A systems biology analysis of apoptosome formation and apoptosis execution supports allosteric procaspase-9 activation

Activation of caspase-9 on the apoptosome as studied by methyl-TROSY NMR

Mitochondrial apoptotic signaling cascades lead to the formation of the apoptosome, a 1.1-MDa heptameric protein scaffold that recruits and activates the caspase-9 protease. Once activated, caspase-9 cleaves and activates downstream effector caspases, triggering the onset of cell death through caspase-mediated proteolysis of cellular proteins. Failure to activate caspase-9 enables the evasion of programmed cell death, which occurs in various forms of cancer. Despite the critical apoptotic function of caspase-9, the structural mechanism by which it is activated on the apoptosome has remained elusive. Here, we used a combination of methyl-transverse relaxation-optimized NMR spectroscopy, protein engineering, and biochemical assays to study the activation of caspase-9 bound to the apoptosome. In the absence of peptide substrate, we observed that both caspase-9 and its isolated protease domain (PD) only very weakly dimerize with dissociation constants in the millimolar range. Methyl-NMR spectra of isotope-labeled caspase-9, within the 1.3-MDa native apoptosome complex or an engineered 480-kDa apoptosome mimic, reveal that the caspase-9 PD remains monomeric after recruitment to the scaffold. Binding to the apoptosome, therefore, organizes caspase-9 PDs so that they can rapidly and extensively dimerize only when substrate is present, providing an important layer in the regulation of caspase-9 activation. Our work highlights the unique role of NMR spectroscopy to structurally characterize protein domains that are flexibly tethered to large scaffolds, even in cases where the molecular targets are in excess of 1 MDa, as in the present example.

Biological evaluation of the novel 3,3'-((4-nitrophenyl)methylene)bis(4-hydroxy-2H-chromen-2-one) derivative as potential anticancer agents via the selective induction of reactive oxygen species-mediated apoptosis

Selective initiation of programmed cell death in cancer cells than normal cells is reflected as an attractive chemotherapeutic strategy. In the current study, a series of synthetic bis-coumarin derivatives were synthesized possessing reactive oxygen species (ROS) modulating functional groups and examined in four cancerous and two normal cell lines for their cytotoxic ability using MTT assay. Among these compounds, 3 l emerged as the most promising derivative in persuading apoptosis in human renal carcinoma cells (SKRC-45) among diverse cancer cell lines. 3 l causes significantly less cytotoxicity to normal kidney cells compared to cisplatin. This compound was able to induce apoptosis and cell-cycle arrest by modulating the p53 mediated apoptotic pathways via the generation of ROS, decreasing mitochondrial membrane potential, and causing DNA fragmentation. Unlike cisplatin, the 3 l derivative was found to inhibit the nuclear localisation of NF-κB in SKRC-45 cells. It was also found to reduce the proliferation, survival and migration ability of SKRC-45 cells by downregulating COX-2/ PTGES2 cascade and MMP-2. In an in vivo tumor model, 3 l showed an anticancer effect by reducing the mean tumor mass, volume and inducing caspase-3 activation, without affecting kidney function. Further studies are needed to establish 3 l as a promising anti-cancer drug candidate.

Current trends in luminescence-based assessment of apoptosis

Apoptosis, the most extensively studied type of cell death, is known to play a crucial role in numerous processes such as elimination of unwanted cells or cellular debris, growth, control of the immune system, and prevention of malignancies. Defective regulation of apoptosis can trigger various diseases and disorders including cancer, neurological conditions, autoimmune diseases and developmental disorders. Knowing the nuances of the cell death type induced by a compound can help decipher which therapy is more effective for specific diseases. The detection of apoptotic cells using classic methods has brought significant contribution over the years, but innovative methods are quickly emerging and allow more in-depth understanding of the mechanisms, aside from a simple quantification. Due to increased sensitivity, time efficiency, pathway specificity and negligible cytotoxicity, these innovative approaches have great potential for both in vitro and in vivo studies. This review aims to shed light on the importance of developing and using novel nanoscale methods as an alternative to the classic apoptosis detection techniques.

