The effect of 1,25(OH)2D3 on alkaline phosphatase in osteoblastic osteosarcoma cells

The effects of 1,25-dihydroxyvitamin D3 on growth, cell protein, and alkaline phosphatase were examined in cultures of a clonal osteoblast-like rat osteosarcoma cell line at different stages of phenotypic maturation, defined by the level of alkaline phosphatase activity. In "immature" cultures with low alkaline phosphatase specific activity, 1,25-dihydroxyvitamin D3 at 10(-8)-10(-7) M inhibited cell growth and elevated alkaline phosphatase activity; lower hormone concentrations had no effect. In cultures with higher alkaline phosphatase activity, the hormone had biphasic effects: at concentrations of 10(-8)-10(-7) M, 1,25-dihydroxyvitamin D3 inhibited growth and elevated alkaline phosphatase and total cell protein; however, lower hormone concentrations (10(-10) and 10(-9) M) reduced alkaline phosphatase activity. This effect was more pronounced in more "mature" cultures where half-maximum inhibition of alkaline phosphatase activity occurred near 10(-11) M. At this stage, the hormone caused a small increase in cell number. The data suggest that 1,25-dihydroxyvitamin D3 exerts differential effects on skeletal cells depending on their state of growth or maturation. This finding may offer new insights into the effects of 1,25-dihydroxyvitamin D3 on bone and may help resolve apparently conflicting experimental findings on the effects of the hormone on osteoblast-like cells in culture.