CYP24A1-deficiency does not affect bone regeneration in distraction osteogenesis

Optimal bone fracture repair requires 24R,25-dihydroxyvitamin D3 and its effector molecule FAM57B2

The biological activity of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] remains controversial, but it has been suggested that it contributes to fracture healing. Cyp24a1-/- mice, synthesizing no 24R,25(OH)2D3, show suboptimal endochondral ossification during fracture repair, with smaller callus and reduced stiffness. These defects were corrected by 24R,25(OH)2D3 treatment, but not by 1,25-dihydroxyvitamin D3. Microarrays with Cyp24a1-/- callus mRNA identified FAM57B2 as a mediator of the 24R,25(OH)2D3 effect. FAM57B2 produced lactosylceramide (LacCer) upon specific binding of 24R,25(OH)2D3. Fam57b inactivation in chondrocytes (Col2-Cre Fam57bfl/fl) phenocopied the callus formation defect of Cyp24a1-/- mice. LacCer or 24R,25(OH)2D3 injections restored callus volume, stiffness, and mineralized cartilage area in Cyp24a1-null mice, but only LacCer rescued Col2-Cre Fam57bfl/fl mice. Gene expression in callus tissue suggested that the 24R,25(OH)2D3/FAM57B2 cascade affects cartilage maturation. We describe a previously unrecognized pathway influencing endochondral ossification during bone repair through LacCer production upon binding of 24R,25(OH)2D3 to FAM57B2. Our results identify potential new approaches to ameliorate fracture healing.

Vitamin D Activity and Metabolism in Bone

PURPOSE OF REVIEW

In addition to the actions of the endocrine hormone, 1alpha,25-dihydroxyvitamin D (1,25(OH)₂D) in stimulating intestinal calcium absorption, the regulation of bone mineral metabolism by 1,25(OH)₂D is also considered an important contributor to calcium homeostasis. However, recent evidence suggest that 1,25(OH)₂D acting either via endocrine or autocrine pathways plays varied roles in bone, which suggests that vitamin D contributes to the maintenance of bone mineral in addition to its catabolic roles. This review highlights the contrasting evidence for the direct action for vitamin D metabolism and activity in bone.

RECENT FINDINGS

Numerous cells within bone express vitamin D receptor (VDR), synthesise and catabolise 1,25(OH)₂D via 25-hydroxyvitamin D 1alpha-hydroxylase (CYP27B1), and 25-hydroxyvitamin D 24-hydroxylase (CYP24A1) enzymes, respectively. Recent evidence suggests that all three genes are required to regulate processes of bone resorption, mineralization and fracture repair. The actions of vitamin D in bone appear to negatively or positively regulate bone mineral depending on the physiological and pathological circumstances, suggesting that vitamin D plays pleiotropic roles in bone.