Smac mimetic sensitizes glioblastoma cells to Temozolomide-induced apoptosis in a RIP1- and NF-κB-dependent manner

SMAC mimetic drives microglia phenotype and glioblastoma immune microenvironment

Tumor-associated macrophages/microglia (TAMs) are highly plastic and heterogeneous immune cells that can be immune-supportive or tumor-supportive depending of the microenvironment. TAMs are the most abundant immune cells in glioblastoma (GB), and play a key role in immunosuppression. Therefore, TAMs reprogramming toward immune-supportive cells is a promising strategy to overcome immunosuppression. By leveraging scRNAseq human GB databases, we identified that Inhibitor of Apoptosis Proteins (IAP) were expressed by TAMs. To investigate their role in TAMs-related immunosuppression, we antagonized IAP using the central nervous system permeant SMAC mimetic GDC-0152 (SMg). On explants and cultured immune cells isolated from human GB samples, SMg modified TAMs activity. We showed that SMg treatment promoted microglia pro-apoptotic and anti-tumoral function via caspase-3 pro-inflammatory cleavage and the inhibition of tumoroids growth. Then we designed a relevant immunogenic mouse GB model to decipher the spatio-temporal densities, distribution, phenotypes and function of TAMs with or without SMg treatment. We used 3D imaging techniques, a transgenic mouse with fluorescent TAM subsets and mass cytometry. We confirmed that SMg promoted microglia activation, antigen-presenting function and tumor infiltration. In addition, we observed a remodeling of blood vessels, a decrease in anti-inflammatory macrophages and an increased level of monocytes and their mo-DC progeny. This remodeling of the TAM landscape is associated with an increase in CD8 T cell density and activation. Altogether, these results demonstrated that SMg drives the immunosuppressive basal microglia toward an active phenotype with pro-apoptotic and anti-tumoral function and modifies the GB immune landscape. This identifies IAP as targets of choice for a potential mechanism-based therapeutic strategy and SMg as a promising molecule for this application.

Astrocytes and the tumor microenvironment inflammatory state dictate the killing of glioblastoma cells by Smac mimetic compounds

Smac mimetic compounds (SMCs) are small molecule drugs that sensitize cancer cells to TNF-α-induced cell death and have multiple immunostimulatory effects through alterations in NF-κB signaling. The combination of SMCs with immunotherapies has been reported to result in durable cures of up to 40% in syngeneic, orthotopic murine glioblastoma (GBM) models. Herein, we find that SMC resistance is not due to a cell-intrinsic mechanism of resistance. We thus evaluated the contribution of GBM and brain stromal components to identify parameters leading to SMC efficacy and resistance. The common physiological features of GBM tumors, such as hypoxia, hyaluronic acid, and glucose deprivation were found not to play a significant role in SMC efficacy. SMCs induced the death of microglia and macrophages, which are the major immune infiltrates in the tumor microenvironment. This death of microglia and macrophages then enhances the ability of SMCs to induce GBM cell death. Conversely, astrocytes promoted GBM cell growth and abrogated the ability of SMCs to induce death of GBM cells. The astrocyte-mediated resistance can be overcome in the presence of exogenous TNF-α. Overall, our results highlight that SMCs can induce death of microglia and macrophages, which then provides a source of death ligands for GBM cells, and that the targeting of astrocytes is a potential mechanism for overcoming SMC resistance for the treatment of GBM.

Involvement of the tumour necrosis factor receptor system in glioblastoma cell death induced by palbociclib-heptamethine cyanine dye conjugate

Glioblastoma is the most common and aggressive primary brain tumour in adults. The development of anti-brain cancer agents are challenged by the blood-brain barrier and the resistance conferred by the local tumour microenvironment. Heptamethine cyanine dyes (HMCDs) are a class of near-infrared fluorescence compounds that have recently emerged as promising agents for drug delivery. We conjugated palbociclib, a cyclin-dependent kinase (CDK) 4/6 inhibitor, to an HMCD, MHI-148, and conducted drug activity analysis on primary patient-derived glioblastoma cell lines. In addition to the expected cytostatic activity, our in vitro studies revealed that palbociclib-MHI-148 conjugate resulted in an almost 100-fold increase in cytotoxicity compared to palbociclib alone. This shift of palbociclib from cytostatic to cytotoxic when conjugated to MHI-148 was due to increased DNA damage, as indicated by an increase in γH2AX foci, followed by an increased expression of key extrinsic apoptosis genes, including TP53, TNFR1, TRAIL, FADD and caspase 8. In addition, we observed a time-dependent increase in the cell surface expression of TNFR1, consistent with an observed increase in the secretion TNFα, followed by TNFR1 endocytosis at 48 h. The treatment of patient GBM cells with the palbociclib-MHI-148 conjugate prevented TNFα-induced NFκB translocation, suggesting conjugate-induced TNFR1 signalling favoured the TNFR1-mediated apoptotic response rather than the pro-inflammatory response pathway. Notably, pharmacological inhibition of endocytosis of TNFR1, and siRNA-knockdown of TNFR1 reversed the palbociclib-MHI-148-induced cell death. These results show a novel susceptibility of glioblastoma cells to TNFR1-dependent apoptosis, dependent on inhibition of canonical NFκB signalling using our previously reported palbociclib-HMCD conjugate. Video Abstract.

Induction of Breast Cancer Cell Apoptosis by TRAIL and Smac Mimetics: Involvement of RIP1 and cFLIP

Smac mimetics are a group of compounds able to facilitate cell death in cancer cells. TNF-related apoptosis-inducing ligand (TRAIL) is a death receptor ligand currently explored in combination with Smac mimetics. The molecular mechanisms determining if the combination treatment results in apoptosis are however not fully understood. In this study, we aimed to shed light on these mechanisms in breast cancer cells. Three breast cancer cell lines, MDA-MB-468, CAMA-1 and MCF-7, were used to evaluate the effects of Smac mimetic LCL-161 and TRAIL using cell death assays and Western blot. The combination treatment induces apoptosis and caspase-8 cleavage in MDA-MB-468 and CAMA-1 but not in MCF-7 cells and downregulation of caspase-8 blocked apoptosis. Downregulation, but not kinase inhibition, of receptor-interacting protein 1 (RIP1) suppressed apoptosis in CAMA-1. Apoptosis is preceded by association of RIP1 with caspase-8. Downregulating cellular FLICE-like inhibitory protein (c-FLIP) resulted in increased caspase cleavage and some induction of apoptosis by TRAIL and LCL-161 in MCF-7. In CAMA-1, c-FLIP depletion potentiated TRAIL-induced caspase cleavage and LCL-161 did not increase it further. Our results lend further support to a model where LCL-161 enables the formation of a complex including RIP1 and caspase-8 and circumvents c-FLIP-mediated inhibition of caspase activation.

