Analysis of CD95 threshold signaling: triggering of CD95 (FAS/APO-1) at low concentrations primarily results in survival signaling

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.

A mathematical framework for the emergence of winners and losers in cell competition

Cell competition is a process in multicellular organisms where cells interact with their neighbours to determine a "winner" or "loser" status. The loser cells are eliminated through programmed cell death, leaving only the winner cells to populate the tissue. Cell competition is context-dependent; the same cell type can win or lose depending on the cell type it is competing against. Hence, winner/loser status is an emergent property. A key question in cell competition is: how do cells acquire their winner/loser status? In this paper, we propose a mathematical framework for studying the emergence of winner/loser status based on a set of quantitative criteria that distinguishes competitive from non-competitive outcomes. We apply this framework in a cell-based modelling context, to both highlight the crucial role of active cell death in cell competition and identify the factors that drive cell competition.

CD95L concatemers highlight different stoichiometries of CD95-mediated apoptotic and nonapoptotic pathways

To better understand the stoichiometry of CD95L required to trigger apoptotic and nonapoptotic signals, we generated several CD95L concatemers from dimer to hexamer conjugated via a flexible link (GGGGS)2 . These ligands reveal that although the hexameric structure is the best stoichiometry to trigger cell death, a dimer is sufficient to induce the apoptotic response in CD95-sensitive Jurkat cells. Interestingly, only trimeric and hexameric forms can implement a potent Ca2+ response, suggesting that while CD95 aggregation controls the implementation of the apoptotic signal, both aggregation and conformation are required to implement the Ca2+ pathway.

Glucose oxidation-dependent survival of activated B cells provides a putative novel therapeutic target for lupus treatment

Aberrant metabolic demand is observed in immune/inflammatory disorders, yet the role in pathogenesis remains unclear. Here, we discover that in lupus, activated B cells, including germinal center B (GCB) cells, have remarkably high glycolytic requirement for survival over T cell populations, as demonstrated by increased metabolic activity in lupus-activated B cells compared to immunization-induced cells. The augmented reliance on glucose oxidation makes GCB cells vulnerable to mitochondrial ROS-induced oxidative stress and apoptosis. Short-term glycolysis inhibition selectively reduces pathogenic activated B in lupus-prone mice, extending their lifespan, without affecting T follicular helper cells. Particularly, BCMA-expressing GCB cells rely heavily on glucose oxidation. Depleting BCMA-expressing activated B cells with APRIL-based CAR-T cells significantly prolongs the lifespan of mice with severe autoimmune disease. These results reveal that glycolysis-dependent activated B and GCB cells, especially those expressing BCMA, are potentially key lupus mediators, and could be targeted to improve disease outcomes.

Increased Expression of CD95 in CD4+ Effector Memory T Cells Promotes Th17 Response in Patients with Myasthenia Gravis

Emerging data have revealed that CD95 can evoke non-apoptotic signals, thereby promoting pro-inflammatory functions that link to the severity of autoimmune disorders. Here, we reported that the expression of CD95 in CD4⁺ effector memory T (CD4⁺ TEM) cells was increased in myasthenia gravis (MG) patients. We also found increased expression of CD95 in CD4⁺ TEM cells from MG patients correlated positively with clinical severity scores (QMGs), serum IL-17 levels and plasma cells (PCs) frequencies. Conventional treatment, such as glucocorticoid, could down-regulate the expression of CD95 in CD4⁺ TEM cells, QMGs, serum IL-17 levels and PCs frequencies from MG patients. In vitro, low-dose of agonistic anti-CD95 mAb could promote Th17 cell development. This effect was reversed by CD95 siRNA. Moverover, CD95 stimulation induced the phosphorylation of p38 and Erk1/2 and Th17 cell differentiation, and p38 specific inhibitor SB203580 or Erk1/2 specific inhibitor PD98059 could induce opposite changes. However, SB203580 or PD98059 do not abrogate the increase of CCR6⁺IL-17A⁺ cells, ROR-γt and IL-17 expression induced by CD95 triggering relatively to each corresponding control. This suggests that p38 and Erk1/2 MAPK pathway plays a role in expression of CCR6⁺IL-17A⁺ cells, ROR-γt and IL-17, but not in their increase induced by CD95 triggering. Taken together, this study revealed that increased expression of CD95 in CD4⁺ TEM cells promotes Th17 response under the microenvironment of MG.

CD95 gene deletion may reduce clonogenic growth and invasiveness of human glioblastoma cells in a CD95 ligand-independent manner

CD95 (Fas/APO-1) is a multifunctional cell surface receptor with antithetic roles. First described to mediate cell death, interactions of CD95 with its natural ligand, CD95L, have also been described to induce tumor-promoting signaling leading to proliferation, invasion and stem cell maintenance, mainly in cancer cells that are resistant to CD95-mediated apoptosis. While activation of CD95-mediated apoptosis in cancer cells may not be clinically practicable due to toxicity, inhibition of tumor-promoting CD95 signaling holds therapeutic potential. In the present study, we characterized CD95 and CD95L expression in human glioma-initiating cells (GIC), a glioblastoma cell population with stem cell features, and investigated the consequences of CRISPR-Cas9-mediated CD95 or CD95L gene deletion. In vitro, GIC expressed CD95 but not CD95L and were sensitive to CD95-mediated apoptosis. Upon genetic deletion of CD95, GIC acquired resistance to CD95L-induced apoptosis but exhibited inferior clonogenic growth, sphere-forming capacity, and invasiveness compared with control cells, suggesting the existence of CD95L-independent constitutive CD95 signaling with tumor-promoting properties in GIC. In vivo, GIC expressed CD95 and a non-canonical form of CD95L lacking the CD95-binding region. CD95 genetic deletion did not prolong survival in immunocompromised GIC-bearing mice. Altogether, these data indicate that canonical CD95L may not be expressed in human GIC and suggest the existence of a CD95L-independent CD95-signaling pathway that maintains some malignancy traits of GIC. The lack of altered survival of tumor-bearing mice after genetic deletion of CD95 suggests that CD95 signaling is not essential to maintain the growth of human GIC xenografted into the brains of nude mice. The ligand-independent tumor-promoting role of constitutive CD95 in our GIC models in vitro highlights the complexity and challenges associated with targeting CD95 with therapeutic intent.

Impact of human CD95 mutations on cell death and autoimmunity: a model

CD95/Fas/APO-1 can trigger apoptotic as well as nonapoptotic pathways in immune cells. CD95 signaling in humans can be inhibited by several mechanisms, including mutations in the gene encoding CD95. CD95 mutations lead to autoimmune disorders, such as autoimmune lymphoproliferative syndrome (ALPS). Gaining further insight into the reported mutations of CD95 and resulting alterations of its signaling networks may provide further understanding of their presumed role in certain autoimmune diseases. For illustrative purposes and to better understand the potential outcomes of CD95 mutations, here we assign their positions to the recently determined 3D structures of human CD95. Based on this, we make certain predictions and speculate on the putative role of CD95 mutation defects in CD95-mediated signaling for certain autoimmune diseases.

