Activation of phospholipase D-2 by P2X7 agonists in rat submandibular gland acini

Abstract

Exogenous ATP stimulated phospholipase D (PLD), but not sphingomyelinase in rat submandibular gland (SMG) acini. PLD activation was dependent upon extracellular Ca2+ and did not involve intracellular Ca2+ mobilization or phosphoinositide-specific phospholipase C activation. ATP-stimulated PLD was attenuated by inhibition or downregulation of protein kinase C (PKC). PLD activation was fully blocked by the cytosolic phospholipase A2 (PLA2) inhibitor ONO-RS-082 and partially attenuated by the selective Ca2+-dependent cytosolic PLA2 inhibitor, arachidonyl trifluoromethylketone (AACOCF3), or by bromoenol lactone, an inhibitor of Ca2+-independent cytosolic PLA2. Magnesium, which decreases the concentration of ATP4-, and nickel, which blocks nonspecific cation channels coupled to purinergic receptors, inhibited PLD activation by ATP. Using reverse transcription-polymerase chain reaction and Northern blotting techniques, we demonstrated that the PLD isoform stimulated by ATP was PLD-2. Among various ATP analogs, only the P2Z/P2X7 purinergic receptor agonist benzoyl-benzoyl ATP stimulated PLD-2. The response to ATP was inhibited by the nonselective P2X purinergic antagonist suramin and by oxidized ATP, a potent P2Z/P2X7 receptor antagonist. It is concluded that in rat SMG acinar cells, PLD-2 is upregulated by exogenous ATP through a mechanism involving Ca2+ influx, cytosolic PLA2, and PKC. Also, the data suggest an involvement of P2X7 receptors in PLD-2 stimulation by ATP.