Phosphorylation-driven assembly of the RIP1-RIP3 complex regulates programmed necrosis and virus-induced inflammation

Programmed cell death pathways in cancer: a review of apoptosis, autophagy and programmed necrosis

Programmed cell death (PCD), referring to apoptosis, autophagy and programmed necrosis, is proposed to be death of a cell in any pathological format, when mediated by an intracellular program. These three forms of PCD may jointly decide the fate of cells of malignant neoplasms; apoptosis and programmed necrosis invariably contribute to cell death, whereas autophagy can play either pro-survival or pro-death roles. Recent bulk of accumulating evidence has contributed to a wealth of knowledge facilitating better understanding of cancer initiation and progression with the three distinctive types of cell death. To be able to decipher PCD signalling pathways may aid development of new targeted anti-cancer therapeutic strategies. Thus in this review, we present a brief outline of apoptosis, autophagy and programmed necrosis pathways and apoptosis-related microRNA regulation, in cancer. Taken together, understanding PCD and the complex interplay between apoptosis, autophagy and programmed necrosis may ultimately allow scientists and clinicians to harness the three types of PCD for discovery of further novel drug targets, in the future cancer treatment.

Prevention of cellular suicide by cytomegaloviruses

As intracellular parasites, viruses rely on many host cell functions to ensure their replication. The early induction of programmed cell death (PCD) in infected cells constitutes an effective antiviral host mechanism to restrict viral spread within an organism. As a countermeasure, viruses have evolved numerous strategies to interfere with the induction or execution of PCD. Slowly replicating viruses such as the cytomegaloviruses (CMVs) are particularly dependent on sustained cell viability. To preserve viability, the CMVs encode several viral cell death inhibitors that target different key regulators of the extrinsic and intrinsic apoptosis pathways. The best-characterized CMV-encoded inhibitors are the viral inhibitor of caspase-8-induced apoptosis (vICA), viral mitochondrial inhibitor of apoptosis (vMIA), and viral inhibitor of Bak oligomerization (vIBO). Moreover, a viral inhibitor of RIP-mediated signaling (vIRS) that blocks programmed necrosis has been identified in the genome of murine CMV (MCMV), indicating that this cell death mode is a particularly important part of the antiviral host response. This review provides an overview of the known cell death suppressors encoded by CMVs and their mechanisms of action.

Apoptotic and nonapoptotic caspase functions in animal development

A developing animal is exposed to both intrinsic and extrinsic stresses. One stress response is caspase activation. Caspase activation not only controls apoptosis but also proliferation, differentiation, cell shape, and cell migration. Caspase activation drives development by executing cell death or nonapoptotic functions in a cell-autonomous manner, and by secreting signaling molecules or generating mechanical forces, in a noncell autonomous manner.

Palmitate induces RIP1-dependent necrosis in RAW 264.7 cells

OBJECTIVE

The kinase receptor-interacting protein (RIP) 1, a serine/threonine protein kinase, is a key signaling molecule for necrosis. The possible involvement of RIP1 in palmitate-induced macrophage death and its underlying molecular mechanism was investigated in this study.

METHODS

Cell viability was measured by an MTT reduction assay. The type of cell death was determined by staining with annexin V, propidium iodide (PI) and the APOPercentage dye, and by examining cell morphology using transmission electron microscopy. The down-regulation of RIP1 was performed by siRNA transfection. Intracellular reactive oxygen species (ROS) were measured by staining with H(2)DCF-DA.

RESULTS

Palmitate largely induced necrosis in RAW 264.7 cells, whereas C2-ceramide induced apoptosis. Palmitate-induced necrosis was inhibited by Necrostatin-1, an inhibitor of RIP1, and by RIP1 siRNA transfection, whereas ordinary cell death was not inhibited by z-VAD-fmk. In addition, the presence of palmitate caused a significant increase in intracellular ROS levels compared to control cells. Pre-treatment with Tempol, a cell permeable ROS scavenger, and MnTBAP, an inhibitor of mitochondrial oxidative stress, protected cells from palmitate-induced cell death. Furthermore, the down-regulation of RIP1 by siRNA transfection significantly decreased palmitate-induced ROS generation compared to control cells.

CONCLUSION

The findings reported herein indicate that palmitate induces necrotic cell death via RIP1-dependent ROS generation in RAW 264.7 cells. These findings may provide a new mechanism that explains the link between elevated levels of free fatty acids (FFAs) and macrophage death.

New insights into the mechanisms for photodynamic therapy-induced cancer cell death

Photodynamic therapy (PDT) is a promising therapeutic modality for cancer treatment; however, a more detailed understanding is needed to improve the clinical use of this therapy. PDT induces cancer cell death by apoptosis, necrosis, and autophagy, and these mechanisms can be concurrently occurred. PDT destroys cancer cells by inducing apoptosis through diverse signaling pathways coupled with Bcl-2 family members, caspases, and apopotosis-inducing factor. When the apoptotic pathway is unavailable, PDT can cause cancer cell death through induction of a necrotic or autophagic mechanism. Autophagy is occurred in a Bax-independent manner and can be stimulated in parallel with apoptosis. PDT directly destroys cancer cells by inducing either apoptotic or necrotic death. PDT also can induce autophagy as a death or a survival mechanism. These mechanisms are dependent on a variety of parameters including the nature of the photosensitizer, PDT dose, and cell genotype. Understanding the complex cross talk between these pathways may improve the effectiveness of PDT. Here, we discuss the interplay between these mechanisms based on recent evidence and suggest prospects with regard to advances in PDT.

An emerging role for LRRK2 in the immune system

Missense mutations in LRRK2 (leucine-rich repeat kinase 2) contribute significantly to autosomal dominant PD (Parkinson's disease). Genome-wide association studies have suggested further that mutations in LRRK2 comprise a risk factor for sporadic PD. How LRRK2 contributes to PD, however, is largely unknown. Recent work has shown that LRRK2 is highly expressed in tissue and circulating immune cells and is suggestive of a potential role for LRRK2 in innate immunity. These studies and their potential implications for PD are discussed in the present paper.

Investigation of receptor interacting protein (RIP3)-dependent protein phosphorylation by quantitative phosphoproteomics

Receptor interacting protein 3 (RIP3) is a protein kinase that plays a key role in programmed necrosis. Despite the importance of RIP3-dependent necrosis in many pathological processes, current knowledge on the function of RIP3 is very limited. Here we present the results of a proteome-wide analysis of RIP3-regulated phosphorylation sites using cells from wildtype (RIP3(+/+)) and RIP3 knockout (RIP3(-/-)) mice. Because the activation of RIP3 requires stimulation by certain extracellular stimuli such as ligands of death receptors or Toll-like receptors, we compared the phosphorylation sites of lipopolysaccharide (LPS)-treated peritoneal macrophages from RIP3(+/+) and RIP3(-/-) mice and the phosphorylation sites of tumor necrosis factor (TNF)-treated RIP3(+/+) and RIP3(-/-) mouse embryonic fibroblast (MEF) cells. Stable isotope labeling by amino acids in cell culture and spike-in stable isotope labeling by amino acids in cell culture were used in the analyses of the MEFs and macrophages, respectively. Proteomic analyses using stable isotope labeling by amino acids in cell culture coupled with immobilized metal affinity chromatography-hydrophilic interaction liquid chromatography fractionation and nanoLC MS/MS identified 14,057 phosphopeptides in 4306 proteins from the macrophages and 4732 phosphopeptides in 1785 proteins from the MEFs. Analysis of amino acid sequence motifs among the phosphopeptides identified a potential motif of RIP3 phosphorylation. Among the phosphopeptides identified, 73 were found exclusively in RIP3(+/+) macrophages, 121 were detected exclusively from RIP3(+/+) MEFs, 286 phosphopeptides were induced more in RIP3(+/+) macrophages than in RIP3(-/-) macrophages and 26 phosphopeptides had higher induction in RIP3(+/+) MEFs than in RIP3(-/-) cells. Many of the RIP3 regulated phosphoproteins from the macrophages and MEF cells are functionally associated with the cell cycle; the rest, however, appear to have diverse functions in that a number of metabolism related proteins were phosphorylated in macrophages and development related phosphoproteins were induced in MEFs. The results of our phosphoproteomic analysis suggest that RIP3 might function beyond necrosis and that cell type specific function of RIP3 exists.

