Structural and biochemical basis of apoptotic activation by Smac/DIABLO

Role of Smac in determining the chemotherapeutic response of esophageal squamous cell carcinoma

PURPOSE

Second mitochondria-derived activator of caspase (Smac) regulates chemotherapy-induced apoptosis. Smac mimetics have been tested in clinical trials as chemosensitizers. We determined the role of Smac in modulating the chemosensitivity of esophageal squamous cell carcinoma (ESCC).

EXPERIMENTAL DESIGN

Smac expression was evaluated in tissues from ESCC patients with differential chemotherapeutic responses. The effects of Smac knockdown and Smac mimetics on the chemosensitivity of ESCC cells and the molecular mechanisms by which Smac and Smac mimetics modulate chemosensitivity were determined. The therapeutic responses of ESCC cells with different Smac statuses were compared using xenograft models.

RESULTS

We found that Smac was significantly downregulated in most ESCC samples (36.8%, 25/68, P = 0.001), and Smac expression differed significantly (P < 0.05) between chemosensitive and chemoresistant tumors. The associations of tested factors and their responses were examined using logistic regression analysis. In ESCC cells treated with cisplatin, a common chemotherapeutic drug, Smac and cytochrome c were released from mitochondria, and caspase-3 and caspase-9 were activated. Knockdown of Smac abrogated cisplatin-induced apoptosis, mitochondrial dysfunction, cytochrome c release, and caspase activation. Smac deficiency also reduced the effect of cisplatin on long-term cell viability, and led to cisplatin resistance in xenograft tumors in vivo. LBW242, a small molecule Smac mimetic, enhanced cisplatin-induced apoptosis and caspase activation and restored cisplatin sensitivity in Smac-deficient cells.

CONCLUSION

Our data suggested that downregulation of Smac may be a chemoresistance mechanism in ESCC. Combinations of Smac mimetics with chemotherapeutic agents may have therapeutic benefits for the treatment of esophageal cancer.

Caspase cleavage of cytochrome c1 disrupts mitochondrial function and enhances cytochrome c release

Mitochondrial catastrophe can be the cause or consequence of apoptosis and is associated with a number of pathophysiological conditions. The exact relationship between mitochondrial catastrophe and caspase activation is not completely understood. Here we addressed the underlying mechanism, explaining how activated caspase could feedback to attack mitochondria to amplify further cytochrome c (cyto.c) release. We discovered that cytochrome c1 (cyto.c1) in the bc1 complex of the mitochondrial respiration chain was a novel substrate of caspase 3 (casp.3). We found that cyto.c1 was cleaved at the site of D106, which is critical for binding with cyto.c, following apoptotic stresses or targeted expression of casp.3 into the mitochondrial intermembrane space. We demonstrated that this cleavage was closely linked with further cyto.c release and mitochondrial catastrophe. These mitochondrial events could be effectively blocked by expressing non-cleavable cyto.c1 (D106A) or by caspase inhibitor z-VAD-fmk. Our results demonstrate that the cleavage of cyto.c1 represents a critical step for the feedback amplification of cyto.c release by caspases and subsequent mitochondrial catastrophe.

Pharmacological interventions for spinal cord injury: where do we stand? How might we step forward?

Despite numerous studies reporting some measures of efficacy in the animal literature, there are currently no effective therapies for the treatment of traumatic spinal cord injuries (SCI) in humans. The purpose of this review is to delineate key pathophysiological processes that contribute to neurological deficits after SCI, as well as to describe examples of pharmacological approaches that are currently being tested in clinical trials, or nearing clinical translation, for the therapeutic management of SCI. In particular, we will describe the mechanistic rationale to promote neuroprotection and/or functional recovery based on theoretical, yet targeted pathological events. Finally, we will consider the clinical relevancy for emerging evidence that pharmacologically targeting mitochondrial dysfunction following injury may hold the greatest potential for increasing tissue sparing and, consequently, the extent of functional recovery following traumatic SCI.

A potent and orally active antagonist (SM-406/AT-406) of multiple inhibitor of apoptosis proteins (IAPs) in clinical development for cancer treatment

We report the discovery and characterization of SM-406 (compound 2), a potent and orally bioavailable Smac mimetic and an antagonist of the inhibitor of apoptosis proteins (IAPs). This compound binds to XIAP, cIAP1, and cIAP2 proteins with K(i) of 66.4, 1.9, and 5.1 nM, respectively. Compound 2 effectively antagonizes XIAP BIR3 protein in a cell-free functional assay, induces rapid degradation of cellular cIAP1 protein, and inhibits cancer cell growth in various human cancer cell lines. It has good oral bioavailability in mice, rats, non-human primates, and dogs, is highly effective in induction of apoptosis in xenograft tumors, and is capable of complete inhibition of tumor growth. Compound 2 is currently in phase I clinical trials for the treatment of human cancer.

Targeting mitochondrial function for the treatment of acute spinal cord injury

Traumatic injury to the mammalian spinal cord is a highly dynamic process characterized by a complex pattern of pervasive and destructive biochemical and pathophysiological events that limit the potential for functional recovery. Currently, there are no effective therapies for the treatment of spinal cord injury (SCI) and this is due, in part, to the widespread impact of the secondary injury cascades, including edema, ischemia, excitotoxicity, inflammation, oxidative damage, and activation of necrotic and apoptotic cell death signaling events. In addition, many of the signaling pathways associated with these cascades intersect and initiate other secondary injury events. Therefore, it can be argued that therapeutic strategies targeting a specific biochemical cascade may not provide the best approach for promoting functional recovery. A "systems approach" at the subcellular level may provide a better strategy for promoting cell survival and function and, as a consequence, improve functional outcomes following SCI. One such approach is to study the impact of SCI on the biology and function of mitochondria, which serve a major role in cellular bioenergetics, function, and survival. In this review, we will briefly describe the importance and unique properties of mitochondria in the spinal cord, and what is known about the response of mitochondria to SCI. We will also discuss a number of strategies with the potential to promote mitochondrial function following SCI.

