Structural and biochemical basis of apoptotic activation by Smac/DIABLO

SM-164: a novel, bivalent Smac mimetic that induces apoptosis and tumor regression by concurrent removal of the blockade of cIAP-1/2 and XIAP

Small-molecule Smac mimetics are being developed as a novel class of anticancer drugs. Recent studies have shown that Smac mimetics target cellular inhibitor of apoptosis protein (cIAP)-1/2 for degradation and induce tumor necrosis factor-alpha (TNFalpha)-dependent apoptosis in tumor cells. In this study, we have investigated the mechanism of action and therapeutic potential of two different types of novel Smac mimetics, monovalent SM-122 and bivalent SM-164. Our data showed that removal of cIAP-1/2 by Smac mimetics or small interfering RNA is not sufficient for robust TNFalpha-dependent apoptosis induction, and X-linked inhibitor of apoptosis protein (XIAP) plays a critical role in inhibiting apoptosis induction. Although SM-164 is modestly more effective than SM-122 in induction of cIAP-1/2 degradation, SM-164 is 1,000 times more potent than SM-122 as an inducer of apoptosis in tumor cells, which is attributed to its much higher potency in binding to and antagonizing XIAP. SM-164 induces rapid cIAP-1 degradation and strong apoptosis in the MDA-MB-231 xenograft tumor tissues and achieves tumor regression, but has no toxicity in normal mouse tissues. Our study provides further insights into the mechanism of action for Smac mimetics and regulation of apoptosis by inhibitor of apoptosis proteins. Furthermore, our data provide evidence that SM-164 is a promising new anticancer drug for further evaluation and development.

IAP-targeted therapies for cancer

DNA damage, chromosomal abnormalities, oncogene activation, viral infection, substrate detachment and hypoxia can all trigger apoptosis in normal cells. However, cancer cells acquire mutations that allow them to survive these threats that are part and parcel of the transformation process or that may affect the growth and dissemination of the tumor. Eventually, cancer cells accumulate further mutations that make them resistant to apoptosis mediated by standard cytotoxic chemotherapy or radiotherapy. The inhibitor of apoptosis (IAP) family members, defined by the presence of a baculovirus IAP repeat (BIR) protein domain, are key regulators of cytokinesis, apoptosis and signal transduction. Specific IAPs regulate either cell division, caspase activity or survival pathways mediated through binding to their BIR domains, and/or through their ubiquitin-ligase RING domain activity. These protein-protein interactions and post-translational modifications are the subject of intense investigations that shed light on how these proteins contribute to oncogenesis and resistance to therapy. In the past several years, we have seen multiple approaches of IAP antagonism enter the clinic, and the rewards of such strategies are about to reap benefit. Significantly, small molecule pan-IAP antagonists that mimic an endogenous inhibitor of the IAPs, called Smac, have demonstrated an unexpected ability to sensitize cancer cells to tumor necrosis factor-alpha and to promote autocrine or paracrine production of this cytokine by the tumor cell and possibly, other cells too. This review will focus on these and other developmental therapeutics that target the IAPs in cancer.

Design of small-molecule peptidic and nonpeptidic Smac mimetics

Smac/DIABLO is a protein released from mitochondria into the cytosol in response to apoptotic stimuli. Smac promotes apoptosis at least in part through antagonizing inhibitor of apoptosis proteins (IAPs), including XIAP, cIAP-1, and cIAP-2. Smac interacts with these IAPs via its N-terminal AVPI binding motif. There has been an enormous interest in academic laboratories and pharmaceutical companies in the design of small-molecule Smac mimetics as potential anticancer agents. This task is particularly challenging because it involves targeting protein-protein interactions. Nevertheless, intense research has now generated potent, specific, cell-permeable small-molecule peptidomimetics and nonpeptidic mimetics. To date, two types of Smac mimetics have been reported, namely, monovalent and bivalent Smac mimetics. The monovalent compounds are designed to mimic the binding of a single AVPI binding motif to IAP proteins, whereas the bivalent compounds contain two AVPI binding motif mimetics tethered together through a linker. Studies from several groups have clearly demonstrated that both monovalent and bivalent Smac mimetics not only enhance the antitumor activity of other anticancer agents but also can induce apoptosis as single agents in a subset of human cancer cell lines in vitro and are capable of achieving tumor regression in animal models of human cancer. In general, bivalent Smac mimetics are 100-1000 times more potent than their corresponding monovalent Smac mimetics in induction of apoptosis in tumor cells. However, properly designed monovalent Smac mimetics can achieve oral bioavailability and may have major advantages over bivalent Smac mimetics as potential drug candidates. In-depth insights on the molecular mechanism of action of Smac mimetics have been provided by several independent studies. It was shown that Smac mimetics induce apoptosis in tumor cells by targeting cIAP-1/-2 for the rapid degradation of these proteins, which leads to activation of nuclear factor kappaB (NF-kappaB) and production and secretion of tumor necrosis factor alpha (TNFalpha). TNFalpha promotes formation of a receptor-interacting serine-threonine kinase 1 (RIPK1)-dependent caspase-8-activating complex, leading to activation of caspase-8 and -3/-7 and ultimately to apoptosis. For the most efficient apoptosis induction, Smac mimetics also need to remove the inhibition of XIAP to caspase-3/-7. Hence, Smac mimetics induce apoptosis in tumor cells by targeting not only cIAP-1/-2 but also XIAP. The employment of potent, cell-permeable, small-molecule Smac mimetics has yielded important insights into the regulation of apoptosis by IAP proteins. To date, at least one Smac mimetic has been advanced into clinical development. Several other Smac mimetics are in an advanced preclinical development stage and are expected to enter human clinical testing for the treatment of cancer in the near future.

Role of Smac/DIABLO in cancer progression

Second mitochondria-derived activator of caspase/direct inhibitor of apoptosis-binding protein with low pI (Smac/DIABLO) is a proapoptogenic mitochondrial protein that is released to the cytosol in response to diverse apoptotic stimuli, including commonly used chemotherapeutic drugs. In the cytosol, Smac/DIABLO interacts and antagonizes inhibitors of apoptosis proteins (IAPs), thus allowing the activation of caspases and apoptosis. This activity has prompted the synthesis of peptidomimetics that could potentially be used in cancer therapy. For these reasons, several authors have analyzed the expression levels of Smac/DIABLO in samples of patients from different tumors. Although dissimilar results have been found, a tissue-specific role of this protein emerges from the data. The objective of this review is to present the current knowledge of the Smac/DIABLO role in cancer and its possible use as a marker or therapeutic target for drug design.

