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DNA damage, chromosomal abnormalities, oncogene activation, viral infection, substrate detachment and hypoxia can all trigger apoptosis in normal cells. However, cancer cells acquire mutations that allow them to survive these threats that are part and parcel of the transformation process or that may affect the growth and dissemination of the tumor. Eventually, cancer cells accumulate further mutations that make them resistant to apoptosis mediated by standard cytotoxic chemotherapy or radiotherapy. The inhibitor of apoptosis (IAP) family members, defined by the presence of a baculovirus IAP repeat (BIR) protein domain, are key regulators of cytokinesis, apoptosis and signal transduction. Specific IAPs regulate either cell division, caspase activity or survival pathways mediated through binding to their BIR domains, and/or through their ubiquitin-ligase RING domain activity. These protein–protein interactions and post-translational modifications are the subject of intense investigations that shed light on how these proteins contribute to oncogenesis and resistance to therapy. In the past several years, we have seen multiple approaches of IAP antagonism enter the clinic, and the rewards of such strategies are about to reap benefit. Significantly, small molecule pan-IAP antagonists that mimic an endogenous inhibitor of the IAPs, called Smac, have demonstrated an unexpected ability to sensitize cancer cells to tumor necrosis factor-α and to promote autocrine or paracrine production of this cytokine by the tumor cell and possibly, other cells too. This review will focus on these and other developmental therapeutics that target the IAPs in cancer.Oncogene (2008) 27, 6252–6275; doi:10.1038/onc.2008.302 [ABSTRACT FROM AUTHOR]

Fifty outbred Syrian golden hamsters were equally divided into three experimental groups and two control groups. The pouches of the experimental groups were painted bilaterally with a 0.5% 7,12-dimethylbenz[a]anthracene (DMBA) solution thrice a week for 3, 7 and 14 weeks. One of the control groups was applied with mineral oil while another control group remained untreated throughout the experiment. Neither survivin nor cIAP2 could be detected in any of the control tissues, whereas survivin and cIAP2 were found to be significantly increased in 3-, 7- and 14-week DMBA-treated pouches compared with the control pouches. Expression of XIAP, cIAP1 and NAIP were noted for both the control and 3-, 7- and 14-week DMBA-treated pouches, but levels were found to be significantly elevated in the experimental groups compared with the control pouches. p53 was not detected in any control tissues, but was significantly increased in 3-, 7- and 14-week DMBA-treated pouches. Direct sequencing revealed a point mutation (C→G) of p53 for pouch tissues treated with DMBA for 3 and 7 weeks, and there was a wide variation in the p53 sequence of the 14-week DMBA-treated pouch tissues, as compared with the control tissues. The control tissues had a survivin- and cIAP2-methylated allele, whereas the DMBA-treated tissues showed no evidence of survivin- and cIAP2-methylation. Neither the control nor DMBA-treated pouches showed evidence of XIAP-, cIAP1- or NAIP-methylation. Our results suggest that the expression of inhibitors of apoptosis family in DMBA-induced hamster buccal-pouch squamous-cell carcinogenesis may be modulated by both genetic (mutant p53) and epigenetic mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: Overexpression of cIAP1 correlates with resistance to radiotherapy and chemotherapy in various cancers. Recently, we reported that a class of bestatin ester analogs represented by MeBS (2) destabilized and promoted the degradation of cIAP1 through auto-ubiquitination, and thereby sensitized cancer cells to apoptosis. Herein, we present chemical evidence that bestatin ester analogs directly interact with the cIAP1-BIR3 domain by means of fluorescence polarization assay and photoaffinity labeling assay using fluorescent probes. [Copyright &y& Elsevier]

Abstract: A series of hybrid compounds of bestatin (1) and actinonin (3), which promote degradation of cellular inhibitor of apoptosis protein 1 (cIAP1), were designed and synthesized. Structure–activity relationship studies indicated that absolute configuration, hydrophobicity at the α-position of the internal amide carbonyl group, and the presence of a small substituent at the α-position of the ester group are important factors for the expression of potent cIAP1 degradation-promoting activity. HAB-5A (30b) showed the most potent activity (IC50 =0.53μM) among the compounds prepared. [Copyright &y& Elsevier]

