODS

The human papillomavirus (HPV) type 18 E7 and eIF4E mRNAs were measured following knock down or overexpression of E7 gene by RT-PCR and real-time PCR. Cell counting kit-8 assay was used to determine the cell proliferation. Flow cytometry was used to analyze the cell cycle and apoptosis. Transwell system was employed to determine the cell migration.

RESULTS

Overexpression of E7 gene increased eIF4E mRNA level by 24.3% (P<0.01) in HPV negative C33A cells. Knock down of E7 decreased markedly eIF4E mRNA by 73% (P<0.01) in HPV18 positive HeLa cells. Under the state of high expression of E7, 1) up-regulation of eIF4E drastically promoted the cell proliferation, cell cycle and cell migration, and inhibited the cell apoptosis. 2) down-regulation of eIF4E significantly inhibited the cell proliferation, cell cycle and the ability of cell migration, and also promoted the apoptosis of cervical cancer cells.

CONCLUSION

HPV E7 induced eIF4E gene over transcription which might be a new marker for CC. The finding broadens the understanding of the CC carcinogenesis.

N-Benzylcinnamide induces apoptosis in HPV16 and HPV18 cervical cancer cells via suppression of E6 and E7 protein expression

Seventy percent of all cervical cancers are caused by human papillomavirus (HPV) infections. Natural products are being extensively explored for their potential ability to prevent and treat cervical cancers. N-benzylcinnamide (PT-3) is a natural product purified from Piper submultinerve. Whether or not PT-3 has an effect on cervical cancer cells is as yet unknown. Therefore, we set out to explore the mechanism of action behind PT-3 and how it affects cells that either contain or lack HPV DNA. Our results demonstrate that PT-3 slows the growth kinetics of CaSki (HPV-16 positive) and HeLa (HPV-18 positive) cells in a dose-dependent manner, but does not slows HPV-negative cells. Importantly, we also found that PT-3 induces apoptosis by suppressing expression of E6 and E7 viral oncogenes in HPV-infected cervical cancer CaSki and HeLa cells. Moreover, we found that suppression of E6 and E7 expression leads to modulations in p53 and protein retinoblastomas, which are not changed in HPV-negative cervical cancer C33A cells. These findings demonstrate that PT-3 can effectively promote apoptosis by downregulating expression of E6 and E7.

HPV-16 impairs the subcellular distribution and levels of expression of protein phosphatase 1γ in cervical malignancy

Background

The high risk Human Papillomavirus (HPV) E6 oncoproteins play an essential role in the development of cervical malignancy. Important cellular targets of E6 include p53 and the PDZ domain containing substrates such as hScrib and Dlg. We recently showed that hScrib activity was mediated in part through recruitment of protein phosphatase 1γ (PP1γ).

Methods

Expression patterns of hScrib and PP1γ were assessed by immunohistochemistry of HPV-16 positive cervical intraepithelial neoplasm (CIN), classified as CIN1 (n = 4), CIN2 (n = 8), CIN3 (n = 8), cervical carcinoma tissues (n = 11), and HPV-negative cervical tissues (n = 8), as well as by subfractionation assay of the HPV-16 positive cervical cancer cell lines, CaSki and SiHa. To explore the effects of the HPV-16 oncoproteins, we have performed siRNA knockdown of E6/E7 expression, and monitored the effects on the expression patterns of hScrib and PP1γ.

Results

We show that PP1γ levels in HPV-16 positive tumour cells are reduced in an E6/E7 dependent manner. Residual PP1γ in these cells is found mostly in the cytoplasm as opposed to the nucleus where it is predominantly found in normal cells. We have found a striking concordance with redistribution in the pattern of expression (9/11; 81.8%) and loss of PP1γ expression in HPV-16 positive cervical tumours (2/11; 18.2%). Furthermore, this loss of PP1γ expression and redistribution in the pattern of expression occurs progressively as the lesions develop (8/8; 100%).

Conclusion

Together, these results suggest that PP1γ may be a novel target of the HPV-16 oncoproteins and indicate that it might be a potential novel biomarker for HPV-16 induced malignancy.

The retinoblastoma tumor suppressor promotes efficient human cytomegalovirus lytic replication

The retinoblastoma (Rb) tumor suppressor controls cell cycle, DNA damage, apoptotic, and metabolic pathways. DNA tumor virus oncoproteins reduce Rb function by either inducing Rb degradation or physically disrupting complexes between Rb and its myriad binding proteins. Human cytomegalovirus (HCMV), a betaherpesvirus being investigated for potential roles in human cancers, encodes multiple lytic-phase proteins that inactivate Rb in distinct ways, leading to the hypothesis that reduced Rb levels and/or activity would benefit HCMV lytic infection. Paradoxically, we found that Rb knockdown prior to infection, whether transient or constitutive, impaired HCMV lytic infection at multiple stages, notably viral DNA replication, late protein expression, and infectious virion production. The existence of differentially modified forms of Rb, the temporally and functionally distinct means by which HCMV proteins interact with Rb, and the necessity of Rb for efficient HCMV lytic replication combine to highlight the complex relationship between the virus and this critical tumor suppressor.

