In recent years, there has been great interest in the study of phospholipid metabolism in intact cell systems. Such an interest arises mainly from the discovery that cellular membrane phospholipids serve not only in structural roles, but are also reservoirs of preformed second messenger molecules with key roles in cellular signaling. These second messenger molecules are generated by agonist-induced activation and secretion of intracellular and extracellular phospholipases, respectively, i.e. enzymes that cleave ester bonds within phospholipids. Prominent members of the large collection of signal-activated phospholipases are the phospholipase A2s. These enzymes hydrolyze the sn-2 ester bond of phospholipids, releasing a free fatty acid and a lysophospholipid, both of which may alter cell function. In addition to its role in cellular signaling, phospholipase A2 has recently been recognized to be involved in a wide number of pathophysiological situations, ranging from systemic and acute inflammatory conditions to cancer. A growing number of pharmacologic inhibitors will help define the role of particular phospholipase A2s in signaling cascades.

A major issue in lipid signaling relates to the role of particular phospholipase A 2 isoforms in mediating receptor-triggered responses. This has been difficult to study because of the lack of isoform-specific inhibitors. Based on the use of the Group VI Ca 2؉ -independent phospholipase A 2 (iPLA 2 ) inhibitor bromoenol lactone (BEL), we previously suggested a role for the iPLA 2 in mediating phospholipid fatty acid turnover (Balsinde, J., Bianco, I. D., Ackermann, E. J., Conde-Frieboes, K., and Dennis, E. A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92: 8527-8531). We have now further evaluated the role of the iPLA 2 in phospholipid remodeling by using antisense RNA technology. We show herein that inhibition of iPLA 2 expression by a specific antisense oligonucleotide decreases both the steady-state levels of lysophosphatidylcholine and the capacity of the cell to incorporate arachidonic acid into membrane phospholipids. These effects correlate with a decrease in both iPLA 2 activity and protein in the antisense-treated cells. Collectively these data provide further evidence that the iPLA 2 plays a major role in regulating phospholipid fatty acyl turnover in P388D 1 macrophages. In stark contrast, experiments with activated cells confirmed that the iPLA 2 does not play a significant role in receptor-coupled arachidonate mobilization in these cells, as manifested by the lack of an effect of the iPLA 2 antisense oligonucleotide on PAF-stimulated arachidonate release.

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Arachidonic acid (AA) and its oxygenated derivatives, collectively known as the eicosanoids, are key mediators of a wide variety of physiological and pathophysiological states. AA, obtained from the diet or synthesized from linoleic acid, is rapidly incorporated into cellular phospholipids by the concerted action of arachidonoyl-CoA synthetase and lysophospholipid acyltransferases. Under the appropriate conditions, AA is liberated from its phospholipid storage sites by the action of one or various phospholipase A(2) enzymes. Thus, cellular availability of AA, and hence the amount of eicosanoids produced, depends on an exquisite balance between phospholipid reacylation and hydrolysis reactions. This review focuses on the enzyme families that are involved in these reactions in resting and stimulated cells.