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1.  FLIP Protects against Hypoxia/Reoxygenation-Induced Endothelial Cell Apoptosis by Inhibiting Bax Activation 
Molecular and Cellular Biology  2005;25(11):4742-4751.
Hypoxia/reoxygenation causes cell death, yet the underlying regulatory mechanisms remain partially understood. Recent studies demonstrate that hypoxia/reoxygenation can activate death receptor and mitochondria-dependent apoptotic pathways, involving Bid and Bax mitochondrial translocation and cytochrome c release. Using mouse lung endothelial cells (MLEC), we examined the role of FLIP, an inhibitor of caspase 8, in hypoxia/reoxygenation-induced cell death. FLIP protected MLEC against hypoxia/reoxygenation by blocking both caspase 8/Bid and Bax/mitochondrial apoptotic pathways. FLIP inhibited Bax activation in wild-type and Bid−/− MLEC, indicating independence from the caspase 8/Bid pathway. FLIP also inhibited the expression and activation of protein kinase C (PKC) (α, ζ) during hypoxia/reoxygenation and promoted an association of inactive forms of PKC with Bax. Surprisingly, FLIP expression also inhibited death-inducing signal complex (DISC) formation in the plasma membrane and promoted the accumulation of the DISC in the Golgi apparatus. FLIP expression also upregulated Bcl-XL, an antiapoptotic protein. In conclusion, FLIP decreased DISC formation in the plasma membrane by blocking its translocation from the Golgi apparatus and inhibited Bax activation through a novel PKC-dependent mechanism. The inhibitory effects of FLIP on Bax activation and plasma membrane DISC formation may play significant roles in protecting endothelial cells from the lethal effects of hypoxia/reoxygenation.
doi:10.1128/MCB.25.11.4742-4751.2005
PMCID: PMC1140634  PMID: 15899875
2.  Induction of BIM Is Essential for Apoptosis Triggered by EGFR Kinase Inhibitors in Mutant EGFR-Dependent Lung Adenocarcinomas 
PLoS Medicine  2007;4(10):e294.
Background
Mutations in the epidermal growth factor receptor (EGFR) gene are associated with increased sensitivity of lung cancers to kinase inhibitors like erlotinib. Mechanisms of cell death that occur after kinase inhibition in these oncogene-dependent tumors have not been well delineated. We sought to improve understanding of this process in order to provide insight into mechanisms of sensitivity and/or resistance to tyrosine kinase inhibitors and to uncover new targets for therapy.
Methods and Findings
Using a panel of human lung cancer cell lines that harbor EGFR mutations and a variety of biochemical, molecular, and cellular techniques, we show that EGFR kinase inhibition in drug-sensitive cells provokes apoptosis via the intrinsic pathway of caspase activation. The process requires induction of the proapoptotic BH3-only BCL2 family member BIM (i.e., BCL2-like 11, or BCL2L11); erlotinib dramatically induces BIM levels in sensitive but not in resistant cell lines, and knockdown of BIM expression by RNA interference virtually eliminates drug-induced cell killing in vitro. BIM status is regulated at both transcriptional and posttranscriptional levels and is influenced by the extracellular signal-regulated kinase (ERK) signaling cascade downstream of EGFR. Consistent with these findings, lung tumors and xenografts from mice bearing mutant EGFR-dependent lung adenocarcinomas display increased concentrations of Bim after erlotinib treatment. Moreover, an inhibitor of antiapoptotic proteins, ABT-737, enhances erlotinib-induced cell death in vitro.
Conclusions
In drug-sensitive EGFR mutant lung cancer cells, induction of BIM is essential for apoptosis triggered by EGFR kinase inhibitors. This finding implies that the intrinsic pathway of caspase activation may influence sensitivity and/or resistance of EGFR mutant lung tumor cells to EGFR kinase inhibition. Manipulation of the intrinsic pathway could be a therapeutic strategy to enhance further the clinical outcomes of patients with EGFR mutant lung tumors.
Using a panel of human drug-sensitive EGFR mutant lung cancer cells, William Pao and colleagues show that induction of BIM, a member of the BCL2 family, is essential for apoptosis triggered by EGFR kinase inhibitors.
Editors' Summary
Background.
Lung cancer, a common type of cancer, has a very low cure rate. Like all cancers, it occurs when cells begin to divide uncontrollably because of changes (mutations) in their genes. Chemotherapy drugs kill these rapidly dividing cells but, because some normal tissues are sensitive to these agents, it is hard to destroy the cancer without causing serious side effects. Recently, “targeted” therapies have brought new hope to some patients with cancer. These therapies attack the changes in cancer cells that allow them to divide uncontrollably but leave normal cells unscathed. One of the first molecules for which a targeted therapy was developed was the epidermal growth factor receptor (EGFR). In normal cells, messenger proteins bind to EGFR and activate its “tyrosine kinase,” an enzyme that sticks phosphate groups on tyrosine (an amino acid) in other proteins. These proteins then tell the cell to divide. Alterations to this signaling system drive uncontrolled cell division in some cancers so blocking the EGFR signaling pathway should stop these cancers growing. Indeed, some lung cancers with mutations in the tyrosine kinase of EGFR shrink dramatically when treated with gefitinib or erlotinib, two tyrosine kinase inhibitors (TKIs).
Why Was This Study Done?
TKI-sensitive lung cancers shrink when treated with TKIs because of drug-induced cell death, but what are the molecular mechanisms underlying this death? A better understanding of how TKIs kill cancer cells might provide new insights into why not all cancer cells with mutations in EGFR (the gene from which EGFR is made) are sensitive to TKIs. It might also uncover new targets for therapy. TKIs do not completely kill lung cancers, but if the mechanism of TKI-induced cell death were understood, it might be possible to enhance their effects. In this study, the researchers have investigated how cell death occurs after kinase inhibition in a panel of human lung cancer cell lines (cells isolated from human tumors that grow indefinitely in dishes) that carry EGFR mutations.
What Did the Researchers Do and Find?
The researchers show, first, that erlotinib induces a type of cell death called apoptosis in erlotinib-sensitive cell lines but not in resistant cell lines. Apoptosis can be activated by two major pathways. In this instance, the researchers report, the so-called “intrinsic” pathway activates apoptosis. This pathway is stimulated by proapoptotic members of the BCL2 family of proteins and is blocked by antiapoptotic members, so the researchers examined the effect of erlotinib treatment on the expression of BCL2 family members in the EGFR mutant cell lines. Erlotinib treatment increased the expression of the proapoptotic protein BIM in sensitive but not in resistant cell lines. It also removed phosphate groups from BIM—dephosphorylated BIM is a more potent proapoptotic protein. Conversely, blocking BIM expression using a technique called RNA interference virtually eliminated the ability of erlotinib to kill EGFR mutant cell lines. The researchers also report that erlotinib treatment increased BIM expression in erlotinib-sensitive lung tumors growing in mice and that an inhibitor of the anti-apoptotic protein BCL2 enhanced erlotinib-induced death in drug-sensitive cells growing in dishes.
What Do These Findings Mean?
These findings indicate that BIM activity is essential for the apoptosis triggered by TKIs in drug-sensitive lung cancer cells that carry EGFR mutations, and that treatment of these cells with TKIs induces both the expression and dephosphorylation of BIM. The finding that the intrinsic pathway of apoptosis activation is involved in TKI-induced cell death suggests that changes in this pathway (possibly mutations in some of its components) might influence the sensitivity of EGFR mutant lung cancers to TKIs. Finally, these findings suggest that giving drugs that affect the intrinsic pathway of apoptosis activation at the same time as TKIs might further improve the clinical outcome for patients with EGFR mutant tumors. Such combinations will have to be tested in clinical trials before being used routinely.
Additional Information.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0040294.