IMPORTANCE

Initial work examining viral protein modulation of cell cycle progression and oncogenic transformation revealed that these proteins inactivated the function of cellular tumor suppressor proteins. However, subsequent work, including experiments described here using human cytomegalovirus, demonstrate a more nuanced interaction that includes the necessity of cellular tumor suppressors for efficient viral replication. Understanding the positive impacts that cellular tumor suppressors have on viral infections may reveal new activities of these well-studied yet incompletely understood proteins. The basis for oncolytic viral therapy is the selective replication of viruses in transformed cells in which tumor suppressor function may be compromised. Understanding how tumor suppressors support viral infections may allow for the generation of modified oncolytic viruses with greater selective tumor cell replication and killing.

Human Beta-papillomavirus infection and keratinocyte carcinomas

Although the role of oncogenic human Alpha-papillomaviruses (HPVs) in the development of mucosal carcinomas at different body sites (eg cervix, anus, oropharynx) is fully recognized, a role for HPV in keratinocyte carcinomas (KCs; basal and squamous cell carcinomas) of the skin is not yet clear. KCs are the most common cancers in Caucasians, with the major risk factor being ultraviolet (UV) light exposure. A possible role for Beta-HPV types (BetaPV) in the development of KC was suggested several decades ago, supported by a number of epidemiological studies. Our current review summarizes the recent molecular and histopathological evidence in support of a causal association between BetaPV and the development of KC, and outlines the suspected synergistic effect of viral gene expression with UV radiation and immune suppression. Further insights into the molecular pathways and protein interactions used by BetaPV and the host cell is likely to extend our understanding of the role of BetaPV in KC.

Emerging biomarkers in head and neck cancer in the era of genomics

Head and neck cancer (HNC) broadly includes carcinomas arising from the mucosal epithelia of the head and neck region as well as various cell types of salivary glands and the thyroid. As reflected by the multiple sites and histologies of HNC, the molecular characteristics and clinical outcomes of this disease vary widely. In this Review, we focus on established and emerging biomarkers that are most relevant to nasopharyngeal carcinoma and head and neck squamous-cell carcinoma (HNSCC), which includes primary sites in the oral cavity, oropharynx, hypopharynx and larynx. Applications and limitations of currently established biomarkers are discussed along with examples of successful biomarker development. For emerging biomarkers, preclinical or retrospective data are also described in the context of recently completed comprehensive molecular analyses of HNSCC, which provide a broad genetic landscape and molecular classification beyond histology and clinical characteristics. We will highlight the ongoing effort that will see a shift from prognostic to predictive biomarker development in HNC with the goal of delivering individualized cancer therapy.

Cullin E3 ligases and their rewiring by viral factors

The ability of viruses to subvert host pathways is central in disease pathogenesis. Over the past decade, a critical role for the Ubiquitin Proteasome System (UPS) in counteracting host immune factors during viral infection has emerged. This counteraction is commonly achieved by the expression of viral proteins capable of sequestering host ubiquitin E3 ligases and their regulators. In particular, many viruses hijack members of the Cullin-RING E3 Ligase (CRL) family. Viruses interact in many ways with CRLs in order to impact their ligase activity; one key recurring interaction involves re-directing CRL complexes to degrade host targets that are otherwise not degraded within host cells. Removal of host immune factors by this mechanism creates a more amenable cellular environment for viral propagation. To date, a small number of target host factors have been identified, many of which are degraded via a CRL-proteasome pathway. Substantial effort within the field is ongoing to uncover the identities of further host proteins targeted in this fashion and the underlying mechanisms driving their turnover by the UPS. Elucidation of these targets and mechanisms will provide appealing anti-viral therapeutic opportunities. This review is focused on the many methods used by viruses to perturb host CRLs, focusing on substrate sequestration and viral regulation of E3 activity.

Prevalence and prognostic value of human papillomavirus genotypes in tonsillar squamous cell carcinoma: a Korean multicenter study

BACKGROUND

This study was aimed at investigating the change in the prevalence of human papillomavirus (HPV) genotypes in tonsillar squamous cell carcinoma (TSCC) and the association of the HPV genotype with the prognosis.

METHODS

This multicenter study included 175 patients with TSCC from 3 general hospitals between 1991 and 2009. HPV DNA was detected in paraffin-embedded tissues with genotyping chips. A survival analysis that considered clinicopathological factors, the HPV genotype, and the expression of p53, retinoblastoma protein, p16, and epidermal growth factor receptor (assessed with immunohistochemistry) was performed with Cox regression analysis.

RESULTS

High-risk HPV types were found in 23.4% of the cases. The prevalence of HPV-18 (10.3%) was as high as that of HPV-16 (10.3%). The proportion of high-risk HPV-positive tumors increased from 5.9% in 1991 to 31.6% in 2009. HPV-16 positivity was associated with an advanced stage and lymph node metastasis, whereas HPV-18 positivity was associated with old age and an advanced T stage. The survival analysis showed that old age and T classification were poor prognostic factors, whereas the expressions of various biomarkers were not associated with prognosis. HPV-18-positive cases had a poorer prognosis than HPV-16-positive cases and non-HPV-related TSCC cases. A multivariate analysis revealed that HPV-18 positivity, old age, and an advanced T stage were independent prognostic factors for predicting poor outcomes for patients with TSCC.

CONCLUSIONS

The proportion of HPV-positive tonsillar cancer cases has increased during the last 20 years in the Republic of Korea. The presence of HPV-18 may serve as a biomarker for a poor prognosis.

