1,25-(OH) 2D 3 and 24,25-(OH) 2D 3 regulation of arachidonic acid turnover in chondrocyte cultures is cell maturation-specific and may involve direct effects on phospholipase A 2

Abstract

Previous studies have shown that 1,25-(OH) 2D 3 stimulates phospholipase A 2(PA 2) activity in growth zone chondrocytes (GC), but has no effect on the resting zone chondrocyte (RC) enzyme activity. 24,25-(OH) 2D 3 inhibits the RC enzyme but has no effect on the GC. This study examined whether the vitamin D metabolites affect arachidonic acid turnover in their contra-target cell populations. Incorporation and release of [ 14C]arachidonate was measured at various times following addition of hormone. Acylation and reacylation were measured independently by incubating with p-chloromercuribenzoate. The results demonstrated that 1,25-(OH) 2D 3 has no effect on arachidonic acid turnover in RC, but stimulates turnover in GC. In contrast, 24,25-(OH) 2D 3 stimulates arachidonic acid turnover in RC, but inhibits both incorporation and release in GC. To determine whether direct interaction with PA 2 is one mechanism by which 1,25-(OH) 2D 3 and 24,25-(OH) 2D 3 regulate arachidonic acid release, snake venom ( Niger niger) PA 2 was incubated with the vitamin D metabolites. Enzyme specific activity was inhibited by 24,25-(OH) 2D 3 and stimulated by 1,25-(OH) 2D 3 in a time- and dose-dependent manner. These results suggest that at least part of the direct effect of vitamin D 3 metabolites on cell membranes may be related to changes in PA 2 activity. The regulation is related to the stage of differentiation of the target cell population. Changes in fatty acid acylation and reacylation may be one mode of vitamin D 3 action in cartilage.