LIPIDS DROPLETS: A NEW INSIGHT INTO STRUCTURE, BIOGENESIS AND FUNCTIONS

Lipid droplets are usually spherical organelles. A core of lipid consists of neutral lipids surrounded by a phospholipid monolayer. Many proteins bind to lipid droplets. Some of them are involved in lipid metabolism and belong to PAT family. Others are well known from different cell compartments, where they play roles not associated with lipid metabolism. Mechanisms of lipid biogenesis and growth are not clear. Several alternative models of lipid droplet formation in eukaryotic cells have been proposed and all of them agree that endoplasmic reticulum plays a key role in lipid droplet biogenesis. Essential differences among the models pertain to the sites of lipid droplet formation (between two leaflets of endoplasmic reticulum membrane or in close neighborhood with endoplasmic reticulum) as well as the mechanisms of lipid droplet detachment from the endoplasmic reticulum membrane. Growth of lipid droplets may result from homotypic fusion or supply of lipid esters and phospholipids to the existing lipid droplet. Lipid droplets interact with organelles (endoplasmic reticulum, mitochondria, peroxisomes, endosomes and phagosomes) and cytoskeleton elements. They are mobile organelles. Intracellular transport of lipid droplets is based mainly on microtubules and their motor proteins. The major roles of lipid droplets are lipid storage and release. Moreover, they mediate intracellular lipid and phospholipid traffic. The ability to bind proteins is a newly discovered function of lipid droplets. The proteins associated with lipid droplets are inactivated and/or destined for degradation. The presence of ribosomes and RNA binding proteins in lipid droplets indicates that they might be capable of RNA binding. Much interest focuses on the role of lipid droplets in pathological states of the cell caused by inflammatory processes or neoplasia. The diverse contents of lipid droplets, their mobility and interactions with many organelles, suggest that they are dynamic and multifunctional organelles.