MODULATION OF ECTO-NUCLEOSIDE TRIPHOSPHATE PYROPHOSPHATASE ACTIVITY OF HUMAN OSTEOBLAST-LIKE BONE-CELLS BY 1-ALPHA,25-DIHYDROXYVITAMIN D-3, 24R,25-DIHYDROXYVITAMIN D-3, PARATHYROID-HORMONE, AND DEXAMETHASONE

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)(2)D-3,24,25(OH)(2)D-3, dexamethasone, and parathyroid hormone (PTH) on the activity of this enzyme in cultured human trabecular bone-derived osteoblast-like cells. 1,25(OH)(2)D-3 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)(2)D-3 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)(2)D-3 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 D-3 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)(2)D-3. These data suggest a possible role for both vitamin D-3 metabolites and glucocorticoids in the regulation of the mineralization process in vivo via modulation of PPi production.