Necrostatin-1 protects photoreceptors from cell death and improves functional outcome after experimental retinal detachment

Necroptosis is a recently discovered programmed necrosis. Evidence demonstrated the importance of necroptosis in neuronal cell death. Necrostatin-1 is a specific inhibitor of necroptosis. In this study, we investigated the role of necrostatin-1 on photoreceptor survival and functional protection after experimental retinal detachment (RD) in rats. Necrostatin-1/inactive analogue of necrostatin-1 was introduced into the subretinal space at RD induction and 6 hours afterward, respectively. We found that necrostatin-1 attenuated retinal histopathological damage and reduced plasma membrane breakdown (a morphological hallmark of necroptosis) in outer retinal layers. Transmission electron microscopy showed that necrostatin-1 directly protected neurons by inhibiting necroptotic, not apoptotic, cell death. Treatment with necrostatin-1 inhibited the induction of receptor-interacting protein kinase phosphorylation after RD (a biomarker of necroptosis). Finally, electroretinographic recording proved that necrostatin-1 contributed to objective functional improvement after RD. These findings indicate that necrostatin-1 is a promising therapeutic agent that protects photoreceptors from necroptosis and improves functional outcome.

Type I interferon induces necroptosis in macrophages during infection with Salmonella enterica serovar Typhimurium

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a virulent pathogen that induces rapid host death. Here we observed that host survival after infection with S. Typhimurium was enhanced in the absence of type I interferon signaling, with improved survival of mice deficient in the receptor for type I interferons (Ifnar1(-/-) mice) that was attributed to macrophages. Although there was no impairment in cytokine expression or inflammasome activation in Ifnar1(-/-) macrophages, they were highly resistant to S. Typhimurium-induced cell death. Specific inhibition of the kinase RIP1 or knockdown of the gene encoding the kinase RIP3 prevented the death of wild-type macrophages, which indicated that necroptosis was a mechanism of cell death. Finally, RIP3-deficient macrophages, which cannot undergo necroptosis, had similarly less death and enhanced control of S. Typhimurium in vivo. Thus, we propose that S. Typhimurium induces the production of type I interferon, which drives necroptosis of macrophages and allows them to evade the immune response.

Receptor interacting protein kinase mediates necrotic cone but not rod cell death in a mouse model of inherited degeneration

Retinitis pigmentosa comprises a group of inherited retinal photoreceptor degenerations that lead to progressive loss of vision. Although in most cases rods, but not cones, harbor the deleterious gene mutations, cones do die in this disease, usually after the main phase of rod cell loss. Rod photoreceptor death is characterized by apoptotic features. In contrast, the mechanisms and features of subsequent nonautonomous cone cell death remain largely unknown. In this study, we show that receptor-interacting protein (RIP) kinase mediates necrotic cone cell death in rd10 mice, a mouse model of retinitis pigmentosa caused by a mutation in a rod-specific gene. The expression of RIP3, a key regulator of programmed necrosis, was elevated in rd10 mouse retinas in the phase of cone but not rod degeneration. Although rd10 mice lacking Rip3 developed comparable rod degeneration to control rd10 mice, they displayed a significant preservation of cone cells. Ultrastructural analysis of rd10 mouse retinas revealed that a substantial fraction of dying cones exhibited necrotic morphology, which was rescued by Rip3 deficiency. Additionally, pharmacologic treatment with a RIP kinase inhibitor attenuated histological and functional deficits of cones in rd10 mice. Thus, necrotic mechanisms involving RIP kinase are crucial in cone cell death in inherited retinal degeneration, suggesting the RIP kinase pathway as a potential target to protect cone-mediated central and peripheral vision loss in patients with retinitis pigementosa.

Formation of the death domain complex between FADD and RIP1 proteins in vitro

Fas-associated death domain (FADD) protein is an adapter molecule that bridges the interactions between membrane death receptors and initiator caspases. The death receptors contain an intracellular death domain (DD) which is essential to the transduction of the apoptotic signal. The kinase receptor-interacting protein 1 (RIP1) is crucial to programmed necrosis. The cell type interplay between FADD and RIP1, which mediates both necrosis and NF-κB activation, has been evaluated in other studies, but the mechanism of the interaction of the FADD and RIP1 proteins remain poorly understood. Here, we provided evidence indicating that the DD of human FADD binds to the DD of RIP1 in vitro. We developed a molecular docking model using homology modeling based on the structures of FADD and RIP1. In addition, we found that two structure-based mutants (G109A and R114A) of the FADD DD were able to bind to the RIP1 DD, and two mutations (Q169A and N171A) of FADD DD and four mutations (G595, K596, E620, and D622) of RIP1 DD disrupted the FADD-RIP1 interaction. Six mutations (Q169A, N171A, G595, K596, E620, and D622) lowered the stability of the FADD-RIP1 complex and induced aggregation that structurally destabilized the complex, thus disrupting the interaction.

RIP3 expression induces a death profile change in U2OS osteosarcoma cells after 5-ALA-PDT

BACKGROUND AND OBJECTIVE

The receptor-interacting protein 3 (RIP3) has recently been outlined as a key necrosis mediator but is also thought to participate in the regulation of apoptosis. The aim of this study is to compare the cell death profile induced by 5-aminolevulic acid (5-ALA)-mediated photodynamic therapy (PDT) in the RIP3-deficient cell line U2OS and in U2OS cells in which the expression of RIP3 was restored.

MATERIALS AND METHODS

RIP3-expressing U2OS cells (RIP3-U2OS) were obtained after transfection and antibiotic selection. Wild type and RIP3-U2OS cells were treated by 5-ALA-PDT. Overall cell viability was evaluated and different parameters characteristic of apoptosis, autophagy, and necrosis were studied.

RESULTS

Surprisingly, the survival of RIP3-U2OS cells was higher compared to that of the wild type cells. In addition, RIP3-U2OS cell death was decreased by a zVAD-fmk pre-treatment. A higher cleavage of caspase-3, 7, 8, 9, and PARP was also detected in these cells, pointing out to the activation of caspase-dependent apoptosis. In parallel, a thrust of autophagy was clearly identified in the RIP3-U2OS cells. Conversely, RIP3-U2OS exhibited a lower level of necrosis than the wild types. Interestingly, necrostatin-1 efficiently decreased necrosis level in RIP3-U2OS but not in wild type cells.

