Structural and biochemical basis of apoptotic activation by Smac/DIABLO

AID binds to transcription-induced structures in c-MYC that map to regions associated with translocation and hypermutation

Translocation and aberrant hypermutation of c-MYC are common in B-cell lymphomas. Activation-induced Cytidine Deaminase (AID) initiates switch recombination and somatic hypermutation in B cells by targeted deamination of transcribed genes. We show that transcription of the immunoglobulin S regions and c-MYC results in formation of similar DNA structures, 'G-loops', which contain a cotranscriptional RNA: DNA hybrid on the C-rich strand and single-stranded regions and G4 DNA on the G-rich strand. AID binds specifically to G-loops within transcribed S regions and c-MYC, and G-loops in c-MYC map to the regions associated with translocation breakpoints and aberrant hypermutation in B-cell lymphomas. Aberrant targeting of AID to DNA structures formed upon c-MYC transcription may therefore contribute to the genetic instability of c-MYC in B-cell malignancies.

Mathematical modeling of the regulation of caspase-3 activation and degradation

Caspases are thought to be important players in the execution process of apoptosis. Inhibitors of apoptosis (IAPs) are able to block caspases and therefore apoptosis. The fact that a subgroup of the IAP family inhibits active caspases implies that not each caspase activation necessarily leads to apoptosis. In such a scenario, however, processed and enzymically active caspases should somehow be removed. Indeed, IAP-caspase complexes covalently bind ubiquitin, resulting in degradation by the 26S proteasome. Following release from mitochondria, IAP antagonists (e.g. second mitochondrial activator of caspases (Smac)) inactivate IAPs. Moreover, although pro-apoptotic factors such as irradiation or anti-cancer drugs may release Smac from mitochondria in tumor cells, high cytoplasmic survivin and ML-IAP levels might be able to neutralize it and, consequently, IAPs would further be able to bind activated caspases. Here, we propose a simple mathematical model, describing the molecular interactions between Smac deactivators, Smac, IAPs, and caspase-3, including the requirements for both induction and prevention of apoptosis, respectively. In addition, we predict a novel mechanism of caspase-3 degradation that might be particularly relevant in long-living cells.

Mitochondrial mechanism of heat stress-induced injury in rat cardiomyocyte

Heat stress results in cardiac dysfunction and even cardiac failure. To elucidate the cellular and molecular mechanism of cardiomyocyte injury induced by heat stress, the changes of structure and function in cardiac mitochondria of heat-exposed Wistar rats and its role in cardiomyocyte injury were investigated. Heat stress induced apoptosis and necrosis of cardiomyocytes in a time- and dose-dependent fashion. In the mitochondria of heat-stressed cardiomyocytes, the respiratory control rate and oxidative phosphorylation efficiency (P:O) were decreased gradually with the rise of rectal temperature. The Ca2+ -adenosine triphosphatase activity and Ca2+ content were also reduced. Exposing isolated mitochondria to the heat stress induced special internal environmental states including Ca2+ overload, oxidative stress, and altered mitochondrial membrane permeability transition (MPT). In vivo, the heat stress-induced mitochondrial MPT alteration was also found. The changes of mitochondrial MPT resulted in the release of cytochrome c from mitochondria into the cytosol, and in turn, caspase-3 was activated. Transfection of bcl-2 caused Bcl-2 overexpression in cardiomyocyte, which protected the mitochondria and reduced the heat stress-induced cardiomyocyte injury. In conclusion, it appears that the destruction of mitochondrial structure and function not only resulted in the impairment of physiological function of cardiomyocytes under heat stress but may also further lead to severe cellular injury and even cell death. These findings underline the contribution of mitochondria to the injury process in cardiomyocytes under heat stress.

Dual role of BRUCE as an antiapoptotic IAP and a chimeric E2/E3 ubiquitin ligase

Apoptotic cell death and survival is controlled by pro- and antiapoptotic proteins. Because these proteins act on each other, cell fate is dictated by the relative activity of pro- versus antiapoptotic proteins. Here we report that BRUCE, a conserved 528 kDa peripheral membrane protein of the trans-Golgi network, protects cells against apoptosis and functions as an inhibitor of apoptosis (IAP). By using wild-type and mutant forms we show that BRUCE inhibits caspase activity and apoptosis depending on its BIR domain. Upon apoptosis induction, BRUCE is antagonized by three mechanisms: first, through binding to Smac; second, by the protease HtrA2; and third, by caspase-mediated cleavage. In addition to its IAP activity BRUCE has the distinctive property of functioning as a chimeric E2/E3 ubiquitin ligase with Smac being a substrate. Our work suggests that, owing to its two activities and its localization, BRUCE may function as a specialized regulator of cell death pathways.

Mature DIABLO/Smac is produced by the IMP protease complex on the mitochondrial inner membrane

DIABLO/Smac is a mitochondrial protein that can promote apoptosis by promoting the release and activation of caspases. To do so, DIABLO/Smac must first be processed by a mitochondrial protease and then released into the cytosol, and we show this in an intact cellular system. We propose that the precursor form of DIABLO/Smac enters the mitochondria through a stop-transfer pathway and is processed to its active form by the inner membrane peptidase (IMP) complex. Catalytic subunits of the mammalian IMP complex were identified based on sequence conservation and functional complementation, and the novel sequence motif RX(5)P in Imp1 and NX(5)S in Imp2 distinguish the two catalytic subunits. DIABLO/Smac is one of only a few specific proteins identified as substrates for the IMP complex in the mitochondrial intermembrane space.

Inhibition of Wnt-2-mediated signaling induces programmed cell death in non-small-cell lung cancer cells

In this report, we have demonstrated that Wnt-2 protein is overexpressed in freshly resected human non-small-cell lung cancer (NSCLC) tissues. We have also developed a monoclonal antibody against the N-terminus of human Wnt-2 protein. This monoclonal antibody induces apoptosis in human NSCLC cell lines that overexpress Wnt-2 protein. Incubation of this antibody with normal human airway cells lacking Wnt-2 expression does not induce apoptosis. Wnt-2 signaling blockade by the anti-Wnt-2 antibody is confirmed by downregulation of cytosolic beta-catenin and reduction in TCF-dependent transcriptional activity (TOPFLASH assay). In addition, Wnt-2-specific small interfering RNA (siRNA) treatment in the NSCLC cell line A549 also downregulated cytosolic beta-catenin and induced apoptosis. Moreover, downregulation of an inhibitor of apoptosis family protein, Survivin, was noticed both in the Wnt-2 antibody- and siRNA-treated NSCLC cells, suggesting that inhibition of Wnt-2-mediated signaling induces apoptosis through inactivating Survinin.