Roles of receptor-interacting protein kinase 1 in SH-SY5Y cells with beta amyloid-induced neurotoxicity

Alzheimer's disease (AD), the major cause of dementia, affects the elderly population worldwide. Previous studies have shown that depletion of receptor‐interacting protein kinase 1 (RIPK1) expression reverted the AD phenotype in murine AD models. Necroptosis, executed by mixed lineage kinase domain‐like (MLKL) protein and activated by RIPK1 and RIPK3, has been shown to be involved in AD. However, the role of RIPK1 in beta‐amyloid (Aβ)‐induced necroptosis is not yet fully understood. In this study, we explored the role of RIPK1 in the SH‐SY5Y human neuroblastoma cells treated with Aβ 1–40 or Aβ 1–42. We showed that Aβ‐induced neuronal cell death was independent of apoptosis and autophagy pathways. Further analyses depicted that activation of RIPK1/MLKL‐dependant necroptosis pathway was observed in vitro. We demonstrated that inhibition of RIPK1 expression rescued the cells from Aβ‐induced neuronal cell death and ectopic expression of RIPK1 was found to enhance the stability of the endogenous APP. In summary, our findings demonstrated that Aβ can potentially drive necroptosis in an RIPK1‐MLKL‐dependent manner, proposing that RIPK1 plays an important role in the pathogenesis of AD.

BH3-mimetics: recent developments in cancer therapy

The hopeful outcomes from 30 years of research in BH3-mimetics have indeed served a number of solid paradigms for targeting intermediates from the apoptosis pathway in a variety of diseased states. Not only have such rational approaches in drug design yielded several key therapeutics, such outputs have also offered insights into the integrated mechanistic aspects of basic and clinical research at the genetics level for the future. In no other area of medical research have the effects of such work been felt, than in cancer research, through targeting the BAX-Bcl-2 protein-protein interactions. With these promising outputs in mind, several mimetics, and their potential therapeutic applications, have also been developed for several other pathological conditions, such as cardiovascular disease and tissue fibrosis, thus highlighting the universal importance of the intrinsic arm of the apoptosis pathway and its input to general tissue homeostasis. Considering such recent developments, and in a field that has generated so much scientific interest, we take stock of how the broadening area of BH3-mimetics has developed and diversified, with a focus on their uses in single and combined cancer treatment regimens and recently explored therapeutic delivery methods that may aid the development of future therapeutics of this nature.

Trilobatin Induces Apoptosis and Attenuates Stemness Phenotype of Acquired Gefitinib Resistant Lung Cancer Cells via Suppression of NF-κB Pathway

Trilobatin is a common type of flavonoids compounds derived from Lithocarpus polystachyus Rehd leaves. Previous report suggests that trilobatin was potentially involved in pro-and anticancer, antioxidative and anti-hyperglycemic activities. Here, we investigated the anticancer efficiency of trilobatin on gefitinib resistant lung cancer cells. In this study, MTT assays, EdU incorporation assays, DAPI staining, tumor sphere formation assays, immunofluorescent staining and Western blot analysis were performed to explore the functional role of trilobatin on gefitinib resistant lung cancer cells. The results showed that trilobatin inhibits proliferation of gefitinib resistant lung cancer cells. In addition, the proportions of apoptotic cells were increased along with down-regulated expression levels of Bcl-2 and mitochondrial Cytochrome C while up-regulated Bax, Cleaved Caspase-3, -9, and cytosolic Cytochrome C expression. Moreover, trilobatin decreased tumor sphere formation and expression levels of multiple stemness markers (ALDH1, CD133, Nanog, and ABCG2) in gefitinib resistant lung cancer cells. Furthermore, investigation of the mechanism indicated that trilobatin suppressed activity of NF-κB via decreasing constitutive phosphorylation of NF-κB p65 and IκB-α in gefitinib resistant lung cancer cells. All these results indicate that trilobatin induces apoptosis and attenuates stemness phenotype of gefitinib resistant lung cancer cells, involved with, or partly, the suppression of NF-κB activity.

Recent insight into the role of RING-finger E3 ligases in glioma

The ubiquitin proteasome system (UPS) serves as the major posttranslational modification system for the maintenance of protein homeostasis. The ubiquitin ligases (E3s) are responsible for the recognition and recruitment of specific substrate proteins for polyubiquitination. Really interesting new gene (RING) finger E3s account for the majority of E3s. The human genome encodes more than 600 RING E3s, which are divided into three subclasses: single polypeptide E3s, cullin-RING ligases (CRLs) and other multisubunit E3s. The abnormal regulation of RING E3s has been reported to disrupt normal biological processes and induce the occurrence of many human malignancies. Glioma is the most common type of malignant primary brain tumor. In the last few decades, patient prognosis has improved as novel targeted therapeutic agents have developed. In this review, we will summarize the current knowledge about the dysregulation of RING E3s and the altered stability of their substrates in glioma. We will further introduce and discuss the current status and future perspectives of the application of small inhibitors and proteolysis-targeting chimeric molecules (PROTACs) interfering with RING E3s as potential anticancer agents for glioma.

The role of E3 ubiquitin ligases in the development and progression of glioblastoma

Despite recent advances in our understanding of the disease, glioblastoma (GB) continues to have limited treatment options and carries a dismal prognosis for patients. Efforts to stratify this heterogeneous malignancy using molecular classifiers identified frequent alterations in targetable proteins belonging to several pathways including the receptor tyrosine kinase (RTK) and mitogen-activated protein kinase (MAPK) signalling pathways. However, these findings have failed to improve clinical outcomes for patients. In almost all cases, GB becomes refractory to standard-of-care therapy, and recent evidence suggests that disease recurrence may be associated with a subpopulation of cells known as glioma stem cells (GSCs). Therefore, there remains a significant unmet need for novel therapeutic strategies. E3 ubiquitin ligases are a family of >700 proteins that conjugate ubiquitin to target proteins, resulting in an array of cellular responses, including DNA repair, pro-survival signalling and protein degradation. Ubiquitin modifications on target proteins are diverse, ranging from mono-ubiquitination through to the formation of polyubiquitin chains and mixed chains. The specificity in substrate tagging and chain elongation is dictated by E3 ubiquitin ligases, which have essential regulatory roles in multiple aspects of brain cancer pathogenesis. In this review, we begin by briefly summarising the histological and molecular classification of GB. We comprehensively describe the roles of E3 ubiquitin ligases in RTK and MAPK, as well as other, commonly altered, oncogenic and tumour suppressive signalling pathways in GB. We also describe the role of E3 ligases in maintaining glioma stem cell populations and their function in promoting resistance to ionizing radiation (IR) and chemotherapy. Finally, we consider how our knowledge of E3 ligase biology may be used for future therapeutic interventions in GB, including the use of blood-brain barrier permeable proteolysis targeting chimeras (PROTACs).

Data-driven prioritization and preclinical evaluation of therapeutic targets in glioblastoma

Background

Patients with glioblastoma (GBM) have a dismal prognosis, and there is an unmet need for new therapeutic options. This study aims to identify new therapeutic targets in GBM.

Methods

mRNA expression data of patient-derived GBM (n = 1279) and normal brain tissue (n = 46) samples were collected from Gene Expression Omnibus and The Cancer Genome Atlas. Functional genomic mRNA profiling was applied to capture the downstream effects of genomic alterations on gene expression levels. Next, a class comparison between GBM and normal brain tissue was performed. Significantly upregulated genes in GBM were further prioritized based on (1) known interactions with antineoplastic drugs, (2) current drug development status in humans, and (3) association with biologic pathways known to be involved in GBM. Antineoplastic agents against prioritized targets were validated in vitro and in vivo.

Results

We identified 712 significantly upregulated genes in GBM compared to normal brain tissue, of which 27 have a known interaction with antineoplastic agents. Seventeen of the 27 genes, including EGFR and VEGFA, have been clinically evaluated in GBM with limited efficacy. For the remaining 10 genes, RRM2, MAPK9 (JNK2, SAPK1a), and XIAP play a role in GBM development. We demonstrated for the MAPK9 inhibitor RGB-286638 a viability loss in multiple GBM cell culture models. Although no overall survival benefit was observed in vivo, there were indications that RGB-286638 may delay tumor growth.