Pro-apoptotic complexes of BAX and BAK on the outer mitochondrial membrane

In multicellular organisms the regulated cell death apoptosis is critically important for both ontogeny and homeostasis. Mitochondria are indispensable for stress-induced apoptosis. The BCL-2 protein family controls mitochondrial apoptosis and initiates cell death through the pro-apoptotic activities of BAX and BAK at the outer mitochondrial membrane (OMM). Cellular survival is ensured by the retrotranslocation of mitochondrial BAX and BAK into the cytosol by anti-apoptotic BCL-2 proteins. BAX/BAK-dependent OMM permeabilization releases the mitochondrial cytochrome c (cyt c), which initiates activation of caspase-9. The caspase cascade leads to cell shrinkage, plasma membrane blebbing, chromatin condensation, and apoptotic body formation. Although it is clear that ultimately complexes of active BAX and BAK commit the cell to apoptosis, the nature of these complexes is still enigmatic. Excessive research has described a range of complexes, varying from a few molecules to several 10,000, in different systems. BAX/BAK complexes potentially form ring-like structures that could expose the inner mitochondrial membrane. It has been suggested that these pores allow the efflux of small proteins and even mitochondrial DNA. Here we summarize the current state of knowledge for mitochondrial BAX/BAK complexes and the interactions between these proteins and the membrane.

Lycopene Alleviates Titanium Dioxide Nanoparticle-Induced Testicular Toxicity by Inhibiting Oxidative Stress and Apoptosis in Mice

PURPOSE

The research was carried out to investigate the possible ameliorative effect of lycopene on TiO₂ NPs-induced male reproductive toxicity and explore the possible mechanism.

METHODS

Ninety-six healthy male Institute of Cancer Research (ICR) mice were equally divided into eight groups (control group, 50 mg/kg TiO₂ NPs group, 5 mg/kg LYC group, 20 mg/kg LYC group, 40 mg/kg LYC group, 50 mg/kg TiO₂ NPs + 5 mg/kg LYC group, 50 mg/kg TiO₂ NPs + 20 mg/kg LYC group, 50 mg/kg TiO₂ NPs + 40 mg/kg LYC group), and the mice were treated by intragastric administration every day for 30 days in this research. Sperm parameters, testicular histopathology, oxidant and antioxidant enzymes, and cell apoptosis-related protein expression in the testicular tissue were analyzed.

RESULTS

The results showed that TiO₂ NPs exposure significantly decreased sperm count and motility, and TiO₂ NPs also increased sperm malformation in the epididymis; these characteristics were improved when co-administration with LYC. Testicular histopathological lesions like disorder of germ cells arrange, detachment, atrophy, and vacuolization were observed after TiO₂ NPs exposure, and these abnormalities were effectively ameliorated by co-administration with LYC. Oxidative stress was induced by TiO₂ NPs exposure as evidenced by increased the level of MDA and decreased the activity of SOD as well as the level of anti-O₂^-, and these alterations were effectively prevented by co-administration with LYC. LYC also alleviated TiO₂ NPs-induced germ cell apoptosis by inhibiting mitochondrial apoptotic pathway, as shown by the upregulation of Bcl-2, the downregulation of Bax, Cleaved Caspase 3, and Cleaved Caspase 9.

CONCLUSION

LYC could ameliorate TiO₂ NPs-induced testicular damage via inhibiting oxidative stress and apoptosis, which could be used to alleviate the testicular toxicity associated with TiO₂ NPs intake.

YQFM alleviated cardiac hypertrophy by apoptosis inhibition and autophagy regulation via PI3K/AKT/mTOR pathway

ETHNOPHARMACOLOGICAL RELEVANCE

As a traditional compound preparation of Chinese medicine, Yiqi Fumai lyophilized injection (YQFM) has protective effects on various cardiac diseases including cardiac hypertrophy, which is the primary cause of arrhythmia. However, the involved mechanism remains unclear.

AIM OF THE STUDY

This study was projected to investigate whether YQFM could prevent cardiac hypertrophy and arrhythmia concurrence.

