ACTIVATION OF CYTOSOLIC PHOSPHOLIPASE A(2) BY PLATELET-DERIVED GROWTH-FACTOR IS ESSENTIAL FOR CYCLOOXYGENASE-2-DEPENDENT PROSTAGLANDIN E(2)SYNTHESIS IN MOUSE OSTEOBLASTS CULTURED WITH INTERLEUKIN-1

The synthesis of prostaglandins (PGs) is regulated by the arachidonic acid release by phospholipase A(2) (PLA(2)) and its conversion to PGs by cyclooxygenase (COX). In the present study, we examined the regulat ion of PG synthesis by interleukin (IL)-1 alpha in primary mouse osteo blastic cells isolated from mouse calvaria. Although IL-1 alpha greatl y enhanced cox-2 mRNA expression and its protein levels, PGE(2) was no t produced until 24 h. When arachidonic acid was added to osteoblastic cells precultured with IL-1 alpha for 24 h, PGE(2) was produced withi n 10 min, Of several growth factors tested, platelet-derived growth fa ctor (PDGF) specifically initiated the rapid synthesis of PGE(2), whic h was markedly suppressed by a selective inhibitor of cox-2 (NS-398), In mouse osteoblastic cells, cytosolic PLA(2) (cPLA(2)) mRNA and its p rotein were constitutively expressed and increased approximately 2-fol d by IL-1 alpha, but secretory PLA(2) mRNA was not detected, PDGF rapi dly stimulated PLA(2) activity, which was blocked completely by a cPLA (2) inhibitor (arachidonyltrifluoromethyl ketone). The PDGF-induced cP LA(2) activation was accompanied by phosphorylation of its protein, Th ese results indicate that cox-2 induction by IL-1 alpha is not suffici ent, but cPLA(2) activation by PDGF is crucial for IL-1 alpha-induced PGE(2) synthesis in mouse osteoblasts.