Targeting inhibitor of apoptosis proteins for therapeutic intervention. Future Med Chem. 2009;1:1509-1525. [PMID: 21426063 DOI: 10.4155/fmc.09.116]

Estrogen Receptor-α Targeting: PROTACs, SNIPERs, Peptide-PROTACs, Antibody Conjugated PROTACs and SNIPERs

Targeting selective estrogen subtype receptors through typical medicinal chemistry approaches is based on occupancy-driven pharmacology. In occupancy-driven pharmacology, molecules are developed in order to inhibit the protein of interest (POI), and their popularity is based on their virtue of faster kinetics. However, such approaches have intrinsic flaws, such as pico-to-nanomolar range binding affinity and continuous dosage after a time interval for sustained inhibition of POI. These shortcomings were addressed by event-driven pharmacology-based approaches, which degrade the POI rather than inhibit it. One such example is PROTACs (Proteolysis targeting chimeras), which has become one of the highly successful strategies of event-driven pharmacology (pharmacology that does the degradation of POI and diminishes its functions). The selective targeting of estrogen receptor subtypes is always challenging for chemical biologists and medicinal chemists. Specifically, estrogen receptor α (ER-α) is expressed in nearly 70% of breast cancer and commonly overexpressed in ovarian, prostate, colon, and endometrial cancer. Therefore, conventional hormonal therapies are most prescribed to patients with ER + cancers. However, on prolonged use, resistance commonly developed against these therapies, which led to selective estrogen receptor degrader (SERD) becoming the first-line drug for metastatic ER + breast cancer. The SERD success shows that removing cellular ER-α is a promising approach to overcoming endocrine resistance. Depending on the mechanism of degradation of ER-α, various types of strategies of developed.

Recent advances in IAP-based PROTACs (SNIPERs) as potential therapeutic agents

Proteolytic targeting chimaeras (PROTACs) have been developed as an effective technology for targeted protein degradation. PROTACs are heterobifunctional molecules that can trigger the polyubiquitination of proteins of interest (POIs) by recruiting the ubiquitin-proteasome system, thereby inhibiting the intracellular level of POIs. To date, a variety of small-molecule PROTACs (CRBN, VHL, IAP, and MDM2-based PROTACs) have been developed. IAP-based PROTACs, also known as specific and nongenetic IAP-dependent protein erasers (SNIPERs), are used to degrade the target proteins closely related to diseases. Their structures consist of three parts, including target protein ligand, E3 ligase ligand, and the linker between them. So far, many SNIPERs have been extensively studied worldwide and have performed well in multiple diseases, especially cancer. In this review, we will present the most relevant advances in the field of SNIPERs and provide our perspective on the opportunities and challenges for SNIPERs to become therapeutic agents.

Melatonin increases 5-flurouracil-mediated apoptosis of colorectal cancer cells through enhancing oxidative stress and downregulating survivin and XIAP

Introduction: Colorectal cancer (CRC) is one of the most lethal human malignancies with a global alarming rate of incidence. The development of resistance against common chemotherapeutics such as 5-fluorouracil (5-FU) remains a big burden for CRC therapy. Therefore, we investigated the effects of melatonin on the increasing 5-FU- mediated apoptosis and its underlying mechanism in SW-480 CRC cell line.

Methods: The effects of melatonin and 5- FU, alone or in combination, on cell proliferation were evaluated using an MTT assay. Further, Annexin-V Flow cytometry was used for determining the effects of melatonin and 5-FU on the apoptosis of SW-480 cell lines. The expression levels of Bax, Bcl-2, pro-caspase-3/activated caspase 3, X-linked inhibitor of apoptosis proteins (XIAP), and survivin were measured after 48 hours incubation with drugs. Cellular levels of reactive oxygen species (ROS), catalase, superoxide dismutase and glutathione peroxidase were also evaluated.

Results: Melatonin and 5-FU significantly decreased the cell proliferation of SW-480 cells. Combination of 5-FU with melatonin significantly decreased the IC50 value of 5-FU from 100 μM to 50 μM. Moreover, combination therapy increased intracellular levels of ROS and suppressed antioxidant enzymatic activities (P < 0.05). Treatment with either melatonin or 5-FU resulted in the induction of apoptosis in comparison to control (P > 0.05). XIAP and survivin expression levels potently decreased after combination treatment with melatonin and 5-FU (P  < 0.05).

