Structural basis of IAP recognition by Smac/DIABLO

Discovery of aminopiperidine-based Smac mimetics as IAP antagonists

A series of structurally unique Smac mimetics that act as antagonists of inhibitor of apoptosis proteins (IAPs) has been discovered. While most previously described Smac mimetics contain the proline ring (or a similar cyclic motif) found in Smac, a key feature of the compounds described herein is that this ring has been removed. Despite this, compounds in this series potently bind to cIAP1 and elicit the expected phenotype of cIAP1 inhibition in cancer cells. Marked selectivity for cIAP1 over XIAP is observed for these compounds, which is attributed to a slight difference in the binding groove between the two proteins and the resulting steric interactions with the inhibitors. XIAP binding can be improved by constraining the inhibitor so that these unfavorable steric interactions are minimized.

The structural basis of mode of activation and functional diversity: a case study with HtrA family of serine proteases

HtrA (High temperature requirement protease A) proteins that are primarily involved in protein quality control belong to a family of serine proteases conserved from bacteria to humans. HtrAs are oligomeric proteins that share a common trimeric pyramidal architecture where each monomer comprises a serine protease domain and one or two PDZ domains. Although the overall structural integrity is well maintained and they exhibit similar mechanism of activation, subtle conformational changes and structural plasticity especially in the flexible loop regions and domain interfaces lead to differences in their active site conformation and hence in their specificity and functions.

Bivalent Smac mimetics with a diazabicyclic core as highly potent antagonists of XIAP and cIAP1/2 and novel anticancer agents

Nonpeptidic, bivalent Smac mimetics designed based upon monovalent Smac mimetics with a diazabicyclic core structure bind to XIAP, cIAP1, and cIAP2 with low to subnanomolar affinities and are highly effective in antagonizing XIAP in cell-free functional assays. They efficiently induce the degradation of cIAP1 and cIAP2 in cancer cells at concentrations as low as 1 nM, activate caspase-3 and -8, and cleave PARP at 3-10 nM. The most potent compounds in the series have IC(50) of 3-5 nM in inhibition of cell growth in both MDA-MB-231 and SK-OV-3 cell lines and are promising lead compounds for the development of a new class of cancer therapy.

ARTS binds to a distinct domain in XIAP-BIR3 and promotes apoptosis by a mechanism that is different from other IAP-antagonists

ARTS (Sept4_i2), is a pro-apoptotic protein localized at the mitochondria of living cells. In response to apoptotic signals, ARTS rapidly translocates to the cytosol where it binds and antagonizes XIAP to promote caspase activation. However, the mechanism of interaction between these two proteins and how it is regulated remained to be explored. In this study, we show that ARTS and XIAP bind directly to each other, as recombinant ARTS and XIAP proteins co-immunoprecipitate together. We also show that over expression of ARTS alone is sufficient to induce a strong down-regulation of XIAP protein levels and that this reduction occurs through the ubiquitin proteasome system (UPS). Using various deletion and mutation constructs of XIAP we show that ARTS specifically binds to the BIR3 domain in XIAP. Moreover, we found that ARTS binds to different sequences in BIR3 than other IAP antagonists such as SMAC/Diablo. Computational analysis comparing the location of the putative ARTS interface in BIR3 with the known interfaces of SMAC/Diablo and caspase 9 support our results indicating that ARTS interacts with residues in BIR3 that are different from those involved in binding SMAC/Diablo and caspase 9. We therefore suggest that ARTS binds and antagonizes XIAP in a way which is distinct from other IAP-antagonists to promote apoptosis.

X-linked inhibitor of apoptosis protein and active caspase-3 expression patterns in antral follicles in the sheep ovary

X-linked inhibitor of apoptosis protein (XIAP) interacts with caspases to inhibit their activity, thereby providing a potential mechanism for regulation of granulosa cell apoptosis occurring during follicular atresia. The aim of this study was to determine the presence and localization of XIAP mRNA and protein content in the sheep ovary and compare these expression patterns with active caspase-3 protein in the same antral follicles. Romney ewe estrous cycles (n=25) were synchronized with 2–3 Estrumate injections and ovarian tissue collected during the luteal and follicular phases of the cycle. The presence ofXIAPmRNA was confirmed by RT-PCR using laser capture microdissected ovarian cell samples.XIAPmRNA was subsequently localized byin situhybridization histochemistry and XIAP and active caspase-3 protein visualized by immunohistochemistry. In antral follicles extensive XIAP localization was evident in both granulosa and thecal cells. In contrast, mRNA expression was widespread in granulosa cells and only detected in thecal tissue from a small proportion of antral follicles. Active caspase-3 and XIAP comparative expression analysis showed positiveXIAPmRNA expression in all late luteal phase (day 14) follicles, despite varying levels of active caspase-3 protein. A proportion of follicular phase (days 15 and 16) follicles, however, showed an inverse expression relationship at the protein and mRNA levels in both granulosa and thecal tissue, as did XIAP protein in day 14 follicles. These results suggest high XIAP may prevent activation of caspase-3, thereby regulating follicular atresia in antral follicles and could potentially be utilized as a marker of follicular health.

