Cellular FLICE-like inhibitory protein secures intestinal epithelial cell survival and immune homeostasis by regulating caspase-8

The Function of Necroptosis and Its Treatment Target in IBD

Inflammatory bowel disease (IBD), which encompasses Crohn's disease (CD) and ulcerative colitis (UC), is a complicated illness whose exact cause is yet unknown. Necroptosis is associated with IBD pathogenesis, leading to intestinal barrier abnormalities and uncontrolled inflammation. Molecules involved in necroptosis, however, exhibit different expression levels in IBD and its associated colorectal cancer. Multiple studies have shown that inhibiting these molecules alleviates necroptosis-induced IBD. Moreover, due to the severe scarcity of clinical medications for treating IBD caused by necroptosis, we review the various functions of crucial necroptosis molecules in IBD, the stimuli regulating necroptosis, and the current emerging therapeutic strategies for treating IBD-associated necroptosis. Eventually, understanding the pathogenesis of necroptosis in IBD will enable the development of additional therapeutic approaches for the illness.

The importance of murine phospho-MLKL-S345 in situ detection for necroptosis assessment in vivo

Necroptosis is a caspase-independent modality of cell death implicated in many inflammatory pathologies. The execution of this pathway requires the formation of a cytosolic platform that comprises RIPK1 and RIPK3 which, in turn, mediates the phosphorylation of the pseudokinase MLKL (S345 in mouse). The activation of this executioner is followed by its oligomerisation and accumulation at the plasma-membrane where it leads to cell death via plasma-membrane destabilisation and consequent permeabilisation. While the biochemical and cellular characterisation of these events have been amply investigated, the study of necroptosis involvement in vivo in animal models is currently limited to the use of Mlkl-/- or Ripk3-/- mice. Yet, even in many of the models in which the involvement of necroptosis in disease aetiology has been genetically demonstrated, the fundamental in vivo characterisation regarding the question as to which tissue(s) and specific cell type(s) therein is/are affected by the pathogenic necroptotic death are missing. Here, we describe and validate an immunohistochemistry and immunofluorescence-based method to reliably detect the phosphorylation of mouse MLKL at serine 345 (pMLKL-S345). We first validate the method using tissues derived from mice in which Caspase-8 (Casp8) or FADD are specifically deleted from keratinocytes, or intestinal epithelial cells, respectively. We next demonstrate the presence of necroptotic activation in the lungs of SARS-CoV-infected mice and in the skin and spleen of mice bearing a Sharpin inactivating mutation. Finally, we exclude necroptosis occurrence in the intestines of mice subjected to TNF-induced septic shock. Importantly, by directly comparing the staining of pMLKL-345 with that of cleaved Caspase-3 staining in some of these models, we identify spatio-temporal and functional differences between necroptosis and apoptosis supporting a role of RIPK3 in inflammation independently of MLKL versus the role of RIPK3 in activation of necroptosis.

Activation of AMPK ameliorates acute severe pancreatitis by suppressing pancreatic acinar cell necroptosis in obese mice models

Obese people with acute pancreatitis (AP) have an increased risk of developing severe acute pancreatitis (SAP), which prolongs the length of hospital stay and increases mortality. Thus, elucidation of the mechanisms through which SAP occurs in obese individuals will provide clues for possible treatment targets. Differences in early events in obese or lean patients with AP have not been conclusively reported. We selected C57BL/6 mice as lean mice models, ob/ob mice or diet induced obese (DIO) mice as obese mice models and then induced experimental AP in mice via injections of caerulein. There were suppressed p-AMPK expressions in the pancreas of obese mice, compared with same-age lean C57BL/6 mice, which were further reduced in AP mice models. Obese AP mice were treated using AICAR, a direct AMPK agonist, which prevented pancreatic damage and cell death, suppressed pancreatic enzyme levels in serum, reduced the areas of fat saponification in the peritoneal cavity, prevented injury in other organs and decreased mice mortality rate. Further assays showed that AICAR activates p-AMPK to stabilize pro-caspase-8. Pro-caspase-8 enhances RIPK3 degradation, inhibits pancreatic acinar cell necroptosis, and downregulates the release of pancreatic enzymes. Thus, activation of AMPK by AICAR alleviates pancreatic acinar cell necroptosis and converts SAP to mild acute pancreatitis in obese mice.

Cleavage of cFLIP restrains cell death during viral infection and tissue injury and favors tissue repair

Cell death coordinates repair programs following pathogen attack and tissue injury. However, aberrant cell death can interfere with such programs and cause organ failure. Cellular FLICE-like inhibitory protein (cFLIP) is a crucial regulator of cell death and a substrate of Caspase-8. However, the physiological role of cFLIP cleavage by Caspase-8 remains elusive. Here, we found an essential role for cFLIP cleavage in restraining cell death in different pathophysiological scenarios. Mice expressing a cleavage-resistant cFLIP mutant, CflipD377A, exhibited increased sensitivity to severe acute respiratory syndrome coronavirus (SARS-CoV)-induced lethality, impaired skin wound healing, and increased tissue damage caused by Sharpin deficiency. In vitro, abrogation of cFLIP cleavage sensitizes cells to tumor necrosis factor(TNF)-induced necroptosis and apoptosis by favoring complex-II formation. Mechanistically, the cell death-sensitizing effect of the D377A mutation depends on glutamine-469. These results reveal a crucial role for cFLIP cleavage in controlling the amplitude of cell death responses occurring upon tissue stress to ensure the execution of repair programs.

Cell death checkpoints in the TNF pathway

Tumor necrosis factor (TNF) plays a central role in orchestrating mammalian inflammatory responses. It promotes inflammation either directly by inducing inflammatory gene expression or indirectly by triggering cell death. TNF-mediated cell death-driven inflammation can be beneficial during infection by providing cell-extrinsic signals that help to mount proper immune responses. Uncontrolled cell death caused by TNF is instead highly detrimental and is believed to cause several human autoimmune diseases. Death is not the default response to TNF sensing. Molecular brakes, or cell death checkpoints, actively repress TNF cytotoxicity to protect the organism from its detrimental consequences. These checkpoints therefore constitute essential safeguards against inflammatory diseases. Recent advances in the field have revealed the existence of several new and unexpected brakes against TNF cytotoxicity and pathogenicity.

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Endothelial Caspase-8 prevents fatal necroptotic hemorrhage caused by commensal bacteria

Caspase-8 transduces signals from death receptor ligands, such as tumor necrosis factor, to drive potent responses including inflammation, cell proliferation or cell death. This is a developmentally essential function because in utero deletion of endothelial Caspase-8 causes systemic circulatory collapse during embryogenesis. Whether endothelial Caspase-8 is also required for cardiovascular patency during adulthood was unknown. To address this question, we used an inducible Cre recombinase system to delete endothelial Casp8 in 6-week-old conditionally gene-targeted mice. Extensive whole body vascular gene targeting was confirmed, yet the dominant phenotype was fatal hemorrhagic lesions exclusively within the small intestine. The emergence of these intestinal lesions was not a maladaptive immune response to endothelial Caspase-8-deficiency, but instead relied upon aberrant Toll-like receptor sensing of microbial commensals and tumor necrosis factor receptor signaling. This lethal phenotype was prevented in compound mutant mice that lacked the necroptotic cell death effector, MLKL. Thus, distinct from its systemic role during embryogenesis, our data show that dysregulated microbial- and death receptor-signaling uniquely culminate in the adult mouse small intestine to unleash MLKL-dependent necroptotic hemorrhage after loss of endothelial Caspase-8. These data support a critical role for Caspase-8 in preserving gut vascular integrity in the face of microbial commensals.

Death by TNF: a road to inflammation

Tumour necrosis factor (TNF) is a central cytokine in inflammatory reactions, and biologics that neutralize TNF are among the most successful drugs for the treatment of chronic inflammatory and autoimmune pathologies. In recent years, it became clear that TNF drives inflammatory responses not only directly by inducing inflammatory gene expression but also indirectly by inducing cell death, instigating inflammatory immune reactions and disease development. Hence, inhibitors of cell death are being considered as a new therapy for TNF-dependent inflammatory diseases.

