Phosphorylation of the proapoptotic BH3-only protein bid primes mitochondria for apoptosis during mitotic arrest

Canonical and Noncanonical Functions of the BH3 Domain Protein Bid in Apoptosis, Oncogenesis, Cancer Therapeutics, and Aging

Effective cancer therapy with limited adverse effects is a major challenge in the medical field. This is especially complicated by the development of acquired chemoresistance. Understanding the mechanisms that underlie these processes remains a major effort in cancer research. In this review, we focus on the dual role that Bid protein plays in apoptotic cell death via the mitochondrial pathway, in oncogenesis and in cancer therapeutics. The BH3 domain in Bid and the anti-apoptotic mitochondrial proteins (Bcl-2, Bcl-XL, mitochondrial ATR) it associates with at the outer mitochondrial membrane provides us with a viable target in cancer therapy. We will discuss the roles of Bid, mitochondrial ATR, and other anti-apoptotic proteins in intrinsic apoptosis, exploring how their interaction sustains cellular viability despite the initiation of upstream death signals. The unexpected upregulation of this Bid protein in cancer cells can also be instrumental in explaining the mechanisms behind acquired chemoresistance. The stable protein associations at the mitochondria between tBid and anti-apoptotic mitochondrial ATR play a crucial role in maintaining the viability of cancer cells, suggesting a novel mechanism to induce cancer cell apoptosis by freeing tBid from the ATR associations at mitochondria.

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.

Kinase signalling adaptation supports dysfunctional mitochondria in disease

Mitochondria form a critical control nexus which are essential for maintaining correct tissue homeostasis. An increasing number of studies have identified dysregulation of mitochondria as a driver in cancer. However, which pathways support and promote this adapted mitochondrial function? A key hallmark of cancer is perturbation of kinase signalling pathways. These pathways include mitogen activated protein kinases (MAPK), lipid secondary messenger networks, cyclic-AMP-activated (cAMP)/AMP-activated kinases (AMPK), and Ca2+/calmodulin-dependent protein kinase (CaMK) networks. These signalling pathways have multiple substrates which support initiation and persistence of cancer. Many of these are involved in the regulation of mitochondrial morphology, mitochondrial apoptosis, mitochondrial calcium homeostasis, mitochondrial associated membranes (MAMs), and retrograde ROS signalling. This review will aim to both explore how kinase signalling integrates with these critical mitochondrial pathways and highlight how these systems can be usurped to support the development of disease. In addition, we will identify areas which require further investigation to fully understand the complexities of these regulatory interactions. Overall, this review will emphasize how studying the interaction between kinase signalling and mitochondria improves our understanding of mitochondrial homeostasis and can yield novel therapeutic targets to treat disease.

BID expression determines the apoptotic fate of cancer cells after abrogation of the spindle assembly checkpoint by AURKB or TTK inhibitors

BACKGROUND

Drugs targeting the spindle assembly checkpoint (SAC), such as inhibitors of Aurora kinase B (AURKB) and dual specific protein kinase TTK, are in different stages of clinical development. However, cell response to SAC abrogation is poorly understood and there are no markers for patient selection.

METHODS

A panel of 53 tumor cell lines of different origins was used. The effects of drugs were analyzed by MTT and flow cytometry. Copy number status was determined by FISH and Q-PCR; mRNA expression by nCounter and RT-Q-PCR and protein expression by Western blotting. CRISPR-Cas9 technology was used for gene knock-out (KO) and a doxycycline-inducible pTRIPZ vector for ectopic expression. Finally, in vivo experiments were performed by implanting cultured cells or fragments of tumors into immunodeficient mice.

RESULTS

Tumor cells and patient-derived xenografts (PDXs) sensitive to AURKB and TTK inhibitors consistently showed high expression levels of BH3-interacting domain death agonist (BID), while cell lines and PDXs with low BID were uniformly resistant. Gene silencing rendered BID-overexpressing cells insensitive to SAC abrogation while ectopic BID expression in BID-low cells significantly increased sensitivity. SAC abrogation induced activation of CASP-2, leading to cleavage of CASP-3 and extensive cell death only in presence of high levels of BID. Finally, a prevalence study revealed high BID mRNA in 6% of human solid tumors.

