Assembling the building blocks: structure and function of inhibitor of apoptosis proteins

The differential interactomes of the KRAS splice variants identify BIRC6 as a ubiquitin ligase for KRAS4A

Transcripts of the KRAS locus are alternatively spliced to generate two proteins, KRAS4A and KRAS4B, which differ in their membrane-targeting sequences. These splice variants have been conserved for more than 450 million years, suggesting non-overlapping functions driven by differential membrane association. Here, we use proximity labeling to map the differential interactomes of the KRAS splice variants. We find 24 and 10 proteins that interact specifically with KRAS4A or KRAS4B, respectively. The KRAS interacting protein most specific to KRAS4A is BIRC6, a large member of the inhibitor of apoptosis protein family unique in possessing E2/E3 ubiquitin ligase activity. We find that this interaction takes place on the Golgi apparatus and results in the mono- and di-ubiquitination of KRAS4A at lysines 128 and 147. Silencing BIRC6 diminishes GTP loading of and growth stimulation by KRAS4A but not KRAS4B. Thus, BIRC6 is a ubiquitin ligase that inhibits apoptosis and also modifies KRAS4A.

Towards New Anti-Inflammatory Agents: Design, Synthesis and Evaluation of Molecules Targeting XIAP-BIR2

The X-chromosome-linked inhibitor of apoptosis protein (XIAP) plays a crucial role in controlling cell survival across multiple regulated cell death pathways and coordinating a range of inflammatory signalling events. The discovery of selective inhibitors for XIAP-BIR2, able to disrupt the direct physical interaction between XIAP and RIPK2, offer promising therapeutic options for NOD2-mediated diseases like Crohn's disease, sarcoidosis, and Blau syndrome. The objective of this study was to design, synthesize, and evaluate small synthetic molecules with binding selectivity to XIAP-BIR2 domain. To achieve this, we applied an interdisciplinary drug design approach and firstly we have synthesized an initial fragment library to achieve a first XIAP inhibition activity. Then using a growing strategy, larger compounds were synthesized and one of them presents a good selectivity for XIAP-BIR2 versus XIAP-BIR3 domain, compound 20 c. The ability of compound 20 c to block the NOD1/2 pathway was confirmed in cell models. These data show that we have synthesized molecules capable of blocking NOD1/2 signalling pathways in cellulo, and ultimately leading to new anti-inflammatory compounds.

Overcoming Cancer Drug Resistance with Nanoparticle Strategies for Key Protein Inhibition

Drug resistance remains a critical barrier in cancer therapy, diminishing the effectiveness of chemotherapeutic, targeted, and immunotherapeutic agents. Overexpression of proteins such as B-cell lymphoma 2 (Bcl-2), inhibitor of apoptosis proteins (IAPs), protein kinase B (Akt), and P-glycoprotein (P-gp) in various cancers leads to resistance by inhibiting apoptosis, enhancing cell survival, and expelling drugs. Although several inhibitors targeting these proteins have been developed, their clinical use is often hampered by systemic toxicity, poor bioavailability, and resistance development. Nanoparticle-based drug delivery systems present a promising solution by improving drug solubility, stability, and targeted delivery. These systems leverage the Enhanced Permeation and Retention (EPR) effect to accumulate in tumor tissues, reducing off-target toxicity and increasing therapeutic efficacy. Co-encapsulation strategies involving anticancer drugs and resistance inhibitors within nanoparticles have shown potential in achieving coordinated pharmacokinetic and pharmacodynamic profiles. This review discusses the mechanisms of drug resistance, the limitations of current inhibitors, and the advantages of nanoparticle delivery systems in overcoming these challenges. By advancing these technologies, we can enhance treatment outcomes and move towards more effective cancer therapies.

Small Molecule Inhibitors Targeting the "Undruggable" Survivin: The Past, Present, and Future from a Medicinal Chemist's Perspective

Survivin, a homodimeric protein and a member of the IAP family, plays a vital function in cell survival and cycle progression by interacting with various proteins and complexes. Its expression is upregulated in cancers but not detectable in normal tissues. Thus, it has been regarded and validated as an ideal cancer target. However, survivin is "undruggable" due to its lack of enzymatic activities or active sites for small molecules to bind/inhibit. Academic and industrial laboratories have explored different strategies to overcome this hurdle over the past two decades, with some compounds advanced into clinical testing. These strategies include inhibiting survivin expression, its interaction with binding partners and homodimerization. Here, we provide comprehensive analyses of these strategies and perspective on different small molecule survivin inhibitors to help drug discovery targeting "undruggable" proteins in general and survivin specifically with a true survivin inhibitor that will prevail in the foreseeable future.

Structure shows that the BIR2 domain of E3 ligase XIAP binds across the RIPK2 kinase dimer interface

RIPK2 is an essential adaptor for NOD signalling and its kinase domain is a drug target for NOD-related diseases, such as inflammatory bowel disease. However, recent work indicates that the phosphorylation activity of RIPK2 is dispensable for signalling and that inhibitors of both RIPK2 activity and RIPK2 ubiquitination prevent the essential interaction between RIPK2 and the BIR2 domain of XIAP, the key RIPK2 ubiquitin E3 ligase. Moreover, XIAP BIR2 antagonists also block this interaction. To reveal the molecular mechanisms involved, we combined native mass spectrometry, NMR, and cryo-electron microscopy to determine the structure of the RIPK2 kinase BIR2 domain complex and validated the interface with in cellulo assays. The structure shows that BIR2 binds across the RIPK2 kinase antiparallel dimer and provides an explanation for both inhibitory mechanisms. It also highlights why phosphorylation of the kinase activation loop is dispensable for signalling while revealing the structural role of RIPK2-K209 residue in the RIPK2-XIAP BIR2 interaction. Our results clarify the features of the RIPK2 conformation essential for its role as a scaffold protein for ubiquitination.

Computational Tool to Design Small Synthetic Inhibitors Selective for XIAP-BIR3 Domain

X-linked inhibitor of apoptosis protein (XIAP) exercises its biological function by locking up and inhibiting essential caspase-3, -7 and -9 toward apoptosis execution. It is overexpressed in multiple human cancers, and it plays an important role in cancer cells' death skipping. Inhibition of XIAP-BIR3 domain and caspase-9 interaction was raised as a promising strategy to restore apoptosis in malignancy treatment. However, XIAP-BIR3 antagonists also inhibit cIAP1-2 BIR3 domains, leading to serious side effects. In this study, we worked on a theoretical model that allowed us to design and optimize selective synthetic XIAP-BIR3 antagonists. Firstly, we assessed various MM-PBSA strategies to predict the XIAP-BIR3 binding affinities of synthetic ligands. Molecular dynamics simulations using hydrogen mass repartition as an additional parametrization with and without entropic term computed by the interaction entropy approach produced the best correlations. These simulations were then exploited to generate 3D pharmacophores. Following an optimization with a training dataset, five features were enough to model XIAP-BIR3 synthetic ligands binding to two hydrogen bond donors, one hydrogen bond acceptor and two hydrophobic groups. The correlation between pharmacophoric features and computed MM-PBSA free energy revealed nine residues as crucial for synthetic ligand binding: Thr308, Glu314, Trp323, Leu307, Asp309, Trp310, Gly306, Gln319 and Lys297. Ultimately, and three of them seemed interesting to use to improve XIAP-BR3 versus cIAP-BIR3 selectivity: Lys297, Thr308 and Asp309.

