RNA virus modulation of IFN, PI3K and apoptosis

Abstract

Interferon (IFN) and phosphatidylinositol 3-kinase (PI3K) are apoptosis regulators that are targeted by viruses to promote survival of infected cells. Significant crosstalk exists between IFN and PI3K, and this study sought to investigate the relationships between IFN, PI3K and apoptosis during virus infection. Parainfluenza virus 5 (PIV5) and influenza A virus (IAV) are both negative-sense single-stranded RNA viruses that encode multifunctional proteins in order to maximise their genome coding capacity. The PIV5 V and IAV NS1 proteins are well-studied as IFN antagonists and, in addition, both are reported to modulate PI3K signalling. Less well-studied is the role of these proteins in apoptosis regulation; the ability of V and NS1 to inhibit apoptosis was therefore investigated. PIV5/V was found to limit cell death in response to a number of apoptosis inducers in a manner that required its STAT1- degradative activity and also inhibited activation of the PI3K downstream target, Akt. IAV/NS1 binds directly to PI3K to stimulate its activity, and this is reported to mediate anti-apoptotic signalling during IAV infection. However, a virus expressing an NS1 unable to bind PI3K did not induce more apoptosis than wt virus. NS1 expression, either in a stable cell-line or during virus infection, was also unable to protect cells from pro-apoptotic stimuli. NS1-mediated PI3K activation similarly had no effect on IFN production or ISG expression in infected cells. In contrast, other NS1 mutant viruses induced large amounts of apoptosis. These viruses also induced significant levels of IFN and were unable to cause apoptosis in IFN-deficient cells, indicating that NS1 limits apoptosis induction through its IFN antagonist functions. The implications of this work for anti-cancer and anti-viral therapies are discussed.