Dimerization of Smac is crucial for its mitochondrial retention by XIAP subsequent to mitochondrial outer membrane permeabilization

Following the apoptotic permeabilization of the outer mitochondrial membrane, the inter-membrane space protein second mitochondria-derived activator of caspases (Smac) is released into the cytosol. Smac efficiently promotes apoptosis by antagonizing x-linked inhibitor of apoptosis protein (XIAP), an inhibitor of caspases-9, -3, and -7, via a short NH(2)-terminal inhibitor of apoptosis protein (IAP) binding motif (AVPI). Native Smac dimerizes to form a highly stable and inflexible elongated arch, however, a functional role for this outstretched structure so far remained unknown. Using time-lapse single-cell imaging of DLD-1 and HCT-116 colon cancer cells, we here demonstrate that upon mitochondrial outer membrane permeabilization physiological expression levels of XIAP are sufficient to selectively prolong the release of dimeric but not monomeric Smac. Elevating the expression of XIAP further extended the release duration of dimeric Smac and resulted in the mitochondrial retention of a significant proportion of the Smac pool. In contrast, monomeric Smac was always fully released and the release kinetics were not affected by altered XIAP expression. Our findings therefore indicate that the dimerization of Smac is critical for the XIAP-mediated retention of Smac at or inside the mitochondria. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Smac mimetic-derived augmentation of chemotherapeutic response in experimental pancreatic cancer

Background

Pancreatic ductal adenocarcinoma (PDAC) is highly resistant to conventional chemotherapy, in part due to the overexpression of inhibitors of apoptosis proteins (IAPs). Smac is an endogenous IAP-antagonist, which renders synthetic Smac mimetics attractive anticancer agents. We evaluated the benefits of combining a Smac mimetic, JP1201 (JP), with conventional chemotherapy agents used for PDAC management.

Methods

Cell viability assays and protein expression analysis were performed using WST-1 reagent and Western blotting, respectively. Apoptosis was detected by annexin V/propidium iodide staining. In vivo tumor growth and survival studies were performed in murine PDAC xenografts.

Results

JP and gemcitabine (Gem) inhibited PDAC cell proliferation with additive effects in combination. The percentage of early apoptotic cells in controls, JP, Gem and JP + Gem was 17%, 26%, 26% and 38%, respectively. JP-induced apoptosis was accompanied by PARP-1 cleavage. Similar additive anti-proliferative effects were seen for combinations of JP with doxorubicin (Dox) and docetaxel (DT). The JP + Gem combination caused a 30% decrease in tumor size in vivo compared to controls. Median animal survival was improved significantly in mice treated with JP + Gem (38 d) compared to controls (22 d), JP (28 d) or Gem (32 d) (p = 0.01). Animal survival was also improved with JP + DT treatment (32 d) compared to controls (16 d), JP (21 d) or DT alone (27 d).

Conclusions

These results warrant further exploration of strategies that promote chemotherapy-induced apoptosis of tumors and highlight the potential of Smac mimetics in clinical PDAC therapy.

Novel X-linked inhibitor of apoptosis inhibiting compound as sensitizer for TRAIL-mediated apoptosis in chronic lymphocytic leukaemia with poor prognosis

Given that aggressive DNA damaging chemotherapy shows suboptimal efficacy in chronic lymphocytic leukaemia (CLL), alternative therapeutic approaches are needed. Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) is able to induce tumour-specific apoptosis. However, apoptosis might be inhibited by elevated levels of X-linked inhibitor of apoptosis (XIAP). Use of XIAP-inhibiting compounds might sensitize primary CLL cells towards TRAIL-mediated apoptosis. A novel small molecule, compound A (CA), an inhibitor of XIAP, was used in combination with TRAIL to induce apoptosis in primary CLL cells (n = 48). XIAP was significantly more highly expressed in primary CLL cells (n = 28) compared to healthy B cells (n = 16) (P = 0·02). Our data obtained by specific knock-down of XIAP by siRNA identified XIAP as the key factor conferring resistance to TRAIL in CLL. Combined treatment with CA/TRAIL significantly increased apoptosis compared to untreated (P = 8·5 × 10⁻¹⁰), solely CA (P = 4·1 × 10⁻¹²) or TRAIL treated (P = 4·8 × 10⁻¹⁰) CLL cells. CA rendered 40 of 48 (83·3%) primary CLL samples susceptible to TRAIL-mediated apoptosis. In particular, cells derived from patients with poor prognosis CLL (ZAP-70(+) , IGHV unmutated, 17p-) were highly responsive to this drug combination. Our highly-effective XIAP inhibitor CA, in concert with TRAIL, shows potential for the treatment of CLL cases with poor prognosis and therefore warrants further clinical investigation.

Smac mimetic reverses resistance to TRAIL and chemotherapy in human urothelial cancer cells

PURPOSE

inhibitors of apoptosis proteins (IAPs) have been shown to contribute to resistance of neoplastic cells to chemotherapy and to biologic antineoplastic agents. Consequently, new agents are being developed targeting this family of proteins. In a panel of bladder cancer cell lines, we evaluated a Smac mimetic that antagonizes several IAPs for its suitability for bladder cancer therapy. Experimental design: A panel of seven bladder cancer cell lines were evaluated for sensitivity to the Smac mimetic compound-A alone, TRAIL alone, chemotherapy alone, compound-A plus TRAIL, and compound-A plus chemotherapy by DNA fragmentation analysis. IAP levels and caspase activation were examined by western blotting. Release of caspase-3 from X-linked inhibitor of apoptosis protein (XIAP), the most effective IAP, was assessed by immunoprecipitation and western blotting. Finally, siRNA knockdown of XIAP was correlated with the sensitivity of cells to apoptosis induced by compound-A plus TRAIL by DNA fragmentation and western blotting.

