Abrogation of a mitotic checkpoint by E2 proteins from oncogenic human papillomaviruses correlates with increased turnover of the p53 tumor suppressor protein

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

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

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

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

The Replicative Consequences of Papillomavirus E2 Protein Binding to the Origin Replication Factor ORC2

The origin recognition complex (ORC) coordinates a series of events that lead to initiation of DNA strand duplication. As a nuclear double stranded DNA plasmid, the papillomavirus (PV) genome resembles a mini-chromosome in infected cells. To initiate its replication, the viral E2 protein binds to and recruits the E1 DNA helicase at the viral origin. PV genome replication program exhibits three stages: initial amplification from a single genome upon infection to a few copies per cell, a cell cycle linked maintenance phase, and a differentiation dependent late stage where the genome is amplified to thousands of copies. Involvement of ORC or other pre-replication complex (pre-RC) factors has not been described. We report that human PV (HPV) and bovine PV (BPV-1) E2 proteins bind to ORC2, however, ORC2 was not detected at the viral origin. Depletion of ORC2 enhanced PV replication in a transient replication model and in keratinocytes stably maintaining viral episomes, while there was no effect on copy number in a cell line with integrated HPV genomes. Consistent with this, occupancy of E1 and E2 at the viral origin increased following ORC2 silencing. These data imply that ORC2 is not necessary for activation of the PV origin by E1 and E2 but instead suppresses E2 replicative function. Furthermore, we observed that over-expression of HPV E2 decreased ORC2 occupation at two known mammalian origins of replication, suggesting that E2 restricts pre-ORC assembly that could otherwise compete for host replication complexes necessary for viral genome amplification. We infer that the ORC2 complex with E2 restricts viral replication in the maintenance phase of the viral replication program and that elevated levels of E2 that occur during the differentiation dependent amplification stage subvert ORC loading and hence DNA synthesis at cellular origins.

Papillomavirus genome replication occurs during three distinct stages that are linked to the differentiation state of the infected epithelium. The viral proteins E1 and E2 recognize the viral origin and initiate a process that attracts host DNA replication factors. The origin recognition complex (ORC) coordinates initiation of chromosome duplication. While ORC2 binds to the E2 protein, its depletion does not impair PV genome replication. Instead, depletion of ORC2 stimulates viral replication, while over-expression of E2 protein decreases ORC2 occupancy at mammalian origins. We propose that the relative abundance of E2 and ORC2 in complex regulates viral and cellular origin licensing.

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

Objective

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

Methods and Materials

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

Results

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

Conclusions

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

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

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

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

Human papillomavirus oncoproteins and apoptosis (Review)

The aim of this study was to review the literature and identify the association between human papillomavirus (HPV) oncoproteins and apoptosis. HPV-associated apoptosis may be primarily blocked by a number of oncoproteins, including E5, E6 and E7. E5 protein protects cells from tumor necrosis factor-associated apoptosis; the oncoprotein E6 predominantly inhibits apoptosis through the p53 pathway; and oncoprotein E7 is involved in apoptosis activation and inhibition. In addition, HPV oncoproteins are involved in activating or repressing the transcription of E6/E7. In conclusion, HPV oncoproteins, including E5, E6 and E7 protein, may interfere with apoptosis via certain regulatory principles.

The modulation of apoptosis by oncogenic viruses

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

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

Human papillomavirus early proteins and apoptosis

INTRODUCTION

The human papillomavirus (HPV) associated apoptosis can be primarily attributed to some early proteins, such as E2, E5, E6, E7, and so on. Though these proteins have a low molecular size, they are capable to interact with a series of host cellular regulation proteins to induce or inhibit apoptosis. The oncoproteins E6 can inhibit the apoptosis mainly through p53 pathway. The E5 protein can protect cells from tumor necrosis factor-related apoptosis. The protein E2 protein have regulatory functions in viral transcription and induction of apoptosis. The oncoprotein E7 plays the role in both apoptosis activation and inhibition. In addition, the HPV full-length E2 proteins involve in activating or repressing the transcription of E6/E7, so as to regulating the apoptosis caused by E6 and E7.

MATERIALS AND METHODS

We search major databases (such as Elsevier) with the following selection criteria: HPV, early protein, apoptosis.

CONCLUSIONS

In this review, we summary the literature related with E2, E5, E6, and E7 proteins, and describe the regulatory principles and specific mechanism by which HPV early proteins can interfere with apoptosis and trigger gynaecopathias for women.

Identification and analysis of papillomavirus E2 protein binding sites in the human genome

Papillomavirus E2 protein is required for the replication and maintenance of viral genomes and transcriptional regulation of viral genes. E2 functions through sequence-specific binding to 12-bp DNA motifs-E2 binding sites (E2BS)-in the virus genome. Papillomaviruses are able to establish persistent infection in their host and have developed a long-term relationship with the host cell in order to guarantee the propagation of the virus. In this study, we have analyzed the occurrence and functionality of E2BSs in the human genome. Our computational analysis indicates that most E2BSs in the human genome are found in repetitive DNA regions and have G/C-rich spacer sequences. Using a chromatin immunoprecipitation approach, we show that human papillomavirus type 11 (HPV11) E2 interacts with a subset of cellular E2BSs located in active chromatin regions. Two E2 activities, sequence-specific DNA binding and interaction with cellular Brd4 protein, are important for E2 binding to consensus sites. E2 binding to cellular E2BSs has a moderate or no effect on cellular transcription. We suggest that the preference of HPV E2 proteins for E2BSs with A/T-rich spacers, which are present in the viral genomes and underrepresented in the human genome, ensures E2 binding to specific binding sites in the virus genome and may help to prevent extensive and possibly detrimental changes in cellular transcription in response to the viral protein.

Characterization of the methylation patterns in human papillomavirus type 16 viral DNA in head and neck cancers

Human papillomavirus (HPV) type 16 can integrate into the host genome, thereby rendering the viral coding genes susceptible to epigenetic modification. Using bisulfite genomic sequencing, we determined the methylation status of all 110 CpG sites within the viral epigenome in advanced stage III/IV HPV-16-associated head and neck cancers. We found that the viral genome was hypomethylated in the majority of head and neck cancers, in particular within the viral regulatory region, long control region (LCR), which controls transcription of the E6 and E7 oncogenes. The hypomethylation status of LCR correlated with detectable levels of E6 and E7 expression, which suggests that the tumors may still be dependent on these viral oncogenes to maintain the malignant phenotype. In addition to the methylation status of LCR, we report other potential factors which may influence intratumoral E6 and E7 expression including viral copy number and integration site. We were able to detect the viral epigenetic alterations in sampled body fluids, such as serum and saliva, which correlated with the changes observed in the primary tumors. Because viral epigenetic changes occur in the setting of viral integration into the human genome, the detection of methylated HPV genes in the serum and/or saliva may have diagnostic potential for early detection strategies of viral integration and assessment of risk for cancer development in high-risk individuals. Our findings also support continued targeting of the E6 and/or E7 antigens through various vaccine strategies against HPV-associated cancers.

