Vitali Matyash, Gerhard Liebisch, Teymuras V. Kurzchalia, Andrej Shevchenko, Dominik Schwudke

Gerhard Liebisch, Eoin Fahy, Junken Aoki, Edward A. Dennis, Thierry Durand, Christer S. Ejsing, Maria Fedorova, Ivo Feussner, William J. Griffiths, Harald Köfeler, Alfred H. Merrill Jr., Robert C. Murphy, Valerie B. O’Donnell, Olga Oskolkova, Shankar Subramaniam, Michael J. O. Wakelam, Friedrich Spener

There is a need for a standardized, practical annotation for structures of lipid species derived from mass spectrometric approaches; i.e., for high-throughput data obtained from instruments operating in either high- or low-resolution modes. This proposal is based on common, officially accepted terms and builds upon the LIPID MAPS terminology. It aims to add defined levels of information below the LIPID MAPS nomenclature, as detailed chemical structures, including stereochemistry, are usually not automatically provided by mass spectrometric analysis. To this end, rules for lipid species annotation were developed that reflect the structural information derived from the analysis. For example, commonly used head group-specific analysis of glycerophospholipids (GP) by low-resolution instruments is neither capable of differentiating the fatty acids linked to the glycerol backbone nor able to define their bond type (ester, alkyl-, or alk-1-enyl-ether). This and other missing structural information is covered by the proposed shorthand notation presented here. Beyond GPs, we provide shorthand notation for fatty acids/acyls (FA), glycerolipids (GL), sphingolipids (SP), and sterols (ST). In summary, this defined shorthand nomenclature provides a standard methodology for reporting lipid species from mass spectrometric analysis and for constructing databases.

Excessive uptake of atherogenic lipoproteins such as modified low-density lipoprotein complexes by vascular macrophages leads to foam cell formation, a critical step in atherogenesis. Cholesterol efflux mediated by high-density lipoproteins (HDL) constitutes a protective mechanism against macrophage lipid overloading. The molecular mechanisms underlying this reverse cholesterol transport process are currently not fully understood. To identify effector proteins that are involved in macrophage lipid uptake and release, we searched for genes that are regulated during lipid influx and efflux in human macrophages using a differential display approach. We report here that the ATP-binding cassette (ABC) transporter ABCG1 (ABC8) is induced in monocyte-derived macrophages during cholesterol influx mediated by acetylated low-density lipoprotein. Conversely, lipid efflux in cholesterol-laden macrophages, mediated by the cholesterol acceptor HDL₃, suppresses the expression of ABCG1. Immunocytochemical and flow cytometric analyses revealed that ABCG1 is expressed on the cell surface and in intracellular compartments of cholesterol-laden macrophages. Inhibition of ABCG1 protein expression using an antisense strategy resulted in reduced HDL₃-dependent efflux of cholesterol and choline-phospholipids. In a comprehensive analysis of the expression and regulation of all currently known human ABC transporters, we identified an additional set of ABC genes whose expression is regulated by cholesterol uptake or HDL₃-mediated lipid release, suggesting a potential function for these transporters in macrophage lipid homeostasis. Our results demonstrating a regulator function for ABCG1 in cholesterol and phospholipid transport define a biologic activity for ABC transporters in macrophages.