Function Of Inhibitor Of Apoptosis Proteins Research Articles

Molecular and Functional Characterization of Inhibitor of Apoptosis Proteins (IAP, BIRP) in Echinococcus granulosus.

The larval stage of Echinococcus granulosus sensu lato, resulting in cystic echinococcosis, a parasitic zoonosis, causes huge economic losses to the livestock industry and poses a threat to public health. Inhibitor of apoptosis proteins (IAPs) is a class of endogenous anti-apoptotic family, which plays a significant functional role in the regulation of organism’s development. Herein, to explore potential functions of IAPs in E. granulosus, two members of IAPs from E. granulosus (Eg-IAP and Eg-BIRP) were cloned, expressed, and molecularly characterized. Eg-IAP and Eg-BIRP encoded putative 331 and 168 residue proteins, respectively. Bioinformatic analysis showed that both proteins contained a type II BIR domain-the essential functional domain of IAPs. Fluorescence immunohistochemistry revealed that both proteins were ubiquitously localized in all life-cycle stages of E. granulosus. Our fluorescent quantitative PCR (RT-qPCR) results revealed relatively higher transcription levels of two Eg-IAPs in protoscoleces (PSCs) compared to the 18-day strobilated worms. We further used different concentrations of LCL161, a Smac-mimetic pan-IAPs inhibitor, to induce the apoptosis in PSCs in vitro, and revealed that the survival rate of PSCs and transcription levels of both genes were negatively correlated with the concentration of LCL161. While the results of light microscopy, transmission electron microscopy (TEM), and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay also showed a higher apoptotic rate in PSCs with the increasing concentrations of LCL161. Taken together, our findings provide the reasonable evidence that both Eg-IAP and Eg-BIRP have potential implication in critical anti-apoptotic roles during the development of E. granulosus.

Biomarker Profile for Prediction of Patient Response to Smac Mimetic Monotherapy in Pediatric Precursor B-Cell Acute Lymphoblastic Leukemia

Acute lymphoblastic leukemia (ALL) is the most common malignancy in childhood. While improved multi-agent chemotherapy regimens with individualized risk stratification have led to increased survival rates of approximately 80 percent, 20 percent of patients respond poorly to therapy or relapse. Therefore, novel therapeutic avenues are urgently needed to improve treatment outcome, overcome resistance and reduce side effects. Failure to undergo cell death represents a key survival mechanism of cancer cells and results in drug resistance and clonal escape. Since inhibitor of apoptosis proteins (IAPs) are often overexpressed in malignant cells and their overexpression correlates with inferior survival rates, they provide an attractive molecular target for therapeutic intervention. Small molecule inhibitors have been developed that act as SMAC mimetics (SMs) to counteract the cell death inhibitory function of IAPs. SMs can activate and/or modulate cell death pathways, and are currently being evaluated in clinical trials. Their successful therapeutic implementation requires identification of patients who could benefit from a SM-based treatment regimen ideally before start of therapy. Here, we analyzed the intrinsic activity of two monovalent (AT406 and LCL161) and two bivalent (Birinapant or BV6) SMs on 29 unselected patient-derived pediatric precursor B-cell (BCP)-ALL samples and identified a subset of BCP-ALL primografts to be sensitive to SM treatment (n=8). When we compared gene expression of SM-sensitive (n=8) and SM-insensitive (n=6) patient-derived BCP-ALL samples, we identified a characteristic gene expression signature with 127 differentially regulated genes, amongst them upregulation of TNFRSF1A (TNFR1) in the SM-sensitive subset. In line with previous reports, we confirmed a critical role of the TNF/TNFR1-axis for SM-induced cell death in BCP-ALL by functional analysis. Expression of TNFRSF1A alone, however, did not correlate with sensitivity to SM-induced cell death indicating that TNFR1 is not the only factor regulating cell fate decisions in response to SM treatment. To identify potential biomarker genes for prediction of patient response to SM monotherapy in BCP-ALL, we compared differentially regulated genes of SM responders and non-responders from our cohort with data from a published cohort. Interestingly, we found 4 genes to overlap between these two cohorts. Of these 4 genes TSPAN7, FAM69C, and TNFRSF1A were upregulated whereas MTX2 was downregulated in SM-sensitive samples. The signature identified may reflect a particular TNF network. Analysis of expression levels of these 4 genes in BCP-ALL cell lines (Nalm6, Reh, UoCB6 and RS4;11) revealed that Reh cells, sensitive to SM-induced cell death, exhibited the biomarker profile of primograft sensitivity, i.e. upregulation of TSPAN7, FAM69C, TNFRSF1A and downregulation of MTX2. Nalm6 cells resembled the expression pattern of SM-insensitive samples with a downregulation of TSPAN7, FAM69C, TNFRSF1A and an upregulation of MTX2 and were resistant to SM-induced cell death. RS4;11 and UoCB6 cells showed no pattern. Based on these findings we hypothesized that the respective expression patterns of TSPAN7, FAM69C, TNFRSF1A and MTX2 could predict sensitivity to SMs. An extended screen of additional primary BCP-ALL samples for their expression levels of TSPAN7, FAM69C, TNFRSF1A and MTX2 and response to SMs substantiated this hypothesis. In summary, the subset of primary BCP-ALL samples with sensitivity to SMs is characterized by a gene signature with MTX2 low and TSPAN7, FAM69C and TNFRSF1A high. By using this expression profile, sensitivity to SMs in BCP-ALL could be identified in cell lines and additional primografts. Based on these results, we suggest the identified gene expression pattern as a biomarker for selecting patients to be treated by SM monotherapy in clinical trials. Disclosures No relevant conflicts of interest to declare.