STUDY ON THE INTERACTIONS OF Smac MIMETICS WITH XIAP-BIR3 DOMAIN BY DOCKING AND MOLECULAR DYNAMICS SIMULATIONS

Upon receiving an apoptotic stimulus, the mature mitochondrial protein second mitochondria-derived activator of caspases (Smac)/direct IAP-binding protein with low PI (DIABLO), which could be released from mitochondria into the cytosol together with cytochrome C , specifically binds to inhibitor of apoptosis proteins (IAPs) and relieves the inhibitory effect of caspase, thus promotes cell death. Some artificial small molecules (called Smac mimetics) can mimic the N-terminal four residues Ala1-Val2-Pro3-Ile4 (AVPI) sequence of mitochondrial protein Smac, and competitively bind to X-linked inhibitor of apoptosis protein baculoviral IAP repeats (XIAP-BIR3) domain with caspase-9, which leads to the removal of the inhibition of caspase-9 by XIAP and induce apoptosis. To gain an insight into the nature of XIAP-BIR3 domain recognizing Smac mimetics, we used docking and molecular dynamics simulations methods to study four representative Smac mimetics. The docking results show that the orientations of these backbones of ligands are identical with that of AVPI in the binding pocket. Each ligand corresponds to two competitive conformations, which are called extended and bended conformations. The results of molecular dynamics simulations show that the extended conformation is more stable, and the calculations of energy decomposition reveal that the residue Thr308 makes the strongest interaction with XIAP-BIR3. In addition, Asp309, Glu314, and Trp323 are indispensable for XIAP-BIR3 recognizing and binding Smac mimetics.

Solid-phase synthesis of smac peptidomimetics incorporating triazoloprolines and biarylalanines

Apoptotic induction mechanisms are of crucial importance for the general homeostasis of multicellular organisms. In cancer the apoptotic pathways are downregulated, which, at least partly, is due to an abundance of inhibitors of apoptosis proteins (IAPs) that block the apoptotic cascade by deactivating proteolytic caspases. The Smac protein has an antagonistic effect on IAPs, thus providing structural clues for the synthesis of new pro-apoptotic compounds. Herein, we report a solid-phase approach for the synthesis of Smac-derived tetrapeptide libraries. On the basis of a common (N-Me)AVPF sequence, peptides incorporating triazoloprolines and biarylalanines were synthesized by means of Cu(I)-catalyzed azide-alkyne cycloaddition and Pd-catalyzed Suzuki cross-coupling reactions. Solid-phase procedures were optimized to high efficiency, thus accessing all products in excellent crude purities and yields (both typically above 90%). The peptides were subjected to biological evaluation in a live/dead cellular assay which revealed that structural decorations on the AVPF sequence indeed are highly important for cytotoxicity toward HeLa cells.

Effect of smoking status on seminal parameters and apoptotic markers in infertile men

PURPOSE

We assessed semen parameters, sperm apoptotic markers and seminal plasma cotinine in infertile smokers.

MATERIALS AND METHODS

A total of 160 men were divided into 4 equal groups, including fertile smokers, fertile nonsmokers, infertile smokers and infertile nonsmokers. Smoking was classified as mild--fewer than 10, moderate--10 to 20 or heavy--more than 20 cigarettes daily. All men underwent semen analysis, and assessment of sperm caspase-9, Smac/DIABLO, DNA fragmentation and seminal plasma cotinine.

RESULTS

Infertile men, particularly smokers, have significantly lower semen variables and significantly higher sperm Smac/DIABLO, caspase-9 activity, the percent of DNA fragmentation and seminal plasma cotinine. The mean number of cigarettes smoked daily and smoking duration significantly correlated positively with sperm Smac/DIABLO, caspase-9 activity, the percent of DNA fragmentation and seminal plasma cotinine, and significantly correlated negatively with tested semen variables. Heavy smoking was associated with a significant increase in sperm Smac/DIABLO, caspase-9 activity and seminal plasma cotinine, and with a significant decrease in tested semen variables compared with those in moderate or mild smokers.

CONCLUSIONS

Smoking has a negative impact on semen variables. It is associated with increased sperm caspase-9, Smac/DIABLO and the percent of DNA fragmentation, especially in infertile heavy smokers.

