Localization and analysis of bovine papillomavirus type 1 transforming functions

Molecular Mechanisms of MmuPV1 E6 and E7 and Implications for Human Disease

Human papillomaviruses (HPVs) cause a substantial amount of human disease from benign disease such as warts to malignant cancers including cervical carcinoma, head and neck cancer, and non-melanoma skin cancer. Our ability to model HPV-induced malignant disease has been impeded by species specific barriers and pre-clinical animal models have been challenging to develop. The recent discovery of a murine papillomavirus, MmuPV1, that infects laboratory mice and causes the same range of malignancies caused by HPVs provides the papillomavirus field the opportunity to test mechanistic hypotheses in a genetically manipulatable laboratory animal species in the context of natural infections. The E6 and E7 proteins encoded by high-risk HPVs, which are the HPV genotypes associated with human cancers, are multifunctional proteins that contribute to HPV-induced cancers in multiple ways. In this review, we describe the known activities of the MmuPV1-encoded E6 and E7 proteins and how those activities relate to the activities of HPV E6 and E7 oncoproteins encoded by mucosal and cutaneous high-risk HPV genotypes.

Manipulation of Epithelial Differentiation by HPV Oncoproteins

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

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

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

Structural Insights in Multifunctional Papillomavirus Oncoproteins

Since their discovery in the mid-eighties, the main papillomavirus oncoproteins E6 and E7 have been recalcitrant to high-resolution structure analysis. However, in the last decade a wealth of three-dimensional information has been gained on both proteins whether free or complexed to host target proteins. Here, we first summarize the diverse activities of these small multifunctional oncoproteins. Next, we review the available structural data and the new insights they provide about the evolution of E6 and E7, their multiple interactions and their functional variability across human papillomavirus (HPV) species.

High-Risk Human Papillomavirus E7 Proteins Target PTPN14 for Degradation

The major transformation activity of the high-risk human papillomaviruses (HPV) is associated with the E7 oncoprotein. The interaction of HPV E7 with retinoblastoma family proteins is important for several E7 activities; however, this interaction does not fully account for the high-risk E7-specific cellular immortalization and transformation activities. We have determined that the cellular non-receptor protein tyrosine phosphatase PTPN14 interacts with HPV E7 from many genus alpha and beta HPV types. We find that high-risk genus alpha HPV E7, but not low-risk genus alpha or beta HPV E7, is necessary and sufficient to reduce the steady-state level of PTPN14 in cells. High-risk E7 proteins target PTPN14 for proteasome-mediated degradation, which requires the ubiquitin ligase UBR4, and PTPN14 is degraded by the proteasome in HPV-positive cervical cancer cell lines. Residues in the C terminus of E7 interact with the C-terminal phosphatase domain of PTPN14, and interference with the E7-PTPN14 interaction restores PTPN14 levels in cells. Finally, PTPN14 degradation correlates with the retinoblastoma-independent transforming activity of high-risk HPV E7.

High-risk human papillomaviruses (HPV) are the cause of cervical cancer, some other anogenital cancers, and a growing fraction of oropharyngeal carcinomas. The high-risk HPV E6 and E7 oncoproteins enable these viruses to cause cancer, and the mechanistic basis of their carcinogenic activity has been the subject of intense study. The high-risk E7 oncoprotein is especially important in the immortalization and transformation of human cells, which makes it a central component of HPV-associated cancer development. E7 oncoproteins interact with retinoblastoma family proteins, but for several decades, it has been recognized that high-risk HPV E7 oncoproteins have additional cancer-associated activities. We have determined that high-risk E7 proteins target the proteolysis of the cellular protein tyrosine phosphatase PTPN14 and find that this activity is correlated with the retinoblastoma-independent transforming activity of E7.

In vitro and Animal Models for Antiviral Therapy in Papillomavirus Infections

The need for antiviral therapies for papillomavirus infections is well recognized but the difficulties of reproducing the infectious cycle of papillomaviruses in vitro has hindered our understanding of virus-cell interactions and the regulation of viral gene expression during permissive growth. Recent advances in understanding the temporal expression and function of papillomavirus proteins has enabled consideration of a targeted approach to papillomavirus chemotherapy and in particular the inhibition of viral replication by targeting the E1 and E2 proteins. There are in vitro culture systems available for the screening of new chemotherapeutic agents, since significant advances have been made with culture systems which promote epithelial differentiation in vitro. However, to date, there are no published data which show that virions generated in vitro can infect keratinocytes and initiate another round of replication in vitro. In vivo animal models are therefore necessary to assess the efficacy of antivirals in preventing and treating viral infection, particularly for the low-risk genital viruses which are on the whole refractory to culture in vitro. Although papillomaviruses affect a wide variety of hosts in a species-specific manner, the animals most useful for modelling papillomavirus infections include the rabbit, ox, mouse, dog, horse, primate and sheep. The ideal animal model should be widely available, easy to house and handle, be large enough to allow for adequate tissue sampling, develop lesions on anatomical sites comparable with those in human diseases and these lesions should be readily accessible for monitoring and ideally should yield large amounts of infectious virus particles for use in both in vivo and in vitro studies. The relative merits of the various papillomavirus animal models available in relation to these criteria are discussed.

Expression and in Silico analysis of the recombinant bovine papillomavirus E6 protein as a model for viral oncoproteins studies

Bovine papillomaviruses (BPVs) are recognized as the causal agents of economical relevant diseases in cattle, associated with the development of tumors in skin and mucosa. The oncogenesis process is mainly associated with different viral oncoprotein expressions, which are involved in cell transformation. The expression and characterization of recombinant viral oncoproteins represent an attractive strategy to obtain biotechnological products as antibodies and potential vaccines, Thus, the aim of this work was to clone and express the BPV-1 and BPV-2 E6 recombinant proteins and perform in silico analysis in order to develop a strategy for the systematic study of other papillomaviruses oncoproteins. The results demonstrated that BPV-1 and BPV-2 E6 recombinant proteins were expressed and purified from bacterial system as well as its in silico analysis was performed in order to explore and predict biological characteristics of these proteins.

Bovine papillomavirus E5 and E7 oncoproteins in naturally occurring tumors: are two better than one?

