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The intracellular storage and utilization of lipids are critical to maintain cellular energy homeostasis. During nutrient deprivation, cellular lipids stored as triglycerides in lipid droplets are hydrolysed into fatty acids for energy. A second cellular response to starvation is the induction of autophagy, which delivers intracellular proteins and organelles sequestered in double-membrane vesicles (autophagosomes) to lysosomes for degradation and use as an energy source. Lipolysis and autophagy share similarities in regulation and function but are not known to be interrelated. Here we show a previously unknown function for autophagy in regulating intracellular lipid stores (macrolipophagy). Lipid droplets and autophagic components associated during nutrient deprivation, and inhibition of autophagy in cultured hepatocytes and mouse liver increased triglyceride storage in lipid droplets. This study identifies a critical function for autophagy in lipid metabolism that could have important implications for human diseases with lipid over-accumulation such as those that comprise the metabolic syndrome.
Free fatty acids (FFAs) are taken up by hepatocytes and converted into triglycerides (TGs) for storage with cholesterol in lipid droplets (LDs)1. LD-sequestered TGs continually undergo hydrolysis, generating FFAs that are predominantly re-esterified back into TGs for storage1,2. Nutrient deprivation upregulates TG hydrolysis to supply FFAs for oxidation to meet cellular energy demands3. An alternative energy source in times of nutrient scarcity is provided by the breakdown of cellular components by autophagy4,5. Both macroautophagy (the type of autophagy quantitatively more important and subsequently referred to as autophagy) and lipolysis are regulated hormonally by insulin and glucagon6 and are increased during starvation. Except for the processing of endocytosed lipoproteins, no direct involvement of the lysosomal degradation pathway in lipid metabolism has been established. The regulatory and functional similarities between autophagy and lipolysis, along with the capability of lysosomes to degrade lipids, indicated that autophagy may contribute to LD and TG breakdown (Supplementary Fig. 1).
Inhibition of autophagy increases lipid storage
Pharmacological inhibition of autophagy with 3-methyladenine (3MA)7 significantly increased hepatocyte TG content in the absence or presence of exogenous lipid supplementation with oleate (Fig. 1a). A knockdown of the autophagy gene Atg5 in hepatocytes (siAtg5 cells; Supplementary Fig. 2a) also increased TG levels with oleate or a second endogenous stimulus for TG formation-culture in methionine- and choline-deficient medium (MCDM)8,9 (Fig. 1b and Supplementary Fig. 2b...