Jolkinolide B from Euphorbia fischeriana Steud induces apoptosis in human leukemic U937 cells through PI3K/Akt and XIAP pathways

Jolkinolide B, a bioactive diterpene isolated from the roots of Euphorbia fischeriana Steud, is known to induce apoptosis in cancer cells. However, the molecular mechanism of its anti-cancer activity has not been fully elucidated. In the present study, we found that Jolkinolide B reduced cell viability and induced apoptosis in a dose- and time-dependent manner in human leukemic U937. The induction of apoptosis was also accompanied by the downregulation of PI3K/Akt and the inhibitor of apoptosis protein (IAP) family proteins. Moreover, we observed that Jolkinolide B treatment resulted in activation of caspase-3 and -9, which may partly explain the anti-cancer activity of Jolkinolide B. Taken together, our study for the first time suggest that Jolkinolide B is able to enhance apoptosis of U937 cells, at least in part, through downregulation of PI3K/Akt and IAP family proteins. Moreover, triggering of caspase-3 and -9 activation mediated apoptotic induction.

BV6, an IAP antagonist, activates apoptosis and enhances radiosensitization of non-small cell lung carcinoma in vitro

INTRODUCTION

Defects in the apoptosis pathway limit the effectiveness of radiation in non-small cell lung cancer (NSCLC) therapy. BV6 is an antagonist of cIAP1 and XIAP, members of the inhibitors of apoptosis (IAP) family. We investigated the potential of BV6 to sensitize NSCLC cell lines to radiation.

METHODS

HCC193 and H460 lung cancer cell lines were treated with BV6 to investigate the effects of drug administration on cell proliferation, apoptosis, inhibition of XIAP and cIAP1, and radiosensitivity. Subsequent immunoblotting and Hoechst staining were used to determine the role of apoptosis in radiosensitization. Finally, the pathway of apoptosis was characterized by Western blot analysis for cleaved caspase-8 and cleaved caspase-9 and enzyme-linked immunosorbent assays for TNF-α.

RESULTS

HCC193 was found to be more sensitive than H460 to BV6-induced apoptosis in a concentration-dependent and time-dependent manner. BV6 significantly sensitized both cell lines to radiation (HCC193-DER = 1.38, p < 0.05 at 1 μM BV6; H460-DER = 1.42, p < 0.05 at 5 μM BV6), but a higher concentration of and longer incubation time with BV6 was necessary for H460 cells. The BV6-induced radiosensitization of HCC193 favored the extrinsic pathway of apoptosis, whereas that of H460 favored the intrinsic pathway.

CONCLUSIONS

BV6, an IAP antagonist, significantly enhanced the radiosensitization of HCC193 and H460 cells in vitro. More research is warranted to test the mechanism of action of BV6 and to assess its potential in vivo and in the clinical setting.

Inhibitor of apoptosis proteins: fascinating biology leads to attractive tumor therapeutic targets

Cell death inhibition is a very successful strategy that cancer cells employ to combat the immune system and various anticancer therapies. Inhibitor of apoptosis (IAP) proteins possess a wide range of biological activities that promote cancer survival and proliferation. One of them, X-chromosome-linked IAP is a direct inhibitor of proapoptotic executioners, caspases. Cellular IAP proteins regulate expression of antiapoptotic molecules and prevent assembly of proapoptotic protein signaling complexes, while survivin regulates cell division. In addition, amplifications, mutations and chromosomal translocations of IAP genes are associated with various malignancies. Several therapeutic strategies have been designed to target IAP proteins, including a small-molecule approach that is based on mimicking the IAP-binding motif of an endogenous IAP antagonist - the second mitochondrial activator of caspases. Other strategies involve antisense nucleotides and transcriptional repression. The main focus of this article is to provide an update on IAP protein biology and perspectives on the development of IAP-targeting therapeutics.

Structural basis for the recognition of phosphorylated histone h3 by the survivin subunit of the chromosomal passenger complex

Localization of the chromosomal passenger complex (CPC) at centromeres during early mitosis is essential for accurate chromosome segregation and is dependent on the phosphorylation of histone H3. We report the 2.7 Å resolution structure of the CPC subunit Survivin bound to the N-terminal tail of histone H3 carrying the Thr3 phosphorylation mark (Thr3ph). The BIR domain of Survivin recognizes the Ala1-Arg2-Thr3ph-Lys4 sequence, decoding the modification state and the free N terminus of histone H3 by a strategy similar to that used by PHD fingers. The structural analysis permitted the identification of putative Survivin-binding epitopes in other mitotic proteins, including human Shugoshin 1. Using biophysical and structural data, we show that a phospho-mimic N-terminal sequence such as that of hSgo1 (Ala1-Lys2-Glu3-Arg4) contains the specificity determinants to bind Survivin. Our findings suggest that the CPC engages in mutually exclusive interactions with other constituents of the mitotic machinery and a histone mark in chromatin.

