Singh R, Kaushik S, Wang Y, Xiang Y, Novak I, Komatsu M, et al. Autophagy regulates lipid metabolism. Nature 2009;458:1131–1135. www.nature.com (Reprinted with permission.)
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 vesicle (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.
The liver plays an important role in the regulation of fat metabolism. During feeding and fasting, the liver actively takes up fat in the form of triglyceride (TG)-containing remnant particles and circulating free fatty acids. Furthermore, the liver synthesizes fatty acids from glucose via de novo lipogenesis. The incoming fatty acids are either partially or completely oxidized or are converted into TGs, which can be exported into the bloodstream in the form of very low density lipoprotein particles. Under conditions when excess hepatic uptake of fatty acids cannot be properly compensated for by increased fatty acid breakdown or increased TG secretion, fatty liver or steatosis ensues. Fatty liver is characterized by the presence of numerous lipid droplets dispersed throughout the cytoplasm of parenchymal cells. This abnormal fat storage represents the early stage of nonalcoholic fatty liver disease (NAFLD), which is the most common liver pathology in Western countries and is considered the hepatic manifestation of the metabolic syndrome. For reasons that are still not fully understood in certain individuals, the hepatic steatosis progresses toward the more severe nonalcoholic steatohepatitis (NASH), which could eventually lead to cirrhosis, liver failure, and even hepatocellular carcinoma.
Although fluctuations in TG levels are a normal response to changes in nutritional status, chronically elevated hepatic lipid storage clearly represents a pathological phenomenon. Elevated TG stores can also be actively dissipated, for example by agonists of the peroxisome proliferator-activated receptor alpha (PPARα), which stimulate oxidation of fatty acids in mitochondria and peroxisomes. Conversely, inhibition of fatty acid oxidation leads to pronounced steatosis.1, 2 Although the process of fatty acid oxidation has been extensively studied, much less is known about TG hydrolysis and the breakdown of lipid droplets.
In an interesting report published in Nature, the group led by Mark Czaja demonstrate that degradation of lipid droplets in liver is mediated by the process of autophagy,3 which is a fundamental function of eukaryotic cells that represents one of the main catabolic pathways for degradation of cellular components. Autophagy mainly targets damaged or superfluous proteins and organelles, which are taken up into large double-membrane vesicles called autophagosomes and then delivered into the interior of lysosomes or vacuoles, where they are digested by hydrolases.4 In the liver, autophagy is important for numerous functions including the removal of misfolded proteins and the turnover of subcellular organelles such as mitochondria, peroxisomes, and the endoplasmic reticulum.5 Defects in autophagy are suspected to be involved in several liver pathologies including alpha 1-antitrypsin deficiency and alcohol-induced liver damage, and may contribute to the age-related decline in organ function. So far, however, no links have been reported between autophagy and hepatic lipid metabolism.
Research by Singh et al. shows that autophagy directly impacts intracellular lipid metabolism in the liver.3 They observed that pharmacological inhibition of autophagy led to increased TG content in cultured hepatocytes. A similar increase in hepatic TG was observed upon knockdown of the Atg5 (autophagy-related 5 homolog) gene, which is involved in the formation of autophagosomes. Consistent with a role of lysosomes in intracellular TG breakdown, hepatocyte lipid content and lipid droplet accumulation increased upon inhibition of lysosomal hydrolysis. A crucial piece of evidence was the observation that typical markers of the autophagic and lysosomal pathway are colocalized with lipid droplets. This colocalization was induced by activators of autophagy and by serum depletion, and reduced by Atg5 knockdown and chemical inhibition of autophagosome formation and autophagosome-lysosome fusion. These data support a scenario in which the content of lipid droplets is delivered to lysosomes for degradation via autophagosomes.
Probably the most important stimulus for autophagy is cellular starvation. Following starvation, autophagy is activated to release cellular constituents that can serve as an alternative energy source.4 In agreement with stimulation of lipophagy by fasting, Singh and colleagues observed that fasting enhanced colocalization between the autophagic marker and lipid droplets in mouse liver and also stimulated the presence of lipid droplet proteins and lipids in autophagic vacuoles and lysosomes. The importance of autophagy in determining liver TG content in vivo was verified using mice with a hepatocyte-specific deletion of the Atg7 gene, which encodes another autophagic gene involved in the formation of autophagosome. The authors observed that Atg7−/− mice exhibit elevated hepatic TG during both feeding and fasting.
Interestingly, it was already demonstrated more than 30 years ago that lysosomes play a major role in the breakdown of hepatic TG stores.6 Subsequent studies indicated TG hydrolysis is exclusively achieved via lysosomal acid lipase, which is encoded by the Lipa gene. The importance of lysosomes in TG hydrolysis is underscored by the excessive lipid accumulation in liver of patients that carry a mutation in the LIPA gene,7 and in Lipa knockout mice.8 However, it was never fully clear how lipid droplets actually end up in lysosomes. The new research suggests that the TG-containing lipid droplets first fuse with autophagosomes, which in turn transport the TG toward lysosomal degradation.
Although autophagy-mediated lipolysis (lipophagy) likely represents a major pathway for hepatic breakdown of TG, an important question is how lipophagy relates to other pathways of TG hydrolysis. Recent studies have suggested a role for adipose TG lipase (patatin-like phospholipase domain containing 2 [Pnpla2]) and possibly hormone-sensitive lipase (Lipe) in hepatic TG breakdown.9–11 One could speculate that fatty acids released upon lysosomal lipolysis may be directed into distinct metabolic pathways compared to fatty acids liberated by cytosolic lipases. Clearly, future research should explore this concept in more detail and address the relative importance of lysosomal versus cytosolic lipolysis.
A major substrate for autophagy is liver glycogen, which is broken down in lysosomes via a specific acid glucosidase encoded by the GAA gene.5 Although induction of glycogen autophagy during fasting makes sense in order to maintain plasma glucose levels, the physiological purpose of fasting-induced lipophagy is more ambiguous, because during fasting the liver already receives plenty of fatty acids originating from adipose tissue lipolysis. It is conceivable that during the initial stages of fasting, fatty acid oxidation cannot keep up with fatty acid uptake, leading to a fatty liver. During prolonged fasting, however, a vast PPARα-mediated increase in fatty acid oxidation capacity may allow for the additional burning of stored fat to comply with the high demands for ATP generation and ketogenesis.
The importance of autophagy in the degradation of lipid droplets raises the possibility that dysfunctional autophagy may contribute to development of a fatty liver. In the metabolic syndrome, hepatic steatosis is believed to be mainly caused by excess release of fatty acids from fat tissue consequent to insulin resistance. Interestingly, insulin is known to have an inhibitory effect on autophagy in the liver.5 Accordingly, one would expect hepatic insulin resistance associated with the metabolic syndrome to reduce fatty liver by stimulating lipophagy. Contrary to this notion, the authors observed that starvation-induced lipophagy was reduced in mice chronically fed a high-fat diet, which induces insulin resistance. These data actually suggest that insulin resistance may be associated with inhibition of lipophagy, thereby aggravating existing steatosis. It is recommended that future studies address whether the insulin-dependent signaling pathway(s) that mediates inhibition of autophagy by insulin in liver exhibit resistance to insulin in the context of the metabolic syndrome. In addition, the impact of hepatic insulin resistance on lipophagy deserves further investigation.
These are exciting times for the study of hepatic steatosis. In recent years, we have gained important new insights into the mechanism of hepatic fat storage and breakdown. The new data on lipophagy represent a further breakthrough in our understanding of how lipid droplets are cleared. These novel mechanistic insights are expected to pave the way for novel therapeutic strategies toward NAFLD.