Caspase-8 in endothelial cells maintains gut homeostasis and prevents small bowel inflammation in mice

The gut has a specific vascular barrier that controls trafficking of antigens and microbiota into the bloodstream. However, the molecular mechanisms regulating the maintenance of this vascular barrier remain elusive. Here, we identified Caspase-8 as a pro-survival factor in mature intestinal endothelial cells that is required to actively maintain vascular homeostasis in the small intestine in an organ-specific manner. In particular, we find that deletion of Caspase-8 in endothelial cells results in small intestinal hemorrhages and bowel inflammation, while all other organs remained unaffected. We also show that Caspase-8 seems to be particularly needed in lymphatic endothelial cells to maintain gut homeostasis. Our work demonstrates that endothelial cell dysfunction, leading to the breakdown of the gut-vascular barrier, is an active driver of chronic small intestinal inflammation, highlighting the role of the intestinal vasculature as a safeguard of organ function.

Executioner caspases 3 and 7 are dispensable for intestinal epithelium turnover and homeostasis at steady state

Historically, programmed cell death by apoptosis is considered crucial for proper intestinal organogenesis and gut homeostasis. To challenge this concept, we generated caspase-3 and -7 double knockout mice specifically in intestinal epithelial cells (IECs). However, absence of apoptosis in IECs elicits neither morphological and inflammatory changes nor intestinal dysbiosis during gut homeostasis at steady state. This demonstrates the robustness of intestinal homeostasis at steady state for the absence of caspase-3/7 and shows that in contrast to caspase-8, which keeps necroptosis and associated inflammation in check, caspase-3/7–dependent apoptosis of IECs in homeostatic conditions is dispensable for normal intestinal development, immune cell composition, and microbiome control.

Apoptosis is widely believed to be crucial for epithelial cell death and shedding in the intestine, thereby shaping the overall architecture of the gastrointestinal tract, but also regulating tolerance induction, pinpointing a role of apoptosis intestinal epithelial cell (IEC) turnover and maintenance of barrier function, and in maintaining immune homeostasis. To experimentally address this concept, we generated IEC-specific knockout mice that lack both executioner caspase-3 and caspase-7 (Casp3/7^ΔIEC), which are the converging point of the extrinsic and intrinsic apoptotic pathway. Surprisingly, the overall architecture, cellular landscape, and proliferation rate remained unchanged in these mice. However, nonapoptotic cell extrusion was increased in Casp3/7^ΔIEC mice, compensating apoptosis deficiency, maintaining the same physiological level of IEC shedding. Microbiome richness and composition stayed unaffected, bearing no sign of dysbiosis. Transcriptome and single-cell RNA sequencing analyses of IECs and immune cells revealed no differences in signaling pathways of differentiation and inflammation. These findings demonstrate that during homeostasis, apoptosis per se is dispensable for IEC turnover at the top of intestinal villi intestinal tissue dynamics, microbiome, and immune cell composition.

Harnessing murine models of Crohn's disease ileitis to advance concepts of pathophysiology and treatment

Crohn's disease (CD) and ulcerative colitis (UC) are both characterized by chronic inflammation and severe dysfunction of the gastrointestinal tract. These two forms of inflammatory bowel disease (IBD) represent distinct clinical disorders with diverse driving mechanisms; however, this divergence is not reflected in currently approved therapeutics that commonly target general proinflammatory pathways. A compelling need therefore remains to understand factors that differentiate the topology and the distinct clinical manifestations of CD versus UC, in order to develop more effective and specialized therapies. Animal models provide valuable platforms for studying IBD heterogeneity and deciphering disease-specific mechanisms. Both the established and the newly developed ileitis mouse models are characterized by various disease initiating mechanisms and diverse phenotypic outcomes that reflect the complexity of human CD-ileitis. Microbial dysbiosis, destruction of epithelial barrier integrity, immune cell deregulation, as well as the recently described genome instability and stromal cell activation have all been proposed as the triggering factors for the development of ileitis-associated pathology. In this review, we aim to critically evaluate the mechanistic underpinnings of murine models of CD-ileitis, discuss their phenotypic similarities to human disease, and envisage their further exploitation for the development of novel targeted and personalized therapeutics.

TNF-α synergises with IFN-γ to induce caspase-8-JAK1/2-STAT1-dependent death of intestinal epithelial cells

Rewiring of host cytokine networks is a key feature of inflammatory bowel diseases (IBD) such as Crohn's disease (CD). Th1-type cytokines-IFN-γ and TNF-α-occupy critical nodes within these networks and both are associated with disruption of gut epithelial barrier function. This may be due to their ability to synergistically trigger the death of intestinal epithelial cells (IECs) via largely unknown mechanisms. In this study, through unbiased kinome RNAi and drug repurposing screens we identified JAK1/2 kinases as the principal and nonredundant drivers of the synergistic killing of human IECs by IFN-γ/TNF-α. Sensitivity to IFN-γ/TNF-α-mediated synergistic IEC death was retained in primary patient-derived intestinal organoids. Dependence on JAK1/2 was confirmed using genetic loss-of-function studies and JAK inhibitors (JAKinibs). Despite the presence of biochemical features consistent with canonical TNFR1-mediated apoptosis and necroptosis, IFN-γ/TNF-α-induced IEC death was independent of RIPK1/3, ZBP1, MLKL or caspase activity. Instead, it involved sustained activation of JAK1/2-STAT1 signalling, which required a nonenzymatic scaffold function of caspase-8 (CASP8). Further modelling in gut mucosal biopsies revealed an intercorrelated induction of the lethal CASP8-JAK1/2-STAT1 module during ex vivo stimulation of T cells. Functional studies in CD-derived organoids using inhibitors of apoptosis, necroptosis and JAKinibs confirmed the causative role of JAK1/2-STAT1 in cytokine-induced death of primary IECs. Collectively, we demonstrate that TNF-α synergises with IFN-γ to kill IECs via the CASP8-JAK1/2-STAT1 module independently of canonical TNFR1 and cell death signalling. This non-canonical cell death pathway may underpin immunopathology driven by IFN-γ/TNF-α in diverse autoinflammatory diseases such as IBD, and its inhibition may contribute to the therapeutic efficacy of anti-TNFs and JAKinibs.

An IFN-STAT Axis Augments Tissue Damage and Inflammation in a Mouse Model of Crohn's Disease

Blocking interferon-function by therapeutic intervention of the JAK-STAT-axis is a novel promising treatment option for inflammatory bowel disease (IBD). Although JAK inhibitors have proven efficacy in patients with active ulcerative colitis (UC), they failed to induce clinical remission in patients with Crohn's disease (CD). This finding strongly implicates a differential contribution of JAK signaling in both entities. Here, we dissected the contribution of different STAT members downstream of JAK to inflammation and barrier dysfunction in a mouse model of Crohn's disease like ileitis and colitis (Casp8^ΔIEC mice). Deletion of STAT1 in Casp8^ΔIEC mice was associated with reduced cell death and a partial rescue of Paneth cell function in the small intestine. Likewise, organoids derived from the small intestine of these mice were less sensitive to cell death triggered by IBD-key cytokines such as TNFα or IFNs. Further functional in vitro and in vivo analyses revealed the impairment of MLKL-mediated necrosis as a result of deficient STAT1 function, which was in turn associated with improved cell survival. However, a decrease in inflammatory cell death was still associated with mild inflammation in the small intestine. The impact of STAT1 signaling on gastrointestinal inflammation dependent on the localization of inflammation, as STAT1 is essential for intestinal epithelial cell death regulation in the small intestine, whereas it is not the key factor for intestinal epithelial cell death in the context of colitis. Of note, additional deletion of STAT2 was not sufficient to restore Paneth cell function but strongly ameliorated ileitis. In summary, we provide here compelling molecular evidence that STAT1 and STAT2, both contribute to intestinal homeostasis, but have non-redundant functions. Our results further demonstrate that STATs individually affect the distinct pathophysiology of inflammation in the ileum and colon, respectively, which might explain the diverse outcome of JAK inhibitors on inflammatory bowel diseases.

