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. 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. 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, metabolic disorders such as type 2 diabetes mellitus and obesity, cancer, and atherosclerosis.

In non-adipocytes, lipid droplets are known to play a role in protection from lipotoxicity by storing free fatty acids through esterification. They 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.

Structure
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, mainly members of the PAT family of proteins which are involved in the regulation of lipid metabolism. These include perilipin (PLIN1), ADRP (PLIN2), TIP47 (PLIN3), S3-12 (PLIN4) and OXPAT/LSDP5 (PLIN5). 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.

Lipid droplets vary greatly in size, ranging from 20-40 nm to 100 um. In adipocytes, lipid bodies tend to be larger and they may compose the majority of the cell, while in other cells they may only be induced under certain conditions and are considerably smaller in size.

Formation
Although the exact mechanism of formation of lipid droplets is still unknown, it is proposed that they bud off the membrane of the endoplasmic reticulum as TAGs are collected between its phospholipid bilayer. Lipid droplet growth can consequently happen through direct diffusion of fatty acids, endocytosis of sterols or by fusion of smaller lipid droplets through the aid of SNARE proteins. In non-adipocytes, lipid droplets can be induced by various stimuli including growth factors, long-chain unsaturated fatty acids (including oleic acid and arachidonic acid), oxidative stress and inflammatory stimuli such bacterial lipopolysaccharides, various microbial pathogens, platelet-activating factor, eicosanoids, and cytokines.