The role of ubiquitin and ubiquitin-like modification systems in papillomavirus biology

Human papillomaviruses (HPVs) are small DNA viruses that are important etiological agents of a spectrum of human skin lesions from benign to malignant. Because of their limited genome coding capacity they express only a small number of proteins, only one of which has enzymatic activity. Additionally, the HPV productive life cycle is intimately tied to the epithelial differentiation program and they must replicate in what are normally non-replicative cells, thus, these viruses must reprogram the cellular environment to achieve viral reproduction. Because of these limitations and needs, the viral proteins have evolved to co-opt cellular processes primarily through protein-protein interactions with critical host proteins. The ubiquitin post-translational modification system and the related ubiquitin-like modifiers constitute a widespread cellular regulatory network that controls the levels and functions of thousands of proteins, making these systems an attractive target for viral manipulation. This review describes the interactions between HPVs and the ubiquitin family of modifiers, both to regulate the viral proteins themselves and to remodel the host cell to facilitate viral survival and reproduction.

Oncogenes and RNA splicing of human tumor viruses

Approximately 10.8% of human cancers are associated with infection by an oncogenic virus. These viruses include human papillomavirus (HPV), Epstein–Barr virus (EBV), Merkel cell polyomavirus (MCV), human T-cell leukemia virus 1 (HTLV-1), Kaposi's sarcoma-associated herpesvirus (KSHV), hepatitis C virus (HCV) and hepatitis B virus (HBV). These oncogenic viruses, with the exception of HCV, require the host RNA splicing machinery in order to exercise their oncogenic activities, a strategy that allows the viruses to efficiently export and stabilize viral RNA and to produce spliced RNA isoforms from a bicistronic or polycistronic RNA transcript for efficient protein translation. Infection with a tumor virus affects the expression of host genes, including host RNA splicing factors, which play a key role in regulating viral RNA splicing of oncogene transcripts. A current prospective focus is to explore how alternative RNA splicing and the expression of viral oncogenes take place in a cell- or tissue-specific manner in virus-induced human carcinogenesis.

Human papillomavirus and host genetic polymorphisms in carcinogenesis: a systematic review and meta-analysis

BACKGROUND

As the role of human papillomavirus (HPV) in carcinogenesis continues to rise, the role of genetic factors that modify this risk have become increasingly important. In this study, we reviewed the literature for associations between polymorphisms and HPV in carcinogenesis.

OBJECTIVE

To identify any associations of genetic polymorphisms with oncogenic HPV in carcinogenesis and to evaluate the methodology used.

STUDY DESIGN

Systematic literature review of HPV, genetic polymorphisms, and cancer risk. Odds ratios (OR), interaction terms, and p-values were tabulated. Meta-analyses and measures of heterogeneity were estimated using RevMan 5.1.

RESULTS

The cervix was the most frequently studied cancer site followed by the head and neck. Overall risk of cancer (cancer vs. control) was the most common comparison, whereas reports of initiation (pre-cancer vs. control) and progression (cancer vs. pre-cancer) were rare. Case-series and joint-effect of HPV and genotype on risk was evaluated frequently, but the independent effect of either risk factor alone was rarely provided. P53-Arg72Pro was the most commonly studied polymorphism studied. No consistent interaction was detected by meta-analysis in the HPV(+) [OR 0.98 (0.55-1.76)] or the HPV(-) [OR 1.10 (0.76-1.60)] subsets in head and neck cancer risk. Polymorphisms in genes known to encode proteins that physically interact with HPV were infrequently studied.

CONCLUSION

No consistent polymorphism-HPV interactions were observed. Study design, choice of candidate polymorphisms/genes, and a focus on overall risk rather than any specific portions of the carcinogenic pathway may have contributed to lack of significant findings.

Analysis of the Cullin binding sites of the E4orf6 proteins of human adenovirus E3 ubiquitin ligases

E4orf6 proteins of human adenoviruses form Cullin-based E3 ubiquitin ligase complexes that degrade cellular proteins, which impedes efficient viral replication. These complexes also include the viral E1B55K product, which is believed to recruit most substrates for ubiquitination. Heterogeneity in the composition of these ligases exists, as serotypes representing some species form Cul5-based complexes (species B2, C, D, and E), whereas others utilize Cul2 (species A and F). Adenovirus type 16 (Ad16; species B1) binds significant levels of both. In this report, we show that the Cul2 binding sequence in E4orf6 of Ad12 (species A) and Ad40 (species F) resembles the cellular consensus Cul2 box. Mutation within this Cul2 box prevents binding not only of Cul2 but also in some cases Elongin C and reduces the ability to degrade target proteins, such as Mre11 and p53. A comparable Cul2 box is not present in E4orf6 of Ad5 and other serotypes that bind Cul5; however, creation of this Cul2 box sequence in Ad5 E4orf6 promoted binding to Cul2 and Cul2-dependent degradation of Mre11. E4orf6 of Ad16 also binds Cul2; however, unlike Ad40, it does not contain an Ad12-like Cul2 box, suggesting that Ad16 binds Cul2 in a unique but perhaps nonfunctional manner, as only Cul5 binding complexes appeared able to degrade Mre11. Our extensive analyses have thus far failed to identify a consensus Cul5 binding sequence, suggesting that association occurs via a novel and perhaps complex pattern of protein-protein interactions. Nevertheless, the identification of the Cul2 box may allow prediction of Cullin specificity for all E4orf6-containing Adenoviridae.

