The NOXA-MCL1-BIM axis defines lifespan on extended mitotic arrest

Chronic spindle assembly checkpoint activation causes myelosuppression and gastrointestinal atrophy

Interference with microtubule dynamics in mitosis activates the spindle assembly checkpoint (SAC) to prevent chromosome segregation errors. The SAC induces mitotic arrest by inhibiting the anaphase-promoting complex (APC) via the mitotic checkpoint complex (MCC). The MCC component MAD2 neutralizes the critical APC cofactor, CDC20, preventing exit from mitosis. Extended mitotic arrest can promote mitochondrial apoptosis and caspase activation. However, the impact of mitotic cell death on tissue homeostasis in vivo is ill-defined. By conditional MAD2 overexpression, we observe that chronic SAC activation triggers bone marrow aplasia and intestinal atrophy in mice. While myelosuppression can be compensated for, gastrointestinal atrophy is detrimental. Remarkably, deletion of pro-apoptotic Bim/Bcl2l11 prevents gastrointestinal syndrome, while neither loss of Noxa/Pmaip or co-deletion of Bid and Puma/Bbc3 has such a protective effect, identifying BIM as rate-limiting apoptosis effector in mitotic cell death of the gastrointestinal epithelium. In contrast, only overexpression of anti-apoptotic BCL2, but none of the BH3-only protein deficiencies mentioned above, can mitigate myelosuppression. Our findings highlight tissue and cell-type-specific survival dependencies in response to SAC perturbation in vivo.

Albendazole inhibits colon cancer progression and therapy resistance by targeting ubiquitin ligase RNF20

BACKGROUND

The repurposing of FDA-approved drugs for anti-cancer therapies is appealing due to their established safety profiles and pharmacokinetic properties and can be quickly moved into clinical trials. Cancer progression and resistance to conventional chemotherapy remain the key hurdles in improving the clinical management of colon cancer patients and associated mortality.

METHODS

High-throughput screening (HTS) was performed using an annotated library of 1,600 FDA-approved drugs to identify drugs with strong anti-CRC properties. The candidate drug exhibiting most promising inhibitory effects in in-vitro studies was tested for its efficacy using in-vivo models of CRC progression and chemoresistance and patient derived organoids (PTDOs).

RESULTS

Albendazole, an anti-helminth drug, demonstrated the strongest inhibitory effects on the tumorigenic potentials of CRC cells, xenograft tumor growth and organoids from mice. Also, albendazole sensitized the chemoresistant CRC cells to 5-fluorouracil (5-FU) and oxaliplatin suggesting potential to treat chemoresistant CRC. Mechanistically, Albendazole treatment modulated the expression of RNF20, to promote apoptosis in CRC cells by delaying the G2/M phase and suppressing anti-apoptotic-Bcl2 family transcription.

CONCLUSIONS

Albendazole, an FDA approved drug, carries strong therapeutic potential to treat colon cancers which are aggressive and potentially resistant to conventional chemotherapeutic agents. Our findings also lay the groundwork for further clinical testing.

PBRM1 loss is associated with increased sensitivity to MCL1 and CDK9 inhibition in clear cell renal cancer

MCL1 is a member of the BCL2 family of apoptosis regulators, which play a critical role in promoting cancer survival and drug resistance. We previously described PRT1419, a potent, MCL1 inhibitor with anti-tumor efficacy in various solid and hematologic malignancies. To identify novel biomarkers that predict sensitivity to MCL1 inhibition, we conducted a gene essentiality analysis using gene dependency data generated from CRISPR/Cas9 cell viability screens. We observed that clear cell renal cancer (ccRCC) cell lines with damaging PBRM1 mutations displayed a strong dependency on MCL1. PBRM1 (BAF180), is a chromatin-targeting subunit of mammalian pBAF complexes. PBRM1 is frequently altered in various cancers particularly ccRCC with ~40% of tumors harboring damaging PBRM1 alterations. We observed potent inhibition of tumor growth and induction of apoptosis by PRT1419 in various preclinical models of PBRM1-mutant ccRCC but not PBRM1-WT. Depletion of PBRM1 in PBRM1-WT ccRCC cell lines induced sensitivity to PRT1419. Mechanistically, PBRM1 depletion coincided with increased expression of pro-apoptotic factors, priming cells for caspase-mediated apoptosis following MCL1 inhibition. Increased MCL1 activity has been described as a resistance mechanism to Sunitinib and Everolimus, two approved agents for ccRCC. PRT1419 synergized with both agents to potently inhibit tumor growth in PBRM1-loss ccRCC. PRT2527, a potent CDK9 inhibitor which depletes MCL1, was similarly efficacious in monotherapy and in combination with Sunitinib in PBRM1-loss cells. Taken together, these findings suggest PBRM1 loss is associated with MCL1i sensitivity in ccRCC and provide rationale for the evaluation of PRT1419 and PRT2527 for the treatment for PBRM1-deficient ccRCC.

Systematic characterization of therapeutic vulnerabilities in Multiple Myeloma with Amp1q reveals increased sensitivity to the combination of MCL1 and PI3K inhibitors

The development of targeted therapy for patients with Multiple Myeloma (MM) is hampered by the low frequency of actionable genetic abnormalities. Gain or amplification of chr1q (Amp1q) is the most frequent arm-level copy number gain in patients with MM, and it is associated with higher risk of progression and death despite recent advances in therapeutics. Thus, developing targeted therapy for patients with MM and Amp1q stands to benefit a large portion of patients in need of more effective management. Here, we employed large-scale dependency screens and drug screens to systematically characterize the therapeutic vulnerabilities of MM with Amp1q and showed increased sensitivity to the combination of MCL1 and PI3K inhibitors. Using single-cell RNA sequencing, we compared subclones with and without Amp1q within the same patient tumors and showed that Amp1q is associated with higher levels of MCL1 and the PI3K pathway. Furthermore, by isolating isogenic clones with different copy number for part of the chr1q arm, we showed increased sensitivity to MCL1 and PI3K inhibitors with arm-level gain. Lastly, we demonstrated synergy between MCL1 and PI3K inhibitors and dissected their mechanism of action in MM with Amp1q.

Drug-repurposing screen on patient-derived organoids identifies therapy-induced vulnerability in KRAS-mutant colon cancer

Patient-derived organoids (PDOs) are widely heralded as a drug-screening platform to develop new anti-cancer therapies. Here, we use a drug-repurposing library to screen PDOs of colorectal cancer (CRC) to identify hidden vulnerabilities within therapy-induced phenotypes. Using a microscopy-based screen that accurately scores drug-induced cell killing, we have tested 414 putative anti-cancer drugs for their ability to switch the EGFRi/MEKi-induced cytostatic phenotype toward cytotoxicity. A majority of validated hits (9/37) are microtubule-targeting agents that are commonly used in clinical oncology, such as taxanes and vinca-alkaloids. One of these drugs, vinorelbine, is consistently effective across a panel of >25 different CRC PDOs, independent of RAS mutational status. Unlike vinorelbine alone, its combination with EGFR/MEK inhibition induces apoptosis at all stages of the cell cycle and shows tolerability and effective anti-tumor activity in vivo, setting the basis for a clinical trial to treat patients with metastatic RAS-mutant CRC.

MARCH5 regulates mitotic apoptosis through MCL1-dependent and independent mechanisms

The anti-apoptotic MCL1 is critical for delaying apoptosis during mitotic arrest. MCL1 is degraded progressively during mitotic arrest, removing its anti-apoptotic function. We found that knockout of components of ubiquitin ligases including APC/C, SCF complexes, and the mitochondrial ubiquitin ligase MARCH5 did not prevent mitotic degradation of MCL1. Nevertheless, MARCH5 determined the initial level of MCL1-NOXA network upon mitotic entry and hence the window of time during MCL1 was present during mitotic arrest. Paradoxically, although knockout of MARCH5 elevated mitotic MCL1, mitotic apoptosis was in fact enhanced in a BAK-dependent manner. Mitotic apoptosis was accelerated after MARCH5 was ablated in both the presence and absence of MCL1. Cell death was not altered after disrupting other MARCH5-regulated BCL2 family members including NOXA, BIM, and BID. Disruption of the mitochondrial fission factor DRP1, however, reduced mitotic apoptosis in MARCH5-disrupted cells. These data suggest that MARCH5 regulates mitotic apoptosis through MCL1-independent mechanisms including mitochondrial maintenance that can overcome the stabilization of MCL1.

