Lipid droplets are dynamic organelles with a central role in fatty acid metabolism. They protect cells from lipotoxicity by sequestering excess fatty acids but also provide fatty acids for metabolic reactions and signalling events. Here we show that lipid droplet turnover in cancer cells is required for production of ω-3 and ω-6 polyunsaturated fatty acid (PUFA)-derived inflammatory lipid mediators, including eicosanoids and specialised pro-resolving mediators. We show that incorporation of PUFAs into triglycerides mediated by diacylglycerol acyltransferase 1 (DGAT1), and their release by adipose triglyceride lipase (ATGL), are required for cyclooxygenase- and lipoxygenase-dependent lipid mediator production and cancer cell proliferation. The human group X secreted phospholipase A2 (hGX sPLA2) drives the delivery of membrane-derived PUFAs into lipid droplets, while ATGL promotes the incorporation of lipid droplet-derived PUFAs into phospholipids. The group IVA cytosolic PLA2 (cPLA2α) acts on membrane phospholipids and complements ATGL in the regulation of PUFA trafficking between phospholipids and triglycerides. This study identifies lipid droplets as essential cellular hubs that control PUFA availability for production of lipid mediators involved in inflammation and tumorigenesis.SynopsisThis study shows that lipid droplets in cancer cells control the supply of ω-3 and ω-6 polyunsaturated fatty acids (PUFAs) for the production of lipid mediators, which in turn drive cancer cell proliferation. The esterification of PUFAs into triacylglycerols (TAGs) and their release from lipid droplets are necessary for PUFA entry into lipid mediator production pathways. Lipid mediator production induced by the human group X secreted phospholipase A2 (hGX sPLA2), which releases PUFAs from the plasma membrane and serum lipoproteins, depends on diacylglycerol acyltransferase 1 (DGAT1)-mediated TAG synthesis.Adipose triglyceride lipase (ATGL) liberates ω-3 and ω-6 PUFAs from TAGs and drives lipid mediator production via cyclooxygenase (COX) and lipoxygenase (LOX) pathways.ATGL promotes the incorporation of lipid droplet-derived PUFAs into phospholipids, which are targeted by the group IVA cytosolic PLA2 (cPLA2α), thereby selectively supplying arachidonic acid for lipid mediator production.Lipid droplets are required for cPLA2α-induced lipid mediator production also in cells that do not depend on ATGL for the supply PUFAs into lipid mediator pathways.

Lipid droplets are fat storage organelles ubiquitously distributed across the eukaryotic kingdom. They have a central role in regulating lipid metabolism and undergo a dynamic turnover of biogenesis and breakdown to meet cellular requirements for fatty acids, including polyunsaturated fatty acids. Polyunsaturated fatty acids esterified in membrane phospholipids define membrane fluidity and can be released by the activity of phospholipases A2 to act as ligands for nuclear receptors or to be metabolized into a wide spectrum of lipid signaling mediators. Polyunsaturated fatty acids in membrane phospholipids are also highly susceptible to lipid peroxidation, which if left uncontrolled leads to ferroptotic cell death. On the one hand, lipid droplets act as antioxidant organelles that control polyunsaturated fatty acid storage in triglycerides in order to reduce membrane lipid peroxidation, preserve organelle function and prevent cell death, including ferroptosis. On the other hand, lipid droplet breakdown fine-tunes the delivery of polyunsaturated fatty acids into metabolic and signaling pathways, but unrestricted lipid droplet breakdown may also lead to the release of lethal levels of polyunsaturated fatty acids. Precise regulation of lipid droplet turnover is thus essential for polyunsaturated fatty acid distribution and cellular homeostasis. In this review, we focus on emerging aspects of lipid droplet-mediated regulation of polyunsaturated fatty acid trafficking, including the management of membrane lipid peroxidation, ferroptosis and lipid mediator signaling.