NFκB and ubiquitination: partners in disarming RIPK1-mediated cell death

Nanomedicine-mediated regulated cell death in cancer immunotherapy

Immunotherapy has attracted widespread attention in cancer treatment and has achieved considerable success in the clinical treatment of some tumors, but it has a low response rate in most tumors. To achieve sufficient activation of the immune response, significant efforts using nanotechnology have been made to enhance cancer immune response. In recent years, the induction of various regulated cell death (RCD) has emerged as a potential antitumor immuno-strategy, including processes related to apoptosis, autophagy, necroptosis, pyroptosis, ferroptosis, and cuproptosis. In particular, damage-associated molecular patterns (DAMPs) released from the damaged membrane of dying cells act as in situ adjuvants to trigger antigen-specific immune responses by the exposure of an increased antigenicity. Thus, RCD-based immunotherapy offers a new approach for enhancing cancer treatment efficacy. Furthermore, incorporation with multimodal auxiliary therapies in cell death-based immunotherapy can trigger stronger immune responses, resulting in more efficient therapeutic outcome. This review discusses different RCD modalities and summarizes recent nanotechnology-mediated RCDs in cancer immunotherapy.

p53 at the crossroad between mitochondrial reactive oxygen species and necroptosis

p53 is a redox-sensitive transcription factor that can regulate multiple cell death programs through different signaling pathways. In this review, we assess the role of p53 in the regulation of necroptosis, a programmed form of lytic cell death highly involved in the pathophysiology of multiple diseases. In particular, we focus on the role of mitochondrial reactive oxygen species (mtROS) as essential contributors to modulate necroptosis execution through p53. The enhanced generation of mtROS during necroptosis is critical for the correct interaction between receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and 3 (RIPK3), two key components of the functional necrosome. p53 controls the occurrence of necroptosis by modulating the levels of mitochondrial H2O2 via peroxiredoxin 3 and sulfiredoxin. Furthermore, in response to increased levels of H2O2, p53 upregulates the long non-coding RNA necrosis-related factor, favoring the translation of RIPK1 and RIPK3. In parallel, a fraction of cytosolic p53 migrates into mitochondria, a process notably involved in necroptosis execution via its interaction with the mitochondrial permeability transition pore. In conclusion, p53 is located at the intersection between mtROS and the necroptosis machinery, making it a key protein to orchestrate redox signaling during necroptosis.

Microglial Dysfunction in Neurodegenerative Diseases via RIPK1 and ROS

Microglial dysfunction is a major contributor to the pathogenesis of multiple neurodegenerative diseases. The neurotoxicity of microglia associated with oxidative stress largely depends on NF-κB pathway activation, which promotes the production and release of microglial proinflammatory cytokines and chemokines. In this review, we discuss the current literature on the essential role of the NF-κB pathway on microglial activation that exacerbates neurodegeneration, with a particular focus on RIPK1 kinase activity-dependent microglial dysfunction. As upregulated RIPK1 kinase activity is associated with reactive oxygen species (ROS) accumulation in neurodegenerative diseases, we also discuss the current knowledge about the mechanistic links between RIPK1 activation and ROS generation. Given RIPK1 kinase activity and oxidative stress are closely regulated with each other in a vicious cycle, future studies are required to be conducted to fully understand how RIPK1 and ROS collude together to disturb microglial homeostasis that drives neurodegenerative pathogenesis.

The resurrection of RIP kinase 1 as an early cell death checkpoint regulator-a potential target for therapy in the necroptosis era

