Merkel cell carcinoma (MCC) is a rare, but aggressive form of skin cancer with rising incidence and high mortality rate. Approximately 80% of MCC tumors are positive for Merkel cell polyomavirus (MCPyV), suggesting a causative relationship between MCPyV and MCC. Exosomes are 30-150 nm vesicles that contain proteins, lipids, mRNAs, lncRNAs, and miRNAs. Exosomes from virus-infected cells comprise also viral nucleic sequences and proteins. Because tumor cell-derived exosomes may contribute to cancer, we hypothesized that MCPyV may affect the composition of exosomes and play a role in tumorigenesis. Therefore, we compared the proteins in exosomes produced by MCPyV-positive MKL1 and MKL2 and MCPyV-negative MCC13 and MCC26 MCC cell lines.

Our proteomic analysis of exosomes originating from polyomavirus-negative and polyomavirus-positive MCC cell lines revealed the presence of the oncogenic proteins periostin and thrombospondin. Western blot analysis of exosomes and lysates of these MCC cells confirmed the presence of these proteins in exosomes from all cell lines. Thrombospondin, but not periostin was detectable in cell lysates, and an enrichment of both proteins was detected in exosomes of all cell lines. The effect of MCPyV large T-antigen (LT-ag) and small t-antigen (st-ag) on the periostin and thrombospondin promoters was examined by transient transfection studies with luciferase reporter plasmids. Transfection experiments in MCC13 revealed that LT-ag, but not st-ag significantly increased the activity of the thrombospondin and periostin promoters. In MCC26 cells, neither LT-ag nor st-ag had a significant effect on the thrombospondin promoter activity, while both proteins alone or in combination significantly stimulated the periostin promoter strength. Our results suggest that MCPyV proteins may contribute to tumorigenesis by enhancing the expression of the oncoproteins thrombospondin and periostin and promote their secretion via exosomes.

Citation Format: Aelita Konstantinell, Ida Sofie Furuholmen, Baldur Sveinbjørnsson, Ugo Moens. Effect of Merkel cell polyomavirus large and small T-antigen on the thrombospondin and the periostin promoter [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4808. doi:10.1158/1538-7445.AM2017-4808