Reciprocal transactivation of Merkel cell polyomavirus and high-risk human papillomavirus promoter activities and increased expression of their oncoproteins

Background: Approximately 15% of human cancers are attributed to viruses. Numerous studies have shown that high-risk human polyomaviruses (HR-HPV) and Merkel cell polyomavirus (MCPyV) are two human tumor viruses associated with anogenetal and oropharyngeal cancers, and with Merkel cell carcinoma, respectively. MCPyV has been found in HR-HPV positive anogenetal and oropharyngeal tumors, suggesting that MCPyV can act as a co-factor in HR-HPV induced oncogenesis. This prompted us to investigate whether the oncoproteins large T-antigen (LT) and small antigen (sT) of MCPyV could affect the transcriptional activity HPV16 and HPV18 and vice versa whether HPV16 and HPV18 E6 and E7 oncoproteins affected the expression of MCPyV LT and sT. Reciprocal stimulation of these viral oncoproteinscould enhance the oncogenic processes triggered by these tumor viruses.

Methods: Transient co-transfection studies using a luciferase reporter plasmid with the long control region of HPV16 or HPV18, or the early or late promoter of MCPyV and expression plasmids for LT and sT, or E6 and E7, respectively were performed in the HPV-negative cervical cancer cell line C33A, in the keratinocyte cell line HaCaT, and in the oral squamous cell carcinoma cell line HSC-3. Transfections were also performed with deletion mutants of all these promoters and with mutants of all four oncoproteins. Finally, the effect of E6 and E7 on LT and sT expression in the MCPyV-positive Merkel cell carcinoma cell line WaGa and the effect of LT and sT on the expression of E6 and E7 was monitored by Western blotting.

Results: LT and sT stimulated the transcriptional activity of the HPV16 and HPV18 LCR and v.v. E6 and E7 potentiated the MCPyV early and late promoter in all cell lines. Induction by E6 and E7 was p53- and pRb-independent, and transactivation by LT did not require DNA binding, nuclear localization and HSC70/pRb interaction, whereas sT stimulated the HPV16/18 LCR activity in a PP2A- and DnaJ-independent manner.

Conclusions: These results indicate that the co-infection of MCPyV may act as a co-factor in the initiation and/or progression of HPV-induced cancers.