Hepatocellular carcinoma and the ubiquitin-proteasome system

Hepatocellular carcinoma is one of the largest causes of cancer-related deaths worldwide for which there are very limited treatment options that are currently effective. The ubiquitin-proteasome system has rapidly become acknowledged as both critical for normal cellular function and a frequent target of de-regulation leading to disease. This review appraises the evidence linking the ubiquitin-proteasome system with this devastatingly intractable cancer and asks whether it may prove to be fertile ground for the development of novel therapeutic interventions against hepatocellular carcinoma.

EGF Mediates Survival of Rat Cochlear Sensory Cells via an NF-κB Dependent Mechanism In Vitro

The survival of cochlear epithelial cells is of considerable importance, biologically. However, little is known about the growth factor(s) that are involved in the survival of cochlear sensory epithelial cells. In this study, we demonstrated that epidermal growth factor (EGF) plays a role in the survival of cochlear epithelial cells. Firstly, the presence of the EGF signaling pathway was demonstrated in the developing cochlear tissues of rats and a sensory epithelial cell line (OC1): -- epidermal growth factor receptor (EGFR), mitogen-activated protein kinase kinase (MAPKK), I kappa B alpha (IκBα), nuclear factor kappa B (NF-κB), and B cell lymphoma 2 (Bcl-2). Secondly, the addition of EGF to OC1 increased the promoter activity of NF-κB and cell viability but not cell cycle progression and cell number increase -- which suggests that EGF is for cellular survival rather than cell proliferation of OC1. Finally, pyrrolidine dithiocarbamate (PDTC, an inhibitor of NF-κB) and inhibitor kappa B alpha (IκBα) mutant (IκBαM, a specific inhibitor of NF-κB) abrogated the EGF-induced NF-κB activity and cell survival. These data suggest that EGF plays a role in the survival of cochlear sensory epithelial cells through the EGFR/MAPKK/IκBα/NF-κB/Bcl-2 pathway.

TNF-alpha induces two distinct caspase-8 activation pathways

The inflammatory response of mammalian cells to TNF-alpha can be switched to apoptosis either by cotreatment with a protein synthesis inhibitor, cycloheximide, or Smac mimetic, a small molecule mimic of Smac/Diablo protein. Cycloheximide promotes caspase-8 activation by eliminating endogenous caspase-8 inhibitor, c-FLIP, while Smac mimetic does so by triggering autodegradation of cIAP1 and cIAP2 (cIAP1/2), leading to the release of receptor interacting protein kinase (RIPK1) from the activated TNF receptor complex to form a caspase-8-activating complex consisting of RIPK1, FADD, and caspase-8. This process also requires the action of CYLD, a RIPK1 K63 deubiquitinating enzyme. RIPK1 is critical for caspase-8 activation-induced by Smac mimetic but dispensable for that triggered by cycloheximide. Moreover, Smac mimetic-induced caspase-8 activation is not blocked by endogenous c-FLIP. These findings revealed that TNF-alpha is able to induce apoptosis via two distinct caspase-8 activation pathways that are differentially regulated by cIAP1/2 and c-FLIP.

The interaction of DIAP1 with dOmi/HtrA2 regulates cell death in Drosophila

Mitochondrial proteins such as cytochrome c, Smac/DIABLO and Omi/HtrA2 play important roles in the cell death pathways of mammalian cells. In Drosophila, the role of mitochondria in cell death is less clear. Here, we report the identification and characterization of the Drosophila ortholog of human Omi/HtrA2. We show that Drosophila Omi/HtrA2 is imported into the mitochondria where it undergoes proteolytic maturation to yield two isoforms, dOmi-L and dOmi-S. dOmi-L contains a canonical N-terminal IAP-binding motif (AVVS), whereas dOmi-S contains a distinct N-terminal motif (SKMT). DIAP1 was able to bind to both isoforms via its BIR1 and BIR2 domains. This resulted in cleavage of the linker region of DIAP1 between the BIR1 and BIR2 domains and further degradation of the BIR1 domain by the proteolytic activity of dOmi. The binding of DIAP1 to dOmi also resulted in DIAP1-mediated polyubiquitination of dOmi, suggesting that DIAP1 could target dOmi for proteasomal degradation. Consistent with this, expression of DIAP1 in Drosophila eye discs protected them from dOmi-induced eye ablation, indicating that DIAP1 plays an important role in protecting cells from the potentially lethal effects of dOmi. The ability of IAPs to bind to and ubiquitinate mitochondrial proteins such as dOmi may be a key conserved function to counterbalance the lethal effects of these proteins if accidentally released into the cytosol.

Angiopoietin-1 inhibits intrinsic apoptotic signaling and vascular hyperpermeability following hemorrhagic shock

Studies from our laboratory demonstrated the involvement of intrinsic apoptotic signaling in hyperpermeability following hemorrhagic shock (HS). Angiopoietin 1 (Ang-1), a potent inhibitor of hyperpermeability, was recently shown to inhibit apoptosis. The purpose of our study was to determine the effectiveness of Ang-1 in attenuating HS-induced hyperpermeability and its relationship to apoptotic signaling. HS was induced in rats by withdrawing blood to reduce the mean arterial pressure to 40 mmHg for 1 h, followed by reperfusion. Mesenteric postcapillary venules were examined for changes in hyperpermeability by intravital microscopy. Mitochondrial release of second mitochondrial derived activator of caspases (smac) and cytochrome c were determined by Western blot and ELISA, respectively. Caspase-3 activity was determined by fluorometric assay. Parallel studies were performed in rat lung microvascular endothelial cell (RLMEC) monolayers, utilizing HS serum and the proapoptotic Bcl-2 homologous antagonist/killer [BAK (BH3)] peptide as inducers of hyperpermeability. In rats, Ang-1 (200 ng/ml) attenuated HS-induced hyperpermeability versus the HS group ( P < 0.05). Ang-1 prevented HS-induced collapse of mitochondrial transmembrane potential (ΔΨm), smac and cytochrome c release, and caspase-3 activity ( P < 0.05). In RLMEC monolayers, HS serum and BAK (BH3) peptide both induced hyperpermeability that was inhibited by Ang-1 ( P < 0.05). Ang-1 attenuated HS and BAK (BH3) peptide-induced collapse of ΔΨm, smac release, cytochrome c release, activation of caspase-3, and vascular hyperpermeability. In vivo, BAK (BH3) induced vascular hyperpermeability that was attenuated by Ang-1 ( P < 0.05). These findings suggest that Ang-1's role in maintaining microvascular endothelial barrier integrity involves the intrinsic apoptotic signaling cascade.

