Characterization of events during the late stages of HPV16 infection in vivo using high-affinity synthetic Fabs to E4

Biology of papillomavirus replication in infected epithelium

Human papillomaviruses complete their life cycle in differentiating epithelial cells that would not normally be competent for either cellular or viral DNA replication. To overcome this, papillomaviruses encode two groups of proteins that work together in the upper epithelial layers to amplify viral genomes. The E6 and E7 proteins play a critical role in driving differentiating epithelial cells that have left the basal layer, back into the cell cycle, in order to produce a replication-competent environment that can be used by the virus for genome amplification. Papillomavirus replication is heavily dependent on cellular replication proteins, but in addition needs the viral E1 and E2 proteins, which act to unwind viral DNA around the origin of replication, and to recruit essential cellular proteins to the replication site. Recent work using mutant viral genomes has suggested that two other viral proteins, E4 and E5, contribute to efficient replication in the upper epithelial layers, although the mechanisms by which they do this have not yet been clearly established. Genome amplification in the upper epithelial layers differs from maintenance replication in the basal layer, where viral genome replication appears coupled to that of the cellular genome. The onset of genome amplification during differentiation is thought to be triggered at least in part by an increase in E1 and E2 levels, and possibly also by a change in the relative levels of the two proteins. The role of E6 and E7 in basal cell replication is, however, uncertain and there is even some question as to the exact requirement for E1. Although similarities in papillomavirus lifecycle organization and protein function suggest a common mechanism by which viral DNA replication is regulated, differences in the site of infection and transmission route appear to manifest themselves as differences in the timing and extent of genome amplification. Understanding the patterns of protein expression seen during natural infection will be important in fully understanding how these differences arise.

Profiling microdissected epithelium and stroma to model genomic signatures for cervical carcinogenesis accommodating for covariates

This study is the first comprehensive, integrated approach to examine grade-specific changes in gene expression along the entire neoplastic spectrum of cervical intraepithelial neoplasia (CIN) in the process of cervical carcinogenesis. This was accomplished by identifying gene expression signatures of disease progression using cDNA microarrays to analyze RNA from laser-captured microdissected epithelium and underlying stroma from normal cervix, graded CINs, cancer, and patient-matched normal cervical tissues. A separate set of samples were subsequently validated using a linear mixed model that is ideal to control for interpatient gene expression profile variation, such as age and race. These validated genes were ultimately used to propose a genomically based model of the early events in cervical neoplastic transformation. In this model, the CIN 1 transition coincides with a proproliferative/immunosuppression gene signature in the epithelium that probably represents the epithelial response to human papillomavirus infection. The CIN 2 transition coincides with a proangiogenic signature, suggesting a cooperative signaling interaction between stroma and tumor cells. Finally, the CIN 3 and squamous cell carcinoma antigen transition coincide with a proinvasive gene signature that may be a response to epithelial tumor cell overcrowding. This work strongly suggests that premalignant cells experience a series of microenvironmental stresses at the epithelium/stroma cell interface that must be overcome to progress into a transformed phenotype and identifies the order of these events in vivo and their association with specific CIN transitions.

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.

Efficacy of a prophylactic adjuvanted bivalent L1 virus-like-particle vaccine against infection with human papillomavirus types 16 and 18 in young women: an interim analysis of a phase III double-blind, randomised controlled trial

BACKGROUND

The aim of this interim analysis of a large, international phase III study was to assess the efficacy of an AS04 adjuvanted L1 virus-like-particle prophylactic candidate vaccine against infection with human papillomavirus (HPV) types 16 and 18 in young women.

METHODS

18,644 women aged 15-25 years were randomly assigned to receive either HPV16/18 vaccine (n=9319) or hepatitis A vaccine (n=9325) at 0, 1, and 6 months. Of these women, 88 were excluded because of high-grade cytology and 31 for missing cytology results. Thus, 9258 women received the HPV16/18 vaccine and 9267 received the control vaccine in the total vaccinated cohort for efficacy, which included women who had prevalent oncogenic HPV infections, often with several HPV types, as well as low-grade cytological abnormalities at study entry and who received at least one vaccine dose. We assessed cervical cytology and subsequent biopsy for 14 oncogenic HPV types by PCR. The primary endpoint--vaccine efficacy against cervical intraepithelial neoplasia (CIN) 2+ associated with HPV16 or HPV18--was assessed in women who were seronegative and DNA negative for the corresponding vaccine type at baseline (month 0) and allowed inclusion of lesions with several oncogenic HPV types. This interim event-defined analysis was triggered when at least 23 cases of CIN2+ with HPV16 or HPV18 DNA in the lesion were detected in the total vaccinated cohort for efficacy. Analyses were done on a modified intention-to-treat basis. This trial is registered with the US National Institutes of Health clinical trial registry, number NCT00122681.

FINDINGS

Mean length of follow-up for women in the primary analysis for efficacy at the time of the interim analysis was 14.8 (SD 4.9) months. Two cases of CIN2+ associated with HPV16 or HPV18 DNA were seen in the HPV16/18 vaccine group; 21 were recorded in the control group. Of the 23 cases, 14 (two in the HPV16/18 vaccine group, 12 in the control group) contained several oncogenic HPV types. Vaccine efficacy against CIN2+ containing HPV16/18 DNA was 90.4% (97.9% CI 53.4-99.3; p<0.0001). No clinically meaningful differences were noted in safety outcomes between the study groups.

INTERPRETATION

The adjuvanted HPV16/18 vaccine showed prophylactic efficacy against CIN2+ associated with HPV16 or HPV18 and thus could be used for cervical cancer prevention.

Analysis of novel human papillomavirus type 16 late mRNAs in differentiated W12 cervical epithelial cells

The life cycle of human papillomavirus type 16 (HPV16) is intimately linked to differentiation of the epithelium it infects, and late events in the life cycle are restricted to the suprabasal layers. Here we have used 5′RACE of polyadenylated RNA isolated from differentiated W12 cells (cervical epithelial cells containing episomal copies of the HPV16 genome) that express virus late proteins to map virus late mRNAs. Thirteen different transcripts were identified. Extensive alternative splicing and use of two late polyadenylation sites were noted. A novel promoter located in the long control region was detected as well as P₉₇ and P_late. Promoters in the E4 and E5 open reading frames were active yielding transcripts where L1 or L2 respectively are the first open reading frames. Finally, mRNAs that could encode novel proteins E6*^*E7, E6*^E4, E1^*E4 and E1^E2C (putative repressor E2) were identified, indicating that HPV16 may encode more late proteins than previously accepted.

Frequency of antibodies against E4 and E7 from human papillomavirus type 16 in Mexican soldiers

The high prevalence of HPV in men's genitalia and the low frequency of virus-associated lesions gave rise to questions on the influence of infection-site on the HPV antibody profile. In a cross-sectional study, HPV infection in penis and urethra, and serum antibodies against HPV-16 E4 and E7 proteins were evaluated in 288 Mexican soldiers. The results showed that HPV prevalence was 31% (51% in penis, 11% in urethra and 38% in both sites), while 47% were multiple infections. Overall, seroprevalence was 13% for anti-E4 antibodies and 6% for anti-E7. However, the highest prevalence of anti-E4 antibodies was observed in men with HPV infection in urethra (30%), while for E7 antibodies, the highest prevalence (10%) was found in men who tested positive for HPV in penis. The prevalence of IgG and IgA anti-E4 was related to HPV-16 urethral infection, while detection of HPV-16 in penis was related to IgG anti-E7 prevalence. In conclusion, the high-risk sexual behavior observed in this population might be responsible for high HPV prevalence and multiple infections. However, the seroprevalence of E4 and E7 was similar to that observed in healthy Mexican women. These results suggest that the humoral immune response against HPV infection in men differs, depending on the site of infection.

