title: Ghrelin Increases Beta-Catenin Level through Protein Kinase A Activation and Regulates OPG Expression in Rat Primary Osteoblasts.
abstract: Ghrelin, by binding growth hormone secretagogue receptor (GHS-R), promotes osteoblast proliferation but the signaling mechanism of GHS-R on these cells remains unclear. Since canonical Wnt/β-catenin pathway is critically associated with bone homeostasis, we investigated its involvement in mediating ghrelin effects in osteoblasts and in osteoblast-osteoclast cross talk. Ghrelin (10(-10)M) significantly increased β-catenin levels in rat osteoblasts (rOB). This stimulatory action on β-catenin involves a specific interaction with GHS-R1a, as it is prevented by the selective GHS-R1a antagonist, D-Lys(3)-GHRP-6 (10(-7)M). The effect of ghrelin on β-catenin involves the phosphorylation and inactivation of GSK-3β via protein kinase A (PKA). Inhibition of PKA activity reduces the facilitatory action of ghrelin on β-catenin stabilization. Ghrelin treatment of rOB significantly increases the expression of osteoprotegerin (OPG), which plays an important role in the regulation of osteoclastogenesis, and this effect is blocked by D-Lys(3)-GHRP-6. Furthermore, ghrelin reduced RANKL/OPG ratio thus contrasting osteoclastogenesis. Accordingly, conditioned media from rOB treated with ghrelin decreased the number of multinucleated TRAcP+ cells as compared with the conditioned media from untreated-control rOB. Our data suggest new roles for ghrelin in modulating bone homeostasis via a specific interaction with GHSR-1a in osteoblasts with subsequent enhancement of both β-catenin levels and OPG expression.
authors: Department of Medical Biotechnology and Translational Medicine, Medical Pharmacology Unit, Università degli Studi di Milano, Via Vanvitelli 32, 20129 Milano, Italy.
authors: Emanuela
authors: E
authors: Mrak
authors: Department of Medical Biotechnology and Translational Medicine, Medical Pharmacology Unit, Università degli Studi di Milano, Via Vanvitelli 32, 20129 Milano, Italy.
authors: Lavinia
authors: L
authors: Casati
authors: Department of Medical Biotechnology and Translational Medicine, Medical Pharmacology Unit, Università degli Studi di Milano, Via Vanvitelli 32, 20129 Milano, Italy.
authors: Francesca
authors: F
authors: Pagani
authors: Bone Metabolism Unit, Scientific Institute San Raffaele, Via Olgettina 60, 20132 Milano, Italy.
authors: Alessandro
authors: A
authors: Rubinacci
authors: Department of Medical and Surgical Specialties, Radiological Sciences and Public Health, University of Brescia, Viale Europa 11, 25123 Brescia, Italy.
authors: Guido
authors: G
authors: Zarattini
authors: Department of Medical Biotechnology and Translational Medicine, Medical Pharmacology Unit, Università degli Studi di Milano, Via Vanvitelli 32, 20129 Milano, Italy.
authors: Valeria
authors: V
authors: Sibilia