Inhibitor of apoptosis proteins as therapeutic targets in bladder cancer

Evasion from apoptosis is a hallmark of cancer. Inhibitor of apoptosis proteins (IAPs) contribute to this hallmark by suppressing the induction of cell death. IAPs were found to be overexpressed in cancerous tissues and to contribute to therapeutic resistance. The present review focuses on the IAP members cIAP1, cIAP2, XIAP, Survivin and Livin and their importance as potential therapeutic targets in bladder cancer.

Structure of the pre-mRNA leakage 39-kDa protein reveals a single domain of integrated zf-C3HC and Rsm1 modules

In Saccharomyces cerevisiae, the pre-mRNA leakage 39-kDa protein (ScPml39) was reported to retain unspliced pre-mRNA prior to export through nuclear pore complexes (NPCs). Pml39 homologs outside the Saccharomycetaceae family are currently unknown, and mechanistic insight into Pml39 function is lacking. Here we determined the crystal structure of ScPml39 at 2.5 Å resolution to facilitate the discovery of orthologs beyond Saccharomycetaceae, e.g. in Schizosaccharomyces pombe or human. The crystal structure revealed integrated zf-C3HC and Rsm1 modules, which are tightly associated through a hydrophobic interface to form a single domain. Both zf-C3HC and Rsm1 modules belong to the Zn-containing BIR (Baculovirus IAP repeat)-like super family, with key residues of the canonical BIR domain being conserved. Features unique to the Pml39 modules refer to the spacing between the Zn-coordinating residues, giving rise to a substantially tilted helix αC in the zf-C3HC and Rsm1 modules, and an extra helix αAB' in the Rsm1 module. Conservation of key residues responsible for its distinct features identifies S. pombe Rsm1 and Homo sapiens NIPA/ZC3HC1 as structural orthologs of ScPml39. Based on the recent functional characterization of NIPA/ZC3HC1 as a scaffold protein that stabilizes the nuclear basket of the NPC, our data suggest an analogous function of ScPml39 in S. cerevisiae.

Impact of Group II Baculovirus IAPs on Virus-Induced Apoptosis in Insect Cells

Apoptosis plays an important role in virus-host interactions and is a major element of the insect immune response. Exploring the regulatory mechanisms of virus-induced apoptosis through the expression of apoptotic genes holds important research and application value. Functional research on the reported inhibitor of apoptosis proteins (IAPs) mainly focuses on the group I baculovirus, while the functions of the group II baculovirus IAPs remains unclear. To explore its role in the regulation of the apoptosis of insect cells, we constructed the transient expression vector (pIE1 vectors) and the recombinant baculovirus expressing Bsiap genes (from the Buzura suppressaria nucleopolyhedrovirus) of the group II baculovirus. Apoptosis gene expression results and the virus-induced apoptosis rate show that the overexpression of BsIAP1 could promote apoptosis in insect cells. However, the overexpression of BsIAP2 and BsIAP3 decreases the expression of apoptotic genes, revealing an inhibitory effect. Results on the impact of baculovirus-induced apoptosis also confirm that BsIAP1 reduces viral nucleocapsid expression and the baculovirus titer, while BsIAP2 and BsIAP3 increase them significantly. Furthermore, compared with single expression, the co-expression of BsIAP2 and BsIAP3 significantly reduces the rate of virus-induced apoptosis and improves the expression of nucleocapsids and the titer of offspring virus, indicating the synergistic effect on BsIAP2 and BsIAP3. In addition, combined expression of all three BsIAPs significantly reduced levels of intracellular apoptosis-related genes (including apoptosis and anti-apoptosis genes), as well as apoptosis rate and progeny virus titer, indicating that life activities in insect cells are also inhibited. These findings reveal the relationship between apoptosis and group II baculovirus IAP, which provide an experimental and theoretical basis for further exploration of the molecular mechanism between group II baculoviruses and insect cells.

Therapeutics Targeting the Core Apoptotic Machinery

Simple Summary

Cancer develops when the balance between cell death and cell division in tissues is dysregulated. A key focus of cancer drug discovery is identifying therapeutic agents which will selectively kill and eliminate cancer cells from the body. A number of proteins can prevent the death of cancer cells and developing inhibitors against these proteins to promote cancer cell death is a focus of recent drug discovery efforts. This review aims to summarize the key targets being explored, the drug development approaches being adopted, and the success or limitations of agents currently approved or in clinical development.

Abstract

Therapeutic targeting of the apoptotic pathways for the treatment of cancer is emerging as a valid and exciting approach in anti-cancer therapeutics. Accumulating evidence demonstrates that cancer cells are typically “addicted” to a small number of anti-apoptotic proteins for their survival, and direct targeting of these proteins could provide valuable approaches for directly killing cancer cells. Several approaches and agents are in clinical development targeting either the intrinsic mitochondrial apoptotic pathway or the extrinsic death receptor mediated pathways. In this review, we discuss the main apoptosis pathways and the key molecular targets which are the subject of several drug development approaches, the clinical development of these agents and the emerging resistance factors and combinatorial treatment approaches for this class of agents with existing and emerging novel targeted anti-cancer therapeutics.

Evolutionary novelty in the apoptotic pathway of aphids

Apoptotic processes play an important role in the development and physiology of almost all metazoan clades. In the highly diverse group of insects, apoptotic pathways have been characterized in only a few dipteran and lepidopteran species, which may not be representative of all insect species. Here, we report the first complete annotation of the apoptotic pathway in a hemipteran insect, the pea aphid Acyrthosiphon pisum. We showed that its apoptotic pathway is rewired compared to other insects, with a significant increase in the number of inhibitors of apoptosis (IAPs) and evidence for functional diversification and structural modularity of this protein family. These novelties are widespread in the aphid lineage, suggesting a yet not understood novel aphid-specific function of IAPs.

Apoptosis, a conserved form of programmed cell death, shows interspecies differences that may reflect evolutionary diversification and adaptation, a notion that remains largely untested. Among insects, the most speciose animal group, the apoptotic pathway has only been fully characterized in Drosophila melanogaster, and apoptosis-related proteins have been studied in a few other dipteran and lepidopteran species. Here, we studied the apoptotic pathway in the aphid Acyrthosiphon pisum, an insect pest belonging to the Hemiptera, an earlier-diverging and distantly related order. We combined phylogenetic analyses and conserved domain identification to annotate the apoptotic pathway in A. pisum and found low caspase diversity and a large expansion of its inhibitory part, with 28 inhibitors of apoptosis (IAPs). We analyzed the spatiotemporal expression of a selected set of pea aphid IAPs and showed that they are differentially expressed in different life stages and tissues, suggesting functional diversification. Five IAPs are specifically induced in bacteriocytes, the specialized cells housing symbiotic bacteria, during their cell death. We demonstrated the antiapoptotic role of these five IAPs using heterologous expression in a tractable in vivo model, the Drosophila melanogaster developing eye. Interestingly, IAPs with the strongest antiapoptotic potential contain two BIR and two RING domains, a domain association that has not been observed in any other species. We finally analyzed all available aphid genomes and found that they all show large IAP expansion, with new combinations of protein domains, suggestive of evolutionarily novel aphid-specific functions.