Fas-threshold signalling in MSCs promotes pancreatic cancer progression and metastasis

Mesenchymal stem cells (MSCs) belong to the tumour microenvironment and have been implicated in tumour progression. We found that the number of MSCs significantly increased in tumour-burdened mice driven by Fas-threshold signalling. Consequently, MSCs lacking Fas lost their ability to induce metastasis development in a pancreatic cancer model. Mixing of MSCs with pancreatic cancer cells led to sustained production of the pro-metastatic cytokines CCL2 and IL6 by the stem cells. The levels of these cytokines were dependent on the number of MSCs, linking Fas-mediated MSC-proliferation to their capacity to promote tumour progression. Furthermore, we discovered that CCL2 and IL6 were induced by pancreatic cancer cell-derived IL1. Importantly, analysis of patient transcriptomic data revealed that high FasL expression correlates with high levels of MSC markers as well as increased IL6 and CCL2 levels in pancreatic tumours. Moreover, both FasL and CCL2 are linked to elevated levels of markers specific for monocytes known to possess further pro-metastatic activities. These results confirm our experimental findings of a FasL-MSC-IL1-CCL2/IL6 axis in pancreatic cancer and highlights the role of MSCs in tumour progression.

Modulation of CD95-mediated signaling by post-translational modifications: towards understanding CD95 signaling networks

CD95 is a member of the death receptor family and is well-known to promote apoptosis. However, accumulating evidence indicates that in some context CD95 has not only the potential to induce apoptosis but also can trigger non-apoptotic signal leading to cell survival, proliferation, cancer growth and metastasis. Despite extensive investigations focused on alterations in the expression level of CD95 and associated signal molecules, very few studies, however, have investigated the effects of post-translational modifications such as glycosylation, phosphorylation, palmitoylation, nitrosylation and glutathionylation on CD95 function. Post-translational modifications of CD95 in mammalian systems are likely to play a more prominent role than anticipated in CD95 induced cell death. In this review we will focus on the alterations in CD95-mediated signaling caused by post-translational modifications of CD95.

System-based approaches as prognostic tools for glioblastoma

BACKGROUND

The evasion of apoptosis is a hallmark of cancer. Understanding this process holistically and overcoming apoptosis resistance is a goal of many research teams in order to develop better treatment options for cancer patients. Efforts are also ongoing to personalize the treatment of patients. Strategies to confirm the therapeutic efficacy of current treatments or indeed to identify potential novel additional options would be extremely beneficial to both clinicians and patients. In the past few years, system medicine approaches have been developed that model the biochemical pathways of apoptosis. These systems tools incorporate and analyse the complex biological networks involved. For their successful integration into clinical practice, it is mandatory to integrate systems approaches with routine clinical and histopathological practice to deliver personalized care for patients.

RESULTS

We review here the development of system medicine approaches that model apoptosis for the treatment of cancer with a specific emphasis on the aggressive brain cancer, glioblastoma.

CONCLUSIONS

We discuss the current understanding in the field and present new approaches that highlight the potential of system medicine approaches to influence how glioblastoma is diagnosed and treated in the future.

IL-1β and TNFα Differentially Influence NF-κB Activity and FasL-Induced Apoptosis in Primary Murine Hepatocytes During LPS-Induced Inflammation

Macrophage-derived cytokines largely influence the behavior of hepatocytes during an inflammatory response. We previously reported that both TNFα and IL-1β, which are released by macrophages upon LPS stimulation, affect Fas ligand (FasL)-induced apoptotic signaling. Whereas TNFα preincubation leads to elevated levels of caspase-3 activity and cell death, pretreatment with IL-1β induces increased caspase-3 activity but keeps cells alive. We now report that IL-1β and TNFα differentially influence NF-κB activity resulting in a differential upregulation of target genes, which may contribute to the distinct effects on cell viability. A reduced NF-κB activation model was established to further investigate the molecular mechanisms which determine the distinct cell fate decisions after IL-1β and TNFα stimulation. To study this aspect in a more physiological setting, we used supernatants from LPS-stimulated bone marrow-derived macrophages (BMDMs). The treatment of hepatocytes with the BMDM supernatant, which contains both IL-1β and TNFα, sensitized to FasL-induced caspase-3 activation and cell death. However, when TNFα action was blocked by neutralizing antibodies, cell viability after stimulation with the BMDM supernatant and FasL increased as compared to single FasL stimulation. This indicates the important role of TNFα in the sensitization of apoptosis in hepatocytes. These results give first insights into the complex interplay between macrophages and hepatocytes which may influence life/death decisions of hepatocytes during an inflammatory reaction of the liver in response to a bacterial infection.

Betulinic acid as apoptosis activator: Molecular mechanisms, mathematical modeling and chemical modifications

A natural product betulinic acid (BA) has gained a huge significance in the recent years for its strong cytotoxicity. Surprisingly, in spite of being an interesting cancer protecting agent on a variety of tumor cells, the normal cells and tissues are rarely affected by BA. Betulinic acid and analogues (BAs) generally exert through the mechanisms that provokes an event of direct cell death and bypass the resistance to normal chemotherapeutics. Although the major mechanism associated with its ability to induce direct cell death is mitochondrial apoptosis, there are several other mechanisms explored recently. Importantly, mathematical modeling of apoptosis has been an important tool to explore the precise mechanism involved in mitochondrial apoptosis. Thus, this review is an endeavor to sum up the molecular mechanisms underlying the action of BA and future directions to apply mathematical modeling technique to better understand the precise mechanism of BA-induced apoptosis. The last section of the review encompasses the plausible structural modifications and formulations to enhance the therapeutic efficacy of BA.

Cisplatin-induced renal inflammation is ameliorated by cilastatin nephroprotection

Background

Cisplatin is a potent chemotherapeutic drug whose nephrotoxic effect is a major complication and a dose-limiting factor for antitumoral therapy. There is much evidence that inflammation contributes to the pathogenesis of cisplatin-induced nephrotoxicity. We found that cilastatin, a renal dehydropeptidase-I inhibitor, has protective effects in vitro and in vivo against cisplatin-induced renal damage by inhibiting apoptosis and oxidation. Here, we investigated the potential use of cilastatin to protect against cisplatin-induced kidney injury and inflammation in rats.

Methods

Male Wistar rats were divided into four groups: control, cilastatin-control, cisplatin and cilastatin-cisplatin. Nephrotoxicity was assessed 5 days after administration of cisplatin based on blood urea nitrogen, creatinine, glomerular filtration rate (GFR), kidney injury molecule (KIM)-1 and renal morphology. Inflammation was measured using the electrophoretic mobility shift assay, immunohistochemical studies and evaluation of inflammatory mediators.

Results

Compared with the control rats, cisplatin-administered rats were affected by significant proximal tubule damage, decreased GFR, increased production of inflammatory mediators and elevations in urea, creatinine and tissue KIM-1 levels. Cilastatin prevented these changes in renal function and ameliorated histological damage in cisplatin-administered animals. Cilastatin also reduced pro-inflammatory cytokine levels, activation of nuclear factor-κB and CD68-positive cell concentrations.