The human papillomavirus type 11 E1/\E4 protein is not essential for viral genome amplification

An abundant human papillomavirus (HPV) protein E1/\E4 is expressed late in the virus life cycle in the terminally differentiated layers of epithelia. The expression of E1/\E4 usually coincides with the onset of viral DNA amplification. However, the function of E1/\E4 in viral life cycle is not completely understood. To examine the role of E1/\E4 in the virus life cycle, we introduced a single nucleotide change in the HPV-11 genome to result in a truncation of E1/\E4 protein without affecting the E2 amino acid sequence. This mutated HPV-11 genome was introduced into a human foreskin keratinocyte cell line immortalized by the catalytic subunit of human telomerase, deficient in p16(INK4a) expression, and previously shown to support the HPV-11 life cycle when grown in organotypic raft culture. We have demonstrated that E1/\E4 is dispensable for HPV-11 viral DNA amplification in the late stages of the viral life cycle.

HPV16 E1^E4 protein is phosphorylated by Cdk2/cyclin A and relocalizes this complex to the cytoplasm

The human papillomavirus type 16 E1--E4 protein is expressed abundantly in cells supporting viral DNA amplification, but its expression is lost during malignant progression. In cell culture, 16E1--E4 causes G2 cell cycle arrest by associating with and preventing the nuclear entry of Cdk1/cyclin B1 complexes. Here, we show that 16E1--E4 is also able to associate with cyclin A and Cdk2 during the G2 phase of the cell cycle. Only a weak association was apparent during S-phase, and progression through S-phase appeared unaffected. As with cyclin B1, the interaction of 16E1--E4 with cyclin A is dependent on residues T22/T23 and results in the accumulation of cyclin A in the cytoplasm where it colocalizes with 16E1--E4. 16E1--E4 serine 32 was found to be phosphorylated by Cdk2/cyclin A. We hypothesize that the interaction of 16E1--E4 with cyclin A may serve to increase the efficiency with which 16E1--E4 is able to prevent mitotic entry.

Human papillomaviruses: basic mechanisms of pathogenesis and oncogenicity

Human papillomaviruses (HPVs) are small double-stranded DNA viruses that infect the cutaneous and mucosal epithelium. Infection by specific HPV types has been linked to the development of cervical carcinoma. HPV infects epithelial cells that undergo terminal differentiation and so encode multiple mechanisms to override the normal regulation of differentiation to produce progeny virions. Two viral proteins, E6 and E7, alter cell cycle control and are the main arbitrators of HPV-induced oncogenesis. Recent data suggest that E6 and E7 also play a major role in the inhibition of the host cell innate immune response to HPV. The E1 and E2 proteins, in combination with various cellular factors, mediate viral replication. In addition, E2 has been implicated in both viral and cellular transcriptional control. Despite decades of research, the function of other viral proteins still remains unclear. While prophylactic vaccines to block genital HPV infection will soon be available, the widespread nature of HPV infection requires greater understanding of both the HPV life cycle as well as the mechanisms underlying HPV-induced carcinogenesis.

Elevation of cyclin B1, active cdc2, and HuR in cervical neoplasia with human papillomavirus type 18 infection

Over 30 cervical epitheliotrophic HPV types may lead to altered biological functions that affect the clinical outcome of HPV infection. In order to determine the regulatory mechanism and effect of different HPV subtypes, we performed functional assays on cdc2, cyclinB1 and HuR in human uterine cervical samples. After confirming 22 HPV types among 95 cervical swabs, 10 cervical tissues, and seven established cell lines using a DNA chip, we evaluated the functional activities of G2 molecules assays, that included; western blotting for cyclin B1, cdc2 and phospho-cdc2 (Y15 and T161), immunoprecipitation for cdc2, a nuclear extraction fractional assay, and RT-PCR for cyclin B1. The expression of cyclin B1 was found to be dependent on HPV type, and was particularly overexpressed in high-risk types, whereas cdc2 was ubiquitously expressed irrespective of HPV type. Phospho-cdc2 and cyclin B1, however, were most intense in HPV18 infected cervical samples. Furthermore, the HuR stabilizing factor of the cyclin B1 transcript was upregulated in HPV 18 infected swabs. Moreover, SiHa cell line showed weaker G2 functional activity than the HeLa cell line. This study demonstrates that HPV-18 decreases the fidelity of mitotic checkpoints and increases cdc2-associated histone H1 kinase activity relative to control populations, and further shows that the G2 checkpoint is aberrant by virtue of the stabilization of cyclin B1 mRNA through the upregulation of HuR protein and the functional form of cdc2, especially in cases with HPV 18 infected cervical lesions.

Inhibition of Papillomavirus Protein Function in Cervical Cancer Cells by Intrabody Targeting

Papillomaviruses (HPVs) are a major cause of human disease, and are responsible for approximately half a million cases of cervical cancer each year. HPVs also cause genital warts, and are the most common sexually transmitted disease in many countries. Despite their importance, there are currently no specific antivirals that are active against HPVs. Papillomavirus protein function is mediated largely by protein-protein interactions, which are difficult to inhibit using conventional approaches. To circumvent these problems, we have prepared an scFv library, and have used this to isolate high-affinity binding molecules that may stearically hinder the association of E6 with p53 and prevent E6-mediated p53 degradation in cervical cancer cells. One of the molecules isolated from the library (GTE6-1), had an affinity for 16E6 of 60nM, and bound within the first zinc finger of the protein. GTE6-1 was able to associate with non-denatured E6 following expression in mammalian cells and could inhibit E6-mediated p53 degradation in in vitro assays. E6-mediated p53 degradation is essential for the continuous growth of cervical cancer cells caused by HPV16. To examine the potential of GTE6-1 as an inhibitor of E6 function in such cells, the molecule was expressed in scFv, diabody and triabody formats in a number of cell lines that are driven to proliferate by the HPV16 oncogenes E6 and E7, including the cervical cancer cell line SiHa. In contrast to small E6-binding peptides containing the ELLG E6-binding motif, GTE6-1 expression lead to changes in nuclear structure, the appearance of apoptosis markers, and an elevation in the levels of p53. No effects were seen with a control scFv molecule, or when GTE6-1 was expressed in cells that are driven to proliferate by simian virus 40 (SV40) T-antigen. Given the accessibility of HPV-associated lesions to topical therapy, our results suggest that large interfering molecules such as intrabodies may be useful inhibitors of viral protein-protein interactions and be particularly appropriate for the treatment of HPV-associated disease.

Human papillomavirus type 16 E1circumflexE4 contributes to multiple facets of the papillomavirus life cycle

The life cycle of human papillomaviruses (HPVs) is tightly linked to the differentiation program of the host's stratified epithelia that it infects. E1(circumflex)E4 is a viral protein that has been ascribed multiple biochemical properties of potential biological relevance to the viral life cycle. To identify the role(s) of the viral E1(circumflex)E4 protein in the HPV life cycle, we characterized the properties of HPV type 16 (HPV16) genomes harboring mutations in the E4 gene in NIKS cells, a spontaneously immortalized keratinocyte cell line that when grown in organotypic raft cultures supports the HPV life cycle. We learned that E1(circumflex)E4 contributes to the replication of the viral plasmid genome as a nuclear plasmid in basal cells, in which we also found E1(circumflex)E4 protein to be expressed at low levels. In the suprabasal compartment of organotypic raft cultures harboring E1(circumflex)E4 mutant HPV16 genomes there were alterations in the frequency of suprabasal cells supporting DNA synthesis, the levels of viral DNA amplification, and the degree to which the virus perturbs differentiation. Interestingly, the comparison of the phenotypes of various mutations in E4 indicated that the E1(circumflex)E4 protein-encoding requirements for these various processes differed. These data support the hypothesis that E1(circumflex)E4 is a multifunctional protein and that the different properties of E1(circumflex)E4 contribute to different processes in both the early and late stages of the virus life cycle.

