Caspase 3 attenuates XIAP (X-linked inhibitor of apoptosis protein)-mediated inhibition of caspase 9

Evolution of Caspases and the Invention of Pyroptosis

The protein scaffold that includes the caspases is ancient and found in all domains of life. However, the stringent specificity that defines the caspase biologic function is relatively recent and found only in multicellular animals. During the radiation of the Chordata, members of the caspase family adopted roles in immunity, events coinciding with the development of substrates that define the modern innate immune response. This review focuses on the switch from the non-inflammatory cellular demise of apoptosis to the highly inflammatory innate response driven by distinct members of the caspase family, and the interplay between these two regulated cell death pathways.

TRAIL-induced apoptosis and proteasomal activity - Mechanisms, signalling and interplay

Programmed cell death, in particular apoptosis, is essential during development and tissue homeostasis, and also is the primary strategy to induce cancer cell death by cytotoxic therapies. Precision therapeutics targeting TRAIL death receptors are being evaluated as novel anti-cancer agents, while in parallel highly specific proteasome inhibitors have gained approval as drugs. TRAIL-dependent signalling and proteasomal control of cellular proteostasis are intricate processes, and their interplay can be exploited to enhance therapeutic killing of cancer cells in combination therapies. This review provides detailed insights into the complex signalling of TRAIL-induced pathways and the activities of the proteasome. It explores their core mechanisms of action, pharmaceutical druggability, and describes how their interplay can be strategically leveraged to enhance cell death responses in cancer cells. Offering this comprehensive and timely overview will allow to navigate the complexity of the processes governing cell death mechanisms in TRAIL- and proteasome inhibitor-based treatment conditions.

Apoptotic signaling: Beyond cell death

Apoptosis is the best described form of regulated cell death, and was, until relatively recently, considered irreversible once particular biochemical points-of-no-return were activated. In this manuscript, we examine the mechanisms cells use to escape from a self-amplifying death signaling module. We discuss the role of feedback, dynamics, propagation, and noise in apoptotic signaling. We conclude with a revised model for the role of apoptosis in animal development, homeostasis, and disease.

Death and survival from executioner caspase activation

Executioner caspases are evolutionarily conserved regulators of cell death under apoptotic stress. Activated executioner caspases drive apoptotic cell death through cleavage of diverse protein substrates or pyroptotic cell death in the presence of gasdermin E. On the other hand, activation of executioner caspases can also trigger pro-survival and pro-proliferation signals. In recent years, a growing body of studies have demonstrated that cells can survive from executioner caspase activation in response to stress and that the survivors undergo molecular and phenotypic alterations. This review focuses on death and survival from executioner caspase activation, summarizing the role of executioner caspases in apoptotic and pyroptotic cell death and discussing the potential mechanism and consequences of survival from stress-induced executioner caspase 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.

Caspase-9 inhibition confers stronger neuronal and vascular protection compared to VEGF neutralization in a mouse model of retinal vein occlusion

Purpose

Retinal vein occlusion (RVO) is a sight-threatening condition typically treated with intravitreal injection of vascular endothelial growth factor (VEGF) antagonists. Treatment response to anti-VEGF therapies is highly variable, with poor visual outcomes and treatment response in patients with significant retinal nonperfusion following RVO. Recently, caspase-9 has been identified as a potent regulator of edema, gliosis, and neuronal dysfunction during acute retinal hypoxia. The purpose of this study was to compare the therapeutic effect of caspase-9 inhibition against VEGF-neutralization in an established mouse model of RVO.

Methods

Adult male C57Bl/6 J mice were randomized to induction of RVO and treatment with either vehicle, intravitreal injection of anti-VEGF antibody, topical administration of a selective caspase-9 inhibitor (Pen1-XBir3), or a combination therapy. Animals were followed on days 1, 2, and 8 after RVO with fundus retinal imaging, and with optical coherence tomography (OCT) to capture retinal swelling, capillary nonperfusion (measured by disorganization of retinal inner layers, DRIL), hyperreflective foci (HRF), and retinal atrophy. Focal electroretinography (ERG) measurements were performed on day 7. Histology was performed on retinal sections from day 8.

Results

Both VEGF neutralization and caspase-9 inhibition showed significant retinal protection from RVO compared to vehicle treatment arm. Retinal reperfusion of occluded veins was accelerated in eyes receiving caspase-9 inhibitor, but not significantly different from vehicle in the anti-VEGF group. Retinal edema was suppressed in all treatment groups, with approximately 2-fold greater edema reduction with caspase-9 inhibition compared to VEGF neutralization. HRF were reduced similarly across all treatment groups compared to vehicle. Retinal detachment was reduced only in eyes treated with caspase-9 inhibitor monotherapy. Caspase-9 inhibition reduced retinal atrophy and preserved ERG response; VEGF neutralization did not prevent neurodegeneration following RVO.

Conclusion

Caspase-9 inhibition confers stronger neuronal and vascular protection compared to VEGF neutralization in the mouse laser-induced model of RVO.

Caspase 3-specific cleavage of MEK1 suppresses ERK signaling and sensitizes cells to stress-induced apoptosis

Proper regulation of apoptotic cell death is crucial for normal development and homeostasis in multicellular organisms and is achieved by the balance between pro-apoptotic processes, such as caspase activation, and pro-survival signaling, such as extracellular signal-regulated kinase (ERK) activation. However, the functional interplay between these opposing signaling pathways remains incompletely understood. Here, we identified MAPK/ERK kinase (MEK) 1, a central component of the ERK pathway, as a specific substrate for the executioner caspase-3. During apoptosis, MEK1 is cleaved at an evolutionarily conserved Asp282 residue in the kinase domain, thereby losing its catalytic activity. Gene knockout experiments showed that MEK1 cleavage was mediated by caspase-3, but not by the other executioner caspases, caspase-6 or -7. Following exposure of cells to osmotic stress, elevated ERK activity gradually decreased, and this was accompanied by increased cleavage of MEK1. In contrast, the expression of a caspase-uncleavable MEK1(D282N) mutant in cells maintained stress-induced ERK activity and thereby attenuated apoptotic cell death. Thus, caspase-3-mediated, proteolytic inhibition of MEK1 sensitizes cells to apoptosis by suppressing pro-survival ERK signaling. Furthermore, we found that a RASopathy-associated MEK1(Y130C) mutation prevented this caspase-3-mediated proteolytic inactivation of MEK1 and efficiently protected cells from stress-induced apoptosis. Our data reveal the functional crosstalk between ERK-mediated cell survival and caspase-mediated cell death pathways and suggest that its dysregulation by a disease-associated MEK1 mutation is at least partly involved in the pathophysiology of congenital RASopathies.

Inhibition of TERC inhibits neural apoptosis and inflammation in spinal cord injury through Akt activation and p-38 inhibition via the miR-34a-5p/XBP-1 axis

This study investigated the function of telomerase RNA component (TERC) in spinal cord injury (SCI). SCI models were established in rats via laminectomy and PC-12 cells were treated with lipopolysaccharide (LPS). TERC and miR-34a-5p expressions in cells and rat spinal cords were detected by quantitative reverse transcription polymerase chain reaction, followed by overexpression/knockdown of TERC/miR-34a-5p. Spinal cord histopathological changes were examined via hematoxylin-eosin staining. miR-34a-5p' relation with TERC and XBP-1 was predicted by TargetScan and checked by dual-luciferase reporter/RNA immunoprecipitation assays. Cell biological behaviors were assessed by Cell counting kit-8, wound healing, Transwell, and flow cytometry assays. XBP-1 and inflammation/apoptosis-related protein expressions were analyzed by western blot. TERC was upregulated and miR-34a-5p was low-expressed in SCI tissues and LPS-induced PC-12 cells. TERC-knockdown alleviated histopathological abnormalities yet upregulated miR-34a-5p in SCI tissues. In LPS-induced PC-12 cells, TERC knockdown promoted cell viability, migration, invasion, and inhibited apoptosis, while TERC overexpression ran oppositely. TERC knockdown downregulated the XBP-1, IL-6, TNF-α, Bax, p-p38/t-p38, and cleaved caspase-9/-3, but upregulated Bcl-2 and p-Akt/t-Akt. TERC targeted miR-34a-5p, which further targeted XBP-1. miR-34a-5p downregulation exerted effects opposite to and offset TERC knockdown-induced effects. TERC knockdown facilitated the regeneration of neuron tissues yet inhibited inflammation in SCI through Akt activation and p-38 inhibition via the miR-34a-5p/XBP-1 axis.