MATERIALS AND METHODS

The cardiac hypertrophy rats were established by transverse aortic ligation and the H9c2 hypertrophy cardiomyocyte was induced by angiotensin II (AngII). The electrocardiogram (ECG) was conducted to estimate the arrhythmia occurrence of cardiac hypertrophy rats under isoprenaline (iso) treatment. The cardiac related indicators and histopathology were also detected. The protective effects of YQFM on H9c2 hypertrophy cardiomyocyte were determined by the cell size measurement, apoptosis detection and mitochondrial membrane potential measurement. The cardiac hypertrophy relative proteins (ANP and BNP), autophagy related factors (LC3II, p62 and Beclin-1), apoptosis related markers (p53, caspase 3, Bax and Bcl-2) and the PI₃K/AKT/mTOR pathway expressions were all measured by Western blot.

RESULTS

YQFM decreased the arrhythmia occurrence and improved cardiac function in cardiac hypertrophy rats. YQFM also reduced the H9c2 cardiomyocyte size and alleviated the cardiomyocyte apoptosis induced by AngII. In addition, YQFM inhibited cell apoptosis by increasing Bcl-2/Bax ratio and decreasing caspase 3 and p53 expressions in vitro and vivo. Meanwhile, YQFM regulated the autophagy pathway by down-regulating of LC3II and Beclin-1 expressions, as well as up-regulating of p62 expression. Finally, the results showed that YQFM could activate the PI₃K/AKT/mTOR pathway by enhancing the p-AKT, p-PI₃K and p-mTOR expressions.

CONCLUSION

Our results displayed that YQFM attenuated the cardiac hypertrophy by apoptosis inhibition and autophagy regulation via PI₃K/AKT/mTOR pathway.

Mechanisms of Survival of Cytomegalovirus-Infected Tumor Cells

Human cytomegalovirus (HCMV) DNA and proteins are often detected in malignant tumors, warranting studies of the role that HCMV plays in carcinogenesis and tumor progression. HCMV proteins were shown to regulate the key processes involved in tumorigenesis. While HCMV as an oncogenic factor just came into focus, its ability to promote tumor progression is generally recognized. The review discusses the viral factors and cell molecular pathways that affect the resistance of cancer cells to therapy. CMV inhibits apoptosis of tumor cells, that not only promotes tumor progression, but also reduces the sensitivity of cells to antitumor therapy. Autophagy was found to facilitate either cell survival or cell death in different tumor cells. In leukemia cells, HCMV induces a "protective" autophagy that suppresses apoptosis. Viral factors that mediate drug resistance and their interactions with key cell death pathways are necessary to further investigate in order to develop agents that can restore the tumor sensitivity to anticancer drugs.

In ovo feeding of vitamin C regulates splenic development through purine nucleotide metabolism and induction of apoptosis in broiler chickens

Nutrition in early life has a long-term influence on later health. In order to the explore effects of in ovo feeding (IOF) of vitamin C on splenic development, splenic metabolism and apoptosis were detected in embryo, adult chickens and in vitro. A total of 360 fertile eggs were selected and randomly assigned to control (CON) and vitamin C (VC) groups which were injected with saline and vitamin C on embryonic day 11, respectively. Functional enrichment of differentially expressed genes by transcriptome on embryonic day 19 suggested that purine nucleotide metabolism might be a potential pathway for the IOF of vitamin C to regulate spleen development. Additionally, the IOF of vitamin C significantly increased splenic vitamin C content on post-hatch day 21. Meanwhile, the splenic expression of adenosine deaminase, serine/threonine kinase 1 and proliferating cell nuclear antigen was down-regulated, whereas the expression of cysteinyl aspartate specific proteinase 9 was up-regulated in the VC group. On post-hatch day 42, the IOF of vitamin C significantly down-regulated the splenic expression of B-cell lymphoma 2 and increased the mRNA level of cysteinyl aspartate specific proteinase 9. The IOF of vitamin C could regulate the expression of genes related to adenylate metabolism and increased the apoptosis rate in vitro, which is consistent with the result in vivo. In conclusion, the IOF of vitamin C regulated splenic development and maturation by affecting purine nucleotide metabolism pathway and promoting apoptosis.