The human papillomavirus type 11 E1--E4 protein is a transglutaminase 3 substrate and induces abnormalities of the cornified cell envelope

The human papillomavirus (HPV) E1--E4 protein is detected in the cytoplasm of differentiated keratinocytes, near the cornified cell envelope. HPV does not induce lysis of the infected keratinocyte, and the normally durable cornified cell envelope that forms during keratinocyte differentiation would seemingly inhibit viral egress. HPV infection induces abnormalities of the cornified cell envelope, but the exact mechanisms involved are not well understood. We tested whether the HPV 11 E1--E4 protein, which co-localizes the cell envelope and co-purifies with cell envelope fragments, could serve as an in vitro substrate for transglutaminases. We found evidence of E1--E4 cross-linking by endogenous transglutaminases in an in situ assay using frozen sections of human foreskin, and in addition, E1--E4 protein was cross-linked by recombinant transglutaminase 3 (but not transglutaminase 1) in an in vitro cross-linking assay. We also tested whether expression of E1--E4 in differentiated keratinocytes would induce morphologic alterations of cornified cell envelopes. Differentiated keratinocytes expressing E1--E4 were disorganized and pleomorphic compared to control cells, and cell envelopes purified from E1--E4-expressing cells were small, fragmented, and rough bordered compared to the round, smooth bordered cell envelopes from control cells. We conclude from these in vitro experiments that the E1--E4 protein is cross-linked by transglutaminase 3, and that E1--E4 expression in differentiated keratinocytes induces morphologic abnormalities of the cornified cell envelope.

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.

Human papillomavirus gene expression in cutaneous squamous cell carcinomas from immunosuppressed and immunocompetent individuals

Epidermodysplasia verruciformis (EV)-type human papillomavirus (HPV) DNA have been detected by PCR in squamous cell carcinomas (SCC) from both organ transplant recipients (OTR) and immunocompetent individuals. Their role in skin cancer remains unclear, and previous studies have not addressed whether the viruses are transcriptionally active. We have used in situ hybridization to investigate the transcriptional activity and DNA localization of HPV. EV-HPV gene transcripts were demonstrated in four of 11 (36%) OTR SCC, one of two (50%) IC SCC, and one of five (20%) OTR warts positive by PCR. Viral DNA co-localized with E2/E4 early region gene transcripts in the middle or upper epidermal layers. Non-EV cutaneous HPV gene transcripts were demonstrated in one of five (20%) OTR SCC and four of 10 (40%) OTR warts. In mixed infections transcripts for both types were detected in two of six (33%) cases. Our results provide evidence of EV-HPV gene expression in SCC; although only a proportion of tumors were positive, the similarly low transcriptional activity in warts suggests this is an underestimate. These observations, together with emerging epidemiological and functional data, provide further reason to focus on the contribution of EV-HPV types to the pathogenesis of cutaneous SCC.

The papillomavirus life cycle

Papillomaviruses infect epithelial cells, and depend on epithelial differentiation for completion of their life cycle. The expression of viral gene products is closely regulated as the infected basal cell migrates towards the epithelial surface. Expression of E6 and E7 in the lower epithelial layers drives cells into S-phase, which creates an environment that is conducive for viral genome replication and cell proliferation. Genome amplification, which is necessary for the production of infectious virions, is prevented until the levels of viral replication proteins rise, and depends on the co-expression of several viral proteins. Virus capsid proteins are expressed in cells that also express E4 as the infected cell enters the upper epithelial layers. The timing of these events varies depending on the infecting papillomavirus, and in the case of the high-risk human papillomaviruses (HPVs), on the severity of neoplasia. Viruses that are evolutionarily related, such as HPV1 and canine oral papillomavirus (COPV), generally organize their productive cycle in a similar way, despite infecting different hosts and epithelial sites. In some instances, such as following HPV16 infection of the cervix or cottontail rabbit papillomavirus (CRPV) infection of domestic rabbits, papillomaviruses can undergo abortive infections in which the productive cycle of the virus is not completed. As with other DNA tumour viruses, such abortive infections can predispose to cancer.

T-cell responses to human papillomavirus type 16 among women with different grades of cervical neoplasia

Infection with high-risk genital human papillomavirus (HPV) types is a major risk factor for the development of cervical intraepithelial neoplasia (CIN) and invasive cervical carcinoma. The design of effective immunotherapies requires a greater understanding of how HPV-specific T-cell responses are involved in disease clearance and/or progression. Here, we have investigated T-cell responses to five HPV16 proteins (E6, E7, E4, L1 and L2) in women with CIN or cervical carcinoma directly ex vivo. T-cell responses were observed in the majority (78%) of samples. The frequency of CD4+ responders was far lower among those with progressive disease, indicating that the CD4+ T-cell response might be important in HPV clearance. CD8+ reactivity to E6 peptides was dominant across all disease grades, inferring that E6-specific CD8+ T cells are not vitally involved in disease clearance. T-cell responses were demonstrated in the majority (80%) of cervical cancer patients, but are obviously ineffective. Our study reveals significant differences in HPV16 immunity during progressive CIN. We conclude that the HPV-specific CD4+ T-cell response should be an important consideration in immunotherapy design, which should aim to target preinvasive disease.

Multinucleation of koilocytes is in fact multilobation and is related to aberration of the G2 checkpoint

AIMS

To clarify the fine structure of koilocytes and correlate this with genetic aberration of the G2 checkpoint.

METHODS

Three dimensional reconstruction from confocal fluorescent images, together with functional assays for key molecules of the G2 checkpoint-cdc2 and cyclin B1-was performed in human uterine cervical samples. After confirming 22 human papillomavirus (HPV) types using a DNA chip from 30 cervical swabs, previously confirmed as 15 cervical low grade and 15 high grade intraepithelial lesions, the activity of molecules involved in the G2 checkpoint was evaluated using western blotting for cyclin B1, cdc2, and phospho-cdc2 (Y15 and T161), a nuclear extraction fractional assay, and a reverse transcription polymerase chain reaction assay. In addition, three dimensional confocal image restoration was performed on confirmed cervical intraepithelial neoplasia tissue samples.

RESULTS

T161 phospho-cdc2 and cyclin B1 expression was higher in HPV infected cervical lesions than in normal samples. Immunofluorescence, revealed that cyclin B1 was present predominantly in the nuclei of HPV infected cells, confirming the results of the nuclear fractional assay. On restoration of three dimensional confocal images, the multinucleation of koilocytes was revealed to be multilobation of a single nucleus, rather than true multinucleation. This multilobation appeared to be associated with chromosomal instability and aberration of the G2 checkpoint.

CONCLUSIONS

The multiple nuclei of koilocytes are in fact multilobation of a single nucleus, and this phenomenon is associated with upregulation of gene products related to the G2 checkpoint.

Role of the E1--E4 protein in the differentiation-dependent life cycle of human papillomavirus type 31

The most highly expressed protein in the productive life cycle of human papillomaviruses (HPVs) is E1--E4, but its function is not well understood. To investigate the role of E1--E4, we undertook a genetic analysis in the context of the complete HPV type 31 (HPV31) genome. A mutant HPV31 genome (E4M9) was constructed that contained a stop codon in the E4 open reading frame at amino acid 9 and was silent in the overlapping E2 coding sequence. Wild-type and mutant genomes were transfected into normal human foreskin keratinocytes (HFKs) and selected for drug resistance, and pooled cultures were examined for effects of E1--E4 on viral functions. Southern blot analyses of transfected HFKs demonstrated that cells carrying the E4M9 mutant genomes were maintained as episomes at copy numbers similar to those in keratinocytes transfected with wild-type HPV31. Both sets of cells grew at similar rates, exhibited comparable extensions of life spans, and had equivalent levels of early transcripts. Following suspension of the cells in a semisolid medium, differentiation-dependent genome amplification and late gene expression were significantly decreased in cells maintaining the E4M9 mutant genome compared to those with wild-type HPV31. One explanation for these effects could be a reduction in the number of cells harboring mutant genomes that enter S phase upon differentiation. An analysis of cells containing E4M9 mutant genomes in organotypic raft cultures indicated a reduction in bromodeoxyuridine incorporation in differentiated suprabasal cells compared to that seen in wild-type rafts. Our results indicate that the HPV31 E1--E4 protein plays a significant role in promoting HPV genome amplification and S phase maintenance during differentiation.

