Inhibitor Of Apoptosis Family Members Research Articles

Comprehensive analysis of prognostic value and immune infiltration of IAPs family in hepatocellular carcinoma.

Hepatocellular carcinoma (HCC) is a malignant tumor with high morbidity and mortality rates. The inhibitors of apoptosis (IAP) family act as oncogenes in various tumor types; however, their functions in HCC remain unclear. Here, we used integrated bioinformatics analysis and experimental verification to assess the expression and the prognostic and clinical value of the IAP family in HCC. Using the University of Alabama at Birmingham Cancer Data Analysis Portal (UALCAN) and the Tumor Immune Estimation Resource (TIMER), we analyzed the expression profiles of IAP family members in HCC tissue, normal tissues, and in patients with different stages and grades of HCC. We further verified the expression level of BIRC2 in 25 HCC samples and matched adjacent normal tissues using quantitative real-time PCR (qRT-PCR), and analyzed its correlation with the marker gene of T-helper type 1 cells (Th1)-STAT1. Meanwhile, the association between BIRC2 and the immunotherapeutic response or immunomodulators was confirmed using the Biomarker Exploration of Solid Tumors (BEST) database. The results showed that NAIP, BIRC2, BIRC3, XIAP, BIRC5, and BIRC6 mRNAs were overexpressed in HCC. The clinical stages, pathological grades, and other clinicopathological features of HCC were closely related to the expression levels of the IAP family members, especially the BIRC2 and BIRC5, which were found to be potential prognostic biomarkers for HCC. Expression of the IAPs was strongly associated with immune cell infiltration. Based on the infiltrative status of various immune cells, HCC patients with high BIRC2 and BIRC5 expression demonstrated poor overall survival (OS) rates. In patients with HCC, BIRC2 expression was noticeably elevated. Concurrently, the expression levels of BIRC2 and STAT1 showed a favorable correlation. BEST database analysis revealed that BIRC2 was a negative predictor of responsiveness to anti-programmed cell death ligand 1 (PD-L1)/cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) inhibitor treatment in HCC, and BIRC2 mRNA expression levels were positively correlated with the expression levels of the immune checkpoint genes programmed cell death protein 1 (PD-1), PD-L1, and CTLA-4 in HCC. Consequently, the IAP family may play a role in carcinogenesis and cancer-immune system interactions in HCC. Our results demonstrate that IAP family members may be viable predictive biomarkers and therapeutic targets for HCC.

Exploring MicroRNA and Exosome Involvement in Malignant Pleural Mesothelioma Drug Response.

Simple SummaryMalignant pleural mesothelioma (MPM) is a deadly thoracic malignancy with limited treatment options. Chemotherapy remains the most widely used first-line treatment for unresectable MPM but is hampered by drug resistance issues. Small molecule inhibitors and microRNA mimics have shown promising potential for the treatment of MPM in preclinical studies, but are yet to be successfully implemented in the clinical setting. Our study aims to provide an understanding of the molecular mechanism(s) that mediate drug response in MPM. The inhibitor of apoptosis family member, survivin, has been reported to be over-expressed in MPM and is associated with drug resistance. Therefore, we particularly focused on determining the cellular mechanism(s) that contribute to MPM cell response to a survivin small molecule inhibitor, YM155. Our study provides key information to facilitate a prediction of the potential utility of small molecule inhibitors and microRNA mimics as treatment options for MPM.Malignant pleural mesothelioma (MPM) is a deadly thoracic malignancy and existing treatment options are limited. Chemotherapy remains the most widely used first-line treatment regimen for patients with unresectable MPM, but is hampered by drug resistance issues. The current study demonstrated a modest enhancement of MPM cell sensitivity to chemotherapy drug treatment following microRNA (miRNA) transfection in MPM cell lines, albeit not for all tested miRNAs. This effect was more pronounced for FAK (PND-1186) small molecule inhibitor treatment; consistent with previously published data. We previously established that MPM response to survivin (YM155) small molecule inhibitor treatment is unrelated to basal survivin expression. Here, we showed that MPM response to YM155 treatment is enhanced following miRNA transfection of YM155-resistant MPM cells. We determined that YM155-resistant MPM cells secrete a higher level of exosomes in comparison to YM155-sensitive MPM cells. Despite this, an exosome inhibitor (GW4896) did not enhance MPM cell sensitivity to YM155. Additionally, our study showed no evidence of a correlation between the mRNA expression of inhibitor of apoptosis (IAP) gene family members and MPM cell sensitivity to YM155. However, two drug transporter genes, ABCA6 and ABCA10, were upregulated in the MPM cell lines and correlated with poor sensitivity to YM155.