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

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

The dynamic DNA methylomes of double-stranded DNA viruses associated with human cancer

The natural history of cancers associated with virus exposure is intriguing, since only a minority of human tissues infected with these viruses inevitably progress to cancer. However, the molecular reasons why the infection is controlled or instead progresses to subsequent stages of tumorigenesis are largely unknown. In this article, we provide the first complete DNA methylomes of double-stranded DNA viruses associated with human cancer that might provide important clues to help us understand the described process. Using bisulfite genomic sequencing of multiple clones, we have obtained the DNA methylation status of every CpG dinucleotide in the genome of the Human Papilloma Viruses 16 and 18 and Human Hepatitis B Virus, and in all the transcription start sites of the Epstein-Barr Virus. These viruses are associated with infectious diseases (such as hepatitis B and infectious mononucleosis) and the development of human tumors (cervical, hepatic, and nasopharyngeal cancers, and lymphoma), and are responsible for 1 million deaths worldwide every year. The DNA methylomes presented provide evidence of the dynamic nature of the epigenome in contrast to the genome. We observed that the DNA methylome of these viruses evolves from an unmethylated to a highly methylated genome in association with the progression of the disease, from asymptomatic healthy carriers, through chronically infected tissues and pre-malignant lesions, to the full-blown invasive tumor. The observed DNA methylation changes have a major functional impact on the biological behavior of the viruses.

Human papillomaviruses: a growing field

A combination of functional studies on human papillomavirus (HPV) oncoproteins and epidemiological studies on persistence of HPV infection firmly established a role for HPV in the etiology of cervical cancers. Understanding the viral life cycle of HPVs has been more difficult. In this issue of Genes & Development, Wang et al. (pp. 181 - 194) describe an efficient method to propagate infectious HPV in differentiating epithelium, providing clear evidence for temporal separation of viral and cellular replication.

Replication and partitioning of papillomavirus genomes

Papillomaviruses establish persistent infection in the dividing, basal epithelial cells of the host. The viral genome is maintained as a circular, double-stranded DNA, extrachromosomal element within these cells. Viral genome amplification occurs only when the epithelial cells differentiate and viral particles are shed in squames that are sloughed from the surface of the epithelium. There are three modes of replication in the papillomavirus life cycle. Upon entry, in the establishment phase, the viral genome is amplified to a low copy number. In the second maintenance phase, the genome replicates in dividing cells at a constant copy number, in synchrony with the cellular DNA. And finally, in the vegetative or productive phase, the viral DNA is amplified to a high copy number in differentiated cells and is destined to be packaged in viral capsids. This review discusses the cis elements and protein factors required for each stage of papillomavirus replication.

Evolutionary and biophysical relationships among the papillomavirus E2 proteins

Infection by human papillomavirus (HPV) may result in clinical conditions ranging from benign warts to invasive cancer. The HPV E2 protein represses oncoprotein transcription and is required for viral replication. HPV E2 binds to palindromic DNA sequences of highly conserved four base pair sequences flanking an identical length variable 'spacer'. E2 proteins directly contact the conserved but not the spacer DNA. Variation in naturally occurring spacer sequences results in differential protein affinity that is dependent on their sensitivity to the spacer DNA's unique conformational and/or dynamic properties. This article explores the biophysical character of this core viral protein with the goal of identifying characteristics that associated with risk of virally caused malignancy. The amino acid sequence, 3d structure and electrostatic features of the E2 protein DNA binding domain are highly conserved; specific interactions with DNA binding sites have also been conserved. In contrast, the E2 protein's transactivation domain does not have extensive surfaces of highly conserved residues. Rather, regions of high conservation are localized to small surface patches. Implications to cancer biology are discussed.

P53 represses human papillomavirus type 16 DNA replication via the viral E2 protein

BACKGROUND

Human papillomavirus (HPV) DNA replication can be inhibited by the cellular tumour suppressor protein p53. However, the mechanism through which p53 inhibits viral replication and the role that this might play in the HPV life cycle are not known. The papillomavirus E2 protein is required for efficient HPV DNA replication and also regulates viral gene expression. E2 represses transcription of the HPV E6 and E7 oncogenes and can thereby modulate indirectly host cell proliferation and survival. In addition, the E2 protein from HPV 16 has been shown to bind p53 and to be capable of inducing apoptosis independently of E6 and E7.

RESULTS

Here we use a panel of E2 mutants to confirm that mutations which block the induction of apoptosis via this E6/E7-independent pathway, have little or no effect on the induction of apoptosis by the E6/E7-dependent pathway. Although these mutations in E2 do not affect the ability of the protein to mediate HPV DNA replication, they do abrogate the repressive effects of p53 on the transcriptional activity of E2 and prevent the inhibition of E2-dependent HPV DNA replication by p53.

CONCLUSION

These data suggest that p53 down-regulates HPV 16 DNA replication via the E2 protein.

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

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

Molecular biology of cervical cancer

Cervical cancer is a virus-induced disease that is caused by the integration of high-risk infecting human papillomaviruses (HPV) in the host genome. For this reason, the carcinogenesis process of cervical cancer is associated to the expression of the viral oncogenic proteins E6 and E7. These proteins are capable of inactivating p53 and pRb, which induces a continuous cell proliferation with the increasing risk of accumulation of DNA damage that eventually leads to cancer. Moreover, cervical cancer can be prevented by prophylactic HPV vaccines; their molecular characteristics and mechanism of action are reviewed. Ultimately, new molecular targets for cervical cancer like proteasome, the EGFR family and IGF family are exposed.

p53 and hTERT determine sensitivity to viral apoptosis

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

A cancer cell-specific inducer of apoptosis

Human papillomavirus (HPV) DNA is found in virtually all cervical cancers, strongly suggesting that these viruses are necessary to initiate this disease. The HPV E2 protein is required for viral replication, but E2 expression is usually lost in HPV-transformed cells because of the integration of viral DNA into the host chromosome. Several studies have shown that the reintroduction of E2 into HPV-transformed cells can induce growth arrest and apoptotic cell death. This raises the possibility that E2 could be useful in the treatment of HPV-induced disease. However, the effects of E2 on cell proliferation are not limited to HPV-transformed cells. The E2 protein from HPV type 16 can induce apoptosis via at least two pathways. One pathway involves the binding of E2 to p53 and operates in HPV-transformed cells, many non-HPV-transformed cell lines, and untransformed normal cells. The second pathway requires the binding of E2 to the viral genome and operates only in HPV-transformed cells. A mutation in E2 that significantly reduces the binding of this protein to p53 abrogates the induction of apoptosis in non-HPV-transformed cells and normal cells, but has no effect on the ability of the mutated protein to induce apoptosis in HPV-transformed cells. Here we show that a chimeric protein consisting of this mutant of E2, fused to the herpes simplex virus type 1 VP22 protein, can traffic between cells in a three-dimensional tumor model and induce apoptosis in HPV-transformed cells with high specificity. This cancer cell-specific inducer of apoptosis may be useful in the treatment of cervical cancer and other HPV-induced diseases.

