Smac mimetic-induced upregulation of interferon-β sensitizes glioblastoma to temozolomide-induced cell death

Cell death in glioblastoma and the central nervous system

Glioblastoma is the commonest and deadliest primary brain tumor. Glioblastoma is characterized by significant intra- and inter-tumoral heterogeneity, resistance to treatment and dismal prognoses despite decades of research in understanding its biological underpinnings. Encompassed within this heterogeneity and therapy resistance are severely dysregulated programmed cell death pathways. Glioblastomas recapitulate many neurodevelopmental and neural injury responses; in addition, glioblastoma cells are composed of multiple different transformed versions of CNS cell types. To obtain a greater understanding of the features underlying cell death regulation in glioblastoma, it is important to understand the control of cell death within the healthy CNS during homeostatic and neurodegenerative conditions. Herein, we review apoptotic control within neural stem cells, astrocytes, oligodendrocytes and neurons and compare them to glioblastoma apoptotic control. Specific focus is paid to the Inhibitor of Apoptosis proteins, which play key roles in neuroinflammation, CNS cell survival and gliomagenesis. This review will help in understanding glioblastoma as a transformed version of a heterogeneous organ composed of multiple varied cell types performing different functions and possessing different means of apoptotic control. Further, this review will help in developing more glioblastoma-specific treatment approaches and will better inform treatments looking at more direct brain delivery of therapeutic agents.

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.

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.

The Smac mimetic BV6 cooperates with STING to induce necroptosis in apoptosis-resistant pancreatic carcinoma cells

Pancreatic cancer (PC) still remains a major cause of cancer-related death worldwide and alternative treatments are urgently required. A common problem of PC is the development of resistance against apoptosis that limits therapeutic success. Here we demonstrate that the prototypical Smac mimetic BV6 cooperates with the stimulator of interferon (IFN) genes (STING) ligand 2',3'-cyclic guanosine monophosphate-adenosine monophosphate (2'3'-cGAMP) to trigger necroptosis in apoptosis-deficient PC cells. Pharmacological inhibition of key components of necroptosis signaling, such as receptor-interacting protein 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL), significantly rescues PC cells from 2'3'-cGAMP/BV6/zVAD.fmk-mediated cell death, suggesting the induction of necroptosis. Consistently, 2'3'-cGAMP/BV6 co-treatment promotes phosphorylation of MLKL. Furthermore, we show that 2'3'-cGAMP stimulates the production of type I IFNs, which cooperate with BV6 to trigger necroptosis in apoptosis-deficient settings. STING silencing via siRNA or CRISPR/Cas9-mediated gene knockout protects PC cells from 2'3'-cGAMP/BV6/zVAD.fmk-mediated cell death. Interestingly, we demonstrate that nuclear factor-κB (NF-κB), tumor necrosis factor-α (TNFα), and IFN-regulatory factor 1 (IRF1) signaling are involved in triggering 2'3'-cGAMP/BV6/zVAD.fmk-induced necroptosis. In conclusion, we show that activated STING and BV6 act together to exert antitumor effects on PC cells with important implications for the design of new PC treatment concepts.

Targeting c-IAP1, c-IAP2, and Bcl-2 Eliminates Senescent Glioblastoma Cells Following Temozolomide Treatment

Simple Summary

Despite extensive research, malignant glioma remains the most aggressive and fatal type of brain tumor. Following resection, therapy is based on radiation concomitant with the methylating agent temozolomide (TMZ), followed by adjuvant high-dose TMZ. In previous work, we showed that following TMZ exposure, most glioma cells evade apoptosis and enter a senescent state and are thereby protected against anticancer therapy. Senescent cells may escape from senescence, contributing to the formation of recurrences or can induce the senescence-associated secretory phenotype (SASP), which may impact therapy success. Therefore, direct targeting of senescent cells might be favorable to improve the effect of TMZ-based anticancer therapy. Here we show that during TMZ-induced senescence in glioblastoma cells, the antiapoptotic factors c-IAP2 and Bcl-2 are responsible for the prevention of cell death and that inhibition of these factors by BV6 and venetoclax effectively kills senescent glioblastoma cells.

Abstract

Therapy of malignant glioma depends on the induction of O⁶-methylguanine by the methylating agent temozolomide (TMZ). However, following TMZ exposure, most glioma cells evade apoptosis and become senescent and are thereby protected against further anticancer therapy. This protection is thought to be dependent on the senescent cell anti-apoptotic pathway (SCAP). Here we analyzed the factors involved in the SCAP upon exposure to TMZ in glioblastoma cell lines (LN-229, A172, U87MG) and examined whether inhibition of these factors could enhance TMZ-based toxicity by targeting senescent cells. We observed that following TMZ treatment, c-IAP2 and Bcl-2 were upregulated. Inhibition of these SCAP factors using non-toxic concentrations of the small molecule inhibitors, BV6 and venetoclax, significantly increased cell death, as measured 144 h after TMZ exposure. Most importantly, BV6 and venetoclax treatment of senescent cells strongly increased cell death after an additional 120 h. Moreover, Combenefit analyses revealed a significant synergy combining BV6 and venetoclax. In contrast to BV6 and venetoclax, AT406, embelin, and TMZ itself, teniposide and the PARP inhibitor pamiparib did not increase cell death in senescent cells. Based on these data, we suggest that BV6 and venetoclax act as senolytic agents in glioblastoma cells upon TMZ exposure.