RESULTS

single-agent compound-A had little effect, but compound-A augmented TRAIL- and chemotherapy-induced apoptosis. Immunoblotting showed that combination treatment with compound-A and TRAIL resulted in cleavage of procaspase-3 and procaspase-7, activation of which irreversibly commits cells to apoptosis. Immunoprecipitation of XIAP showed displacement of active caspase-3 fragments from XIAP, supporting the proposed mechanism of action. Furthermore, siRNA-mediated silencing of XIAP similarly sensitized these cells to apoptosis.

EXPERIMENTAL DESIGN

a panel of seven bladder cancer cell lines were evaluated for sensitivity to the Smac mimetic compound-Alone, TRAIL alone, Chemotherapy alone, compound-A plus TRAIL and compound-A plus chemotherapy by DNA fragmentation analysis. IAP levels and caspase activation were examined by western blotting. Release of caspase-3 from X-linked inhibitor of apoptosis protein (XIAP), the most effective IAP, was assessed by immunoprecipitation and western blotting. Finally siRNA knockdown of XIAP was correlated with the sensitivity of cells to apoptosis induced by compound-A plus TRAIL by DNA fragmentation and western blotting.

CONCLUSION

our results suggest that targeting of XIAP with the Smac mimetic compound-A has the potential to augment the effects of a variety of chemotherapeutic and biologic therapies in bladder cancer.

Monomerization of cytosolic mature smac attenuates interaction with IAPs and potentiation of caspase activation

The four residues at the amino-terminus of mature Smac/DIABLO are an IAP binding motif (IBM). Upon exit from mitochondria, mature Smac interacts with inhibitor of apoptosis proteins (IAPs), abrogating caspase inhibition. We used the ubiquitin fusion model to express mature Smac in the cytosol. Transiently expressed mature Smac56-239 (called Smac56) and Smac60-239 (called Smac60), which lacks the IBM, interacted with X-linked inhibitor of apoptosis protein (XIAP). However, stable expression produced wild type Smac56 that failed to homodimerize, interact with XIAP, and potentiate caspase activation. Cytosolic Smac60 retained these functions. Cytosolic Smac56 apparently becomes posttranslationally modified at the dimer interface region, which obliterated the epitope for a monoclonal antibody. Cytosolic Smacδ, which has the IBM but lacks amino acids 62–105, homodimerized and weakly interacted with XIAP, but failed to potentiate apoptosis. These findings suggest that the IBM of Smac is a recognition point for a posttranslational modification(s) that blocks homodimerization and IAP interaction, and that amino acids 62–105 are required for the proapoptotic function of Smac.

Attenuating Smac mimetic compound 3-induced NF-kappaB activation by luteolin leads to synergistic cytotoxicity in cancer cells

Smac mimetics are potential anticancer therapeutics selectively killing cancer cells through autocrine tumor necrosis factor (TNF)-mediated apoptosis pathway. Our recent study reveal that the Smac mimetic compound 3 (SMC3)-activated NF-kappaB protects cancer cells against apoptosis, thus blunting SMC3's anticancer activity. Based on our previous observations that the nutrient flavonoid luteolin potently blocks TNF-induced NF-kappaB activation in cancer cells, we investigated if the combination of SMC3 and luteolin would achieve a synergistic anticancer activity. The results show that luteolin had no effect on autocrine TNF but it effectively blocked SMC3-induced nuclear factor kappa B (NF-kappaB) activation and expression of anti-apoptotic NF-kappaB targets. When SMC3 and luteolin were combined in treating cancer cells derived from lung and liver tumors, the activation of TNF-dependent apoptosis was markedly sensitized and a synergistic cytotoxic effect was achieved. In addition, the SMC3 and luteolin co-treatment had marginal effect on immortalized normal human bronchial epithelial cells. The results suggest that combination of SMC3 and luteolin is an effective approach for improving the anticancer value of SMC3, which has implications in cancer prevention and therapy.

XIAP impairs Smac release from the mitochondria during apoptosis

X-linked inhibitor of apoptosis protein (XIAP) is a potent inhibitor of caspases 3, 7 and 9, and mitochondrial Smac (second mitochondria-derived activator of caspase) release during apoptosis inhibits the activity of XIAP. In this study we show that cytosolic XIAP also feeds back to mitochondria to impair Smac release. We constructed a fluorescent XIAP-fusion protein by labelling NH₂- and COOH-termini with Cerulean fluorescent protein (C-XIAP-C). Immunoprecipitation confirmed that C-XIAP-C retained the ability to interact with Smac and impaired extrinsically and intrinsically activated apoptosis in response to tumour necrosis factor-related apoptosis-inducing ligand/cycloheximide and staurosporine. In C-XIAP-C-expressing cells, cytochrome c release from mitochondria proceeded normally, whereas Smac release was significantly prolonged and incomplete. In addition, physiological expression of native XIAP prolonged or limited Smac release in HCT-116 colon cancer cells and primary mouse cortical neurons. The Smac-binding capacity of XIAP, but not caspase inhibition, was central for mitochondrial Smac retention, as evidenced in experiments using XIAP mutants that cannot bind to Smac or effector caspases. Similarly, the release of a Smac mutant that cannot bind to XIAP was not impaired by C-XIAP-C expression. Full Smac release could however be provoked by rapid cytosolic C-XIAP-C depletion upon digitonin-induced plasma membrane permeabilization. Our findings suggest that although mitochondria may already contain pores sufficient for cytochrome c release, elevated amounts of XIAP can selectively impair and limit the release of Smac.