Neurotoxic consequences of chronic alcohol withdrawal: expression profiling reveals importance of gender over withdrawal severity

While women are more vulnerable than men to many of the medical consequences of alcohol abuse, the role of sex in the response to ethanol is controversial. Neuroadaptive responses that result in the hyperexcitability associated with withdrawal from chronic ethanol likely reflect gene expression changes. We have examined both genders for the effects of withdrawal on brain gene expression using mice with divergent withdrawal severity that have been selectively bred from a genetically heterogeneous population. A total of 295 genes were identified as ethanol regulated from each gender of each selected line by microarray analyses. Hierarchical cluster analysis of the arrays revealed that the transcriptional response correlated with sex rather than with the selected withdrawal phenotype. Consistent with this, gene ontology category over-representation analysis identified cell death and DNA/RNA binding as targeted classes of genes in females, while in males, protein degradation, and calcium ion binding pathways were more altered by alcohol. Examination of ethanol-regulated genes and these distinct signaling pathways suggested enhanced neurotoxicity in females. Histopathological analysis of brain damage following ethanol withdrawal confirmed elevated cell death in female but not male mice. The sexually dimorphic response was observed irrespective of withdrawal phenotype. Combined, these results indicate a fundamentally distinct neuroadaptive response in females compared to males during chronic ethanol withdrawal and are consistent with observations that female alcoholics may be more vulnerable than males to ethanol-induced brain damage associated with alcohol abuse.

Inhibition of fibroblast growth factor 2-induced apoptosis involves survivin expression, protein kinase C alpha activation and subcellular translocation of Smac in human small cell lung cancer cells

To investigate the mechanism by which fibroblast growth factor 2 (FGF-2) inhibits apoptosis in the human small cell lung cancer cell line H446 subjected to serum starvation, apoptosis was evaluated by flow cytometry, Hoechst 33258 staining, caspase-3 activity, and DNA fragmentation. Survivin expression induced by FGF-2 and protein kinase C alpha (PKC alpha) translocation was detected by subcellular fractionation and Western blot analysis. In addition, FGF-2-induced release of Smac from mitochondria to the cytoplasm was analyzed by Western blotting and immunofluorescence. FGF-2 reduced apoptosis induced by serum starvation and up-regulated survivin expression in H446 cells in a dose-dependent and time-dependent manner, and inhibited caspase-3 activity. FGF-2 also inhibited the release of Smac from mitochondria to the cytoplasm induced by serum starvation and increased PKC alpha translocation from the cytoplasm to the cell membrane. In addition, PKC inhibitor inhibited the expression of survivin. FGF-2 up-regulates the expression of survivin protein in H446 cells and blocks the release of Smac from mitochondria to the cytoplasm. PKC alpha regulated FGF-2-induced survivin expression. Thus, survivin, Smac, and PKC alpha might play important roles in the inhibition of apoptosis by FGF-2 in human small cell lung cancer cells.

Detecting mitochondrial RNA and other cellular events in living cells

Intracellular signaling can be monitored in vivo in living cells by genetically encoded intracellular fluorescent probes. In this review, three aspects of these probes are introduced: 1) the imaging dynamics of endogenous mitochondrial RNA; 2) nuclear receptor and coactivator/corepressor interactions, and; 3) the signal sequence in mitochondrial intermembrane space. These probes are generally applicable to fundamental biological studies as well as for assaying and screening possible pharmaceutical or toxic chemicals that facilitate or inhibit cellular signaling pathways.

Apoptosis signaling pathways and lymphocyte homeostasis

It has been almost three decades since the term "apoptosis" was first coined to describe a unique form of cell death that involves orderly, gene-dependent cell disintegration. It is now well accepted that apoptosis is an essential life process for metazoan animals and is critical for the formation and function of tissues and organs. In the adult mammalian body, apoptosis is especially important for proper functioning of the immune system. In recent years, along with the rapid advancement of molecular and cellular biology, great progress has been made in understanding the mechanisms leading to apoptosis. It is generally accepted that there are two major pathways of apoptotic cell death induction: extrinsic signaling through death receptors that leads to the formation of the death-inducing signaling complex (DISC), and intrinsic signaling mainly through mitochondria which leads to the formation of the apoptosome. Formation of the DISC or apoptosome, respectively, activates initiator and common effector caspases that execute the apoptosis process. In the immune system, both pathways operate; however, it is not known whether they are sufficient to maintain lymphocyte homeostasis. Recently, new apoptotic mechanisms including caspase-independent pathways and granzyme-initiated pathways have been shown to exist in lymphocytes. This review will summarize our understanding of the mechanisms that control the homeostasis of various lymphocyte populations.

Targeting tumors with peptides from natural sources

Peptide-based therapies offer the potential for non-genotoxic, genotype-specific alternatives, or adjuvants, to the current range of traditional cancer treatments. Such a patient-tailored cancer-cell-directed therapeutic approach should have fewer side effects and could well be more effective than the current drug- or combination-based regimens. Here, we review the potential of novel natural anticancer peptides such as necrotic peptides, apoptotic peptides, function-blocking peptides, antiangiogenic peptides and immunostimulatory peptides in the context of their ability to induce tumor regression. We focus on the therapeutic prospects of anticancer peptides and their possible application in tumor therapy.

IAP antagonists target cIAP1 to induce TNFalpha-dependent apoptosis

XIAP prevents apoptosis by binding to and inhibiting caspases, and this inhibition can be relieved by IAP antagonists, such as Smac/DIABLO. IAP antagonist compounds (IACs) have therefore been designed to inhibit XIAP to kill tumor cells. Because XIAP inhibits postmitochondrial caspases, caspase 8 inhibitors should not block killing by IACs. Instead, we show that apoptosis caused by an IAC is blocked by the caspase 8 inhibitor crmA and that IAP antagonists activate NF-kappaB signaling via inhibtion of cIAP1. In sensitive tumor lines, IAP antagonist induced NF-kappaB-stimulated production of TNFalpha that killed cells in an autocrine fashion. Inhibition of NF-kappaB reduced TNFalpha production, and blocking NF-kappaB activation or TNFalpha allowed tumor cells to survive IAC-induced apoptosis. Cells treated with an IAC, or those in which cIAP1 was deleted, became sensitive to apoptosis induced by exogenous TNFalpha, suggesting novel uses of these compounds in treating cancer.