Induction of apoptosis through inactivation of ROS-dependent PI3K/Akt signaling pathway by platycodin D in human bladder urothelial carcinoma cells.

Platycodin D (PD) is a triterpenoid saponin, a major bioactive constituent of the roots of Platycodon grandiflorum, which is well known for possessing various pharmacological properties. However, the anti-cancer mechanism of PD in bladder cancer cells remains poorly understood. In the current study, we investigated the effect of PD on the growth of human bladder urothelial carcinoma cells. PD treatment significantly reduced the cell survival of bladder cancer cells associated with induction of apoptosis and DNA damage. PD inhibited the expression of inhibitor of apoptosis family members, activated caspases, and induced cleavage of poly (ADP-ribose) polymerase. PD also increased the release of cytochrome c into the cytoplasm by disrupting the mitochondrial membrane potential while upregulating the expression ratio of Bax to Bcl-2. The PD-mediated anti-proliferative effect was significantly inhibited by pre-treatment with a pancaspase inhibitor, but not by an inhibitor of necroptosis. Moreover, PD suppressed the phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling pathway, and the apoptosis-inducing effect of PD was further enhanced by a PI3K inhibitor. In addition, PD increased the accumulation of reactive oxygen species (ROS), whereas N-acetyl cysteine (NAC), an ROS inhibitor, significantly attenuated the growth inhibition and inactivation of the PI3K/Akt/mTOR signaling caused by PD. Furthermore, NAC significantly suppressed apoptosis, DNA damage, and decreased cell viability induced by PD treatment. Collectively, our findings indicated that PD blocked the growth of bladder urothelial carcinoma cells by inducing ROS-mediated inactivation of the PI3K/Akt/mTOR signaling.

Abstract B070: Mitotic kinases differentially phosphorylate survivin in African American TNBC: Clues for racial disparity

Abstract Protein diversity arising from alternative mRNA splicing or post-translational modifications (PTM) such as phosphorylation, methylation etc. plays a vital role in the modulation of cellular functions. Mitotic kinases such as polo-like kinase 1 (PLK1) and aurora B (AURKB) are known to phosphorylate survivin, an inhibitor of apoptosis family member, to promote cell survival and proliferation. This triad is significantly overexpressed in triple negative breast cancer (TNBC), an aggressive breast cancer (BC) subtype, associated with high proliferative capacity, propensity for distant metastasis and chemotherapeutic resistance. TNBC disproportionately afflicts African-American (AA) women relative to European-American (EA) women. AAs demonstrate a much more aggressive disease course compared to EAs. Thus far, efforts to examine molecular underpinnings of racial disparity in TNBC have largely focused on differences in gene expression and epigenetic aspects. Herein, we demonstrate the role of survivin phosphorylation, an important PTM, in enhancing cell proliferation and reducing apoptosis in AA compared to EA TNBC cells. Using the TCGA breast cancer dataset, our in silico analysis of various mitotic kinases yielded a significantly higher gene expression of PLK1 (p=0.061) and AURKB (p=0.003) in AA (n=41) compared to EA TNBC (n=86) patients. Surprisingly, we found no race-related differences in survivin gene expression in this dataset (n=127, p=0.343). In addition, there was no statistical difference in protein levels of survivin in TNBC clinical samples (n=142, p=0.45) as measured immunohistochemically. These findings led us to rationalize that the differential activation of survivin in AA vs. EA TNBC cells is based on PTM, and not due to its differential gene/protein expression levels. We hypothesize that differential phosphorylation of survivin by mitotic kinases underlies enhanced proliferative and anti-apoptotic activity in AA relative to EA TNBC. Survivin phosphorylation at Ser-20 and Thr-117 is known to regulate the activity of the chromosome passenger complex facilitating mitosis and cell proliferation. Moreover, p-survivin Thr-117 inhibits X-linked inhibitor of apoptosis-Smac interaction, which in turn prevents caspase-9 activation, leading to anti-apoptosis. We observed an upregulation of phosphorylated substrates of PLK1 and AURKB viz. p-survivin Ser-20 & p-survivin Thr-117, respectively, in AA compared to EA TNBC cells. Survivin ablation significantly attentuated cell proliferation, cell cycle progression, and increased cell apoptosis in AA compared to EA TNBC cells. These data strongly support our premise that higher survivin phosphorylation confers enhanced proliferative capacity and anti-apoptosis in AA compared to EA TNBC cells, suggesting that modulating survivin PTM may be a viable therapeutic strategy for AA TNBC patients. Further studies intended to tease out the role of survivin phosphorylation in TNBC racial disparity are currently underway in our lab. Citation Format: Shriya Joshi, Chakravarthy Garlapati, Luciane Cavalli, Uma Krishnamurti, Shobhna Kapoor, Ritu Aneja. Mitotic kinases differentially phosphorylate survivin in African American TNBC: Clues for racial disparity [abstract]. In: Proceedings of the Twelfth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2019 Sep 20-23; San Francisco, CA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(6 Suppl_2):Abstract nr B070.