The functional and neuroprotective actions of Neu2000, a dual-acting pharmacological agent, in the treatment of acute spinal cord injury

The goal of the present study was to examine the neuroprotective and functional significance of targeting both N-methyl-D-aspartate (NMDA) receptor-mediated excitotoxicity and oxidative stress using a dual-acting compound, Neu2000, in rat model of moderate spinal cord injury (SCI). An initial set of experiments was conducted in uninjured rats to study the pharmacokinetic profile of Neu2000 following intraperitoneal and intravenous administration. A second experiment measured free radical production in mitochondria isolated from sham or injured spinal cords of animals receiving vehicle or Neu2000 treatment. A third set of animals was divided into three treatment groups consisting of vehicle treatment, a single dose of Neu2000 (50 mg/kg) administered at 10 min following injury, or a repeated treatment paradigm consisting of a single bolus of Neu2000 at 10 min following injury (50 mg/kg) plus a maintenance dose (25 mg/kg) administered every 24 h for an additional 6 days. Animals were tested once a week for a period of 6 weeks for evidence of locomotor recovery in an open field and kinematic analysis of fine motor control using the DigiGait Image Analysis System. At the end of the testing period, spinal cord reconstruction was performed to obtain nonbiased stereological measures of tissue sparing. The results of this study demonstrate that Neu2000 treatment significantly reduced the production of mitochondrial free radicals and improved locomotor outcomes that were associated with a significant increase in the volume of spared spinal cord tissue.

Tumor necrosis factor-mediated cell death: to break or to burst, that's the question

In this review, we discuss the signal-transduction pathways of three major cellular responses induced by tumor necrosis factor (TNF): cell survival through NF-kappaB activation, apoptosis, and necrosis. Recruitment and activation of caspases plays a crucial role in the initiation and execution of TNF-induced apoptosis. However, experimental inhibition of caspases reveals an alternative cell death pathway, namely necrosis, also called necroptosis, suggesting that caspases actively suppress the latter outcome. TNF-induced necrotic cell death crucially depends on the kinase activity of receptor interacting protein serine-threonine kinase 1 (RIP1) and RIP3. It was recently demonstrated that ubiquitination of RIP1 determines whether it will function as a pro-survival or pro-cell death molecule. Deeper insight into the mechanisms that control the molecular switches between cell survival and cell death will help us to understand why TNF can exert so many different biological functions in the etiology and pathogenesis of human diseases.

Small-molecule pan-IAP antagonists: a patent review

IMPORTANCE OF THE FIELD

The inhibitor of apoptosis (IAP) proteins are critical regulators of cancer cell survival that have become important targets for therapeutic intervention in human malignancies. One strategy for targeting IAP proteins involves agents that mimic the amino terminus of the endogenous IAP protein antagonist second mitochondria-derived activator of caspases (Smac)/direct IAP-binding protein with low pI (DIABLO) and thus block critical IAP protein interactions.

AREAS COVERED IN THIS REVIEW

This review of the IAP antagonist patent literature covers the period from 2000 to mid-2009. Over 50 patents and patent applications pertaining to IAP antagonists have been published over the past 10 years. In the case of several filings, only the original source is reviewed in this analysis.

WHAT THE READER WILL GAIN

Readers will gain an overview of IAP protein antagonist scaffolds, with representative examples including monovalent and bivalent Smac mimetics, and an understanding of their structure-activity relationships.

TAKE HOME MESSAGE

The feasibility of disrupting IAP protein interactions with pro-apoptotic proteins using monovalent and bivalent Smac-derived peptidomimetic compounds has been broadly established. Four such compounds have entered or been approved to enter human clinical trials, which will hopefully allow the utility of this potential therapeutic approach to be evaluated in cancer patients.

The role of cyclooxygenase-2 in cell proliferation and cell death in human malignancies

It is well admitted that the link between chronic inflammation and cancer involves cytokines and mediators of inflammatory pathways, which act during the different steps of tumorigenesis. The cyclooxygenases (COXs) are a family of enzymes, which catalyze the rate-limiting step of prostaglandin biosynthesis. This family contains three members: ubiquitously expressed COX-1, which is involved in homeostasis; the inducible COX-2 isoform, which is upregulated during both inflammation and cancer; and COX-3, expressed in brain and spinal cord, whose functions remain to be elucidated. COX-2 was described to modulate cell proliferation and apoptosis mainly in solid tumors, that is, colorectal, breast, and prostate cancers, and, more recently, in hematological malignancies. These findings prompt us to analyze here the effects of a combination of COX-2 inhibitors together with different clinically used therapeutic strategies in order to further improve the efficiency of future anticancer treatments. COX-2 modulation is a promising field investigated by many research groups.

Molecular interaction between HAX-1 and XIAP inhibits apoptosis

Caspase-3 is an important executor caspase that plays an essential role in apoptosis. Recently, HS1-associated protein X1 (HAX-1) was found to be a substrate of caspase-3. Although HAX-1 has serve multifunctional roles in cellular functions such as cell survival and calcium homeostasis, the detailed functional mechanism of HAX-1 remains still unclear. In this study, we performed proteomic experiments to identify the HAX-1 interactome. Through immunoprecipitation and 2D gel electrophoresis, we identified X-linked inhibitor of apoptosis protein (XIAP) as a novel HAX-1-interacting protein. By performing the GST pull-down assay, we defined the interaction domains in HAX-1 and XIAP, showing that HAX-1 binds to the BIR2 and BIR3 domains of XIAP whereas XIAP binds to the C-terminal domain of HAX-1. In addition, surface plasma resonance experiments showed that both BIR2 and BIR3 domains of XIAP bind to HAX-1 with affinity similar to that of full-length XIAP, indicating that either domain is necessary and sufficient for tight binding to HAX-1. Taken together with the observation that HAX-1 suppresses the polyubiquitination of XIAP, the cell viability assay results suggest that the formation of the HAX-1-XIAP complex inhibits apoptosis by enhancing the stability of XIAP against proteosomal degradation.