Bovine papillomaviruses (BPVs) are oncogenic DNA viruses, which mainly induce benign lesions of cutaneous and/or mucosal epithelia in cattle. Thirteen (BPV 1-13) different viral genotypes have been characterized so far. BPVs are usually species-specific but BPV 1/2 may also infect equids as well as buffaloes and bison and cause tumors in these species. BPV-induced benign lesions usually regress, however occasionally they develop into cancer particularly in the presence of environmental carcinogenic co-factors. The major transforming protein of BPV is E5, a very short hydrophobic, transmembrane protein with many oncogenic activities. E5 contributes to cell transformation through the activation of the cellular β receptor for the platelet-derived growth factor (PDGFβ-r), it also decreases cell surface expression of major histocompatibility complex class I (MHCI) causing viral escape from immunosurveillance, and plays a role in the inhibition of the intracellular communication by means of aberrant connexin expression. E7 is considered as a weak transforming gene, it synergies with E5 in cell transformation during cancer development. E7 expression correlates in vivo with the over-expression of β1-integrin, which plays a role in the regulation of keratinocytes proliferation and differentiation. Additionally, E7 is involved in cell-mediated immune responses leading to tumour rejection, in anoikis process by direct binding to p600, and in invasion process by upregulation of Matrix metalloproteinase1 (MMP-1) expression. Studies on the role of BPV E5 and E7 oncoproteins in naturally occurring tumours are of scientific value, as they may shed new light on the biological role of these two oncogenes in cell transformation.

Bovine papillomavirus E7 oncoprotein binds to p600 in naturally occurring equine sarcoids

Studies regarding the functions of the bovine papillomavirus (BPV) E7 oncoprotein in vivo are lacking and no E7-mediated mechanism underlying mesenchymal carcinogenesis is known. Here, we show that the interaction between the 600 kDa retinoblastoma protein-associated factor (p600) and BPV E7, described in vitro in cultured cells, takes place in vivo in naturally occurring equine sarcoids. In these cancers we detect the expression of E7 and p600, and demonstrate that E7 and p600 co-localize and physically interact. Furthermore, intracellular signals involved in p600 functional activity are found not to be overexpressed, suggesting a different functional activity of p600 in naturally occurring carcinogenesis. Our results demonstrate, for the first time, that E7-p600 interaction occurs during the natural history of BPV-induced equine tumours, suggesting an important role for E7 in carcinogenesis. Finally, the system provides a suitable animal model of papillomavirus-associated cancer to test therapeutic vaccination against E7.

Bovine papillomaviruses, papillomas and cancer in cattle

Bovine papillomaviruses (BPV) are DNA oncogenic viruses inducing hyperplastic benign lesions of both cutaneous and mucosal epithelia in cattle. Ten (BPV 1-10) different viral genotypes have been characterised so far. BPV 1-10 are all strictly species-specific but BPV 1/2 may also infect equids inducing fibroblastic tumours. These benign lesions generally regress but may also occasionally persist, leading to a high risk of evolving into cancer, particularly in the presence of environmental carcinogenic co-factors. Among these, bracken fern is the most extensively studied. The synergism between immunosuppressants and carcinogenic principles from bracken fern and the virus has been experimentally demonstrated for both urinary bladder and alimentary canal cancer in cows whose diets were based on this plant. BPV associated tumours have veterinary and agricultural relevance in their own right, although they have also been studied as a relevant model of Human papillomavirus (HPV). Recent insights into BPV biology have paved the way to new fields of speculation on the role of these viruses in neoplastic transformation of cells other than epithelial ones. This review will briefly summarise BPV genome organization, will describe in greater detail the functions of viral oncoproteins, the interaction between the virus and co-carcinogens in tumour development; relevant aspects of immunity and vaccines will also be discussed.

Production of spliced DNA copies of the cottontail rabbit papillomavirus genome in a retroviral vector

The early region of the cottontail rabbit papillomavirus (CRPV) genome has been introduced into a retroviral vector and recombinant retroviruses, produced upon transfection of the psi 2 packaging cell line, have been used to infect NIH 3T3 cells. Spliced derivatives of the CRPV early region can be rescued from the infected cells. Sequence analysis demonstrates that the major splicing event observed in RNA in tumours is faithfully reproduced in this system. This splice generates a polycistronic mRNA that contains in its 5' portion the E7 open reading frame, or both E6 and E7, and at its 3' end a reading frame with codons for three amino acids from the N-terminus of E1 linked to codons for 100 amino acids from the C-terminus of the E4 region. Recombinant retroviruses containing intact or spliced CRPV sequences can now be used to introduce the viral genes efficiently into a variety of cell lines.

Criteria for establishing that a virus is oncogenic

To prove that a particular infectious agent causes a disease is much more difficult in human subjects than in other animals for both ethical and practical reasons. Where the disease is a malignant tumour with a long latent period the situation is even more difficult. For these reasons, it is often necessary to concentrate in the first instance on association of the virus with the disease, and this is discussed in the context of papillomaviruses. Association of a virus with a tumour may occur for a variety of reasons other than the virus being the cause of the tumour. This is illustrated by several examples of parvoviruses and DNA tumour viruses. Conversely, the absence of any sign of virus or viral nucleic acid in a tumour does not prove that the tumour was not induced by a virus. Apart from association of a virus with a tumour it is also necessary to show that the virus in question is oncogenic. Again this cannot normally be done directly, so that indirect evidence from animal experiments or from in vitro transformation is likely to be the best available alternative. In the final analysis the best proof of oncogenicity may be the effectiveness of intervention directed at the virus.

Bovine papillomavirus E7 oncoprotein inhibits anoikis

The bovine papillomavirus type 1 (BPV-1) E7 oncoprotein is required for the full transformation activity of the virus. Although BPV-1 E7 by itself is not sufficient to induce cellular transformation, it enhances the abilities of the other BPV-1 oncogenes to induce anchorage independence. We have been exploring the mechanisms by which E7 might affect the transformation efficiency of other viral oncoproteins and in particular whether it might protect cells from apoptosis. We report here that BPV-1 E6 and E7 can each independently inhibit anoikis, a type of apoptosis that is induced upon cell detachment. Using site-directed mutagenesis, we determined regions of the E7 protein that were essential for its antiapoptotic activity. The ability of E7 to inhibit anoikis did partially correlate with an ability to enhance anchorage independence of BPV-1 E6-transformed cells. In addition, the antiapoptotic activity of E7 also only partially correlated with its ability to bind p600, a cellular protein that has previously been reported to play a role in anoikis. We conclude that the contribution of E7 to BPV-induced cellular transformation may involve its ability to inhibit anoikis but that additional functional activities must also be involved.

