Studies on 24R,25-dihydroxyvitamin D3: evidence for a nonnuclear membrane receptor in the chick tibial fracture-healing callus.
The effect(s) of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] on fracture healing was studied in a vitamin D-depleted chick model. 24R,25(OH)2D3, together with another hormonally active vitamin D metabolite, 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3], improved bone mechanical strength parameters (torsional strength, angular deformation, and stiffness) and the ash content. The synthetic epimer 24S,25-dihydroxyvitamin D3 [24S,25(OH)2D3] was not as potent as the natural 24R,25(OH)2D3. In light of the ability of the fracture-healing callus to discriminate between 24R,25(OH)2D3 and 24S,25(OH)2D3, a search was initiated in fracture-healing callus tissue for the presence of a specific 24R,25(OH)2D3 receptor. No evidence was obtained for a classical nuclear/cytosol receptor for 24R,25(OH)2D3 in the fracture-healing callus. A specific receptor/binding protein for 24R,25(OH)2D3 was found in the callus membrane fraction, which showed different ligand binding affinities [KD = 18.3 +/- 1.9 nmol/L, Bmax = 43.9 +/- 6.0 fmol/mg; relative competitive index (RCI) for 24R,25(OH)2D3/24S,25(OH)2D3/25(OH)D3/1alpha,25(OH)2D3 = 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 1alpha,25(OH)2D3 was detected with a KD = 0.83 +/- 0.35 nmol/L and a Bmax = 35.5 +/- 5.2 fmol/mg. Thus, we have demonstrated a biological role for 24R,25(OH)2D3 in fracture healing and described the presence of its receptor/binding protein in a callus membrane fraction.
Kato
A
A
Department of Biochemistry, University of California, Riverside 92521, USA.
Seo
E G
EG
Einhorn
T A
TA
Bishop
J E
JE
Norman
A W
AW
D015650:24,25-Dihydroxyvitamin D 3 / Q000494:pharmacology*; D000818:Animals; D002146:Bony Callus / Q000187:drug effects* / Q000378:metabolism; D002112:Calcifediol / Q000494:pharmacology; D002117:Calcitriol / Q000494:pharmacology; D002645:Chickens; D017102:Fracture Healing / Q000187:drug effects*; D008297:Male; D018167:Receptors, Calcitriol / Q000032:analysis*; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D013978:Tibial Fractures / Q000188:drug therapy*; D014808:Vitamin D Deficiency
D018167:Receptors, Calcitriol; D015650:24,25-Dihydroxyvitamin D 3; D002117:Calcitriol; D002112:Calcifediol