Targeting RIP Kinases in Chronic Inflammatory Disease

Chronic inflammatory disorders are characterised by aberrant and exaggerated inflammatory immune cell responses. Modes of extrinsic cell death, apoptosis and necroptosis, have now been shown to be potent drivers of deleterious inflammation, and mutations in core repressors of these pathways underlie many autoinflammatory disorders. The receptor-interacting protein (RIP) kinases, RIPK1 and RIPK3, are integral players in extrinsic cell death signalling by regulating the production of pro-inflammatory cytokines, such as tumour necrosis factor (TNF), and coordinating the activation of the NOD-like receptor protein 3 (NLRP3) inflammasome, which underpin pathological inflammation in numerous chronic inflammatory disorders. In this review, we firstly give an overview of the inflammatory cell death pathways regulated by RIPK1 and RIPK3. We then discuss how dysregulated signalling along these pathways can contribute to chronic inflammatory disorders of the joints, skin, and gastrointestinal tract, and discuss the emerging evidence for targeting these RIP kinases in the clinic.

Interferons at the crossroad of cell death pathways during gastrointestinal inflammation and infection

Interferons (IFNs) are pleiotropic immune-modulatory cytokines that are well known for their essential role in host defense against viruses, bacteria, and other pathogenic microorganisms. They can exert both, protective or destructive functions depending on the microorganism, the targeted tissue and the cellular context. Interferon signaling results in the induction of IFN-stimulated genes (ISGs) influencing different cellular pathways including direct anti-viral/anti-bacterial response, immune-modulation or cell death. Multiple pathways leading to host cell death have been described, and it is becoming clear that depending on the cellular context, IFN-induced cell death can be beneficial for both: host and pathogen. Accordingly, activation or repression of corresponding signaling mechanisms occurs during various types of infection but is also an important pathway for gastrointestinal inflammation and tissue damage. In this review, we summarize the role of interferons at the crossroad of various cell death pathways in the gut during inflammation and infection.

Panax quinquefolium saponins protect against cisplatin evoked intestinal injury via ROS-mediated multiple mechanisms

BACKGROUND

Cisplatin is one of the most common chemotherapeutic drugs. Cisplatin-induced toxicity gives rise to gastrointestinal cell damage, subsequent diarrhea and vomiting, leading to the discontinuation of its clinical application in long-term cancer chemotherapy. Panax quinquefolium L., also known as American ginseng, has many pharmacological activities such as improving immunity, anti-tumor, anti-radiation and blood sugar lowering.

PURPOSE

Previously, our laboratory reported that American ginseng berry extract could alleviate chemotherapeutic agents-induced renal damage caused by cisplatin. Hence, this study further explored the protective effect of P. quinquefolium saponins (PQS) on cisplatin-induced intestinal injury in mice and the possible molecular mechanisms.

METHODS

Biochemical markers, levels of inflammatory factors, histopathological staining and western blotting were used to analyze intestinal injury based on various molecular mechanisms.

RESULTS

We demonstrated the destruction of the intestinal barrier caused by cisplatin exposure by detecting the activity of diamine oxidase (DAO) and the expression of tight junction proteins zonula occludens-1 (ZO-1) and occludin. Meanwhile, cisplatin exposure changed SOD and MDA levels in the small intestine, causing oxidative damage to the intestinal mucosa. The inflammation associated-intestinal damage was further explored by the measurement of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and analysis of nuclear factor-kappa B (NF-κB) inflammatory pathway protein expression. Moreover, apoptotic cells labeled with TUNEL staining-positive cells and activated caspase family proteins suggest that cisplatin induces intestinal apoptosis. Interestingly, PQS pretreatment significantly reversed these situations.

CONCLUSION

These evidences clearly suggest that PQS can alleviate cisplatin-induced intestinal damage by inhibiting oxidative stress, reducing the occurrence of inflammation and apoptosis, and improving intestinal barrier function.

Gut epithelial TSC1/mTOR controls RIPK3-dependent necroptosis in intestinal inflammation and cancer

Although Western diet and dysbiosis are the most prominent environmental factors associated with inflammatory bowel diseases (IBDs), the corresponding host factors and cellular mechanisms remain poorly defined. Here we report that the TSC1/mTOR pathway in the gut epithelium represents a metabolic and innate immune checkpoint for intestinal dysfunction and inflammation. mTOR hyperactivation triggered by Western diet or Tsc1 ablation led to epithelium necroptosis, barrier disruption, and predisposition to dextran sulfate sodium-induced colitis and inflammation-associated colon cancer. Mechanistically, our results uncovered a critical role for TSC1/mTOR in restraining the expression and activation of RIPK3 in the gut epithelium through TRIM11-mediated ubiquitination and autophagy-dependent degradation. Notably, microbiota depletion by antibiotics or gnotobiotics attenuated RIPK3 expression and activation, thereby alleviating epithelial necroptosis and colitis driven by mTOR hyperactivation. mTOR primarily impinged on RIPK3 to potentiate necroptosis induced by TNF and by microbial pathogen-associated molecular patterns (PAMPs), and hyperactive mTOR and aberrant necroptosis were intertwined in human IBDs. Together, our data reveal a previously unsuspected link between the Western diet, microbiota, and necroptosis and identify the mTOR/RIPK3/necroptosis axis as a driving force for intestinal inflammation and cancer.

Cell death in the gut epithelium and implications for chronic inflammation

The intestinal epithelium has one of the highest rates of cellular turnover in a process that is tightly regulated. As the transit-amplifying progenitors of the intestinal epithelium generate ~300 cells per crypt every day, regulated cell death and sloughing at the apical surface keeps the overall cell number in check. An aberrant increase in the rate of intestinal epithelial cell (IEC) death underlies instances of extensive epithelial erosion, which is characteristic of several intestinal diseases such as inflammatory bowel disease and infectious colitis. Emerging evidence points to a crucial role of necroptosis, autophagy and pyroptosis as important modes of programmed cell death in the intestine in addition to apoptosis. The mode of cell death affects tissue restitution responses and ultimately the long-term risks of intestinal fibrosis and colorectal cancer. A vicious cycle of intestinal barrier breach, misregulated cell death and subsequent inflammation is at the heart of chronic inflammatory and infectious gastrointestinal diseases. This Review discusses the underlying molecular and cellular underpinnings that control programmed cell death in IECs, which emerge during intestinal diseases. Translational aspects of cell death modulation for the development of novel therapeutic alternatives for inflammatory bowel diseases and colorectal cancer are also discussed.

Is Polymorphism in the Apoptosis and Inflammatory Pathway Genes Associated With a Primary Response to Anti-TNF Therapy in Crohn's Disease Patients?

Anti-tumor necrosis factor (TNF) therapy is used for the induction and maintenance of remission in Crohn’s disease (CD) patients. However, primary nonresponders to initial treatment constitute 20%–40% of cases. The causes of this phenomenon are still unknown. In this study, we aimed to determine the genetic predictors of the variable reactions of CD patients to anti-TNF therapy. Using long-range PCR libraries and the next-generation sequencing (NGS) method, we performed broad pharmacogenetic studies including a panel of 23 genes (TNFRSF1A, TNFRSF1B, CASP9, FCGR3A, LTA, TNF, FAS, ADAM17, IL17A, IL6, MMP1, MMP3, S100A8, S100A9, S100A12, TLR2, TLR4, TLR9, CD14, IL23R, IL23, IL1R, and IL1B) in a group of 107 diagnosed and clinically characterized CD patients following anti-TNF therapy. In the studied group, we indicated, in total, 598 single nucleotide variants for all analyzed genomic targets. Twelve patients (11.2%) did not respond to the induction therapy, which was associated with alleles in 11 loci located in FCGR3A (rs7539036, rs6672453, rs373184583, and rs12128686), IL1R (rs2041747), TNFRSF1B (rs5746053), IL1B (rs1071676, rs1143639, rs1143637, and rs1143634), and FAS (rs7896789) genes. After multiple comparison corrections, the results were not statistically significant, however for nonresponders the alleles distribution for those loci presented large differences and specified scheme compared to responders and populations. These findings require further investigation in an independent larger cohort before introducing them for a clinical setting, however, we identified an interesting direction. Polymorphism of the FCGR3A, IL1R, TNFRSF1B, IL1B, and FAS genes could be a predictor of the primary response to anti-TNF therapy in CD patients.