Qingda granule attenuates angiotensin II-induced cardiac hypertrophy and apoptosis and modulates the PI3K/AKT pathway

Qingda granule (QDG), simplified from Qingxuan Jiangya Decoction, is a well-known traditional Chinese medicine formula that has been used for decades to treat hypertension. However, the cardioprotective effects of QDG on Ang II-induced hypertension remain unknown. This study aimed to investigate the effects of QDG on hypertension-induced cardiac hypertrophy and apoptosis, as well as explore its underlying mechanisms. Mice were infused with Ang II (500 ng/kg/min) or saline solution as control, then administered oral QDG (1.145 g/kg/day) or saline for two weeks. QDG treatment attenuated the elevation in blood pressure caused by Ang II, as well as the decreased left ventricle ejection fractions and fractional shortening. Moreover, QDG treatment significantly alleviated the Ang II-induced elevation of the ratio of heart weight to tibia length, as well as cardiac injury, hypertrophy, and apoptosis. In cultured H9C2 cells stimulated with Ang II, QDG partially reversed the increase in cell surface area and number of apoptotic cells, up-regulation of hypertrophy markers ANP and BNP, and activation of caspases-9 and -3. QDG also partially reversed Ang II-induced accumulation of reactive oxygen species (ROS), depolarization of the mitochondrial membrane, release of cytochrome C, up-regulation of Bax, and decrease in levels of p-PI3K, p-AKT, and Bcl-2. These results suggest that QDG can significantly attenuate Ang II-induced hypertension, cardiac hypertrophy and apoptosis, and it may exert these effects in part by suppressing ROS production and activating the PI3K/AKT signaling pathway.

The Lumiptosome, an engineered luminescent form of the apoptosome can report cell death by using the same Apaf-1 dependent pathway

Detection of the apoptosis signature becomes central in understanding cell death modes. We present here a whole-cell biosensor that detects Apaf-1 association and apoptosome formation using a split-luciferase complementary assay. Fusion of N-terminal (Nluc) and C-terminal (Cluc)-fragments of firefly luciferase to the N-terminus of human Apaf-1 was performed in HEK293 cells by using CRISPR-Cas9 technology. This resulted in a luminescent form of the apoptosome that we named ‘Lumiptosome’. During Apaf-1 gene editing, a high number of knock-in events were observed without selection, suggesting that the Apaf-1 locus is important for the integration of exogenous transgenes. Since activation of caspase-9 is directly dependent on the apoptosome formation, measured reconstitution of luciferase activity should result from the cooperative association of Nluc-Apaf-1 and Cluc-Apaf-1. Time-response measurements also confirmed that formation of the apoptosome occurs prior to activation of caspase-3. Additionally, overexpression of the Bcl2 apoptosis regulator in transgenic and normal HEK293 cells confirmed that formation of the Lumiptosome depends on release of cytochrome c. Thus, HEK293 cells that stably express the Lumiptosome can be utilized to screen pro- and anti-apoptotic drugs, and to examine Apaf-1-dependent cellular pathways.

The apoptosome molecular timer synergises with XIAP to suppress apoptosis execution and contributes to prognosticating survival in colorectal cancer

The execution phase of apoptosis is a critical process in programmed cell death in response to a multitude of cellular stresses. A crucial component of this pathway is the apoptosome, a platform for the activation of pro-caspase 9 (PC9). Recent findings have shown that autocleavage of PC9 to Caspase 9 (C9) p35/p12 not only permits XIAP-mediated C9 inhibition but also temporally shuts down apoptosome activity, forming a molecular timer. In order to delineate the combined contributions of XIAP and the apoptosome molecular timer to apoptosis execution we utilised a systems modelling approach. We demonstrate that cooperative recruitment of PC9 to the apoptosome, based on existing PC9-apoptosome interaction data, is important for efficient formation of PC9 homodimers, autocatalytic cleavage and dual regulation by XIAP and the molecular timer across biologically relevant PC9 and APAF1 concentrations. Screening physiologically relevant concentration ranges of apoptotic proteins, we discovered that the molecular timer can prevent apoptosis execution in specific scenarios after complete or partial mitochondrial outer membrane permeabilisation (MOMP). Furthermore, its ability to prevent apoptosis is intricately tied to a synergistic combination with XIAP. Finally, we demonstrate that simulations of these processes are prognostic of survival in stage III colorectal cancer and that the molecular timer may promote apoptosis resistance in a subset of patients. Based on our findings, we postulate that the physiological function of the molecular timer is to aid XIAP in the shutdown of caspase-mediated apoptosis execution. This shutdown potentially facilitates switching to pro-inflammatory caspase-independent responses subsequent to Bax/Bak pore formation.

The FLAME-accelerated signalling tool (FaST) for facile parallelisation of flexible agent-based models of cell signalling

Agent-based modelling is particularly adept at modelling complex features of cell signalling pathways, where heterogeneity, stochastic and spatial effects are important, thus increasing our understanding of decision processes in biology in such scenarios. However, agent-based modelling often is computationally prohibitive to implement. Parallel computing, either on central processing units (CPUs) or graphical processing units (GPUs), can provide a means to improve computational feasibility of agent-based applications but generally requires specialist coding knowledge and extensive optimisation. In this paper, we address these challenges through the development and implementation of the FLAME-accelerated signalling tool (FaST), a software that permits easy creation and parallelisation of agent-based models of cell signalling, on CPUs or GPUs. FaST incorporates validated new agent-based methods, for accurate modelling of reaction kinetics and, as proof of concept, successfully converted an ordinary differential equation (ODE) model of apoptosis execution into an agent-based model. We finally parallelised this model through FaST on CPUs and GPUs resulting in an increase in performance of 5.8× (16 CPUs) and 53.9×, respectively. The FaST takes advantage of the communicating X-machine approach used by FLAME and FLAME GPU to allow easy alteration or addition of functionality to parallel applications, but still includes inherent parallelisation optimisation. The FaST, therefore, represents a new and innovative tool to easily create and parallelise bespoke, robust, agent-based models of cell signalling.

Transit of Procaspase-9 towards its activation. New mechanistic insights from molecular dynamics simulations

Caspases are cysteine proteases that perform a wide variety of roles in lethal intracellular signaling and cell-death regulation. Caspase-9, the primary initiator caspase of the intrinsic apoptotic pathway, is produced as a scarcely active zymogen (Procaspase-9). Here, we describe, for the first time, at the atomistic level, conformational changes which might be correlated to the activation of Procaspase-9. Molecular dynamics simulations performed at two temperatures (310 and 410 K) provide insights about the conformational space and the time-course evolution of the geometrical and structural characteristics of Procaspase-9. At both temperatures studied, the extremal globular domains of the protein approach each other, contracting the disordered region. In both temperatures, the compact conformations hide more than 40 nm² (about 20% of the total solvent-accessible surface area), and their radius of gyration are reduced by about 40% from the original values. At each temperature, the pathway of contraction is different, as well as the compact structures reached. In consequence, the network of stabilizing interactions at the final conformations is dissimilar. Both final conformations were evaluated in their structural compatibility with the activation models described so far. In this work, we describe mechanistically how and why the activation of Procaspase-9 is favored by apoptosome recruitment via the Caspase Activation Recruitment Domain (CARD), as it has been proposed recently by in vitro experiments.

Anticancer activity of a novel methylated analogue of L-mimosine against an in vitro model of human malignant melanoma

The anticancer activity of a series of novel synthesized, hydroxypyridone-based metal chelators (analogues of L-mimosine) was evaluated in an in vitro model of melanoma consisting of malignant melanoma (A375), non-melanoma epidermoid carcinoma (A431) and immortalized non-malignant keratinocyte (HaCaT) cells. More specifically, we have demonstrated that the L-enantiomer of a methylated analogue of L-mimosine (compound 22) can exert a potent anticancer effect in A375 cells when compared to either A431 or HaCaT cells. Moreover, we have demonstrated that this analogue has the ability to i) promote increased generation of reactive oxygen species (ROS), ii) activate both intrinsic and extrinsic apoptosis and iii) induce perturbations in cell cycle growth arrest. Our data highlights the potential of compound 22 to act as a promising therapeutic agent against an in vitro model of human malignant melanoma.