CONCLUSION

Expression of RIP3 in U2OS cells led to a better survival but also to a death profile change in response to PDT. The apoptotic and autophagic pathways were clearly up-regulated compared to the RIP3-deficient wild type cells. However, induction of necrosis was weaker in the RIP3-U2OS cells. In this context, autophagy is likely to play a protective role against PDT-induced cell death and to allow a better survival of RIP3-U2OS cells. This work also highlights the important role played by RIP3 in the apoptotic pathway, although the modalities are still widely unknown.

Autophagy Contributes to the Death/Survival Balance in Cancer PhotoDynamic Therapy

Autophagy is an important cellular program with a “double face” role, since it promotes either cell survival or cell death, also in cancer therapies. Its survival role occurs by recycling cell components during starvation or removing stressed organelles; when damage becomes extensive, autophagy provides another programmed cell death pathway, known as Autophagic Cell Death (ACD). The induction of autophagy is a common outcome in PhotoDynamic Therapy (PDT), a two-step process involving the irradiation of photosensitizer (PS)-loaded cancer cells. Upon tissue oxygen interaction, PS provokes immediate and direct Reactive Oxygen Species (ROS)-induced damage to Endoplasmic Reticulum (ER), mitochondria, plasma membrane, and/or lysosomes. The main biological effects carried out in cancer PDT are direct cytotoxicity to tumor cells, vasculature damage and induction of inflammatory reactions stimulating immunological responses. The question about the role of autophagy in PDT and its putative immunological impact is hotly controversial and largely studied in recent times. This review deals with the induction of autophagy in PDT protocols and its dual role, also considering its interrelationship with apoptosis, the preferential cell death program triggered in the photodynamic process.

A liver full of JNK: signaling in regulation of cell function and disease pathogenesis, and clinical approaches

c-Jun-N-terminal kinase (JNK) is a mitogen-activated protein kinase family member that is activated by diverse stimuli, including cytokines (such as tumor necrosis factor and interleukin-1), reactive oxygen species (ROS), pathogens, toxins, drugs, endoplasmic reticulum stress, free fatty acids, and metabolic changes. Upon activation, JNK induces multiple biologic events through the transcription factor activator protein-1 and transcription-independent control of effector molecules. JNK isozymes regulate cell death and survival, differentiation, proliferation, ROS accumulation, metabolism, insulin signaling, and carcinogenesis in the liver. The biologic functions of JNK are isoform, cell type, and context dependent. Recent studies using genetically engineered mice showed that loss or hyperactivation of the JNK pathway contributes to the development of inflammation, fibrosis, cancer growth, and metabolic diseases that include obesity, hepatic steatosis, and insulin resistance. We review the functions and pathways of JNK in liver physiology and pathology and discuss findings from preclinical studies with JNK inhibitors.

Estrogen receptor beta (ERβ) produces autophagy and necroptosis in human seminoma cell line through the binding of the Sp1 on the phosphatase and tensin homolog deleted from chromosome 10 (PTEN) promoter gene

Testicular germ cell tumors are the most common tumor in male and the least studied. We focused on human seminoma using the TCAM2 cell line. Through ERβ, 10 nM estradiol (E2) was able to induce PTEN gene expression and promoter transactivation. Transient transfections, ChIP and EMSA assays evidenced the 5'-flanking region of PTEN gene promoter E2-responsive. The ERβ binding to the Sp1 on PTEN promoter decreased cell survival. The presence of ERβ or PTEN is necessary to induce the loss of cell survival upon E2, addressing their cooperation in this action. pAKT and AKT expression decreased under E2 and DPN, while known apoptotic markers appeared to be unchanged. The PI3K/AKT pathway inhibition also leads to autophagy: E2 and DPN enhanced the expression of autophagy-related markers such as PI3III, Beclin 1, AMBRA and UVRAG. MDC and TEM assays confirmed E2-induced autophagy. The absence of DNA fragmentation, caspase 9 and PARP1 cleavages suggested that necroptosis and/or parthanatos may occur. FACS analysis, LDH assay and RIP1 expression attested this hypothesis. Our study reveals a unique mechanism through which ERβ/PTEN signaling induces cell death in TCAM2 by autophagy and necroptosis. These data, supporting estrogen-dependency of human seminoma, propose ERβ ligands for therapeutic use in the treatment of this pathological condition.

Staurosporine induces necroptotic cell death under caspase-compromised conditions in U937 cells

For a long time necrosis was thought to be an uncontrolled process but evidences recently have revealed that necrosis can also occur in a regulated manner. Necroptosis, a type of programmed necrosis is defined as a death receptor-initiated process under caspase-compromised conditions. The process requires the kinase activity of receptor-interacting protein kinase 1 and 3 (RIPK1 and RIPK3) and mixed lineage kinase domain-like protein (MLKL), as a substrate of RIPK3. The further downstream events remain elusive. We applied known inhibitors to characterize the contributing enzymes in necroptosis and their effect on cell viability and different cellular functions were detected mainly by flow cytometry. Here we report that staurosporine, the classical inducer of intrinsic apoptotic pathway can induce necroptosis under caspase-compromised conditions in U937 cell line. This process could be hampered at least partially by the RIPK1 inhibitor necrotstin-1 and by the heat shock protein 90 kDa inhibitor geldanamycin. Moreover both the staurosporine-triggered and the classical death ligand-induced necroptotic pathway can be effectively arrested by a lysosomal enzyme inhibitor CA-074-OMe and the recently discovered MLKL inhibitor necrosulfonamide. We also confirmed that the enzymatic role of poly(ADP-ribose)polymerase (PARP) is dispensable in necroptosis but it contributes to membrane disruption in secondary necrosis. In conclusion, we identified a novel way of necroptosis induction that can facilitate our understanding of the molecular mechanisms of necroptosis. Our results shed light on alternative application of staurosporine, as a possible anticancer therapeutic agent. Furthermore, we showed that the CA-074-OMe has a target in the signaling pathway leading to necroptosis. Finally, we could differentiate necroptotic and secondary necrotic processes based on participation of PARP enzyme.

NEMO inhibits programmed necrosis in an NFκB-independent manner by restraining RIP1

TNF can trigger two opposing responses: cell survival and cell death. TNFR1 activates caspases that orchestrate apoptosis but some cell types switch to a necrotic death when treated with caspase inhibitors. Several genes that are required to orchestrate cell death by programmed necrosis have been identified, such as the kinase RIP1, but very little is known about the inhibitory signals that keep this necrotic cell death pathway in check. We demonstrate that T cells lacking the regulatory subunit of IKK, NFκB essential modifier (NEMO), are hypersensitive to programmed necrosis when stimulated with TNF in the presence of caspase inhibitors. Surprisingly, this pro-survival activity of NEMO is independent of NFκB-mediated gene transcription. Instead, NEMO inhibits necrosis by binding to ubiquitinated RIP1 to restrain RIP1 from engaging the necrotic death pathway. In the absence of NEMO, or if ubiquitination of RIP1 is blocked, necrosis ensues when caspases are blocked. These results indicate that recruitment of NEMO to ubiquitinated RIP1 is a key step in the TNFR1 signaling pathway that determines whether RIP1 triggers a necrotic death response.