Apoptosis as a Barrier against CIN and Aneuploidy

Aneuploidy is the gain or loss of entire chromosomes, chromosome arms or fragments. Over 100 years ago, aneuploidy was described to be a feature of cancer and is now known to be present in 68-90% of malignancies. Aneuploidy promotes cancer growth, reduces therapy response and frequently worsens prognosis. Chromosomal instability (CIN) is recognized as the main cause of aneuploidy. CIN itself is a dynamic but stochastic process consisting of different DNA content-altering events. These can include impaired replication fidelity and insufficient clearance of DNA damage as well as chromosomal mis-segregation, micronuclei formation, chromothripsis or cytokinesis failure. All these events can disembogue in segmental, structural and numerical chromosome alterations. While low levels of CIN can foster malignant disease, high levels frequently trigger cell death, which supports the "aneuploidy paradox" that refers to the intrinsically negative impact of a highly aberrant karyotype on cellular fitness. Here, we review how the cellular response to CIN and aneuploidy can drive the clearance of karyotypically unstable cells through the induction of apoptosis. Furthermore, we discuss the different modes of p53 activation triggered in response to mitotic perturbations that can potentially trigger CIN and/or aneuploidy.

BCL-xL inhibition potentiates cancer therapies by redirecting the outcome of p53 activation from senescence to apoptosis

Cancer therapies trigger diverse cellular responses, ranging from apoptotic death to acquisition of persistent therapy-refractory states such as senescence. Tipping the balance toward apoptosis could improve treatment outcomes regardless of therapeutic agent or malignancy. We find that inhibition of the mitochondrial protein BCL-xL increases the propensity of cancer cells to die after treatment with a broad array of oncology drugs, including mitotic inhibitors and chemotherapy. Functional precision oncology and omics analyses suggest that BCL-xL inhibition redirects the outcome of p53 transcriptional response from senescence to apoptosis, which likely occurs via caspase-dependent down-modulation of p21 and downstream cytostatic proteins. Consequently, addition of a BCL-2/xL inhibitor strongly improves melanoma response to the senescence-inducing drug targeting mitotic kinase Aurora kinase A (AURKA) in mice and patient-derived organoids. This study shows a crosstalk between the mitochondrial apoptotic pathway and cell cycle regulation that can be targeted to augment therapeutic efficacy in cancers with wild-type p53.

Bcl-xL activity influences outcome of the mitotic arrest

Microtubule-targeting (MT) drugs taxanes and vinca alkaloids are widely used as chemotherapeutic agents against different tumors for more than 30 years because of their ability to block mitotic progression by disrupting the mitotic spindle and activating the spindle assembly checkpoint (SAC) for a prolonged period of time. However, responses to mitotic arrest are different—some cells die during mitotic arrest, whereas others undergo mitotic slippage and survive becoming able for proliferation. Using normal fibroblasts and several cancer cell types we determined two critical doses, T1 and T2, of mitotic inhibitors (nocodazole, Taxol, and vinorelbine). T1 is the maximal dose cells can tolerate undergoing normal division, and T2 is the minimal mitostatic dose, wherein > 90% of mitotic cells are arrested in mitosis. In all studied cell lines after treatment with mitotic inhibitors in a dose above T2 cells had entered mitosis either die or undergo mitotic slippage. We show that for all three drugs used cell death during mitotic arrest and after slippage proceeded via mitochondria-dependent apoptosis. We determined two types of cancer cells: sensitive to mitotic arrest, that is, undergoing death in mitosis (DiM) frequently, and resistant to mitotic arrest, that is, undergoing mitotic slippage followed by prolonged survival. We then determined that inhibition of Bcl-xL, but not other anti-apoptotic proteins of the Bcl-2 group that regulate MOMP, make resistant cells susceptible to DiM induced by mitotic inhibitors. Combined treatment with MT drugs and highly specific Bcl-xL inhibitors A-1155643 or A-1331852 allows achieving 100% DiM in a time significantly shorter than maximal duration of mitotic arrest in all types of cultured cells tested. We further examined efficacy of sequential treatment of cultured cells using mitotic inhibitors followed by inhibitors of Bcl-xL anti-apoptotic protein and for the first time show that sensitivity to Bcl-xL inhibitors rapidly declines after mitotic slippage. Thus sequential use of mitotic inhibitors and inhibitors of Bcl-xL anti-apoptotic protein will be efficient only if the Bcl-xL inhibitor will be added before mitotic slippage occurs or soon afterward. The combined treatment proposed might be an efficient approach to anti-cancer therapy.

New Therapeutic Strategies for Adult Acute Myeloid Leukemia

Simple Summary

For almost 40 years, the combination of anthracyclines and cytarabine, called 3 + 7, has been the standard of induction chemotherapy for the treatment of acute myeloid leukemia (AML). However, with the advent of new drugs in recent years, it has become possible to improve the prognosis of patients with AML harboring certain genetic mutations. Additionally, immunotherapies and therapies targeting cell-surface antigens, which are highly expressed in AML, are emerging. Herein, we review new therapeutic strategies for AML that are evolving with the introduction of these drugs.

Abstract

Acute myeloid leukemia (AML) is a genetically heterogeneous hematological malignancy. Chromosomal and genetic analyses are important for the diagnosis and prognosis of AML. Some patients experience relapse or have refractory disease, despite conventional cytotoxic chemotherapies and allogeneic transplantation, and a variety of new agents and treatment strategies have emerged. After over 20 years during which no new drugs became available for the treatment of AML, the CD33-targeting antibody–drug conjugate gemtuzumab ozogamicin was developed. This is currently used in combination with standard chemotherapy or as a single agent. CPX-351, a liposomal formulation containing daunorubicin and cytarabine, has become one of the standard treatments for secondary AML in the elderly. FMS-like tyrosine kinase 3 (FLT3) inhibitors and isocitrate dehydrogenase 1/2 (IDH 1/2) inhibitors are mainly used for AML patients with actionable mutations. In addition to hypomethylating agents and venetoclax, a B-cell lymphoma-2 inhibitor is used in frail patients with newly diagnosed AML. Recently, tumor protein p53 inhibitors, cyclin-dependent kinase inhibitors, and NEDD8 E1-activating enzyme inhibitors have been gaining attention, and a suitable strategy for the use of these drugs is required. Antibody drugs targeting cell-surface markers and immunotherapies, such as antibody–drug conjugates and chimeric antigen receptor T-cell therapy, have also been developed for AML.

MEK and MCL-1 sequential inhibition synergize to enhance rhabdomyosarcoma treatment

Targeted agents have emerged as promising molecules for cancer treatment, but most of them fail to achieve complete tumor regression or attain durable remissions due to tumor adaptations. We used dynamic BH3 profiling to identify targeted agents effectiveness and anti-apoptotic adaptations upon targeted treatment in rhabdomyosarcoma. We focused on studying the use of BH3 mimetics to specifically inhibit pro-survival BCL-2 family proteins, overwhelm resistance to therapy and prevent relapse. We observed that the MEK1/2 inhibitor trametinib rapidly depleted the pro-apoptotic protein NOXA, thus increasing MCL-1 availability. Indeed, we found that the MCL-1 inhibitor S63845 synergistically enhanced trametinib cytotoxicity in rhabdomyosarcoma cells in vitro and in vivo. In conclusion, our findings indicate that the combination of a BH3 mimetic targeting MCL-1 with trametinib improves efficiency on rhabdomyosarcoma by blocking tumor adaptation to treatment.

