INTERLEUKIN-1-ALPHA CAUSES RAPID ACTIVATION OF CYTOSOLIC PHOSPHOLIPASE A(2) BY PHOSPHORYLATION IN RAT MESANGIAL CELLS

We have shown previously that interleukin 1 (IL-1) stimulates eicosano id production in glomerular mesangial cells (MC) by de novo synthesis of a 14-kD, group II phospholipase A(2) (PLA(2)). IL-1-stimulated pros taglandin E(2) synthesis precedes espression of this enzyme, suggestin g that another PLA(2) isoform must be more rapidly activated. In the p resence but not absence of calcium ionophore, [H-3]arachidonate releas e is increased significantly as early as 5 min after addition of IL-1, and IL-1 concurrently stimulates a Ca2+-dependent phospholipase activ ity, 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 posttr anslational modification. IL-1 treatment for 30 min doubles P-32 incor poration 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 significant ly reduces cytokine-activated cPLA(2) activity, further indicating tha t 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 ph enotypic responses induced in target cells by this cytokine.