US National Cancer Institute information for patients and professionals on lung cancer (in English and Spanish)
Information for patients from Cancer Research UK on lung cancer including information on treatment with TKIs
Wikipedia pages on apoptosis, epidermal growth factor receptor, and BCL-2 proteins (note that Wikipedia is a free online encyclopedia that anyone can edit; available in several languages)
Information for patients from Cancerbackup on erlotinib and gefitinib
doi:10.1371/journal.pmed.0040294
PMCID: PMC2001209  PMID: 17927446
3.  A Protective Hsp70-TLR4 Pathway in Lethal Oxidant Lung Injury 
Administering high levels of inspired oxygen, or hyperoxia, is commonly used as a life-sustaining measure in critically ill patients. However, prolonged exposures can exacerbate respiratory failure. Our previous study showed that toll-like receptor 4 (TLR4) confers protection against hyperoxia-induced lung injury and mortality. Hsp70 has potent cytoprotective properties and has been described as a TLR4 ligand in cell lines. We sought to elucidate the relationship between TLR4 and Hsp70 in hyperoxia-induced lung injury in vitro and in vivo and to define the signaling mechanisms involved. Wild type, TLR4−/− and Trif−/− (a TLR4 adapter protein) murine lung endothelial cells (MLEC) were exposed to hyperoxia. We found markedly elevated levels of intracellular and secreted Hsp70 from mice lung and MLEC after hyperoxia. We confirmed that Hsp70 and TLR4 co-immunoprecipitate in lung tissue and MLEC. Hsp70-mediated NFκB activation appears to depend upon TLR4. In the absence of TLR4, Hsp70 loses its protective effects in endothelial cells. Furthermore, these protective properties of Hsp70 are TLR4 adapter Trif-dependent, MyD88-independent. Hsp70-deficient mice have increased mortality during hyperoxia and lung-targeted adenoviral delivery of Hsp70 effectively rescues both Hsp70-deficient and wild type mice. Our studies are the first to define an Hsp70-TLR4-Trif cytoprotective axis in the lung and endothelial cells. This pathway is a potential therapeutic target against a range of oxidant-induced lung injuries.
doi:10.4049/jimmunol.1300052
PMCID: PMC3730854  PMID: 23817427
4.  Dynamics within the CD95 death-inducing signaling complex decide life and death of cells 
CD95-mediated apoptotic and NF-κB signaling were described by a simple kinetic model. We used a model reduction technique to reduce the number of reactions from 92 to 23 while maintaining a good model fit.p43-FLIP, which is generated at the CD95 DISC by procaspase-8 cleavage, was found to be the link between the CD95 DISC and the NF-κB pathway. P43-FLIP interacts with the IKK complex and leads to its activation.The CD95 DISC complex acts as a signal processor that diverges signals into the apoptotic and NF-κB pathways depending on the amounts of specific DISC proteins.Life/death decisions in CD95 signaling are determined by c-FLIPL and procaspase-8 in a non-linear way.
The CD95 protein (APO-1/Fas; Krammer et al, 2007) is a member of the death receptor family. Signal transduction of CD95 starts with the formation of the death-inducing signaling complex (DISC) detectable within seconds after receptor stimulation (Kischkel et al, 1995). The DISC consists of CD95, the adaptor molecule FADD, procaspase-8/10 and c-FLIPL/S/R (Muzio et al, 1996; Scaffidi et al, 1999; Sprick et al, 2002; Golks et al, 2005; Krammer et al, 2007). Procaspase-8 is converted at the DISC, in a series of autoproteolytic cleavage steps, to p43/p41 and p18, which leads to the activation of effector caspase-3 and demolition of the cell. Recently, experiments have demonstrated that CD95L also activates the induction of transcription factor NF-κB (Barnhart et al, 2004; Kreuz et al, 2004; Peter et al, 2007). It was shown that DED-containing proteins at the DISC, such as procaspase-8 and c-FLIP have a complex role in NF-κB activation (Chaudhary et al, 2000; Hu et al, 2000; Kreuz et al, 2004; Dohrman et al, 2005; Su et al, 2005). These findings motivated our systems biology approach and prompted us to determine whether CD95-mediated signaling should be considered a dynamic system, resulting in life/death decisions.
We observed simultaneous apoptosis and NF-κB induction on CD95 stimulation in HeLa cells stably overexpressing CD95–GFP (HeLa-CD95) using biochemical approaches and live-cell imaging. To understand the crosstalk between CD95-mediated apoptosis and NF-κB activation, we created a mathematical model of CD95 signaling. Our model assumes a trimerized ligand (L) that binds to a trimerized CD95 receptor (R) that can recruit three copies of FADD (F) leading to the DISC formation. Subsequently, DED-containing proteins, such as procaspase-8 (C8), c-FLIPL (FL) and c-FLIPS (FS) can bind to FADD. The order of protein binding gives rise to a combinatorial variety of intermediates, resulting either in the formation of the cleavage product of procaspase-8: p43/p41, or in the formation of the cleavage product of c-FLIPL: p43-FLIP. p43/p41 gives rise to signaling in the apoptotic branch of the model, whereas the cleavage product p43-FLIP triggers the activation of NF-κB. The model postulates that p43-FLIP interacts with the IKK complex leading to the phosphorylation of IκB (NF-κB·IκB·P), which entails its degradation and the translocation of p65 to the nucleus (NF-κB*). As a validation of the model topology, we confirmed experimentally that p43-FLIP interacts with the IKK complex and subsequently leads to its activation.
The complete model could be fitted well to a data set derived from quantitative western blots of a number of key proteins of the apoptotic and NF-κB pathways. However, we tested whether all the 92 reactions were required to reproduce the observed dynamics, as a small model would yield more reliable parameter estimates, which in turn would increase its usefulness as a predictive tool. To determine the most important interactions, we simplified the complete model in a step-wise manner obtaining a model of considerably lower complexity (Figure 5A, simplification steps are listed in Figure 5B). The final reduced model still approximated well the experimental data set (Figure 5D), whereas the number of reactions decreased from 92 to 23 (Figure 5C).
To better understand the interplay of DISC proteins in the determination of cell fate, we analyzed the activity of caspase-3 and NF-κB as a function of procaspase-8 and c-FLIPL levels (Figure 8A). We observed in our simulations that the decision over apoptosis and NF-κB is controlled by both proteins. Different scenarios occur that show combination or absence of either caspase-3 or NF-κB activity. The phase diagram shown in Figure 8A predicts that either increasing or decreasing the amount of c-FLIPL leads to a different signaling mode. We sought to validate this prediction by downregulating or overexpressing procaspase-8 and c-FLIPL, respectively, in HeLa-CD95 cells and measuring CD95-mediated signaling. In agreement with the phase diagram (Figure 8A), we observed that c-FLIPL overexpression resulted in a strong reduction of apoptosis (Figure 8D). Furthermore, we could further confirm by western blot analysis that the stable knockdown of c-FLIPL and procaspase-8 led to a reduction of the levels of p43-FLIP and phosphorylated IκBα after receptor stimulation (Figure 8C and D). In addition, to control the specificity of c-FLIP downregulation and further confirm the requirement of cleavage of c-FLIPL to p43-FLIP, we performed a reconstitution experiment in HeLa-CD95–c-FLIP-deficient cells (Figure 8E). Cells reconstituted with WT c-FLIPL were able to generate p43-FLIP and increased IκBα phosphorylation on CD95 stimulation. In contrast, cells reconstituted with the noncleavable mutant of c-FLIPL (D376E) did not show processing to p43-FLIP (Figure 8E; Supplementary Figure S9). Noticeably, as postulated by the model, this resulted in a strong reduction of the levels of IκBα phosphorylation on CD95 stimulation. Hence, by perturbing the ratio of procaspase-8 to c-FLIPL at the DISC, we directed the induction of apoptosis and NF-κB activation as predicted by our model. Taken together, we found that the DISC protein levels determine cell fate in a nonlinear manner, highlighting the role of signal processing within the DISC.
In this study, we propose, to the best of our knowledge, the first integrated kinetic model of CD95-mediated apoptosis and NF-κB signaling. This was achieved by integrating mechanistic knowledge of DISC assembly and caspase activation with a simple scheme of NF-κB activation. We observed that c-FLIPL levels crucially determine the balance between apoptotic and NF-κB signaling by shaping the dynamics of DISC assembly. Although this finding is based on experiments performed in cell lines, we expect that the nonlinear dynamics of DISC assembly is a generic systems property of life/death decision making in CD95 signaling pathways. This is especially important for understanding the regulation of cell death in physiologically relevant cells, such as cancer cells often showing resistance against death receptor-induced apoptosis.
This study explores the dilemma in cellular signaling that triggering of CD95 (Fas/APO-1) in some situations results in cell death and in others leads to the activation of NF-κB. We established an integrated kinetic mathematical model for CD95-mediated apoptotic and NF-κB signaling. Systematic model reduction resulted in a surprisingly simple model well approximating experimentally observed dynamics. The model postulates a new link between c-FLIPL cleavage in the death-inducing signaling complex (DISC) and the NF-κB pathway. We validated experimentally that CD95 stimulation resulted in an interaction of p43-FLIP with the IKK complex followed by its activation. Furthermore, we showed that the apoptotic and NF-κB pathways diverge already at the DISC. Model and experimental analysis of DISC formation showed that a subtle balance of c-FLIPL and procaspase-8 determines life/death decisions in a nonlinear manner. We present an integrated model describing the complex dynamics of CD95-mediated apoptosis and NF-κB signaling.
doi:10.1038/msb.2010.6
PMCID: PMC2858442  PMID: 20212524
apoptosis; CD95 signaling; DISC; model reduction; NF-κB
5.  Human Cytomegalovirus Causes Endothelial Injury Through the Ataxia Telangiectasia Mutant and p53 DNA Damage Signaling Pathways 
Circulation research  2004;94(10):1310-1317.