Conclusions

The MAPK9 inhibitor RGB-286638 showed promising in vitro results. Furthermore, in vivo target engagement studies and combination therapies with this compound warrant further exploration.

Combination therapy using Smac peptide and doxorubicin-encapsulated MUC 1-targeted polymeric nanoparticles to sensitize cancer cells to chemotherapy: An in vitro and in vivo study

Targeting inhibitors of apoptosis proteins (IAPs) family comprising high level expression in many cancer cells, could sensitize tumor cells to conventional chemotherapies. In the present study, we designed both doxorubicin and SmacN6 (an antagonist of the IAPs) encapsulated polymeric nanoparticles (NPs) and investigated their synergistic effect of combination therapy in vitro and in vivo. According to the results, NPs-SmacN6 significantly enhanced the cytotoxicity effect of NPs-DOX and reduced its IC₅₀ in MCF-7, 4T1 and C26 cancer cells. Western blot analysis confirmed mechanism of cell apoptosis via caspase activation through intrinsic and also extrinsic pathways. Moreover, 5TR1 aptamer-modified NPs could effectively deliver DOXor SmacN6 to C26 cancer cells (MUC1 positive) in comparison with the non-targeted one (p < 0.001). However, they could not be efficiently internalized into CHO cells (MUC1 negative), showing less cytotoxicity in this cell line. In vivo experiments in BALB/c mice bearing C26 tumor indicated that Apt-NPs-DOX in combination with Apt-NPs-SmacN6 had significant tumor growth inhibition in comparison with mice receiving either free DOX or Apt-NPs-DOX with p < 0.0001 and p < 0.05, respectively. Our results revealed that combination therapy of DOX and SmacN6 via Apt-modified nanoparticles can lead to improvement of therapeutic index of DOX in MUC1 positive cancer cells.

NF-κB contributes to Smac mimetic-conferred protection from tunicamycin-induced apoptosis

Smac mimetics that deplete cellular inhibitor of apoptosis (cIAP) proteins have been shown to activate Nuclear Factor-kappa B (NF-κB). Here, we report that Smac mimetic-mediated activation of NF-κB contributes to the rescue of cancer cells from tunicamycin (TM)-triggered apoptosis. The prototypic Smac mimetic BV6 activates non-canonical and canonical NF-κB pathways, while TM has little effect on NF-κB signaling. Importantly, ectopic expression of dominant-negative IκBα superrepressor (IκBα-SR), which inhibits canonical and non-canonical NF-κB activation, significantly reversed this BV6-imposed protection against TM. Similarly, transient or stable knockdown of NF-κB-inducing kinase, which accumulated upon exposure to BV6 alone and in combination with TM, significantly counteracted BV6-mediated inhibition of TM-induced apoptosis. Interestingly, while cIAP2 was initially degraded upon BV6 treatment, it was subsequently upregulated in an NF-κB-dependent manner, as this restoration of cIAP2 expression was abolished in IκBα-SR-overexpressing cells. Interestingly, upon exposure to TM/BV6 apoptosis was significantly increased in cIAP2 knockdown cells. Furthermore, NF-κB inhibition partially prevented BV6-stimulated expression of Mcl-1 upon TM treatment. Consistently, Mcl-1 silencing significantly inhibited BV6-mediated protection from TM-induced apoptosis. Thus, NF-κB activation by Smac mimetic contributes to Smac mimetic-mediated protection against TM-induced apoptosis.

Key necroptotic proteins are required for Smac mimetic-mediated sensitization of cholangiocarcinoma cells to TNF-α and chemotherapeutic gemcitabine-induced necroptosis

Cholangiocarcinoma (CCA), a malignant tumor originating in the biliary tract, is well known to be associated with adverse clinical outcomes and high mortality rates due to the lack of effective therapy. Evasion of apoptosis is considered a key contributor to therapeutic success and chemotherapy resistance in CCA, highlighting the need for novel therapeutic strategies. In this study, we demonstrated that the induction of necroptosis, a novel regulated form of necrosis, could potentially serve as a novel therapeutic approach for CCA patients. The RNA sequencing data in The Cancer Genome Atlas (TCGA) database were analyzed and revealed that both receptor-interacting protein kinase 3 (RIPK3) and mixed lineage kinase domain-like (MLKL), two essential mediators of necroptosis, were upregulated in CCA tissues when compared with the levels in normal bile ducts. We demonstrated in a panel of CCA cell lines that RIPK3 was differentially expressed in CCA cell lines, while MLKL was more highly expressed in CCA cell lines than in nontumor cholangiocytes. We therefore showed that treatment with both tumor necrosis factor-α (TNF-α) and Smac mimetic, an inhibitor of apoptosis protein (IAP) antagonist, induced RIPK1/RIPK3/MLKL-dependent necroptosis in CCA cells when caspases were blocked. The necroptotic induction in a panel of CCA cells was correlated with RIPK3 expression. Intriguingly, we demonstrated that Smac mimetic sensitized CCA cells to a low dose of standard chemotherapy, gemcitabine, and induced necroptosis in an RIPK1/RIPK3/MLKL-dependent manner upon caspase inhibition but not in nontumor cholangiocytes. We further demonstrated that Smac mimetic and gemcitabine synergistically induced an increase in TNF-α mRNA levels and that Smac mimetic reversed gemcitabine-induced cell cycle arrest, leading to cell killing. Collectively, our present study demonstrated that TNF-α and gemcitabine induced RIPK1/RIPK3/MLKL-dependent necroptosis upon IAP depletion and caspase inhibition; therefore, our findings have pivotal implications for designing a novel necroptosis-based therapeutic strategy for CCA patients.

Understanding Apoptosis and Apoptotic Pathways Targeted Cancer Therapeutics

Various physiological processes involve appropriate tissue developmental process and homeostasis - the pathogenesis of several diseases connected with deregulatory apoptosis process. Apoptosis plays a crucial role in maintaining a balance between cell death and division, evasion of apoptosis results in the uncontrolled multiplication of cells leading to different diseases such as cancer. Currently, the development of apoptosis targeting anticancer drugs has gained much interest since cell death induced by apoptosis causes minimal inflammation. The understanding of complexities of apoptosis mechanism and how apoptosis is evolved by tumor cells to oppose cell death has focused research into the new strategies designed to induce apoptosis in cancer cells. This review focused on the underlying mechanism of apoptosis and the dysregulation of apoptosis modulators involved in the extrinsic and intrinsic apoptotic pathway, which include death receptors (DRs) proteins, cellular FLICE inhibitory proteins (c-FLIP), anti-apoptotic Bcl-2 proteins, inhibitors of apoptosis proteins (IAPs), tumor suppressor (p53) in cancer cells along with various current clinical approaches aimed to selectively induce apoptosis in cancer cells.

