Lipid droplets and cellular lipid metabolism

Abstract

Among organelles, lipid droplets (LDs) uniquely constitute a hydrophobic phase in the aqueous environment of the cytosol. Their hydrophobic core of neutral lipids stores metabolic energy and membrane components, making LDs hubs for lipid metabolism. In addition, LDs are implicated in a number of other cellular functions, ranging from protein storage and degradation to viral replication. These processes are functionally linked to many physiological and pathological conditions, including obesity and related metabolic diseases. Despite their important functions and nearly ubiquitous presence in cells, many aspects of LD biology are unknown. In the past few years, the pace of LD investigation has increased, providing new insights. Here, we review the current knowledge of LD cell biology and its translation to physiology.

Figure 1

Examples of lipid droplets (LDs). Shown are a schematic overview of LD structure, LDs in Drosophila S2 cells (middle panel), and in murine adipocytes. Images courtesy of Natalie Krahmer and Caroline Mrejen.

Figure 2

Models of lipid droplet (LD) formation and expansion. For formation, the budding model is shown, with the LD either remaining attached to the endoplasmic reticulum (ER) or detached. The neutral lipid synthesis enzymes, ACAT or DGAT, catalyze the formation of lipids that fill the core. For lipid droplet expansion, the models of local synthesis of triacylglycerol (TG) at the LD and of LD fusion (coalescence) are shown. Abbreviations: DAG, diacylglycerol; FA-CoA, fatty acyl-coenzyme A; SE, sterol ester; PC, phosphatidylcholine.

Figure 3

Models of proteins that target lipid droplets (LDs) and the pathways for their targeting. (a) Some examples of the types of proteins that target the phospholipid monolayer of a LD surface are shown. (b) Several mechanisms by which proteins may target the surfaces of LDs or adjacent membranes are shown. Abbreviation: ER, endoplasmic reticulum.