Lipidomics reveals that adiposomes store ether lipids and mediate phospholipid traffic,

Abstract

Lipid droplets are accumulations of neutral lipids surrounded by a monolayer of phospholipids and associated proteins. Recent proteomic analysis of isolated droplets suggests that they are part of a dynamic organelle system that is involved in membrane traffic as well as packaging and distributing lipids in the cell. To gain a better insight into the function of droplets, we used a combination of mass spectrometry and NMR spectroscopy to characterize the lipid composition of this compartment. In addition to cholesteryl esters and triacylglycerols with mixed fatty acid composition, we found that ?10-20% of the neutral lipids were the ether lipid monoalk(en)yl diacylglycerol. Although lipid droplets contain only 1-2% phospholipids by weight, .160 molecular species were identified and quantified. Phosphatidylcholine (PC) was the most abundant class, followed by phosphatidylethanolamine (PE), phosphatidylinositol, and ether-linked phosphatidylcholine (ePC). Relative to total membrane, droplet phospholipids were enriched in lysoPE, lysoPC, and PC but deficient in sphingomyelin, phosphatidylserine, and phosphatidic acid. These results suggest that droplets play a central role in ether lipid metabolism and intracellular lipid traffic. Lipid droplets are recognized by their conserved structural organization, which consists of a hydrophobic matrix of neutral lipid covered by a monolayer of phospholipids and associated proteins (1). Although traditionally regarded as a simple repository for stored carbon reserves, emerging evidence suggests that droplets function as dynamic organelles with a central role in cellular lipid metabolism, membrane trafficking, and cell signaling (2). Because lipid droplets can be found in bacteria, yeast, plant, and animal cells, over the years they have acquired a variety of names. Recently, we proposed that this diverse collection of names be replaced with the designation adiposome (3). Thus, an adiposome is an organelle that is specialized for packaging and distributing lipids in cells. In this nomenclature, the droplet is simply the most visible stage in the complex life cycle of an adiposome.During the past few years, a number of reports have focused on the protein composition of lipid droplets isolated from yeast (4), plant (5), and animal (3, 6, 7) cells. A consensus view from these studies is that droplets contain structural proteins, proteins involved in the biosynthesis and breakdown of lipids, and proteins that mediate membrane traffic. Thus, the proteome indicates that droplets are actively engaged in membrane traffic, perhaps for the purpose of maintaining the proper lipid composition of different membrane compartments. In contrast to the proteins, surprisingly little is known about the lipid composition of animal cell droplets. Generally, droplets are rich in neutral lipids such as triacylglycerol (TAG) and cholesteryl esters that have a diverse population of esterified fatty acids (8). Here, we report an analysis of the lipid composition of droplets purified from variou...

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