Receptor-interacting serine threonine protein kinase 1 (RIPK1) has emerged as a central molecular switch in controlling the balance between cell survival and cell death. The pro-survival role of RIPK1 in maintaining cell survival is achieved via its ability to induce NF-κB-dependent expression of anti-apoptotic genes. However, recent advances have identified the pro-death function of RIPK1: posttranslational modifications of RIPK1 in the tumor necrosis factor receptor 1 (TNFR1)-associated complex-I, in the cytosolic complex-IIb or in necrosomes regulate the cytotoxic potential of RIPK1, forming an early cell death checkpoint. Since the kinase activity of RIPK1 is indispensable in RIPK3- and MLKL-mediated necroptosis induction, while it is dispensable in apoptosis, a better understanding of this early cell death checkpoint via RIPK1 might lead to new insights into the molecular mechanisms controlling both apoptotic and necroptotic modes of cell death and help develop novel therapeutic approaches for cancer. Here, we present an emerging view of the regulatory mechanisms for RIPK1 activity, especially with respect to the early cell death checkpoint. We also discuss the impact of dysregulated RIPK1 activity in pathophysiological settings and highlight its therapeutic potential in treating human diseases.

Improved understanding of the molecular mechanisms that allow a protein to control the balance between cell survival or early death could reveal new approaches to treating conditions including chronic inflammatory disease and cancer. Gang Min Hur and colleagues at Chungnam National University in Daejeon, South Korea, with Han-Ming Shen at the University of Macau in China, review emerging evidence about how the protein called receptor-interacting serine/threonine-protein kinase 1 (RIPK1) influences whether cells move towards death or survival at a key ‘checkpoint’ in cell development. Cells can undergo a natural process of programmed cell death called apoptosis, die abnormally in a disease process called necroptosis, or survive. RIPK1 appears able to influence which path is chosen depending on which genes it regulates and which proteins it interacts with. Many details are still unclear, and need further investigation.

The RIPK family: expression profile and prognostic value in lung adenocarcinoma

Receptor interacting protein kinases (RIPKs) are a family of serine/threonine kinases which are supposed to regulate tumor generation and progression. Rare study illustrates the roles and functions of RIPKs family in lung adenocarcinoma (LUAD) comprehensively. Our results indicated that the expression of RIPK2 higher in LUAD patients while RIPK5 (encoded by gene DSTYK) expression was lower. Only RIPK2 had a strong correlation with pathological stage in LUAD patients. Kaplan-Meier plotter revealed that LUAD patients with low RIPK2 or RIPK3 level showed better overall survival (OS), but worse when LUAD patients with high RIPK5. Further, lower expression of RIPK2 and higher expression of RIPK1, RIPK4 and RIPK5 prompted a longer disease free survival (DFS). Genetic alterations based on cBioPortal revealing 16% alteration rates of RIPK2, as well as RIPK5. We also found that the functions of RIPKs family were linked to cellular senescence, protein serine/threonine kinase activity, apoptosis process et al. TIMER database indicated that the RIPKs family members had distinct relationships with the infiltration of six types of immune cells (macrophages, neutrophils, CD8+ T-cells, B-cells, CD4+ T-cells and dendritic cells). Moreover, RIPK2 could be observed as an independent prognostic factor with Cox proportional hazard model analysis. DiseaseMeth databases revealed that the global methylation levels of RIPK2 increased in LUAD patients. Thus, the findings above will enhance the understanding of RIPKs family in LUAD pathology and progression, providing novel insights into RIPKs-core therapy for LUAD patients.

Neuron-specific activation of necroptosis signaling in multiple sclerosis cortical grey matter

