Viral proteins of bovine papillomavirus type 4 during the development of alimentary canal tumours

The serological detection of Bovine papillomavirus's E5 oncoprotein antibodies in horses

Equine sarcoids (ES) are known globally as the most frequent skin tumour affecting horses. These tumours affect the horse's monetary value, they can affect the horse's welfare and can be difficult and expensive to treat. Bovine papillomavirus (BPV) is considered to be the aetiological agent of this tumour, as BPV 1, 2 and 13 have been detected in ES. This is the only known natural cross species infection by a papillomavirus. The BPV genome can be divided into two coding regions: The early region E which encodes the transforming proteins E5, E6 and E7 as well as the replication and transcription regulatory proteins E1 and E2 and the late region encoding the structural proteins of the virus L1 and L2. The E5 oncoprotein is believed to downregulate MHC 1 and as a result, escapes an immune reaction with affecting the cells cycle and eventually allows the viral affected cells to proliferate into ES. We have constructed an ELISA test by utilising the C terminal peptide of the E5 oncoprotein and explored the possibility of IgG antibodies existence in horses to the E5 oncoprotein. For this study we have examined 136 horses, some showing ES lesions (80 horses) and some without ES lesions (56 horses). By using our ELISA test, we have shown that antibodies to the E5 oncoprotein are in fact present and that from a certain level seem to be found only in ES positive horses. Therefore, proving that an immune response to this protein can be expected.

Reprint of: cancer "causation" by infections--individual contributions and synergistic networks

The search for infectious agents playing a role in human carcinogenesis and their identification remain important issues. This could provide clues for a broader spectrum of cancers preventable by vaccination and accessible to specific therapeutic regimens. Yet, the various ways of interacting among different factors functioning synergistically and their different modes of affecting individual cells should bring to question the validity of the term "causation". It also should put a word of caution into all attempts to summarize criteria for "causality" of infectious agents in cancer development. At least in the opinion of these authors, we would be much better off avoiding these terms, replacing "causal factor" by "risk factor" and grading them according to their contribution to an individual's cancer risk.

Cancer "causation" by infections--individual contributions and synergistic networks

The search for infectious agents playing a role in human carcinogenesis and their identification remain important issues. This could provide clues for a broader spectrum of cancers preventable by vaccination and accessible to specific therapeutic regimens. Yet, the various ways of interacting among different factors functioning synergistically and their different modes of affecting individual cells should bring to question the validity of the term "causation". It also should put a word of caution into all attempts to summarize criteria for "causality" of infectious agents in cancer development. At least in the opinion of these authors, we would be much better off avoiding these terms, replacing "causal factor" by "risk factor" and grading them according to their contribution to an individual's cancer risk.

Bovine and Human Papillomaviruses

Fifty years ago, inoculation with bovine papillomavirus (BPV) was found to cause mesenchymal tumors of the skin in cattle and horses, as well as tumors of the bladder in cattle. Subsequent to these studies of BPVs, human papillomaviruses (HPVs) were found to cause cervical cancer resulting in intense research into papillomaviruses. During the past 50 years, the ways that HPVs and BPVs cause disease have been investigated, and both HPVs and BPVs have been associated with an increasingly diverse range of diseases. Herein, the biology, oncogenic mechanisms, and diseases associated with BPVs are compared with those of HPVs. As reviewed, there are currently significant differences between BPVs and HPVs. However, research 50 years ago into BPVs formed a prologue for the recognition that papillomaviruses have a significant role in human disease, and it is possible that future research may similarly reveal that BPVs are less different from HPVs than is currently recognized.

The E5 proteins

The E5 proteins are short transmembrane proteins encoded by many animal and human papillomaviruses. These proteins display transforming activity in cultured cells and animals, and they presumably also play a role in the productive virus life cycle. The E5 proteins are thought to act by modulating the activity of cellular proteins. Here, we describe the biological activities of the best-studied E5 proteins and discuss the evidence implicating specific protein targets and pathways in mediating these activities. The primary target of the 44-amino acid BPV1 E5 protein is the PDGF β receptor, whereas the EGF receptor appears to be an important target of the 83-amino acid HPV16 E5 protein. Both E5 proteins also bind to the vacuolar ATPase and affect MHC class I expression and cell-cell communication. Continued studies of the E5 proteins will elucidate important aspects of transmembrane protein-protein interactions, cellular signal transduction, cell biology, virus replication, and tumorigenesis.