Atherosclerosis is the leading cause of death in the United States, and human cytomegalovirus (HCMV), a member of the herpes virus family, may play a role in the development of the disease. We previously showed that HCMV regulated endothelial apoptosis. In this study, we investigated the induction of apoptosis and signal transduction pathways regulating this process in HCMV-infected endothelial cells. As observed previously, HCMV induced a typical cytopathic effect in human aortic endothelial cells (HAECs), ie, the formation of single nucleated or multinucleated giant cells. Although infected HAECs were resistant to apoptosis at earlier stages of infection, they became apoptotic with prolonged infection as demonstrated by positive staining using terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL). This apoptotic process was mediated by the caspase-dependent mitochondrial apoptotic pathway as indicated by increased expression and cleavage of caspases 3 and 9 as well as increased expressions of pro-apoptotic molecules Bax and Bak. Blocking caspases 3 or 9 significantly inhibited the HCMV-induced apoptosis. Further exploration of the upstream pathway demonstrated upregulation of the tumor suppressor p53 gene and activation of the ataxia telangiectasia mutant (ATM) pathway in the infected cells. Blocking p53 inhibited HCMV-stimulated Bax and Bak expression as well as caspase-3 activation and blocking the ATM pathway inhibited HCMV-stimulated p53 activation. Although early infection may render cells antiapoptotic, prolonged infection, however, induced endothelial apoptosis through ATM and p53-dependent activation of the mitochondrial death pathway. This proapoptotic effect may be relevant to endothelial dysfunction and HCMV-associated vascular diseases.
doi:10.1161/01.RES.0000129180.13992.43
PMCID: PMC1350949  PMID: 15105295
cytomegalovirus; endothelium; ATM; p53; apoptosis
6.  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.
doi:10.1038/msb.2011.85
PMCID: PMC3261706  PMID: 22108795
apoptosis; caspases; dynamical systems analysis; kinetic modeling; XIAP
7.  Notochordal cell disappearance and modes of apoptotic cell death in a rat tail static compression-induced disc degeneration model 
Introduction
The intervertebral disc has a complex structure originating developmentally from both the mesenchyme and notochord. Notochordal cells disappear during adolescence, which is also when human discs begin to show degenerative signs. During degeneration later in life, disc cells decline because of apoptosis. Although many animal models have been developed to simulate human disc degeneration, few studies have explored the long-term changes in cell population and phenotype. Our objective was to elucidate the time-dependent notochordal cell disappearance and apoptotic cell death in a rat tail static compression-induced disc degeneration model.
Methods
Twenty-four 12-week-old male Sprague–Dawley rat tails were instrumented with an Ilizarov-type device and loaded statically at 1.3 MPa for up to 56 days. Loaded and distal-unloaded discs were harvested. Changes in cell number and phenotype were assessed with histomorphology and immunofluorescence. Apoptosis involvement was determined with terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining and immunohistochemistry.
Results
The number of disc nucleus pulposus and annulus fibrosus cells decreased with the loading period; particularly, the decrease was notable at day 7 in larger, vacuolated, cytokeratin-8- and galectin-3-co-positive cells, indicating notochordal origin. Subsequently, the proportion of cells positive for TUNEL and cleaved caspase-3, markers of apoptosis induction, increased from day 7 through day 56. Although the percentage of cells immunopositive for cleaved caspase-8, a marker of apoptosis initiation through the death-receptor pathway, increased only at day 7, the percentage of cells immunopositive for cleaved caspase-9 and p53-regulated apoptosis-inducing protein 1 (p53AIP1), markers of apoptosis initiation through the p53-mediated mitochondrial pathway, increased from day 7 through day 56. The percentage of cells immunopositive for B-cell lymphoma 2 (Bcl-2) and silent mating type information regulation 2 homolog 1 (SIRT1), antiapoptotic proteins, decreased consistently with compression.
Conclusions
This rat tail model mimics notochordal cell disappearance and apoptotic cell death in human disc aging and degeneration. Sustained static compression induces transient activation of apoptosis through the death-receptor pathway and persistent activation of apoptosis through the p53-mediated mitochondrial pathway in disc cells. The increased proapoptotic and decreased antiapoptotic proteins observed at all time points signify static compression-induced disc cell death and degeneration.
doi:10.1186/ar4460
PMCID: PMC3979117  PMID: 24472667
8.  Analysis of apoptotic and antiapoptotic signalling pathways induced by Helicobacter pylori 
Molecular Pathology  2002;55(5):286-293.
Background and aims: Although it is reported that Helicobacter pylori induces apoptosis on gastric epithelial cells, the mechanism remains unknown. Antiapoptotic effects generated by H pylori have not yet been evaluated.
Methods: (1) H pylori strains (type 1 wild, TN2-ΔcagE, TN2-ΔvacA) were cocultured with MKN45, TMK1, and HeLa cells, and cell viability and apoptosis were assessed by trypan blue exclusion and DNA laddering, respectively. (2) Activation of caspases-3, 7, and 8, cytochrome c release from the mitochondria, and Fas, Fas associated death domain protein (FADD), Bax, Bak, and Bcl-X expression were evaluated by immunoblot analysis. (3) To investigate whether nuclear factor kappa B (NFκB) activation induced by cag pathogenicity island (PAI) positive H pylori affects antiapoptosis, MKN45 cells stably expressing super-repressor IκBα were cocultured with H pylori, and cell viability and caspase activation were evaluated. NFκB regulated gene expression was also evaluated by ribonuclease protection assay.
Results: (1) Wild-type and ΔvacA mutant H pylori induced apoptosis more potently than the ΔcagE mutant. Inhibition of cell contact between H pylori and cancer cells and heat killing H pylori diminished cell death. (2) Caspases-3, 7, and 8 were activated time dependently by H pylori as well as by the agonist anti-Fas. Cytochrome c release from mitochondria was observed and was not inhibited by caspase-8 inhibitor. Although protein expression of Fas, FADD, Bax, Bak, and Bcl-X in the whole cell lysates was not changed by H pylori, Bax was decreased from mitochondria free cytosol suggesting that Bax was translocated into mitochondria. (3) Cell death and the activities of caspases-3 and 8 were promoted in MKN45 cells stably expressing super-repressor IκBα that inhibits NFκB activation. Antiapoptotic proteins c-IAP1 and c-IAP2 were upregulated by the wild-type strains.
Conclusion: cag PAI positive H pylori is capable of inducing apoptotic effects mainly through the mitochondrial pathway. Antiapoptotic effects mediated by NFκB activation were also observed.
PMCID: PMC1187257  PMID: 12354930
Helicobacter pylori; apoptosis; antiapoptosis; signalling pathway
9.  Analysis of apoptotic and antiapoptotic signalling pathways induced by Helicobacter pylori 
Gut  2002;50(6):771-778.
Background and aims: Although it is reported that Helicobacter pylori induces apoptosis on gastric epithelial cells, the mechanism remains unknown. Antiapoptotic effects generated by H pylori have not yet been evaluated.
Methods: (1) H pylori strains (type 1 wild, TN2-ΔcagE, TN2-ΔvacA) were cocultured with MKN45, TMK1, and HeLa cells, and cell viability and apoptosis were assessed by trypan blue exclusion and DNA laddering, respectively. (2) Activation of caspases-3, 7, and 8, cytochrome c release from the mitochondria, and Fas, Fas associated death domain protein (FADD), Bax, Bak, and Bcl-X expression were evaluated by immunoblot analysis. (3) To investigate whether nuclear factor kappa B (NFκB) activation induced by cag pathogenicity island (PAI) positive H pylori affects antiapoptosis, MKN45 cells stably expressing super-repressor Iκβα were cocultured with H pylori, and cell viability and caspase activation were evaluated. NFκB regulated gene expression was also evaluated by ribonuclease protection assay.
Results: (1) Wild-type and ΔvacA mutant H pylori induced apoptosis more potently than the ΔcagE mutant. Inhibition of cell contact between H pylori and cancer cells and heat killing H pylori diminished cell death. (2) Caspases-3, 7, and 8 were activated time dependently by H pylori as well as by the agonist anti-Fas. Cytochrome c release from mitochondria was observed and was not inhibited by caspase-8 inhibitor. Although protein expression of Fas, FADD, Bax, Bak, and Bcl-X in the whole cell lysates was not changed by H pylori, Bax was decreased from mitochondria free cytosol suggesting that Bax was translocated into mitochondria. (3) Cell death and the activities of caspases-3 and 8 were promoted in MKN45 cells stably expressing super-repressor Iκβα that inhibits NFκB activation. Antiapoptotic proteins c-IAP1 and c-IAP2 were upregulated by the wild-type strains.