Caspase activation, inhibition, and reactivation: a mechanistic view

Caspases, a unique family of cysteine proteases, execute programmed cell death (apoptosis). Caspases exist as inactive zymogens in cells and undergo a cascade of catalytic activation at the onset of apoptosis. The activated caspases are subject to inhibition by the inhibitor-of-apoptosis (IAP) family of proteins. This inhibition can be effectively removed by diverse proteins that share an IAP-binding tetrapeptide motif. Recent structural and biochemical studies have revealed the underlying molecular mechanisms for these processes in mammals and in Drosophila. This paper reviews these latest advances.

Non-peptidic small molecule inhibitors of XIAP

Non-peptidic small molecule SMAC mimetics were designed and synthesized that bind to the BIR3 domain of XIAP using structure-based design. Substituted five-membered heterocycles such as thiazoles and imidazoles were identified that serve as replacements for peptide fragments of the lead.

Bench-to-bedside review: Apoptosis/programmed cell death triggered by traumatic brain injury

Apoptosis, or programmed cell death, is a physiological form of cell death that is important for normal embryologic development and cell turnover in adult organisms. Cumulative evidence suggests that apoptosis can also be triggered in tissues without a high rate of cell turnover, including those within the central nervous system (CNS). In fact, a crucial role for apoptosis in delayed neuronal loss after both acute and chronic CNS injury is emerging. In the current review we summarize the growing evidence that apoptosis occurs after traumatic brain injury (TBI), from experimental models to humans. This includes the identification of apoptosis after TBI, initiators of apoptosis, key modulators of apoptosis such as the Bcl-2 family, key executioners of apoptosis such as the caspase family, final pathways of apoptosis, and potential therapeutic interventions for blocking neuronal apoptosis after TBI.

Coated microwell plate-based affinity purification of antigens

Antibody-based affinity capture of antigens is widely used in the isolation of antigens from complex mixtures. Antibody and the corresponding antigen are allowed to interact with each other to form immunocomplexes which are then typically captured by protein A or protein G immobilized on beaded support. Antigen capture performed using this method generally requires multiple centrifugation steps and careful pipetting to avoid loss of the bead-bound complexes. This traditional procedure is tedious and not easily reproducible, especially when working with multiple samples. To address these issues we have demonstrated that antigens can be captured with protein A/G, protein G, and high binding-capacity streptavidin 96-well strip-coated plates. The coated plate method of antigen purification is reproducible, within the same experiment and between experiments, due to the uniform binding capacity of the plates and wells. Here we report the use of coated microwell plates for antigen capture and for protein-protein interaction studies with the well-characterized BIR2-SMAC, transferrin receptor/ transferrin, and other systems.

Chrysin-induced apoptosis is mediated through caspase activation and Akt inactivation in U937 leukemia cells

Chrysin is a natural, biologically active compound extracted from many plants, honey, and propolis. It possesses potent anti-inflammation, anti-cancer, and anti-oxidation properties. The mechanism by which chrysin initiates apoptosis remains poorly understood. In the present report, we investigated the effect of chrysin on the apoptotic pathway in U937 human promonocytic cells. We show that chrysin induces apoptosis in association with the activation of caspase 3 and that Akt signal pathway plays a crucial role in chrysin-induced apoptosis in U937 cells. Furthermore, we have shown that inhibition of Akt phosphorylation in U937 cells by the specific PI3K inhibitor, LY294002 significantly, enhanced apoptosis. Overexpression of a constitutively active Akt (myr-Akt) in U937 cells inhibited the induction of apoptosis, activation of caspase 3, and PLC-gamma1 cleavage by chrysin. Together, these findings suggest that the Akt pathway plays a major role in regulating the apoptotic response of human leukemia cells to chrysin and raise the possibility that combined interruption of chrysin and PI3K/Akt-related pathways may represent a novel therapeutic strategy in hematological malignancies.

XIAP inhibits caspase-3 and -7 using two binding sites: evolutionarily conserved mechanism of IAPs

The X-linked inhibitor of apoptosis protein (XIAP) uses its second baculovirus IAP repeat domain (BIR2) to inhibit the apoptotic executioner caspase-3 and -7. Structural studies have demonstrated that it is not the BIR2 domain itself but a segment N-terminal to it that directly targets the activity of these caspases. These studies failed to demonstrate a role of the BIR2 domain in inhibition. We used site-directed mutagenesis of BIR2 and its linker to determine the mechanism of executioner caspase inhibition by XIAP. We show that the BIR2 domain contributes substantially to inhibition of executioner caspases. A surface groove on BIR2, which also binds to Smac/DIABLO, interacts with a neoepitope generated at the N-terminus of the caspase small subunit following activation. Therefore, BIR2 uses a two-site interaction mechanism to achieve high specificity and potency for inhibition. Moreover, for caspase-7, the precise location of the activating cleavage is critical for subsequent inhibition. Since apical caspases utilize this cleavage site differently, we predict that the origin of the death stimulus should dictate the efficiency of inhibition by XIAP.

Apoptotic Pathways and Therapy Resistance in Human Malignancies

Apoptosis and necrosis are two morphologically distinct forms of cell death that are important for maintaining of cellular homeostasis. Almost all agents can provoke either response when applied to cells; however, the duration of treatment and the dose of the used agents determine which type of death (apoptosis or necrosis) is initiated. The response of tumors to chemo-, radio-, and hormone therapy or to treatment with biologically active agents may depend at least in part on the propensity of these tumors to undergo cell death. Some tumors, e.g., leukemias, small cell lung cancer, and seminomas, respond quickly to first-line therapy; this fast response is thought to result from induction of apoptosis. Solid tumors, on the other hand, usually respond slowly and less effectively, with cell death characterized not only by apoptosis but also by necrosis, or mitotic catastrophe. It is likely that resistance of tumors to treatment might be associated with defects in, or dysregulation of, different steps of the apoptotic pathways. Several attempts were undertaken to use the knowledge of these defects to design new drugs, which might either activate or re-activate the apoptotic machinery of tumor cells. Here we discuss the apoptotic pathways and their role in therapy resistance of human malignancies. Although such studies are still in progress, they offer great promise for future cancer therapy. We hope that some of these agents will turn out to be valuable additions to the future therapeutic arsenal, which will most probably include a combination of conventional cytotoxic drugs and molecular target-based pro-apoptotic drugs.

Molecular mechanisms of caspase regulation during apoptosis

Caspases, which are the executioners of apoptosis, comprise two distinct classes, the initiators and the effectors. Although general structural features are shared between the initiator and the effector caspases, their activation, inhibition and release of inhibition are differentially regulated. Biochemical and structural studies have led to important advances in understanding the underlying molecular mechanisms of caspase regulation. This article reviews these latest advances and describes our present understanding of caspase regulation during apoptosis.