Impact of inhibitor of apoptosis proteins on immune modulation and inflammation.

The routes leading to programmed cell death are as tightly regulated as those of cellular growth and proliferation, and a finely synchronized balance between the life and death of cells ensures proper organ size and function. Inhibitors of apoptosis (IAPs) proteins were initially characterized by their ability to directly bind and inhibit apoptotic caspases. However, recent studies have clarified that IAPs are much more functionally versatile, modulating a vast range of signaling pathways that have an impact on antimicrobial responses, tumorigenesis, metastasis and cellular migration. A significant contribution of IAPs in tumorigenesis is their inherent function as E3 ubiquitin ligases to modulate cellular signaling downstream of death receptors or certain pattern recognition receptors. In this review, we focus on modulation of the innate and adaptive immune systems, macrophage plasticity and inflammatory responses by IAP family members. We also explore the rationale to target IAPs pharmacologically for the treatment of a number of inflammatory diseases and cancer.

Characterization of Potent SMAC Mimetics that Sensitize Cancer Cells to TNF Family-Induced Apoptosis.

Members of the Inhibitor of APoptosis (IAP) protein family suppress apoptosis within tumor cells, particularly in the context of immune cell-mediated killing by the tumor necrosis factor (TNF) superfamily cytokines. Most IAPs are opposed endogenously by the second mitochondrial activator of caspases (SMAC), which binds to selected baculovirus IAP repeat (BIR) domains of IAPs to displace interacting proteins. The development of SMAC mimetics as novel anticancer drugs has gained impetus, with several agents now in human clinical trials. To further understand the cellular mechanisms of SMAC mimetics, we focused on IAP family members cIAP1 and cIAP2, which are recruited to TNF receptor complexes where they support cell survival through NF-κB activation while suppressing apoptosis by preventing caspase activation. We established fluorescence polarization (FP) assays for the BIR2 and BIR3 domains of human cIAP1 and cIAP2 using fluorochrome-conjugated SMAC peptides as ligands. A library of SMAC mimetics was profiled using the FP assays to provide a unique structure activity relationship (SAR) analysis compared to previous assessments of binding to XIAP. Potent compounds displayed mean inhibitory binding constants (Ki) of 9 to 27 nM against the BIR3 domains of cIAP1 and cIAP2, respectively. Selected compounds were then characterized using cytotoxicity assays in which a cytokine-resistant human tumor cell line was sensitized to either TNF or lymphotoxin-α (LT-α). Cytotoxicity correlated closely with cIAP1 and cIAP2 BIR3 binding activity with the most potent compounds able to reduce cell viability by 50%. Further testing demonstrated that active compounds also inhibit RIP1 binding to BIR3 of cIAP1 and cIAP2 in vitro and reduce steady-state cIAP1 protein levels in cells. Altogether, these data inform the SAR for our SMAC mimetics with respect to cIAP1 and cIAP2, suggesting that these IAP family members play an important role in tumor cell resistance to cytotoxicity mediated by TNF and LT-α.

Tumor necrosis factor-α-induced apoptosis of gastric cancer MKN28 cells: Accelerated degradation of the inhibitor of apoptosis family members

The role of the inhibitor of apoptosis (IAP) family members in tumor necrosis factor-α (TNF-α)-induced apoptosis of human gastric cancer MKN28 cells was explored. TNF-α induced up-regulation of cIAP2, whereas cycloheximide (CHX) induced down-regulation of XIAP and survivin. Degradation of cIAP1 and XIAP, but not survivin, was accelerated by co-treatment of cells with TNF-α and CHX, and TNF-α-induced up-regulation of cIAP2 was inhibited by BMS-345541 (NF-κB inhibitor). Treatment of MKN28 cells with TNF-α plus CHX induced degradation of survivin and activation of caspase-8 and -3, followed by degradation of cIAP1 and XIAP and apoptosis. Proteasome inhibitors (MG132 and epoxomicin) suppressed TNF-α plus CHX-induced degradation of survivin, cIAP1, and XIAP as well as apoptosis. A caspase inhibitor (z-VAD-fmk) suppressed TNF-α plus CHX-induced apoptosis, but allowed degradation of survivin, cIAP1 and XIAP. TNF-α receptor 1 and 2 were expressed on MKN28 cells. The magnitude of apoptosis induced by TNF-α plus BMS-345541 was much less than that induced by TNF-α plus CHX. These findings suggest that TNF-α plus CHX-induced apoptosis of gastric cancer MKN28 cells may be caused by accelerated degradation of the IAP family members (survivin, cIAP1, and XIAP), in addition to inhibition of NF-κB-dependent synthesis of anti-apoptotic molecules.

