Dissertations / Theses on the topic 'Apoptosomes'

The apoptosome is a caspase-activating complex consisting of Apaf-1, caspase-9 and cytochrome c, which is essential for the induction of stress-mediated apoptosis. This complex ranges in size from ~ 700 kDa to ~ 1.4 MDa, possibly due to the stable association of modulatory proteins. In the current study I employed two strategies to isolate and characterise the active ~ 700 kDa apoptosome in vitro. Firstly, I used GST-Casp91-130, which binds to the CARD domain of Apaf-1 in a dATP and cytochrome c-dependent manner, to affinity-purify an apoptosome containing only Apaf-1XL and cytochrome c. This result was confirmed by the second approach, which used an antibody to the caspase-9 to immunoprecipitate of the native apoptosome, which contained Apaf-1 and caspase-9 (p34/p35). However, in the absence of SMAC and Omi, the native apoptosome also contained caspase-3 and XIAP, both of which associated via the catalytic domains of caspase-9. When isolating the apoptosome from apoptotic cells using TAP-tagged caspase-9 variants, I discovered that the location of the TAP-tag can affect the ability of caspase-9 to interact with known binding partners and consequently can influence the induction of cell death. I have also studied the role of the apoptosome in the caspase-3 null cell line, MCF-7, and have demonstrated that an active ~ 700 kDa apoptosome complex is formed in both dATP-activated cell lysates and an apoptotic MCF-7 cells. Furthermore, the active apoptosome can directly process and activate caspase-7.

Apoptosis or programmed cell death is a process involving the regulated self-destruction of a cell. Execution of the apoptotic programme requires the hierarchical activation of a family of proteases, the caspases. Here, I describe the characterisation and purification of the apoptosome, a ~700kDa caspase activating complex. Gel filtration of dATP-activated THP.1 cell lysates showed that effector caspase activity is assessed with a large complex, the apoptosome, which contains Apaf-1 and active caspases-9, -3 and -7. Further analysis revealed two large complexes, a biologically inactive ~1.4MDa and a biologically active ~700kDa complex to be formed very rapidly. Experiments involving the use of the caspase inhibitor z-VAD.FMK demonstrated that complex formation is caspase-independent, however, release of processed caspase-9 from the complex and the recruitment of other procaspases to the complex, is caspase activity-dependent. In apoptotic cells, the ~700kDa apoptosome complex predominates and is therefore suggested to be the biologically relevant complex. Normal intracellular K+ concentrations inhibited formation of the ~700kDa apoptosome complex in lysates and in a reconstituted system. Increasing concentrations of cytochrome c partially reversed inhibition. A mechanism is proposed for the inhibitory effect of K+ during dATP-dependent caspase activation in cellular lysates. The ~700kDa apoptosome complex was purified using multi-step purification procedures. Several proteins were identified in the purified apoptosome fraction and these included Apaf-1 and capase-9 as well as XIAP, b-actin, CAP-1, CAZ1/CAZ2 and possibly Hsc70 are all involved with the cytoskeleton and are capable of binding actin, whereas rabaptin-5 and IKKg (NEMO) are known to be involved in endosomal transport and the NFkB pathway, respectively. The possible involvement of these proteins in apoptosome assembly and function is discussed.