FADD and Caspase-8 Regulate Gut Homeostasis and Inflammation by Controlling MLKL- and GSDMD-Mediated Death of Intestinal Epithelial Cells

Pathways controlling intestinal epithelial cell (IEC) death regulate gut immune homeostasis and contribute to the pathogenesis of inflammatory bowel diseases. Here, we show that caspase-8 and its adapter FADD act in IECs to regulate intestinal inflammation downstream of Z-DNA binding protein 1 (ZBP1)- and tumor necrosis factor receptor-1 (TNFR1)-mediated receptor interacting protein kinase 1 (RIPK1) and RIPK3 signaling. Mice with IEC-specific FADD or caspase-8 deficiency developed colitis dependent on mixed lineage kinase-like (MLKL)-mediated epithelial cell necroptosis. However, MLKL deficiency fully prevented ileitis caused by epithelial caspase-8 ablation, but only partially ameliorated ileitis in mice lacking FADD in IECs. Our genetic studies revealed that caspase-8 and gasdermin-D (GSDMD) were both required for the development of MLKL-independent ileitis in mice with epithelial FADD deficiency. Therefore, FADD prevents intestinal inflammation downstream of ZBP1 and TNFR1 by inhibiting both MLKL-induced necroptosis and caspase-8-GSDMD-dependent pyroptosis-like death of epithelial cells.

A20 Promotes Ripoptosome Formation and TNF-Induced Apoptosis via cIAPs Regulation and NIK Stabilization in Keratinocytes

The ubiquitin-editing protein A20 (TNFAIP3) is a known key player in the regulation of immune responses in many organs. Genome-wide associated studies (GWASs) have linked A20 with a number of inflammatory and autoimmune disorders, including psoriasis. Here, we identified a previously unrecognized role of A20 as a pro-apoptotic factor in TNF-induced cell death in keratinocytes. This function of A20 is mediated via the NF-κB-dependent alteration of cIAP1/2 expression. The changes in cIAP1/2 protein levels promote NIK stabilization and subsequent activation of noncanonical NF-κB signaling. Upregulation of TRAF1 expression triggered by the noncanonical NF-κB signaling further enhances the NIK stabilization in an autocrine manner. Finally, stabilized NIK promotes the formation of the ripoptosome and the execution of cell death. Thus, our data demonstrate that A20 controls the execution of TNF-induced cell death on multiple levels in keratinocytes. This signaling mechanism might have important implications for the development of new therapeutic strategies for the treatment of A20-associated skin diseases.

Viral FLIP blocks Caspase-8 driven apoptosis in the gut in vivo

A strict cell death control in the intestinal epithelium is indispensable to maintain barrier integrity and homeostasis. In order to achieve a balance between cell proliferation and cell death, a tight regulation of Caspase-8, which is a key player in controlling apoptosis, is required. Caspase-8 activity is regulated by cellular FLIP proteins. These proteins are expressed in different isoforms (cFLIP_long and cFLIP_short) which determine cell death and survival. Interestingly, several viruses encode FLIP proteins, homologous to cFLIP_short, which are described to regulate Caspase-8 and the host cell death machinery. In the current study a mouse model was generated to show the impact of viral FLIP (vFLIP) from Kaposi’s Sarcoma-associated Herpesvirus (KSHV)/ Human Herpesvirus-8 (HHV-8) on cell death regulation in the gut. Our results demonstrate that expression of vFlip in intestinal epithelial cells suppressed cFlip expression, but protected mice from lethality, tissue damage and excessive apoptotic cell death induced by genetic cFlip deletion. Finally, our model shows that vFlip expression decreases cFlip mediated Caspase-8 activation in intestinal epithelial cells. In conclusion, our data suggests that viral FLIP neutralizes and compensates for cellular FLIP, efficiently counteracting host cell death induction and facilitating further propagation in the host organism.

Environmental Microbial Factors Determine the Pattern of Inflammatory Lesions in a Murine Model of Crohn's Disease-Like Inflammation

Crohn's disease (CD) patients can be grouped into patients suffering from ileitis, ileocolitis, jejunoileitis, and colitis. The pathophysiological mechanism underlying this regional inflammation is still unknown. Although most murine models of inflammatory bowel disease (IBD) develop inflammation in the colon, there is an unmet need for novel models that recapitulate the spontaneous and fluctuating nature of inflammation as seen in CD. Recently, mice with an intestinal epithelial cell-specific deletion for Caspase-8 (Casp8ΔIEC mice), which are characterized by cell death-driven ileitis and disrupted Paneth cell homeostasis, have been identified as a novel model of CD-like ileitis. Here we uncovered that genetic susceptibility alone is sufficient to drive ileitis in Casp8ΔIEC mice. In sharp contrast, environmental factors, such as a disease-relevant microbial flora, determine colonic inflammation. Accordingly, depending on the microbial environment, isogenic Casp8ΔIEC mice either exclusively developed ileitis or suffered from pathologies in several parts of the gastrointestinal tract. Colitis in these mice was characterized by massive epithelial cell death, leading to spread of commensal gut microbes to the extra-intestinal space and hence an aberrant activation of the systemic immunity. We further uncovered that Casp8ΔIEC mice show qualitative and quantitative changes in the intestinal microbiome associated with an altered mucosal and systemic immune response. In summary, we identified that inflammation in this murine model of CD-like inflammation is characterized by an immune reaction, presumably directed against a disease-relevant microbiota in a genetically susceptible host, with impaired mucosal barrier function and bacterial clearance at the epithelial interface.

Interferon Lambda Promotes Paneth Cell Death Via STAT1 Signaling in Mice and Is Increased in Inflamed Ileal Tissues of Patients With Crohn's Disease

BACKGROUND & AIMS

Interferon lambda (IFNL) is expressed at high levels by intestinal epithelial cells (IECs) and mucosal immune cells in response to infection and inflammation. We investigated whether IFNL might contribute to pathogenesis of Crohn's disease (CD).

METHODS

We obtained serum samples and terminal ileum biopsies from 47 patients with CD and 16 healthy individuals (controls). We measured levels of IFNL by enzyme-linked immunosorbent assay and immunohistochemistry and location of expression by confocal microscopy. Activation of IFNL signaling via STAT1 was measured in areas of no, mild, moderate, and severe inflammation and correlated with Paneth cell homeostasis and inflammation. IFNL expression and function were studied in wild-type mice and mice with intestinal epithelial cell-specific (ΔIEC) disruption or full-body disruption of specific genes (Mlkl^-/-, Stat1^ΔIEC, Casp8^ΔIEC, Casp8^ΔIECRipk3^-/-, Casp8^ΔIECTnfr^-/-, Casp8^ΔIECMlkl^-/-, and Nod2^-/- mice). Some mice were given tail vein injections of a vector encoding a secreted form of IFNL. Intestinal tissues were collected from mice and analyzed by immunohistochemistry and immunoblots. We generated 3-dimensional small intestinal organoids from mice and studied the effects of IFNL and inhibitors of STAT-signaling pathway.

RESULTS

Patients with CD had significant increases in serum and ileal levels of IFNL compared with controls. Levels of IFNL were highest in ileum tissues with severe inflammation. High levels of IFNL associated with a reduced number of Paneth cells and increased cell death at the crypt bottom in inflamed ileum samples. Intestinal tissues from the ileum of wild-type mice injected with a vector expressing IFNL had reduced numbers of Paneth cells. IFNL-induced death of Paneth cells in mice did not occur via apoptosis, but required Mixed Lineage Kinase Domain Like (MLKL) and activation of Signal transducer and activator of transcription 1 (STAT1). In organoids, inhibitors of Janus kinase (JAK) signaling via STAT1 (glucocorticoids, tofacitinib, or filgotinib) reduced expression of proteins that mediate cell death and prevented Paneth cell death.

