PHOSPHOLIPASE D-MEDIATED HYDROLYSIS OF PHOSPHATIDYLCHOLINE - ROLE IN CELL SIGNALING

Studies carried out in many laboratories have demonstrated the activat ion of phospholipase D (PLD) by a variety of receptor agonists and in many cell types. The signal-dependent formation of phosphatidic acid ( PA), by PLD-catalyzed hydrolysis of phosphatidylcholine (PC), may repr esent a novel and ubiquitous signal transduction pathway in mammalian cells. The mode(s) of coupling between agonist receptors and PLD activ ation are not well understood. Studies utilizing NIH-3T3 fibroblasts i ndicated that PLD activation by different mitogens involves distinct m echanisms. Protein kinase C (PKC) seems to play a role both as a media tor and as a modulator of PLD activation. The role of PKC was further examined in Swiss/3T3-derived fibroblasts which stably overexpress PKC -alpha. In these cells, both basal and agonist-stimulated PLD activity are higher than in control cells. In vitro analysis of PLD activity i n detergent-solubilized cell membranes, utilizing exogenous C-6-NBD-PC as fluorescent substrate, showed nearly 2-fold higher activity in mem branes from cells that overexpress PKC-alpha. These results suggest th at PKC-alpha may play a role in regulating PLD expression. The PLD pro duct PA was identified as a precursor of 'late phase' diacylglycerol w hich, at least in some cases, was temporally correlated and causally r elated to the sustained activation of PKC. However, PA may itself act as an intracellular messenger in its own right, although immediate tar gets for its action have not yet been identified. Activation of phosph oinositide-phospholipase C, PLD and phospholipase A(2) seems to compri se a signaling cascade which is typically utilized by most (if not all ) Ca2+-mobilizing agonists.