Prostaglandin D2 (PGD2) stimulated the formation of choline in a dose-dependent manner in the range between 10 nM and 10 μM. The effect of PGD2 on the formation of inositol phosphates (EC50 was 20 nM) was more potent than that on the formation of choline (EC50 was 0.5 μM). The formation of choline stimulated by a combination of PGD2 and 12-O-tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C, was additive. Staurosporine, an inhibitor for protein kinases, enhanced the PGD2-induced formation of choline, but H-7, another inhibitor for protein kinases, had little effect. PGD2 stimulated Ca2+ influx from extracellular space dose-dependently. The depletion of extracellular Ca2+ by EGTA reduced the PGD2-induced formation of choline. W-7 and trifluoperazine dihydrochloride, antagonists of calmodulin, dose-dependently inhibited the PGD2-induced choline formation. These results strongly suggest that PGD2 activates phospholipase D in a Ca2+/calmodulin dependent manner in osteoblast-like cells, and that protein kinase C is not essential for the PGD2-induced activation of phospholipase D.
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