Conclusions

Cilastatin reduces cisplatin-induced nephrotoxicity, which is associated with decreased inflammation in vivo. Although the exact role of decreased inflammation in nephroprotection has not been fully elucidated, treatment with cilastatin could be a novel strategy for the prevention of cisplatin-induced acute kidney injury.

A20 Curtails Primary but Augments Secondary CD8+ T Cell Responses in Intracellular Bacterial Infection

The ubiquitin-modifying enzyme A20, an important negative feedback regulator of NF-κB, impairs the expansion of tumor-specific CD8⁺ T cells but augments the proliferation of autoimmune CD4⁺ T cells. To study the T cell-specific function of A20 in bacterial infection, we infected T cell-specific A20 knockout (CD4-Cre A20^fl/fl) and control mice with Listeria monocytogenes. A20-deficient pathogen-specific CD8⁺ T cells expanded stronger resulting in improved pathogen control at day 7 p.i. Imaging flow cytometry revealed that A20-deficient Listeria-specific CD8⁺ T cells underwent increased apoptosis and necroptosis resulting in reduced numbers of memory CD8⁺ T cells. In contrast, the primary CD4⁺ T cell response was A20-independent. Upon secondary infection, the increase and function of pathogen-specific CD8⁺ T cells, as well as pathogen control were significantly impaired in CD4-Cre A20^fl/fl mice. In vitro, apoptosis and necroptosis of Listeria-specific A20-deficient CD8⁺ T cells were strongly induced as demonstrated by increased caspase-3/7 activity, RIPK1/RIPK3 complex formation and more morphologically apoptotic and necroptotic CD8⁺ T cells. In vitro, A20 limited CD95L and TNF-induced caspase3/7 activation. In conclusion, T cell-specific A20 limited the expansion but reduced apoptosis and necroptosis of Listeria-specific CD8⁺ T cells, resulting in an impaired pathogen control in primary but improved clearance in secondary infection.

Cell death-independent activities of the death receptors CD95, TRAILR1, and TRAILR2

Since their identification more than 20 years ago, the death receptors CD95, TRAILR1, and TRAILR2 have been intensively studied with respect to their cell death-inducing activities. These receptors, however, can also trigger a variety of cell death-independent cellular responses reaching from the activation of proinflammatory gene transcription programs over the stimulation of proliferation and differentiation to induction of cell migration. The cell death-inducing signaling mechanisms of CD95 and the TRAIL death receptors are well understood. In contrast, despite the increasing recognition of the biological and pathophysiological relevance of the cell death-independent activities of CD95, TRAILR1, and TRAILR2, the corresponding signaling mechanisms are less understood and give no fully coherent picture. This review is focused on the cell death-independent activities of CD95 and the TRAIL death receptors and addresses mainly three questions: (a) how are these receptors linked to noncell death pathways at the molecular level, (b) which factors determine the balance of cell death and cell death-independent activities of CD95 and the TRAIL death receptors at the cellular level, and (c) what are the consequences of the cell death-independent functions of these receptors for their role in cancer and inflammatory diseases.

Fas expression is downregulated in gastric cancer

The aim of the present study was to investigate Fas expression in tumor samples from patients with gastric cancer, in order to determine the involvement of the Fas signaling pathway. The protein expression levels of Fas, caspase-8, caspase-3 and poly (adenosine diphosphate-ribose) polymerase 1 (PARP1) were examined in gastric cancer specimens and their associations with clinical pathological parameters were analyzed with immunohistochemical staining and western blot analysis. The mRNA expression was quantified with quantitative PCR and apoptosis was examined with a FACScan flow cytometer. The results demonstrated that the downregulation of Fas expression was correlated with less histological differentiation, gender (male), and increased lymph node and distant metastases (P<0.05). In the AGS established gastric cancer cell line, upregulation of the Fas signaling pathway promoted the apoptosis of gastric cancer cells by upregulating the expression of caspase-8 and caspase-3, and downregulating the expression of PARP1. The present study demonstrated that Fas was associated with gastric cancer and promoted the apoptosis of gastric cancer cells via caspase-8, caspase-3 and PARP1. These results suggested that caspase-8, caspase-3 and PARP1 may be triggers of gastric cancer, and upregulation of caspase-8 and caspase-3 expression, or inhibition of PARP1 expression may improve the therapeutic outcome in patients with gastric cancer.

Inferring catalysis in biological systems

In systems biology, one is often interested in the communication patterns between several species, such as genes, enzymes or proteins. These patterns become more recognisable when temporal experiments are performed. This temporal communication can be structured by reaction networks such as gene regulatory networks or signalling pathways. Mathematical modelling of data arising from such networks can reveal important details, thus helping to understand the studied system. In many cases, however, corresponding models still deviate from the observed data. This may be due to unknown but present catalytic reactions. From a modelling perspective, the question of whether a certain reaction is catalysed leads to a large increase of model candidates. For large networks the calibration of all possible models becomes computationally infeasible. We propose a method which determines a substantially reduced set of appropriate model candidates and identifies the catalyst of each reaction at the same time. This is incorporated in a multiple-step procedure which first extends the network by additional latent variables and subsequently identifies catalyst candidates using similarity analysis methods. Results from synthetic data examples suggest a good performance even for non-informative data with few observations. Applied on CD95 apoptotic pathway our method provides new insights into apoptosis regulation.

Parameter identification using stochastic simulations reveals a robustness in CD95 apoptotic response

A number of mathematical models of apoptosis generated recently allowed us to understand intrinsic mechanisms of life/death decisions in a cell. Nevertheless, the parameters for the mathematical models are often experimentally difficult to obtain and there is an emerging need for the development of efficient approaches for parameter estimation. In this study we suggest a new method for parameter estimation, which is based on stochastic simulations and can be used when the number of molecules in the system is small. Our approach comprised the following steps: we start from the selection of parameters that lead to a good ordinary differential equation (ODE) fit. We continued by carrying out stochastic simulations for each of these parameters. Comparing the correlation structure of these simulations with the data, we finally could identify the best parameter set. The method was applied for a model of CD95-induced apoptosis, the new best identified parameters fit well to the experimental data. The best parameter set allowed us to get new insights into CD95 apoptosis regulation and can be applied for the comprehensive analysis of other signaling networks. The modeling approach allowed us to get new insights into network regulation, in particular, to identify robustness in CD95 apoptotic response. Taken together, this new method provides valuable predictions and can be applied for the analysis of other signaling networks.

Death receptor 6 (DR6) is required for mouse B16 tumor angiogenesis via the NF-κB, P38 MAPK and STAT3 pathways

Although death receptor 6 (DR6) is aberrantly expressed in certain cancer cell lines, its function, signaling pathway and potential clinical significance in tumor progression are not well characterized. We report here that knocking down DR6 in the mouse B16 cell line has no effect on B16 cell death in vitro but suppresses xenograft B16 tumor growth by preventing tumor blood vessel formation in vivo. Deficiency of DR6 changes cytokine expression and secretion; in particular, it inhibits the proinflammatory cytokine interleukin-6 (IL-6), which is able to induce the expression of the angiogenesis-related factors: vascular endothelial growth factor-A, platelet-derived growth factor-β, vascular endothelial growth factor-D and platelet-derived growth factor receptor-α. Further experiments demonstrate that DR6-dependent angiogenesis is involved in the IL-6/P38 MAPK and IL-6/STAT3 pathways. Our novel findings demonstrate for the first time that DR6 expression in B16 cells facilitates tumor growth by accelerating tumor angiogenesis. Moreover, these results suggest that DR6 is involved in three important intracellular pathways that lead to homeostatic angiogenesis in tumor growth.