Conclusion: We demonstrated that melatonin exerts a reversing effect on the resistance to apoptosis by targeting oxidative stress, XIAP and survivin in CRC cells. Therefore, more studies need for better understanding of underlying mechanisms for beneficial effects of combination of melatonin and 5-FU.

Specific non-genetic IAP-based protein erasers (SNIPERs) as a potential therapeutic strategy

As a newly emerged technology, PROTAC (proteolysis targeting chimera) is a promising therapeutic strategy for varieties of diseases. Unlike small molecule inhibitors, PROTACs catalytically induce target proteins degradation, including currently "undruggable" target proteins. In addition, PROTACs can be a potentially successful strategy to overcome drug resistance. IAPs can inhibit apoptosis by inhibiting caspase, and also exhibits the activity of E3 ubiquitin ligase. Specific and nongenetic IAP-based protein erasers (SNIPERs) are hybrid molecules that designed based on IAPs, and used to degrade the target proteins closely associated with diseases. Their structures consist of three parts, including target protein ligand, E3 ligase ligand and the linker between them. SNIPERs (PROTACs) degrade diseases-associated proteins through human inherent ubiquitin-proteasome system. So far, many SNIPERs have been developed to treat diseases that difficult to handle by traditional methods, such as radiotherapy, chemotherapy and small molecule inhibitors, and showed promising prospects in application. In this paper, the recent advances of SNIPERs were summarized, and the chances and challenges associated with this area were also highlighted.

Immunoblot Analysis of the Regulation of TNF Receptor Family-Induced NF-κB Signaling by c-IAP Proteins

Proper maintenance of organismal homeostasis, development, and immune defense requires precise regulation of survival and signaling pathways. Inhibitor of apoptosis (IAP) proteins are evolutionarily conserved regulators of cell death and immune signaling that impact numerous cellular processes. Although initially characterized as inhibitors of apoptosis, the ubiquitin ligase activity of IAP proteins is critical for modulating various signaling pathways (e.g., NF-κB, MAPK) and cell survival. Cellular IAP1 and 2 regulate the pro-survival canonical NF-κB pathway by ubiquitinating RIP1 and themselves thus enabling recruitment of kinase (IKK) and E3 ligase (LUBAC) complexes. On the other hand, c-IAP1 and c-IAP2 are negative regulators of noncanonical NF-κB signaling by promoting ubiquitination and consequent proteasomal degradation of the NF-κB-inducing kinase NIK. Here we describe the involvement of c-IAP1 and c-IAP2 in NF-κB signaling and provide detailed methodology for examining functional roles of c-IAPs in these pathways.

Regulation of Cell Death and Immunity by XIAP

X-chromosome-linked inhibitor of apoptosis protein (XIAP) controls cell survival in several regulated cell death pathways and coordinates a range of inflammatory signaling events. Initially identified as a caspase-binding protein, it was considered to be primarily involved in blocking apoptosis from both intrinsic as well as extrinsic triggers. However, XIAP also prevents TNF-mediated, receptor-interacting protein 3 (RIPK3)-dependent cell death, by controlling RIPK1 ubiquitylation and preventing inflammatory cell death. The identification of patients with germline mutations in XIAP (termed XLP-2 syndrome) pointed toward its role in inflammatory signaling. Indeed, XIAP also mediates nucleotide-binding oligomerization domain-containing 2 (NOD2) proinflammatory signaling by promoting RIPK2 ubiquitination within the NOD2 signaling complex leading to NF-κB and MAPK activation and production of inflammatory cytokines and chemokines. Overall, XIAP is a critical regulator of multiple cell death and inflammatory pathways making it an attractive drug target in tumors and inflammatory diseases.

