ID: 242: Combinatorial association and abundance of ISGF3 components dictate selectivity and kinetics of interferon responses

IFN α induces ISG expression by phosphorylating STAT1 and STAT2. STAT1 homodimers facilitate transcriptional responses by directly activating ISGs containing GAS element. Combinatorial association of STAT1 and STAT2 with the additional DNA binding protein IRF9 (forming ISGF3) induce ISRE-containing ISGs. Evidence is accumulating for the existence of a STAT2/IRF9-dependent, STAT1-independent IFN α signaling pathway that generates ISGF3-like response. However, no detailed insight exists in the genome-wide transcriptional regulation and the biological implications of STAT2/IRF9 dependent IFN α signaling as compared to ISGF3 and how WT- and alternative responses are dictated. In hST2-U3C IFN α -induced expression of classical ISGs correlated with the kinetics of STAT2 phosphorylation, and the presence of STAT2/IRF9 complex. Expression profiling of IFN α treated 2fTGH and hST2-U3C identified 120 known antiviral ISRE-containing ISGs commonly up-regulated by STAT2/IRF9 and ISGF3. The STAT2/IRF9 directed expression profile of these ISGs was prolonged as compared to the transient response mediated by ISGF3. ChIP-seq analysis confirmed binding of STAT2 to the ISG promoters, in IFN α -dependent STAT1-independent manner. STAT2/IRF9 was able to generate a sustained antiviral response upon EMCV and VSV. Interestingly, IFN α -treated NB4 cells that display transient phosphorylation of STAT1 and prolonged phosphorylation of STAT2, revealed transient STAT1-mediated ISG expression and prolonged expression of STAT2/IRF9-dependent genes. Our results prove that IFN α -activated STAT2/IRF9 induces prolonged ISGF3-like transcriptome and generates an antiviral response in the absence of STAT1. We provide additional evidence that in the presence of STAT1, abundance and phosphorylation kinetics of ISGF3 components dictates the nature of IFN α responses.