TRAIL signalling: decisions between life and death

The TNF-related apoptosis-inducing ligand, TRAIL, has been shown to selectively kill tumour cells. This property has made TRAIL and agonistic antibodies against its death inducing receptors (TRAIL-R1 and TRAIL-R2) to some of the most promising novel biotherapeutic agents for cancer therapy. Here we review the signalling pathways initiated by the apoptosis- as well as the non-apoptosis-inducing receptors, TRAIL-R3 and TRAIL-R4. The TRAIL "death-inducing signalling complex" (DISC) transmits the apoptotic signal. DISC formation leads to activation of a protease cascade, finally resulting in cell death. The TRAIL death receptor-mediated "extrinsic" pathway and the "intrinsic" pathway, which is controlled by the interaction of members of the Bcl-2 family, interact with each other in the decision about life or death of a cell. Apoptotic and non-apoptotic signalling is influenced by the NF-kappaB, PKB/Akt and the MAPK signalling pathways. In this review we intend to summarise the most important findings on the TRAIL signalling network and the interplay in the decisions between life and death of a tumor cell.     

Lipid raft connection between extrinsic and intrinsic apoptotic pathways

Apoptosis in mammalian cells is modulated by extrinsic and intrinsic signaling pathways through the formation of death receptor-mediated death-inducing signaling complex (DISC) and mitochondrial-derived apoptosome, respectively. We found by ultrastructural approaches that the antitumor drug edelfosine induced aggregates of lipid rafts containing Fas/CD95 receptor and Fas-associated death domain-containing protein in leukemic cells. Death receptors together with DISC and apoptosome constituents were recruited in rafts during edelfosine treatment in multiple myeloma cells. This apoptotic response involved caspases-8/-9/-10 that were translocated to rafts. Lipid raft disruption by cholesterol depletion inhibited loss of mitochondrial transmembrane potential, caspase activation and apoptosis, whereas cholesterol replenishment restored these responses. Our data indicate that rafts act as scaffolds where extrinsic and intrinsic apoptotic signaling pathways concentrate, forming clusters of apoptotic signaling molecule-enriched rafts (CASMER), which function as novel supramolecular entities in the triggering of apoptosis, and play an important role in edelfosine-induced apoptosis in blood cancer cells.     

Caspase-8 inactivation in T cells increases necroptosis and suppresses autoimmunity in Bim-/- mice

Dysregulation of either the extrinsic or intrinsic apoptotic pathway can lead to various diseases including immune disorders and cancer. In addition to its role in the extrinsic apoptotic pathway, caspase-8 plays nonapoptotic functions and is essential for T cell homeostasis. The pro-apoptotic BH3-only Bcl-2 family member Bim is important for the intrinsic apoptotic pathway and its inactivation leads to autoimmunity that is further exacerbated by loss of function of the death receptor Fas. We report that inactivation of caspase-8 in T cells of Bim(-/-) mice restrained their autoimmunity and extended their life span. We show that, similar to caspase-8(-/-) T cells, Bim(-/-) T cells that also lack caspase-8 displayed elevated levels of necroptosis and that inhibition of this cell death process fully rescued the survival and proliferation of these cells. Collectively, our data demonstrate that inactivation of caspase-8 suppresses the survival and proliferative capacity of Bim(-/-) T cells and restrains autoimmunity in Bim(-/-) mice.     

Silibinin triggers apoptotic signaling pathways and autophagic survival response in human colon adenocarcinoma cells and their derived metastatic cells