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.

Expression of connexin 26 and bovine papillomavirus E5 in cutaneous fibropapillomas of cattle

Bovine papillomaviruses (BPVs) can infect epithelial cells and fibroblasts, inducing fibropapillomas in cattle. Gap junctions are communication channels between cells composed of connexins (Cxs). This study evaluated expression of Cx26 and the major BPV oncoprotein E5 in bovine cutaneous fibropapillomas. BPV DNA was amplified from 20/20 fibropapillomas and 3/3 samples of normal skin. All fibropapillomas (20/20) were positive by immunostaining for E5, whereas the three normal skin samples were negative. Cx26 was expressed faintly in the normal skin epithelium. Positive cytoplasmic and juxtanuclear immunoreactivity for Cx26 was evident in 18/20 (90%) fibropapillomas. Western blot analysis demonstrated higher expression of Cx26 in 6/6 fibropapillomas compared to normal skin samples.

Role of human papillomaviruses in esophageal squamous cell carcinoma

Esophageal cancer (EC) is responsible for almost half a million deaths worldwide annually and has a multifactorial etiology, which may account for its geographical variation in incidence. In the last 30 years the potential of human papillomaviruses (HPV) as oncogenes or co-factors in the tumorigenic process of esophageal squamous cell carcinoma (ESCC) has been widely studied. While the etiology of HPV in cervical and certain other anogenital and aerodigestive cancers has been established, results regarding its role in EC have been largely inconclusive. A causal association can be evaluated only with a case-control study, where normal controls are compared to ESCC cases for the presence of HPV. We reviewed all studies investigating ESCC tissue for HPV DNA and identified 139 that met our inclusion criteria, of which only 22 were case-control studies. Our results support previous findings of higher levels of HPV detection in high-risk ESCC regions than in areas of low risk. In addition, we confirm that the role of HPV in ESCC remains unclear, despite an accumulation of studies on the subject. The variations in investigative technique, study design and sample types tested may account for the lack of consistency in results. There is a need for a meta-analysis of all case-control studies to date, and for large, well-designed case-control studies with adequate power to investigate the association. The potential benefits of prophylactic HPV vaccines could be evaluated if HPV is identified as an etiological factor in EC, highlighting the need for further research in this area.

Relação entre a linfopenia e a persistência da papilomatose alimentar em bovinos intoxicados crônica e espontaneamente por samambaia (Pteridium aquilinum)

Papilomavírus bovino tipo 4 (BPV-4) é responsável pelo desenvolvimento de papilomas no trato alimentar superior (TAS) de bovinos. Os passos necessários para o crescimento, o desenvolvimento e a regressão dos papilomas estão intimamente relacionados com o estado imunológico do animal. A ingestão de samambaia (Pteridium aquilinum) tem sido relacionada como o principal fator envolvido na persistência da infecção pelo BPV-4 no TAS. A teoria que estabelece a relação entre papilomatose alimentar e a formação de CCEs sugere a produção de um estado imunossupressivo crônico pela planta, permitindo a persistência dos papilomas no TAS. Os papilomas serviriam então como sítios de desenvolvimento dos CCEs através da interação entre as proteínas do BPV-4 e os carcinógenos da samambaia. O objetivo deste estudo foi avaliar a relação entre a quantidade de linfócitos circulantes e a papilomatose alimentar em casos de intoxicação espontânea crônica por P. aquilinum em bovinos com CCE no TAS. Quarenta bovinos com CCEs no TAS foram avaliados quanto à idade, à intensidade da papilomatose alimentar no TAS e ao leucograma. Três bovinos tinham leucopenia e um apresentava neutrofilia. A média de linfócitos foi de 5.395 (±1.696) na papilomatose leve, 4.560 (±1.561) na moderada e 5.007 (±1.786) na acentuada. Não houve diferença estatisticamente significativa entre o grau de papilomatose, a idade e a quantidade de linfócitos circulantes. Imunossupressão por linfopenia foi um achado esporádico (três casos) neste estudo. Os resultados indicam que a persistência da papilomatose alimentar em casos espontâneos de intoxicação crônica por samambaia em bovinos não tem relação com a quantidade de linfócitos circulantes e que talvez esteja relacionada a outros fatores imunológicos.