SMAC mimetics sensitize nonsteroidal anti-inflammatory drug-induced apoptosis by promoting caspase-3-mediated cytochrome c release

Nonsteroidal anti-inflammatory drugs (NSAID) are effective in suppressing the formation of colorectal tumors. However, the mechanisms underlying the antineoplastic effects of NSAIDs remain unclear. The effects of NSAIDs are incomplete, and resistance to NSAIDs is often developed. Growing evidence has indicated that the chemopreventive activity of NSAIDs is mediated by induction of apoptosis. Our previous studies showed that second mitochondria-derived activator of caspase (SMAC)/Diablo, a mitochondrial apoptogenic protein, plays an essential role in NSAID-induced apoptosis in colon cancer cells. In this study, we found that SMAC mediates NSAID-induced apoptosis through a feedback amplification mechanism involving interactions with inhibitor of apoptosis proteins, activation of caspase-3, and induction of cytosolic release of cytochrome c. Small-molecule SMAC mimetics at nanomolar concentrations significantly sensitize colon cancer cells to NSAID-induced apoptosis by promoting caspase-3 activation and cytochrome c release. Furthermore, SMAC mimetics overcome NSAID resistance in Bax-deficient or SMAC-deficient colon cancer cells by restoring caspase-3 activation and cytochrome c release. Together, these results suggest that SMAC is useful as a target for the development of more effective chemopreventive strategies and agents.

Programmed cell death and new discoveries in the genetics of parkinsonism

The concept that activation of cellular pathways of programmed cell death (PCD) may lead to the death of neurons has been an important hypothesis for adult neurodegenerative diseases. For Parkinson's disease (PD), up until now, the evidence for this hypothesis has largely been of two types: clear evidence of a role for PCD in neurotoxin models of the disease, and somewhat controversial evidence from human postmortem studies. With the rapid pace of discoveries in recent years of the genetic basis of PD, a new form of evidence has emerged. The prevailing concept of the role for PCD in PD has been that its mediators are 'downstream' effectors of more proximate and specific causes related to genetic or environmental factors. However, recent studies of three genes which cause autosomal recessive forms of parkinsonism, parkin, PTEN-induced kinase, and DJ-1, suggest that they may have more intimate relationships with the mediators of PCD and that loss-of-function mutations may result in an increased propensity for neurons to die. Intriguingly, independent studies of the function of these genes have suggested that they may share roles in regulating survival signaling pathways, such as those mediated by the survival signaling kinase Akt. Further elucidation of these relationships will have implications for the pathogenesis and neuroprotective treatment of PD.

Resveratrol imparts photoprotection of normal cells and enhances the efficacy of radiation therapy in cancer cells

Solar radiation spans a whole range of electromagnetic spectrum including UV radiation, which are potentially harmful to normal cells as well as ionizing radiations which are therapeutically beneficial towards the killing of cancer cells. UV radiation is an established cause of a majority of skin cancers as well as precancerous conditions such as actinic keratosis. However, despite efforts to educate people about the use of sunscreens and protective clothing as preventive strategies, the incidence of skin cancer and other skin-related disorders are on the rise. This has generated an enormous interest towards finding alternative approaches for management of UV-mediated damages. Chemoprevention via nontoxic agents, especially botanical antioxidants, is one such approach that is being considered as a plausible strategy for prevention of photodamages including photocarcinogenesis. In this review, we have discussed the photoprotective effects of resveratrol, an antioxidant found in grapes and red wine, against UVB exposure-mediated damages in vitro and in vivo. In addition, we have also discussed studies showing that resveratrol can act as a sensitizer to enhance the therapeutic effects of ionizing radiation against cancer cells. Based on available literature, we suggest that resveratrol may be useful for (1) prevention of UVB-mediated damages including skin cancer and (2) enhancing the response of radiation therapies against hyperproliferative, precancerous and neoplastic conditions.

Modulation of cellular apoptosis with apoptotic protease-activating factor 1 (Apaf-1) inhibitors

The programmed cell death or apoptosis plays both physiological and pathological roles in biology. Anomalous activation of apoptosis has been associated with malignancies. The intrinsic mitochondrial pathway of apoptosis activation occurs through a multiprotein complex named the apoptosome. We have discovered molecules that bind to a central protein component of the apoptosome, Apaf-1, and inhibits its activity. These new first-in-class apoptosome inhibitors have been further improved by modifications directed to enhance their cellular penetration to yield compounds that decrease cell death, both in cellular models of apoptosis and in neonatal rat cardiomyocytes under hypoxic conditions.

Molecular cloning and characterization of an inhibitor of apoptosis protein (IAP) from the tiger shrimp, Penaeus monodon

The inhibitor of apoptosis proteins (IAPs) play important roles in both apoptosis and innate immunity. Here, we report the first cloning and characterization of a novel IAP family member, PmIAP, from Penaeus monodon. The full-length PmIAP cDNA is 4769bp, with an ORF encoding a protein of 698 amino acids. The PmIAP protein contains three BIR domains and a C-terminal RING domain, and its mRNA was expressed in all analyzed tissues. In insect cells, PmIAP, together with Spodoptera frugiperda IAP, AcMNPV P35, and WSSV449 (or ORF390, an anti-apoptosis protein encoded by white spot syndrome virus), could all block the apoptosis induced by Drosophila Reaper protein (Rpr), whereas only P35 and WSSV449 could block the apoptosis induced by actinomycin D. Co-immunoprecipitation showed that PmIAP physically interacted with Rpr, and in an immunofluorescent analysis the two proteins produced co-localized punctate signals in the cytoplasm. Deletion analysis revealed that both the BIR2 and BIR3 domains of PmIAP could independently bind to and inhibit Rpr, whereas the BIR1 domain could not. These results strongly suggest that PmIAP blocks Rpr's pro-apoptotic activity through mechanisms that are evolutionarily conserved across crustaceans, insects, and mammals.

Optical probes for molecular processes in live cells

In this review, I summarize the development over the past several years of fluorescent and/or bioluminescent indicators to pinpoint cellular processes in living cells. These processes involve second messengers, protein phosphorylations, protein-protein interactions, protein-ligand interactions, nuclear receptor-coregulator interactions, nucleocytoplasmic trafficking of functional proteins, and protein localization.

