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

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

Requirement of both the second and third BIR domains for the relief of X-linked inhibitor of apoptosis protein (XIAP)-mediated caspase inhibition by Smac

The inhibitor of apoptosis proteins (IAP) are endogenous caspase inhibitors in the metazoan and characterized by the presence of baculoviral IAP repeats (BIR). X-linked IAP (XIAP) contains three BIR domains and directly inhibits effector caspases such as caspase-7 via a linker_BIR2 fragment and initiator caspases such as caspase-9 via the BIR3 domain. A mitochondrial protein Smac/DIABLO, which is released during apoptosis, antagonizes XIAP-mediated caspase inhibition by interacting directly with XIAP. Here, using glutathione S-transferase pulldown and caspase activity assay, we show that Smac is ineffective in relieving either caspase-7 or caspase-9 inhibition by XIAP domain fragments. In addition, Smac forms a ternary complex with caspase-7 and linker_BIR2, suggesting that Smac/linker_BIR2 interaction does not sterically exclude linker_BIR2/caspase-7 interaction. However, Smac is effective in removing caspase-7 and caspase-9 inhibition by XIAP fragments containing both the BIR2 and BIR3 domains. Surface plasmon resonance measurements show that Smac interacts with the BIR2 or BIR3 domain in micromolar dissociation constants. On the other hand, Smac interacts with an XIAP construct containing both BIR2 and BIR3 domains in a subnanomolar dissociation constant by the simultaneous interaction of the Smac dimer with the BIR2 and BIR3 domains of a single XIAP molecule. This 2:1 Smac/XIAP interaction not only possesses enhanced affinity but also sterically excludes XIAP/caspase-7 interaction, demonstrating the requirement of both BIR2 and BIR3 domains for Smac to relieve XIAP-mediated caspase inhibition.     

X-linked inhibitor of apoptosis protein (XIAP) inhibits caspase-3 and -7 in distinct modes

The inhibitor of apoptosis proteins (IAP) regulates cell death by inhibiting caspases. The region of X-linked (X) IAP containing the second baculovirus IAP repeat domain (BIR2) is sufficient for inhibiting caspase-3 and -7. In this study, we found that the modes of inhibition of these two caspases were different: caspase-3 is inhibited in a competitive manner whereas caspase-7 inhibition occurs through a mixed competitive and noncompetitive mechanism. Binding assays revealed that the inhibition of caspase-3 by XIAP was totally dependent on the interaction between the active site of caspase-3 and the linker region between the BIR1 and BIR2 domains of XIAP. In contrast, the active site and the NH(2)-terminal region of caspase-7 bound to the linker region and the BIR2, respectively. Moreover the BIR2 with a mutated linker region, which inhibited caspase-3 very weakly, still bound to and inhibited caspase-7. Furthermore, a chimeric caspase-7/3 comprising the NH(2)-terminal portion of caspase-7 and COOH-terminal portion of caspase-3 was inhibited by XIAP by a mixed competitive and noncompetitive mechanism. Our results suggest that the linker region between BIR1 and BIR2 domains is responsible for active site-directed, competitive inhibition of both caspase-3 and -7, whereas the BIR2 itself is involved in noncompetitive inhibition of caspase-7.     

Design and characterization of bivalent Smac-based peptides as antagonists of XIAP and development and validation of a fluorescence polarization assay for XIAP containing both BIR2 and BIR3 domains

XIAP (X-chromosome-linked inhibitor of apoptosis protein) is an inhibitor of apoptosis by binding to and inhibition of caspase-3 and caspase-7 through its BIR2 domain and caspase-9 through its BIR3 domain. Smac (second mitochondria-derived activator of caspases) protein is an endogenous antagonist of XIAP. Smac forms a dimer and concurrently binds both the BIR2 and BIR3 domains in XIAP, functioning as a highly efficient and potent cellular inhibitor of XIAP. In this article, we have designed and synthesized a bivalent Smac-based ligand (Smac-1) and its fluorescent labeled analogue (Smac-1F) and characterized their interaction with different constructs of XIAP. Our study demonstrates that bivalent Smac-based ligands bind concurrently to both the BIR2 and BIR3 domains of XIAP and are more than 500 times more potent than the corresponding monovalent Smac-based ligands. Bivalent Smac-based ligands also function as much more potent antagonists of XIAP than do the corresponding monovalent Smac-based ligands in cell-free functional assays. Using Smac-1F and XIAP containing both BIR2 and BIR3 domains, we also developed and validated a new fluorescence polarization-based assay. Hence, our designed bivalent Smac-based peptides mimic the mode of dimeric Smac protein in their interaction with XIAP containing both BIR2 and BIR3 domains and achieve extremely high potency in binding and functional assays. Our study provides new insights into the mode of action of bivalent Smac ligands targeting XIAP and a basis for the design and development of cell-permeable, bivalent Smac mimetics.     