Smac mimetic suppresses tunicamycin-induced apoptosis via resolution of ER stress

Since Inhibitor of Apoptosis (IAP) proteins have been implicated in cellular adaptation to endoplasmic reticulum (ER) stress, we investigated the regulation of ER stress-induced apoptosis by small-molecule second mitochondria-derived activator of caspase (Smac) mimetics that antagonize IAP proteins. Here, we discover that Smac mimetic suppresses tunicamycin (TM)-induced apoptosis via resolution of the unfolded protein response (UPR) and ER stress. Smac mimetics such as BV6 selectively inhibit apoptosis triggered by pharmacological or genetic inhibition of protein N-glycosylation using TM or knockdown of DPAGT1, the enzyme that catalyzes the first step of protein N-glycosylation. In contrast, BV6 does not rescue cell death induced by other typical ER stressors (i.e., thapsigargin (TG), dithiothreitol, brefeldin A, bortezomib, or 2-deoxyglucose). The protection from TM-triggered apoptosis is found for structurally different Smac mimetics and for genetic knockdown of cellular IAP (cIAP) proteins in several cancer types, underlining the broader relevance. Interestingly, lectin microarray profiling reveals that BV6 counteracts TM-imposed inhibition of protein glycosylation. BV6 consistently abolishes TM-stimulated accumulation of ER stress markers such as glucose-regulated protein 78 (GRP78) and C/EBP homologous protein (CHOP) and reduces protein kinase RNA-like ER kinase (PERK) phosphorylation and X box-binding protein 1 (XBP1) splicing upon TM treatment. BV6-stimulated activation of nuclear factor-κB (NF-κB) contributes to the resolution of ER stress, since NF-κB inhibition by overexpression of dominant-negative IκBα superrepressor counteracts the suppression of TM-stimulated transcriptional activation of CHOP and GRP78 by BV6. Thus, our study is the first to show that Smac mimetic protects from TM-triggered apoptosis by resolving the UPR and ER stress. This provides new insights into the regulation of cellular stress responses by Smac mimetics.

The transcription factor SP3 drives TNF-α expression in response to Smac mimetics

The controlled production and downstream signaling of the inflammatory cytokine tumor necrosis factor-α (TNF-α) are important for immunity and its anticancer effects. Although chronic stimulation with TNF-α is detrimental to the health of the host in several autoimmune and inflammatory disorders, TNF-α-contrary to what its name implies-leads to cancer formation by promoting cell proliferation and survival. Smac mimetic compounds (SMCs), small-molecule antagonists of inhibitor of apoptosis proteins (IAPs), switch the TNF-α signal from promoting survival to promoting death in cancer cells. Using a genome-wide siRNA screen to identify factors required for SMC-to-TNF-α-mediated cancer cell death, we identified the transcription factor SP3 as a critical molecule in both basal and SMC-induced production of TNF-α by engaging the nuclear factor κB (NF-κB) transcriptional pathway. Moreover, the promotion of TNF-α expression by SP3 activity confers differential sensitivity of cancer versus normal cells to SMC treatment. The key role of SP3 in TNF-α production and signaling will help us further understand TNF-α biology and provide insight into mechanisms relevant to cancer and inflammatory disease.

Targeting the BIR Domains of Inhibitor of Apoptosis (IAP) Proteins in Cancer Treatment

Inhibitor of apoptosis (IAP) proteins are characterized by the presence of the conserved baculoviral IAP repeat (BIR) domain that is involved in protein-protein interactions. IAPs were initially thought to be mainly responsible for caspase inhibition, acting as negative regulators of apoptosis, but later works have shown that IAPs also control a plethora of other different cellular pathways. As X-linked IAP (XIAP), and other IAP, levels are often deregulated in cancer cells and have been shown to correlate with patients' prognosis, several approaches have been pursued to inhibit their activity in order to restore apoptosis. Many small molecules have been designed to target the BIR domains, the vast majority being inspired by the N-terminal tetrapeptide of Second Mitochondria-derived Activator of Caspases/Direct IAp Binding with Low pI (Smac/Diablo), which is the natural XIAP antagonist. These compounds are therefore usually referred to as Smac mimetics (SMs). Despite the fact that SMs were intended to specifically target XIAP, it has been shown that they also interact with cellular IAP-1 (cIAP1) and cIAP2, promoting their proteasome-dependent degradation. SMs have been tested in combination with several cytotoxic compounds and are now considered promising immune modulators which can be exploited in cancer therapy, especially in combination with immune checkpoint inhibitors. In this review, we give an overview of the structural hot-spots of BIRs, focusing on their fold and on the peculiar structural patches which characterize the diverse BIRs. These structures are exploited/exploitable for the development of specific and active IAP inhibitors.

Smac mimetics as novel promising modulators of apoptosis in the treatment of breast cancer

Breast cancer is the most prevalent cancer in women. Despite improvements in treatment, the rate of breast cancer-related deaths is still high, and this issue needs further, accurate investigations. Although several treatment options are available, none of them are efficient for complete remission, particularly in advanced stages of the disease. It is known that cancerous cells have dysregulated apoptosis-related pathways, by which they can remain alive for a long time, expand freely, and escape from apoptosis-inducing drugs or antitumor immune responses. Therefore, modulation of apoptosis resistance in cancer cells may be an efficient strategy to overcome current problems faced in the development of immunotherapeutic approaches for the treatment of breast cancer. The inhibitors of apoptosis protein (IAPs) are important targets for cancer therapy because it has been shown that these molecules are overexpressed and highly active in various cancer cells and suppress apoptosis process in malignant cells by blockage of caspase proteins. There is evidence of Smac mimetics efficacy as a single agent; however, recent studies have indicated the efficacy of current anticancer immunotherapeutic approaches when combined with Smac mimetics, which are potent inhibitors of IAPs and synthesized mimicking Smac/Diablo molecules. In this review, we are going to discuss the efficacy of treatment of breast cancer by Smac mimetics alone or in combination with other therapeutics.

Mutagenic assessment of chemotherapy and Smac mimetic drugs in cells with defective DNA damage response pathways

DNA damaging therapies can spur the formation of therapy-related cancers, due to mis-repair of lesions they create in non-cancerous cells. This risk may be amplified in patients with impaired DNA damage responses. We disabled key DNA damage response pathways using genetic and pharmacological approaches, and assessed the impact of these deficiencies on the mutagenicity of chemotherapy drugs or the "Smac mimetic" GDC-0152, which kills tumor cells by targeting XIAP, cIAP1 and 2. Doxorubicin and cisplatin provoked mutations in more surviving cells deficient in ATM, p53 or the homologous recombination effector RAD51 than in wild type cells, but suppressing non-homologous end joining (NHEJ) by disabling DNA-PKcs prevented chemotherapy-induced mutagenesis. Vincristine-induced mutagenesis required p53 and DNA-PKcs but was not affected by ATM status, consistent with it provoking ATM-independent p53-mediated activation of caspases and CAD, which creates DNA lesions in surviving cells that could be mis-repaired by NHEJ. Encouragingly, GDC-0152 failed to stimulate mutations in cells with proficient or defective DNA damage response pathways. This study highlights the elevated oncogenic risk associated with treating DNA repair-deficient patients with genotoxic anti-cancer therapies, and suggests a potential advantage for Smac mimetic drugs over traditional therapies: a reduced risk of therapy-related cancers.

Inhibitors of apoptosis: clinical implications in cancer

Inhibitor of apoptosis (IAP) family comprises a group of endogenous proteins that function as main regulators of caspase activity and cell death. They are considered the main culprits in evasion of apoptosis, which is a fundamental hallmark of carcinogenesis. Overexpression of IAP proteins has been documented in various solid and hematological malignancies, rendering them resistant to standard chemotherapeutics and radiation therapy and conferring poor prognosis. This observation has urged their exploitation as therapeutic targets in cancer with promising pre-clinical outcomes. This review describes the structural and functional features of IAP proteins to elucidate the mechanism of their anti-apoptotic activity. We also provide an update on patterns of IAP expression in different tumors, their impact on treatment response and prognosis, as well as the emerging investigational drugs targeting them. This aims at shedding the light on the advances in IAP targeting achieved to date, and encourage further development of clinically applicable therapeutic approaches.