Dual mTORC1/2 inhibition compromises cell defenses against exogenous stress potentiating Obatoclax-induced cytotoxicity in atypical teratoid/rhabdoid tumors

Atypical teratoid/rhabdoid tumors (AT/RT) are the most common malignant brain tumors of infancy and have a dismal 4-year event-free survival (EFS) of 37%. We have previously shown that mTOR activation contributes to AT/RT’s aggressive growth and poor survival. Targeting the mTOR pathway with the dual mTORC1/2 inhibitor TAK-228 slows tumor growth and extends survival in mice bearing orthotopic xenografts. However, responses are primarily cytostatic with limited durability. The aim of this study is to understand the impact of mTOR inhibitors on AT/RT signaling pathways and design a rational combination therapy to drive a more durable response to this promising therapy. We performed RNASeq, gene expression studies, and protein analyses to identify pathways disrupted by TAK-228. We find that TAK-228 decreases the expression of the transcription factor NRF2 and compromises AT/RT cellular defenses against oxidative stress and apoptosis. The BH3 mimetic, Obatoclax, is a potent inducer of oxidative stress and apoptosis in AT/RT. These complementary mechanisms of action drive extensive synergies between TAK-228 and Obatoclax slowing AT/RT cell growth and inducing apoptosis and cell death. Combination therapy activates the integrative stress response as determined by increased expression of phosphorylated EIF2α, ATF4, and CHOP, and disrupts the protective NOXA.MCL-1.BIM axis, forcing stressed cells to undergo apoptosis. Combination therapy is well tolerated in mice bearing orthotopic xenografts of AT/RT, slows tumor growth, and extends median overall survival. This novel combination therapy could be added to standard upfront therapies or used as a salvage therapy for relapsed disease to improve outcomes in AT/RT.

Cell cycle progression and transmitotic apoptosis resistance promote escape from extrinsic apoptosis

Extrinsic apoptosis relies on TNF-family receptor activation by immune cells or receptor-activating drugs. Here, we monitored cell cycle progression at a resolution of minutes to relate apoptosis kinetics and cell-to-cell heterogeneities in death decisions to cell cycle phases. Interestingly, we found that cells in S phase delay TRAIL receptor-induced death in favour of mitosis, thereby passing on an apoptosis-primed state to their offspring. This translates into two distinct fates, apoptosis execution post mitosis or cell survival from inefficient apoptosis. Transmitotic resistance is linked to Mcl-1 upregulation and its increased accumulation at mitochondria from mid-S phase onwards, which allows cells to pass through mitosis with activated caspase-8, and with cells escaping apoptosis after mitosis sustaining sublethal DNA damage. Antagonizing Mcl-1 suppresses cell cycle-dependent delays in apoptosis, prevents apoptosis-resistant progression through mitosis and averts unwanted survival after apoptosis induction. Cell cycle progression therefore modulates signal transduction during extrinsic apoptosis, with Mcl-1 governing decision making between death, proliferation and survival. Cell cycle progression thus is a crucial process from which cell-to-cell heterogeneities in fates and treatment outcomes emerge in isogenic cell populations during extrinsic apoptosis. This article has an associated First Person interview with the first author of the paper.

The BH3-only protein NOXA serves as an independent predictor of breast cancer patient survival and defines susceptibility to microtubule targeting agents

Breast cancer (BC) treatment frequently involves microtubule-targeting agents (MTAs), such as paclitaxel, that arrest cells in mitosis. Sensitivity to MTAs is defined by a subset of pro- and anti-apoptotic BCL2 family proteins controlling mitochondrial apoptosis. Here, we aimed to determine their prognostic value in primary tumour samples from 92 BC patients. Our analysis identified high NOXA/PMAIP mRNA expression levels as an independent prognostic marker for improved relapse-free survival (RFS) and overall survival (OS) in multivariate analysis in BC patients, independent of their molecular subtype. Analysis of available TCGA datasets of 1060 BC patients confirmed our results and added a clear predictive value of NOXA mRNA levels for patients who received MTA-based therapy. In this TCGA cohort, 122 patients received MTA-treatment and high NOXA mRNA levels correlated with their progression-free interval (PFI) and OS. Our follow-up analyses in a panel of BC cell lines of different molecular subtypes identified NOXA protein expression as a key determinant of paclitaxel sensitivity in triple-negative breast cancer (TNBC) cells. Moreover, we noted highest additive effects between paclitaxel and chemical inhibition of BCLX, but not BCL2 or MCL1, documenting dependence of TNBC cells on BCLX for survival and paclitaxel sensitivity defined by NOXA expression levels.

Do Lipid-based Nanoparticles Hold Promise for Advancing the Clinical Translation of Anticancer Alkaloids?

Since the commercialization of morphine in 1826, numerous alkaloids have been isolated and exploited effectively for the betterment of mankind, including cancer treatment. However, the commercialization of alkaloids as anticancer agents has generally been limited by serious side effects due to their lack of specificity to cancer cells, indiscriminate tissue distribution and toxic formulation excipients. Lipid-based nanoparticles represent the most effective drug delivery system concerning clinical translation owing to their unique, appealing characteristics for drug delivery. To the extent of our knowledge, this is the first review to compile in vitro and in vivo evidence of encapsulating anticancer alkaloids in lipid-based nanoparticles. Alkaloids encapsulated in lipid-based nanoparticles have generally displayed enhanced in vitro cytotoxicity and an improved in vivo efficacy and toxicity profile than free alkaloids in various cancers. Encapsulated alkaloids also demonstrated the ability to overcome multidrug resistance in vitro and in vivo. These findings support the broad application of lipid-based nanoparticles to encapsulate anticancer alkaloids and facilitate their clinical translation. The review then discusses several limitations of the studies analyzed, particularly the discrepancies in reporting the pharmacokinetics, biodistribution and toxicity data. Finally, we conclude with examples of clinically successful encapsulated alkaloids that have received regulatory approval and are undergoing clinical evaluation.

The multiple mechanisms of MCL1 in the regulation of cell fate

MCL1 (myeloid cell leukemia-1) is a widely recognized pro-survival member of the Bcl-2 (B-cell lymphoma protein 2) family and a promising target for cancer therapy. While the role MCL1 plays in apoptosis is well defined, its participation in emerging non-apoptotic signaling pathways is only beginning to be appreciated. Here, we synthesize studies characterizing MCL1s influence on cell proliferation, DNA damage response, autophagy, calcium handling, and mitochondrial quality control to highlight the broader scope that MCL1 plays in cellular homeostasis regulation. Throughout this review, we discuss which pathways are likely to be impacted by emerging MCL1 inhibitors, as well as highlight non-cancerous disease states that could deploy Bcl-2 homology 3 (BH3)-mimetics in the future.

In this review Widden and Placzek synthesize studies characterizing the influence that myeloid cell leukemia-1 (MCL1) has on cell proliferation, DNA damage response, autophagy, calcium handling, and mitochondrial quality control to highlight the broader scope that it plays in cellular homeostasis regulation. They discuss which pathways are likely to be impacted by emerging MCL1 inhibitors, as well as highlight non-cancerous disease states that could deploy BH3-mimetics in the future.

Transcriptional CDK inhibitors, CYC065 and THZ1 promote Bim-dependent apoptosis in primary and recurrent GBM through cell cycle arrest and Mcl-1 downregulation

Activation of cyclin-dependent kinases (CDKs) contributes to the uncontrolled proliferation of tumour cells. Genomic alterations that lead to the constitutive activation or overexpression of CDKs can support tumourigenesis including glioblastoma (GBM), the most common and aggressive primary brain tumour in adults. The incurability of GBM highlights the need to discover novel and more effective treatment options. Since CDKs 2, 7 and 9 were found to be overexpressed in GBM, we tested the therapeutic efficacy of two CDK inhibitors (CKIs) (CYC065 and THZ1) in a heterogeneous panel of GBM patient-derived cell lines (PDCLs) cultured as gliomaspheres, as preclinically relevant models. CYC065 and THZ1 treatments suppressed invasion and induced viability loss in the majority of gliomaspheres, irrespective of the mutational background of the GBM cases, but spared primary cortical neurons. Viability loss arose from G2/M cell cycle arrest following treatment and subsequent induction of apoptotic cell death. Treatment efficacies and treatment durations required to induce cell death were associated with proliferation velocities, and apoptosis induction correlated with complete abolishment of Mcl-1 expression, a cell cycle-regulated antiapoptotic Bcl-2 family member. GBM models generally appeared highly dependent on Mcl-1 expression for cell survival, as demonstrated by pharmacological Mcl-1 inhibition or depletion of Mcl-1 expression. Further analyses identified CKI-induced Mcl-1 loss as a prerequisite to establish conditions at which the BH3-only protein Bim can efficiently induce apoptosis, with cellular Bim amounts strongly correlating with treatment efficacy. CKIs reduced proliferation and promoted apoptosis also in chick embryo xenograft models of primary and recurrent GBM. Collectively, these studies highlight the potential of these novel CKIs to suppress growth and induce cell death of patient-derived GBM cultures in vitro and in vivo, warranting further clinical investigation.

Age-dependent regulation of SARS-CoV-2 cell entry genes and cell death programs correlates with COVID-19 severity

Novel coronavirus disease 2019 (COVID-19) severity is highly variable, with pediatric patients typically experiencing less severe infection than adults and especially the elderly. The basis for this difference is unclear. We find that mRNA and protein expression of angiotensin-converting enzyme 2 (ACE2), the cell entry receptor for the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that causes COVID-19, increases with advancing age in distal lung epithelial cells. However, in humans, ACE2 expression exhibits high levels of intra- and interindividual heterogeneity. Further, cells infected with SARS-CoV-2 experience endoplasmic reticulum stress, triggering an unfolded protein response and caspase-mediated apoptosis, a natural host defense system that halts virion production. Apoptosis of infected cells can be selectively induced by treatment with apoptosis-modulating BH3 mimetic drugs. Notably, epithelial cells within young lungs and airways are more primed to undergo apoptosis than those in adults, which may naturally hinder virion production and support milder COVID-19 severity.

Myeloid cell leukemia 1 (MCL-1): Structural characteristics and application in cancer therapy

Apoptosis, a major hallmark of cancer cells, regulates cellular fate and homeostasis. BCL-2 (B-cell CLL/Lymphoma 2) protein family is popularly known to mediate the intrinsic mode of apoptosis, of which MCL-1 is a crucial member. Myeloid cell leukemia 1 (MCL-1) is an anti-apoptotic oncoprotein and one of the most investigated members of the BCL-2 family. It is commonly known to be genetically altered, aberrantly overexpressed, and primarily associated with drug resistance in various human cancers. Recent advancements in the development of selective MCL-1 inhibitors and evaluating their effectiveness in cancer treatment establish its popularity as a molecular target. The overall aim is the selective induction of apoptosis in cancer cells by using a single or combination of BCL-2 family inhibitors. Delineating the precise molecular mechanisms associated with MCL-1-mediated cancer progression will certainly improve the efficacy of clinical interventions aimed at MCL-1 and hence patient survival. This review is structured to highlight the structural characteristics of MCL-1, its specific interactions with NOXA, MCL-1-regulatory microRNAs, and at the same time focus on the emerging therapeutic strategies targeting our protein of interest (MCL-1), alone or in combination with other treatments.

Identification of a novel Bax-Cdk1 signalling complex that links activation of the mitotic checkpoint to apoptosis

In eukaryotes, entry into and exit from mitosis is regulated, respectively, by the transient activation and inactivation of Cdk1. Taxol, an anti-microtubule anti-cancer drug, prevents microtubule-kinetochore attachments to induce spindle assembly checkpoint (SAC; also known as the mitotic checkpoint)-activated mitotic arrest. SAC activation causes mitotic arrest by chronically activating Cdk1. One consequence of prolonged Cdk1 activation is cell death. However, the cytoplasmic signal(s) that link SAC activation to the initiation of cell death remain unknown. We show here that activated Cdk1 forms a complex with the pro-apoptotic proteins Bax and Bak (also known as BAK1) during SAC-induced apoptosis. Bax- and Bak-mediated delivery of activated Cdk1 to the mitochondrion is essential for the phosphorylation of the anti-apoptotic proteins Bcl-2 and Bcl-xL (encoded by BCL2L1) and the induction of cell death. The interactions between a key cell cycle control protein and key pro-apoptotic proteins identify the Cdk1-Bax and Cdk1-Bak complexes as the long-sought-after cytoplasmic signal that couples SAC activation to the induction of apoptotic cell death.

Antagonizing the spindle assembly checkpoint silencing enhances paclitaxel and Navitoclax-mediated apoptosis with distinct mechanistic

Antimitotic drugs arrest cells in mitosis through chronic activation of the spindle assembly checkpoint (SAC), leading to cell death. However, drug-treated cancer cells can escape death by undergoing mitotic slippage, due to premature mitotic exit. Therefore, overcoming slippage issue is a promising chemotherapeutic strategy to improve the effectiveness of antimitotics. Here, we antagonized SAC silencing by knocking down the MAD2-binding protein p31^comet, to delay mitotic slippage, and tracked cancer cells treated with the antimitotic drug paclitaxel, over 3 days live-cell time-lapse analysis. We found that in the absence of p31^comet, the duration of mitotic block was increased in cells challenged with nanomolar concentrations of paclitaxel, leading to an additive effects in terms of cell death which was predominantly anticipated during the first mitosis. As accumulation of an apoptotic signal was suggested to prevent mitotic slippage, when we challenged p31^comet-depleted mitotic-arrested cells with the apoptosis potentiator Navitoclax (previously called ABT-263), cell fate was shifted to accelerated post-mitotic death. We conclude that inhibition of SAC silencing is critical for enhancing the lethality of antimitotic drugs as well as that of therapeutic apoptosis-inducing small molecules, with distinct mechanisms. The study highlights the potential of p31^comet as a target for antimitotic therapies.

Depletion of Survivin suppresses docetaxel-induced apoptosis in HeLa cells by facilitating mitotic slippage

The anticancer effects of taxanes are attributed to the induction of mitotic arrest through activation of the spindle assembly checkpoint. Cell death following extended mitotic arrest is mediated by the intrinsic apoptosis pathway. Accordingly, factors that influence the robustness of mitotic arrest or disrupt the apoptotic machinery confer drug resistance. Survivin is an inhibitor of apoptosis protein. Its overexpression is associated with chemoresistance, and its targeting leads to drug sensitization. However, Survivin also acts specifically in the spindle assembly checkpoint response to taxanes. Hence, the failure of Survivin-depleted cells to arrest in mitosis may lead to taxane resistance. Here we show that Survivin depletion protects HeLa cells against docetaxel-induced apoptosis by facilitating mitotic slippage. However, Survivin depletion does not promote clonogenic survival of tumor cells but increases the level of cellular senescence induced by docetaxel. Moreover, lentiviral overexpression of Survivin does not provide protection against docetaxel or cisplatin treatment, in contrast to the anti-apoptotic Bcl-xL or Bcl-2. Our findings suggest that targeting Survivin may influence the cell response to docetaxel by driving the cells through aberrant mitotic progression, rather than directly sensitizing cells to apoptosis.