Abstract 1108: B-cell lymphoma-derived exosomes are reservoirs of inhibitors of apoptosis

Abstract B-cells, along with other normal and cancerous cells, secrete 50-150 nm vesicles called exosomes. Exosomes carry proteins, mRNA, and miRNA that can be functional upon cellular uptake and are often specific to cell of origin, offering potential as disease biomarkers. Our lab has identified within exosomes from solid tumor cells a cancer-specific protein, Survivin, which is a member of the Inhibitors of Apoptosis (IAP) family. Whether Survivin and other IAP family members such as XIAP, cIAP1 or cIAP2 are also expressed in exosomes from hematopoietic malignancies is yet to be determined. In this study, we aim to investigate the exosomes released from B-cell lymphoma cells and determine the IAP content under basal and sublethal stress conditions. Three human non EBV-transformed B-cell lymphoma lines developed at Wayne State University were used. WSU-FSCCL (follicular small cleaved cell), WSU-DLCL-2 (diffuse large cell), and WSU-WM (Waldenstrom Macroglobulinemia) represent different levels of aggressiveness. Cells were cultured for 24 or 48 hours before exosome isolation from the conditioned media using either ultracentrifugation on a sucrose cushion or ExoQuick (System Biosciences). Sublethal levels of UV irradiation were determined for each cell line by analyzing DNA content by propidium iodide staining with a FACSCalibur. Protein quantity of exosomes was measured using a BCA assay, and protein content was studied using Western blotting. Corresponding gene expression was identified using RT-PCR. Here we report IAP protein and mRNA (except XIAP) within exosomes from basal cells. We are currently probing exosomes from stressed cells for comparison. We hypothesize that the IAP exosomal profile may change under stressed conditions. The specificity and variability in exosomal content offers capabilities not only as disease biomarkers, but also in the immunomodulating activity of exosomes which have shown roles in both stimulating and inhibiting the immune system, We have found evidence that exosomal Survivin plays an immune role in skewing Th1/Th2 populations and decreasing cytotoxic function. Changes in exosome content between basal and stressed cells may have roles in the immunomodulation of the tumor microenvironment which facilitates the evasion of lymphoma cells to immune destruction, as well as possible prognostic benefit as to whether treatments are being effective or detrimental. Citation Format: Heather R. Ferguson Bennit, Malyn M. Asuncion Valenzuela, Jessica S. Jutzy, Nathan R. Wall. B-cell lymphoma-derived exosomes are reservoirs of inhibitors of apoptosis. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1108. doi:10.1158/1538-7445.AM2014-1108

The BIRC6 gene as a novel target for therapy of prostate cancer: dual targeting of inhibitors of apoptosis.

Treatment resistance, the major challenge in the management of advanced prostate cancer, is in part based on resistance to apoptosis. The Inhibitor of Apoptosis (IAP) family is thought to play key roles in survival and drug resistance of cancer via inhibition of apoptosis. Of the IAP family members, cIAP1, cIAP2, XIAP and survivin are known to be up-regulated in prostate cancer. BIRC6, a much less studied IAP member, was recently shown to be elevated in castration-resistant prostate cancer (CRPC). In the present study, we showed a correlation between elevated BIRC6 expression in clinical prostate cancer specimens and poor patient prognostic factors, as well as co-upregulation of certain IAP members. In view of this, we designed antisense oligonucleotides that simultaneously target BIRC6 and another co-upregulated IAP member (dASOs). Two dASOs, targeting BIRC6+cIAP1 and BIRC6+survivin, showed substantial inhibition of CRPC cells proliferation, exceeding that obtained with single BIRC6 targeting. The growth inhibition was associated with increased apoptosis, cell cycle arrest and suppression of NFkB activation. Moreover, treatment with both dASOs led to significantly lower viable tumor volume in vivo, without major host toxicity. This study shows that BIRC6-based dual IAP-targeting ASOs represent potential novel therapeutic agents against advanced prostate cancer.

