Interactions with pocket proteins contribute to the role of human papillomavirus type 16 E7 in the papillomavirus life cycle

Retinoblastoma Protein Is Required for Epstein-Barr Virus Replication in Differentiated Epithelia

Coinfection of human papillomavirus (HPV) and Epstein-Barr virus (EBV) has been detected in oropharyngeal squamous cell carcinoma. Although HPV and EBV replicate in differentiated epithelial cells, we previously reported that HPV epithelial immortalization reduces EBV replication within organotypic raft culture and that the HPV16 oncoprotein E7 was sufficient to inhibit EBV replication. A well-established function of HPV E7 is the degradation of the retinoblastoma (Rb) family of pocket proteins (pRb, p107, and p130). Here, we show that pRb knockdown in differentiated epithelia and EBV-positive Burkitt lymphoma (BL) reduces EBV lytic replication following de novo infection and reactivation, respectively. In differentiated epithelia, EBV immediate early (IE) transactivators were expressed, but loss of pRb blocked expression of the early gene product, EA-D. Although no alterations were observed in markers of epithelial differentiation, DNA damage, and p16, increased markers of S-phase progression and altered p107 and p130 levels were observed in suprabasal keratinocytes after pRb knockdown. In contrast, pRb interference in Akata BX1 Burkitt lymphoma cells showed a distinct phenotype from differentiated epithelia with no significant effect on EBV IE or EA-D expression. Instead, pRb knockdown reduced the levels of the plasmablast differentiation marker PRDM1/Blimp1 and increased the abundance of c-Myc protein in reactivated Akata BL with pRb knockdown. c-Myc RNA levels also increased following the loss of pRb in epithelial rafts. These results suggest that pRb is required to suppress c-Myc for efficient EBV replication in BL cells and identifies a mechanism for how HPV immortalization, through degradation of the retinoblastoma pocket proteins, interferes with EBV replication in coinfected epithelia. IMPORTANCE Terminally differentiated epithelium is known to support EBV genome amplification and virion morphogenesis following infection. The contribution of the cell cycle in differentiated tissues to efficient EBV replication is not understood. Using organotypic epithelial raft cultures and genetic interference, we can identify factors required for EBV replication in quiescent cells. Here, we phenocopied HPV16 E7 inhibition of EBV replication through knockdown of pRb. Loss of pRb was found to reduce EBV early gene expression and viral replication. Interruption of the viral life cycle was accompanied by increased S-phase gene expression in postmitotic keratinocytes, a process also observed in E7-positive epithelia, and deregulation of other pocket proteins. Together, these findings provide evidence of a global requirement for pRb in EBV lytic replication and provide a mechanistic framework for how HPV E7 may facilitate a latent EBV infection through its mediated degradation of pRb in copositive epithelia.

YAP1 activation by human papillomavirus E7 promotes basal cell identity in squamous epithelia

Persistent human papillomavirus (HPV) infection of stratified squamous epithelial cells causes nearly 5% of cancer cases worldwide. HPV-positive oropharyngeal cancers harbor few mutations in the Hippo signaling pathway compared to HPV-negative cancers at the same anatomical site, prompting the hypothesis that an HPV-encoded protein inactivates the Hippo pathway and activates the Hippo effector yes-associated protein (YAP1). The HPV E7 oncoprotein is required for HPV infection and for HPV-mediated oncogenic transformation. We investigated the effects of HPV oncoproteins on YAP1 and found that E7 activates YAP1, promoting YAP1 nuclear localization in basal epithelial cells. YAP1 activation by HPV E7 required that E7 binds and degrades the tumor suppressor protein tyrosine phosphatase non-receptor type 14 (PTPN14). E7 required YAP1 transcriptional activity to extend the lifespan of primary keratinocytes, indicating that YAP1 activation contributes to E7 carcinogenic activity. Maintaining infection in basal cells is critical for HPV persistence, and here we demonstrate that YAP1 activation causes HPV E7 expressing cells to be retained in the basal compartment of stratified epithelia. We propose that YAP1 activation resulting from PTPN14 inactivation is an essential, targetable activity of the HPV E7 oncoprotein relevant to HPV infection and carcinogenesis.

The ‘epithelial’ cells that cover our bodies are in a constant state of turnover. Every few weeks, the outermost layers die and are replaced by new cells from the layers below. For scientists, this raises a difficult question. Cells infected by human papillomaviruses, often known as HPV, can become cancerous over years or even decades. How do infected cells survive while the healthy cells around them mature and get replaced?

One clue could lie in PTPN14, a human protein which many papillomaviruses eliminate using their viral E7 protein; this mechanism could be essential for the virus to replicate and cause cancer. To find out the impact of losing PTPN14, Hatterschide et al. used human epithelial cells to make three-dimensional models of infected tissues. These experiments showed that, when papillomaviruses destroy PTPN14, a human protein called YAP1 turns on in the lowest, most long-lived layer of the tissue. Cells in which YAP1 is activated survive while those that carry the inactive version mature and die. This suggests that papillomaviruses turn on YAP1 to remain in tissues for long periods.

Papillomaviruses cause about five percent of all human cancers. Finding ways to stop them from activating YAP1 has the potential to prevent disease. Overall, the research by Hatterschide et al. also sheds light on other epithelial cancers which are not caused by viruses.

