Paving TRAIL's Path with Ubiquitin

Arginine methylation of caspase-8 controls life/death decisions in extrinsic apoptotic networks

Procaspase-8 is a key mediator of death receptor (DR)-mediated pathways. Recently, the role of post-translational modifications (PTMs) of procaspase-8 in controlling cell death has received increasing attention. Here, using mass spectrometry screening, pharmacological inhibition and biochemical assays, we show that procaspase-8 can be targeted by the PRMT5/RIOK1/WD45 methylosome complex. Furthermore, two potential methylation sites of PRMT5 on procaspase-8, R233 and R435, were identified in silico. R233 and R435 are highly conserved in mammals and their point mutations are among the most common mutations of caspase-8 in cancer. The introduction of mutations at these positions resulted in inhibitory effects on CD95L-induced caspase-8 activity, effector caspase activation and apoptosis. In addition, we show that procaspase-8 can undergo symmetric di-methylation. Finally, the pharmacological inhibition of PRMT5 resulted in the inhibitory effects on caspase activity and apoptotic cell death. Taken together, we have unraveled the additional control checkpoint in procaspase-8 activation and the arginine methylation network in the extrinsic apoptosis pathway.

Oxidative Stress in Sepsis: A Focus on Cardiac Pathology

This study aims to analyze post-mortem human cardiac specimens, to verify and evaluate the existence or extent of oxidative stress in subjects whose cause of death has been traced to sepsis, through immunohistological oxidative/nitrosative stress markers. Indeed, in the present study, i-NOS, NOX2, and nitrotyrosine markers were higher expressed in the septic death group when compared to the control group, associated with also a significant increase in 8-OHdG, highlighting the pivotal role of oxidative stress in septic etiopathogenesis. In particular, 70% of cardiomyocyte nuclei from septic death specimens showed positivity for 8-OHdG. Furthermore, intense and massive NOX2-positive myocyte immunoreaction was noticed in the septic group, as nitrotyrosine immunostaining intense reaction was found in the cardiac cells. These results demonstrated a correlation between oxidative and nitrosative stress imbalance and the pathophysiology of cardiac dysfunction documented in cases of sepsis. Therefore, subsequent studies will focus on the expression of oxidative stress markers in other organs and tissues, as well as on the involvement of the intracellular pattern of apoptosis, to better clarify the complex pathogenesis of multi-organ failure, leading to support the rationale for including therapies targeting redox abnormalities in the management of septic patients.

The E3 ubiquitin ligase Itch regulates death receptor and cholesterol trafficking to affect TRAIL-mediated apoptosis

The activation of apoptosis signalling by TRAIL (TNF-related apoptosis-inducing ligand) through receptor binding is a fundamental mechanism of cell death induction and is often perturbed in cancer cells to enhance their cell survival and treatment resistance. Ubiquitination plays an important role in the regulation of TRAIL-mediated apoptosis, and here we investigate the role of the E3 ubiquitin ligase Itch in TRAIL-mediated apoptosis in oesophageal cancer cells. Knockdown of Itch expression results in resistance to TRAIL-induced apoptosis, caspase-8 activation, Bid cleavage and also promotes cisplatin resistance. Whilst the assembly of the death-inducing signalling complex (DISC) at the plasma membrane is not perturbed relative to the control, TRAIL-R2 is mis-localised in the Itch-knockdown cells. Further, we observe significant changes to mitochondrial morphology alongside an increased cholesterol content. Mitochondrial cholesterol is recognised as an important anti-apoptotic agent in cancer. Cells treated with a drug that increases mitochondrial cholesterol levels, U18666A, shows a protection from TRAIL-induced apoptosis, reduced caspase-8 activation, Bid cleavage and cisplatin resistance. We demonstrate that Itch knockdown cells are less sensitive to a Bcl-2 inhibitor, show impaired activation of Bax, cytochrome c release and an enhanced stability of the cholesterol transfer protein STARD1. We identify a novel protein complex composed of Itch, the mitochondrial protein VDAC2 and STARD1. We propose a mechanism where Itch regulates the stability of STARD1. An increase in STARD1 expression enhances cholesterol import to mitochondria, which inhibits Bax activation and cytochrome c release. Many cancer types display high mitochondrial cholesterol levels, and oesophageal adenocarcinoma tumours show a correlation between chemotherapy resistance and STARD1 expression which is supported by our findings. This establishes an important role for Itch in regulation of extrinsic and intrinsic apoptosis, mitochondrial cholesterol levels and provides insight to mechanisms that contribute to TRAIL, Bcl-2 inhibitor and cisplatin resistance in cancer cells.