Molecules that modulate Apaf-1 activity

Programmed cell death, apoptosis, is a highly regulated cellular pathway, responsible for the elimination of cells in the organism that are no longer needed or extensively damaged. Defects in the regulation of apoptosis could be at the molecular basis of different diseases, either when it is insufficient or excessive. The formation of the macromolecular complex, apoptosome, is a key event in this pathway, which has also been defined as the intrinsic apoptosis pathway. The apoptosome is a holoenzyme multiprotein complex formed by cytochrome c-activated apoptotic protease-activating factor (Apaf-1), dATP, and procaspase-9. Recent studies have produced a wealth of information about the regulation and functions of Apaf-1, but additional studies aimed at elucidating its role as a signaling device at the crosstalk between different signaling pathways are needed to take advantage for the development of modulators of apoptosis pathways and possible therapeutic applications.

Design of small-molecule Smac mimetics as IAP antagonists

Smac/DIABLO, discovered in 2000 as a protein released from mitochondria into the cytosol in response to apoptotic stimuli, functions as an endogenous antagonist of X-linked inhibitor of apoptosis protein (XIAP) and several other IAP proteins through direct binding. The interaction between Smac and IAPs involves the AVPI tetrapeptide binding motif on the N-terminus of Smac and a well-defined groove on the surface of these IAP proteins, providing an ideal site for the design of small-molecule Smac mimetics. Potent and cell-permeable small-molecule Smac mimetics have provided powerful pharmacological tools for study of the regulation of apoptosis by IAP proteins, and several such compounds are now in early clinical trials as new anticancer agents.

Therapeutic peptides for cancer therapy. Part II - cell cycle inhibitory peptides and apoptosis-inducing peptides

BACKGROUND

Therapeutic peptides have great potential as anticancer agents owing to their ease of rational design and target specificity. However, their utility in vivo is limited by low stability and poor tumor penetration.

OBJECTIVE

The authors review the development of peptide inhibitors with potential for cancer therapy. Peptides that arrest the cell cycle by mimicking CDK inhibitors or induce apoptosis directly are discussed.

METHODS

The authors searched Medline for articles concerning the development of therapeutic peptides and their delivery.

RESULTS/CONCLUSION

Inhibition of cancer cell proliferation directly using peptides that arrest the cell cycle or induce apoptosis is a promising strategy. Peptides can be designed that interact very specifically with cyclins and/or cyclin-dependent kinases and with members of apoptotic cascades. Use of these peptides is not limited by their design, as a rational approach to peptide design is much less challenging than the design of small molecule inhibitors of specific protein-protein interactions. However, the limitations of peptide therapy lie in the poor pharmacokinetic properties of these large, often charged molecules. Therefore, overcoming the drug delivery hurdles could open the door for effective peptide therapy, thus making an entirely new class of molecules useful as anticancer drugs.

NGFI-B targets mitochondria and induces cardiomyocyte apoptosis in restraint-stressed rats by mediating energy metabolism disorder

NGFI-B/Nur77/TR3, originally identified as an immediate-early gene rapidly induced by serum and growth factors, is a member of the steroid hormone nuclear receptor superfamily with no identified endogenous ligand. NGFI-B induces apoptosis in a number of cell lineages exposed to proapoptotic stimuli by directly targeting the mitochondria, inducing cytochrome c release. The present study was designed to determine the role of NGFI-B in cardiomyocytes of restraint-stressed rats. The NGFI-B content was increased in mitochondria and reduced in plasma as apoptosis increased. Analysis showed that NGFI-B induces cardiomyocyte apoptosis in restraint-stressed rats by mediating mitochondrial energy metabolism disorder. Several novel mitochondrial proteins, which correlate with NGFI-B, were reported in cardiomyocyte apoptosis of restraint-stressed rats. Five proteins associated with NGFI-B participate directly in mitochondrial energy metabolism. Studies of mitochondrial respiratory efficiency and ATP synthase activity strongly support the findings. These results provide significant information for comprehensively understanding the cellular mechanism of cardiovascular diseases.

Smac mimetics as new cancer therapeutics

The recent discovery of Smac and the elucidation of its structure and function have led to the rapid development of Smac mimetics, comprising Smac derivative and mimicking molecules, for use in cancer treatment. Smac is an endogenous proapoptotic protein that resides in the mitochondria and is released when a cell is triggered to undergo programmed cell death. One of the mechanisms by which Smac promotes apoptosis is through its ability to inhibit inhibitors of apoptosis (IAPs), by direct inhibition and/or proteasomal degradation of some members of the IAP family, and therefore disinhibit caspases. Thus, the use of Smac mimetics as anticancer agents follows a rational approach in cancer therapeutics. This approach directly targets dysregulated, neoplastic cells that overexpress IAPs or underexpress Smac. Although Smac mimetics are able to elicit an anticancer response when used alone, these molecules can also function effectively and synergistically when combined with other therapeutic agents. A variety of Smac mimetic types comprising peptides, polynucleotides, and compounds have been studied both in vitro and in vivo. This discussion addresses the current status of Smac mimetics in cancer research.

Caspase-9 activation by the apoptosome is not required for fas-mediated apoptosis in type II Jurkat cells

Activation of executioner caspases during receptor-mediated apoptosis in type II cells requires the engagement of the mitochondrial apoptotic pathway. Although it is well established that recruitment of mitochondria in this context involves the cleavage of Bid to truncated Bid (tBid), the precise post-mitochondrial signaling responsible for executioner caspase activation is controversial. Here, we used distinct clones of type II Jurkat T-lymphocytes in which the mitochondrial apoptotic pathway had been inhibited to investigate the molecular requirements necessary for Fas-induced apoptosis. Cells overexpressing either Bcl-2 or Bcl-x(L) were protected from apoptosis induced by agonistic anti-Fas antibody. By comparison, Apaf-1-deficient Jurkat cells were sensitive to anti-Fas, exhibiting Bid cleavage, Bak activation, the release of cytochrome c and Smac, and activation of executioner caspase-3. Inhibiting downstream caspase activation with the pharmacological inhibitor Z-DEVD-fmk or by expressing the BIR1/BIR2 domains of X-linked inhibitor of apoptosis protein (XIAP) decreased all anti-Fas-induced apoptotic changes. Additionally, pretreatment of Bcl-x(L)-overexpressing cells with a Smac mimetic sensitized these cells to Fas-induced apoptosis. Combined, our findings strongly suggest that Fas-mediated activation of executioner caspases and induction of apoptosis do not depend on apoptosome-mediated caspase-9 activation in prototypical type II cells.

