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cAMP activation by PACAP/VIP stimulates IL‐6 release and inhibits osteoblastic differentiation through VPAC2 receptor in osteoblastic MC3T3 cells

The neuropeptide pituitary adenylate cyclase‐activating polypeptide (PACAP), a member of the glucagon/vasoactive intestinal peptide (VIP) superfamily, stimulates cyclic AMP accumulation initiating a variety of biological processes such as: neurotropic actions, immune and pituitary function, learning and memory, catecholamine biosynthesis and regulation of cardiopulmonary function. Both osteoclasts and osteoblasts have been shown to express receptors for PACAP/VIP implicated in their role in bone metabolism. To further understand the role of PACAP/VIP family in controlling bone metabolism, we investigated differentiation model of MC3T3‐E1 cells, an osteoblastic cell line derived from mouse calvaria. Quantitative RT‐PCR analysis demonstrated that MC3T3‐E1 cells expressed only VPAC2 receptor and its expression was upregulated during osteoblastic differentiation, whereas VPAC1 and PAC1 receptors were not expressed. Consistent with expression of receptor subtype, both PACAP and VIP stimulate cAMP accumulation in a time‐ and dose‐dependent manner with the similar potency in undifferentiated and differentiated cells, while Maxadilan, a specific agonist for PAC1‐R, did not. Furthermore, downregulation of VPAC2‐R by siRNA completely blocked cAMP response mediated by PACAP and VIP. Importantly, PACAP/VIP as well as forskolin markedly suppressed the induction of alkaline phosphatase mRNA upon differentiation and the pretreatment with 2′,5′‐dideoxyadenosine, a cAMP inhibitor, restored its inhibitory effect of PACAP. We also found that PACAP and VIP stimulated IL‐6 release, a stimulator of bone resorption, and VPAC2‐R silencing inhibited IL‐6 production. Thus, PACAP/VIP can activate adenylate cyclase response and regulate IL‐6 release through VPAC2 receptor with profound functional consequences for the inhibition of osteoblastic differentiation in MC3T3‐E1 cells. J. Cell. Physiol. 221: 75–83, 2009. © 2009 Wiley‐Liss, Inc