Silibinin, a flavonolignan isolated from the milk thistle plant (Silybum marianum), possesses anti-neoplastic properties. In vitro and in vivo studies have recently shown that silibinin inhibits the growth of colorectal cancer (CRC). The present study investigates the mechanisms of silibinin-induced cell death using an in vitro model of human colon cancer progression, consisting of primary tumor cells (SW480) and their derived metastatic cells (SW620) isolated from a metastasis of the same patient. Silibinin induced apoptotic cell death evidenced by DNA fragmentation and activation of caspase-3 in both cell lines. Silibinin enhanced the expression (protein and mRNA) of TNF-related apoptosis-inducing ligand (TRAIL) death receptors (DR4/DR5) at the cell surface in SW480 cells, and induced their expression in TRAIL-resistant SW620 cells normally not expressing DR4/DR5. Caspase-8 and -10 were activated demonstrating the involvement of the extrinsic apoptotic pathway in silibinin-treated SW480 and SW620 cells. The protein Bid was cleaved in SW480 cells indicating a cross-talk between extrinsic and intrinsic apoptotic pathway. We demonstrated that silibinin activated also the intrinsic apoptotic pathway in both cell lines, including the perturbation of the mitochondrial membrane potential, the release of cytochrome c into the cytosol and the activation of caspase-9. Simultaneously to apoptosis, silibinin triggered an autophagic response. The inhibition of autophagy with a specific inhibitor enhanced cell death, suggesting a cytoprotective function for autophagy in silibinin-treated cells. Taken together, our data show that silibinin initiated in SW480 and SW620 cells an autophagic-mediated survival response overwhelmed by the activation of both the extrinsic and intrinsic apoptotic pathways.     

Toll-like receptor 3 (TLR3) induces apoptosis via death receptors and mitochondria by up-regulating the transactivating p63 isoform alpha (TAP63alpha)

Toll-like receptor 3 (TLR3), a member of the pathogen recognition receptors, is widely expressed in various cells and has been shown to activate immune signaling pathways by recognizing viral double-stranded RNA. Recently, it was reported that the activation of TLR3 induced apoptosis in some cells, but the detailed molecular mechanism is not fully understood. In this study, we found that in endothelial cells polyinosinic-polycytidylic acid (poly(I-C)) induced dose- and time-dependent cell apoptosis, which was elicited by TLR3 activation, as TLR3 neutralization and down-regulation repressed the apoptosis. Poly(I-C) induced the activation of both caspases 8 and 9, indicating that TLR3 triggered the signaling of both the extrinsic and intrinsic apoptotic pathways. Poly(I-C) up-regulated tumor necrosis factor-related apoptosis-inducing ligand and its receptors, death receptors 4/5, resulting in initiating the extrinsic pathway. Furthermore, poly(I-C) down-regulated anti-apoptotic protein, B cell lymphoma 2 (Bcl-2), and up-regulated Noxa, a key Bcl-2 homology 3-only antagonist of Bcl-2, leading to the priming of the intrinsic pathway. A p53-related protein, the transactivating p63 isoform α (TAp63α), was induced by TLR3 activation and contributed to the activation of both the intrinsic and extrinsic apoptotic pathways. Both the cells deficient in p63 gene expression by RNA interference and cells that overexpressed the N-terminally truncated p63 isoform α (ΔNp63α), a dominant-negative variant of TAp63α, by gene transfection, survived TLR3 activation. Taken together, TAp63α is a crucial regulator downstream of TLR3 to induce cell death via death receptors and mitochondria.     

Evidence for Two Modes of Synergistic Induction of Apoptosis by Mapatumumab and Oxaliplatin in Combination with Hyperthermia in Human Colon Cancer Cells

Colorectal cancer is the third leading cause of cancer-related mortality in the world-- the main cause of death from colorectal cancer is hepatic metastases, which can be treated with isolated hepatic perfusion (IHP). Searching for the most clinically relevant approaches for treating colorectal metastatic disease by isolated hepatic perfusion (IHP), we developed the application of oxaliplatin concomitantly with hyperthermia and humanized death receptor 4 (DR4) antibody mapatumumab (Mapa), and investigated the molecular mechanisms of this multimodality treatment in human colon cancer cell lines CX-1 and HCT116 as well as human colon cancer stem cells Tu-12, Tu-21 and Tu-22. We showed here, in this study, that the synergistic effect of the multimodality treatment-induced apoptosis was caspase dependent and activated death signaling via both the extrinsic apoptotic pathway and the intrinsic pathway. Death signaling was activated by c-Jun N-terminal kinase (JNK) signaling which led to Bcl-xL phosphorylation at serine 62, decreasing the anti-apoptotic activity of Bcl-xL, which contributed to the intrinsic pathway. The downregulation of cellular FLICE inhibitory protein long isoform (c-FLIP_L) in the extrinsic pathway was accomplished through ubiquitination at lysine residue (K) 195 and protein synthesis inhibition. Overexpression of c-FLIP_L mutant (K195R) and Bcl-xL mutant (S62A) completely abrogated the synergistic effect. The successful outcome of this study supports the application of multimodality strategy to patients with colorectal hepatic metastases who fail to respond to standard chemoradiotherapy that predominantly targets the mitochondrial apoptotic pathway.     