Promising Anti-Mitochondrial Agents for Overcoming Acquired Drug Resistance in Multiple Myeloma

Multiple myeloma (MM) remains an incurable tumor due to the high rate of relapse that still occurs. Acquired drug resistance represents the most challenging obstacle to the extension of survival and several studies have been conducted to understand the mechanisms of this phenomenon. Mitochondrial pathways have been extensively investigated, demonstrating that cancer cells become resistant to drugs by reprogramming their metabolic assessment. MM cells acquire resistance to proteasome inhibitors (PIs), activating protection programs, such as a reduction in oxidative stress, down-regulating pro-apoptotic, and up-regulating anti-apoptotic signals. Knowledge of the mechanisms through which tumor cells escape control of the immune system and acquire resistance to drugs has led to the creation of new compounds that can restore the response by leading to cell death. In this scenario, based on all literature data available, our review represents the first collection of anti-mitochondrial compounds able to overcome drug resistance in MM. Caspase-independent mechanisms, mainly based on increased oxidative stress, result from 2-methoxyestradiol, Artesunate, ascorbic acid, Dihydroartemisinin, Evodiamine, b-AP15, VLX1570, Erw-ASNase, and TAK-242. Other agents restore PIs' efficacy through caspase-dependent tools, such as CDDO-Im, NOXA-inhibitors, FTY720, GCS-100, LBH589, a derivative of ellipticine, AT-101, KD5170, SMAC-mimetics, glutaminase-1 (GLS1)-inhibitors, and thenoyltrifluoroacetone. Each of these substances improved the efficacy rates when employed in combination with the most frequently used antimyeloma drugs.

Differential involvement of TAK1, RIPK1 and NF-κB signaling in Smac mimetic-induced cell death in breast cancer cells

Smac mimetics (SMs) are considered promising cancer therapeutics. However, the mechanisms responsible for mediating cell death by SMs are still only partly understood. Therefore, in this study, we investigated signaling pathways upon treatment with the bivalent SM BV6 using two SM-sensitive breast cancer cell lines as models. Interestingly, genetic silencing of transforming growth factor (TGF)β activated kinase (TAK)1, an upstream activator of the nuclear factor-kappaB (NF-κB) subunit RelA (p65), increased BV6-induced cell death only in EVSA-T cells, although it reduced BV6-mediated upregulation of tumor necrosis factor (TNF)α in both EVSA-T and MDA-MB-231 cells. By comparison, genetic silencing of p65, a key component of canonical NF-κB signaling, blocked BV6-induced cell death in MDA-MB-231 but not in EVSA-T cells. Similarly, knockdown of NF-κB-inducing kinase (NIK) rescued MDA-MB-231 cells from BV6-induced cell death, while it failed to do so in EVSA-T cells. Consistently, silencing of p65 or NIK reduced BV6-stimulated upregulation of TNFα in MDA-MB-231 cells. In conclusion, TAK1, receptor-interacting kinase 1 (RIPK1) as well as canonical and non-canonical NF-κB signaling are differentially involved in SM-induced cell death in breast cancer cells. These findings contribute to a better understanding of SM-induced signaling pathways.

Cell cycle arrest in mitosis promotes interferon-induced necroptosis

Resistance to apoptosis is a hallmark of cancer and deregulation of apoptosis often leads to chemoresistance. Therefore, new approaches to target apoptosis-resistant cancer cells are crucial for the development of directed cancer therapies. In the present study, we investigated the effect of cell cycle regulators on interferon (IFN)-induced necroptosis as an alternative cell death mechanism to overcome apoptosis resistance. Here, we report a novel combination treatment of IFNs with cell cycle arrest-inducing compounds that induce necroptosis in apoptosis-resistant cancer cells and elucidate the underlying molecular mechanisms. Combination treatment of IFNs (i.e. IFNβ) with inhibitors of the cell cycle (e.g. vinorelbine (VNR), nocodazole (Noc), polo-like kinase-1 (Plk-1) inhibitor BI 6727) co-operate to induce necroptotic cell death upon caspase inactivation. The mode of cell death was confirmed by pharmacological inhibition and siRNA-mediated downregulation of the key necroptotic factors receptor-interacting protein (RIP) kinase 3 (RIP3) and mixed-lineage kinase-like (MLKL) in various cell lines. Mechanistically, we show that necroptosis upon VNR/IFNβ/zVAD.fmk treatment is RIP1-independent but relies on IFNβ-induced gene expression of Z-DNA-binding protein 1 (ZBP1) as shown by quantitative RT-PCR and genetic knockdown experiments. Interestingly, we find that RIP3 is phosphorylated in response to compounds that trigger mitotic arrest, even in the absence of IFNβ signaling and necroptosis induction. Together, the identification of a novel combination treatment that triggers necroptosis has implications for the development of molecular-targeted therapies to circumvent apoptosis resistance and point to an underestimated role of cell cycle regulation in cell death signaling.

