RNA-specific Adenosine Deaminase ADAR1 Suppresses Measles Virus-induced Apoptosis and Activation of Protein Kinase PKR*

ADAR1 (adenosine deaminase acting on RNA) catalyzes the conversion of adenosine to inosine, a process known as A-to-I editing. Extensive A-to-I editing has been described in viral RNAs isolated from the brains of patients persistently infected with measles virus, although the precise role of ADAR during measles virus infection remains unknown. We generated human HeLa cells stably deficient in ADAR1 (“ADAR1kd cells”) through short hairpin RNA-mediated knockdown, and using these cells, we tested the effect of ADAR1 deficiency on measles virus (MVvac strain) growth and virus-induced cell death. We found that the growth of mutant viruses lacking expression of the viral accessory proteins V and C (Vko and Cko, respectively) was decreased in ADAR1-deficient cells compared with ADAR1-sufficient cells. In addition, apoptosis was enhanced in ADAR1-deficient cells following infection with wild type and Vko virus but not following infection with Cko virus or treatment with tumor necrosis factor-α or staurosporine. Furthermore, in Cko-infected ADAR1-sufficient cells when ADAR1 did not protect against apoptosis, caspase cleavage of the ADAR1 p150 protein was detected. Finally, enhanced apoptosis in ADAR1kd cells following infection with wild type and Vko virus correlated with enhanced activation of PKR kinase and interferon regulatory factor IRF-3. Taken together, these results demonstrate that ADAR1 is a proviral, antiapoptotic host factor in the context of measles virus infection and suggest that the antiapoptotic activity of ADAR1 is achieved through suppression of activation of proapoptotic and double-stranded RNA-dependent activities, as exemplified by PKR and IRF-3.