IMPORTANCE

The work described in this paper is a continuation of our in-depth studies on the Cullin-based E3 ligase complexes formed by the viral E4orf6 and E1B55K proteins of all human adenoviruses. This complex induces the degradation of a growing series of cellular proteins that impede efficient viral replication. Some human adenovirus species utilize Cul5, whereas others bind Cul2. In this paper, we are the first to identify the E4orf6 Cul2 binding site, which conforms in sequence to a classic cellular Cul2 box. Ours is the first detailed biochemical and genetic analysis of a Cul2-based adenovirus ligase and provides insights into both the cooperative interactions in forming Cullin-based ligases as well as the universality of formation of all adenovirus ligase complexes. Our work now permits future analysis of the evolutionary significance of the ligase complex, work that is currently in progress in our lab.

E3 ubiquitin ligases and human papillomavirus-induced carcinogenesis

Human papillomavirus (HPV) infection is clinically very common. It is usually a major risk factor in the development of cutaneous benign lesions, cervical cancer and a variety of other malignancies. The biological function of ubiquitination as an intracellular proteasomal-mediated form of protein degradation and an important modulator in the regulation of many fundamental cellular processes has been increasingly recognized over the last decade. HPV proteins have been demonstrated to evolve different strategies to utilize the ubiquitin system for their own purposes. The putative roles of E3 ubiquitin ligases in HPV-induced carcinogenesis have become increasingly apparent, although the mechanisms remain unclear. In this review we provide an update on the mechanisms of the involvement of E3 ubiquitin ligases in HPV-induced carcinogenesis, focusing on their interaction with HPV proteins and their roles in several signalling pathways. Targeting the E3 ubiquitin ligases might offer potential therapeutic strategies for HPV-related diseases in future.

Increased expression of RRM2 by human papillomavirus E7 oncoprotein promotes angiogenesis in cervical cancer

Background:

The purpose of this study was to confirm that RRM2 as a novel target of HPVE7 involved in cervical cancer angiogenesis.

Methods:

Gene expression was analysed by RT-qPCR, western blot and immunohistochemistry in cervical cancer tissue and cell lines. Luciferase reporter assay was used to determine the activities of various RRM2 promoters. Secreted VEGF was measured by ELISA. RRM2-mediated capillary tube formation induced by HPVE7 in cervical cancer cells were evaluated using human umbilical vein endothelial cells in vitro. ROS induced by RRM2 in cercal cancer cells was confirmed by flow cytometry. The growth of cervical cancer cell overexpression RRM2 was examined by nude mouse xenograft.

Results:

RRM2 as a novel downstream target for HPVE7 was upregulated by it at the transcriptional level through the E7-pRb interaction and binding of E2F to the RRM2 promoter region. Immunohistochemical analysis showed that the level of RRM2 positively correlated with the HPVE7 level in human cervical cancer. Functionally, overexpression of RRM2 enhanced the expression of HIF-1α and VEGF via activation of the ERK1/2 signalling pathway in cervical cancer cells, and significantly associated with increased microvessel densities in cervical cancer tissues. In vitro, HPVE7 stimulated RRM2-dependent capillary tube formation by HUVECs, and RRM2-enhanced angiogenesis was VEGF dependent. RRM2-activated ERK1/2 pathway was mediated through production of ROS. In the xenograft mouse model, overexpression of RRM2 in cervical cancer cells enhanced tumour growth as well as microvessel densities.

Conclusion:

HPVE7 induces upregulation of RRM2, which then promotes cervical carcinogenesis via ROS-ERK1/2-HIF-1α-VEGF-induced angiogenesis. Thus, the inhibition of RRM2 activity may be a novel therapeutic strategy for human cervical cancer.

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.

The human papillomavirus family and its role in carcinogenesis

Human papillomaviruses (HPVs) are a family of small double-stranded DNA viruses that have a tropism for the epithelia of the genital and upper respiratory tracts and for the skin. Approximately 150 HPV types have been discovered so far, which are classified into several genera based on their DNA sequence. Approximately 15 high-risk mucosal HPV types are clearly associated with cervical cancer; HPV16 and HPV18 are the most carcinogenic since they are responsible for approximately 50% and 20% of all cervical cancers worldwide, respectively. It is now also clear that these viruses are linked to a subset of other genital cancers, as well as head and neck cancers. Due to their high level of carcinogenic activity, HPV16 and HPV18 are the most studied HPV types so far. Biological studies have highlighted the key roles in cellular transformation of the products of two viral early genes, E6 and E7. Many of the mechanisms of E6 and E7 in subverting the regulation of fundamental cellular events have been fully characterized, contributing not only to our knowledge of how the oncogenic viruses promote cancer development but also to our understanding of basic cell biology. Despite HPV research resulting in extraordinary achievements in the last four decades, significantly improving the screening and prophylaxis of HPV-induced lesions, additional research is necessary to characterize the biology and epidemiology of the vast number of HPV types that have been poorly investigated so far, with a final aim of clarifying their potential roles in other human diseases.

Human papillomavirus type 16 E7 perturbs DREAM to promote cellular proliferation and mitotic gene expression

The study of the small DNA tumor viruses continues to provide valuable new insights into oncogenesis and fundamental biological processes. Although much has already been revealed about how the human papillomaviruses (HPVs) can transform cells and contribute to cervical and oropharyngeal cancer, there clearly is much more to learn. In this issue of Oncogene, Pang et al., doi:10.1038/onc.2013.426, demonstrate that the high-risk HPV16 E7 oncogene can promote cellular proliferation by interacting with the DREAM (DP, RB-like, E2F and MuvB) complex at two distinct phases of the cell cycle. Consistent with earlier work, HPV16 E7 can bind to the retinoblastoma tumor suppressor (RB) family member p130 (RBL2) protein and promote its proteasome-mediated destruction thereby disrupting the DREAM complex and can prevent exit from the cell cycle into quiescence. In addition, they demonstrate that HPV16 E7 can bind to MuvB core complex in association with BMYB and FOXM1 and activate gene expression during the G2 and M phase of the cell cycle. Thus, HPV16 E7 acts to prevent exit from the cell cycle entry and promotes mitotic proliferation and may account for the high levels of FOXM1 often observed in poor-risk cervical cancers.