Characterization of a heterodimeric Smac-based peptide that features sequences specific to both the BIR2 and BIR3 domains of the X-linked inhibitor of apoptosis protein

XIAP, an important regulator of apoptosis, has emerged as a target for the development of cancer therapeutics. The homodimeric Smac protein simultaneously binds to both the BIR2 and BIR3 domains of XIAP. Peptide-based dimeric compounds that mimic the binding mode of Smac show promise as XIAP antagonists. Herein we characterize the first example of a Smac mimetic that incorporates a peptide sequence specific for BIR2. We show that the tetrapeptide motif Ala-Glu-Ala-Val has a higher affinity for BIR2 than the BIR3-specific sequence Ala-Val-Pro-Phe, and we compare the binding characteristics of a heterodimeric peptide containing both tetrapeptide motifs to those of a homodimeric peptide featuring only AVPF. Despite the enhanced affinity of AEAV (relative to AVPF) for BIR2, the heterodimeric peptide displays only a slightly higher affinity for XIAP relative to its homodimeric counterpart. Enhanced affinity of both dimers relative to the tetrapeptide AVPF is largely maintained even when the BIR2 binding groove is modified, implying that hydrophobic contacts afforded by the second peptide motif need not necessarily be made at the BIR2 binding groove to contribute substantial binding energy. Finally, we use mutagenesis to show that the difference in sequence specificity observed between the two domains is primarily owing to steric bulk introduced at the BIR2 site by lysine 206. Replacement of K206 at BIR2 with glycine, the corresponding residue in BIR3, restores the majority of the affinity for the AVPF motif exhibited by BIR3. The implications of these finding in the development of XIAP antagonists are discussed. © 2011 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 98: 122-130, 2012.

Expression of apoptosis-related proteins and its clinical implication in surgically resected gastric carcinoma

Apoptosis, via caspase cascade, is involved in tumorigenesis and progression, and thus, altered apoptosis-related protein expressions have clinical and prognostic significance. Moreover, the apoptosis pathway is highlighted due to the recent introduction of apoptosis-targeted therapy for several genes such as the X-linked inhibitor of apoptosis protein (XIAP). XIAP is the most potent direct inhibitor of caspase, and XIAP-associated factor 1 (XAF1) and secondary mitochondrial activator of caspase/direct IAP-binding protein with low PI (Smac/DIABLO) are negative regulators of XIAP. In this study, we evaluated the expression of these proteins and investigated their clinical and prognostic significance in gastric carcinomas. Immunohistochemical analysis by using the tissue array method was performed for XIAP, survivin, Bcl-2, XAF1, Smac/DIABLO, and cleaved caspase-3 proteins in 1,162 surgically resected gastric carcinoma cases. XIAP expression was related to the advanced stage. The expression of XIAP showed negative relationship with XAF1 and Smac/DIABLO expressions. In addition, XIAP expression was associated with a poor prognosis and was also proved to be an independent prognostic factor. Cleaved caspase-3 expression was related to the early stage. In addition, cleaved caspase-3 expression was associated with a favorable prognosis and was also proved to be an independent prognostic factor. The expression of XIAP showed an inverse relationship with cleaved caspase-3. In addition, the expression of XAF1 and Smac/DIABLO had a positive relationship with cleaved caspase-3. These findings are consistent with their known functions, and they may help to identify individuals best suited for apoptosis-targeted therapy as a baseline data in gastric carcinoma.

Smac-mimetic compound SM-164 induces radiosensitization in breast cancer cells through activation of caspases and induction of apoptosis

Radiotherapy is a treatment choice for local control of breast cancer, particularly after the removal of tumor tissues by surgery. However, intrinsic radioresistance of cancer cells limits therapeutic efficacy. Here, we determined in breast cancer cells the potential radiosensitizing activity of SM-164, a small molecule compound, that mimics the activity of SMAC, a mitochondrial protein released during apoptosis to activate caspases by inhibiting cellular inhibitor of apoptosis proteins, cIAP-1, and XIAP. We found that SM-164 at nanomolar concentrations promoted degradation of cIAP-1, disrupted the inhibitory binding of XIAP to active caspase-9, and sensitized breast cancer cells to radiation with a sensitization enhancement ratio (SER) of 1.7-1.8. In one line of breast cancer cells resistant to SM-164 as a single agent, SM-164 radiosensitization was mediated by intrinsic apoptosis pathway through activation of caspases-9 and -3. In a line of breast cancer cells sensitive to SM-164 as a single agent, SM-164 radiosensitization was mediated by both extrinsic and intrinsic apoptosis pathways through activation of caspases-9, -8, and -3. Consistently, blockage of caspase activation, through siRNA knockdown or treatment with a pan-caspase inhibitor z-VAD-fmk, inhibited apoptosis and abrogated SM-164 radiosensitization. Our study demonstrates that IAPs are valid radiosensitizing targets in breast cancer cells and SM-164 could be further developed as a novel class of radiosensitizers for the treatment of radioresistant breast cancer.