CONCLUSIONS

The fate of tumor cells after SAC abrogation is driven by an AURKB/ CASP-2 signaling mechanism, regulated by BID levels. Our results pave the way to clinically explore SAC-targeting drugs in tumors with high BID expression.

Select Early-Life Environmental Exposures and DNA Methylation in the Placenta

PURPOSE OF REVIEW

To summarize recent literature relating early-life environmental exposures on DNA methylation in the placenta, to identify how variation in placental methylation is regulated in an exposure-specific manner, and to encourage additional work in this area.

RECENT FINDINGS

Multiple studies have evaluated associations between prenatal environmental exposures and placental methylation in both gene-specific and epigenome-wide frameworks. Specific exposures lead to unique variability in methylation, and cross-exposure assessments have uncovered certain genes that demonstrate consistency in differential placental methylation. Exposure studies that assess methylation effects in a trimester-specific approach tend to find larger effects during the 1st trimester exposure. Earlier studies have more targeted gene-specific approaches to methylation, while later studies have shifted towards epigenome-wide, array-based approaches. Studies focusing on exposures such as air pollution, maternal smoking, environmental contaminants, and trace metals appear to be more abundant, while studies of socioeconomic adversity and circadian disruption are scarce but demonstrate remarkable effects. Understanding the impacts of early-life environmental exposures on placental methylation is critical to establishing the link between the maternal environment, epigenetic variation, and long-term health. Future studies into this field should incorporate repeated measures of exposure throughout pregnancy, in order to determine the critical windows in which placental methylation is most heavily affected. Additionally, the use of methylation-based scores and sequencing technology could provide important insights into epigenetic gestational age and uncovering more genomic regions where methylation is affected. Studies examining the impact of other exposures on methylation, including pesticides, alcohol, and other chemicals are also warranted.

Role of Caspase-10-P13tBID axis in erythropoiesis regulation

Red blood cell production is negatively controlled by the rate of apoptosis at the stage of CFU-E/pro-erythroblast differentiation, depending on the balance between erythropoietin (EPO) levels and activation of the Fas/FasL pathway. At this stage, activation of transient caspases through depolarization via mitochondrial outer membrane permeabilization (MOMP) is also required for terminal erythroid differentiation. Molecular mechanisms regulating the differential levels of MOMP during differentiation and apoptosis, however, remain poorly understood. Here we show a novel and essential role for the caspase-10-P13-tBID axis in erythroid terminal differentiation. Caspase-10 (but not caspase-8, which is activated during apoptosis) is activated at the early stages of erythroid terminal differentiation leading to the cleavage of P22-BID into P18-tBID, and later into P13-tBID. Erythropoietin (EPO) by inducing casein kinase I alpha (CKIα) expression, which in turn phosphorylates P18-tBID, prevents the generation of MYR-P15-tBID (leading to apoptosis) and allows the generation of P13-tBID by caspase-10. Unlike P15-tBID, P13-tBID is not myristoylated and as such, does not irreversibly anchor the mitochondrial membrane resulting in a transient MOMP. Likewise, transduction of a P13-tBID fragment induces rapid and strong erythroid terminal differentiation. Thus, EPO modulates the pattern of BID cleavage to control the level of MOMP and determines the fate of erythroblasts between apoptosis and differentiation. This pathway is impaired in 5q- myelodysplastic syndromes because of CK1α haplo-insufficiency and may contribute to erythroid differentiation arrest and high sensitivity of this disease to lenalidomide (LEN).

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.

Roles of calpain in the apoptosis of Eimeria tenella host cells at the middle and late developmental stages

This study investigated the role of calpain in Eimeria tenella-induced host cell apoptosis. Chick embryo cecal epithelial cell culture technology, flow cytometry, enzyme-linked immunosorbent assays, and fluorescence quantitative PCR were used to detect the E. tenella host cell apoptotic rate, Bax and Bid expression levels, and calpain activity. The results demonstrated that Bax, Bid, and calpain levels were upregulated and apoptosis was increased following E. tenella infection at 24-120 h. Calpain levels were reduced by pharmacological inhibition of calpain using SJA6017 or by blocking Ca²⁺ entry into the cell using BAPTA/AM at 24-120 h. The mRNA and protein levels of Bax and Bid, the E. tenella infection rate, and the early apoptotic and late apoptotic (necrosis) rates were decreased by using SJA6017 at 24-120 h. These results indicated that E. tenella-promoted host cell apoptosis is regulated by calpain via Bid and Bax at 24-120 h. Thus, manipulation of calpain levels could be used to manage E. tenella infection in chickens in the middle and late developmental stages.