Mice lacking functional CD95-ligand display reduced proliferation of the intestinal epithelium without gross homeostatic alterations

Homeostasis of the continuously self-renewing intestinal tract involves cell proliferation, migration, differentiation along the crypt-villus-axis and shedding of cells into the gut lumen. CD95-ligand (FAS-ligand, CD95L) is a cytokine that is known for its capacity to induce apoptosis by binding its cognate receptor, CD95 (Fas). More recently, it was discovered that CD95L can also induce other cellular responses, such as proliferation, differentiation and cell migration. CD95L is highly expressed in Paneth cells of the small intestine which are in close contact with intestinal stem cells. This suggests a potential role for CD95L in controlling stem cell function and, possibly, intestinal homeostasis. We analyzed the intestines of mice deficient for functional CD95L (gld) for potential alterations in the diversity of stem-cell-lineages and parameters of intestinal homeostasis. Stem cell diversity was assessed by analyzing methylation patterns of the non-transcribed mMYOD gene. Proliferation was analyzed by BrdU labeling and differentiation was assessed by immunohistochemistry. Of all parameters analyzed, only epithelial cell proliferation was significantly reduced in the small intestines of gld-mice, but not in their colons which lack CD95L expression. We conclude that CD95L has a proliferation-stimulating role during normal turnover of the small intestine, but has a marginal effect on overall intestinal homeostasis.

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 impact of multi-walled carbon nanotubes with different amount of metallic impurities on immunometabolic parameters in healthy volunteers

The impact of two types of multi-walled carbon nanotubes (MWCNTs) (12-14 nm) with different content of metallic impurities (purified and unpurified nanotubes) on peroxidation processes, the status of immune cells in healthy volunteers and gene expression combined to pathway analysis was studied in vitro. From the study it was shown that the main mechanism of action for both types of MWCNTs is induction of oxidative stress, the intensity of which is directly related to the amount of metallic impurities. Unpurified MWCNTs produced twice as high levels of oxidation than the purified CNTs inducing thus more intense mitochondrial dysfunction. All the above were also verified by gene expression analysis of 2 different human cellular cultures (lung epithelium and keratinoma cells) and the respective pathway analysis; modulation of genes activating the NFkB pathway is associated to inflammatory responses. This may cause a perturbation in the IL-6 signaling pathway in order to regulate inflammatory processes and compensate for apoptotic changes. A plausible hypothesis for the immunological effects observed in vivo, are considered as the result of the synergistic effect of systemic (mediated by cells of the routes of exposure) and local inflammation (blood cells).

Molecular architecture of the DED chains at the DISC: regulation of procaspase-8 activation by short DED proteins c-FLIP and procaspase-8 prodomain

The CD95/Fas/APO-1 death-inducing signaling complex (DISC), comprising CD95, FADD, procaspase-8, procaspase-10, and c-FLIP, has a key role in apoptosis induction. Recently, it was demonstrated that procaspase-8 activation is driven by death effector domain (DED) chains at the DISC. Here, we analyzed the molecular architecture of the chains and the role of the short DED proteins in regulating procaspase-8 activation in the chain model. We demonstrate that the DED chains are largely composed of procaspase-8 cleavage products and, in particular, of its prodomain. The DED chain also comprises c-FLIP and procaspase-10 that are present in 10 times lower amounts compared with procaspase-8. We show that short c-FLIP isoforms can inhibit CD95-induced cell death upon overexpression, likely by forming inactive heterodimers with procaspase-8. Furthermore, we have addressed mechanisms of the termination of chain elongation using experimental and mathematical modeling approaches. We show that neither c-FLIP nor procaspase-8 prodomain terminates the DED chain, but rather the dissociation/association rates of procaspase-8 define the stability of the chain and thereby its length. In addition, we provide evidence that procaspase-8 prodomain generated at the DISC constitutes a negative feedback loop in procaspase-8 activation. Overall, these findings provide new insights into caspase-8 activation in DED chains and apoptosis initiation.

Agonist mobility on supported lipid bilayers affects Fas mediated death response

Extrinsic apoptosis is initiated by recognition and clustering of the single-pass transmembrane proteins Fas ligand and Fas expressed at the surface of closely apposed lymphocytes and target cells, respectively. Since Fas-mediated death response was mainly studied with soluble antibodies, the mobility constraints for receptor activation by a membrane embedded agonist is not well understood. We explored this influence by stimulating apoptosis on functionalized supported lipid bilayers, where we quantified agonist mobility by z-scan fluorescence correlation spectroscopy. Using different lipid compositions, we show that the apoptotic response correlates with increased lateral mobility of the agonist in the lipid bilayer.

Surviving apoptosis: life-death signaling in single cells

Tissue development and homeostasis are regulated by opposing pro-survival and pro-death signals. An interesting feature of the Tumor Necrosis Factor (TNF) family of ligands is that they simultaneously activate opposing signals within a single cell via the same ligand-receptor complex. The magnitude of pro-death events such as caspase activation and pro-survival events such as Nuclear Factor (NF)-κB activation vary not only from one cell type to the next but also among individual cells of the same type due to intrinsic and extrinsic noise. The molecules involved in these pro-survival and/or pro-death pathways, and the different phenotypes that result from their activities, have been recently reviewed. Here we focus on the impact of cell-to-cell variability in the strength of these opposing signals on shaping cell fate decisions.

Fas and TRAIL 'death receptors' as initiators of inflammation: Implications for cancer

Fas (CD95/APO-1) and TRAIL (CD253, TNFSF10, APO2) are members of a subset of the TNF receptor superfamily known as 'death receptors'. To date, the overwhelming majority of studies on Fas and TRAIL (TNF-related apoptosis-inducing ligand) have explored the role of these receptors as initiators of apoptosis. However, sporadic reports also suggest that engagement of the Fas and TRAIL receptors can lead to other outcomes such as cytokine and chemokine production, cell proliferation, cell migration and differentiation. Indeed, although transformed cells frequently express Fas and TRAIL, most do not undergo apoptosis upon engagement of these receptors and significant effort has been devoted toward exploring how to sensitize such cells to the pro-apoptotic effects of 'death receptor' stimulation. Moreover, the expression of Fas and TRAIL receptors is greatly elevated in many cancer types such as hepatocellular carcinoma, renal carcinoma and ovarian cancer, suggesting that such tumors benefit from the expression of these receptors. Furthermore, several studies have shown that tumor proliferation, progression and invasion can be impaired through blocking or downregulation of Fas expression, but the mechanistic basis for these effects is largely unknown. Thus, the characterization of Fas and TRAIL as 'death receptors' is a gross oversimplification, especially in the context of cancer. It is becoming increasingly clear that 'death receptor' engagement can lead to outcomes, other than apoptosis, that become subverted by certain tumors to their benefit. Here we will discuss death-independent outcomes of Fas and TRAIL signaling and their implications for cancer.