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.

MAP30 promotes apoptosis of U251 and U87 cells by suppressing the LGR5 and Wnt/β-catenin signaling pathway, and enhancing Smac expression

Significant antitumor activity of Momordica anti-human immunodeficiency virus protein of 30 kDa (MAP30) purified from Momordica charantia has been the subject of previous research. However, the effective mechanism of MAP30 on malignant glioma cells has not yet been clarified. The aim of the present study was to investigate the effects and mechanism of MAP30 on U87 and U251 cell lines. A Cell Counting Kit-8 assay, wound healing assay and Transwell assay were used to detect the effects on U87 and U251 cells treated with different concentrations of MAP30 (0.5, 1, 2, 4, 8 and 16 µM) over different periods of time. Proliferation, migration and invasion of each cell line were markedly inhibited by MAP30 in a dose- and time-dependent manner. Flow cytometry and fluorescence staining demonstrated that apoptosis increased and the cell cycle was arrested in S-phase in the two investigated cell lines following MAP30 treatment. Western blot analysis demonstrated that leucine-rich-repeat-containing G-protein-coupled receptor 5 (LGR5) expression and key proteins in the Wnt/β-catenin signaling pathway were apparently decreased, whereas second mitochondria-derived activator of caspase (Smac) protein expression significantly increased with MAP30 treatment in the same manner. These results suggest that MAP30 markedly induces apoptosis in U87 and U251 cell lines by suppressing LGR5 and the Wnt/β-catenin signaling pathway, and enhancing Smac expression in a dose- and time-dependent manner.

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.

PD-L1 and IAPs co-operate to protect tumors from cytotoxic lymphocyte-derived TNF

Smac-mimetics are emerging as promising anti-cancer agents and are being evaluated in clinical trials for a variety of malignancies. Smac-mimetics can induce TNF production from a subset of tumor cells and simultaneously sensitize them to TNF-induced apoptosis. However, TNF derived from other cellular sources, such as cytotoxic lymphocytes (CLs) within the tumor, may also contribute to the anti-tumor activity of SMs. Here, we show that CD8⁺ T cells and NK cells potently kill tumor cells in the presence of the SM, birinapant. Enhanced CL killing occurred through TNF secretion upon tumor antigen recognition or NK-activating receptor ligation. Importantly, the perforin/granzyme route to CL-mediated tumor cell killing was dispensable for the efficacy of birinapant, emphasizing the importance of the TNF-mediated apoptosis pathway. Time-lapse microscopy revealed that birinapant sensitized tumor cells to apoptosis as bystanders and to membrane-bound TNF delivered to tumor cells within the immunological synapse. Furthermore, PD-L1 expression on tumor cells suppressed antigen-driven TNF production by CD8⁺ T cells, which could be antagonized through PD-1 blockade. Importantly, the elevated levels of TNF produced upon PD-1 blockade further enhanced tumor cell killing when combined with birinapant. The combined anti-tumor activity of IAP antagonism and PD-1 blockade occurred independently of perforin-mediated tumor cell death. Taken together, we identify CL-derived TNF as a potent effector of birinapant mediated anti-tumor immunity and opportunity for combination therapy through co-inhibition of immune checkpoints.

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.

The SMAC mimetic BV6 induces cell death and sensitizes different cell lines to TNF-α and TRAIL-induced apoptosis

The inhibitors of apoptosis proteins are implicated in promoting cancer cells survival and resistance toward immune surveillance and chemotherapy. Second mitochondria-derived activator of caspases (SMAC) mimetics are novel compounds developed to mimic the inhibitory effect of the endogenous SMAC/DIABLO on these IAPs. Here, we examined the potential effects of the novel SMAC mimetic BV6 on different human cancer cell lines. Our results indicated that BV6 was able to induce cell death in different human cancer cell lines. Mechanistically, BV6 dose dependently induced degradation of IAPs, including cIAP1 and cIAP2. This was coincided with activating the non-canonical NF -kappa B (NF-κB) pathway, as indicated by stabilizing NF-κB-inducing kinase (NIK) for p100 processing to p52. More interestingly, BV6 was able to sensitize some of the resistant cancer cell lines to apoptosis induced by the death ligands tumor necrosis factor-α (TNF-α) and TNF-related apoptosis-inducing ligand (TRAIL) that are produced by different cells of the immune system. Such cell death enhancement was mediated by inducing an additional cleavage of caspase-9 to augment that of caspase-8 induced by death ligands. This eventually led to more processing of the executioner caspase-3 and poly (ADP-ribose) polymerase (PARP). In conclusion, therapeutic targeting of IAPs by BV6 might be an effective approach to enhance cancer regression induced by immune system. Our data also open up the future possibility of using BV6 in combination with other antitumor therapies to overcome cancer drug resistance.