Design, synthesis and evaluation of monovalent Smac mimetics that bind to the BIR2 domain of the anti-apoptotic protein XIAP

We report the systematic rational design and synthesis of new monovalent Smac mimetics that bind preferentially to the BIR2 domain of the anti-apoptotic protein XIAP. Characterization of compounds in vitro (including 9i; ML101) led to the determination of key structural requirements for BIR2 binding affinity. Compounds 9h and 9j sensitized TRAIL-resistant breast cancer cells to apoptotic cell death, highlighting the value of these probe compounds as tools to investigate the biology of XIAP.

Sensitization of human bladder tumor cells to TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis with a small molecule IAP antagonist

Urothelial carcinoma of the bladder accounts for approximately 5% of all cancer deaths in humans. The large majority of bladder tumors are non-muscle invasive at diagnosis, but even after local surgical therapy there is a high rate of local tumor recurrence and progression. Current treatments extend time to recurrence but do not significantly alter disease survival. The objective of the present study was to investigate the tumoricidal potential of combining the apoptosis-inducing protein TNF-related apoptosis-inducing ligand (TRAIL) with a small molecule inhibitor of apoptosis proteins (IAP) antagonist to interfere with intracellular regulators of apoptosis in human bladder tumor cells. Our results demonstrate that the IAP antagonist Compound A exhibits high binding affinity to the XIAP BIR3 domain. When Compound A was used at nontoxic concentrations in combination with TRAIL, there was a significant increase in the sensitivity of TRAIL-sensitive and TRAIL-resistant bladder tumor lines to TRAIL-mediated apoptosis. In addition, modulation of TRAIL sensitivity in the TRAIL-resistant bladder tumor cell line T24 with Compound A was reciprocated by XIAP small interfering RNA-mediated suppression of XIAP expression, suggesting the importance of XIAP-mediated resistance to TRAIL in these cells. These results suggest the potential of combining Compound A with TRAIL as an alternative therapy for bladder cancer.

Targeting inhibitor of apoptosis proteins for therapeutic intervention

The inhibitors of apoptosis (IAP) proteins have emerged over the last decade as important targets for therapeutic intervention in human malignancies. Overexpression of IAPs has been implicated in cell survival and resistance against stress-induced apoptosis brought on by radiation and/or chemotherapeutics (currently the standard-of-care in a variety of different cancer diseases). In addition, evasion from death receptor-mediated apoptosis and regulation of NF-κB pathways and cell division have also been associated with IAP proteins. Efforts to target IAP proteins in tumors have focused mainly on designing small molecules that mimic the IAP-binding motif of the endogenous IAP antagonist, second mitochondrial activator of caspases. In addition, several other IAP-targeting strategies, including antisense oligonucleotides and transcriptional repression, have also been initiated, with the hope of providing therapeutic benefit to cancer patients.

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.

Silencing survivin splice variant 2B leads to antitumor activity in taxane--resistant ovarian cancer

PURPOSE

To study the role of survivin and its splice variants in taxane-resistant ovarian cancer.

EXPERIMENTAL DESIGN

We assessed the mRNA levels of survivin splice variants in ovarian cancer cell lines and ovarian tumor samples. siRNAs targeting survivin were designed to silence all survivin splice variants (T-siRNA) or survivin 2B (2B-siRNA) in vitro and orthotopic murine models of ovarian cancer. The mechanism of cell death was studied in taxane-resistant ovarian cancer cells and in tumor sections obtained from different mouse tumors.

RESULTS

Taxane-resistant ovarian cancer cells express higher survivin mRNA levels than their taxane-sensitive counterparts. Survivin 2B expression was significantly higher in taxane-resistant compared with -sensitive cells. Silencing survivin 2B induced growth inhibitory effects similar to silencing total survivin in vitro. In addition, survivin 2B-siRNA incorporated into DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) nanoliposomes resulted in significant reduction in tumor growth (P < 0.05) in orthotopic murine models of ovarian cancer, and these effects were similar to T-siRNA-DOPC. The antitumor effects were further enhanced in combination with docetaxel chemotherapy (P < 0.01). Finally, we found a significant association between survivin 2B expression and progression-free survival in 117 epithelial ovarian cancers obtained at primary debulking surgery.

CONCLUSIONS

These data identify survivin 2B as an important target in ovarian cancer and provide a translational path forward for developing new therapies against this target.

DNA-instructed acyl transfer reactions for the synthesis of bioactive peptides

We present a method which allows for the translation of nucleic acid information into the output of molecules that interfere with disease-related protein-protein interactions. The method draws upon a nucleic acid-templated reaction, in which adjacent binding of reactive conjugates triggers the transfer of an aminoacyl or peptidyl group from a donating thioester-linked PNA-peptide hybrid to a peptide-PNA acceptor. We evaluated the influence of conjugate structures on reactivity and sequence specificity. The DNA-triggered peptide synthesis proceeded sequence specifically and showed catalytic turnover in template. The affinity of the formed peptide conjugates for the BIR3 domain of the X-linked inhibitor of apoptosis protein (XIAP) is discussed.

