Increased expression of interleukin-6 by vasoactive intestinal peptide is associated with regulation of CREB, AP-1 and C/EBP, but not NF-κB, in mouse calvarial osteoblasts

Abstract

Interleukin-6 (IL-6), and the related cytokines IL-11, leukemia inhibitory factor (LIF) and oncostatin M (OSM), are potent stimulators of osteoclastic bone resorption. In the present study, we have addressed the possibility that the neuropeptide vasoactive intestinal peptide (VIP) may regulate the production of and/or sensitivity to the IL-6 family of cytokines in mouse calvarial osteoblasts. VIP stimulated IL-6 mRNA expression and protein release in a time- and concentration-dependent manner, whereas mRNA expression of the IL-6 receptor, as well as mRNA expressions of IL-11, LIF, OSM and their cognate receptors, were unaffected by VIP. In cells transfected with the IL-6 promoter coupled to luciferase, VIP increased transcriptional activity. The effects of VIP were shared by the related neuropeptide PACAP-38, belonging to the same superfamily of neuropeptides, whereas secretin did not have any effect, indicating that the effects were mediated by VPAC 2 receptors. The effects of VIP were potentiated by the cyclic AMP phosphodiesterase inhibitor rolipram and mimicked by forskolin, indicating the involvement of the cyclic AMP/protein kinase A pathway. This was further demonstrated by the facts that the stimulatory effect of VIP on luciferase activity could be reversed by the PKA inhibitors H-89 and KT5720 and was mimicked by cyclic AMP analogues selective for PKA, but not by those selective for Epac. In addition, VIP enhanced the phosphorylation of CREB, as assessed by both immunocytochemical analysis and Western blot. The DNA binding activity of nuclear extracts to C/EBP was increased by VIP, whereas binding to AP-1 was decreased. In contrast, DNA binding to NF-κB, as well as nuclear translocation of NF-κB and C/EBP, were unaffected by VIP. The mRNA expressions of C/EBPβ, C/EBPδ, C/EBPγ, c-Jun, JunB, c-Fos, Fra-1 and IκBα and protein level of IκBα were all unaffected by VIP. These observations, together, demonstrate that VIP stimulates IL-6 production in osteoblasts by a mechanism likely to be mediated by VPAC 2 receptors and dependent on cyclic AMP/protein kinase A/CREB activation and also involving the transcription factors C/EBP and AP-1.