Novel cellular pathways inactivated by the HPV E6 transforming protein

The human papillomavirus (HPV) E6 is one of three oncoproteins encoded by the virus. It has long been recognised as a potent oncogene and is intimately associated with the events that result in the malignant conversion of virally infected cells. The aim of this thesis is to investigate how E6 impinges upon diverse cellular pathways, whose inactivation appears central to tumour progression. Firstly, the activation of the p53 tumour suppressor was examined in HPV-positive cancer cells, in conditions where E6-mediated degradation was blocked, with the aim of studying whether E6 interferes with the functions of p53 independently of targeting it for degradation. It was found that inhibition of proteasome activity in HPV-containing tumour cells does not always result in activation of p53, moreover, in E6 expressing cells the nuclear localisation of p53 in response to DNA damage is also blocked, consistent with previous observations which indicate that degradation of p53 is not essential for E6 mediated inhibition of p53 functions. Several PDZ-containing proteins have recently been shown to be crucial targets for the tumour-promoting activities of high-risk E6. Therefore, the activity of HPV E6 towards the multi-PDZ tight junction protein MUPP-1 was analysed, demonstrating that high-risk HPV E6 binds MUPPl through its C-terminal PDZ-binding motif and targets it for ubiquitin-mediated degradation. Finally, since high-risk HPV E6 has been demonstrated to target the putative tumour suppressor protein Dlg for degradation, the cellular mechanisms which regulate the stability of this protein were studied in both HPV-positive and negative epithelial cells. This study highlighted a complex pattern of hDlg regulation through both phosphorylation and ubiquitin-mediated degradation in response to cell-cell contact. The use of models of tumour progression in vitro also demonstrated that loss of this regulation is likely to represent a significant step in the development of malignancy. These results have important implications for any potential therapies which might aim to block the interactions of HPV E6 with some of its cellular targets.