EFFECT OF PROSTAGLANDIN E(2) ON PHOSPHOLIPASE-D ACTIVITY IN OSTEOBLAST-LIKE MC3T3-E1 CELLS

Recent evidence indicates that phosphatidylcholine breakdown by phosph olipase D (PLD) is an important cellular control mechanism, We investi gated the signaling pathway participating in prostaglandin E(2) (PGE(2 ))-induced PLD activation in osteoblast-like MC3T3-E1 cells, PGE(2) st imulated PLD activity, as measured by choline generated from phosphati dylcholine, just after the stimulation. The reaction reached a plateau 15 minutes later, PGE(2) stimulated PLD activity in a dose-related ma nner and also increased inositol phosphate (IP) formation, However, th e EC(50) value for PGE(2)-induced IP formation is lower than that for PLD activation, 12-O-Tetradecanoylphorbol-13-acetate (TPA), a protein kinase C (PKC) activator, stimulated PLD activity, and a combination o f PGE(2) and TPA potentiated it in an additive manner, Although NaF, a heterotrimeric GTP-binding protein activator, significantly stimulate d PLD activity, this effect was not augmented by combination with PGE( 2). PGE(2)-induced PLD activity was markedly suppressed by either chel ating extracellular Ca2+ by EGTA or pertussis toxin. These findings su ggest that osteoblasts might have at least two PLD activation mechanis ms which involve PKC-dependent or -independent pathways. However, pres ent results indicate that PKC is unlikely to be essential to PGE(2)-in duced PLD activation, On the contrary, pertussis toxin-sensitive GTP-b inding protein and extracellular Ca2+ might play important roles in th e pathway of PGE(2)-induced PLD activation.