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 regulat ion of mineralization, and there is evidence that the cell surface enz yme, NTP pyrophosphatase, is a major source of this metabolite in bone . Osteotrophic agents that influence bone turnover may exert their eff ects, 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-de pendent increase in activity (at 96 h; maximum 10(-9) M, p < 0.001), w hereas higher concentrations (10(-8) and 10(-7) M) had no effect. In c ontrast, 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-depend ent decrease in ecto-NTP pyrophosphatase activity (10(-7) M, p < 0.001 ); concentrations higher than 10(-7) M did not evoke greater inhibitio n. This effect became apparent by 48 h and was significantly enhanced after 72 h. The response to dexamethasone was attenuated by cyclohexim ide, indicating a requirement for de novo protein synthesis. Interesti ngly, the stimulatory effect of 10(-9) M 1,25(OH)(2)D-3 on ecto-NTP py rophosphatase activity was significantly enhanced in the presence of d examethasone (10(-9)-10(-7) M). Human PTH(1-34) and bovine PTH(1-34) i n the range 10(-10)-10(-7) M had no effect on enzyme activity over a 7 2 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 ec to-NTP pyrophosphatase activity of subject-matched skin-derived fibrob lasts showed no sensitivity to 1,25(OH)(2)D-3. These data suggest a po ssible role for both vitamin D-3 metabolites and glucocorticoids in th e regulation of the mineralization process in vivo via modulation of P Pi production.