La apoptosis es un proceso importante en una amplia variedad sistemas biológicos, incluyendo el recambio celular normal, el sistema inmunológico y el desarrollo embrionario. La apoptosis inadecuada está implicada en muchas enfermedades incluyendo enfermedades neurodegenerativas tales como las enfermedades de Alzheimer y Huntington, isquemia, desórdenes autoinmunes y varias formas de cáncer. La familia de proteínas Bcl-2 abarca una clase de estructuras homólogas que sirven para inhibir o para activar la apoptosis en un proceso intrincado y a su vez, bien orquestado. Los estímulos apoptósicos inducen la translocación de miembros pro-apoptósicos de la familia de Bcl-2 a la membrana mitochondrial externa donde forman canales iónicos que pueden contribuir a disipar el potencial transmembrana de la mitocondria y favorecer la liberación de citocromo c. En el citosol, esta proteína se une al factor activador de apoptosis (Apaf-1) para formar el complejo denominado apoptosoma que activa una familia de proteasas denominada caspasas. Las caspasas hidrolizan una serie de proteínas clave para la supervivencia celular y la célula muere de forma no necrótica. Estas moléculas moduladoras podrían ser consideradas como agentes "cabeza de serie" para el desarrollo de compuestos que puedan inhibir el crecimiento de células tumorales y/o paliar o aminorar los daños celulares asociados a las enfermedades neurodegenerativas En la tesis doctoral se utiliza la Química Combinatoria para la identificación de moléculas de interés biomédico moduladoras del apoptosoma. Para ello, en primer lugar es necesario poner a punto un ensayo de alto rendimiento que permita utilizar la diana terapéutica, el apoptosoma, y que permita el cribado funcional de un elevado número de moléculas con el objetivo de identificar efectores artificiales del sistema Apaf-1-caspasas. El trabajo presentado en esta tesis incorpora como novedad la utilización de los componentes básicos recombinantes purificados del apoptosoma y la reconstitución de su actividad in vitro para el cribado. Con este ensayo y formato de cribado, lo que se consigue una mejor definición de la diana molecular de búsqueda de moléculas moduladoras. Por otro lado, la posibilidad de manipulación de los distintos componentes del apoptosoma en las reacciones de reconstitución permite llevar a cabo la caracterización del mecanismo de acción de los compuestos actuvos identificados y su sitio de unión. Finalmente se llevo a cabo unestudio in vivo de los compuestos identificados como moduladores.
Protein-protein interactions represent points of chemical intervention for therapeutic gain in the biological processes associated with disease. Apoptosis is an interesting biological process because its importance in a wide variety of biological systems. Inappropriate apoptosis is involved in many human pathologies, including neurodegenerative diseases such as Alzheimer's and Huntington's, ischaemia, autoimmune disorders and several forms of cancer. Diverse apoptotic stimuli, including activation of cell surface death receptors, anticancer agents, irradiation, lack of survival factors, and ischemia induce signaling cascades that all activate a family of cysteine aspartyl proteases called caspases. It is these proteases that execute the apoptotic process. Effector caspases are responsible for the disassembly of cellular components while initiator caspases are responsible for activation of the effector caspases. Because of the critical consequences of apoptosis malfunctioning, the activation of caspases is scrupulously controlled. Some apoptotic signals activate the mitochondria-mediated or intrinsic pathway that utilizes caspase-9 as its initiator. Caspase-9 activation is triggered by the release to the cytosol of proapoptotic proteins from the mitochondrial inter-membrane space, in particular cytochrome c. The formation of the macromolecular complex named apoptosome is a key event in this pathway. The apoptosome is a holoenzyme multiprotein complex formed by cytochrome c-activated Apaf-1 (apoptotic protease-activating factor), dATP and procaspase-9. In this macromolecular complex apoptosome-associated caspase-9 is activated and then, in turn, activate effector caspases. To identify molecules that could ameliorate disease-associated apoptosis, drug discovery efforts have initially targeted caspase activity rather than activation. Nevertheless, protein-protein interactions upstream of caspase activation can be also relevant points of intervention for the development of modulators of apoptosis pathways. In particular, recent data propose the formation of the apoptosome as an interesting target for the development of apoptotic modulators. In the absence of detailed structural information, the conventional methods used for the identification of modulators of the apoptosome have been based in indirect measurements of the cytochrome c- and dATP-induced activation of caspase-3-like activity on defined cytosolic extracts. Using this methodology Lademann et al. have identified inhibitors of the apoptosome through the screening of small molecules using cytosolic extracts of selected cells.We have carried out a discovery program employing an in vitro reconstituted active apoptosome assembled from its recombinant constituent proteins. Here we describe the identification of compounds that inhibit the apoptosome-mediated activation of procaspase-9 from the screening of a diversity-oriented chemical library of N-alkylglycines. The active compounds rescued from the library were chemically optimized to obtain molecules that bind to both recombinant and human endogenous Apaf-1 and decrease the apoptotic phenotype in mitochondrial-mediated models of cellular apoptosis.

