Arachidonic acid inhibits osteoblast differentiation through cytosolic phospholipase A(2)-dependent pathway

OBJECTIVE Arachidonic acid, a precursor of prostaglandins (PGs), is released by phospholipase A(2) (PLA(2)) and plays an important role in biological reactions. We examined the roles of arachidonic acid on the pathway of PG synthesis and osteoblast differentiation by using clone MC3T3-E1 cells. MATERIALS AND METHODS: The effect of arachidonic acid was evaluated by the measurement of alkaline phosphatase activity, cells shape, production of arachidonic acid and the expression of cyclooxygenase (COX). RESULTS: Arachidonic acid dose dependently decreased alkaline phosphatase activity and increased PGE(2) production in MC3T3-E1 cells. The cell shape changed from polygonal to fibroblastic following treatment with arachidonic acid. These effects were recovered by the treatment of NS-398 and indomethacin. Arachidonic acid increased the expression of COX-2 mRNA and the PGE(2) production. The exogenous arachidonic acid induced the release of cellular arachidonic acid in MC3T3-E1 cells. Moreover, methylarachidonyl fluorophosphonate suppressed the arachidonic acid release and the expression of COX-2 mRNA. CONCLUSIONS: The present results indicate that exogenous arachidonic acid stimulated the activity of PLA(2), leading to the new release of membranous arachidonic acid. The amplified arachidonic acid enhanced PGE(2) production by COX-2, which inhibits the differentiation of MC3T3-E1 cells. Our results provide a new insight into the molecular mechanisms by which exogenous arachidonic acid plays a role as a paracrine/autocrine amplifier of PGE(2) biosynthesis by coupling with PLA(2) and COX-2.