Inhibitor of apoptosis proteins in human health and disease.

The inhibitor of apoptosis proteins (IAPs) are best known for their ability to regulate cell survival and death processes. However, in addition to cell death, IAPs also act as innate immune sensors and modulate multiple pathways, such as autophagy and cell division. Many of these IAP functions are non-redundant even though they are based on the same molecular mechanism of action. These distinct functions of IAPs derive from their capacity to target specific substrates for ubiquitination and/or proteolytic cleavage. The unique functions of IAPs also derives from their unique cellular localizations, cell type and tissue-specific expression patterns. The diverse roles of IAPs are reflected by the fact that in humans the IAP family comprises eight distinct members. Genetic evidence from human pathologies also attests to the non-redundant functions of the IAPs since very diverse diseases arise upon aberrant IAP expression. In this review, we give an overview of the known mechanisms of action of the various IAPs, and focus on their specific roles in mediating innate immunity. We also look at the distinct phenotypes related to the dysregulation of the IAPs, and the human pathologies associated with each human IAP.

Cutting Edge: Blockade of Inhibitor of Apoptosis Proteins Sensitizes Neutrophils to TNF- but Not Lipopolysaccharide-Mediated Cell Death and IL-1β Secretion.

The mammalian inhibitor of apoptosis proteins (IAPs) are key regulators of cell death and inflammation. A major function of IAPs is to block the formation of a cell death-inducing complex, termed the ripoptosome, which can trigger caspase-8-dependent apoptosis or caspase-independent necroptosis. Recent studies report that upon TLR4 or TNF receptor 1 (TNFR1) signaling in macrophages, the ripoptosome can also induce NLRP3 inflammasome formation and IL-1β maturation. Whether neutrophils have the capacity to assemble a ripoptosome to induce cell death and inflammasome activation during TLR4 and TNFR1 signaling is unclear. In this study, we demonstrate that murine neutrophils can signal via TNFR1-driven ripoptosome assembly to induce both cell death and IL-1β maturation. However, unlike macrophages, neutrophils suppress TLR4-dependent cell death and NLRP3 inflammasome activation during IAP inhibition via deficiencies in the CD14/TRIF arm of TLR4 signaling.

Smac13-Tat Fusion Peptide Induces Cell Death and Sensitizes HeLa Cells to Chemotherapeutic Drugs

Background: The inhibitor of apoptosis proteins (IAPs) are overexpressed in a variety of cancer cells and play an important role in the inhibition of caspases, thereby suppressing programed cell death and leading to chemoresistance of cancer cells. However, the anti-apoptotic function of IAPs could be suppressed by Smac, the second mitochondria-derived activator of caspase through its direct interaction with IAPs. Smac can interact with IAPs through the N-terminal Ala-Val-Pro-Ile tetrapeptide domain. Methods: In the present study, the first 7 amino acid sequence (residues 54-60), Smac7,and the first 13 amino acid sequence (residues 54-66), Smac13, which are conserved among mammalian homologues were transduced into cells by bonding to the HIV-1 Tat-derived cell-penetrating peptide. Results: The Smac-Tat peptides induced cell death and increased the sensitivity to chemotherapeutic agents in HeLa cells, whereas they did not affect cell survival in normal lymphocytes. The Smac7Tat peptide could bind to Livin but not Survivin. Conclusion: We demonstrate here that Smac13-Tat is more effective for chemosensitization than Smac7-Tat and that its use might serve as a potent molecular targeted therapy for chemoresistant cervical cancer.