Activated platelet-derived growth factor beta receptor expression, PI3K-AKT pathway molecular analysis, and transforming signals in equine sarcoids

The equine sarcoid is the most common dermatologic neoplasm reported in horses. Bovine papillomavirus (BPV) types 1 and 2 are associated with sarcoids, in which the expression of the major transforming oncoprotein (E5) is often recorded. The transformation activity of the virus is due to the binding of the E5 to the platelet-derived growth factor beta receptor (PDGFbeta-r). In the present study, we show by Western blot in 4 sarcoid samples and 3 normal equine skin samples that the PDGFbeta-r is more phosphorylated in sarcoid tissue than in normal skin (P < .001). Furthermore, the physical interaction between the activated receptor and the 85-kDa regulatory subunit (p85) of phosphatidylinositol-3-kinase (PI3K) is shown by coimmunoprecipitation. The PI3K-AKT-cyclin D3 molecular pathway downstream to the activation of the PDGFbeta-r is shown to be expressed, and the amount of the investigated molecules is higher than normal (P < .001), suggesting an activation of these effectors in sarcoids. Further, we demonstrate that phospho-JNK and phospho-JUN are more expressed in sarcoids than in normal skin. Our results provide new insights into the pathogenesis of equine sarcoids and support the validity of this in-vivo model to further characterize the molecular pathways underlying BPV E5-induced carcinogenesis.

Bovine papillomavirus type 1 DNA and E5 oncoprotein expression in water buffalo fibropapillomas

Papillomas and fibropapillomas may occur in the skin and in different organs in animals. Ten different genotypes of bovine papillomavirus (BPV) have been identified. BPV-1 through BPV-10 are all strictly species-specific, but BPV-1/2 may also infect other species such as equids, inducing fibroblastic tumors. BPV-1 and BPV-2 are associated with fibropapillomas in cattle; these tumors are formed by excessive proliferation of virus-infected dermal fibroblasts and epidermal keratinocytes. Nine water buffalo (Bubalus bubalis) were examined for the presence of multiple cutaneous and perivulvar tumors. Cutaneous and perivulvar fibropapillomatosis were confirmed histologically. Negative-stain transmission electron microscopic examination revealed papillomavirus-like particles in the fibropapillomas, and papillomaviral DNA was also detected by the polymerase chain reaction. The amplified long control region (LCR) DNA sequence was identical to that of BPV-1. The BPV-1 E5 oncoprotein was strongly expressed in the tumor cells thus confirming a causal role of the virus. This article represents the first report of cutaneous, perivulvar, and vulvar fibropapilloma associated with BPV-1 infection in the water buffalo and describes another example of cross-species infection by BPV-1.

Bovine papillomaviruses: their role in the aetiology of cutaneous tumours of bovids and equids

Bovine papillomavirus (BPV) is perhaps the most extensively studied animal papillomavirus. In cattle BPVs induce benign tumours of cutaneous or mucosal epithelia, called papillomas or warts. Cattle papillomas are benign tumours and generally regress without eliciting any serious clinical problems in the host, but occasionally persist and provide the focus for malignant transformation to squamous cell carcinoma, as in the case of cancer of the urinary bladder and cancer of the upper alimentary canal. BPV is the only papillomavirus that jumps species: the virus also infects equids, and gives rise to fibroblastic tumours called sarcoids. Sarcoids very rarely regress, more often they persist and can be locally aggressive. These tumours are the most common dermatological tumour of equids worldwide. The purpose of this review is to discuss the biology of BPV, the biology of bovine tumours and equine sarcoids, and present the current understanding of BPV in tumour pathogenesis in its natural host, cattle, and in its heterologous host, equids. Finally, the use of anti-BPV vaccines as a therapy for equine sarcoids will be discussed. Only limited information on the clinical or pathological aspects of either bovine or equine tumours will be provided as this subject has been extensively addressed previously.

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.