Conclusion: cag PAI positive H pylori is capable of inducing apoptotic effects mainly through the mitochondrial pathway. Antiapoptotic effects mediated by NFκB activation were also observed.
PMCID: PMC1773255  PMID: 12010877
Helicobacter pylori; apoptosis; antiapoptosis; signalling pathway
10.  Heme Oxygenase-1, a Critical Arbitrator of Cell Death Pathways in Lung Injury and Disease 
Increases in cell death by programmed (ie., apoptosis, autophagy) or non-programmed mechanisms (ie., necrosis) occur during tissue injury, and may contribute to the etiology of several pulmonary or vascular disease states. The low molecular weight stress protein heme oxygenase-1 (HO-1) confers cytoprotection against cell death in various models of lung and vascular injury by inhibiting apoptosis, inflammation, and cell proliferation. HO-1 serves a vital metabolic function as the rate-limiting step in the heme degradation pathway and in the maintenance of iron homeostasis. The transcriptional induction of HO-1 occurs in response to multiple forms of chemical and physical cellular stress. The cytoprotective functions of HO-1 may be attributed to heme turnover, as well as to beneficial properties of its enzymatic reaction products: biliverdin-IXα, iron, and carbon monoxide (CO). Recent studies have demonstrated that HO-1 or CO inhibits stress-induced extrinsic and intrinsic apoptotic pathways in vitro. A variety of signaling molecules have been implicated in the cytoprotection conferred by HO-1/CO, including autophagic proteins, p38 mitogen activated protein kinase, signal transducer and activator of transcription proteins, nuclear factor-κB, phosphatydylinositol-3-kinase/Akt, and others. Enhanced HO-1 expression or the pharmacological application of HO end-products affords protection in preclinical models of tissue injury, including experimental and transplant-associated ischemia/reperfusion injury, promising potential future therapeutic applications.
doi:10.1016/j.freeradbiomed.2009.04.007
PMCID: PMC3078523  PMID: 19362144
11.  FAK and p38-MAP Kinase-Dependent Activation of Apoptosis and Caspase-3 in Retinal Endothelial Cells by α1(IV)NC1 
Purpose
To determine the impact of the antiangiogenic factor α1(IV)NC1 on vascular endothelial growth factor mediated proangiogenic activity in mouse retinal endothelial cell (MLEC).
Methods
Primary culture of mouse retinal endothelial cells were established as previously described and used to determine the effects of α1(IV)NC1 on proangiogenic activity of VEGF. Cell proliferation was evaluated using [H3] thymidine incorporation and 3,(4,5-dimethylthiazol-2-yl)-2,5- diphenyl-tetrazolium bromide colorimetric assays. Cell migration was determined using modified Boyden chamber and scratch wound assays and tube formation was assessed on Matrigel. The intracellular signaling events Bcl-2/Bcl-xL and caspase-3/poly (ADP-ribose) polymerase (PARP) activities were evaluated in cells stimulated with VEGF and plated on type IV collagen coated dishes. Apoptosis was assessed by measuring different caspases activity as well as quantitative fluorescence analysis using fluorescence-activated cell sorting assay. Subcutaneously injected VEGF induced in-vivo neovascularization was studied using Matrigel plug assay.
Results
VEGF induced sub-confluent MREC proliferation, migration, and tube formation was significantly inhibited by α1(IV)NC1 at 1.0µM (P<0.001). α1(IV)NC1 induced MREC apoptosis mediating through by inhibition of Bcl-2 and Bcl-xL expressions and activation of caspase-3/PARP through FAK/p38-MAPK signaling. In addition, α1(IV)NC1 dose dependently inhibited VEGF-mediated neovascularization in-vivo.
Conclusions
α1(IV)NC1 inhibited VEGF-mediated angiogenesis by promoting apoptosis, caspase-3/PARP activation and negatively impacting FAK/p38-MAPK phosphorylation, Bcl-2 and Bcl-xL expressions leading to MREC death. The endothelial specific inhibitory actions of recombinant α1(IV)NC1 may be of benefit in the treatment of a variety of eye diseases with a neovascular component.
doi:10.1167/iovs.09-3473
PMCID: PMC2795568  PMID: 19443723
12.  Bim and Bmf Synergize To Induce Apoptosis in Neisseria Gonorrhoeae Infection 
PLoS Pathogens  2009;5(3):e1000348.
Bcl-2 family proteins including the pro-apoptotic BH3-only proteins are central regulators of apoptotic cell death. Here we show by a focused siRNA miniscreen that the synergistic action of the BH3-only proteins Bim and Bmf is required for apoptosis induced by infection with Neisseria gonorrhoeae (Ngo). While Bim and Bmf were associated with the cytoskeleton of healthy cells, they both were released upon Ngo infection. Loss of Bim and Bmf from the cytoskeleton fraction required the activation of Jun-N-terminal kinase-1 (JNK-1), which in turn depended on Rac-1. Depletion and inhibition of Rac-1, JNK-1, Bim, or Bmf prevented the activation of Bak and Bax and the subsequent activation of caspases. Apoptosis could be reconstituted in Bim-depleted and Bmf-depleted cells by additional silencing of antiapoptotic Mcl-1 and Bcl-XL, respectively. Our data indicate a synergistic role for both cytoskeletal-associated BH3-only proteins, Bim, and Bmf, in an apoptotic pathway leading to the clearance of Ngo-infected cells.
Author Summary
A variety of physiological death signals, as well as pathological insults, trigger apoptosis, a genetically programmed form of cell death. Pathogens often induce host cell apoptosis to establish a successful infection. Neisseria gonorrhoeae (Ngo), the etiological agent of the sexually transmitted disease gonorrhoea, is a highly adapted obligate human-specific pathogen and has been shown to induce apoptosis in infected cells. Here we unveil the molecular mechanisms leading to apoptosis of infected cells. We show that Ngo-mediated apoptosis requires a special subset of proapoptotic proteins from the group of BH3-only proteins. BH3-only proteins act as stress sensors to translate toxic environmental signals to the initiation of apoptosis. In a siRNA-based miniscreen, we found Bim and Bmf, BH3-only proteins associated with the cytoskeleton, necessary to induce host cell apoptosis upon infection. Bim and Bmf inactivated different inhibitors of apoptosis and thereby induced cell death in response to infection. Our data unveil a novel pathway of infection-induced apoptosis that enhances our understanding of the mechanism by which BH3-only proteins control apoptotic cell death.
doi:10.1371/journal.ppat.1000348
PMCID: PMC2654407  PMID: 19300516
13.  Isolation and Culture of Pulmonary Endothelial Cells from Neonatal Mice 
Endothelial cells provide a useful research model in many areas of vascular biology. Since its first isolation 1, human umbilical vein endothelial cells (HUVECs) have shown to be convenient, easy to obtain and culture, and thus are the most widely studied endothelial cells. However, for research focused on processes like angiogenesis, permeability or many others, microvascular endothelial cells (ECs) are a much more physiologically relevant model to study 2. Furthermore, ECs isolated from knockout mice provide a useful tool for analysis of protein function ex vivo. Several approaches to isolate and culture microvascular ECs of different origin have been reported to date 3-7, but consistent isolation and culture of pure ECs is still a major technical problem in many laboratories. Here, we provide a step-by-step protocol on a reliable and relatively simple method of isolating and culturing mouse lung endothelial cells (MLECs). In this approach, lung tissue obtained from 6- to 8-day old pups is first cut into pieces, digested with collagenase/dispase (C/D) solution and dispersed mechanically into single-cell suspension. MLECS are purified from cell suspension using positive selection with anti-PECAM-1 antibody conjugated to Dynabeads using a Magnetic Particle Concentrator (MPC). Such purified cells are cultured on gelatin-coated tissue culture (TC) dishes until they become confluent. At that point, cells are further purified using Dynabeads coupled to anti-ICAM-2 antibody. MLECs obtained with this protocol exhibit a cobblestone phenotype, as visualized by phase-contrast light microscopy, and their endothelial phenotype has been confirmed using FACS analysis with anti-VE-cadherin 8 and anti-VEGFR2 9 antibodies and immunofluorescent staining of VE-cadherin. In our hands, this two-step isolation procedure consistently and reliably yields a pure population of MLECs, which can be further cultured. This method will enable researchers to take advantage of the growing number of knockout and transgenic mice to directly correlate in vivo studies with results of in vitro experiments performed on isolated MLECs and thus help to reveal molecular mechanisms of vascular phenotypes observed in vivo.