Targeting the Fas/FasL system in Rheumatoid Arthritis therapy: Promising or risky?

Rheumatoid Arthritis (RA) is a chronic inflammatory disease affecting synovial joints. Tumor necrosis factor (TNF) α is a key component of RA pathogenesis and blocking this cytokine is the most common strategy to treat the disease. Though TNFα blockers are very efficient, one third of the RA patients are unresponsive or present side effects. Therefore, the development of novel therapeutic approaches is required. RA pathogenesis is characterized by the hyperplasia of the synovium, closely associated to the pseudo-tumoral expansion of fibroblast-like synoviocytes (FLS), which invade and destroy the joint structure. Hence, depletion of RA FLS has been proposed as an alternative therapeutic strategy. The TNF family member Fas ligand (FasL) was reported to trigger apoptosis in FLS of arthritic joints by binding to its receptor Fas and therefore suggested as a promising candidate for targeting the hyperplastic synovial tissue. However, this cytokine is pleiotropic and recent data from the literature indicate that Fas activation might have a disease-promoting role in RA by promoting cell proliferation. Therefore, a FasL-based therapy for RA requires careful evaluation before being applied. In this review we aim to overview what is known about the apoptotic and non-apoptotic effects of Fas/FasL system and discuss its relevance in RA.

Perforin, CD28 and CD95 expression in circulating CD4 and CD8 cells as predictors of head and neck (H&N) cancer patient survival

Long-term survival of H&N cancer patients has not improved significantly over the last 30 years. The possibility of using circulating blood cell phenotypes as a prognostic biomarker of H&N cancer patient was investigated in this study. Pre-treatment, circulating T lymphocyte subpopulations as well as the survival time of the patients in question were studied. Upregulated CD4+ perforin+ and CD8+ CD95+ but downregulated CD4+ CD28+ (p < 0.001) were detected in H&N cancer patients. With 3 years of follow-up time, an increase in the frequency of the pre-treatment, circulating CD4+ perforin+ cells and CD8+ perforin+ cells was showed to have reverse effects on the survival time in H&N cancer patients (p < 0.01). Detection of perforin+ frequency in CD4+ and CD8+ lymphocyte by FACS is fast, simple and cost-effective. A potential role of perforin expression in CD4+ and CD8+ cells as a prognostic biomarker for H&N cancer patient in the clinical setting was suggested.

Systems biology of death receptor networks: live and let die

The extrinsic apoptotic pathway is initiated by death receptor activation. Death receptor activation leads to the formation of death receptor signaling platforms, resulting in the demolition of the cell. Despite the fact that death receptor-mediated apoptosis has been studied to a high level of detail, its quantitative regulation until recently has been poorly understood. This situation has dramatically changed in the last years. Creation of mathematical models of death receptor signaling led to an enormous progress in the quantitative understanding of the network regulation and provided fascinating insights into the mechanisms of apoptosis control. In the following sections, the models of the death receptor signaling and their biological implications will be addressed. Central attention will be given to the models of CD95/Fas/APO-1, an exemplified member of the death receptor signaling pathways. The CD95 death-inducing signaling complex (DISC) and regulation of CD95 DISC activity by its key inhibitor c-FLIP, have been vigorously investigated by modeling approaches, and therefore will be the major topic here. Furthermore, the non-linear dynamics of the DISC, positive feedback loops and bistability as well as stoichiometric switches in extrinsic apoptosis will be discussed. Collectively, this review gives a comprehensive view how the mathematical modeling supported by quantitative experimental approaches has provided a new understanding of the death receptor signaling network.

The CD95/CD95L signaling pathway: a role in carcinogenesis

Apoptosis is a fundamental process that contributes to tissue homeostasis, immune responses, and development. The receptor CD95, also called Fas, is a member of the tumor necrosis factor receptor (TNF-R) superfamily. Its cognate ligand, CD95L, is implicated in immune homeostasis and immune surveillance, and various lineages of malignant cells exhibit loss-of-function mutations in this pathway; therefore, CD95 was initially classified as a tumor suppressor gene. However, more recent data indicate that in different pathophysiological contexts, this receptor can transmit non-apoptotic signals, promote inflammation, and contribute to carcinogenesis. A comparison with the initial molecular events of the TNF-R signaling pathway leading to non-apoptotic, apoptotic, and necrotic pathways reveals that CD95 is probably using different molecular mechanisms to transmit its non-apoptotic signals (NF-κB, MAPK, and PI3K). As discussed in this review, the molecular process by which the receptor switches from an apoptotic function to an inflammatory role is unknown. More importantly, the biological functions of these signals remain elusive.

Harnessing system models of cell death signalling for cytotoxic chemotherapy: towards personalised medicine approaches?

Most cytotoxic chemotherapeutics are believed to kill cancer cells by inducing apoptosis. Understanding the factors that contribute to impairment of apoptosis in cancer cells is therefore critical for the development of novel therapies that circumvent the widespread chemoresistance. Apoptosis, however, is a complex and tightly controlled process that can be induced by different classes of chemotherapeutics targeting different signalling nodes and pathways. Moreover, apoptosis initiation and apoptosis execution strongly depend on patient-specific, genomic and proteomic signatures. Here, we will review recent translational studies that suggest a critical link between the sensitivity of cancer cells to initiate apoptosis and clinical outcome. Next we will discuss recent advances in the field of system modelling of apoptosis pathways for the prediction of treatment responses. We propose that initiation of mitochondrial apoptosis, defined as the process of mitochondrial outer membrane permeabilisation (MOMP), is a dose-dependent decision process that allows for a prediction of individual therapy responses and therapeutic windows. We provide evidence in contrast that apoptosis execution post-MOMP may be a binary decision that dictates whether apoptosis is executed or not. We will discuss the implications of this concept for the future use of novel adjuvant therapeutics that specifically target apoptosis signalling pathways or which may be used to reduce the impact of cell-to-cell heterogeneity on therapy responses. Finally, we will discuss the technical and regulatory requirements surrounding the use and implications of system-based patient stratification tools for the future of personalised oncology.

Life in the Fas lane: differential outcomes of Fas signaling

Fas, also known as CD95 or APO-1, is a member of the tumor necrosis factor/nerve growth factor superfamily. Although best characterized in terms of its apoptotic function, recent studies have identified several other cellular responses emanating from Fas. These responses include migration, invasion, inflammation, and proliferation. In this review, we focus on the diverse cellular outcomes of Fas signaling and the molecular switches identified to date that regulate its pro- and anti-apoptotic functions. Such switches occur at different levels of signal transduction, ranging from the receptor through to cross-talk with other signaling pathways. Factors identified to date including other extracellular signals, proteins recruited to the death-inducing signaling complex, and the availability of different intracellular components of signal transduction pathways. The success of therapeutically targeting Fas will require a better understanding of these pathways, as well as the regulatory mechanisms that determine cellular outcome following receptor activation.