Bovine papillomavirus type-2 DNA and expression of E5 and E7 oncoproteins in vascular tumours of the urinary bladder in cattle

Cattles suffering from chronic enzootic haematuria frequently develop urinary bladder tumours of both epithelial and mesenchymal origin mainly haemangioma and its malignant counterpart. The role of the bovine papillomavirus type-2 (BPV-2) and of its major transforming oncoprotein in naturally occurring urothelial carcinogenesis has been recently clarified. E5 interacts in vivo as in vitro with the beta receptor for the platelet-derived growth factor (PDGF). However, studies regarding tumours of mesenchymal origin such as those arising from blood vessels are lacking. We show that the BPV-2 is present in 100% of the vascular tumours of the urinary bladder examined. Twenty-six out of twenty-seven tumour samples (96%) expressed E5 while 20 out of 27 (74%) tumour samples expressed E7. The two viral oncoproteins were not expressed in normal endothelial cells. Additionally, they co-localize in neoplastic endothelial cells as demonstrated by confocal immunofluorescence. PDGFbeta receptor was also shown to be expressed and co-localizes with E5 in neoplastic blood vessels. Our results demonstrate, for the first time, that the BPV-2 is present in high percentage in tumours of mesenchymal origin arising in its natural host. Furthermore, the expression of the two viral oncoproteins confirm that the virus may have a causative role in the neoplastic process.

The E5 protein of BPV-4 interacts with the heavy chain of MHC class I and irreversibly retains the MHC complex in the Golgi apparatus

BPV-4 E5 inhibits transcription of the bovine MHC class I heavy chain (HC) gene, increases degradation of HC and downregulates surface expression of MHC class I by retaining the complex in the Golgi apparatus (GA). Here we report that transcription inhibition can be alleviated by interferon treatment and the degradation of HC can be reversed by treatment with inhibitors of proteasomes and lysosomes. However, the inhibition of transport of MHC class I to the cell surface is irreversible. We show that E5 is capable of physically interacting with HC. Together with the inhibition of the vacuolar ATPase (due to the interaction between E5 and 16k subunit c), the interaction between E5 and HC is likely to be responsible for retention of MHC class I in the GA. C-terminus deletion mutants of E5 are incapable of either downregulating surface MHC class I or interacting with HC, establishing that the C-terminus domain of E5 is important in the inhibition of MHC class I.

Bovine papillomavirus E5 oncoprotein binds to the activated form of the platelet-derived growth factor beta receptor in naturally occurring bovine urinary bladder tumours

Studies regarding the functions of the bovine papillomavirus (BPV) E5 oncoprotein in vivo are lacking and no E5-mediated mechanism underlying epithelial carcinogenesis is known. We have shown that BPV-2 DNA is present in the majority of naturally occurring urinary bladder tumours of cattle and that E5 is expressed in the cancer cells. Here we show that the interaction between the platelet-derived growth factor (PDGF) beta receptor and BPV E5, described in vitro in cultured cells, takes place in vivo in bovine urinary bladder cancers. In these cancers, E5 and PDGF beta receptor colocalize, as shown by confocal microscopy, and physically interact, as shown by coimmunoprecipitation. Furthermore, the PDGF beta receptor associated with E5 is highly phosphorylated, suggesting the functional activation of the receptor upon E5 interaction. Our results demonstrate, for the first time, that E5-PDGF beta receptor interaction occurs during the natural history of bovine urinary bladder tumours, suggesting an important role for E5 in carcinogenesis. Finally, the system provides a suitable animal model of papillomavirus-associated cancer to test therapeutic vaccination against E5. Successful bladder tumour regression would provide a valuable model for therapeutic vaccination against papillomavirus-associated tumours.

