DIFFERENTIAL REGULATION OF PROSTAGLANDIN-E2 SYNTHESIS AND PHOSPHOLIPASE-A2 ACTIVITY BY 1,25-(OH)2D3 IN 3 OSTEOBLAST-LIKE CELL-LINES (MC-3T3-E1, ROS17/2.8, AND MG-63)

Both 1,25-(OH)2D3 and prostaglandin E2 (PGE2) stimulate alkaline phosphatase activity in MC-3T3-E1 cells. Previous studies, demonstrating a correlation between 1,25-(OH)2D3-dependent alkaline phosphatase and phospholipase A2 activities in matrix vesicles isolated from growth cartilage chondrocyte cultures, suggest that one mechanism of vitamin D action may be via autocrine or paracrine action of PGE2. Since most PGE2 is derived from arachidonic acid released by the action of phospholipase A2, we examined whether 1,25-(OH)2D3 stimulates phospholipase A2 activity in three osteoblastic cell lines: ROS 17/2.8 cells, MC-3T3-E1 cells, and MG-63 cells. 1,25-(OH)2D3-dependent alkaline phosphatase and phospholipase A2 activity were correlated with production of PGE2 and PGE1 in the MC-3T3-E1 cells. Alkaline phosphatase specific activity was enriched in the matrix vesicles produced by all three cell types and was stimulated by 1,25-(OH)2D3 at 10(-8) to 10(-7) M. Although phospholipase A2 Specific activity was enriched in the matrix vesicles produced only by the ROS 17/2.8 cell cultures, stimulation of this enzyme activity was observed only in the MC-3T3-E1 cell cultures. The effects of 1,25-(OH)2D3 on phospholipase A2 were dose-dependent and were significant at 10(-8) to 10(-7) M. There was a significant increase in PGE2 production in the MC-3T3-E1 cell cultures only. Indomethacin reduced PGE2 production to base line values. Even at baseline, MC-3T3-E1 cells produced ten times more PGE2 than did the ROS 17/2.8 or MG-63 cell cultures. The effects of 1,25-(OH)2D3 on PGE1 were comparable to those on PGE2. While addition of indomethacin alone to MC-3T3-E1 cells had no effect on alkaline phosphatase activity, it inhibited the 1,25-(OH)2D3-dependent stimulation of this enzyme. These data suggest that one mechanism by which 1,25-(OH)2D3 elicits its effects in osteoblasts is via activation of phospholipase A2, prostaglandin production, and the subsequent autocrine (MC-3T3-E1) or paracrine (ROS 17/2.8, MG-63) action of PGE2 on the cell membrane. Variation in bone cell response may reflect species differences, differences in the mechanism of cell line immortalization, or differences in the degree of osteoblastic differentiation.