G2/M cell cycle arrest in the life cycle of viruses

There is increasing evidence that viral infection, expression of viral protein or the presence of viral DNA causes the host cell cycle to arrest during G2/M. The mechanisms used by viruses to cause arrest vary widely; some involve the activation of the cellular pathways that induce arrest in response to DNA damage, while others use completely novel means. The analysis of virus-mediated arrest has not been proven easy, and in most cases the consequences of arrest for the virus life cycle are not well defined. However, a number of effects of arrest are being investigated and it will be interesting to see to what extent perturbation of the G2/M transition is involved in viral infections.

Development of a topical protein therapeutic for human papillomavirus and associated cancers

Human papillomaviruses (HPVs) are the causative agents of several disease states, including genital warts and cervical cancer. There are around 500 million cases of genital warts per annum worldwide and around 450,000 cases of cervical cancer. Although HPV vaccines should eventually reduce the incidence of these diseases, new and effective treatments are still urgently required. The E2 (early) proteins from some HPV types induce growth arrest and apoptosis, and these proteins could be used as therapeutics for HPV-induced disease. A major obstacle to this approach concerns the delivery of the protein to HPV-transformed cells and/or HPV-infected cells in vivo. One possible solution is to use recombinant viruses to deliver E2. Another possible solution is to use purified E2 proteins or E2 fusion proteins. The herpes simplex virus VP22 protein is one of a small number of proteins that have been shown to cross the cell membrane with high efficiency. VP22-E2 fusion proteins produced in bacterial cells are able to enter mammalian cells and induce apoptosis. This suggests that VP22-E2 fusion proteins could be topically applied as a treatment for HPV-induced diseases, most probably post-surgery. In this review, we discuss this and other approaches to the topical delivery of selective therapeutic agents against HPV-associated conditions.

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

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

Molecular biology of human papillomavirus infection and cervical cancer

HPVs (human papillomaviruses) infect epithelial cells and cause a variety of lesions ranging from common warts/verrucas to cervical neoplasia and cancer. Over 100 different HPV types have been identified so far, with a subset of these being classified as high risk. High-risk HPV DNA is found in almost all cervical cancers (>99.7%), with HPV16 being the most prevalent type in both low-grade disease and cervical neoplasia. Productive infection by high-risk HPV types is manifest as cervical flat warts or condyloma that shed infectious virions from their surface. Viral genomes are maintained as episomes in the basal layer, with viral gene expression being tightly controlled as the infected cells move towards the epithelial surface. The pattern of viral gene expression in low-grade cervical lesions resembles that seen in productive warts caused by other HPV types. High-grade neoplasia represents an abortive infection in which viral gene expression becomes deregulated, and the normal life cycle of the virus cannot be completed. Most cervical cancers arise within the cervical transformation zone at the squamous/columnar junction, and it has been suggested that this is a site where productive infection may be inefficiently supported. The high-risk E6 and E7 proteins drive cell proliferation through their association with PDZ domain proteins and Rb (retinoblastoma), and contribute to neoplastic progression, whereas E6-mediated p53 degradation prevents the normal repair of chance mutations in the cellular genome. Cancers usually arise in individuals who fail to resolve their infection and who retain oncogene expression for years or decades. In most individuals, immune regression eventually leads to clearance of the virus, or to its maintenance in a latent or asymptomatic state in the basal cells.

E2 proteins from high- and low-risk human papillomavirus types differ in their ability to bind p53 and induce apoptotic cell death

The E2 proteins from oncogenic (high-risk) human papillomaviruses (HPVs) can induce apoptotic cell death in both HPV-transformed and non-HPV-transformed cells. Here we show that the E2 proteins from HPV type 6 (HPV6) and HPV11, two nononcogenic (low-risk) HPV types, fail to induce apoptosis. Unlike the high-risk HPV16 E2 protein, these low-risk E2 proteins fail to bind p53 and fail to induce p53-dependent transcription activation. Interestingly, neither the ability of p53 to activate transcription nor the ability of p53 to bind DNA, are required for HPV16 E2-induced apoptosis in non-HPV-transformed cells. However, mutations that reduce the binding of the HPV16 E2 protein to p53 inhibit E2-induced apoptosis in non-HPV-transformed cells. In contrast, the interaction between HPV16 E2 and p53 is not required for this E2 protein to induce apoptosis in HPV-transformed cells. Thus, our data suggest that this high-risk HPV E2 protein induces apoptosis via two pathways. One pathway involves the binding of E2 to p53 and can operate in both HPV-transformed and non-HPV-transformed cells. The second pathway requires the binding of E2 to the viral genome and can only operate in HPV-transformed cells.

Impact of viral E2-gene status on outcome after radiotherapy for patients with human papillomavirus 16-positive cancer of the uterine cervix

PURPOSE

Integration of high-risk papillomavirus DNA has been considered an important step in oncogenic progression to cervical carcinoma. Disruption of the human papillomavirus (HPV) genome within the E2 gene is frequently a consequence. This study investigated the influence of episomal viral DNA on outcome in patients with advanced cervical cancer treated with primary radiotherapy.

METHODS AND MATERIALS

Paraffin-embedded biopsies of 82 women with locally advanced cervical cancer could be analyzed for HPV infection by multiplex polymerase chain reaction (PCR) by use of SPF1/2 primers. E2-gene intactness of HPV-16-positive samples was analyzed in 3 separate amplification reactions by use of the E2A, E2B, E2C primers. Statistical analyses (Kaplan-Meier method; log-rank test) were performed for overall survival (OS), disease-free survival (DFS), local progression-free survival (LPFS), and distant metastases-free survival (DMFS).