BC-box protein domain-related mechanism for VHL protein degradation

The tumor suppressor VHL (von Hippel-Lindau) protein is a substrate receptor for Ubiquitin Cullin Ring Ligase complexes (CRLs), containing a BC-box domain that associates to the adaptor Elongin B/C. VHL targets hypoxia-inducible factor 1α to proteasome-dependent degradation. Gam1 is an adenoviral protein, which also possesses a BC-box domain that interacts with the host Elongin B/C, thereby acting as a viral substrate receptor. Gam1 associates with both Cullin2 and Cullin5 to form CRL complexes targeting the host protein SUMO enzyme SAE1 for proteasomal degradation. We show that Gam1 protein expression induces VHL protein degradation leading to hypoxia-inducible factor 1α stabilization and induction of its downstream targets. We also characterize the CRL-dependent mechanism that drives VHL protein degradation via proteasome. Interestingly, expression of Suppressor of Cytokine Signaling (SOCS) domain-containing viral proteins and cellular BC-box proteins leads to VHL protein degradation, in a SOCS domain-containing manner. Our work underscores the exquisite ability of viral domains to uncover new regulatory mechanisms by hijacking key cellular proteins.

Interaction of HPV E6 oncoproteins with specific proteasomal subunits

The Human Papillomavirus E6 oncoproteins have the capacity to target several of their cellular interacting partners for proteasome mediated degradation, and recent proteomic analyses suggest a close involvement of E6 with the cellular proteasome machinery. In this study we have performed an extensive analysis of the capacity of different E6 oncoproteins to interact with specific proteasome components. We demonstrate that multiple subunits of the proteasome can be bound by different HPV E6 oncoproteins. Furthermore, whilst most of these interactions appear independent of the E6AP ubiquitin ligase, the association of E6 with the major ubiquitin-accepting proteasome subunit, S5a, does require the presence of E6AP. One consequence of the interaction between E6/E6AP and S5a is enhanced ubiquitination of this proteasome subunit. These results suggest a complex interplay between E6 and the proteasome, only some aspects of which are dependent upon the E6AP ubiquitin ligase.

Nuclear import of high risk HPV16 E7 oncoprotein is mediated by its zinc-binding domain via hydrophobic interactions with Nup62

We previously discovered that nuclear import of high risk HPV16 E7 is mediated by a cNLS located within the zinc-binding domain via a pathway that is independent of karyopherins/importins (Angeline et al., 2003; Knapp et al., 2009). In this study we continued our characterization of the cNLS and nuclear import pathway of HPV16 E7. We find that an intact zinc-binding domain is essential for the cNLS function in mediating nuclear import of HPV16 E7. Mutagenesis of cysteine residues to alanine in each of the two CysXXCys motifs involved in zinc-binding changes the nuclear localization of the EGFP-16E7 and 2xEGFP-16E7 mutants. We further discover that a patch of hydrophobic residues, 65LRLCV69, within the zinc-binding domain of HPV16 E7 mediates its nuclear import via hydrophobic interactions with the FG domain of the central channel nucleoporin Nup62.

Human papillomavirus type 16 E7 oncoprotein inhibits the anaphase promoting complex/cyclosome activity by dysregulating EMI1 expression in mitosis

The anaphase promoting complex/cyclosome (APC/C) is a ubiquitin ligase complex that orchestrates mitotic progression by targeting key mitotic regulators for proteasomal degradation. APC/C dysfunction is a frequent event during cancer development and can give rise to genomic instability. Here we report that the HPV16 E7 oncoprotein interferes with the degradation of APC/C substrates and that the APC/C inhibitor, EMI1, is expressed at higher levels in HPV16 E7-expressing mitotic cells. HPV16 E7 expression causes increased EMI1 mRNA expression and also inhibits EMI1 degradation. The resulting abnormally high EMI1 levels in HPV16 E7-expressing mitotic cells may inhibit degradation of APC/C substrates and cause the prometaphase delay that we have previously observed in such cells.

The aqueous extract of Ficus religiosa induces cell cycle arrest in human cervical cancer cell lines SiHa (HPV-16 Positive) and apoptosis in HeLa (HPV-18 positive)

Natural products are being extensively explored for their potential to prevent as well as treat cancer due to their ability to target multiple molecular pathways. Ficus religiosa has been shown to exert diverse biological activities including apoptosis in breast cancer cell lines. In the present study, we report the anti-neoplastic potential of aqueous extract of F. religiosa (FRaq) bark in human cervical cancer cell lines, SiHa and HeLa. FRaq altered the growth kinetics of SiHa (HPV-16 positive) and HeLa (HPV-18 positive) cells in a dose-dependent manner. It blocked the cell cycle progression at G1/S phase in SiHa that was characterized by an increase in the expression of p53, p21 and pRb proteins with a simultaneous decrease in the expression of phospho Rb (ppRb) protein. On the other hand, in HeLa, FRaq induced apoptosis through an increase in intracellular Ca(2+) leading to loss of mitochondrial membrane potential, release of cytochrome-c and increase in the expression of caspase-3. Moreover, FRaq reduced the migration as well as invasion capability of both the cervical cancer cell lines accompanied with downregulation of MMP-2 and Her-2 expression. Interestingly, FRaq reduced the expression of viral oncoproteins E6 and E7 in both the cervical cancer cell lines. All these data suggest that F. religiosa could be explored for its chemopreventive potential in cervical cancer.