New insights into apoptosome structure and function

The apoptosome is a platform that activates apical procaspases in response to intrinsic cell death signals. Biochemical and structural studies in the past two decades have extended our understanding of apoptosome composition and structure, while illuminating the requirements for initiator procaspase activation. A number of studies have now provided high-resolution structures for apoptosomes from C. elegans (CED-4), D. melanogaster (Dark), and H. sapiens (Apaf-1), which define critical protein interfaces, including intra and interdomain interactions. This work also reveals interactions of apoptosomes with their respective initiator caspases, CED-3, Dronc and procaspase-9. Structures of the human apoptosome have defined the requirements for cytochrome c binding, which triggers the conversion of inactive Apaf-1 molecules to an extended, assembly competent state. While recent data have provided a detailed understanding of apoptosome formation and procaspase activation, they also highlight important evolutionary differences with functional implications for caspase activation. Comparison of the CARD/CARD disks and apoptosomes formed by CED-4, Dark and Apaf-1. Cartoons of the active states of the CARD-CARD disks, illustrating the two CED-4 CARD tetrameric ring layers (CED4a and CED4b; top row) and the binding of 8 Dronc CARDs and between 3-4 pc-9 CARDs, to the Dark and Apaf-1 CARD disk respectively (middle and lower rows). Ribbon diagrams of the active CED-4, Dark and Apaf-1 apoptosomes are shown (right column).

Optimal pathways for the assembly of the Apaf-1·cytochrome c complex into apoptosome

The formation of a heptameric apoptosome is a crucial event in the intrinsic cell death pathway. Considerable progress has been made towards unraveling the constituents and the structure of the apoptosome as well as the mechanism of apoptosome-mediated caspase-9 activation. However, a significant gap remains in the understanding of this process, i.e., how seven Apaf-1·cytochrome c complexes stepwisely assemble into an apoptosome. Here, we construct a biophysical model that incorporates current biochemical knowledge about the formation of apoptosome. We propose 11 elementary routes and enumerate all 2047 possible assembly pathways from the Apaf-1·cytochrome c complex to the heptameric apoptosome. By combining mathematical analysis and numerical simulation, we find that two elementary routes are the most favorable biochemical reaction routes and there are 52 optimal assembly pathways which are economical and relatively fast. Our study yields the first comprehensive analysis of apoptosome assembly and provides insights into complex assembly pathways.

Molecular interplays in hepatic stellate cells: apoptosis, senescence, and phenotype reversion as cellular connections that modulate liver fibrosis

Liver fibrosis is a pathophysiological process correlated with intense repair and cicatrization mechanisms in injured liver, and over the past few years, the characterization of the fine-tuning of molecular interconnections that support the development of liver fibrosis has been investigated. In this cellular process, the hepatic stellate cells (HSCs) support the organ fibrogenesis. The HSCs are found in two distinct morpho-physiological states: quiescent and activated. In normal liver, most HSCs are found in quiescent state, presenting a considerable amount of lipid droplets in the cytoplasm, while in injured liver, the activated phenotype of HSCs is a myofibroblast, that secrete extracellular matrix elements and contribute to the establishment of the fibrotic process. Studies on the molecular mechanisms by which HSCs try to restore their quiescent state have been performed; however, no effective treatment to reverse fibrosis has been so far prescribed. Therefore, the elucidation of the cellular and molecular mechanisms of apoptosis, senescence, and the cell reversion phenotype process from activate to quiescent state will certainly contribute to the development of effective therapies to treat hepatic fibrosis. In this context, this review aimed to address central elements of apoptosis, senescence, and reversal of HSC phenotype in the control of hepatic fibrogenesis, as a guide to future development of therapeutic strategies.