CDIP, a novel pro-apoptotic gene, regulates TNFalpha-mediated apoptosis in a p53-dependent manner

We have identified a novel pro-apoptotic p53 target gene named CDIP (Cell Death Involved p53-target). Inhibition of CDIP abrogates p53-mediated apoptotic responses, demonstrating that CDIP is an important p53 apoptotic effector. CDIP itself potently induces apoptosis that is associated with caspase-8 cleavage, implicating the extrinsic cell death pathway in apoptosis mediated by CDIP. siRNA-directed knockdown of caspase-8 results in a severe impairment of CDIP-dependent cell death. In investigating the potential involvement of extrinsic cell death pathway in CDIP-mediated apoptosis, we found that TNF-alpha expression tightly correlates with CDIP expression, and that inhibition of TNF-alpha signaling attenuates CDIP-dependent apoptosis. We also demonstrate that TNF-alpha is upregulated in response to p53 and p53 inducing genotoxic stress, in a CDIP-dependent manner. Consistently, knockdown of TNF-alpha impairs p53-mediated stress-induced apoptosis. Together, these findings support a novel p53 --> CDIP --> TNF-alpha apoptotic pathway that directs apoptosis after exposure of cells to genotoxic stress. Thus, CDIP provides a new link between p53-mediated intrinsic and death receptor-mediated extrinsic apoptotic signaling, providing a novel target for cancer therapeutics aimed at maximizing the p53 apoptotic response of cancer cells to drug therapy.     

Resilience of death: intrinsic disorder in proteins involved in the programmed cell death

It is recognized now that intrinsically disordered proteins (IDPs), which do not have unique 3D structures as a whole or in noticeable parts, constitute a significant fraction of any given proteome. IDPs are characterized by an astonishing structural and functional diversity that defines their ability to be universal regulators of various cellular pathways. Programmed cell death (PCD) is one of the most intricate cellular processes where the cell uses specialized cellular machinery and intracellular programs to kill itself. This cell-suicide mechanism enables metazoans to control cell numbers and to eliminate cells that threaten the animal''s survival. PCD includes several specific modules, such as apoptosis, autophagy, and programmed necrosis (necroptosis). These modules are not only tightly regulated but also intimately interconnected and are jointly controlled via a complex set of protein–protein interactions. To understand the role of the intrinsic disorder in controlling and regulating the PCD, several large sets of PCD-related proteins across 28 species were analyzed using a wide array of modern bioinformatics tools. This study indicates that the intrinsic disorder phenomenon has to be taken into consideration to generate a complete picture of the interconnected processes, pathways, and modules that determine the essence of the PCD. We demonstrate that proteins involved in regulation and execution of PCD possess substantial amount of intrinsic disorder. We annotate functional roles of disorder across and within apoptosis, autophagy, and necroptosis processes. Disordered regions are shown to be implemented in a number of crucial functions, such as protein–protein interactions, interactions with other partners including nucleic acids and other ligands, are enriched in post-translational modification sites, and are characterized by specific evolutionary patterns. We mapped the disorder into an integrated network of PCD pathways and into the interactomes of selected proteins that are involved in the p53-mediated apoptotic signaling pathway.     