Decreased apoptosome activity with neuronal differentiation sets the threshold for strict IAP regulation of apoptosis

Despite the potential of the inhibitor of apoptosis proteins (IAPs) to block cytochrome c-dependent caspase activation, the critical function of IAPs in regulating mammalian apoptosis remains unclear. We report that the ability of endogenous IAPs to effectively regulate caspase activation depends on the differentiation state of the cell. Despite being expressed at equivalent levels, endogenous IAPs afforded no protection against cytochrome c-induced apoptosis in naive pheochromocytoma (PC12) cells, but were remarkably effective in doing so in neuronally differentiated cells. Neuronal differentiation was also accompanied with a marked reduction in Apaf-1, resulting in a significant decrease in apoptosome activity. Importantly, this decrease in Apaf-1 protein was directly linked to the increased ability of IAPs to stringently regulate apoptosis in neuronally differentiated PC12 and primary cells. These data illustrate specifically how the apoptotic pathway acquires increased regulation with cellular differentiation, and are the first to show that IAP function and apoptosome activity are coupled in cells.

The protein structures that shape caspase activity, specificity, activation and inhibition

The death morphology commonly known as apoptosis results from a post-translational pathway driven largely by specific limited proteolysis. In the last decade the structural basis for apoptosis regulation has moved from nothing to 'quite good', and we now know the fundamental structures of examples from the initiator phase, the pre-mitochondrial regulator phase, the executioner phase, inhibitors and their antagonists, and even the structures of some substrates. The field is as well advanced as the best known of proteolytic pathways, the coagulation cascade. Fundamentally new mechanisms in protease regulation have been disclosed. Structural evidence suggests that caspases have an unusual catalytic mechanism, and that they are activated by apparently unrelated events, depending on which position in the apoptotic pathway they occupy. Some naturally occurring caspase inhibitors have adopted classic inhibition strategies, but other have revealed completely novel mechanisms. All of the structural and mechanistic information can, and is, being applied to drive therapeutic strategies to combat overactivation of apoptosis in degenerative disease, and underactivation in neoplasia. We present a comprehensive review of the caspases, their regulators and inhibitors from a structural and mechanistic point of view, and with an aim to consolidate the many threads that define the rapid growth of this field.

Dissection of DIAP1 Functional Domains via a Mutant Replacement Strategy

Inhibitor of apoptosis proteins (IAPs) act as endogenous inhibitors of active caspases. Drosophila IAP1 (DIAP1) activity is required to keep cells from undergoing apoptosis. The central cell death regulators Reaper and Hid induce apoptosis very rapidly by inhibiting DIAP1 function. We have developed a system for replacing endogenous DIAP1 with mutant forms of the protein, allowing us to examine the roles of various domains of the protein in living and dying cells. We found that DIAP1 is cleaved by a caspase early after the initiation of apoptosis. This cleavage is required for DIAP1 degradation, but Rpr and Hid can still initiate apoptosis in the absence of cleavage. The cleavage of DIAP1 promotes DIAP1 degradation in a manner dependent on the function of the ubiquitin ligase function of the DIAP1 ring domain. This ring domain function is required for Hid-induced apoptosis. We propose a model that synthesizes our data with those of other laboratories and provide a consistent model for DIAP1 function in living and dying cells.

SMAC/Diablo-dependent apoptosis induced by nonsteroidal antiinflammatory drugs (NSAIDs) in colon cancer cells

Nonsteroidal antiinflammatory drugs (NSAIDs) form a paradigm for the chemoprevention of cancer, preventing colonic tumor progression in both experimental animals and humans. However, the mechanisms underlying the antineoplastic effects of NSAIDs are currently unclear. We found that the mitochondrial second mitochondrial-derived activator of caspase (SMAC)/direct inhibitor of apoptosis protein-binding protein with low pI (Diablo) protein translocates into the cytosol during NSAID-induced apoptosis in colon cancer cells. When SMAC/Diablo is disrupted by homologous recombination and RNA interference in these cells, the NSAID-induced apoptosis is abrogated. Biochemical markers of apoptosis, such as caspase activation, cytosolic release of cytochrome c and apoptosis-inducing factor, and mitochondrial membrane potential change, are accordingly decreased. These results establish that SMAC/Diablo is essential for the apoptosis induced by NSAIDs in colon cancer cells.

SMAC is expressed de novo in a subset of cervical cancer tumors

Background

Smac/Diablo is a recently identified protein that is released from mitochondria after apoptotic stimuli. It binds IAPs, allowing caspase activation and cell death. In view of its activity it might participate in carcinogenesis. In the present study, we analyzed Smac expression in a panel of cervical cancer patients.

Methods

We performed semi quantitative RT-PCR on 41 cervical tumor and 6 normal tissue samples. The study included 8 stage I cases; 16 stage II; 17 stage III; and a control group of 6 samples of normal cervical squamous epithelial tissue.

Results

Smac mRNA expression was below the detection limit in the normal cervical tissue samples. In contrast, 13 (31.7%) of the 41 cervical cancer biopsies showed detectable levels of this transcript. The samples expressing Smac were distributed equally among the stages (5 in stage I, 4 in stage II and 4 in stage III) with similar expression levels. We found no correlation between the presence of Smac mRNA and histology, menopause, WHO stage or disease status.

Conclusions

Smac is expressed de novo in a subset of cervical cancer patients, reflecting a possible heterogeneity in the pathways leading to cervical cancer. There was no correlation with any clinical variable.

Cellular, Biochemical, and Genetic Analysis of Mechanism of Small Molecule IAP Inhibitors

XIAP is member of the IAP family of anti-apoptotic proteins and is known for its ability to bind and suppress caspase family cell death proteases. A phenylurea series of chemical inhibitors of XIAP was recently generated by our laboratories (Schimmer, A. D., Welsh, K., Pinilla, C., Bonneau, M., Wang, Z., Pedersen, I. M., Scott, F. L., Glinsky, G. V., Scudiero, D. A., Sausville, E., Salvesen, G., Nefzi, A., Ostresh, J. M., Houghten, R. A., and Reed, J. C. (2004) Cancer Cell 5, 25-35). We examined the mechanisms of action of these chemical compounds using biochemical, molecular biological, and genetic methods. Active phenylurea-based compounds dissociated effector protease caspase-3 but not initiator protease caspase-9 from XIAP in vitro and restored caspase-3 but not caspase-9 enzymatic activity. When applied to tumor cell lines in culture, active phenylurea-based compounds induced apoptosis in a rapid, concentration-dependent manner, associated with activation of cellular caspases. Apoptosis induced by active phenylurea-based compounds was blocked by chemical inhibitors of caspases, with inhibitors of downstream effector caspases displaying more effective suppression than inhibitors of upstream initiator caspases. Phenylurea-based XIAP antagonists induced apoptosis (defined by annexin V staining) prior to mitochondrial membrane depolarization, in contrast to cytotoxic anticancer drugs. Consistent with these findings, apoptosis induced by phenylurea-based compounds was not altered by genetic alterations in the expression of Bcl-2 family proteins that control mitochondria-dependent cell death pathways, including over-expression of anti-apoptotic proteins Bcl-2 or Bcl-X(L) and genetic ablation of pro-apoptotic proteins Bax and Bak. Conversely, conditional over-expression of an active fragment of XIAP or genetic ablation of XIAP expression altered the apoptosis dose-response of the compounds. Altogether, these findings indicate that phenylurea-based XIAP antagonists block interaction of downstream effector caspases with XIAP, thus inducing apoptosis of tumor cell lines through a caspase-dependent, Bcl-2/Bax-independent mechanism.