The inhibitor of apoptosis proteins as therapeutic targets in cancer

Apoptosis is a cell suicide process with a major role in development and homeostasis in vertebrates and invertebrates. Inhibition of apoptosis enhances the survival of cancer cells and facilitates their escape from immune surveillance and cytotoxic therapies. Among the principal molecules contributing to this phenomenon are the inhibitor of apoptosis (IAP) proteins, a family of antiapoptotic regulators that block cell death in response to diverse stimuli through interactions with inducers and effectors of apoptosis. IAP proteins are expressed in the majority of human malignancies at elevated levels and play an active role in promoting tumor maintenance through the inhibition of cellular death and participation in signaling pathways associated with malignancies. Here, we discuss the role of IAP proteins in cancer and options for targeting IAP proteins for therapeutic intervention.

Design, synthesis, and characterization of a potent, nonpeptide, cell-permeable, bivalent Smac mimetic that concurrently targets both the BIR2 and BIR3 domains in XIAP

XIAP is a central apoptosis regulator that inhibits apoptosis by binding to and inhibiting the effectors caspase-3/-7 and an initiator caspase-9 through its BIR2 and BIR3 domains, respectively. Smac protein in its dimeric form effectively antagonizes XIAP by concurrently targeting both its BIR2 and BIR3 domains. We report the design, synthesis, and characterization of a nonpeptide, cell-permeable, bivalent small-molecule (SM-164) which mimics Smac protein for targeting XIAP. Our study shows that SM-164 binds to XIAP containing both BIR domains with an IC50 value of 1.39 nM, being 300 and 7000 times more potent than its monovalent counterparts and the natural Smac AVPI peptide, respectively. SM-164 concurrently interacts with both BIR domains in XIAP and functions as an ultrapotent antagonist of XIAP in both cell-free functional and cell-based assays. SM-164 targets cellular XIAP and effectively induces apoptosis at concentrations as low as 1 nM in the HL-60 leukemia cell line. The potency of bivalent SM-164 in binding, functional, and cellular assays is 2-3 orders of magnitude higher than its corresponding monovalent Smac mimetics.

Enhanced sensitivity of hepatocellular carcinoma cells to chemotherapy with a Smac-armed oncolytic adenovirus

AIM

The aim of the present study was to further improve the therapeutic effects for human hepatocellular carcinoma (HCC) and reduce the damage in normal cells using a novel chemo-gene-virotherapeutic strategy.

METHODS

An oncolytic adenoviral vector (ZD55) similar to the typical oncolytic adenovirus ONYX-015, with a deletion of E1B-55K gene, was employed to express the second mitochondria-derived activator of caspases (Smac) protein by constructing a recombinant virus ZD55-Smac. The enhanced cytotoxicity of the combined treatment of ZD55-Smac with cisplatin or 5-fluorouracil (5-FU) was evaluated in several HCC cell lines. Moreover, the negative effects on normal cells have been tested in human normal liver cell lines L-02 and QSG-7701 cell lines by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and apoptotic cell staining.

RESULTS

According to our observation, ZD55-Smac is superior to ONYX-015 in sensitizing chemotherapy, ZD55-Smac used in conjunction with chemotherapy was found to exhibit obviously enhanced cytotoxicity in HCC cells, yet significantly abolished the negative toxicity in normal cells by utilizing the tumor selective replication vector and reducing the dosage.

CONCLUSION

This chemo-gene-virotherapeutic (cisplatin or 5-FU+ZD55-Smac) strategy is superior to the conventional chemo-gene or chemo-viro approach.

Autocrine TNFalpha signaling renders human cancer cells susceptible to Smac-mimetic-induced apoptosis

A small-molecule mimetic of Smac/Diablo that specifically counters the apoptosis-inhibiting activity of IAP proteins has been shown to enhance apoptosis induced by cell surface death receptors as well as chemotherapeutic drugs. Survey of a panel of 50 human non-small-cell lung cancer cell lines has revealed, surprisingly, that roughly one-quarter of these lines are sensitive to the treatment of Smac mimetic alone, suggesting that an apoptotic signal has been turned on in these cells and is held in check by IAP proteins. This signal has now been identified as the autocrine-secreted cytokine tumor necrosis factor alpha (TNFalpha). In response to autocrine TNFalpha signaling, the Smac mimetic promotes formation of a RIPK1-dependent caspase-8-activating complex, leading to apoptosis.

Apoptosis-inducing factor is a target for ubiquitination through interaction with XIAP

X-linked inhibitor of apoptosis (XIAP) is an inhibitor of apoptotic cell death that protects cells by caspase-dependent and independent mechanisms. In a screen for molecules that participate with XIAP in regulating cellular activities, we identified apoptosis-inducing factor (AIF) as an XIAP binding protein. Baculoviral IAP repeat 2 of XIAP is sufficient for the XIAP/AIF interaction, which is disrupted by Smac/DIABLO. In healthy cells, mature human AIF lacks only the first 54 amino acids, differing significantly from the apoptotic form, which lacks the first 102 amino-terminal residues. Fluorescence complementation and immunoprecipitation experiments revealed that XIAP interacts with both AIF forms. AIF was found to be a target of XIAP-mediated ubiquitination under both normal and apoptotic conditions, and an E3 ubiquitin ligase-deficient XIAP variant displayed a more robust interaction with AIF. Expression of either XIAP or AIF attenuated both basal and antimycin A-stimulated levels of reactive oxygen species (ROS), and when XIAP and AIF were expressed in combination, a cumulative decrease in ROS was observed. These results identify AIF as a new XIAP binding partner and indicate a role for XIAP in regulating cellular ROS.

The inhibitors of apoptosis (IAPs) as cancer targets

Apoptosis has been accepted as a fundamental component in the pathogenesis of cancer, in addition to other human diseases including neurodegeneration, coronary disease and diabetes. The origin of cancer involves deregulated cellular proliferation and the suppression of apoptotic processes, ultimately leading to tumor establishment and growth. Several lines of evidence point toward the IAP family of proteins playing a role in oncogenesis, via their effective suppression of apoptosis. The central mechanisms of IAP apoptotic suppression appear to be through direct caspase and pro-caspase inhibition (primarily caspase 3 and 7) and modulation of, and by, the transcription factor NF-kappaB. Thus, when the IAPs are over-expressed or over-active, as is the case in many cancers, cells are no longer able to die in a physiologically programmed fashion and become increasingly resistant to standard chemo- and radiation therapies. To date several approaches have been taken to target and eliminate IAP function in an attempt to re-establish sensitivity, reduce toxicity, and improve efficacy of cancer treatment. In this review, we address IAP proteins as therapeutic targets for the treatment of cancer and emphasize the importance of novel therapeutic approaches for cancer therapy. Novel targets of IAP function are being identified and include gene therapy strategies and small molecule inhibitors that are based on endogenous IAP antagonists. As well, molecular mechanistic approaches, such as RNAi to deplete IAP expression, are in development.