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.

Stress Relief Techniques: p38 MAPK Determines the Balance of Cell Cycle and Apoptosis Pathways

Protein signaling networks are formed from diverse and inter-connected cell signaling pathways converging into webs of function and regulation. These signaling pathways both receive and conduct molecular messages, often by a series of post-translation modifications such as phosphorylation or through protein-protein interactions via intrinsic motifs. The mitogen activated protein kinases (MAPKs) are components of kinase cascades that transmit signals through phosphorylation. There are several MAPK subfamilies, and one subfamily is the stress-activated protein kinases, which in mammals is the p38 family. The p38 enzymes mediate a variety of cellular outcomes including DNA repair, cell survival/cell fate decisions, and cell cycle arrest. The cell cycle is itself a signaling system that precisely controls DNA replication, chromosome segregation, and cellular division. Another indispensable cell function influenced by the p38 stress response is programmed cell death (apoptosis). As the regulators of cell survival, the BCL2 family of proteins and their dynamics are exquisitely sensitive to cell stress. The BCL2 family forms a protein-protein interaction network divided into anti-apoptotic and pro-apoptotic members, and the balance of binding between these two sides determines cell survival. Here, we discuss the intersections among the p38 MAPK, cell cycle, and apoptosis signaling pathways.

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.

Prenatal fine particulate matter exposure, placental DNA methylation changes, and fetal growth

This study was designed to examine the impact of prenatal fine particulate matter (PM_2.5) exposure on fetal growth and the underlying placental epigenetic mechanism in a cohort of Chinese women. Within the prospective Shanghai Mother-Child Pairs cohort (Shanghai MCPC), 329 women carrying singleton pregnancy with a due date in 2018 were recruited between 2017 and 2018. Maternal PM_2.5 exposure levels were estimated using gestational exposure prediction model combining satellite-driven ambient concentrations and personal air sampling. Fetal growth characteristics were evaluated by prenatal ultrasound examinations and anthropometric measurements at birth. In a discovery phase, whole-genome DNA methylation analysis was performed using the Infinium 850 K array. In a validation phase, placental DNA methylation was measured using bisulfite pyrosequencing for five candidate genes that showed the most significant alterations and function relevance in our methylation array screen, including BID (BH3 interacting domain death agonist), FOXN3 (Forkhead box N3), FOXP1 (Forkhead box P1), IGF2 (Insulin-like growth factor 2) and HSD11B2 (Hydroxysteroid 11-beta dehydrogenase 2). Multivariate linear regression models were applied to examine the associations among PM_2.5 exposure, fetal growth characteristics and DNA methylation on placental candidate genes. Sobel tests were used to evaluate the mediating role of DNA methylation in multivariable models. After excluding women who withdrew or failed to provide placenta, a total of 287 pregnant women with an average age of 30 entered the final analysis. Increased PM_2.5 exposure was significantly associated with reduced biparietal diameter (BPD) (β: -0.136 mm, 95% CI: -0.228 to -0.043), head circumference (HC) (β: -0.462 mm, 95% CI: -0.782 to -0.142), femur length (FL) (β: -0.113 mm, 95% CI: -0.185 to -0.041) and abdominal circumference (AC) (β: -0.371 mm, 95% CI: -0.672 to -0.071) in the second trimester and birth length (β: -0.013 cm, 95% CI: -0.025 to -0.001). Prenatal PM_2.5 exposure could lead to aberrant changes in DNA methylation profile of placenta genome, which were mainly enriched in reproductive development, energy metabolism and immune response. DNA methylation of IGF2 and BID showed significant associations with PM_2.5 exposures during all exposure windows. In addition, BID methylation was negatively correlated with HC (β: -1.396 mm, 95% CI: -2.582 to -0.209) and BPD (β: -0.330 mm, 95% CI: -0.635 to -0.026) in the second trimester. Further mediation analysis indicated that BID methylation mediated about 30% of the effects of PM_2.5 exposure on HC. These findings collectively suggested that prenatal PM_2.5 exposure may cause adverse effects on fetal growth by modifying placental DNA methylation.

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.

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.

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.

A novel method to investigate the effects of gene mutations at the cellular level using a dual expression lentiviral vector

One of the conventional methods to study the effects of gene mutations is that gene mutants are transfected into mammalian cells, and the dominant effects of gene mutants in the cells are examined. However, the result obtained using this method is not always satisfactory due to the interference of endogenous expression. Whether there is a better method to investigate the effects of gene mutations in cells remains to be examined. In the present study, a novel dual expression lentiviral vector was constructed using a shRNA-expressing lentiviral vector and combined techniques. Using this dual expression system, the vectors expressing both transcription factor IIA γ (TFIIAγ) shRNA and HA-TFIIAγ or its mutants were generated, and the effects of TFIIAγ gene mutations on transcription and protein–DNA interaction were investigated. We show that the transfection of the vector expressing TFIIAγ shRNA and HA-TFIIAγ fusion gene was able to silence the expression of endogenous TFIIAγ gene but not affect that of exogenous HA-TFIIAγ fusion gene in either transiently transfected cells or stable cell lines. Mutations in the conservative domain between AA₆₂ and AA₆₉ in TFIIAγ inhibit the activities of promoters and endogenous gene expression, and reduce TFIIAγ binding to AdML core promoter compared with wild-type (WT) TFIIAγ. ChIP-qPCR data suggest that the TFIIAγ N63A mutant inhibits insulin-like growth factor 2 (IGF2) transcription by reducing the recruitments of TFIIAγ, polymerase II (Pol II), TATA box-binding protein (TBP), and TBP associated factor 1 (250 kDa) (TAF1) at its promoter. Our study provides a novel method that is used to investigate the effects of gene mutations at the cellular level.

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.

Bid maintains mitochondrial cristae structure and function and protects against cardiac disease in an integrative genomics study

Bcl-2 family proteins reorganize mitochondrial membranes during apoptosis, to form pores and rearrange cristae. In vitro and in vivo analysis integrated with human genetics reveals a novel homeostatic mitochondrial function for Bcl-2 family protein Bid. Loss of full-length Bid results in apoptosis-independent, irregular cristae with decreased respiration. Bid-/- mice display stress-induced myocardial dysfunction and damage. A gene-based approach applied to a biobank, validated in two independent GWAS studies, reveals that decreased genetically determined BID expression associates with myocardial infarction (MI) susceptibility. Patients in the bottom 5% of the expression distribution exhibit >4 fold increased MI risk. Carrier status with nonsynonymous variation in Bid’s membrane binding domain, Bid^M148T, associates with MI predisposition. Furthermore, Bid but not Bid^M148T associates with Mcl-1^Matrix, previously implicated in cristae stability; decreased MCL-1 expression associates with MI. Our results identify a role for Bid in homeostatic mitochondrial cristae reorganization, that we link to human cardiac disease.

Cells contain specialized structures called mitochondria, which help to convert fuel into energy. These tiny energy factories have a unique double membrane, with a smooth outer and a folded inner lining. The folds, called cristae, provide a scaffold for the molecular machinery that produces chemical energy that the cell can use. The cristae are dynamic, and can change shape, condensing to increase energy output.

Mitochondria also play a role in cell death. In certain situations, cristae can widen and release the proteins held within their folds. This can trigger a program of self-destruction in the cell. A family of proteins called Bcl-2 control such a ‘programmed cell death’ through the release of mitochondrial proteins. Some family members, including a protein called Bid, can reorganize cristae to regulate this cell-death program. When cells die, Bid proteins that had been split move to the mitochondria. But, even when cells are healthy, Bid molecules that are intact are always there, suggesting that this form of the protein may have another purpose.

To investigate this further, Salisbury-Ruf, Bertram et al. used mice with Bid, and mice that lacked the protein. Without Bid, cells – including heart cells – struggled to work properly and used less oxygen than their normal counterparts. A closer look using electron microscopy revealed abnormalities in the cristae. However, adding ‘intact’ Bid proteins back in to the deficient cells restored them to normal.

Moreover, without Bid, the mice hearts were less able to respond to an increased demand for energy. This decreased their performance and caused the formation of scars in the heart muscle called fibrosis, similar to a pattern observed in human patients following a heart attack.

DNA data from an electronic health record database revealed a link between low levels of Bid genes and heart attack in humans, which was confirmed in further studies. In addition, a specific mutation in the Bid gene was found to affect its ability to regulate the formation of proper cristae.

Combining evidence from mice with human genetics revealed new information about heart diseases. Mitochondrial health may be affected by a combination of specific variations in genes and changes in the Bid protein, which could affect heart attack risk. Understanding more about this association could help to identify and potentially reduce certain risk factors for heart attack.

Perturbing mitosis for anti-cancer therapy: is cell death the only answer?

Interfering with mitosis for cancer treatment is an old concept that has proven highly successful in the clinics. Microtubule poisons are used to treat patients with different types of blood or solid cancer since more than 20 years, but how these drugs achieve clinical response is still unclear. Arresting cells in mitosis can promote their demise, at least in a petri dish. Yet, at the molecular level, this type of cell death is poorly defined and cancer cells often find ways to escape. The signaling pathways activated can lead to mitotic slippage, cell death, or senescence. Therefore, any attempt to unravel the mechanistic action of microtubule poisons will have to investigate aspects of cell cycle control, cell death initiation in mitosis and after slippage, at single-cell resolution. Here, we discuss possible mechanisms and signaling pathways controlling cell death in mitosis or after escape from mitotic arrest, as well as secondary consequences of mitotic errors, particularly sterile inflammation, and finally address the question how clinical efficacy of anti-mitotic drugs may come about and could be improved.

The kinesin spindle protein inhibitor filanesib enhances the activity of pomalidomide and dexamethasone in multiple myeloma

Kinesin spindle protein inhibition is known to be an effective therapeutic approach in several malignancies. Filanesib (ARRY-520), an inhibitor of this protein, has demonstrated activity in heavily pre-treated multiple myeloma patients. The aim of the work herein was to investigate the activity of filanesib in combination with pomalidomide plus dexamethasone backbone, and the mechanisms underlying the potential synergistic effect. The ability of filanesib to enhance the activity of pomalidomide plus dexamethasone was studied in several in vitro and in vivo models. Mechanisms of this synergistic combination were dissected by gene expression profiling, immunostaining, cell cycle and short interfering ribonucleic acid studies. Filanesib showed in vitro, ex vivo, and in vivo synergy with pomalidomide plus dexamethasone treatment. Importantly, the in vivo synergy observed in this combination was more evident in large, highly proliferative tumors, and was shown to be mediated by the impairment of mitosis transcriptional control, an increase in monopolar spindles, cell cycle arrest and the induction of apoptosis in cells in proliferative phases. In addition, the triple combination increased the activation of the proapoptotic protein BAX, which has previously been associated with sensitivity to filanesib, and could potentially be used as a predictive biomarker of response to this combination. Our results provide preclinical evidence for the potential benefit of the combination of filanesib with pomalidomide and dexamethasone, and supported the initiation of a recently activated trial being conducted by the Spanish Myeloma group which is investigating this combination in relapsed myeloma patients.

Mitosis and mitochondrial priming for apoptosis

Cell division is a period of danger for cells, as inaccurate segregation of chromosomes can lead to loss of cell viability or aneuploidy. In order to protect against these dangers, cells ultimately initiate mitochondrial apoptosis if they are unable to correctly exit mitosis. A number of important chemotherapeutics exploit this response to delayed mitotic exit, but despite this, the molecular mechanism of the apoptotic timer in mitosis has proved elusive. Some recent studies have now shed light on this, showing how passage through the cell cycle fine-tunes a cell's apoptotic sensitivity such that it can respond appropriately when errors arise.