Harnessing TRAIL-induced cell death for cancer therapy: a long walk with thrilling discoveries

Tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL) can induce apoptosis in a wide variety of cancer cells, both in vitro and in vivo, importantly without killing any essential normal cells. These findings formed the basis for the development of TRAIL-receptor agonists (TRAs) for cancer therapy. However, clinical trials conducted with different types of TRAs have, thus far, afforded only limited therapeutic benefit, as either the respectively chosen agonist showed insufficient anticancer activity or signs of toxicity, or the right TRAIL-comprising combination therapy was not employed. Therefore, in this review we will discuss molecular determinants of TRAIL resistance, the most promising TRAIL-sensitizing agents discovered to date and, importantly, whether any of these could also prove therapeutically efficacious upon cancer relapse following conventional first-line therapies. We will also discuss the more recent progress made with regards to the clinical development of highly active non-immunogenic next generation TRAs. Based thereupon, we next propose how TRAIL resistance might be successfully overcome, leading to the possible future development of highly potent, cancer-selective combination therapies that are based on our current understanding of biology TRAIL-induced cell death. It is possible that such therapies may offer the opportunity to tackle one of the major current obstacles to effective cancer therapy, namely overcoming chemo- and/or targeted-therapy resistance. Even if this were achievable only for certain types of therapy resistance and only for particular types of cancer, this would be a significant and meaningful achievement.

Polyethyleneglycol-Betulinic Acid (PEG-BA) Polymer-Drug Conjugate Induces Apoptosis and Antioxidation in a Biological Model of Pancreatic Cancer

Pancreatic cancer (PC) is one of the most aggressive solid malignancies with poor treatment response and low survival rates. Herbal medicines such as betulinic acid (BA) have shown potential in treating various solid tumours, but with limitations that can be circumvented by polymer-drug conjugation. Polyethylene glycol-BA (PEG-BA) polymer-drug conjugate has previously shown selective anticancer activity against PC cells. Here, we elucidate the mechanism of cell death and the cell death pathway, anti-inflammatory and antioxidant activities of PEG-BA. PEG-BA induced apoptotic cell death by arresting MIA-PaCa-2 cells in the Sub-G1 phase of the cell cycle compared with BA and untreated cells (39.50 ± 5.32% > 19.63 ± 4.49% > 4.57 ± 0.82%). NFκB/p65 protein expression was moderately increased by PEG-BA (2.70 vs. 3.09 ± 0.42 ng/mL; p = 0.1521). However, significant (p < 0.05) overexpression of the proapoptotic genes TNF (23.72 ± 1.03) and CASPASE 3 (12,059.98 ± 1.74) compared with untreated cells was notable. The antioxidant potential of PEG-BA was greater (IC50 = 15.59 ± 0.64 µM) compared with ascorbic acid (25.58 ± 0.44 µM) and BA-only (>100 µM) and further confirmed with the improved reduction of hydroperoxide levels compared with BA-only (518.80 ± 25.53 µM vs. 542.43 ± 9.70 µM). In conclusion, PEG-BA activated both the intrinsic and extrinsic pathways of apoptosis and improved antioxidant activities in PC cells, suggesting enhanced anticancer activity upon conjugation.

Ovarian tumor deubiquitinase 6A regulates cell proliferation via deubiquitination of nucleolin and caspase‑7

Most proteins maintain protein homeostasis via post‑translational modifications, including the ubiquitin‑proteasome system. Deubiquitinating enzymes (DUBs) have essential intercellular roles, such as responses to DNA damage, proteolysis and apoptosis. Therefore, it is important to understand DUB‑related diseases to identify DUBs that target abnormally regulated proteins in cells. Ovarian tumor deubiquitinase 6A (OTUD6A) was previously reported as a downregulated DUB in HCT116 cells with p53 knockdown. Therefore, it was expected that the relationship between OTUD6A and p53 would affect cell proliferation. In the present study, putative substrates of OTUD6A related to the p53 signaling pathway were identified. Application of liquid chromatography‑tandem mass spectrometry and proteomic analysis led to the identification of nucleolin (known to bind p53) as a binding protein. In addition, immunoprecipitation studies determined that caspase‑7, an apoptotic protein, is associated with p53 signaling and is regulated by OTUD6A. It was further identified that OTUD6A regulates the protein stability of nucleolin, but not caspase‑7. It was also demonstrated that OTUD6A acts as a respective DUB through the deubiquitination of K48‑linked polyubiquitin chain of nucleolin and the K63‑linked polyubiquitin chain of caspase‑7. Furthermore, overexpression of OTUD6A induced cell proliferation via enhancing cell cycle progression of MCF7 cells. Taken together, OTUD6A may be proposed as a target for anticancer therapy.