Association between promoter methylation and gene expression of CGB3 and NOP56 in HPV-infected cervical cancer cells

Overexpression of the E7 gene of human papillomavirus (HPV) type 16 is one of the primary causes of cervical cancer. The E7 protein can bind with DNA methyltransferase I and induce methylation of tumor suppressor genes, such as cyclin-A1 (CCNA1), leading to suppression of their expression, and thus, cancer progression. In the present study, the confirmation of methylation-related expression of chorionic gonadotropin subunit 3 (CGB3) and nucleolar protein 56 (NOP56) genes in 5-Azacytidine (5'-aza)-treated HPV16-positive SiHa and HPV16-negative C33A cell lines was shown. Using methylation-specific-PCR and quantitative PCR, the results showed that CGB3 and NOP56 methylation significantly decreased as the 5'-aza concentration was increased, and this was inversely associated with their expression. Moreover, overexpression of E7 contributed to the augmentation of CGB3 and NOP56 methylation levels in C33A cells, resulting in a decrease in their expression. This study extends on previous observations of E7 HPV16 oncogenic function in terms of methylation-repressing expression in more genes, which may be wholly applied to gene therapy in cervical cancer prevention.

Proteasomal degradation of p130 facilitate cell cycle deregulation and impairment of cellular differentiation in high-risk Human Papillomavirus 16 and 18 E7 transfected cells

The High-Risk Human Papillomaviruses (HR-HPVs) 16 and 18 are known to cause cervical cancer, which is primarily attributed to E6 and E7 oncoproteins. In addition, recent studies have focused on the vital role of the p130 pocket protein as an oncosuppressor to limit the expression of E2F transcription factors required for cell cycle progression. In view of this, the current study was conducted to investigate the mechanism by which transfection with HPV16/18 E7 leads to the deregulation of the host cell cycle, altering the localisation of p130, and expression of differentiation genes in Human Keratinocytes (HaCaT) cells. Co-immunoprecipitation, Western blot analysis, immunofluorescence microscopy, flow cytometry, quantitative-Polymerase Chain Reaction (qPCR), and the inhibition of p130 by MG132 inhibitor were employed to investigate the loss of p130 and its disruption in HPV 16/18 E7-transfected HaCaT cells. The HPV16- and HPV18-transformed cells, known as CaSki and HeLa, respectively, were also used to complement the ectopic expressions of E7 in HaCaT cells. Normal keratinocytes displayed higher level of p130 expression than HPV-transformed cells. In addition, the immunofluorescence analysis revealed that both HPV 16/18 E7-transfected HaCaT and HPV-transformed cells exhibited higher level of cytoplasmic p130 compared to nuclear p130. A significant increase in the number of S/G2 phase cells in HPV-transformed cells was also recorded since E7 has been shown to stimulate proliferation through the deactivation of Retinoblastoma Protein (pRB)-dependent G1/S checkpoint. Furthermore, the findings recorded the down-regulation of keratinocyte differentiation markers, namely p130, keratin10, and involucrin. The proteasomal degradation of the exported p130 confirmed the cellular localisation pattern of p130, which was commonly observed in cancerous cells. The findings provide strong evidence that the localisation of nuclear p130 nuclear was disrupted by HPV16/18 E7 led to the deregulation of the cell cycle and the impairment of cellular differentiation ultimately lead to cellular transformation.

Metabolic Control by DNA Tumor Virus-Encoded Proteins

Viruses co-opt a multitude of host cell metabolic processes in order to meet the energy and substrate requirements for successful viral replication. However, due to their limited coding capacity, viruses must enact most, if not all, of these metabolic changes by influencing the function of available host cell regulatory proteins. Typically, certain viral proteins, some of which can function as viral oncoproteins, interact with these cellular regulatory proteins directly in order to effect changes in downstream metabolic pathways. This review highlights recent research into how four different DNA tumor viruses, namely human adenovirus, human papillomavirus, Epstein-Barr virus and Kaposi's associated-sarcoma herpesvirus, can influence host cell metabolism through their interactions with either MYC, p53 or the pRb/E2F complex. Interestingly, some of these host cell regulators can be activated or inhibited by the same virus, depending on which viral oncoprotein is interacting with the regulatory protein. This review highlights how MYC, p53 and pRb/E2F regulate host cell metabolism, followed by an outline of how each of these DNA tumor viruses control their activities. Understanding how DNA tumor viruses regulate metabolism through viral oncoproteins could assist in the discovery or repurposing of metabolic inhibitors for antiviral therapy or treatment of virus-dependent cancers.

Manipulation of Epithelial Differentiation by HPV Oncoproteins

Papillomaviruses replicate and cause disease in stratified squamous epithelia. Epithelial differentiation is essential for the progression of papillomavirus replication, but differentiation is also impaired by papillomavirus-encoded proteins. The papillomavirus E6 and E7 oncoproteins partially inhibit and/or delay epithelial differentiation and some of the mechanisms by which they do so are beginning to be defined. This review will outline the key features of the relationship between HPV infection and differentiation and will summarize the data indicating that papillomaviruses alter epithelial differentiation. It will describe what is known so far and will highlight open questions about the differentiation-inhibitory mechanisms employed by the papillomaviruses.

PTPN14 degradation by high-risk human papillomavirus E7 limits keratinocyte differentiation and contributes to HPV-mediated oncogenesis

High-risk human papillomavirus (HPV) E7 proteins enable oncogenic transformation of HPV-infected cells by inactivating host cellular proteins. High-risk but not low-risk HPV E7 target PTPN14 for proteolytic degradation, suggesting that PTPN14 degradation may be related to their oncogenic activity. HPV infects human keratinocytes but the role of PTPN14 in keratinocytes and the consequences of PTPN14 degradation are unknown. Using an HPV16 E7 variant that can inactivate retinoblastoma tumor suppressor (RB1) but cannot degrade PTPN14, we found that high-risk HPV E7-mediated PTPN14 degradation impairs keratinocyte differentiation. Deletion of PTPN14 from primary human keratinocytes decreased keratinocyte differentiation gene expression. Related to oncogenic transformation, both HPV16 E7-mediated PTPN14 degradation and PTPN14 deletion promoted keratinocyte survival following detachment from a substrate. PTPN14 degradation contributed to high-risk HPV E6/E7-mediated immortalization of primary keratinocytes and HPV⁺ but not HPV^- cancers exhibit a gene-expression signature consistent with PTPN14 inactivation. We find that PTPN14 degradation impairs keratinocyte differentiation and propose that this contributes to high-risk HPV E7-mediated oncogenic activity independent of RB1 inactivation.