Interaction of a cyclic, bivalent smac mimetic with the x-linked inhibitor of apoptosis protein

We have designed and synthesized a cyclic, bivalent Smac mimetic (compound 3) and characterized its interaction with the X-linked inhibitor of apoptosis protein (XIAP). Compound 3 binds to XIAP containing both BIR2 and BIR3 domains with a biphasic dose-response curve representing two binding sites with IC 50 values of 0.5 and 406 nM, respectively. Compound 3 binds to XIAPs containing the BIR3-only and BIR2-only domain with K i values of 4 nM and 4.4 microM, respectively. Gel filtration experiments using wild-type and mutated XIAPs showed that 3 forms a 1:2 stoichiometric complex with XIAP containing the BIR3-only domain. However, it forms a 1:1 stoichiometric complex with XIAP containing both BIR2 and BIR3 domains, and both BIR domains are involved in the binding. Compound 3 efficiently antagonizes inhibition of XIAP in a cell-free functional assay and is >200 times more potent than its corresponding monovalent compound 2. Determination of the crystal structure of 3 in complex with the XIAP BIR3 domain confirms that 3 induces homodimerization of the XIAP BIR3 domain and provides a structural basis for the cooperative binding of one molecule of compound 3 to two XIAP BIR3 molecules. On the basis of this crystal structure, a binding model of XIAP containing both BIR2 and BIR3 domains and 3 was constructed, which sheds light on the ability of 3 to relieve the inhibition of XIAP with not only caspase-9 but also caspase-3/-7. Compound 3 is cell-permeable, effectively activates caspases in whole cells, and potently inhibits cancer cell growth. Compound 3 is a useful biochemical and pharmacological tool for further elucidating the role of XIAP in regulation of apoptosis and represents a promising lead compound for the design of potent, cell-permeable Smac mimetics for cancer treatment.     

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

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

Peptides targeting caspase inhibitors

Here we report on the identification of peptides targeting the X-inhibitor of apoptosis protein (XIAP). XIAP functions as a caspase inhibitor and is a member of the inhibitors of apoptosis (IAP) family of proteins. IAPs are often overexpressed in cancers and leukemias and are associated with an unfavorable clinical prognosis. We have selected peptides from a phage library by using recombinant full-length human XIAP or a fragment containing only the baculovirus IAP repeat 2 (BIR2) domain. A consensus motif, C(D/E/P)(W/F/Y)-acid/basic-XC, was recovered from two independent screenings by using different libraries. Phage-displaying variations of the consensus sequence bound specifically to the BIR2 domain of XIAP but not to other IAPs. The interaction was specific as it could be blocked by the cognate synthetic peptides in a dose-dependent manner. Phage displaying the XIAP-binding motif CEFESC bound to the BIR2 domain of XIAP with an estimated dissociation constant of 1.8 nm as determined by surface plasmon resonance. Protein-protein interaction assays revealed that caspase-3 and caspase-7 (but not caspase-8) blocked the binding of the CEFESC phage to XIAP, indicating that this peptide targets a domain within XIAP that is related to the caspase-binding site. In fact, the sequence EFES is homologous to a loop unique to the executioner caspase-3 and caspase-7 that are targeted by XIAP. Finally, we demonstrated that an internalizing version of the XIAP-binding peptide identified in our screenings (PFKQ) can induce programmed cell death in leukemia cells. Peptides interacting with XIAP could serve as prototypes for the design of low molecular weight modulators of apoptosis.     

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

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

Building homogeneous time-resolved fluorescence resonance energy transfer assays for characterization of bivalent inhibitors of an inhibitor of apoptosis protein target

XIAP (X-chromosome-linked inhibitor of apoptosis protein) is a central apoptosis regulator that blocks cell death by inhibiting caspase-3, caspase-7, and caspase-9 via binding interactions with the XIAP BIR2 and BIR3 domains (where BIR is baculovirus IAP repeat). Smac protein, in its dimeric form, effectively antagonizes XIAP by concurrently targeting both its BIR2 and BIR3 domains. Here we describe the development of highly sensitive homogeneous time-resolved fluorescence resonance energy transfer (HTRF) assays to measure binding affinities of potent bivalent peptidomimetic inhibitors of XIAP. Our results indicate that these assays can differentiate Smac-mimetic inhibitors with a wide range of binding affinities down to the picomolar range. Furthermore, we demonstrate the utility of these fluorescent tools for characterization of inhibitor off-rates, which as a crucial determinant of target engagement and cellular potency is another important parameter to guide optimization in a structure-based drug discovery effort. Our study also explores how increased inhibitor valency can lead to enhanced potency at multimeric proteins such as IAP.     