Similarities and differences between the E5 oncoproteins of bovine papillomaviruses type 1 and type 4: cytoskeleton, motility and invasiveness in E5-transformed bovine and mouse cells

Bovine papillomaviruses (BPVs) are oncogenic viruses. In cattle, BPV-1/2 is associated with urinary bladder cancer and BPV-4 with upper GI tract cancer. BPV E5 is a small hydrophobic protein localised in the endoplasmic reticulum (ER) and Golgi apparatus (GA). E5 is the major transforming protein of BPVs, capable of inducing cell transformation in cultured mouse fibroblasts and, in cooperation with E7, in primary bovine cells. E5-induced cell transformation is accompanied by activation of several cellular protein kinases, including growth factor receptors, and alkalinisation of endosomes and GA. We have reported that BPV E5 causes swelling and fragmentation of the GA and extensive vacuolisation of the cytoplasm. We now show that E5 from both BPV-1 and BPV-4 disturbs the actin cytoskeleton and focal adhesions in transformed bovine cells, where these morphological and behavioural characteristics are accompanied by hyperphosphorylation of the cellular phosphotyrosine kinase c-src. Both BPV-1 and BPV-4 E5 increase the motility of transformed mouse cells, but only BPV-1 E5 causes transformed mouse cells to penetrate a matrigel matrix. BPV-1 transformed mouse cells, but not BPV-4 transformed mouse cells, have hyperhpsphorylated c-src.

Downregulation of major histocompatibility complex class I in bovine papillomas

Bovine papillomavirus (BPV) induces papillomas in cattle; in the great majority of cases, these regress due to the host immune response, but they can persist and progress to malignancy. Even in the absence of malignant transformation, BPV infection persists for a significant period of time before activation of the host immune system, suggesting that the host immune system is unaware of, or disabled by, BPV. E5 is the major oncoprotein of BPV, which, in addition to its transforming properties, downregulates the expression and transport to the cell surface of major histocompatibility complex class I (MHC I). Here, it is shown that co-expression of MHC I and E5 in papillomas caused by BPV-4 infection is mutually exclusive, in agreement with the inhibition of surface MHC I expression by E5 that is observed in vitro. The inhibition of MHC expression in E5-expressing papilloma cells could explain the long period that is required for activation of the immune response and has implications for the progression of papillomas to the malignant stage; absence of peptide presentation by MHC I to cytotoxic T lymphocytes would allow the infected cells to evade the host cellular immune response and allow the lesions to persist.

Presence of bovine papillomavirus type 2 DNA and expression of the viral oncoprotein E5 in naturally occurring urinary bladder tumours in cows

Samples of neoplastic and normal urothelium were obtained from cows originating from areas of southern Italy, a region in which chronic enzootic haematuria is endemic and bracken fern infestation is widespread. Specimens were analysed for bovine papillomavirus type 2 (BPV-2) DNA, BPV-2 E5 expression and telomerase activity. A total of 46 of 60 tumours and 17 of 34 normal bladder mucosa samples harboured BPV-2 DNA. Analysis of a subset of samples showed E5 protein expression and telomerase activity in tumour tissue only. No normal samples positive for BPV DNA showed E5 protein expression or telomerase activity, suggesting the presence of DNA in a latent state. Taken together, these data on naturally occurring bovine bladder tumours corroborate the hypothesis of their virus origin.

The bovine papillomavirus oncoprotein E5 retains MHC class I molecules in the Golgi apparatus and prevents their transport to the cell surface

During papillomavirus infection, the E5 protein localizes in the cell Golgi apparatus and other endomembrane compartments. Cells transformed by E5 do not express major histocompatibility class I complex (MHC I) on the cell surface, while cells transformed by the other transforming proteins E6 and E7 do. In addition, the total amount of both MHC I protein and mRNA is reduced in E5-transformed cells. Here we show that expression of bovine papillomavirus E5 causes the retention of MHC I in the Golgi apparatus, thus preventing its transport to the cell surface. We ascribe this effect to a failure of acidification of the Golgi apparatus, as similar effects are observed in control cells treated with the ionophore monensin. Treatment of E5-transformed cells with either beta- or gamma-interferon increases the synthesis of MHC I, showing that inhibition of MHC I expression by E5 is not irreversible. However, even after interferon treatment, MHC I, although increased in quantity, is not transported to the cell surface. E5 therefore affects MHC I at several levels, but prevention of MHC I transport to the cell surface appears to be the dominant effect. Lack of surface MHC I would have profound consequences for presentation of viral peptides to the immune system.