Potent bivalent Smac mimetics: effect of the linker on binding to inhibitor of apoptosis proteins (IAPs) and anticancer activity

We have synthesized and evaluated a series of nonpeptidic, bivalent Smac mimetics as antagonists of the inhibitor of apoptosis proteins and new anticancer agents. All these bivalent Smac mimetics bind to full-length XIAP with low nanomolar affinities and function as ultrapotent antagonists of XIAP. While these Smac mimetics bind to cIAP1/2 with similar low nanomolar affinities, their potencies to induce degradation of cIAP1/2 proteins in cells differ by more than 100-fold. The most potent bivalent Smac mimetics inhibit cell growth with IC(50) from 1 to 3 nM in the MDA-MB-231 breast cancer cell line and are 100 times more potent than the least potent compounds. Determination of intracellular concentrations for several representative compounds showed that the linkers in these bivalent Smac mimetics significantly affect their intracellular concentrations and hence the overall cellular activity. Compound 27 completely inhibits tumor growth in the MDA-MB-231 xenografts while causing no signs of toxicity in the animals.

Gene discovery, comparative analysis and expression profile reveal the complexity of the Crassostrea gigas apoptosis system

Apoptosis system was reported to play important role in organism immunity, but it was a currently understudied respect in molluscan immunity studies. Base on the recent generation of ESTs in the pacific oyster, Crassostrea gigas, a survey of apoptosis-related molecules was conducted in the assembled unigenes, we found that the basic genes and domains in apoptosis-associated proteins were conserved, the overall apoptotic machinery was complex in C. gigas and that the organism had an expanded number of putative baculovirus inhibitor of apoptosis repeat domains. Moreover, four typical apoptosis-related genes were cloned in C. gigas and compared with the sequences of these genes in Drosophila melanogaster and Homo sapiens. The expression level of these four apoptosis-related genes in the hemolymph increased dramatically in the presence of the bacteria, Vibrio anguillarum, indicating their role in bacterial defense. Our results suggest that the oyster apoptosis system is not simple and cannot be represented by model invertebrates.

Smac modulates chemosensitivity in head and neck cancer cells through the mitochondrial apoptotic pathway

PURPOSE

Overexpression of inhibitors of apoptosis proteins (IAP) contributes to therapeutic resistance. Second mitochondria-derived activator of caspase (Smac) promotes caspase activation by binding to IAPs upon release from the mitochondria. IAP antagonists, also called SMAC mimetics, are promising anticancer agents modeled after this mechanism. We investigated the role and mechanisms of Smac- and Smac mimetic-mediated chemosensitization in head and neck squamous cell carcinoma (HNSCC) cells.

EXPERIMENTAL DESIGN

The effects of SMAC knockdown, SMAC overexpression, and a small molecule Smac mimetic on the chemosensitivities of HNSCC cells were determined. The mechanisms of Smac- and Smac mimetic-mediated chemosensitization were investigated by analyzing growth suppression, the mitochondrial apoptotic pathway, caspase activation, and IAP proteins. The therapeutic responses of HNSCC cells with different levels of Smac were compared in xenograft models.

RESULTS

We found that Smac mediates apoptosis induced by several classes of therapeutic agents through the mitochondrial pathway. SMAC knockdown led to impaired caspase activation, mitochondrial membrane depolarization, and release of cytochrome c. A small molecule Smac mimetic, at nanomolar concentrations, significantly sensitized HNSCC cells to gemcitabine-induced apoptosis and restored gemcitabine sensitivity in SMAC knockdown cells, through caspase activation, X-linked IAP dissociation, and mitochondria-associated events, but not the TNF-α pathway. Furthermore, Smac levels modulated the therapeutic response of HNSCC cells to gemcitabine in xenograft models.

CONCLUSIONS

Our results establish a critical role of Smac in mediating therapeutic responses of HNSCC cells and provide a strong rationale for combining Smac mimetics with other anticancer agents to treat HNSCC.

SMG1 and NIK regulate apoptosis induced by Smac mimetic compounds

Smac mimetic compounds (SMCs) are experimental small molecules that induce tumour necrosis factor alpha (TNFα)-dependent cancer cell death by targeting the inhibitor of apoptosis proteins. However, many cancer cell lines are resistant to SMC-mediated apoptosis despite the presence of TNFα. To add insight into the mechanism of SMC-resistance, we used functional siRNA-based kinomic and focused chemical screens and identified suppressor of morphogenesis in genitalia-1 (SMG1) and NF-κB-inducing kinase (NIK) as novel protective factors. Both SMG1 and NIK prevent SMC-mediated apoptosis likely by maintaining FLICE inhibitory protein (c-FLIP) levels to suppress caspase-8 activation. In SMC-resistant cells, the accumulation of NIK upon SMC treatment enhanced the activity of both the classical and alternative nuclear factor-κB pathways, and increased c-FLIP mRNA levels. In parallel, persistent SMG1 expression in SMC-resistant cells repressed SMC-mediated TNFα-induced JNK activation and c-FLIP levels were sustained. Importantly, SMC-resistance is overcome by depleting NIK and SMG1, which appear to facilitate the downregulation of c-FLIP in response to SMC and TNFα treatment, leading to caspase-8-dependent apoptosis. Collectively, these data show that SMG1 and NIK function as critical repressors of SMC-mediated apoptosis by potentially converging on the regulation of c-FLIP metabolism.

