Interleukin-1α stimulates the release of prostaglandin E2 and phospholipase A2 from fetal rat calvarial cells in vitro: Relationship to bone nodule formation

Abstract

We have shown previously that interleukin-1 (IL-1) has biphasic effects on the formation of bone nodules in long-term cultures of fetal rat calvarial (RC) cells (Ellies and Aubin, Cytokine 2:430–437, 1990). To determine the role of arachidonic acid metabolism in this process, we have examined the release of prostaglandin E₂ (PGE₂) and phospholipase A₂ (PLA₂) from RC cells under conditions that allowed concomitant analysis of the formation of bone nodules. Recombinant human IL-1α (rhIL-1α) stimulated PGE₂ and PLA₂ release in a time- and dose-dependent manner. PGE₂ release was highest in preconfluent cultures (days 1–6) and was stimulated up to 8.5-fold in response to 50 U/ml of rhIL-1α. In contrast, extracellular PLA₂ activity was maximal in postconfluent cultures, with 50 U/ml of rhIL-1α causing a 20-fold increase by day 15. PLA₂ release by RC cells was not significantly affected by PGE₂, the glucocorticoid dexamethasone, or the cyclooxygenase inhibitor indomethacin. Indomethacin partially blocked the inhibition of bone nodule formation caused by rhIL-1α, and exogenous PGE₂ reversed this effect. Addition of group I PLA₂ from Naja naja venom to RC cells had no effect on bone nodule development; however, group II PLA₂ from Crotalus adamanteus venom inhibited the formation of bone nodules in a dose range similar to that induced by rhIL-1α. These results indicate that PGE₂ release does not have a direct temporal correlation with increases in PLA₂ activity. In addition, the data show that only part of the inhibition of bone formation seen with rhIL-1α is mediated by PGE₂ and suggest that extracellular PLA₂ also accounts for part of the inhibition.