title: The role of the vitamin D receptor in regulating vitamin D metabolism: a study of vitamin D-dependent rickets, type II.
abstract: In vitro studies and animal experiments suggest that the production of 1,25-dihydroxyvitamin D [1,25-(OH)(2)D] and 24,25-(OH)(2)D is reciprocally controlled by 1,25-(OH)(2)D. To investigate the role of the vitamin D receptor (VDR) in controlling vitamin D metabolism in humans, we studied 10 patients with vitamin D-dependent rickets type II due to a defective VDR. After a period of high dose calcium therapy, 7 of the patients had normal serum calcium, phosphorus, alkaline phosphatase, and plasma PTH levels (PTH-N), and 3 showed increased serum alkaline phosphatase and plasma PTH (PTH-H). Serum calcium, phosphorus, alkaline phosphatase, PTH, vitamin D metabolites, urinary calcium/creatinine, and renal phosphate threshold concentration were compared with unaffected family members that comprised the control group. Vitamin D metabolites were measured before and after an oral load of 50,000 U/m(2) cholecalciferol. Compared with the control group, 1,25-(OH)(2)D levels were significantly higher and 24,25-(OH)(2)D levels were lower in the PTH-N group and even more so in the PTH-H group. 1alpha-Hydroxylase (1-OHase) and 24-OHase activities were estimated by the product/substrate ratio. In the PTH-N group, 1-OHase activity was higher and 24-OHase activity was lower than in controls. In the PTH-H group, 1-OHase activity was even higher, probably due to an additive effect of PTH. Thus, 1,25-(OH)(2)D-liganded VDR is a major control mechanism for vitamin D metabolism, and PTH exerts an additive effect. Assessment of the influence of 1,25-(OH)(2)D shows reciprocal control of enzyme activity in man, suppressing 1-OHase and stimulating 24-OHase activity.
authors.affiliation: Department of Pediatrics, Rambam Medical Center, Haifa 31096, Israel. d_tiosano@rambam.health.gov.il
authors.forename: D
authors.initials: D
authors.lastname: Tiosano
authors.forename: Y
authors.initials: Y
authors.lastname: Weisman
authors.forename: Z
authors.initials: Z
authors.lastname: Hochberg
mesh_terms: D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase / Q000378:metabolism; D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D005260:Female; D006801:Humans; D008297:Male; D010281:Parathyroid Hormone / Q000097:blood; D018167:Receptors, Calcitriol / Q000502:physiology*; D012279:Rickets / Q000378:metabolism*; D013250:Steroid Hydroxylases / Q000378:metabolism; D014807:Vitamin D / Q000378:metabolism*; D065668:Vitamin D3 24-Hydroxylase
chemical_list: D010281:Parathyroid Hormone; D018167:Receptors, Calcitriol; D014807:Vitamin D; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase; D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase