RIPK1 blocks early postnatal lethality mediated by caspase-8 and RIPK3. Cell. 2014;157:1189-1202. [PMID: 24813850 DOI: 10.1016/j.cell.2014.04.018]

RIP3 Regulates Autophagy and Promotes Coxsackievirus B3 Infection of Intestinal Epithelial Cells

Receptor interacting protein kinase-3 (RIP3) is an essential kinase for necroptotic cell death signaling and has been implicated in antiviral cell death signaling upon DNA virus infection. Here, we performed high-throughput RNAi screening and identified RIP3 as a positive regulator of coxsackievirus B3 (CVB) replication in intestinal epithelial cells (IECs). RIP3 regulates autophagy, a process utilized by CVB for viral replication factory assembly, and depletion of RIP3 inhibits autophagic flux and leads to the accumulation of autophagosomes and amphisomes. Additionally, later in infection, RIP3 is cleaved by the CVB-encoded cysteine protease 3C(pro), which serves to abrogate RIP3-mediated necrotic signaling and induce a nonnecrotic form of cell death. Taken together, our results show that temporal targeting of RIP3 allows CVB to benefit from its roles in regulating autophagy while inhibiting the induction of necroptotic cell death.

Caspase-8 scaffolding function and MLKL regulate NLRP3 inflammasome activation downstream of TLR3

TLR2 promotes NLRP3 inflammasome activation via an early MyD88-IRAK1-dependent pathway that provides a priming signal (signal 1) necessary for activation of the inflammasome by a second potassium-depleting signal (signal 2). Here we show that TLR3 binding to dsRNA promotes post-translational inflammasome activation through intermediate and late TRIF/RIPK1/FADD-dependent pathways. Both pathways require the scaffolding but not the catalytic function of caspase-8 or RIPK1. Only the late pathway requires kinase competent RIPK3 and MLKL function. Mechanistically, FADD/caspase-8 scaffolding function provides a post-translational signal 1 in the intermediate pathway, whereas in the late pathway it helps the oligomerization of RIPK3, which together with MLKL provides both signal 1 and 2 for inflammasome assembly. Cytoplasmic dsRNA activates NLRP3 independent of TRIF, RIPK1, RIPK3 or mitochondrial DRP1, but requires FADD/caspase-8 in wildtype macrophages to remove RIPK3 inhibition. Our study provides a comprehensive analysis of pathways that lead to NLRP3 inflammasome activation in response to dsRNA.

Inflammasome activation requires a complex and incompletely understood network of signalling events. Here the authors characterize step-by-step contributions of TLR3, caspase-8, RIPK3 and MLKL to the activation of NLRP3 inflammasome in response to double-stranded RNA.

Characterization of RIPK3-mediated phosphorylation of the activation loop of MLKL during necroptosis

Mixed lineage kinase domain-like pseudokinase (MLKL) mediates necroptosis by translocating to the plasma membrane and inducing its rupture. The activation of MLKL occurs in a multimolecular complex (the 'necrosome'), which is comprised of MLKL, receptor-interacting serine/threonine kinase (RIPK)-3 (RIPK3) and, in some cases, RIPK1. Within this complex, RIPK3 phosphorylates the activation loop of MLKL, promoting conformational changes and allowing the formation of MLKL oligomers, which migrate to the plasma membrane. Previous studies suggested that RIPK3 could phosphorylate the murine MLKL activation loop at Ser345, Ser347 and Thr349. Moreover, substitution of the Ser345 for an aspartic acid creates a constitutively active MLKL, independent of RIPK3 function. Here we examine the role of each of these residues and found that the phosphorylation of Ser345 is critical for RIPK3-mediated necroptosis, Ser347 has a minor accessory role and Thr349 seems to be irrelevant. We generated a specific monoclonal antibody to detect phospho-Ser345 in murine cells. Using this antibody, a series of MLKL mutants and a novel RIPK3 inhibitor, we demonstrate that the phosphorylation of Ser345 is not required for the interaction between RIPK3 and MLKL in the necrosome, but is essential for MLKL translocation, accumulation in the plasma membrane, and consequent necroptosis.

