Inhibition of IFN-γ-induced Janus kinase-1-STAT1 activation in macrophages by vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide

The vasoactive intestinal peptide (VIP) and the pituitary adenylate cyclase-activating polypeptide (PACAP), two immunomodulatory neuropeptides that affect both innate and acquired immunity, down-regulate IL-12 p40 and inducible NO synthase expression in LPS/IFN-gamma -stimulated macrophages. We showed previously that VIP/PACAP inhibit NF-kappaB nuclear translocation through the stabilization of I kappaB and reduce IFN regulatory factor-1 (IRF-1) binding to the regulatory elements found in the IL-12 p40 and inducible NO synthase promoters. In this paper we studied the molecular mechanisms involved in the VIP/PACAP regulation of IRF-1 transactivating activity, Our studies indicate that the inhibition in IRF-1 binding correlates with a reduction in IRF-1 protein and mRNA in IFN-gamma -treated Raw 264.7 macrophages. In agreement with the described Janus kinase (Jak)1/ Jak2/STAT1/IRF-1 activation pathway, VIP/PACAP inhibit Jak1/Jak2, STAT1 phosphorylation, and the binding of STAT1 to the GAS sequence motif in the IRF-1 promoter. The effects of VIP/PACAP are mediated through the specific VIP/PACAP receptor-1 and the cAMP/protein kinase A (PKA) transduction pathway, but not through the induction of suppressor of cytokine signaling-1 or suppressor of cytokine signaling-3, Because IFN-gamma is a major stimulator of innate immune responses in vivo, the downregulation of IFN-gamma -induced gene expression by VIP and PACAP could represent a significant element in the regulation of the inflammatory response by endogenous neuropeptides.