Targeting antisense mitochondrial noncoding RNAs induces bladder cancer cell death and inhibition of tumor growth through reduction of survival and invasion factors

Knockdown of the antisense noncoding mitochondrial RNAs (ASncmtRNAs) induces apoptotic death of several human tumor cell lines, but not normal cells, supporting a selective therapy against different types of cancer. In this work, we evaluated the effects of knockdown of ASncmtRNAs on bladder cancer (BCa). We transfected the BCa cell lines UMUC-3, RT4 and T24 with the specific antisense oligonucleotide Andes-1537S, targeted to the human ASncmtRNAs. Knockdown induced a strong inhibition of cell proliferation and increase in cell death in all three cell lines. As observed in UMUC-3 cells, the treatment triggered apoptosis, evidenced by loss of mitochondrial membrane potential and Annexin V staining, along with activation of procaspase-3 and downregulation of the anti-apoptotic factors survivin and Bcl-xL. Treatment also inhibited cell invasion and spheroid formation together with inhibition of N-cadherin and MMP 11. In vivo treatment of subcutaneous xenograft UMUC-3 tumors in NOD/SCID mice with Andes-1537S induced inhibition of tumor growth as compared to saline control. Similarly, treatment of a high-grade bladder cancer PDX with Andes-1537S resulted in a strong inhibition of tumor growth. Our results suggest that ASncmtRNAs could be potent targets for bladder cancer as adjuvant therapy.

Conformational characterization of full-length X-chromosome-linked inhibitor of apoptosis protein (XIAP) through an integrated approach

The X-chromosome-linked inhibitor of apoptosis protein (XIAP) is a multidomain protein whose main function is to block apoptosis by caspase inhibition. XIAP is also involved in other signalling pathways, including NF-κB activation and copper homeostasis. XIAP is overexpressed in tumours, potentiating cell survival and resistance to chemotherapeutics, and has therefore become an important target for the treatment of malignancy. Despite the fact that the structure of each single domain is known, the conformation of the full-length protein has never been determined. Here, the first structural model of the full-length XIAP dimer, determined by an integrated approach using nuclear magnetic resonance, small-angle X-ray scattering and electron paramagnetic resonance data, is presented. It is shown that XIAP adopts a compact and relatively rigid conformation, implying that the spatial arrangement of its domains must be taken into account when studying the interactions with its physiological partners and in developing effective inhibitors.

Smac mimetics as novel promising modulators of apoptosis in the treatment of breast cancer

Breast cancer is the most prevalent cancer in women. Despite improvements in treatment, the rate of breast cancer-related deaths is still high, and this issue needs further, accurate investigations. Although several treatment options are available, none of them are efficient for complete remission, particularly in advanced stages of the disease. It is known that cancerous cells have dysregulated apoptosis-related pathways, by which they can remain alive for a long time, expand freely, and escape from apoptosis-inducing drugs or antitumor immune responses. Therefore, modulation of apoptosis resistance in cancer cells may be an efficient strategy to overcome current problems faced in the development of immunotherapeutic approaches for the treatment of breast cancer. The inhibitors of apoptosis protein (IAPs) are important targets for cancer therapy because it has been shown that these molecules are overexpressed and highly active in various cancer cells and suppress apoptosis process in malignant cells by blockage of caspase proteins. There is evidence of Smac mimetics efficacy as a single agent; however, recent studies have indicated the efficacy of current anticancer immunotherapeutic approaches when combined with Smac mimetics, which are potent inhibitors of IAPs and synthesized mimicking Smac/Diablo molecules. In this review, we are going to discuss the efficacy of treatment of breast cancer by Smac mimetics alone or in combination with other therapeutics.

Butein induces apoptotic cell death of human cervical cancer cells

Butein is a chalcone, a flavonoid that is widely biosynthesized in plants. Butein has been identified to possess varied pharmacological activity and is extractable from traditional Chinese medicinal herbs, therefore applicable for disease treatment. Recently, in vitro and in vivo studies have shown that butein may induce apoptotic cell death in various human cancer cells. In this study we investigated the apoptotic effect of butein and the underlying mechanisms in human cervical cancer cells. Two cell lines, C-33A and SiHa cells, were treated with butein at different dosages for different durations. The effect of butein on cell viability was assessed by MTT assay, which revealed that butein exerted cytotoxicity in both cervical cancer cells in a dose- and time-dependent fashion. Apoptotic pathway-related factors in the butein-treated cervical cancer cells were then examined. JC-1 flow cytometry, cytochrome c assay, and caspase activity assays demonstrated that butein disturbed mitochondrial transmembrane potential, and increased cytosolic cytochrome c levels and caspase activities in both cervical cancer cells. Western blot analysis revealed that butein downregulated anti-apoptotic protein Bcl-xL and led to proteolytic cleavage of poly (ADP-ribose) polymerase. In addition, butein decreased expressions of the inhibitor of apoptosis (IAP) proteins, including X-linked IAP, survivin, and cellular IAP-1. The findings of this study suggest that butein can decrease cervical cancer cell viability via a pro-apoptotic effect, which involves inhibition of the IAP proteins and activation of both extrinsic and intrinsic pro-apoptotic pathways. Therefore, butein may be applicable for cervical cancer treatment.

Targeting antisense mitochondrial ncRNAs inhibits murine melanoma tumor growth and metastasis through reduction in survival and invasion factors

We reported that knockdown of the antisense noncoding mitochondrial RNAs (ASncmtRNAs) induces apoptotic death of several human tumor cell lines, but not normal cells, suggesting this approach for selective therapy against different types of cancer. In order to translate these results to a preclinical scenario, we characterized the murine noncoding mitochondrial RNAs (ncmtRNAs) and performed in vivo knockdown in syngeneic murine melanoma models. Mouse ncmtRNAs display structures similar to the human counterparts, including long double-stranded regions arising from the presence of inverted repeats. Knockdown of ASncmtRNAs with specific antisense oligonucleotides (ASO) reduces murine melanoma B16F10 cell proliferation and induces apoptosis in vitro through downregulation of pro-survival and metastasis markers, particularly survivin. For in vivo studies, subcutaneous B16F10 melanoma tumors in C57BL/6 mice were treated systemically with specific and control antisense oligonucleotides (ASO). For metastasis studies, tumors were resected, followed by systemic administration of ASOs and the presence of metastatic nodules in lungs and liver was assessed. Treatment with specific ASO inhibited tumor growth and metastasis after primary tumor resection. In a metastasis-only assay, mice inoculated intravenously with cells and treated with the same ASO displayed reduced number and size of melanoma nodules in the lungs, compared to controls. Our results suggest that ASncmtRNAs could be potent targets for melanoma therapy. To our knowledge, the ASncmtRNAs are the first potential non-nuclear targets for melanoma therapy.