Autophagy suppresses RIP kinase-dependent necrosis enabling survival to mTOR inhibition

mTOR inhibitors are used clinically to treat renal cancer but are not curative. Here we show that autophagy is a resistance mechanism of human renal cell carcinoma (RCC) cell lines to mTOR inhibitors. RCC cell lines have high basal autophagy that is required for survival to mTOR inhibition. In RCC4 cells, inhibition of mTOR with CCI-779 stimulates autophagy and eliminates RIP kinases (RIPKs) and this is blocked by autophagy inhibition, which induces RIPK- and ROS-dependent necroptosis in vitro and suppresses xenograft growth. Autophagy of mitochondria is required for cell survival since mTOR inhibition turns off Nrf2 antioxidant defense. Thus, coordinate mTOR and autophagy inhibition leads to an imbalance between ROS production and defense, causing necroptosis that may enhance cancer treatment efficacy.

Evidence for multiple cell death pathways during development of experimental cytomegalovirus retinitis in mice with retrovirus-induced immunosuppression: apoptosis, necroptosis, and pyroptosis

AIDS-related human cytomegalovirus (HCMV) retinitis remains a major ophthalmologic problem worldwide. Although this sight-threatening disease is well characterized clinically, many pathogenic issues remain unresolved, among them a basic understanding of the relative roles of cell death pathways during development of retinal tissue destruction. Using an established model of experimental murine cytomegalovirus (MCMV) retinitis in mice with retrovirus-induced immunosuppression (MAIDS), we initially investigated MCMV-infected eyes for evidence of apoptosis-associated molecules in mice with MAIDS of 4 weeks' (MAIDS-4) and 10 weeks' (MAIDS-10) duration, which were resistant and susceptible to retinal disease, respectively, but which harbored equivalent amounts of infectious MCMV. Whereas MCMV-infected eyes of MAIDS-4 mice showed little evidence of apoptosis-associated molecules, MCMV-infected eyes of MAIDS-10 mice showed significant amounts of tumor necrosis factor alpha (TNF-α), TNF receptors 1 and 2, active caspase 8, active caspase 3, TNF-related apoptosis-inducing ligand (TRAIL), TRAIL-R(DR5), Fas, and Fas ligand mRNAs and/or proteins, all detected at peak amounts prior to development of most severe retinal disease. Immunohistochemical staining showed macrophages, granulocytes (neutrophils), Müller cells, and microglial cells as TNF-α sources. Remarkably, quantification of apoptosis by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) assay suggested that apoptosis contributed minimally to retinal disease in MCMV-infected eyes of MAIDS-10 mice. Subsequent studies demonstrated that MCMV-infected eyes of MAIDS-10 mice, but not MAIDS-4 mice, showed evidence of significant increases in molecules associated with two additional cell death pathways, necroptosis (receptor-interacting protein 1 [RIP1] and RIP3 mRNAs) and pyroptosis (caspase 1, interleukin 1β [IL-1β], and IL-18 mRNAs). We conclude that apoptosis, necroptosis, and pyroptosis participate simultaneously during MAIDS-related MCMV retinitis, and all may play a role during AIDS-related HCMV retinitis.

TRAIL induces necroptosis involving RIPK1/RIPK3-dependent PARP-1 activation

Although TRAIL (tumor necrosis factor (TNF)-related apoptosis inducing ligand) is a well-known apoptosis inducer, we have previously demonstrated that acidic extracellular pH (pHe) switches TRAIL-induced apoptosis to regulated necrosis (or necroptosis) in human HT29 colon and HepG2 liver cancer cells. Here, we investigated the role of RIPK1 (receptor interacting protein kinase 1), RIPK3 and PARP-1 (poly (ADP-ribose) polymerase-1) in TRAIL-induced necroptosis in vitro and in concanavalin A (Con A)-induced murine hepatitis. Pretreatment of HT29 or HepG2 with pharmacological inhibitors of RIPK1 or PARP-1 (Nec-1 or PJ-34, respectively), or transient transfection with siRNAs against RIPK1 or RIPK3, inhibited both TRAIL-induced necroptosis and PARP-1-dependent intracellular ATP depletion demonstrating that RIPK1 and RIPK3 were involved upstream of PARP-1 activation and ATP depletion. In the mouse model of Con A-induced hepatitis, where death of mouse hepatocytes is dependent on TRAIL and NKT (Natural Killer T) cells, PARP-1 activity was positively correlated with liver injury and hepatitis was prevented both by Nec-1 or PJ-34. These data provide new insights into TRAIL-induced necroptosis with PARP-1 being active effector downstream of RIPK1/RIPK3 initiators and suggest that pharmacological inhibitors of RIPKs and PARP-1 could be new treatment options for immune-mediated hepatitis.

The RIP1/RIP3 necrosome forms a functional amyloid signaling complex required for programmed necrosis

RIP1 and RIP3 kinases are central players in TNF-induced programmed necrosis. Here, we report that the RIP homotypic interaction motifs (RHIMs) of RIP1 and RIP3 mediate the assembly of heterodimeric filamentous structures. The fibrils exhibit classical characteristics of β-amyloids, as shown by Thioflavin T (ThT) and Congo red (CR) binding, circular dichroism, infrared spectroscopy, X-ray diffraction, and solid-state NMR. Structured amyloid cores are mapped in RIP1 and RIP3 that are flanked by regions of mobility. The endogenous RIP1/RIP3 complex isolated from necrotic cells binds ThT, is ultrastable, and has a fibrillar core structure, whereas necrosis is partially inhibited by ThT, CR, and another amyloid dye, HBX. Mutations in the RHIMs of RIP1 and RIP3 that are defective in the interaction compromise cluster formation, kinase activation, and programmed necrosis in vivo. The current study provides insight into the structural changes that occur when RIP kinases are triggered to execute different signaling outcomes and expands the realm of amyloids to complex formation and signaling.

Ubiquitination in signaling to and activation of IKK

A role for polyubiquitination in the activation of inhibitor of NF-κB (IκB) kinase (IKK) through a proteasome-independent mechanism was first reported in 1996, but the physiological significance of this finding was not clear until 2000 when TRAF6 was found to be a ubiquitin E3 ligase that catalyzes lysine-63 (K63) polyubiquitination. Since then, several proteins known to regulate IKK have been linked to the ubiquitin pathway. These include the deubiquitination enzymes CYLD and A20 that inhibit IKK, and the ubiquitin binding proteins NEMO and TAB2 which are the regulatory subunits of IKK and TAK1 kinase complexes, respectively. Now accumulating evidence strongly supports a central role of K63 polyubiquitination in IKK activation by multiple immune and inflammatory pathways. Interestingly, recent research suggests that some alternative ubiquitin chains such as linear or K11 ubiquitin chains may also play a role in certain pathways such as the TNF pathway. Here I present a historical narrative of the discovery of the role of ubiquitin in IKK activation, review recent advances in understanding the role and mechanism of ubiquitin-mediated IKK activation, and raise some questions to be resolved in future research.

The phosphorylation-dependent regulation of mitochondrial proteins in stress responses

To maintain cellular homeostasis, cells are equipped with precise systems that trigger the appropriate stress responses. Mitochondria not only provide cellular energy but also integrate stress response signaling pathways, including those regulating cell death. Several lines of evidence suggest that the mitochondrial proteins that function in this process, such as Bcl-2 family proteins in apoptosis and phosphoglycerate mutase family member 5 (PGAM5) in necroptosis, are regulated by several kinases. It has also been suggested that the phosphorylation-dependent regulation of mitochondrial fission machinery, dynamin-related protein 1 (Drp1), facilitates appropriate cellular stress responses. However, mitochondria themselves are also damaged by various stresses. To avoid the deleterious effects exerted by damaged mitochondria, cells remove these mitochondria in a selective autophagic degradation process called mitophagy. Interestingly, several kinases, such as PTEN-induced putative kinase 1 (PINK1) in mammals and stress-responsive mitogen-activated protein (MAP) kinases in yeast, have recently been shown to be involved in mitophagy. In this paper, we focus on the phosphorylation-dependent regulation of mitochondrial proteins and discuss the roles of this regulation in the mitochondrial and cellular stress responses.

DAI/ZBP1/DLM-1 complexes with RIP3 to mediate virus-induced programmed necrosis that is targeted by murine cytomegalovirus vIRA

Programmed necrosis, like apoptosis, eliminates pathogen-infected cells as a component of host defense. Receptor-interacting protein kinase (RIP) 3 (also called RIPK3) mediates RIP homotypic interaction motif (RHIM)-dependent programmed necrosis induced by murine cytomegalovirus (MCMV) infection or death receptor activation and suppressed by the MCMV-encoded viral inhibitor of RIP activation (vIRA). We find that interferon-independent expression of DNA-dependent activator of interferon regulatory factors (DAI, also known as ZBP1 or DLM-1) sensitizes cells to virus-induced necrosis and that DAI knockdown or knockout cells are resistant to this death pathway. Importantly, as with RIP3(-/-) mice, vIRA mutant MCMV pathogenesis is restored in DAI(-/-) mice, consistent with a DAI-RIP3 complex being the natural target of vIRA. Thus, DAI interacts with RIP3 to mediate virus-induced necrosis analogous to the RIP1-RIP3 complex controlling death receptor-induced necroptosis. These studies unveil a role for DAI as the RIP3 partner mediating virus-induced necrosis.

Calpain activation by the Shigella flexneri effector VirA regulates key steps in the formation and life of the bacterium's epithelial niche

The enteropathogen Shigella flexneri invades epithelial cells, leading to inflammation and tissue destruction. We report that Shigella infection of epithelial cells induces an early genotoxic stress, but the resulting p53 response and cell death are impaired due to the bacterium's ability to promote p53 degradation, mainly through calpain protease activation. Calpain activation is promoted by the Shigella virulence effector VirA and dependent on calcium flux and the depletion of the endogenous calpain inhibitor calpastatin. Further, although VirA-induced calpain activity is critical for regulating cytoskeletal events driving bacterial uptake, calpain activation ultimately leads to necrotic cell death, thereby restricting Shigella intracellular growth. Therefore, calpains work at multiple steps in regulating Shigella pathogenesis by disrupting the p53-dependent DNA repair response early during infection and regulating both formation and ultimate death of the Shigella epithelial replicative niche.

Tropism and pathogenicity of rickettsiae

Rickettsiae are obligate intracellular parasitic bacteria that cause febrile exanthematous illnesses such as Rocky Mountain spotted fever, Mediterranean spotted fever, epidemic, and murine typhus, etc. Although the vector ranges of each Rickettsia species are rather restricted; i.e., ticks belonging to Arachnida and lice and fleas belonging to Insecta usually act as vectors for spotted fever group (SFG) and typhus group (TG) rickettsiae, respectively, it would be interesting to elucidate the mechanisms controlling the vector tropism of rickettsiae. This review discusses the factors determining the vector tropism of rickettsiae. In brief, the vector tropism of rickettsiae species is basically consistent with their tropism toward cultured tick and insect cells. The mechanisms responsible for rickettsiae pathogenicity are also described. Recently, genomic analyses of rickettsiae have revealed that they possess several genes that are homologous to those affecting the pathogenicity of other bacteria. Analyses comparing the genomes of pathogenic and non-pathogenic strains of rickettsiae have detected many factors that are related to rickettsial pathogenicity. It is also known that a reduction in the rickettsial genome has occurred during the course of its evolution. Interestingly, Rickettsia species with small genomes, such as Rickettsia prowazekii, are more pathogenic to humans than those with larger genomes. This review also examines the growth kinetics of pathogenic and non-pathogenic species of SFG rickettsiae (SFGR) in mammalian cells. The growth of non-pathogenic species is restricted in these cells, which is mediated, at least in part, by autophagy. The superinfection of non-pathogenic rickettsiae-infected cells with pathogenic rickettsiae results in an elevated yield of the non-pathogenic rickettsiae and the growth of the pathogenic rickettsiae. Autophagy is restricted in these cells. These results are discussed in this review.

Fas ligand-Fas signaling participates in light-induced apoptotic death in photoreceptor cells

PURPOSE

We investigated the function of Fas in photoreceptors.

METHODS

Postmortem human eyes and mouse-derived photoreceptor cells (661W) were examined for Fas expression by in situ hybridization and immunofluorescence. 661W cells were treated with FasL or Fas agonistic antibody, or exposed to light with/without pharmacological manipulation of Fas signaling, followed by apoptosis detection by TUNEL, immunofluorescence and fluorescence activated cell scanning (FACS). Fractionated cellular extracts were used to detect protein expression or protein phosphorylation after immunoprecipitation by Western blot.

RESULTS

Fas was expressed in the photoreceptor layer of human retina. Fas and a cleaved form of FasL were found on the cell surface of 661W cells. Treatment with FasL or Fas agonistic antibody induced apoptosis in 661W cells. Blocking the activity of FasL or administration of caspase-8 inhibitor z-IETD inhibited light-induced apoptosis. However, it simultaneously caused induction of necroptosis, which could be blocked by the receptor-interacting protein 1 (RIP1) inhibitor, necrostatin-1. Light exposure in the presence of z-IETD caused hyper-phosphorylation of RIP1. Light exposure did not elevate the expression of Fas, FasL, or the Fas-associated death domain adaptor protein (FADD). Cells or conditioned medium after light exposure induced apoptosis in dark-adapted cells, which could be attenuated by blockade of Fas.

CONCLUSIONS

Fas has a pro-apoptotic role in photoreceptors. Under light stress, soluble and membrane-bound FasL can bind to Fas, inducing apoptosis via a paracrine mechanism. Although blocking Fas signaling inhibits apoptosis, it does not improve the overall photoreceptor survival due to a compensatory activation of necroptosis. Hence, prevention of photoreceptor loss from retinal photo-oxidative stress should target Fas and RIP1.

Inhibition of autophagy rescues palmitic acid-induced necroptosis of endothelial cells

Accumulation of palmitic acid (PA) in cells from nonadipose tissues is known to induce lipotoxicity resulting in cellular dysfunction and death. The exact molecular pathways of PA-induced cell death are still mysterious. Here, we show that PA triggers autophagy, which did not counteract but in contrast promoted endothelial cell death. The PA-induced cell death was predominantly necrotic as indicated by annexin V and propidium iodide (PI) staining, absence of caspase activity, low levels of DNA hypoploidy, and an early ATP depletion. In addition PA induced a strong elevation of mRNA levels of ubiquitin carboxyl-terminal hydrolase (CYLD), a known mediator of necroptosis. Moreover, siRNA-mediated knockdown of CYLD significantly antagonized PA-induced necrosis of endothelial cells. In contrast, inhibition and knockdown of receptor interacting protein kinase 1 (RIPK1) had no effect on PA-induced necrosis, indicating the induction of a CYLD-dependent but RIPK1-independent cell death pathway. PA was recognized as a strong and early inducer of autophagy. The inhibition of autophagy by both pharmacological inhibitors and genetic knockdown of the autophagy-specific genes, vacuolar protein sorting 34 (VPS34), and autophagy-related protein 7 (ATG7), could rescue the PA-induced death of endothelial cells. Moreover, the initiation of autophagy and cell death by PA was reduced in endothelial cells loaded with the Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-(acetoxymethyl) ester (BAPTA-AM), indicating that Ca(2+) triggers the fatal signaling of PA. In summary, we introduce an unexpected mechanism of lipotoxicity in endothelial cells and provide several novel strategies to counteract the lipotoxic signaling of PA.

A novel role for RIP1 kinase in mediating TNFα production

Receptor-interacting protein 1 (RIP1) is a Ser/Thr kinase with both kinase-dependent and kinase-independent roles in death receptor signaling. The kinase activity of RIP1 is required for necroptosis, a caspase-independent pathway of programmed cell death. In some cell types, the inhibition of caspases leads to autocrine production of TNFα, which then activates necroptosis. Here, we describe a novel role for RIP1 kinase in regulating TNFα production after caspase inhibition. Caspase inhibitors activate RIP1 kinase and another protein, EDD, to mediate JNK signaling, which stimulates Sp1-dependent transcription of TNFα. This pathway is independent of nuclear factor κB and also occurs after Smac mimetic/IAP antagonist treatment or the loss of TNF receptor-associated factor 2 (Traf2). These findings implicate cIAP1/2 and Traf2 as negative regulators of this RIP1 kinase-dependent TNFα production pathway and suggest a novel role for RIP1 kinase in mediating TNFα production under certain conditions.

Nec-1 enhances shikonin-induced apoptosis in leukemia cells by inhibition of RIP-1 and ERK1/2

Necrostatin-1 (Nec-1) inhibits necroptosis by allosterically inhibiting the kinase activity of receptor-interacting protein 1 (RIP1), which plays a critical role in necroptosis. RIP1 is a crucial adaptor kinase involved in the activation of NF-κB, production of reactive oxygen species (ROS) and the phosphorylation of mitogen activated protein kinases (MAPKs). NF-κB, ROS and MAPKs all play important roles in apoptotic signaling. Nec-1 was regarded as having no effect on apoptosis. Here, we report that Nec-1 increased the rate of nuclear condensation and caspases activation induced by a low concentration of shikonin (SHK) in HL60, K562 and primary leukemia cells. siRNA-mediated knockdown of RIP1 significantly enhanced shikonin-induced apoptosis in K562 and HL60 cells. Shikonin treatment alone could slightly inhibit the phosphorylation of ERK1/2 in leukemia cells, and the inhibitory effect on ERK1/2 was significantly augmented by Nec-1. We also found that Nec-1 could inhibit NF-κB p65 translocation to the nucleus at a later stage of SHK treatment. In conclusion, we found that Nec-1 can promote shikonin-induced apoptosis in leukemia cells. The mechanism by which Nec-1 sensitizes shikonin-induced apoptosis appears to be the inhibition of RIP1 kinase-dependent phosphorylation of ERK1/2. To our knowledge, this is the first study to document Nec-1 sensitizes cancer cells to apoptosis.

A death effector domain chain DISC model reveals a crucial role for caspase-8 chain assembly in mediating apoptotic cell death

Formation of the death-inducing signaling complex (DISC) is a critical step in death receptor-mediated apoptosis, yet the mechanisms underlying assembly of this key multiprotein complex remain unclear. Using quantitative mass spectrometry, we have delineated the stoichiometry of the native TRAIL DISC. While current models suggest that core DISC components are present at a ratio of 1:1, our data indicate that FADD is substoichiometric relative to TRAIL-Rs or DED-only proteins; strikingly, there is up to 9-fold more caspase-8 than FADD in the DISC. Using structural modeling, we propose an alternative DISC model in which procaspase-8 molecules interact sequentially, via their DED domains, to form a caspase-activating chain. Mutating key interacting residues in procaspase-8 DED2 abrogates DED chain formation in cells and disrupts TRAIL/CD95 DISC-mediated procaspase-8 activation in a functional DISC reconstitution model. This provides direct experimental evidence for a DISC model in which DED chain assembly drives caspase-8 dimerization/activation, thereby triggering cell death.

Fluorescence polarization assay for inhibitors of the kinase domain of receptor interacting protein 1

Necrotic cell death is prevalent in many different pathological disease states and in traumatic injury. Necroptosis is a form of necrosis that stems from specific signaling pathways, with the key regulator being receptor interacting protein 1 (RIP1), a serine/threonine kinase. Specific inhibitors of RIP1, termed necrostatins, are potent inhibitors of necroptosis. Necrostatins are structurally distinct from one another yet still possess the ability to inhibit RIP1 kinase activity. To further understand the differences in the binding of the various necrostatins to RIP1 and to develop a robust high-throughput screening (HTS) assay, which can be used to identify new classes of RIP1 inhibitors, we synthesized fluorescein derivatives of Necrostatin-1 (Nec-1) and Nec-3. These compounds were used to establish a fluorescence polarization (FP) assay to directly measure the binding of necrostatins to RIP1 kinase. The fluorescein-labeled compounds are well suited for HTS because the assays have a dimethyl sulfoxide (DMSO) tolerance up to 5% and Z' scores of 0.62 (fluorescein-Nec-1) and 0.57 (fluorescein-Nec-3). In addition, results obtained from the FP assays and ligand docking studies provide insights into the putative binding sites of Nec-1, Nec-3, and Nec-4.

Novel Ser/Thr protein phosphatases in cell death regulation

Cell death is regulated by a myriad of intracellular molecular pathways, with many involving protein phosphorylation and dephosphorylation. In this review, we will focus on Ser/Thr phosphatases-mediated regulation in cell apoptosis as well as on their potential roles in cell necrosis. The emerging functional importance of Ser/Thr protein phosphatases in cell death regulation adds new dimension to the signaling mechanisms of cellular function, physiology, and diseases.

Programmed Necrosis: A Prominent Mechanism of Cell Death following Neonatal Brain Injury

Despite the introduction of therapeutic hypothermia, neonatal hypoxic ischemic (HI) brain injury remains a common cause of developmental disability. Development of rational adjuvant therapies to hypothermia requires understanding of the pathways of cell death and survival modulated by HI. The conceptualization of the apoptosis-necrosis “continuum” in neonatal brain injury predicts mechanistic interactions between cell death and hydrid forms of cell death such as programmed or regulated necrosis. Many of the components of the signaling pathway regulating programmed necrosis have been studied previously in models of neonatal HI. In some of these investigations, they participate as part of the apoptotic pathways demonstrating clear overlap of programmed death pathways. Receptor interacting protein (RIP)-1 is at the crossroads between types of cellular death and survival and RIP-1 kinase activity triggers formation of the necrosome (in complex with RIP-3) leading to programmed necrosis. Neuroprotection afforded by the blockade of RIP-1 kinase following neonatal HI suggests a role for programmed necrosis in the HI injury to the developing brain. Here, we briefly review the state of the knowledge about the mechanisms behind programmed necrosis in neonatal brain injury recognizing that a significant proportion of these data derive from experiments in cultured cell and some from in vivo adult animal models. There are still more questions than answers, yet the fascinating new perspectives provided by the understanding of programmed necrosis in the developing brain may lay the foundation for new therapies for neonatal HI.

