Functions of autophagy in normal and diseased liver

Abstract

Autophagy has emerged as a critical lysosomal pathway that maintains cell function and survival through the degradation of cellular components such as organelles and proteins. Investigations specifically employing the liver or hepatocytes as experimental models have contributed significantly to our current knowledge of autophagic regulation and function. The diverse cellular functions of autophagy, along with unique features of the liver and its principal cell type the hepatocyte, suggest that the liver is highly dependent on autophagy for both normal function and to prevent the development of disease states. However, instances have also been identified in which autophagy promotes pathological changes such as the development of hepatic fibrosis. Considerable evidence has accumulated that alterations in autophagy are an underlying mechanism of a number of common hepatic diseases including toxin-, drug- and ischemia/reperfusion-induced liver injury, fatty liver, viral hepatitis and hepatocellular carcinoma. This review summarizes recent advances in understanding the roles that autophagy plays in normal hepatic physiology and pathophysiology with the intent of furthering the development of autophagy-based therapies for human liver diseases.

Keywords: autophagy; drug toxicity; hepatitis; hepatocellular carcinoma; hepatocyte; hepatotoxin; ischemia/reperfusion; liver; liver injury.

Figure 1. Type 1 and type 2 mitophagy. In (A and B), GFP-LC3 transgenic hepatocytes were loaded with red-fluorescing tetramethylrhodamine methylester, an indicator of mitochondrial polarization. (A) Nutrient deprivation-induced (type 1) mitophagy in a GFP-LC3 transgenic hepatocyte. Note the presence of a pre-autophagic structure (arrow), phagophores forming around mitochondria (double arrows) and a mitophagosome containing a red-fluorescing polarized mitochondria (asterisk). (B) Photodamage-induced (type 2) mitophagy in a wortmannin-treated GFP-LC3 transgenic hepatocyte. In this form of mitophagy, mitophagosomes indicated by green rings contain depolarized mitochondria, which therefore lack fluorescence. (C) Scheme of type 1 and 2 mitophagy. In type 1 mitophagy induced by nutrient deprivation, PtdIns3K-BECN1 activation leads to formation of a GFP-LC3-labeled phagophore, which sequesters a polarized mitochondrion into a mitophagosome, often in coordination with mitochondrial fission. Mitochondrial depolarization follows sequestration, which can be blocked by inhibitors of the MPT. The mitophagosome then undergoes PtdIns3K-dependent fusion with lysosomes, and hydrolytic digestion of the entrapped mitochondrion occurs. In Type 2 mitophagy induced by photodamage, photoirradiation causes MPT onset and sustained mitochondrial depolarization. GFP-LC3 attaches to the depolarized mitochondrion and by coalescence forms a mitophagosome in a PtdIns3K-independent fashion. Further mitophagosome processing occurs identically to the type 1 pathway.

Figure 2. Circadian regulation of autophagy. The expression of autophagy genes is regulated by the biological clock through a CEBPB-mediated transcriptional pathway. In parallel, autophagy activity is modulated by nutrient- and energy-sensing pathways to drive rhythmic autophagy induction that maintains homeostasis. These pathways include the AMPK and MTOR pathways as well as the transcription factors TFEB, FOXO3 and SREBF2.

Figure 3. Proposed model for the role of autophagy in alcohol- and APAP-induced liver injury. Both ethanol and APAP are first metabolized in the liver by the enzymes CYP2E1 (ethanol and APAP) and ADH (ethanol). The metabolism of APAP generates reactive metabolites which deplete hepatic GSH and bind to cellular and mitochondrial proteins to initiate mitochondrial damage. Consequently, the metabolism of both ethanol and APAP lead to increased ROS production and damaged mitochondria. Damaged mitochondria can lead to necrotic/apoptotic cell death and further ROS production. ROS may inactivate MTOR to trigger autophagy, which helps to remove ethanol-induced excessive lipid droplets and damaged mitochondria and in turn attenuate alcohol-induced liver injury. Pharmacological induction of autophagy by rapamycin and Torin 1 significantly protects against ethanol- and APAP-induced liver injury in mice. In hepatocytes exposed to APAP, damaged mitochondria can be removed by canonical mitophagy resulting in reduced necrosis. A portion of damaged mitochondria can also form mitochondrial spheroids which may also attenuate APAP-induced liver injury.

Figure 4. Functions of autophagy in tumorigenesis. Autophagy has a bifunctional role in the development of liver cancer. Prior to cellular transformation, autophagy functions to suppress tumor formation by preventing genomic instability, promoting cell senescence and limiting inflammation. After transformation has occurred, autophagy promotes tumorigenesis by satisfying the high metabolic demands of the tumor cells and aiding their survival.