HPV infections and oesophageal cancer

The first reports suggesting an involvement of human papillomavirus (HPV) in the development of both benign and malignant squamous cell tumours of the oesophagus date back to 1982. Since then, a substantial amount of literature has accumulated on this subject, summarised in this review. To date, 239 oesophageal squamous cell papillomas have been analysed in 29 separate studies using different HPV detection methods, with HPV being detected in 51 (21.3%) cases. Many more squamous cell carcinomas have been analysed: of the 1485 squamous cell carcinomas analysed by in situ hybridisation, 22.9% were positive for HPV DNA, as were 15.2% of the 2020 cases tested by the polymerase chain reaction. In addition, evidence derived from large scale serological studies, animal experiments, and in vitro studies is discussed in the light of the highly variable geographical incidence rates of oesophageal carcinoma worldwide. It may be that the (multifactorial) aetiology of oesophageal cancer differs greatly between those geographical areas with a low risk and those with a high risk for this disease. Oncogenic HPV types seem to play an important causal role, particularly in high risk areas.

HPV infections and oesophageal cancer

The first reports suggesting an involvement of human papillomavirus (HPV) in the development of both benign and malignant squamous cell tumours of the oesophagus date back to 1982. Since then, a substantial amount of literature has accumulated on this subject, summarised in this review. To date, 239 oesophageal squamous cell papillomas have been analysed in 29 separate studies using different HPV detection methods, with HPV being detected in 51 (21.3%) cases. Many more squamous cell carcinomas have been analysed: of the 1485 squamous cell carcinomas analysed by in situ hybridisation, 22.9% were positive for HPV DNA, as were 15.2% of the 2020 cases tested by the polymerase chain reaction. In addition, evidence derived from large scale serological studies, animal experiments, and in vitro studies is discussed in the light of the highly variable geographical incidence rates of oesophageal carcinoma worldwide. It may be that the (multifactorial) aetiology of oesophageal cancer differs greatly between those geographical areas with a low risk and those with a high risk for this disease. Oncogenic HPV types seem to play an important causal role, particularly in high risk areas.

Evasion of host immunity directed by papillomavirus-encoded proteins

The nature of the interaction between papillomaviruses (PV) and their infected host has led to the identification of ways in which the viral oncoproteins can transform the infected host cells into cancer cells. As viral persistence is required for malignancy, and persistence requires avoidance of immune attack by the host, defining the relationship between PV and the immune system is also paramount in understanding tumorigenesis. It has emerged that PV have evolved several ways in which to prevent clearance by the host immune system. The limitation of the PV replication cycle to the epithelium, together with low level expression of the virus proteins and an absence of inflammation, minimises the exposure of virus to immune cells. In addition, more recently it has been shown that, like many other viruses, PV can directly subvert the immune response, including interference with the interferon pathway, modulation of antigen presentation, inhibition of interleukin-18 activity and down-regulation of major histocompatibility class I on infected cells. Collectively these mechanisms explain how PV lesions are able to persist for long periods of time in immunocompetent hosts.

Down-regulation of MHC class I by bovine papillomavirus E5 oncoproteins

The papillomavirus E5 protein is localized in the endoplasmic reticulum (ER) and Golgi apparatus (GA) of the host cell. Transformed bovine fibroblasts expressing bovine papillomavirus (BPV) E5 are highly vacuolated and have a much enlarged, distorted and fragmented GA. Major histocompatibility complex class I (MHC I) is processed and transported to the cell surface through the GA. Given the cellular localization of E5 in the GA and the morphologically abnormal GA, we investigated the expression of MHC I in cells transformed by E5 from BPV-1 and BPV-4. Two cell lines were used: bovine cells that also express E6, E7 and activated ras, and NIH3T3 cells that express only E5. In addition, PalF cells acutely infected with a recombinant retrovirus expressing E5 were also examined. In contrast to non-transformed normal cells, or transformed cells expressing other papillomavirus proteins, cells expressing E5 do not express MHC I on their surface, but retain it intracellularly, independently of the presence of other viral or cellular oncogenes, or of whether the cells are long-term transformants or acutely infected. We conclude that expression of E5 prevents expression of MHC I to the cell surface and causes its retention within the cell. In addition, lower amounts of total MHC I heavy chain and of heavy chain RNA are detected in E5-transformed cells than in control cells. As surface expression of another glycosylated membrane protein, the transferrin receptor, is not affected, it appears that E5 targets MHC I with at least a degree of specificity. In papillomavirus lesions this effect would have important implications for antigen presentation by, and immunosurveillance of, virally infected 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.

