RB1 is the crucial target of the Merkel cell polyomavirus Large T antigen in Merkel cell carcinoma cells
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Sonja Hesbacher1,*, Lisa Pfitzer1,2,*, Katharina Wiedorfer1, Sabrina Angermeyer1, Andreas Borst1, Sebastian Haferkamp3, Claus-Jürgen Scholz4, Marion Wobser1, David Schrama1, Roland Houben1
1Department of Dermatology, Venereology and Allergology, University Hospital Würzburg, Würzburg, Germany
2Department of Pharmacy, Center for Drug Research, University of Munich (Ludwigs-Maximilians-Universität), Munich, Germany
3Department of Dermatology, University of Regensburg, Regensburg, Germany
4Core Unit Systems Medicine, University of Würzburg, Würzburg, Germany
*These authors have contributed equally to this work
Roland Houben, email: Houben_R@ukw.de
Keywords: Merkel cell carcinoma, polyomavirus, Large T antigen, retinoblastoma protein, viral carcinogenesis
Received: November 09, 2015 Accepted: March 28, 2016 Published: April 18, 2016
The pocket protein (PP) family consists of the three members RB1, p107 and p130 all possessing tumor suppressive properties. Indeed, the PPs jointly control the G1/S transition mainly by inhibiting E2F transcription factors. Notably, several viral oncoproteins are capable of binding and inhibiting PPs. Merkel cell polyomavirus (MCPyV) is considered as etiological factor for Merkel cell carcinoma (MCC) with expression of the viral Large T antigen (LT) harboring an intact PP binding domain being required for proliferation of most MCC cells. Therefore, we analyzed the interaction of MCPyV-LT with the PPs. Co-IP experiments indicate that MCPyV-LT binds potently only to RB1. Moreover, MCPyV-LT knockdown-induced growth arrest in MCC cells can be rescued by knockdown of RB1, but not by p107 or p130 knockdown. Accordingly, cell cycle arrest and E2F target gene repression mediated by the single PPs can only in the case of RB1 be significantly reverted by MCPyV-LT expression. Moreover, data from an MCC patient indicate that loss of RB1 rendered the MCPyV-positive MCC cells LT independent. Thus, our results suggest that RB1 is the dominant tumor suppressor PP in MCC, and that inactivation of RB1 by MCPyV-LT is largely sufficient for its growth supporting function in established MCPyV-positive MCC cells.
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