Pyrrolo[2',3':3,4]cyclohepta[1,2-d][1,2]oxazoles, a New Class of Antimitotic Agents Active against Multiple Malignant Cell Types

A new class of pyrrolo[2',3':3,4]cyclohepta[1,2-d][1,2]oxazoles was synthesized for the treatment of hyperproliferative pathologies, including neoplasms. The new compounds were screened in the 60 human cancer cell lines of the NCI drug screen and showed potent activity with GI50 values reaching the nanomolar level, with mean graph midpoints of 0.08-0.41 μM. All compounds were further tested on six lymphoma cell lines, and eight showed potent growth inhibitory effects with IC50 values lower than 500 nM. Mechanism of action studies showed the ability of the new [1,2]oxazoles to arrest cells in the G2/M phase in a concentration dependent manner and to induce apoptosis through the mitochondrial pathway. The most active compounds inhibited tubulin polymerization, with IC50 values of 1.9-8.2 μM, and appeared to bind to the colchicine site. The G2/M arrest was accompanied by apoptosis, mitochondrial depolarization, generation of reactive oxygen species, and PARP cleavage.

BioID-based proteomic analysis of the Bid interactome identifies novel proteins involved in cell-cycle-dependent apoptotic priming

Apoptotic priming controls the commitment of cells to apoptosis by determining how close they lie to mitochondrial permeabilisation. Variations in priming are important for how both healthy and cancer cells respond to chemotherapeutic agents, but how it is dynamically coordinated by Bcl-2 proteins remains unclear. The Bcl-2 family protein Bid is phosphorylated when cells enter mitosis, increasing apoptotic priming and sensitivity to antimitotic drugs. Here, we report an unbiased proximity biotinylation (BioID) screen to identify regulators of apoptotic priming in mitosis, using Bid as bait. The screen primarily identified proteins outside of the canonical Bid interactome. Specifically, we found that voltage-dependent anion-selective channel protein 2 (VDAC2) was required for Bid phosphorylation-dependent changes in apoptotic priming during mitosis. These results highlight the importance of the wider Bcl-2 family interactome in regulating the temporal control of apoptotic priming.

Age-dependent regulation of SARS-CoV-2 cell entry genes and cell death programs correlates with COVID-19 disease severity

Angiotensin-converting enzyme 2 (ACE2) maintains cardiovascular and renal homeostasis but also serves as the entry receptor for the novel severe acute respiratory syndrome coronavirus (SARS-CoV-2), the causal agent of novel coronavirus disease 2019 (COVID-19). COVID-19 disease severity is typically lower in pediatric patients than adults (particularly the elderly), but higher rates of hospitalizations requiring intensive care are observed in infants than in older children - the reasons for these differences are unknown. ACE2 is expressed in several adult tissues and cells, including alveolar type 2 cells of the distal lung epithelium, but expression at other ages is largely unexplored. Here we show that ACE2 transcripts are expressed in the lung and trachea shortly after birth, downregulated during childhood, and again expressed at high levels in late adulthood. Notably, the repertoire of cells expressing ACE2 protein in the mouse lung and airways shifts during key phases of lung maturation. In particular, podoplanin-positive cells, which are likely alveolar type I cells responsible for gas exchange, express ACE2 only in advanced age. Similar patterns of expression were evident in analysis of human lung tissue from over 100 donors, along with extreme inter- and intra-individual heterogeneity in ACE2 protein expression in epithelial cells. Furthermore, we find that apoptosis, which is a natural host defense system against viral infection, is dynamically regulated during lung maturation, resulting in periods of heightened apoptotic priming and dependence on pro-survival BCL-2 family proteins including MCL-1. Infection of human lung cells with SARS-CoV-2 triggers an unfolded protein stress response and upregulation of the endogenous MCL-1 inhibitor Noxa; in young individuals, MCL-1 inhibition is sufficient to trigger apoptosis in lung epithelial cells and may thus limit virion production and inflammatory signaling. Overall, we identify strong and distinct correlates of COVID-19 disease severity across lifespan and advance our understanding of the regulation of ACE2 and cell death programs in the mammalian lung. Furthermore, our work provides the framework for translation of apoptosis modulating drugs as novel treatments for COVID-19.

Simultaneously Inhibiting BCL2 and MCL1 Is a Therapeutic Option for Patients with Advanced Melanoma

There is an urgent need to develop treatments for patients with melanoma who are refractory to or ineligible for immune checkpoint blockade, including patients who lack BRAF-V600E/K mutations. This is often the case in patients diagnosed with rare melanoma subtypes such as mucosal and acral melanoma. Here, we analyzed data from the cutaneous melanoma The Cancer Genome Atlas Network (TCGA) transcriptomic and proteomic databases for differential expression of apoptosis molecules between melanomas with or without BRAF hotspot mutations. Our data indicated higher B-cell CLL/lymphoma 2 (BCL2) expression in melanoma without BRAF hotspot mutations, suggesting that BH3 mimetics, such as ABT-199 (venetoclax, a small molecule against BCL2), may be a potential therapeutic option for these patients. We explored the efficacy of combining two BH3 mimetics, ABT-199 and a myeloid cell leukemia sequence 1 (MCL1) inhibitor (S63845 or S64315/MIK665) in cutaneous, mucosal and acral melanomas, in vitro and in vivo. Our data indicate this combination induced cell death in a broad range of melanoma cell lines, including melanoma initiating cell populations, and was more potent in melanoma cells without BRAF-V600E/K mutations. Our knockdown/knockout experiments suggest that several pro-apoptotic BCL2 family members, BCL2-like 11 (apoptosis facilitator) (BIM), phorbol-12-myristate-13-acetate-induced protein 1 (NOXA) or BID, play a role in the combination-induced effects. Overall, our study supports the rationale for combining an MCL1 inhibitor with a BCL2 inhibitor as a therapeutic option in patients with advanced melanoma.

Targeting of BCL-2 Family Members during Anticancer Treatment: A Necessary Compromise between Individual Cell and Ecosystemic Responses?

The imbalance between BCL-2 homologues and pro-death counterparts frequently noted in cancer cells endows them with a cell autonomous survival advantage. To eradicate ectopic cells, inhibitors of these homologues (BH3 mimetics) were developed to trigger, during anticancer treatment, full activation of the canonical mitochondrial apoptotic pathway and related caspases. Despite efficiency in some clinical settings, these compounds do not completely fulfill their initial promise. We herein put forth that a growing body of evidence indicates that mitochondrial integrity, controlled by BCL-2 family proteins, and downstream caspases regulate other cell death modes and influence extracellular signaling by committed cells. Moreover, intercellular communications play a key role in spreading therapeutic response across cancer cell populations and in engaging an immune response. We thus advocate that BH3 mimetics administration would be more efficient in the long term if it did not induce apoptosis in all sensitive cells at the same time, but if it could instead allow (or trigger) death signal production by non-terminally committed dying cell populations. The development of such a trade-off strategy requires to unravel the effects of BH3 mimetics not only on each individual cancer cell but also on homotypic and heterotypic cell interactions in dynamic tumor ecosystems.

Application of the In Vitro HoxB8 Model System to Characterize the Contributions of Neutrophil-LPS Interaction to Periodontal Disease

(1) Background: Studying neutrophils in vitro is difficult since these cells are terminally differentiated and are easily activated during isolation. At the same time, most of the available model cell lines are associated with certain limitations, such as functional deficiency or a lack of expression of surface markers characteristic of neutrophils. P. gingivalis is a periodontopathogen that causes dysbiosis in subgingival bacterial biofilm. This triggers the accumulation of functional neutrophils in the periodontium. However, until now, the specific effects of P. gingivalis-derived lipopolysaccharide on neutrophil functions have not been analyzed. (2) Methods: The impact of two variants of commercially available P. gingivalis endotoxin on neutrophil functions was tested using the HoxB8 in vitro system that is well suited to analyze neutrophil response to different stimuli in a controlled manner. (3) Results: The Standard P. gingivalis lipopolysaccharide (LPS), known to activate cells through Toll-like receptor 2 (TLR2)- and Toll-like receptor 4 (TLR4)-dependent pathways, prolonged neutrophil survival and exhibited pro-inflammatory effects. In contrast, Ultrapure LPS, binding exclusively to TLR4, neither protected neutrophils from apoptosis, nor induced an inflammatory response. (4) Conclusion: Two variants of P. gingivalis-derived LPS elicited effects on neutrophils and, based on the obtained results, we concluded that the engagement of both TLR2 and TLR4 is required for the manipulation of survival and the stimulation of immune responses of HoxB8 neutrophils.