The Sensitization of Melatonin in Osteosarcoma Cells by Suppression of Anti-Apoptotic Proteins

Background: Investigation of anti-cancer agents with desirable selective toxicity is critical for cancer therapy. The use of natural adjuvants can be a promising option in reducing the toxicity of the anti-cancer agent. The aim of this study was to investigate the potential application of melatonin (MLT) as a natural adjuvant molecule along with doxorubicin (DOX) to induce cytotoxicity in osteosarcoma (OS) cells. Methods: Human OS cell lines included Saos-2, MG-63, and Human Bone Marrow Mesenchymal Stem Cells (hBM-MSCs) were treated with free DOX, free MLT, DOX-loaded NPs (DOX-NPs), MLT-loaded NPs (MLT-NPs), combination of DOX and MLT (DOX-MLT) and combination of DOX and MLT-loaded NPs (DOX-MLT-NPs) in separated cell culture. Cell proliferation of experiments were evaluated by MTT assay after 24 h. Total protein levels were determined by enzyme immunoassay ELISA. Results: Herein, we found the combination of MLT with DOX, especially formulated in nano-form, is resulted in a significant reduction in the protein levels of both X-linked Inhibitor of Apoptosis (XIAP) and Survivin (p<0.0001). Indeed, there was a significant decrease in the expression of XIAP and Survivin when MLT is combined with DOX compared to the individual treatments. Conclusion: Our findings indicated the synergism of the antitumor effect could be due to the down-regulation of XIAP and Survivin in the levels of protein.

Medicinal Chemistry of Inhibiting RING-Type E3 Ubiquitin Ligases

The ubiquitin proteasome system (UPS) presents many opportunities for pharmacological intervention. Key players in the UPS are E3 ubiquitin ligases, responsible for conjugation of ubiquitin to specific cognate substrates. Numbering more than 600 members, these ligases represent the most selective way to intervene within this physiologically important system. This Perspective highlights some of the dedicated medicinal chemistry efforts directed at inhibiting the function of specific single-protein and multicomponent RING-type E3 ubiquitin ligases. We present opportunities and challenges associated with targeting this important class of enzymes.

NO-Donor Nitrosyl Iron Complex with 2-Aminophenolyl Ligand Induces Apoptosis and Inhibits NF-κB Function in HeLa Cells

NO donating iron nitrosyl complex with 2-aminothiophenyl ligand (2-AmPh complex) was studied for its ability to cause cell death and affect nuclear factor kappa B (NF-κB) signaling. The complex inhibited viability of HeLa cells and induced cell death that was accompanied by loss of mitochondrial membrane potential and characteristic for apoptosis phosphatidylserine externalization. At IC50, 2-AmPh caused decrease in nuclear content of NF-κB p65 polypeptide and mRNA expression of NF-κB target genes encoding interleukin-8 and anti-apoptotic protein BIRC3. mRNA levels of interleukin-6 and anti-apoptotic protein BIRC2 encoding genes were not affected. Our data demonstrate that NO donating iron nitrosyl complex 2-AmPh can inhibit tumor cell viability and induce apoptosis that is preceded by impairment of NF-κB function and suppression of a subset of NF-κB target genes.

Survivin and XIAP expression in distinct tumor compartments of surgically resected gastric cancer: XIAP as a prognostic marker in diffuse and mixed type adenocarcinomas

There is considerable evidence that the inhibitor of apoptosis protein (IAP) family serves a role in tumorigenesis. The most studied IAP family members, survivin and X-linked inhibitor of apoptosis (XIAP), have been demonstrated to serve as biomarkers in distinct tumor entities. Thus, the present study aimed to investigate the expression levels of both IAPs in the tumor center, invasion front and lymph node metastases of surgically resected gastric cancer (GC) specimens. Tissue microarrays containing samples from 201 primary GCs were analyzed. IAP expression was detected using immunohistochemistry in different tumor compartments, normal mucosa and lymph node metastases. In addition, the association between the expression levels of these proteins, and clinicopathological parameters and overall survival was investigated. High levels of survivin and XIAP were evident in GC, when compared with normal mucosa, and were correlated with intestinal-type and well-differentiated GC, as well as low International Union Against Cancer stages. Increased XIAP expression was detected in lymph node metastases as compared with corresponding primary tumors. XIAP overexpression was identified to be an independent negative prognostic marker in diffuse and mixed type GC. These results suggest a potential role of survivin and XIAP in the early phase of gastric carcinogenesis. In addition, increased XIAP expression in lymph node metastases supports the observation that IAPs serve an essential role in metastatic tumor disease. Since XIAP expression was identified to be associated with poor survival in diffuse and mixed type GC, XIAP may serve as a novel therapeutic target in these types of GC.