Caspase mechanisms in the regulation of inflammation

Regulated cell death is defined as genetically encoded pathways that lead towards the demise of cells. In mammals, cell demise can be either inflammatory or non-inflammatory, depending on whether the mechanism of death results in cell rupture or not. Inflammatory cell death can lead towards acute and chronic disease. Therefore, it becomes important to distinguish the mechanisms that result in these different inflammatory cell death outcomes. Apoptosis is a non-inflammatory form of cell death where cells resist rupture. In contrast, pyroptosis and necroptosis are inflammatory forms of cell death principally because of release of pro-inflammatory mediators from cells undergoing lysis. This review focusses on the mechanisms of these different cell death outcomes with specific emphasis on the caspase family of proteolytic enzymes.

Propofol postconditioning ameliorates hypoxia/reoxygenation induced H9c2 cell apoptosis and autophagy via upregulating forkhead transcription factors under hyperglycemia

BACKGROUND

Administration of propofol, an intravenous anesthetic with antioxidant property, immediately at the onset of post-ischemic reperfusion (propofol postconditioning, P-PostC) has been shown to confer cardioprotection against ischemia-reperfusion injury, while the underlying mechanism remains incompletely understood. The FoxO transcription factors are reported to play critical roles in activating cardiomyocyte survival signaling throughout the process of cellular injuries induced by oxidative stress and are also involved in hypoxic postconditioning mediated neuroprotection, however, the role of FoxO in postconditioning mediated protection in the heart and in particular in high glucose condition is unknown.

METHODS

Rat heart-derived H9c2 cells were exposed to high glucose (HG) for 48 h (h), then subjected to hypoxia/reoxygenation (H/R, composed of 8 h of hypoxia followed by 12 h of reoxygenation) in the absence or presence of postconditioning with various concentrations of propofol (P-PostC) at the onset of reoxygenation. After having identified the optical concentration of propofol, H9c2 cells were subjected to H/R and P-PostC in the absence or presence of FoxO1 or FoxO3a gene silencing to explore their roles in P-PostC mediated protection against apoptotic and autophagic cell deaths under hyperglycemia.

RESULTS

The results showed that HG with or without H/R decreased cell viability, increased lactate dehydrogenase (LDH) leakage and the production of reactive oxygen species (ROS) in H9c2 cells, all of which were significantly reversed by propofol (P-PostC), especially at the concentration of 25 µmol/L (P25) (all P < 0.05, NC vs. HG; HG vs. HG + HR; HG + HR + P12.5 or HG + HR + P25 or HG + HR + P50 vs. HG + HR). Moreover, we found that propofol (P25) decreased H9c2 cells apoptosis and autophagy that were concomitant with increased FoxO1 and FoxO3a expression (all P < 0.05, HG + HR + P25 vs. HG + HR). The protective effects of propofol (P25) against H/R injury were reversed by silencing FoxO1 or FoxO3a (all P < 0.05, HG + HR + P25 vs. HG + HR + P25 + siRNA-1 or HG + HR + P25 + siRNA-5).

CONCLUSION

It is concluded that propofol postconditioning attenuated H9c2 cardiac cells apoptosis and autophagy induced by H/R injury through upregulating FoxO1 and FoxO3a under hyperglycemia.

Protein Transnitrosylation Signaling Networks Contribute to Inflammaging and Neurodegenerative Disorders

Significance: Physiological concentrations of nitric oxide (NO^•) and related reactive nitrogen species (RNS) mediate multiple signaling pathways in the nervous system. During inflammaging (chronic low-grade inflammation associated with aging) and in neurodegenerative diseases, excessive RNS contribute to synaptic and neuronal loss. "NO signaling" in both health and disease is largely mediated through protein S-nitrosylation (SNO), a redox-based posttranslational modification with "NO" (possibly in the form of nitrosonium cation [NO⁺]) reacting with cysteine thiol (or, more properly, thiolate anion [R-S^-]). Recent Advances: Emerging evidence suggests that S-nitrosylation occurs predominantly via transnitros(yl)ation. Mechanistically, the reaction involves thiolate anion, as a nucleophile, performing a reversible nucleophilic attack on a nitroso nitrogen to form an SNO-protein adduct. Prior studies identified transnitrosylation reactions between glyceraldehyde-3-phosphate dehydrogenase (GAPDH)-nuclear proteins, thioredoxin-caspase-3, and X-linked inhibitor of apoptosis (XIAP)-caspase-3. Recently, we discovered that enzymes previously thought to act in completely disparate biochemical pathways can transnitrosylate one another during inflammaging in an unexpected manner to mediate neurodegeneration. Accordingly, we reported a concerted tricomponent transnitrosylation network from Uch-L1-to-Cdk5-to-Drp1 that mediates synaptic damage in Alzheimer's disease. Critical Issues: Transnitrosylation represents a critical chemical mechanism for transduction of redox-mediated events to distinct subsets of proteins. Although thousands of thiol-containing proteins undergo S-nitrosylation, how transnitrosylation regulates a myriad of neuronal attributes is just now being uncovered. In this review, we highlight recent progress in the study of the chemical biology of transnitrosylation between proteins as a mechanism of disease. Future Directions: We discuss future areas of study of protein transnitrosylation that link our understanding of aging, inflammation, and neurodegenerative diseases. Antioxid. Redox Signal. 35, 531-550.

Selective striatal cell loss is ameliorated by regulated autophagy of the cortex

AIMS

The harmful cellular environment leads to brain damage, and each brain subregion exhibits a differential vulnerability to its effects. This study investigated the causes of selectively striatal cell loss in systemic 3-nitropropionic acid (3-NP) infused mice.

MAIN METHODS

This study was performed in the neuronal cell line, primary neuron, cultured mouse brain, and mice brain tissues. The 3-NP solution was delivered using an osmotic mini-pump system for 7 days. ROS in brain tissue were detected and evaluated with the signals of CM-H2DCFDA for total cellular ROS and MitoSOX Red for mitochondrial ROS. Cellular ROS and the functional status of mitochondria were assessed with a detection kit and analyzed using flow cytometry. To quantify oxidative damaged DNA, apurinic/apyrimidinic (AP) site numbers in DNA were measured. The protein expression level was assessed using Western blotting, and immunohistochemistry was performed. Cleaved caspase-3 activities were measured by using an enzyme-linked immunosorbent assay (ELISA) kit.

KEY FINDINGS

By 3-NP, mitochondrial dysfunction was higher in the striatum than in the cortex, and mitochondria-derived ROS levels were higher in the striatum than in the cortex. However, autophagy that may restore the energy depletion resulting from mitochondrial dysfunction occurred comparably less in the striatum than in the cortex. Inhibition of ASK1 by NQDI1 regulates MAPK signaling, apoptosis, and autophagy. Regulated autophagy of the cortex improved non-cell autonomously striatal damaged condition.

SIGNIFICANCE

This study illustrated that the different vulnerabilities of the brain subregions, striatum or cortex, against 3-NP are rooted in different mitochondria-derived ROS amounts and autophagic capacity.