Abstract: Previously, this laboratory demonstrated that ethanol reduces the number of developing serotonin (5-HT)-containing neurons by increasing apoptosis. We also found that 5-HT1A agonists attenuate the proapoptotic effects of ethanol and the ethanol-mediated reduction of fetal 5-HT neurons. These neuroprotective effects are mediated in part by the ability of 5-HT1A agonists to activate the phosphatidyl 3′-kinase (PI-3K) prosurvival pathway. NF-κB is one of the downstream effectors activated by this pathway. In the present study, we used quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR) to determine the effects of 50mM ethanol and 100nM of ipsapirone, a 5-HT1A agonist, on the expression of several NF-κB-dependent antiapoptotic genes: X-linked inhibitor of apoptosis protein (XIAP), cIAP1, cIAP2, Bcl-2, and Bcl-xl. We also investigated the effects of ethanol and ipsapirone on the expression of the gene encoding the 5-HT1A receptor. The results demonstrate that ethanol reduces the expression of several prosurvival genes: XIAP, cIAP1, cIAP2, Bcl-2, and Bcl-xl. Importantly, the ethanol-mediated reduction in the expression of XIAP and Bcl-xl was prevented by co-treatment with ipsapirone. Thus, the damaging effects of ethanol are likely to involve a reduction in several prosurvival proteins. Moreover, the protective effects of ipsapirone on ethanol-treated neurons might involve their ability to prevent the reduction of XIAP and Bcl-xl. Although ipsapirone treatment decreased the expression of cIAP1, Bcl-2, and Bcl-xl in control neurons, our prior studies suggest that their survival is not reduced by ipsapirone. We also observed an increased expression of the 5-HT1A receptor in ipsapirone-treated control neurons. [Copyright &y& Elsevier]

Abstract: Human cIAP1 protein is a member of the inhibitor of apoptosis proteins (IAPs) that are involved in apoptosis regulation and an increasing number of other functions, including cell cycle and intracellular signal transduction. In order to identify novel proteins involved in cIAP1 regulation, we performed a yeast two-hybrid screen and identified an F-box protein Fbxo7 as a cIAP1 interacting protein. Co-immunoprecipitation assay showed that cIAP1 can interact with Fbxo7 in human cells. When co-expressed in cells, cIAP1 and Fbxo7 co-localized remarkably both in the cytoplasm and nucleus, and considerable amounts of these often co-localized at one or few distinct Golgi-like structures close to the nucleus. Furthermore, we showed that overexpression of Fbxo7 promotes the ubiquitination of cIAP1. Since F-box proteins are specificity determining subunits of SCF ubiquitin protein ligases, our results suggest that Fbxo7 can mediate the ubiquitination of cIAP1 by SCF ubiquitin protein ligase and thus have important implication in the regulation of cIAP1 function. [Copyright &y& Elsevier]

Abstract: cIAP-1, an apoptosis inhibiting protein, has been suggested to play important roles in the development of cervical and esophageal squamous cell carcinomas (SCCs). In order to clarify the subcellular localization of cIAP-1 and to investigate its clinicopathological significance in head and neck SCCs (HNSCCs), we examined cIAP-1 expression in four oral SCC cell lines by immunocytochemistry and Western blot. Expressions of nuclear and cytoplasmic cIAP-1, caspase-3, and Smac/DIABLO were also examined immunohistochemically in 57 cases of the HNSCCs. cIAP-1 expression was detected in HSC-2, HSC-3, and HSC-4 cells by immunohistochemistry and Western blot. In HSC-2 and HSC-4 cells, cIAP-1 was detected in both the nuclear and cytoplasmic fractions. Nuclear cIAP-1 expression was positive in 17 (30%) of HNSCCs, was correlated with lymph node metastasis (P=0.020) and advanced disease stage (P=0.032), and tended to be correlated with poor patient prognosis (P=0.059). Cytoplasmic cIAP-1 expression showed similar but weaker clinicopathological correlations. Nuclear cIAP-1 expression was inversely correlated with caspase-3 expression, but was correlated with Smac/DIABLO expression. Nuclear cIAP-1 expression appears to be a useful marker for predicting poor patient prognosis in HNSCCs, and may play roles in HNSCCs through the signaling pathway mediated by Smac/DIABLO and caspase-3. [Copyright &y& Elsevier]