Antagonists of IAP proteins: novel anti-tumor agents.

Evasion of apoptosis is an important reason for tumor cells to resist the anticancer drugs in cancer therapy. As a critical regulator, the inhibitor of apoptosis proteins (IAPs) can block the apoptosis by inhibiting the activities of caspases. Scientists find that IAPs are over-expressed in many cancer cells, such as leukemia and B-cell lymphoma, which elucidate that high levels of IAPs are closely related to tumorigenesis and cancer development. Thus, targeting IAPs may be an attractive strategy for anti-tumor treatment. As an endogenous antagonist of IAPs, second mitochondria-derived activator of caspases (Smac) can suppress their activities through directly binding to IAPs. Based on structural biology study, Smac interacts with IAPs through the Ala-Val-Pro-Ile (AVPI) tetra-peptide of Smac. Therefore, many agents have been studied to suppress the IAPs which result in the activation of caspases and subsequently induce the apoptosis of tumor cells based on mimicking AVPI peptide strategy. In this review, the functions of IAPs in apoptosis and the recent advance of IAPs antagonists will be discussed.

X-linked inhibitor of apoptosis protein - a critical death resistance regulator and therapeutic target for personalized cancer therapy.

Defects in apoptosis regulation are one main cause of cancer development and may result from overexpression of anti-apoptotic proteins such as inhibitor of apoptosis proteins (IAPs). IAPs are cell death regulators that, among other functions, bind caspases, and interfere with apoptotic signaling via death receptors or intrinsic cell death pathways. All IAPs share one to three common structures, the so called baculovirus-IAP-repeat (BIR)-domains that allow them to bind caspases and other proteins. X-linked inhibitor of apoptosis protein (XIAP) is the most potent and best-defined anti-apoptotic IAP family member that directly neutralizes caspase-9 via its BIR3 domain and the effector caspases-3 and -7 via its BIR2 domain. A natural inhibitor of XIAP is SMAC/Diablo, which is released from mitochondria in apoptotic cells and displaces bound caspases from the BIR2/BIR3 domains of XIAP thereby reactivating cell death execution. The central apoptosis-inhibitory function of XIAP and its overexpression in many different types of advanced cancers have led to significant efforts to identify therapeutics that neutralize its anti-apoptotic effect. Most of these drugs are chemical derivatives of the N-terminal part of SMAC/Diablo. These “SMAC-mimetics” either specifically induce apoptosis in cancer cells or act as drug-sensitizers. Several “SMAC-mimetics” are currently tested by the pharmaceutical industry in Phase I and Phase II trials. In this review, we will discuss recent advances in understanding the function of IAPs in normal and malignant cells and focus on approaches to specifically neutralize XIAP in cancer cells.

Targeting Inhibitor of Apoptosis Proteins (IAPs) for Cancer Therapy

Since cell death by apoptosis plays a key role in the regulation of tissue homeostasis, dysregulation of the cell's intrinsic death program may foster tumor formation and progression. "Inhibitor of apoptosis proteins" (IAPs) block apoptosis at the core of the apoptotic machinery by inhibiting effector caspases. Aberrant expression and/or function of IAPs are found in many human cancers and have been implied in resistance to current treatment approaches. Recent insights into the role of IAPs have provided the basis for various exciting discoveries that aim at modulating expression or function of IAPs. Thus, targeting IAPs, e.g. by antisense approaches or small molecule inhibitors, presents a promising novel approach for future drug development and may proof to be a successful strategy to overcome apoptosis resistance of human cancers.

New Insights into the Function of IAP Proteins: Modulation of the MYC/MAX/MAD Network

A growing number of studies have revealed that the inhibitor of apoptosis (IAP) proteins play a variety of cellular roles in addition to suppression of apoptosis. A recent study in Molecular Cell demonstrates that one IAP member, c-IAP1, functions to potentiate the activity of Myc by triggering the ubiquitination and proteasomal degradation of the Myc inhibitory protein Mad1.