RESULTS

Sixty-one (75%) of 82 carcinomas were HPV positive, 44 of them for HPV-16 (72%). Seventeen of the 44 HPV-16-positive tumors (39%) had an intact E2 gene. Patients with a HPV-16-positive tumor and an intact E2 gene showed a trend for a better DFS (58% vs. 38%, p = 0.06) compared with those with a disrupted E2 gene. A nonsignificant difference occurred regarding OS (87% vs. 66%, p = 0.16) and DMFS (57% vs. 48%, p = 0.15).

CONCLUSION

E2-gene status may be a promising new target, but more studies are required to elucidate the effect of the viral E2 gene on outcome after radiotherapy in HPV-positive tumors.

Adeno-associated virus type 2 increases proteosome-dependent degradation of p21WAF1 in a human papillomavirus type 31b-positive cervical carcinoma line

Adeno-associated virus type 2 (AAV2) seropositivity is negatively correlated with the development of human papillomavirus (HPV)-associated cervical cancer. We have begun analysis of the molecular mechanisms underlying AAV2-mediated onco-suppression through cell cycle regulation in HPV-infected keratinocytes isolated from a low-grade cervical lesion. AAV2 superinfection of HPV type 31b (HPV31b)-positive cells at early times postinfection resulted in degradation of the cyclin-dependent kinase (CDK) inhibitor p21(WAF1) protein in a proteosome-dependent manner. Downstream consequences of lowering p21(WAF1) levels included a proportional loss of cyclin E/CDK2 complexes bound to p21(WAF1). The loss of stable p21(WAF1)/cyclin E/CDK2 complexes coincided with an increase in CDK2-associated kinase activity and cyclin E levels. Both events have the potential to enhance the G(1)/S transition point mediated by active cyclin E/CDK2 complexes. Concurrently, cyclin A and E2F levels were decreased, conditions reminiscent of delayed entrance into the S phase of the cell cycle. On the other hand, infection of primary human foreskin keratinocytes with AAV2 resulted in upregulation of p21(WAF1) protein levels, reminiscent of a block in G(1) phase progression. We propose that by down regulating p21(WAF1), AAV2 initiates cell cycle activities leading to enhanced G(1)/S phase-like conditions which may be favorable for AAV2-specific functions and may lead to downstream interference with HPV-associated cervical cancer progression.

A splicing enhancer in the E4 coding region of human papillomavirus type 16 is required for early mRNA splicing and polyadenylation as well as inhibition of premature late gene expression

Successful inhibition of human papillomavirus type 16 (HPV-16) late gene expression early in the life cycle is essential for persistence of infection, the highest risk factor for cervical cancer. Our study aimed to locate regulatory RNA elements in the early region of HPV-16 that influence late gene expression. For this purpose, subgenomic HPV-16 expression plasmids under control of the strong human cytomegalovirus immediate early promoter were used. An exonic splicing enhancer that firmly supported the use of the E4 3' splice site at position 3358 in the early region of the HPV-16 genome was identified. The enhancer was mapped to a 65-nucleotide AC-rich sequence located approximately 100 nucleotides downstream of the position 3358 3' splice site. Deletion of the enhancer caused loss of both splicing at the upstream position 3358 3' splice site and polyadenylation at the early polyadenylation signal, pAE. Direct splicing occurred at the competing L1 3' splice site at position 5639 in the late region. Optimization of the position 3358 3' splice site restored splicing to that site and polyadenylation at pAE. Additionally, a sequence of 40 nucleotides with a negative effect on late mRNA production was located immediately downstream of the enhancer. As the E4 3' splice site is employed by both early and late mRNAs, the enhancer constitutes a key regulator of temporal HPV-16 gene expression, which is required for early mRNA production as well as for the inhibition of premature late gene expression.

Repression of the human papillomavirus E6 gene initiates p53-dependent, telomerase-independent senescence and apoptosis in HeLa cervical carcinoma cells

Cervical cancer cells express high-risk human papillomavirus (HPV) E6 and E7 proteins. When both HPV oncogenes are repressed in HeLa cervical carcinoma cells, the dormant p53 and retinoblastoma (Rb) tumor suppressor pathways are activated, and the cells undergo senescence in the absence of apoptosis. When the E6 gene is repressed in cells that continue to express an E7 gene, the p53 pathway, but not the Rb pathway, is activated, and both senescence and apoptosis are triggered. To determine the role of p53 signaling in senescence or apoptosis after repression of HPV oncogenes, we introduced a dominant-negative allele of p53 into HeLa cells. Dominant-negative p53 prevented senescence and apoptosis when E6 alone was repressed but did not inhibit senescence when both E6 and E7 were repressed. To determine whether reduced telomerase activity was involved in senescence or apoptosis after E6 repression, we generated HeLa cells stably expressing an exogenous hTERT gene, which encodes the catalytic subunit of telomerase. Although these cells contained markedly elevated telomerase activity and elongated telomeres, hTERT expression did not prevent senescence and apoptosis when E6 alone was repressed. These results demonstrate that when the Rb tumor suppressor pathway is inactivated by the E7 protein, E6 repression activates p53 signaling, which in turn is required for growth inhibition, senescence, and apoptosis. Thus, sustained inactivation of the p53 pathway by the E6 protein is required for maintenance of the proliferative phenotype of HeLa cervical carcinoma cells.

Squamous metaplasia induced by transfection of human papillomavirus DNA into cultured adenocarcinoma cells

BACKGROUND/AIM

It has been reported previously in cases of adenosquamous carcinoma of the lung in Okinawa, a subtropical island 2000 km south of mainland Japan, that the squamous cell carcinoma components were positive for human papillomavirus (HPV) by non-isotopic in situ hybridisation (NISH). The adenocarcinoma cells adjacent to the squamous cell carcinoma components were enlarged and also positive for HPV. This is thought to indicate that after adenocarcinoma cells are infected with HPV, they undergo morphological changes, and that "squamous metaplasia" follows. In this present study, the effects of HPV transfection into adenocarcinoma cells were examined. The relation between the region expressing the HPV gene and squamous metaplasia was also studied.

METHODS

Plasmid pBR322 containing HPV type 16 (HPV-16) was transfected into cultured colonic adenocarcinoma (DLD-1) and lung adenocarcinoma (PC-14) cells using the calcium phosphate method. Neomycin was used as a selection marker. The presence of HPV E1, E2, E4, E5, E6, E7, L1, and L2 mRNAs and also transglutaminase 1, involucrin, cyclin dependent kinases (CDKs), cyclins, caspases, apoptosis inducing factor, DNase gamma, Fas, and Fas ligand mRNAs in HPV transfected cells was investigated by means of reverse transcription polymerase chain reaction (RT-PCR). The G0-G1 cell population was analysed by flow cytometry. Morphological examination under light and electron microscopes was also carried out.