A Novel In Vivo Infection Model To Study Papillomavirus-Mediated Disease of the Female Reproductive Tract

Papillomaviruses exhibit species-specific tropism, thereby limiting understanding and research of several aspects of HPV infection and carcinogenesis. The discovery of a murine papillomavirus (MmuPV1) provides the opportunity to study papillomavirus infections in a tractable, in vivo laboratory model. MmuPV1 infects and causes disease in the cutaneous epithelium, as well as the mucosal epithelia of the oral cavity and anogenital tract. In this report, we describe a murine model of MmuPV1 infection and neoplastic disease in the female reproductive tracts of wild-type immunocompetent FVB mice. Low-grade dysplastic lesions developed in reproductive tracts of FVB mice infected with MmuPV1 for 4 months, and mice infected for 6 months developed significantly worse disease, including squamous cell carcinoma (SCC). We also tested the contribution of estrogen and/or UV radiation (UVR), two cofactors we previously identified as being involved in papillomavirus-mediated disease, to cervicovaginal disease. Similar to HPV16 transgenic mice, exogenous estrogen treatment induced high-grade precancerous lesions in the reproductive tracts of MmuPV1-infected mice by 4 months and together with MmuPV1 efficiently induced SCC by 6 months. UV radiation and exogenous estrogen cooperated to promote carcinogenesis in MmuPV1-infected mice. This murine infection model represents the first instance of de novo papillomavirus-mediated carcinogenesis in the female reproductive tract of wild-type mice resulting from active virus infection and is also the first report of the female hormone estrogen contributing to this process. This model will provide an additional platform for fundamental studies on papillomavirus infection, cervicovaginal disease, and the role of cellular cofactors during papillomavirus-induced carcinogenesis.IMPORTANCE Tractable and efficient models of papillomavirus-induced pathogenesis are limited due to the strict species-specific and tissue-specific tropism of these viruses. Here, we report a novel preclinical murine model of papillomavirus-induced cervicovaginal disease in wild-type, immunocompetent mice using the recently discovered murine papillomavirus, MmuPV1. In this model, MmuPV1 establishes persistent viral infections in the mucosal epithelia of the female reproductive tract, a necessary component needed to accurately mimic HPV-mediated neoplastic disease in humans. Persistent MmuPV1 infections were able to induce progressive neoplastic disease and carcinogenesis, either alone or in combination with previously identified cofactors of papillomavirus-induced disease. This new model will provide a much-needed platform for basic and translational studies on both papillomavirus infection and associated disease in immunocompetent mice.

Inhibition of Epstein-Barr Virus Replication in Human Papillomavirus-Immortalized Keratinocytes

Epstein-Barr virus (EBV) is implicated in the pathogenesis of human papillomavirus (HPV)-associated oropharyngeal squamous cell carcinoma (OSCC). EBV-associated cancers harbor a latent EBV infection characterized by a lack of viral replication and the expression of viral oncogenes. Cellular changes promoted by HPV are comparable to those shown to facilitate EBV latency, though whether HPV-positive cells support a latent EBV infection has not been demonstrated. Using a model of direct EBV infection into HPV16-immortalized tonsillar cells grown in organotypic raft culture, we showed robust EBV replication in HPV-negative rafts but little to no replication in HPV-immortalized rafts. The reduced EBV replication was independent of immortalization, as human telomerase-immortalized normal oral keratinocytes supported robust EBV replication. Furthermore, we observed reduced EBV lytic gene expression and increased expression of EBER1, a noncoding RNA highly expressed in latently infected cells, in the presence of HPV. The use of human foreskin keratinocyte rafts expressing the HPV16 E6 and/or E7 oncogene(s) (HPV E6 and E7 rafts) showed that E7 was sufficient to reduce EBV replication. EBV replication is dependent upon epithelial differentiation and the differentiation-dependent expression of the transcription factors KLF4 and PRDM1. While KLF4 and PRDM1 levels were unaltered, the expression levels of KLF4 transcriptional targets, including late differentiation markers, were reduced in HPV E6 and E7 rafts compared to their levels in parental rafts. However, the HPV E7-mediated block in EBV replication correlated with delayed expression of early differentiation markers. Overall, this study reveals an HPV16-mediated block in EBV replication, through E7, that may facilitate EBV latency and long-term persistence in the tumor context.IMPORTANCE Using a model examining the establishment of EBV infection in HPV-immortalized tissues, we showed an HPV-induced interruption of the normal EBV life cycle reminiscent of a latent EBV infection. Our data support the notion that a persistent EBV epithelial infection depends upon preexisting cellular alterations and suggest the ability of HPV to promote such changes. More importantly, these findings introduce a model for how EBV coinfection may influence HPV-positive (HPV-pos) OSCC pathogenesis. Latently EBV-infected epithelial cells, as well as other EBV-associated head-and-neck carcinomas, exhibit oncogenic phenotypes commonly seen in HPV-pos OSCC. Therefore, an HPV-induced shift in the EBV life cycle toward latency would not only facilitate EBV persistence but also provide additional viral oncogene expression, which can contribute to the rapid progression of HPV-pos OSCC. These findings provide a step toward defining a role for EBV as a cofactor in HPV-positive oropharyngeal tumors.