doi:10.3791/2316
PMCID: PMC3278331  PMID: 21178973
14.  Mechanism(s) Involved in Carbon Monoxide-releasing Molecule-2-mediated Cardioprotection During Ischaemia-reperfusion Injury in Isolated Rat Heart 
The purpose of the present study was to determine the mechanism(s) involved in carbon monoxide-releasing molecule-2, carbon monoxide-releasing molecule-2-induced cardioprotection. We used the transition metal carbonyl compound carbon monoxide-releasing molecule-2 that can act as carbon monoxide donor in cardiac ischaemia-reperfusion injury model using isolated rat heart preparation. Langendorff's perfused rat hearts when treated with carbon monoxide-releasing molecule-2 (50 μM) for 10 min before global ischaemia exhibited significant reduction in postischaemic levels of myocardial injury markers, creatine kinase and lactate dehydrogenase in coronary effluent. Similarly, pretreatment with carbon monoxide-releasing molecule-2 showed significantly improved postischaemic recovery of heart rate, coronary flow rate, cardiodynamic parameters and reduced infarct size as compared to vehicle control hearts. Perfusion with p38 mitogen-activated protein kinase inhibitor, SB203580, a specific inhibitor of α and β isoform, before and concomitantly with carbon monoxide-releasing molecule-2 treatment abolished carbon monoxide-releasing molecule-2-induced cardioprotection. However, p38 mitogen-activated protein kinase alpha inhibitor, SCIO-469, was unable to inhibit the cardioprotective effect of carbon monoxide-releasing molecule-2. Furthermore, protective effect of carbon monoxide-releasing molecule-2 was significantly inhibited by the protein kinase C inhibitor, chelerythrine, when added before and concomitantly with carbon monoxide-releasing molecule-2. It was also observed that, perfusion with phosphatidylinositol 3-kinase inhibitor, wortmannin, before and concomitantly with carbon monoxide-releasing molecule-2 was not able to inhibit carbon monoxide-releasing molecule-2-induced cardioprotection. Interestingly, we observed that wortmannin perfusion before ischaemia and continued till reperfusion significantly inhibited carbon monoxide-releasing molecule-2-mediated cardioprotection. Our findings suggest that the carbon monoxide-releasing molecule-2 treatment may activate the p38 mitogen-activated protein kinase β and protein kinase C pathways before ischaemia and phosphatidylinositol 3-kinase pathway during reperfusion which may be responsible for the carbon monoxide-releasing molecule-2-mediated cardioprotective effect.
doi:10.4103/0250-474X.107047
PMCID: PMC3630723  PMID: 23626383
Carbon monoxide releasing molecule-2; cardioprotection; ischaemia-reperfusion; Langendorff's heart
15.  Gefitinib-Induced Killing of NSCLC Cell Lines Expressing Mutant EGFR Requires BIM and Can Be Enhanced by BH3 Mimetics 
PLoS Medicine  2007;4(10):e316.
Background
The epidermal growth factor receptor (EGFR) plays a critical role in the control of cellular proliferation, differentiation, and survival. Abnormalities in EGF-EGFR signaling, such as mutations that render the EGFR hyperactive or cause overexpression of the wild-type receptor, have been found in a broad range of cancers, including carcinomas of the lung, breast, and colon. EGFR inhibitors such as gefitinib have proven successful in the treatment of certain cancers, particularly non-small cell lung cancers (NSCLCs) harboring activating mutations within the EGFR gene, but the molecular mechanisms leading to tumor regression remain unknown. Therefore, we wished to delineate these mechanisms.
Methods and Findings
We performed biochemical and genetic studies to investigate the mechanisms by which inhibitors of EGFR tyrosine kinase activity, such as gefitinib, inhibit the growth of human NSCLCs. We found that gefitinib triggered intrinsic (also called “mitochondrial”) apoptosis signaling, involving the activation of BAX and mitochondrial release of cytochrome c, ultimately unleashing the caspase cascade. Gefitinib caused a rapid increase in the level of the proapoptotic BH3-only protein BIM (also called BCL2-like 11) through both transcriptional and post-translational mechanisms. Experiments with pharmacological inhibitors indicated that blockade of MEK–ERK1/2 (mitogen-activated protein kinase kinase–extracellular signal-regulated protein kinase 1/2) signaling, but not blockade of PI3K (phosphatidylinositol 3-kinase), JNK (c-Jun N-terminal kinase or mitogen-activated protein kinase 8), or AKT (protein kinase B), was critical for BIM activation. Using RNA interference, we demonstrated that BIM is essential for gefitinib-induced killing of NSCLC cells. Moreover, we found that gefitinib-induced apoptosis is enhanced by addition of the BH3 mimetic ABT-737.
Conclusions
Inhibitors of the EGFR tyrosine kinase have proven useful in the therapy of certain cancers, in particular NSCLCs possessing activating mutations in the EGFR kinase domain, but the mechanisms of tumor cell killing are still unclear. In this paper, we demonstrate that activation of the proapoptotic BH3-only protein BIM is essential for tumor cell killing and that shutdown of the EGFR–MEK–ERK signaling cascade is critical for BIM activation. Moreover, we demonstrate that addition of a BH3 mimetic significantly enhances killing of NSCLC cells by the EGFR tyrosine kinase inhibitor gefitinib. It appears likely that this approach represents a paradigm shared by many, and perhaps all, oncogenic tyrosine kinases and suggests a powerful new strategy for cancer therapy.
Andreas Strasser and colleagues demonstrate that activation of the proapoptotic BH3-only protein BIM is essential for tumor cell killing and that shutdown of the EGFR−MEK−ERK signaling cascade is critical for BIM activation.
Editors' Summary
Background.
Normally, cell division (which produces new cells) and cell death are finely balanced to keep the human body in good working order. But sometimes cells acquire changes (mutations) in their genetic material that allow them to divide uncontrollably to form cancers—life-threatening, disorganized masses of cells. One protein with a critical role in cell division that is often mutated in tumors is the epidermal growth factor receptor (EGFR). In normal cells, protein messengers bind to EGFR and activate its tyrosine kinase. This enzyme then adds phosphate groups to tyrosine (an amino acid) in proteins that form part of signaling cascades (for example, the MEK–ERK signaling cascade) that tell the cell to divide. In cancers that have mutations in EGFR, signaling is overactive so the cancer cells divide much more than they should. Some non-small cell lung cancers (NSCLC, the commonest type of lung cancer), for example, have activating mutations within the EGFR tyrosine kinase. Treatment with EGFR tyrosine kinase inhibitors (TKIs) such as gefitinib and erlotinib induces the cells in these tumors to stop growing and die. This cell death causes tumor shrinkage (regression) and increases the life expectancy of patients with this type of NSCLC.
Why Was This Study Done?
Unfortunately, treatment with TKIs rarely cures NSCLC, so it would be useful to find a way to augment the effect that TKIs have on cancer cells. To do this, the molecular mechanisms that cause cancer-cell death and tumor regression in response to these drugs need to be fully understood. In this study, the researchers have used a combination of biochemical and genetic approaches to investigate how gefitinib kills NSCLC cells with mutated EGFR.
What Did the Researchers Do and Find?
The researchers first measured the sensitivity of NSCLC cell lines (tumor cells that grow indefinitely in dishes) to gefitinib-induced apoptosis. Gefitinib caused extensive apoptosis in two cell lines expressing mutant EGFR but not in one expressing normal EGFR. Next, they investigated the mechanism of gefitinib-induced apoptosis in the most sensitive cell line (H3255). Apoptosis is activated via two major pathways. Hallmarks of the “intrinsic” pathway include activation of a protein called BAX and cytochrome c release from subcellular compartments known as mitochondria. Gefitinib treatment induced both these events in H3255 cells. BAX (a proapoptotic member of the BCL-2 family of proteins) is activated when proapoptotic BH3-only BCL-2 proteins (for example, BIM; “BH3-only” describes the structure of these proteins) bind to antiapoptotic BCL2 proteins. Gefitinib treatment rapidly increased BIM activity in H3255 and HCC827 cells (but not in gefitinib-resistant cells) by increasing the production of BIM protein and the removal of phosphate groups from it, which increases BIM activity. Pharmacological blockade of the MEK–ERK signaling cascade, but not of other EGFR signaling cascades, also caused the accumulation of BIM. By contrast, blocking BIM expression using a technique called RNA interference reduced gefitinib-induced apoptosis. Finally, a combination of gefitinib and a BH3-mimicking compound called ABT-737 (which, like BIM, binds to antiapoptotic BCL-2 proteins) caused more apoptosis than gefitinib alone.
What Do These Findings Mean?