RESULTS

The virus transfected cells showed squamous metaplasia when they were injected into severe combined immunodeficient mice, expressing the high molecular weight keratin (Moll's number 1 keratin) and involucrin molecules immunohistochemically, and involucrin and transglutaminase I mRNAs by RT-PCR. The squamous metaplasia was most conspicuous in the HPV transfected DLD-1 cell when compared with HPV transfected PC-14 cells. Squamous metaplasia was most clearly demonstrated in one HPV transfected DLD-1 cell clone, which expressed not only E2 but also E6-E7 fusion gene mRNA. Viral L1 mRNA expression was absent in HPV transfected cell clones, and was not related to squamous metaplasia. The growth rate of HPV transfected cells was reduced. Transfection of the virus into the cultured adenocarcinoma cells increased the G0-G1 cell population greatly, as assessed by flow cytometer analysis. Furthermore, in the virus transfected cells, apoptosis was also observed by means of the terminal deoxynucleotidyl transferase mediated dUTP biotin nick end labelling method.

CONCLUSION

HPV transfection into adenocarcinoma cells induced clear squamous metaplasia. One of the HPV transfected cell clones that expressed E2 and E6-E7 fusion gene mRNA showed the squamous metaplasia particularly clearly, and apoptosis was also demonstrated.

Endogenous human papillomavirus E6 and E7 proteins differentially regulate proliferation, senescence, and apoptosis in HeLa cervical carcinoma cells

Cervical cancer cells express high-risk human papillomavirus (HPV) E6 and E7 proteins, and repression of HPV gene expression causes the cells to cease proliferation and undergo senescence. However, it is not known whether both HPV proteins are required to maintain the proliferative state of cervical cancer cells, or whether mutations that accumulate during carcinogenesis eliminate the need for one or the other of them. To address these questions, we used the bovine papillomavirus E2 protein to repress the expression of either the E6 protein or the E7 protein encoded by integrated HPV18 DNA in HeLa cervical carcinoma cells. Repression of the E7 protein activated the Rb pathway but not the p53 pathway and triggered senescence, whereas repression of the E6 protein activated the p53 pathway but not the Rb pathway and triggered both senescence and apoptosis. Telomerase activity, cyclin-dependent kinase activity, and expression of c-myc were markedly inhibited by repression of either E6 or E7. These results demonstrate that continuous expression of both the E6 and the E7 protein is required for optimal proliferation of cervical carcinoma cells and that the two viral proteins exert distinct effects on cell survival and proliferation. Therefore, strategies that inhibit the expression or activity of either viral protein are likely to inhibit the growth of HPV-associated cancers.

Transcriptional activity among high and low risk human papillomavirus E2 proteins correlates with E2 DNA binding

The full-length E2 protein, encoded by human papillomaviruses (HPVs), is a sequence-specific transcription factor found in all HPVs, including cancer-causing high risk HPV types 16 and 18 and wart-inducing low risk HPV types 6 and 11. To investigate whether E2 proteins encoded by high risk HPVs may function differentially from E2 proteins encoded by low risk HPVs and animal papillomaviruses, we conducted comparative DNA-binding and transcription studies using electrophoretic mobility shift assays and cell-free transcription systems reconstituted with purified general transcription factors, cofactor, RNA polymerase II, and with E2 proteins encoded by HPV-16, HPV-18, HPV-11, and bovine papillomavirus type 1 (BPV-1). We found that although different types of E2 proteins all exhibited transactivation and repression activities, depending on the sequence context of the E2-binding sites, HPV-16 E2 shows stronger transcription activity and greater DNA-binding affinity than those displayed by the other E2 proteins. Surprisingly, HPV-18 E2 behaves more similarly to BPV-1 E2 than HPV-16 E2 in its functional properties. Our studies thus categorize HPV-18 E2 and BPV-1 E2 in the same protein family, a finding consistent with the available E2 structural data that separate the closely related HPV-16 and HPV-18 E2 proteins but classify together the more divergent BPV-1 and HPV-18 E2 proteins.

Identification of a G(2) arrest domain in the E1 wedge E4 protein of human papillomavirus type 16

Human papillomavirus type 16 (HPV16) is the most common cause of cervical carcinoma. Cervical cancer develops from low-grade lesions that support the productive stages of the virus life cycle. The 16E1 wedge E4 protein is abundantly expressed in such lesions and can be detected in cells supporting vegetative viral genome amplification. Using an inducible mammalian expression system, we have shown that 16E1 wedge E4 arrests HeLa cervical epithelial cells in G(2). 16E1 wedge E4 also caused a G(2) arrest in SiHa, Saos-2 and Saccharomyces pombe cells and, as with HeLa cells, was found in the cytoplasm. However, whereas 16E1 wedge E4 is found on the keratin networks in HeLa and SiHa cells, in Saos-2 and S. pombe cells that lack keratins, 16E1 wedge E4 had a punctate distribution. Mutagenesis studies revealed a proline-rich region between amino acids 17 and 45 of 16E1 wedge E4 to be important for arrest. This region, which we have termed the "arrest domain," contains a putative nuclear localization signal, a cyclin-binding motif, and a single cyclin-dependent kinase (Cdk) phosphorylation site. A single point mutation in the putative Cdk phosphorylation site (T23A) abolished 16E1 wedge E4-mediated G(2) arrest. Arrest did not involve proteins regulating the phosphorylation state of Cdc2 and does not appear to involve the activation of the DNA damage or incomplete replication checkpoint. G(2) arrest was also mediated by the E1 wedge E4 protein of HPV11, a low-risk mucosal HPV type that also causes cervical lesions. The E1 wedge E4 protein of HPV1, which is more distantly related to that of HPV16, did not cause G(2) arrest. We conclude that, like other papillomavirus proteins, 16E1 wedge E4 affects cell cycle progression and that it targets a conserved component of the cell cycle machinery.