CONCLUSIONS

Levels of IFNL are increased in serum and inflamed ileal tissues from patients with CD and associated with a loss of Paneth cells. Expression of a secreted form of IFNL in mice results in loss of Paneth cells from intestinal tissues, via STAT1 and MLKL, controlled by caspase 8. Strategies to reduce IFNL or block its effects might be developed for treatment of patients with CD affecting the terminal ileum.

Tumour Necrosis Factor Alpha in Intestinal Homeostasis and Gut Related Diseases

The intestinal epithelium constitutes an indispensable single-layered barrier to protect the body from invading pathogens, antigens or toxins. At the same time, beneficial nutrients and water have to be absorbed by the epithelium. To prevent development of intestinal inflammation or tumour formation, intestinal homeostasis has to be tightly controlled and therefore a strict balance between cell death and proliferation has to be maintained. The proinflammatory cytokine tumour necrosis factor alpha (TNFα) was shown to play a striking role for the regulation of this balance in the gut. Depending on the cellular conditions, on the one hand TNFα is able to mediate cell survival by activating NFκB signalling. On the other hand, TNFα might trigger cell death, in particular caspase-dependent apoptosis but also caspase-independent programmed necrosis. By regulating these cell death and survival mechanisms, TNFα exerts a variety of beneficial functions in the intestine. However, TNFα signalling is also supposed to play a critical role for the pathogenesis of inflammatory bowel disease (IBD), infectious diseases, intestinal wound healing and tumour formation. Here we review the literature about the physiological and pathophysiological role of TNFα signalling for the maintenance of intestinal homeostasis and the benefits and difficulties of anti-TNFα treatment during IBD.

RIP Kinases in Liver Cell Death, Inflammation and Cancer

Cell death is intrinsically linked to inflammatory liver disease and cancer development. Recent genetic studies have suggested that receptor-interacting protein kinase (RIPK)1 is implicated in liver disease pathogenesis by regulating caspase-dependent hepatocyte apoptosis induced by tumor necrosis factor (TNF) or other stimuli. In contrast, the contribution of caspase-independent RIPK3/mixed lineage kinase like (MLKL)-mediated hepatocyte necroptosis remains debatable. Hepatocyte apoptosis depends on the balance between RIPK1 prosurvival scaffolding functions and its kinase-activity-mediated proapoptotic function. Several regulatory steps promote the prosurvival role of RIPK1, including phosphorylation and ubiquitination of RIPK1 itself and other molecules involved in RIPK1 signaling. Pharmacological inhibition of liver damage by targeting RIPK1 signaling emerges as a potential therapeutic strategy to prevent chronic liver inflammation and hepatocarcinogenesis.

Chronic intestinal inflammation in mice expressing viral Flip in epithelial cells

Viruses are present in the intestinal microflora and are currently discussed as a potential causative mechanism for the development of inflammatory bowel disease. A number of viruses, such as Human Herpesvirus-8, express homologs to cellular FLIPs, which are major contributors for the regulation of epithelial cell death. In this study we analyzed the consequences of constitutive expression of HHV8-viral FLIP in intestinal epithelial cells (IECs) in mice. Surprisingly, expression of vFlip disrupts tissue homeostasis and induces severe intestinal inflammation. Moreover vFlip^IEC-tg mice showed reduced Paneth cell numbers, associated with excessive necrotic cell death. On a molecular level vFlip expression altered classical and alternative NFκB activation. Blocking of alternative NFκB signaling by deletion of Ikka in vivo largely protected mice from inflammation and Paneth cell loss induced by vFLIP. Collectively, our data provide functional evidence that expression of a single viral protein in IECs can be sufficient to disrupt epithelial homeostasis and to initiate chronic intestinal inflammation.

Klebsiella pneumoniae infection of murine neutrophils impairs their efferocytic clearance by modulating cell death machinery

Neutrophils are the first infiltrating cell type essential for combating pneumoseptic infections by bacterial pathogens including Klebsiella pneumoniae (KPn). Following an infection or injury, removal of apoptotic infiltrates via a highly regulated process called efferocytosis is required for restoration of homeostasis, but little is known regarding the effect of bacterial infection on this process. Here we demonstrate that KPn infection impedes the efferocytic uptake of neutrophils in-vitro and in-vivo in lungs by macrophages. This impaired efferocytosis of infected neutrophils coincides with drastic reduction in the neutrophil surface exposure of apoptosis signature phospholipid phosphatidyserine (PS); and increased activity of phospholipid transporter flippases, which maintain PS in the inner leaflet of plasma membrane. Concomitantly, pharmacological inhibition of flippase activity enhanced PS externalization and restored the efferocytosis of KPn infected neutrophils. We further show that KPn infection interferes with apoptosis activation and instead activates non-apoptotic programmed cell death via activation of necroptosis machinery in neutrophils. Accordingly, pharmacological inhibition of necroptosis by RIPK1 and RIPK3 inhibitors restored the efferocytic uptake of KPn infected neutrophils in-vitro. Importantly, treatment of KPn infected mice with necroptosis inhibitor improved the disease outcome in-vivo in preclinical mouse model of KPn pneumonia. To our knowledge, this is the first report of neutrophil efferocytosis impairment by KPn via modulation of cell death pathway, which may provide novel targets for therapeutic intervention of this infection.

Inflammatory diseases caused by infectious or sterile injuries are often characterized by pathological accumulation of dead or dying infiltrating cells. Pneumonic sepsis caused by Klebsiella pneumoniae (KPn), an opportunistic pathogen, has similar etiology, however, the underlying mechanism remains unknown. Here we report that KPn infection subverts a protective host process termed efferocytosis, by which the phagocytic cells engulf and clear dead/dying cells thereby resolving inflammation and infection. Our results show that KPn infected neutrophils are cleared less efficiently via efferocytosis than the uninfected neutrophils. Mechanistic studies implicated a reduced exposure of “eat me” signal phosphatidyleserine (PS) via increased flippase activity and skewing of cell death pathway toward necroptosis in impaired efferocytosis of infected neutrophils. Accordingly, pharmacological reversal of PS exposure by flippase inhibition, treatment with necroptosis inhibitors restored the efferocytic clearance of KPn infected neutrophils, and improved the disease outcome in a preclinical model of pneumonic sepsis. To our knowledge this is the first report of KPn subversion of efferocytic clearance of neutrophils by impairing pro-efferocytic apoptotic signatures and activation of necroptosis machinery. This could lead to novel therapeutic targets against KPn infection and associated inflammation in pneumonic sepsis.

Differentially Tolerized Mouse Antigen Presenting Cells Share a Common miRNA Signature Including Enhanced mmu-miR-223-3p Expression Which Is Sufficient to Imprint a Protolerogenic State

Dendritic cells (DCs) are pivotal for the induction and maintenance of antigen-specific tolerance and immunity. miRNAs mediate post-transcriptional gene regulation and control in part the differentiation and stimulation-induced immunogenic function of DCs. However, the relevance of miRNAs for the induction and maintenance of a tolerogenic state of DCs has scarcely been highlighted yet. We differentiated mouse bone marrow cells to conventional/myeloid DCs or to tolerogenic antigen presenting cells (APCs) by using a glucocorticoid (dexamethasone) or interleukin-10, and assessed the miRNA expression patterns of unstimulated and LPS-stimulated cell populations by array analysis and QPCR. Differentially tolerized mouse APCs convergingly down-regulated a set of miRNA species at either state of activation as compared with the corresponding control DC population (mmu-miR-9-5p, mmu-miR-9-3p, mmu-miR-155-5p). These miRNAs were also upregulated in control DCs in response to stimulation. In contrast, miRNAs that were convergingly upregulated in both tolerized APC groups at stimulated state (mmu-miR-223-3p, mmu-miR-1224-5p) were downregulated in control DCs in response to stimulation. Overexpression of mmu-miR-223-3p in DCs was sufficient to prevent stimulation-associated acquisition of potent T cell stimulatory capacity. Overexpression of mmu-miR-223-3p in a DC line resulted in attenuated expression of known (Cflar, Rasa1, Ras) mRNA targets of this miRNA species shown to affect pathways that control DC activation. Taken together, we identified sets of miRNAs convergingly regulated in differentially tolerized APCs, which may contribute to imprint stimulation-resistant tolerogenic function as demonstrated for mmu-miR-223-3p. Knowledge of miRNAs with protolerogenic function enables immunotherapeutic approaches aimed to modulate immune responses by regulating miRNA expression.