The mitochondrial pathway of apoptosis is an important mediator of cell death in many human diseases. A key event in this pathway is permeabilisation of the outer mitochondrial membrane and subsequent release of the heme-protein cytochrome c from mitochondria into the cytosol. Thereafter, cytosolic cytochrome c mediates the formation of the apoptosome complex, which cleaves the initiator caspase 9, thereby activating the caspase signalling cascade. Apoptosome formation is thought to require cytochrome c which has its heme iron in the ferric form. Neuroglobin, a recently discovered globin, protects various cell lines and tissues from apoptosis. Previously, neuroglobin has been shown to efficiently reduce the heme iron of cytochrome c in vitro. We hypothesise that neuroglobin prevents apoptosome assembly by reducing cytochrome c. The work carried out in this thesis aimed to investigate the properties of the neuroglobin-cytochrome c complex and identify its role in the regulation of apoptosome formation. Surface Plasmon Resonance experiments have been used to explore the nature of the complex and have revealed that neuroglobin binds to cytochrome c with weak affinity (KD = 27-56 μM), forming a transient protein complex that is mediated largely by electrostatic interactions. A putative specific complex structure has been predicted by computer modelling. The interacting residues on the neuroglobin interface in the putative complex have been mutated and changes in binding affinity were measured with SPR. The results from these studies are consistent with the neuroglobin-cytochrome c interaction occurring via an encounter complex mechanism. Furthermore, cell-free in vitro assay systems have been developed to measure caspase activation and apoptosome formation induced by exogenous cytochrome c. In keeping with previous studies, ferric cytochrome c efficiently initiated apoptosome formation, while ferrous cytochrome c exhibited very little of this activity. Ferrous neuroglobin was capable of efficiently inhibiting apoptosome formation initiated by ferric cytochrome c, whereas ferric neuroglobin was significantly less effective. Taken together, these experiments suggest a role for ferrous neuroglobin in the prevention of apoptosome formation.

La mort cellulaire programmée ou apoptose est un mécanisme conservé chez les eucaryotes multicellulaires qui contribue au développement embryonnaire et à l'homéostasie cellulaire des organismes. Dans les cellules vivantes, l'activité des protéases qui exécutent le programme de mort cellulaire, les caspases, est contrôlée par des signaux de survie provenant de l'environnement cellulaire. Les caspases initiatrices de l'apoptose régulée par l'environnement, la caspase 9 et la caspase 8 sont activées respectivement par l'apoptosome et par les récepteurs de mort. Les signaux environnementaux, parmi lesquels le contact avec la matrice extracellulaire ou la présence de facteurs de croissance, activent des voies de signalisation contrôlant la machinerie de mort cellulaire. La voie des MAPK1/3 est une voie de signalisation contrôlée par le proto-oncogènes Ras et comportant les kinases Raf, MEK1/2 et MAPK1/3 (ERK2/1 ou p42/p44). La voie des MAPK1/3, qui est impliquée dans la prolifération et la différentiation cellulaire, joue un rôle essentiel dans la survie cellulaire. L'objectif de cette thèse a été de caractériser les mécanismes moléculaires impliqués dans le contrôle de la mort cellulaire par la voie des MAPK1/3. Ce travail est basé sur l'utilisation d'une forme active et inductible de la kinase Raf-1 (DRaf-1:ER) dont l'activation forte et prolongée correspond à une induction pathologique de la voie des MAPK1/3. Nous avons montré que, selon le type cellulaire, l'activation de deltaRaf-1:ER favorise la survie ou la mort cellulaire. Dans les cellules fibroblastiques CCL39, l'activation de deltaRaf-1:ER protège de la mort cellulaire mitochondriale induite par la privation en sérum du milieu de culture. Dans ces conditions, nous avons montré que la stimulation de Raf-1 :ER bloque l'activation de la caspase-9 mais n'empêche pas la délocalisation du cytochrome c, la multimérisation d'APAF1 ni le recrutement de la procaspase 9 dans l'apoptosome. Ce mécanisme post mitochondrial de protection contre la mort cellulaire dépend de la néo-synthèse des protéines et nécessite une activité continue de la kinase MEK. A l'inverse, dans les cellules HEK 293 issues de rein embryonnaire et présentant des caractéristiques neuronales, nous avons montré que l'activation soutenue de la voie des MAPK1/3 par DRaf1-ER induit une mort cellulaire massive. Celle-ci est caractérisée par l'activation des caspases et la fragmentation de l'ADN. La mort cellulaire est détectée plus de 24 heures après l'activation de Raf1-ER, elle est maximale à 48h. L'induction de la mort cellulaire ne requière la synthèse protéique que durant la phase précoce d'activation mais nécessite l'activité continue du module MEK/MAPK. La mort cellulaire résulte de l'activation de la caspase 8 et n'implique pas la voie mitochondriale, elle est caractérisée par une vacuolisation importante du cytoplasme des cellules qui l'apparente à une forme particulière d'apoptose. L'inactivation des fonctions du récepteur fas et de son adaptateur FADD indique que le processus d'activation de la caspase 8 est indépendant de la voie des récepteurs de mort. L'ensemble de ces travaux apporte des connaissances nouvelles sur le contrôle de la mort cellulaire par la voie Raf/MAPK1/3. Nous avons montré que la voie de signalisation peut, selon le contexte cellulaire, favoriser la survie cellulaire ou induire la mort. Dans les deux cas, le contrôle de la mort cellulaire dépend à la fois de la synthèse protéique et de mécanismes post-traductionnels. Les mécanismes moléculaires affectés par l'activation prolongée des MAPK1/3 seraient impliqués aussi bien dans la résistance des cellules tumorales aux traitements proapoptotiques que dans le développement des maladies neurodégénératives.