Overcoming chemotherapy drug resistance by targeting inhibitors of apoptosis proteins (IAPs)

Inhibitors of apoptosis (IAPs) are a family of proteins that play a significant role in the control of programmed cell death (PCD). PCD is essential to maintain healthy cell turnover within tissue but also to fight disease or infection. Uninhibited, IAPs can suppress apoptosis and promote cell cycle progression. Therefore, it is unsurprising that cancer cells demonstrate significantly elevated expression levels of IAPs, resulting in improved cell survival, enhanced tumor growth and subsequent metastasis. Therapies to target IAPs in cancer has garnered substantial scientific interest and as resistance to anti-cancer agents becomes more prevalent, targeting IAPs has become an increasingly attractive strategy to re-sensitize cancer cells to chemotherapies, antibody based-therapies and TRAIL therapy. Antagonism strategies to modulate the actions of XIAP, cIAP1/2 and survivin are the central focus of current research and this review highlights advances within this field with particular emphasis upon the development and specificity of second mitochondria-derived activator of caspase (SMAC) mimetics (synthetic analogs of endogenously expressed inhibitors of IAPs SMAC/DIABLO). While we highlight the potential of SMAC mimetics as effective single agent or combinatory therapies to treat cancer we also discuss the likely clinical implications of resistance to SMAC mimetic therapy, occasionally observed in cancer cell lines.

Cell death-based treatment of glioblastoma

Cancer cells including glioblastoma have typically evolved multiple mechanisms to escape programmed cell death in order to maintain their survival. Defects in cell death mechanisms not only facilitate tumorigenesis but also ensure resistance to current anticancer therapies. This emphasizes that targeting cell death pathways may provide a means to tackle one of the Achilles' heels of cancer. Over the last decades several approaches have been developed to selectively target cell death pathways for therapeutic purposes. Some of these concepts have already been transferred into clinical application in oncology and may open new perspectives for the treatment of cancer.

Therapeutic opportunities based on caspase modulation

Caspases are a family of proteolytic enzymes that play a critical role in the regulation of programmed cell death via apoptosis. Activation of caspases is frequently impaired in human cancers, contributing to cancer formation, progression and therapy resistance. A better understanding of the molecular mechanisms regulating caspase activation in cancer cells is therefore highly important. Thus, targeted modulation of caspase activation and apoptosis represents a promising approach for the development of new therapeutic options to elucidate cancer cell death.

Glioblastoma and acute myeloid leukemia: malignancies with striking similarities

Acute myeloid leukemia (AML) and glioblastoma (GB) are two malignancies associated with high incidence of treatment refractoriness and generally, uniformly poor survival outcomes. While the former is a hematologic (i.e. a "liquid") malignancy and the latter a solid tumor, the two diseases share both clinical and biochemical characteristics. Both diseases exist predominantly in primary (de novo) forms, with only a small subset of each progressing from precursor disease states like the myelodysplastic syndromes or diffuse glioma. More importantly, the primary and secondary forms of each disease are characterized by common sets of mutations and gene expression abnormalities. The primary versions of AML and GB are characterized by aberrant RAS pathway, matrix metalloproteinase 9, and Bcl-2 expression, and their secondary counterparts share abnormalities in TP53, isocitrate dehydrogenase, ATRX, inhibitor of apoptosis proteins, and survivin that both influence the course of the diseases themselves and their progression from precursor disease. An understanding of these shared features is important, as it can be used to guide both the research about and treatment of each.

Apoptotic Signaling Pathways in Glioblastoma and Therapeutic Implications

Glioblastoma multiforme (GBM) is the most hostile type of brain cancer. Its aggressiveness is due to increased invasion, migration, proliferation, angiogenesis, and a decreased apoptosis. In this review, we discuss the role of key regulators of apoptosis in GBM and glioblastoma stem cells. Given their importance in the etiology and pathogenesis of GBM, these signaling molecules may represent potential therapeutic targets.

Sensitization of neuroblastoma for vincristine-induced apoptosis by Smac mimetic LCL161 is attended by G2 cell cycle arrest but is independent of NFκB, RIP1 and TNF-α

We demonstrated sensitization for chemotherapy by Smac mimetic (SM) LCL161, a potent antagonist of inhibitor of apoptosis proteins (IAP), in neuroblastoma (NB). Vinca alkaloids, particularly vincristine (VCR), displayed the strongest impact on inhibition of proliferation and apoptosis induction in combination with LCL161. The underlying signaling pathways remain elusive, though. LCL161 induces a quick degradation of cellular IAP 1 (cIAP-1). Combination of LCL161 with VCR had only marginal effects on X-linked IAP (XIAP) protein expression. Cell death is accompanied by activation of intrinsic (caspase-9 and MMP) and extrinsic (caspase-8) pathways of apoptosis, repression of migratory potential and cell cycle arrest in G2 phase.

LCL161-induced cIAP degradation leads to activation of non-canonical and blockade of canonical NF-κB pathways but not induction of apoptosis. Surprisingly NF-κB and TNF-α signaling is negligible for VCR- and VCR/LCL161-induced apoptosis since chemical inhibition of NF-κB using BAY-7085 and PBS-1086, as well as application of TNF-α blocking antibody Humira (adalimumab) has no relevant effect on cell death. Recently formation of a TNF-α-independent complex (ripoptosome) consisting of RIP1, FADD and caspase-8 following IAP inhibition by SM has been described. However, targeting of RIP1 by Necrostatin was not sufficient to influence apoptosis induced by VCR/LCL161.

Examining the In Vitro Efficacy of the IAP Antagonist Birinapant as a Single Agent or in Combination With Dacarbazine to Induce Melanoma Cell Death

Antagonists of inhibitors of apoptosis proteins (IAPs), alone or in combination with genotoxic therapeutics, have been shown to efficiently induce cell death in various solid tumors. The IAP antagonist birinapant is currently being tested in phase II clinical trials. We herein aimed to investigate the antitumor efficacy of dacarbazine in vitro, both as a single agent and in combination with birinapant, in melanoma cell lines. Covering clinically relevant drug concentration ranges, we conducted a total of 5,400 measurements in a panel of 12 human melanoma cell lines representing different stages of disease progression. Surprisingly, functionally relevant synergies or response potentiation in combination treatments was not observed, and only one cell line modestly responded to birinapant single treatment (approximately 16% cell death). Although we did not study the underlying resistance mechanisms or more complex in vivo scenarios in which dacarbazine/birinapant response synergies may still possibly manifest, our findings are nevertheless noteworthy because IAP antagonists were demonstrated to strongly enhance responses to DNA-damaging agents in cell lines of other cancer types under comparable experimental conditions in vitro.

