We have shown previously that interleukin 1 (IL-1) stimulates eicosanoid production in glomerular mesangial cells (MC) by de novo synthesis of a 14-kD, group II phospholipase A(2) (PLA(2)). IL-1-stimulated prostaglandin E(2) synthesis precedes espression of this enzyme, suggesting that another PLA(2) isoform must be more rapidly activated. In the presence but not absence of calcium ionophore, [H-3]arachidonate release is increased significantly as early as 5 min after addition of IL-1, and IL-1 concurrently stimulates a Ca2+-dependent phospholipase activity, which was characterized as the cytosolic form of PLA(2) (cPLA(2)). IL-1 does not alter either cPLA(2) mRNA expression or mass in serum-stimulated MC, suggesting that cPLA, activity is increased by a posttranslational modification. IL-1 treatment for 30 min doubles P-32 incorporation into immunoprecipitable cPLA(2), protein, concordant with the increase in enzyme activity. Immunoblot analysis of extracts derived from IL-1-treated (30 min) cells demonstrates a decreased mobility of cPLA(2), and treatment of MC lysates with acid phosphatase significantly reduces cytokine-activated cPLA(2) activity, further indicating that IL-1 stimulates phosphorylation of the enzyme. IL-1 treatment (24 h) of serum-deprived MC doubled cPLA(2) mRNA, protein, and activity. In summary, IL-1 increases cPLA(2) activity in a biphasic, time-dependent manner both by posttranslational modification and de novo synthesis. We consider cPLA(2) activation a key step in IL-1-stimulated synthesis of pro-inflammatory, lipid mediators, and an integral event in the phenotypic responses induced in target cells by this cytokine.
