1ɑ,25-Dihydroxyvitamin D promotes osteogenesis by down-regulating FGF23 in diabetic mice.

1ɑ,25-dihydroxyvitamin D3 (1,25D) and fibroblast growth factor 23 (FGF23) play important roles in bone metabolism through mutual regulation. However, the underlying mechanism between 1,25D and FGF23 in diabetes-induced bone metabolism disorders has not yet been elucidated. In this study, we investigated the effect of 1,25D on FGF23 under diabetic condition both in vitro and in vivo. The results showed that 1,25D down-regulated the expression of FGF23 in osteoblast significantly though a dose-dependent manner in vitro within high glucose environment. Western blot and immunofluorescence analysis indicated that 1,25D activated PI3K/Akt signalling through binding to vitamin D receptor (VDR), which inhibited the phosphorylation of the transcription factor Forkhead Box O1 (FOXO1). Decreased phosphorylation of FOXO1 down-regulated the expression Dickkopf-1 (DKK1), a well-known inhibitor of Wnt signalling. In addition, we observed that 1,25D remarkably ameliorated osteogenic phenotypic markers such as Ocn and Runx2 and rescued diabetes-induced bone loss in vivo. Our results suggested that 1,25D could promote osteogenesis though down-regulating FOXO1/FGF23 in diabetes.