Caspase-9: A Multimodal Therapeutic Target With Diverse Cellular Expression in Human Disease

Caspase-9, a cysteine-aspartic protease known for its role as an initiator of intrinsic apoptosis, regulates physiological cell death and pathological tissue degeneration. Its nonapoptotic functions, including regulation of cellular differentiation/maturation, innate immunity, mitochondrial homeostasis, and autophagy, reveal a multimodal landscape of caspase-9 functions in health and disease. Recent work has demonstrated that caspase-9 can drive neurovascular injury through nonapoptotic endothelial cell dysfunction. CASP9 polymorphisms have been linked with various cancers, neurological disorders, autoimmune pathologies and lumbar disc disease. Clinical reports suggest alterations in caspase-9 expression, activity or function may be associated with acute and chronic neurodegeneration, retinal neuropathy, slow-channel myasthenic syndrome, lumbar disc disease, cardiomyopathies, atherosclerosis and autoimmune disease. Healthy tissues maintain caspase-9 activity at low basal levels, rendering supraphysiological caspase-9 activation a tractable target for therapeutic interventions. Strategies for selective inhibition of caspase-9 include dominant negative caspase-9 mutants and pharmacological inhibitors derived from the XIAP protein, whose Bir3 domain is an endogenous highly selective caspase-9 inhibitor. However, the mechanistic implications of caspase-9 expression and activation remain indeterminate in many pathologies. By assembling clinical reports of caspase-9 genetics, signaling and cellular localization in human tissues, this review identifies gaps between experimental and clinical studies on caspase-9, and presents opportunities for further investigations to examine the consequences of caspase activity in human disease.

Spiraeoside extracted from red onion skin ameliorates apoptosis and exerts potent antitumor, antioxidant and enzyme inhibitory effects

Red onion skin waste (ROSW) was analyzed for extraction of naturally occurring 4'-O-glucoside of quercetin, spiraeoside (SPI) with promising biological activities. Reversed-phase high-performance liquid chromatography was used to determine the SPI content in three different solvent extracts of ROSW: water (12.2 mg/g), methanol (27.6 mg/g), and ethanol (32.5 mg/g). The ethanol extract and SPI showed significant radical-scavenging and anti-inflammatory activities. In addition, the anti-cancer effects of SPI on a HeLa cells was investigated. The results indicated that SPI treatment significantly inhibited cell growth, and the dose of 50 μg/mL exhibited the highest anti-cancer activity. SPI inhibited the expression of B-cell lymphoma 2 and BH3-interacting domain-death agonist and promoted apoptosis by activating caspase-9/-3 expression. Notably, SPI inhibited the expression of mu-2-related death-inducing gene, a molecule involved in death receptor-mediated apoptotic signaling. Cyclin-dependent kinase 2-cyclin-E expression was also inhibited after SPI treatment, particularly at the G₂/M checkpoint. Our findings provide novel insights into the apoptotic potential with promising anticancer and enzyme inhibitory effects of ROSW SPI.

Endothelial activation of caspase-9 promotes neurovascular injury in retinal vein occlusion

Central nervous system ischemic injury features neuronal dysfunction, inflammation and breakdown of vascular integrity. Here we show that activation of endothelial caspase-9 after hypoxia-ischemia is a critical event in subsequent dysfunction of the blood-retina barrier, using a panel of interrelated ophthalmic in vivo imaging measures in a mouse model of retinal vein occlusion (RVO). Rapid nonapoptotic activation of caspase-9 and its downstream effector caspase-7 in endothelial cells promotes capillary ischemia and retinal neurodegeneration. Topical eye-drop delivery of a highly selective caspase-9 inhibitor provides morphological and functional retinal protection. Inducible endothelial-specific caspase-9 deletion phenocopies this protection, with attenuated retinal edema, reduced inflammation and preserved neuroretinal morphology and function following RVO. These results reveal a non-apoptotic function of endothelial caspase-9 which regulates blood-retina barrier integrity and neuronal survival, and identify caspase-9 as a therapeutic target in neurovascular disease.

Retinal vein occlusion can cause blindness, and features neuronal dysfunction, inflammation and breakdown of vascular integrity. Here the authors report a non-apoptotic role of endothelial caspase-9 in regulating blood-retina barrier integrity and neuronal survival, which can be therapeutically targeted in a mouse model of retinal vein occlusion.

A novel derivative of valepotriate inhibits the PI3K/AKT pathway and causes Noxa-dependent apoptosis in human pancreatic cancer cells

Natural compound valepotriate exhibits inhibitory activity against a number of cancers, but the effect of valepotriate against pancreatic cancer is unclear, and the structure–activity relationship of valepotriate has not been characterized. In this study, we performed a structure-based similarity search and found 16 hit compounds. Among the 16 hits, (1S,6S,7R)-6-(acetyloxy)-1-[(3-methylbutanoyl)oxy]-4a,5,6,7a-tetrahydro-1H-spiro[cyclopenta[c]pyran-7,2’-oxiran]-4-ylmethyl 3-methylbutanoate (denoted as Amcp) exhibited superior anticancer activity against human pancreatic cancer BxPC-3 and SW1990 cells. The anti-proliferation activity of Amcp was validated in human pancreatic cancer BxPC-3 and SW1990 cells in vitro. Amcp more effectively induced apoptosis in BxPC-3 and SW1990 cells than gemcitabine. At a concentration of 15 μM, Amcp significantly suppressed the PI3K/AKT pathway and disrupted the mitochondrial membrane equilibrium through modulation of Noxa and Mcl-1 balance in both cell lines. Meanwhile, knockdown of Noxa substantially attenuated Amcp-induced reduction of cell viability and anti-apoptotic protein Mcl-1 level in BxPC-3 cells. In addition, Amcp showed synergistic anticancer effects when combined with gemcitabine in BxPC-3 cells. To conclude, this work not only suggests that Amcp possesses a dual-inhibitory activity towards PI3K/AKT pathway and Mcl-1, but also enlightens further development of bioactive valepotriate derivatives.

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.

Understanding MAPK Signaling Pathways in Apoptosis

MAPK (mitogen-activated protein kinase) signaling pathways regulate a variety of biological processes through multiple cellular mechanisms. In most of these processes, such as apoptosis, MAPKs have a dual role since they can act as activators or inhibitors, depending on the cell type and the stimulus. In this review, we present the main pro- and anti-apoptotic mechanisms regulated by MAPKs, as well as the crosstalk observed between some MAPKs. We also describe the basic signaling properties of MAPKs (ultrasensitivity, hysteresis, digital response), and the presence of different positive feedback loops in apoptosis. We provide a simple guide to predict MAPKs’ behavior, based on the intensity and duration of the stimulus. Finally, we consider the role of MAPKs in osmostress-induced apoptosis by using Xenopus oocytes as a cell model. As we will see, apoptosis is plagued with multiple positive feedback loops. We hope this review will help to understand how MAPK signaling pathways engage irreversible cellular decisions.

TRAIL receptor signaling: From the basics of canonical signal transduction toward its entanglement with ER stress and the unfolded protein response

The cytokine tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the large TNF superfamily that can trigger apoptosis in transformed or infected cells by binding and activating two receptors, TRAIL receptor 1 (TRAILR1) and TRAIL receptor 2 (TRAILR2). Compared to other death ligands of the same family, TRAIL induces apoptosis preferentially in malignant cells while sparing normal tissue and has therefore been extensively investigated for its suitability as an anti-cancer agent. Recently, it was noticed that TRAIL receptor signaling is also linked to endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). The role of TRAIL receptors in regulating cellular apoptosis susceptibility therefore is broader than previously thought. Here, we provide an overview of TRAIL-induced signaling, covering the core signal transduction during extrinsic apoptosis as well as its link to alternative outcomes, such as necroptosis or NF-κB activation. We discuss how environmental factors, transcriptional regulators, and genetic or epigenetic alterations regulate TRAIL receptors and thus alter cellular TRAIL susceptibility. Finally, we provide insight into the role of TRAIL receptors in signaling scenarios that engage the unfolded protein response and discuss how these findings might be translated into new combination therapies for cancer treatment.