Oxidative stress in female cancers

Breast, cervical and ovarian cancers are highly prevalent in women worldwide. Environmental, hormonal and viral-related factors are especially relevant in the development of these tumors. These factors are strongly related to oxidative stress (OS) through the generation of reactive oxygen species (ROS). The OS is caused by an imbalance in the redox status of the organism and is literally defined as "an imbalance between ROS generation and its detoxification by biological system leading to impairment of damage repair by cell/tissue". The multistep progression of cancer suggests that OS is involved in cancer initiation, promotion and progression. In this review, we described the role of OS and the interplay with environmental, host and viral factors related to breast, cervical and ovarian cancers initiation, promotion and progression. In addition, the role of the natural antioxidant compound curcumin and other compounds for breast, cervical and ovarian cancers prevention/treatment is discussed.

Generation of affibody molecules specific for HPV16 E7 recognition

Cervical cancer caused by infection with high-risk human papillomavirus remains to be the most deadly gynecologic malignancy worldwide. It is well documented that persistent expression of two oncogenes (E6/E7) plays the key roles in cervical cancer. Thus, in vivo detection of the oncoproteins is very important for the diagnosis of the cancer. Recently, affibody molecules have been demonstrated to be a powerful targeting probe for tumor-targeted imaging and diagnosis. In this study, four HPV16 E7-binding affibody molecules (Z HPV16 E7127, Z HPV16E7301, Z HPV16E7384 and Z HPV16E7745) were screened from a phage-displayed peptide library and used for molecular imaging in tumor-bearing mice. Biosensor binding analyses showed first that the four affibody molecules bound to HPV16 E7 with very high affinity and specificity. They co-localized with E7 protein only in two HPV16-positive cancer cells (SiHa and CaSki). Furthermore, affibody ZHPV16E7384 was conjugated with Dylight755 and used for in vivo tumor-imaging. Strongly high-contrast tumor retention of this affibody only occurred in HPV16-derived tumors of mice as early as 30 min post-injection, not in HPV-negative and HPV18-derived tumors. The accumulation of Dylight755-conjugated ZHPV16E7384 in tumor was achieved over a longer time period (24 h). The data here provide strong evidence that E7-specific affibody molecules have great potential used for molecular imaging and diagnosis of HPV-induced cancers.

The human papillomavirus E7 oncoprotein as a regulator of transcription

High-risk human papillomaviruses (HPVs) encode oncoproteins which manipulate gene expression patterns in the host keratinocytes to facilitate viral replication, regulate viral transcription, and promote immune evasion and persistence. In some cases, oncoprotein-induced changes in host cell behavior can cause progression to cancer, but a complete picture of the functions of the viral oncoproteins in the productive HPV life cycle remains elusive. E7 is the HPV-encoded factor most responsible for maintaining cell cycle competence in differentiating keratinocytes. Through interactions with dozens of host factors, E7 has an enormous impact on host gene expression patterns. In this review, we will examine the role of E7 specifically as a regulator of transcription. We will discuss mechanisms of regulation of cell cycle-related genes by E7 as well as genes involved in immune regulation, growth factor signaling, DNA damage responses, microRNAs, and others pathways. We will also discuss some unanswered questions about how transcriptional regulation by E7 impacts the biology of HPV in both benign and malignant conditions.

Using Organotypic Epithelial Tissue Culture to Study the Human Papillomavirus Life Cycle

Human papillomaviruses (HPVs) are small double-stranded DNA viruses that are associated with greater than 95% of cervical cancers and 20% of head and neck cancers. These cancers arise from persistent infections in which there is continued expression of the HPV E6 and E7 oncogenes, often as a consequence of integration of HPV DNA into the host genome. Such cancers represent "dead ends" for the virus as integration disrupts the viral genome and because the cancers are defective in normal epithelial differentiation, which is required for production of progeny papillomavirus. In order to study the full viral life cycle, from the establishment to maintenance to productive stages, our lab makes use of the organotypic epithelial tissue culture system. This system allows us to mimic the three-dimensional structure of epithelia whose differentiation is tightly linked to the completion of the HPV viral life cycle. In this chapter we describe how various aspects of the HPV life cycle are monitored in raft cultures making use of an immortalized keratinocyte cell line. © 2016 by John Wiley & Sons, Inc.

The association of mammalian DREAM complex and HPV16 E7 proteins

The mammalian DREAM (Drosophila, RB, E2F, and Myb) complex was discovered in 2004 by several research groups. It was initially identified in Drosophila followed by Caenorhaditis elegans and later in mammalian cells. The composition of DREAM is temporally regulated during cell cycle; being associated with E2F-4 and either p107 or p130 in G0/G1 (repressive DREAM complexes) and with B-myb transcription factor in S/G2 (activator DREAM complex). High risk human papillomavirus (HPV) E6 and E7 oncoproteins expression are important for malignant transformation of cervical cancer cells. In particular, the E7 of high risk HPV binds to pRB family members (pRB, p107 and p130) for degradation. It has recently been discovered that the p107 and p130 'pocket proteins' are members of mammalian DREAM complexes. With this understanding, we would like to hypothesise the mammalian DREAM complex could plays a critical role for malignant transformation in cervical cancer cells.

Deficiencies in Cellular Processes Modulated by the Retinoblastoma Protein Do Not Account for Reduced Human Cytomegalovirus Replication in Its Absence

Despite encoding multiple viral proteins that modulate the retinoblastoma (Rb) protein in a manner classically defined as inactivation, human cytomegalovirus (HCMV) requires the presence of the Rb protein to replicate efficiently. In uninfected cells, Rb controls numerous pathways that the virus also commandeers during infection. These include cell cycle progression, senescence, mitochondrial biogenesis, apoptosis, and glutaminolysis. We investigated whether a potential inability of HCMV to regulate these Rb-controlled pathways in the absence of the Rb protein was the reason for reduced viral productive replication in Rb knockdown cells. We found that HCMV was equally able to modulate these pathways in the parental Rb-expressing and Rb-depleted cells. Our results suggest that Rb may be required to enhance a specific viral process during HCMV productive replication.