Chapter Ten - Use of E2~ Ubiquitin Conjugates for the Characterization of Ubiquitin Transfer by RING E3 Ligases Such as the Inhibitor of Apoptosis Proteins

Ubiquitylation of proteins is a versatile posttranslational modification that can serve to promote protein degradation, or it can have nondegradative roles, such as mediating protein-protein interactions. The Inhibitor of APoptosis (IAP) proteins are important regulators of pathways that control cell death, proliferation, and differentiation. A number of IAP family members are RING E3 ubiquitin-protein ligases, which promote direct transfer of ubiquitin from charged E2 enzymes, or E2~ubiquitin (E2~Ub) conjugates, to substrate proteins. This results in the attachment of nondegradative ubiquitin signals to other proteins, or the autoubiquitylation and degradation of IAPs. Modulating ubiquitin transfer by IAPs is the focus of a number of drug development initiatives and these studies require a detailed understanding of ubiquitylation. Here, we describe preparation of stable E2~Ub conjugates that can be used in biochemical and biophysical experiments to examine RING domain function. In the last 2 years, the availability of these conjugates has helped unveil a molecular understanding of the process of ubiquitin transfer by IAPs. The approaches described here will be suitable for studying other RING E3 ligases.

Abstract 3336: The Smac Mimetic Birinapant Synergistically Induces Apoptosis in Combination with Type I Interferons and GM-CSF.

Abstract The Inhibitor of Apoptosis (IAP) protein family is able to inhibit cell death and promote survival signaling triggered by a wide variety of stimuli. Two members of this family in particular, cIAP1 and cIAP2, play key roles in enabling tumor necrosis factor alpha (TNFα)-induced nuclear factor-kappa beta (NF-kB) activation and inflammatory responses. Birinapant (TL32711) is a bivalent Smac Mimetic that antagonizes multiple IAP family members resulting in the conversion of TNFα-induced survival signaling into an apoptotic response, an effect which is more prominent in tumor cells than their normal counterparts. Combining birinapant with agents capable of increasing the levels of TNFα in the tumor microenvironment represents a novel approach to cancer treatment. We have tested the ability of interferon-alpha/-beta (IFN-α, IFN-β) and granulocyte macrophage colony stimulating factor (GM-CSF), both known to be immunomodulatory agents, for their ability to stimulate TNFα production by peripheral blood mononuclear cells (PBMC). Treatment with each cytokine induced the production of TNFα by cultured PBMC as measured by ELISA. Supernatants of IFN-α/-β and GM-CSF-treated PBMC cultures were able to sensitize resistant tumor cells to Smac Mimetics in a TNFα-dependent manner. Furthermore, the combination of birinapant and either IFN-β or GM-CSF synergistically inhibited the development of RENCA tumors in a syngeneic mouse prevention model. Due to the highly aggressive nature of the syngeneic tumors, this model is performed by simultaneous implantation of tumor cells and initiation of dosing of test agents. Activity in this model is measured by prevention of tumor outgrowth. Birinapant in combination with IFN-β resulted in only 5 of 18 measurable tumors, versus 19 of 20 tumors in vehicle-treated mice, 16 of 20 tumors in IFN-β alone-treated mice and 13 of 20 tumors in birinapant alone-treated mice. In addition, when tumor growth occurred it was delayed in mice treated with the combination. Similarly, the birinapant and GM-CSF combination treatment resulted in 9 of 20 tumors, versus 20 of 20 tumors in the vehicle-treated mice, 20 of 20 tumors in the GM-CSF alone-treated mice, and 16 of 20 tumors in the birinapant alone-treated mice. In addition when tumors did develop in the combination-treated mice, their growth was substantially delayed: 4 of 10 mice treated with the combination of birinapant and GM-CSF were still on study on day 85 post initiation - approximately 40 days post dosing - with no measureable tumor. The combination of birinapant with immunomodulatory agents that induce TNF represents a potential novel therapeutic strategy. Citation Format: Christopher A. Benetatos, Jennifer M. Burns, Ernest C. Borden, Daniel Lindner, Yasuhiro Mitsuuchi, Mark A. Mckinlay, Gurpreet Singh Kapoor, Eric M. Neiman, Martin E. Seipel, Guangyao Yu, Martin Graham, David Weng, Stephen M. Condon, C. Glenn Begley, Srinivas Chunduru. The Smac Mimetic Birinapant Synergistically Induces Apoptosis in Combination with Type I Interferons and GM-CSF. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3336. doi:10.1158/1538-7445.AM2013-3336

Role of Melanoma Inhibitor of Apoptosis (ML-IAP) Protein, a Member of the Baculoviral IAP Repeat (BIR) Domain Family, in the Regulation of C-RAF Kinase and Cell Migration

Inhibitor of apoptosis (IAPs) proteins are characterized by the presence of evolutionarily conserved baculoviral inhibitor of apoptosis repeat (BIR) domains, predominantly known for their role in inhibiting caspases and, thereby, apoptosis. We have shown previously that multi-BIR domain-containing IAPs, cellular IAPs, and X-linked IAP can control tumor cell migration by directly regulating the protein stability of C-RAF kinase. Here, we extend our observations to a single BIR domain containing IAP family member melanoma-IAP (ML-IAP). We show that ML-IAP can directly bind to C-RAF and that ML-IAP depletion leads to an increase in C-RAF protein levels, MAPK activation, and cell migration in melanoma cells. Thus, our results unveil a thus far unknown role for ML-IAP in controlling C-RAF stability and cell migration.

