STAT2 and IRF9: Beyond ISGF3

Abstract

Cytokine signaling is mediated by the combinatorial usage of seven STAT proteins that form homo- or heterodimers involved in the regulation of specific transcriptional programs. Among STATs, STAT2 is classically known to dimerize with STAT1 and together with IRF9 forms the ISGF3 transcription factor complex that has long been considered a hallmark of activation by type I and type III interferons. However, accumulating evidence reveal distinct facets of STAT2 and IRF9 activity mediated by the segregation in alternative STAT1-independent complexes/pathways that are thought to trigger different transcriptional programs. The goal of this review is to summarize our current knowledge of the stimuli, regulatory mechanisms, and function of these alternative pathways.

Keywords: IRF9; ISGF3; ISGs; STAT; TNFα; gene expression; interferon; signaling; synergism.

Figure 1. STAT2- and/or IRF9-containing complexes. The canonical type I and type III IFN signaling leads to the formation of the classical ISGF3 complex composed of STAT1, STAT2, and IRF9. In this complex, STAT1 is phosphorylated on Tyr701 (green) and STAT2 is phosphorylated on Tyr690 (violet) as a result of JAK1 and Tyk2 activation. ISGF3 regulates the expression of hundreds of ISGs containing an interferon stimulated response element (ISRE) in their promoter region. In the recent years, alternative complexes containing STAT2 and/or IRF9 have been described in various contexts either in combination with STAT1 (STAT1-dependent) or without STAT1 (STAT1-independent). Some STAT1-dependent complexes are distinct from the ISGF3 complex by their phosphorylation. For instance, in the ISGF3^II complex STAT2 is not phosphorylated on Tyr690. In the ISGF3^III complex, STAT1 is phosphorylated on Ser708 (pink). Additionally, alternative complexes are formed by association with different STAT partners, including STAT6 and STAT3. In some STAT6-containing complexes, STAT6 is also phosphorylated on a yet unknown residue (orange). These different complexes regulate gene expression though diverse consensus sequences, including ISRE, IFNγ-activated sequence (GAS), GAS-like, and palindromic IFN response element (pIRE). References are indicated in brackets.

Figure 2. Canonical and non-canonical functions of STAT2 and IRF9 in the antiviral response. Cells respond to virus infection through the activation of multiple signaling pathways, leading to the activation of AP-1 (ATF-2/c-jun), NFκB and interferon regulatory factor 3 (IRF3). These transcription factors regulate the expression of various proinflammatory and antiviral cytokines and chemokines, including IFNβ and TNFα. Binding of IFNβ to its cognate receptor activates the “classical” antiviral pathway through activation of Tyk2 and JAK1 kinases ultimately leading to the formation of ISGF3. ISGF3 regulates the expression of multiple interferon stimulated genes (ISGs). Additionally, the synergism between IFNβ and TNFα induces late gene expression through a non-canonical antiviral STAT2- and IRF9-dependent, but STAT1-independent, pathway. This pathway is dependent on Tyk2 kinase activity and requires phosphorylation of STAT2 on Tyr690. This pathway triggers the delayed expression of an alternative ISG panel and, at least in airway epithelial cells, of the DUOX2/DUOXA2 genes encoding for a functional NADPH oxidase enzyme.