Balance of apoptotic and anti-apoptotic marker and perforin granule release in squamous intraepithelial lesions. HIV infection leads to a decrease in perforin degranulation

Cell-mediated cytotoxicity plays an important role in the regulation to HPV-associated cervical intraepithelial neoplasia. HIV co-infection is related to poorer prognosis and more rapid clinical progression to cancer. We evaluated the presence of cervical inflammatory cells, apoptotic (Bax, Bcl-2, FasL, NOS2, perforin) markers and the degranulating expressing cell marker (CD107a) in low and high squamous intraepithelial lesions (LSIL and HSIL, respectively) from HIV-negative and -positive women. Higher percentage of cervical CD4(+), CD8(+) T cells and macrophage were observed in LSIL and HSIL groups when compared with control, especially in epithelium and basal layer of epithelium. However, progression from LSIL to HSIL did not change the frequency of inflammatory cells. HIV-infection lead to a reduction on cervical CD4(+) T cell infiltration and an increased CD8(+) T cell distribution in LSIL groups. A balance between pro- and anti-apoptotic protein expressions was verified. Bax-expressing cells were present in all groups and were rarely expressed in keratinocytes in the epithelium in LSIL and control groups, but notably decreased in HSIL group. However, its frequency was enhanced in the basal layer of the epithelium meanly in LSIL group. Bcl2-expressing cells in the epithelium and the stroma were enhanced in HSIL group when compared with LSIL group. HIV-infection did not interfere in both expressions NOS2 expression was located on keratinocytes in both LSIL and HSIL groups when compared with control group. There were few FasL cervical expressing cells in all groups. Indeed, perforin was identified in few cervical cells. However, CD107a, a surface marker for cellular degranulation was significantly higher in epithelium, basal layer of epithelium and stroma in LSIL and HSIL, respectively, when compared with control group. These results support that HIV infection may induce reduction on inflammatory cervical cell degranulation corroborating to carcinogenesis process. This is the first description on the role of HIV in downregulation of perforin degranulation in the cervical lesions and it might be related to carcinogenesis.

Nip the HPV encoded evil in the cancer bud: HPV reshapes TRAILs and signaling landscapes

HPV encoded proteins can elicit ectopic protein–protein interactions that re-wire signaling pathways, in a mode that promotes malignancy. Moreover, accumulating data related to HPV is now providing compelling substantiation of a central role played by HPV in escaping immunosurveillance and impairment of apoptotic response. What emerges is an intricate network of Wnt, TGF, Notch signaling cascades that forms higher-order ligand–receptor complexes routing downstream signaling in HPV infected cells. These HPV infected cells are regulated both extracellularly by ligand receptor axis and intracellularly by HPV encoded proteins and impair TRAIL mediated apoptosis. We divide this review into different sections addressing how linear signaling pathways integrate to facilitate carcinogenesis and compounds that directly or indirectly reverse these aberrant interactions offer new possibilities for therapy in cancer. Although HPV encoded proteins mediated misrepresentation of pathways is difficult to target, improved drug-discovery platforms and new technologies have facilitated the discovery of agents that can target dysregulated pathways in HPV infected cervical cancer cells, thus setting the stage for preclinical models and clinical trials.

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.

Papillomavirus E6 oncoproteins

Papillomaviruses induce benign and malignant epithelial tumors, and the viral E6 oncoprotein is essential for full transformation. E6 contributes to transformation by associating with cellular proteins, docking on specific acidic LXXLL peptide motifs found on these proteins. This review examines insights from recent studies of human and animal E6 proteins that determine the three-dimensional structure of E6 when bound to acidic LXXLL peptides. The structure of E6 is related to recent advances in the purification and identification of E6 associated protein complexes. These E6 protein-complexes, together with other proteins that bind to E6, alter a broad array of biological outcomes including modulation of cell survival, cellular transcription, host cell differentiation, growth factor dependence, DNA damage responses, and cell cycle progression.

Folding of a cyclin box: linking multitarget binding to marginal stability, oligomerization, and aggregation of the retinoblastoma tumor suppressor AB pocket domain

The retinoblastoma tumor suppressor (Rb) controls the proliferation, differentiation, and survival of cells in most eukaryotes with a role in the fate of stem cells. Its inactivation by mutation or oncogenic viruses is required for cellular transformation and eventually carcinogenesis. The high conservation of the Rb cyclin fold prompted us to investigate the link between conformational stability and ligand binding properties of the RbAB pocket domain. RbAB unfolding presents a three-state transition involving cooperative secondary and tertiary structure changes and a partially folded intermediate that can oligomerize. The first transition corresponds to unfolding of the metastable B subdomain containing the binding site for the LXCXE motif present in cellular and viral targets, and the second transition corresponds to the stable A subdomain. The low thermodynamic stability of RbAB translates into a propensity to rapidly oligomerize and aggregate at 37 °C (T50 = 28 min) that is suppressed by human papillomavirus E7 and E2F peptide ligands, suggesting that Rb is likely stabilized in vivo through binding to target proteins. We propose that marginal stability and associated oligomerization may be conserved for function as a "hub" protein, allowing the formation of multiprotein complexes, which could constitute a robust mechanism to retain its cell cycle regulatory role throughout evolution. Decreased stability and oligomerization are shared with the p53 tumor suppressor, suggesting a link between folding and function in these two essential cell regulators that are inactivated in most cancers and operate within multitarget signaling pathways.