The membrane-associated inhibitor of apoptosis protein, BRUCE/Apollon, antagonizes both the precursor and mature forms of Smac and caspase-9

Smac/DIABLO, HtrA2/Omi, and caspase-9 play key roles in the initiation of apoptosis. The inhibitor of apoptosis proteins (IAPs) are believed to bind to the N-terminal IAP binding motifs of the mature (proteolytically processed) forms of Smac, HtrA2, and caspase-9. However, we show here that BRUCE/Apollon, a 528-kDa IAP whose degradation promotes apoptosis, associates with their precursors as well as the mature forms by binding to regions in addition to the IAP binding motif. Through these associations, BRUCE promotes the degradation of Smac and inhibits the activity of caspase-9 but not the effector caspase, caspase-3. In response to apoptotic stimuli, BRUCE is degraded by proteasomes and/or cleaved by caspases and HtrA2 depending on the specific stimulus and the cell type. These results suggest that the ability of BRUCE to antagonize both the precursor and mature forms of Smac and caspase-9 is an important mechanism for the prevention of apoptosis under normal conditions.

Linking JNK signaling to NF-kappaB: a key to survival

In addition to marshalling immune and inflammatory responses, transcription factors of the NF-kappaB family control cell survival. This control is crucial to a wide range of biological processes, including B and T lymphopoiesis, adaptive immunity, oncogenesis and cancer chemoresistance. During an inflammatory response, NF-kappaB activation antagonizes apoptosis induced by tumor necrosis factor (TNF)-alpha, a protective activity that involves suppression of the Jun N-terminal kinase (JNK) cascade. This suppression can involve upregulation of the Gadd45-family member Gadd45beta/Myd118, which associates with the JNK kinase MKK7/JNKK2 and blocks its catalytic activity. Upregulation of XIAP, A20 and blockers of reactive oxygen species (ROS) appear to be important additional means by which NF-kappaB blunts JNK signaling. These recent findings might open up entirely new avenues for therapeutic intervention in chronic inflammatory diseases and certain cancers; indeed, the Gadd45beta-MKK7 interaction might be a key target for such intervention.

Cytochrome C-mediated apoptosis

Apoptosis, or programmed cell death, is involved in development, elimination of damaged cells, and maintenance of cell homeostasis. Deregulation of apoptosis may cause diseases, such as cancers, immune diseases, and neurodegenerative disorders. Apoptosis is executed by a subfamily of cysteine proteases known as caspases. In mammalian cells, a major caspase activation pathway is the cytochrome c-initiated pathway. In this pathway, a variety of apoptotic stimuli cause cytochrome c release from mitochondria, which in turn induces a series of biochemical reactions that result in caspase activation and subsequent cell death. In this review, we focus on the recent progress in understanding the biochemical mechanisms and regulation of the pathway, the roles of the pathway in physiology and disease, and their potential therapeutic values.

A small molecule Smac mimic potentiates TRAIL- and TNFalpha-mediated cell death

We describe the synthesis and properties of a small molecule mimic of Smac, a pro-apoptotic protein that functions by relieving inhibitor-of-apoptosis protein (IAP)-mediated suppression of caspase activity. The compound binds to X chromosome- encoded IAP (XIAP), cellular IAP 1 (cIAP-1), and cellular IAP 2 (cIAP-2) and synergizes with both tumor necrosis factor alpha (TNFalpha) and TNF-related apoptosis-inducing ligand (TRAIL) to potently induce caspase activation and apoptosis in human cancer cells. The molecule has allowed a temporal, unbiased evaluation of the roles that IAP proteins play during signaling from TRAIL and TNF receptors. The compound is also a lead structure for the development of IAP antagonists potentially useful as therapy for cancer and inflammatory diseases.

Inhibitor of apoptosis proteins: new therapeutic targets in hematological cancer?

Apoptosis is an essential process for the selection and survival of lymphocytes. Resistance to apoptosis can promote malignant transformation of hematopoietic cells. Proteins that regulate apoptosis may therefore be critically involved in the development of hematological cancer. A delicate balance between pro- and antiapoptotic mechanisms determines whether a cell death signal can activate the execution of the apoptotic cell death program. The family of inhibitor of apoptosis (IAP) proteins is a recently identified, novel category of apoptosis-regulatory proteins. IAPs can inhibit the activation of caspases that are the executioners of apoptosis, activated by both the extrinsic and intrinsic pathway. IAPs may thereby set the threshold for apoptosis-activation and play a key role in the regulation of apoptotic cell death. IAPs themselves are also subject to strict regulation through feedback mechanisms. This paper focuses on the role of IAP family proteins in the regulation of apoptosis and discusses implications for their involvement in cancer and possible use for cancer therapy, especially in leukemias and lymphomas.

Discovery of potent antagonists of the antiapoptotic protein XIAP for the treatment of cancer

Inhibitor of apoptosis (IAP) proteins are overexpressed in many cancers and have been implicated in tumor growth, pathogenesis, and resistance to chemo- or radiotherapy. On the basis of the NMR structure of a SMAC peptide complexed with the BIR3 domain of X-linked IAP (XIAP), a novel series of XIAP antagonists was discovered. The most potent compounds in this series bind to the baculovirus IAP repeat 3 (BIR3) domain of XIAP with single-digit nanomolar affinity and promote cell death in several human cancer cell lines. In a MDA-MB-231 breast cancer mouse xenograft model, these XIAP antagonists inhibited the growth of tumors. Close structural analogues that showed only weak binding to the XIAP-BIR3 domain were inactive in the cellular assays and showed only marginal in vivo activity. Our results are consistent with a mechanism in which ligands for the BIR3 domain of XIAP induce apoptosis by freeing up caspases. The present study validates the BIR3 domain of XIAP as a target and supports the use of small molecule XIAP antagonists as a potential therapy for cancers that overexpress XIAP.