Concordant p53 and mdm-2 protein expression in vulvar squamous cell carcinoma and adjacent lichen sclerosus

To determine if carcinogenic events in vulvar skin precede the onset of morphologic atypia, the authors investigated for derangements in DNA content, cell proliferation, and cell death in vulvar carcinomas and surrounding skin in 140 samples of tumor and surrounding skin collected from 35 consecutive vulvectomy specimen for squamous cell carcinoma (SCC) or vulvar intraepithelial neoplasia (VIN) 3. Vulvar non-cancer excisions were used as controls. Investigations consisted of histologic classification and measurement of 9 variables--epidermal thickness (acanthosis and rete ridge length), immunolabeling index (LI) for 3 proteins (p53 protein, Ki-67, and mdm-2), pattern of p53 expression (dispersed vs. compact), DNA content index, and presence of aneuploidy by image analysis and apoptotic rate by Apotag labeling. Significant positive correlations were found for all nine variables studied versus increasing histologic severity in two proposed histologic stepwise models of vulvar carcinogenesis (lichen sclerosus (LS) and VIN 3 undifferentiated associated SCC groups). High p53 LI (>25) and the compact pattern of p53 expression (suspected oncoprotein) significantly correlated with LS and its associated vulvar samples compared with samples not associated with LS (P < or = 0.001). Furthermore, p53 LI, mdm-2 LI, and pattern of p53 expression were concordant between patient matched samples of LS and SCC. In addition, mdm-2 LI significantly correlated with dispersed pattern p53 LI suggesting a response to wild-type p53 protein accumulation. These findings support the hypothesis that neoplastic transformation occurs in sequential steps and compromises proteins involved in the cell cycle control. Concordance of p53 and mdm-2 protein expression in LS and adjacent SCC provides evidence that LS can act as a precursor lesion in the absence of morphologic atypia. Overexpression of mdm-2 with stabilization and inactivation of p53 protein may provide an alternate pathway for vulvar carcinogenesis.

Oestrogen and progesterone increase the levels of apoptosis induced by the human papillomavirus type 16 E2 and E7 proteins

Human papillomavirus (HPV) type 16 infects the genital tract and is generally acknowledged to be a causative agent of cervical cancer. HPV infection alone is not sufficient to induce cervical cancer and other factors such as steroid hormones are thought to play a role in the establishment and/or progression of this disease. The HPV-16 E2 protein is required for virus replication and modulates viral gene expression whereas the HPV-16 E7 protein is required for cell transformation. We and others have shown that both the E2 and E7 proteins can induce apoptotic cell death in HPV-transformed and non-HPV transformed cell lines. Here we show that the steroid hormones oestrogen and progesterone can both increase the levels of E2- and E7-induced apoptosis. The oestrogen metabolite 16 alpha-hydroxyoestrone also increases E2- and E7-induced cell death and the dietary component indole-3-carbinol, which reduces the formation of 16alpha-hydroxyoestrone from oestrogen, blocks the effects of oestrogen. Thus the metabolism of oestrogen to 16 alpha-hydroxyoestrone appears to be required for the effects of this hormone on E2- and E7-induced cell death. We also show that the oestrogen receptor antagonist 3-hydroxytamoxifen blocks the effects of oestrogen on E2- and E7-induced cell death, whereas the anti-progesterone RU486 blocks the effects of both progesterone and oestrogen. We discuss these results in terms of the origin and progression of cervical cancer.

Repression of human papillomavirus oncogenes in HeLa cervical carcinoma cells causes the orderly reactivation of dormant tumor suppressor pathways

Most cervical carcinomas express high-risk human papillomaviruses (HPVs) E6 and E7 proteins, which neutralize cellular tumor suppressor function. To determine the consequences of removing the E6 and E7 proteins from cervical cancer cells, we infected HeLa cells, a cervical carcinoma cell line that contains HPV18 DNA, with a recombinant virus that expresses the bovine papillomavirus E2 protein. Expression of the E2 protein resulted in rapid repression of HPV E6 and E7 expression, followed approximately 12 h later by profound inhibition of cellular DNA synthesis. Shortly after E6/E7 repression, there was dramatic posttranscriptional induction of p53. Two p53-responsive genes, mdm2 and p21, were induced with slightly slower kinetics than p53 and appeared to be functional, as assessed by inhibition of cyclin-dependent kinase activity and p53 destabilization. There was also dramatic posttranscriptional induction of p105(Rb) and p107 after E6/E7 repression, followed shortly thereafter by induction of p130. By 24 h after infection, only hypophosphorylated p105(Rb) was detectable and transcription of several Rb/E2F-regulated genes was dramatically repressed. Constitutive expression of the HPV16 E6/E7 genes alleviated E2-induced growth inhibition and impaired activation of the Rb pathway and repression of E2F-responsive genes. This dynamic response strongly suggests that the p53 and Rb tumor suppressor pathways are intact in HeLa cells and that repression of HPV E6 and E7 mobilizes these pathways in an orderly fashion to deliver growth inhibitory signals to the cells. Strikingly, the major alterations in the cell cycle machinery underlying cervical carcinogenesis can be reversed by repression of the endogenous HPV oncogenes.

Chromosome 17 aneusomy detected by fluorescence in situ hybridization in vulvar squamous cell carcinomas and synchronous vulvar skin

Vulvar squamous cell carcinoma (SCC) affects a spectrum of women with granulomatous vulvar diseases, human papillomavirus (HPV) infections, and chronic inflammatory vulvar dermatoses. To determine whether there is evidence of chromosomal instability occurring in synchronous skin surrounding vulvar SCCs, we investigated abnormalities in chromosome 17 copy number. Samples of SCC, vulvar intraepithelial neoplasia (VIN), and surrounding vulvar skin were obtained from all vulvar excisions performed for squamous neoplasia at Albany Medical College from 1996 to 1997. Histological categorization, fluorescent in situ hybridization (FISH) for the alpha satellite region of chromosome 17, DNA content by image analysis, and Ki-67 labeling were evaluated. Controls of normal vulvar skin not associated with cancer were used for comparison. One hundred ten specimens were obtained from 33 patients with either SCC or VIN 3 and consisted of 49 neoplastic, 52 nonneoplastic, and 9 histologically normal vulvar skin samples. The majority of SCCs (88%) and a minority (18%) of VIN 3 excisions were associated with lichen sclerosus. Normal vulvar skin controls did not exhibit chromosome 17 polysomy (cells with more than four FISH signals), whereas 56% of normal vulvar skin associated with cancer did. Moreover, the frequency of polysomy significantly increased as the histological classification progressed from normal to inflammatory to neoplastic lesions. The largest mean value and variance for chromosome 17 copy number was identified in SCCs (2.4 +/- 1.0) with intermediate values identified, in decreasing order, for SCC in situ (2.1 +/- 1.0), VIN 2 (2.1 +/- 0.8), lichen sclerosus (2.0 +/- 0.5), lichen simplex chronicus (1.9 +/- 0.4), and normal skin associated with SCC (1.8 +/- 0.4) compared with control vulvar skin (1.5 +/- 0. 05). Concordance of chromosome 17 aneusomy between cancers and synchronous skin lesions was found in 48% of patients. Loss of chromosome 17 was identified 5% of all samples and was significantly associated with women with SCC in situ (HPV-related). Both DNA content and Ki-67 labeling positively and significantly correlated with mean chromosome 17 copy number (r = 0.1, P: = 0.007). A high degree of genetic instability (aneuploidy) occurs in the skin surrounding vulvar carcinomas. As these events could be detected in histologically normal skin and inflammatory lesions (lichen sclerosus), chromosomal abnormalities may be a driving force in the early stages of carcinogenesis. Differences in chromosomal patterns (loss or gain) support the concept of at least two pathways in vulvar carcinogenesis.