An inhibitor of apoptosis protein antagonist T-3256336 potentiates the antitumor efficacy of the Nedd8-activating enzyme inhibitor pevonedistat (TAK-924/MLN4924)

Inhibitors of apoptosis proteins (IAPs) are antiapoptotic regulators that block cell death, and are frequently overexpressed in several human cancers, where they facilitate evasion of apoptosis and promote cell survival. IAP antagonists are also known as second mitochondria-derived activator of caspase (SMAC)-mimetics, and have recently been considered as novel therapeutic agents for inducing apoptosis, alone and in combination with other anticancer drugs. In this study, we showed that T-3256336, the orally available IAP antagonist has synergistically enhances the antiproliferative effects of the NEDD8-activating enzyme (NAE) inhibitor pevonedistat (TAK-924/MLN4924), and these effects were attenuated by a TNFα-neutralizing antibody. In the present mechanistic analyses, pevonedistat induced TNFα mRNA and triggered IAP antagonist-dependent extrinsic apoptotic cell death in cancer cell lines. Furthermore, synergistic effects of the combination of T-3256336 and pevonedistat were demonstrated in a HL-60 mouse xenograft model. Our findings provide mechanistic evidence of the effects of IAP antagonists in combination with NAE inhibitors, and demonstrate the potential of a new combination therapy for cancer.

T-3256336, a novel and orally available small molecule IAP antagonist, induced tumor cell death via induction of systemic TNF alpha production

Inhibitors of apoptosis proteins (IAPs) are a family of antiapoptotic regulators that have attracted attention as potential targets for cancer therapeutics. Although recent studies have revealed that small-molecule IAP antagonists induce tumor selective cell death in an autocrine tumor necrosis factor (TNF)α-dependent manner, the single-agent efficacy of IAP antagonists is restricted to a small subset of cancer cells. In this study, we showed that the single-agent activity of T-3256336 was limited to a few cancer cell lines in vitro, and these cell lines were defined by relatively high levels of TNFα mRNA expression. However, some other cancer cells, including PANC-1 cells, become drastically sensitive to T-3256336 when costimulated with exogenous TNFα. In PANC-1 mouse xenograft models, the administration of T-3256336 increased levels of several cytokines including TNFα and lead to tumor regression as a single agent, which was attenuated by the neutralization of circulating mouse TNFα with an antibody. These results suggest dual roles of IAP antagonists, increase systemic cytokines including TNFα, and sensitization of tumors to IAP antagonist-induced death.

Targeting Cell Survival Proteins for Cancer Cell Death

Escaping from cell death is one of the adaptations that enable cancer cells to stave off anticancer therapies. The key players in avoiding apoptosis are collectively known as survival proteins. Survival proteins comprise the Bcl-2, inhibitor of apoptosis (IAP), and heat shock protein (HSP) families. The aberrant expression of these proteins is associated with a range of biological activities that promote cancer cell survival, proliferation, and resistance to therapy. Several therapeutic strategies that target survival proteins are based on mimicking BH3 domains or the IAP-binding motif or competing with ATP for the Hsp90 ATP-binding pocket. Alternative strategies, including use of nutraceuticals, transcriptional repression, and antisense oligonucleotides, provide options to target survival proteins. This review focuses on the role of survival proteins in chemoresistance and current therapeutic strategies in preclinical or clinical trials that target survival protein signaling pathways. Recent approaches to target survival proteins-including nutraceuticals, small-molecule inhibitors, peptides, and Bcl-2-specific mimetic are explored. Therapeutic inventions targeting survival proteins are promising strategies to inhibit cancer cell survival and chemoresistance. However, complete eradication of resistance is a distant dream. For a successful clinical outcome, pretreatment with novel survival protein inhibitors alone or in combination with conventional therapies holds great promise.

