Tyrosine kinase regulates phospholipase D activation at a point downstream from protein kinase C in osteoblast‐like cells

It has recently been shown that the activation of protein kinase C (PKC) induces protein tyrosine phosphorylation in osteoblast‐like MC3T3‐E1 cells. We previously reported that the activation of PKC stimulates phosphatidylcholine‐hydrolyzing phospholipase D in these cells. In this study, we examined whether protein tyrosine kinase is involved in the PKC‐induced activation of phospholipase D in MC3T3‐E1 cells. Genistein, an inhibitor of protein tyrosine kinases, which by itself had little effect on choline formation, significantly suppressed the formation of choline induced by 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA), an activator of PKC, in a dose‐dependent manner. Tyrphostin, an inhibitor of protein tyrosine kinases chemically distinct from genistein, also dose‐dependently suppressed the TPA‐induced formation of choline. Sodium orthovandate, an inhibitor of protein tyrosine phosphatases, significantly enhanced the TPA‐induced formation of choline in a dose‐dependent manner. These results strongly suggest that protein tyrosine kinase regulates phospholipase D activity at a point downstream from PKC in osteoblast‐like cells.