Studies on 24R,25-dihydroxyvitamin D 3: evidence for a nonnuclear membrane receptor in the chick tibial fracture-healing callus

• Published in: Bone (New York, N.Y.) Vol. 23; no. 2; pp. 141 - 146
• Main Authors: Kato, A, Seo, E.-G, Einhorn, T.A, Bishop, J.E, Norman, A.W
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.08.1998
• Subjects: 1α,25-dihydroxyvitamin D 3; 24R,25-dihydroxyvitamin D 3; Bone strength; Fracture healing; Membrane receptor/binding protein; Fracture healing; Membrane receptor/binding protein; 1α,25-dihydroxyvitamin D 3; 24R,25-dihydroxyvitamin D 3; Bone strength
• ISSN: 8756-3282; 1873-2763
• DOI: 10.1016/S8756-3282(98)00085-4

The effect(s) of 24R,25-dihydroxyvitamin D 3 [24R,25(OH) 2D 3] on fracture healing was studied in a vitamin D-depleted chick model. 24R,25(OH) 2D 3, together with another hormonally active vitamin D metabolite, 1α,25-dihydroxyvitamin D 3 [1α,25(OH) 2D 3], improved bone mechanical strength parameters (torsional strength, angular deformation, and stiffness) and the ash content. The synthetic epimer 24S,25-dihydroxyvitamin D 3 [24S,25(OH) 2D 3] was not as potent as the natural 24R,25(OH) 2D 3. In light of the ability of the fracture-healing callus to discriminate between 24R,25(OH) 2D 3 and 24S,25(OH) 2D 3, a search was initiated in fracture-healing callus tissue for the presence of a specific 24R,25(OH) 2D 3 receptor. No evidence was obtained for a classical nuclear/cytosol receptor for 24R,25(OH) 2D 3 in the fracture-healing callus. A specific receptor/binding protein for 24R,25(OH) 2D 3 was found in the callus membrane fraction, which showed different ligand binding affinities [ K D = 18.3 ± 1.9 nmol/L, B max = 43.9 ± 6.0 fmol/mg; relative competitive index (RCI) for 24R,25(OH) 2D 3/24S,25(OH) 2D 3/25(OH)D 3/1α,25(OH) 2D 3 = 100/37/401/2.0] compared with the ubiquitous serum vitamin D-binding protein (RCI = 100/99/219/5). Also, a callus membrane-binding protein/receptor for 1α,25(OH) 2D 3 was detected with a K D = 0.83 ± 0.35 nmol/L and a B max = 35.5 ± 5.2 fmol/mg. Thus, we have demonstrated a biological role for 24R,25(OH) 2D 3 in fracture healing and described the presence of its receptor/binding protein in a callus membrane fraction.