Inhibitor of Apoptosis Proteins Limit RIP3 Kinase-Dependent Interleukin-1 Activation

Interleukin-1β (IL-1β) is a potent inflammatory cytokine that is usually cleaved and activated by inflammasome-associated caspase-1. To determine whether IL-1β activation is regulated by inhibitor of apoptosis (IAP) proteins, we treated macrophages with an IAP-antagonist "Smac mimetic" compound or genetically deleted the genes that encode the three IAP family members cIAP1, cIAP2, and XIAP. After Toll-like receptor priming, IAP inhibition triggered cleavage of IL-1β that was mediated not only by the NLRP3-caspase-1 inflammasome, but also by caspase-8 in a caspase-1-independent manner. In the absence of IAPs, rapid and full generation of active IL-1β by the NLRP3-caspase-1 inflammasome, or by caspase-8, required the kinase RIP3 and reactive oxygen species production. These results demonstrate that activation of the cell death-inducing ripoptosome platform and RIP3 can generate bioactive IL-1β and implicate them as additional targets for the treatment of pathological IL-1-driven inflammatory responses.

Inhibitor of Apoptosis (IAP)-like Protein Lacks a Baculovirus IAP Repeat (BIR) Domain and Attenuates Cell Death in Plant and Animal Systems

A novel Arabidopsis thaliana inhibitor of apoptosis was identified by sequence homology to other known inhibitor of apoptosis (IAP) proteins. Arabidopsis IAP-like protein (AtILP) contained a C-terminal RING finger domain but lacked a baculovirus IAP repeat (BIR) domain, which is essential for anti-apoptotic activity in other IAP family members. The expression of AtILP in HeLa cells conferred resistance against tumor necrosis factor (TNF)-α/ActD-induced apoptosis through the inactivation of caspase activity. In contrast to the C-terminal RING domain of AtILP, which did not inhibit the activity of caspase-3, the N-terminal region, despite displaying no homology to known BIR domains, potently inhibited the activity of caspase-3 in vitro and blocked TNF-α/ActD-induced apoptosis. The anti-apoptotic activity of the AtILP N-terminal domain observed in plants was reproduced in an animal system. Transgenic Arabidopsis lines overexpressing AtILP exhibited anti-apoptotic activity when challenged with the fungal toxin fumonisin B1, an agent that induces apoptosis-like cell death in plants. In AtIPL transgenic plants, suppression of cell death was accompanied by inhibition of caspase activation and DNA fragmentation. Overexpression of AtILP also attenuated effector protein-induced cell death and increased the growth of an avirulent bacterial pathogen. The current results demonstrated the existence of a novel plant IAP-like protein that prevents caspase activation in Arabidopsis and showed that a plant anti-apoptosis gene functions similarly in plant and animal systems.