L'apoptosi o mort cel.lular programada és un procés central al desenvolupament, homeòstasi cel.lular, la resposta a l'estrés, l'envelliment i diferents malalties, als organismes eucariotes pluricel.lulars. Una de les vies que regula l'activació de l'apoptosi és la via intrinseca mitocondrial, a la que el citocrom c (Cit c), alliberat des de la mitocòndria en reposta a un estímul proapoptòtic, en presència de dATP indueix l'oligomerizació d'Apaf-1 (Apoptotic protease activating factor-1) i posterior reclutament de la caspasa-9, conduint a la formació del complex macromolecular activador de l'apoptosi anomenat apoptosoma. Citocroms de diferents vertebrats són equivalents en quant a la seva capacitat d'induir apoptosi a sistemes in vitro, mentre que el de llevat (YCit c) no presenta aquesta activitat. En concret s'ha determinat a partir d'estudis amb mutants derivats de HHCit c (citocrom c de cor de cavall) i YCit c, que els residus de Lys 7, 25, 39 i 72, així com els residus 62-65 dels citocroms de mamífer participen a la interacció amb Apaf-1. En base a aquests antecedents, i dins de la línia de treball iniciada al nostre laboratori encaminada al descobriment de molècules d'interès terapèutic relacionades amb l'apoptosi, es va plantejar com objetiu d'aquesta tesi l'estudi de la interacció entre Cit c i Apaf-1, com un pas preliminar per a l'obtenció de compostos orgànics capaços de modular aquesta interacció. En base als antecedents citats, es va proposar la generació de mutants de YCit c que incorporessin modificacions a la seva regió N-terminal que aproximessin la seva seqüència a la de HHCit c, per dotar-los d'activitat apoptòtica, així com pèptids derivats dels mateixos que mantinguessin unit el grup prostètic hemo. Les mutacions plantejades comprenien la delecció de la cua de cinc residus de l'extrem N-terminal de la seqüència de YCit c (Delta), i la substitució dels residus Ala 7 i Pro 25 d'aquest, per dos residus de Lys (A7K, P25K). Addicionalment es va introduir una Met a la possició 27 (K27M), que permetria generar els hemopèptids derivats dels mutants anteriors per degradació química amb BrCN. Per a la realització dels estudis de funcionalitat d'aquests derivats de YCit c, que comprenien assaigs in vitro d'activació de la caspasa -9 i -3, així com estudis de polarització de la fluorescència, es requeria disposar de la proteïna Apaf-1 en quantitat suficient, pel que es plantejà l'obtenció de dues construccions derivades d'Apaf-1-XL que poguessin expresar-se a E.coli, corresponents als dominis N-terminals CARD- NOD, constitutivament actius, i al C-terminal WDR, responsable de la interacció amb Cit c. Així, es van generar i caracteritzar mitjançant diferents tècniques espectroscòpiques, els mutants derivats de YCit c, YCit c (Delta/A7K/K27M), YCit c (Delta/P25K/K27M) i YCit c (Delta/A7K/P25K/K27M), així com l'hemopèptid derivat d'aquest, HP 7K25K 1-27. Igualment es van obtenir construccions derivades d'Apaf-1-XL que comprenien els residus 1-570 i 1-572, corresponents als dominis N-terminals, així com les que comprenien els residus 570-1248 i 583-1248, corresponents al domini C-terminal. Els estudis d'activació de caspases en cap cas van posar de manifest una capacitat proapoptòtica dels mutants generats. El mateix resultat negatiu es va obtenir per a l'hemopèptid anterior i pèptids sintètics derivats de la seqüència de HHCit c, sintetitzats al laboratori del grup col.laborador del Dr. E. Pérez- Payá (Centro de investigación Príncipe Felipe, València). No obstant, els estudis de polarització de la fluorescència van posar de manifest la capacitat del mutant YCit c (Delta/A7K/P25K/K27M) d'interaccionar amb Apaf-1. El conjunt d'aquests resultats indicava que les mutacions introduïdes a la seqüència de YCit c eren suficients per induir en aquest la capacitat per interaccionar amb Apaf-1, però no suficients per induir la formació de l'apoptosoma actiu. Per a l'hemopèptid es va determinar una potencial capacitat inhibidora de la formació de l'apoptosoma, que quedava emmascarada per l'efecte oxidatiu del grup prostètic hemo. Aquests mateixos assaigs van permetre establir la funcionalitat per a totes les proteïnes recombinants derivades d'Apaf-1-XL. Amb motiu d'una estada al laboratori del Profesor G. Núñez (University of Michigan, Ann Arbor, EUA) es plantejà un objetiu addicional que va consistir en la determinació dels components del complex de reconeixement del component bacterià muramil dipèptid (MDP), responsable de l'activació de la resposta inflamatòria mediada per Nod2, proteïna de la família NOD, a la que pertany igualment Apaf-1. Així, assaigs d'immunoprecipitació amb derivats del MDP, van indicar una interacció no directa entre aquest i Nod2. Mitjançant cromatografia d'afinitat i posterior anàlisi per espectrometria de masses s'han determinat potencials components del complex de reconeixement del MDP. Addicionalment, estudis preliminars de localizació cel.lular del MDP ha indicat que aquest seria incorporat per les cèl.lules de macròfag per la via endocítica.