IMPORTANCE

The retinoblastoma (Rb) tumor suppressor is well established as a repressor of E2F-dependent transcription. Rb hyperphosphorylation, degradation, and binding by viral oncoproteins are also codified. Recent reports indicate Rb can be monophosphorylated, repress the transcription of antiviral genes in association with adenovirus E1A, modulate cellular responses to polycomb-mediated epigenetic methylations in human papillomavirus type 16 E7 expressing cells, and increase the efficiency of human cytomegalovirus (HCMV) productive replication. Since Rb function also now extends to regulation of mitochondrial function (apoptosis, metabolism), it is clear that our current understanding of this protein is insufficient to explain its roles in virus-infected cells and tumors. Work here reinforces this concept, showing the known roles of Rb are insufficient to explain its positive impact on HCMV replication. Therefore, HCMV, along with other viral systems, provide valuable tools to probe functions of Rb that might be modulated with therapeutics for cancers with viral or nonviral etiologies.

Bridging Links between Long Noncoding RNA HOTAIR and HPV Oncoprotein E7 in Cervical Cancer Pathogenesis

Human Papillomavirus (HPV) type 16 oncoprotein E7 plays a major role in cervical carcinogenesis by interacting with and functionally inactivating various host regulatory molecules. Long noncoding RNA (lncRNA) HOTAIR is one such regulator that recruits chromatin remodelling complex PRC2, creating gene silencing H3K27 me3 marks. Hence, we hypothesized that HOTAIR could be a potential target of E7, in HPV16 related cervical cancers (CaCx). We identified significant linear trend of progressive HOTAIR down-regulation through HPV negative controls, HPV16 positive non-malignants and CaCx samples. Majority of CaCx cases portrayed HOTAIR down-regulation in comparison to HPV negative controls, with corresponding up-regulation of HOTAIR target, HOXD10, and enrichment of cancer related pathways. However, a small subset had significantly higher HOTAIR expression, concomitant with high E7 expression and enrichment of metastatic pathways. Expression of HOTAIR and PRC2-complex members (EZH2 and SUZ12), showed significant positive correlation with E7 expression in CaCx cases and E7 transfected C33A cell line, suggestive of interplay between E7 and HOTAIR. Functional inactivation of HOTAIR by direct interaction with E7 could also be predicted by in silico analysis and confirmed by RNA-Immunoprecipitation. Our study depicts one of the causal mechanisms of cervical carcinogenesis by HPV16 E7, through modulation of HOTAIR expression and function.

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.

High incidence of female reproductive tract cancers in FA-deficient HPV16-transgenic mice correlates with E7's induction of DNA damage response, an activity mediated by E7's inactivation of pocket proteins

Fanconi anemia (FA) is a rare genetic disorder caused by defects in a DNA damage repair system, the FA pathway. FA patients frequently develop squamous cell carcinoma (SCC) at sites that are associated with human papillomavirus (HPV)-driven cancer including the female reproductive tract. To assess experimentally whether FA deficiency increases susceptibility to HPV-associated cervical/vaginal cancer, we monitored cancer incidence in the female lower reproductive tract of FA-deficient mice expressing HPV16 oncogenes, E6 and/or E7. FA deficiency specifically increased the incidence of cancers in mice expressing E7; but this effect was not observed in mice just expressing E6. We also observed that E7, but not E6, induced DNA damage as scored by induction of γ-H2AX and 53BP1 (p53 binding protein 1) nuclear foci, and this induction was heightened in FA-deficient tissue. Finally, we discovered that this induction of DNA damage responses was recapitulated in mice deficient in expression of 'pocket' proteins, pRb, p107 and p130, which are established targets of E7. Our findings support the hypothesis that E7 induces cancer by causing DNA damage at least in part through the inactivation of pocket proteins. This hypothesis explains why a deficiency in DNA damage repair would increase susceptibility to E7-driven cancer.

Papillomavirus E6 PDZ interactions can be replaced by repression of p53 to promote episomal human papillomavirus genome maintenance

Cancer-associated human papillomaviruses (HPVs) express E6 oncoproteins that target the degradation of p53 and have a carboxy-terminal PDZ ligand that is required for stable episomal maintenance of the HPV genome. We find that the E6 PDZ ligand can be deleted and the HPV genome stably maintained if cellular p53 is inactivated. This indicates that the E6-PDZ interaction promotes HPV genome maintenance at least in part by neutralization of an activity that can arise from residual undegraded p53.

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.

Inactivating all three rb family pocket proteins is insufficient to initiate cervical cancer

Human papillomavirus-16 (HPV-16) is associated etiologically with many human cervical cancers. It encodes 3 oncogenes E5, E6, and E7. Of these oncogenes, E7 has been found to be the dominant driver of cervical cancer in mice. More than 100 cellular proteins have been reported to associate with HPV-16 E7, which is thought to dysregulate the cell cycle in part by binding and inducing the degradation of pRb and its related pocket protein family members, p107 and p130. The ability of E7 to inactivate the pRb family correlates with its ability to induce head and neck cancers in mice. We previously showed that the inactivation of pRb is itself not sufficient to recapitulate the oncogenic properties of E7 in cervical carcinogenesis. In this study, we evaluated mice that were deficient in multiple pocket proteins, including mice that lacked pRb, p107, and p130. Strikingly, combined loss of two or all 3 pocket proteins resulted in development of high-grade cervical intraepithelial neoplasia, but not frank cervical carcinoma. These findings strongly argue that the oncogenic properties of HPV-16 E7 in human cervical carcinogenesis may involve disruption of E7 binding proteins beyond simply the pRb family members.