Dynamic subcellular localization is an important regulatory mechanism for many proteins. cIAP1 and cIAP2 are two closely related members of inhibitor of apoptosis (IAP) family that play a role both as caspase inhibitors and as mediators of tumor necrosis factor (TNF) receptor signaling. Here, we report that cIAP1 and cIAP2 are nuclear shuttling proteins, whose subcellular localization is mediated by the CRM1-dependent nuclear export pathway. Blocking export with leptomycin B induces accumulation of both endogenous cIAP1 and epitope-tagged cIAP1 and cIAP2 in the nucleus of human cancer cells. We have identified a new CRM1-dependent leucine-rich nuclear export signal (NES) in the linker region between cIAP1 BIR2 and BIR3 repeats. Mutational inactivation of the NES, which is not conserved in cIAP2, reduces cIAP1 nuclear export. Forced relocation of cIAP1 to the nucleus did not significantly alter its ability to prevent apoptosis. Interestingly, co-expression experiments showed that the cIAP1 and cIAP2-interacting protein TNF receptor-associated factor 2 (TRAF2) plays an important role as regulator of IAP nucleocytoplasmic localization, by preventing nuclear translocation of cIAP1 and cIAP2. TRAF2-mediated cytoplasmic retention of cIAP1 was reduced upon TNFα treatment. Our results identify molecular mechanisms that contribute to regulate the subcellular localization of cIAP1 and cIAP2. Translocation between different cell compartments may add a further level of control for cIAP1 and cIAP2 activity. [Copyright &y& Elsevier]

Cellular inhibitor of apoptosis protein‑1 (cIAP1) is a key regulator of programmed cell death and is known to be associated with chemotherapeutic resistance. The present study aimed to investigate the antitumor efficacy of birinapant, a novel selective inhibitor of cIAP1, against cisplatin (CDDP)‑resistant hepatoblastoma (HB) cells. Western blot analysis was used to investigate the antitumor effect of birinapant on cIAP1 expression in Huh6 cells at the protein level. A WST‑8 assay was performed to evaluate the tumor growth inhibitory effect of birinapant on the human HB cell lines, Huh6 and HepG2. Huh6 cells were exposed to CDDP and/or birinapant in order to confirm tumor growth inhibition. The antitumor efficacy of birinapant plus CDDP combination therapy was significantly higher than that of CDDP monotherapy in a dose‑dependent manner (P=0.035). The study also investigated the antitumor efficacy of birinapant plus CDDP combination therapy in an established xenograft model of SCID mice. Compared with CDDP monotherapy, birinapant combined with CDDP showed better inhibition of tumor growth (P=0.121). It was observed that the mRNA expression of cIAP1 in tumors was significantly enriched in the CDDP monotherapy group compared with that in the untreated group. Furthermore, immunohistochemical staining was performed to compare cIAP1 expression in pre‑ and post‑chemotherapy specimens in patients with HB, and a significant increase was observed in the post‑chemotherapy specimens (P<0.001). CDDP‑resistant Huh6 (Huh6‑CDDP R ) cells were also established following repeated exposure to CDDP. Birinapant was substantially more effective against the Huh6‑CDDP R cells than against the Huh6 wild‑type cells. Taken together, these findings suggest that repeated exposure to CDDP enhances cIAP1 expression in HB cells and that birinapant is a promising therapeutic drug for CDDP‑resistant HB.

The N-terminal sequence of the Smac (second-mitochondria derived activator) protein is known to be involved in binding to the BIR3 (Baculovirus IAP repeat) domain of the IAPs (inhibitors of apoptosis proteins), and antagonized their function. Short peptides derived from N-terminal residues of Smac have shown to sensitize cancer cells to chemotherapeutic agents. In this regard, small library including 6-mer peptides were designed using docking to the BIR3 domain of cIAP1 in silico . Molecular dynamics simulation studies were also done on top-scored hits (SmacAQ, SmacIQ) using Desmond 2017-2 for 150 ns simulation time. These two peptides were conveniently synthesized using solid phase peptide synthesis on Fmoc-Gln (Trt)-Wang resin. Furthermore, we encapsulated DOX (doxorubicin) and synthesized peptides in PLGA: PLGA-PEG (9:1) NPs (nanoparticles) followed by MD (molecular dynamic) studies to understand the NP structure and the interactions between either DOX or peptide with polymeric nanoparticles during 100 ns simulation. Finally, the cytotoxic activity of these peptides in combination with DOX against two cancer cell lines including MCF7 and C26 were investigated. As a result, we found that DOX or peptide-loaded NPs had stable structure during the simulation. MD simulation also showed that alanine at N-terminal of Smac could be replaced with isoleucine without alternation of biological activity which was in agreement with in vitro experiments. Moreover, NPs-SmacIQ and NPs-SmacAQ significantly enhanced the cytotoxicity effect of NPs-DOX in vitro ( p  < 0.001).Communicated by Ramaswamy H. Sarma.

