Chemical Inhibition of RNA Viruses Reveals REDD1 as a Host Defense Factor
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Influenza (flu) is a contagious infectious respiratory illness. The flu can cause from mild to life-threatening illness. The current therapeutic intervention strategies to prevent or treat influenza infection are not sufficient in the event that a pathogenic virus strains reaches pandemic proportions. Therefore, the development of anti-influenza therapeutic modalities is critical to respond to a future influenza pandemic. In this study, a chemical genetics approach was taken to identify inhibitors of NS1, a major influenza A virus virulence factor that inhibits host gene expression. A high-throughput screen of 200,000 synthetic compounds identified small molecules that reversed NS1-mediated inhibition of host gene expression. A counterscreen for suppression of influenza virus cytotoxicity identified naphthalimides that inhibited replication of influenza virus and vesicular stomatitis virus (VSV). The mechanism of action occurs through activation of REDD1 expression and concomitant inhibition of mammalian target of rapamycin complex 1 (mTORC1) via TSC1–TSC2 complex. The antiviral activity of naphthalimides was abolished in REDD1−/− cells. Inhibition of REDD1 expression by viruses resulted in activation of the mTORC1 pathway. REDD1-/- cells prematurely upregulated viral proteins via mTORC1 activation and were permissive to virus replication. In contrast, cells conditionally expressing high concentrations of REDD1 downregulated the amount of viral protein. Whole animal studies revealed REDD1-/- mice are highly susceptible to virus infection. Influenza infection of REDD1-/- mice results in decreased TLR7 and MHC class II expression by dendritic cells and macrophages. In addition, excessive inflammatory cell infiltration in the lungs of REDD1-/- infected mice was observed. Preliminary evidence suggests a potential defect in NF-κB signaling upon influenza virus infection in REDD1 deficient mice. Thus, REDD1 is a new host defense factor, and chemical activation of REDD1 expression represents a potent antiviral intervention strategy. Our studies also reveal passage immortalization of REDD1-/- MEFs require loss of the type I IFN response pathway as these cells are unable to induce the expression of interferon genes and interferon inducible genes when challenged with synthetic dsRNA. In contrast, primary or SV40 large T antigen transformed REDD1-/- MEFs activate a type I IFN response when exposed to synthetic dsRNA.