HPV-16 E2 gene disruption and sequence variation in CIN 3 lesions and invasive squamous cell carcinomas of the cervix: relation to numerical chromosome abnormalities

AIM

To test the hypothesis that, because the human papillomavirus (HPV) E2 protein represses viral early gene transcription, E2 gene sequence variation or disruption could play a part in the induction of the numerical chromosome abnormalities that have been described in squamous cervical lesions.

METHODS

The integrity and sequence of the E2 gene from 11 cervical intraepithelial neoplasia (CIN) grade 3 lesions and 14 invasive squamous cell carcinomas, all of which contained HPV-16, were analysed by the polymerase chain reaction (PCR). The E2 gene was amplified in three overlapping fragments and PCR products sequenced directly. Chromosome abnormalities were identified by interphase cytogenetics using chromosome specific probes for chromosomes 1, 3, 11, 17, 18, and X.

RESULTS

E2 gene disruption was present in significantly more invasive carcinomas (eight of 14) than CIN 3 lesions (one of 11) (p = 0.03). No association was found between E2 disruption and the presence of a numerical chromosome abnormality. The E2 gene from the non-disrupted isolates was sequenced and wild-type (n = 5) and variant (n = 11) sequences identified. Variant sequences belonged to European and African classes and contained from one to 15 amino acid substitutions. Although numerical chromosome abnormalities were significantly more frequent in invasive squamous cell carcinoma than CIN 3 (p = 0.04), there was no significant relation between the presence of sequence variation and either histological diagnosis or chromosome abnormality.

CONCLUSIONS

These data do not support the hypothesis that E2 gene disruption or variation is important in the induction of chromosome imbalance in these lesions. However, there is a relation between E2 gene disruption and the presence of invasive disease.

Proteasome-mediated degradation of the papillomavirus E2-TA protein is regulated by phosphorylation and can modulate viral genome copy number

The bovine papillomavirus E2 proteins regulate viral transcription, replication, and episomal genome maintenance. We have previously mapped the major phosphorylation sites of the E2 proteins to serine residues 298 and 301 and shown that mutation of serine residue 301 to alanine leads to a dramatic (10- to 20-fold) increase in viral DNA copy number. In this study we analyzed how phosphorylation regulates E2 protein function. S301 is located in a PEST sequence; these sequences are often found in proteins with a short half-life and can be regulated by phosphorylation. We show here that the E2 protein is ubiquitinated and degraded by the proteasome. Mutation of serine 301 to alanine increases the half-life of E2 from approximately 50 min to 160 min. Furthermore, the A301 E2 protein shows greatly reduced ubiquitination and degradation by the proteasome. These results suggest that the E2 protein level is regulated by phosphorylation, which in turn determines viral episomal copy number.

Disruption of cell cycle control by human papillomaviruses with special reference to cervical carcinoma

Human papillomaviruses (HPVs) play a major role in neoplastic transformation of squamous epithelial cells. The viral genome is small in size and only encodes a limited number of proteins, so one of the major functions of the viral proteins is to modulate the function of key cellular proteins involved in cell cycle control and DNA replication. During this process important host cell cycle checkpoints are lost which may lead to the accumulation of genetic abnormalities and eventual malignant transformation. This review briefly describes the normal cell cycle and also the mechanisms by which HPVs interfere with cell cycle control both as part of their productive life cycle and in the process of neoplastic transformation.

Productive replication of adeno-associated virus can occur in human papillomavirus type 16 (HPV-16) episome-containing keratinocytes and is augmented by the HPV-16 E2 protein

We used a sensitive assay to test whether an adeno-associated virus (AAV) productive replication cycle can occur in immortalized human keratinocytes carrying episomal human papillomavirus type 16 (HPV-16) DNA. Following transfection with cloned AAV DNA, infectious AAV was produced, and the infectivity was blocked by anti-AAV antiserum. The HPV-16 E2 protein substantially increased the yield of AAV. Other HPV early proteins did not, in our experiments, show this ability. E2 has been shown to be able to affect p53 levels and to block cell cycle progression at mitosis. We tested the effect of changes in p53 expression on AAV replication and found that large differences in the level of p53 did not alter AAV DNA replication. In extension of this, we found that cellular help for AAV in response to stress was also independent of p53. To test if a mitotic block could trigger AAV DNA replication, we treated the cells with the mitotic inhibitor nocodazole. AAV DNA replication was stimulated by the presence of nocodazole in these and a number of other cell types tested. Yields of infectious virus, however, were not increased by this treatment. We conclude that the HPV-16 E2 protein stimulates AAV multiplication in these cells and propose that this occurs independently of the effects of E2 on p53 and cell cycle progression. Since the effect of E2 was not seen in keratinocytes lacking the HPV-16 episome, we suggest that E2 can help AAV by working in concert with other HPV-16 proteins.

The human papillomavirus (HPV) 16 E2 protein induces apoptosis in the absence of other HPV proteins and via a p53-dependent pathway

The human papillomavirus (HPV) E2 protein regulates viral gene expression and is also required for viral replication. HPV-transformed cells often contain chromosomally integrated copies of the HPV genome in which the viral E2 gene is disrupted. We have shown previously that re-expression of the HPV 16 E2 protein in HPV 16-transformed cells results in cell death via apoptosis. Here we show that the HPV 16 E2 protein can induce apoptosis in both HPV-transformed and non-HPV-transformed cell lines. E2-induced apoptosis is abrogated by a trans-dominant negative mutant of p53 or by overexpression of the HPV 16 E6 protein, but is increased by overexpression of wild-type p53. We show that mutations that block the DNA binding activity of E2 do not impair the ability of this protein to induce apoptosis. In contrast, removal of both N-terminal domains from the E2 dimer completely blocks E2-induced cell death. Heterodimers formed between wild-type E2 and N-terminally deleted E2 proteins also fail to induce cell death. Our data suggest that neither the DNA binding activity of E2 nor other HPV proteins are required for the induction of apoptosis by E2 and that E2-induced cell death occurs via a p53-dependent pathway.