An anti-Wnt-2 monoclonal antibody induces apoptosis in malignant melanoma cells and inhibits tumor growth

Activation of the Wnt/beta-catenin signaling pathway has been associated with human cancers. To test whether Wnt-2 signal is a survival factor in human melanoma cells and thus represents a potential therapeutic target, we investigated the effects of inhibition of Wnt-2 signaling in human melanoma cell lines. We have developed a novel monoclonal antibody against the NH(2) terminus of the human Wnt-2 ligand that induces apoptosis in human melanoma cells overexpressing Wnt-2. Whereas incubation of this antibody with normal cells lacking Wnt-2 expression does not induce apoptosis, Wnt-2 signaling blockade by the ligand-binding antibody is confirmed by down-regulation of Dishevelled and beta-catenin. Wnt-2 small interfering RNA treatment in these cells yielded similar apoptotic effects and downstream changes. Down-regulation of an inhibitor of apoptosis family protein, survivin, was observed in both the Wnt-2 antibody-treated and small interfering RNA-treated melanoma cell lines, suggesting that the antibody induces apoptosis by inactivating survivin. In an in vivo study, this monoclonal anti-Wnt-2 antibody suppresses tumor growth in a xenograft model. These findings suggest that the anti-Wnt-2 monoclonal antibody may be useful for the treatment of patients with malignant melanoma.

Upstream regulatory role for XIAP in receptor-mediated apoptosis

X-linked inhibitor of apoptosis (XIAP) is an endogenous inhibitor of cell death that functions by suppressing caspases 3, 7, and 9. Here we describe the establishment of Jurkat-derived cell lines stably overexpressing either full-length XIAP or a truncation mutant of XIAP that can only inhibit caspase 9. Characterization of these cell lines revealed that following CD95 activation full-length XIAP supported both short- and long-term survival as well as proliferative capacity, in contrast to the truncation mutant but similar to Bcl-x(L). Full-length XIAP was also able to inhibit CD95-mediated caspase 3 processing and activation, the mitochondrial release of cytochrome c and Smac/DIABLO, and the loss of mitochondrial membrane potential, whereas the XIAP truncation mutant failed to prevent any of these cell death events. Finally, suppression of XIAP levels by RNA interference sensitized Bcl-x(L)-overexpressing cells to death receptor-induced apoptosis. These data demonstrate for the first time that full-length XIAP inhibits caspase activation required for mitochondrial amplification of death receptor signals and that, by acting upstream of mitochondrial activation, XIAP supports the long-term proliferative capacity of cells following CD95 stimulation.

Autocatalytic Processing of HtrA2/Omi Is Essential for Induction of Caspase-dependent Cell Death through Antagonizing XIAP

A mature form of nuclear-encoded mitochondrial serine protease HtrA2/Omi is pivotal in regulating apoptotic cell death; however, the underlying mechanism of the processing event of HtrA2/Omi and its relevant biological function remain to be clarified. Here, we describe that HtrA2/Omi is autocatalytically processed to the 36-kDa protein fragment, which is required for the cytochrome c-dependent caspase activation along with neutralizing XIAP-mediated inhibition of caspases through interaction with XIAP, eventually promoting apoptotic cell death. We have shown that the autocatalytic processing of HtrA2/Omi occurs via an intermolecular event, demonstrated by incubating an in vitro translated HtrA2/Omi (S306A) mutant with the enzymatically active glutathione S-transferase-HtrA2/Omi protein. Using N-terminal amino acid sequencing and mutational analysis, we identified that the autocatalytic cleavage site is the carboxyl side of alanine 133 of HtrA2/Omi, resulting in exposure of an inhibitor of apoptosis protein binding motif in its N terminus. Our study provides evidence that the autocatalytic processing of HtrA2/Omi is crucial for regulating HtrA2/Omi-mediated apoptotic cell death.

Development and optimization of a binding assay for the XIAP BIR3 domain using fluorescence polarization

The X-linked inhibitor of apoptosis protein (XIAP) is a potent cellular inhibitor of apoptosis. Designing small-molecule inhibitors that target the BIR3 domain of XIAP, where Smac/DIABLO (second mitochondria-derived activator of caspase/direct IAP-binding protein with low pI) and caspase-9 bind, is a promising strategy for inhibiting the antiapoptotic activity of XIAP and for overcoming apoptosis resistance of cancer cells mediated by XIAP. Herein, we report the development of a homogeneous high-throughput assay based on fluorescence polarization for measuring the binding affinities of small-molecule inhibitors to the BIR3 domain of XIAP. Among four fluorescent probes tested, a mutated N-terminal Smac peptide (AbuRPFK-(5-Fam)-NH(2)) showed the highest affinity (Kd =17.92 nM) and a large dynamic range (deltamP = 231 +/- 0.9), and was selected as the most suitable probe for the binding assay. The binding conditions (DMSO tolerance and stability) have been investigated. Under optimized conditions, a Z' factor of 0.88 was achieved in a 96-well format for high-throughput screening. It was found that the popular Cheng-Prusoff equation is invalid for the calculation of the competitive inhibition constants (Ki values) for inhibitors in the FP-based competitive binding assay conditions, and accordingly, a new mathematical equation was developed, validated, and used to compute the Ki values. An associated Web-based computer program was also developed for this task. Several known Smac peptides with high and low affinities have been evaluated under the assay conditions and the results obtained indicated that the FP-based competitive binding assay performs correctly as designed: it can quantitatively and accurately determine the binding affinities of Smac-based peptide inhibitors with a wide range of affinities, and is suitable for high-throughput screening of inhibitors binding to the XIAP BIR3 domain.

Chemical-induced apoptosis: formation of the Apaf-1 apoptosome

Many environmental and therapeutic agents initiate apoptotic cell death by inducing the release of cytochrome c from the mitochondria, which activates Apaf-1 (apoptotic protease-activating factor-1). This large (approximately 130kD) protein is a mammalian homologue of CED-4, an essential protein involved in programmed cell death in the nematode C. elegans. Cytochrome c activates Apaf-1, which oligomerizes to form an approximately 700-1400-kDa caspase-activating complex known as the Apaf-1 apoptosome. Caspase-9, an initiator caspase, is then recruited to the complex by binding to Apaf-1 through CARD-CARD (caspase recruitment domain) interactions to form a holoenzyme complex. Subsequently, the Apaf-1/caspase-9 holoenzyme complex recruits the effector caspase-3 via an interaction between the active site cysteine in caspase-9 and the critical aspartate, which is the cleavage site for generating the large and small subunits of caspase-3 that constitute the activated form of caspase-3. This initiates the caspase cascade that is responsible for the execution phase of apoptosis. Intracellular levels of K+, XIAP an inhibitor of apoptosis protein, and at least two mitochondrial released proteins, Smac/DIABLO and Omi/Htra 2 a serine protease, tightly regulate formation and function of the apoptosome. Thus, a number of physiological mechanisms ensure that the apoptosome complex is only fully assembled and functional when the cell is irrevocably committed to die. It is interesting that more recent studies show that a variety of small molecules can directly activate or inhibit caspase activation by interfering with the formation and function of the apoptosome complex. The cytotoxicity of many conventional chemotherapeutic drugs rests on their ability to induce apoptosome formation and apoptosis. Defects in this pathway can result in drug resistance, and the discovery that small molecules can directly activate or inhibit the apoptosome may provide new alternative treatments for cancer.