The present study investigated the effects and molecular mechanism of the second mitochondria‑derived activator of caspase (SMAC) mimetic birinapant on the proliferation and apoptotic rate of liver cancer cells. Western blotting and reverse transcription‑quantitative PCR were used to detect the protein and mRNA expression levels of cellular inhibitor of apoptosis 1 (cIAP1) and tumor necrosis factor receptor‑associated factor 3 (TRAF3) in the liver cancer cell lines Huh7, H22 and HepG2, and the hepatocyte line AML12. Annexin V‑FITC and Transwell assays were used to assess the effect of birinapant pretreatment on the apoptotic rate and invasive ability of liver cancer cells. Lentivirus‑mediated silencing of TRAF3 was performed in liver cancer cells. Western blotting was used to detect the lentivirus silencing efficiency. A subcutaneous hepatocellular carcinoma model was established in nude mice and 15 days after tumor induction the subcutaneous tumors were measured in each group. Immunohistochemistry assays were used to detect the protein expression levels of proliferating cell nuclear antigen and caspase‑3. The results suggested that the expression levels of cIAP1 and TRAF3 were lower in Huh7, H22 and HepG2 cells compared with AML12 cells. Pretreatment with birinapant promoted apoptosis and inhibited invasion of liver cancer cells by activating the cIAP1/TRAF3 axis. Birinapant also promoted apoptosis and inhibited the growth of subcutaneous hepatocellular carcinoma tumors in nude mice. The present results suggested that the SMAC mimetic birinapant may promote apoptosis, and inhibit the proliferation and invasion of liver cancer cells. The molecular mechanism responsible for the effects of birinapant may be related to activation of the cIAP1/TRAF3 signaling pathway by birinapant in liver cancer cells.

Inhibitor of apoptosis proteins (IAPs) are important regulators of apoptosis, and protein targets for the development of anti-cancer drugs. Cellular inhibitor of apoptosis protein-1 (cIAP1) is an important member of IAPs. Peptides or small-molecular antagonists can induce the dimerization, auto-ubiquitination, and proteasomal degradation of the cellular inhibitor of apoptosis protein-1 (cIAP1). While in the absence of antagonists, several mutations of the cIAP1 protein also lead to its dimerization and auto-ubiquitination. Even though the crystal structure of cIAP1 protein has been determined, the intrinsic mechanism of its dimerization remains unexplored. Accumulating evidence indicated that intrinsic conformational change existed during the binding of antagonists with cIAP1 protein, or introduction of mutations. To reveal this intrinsic conformational change, molecular dynamics simulations at microsecond scale were applied for the wild-type and mutant-type cIAP1 proteins. Compared to the crystal structure, significant conformational change was observed during the simulations, which could explain the importance of previously identified key mutations. To validate these findings revealed by our simulations, a new mutation D303A was constructed and the following native polyacrylamide gel electrophoresis (native-PAGE) assay observed a proportion of spontaneous dimerization, in comparison with the wild-type control. Taken together, these computational and experimental results revealed the intrinsic conformational change of cIAP1, which could not only explain previously identified key mutations, but also be exploited for further design and development of anti-tumor compounds that target the cIAP1 protein.Communicated by Ramaswamy H. Sarma.

