RIP1/RIP3-regulated necroptosis as a target for multifaceted disease therapy (Review)

Abstract

Necroptosis is a type of programmed cell death with necrotic morphology, occurring in a variety of biological processes, including inflammation, immune response, embryonic development and metabolic abnormalities. The current nomenclature defines necroptosis as cell death mediated by signal transduction from receptor‑interacting serine/threonine kinase (RIP) 1 to RIP3 (hereafter called RIP1/RIP3). However, RIP3‑dependent cell death would be a more precise definition of necroptosis. RIP3 is indispensable for necroptosis, while RIP1 is not consistently involved in the signal transduction. Notably, deletion of RIP1 even promotes RIP3‑mediated necroptosis under certain conditions. Necroptosis was previously thought as an alternate process of cell death in case of apoptosis inhibition. Currently, necroptosis is recognized to serve a pivotal role in regulating various physiological processes. Of note, it mediates a variety of human diseases, such as ischemic brain injury, immune system disorders and cancer. Targeting and inhibiting necroptosis, therefore, has the potential to be used for therapeutic purposes. To date, research has elucidated the suppression of RIP1/RIP3 via effective inhibitors and highlighted their potential application in disease therapy. The present review focused on the molecular mechanisms of RIP1/RIP3‑mediated necroptosis, explored the functions of RIP1/RIP3 in necroptosis, discussed their potential as a novel therapeutic target for disease therapy, and provided valuable suggestions for further study in this field.

Figure 1

Structural diagrams of RIP1 and RIP3. (A) Schematic of functional domains of RIP1 and RIP3. (B) Protein tertiary structures of RIP1 and RIP3. RIP, receptor-interacting serine/threonine kinase; ID, intermediate domain; RHIM, RIP homotypic interaction motif; DD, death domain.

Figure 2

TNFR1-mediated signaling pathways. After the binding of TNF to its receptor, TNFR1 undergoes a conformational change and recruits multiple proteins to form complex I, consisting of TRADD, TRAF2/5, RIP1 and cIAP1/2. The K63-linked ubiquitination of RIP1 by cIAP1/2 promotes the formation and activation of the TAK1/TAB complex and the IKKα/IKKβ/NEMO complex, which induced the NF-κB pathway and cell survival. Destabilization of complex I results in the formation of complex IIa, that contains TRADD, FADD and caspase-8. When cIAPs are blocked and RIP1 deubiquitylated by CYLD, complex IIb is formed. This consists of RIP1, RIP3, FADD, caspase-8 and FLIPL. Both IIa and IIb can initiate apoptosis. When caspase-8 is inhibited by chemical caspase inhibitors, RIP1 binds to RIP3, resulting in the formation of RIP1/RIP3 by intramolecular auto- and trans-phosphorylation. Then, RIP3 recruits and phosphorylates MLKL to form complex IIc, conventionally referred to as the necrosome. The phosphorylated MLKL then translocates from the cytosol to the plasma and intracellular membranes. The oligomerization of MLKL results in membrane pore formation, causing membrane rupture and eventually necroptosis. TNFR1, TNF receptor 1; TNF, tumor necrosis factor; TRADD, TNF-receptor-associated death domain; TRAF, TNF-receptor-associated factor; RIP, receptor-interacting serine/threonine kinase; cIAP, cellular inhibitor of apoptosis 1; TAK1, transforming growth factor-activated kinase 1; TAB, TAK1-binding protein; IKK, inhibitor of NF-κB kinase; NEMO, NF-κB essential modulator; FADD, Fas-associated protein with death domain; CYLD, CYLD lysine 63 deubiquitinase; FLIPL, FLICE-like inhibitory protein long form; MLKL, mixed lineage kinase domain like protein.

Figure 3

Other stimuli leading to necroptosis. In addition to the TNF-α-mediated necroptosis pathway, multiple other necroptosis triggers have been identified. These involve the canonical pathway that requires RIP1 kinase activity, as well as the non-canonical pathway that is dependent on either the TRIF adaptor or the DAI sensor. RIP1-dependent stimuli include TNF-α, CD95L (also known as FASL), APO-1L, TRAIL (also known as APO-2L) and IFN-α/β. RIP1-independent stimuli include viruses (such as CMV), viral DNA, LPS and polycytidylic acid. After various necroptotic stimuli induce necroptosis, a necrosome is formed. Phosphorylated MLKL by RIP3 transfers from the cytosol to the plasma and intracellular membranes, causing destruction of membrane integrity and eventually necrotic death. TNF, tumor necrosis factor; RIP, receptor-interacting serine/threonine kinase; TRIF, TIR domain-containing interferon-β; DAI, DNA-dependent activator of IFN regulatory factors; FASL, Fas ligand; TRAIL, TNF-related apoptosis inducing ligand; IFN, interferon; CMV, cytomegalovirus; LPS, lipopolysaccharide; MLKL, mixed lineage kinase domain like protein.

Figure 4

Necroptosis-associated pathological mechanisms. (A) Inflammatory factors include TNF-α and IL-1β. Certain viral infections can induce RIP1/RIP3-mediated necroptosis, which can result in the disruption of the plasma membrane and in the release of endogenous molecules, also known as DAMPs. Inflammasomes are then activated by DAMPs, promoting inflammatory cell recruitment and virus-specific T cell responses to induce inflammatory diseases. (B) DRs can regulate the necroptosis of T and B cells. When either caspase-8 or FADD is deficient, T cells undergo hyper-autophagy and generate a RIP1/RIP3-mediated necrosome, triggering necroptosis. BCR mediates necroptosis via reaction with TLR7. Necroptosis of T lymphocytes and B lymphocytes can cause immunodeficiency. (C) RIP1 prevents embryonic and postnatal lethality by blocking two different cell death pathways: FADD/caspase-8-mediated apoptosis and RIP3-mediated necroptosis. TNF, tumor necrosis factor; IL, interleukin; RIP, receptor-interacting serine/threonine kinase; DAMPs, damage-associated molecular patterns; DR, death receptor; FADD, Fas-associated protein with death domain; BCR, B cell receptor; TLR, toll-like receptor; MLKL, mixed lineage kinase domain like protein; TNFR1, TNF receptor 1.