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 regulation of PG synthesis by interleukin (IL)-1 alpha in primary mouse osteoblastic cells isolated from mouse calvaria. Although IL-1 alpha greatly enhanced cox-2 mRNA expression and its protein levels, PGE(2) was not produced until 24 h. When arachidonic acid was added to osteoblastic cells precultured with IL-1 alpha for 24 h, PGE(2) was produced within 10 min, Of several growth factors tested, platelet-derived growth factor (PDGF) specifically initiated the rapid synthesis of PGE(2), which was markedly suppressed by a selective inhibitor of cox-2 (NS-398), In mouse osteoblastic cells, cytosolic PLA(2) (cPLA(2)) mRNA and its protein were constitutively expressed and increased approximately 2-fold by IL-1 alpha, but secretory PLA(2) mRNA was not detected, PDGF rapidly stimulated PLA(2) activity, which was blocked completely by a cPLA(2) inhibitor (arachidonyltrifluoromethyl ketone). The PDGF-induced cPLA(2) activation was accompanied by phosphorylation of its protein, These results indicate that cox-2 induction by IL-1 alpha is not sufficient, but cPLA(2) activation by PDGF is crucial for IL-1 alpha-induced PGE(2) synthesis in mouse osteoblasts.