Sequential activation of protein kinase C δ and JNK is required for interferon-α-induced expression of IFIT4

A multitude of interferon (IFN)-inducible genes (IFIGs) are coordinately expressed in peripheral blood mononuclear cells (PBMCs) of patients with systemic lupus erythematosus (SLE), emphasising the globle activating of signal pathway mediated by IFN-I in SLE. In this study, we investigated the mechanisms of expression regulation of IFIT4 (interferon induced protein with tetratricopeptide repeats 4) by IFN-α. We found that IFN-α failed in inducing IFIT4 in STAT1-negative U3A cells. Ectopic expression of STAT1, but not mutant STAT1-S727A, almost completely restored IFN-α2a-induced IFIT4 expression. IFN-α induced the expression of IFIT4 and STAT1 in THP-1 cells, and this process was significantly antagonized by the specific inhibitors of both PKCδ and JNK or their dominant negative mutants respectively. The inhibition of JNK activity by its specific inhibitor or its dominant negative mutant suppressed both IFIT4 expression and serine phosphorylation of STAT1 but not the activation of PKCδ, while inhibition of PKCδ suppressed activation of IFIT4, STAT1, and JNK. Our results suggest that the induction of IFIT4 transcription by IFN-α depends upon sequential activation of PKCδ, JNK and STAT1, and that the influence of PKCδ or JNK on IFN-α-mediated induction of IFIT4 is dependent upon the phosphorylation of STAT1 at Ser-727. The results in our experiment provide an in vitro model of the signaling mechanisms of IFIGs regulated by IFN-α, that is putatively thought to occur in vivo as the one of pathogenesis of SLE.