The regulation of phospholipid metabolism in mammalian tissues

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Wong, Jason T.
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The purpose of this study was to investigate the regulation of phospholipid biosynthesis and catabolism in mammalian tissues. Phospholipids form a fluid bilayer that provides both a selective permeability barrier and a matrix with which membrane proteins are associated. In addition to their structural role, phospholipids can be hydrolyzed to provide biological signalling molecules, such as arachidonic acid and its metabolites, known collectively as eicosanoids. The release ofarachidonic acid is the rate-limiting ste in eicosanoid synthesis. We hypothesize that there are at least two levels of control of phospholipid synthesis in the heart: (a) the activities of key biosynthetic enzymes; and (b) the energy status of the heart. Lidocaine was used as an agent to modulate phospholipid synthesis, while hearts were perfused under hypoxic conditions in order to alter the energy status of the heart. Perfusion of hearts in the presence of lidocaine enhanced the incorporation of ($\sp3$H) glycerol into lysophosphatidic acid, phosphatidic acid, phosphatidylinositol, phosphatidylethanolamine, phosphatidylserine, and diacylglycerol. Phosphatidylcholine synthesis was reduced. Enzyme assays after heart perfusion showed that lidocaine stimulated phosphatidic acid phosphatase, which catalyzes the formation of 1,2-diacyl-sn-glycerol. In separate experiments, the addition of lidocaine to the assay mixtures did not directly affect this enzyme. In contrast, acyl-CoA:sn-glycerol-3-phosphate acyltransferase, which catalyzes the formation of lysophosphatidic acid, was stimulated by the direct addition of lidocaine, whereas CDP-choline:1,2-diacyl-sn-glycerol cholinephosphotransferase, which catalyzes the final step in phosphatidylcholine synthesis, was inhibited. Perfusion of hearts under hypoxic conditions caused a general decrease in the biosynthesis of phospholipids. Lidocaine partially restored the synthesis of most phospholipids examined, with the exception of phosphatidylcholine. ATP and CTP levels were reduced under hypoxia and were not restored by lidocaine. CTP:phosphatidic acid cytidylyltransferase, which catalyzes the rate-limiting step in phosphatidylinositol synthesis, was stimulated under hypoxia, and was further stimulated by lidocaine under hypoxia. We conclude that lidocaine affects the regulation of phospholipid biosynthesis in the heart via both direct and indirect modulation of key enzymes. Furthermore, hypoxia lowered the energy status of the heart, which lead to depressed levels of the metabolites ATP and CTP, ultimately resulting in decreased phospholipid synthesis. Lysophosphatidylcholine is a product of phosphatidylcholine hydrolysis by phospholipase A$\sb2$ (PLA$\sb2$), and is present in cell membranes, oxidized lipoproteins, and atherosclerotic tissues. It has the ability to alter endothelial functions and is regarded as a casual agent in atherogenesis. In this study, the modulation of arachidonic acid release by lysophosphatidylcholine in human umbilical vein endothelial cells was examined. The results of this study show that lysophosphatidylcholine caused the elevation of cellular Ca$\sp{2+}$ and the activation of PKC, which stimulated cPLA$\sb2$ in an indirect manner and resulted in an enhanced release of arachidonic acid. (Abstract shortened by UMI.)