Recent evidence indicates that phosphatidylcholine breakdown by phospholipase D (PLD) is an important cellular control mechanism. We investigated the signaling pathway participating in prostaglandin E2 (PGE2)–induced PLD activation in osteoblast‐like MC3T3‐E1 cells. PGE2 stimulated PLD activity, as measured by choline generated from phosphatidylcholine, just after the stimulation. The reaction reached a plateau 15 minutes later. PGE2 stimulated PLD activity in a dose‐related manner and also increased inositol phosphate (IP) formation. However, the EC50 value for PGE2‐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 of PGE2 and TPA potentiated it in an additive manner. Although NaF, a heterotrimeric GTP‐binding protein activator, significantly stimulated PLD activity, this effect was not augmented by combination with PGE2. PGE2‐induced PLD activity was markedly suppressed by either chelating extracellular Ca2+ by EGTA or pertussis toxin. These findings suggest that osteoblasts might have at least two PLD activation mechanisms which involve PKC‐dependent or ‐independent pathways. However, present results indicate that PKC is unlikely to be essential to PGE2‐induced PLD activation. On the contrary, pertussis toxin‐sensitive GTP‐binding protein and extracellular Ca2+ might play important roles in the pathway of PGE2‐induced PLD activation.
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