This information is current as Signaling by Stabilizing STAT 1 Ubiquitin-Specific Protease 13 Regulates IFN

The IFN immune system comprises type I, II, and III IFNs, signals through the JAK-STAT pathway, and plays central roles in host defense against viral infection. Posttranslational modifications such as ubiquitination regulate diverse molecules in the IFN pathway. To search for the deubiquitinating enzymes (DUBs) involved in the antiviral activity of IFN, we used RNA interference screening to identify a human DUB, ubiquitin-specific protease (USP) 13, whose expression modulates the antiviral activity of IFN-a against dengue virus serotype 2 (DEN-2). The signaling events and anti–DEN-2 activities of IFN-a and IFN-g were reduced in cells with USP13 knockdown but enhanced with USP13 overexpression. USP13 may regulate STAT1 protein because the protein level and stability of STAT1 were increased with USP13 overexpression. Furthermore, STAT1 ubiquitination was reduced in cells with USP13 overexpression and increased with USP13 knockdown regardless of with or without IFN-a treatment. Thus, USP13 positively regulates type I and type II IFN signaling by deubiquitinating and stabilizing STAT1 protein. Overall, to our knowledge, USP13 is the first DUB identified to modulate STAT1 and play a role in the antiviral activity of IFN against DEN-2 replication. T he cytokines IFNs play important roles in antiviral, anti-proliferative, and antitumor activities. There are three IFN families: type I IFNs (including IFN-b and diverse species of IFN-a) are produced by numerous cell types, and type II (IFN-g) and type III (IFN-l) IFNs are largely restricted to immune and epithelial cells, respectively (1, 2). After binding to IFN receptors, IFNs primarily signal through receptor-associated JAK kinases, which leads to activation of STAT proteins (3). With type I IFN stimulation, phosphorylated STAT1 and STAT2 di-merize and associate with IFN regulatory factor 9 to form IFN-stimulated gene factor 3, which then binds to specific DNA sequences named IFN-stimulated response elements (ISREs). Type II IFN triggers the homodimer formation of phosphorylated STAT1 and activates IFN-stimulated genes by binding to IFN-g activation sequence (GAS) (4, 5). The IFN pathway is tightly regulated by several posttranslation-al mechanisms such as phosphorylation, acetylation, and ubiq-uitination (6–8). Ubiquitin, a 76-aa polypeptide, can be covalently attached to cellular proteins by an enzymatic cascade involving three classes of enzymes, E1, E2, and E3 (9). Ubiquitin forms an isopeptide linkage with substrate protein via the C-terminal glycine residue of ubiquitin and the lysine residue on the substrate protein. and K63); each can form an isopeptide bond with the C-terminal glycine residue of the following ubiquitin. Monoubiquitination …