Human papillomavirus type 16 E1 E4-induced G2 arrest is associated with cytoplasmic retention of active Cdk1/cyclin B1 complexes

Human papillomavirus type 16 (HPV16) can cause cervical cancer. Expression of the viral E1 E4 protein is lost during malignant progression, but in premalignant lesions, E1 E4 is abundant in cells supporting viral DNA amplification. Expression of 16E1 E4 in cell culture causes G2 cell cycle arrest. Here we show that unlike many other G2 arrest mechanisms, 16E1 E4 does not inhibit the kinase activity of the Cdk1/cyclin B1 complex. Instead, 16E1 E4 uses a novel mechanism in which it sequesters Cdk1/cyclin B1 onto the cytokeratin network. This prevents the accumulation of active Cdk1/cyclin B1 complexes in the nucleus and hence prevents mitosis. A mutant 16E1 E4 (T22A, T23A) which does not bind cyclin B1 or alter its intracellular location fails to induce G2 arrest. The significance of these results is highlighted by the observation that in lesions induced by HPV16, there is evidence for Cdk1/cyclin B1 activity on the keratins of 16E1 E4-expressing cells. We hypothesize that E1 E4-induced G2 arrest may play a role in creating an environment optimal for viral DNA replication and that loss of E1 E4 expression may contribute to malignant progression.

Intracellular expression patterns of the human papillomavirus type 59 E1/E4 protein in COS cells, keratinocytes, and genital epithelium

OBJECTIVE

To compare and contrast the intracellular distribution pattern of the human papillomavirus type 59 (HPV 59) E1/E4 protein in COS cells, human keratinocytes, and naturally infected genital epithelium.

METHODS

The HPV 59 E1/E4 protein was expressed in COS cells and NIKS cells (immortalized human keratinocytes). A subset of NIKS cells was induced to differentiate. The intracellular distribution pattern of E1/E4 and the effects of E1/E4 expression on the cytoskeleton network were compared for COS and NIKS cells. Expression of E1/E4 was examined in HPV 59-infected foreskin xenografts grown in athymic mice and in a natural HPV 59-infected genital lesion.

RESULTS

The HPV 59 E1/E4 protein formed dense perinuclear inclusions in COS cells, similar to those reported for the HPV 16 E1/E4 protein. In contrast, the E1/E4 protein was diffusely cytoplasmic in undifferentiated NIKS cells, co-localizing with an intact cytokeratin filament network. The E1/E4 protein was concentrated in the region of the cornified cell envelope (CCE) of differentiated NIKS cells, co-localizing with involucrin, a CCE component. A similar distribution in the region of the CCE was observed for E1/E4 protein in HPV 59-infected human epithelial tissues.

CONCLUSIONS

The HPV 59 E1/E4 protein is cytoplasmic and co-localizes with an intact cytokeratin filament network in undifferentiated keratinocytes. The E1/E4 protein is distributed in the region of the CCE and co-localizes with involucrin in differentiated human keratinocytes, consistent with the intracellular distribution pattern observed in HPV 59-infected epithelium.

Mitochondria as Functional Targets of Proteins Coded by Human Tumor Viruses

Molecular analyses of tumor virus-host cell interactions have provided key insights into the genes and pathways involved in neoplastic transformation. Recent studies have revealed that the human tumor viruses Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), human papillomavirus (HPV), hepatitis B virus (HBV), hepatitis C virus (HCV), and human T-cell leukemia virus type 1 (HTLV-1) express proteins that are targeted to mitochondria. The list of these viral proteins includes BCL-2 homologues (BHRF1 of EBV; KSBCL-2 of KSHV), an inhibitor of apoptosis (IAP) resembling Survivin (KSHV K7), proteins that alter mitochondrial ion permeability and/or membrane potential (HBV HBx, HPV E[wedge]14, HCV p7, and HTLV-1 p13(II)), and K15 of KSHV, a protein with undefined function. Consistent with the central role of mitochondria in energy production, cell death, calcium homeostasis, and redox balance, experimental evidence indicates that these proteins have profound effects on host cell physiology. In particular, the viral BCL-2 homologues BHRF1 and KSBCL-2 inhibit apoptosis triggered by a variety of stimuli. HBx, p7, E1[wedge]4, and p13(II) exert powerful effects on mitochondria either directly due to their channel-forming activity or indirectly through interactions with endogenous channels. Further investigation of these proteins and their interactions with mitochondria will provide important insights into the mechanisms of viral replication and tumorigenesis and could aid in the discovery of new targets for anti-tumor therapy.

Cooperation between different forms of the human papillomavirus type 1 E4 protein to block cell cycle progression and cellular DNA synthesis

Posttranslational modification-oligomerization, phosphorylation, and proteolytic cleavage-of the human papillomavirus (HPV) E4 protein occurs as the infected keratinocytes migrate up through the suprabasal wart layers. It has been postulated that these events modify E4 function during the virus life cycle. In HPV type 1 (HPV1)-induced warts, N-terminal sequences are progressively cleaved from the full-length E4 protein (E1(wedge)E4) of 17 kDa to produce a series of polypeptides of 16, 11 and 10 kDa. Here, we have shown that in human keratinocytes, a truncated protein (E4-16K), equivalent to the 16-kDa species, mediated a G(2) arrest in the cell cycle that was dependent on a threonine amino acid in a proline-rich domain of the protein. Reconstitution of cyclin B1 expression in E4-16K cells reversed the G(2) arrest. Expression of E4-16K also induced chromosomal rereplication, and this was associated with aberrant nuclear morphology. Perturbation of the mitotic cell cycle was a biological activity specific to the truncated protein. However, coexpression of the full-length E1(wedge)E4 protein and the truncated E4-16K protein inhibited normal cellular proliferation and cellular DNA rereplication but did not prevent cells from arresting in G(2). Our findings provide the first evidence to support the hypothesis that proteolytic cleavage of the E1(wedge)E4 protein modifies its function. Also, different forms of the HPV1 E4 protein cooperate to negatively influence keratinocyte proliferation. We predict that these distinct biological activities of E4 act to support efficient amplification of the viral genome in suprabasal keratinocytes.

Hpv Proteins as Targets for Therapeutic Intervention

Human papillomaviruses (HPV) are the aetiological agents of several types of anogenital tumours, particularly cervical carcinoma. Recent evidence also suggests a role for HPV in the development of squamous cell carcinomas of the skin, especially in immunocompromised individuals. HPV infection also produces a number of non-malignant, but nonetheless cosmetically unpleasant lesions. Therefore, any effective therapeutic treatment for HPV-induced diseases would be extremely beneficial both on humanitarian grounds as well as being economically very attractive. In this review, we will discuss the functions of the viral proteins that appear to be the most appropriate for the development of therapeutics aimed at the treatment of viral infection and virus-induced cancers.

The viral E4 protein is required for the completion of the cottontail rabbit papillomavirus productive cycle in vivo

Expression of the papillomavirus E4 protein correlates with the onset of viral DNA amplification. Using a mutant cottontail rabbit papillomavirus (CRPV) genome incapable of expressing the viral E4 protein, we have shown that E4 is required for the productive stage of the CRPV life cycle in New Zealand White and cottontail rabbits. In these lesions, E4 was not required for papilloma development, but the onset of viral DNA amplification and L1 expression were abolished. Viral genome amplification was partially restored when mutant genomes able to express longer forms of E4 were used. These findings suggest that efficient amplification of the CRPV genome is dependent on the expression of a full-length CRPV E4 protein.

