Arachidonic acid release by basophilic leukemia cells and macrophages stimulated by Ca²⁺ ionophores, antigen and diacylglycerol: essential role for protein kinase C and prevention by glucocorticosteroids

The role of protein kinase C in phospholipase A₂ (PLA₂) activation in rat basophilic leukemia cells (RBL-2H3) and macrophages was investigated. 12-O-Tetradecanoyl phorbol 13-acetate (TPA) doubled ionomycin-induced PLA₂ activity, assessed by [³H]arachidonate release. Protein kinase C inhibitors, staurosporine and K252a (100 nM) or H-7 (15 μg/ml) inhibited ionomycin-stimulation of PLA₂ activity by 62, 75 and 80%, respectively. Down-regulation of protein kinase C by prolonged treatment with TPA inhibited Ca²⁺-ionophore A23187 or antigen-stimulation of [³H]arachidonate release by 80%. We examined whether the inhibitory effect of dexamethasone (DEX) on PLA₂ activity is related to modulation of protein kinase C activity. The 50% inhibition by DEX of ionomycin elevation of [³H]arachidonate release was almost overcome by addition of TPA. The Ca²⁺ ionophore and antigen-induced increase in [³H]TPA binding to intact RBL cells was not impaired by DEX. However, DEX markedly reduced phosphorylation of several proteins. 1-Oleoyl-2-acetyl-glycerol (OAG) had a sustained stimulatory effect on PLA₂ activity in isolated plasma membranes derived from treated bone-marrow intact mouse macrophages, while both DEX and staurosporine reduced elevated PLA₂ activity by 68 and 84%, respectively. The results support an essential role for protein kinase C in regulation of PLA₂ activity.