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.

p63 is necessary for the activation of human papillomavirus late viral functions upon epithelial differentiation

The late phase of the human papillomavirus (HPV) life cycle is linked to epithelial differentiation, and we investigated the factors that regulate this process. One potential regulator is p63, a member of the p53 family of proteins, which modulates epithelial development, as well as proliferation capability, in stem cells. In this study, we examined the role of p63 in the HPV life cycle using a lentiviral knockdown system for p63. In epithelial cells, the ΔN truncated isoforms of p63 predominate, while the full-length TA isoforms are present at very low levels. Upon the differentiation of normal keratinocytes, p63 levels rapidly decreased while higher levels were retained in HPV-positive cells. Our studies indicate that reducing p63 levels in differentiated HPV-positive cells resulted in the loss of viral genome amplification and late gene expression. p63 regulates the expression of cell cycle regulators, and we determined that cyclin A, cyclin B1, cdk1, and cdc25c were reduced in p63-deficient, HPV-positive keratinocytes, which suggests a possible mechanism of action. In addition, activation of the DNA repair pathway is necessary for genome amplification, and the expression of two members, BRCA2 and RAD51, was altered in the absence of p63 in HPV-positive cells. Our studies indicate that p63 is necessary for the activation of differentiation-dependent HPV late viral functions and provide insights into relevant cellular targets.

The E7 open reading frame acts in cis and in trans to mediate differentiation-dependent activities in the human papillomavirus type 16 life cycle

Human papillomaviruses (HPVs) are the causative agents of several important genital and other mucosal cancers. The HPV16 E7 gene encodes a viral oncogene that is necessary for the continued growth of cancer cells, but its role in the normal, differentiation-dependent life cycle of the virus is not fully understood. The function of E7 in the viral life cycle was examined using a series of mutations of E7 created in the context of the complete HPV16 genome. The effect of these E7 mutations on key events of the viral life cycle, including immortalization, episomal maintenance, late promoter activation, and infectious virion synthesis, was examined. Our studies show that the pRb binding domain is indispensable for early viral activities, whereas the C-terminal zinc finger domain contributed primarily to very late events. Mutations of the casein kinase II phosphorylation site caused a complex phenotype involving both the function of E7 protein and a cis element necessary for the activation of the late promoter, identifying for the first time a promoter element important for late promoter function in the context of the viral genome. All mutant genomes tested showed reduced viral titers following growth in organotypic raft cultures. These studies clarify the role of E7 as a regulator of late events in the differentiation-dependent HPV life cycle.

Low- and high-risk human papillomavirus E7 proteins regulate p130 differently

The E7 protein of high-risk human papillomaviruses (HR HPVs) targets pRb family members (pRb, p107 and p130) for degradation; low-risk (LR) HPV E7 only targets p130 for degradation. The effect of HR HPV 16 E7 and LR HPV 6 E7 on p130 intracellular localization and half-life was examined. Nuclear/cytoplasmic fractionation and immunofluorescence showed that, in contrast to control and HPV 6 E7-expressing cells, a greater amount of p130 was present in the cytoplasm in the presence of HPV 16 E7. The half-life of p130, relative to control cells, was decreased in the cytoplasm in the presence of HPV 6 E7 or HPV 16 E7, but only decreased by HPV 6 E7 in the nucleus. Inhibition of proteasomal degradation extended the half-life of p130, regardless of intracellular localization. These results suggest that there may be divergent mechanisms by which LR and HR HPV E7 target p130 for degradation.

The human papillomavirus E7 oncoprotein

The human papillomavirus (HPV) E7 oncoprotein shares functional similarities with such proteins as adenovirus E1A and SV40 large tumor antigen. As one of only two viral proteins always expressed in HPV-associated cancers, E7 plays a central role in both the viral life cycle and carcinogenic transformation. In the HPV viral life cycle, E7 disrupts the intimate association between cellular differentiation and proliferation in normal epithelium, allowing for viral replication in cells that would no longer be in the dividing population. This function is directly reflected in the transforming activities of E7, including tumor initiation and induction of genomic instability.

Different Isoforms of HPV-16 E7 Protein are Present in Cytoplasm and Nucleus

The E7 protein of high risk HPV types has been found with different molecular weights, mainly because of phosphorylation, an event that changes protein charge and mobility in SDS-PAGE. Distribution of E7 protein in the cellular compartments has also been subject of debate as some groups report the protein in nucleus and others in cytoplasm. The different subcellular distribution and molecular weights reported for the E7 protein suggest the presence of isoforms. We examined this possibility by using several antibodies that recognize different epitopes on the HPV-16 E7 protein. We showed that E7 is processed in 3 isoforms with different molecular weights and isoelectric points (IEP), and described as E7a1 (17.5 kDa, IEP 4.68), E7a (17 kDa, IEP 6.18) and E7b (16 kDa, IEP 6.96). The immunofluorescense results also showed that E7 is distributed into different compartments (ER, Golgi and nucleus), which suggest the presence of other posttranslational modifications, besides phosphorylation.