Cell-Cycle Cross Talk with Caspases and Their Substrates

Caspases play central roles in mediating both cell death and inflammation. It has more recently become evident that caspases also drive other biological processes. Most prominently, caspases have been shown to be involved in differentiation. Several stem and progenitor cell types rely on caspases to initiate and execute their differentiation processes. These range from neural and glial cells, to skeletal myoblasts and osteoblasts, and several cell types of the hematopoietic system. Beyond differentiation, caspases have also been shown to play roles in other "noncanonical" processes, including cell proliferation, arrest, and senescence, thereby contributing to the mechanisms that regulate tissue homeostasis at multiple levels. Remarkably, caspases directly influence the course of the cell cycle in both a positive and negative manner. Caspases both cleave elements of the cell-cycle machinery and are themselves substrates of cell-cycle kinases. Here we aim to summarize the breadth of interactions between caspases and cell-cycle regulators. We also highlight recent developments in this area.

Albendazole-Induced SIRT3 Upregulation Protects Human Leukemia K562 Cells from the Cytotoxicity of MCL1 Suppression

Previous studies have shown that MCL1 stabilization confers cancer cells resistance to microtubule targeting agents (MTAs) and functionally extends the lifespan of MTA-triggered mitotically arrested cells. Albendazole (ABZ), a benzimidazole anthelmintic, shows microtubule-destabilizing activity and has been repositioned for cancer therapies. To clarify the role of MCL1 in ABZ-induced apoptosis, we investigated the cytotoxicity of ABZ on human leukemia K562 cells. Treatment with ABZ for 24 h did not appreciably induce apoptosis or mitochondrial depolarization in K562 cells, though it caused the mitotic arrest of K562 cells. ABZ-evoked p38 MAPK activation concurrently suppressed Sp1-mediated MCL1 expression and increased SIRT3 mRNA stability and protein expression. ABZ and A-1210477 (an MCL1 inhibitor) enhanced the cytotoxicity of ABT-263 (a BCL2/BCL2L1 inhibitor) to their effect on MCL1 suppression. Unlike ABZ, A-1210477 did not affect SIRT3 expression and reduced the survival of K562 cells. Overexpression of SIRT3 attenuated the A-1210477 cytotoxicity on K562 cells. ABZ treatment elicited marked apoptosis and ΔΨm loss in ABT-263-resistant K562 (K562/R) cells, but did not alter SIRT3 expression. Ectopic expression of SIRT3 alleviated the cytotoxicity of ABZ on K562/R cells. Collectively, our data demonstrate that ABZ-induced SIRT3 upregulation delays the apoptosis-inducing effect of MCL1 suppression on apoptosis induction in K562 cells.

Separase-triggered apoptosis enforces minimal length of mitosis

Prolonged mitosis often results in apoptosis¹. Shortened mitosis causes tumorigenic aneuploidy, but it is unclear whether it also activates the apoptotic machinery². Separase, a cysteine protease and trigger of all eukaryotic anaphases, has a caspase-like catalytic domain but has not previously been associated with cell death^3,4. Here we show that human cells that enter mitosis with already active separase rapidly undergo death in mitosis owing to direct cleavage of anti-apoptotic MCL1 and BCL-XL by separase. Cleavage not only prevents MCL1 and BCL-XL from sequestering pro-apoptotic BAK, but also converts them into active promoters of death in mitosis. Our data strongly suggest that the deadliest cleavage fragment, the C-terminal half of MCL1, forms BAK/BAX-like pores in the mitochondrial outer membrane. MCL1 and BCL-XL are turned into separase substrates only upon phosphorylation by NEK2A. Early mitotic degradation of this kinase is therefore crucial for preventing apoptosis upon scheduled activation of separase in metaphase. Speeding up mitosis by abrogation of the spindle assembly checkpoint results in a temporal overlap of the enzymatic activities of NEK2A and separase and consequently in cell death. We propose that NEK2A and separase jointly check on spindle assembly checkpoint integrity and eliminate cells that are prone to chromosome missegregation owing to accelerated progression through early mitosis.

Drp1 modulates mitochondrial stress responses to mitotic arrest

Antimitotic drugs are extensively used in the clinics to treat different types of cancer. They can retain cells in a prolonged mitotic arrest imposing two major fates, mitotic slippage, or mitotic cell death. While the former is molecularly well characterized, the mechanisms that control mitotic cell death remain poorly understood. Here, we performed quantitative proteomics of HeLa cells under mitotic arrest induced with paclitaxel, a microtubule-stabilizer drug, to identify regulators of such cell fate decision. We identified alterations in several apoptosis-related proteins, among which the mitochondrial fission protein Drp1 presented increased levels. We found that Drp1 depletion during prolonged mitotic arrest led to strong mitochondrial depolarization and faster mitotic cell death as well as enhanced mitophagy, a mechanism to remove damaged mitochondria. Our findings support a new role of Drp1 in orchestrating the cellular stress responses during mitosis, where mitochondrial function and distribution into the daughter cells need to be coordinated with cell fate. This novel function of Drp1 in the cell cycle becomes best visible under conditions of prolonged mitotic arrest.

MCL1 binding to the reverse BH3 motif of P18INK4C couples cell survival to cell proliferation

Commitment to cell cycle entry and cellular duplication is a tightly coordinated and regulated process. Once initiated, a series of multiple checkpoints ensure both accurate genomic replication and chromosomal separation. In the event of unsuccessful cell division, parallel pathways exist that induce the cell to undergo programmed cell death, or apoptosis. At the center of such stress-induced, intrinsic apoptotic regulation lies the BCL2 family of pro- and anti-apoptotic regulatory proteins. In a proliferative state the balance of pro- and anti-apoptotic signaling proteins would be expected to favor an excess population of anti-apoptotic members. While the anti-apoptotic BCL2 family member, MCL1, has been identified to oversee mitotic progression, direct communication between the BCL2 family and cell proliferation has not been observed. In this study, we demonstrate a direct protein–protein interaction between MCL1 and the G₁/S checkpoint protein, P18INK4C. This interaction is mediated by a reverse BH3 (rBH3) motif located in P18INK4C’s C-terminal ankyrin repeat. MCL1 is further shown to decrease P18INK4C expression and thereby regulate cell cycle entry in a retinoblastoma (RB1)-dependent manner. Our findings establish a mechanism for translation independent and direct communication between the BCL2 family regulation of apoptosis and CDK4/6-RB regulation of early G₁/S transition during cellular division/growth.

MARCH5-dependent degradation of MCL1/NOXA complexes defines susceptibility to antimitotic drug treatment

Cells experiencing delays in mitotic progression are prone to undergo apoptosis unless they can exit mitosis before proapoptotic factors reach a critical threshold. Microtubule targeting agents (MTAs) arrest cells in mitosis and induce apoptotic cell death engaging the BCL2 network. Degradation of the antiapoptotic BCL2 family member MCL-1 is considered to set the time until onset of apoptosis upon MTA treatment. MCL1 degradation involves its interaction with one of its key binding partners, the proapoptotic BH3-only protein NOXA. Here, we report that the mitochondria-associated E3-ligase MARCH5, best known for its role in mitochondrial quality control and regulation of components of the mitochondrial fission machinery, controls the levels of MCL1/NOXA protein complexes in steady state as well as during mitotic arrest. Inhibition of MARCH5 function sensitizes cancer cells to the proapoptotic effects of MTAs by the accumulation of NOXA and primes cancer cells that may undergo slippage to escape death in mitosis to cell death in the next G1 phase. We propose that inhibition of MARCH5 may be a suitable strategy to sensitize cancer cells to antimitotic drug treatment.