Glucosamine induces activated T cell apoptosis through reduced T cell receptor

Glucosamine (GlcN), like N-acetylglucosamine (GlcNAc), is salvaged into the hexosamine pathway and is converted to UDP-GlcNAc. Golgi N-glycan branching enzymes produce N-glycans, using UDP-GlcNAc as a substrate, which attach to the T cell receptor (TCR) and cytotoxic T-lymphocyte antigen-4 (CTLA-4). These findings suggest that GlcN exerts the immunoregulation through TCR signalling, which could be involved not only in cytokine production but also activated T cell apoptosis. In fact, a preliminary study showed that GlcN reduced the number of CD3+ T cells of NC/Nga mice with AD-like skin lesions. Therefore, whether apoptosis of T cells would be one of the potential molecular mechanisms of GlcN-induced immunosuppression was investigated. Cultured human primary along with Jurkat T cells and purified T cells from NC/Nga mice with or without Df-induced AD-like skin lesion were used for the study. Glucosamine treatment increased the number of T cells expressing β1,6GlcNAc-branched N-glycans, with reduced ZAP-70 phosphorylation and enhanced CTLA-4 expression. Glucosamine treatment reduced the number of activated T cells from both the human primary and Jurkat cells and the dermatitis-induced mice. The expression of FasL and activated caspases, particularly caspase-3, was increased, whereas the phosphorylation of PI3K, Akt and NF-κB was decreased by GlcN treatment. Therefore, in addition to down-regulating TCR signalling and promoting CTLA-4 expression, GlcN may also suppress T cell function by enhancing apoptosis of activated T cells, through both extrinsic and intrinsic apoptotic signalling pathways, which were regulated by the inhibition of PI3K/Akt and NF-κB phosphorylation.

Mathematical modeling of apoptosis

Apoptosis is a form of programmed cell death, which is fundamental to all multicellular organisms. Deregulation of apoptosis leads to a number of severe diseases including cancer. Apoptosis is initiated either by extrinsic signals via stimulation of receptors at the cellular surface or intrinsic signals, such as DNA damage or growth factor withdrawal. Apoptosis has been extensively studied using systems biology which substantially contributed to the understanding of this death signaling network. This review gives an overview of mathematical models of apoptosis and the potential of systems biology to contribute to the development of novel therapies for cancer or other apoptosis-related diseases.

Cells surviving fractional killing by TRAIL exhibit transient but sustainable resistance and inflammatory phenotypes

Cells that survive fractional killing by TRAIL or FasR agonists enter a state of resistance accompanied by inflammatory phenotypes. This state is transient, decaying over the course of several days, but can be sustained by periodic TRAIL treatments. This finding has implications for optimal dosing strategies of extrinsic cell death agents.

When clonal populations of human cells are exposed to apoptosis-inducing agents, some cells die and others survive. This fractional killing arises not from mutation but from preexisting, stochastic differences in the levels and activities of proteins regulating apoptosis. Here we examine the properties of cells that survive treatment with agonists of two distinct death receptors, tumor necrosis factor–related apoptosis-inducing ligand (TRAIL) and anti-FasR antibodies. We find that “survivor” cells are highly resistant to a second ligand dose applied 1 d later. Resistance is reversible, resetting after several days of culture in the absence of death ligand. “Reset” cells appear identical to drug-naive cells with respect to death ligand sensitivity and gene expression profiles. TRAIL survivors are cross-resistant to activators of FasR and vice versa and exhibit an NF-κB–dependent inflammatory phenotype. Remarkably, reversible resistance is induced in the absence of cell death when caspase inhibitors are present and can be sustained for 1 wk or more, also without cell death, by periodic ligand exposure. Thus stochastic differences in cell state can have sustained consequences for sen­sitivity to prodeath ligands and acquisition of proinflammatory phenotypes. The important role played by periodicity in TRAIL exposure for induction of opposing apoptosis and survival mechanisms has implications for the design of optimal therapeutic agents and protocols.

Systems analysis of cancer cell heterogeneity in caspase-dependent apoptosis subsequent to mitochondrial outer membrane permeabilization

Deregulation of apoptosis is a hallmark of carcinogenesis. We here combine live cell imaging and systems modeling to investigate caspase-dependent apoptosis execution subsequent to mitochondrial outer membrane permeabilization (MOMP) in several cancer cell lines. We demonstrate that, although most cell lines that underwent MOMP also showed robust and fast activation of executioner caspases and apoptosis, the colorectal cancer cell lines LoVo and HCT-116 Smac(-/-), similar to X-linked inhibitor of apoptosis protein (XIAP)-overexpressing HeLa (HeLa XIAP(Adv)) cells, only showed delayed and often no caspase activation, suggesting apoptosis impairment subsequent to MOMP. Employing APOPTO-CELL, a recently established model of apoptosis subsequent to MOMP, this impairment could be understood by studying the systemic interaction of five proteins that are present in the apoptosis pathway subsequent to MOMP. Using APOPTO-CELL as a tool to study detailed molecular mechanisms during apoptosis execution in individual cell lines, we demonstrate that caspase-9 was the most important regulator in DLD-1, HCT-116, and HeLa cells and identified additional cell line-specific co-regulators. Developing and applying a computational workflow for parameter screening, systems modeling identified that apoptosis execution kinetics are more robust against changes in reaction kinetics in HCT-116 and HeLa than in DLD-1 cells. Our systems modeling study is the first to draw attention to the variability in cell specific protein levels and reaction rates and to the emergent effects of such variability on the efficiency of apoptosis execution and on apoptosis impairment subsequent to MOMP.

Apoptotic potential of Fas-associated death domain on regulation of cell death regulatory protein cFLIP and death receptor mediated apoptosis in HEK 293T cells

Fas-associated death domain (FADD) is a common adaptor molecule which plays an important role in transduction of death receptor mediated apoptosis. The FADD provides DED motif for binding to both procaspase-8 and cFLIP molecules which executes death receptor mediated apoptosis. Dysregulated expression of FADD and cFLIP may contribute to inhibition of apoptosis and promote cell survival in cancer. Moreover elevated intracellular level of cFLIP competitively excludes the binding of procaspase-8 to the death effector domain (DED) of FADD at the DISC to block the activation of death receptor signaling required for apoptosis. Increasing evidence shows that defects in FADD protein expression are associated with progression of malignancies and resistance to apoptosis. Therefore, improved expression and function of FADD may provide new paradigms for regulation of cell proliferation and survival in cancer. In the present study, we have examined the potential of FADD in induction of apoptosis by overexpression of FADD in HEK 293T cells and validated further its consequences on the expression of pro and anti-apoptotic proteins besides initiation of death receptor mediated signaling. We have found deficient expression of FADD and elevated expression of cFLIP(L) in HEK 293T cells. Our results demonstrate that over expression of FADD attenuates the expression of anti-apoptotic protein cFLIP and activates the cascade of extrinsic caspases to execution of apoptosis in HEK 293T cells.