Structural basis of caspase-7 inhibition by XIAP

The inhibitor of apoptosis (IAP) proteins suppress cell death by inhibiting the catalytic activity of caspases. Here we present the crystal structure of caspase-7 in complex with a potent inhibitory fragment from XIAP at 2.45 A resolution. An 18-residue XIAP peptide binds the catalytic groove of caspase-7, making extensive contacts to the residues that are essential for its catalytic activity. Strikingly, despite a reversal of relative orientation, a subset of interactions between caspase-7 and XIAP closely resemble those between caspase-7 and its tetrapeptide inhibitor DEVD-CHO. Our biochemical and structural analyses reveal that the BIR domains are dispensable for the inhibition of caspase-3 and -7. This study provides a structural basis for the design of the next-generation caspase inhibitors.     

Structural basis of caspase inhibition by XIAP: differential roles of the linker versus the BIR domain

The inhibitor of apoptosis proteins (IAPs) represent the only endogenous caspase inhibitors and are characterized by the presence of baculoviral IAP repeats (BIRs). Here, we report the crystal structure of the complex between human caspase-7 and XIAP (BIR2 and the proceeding linker). The structure surprisingly reveals that the linker is the only contacting element for the caspase, while the BIR2 domain is invisible in the crystal. The linker interacts with and blocks the substrate groove of the caspase in a backward fashion, distinct from substrate recognition. Structural analyses suggest that the linker is the energetic and specificity determinant of the interaction. Further biochemical characterizations clearly establish that the linker harbors the major energetic determinant, while the BIR2 domain serves as a regulatory element for caspase binding and Smac neutralization.     

Molecular interaction between HAX-1 and XIAP inhibits apoptosis

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

Mechanism of XIAP-mediated inhibition of caspase-9

The inhibitor of apoptosis (IAP) proteins potently inhibit the catalytic activity of caspases. While profound insight into the inhibition of the effector caspases has been gained in recent years, the mechanism of how the initiator caspase-9 is regulated by IAPs remains enigmatic. This paper reports the crystal structure of caspase-9 in an inhibitory complex with the third baculoviral IAP repeat (BIR3) of XIAP at 2.4 A resolution. The structure reveals that the BIR3 domain forms a heterodimer with a caspase-9 monomer. Strikingly, the surface of caspase-9 that interacts with BIR3 also mediates its homodimerization. We demonstrate that monomeric caspase-9 is catalytically inactive due to the absence of a supporting sequence element that could be provided by homodimerization. Thus, XIAP sequesters caspase-9 in a monomeric state, which serves to prevent catalytic activity. These studies, in conjunction with other observations, define a unified mechanism for the activation of all caspases.     

Smac3, a novel Smac/DIABLO splicing variant, attenuates the stability and apoptosis-inhibiting activity of X-linked inhibitor of apoptosis protein

X-linked inhibitor of apoptosis protein (XIAP), the most potent member of the inhibitor of apoptosis protein (IAP) family, plays a crucial role in the regulation of apoptosis. XIAP is structurally characterized by three baculovirus IAP repeat (BIR) domains that mediate binding to and inhibition of caspases and a RING domain that confers ubiquitin ligase activity. The caspase inhibitory activity of XIAP can be eliminated by the second mitochondria-derived activator of caspases (Smac)/direct IAP-binding protein with low pI (DIABLO) during apoptosis. Here we report the identification and characterization of a novel isoform of Smac/DIABLO named Smac3, which is generated by alternative splicing of exon 4. Smac3 contains an NH2-terminal mitochondrial targeting sequence required for mitochondrial targeting of Smac3 and an IAP-binding motif essential for Smac3 binding to XIAP. Smac3 is released from mitochondria into the cytosol in response to apoptotic stimuli, where it interacts with the second and third BIR domains of XIAP. Smac3 disrupts processed caspase-9 binding to XIAP, promotes caspase-3 activation, and potentiates apoptosis. Strikingly, Smac3, but not Smac/DIABLO, accelerates XIAP auto-ubiquitination and destruction. Smac3-stimulated XIAP ubiquitination is contingent upon the physical association of XIAP with Smac3 and an intact RING domain of XIAP. Smac3-accelerated XIAP destabilization is, at least in part, attributed to its ability to enhance XIAP ubiquitination. Our study demonstrates that Smac3 is functionally additive to, but independent of, Smac/DIABLO.     

