Bone Abstracts

Objectives: A role for glucose-dependent insulinotropic polypeptide (GIP) in controlling extracellular bone matrix deposition and maturation has previously been evidenced in GIP receptor knock-out mice. However, as the GIP receptor is expressed in several tissues other than bone it was difficult to ascertain whether the modifications of extracellular bone matrix were due to direct effects on osteoblasts or indirect through regulation of signals originating from other tissues. The aim of the present study was to assess in osteoblast cultures in vitro whether GIP could affect collagen deposition and maturation.

Materials and methods: The murine MC3T3-E1 cells were used as the source of osteoblasts. A GIP mimetic, the D-ala(2)-GIP (GIP) was synthesized using standard solid phase Fmoc protocols. MC3T3-E1 cells were differentiated in the presence of ascorbic acid (50 μg/ml) and various concentrations of GIP ranged between 10–100 pM. Amine oxidase activities were assessed in supernatant of differentiated cells in response to GIP by a fluorometric assay. Collagen deposition and maturation were assessed by transmitting electron microscopy and Fourier-transformed infrared microscopy (1660/1690 per cm ratio) respectively. Non-parametric Mann–Whitney U-test was used to compare differences between groups.

Results: Compared to untreated cells, GIP dose-dependently stimulated the production of cAMP by osteoblasts. Furthermore, the amine oxidase activity was dose-dependently increase by GIP treatment to reach a 95% augmentation at 100 pM GIP. Furthermore, GIP affected collagen deposition as evidenced by significant decreases in collagen fibril diameters. Collagen maturity was also affected with significant augmentation of the 1660/1690 percm ratio. The use of 2′,5′-dideoxyadenosine, an adenylyl cyclase inhibitor, markedly blocked the action of GIP on amine oxidase activity, collagen deposition and maturation.

Conclusions: Overall, this study provides evidences that GIP acts on osteoblasts through an andenylyl cyclase-cAMP pathway and directly affects collagen deposition and maturation.