Targeting Cell Survival Proteins for Cancer Cell Death

Escaping from cell death is one of the adaptations that enable cancer cells to stave off anticancer therapies. The key players in avoiding apoptosis are collectively known as survival proteins. Survival proteins comprise the Bcl-2, inhibitor of apoptosis (IAP), and heat shock protein (HSP) families. The aberrant expression of these proteins is associated with a range of biological activities that promote cancer cell survival, proliferation, and resistance to therapy. Several therapeutic strategies that target survival proteins are based on mimicking BH3 domains or the IAP-binding motif or competing with ATP for the Hsp90 ATP-binding pocket. Alternative strategies, including use of nutraceuticals, transcriptional repression, and antisense oligonucleotides, provide options to target survival proteins. This review focuses on the role of survival proteins in chemoresistance and current therapeutic strategies in preclinical or clinical trials that target survival protein signaling pathways. Recent approaches to target survival proteins-including nutraceuticals, small-molecule inhibitors, peptides, and Bcl-2-specific mimetic are explored. Therapeutic inventions targeting survival proteins are promising strategies to inhibit cancer cell survival and chemoresistance. However, complete eradication of resistance is a distant dream. For a successful clinical outcome, pretreatment with novel survival protein inhibitors alone or in combination with conventional therapies holds great promise.

Terazosin activates Pgk1 and Hsp90 to promote stress resistance

Drugs that can protect against organ damage are urgently needed, especially for diseases such as sepsis and brain stroke. We discovered that terazosin (TZ), a widely marketed α1-adrenergic receptor antagonist, alleviated organ damage and improved survival in rodent models of stroke and sepsis. Through combined studies of enzymology and X-ray crystallography, we discovered that TZ binds a new target, phosphoglycerate kinase 1 (Pgk1), and activates its enzymatic activity, probably through 2,4-diamino-6,7-dimethoxyisoquinoline's ability to promote ATP release from Pgk1. Mechanistically, the ATP generated from Pgk1 may enhance the chaperone activity of Hsp90, an ATPase known to associate with Pgk1. Upon activation, Hsp90 promotes multistress resistance. Our studies demonstrate that TZ has a new protein target, Pgk1, and reveal its corresponding biological effect. As a clinical drug, TZ may be quickly translated into treatments for diseases including stroke and sepsis.

Roles of c-IAP proteins in TNF receptor family activation of NF-κB signaling

Precise regulation of survival and signaling pathways is essential for proper maintenance of organismal homeostasis, development, and immune defense. Inhibitor of apoptosis (IAP) proteins are evolutionarily conserved regulators of cell death and immune signaling that impact numerous cellular processes. Initially characterized as inhibitors of apoptosis, the ubiquitin ligase activity of IAP proteins is critical for modulating various signaling pathways (e.g., NF-κB, MAPK) and cellular fate. Cellular IAP1 and IAP2 regulate the pro-survival canonical NF-κB pathway by ubiquitinating RIP1 and enabling recruitment of kinase (IKK) and E3 ligase (LUBAC) complexes. On the other hand, c-IAP1 and c-IAP2 are negative regulators of noncanonical NF-κB signaling by promoting ubiquitination and consequent degradation of the NF-κB-inducing kinase NIK. In this article, we describe the involvement of c-IAP1 and c-IAP2 in NF-κB signaling and provide detailed methodology for examining how c-IAPs exert their functional roles.

Melatonin overcomes apoptosis resistance in human hepatocellular carcinoma by targeting survivin and XIAP

Apoptosis resistance in hepatocellular carcinoma (HCC) is one of the most significant factors for hepatocarcinogenesis and tumor progression, and leads to resistance to conventional chemotherapy. It is well known that inhibitor of apoptosis proteins (IAPs) play key roles in apoptosis resistance, it has become an important target for antitumor therapy. In this study, we examined if melatonin, the main secretory product of the pineal gland, targeted IAPs, leading to the inhibition of apoptosis resistance. To accomplish this, we first observed that four members of IAPs (cIAP-1, cIAP-2, survivin, and XIAP) were overexpressed in human HCC tissue. Interestingly, melatonin significantly inhibited the growth of HepG2 and SMMC-7721 cells and promoted apoptosis along with the downregulation of survivin and XIAP, but had no effect on the expression of cIAP-1 and cIAP-2. These data suggest that the inhibition of survivin and XIAP by melatonin may play an important part in reversing apoptosis resistance. Notably, cIAP-1, survivin and XIAP were significantly associated with the coexpression of COX-2 in human HCC specimens. Melatonin also reduced the expression of COX-2 and inhibited AKT activation in HepG2 and SMMC-7721 cells. Inhibition of COX-2 activity with the selective inhibitor, NS398, and inhibition of AKT activation using the PI3K inhibitor, LY294002, in tumor cells confirmed that melatonin-induced apoptosis was COX-2/PI3K/AKT-dependent, suggesting that the COX-2/PI3K/AKT pathway plays a role in melatonin inhibition of IAPs. Taken together, these results suggest that melatonin overcomes apoptosis resistance by the suppressing survivin and XIAP via the COX-2/PI3K/AKT pathway in HCC cells.

