Lipid droplet

Lipid droplets, also referred to as lipid bodies, oil bodies or adiposomes, are lipid-rich cellular organelles that regulate the storage and hydrolysis of neutral lipids and are found largely in the adipose tissue. They also serve as a reservoir for cholesterol and acyl-glycerols for membrane formation and maintenance. Lipid droplets are found in all eukaryotic organisms and store a large portion of lipids in mammalian adipocytes. Initially, these lipid droplets were considered to merely serve as fat depots, but since the discovery in the nineties of proteins in the lipid droplet coat that regulate lipid droplet dynamics and lipid metabolism, lipid droplets are seen as highly dynamic organelles that play a very important role in the regulation of intracellular lipid storage and lipid metabolism.The role of lipid droplets outside of lipid and cholesterol storage has recently begun to be elucidated and includes a close association to inflammatory responses through the synthesis and metabolism of eicosanoids and to metabolic disorders such as obesity, cancer, and atherosclerosis. In non-adipocytes, lipid droplets are known to play a role in protection from lipotoxicity by storage of fatty acids in the form of neutral triacylglycerol, which consists of three fatty acids bound to glycerol. Alternatively, fatty acids can be converted to lipid intermediates like diacylglycerol (DAG), ceramides and fatty acyl-CoAs. These lipid intermediates can impair insulin signaling, which is referred to as lipid-induced insulin resistance and lipotoxicity. Lipid droplets also serve as platforms for protein binding and degradation. Finally, lipid droplets are known to be exploited by pathogens such as the hepatitis C virus, the dengue virus and chlamydia trachomatis among others.

Lipid droplets are composed of a neutral lipid core consisting mainly of triacylglycerols (TAGs) and cholesteryl esters surrounded by a phospholipid monolayer. The surface of lipid droplets is decorated by a number of proteins which are involved in the regulation of lipid metabolism. The first and best-characterized family of lipid droplet coat proteins is the perilipin protein family, consisting of five proteins. These include perilipin 1 perilipin/ PLIN1, perilipin 2 (PLIN2/ ADRP), perilipin 3 (PLIN3/ TIP47), perilipin 4 (PLIN4/ S3-12) and perilipin 5 (PLIN5/ OXPAT/ LSDP5/ MLDP).Proteomics studies have elucidated the association of many other families of proteins to the lipid surface including proteins involved in membrane trafficking, vesicle docking, endocytosis and exocytosis. Analysis of the lipid composition of lipid droplets has revealed the presence of a diverse set of phospholipid species;phosphatidylcholine and phosphatidylethanolamine are the most abundant, followed by phosphatidylinositol.

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