Nerve growth factor stimulates diacylglycerol de novo synthesis and phosphatidylinositol hydrolysis in pheochromocytoma cells

Abstract

Induction of neurite outgrowth by treating pheochromocytoma cells (PC12 cells) with nerve growth factor (NGF) is associated with major increases in cellular levels of diacylglycerol (DAG), an essential and probably limiting precursor in phosphatidylcholine (PC) and phosphatidylethanolamine (PE) syntheses. To identify the sources of this DAG we examined the effects of NGF treatment on the conversion of [ 3 H ]oleic acid (OA) or [ 3 H ]glycerol to [ 3 H ]glycerolipids, and the turnover of these products in PC12 cells. In kinetic studies on [ 3 H ]OA incorporation, most of the radioactivity in the cells initially was free [ 3 H ]OA; then it appeared predominantly as [ 3 H ]DAG and, eventually, as large amounts of [ 3 H ]phospholipids (PLs). In NGF pre-treated cells, the increases in the levels of [ 3 H ]DAG (which were most prominent) and PLs were similar to those in unlabeled DAG and PLs. These effects of NGF could be partially blocked by an inhibitor (triacsin C) of long chain acyl-CoA synthetase. NGF pre-treatment also significantly enhanced the incorporation of [ 3 H ]glycerol into lipids, a pathway for de novo synthesis of glycerolipids. In studies on the degradation of [ 3 H ]OA-labeled lipids, the disappearance of [ 3 H ]OA-labeled neutral lipids exhibited an initial rapid phase and a subsequent stable phase. NGF treatment transiently promoted the hydrolysis of [ 3 H ]PI to [ 3 H ]DAG. These data suggest that the increases in DAG levels observed in PC12 cells exposed to NGF derive mainly from de novo synthesis and, to a lesser and transient extent, from the hydrolysis of [ 3 H ]PI.