Abstract
The primary risk factor for the development of cervical cancer is a persistent infection with certain high-risk human papillomavirus (HPV) types such as 16 and 18. The ability of these viruses to cause such persistent infections within the host can be attributed to several immune evasion mechanisms. One such mechanism involves adversely affecting the amount of viral antigen presented to the host immune system. It was previously shown that the E6 oncoprotein of HPV16 is capable of actively down-regulating expression of epithelial (E)-cadherin expression which directly correlates with depletion in the number of immune Langerhans cells (LCs) in the infected tissue. In this manner, HPV16 E6 is able to contribute to viral survival and protection.
The aims of this study were firstly, to investigate the mechanism of E6 regulation of E-cadherin; secondly, to identify a region of E6 responsible for regulation of E-cadherin; thirdly, to investigate the roles of the other HPV16 oncoproteins in E-cadherin regulation and fourthly, to identify potential therapeutic interventions for early HPV16 infections.
The E6 oncoprotein from HPV16 was expressed in HCT116 cells and its ability to regulate E-cadherin promoter, transcript and surface protein was investigated. E6 was found to regulate E-cadherin at a transcriptional level. The E-cadherin promoter was found to be repressed in HCT116 E6 cells through a methylation dependent mechanism that is independent of direct methylation of the E-cadherin promoter. In order to identify the region of E6 oncoprotein important for E-cadherin regulation, a range of HPV16 E6 mutants were tested for their abilities to decrease transcription of E-cadherin. Mutational analysis of HPV16 E6 led to the identification of residues 118-122 and the nearby R124 residue, located within the C-terminal zinc-finger binding domain to be important for transcriptional regulation of E-cadherin by E6. In addition, the other major oncoproteins of HPV16, namely E5 and E7 were co-expressed with E6 and their abilities to regulate surface E-cadherin were assessed. E7 was also found to be effective in decreasing surface E-cadherin levels, and this effect was enhanced when the two oncoproteins were co-expressed in HCT116 cells. Furthermore, regulation of E-cadherin by HPV16 oncoproteins was assessed under conditions of differentiation using a simple monolayer model of differentiation and a three-dimensional organotypic raft culture system. Lastly, in order to identify potential therapeutic interventions for early HPV16 infections, rational design was used to design peptide antagonists of E6 that could potentially interfere with E6 regulation of E-cadherin. Despite efficient delivery of peptide inhibitors of E6 into target E6 expressing cells, no restoration of E-cadherin levels were detected. However, another potential therapeutic compound, Indole-3-carbinol, was identified.
This research gives insight into the mechanism of transcriptional regulation of E-cadherin by HPV16 E6. The data presented here suggest that a specific regulatory region of the protein can directly affect regulation of E-cadherin. These data may provide the foundation for an effective HPV16 inhibitor, which would help decrease the burden of HPV-related disease that affects millions of people each year.