Bovine papillomavirus type 1 infection is mediated by SNARE syntaxin 18

Events that lead to viral infections include the binding of the virus to the target cells, internalization of the virus into the cells, and the ability of the viral genome to be expressed. These steps are mediated by cellular and viral proteins and are temporally regulated. The papillomavirus capsid consists of two virally encoded capsid proteins, L1 and L2. Much is known about the role of the major capsid protein L1 compared to what is known of the role of the L2 protein. We identified the interaction of the L2 protein with SNARE protein syntaxin 18, which mediates the trafficking of vesicles and their cargo between the endoplasmic reticulum, the cis-Golgi compartment, and possibly the plasma membrane. Mutations of L2 residues 41 to 44 prevented the interaction of L2 protein with syntaxin 18 in cotransfection experiments and resulted in noninfectious pseudovirions. In this paper, we describe that syntaxin 18 colocalizes with infectious bovine papillomavirus type 1 (BPV1) pseudovirions during infection but does not colocalize with the noninfectious BPV1 pseudovirions made with an L2 mutant at residues 41 to 44. We show that an antibody against BPV1 L2 residues 36 to 49 (alpha L2 36-49) binds to in vitro-generated BPV1 pseudoviral capsids and does not coimmunoprecipitate syntaxin 18- and BPV1 L2-transfected proteins. alpha L2 36-49 was able to partially or completely neutralize infection of BPV1 pseudovirions and genuine virions. These results support the dependence of syntaxin 18 during BPV1 infection and the ability to interfere with infection by targeting the L2-syntaxin 18 interaction and further define the infectious route of BPV1 mediated by the L2 protein.

Bovine papillomavirus E7 transformation function correlates with cellular p600 protein binding

The E7 oncoprotein of bovine papillomavirus type 1 (BPV-1) is required for the full transformation activity of the virus. However, the mechanism by which E7 contributes to cellular transformation is unknown. To address this question, we used the proteomic approach of tandem affinity purification to identify cellular proteins that are in complex with E7, and identified the 600-kDa protein, p600, as a binding partner of E7. The ability of E7 to complex with p600 correlated with its ability to enhance anchorage independence of BPV-1 E6-expressing cells. Furthermore, E7 mutant proteins impaired in their ability to bind p600 were transformation defective. Additionally, knockdown of p600 reduced transformation of cells expressing both BPV-1 E6 and E7, as well as E6 alone, suggesting that the ability of E7 to transformed cells is mediated, at least in part, through its ability to bind p600. These data complement work that shows that HPV16 E7 also interacts with p600, and that this interaction correlates with the ability of HPV16 E7 to transform cells. These studies thus identify p600 as a shared target of the E7 proteins of multiple papillomaviruses.

A transactivator function of cottontail rabbit papillomavirus e2 is essential for tumor induction in rabbits

Infection of domestic rabbits with cottontail rabbit papillomavirus (CRPV) causes local papillomas which progress to carcinomas in more than 80% of cases. This animal model system therefore allows the identification of molecular mechanisms required for the induction and progression of epithelial tumors. The viral E2 protein stimulates both viral DNA replication and transcription, and these functions can be genetically separated. We introduced the respective mutations into CRPV E2 and found, in line with published data for other papillomavirus E2 proteins, that mutation of the highly conserved amino acid 37 or 73 resulted in replication-competent but transactivation-deficient E2 proteins, whereas E2 proteins with mutations at residue 39 were replication deficient and transactivation competent. The R37A, I73L, and I73A E2 mutants, showing a loss of transactivation function, and the R37K E2 mutant, which is still transactivation competent, were introduced into the whole genome of CRPV, which was then injected into the skin of rabbits. Strikingly, the ability to induce tumors within 6 weeks was abolished by each of the E2 mutations, in contrast to the tumor induction rate (93%) obtained with wild-type CRPV DNA. Two small papillomas induced by mutant E2 I73A CRPV DNA appeared as late as 12 or 24 weeks postinjection, were significantly smaller, and showed no further extension of growth. These data suggest that functionally conserved amino acids in the transactivation domain of E2 are also required for the induction and growth of epithelial tumors in rabbits infected with CRPV.

Papillomavirus E1 proteins: form, function, and features

The E1 proteins are the essential origin recognition proteins for papillomavirus (PV) replication. E1 proteins bind to specific DNA elements in the viral origin of replication and assemble into hexameric helicases with the aid of a second viral protein, E2. The resultant helicase complex initiates origin DNA unwinding to provide the template for subsequent syntheses of progeny DNA. In addition to ATP-dependent helicase activity, E1 proteins interact with and recruit several host cell replication proteins to viral origin, including DNA polymerase alpha and RPA. This review will compare the basic structures and features of the human (HPV) and bovine (BPV1) papillomaviruses with an emphasis on mechanisms of replication function.

Human papillomavirus: a review

Human papillomavirus infection remains a great source of morbidity and mortality. Progress in understanding the structure of HPV and its pathogenesis has led to a wide variety of possible new treatment modalities to combat HPV-related disease. Most HPV infections (whether high risk or low risk) resolve without any medical intervention. Persistent or progressive disease, however, remains difficult to treat. Although currently available therapies have proved efficacious and tolerable in the treatment of nongenital and genital warts, no single therapy is uniformly effective in eradicating persistent HPV infection. Cytodestructive methods, such as cryotherapy, remain the primary treatment modality for nongenital warts. Immune response modifiers, such as imiquimod, currently show the greatest promise in treating HPV-induced anogenital lesions, both with respect to complete response and in preventing recurrence. Human papillomavirus infection is one of the most common sexually transmitted diseases in the world, and cervical cancer still causes significant morbidity and mortality. Pap smear tests have greatly reduced the incidence and mortality of cervical cancer in developed countries. Additional research will focus on primary and secondary prevention strategies. Vaccines against high-risk HPV types are promising modalities currently under investigation to prevent HPV infections and possibly to treat them.

E1 protein of bovine papillomavirus 1 is not required for the maintenance of viral plasmid DNA replication

Derivatives of bovine papillomavirus 1 (BPV1) with temperature-sensitive and dominant-negative mutation the E1 gene were used to determine the requirement for E1 in the maintenance of viral plasmid DNA replication. The abilities of these mutant BPV1 genomes to replicate as nuclear plasmids were monitored at permissive (32 degrees C) and nonpermissive (37 degrees C) temperatures in mouse C127 cells. We found that the temperature-sensitive E1 mutant BPV1 genomes replicate as nuclear plasmids as efficiently as does wild-type BPV1 in C127 cells after shifting to the nonpermissive temperature. These findings indicate that BPV1 does not require E1 for the maintenance of viral plasmids.