Sustained exposure to pro-inflammatory cytokines in the leptomeninges is thought to play a major role in the pathogenetic mechanisms leading to cortical pathology in multiple sclerosis (MS). Although the molecular mechanisms underlying neurodegeneration in the grey matter remain unclear, several lines of evidence suggest a prominent role for tumour necrosis factor (TNF). Using cortical grey matter tissue blocks from post-mortem brains from 28 secondary progressive MS subjects and ten non-neurological controls, we describe an increase in expression of multiple steps in the TNF/TNF receptor 1 signaling pathway leading to necroptosis, including the key proteins TNFR1, FADD, RIPK1, RIPK3 and MLKL. Activation of this pathway was indicated by the phosphorylation of RIPK3 and MLKL and the formation of protein oligomers characteristic of necrosomes. In contrast, caspase-8 dependent apoptotic signaling was decreased. Upregulation of necroptotic signaling occurred predominantly in macroneurons in cortical layers II–III, with little expression in other cell types. The presence of activated necroptotic proteins in neurons was increased in MS cases with prominent meningeal inflammation, with a 30-fold increase in phosphoMLKL+ neurons in layers I–III. The density of phosphoMLKL+ neurons correlated inversely with age at death, age at progression and disease duration. In vivo induction of chronically elevated TNF and INFγ levels in the CSF in a rat model via lentiviral transduction in the meninges, triggered inflammation and neurodegeneration in the underlying cortical grey matter that was associated with increased neuronal expression of TNFR1 and activated necroptotic signaling proteins. Exposure of cultured primary rat cortical neurons to TNF induced necroptosis when apoptosis was inhibited. Our data suggest that neurons in the MS cortex are dying via TNF/TNFR1 stimulated necroptosis rather than apoptosis, possibly initiated in part by chronic meningeal inflammation. Neuronal necroptosis represents a pathogenetic mechanism that is amenable to therapeutic intervention at several points in the signaling pathway.

Overexpressed microRNA-494 represses RIPK1 to attenuate hippocampal neuron injury in epilepsy rats by inactivating the NF-κB signaling pathway

OBJECTIVE

The effects of microRNAs (miRNAs) have been identified in epilepsy (Ep) in recent years, our research was focused on the functions of miR-494 in Ep and its inner mechanisms.

METHODS

The Ep modeled rats induced by lithium chloride-pilocarpine were treated with agomir-miR-494 or RIPK1-siRNA. The pathology of rat hippocampal tissues was observed. Expression of miR-494, receptor-interacting protein kinase 1 (RIPK1) and nuclear factor-kappaB (NF-κB) p65 was assessed by RT-qPCR and Western blot analysis. The hippocampal neurons of epileptic rats were successfully modeled, which were transfected with miR-494 mimics or RIPK1-siRNA to determine neurons' proliferation ability and cell apoptosis. The target relation between miR-494 and RIPK1 was measured by bioinformatics website and dual luciferase gene reporter assay.

RESULTS

The expression of miR-494 was reduced, while the expression of RIPK1 and NF-κB p65 was amplified in hippocampus of Ep rats. Elevated miR-494 repressed the expression of RIPK1 to ameliorate the hippocampal neuron injury, accelerate neuronal proliferation, and restrain neuronal apoptosis via inactivating the NF-κB signaling pathway, causing a deceleration of Ep development. Furthermore, amplified RIPK1 was able to reverse the amelioration of neuronal injury in Ep rats which was contributed by upregulated miR-494.

CONCLUSION

We found in this study that elevated miR-494 repressed RIPK1, causing an inactivation of the NF-κB signaling pathway and acceleration of cell proliferation, and suppression of apoptosis of hippocampal neurons in Ep rats, thereby attenuating the neuron injury and Ep development. Our research may provide novel targets for the therapy of Ep.

Necroptosis: Modules and molecular switches with therapeutic implications

Among the various programmed cell death (PCD) pathways, "Necroptosis" has gained much importance as a novel paradigm of cell death. This pathway has emerged as a backup mechanism when physiologically conserved PCD (apoptosis) is non-functional either genetically or pathogenically. The expanding spectrum of necroptosis from physiological development to diverse patho-physiological disorders, including xenobiotics-mediated toxicity has now grabbed the attention worldwide. The efficient role of necroptosis regulators in disease development and management are under constant examination. In fact, few regulators (e.g. MLKL) have already paved their way towards clinical trials and others are in queue. In this review, emphasis has been paid to the various contributing factors and molecular switches that can regulate necroptosis. Here we linked the overview of current knowledge of this enigmatic signaling with magnitude of therapeutics that may underpin the opportunities for novel therapeutic approaches to suppress the pathogenesis of necroptosis-driven disorders.

