ARACHIDONIC-ACID IS AN AUTOCOID MEDIATOR OF THE DIFFERENTIAL ACTION OF 1,25-(OH)(2)D-3 AND 24,25-(OH)(2)D-3 ON GROWTH-PLATE CHONDROCYTES

Prior studies have shown that 24,25-(OH)(2)D-3 and 1,25-(OH)(2)D-3 reg ulate protein kinase C (PKC) in costochondral chondrocytes in a cell m aturation-dependent manner, with 1,25-(OH)(2)D-3 affecting primarily g rowth zone (GC) cells and 24,25-(OH)(2)D-3 affecting primarily resting zone (RC) cells. In addition, 1,25-(OH)(2)D-3 has been shown to incre ase phospholipase A(2) activity in GC, while 24,25-(OH)(2)D-3 has been shown to decrease phospholipase A(2) activity in RC. Stimulation of p hospholipase A(2) in CC caused an increase in PKC, whereas inhibition of phospholipase A, activity in RC cultures increased both basal and 2 4,25-(OH)(2)D-3-induced PKC activity, suggesting that phospholipase A( 2) may play a central role in mediating the effects of the vitamin D m etabolites on PKC. To test this hypothesis, RC and CC cells were cultu red in the presence and absence of phospholipase A(2) inhibitors (quin acrine and oleyloxyethyl-phosphorylcholine [OEPC]), phospholipase A(2) activators (melittin and mastoparan), or arachidonic acid alone or in the presence of the target cell-specific vitamin D metabolite. PKC sp ecific activity in the cell layer was determined as a function of time . Phospholipase A(2) inhibitors decreased both basal and 1,25-(OH)(2)D -3 induced PKC activity in GC. When phospholipase A(2) activity was ac tivated by inclusion of melittin or mastoparan in the cultures, basal PKC activity in RC was reduced, while that in GC was increased. Simila rly, melittin and mastoparan decreased 24,25(OH)(2)D-3-induced PKC act ivity in RC and increased 1,25-(OH)(2)D-3-induced PKC activity in GC. For both cell types, the addition of arachidonic acid to the culture m edia produced an effect on PKC activity that was similar to that obser ved when phospholipase A(2) activators were added to the cells. These results demonstrate that vitamin D metabolite-induced changes in phosp holipase A(2) activity are directly related to changes in PKC activity . Similarly, exogenous arachidonic acid affects PKC in a manner consis tent with activation of phospholipase A(2). These effects are cell mat uration- and time-dependent and metabolite-specific. J. Cell. Physiol. 176.516-524, 1998. (C) 1998 Wiley-Liss Inc.