DNA methylation profiling in peripheral lung tissues of smokers and patients with COPD

BACKGROUND

Epigenetics changes have been shown to be affected by cigarette smoking. Cigarette smoke (CS)-mediated DNA methylation can potentially affect several cellular and pathophysiological processes, acute exacerbations, and comorbidity in the lungs of patients with chronic obstructive pulmonary disease (COPD). We sought to determine whether genome-wide lung DNA methylation profiles of smokers and patients with COPD were significantly different from non-smokers. We isolated DNA from parenchymal lung tissues of patients including eight lifelong non-smokers, eight current smokers, and eight patients with COPD and analyzed the samples using Illumina's Infinium HumanMethylation450 BeadChip.

RESULTS

Our data revealed that the differentially methylated genes were related to top canonical pathways (e.g., G beta gamma signaling, mechanisms of cancer, and nNOS signaling in neurons), disease and disorders (organismal injury and abnormalities, cancer, and respiratory disease), and molecular and cellular functions (cell death and survival, cellular assembly and organization, cellular function and maintenance) in patients with COPD. The genome-wide DNA methylation analysis identified suggestive genes, such as NOS1AP, TNFAIP2, BID, GABRB1, ATXN7, and THOC7 with DNA methylation changes in COPD lung tissues that were further validated by pyrosequencing. Pyrosequencing validation confirmed hyper-methylation in smokers and patients with COPD as compared to non-smokers. However, we did not detect significant differences in DNA methylation for TNFAIP2, ATXN7, and THOC7 genes in smokers and COPD groups despite the changes observed in the genome-wide analysis.

CONCLUSIONS

Our study suggests that DNA methylation in suggestive genes, such as NOS1AP, BID, and GABRB1 may be used as epigenetic signatures in smokers and patients with COPD if the same is validated in a larger cohort. Future studies are required to correlate DNA methylation status with transcriptomics of selective genes identified in this study and elucidate their role and involvement in the progression of COPD and its exacerbations.

Paclitaxel-induced aberrant mitosis and mitotic slippage efficiently lead to proliferative death irrespective of canonical apoptosis and p53

Spindle poisons elicit various cellular responses following metaphase arrest, but how they relate to long-term clonogenicity has remained unclear. We prepared several HeLa lines in which the canonical apoptosis pathway was attenuated, and compared their acute responses to paclitaxel, as well as long-term fate, with the parental line. Three-nanomolar paclitaxel induced brief metaphase arrest (<5 h) often followed by aberrant mitosis, and about 90% of the cells of each line had lost their clonogenicity after 48 h of the treatment. A combination of the same concentration of paclitaxel with the kinesin-5 inhibitor, S-trityl-L-cysteine (STLC), at 1 µM led to much longer arrest (∼20 h) and predominance of subsequent line-specific responses: mitochondrial outer membrane permeabilization (MOMP) in the apoptosis-prone line, or mitotic slippage without obvious MOMP in the apoptosis-reluctant lines. In spite of this, combination with STLC did not lead to a marked difference in clonogenicity between the apoptosis-prone and -reluctant lines, and intriguingly resulted in slightly better clonogenicity than that of cells treated with 3 nM paclitaxel alone. This indicates that changes in the short-term response within 3 possible scenarios - acute MOMP, mitotic slippage or aberrant mitosis - has only a weak impact on clonogenicity. Our results suggest that once cells have committed to slippage or aberrant mitosis they eventually undergo proliferative death irrespective of canonical apoptosis or p53 function. Consistent with this, cells with irregular DNA contents originating from mitotic slippage or aberrant mitosis were mostly eliminated from the population within several rounds of division after the drug treatment.

Inhibition of Bcl-xL sensitizes cells to mitotic blockers, but not mitotic drivers

Cell fate in response to an aberrant mitosis is governed by two competing networks: the spindle assembly checkpoint (SAC) and the intrinsic apoptosis pathway. The mechanistic interplay between these two networks is obscured by functional redundancy and the ability of cells to die either in mitosis or in the subsequent interphase. By coupling time-lapse microscopy with selective pharmacological agents, we systematically probe pro-survival Bcl-xL in response to various mitotic perturbations. Concentration matrices show that BH3-mimetic-mediated inhibition of Bcl-xL synergises with perturbations that induce an SAC-mediated mitotic block, including drugs that dampen microtubule dynamics, and inhibitors targeting kinesins and kinases required for spindle assembly. By contrast, Bcl-xL inhibition does not synergize with drugs which drive cells through an aberrant mitosis by overriding the SAC. This differential effect, which is explained by compensatory Mcl-1 function, provides opportunities for patient stratification and combination treatments in the context of cancer chemotherapy.