Apoptosis as anticancer mechanism: function and dysfunction of its modulators and targeted therapeutic strategies

Apoptosis is a form of programmed cell death that results in the orderly and efficient removal of damaged cells, such as those resulting from DNA damage or during development. Apoptosis can be triggered by signals from within the cell, such as genotoxic stress, or by extrinsic signals, such as the binding of ligands to cell surface death receptors. Deregulation in apoptotic cell death machinery is an hallmark of cancer. Apoptosis alteration is responsible not only for tumor development and progression but also for tumor resistance to therapies. Most anticancer drugs currently used in clinical oncology exploit the intact apoptotic signaling pathways to trigger cancer cell death. Thus, defects in the death pathways may result in drug resistance so limiting the efficacy of therapies. Therefore, a better understanding of the apoptotic cell death signaling pathways may improve the efficacy of cancer therapy and bypass resistance. This review will highlight the role of the fundamental regulators of apoptosis and how their deregulation, including activation of anti-apoptotic factors (i.e., Bcl-2, Bcl-xL, etc) or inactivation of pro-apoptotic factors (i.e., p53 pathway) ends up in cancer cell resistance to therapies. In addition, therapeutic strategies aimed at modulating apoptotic activity are briefly discussed.

APOPTOSIS INDUCTION OF EPIFRIEDELINOL ON HUMAN CERVICAL CANCER CELL LINE

Background:

Present investigation evaluates the antitumor activity of epifriedelinol for the management of cervical cancer by inducing process of apoptosis.

Methods:

Human Cervical Cancer Cell Line, C33A and HeLa were selected for study and treated with epifriedelinol at a concentration of (50-1000 µg/ml). Cytotoxicity of epifriedelinol was estimated by MTT assay and induction of apoptosis was assessed by estimating the activity of caspase 3, 8 and 9 enzyme, apoptosis assay and translocation of cytochrome c. Moreover an expression of several proteins that plays role in the apoptosis process was estimated by western blot method.

Results:

Result of the study suggested that treatment with epifriedelinol significantly decrease the viability count of cancerous cell in a dose perndent manner and also enhances the formation of oligonucleosome in both the cell lines. However activity of caspase enzymes and translocation of cytochrome c were enhanced after treatment with epifriedelinol. It was also observed that epifriedelinol treatment alters the ratio of pro-apoptotic to anti-apoptotic proteins and enhances the expressions of inhibitor of apoptosis proteins (IAP).

Conclusion:

Result of our study proves the anticancer activity of epifriedelinol in cervical cancer by inducing apoptosis as treatment with it enhances the production of oligonucleosomes, translocation of cytochrome c and activity caspase enzymes.

The Culicoides sonorensis inhibitor of apoptosis 1 protein protects mammalian cells from apoptosis induced by infection with African horse sickness virus and bluetongue virus

African horse sickness virus (AHSV) and bluetongue virus (BTV) are arboviruses of the genus Orbivirus that are transmitted to their vertebrate hosts by Culicoides biting midges. These orbiviruses exhibit lytic infection (apoptosis) in mammalian cells, but cause persistent infection with no cytopathic effects in Culicoides sonorensis cells. Although regulation of apoptosis could thus be integral for establishing persistent virus infection in midge cells, nothing is known about the presence and function of apoptosis pathways in Culicoides midges and their derived cell lines. Here, we report the cloning and functional characterization of an inhibitor of apoptosis protein (IAP), designated CsIAP1, from C. sonorensis cells. The CsIAP1 protein contains two baculoviral IAP repeat (BIR) domains and a RING domain. Silencing of the Cs iap1 gene in C. sonorensis cells caused apoptosis, indicating that CsIAP1 plays a role in cell survival. Stable expression of the CsIAP1 protein in BSR mammalian cells suppressed apoptosis induced by AHSV-4 and BTV-10 infection, and biochemical data indicated that CsIAP1 is an inhibitor of mammalian caspase-9, an initiator caspase in the intrinsic apoptotic pathway. Mutagenesis studies indicated that the BIR2 and RING domains are required for the anti-apoptotic activity of CsIAP1. The results suggest that the mechanism by which CsIAP1 suppresses apoptosis in insect cells may involve inhibition of a Culicoides caspase-9 homologue through a mechanism that requires both the BIR2 and RING domains. This study provides the first evidence that the CsIAP1 protein is a key negative regulator of apoptosis in C. sonorensis cells.

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

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

Recurrent phagocytosis-induced apoptosis in the cyclical generation change of the compound ascidian Botryllus schlosseri

Colonies of the marine, filter-feeding ascidian Botryllus schlosseri undergo cyclical generation changes or takeovers. These events are characterised by the progressive resorption of adult zooids and their replacement by their buds that grow to adult size, open their siphons and start filtering. During the take-over, tissues of adult zooids undergo extensive apoptosis; circulating, spreading phagocytes enter the effete tissues, ingest dying cells acquiring a giant size and a round morphology. Then, phagocytes re-enter the circulation where they represent a considerable fraction (more than 20%) of circulating haemocytes. In this study, we evidence that most of these circulating phagocytes show morphological and biochemical signs of apoptosis. Accordingly, these phagocytes express transcripts of orthologues of the apoptosis-related genes Bax, AIF1 and PARP1. Electron microscopy shows that giant phagocytes contain apoptotic phagocytes inside their own phagocytic vacuole. The transcript of the orthologues of the anti-apoptotic gene IAP7 was detected only in spreading phagocytes, mostly abundant in phases far from the take-over. Therefore, the presented data suggest that, at take-over, phagocytes undergo phagocytosis-induced apoptosis (PIA). In mammals, PIA is assumed to be a process assuring the killing and the complete elimination of microbes, by promoting the disposal of terminally differentiated phagocytes and the resolution of infection. In B. schlosseri, PIA assumes a so far undescribed role, being required for the control of asexual development and colony homeostasis.

Baculovirus Inhibitor-of-Apoptosis Op-IAP3 Blocks Apoptosis by Interaction with and Stabilization of a Host Insect Cellular IAP

Baculovirus-encoded inhibitor of apoptosis (IAP) proteins likely evolved from their host cell IAP homologs, which function as critical regulators of cell death. Despite their striking relatedness to cellular IAPs, including the conservation of two baculovirus IAP repeat (BIR) domains and a C-terminal RING, viral IAPs use an unresolved mechanism to suppress apoptosis in insects. To define this mechanism, we investigated Op-IAP3, the prototypical IAP from baculovirus OpMNPV. We found that Op-IAP3 forms a stable complex with SfIAP, the native, short-lived IAP of host insect Spodoptera frugiperda. Long-lived Op-IAP3 prevented virus-induced SfIAP degradation, which normally causes caspase activation and apoptosis. In uninfected cells, Op-IAP3 also increased SfIAP steady-state levels and extended SfIAP's half-life. Conversely, SfIAP stabilization was lost or reversed in the presence of mutated Op-IAP3 that was engineered for reduced stability. Thus, Op-IAP3 stabilizes SfIAP and preserves its antiapoptotic function. In contrast to SfIAP, Op-IAP3 failed to bind or inhibit native Spodoptera caspases. Furthermore, BIR mutations that abrogate binding of well-conserved IAP antagonists did not affect Op-IAP3's capacity to prevent virus-induced apoptosis. Remarkably, Op-IAP3 also failed to prevent apoptosis when endogenous SfIAP was ablated by RNA silencing. Thus, Op-IAP3 requires SfIAP as a cofactor. Our findings suggest a new model wherein Op-IAP3 interacts directly with SfIAP to maintain its intracellular level, thereby suppressing virus-induced apoptosis indirectly. Consistent with this model, Op-IAP3 has evolved an intrinsic stability that may serve to repress signal-induced turnover and autoubiquitination when bound to its targeted cellular IAP.