Apollon ubiquitinates SMAC and caspase-9, and has an essential cytoprotection function

Apollon (also known as BRUCE or BIRC6) is a large protein containing baculoviral-IAP-repeat (BIR) and ubiquitin-conjugating enzyme (UBC) domains at the amino- and carboxy termini, respectively. Apollon inhibits apoptosis, but its molecular and physiological function remains unclear. Here we report that Apollon binds to, ubiquitinates and facilitates proteasomal degradation of SMAC and caspase-9, which both contain IAP-binding motifs. Targeted disruption of Apollon in mice caused embryonic and neonatal lethality. Notably, SMAC induced apoptosis in Apollon-deficient cells, but not in Apollon-expressing cells. Furthermore, the IAP-binding motif of SMAC was required to induce apoptosis in Apollon-deficient cells. These results suggest that Apollon has an essential function in preventing SMAC-induced apoptosis.

Direct interaction of Smac with NADE promotes TRAIL-induced apoptosis

Second mitochondria-derived activator of caspase (Smac) has been implicated in the activation of apoptosis in response to cell stress. We screened for Smac/DIABLO-binding protein for further understanding of Smac-mediated apoptosis. We identified NADE, previously known as p75NTR-associated cell death executor, as a Smac-binding protein. Smac-NADE interaction was mapped to the N-terminal region of Samc and the C-terminal region of NADE. Co-expression of NADE and Smac promotes TRAIL-induced apoptosis in MCF-7 cells. Interestingly, the co-presence of Smac and NADE inhibits XIAP-mediated Smac ubiquitination. In conclusion, our results provide the first evidence that the interaction between Smac and NADE regulates apoptosis through the inhibition of Smac ubiquitination.

Smac induces cytochrome c release and apoptosis independently from Bax/Bcl-x(L) in a strictly caspase-3-dependent manner in human carcinoma cells

The mitochondrial apoptosis pathway mediates cell death through the release of various pro-apoptotic factors including cytochrome c and Smac, the second mitochondrial activator of caspases, into the cytosol. Smac was shown previously to inhibit IAP proteins and to facilitate initiation of the caspase cascade upon cytochrome c release. To investigate Smac function during apoptosis and to explore Smac as an experimental cancer therapeutic, we constructed an expression system based on a single adenoviral vector containing Smac under control of the Tet-off system supplied in cis. Conditional expression of Smac induced apoptosis in human HCT116 and DU145 carcinoma cells regardless of the loss of Bax or overexpression of Bcl-x(L). Nevertheless, apoptosis induced by Smac was associated with cytochrome c release and breakdown of the mitochondrial membrane potential. This indicates that Smac acts independently of Bax and Bcl-x(L) during initiation of apoptosis and triggers a positive feedback loop that results in Bax/Bcl-x(L)-independent activation of mitochondria. In caspase-proficient cells, Smac-induced apoptosis could be inhibited partially by cell-permeable LEHD (caspase-9 inhibitor) and DEVD (caspase-3 inhibitor) peptides. Furthermore, loss of caspase-3 expression in MCF-7 cells carrying a caspase-3 null mutation completely abrogated the sensitivity for Smac-induced apoptotic or nonapoptotic, necrosis-like cell death, while re-expression of caspase-3 conferred sensitivity. Altogether, caspase-3 but not caspase-9 activation was necessary for execution of Smac-induced cell death. Notably, Smac did not induce caspase-9 processing in the absence of caspase-3. Thus, caspase-9 processing occurs secondary to caspase-3 activation during Smac-induced apoptosis. Altogether, Smac is capable of circumventing defects in mitochondrial apoptosis signaling such as loss of Bax or overexpression of Bcl-x(L) that are frequently observed in tumor cells resistant to anticancer therapy. Consequently, Smac appears to be a promising therapeutic target in anticancer treatment.

The domains of apoptosis: a genomics perspective

Apoptosis plays important roles in many facets of normal physiology in animal species, including programmed cell death associated with fetal development or metamorphosis, tissue homeostasis, immune cell education, and some aspects of aging. Defects in the regulation of apoptosis contribute to multiple diseases associated with either inappropriate cell loss or pathological cell accumulation. Host-pathogen interactions have additionally provided evolutionary pressure for apoptosis as a defense mechanism against viruses and microbes, sometimes linking apoptosis mechanisms with inflammatory responses. To a large extent, the apoptosis machinery can be viewed as a network, with different nodes connected by physical interactions of evolutionarily conserved domains. These domains can serve as signatures for identification of proteins involved in the network. In particular, the caspase recruitment domains (CARDs); death effector domains (DEDs); death domains (DDs); BIR (baculovirus IAP repeat) domains of inhibitor of apoptosis proteins (IAPs); Bcl-2 family proteins; caspase protease domains; and endonuclease-associated CIDE (cell death-inducing DFF45-like effector) domains are found in common in proteins involved in apoptosis. In the genomes of mammals, genes encoding proteins that carry one or more of these signature domains are often present in multiple copies, making up diverse gene families that permit tissue-specific and highly regulated control of cell life and death decisions through combinations of stimulus-specific gene expression and complex protein interaction networks. In this Review, we organize the repertoire of apoptosis proteins of humans into domain families, drawing comparisons with homologs in other vertebrate and invertebrate animal species, and discuss some of the functional implications of these findings.

Functional evaluation of the role of inhibitor of apoptosis proteins in chronic lymphocytic leukemia

OBJECTIVE

The slow accumulation of malignant cells in chronic lymphocytic leukemia (CLL) suggests a defect in the induction of apoptosis in these cells. Previous studies have found sporadic alterations in the apoptotic apparatus in CLL cells, but a widespread defect has not been detected until now. A crucial checkpoint in the progression of apoptosis is the activity of inhibitor of apoptosis proteins (IAP) that control the activity of caspases upon the release of cytochrome c from mitochondria. The aim of this study was to evaluate the role of IAP in the regulation of apoptosis in CLL cells.

MATERIALS AND METHODS

Lysates from CLL cells were prepared, and the regular function of components of the cytochrome c-dependent caspase-activating machinery (the apoptosome) was investigated. The effect of IAP in caspase-inhibition was tested using a peptide derived from the mitochondrial IAP antagonist Smac/DIABLO. Regulation of expression as well as inhibitory function of the X-linked IAP (XIAP) by cytokines was analyzed.

RESULTS

The apoptosome was found to be structurally and functionally competent in CLL. XIAP expression was enhanced by culture in the presence of cytokines. Smac/DIABLO was easily detectable in CLL cells and was released into the cytosol during apoptosis. No inhibitory effect of IAP was detected in CLL, irrespective of XIAP levels and culture conditions.

