Modulation of ecto-nucleoside triphosphate pyrophosphatase activity of human osteoblast-like bone cells by 1α,25-dihydroxyvitamin D3, 24R, 25-dihydroxyvitamin D3, parathyroid hormone, and dexamethasone

Authors

  • Babatunde O. Oyajobi Ph.D.,

    Corresponding author
    1. Department of Human Metabolism and Clinical Biochemistry, University of Sheffield Medical School, Sheffield, England
    • Department of Human Metabolism and Clinical Biochemistry University of Sheffield Medical School Beech Hill Road Sheffield S10 2RX, UK
    Search for more papers by this author
  • R. Graham G. Russell,

    1. Department of Human Metabolism and Clinical Biochemistry, University of Sheffield Medical School, Sheffield, England
    Search for more papers by this author
  • Alison M. Caswell

    1. Department of Biochemistry and Molecular Biology, University of Leeds, Leeds, England
    Search for more papers by this author

Abstract

Extracellular inorganic pyrophosphate (PPi) is involved in the regulation of mineralization, and there is evidence that the cell surface enzyme, NTP pyrophosphatase, is a major source of this metabolite in bone. Osteotrophic agents that influence bone turnover may exert their effects, in part, by modulating the activity of ecto-NTP pyrophosphatase in bone cells. We investigated the effect of 1,25(OH)2D3,24,25(OH)2D3, dexamethasone, and parathyroid hormone (PTH) on the activity of this enzyme in cultured human trabecular bone-derived osteoblast-like cells. 1,25(OH)2D3 at 10−11-10−9 M induced a dose- and time-dependent increase in activity (at 96 h; maximum 10−9 M, p > 0.001), whereas higher concentrations (10−8 and 10−7 M) had no effect. In contrast, 24,25(OH)2D3 was effective only at 10−8 and 10−6 M (at 96 h; p > 0.01). Dexamethasone (10−9-10−7 M) caused a dose-dependent decrease in ecto-NTP pyrophosphatase activity (10−7 M, p > 0.001); concentrations higher than 10−7 M did not evoke greater inhibition. This effect became apparent by 48 h and was significantly enhanced after 72 h. The response to dexamethasone was attenuated by cycloheximide, indicating a requirement for de novo protein synthesis. Interestingly, the stimulatory effect of 10−9 M 1,25(OH)2D3 on ecto-NTP pyrophosphatase activity was significantly enhanced in the presence of dexamethasone (10−9-10−7 M). Human PTH(1-34) and bovine PTH(1-34) in the range 10−10-10−7 M had no effect on enzyme activity over a 72 h period. The effects of vitamin D3 on the expression of bone ecto-NTP pyrophosphatase may be tissue or cell type specific because the ecto-NTP pyrophosphatase activity of subject-matched skin-derived fibroblasts showed no sensitivity to 1,25(OH)2D3. These data suggest a possible role for both vitamin D3 metabolites and glucocorticoids in the regulation of the mineralization process in vivo via modulation of PPi production.

Ancillary