2α-Substituted Vitamin D Derivatives Effectively Enhance the Osteoblast Differentiation of Dedifferentiated Fat Cells

The active form of vitamin D₃, 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], is a principal regulator of calcium homeostasis through activation of the vitamin D receptor (VDR). Previous studies have shown that 2α-(3-hydroxypropyl)-1,25D₃ (O1C3) and 2α-(3-hydroxypropoxy)-1,25D₃ (O2C3), vitamin D derivatives resistant to inactivation enzymes, can activate VDR, induce leukemic cell differentiation, and increase blood calcium levels in rats more effectively than 1,25(OH)₂D₃. In this study, to further investigate the usefulness of 2α-substituted vitamin D derivatives, we examined the effects of O2C3, O1C3, and their derivatives on VDR activity in cells and mouse tissues and on osteoblast differentiation of dedifferentiated fat (DFAT) cells, a cell type with potential therapeutic application in regenerative medicine. In cell culture experiments using kidney-derived HEK293 cells, intestinal mucosa-derived CaCO₂ cells, and osteoblast-derived MG63 cells, and in mouse experiments, O2C2, O2C3, O1C3, and O1C4 had a weaker effect than or equivalent effect to 1,25(OH)₂D₃ in VDR transactivation and induction of the VDR target gene CYP24A1, but they enhanced osteoblast differentiation in DFAT cells equally to or more effectively than 1,25(OH)₂D₃. In long-term treatment with the compound without the medium change (7 days), the derivatives enhanced osteoblast differentiation more effectively than 1,25(OH)₂D₃. O2C3 and O1C3 were more stable than 1,25(OH)₂D₃ in DFAT cell culture. These results indicate that 2α-substituted vitamin D derivatives, such as inactivation-resistant O2C3 and O1C3, are more effective than 1,25(OH)₂D₃ in osteoblast differentiation of DFAT cells, suggesting potential roles in regenerative medicine with DFAT cells and other multipotent cells.