The Dynamic Cell 981 The assembly of lipid droplets and its relation to cellular insulin sensitivity Pontus Bostr ¨ om*, Linda Andersson*, Lu Li*, Rosie Perkins*, Kurt Højlund†, Jan Bor ´ en* and Sven-Olof Olofsson* 1 *Sahlgrenska Center for Cardiovascular and Metabolic Research, Wallenberg Laboratory, Sahlgrenska University Hospital, SE-413 45 Gothenburg, Sweden, and †Diabetes Research Centre, Department of Endocrinology, Odense University Hospital, Odense, Denmark Abstract The assembly of lipid droplets is dependent on PtdIns(4,5)P 2 that activates PLD 1 (phospholipase D 1 ), which is important for the assembly process. ERK2 (extracellular-signal-regulated kinase 2) phosphorylates the motor protein dynein and sorts it to lipid droplets, allowing them to be transported on microtubules. Lipid droplets grow in size by fusion, which is dependent on dynein and the transfer on microtubules, and is catalysed by the SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) proteins SNAP-23 (23 kDa synaptosome-associated protein), syntaxin-5 and VAMP-4 (vesicle-associated protein 4). SNAP-23 is also involved in the insulin-dependent translocation of the glucose transporter GLUT4 to the plasma membrane. Fatty acids induce a missorting of SNAP-23, from the plasma membrane to the interior of the cell, resulting in cellular insulin resistance that can be overcome by increasing the levels of SNAP-23. The same missorting of SNAP-23 occurs in vivo in skeletal-muscle biopsies from patients with T2D (Type 2 diabetes). Moreover, there was a linear relation between the amount of SNAP-23 in the plasma membrane from human skeletal-muscles biopsies and the systemic insulin-sensitivity. Syntaxin-5 is low in T2D patients, which leads to a decrease in the insulin-dependent phosphorylation of Akt (also known as protein kinase B). Thus both SNAP-23 and syntaxin-5 are highly involved in the development of insulin resistance. Introduction Neutral lipids, such as triacylglycerols or cholesteryl esters, are stored in lipid droplets in the cytosol. This phenomenon is preserved throughout evolution and is present in most mammalian cells [1–4]. For many years, lipid droplets were considered to be only static fat depots. However, their role has recently been re-evaluated after observations that they have a complex surface [4], move in the microtubule network [5,6] and interact with other organelles such as mitochondria [7], peroxisomes [8] and the ER (endoplasmic reticulum) [9]. They are thus now regarded as highly dynamic organelles that play a role in several cellular processes. Organization of the lipid droplet Lipid droplets consist of a core of neutral lipids, surrounded by a monolayer of amphipathic lipids, such as phospholipids and unesterified cholesterol [2–4]. A number of proteins are also associated with this monolayer: the best described are the PAT-domain proteins, which include perilipin, ADFP (adipocyte differentiation-related protein), TIP47 (47 kDa Key words: fusion, insulin resistance, 23 kDa synaptosome-associated protein (SNAP-23), lipid droplet assembly, soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor (SNARE), syntaxin-5. Abbreviations used: ADFP, adipocyte differentiation-related protein; ARF, ADP-ribosylation factor; cPLA2, cytosolic phospholipase A2; DGAT, diacylglycerol acyltransferase; ER, endoplasmic reticulum; ERK2, extracellular-signal-regulated kinase 2; NSF, N-ethylmaleimide-sensitive factor; PLD1, phospholipase D1; SNARE, soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor; α-SNAP, α-soluble NSF-attachment protein; SNAP-23, 23 kDa synaptosome- associated protein; T2D, Type 2 diabetes; VAMP-4, vesicle-associated protein 4. 1 To whom correspondence should be addressed (email Sven-Olof.Olofsson@wlab.gu.se). tail-interacting protein), LSDP5 (lipid droplet storage protein 5) and S3-12. Perilipin is expressed only in adipose tissue and has a dual role: it protects the triacylglycerols from hydrolysis in its non-phosphorylated state and, in addition, promotes hydrolysis once phosphorylated (reviewed in [4]). ADFP has also been suggested to protect the turnover of triacylglycerols in lipid droplets: overexpression of ADFP in liver cells prevents fatty acids from entering into other metabolic pathways such as the formation of very low density lipoproteins ([10], reviewed in [3,4]). Several other proteins have also been described in lipid droplets. They are involved in processes such as sorting/transport [e.g. Rab, ARF (ADP-ribosylation factor) and motor proteins] and turnover of lipids [e.g. ATGL (adipose triacylglycerol lipase) and its co-activator CGI-58, HSL (hormone-sensitive lipase), DGAT (diacylglycerol acyltransferase), acyl-CoA synthase and cPLA 2 (cytosolic phospholipase A 2 )] (reviewed in [4]). Assembly of lipid droplets (Figure 1) Lipid droplets are formed from the microsomal membranes by a process that is dependent on triacylglycerol biosynthesis [11]. However, several other factors are also of importance for the assembly process, including PLD 1 (phospholipase D 1 ) [12,13] and PtdIns(4,5)P 2 (L. Li, B. Liu, L. Andersson, E. Lu and S.-O. Olofsson, unpublished work). PLD 1 catalyses the formation of phosphatidic acid, a lipid that is essential for lipid droplet assembly [12,13]. In addition, the absolute importance of PtdIns(4,5)P 2 for the assembly Biochem. Soc. Trans. (2009) 37, 981–985; doi:10.1042/BST0370981 C  The Authors Journal compilation C  2009 Biochemical Society