Functional analysis of the human papillomavirus type 16 E1=E4 protein provides a mechanism for in vivo and in vitro keratin filament reorganization

High-risk human papillomaviruses, such as human papillomavirus type 16 (HPV16), are the primary cause of cervical cancer. The HPV16 E1=E4 protein associates with keratin intermediate filaments and causes network collapse when expressed in epithelial cells in vitro. Here, we show that keratin association and network reorganization also occur in vivo in low-grade cervical neoplasia caused by HPV16. The 16E1=E4 protein binds to keratins directly and interacts strongly with keratin 18, a member of the type I intermediate-filament family. By contrast, 16E1=E4 bound only weakly to keratin 8, a type II intermediate-filament protein, and showed no detectable affinity for the type III protein, vimentin. The N-terminal 16 amino acids of the 16E1=E4 protein, which contains the YPLLXLL motif that is conserved among supergroup A viruses, were sufficient to target green fluorescent protein to the keratin network. When expressed in the SiHa cervical epithelial cell line, the full-length 16E1=E4 protein caused an almost total inhibition of keratin dynamics, despite the phosphorylation of keratin 18 at serine 33, which normally leads to 14-3-3-mediated keratin solubilization. Mutant 16E1=E4 proteins which lack the LLKLL motif, or which have lost amino acids from their C termini, and which were compromised in the ability to associate with keratins did not disturb normal keratin dynamics. 16E1=E4 was found to exist as dimers and hexamers, whereas a C-terminal deletion mutant (16E1=E4Delta87-92) existed as monomers and formed multimeric structures only poorly. Considered together, our results suggest that by associating with keratins through its N terminus, and by associating with itself through its C terminus, 16E1=E4 may act as a keratin cross-linker and prevent the movement of keratins between the soluble and insoluble compartments. The increase in avidity associated with multimeric binding may contribute to the ability of 16E1=E4 to sequester its cellular targets in the cytoplasm.

Organization of human papillomavirus productive cycle during neoplastic progression provides a basis for selection of diagnostic markers

The productive cycle of human papillomaviruses (HPVs) can be divided into discrete phases. Cell proliferation and episomal maintenance in the lower epithelial layers are followed by genome amplification and the expression of capsid proteins. These events, which occur in all productive infections, can be distinguished by using antibodies to viral gene products or to surrogate markers of their expression. Here we have compared precancerous lesions caused by HPV type 16 (HPV16) with lesions caused by HPV types that are not generally associated with human cancer. These include HPV2 and HPV11, which are related to HPV16 (supergroup A), as well as HPV1 and HPV65, which are evolutionarily divergent (supergroups E and B). HPV16-induced low-grade squamous intraepithelial lesions (CIN1) are productive infections which resemble those caused by other HPV types. During progression to cancer, however, the activation of late events is delayed, and the thickness of the proliferative compartment is progressively increased. In many HPV16-induced high-grade squamous intraepithelial lesions (CIN3), late events are restricted to small areas close to the epithelial surface. Such heterogeneity in the organization of the productive cycle was seen only in lesions caused by HPV16 and was not apparent when lesions caused by other HPV types were compared. By contrast, the order in which events in the productive cycle were initiated was invariant and did not depend on the infecting HPV type or the severity of disease. The distribution of viral gene products in the infected cervix depends on the extent to which the virus can complete its productive cycle, which in turn reflects the severity of cervical neoplasia. It appears from our work that the presence of such proteins in cells at the epithelial surface allows the severity of the underlying disease to be predicted and that markers of viral gene expression may improve cervical screening.

mRNA splicing regulates human papillomavirus type 11 E1 protein production and DNA replication

The papillomavirus replicative helicase E1 and the origin recognition protein E2 are required for efficient viral DNA replication. We fused the green fluorescent protein (GFP) to the human papillomavirus type 11 E1 protein either in a plasmid with the E1 coding region alone (nucleotides [nt] 832 to 2781) (pGFP-11E1) or in a plasmid containing both the E1 and E2 regions (nt 2723 to 3826) and the viral origin of replication (ori) (p11Rc). The former supported transient replication of an ori plasmid, whereas the latter was a self-contained replicon. Unexpectedly, these plasmids produced predominantly a cytoplasmic variant GFP or a GFP-E1 E4 protein, respectively. The majority of the mRNAs had an intragenic or intergenic splice from nt 847 to nt 2622 or from nt 847 to nt 3325, corresponding to the E2 or E1 E4 messages. pGFP-11E1dm and p11Rc-E1dm, mutated at the splice donor site, abolished these splices and increased GFP-E1 protein expression. Three novel, alternatively spliced, putative E2 mRNAs were generated in higher abundance from the mutated replicon than from the wild type. Relative to pGFP-11E1, low levels of pGFP-11E1dm supported more efficient replication, but high levels had a negative effect. In contrast, elevated E2 levels always increased replication. Despite abundant GFP-E1 protein, p11Rc-E1dm replicated less efficiently than the wild type. Collectively, these observations show that the E1/E2 ratio is as important as the E1 and E2 concentrations in determining the replication efficiency. These findings suggest that alternative mRNA splicing could provide a mechanism to regulate E1 and E2 protein expression and DNA replication during different stages of the virus life cycle.

Human papillomaviruses: targeting differentiating epithelial cells for malignant transformation

Human papillomavirus (HPV) infections play a crucial role in the pathogenesis of cervical neoplasia. Insights into the mechanisms by which HPV infection can, in a small numbers of cases, result in malignancy, comes from the observation that three proteins encoded by high-risk genital HPVs, E6, E7 and to a lesser extent E5, target factors that control the cell cycle and proliferation. These interactions result in abrogation of cell cycle control, chromosomal alterations, telomerase activation, and eventual cell immortalization. In this review, we discuss the functions of E6, E7, and E5 proteins that are most relevant to the malignant progression of HPV-transformed cells.

Depletion of Langerhans cells in human papillomavirus type 16-infected skin is associated with E6-mediated down regulation of E-cadherin

Human papillomavirus type 16 (HPV16) is an oncogenic virus that causes persistent infections in cervical epithelium. The chronic nature of HPV16 infections suggests that this virus actively evades the host immune response. Intraepithelial Langerhans cells (LC) are antigen-presenting cells that are critical in T-cell priming in response to viral infections of the skin. Here we show that HPV16 infection is directly associated with a reduction in the numbers of LC in infected epidermis. Adhesion between keratinocytes (KC) and LC, mediated by E-cadherin, is important in the retention of LC in the skin. Cell surface E-cadherin is reduced on HPV16-infected basal KC, and this is directly associated with the reduction in numbers of LC in infected epidermis. Expression of a single viral early protein, HPV16 E6, in KC reduces levels of cell surface E-cadherin thereby interfering with E-cadherin-mediated adhesion. Through this pathway, E6 expression in HPV16-infected KC may limit presentation of viral antigens by LC to the immune system, thus preventing the initiation of a cell-mediated immune response and promoting survival of the virus.

Infection with the oncogenic human papillomavirus type 59 alters protein components of the cornified cell envelope

Infection of the genital tract with human papillomaviruses (HPVs) leads to proliferative and dysplastic epithelial lesions. The mechanisms used by the virus to escape the infected keratinocyte are not well understood. Infection of keratinocytes with HPV does not cause lysis, the mechanism used by many viruses to release newly formed virions. For HPV 11, a type associated with a low risk of neoplastic disease, the cornified cell envelope (CCE) of infected keratinocytes is thin and fragile, and transcription of loricrin, the major CCE protein, is reduced. The effects of high-risk HPV infection on components of the CCE have not been previously reported. HPV 59, an oncogenic genital type related to HPV types 18 and 45 was identified in a condylomata acuminata lesion. An extract of this lesion was used to infect human foreskin fragments, which were grown in athymic mice as xenografts. Continued propagation using extracts of xenografts permitted growth of additional HPV 59-infected xenografts. CCEs purified from HPV 59-infected xenografts displayed subtle morphologic abnormalities compared to those derived from uninfected xenografts. HPV 59-infected xenografts revealed dysplastic-appearing cells with mitotic figures. Detection of loricrin, involucrin, and cytokeratin 10 was reduced in HPV 59-infected epithelium, while small proline-rich protein 3 (SPR3) was increased. Reduction in loricrin was most apparent in regions of epithelium containing abundant HPV 59 DNA. Compared to uninfected epithelium, loricrin transcription was decreased in HPV 59-infected epithelium. We conclude that HPV 59 shares with HPV 11 the ability to alter CCE components and to specifically reduce transcription of the loricrin gene. Because loricrin is the major CCE protein, a reduction in this component could alter the physical properties of the CCE, thus facilitating virion release.

