Apoptosis and necroptosis in the liver: a matter of life and death

Abstract

Cell death represents a basic biological paradigm that governs outcomes and long-term sequelae in almost every hepatic disease condition. Acute liver failure is characterized by massive loss of parenchymal cells but is usually followed by restitution ad integrum. By contrast, cell death in chronic liver diseases often occurs at a lesser extent but leads to long-term alterations in organ architecture and function, contributing to chronic hepatocyte turnover, the recruitment of immune cells and activation of hepatic stellate cells. These chronic cell death responses contribute to the development of liver fibrosis, cirrhosis and cancer. It has become evident that, besides apoptosis, necroptosis is a highly relevant form of programmed cell death in the liver. Differential activation of specific forms of programmed cell death might not only affect outcomes in liver diseases but also offer novel opportunities for therapeutic intervention. Here, we summarize the underlying molecular mechanisms and open questions about disease-specific activation and roles of programmed cell death forms, their contribution to response signatures and their detection. We focus on the role of apoptosis and necroptosis in acute liver injury, nonalcoholic fatty liver disease (NAFLD), nonalcoholic steatohepatitis (NASH) and liver cancer, and possible translations into clinical applications.

Figure 1.. Mediators of TNF-dependent programmed cell death (simplified scheme).

Activation of distinct cell death pathways in response to TNF signalling is regulated by diverse post-transcriptional modification steps, including phosphorylation and ubiquitylation. Upon ligation of TNF to its receptor, TNF receptor 1 (TNFR1), distinct signalling complexes can be formed, which is mainly orchestrated through ubiquitylation events that influence cell fate towards survival or cell death. The first complex that forms upon TNF stimulation is complex I, consisting of the adaptor protein TNFRSF1A-associated via death domain (TRADD), receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and the E3 ligases TNF receptor-associated factor 2 (TRAF2), cellular inhibitor of apoptosis 1 (CIAP1) and CIAP2, which together mediate the main ubiquitylation events (for example, ubiquitylation of K63 and K48), thereby enabling the further recruitment of the linear ubiquitin chain assembly complex (LUBAC), the inhibitor of NF-κB (IκB) kinase (IKK) complex (comprising IKK subunit-α (IKKα), IKKβ and NF-κB essential modulator (NEMO)), orphan nuclear receptor TAK1 and its adaptor proteins (TAK1-binding protein 1 (TAB1), TAB2 and TAB3) and subsequent activation of the pro-survival transcription factor nuclear factor-κB (NF-κB). Upon genetic or pharmacological inactivation of NF-κB or other factors that drive NF-κB activation (such as TAK1 or NEMO), deubiquitylation and release of RIPK1 from complex I can promote the formation of other complexes that tip the balance of TNF signalling towards cell death. Complexes containing FAS-associated death domain protein (FADD), caspase 8 and RIPK1 (complex IIb) or alternatively TRADD (complex IIa) typically trigger apoptosis. By contrast, complexes containing RIPK1 and RIPK3 (complex IIc) typically activate necroptosis, a form of programmed necrosis, via RIPK3-mediated phosphorylation of mixed lineage kinase domain-like protein (MLKL). Complex formation is further modified by deubiquitinases such as zinc-finger protein A20, ubiquitin carboxyl-terminal hydrolase CYLD or ubiquitin carboxyl-terminalhydrolase 2. c-FLIP, cellular FLICE-like inhibitory protein; p50, nuclea rfactor NF-κB subunit p50; p65, nuclear factor NF-κB subunit p65.

Figure 2.. Mechanisms of death receptor-induced apoptosis.

Following activation of apoptosis-mediating surface antigen FAS, TNF-related apoptosis-inducing ligand (TRAIL) receptors death receptor 4 (DR4; also known as TNFRSF10A) or DR5 (also known as TNFRSF10B) or TNF receptor 1 (TNFR1), caspase 8 becomes activated (Fig. 1). In type I cells (which induce apoptosis independent of mitochondria), caspase 8 activation is sufficient to trigger caspase 3 activation and apoptosis. In type II cells (in which apoptosis is mitochondria-dependent), caspase 8 cleaves BH3-interacting domain death agonist (BID), thereby triggering mitochondrial outer membrane permeabilization (MOMP) and release of cytochrome c and second mitochondria-derived activator of caspase (SMAC). SMAC neutralizes E3ubiquitin-protein ligase XIAP, thereby allowing the cytochrome c–apoptotic protease-activating factor 1 (APAF1) complex to trigger caspase 9 activation, which in turn triggers activation of caspase 8. JUN N-terminal kinase (JNK) activation amplifies this mitochondrial amplification pathway (dashed line) in TNFR1-induced apoptosis. FADD, FAS-associated death domain protein; FASL, FAS antigen ligand; TRADD, TNFRSF1A-associated via death domain.

Figure 3.. Apoptosis and necroptosis and the development of liver cirrhosis and HCC.

Apoptosis and necroptosis, triggered by chronic liver diseases such as viral hepatitis, nonalcoholic steatohepatitis and alcoholic steatohepatitis, can trigger hepatic regeneration, inflammation and fibrogenesis. Apoptosis is considered a less reactive form of cell death than necroptosis, which is explained by the rapid phagocytosis of apoptotic cells and the caspase 8-mediated inhibition of the NACHT, LRR and PYD domains-containing protein 3 (NLRP3) inflammasome. However, it seems that apoptotic cells can nonetheless release selective DAMPs or low levels of DAMPs and thereby also trigger inflammation in specific settings (dashed line). Necroptosis on the other hand not only results in increased cellular leakage and damage-associated molecular pattern (DAMP) release but also can contribute to inflammation via receptor-interacting serine/threonine-protein kinase 3 (RIPK3)-mediated NLRP3 inflammasome activation. Chronic regeneration, inflammation and fibrogenesis contribute to progression to cirrhosis and hepatocellular carcinoma (HCC). In specific settings, apoptosis and necroptosis can trigger immunogenic cell death (ICD). ICD requires simultaneous release of DAMPs and antigens or tumour-associated antigens (TAAs) in sufficient amounts and probably inhibits HCC development and might also contribute to antiviral immunity. In the absence of sufficient DAMP and TAA release, tolerogenic cell death (TCD) can occur.