Abstract
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.
Highlights
Papillomaviruses (PVs) are small oncogenic DNA viruses infecting humans as well as different animal species, causing benign hyperproliferative lesions of both mucosal and cutaneous epithelia or malignant cancers
Bovine papillomaviruses (BPVs) E7 is considered as a transforming gene very little is known about E7-mediated mechanisms underlying carcinogenesis, so far
Both proteins are expressed in virally induced alimentary canal tumours: E5 is mostly expressed in early development stages, on the contrary E7 is produced throughout all stages of the fibropapilloma, suggesting its pivotal role in the maintenance of the proliferative state of the epithelia and its necessary role in cell transformation [38]
Summary
Papillomaviruses (PVs) are small oncogenic DNA viruses infecting humans as well as different animal species, causing benign hyperproliferative lesions of both mucosal and cutaneous epithelia or malignant cancers. BPV-4 E7 unlike BPV-1 E7 possesses the canonical Cys-X-X-Cys motif and the pRb interaction domain, that is crucial for its transforming ability [25] Both proteins are expressed in virally induced alimentary canal tumours: E5 is mostly expressed in early development stages, on the contrary E7 is produced throughout all stages of the fibropapilloma, suggesting its pivotal role in the maintenance of the proliferative state of the epithelia and its necessary role in cell transformation [38]. The levels of E5 m-RNA are higher in the nodular type of sarcoids as compared with the other clinical types, a recent report indicates that the highest levels of E5 DNA are found in the lesions of horses suffering from an aggressive form of disease i.e. multiple, rapidly growing tumours of various type [72,73] These data provide all evidence of active BPV infection in sarcoids supporting the role of E5 in cell transformation. These findings suggest that E7 may contribute to sarcoid cell invasion process through upregulation of MMP
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