Transient pockets on XIAP-BIR2: toward the characterization of putative binding sites of small-molecule XIAP inhibitors

Protein-protein interactions are abundant in signal transduction pathways and thus of crucial importance in the regulation of apoptosis. However, designing small-molecule inhibitors for these potential drug targets is very challenging as such proteins often lack well-defined binding pockets. An example for such an interaction is the binding of the anti-apoptotic BIR2 domain of XIAP to the pro-apoptotic caspase-3 that results in the survival of damaged cells. Although small-molecule inhibitors of this interaction have been identified, their exact binding sites on XIAP are not known as its crystal structures reveal no suitable pockets. Here, we apply our previously developed protocol for identifying transient binding pockets to XIAP-BIR2. Transient pockets were identified in snapshots taken during four different molecular dynamics simulations that started from the caspase-3:BIR2 complex or from the unbound BIR2 structure and used water or methanol as solvent. Clustering of these pockets revealed that surprisingly many pockets opened in the flexible linker region that is involved in caspase-3 binding. We docked three known inhibitors into these transient pockets and so determined five putative binding sites. In addition, by docking two inactive compounds of the same series, we show that this protocol is also able to distinguish between binders and nonbinders which was not possible when docking to the crystal structures. These findings represent a first step toward the understanding of the binding of small-molecule XIAP-BIR2 inhibitors on a molecular level and further highlight the importance of considering protein flexibility when designing small-molecule protein-protein interaction inhibitors.     

XIAP inhibition of caspase-3 preserves its association with the Apaf-1 apoptosome and prevents CD95- and Bax-induced apoptosis

Ligation of death receptors or formation of the Apaf-1 apoptosome results in the activation of caspases and execution of apoptosis. We recently demonstrated that X-linked inhibitor-of-apoptosis protein (XIAP) associates with the apoptosome in vitro. By utilizing XIAP mutants, we now report that XIAP binds to the 'native' apoptosome complex via a specific interaction with the small p12 subunit of processed caspase-9. Indeed, we provide the first direct evidence that XIAP can simultaneously bind active caspases-9 and -3 within the same complex and that inhibition of caspase-3 by the Linker-BIR2 domain prevents disruption of BIR3-caspase-9 interactions. Recent studies suggest that inhibition of caspase-3 is dispensable for its anti-apoptotic effects. However, we clearly demonstrate that inhibition of caspase-3 is required to inhibit CD95 (Fas/Apo-1)-mediated apoptosis, whereas inhibition of either caspase-9 or caspase-3 prevents Bax-induced cell death. Finally, we illustrate for the first time that XIAP mutants, which are incapable of binding to caspases-9 and -3 are completely devoid of anti-apoptotic activity. Thus, XIAP's capacity to maintain inhibition of caspase-9 within the Apaf-1 apoptosome is influenced by its ability to simultaneously inhibit active caspase-3, and depending upon the apoptotic stimulus, inhibition of caspase-9 or 3 is essential for XIAP's anti-apoptotic activity.     

W323S variant of Xiap-Bir3 binds to SMAC but not caspase-9

The ability of the wild-type XIAP BIR3 domain as well as its Trp323Ser variant in inhibition of human caspase-9, binding to AVPFVASLPN (SMAC-peptide), SMAC protein, and mature caspase-9 was investigated. In order to investigate the role of W323 on these interactions, this residue was mutated to Serine. Circular dichroism as well as thermal denaturation studies showed that W323S mutation did not hamper proper folding of the protein. The dissociation constants for the interaction of the wild type BIR3 as well as its mutant to Smac-type peptide were found to be 1.8 and 27 muM, respectively. The inhibition of and binding to caspase-9 by wild-type BIR3 and its mutant were also compared. While the wild-type protein potently inhibited the enzyme, the mutant failed to do so. The lack of caspase-9 inhibition was due to absence of interaction of the mutant BIR3 with mature caspase-9. These results indicate that Trp323 of BIR3 plays a pivotal role both in maintaining necessary conformation for caspase-9 interaction and to a lesser extent, recognition of Smac-type peptide. Moreover, decreased stability of the mutant compared with the wild type indicates that W323 is essential for maintaining the stability BIR3-Smac-peptide complex.     