Proteomic approaches to the study of papillomavirus-host interactions

The identification of interactions between viral and host cellular proteins has provided major insights into papillomavirus research, and these interactions are especially relevant to the role of papillomaviruses in the cancers with which they are associated. Recent advances in mass spectrometry technology and data processing now allow the systematic identification of such interactions. This has led to an improved understanding of the different pathologies associated with the many papillomavirus types, and the diverse nature of these viruses is reflected in the spectrum of interactions with host proteins. Here we review a history of proteomic approaches, particularly as applied to the papillomaviruses, and summarize current techniques. Current proteomic studies on the papillomaviruses use yeast-two-hybrid or affinity purification-mass spectrometry approaches. We detail the advantages and disadvantages of each and describe current examples of papillomavirus proteomic studies, with a particular focus on the HPV E6 and E7 oncoproteins.

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.

Biology of human papillomavirus-related oropharyngeal cancer

Recent data show that human papillomavirus-positive oropharyngeal cancer (OPC) has a distinct biological and clinical behavior compared with human papillomavirus-negative OPC. As this subset of head and neck cancer represents an increasing public health concern, a thorough understanding of the causative and mechanistic differences between these diseases and how these distinctions impact clinical treatment is required. In this review, we will summarize recent data in epidemiology, the mechanism of viral carcinogenesis and differences in tumor biology that may provide insights to improve the clinical management of patients with human papillomavirus-positive OPC.

Conserved region 3 of human papillomavirus 16 E7 contributes to deregulation of the retinoblastoma tumor suppressor

The human papillomavirus (HPV) E7 oncoprotein binds cellular factors, preventing or retargeting their function and thereby making the infected cell conducive for viral replication. A key target of E7 is the product of the retinoblastoma susceptibility locus (pRb). This interaction results in the release of E2F transcription factors and drives the host cell into the S phase of the cell cycle. E7 binds pRb via a high-affinity binding site in conserved region 2 (CR2) and also targets a portion of cellular pRb for degradation via the proteasome. Evidence suggests that a secondary binding site exists in CR3, and that this interaction influences pRb deregulation. Additionally, evidence suggests that CR3 also participates in the degradation of pRb. We have systematically analyzed the molecular mechanisms by which CR3 contributes to deregulation of the pRb pathway by utilizing a comprehensive series of mutations in residues predicted to be exposed on the surface of HPV16 E7 CR3. Despite differences in the ability to interact with cullin 2, all CR3 mutants degrade pRb comparably to wild-type E7. We identified two specific patches of residues on the surface of CR3 that contribute to pRb binding independently of the high-affinity CR2 binding site. Mutants within CR3 that affect pRb binding are less effective than the wild-type E7 in overcoming pRb-induced cell cycle arrest. This demonstrates that the interaction between HPV16 E7 CR3 and pRb is functionally important for alteration of the cell cycle.

Human papillomaviruses and the specificity of PDZ domain targeting

The human papillomavirus (HPV) E6 oncoprotein is fundamental to the ability of these viruses to induce human malignancy. A defining characteristic of the HPV E6 oncoproteins found in cancer-causing HPV types is the presence of a PDZ binding motif at their extreme C-terminus. Through this motif, E6 is able to interact with a large number of cellular proteins that contain PDZ domains. Many of these cellular proteins are involved in regulation of processes associated with the control of cell attachment, cell proliferation, cell polarity and cell signaling. How E6 targets multiple proteins containing the same recognition domain is still an open question. In this review, we highlight aspects of E6 function and biology that help to answer this question, and thereby provide insight into the role of these substrates during development of HPV-induced malignancy.

Antiviral activity of Bifidobacterium adolescentis SPM1005-A on human papillomavirus type 16

BACKGROUND

Probiotic lactic acid bacteria (LAB) support a functional and balanced immune system, and contribute to immune modulatory effects in combatting microbial pathogens, including viruses. Most cervical cancers are associated with anogenital region infection with high-risk (HR) human papillomavirus (HPV). In this study, we analyzed the antiviral activity of Bifidobacterium adolescentis SPM1005-A in the SiHa cervical cancer cell line expressing HPV type 16.

METHODS

We assessed the cellular toxicity of B. adolescentis SPM1005-A in SiHa cells by the Trypan blue dye exclusion assay. Cells (3.6 × 105) in culture plates with or without B. adolescentis SPM1005-A in the same type of medium, were incubated with HPV type 16 at a concentration of 5.1 × 107 cfu/ml. For antiviral analysis, we performed quantitative real-time PCR (qRT-PCR) for E6 and E7 oncogene expressions and observed protein levels by immunoblotting.

RESULTS

The qRT-PCR results showed that E6 and E7 mRNA levels decreased simultaneously. Western blot analysis revealed that the E6 protein expression slightly decreased after 24 and 48 h, but the level of E7 protein expression appear unaffected compared with that in the control. Decreased HPV16 E6 and E7 mRNA transcript and protein levels were not associated with cell morphology or significant cytotoxic effects.