Efficient apoptosis requires feedback amplification of upstream apoptotic signals by effector caspase-3 or -7

Apoptosis is a complex multi-step process driven by caspase-dependent proteolytic cleavage cascades. Dysregulation of apoptosis promotes tumorigenesis and limits the efficacy of chemotherapy. To assess the complex interactions among caspases during apoptosis, we disrupted caspase-8, -9, -3, -7, or -6 and combinations thereof, using CRISPR-based genome editing in living human leukemia cells. While loss of apical initiator caspase-8 or -9 partially blocked extrinsic or intrinsic apoptosis, respectively, only combined loss of caspase-3 and -7 fully inhibited both apoptotic pathways, with no discernible effect of caspase-6 deficiency alone or in combination. Caspase-3/7 double knockout cells exhibited almost complete inhibition of caspase-8 or -9 activation. Furthermore, deletion of caspase-3 and -7 decreased mitochondrial depolarization and cytochrome c release upon apoptosis activation. Thus, activation of effector caspase-3 or -7 sets off explosive feedback amplification of upstream apoptotic events, which is a key feature of apoptotic signaling essential for efficient apoptotic cell death.

Proteases: Pivot Points in Functional Proteomics

Proteases drive the life cycle of all proteins, ensuring the transportation and activation of newly minted, would-be proteins into their functional form while recycling spent or unneeded proteins. Far from their image as engines of protein digestion, proteases play fundamental roles in basic physiology and regulation at multiple levels of systems biology. Proteases are intimately associated with disease and modulation of proteolytic activity is the presumed target for successful therapeutics. "Proteases: Pivot Points in Functional Proteomics" examines the crucial roles of proteolysis across a wide range of physiological processes and diseases. The existing and potential impacts of proteolysis-related activity on drug and biomarker development are presented in detail. All told the decisive roles of proteases in four major categories comprising 23 separate subcategories are addressed. Within this construct, 15 sets of subject-specific, tabulated data are presented that include identification of proteases, protease inhibitors, substrates, and their actions. Said data are derived from and confirmed by over 300 references. Cross comparison of datasets indicates that proteases, their inhibitors/promoters and substrates intersect over a range of physiological processes and diseases, both chronic and pathogenic. Indeed, "Proteases: Pivot Points …" closes by dramatizing this very point through association of (pro)Thrombin and Fibrin(ogen) with: hemostasis, innate immunity, cardiovascular and metabolic disease, cancer, neurodegeneration, and bacterial self-defense.

Caspase selective reagents for diagnosing apoptotic mechanisms

Apical caspases initiate and effector caspases execute apoptosis. Reagents that can distinguish between caspases, particularly apical caspases-8, 9, and 10 are scarce and generally nonspecific. Based upon a previously described large-scale screen of peptide-based caspase substrates termed HyCoSuL, we sought to develop reagents to distinguish between apical caspases in order to reveal their function in apoptotic cell death paradigms. To this end, we selected tetrapeptide-based sequences that deliver optimal substrate selectivity and converted them to inhibitors equipped with a detectable tag (activity-based probes-ABPs). We demonstrate a strong relationship between substrate kinetics and ABP kinetics. To evaluate the utility of selective substrates and ABPs, we examined distinct apoptosis pathways in Jurkat T lymphocyte and MDA-MB-231 breast cancer lines triggered to undergo cell death via extrinsic or intrinsic apoptosis. We report the first highly selective substrate appropriate for quantitation of caspase-8 activity during apoptosis. Converting substrates to ABPs promoted loss-of-activity and selectivity, thus we could not define a single ABP capable of detecting individual apical caspases in complex mixtures. To overcome this, we developed a panel strategy utilizing several caspase-selective ABPs to interrogate apoptosis, revealing the first chemistry-based approach to uncover the participation of caspase-8, but not caspase-9 or -10 in TRAIL-induced extrinsic apoptosis. We propose that using select panels of ABPs can provide information regarding caspase-8 apoptotic signaling more faithfully than can single, generally nonspecific reagents.

LncRNA HULC mediates radioresistance via autophagy in prostate cancer cells

Prostate cancer (PCa) is the second leading cause of cancer death in men. Irradiation is one of the available options for treatment of PCa, however, approximately 10-45% of PCa are resistant to irradiation. We aimed to explore the role of long non-coding RNA highly upregulated in liver cancer (HULC) in the sensitivity of PCa cells to irradiation. Survival rate, cell apoptosis, cycle, expressions of related proteins, and caspase-3 activity were assessed to explore the effects of HULC on sensitivity of PCa cells to irradiation. Expression of HULC in DU-145, PC3, LNCaP, and RWPE-1 cells was determined and the influence of HULC on DU-145 cells was explored. Then, PC3 cells aberrantly expressing HULC were implanted into NOD-SCID mice for tumor xenograft study. Changes of autophagy after aberrant expression of HULC in vivo and in vitro were tested. Furthermore, the interacted protein of HULC and involved signaling pathway were investigated. In PC3 and LNCaP cells under irradiation, survival rate and cell cycle were decreased and apoptosis was increased by HULC knockdown. HULC knockdown arrested PC3 cells at G0/G1 phase. DU-145 was sensitive to irradiation, and resistance to irradiation of DU-145 cells was enhanced by HULC overexpression. Moreover, HULC knockdown enhanced the sensitivity of PC3 xenografts to irradiation. HULC knockdown promoted autophagy through interaction with Beclin-1 and inhibition of mTOR, resulting in increased apoptosis. HULC knockdown improved sensitivity of PCa cells to irradiation both in vivo and in vitro. HULC suppressed Beclin-1 phosphorylation, thereby reduced autophagy, involving the mTOR pathway.

Toxicity study of reclaimed water on human embryonic kidney cells

The importance of evaluating the toxic effects associated with the use of reclaimed water has been increasing. The purpose of this research was to investigate the cytotoxicity and molecular toxicity of reclaimed water on the human embryonic kidney 293 (HEK293) cells. The culture medium was synthesized using the reclaimed water samples. Wastewater treatment plant influent (WTI) and effluent (WTE), containing micropollutants at the nanogram per liter level, decreased cell proliferation (93.4-98.9% and 91.5-96.6% of the control, respectively) and increased cell damage (103.6-117.5% and 100.7-109% of the control, respectively) at all exposure times, except for a decrease in cell damage observed after an 8-h exposure to WTE. Membrane bioreactor permeate (MBRP) increased cell proliferation (102.1-106.7% of the control) and decreased cell damage at 8 and 12 h (92.4 and 98.4% of the control, respectively), but slightly increased cell damage at 24 h and later time points (101.1-104.9% of the control). All three water samples induced cell apoptosis (120.9-123.4% of the control). They also affected the expression of cell-cycle regulatory proteins (p16^INK4a, p27^Kip1, cyclin-dependent kinases 2 and 4, cyclin D1, and cyclin E) and apoptosis-related regulatory proteins (p-JNK, Bcl-2, caspase-9, and caspase-3). In conclusion, all three water samples showed cytotoxicity and molecular toxicity in the HEK293 cells, and the results of the cell-cycle and apoptosis regulatory protein expression after WTI and WTE treatments were consistent with the results of the cytotoxicity.

Blue light induced reactive oxygen species from flavin mononucleotide and flavin adenine dinucleotide on lethality of HeLa cells

Photodynamic therapy (PDT) is a safe and non-invasive treatment for cancers and microbial infections. Various photosensitizers and light sources have been developed for clinical cancer therapies. Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are the cofactor of enzymes and are used as photosensitizers in this study. Targeting hypoxia and light-triggering reactive oxygen species (ROS) are experimental strategies for poisoning tumor cells in vitro. HeLa cells are committed to apoptosis when treated with FMN or FAD and exposed to visible blue light (the maximum emitted wavelength of blue light is 462nm). Under blue light irradiation at 3.744J/cm² (=0.52mW/cm² irradiated for 2h), the minimal lethal dose is 3.125μM and the median lethal doses (LD₅₀) for FMN and FAD are 6.5μM and 7.2μM, respectively. Individual exposure to visible blue light irradiation or riboflavin photosensitizers does not produce cytotoxicity and no side effects are observed in this study. The western blotting results also show that an intrinsic apoptosis pathway is activated by the ROS during photolysis of riboflavin analogues. Blue light triggers the cytotoxicity of riboflavins on HeLa cells in vitro. Based on these results, this is a feasible and efficient of PDT with an intrinsic photosensitizer for cancer research.