Negative Regulators of JAK/STAT Signaling in Rheumatoid Arthritis and Osteoarthritis

Elevated levels of pro-inflammatory cytokines are generally thought to be responsible for driving the progression of synovial joint inflammation in rheumatoid arthritis (RA) and osteoarthritis (OA). These cytokines activate several signal transduction pathways, including the Janus kinase/Signal Transducers and Activators of Transcription (JAK/STAT), Stress-Activated/Mitogen-Activated Protein Kinase (SAPK/MAPK) and phosphatidylinositol-3-kinase/Akt/mechanistic target of rapamycin (PI3K/Akt/mTOR) pathways which regulate numerous cellular responses. However, cytokine gene expression, matrix metalloproteinase gene expression and aberrant immune cell and synoviocyte survival via reduced apoptosis are most critical in the context of inflammation characteristic of RA and OA. Negative regulation of JAK/STAT signaling is controlled by Suppressor of Cytokine Signaling (SOCS) proteins. SOCS is produced at lower levels in RA and OA. In addition, gaining further insight into the role played in RA and OA pathology by the inhibitors of the apoptosis protein family, cellular inhibitor of apoptosis protein-1, -2 (c-IAP1, c-IAP2), X (cross)-linked inhibitor of apoptosis protein (XIAP), protein inhibitor of activated STAT (PIAS), and survivin (human) as well as SOCS appears to be a worthy endeavor going forward.

Selective activation of TNFR1 and NF-κB inhibition by a novel biyouyanagin analogue promotes apoptosis in acute leukemia cells

BACKGROUND

Acquired resistance towards apoptosis is a hallmark of cancer. Elimination of cells bearing activated oncogenes or stimulation of tumor suppressor mediators may provide a selection pressure to overcome resistance. KC-53 is a novel biyouyanagin analogue known to elicit strong anti-inflammatory and anti-viral activity. The current study was designed to evaluate the anticancer efficacy and molecular mechanisms of KC-53 against human cancer cells.

METHODS

Using the MTT assay we examined initially how KC-53 affects the proliferation rates of thirteen representative human cancer cell lines in comparison to normal peripheral blood mononuclear cells (PBMCs) and immortalized cell lines. To decipher the key molecular events underlying its mode of action we selected the human promyelocytic leukemia HL-60 and the acute lymphocytic leukemia CCRF/CEM cell lines that were found to be the most sensitive to the antiproliferative effects of KC-53.

RESULTS

KC-53 promoted rapidly and irreversibly apoptosis in both leukemia cell lines at relatively low concentrations. Apoptosis was characterized by an increase in membrane-associated TNFR1, activation of Caspase-8 and proteolytic inactivation of the death domain kinase RIP1 indicating that KC-53 induced mainly the extrinsic/death receptor apoptotic pathway. Regardless, induction of the intrinsic/mitochondrial pathway was also achieved by Caspase-8 processing of Bid, activation of Caspase-9 and increased translocation of AIF to the nucleus. FADD protein knockdown restored HL-60 and CCRF/CEM cell viability and completely blocked KC-53-induced apoptosis. Furthermore, KC-53 administration dramatically inhibited TNFα-induced serine phosphorylation on TRAF2 and on IκBα hindering therefore p65/NF-κΒ translocation to nucleus. Reduced transcriptional expression of pro-inflammatory and pro-survival p65 target genes, confirmed that the agent functionally inhibited the transcriptional activity of p65.

CONCLUSIONS

Our findings demonstrate, for the first time, the selective anticancer properties of KC-53 towards leukemic cell lines and provide a detailed understanding of the molecular events underlying its dual anti-proliferative and pro-apoptotic properties. These results provide new insights into the development of innovative and targeted therapies for the treatment of some forms of leukemia.

Poly-ubiquitination in TNFR1-mediated necroptosis

Tumor necrosis factor (TNF) is a master pro-inflammatory cytokine, and inappropriate TNF signaling is implicated in the pathology of many inflammatory diseases. Ligation of TNF to its receptor TNFR1 induces the transient formation of a primary membrane-bound signaling complex, known as complex I, that drives expression of pro-survival genes. Defective complex I activation results in induction of cell death, in the form of apoptosis or necroptosis. This switch occurs via internalization of complex I components and assembly and activation of secondary cytoplasmic death complexes, respectively known as complex II and necrosome. In this review, we discuss the crucial regulatory functions of ubiquitination—a post-translational protein modification consisting of the covalent attachment of ubiquitin, and multiples thereof, to target proteins—to the various steps of TNFR1 signaling leading to necroptosis.