Lewis glycosphingolipids as critical determinants of TRAIL sensitivity in cancer cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces cancer cell death and contributes to tumor rejection by cytotoxic lymphocytes in cancer immunosurveillance and immunotherapy. TRAIL and TRAIL receptor agonists have garnered wide popularity as promising agents for cancer therapy. We previously demonstrated that the loss of fucosylation in cancer cells impairs TRAIL sensitivity; however, the precise structures of the fucosylated glycans that regulate TRAIL sensitivity and their carrier molecules remain elusive. Herein, we observed that Lewis glycans among various fucosylated glycans positively regulate TRAIL-induced cell death. Specifically, Lewis glycans on lacto/neolacto glycosphingolipids, but not glycoproteins including TRAIL receptors, enhanced TRAIL-induced formation of the cytosolic caspase 8 complex, without affecting the formation of the membranous receptor complex. Furthermore, type I Lewis glycan expression in colon cancer cell lines and patient-derived cancer organoids was positively correlated with TRAIL sensitivity. These findings provide novel insights into the regulatory mechanism of TRAIL-induced cell death and facilitate the identification of novel predictive biomarkers for TRAIL-related cancer therapies in future.

Cardiomyocyte death in sepsis: Mechanisms and regulation (Review)

Sepsis‑induced cardiac dysfunction is one of the most common types of organ dysfunction in sepsis; its pathogenesis is highly complex and not yet fully understood. Cardiomyocytes serve a key role in the pathophysiology of cardiac function; due to the limited ability of cardiomyocytes to regenerate, their loss contributes to decreased cardiac function. The activation of inflammatory signalling pathways affects cardiomyocyte function and modes of cardiomyocyte death in sepsis. Prevention of cardiomyocyte death is an important therapeutic strategy for sepsis‑induced cardiac dysfunction. Thus, understanding the signalling pathways that activate cardiomyocyte death and cross‑regulation between death modes are key to finding therapeutic targets. The present review focused on advances in understanding of sepsis‑induced cardiomyocyte death pathways, including apoptosis, necroptosis, mitochondria‑mediated necrosis, pyroptosis, ferroptosis and autophagy. The present review summarizes the effect of inflammatory activation on cardiomyocyte death mechanisms, the diversity of regulatory mechanisms and cross‑regulation between death modes and the effect on cardiac function in sepsis to provide a theoretical basis for treatment of sepsis‑induced cardiac dysfunction.

Proscillaridin A Sensitizes Human Colon Cancer Cells to TRAIL-Induced Cell Death

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytotoxic cytokine that induces cancer cell death by binding to TRAIL receptors. Because of its selective cytotoxicity toward cancer cells, TRAIL therapeutics, such as recombinant TRAIL and agonistic antibodies targeting TRAIL receptors, have garnered attention as promising cancer treatment agents. However, many cancer cells acquire resistance to TRAIL-induced cell death. To overcome this issue, we searched for agents to sensitize cancer cells to TRAIL-induced cell death by screening a small-molecule chemical library consisting of diverse compounds. We identified a cardiac glycoside, proscillaridin A, as the most effective TRAIL sensitizer in colon cancer cells. Proscillaridin A synergistically enhanced TRAIL-induced cell death in TRAIL-sensitive and -resistant colon cancer cells. Additionally, proscillaridin A enhanced cell death in cells treated with TRAIL and TRAIL sensitizer, the second mitochondria-derived activator of caspase mimetic. Proscillaridin A upregulated TRAIL receptor expression, while downregulating the levels of the anti-cell death molecules, cellular FADD-like IL-1β converting enzyme-like inhibitor protein and Mcl1, in a cell type-dependent manner. Furthermore, proscillaridin A enhanced TRAIL-induced cell death partly via O-glycosylation. Taken together, our findings suggest that proscillaridin A is a promising agent that enhances the anti-cancer efficacy of TRAIL therapeutics.

Therapeutic approaches targeting CD95L/CD95 signaling in cancer and autoimmune diseases

Cell death plays a pivotal role in the maintenance of tissue homeostasis. Key players in the controlled induction of cell death are the Death Receptors (DR). CD95 is a prototypic DR activated by its cognate ligand CD95L triggering programmed cell death. As a consequence, alterations in the CD95/CD95L pathway have been involved in several disease conditions ranging from autoimmune diseases to inflammation and cancer. CD95L-induced cell death has multiple roles in the immune response since it constitutes one of the mechanisms by which cytotoxic lymphocytes kill their targets, but it is also involved in the process of turning off the immune response. Furthermore, beyond the canonical pro-death signals, CD95L, which can be membrane-bound or soluble, also induces non-apoptotic signaling that contributes to its tumor-promoting and pro-inflammatory roles. The intent of this review is to describe the role of CD95/CD95L in the pathophysiology of cancers, autoimmune diseases and chronic inflammation and to discuss recently patented and emerging therapeutic strategies that exploit/block the CD95/CD95L system in these diseases.