Signaling active CD95 receptor molecules trigger co-translocation of inactive CD95 molecules into lipid rafts

The capability of soluble CD95L trimers to trigger CD95-associated signaling pathways is drastically increased by oligomerization. The latter can be achieved, for example, by antibodies recognizing a N-terminal epitope tag in recombinant CD95L variants or by genetic engineering-enforced formation of hexamers. Using highly sensitive and accurate binding studies with recombinant CD95L variants equipped with a Gaussia princeps luciferase reporter domain, we found that oligomerization of CD95L has no major effect on CD95 occupancy. This indicates that the higher activity of oligomerized CD95L trimers is not related to an avidity-related increase in apparent affinity and points instead to a crucial role of aggregation of initially formed trimeric CD95L-CD95 complexes in CD95 activation. Furthermore, binding of soluble CD95L trimers was found to be insufficient to increase the association of CD95 with the lipid raft-containing membrane fraction. However, when Gaussia princeps luciferase-CD95L trimers were used as tracers to "mark" inactive CD95 molecules, increased association of these inactive receptors was observed upon activation of the remaining CD95 molecules by help of highly active hexameric Fc-CD95L or membrane CD95L. Moreover, in cells expressing endogenous CD95 and chimeric CD40-CD95 receptors, triggering of CD95 signaling via endogenous CD95 resulted in co-translocation of CD40-CD95 to the lipid raft fraction, whereas vice versa activation of CD95-associated pathways with Fc-CD40L via CD40-CD95 resulted in co-translocation of endogenous CD95. In sum, this shows that signaling-active CD95 molecules not only enhance their own association with the lipid raft-containing membrane fraction but also those of inactive CD95 molecules.

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

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

Death receptors DR6 and TROY regulate brain vascular development

Signaling events that regulate central nervous system (CNS) angiogenesis and blood-brain barrier (BBB) formation are only beginning to be elucidated. By evaluating the gene expression profile of mouse vasculature, we identified DR6/TNFRSF21 and TROY/TNFRSF19 as regulators of CNS-specific angiogenesis in both zebrafish and mice. Furthermore, these two death receptors interact both genetically and physically and are required for vascular endothelial growth factor (VEGF)-mediated JNK activation and subsequent human brain endothelial sprouting in vitro. Increasing beta-catenin levels in brain endothelium upregulate DR6 and TROY, indicating that these death receptors are downstream target genes of Wnt/beta-catenin signaling, which has been shown to be required for BBB development. These findings define a role for death receptors DR6 and TROY in CNS-specific vascular development.

Depletion of the receptor for advanced glycation end products (RAGE) sensitizes towards apoptosis via p53 and p73 posttranslational regulation

The receptor for advanced glycation endproduct (RAGE) is involved in diabetic complications and chronic inflammation, conditions known to affect the sensitivity towards apoptosis. Here, we studied the effect of genetically depleting RAGE on the susceptibility towards apoptosis. In murine osteoblastic cells, RAGE knockout increased both spontaneous and induced apoptosis. Decreased levels of B-cell lymphoma 2 protein and increased intrinsic apoptosis were observed in Rage(-/-) cells. Furthermore, loss of RAGE increased expression of the death receptor CD95 (Fas, Apo-1), CD95-dependent caspase activation and extrinsic apoptosis, whereas NF-kB-p65 nuclear translocation was diminished. Importantly, depletion of RAGE reduced the ubiquitination and degradation of p53 and p73 and increased their nuclear translocation. The increase of p53 and p73 transactivational activity was essential for the RAGE-dependent regulation of apoptosis, because knockdown of p53 and p73 significantly decreased apoptosis in RAGE-deficient but not in RAGE-expressing cells. Thus, the RAGE-mediated posttranslational regulation of p53 and p73 orchestrates a sequence of events culminating in control of intrinsic and extrinsic apoptosis signaling pathways.

Fas signaling promotes motility and metastasis through epithelial-mesenchymal transition in gastrointestinal cancer

Fas signaling was reported to participate in cell apoptosis. However, this pathway has also been shown to promote tumor cell motility, leading to the hypothesis that Fas signaling may induce epithelial-mesenchymal transition (EMT) to promote metastasis. The effects of Fas-ligand (FasL) treatment and inhibition of Fas signaling on colorectal and gastric cancer cells were tested using motility assay, immunofluorescence, RT-PCR and immunoblot analyses. Fas signaling downregulated epithelial markers, upregulated mesenchymal markers and promoted motility in gastrointestinal (GI) cancer cells. FasL treatment also increased the expression of EMT transcriptional factors in the nucleus and induced a spindle shape cell morphology in these cells. Knockdown of Snail or Twist expression significantly decreased FasL-induced motility. The ERK1/2 pathway was activated by Fas signaling and is required for FasL-induced EMT and motility. Moreover, oxaliplatin, a chemotherapeutic agent, induced EMT partly through Fas signaling. Evaluation of human GI clinical specimens showed that FasL expression increased whereas E-cadherin expression decreased during GI cancer progression. Both markers were significantly inversely correlated. Tissue samples with a non-EMT phenotype were mainly distributed in patients with early cancer stages, whereas samples with an EMT phenotype were mostly distributed in patients with advanced cancer stages. A non-EMT phenotype significantly correlated with better prognosis. Altogether, these data indicate that Fas signaling may induce EMT to promote tumor motility and metastasis in GI cancer in vivo and in vitro.

CD95 signaling in colorectal cancer

CD95 and its ligand (CD95L) are widely expressed in colorectal tumors, but their role in shaping tumor behavior is unclear. CD95 activation on tumor cells can lead to apoptosis, while CD95L attracts neutrophils, suggesting a function in tumor suppression. However, CD95 can also promote tumorigenesis, at least in part by activating non-apoptotic signaling pathways that stimulate tumor cell proliferation, invasion and survival. In addition, CD95 signaling in stromal cells and tumor-infiltrating inflammatory cells has to be taken into account when addressing the function of CD95 and its ligand in colorectal tumor biology. We present a model in which the tumor-suppressing and tumor-promoting activities of CD95/CD95L together determine colorectal tumor behavior. We also discuss how these multiple activities are changing our view of CD95 and CD95L as potential therapeutic targets in the treatment of colorectal cancer. We conclude that locking CD95 in apoptosis-mode may be a more promising anti-cancer strategy than simply inhibiting or stimulating CD95.

Lyapunov exponents and phase diagrams reveal multi-factorial control over TRAIL-induced apoptosis

Kinetic modeling, phase diagrams analysis, and quantitative single-cell experiments are combined to investigate how multiple factors, including the XIAP:caspase-3 ratio and ligand concentration, regulate receptor-mediated apoptosis.

Based on protein expression levels, Lyapunov-based phase diagrams predict which pathways are required for a cell to undergo receptor-mediated cell death.

Multiple inter-dependent factors, including the XIAP:caspase-3 ratio and ligand concentration, regulate the requirement for mitochondrial outer membrane permeabilization during receptor-mediated apoptosis.

The E3 ubiquitin ligase activity of XIAP is essential for maintaining the ‘snap-action' regulation of effector caspase activity.

Cell-to-cell variability in protein expression gives rise to mixed phenotypes in cell lines that map close to boundaries (separatrices) identified by Lyapunov exponent analysis.

