CHARACTERIZATION OF PHOSPHOLIPASE-D ACTIVATION BY MUSCARINIC RECEPTORS IN HUMAN NEUROBLASTOMA SH-SY5Y CELLS

The cholinergic regulation of phospholipase D activity was studied in SH-SY5Y human neuroblastoma cells with phosphatidylethanol formation a s a specific marker for the enzyme activity. The muscarinic antagonist s, hexahydrosiladifenidol and pirenzepine, inhibited carbachol-induced phosphatidylethanol formation in a concentration-dependent manner and the inhibitory constants indicated that muscarinic M-1 receptors are responsible for the major part of the phospholipase D activation. The mechanism of receptor-mediated phospholipase D activation varies betwe en different cell types and receptors. In SH-SY5Y cells, the carbachol -induced phospholipase D activity was inhibited by protein kinase C in hibitors. Since both phospholipases D and C are activated by muscarini c stimulation in SH-SY5Y cells, most of the phospholipase D activation is probably secondary to the protein kinase C activation that follows phospholipase C-mediated increase in diacylglycerols. Other kinases m ay be involved in the regulation since also a tyrosine kinase inhibito r decreased the phosphatidylethanol formation. Stimulation of G-protei n(s) and increase in the intracellular Ca2+ concentration activated ph ospholipase D and may be additional mechanisms for the muscarinic regu lation of phospholipase D in SH-SY5Y cells. Propranolol, an inhibitor of phosphatidic acid phosphohydrolase, increased the carbachol-induced formation of phosphatidic acid at the expense of 1,2-diacylglycerol. This indicates that phospholipase D contributes to the formation of 1, 2-diacylglycerol after carbachol stimulation in SH-SY5Y cells. (C) 199 7 Elsevier Science Ltd.