Apoptosis is a programmed cell death mechanism that is evolutionary conserved from worms to humans. Apoptosis is mediated by initiator and effector caspases. The initiator caspases carry long pro-domains for their interaction with scaffolding proteins to form a cell-death platform, which is essential for their activation. Activated initiator caspases then cleave effector caspases that execute cell death through cleaving downstream targets. In addition to their apoptotic function, caspases also participate in events where caspase activity is not required for cell killing, but for regulating other functions, so-called non-apoptotic functions of caspases. The Drosophila initiator caspase Dronc, the ortholog of mammalian caspase-2 and caspase-9 has a CARD domain that is essential for its interaction with the scaffolding protein Dark to form the apoptosome. Apoptosome formation is crucial for activation of Dronc. Activity of both initiator and effector caspases are further kept in control by the ubiquitin system to avoid inappropriate caspase activity. However, mechanistic details of how the ubiquitin system regulates activation of Dronc are not clear. Therefore, I investigated the ubiquitylation status of Dronc and its function in Drosophila. I found that Dronc is mono-ubiquitylated at Lys78 (K78) in its CARD domain, which blocks its interaction with Dark and formation of the apoptosome. Furthermore, I demonstrated that K78 mono-ubiquitylation plays an inhibitory role in Dronc’s non-apoptotic functions, which may not require its catalytic activity but may be important for the survival of the fly. This thesis study unveils the link between the ubiquitin system and caspases through a regulatory mechanism where a single mono-ubiquitylation event could inhibit both apoptotic and non-apoptotic functions of a caspase.

Apoptosis is a programmed cell death mechanism that is evolutionary conserved from worms to humans. Apoptosis is mediated by initiator and effector caspases. The initiator caspases carry long pro-domains for their interaction with scaffolding proteins to form a cell-death platform, which is essential for their activation. Activated initiator caspases then cleave effector caspases that execute cell death through cleaving downstream targets. In addition to their apoptotic function, caspases also participate in events where caspase activity is not required for cell killing, but for regulating other functions, so-called non-apoptotic functions of caspases. The Drosophila initiator caspase Dronc, the ortholog of mammalian caspase-2 and caspase-9 has a CARD domain that is essential for its interaction with the scaffolding protein Dark to form the apoptosome. Apoptosome formation is crucial for activation of Dronc. Activity of both initiator and effector caspases are further kept in control by the ubiquitin system to avoid inappropriate caspase activity. However, mechanistic details of how the ubiquitin system regulates activation of Dronc are not clear. Therefore, I investigated the ubiquitylation status of Dronc and its function in Drosophila. I found that Dronc is mono-ubiquitylated at Lys78 (K78) in its CARD domain, which blocks its interaction with Dark and formation of the apoptosome. Furthermore, I demonstrated that K78 mono-ubiquitylation plays an inhibitory role in Dronc’s non-apoptotic functions, which may not require its catalytic activity but may be important for the survival of the fly. This thesis study unveils the link between the ubiquitin system and caspases through a regulatory mechanism where a single mono-ubiquitylation event could inhibit both apoptotic and non-apoptotic functions of a caspase.

