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 1α,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 ≧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 t½ 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. Conclusion: 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.

Journal

Journal ID (publisher-id): NEP

Journal ID (nlm-ta): Nephron Physiol

Journal ID (doi): 10.1159/issn.1660-2137

Title: Nephron Physiology

Publisher: S. Karger AG

ISSN (Electronic): 1660-2137

Publication date Subscription-year: 2003

Publication date (Print): August 2003

Publication date (Electronic): 12 September 2003

Volume: 94

Issue: 4

Pages: p62-p73

Affiliations

Departments of ^aEndocrinology and ^bExperimental Medicine and Nephrology and ^cClinical Biochemistry, St. Bartholomew’s and the Royal London School of Medicine and Dentistry, London, UK

Article

Publisher ID: 72519 Other ID: Nephron Physiol 2003;94:p62–p73

DOI: 10.1159/000072519

PubMed ID: 12972708

Copyright statement: © 2003 S. Karger AG, Basel

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Page count

Figures: 6, References: 43, Pages: 1

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Categories

Subject: Original Paper