Survivin in Multiple Myeloma: Prognostic and Therapeutic Implications

Abstract 137The IAP (inhibitor-of-apoptosis) family member, survivin, is one of the most significantly over-expressed genes in malignant cells. Survivin has been reported to inhibit apoptosis and regulate mitosis as well as cytokinesis. We therefore first determined the expression of survivin in CD138-purified multiple myeloma (MM) cells from previously untreated patients at our centers’ trial group (TG) (n=246) and the UAMS Arkansas (n=345) validation group (VG). Using the PANP algorithm, survivin is aberrantly expressed in 27% (TG) of MM cell samples. It is not expressed in normal bone marrow plasma cell samples (n=7) while expression increases significantly from MM stage I-III (P<.001). Survivin expression correlates with proliferation as assessed by gene expression- (r=.8, P<.001) or propidium iodide (r=.7, P<.001). Presence of survivin expression correlates with inferior event-free and overall survival in patients undergoing high-dose chemotherapy in the TG (22.6 vs. 35.4 months, P<.001, 52.9 vs. n.r., P=.002) as well as in the VG (12.3 vs. 54.1 months, P<.001, and 17.4 vs. n.r., respectively). These results support the further evaluation of survivin as a therapeutic target in MM. We next assessed the effects of siRNA-mediated knock-down of survivin in vitro. Suppression of survivin by siRNA induced cell cycle arrest and apoptosis in MM cell lines. The small molecule suppressant of survivin, YM155, is currently in clinical development for the treatment of solid tumors. Here, we investigated YM155 for its anti-MM activity. YM155 abrogated proliferation and induced apoptosis in a panel of 10 human MM cell lines and MM cells isolated from multidrug-resistant patients at an IC50 of 4–50nM while the IC50 was not reached in primary bone marrow stromal cells up to 500nM. YM155 was also able to overcome the protective effect of IL-6, IGF-1 and the presence of bone marrow stromal cells, respectively. The induction of apoptosis by YM155 closely correlated with down-regulation of intracellular survivin protein expression within 24 to 36h of treatment with 50–100nM of YM155. However, inhibition of cell proliferation is already detectable at 12h at 5–10nM, suggesting two different dose- and time-dependent mechanisms of action. We therefore performed gene expression and protein profiling on YM155-treated MM cells. Strikingly, these data revealed early up-regulation of the ER stress response (PERK, phospho-eIF2a, ATF4, ATF3) followed by increased CHOP expression and a profound abrogation of proliferation. This appeared to be independent of cellular survivin levels, indicating that the early proliferation arrest at very low nanomolar concentrations is mediated primarily by the ER stress response. Moreover, gene signatures regulated by the IL-6/STAT pathway (CCND1, BCL2L1, MCL1, BIRC5A) were markedly altered upon YM155 treatment. Importantly, IL-6 profoundly sensitized IL-6 responsive MM cell lines to treatment with YM155. We therefore hypothesized that YM155 might abrogate upstream regulatory signaling pathways of survivin expression. Indeed, YM155 abrogated constitutive as well as IL-6 induced phosphorylation of STAT3, an important transcription factor for survivin expression in MM cells. In contrast, phosphorylation of ERK1/2 and AKT remained unchanged. Dephosphorylation of STAT3 closely correlated with the loss of intracellular survivin. In conclusion, we have demonstrated the prognostic significance of survivin expression and a potential therapeutic role for the small molecule suppressant of survivin YM155 in MM. Disclosures:No relevant conflicts of interest to declare.

The RNA-binding protein HuR stabilizes survivin mRNA in human oesophageal epithelial cells

Overexpression of survivin, a member of the IAP (inhibitor of apoptosis) family, has been correlated with poorer outcomes in multiple malignancies, including oesophageal cancer. The regulatory mechanisms, particularly at the post-transcriptional level, involved in survivin overexpression are not well understood. Previous work from our group has shown that the RNA-binding protein HuR (Hu antigen R), which is also overexpressed in several malignancies, stabilizes the mRNA of XIAP (X-linked IAP), another IAP family member. In the present study, we demonstrate the binding of HuR to a 288bp fragment in the 3'-UTR (untranslated region) of survivin mRNA in human oesophageal epithelial cells. Unexpectedly, overexpression of HuR led to a decrease in survivin expression. This was associated with decreased survivin mRNA and promoter activity, suggesting a decrease in transcription. Levels of p53, a negative transcriptional regulator of survivin, increased following HuR overexpression, in conjunction with enhanced p53 mRNA stability. Silencing p53 prior to HuR overexpression resulted in increased survivin protein and mRNA stability. These results demonstrate that, in the absence of p53, HuR overexpression results in increased survivin mRNA stability and protein expression. This provides an additional explanation for the increased survivin expression observed in oesophageal cancer cells that have lost p53.

Abstract 4409: A quantitative MSD assay to measure c-IAP1 degradation in PBMC, cell lines and tumor cells as a pharmacodynamic biomarker following antagonism of IAP family members

Abstract The Inhibitor of Apoptosis (IAP) protein family members are widely expressed in many cancers, providing a mechanism to protect these cancer cells from apoptosis. The IAP proteins block activation of the canonical and non-canonical NF-kB pathway, the subsequent expression of TNFα and its initiation of the extrinsic apoptosis pathway. IAP proteins also directly bind and inhibit caspases activated during the induction of apoptosis. Using MDA-MB-231 cells as a model system to assess the effect of candidate IAP antagonists on markers of extrinsic and intrinsic pathway inhibition, we observe that the antagonism of the major IAP family members results in apoptosis in vitro and inhibits their tumor growth in nude mice in vivo. These effects are coincident with changes in the cellular levels of IAP family members such as c-IAP1, detected by Western analysis. These data are consistent with previously described autoubiquitination and subsequent proteasomal degradation of c-IAP1 following IAP antagonism. We have developed a high-throughput, quantifiable and sensitive assay to measure c-IAP1 levels in multiple cell lines, xenografts and in PBMCs using the MesoScale Discovery (MSD) Platform. The c-IAP1 MSD assay has a lower limit of quantification of 1.6 ng/ml of cell lysate, has precision of &amp;lt;10% CV and is linear down to at least a 4-fold dilution of lysate. Quantifiable changes in cellular c-IAP1 protein may serve as an ideal proximal pharmacodynamic (PD) biomarker to measure IAP family inhibition and degradation in the cell, particularly in the evaluation of candidate inhibitors targeting the IAP pathway in the clinical setting. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4409. doi:10.1158/1538-7445.AM2011-4409