Production of infectious bovine papillomavirus from cloned viral DNA by using an organotypic raft/xenograft technique

Bovine papillomavirus type 1 (BPV-1) induces fibropapillomas in its natural host and can transform fibroblasts in culture. The viral genome is maintained as an episome within fibroblasts, which has allowed extensive genetic analyses of the viral functions required for DNA replication, gene expression, and transformation. Much less is known about BPV-1 gene expression and replication in bovine epithelial cells because the study of the complete viral life cycle requires an experimental system capable of generating a fully differentiated stratified bovine epithelium. Using a combination of organotypic raft cultures and xenografts on nude mice, we have developed a system in which BPV-1 can replicate and produce infectious viral particles. Organotypic cultures were established with bovine keratinocytes plated on a collagen raft containing BPV-1-transformed fibroblasts. These keratinocytes were infected with virus particles isolated from a bovine wart or were transfected with cloned BPV-1 DNA. Several days after the rafts were lifted to the air interface, they were grafted on nude mice. After 6-8 weeks, large xenografts were produced that exhibited a hyperplastic and hyperkeratotic epithelium overlying a large dermal fibroma. These lesions were strikingly similar to a fibropapilloma caused by BPV-1 in the natural host. Amplified viral DNA and capsid antigens were detected in the suprabasal cells of the epithelium. Moreover, infectious virus particles could be isolated from these lesions and quantitated by a focus formation assay on mouse cells in culture. Interestingly, analysis of grafts produced with infected and uninfected fibroblasts indicated that the fibroma component was not required for productive infection or morphological changes characteristic of papillomavirus-infected epithelium. This system will be a powerful tool for the genetic analysis of the roles of the viral gene products in the complete viral life cycle.

Transactivation by the E2 protein of oncogenic human papillomavirus type 31 is not essential for early and late viral functions

The activation of transcription and of DNA replication are, in some cases, mediated by the same proteins. A prime example is the E2 protein of human papillomaviruses (HPVs), which binds ACCN6GGT sequences and activates heterologous promoters from multimerized binding sites. The E2 protein also has functions in replication, where it complexes with the virally encoded origin recognition protein, E1. Much of the information on these activities is based on transient-transfection assays as well as biochemical analyses; however, their importance in the productive life cycle of oncogenic HPVs remains unclear. To determine the contributions of these E2 functions to the HPV life cycle, a genetic analysis was performed by using an organotypic tissue culture model. HPV type 31 (HPV31) genomes that contained mutations in the N terminus of E2 (amino acid 73) were constructed; these mutants retained replication activities but were transactivation defective. Following transfection of normal human keratinocytes, these mutant genomes were established as stable episomes and expressed early viral transcripts at levels similar to those of wild-type HPV31. Upon differentiation in organotypic raft cultures, the induction of late gene expression and amplification of viral DNA were detected in cell lines harboring mutant genomes. Interestingly, only a modest reduction in late gene expression was observed in the mutant lines. We conclude that the transactivation function of E2 is not essential for the viral life cycle of oncogenic HPVs, although it may act to moderately augment late expression. Our studies suggest that the primary positive role of E2 in the viral life cycle is as a replication factor.

The papillomavirus E6 proteins

Specific types of human papillomaviruses (HPV) are strongly associated with the development of cervical cancer. The E6 gene from cancer-related HPVs has exhibited functions in tumorigenesis, regulation of transcription, telomerase, and apoptosis. Cancer-related HPVs E6 proteins bind the tumor suppressor p53 and promotes its degradation through an ubiquitin-dependent pathway. Several additional cellular E6-binding proteins have recently been identified and implicated in playing roles in p53-independent functions of E6.

The bovine papillomavirus E6 oncoprotein interacts with paxillin and disrupts the actin cytoskeleton

The E6 oncoprotein of bovine papillomavirus type 1 (BPV-1) has been shown to transform cells through a p53-independent pathway, but its transforming mechanism is unknown. Here we demonstrate in vitro and in vivo interactions between BPV-1 E6 and the focal adhesion protein paxillin. The ability of BPV-1 E6 to complex with paxillin correlated with its ability to transform; E6 mutant proteins impaired in their transformation function also were impaired in their abilities to bind paxillin. E6 binding to paxillin also may contribute to the carcinogenic potential of the human papillomavirus (HPV); we were able to show in vitro binding of paxillin to the E6 proteins of the cancer-associated type HPV 16 but not of the nononcogenic types 6 and 11. The association of E6 with paxillin was affected by depolymerization of the actin fiber network, and overexpression of BPV-1 E6 led to disruption of actin fiber formation. Disruption of the actin cytoskeleton is a characteristic of many transformed cells, and, in BPV-1 transformed cells, may be mediated by BPV-1 E6 through its interaction with paxillin.

Bovine papillomavirus type 1 E2 transcriptional regulators directly bind two cellular transcription factors, TFIID and TFIIB

The bovine papillomavirus type 1 (BPV-1) E2 translational open reading frame encodes three proteins that regulate viral transcription and DNA replication: the E2 transcriptional activator (E2TA), the E2 transcriptional repressor (E2TR) and the E8/E2 transcriptional repressor (E8/E2TR). E2TA is a strong activator of papillomaviral promoters and is required for viral DNA replication. E2TR and E8/E2TR inhibit the activities of E2TA but also possess weak transactivational properties of their own. Two components of the cellular transcription apparatus, TFIID and TFIIB, have previously been shown to associate with other viral and cellular transcriptional activators. We present evidence here that E2TA, the full-length E2 open reading frame gene product, directly binds both of these transcription factors in vitro. Glutathione S-transferase E2TA (GST-E2TA) fusion protein bound in vitro-synthesized TATA-box-binding protein (TBP), a component of TFIID, and in vitro-synthesized TFIIB. Likewise, GST-E2TA bound TFIID and TFIIB present in a nuclear extract from the human cervical cancer-derived cell line, HeLa. The binding of GST-E2TA to TBP and TFIIB required no additional mammalian factors, as shown by direct binding of GST-E2TA to bacterially synthesized recombinant TBP and recombinant TFIIB. The domain of E2TA required for its interaction with both TBP and TFIIB was localized to the C terminus of E2TA, a region also present in E2TR and E8/E2TR. This domain lies within the region of E2TA previously shown to confer cooperative DNA binding by E2TA and TBP and overlaps with the region of E2TA required for DNA binding and dimerization. Our findings, taken in context with previous studies, lead us to conclude that (i) cooperative DNA binding by E2 proteins and TBP is likely mediated by the direct binding of E2 proteins to TBP, (ii) the weak transcriptional transactivation by E2TR and E8/E2TR may result as a consequence of direct TBP and TFIIB binding by these proteins, and (iii) TBP and/or TFIIB binding may be required but is not sufficient for E2TA's strong transactivational activity.