Sulforaphane-cysteine-induced apoptosis via phosphorylated ERK1/2-mediated maspin pathway in human non-small cell lung cancer cells

Sulforaphane (SFN) was demonstrated to induce apoptosis in a variety of cancers via multiple mechanisms. However, owing to a short half-life in circulation, SFN was not used for clinical treatment yet. Interestingly, SFN analog, sulforaphane-cysteine (SFN-Cys) has a longer half-life in metabolism, and we previously demonstrated that SFN-Cys inhibited invasion in human prostate cancer cells. Here, we would investigate whether SFN-Cys induces apoptosis and find the underlying mechanisms in human non-small cell lung cancer (NSCLC) cells. Western blots were used to test the molecular linkages among extracellular signal-regulated kinases 1/2 (ERK1/2) and downstream signal molecules. Flow cytometry and fluorescence microscopy were used to detect cell death. Cell proliferation assay showed that SFN-Cys inhibited cell viability following a dose-dependent manner. Abnormal cell morphology was viewed after the cells were exposed to SFN-Cys. Flow cytometry showed that SFN-Cys induced cell apoptosis via a dose-dependent manner. Further, SFN-Cys triggered the activation of ERK1/2, which resulted in the upregulation of maspin, Bax, cleaved caspase-3 and downregulation of pro-caspase-3, Bcl-2, α-tubulin. Meanwhile, we demonstrated that recombinant caspase-3 cleaved α-tubulin in the lysate of cells, which were treated by SFN-Cys. These data indicated that SFN-Cys activated the ERK1/2-mediated mitochondria signaling pathway with maspin upregulation and α-tubulin downregulation leading to apoptosis. These findings will help to develop a novel therapeutic to target NSCLC cells.

Apaf-1: Regulation and function in cell death

Apoptosis, a form of programmed cell death, is responsible for eliminating damaged or unnecessary cells in multicellular organisms. Various types of intracellular stress trigger apoptosis by induction of cytochrome c release from mitochondria into the cytosol. Apoptotic protease activating factor-1 (Apaf-1) is a key molecule in the intrinsic or mitochondrial pathway of apoptosis, which oligomerizes in response to cytochrome c release and forms a large complex known as apoptosome. Procaspase-9, an initiator caspase in the mitochondrial pathway, is recruited and activated by the apoptosome leading to downstream caspase-3 processing. Various cellular proteins and small molecules can modulate apoptosome formation and function directly or indirectly. Despite recent progress in understanding the mitochondrial pathway of apoptosis, numerous questions such as the molecular mechanism of Apaf-1 oligomerization and caspase-9 activation remain poorly understood. In addition, reports have emerged showing non-apoptotic functions for Apaf-1. The current review summarizes the latest findings regarding structure-function relationship of Apaf-1 as well as its modifiers.

ASC Induces Apoptosis via Activation of Caspase-9 by Enhancing Gap Junction-Mediated Intercellular Communication

ASC (apoptosis-associated speck-like protein containing a CARD) is a key adaptor molecule of inflammasomes that mediates inflammatory and apoptotic signals. Aberrant methylation-induced silencing of ASC has been observed in a variety of cancer cells, thus implicating ASC in tumor suppression, although this role remains incompletely defined especially in the context of closely neighboring cell proliferation. As ASC has been confirmed to be silenced by abnormal methylation in HT1080 fibrosarcoma cells as well, this cell line was investigated to characterize the precise role and mechanism of ASC in tumor progression. The effects of ASC were examined using in vitro cell cultures based on comparisons between low and high cell density conditions as well as in a xenograft murine model. ASC overexpression was established by insertion of the ASC gene into pcDNA3 and pMX-IRES-GFP vectors, the latter being packed into a retrovirus and subjected to reproducible competitive assays using parental cells as an internal control, for evaluation of cell viability. p21 and p53 were silenced using shRNA. Cell viability was suppressed in ASC-expressing transfectants as compared with control cells at high cell density conditions in in vitro culture and colony formation assays and in in vivo ectopic tumor formation trials. This suppression was not detected in low cell density conditions. Furthermore, remarkable progression of apoptosis was observed in ASC-introduced cells at a high cell density, but not at a low one. ASC-dependent apoptosis was mediated not by p21, p53, or caspase-1, but rather by cleavage of caspase-9 as well as by suppression of the NF-κB-related X-linked inhibitor-of-apoptosis protein. Caspase-9 cleavage was observed to be dependent on gap junction formation. The remarkable effect of ASC on the induction of apoptosis through caspase-9 and gap junctions revealed in this study may lead to promising new approaches in anticancer therapy.