CONCLUSION

Although XIAP is present in CLL cells and is up-regulated in conditions where apoptosis is prevented, no caspase-inhibiting anti-apoptotic effect of IAP is detectable. This is likely due to the high expression of Smac/DIABLO in CLL cells that is sufficient to overcome the caspase-inhibiting effect of IAP.

Apoptosis as a novel target for cancer chemoprevention

Cancer chemopreventive agents are typically natural products or their synthetic analogs that inhibit the transformation of normal cells to premalignant cells or the progression of premalignant cells to malignant cells. These agents are believed to function by modulating processes associated with xenobiotic biotransformation, with the protection of cellular elements from oxidative damage, or with the promotion of a more differentiated phenotype in target cells. However, an increasing number of chemopreventive agents (e.g., certain retinoids, nonsteroidal anti-inflammatory drugs, polyphenols, and vanilloids) have been shown to stimulate apoptosis in premalignant and malignant cells in vitro or in vivo. Apoptosis is arguably the most potent defense against cancer because it is the mechanism used by metazoans to eliminate deleterious cells. Many chemopreventive agents appear to target signaling intermediates in apoptosis-inducing pathways. Inherently, the process of carcinogenesis selects against apoptosis to initiate, promote, and perpetuate the malignant phenotype. Thus, targeting apoptosis pathways in premalignant cells--in which these pathways are still relatively intact--may be an effective method of cancer prevention. In this review, we construct a paradigm supporting apoptosis as a novel target for cancer chemoprevention by highlighting recent studies of several chemopreventive agents that engage apoptosis pathways.

Cotreatment with histone deacetylase inhibitor LAQ824 enhances Apo-2L/tumor necrosis factor-related apoptosis inducing ligand-induced death inducing signaling complex activity and apoptosis of human acute leukemia cells

Present studies demonstrate that treatment with the histone deacetylases inhibitor LAQ824, a cinnamic acid hydroxamate, increased the acetylation of histones H3 and H4, as well as induced p21(WAF1) in the human T-cell acute leukemia Jurkat, B lymphoblast SKW 6.4, and acute myelogenous leukemia HL-60 cells. This was associated with increased accumulation of the cells in the G(1) phase of the cell cycle, as well as accompanied by the processing and activity of caspase-9 and -3, and apoptosis. Exposure to LAQ824 increased the mRNA and protein expressions of the death receptors DR5 and/or DR4, but reduced the mRNA and protein levels of cellular FLICE-inhibitory protein (c-FLIP). As compared with treatment with Apo-2L/tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) or LAQ824 alone, pretreatment with LAQ824 increased the assembly of Fas-associated death domain and caspase-8, but not of c-FLIP, into the Apo-2L/TRAIL-induced death-inducing signaling complex. This increased the processing of caspase-8 and Bcl-2 interacting domain (BID), augmented cytosolic accumulation of the prodeath molecules cytochrome-c, Smac and Omi, as well as led to increased activity of caspase-3 and apoptosis. Treatment with LAQ824 also down-regulated the levels of Bcl-2, Bcl-x(L), XIAP, and survivin. Partial inhibition of apoptosis due to LAQ824 or Apo-2L/TRAIL exerted by Bcl-2 overexpression was reversed by cotreatment with LAQ824 and Apo-2L/TRAIL. Significantly, cotreatment with LAQ824 increased Apo-2L/TRAIL-induced apoptosis of primary acute myelogenous leukemia blast samples isolated from 10 patients with acute myelogenous leukemia. Taken together, these findings indicate that LAQ824 may have promising activity in augmenting Apo-2L/TRAIL-induced death-inducing signaling complex and apoptosis of human acute leukemia cells.

Caspase activation - stepping on the gas or releasing the brakes? Lessons from humans and flies

The central components of the execution phase of apoptosis in worms, flies, and humans are members of the caspase protease family. Work in Drosophila and mammalian systems has revealed a web of interactions that govern the activity of these proteases, and two fundamental control points have been identified. These are zymogen activation - the process that converts a latent caspase into its active form, and inhibition of the resulting active protease. In humans, the driving force for caspase activity is activation of the zymogens, but in Drosophila, a major thrust is derepression of caspase inhibitors. In this review, we consider evidence for these two distinct events in terms of the regulation of caspase activity. This sets the scene for therapy to reinstate the normal death mechanisms that have been overcome in a cancer cell's quest for immortality.

Molecular mechanisms of DrICE inhibition by DIAP1 and removal of inhibition by Reaper, Hid and Grim

The Drosophila melanogaster inhibitor of apoptosis protein DIAP1 suppresses apoptosis in part through inhibition of the effector caspase DrICE. The pro-death proteins Reaper, Hid and Grim (RHG) induce apoptosis by antagonizing DIAP1 function. However, the underlying molecular mechanisms remain unknown. Here we demonstrate that DIAP1 directly inhibits the catalytic activity of DrICE through its BIR1 domain and this inhibition is countered effectively by the RHG proteins. Inhibition of DrICE by DIAP1 occurs only after the cleavage of its N-terminal 20 amino acids and involves a conserved surface groove on BIR1. Crystal structures of BIR1 bound to the RHG peptides show that the RHG proteins use their N-terminal IAP-binding motifs to bind to the same surface groove, hence relieving DIAP1-mediated inhibition of DrICE. These studies define novel molecular mechanisms for the inhibition and activation of a representative D. melanogaster effector caspase.

Smac/Diablo antagonizes ubiquitin ligase activity of inhibitor of apoptosis proteins

Inhibitor of apoptosis proteins (IAPs) can block apoptosis through binding to active caspases and antagonizing their function. IAP function can be neutralized by Smac/Diablo, an IAP-binding protein that is released from mitochondria during apoptosis. In addition to their ability to interact with caspases, certain IAPs also display ubiquitin-protein isopeptide ligase activity because of the presence of a RING domain. However, it is not known whether the ubiquitin-protein isopeptide ligase activities of human IAPs contribute to their apoptosis inhibitory activity or whether this IAP property can be modulated through association with Smac/Diablo. Here we demonstrate that the ubiquitin ligase activities of XIAP, and to a lesser extent c-IAP-1 and c-IAP2, are potently repressed through binding to Smac/Diablo. We also show that mutation of the XIAP RING domain rendered this IAP a less effective inhibitor of apoptosis, suggesting that the ubiquitin ligase activity of XIAP contributes to its anti-apoptotic function. These data suggest that Smac/Diablo potentiates apoptosis by simultaneously antagonizing caspase-IAP interactions and repressing IAP ubiquitin ligase activities.