IKKbeta-mediated nuclear factor-kappaB activation attenuates smac mimetic-induced apoptosis in cancer cells

Smac mimetics (SM) have been recently reported to kill cancer cells through the extrinsic apoptosis pathway mediated by autocrine tumor necrosis factor (TNF). SM also activates nuclear factor-kappaB (NF-kappaB). However, how SM induces NF-kappaB and the role of NF-kappaB in SM-induced cancer cell death has not been well elucidated. We found that effective blockage of NF-kappaB had no detectable effect on SM compound 3 (SMC3)-induced TNF secretion, suggesting that the induction of TNF by SMC3 is independent of NF-kappaB. Conversely, SMC3-induced NF-kappaB activation was found to be mediated by autocrine TNF because this effect of SMC3 was effectively inhibited when TNF was blocked with either a TNF neutralizing antibody or TNF small interfering RNA. In addition, although SMC3 dramatically reduced c-IAP1 level, it had marginal effect on c-IAP2 expression, TNF-induced RIP modification, NF-kappaB activation, and downstream antiapoptosis NF-kappaB target expression. Furthermore, blocking NF-kappaB by targeting IKKbeta or RelA substantially potentiated SMC3-induced cytotoxicity, suggesting that the NF-kappaB pathway inhibits SMC3-induced apoptosis in cancer cells. Our results show that through TNF autocrine, SM induces an IKKbeta-mediated NF-kappaB activation pathway that protects cancer cells against SM-induced apoptosis, and thus, NF-kappaB blockage could be an effective approach for improving the anticancer value of SM.

Expression of Livin, Smac/DIABLO and PTEN and their relationships with human gastric adenocarcinoma

AIM: To investigate the regulating role and significance of protein Livin, mitochondrial apoptosis-promoting protein Smac/DIABLO and PTEN in carcinogenesis and progression mechanism.

METHODS: Real-time RT PCR was used to examine the expression of Livin mRNA and Smac/DIABLO mRNA in 75 gastric carcinoma specimens, 20 normal gastric tissues and 20 adjacent tissues. The expression and location of Livin, Smac/DIABLO and PTEN were detected using Western blot combined with immunohistochemistry (SP).

RESULTS: The expression of Livin mRNA was significantly up-regulated in gastric carcinoma specimens (6.374 ± 4.759), however, no expression was found in normal or adjacent tissues. There was a significance in expression between low differentiated carcinoma group and lymph node metastases group (χ² = 9.60, 5.51, P < 0.01 or 0.05). The expression of Livin mRNA had no correlation with tumor size, invasion of nerve or TNM stage. The expression of Smac/DIABLO mRNA was lower in gastric cancer tissues than in normal gastric tissues and adjacent tissues, but there was no significant difference (0.731 ± 0.420 vs 1.104 ± 0.276, 1.061 ± 0.737, all P > 0.05). The expression of Smac/DIABLO mRNA in gastric cancer tissues had no correlation with clinical pathological factors of gastric carcinoma. The expression of Smac/DIABLO held significant difference from intestinal-type gastric carcinoma to diffuse-type gastric carcinoma (χ² = 5.06, P < 0.05). The expression of PTEN was not determined in gastric carcinoma tissues and normal tissues.

CONCLUSION: There is a diversity of expression of Livin, Smac/DIABLO and PTEN in different stages and pathological types of gastric carcinoma. Real-time RT PCR and the expression of Livin and Smac/DIABLO could be beneficial to diagnosis of gastric carcinogenesis, severity of differentiation, and chemotherapy sensitivity.

Patterns of variation in the inhibitor of apoptosis 1 gene of Aedes triseriatus, a transovarial vector of La Crosse virus

Aedes triseriatus mosquitoes transovarially transmit (TOT) La Crosse virus (LACV) to their offspring with minimal damage to infected ovaries. Ae. triseriatus inhibitor of apoptosis 1 (AtIAP1) is a candidate gene conditioning the ability to vertically transmit LACV. AtIAP1 was amplified and sequenced in adult mosquitoes reared from field-collected eggs. Sequence analysis showed that AtIAP1 has much higher levels of genetic diversity than genes found in other mosquitoes. Despite this large amount of diversity, strong purifying selection of polymorphisms located in the Baculovirus inhibitor of apoptosis repeat (BIR) domains and, to a lesser extent, in the 5' untranslated region seems to indicate that these portions of AtIAP1 are the most important. These results indicate that the 5'UTR plays an important role in transcription and translation and that the BIR domains are important functional domains in the protein. Single nucleotide polymorphisms (SNPs) were compared between LACV-positive and -negative mosquitoes to test for associations between segregating sites and the ability to be transovarially infected with LACV. Initial results indicated that five SNPs were associated with TOT of LACV; however, these results were not replicable with larger sample sizes.