Inhibitor of apoptosis proteins as targets for anticancer therapy

Cell death by apoptosis plays a critical role in regulating the subtle balance between cell death and proliferation to maintain tissue homeostasis. Accordingly, tipping the balance in either direction may cause human disease. Too little cell death may promote tumor formation and progression. In addition, killing of cancer cells by current therapies is largely due to induction of apoptosis in tumor cells. Since a hallmark of human cancers is their resistance to apoptosis, there is a demand to develop novel strategies that restore the apoptotic machinery in order to overcome cancer resistance. Inhibitor of apoptosis proteins (IAPs) block apoptosis at the core of the apoptotic machinery by inhibiting caspases. Elevated levels of IAPs are found in many human cancers and have been associated with poor prognosis. Recent insights into the role of IAPs have provided the basis for various exciting developments that aim to modulate the expression or function of IAPs in human cancers. Targeting IAPs (e.g., by antisense approaches or small-molecule inhibitors) presents a promising novel approach to either directly trigger apoptosis or to potentiate the efficacy of cytotoxic therapies in cancer cells. Thus, inhibition of IAPs such as X chromosome-linked IAP may prove to be a successful strategy to overcome apoptosis resistance of human cancers that deserves further exploitation.

Targeting Inhibitor of Apoptosis Proteins (IAPs) for Diagnosis and Treatment of Human Diseases

Since cell death by apoptosis plays a key role in the regulation of tissue homeostasis, any defect in this intrinsic death program may result in tumor formation. "Inhibitor of apoptosis proteins" (IAPs) block apoptosis at the core of the apoptotic machinery by inhibiting effector caspases. Aberrant expression and/or function of IAPs have been implied to be involved in the pathogenesis and progression of various human diseases including cancer, autoimmune disorders or neurodegeneration. Recent insights into the regulation of IAPs have provided the basis for various exciting discoveries aimed at modulating expression or dysfunction of IAPs. Thus, targeting IAPs, e.g. by antisense approaches, RNA interference or small molecules, may proof to be a novel strategy for the diagnosis and treatment of human diseases.

IAPs, their Antagonists and their Role in Neurological Disease and Cancer

Apoptosis is a noninflammatory process used by multicellular organisms to eliminate unwanted cells. It is implemented by a family of cysteine proteases called caspases through their cleavage of cellular proteins. The upstream events leading to caspase activation are controlled at multiple levels by an extensive array of proteins, including death receptors, adapters, transcription factors and Bcl-2 family members, while acting downstream to curb caspase activity are the inhibitor of apoptosis proteins (IAPs). Since their initial discovery much has been learned about how IAPs regulate cell death and are themselves regulated. Structural studies have given us insight into how IAPs interact with and neutralise caspase activity, proteomic approaches have uncovered IAP interacting molecules, such as DIABLO/SMAC, that can antagonise IAP function, and additional modes of IAP regulation at the transcriptional and post translational level have been identified. Their potential involvement in conferring cancer cell resistance to apoptotic stimuli, e.g. chemotherapeutic drugs, has presented them as plausible targets for cancer therapy. Conversely, they may be beneficial in protecting neuronal cells from inappropriate apoptosis observed in many different neurological diseases. What follows is a discussion of the various mammalian IAPs and other BIR domain containing proteins (BIRPs), focussing on their structure, function and regulation by antagonists as well as their possible involvement in disease processes.

Decreased apoptosome activity with neuronal differentiation sets the threshold for strict IAP regulation of apoptosis

Despite the potential of the inhibitor of apoptosis proteins (IAPs) to block cytochrome c–dependent caspase activation, the critical function of IAPs in regulating mammalian apoptosis remains unclear. We report that the ability of endogenous IAPs to effectively regulate caspase activation depends on the differentiation state of the cell. Despite being expressed at equivalent levels, endogenous IAPs afforded no protection against cytochrome c–induced apoptosis in naïve pheochromocytoma (PC12) cells, but were remarkably effective in doing so in neuronally differentiated cells. Neuronal differentiation was also accompanied with a marked reduction in Apaf-1, resulting in a significant decrease in apoptosome activity. Importantly, this decrease in Apaf-1 protein was directly linked to the increased ability of IAPs to stringently regulate apoptosis in neuronally differentiated PC12 and primary cells. These data illustrate specifically how the apoptotic pathway acquires increased regulation with cellular differentiation, and are the first to show that IAP function and apoptosome activity are coupled in cells.

Use of smac peptidomimetic compounds as effective radiosensitizers of human cancer cell lines

Purpose/Objective: IAPs (inhibitor of apoptosis proteins) comprise a family of well-conserved eukaryotic anti-apoptotic proteins. It has been shown that IAP over-expression is one of the mechanisms cancer cells use to counteract the induction of apoptosis by ionizing radiation. Smac/DIABLO (second mitochondria-derived activator of caspase or direct IAP binding protein with low pI) was recently identified as a protein released from mitochondria in response to apoptotic stimuli. Smac was shown to promote apoptosis by directly binding to and displacing caspase-9 from XIAP (X-linked inhibitor of apoptosis protein) and other IAPs. Hence, Smac appears to functions as an endogenous pro-apoptotic protein by directly inhibiting the function of IAPs. We have designed and synthesized a series of potent peptidomimetics with high affinity to the BIR3 domain of the XIAP protein by mimicking the conserved IAP binding motifs in Smac and caspase-9 proteins. In this study, we evaluated the effectiveness of these peptidomimetics to act as potent radiosensitizers.