IMPORTANCE

The IAPs were first discovered in baculoviruses because of their potency for preventing apoptosis. However, the antiapoptotic mechanism of viral IAPs in host insects has been elusive. We show here that the prototypical viral IAP, Op-IAP3, blocks apoptosis indirectly by associating with unstable, autoubiquitinating host IAP in such a way that cellular IAP levels and antiapoptotic activities are maintained. This mechanism explains Op-IAP3's requirement for native cellular IAP as a cofactor and the dispensability of caspase inhibition. Viral IAP-mediated preservation of the host IAP homolog capitalizes on normal IAP-IAP interactions and is likely the result of viral IAP evolution in which degron-mediated destabilization and ubiquitination potential have been reduced. This mechanism illustrates another novel means by which DNA viruses incorporate host death regulators that are modified for resistance to host regulatory controls for the purpose of suppressing host cell apoptosis and acquiring replication advantages.

Butein Shows Cytotoxic Effects and Induces Apoptosis in Human Ovarian Cancer Cells

Butein is a polyphenol, one of the compounds of chalcones, which are flavonoids that are widely biosynthesized in plants, and exhibits different pharmacological activities. Plants containing butein have been used in Chinese traditional medicine. Recently, it has been reported that butein suppresses proliferation and triggers apoptosis in various human cancer cells in vitro and in vivo. The aim of this study was to investigate its pro-apoptotic effect and mechanisms in two cultured human ovarian cancer cells (ES-2 and TOV-21G). The effects of butein on cell viability were assessed by a MTT assay at 3, 10, 30, and 100 μ/M. The apoptotic pathway related factors, including the mitochondrial transmembrane potential (MTP), cytochrome c, caspase cascade, and Bcl-2 family proteins, were examined. MTT assay revealed that butein was cytotoxic to both ovarian cancer cells in a dose- and time-dependent manner. JC-1 flow cytometry, cytochrome c, and caspase activity assays revealed that butein damaged the MTP, increased the level of cytosol cytochrome c and the activities of caspase-3, -8, and -9 in the two ovarian cancer cells. Western blot analysis revealed that butein down-regulated the anti-apoptotic proteins Bcl-2 and Bcl-xL and increased the pro-apoptotic proteins Bax and Bad. These findings suggest that butein-induced apoptosis in ovarian cancer cells via the activation of both extrinsic and intrinsic pathways. In addition, butein also down-regulated the expressions of the inhibitor of apoptosis (IAP) proteins, XIAP, survivin, CIAP-1, and CIAP-2. This indicates that the inhibition of IAP proteins was also involved in butein-induced apoptosis. The results of our study suggest that butein may be a promising anticancer agent in treating ovarian cancer.

Insect inhibitor-of-apoptosis (IAP) proteins are negatively regulated by signal-induced N-terminal degrons absent within viral IAP proteins

Inhibitor-of-apoptosis (IAP) proteins are key regulators of the innate antiviral response by virtue of their capacity to respond to signals affecting cell survival. In insects, wherein the host IAP provides a primary restriction to apoptosis, diverse viruses trigger rapid IAP depletion that initiates caspase-mediated apoptosis, thereby limiting virus multiplication. We report here that the N-terminal leader of two insect IAPs, Spodoptera frugiperda SfIAP and Drosophila melanogaster DIAP1, contain distinct instability motifs that regulate IAP turnover and apoptotic consequences. Functioning as a protein degron, the cellular IAP leader dramatically shortened the life span of a long-lived viral IAP (Op-IAP3) when fused to its N terminus. The SfIAP degron contains mitogen-activated kinase (MAPK)-like regulatory sites, responsible for MAPK inhibitor-sensitive phosphorylation of SfIAP. Hyperphosphorylation correlated with increased SfIAP turnover independent of the E3 ubiquitin-ligase activity of the SfIAP RING, which also regulated IAP stability. Together, our findings suggest that the SfIAP phospho-degron responds rapidly to a signal-activated kinase cascade, which regulates SfIAP levels and thus apoptosis. The N-terminal leader of dipteran DIAP1 also conferred virus-induced IAP depletion by a caspase-independent mechanism. DIAP1 instability mapped to previously unrecognized motifs that are not found in lepidopteran IAPs. Thus, the leaders of cellular IAPs from diverse insects carry unique signal-responsive degrons that control IAP turnover. Rapid response pathways that trigger IAP degradation and initiate apoptosis independent of canonical prodeath gene (Reaper-Grim-Hid) expression may provide important innate immune advantages. Furthermore, the elimination of these response motifs within viral IAPs, including those of baculoviruses, explains their unusual stability and their potent antiapoptotic activity.

IMPORTANCE

Apoptosis is an effective means by which a host controls virus infection. In insects, inhibitor-of-apoptosis (IAP) proteins act as regulatory sentinels by responding to cellular signals that determine the fate of infected cells. We discovered that lepidopteran (moth and butterfly) IAPs, which are degraded upon baculovirus infection, are controlled by a conserved phosphorylation-sensitive degron within the IAP N-terminal leader. The degron likely responds to virus-induced kinase-specific signals for degradation through SKP1/Cullin/F-box complex-mediated ubiquitination. Such signal-induced destruction of cellular IAPs is distinct from degradation caused by well-known IAP antagonists, which act to expel IAP-bound caspases. The major implication of this study is that insects have multiple signal-responsive mechanisms by which the sentinel IAPs are actively degraded to initiate host apoptosis. Such diversity of pathways likely provides insects with rapid and efficient strategies for pathogen control. Furthermore, the absence of analogous degrons in virus-encoded IAPs explains their relative stability and antiapoptotic potency.

NPD1-mediated stereoselective regulation of BIRC3 expression through cREL is decisive for neural cell survival

Neuroprotectin D1 (NPD1), a docosahexaenoic acid (DHA)-derived mediator, induces cell survival in uncompensated oxidative stress (OS), neurodegenerations or ischemic stroke. The molecular principles underlying this protection remain unresolved. We report here that, in retinal pigment epithelial cells, NPD1 induces nuclear translocation and cREL synthesis that, in turn, mediates BIRC3 transcription. NPD1 activates NF-κB by an alternate route to canonical signaling, so the opposing effects of TNFR1 and NPD1 on BIRC3 expression are not due to interaction/s between NF-κB pathways. RelB expression follows a similar pattern as BIRC3, indicating that NPD1 also is required to activate cREL-mediated RelB expression. These results suggest that cREL, which follows a periodic pattern augmented by the lipid mediator, regulates a cluster of NPD1-dependent genes after cREL nuclear translocation. BIRC3 silencing prevents NPD1 induction of survival against OS. Moreover, brain NPD1 biosynthesis and selective neuronal BIRC3 abundance are increased by DHA after experimental ischemic stroke followed by remarkable neurological recovery. Thus, NPD1 bioactivity governs key counter-regulatory gene transcription decisive for retinal and brain neural cell integrity when confronted with potential disruptions of homeostasis.