Apoptosis is a cellular suicide program that can be initiated by various genotoxic and cytotoxic stimuli. In many cases, such cell damaging agents promote cell death through the intrinsic apoptotic pathway by triggering mitochondrial cytochrome c release and subsequent caspase activation. Cytosolic cytochrome c is directly responsible for initiating formation of the caspase-activating apoptosome, which plays a crucial role in the apoptotic process. Given the importance of cellular fate, apoptosis is tightly controlled by a balance between survival and death signals. It has been shown that activated cell survival pathways, including the mitogen-activated protein kinase (MAPK) cascade and the PI3K/Akt signaling, enhance cell viability by conferring resistance to apoptotic cell death. However, the underlying mechanism(s) that lead to inhibition of functional apoptosome formation (and caspase activation) has yet to be elucidated. In the studies that are described in this dissertation, I have investigated the regulation of apoptosis downstream of mitochondrial cytochrome c release with the goal of understanding how survival signaling can alter the apoptotic program, contributing to human malignancies.

First, we describe a mechanism for the inhibition of cytochrome c-induced caspase activation by MAPK signaling, identifying a novel mode of apoptotic regulation exerted through Apaf-1 phosphorylation by the 90-kDa ribosomal S6 kinase (Rsk). We have found that recruitment of 14-3-3ε to phosphorylated Ser268 impedes the ability of cytochrome c to nucleate apoptosome formation and activate downstream caspases. High endogenous levels of Rsk in PC3 prostate cancer cells or Rsk activation in other cell types promoted 14-3-3ε binding to Apaf-1 and rendered the cells insensitive to cytochrome c, suggesting a role for Rsk signaling in apoptotic resistance of prostate cancers and other cancers with elevated Rsk activity. These results identify a novel locus of apoptosomal regulation wherein MAPK signaling promotes Rsk-catalyzed Apaf-1 phosphorylation and consequent binding of 14-3-3ε, resulting in decreased cellular responsiveness to cytochrome c.

In the second part, we examine how apoptosis is inhibited by oncogenic tyrosine kinase signaling by using leukemogenic tyrosine kinase-induced leukemia model systems. We have demonstrated that protein phosphatase 5 (PP5) is responsible for Hsp90β hypophosphorylation, which can contribute to impaired cell death in leukemia expressing oncogenic tyrosine kinases. Loss of PP5 results in an increase of Hsp90β phosphorylation, raising leukemic cells' responsiveness to imatinib, a BCR-ABL kinase inhibitor. Further we have discovered that acetylation regulates PP5 activity on Hsp90β. Mutational study showed that K144 acetylation on PP5, which was diminished in leukemic conditions, inhibited PP5 binding to Hsp90β, causing Hsp90β hyperphosphorylation and subsequently potentiating cells to apoptosis. These studies reveal a molecular mechanism by which agents enhancing PP5 acetylation may be a potential treatment for leukemias. Collectively, this work provides new insight into mechanisms of regulation of apoptosome formation/function, helping us understand how the evasion of apoptotic cell death contributes to cancer cell survival. Further, this finding implicates cytochrome c-induced apoptotic signaling in the context of cancer cell responsiveness to chemotherapeutic treatments.

