STAT2 Is a Pervasive Cytokine Regulator due to Its Inhibition of STAT1 in Multiple Signaling Pathways.

STAT2 is the quintessential transcription factor for type 1 interferons (IFNs), where it functions as a heterodimer with STAT1. However, the human and murine STAT2-deficient phenotypes suggest important additional and currently unidentified type 1 IFN-independent activities. Here, we show that STAT2 constitutively bound to STAT1, but not STAT3, via a conserved interface. While this interaction was irrelevant for type 1 interferon signaling and STAT1 activation, it precluded the nuclear translocation specifically of STAT1 in response to IFN-γ, interleukin-6 (IL-6), and IL-27. This is explained by the dimerization between activated STAT1 and unphosphorylated STAT2, whereby the semiphosphorylated dimers adopted a conformation incapable of importin-α binding. This, in turn, substantially attenuated cardinal IFN-γ responses, including MHC expression, senescence, and antiparasitic immunity, and shifted the transcriptional output of IL-27 from STAT1 to STAT3. Our results uncover STAT2 as a pervasive cytokine regulator due to its inhibition of STAT1 in multiple signaling pathways and provide an understanding of the type 1 interferon-independent activities of this protein.

Evidence against a role for ß-arrestin1 in STAT1 dephosphorylation and the inhibition of interferon-γ signaling.

Signal transducer and activator of transcription 1 (STAT1) is activated by tyrosine phosphorylation upon interferon-γ (IFNγ) stimulation, which results in the expression of genes with antiproliferative and immunomodulatory functions. The inactivation of STAT1 occurs through tyrosine dephosphorylation by the tyrosine phosphatase TC45. It was proposed that recruitment of TC45 required the direct interaction of STAT1 with the scaffold protein ß-arrestin1, making ß-arrestin1 an essential negative regulator of STAT1 and IFNγ signaling (Mo et al., 2008). We tested the relevance of ß-arrestin1 for STAT1 activity. Our results do not confirm ß-arrestin1 as a STAT1-interacting protein. The STAT1 phosphorylation/dephosphorylation cycle was found to be unaffected by both the overexpression and the genetic deletion of ß-arrestin1. Accordingly, ß-arrestin1 did not inhibit STAT1 transcriptional activity or the induction of IFNγ target genes in response to IFNγ. Our data indicate that ß-arrestin1 is dispensable for STAT1 dephosphorylation and the termination of IFNγ signaling.