Phosphorylation of Smac by JNK3 attenuates its interaction with XIAP

Here we demonstrate that JNK3 can phosphorylate Smac. Smac phosphorylation attenuates its ability to activate apoptosome activity in HeLa S-100 cell lysates. Addition of the X-linked inhibitor of apoptosis protein (XIAP) to the S-100 markedly suppresses apoptosome activity, and this suppressive effect of XIAP is neutralized by adding unphosphorylated Smac, but not phosphorylated Smac. Furtherover, JNK3-mediated phosphorylation of Smac markedly attenuates the interaction between Smac and XIAP, as measured by BIACORE assays and non-denaturing gel shift assays. When JNK3 activity is down-regulated in etoposide-induced HeLa cells by transiently overexpressing a dominant negative version of JNK3 (DN-JNK3), the caspase-3 activity as well as PARP cleavages are markedly enhanced. And the interaction of Smac with XIAP also increases by down-regulating JNK3 activity under the same conditions. These results suggest that JNK3 activity can attenuate the progression of apoptosis through a novel mechanism of action, the down-regulation of interaction between Smac and XIAP.

Crystal structure of the BIR1 domain of XIAP in two crystal forms

X-linked inhibitor of apoptosis (XIAP) is a potent negative regulator of apoptosis. It also plays a role in BMP signaling, TGF-beta signaling, and copper homeostasis. Previous structural studies have shown that the baculoviral IAP repeat (BIR2 and BIR3) domains of XIAP interact with the IAP-binding-motifs (IBM) in several apoptosis proteins such as Smac and caspase-9 via the conserved IBM-binding groove. Here, we report the crystal structure in two crystal forms of the BIR1 domain of XIAP, which does not possess this IBM-binding groove and cannot interact with Smac or caspase-9. Instead, the BIR1 domain forms a conserved dimer through the region corresponding to the IBM-binding groove. Structural and sequence analyses suggest that this dimerization of BIR1 in XIAP may be conserved in other IAP family members such as cIAP1 and cIAP2 and may be important for the action of XIAP in TGF-beta and BMP signaling and the action of cIAP1 and cIAP2 in TNF receptor signaling.

Intracellular K(+) inhibits apoptosis by suppressing the Apaf-1 apoptosome formation and subsequent downstream pathways but not cytochrome c release

Cellular ionic homeostasis, fundamentally K(+) homeostasis, has been implicated as a critical regulator of apoptosis. The intracellular K(+) efflux on apoptotic insult and suppression of apoptosis by high concentration of extracellular K(+) or after inhibition of this efflux by K(+) channel blockers have established the crucial role of K(+) in turning on the apoptotic machinery. Several contrasting observations have reported the antiapoptotic effect of intracellular K(+) concentration to be the result of inhibition of cytochrome c release from mitochondria, but the exact inhibitory mechanism remains obscure. However, here we show the blockage of K(+) efflux during apoptosis did not affect cytochrome c release from the mitochondria, still completely inhibited the formation of the apoptosome comprising Apaf-1, cytochrome c, caspase-9 and other accessories. As a consequence of this event, procaspase-9, -3, -8 and other death-related proteins were not processed. Furthermore, physiological concentrations of K(+) also inhibited the processing of procaspase-3 by purified caspase-8 or -9, the nucleosomal DNA fragmentation by purified DFF40/CAD and the nuclear fragmentation to varying extents. Altogether, these findings suggest that the efflux of K(+) is prerequisite not only for the formation of the apoptosome but also for the downstream apoptotic signal-transduction pathways.

BCL-2 family regulation by the 20S proteasome inhibitor bortezomib

Bortezomib (Velcade, PS341) was licensed in 2003 as a first-in-class 20S proteasome inhibitor indicated for treatment of multiple myeloma, and is currently being evaluated clinically in a range of solid tumours. The mechanisms underlying its cancer cell toxicity are complex. A growing body of evidence suggests proteasome inhibition-dependent regulation of the BCL-2 family is a critical requirement. In particular, the stabilization of BH3-only proteins BIK, NOXA and BIM, appear to be essential for effecting BAX- and BAK-dependent cell death. These mechanisms are reviewed and the implications for favourable novel drug interactions are highlighted.

A dimeric Smac/diablo peptide directly relieves caspase-3 inhibition by XIAP. Dynamic and cooperative regulation of XIAP by Smac/Diablo

Caspase activation, the executing event of apoptosis, is under deliberate regulation. IAP proteins inhibit caspase activity, whereas Smac/Diablo antagonizes IAP. XIAP, a ubiquitous IAP, can inhibit both caspase-9, the initiator caspase of the mitochondrial apoptotic pathway, and the downstream effector caspases, caspase-3 and caspase-7. Smac neutralizes XIAP inhibition of caspase-9 by competing for binding of the BIR3 domain of XIAP with caspase-9, whereas how Smac liberates effector caspases from XIAP inhibition is not clear. It is generally believed that binding of Smac with IAP generates a steric hindrance that prevents XIAP from inhibiting effector caspases, and therefore small molecule mimics of Smac are not able to reverse inhibition of the effector caspases. Surprisingly, we show here that binding of a dimeric Smac N-terminal peptide with the BIR2 domain of XIAP effectively antagonizes inhibition of caspase-3 by XIAP. Further, we defined the dynamic and cooperative interaction of Smac with XIAP: binding of Smac with the BIR3 domain anchors the subsequent binding of Smac with the BIR2 domain, which in turn attenuates the caspase-3 inhibitory function of XIAP. We also show that XIAP homotrimerizes via its C-terminal Ring domain, making its inhibitory activity toward caspase-3 more susceptible to Smac.

