1,25-Dihydroxyvitamin D Protects Against Age-related Osteoporosis by a Novel VDR-Ezh2-p16 Signal Axis

To determine whether 1,25-dihydroxyvitamin D (1,25(OH) D) can exert an anti-osteoporosis role through anti-aging mechanisms, we analyzed the bone phenotype of mice with 1,25(OH) D deficiency due to deletion of the enzyme, 25-hydroxyvitamin D 1α-hydroxylase, while on a rescue diet. 1,25(OH) D deficiency accelerated age-related bone loss by activating the p16/p19 senescence signaling pathway, inhibiting osteoblastic bone formation, and stimulating osteoclastic bone resorption, osteocyte senescence, and senescence-associated secretory phenotype (SASP). Supplementation of exogenous 1,25(OH) D corrected the osteoporotic phenotype caused by 1,25(OH) D deficiency or natural aging by inhibiting the p16/p19 pathway. The proliferation, osteogenic differentiation, and ectopic bone formation of bone marrow mesenchymal stem cells derived from mice with genetically induced deficiency of the vitamin D receptor (VDR) were significantly reduced by mechanisms including increased oxidative stress, DNA damage, and cellular senescence. We also demonstrated that p16 deletion largely rescued the osteoporotic phenotype caused by 1,25(OH) D deficiency, whereas 1,25(OH) D could up-regulate the enzyme Ezh2 via VDR-mediated transcription thereby enriching H3K27me3 and repressing p16/p19 transcription. Finally, we demonstrated that treatment with 1,25(OH) D improved the osteogenic defects of human BM-MSCs caused by repeated passages by stimulating their proliferation and inhibiting their senescence via the VDR-Ezh2-p16 axis. The results of this study therefore indicate that 1,25(OH) D plays a role in preventing age-related osteoporosis by up-regulating Ezh2 via VDR-mediated transcription, increasing H3K27me3 and repressing p16 transcription, thus promoting the proliferation and osteogenesis of BM-MSCs and inhibiting their senescence, while also stimulating osteoblastic bone formation, and inhibiting osteocyte senescence, SASP, and osteoclastic bone resorption.