The Smac Mimetic BV6 Improves NK Cell-Mediated Killing of Rhabdomyosarcoma Cells by Simultaneously Targeting Tumor and Effector Cells

Rhabdomyosarcoma (RMS), the most common cancer of connective tissues in pediatrics, is often resistant to conventional therapies. One underlying mechanism of this resistance is the overexpression of Inhibitor of Apoptosis (IAP) proteins, leading to a dysfunctional cell death program within tumor cells. Smac mimetics (SM) are small molecules that can reactivate the cell death program by antagonizing IAP proteins and thereby compensating their overexpression. Here, we report that SM sensitize two RMS cell lines (RD and RH30) toward natural killer (NK) cell-mediated killing on the one hand, and increase the cytotoxic potential of NK cells on the other. The SM-induced sensitization of RH30 cells toward NK cell-mediated killing is significantly reduced through blocking tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) on NK cells prior to coculture. In addition, the presence of zVAD.fmk, a pancaspase inhibitor, rescues tumor cells from the increase in killing, indicating an apoptosis-dependent cell death. On the NK cell side, the presence of SM in addition to IL-2 during the ex vivo expansion leads to an increase in their cytotoxic activity against RH30 cells. This effect is mainly TNFα-dependent and partially mediated by NK cell activation, which is associated with transcriptional upregulation of NF-κB target genes such as IκBα and RelB. Taken together, our findings implicate that SM represent a novel double-hit strategy, sensitizing tumor and activating NK cells with one single drug.

Complex Pathologic Roles of RIPK1 and RIPK3: Moving Beyond Necroptosis

A process of regulated necrosis, termed necroptosis, has been recognized as a major contributor to cell death and inflammation occurring under a wide range of pathologic settings. The core event in necroptosis is the formation of the detergent-insoluble 'necrosome' complex of homologous Ser/Thr kinases, receptor protein interacting kinase 1 (RIPK1) and receptor interacting protein kinase 3 (RIPK3), which promotes phosphorylation of a key prodeath effector, mixed lineage kinase domain-like (MLKL), by RIPK3. Core necroptosis mediators are under multiple controls, which have been a subject of intense investigation. Additional, non-necroptotic functions of these factors, primarily in controlling apoptosis and inflammatory responses, have also begun to emerge. This review will provide an overview of the current understanding of the human disease relevance of this pathway, and potential therapeutic strategies, targeting necroptosis mediators in various pathologies.

Smac Mimetics to Therapeutically Target IAP Proteins in Cancer

Inhibitor of Apoptosis (IAP) proteins are overexpressed in a variety of human cancers. Therefore, they are considered as promising targets for the design of therapeutic strategies. Smac mimetics mimic the endogenous mitochondrial protein Smac that antagonizes IAP proteins upon its release into the cytosol. Multiple preclinical studies have documented the ability of Smac mimetics to either directly induce cell death of cancer cells or to prime them to agents that trigger cell death. At present, several Smac mimetics are being evaluated in early clinical trials. The current review provides an overview on the potential of Smac mimetics as cancer therapeutics to target IAP proteins for cancer therapy.

Reactive oxygen species contribute toward Smac mimetic/temozolomide-induced cell death in glioblastoma cells

Small-molecule inhibitors of Inhibitor of Apoptosis proteins such as Smac mimetics have been reported to provide a promising tool to sensitize glioblastoma (GBM) cells to cytotoxic therapies including chemotherapeutic drugs. However, the underlying molecular mechanisms of action have not yet been fully unraveled. In the present study, we therefore investigated the role of reactive oxygen species (ROS) in the regulation of Smac mimetic/temozolomide (TMZ)-induced cell death in GBM cells. Here, we show that the Smac mimetic BV6 and TMZ act in concert to stimulate the production of both cytosolic and mitochondrial ROS. This accumulation of ROS contributes toward the activation of the proapoptotic factor BAX upon BV6/TMZ cotreatment as several ROS scavengers (i.e. N-acetyl-L-cysteine, MnTBAP, or α-tocopherol) protect GBM cells against BV6/TMZ-mediated BAX activation. In addition, ROS scavengers significantly rescue GBM cells from BV6/TMZ-triggered cell death, indicating that ROS generation is required for the induction of cell death. By showing that ROS play an important role in the regulation of Smac mimetic/TMZ-induced cell death, our work sheds light on the crucial role of the oxidative system in the cooperative antitumor activity of Smac mimetic/TMZ combination therapy against GBM cells.

Capsaicin and dihydrocapsaicin induce apoptosis in human glioma cells via ROS and Ca2+‑mediated mitochondrial pathway

Human glioma is the most common type of primary brain tumor and one of the most invasive and aggressive tumors, which, even with treatments including surgery, radiotherapy and chemotherapy, often relapses and exhibits resistance to conventional treatment methods. Developing novel strategies to control human glioma is, therefore, an important research focus. The present study investigated the mechanism of apoptosis induction in U251 human glioma cells by capsaicin (Cap) and dihydrocapsaicin (DHC), the major pungent ingredients of red chili pepper, using the Cell Counting Kit‑8 assay, transmission electron microscopy analysis, flow cytometry analysis, laser scanning confocal microscope analysis and immunohistochemical staining. Treatment of U251 glioma cells with Cap and DHC resulted in a dose‑ and time‑dependent inhibition of cell viability and induction of apoptosis, whereas few effects were observed on the viability of L929 normal murine fibroblast cells. The apoptosis‑inducing effects of Cap and DHC in U251 cells were associated with the generation of reactive oxygen species, increased Ca2+ concentrations, mitochondrial depolarization, release of cytochrome c into the cytosol and activation of caspase‑9 and ‑3. These effects were further confirmed by observations of the anti‑tumor effects of Cap and DHC in vivo in a U251 cell murine tumor xenograft model. These results demonstrate that Cap and DHC are effective inhibitors of in vitro and in vivo survival of human glioma cells, and provide the rationale for further clinical investigation of Cap and DHC as treatments for human glioma.

CXCR4-targeted modular peptide carriers for efficient anti-VEGF siRNA delivery

The application of small interfering RNA (siRNA) for specific gene inhibition is a promising strategy in gene therapy treatments. The efficient cellular delivery of therapeutic siRNA is a critical step in RNA interference (RNAi) application. Highly efficient siRNA carriers should be developed for specific cellular uptake, stable RNA-complexes formation and intracellular RNA release. To study these features, we evaluated modular peptide carriers bearing CXCR4 targeting ligand for their ability to condense siRNA, facilitate endosomal escape and VEGFA gene silencing in CXCR4-expressing endothelial and glioblastoma cells. Peptide carriers were shown to condense and protect siRNA from RNAse degradation. Various N/P ratios were used for physicochemical characterization to optimize siRNA/peptide complexes for in vitro studies. On average, cytotoxicity of siRNA-polyplexes depended on cell type and was not higher than that of PEI/siRNA complexes. VEGFA gene knockdown was significantly improved with CXCR4-targeted carriers in contrast to nontargeted peptides. siRNA delivery by means of ligandconjugated carriers resulted in 2.5-3-fold decrease of VEGF expression in glioblastoma cells and in 1.5-2-fold decrease of VEGF expression in endothelial cells. Delivery of siRNA/peptide complexes resulted in 2-6- fold decrease in VEGF protein yield and in significant inhibition of endothelial cells migration. The study shows that implication of peptide carriers modified with CXCR4 ligand is a promising approach to develop targeted siRNA delivery system into CXCR4-expressing cancer and endothelial cells.

LCL161 increases paclitaxel-induced apoptosis by degrading cIAP1 and cIAP2 in NSCLC

BACKGROUND

LCL161, a novel Smac mimetic, is known to have anti-tumor activity and improve chemosensitivity in various cancers. However, the function and mechanisms of the combination of LCL161 and paclitaxel in non-small cell lung cancer (NSCLC) remain unknown.