Intestinal epithelial Caspase-8 signaling is essential to prevent necroptosis during Salmonella Typhimurium induced enteritis

Although induction of host cell death is a pivotal step during bacteria-induced gastroenteritis, the molecular regulation remains to be fully characterized. To expand our knowledge, we investigated the role of the central cell death regulator Caspase-8 in response to Salmonella Typhimurium. Here, we uncovered that intestinal salmonellosis was associated with strong upregulation of members of the host cell death machinery in intestinal epithelial cells (IECs) as an early event, suggesting that elimination of infected IECs represents a host defense strategy. Indeed, Casp8^∆IEC mice displayed severe tissue damage and high lethality after infection. Additional deletion of Ripk3 or Mlkl rescued epithelial cell death and lethality of Casp8^∆IEC mice, demonstrating the crucial role of Caspase-8 as a negative regulator of necroptosis. While Casp8^∆IECTnfr1^-/- mice showed improved survival after infection, tissue destruction was similar to Casp8^∆IEC mice, indicating that necroptosis partially depends on TNF-α signaling. Although there was no impairment in antimicrobial peptide secretion during the early phase of infection, functional Caspase-8 seems to be required to control pathogen colonization. Collectively, these results demonstrate that Caspase-8 is essential to prevent Salmonella Typhimurium induced enteritis and to ensure host survival by two different mechanisms: maintenance of intestinal barrier function and restriction of pathogen colonization.

FLIP as a therapeutic target in cancer

One of the classic hallmarks of cancer is disruption of cell death signalling. Inhibition of cell death promotes tumour growth and metastasis, causes resistance to chemo- and radiotherapies as well as targeted agents, and is frequently due to overexpression of antiapoptotic proteins rather than loss of pro-apoptotic effectors. FLIP is a major apoptosis-regulatory protein frequently overexpressed in solid and haematological cancers, in which its high expression is often correlated with poor prognosis. FLIP, which is expressed as long (FLIP(L)) and short (FLIP(S)) splice forms, achieves its cell death regulatory functions by binding to FADD, a critical adaptor protein which links FLIP to the apical caspase in the extrinsic apoptotic pathway, caspase-8, in a number of cell death regulating complexes, such as the death-inducing signalling complexes (DISCs) formed by death receptors. FLIP also plays a key role (together with caspase-8) in regulating another form of cell death termed programmed necrosis or 'necroptosis', as well as in other key cellular processes that impact cell survival, including autophagy. In addition, FLIP impacts activation of the intrinsic mitochondrial-mediated apoptotic pathway by regulating caspase-8-mediated activation of the pro-apoptotic Bcl-2 family member Bid. It has been demonstrated that FLIP can not only inhibit death receptor-mediated apoptosis, but also cell death induced by a range of clinically relevant chemotherapeutic and targeted agents as well as ionizing radiation. More recently, key roles for FLIP in promoting the survival of immunosuppressive tumour-promoting immune cells have been discovered. Thus, FLIP is of significant interest as an anticancer therapeutic target. In this article, we review FLIP's biology and potential ways of targeting this important tumour and immune cell death regulator.

Inflammatory cell death in intestinal pathologies

The intestinal tract is a site of intense immune cell activity that is poised to mount an effective response against a pathogen and yet maintain tolerance toward commensal bacteria and innocuous dietary antigens. The role of cell death in gut pathologies is particularly important as the intestinal epithelium undergoes self-renewal every 4-7 days through a continuous process of cell death and cell division. Cell death is also required for removal of infected, damaged, and cancerous cells. Certain forms of cell death trigger inflammation through release of damage-associated molecular patterns. Further, molecules involved in cell death decisions also moonlight as critical nodes in immune signaling. The manner of cell death is, therefore, highly instructive of the immunological consequences that ensue. Perturbations in cell death pathways can impact the regulation of the immune system with deleterious consequences. In this review, we discuss the various forms of cell death with a special emphasis on lytic cell death pathways of pyroptosis and necroptosis and their implications in inflammation and cancer in the gut. Understanding the implications of distinct cell death pathways will help in the development of therapeutic interventions in intestinal pathologies.

Blockade of TNF receptor superfamily 1 (TNFR1)-dependent and TNFR1-independent cell death is crucial for normal epidermal differentiation

BACKGROUND

A delicate balance between cell death and keratinocyte proliferation is crucial for normal skin development. Previous studies have reported that cellular FLICE (FADD-like ICE)-inhibitory protein plays a crucial role in prevention of keratinocytes from TNF-α-dependent apoptosis and blocking of dermatitis. However, a role for cellular FLICE-inhibitory protein in TNF-α-independent cell death remains unclear.

OBJECTIVE

We investigated contribution of TNF-α-dependent and TNF-α-independent signals to the development of dermatitis in epidermis-specific Cflar-deficient (Cflar^E-KO) mice.

METHODS

We examined the histology and expression of epidermal differentiation markers and inflammatory cytokines in the skin of Cflar^E-KO;Tnfrsf1a^+/- and Cflar^E-KO;Tnfrsf1a^-/- mice. Mice were treated with neutralizing antibodies against Fas ligand and TNF-related apoptosis-inducing ligand to block TNF-α-independent cell death of Cflar^E-KO;Tnfrsf1a^-/- mice.

RESULTS

Cflar^E-KO;Tnfrsf1a^-/- mice were born but experienced severe dermatitis and succumbed soon after birth. Cflar^E-KO;Tnfrsf1a^+/- mice exhibited embryonic lethality caused by massive keratinocyte apoptosis. Although keratinocytes from Cflar^E-KO;Tnfrsf1a^-/- mice still died of apoptosis, neutralizing antibodies against Fas ligand and TNF-related apoptosis-inducing ligand substantially prolonged survival of Cflar^E-KO;Tnfrsf1a^-/- mice. Expression of inflammatory cytokines, such as Il6 and Il17a was increased; conversely, expression of epidermal differentiation markers was severely downregulated in the skin of Cflar^E-KO;Tnfrsf1a^-/- mice. Treatment of primary keratinocytes with IL-6 and, to a lesser extent, IL-17A suppressed expression of epidermal differentiation markers.

CONCLUSION

TNF receptor superfamily 1 (TNFR1)-dependent or TNFR1-independent apoptosis of keratinocytes promotes inflammatory cytokine production, which subsequently blocks epidermal differentiation. Thus blockade of both TNFR1-dependent and TNFR1-independent cell death might be an alternative strategy to treat skin diseases when treatment with anti-TNF-α antibody alone is not sufficient.