Cooperative transformation and coexpression of bovine papillomavirus type 1 E5 and E7 proteins

Productively infected bovine fibropapillomas were examined for bovine papillomavirus type 1 (BPV-1) E7 localization. BPV-1 E7 was observed in the cytoplasm of basal and lower spinous epithelial cells, coexpressed in the cytoplasm of basal cells with the E5 oncoprotein. E7 was also observed in nucleoli throughout the basal and spinous layers but not in the granular cell layer. Ectopic expression of E7 in cultured epithelial cells gave rise to localization similar to that seen in productive fibropapillomas, with cytoplasmic and nucleolar expression observed. Consistent with the coexpression of E7 and E5 in basal keratinocytes, BPV-1 E7 cooperated with E5 as well as E6 in an anchorage independence transformation assay. While E5 is expressed in both basal and superficial differentiating keratinocytes, BPV-1 E7 is only observed in basal and lower spinous epithelial cells. Therefore, BPV-1 E7 may serve to modulate the cellular response of basal epithelial cells to E5 expression.

Binding of bovine papillomavirus type 4 E8 to ductin (16K proteolipid), down-regulation of gap junction intercellular communication and full cell transformation are independent events

The E8 open reading frame of bovine papillomavirus type 4 encodes a small hydrophobic polypeptide that contributes to primary cell transformation by conferring to cells the ability to form foci and to grow in low serum and in suspension. Wild-type E8 binds in vitro to ductin, a component of gap junctions, and this binding is accompanied by a loss of gap junction intercellular communication in transformed bovine fibroblasts. However, through the analysis of a panel of E8 mutants, we show here that binding of E8 to ductin is not sufficient for down-regulation of gap junction communication and that there is no absolute correlation between down-regulation of gap junction communication and the transformed phenotype.

A mutational analysis of the transforming functions of the E8 protein of bovine papillomavirus type 4

The E8 protein of BPV-4 contributes to transformation of primary bovine cells (PalFs) by inducing anchorage-independent growth and by down-regulating gap junction intercellular communication, likely due to its binding to 16K ductin. We show here that, in addition, E8 confers on PalF cells the ability to grow in low serum and to escape from contact inhibition (focus formation). E8 also transactivates an exogenous human cyclin A gene promoter, suggesting that overexpression of cyclin A is responsible for the transformed phenotype. Mutant forms of E8 were generated to establish whether the transforming functions of the protein could be segregated. Mutations were introduced both in the hydrophobic domain and in the hydrophilic C-terminal "tail", and chimeras with BPV-1 E5 were constructed. Cells expressing either wild-type E8 or mutant forms were analyzed for their ability to grow in low serum and in suspension and to form foci. Wild-type E8 and its mutants were also analyzed for their ability to transactivate the cyclin A promoter. We show here that the transforming functions of E8 can be segregated and that both the hydrophilic C-terminal tail and the residue at position 17 in the hydrophobic domain are crucial for E8 functions and for the transactivation of the cyclin A promoter. These results support the hypothesis that the different aspects of cellular transformation brought about by E8 might be due to interaction with different cellular targets. They suggest that E8 might function differently from BPV-1 E5 and demonstrate that the separate domains of E5 and E8 are not functionally interchangeable.

Bovine papillomavirus and cancer

Bovine papillomavirus (BPV) induces papillomas of cutaneous or mucosal epithelia in cattle. The papillomas are benign tumours and generally regress, but occasionally persist and provide the focus for malignant transformation to squamous cell carcinoma, particularly in the presence of environmental cofactors. This has been experimentally demonstrated for BPV-2 and cancer of the urinary bladder, and BPV-4 and cancer of the upper alimentary canal in cattle feeding on bracken fern. In this review, several aspects of the biology of the virus are described including viral genome structure, regulation of transcription of the viral oncogenes, function of the viral oncoproteins, cooperation between virus and chemical cofactors in carcinogenesis, virus latency and prophylactic and therapeutic vaccination programmes.