Negative regulation of the bovine papillomavirus E5, E6, and E7 oncogenes by the viral E1 and E2 genes

Papillomaviruses induce benign squamous epithelial lesions that infrequently are associated with uncontrolled growth or malignant conversion. The virus-encoded oncogenes are clearly under negative regulation since papillomaviruses can latently infect cells and since different levels of viral oncogene expression are seen within the layers of differentiating infected epitheliomas. We used bovine papillomavirus type 1 (BPV-1) to investigate the mechanisms involved in the negative regulation of transformation. We found that the following two distinct and interacting mechanisms negatively regulate BPV-1 transformation effected by virally encoded trans-acting factors: (i) E2 repressors suppress transformation by the E6 and E7 oncogenes, and (ii) E1 and the E2 transactivator suppress transformation by the E6, E7, and E5 oncogenes. These systems interact in that the E2 repressors function to relieve the transformation suppression effected by the E1 and E2 transactivator genes. A BPV-1 mutant that lacked E2 repressors and E1 had greatly augmented transformation capacity. Analysis of this mutant revealed that the enhanced transformation was due to expression of the E6 and E7 genes in the absence of E5, revealing a previously unappreciated potency and synergy for the BPV-1 E6 and E7 oncogenes.

Kinetic and equilibrium binding studies of the human papillomavirus type-16 transcription regulatory protein E2 interacting with core enhancer elements

The human papillomaviruses (HPVs) are a family of DNA viruses which cause benign tumours of the skin and mucosa that infrequently progress to malignant carcinoma. The E2 open reading frame of HPV is thought to encode a papillomavirus-specific transcription factor which also has a role in viral replication. The E2 proteins of all papillomaviruses studied to date have been shown to bind specifically to the common conserved sequence ACC(N)6GGT found at multiple locations in their genomes. In the case of HPV-16, a 'high risk' genital papillomavirus, the E2 protein is thought to negatively regulate expression of the major viral transforming genes E6 and E7, which have been directly implicated in the oncogenic process. However, little information exists concerning the relative or absolute affinities of the native HPV-16 protein for its palindromic recognition sequences; moreover, interpretation of any transcription or replication phenomena attributed to this protein is more complicated in the absence of such data. Here we describe the overexpression, purification and characterisation of the C-terminal 89 amino acids of the protein encompassing the DNA binding/dimerisation domain. We show that the recombinant protein purified from E.coli by a combination of non-group-specific chromatography steps retains high biological activity and is able to bind to all sites in the HPV-16 genome with high affinity (approximately 8 x 10(-11) M). In addition, kinetic studies show that the E2-DNA complexes are very stable, with half-lives ranging from 2.15 to greater than 240 min, and that nucleotides internal and external to the conserved palindrome appear to influence stability.

Role of transcriptional repressors in transformation by bovine papillomavirus type 1

Transformation of rodent cells by bovine papillomavirus type 1 (BPV-1) has been shown to require the direct contribution of the viral oncogenes encoded by the E5, E6, and E7 translational open reading frames (ORFs). It is also known that the viral E1 and E2 ORFs contribute indirectly to cellular transformation through their transcriptional modulation of these viral oncogenes. A mutant BPV-1 disrupted in two of the proteins encoded by the E2 ORF, the E2 transcriptional repressors, has a complex transformation phenotype. In this paper, we provide evidence that (i) this phenotype is not attributable to the absence of other viral genes that overlap the E2 repressor genes, (ii) the acquisition of transformation over time in cells harboring the E2 double-repressor mutant correlates with reversions of the mutations that disrupt one of the E2 repressor genes, E8/E2TR, and (iii) the initial transformation defect in the E2 double-repressor mutant can be rescued by disruption of the full-length protein encoded by the E1 ORF, the E1 transcriptional repressor. We propose a model by which the combination of E1 and E2 transcriptional repressors functions to modulate the transforming capacity of the virus.

Bovine papillomavirus E1 protein affects the host cell cycle phase fractions

C127 murine fibroblast cells were electroporated with a bovine papillomavirus E1 protein expression vector and examined by flow cytometry. E1 expressing cells (E1+) within the total cell population were distinguished from nonexpressing cells (E1-) by immunofluorescent staining with anti-E1 serum and a fluorescein-conjugated second antibody. Under conditions of saturation with the first and second antibodies, the specific green fluorescence reflected the level of intracellular E1 protein. Simultaneous staining with a DNA-specific dye, propidium iodide (PI), enabled the cell cycle distributions for the E1+ and E1- cell populations to be determined. It was found that the E1+ subpopulation had a reduced percentage of cells in G1 phase and an increased percentage of G2+M phase cells, compared to the E1- subpopulation. There was no significant difference in overall doubling time or percentage of noncycling cells in the E1+ vs. E1- populations, indicating that the change in cell cycle distribution was not due to a general activation or inhibition of cell growth by E1. Direct measurement of cell cycle phase fractions confirmed that the G1 phase was decreased and the G2+M phase was increased in E1 expressing cells. As these observations were made in the absence of other viral proteins or viral DNA replication, it suggests that the E1 protein exerts an effect on the host cell independent of its direct role in viral DNA replication. Thus, E1 may interact directly with the host cell cycle regulatory machinery.

Importance of the bovine papillomavirus P2443 promoter in the regulation of E2 and E5 expression

The full-length bovine papillomavirus E2 gene product (E2TA), which has a direct role in DNA replication and functions as a transcriptional activator, can be expressed from an unspliced mRNA transcribed from the P2443 promoter or from spliced mRNAs transcribed from other upstream promoters. The regulation of E2 expression from these promoters is still in question. In the background of wild-type protein coding sequences, this study identified the P2443 promoter as the major source of E2TA as well as E5 expression in C127 cells.

