Cooperative Transformation and Coexpression of Bovine Papillomavirus Type 1 E5 and E7 Proteins

Abstract

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.Papillomaviruses are causative agents for a variety of epithelial neoplasms in vertebrates. Typically, virus-induced epitheliomas are benign lesions containing episomal viral DNA in proliferative basal epithelial cells, producing virus within superficial differentiated epithelial cells. The role of virus-encoded oncogenes (E5, E6, and E7) in the maintenance of viral DNA within proliferative basal epithelial cells and/or the role of these oncogenes in the transition to vegetative viral DNA amplification and virus production has yet to be fully defined. The separation of papillomavirus oncogenes into at least three separate polypeptides (E5, E6, and E7) may reflect a requirement for separate regulation of expression and activity of each product in the virus life cycle. One area of uncertainty in papillomavirus biology has been the location of each oncoprotein within the stratified epithelium of a virus-induced epithelioma.While bovine papillomavirus type 1 (BPV-1) encodes E5, E6, and E7 oncoproteins, only mutation of BPV-1 E5 decreases transformed cell focus formation by BPV-1 viral DNA. Mutations in the BPV-1 E6 or E7 genes have little effect upon focus formation or viral DNA replication, although there is a decrease in anchorage-independent growth and tumorigenicity (12,15,20). The modest oncogenic potency of BPV-1 E6 and BPV-1 E7 expressed from the wild-type BPV-1 genome is due to repression by two separate repressor mechanisms that, when mutated, reveal cooperative transformation by BPV-1 E6 and BPV-1 E7 (35). Since both BPV-1 E6 and BPV-1 E7 are thought to be translated from the same mRNA transcript, it was unproven if BPV-1 E7 had a direct role in transformation together with BPV-1 E6. Correlating with this observation, BPV-1 E6 strongly transforms murine C127 cells when expressed from retroviral long terminal repeats, whereas E7 did not (27).The E6 proteins of human papillomavirus type 16 (HPV-16) (16E6) and BPV-1 E6 bind to the cellular targets E6AP, ERC-55, and paxillin through interaction with homologous peptide sequences found on the t...

Mechanisms of cell transformation by papillomavirus E5 proteins

“…In bovine warts, the BPV E5 protein is expressed in epidermal keratinocytes as well as in dermal ®broblasts (Bohl et al, 2001;Burnett et al, 1992). Moreover, the BPV E5 protein can cause tumorigenic transformation of established murine keratinocytes (Leptak et al, 1991).…”

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

“…However, its function in vivo is still unknown. Recent reports show E5 expression in both equine and bovine BPV-infected cancers (Bohl et al, 2001;Carr et al, 2001), suggesting a possible role of this viral protein in malignant transformation in vivo. A total of 36 BPV-positive (including pre-neoplastic lesions) and 10 BPV-negative tumours were analysed together with 15 BPV-positive and 10 BPV-negative normal mucosa samples.…”

Lack of the canonical pRB-binding domain in the E7 ORF of artiodactyl papillomaviruses is associated with the development of fibropapillomas

“…This correlation extends beyond the phylogenetic topology and pathology. In BPV-1 fibropapillomas, it has been shown that E5 and E7 co-localize within the cytoplasm of undifferentiated basal epithelial cells, and that this co-expression is the basis of cooperative transformation between E5 and an E7 lacking the pRB-binding motif (Bohl et al, 2001). …”