A near atomic structure of the active human apoptosome

In response to cell death signals, an active apoptosome is assembled from Apaf-1 and procaspase-9 (pc-9). Here we report a near atomic structure of the active human apoptosome determined by cryo-electron microscopy. The resulting model gives insights into cytochrome c binding, nucleotide exchange and conformational changes that drive assembly. During activation an acentric disk is formed on the central hub of the apoptosome. This disk contains four Apaf-1/pc-9 CARD pairs arranged in a shallow spiral with the fourth pc-9 CARD at lower occupancy. On average, Apaf-1 CARDs recruit 3 to 5 pc-9 molecules to the apoptosome and one catalytic domain may be parked on the hub, when an odd number of zymogens are bound. This suggests a stoichiometry of one or at most, two pc-9 dimers per active apoptosome. Thus, our structure provides a molecular framework to understand the role of the apoptosome in programmed cell death and disease.

DOI:http://dx.doi.org/10.7554/eLife.17755.001

An adult human loses around 50–70 billion cells every day via a process termed apoptosis. The term arises from the Greek word that describes leaves “falling off” a tree, and the process entails damaged or unwanted cells essentially committing suicide in a controlled manner. As such, apoptosis keeps the number of cells in tissues and organs in check. It also allows components of old cells to be recycled to make new ones.

In cells that are targeted to die, a protein called cytochrome c interacts with another protein, named Apaf-1, together with a nucleotide triphosphate molecule. These steps work in concert to trigger the assembly of the apoptosome: a large wheel-like complex that contains seven copies each of Apaf-1 and cytochrome c. The central hub of the wheel then recruits a specific protein-cutting enzyme, which once activated sets in motion a cascade of events that dismantle the cell from the inside out.

Cheng et al. now use an electron microscope to reveal the three-dimensional structure of the active human apoptosome, in enough detail to determine the positions of many of the amino acids that make up the complex. The three dimensional model provides new insights into how Apaf-1 changes shape as the complex assembles in the presence of cytochrome c and nucleotide triphosphate. Cheng et al. went on to reveal a disk-like structure made from the parts of four Apaf-1 proteins that interact with the protein-cutting enzymes. This disk forms a spiral that sits atop the central hub of the wheel-like apoptosome. Finally, the findings suggest that, although the wheel like complex has seven spokes, at any one time the active apoptosome may only need two (or at most four) copies of the protein-cutting enzyme to trigger the cascade of events that lead to cell death

In the future, emerging technologies may provide high enough resolution to visualize fine details of the interactions between cytochrome c and Apaf-1, and reveal more about the disk-like spiral as well. This in turn will give a better understanding of how the apoptosome assembles and how the protein-cutting enzyme becomes activated.

DOI:http://dx.doi.org/10.7554/eLife.17755.002

Necroptosis: A new way of dying?

This review embarks upon a cell death journey from the discovery of apoptosis and necrosis through to the coalescence of these: necroptosis. The mechanisms of 2 emerging necrotic cell death pathways, pyroptosis and ferroptosis, will be explored before delving into apoptotic and necroptotic signaling cascades, highlighting the complex interplay between molecular players. The involvement of the ripoptosome, interferon signaling and DNA damage in necroptosis will be discussed briefly. The major focus is on necroptosis initiation by tumor necrosis factor-α (TNFα) and its cognate receptor TNFR1, caspase-independent RIP1/RIP3/MLKL necrosome activation and cell death propagation by damage-associated molecular pattern (DAMP) release. Finally, the implications of a complex cell death signaling network will be revealed in the context of cancer biology and therapy. The clinical contribution of the discovery of necroptosis as an unequivocally new way of dying is monumental and could drastically alter cancer therapy strategies in the future.