BAD sensitizes breast cancer cells to docetaxel with increased mitotic arrest and necroptosis

Breast cancer patients are commonly treated with taxane (e.g. docetaxel) chemotherapy, despite poor outcomes and eventual disease relapse. We previously identified the Bcl-2-associated death promoter (BAD) as a prognostic indicator of good outcome in taxane-treated breast cancer patients. We also demonstrated that BAD expression in human breast carcinoma cells generated larger tumors in mouse xenograft models. These paradoxical results suggest that BAD-expressing tumors are differentially sensitive to taxane treatment. We validated this here and show that docetaxel therapy preferentially reduced growth of BAD-expressing xenograft tumors. We next explored the cellular mechanism whereby BAD sensitizes cells to docetaxel. Taxanes are microtubule inhibiting agents that cause cell cycle arrest in mitosis whereupon the cells either die in mitosis or aberrantly exit (mitotic slippage) and survive as polyploid cells. In response to docetaxel, BAD-expressing cells had lengthened mitotic arrest with a higher proportion of cells undergoing death in mitosis with decreased mitotic slippage. Death in mitosis was non-apoptotic and not dependent on Bcl-XL interaction or caspase activation. Instead, cell death was necroptotic, and dependent on ROS. These results suggest that BAD is prognostic for favourable outcome in response to taxane chemotherapy by enhancing necroptotic cell death and inhibiting the production of potentially chemoresistant polyploid cells.

STING-dependent paracriny shapes apoptotic priming of breast tumors in response to anti-mitotic treatment

A fascinating but uncharacterized action of antimitotic chemotherapy is to collectively prime cancer cells to apoptotic mitochondrial outer membrane permeabilization (MOMP), while impacting only on cycling cell subsets. Here, we show that a proapoptotic secretory phenotype is induced by activation of cGAS/STING in cancer cells that are hit by antimitotic treatment, accumulate micronuclei and maintain mitochondrial integrity despite intrinsic apoptotic pressure. Organotypic cultures of primary human breast tumors and patient-derived xenografts sensitive to paclitaxel exhibit gene expression signatures typical of type I IFN and TNFα exposure. These cytokines induced by cGAS/STING activation trigger NOXA expression in neighboring cells and render them acutely sensitive to BCL-xL inhibition. cGAS/STING-dependent apoptotic effects are required for paclitaxel response in vivo, and they are amplified by sequential, but not synchronous, administration of BH3 mimetics. Thus anti-mitotic agents propagate apoptotic priming across heterogeneously sensitive cancer cells through cytosolic DNA sensing pathway-dependent extracellular signals, exploitable by delayed MOMP targeting.

Importin-β/karyopherin-β1 modulates mitotic microtubule function and taxane sensitivity in cancer cells via its nucleoporin-binding region

The nuclear transport receptor importin-β/karyopherin-β1 is overexpressed in cancers that display genomic instability. It is regarded as a promising cancer target and inhibitors are being developed. In addition to its role in nucleo-cytoplasmic transport, importin-β regulates mitosis, but the programmes and pathways in which it operates are defined only in part. To unravel importin-β's mitotic functions we have developed cell lines expressing either wild-type or a mutant importin-β form in characterised residues required for nucleoporin binding. Both forms similarly disrupted spindle pole organisation, while only wild-type importin-β affected microtubule plus-end function and microtubule stability. A proteome-wide search for differential interactors identified a set of spindle regulators sensitive to mutations in the nucleoporin-binding region. Among those, HURP (hepatoma up-regulated protein) is an importin-β interactor and a microtubule-stabilising factor. We found that induction of wild type, but not mutant importin-β, under the same conditions that destabilise mitotic microtubules, delocalised HURP, indicating that the spatial distribution of HURP along the spindle requires importin-β's nucleoporin-binding residues. Concomitantly, importin-β overexpression sensitises cells to taxanes and synergistically increases mitotic cell death. Thus, the nucleoporin-binding domain is dispensable for importin-β function in spindle pole organisation, but regulates microtubule stability, at least in part via HURP, and renders cells vulnerable to certain microtubule-targeting drugs.

Evaluating the effects of fluorine on biological properties and metabolic stability of some antitubulin 3-substituted 7-phenyl-pyrroloquinolinones

A small number of fluorinated 7-phenyl-pyrroloquinolinone (7-PPyQ) derivatives was synthesized in an attempt to improve the metabolic stability of 3N-ethyl-7-PPyQ and 3N-benzoyl-7-PPyQ. The possible impacts of the fluorine-hydrogen isosterism on both biological activity and metabolic stability were evaluated. Introduction of a fluorine atom in the 2 or 3 position of the 7-phenyl ring yielded the 7-PPyQ derivatives 12, 13 and 15, which showed potent cytotoxicity (low micromolar and sub-nanomolar GI₅₀s) both in human leukemic and solid tumor cell lines. None of them induced significant cell death in quiescent and proliferating human lymphocytes. Moreover, 12, 13 and 15 exhibited remarkable cytotoxic activity in the multidrug-resistant cell line CEM^Vbl100, suggesting that they are not substrates for P-glycoprotein. All compounds inhibited tubulin assembly and the binding of [³H]colchicine to tubulin, with the best activity occurring with compound 15. Mechanistic studies carried out on compound 12 indicated that it caused (a) a strong G2/M arrest; (b) apoptosis in a time- and concentration-dependent manner; (c) a significant production of ROS (in good agreement with the observed mitochondrial depolarization); (d) caspase-3 and poly (ADP-ribose) polymerase activation; and (e) a decrease in the expression of anti-apoptotic proteins. In vivo experiments in a murine syngeneic tumor model demonstrated that compounds 12 and 15 significantly reduced tumor mass at doses four times lower than that required for the reference compound combretastatin A-4 phosphate. Neither monofluorination of the 7-phenyl ring of 3N-ethyl-7-PPyQ nor replacement of the benzoyl function of 3N-benzoyl-7-PPyQ with a 2-fluorobenzoyl moiety led to any improvement in the metabolic stability.

Regulating the BCL2 Family to Improve Sensitivity to Microtubule Targeting Agents

Chemotherapeutic targeting of microtubules has been the standard of care in treating a variety of malignancies for decades. During mitosis, increased microtubule dynamics are necessary for mitotic spindle formation and successful chromosomal segregation. Microtubule targeting agents (MTAs) disrupt the dynamics necessary for successful spindle assembly and trigger programmed cell death (apoptosis). As the critical regulators of apoptosis, anti-apoptotic BCL2 family members are often amplified during carcinogenesis that can result in MTA resistance. This review outlines how BCL2 family regulation is positioned within the context of MTA treatment and explores the potential of combination therapy of MTAs with emerging BCL2 family inhibitors.

Imbalance of the spindle-assembly checkpoint promotes spindle poison-mediated cytotoxicity with distinct kinetics

Disrupting microtubule dynamics with spindle poisons activates the spindle-assembly checkpoint (SAC) and induces mitotic cell death. However, mitotic exit can occur prematurely without proper chromosomal segregation or cytokinesis by a process termed mitotic slippage. It remains controversial whether mitotic slippage increases the cytotoxicity of spindle poisons or the converse. Altering the SAC induces either mitotic cell death or mitotic slippage. While knockout of MAD2-binding protein p31^comet strengthened the SAC and promoted mitotic cell death, knockout of TRIP13 had the opposite effect of triggering mitotic slippage. We demonstrated that mitotic slippage prevented mitotic cell death caused by spindle poisons, but reduced subsequent long-term survival. Weakening of the SAC also reduced cell survival in response to spindle perturbation insufficient for triggering mitotic slippage, of which mitotic exit was characterized by displaced chromosomes during metaphase. In either mitotic slippage or mitotic exit with missegregated chromosomes, cell death occurred only after one cell cycle following mitotic exit and increased progressively during subsequent cell cycles. Consistent with these results, transient inhibition of the SAC using an MPS1 inhibitor acted synergistically with spindle perturbation in inducing chromosome missegregation and cytotoxicity. The specific temporal patterns of cell death after mitotic exit with weakened SAC may reconcile the contradictory results from many previous studies.