Killing a cancer: what are the alternatives?

Evading programmed cell death is one of the hallmarks of cancer. Conversely, inducing cell death by pharmacological means is the basis of almost every non-invasive cancer therapy. Research over the past decade has greatly increased our understanding of non-apoptotic programmed cell death events, such as lysosomal-mediated cell death, necroptosis and cell death with autophagy. It is becoming clear that an intricate effector network connects many of these classical and non-classical death pathways. In this Review, we discuss converging and diverging features of these pathways, as well as attempts to exploit this newly gained knowledge pharmacologically to provide therapeutics for cancer.

Necrostatin-1 attenuates mitochondrial dysfunction in neurons and astrocytes following neonatal hypoxia-ischemia

Receptor interacting protein (RIP)-1 kinase activity mediates a novel pathway that signals for regulated necrosis, a form of cell death prominent in traumatic and ischemic brain injury. Recently, we showed that an allosteric inhibitor of RIP-1 kinase activity, necrostatin-1 (Nec-1), provides neuroprotection in the forebrain following neonatal hypoxia-ischemia (HI). Because Nec-1 also prevents early oxidative injury, we hypothesized that mechanisms involved in this neuroprotection may involve preservation of mitochondrial function and prevention of secondary energy failure. Therefore, our objective was to determine if Nec-1 treatment following neonatal HI attenuates oxidative stress and mitochondrial injury. Postnatal day (p) 7 mice exposed to HI were injected intracerebroventricularly with 0.1 μL (80 μmol) of Nec-1 or vehicle. Nec-1 treatment prevented nitric oxide (NO•), inducible nitric oxide synthase (iNOS) and 3-nitrotyrosine increase, and attenuated glutathione oxidation that was found in vehicle-treated mice at 3h following HI. Similarly, Nec-1 following HI prevented: (i) up-regulation of hypoxia inducible factor-1 alpha (HIF-1α) and BCL2/adenovirus E1B 19 kDa protein-interacting protein 3 (BNIP3) expression, (ii) decline in mitochondrial complex-I activity, (iii) decrease in ATP levels, and (iv) mitochondrial structural pathology in astrocytes and in neurons. Up-regulation of glial fibrillary acidic protein (GFAP) following HI was also prevented by Nec-1 treatment. No differences by gender were observed. We conclude that Nec-1 immediately after HI, is strongly mitoprotective and prevents secondary energy failure by blocking early NO• accumulation, glutathione oxidation and attenuating mitochondrial dysfunction.

Dichotomy between RIP1- and RIP3-mediated necroptosis in tumor necrosis factor-α-induced shock

Tumor necrosis factor receptor (TNFR) signaling may result in survival, apoptosis or programmed necrosis. The latter is called necroptosis if the receptor-interacting protein 1 (RIP1) inhibitor necrostatin-1 (Nec-1) or genetic knockout of RIP3 prevents it. In the lethal mouse model of TNFα-mediated shock, addition of the pan-caspase inhibitor zVAD-fmk (zVAD) accelerates time to death. Here, we demonstrate that RIP3-deficient mice are protected markedly from TNFα-mediated shock in the presence and absence of caspase inhibition. We further show that the fusion protein TAT-crmA, previously demonstrated to inhibit apoptosis, also prevents necroptosis in L929, HT29 and FADD-deficient Jurkat cells. In contrast to RIP3-deficient mice, blocking necroptosis by Nec-1 or TAT-crmA did not protect from TNFα/zVAD-mediated shock, but further accelerated time to death. Even in the absence of caspase inhibition, Nec-1 application led to similar kinetics. Depletion of macrophages, natural killer (NK) cells, granulocytes or genetic deficiency for T lymphocytes did not influence this model. Because RIP3-deficient mice are known to be protected from cerulein-induced pancreatitis (CIP), we applied Nec-1 and TAT-crmA in this model and demonstrated the deterioration of pancreatic damage upon addition of these substances. These data highlight the importance of separating genetic RIP3 deficiency from RIP1 inhibition by Nec-1 application in vivo and challenge the current definition of necroptosis.

Inhibition of RIP1-dependent necrosis prevents adverse cardiac remodeling after myocardial ischemia-reperfusion in vivo

Accumulating evidence indicates that programmed necrosis plays a critical role in cell death during ischemia-reperfusion. Necrostatin-1 (Nec-1), a small molecule capable of inhibiting a key regulator of programmed necrosis (RIP1), was shown to prevent necrotic cell death in experimental models including cardiac ischemia. However, no functional follow-up was performed and the action of Nec-1 remains unclear. Here, we studied whether Nec-1 inhibits RIP1-dependent necrosis and leads to long-term improvements after ischemia-reperfusion in vivo. Mice underwent 30 min of ischemia and received, 5 min before reperfusion, 3.3 mg/kg Nec-1 or vehicle treatment, followed by reperfusion. Nec-1 administration reduced infarct size to 26.3 ± 1.3% (P = 0.001) compared to 38.6 ± 1.7% in vehicle-treated animals. Furthermore, Nec-1 inhibited RIP1/RIP3 phosphorylation in vivo and significantly reduced necrotic cell death, while apoptotic cell death remained constant. By using MRI, cardiac dimensions and function were assessed before and 28 days after surgery. Nec-1-treated mice displayed less adverse remodeling (end-diastolic volume 63.5 ± 2.8 vs. 74.9 ± 2.8 μl, P = 0.031) and preserved cardiac performance (ejection fraction 45.81 ± 2.05 vs. 36.03 ± 2.37%, P = 0.016). Nec-1 treatment significantly reduced inflammatory influx, tumor necrosis factor-α mRNA levels and oxidative stress levels. Interestingly, this was accompanied by significant changes in the expression signature of oxidative stress genes. Administration of Nec-1 at the onset of reperfusion inhibits RIP1-dependent necrosis in vivo, leading to infarct size reduction and preservation of cardiac function. The cardioprotective effect of Nec-1 highlights the importance of necrotic cell death in the ischemic heart, thereby opening a new direction for therapy in patients with myocardial infarction.

The DC receptor DNGR-1 mediates cross-priming of CTLs during vaccinia virus infection in mice

In order to prime T cells, DCs integrate signals emanating directly from pathogens and from their noxious action on the host. DNGR-1 (CLEC9A) is a DC-restricted receptor that detects dead cells. Therefore, we investigated the possibility that DNGR-1 affects immunity to cytopathic viruses. DNGR-1 was essential for cross-presentation of dying vaccinia virus-infected (VACV-infected) cells to CD8(+) T cells in vitro. Following injection of VACV or VACV-infected cells into mice, DNGR-1 detected the ligand in dying infected cells and mediated cross-priming of anti-VACV CD8(+) T cells. Loss of DNGR-1 impaired the CD8+ cytotoxic response to VACV, especially against those virus strains that are most dependent on cross-presentation. The decrease in total anti-VACV CTL activity was associated with a profound increase in viral load and delayed resolution of the primary lesion. In addition, lack of DNGR-1 markedly diminished protection from infection induced by vaccination with the modified vaccinia Ankara (MVA) strain. DNGR-1 thus contributes to anti-VACV immunity, following both primary infection and vaccination. The non-redundant ability of DNGR-1 to regulate cross-presentation of viral antigens suggests that this form of regulation of antiviral immunity could be exploited for vaccination.