Interaction between the HPV-16 E2 transcriptional activator and p53

The HPV-16 E2 protein is a major regulator of viral DNA replication and gene expression. Through interactions with the viral origin binding protein, E1, it localizes E1 to the origin of replication and stimulates the initiation of viral DNA replication. However, several recent reports have described a number of diverse activities of E2 relating to the induction of apoptosis through both p53 dependent and independent mechanisms, and to induction of growth arrest in both the G1 and G2M phases of the cell cycle. Recent studies have also shown that p53 can specifically inhibit HPV DNA replication, albeit through an unknown mechanism. Since p53 has been described in the replication centres of Herpes Viruses, Adenovirus and SV40 we decided to investigate whether any of the above activities of E2 may be related to an association with p53. We show, in a series of in vitro assays, specific interaction between p53 and HPV-16 E2 via residues in the carboxy terminal half of the E2 protein. Mutational analysis of p53 indicates that sequences in both the DNA binding and oligomerization domains are essential for the interaction, and a mutant of p53 which is unable to bind E2 is also unable to inhibit HPV DNA replication. Finally, using an inducible system of p53 expression we also show that E2 will complex with p53 in vivo. These results raise the intriguing possibility that p53 may also be involved in HPV DNA replication centres, and also provides explanations for some of the diverse activities reported for the HPV E2 proteins.

The molecular genetics of cervical carcinoma

In the pathogenesis of cervical carcinoma there are three major components, two of them related to the role of human papillomaviruses (HPV). First, the effect of viral E6 and E7 proteins. Second, the integration of viral DNA in chromosomal regions associated with well known tumour phenotypes. Some of these viral integrations occur recurrently at specific chromosomal locations, such as 8q24 and 12q15, both harbouring HPV18 and HPV16. And third, there are other recurrent genetic alterations not linked to HPV. Recurrent losses of heterozygosity (LOH) have been detected in chromosome regions 3p14-22, 4p16, 5p15, 6p21-22, 11q23, 17p13.3 without effect on p53, 18q12-22 and 19q13, all of them suggesting the alteration of putative tumour suppressor genes not yet identified. Recurrent amplification has been mapped to 3q+ arm, with the common region in 3q24-28 in 90% of invasive carcinomas. The mutator phenotype, microsatellite instability, plays a minor role and is detected in only 7% of cervical carcinomas. The development of cervical carcinoma requires the sequential occurrence and selection of several genetic alterations. The identification of the specific genes involved, and their correlation with specific tumour properties and stages could improve the understanding and perhaps the management of cervical carcinoma.

The mitotic machinery as a source of genetic instability in cancer

Development and growth of all organisms involves the faithful reproduction of cells and requires that the genome be accurately replicated and equally partitioned between two cellular progeny. In human cells, faithful segregation of the genome is accomplished by an elaborate macromolecular machine, the mitotic spindle. It is not difficult to envision how defects in components of this complex machine molecules that control its organization and function and regulators that temporally couple spindle operation to other cell cycle events could lead to chromosome missegregation. Recent evidence indicates that the persistent missegregation of chromosomes result in gains and losses of chromosomes and may be an important cause of aneuploidy. This form of chromosome instability may contribute to tumor development and progression by facilitating loss of heterozygocity (LOH) and the phenotypic expression of mutated tumor suppressor genes, and by favoring polysomy of chromosomes that harbor oncogenes. In this review, we will discuss mitotic defects that cause chromosome missegregation, examine components and regulatory mechanisms of the mitotic machine implicated in cancer, and explore mechanisms by which chromosome missegregation could lead to cancer.

Expression of human papillomavirus 16 E2 protein in Schizosaccharomyces pombe delays the initiation of mitosis

Infection by some types of human papillomavirus (HPV) is associated with the development of cervical cancer. Analysis of viral DNA from cervical tumours shows that the E2 gene is frequently disrupted during integration into the host cell's DNA. It has therefore been suggested that loss of E2p is an important step in malignant transformation. Expression of E2p in the fission yeast Schizosaccharomyces pombe retards the G2-M transition, by delaying activation of Cdc2p kinase. In contrast, S phase progression, and commitment to cell division in late G1 are not affected. The delay is independent of the transcriptional trans-activation function of E2p, and does not result from E2p DNA binding mimicking DNA damage. Increased expression of E2p also delays mitotic initiation in mammalian cells. S. pombe may thus provide a simple model for the analysis of E2p function.

Cyclin E in human cancers

Regulators of the cell cycle such as cyclin E play an important part in neoplasia. The cyclin E protein forms a partnership with a specific protein kinase. This complex phosphorylates key substrates to initiate DNA synthesis. Cyclin-dependent kinase inhibitors (CKIs) are able to suppress the activity of cyclin E. Various substances (including proteins produced by oncogenic viruses) affect cyclin E directly or indirectly through an interaction with CKIs. These interactions are important in elucidating the mechanisms of neoplasia. They may also provide prognostic information in a wide range of common cancers. Cyclin E may even be a target for treatment of cancers in the future.

Expression patterns of the human papillomavirus type 16 transcription factor E2 in low- and high-grade cervical intraepithelial neoplasia

Specific antibodies against the C-terminus of E2, produced by affinity purification of polyclonal antisera, have been used to identify the cellular populations which express the HPV 16 E2 transcription factor, in a series of formalin-fixed, paraffin-embedded cervical tissues. Cases were selected for both the presence of HPV 16 DNA (confirmed by multiple gene-specific PCR detections) and the presence of multiple grades of cervical intraepithelial neoplasia (CIN). The data indicate that E2 expression is highest in CIN I and in koilocytic lesions. Lower expression was observed in CIN II and little in CIN III lesions. In contrast, there was some restoration of E2 expression in invasive carcinomas, although the intracellular distribution was much more diffuse. The location of E2 expression to the superficial layers of the cervical epithelium, as well as the occurrence of some basal expression in CIN I, suggests that antibodies against HPV 16 E2 could be a useful adjunct to standard histological techniques for the detection of 'at-risk' patients as part of a cervical screening programme.

Apoptosis regulators from DNA viruses

The past year has seen an considerable expansion in knowledge about the field of apoptosis modulators expressed by DNA viruses. These diverse classes of virus-encoded regulators include caspase inhibitors, signal transduction effectors, Bcl-2 homologs, cell cycle control proteins, transcriptional regulators, reactive oxide scavengers, kinases, 'death factors' and novel host-range proteins.