Interaction with the nuclear matrix of a chimeric construct containing a replication origin and a transcription unit

We have studied the interaction of a chimeric construct containing an origin of replication (from bovine papilloma virus) and a hormonally regulated transcription unit (long terminal repeat from the mouse mammary tumor virus, driving the v-Ha-ras gene) with the nuclear scaffold and matrix from mouse fibroblasts. We used two experimental approaches because the nuclear matrix protein composition depends largely on the isolation conditions, making its definition mostly operational. In situ studies and in vitro experiments performed in 1361.5 cells, a cell line in which multiple copies of the construct have been established, indicate that two interesting regions of the construct interact with the nuclear matrix. The first region is located in the v-Ha-ras gene 5'-flanking sequences. These sequences come from the Harvey virus and contain a piece of the virus like 30S (VL30) sequences in which the v-Ha-ras gene is embedded. This DNA fragment was coupled to the thymidine kinase (TK) promoter driving the reporter luciferase gene and assayed in transient transfection experiments. Its insertion, in the sense orientation, upstream of the TK promoter resulted in a moderate enhancement (2-3-fold) of the luciferase activity. The second region is the most interesting from a physiological point of view. It contains the plasmid maintenance sequence 1 (PMS-1) and the core origin of replication of the bovine papilloma virus. Differences in the results from in situ (nuclear scaffold) and in vitro (nuclear matrix) experiments suggest that the components involved in the interaction with PMS-1 and the viral origin of replication are different. This may be of importance in the context of the recently proposed view that PMS-1 could be part of a composite origin of replication and provide information at a distance.

Transient replication of human papillomavirus DNAs

Information on papillomavirus DNA replication has primarily derived from studies with bovine papillomavirus type 1 (BPV-1). Our knowledge of DNA replication of the human papillomaviruses (HPVs) is quite limited, in part because of the lack of a cell culture system capable of supporting the stable replication of HPV DNA. This study demonstrates that the full-length genomic DNAs of HPV types 11 and 18 (HPV-11 and HPV-18), but not HPV-16, are able to replicate transiently after transfection into several different human squamous cell carcinoma cell lines. This system was used to identify the viral cis and trans elements required for DNA replication. The viral origins of replication were localized to a region of the viral long control region. Like BPV-1, E1 and E2 were the only viral factors required in trans for the replication of plasmids containing the origin. Cotransfection of a plasmid expressing the E1 open reading frame (ORF) from HPV-11 with a plasmid that expresses the E2 ORF from HPV-6, HPV-11, HPV-16, or HPV-18 supported the replication of plasmid DNAs containing the origin regions of HPV-11, HPV-16, or HPV-18, indicating that there are functions shared among the corresponding E1 and E2 proteins and origins of these viruses. Although HPV-16 genomic DNA did not replicate by itself under experimental conditions that supported the replication of HPV-11 and HPV-18 genomic DNAs, expression of the HPV-16 early region functions from a strong heterologous promoter supported the replication of a cotransfected plasmid containing the HPV-16 origin of replication. This finding suggests that the inability of the HPV-16 genomic DNA to replicate transiently in the cell lines tested was most likely due to insufficient expression of the viral E1 and/or E2 genes required for DNA replication.

The bovine papillomavirus constitutive enhancer is essential for viral transformation, DNA replication, and the maintenance of latency

Bovine papillomavirus type 1 (BPV-1) has served as the prototype papillomavirus for the study of viral transcription, DNA replication, and latency. However, no cis essential transcription control regions which are necessary for both transformation and replication of BPV-1 or any other papillomavirus have yet been defined. We have found that BPV-1 mutants with deletions in the long control region were defective for transformation and replication, with the essential region in the 5' long control region corresponding to the previously defined BPV-1 constitutive enhancer (S. B. Vande Pol and P. M. Howley, J. Virol. 64:5420-5429, 1990). BPV-1 mutants deleted of the constitutive enhancer could be complemented in trans by the full-length virally encoded E2 transactivator and replication factor (E2TA) and in cis by the simian virus 40 enhancer. The constitutive enhancer induced the production of E2TA by activating all the major viral early promoters upstream of the E2 open reading frame. Complementation experiments using a temperature-sensitive E2TA mutant indicated that the constitutive enhancer was necessary for the maintenance of viral DNA replication within latently infected cells and implied that viral transcription under the regulation of the constitutive enhancer may be controlled during the cell cycle. The constitutive enhancer is a master regulatory control region for establishing and maintaining BPV-1 latency, and its characteristics reveal some analogies with cell type-specific enhancer elements recognized in the human papillomaviruses.

In vitro expressed HPV 8 E6 protein does not bind p53

The oncogene E6 of human papillomavirus 8, which is associated with skin cancers in epidermodysplasia verruciformis, was transcribed and translated in vitro. The resulting 17 kDa protein did not bind to the cellular p53 in contrast to E6 of HPV16.

Bovine papillomavirus with a mutation in the E2 serine 301 phosphorylation site replicates at a high copy number

The E2 open reading frame of bovine papillomavirus type 1 (BPV-1) encodes at least three proteins with transcriptional regulatory properties. The full-length E2 open reading frame encodes a transcriptional transactivator, and the 3' region encodes two smaller polypeptides that repress E2-mediated transactivation. The full-length gene product is also required for viral DNA replication. We have demonstrated that the BPV-1 E2 polypeptides are phosphorylated primarily on two serine residues at a site adjacent to the carboxy-terminal DNA binding domain, which is common to all three E2 proteins (A. A. McBride, J. B. Bolen, and P. M. Howley, J. Virol. 63:5076-5085, 1989). These serine residues, at amino acid positions 298 and 301, were substituted with alanine residues in the context of the entire BPV-1 genome. The mutated BPV-1 genomes were introduced into rodent cell lines and assayed for focus formation, viral gene expression, and extrachromosomal viral DNA replication. Viral DNAs containing the E2 serine-to-alanine substitution mutants transformed C127 cells with efficiencies comparable to that of wild-type BPV-1. However, the viral genome containing the serine-to-alanine substitution at position 301 of the E2 polypeptide replicated to a copy number 20-fold higher than that of wild-type DNA.

Presence of human papilloma virus type 16 DNA sequences in human nonmelanoma skin cancers

The presence of human papillomaviruses (HPV) has been shown to be associated with the development and progression of invasive cancers of the genital tract, skin, and head and neck. In this study we analyzed 37 human nonmelanoma skin cancers (21 squamous cell carcinomas and 16 basal cell carcinomas) for the presence of HPV sequences. The polymerase chain reaction (PCR) was employed using primers designed to amplify DNA encoding the E6-E7 region of HPV types 6b/11, 16, and 18. HPV type 6b/11 and 18 sequences were absent from the DNA of all 37 tumors examined. However, HPV type 16 sequences were detected in four of 21 squamous cell carcinomas (SCC) (19%) and three of 16 basal cell carcinomas (BCC) (19%) as indicated on agarose gel electrophoresis by the presence of a single specific DNA band of predicted length. Furthermore, HPV type 16 sequences were absent in the DNA of normal skin from these seven skin cancer patients. The presence of HPV type 16 sequences in the seven skin tumors was confirmed by dot blot hybridization of PCR-amplified material to 32P-labeled HPV type 16, but not to HPV type 6/11 or 18-specific probes. These data indicate that HPV type 16, but not 6b/11 or 18, is associated with development of some human nonmelanoma skin cancers.