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

During apoptosis, the initiator caspase 9 is activated at the apoptosome after which it activates the executioner caspases 3 and 7 by proteolysis. During this process, caspase 9 is cleaved by caspase 3 at Asp(330), and it is often inferred that this proteolytic event represents a feedback amplification loop to accelerate apoptosis. However, there is substantial evidence that proteolysis per se does not activate caspase 9, so an alternative mechanism for amplification must be considered. Cleavage at Asp(330) removes a short peptide motif that allows caspase 9 to interact with IAPs (inhibitors of apoptotic proteases), and this event may control the amplification process. We show that, under physiologically relevant conditions, caspase 3, but not caspase 7, can cleave caspase 9, and this does not result in the activation of caspase 9. An IAP antagonist disrupts the inhibitory interaction between XIAP (X-linked IAP) and caspase 9, thereby enhancing activity. We demonstrate that the N-terminal peptide of caspase 9 exposed upon cleavage at Asp330 cannot bind XIAP, whereas the peptide generated by autolytic cleavage of caspase 9 at Asp315 binds XIAP with substantial affinity. Consistent with this, we found that XIAP antagonists were only capable of promoting the activity of caspase 9 when it was cleaved at Asp315, suggesting that only this form is regulated by XIAP. Our results demonstrate that cleavage by caspase 3 does not activate caspase 9, but enhances apoptosis by alleviating XIAP inhibition of the apical caspase.

XIAP: cell death regulation meets copper homeostasis

X-linked inhibitor of apoptosis (XIAP), traditionally known as an anti-apoptotic protein, has recently been shown to be involved in copper homeostasis. XIAP promotes the ubiquitination and degradation of COMMD1, a protein that promotes the efflux of copper from the cell. Through its effects on COMMD1, XIAP can regulate copper export from the cell and potentially represents an additional intracellular sensor for copper levels. XIAP binds copper directly and undergoes a substantial conformational change in the copper-bound state. This in turn destabilizes XIAP, resulting in lowered steady-state levels of the protein. Furthermore, copper-bound XIAP is unable to inhibit caspases and cells that express this form of the protein exhibit increased rates of cell death in response to apoptotic stimuli. These events take place in the setting of excess intracellular copper accumulation as seen in copper toxicosis disorders such as Wilson's disease and establish a new relationship between copper levels and the regulation of cell death via XIAP. These findings raise important questions about the role of XIAP in the development of copper toxicosis disorders and may point to XIAP as a potential therapeutic target in these disease states.

Caspase-9 cleavage, do you need it?

Caspase-9, which is activated by association with the Apaf-1 (apoptotic protease-activating factor-1) apoptosome complex, cleaves and activates the downstream effector caspases-3 and -7, thereby executing the caspase-cascade and cell-death programme. Although caspase-9 does not need to be cleaved to be active, apoptotic cell death is always accompanied by autocatalytic cleavage and by further downstream effector caspase-dependent cleavage of caspase-9. In this issue of the Biochemical Journal, Denault and co-workers evaluate the role of caspase-3-dependent cleavage of caspase-9 and conclude that this mechanism mainly serves to enhance apoptosis by alleviating XIAP (X-linked inhibitor of apoptosis) inhibition of the apical caspase.

Polymer-drug conjugates as modulators of cellular apoptosis

The successful clinical application of polymer-protein conjugates (PEGylated enzymes and cytokines) and the promising results arising from clinical trials with polymer-bound chemotherapy (eg, doxorubicin or paclitaxel) have established their potential to reduce toxicity and improve activity in chemotherapy-refractory patients. Furthermore, and more important, they have also provided a firm foundation for more sophisticated second-generation constructs that deliver the newly emerging target-directed bioactive agents (eg, modulators of apoptosis, cell cycle, anti-angiogenic drugs) in addition to polymer-based drug combinations (eg, endocrine therapy and chemotherapy). This review will focus on polymer-drug conjugate modulators of cellular apoptosis to be used as single pro-apoptotic (eg, cancer) or anti-apoptotic (eg, ischemia) agents or as a combination therapy.

XIAP induces NF-kappaB activation via the BIR1/TAB1 interaction and BIR1 dimerization

In addition to caspase inhibition, X-linked inhibitor of apoptosis (XIAP) induces NF-kappaB and MAP kinase activation during TGF-b and BMP receptor signaling and upon overexpression. Here we show that the BIR1 domain of XIAP, which has no previously ascribed function, directly interacts with TAB1 to induce NF-kappaB activation. TAB1 is an upstream adaptor for the activation of the kinase TAK1, which in turn couples to the NF-kappaB pathway. We report the crystal structures of BIR1, TAB1, and the BIR1/TAB1 complex. The BIR1/TAB1 structure reveals a striking butterfly-shaped dimer and the detailed interaction between BIR1 and TAB1. Structure-based mutagenesis and knockdown of TAB1 show unambiguously that the BIR1/TAB1 interaction is crucial for XIAP-induced TAK1 and NF-kappaB activation. We show that although not interacting with BIR1, Smac, the antagonist for caspase inhibition by XIAP, also inhibits the XIAP/TAB1 interaction. Disruption of BIR1 dimerization abolishes XIAP-mediated NF-kappaB activation, implicating a proximity-induced mechanism for TAK1 activation.

Cloning and characterization of Xenopus laevis Smac/DIABLO

Mitochondria-mediated apoptosis plays a central role in animal development and tissue homeostasis, and mitochondria contain several pro-apoptotic proteins that have key roles in apoptosis. Smac/DIABLO was identified as a mitochondrial protein that is released into the cytosol following apoptotic stimuli, subsequently blocking the anti-apoptotic activity of inhibitor of apoptosis proteins. Through expressed sequence tag (EST) analysis we detected evidence for the presence of a number of Xenopus counterparts to mammalian mitochondrial pro-apoptotic proteins. EST and genome sequencing provides evidence for the presence of endonuclease G, AIF, HtrA/Omi and Smac/DIABLO in Xenopus laevis and tropicalis. Here we report the cloning and characterization of X. laevis Smac/DIABLO (XSmac/DIABLO). In this study degenerate primers based on conserved regions of human, mouse and an EST predicted Smac from X. tropicalis were used to amplify cDNA templates from X. laevis. The full length cDNA of Xenopus Smac contained a complete open reading frame of 732 bp, encoding 244 amino acids, that when expressed is observed to be approximately 27 kDa in size. The protein sequence is 49% identical and 71% similar to human Smac, and includes the motifs involved in mitochondrial targeting, and IAP-binding (AIPV). Smac expression was detected throughout early development with multiple transcripts being detected by Northern blot analysis, suggesting the presence of alternatively spliced isoforms. Exogenous expression of Xenopus Smac enhances gamma-irradiation-induced apoptosis in HeLa cells, demonstrating its functional equivalence with mammalian forms. Our study has identified the third vertebrate homologue of Smac/DIABLO, with its structural and functional similarities to mammalian Smac/DIABLO further illustrating the evolutionary conservation of apoptotic pathways across vertebrate species.