Resistance to anti-microtubule drug-induced cell death is determined by regulation of BimEL expression

Anti-microtubule agents are frequently used as anticancer therapeutics. Cell death induced by these agents is considered to be due to sustained mitotic arrest caused by the activation of spindle assembly checkpoint (SAC). However, some cell types are resistant to mitotic cell death. Cells' ability to escape mitotic arrest (mitotic slippage) is thought to be a major mechanism contributing to this resistance. Here, we show that resistance to cell death induced by anti-mitotic agents is not linked to cells' capacity to undergo mitotic slippage as generally believed but is dependent on the state of BimEL regulation during mitosis. While transcriptional repression of BimEL in the mitotic death-resistant cells involves polycomb repressive complex 2 (PRC2)-mediated histone trimethylation, the BimEL protein is destabilized by cullin 1/4A-βTrCP-dependent degradation involving activation of cullin 1/4A by neddylation. These results imply that pharmacological augmentation of BimEL activity in anti-microtubule drug-resistant tumors may have important therapeutic implications.

Anti-apoptotic Mutations Desensitize Human Pluripotent Stem Cells to Mitotic Stress and Enable Aneuploid Cell Survival

Human pluripotent stem cells (hPSCs) are susceptible to numerical and structural chromosomal alterations during long-term culture. We show that mitotic errors occur frequently in hPSCs and that prometaphase arrest leads to very rapid apoptosis in undifferentiated but not in differentiated cells. hPSCs express high levels of proapoptotic protein NOXA in undifferentiated state. Knocking out NOXA by CRISPR or upregulation of the anti-apoptosis gene BCL-XL significantly reduced mitotic cell death, allowing the survival of aneuploid cells and the formation of teratomas significantly larger than their wild-type parental hPSCs. These results indicate that the normally low threshold of apoptosis in hPSCs can safeguard their genome integrity by clearing cells undergoing abnormal division. The amplification of BCL2L1 on chromosome 20q11.21, a frequent mutation in hPSCs, although not directly oncogenic, reduces the sensitivity of hPSCs to damage caused by erroneous mitosis and increases the risk of gaining aneuploidy.

Low glucose and metformin-induced apoptosis of human ovarian cancer cells is connected to ASK1 via mitochondrial and endoplasmic reticulum stress-associated pathways

Background

Metformin, a first-line drug for type 2 diabetes, could induce apoptosis in cancer cells. However, the concentration of glucose affects the effect of metformin, especially low glucose in the culture medium can enhance the cytotoxicity of metformin on cancer cells. Since mitochondria and endoplasmic reticulum is vital for maintaining cell homeostasis, we speculate that low glucose and metformin-induced cell apoptosis may be associated with mitochondria and endoplasmic reticulum. ASK1, as apoptosis signaling regulating kinase 1, is associated with cell apoptosis and mitochondrial damage. This study was designed to investigate the functional significance of ASK1, mitochondria and endoplasmic reticulum and underlying mechanism in low glucose and metformin-induced cell apoptosis.

Methods

An MTT assay was used to evaluate cell viability in SKOV3, OVCAR3 and HO8910 human ovarian cancer cells. Cell apoptosis was analyzed by flow cytometry. The expression of ASK1 was inhibited using a specific pharmacological inhibitor or ASK1-siRNA. Immunofluorescence was used to detect mitochondrial damage and ER stress. Nude mouse xenograft models were given metformin or/and NQDI-1, and ASK1 expression was detected using immunoblotting. In addition, subcellular fractionation of mitochondria was performed to assay the internal connection between ASK1 and mitochondria.

Results

The present study found that low glucose in culture medium enhanced the anticancer effect of metformin in human ovarian cancer cells. Utilization of a specific pharmacological inhibitor or ASK1-siRNA identified a potential role for ASK1 as an apoptotic protein in the regulation of low glucose and metformin-induced cell apoptosis via ASK1-mediated mitochondrial damage through the ASK1/Noxa pathway and via ER stress through the ROS/ASK1/JNK pathway. Moreover, ASK1 inhibition weakened the antitumor activity of metformin in vivo. Thus, mitochondrial damage and ER stress play a crucial role in low glucose–enhanced metformin cytotoxicity in human ovarian cancer cells.

Conclusions

These data suggested that low glucose and metformin induce cell apoptosis via ASK1-mediated mitochondrial damage and ER stress. These findings indicated that the effect of metformin in anticancer treatment may be related to cell culture conditions.

Mapping Mitotic Death: Functional Integration of Mitochondria, Spindle Assembly Checkpoint and Apoptosis

Targeting the mitotic pathways of rapidly proliferating tumor cells has been an effective strategy in traditional cancer therapy. Chemotherapeutics such as taxanes and vinca alkaloids, which disrupt microtubule function, have enjoyed clinical success; however, the accompanying side effects, toxicity and multi drug resistance remain as serious concerns. The emerging classes of inhibitors targeting mitotic kinases and proteasome face their own set of challenges. It is hoped that elucidation of the regulatory interface between mitotic checkpoints, mitochondria and mitotic death will aid the development of more efficacious anti-mitotic agents and improved treatment protocols. The links between the spindle assembly checkpoint (SAC) and mitochondrial dynamics that control the progression of anti-mitotic agent-induced apoptosis have been under investigation for several years and the functional integration of these various signaling networks is now beginning to emerge. In this review, we highlight current research on the regulation of SAC, the death pathway and mitochondria with particular focus on their regulatory interconnections.

Mitotic slippage: an old tale with a new twist

Targeting the mitotic machinery using anti-mitotic drugs for elimination of cancer cells is a century-old concept, which continues to be routinely used as a first line of treatment in the clinic. However, patient response remains unpredictable and drug resistance limits effectiveness of these drugs. Cancer cells exit from drug-induced mitotic arrest (mitotic slippage) to avoid subsequent cell death which is thought to be a major mechanism contributing to this resistance. The tumor cells that acquire resistance to anti-mitotic drugs have chromosomal instability (CIN) and are often aneuploid. In this review, we outline the key mechanisms involved in dictating the cell fate during perturbed mitosis and how these processes impede the efficacy of anti-mitotic therapies. Further, we emphasize the recent work from our laboratory, which highlights the functional role of CEP55 in protecting aneuploid cells from death. We also discuss the rationale of targeting CEP55 in vivo, which could prove to be a novel and effective therapeutic strategy for sensitizing cells to microtubule inhibitors and might offer significantly improved patient outcome. Abbreviations: APC/C: Anaphase-Promoting Complex/Cyclosome; BAD: BCL2-Associated agonist of cell Death; BAK1: BCL2 Antagonist Kinase1; BAX: BCL2-Associated X; BCL2: B-cell Chronic Lymphocytic Leukaemia (CLL)/Lymphoma 2; BH: BCL2 Homology Domain; BID: BH3-Interacting domain Death agonist; BIM: BCL2-Interacting Mediator of cell death; BUB: Budding Uninhibited by Benzimidazoles; CDC: Cell Division Cycle; CDH1: Cadherin-1; CDK1: Cyclin-Dependent Kinase 1; CEP55: Centrosomal Protein (55 KDa): CIN: Chromosomal Instability; CTA: Cancer Testis Antigen; EGR1: Early Growth Response protein 1; ERK: Extracellular Signal-Regulated Kinase; ESCRT: Endosomal Sorting Complexes Required for Transport; GIN: Genomic Instability; MAD2: Mitotic Arrest Deficient 2; MCL1: Myeloid Cell Leukemia sequence 1; MPS1: Monopolar Spindle 1 Kinase; MYT1: MYelin Transcription factor 1; PLK1: Polo Like Kinase 1; PUMA: p53-Upregulated Mediator of Apoptosis; SAC: Spindle Assembly Checkpoint; TAA: Tumor-Associated Antigen.