The role of IAP antagonist proteins in the core apoptosis pathway of the mosquito disease vector Aedes aegypti

While apoptosis regulation has been studied extensively in Drosophila melanogaster, similar studies in other insects, including disease vectors, lag far behind. In D. melanogaster, the inhibitor of apoptosis (IAP) protein DIAP1 is the major negative regulator of caspases, while IAP antagonists induce apoptosis, in part, by binding to DIAP1 and inhibiting its ability to regulate caspases. In this study, we characterized the roles of two IAP antagonists, Michelob_x (Mx) and IMP, in apoptosis in the yellow fever mosquito Aedes aegypti. Overexpression of Mx or IMP caused apoptosis in A. aegypti Aag2 cells, while silencing expression of mx or imp attenuated apoptosis. Addition of recombinant Mx or IMP, but not cytochrome c, to Aag2 cytosolic extract caused caspase activation. Consistent with this finding, AeIAP1 bound and inhibited both initiator and effector caspases from A. aegypti, and Mx and IMP competed with caspases for binding to AeIAP1. However, a difference was observed in the BIR domains responsible for Dronc binding by AeIAP1 versus DIAP1. These findings demonstrate that the mechanisms by which IAP antagonists regulate apoptosis are largely conserved between A. aegypti and D. melanogaster, although subtle differences exist.

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.

Novel X-linked inhibitor of apoptosis protein inhibitors as probes of apoptosis in biology and medicine

We report here a series of new inhibitors of the X-linked inhibitor of apoptosis protein (XIAP protein) based on the SMAC-tetrapeptide AVPI. The structural novelty of these molecules is the presence of the proline mimetic exo-2-azabicyclo[2.2.1]heptane-3-carboxylic acid, leading to analogs with similar activity to the natural ligand peptide. The structure–activity relationship and computational docking studies support the convenience of this unnatural amino acid as a building block to develop new peptides or small molecules targeting the XIAP-BIR3 domain.

Inhibitor of apoptosis protein family as diagnostic markers and therapeutic targets of colorectal cancer

The apoptosis and antiapoptotic signaling pathways are important for regulating carcinogenesis and cancer progression, and for determining prognosis. Molecules involved in apoptosis represent potential cancer diagnostic markers and therapeutic targets. The inhibitor of apoptosis protein (IAP) family includes several important molecules involved in apoptosis that might represent such targets. Increasing evidence has demonstrated that the IAP family of proteins is integral for antiapoptotic and nuclear factor-κB signal transduction, and enhanced expression of IAPs contributes to colon carcinogenesis and its poor prognosis, as well as to drug resistance of tumors. X-linked IAP, cIAP1, cIAP2, and survivin are prognostic markers of colorectal cancer, and survivin and cIAP2 are also utilized to predict the effect of anticancer treatment in colorectal cancer patients. Novel therapies such as YM155 and LY2181308 targeting survivin, AEG35156 and phenoxodiol targeting X-linked IAP, AT-406 as a Smac mimetic, and survivin peptides are currently being evaluated in clinical trials. This report reviews the involvement of the IAP family in colorectal adenocarcinoma in order to summarize the role of the IAP family members as diagnostic and therapeutic targets, and to provide an overview of the future course of research in this area.

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.

In silico discovery of acylated flavonol monorhamnosides from Eriobotrya japonica as natural, small-molecular weight inhibitors of XIAP BIR3

Targeting the baculoviral inhibitor of apoptosis proteins repeat (BIR) 3 of X-linked inhibitor of apoptosis proteins (XIAP) represents an innovative strategy for the design of chemosensitizers. Acylated flavonol monorhamnosides (AFMR) from Eriobotrya japonica Lindl. (Rosaceae) were virtually predicted as ligands of the XIAP BIR3 domain by using a previously generated pharmacophore model. From the methanol leaf extract of E. japonica an enriched mixture of AFMR was obtained showing chemosensitizing potential in combination with etoposide in XIAP-overexpressing Jurkat cells. The HPLC-SPE-NMR hyphenated technique facilitated the structure elucidation of three known and two new natural AFMR. The main constituent and virtual hit, kaempferol-3-O-α-l-(2″,4″-di-E-p-coumaroyl)-rhamnoside (3) was isolated from the enriched fraction. Applying a fluorescence polarization based binding assay, 3 was identified as XIAP BIR3 ligand with a dose-dependent affinity (IC₅₀ 10.4 μM). Further, 3 induced apoptosis in XIAP-overexpressing Jurkat cells and activated caspase-9 in combination with etoposide. Docking experiments revealed a major impact of the coumaric acid and sugar moieties of 3 on XIAP BIR3 binding, which was experimentally confirmed. To conclude, this study elucidates 3 as natural, small-molecular weight XIAP BIR3 inhibitor using a combination of in silico and HPLC-SPE-NMR hyphenated techniques.

