Phosphatidic acid effects on cytosolic calcium and proliferation in osteoblastic cells

Our previous studies show that epidermal growth factor (EGF) stimulates phospholipase D (PLD)-induced phosphatidic acid (PA) formation in rat calvarial osteoblastic cells. This study investigated the effects of PA on cytosolic calcium ([Ca²⁺](i)) and proliferation, and the possible involvement of the PLD pathway in EGF effects on [Ca²⁺](i) and proliferation in rat calvarial osteoblastic cells. PA markedly increased [Ca²⁺](i). This response was unaffected by thapsigargin, which depletes [Ca²⁺](i) pools, blocked by verapamil, a calcium channel blocker, and enhanced by propanolol, an inhibitor of PA-phosphohydrolase. PA also reduced the EGF dependent-[Ca²⁺](i) increase by 60%, while a PLD inhibitor blocked these effects. Furthermore, PA significantly increased cell proliferation (P < 0.05) which was Inhibited by verapamil and enhanced 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 mobilization of [Ca²⁺](i) using extracellular pools, and EGF's mitogenic effect on these cells requires activation of PLD.