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

Abstract

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.

A chromatin-based mechanism controls differential regulation of the cytochrome P450 gene Cyp24a1 in renal and non-renal tissues

“…New metabolite profiling methods based on LC-tandem MS are now able to detect a range of serum metabolites formed by CYP24A1, including 24,25(OH) 2 D 3 , 1,24,25(OH) 3 D 3 , and 25(OH)D 3 -26,23-lactone. It is questionable whether any of these metabolites are biologically active, although recent studies by St-Arnaud and colleagues (6) have suggested that 24,25(OH) 2 D 3 may act as an allosteric activator of FAM57B2 that is involved in the synthesis of lactosylceramide, providing a potential mechanism for an early hypothesized role for 24,25(OH) 2 D 3 in bone fracture healing. 1,24,25(OH) 3 D 3 is also of particular interest because recent estimates of its circulating level in mice suggest that this metabolite might contribute to the net biological activity of vitamin D 3 (7)(8)(9)(10).…”

Vitamin D Metabolism Revised: Fall of Dogmas

“…CYP24A1 null mice also have a problem with fracture repair as 24R,25(OH) 2 D is able to bind to a GPCR, Fam57B2, and thereby stimulates lactosylceramide production and fracture repair . Whether this also applies to humans with bi‐allelic mutations has, however, so far not been reported .…”

“…(76,77) CYP24A1 null mice also have a problem with fracture repair as 24R,25(OH) 2 D is able to bind to a GPCR, Fam57B2, and thereby stimulates lactosylceramide production and fracture repair. (78) Whether this also applies to humans with bi-allelic mutations has, however, so far not been reported. (77) Polymorphism of the CYP24A1 gene is responsible for modest genetic variability of serum 25OHD (as one of the 8 genes known so far to result in genetically predisposed higher or lower serum 25OHD concentrations).…”

Update on pharmacologically‐relevant vitamin D analogues

“…However, a very recent publication has provided, for the first time, evidence for a possible novel mechanism to explain acceleration of bone‐fracture repair. Matrineau et al .…”

“…have identified and cloned an effector molecule named FAM57B2 that binds 24,25‐(OH) 2 D 3 and produces a lipid‐signalling molecule lactosyl ceramide (LacCer) thatmediates endochondral ossification during bone callus formation . Ablation of CYP24A1 gene or inactivation of Fam57b2 gene in chondrocytes caused loss of the bone‐healing effect while treatment with exogenous synthetic 24R,25‐(OH) 2 D 3 or lactosyl ceramide, but not 1,25‐(OH) 2 D 3 , restores the effect . It is not yet clear if other 24‐hydroxylated vitamin D analogues or even other vitamin D analogues in general can reproduce the same bone‐healing effect as 24R,25‐(OH) 2 D 3 at equivalent doses to those used here.…”