Polyoma Small T antigen Induces Apoptosis In Mammalian Cells Through UNC5B Pathway In A PP2A Dependent Manner

: UNC5B is a dependence receptor that promotes survival in the presence of its ligand netrin-1 while inducing cell death in its absence. The receptor has an important role in 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 understood. We have utilized murine polyoma virus small T antigen (PyST) as a tool to study UNC5B mediated apoptosis in mammalian cells. PyST is known to induce mitotic arrest followed by extensive cell death in mammalian cells. Our results show that the expression of PyST increases mRNA levels of UNC5B by about 3 fold in osteosarcoma cells (U2OS) and also stabilizes UNC5B at the post-translational level. Furthermore, UNC5B is up-regulated predominantly in those cells that undergo mitotic arrest upon PyST expression. Interestingly, although its expression has been previously reported to be regulated by p53, our data showed that the increase in UNC5B levels by PyST is p53 independent. Rather, the post-translational 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 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 a 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 up-regulated in many cancers. Here we show that UNC5B mediated apoptosis can occur independently of p53, but in a PP2A dependent manner. 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 conventional drugs leads to the up-regulation of netrin-1 through activated p53, which is counter-intuitive. Therefore, understanding the p53 independent mechanisms of Netrin-UNC5B axis, such as those involving PP2A, assume greater clinical significance. Anticancer strategies utilizing anti-netrin-1 antibody treatment are already in clinical trials.