Upregulation of miR‑185 promotes apoptosis of the human gastric cancer cell line MGC803

MicroRNA (miR)-185, which has been reported to be abnormally expressed in some types of cancer, exerts significant effects on the proliferation, apoptosis, drug resistance and metastasis of cancer cells. The present study aimed to explore the effects and underlying molecular mechanisms of miR‑185 upregulation on the apoptosis of gastric cancer (GC) cells. Quantitative polymerase chain reaction (qPCR) and western blotting were used to detect the expression levels of miR‑185 in GC and adjacent normal tissues. In addition, miR‑185 expression was detected in the following GC cell lines: MKN74, SGC7901, BGC823, MGC803, as well as in the gastric epithelial cell line GES‑1. Subsequently, miR‑185 mimics were transfected into MGC803 cells. Post‑transfection, the following experiments were conducted: MTT assay was applied to test cell viability; flow cytometry (FCM) was used to determine the apoptotic rate of the cells; and qPCR and western blotting were conducted to detect the expression levels of the following apoptosis‑associated factors: B‑cell lymphoma 2 (Bcl‑2), Bcl‑2‑associated X protein (Bax), survivin, X‑linked inhibitor of apoptosis protein (XIAP), livin, caspase‑3 and caspase‑8. The results demonstrated that miR‑185 was downregulated in GC tissues compared with the adjacent tissues. In cell lines, miR‑185 expression was higher in GES‑1 cells compared with in the GC cell lines; in the 4 GC cell lines, the strongest miR‑185 expression was in MKN74 cells, followed by SGC7901 and BGC823 cells, and the weakest was in MGC803 cells (P<0.05). Expression of miR‑185 was associated with tumor size, differentiation and lymphatic metastasis. Post-transfection with miR‑185 mimics, miR‑185 expression was significantly increased in a time‑ and concentration‑dependent manner. MGC803 cell viability was significantly decreased following miR‑185 mimics transfection. The results of FCM demonstrated that post‑transfection with miR‑185 mimics, the apoptotic rate of MGC803 cells was significantly increased. Post‑transfection with miR‑185 mimics, the expression levels of Bcl‑2, survivin and XIAP were significantly decreased in MGC803 cells, whereas the expression levels of Bax and livin were not altered, and caspase‑3 and caspase‑8 expression was significantly increased. Spectrophotometry indicated that caspase‑3 and caspase‑8 activity was significantly increased in MGC803 cells following transfection with miR‑185 mimics. In conclusion, the present study suggested that miR‑185 upregulation in GC cells may promote apoptosis of tumor cells via gene expression regulation.

Anti-leukemia activity of NSC-743380 in SULT1A1-expressing acute myeloid leukemia cells is associated with inhibitions of cFLIP expression and PI3K/AKT/mTOR activities

Our recent study showed that acute myeloid leukemia (AML) cells expressing SULT1A1 are highly sensitive to NSC-743380, a small molecule that inhibits STAT3 activity and induces SULT1A1-dependent apoptosis of various cancer cell lines. In this study, we characterized the molecular mechanisms of NSC-743380-mediated anti-leukemia activity in AML cell lines and antileukemia activity of NSC-743380 in patient-derived primary leukemia cells from AML patients. Our results showed that treatment with NSC-743380 triggered robust apoptosis in SULT1A1-positive AML cells. Treatment with NSC-743380 did not increase intracellular reactive oxygen species or change of STAT3 activity in AML cells, but did dramatically and rapidly decrease cFLIP expression. Proteomic analysis with reverse phase protein microarray revealed that treatment of U937 and THP-1 AML cells with NSC-743380 led to drastic and time-dependent suppression of phosphorylation of several key nodes in the PI3K/AKT/mTOR pathway, including AKT and mTOR. Moreover, primary AML cells expressed SULT1A1 were highly sensitive to treatment with NSC-743380, which was not affected by co-culture with bone marrow mesenchymal stem cells. Thus, our results provide proof-of-concept evidence that AML cells expressing SULT1A1 can be targeted by small molecules that induce apoptosis through inhibiting the expression or activities of multiple targets.

Regression of apoptosis-resistant colorectal tumors by induction of necroptosis in mice

He et al. demonstrate that SMAC mimetic is efficient to target caspase-8–deficient colorectal cancer by induction of necroptosis. This study represents an attractive strategy for overcoming apoptosis resistance in colorectal cancer for the development of more effective personalized therapy.

Cancer cells often acquire capabilities to evade cell death induced by current chemotherapeutic treatment approaches. Caspase-8, a central initiator of death receptor–mediated apoptosis, for example, is frequently inactivated in human cancers via multiple mechanisms such as mutation. Here, we show an approach to overcome cell death resistance in caspase-8–deficient colorectal cancer (CRC) by induction of necroptosis. In both a hereditary and a xenograft mouse model of caspase-8–deficient CRC, second mitochondria-derived activator of caspase (SMAC) mimetic treatment induced massive cell death and led to regression of tumors. We further demonstrate that receptor-interacting protein kinase 3 (RIP3), which is highly expressed in mouse models of CRC and in a subset of human CRC cell lines, is the deciding factor of cancer cell susceptibility to SMAC mimetic–induced necroptosis. Thus, our data implicate that it may be worthwhile to selectively evaluate the efficacy of SMAC mimetic treatment in CRC patients with caspase-8 deficiency in clinical trials for the development of more effective personalized therapy.

Enhancement of Drug Sensitivity by Knockdown of HIF-1α in Gastric Carcinoma Cells

In this study, the effects of hypoxia-inducible factor-1α (HIF-1α) on gastric carcinoma (GC) drug resistance through apoptosis-related genes are investigated. First, HIF-1α-specific siRNA was synthetized and transfected into drug-resistant GC cell line OCUM-2MD3/L-OHP. Then MTT assay was applied to test the inhibition rate of GC cells by 5-fluorouracil (5-FU) and oxaliplatin (L-OHP). After that, flow cytometry (FCM) was applied to measure apoptosis rate. qPCR and Western blot assay were employed to detect HIF-1α and apoptosis-related genes. Results showed that HIF-1α in OCUM-2MD3/L-OHP cells was higher than that in OCUM-2MD3 and gastric epithelial cells. After HIF-1α-siRNA transfection, inhibition rates of 5-FU and L-OHP to tumor cells increased significantly. FCM results showed that apoptosis rate of OCUM-2MD3/L-OHP cells increased significantly. After HIF-1α-siRNA transfection, survivin and Bcl-2 decreased, whereas Bax, caspase 3, and caspase 8 increased significantly. Results from this study seem to confirm that HIF-1α getting involved in GC drug resistance is possibly due to its regulation of some apoptosis-related genes. HIF-1α may be a potential target to reverse drug resistance of GC.

Regulation of antiapoptotic and cytoprotective pathways in colonic epithelial cells in ulcerative colitis

Ulcerative colitis is an inflammatory bowel disease involving the colon resulting in bloody diarrhea and increased risk of colorectal cancer in certain patient subgroups. Increased apoptosis in the epithelial cell layer causes increased permeability, especially during flares; this leads to translocation of luminal pathogens resulting in a continued inflammatory drive. The present work investigates how epithelial apoptosis is regulated in ulcerative colitis. The main results are that Fas mediated apoptosis is inhibited during flares of ulcerative colitis, probably by an upregulation of cellular inhibitor of apoptosis protein 2 (cIAP2) and cellular FLICE-like inhibitory protein. cIAP2 is upregulated in regenerative epithelial cells both in ulcerative colitis and in experimental intestinal wounds. Inhibition of cIAP2 decreases wound healing in vitro possibly through inhibition of migration. Altogether, it is shown that epithelial cells in ulcerative colitis responds to the hostile microenvironment by activation of cytoprotective pathways that tend to counteract the cytotoxic effects of inflammation. However, the present studies also show that epithelial cells produce increased amounts of reactive oxygen species during stimulation with tumor necrosis factor-α and interferon-γ resulting in DNA instability. The combined effect of increased DNA-instability and decreased apoptosis responses could lead to neoplasia.

Intrinsic Defense Mechanisms of the Intestinal Epithelium

The intestinal epithelium is a single cell layer that facilitates the absorption of nutrients but also provides a tight barrier to prevent pathogen invasion and dissemination of commensal microbes. Specialized epithelial cells of the gastrointestinal tract achieve this frontline defense by working in concert with lymphoid, myeloid, and stromal cells to secrete an array of factors that limit direct contact between the epithelium and infectious agents. The importance of these mechanisms is underscored by the ability of enteric pathogens to target these mechanisms to achieve invasion and dissemination. This review highlights recent advances in our understanding of these intricate molecular and cellular mechanisms adopted by these cells to promote spatial segregation and barrier maintenance.

