Effect of viral early proteins, mutations and IL-17F on the transcriptional activity of the Merkel cell polyomavirus promoter in different cell lines

Abstract

Merkel cell polyomavirus (MCPyV) is common in the human population with a seropositivity of approximately 60%. The virus is chronically shed from healthy skin, but the genuine host cell remains unknown and a permissive cell culture system is lacking. The viral genome is in an episomal state in cells where MCPyV has been found. The virus is not harmful in healthy individuals, but it is involved in the pathogenesis of Merkel cell carcinoma (MCC) in elderly and immunosuppressed individuals. Approximately 80% of all examined MCC specimens are MCPyV-positive. Two hallmarks of virus-positive MCCs are integrated viral genome and expression of truncated large T-antigen (tLT-ag). The non-coding control region (NCCR), encompassing the origin of replication and the promoter/enhancer controlling the expression of the early and late viral genes, of most MCPyV isolates are quasi identical to the reference strain MCC350. However, the NCCR of MCPyV isolated from healthy skin (strain 16b), feces (strain HB039C), and a Kaposi’s sarcoma sample (strain TKS) is ~25 bp longer due to a repeated sequence. In this study the relative MCPyV promoter strength of variants MCC350 and 16b was compared in three different cell lines (HEK 293, MCC13 and C33A). The effect of the early proteins: large T antigen (LT-ag) and small t-antigen (st-ag) on promoter activity was examined. Our result demonstrated that early and late promoter strength of MCPyV16b variant is higher than that of MCC350 in HEK293 and MCC13 cells but similar in C33A cell. MCPyV LT-ag and st-ag regulated the expression of the viral promoter and the differences in promoter architecture affect their effect on transcriptional activity in a cell-dependent manner. Because expression of interleukin-17F (IL-17F), a pro-tumorigenic cytokine, is upregulated in MCPyV-positive MCC compared to MCPyV-negative MCC, we investigated the effect of LT-ag and st-ag on the IL-17F promoter, as well as the effect of IL-17F on the MCPyV promoter activity. MCPyV LT-ag stimulated the expression of IL-17F and vice versa, IL-17F enhanced the MCPyV promoter. In conclusion, mutations in the MCPyV promoter changes its activity and may affect cell tropism and virulence. The reciprocal interaction between IL-17 and MCPyV may contribute to the development of MCC.