Purpose: We assessed whether 1,25(OH)2D plays a role in prevention of mandibular osteoporosis via the NAD-dependent deacetylase sirtuin-1 (Sirt1).
Methods: 1α(OH)ase-/- mice with Sirt1 overexpression in MSCs were generated for comparing mandibular phenotypes with Prx1-driven Sirt1 transgenic, 1α(OH)ase-/-, and WT mice using imaging, histopathology, and cellular molecular biology methods. Chromatin immunoprecipitation (ChIP), electrophoretic mobility shift assay (EMSA) and luciferase assays were used to assess 1,25(OH)2D regulation of Sirt1.
Results: We first demonstrated that 1,25(OH)2D3 promoted osteogenic differentiation of bone marrow mesenchymal stem cells (BM-MSCs) isolated from mandibles, with up-regulation of Sirt1 expression. Bioinformatic analysis suggested the presence of a VDR response element-like sequence in the Sirt1 promoter. ChIP and EMSA demonstrated that the VDR could physically bind the Sirt1 promoter. Luciferase activity increased significantly in BM-MSCs transfected with a Sirt1-GV238 plasmid compared with the empty plasmid or a mutant plasmid, and activity was further increased in the presence of 1,25(OH)2D. Overexpression of Sirt1 in MSCs on a 1α(OH)ase-/- background corrected 1,25(OH)2D deficiency-induced mandibular osteoporosis by enhancing osteoblastic bone formation, reducing osteoclastic bone resorption, and increasing alveolar bone mass and mandibular BMD. Overexpression of Sirt1 in MSCs of 1α(OH)ase-/- mice also corrected 1,25(OH)2D deficiency-induced oxidative stress, cellular senescence and senescence associated secretory phenotypes (SASP) with up-regulation of the expression levels of forkhead box protein O3 (FOXO3) and superoxide dismutase (SOD)1/2, reduction of the percentage of p16, p21 and β-Gal positive cells and down-regulation of p16, p53, p21, IL-1/6, and MMP3/13 expression levels. The Sirt1 agonist resveratrol promoted the osteogenic differentiation of BM-MSCs isolated from mandibles, by up-regulating Sirt1 and FOXO3 expression levels, increasing the interaction between Sirt1 and FOXO3, reducing the level of FOXO3 acetylation and promoting its nuclear translocation.
Conclusions: 1,25(OH)2D can up-regulate Sirt1 expression through VDR-mediated transcription, increase FOXO3 deacetylation and nuclear translocation, reduce oxidative stress, cellular senescence and SASP, and promote osteogenic differentiation. Sirt1 may therefore mediate the anti-mandibular osteoporosis effects of 1,25(OH)2D.