The role of Ubiquitination in Apoptosis and Necroptosis

Cell death pathways have evolved to maintain tissue homoeostasis and eliminate potentially harmful cells from within an organism, such as cells with damaged DNA that could lead to cancer. Apoptosis, known to eliminate cells in a predominantly non-inflammatory manner, is controlled by two main branches, the intrinsic and extrinsic apoptotic pathways. While the intrinsic pathway is regulated by the Bcl-2 family members, the extrinsic pathway is controlled by the Death receptors, members of the tumour necrosis factor (TNF) receptor superfamily. Death receptors can also activate a pro-inflammatory type of cell death, necroptosis, when Caspase-8 is inhibited. Apoptotic pathways are known to be tightly regulated by post-translational modifications, especially by ubiquitination. This review discusses research on ubiquitination-mediated regulation of apoptotic signalling. Additionally, the emerging importance of ubiquitination in regulating necroptosis is discussed.

TRAIL Conjugated Silver Nanoparticle Synthesis, Characterization and Therapeutic Effects on HT-29 Colon Cancer Cells

Colon cancer is one of the most prominent causes of cancer-related morbidity and mortality and curable if detected in the early stages. TNF-related apoptosis-inducing ligand (TRAIL) is a therapeutic protein and has a potential anti-cancer activity that is widely used for the treatment of several cancers. In this study, we aimed to develop a silver nanoparticle system conjugated with TRAIL and coated with PEG (AgCTP NPs) to improve the therapeutic effects of colon cancer. AgCTP NPs were characterized by UV spectrum, FTIR and zetasizer. Cytotoxicity, hemolysis assay and apoptotic effects of nanoparticles were investigated using a colon cancer cell line (HT-29) in-vitro. Treatment with AgCTP NPs effectively inhibited proliferation and colony formation of HT-29 cells. The apoptotic effects of nanoparticles on HT-29 cells were determined as Bax, Bcl-2, PARP and clv-PARP protein expression levels using Western blot. Apoptotic proteins were upregulated by AgCTP NPs. In this study, we demonstrated that AgCTP NPs had an anti-cancer effect by activating cell death. Thus, we have confirmed that silver nanoparticles can be selected as a good carrier for TRAIL therapeutic proteins that can be used to treat colon cancer.

Regulation of Cancer Metabolism by Deubiquitinating Enzymes: The Warburg Effect

Cancer is a disorder of cell growth and proliferation, characterized by different metabolic pathways within normal cells. The Warburg effect is a major metabolic process in cancer cells that affects the cellular responses, such as proliferation and apoptosis. Various signaling factors down/upregulate factors of the glycolysis pathway in cancer cells, and these signaling factors are ubiquitinated/deubiquitinated via the ubiquitin-proteasome system (UPS). Depending on the target protein, DUBs act as both an oncoprotein and a tumor suppressor. Since the degradation of tumor suppressors and stabilization of oncoproteins by either negative regulation by E3 ligases or positive regulation of DUBs, respectively, promote tumorigenesis, it is necessary to suppress these DUBs by applying appropriate inhibitors or small molecules. Therefore, we propose that the DUBs and their inhibitors related to the Warburg effect are potential anticancer targets.

Selective HSP90β inhibition results in TNF and TRAIL mediated HIF1α degradation

Signaling via TNF-R1 mediates pleiotropic biological outcomes ranging from inflammation and proliferation to cell death. Previous reports demonstrated that pro-survival signaling emanates from membrane resident TNF-R1 complexes (complex I) while only internalized TNF-R1 complexes are capable for DISC formation (complex II) and thus, apoptosis induction. Internalized TNF-R1 containing endosomes undergo intracellular maturation towards lysosomes, resulting in activation and release of Cathepsin D (CtsD) into the cytoplasm. We recently revealed HSP90 as target for proteolytic cleavage by CtsD, resulting in cell death amplification. In this study, we show that extrinsic cell death activation via TNF or TRAIL results in HSP90β degradation. Co-incubation of cells with either TNF or TRAIL in combination with the HSP90β inhibitor KUNB105 but not HSP90α selective inhibition promotes apoptosis induction. In an attempt to reveal further downstream targets of combined TNF-R1 or TRAIL-R1/-R2 activation with HSP90β inhibition, we identify HIF1α and validate its ligand:inhibitor triggered degradation. Together, these findings suggest that selective inhibition of HSP90 isoforms together with death ligand stimulation may provide novel strategies for therapy of inflammatory diseases or cancer, in future.

