Necroptosis: a crucial pathogenic mediator of human disease

Abstract

Necroptosis is a genetically regulated form of necrotic cell death that has emerged as an important pathway in human disease. The necroptosis pathway is induced by a variety of signals, including death receptor ligands, and regulated by receptor-interacting protein kinases 1 and 3 (RIPK1 and RIPK3) and mixed-lineage kinase domain-like pseudokinase (MLKL), which form a regulatory necrosome complex. RIPK3-mediated phosphorylation of MLKL executes necroptosis. Recent studies, using animal models of tissue injury, have revealed that RIPK3 and MLKL are key effectors of injury propagation. This Review explores the functional roles of RIPK3 and MLKL as crucial pathogenic determinants and markers of disease progression and severity in experimental models of human disease, including acute and chronic pulmonary diseases; renal, hepatic, cardiovascular, and neurodegenerative diseases; cancer; and critical illness.

Figure 1. Modes of cell death.

Cell death pathways include those that are regulated by genetic programs, which are designated regulated cell death (RCD), and those that are not regulated, which are designated accidental cell death (ACD). Apoptosis is a form of RCD that requires activation of caspases, leading to DNA fragmentation. The initiation of apoptosis may be receptor dependent (extrinsic pathway) or triggered by injurious stimuli (intrinsic pathway), resulting in mitochondrial dysfunction. Apoptosis culminates in nonlytic cell death, which is noninflammatory. Pyroptosis is a form of lytic cell death that occurs in inflammatory cells in response to proinflammatory stimuli. A cardinal feature of pyroptosis is the requirement for inflammasome-dependent caspase-1 activation, which regulates the maturation and secretion of proinflammatory cytokines. Pyroptosis occurs as the result of gasdermin D–regulated (GSDMD-regulated) membranous pore formation and features cytoplasmic swelling and cytosolic content leakage. Necroptosis represents a form of RCD that is activated by RIPK1 and requires RIPK3-dependent phosphorylation of MLKL. MLKL oligomerization results in plasma membrane rupture, leading to a lytic form of cell death associated with release of DAMPs. Ferroptosis is an RCD mode that is distinct from necroptosis. In ferroptosis, iron-dependent lipid peroxidation causes lytic cell death, which can be inhibited by glutathione peroxidase 4 (GPX4). Necrosis is a lytic form of ACD that results in DAMP release and propagation of inflammation. A variant of necrosis that involves mitochondrial permeability transition (MPT) has also been described. DAMPs, damage-associated molecular patterns; MLKL, mixed-lineage kinase domain–like pseudokinase; NLRP3, nucleotide-binding oligomerization domain–, leucine rich repeat–, and pyrin domain–containing protein 3; PYCARD, PYRIN-PAAD-DAPIN domain– and C-terminal caspase-recruitment domain–containing protein; RIPK1, receptor-interacting protein kinase 1; RIPK3, receptor-interacting protein kinase 3.

Figure 2. Regulation of necroptosis, apoptosis, and inflammation.

Stimulation of TNFR1 by TNF results in differential activation of proinflammatory and proapoptosis pathways. TNFR1 activation results in complex formation between RIPK1 and TNFR1-associated death domain (TRADD) protein and other accessory proteins. Ubiquitinylation of RIPK1 by cellular inhibitor of apoptosis proteins (cIAPs) or linear ubiquitin chain assembly complex (LUBAC) results in recruitment of the inhibitor of I-κB kinase (IKK) complex, leading to nuclear translocation and activation of NF-κB. NF-κB regulates transcriptional activation of genes important for cell survival, proinflammatory cytokines, and apoptosis-related proteins. Alternatively, a second deubiquitinylated complex of RIPK1 associates with Fas-associated death domain (FADD) protein and pro–caspase-8. The resulting caspase-8 activation leads to activation of the extrinsic apoptosis pathway, and activation of effector caspase-3 and -7, culminating in apoptosis. Under conditions of caspase-8 depletion or cIAP deficiency, stimulation of TNFR1 promotes the necroptosis pathway. Necrosome formation involves RIPK1/RIPK3 interaction and activation of MLKL. Alternative receptor/ligand interactions, such as stimulation of TLR3/TLR4 by dsRNA or LPS or sensing of viral DNA by DNA-dependent activator of IFN-regulatory factors (DAI), leads to the formation of a TRIF-RIPK3 complex or DAI-RIPK3 complex and RIPK1-independent necrosome formation. Phospho-MLKL (p-MLKL) oligomerizes and translocates into the plasma membrane, where it causes membrane rupture and release of DAMPs. TRAF, TNF receptor associated factor; TRAIL, TNF-related apoptosis-inducing ligand; TRIF, Toll/IL-1 receptor domain–containing adapter-inducing IFN-β.

Figure 3. Central role of necroptosis in human disease.

The necroptosis pathway and its regulator proteins (RIPK1, RIPK3, MLKL) have been implicated in various human diseases, including cardiovascular, neurodegenerative, infectious, hepatic, pulmonary, and renal diseases and cancer. Necroptotic cell death mediated by RIPK1/RIPK3 and/or MLKL may play a role in human diseases. In the context of infectious disease, necroptotic cell death may have a beneficial role in removing infected cells. In addition, RIPK3-dependent, but MLKL-independent, phenotypes have been observed in diseases such as acute lung and kidney injury. RIPK1-dependent necroinflammation, independent of necroptosis, has been implicated in neurodegenerative, hepatic, and renal diseases and sepsis. MLKL, mixed-lineage kinase domain–like pseudokinase; NAFLD, nonalcoholic fatty liver disease; NASH, nonalcoholic steatohepatitis; RIPK1, receptor-interacting protein kinase 1; RIPK3, receptor-interacting protein kinase 3.