Crosstalk between microbiota, pathogens and the innate immune responses

Research in the last decade has convincingly demonstrated that the microbiota is crucial in order to prime and orchestrate innate and adaptive immune responses of their host and influence barrier function as well as multiple developmental and metabolic parameters of the host. Reciprocally, host reactions and immune responses instruct the composition of the microbiota. This review summarizes recent evidence from experimental and human studies which supports these arms of mutual relationship and crosstalk between host and resident microbiota, with a focus on innate immune responses in the gut, the role of cell death pathways and antimicrobial peptides. We also provide some recent examples on how dysbiosis and pathogens can act in concert to promote intestinal infection, inflammatory pathologies and cancer. The future perspectives of these combined research efforts include the discovery of protective species within the microbiota and specific traits and factors of microbes that weaken or enforce host intestinal homeostasis.

Inhibition of Vav3 gene can promote apoptosis of human gastric cancer cell line MGC803 by regulating ERK pathway

Previous studies proved that Vav3 gene was overexpressed in cancers. However, the molecular mechanism of Vav3 in apoptosis still keeps unclear; therefore, the relationship between Vav3 gene and apoptosis of gastric cancer (GC) was explored in the present study. Vav3-siRNA was transfected into MGC803 cells, and then cell activity and apoptosis rate were tested with MTT and FCM; apoptosis-related genes and proteins in MAPK signaling pathway were also tested. Results showed that Vav3 was overexpressed in GC than in adjacent normal tissues (all P < 0.05), and expression of Vav3 was related to degree of histological differentiation, cancer invasion depth, and lymphatic metastasis (Χ (2)  = 7.185, P = 0.007; Χ (2)  = 18.654, P < 0.001; Χ (2)  = 5.058, P = 0.025). Vav3 silencing inhibited activity of MGC803 cells, and apoptosis rate of cells was affected. Vav3-siRNA transfection led to changes of apoptosis-related genes such as Survivin, xIAP, Bcl-2, caspase-3, and Bax (all P < 0.01). After transfection, ratio of phosphorylation of ERK significantly reduced. We concluded that Vav3 inhibition can suppress cell activity and promote apoptosis by regulating the apoptosis-related genes through the ERK pathway.

FLIP the Switch: Regulation of Apoptosis and Necroptosis by cFLIP

cFLIP (cellular FLICE-like inhibitory protein) is structurally related to caspase-8 but lacks proteolytic activity due to multiple amino acid substitutions of catalytically important residues. cFLIP protein is evolutionarily conserved and expressed as three functionally different isoforms in humans (cFLIP_L, cFLIP_S, and cFLIP_R). cFLIP controls not only the classical death receptor-mediated extrinsic apoptosis pathway, but also the non-conventional pattern recognition receptor-dependent apoptotic pathway. In addition, cFLIP regulates the formation of the death receptor-independent apoptotic platform named the ripoptosome. Moreover, recent studies have revealed that cFLIP is also involved in a non-apoptotic cell death pathway known as programmed necrosis or necroptosis. These functions of cFLIP are strictly controlled in an isoform-, concentration- and tissue-specific manner, and the ubiquitin-proteasome system plays an important role in regulating the stability of cFLIP. In this review, we summarize the current scientific findings from biochemical analyses, cell biological studies, mathematical modeling, and gene-manipulated mice models to illustrate the critical role of cFLIP as a switch to determine the destiny of cells among survival, apoptosis, and necroptosis.

The endogenous caspase-8 inhibitor c-FLIPL regulates ER morphology and crosstalk with mitochondria

Components of the death receptor-mediated pathways like caspase-8 have been identified in complexes at intracellular membranes to spatially restrict the processing of local targets. In this study, we report that the long isoform of the cellular FLICE-inhibitory protein (c-FLIP(L)), a well-known inhibitor of the extrinsic cell death initiator caspase-8, localizes at the endoplasmic reticulum (ER) and mitochondria-associated membranes (MAMs). ER morphology was disrupted and ER Ca(2+)-release as well as ER-mitochondria tethering was decreased in c-FLIP(-/-) mouse embryonic fibroblasts (MEFs). Mechanistically, c-FLIP ablation resulted in enhanced basal caspase-8 activation and in caspase-mediated processing of the ER-shaping protein reticulon-4 (RTN4) that was corrected by re-introduction of c-FLIP(L) and caspase inhibition, resulting in the recovery of a normal ER morphology and ER-mitochondria juxtaposition. Thus, the caspase-8 inhibitor c-FLIP(L) emerges as a component of the MAMs signaling platforms, where caspases appear to regulate ER morphology and ER-mitochondria crosstalk by impinging on ER-shaping proteins like the RTN4.

Caspase-8 controls the gut response to microbial challenges by Tnf-α-dependent and independent pathways

OBJECTIVES

Intestinal epithelial cells (IEC) express toll-like receptors (TLR) that facilitate microbial recognition. Stimulation of TLR ligands induces a transient increase in epithelial cell shedding, a mechanism that serves the antibacterial and antiviral host defence of the epithelium and promotes elimination of intracellular pathogens. Although activation of the extrinsic apoptosis pathway has been described during inflammatory shedding, its functional involvement is currently unclear.

DESIGN

We investigated the functional involvement of caspase-8 signalling in microbial-induced intestinal cell shedding by injecting Lipopolysaccharide (LPS) to mimic bacterial pathogens and poly(I:C) as a probe for RNA viruses in vivo.

RESULTS

TLR stimulation of IEC was associated with a rapid activation of caspase-8 and increased epithelial cell shedding. In mice with an epithelial cell-specific deletion of caspase-8 TLR stimulation caused Rip3-dependent epithelial necroptosis instead of apoptosis. Mortality and tissue damage were more severe in mice in which IECs died by necroptosis than apoptosis. Inhibition of receptor-interacting protein (Rip) kinases rescued the epithelium from TLR-induced gut damage. TLR3-induced necroptosis was directly mediated via TRIF-dependent pathways, independent of Tnf-α and type III interferons, whereas TLR4-induced tissue damage was critically dependent on Tnf-α.

CONCLUSIONS

Together, our data demonstrate an essential role for caspase-8 in maintaining the gut barrier in response to mucosal pathogens by permitting inflammatory shedding and preventing necroptosis of infected cells. These data suggest that therapeutic strategies targeting the cell death machinery represent a promising new option for the treatment of inflammatory and infective enteropathies.

Withanolide E sensitizes renal carcinoma cells to TRAIL-induced apoptosis by increasing cFLIP degradation

Withanolide E, a steroidal lactone from Physalis peruviana, was found to be highly active for sensitizing renal carcinoma cells and a number of other human cancer cells to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis. Withanolide E, the most potent and least toxic of five TRAIL-sensitizing withanolides identified, enhanced death receptor-mediated apoptotic signaling by a rapid decline in the levels of cFLIP proteins. Other mechanisms by which TRAIL sensitizers have been reported to work: generation of reactive oxygen species (ROS), changes in pro-and antiapoptotic protein expression, death receptor upregulation, activation of intrinsic (mitochondrial) apoptotic pathways, ER stress, and proteasomal inhibition proved to be irrelevant to withanolide E activity. Loss of cFLIP proteins was not due to changes in expression, but rather destabilization and/or aggregation, suggesting impairment of chaperone proteins leading to degradation. Indeed, withanolide E treatment altered the stability of a number of HSP90 client proteins, but with greater apparent specificity than the well-known HSP90 inhibitor geldanamycin. As cFLIP has been reported to be an HSP90 client, this provides a potentially novel mechanism for sensitizing cells to TRAIL. Sensitization of human renal carcinoma cells to TRAIL-induced apoptosis by withanolide E and its lack of toxicity were confirmed in animal studies. Owing to its novel activity, withanolide E is a promising reagent for the analysis of mechanisms of TRAIL resistance, for understanding HSP90 function, and for further therapeutic development. In marked contrast to bortezomib, among the best currently available TRAIL sensitizers, withanolide E's more specific mechanism of action suggests minimal toxic side effects.

Necroptosis and its role in inflammation

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