Ischemic Preconditioning Upregulates Decoy Receptors to Protect SH-SY5Y Cells from OGD Induced Cellular Damage by Inhibiting TRAIL Pathway and Agitating PI3K/Akt Pathway

As ischemic preconditioning (IPC) represents a potential therapy against cerebral ischemia, the purpose of the present study is to explore the molecular mechanisms of ischemic preconditioning induced cerebral protective effect. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor superfamily, which induces apoptosis through binding to its death receptors (DR4 and DR5). When TRAIL binds to decoy receptors (DcR1 and DcR2), as DcRs lack intact cytoplasmic death domain, TRAIL fails to induce neuronal apoptosis. In the present study, we demonstrated that ischemic preconditioning upregulated DcR1 and DcR2, which subsequently inhibited oxygen glucose deprivation-induced cellular apoptosis. Then, we investigated the protective molecular mechanism of DcRs after ischemic preconditioning treatment. Results showed that DcR1 could competitively bind to TRAIL and partially inhibit TRAIL-induced cellular apoptosis. On the other hand, DcR2 could disturb DRs-associated death-inducing signaling complex formation (DISC), which further inhibited capase-8 activation. Besides, we also found that ischemic preconditioning activated IPC-induced Akt phosphorylation via regulating DcR2 level. Thus, ischemic preconditioning upregulated decoy receptors, which protected cells from oxygen glucose deprivation-induced cellular damage by inhibiting TRAIL-induced apoptosis and agitating PI3K/Akt pathway. Our data complemented the knowledge of neuroprotective mechanism of ischemic preconditioning and provided new evidence for supporting its clinical application.

Role of Extrinsic Apoptotic Signaling Pathway during Definitive Erythropoiesis in Normal Patients and in Patients with β-Thalassemia

Apoptosis is a process of programmed cell death which has an important role in tissue homeostasis and in the control of organism development. Here, we focus on information concerning the role of the extrinsic apoptotic pathway in the control of human erythropoiesis. We discuss the role of tumor necrosis factor α (TNFα), tumor necrosis factor ligand superfamily member 6 (FasL), tumor necrosis factor-related apoptosis-inducing (TRAIL) and caspases in normal erythroid maturation. We also attempt to initiate a discussion on the observations that mature erythrocytes contain most components of the receptor-dependent apoptotic pathway. Finally, we point to the role of the extrinsic apoptotic pathway in ineffective erythropoiesis of different types of β-thalassemia.

Stress Management: Death Receptor Signalling and Cross-Talks with the Unfolded Protein Response in Cancer

Throughout tumour progression, tumour cells are exposed to various intense cellular stress conditions owing to intrinsic and extrinsic cues, to which some cells are remarkably able to adapt. Death Receptor (DR) signalling and the Unfolded Protein Response (UPR) are two stress responses that both regulate a plethora of outcomes, ranging from proliferation, differentiation, migration, cytokine production to the induction of cell death. Both signallings are major modulators of physiological tissue homeostasis and their dysregulation is involved in tumorigenesis and the metastastic process. The molecular determinants of the control between the different cellular outcomes induced by DR signalling and the UPR in tumour cells and their stroma and their consequences on tumorigenesis are starting to be unravelled. Herein, I summarize the main steps of DR signalling in relation to its cellular and pathophysiological roles in cancer. I then highlight how the UPR and DR signalling control common cellular outcomes and also cross-talk, providing potential opportunities to further understand the development of malignancies.

The SCFSkp2 ubiquitin ligase complex modulates TRAIL-R2-induced apoptosis by regulating FLIP(L)

