title: Effects of 'non-calcaemic' vitamin D analogues on 24-hydroxylase expression in MG-63 osteoblast-like cells.
abstract: BACKGROUND
  New 'non-calcaemic' analogues of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are entering the clinical arena and some of them have been shown to have differential effects in bone. This may have a bearing on the evolution of bone lesions in uraemic patients receiving vitamin D therapies. A potential mechanism for differential effects of analogues lies in their target cell inactivation.
  
  
  METHODS
  Using a human osteoblastic cell line, MG-63, three analogues, 22-oxacalcitriol (OCT), 19-nor-1,25-dihydroxyvitamin D2 (paricalcitol) and 1alpha,25-dihydroxydihydrotachysterol2(1,25(OH)2DHT2), were compared with 1,25(OH)2D3 for (1) their affinity for the vitamin D receptor (VDR) by competitive displacement of tritiated 1,25(OH)2D3 from calf thymus VDR; (2) effects on 24-hydroxylase mRNA expression using comparative RT-PCR, and (3) rates of metabolism, using high performance liquid chromatography, over a 24-hour time course.
  
  
  RESULTS
  Relative VDR-binding affinities (IC50) were 1,25(OH)2D3 (100%), OCT (25%), paricalcitol (14%) and 1,25(OH)2DHT2 (0.3%). A > or =3-fold increase in 24-hydroxylase mRNA expression was observed for all compounds at 2 h peaking at 7- to 8-fold above control levels by 12 h, with no significant difference between the analogues and 1,25(OH)2D3. Differences in their rates of metabolism were observed [calculated t(1/2) values = OCT (1.2 h) > paricalcitol (2.3 h) > 1,25(OH)2D3 (2.6 h) > 1,25(OH)2DHT2 (3.4 h)], with OCT having a significantly shorter half-life.
  
  
  CONCLUSIONS
  In MG-63 cells these analogues up-regulate 24-hydroxylase mRNA expression with similar potency, in each case accelerating ligand inactivation, despite significant differences in VDR affinity. VDR affinity did not correspond to either 24-hydroxylase mRNA expression or the rates of ligand disappearance, suggesting cellular metabolism is one of several factors that determine the analogue specificity of these agents in bone.
authors.affiliation: Department of Endocrinology, St. Bartholomew's and the Royal London School of Medicine and Dentistry, London, UK.
authors.forename: Neil J
authors.initials: NJ
authors.lastname: Schroeder
authors.forename: Jacky M
authors.initials: JM
authors.lastname: Burrin
authors.forename: Kate
authors.initials: K
authors.lastname: Noonan
authors.forename: Hugh L J
authors.initials: HL
authors.lastname: Makin
authors.forename: John
authors.initials: J
authors.lastname: Cunningham
mesh_terms: D000818:Animals; D002117:Calcitriol / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology*; D002417:Cattle; D002460:Cell Line; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D004097:Dihydrotachysterol / Q000737:chemistry / Q000378:metabolism; D004872:Ergocalciferols / Q000378:metabolism / Q000494:pharmacology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D006801:Humans; D006900:Hydroxylation / Q000187:drug effects; D006899:Mixed Function Oxygenases / Q000378:metabolism; D010006:Osteoblasts / Q000737:chemistry / Q000187:drug effects* / Q000201:enzymology*; D011485:Protein Binding / Q000187:drug effects; D018167:Receptors, Calcitriol / Q000378:metabolism; D013250:Steroid Hydroxylases / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D013950:Thymus Gland / Q000737:chemistry / Q000201:enzymology / Q000378:metabolism; D065668:Vitamin D3 24-Hydroxylase
chemical_list: D004872:Ergocalciferols; D018167:Receptors, Calcitriol; C084656:paricalcitol; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase; D002117:Calcitriol; C051883:maxacalcitol; D004097:Dihydrotachysterol