C-IAP1 and UbcH5 promote K11-linked polyubiquitination of RIP1 in TNF signalling

Ubiquitin ligases are critical components of the ubiquitination process that determine substrate specificity and, in collaboration with E2 ubiquitin-conjugating enzymes, regulate the nature of polyubiquitin chains assembled on their substrates. Cellular inhibitor of apoptosis (c-IAP1 and c-IAP2) proteins are recruited to TNFR1-associated signalling complexes where they regulate receptor-stimulated NF-κB activation through their RING domain ubiquitin ligase activity. Using a directed yeast two-hybrid screen, we found several novel and previously identified E2 partners of IAP RING domains. Among these, the UbcH5 family of E2 enzymes are critical regulators of the stability of c-IAP1 protein following destabilizing stimuli such as TWEAK or CD40 signalling or IAP antagonists. We demonstrate that c-IAP1 and UbcH5 family promote K11-linked polyubiquitination of receptor-interacting protein 1 (RIP1) in vitro and in vivo. We further show that TNFα-stimulated NF-κB activation involves endogenous K11-linked ubiquitination of RIP1 within the TNFR1 signalling complex that is c-IAP1 and UbcH5 dependent. Lastly, NF-κB essential modifier efficiently binds K11-linked ubiquitin chains, suggesting that this ubiquitin linkage may have a signalling role in the activation of proliferative cellular pathways.

DHA Alters Expression of Target Proteins of Cancer Therapy in Chemotherapy Resistant SW620 Colon Cancer Cells

Diets rich in n-3 polyunsaturated fatty acids (PUFAs) have been associated with a reduced risk of several types of cancer. Recent reports have suggested that these PUFAs enhance the cytotoxic effect of cancer chemoradiotherapy. The effect of docosahexaenoic acid (DHA) on key cell cycle regulators and target proteins of cancer therapy was investigated in the human malign colon cancer cell line SW620. Cell cycle check point proteins such as p21 and stratifin (14-3-3 sigma) increased at mRNA and protein level, whereas cell cycle progression proteins such as cell division cycle 25 homolog and cyclin-dependent kinase 1 decreased after DHA treatment. Protein levels of inhibitors of apoptosis family members associated with chemotherapy resistance and cancer malignancy, survivin and livin, decreased after the same treatment: likewise the expression of NF-κB. Levels of the proapoptotic proteins phosphorylated p38 MAPK and growth arrest-inducible and DNA damage-inducible gene 153/C/EBP-homologous protein (CHOP) increased. The results indicate that DHA treatment causes simultaneous cell cycle arrest in both the G1 and G2 phase. In conclusion, DHA affects several target proteins of chemotherapy in a favorable way. This may explain the observed enhanced chemosensitivity in cancer cells supplemented with n-3 PUFAs and encourage further studies investigating the role of n-3 PUFAs as adjuvant to chemotherapy and radiotherapy in vivo.

Transgenic expression of an insect inhibitor of apoptosis gene, SfIAP, confers abiotic and biotic stress tolerance and delays tomato fruit ripening

SfIAP, an IAP ( inhibitor of apoptosis) family member from the insect Spodoptera frugiperda, was introduced into tobacco and tomato plants. Expression of SfIAP conferred tolerance to several abiotic stresses including heat, salt, and fumonisin B1 (FB1) as well as resistance to the necrotrophic fungus Alternaria alternata. In wild type tomato, these stresses induced DNA fragmentation and formation of apoptotic-like bodies prior to cell death. Transgenic expression of SfIAP inhibited these features and cell death did not occur. In addition, tomato fruit ripening was delayed in plants harboring SfIAP. Expression of EIN3, a key transcription factor mediating ethylene responses, was suppressed in transgenic plants during fruit ripening and in response to fungal challenge. Treatment of transgenic plants with ethephon restored normal fruit ripening and disease susceptibility. The expression of ethylene-associated genes during the process of fruit ripening was blocked in SfIAP-expressing plants. The phenotypes associated with expression of SfIAP were not correlated with the expression of pathogenesis-related proteins. Taken together, these data suggest that the insect anti-apoptotic gene SfIAP, has a conserved function that inhibits stress-induced cell death both in plants and animals.