API2-MALT1 oncoprotein promotes lymphomagenesis via unique program of substrate ubiquitination and proteolysis

Lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma) is the most common extranodal B cell tumor and accounts for 8% of non-Hodgkin’s lymphomas. Gastric MALT lymphoma is the best-studied example and is a prototypical neoplasm that occurs in the setting of chronic inflammation brought on by persistent infection or autoimmune disease. Cytogenetic abnormalities are commonly acquired during the course of disease and the most common is chromosomal translocation t(11;18)(q21;q21), which creates the API2-MALT1 fusion oncoprotein. t(11;18)-positive lymphomas can be clinically aggressive and have a higher rate of dissemination than t(11;18)-negative tumors. Many cancers, including MALT lymphomas, characteristically exhibit deregulated over-activation of cellular survival pathways, such as the nuclear factor-κB (NF-κB) pathway. Molecular characterization of API2-MALT1 has revealed it to be a potent activator of NF-κB, which is required for API2-MALT1-induced cellular transformation, however the mechanisms by which API2-MALT1 exerts these effects are only recently becoming apparent. The API2 moiety of the fusion binds tumor necrosis factor (TNF) receptor associated factor (TRAF) 2 and receptor interacting protein 1 (RIP1), two proteins essential for TNF receptor-induced NF-κB activation. By effectively mimicking ligand-bound TNF receptor, API2-MALT1 promotes TRAF2-dependent ubiquitination of RIP1, which then acts as a scaffold for nucleating and activating the canonical NF-κB machinery. Activation occurs, in part, through MALT1 moiety-dependent recruitment of TRAF6, which can directly modify NF-κB essential modulator, the principal downstream regulator of NF-κB. While the intrinsic MALT1 protease catalytic activity is dispensable for this canonical NF-κB signaling, it is critical for non-canonical NF-κB activation. In this regard, API2-MALT1 recognizes NF-κB inducing kinase (NIK), the essential upstream regulator of non-canonical NF-κB, and cleaves it to generate a stable, constitutively active fragment. Thus, API2-MALT1 harnesses multiple unique pathways to achieve deregulated NF-κB activation. Emerging data from our group and others have also detailed additional gain-of-function activities of API2-MALT1 that extend beyond NF-κB activation. Specifically, API2-MALT1 recruits and subverts multiple other signaling factors, including LIM domain and actin-binding protein 1 (LIMA1) and Smac/DIABLO. Like NIK, LIMA1 represents a unique substrate for API2-MALT1 protease activity, but unlike NIK, its cleavage sets in motion a major NF-κB-independent pathway for promoting oncogenesis. In this review, we highlight the most recent results characterizing these unique and diverse gain-of-function activities of API2-MALT1 and how they contribute to lymphomagenesis.

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.

Effect of the TLR2/MyD88/NF-κB axis on corneal allograft rejection after penetrating keratoplasty

PURPOSE

To evaluate the effect of the TLR2 (Toll-like receptor 2)/MyD88/NF-κB axis on the allograft rejection after penetrating keratoplasty (PK).

METHODS

The PK rat models were randomly divided into four groups: allograft group, dexamethasone group, PDTC group and isograft group. The mean survival time (MST) and rejection index of corneal grafts were observed. The immunohistochemical staining of TGF-α was performed on day 15. The messenger RNA (mRNA) and protein expression of TLR2, MyD88 and NF-κB p65 in corneal grafts were detected by reverse transcription-polymerase chain reaction (RT-PCR) and western blotting.