Theoretical studies on the interactions of XIAP-BIR3 domain with bicyclic and tricyclic core monovalent Smac mimetics

X-linked IAP can bind caspase-9 and inhibit its activity. Mitochondrial protein Smac/DIABLO can also interact with XIAP and relieve the inhibition on caspase-9 to induce apoptosis. A series of artificial Smac mimetics have been used to mimic the Smac N-terminal tetrapeptide AVPI to bind to XIAP-BIR3, but these structural diverse mimetics exhibited distinct binding affinities. To get an insight into the binding nature and optimize the structures, molecular docking and dynamics simulations were used to study the binding of XIAP-BIR3 with three groups of Smac mimetics. The docking results reveal that these Smac mimetics anchored on the surface groove of XIAP-BIR3 and superimposed well with AVPI. The modifications on the seven-membered ring of bicyclic core segment do not strengthen the binding affinity, while a benzyl introduced to the five-membered ring is favorable to the binding. Molecular dynamics simulations on three typical systems show that these complexes are very stable. Hydrogen bonds between the bicyclic core segment and Thr308 play critical roles in maintaining the stability of complex. The binding free energies calculated by MM_PBSA method are consistent with the experimental results.

Survivin reads phosphorylated histone H3 threonine 3 to activate the mitotic kinase Aurora B

A hallmark of mitosis is the appearance of high levels of histone phosphorylation, yet the roles of these modifications remain largely unknown. Here, we demonstrate that histone H3 phosphorylated at threonine 3 is directly recognized by an evolutionarily conserved binding pocket in the BIR domain of Survivin, which is a member of the chromosomal passenger complex (CPC). This binding mediates recruitment of the CPC to chromosomes and the resulting activation of its kinase subunit Aurora B. Consistently, modulation of the kinase activity of Haspin, which phosphorylates H3T3, leads to defects in the Aurora B-dependent processes of spindle assembly and inhibition of nuclear reformation. These findings establish a direct cellular role for mitotic histone H3T3 phosphorylation, which is read and translated by the CPC to ensure accurate cell division.

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.

Smac mimetics: implications for enhancement of targeted therapies in leukemia

Drug resistance is a growing concern with clinical use of tyrosine kinase inhibitors. Utilizing in vitro models of intrinsic drug resistance and stromal-mediated chemoresistance, as well as functional mouse models of progressive and residual disease, we attempted to develop a potential therapeutic approach designed to suppress leukemia recurrence following treatment with selective kinase inhibitors. The novel IAP inhibitor, LCL161, [corrected] was observed to potentiate the effects of tyrosine kinase inhibition against leukemic disease both in the absence and presence of a stromal-protected [corrected] environment. LCL161 enhanced the proapoptotic effects of nilotinib and PKC412, against leukemic disease in vitro and potentiated the activity of both kinase inhibitors against leukemic disease in vivo. In addition, LCL161 synergized in vivo with nilotinib to reduce leukemia burden significantly below the baseline level suppression exhibited by a moderate-to-high dose of nilotinib. Finally, LCL161 displayed antiproliferative effects against cells characterized by intrinsic resistance to tyrosine kinase inhibitors as a result of expression of point mutations in the protein targets of drug inhibition. These results support the idea of using IAP inhibitors in conjunction with targeted tyrosine kinase inhibition to override drug resistance and suppress or eradicate residual disease.

Drosophila IAP antagonists form multimeric complexes to promote cell death

Self- and hetero-association of the pro-apoptotic proteins Reaper, Hid, and Grim is required for efficient induction of the cell death program.

Apoptosis is a specific form of cell death that is important for normal development and tissue homeostasis. Caspases are critical executioners of apoptosis, and living cells prevent their inappropriate activation through inhibitor of apoptosis proteins (IAPs). In Drosophila, caspase activation depends on the IAP antagonists, Reaper (Rpr), Head involution defective (Hid), and Grim. These proteins share a common motif to bind Drosophila IAP1 (DIAP1) and have partially redundant functions. We now show that IAP antagonists physically interact with each other. Rpr is able to self-associate and also binds to Hid and Grim. We have defined the domain involved in self-association and demonstrate that it is critical for cell-killing activity in vivo. In addition, we show that Rpr requires Hid for recruitment to the mitochondrial membrane and for efficient induction of cell death in vivo. Both targeting of Rpr to mitochondria and forced dimerization strongly promotes apoptosis. Our results reveal the functional importance of a previously unrecognized multimeric IAP antagonist complex for the induction of apoptosis.