IAPs: Modular regulators of cell signalling

Members of the inhibitor of apoptosis (IAP) family are characterised by the presence of at least one baculoviral IAP repeat (BIR) domain. However, during the course of evolution, other globular modules have been adopted to perform distinct functions. Consequently, the IAP family is now recognised as consisting of members that perform critical functions in different aspects of cellular regulation. In this review, the structural diversity present within the IAP protein family is presented. Known structures of individual domains are discussed and their properties are described in light of recent data. In particular the plasticity of BIR domains and their ability to accommodate different binding partners is highlighted, as well as the importance of communication between the domains in regulating the covalent attachment of ubiquitin.

Toxicity of sulcotrione and grape marc on Vicia faba cells

The cell toxicity of sulcotrione, a selective triketone herbicide, was evaluated on Vicia faba. Sulcotrione, trademark Mikado, grape marc, and mixtures of sulcotrione or Mikado with grape marc induced cell death. Addition of grape marc to either sulcotrione or Mikado enhanced cell death, especially with Mikado. Addition of grape marc to herbicides, sulcotrione, or Mikado resulted in different expression of genes usually associated with cell stress. Mixtures of grape marc and herbicides enhanced transcript accumulation for ubiquitin, hsp 70, and cytosolic superoxide dismutase, but did not change ascorbate peroxidase transcript accumulation. The results thus provide evidence that sulcotrione, Mikado, and mixtures with grape marc can trigger cell death and specific gene expressions. Cocktails of products with sulcotrione, such as commercial additives and grape marc, can modify biological features of pesticide. Moreover, grape marc differently enhanced cell toxicity of sulcotrione and Mikado, suggesting a synergy between pesticide products and grape marc.

Small-molecule SMAC mimetics as new cancer therapeutics

Apoptosis is a tightly regulated cellular process and faulty regulation of apoptosis is a hallmark of human cancers. Targeting key apoptosis regulators with the goal to restore apoptosis in tumor cells has been pursued as a new cancer therapeutic strategy. XIAP, cIAP1, and cIAP2, members of inhibitor of apoptosis (IAP) proteins, are critical regulators of cell death and survival and are attractive targets for new cancer therapy. The SMAC/DIABLO protein is an endogenous antagonist of XIAP, cIAP1, and cIAP2. In the last decade, intense research efforts have resulted in the design and development of several small-molecule SMAC mimetics now in clinical trials for cancer treatment. In this review, we will discuss the roles of XIAP, cIAP1, and cIAP2 in regulation of cell death and survival, and the design and development of small-molecule SMAC mimetics as novel cancer treatments.

Selective IAP inhibition results in sensitization of unstimulated but not CD40-stimulated chronic lymphocytic leukaemia cells to TRAIL-induced apoptosis

Despite recent advances in therapy, chronic lymphocytic leukaemia (CLL) remains incurable and new treatment strategies are therefore urgently required. Inhibitor of apoptosis proteins (IAPs) are over-expressed in CLL, suggesting both a role in disease pathogenesis and the potential for therapeutic targeting. To explore these questions, we evaluated the effects on primary CLL cells of AZD5582, a novel potent and selective inhibitor of IAPs. AZD5582 at nanomolar concentrations induced extensive degradation of cIAP-1 and cIAP-2, but minimally of X chromosome-linked IAP (XIAP). However, these effects of AZD5582 produced little or no direct cytotoxicity, nor did they sensitize CLL cells to p53-dependent killing by fludarabine or p53-independent killing by dexamethasone. In contrast, AZD5582 significantly enhanced apoptosis induced by the death receptor (DR) agonist tumour necrosis factor-related apoptosis-inducing ligand (TRAIL). Importantly, killing by TRAIL plus AZD5582 was independent of adverse prognostic features including TP53 deletion which is strongly associated with chemoresistance in CLL. Coculture experiments involving transfected mouse fibroblasts expressing human CD40L (CD154) to mimic the effect of T cells at sites of tissue involvement showed that CD40 stimulation almost completely prevented the killing of CLL cells by TRAIL plus AZD5582 despite up-regulating TRAIL receptors 1 and 2. In conclusion, our findings confirm the rate-limiting, upstream involvement of IAPs in the extrinsic but not intrinsic apoptotic pathway of CLL cells and suggest that drug combinations that simultaneously activate DRs and inhibit IAPs may have therapeutic potential in patients with CLL who have failed T-cell-depleting chemotherapy.

Down-regulation of the antisense mitochondrial non-coding RNAs (ncRNAs) is a unique vulnerability of cancer cells and a potential target for cancer therapy

Hallmarks of cancer are fundamental principles involved in cancer progression. We propose an additional generalized hallmark of malignant transformation corresponding to the differential expression of a family of mitochondrial ncRNAs (ncmtRNAs) that comprises sense and antisense members, all of which contain stem-loop structures. Normal proliferating cells express sense (SncmtRNA) and antisense (ASncmtRNA) transcripts. In contrast, the ASncmtRNAs are down-regulated in tumor cells regardless of tissue of origin. Here we show that knockdown of the low copy number of the ASncmtRNAs in several tumor cell lines induces cell death by apoptosis without affecting the viability of normal cells. In addition, knockdown of ASncmtRNAs potentiates apoptotic cell death by inhibiting survivin expression, a member of the inhibitor of apoptosis (IAP) family. Down-regulation of survivin is at the translational level and is probably mediated by microRNAs generated by dicing of the double-stranded stem of the ASncmtRNAs, as suggested by evidence presented here, in which the ASncmtRNAs are bound to Dicer and knockdown of the ASncmtRNAs reduces reporter luciferase activity in a vector carrying the 3′-UTR of survivin mRNA. Taken together, down-regulation of the ASncmtRNAs constitutes a vulnerability or Achilles' heel of cancer cells, suggesting that the ASncmtRNAs are promising targets for cancer therapy.

tLivin displays flexibility by promoting alternative cell death mechanisms

Livin is a member of the Inhibitor of Apoptosis (IAP) protein family that inhibits apoptosis triggered by a variety of stimuli. We previously demonstrated that while Livin inhibits caspase activity, caspases can cleave Livin to produce a truncated protein, tLivin and that this newly formed tLivin paradoxically induces cell death. However to date, the mechanism of tLivin-induced cell death is not fully understood. In this study, we set out to characterize the form of cell death mediated by tLivin. Here we demonstrate that, unlike most death-promoting proteins, tLivin is a flexible inducer of cell death capable of promoting necrosis or apoptosis in different cell lines. The unusual flexibility of tLivin is displayed by its ability to activate an alternative form of cell death when apoptosis is inhibited. Thus, tLivin can promote more than one form of cell death in the same cell type. Interestingly, in cells where tLivin induces necrosis, deletion of the caspase binding BIR domain results in tLivin-induced apoptosis, suggesting the BIR domain can potentially hamper the ability of tLivin to induce apoptosis. We further elucidate that tLivin activates the JNK pathway and both tLivin-induced apoptosis and necrosis are partially mediated by JNK activity. Acquired resistance to apoptosis, common in many tumors, impinges on the efficiency of conventional anti-cancer agents that function primarily by inducing apoptosis. The ability of tLivin to induce death of apoptosis-compromised cells makes it an attractive candidate for targeted cancer therapy.