These findings (and those reported by Gong et al. and Costa et al.) indicate that activation of the proapoptotic BH3-only protein BIM is essential for gefitinib-induced killing of NSCLC cells that carry EGFR tyrosine kinase mutations. They also show that inhibition of the EGFR–MEK–ERK signaling cascade by gefitinib is essential for BIM activation. Because these findings come from studies on NSCLC cell lines, they need confirming in freshly isolated tumor cells and in tumors growing in people. However, the demonstration that a compound that mimics BH3 action enhances gefitinib-induced killing of NSCLC cells suggests that combinations of TKIs and drugs that affect the intrinsic pathway of apoptosis activation might provide a powerful strategy for treating cancers in which tyrosine kinase mutations drive tumor growth.
Additional Information.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0040316.
A perspective by Ingo Mellinghoff discusses this article and two related research articles
Wikipedia pages on epidermal growth factor receptor, apoptosis, and BCL2 proteins (note that Wikipedia is a free online encyclopedia that anyone can edit; available in several languages)
CancerQuest provides information on all aspects of cancer from Emory University (in several languages)
US National Cancer Institute information for patients and professionals on lung cancer (in English and Spanish)
Information for patients from Cancer Research UK on lung cancer including information on treatment with TKIs
Information for patients from Cancerbackup on erlotinib and gefitinib
doi:10.1371/journal.pmed.0040316
PMCID: PMC2043013  PMID: 17973573
16.  Characterization of the anti-angiogenic properties of arresten, an α1β1 integrin dependent collagen-derived tumor suppressor 
Experimental cell research  2008;314(18):3292-3305.
Physiological and pathological turnover of basement membranes liberates biologically active cryptic molecules. Several collagen-derived fragments possess anti-angiogenic activity. Arresten is the 26-kDa non-collagenous domain of type IV collagen α1 chain. It functions as an efficient inhibitor of angiogenesis and tumor growth in mouse models, but its anti-angiogenic mechanism is not completely known. Here we show that arresten significantly increases apoptosis of endothelial cells in vitro by decreasing the amount of anti-apoptotic molecules of the Bcl-family, Bcl-2 and Bcl-xL. Although the pro-apoptotic effect of arresten is endothelial cell specific in vitro, in mouse tumors arresten induced apoptosis both in endothelial and tumor cells. The tumor cell apoptosis is likely an indirect effect due to the inhibition of blood vessel growth into the tumor. The active site of arresten was localized by deletion mutagenesis within the C-terminal half of the molecule. We have previously shown that arresten binds toα1β1 integrin on human umbilical vein endothelial cells. However, the microvascular endothelial cells (MLECs) are more important in the context of tumor vasculature. We show here that arresten binds also to the microvascular endothelial cells via α1β1 integrin. Furthermore, it has no effect on Matrigel neovascularization or the viability of integrin α1 null MLECs. Tumors implanted on integrin α1 deficient mice show no integrin α1 expression in the host-derived vascular endothelium, and thus arresten does not inhibit the tumor growth. Collectively, this data sheds more light into the anti-angiogenic mechanism of arresten.
doi:10.1016/j.yexcr.2008.08.011
PMCID: PMC2613512  PMID: 18775695
angiogenesis; apoptosis; arresten; collagen IV; basement membrane; integrin
17.  Carbon monoxide induces a late preconditioning-mimetic cardioprotective and antiapoptotic milieu in the myocardium 
A growing body of evidence indicates that carbon monoxide (CO), once perceived merely as a poisonous gas, exerts antiapoptotic and cytoprotective effects. Using a water-soluble CO-releasing molecule (CORM) tricarbonylchloro(glycinato)ruthenium(II) (CORM-3), we previously reported that CO induces a delayed protection against myocardial infarction similar to that observed in the late phase of ischemic preconditioning (PC). In the current study, we investigated the molecular mechanisms underlying this cardioprotective effect. The impact on apoptotic signaling pathways was first examined in the setting of ischemia/reperfusion injury. Mice were pretreated with CORM-3 or iCORM-3 (which does not release CO) and subjected to coronary occlusion/reperfusion 24 h later. In mice that received CORM-3, there was a significant reduction in markers of apoptosis (cleaved lamin A, cleaved caspase-3, and cleaved PARP-1) after ischemia/reperfusion injury. To elucidate the mechanism of CORM-3-induced cardioprotection we further examined the activation of transcription factors and induction of cardioprotective and apoptosis modulating proteins. Infusion of CORM-3 rapidly activated the stress-responsive transcription factors nuclear factor kappaB (NF-κB), signal transducers and activators of transcription (STAT)1, STAT3, and NF-E2-related factor-2 (Nrf2). This was followed 24 h later by upregulation of cardioprotective proteins (heme oxygenase-1 [HO-1], cyclooxygenase-2 [COX-2], and extracellular superoxide dismutase [Ec-SOD]) and antiapoptotic proteins involving both the mitochondria-mediated (Mcl-1) and the death receptor-mediated (c-FLIPS, and c-FLIPL) apoptosis pathways. We conclude that CO released by CORM-3 triggers a cardioprotective signaling cascade that recruits the transcription factors NF-κB, STAT1/3, and Nrf2 with a subsequent increase in cardioprotective and antiapoptotic molecules in the myocardium leading to the late PC-mimetic infarct-sparing effects.
doi:10.1016/j.yjmcc.2011.11.005
PMCID: PMC3679555  PMID: 22119801
carbon monoxide-releasing molecules; late preconditioning; myocardial infarction; apoptosis; NF-κB; STAT1/3; Nrf2
18.  PARP-1 regulates resistance of pancreatic cancer to TRAIL therapy 
Purpose
Activating extrinsic apoptotic pathways targeting death receptors (DR) using agonistic antibodies or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is promising for cancer therapy. However, most pancreatic cancers are resistant to TRAIL therapy. The present studies aimed to identify combination therapies that enhance the efficacy of TRAIL therapy; and to investigate the underlying mechanisms.
Experimental Design
A xenograft model in nude mice was used to determine pancreatic cancer tumorigenesis and therapeutic efficacy of TRA-8, a monoclonal agonistic antibody for DR5. Pancreatic cancer cells were used to characterize mechanisms underlying poly(ADP-ribose) polymerase-1 (PARP-1) in regulating TRA-8-induced apoptosis in vitro.
Results
PARP-1 was found highly expressed in the TRA-8-resistant PANC-1 and Suit-2 cells, compared with TRA-8-sensitive BxPc-3 and MiaPaca-2. Inhibition of PARP-1 with a pharmacologic inhibitor sensitized PANC-1 and Suit2 cells to TRA-8 induced apoptosis in a dose-dependent manner. Furthermore, small interfering RNAs specifically knocking down PARP-1 markedly enhanced TRA-8-induced apoptosis in vitro, and augmented the efficacy of TRA-8 therapy on tumorigenesis in vivo. PARP-1 knockdown increased TRA-8-induced activation of caspase-8 in the death-induced signaling complex (DISC). Immuoprecipitation with DR5 antibody identified the recruitment of PARP-1 and PARP-1-mediated protein poly-ADP-ribosylation(pADPr) modification in the DR5-associated DISC. Further characterization revealed that PARP-1-mediated pADPr modification of caspase-8 inhibited caspase-8 activation, which may contribute to its function in regulating TRA-8 resistance.
Conclusions
Our studies not only provide novel molecular insights into the function of PARP-1 in regulating the extrinsic apoptosis machinery, but also support interventions combining PARP-1 inhibitors with death receptor agonists for pancreatic cancer therapy.
doi:10.1158/1078-0432.CCR-13-0516
PMCID: PMC4050702  PMID: 23833311
Pancreatic Cancer; resistance; death receptor; apoptosis; PARP-1
19.  Trauma induces apoptosis in human thoracolumbar intervertebral discs 
Background
Vertebral fractures resulting from high energy trauma often comprise the risk of posttraumatic degenerative changes in the affected intervertebral discs (IVD). Particularly in conservatively treated patients, or in cases after implant removal of an exclusively posterior stabilization, consecutive disc degeneration and the associated functional losing of the spinal segment clearly represent detrimental treatment results. In this regard, apoptosis of IVD cells has been suggested to be involved in the critical changes of the extracellular matrix.
Methods
To investigate whether fractures of the vertebrae induce apoptosis in the affected IVD, disc tissue from patients (n = 17) undergoing open reduction and internal fixation of thoracolumbar spine fractures were analysed in regards to caspase activity, apoptosis-receptor expression levels and gene expression of apoptosis-regulating proteins such as Bax and Bcl-2. Healthy IVD tissue (n = 3) obtained from patients undergoing surgical resection of adjacent vertebrae were used as control samples.