Cellular inhibitor of apoptosis proteins cIAP1 and cIAP2 ubiquitinate nuclear factor κB (NF-κB)-inducing kinase (NIK) to suppress non-canonical NF-κB signaling and substrates such as receptor interacting protein kinase 1 (RIPK1) to promote cell survival. We investigate how these functions contribute to homeostasis by eliminating cIap2 from adult cIap1 -deficient mice. cIAP1 and cIAP2 (cIAP1/2) deficiency causes rapid weight loss and inflammation, with aberrant cell death, indicated by cleaved caspases-3 and -8, prevalent in intestine and liver. Deletion of Casp8 and Ripk3 prevents this aberrant cell death, reduces the inflammation, and prolongs mouse survival, whereas Ripk3 loss alone offers little benefit. Residual inflammation in mice lacking cIap1/2 , Casp8 , and Ripk3 is reduced by inhibition of NIK. Loss of Casp8 and Mlkl (mixed lineage kinase domain-like), but not Mlkl loss alone, also prevents cIAP1/2-deficient mice from dying around embryonic day 11. Therefore, a major function of cIAP1/2 in vivo is to suppress caspase-8-dependent cell death. • Acute cIAP1/2 deficiency in adult mice unleashes apoptosis and inflammation • cIAP1/2 limit inflammation by suppressing the activation of caspase-8 • cIAP1/2 also prevent embryonic lethality by limiting caspase-8-dependent cell death Zhang et al. use mouse genetics to show that the ubiquitin ligases cIAP1 and cIAP2 are critical for suppressing pro-inflammatory caspase-8-dependent cell death. Loss of caspase-8 (in combination with RIPK3 or MLKL loss) prevents embryonic lethality due to cIAP1/2 deficiency and ameliorates inflammation when cIAP1/2 are deleted from adult mice. [ABSTRACT FROM AUTHOR]

A genome-wide small-interfering RNA-based screen identified the transcription factor Specificity Protein 3 (SP3) as a critical factor for Second mitochondrial-derived activator of caspase (Smac) mimetic-mediated killing of cancer cells. In concert with Nuclear Factor kappa B (NF-κB,) SP3 is required for the expression of the cytokine Tumor Necrosis Factor alpha (TNF-α) under basal and Smac mimetic-stimulated conditions.

Ubiquitin (Ub)-conjugating enzymes and Ub ligases control protein degradation and regulate many cellular processes in eukaryotes. Cellular inhibitor of apoptosis protein-1 (cIAP1) plays a central role in apoptosis and tumor necrosis factor signaling. It harbors a C-terminal RING domain that homodimerizes to recruit E2∼Ub (where ∼ denotes a thioester bond) complex to catalyze Ub transfer. Noncovalent Ub binding to the backside of the E2 Ub-conjugating enzyme UbcH5 has previously been shown to enhance RING domain activity, but the molecular basis for this enhancement is unclear. To investigate how dimeric cIAP1 RING activates E2∼Ub for Ub transfer and what role noncovalently bound Ub has in Ub transfer, here we determined the crystal structure of the cIAP1 RING dimer bound to both UbcH5B covalently linked to Ub (UbcH5B-Ub) and a noncovalent Ub to 1.7 Å resolution. The structure along with biochemical analyses revealed that the cIAP1 RING domain interacts with UbcH5B-Ub and thereby promotes the formation of a closed UbcH5B-Ub conformation that primes the thioester bond for Ub transfer. We observed that the noncovalent Ub binds to the backside of UbcH5B and abuts UbcH5B's α1β1-loop, which, in turn, stabilizes the closed UbcH5B-Ub conformation. Our results disclose the mechanism by which cIAP1 RING dimer activates UbcH5B∼Ub and indicate that noncovalent Ub binding further stabilizes the cIAP1-UbcH5B∼Ub complex in the active conformation to stimulate Ub transfer., (© 2019 Patel et al.)

Protooncogene c-MYC , a master transcription factor, is a major driver of human tumorigenesis. Development of pharmacological agents for inhibiting c-MYC as an anticancer therapy has been a longstanding but elusive goal in the cancer field. E3 ubiquitin ligase cIAP1 has been shown to mediate the activation of c-MYC by destabilizing MAD1, a key antagonist of c-MYC. Here we developed a high-throughput assay for cIAP1 ubiquitination and identified D19, a small-molecule inhibitor of E3 ligase activity of cIAP1. We show that D19 binds to the RING domain of cIAP1 and inhibits the E3 ligase activity of cIAP1 by interfering with the dynamics of its interaction with E2. Blocking cIAP1 with D19 antagonizes c-MYC by stabilizing MAD1 protein in cells. Furthermore, we show that D19 and an improved analog (D19-14) promote c-MYC degradation and inhibit the oncogenic function of c-MYC in cells and xenograft animal models. In contrast, we show that activating E3 ubiquitin ligase activity of cIAP1 by Smac mimetics destabilizes MAD1, the antagonist of MYC, and increases the protein levels of c-MYC. Our study provides an interesting example using chemical biological approaches for determining distinct biological consequences from inhibiting vs. activating an E3 ubiquitin ligase and suggests a potential broad therapeutic strategy for targeting c-MYC in cancer treatment by pharmacologically modulating cIAP1 E3 ligase activity., Competing Interests: Conflict of interest statement: J.Y. and Eileen White are coauthors of a 2016 paper on guidelines for autophagy.

