Polyomavirus Small T Antigen Induces Apoptosis in Mammalian Cells through the UNC5B Pathway in a PP2A- Dependent Manner Sameer Ahmed Bhat, a Zarka Sarwar, b Syed Qaaifah Gillani, b Misbah Un Nisa, b Irfana Reshi, a Nusrat Nabi, b Shaozhen Xie, c Khalid M. Fazili, a Thomas M. Roberts, c Shaida Andrabi b a Department of Biotechnology, University of Kashmir, Srinagar, India b Department of Biochemistry, University of Kashmir, Srinagar, India c Department of Cancer Biology, Dana Farber Cancer Institute, Harvard Medical School, Boston, Massachusetts, USA ABSTRACT UNC5B is a dependence receptor that promotes survival in the pres- ence of its ligand, netrin-1, while inducing cell death in its absence. The receptor has an important role in the development of the nervous and vascular systems. It is also involved in the normal turnover of intestinal epithelium. Netrin-1 and UNC5B are deregulated in multiple cancers, including colorectal, neuroblastoma, and breast tumors. However, the detailed mechanism of UNC5B function is not fully under- stood. We have utilized the murine polyomavirus small T antigen (PyST) as a tool to study UNC5B-mediated apoptosis. PyST is known to induce mitotic arrest fol- lowed by extensive cell death in mammalian cells. Our results show that the ex- pression of PyST increases mRNA levels of UNC5B by approximately 3-fold in os- teosarcoma cells (U2OS) and also stabilizes UNC5B at the posttranslational level. Furthermore, UNC5B is upregulated predominantly in those cells that undergo mitotic arrest upon PyST expression. Interestingly, although its expression was previously reported to be regulated by p53, our data show that the increase in UNC5B levels by PyST is p53 independent. The posttranslational stabilization of UNC5B by PyST is regulated by the interaction of PyST with PP2A. We also show that netrin-1 expression, which is known to inhibit UNC5B apoptotic activity, promotes survival of PyST-expressing cells. Our results thus suggest an important role of UNC5B in small-T antigen-induced mitotic catastrophe that also requires PP2A. IMPORTANCE UNC5B, PP2A, and netrin-1 are deregulated in a variety of cancers. UNC5B and PP2A are regarded as tumor suppressors, as they promote apoptosis and are deleted or mutated in many cancers. In contrast, netrin-1 promotes survival by inhibiting dependence receptors, including UNC5B, and is upregulated in many cancers. Here, we show that UNC5B-mediated apoptosis can occur independently of p53 but in a PP2A-dependent manner. A substantial percentage of cancers arise due to p53 mutations and are insensitive to chemotherapeutic treatments that activate p53. Unexpectedly, treatment of cancers having functional p53 with many conven- tional drugs leads to the upregulation of netrin-1 through activated p53, which is counterintuitive. Therefore, understanding the p53-independent mechanisms of the netrin-UNC5B axis, such as those involving PP2A, assumes greater clinical signifi- cance. Anticancer strategies utilizing anti-netrin-1 antibody treatment are already in clinical trials. KEYWORDS netrin 1, PP2A, polyomavirus, UNC5B, apoptosis, mitotic arrest, small T antigen Citation Bhat SA, Sarwar Z, Gillani SQ, Un Nisa M, Reshi I, Nabi N, Xie S, Fazili KM, Roberts TM, Andrabi S. 2020. Polyomavirus small T antigen induces apoptosis in mammalian cells through the UNC5B pathway in a PP2A-dependent manner. J Virol 94:e02187-19. https://doi.org/ 10.1128/JVI.02187-19. Editor Lawrence Banks, International Centre for Genetic Engineering and Biotechnology Copyright © 2020 American Society for Microbiology. All Rights Reserved. Address correspondence to Shaida Andrabi, shaida.andrabi@uok.edu.in. Received 10 January 2020 Accepted 2 May 2020 Accepted manuscript posted online 13 May 2020 Published TRANSFORMATION AND ONCOGENESIS crossm July 2020 Volume 94 Issue 14 e02187-19 jvi.asm.org 1 Journal of Virology 1 July 2020 Downloaded from https://journals.asm.org/journal/jvi on 23 February 2022 by 18.234.69.63.