Human papillomavirus type 31 E5 protein supports cell cycle progression and activates late viral functions upon epithelial differentiation

The function of the E5 protein of human papillomaviruses (HPV) is not well characterized, and controversies exist about its role in the viral life cycle. To determine the function of E5 within the life cycle of HPV type 31 (HPV31) we first constructed HPV31 mutant genomes that contained an altered AUG initiation codon or stop codons in E5. Cell lines were established which harbored transfected wild-type or E5 mutant HPV31 genomes. These cell lines all maintained episomal copies of HPV31 and revealed similar phenotypes with respect to growth rate, early gene expression, and viral copy number in undifferentiated monolayer cultures. Following epithelial differentiation, genome amplification and differentiation-dependent late gene expression were observed in mutant cell lines, but at a rate significantly reduced from that observed in cells containing the wild-type genomes. Organotypic raft cultures indicated that E5 does not effect the expression of differentiation markers but does reduce expression of late viral proteins. Western analysis and immunofluorescence staining for cyclins during epithelial differentiation revealed a decreased expression of cyclin A and B in E5 mutant cells compared to HPV wild-type cells. Using a replating assay, a significant reduction in colony-forming ability was detected in the absence of E5 expression when cells containing wild-type or E5 mutant HPV genomes were allowed to proliferate following 24 h in suspension-induced differentiation. This suggests that HPV E5 modifies the differentiation-induced cell cycle exit and supports the ability of HPV31-positive keratinocytes to retain proliferative competence. In these studies, E5 was found to have little effect on the levels of the epidermal growth factor receptor (EGFR) or on its phosphorylation status. This indicates that EGFR is not a target of E5 action. Our results propose a role for high risk HPV E5 in modulation of late viral functions through activation of proliferative capacity in differentiated cells. We suspect that the primary target of E5 is a membrane protein or receptor that then acts to alter the levels or activities of cell cycle regulators.

The ND10 component promyelocytic leukemia protein relocates to human papillomavirus type 1 E4 intranuclear inclusion bodies in cultured keratinocytes and in warts

Human papillomavirus type 1 (HPV1) E4 protein is associated with cytoplasmic and nuclear inclusions in productively infected keratinocytes. Here we have used transient expression of HPV1 E4 (also known as E1E4) protein in keratinocytes to reproduce formation of E4 inclusions. Immunofluorescence analysis showed that progressive formation of inclusions correlated with diminished colocalization between E4 and keratin intermediate filaments (IFs). Our results support a model in which the HPV1 E4-keratin IF association is transient, occurring only at an early stage of inclusion formation. We also demonstrate that E4 induces relocation of the promyelocytic leukemia protein (PML) from multiple intranuclear speckles (ND10 bodies) to the periphery of nuclear E4 inclusions and that this activity is specific to full-length E4 protein. Analysis of HPV1-induced warts demonstrated that nuclear PML-E4 inclusions were present in productively infected keratinocytes, indicating that reorganization of PML occurs during the virus's replication cycle. It has been suggested that ND10 bodies are the sites for papillomavirus genome replication and virion assembly. Our finding that E4 induces reorganization of ND10 bodies in vitro and in vivo is further strong evidence that these domains play an important role in the papillomavirus life cycle. This study indicates that HPV1 is analogous to other DNA viruses that disrupt or reorganize ND10 domains, possibly to increase efficiency of virus infection. We hypothesize that HPV1 E4-induced reorganization of PML is necessary for efficient replication of the virus during the virus-producing phase.

New markers for cervical dysplasia to visualise the genomic chaos created by aberrant oncogenic papillomavirus infections

Extensive research over the past 20 years provided strong evidence that persistent infections with high risk type human papillomaviruses (HR-HPVs) cause cervical cancer. However, depending on their age, more than 20% of normal women are infected with these viruses and only very few develop clinically relevant dysplastic lesions or even cancer. During an acute HPV infection, expression of viral genes, in particular the viral E6 and E7 oncogenes is restricted to differentiated epithelial cells, which lost the capability to replicate their genomes and are therefore at no further risk for acquiring functionally relevant mutations upon genotoxic damage. High grade cervical dysplasia, however, is initiated by deregulated expression of viral oncogenes in replicating epithelial stem cells. Here, the E6-E7 gene products submerge control of the cell cycle and mitotic spindle pole formation through complex interactions with various cellular protein complexes and induce severe chromosomal instability. The detailed molecular analysis of these interactions allowed to define new biomarkers for dysplastic cervical cells. E7 for example induces increasing expression of the cyclin dependent kinase inhibitor p16(ink4a) in dysplastic cells. This can be used to identify dysplastic cells in histological slides, cytological smears or samples taken for biochemical analyses with an yet unmet fidelity. Detection of specific viral mRNAs derived from integrated HPV genomes in advanced precancers can be used to identify lesions with a particularly high risk for progression into invasive carcinomas (APOT assay). These new markers will result in a modified classification of cervical precancers and improved screening assays. Here, we review the basic concept and potential clinical applications of these new developments.

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.

Life cycle heterogeneity in animal models of human papillomavirus-associated disease

Animal papillomaviruses are widely used as models to study papillomavirus infection in humans despite differences in genome organization and tissue tropism. Here, we have investigated the extent to which animal models of papillomavirus infection resemble human disease by comparing the life cycles of 10 different papillomavirus types. Three phases in the life cycles of all viruses were apparent using antibodies that distinguish between early events, the onset of viral genome amplification, and the expression of capsid proteins. The initiation of these phases follows a highly ordered pattern that appears important for the production of virus particles. The viruses examined included canine oral papillomavirus, rabbit oral papillomavirus (ROPV), cottontail rabbit papillomavirus (CRPV), bovine papillomavirus type 1, and human papillomavirus types 1, 2, 11, and 16. Each papillomavirus type showed a distinctive gene expression pattern that could be explained in part by differences in tissue tropism, transmission route, and persistence. As the timing of life cycle events affects the accessibility of viral antigens to the immune system, the ideal model system should resemble human mucosal infection if vaccine design is to be effective. Of the model systems examined here, only ROPV had a tissue tropism and a life cycle organization that resembled those of the human mucosal types. ROPV appears most appropriate for studies of the life cycles of mucosal papillomavirus types and for the development of prophylactic vaccines. The persistence of abortive infections caused by CRPV offers advantages for the development of therapeutic vaccines.

Gene Expression of Genital Human Papillomaviruses and Considerations on Potential Antiviral Approaches

Genital human papillomaviruses (HPVs) are carcinogenic to humans and are associated with most cases of cervical cancer, genital and laryngeal warts, and certain cutaneous neoplastic lesions. Five of the more than 50 known genital HPV types, HPV-6, -11, -16, -18 and -31, have become the models to study gene expression. The comparison of the studies of these five viruses and analyses of the genomic sequences of those genital HPV types that have not been transcriptionally studied make it likely that genital HPVs share most strategies for regulating their transcription. These strategies are quite different from those of unrelated human and animal papillomaviruses. Among these common properties are (i) a specific promoter structure allowing for fine-tuned negative feedback, (ii) a transcriptional enhancer that is specific for epithelial cells, (iii) regulation by progesterone and glucocorticoid hormones, (iv) silencers, whose principal function appears to be transcriptional repression in the basal layer of infected epithelia, (v) specifically positioned nucleosomes that mediate the functions of some enhancer and the silencer factors, (vi) nuclear matrix attachment regions that can, under different conditions, repress or stimulate transcription, and (vii) as yet poorly understood late promoters positioned very remote from the late genes. Most of these properties are controlled by cellular proteins that, due to their simultaneous importance for cellular processes, may not be useful as HPV-specific drug targets. It should be possible, however, to target complex cis-responsive elements unique to these HPV genomes by nucleotide sequence-specific molecules, such as antisense RNA, polyamides and artificial transcription factors. The application of small molecule-based drugs may be restricted to target proteins encoded by the HPV DNA, such as the replication factor E1 and the transcription/replication factor E2.