Characterization of Potent SMAC Mimetics that Sensitize Cancer Cells to TNF Family-Induced Apoptosis

Members of the Inhibitor of APoptosis (IAP) protein family suppress apoptosis within tumor cells, particularly in the context of immune cell-mediated killing by the tumor necrosis factor (TNF) superfamily cytokines. Most IAPs are opposed endogenously by the second mitochondrial activator of caspases (SMAC), which binds to selected baculovirus IAP repeat (BIR) domains of IAPs to displace interacting proteins. The development of SMAC mimetics as novel anticancer drugs has gained impetus, with several agents now in human clinical trials. To further understand the cellular mechanisms of SMAC mimetics, we focused on IAP family members cIAP1 and cIAP2, which are recruited to TNF receptor complexes where they support cell survival through NF-κB activation while suppressing apoptosis by preventing caspase activation. We established fluorescence polarization (FP) assays for the BIR2 and BIR3 domains of human cIAP1 and cIAP2 using fluorochrome-conjugated SMAC peptides as ligands. A library of SMAC mimetics was profiled using the FP assays to provide a unique structure activity relationship (SAR) analysis compared to previous assessments of binding to XIAP. Potent compounds displayed mean inhibitory binding constants (K_i) of 9 to 27 nM against the BIR3 domains of cIAP1 and cIAP2, respectively. Selected compounds were then characterized using cytotoxicity assays in which a cytokine-resistant human tumor cell line was sensitized to either TNF or lymphotoxin-α (LT-α). Cytotoxicity correlated closely with cIAP1 and cIAP2 BIR3 binding activity with the most potent compounds able to reduce cell viability by 50%. Further testing demonstrated that active compounds also inhibit RIP1 binding to BIR3 of cIAP1 and cIAP2 in vitro and reduce steady-state cIAP1 protein levels in cells. Altogether, these data inform the SAR for our SMAC mimetics with respect to cIAP1 and cIAP2, suggesting that these IAP family members play an important role in tumor cell resistance to cytotoxicity mediated by TNF and LT-α.

Tideglusib induces apoptosis in human neuroblastoma IMR32 cells, provoking sub-G0/G1 accumulation and ROS generation

Neuroblastoma is the most common tumor amongst children amounting to nearly 15% of cancer deaths. This cancer is peculiar in its characteristics, exhibiting differentiation, maturation and metastatic transformation leading to poor prognosis and low survival rates among children. Chemotherapy, though toxic to normal cells, has shown to improve the survival of the patient with emphasis given more towards targeting angiogenesis. Recently, Tideglusib was designed as an 'Orphan Drug' to target the neurodegenerative Alzheimer's disease and gained significant momentum in its function during clinical trials. Duffy et al. recently reported a reduction in cell viability of human IMR32 neuroblastoma cells when treated with Tideglusib at varying concentrations. We investigated the effects of Tideglusib, at various concentrations, compared to Lithium chloride at various concentrations, on IMR32 cells. Lithium, a known GSK-3 inhibitor, was used as a standard to compare the efficiency of Tideglusib in a dose-dependent manner. Cell viability was assessed by MTT assay. The stages of apoptosis were evaluated by AO/EB staining and nuclear damage was determined by Hoechst 33258 staining. Reactive oxygen species (ROS) and mitochondrial membrane potential (ΔΨm) were assessed by DCFDA dye and Rhodamine-123 dye, respectively. Tideglusib reported a significant dose-dependent increase in pro-apoptotic proteins (PARP, Caspase-9, Caspase-7, Caspase-3) and tumor-related genes (FasL, TNF-α, Cox-2, IL-8, Caspase-3). Anti-GSK3 β, pGSK3 β, Bcl-2, Akt-1, p-Akt1 protein levels were observed with cells exposed to Tideglusib and Lithium chloride. No significant dose-dependent changes were observed for the mRNA expression of collagenase MMP-2, the tumor suppressor p53, or the cell cycle protein p21. Our study also reports Tideglusib reducing colony formation and increasing the level of sub-G0/G1 population in IMR32 cells. Our investigations report the significance of Tideglusib as a promising apoptotic inducer in human neuroblastoma IMR32 cells. Our study also reports that LiCl reduced cell viability in IMR32 cells inducing apoptosis mediated by ROS generation.

Counter Selection Substrate Library Strategy for Developing Specific Protease Substrates and Probes

Legumain (AEP) is a lysosomal cysteine protease that was first characterized in leguminous seeds and later discovered in higher eukaryotes. AEP upregulation is linked to a number of diseases including inflammation, arteriosclerosis, and tumorigenesis. Thus this protease is an excellent molecular target for the development of new chemical markers. We deployed a hybrid combinatorial substrate library (HyCoSuL) approach to obtain P1-Asp fluorogenic substrates and biotin-labeled inhibitors that targeted legumain. Since this approach led to probes that were also recognized by caspases, we introduced a Counter Selection Substrate Library (CoSeSuL) approach that biases the peptidic scaffold against caspases, thus delivering highly selective legumain probes. The selectivity of these tools was validated using M38L and HEK293 cells. We also propose that the CoSeSuL methodology can be considered as a general principle in the design of selective probes for other protease families where selectivity is difficult to achieve by conventional sequence-based profiling.

Calycosin‑7‑O‑β‑D‑glucoside attenuates ischemia‑reperfusion injury in vivo via activation of the PI3K/Akt pathway

The aim of the present study was to investigate the effects and mechanisms of calycosin‑7‑O‑β‑D‑glucoside (CG) on ischemia‑reperfusion (I/R) injury in vivo. Hemodynamic parameters, including ejection fraction (EF), fractional shortening (FS), left ventricular end‑systolic pressure (LVESP) and left ventricular end‑diastolic pressure (LVEDP) were monitored using an ultrasound system, and infarct size was measured using Evans blue/tetrazolium chloride double staining. The activities of serum creatine kinase (CK), lactate dehydrogenase (LDH) and superoxide dismutase (SOD), and the levels of malondialdehyde (MDA) were determined to assess the degree of myocardial injury and oxidative stress‑induced damage. The protein expression levels of cleaved‑caspase‑3, cleaved‑caspase‑9, phosphorylated (p)‑phosphatidylinositol 3‑kinase (PI3K) p85, PI3K p85, p‑Akt and Akt were determined using western blotting. The results demonstrated that pretreatment with high dose (H)‑CG markedly improved cardiac function, as evidenced by upregulated EF, FS and LVESP, and downregulated LVEDP. In addition, administration of CG resulted in significant decreases in infarct size in the I/R+low dose‑CG and I/R+H‑CG groups, compared with the I/R group. The activities of CK and LDH, and the levels of MDA in the I/R+H‑CG group were reduced, compared with those in the I/R group, whereas SOD activity was elevated. Treatment with CG inhibited the cleavage and activity of caspase‑3 and caspase‑9, and enhanced the phosphorylation of PI3K p85 and Akt. Notably, administration of the PI3K inhibitor, LY294002, markedly lowered the levels of p‑PI3K p85/p‑Akt, and eradicated the inhibitory effects of H‑CG on infarct size, myocardial injury and oxidative stress‑induced damage. Taken together, the results suggested that CG may alleviate I/R injury by activating the PI3K/Akt signaling pathway.