The role of apoptosis in the development and function of T lymphocytes

Apoptosis plays an essential role in T cell biology. Thymocytes expressing nonfunctional or autoreactive TCRs are eliminated by apoptosis during development. Apoptosis also leads to the deletion of expanded effector T cells during immune responses. The dysregulation of apoptosis in the immune system results in autoimmunity, tumorogenesis and immunodeficiency. Two major pathways lead to apoptosis： the intrinsic cell death pathway controlled by Bcl-2 family members and the extrinsic cell death pathway controlled by death receptor signaling. These two pathways work together to regulate T lymphocyte development and function.     

冠状病毒感染调控细胞凋亡机制研究进展

冠状病毒是常见的感染人类和动物并造成健康危害的主要病原性微生物之一,冠状病毒感染细胞后,细胞产生免疫应答,病毒为了在细胞内转录翻译和装配下一代,应对细胞免疫应答的同时,还参与到许多细胞活动中,当细胞特定受体与病毒蛋白结合后,细胞即启动凋亡程序。冠状病毒的许多蛋白在细胞凋亡程序中起促进或抑制凋亡的不同作用,如病毒S蛋白与细胞膜死亡受体作用诱导细胞启动外在凋亡途径,病毒感染细胞后产生的M、S蛋白引起细胞内质网应激、Ca2+失衡,诱导细胞启动内在凋亡途径,而E蛋白则抑制细胞凋亡的发生。本文综述了冠状病毒对侵染细胞的促凋亡或抑制凋亡作用及其作用机制,通过了解病毒不同蛋白在各种凋亡途径中的不同作用,希望为人工干预调控细胞研究提供思路,为冠状病毒感染防控提供理论支持。     

Chemical inhibitor of nonapoptotic cell death with therapeutic potential for ischemic brain injury

The mechanism of apoptosis has been extensively characterized over the past decade, but little is known about alternative forms of regulated cell death. Although stimulation of the Fas/TNFR receptor family triggers a canonical 'extrinsic' apoptosis pathway, we demonstrated that in the absence of intracellular apoptotic signaling it is capable of activating a common nonapoptotic death pathway, which we term necroptosis. We showed that necroptosis is characterized by necrotic cell death morphology and activation of autophagy. We identified a specific and potent small-molecule inhibitor of necroptosis, necrostatin-1, which blocks a critical step in necroptosis. We demonstrated that necroptosis contributes to delayed mouse ischemic brain injury in vivo through a mechanism distinct from that of apoptosis and offers a new therapeutic target for stroke with an extended window for neuroprotection. Our study identifies a previously undescribed basic cell-death pathway with potentially broad relevance to human pathologies.     

Epirubicin induces apoptosis in osteoblasts through death-receptor and mitochondrial pathways

Epirubicin is an anthracycline and is widely used in tumor treatment, but has toxic and undesirable side effects on wide range of cells and hematopoietic stem cells (HSC). Osteoblasts play important roles in bone development and in supporting HSC differentiation and maturation. It remains unknown whether epirubicin-induced bone loss and hematological toxicity are associated with its effect on osteoblasts. In primary osteoblast cell cultures, epirubicin inhibited cell growth and decreased mineralization. Moreover, epirubicin arrested osteoblasts in the G2/M phase, and this arrest was followed by apoptosis in which both the extrinsic (death receptor-mediated) and intrinsic (mitochondrial-mediated) apoptotic pathways were evoked. The factors involved in the extrinsic apoptotic pathway were increased FasL and FADD as well as activated caspase-8. Those involved in the intrinsic apoptotic pathway were decreased Bcl-2; increased reactive oxygen species, Bax, cytochrome c; and activated caspase-9 and caspase-3. These results demonstrate that epirubicin induced osteoblast apoptosis through the extrinsic and intrinsic apoptotic pathways, leading to the destruction of osteoblasts and consequent lessening of their functions in maintaining bone density and supporting hematopoietic stem cell differentiation and maturation.     