TRAIL-R2 (DR5) is a clinically-relevant therapeutic target and a key target for immune effector cells. Herein, we identify a novel interaction between TRAIL-R2 and the Skp1-Cullin-1-F-box (SCF) Cullin-Ring E3 Ubiquitin Ligase complex containing Skp2 (SCF^Skp2). We find that SCF^Skp2 can interact with both TRAIL-R2’s pre-ligand association complex (PLAC) and ligand-activated death-inducing signalling complex (DISC). Moreover, Cullin-1 interacts with TRAIL-R2 in its active NEDDylated form. Inhibiting Cullin-1’s DISC recruitment using the NEDDylation inhibitor MLN4924 (Pevonedistat) or siRNA increased apoptosis induction in response to TRAIL. This correlated with enhanced levels of the caspase-8 regulator FLIP at the TRAIL-R2 DISC, particularly the long splice form, FLIP(L). We subsequently found that FLIP(L) (but not FLIP(S), caspase-8, nor the other core DISC component FADD) interacts with Cullin-1 and Skp2. Importantly, this interaction is enhanced when FLIP(L) is in its DISC-associated, C-terminally truncated p43-form. Prevention of FLIP(L) processing to its p43-form stabilises the protein, suggesting that by enhancing its interaction with SCF^Skp2, cleavage to the p43-form is a critical step in FLIP(L) turnover. In support of this, we found that silencing any of the components of the SCF^Skp2 complex inhibits FLIP ubiquitination, while overexpressing Cullin-1/Skp2 enhances its ubiquitination in a NEDDylation-dependent manner. DISC recruitment of TRAF2, previously identified as an E3 ligase for caspase-8 at the DISC, was also enhanced when Cullin-1’s recruitment was inhibited, although its interaction with Cullin-1 was found to be mediated indirectly via FLIP(L). Notably, the interaction of p43-FLIP(L) with Cullin-1 disrupts its ability to interact with FADD, caspase-8 and TRAF2. Collectively, our results suggest that processing of FLIP(L) to p43-FLIP(L) at the TRAIL-R2 DISC enhances its interaction with co-localised SCF^Skp2, leading to disruption of p43-FLIP(L)’s interactions with other DISC components and promoting its ubiquitination and degradation, thereby modulating TRAIL-R2-mediated apoptosis.

Controlling Cell Death through Post-translational Modifications of DED Proteins

Apoptosis is a form of programmed cell death, deregulation of which occurs in multiple disorders, including neurodegenerative and autoimmune diseases as well as cancer. The formation of a death-inducing signaling complex (DISC) and death effector domain (DED) filaments are critical for initiation of the extrinsic apoptotic pathway. Post-translational modifications (PTMs) of DED-containing DISC components such as FADD, procaspase-8, and c-FLIP comprise an additional level of apoptosis regulation, which is necessary to overcome the threshold for apoptosis induction. In this review we discuss the influence of PTMs of FADD, procaspase-8, and c-FLIP on DED filament assembly and cell death induction, with a focus on the 3D organization of the DED filament.

TRAIL in oncology: From recombinant TRAIL to nano- and self-targeted TRAIL-based therapies

TNF-related apoptosis-inducing ligand (TRAIL) selectively induces the apoptosis pathway in tumor cells leading to tumor cell death. Because TRAIL induction can kill tumor cells, cancer researchers have developed many agents to target TRAIL and some of these agents have entered clinical trials in oncology. Unfortunately, these trials have failed for many reasons, including drug resistance, off-target toxicities, short half-life, and specifically in gene therapy due to the limited uptake of TRAIL genes by cancer cells. To address these drawbacks, translational researchers have utilized drug delivery platforms. Although, these platforms can improve TRAIL-based therapies, they are unable to sufficiently translate the full potential of TRAIL-targeting to clinically viable products. Herein, we first summarize the complex biology of TRAIL signaling, including TRAILs cross-talk with other signaling pathways and immune cells. Next, we focus on known resistant mechanisms to TRAIL-based therapies. Then, we discuss how nano-formulation has the potential to enhance the therapeutic efficacy of TRAIL protein. Finally, we specify strategies with the potential to overcome the challenges that cannot be addressed via nanotechnology alone, including the alternative methods of TRAIL-expressing circulating cells, tumor-targeting bacteria, viruses, and exosomes.

Tumor necrosis factor related apoptosis inducing ligand (TRAIL) regulates deubiquitinase USP5 in tumor cells

The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) pathway has emerged as a cancer therapeutic target. However, clinical trials have proven that most human cancers are resistant to TRAIL. We show that exposure to recombinant TRAIL resulted in the accumulation of ubiquitinated proteins and free ubiquitin polymers, suggesting a link between TRAIL and the ubiquitin (Ub)-proteasome pathway. TRAIL treatment in cancer cells reduced the activity and cleavage of USP5, a deubiquitinase (DUB) previously shown to target unanchored Ub polymers and regulate p53-mediated transcription. TRAIL was effective in suppressing USP5 activity and cleavage in TRAIL-sensitive cells but not resistant cells. Knockdown of USP5 in TRAIL-resistant cells demonstrated that USP5 controls apoptotic responsiveness to TRAIL. USP5 cleavage and ubiquitination were blocked by caspase-8 specific inhibitors. A small-molecule USP5/9× inhibitor (G9) combined with TRAIL enhanced apoptosis and blocked colony growth in highly TRAIL-resistant cell lines. Finally, USP5 protein levels and activity were found to be frequently deregulated in TRAIL-resistant cells. Together, we conclude that activated TRAIL enhances USP5 activity and induces apoptosis in TRAIL-sensitive and -resistant cells. We also suggest that USP5 inhibition may be effective in inducing apoptotic thresholds to enhance responsiveness to TRAIL.