Smac/Diablo Antagonizes Ubiquitin Ligase Activity of Inhibitor of Apoptosis Proteins

Inhibitor of apoptosis proteins (IAPs) can block apoptosis through binding to active caspases and antagonizing their function. IAP function can be neutralized by Smac/Diablo, an IAP-binding protein that is released from mitochondria during apoptosis. In addition to their ability to interact with caspases, certain IAPs also display ubiquitin-protein isopeptide ligase activity because of the presence of a RING domain. However, it is not known whether the ubiquitin-protein isopeptide ligase activities of human IAPs contribute to their apoptosis inhibitory activity or whether this IAP property can be modulated through association with Smac/Diablo. Here we demonstrate that the ubiquitin ligase activities of XIAP, and to a lesser extent c-IAP-1 and c-IAP2, are potently repressed through binding to Smac/Diablo. We also show that mutation of the XIAP RING domain rendered this IAP a less effective inhibitor of apoptosis, suggesting that the ubiquitin ligase activity of XIAP contributes to its anti-apoptotic function. These data suggest that Smac/Diablo potentiates apoptosis by simultaneously antagonizing caspase-IAP interactions and repressing IAP ubiquitin ligase activities.

X-linked inhibitor of apoptosis protein (XIAP) is a nonredundant modulator of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis in human cancer cells.

Although the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to play an important role in the immunosurveillance of neoplasia, apoptotic factors that modulate the sensitivity of cancer cells to TRAIL are poorly understood. The inhibitor of apoptosis proteins (IAPs) have generated considerable interest as potential targets for cancer therapy, but the lack of a phenotype in X-linked IAP (XIAP) knockout mice has generated speculation that IAP function may be redundant. Using gene targeting technology, we show that disruption of the gene encoding XIAP in human cancer cells did not interfere with basal proliferation, but caused a remarkable sensitivity to TRAIL. These results demonstrate that XIAP is a nonredundant modulator of TRAIL-mediated apoptosis and provide a rationale for XIAP as a therapeutic target.

Cell Division Regulation by BIR1, a Member of the Inhibitor of Apoptosis Family in Yeast

The inhibitor of apoptosis (IAP) gene family comprises molecules that block the activity of pro-apoptotic caspase proteases. Paradoxically, yeasts contain IAP proteins but no caspases and no apoptotic program. To determine the function of these proteins in vivo, we disrupted the BIR1 gene, encoding the only known IAP in yeast Saccharomyces cerevisiae. Sporulation of heterozygous diploids yielded no viable mutant haploids, indicating that BIR1 is an essential gene. By flow cytometry, some heterozygous mutants were polyploid accumulating >4 N DNA content. These cells exhibited a 20-40% reduction in growth rate, which was rescued by plasmid-borne over-expression of BIR1 but not by its human counterpart, survivin. Deletion analysis revealed that the N-terminal domain of Bir1, containing the conserved baculovirus IAP repeat, was able to partially complement the cell growth defect caused by BIR1 deletion. Moreover, the full-length and truncated forms of Bir1 accelerated cell division in wild-type cells. Finally, BIR1 heterozygous mutants exhibited grossly altered cell morphology with misshapen or abnormally long buds connected to an unusually large mother cell. These findings identify a novel function of IAP proteins in the pleiotropic control of cell division, in addition to their role in the suppression of apoptosis.

Drosophila homologs of baculovirus inhibitor of apoptosis proteins function to block cell death

Apoptotic cell death is a mechanism by which organisms eliminate superfluous or harmful cells. Expression of the cell death regulatory protein REAPER (RPR) in the developing Drosophila eye results in a small eye owing to excess cell death. We show that mutations in thread (th) are dominant enhancers of RPR-induced cell death and that th encodes a protein homologous to baculovirus inhibitors of apoptosis (IAPs), which we call Drosophila IAP1 (DIAP1). Overexpression of DIAP1 or a related protein, DIAP2, in the eye suppresses normally occurring cell death as well as death due to overexpression of rpr or head involution detective. IAP death-preventing activity localizes to the N-terminal baculovirus IAP repeats, a motif found in both viral and cellular proteins associated with death prevention.