An element in the bovine papillomavirus late 3' untranslated region reduces polyadenylated cytoplasmic RNA levels

Expression of the two bovine papillomavirus type 1 (BPV-1) late genes, L1 and L2, coding for the two capsid proteins, is limited to terminally differentiated keratinocytes in bovine fibropapillomas. This pattern of expression is determined both by the activity of the late promoter and by the inhibition of late region expression in less well differentiated cells. Inhibition of L1 and L2 mRNA production in nonpermissive cells must occur since the late region potentially could be transcribed from early region promoters. Nuclear runoff analysis of the late region has demonstrated that up to 95% of transcripts which are initiated in the early region in nonpermissive cells terminate within the late region upstream of the late polyadenylation site (C. C. Baker and J. Noe, J. Virol. 63:3529-3534, 1989). However, very few of the primary transcripts which include the late polyadenylation site are processed into mRNA. In this study, we have used expression vectors to characterize an inhibitory element active in nonpermissive cells which is located in the late 3' untranslated region (3'UTR). While the late polyadenylation site is functional in these cells, a 53-bp element in the late 3'UTR reduces levels of polyadenylated cytoplasmic RNA. This element inhibited chloramphenicol acetyltransferase (CAT) expression 6- to 10-fold when cloned in the sense orientation into the 3'UTR of a CAT expression vector. No block to expression was seen when the fragment was cloned immediately downstream of the poly(A) site, in an intron upstream of the CAT coding sequence, or in an antisense orientation in the 3'UTR. When the same fragment was deleted from a BPV-1 L1 expression vector, a sixfold increase in mRNA levels was seen. Actinomycin D chase experiments using BPV-1 L1 expression vectors indicated that the element does not destabilize cytoplasmic polyadenylated RNA. Therefore, the element must act before the mature mRNA reaches the cytoplasm. The data presented are consistent with effects on nuclear stability and/or inhibition of polyadenylation or nuclear transport.

Neovascularization is associated with a switch to the export of bFGF in the multistep development of fibrosarcoma

In a transgenic mouse model, dermal fibrosarcomas develop in a pathway comprised of at least three stages: mild fibromatosis, aggressive fibromatosis, and fibrosarcoma. The latter two stages are highly vascularized when compared with both the normal dermis and the initial mild lesion. Analysis of cell cultures derived from biopsies of these lesions has revealed that basic fibroblast growth factor (bFGF) is synthesized in all three stages and in normal dermal fibroblasts derived from the same mice. Unexpectedly, there is a change in the localization of bFGF from its normal cell-associated state to extracellular release in the latter two stages, which is concomitant both with the neovascularization seen in vivo and with the tumorigenicity of these cell lines. Thus, in this multistep tumorigenesis pathway there appears to be a discrete switch to the angiogenic phenotype that correlates with the export of bFGF, a known angiogenic factor.

Formation of the complex of bovine papillomavirus E1 and E2 proteins is modulated by E2 phosphorylation and depends upon sequences within the carboxyl terminus of E1

The 68-kDa bovine papillomavirus (BPV) type 1 replication protein E1 and the 48-kDa transactivator protein E2 form a complex that specifically binds DNA [Mohr, I.J., Clark, R., Sun, S., Androphy, E.J., MacPherson, P. & Botchan, M.R. (1990) Science 250, 1694-1699]. We have confirmed this observation and shown that the E1-E2 complex binds to DNA fragments that contain the BPV plasmid maintenance sequence 1 and a site for the initiation of bidirectional BPV DNA synthesis. The E1 protein was found to bind preferentially to non- or underphosphorylated species of E2, suggesting that the phosphorylation state of E2 modulates the association of the two proteins. Replication-deficient E1 mutants with single amino acid substitutions and deletions in the carboxyl terminus failed to interact with E2, indicating that a region in the E1 carboxyl terminus is required for E1 to interact with E2. Our results suggest that the replication deficiency of some E1 mutants reflects their inability to associate with E2.

Characterization of the cis elements involved in basal and E2-transactivated expression of the bovine papillomavirus P2443 promoter

Transcriptional transactivation and repression by the viral E2 proteins are important regulatory mechanisms for the papillomaviruses. In the bovine papillomavirus type 1 (BPV-1), several viral promoters can be transactivated by E2 through E2-dependent enhancer elements located in the viral long control region (LCR), including promoters involved in E2 expression itself. This report demonstrates that the BPV-1 P2443 promoter is transactivated by E2-responsive elements in the LCR and that this promoter is responsible for a major part of the expression of the E2 and E5 gene products. Characterization of the cis elements involved in P2443 regulation indicated that the single E2-binding site directly upstream of P2443 is not required for either the E2 transactivation or for any E2 repression of the basal or transactivated activity of this promoter. Therefore, cooperative interactions between E2 bound at the LCR and E2 bound near P2443 do not have any role in the regulation of this promoter. Further definition of the cis regulatory elements of this promoter indicated that a binding site for the transcriptional factor Sp1 exists directly upstream of the P2443 TATA box and is critical for the basal level of transcription from this promoter. Disruption of this Sp1 site eliminated P2443 promoter activity in transient expression assays for E2 and E5 and resulted in a loss of transforming activity when introduced into the full viral genome.

The 68-kilodalton E1 protein of bovine papillomavirus is a DNA binding phosphoprotein which associates with the E2 transcriptional activator in vitro

The E1 open reading frame of bovine papillomavirus type 1 encodes factors necessary for extrachromosomal maintenance of the viral genome in transformed cells. To facilitate biochemical characterization of the gene products encoded by this open reading frame, we have expressed the full-length E1 protein in a baculovirus-insect cell system. This protein was found to be phosphorylated and localized to the nucleus of infected cells. The E1 protein alone has affinity for DNA but appears to lack specificity for viral sequences. In addition, we present evidence that the E1 protein interacts with the virally encoded transcriptional activator E2 in vitro. These results are consistent with a model in which the E1 protein, as part of a complex with E2, interacts with specific DNA sequences in the viral genome.