Comparative analysis of the intracellular location of the high- and low-risk human papillomavirus oncoproteins

We have compared the intracellular location of the HPV E6 and E7 proteins from high- and low-risk virus types. While high-risk HPV E7 displays diffuse nuclear expression, low-risk E7 has a nuclear punctuate pattern of expression. Similarly, while high-risk E6 is expressed throughout the cell, low-risk E6 is again predominantly nuclear with a punctuate pattern of expression. Both low-risk viral oncoproteins show colocalization with PML, whereas high-risk viral proteins do not. Finally, inhibition of the proteasome pathway results in a dramatic nuclear accumulation of high-risk E6 protein. These results demonstrate fundamental differences in the localization of these viral oncoproteins within the cell and offer alternative explanations for their respective differences in pathology.

Proteasome-mediated regulation of the hDlg tumour suppressor protein

The Dlg tumour suppressor protein is intimately involved in the control of cell contact and polarity. Previous studies have shown that hDlg is a target for a number of viral transforming proteins. In particular, the high risk human papillomavirus (HPV) E6 proteins target hDlg for proteasome-mediated degradation, an activity that appears to contribute to HPV-induced malignancy. However, little information is available concerning the normal regulation of hDlg. In this study we have investigated the role of the proteasome in the regulation of endogenous hDlg protein levels in epithelial cell lines. We demonstrate that hDlg is, indeed, degraded via the proteasome both in the presence and absence of HPV, in a fashion that is dependent on the ability of the cells to form cell junctions. By western blot and immunofluorescence analysis we show that hDlg is efficiently degraded in isolated cells; however, upon cell-cell contact, hDlg is both hyper-phosphorylated and stabilised. Strikingly, in both transformed rodent cells and undifferentiated cervical cancer cells, this ability to stabilise Dlg upon increased cell density is lost. These results demonstrate a complex pattern of hDlg regulation by phosphorylation and proteasome degradation in response to cell contact. Loss of this regulation probably represents a significant step in the development of malignancy.

The human papillomavirus E6 protein and its contribution to malignant progression

The human papillomavirus (HPV) E6 protein is one of three oncoproteins encoded by the virus. It has long been recognized as a potent oncogene and is intimately associated with the events that result in the malignant conversion of virally infected cells. In order to understand the mechanisms by which E6 contributes to the development of human malignancy many laboratories have focused their attention on identifying the cellular proteins with which E6 interacts. In this review we discuss these interactions in the light of their respective contributions to the malignant progression of HPV transformed cells.

Detection of Viral DNA and E4 Protein in Basal Keratinocytes of Experimental Canine Oral Papillomavirus Lesions

We studied experimental canine oral papillomavirus (COPV) infection by in situ hybridization and immunohistochemistry of weekly biopsies. After 4 weeks, viral DNA in rete ridges suggested a keratinocyte stem cell target. Abundant viral DNA was seen in E4-positive cells only. E4 was predominantly cytoplasmic but also nuclear, being concentrated in the nucleoli during wart formation. Infected cells spread laterally along the basal layer and into the parabasal layers, accompanied by E7 transcription and increased mitoses. Most of the lower epithelium was positive for viral DNA, but, in mature warts, higher levels of E4 expression and genome amplification occurred in only sporadic superficial cells. L1 expression was late and in only a subset of E4-positive cells. During regression, viral DNA was less abundant in deep epithelial layers, suggesting downregulation of replication prior to replacement of infected cells from beneath. Detection of viral DNA in post-regression tissue indicated latent infection.

Association of the human papillomavirus type 11 E1()E4 protein with cornified cell envelopes derived from infected genital epithelium

The cornified cell envelope (CCE) is an insoluble matrix of covalently linked proteins assembled in differentiating keratinocytes, providing a barrier against external insults. CCEs derived from HPV 11-infected tissue are fragile compared to those derived from healthy epithelium. To study a possible role for the E1()E4 protein, HPV 11-infected epithelium was examined for the distribution of this protein and three CCE proteins. CCEs were then purified from genital epithelium, fragmented, washed to remove nonassociated proteins, and analyzed for E1()E4 protein. In HPV 11-infected tissue, the E1()E4 protein was detected in the region of the CCE in differentiated keratinocytes. Loricrin and cytokeratin 10 (K10) were absent in E1()E4-positive cells, and E1()E4 protein was not detected in cells containing these proteins. E1()E4 protein was detected in immunoblots as a 10- to 11-kDa doublet in extracts of intact CCEs from infected tissue and in extracts of CCE fragments prepared without using reducing agents. Extraction with reducing agents eliminated E1()E4 detection, suggesting that disulfide bonding was involved in the association with CCE fragments. In addition, cyanogen bromide degradation experiments, immunofluorescence, and immunoelectron microscopy provided evidence that E1()E4 protein was associated with CCE fragments by covalent bonds other than disulfide bonds. We conclude that E1()E4 protein expression is associated with profound alterations in detection of loricrin and K10 in HPV 11-infected genital epithelium. The E1()E4 protein copurified with CCEs derived from infected epithelium and could be identified in CCE fragments, suggesting a possible role for E1()E4 in the development of CCE abnormalities.

The E1E4 protein of human papillomavirus type 16 associates with a putative RNA helicase through sequences in its C terminus

Human papillomavirus type 16 (HPV16) infects cervical epithelium and is associated with the majority of cervical cancers. The E1E4 protein of HPV16 but not those of HPV1 or HPV6 was found to associate with a novel member of the DEAD box protein family of RNA helicases through sequences in its C terminus. This protein, termed E4-DBP (E4-DEAD box protein), has a molecular weight of 66,000 (66K) and can shuttle between the nucleus and the cytoplasm. It binds to RNA in vitro, including the major HPV16 late transcript (E1E4. L1), and has an RNA-independent ATPase activity which can be partially inhibited by E1E4. E4-DBP was detectable in the cytoplasm of cells expressing HPV16 E1E4 (in vivo and in vitro) and could be immunoprecipitated as an E1E4 complex from cervical epithelial cell lines. In cell lines lacking cytoplasmic intermediate filaments, loss of the leucine cluster-cytoplasmic anchor region of HPV16 E1wedgeE4 resulted in both proteins colocalizing exclusively to the nucleoli. Two additional HPV16 E1E4-binding proteins, of 80K and 50K, were identified in pull-down experiments but were not recognized by antibodies to E4-DBP or the conserved DEAD box motif. Sequence analysis of E4-DBP revealed homology in its E4-binding region with three Escherichia coli DEAD box proteins involved in the regulation of mRNA stability and degradation (RhlB, SrmB, and DeaD) and with the Rrp3 protein of Saccharomyces cerevisiae, which is involved in ribosome biogenesis. The synthesis of HPV16 coat proteins occurs after E1E4 expression and genome amplification and is regulated at the level of mRNA stability and translation. Identification of E4-DBP as an HPV16 E1E4-associated protein indicates a possible role for E1E4 in virus synthesis.