Intracellular nicotinamide adenine dinucleotide promotes TNF-induced necroptosis in a sirtuin-dependent manner

Cellular necrosis has long been regarded as an incidental and uncontrolled form of cell death. However, a regulated form of cell death termed necroptosis has been identified recently. Necroptosis can be induced by extracellular cytokines, pathogens and several pharmacological compounds, which share the property of triggering the formation of a RIPK3-containing molecular complex supporting cell death. Of interest, most ligands known to induce necroptosis (including notably TNF and FASL) can also promote apoptosis, and the mechanisms regulating the decision of cells to commit to one form of cell death or the other are still poorly defined. We demonstrate herein that intracellular nicotinamide adenine dinucleotide (NAD(+)) has an important role in supporting cell progression to necroptosis. Using a panel of pharmacological and genetic approaches, we show that intracellular NAD(+) promotes necroptosis of the L929 cell line in response to TNF. Use of a pan-sirtuin inhibitor and shRNA-mediated protein knockdown led us to uncover a role for the NAD(+)-dependent family of sirtuins, and in particular for SIRT2 and SIRT5, in the regulation of the necroptotic cell death program. Thus, and in contrast to a generally held view, intracellular NAD(+) does not represent a universal pro-survival factor, but rather acts as a key metabolite regulating the choice of cell demise in response to both intrinsic and extrinsic factors.

The Roles of ROS and Caspases in TRAIL-Induced Apoptosis and Necroptosis in Human Pancreatic Cancer Cells

Death signaling provided by tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) can induce death in cancer cells with little cytotoxicity to normal cells; this cell death has been thought to involve caspase-dependent apoptosis. Reactive oxygen species (ROS) are also mediators that induce cell death, but their roles in TRAIL-induced apoptosis have not been elucidated fully. In the current study, we investigated ROS and caspases in human pancreatic cancer cells undergoing two different types of TRAIL-induced cell death, apoptosis and necroptosis. TRAIL treatment increased ROS in two TRAIL-sensitive pancreatic cancer cell lines, MiaPaCa-2 and BxPC-3, but ROS were involved in TRAIL-induced apoptosis only in MiaPaCa-2 cells. Unexpectedly, inhibition of ROS by either N-acetyl-L-cysteine (NAC), a peroxide inhibitor, or Tempol, a superoxide inhibitor, increased the annexin V-/propidium iodide (PI)+ early necrotic population in TRAIL-treated cells. Additionally, both necrostatin-1, an inhibitor of receptor-interacting protein kinase 1 (RIP1), and siRNA-mediated knockdown of RIP3 decreased the annexin V-/PI+ early necrotic population after TRAIL treatment. Furthermore, an increase in early apoptosis was induced in TRAIL-treated cancer cells under inhibition of either caspase-2 or -9. Caspase-2 worked upstream of caspase-9, and no crosstalk was observed between ROS and caspase-2/-9 in TRAIL-treated cells. Together, these results indicate that ROS contribute to TRAIL-induced apoptosis in MiaPaCa-2 cells, and that ROS play an inhibitory role in TRAIL-induced necroptosis of MiaPaCa-2 and BxPC-3 cells, with caspase-2 and -9 playing regulatory roles in this process.

Live to die another way: modes of programmed cell death and the signals emanating from dying cells

All life ends in death, but perhaps one of life's grander ironies is that it also depends on death. Cell-intrinsic suicide pathways, termed programmed cell death (PCD), are crucial for animal development, tissue homeostasis and pathogenesis. Originally, PCD was almost synonymous with apoptosis; recently, however, alternative mechanisms of PCD have been reported. Here, we provide an overview of several distinct PCD mechanisms, namely apoptosis, autophagy and necroptosis. In addition, we discuss the complex signals that emanate from dying cells, which can either trigger regeneration or instruct additional killing. Further advances in understanding the physiological roles of the various mechanisms of cell death and their associated signals will be important to selectively manipulate PCD for therapeutic purposes.

Molecular crosstalk between apoptosis, necroptosis, and survival signaling

Our current knowledge of the molecular mechanisms regulating the signaling pathways leading to cell survival, cell death, and inflammation has shed light on the tight mutual interplays between these processes. Moreover, the fact that both apoptosis and necrosis can be molecularly controlled has greatly increased our interest in the roles that these types of cell death play in the control of general processes such as development, homeostasis, and inflammation. In this review, we provide a brief update on the different cell death modalities and describe in more detail the intracellular crosstalk between survival, apoptotic, necroptotic, and inflammatory pathways that are activated downstream of death receptors. An important concept is that the different cell death processes modulate each other by mutual inhibitory mechanisms, serve as alternative back-up death routes in the case of a defect in the first-line cell death response, and are controlled by multiple feedback loops. We conclude by discussing future perspectives and challenges in the field of cell death and inflammation research.

