Phosphatidylethanol Stimulates Calcium-Dependent Cytosolic Phospholipase A₂ Activity of a Macrophage Cell Line (RAW 264.7)

Abstract

The synthesis of inflammation mediators produced from arachidonic acid is regulated primarily by the cellular concentration of free arachidonic acid. Since intracellular arachidonic acid is almost totally present as phospholipid esters, the concentration of intracellular arachidonic acid is primarily dependent on the balance between the release of arachidonic acid from membrane phospholipids and the uptake of arachidonic acid into membrane phospholipids. Cytosolic phospholipase A₂ is a calciumdependent enzyme that catalyzes the stimulus-coupled hydrolysis of arachidonic acid from membrane phospholipids. Following exposure of macrophages to various foreign or endogenous stimulants, cytosolic phospholipase A₂ is activated. Treatment with these compounds may also stimulate phospholipase D activity, and, in the presence of ethanol, phospholipase D catalyzes the synthesis of phosphatidylethanol. A cell-free system was used to evaluate the effect of phosphatidylethanol on cytosolic phospholipase A₂ activity. Phosphatidylethanol (0.5 μM) added to 1-stearoyl-2-[³H]-arachidonoyl-sn-glycero-3-phosphocholine vesicles stimulated cytosolic phospholipase A₂ activity. However, high concentrations (20–100 μM) of phosphatidylethanol inhibited cytosolic phospholipase A₂ activity. Phosphatidic acid, the normal phospholipase D product, also stimulated cytosolic phospholipase A₂ activity at 0.5 μM, but had an inhibitory effect on cytosolic phospholipase A₂ activity at concentrations of 50 and 100 μM. Ethanol (20–200 mM), the precursor of phosphatidylethanol, added directly to the assay did not alter cytosolic phospholipase A₂ activity. These results suggest that phosphatidylethanol alters the physical properties of the substrate, and at lower concentrations of anionic phospholipids the substrate is more susceptible to hydrolysis. However, at high concentrations, phosphatidylethanol either reverses the alterations in physical properties of the substrate or phosphatidylethanol may be competing as the substrate. Both interactions may result in lower cytosolic phospholipase A₂ activity.