Adenosine deaminase acting on RNA (ADAR1) as a modulator of innate antiviral immunity

Interferon (IFN) inducible adenosine deaminase acting on RNA (ADAR1) binds to double-stranded RNA (dsRNA) and catalyzes adenosine-C6-deamination, resulting in Adenosine-to-Inosine (A-to-I) editing that increases RNA structure instability due to I:U mismatch pairing or genome diversity, as I is decoded as guanosine (G). The edited RNA can be of cellular or viral origin. Overexpression of ADAR1 reduces the activation of protein kinase PKR, an IFN-inducible enzyme that is regulated by dsRNA. Activated PKR phosphorylates protein synthesis initiation factor eIF-2 a subunit and leads to translational inhibition. Using RNAi interference strategy we generated ADAR1-deficient human cell clone, in which we observed enhanced PKR activation following infection with vesicular stomatitis virus (VSV) or measles virus C-knockout mutant (MV-Cko), or treatment with IFN. With VSV infection or IFN treatment, phospho-PKR accumulated in the nucleus in ADAR1-deficient cells. The enhanced activation of PKR led to a further reduction of virus yield of ∼10-fold compared to ADAR1-sufficient cells. Furthermore, the adenovirus VAI deletion mutant dl331, which was not rescued in PKR-deficient cells, was partially rescued in ADAR1-deficient cells. Moreover, in ADAR1-deficient cells, regardless of adenovirus infection, we observed reduced activation of antiviral components involved in both IFN-production and IFN-action signaling pathways, including phospho-STAT1 and STAT2, phospho-IRF3, and ISG15. The reductions were not observed with IFN treatment, or following infection with MV-Cko, a known IFN-inducer. These observations correlated with their IFNbeta inducing capacity. We hypothesize that ADAR1 induction and activation counter-balances that of PKR through competition for common interaction partners. In ADAR1-deficient cells, this competition diminishes, and hence basal PKR activation is enhanced. The ensuing translational inhibition affects protein expression and viral growth. Overall, we found that the expression of ADAR1 in the host cells is important to maintain normal suppression of basal PKR activation, without interfering with a robust innate immune response following viral infection.