Figure 2. Transduction pathways and transcription factors involved in the effects of VIP and PACAP on activated macrophages. (A) Effects on the IFN
-induced transduction pathway. VIP/PACAP bind to VPAC1, induce cAMP, and inhibit Jak1/Jak2 phosphorylation, and subsequent STAT1 phosphorylation and binding to the GAS sequence. As a consequence, IRF-1 transcription and synthesis is inhibited, and VIP/PACAP inhibit IRF-1 binding to sequences from both iNOS and IL12p40 genes. (B) Effects on the AP-1 transcription factor. VIP/PACAP bind to the VPAC1 receptor, induce cAMP, and exert two different effects: (a) inhibition of MEKK1 phosphorylation, and subsequent phosphorylation of MEK4, JNK, and c-Jun; and (b) up-regulation of JunB synthesis and nuclear translocation. As a result, the complexes binding to the AP-1 sites in activated macrophages treated with VIP or PACAP contain JunB/c-Fos instead of c-Jun/c-Fos, the transactivating complexes found in LPS-stimulated macrophages. (C) Effects on NF
B. VIP/PACAP bind to VPAC1 and initiate two transduction pathways, a cAMP-dependent and a cAMP-independent (unidentified yet) pathway. The cAMP-independent pathway is responsible for the inhibition of the activity of the specific IKK kinase, resulting in the stabilization of the IkB inhibitor. As a result, p65 nuclear translocation and binding to the NFB sequence are inhibited. The cAMP-dependent pathway affects two separate transduction pathways: (a) phosphorylation of CREB (CREB-P), which, upon nuclear translocation, has a high affinity for the co-activator CBP and sequesters CBP, preventing its further interaction with NFB; and (b) inhibition of MEKK1 phosphorylation, subsequent phosphorylation of MEK3/6, p38, and TBP. Non-phosphorylated TBP does not bind efficiently to the TATA box, and does not form an active transactivating complex with CBP and NFB. Reduction in the amounts of nuclear p65, CBP, and phosphorylated TBP inhibits the formation of the conformationally active transactivating complex required for the transcription of most cytokine and chemokine genes. (D) Effects on IL-10 gene expression. VIP/PACAP bind to VPAC1 and induce cAMP, which leads to the phosphorylation of CREB. CREB-P, upon nuclear translocation, binds to the CRE sequence and up-regulates IL-10 gene expression following LPS stimulation.
