Prostaglandins mediate the effects of 1,25-(OH) 2D 3 and 24,25-(OH) 2D 3 on growth plate chondrocytes in a metabolite-specific and cell maturation-dependent manner

Abstract

Prior studies have shown that 1,25-(OH) 2D 3 stimulates alkaline phosphatase, phospholipase A 2 (PLA 2), and protein kinase C (PKC)-specific activities, and production of prostaglandin E 2 (PGE 2) in growth zone chondrocytes. In contrast, 24,25-(OH) 2D 3 stimulates alkaline phosphatase and PKC-specific activities but inhibits PLA 2-specific activity and PGE 2 production in resting zone cells. This indicates that different mechanisms are involved in the action of 1,25-(OH) 2D 3 and 24,25-(OH) 2D 3 on their respective target cells. In this study, we examined the hypothesis that differential regulation of prostaglandin production modulates the activity of PKC and alkaline phosphatase. To do this, we examined the effect of the cyclooxygenase inhibitor indomethacin (Indo) on alkaline phosphatase, PLA 2, and PKC-specific activities in growth plate chondrocytes treated with these two vitamin D metabolites. In addition, we examined whether inhibition of PKC altered PGE 2 production. In growth zone cells, Indo inhibited basal alkaline phosphatase and blocked the 1,25-(OH) 2D 3-dependent increase in alkaline phosphatase. This effect was due to inhibition of both plasma membrane and matrix vesicle alkaline phosphatase. In resting zone cells, Indo increased basal alkaline phosphatase activity in a dose-dependent manner, but it did not further enhance the 24,25-(OH) 2D 3-dependent stimulation of this enzyme. The effect of Indo was found in both plasma membranes and matrix vesicles. These data indicate that 1,25-(OH) 2D 3-dependent increases in alkaline phosphatase-specific activity in growth zone cells are mediated through increased prostaglandin production, whereas 24,25-(OH) 2D 3-mediated changes in enzyme activity in resting zone cells are mediated through decreased prostaglandin production. Regulation of PLA 2 by either 1,25-(OH) 2D 3 or 24,25-(OH) 2D 3 in their target cells was unaffected by Indo, indicating that the effect of the vitamin D metabolites on this enzyme is not dependent on changes in PGE 2 production. The rapid increase in 1,25-(OH) 2D 3-dependent PKC-specific activity in growth zone cells was inhibited by Indo, whereas there was a potentiation of the effect of 24,25-(OH) 2D 3 on PKC activity in resting zone cells. In addition, inhibition of PKC blocked the 1,25-(OH) 2D 3-dependent increase in PGE 2 production in growth zone cells and the 24,25-(OH) 2D 3-dependent decrease in PGE 2 production by resting zone cells. These data indicate that prostaglandins are involved in mediating the rapid effects of 1,25-(OH) 2D 3 on growth zone cells, and contribute to the effects of 24,25-(OH) 2D 3 on resting zone cells; in both instances, the vitamin D metabolites exert their effects on PKC through changes in arachidonic acid via the action of PLA 2. In addition, PKC by itself may mediate the production of PGE 2.