Regulation of cell migration, invasion and metastasis by IAP proteins and their antagonists

Inhibitor of apoptosis (IAP) proteins are a family comprised of a total of eight mammalian members that were initially described to act as endogenous inhibitors of caspases. In addition, extensive evidence has been accumulated over the last years showing that IAP proteins can regulate various signal transduction pathways, thereby exerting non-apoptotic functions beyond the inhibition of apoptosis. For example, IAP proteins have been implied in the control of cell motility, migration, invasion and metastasis. However, currently the question is controversially discussed whether or not they positively or negatively control these processes. As small-molecule inhibitors of IAP proteins have entered the stage of clinical evaluation as experimental cancer therapeutics, a better understanding of their various cellular effects will be critical for their rational use in the treatment of human diseases.

Synergistic induction of TRAIL-mediated apoptosis by anisomycin in human hepatoma cells via the BH3-only protein Bid and c-Jun/AP-1 signaling pathway

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF super-family, and it has been shown that many human cancer cell lines are refractory to TRAIL-induced cell death. However, the molecular mechanisms underlying resistance are unclear. In the present study, we show that TRAIL-resistance is reversed in human hepatoma cells by anisomycin, which is known to inhibit protein synthesis and induce ribotoxic stress. Synergistic induction of apoptosis in cells treated with anisomycin plus TRAIL was associated with activation of caspases and cleavage of Bid, a pro-apoptotic BH3-only protein. Silencing of Bid expression by small interfering RNA (siRNA) significantly attenuated the loss of mitochondrial membrane potential (MMP, Δψm) and significantly increased induction of apoptosis in cells treated with anisomycin and TRAIL, confirming that Bid cleavage is required for the response. In addition, c-Jun/AP-1 was rapidly activated upon stimulation with anisomycin; however, the knockdown of c-Jun/AP-1 expression by c-Jun siRNA markedly reduced anisomycin plus TRAIL-induced loss of MMP and apoptosis. Taken together, the findings show that anisomycin sensitizes TRAIL-mediated hepatoma cell apoptosis via the mitochondria-associated pathway, involving the cleavage of Bid and activation of the c-Jun/AP-1 pathway, indicating that this compound can be used as an anti-tumor agent in combination with TRAIL.

Progress in understanding the role of inhibitor of apoptosis proteins in molecular targeted therapy of esophageal cancer

The apoptosis and antiapoptotic signaling pathways play a critical role in the embryonic and lymphocyte development, immune system modulation, and tissue homeostasis, as well as carcinogenesis. As inhibitor of apoptosis proteins (IAPs) are highly expressed in several neoplasms and are closely related to carcinogenesis, cancer progression, radiochemotherapeutic resistance, and prognosis, therapies targeting IAPs have become a research hotspot for molecular targeted therapy of tumors. In recent years, many agents targeting IAPs which are being evaluated in clinical trials are showing promising prospect for neoplastic therapy. As such, the identification of key roles of IAPs in esophageal cancer has revealed their potential value as therapeutic targets. This report reviews the progress in understanding the role of IAPs in molecular targeted therapy of esophageal cancer.

Cellular inhibitors of apoptosis are global regulators of NF-κB and MAPK activation by members of the TNF family of receptors