The Role of TRAIL/DRs in the Modulation of Immune Cells and Responses

Expression of TRAIL (tumor necrosis factor–related apoptosis–inducing ligand) by immune cells can lead to the induction of apoptosis in tumor cells. However, it becomes increasingly clear that the interaction of TRAIL and its death receptors (DRs) can also directly impact immune cells and influence immune responses. Here, we review what is known about the role of TRAIL/DRs in immune cells and immune responses in general and in the tumor microenvironment in particular.

Salmonella Effectors SseK1 and SseK3 Target Death Domain Proteins in the TNF and TRAIL Signaling Pathways

Strains of Salmonella utilize two distinct type three secretion systems to deliver effector proteins directly into host cells. The Salmonella effectors SseK1 and SseK3 are arginine glycosyltransferases that modify mammalian death domain containing proteins with N-acetyl glucosamine (GlcNAc) when overexpressed ectopically or as recombinant protein fusions. Here, we combined Arg-GlcNAc glycopeptide immunoprecipitation and mass spectrometry to identify host proteins GlcNAcylated by endogenous levels of SseK1 and SseK3 during Salmonella infection. We observed that SseK1 modified the mammalian signaling protein TRADD, but not FADD as previously reported. Overexpression of SseK1 greatly broadened substrate specificity, whereas ectopic co-expression of SseK1 and TRADD increased the range of modified arginine residues within the death domain of TRADD. In contrast, endogenous levels of SseK3 resulted in modification of the death domains of receptors of the mammalian TNF superfamily, TNFR1 and TRAILR, at residues Arg376 and Arg293 respectively. Structural studies on SseK3 showed that the enzyme displays a classic GT-A glycosyltransferase fold and binds UDP-GlcNAc in a narrow and deep cleft with the GlcNAc facing the surface. Together our data suggest that salmonellae carrying sseK1 and sseK3 employ the glycosyltransferase effectors to antagonise different components of death receptor signaling.

TRAIL and FasL Functions in Cancer and Autoimmune Diseases: Towards an Increasing Complexity

Tumor Necrosis Factor-Related Apoptosis Inducing Ligand (TRAIL/TNFSF10) and Fas Ligand (FasL/TNFSF6), two major cytokines of the TNF (Tumor Necrosis Factor) superfamily, exert their main functions from the immune system compartment. Mice model studies revealed that TRAIL and FasL-mediated signalling both control the homeostasis of the immune cells, mainly from the lymphoid lineage, and function on cytotoxic cells as effector proteins to eliminate the compromised cells. The first clues in the physiological functions of TRAIL arose from the analysis of TRAIL deficient mice, which, even though they are viable and fertile, are prone to cancer and autoimmune diseases development, revealing TRAIL as an important safeguard against autoimmunity and cancer. The naturally occurring gld (generalized lymphoproliferative disease) and lpr (lymphoproliferation) mutant mice develop lymphadenopathy and lupus-like autoimmune disease. The discovery that they are mutated in the fasl and the fas receptor gene, respectively, demonstrates the critical role of the FasL/Fas system in lymphocyte homeostasis and autoimmunity. This review summarizes the state of current knowledge regarding the key death and non-death immune functions that TRAIL and FasL play in the initiation and progression of cancer and autoimmune diseases.

TAK1 suppresses RIPK1-dependent cell death and is associated with disease progression in melanoma

Melanoma cells are highly resistant to conventional genotoxic agents, and BRAFV600/MEK-targeted therapies as well as immunotherapies frequently remain inefficient. Alternative means to treat melanoma, in particular through the induction of programmed cell death modalities such as apoptosis or necroptosis, therefore still need to be explored. Here, we report that melanoma cell lines expressing notable amounts of RIPK1, RIPK3 and MLKL, the key players of necroptosis signal transduction, fail to execute necroptotic cell death. Interestingly, the activity of transforming growth factor β-activated kinase 1 (TAK1) appears to prevent RIPK1 from contributing to cell death induction, since TAK1 inhibition by (5Z)-7-Oxozeaenol, deletion of MAP3K7 or the expression of inactive TAK1 were sufficient to sensitize melanoma cells to RIPK1-dependent cell death in response to TNFα or TRAIL based combination treatments. However, cell death was executed exclusively by apoptosis, even when RIPK3 expression was high. In addition, TAK1 inhibitor (5Z)-7-Oxozeaenol suppressed intrinsic or treatment-induced pro-survival signaling as well as the secretion of cytokines and soluble factors associated with melanoma disease progression. Correspondingly, elevated expression of TAK1 correlates with reduced disease free survival in patients diagnosed with primary melanoma. Overall, our results therefore demonstrate that TAK1 suppresses the susceptibility to RIPK1-dependent cell death and that high expression of TAK1 indicates an increased risk for disease progression in melanoma.