CONCLUSIONS

This study showed that B. adolescentis SPM1005-A had antiviral activity through suppression E6 and E7 oncogene expression. The results suggest that B. adolescentis SPM1005-A could be potential applications of HPV-associated cervical cancer prevention.

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.

Functional interaction between human papillomavirus type 16 E6 and E7 oncoproteins and cigarette smoke components in lung epithelial cells

The smoking habit is the most important, but not a sufficient cause for lung cancer development. Several studies have reported the human papillomavirus type 16 (HPV16) presence and E6 and E7 transcripts expression in lung carcinoma cases from different geographical regions. The possible interaction between HPV infection and smoke carcinogens, however, remains unclear. In this study we address a potential cooperation between tobacco smoke and HPV16 E6 and E7 oncoproteins for alterations in proliferative and tumorigenic properties of lung epithelial cells. A549 (alveolar, tumoral) and BEAS-2B (bronchial, non-tumoral) cell lines were stably transfected with recombinant pLXSN vectors expressing HPV16 E6 and E7 oncoproteins and exposed to cigarette smoke condensate (CSC) at different concentrations. HPV16 E6 and E7 expression was associated with loss of p53 stability, telomerase (hTERT) and p16(INK4A) overexpression in BEAS-2B cells as demonstrated by quantitative real-time polymerase chain reaction (qRT-PCR) and western blotting (WB). In A549 cells we observed downregulation of p53 but not a significant increase of hTERT transcripts. In addition, the HPV16 E6/E7 transfected cell lines showed an increased proliferation rate and anchorage-independent growth in a HPV16 E6 and E7 expression-dependent manner. Moreover, both HPV16 E6/E7 and mock transfected cells showed an increased proliferation rate and anchorage-independent growth in the presence of 0.1 and 10 µg/mL CSC. However, this increase was significantly greater in HPV16 E6/E7 transfected cells (p<0.001). Data were confirmed by FCSE proliferation assay. The results obtained in this study are suggestive of a functional interaction between tobacco smoke and HPV16 E6/E7 oncoproteins for malignant transformation and tumorigenesis of lung epithelial cells. More studies are warranted in order to dissect the molecular mechanisms involved in this cooperation.

Recent advances in biomarkers and potential targeted therapies in head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is a devastating tumor of the upper aerodigestive tract with no significant change in treatment modality or improvement in survival over the last several decades. Biomarkers are important biological molecules that can be utilized in tumor detection, prognosis, and as targeted therapies. There are several important biomarkers and potential targets in the forefront, including biomarkers of tumorigenesis, signal transduction molecules, proteins involved in angiogenesis, and oncogenic viruses. The clinical applications of these biomarkers are in various states from in vitro and in vivo models, phase II and III clinical trials, to accepted modes of treatment in patients with HNSCC. Given the potential improvement in prognosis that biomarkers and their targeted therapies may have on the treatment of HNSCC, their investigation is both important and essential.

The Role of Elongin BC-Containing Ubiquitin Ligases

The Elongin complex was originally identified as a positive regulator of RNA polymerase II and is composed of a transcriptionally active subunit (A) and two regulatory subunits (B and C). The Elongin BC complex enhances the transcriptional activity of Elongin A. “Classical” SOCS box-containing proteins interact with the Elongin BC complex and have ubiquitin ligase activity. They also interact with the scaffold protein Cullin (Cul) and the RING domain protein Rbx and thereby are members of the Cullin RING ligase (CRL) superfamily. The Elongin BC complex acts as an adaptor connecting Cul and SOCS box proteins. Recently, it was demonstrated that classical SOCS box proteins can be further divided into two groups, Cul2- and Cul5-type proteins. The classical SOCS box-containing protein pVHL is now classified as a Cul2-type protein. The Elongin BC complex containing CRL family is now considered two distinct protein assemblies, which play an important role in regulating a variety of cellular processes such as tumorigenesis, signal transduction, cell motility, and differentiation.

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.

Systematic identification of interactions between host cell proteins and E7 oncoproteins from diverse human papillomaviruses

More than 120 human papillomaviruses (HPVs) have now been identified and have been associated with a variety of clinical lesions. To understand the molecular differences among these viruses that result in lesions with distinct pathologies, we have begun a MS-based proteomic analysis of HPV-host cellular protein interactions and have created the plasmid and cell line libraries required for these studies. To validate our system, we have characterized the host cellular proteins that bind to the E7 proteins expressed from 17 different HPV types. These studies reveal a number of interactions, some of which are conserved across HPV types and others that are unique to a single HPV species or HPV genus. Binding of E7 to UBR4/p600 is conserved across all virus types, whereas the cellular protein ENC1 binds specifically to the E7s from HPV18 and HPV45, both members of genus alpha, species 7. We identify a specific interaction of HPV16 E7 with ZER1, a substrate specificity factor for a cullin 2 (CUL2)-RING ubiquitin ligase, and show that ZER1 is required for the binding of HPV16 E7 to CUL2. We further show that ZER1 is required for the destabilization of the retinoblastoma tumor suppressor RB1 in HPV16 E7-expressing cells and propose that a CUL2-ZER1 complex functions to target RB1 for degradation in HPV16 E7-expressing cells. These studies refine the current understanding of HPV E7 functions and establish a platform for the rapid identification of virus-host interactions.