Dissertation

During stress-induced apoptosis, the initiator caspase-9 is activated by the Apaf-1 apoptosome and must remain bound in order to retain significant catalytic activity. Nevertheless, in apoptotic cells, the vast majority of processed caspase-9 is paradoxically observed outside of the complex. We demonstrate herein that apoptosome-mediated cleavage of procaspase-9 occurs exclusively through a CARD-displacement mechanism, so that unlike the effector procaspase-3, procaspase-9 cannot be processed by the apoptosome as a typical substrate. Indeed, procaspase-9 possessed higher affinity for the apoptosome and could displace processed caspase-9 from the complex, thereby facilitating a continuous cycle of procaspase-9 recruitment/activation, processing, and release from the complex. Due to its rapid autocatalytic cleavage, however, procaspase-9 per se contributed little to the activation of procaspase-3. Thus, the Apaf-1 apoptosome functions as a proteolytic-based “molecular timer”, wherein the intracellular concentration of procaspase-9 sets the overall duration of the timer, procaspase-9 autoprocessing activates the timer, and the rate at which processed caspase-9 dissociates from the complex (and thus loses its capacity to activate procaspase-3) dictates how fast the timer “ticks” over. To understand the physiological relevance of molecular timer in vivo, we are currently generating caspase-9 knock-in mouse models. These mouse models will enhance our understanding of the importance of caspase-9 processing within the apoptosome.
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博士
國立臺灣大學
生化科學研究所
106
Death domain (DD)-fold assemblies play a crucial role in regulating the signaling to cell survival or death. Here we report the crystal structure of the caspase recruitment domain (CARD)-CARD disk of the human apoptosome. The structure surprisingly reveals that three 1:1 Apaf-1: procaspase-9 CARD protomers form a novel helical DD-fold assembly on the heptameric wheel-like platform of the apoptosome. The small-angle X-ray scattering and multi-angle light scattering data also support that three protomers could form an oligomeric complex similar to the crystal structure. Interestingly, the quasi-equivalent environment of CARDs could generate different quaternary CARD assemblies. We also found that the type II interaction is conserved in all DD-fold complexes, whereas the type I interaction is found only in the helical DD-fold assemblies. This study provides crucial insights into the caspase activation mechanism, which is tightly controlled by a sophisticated and highly evolved CARD assembly on the apoptosome, and also enables better understanding of the intricate DD-fold assembly.

Many pro-apoptotic signals trigger mitochondrial cytochrome c release, leading to caspase activation and ultimate cellular breakdown. Cell survival pathways, including the mitogen-activated protein kinase (MAPK) cascade, promote cell viability both by impeding mitochondrial cytochrome c release and by inhibiting subsequent activation of caspases. Cytosolic cytochrome c is directly responsible for initiating formation of the caspase-activating apoptosome, which, in many cell types, plays a crucial role in the apoptotic process. Given the important role of cytochrome c in dismantling the dying cell, we wanted to investigate the process of cytochrome c-induced apoptosis with the goal of understanding how this mechanism is altered in certain malignant conditions.

First, we examined cytochrome c-induced caspase activation in normal and tumorigenic mammary epithelial cells. Although most tumor types have developed mechanisms for evading apoptosis, we surprisingly discovered that breast cancer cells were hypersensitive to cytochrome c when compared with their normal counterpart. Specifically, breast cancer cells show increased binding of caspase-9 to the Apaf-1 caspase recruitment domain. This altered apoptosome formation is mediated by overexpression of the protein PHAPI in the malignant mammary epithelial cells. Immunoblot analysis demonstrated that protein levels of PHAPI are also elevated in human breast tumors. These results suggest a novel paradigm where breast cancer cells are refractory to cytochrome c release in response to certain stimuli, but they are quite sensitive to apoptosis downstream of the mitochondria.

Secondly, we describe a mechanism for the inhibition of cytochrome c-induced caspase activation by MAPK signaling, identifying a novel mode of apoptotic regulation exerted through Apaf-1 phosphorylation by the 90-kDa ribosomal S6 kinase (Rsk). This Apaf-1 phosphorylation results in impaired apoptosome formation, thereby inhibiting caspase activation. The Rsk effect on Apaf-1 is antagonized by protein phosphatase 1 (PP1), which promotes Apaf-1 dephosphorylation. High endogenous levels of Rsk in PC3 prostate cancer cells leads to Apaf-1 phosphorylation and renders them relatively insensitive to cytochrome c, suggesting a role for Rsk signaling in the apoptotic resistance of certain cancers. These results identify a novel locus of apoptosomal regulation wherein MAPK signaling promotes direct Rsk-catalyzed phosphorylation of Apaf-1, resulting in decreased cellular responsiveness to cytochrome c. Collectively, this work provides insight into novel mechanisms of regulation for cytochrome c-induced apoptosis.

Dissertation