Results
In contrast to healthy control IVD tissues, samples from traumatic thoracolumbar IVD showed positive TUNEL staining and a significant increase of caspase-3/7 activity. Interestingly, analyses of the initiator caspase-8 and -9 revealed significantly increased activation levels compared to control values, suggesting the coexistent activation of both the extrinsic (receptor-mediated) and intrinsic (mitochondria-mediated) apoptosis pathway. Accordingly, expression levels of the Fas receptor (FasR) mRNA were significantly increased. Although the TNF receptor I (TNFR I) was only slightly upregulated, corresponding TNFα from trauma IVD presented significantly increased mRNA expression values. Furthermore, traumatic IVD cells demonstrated significantly reduced expression of the mitochondria-bound anti-apoptotic Bcl-2, thereby maintaining baseline transcriptional levels of the pro-apoptotic Bax protein when compared to control IVD cells.
Conclusion
Our data suggest that thoracolumbar fractures induce early caspase-dependent apoptosis in IVD cells of the affected intervertebral disc, in part, by downregulation of the anti-apoptotic protein Bcl-2 (intrinsic apoptosis pathway), as well as signalling via the death receptor complex (TNFR I and FasR).
doi:10.1186/1472-6890-6-5
PMCID: PMC1538608  PMID: 16719914
20.  Glutamate-induced apoptosis in primary cortical neurons is inhibited by equine estrogens via down-regulation of caspase-3 and prevention of mitochondrial cytochrome c release 
BMC Neuroscience  2005;6:13.
Background
Apoptosis plays a key role in cell death observed in neurodegenerative diseases marked by a progressive loss of neurons as seen in Alzheimer's disease. Although the exact cause of apoptosis is not known, a number of factors such as free radicals, insufficient levels of nerve growth factors and excessive levels of glutamate have been implicated. We and others, have previously reported that in a stable HT22 neuronal cell line, glutamate induces apoptosis as indicated by DNA fragmentation and up- and down-regulation of Bax (pro-apoptotic), and Bcl-2 (anti-apoptotic) genes respectively. Furthermore, these changes were reversed/inhibited by estrogens. Several lines of evidence also indicate that a family of cysteine proteases (caspases) appear to play a critical role in neuronal apoptosis. The purpose of the present study is to determine in primary cultures of cortical cells, if glutamate-induced neuronal apoptosis and its inhibition by estrogens involve changes in caspase-3 protease and whether this process is mediated by Fas receptor and/or mitochondrial signal transduction pathways involving release of cytochrome c.
Results
In primary cultures of rat cortical cells, glutamate induced apoptosis that was associated with enhanced DNA fragmentation, morphological changes, and up-regulation of pro-caspase-3. Exposure of cortical cells to glutamate resulted in a time-dependent cell death and an increase in caspase-3 protein levels. Although the increase in caspase-3 levels was evident after 3 h, cell death was only significantly increased after 6 h. Treatment of cells for 6 h with 1 to 20 mM glutamate resulted in a 35 to 45% cell death that was associated with a 45 to 65% increase in the expression of caspase-3 protein. Pretreatment with caspase-3-protease inhibitor z-DEVD or pan-caspase inhibitor z-VAD significantly decreased glutamate-induced cell death of cortical cells. Exposure of cells to glutamate for 6 h in the presence or absence of 17β-estradiol or Δ8, 17β-estradiol (10 nM-10 μM) resulted in the prevention of cell death and was associated with a significant dose-dependent decrease in caspase-3 protein levels, with Δ8, 17β-E2 being more potent than 17β-E2. Protein levels of Fas receptor remained unchanged in the presence of glutamate. In contrast, treatment with glutamate induced, in a time-dependent manner, the release of cytochrome c into the cytosol. Cytosolic cytochrome c increased as early as 1.5 h after glutamate treatment and these levels were 5 fold higher after 6 h, compared to levels in the untreated cells. Concomitant with these changes, the levels of cytochrome c in mitochondria decreased significantly. Both 17β-E2 and Δ8, 17β-E2 reduced the release of cytochrome c from mitochondria into the cytosol and this decrease in cytosolic cytochrome c was associated with inhibition of glutamate-induced cell death.
Conclusion
In the primary cortical cells, glutamate-induced apoptosis is accompanied by up-regulation of caspase-3 and its activity is blocked by caspase protease inhibitors. These effects of glutamate on caspase-3 appear to be independent of changes in Fas receptor, but are associated with the rapid release of mitochondrial cytochrome c, which precedes changes in caspase-3 protein levels leading to apoptotic cell death. This process was differentially inhibited by estrogens with the novel equine estrogen Δ8, 17β-E2 being more potent than 17β-E2. To our knowledge, this is the first study to demonstrate that equine estrogens can prevent glutamate-induced translocation of cytochrome c from mitochondria to cytosol in rat primary cortical cells.
doi:10.1186/1471-2202-6-13
PMCID: PMC555946  PMID: 15730564
21.  Nicotine Induces Resistance to Chemotherapy by Modulating Mitochondrial Signaling in Lung Cancer 
Continued smoking causes tumor progression and resistance to therapy in lung cancer. Carcinogens possess the ability to block apoptosis, and thus may induce development of cancers and resistance to therapy. Tobacco carcinogens have been studied widely; however, little is known about the agents that inhibit apoptosis, such as nicotine. We determine whether mitochondrial signaling mediates antiapoptotic effects of nicotine in lung cancer. A549 cells were exposed to nicotine (1 μM) followed by cisplatin (35 μM) plus etoposide (20 μM) for 24 hours. We found that nicotine prevented chemotherapy-induced apoptosis, improved cell survival, and caused modest increases in DNA synthesis. Inhibition of mitogen-activated protein kinase (MAPK) and Akt prevented the antiapoptotic effects of nicotine and decreased chemotherapy-induced apoptosis. Small interfering RNA MAPK kinase-1 blocked antiapoptotic effects of nicotine, whereas small interfering RNA MAPK kinase-2 blocked chemotherapy-induced apoptosis. Nicotine prevented chemotherapy-induced reduction in mitochondrial membrane potential and caspase-9 activation. Antiapoptotic effects of nicotine were blocked by mitochondrial anion channel inhibitor, 4,4′diisothiocyanatostilbene-2,2′disulfonic acid. Chemotherapy enhanced translocation of proapoptotic Bax to the mitochondria, whereas nicotine blocked these effects. Nicotine up-regulated Akt-mediated antiapoptotic X-linked inhibitor of apoptosis protein and phosphorylated proapoptotic Bcl2-antagonist of cell death. The A549-ρ0 cells, which lack mitochondrial DNA, demonstrated partial resistance to chemotherapy-induced apoptosis, but blocked the antiapoptotic effects of nicotine. Accordingly, we provide evidence that nicotine modulates mitochondrial signaling and inhibits chemotherapy-induced apoptosis in lung cancer. The mitochondrial regulation of nicotine imposes an important mechanism that can critically impair the treatment of lung cancer, because many cancer-therapeutic agents induce apoptosis via the mitochondrial death pathway. Strategies aimed at understanding nicotine-mediated signaling may facilitate the development of improved therapies in lung cancer.
doi:10.1165/rcmb.2007-0277OC
PMCID: PMC2633138  PMID: 18676776
apoptosis; chemotherapy; lung cancer; mitochondria; nicotine
22.  Hepatocyte-specific deletion of the anti-apoptotic protein Mcl-1 triggers proliferation and hepatocarcinogenesis in mice 
Hepatology (Baltimore, Md.)  2010;51(4):1226-1236.
Regulation of hepatocellular apoptosis is crucial for liver homeostasis. Increased sensitivity of hepatocytes towards apoptosis results in chronic liver injury, whereas apoptosis resistance is linked to hepatocarcinogenesis and non-responsiveness to therapy-induced cell death. Recently, we have demonstrated an essential role of the anti-apoptotic Bcl-2 family member Myeloid cell leukemia-1 (Mcl-1) in hepatocyte survival. In mice lacking Mcl-1 specifically in hepatocytes (Mcl-1Δhep) spontaneous apoptosis caused severe liver damage. Here, we demonstrate that chronically increased apoptosis of hepatocytes coincides with strong hepatocyte proliferation resulting in hepatocellular carcinoma (HCC). Liver cell tumor formation was observed in >50% of Mcl-1Δhep mice already by the age of 8 months, whereas 12 month-old wild-type and heterozygous Mcl-1flox/wt mice lacked tumors. Tumors revealed a heterogenous spectrum ranging from small dysplastic nodules to HCC. The neoplastic nature of the tumors was confirmed by histology, expression of the HCC marker glutamine synthetase and chromosomal aberrations. Liver carcinogenesis in Mcl-1Δhep mice was paralleled by markedly increased levels of survivin, an important regulator of mitosis which is selectively overexpressed in common human cancers.