METHODS

Cellular inhibitor of apoptotic protein 1 and 2 (cIAP1&2) expression in NSCLC tissues and adjacent non-tumor tissues were assessed by immunohistochemistry. The correlations between cIAP1&2 expression and clinicopathological characteristics, prognosis were analyzed. Cell viability and apoptosis were measured by MTT assays and Flow cytometry. Western blot and co-immunoprecipitation assay were performed to measure the protein expression and interaction in NF-kB pathway. siRNA-mediated gene silencing and caspases activity assays were applied to demonstrate the role and mechanisms of cIAP1&2 and RIP1 in lung cancer cell apoptosis. Mouse xenograft NSCLC models were used in vivo to determine the therapeutic efficacy of LCL161 alone or in combination with paclitaxel.

RESULTS

The expression of cIAP1 and cIAP2 in Non-small cell lung cancer (NSCLC) tumors was significantly higher than that in adjacent normal tissues. cIAP1 was highly expressed in patients with late TNM stage NSCLC and a poor prognosis. Positivity for both cIAP1 and cIAP2 was an independent prognostic factor that indicated a poorer prognosis in NSCLC patients. LCL161, an IAP inhibitor, cooperated with paclitaxel to reduce cell viability and induce apoptosis in NSCLC cells. Molecular studies revealed that paclitaxel increased TNFα expression, thereby leading to the recruitment of various factors and the formation of the TRADD-TRAF2-RIP1-cIAP complex. LCL161 degraded cIAP1&2 and released RIP1 from the complex. Subsequently, RIP1 was stabilized and bound to caspase-8 and FADD, thereby forming the caspase-8/RIP1/FADD complex, which activated caspase-8, caspase-3 and ultimately lead to apoptosis. In nude mouse xenograft experiments, the combination of LCL161 and paclitaxel degraded cIAP1,2, activated caspase-3 and inhibited tumor growth with few toxic effects.

CONCLUSION

Thus, LCL161 could be a useful agent for the treatment of NSCLC in combination with paclitaxel.

Differential role of RIP1 in Smac mimetic-mediated chemosensitization of neuroblastoma cells

We explored the potential of Smac mimetics, which antagonize Inhibitor of Apoptosis (IAP) proteins, for chemosensitization of neuroblastoma (NB). Here, we report that Smac mimetics, e.g. BV6, prime NB cells for chemotherapeutics including the topoisomerase II inhibitor doxorubicin (DOX) and vinca alkaloids such as Vincristine (VCR), Vinblastine (VBL) and Vinorelbine (VNR). Additionally, BV6 acts in concert with DOX or VCR to suppress long-term clonogenic growth. While BV6 causes rapid downregulation of cellular IAP (cIAP)1 protein and nuclear factor-kappaB (NF-κB) activation, DOX/BV6- or VCR/BV6-induced apoptosis occurs independently of NF-κB or TNFα signaling, since overexpression of dominant-negative IκBα superrepressor or the Tumor Necrosis Factor (TNF)α-blocking antibody Enbrel fail to block cell death. Mechanistic studies reveal that Receptor-interacting protein (RIP)1 is required for DOX/BV6-, but not for VCR/BV6-induced apoptosis, since transient or stable knockdown of RIP1 or the pharmacological RIP1 inhibitor necrostatin-1 significantly reduce apoptosis. By comparison, VCR/BV6-mediated apoptosis critically depends on the mitochondrial pathway. VCR/BV6 cotreatment causes phosphorylation of BCL-2 during mitotic arrest, enhanced activation of BAX and BAK and loss of mitochondrial membrane potential (MMP). Additionally, overexpression of BCL-2 profoundly suppresses VCR/BV6-induced apoptosis. Thus, BV6 sensitizes NB cells to chemotherapy-induced apoptosis via distinct initial signaling mechanisms depending on the chemotherapeutic drug. These findings provide novel mechanistic insights into Smac mimetic-mediated chemosensitization of NB.

Promises and Challenges of Smac Mimetics as Cancer Therapeutics

Inhibitor of Apoptosis (IAP) proteins block programmed cell death and are expressed at high levels in various human cancers, thus making them attractive targets for cancer drug development. Second mitochondrial activator of caspases (Smac) mimetics are small-molecule inhibitors that mimic Smac, an endogenous antagonist of IAP proteins. Preclinical studies have shown that Smac mimetics can directly trigger cancer cell death or, even more importantly, sensitize tumor cells for various cytotoxic therapies, including conventional chemotherapy, radiotherapy, or novel agents. Currently, several Smac mimetics are under evaluation in early clinical trials as monotherapy or in rational combinations (i.e., GDC-0917/CUDC-427, LCL161, AT-406/Debio1143, HGS1029, and TL32711/birinapant). This review discusses the promise as well as some challenges at the translational interface of exploiting Smac mimetics as cancer therapeutics.

Inhibitor of apoptosis protein expression in glioblastomas and their in vitro and in vivo targeting by SMAC mimetic GDC-0152

Glioblastomas (GBMs) are the most aggressive primary brain tumors in adult and remain a therapeutic challenge. Targeting key apoptosis regulators with the ultimate aim to restore apoptosis in tumor cells could be an interesting therapeutic strategy. The inhibitors of apoptosis proteins (IAPs) are regulators of cell death and represent attractive targets, especially because they can be antagonized by SMAC mimetics. In this study, we first investigated the expression of cIAP1, cIAP2, XIAP and ML-IAP in human GBM samples and in four different cell lines. We showed that all GBM samples and GBM cell lines expressed all these IAPs, although the expression of each IAP varied from one case to another. We then showed that high level of ML-IAP predicted worse progression-free survival and overall survival in both univariate and multivariate analyses in two independent cohorts of 58 and 43 primary human GBMs. We then used GDC-0152, a SMAC mimetic that antagonizes these IAPs and confirmed that GDC-0152 treatment in vitro decreased IAPs in all the cell lines studied. It affected cell line viability and triggered apoptosis, although the effect was higher in U87MG and GL261 than in GBM6 and GBM9 cell lines. In vivo, GDC-0152 effect on U87MG orthotopic xenografts was dose dependent; it postponed tumor formation and slowed down tumor growth, significantly improving survival of GBM-bearing mice. This study revealed for the first time that ML-IAP protein expression correlates with GBM patient survival and that its antagonist GDC-0152 improves outcome in xenografted mouse.

Smac Mimetic-Induced Upregulation of CCL2/MCP-1 Triggers Migration and Invasion of Glioblastoma Cells and Influences the Tumor Microenvironment in a Paracrine Manner

Second mitochondria-derived activator of caspase (Smac) mimetics are considered as promising anticancer therapeutics that are currently under investigation in early clinical trials. They induce apoptosis by antagonizing inhibitor of apoptosis proteins, which are frequently overexpressed in cancer. We previously reported that Smac mimetics, such as BV6, additionally exert non-apoptotic functions in glioblastoma (GBM) cells by stimulating migration and invasion in a nuclear factor kappa B (NF-κB)-dependent manner. Because NF-κB target genes mediating these effects are largely unknown, we performed whole-genome expression analyses. Here, we identify chemokine (C-C motif) ligand 2 (CCL2) as the top-listed NF-κB-regulated gene being upregulated upon BV6 treatment in GBM cells. BV6-induced upregulation and secretion of CCL2 are required for migration and invasion of GBM cells because knockdown of CCL2 in GBM cells abolishes these effects. Co-culture experiments of GBM cells with non-malignant astroglial cells reveal that BV6-stimulated secretion of CCL2 by GBM cells into the supernatant triggers migration of astroglial cells toward GBM cells because CCL2 knockdown in BV6-treated GBM cells impedes BV6-stimulated migration of astroglial cells. In conclusion, we identify CCL2 as a BV6-induced NF-κB target gene that triggers migration and invasion of GBM cells and exerts paracrine effects on the GBM's microenvironment by stimulating migration of astroglial cells. These findings provide novel insights into the biological functions of Smac mimetics with important implications for the development of Smac mimetics as cancer therapeutics.