RESULTS

On days 5, 7, 9, 11, 13 and 15, the rejection index in the allograft group was higher than in the other three groups (p < 0.05). The MST in the PDTC group (MST, 23.30 ± 0.42 days, n = 10) and in the dexamethasone group (MST, 24.40 ± 0.50 days, n = 10) were higher than in the allograft group (MST, 14.7 ± 0.70 days, n = 10) (χ(2) = 18.02, p < 0.01; χ(2) = 21.47, p < 0.01). The expression of TNF-α in the PDTC group and in the dexamethasone group decreased compared with the allograft group by immunohistochemistry. On day 15, the mRNA and protein expression of TLR2, MyD88 and NF-κB p65 in the PDTC group and the dexamethasone group were less than in the allograft group (p < 0.05).

CONCLUSIONS

Expression of TLR2, MyD88 and NF-κB p65 in rat corneal graft increased significantly and concurred with the allograft rejection, but were effectively inhibited by the treatment with dexamethasone and PDTC after PK. Dexamethasone could improve corneal allograft survival by the TLR2/MyD88/NF-κB axis. PDTC could suppress corneal graft rejection by inhibiting the activity of NF-κB. The TLR2/MyD88/NF-κB axis maybe a potential therapeutic target for corneal allograft rejection.

Necroptotic signaling in adaptive and innate immunity

The vertebrate immune system is highly dependent on cell death for efficient responsiveness to microbial pathogens and oncogenically transformed cells. Cell death pathways are vital to the function of many immune cell types during innate, humoral and cellular immune responses. In addition, cell death regulation is imperative for proper adaptive immune self-tolerance and homeostasis. While apoptosis has been found to be involved in several of these roles in immunity, recent data demonstrate that alternative cell death pathways are required. Here, we describe the involvement of a programmed form of cellular necrosis called "necroptosis" in immunity. We consider the signaling pathways that promote necroptosis downstream of death receptors, type I transmembrane proteins of the tumor necrosis factor (TNF) receptor family. The involvement of necroptotic signaling through a "RIPoptosome" assembled in response to innate immune stimuli or genotoxic stress is described. We also characterize the induction of necroptosis following antigenic stimulation in T cells lacking caspase-8 or FADD function. While necroptotic signaling remains poorly understood, it is clear that this pathway is an essential component to effective vertebrate immunity.

Akt and mTOR mediate programmed necrosis in neurons

Necroptosis is a newly described form of regulated necrosis that contributes to neuronal death in experimental models of stroke and brain trauma. Although much work has been done elucidating initiating mechanisms, signaling events governing necroptosis remain largely unexplored. Akt is known to inhibit apoptotic neuronal cell death. Mechanistic target of rapamycin (mTOR) is a downstream effector of Akt that controls protein synthesis. We previously reported that dual inhibition of Akt and mTOR reduced acute cell death and improved long term cognitive deficits after controlled-cortical impact in mice. These findings raised the possibility that Akt/mTOR might regulate necroptosis. To test this hypothesis, we induced necroptosis in the hippocampal neuronal cell line HT22 using concomitant treatment with tumor necrosis factor α (TNFα) and the pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone. TNFα/zVAD treatment induced cell death within 4 h. Cell death was preceded by RIPK1–RIPK3–pAkt assembly, and phosphorylation of Thr-308 and Thr473 of AKT and its direct substrate glycogen synthase kinase-3β, as well as mTOR and its direct substrate S6 ribosomal protein (S6), suggesting activation of Akt/mTOR pathways. Pretreatment with Akt inhibitor viii and rapamycin inhibited Akt and S6 phosphorylation events, mitochondrial reactive oxygen species production, and necroptosis by over 50% without affecting RIPK1–RIPK3 complex assembly. These data were confirmed using small inhibitory ribonucleic acid-mediated knockdown of AKT1/2 and mTOR. All of the aforementioned biochemical events were inhibited by necrostatin-1, including Akt and mTOR phosphorylation, generation of oxidative stress, and RIPK1–RIPK3–pAkt complex assembly. The data suggest a novel, heretofore unexpected role for Akt and mTOR downstream of RIPK1 activation in neuronal cell death.