Oncogenic KRAS triggers MAPK-dependent errors in mitosis and MYC-dependent sensitivity to anti-mitotic agents

Oncogenic KRAS induces cell proliferation and transformation, but little is known about its effects on cell division. Functional genetic screens have recently revealed that cancer cell lines expressing oncogenic KRAS are sensitive to interference with mitosis, but neither the mechanism nor the uniformity of anti-mitotic drug sensitivity connected with mutant KRAS expression are yet clear. Here, we report that acute expression of oncogenic KRAS in HeLa cells induces mitotic delay and defects in chromosome segregation through mitogen-activated protein kinase (MAPK) pathway activation and de-regulated expression of several mitosis-related genes. These anomalies are accompanied by increased sensitivity to anti-mitotic agents, a phenotype dependent on the transcription factor MYC and its downstream target anti-apoptotic protein BCL-XL. Unexpectedly, we find no correlation between KRAS mutational status or MYC expression levels and anti-mitotic drug sensitivity when surveying a large database of anti-cancer drug responses. However, we report that the co-existence of KRAS mutations and high MYC expression predicts anti-mitotic drug sensitivity. Our findings reveal a novel function of oncogenic KRAS in regulating accurate mitotic progression and suggest new avenues to therapeutically target KRAS-mutant tumours and stratify patients in ongoing clinical trials of anti-mitotic drugs.

Targeting cell death signalling in cancer: minimising 'Collateral damage'

Targeting apoptosis for the treatment of cancer has become an increasingly attractive strategy, with agents in development to trigger extrinsic apoptosis via TRAIL signalling, or to prevent the anti-apoptotic activity of BCL-2 proteins or inhibitor of apoptosis (IAP) proteins. Although the evasion of apoptosis is one of the hallmarks of cancer, many cancers have intact apoptotic signalling pathways, which if unblocked could efficiently kill cancerous cells. However, it is becoming increasing clear that without a detailed understanding of both apoptotic and non-apoptotic signalling, and the key proteins that regulate these pathways, there can be dose-limiting toxicity and adverse effects associated with their modulation. Here we review the main apoptotic pathways directly targeted for anti-cancer therapy and the unforeseen consequences of their modulation. Furthermore, we highlight the importance of an in-depth mechanistic understanding of both the apoptotic and non-apoptotic functions of those proteins under investigation as anti-cancer drug targets and outline some novel approaches to sensitise cancer cells to apoptosis, thereby improving the efficacy of existing therapies when used in combination with novel targeted agents.

Mcl-1 dynamics influence mitotic slippage and death in mitosis

Microtubule-binding drugs such as taxol are frontline treatments for a variety of cancers but exactly how they yield patient benefit is unclear. In cell culture, inhibiting microtubule dynamics prevents spindle assembly, leading to mitotic arrest followed by either apoptosis in mitosis or slippage, whereby a cell returns to interphase without dividing. Myeloid cell leukaemia-1 (Mcl-1), a pro-survival member of the Bcl-2 family central to the intrinsic apoptosis pathway, is degraded during a prolonged mitotic arrest and may therefore act as a mitotic death timer. Consistently, we show that blocking proteasome-mediated degradation inhibits taxol-induced mitotic apoptosis in a Mcl-1-dependent manner. However, this degradation does not require the activity of either APC/C-Cdc20, FBW7 or MULE, three separate E3 ubiquitin ligases implicated in targeting Mcl-1 for degradation. This therefore challenges the notion that Mcl-1 undergoes regulated degradation during mitosis. We also show that Mcl-1 is continuously synthesized during mitosis and that blocking protein synthesis accelerates taxol induced death-in-mitosis. Modulating Mcl-1 levels also influences slippage; overexpressing Mcl-1 extends the time from mitotic entry to mitotic exit in the presence of taxol, while inhibiting Mcl-1 accelerates it. We suggest that Mcl-1 competes with Cyclin B1 for binding to components of the proteolysis machinery, thereby slowing down the slow degradation of Cyclin B1 responsible for slippage. Thus, modulating Mcl-1 dynamics influences both death-in-mitosis and slippage. However, because mitotic degradation of Mcl-1 appears not to be under the control of an E3 ligase, we suggest that the notion of network crosstalk is used with caution.