Manipulation of apoptosis and necroptosis signaling by herpesviruses

Like apoptosis, necroptosis is an innate immune mechanism that eliminates pathogen-infected cells. Receptor-interacting protein kinase (RIP)3 (also called RIPK3) mediates necrotic death by phosphorylating an executioner protein, MLKL, leading to plasma membrane leakage. The pathway is triggered against viruses that block caspase 8. In murine CMV, the viral inhibitor of caspase 8 activation prevents extrinsic apoptosis but also has the potential to unleash necroptosis. This virus encodes the viral inhibitor of RIP activation to prevent RIP homotypic interaction motif (RHIM)-dependent signal transduction and necroptosis. Recent investigations reveal a similar mechanism at play in the human alpha-herpesviruses, herpes simplex virus (HSV)1 and HSV2, where RHIM competitor function and caspase 8 suppression are carried out by the virus-encoded large subunit of ribonucleotide reductase (R1). In human cells, R1 inhibition of caspase 8 prevents TNF-induced apoptosis, but sensitizes to TNF-induced necroptosis. The RHIM and caspase 8 interaction domains of R1 collaborate to prevent RIP3-dependent steps and enable both herpesviruses to deflect host cell death machinery that would cut short infection. In mouse cells, HSV1 infection by itself triggers necroptosis by driving RIP3 protein kinase activity. HSV1 R1 contributes to the activation of RIP3 adaptor function in mice, a popular host animal for experimental infection. Based on these studies, infection of RIP3-kinase inactive mice should be explored in models of pathogenesis and latency. The necrotic death pathway that is suppressed during infection in the natural host becomes a cross-species barrier to infection in a non-natural host.

Necroptosis: The Trojan horse in cell autonomous antiviral host defense

Herpesviruses suppress cell death to assure sustained infection in their natural hosts. Murine cytomegalovirus (MCMV) encodes suppressors of apoptosis as well as M45-encoded viral inhibitor of RIP activation (vIRA) to block RIP homotypic interaction motif (RHIM)-signaling and recruitment of RIP3 (also called RIPK3), to prevent necroptosis. MCMV and human cytomegalovirus encode a viral inhibitor of caspase (Casp)8 activation to block apoptosis, an activity that unleashes necroptosis. Herpes simplex virus (HSV)1 and HSV2 incorporate both RHIM and Casp8 suppression strategies within UL39-encoded ICP6 and ICP10, respectively, which are herpesvirus-conserved homologs of MCMV M45. Both HSV proteins sensitize human cells to necroptosis by blocking Casp8 activity while preventing RHIM-dependent RIP3 activation and death. In mouse cells, HSV1 ICP6 interacts with RIP3 and, surprisingly, drives necroptosis. Thus, herpesviruses have illuminated the contribution of necoptosis to host defense in the natural host as well as its potential to restrict cross-species infections in nonnatural hosts.

A novel function of RIP1 in postnatal development and immune homeostasis by protecting against RIP3-dependent necroptosis and FADD-mediated apoptosis

RIP1 is an adaptor kinase originally identified as being able to associate with TNFR1 and Fas, and is later shown to be involved in signaling induced by TLRs. Major signaling pathways regulated by RIP1 include necroptosis, apoptosis, and pro-survival/inflammation NF-κB activation. Previous studies show that RIP1 deficiency has no effect on mouse embryogenesis, but blocks postnatal development. This phenotype could not readily be explained, since mice lacking TNFR1, Fas, or TLRs show no apparent developmental defect. Certain types of RIP1-deficient cells are hypersensitive to TNF-induced apoptosis. However, in our previous study, deletion of the apoptotic adaptor protein, FADD, provides marginal improvement of postnatal development of rip1^−/− mice. Remarkably, the current data shows that haploid insufficiency of RIP3, a known mediator of necroptosis, allowed survival of rip1^−/−fadd^−/− mice beyond weaning age, although the resulting rip1^−/−fadd^−/− rip3^+/− mice were significant smaller in size and weight. Moreover, complete absence of RIP3 further improved postnatal development of the resulting rip1^−/−fadd^−/−rip3^−/− mice, which display normal size and weight. In such triple knockout (TKO) mice, lymphocytes underwent normal development, but progressively accumulated as mice age. This lymphoproliferative (lpr) disease in TKO mice is, however, less severe than that of fadd^−/−rip3^−/− double knockout mice. In total, the data show that the postnatal developmental defect in rip1^−/− mice is due in part to FADD-mediated apoptosis as well as RIP3-dependent necroptosis. Moreover, the function of RIP1 contributes to development of lpr diseases.