In mammalian cells, extrinsic (receptor-mediated) apoptosis is triggered by binding of extracellular death ligands such as tumor necrosis factor (TNF) and TRAIL (TNF-related apoptosis-inducing ligand) to cognate receptors. When death receptors are activated, death inducing signaling complexes (DISCs) assemble causing activation and cleavage of initiator pro-caspases-8 and -10, which then cleave effector pro-caspases-3 and -7 in a multi-enzyme cascade (Riedl and Shi, 2004). Active effector caspases digest essential cellular proteins and activate the CAD nucleases that cleave genomic DNA, thereby killing cells. This cascade of DISC assembly followed by initiator and then effector caspase activation is sufficient to kill so-called type I cells (e.g. B lymphocytes), but most cell types exhibit a type II behavior in which mitochondrial outer membrane permeabilization (MOMP) is an essential step in the march to death (Scaffidi et al, 1998; Barnhart et al, 2003; Letai, 2008). Identifying factors that determine whether cells are type I or II is of practical and theoretical interest. From a practical perspective, whether a cell requires MOMP for apoptosis determines the potency of Bcl2 and similar oncogenes, the efficacy of anti-Bcl2 drugs such as navitoclax (ABT-263), and the sensitivity of cells to TRAIL and anti-TRAIL receptor antibodies, which are also investigational anti-cancer drugs (Newsom-Davis et al, 2009). From a theoretical perspective, the type I versus II choice exemplifies a common situation in mammalian cells in which overlapping signaling pathways play a greater or lesser role in controlling cell fate depending on cell type: it is remarkable that a simple three-step (receptor→initiator caspase→effector caspase) process is sufficient to trigger apoptosis in some cell types but that a much more complex route involving MOMP is required in others.

Attempts to understand why some cells require MOMP for cell death and others do not have identified differences in the oligomeric state of death ligand receptors and the efficiency of DISC formation as important variables. In cells in which DISCs form efficiently, initiator caspases are cleaved rapidly and sufficient effector pro-caspases are processed into their active forms to kill cells (type I cells; Scaffidi et al, 1999b). In type II cells, DISC formation seems to be less efficient, and it has been proposed that MOMP is required to amplify weak initiator caspase signals and thereby generate lethal effector caspase levels (Barnhart et al, 2003). However, it has recently become apparent that XIAP also plays a role in type I versus II choice: in XIAP knockout mice, liver cells switch from a type II to a type I phenotype (Jost et al, 2009) and XIAP is observed to be involved in the survival of type I cells treated with death ligands in culture (Maas et al, 2010).

In this paper, we attempt to place these observations in a quantitative context by analyzing a computational model of extrinsic cell death using a method drawn from dynamical system analysis, direct finite-time Lyapunov exponent (DLE) analysis. Our implementation of DLE analysis relates changes in the concentrations of protein in a model to an outcome several hours later. We computed DLEs for six regulators of apoptosis over a range of concentrations determined experimentally to represent a natural range of variation in parental or genetically modified tumor cell lines. This generated a phase space onto which individual cell lines could be mapped using quantitative immunoblotting data. Cell-to-cell variation was estimated by flow cytometry and also mapped onto the phase space. The most interesting regions of the space were those in which a small change in one or more initial protein concentration resulted in a dramatic change in phenotype. Such a boundary or separatrix was observed in slices of phase space corresponding XIAP versus pro-caspase-3 concentration (the [XIAP]:[caspase-3] ratio). In cells in which the ratio is low, a type I phenotype is predicted to occur; when the ratio is high, a type II phenotype is favored; and in cell lines that lie close to the separatrix, cell-to-cell variability is expected, with some cells exhibiting a type I phenotype and others a type II behavior. DLE analysis shows that the [XIAP]:[caspase-3] ratio is not the only controlling factor in type I versus II control: as receptor activity or ligand concentration increase, the position of the separatrix changes so as to expand the region in which the type I phenotype is favored.

We tested these predictions by manipulating XIAP and ligand levels in multiple cell lines and then followed cell death by imaging, flow cytometry, or clonogenic assays. We observed that when XIAP was knocked out (by homologous recombination) in the HCT116 colorectal cancer line, cells shifted from a pure type II to a type I phenotype, as predicted from the DLE phase diagram. SKW6.4 B-cell lymphoma cells were predicted to lie at a position in phase space that is insensitive to XIAP levels (within the range achievable by over-expression) and we confirmed this experimentally. Finally, T47D breast cancer cells were predicted—and observed—to straddle the separatrix and to exhibit cell-to-cell variability in fate, with some cells showing a type I and others a type II phenotype. As the concentration of TRAIL was increased, the ratio of type I to type II T47D cells increased, confirming the prediction that this ratio is controlled in a multi-factorial manner.

To extend our approach to mutations that change protein activity rather than protein level, we simulated the effects of changing rate constants that control ubiquitylation of caspase-3 following its binding to XIAP. We generated cells carrying a truncated form of XIAP that lacks the RING domain (XIAPΔRING) and cannot mediate the ubiquitylation of caspase-3 (this truncation leaves the affinity of XIAP for caspase-3 unchanged). We predicted and demonstrated experimentally that expression of XIAPΔRING disrupts normal snap-action control over caspase-3 activation. Our findings not only advance understanding of extrinsic apoptosis but also constitute a proof of principle for an approach to quantitative modeling of dynamic regulatory processes in diverse cell types.

Receptor-mediated apoptosis proceeds via two pathways: one requiring only a cascade of initiator and effector caspases (type I behavior) and the second requiring an initiator–effector caspase cascade and mitochondrial outer membrane permeabilization (type II behavior). Here, we investigate factors controlling type I versus II phenotypes by performing Lyapunov exponent analysis of an ODE-based model of cell death. The resulting phase diagrams predict that the ratio of XIAP to pro-caspase-3 concentrations plays a key regulatory role: type I behavior predominates when the ratio is low and type II behavior when the ratio is high. Cell-to-cell variability in phenotype is observed when the ratio is close to the type I versus II boundary. By positioning multiple tumor cell lines on the phase diagram we confirm these predictions. We also extend phase space analysis to mutations affecting the rate of caspase-3 ubiquitylation by XIAP, predicting and showing that such mutations abolish all-or-none control over activation of effector caspases. Thus, phase diagrams derived from Lyapunov exponent analysis represent a means to study multi-factorial control over a complex biochemical pathway.

Regulation of CD95/Fas signaling at the DISC

CD95 (APO-1/Fas) is a member of the death receptor (DR) family. Stimulation of CD95 leads to induction of apoptotic and non-apoptotic signaling pathways. The formation of the CD95 death-inducing signaling complex (DISC) is the initial step of CD95 signaling. Activation of procaspase-8 at the DISC leads to the induction of DR-mediated apoptosis. The activation of procaspase-8 is blocked by cellular FLICE-inhibitory proteins (c-FLIP). This review is focused on the role in the CD95-mediated signaling of the death effector domain-containing proteins procaspase-8 and c-FLIP. We discuss how dynamic cross-talk between procaspase-8 and c-FLIP at the DISC regulates life/death decisions at CD95.