PHOSPHATIDIC-ACID EFFECTS ON CYTOSOLIC CALCIUM AND PROLIFERATION IN OSTEOBLASTIC CELLS

Our previous studies show that epidermal growth factor (EGF) stimulate s phospholipase D (PLD)-induced phosphatidic acid (PA) formation in ra t calvarial osteoblastic cells.(28) This study investigated the effect s of PA on cytosolic calcium ([Ca2+](i)) and proliferation, and the po ssible involvement of the PLD pathway in EGF effects on [Ca2+](i) and proliferation in rat calvarial osteoblastic cells. PA markedly increas ed [Ca2+](i). This response was unaffected by thapsigargin, which depl etes [Ca2+](i) pools, blocked by verapamil, a calcium channel blocker, and enhanced by propanolol, an inhibitor of PA-phosphohydrolase. PA a lso reduced the EGF dependent-[Ca2+](i) increase by 60%, while a PLD i nhibitor blocked these effects. Furthermore, PA significantly increase d cell proliferation (P < 0.05) which was inhibited by verapamil and e nhanced by H-7 (PKC inhibitor). The PLD inhibitor significantly (P < 0 .05) reduced the EGF-induced increase in proliferation. In summary, PA stimulates rat calvarial osteoblastic cell proliferation and mobiliza tion of [Ca2+](i) using extracellular pools, and EGF's mitogenic effec t on these cells requires activation of PLD.