1,25-Dihydroxyvitamin D3 stimulates the alkaline phosphatase activity of osteoblast-like cells

Rat osteogenic sarcoma cells which have osteoblast properties including the ability to form bone and to mineralize were recently found to possess specific cytoplasmic receptors for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). We have examined now the effect of 1,25(OH)2D3 and other vitamin D3 metabolites on the alkaline phosphatase of such cell lines. We found in two cell lines cultured in the presence of 10(-7) M 1,25(OH)2D3 a 3-fold increase in intracellular alkaline phosphatase activity and a 6-fold increase in secreted alkaline phosphatase activity. The cellular response occurred in a dose-dependent fashion at a range of 10(-9) to 10(-7) M. In a third cell line, which does not possess the specific receptor for 1,25(OH)2D3, we could not detect stimulation of alkaline phosphatase. Vitamin D3, 25-dihydroxyvitamin D3, and 24,25-dihydroxyvitamin D3 at 10(-7) M had no effect on alkaline phosphatase. The effect of 1,25(OH)2D3 was enhanced in the presence of increased calcium. In view of the postulated role for alkaline phosphatase in calcification, we speculate that the stimulatory effect of 1,25(OH)2D3 on the alkaline phosphatase activity of osteoblast-like cells indicates a direct involvement of 1,25(OH)2D3 in bone mineralization.