Caspase-8 as a regulator of tumor cell motility

The caspases are a family of ubiquitously expressed cysteine proteases best known for their roles in programmed cell death. However, caspases play a number of other roles in vertebrates. In the case of caspase-8, loss of expression is an embryonic lethal phenotype, and caspase-8 plays roles in suppressing cellular necrosis, promoting differentiation and immune signaling, regulating autophagy, and promoting cellular migration. Apoptosis and migration require localization of caspase-8 in the periphery of the cells, where caspase-8 acts as part of distinct biosensory complexes that either promote migration in appropriate cellular microenvironments, or cell death in inappropriate settings. In the cellular periphery, caspase-8 interacts with components of the focal adhesion complex in a tyrosine-kinase dependent manner, promoting both cell migration in vitro and metastasis in vivo. Mechanistically, caspase-8 interacts with components of both focal adhesions and early endosomes, enhancing focal adhesion turnover and promoting rapid integrin recycling to the cell surface. Clinically, this suggests that the expression of caspase-8 may not always be a positive prognostic sign, and that the role of caspase-8 in cancer progression is likely context-dependent.

Regulation of RIP1 kinase signalling at the crossroads of inflammation and cell death

Receptor-interacting protein 1 (RIP1) kinase has emerged as a key upstream regulator that controls inflammatory signalling as well as the activation of multiple cell death pathways, including apoptosis and necroptosis. The ability of RIP1 to modulate these key cellular events is tightly controlled by ubiquitylation, deubiquitylation and the interaction of RIP1 with a class of ubiquitin receptors. The modification of RIP1 may thus provide a unique 'ubiquitin code' that determines whether a cell activates nuclear factor-κB (NF-κB) to promote inflammatory signalling or induces cell death by apoptosis or necroptosis. Targeting RIP1 might be a novel therapeutic strategy for the treatment of both acute and chronic human diseases.

Opposing effect of EGFRWT on EGFRvIII-mediated NF-κB activation with RIP1 as a cell death switch

RIP1 is a central mediator of cell death in response to cell stress but can also mediate cell survival by activating NF-κB. Here, we show that RIP1 acts as a switch in EGFR signaling. EGFRvIII is an oncogenic mutant that does not bind ligand and is coexpressed with EGFRWT in glioblastoma multiforme (GBM). EGFRvIII recruits ubiquitin ligases to RIP1, resulting in K63-linked ubiquitination of RIP1. RIP1 binds to TAK1 and NEMO, forming an EGFRvIII-RIP1 signalosome that activates NF-κB. RIP1 is essential for EGFRvIII-mediated oncogenicity and correlates with NF-κB activation in GBM. Surprisingly, activation of EGFRWT with EGF results in a negative regulation of EGFRvIII, with dissociation of the EGFRvIII-RIP1 signalosome, loss of RIP1 ubiquitination and NF-κB activation, and association of RIP1 with FADD and caspase-8. If EGFRWT is overexpressed with EGFRvIII, the addition of EGF leads to a RIP1 kinase-dependent cell death. The EGFRWT-EGFRvIII-RIP1 interplay may regulate oncogenicity and vulnerability to targeted treatment in GBM.

Viral modulation of programmed necrosis

Apoptosis and programmed necrosis balance each other as alternate first line host defense pathways against which viruses have evolved countermeasures. Intrinsic apoptosis, the critical programmed cell death pathway that removes excess cells during embryonic development and tissue homeostasis, follows a caspase cascade triggered at mitochondria and modulated by virus-encoded anti-apoptotic B cell leukemia (BCL)2-like suppressors. Extrinsic apoptosis controlled by caspase 8 arose during evolution to trigger executioner caspases directly, circumventing viral suppressors of intrinsic (mitochondrial) apoptosis and providing the selective pressure for viruses to acquire caspase 8 suppressors. Programmed necrosis likely evolved most recently as a 'trap door' adaptation to extrinsic apoptosis. Receptor interacting protein (RIP)3 kinase (also called RIPK3) becomes active when either caspase 8 activity or polyubiquitylation of RIP1 is compromised. This evolutionary dialog implicates caspase 8 as a 'supersensor' alternatively activating and suppressing cell death pathways.