HGF/SF regulates expression of apoptotic genes in MCF-10A human mammary epithelial cells

Hepatocyte growth factor/scatter factor (HGF/SF) induces scattering, morphogenesis, and survival of epithelial cells through activation of the MET tyrosine kinase receptor. HGF/SF and MET are involved in normal development and tumor progression of many tissues and organs, including the mammary gland. In order to find target genes of HGF/SF involved in its survival function, we used an oligonucleotide microarray representing 1,920 genes known to be involved in apoptosis, transcriptional regulation, and signal transduction. MCF-10A human mammary epithelial cells were grown in the absence of serum and treated or not with HGF/SF for 2 h. Total RNA was reverse-transcribed to cDNA in the presence of fluorescent Cy3-dUTP or Cy5-dUTP to generate fluorescently labeled cDNA probes. Microarrays were performed and the ratios of Cy5/Cy3 fluorescence were determined. The expression of three apoptotic genes was modified by HGF/SF, with A20 being upregulated, and DAXX and SMAC being downregulated. These changes of expression were confirmed by real-time quantitative PCR. According to current-knowledge, A20 is antiapoptotic and SMAC is proapoptotic, while a pro- or antiapoptotic function of DAXX is controversial. The fact that HGF/SF upregulates an antiapoptotic gene (A20) and downregulates a proapoptotic gene (SMAC) is in agreement with its survival effect in MCF-10A cells. This study identified novel apoptotic genes regulated by HGF/SF, which can contribute to its survival effect.

MUC4 Mucin Potentiates Pancreatic Tumor Cell Proliferation, Survival, and Invasive Properties and Interferes with Its Interaction to Extracellular Matrix Proteins

MUC4, a transmembrane mucin, is aberrantly expressed in pancreatic adenocarcinomas while remaining undetectable in the normal pancreas. Recent studies have shown that the expression of MUC4 is associated with the progression of pancreatic cancer and is inversely correlated with the prognosis of pancreatic cancer patients. In the present study, we have examined the phenotypic and molecular consequences of MUC4 silencing with an aim of establishing the mechanistic basis for its observed role in the pathogenesis of pancreatic cancer. The silencing of MUC4 expression was achieved by stable expression of a MUC4-specific short hairpin RNA in CD18/HPAF, a highly metastatic pancreatic adenocarcinoma cell line. A significant decrease in MUC4 expression was detected in MUC4-knockdown (CD18/HPAF-siMUC4) cells compared with the parental and scrambled short interfering RNA-transfected (CD18/HPAF-Scr) control cells by immunoblot analysis and immunofluorescence confocal microscopy. Consistent with our previous observation, inhibition of MUC4 expression restrained the pancreatic tumor cell growth and metastasis as shown in an orthotopic mouse model. Our in vitro studies revealed that MUC4-associated increase in tumor cell growth resulted from both the enhanced proliferation and reduced cell death. Furthermore, MUC4 expression was also associated with significantly increased invasiveness (P < or = 0.05) and changes in actin organization. The presence of MUC4 on the cell surface was shown to interfere with the tumor cell-extracellular matrix interactions, in part, by inhibiting the integrin-mediated cell adhesion. An altered expression of growth- and metastasis-associated genes (LI-cadherin, CEACAM6, RAC1, AnnexinA1, thrombomodulin, epiregulin, S100A4, TP53, TP53BP, caspase-2, caspase-3, caspase-7, plakoglobin, and neuregulin-2) was also observed as a consequence of the silencing of MUC4. In conclusion, our study provides experimental evidence that supports the functional significance of MUC4 in pancreatic cancer progression and indicates a novel role for MUC4 in cancer cell signaling.

A minimal peptide sequence that targets fluorescent and functional proteins into the mitochondrial intermembrane space

Protein-based fluorescent and functional probes are widely used for real-time visualization, purification, and regulation of a variety of biological molecules. The protein-based probes can generally be targeted into subcellular compartments of eukaryotic cells by a particular short peptide sequence. Little is known, however, about the sequence that targets probes into the mitochondrial intermembrane space (IMS). To identify the IMS-targeting sequence, we developed a simple genetic screening method to discriminate the proteins localized in the IMS from those in the mitochondrial matrix, thereby revealing the minimum requisite sequence for the IMS targeting. An IMS-localized protein, Smac/DIABLO, was randomly mutated, and the mitochondrial localization of each mutant was analyzed. We found that the four residues of Ala-Val-Pro-Ile are required for IMS localization, and a sequence of these four residues fused with matrix-targeting signals is sufficient for targeting the Smac/DIABLO into the IMS. The sequence was shown to readily direct three dissimilar proteins of interest to the IMS, which will open avenues to elucidating the functions of the IMS in live cells.

Mitochondrial membrane permeabilization in cell death

Irrespective of the morphological features of end-stage cell death (that may be apoptotic, necrotic, autophagic, or mitotic), mitochondrial membrane permeabilization (MMP) is frequently the decisive event that delimits the frontier between survival and death. Thus mitochondrial membranes constitute the battleground on which opposing signals combat to seal the cell's fate. Local players that determine the propensity to MMP include the pro- and antiapoptotic members of the Bcl-2 family, proteins from the mitochondrialpermeability transition pore complex, as well as a plethora of interacting partners including mitochondrial lipids. Intermediate metabolites, redox processes, sphingolipids, ion gradients, transcription factors, as well as kinases and phosphatases link lethal and vital signals emanating from distinct subcellular compartments to mitochondria. Thus mitochondria integrate a variety of proapoptotic signals. Once MMP has been induced, it causes the release of catabolic hydrolases and activators of such enzymes (including those of caspases) from mitochondria. These catabolic enzymes as well as the cessation of the bioenergetic and redox functions of mitochondria finally lead to cell death, meaning that mitochondria coordinate the late stage of cellular demise. Pathological cell death induced by ischemia/reperfusion, intoxication with xenobiotics, neurodegenerative diseases, or viral infection also relies on MMP as a critical event. The inhibition of MMP constitutes an important strategy for the pharmaceutical prevention of unwarranted cell death. Conversely, induction of MMP in tumor cells constitutes the goal of anticancer chemotherapy.