Targeting the E1 replication protein to the papillomavirus origin of replication by complex formation with the E2 transactivator

The mechanism by which transcription factors stimulate DNA replication in eukaryotes is unknown. Bovine papillomavirus DNA synthesis requires the products of the viral E1 gene and the transcriptional activator protein encoded by the E2 gene. Experimental data showed that the 68-kilodalton (kD) E1 protein formed a complex with the 48-kD E2 transcription factor. This complex bound specifically to the viral origin of replication, which contains multiple binding sites for E2. Repressor proteins encoded by the E2 open reading frame failed to complex with E1 suggesting that the 162-amino acid region of E2 that participates in transactivation contained critical determinants for interaction with E1. The physical association between a replication protein and a transcription factor suggests that transcriptional activator proteins may function in targeting replication initiator proteins to their respective origins of replication.

Proteins encoded by the bovine papillomavirus E1 open reading frame: expression in heterologous systems and in virally transformed cells

The E1 open reading frame (ORF) of bovine papillomavirus type 1 is required for the persistence of viral genomes as multicopy plasmid molecules in transformed rodent fibroblasts. E1 has been reported to contain two separate complementation groups (M and R, corresponding to N- and C-terminal domains, respectively) which regulate viral replication. However, E1 behaves as a single gene with respect to cell transformation and viral transcription. We examined the proteins translated from the entire ORF by using three antisera raised against E1 peptide or bacterial fusion proteins. The capacity of the whole ORF to encode a 72-kDa protein was demonstrated by translation of synthetic RNA in a reticulocyte lysate system, by microinjection of RNA into Xenopus oocytes, and by expression in recombinant baculoviruses and vaccinia viruses. In eucaryotic cells, this protein was found to be phosphorylated and targeted to the cell nucleus. In vitro translation also produced shorter peptides, containing only the E1 C-terminal domain, because of internal translation starts on the third and fourth methionine codons within E1 ORF. On the other hand, mammalian cells infected by vaccinia E1 recombinant virus contained additional larger E1 phosphoproteins (transient 85-kDa and stable 88-kDa species), likely representing processed forms of the 72-kDa species. The E1 72-kDa nuclear phosphoprotein was detected in bovine papillomavirus type 1-transformed cells. We report the biochemical characteristics of full-sized and truncated E1 proteins: (i) the C-terminal half of E1 ORF contains a phosphorylation site(s); (ii) the full-sized E1, but not the C-terminal protein, binds DNA, without indication for recognition of defined sequences, and critical determinants for this activity are likely confined to an N-terminal domain of the protein; (iii) covalent affinity labeling experiments performed on vaccinia virus-encoded E1 proteins with an ATP analog confirmed our previous observation of sequence similarities between the E1 C-terminal domain and the ATPase domain of simian virus 40 large T antigen.

Mapping of linear epitopes of human papillomavirus type 16: the E1, E2, E4, E5, E6 and E7 open reading frames

Certain types of human papillomavirus (HPV), especially HPV type 16, are associated with proliferative lesions of the cervix uteri that can progress to malignancy. In order to map the linear epitopes of the HPV-encoded proteins, we have synthesized the predicted amino acid sequences of the open reading frames (ORFs) in the early region of HPV 16, as a set of 94 synthetic 20-residue peptides overlapping each other with 5 amino acids. The peptides were tested for reactivity with IgA, IgG and IgM antibodies in the sera of 30 patients with HPV 16-carrying cervical neoplasia. The EI ORF had only low immunoreactivity, but several relatively minor epitopes were identified in the carboxyterminal part. The E2 ORF was found to contain several epitopes that were highly immunoreactive with a majority (up to 87%) of the cervical cancer patients' sera. The E4 ORF had one major, regularly IgA- and IgG-reactive epitope, whereas the E5 and E6 ORFs had only a few minor epitopes. The E7 ORF had several epitopes that were highly immunoreactive, but only with a minority of patients' sera. The 10 most immunoreactive peptides were also analyzed for immunoreactivity with 60 control sera, of which 22 were derived from patients with parotid gland tumors and 38 were derived from healthy volunteers. Most of the peptides were also immunoreactive with the control sera. However, the IgA antibodies, and to some extent the IgG antibodies, were found at much lower levels among the controls.

Determinants of the ability of malignant fibroma virus to induce immune dysfunction and tumor dissemination in vivo

The relationship of virus-induced immunological dysfunction and tumor dissemination was studied using two related tumor-causing leporipoxviruses: malignant fibroma virus (MV) and Shope fibroma virus (SFV). Recombinant viruses, produced by transferring MV's 10.7 kb BamHI C fragment to SFV, replicate in lymphocytes and suppress lymphocyte function in vitro. Those recombinants that replicate in lymphocytes and suppress lymphocyte function in vitro share about 3.5 kb from MV's C fragment. Some recombinants mimic MV in producing immune suppression and disseminated virus infection in vivo. Other recombinants, even some that are highly immunosuppressive in vitro (e.g. R71), only variably induce immune suppression in vivo, and do not cause disseminated disease. A segment of DNA from MV that transfers to Shope fibroma virus almost all of MV's virulence in vivo was identified.

Vaccinia recombinants expressing early bovine papilloma virus (BPV1) proteins: retardation of BPV1 tumour development

Papillomaviruses are aetiological agents of epithelial proliferative diseases in animals and in man. It was previously demonstrated that animals inoculated with live vaccinia recombinants expressing early proteins of polyoma virus resist challenge with polyoma-tumour cells, and this approach has been extended to the development of a vaccine against papillomavirus-transformed cells. Bovine papillomavirus type 1 (BPV1), a virus responsible for dermal lesions in cattle, is a prototype virus of the papillomavirus group. Independent vaccinia recombinant viruses expressing the early E1, E2, E5, E6 or E7 open reading frames of BPV1 were tested for their ability to direct the expression of the corresponding protein in cultured cells. Recombinants were then assessed for their ability to elicit anti-tumour immunity in Fischer rats seeded with BPV1-transformed syngeneic FR3T3 cells. Retardation of tumour growth was observed in animals vaccinated with recombinants expressing E5, E6 or E7.

The E2 protein of human papillomavirus type 16. Over-expression and purification of an active transcriptional regulator

The E2 open reading frame of human papillomavirus type 16 was inserted into the Escherichia coli vector pKK223-3, and expressed to greater than 15% of total cellular protein when induced with isopropyl beta-D-thiogalactopyranoside. The highest expressing clone was grown in bulk and the E2 protein purified to homogeneity by the following procedure: (a) isolation of the insoluble protein fraction; (b) extraction with urea; (c) quaternary amino-ethyl-Sepharose ion-exchange chromatography and (d) renaturation and chromatography on dextran sulphate. That the purified protein was fully functionally active was confirmed by its specific DNA-binding properties and its ability to activate gene transcription by over two orders of magnitude in an in vivo assay.