Calcitonin Gene-Related Peptide (CGRP) Inhibits Apoptosis in Human Osteoblasts by β-Catenin Stabilization

Transgenic mice over-expressing calcitonin gene-related peptide (CGRP) in osteoblasts have increased bone density due to increased bone formation, thus suggesting that CGRP plays a role in bone metabolism. In this study we determined the relationship between CGRP, the canonical Wnt signaling and apoptosis in human osteoblasts (hOBs) in consideration of the well-documented involvement of this pathway in bone cells. Primary cultures of F OBs were treated with CGRP 10 (8) M. Levels of beta-catenin, which is the cytoplasmic protein mediator of canonical Wnt signaling, and mRNA were determined. CGRP increases both the expression and the levels of cytoplasmic beta-catenin by binding to its receptor, as this effect is blocked by the antagonist CGRP(8 37). This facilitatory action on beta-catenin appears to be mediated by the inhibition of the enzyme GSK-3 beta via protein kinase A (PKA) activation. GSK-3 beta is a glycogen synthase kinase that, by phosphorylating beta-catenin, promotes its dagradation by the proteosomal machinery. Moreover, the peptide is able to inhibit hOBs apoptosis stimulated by dexamethasone or by serum deprivation, possibly through the accumulation of beta-catenin, since the inhibitor of PKA activity H89 partially prevents the antiapoptotic effect of the peptide. In conclusion CGRP, released by nerve fibers, exerts its anabolic action on bone cells by stimulating canonical Wnt signaling and by inhibiting hOBs apoptosis, thus favoring local bone regeneration. J. Cell. Physiol. 225: 701-708, 2010. (C) 2010 Wiley-Liss, Inc.