Lytic cell death in metabolic liver disease

Abstract

Regulated cell death is intrinsically associated with inflammatory liver disease and is pivotal in governing outcomes of metabolic liver disease. Different types of cell death may coexist as metabolic liver disease progresses to inflammation, fibrosis, and ultimately cirrhosis. In addition to apoptosis, lytic forms of hepatocellular death, such as necroptosis, pyroptosis and ferroptosis elicit strong inflammatory responses due to cell membrane permeabilisation and release of cellular components, contributing to the recruitment of immune cells and activation of hepatic stellate cells. The control of liver cell death is of fundamental importance and presents novel opportunities for potential therapeutic intervention. This review summarises the underlying mechanism of distinct lytic cell death modes and their commonalities, discusses their relevance to metabolic liver diseases of different aetiologies, and acknowledges the limitations of current knowledge in the field. We focus on the role of hepatocyte necroptosis, pyroptosis and ferroptosis in non-alcoholic fatty liver disease, alcohol-associated liver disease and other metabolic liver disorders, as well as potential therapeutic implications.

Keywords: ASH; Ferroptosis; Gaucher's disease; Hemochromatosis; NAFLD; NASH; Necroptosis; Niemann-Pick disease; Programmed cell death; Pyroptosis.

Figure 1.. RIPK3- and MLKL-dependent necroptosis mediated by various stimuli.

TNF-α or FasL binding to their receptor induces the formation of a receptor proximal complex (complex 1), which consists of adaptor proteins (e.g., TRADD, tumor necrosis factor receptor type 1-associated death domain; FADD, fas-associated protein with death domain; TAB, transforming growth factor beta-binding protein), ubiquitin ligases (e.g., TRAF2, TNF receptor associated factor 2; LUBAC, linear ubiquitin chain assembly complex; cIAPs, cellular inhibitors of apoptosis), and kinases (e.g., TAK, transforming growth factor beta-activated kinase 1; RIPK1, receptor interacting protein kinase 1). The ubiquitination of RIPK1 provides docking sites for the recruitment of key proteins leading to cell survival by nuclear factor κB (NF-κB)-dependent upregulation of pro-survival genes. Then, complex 1 internalization leads to the formation of complex 2a, which ultimately results in caspase-8-dependent apoptosis. Finally, when caspases or cIAPs are inhibited, RIPK1 associates with RIPK3 to form the necrosome (complex 2b), which in turn recruits the pseudokinase MLKL. RIPK3-mediated phosphorylation of MLKL results in MLKL translocation to the plasma membrane and pore formation. Other necroptotic stimuli such as IFNα/β, virus, pathogen-associated molecular patterns (e.g., LPS, poly I:C) are also depicted. Although the nature of these signalling complexes is not completely characterized, the main mediators activating the necrosome are shown. To antagonize necroptosis, ESCRT-III components are recruited to localized sites of MLKL-directed membrane damage to shed broken membrane into blebs.

Figure 2.. Caspase-1-dependent and -independent pyroptosis.

Caspase-1-dependent pyroptosis requires activation of the canonical inflammasomes, including NLRP1b, NLRP3, NLRC4, AIM2 and Pyrin. The NLRs are characterized by the combined presence of a nucleotide-binding and oligomerization domain (NACHT) and a variable number of LRRs. They also contain either a caspase recruitment domain (CARD) or pyrin domain (PYD) in their amino-terminus. The AIM2 protein is composed of a N-terminal PYD and a C-terminal DNA-binding HIN200 domain. The pyrin protein has a N-terminal PYD, a bZIP transcription factor domain, a B-box, a coiled-coil, and a C-terminal B30.2 domain. NLRP1b and NLRC4 recruit caspase-1 via their CARD domain, and the bipartite PYD-CARD adaptor protein ASC is required for assembly of the AIM2, NLRP3 and pyrin inflammasomes. The pyrin protein also binds directly to caspase-1 via its B30.2 domain. Caspase-1-independent pyroptosis requires the activation of caspase-11, which recognizes cytosolic LPS and cleaves GSDMD to initiate pyroptosis. In this pathway, GSDMD^NT also activates the NLRP3 inflammasome and thereby caspase-1-dependent maturation of IL-1β and IL-18. Caspase-3-dependent pyroptosis requires the activation of Gasdermin E (GSDME). Caspase-3 can be activated by the mitochondrial and death receptor pathway. Active caspase-3 cleaves GSDME, to produce GSDME N-fragments (GSDME^NT), which then forms pores in the plasma membrane.

Figure 3.. The core components controlling ferroptosis.

Glutathione (GSH) is a tripeptide essential for preventing damage caused by ROS, such as lipid peroxides. It is synthesized continuously from cysteine, glutamate and glycine. Cellular availability of cysteine is the limiting step for GSH synthesis. The Xc⁻ transporter, which consists of two subunits belonging to the family of solute transporters (SLC3A2 and SLC7A11), is responsible for the uptake of extracellular cystine, the precursor of cysteine. GSH provides electrons to the key regulator of ferroptosis, GPX4, which reduces lipid peroxides (-OOH) in plasma membranes to the corresponding alcohols (-OH). Recently, the enzymes ACSL4 and LPCAT3 that are directly involved in shaping the cellular lipid composition, have been shown to sensitize cells to ferroptosis. Oxidation of lipid bilayers during ferroptosis occurs both in an enzymatic (i.e., LOXs, lipoxygenases) and non-enzymatic (i.e., radical-mediated autoxidative; symbolized by the red arrow) manner. Alternatively, FSP1 can suppress ferroptosis by ubiquinone, the reduced form of ubiquinol, which traps lipid peroxyl radicals. FSP1 catalyses the regeneration of ubiquinone using NAD(P)H. Cellular iron homeostasis, which is dependent on the coordination of iron uptake (i.e., TFRC) and export (i.e., ferroportin), directly regulates ferroptosis through Fenton reactions and generation of ROS. Fe³⁺ is reduced to Fe²⁺ by metalloreductases in the endosome, and the divalent metal transporter 1 (DMT1) mediates the transport of Fe²⁺ from the endosome into a labile iron pool in the cytoplasm. Receptor tyrosine kinase (RTK) activates the oncogene RAS, which is known to induce oxidative stress through generation of ROS. Ferroptosis-inducing drugs are depicted in red, whereas ferroptosis inhibitors are shown in green.