Targeting XIAP for the treatment of malignancy

X-linked inhibitor of apoptosis protein (XIAP) is a member of the inhibitor of apoptosis proteins family of caspase inhibitors that selectively binds and inhibits caspases-3, -7 and -9, but not caspase-8. As such, XIAP blocks a substantial portion of the apoptosis pathway and is an attractive target for novel therapeutic agents for the treatment of malignancy. Antisense oligonucleotides directed against XIAP are effective in vitro and are currently being evaluated in clinical trials. Small molecule XIAP inhibitors that target the baculovirus IAP repeat (BIR) 2 or BIR 3 domain are in preclinical development and are advancing toward the clinic. This review will discuss the progress being made in developing antisense and small-molecule XIAP inhibitors.     

Structural Basis for Bivalent Smac-Mimetics Recognition in the IAP Protein Family

XIAP is an apoptotic regulator protein that binds to the effector caspases -3 and -7 through its BIR2 domain, and to initiator caspase-9 through its BIR3 domain. Molecular docking studies suggested that Smac-DIABLO may antagonize XIAP by concurrently targeting both BIR2 and BIR3 domains; on this basis bivalent Smac-mimetic compounds have been proposed and characterized. Here, we report the X-ray crystal structure of XIAP-BIR3 domain in complex with a two-headed compound (compound 3) with improved efficacy relative to its monomeric form. A small-angle X-ray scattering study of XIAP-BIR2BIR3, together with fluorescence polarization binding assays and compound 3 cytotoxicity tests on HL60 leukemia cell line are also reported. The crystal structure analysis reveals a network of interactions supporting XIAP-BIR3/compound 3 recognition; moreover, analytical gel-filtration chromatography shows that compound 3 forms a 1:1 stoichiometric complex with a XIAP protein construct containing both BIR2 and BIR3 domains. On the basis of the crystal structure and small-angle X-ray scattering, a model of the same BIR2-BIR3 construct bound to compound 3 is proposed, shedding light on the ability of compound 3 to relieve XIAP inhibitory effects on caspase-9 as well as caspases -3 and -7. A molecular modeling/docking analysis of compound 3 bound to cIAP1-BIR3 domain is presented, considering that Smac-mimetics have been shown to kill tumor cells by inducing cIAP1 and cIAP2 ubiquitination and degradation. Taken together, the results reported here provide a rationale for further development of compound 3 as a lead in the design of dimeric Smac mimetics for cancer treatment.     

Targeting the X-linked inhibitor of apoptosis protein through 4-substituted azabicyclo[5.3.0]alkane smac mimetics. Structure, activity, and recognition principles

The X-linked inhibitor of apoptosis protein (XIAP) is overexpressed in several malignant cells where it prevents apoptosis by binding to, and blocking, the activation of caspase-3, -7, and -9. Human XIAP (479 residues) is composed of three tandem-repeated baculoviral IAP repeat (BIR) domains (BIR1-3), and by a C-terminal RING domain. Smac-DIABLO [second mitochondria-derived activator of caspases (Smac)-direct IAP binding protein with low pI (DIABLO)], the natural antagonist of XIAP, binds through its N-terminal sequence AVPI to the same surface groove, in the BIR domains, that binds caspases. Synthetic compounds mimicking such tetrapeptide motif effectively block the interaction between IAP and active caspases, thus triggering apoptosis. Peptidomimetics based on an azabicyclo[x.y.0]alkane scaffolds, have been shown to bind the BIR3 domain of XIAP with micromolar to nanomolar affinities, thus presenting attractive features for drug lead optimization. Here we report a study on three newly synthesized Smac mimetics, which have been characterized in their complexes with XIAP BIR3 domain through X-ray crystallography and molecular modelling/docking simulations. Based on analysis of the crystal structures, we show that specific substitutions at the 4-position of the azabicyclo[5.3.0]alkane scaffold results in sizeable effects on the peptidomimetic-BIR3 domain affinity. By means of functional, biophysical and simulative approaches we also propose that the same Smac mimetics can bind XIAP BIR2 domain at a location structurally related to the BIR3 domain AVPI binding groove. Details of the XIAP-Smac mimetic recognition principles highlighted by this study are discussed in light of the drug-like profile of the three (potentially proapoptotic) compounds developed that show improved performance in ADMET (adsorption, distribution, metabolism, excretion and toxicity) tests.