1,25-(OH)₂D₃ and 24,25-(OH)₂D₃ regulation of arachidonic acid turnover in chondrocyte cultures is cell maturation-specific and may involve direct effects on phospholipase A₂

Previous studies have shown that 1,25-(OH)₂D₃ stimulates phospholipase A₂(PA₂) activity in growth zone chondrocytes (GC), but has no effect on the resting zone chondrocyte (RC) enzyme activity. 24,25-(OH)₂D₃ 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)₂D₃ has no effect on arachidonic acid turnover in RC, but stimulates turnover in GC. In contrast, 24,25-(OH)₂D₃ stimulates arachidonic acid turnover in RC, but inhibits both incorporation and release in GC. To determine whether direct interaction with PA₂ is one mechanism by which 1,25-(OH)₂D₃ and 24,25-(OH)₂D₃ regulate arachidonic acid release, snake venom (Niger niger) PA₂ was incubated with the vitamin D metabolites. Enzyme specific activity was inhibited by 24,25-(OH)₂D₃ and stimulated by 1,25-(OH)₂D₃ in a time- and dose-dependent manner. These results suggest that at least part of the direct effect of vitamin D₃ metabolites on cell membranes may be related to changes in PA₂ 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 D₃ action in cartilage.