Deficiency of vitamin A delays bone healing process in association with reduced BMP2 expression after drill-hole injury in mice

Although it is predicted that vitamin A and its active form, retinoic acid, regulate osteoblast lineage, this has not been elucidated in growing mammalians. To clarify the direct effect of retinoic acid on bone, we observed the process of filling up newly generating bone into a drill-hole of the bone, which is understood as membranous ossification, in vitamin A-deficient mice. Mice were assigned to three groups: a vitamin A-deficient group (VAD), which was fed a diet without vitamin A from the 10th day of gestation to the end of the experiments; a vitamin A-deficient-sufficient group (VADS), which was fed a diet without vitamin A from the 10th day of gestation to 4 weeks of age; and a vitamin A-sufficient group (VAS), which was fed a standard diet to the end of the experiment. In mice at 10 weeks of age (day 0), a drill-hole injury was made with a diameter of 1 mm at the anterior portion of the diaphysis of the bilateral femurs. In VAD, retardation in repairing the drill-hole was demonstrated by in vivo micro-CT and histomorphometry from day7 and after surgery. During repair of the bone defect, increases of bmp2, dlx5, msx2, col1a1, and osteocalcin mRNA expression were suppressed, and runx2-p2 mRNA expression was accelerated in VAD. Implantation of BMP2 in the bone defect of VAD normalized the delayed bone healing and mRNA expressions. We concluded that vitamin A regulates bmp2 mRNA expression and plays a crucial role in osteoblastogenesis and bone formation.