The extrinsic and intrinsic apoptotic pathways are differentially affected by temperature upstream of mitochondrial damage

It is well known that mild hypothermia prevents neuronal cell death following cerebral ischemia, although it can also cause apoptosis in other cell types. Thus, incubation at room temperature (RT) has been shown to induce apoptosis in hematopoietic cells, including Jurkat T leukemia cells. To further understand the apoptotic events that can be activated at RT, we compared the induction of apoptosis by several apoptotic insults in Jurkat cells stimulated at 37°C or RT. Retinoid-related molecules, which induce apoptosis via the intrinsic pathway, failed to induce apoptosis when cells were treated at RT, as determined by various apoptotic parameters including cytochrome c release and activation of caspase 3. In contrast, most apoptotic events were enhanced by lower temperatures when cells were stimulated with anti-Fas antibody via the extrinsic pathway. Ultraviolet radiation produced partial effects at RT, correlating with its capacity to activate both pathways. Our results indicate that the core caspase machinery is operational under mild hypothermia conditions. Experiments using purified recombinant caspases and cell-free assays confirmed that caspases are fully functional at RT. Other hallmark events of apoptosis, such as phosphatidylserine externalization and formation of apoptotic bodies were variably affected by RT in a stimulus-dependent manner, suggesting the existence of critical steps that are sensitive to temperature. Thus, analysis of apoptosis at RT might be useful to (i) discriminate between the extrinsic and intrinsic pathways in Jurkat cells treated with prospective stimuli, and (ii) to unravel temperature-sensitive steps of apoptotic signaling cascades.     

Ethanol-induced neuronal apoptosis in vivo requires BAX in the developing mouse brain

A single episode of ethanol intoxication triggers widespread apoptotic neurodegeneration in the infant rat or mouse brain. The cell death process occurs over a 6-16 h period following ethanol administration, is accompanied by a robust display of caspase-3 enzyme activation, and meets ultrastructural criteria for apoptosis. Two apoptotic pathways (intrinsic and extrinsic) have been described, either of which may culminate in the activation of caspase-3. The intrinsic pathway is regulated by Bax and Bcl-XL and involves Bax-induced mitochondrial dysfunction and release of cytochrome c as antecedent events leading to caspase-3 activation. Activation of caspase-8 is a key event preceding caspase-3 activation in the extrinsic pathway. In the present study, following ethanol administration to infant mice, we found no change in activated caspase-8, which suggests that the extrinsic pathway is not involved in ethanol-induced apoptosis. We also found that ethanol triggers robust caspase-3 activation and apoptotic neurodegeneration in C57BL/6 wildtype mice, but induces neither phenomenon in homozygous Bax-deficient mice. Therefore, it appears that ethanol-induced neuroapoptosis is an intrinsic pathway-mediated phenomenon involving Bax-induced disruption of mitochondrial membranes and cytochrome c release as early events leading to caspase-3 activation.     

Lupulone, a hop bitter acid, activates different death pathways involving apoptotic TRAIL-receptors, in human colon tumor cells and in their derived metastatic cells

Our study aimed to compare death signalling pathways triggered by lupulone in TRAIL-sensitive human colon cancer cells (SW480) and in their derived TRAIL-resistant metastatic cells (SW620). Lupulone (40 μg/ml) up-regulated expression of TRAIL DR4/DR5 death receptors at the cell surface of both cell lines, even in the absence of exogenous TRAIL ligand. Cell death induced by lupulone was inhibited in SW480 and SW620 cells exposed to blocking anti-DR4/DR5 antibodies. In SW480 cells, lupulone triggered cell death through a cross-talk between TRAIL-DR4/DR5 and the mitochondrial (intrinsic) pathways involving caspase-8 activation and Bid protein cleavage. As a consequence mitochondrial cytochrome c was released into the cytosol and activation of caspases-9 and -3 was observed. In the metastatic SW620 cells, lupulone restored the sensibility of these cells to TRAIL ligand and activated the extrinsic apoptotic pathway via DR4/DR5 death receptors and the involvement of the caspase-8/caspase-3 cascade. The demonstration that lupulone is able to activate TRAIL-death signalling pathways even in TRAIL resistant cancer cells highlights the potential of this natural compound for cancer prevention and therapy.     