Small Molecule Active Site Directed Tools for Studying Human Caspases

Caspases are proteases of clan CD and were described for the first time more than two decades ago. They play critical roles in the control of regulated cell death pathways including apoptosis and inflammation. Due to their involvement in the development of various diseases like cancer, neurodegenerative diseases, or autoimmune disorders, caspases have been intensively investigated as potential drug targets, both in academic and industrial laboratories. This review presents a thorough, deep, and systematic assessment of all technologies developed over the years for the investigation of caspase activity and specificity using substrates and inhibitors, as well as activity based probes, which in recent years have attracted considerable interest due to their usefulness in the investigation of biological functions of this family of enzymes.

let-7b suppresses apoptosis and autophagy of human mesenchymal stem cells transplanted into ischemia/reperfusion injured heart 7by targeting caspase-3

Introduction

Mesenchymal stem cells (MSCs) have therapeutic potential for the repair of myocardial injury. The efficacy of MSC therapy for myocardial regeneration mainly depends on the survival of cells after transplantation into the infarcted heart. In the transplanted regions, reactive oxygen species (ROS) can cause cell death, and this process depends on caspase activation and autophagosome formation.

Methods

A Software TargetScan was utilized to search for microRNAs (miRNAs) that target caspase-3 mRNA. Six candidate miRNAs including let-7b were selected and transfected into human MSCs in vitro. Expression of MEK-EKR signal pathways and autophagy-related genes were detected. Using ischemia/reperfusion model (I/R), the effect of MSCs enriched with let-7b was determined after transplantation into infarcted heart area. Miller catheter was used to evaluate cardiac function.

Results

Here, we report that let-7b targets caspase-3 to regulate apoptosis and autophagy in MSCs exposed to ROS. Let-7b-transfected MSCs (let-7b-MSCs) showed high expression of survival-related proteins, including p-MEK, p-ERK and Bcl-2, leading to a decrease in Annexin V/PI- and TUNEL-positive cells under ROS-rich conditions. Moreover, autophagy-related genes, including Atg5, Atg7, Atg12 and beclin-1, were significantly downregulated in let-7b-MSCs. Using a rat model of acute myocardial infarction, we found that intramyocardial injection of let-7b-MSCs markedly enhanced left ventricular (LV) function and microvessel density, in accordance with a reduced infarct size and the expression of caspase-3.

Conclusions

Taken together, these data indicate that let-7b may protect MSCs implanted into infarcted myocardium from apoptosis and autophagy by directly targeting caspase-3 signaling.

The role of death-associated protein 3 in apoptosis, anoikis and human cancer

Death-associated protein 3 (DAP3) is a molecule with a significant role in the control of both apoptosis and anoikis. Apoptosis is the predominant type of programmed cell death (PCD) which may occur in response to irreparable damage to DNA, or in response to induction by inflammatory cells. Anoikis is subset of apoptosis which occurs in epithelial cells in response to detachment from the surrounding matrix. Both apoptosis and anoikis are of interest in the context of carcinogenesis. In this review, we shall discuss apoptosis and anoikis, and the recent literature regarding the role of DAP3 in both these pathways.

Caspases: therapeutic targets in neurologic disease

Specific therapies for neurologic diseases such as Alzheimer's disease provide the potential for better clinical outcomes. Expression of caspases in the brain is developmentally regulated, and dysregulated in neurologic disease, supporting that caspases may be therapeutic targets. The activity of caspases is carefully regulated via binding partners, cleavage, or endogenous inhibitors to prevent spontaneous activation, which could lead to aberrant cell death. This review serves as a brief examination of the current understanding of the regulation and function of caspases, and approaches to specifically target aberrant caspase activity. The use of proper tools to investigate individual caspases is addressed. Moreover, it summarizes the reports of various caspases in Alzheimer's disease studies. A better understanding of specific caspase pathways in heath and neurodegenerative disease is crucial for identifying specific targets for the development of therapeutic interventions.

Glutathione depletion regulates both extrinsic and intrinsic apoptotic signaling cascades independent from multidrug resistance protein 1

Glutathione (GSH) depletion is an important hallmark of apoptosis. We previously demonstrated that GSH depletion, by its efflux, regulates apoptosis by modulation of executioner caspase activity. However, both the molecular identity of the GSH transporter(s) involved and the signaling cascades regulating GSH loss remain obscure. We sought to determine the role of multidrug resistance protein 1 (MRP1) in GSH depletion and its regulatory role on extrinsic and intrinsic pathways of apoptosis. In human lymphoma cells, GSH depletion was stimulated rather than inhibited by pharmacological blockage of MRP1 with MK571. GSH loss was dependent on initiator caspases 8 and 9 activity. Genetic knock-down (>60 %) of MRP1 by stable transfection with short hairpin small interfering RNA significantly reduced MRP1 protein levels, which correlated directly with the loss of MRP1-mediated anion transport. However, GSH depletion and apoptosis induced by both extrinsic and intrinsic pathways were not affected by MRP1 knock-down. Interestingly, stimulation of GSH loss by MK571 also enhanced the initiator phase of apoptosis by stimulating initiator caspase 8 and 9 activity and pro-apoptotic BCL-2 interacting domain cleavage. Our results clearly show that caspase-dependent GSH loss and apoptosis are not mediated by MRP1 proteins and that GSH depletion stimulates the initiation phase of apoptosis in lymphoid cells.

The caspase-8 homolog Dredd cleaves Imd and Relish but is not inhibited by p35

In Drosophila, the Imd pathway is activated by diaminopimelic acid-type peptidoglycan and triggers the humoral innate immune response, including the robust induction of antimicrobial peptide gene expression. Imd and Relish, two essential components of this pathway, are both endoproteolytically cleaved upon immune stimulation. Genetic analyses have shown that these cleavage events are dependent on the caspase-8 like Dredd, suggesting that Imd and Relish are direct substrates of Dredd. Among the seven Drosophila caspases, we find that Dredd uniquely promotes Imd and Relish processing, and purified recombinant Dredd cleaves Imd and Relish in vitro. In addition, interdomain cleavage of Dredd is not required for Imd or Relish processing and is not observed during immune stimulation. Baculovirus p35, a suicide substrate of executioner caspases, is not cleaved by purified Dredd in vitro. Consistent with this biochemistry but contrary to earlier reports, p35 does not interfere with Imd signaling in S2* cells or in vivo.

Zinc chloride inhibits human lens epithelial cell migration and proliferation involved in TGF-β1 and TNF-α signaling pathways in HLE B-3 cells

Zinc is one of the most abundant essential elements in the human body, which is an essential, coenzyme-like component of many enzymes, and is indispensable to their functions. However, high levels of zinc ions can lead to cell damage. In the present study, we explored the effects of high concentrations of zinc chloride (ZnCl2) on lens epithelial cell proliferation and migration and further investigated the effects of different concentrations of ZnCl2 on caspase-9 and caspase-12, transforming growth factor-beta 1 (TGF-β1), and tumor necrosis factor-alpha (TNF-α). We found that ZnCl2 could inhibit human lens epithelial (HLE) B-3 cell migration and induce apoptosis/necrosis. In addition, ZnCl2 can efficiently decrease the expressions of caspase-9 and caspase-12, increase the expression of TNF-α at both gene and protein levels, and thus induces cell death. Taken together, our results indicate that ZnCl2 can inhibit HLE B-3 cell migration and proliferation by decreasing the expression of TGF-β1 and increasing the expression of TNF-α and finally lead to HLE B-3 cell death.

Unnatural amino acids increase sensitivity and provide for the design of highly selective caspase substrates

Traditional combinatorial peptidyl substrate library approaches generally utilize natural amino acids, limiting the usefulness of this tool in generating selective substrates for proteases that share similar substrate specificity profiles. To address this limitation, we synthesized a Hybrid Combinatorial Substrate Library (HyCoSuL) with the general formula of Ac-P4-P3-P2-Asp-ACC, testing the approach on a family of closely related proteases - the human caspases. The power of this library for caspase discrimination extends far beyond traditional PS-SCL approach, as in addition to 19 natural amino acids we also used 110 diverse unnatural amino acids that can more extensively explore the chemical space represented by caspase-active sites. Using this approach we identified and employed peptide-based substrates that provided excellent discrimination between individual caspases, allowing us to simultaneously resolve the individual contribution of the apical caspase-9 and the executioner caspase-3 and caspase-7 in the development of cytochrome-c-dependent apoptosis for the first time.