The roles of ubiquitination in extrinsic cell death pathways and its implications for therapeutics

Regulation of cell survival and death, including apoptosis and necroptosis, is important for normal development and tissue homeostasis, and disruption of these processes can cause cancer, inflammatory diseases, and degenerative diseases. Ubiquitination is a cellular process that induces proteasomal degradation by covalently attaching ubiquitin to the substrate protein. In addition to proteolytic ubiquitination, nonproteolytic ubiquitination, such as M1-linked and K63-linked ubiquitination, has been shown to be important in recent studies, which have demonstrated its function in cell signaling pathways that regulate inflammation and cell death pathways. In this review, we summarize the TRAIL- and TNF-induced death receptor signaling pathways along with recent advances in this field and illustrate how different types of ubiquitination control cell death and survival. In particular, we provide an overview of the different types of ubiquitination, target residues, and modifying enzymes, including E3 ligases and deubiquitinating enzymes. Given the relevance of these regulatory pathways in human disease, we hope that a better understanding of the regulatory mechanisms of cell death pathways will provide insights into and therapeutic strategies for related diseases.

LUBAC and ABIN-1 Modulate TRAIL-Based NF-κB Induction in Human Embryonic Kidney 293 Cells

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is known to activate the canonical NF-κB pathway similar to TNF. The exact mechanism of the entire signaling cascade is still under investigation. The involvement of linear ubiquitylation as upregulating component has already been shown recently in some cell lines, but not in human embryonic kidney 293 (HEK293) cells. The downregulating function of the ABIN-1 (A20 binding and inhibitor of NF-κB) as linear ubiquitylation antagonist has been shown in combination with some NF-κB-inducing pathways, but not with TRAIL. We performed luciferase and western blot assays using HEK293 cells stimulated with either TRAIL (or TNF as a control) to analyze the involvement of linear ubiquitin chain assembly complex (LUBAC) components and the impact of ABIN-1 and ABIN-1-MAD (truncated form without A20 binding site) on NF-κB signaling. For overexpression experiments, we added plasmids of ABIN-1 and ABIN-1-MAD or LUBAC components HOIP, HOIL-1, or SHARPIN (single and combinations). For downregulation experiments five pairs of either SHARPIN, HOIL-1, or HOIP targeting miRNAs or one miRNA for ABIN-1 were designed and added. ABIN-1 and its truncated form ABIN-1-MAD reduced the NF-κB induction significantly indicating its involvement as antagonist (independent of deubiquitinase A20) of linear ubiquitylation in TRAIL-induced NF-κB signaling. In opposition, knockdown of ABIN-1 using a specific ABIN-1 miRNA led a clear increase of NF-κB signaling. Addition of single LUBAC components or combinations (except for SHARPIN with HOIL-1) resulted in clearly stronger NF-κB inductions. MiRNAs targeting LUBAC components significantly reduced NF-κB activation. Thus, in HEK293 cells linear ubiquitylation by LUBAC critically upregulates and ABIN-1 downregulates TRAIL-induced NF-κB signaling and may be interesting targets for future pathological therapies.

Regulation of TNF-Related Apoptosis-Inducing Ligand Signaling by Glycosylation

Tumor necrosis-factor related apoptosis-inducing ligand, also known as TRAIL or APO2L (Apo-2 ligand), is a cytokine of the TNF superfamily acknowledged for its ability to trigger selective apoptosis in tumor cells while being relatively safe towards normal cells. Its binding to its cognate agonist receptors, namely death receptor 4 (DR4) and/or DR5, can induce the formation of a membrane-bound macromolecular complex, coined DISC (death-signaling inducing complex), necessary and sufficient to engage the apoptotic machinery. At the very proximal level, TRAIL DISC formation and activation of apoptosis is regulated both by antagonist receptors and by glycosylation. Remarkably, though, despite the fact that all membrane-bound TRAIL receptors harbor putative glycosylation sites, only pro-apoptotic signaling through DR4 and DR5 has, so far, been found to be regulated by N- and O-glycosylation, respectively. Because putative N-glycosylation sequons and O-glycosylation sites are also found and conserved in all these receptors throughout all animal species (in which these receptors have been identified), glycosylation is likely to play a more prominent role than anticipated in regulating receptor/receptor interactions or trafficking, ultimately defining cell fate through TRAIL stimulation. This review aims to present and discuss these emerging concepts, the comprehension of which is likely to lead to innovative anticancer therapies.