Apoptosome dysfunction in human cancer

Apoptosis is a cell suicide mechanism that enables organisms to control cell number and eliminate cells that threaten survival. The apoptotic cascade can be triggered through two major pathways. Extracellular signals such as members of the tumor necrosis factor (TNF) family can activate the receptor-mediated extrinsic pathway. Alternatively, stress signals such as DNA damage, hypoxia, and loss of survival signals may trigger the mitochondrial intrinsic pathway. In the latter, mitochondrial damage results in cytochrome c release and formation of the apoptosome, a multimeric protein complex containing Apaf-1, cytochrome c , and caspase-9. Once bound to the apoptosome, caspase-9 is activated, and subsequently triggers a cascade of effector caspase activation and proteolysis, leading to apoptotic cell death. Recent efforts have led to the identification of multiple factors that modulate apoptosome formation and function. Alterations in the expression and/or function of these factors may contribute to the pathogenesis of cancer and resistance of tumor cells to chemotherapy or radiation. In this review we discuss how disruption of normal apoptosome formation and function may lead or contribute to tumor development and progression.     

Death receptor activation-induced hepatocyte apoptosis and liver injury

The TNFalpha receptor super-family consists of several members sharing a sequence homology in a unique function domain, the death domain, which is located in the intracellular portion of the receptor. These so-called death receptors, including Fas, TNF-R1 and TRAIL-R1/TRAIL-R2, are expressed on hepatocytes. When stimulated by their ligands, FasL, TNFalpha or TRAIL, respectively, the death receptors can activate multiple death domain-initiated apoptosis programs, including both extrinsic and intrinsic pathways. A cascade of caspases is activated, which cleave proteins important for the cell structure and function. Activation of the intrinsic pathway also leads to mitochondrial release of several apoptotic proteins and mitochondrial dysfunction, which kill the cell through both caspase-dependent and caspase-independent mechanisms. Death receptor-induced hepatocyte apoptosis contributes to the development of a number of liver diseases, including viral hepatitis, inflammatory hepatitis, Wilson's disease, alcoholic liver disease, endotoxiemia-induced liver failure and ischemia/reperfusion-induced liver damage. This article comprehensively reviews the mechanisms of induction and regulation of death receptor-initiated apoptosis in hepatocytes, examines how these molecular events affect our understanding of the pathogenesis of these diseases and further discusses the potential therapeutic application of the knowledge. We hope we can provide a cohesive and integrated perspective on the many aspects of these complicated processes.     

Stress-induced apoptosis: Toward a symmetry with receptor-mediated cell death

Apoptosis is a form of programmed cell death executed by caspases activated along signalling pathways initiated by ligation of cell-surface death receptors ( extrinsic pathway ) or by perturbation of the mithocondrial membrane promoted by physical or chemical stress agents ( intrinsic pathway ). In metazoans, this evolutionary conserved, genetically controlled process has a role in a variety of physiological settings, as development, homeostasis of tissues and maintenance of the organism integrity. When deranged by impaired regulation or inappropriate activation apoptosis contributes to the pathogenesis of diseases as autoimmunity, cancer, restenosis, ischaemia, heart failure and neurodegenerative disorders. In this review we will present a survey of the stress-induced intrinsic, mithochondrial, pathway and, based on recent experimental data, we will propose a view compatible with an emergent conceptual symmetry between the two apoptogenic extrinsic and intrinsic pathways. Elements of symmetry present in both the apoptogenic signalling pathways include: early activation of initiator caspases (feed-forwarded by a direct or post-mitocondrial effector caspase-mediated amplification loop in some cell types) and mitochondrial membrane permeabilization with required release of antagonists of active caspase inhibitors (IAPs) in high-level IAPs-expressing cells and apoptosome-mediated amplification of the caspase cascade more or less needed in different cell types.