Endogenous XIAP, but not other members of the inhibitory apoptosis protein family modulates cerebellar granule neurons survival

Programmed cell death plays a critical role during cerebellar development. In particular, it has been shown in vivo and in vitro that developing cerebellar granule neurons (CGN) die apoptotically. Apoptosis involves a series of morphological changes and the activation of caspases. Inhibitor of apoptosis proteins (IAPs) is implicated in negative regulation of caspase activation and apoptotic cell death. Although apoptotic death of CGN has been extensively studied, there is no information about the role of IAPs in the developing cerebellum. Here, we studied the participation of some members of IAPs in the survival of the developing rat CGN in culture and under physiological conditions. Under these conditions, we found a differential expression pattern of cIAP-1, cIAP-2, XIAP and survivin during cerebellar development in an age-dependent manner, highlighting the significant increase of XIAP levels. We also detected an interaction between XIAP and caspase 3 at postnatal day (P) 12 and 16. On the other hand, we found a significant decrease of XIAP levels in cultured CGN maintained in chronic potassium deprivation, an apoptotic condition, suggesting a possible relationship between XIAP levels and neuronal viability. Under these conditions, we also detected the interaction of XIAP with active caspase-3. The down-regulation of XIAP in CGN cultured under survival conditions (chronic potassium depolarization) induced a reduction of cell viability and an increment of apoptotic cells. These findings support the idea that IAPs could be involved in the survival of CGN and that XIAP might be critical for neuronal survival in cerebellar development and during chronic depolarization in cultured CGN through a mechanism involving caspase inhibition.

Cytotoxic effect and induction of apoptosis in human cervical cancer cells by Antrodia camphorata

Antrodia camphorata is a Chinese herb indigenous to Taiwan. Previous reports demonstrated that it could induce apoptosis in some cancer cells. The purpose of this study was to investigate the apoptotic effect of the crude extract of A. camphorata in cervical cancer cells. Two human cervical cancer cell lines, HeLa and C-33A, were treated with extract of A. camphorata (10-1000 μg/mL). We found that A. camphorata extract was cytotoxic to both cervical cancer cells in a dose- and time-dependent manner as examined by MTT assay. Treatment with A. camphorata extract at 400 μg/mL induced a 2.3- and 4.4-fold increase in oligonucleosome formation from the cleaved chromosomal DNA in HeLa and C-33A cells, respectively. A. camphorata extract also activated caspase-3, -8, and -9 activities and increased the cytosolic level of cytochrome c in both cell lines as the dosage increased. Furthermore, A. camphorata extract increased expressions of Bak, Bad and Bim, while decreasing expressions of Bcl-2 and Bcl-xL of the Bcl-2 family proteins in HeLa and C-33A cells. The expression of IAP proteins, XIAP and survivin, was also decreased in both cervical cancer cells after treatment with A. camphorata. Our in vitro study suggests that A. camphorata is cytotoxic to cervical cancer cells through both extrinsic and intrinsic apoptotic mechanisms. It could be used as a novel phytotherapeutic agent or auxiliary therapy in the treatment of cervical cancer.

New roles for old enzymes: killer caspases as the engine of cell behavior changes

It has become increasingly clear that caspases, far from being merely cell death effectors, have a much wider range of functions within the cell. These functions are as diverse as signal transduction and cytoskeletal remodeling, and caspases are now known to have an essential role in cell proliferation, migration, and differentiation. There is also evidence that apoptotic cells themselves can direct the behavior of nearby cells through the caspase-dependent secretion of paracrine signaling factors. In some processes, including the differentiation of skeletal muscle myoblasts, both caspase activation in differentiating cells as well as signaling from apoptotic cells has been reported. Here, we review the non-apoptotic outcomes of caspase activity in a range of different model systems and attempt to integrate this knowledge.

X-linked inhibitor of apoptosis protein negatively regulates neuronal differentiation through interaction with cRAF and Trk

X-linked Inhibitor of apoptosis protein (XIAP) has been classically identified as a cell death regulator. Here, we demonstrate a novel function of XIAP as a regulator of neurite outgrowth in neuronal cells. In PC12 cells, XIAP overexpression prevents NGF-induced neuronal differentiation, whereas NGF treatment induces a reduction of endogenous XIAP levels concomitant with the induction of neuronal differentiation. Accordingly, downregulation of endogenous XIAP protein levels strongly increases neurite outgrowth in PC12 cells as well as axonal and dendritic length in primary cortical neurons. The effects of XIAP are mediated by the mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinases (ERKs) pathway since blocking this pathway completely prevents the neuritogenesis mediated by XIAP downregulation. In addition, we found that XIAP binds to cRaf and Trk receptors. Our results demonstrate that XIAP plays a new role as a negative regulator of neurotrophin-induced neurite outgrowth and neuronal differentiation in developing neurons.

Cellular inhibitor of apoptosis protein-1 and survival of beta cells undergoing endoplasmic reticulum stress

Pancreatic beta cells rely heavily on the endoplasmic reticulum (ER) to process folding and posttranslational modification of a large amount of insulin and many other proteins and are therefore vulnerable to ER stress. The role of the ER is thus crucial in the regulation of beta cell function and survival through the unfolded protein response (UPR) pathways. However, the UPR can either allow cells to survive by adapting to stress or kill cells through apoptosis in a context-dependent manner. How cell fate is determined following UPR activation remains enigmatic. In this review, we discuss the molecular mechanisms linking ER stress to beta cell survival or apoptosis. Specifically, we focus on the role of the cellular inhibitor of apoptosis protein-1 and propose a new model for understanding survival of beta cells undergoing ER stress.

The HECTD3 E3 ubiquitin ligase facilitates cancer cell survival by promoting K63-linked polyubiquitination of caspase-8

Apoptosis resistance is a hurdle for cancer treatment. HECTD3, a new E3 ubiquitin ligase, interacts with caspase-8 death effector domains and ubiquitinates caspase-8 with K63-linked polyubiquitin chains that do not target caspase-8 for degradation but decrease the caspase-8 activation. HECTD3 depletion can sensitize cancer cells to extrinsic apoptotic stimuli. In addition, HECTD3 inhibits TNF-related apoptosis-inducing ligand (TRAIL)-induced caspase-8 cleavage in an E3 ligase activity-dependent manner. Mutation of the caspase-8 ubiquitination site at K215 abolishes the HECTD3 protection from TRAIL-induced cleavage. Finally, HECTD3 is frequently overexpressed in breast carcinomas. These findings suggest that caspase-8 ubiquitination by HECTD3 confers cancer cell survival.

