GIP receptor reduces osteoclast activity and improves osteoblast survival by activating multiple signaling pathways
Bone is a dynamic tissue that is remodeled throughout life by bone resorbing osteoclasts and bone forming osteoblasts, to adapt to physiological or mechanical demands. These processes are impaired in osteoporosis, and understanding how bone remodeling is regulated could improve anti-osteoporotic treatments. Clinical investigations show that short-term treatment with glucose-dependent insulinotropic polypeptide (GIP) acutely decreases serum markers of bone resorption and may increase bone formation. However, evidence for direct effects of GIP intracellular signaling and functions in mature human osteoclasts and osteoblasts have not been investigated. We report that the GIP receptor (GIPR) is robustly expressed in mature human osteoclasts. Exposure of osteoclasts to GIP inhibits osteoclastogenesis, delays bone resorption, and increases osteoclast apoptosis by acting upon multiple signaling pathways (cAMP, Src, Akt, calcium, p38) to impair nuclear translocation of nuclear factor of activated T cells 1 (NFATc1) and nuclear factor-{kappa}B (NF{kappa}B). Human osteoblasts also express GIPR, and GIP improves osteoblast survival via cAMP and Akt-mediated pathways. GIP treatment of co-cultures of osteoclasts and osteoblasts also decreased bone resorption. Antagonizing GIPR with GIP(3-30)NH2 abolished the effects of GIP on osteoclasts and osteoblasts. This study demonstrates that GIP inhibits bone resorption and improves survival of human osteoblasts, which could increase bone mass and strength, supporting clinical investigations of the effect of GIP on bone. Moreover, this study demonstrates that GIPR agonism could be beneficial in the treatment of disorders of bone remodeling, such as osteoporosis. One-sentence SummaryGIP acts directly on bone cells to regulate bone remodeling