Apoptotic caspases suppress mtDNA-induced STING-mediated type I IFN production

Activated caspases are a hallmark of apoptosis induced by the intrinsic pathway, but they are dispensable for cell death and the apoptotic clearance of cells in vivo. This has led to the suggestion that caspases are activated not just to kill but to prevent dying cells from triggering a host immune response. Here, we show that the caspase cascade suppresses type I interferon (IFN) production by cells undergoing Bak/Bax-mediated apoptosis. Bak and Bax trigger the release of mitochondrial DNA. This is recognized by the cGAS/STING-dependent DNA sensing pathway, which initiates IFN production. Activated caspases attenuate this response. Pharmacological caspase inhibition or genetic deletion of caspase-9, Apaf-1, or caspase-3/7 causes dying cells to secrete IFN-β. In vivo, this precipitates an elevation in IFN-β levels and consequent hematopoietic stem cell dysfunction, which is corrected by loss of Bak and Bax. Thus, the apoptotic caspase cascade functions to render mitochondrial apoptosis immunologically silent.

Necroptosis: Pathway diversity and characteristics

Regulated cell death is a physiological process that controls organismal homeostasis. Deregulation of cell death can lead to the development of a number of human diseases and tissue damage. Apoptosis is a best-known model of caspase-dependent regulated cell death, but recently necroptosis has garnered a lot of attention as a form of regulated cell death not mediated by caspases. Different stimuli can trigger necroptosis, and all of them converge at the activation of the protein kinase RIP3 (receptor-interacting protein 3) and the pseudokinase MLKL (mixed lineage kinase domain-like). Necroptosis activation relies on the unique protein-interaction motif RHIM (RIP homology interaction motif). Different RHIM-containing proteins (RIP1, DAI and TRIF) transduce necroptotic signals from the cell death trigger to the cell death mediators RIP3-MLKL. RIP1 has a particularly important and complex role in necroptotic cell death regulation ranging from cell death activation to inhibition, often in a cell type and context dependent fashion.

RIPK1 and RIPK3: critical regulators of inflammation and cell death

RIPK1 and RIPK3 (receptor-interacting serine/threonine protein kinases 1/3) interact by virtue of their RIP homotypic interaction motifs to mediate a form of cell death called necroptosis, although mice lacking these kinases have very different phenotypes. RIPK1-deficient mice die soon after birth, whereas RIPK3-deficient mice are healthy. Necroptosis involves cell rupture and is triggered by tumor necrosis factor (TNF), Toll-like receptors (TLRs), or the T cell receptor (TCR) when pro-apoptotic caspase-8 is inhibited. Various mouse models of disease are ameliorated by RIPK3 deficiency, suggesting that necroptosis contributes to pathology. Genetic rescue experiments now reveal why RIPK3-deficient are viable but RIPK1-deficient mice are not. These and other experiments indicate unexpected complexity in the regulation of both apoptosis and necroptosis by RIPK1 and RIPK3.

Necroptosis and its role in inflammation

Regulated cell death has essential functions in development and in adult tissue homeostasis. Necroptosis is a newly discovered pathway of regulated necrosis that requires the proteins RIPK3 and MLKL and is induced by death receptors, interferons, toll-like receptors, intracellular RNA and DNA sensors, and probably other mediators. RIPK1 has important kinase-dependent and scaffolding functions that inhibit or trigger necroptosis and apoptosis. Mouse-model studies have revealed important functions for necroptosis in inflammation and suggested that it could be implicated in the pathogenesis of many human inflammatory diseases. We discuss the mechanisms regulating necroptosis and its potential role in inflammation and disease.