A delay prior to mitotic entry triggers caspase 8-dependent cell death in p53-deficient Hela and HCT-116 cells

Stathmin/Oncoprotein 18, a microtubule destabilizing protein, is required for survival of p53-deficient cells. Stathmin-depleted cells are slower to enter mitosis, but whether delayed mitotic entry triggers cell death or whether stathmin has a separate pro-survival function was unknown. To test these possibilities, we abrogated the cell cycle delay by inhibiting Wee1 in synchronized, stathmin-depleted cells and found that apoptosis was reduced to control levels. Synchronized cells treated with a 4 hour pulse of inhibitors to CDK1 or both Aurora A and PLK1 delayed mitotic entry and apoptosis was triggered only in p53-deficient cells. We did not detect mitotic defects downstream of the delayed mitotic entry, indicating that cell death is activated by a mechanism distinct from those activated by prolonged mitotic arrest. Cell death is triggered by initiator caspase 8, based on its cleavage to the active form and by rescue of viability after caspase 8 depletion or treatment with a caspase 8 inhibitor. In contrast, initiator caspase 9, activated by prolonged mitotic arrest, is not activated and is not required for apoptosis under our experimental conditions. P53 upregulates expression of cFLIP_L, a protein that blocks caspase 8 activation. cFLIP_L levels are lower in cells lacking p53 and these levels are reduced to a greater extent after stathmin depletion. Expression of FLAG-tagged cFLIP_L in p53-deficient cells rescues them from apoptosis triggered by stathmin depletion or CDK1 inhibition during G2. These data indicate that a cell cycle delay in G2 activates caspase 8 to initiate apoptosis specifically in p53-deficient cells.

AMPK and PFKFB3 mediate glycolysis and survival in response to mitophagy during mitotic arrest

Blocking mitotic progression has been proposed as an attractive therapeutic strategy to impair proliferation of tumour cells. However, how cells survive during prolonged mitotic arrest is not well understood. We show here that survival during mitotic arrest is affected by the special energetic requirements of mitotic cells. Prolonged mitotic arrest results in mitophagy-dependent loss of mitochondria, accompanied by reduced ATP levels and the activation of AMPK. Oxidative respiration is replaced by glycolysis owing to AMPK-dependent phosphorylation of PFKFB3 and increased production of this protein as a consequence of mitotic-specific translational activation of its mRNA. Induction of autophagy or inhibition of AMPK or PFKFB3 results in enhanced cell death in mitosis and improves the anti-tumoral efficiency of microtubule poisons in breast cancer cells. Thus, survival of mitotic-arrested cells is limited by their metabolic requirements, a feature with potential implications in cancer therapies aimed to impair mitosis or metabolism in tumour cells.

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

Cell death on extended mitotic arrest is considered arguably most critical for the efficacy of microtubule-targeting agents (MTAs) in anticancer therapy. While the molecular machinery controlling mitotic arrest on MTA treatment, the spindle assembly checkpoint (SAC), appears well defined, the molecular components executing cell death, as well as factors connecting both networks remain poorly understood. Here we conduct a mini screen exploring systematically the contribution of individual BCL2 family proteins at single cell resolution to death on extended mitotic arrest, and demonstrate that the mitotic phosphorylation of BCL2 and BCLX represent a priming event for apoptosis that is ultimately triggered by NOXA-dependent MCL1 degradation, enabling BIM-dependent cell death. Our findings provide a comprehensive model for the initiation of apoptosis in cells stalled in mitosis and provide a molecular basis for the increased efficacy of combinatorial treatment of cancer cells using MTAs and BH3 mimetics.

Cells experiencing extended mitotic arrest often undergo cell death as a result of steadily declining levels of the apoptotic inhibitor MCL1, but the mechanism controlling this process is poorly understood. Here, Haschka et al. show that the BH3-only protein NOXA promotes the degradation of MCL1, enabling BIM-dependent cell death.