Casein kinase II motif-dependent phosphorylation of human papillomavirus E7 protein promotes p130 degradation and S-phase induction in differentiated human keratinocytes

The E7 proteins of human papillomaviruses (HPVs) promote S-phase reentry in differentiated keratinocytes of the squamous epithelia to support viral DNA amplification. In this study, we showed that nuclear p130 was present in the differentiated strata of several native squamous epithelia susceptible to HPV infection. In contrast, p130 was below the level of detection in HPV-infected patient specimens. In submerged and organotypic cultures of primary human keratinocytes, the E7 proteins of the high-risk mucosotrophic HPV-18, the benign cutaneous HPV-1, and, to a lesser extent, the low-risk mucosotropic HPV-11 destabilized p130. This E7 activity depends on an intact pocket protein binding domain and a casein kinase II (CKII) phosphorylation motif. Coimmunoprecipitation experiments showed that both E7 domains were important for binding to p130 in extracts of organotypic cultures. Metabolic labeling in vivo demonstrated that E7 proteins were indeed phosphorylated in a CKII motif-dependent manner. Moreover, the efficiencies of the E7 proteins of various HPV types or mutations to induce S-phase reentry in spinous cells correlated with their relative abilities to bind and to destabilize p130. Collectively, these data support the notion that p130 controls the homeostasis of the differentiated keratinocytes and is therefore targeted by E7 for degradation to establish conditions permissive for viral DNA amplification.

Effects of cellular differentiation, chromosomal integration and 5-aza-2'-deoxycytidine treatment on human papillomavirus-16 DNA methylation in cultured cell lines

Human papillomavirus-16 (HPV-16) genomes in cell culture and in situ are affected by polymorphic methylation patterns, which can repress the viral transcription. In order to understand some of the underlying mechanisms, we investigated changes of the methylation of HPV-16 DNA in cell cultures in response to cellular differentiation, to recombination with cellular DNA, and to an inhibitor of methylation. Undifferentiated W12E cells, derived from a precancerous lesion, contained extrachromosomal HPV-16 DNA with a sporadically methylated enhancer-promoter segment. Upon W12E cell differentiation, the viral DNA was demethylated, suggesting a link between differentiation and the epigenetic state of HPV-16 DNA. The viral genomes present in two W12I clones, in which individual copies of the HPV-16 genome have integrated into cellular DNA (type 1 integrants), were unmethylated, akin to that seen in the cervical carcinoma cell line SiHa (also a type 1 integrant). This finding is consistent with hypomethylation being necessary for continued viral gene expression. In contrast, two of three type 2 integrant W12I clones, containing concatemers of HPV-16 genomes integrated into the cellular DNA contained hypermethylated viral DNA, as observed in the cervical carcinoma cell line CaSki (also a type 2 integrant). A third, type 2, W12I clone, interestingly with fewer copies of the viral genome, contained unmethylated HPV-16 genomes. Epithelial differentiation of W12I clones did not lead to demethylation of chromosomally integrated viral genomes as was seen for extrachromosomal HPV-16 DNA in W12E clones. Hypomethylation of CaSki cells in the presence of the DNA methylation inhibitor 5-aza-2'-deoxycytidine reduced the cellular viability, possibly as a consequence of toxic effects of an excess of HPV-16 gene products. Our data support a model wherein (i) the DNA methylation state of extrachromosomal HPV16 replicons and epithelial differentiation are inversely coupled during the viral life cycle, (ii) integration of the viral genome into the host chromosome events leads to an alteration in methylation patterns on the viral genome that is dependent upon the type of integration event and possibly copy number, and (iii) integration universally results in the viral DNA becoming refractory to changes in methylation state upon cellular differentiation that are observed with extrachromosomal HPV-16 genomes.

Role of Rb-dependent and Rb-independent functions of papillomavirus E7 oncogene in head and neck cancer

Infection with high-risk human papillomaviruses (HPV) and in particular the expression of the viral proteins E6 and E7 have been associated with the etiology of a subset of head and neck squamous cell cancer (HNSCC). However, the individual consequences of E6 and E7 expression in an in vivo model have not been examined in these tissues. We have used transgenes that direct expression of the HPV16 E6 and E7 proteins to the head and neck tissues of mice to dissect the contribution of these proteins to head and neck carcinogenesis. We report here that E7 is the major transforming oncogene in HPV-associated HNSCC, whereas E6 is more likely to play a secondary role in contributing to later stages of carcinogenesis. Furthermore, a conditional deletion of Rb, a prominent target for E7, in the same tissues did not recapitulate all E7-mediated phenotypes. Although our results do not preclude an important role for the E7-pRb interaction, they highlight the importance of pRb-independent functions of E7 in head and neck carcinogenesis.

HUMAN PAPILLOMAVIRUS ASSOCIATION WITH HEAD AND NECK CANCERS: UNDERSTANDING VIRUS BIOLOGY AND USING IT IN THE DEVELOPMENT OF CANCER DIAGNOSTICS

The link between human papillomaviruses and human cervical cancers has long been established. However, human papillomaviruses (HPVs) are now being detected in another type of cancer, not previously associated with this virus, head and neck squamous cell carcinoma (HNSCC). This review will focus on experimental data supporting the view that HPVs contribute to the etiology of a subset of HNSCC. We further put forth the argument that HPV-associated HNSCC deserves to be recognized as a distinct disease in the clinic and as such needs to be appropriately diagnosed. We offer an overview of studies that have helped dissect the role of HPVs in HNSCC and that may be helpful in the development of new diagnostic tools for discriminating this type of HNSCC.

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

Human papillomavirus type 16 (HPV16) and other high-risk HPVs are etiologically linked to the development of cervical carcinomas and contribute to a number of other tumors of the anogenital tract, as well as oral cancers. The high-risk HPV E6 and E7 oncoproteins are consistently expressed in cervical cancer cells and are necessary for the induction and maintenance of the transformed phenotype. An important aspect of HPV16 E7's oncogenic activities is destabilization of the retinoblastoma tumor suppressor (pRB) through a ubiquitin/proteasome-dependent mechanism, although the exact molecular mechanism is unknown. Here, we report that HPV16 E7 is associated with an enzymatically active cullin 2 ubiquitin ligase complex and that the HPV16 E7/pRB complex contains cullin 2. Depletion of cullin 2 by RNA interference causes increased steady-state levels and stability of pRB in HPV16 E7-expressing cells, and ectopic expression of HPV16 E7 and the cullin 2 complex leads to pRB ubiquitination in vivo. Hence, we propose that the HPV16 E7-associated cullin 2 ubiquitin ligase complex contributes to aberrant degradation of the pRB tumor suppressor in HPV16 E7-expressing cells.