Novel apoptosis suppressor Apsup from the baculovirus Lymantria dispar multiple nucleopolyhedrovirus precludes apoptosis by preventing proteolytic processing of initiator caspase Dronc

We previously identified a novel baculovirus-encoded apoptosis suppressor, Apsup, from the baculovirus Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV). Apsup inhibits the apoptosis of L. dispar Ld652Y cells triggered by infection with p35-defective Autographa californica MNPV (vAcΔp35) and exposure to actinomycin D or UV light. Here, we examined the functional role of Apsup in apoptosis regulation in insect cells. Apsup prevented apoptosis and the proteolytic processing of L. dispar initiator caspase Dronc (Ld-Dronc) in Ld652Y cells triggered by overexpression of Ld-Dronc, LdMNPV inhibitor-of-apoptosis 3 (IAP3), or Hyphantria cunea MNPV IAP1. In vAcΔp35-infected apoptotic Ld652Y cells, Apsup restricted apoptosis induction and prevented processing of endogenous Ld-Dronc. Conversely, upon RNA interference (RNAi)-mediated silencing of apsup, LdMNPV-infected Ld652Y cells, which typically support high-titer virus replication, underwent apoptosis, accompanied by the processing of endogenous Ld-Dronc. Furthermore, endogenous Ld-Dronc coimmunoprecipitated with transiently expressed Apsup, indicating that Apsup physically interacts with Ld-Dronc. Apsup prevented the apoptosis of Sf9 cells triggered by vAcΔp35 infection but did not inhibit apoptosis or activation of caspase-3-like protease in vAcΔp35-infected Drosophila melanogaster S2 cells. Apsup also inhibited the proteolytic processing of L. dispar effector caspase Ld-caspase-1 in the transient expression assay but did not physically interact with Ld-caspase-1. These results demonstrate that Apsup inhibits apoptosis in Ld652Y cells by preventing the proteolytic processing of Ld-Dronc. Together with our previous findings showing that Apsup prevents the processing of both overexpressed Ld-Dronc and Bombyx mori Dronc, these results also demonstrate that Apsup functions as an effective apoptotic suppressor in various lepidopteran, but not dipteran, insect cells.

Elevated expression of BIRC6 protein in non-small-cell lung cancers is associated with cancer recurrence and chemoresistance

INTRODUCTION

Non-small-cell lung cancer (NSCLC) is an aggressive, highly chemoresistant disease. Reliable prognostic assays and more effective treatments are critically required. BIRC6 (baculoviral inhibitors of apoptosis proteins repeat-containing 6) protein is a member of the inhibitors of apoptosis protein family thought to play an important role in the progression or chemoresistance of many cancers. In this study, we investigated whether BIRC6 expression can be used as a prognostic marker or potential therapeutic target for NSCLC.

METHODS

In a retrospective analysis, BIRC6 protein expression was determined for 78 resected primary NSCLCs and nine benign lung tissues. Twenty-nine chemoresistant or chemosensitive subrenal capsule NSCLC tissue xenografts were assessed for BIRC6 expression, using immunohistochemistry, and 13 of them for BIRC6 gene copy number, using array comparative genomic hybridization analysis. The effect of small interfering RNA-induced BIRC6 knockdown on the growth of human NSCLC cell cultures and apoptosis (in combination with cisplatin) was investigated.

RESULTS

Elevated BIRC6 protein expression in NSCLC tissues was associated with poor 3-year relapse-free patient survival, lymph node involvement, and advanced pathological tumor, node, metastasis stage. In patient-derived lung squamous cell carcinoma xenografts, chemoresistance was associated with elevated BIRC6 expression and increased gene copy number. Small interfering RNA-induced BIRC6 down-regulation inhibited growth of the NSCLC cells and sensitized the cells to cisplatin.

CONCLUSIONS

BIRC6 may play an important role in the malignant progression and chemoresistance of NSCLC. Elevated BIRC6 protein expression may serve as a predictive marker for chemoresistance of NSCLCs and a poor prognostic factor for NSCLC patients. Down-regulation of the BIRC6 gene as a therapeutic approach may be effective, especially in combination with conventional chemotherapeutics.

TNF-α up-regulates cellular inhibitor of apoptosis protein 2 (c-IAP2) via c-Jun N-terminal kinase (JNK) pathway in nasopharyngeal carcinoma

Inhibitor of apoptosis proteins (IAPs) contribute to both tumor progression and tumor metastasis. Here, we show that pro-inflammatory cytokine TNF-α induced the up-regulation of c-IAP2 in the potential metastatic nasopharyngeal carcinoma (NPC) cells in a dose- and time-dependent manner. This up-regulation is tolerant, as the pre-treatment of NPC cells with TNF-α reversed the up-regulation of c-IAP2 induced by TNF-α re-stimulation. TNF-α activated MAKP signals, including ERK, JNK and p38, and NF-κB signal, but only inhibition of JNK signal transduction reversed the induction of c-IAP2, suggesting that JNK signaling contributed to the c-IAP2 induction. The results from in vitro scratch wound-healing assays showed that TNF-α promoted cell invasion, which was reversed by the inhibition of JNK signaling. Taken together, these studies suggested that pro-inflammation cytokine TNF-α may be a promoter for NPC metastasis, and the anti-inflammatory therapy may be of benefit to the prevention of NPC metastasis.

Specificity of archaeal caspase activity in the extreme halophile Haloferax volcanii

Caspase-like proteases are key initiators and executioners of programmed cell death (PCD), which is initiated by environmental stimuli and manifests in organisms ranging from unicellular microbes to higher eukaryotes. Archaea had been absent from the caspase inheritance discussion due to a lack of gene homologues. We recently demonstrated extremely high, basal caspase-like catalytic activity in the model haloarcheon, Haloferax volcanii, which was linked to the cellular stress response and was widespread among diverse Archaea. Here, we rigorously tested the catalytic specificity of the observed archaeal caspase-like activities using hydrolytic assays with a diverse suite of protease substrates and inhibitors compared with known model serine and cysteine proteases (trypsin, cathepsin, papain, and human caspase-8). Our experiments demonstrate that exponentially growing H. volcanii possesses a highly specific caspase-like activity that most closely resembles caspase-4, is preferentially inhibited by the pancaspase inhibitor, zVAD-FMK, and has no crossreactivity with other known protease families. Our findings firmly root the extremely high levels of caspase-like activity as the dominant proteolytic activity in this extreme haloarcheaon, thereby providing further support for housekeeping functions in Haloarchaea. Given the deep archaeal roots of eukaryotes, we suggest that this activity served as a foundation for stress pathways in higher organisms.

Apoptosis deregulation in CLL

The description of apoptosis and the identification of the genes that regulate it have proved pivotal to our understanding of how cancer cells accumulate and ultimately cause morbidity and mortality. It has become increasingly clear that in CLL the balance between the pro- and anti-apoptotic members of the BCL2 family of apoptotic regulatory proteins is critical in the development and clinical progression of CLL. Furthermore, the apoptotic potential of the CLL cell determines chemotherapy sensitivity and ultimately progression-free and overall survival. The unravelling of the BCL2 story in CLL has led to the development of a whole new class of therapeutic agents-the BH3 mimetics-which are significantly more targeted than conventional chemo-immunotherapy and therefore promise potent clinical activity coupled with reduced toxicity.

Inhibitor of Apoptosis Proteins: Promising Targets for Cancer Therapy

Cancer is a disease in which normal physiological processes are imbalanced, leading to tumour formation, metastasis and eventually death. Recent biological advances have led to the advent of targeted therapies to complement traditional chemotherapy and radiotherapy. However, a major problem still facing modern medicine is resistance to therapies, whether targeted or traditional. Therefore, to increase the survival rates of cancer patients, it is critical that we continue to identify molecular targets for therapeutic intervention. The Inhibitor of Apoptosis (IAP) proteins act downstream of a broad range of stimuli, such as cytokines and extracellular matrix interactions, to regulate cell survival, proliferation and migration. These processes are dysregulated during tumourigenesis and are critical to the metastatic spread of the disease. IAPs are commonly upregulated in cancer and have therefore become the focus of much research as both biomarkers and therapeutic targets. Here we discuss the roles that IAPs may play in cancer, and the potential benefits and pitfalls that targeting IAPs could have in the clinic.