This article is available online at http://www.jlr.org developed very early during evolution. Cytosolic lipid droplets (LDs) are the main reservoir of lipids and are common to many if not all eukaryotic cells ( 1 ). LDs have gained much recent interest because of their regulatory role in lipid homeostasis and their implication in metabolic diseases such as obesity and type 2 diabetes ( 2-4 ). They have a unique structure composed of a hydrophobic core surrounded by a phospholipid monolayer containing a specifi c protein composition. Perilipin family proteins and lipid metabolizing enzymes are the most abundant proteins ( 5, 6 ), and proteomic studies have identifi ed many additional constituents ( 7 ). Little is known how proteins are specifi cally targeted to lipid droplets ( 8, 9 ). The biogenesis of LDs likely involves the endoplasmic reticulum, but the mechanism has not been solved and is debated intensely ( 2,3,10 ).Triglycerides are the main species of neutral lipids stored within the LDs of most cell types. The predominant biosynthetic pathway requires three activated fatty acids for each triglyceride molecule. The fatty acids are taken up from the extracellular medium or are derived from endogenous metabolism by fatty acid synthase. In fact, addition of fatty acids is a very effi cient way to induce the formation of LDs ( 4 ). However, fatty acids are chemically quite inert and need to be activated by esterifi cation with CoA ( 11 ). This activation is catalyzed by the family of acylCoA synthetases ( 12 ); physiologically highly relevant are the long chain (ACSL1, -3, -4, -5, -6) and very long chain (ACSVL1, -2, -3, -4, -5, and -6) fatty acyl-CoA synthetase subfamilies ( 13 ). Apart from their obvious enzymatic role, additional functions have been suggested for ACS(V)L family proteins: metabolic channeling of fatty acids toward specifi c metabolic fates [e.g., phospholipid synthesis vs.Abstract Cytosolic lipid droplets (LDs) are storage organelles for neutral lipids derived from endogenous metabolism. Acyl-CoA synthetase family proteins are essential enzymes in this biosynthetic pathway, contributing activated fatty acids. Fluorescence microscopy showed that ACSL3 is localized to the endoplasmic reticulum (ER) and LDs, with the distribution dependent on the cell type and the supply of fatty acids. The N-terminus of ACSL3 was necessary and sufficient for targeting reporter proteins correctly, as demonstrated by subcellular fractionation and confocal microscopy. The N-terminal region of ACSL3 was also found to be functionally required for the enzyme activity. Selective permeabilization and in silico analysis suggest that ACSL3 assumes a hairpin membrane topology, with the N-terminal hydrophobic amino acids forming an amphipathic helix restricted to the cytosolic leafl et of the ER membrane. ACSL3 was effectively translocated from the ER to nascent LDs when neutral lipid synthesis was stimulated by the external addition of fatty acids. Cellular fatty acid uptake was increased by overexpression and reduced b...

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Long chain acyl-CoA synthetases are essential enzymes of lipid metabolism, and have also been implicated in the cellular uptake of fatty acids. It is controversial if some or all of these enzymes have an additional function as fatty acid transporters at the plasma membrane. The most abundant acyl-CoA synthetases in adipocytes are FATP1, ACSVL4/FATP4 and ACSL1. Previous studies have suggested that they increase fatty acid uptake by direct transport across the plasma membrane. Here, we used a gain-of-function approach and established FATP1, ACSVL4/FATP4 and ACSL1 stably expressing 3T3-L1 adipocytes by retroviral transduction. All overexpressing cell lines showed increased acyl-CoA synthetase activity and fatty acid uptake. FATP1 and ACSVL4/FATP4 localized to the endoplasmic reticulum by confocal microscopy and subcellular fractionation whereas ACSL1 was found on mitochondria. Insulin increased fatty acid uptake but without changing the localization of FATP1 or ACSVL4/FATP4. We conclude that overexpressed acyl-CoA synthetases are able to facilitate fatty acid uptake in 3T3-L1 adipocytes. The intracellular localization of FATP1, ACSVL4/FATP4 and ACSL1 indicates that this is an indirect effect. We suggest that metabolic trapping is the mechanism behind the influence of acyl-CoA synthetases on cellular fatty acid uptake.

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The mechanism of fatty acid uptake is of high interest for basic research and clinical interventions. Recently, we showed that mammalian long chain fatty acyl-CoA synthetases (ACS) are not only essential enzymes for lipid metabolism but are also involved in cellular fatty acid uptake. Overexpression, RNAi depletion or hormonal stimulation of ACS enzymes lead to corresponding changes of fatty acid uptake. Remarkably, ACS are not localized to the plasma membrane where fatty acids are entering the cell, but are found instead at the endoplasmic reticulum (ER) or other intracellular organelles like mitochondria and lipid droplets. This is in contrast to current models suggesting that ACS enzymes function in complex with transporters at the cell surface. Drawing on recent insights into non-vesicular lipid transport, we suggest a revised model for the cellular fatty acid uptake of mammalian cells which incorporates trafficking of fatty acids across membrane junctions. Intracellular ACS enzymes are then metabolically trapping fatty acids as acyl-CoA derivatives. These local decreases in fatty acid concentration will unbalance the equilibrium of fatty acids across the plasma membrane, and thus provide a driving force for fatty acid uptake.