Arachidonic acid is an autocoid mediator of the differential action of 1,25-(OH)2D3 and 24,25-(OH)2D3 on growth plate chondrocytes.

Prior studies have shown that 24,25-(OH)2D3 and 1,25-(OH)2D3 regulate protein kinase C (PKC) in costochondral chondrocytes in a cell maturation-dependent manner, with 1,25-(OH)2D3 affecting primarily growth zone (GC) cells and 24,25-(OH)2D3 affecting primarily resting zone (RC) cells. In addition, 1,25-(OH)2D3 has been shown to increase phospholipase A2 activity in GC, while 24,25-(OH)2D3 has been shown to decrease phospholipase A2 activity in RC. Stimulation of phospholipase A2 in GC caused an increase in PKC, whereas inhibition of phospholipase A2 activity in RC cultures increased both basal and 24,25-(OH)2D3-induced PKC activity, suggesting that phospholipase A2 may play a central role in mediating the effects of the vitamin D metabolites on PKC. To test this hypothesis, RC and GC cells were cultured in the presence and absence of phospholipase A2 inhibitors (quinacrine and oleyloxyethylphosphorylcholine [OEPC]), phospholipase A2 activators (melittin and mastoparan), or arachidonic acid alone or in the presence of the target cell-specific vitamin D metabolite. PKC specific activity in the cell layer was determined as a function of time. Phospholipase A2 inhibitors decreased both basal and 1,25-(OH)2D3-induced PKC activity in GC. When phospholipase A2 activity was activated by inclusion of melittin or mastoparan in the cultures, basal PKC activity in RC was reduced, while that in GC was increased. Similarly, melittin and mastoparan decreased 24,25-(OH)2D3-induced PKC activity in RC and increased 1,25-(OH)2D3-induced PKC activity in GC. For both cell types, the addition of arachidonic acid to the culture media produced an effect on PKC activity that was similar to that observed when phospholipase A2 activators were added to the cells. These results demonstrate that vitamin D metabolite-induced changes in phospholipase A2 activity are directly related to changes in PKC activity. Similarly, exogenous arachidonic acid affects PKC in a manner consistent with activation of phospholipase A2. These effects are cell maturation- and time-dependent and metabolite-specific.