Cisplatin-induced apoptosis in Hep3B cells: mitochondria-dependent and -independent pathways

Human hepatoma cell lines undergo apoptosis after treatment with cisplatin (CP), by mechanisms that are not fully understood, although our previous study demonstrated that Fas-dependent or -independent pathways are involved. To elucidate the mechanisms of CP-induced apoptosis in Hep3B cells, which are Fas- and p53-negative, we investigated mitochondria associated pathways, the involvement of NF-kappaB, and p73 activation. Results of Western blot and flow cytometry assay revealed that the translocation of Bax, resulted in the loss of mitochondrial membrane potential (Deltaphi(m)) and the efflux of cytochrome c and of second mitochondria-derived activator of caspase/DIABLO from mitochondria into the cytosol. Caspase-3, -8 and -9 were activated by CP treatment, however, CP-induced apoptosis was not completely blocked by pretreating with the pan-caspase inhibitor, benzyloxycarbonyl-valinyl-alaninyl-aspartyl-(O-methyl)-fluoromethylketone, indicating that caspase-independent apoptotic pathways might also be involved. RNase protection assay confirmed that NF-kappaB downregulation leading to the suppression of its target genes, such as XIAP and TRAF2, and p73 accumulation were also observed in Hep3B cells treated with CP. CP-induced apoptosis was inhibited to some extent by transiently overexpressed p73 dominant negative and XIAP, but not by p73DN or XIAP alone. In conclusion, this study demonstrates that CP-induced apoptosis in Hep3B cells is associated with mitochondrial dysregulation, NF-kappaB downregulation and p73 accumulation.

IAP-antagonists exhibit non-redundant modes of action through differential DIAP1 binding

The Drosophila inhibitor of apoptosis protein DIAP1 ensures cell viability by directly inhibiting caspases. In cells destined to die this IAP-mediated inhibition of caspases is overcome by IAP-antagonists. Genetic evidence indicates that IAP-antagonists are non-equivalent and function synergistically to promote apoptosis. Here we provide biochemical evidence for the non-equivalent mode of action of Reaper, Grim, Hid and Jafrac2. We find that these IAP-antagonists display differential and selective binding to specific DIAP1 BIR domains. Consistently, we show that each DIAP1 BIR region associates with distinct caspases. The differential DIAP1 BIR interaction seen both between initiator and effector caspases and within IAP-antagonist family members suggests that different IAP-antagonists inhibit distinct caspases from interacting with DIAP1. Surprisingly, we also find that the caspase-binding residues of XIAP predicted to be strictly conserved in caspase-binding IAPs, are absent in DIAP1. In contrast to XIAP, residues C-terminal to the DIAP1 BIR1 domain are indispensable for caspase association. Our studies on DIAP1 and caspases expose significant differences between DIAP1 and XIAP suggesting that DIAP1 and XIAP inhibit caspases in different ways.

Expression of Smac/DIABLO in mouse preimplantation embryos and its correlation to apoptosis and fragmentation

Regulation of early embryonal development during fertilization and implantation is crucial for mammalian reproduction. Several studies have described cell death during preimplantation embryogenesis in a range of mammalian species, both in vivo and in vitro. Therefore, apoptosis may be involved in early embryonic arrest and the characteristic cytoplasmic fragments are the equivalents of apoptotic bodies, the end-product of apoptosis. Although apoptosis is expected to associate with fragmentation in early preimplantation embryos, the mechanism through which this fragmentation occurs has not been elucidated. Recently, second mitochondria-derived activator of caspase/Direct IAP Binding Protein with Low pI (Smac/DIABLO) was identified as a mitochondrial protein that is released into the cytosol during apoptosis. Once released, the Smac/DIABLO blocks the anti-apoptotic activity of inhibitor of apoptosis proteins (IAPs). We hypothesized that the Smac/DIABLO may be involved in the fragmentation of mouse preimplantation embryos. Therefore, we investigated the expression of Smac/DIABLO mRNA and protein and its localization in mouse oocytes and preimplantation embryos. Smac/DIABLO mRNA was detected by RT-PCR in the oocytes and the preimplantation embryos. Immunohistochemistry studies showed that the Smac/DIABLO protein localized in mitochondria and was released into the cytosol in both fragmented embryos and embryos in which apoptosis was induced by staurosporine. These observations indicate that the Smac/DIABLO is involved in the fragmentation and apoptosis of preimplantation embryos.

Bifunctional killing activity encoded by conserved reaper proteins

Drosophila activators of apoptosis mapping to the Reaper region function, in part, by antagonizing IAP proteins through a shared RHG motif. We isolated Reaper from the Blowfly L. cuprina, which triggered extensive apoptosis in Drosophila cells. Conserved regions of Reaper were tested in the context of GFP fusions and a second killing activity, distinct from the RHG, was identified. A 20 amino-acid peptide, designated R3, conferred targeting to a focal compartment and promoted membrane blebbing. Killing by the R3 fragment did not correlate with translational suppression or with reduced DIAP1 levels. Likewise, R3-induced cell deaths were only modestly suppressed by silencing of Dronc and involved no detectable association with DIAP1. Instead, a second IAP-binding domain, distinct from the R3, was identified at the C terminus of Reaper that bound to DIAP1 but failed to trigger apoptosis. Collectively, these findings are inconsistent with single effector models for cell killing by Reaper and suggest, instead, that Reaper encodes conserved bifunctional death activities that propagate through distinct effector pathways.

Cell death: critical control points

Programmed cell death is a distinct genetic and biochemical pathway essential to metazoans. An intact death pathway is required for successful embryonic development and the maintenance of normal tissue homeostasis. Apoptosis has proven to be tightly interwoven with other essential cell pathways. The identification of critical control points in the cell death pathway has yielded fundamental insights for basic biology, as well as provided rational targets for new therapeutics.

Smac/DIABLO selectively reduces the levels of c-IAP1 and c-IAP2 but not that of XIAP and livin in HeLa cells

The inhibitor of apoptosis (IAP) proteins bind and inhibit caspases via their baculovirus IAP repeat domains. Some of these IAPs are capable of ubiquitinating themselves and their interacting proteins through the ubiquitin-protein isopeptide ligase activity of their RING domain. The Drosophila IAP antagonists Reaper, Hid, and Grim can accelerate the degradation of Drosophila IAP1 and some mammalian IAPs by promoting their ubiquitin-protein isopeptide ligase activity. Here we show that Smac/DIABLO, a mammalian functional homolog of Reaper/Hid/Grim, selectively causes the rapid degradation of c-IAP1 and c-IAP2 but not XIAP and Livin in HeLa cells, although it efficiently promotes the auto-ubiquitination of them all. Smac binding to c-IAP via its N-terminal IAP-binding motif is the prerequisite for this effect, which is further supported by the findings that Smac N-terminal peptide is sufficient to enhance c-IAP1 ubiquitination, and Smac no longer promotes the ubiquitination of mutant c-IAP1 lacking all three baculovirus IAP repeat domains. In addition, different IAPs require the same ubiquitin-conjugating enzymes UbcH5a and UbcH6 for their ubiquitination. Taken together, Smac may serve as a key molecule in vivo to selectively reduce the protein level of c-IAPs through the ubiquitin/proteasome pathway.