PKC activation sensitizes basal-like breast cancer cell lines to Smac mimetics

There is a need for novel strategies to initiate cancer cell death. One approach is the use of Smac mimetics, which antagonize inhibitor of apoptosis proteins (IAPs). Recent studies have shown that combinations of Smac mimetics such as LBW242 or LCL161 in combination with chemotherapeutic agents increase cancer cell death. Here we show that the protein kinase C (PKC) activator TPA together with the Smac mimetic LBW242 induces cell death in two basal breast cancer cell lines (MDA-MB-468 and BT-549) that are resistant to Smac mimetic as single agent. Ten other LBW242-insensitive cancer cell lines were not influenced by the TPA+LBW242 combination. The TPA+LBW242 effect was suppressed by the PKC inhibitor GF109203X, indicating dependence on PKC enzymatic activity. The PKC effect was mediated via increased synthesis and release of TNFα, which can induce death in the presence of Smac mimetics. The cell death, coinciding with caspase-3 cleavage, was suppressed by caspase inhibition and preceded by the association of RIP1 with caspase-8, as seen in complex II formation. Smac mimetics, but not TPA, induced the non-canonical NF-κB pathway in both MDA-MB-231 and MDA-MB-468 cells. Blocking the canonical NF-κB pathway suppressed TPA induction of TNFα in MDA-MB-468 cells whereas isolated downregulation of either the canonical or non-canonical pathways did not abolish the Smac mimetic induction of the NF-κB driven genes TNFα and BIRC3 in MDA-MB-231 cells although the absolute levels were suppressed. A combined downregulation of the canonical and non-canonical pathways further suppressed TNFα levels and inhibited Smac mimetic-mediated cell death. Our data suggest that in certain basal breast cancer cell lines co-treatment of TPA with a Smac mimetic induces cell death highlighting the potential of using these pathways as molecular targets for basal-like breast cancers.

BIRC3 is a novel driver of therapeutic resistance in Glioblastoma

Genome-wide analysis of glioblastoma (GBM) reveals pervasive aberrations in apoptotic signaling pathways that collectively contribute to therapeutic resistance. Inhibitors of apoptosis proteins (IAP) exert critical control on the terminal segment of apoptosis leading to apoptosis evasion. In this study, we uncover a unique role for BIRC3, as an IAP that is critical in GBM in response to therapy. Using the TCGA dataset of 524 unique samples, we identify BIRC3 is the only IAP whose differential expression is associated with long-term survival in GBM patients. Using patient tissue samples we further show that BIRC3 expression increases with recurrence. When extrapolated to a preclinical model of a human GBM cell line, we find an increase in BIRC3 expression in response to irradiation (RT) and temozolomide (TMZ) treatment. More importantly, we mechanistically implicate STAT3 and PI3K signaling pathways as drivers of RT-induced up-regulation of BIRC3 expression. Lastly, we demonstrate that both in-vivo and in-vitro BIRC3 up-regulation results in apoptosis evasion and therapeutic resistance in GBM. Collectively, our study identifies a novel translational and targetable role for BIRC3 expression as a predictor of aggressiveness and therapeutic resistance to TMZ and RT mediated by STAT3 and PI3K signaling in GBM.

Overexpression of hSNF2H in glioma promotes cell proliferation, invasion, and chemoresistance through its interaction with Rsf-1

hSNF2H partners with Rsf-1 to compose the Rsf complex to regulate gene expression. Recent studies indicated that hSNF2H was overexpressed in several human cancers. However, its expression pattern and biological mechanism in glioma remain unexplored. In this study, we found that hSNF2H was overexpressed in 32 % of glioma specimens. hSNF2H overexpression correlated with advanced tumor grade (p = 0.0338) and Rsf-1 positivity in glioma tissues (p = 0.016). Small interfering RNA (siRNA) knockdown was performed in A172 and U87 cell lines. MTT, colony formation assay, and cell cycle analysis showed that knockdown of hSNF2H inhibited cell proliferation, colony formation ability, and cell cycle transition. Matrigel invasion assay showed that hSNF2H depletion inhibited invasive ability of glioma cells. In addition, we demonstrated that hSNF2H depletion decreased temozolomide resistance of A172 and U87 cell lines and increased temozolomide induced apoptosis. Furthermore, hSNF2H depletion decreased cyclin D1, cyclin E, p-Rb, MMP2, cIAP1, Bcl-2 expression, and phosphorylation of IκBα and p65, suggesting hSNF2H regulates apoptosis through NF-κB pathway. Immunoprecipitation showed that hSNF2H could interact with Rsf-1 in both cell lines. To validate the involvement of Rsf-1, we checked the change of its downstream targets in Rsf-1 depleted cells. In Rsf-1 depleted cells, changes of cyclin E, Bcl-2, and p-IκBα were not significant using hSNF2H siRNA treatment. In conclusion, our study demonstrated that hSNF2H was overexpressed in human gliomas and contributed to glioma proliferation, invasion, and chemoresistance through regulation of cyclin E and NF-κB pathway, which is dependent on its interaction with Rsf-1.

Patient-derived glioblastoma cells show significant heterogeneity in treatment responses to the inhibitor-of-apoptosis-protein antagonist birinapant

Background:

Resistance to temozolomide (TMZ) greatly limits chemotherapeutic effectiveness in glioblastoma (GBM). Here we analysed the ability of the Inhibitor-of-apoptosis-protein (IAP) antagonist birinapant to enhance treatment responses to TMZ in both commercially available and patient-derived GBM cells.

Methods:

Responses to TMZ and birinapant were analysed in a panel of commercial and patient-derived GBM cell lines using colorimetric viability assays, flow cytometry, morphological analysis and protein expression profiling of pro- and antiapoptotic proteins. Responses in vivo were analysed in an orthotopic xenograft GBM model.

Results:

Single-agent treatment experiments categorised GBM cells into TMZ-sensitive cells, birinapant-sensitive cells, and cells that were insensitive to either treatment. Combination treatment allowed sensitisation to therapy in only a subset of resistant GBM cells. Cell death analysis identified three principal response patterns: Type A cells that readily activated caspase-8 and cell death in response to TMZ while addition of birinapant further sensitised the cells to TMZ-induced cell death; Type B cells that readily activated caspase-8 and cell death in response to birinapant but did not show further sensitisation with TMZ; and Type C cells that showed no significant cell death or moderately enhanced cell death in the combined treatment paradigm. Furthermore, in vivo, a Type C patient-derived cell line that was TMZ-insensitive in vitro and showed a strong sensitivity to TMZ and TMZ plus birinapant treatments.

Conclusions:

Our results demonstrate remarkable differences in responses of patient-derived GBM cells to birinapant single and combination treatments, and suggest that therapeutic responses in vivo may be greatly affected by the tumour microenvironment.