Researches indicate that the dysregulation of microRNA (miRNA) is involved in tumorigenesis. Among such dysregulated miRNAs in cancer, miR-145 is reported to be downregulated in multiple cancers. In this study, we demonstrated the downregulation of miR-145 in triple-negative breast cancer (TNBC) tissues and TNBC cell lines by quantitative reverse-transcription polymerase chain reaction (RT-PCR) analysis. Furthermore, we found that the tumor necrosis factor-alpha (TNF-α)-induced apoptosis was expanded by the transfection of miR-145 in MDA-MB-231 which belongs to the TNBC cell lines. We then indicated that the mechanism by which miR-145 promotes the TNF-α-induced apoptosis is dependent on the formation of RIP1-FADD-caspase-8 complex. The cellular inhibitor of apoptosis (cIAP1), which is the inhibitor of apoptosis protein, was found to be a target of miR-145 in MDA-MB-231 cells. As a result of cIAP1 overexpression, the promotion of miR-145 on TNF-α-induced apoptosis was inhibited in MDA-MB-231 cells. Therefore, our results indicate that miR-145 acts as a tumor suppressor in TNBC, suggesting that the miR-145-cIAP1 axis might be a potential therapeutic target for TNBC.

A dual immunotherapy approach employing small-molecule inhibitors of apoptosis (IAP) protein antagonists in combination with innate immune stimuli has proven to be highly synergistic and effective in animal tumor models. This strategy overcomes many of the limitations of either single agent therapy and our results suggest that the combination could be easily and effectively translated to the clinic. [ABSTRACT FROM PUBLISHER]

Background: Treatment of acute myeloid leukemia (AML) remains difficult owing to the development of treatment resistance, which might be overcome through antagonists of inhibitors of apoptosis proteins (IAPs)., Patients and Methods: The present multicenter, open-label, dose-escalation study aimed to evaluate the tolerability, pharmacokinetics (PK), pharmacodynamics (PD), and efficacy of Debio1143 (formerly AT-406), a new IAP antagonist, when given along with a standard "7 plus 3 regimen" of daunorubicin and cytarabine to poor-risk patients with AML during the induction cycle. Consecutive patient cohorts received once-daily 100, 200, 300, or 400 mg of oral Debio1143 on treatment days 1 to 5. Blood samples were collected regularly until hematologic recovery or response was documented. Bone marrow samples were collected on days 0, 14, and 29 and PK and PD samples on days 1, 3, 5, 8, and 10 and 1, 2, and 8, respectively., Results: Of the 29 enrolled patients, 23 completed the study. The most common adverse events of any grade deemed related to treatment were nausea (31% of patients), diarrhea (14%), and febrile neutropenia (14%). Exposure exceeded dose proportionality, without accumulation over 5 days. Inhibition of cellular IAP1 was detectable in the CD34/CD117(+) cells and blasts. A total of 11 patients (38%) achieved complete remission, most in the 100-mg dose cohort. Of these, 6 (56%) developed a relapse within the study period. The patients with a response more frequently showed plasma increases of tumor necrosis factor-α and interleukin-8 after the first dose of Debio1143., Conclusion: Debio1143 ≤ 400 mg/d showed good tolerability in combination with daunorubicin and cytarabine. Additional studies in subsets of patients with AML are warranted., Competing Interests: This work was supported by Ascenta Therapeutics, Malvern, PA, USA. GV, ER and CZ are employed by Debiopharm International SA; JMB is employed by Ascenta Therapeutics; JMS is a founder of Ascenta Therapeutics and has stocks. All remaining authors have declared no conflicts of interest., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Hepatitis B virus (HBV) infection can result in a spectrum of outcomes from immune-mediated control to disease progression, cirrhosis, and liver cancer. The host molecular pathways that influence and contribute to these outcomes need to be defined. Using an immunocompetent mouse model of chronic HBV infection, we identified some of the host cellular and molecular factors that impact on infection outcomes. Here, we show that cellular inhibitor of apoptosis proteins (cIAPs) attenuate TNF signaling during hepatitis B infection, and they restrict the death of infected hepatocytes, thus allowing viral persistence. Animals with a liver-specific cIAP1 and total cIAP2 deficiency efficiently control HBV infection compared with WT mice. This phenotype was partly recapitulated in mice that were deficient in cIAP2 alone. These results indicate that antagonizing the function of cIAPs may promote the clearance of HBV infection.