Skeletal muscle differentiation evokes endogenous XIAP to restrict the apoptotic pathway

Myotube apoptosis occurs normally during muscle development and aging but it can lead to destruction of skeletal muscle in neuromuscular diseases. Therefore, understanding how myotube apoptosis is regulated is important for developing novel strategies for treatment of muscle loss. We investigated the regulation of apoptosis in skeletal muscle and report a striking increase in resistance to apoptosis following differentiation. We find mitotic C2C12 cells (myoblast-like cells) are sensitive to cytosolic cytochrome c microinjection. However, differentiated C2C12 cells (myotube-like cells) and primary myotubes are markedly resistant. This resistance is due to endogenous X-linked inhibitor of apoptotic protein (XIAP). Importantly, the selective difference in the ability of XIAP to block myotube but not myoblast apoptosis is not due to a change in XIAP but rather a decrease in Apaf-1 expression. This decrease in Apaf-1 links XIAP to caspase activation and death. Our findings suggest that in order for myotubes to die, they may degrade XIAP, functionally inactivate XIAP or upregulate Apaf-1. Importantly, we identify a role for endogenous Smac in overcoming XIAP to allow myotube death. However, in postmitotic cardiomyocytes, where XIAP also restricts apoptosis, endogenous Smac was not capable of overcoming XIAP to cause death. These results show that as skeletal muscle differentiate, they become resistant to apoptosis because of the ability of XIAP to regulate caspase activation. The increased restriction of apoptosis in myotubes is presumably important to ensure the long term survival of these postmitotic cells as they play a vital role in the physiology of organisms.

Naphthoimidazoles promote different death phenotypes in Trypanosoma cruzi

SUMMARY

In a screening of 65 derivatives of natural quinones using bloodstream trypomastigotes of Trypanosoma cruzi, the 3 naphthoimidazoles derived from beta-lapachone - N1, N2 and N3--were selected as the most active. Investigation of their mode of action led to the characterization of mitochondrion, reservosomes and DNA as their main targets, and stimulated further studies on death pathways. Ultrastructural analysis revealed both autophagic (autophagosomes) and apoptotic-like (membrane blebbing) phenotypes. Flow cytometry analysis showed, in N2-treated trypomastigotes, a small increase of phosphatidylserine exposure, and a large increase in the percentage of necrosis, caused by N1 or N2. These death phenotypes were not detected in treated epimastigotes. The strong increase in labelling of monodansyl cadaverine, the inhibition of the death process by wortmannin or 3-methyladenine, the overexpression of ATG genes in treated epimastigotes, together with ultrastructural evidence point to autophagy as the predominant phenotype induced by the naphthoimidazoles. However, there are other pathways occurring concomitantly with variable intensities, justifying the need to detail the molecular features involved.

The mitochondrial death pathway: a promising therapeutic target in diseases

The mitochondrial pathway to apoptosis is a major pathway of physiological cell death in vertebrates. The mitochondrial cell death pathway commences when apoptogenic molecules present between the outer and inner mitochondrial membranes are released into the cytosol by mitochondrial outer membrane permeabilization (MOMP). BCL-2 family members are the sentinels of MOMP in the mitochondrial apoptotic pathway; the pro-apoptotic B cell lymphoma (BCL)-2 proteins, BCL-2 associated x protein and BCL-2 antagonist killer 1 induce MOMP whereas the anti-apoptotic BCL-2 proteins, BCL-2, BCL-x_l and myeloid cell leukaemia 1 prevent MOMP from occurring. The release of pro-apoptotic factors such as cytochrome c from mitochondria leads to formation of a multimeric complex known as the apoptosome and initiates caspase activation cascades. These pathways are important for normal cellular homeostasis and play key roles in the pathogenesis of many diseases. In this review, we will provide a brief overview of the mitochondrial death pathway and focus on a selection of diseases whose pathogenesis involves the mitochondrial death pathway and we will examine the various pharmacological approaches that target this pathway.

Cytoprotective effects of IAPs revealed by a small molecule antagonist

Deregulated expression of members of the IAP (inhibitor of apoptosis) family has been identified in a wide variety of neoplastic cells, and synthetic IAP antagonists represent a promising novel class of chemotherapeutic agents. Early work focused on the ability of these compounds to block the caspase-inhibitory function of XIAP (X-linked IAP). However, recent studies have shown that IAP antagonists, although primarily designed to target XIAP, trigger ubiquitin-mediated degradation of two related proteins, c-IAP (cellular IAP) 1 and c-IAP2, and through this process potentiates the death of tumour cells via autocrine cellular-signalling pathways. In this context, the relative contribution of XIAP as a target of this class of compounds is unclear. In the present study, we examine the involvement of XIAP using a recently described synthetic IAP antagonist, AEG40730, and through comparison of a human XIAP-depleted tumour cell line with its isogenic wild-type control line. Treatment with nanomolar concentrations of AEG40730 resulted in the loss of both XIAP and c-IAP1 proteins, albeit with different kinetics. Although XIAP-deficient HCT116 cells retained some sensitivity to external apoptotic stimuli, the results suggest that IAP antagonists, such as AEG40730, exert their apoptosis-enhancing effects through XIAP in addition to the c-IAPs. These results indicate that IAP antagonists can target multiple IAPs to augment distinct pro-apoptotic signalling pathways, thereby revealing the potential for these compounds in cancer therapy and underscoring the promise of IAP-targeted therapies.

Caspase-independent apoptosis in Friend's erythroleukemia cells: role of mitochondrial ATP synthesis impairment in relocation of apoptosis-inducing factor and endonuclease G

Mitochondria have emerged as the central components of both caspase-dependent and independent apoptosis signalling pathways through release of different apoptogenic proteins. We previously documented that parental and differentiated Friend's erythroleukemia cells were induced to apoptosis by oligomycin and H(2)O(2) exposure, showing that the energy impairment occurring in both cases as a consequence of a severe mitochondrial F(0)F(1)ATPsynthase inactivation was a common early feature. Here we provide evidence for AIF and Endo G mitochondrio-nuclear relocation in both cases, as a component of caspase-independent apoptosis pathways. No detectable change in mitochondrial transmembrane potential and no variation in mitochondrial levels of Bcl-2 and Bax are observed. These results point to the osmotic rupture of the mitochondrial outer membrane as occurring in response to cell exposure to the two energy-impairing treatments under conditions preserving the mitochondrial inner membrane. A critical role of the mitochondrial F(0)F(1)ATP synthase inhibition in this process is also suggested.