Differential sensitivity to apoptosome apparatus activation in non-small cell lung carcinoma and the lung

The intrinsic apoptosis pathway represents an important mechanism of stress-induced death of cancer cells. To gain insight into the functional status of the apoptosome apparatus in non-small cell lung carcinoma (NSCLC), we studied its sensitivity to activation, the assembly of apoptosome complexes and stability of their precursors, and the importance of X-linked inhibitor of apoptosis (XIAP) in the regulation of apoptosome activity, using cell-free cytosols from NSCLC cell lines and NSCLC tumours and lungs from 62 surgically treated patients. Treatment of cytosol samples with cytochrome c (cyt-c) and dATP induced proteolytic processing of procaspase-9 to caspase-9, which was followed by procaspase-3 processing to caspase-3, and by generation of caspase-3-like activity in 5 of 7 studied NSCLC cell lines. Further analysis demonstrated formation of high-M_r Apaf-1 complexes associated with cleaved caspase-9 in the (cyt-c + dATP)-responsive COLO-699 and CALU-1 cells. By contrast, in A549 cells, Apaf-1 and procaspase-9 co-eluted in the high-M_r fractions, indicating formation of an apoptosome complex unable of procaspase-9 processing. Thermal pre-treatment of cell-free cytosols in the absence of exogenous cyt-c and dATP lead to formation of Apaf-1 aggregates, unable to recruit and activate procaspase-9 in the presence of cyt-c and dATP, and to generate caspase-3-like activity. Further studies showed that the treatment with cyt-c and dATP induced a substantially higher increase of caspase-3-like activity in cytosol samples from NSCLC tumours compared to matched lungs. Tumour histology, grade and stage had no significant impact on the endogenous and the (cyt-c + dATP)-induced caspase-3-like activity. Upon addition into the cytosol, the XIAP-neutralizing peptides AVPIAQK and ATPFQEG only moderately heightened the (cyt-c + dATP)-induced caspase-3-like activity in some NSCLC tumours. Taken together, the present study provides evidence that the apoptosome apparatus is functional in the majority of NSCLCs and that its sensitivity to the (cyt-c + dATP)-mediated activation is often enhanced in NSCLCs compared to lungs. They also indicate that XIAP does not frequently and effectively suppress the activity of apoptosome apparatus in NSCLCs.

A multipronged approach for compiling a global map of allosteric regulation in the apoptotic caspases

One of the most promising and as yet underutilized means of regulating protein function is exploitation of allosteric sites. All caspases catalyze the same overall reaction, but they perform different biological roles and are differentially regulated. It is our hypothesis that many allosteric sites exist on various caspases and that understanding both the distinct and overlapping mechanisms by which each caspase can be allosterically controlled should ultimately enable caspase-specific inhibition. Here we describe the ongoing work and methods for compiling a comprehensive map of apoptotic caspase allostery. Central to this approach are the use of (i) the embedded record of naturally evolved allosteric sites that are sensitive to zinc-mediated inhibition, phosphorylation, and other posttranslational modifications, (ii) structural and mutagenic approaches, and (iii) novel binding sites identified by both rationally-designed and screening-derived small-molecule inhibitors.

General in vitro caspase assay procedures

One of the most valuable tools that have been developed for the study of apoptosis is the availability of recombinant active caspases. The determination of caspase substrate preference, the design of sensitive substrates and potent inhibitors, the resolution of caspase structures, the elucidation of their activation mechanisms, and the identification of their substrates were made possible by the availability of sufficient amounts of enzymatically pure caspases. The current chapter describes at length the expression, purification, and basic enzymatic characterization of apoptotic caspases.

Tannic Acid preferentially targets estrogen receptor-positive breast cancer

Research efforts investigating the potential of natural compounds in the fight against cancer are growing. Tannic acid (TA) belongs to the class of hydrolysable tannins and is found in numerous plants and foods. TA is a potent collagen cross-linking agent; the purpose of this study was to generate TA-cross-linked beads and assess the effects on breast cancer cell growth. Collagen beads were stable at body temperature following crosslinking. Exposure to collagen beads with higher levels of TA inhibited proliferation and induced apoptosis in normal and cancer cells. TA-induced apoptosis involved activation of caspase 3/7 and caspase 9 but not caspase 8. Breast cancer cells expressing the estrogen receptor were more susceptible to the effects of TA. Taken together the results suggest that TA has the potential to become an anti-ER⁺ breast cancer treatment or preventative agent.

Overexpression of 4EBP1, p70S6K, Akt1 or Akt2 differentially promotes Coxsackievirus B3-induced apoptosis in HeLa cells

Our previous studies have shown that the inhibition of phosphatidylinositol 3-kinase (PI3K) or mTOR complex 1 can obviously promote the Coxsackievirus B3 (CVB3)-induced apoptosis of HeLa cells by regulating the expression of proapoptotic factors. To further illustrate it, Homo sapiens eIF4E-binding protein 1 (4EBP1), p70S6 kinase (p70S6K), Akt1 and Akt2 were transfected to HeLa cells, respectively. And then, we established the stable transfected cell lines. Next, after CVB3 infection, apoptosis in different groups was determined by flow cytometry; the expressions of Bim, Bax, caspase-9 and caspase-3 were examined by real-time fluorescence quantitative PCR and western blot analysis; the expression of CVB3 mRNA and viral capsid protein VP1 were also analyzed by real-time fluorescence quantitative PCR, western blot analysis and immunofluorescence, respectively. At the meantime, CVB3 replication was observed by transmission electron microscope. We found that CVB3-induced cytopathic effect and apoptosis in transfected groups were more obvious than that in controls. Unexpectedly, apoptosis rate in Akt1 group was higher than others at the early stage after viral infection and decreased with the viral-infected time increasing, which was opposite to other groups. Compared with controls, the expression of CVB3 mRNA was increased at 3, 6, 12 and 24 h postinfection (p. i.) in all groups. At the meantime, VP1 expression in 4EBP1 group was higher than control during the process of infection, while the expressions in the other groups were change dynamically. Moreover, overexpression of 4EBP1 did not affect the mRNA expressions of Bim, Bax, caspase-9 and caspase-3; while protein expressions of Bim and Bax were decreased, the self-cleavages of caspase-9 and caspase-3 were stimulated. Meanwhile, overexpression of p70S6K blocked the CVB3-induced Bim, Bax and caspase-9 expressions but promoted the self-cleavage of caspase-9. In the Akt1 group, it is noteworthy that the expressions of Bim protein were higher than controls at 3 and 6 h p. i. but lower at 24 h p. i., and the expression of Bax protein were higher at 6 and 24 h p. i., while their mRNA expressions were all decreased. Furthermore, overexpression of Akt1 stimulated the procaspase-9 and procaspase-3 expression but blocked their self-cleavages. Overexpression of Akt2, however, had little effect on Bim, Bax and caspase-3, while prevented caspase-9 from self-cleavage at the late stage of CVB3 infection. As stated above, our results demonstrated that overexpression of 4EBP1, p70S6K, Akt1 or Akt2 could promote the CVB3-induced apoptosis in diverse degree via different mediating ways in viral replication and proapoptotic factors in BcL-2 and caspase families. As 4EBP1, p70S6K and Akt are the important substrates of PI3K and mammalian target of rapamycin (mTOR), we further illustrated the role of PI3K/Akt/mTOR signaling pathway in the process of CVB3-induced apoptosis.