Recent Advances in the Search for Antiviral Agents against Human Papillomaviruses

Infection by human papillomavirus (HPV) is extremely common and associated with the development of benign warts or malignant lesions of the skin and mucosa. Infection by a high-risk (oncogenic) anogenital HPV type, most often through sexual contacts, is the starting point of virtually all cases of cervical cancers and the majority of anal cancers. The same viral types are also increasingly being linked with a subset of head-and-neck and non-melanoma skin cancers. Although prophylactic vaccines are now available to protect against the four types most commonly found in cervical and anal cancers (HPV16 and HPV18) and anogenital warts (HPV6 and HPV11), these neither protect against all genital HPVs nor are of therapeutic utility for already infected patients. Thus, the need for antiviral agents to treat HPV-associated diseases remains great, but none currently exist. This article reviews the recent progress made towards the development of antiviral agents to treat HPV infections, from target identification and validation to the discovery of lead compounds with therapeutic potential. Emphasis has been placed on novel low-molecular-weight compounds that antagonize HPV proteins or, alternatively, inhibit cellular proteins which have been usurped by papillomaviruses and are mediating their pathogenic effects.

Recent advances in the search for antiviral agents against human papillomaviruses

Infection by human papillomavirus (HPV) is extremely common and associated with the development of benign warts or malignant lesions of the skin and mucosa. Infection by a high-risk (oncogenic) anogenital HPV type, most often through sexual contacts, is the starting point of virtually all cases of cervical cancers and the majority of anal cancers. The same viral types are also increasingly being linked with a subset of head-and-neck and non-melanoma skin cancers. Although prophylactic vaccines are now available to protect against the four types most commonly found in cervical and anal cancers (HPV16 and HPV18) and anogenital warts (HPV6 and HPV11), these neither protect against all genital HPVs nor are of therapeutic utility for already infected patients. Thus, the need for antiviral agents to treat HPV-associated diseases remains great, but none currently exist. This article reviews the recent progress made towards the development of antiviral agents to treat HPV infections, from target identification and validation to the discovery of lead compounds with therapeutic potential. Emphasis has been placed on novel low-molecular-weight compounds that antagonize HPV proteins or, alternatively, inhibit cellular proteins which have been usurped by papillomaviruses and are mediating their pathogenic effects.

Identification of the FHL2 transcriptional coactivator as a new functional target of the E7 oncoprotein of human papillomavirus type 16

We identified the transcriptional coactivator FHL2 as a novel target of the human papillomavirus type 16 (HPV-16) E7 oncoprotein, which plays a major role in cell transformation. The interaction with FHL2 is abolished by mutations in conserved regions 1 and 2 and in the C-terminal zinc finger domain of E7, all required for its transforming potential. We found that E7 impairs the coactivator function of FHL2 on both beta-catenin/LCF-dependent and AP-1-dependent promoters. Thus, the interaction with HPV-16 E7 leads to a promoter-specific impairment of FHL2 function and this may contribute to cell transformation.

Cell Cycle Regulators p105, p107, Rb2/p130, E2F4, p21CIP1/WAF1, Cyclin A in Predicting Cervical Intraepithelial Neoplasia, High-Risk Human Papillomavirus Infections and Their Outcome in Women Screened in Three New Independent States of the Former Soviet Union

BACKGROUND

The growth-controlling functions of the high-risk human papillomaviruses (HPV) depend on their ability to interact with several cellular proteins, including the key regulatory proteins of the cell cycle. We have examined the value of cell cycle regulatory proteins as predictors of the intermediate end point markers in cervical carcinogenesis: (a) grade of cervical intraepithelial neoplasia (CIN), (b) high-risk HPV type, (c) clearance/persistence of high-risk HPV, and (d) disease outcome in women participating in a multicenter follow-up study in three New Independent States countries.

METHODS

Totally, 232 biopsy samples tested high-risk HPV-positive and/or Papanicolaou smear-positive women were immunohistochemically stained for the following cell cycle markers: p105, p107, p130, E2F4, p21(CIP1/WAF1/SDI1), cyclin A, and Ki-67. In addition, apoptotic index (AI) and mitotic index (MI) were determined in H&E-stained sections. Prospective follow-up data were available to disclose the clinical and virological outcome of the lesions.

RESULTS

The expression of Ki-67, p21(CIP1/WAF1/SDI1), and cyclin A and AI and MI values were markedly increased in high-grade lesions, but only MI was an independent predictor of CIN3 in multivariate analysis. Cyclin A was the only independent predictor of high-risk HPV (odds ratio, 1.09; 95% confidence interval, 1.01-1.18; P = 0.021), exceeding the predictive power of CIN grade and high-grade squamous intraepithelial lesion Papanicolaou smears. None of these markers provided any useful predictive information as to the clinical and virological outcomes during the follow-up. Highly significant correlations (P = 0.0001) were found between AI and MI as well as between MI and cyclin A, Ki-67 and p21(CIP1/WAF1/SDI1), Ki-67 and cyclin A, and p21(CIP1/WAF1/SDI1) and cyclin A followed by that between p105 and cyclin A (P = 0.001) and p105 and p130 (P = 0.002).

CONCLUSIONS

All tested factors related to cell cycle were increased, but only MI and cyclin A was an independent predictor of CIN3 and high-risk HPV carriage, respectively.