Lipids, lysosomes, and autophagy

Abstract

Lipids are essential components of a cell providing energy substrates for cellular processes, signaling intermediates, and building blocks for biological membranes. Lipids are constantly recycled and redistributed within a cell. Lysosomes play an important role in this recycling process that involves the recruitment of lipids to lysosomes via autophagy or endocytosis for their degradation by lysosomal hydrolases. The catabolites produced are redistributed to various cellular compartments to support basic cellular function. Several studies demonstrated a bidirectional relationship between lipids and lysosomes that regulate autophagy. While lysosomal degradation pathways regulate cellular lipid metabolism, lipids also regulate lysosome function and autophagy. In this review, we focus on this bidirectional relationship in the context of dietary lipids and provide an overview of recent evidence of how lipid-overload lipotoxicity, as observed in obesity and metabolic syndrome, impairs lysosomal function and autophagy that may eventually lead to cellular dysfunction or cell death.

Keywords: lipid metabolism; lipophagy; lipotoxicity; lysosomal dysfunction; oxidative stress; reactive oxygen species.

Fig. 1.

Overview of dietary TG metabolism. Dietary lipids enter the circulation by three major pathways. 1) Pancreatic lipases (PLs) digest fats to generate FFAs and glycerol, which are reesterified and released into the circulation packaged into chylomicrons (CM). 2) The liver synthesizes VLDL from TGs and CEs and releases them into the circulation. 3) Adipose tissue TG hydrolysis generates FFAs, which are released into the blood stream bound to albumin. Hydrolysis of CM and VLDL by LPL at the endothelial surface generates FFAs that enter the peripheral tissue via FA transporters CD36/FATP. The CM and VLDL remnants generated from LPL-mediated hydrolysis are taken up by liver for further processing. In the fed state, FFAs are esterified into TG and stored as LDs in adipose tissue and liver or undergo β-oxidation to generate ATP in muscle and other peripheral tissues. In addition, de novo lipogenesis from glucose also generates FFA in the fed state. In the fasted state when lipid supply dwindles, TG hydrolysis generates FFAs that are released into the circulation by adipose tissue or used to generate ketone bodies in liver. Albumin-bound FFA (Alb-FA) and ketone bodies serve as the major source of energy during fasting. DGAT, diacylglycerol acyltransferase; GPAT; glycerol-3-phosphate acyltransferase; HSL, hormone-sensitive lipase; LPIN, lipin; MGL, monoacylglycerol lipase.

Fig. 2.

FA overload-induced oxidative stress and lipotoxicity. Oversupply of FFA may overwhelm mitochondrial capacity to oxidize FAs for ATP production, which leads to accumulation of toxic lipid intermediates, such as diacylglycerol (DAG), ceramides, and acyl carnitines. Toxic lipid intermediates impair insulin signaling through insulin receptor substrates and blunt insulin-stimulated glucose uptake. DAG also activates NOX via protein kinase C (PKC) to increase intracellular ROS production. In addition, dysregulation of mitochondrial electron transport chain (ETC) due to excessive FA β-oxidation promotes mitochondrial ROS production. ROS-induced oxidative stress coupled with reduced expression or activity of antioxidant enzymes eventually leads to cellular lipotoxicity manifested by lipid peroxidation, DNA damage, mitochondrial and lysosomal dysfunction, defective autophagy, and activation of inflammatory responses in multiple tissues. FA-CoA, fatty acyl-CoA; CPT I and CPT II, carnitine palmitoyltransferase I and II; FADH₂, reduced flavin adenine dinucleotide.

Fig. 3.

Molecular mechanism of mammalian autophagy. Under nutrient-rich conditions, mTORC1 binds and inactivates Ulk1/2 complex by inhibitory phosphorylation to suppress autophagy. An autophagy stimulus such as starvation relieves inhibition of Ulk1/2 complex by mTORC1. Activation of AMPK upon starvation inhibits mTORC1 and further activates the Ulk1/2 complex to initiate autophagy. The Ulk1/2 complex, in turn, activates Beclin1-Vps34 (PI3K3) complex either by phosphorylating Beclin1 or through Ambra1 phosphorylation. The kinase activity of Vps34 is regulated by Beclin1, which in turn is controlled by its inhibitory interaction with Bcl-2. The catalytic activity of Vps34 results in the formation of phosphatidyl inositol-3 phosphate (PI3P): a key lipid molecule that recruits specific membrane and protein components to the growing isolation membrane (phagophore). In addition, autophagosome expansion and maturation requires two ubiquitin-like conjugation systems that result in ATG16-ATG5-ATG12 complex and lipidation of LC3-I to LC3-II. LC3-II is required for the selection of autophagic substrate, determining autophagosome size and membrane curvature, and for the bidirectional movement of autophagosomes toward lysosomes for fusion. Mature autophagosomes fuse with lysosomes to form autolysosomes. The autophagic substrates are degraded by lysosomal hydrolases into simple substrates which are utilized for energy production and biomolecule synthesis to maintain cellular homeostasis.

Fig. 4.

Degradation of LDs via autophagy. LDs, containing mostly TGs and CEs at the core, undergo CMA of their structural membrane proteins, PLIN2 and PLIN3. PLIN removal activates RAB7 at the exposed LD surface. RAB7 interacts with autophagy proteins, such as p62 and LC3-II, to facilitate the engulfment of the LD by the growing autophagosome. LDs are sequestered by autophagosomes either selectively via lipophagy (A, B) or with other autophagic substrates via nonselective macroautophagy (C). Autophagosomes fuse with lysosomes to form autolysosomes wherein lysosomal hydrolases, such as LALs, degrade LD TGs and CEs into FFAs that are recycled back into the cytosol to support various energetic and structural requirements of the cell.

Fig. 5.

Lysosomal stress-response pathways induced by dietary lipid overload. Overload of FFA supplied exogenously or derived from TG hydrolysis induces ROS production from mitochondrial electron transport chain complexes, the NADPH oxidase 2 (NOX2) complex, and other ROS producing enzymes. The ROS molecules hydrogen peroxide (H₂O₂) and superoxide anion (O₂^•−) undergo Fenton’s reaction in lysosomes to generate hydroxyl radical (OH^•), a highly reactive ROS. ROS-induced peroxidation of lysosomal membrane lipids leads to LMP and release of cathepsins into the cytosol. Cytosolic cathepsins induce apoptotic cell death either by direct activation of caspases or through mitochondrial membrane permeabilization (MMP)-mediated caspase activation. ROS-induced protein oxidation impairs the function of several lysosomal enzymes, such as v-ATPase proton pump and lysosomal hydrolases, which raises lysosomal pH and inhibits degradation of autophagic cargo, respectively. Lipid overload also induces lysosomal dysfunction by ROS-independent pathways. Incubation with excess FFAs alters the membrane composition of lysosomes and autophagosomes that impairs heterotypic fusion between the two organelles. Macrophages loaded with atherogenic lipids exhibited an increase in lysosomal pH that contributed to a defect in cholesterol efflux that also inhibits chaperone mediated autophagy (CMA) by promoting selective degradation of the lysosome-associated membrane protein 2A (LAMP2A). FFA-induced translocation of mTORC1 to the lysosomal membrane has been shown to promote ER stress-dependent apoptosis in podocytes.