Abnormalities of cornified cell envelopes isolated from human papillomavirus type 11-infected genital epithelium

Keratinocytes are the predominant cells in human skin. As keratinocytes differentiate, the nuclei are lost and the cornified cell envelope (CCE) develops, forming a covalently cross-linked, insoluble structure under the cell membrane. Layers of anuclear CCEs in the stratum corneum provide a barrier against water loss and mechanical damage and are a first line of immunologic defense. Infection of keratinocytes with human papillomaviruses (HPVs) induces proliferation and abnormalities including retention of nuclei in the stratum corneum and perinuclear halo formation. For effective transmission, HPV virions must be released from the CCE, a normally very durable structure. Therefore, it is likely that HPV infection affects the CCE in a manner that would facilitate virion release. To investigate the effects of HPV 11 infection on morphology and fragility, CCEs were purified from infected and uninfected epithelium. CCEs isolated from uninfected epithelium were smooth, cuboidal, and sonicated into long coiled structures. In contrast, CCEs from HPV 11-infected epithelium were irregular in size and shape, with rough edges, and sonicated into small fragments. In addition, the thickness of CCEs from HPV 11-infected tissue was 65% that of uninfected epithelium. Immunohistochemical analysis demonstrated that in contrast to uninfected epithelium, loricrin, the major component of the CCE, was abnormally distributed in the differentiated layers of HPV 11-infected epithelium. We conclude that in addition to the previously described epithelial abnormalities induced by HPV, the CCE is also affected by infection in ways that may facilitate transmission of virus from person to person.

Synthesis of viral DNA and late capsid protein L1 in parabasal spinous cell layers of naturally occurring benign warts infected with human papillomavirus type 1

We investigated human papillomavirus type 1 (HPV1)-specific transcription, viral DNA replication, and viral protein expression in naturally occurring benign tumors by in situ hybridization, 5-bromodeoxyuridine (BrdU) incorporation, and immunohistochemistry and obtained results different from other HPV-infected benign tumors characterized so far. Moderate amounts of transcripts with a putative coding potential for E6/E7, E1, and E2 were demonstrated from the first subrabasal cell layer throughout the stratum spinosum and granulosum. In addition very large amounts of E4 and L1 transcripts were present in the same epithelial layers. This finding was substantiated by the demonstration of L1 and E4 protein already in the bottom-most spinous cell layer. Furthermore massive amplification of the viral DNA as measured by BrdU incorporation and different methods of in situ hybridization took place in the lowest 5 to 10 suprabasal cell layers. These findings are in contrast to the assumption that late gene expression and viral DNA synthesis are restricted to the more differentiated cell layers of the epithelium and point to differences in the regulation of the vegetative life cycle between different papillomavirus types.

The role of the E6-p53 interaction in the molecular pathogenesis of HPV

Human papillomaviruses (HPVs) are associated with a number of clinical conditions, of which the most serious is cervical carcinoma. The E6 protein of the oncogenic, mucosal-specific HPV types has been shown to complex with p53 and, as a result, target it for rapid proteasome-mediated degradation. As a consequence, p53's growth-arrest and apoptosis-inducing activities are abrogated. Since p53 is frequently wild type in cervical cancers, unlike other cancers in which it is often mutated, the notion has arisen that E6's activity with respect to p53 is equivalent to an inactivating mutation of p53. In addition, several studies have shown that the pathways both upstream and downstream of p53 are intact in cervical cancers; this suggests the potential importance of the E6 - p53 interaction for therapeutic intervention. However, like all viral oncoproteins, E6 is a multifunctional protein and a plethora of other cellular targets has been identified. Indeed, E6's interactions with some of these additional targets appear to be equally important in the pathogenesis of HPV, and may also represent valid targets for therapeutic intervention.

The interaction between p53 and papillomaviruses

The p53 tumour suppressor is one of the host's principal defences against viral replication and subsequent cell transformation. The human papillomaviruses have evolved an elaborate strategy whereby the viral E6 proteins directly target p53 for ubiquitin mediated degradation and thus overcome the inhibitory effects of p53. However, a more detailed picture of the HPV*b1p53 interaction is now emerging in which there is a complex interplay between both positive and negative effectors of these interactions. This demonstrates the existence of a finely balanced virus*b1host relationship which, on rare occasion, fails and initiates the processes that ultimately lead to malignancy.

Potential strategies utilised by papillomavirus to evade host immunity

The co-evolution of papillomaviruses (PV) and their mammalian hosts has produced mechanisms by which PV might avoid specific and non-specific host immune responses. Low level expression of PV proteins in infected basal epithelial cells, together with an absence of inflammation and of virus-induced cell lysis, restricts the opportunity for effective PV protein presentation to immunocytes by dendritic cells. Additionally, PV early proteins, by a range of mechanisms, may restrict the efficacy of antigen presentation by these cells. Should an immune response be induced to PV antigens, resting keratinocytes (KC) appear resistant to interferon-gamma-enhanced mechanisms of cytotoxic T-lymphocyte (CTL)-mediated lysis, and expression of PV antigens by resting KC can tolerise PV-specific CTL. Thus, KC, in the absence of inflammation, may represent an immunologically privileged site for PV infection. Together, these mechanisms play a part in allowing persistence of PV-induced proliferative skin lesions for months to years, even in immunocompetent hosts.

Generation of native bovine mAbs by phage display

Modeling of disease pathogenesis and immunity often is carried out in large animals that are natural targets for pathogens of human or economic relevance. Although murine mAbs are a valuable tool in identifying certain host/pathogen interactions, progress in comparative immunology would be enhanced by the use of mAbs isolated from the host species. Such antibodies would reflect an authentic host immune response to infection or vaccination, and as they are host derived, would allow the application of in vivo experiments that previously have been unrealizable in large animals because of induction of an antispecies immune response. The advent of antibody phage display technology provides a way of producing host-derived mAbs in animals where the molecular genetics of Ig formation are known. Exploiting recent advances in the molecular immunology of cattle, we report here the design of an optimized phage display vector, pComBov, for the construction of combinatorial libraries of bovine Ig antigen-binding fragments (Fab) of native sequence. By using this system, we initially have generated and characterized a panel of bovine mAbs against a model antigen glutathione S-transferase. The isolated mAbs showed features typical of bovine Igs and recognized glutathione S-transferase with high specificity in ELISA and by Western blotting. The pComBov expression system can be readily adapted for the preparation of libraries from related ruminant species and advances the use of monoclonal reagents derived in this way for comparative studies in animals of economic importance.

Human papillomavirus oncoproteins alter differentiation-dependent cell cycle exit on suspension in semisolid medium

The suspension of keratinocytes containing episomal forms of the human papillomavirus (HPV)-31 genome in semisolid medium results in the induction of viral late functions. In this study, the suspension in semisolid medium was used to analyze how HPV deregulates the process of cell cycle exit during differentiation. In cells that contain the entire HPV-31 genome, induction of late protein synthesis was found to be linked with the expression of cyclin A. Consistent with analyses in organotypic rafts, the expression of the high-risk E7 oncoprotein alone was sufficient to retain cyclin A expression during suspension-induced differentiation. The cyclin-dependent kinase inhibitors (CKIs) p27 and p57 were found to be up-regulated in normal keratinocytes, as well as in the lines that express the HPV oncoproteins. The up-regulation of these CKIs is coincident with the inhibition of cyclin/cdk activity in normal keratinocytes. Cells expressing E7 were found to retain significant cdk2-associated kinase activity, although it was partially inhibited, coincident with CKI induction. When the phosphorylation state of Rb was examined during differentiation, cells expressing E7 retained phosphorylated forms of Rb, whereas Rb in normal keratinocytes was hypophosphorylated. As previously reported, E7-expressing cells were found to contain less Rb protein than normal keratinocytes. Interestingly, the Rb levels decreased during normal keratinocyte differentiation, and this differentiation-dependent reduction in Rb levels was enhanced by EG and E7 expression. This study identified proteins that may be critical for cell cycle regulation during normal epithelial differentiation and demonstrated that HPV oncoproteins alter their activities.