Combined inhibition of FLIP and XIAP induces Bax-independent apoptosis in type II colorectal cancer cells

Death receptors can directly (type I cells) or indirectly induce apoptosis by activating mitochondrial-regulated apoptosis (type II cells). The level of caspase 8 activation is thought to determine whether a cell is type I or II, with type II cells less efficient at activating this caspase following death receptor activation. FLICE-inhibitory protein (FLIP) blocks death receptor-mediated apoptosis by inhibiting caspase 8 activation; therefore, we assessed whether silencing FLIP could convert type II cells into type I. FLIP silencing-induced caspase 8 activation in Bax wild-type and null HCT116 colorectal cancer cells; however, complete caspase 3 processing and apoptosis were only observed in Bax wild-type cells. Bax-null cells were also more resistant to chemotherapy and tumor necrosis factor-related apoptosis inducing ligand and, unlike the Bax wild-type cells, were not sensitized to these agents by FLIP silencing. Further analyses indicated that release of second mitochondrial activator of caspases from mitochondria and subsequent inhibition of X-linked inhibitor of apoptosis protein (XIAP) was required to induce full caspase 3 processing and apoptosis following FLIP silencing. These results indicate that silencing FLIP does not necessarily bypass the requirement for mitochondrial involvement in type II cells. Furthermore, targeting FLIP and XIAP may represent a therapeutic strategy for the treatment of colorectal tumors with defects in mitochondrial-regulated apoptosis.

Smac deficiency affects endoplasmic reticulum stress-induced apoptosis in human colon cancer cells

Thapsigargin (TG) is a sesquiterpen lactone that inhibits the endoplasmic reticulum (ER) calcium ATPases to disrupt calcium homeostasis and consequently induces ER stress. We have previously reported that TG induces apoptosis by engaging the death receptor 5 (DR5) and the intrinsic pathways. Second mitochondrial-derived activator (Smac) is an important modulator of apoptosis that induces activation of caspases by antagonizing inhibitors of apoptosis (IAPs). In this study, we have utilized Smac-proficient and -deficient human colon cancer cells to investigate the effects of Smac deficiency during ER-stress-induced apoptosis. Our results indicate that Smac deficiency considerably affects ER stress-induced apoptosis in human colon cancer cells. For example, ER stress inducing agent TG upregulates DR5, and activates caspases 3, 9 and 8 in Smac-proficient cells. In Smac-deficient cells, although TG-induced DR5 upregulation is not affected, activation of caspases 3, 9 and 8 is affected. Smac deficiency also affects TG-induced cytochrome c release from mitochondria into cytosol suggesting the existence of a potential cross-talk between Smac and cytochrome c. Thus, our results indicate that ER stress-induced apoptosis also engages Smac for transduction of apoptotic signals in human colon cancer cells and that a potential feedback signaling between Smac and cytochrome c appears to modulate the intrinsic pathway of apoptosis.

Effects of endotoxin on liver Smac apoptosis channel

To study the effect of endotoxin on liver apoptosis, L02 liver cells were cultured and passaged in vitro, and then stimulated by endotoxin at 10 mg/mL for 4, 8, 16 and 24 h respectively. Liver apoptosis was flow cytometrically and fluorescently detected. Immunohistochemistry was used to detect the delivery of smac and caspase9. The delivery of liver cell smac and the activity of caspase3 were measured by caspase3 assay kit. The hepatic failure models of rats were established by using D-galactosamine. The blood serum and liver tissues were collected for the detection of the liver function, the level of endotoxin and the activity of caspase3 by using chromogenic substrate limulus amebocyte lysate method (LAL) and caspase3 active assay kit. The expression of smac and caspase9 in liver cells was detected by Western blotting. With in vitro study, the L02 cells stimulated by LPS condensed into conglobation and formed apoptotic bodies. After those cells were stained by hoechst, the apoptotic cells displayed blue color under the fluorescent microscope. The apoptosis rate was increased over time and the apoptosis was mainly of advanced stage. Meanwhile, the rate of smac delivery and activity of caspase9 and caspase3 were increased on L02 cell membrane. In vivo, hepatic failure and obvious endotoxemia were induced by injection of more than 200 mg/kg D-GalN. Hepatic mitochondria smac was reduced with dosage of D-GalN and, on the contrary, the activity of caspase3 was increased. D-GalN at 200 mg/kg increased Caspase9 while D-GalN at 300 mg/kg decreased caspase9. Mitochondria signal channel plays an important role in the endotoxin-induced apoptosis of hepatic cells by promoting the release of smac from mitochondria to cytoplasm and activating caspase9 and caspase3 in its low-level channel.

Induction of apoptosis by withaferin A in human leukemia U937 cells through down-regulation of Akt phosphorylation

Withaferin A, a major chemical constituent of Withania somnifera, has been reported for its tumor cell growth inhibitory activity, antitumor effects, and impairing metastasis and angiogenesis. The mechanism by which withaferin A initiates apoptosis remains poorly understood. In the present report, we investigated the effect of withaferin A on the apoptotic pathway in U937 human promonocytic cells. We show that withaferin A induces apoptosis in association with the activation of caspase-3. JNK and Akt signal pathways play crucial roles in withaferin A-induced apoptosis in U937 cells. Furthermore, we have shown that overexpression of Bcl-2 and active Akt (myr-Akt) in U937 cells inhibited the induction of apoptosis, activation of caspase-3, and PLC-gamma1 cleavage by withaferin A. Taken together, our results indicated that the JNK and Akt pathways and inhibition of NF-kappaB activity were key regulators of apoptosis in response to withaferin A in human leukemia U937 cells.