Conclusion
The present study provides in vivo evidence that increased apoptosis of hepatocytes not only impairs liver homeostasis but is also accompanied by hepatocyte proliferation and hepatocarcinogenesis. Our findings might have implications for understanding apoptosis-related human liver diseases.
The survival of multicellular organisms depends on the maintenance of tissue homeostasis. Under physiological conditions apoptosis contributes to liver homeostasis by removing damaged hepatocytes. Proliferation, growth and programmed hepatocyte cell death are highly coordinated and tightly controlled events in the normal liver (1).
On the one hand, increased apoptosis sensitivity contributes to liver injury. On the other hand, defective apoptosis was demonstrated to lead to excessive hepatocellular survival and has emerged as a major mechanism by which pre-malignant hepatocytes obtain a competitive advantage over normal liver cells (2). Various molecular alterations have been characterized causing an imbalance in the regulation of apoptosis. Among these are alterations in p53 signalling, expression of death receptors, growth factors and mitochondrial integrity (3). Decreased activity of pro-apoptotic signalling as well as increased activity of anti-apoptotic events are associated with HCC development and progression (4).
Among the main cellular changes that trigger apoptosis of hepatocytes is the permeabilization of the outer mitochondrial membrane followed by the release of pro-apoptotic factors (5). The Bcl-2 protein family plays a pivotal role for mitochondrial integrity and the selective interactions between pro- and anti-apoptotic family members regulate mitochondrial activation (6). Bcl-2 family members are similar within the Bcl-2 homology regions (BH1-BH4) and can be divided in pro- and anti-apoptotic Bcl-2 proteins.
Pro-apoptotic Bcl-2 proteins comprise (1) multi-domain members, which lack the BH4 domain (e.g. Bax, Bak), and (2) BH3-only proteins, which lack BH1, 2 and 4 domains (e.g. Bid, Noxa, Puma). BH3-only proteins initiate the mitochondrial signalling cascade by sensing cellular damage (7). After activation, BH3-only proteins are released to neutralise anti-apoptotic Bcl-2 proteins. Subsequently, Bax and Bak trigger mitochondrial membrane leakage and the release of mitochondrial proteins, including cytochrome c, Smac/DIABLO (second mitochondria-derived activator of caspases/direct IAP-binding protein with low pI) and apoptosis-inducing factor (AIF). Smac/DIABLO proteins inactivate the IAP (inhibitors of apoptosis proteins) family, which consists of IAP1/2, BRUCE, NAIP, ILP2, ML-IAP, survivin and XIAP. XIAP is a direct caspase inhibitor. Other IAPs including survivin have several functions apart from caspase inhibition, eg, triggering of ubiquitination processes (8). Anti-apoptotic Bcl-2 family members (eg, Bcl-2, Bcl-xL and Mcl-1), interact with Bax and Bak to inhibit the activation of mitochondria (7).
Both Bcl-xL and Mcl-1 have been identified as major anti-apoptotic Bcl-2 proteins in the liver (9-11). Liver homeostasis is severely disturbed in Mcl-1Δhep mice (10, 11). Spontaneous hepatocyte apoptosis was observed in livers of Mcl-1Δhep mice in profound liver cell damage and increased susceptibility of hepatocytes towards pro-apoptotic stimuli (10). In addition, Mcl-1 has been shown to be highly expressed in a subset of human HCC, contributing to apoptosis resistance of cancer cells (12, 13). Thus, abrogation of the pro-survival function of Mcl-1 (1) either by diminishing its levels or (2) by inactivating its function, have shown promising results with regards to treatment of HCC (12, 13).
In this study, we show that liver-specific depletion of Mcl-1 increases hepatocyte apoptosis, induces hepatocellular proliferation and causes HCC in the absence of overt inflammation.
doi:10.1002/hep.23479
PMCID: PMC2936921  PMID: 20099303
liver; hepatocellular carcinoma; apoptosis; Bcl-2 proteins; survivin
23.  Combined Treatment of Hydroxytyrosol with Carbon Monoxide-Releasing Molecule-2 Prevents TNFα-Induced Vascular Endothelial Cell Dysfunction through NO Production with Subsequent NFκB Inactivation 
BioMed Research International  2013;2013:912431.
This study investigated the atheroprotective properties of olive oil polyphenol, hydroxytyrosol (HT), in combination with carbon monoxide-releasing molecule-2 (CORM-2) that acts as a carbon monoxide donor using vascular endothelial cells (VECs). Our results showed that CORM-2 could strengthen the cytoprotective and anti-apoptotic effects of HT against TNFα-induced cellular damage by enhancing cell survival and the suppression of caspase-3 activation. While HT alone attenuated NFκBp65 phosphorylation and IκBα degradation triggered by TNFα in a dose-dependent manner, combined treatment of HT with CORM-2 but not iCORM-2 nearly completely blocked these TNFα effects. Furthermore, combined action of both compounds results in the inhibition of NFκB nuclear translocation. Results also indicate that both compounds time-dependently increased eNOS phosphorylation levels and the combination of HT with CORM-2 was more effective in enhancing eNOS activation and NO production in VECs. The NOS inhibitor, L-NMMA, significantly suppressed the combined effects of HT and CORM-2 on TNFα-triggered NFκBp65 and IκBα phosphorylation as well as decreased cell viability. Together, these data suggest that carbon monoxide-dependent regulation of NO production by the combination of HT with CORM-2 may provide a therapeutic benefit in the treatment of endothelial dysfunction and atherosclerosis.
doi:10.1155/2013/912431
PMCID: PMC3771260  PMID: 24066302
24.  Carbon Monoxide Generated by Heme Oxygenase 1 Suppresses Endothelial Cell Apoptosis 
The Journal of Experimental Medicine  2000;192(7):1015-1026.
Heme oxygenase 1 (HO-1) inhibits apoptosis by regulating cellular prooxidant iron. We now show that there is an additional mechanism by which HO-1 inhibits apoptosis, namely by generating the gaseous molecule carbon monoxide (CO). Overexpression of HO-1, or induction of HO-1 expression by heme, protects endothelial cells (ECs) from apoptosis. When HO-1 enzymatic activity is blocked by tin protoporphyrin (SnPPIX) or the action of CO is inhibited by hemoglobin (Hb), HO-1 no longer prevents EC apoptosis while these reagents do not affect the antiapoptotic action of bcl-2. Exposure of ECs to exogenous CO, under inhibition of HO-1 activity by SnPPIX, substitutes HO-1 in preventing EC apoptosis. The mechanism of action of HO-1/CO is dependent on the activation of the p38 mitogen-activated protein kinase (MAPK) signaling transduction pathway. Expression of HO-1 or exposure of ECs to exogenous CO enhanced p38 MAPK activation by TNF-α. Specific inhibition of p38 MAPK activation by the pyridinyl imidazol SB203580 or through overexpression of a p38 MAPK dominant negative mutant abrogated the antiapoptotic effect of HO-1. Taken together, these data demonstrate that the antiapoptotic effect of HO-1 in ECs is mediated by CO and more specifically via the activation of p38 MAPK by CO.
PMCID: PMC2193315  PMID: 11015442
apoptosis; endothelial cells; heme oxygenase 1; carbon monoxide; p38 mitogen-activated protein kinase
25.  Identification of an antiapoptotic protein complex at death receptors 
Sun, M | Song, L | Li, Y | Zhou, T | Jope, RS
Cell death and differentiation  2008;15(12):1887-1900.
Stimulation of death receptors activates the extrinsic apoptotic signaling pathway that leads to cell death. Although many steps of this apoptotic signaling cascade are known, few mechanisms that counterbalance the death signal have been described. We identified an antiapoptotic protein complex associated with death receptors that contains glycogen synthase kinase-3 (GSK3), DDX3 and cellular inhibitor of apoptosis protein-1 (cIAP-1). GSK3, DDX3 and cIAP-1 are associated in cells with each other and with death receptors. Blocking the actions of GSK3 or DDX3 potentiated caspase-3 activation induced by stimulation of four different death receptors in several types of cells. GSK3 restrained apoptotic signaling by inhibiting formation of the death-inducing signaling complex and caspase-8 activation. Stimulated death receptors surmount the antiapoptotic complex by causing GSK3 inactivation and cleavage of DDX3 and cIAP-1 to enable progression of the apoptotic signaling cascade, but the antiapoptotic complex remains functional in cancer cells resistant to death receptor stimulation, a resistance that is overcome by GSK3 inhibitors. Thus, an antiapoptotic complex of GSK3, DDX3 and cIAP-1 caps death receptors, providing a checkpoint to counterbalance apoptotic signaling.
doi:10.1038/cdd.2008.124
PMCID: PMC2662711  PMID: 18846110
GSK3; DDX3; cIAP-1; death receptor

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