Inhibitor of apoptosis proteins limit RIP3 kinase-dependent interleukin-1 activation

Interleukin-1β (IL-1β) is a potent inflammatory cytokine that is usually cleaved and activated by inflammasome-associated caspase-1. To determine whether IL-1β activation is regulated by inhibitor of apoptosis (IAP) proteins, we treated macrophages with an IAP-antagonist "Smac mimetic" compound or genetically deleted the genes that encode the three IAP family members cIAP1, cIAP2, and XIAP. After Toll-like receptor priming, IAP inhibition triggered cleavage of IL-1β that was mediated not only by the NLRP3-caspase-1 inflammasome, but also by caspase-8 in a caspase-1-independent manner. In the absence of IAPs, rapid and full generation of active IL-1β by the NLRP3-caspase-1 inflammasome, or by caspase-8, required the kinase RIP3 and reactive oxygen species production. These results demonstrate that activation of the cell death-inducing ripoptosome platform and RIP3 can generate bioactive IL-1β and implicate them as additional targets for the treatment of pathological IL-1-driven inflammatory responses.

RIP1-mediated regulation of lymphocyte survival and death responses

RIP1 is an adaptor serine/threonine kinase associated with the signaling complex of death receptors (DRs) including Fas, TNFR1, and TRAIL-Rs which can initiate apoptosis. While DRs are dispensable throughout development, RIP1 deletion results in perinatal lethality. The developmental defect caused by absence of RIP1 remains unexplained. In previous studies, RIP1-deficient hematopoietic progenitors failed to reconstitute the T cell compartment and our recent data indicate a new role for RIP1 in TCR-induced activation of the pro-survival NF-κB pathway. Here, we show that RIP1 is also critical for B cell development. In addition, RIP1(-/-) B cells stimulated through LPS/TLR4 are impaired in NF-κB activation but have no major defect in the Akt pathway. Recently, RIP1 has also emerged as a critical player in necrosis-like death, necroptosis, in various cell lines. We have demonstrated that RIP1 deficiency can reverse the embryonic and T cell proliferation defects in mice lacking FADD, a caspase adaptor protein, which indicates a potential role for RIP1 in mediating in vivo necroptosis. We provide an overview and discussion of the accumulating data revealing insights into the diverse functions of RIP1 in survival and death signaling in lymphocytes.

The 'complexities' of life and death: death receptor signalling platforms

Cell death is critical to the normal functioning of multi-cellular organisms, playing a central role in development, immunity, inflammation, and cancer progression. Two cell death mechanisms, apoptosis and necroptosis, are dependent on the formation of distinct multi-protein complexes including the DISC, Apoptosome, Piddosome and Necrosome following the induction of cell death by specific stimuli. The role of several of these key multi-protein signalling platforms, namely the DISC, TNFR1 complex I/II, the Necrosome and Ripoptosome, in mediating these pathways will be discussed, as well as the open questions and potential therapeutic benefits of understanding their underlying mechanisms.

D-galactose induces necroptotic cell death in neuroblastoma cell lines

D-Galactose (D-gal) can induce oxidative stress in non-cancer cells and result in cell damage by disturbing glucose metabolism. However, the effect of D-gal on cancer cells is yet to be explored. In this study, we investigated the toxicity of D-gal to malignant cells specifically neuroblastoma cells. As the results, high concentrations of D-gal had significant toxicity to cancer cells, whereas the same concentrations of glucose had no; the viability loss via D-gal treatment was prominent to malignant cells (Neuro2a, SH-SY5Y, PC-3, and HepG2) comparing to non-malignant cells (NIH3T3 and LO(2)). Differing from the apoptosis induced by H(2) O(2), D-gal damaged cells showed the characters of necrotic cell death, such as trypan blue-tangible and early phase LDH leakage. Further experiments displayed that the toxic effect of D-gal can be alleviated by necroptosis inhibitor Necrostatin (Nec-1) and autophagy inhibitor 3-methyladenine (3-MA) but not by caspase inhibitor z-VAD-fmk. D-Gal treatment can transcriptionally up-regulate the genes relevant to necroptosis (Bmf, Bnip3) and autophagy (Atg5, TIGAR) but not the genes related to apoptosis (Caspase3, Bax, and p53). D-Gal did not activate Caspase-3, but prompted puncta-like GFP-LC3 distribution, an indicator for activated autophagy. The involvement of aldose reductase (AR)-mediated polyol pathway was proved because the inhibitor of AR can attenuate the toxicity of D-gal and D-gal treatment elevates the expression of AR. This study demonstrates for the first time that D-gal can induce non-apoptotic but necroptotic cell death in neuroblastoma cells and provides a new clue for developing the strategy against apoptosis-resistant cancers.

Mixed lineage kinase domain-like protein mediates necrosis signaling downstream of RIP3 kinase

The receptor-interacting serine-threonine kinase 3 (RIP3) is a key signaling molecule in the programmed necrosis (necroptosis) pathway. This pathway plays important roles in a variety of physiological and pathological conditions, including development, tissue damage response, and antiviral immunity. Here, we report the identification of a small molecule called (E)-N-(4-(N-(3-methoxypyrazin-2-yl)sulfamoyl)phenyl)-3-(5-nitrothiophene-2-yl)acrylamide--hereafter referred to as necrosulfonamide--that specifically blocks necrosis downstream of RIP3 activation. An affinity probe derived from necrosulfonamide and coimmunoprecipitation using anti-RIP3 antibodies both identified the mixed lineage kinase domain-like protein (MLKL) as the interacting target. MLKL was phosphorylated by RIP3 at the threonine 357 and serine 358 residues, and these phosphorylation events were critical for necrosis. Treating cells with necrosulfonamide or knocking down MLKL expression arrested necrosis at a specific step at which RIP3 formed discrete punctae in cells. These findings implicate MLKL as a key mediator of necrosis signaling downstream of the kinase RIP3.

The mitochondrial phosphatase PGAM5 functions at the convergence point of multiple necrotic death pathways

The programmed necrosis induced by TNF-α requires the activities of the receptor-interacting serine-threonine kinases RIP1 and RIP3 and their interaction with the mixed lineage kinase domain-like protein MLKL. We report the identification of RIP1- and RIP3-containing protein complexes that form specifically in response to necrosis induction. One component of these complexes is the mitochondrial protein phosphatase PGAM5, which presents as two splice variants, PGAM5L (long form) and PGAM5S (short form). Knockdown of either form attenuated necrosis induced by TNF-α as well as reactive oxygen species (ROS) and calcium ionophore, whereas knockdown of RIP3 and MLKL blocked only TNF-α-mediated necrosis. Upon necrosis induction, PGAM5S recruited the mitochondrial fission factor Drp1 and activated its GTPase activity by dephosphorylating the serine 637 site of Drp1. Drp1 activation caused mitochondrial fragmentation, an early and obligatory step for necrosis execution. These data defined PGAM5 as the convergent point for multiple necrosis pathways.

RIP3 finds partners in crime

Programmed necrosis has long been recognized as a crucial component of animal development; however, the signaling pathway beyond the protein kinases RIP1 and RIP3 remains largely unknown. Sun et al. and Wang et al. now identify critical factors downstream of RIP1 and RIP3 in programmed necrosis, extending our understanding of this form of cell death.