Inhibitor of apoptosis proteins (IAPs) as regulatory factors of hepatic apoptosis

IAPs are a group of regulatory proteins that are structurally related. Their conserved homologues have been identified in various organisms. In human, eight IAP members have been recognized based on baculoviral IAP repeat (BIR) domains. IAPs are key regulators of apoptosis, cytokinesis and signal transduction. The antiapoptotic property of IAPs depends on their professional role for caspases. IAPs are functionally non-equivalent and regulate effector caspases through distinct mechanisms. IAPs impede apoptotic process via membrane receptor-dependent (extrinsic) cascade and mitochondrial dependent (intrinsic) pathway. IAP-mediated apoptosis affects the progression of liver diseases. Therapeutic options of liver diseases may depend on the understanding toward mechanisms of the IAP-mediated apoptosis.

Both PI3K- and mTOR-signaling pathways take part in CVB3-induced apoptosis of Hela cells

This work illustrated the mechanism contributing to the process of Phosphatidylinostiol 3-kinase (PI3K)/protein kinase B (PKB)/mammalian target of rapamycin (mTOR) signaling pathway, which has been demonstrated to play an important role in virus-induced apoptosis, which contributes to the Viral Myocarditis (VMC) pathogeneses. We examined the expression of Bax, Bim, caspase-3, caspase-9, and viral replication after Coxsackievirus B3 (CVB3) infection using the mTOR inhibitor and PI3K inhibitor pretreated HeLa cells, respectively. Apoptosis in different groups was determined by flow cytometry. Bax, Bim, caspase-9, and caspase-3 were examined by semiquantitative polymerase chain reaction (PCR) and Western blot analysis. The expression of CVB3 mRNA and viral capsid protein VP1 were analyzed by semiquantitative PCR and Western blot analysis distinctively. We found that rapamycin and LY294002 promote CVB3-induced cytopathic effect (CPE) and apoptosis. CVB3 replication in host cells is mediated in mRNA and protein expression by rapamycin and LY294002. Moreover, comparing with controls, at 12 and 24 h of postinfection (p.i.), Bim and Bax expression increased in cells after treated with rapamycin or LY294002, which also stimulates the activation of procaspase-9, and the CVB3-induced caspase-3 self-cleavage. However, in the meantime, the mRNA expression of caspase-9 and caspase-3 did not have an obvious change. In summary, our results demonstrated that the mTOR-signaling pathway plays an important role in CVB3-induced CPE and apoptosis, which is indispensable in VMC, via regulating Bim, Bax, caspase-9, caspase-3, and viral replication. Our findings may provide a new perspective and a deeper understanding of the mechanism of CVB3-induced apoptosis which, in turn, may help with the development of new therapy for the CVB3 infection.

AIM2 and NLRP3 inflammasomes activate both apoptotic and pyroptotic death pathways via ASC

Inflammasomes are protein complexes assembled upon recognition of infection or cell damage signals, and serve as platforms for clustering and activation of procaspase-1. Oligomerisation of initiating proteins such as AIM2 (absent in melanoma-2) and NLRP3 (NOD-like receptor family, pyrin domain-containing-3) recruits procaspase-1 via the inflammasome adapter molecule ASC (apoptosis-associated speck-like protein containing a CARD). Active caspase-1 is responsible for rapid lytic cell death termed pyroptosis. Here we show that AIM2 and NLRP3 inflammasomes activate caspase-8 and -1, leading to both apoptotic and pyroptotic cell death. The AIM2 inflammasome is activated by cytosolic DNA. The balance between pyroptosis and apoptosis depended upon the amount of DNA, with apoptosis seen at lower transfected DNA concentrations. Pyroptosis had a higher threshold for activation, and dominated at high DNA concentrations because it happens more rapidly. Gene knockdown showed caspase-8 to be the apical caspase in the AIM2- and NLRP3-dependent apoptotic pathways, with little or no requirement for caspase-9. Procaspase-8 localised to ASC inflammasome 'specks' in cells, and bound directly to the pyrin domain of ASC. Thus caspase-8 is an integral part of the inflammasome, and this extends the relevance of the inflammasome to cell types that do not express caspase-1.

A novel thiazolidine compound induces caspase-9 dependent apoptosis in cancer cells

The forward chemogenomics strategy allowed us to identify a potent cytotoxic thiazolidine compound as an apoptosis-inducing agent. Chemical structures were designed around a thiazolidine ring, a structure already noted for its anticancer properties. Initially, we evaluated these novel compounds on liver, breast, colon and endometrial cancer cell lines. The compound 3 (ALC67) showed the strongest cytotoxic activity (IC(50) ∼5 μM). Cell cycle analysis with ALC67 on liver cells revealed SubG1/G1 arrest bearing apoptosis. Furthermore we demonstrated that cytotoxicity of this compound was due to the activation of caspase-9 involved apoptotic pathway, which is death receptor independent.

Caspase-7 uses an exosite to promote poly(ADP ribose) polymerase 1 proteolysis

During apoptosis, hundreds of proteins are cleaved by caspases, most of them by the executioner caspase-3. However, caspase-7, which shares the same substrate primary sequence preference as caspase-3, is better at cleaving poly(ADP ribose) polymerase 1 (PARP) and Hsp90 cochaperone p23, despite a lower intrinsic activity. Here, we identified key lysine residues (K(38)KKK) within the N-terminal domain of caspase-7 as critical elements for the efficient proteolysis of these two substrates. Caspase-7's N-terminal domain binds PARP and improves its cleavage by a chimeric caspase-3 by ∼30-fold. Cellular expression of caspase-7 lacking the critical lysine residues resulted in less-efficient PARP and p23 cleavage compared with cells expressing the wild-type peptidase. We further showed, using a series of caspase chimeras, the positioning of p23 on the enzyme providing us with a mechanistic insight into the binding of the exosite. In summary, we have uncovered a role for the N-terminal domain (NTD) and the N-terminal peptide of caspase-7 in promoting key substrate proteolysis.

The central role of initiator caspase-9 in apoptosis signal transduction and the regulation of its activation and activity on the apoptosome

Key structural and catalytic features are conserved across the entire family of cysteine-dependent aspartate-specific proteases (caspases). Of the caspases involved in apoptosis signal transduction, the initiator caspases-2, -8 and -9 are activated at multi-protein activation platforms, and activation is thought to involve homo-dimerisation of the monomeric zymogens. Caspase-9, the essential initiator caspase required for apoptosis signalling through the mitochondrial pathway, is activated on the apoptosome complex, and failure to activate caspase-9 has profound pathophysiological consequences. Here, we review the pertinent literature on which the currently prevalent understanding of caspase-9 activation is based, extend this view by insight obtained from recent structural and kinetic studies on caspase-9 signalling, and describe an emerging model for the regulation of caspase-9 activation and activity that arise from the complexity of multi-protein interactions at the apoptosome. This integrated view allows us to postulate and to discuss functional consequences for caspase-9 activation and apoptosis execution that may take centre stage in future experimental cell research on apoptosis signalling.

Molecular determinants involved in activation of caspase 7

During apoptosis, initiator caspases (8, 9 and 10) activate downstream executioner caspases (3, 6 and 7) by cleaving the IDC (interdomain connector) at two sites. Here, we demonstrate that both activation sites, site 1 and site 2, of caspase 7 are suboptimal for activation by initiator caspases 8 and 9 in cellulo, and in vitro using recombinant proteins and activation kinetics. Indeed, when both sites are replaced with the preferred motifs recognized by either caspase 8 or 9, we found an up to 36-fold improvement in activation. Moreover, cleavage at site 1 is preferred to site 2 because of its location within the IDC, since swapping sites does not lead to a more efficient activation. We also demonstrate the important role of Ile195 of site 1 involved in maintaining a network of contacts that preserves the proper conformation of the active enzyme. Finally, we show that the length of the IDC plays a crucial role in maintaining the necessity of proteolysis for activation. In fact, although we were unable to generate a caspase 7 that does not require proteolysis for activity, shortening the IDC of the initiator caspase 8 by four residues was sufficient to confer a requirement for proteolysis, a key feature of executioner caspases. Altogether, the results demonstrate the critical role of the primary structure of caspase 7's IDC for its activation and proteolytic activity.