Function of Ca2+ influx in phospholipase D activation induced by prostaglandin F2 alpha in osteoblast-like cells: involvement of tyrosine kinase.

We previously reported that prostaglandin F2 alpha (PGF2 alpha) induces Ca2+ influx from the extracellular space via protein tyrosine kinase in osteoblast-like MC3T3-E1 cells and that PGF2 alpha stimulates phosphatidylcholine-hydrolyzing phospholipase D in these cells (6, 12). In this study, we examined the relationship between the tyrosine kinase-regulated Ca2+ influx by PGF2 alpha and the activation of phospholipase D in MC3T3-E1 cells. The depletion of extracellular Ca2+ by [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) markedly reduced the PGF2 alpha-induced formation of choline. Genistein, an inhibitor of protein tyrosine kinases, which by itself had little effect on choline formation, significantly suppressed the formation of choline induced by PGF2 alpha in a dose-dependent manner. Tyrphostin, an inhibitor of protein tyrosine kinases chemically distinct from genistein, also suppressed the PGF2 alpha-induced formation of choline. Sodium orthovanadate, an inhibitor of protein tyrosine phosphatases, significantly enhanced the PGF2 alpha-induced formation of choline. These results strongly suggest that the phospholipase D activation by PGF2 alpha is dependent on extracellular Ca2+ in osteoblast-like cells and that protein tyrosine kinase is involved in the activation of phospholipase D.