Tumor necrosis factor (TNF) family members are essential for the development and proper functioning of the immune system. TNF receptor (TNFR) signaling is mediated through the assembly of protein signaling complexes that activate the nuclear factor κB (NF-κB) and mitogen-activated protein kinase (MAPK) pathways in a ubiquitin-dependent manner. The cellular inhibitor of apoptosis (c-IAP) proteins c-IAP1 and c-IAP2 are E3 ubiquitin ligases that are recruited to TNFR signaling complexes through their constitutive association with the adaptor protein TNFR-associated factor 2 (TRAF2). We demonstrated that c-IAP1 and c-IAP2 were required for canonical activation of NF-κB and MAPK by members of the TNFR family. c-IAPs were required for the recruitment of inhibitor of κB kinase β (IKKβ), the IKK regulatory subunit NF-κB essential modulator (NEMO), and RBCK1/Hoil1-interacting protein (HOIP) to TNFR signaling complexes and the induction of gene expression by TNF family members. In contrast, TNFRs that stimulated the noncanonical NF-κB pathway triggered translocation of c-IAPs, TRAF2, and TRAF3 from the cytosol to membrane fractions, which led to their proteasomal and lysosomal degradation. Finally, we established that signaling by B cell-activating factor receptor 3 induced the cytosolic depletion of TRAF3, which enabled noncanonical NF-κB activation. These results define c-IAP proteins as critical regulators of the activation of NF-κB and MAPK signaling pathways by members of the TNFR superfamily.

Targeting IAP proteins for therapeutic intervention in cancer

Evasion of apoptosis is one of the crucial acquired capabilities used by cancer cells to fend off anticancer therapies. Inhibitor of apoptosis (IAP) proteins exert a range of biological activities that promote cancer cell survival and proliferation. X chromosome-linked IAP is a direct inhibitor of caspases - pro-apoptotic executioner proteases - whereas cellular IAP proteins block the assembly of pro-apoptotic protein signalling complexes and mediate the expression of anti-apoptotic molecules. Furthermore, mutations, amplifications and chromosomal translocations of IAP genes are associated with various malignancies. Among the therapeutic strategies that have been designed to target IAP proteins, the most widely used approach is based on mimicking the IAP-binding motif of second mitochondria-derived activator of caspase (SMAC), which functions as an endogenous IAP antagonist. Alternative strategies include transcriptional repression and the use of antisense oligonucleotides. This Review provides an update on IAP protein biology as well as current and future perspectives on targeting IAP proteins for therapeutic intervention in human malignancies.

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.

Ubiquitylation in apoptosis: a post-translational modification at the edge of life and death

The proper regulation of apoptosis is essential for the survival of multicellular organisms. Furthermore, excessive apoptosis can contribute to neurodegenerative diseases, anaemia and graft rejection, and diminished apoptosis can lead to autoimmune diseases and cancer. It has become clear that the post-translational modification of apoptotic proteins by ubiquitylation regulates key components in cell death signalling cascades. For example, ubiquitin E3 ligases, such as MDM2 (which ubiquitylates p53) and inhibitor of apoptosis (IAP) proteins, and deubiquitinases, such as A20 and ubiquitin-specific protease 9X (USP9X) (which regulate the ubiquitylation and degradation of receptor-interacting protein 1 (RIP1) and myeloid leukaemia cell differentiation 1 (MCL1), respectively), have important roles in apoptosis. Therapeutic agents that target apoptotic regulatory proteins, including those that are part of the ubiquitin-proteasome system, might afford clinical benefits.

Antagonists of IAP proteins as cancer therapeutics

Inhibitor of apoptosis (IAP) proteins play pivotal roles in cellular survival by blocking apoptosis, modulating signal transduction, and affecting cellular proliferation. Through their interactions with inducers and effectors of apoptosis IAP proteins can effectively suppress apoptosis triggered by diverse stimuli including death receptor signaling, irradiation, chemotherapeutic agents, or growth factor withdrawal. Evasion of apoptosis, in part due to the action of IAP proteins, enhances resistance of cancer cells to treatment with chemotherapeutic agents and contributes to tumor progression. Additionally, IAP genes are known to be subject to amplification, mutation, and chromosomal translocation in human malignancies and autoimmune diseases. In this review we will discuss the role of IAP proteins in cancer and the development of antagonists targeting IAP proteins for cancer treatment.

Antagonists of inhibitor of apoptosis proteins based on thiazole amide isosteres

A series of IAP antagonists based on thiazole or benzothiazole amide isosteres was designed and synthesized. These compounds were tested for binding to the XIAP-BIR3 and ML-IAP BIR using a fluorescence polarization assay. The most potent of these compounds, 19a and 33b, were found to have K(i)'s of 20-30 nM against ML-IAP and 50-60 nM against XIAP-BIR3.