Hsp90 is an essential chaperone for more than 200 client proteins in eukaryotic cells. The human genome encodes two highly similar cytosolic Hsp90 proteins called Hsp90α and Hsp90β. Most of the client proteins can interact with either Hsp90 protein; however, only a handful client proteins and one co-chaperone that interact specifically with one of the Hsp90 isoforms were identified. Structural differences underlying these isoform-specific interactions were not studied. Here we report for the first time that the Hsp90 co-chaperone Aha1 interacts preferentially with Hsp90α. The distinction depends on the middle domain of Hsp90. The middle domain of Hsp90α is also responsible for the slow growth phenotype of yeasts that express this isoform as a sole source of Hsp90. These results suggest that differences in the middle domain of Hsp90α and Hsp90β may be responsible for the isoform-specific interactions with selected proteins. Also shown here within, we determine that preferential chaperoning of cIAP1 by Hsp90β is mediated by the N-terminal domain of this isoform., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Recently, we revealed that ubiquitination of MEKK2 and MEKK3 by inhibitor of apoptosis proteins (IAPs) directly disrupts MEK5/ERK5 interaction and subsequently attenuates ERK5 activation. In addition, loss of XIAP promotes human myogenic differentiation in an ERK5-dependent manner. These results reveal another layer of MAPK regulation and a novel role for XIAP in controlling myogenic differentiation.

There is currently immense interest in understanding the biological consequences of aberrant necroptosis. The recently uncovered role for X-linked inhibitor of apoptosis protein (XIAP) in blocking tumor necrosis factor-dependent necroptosis explains, at least in part, the systemic hyperinflammatory syndrome XLP-2. However, it also points to rather unexpected differences between XIAP and the related proteins baculoviral IAP repeat containing 2 and 3 (cIAP1/2).

The prognosis for patients with acute myeloid leukaemia (AML) is still poor, thus calling for novel treatment strategies. Here, we report that the small-molecule Smac mimetic BV6, which antagonizes Inhibitor of Apoptosis (IAP) proteins, acts in concert with cytarabine (AraC) to trigger cell death in AML cells in a highly synergistic manner (combination index 0.02-0.27). Similarly, BV6 cooperates with AraC to trigger cell death in primary AML samples, underscoring the clinical relevance of our findings. Molecular studies reveal that the TNFα-blocking antibody Enbrel significantly reduces BV6/AraC-induced cell death, demonstrating that an autocrine/paracrine TNFα loop mediates cell death. Furthermore, BV6 and AraC synergize to induce loss of mitochondrial membrane potential, caspase activation and DNA fragmentation, consistent with apoptotic cell death. Nevertheless, the caspase inhibitor zVAD.fmk fails to protect against BV6/AraC-induced cell death. Intriguingly, this cell death upon caspase inhibition is significantly reduced by pharmacological inhibition of two key components of necroptosis signaling, i.e. by RIP1 kinase inhibitor Necrostatin-1 or MLKL inhibitor NSA. Thus, BV6 sensitizes AML cells to AraC-induced cell death and overcomes apoptosis resistance by triggering necroptosis as alternative form of cell death. These findings have important implications for Smac mimetic-based strategies to bypass apoptosis resistance of AML., (Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.)

Apoptosis and necrosis are the two major modes of cell death, the molecular mechanisms of which have been extensively studied. Although initially thought to constitute mutually exclusive cellular states, recent findings reveal cellular contexts that require a balanced interplay between these two modes of cellular demise. Several death initiator and effector molecules, signaling pathways and subcellular sites have been identified as key mediators in both processes, either by constituting common modules or alternatively by functioning as a switch allowing cells to decide which route to take, depending on the specific situation. Importantly, autophagy, which is a predominantly cytoprotective process, has been linked to both types of cell death, serving either a pro-survival or pro-death function. Here we review the recent literature that highlights the intricate interplay between apoptosis, necrosis and autophagy, focusing on the relevance and impact of this crosstalk in normal development and in pathology. This article is part of a Special Section entitled: Cell Death Pathways., (Copyright © 2013 Elsevier B.V. All rights reserved.)