BID preferentially activates BAK while BIM preferentially activates BAX, affecting chemotherapy response

Comparative analysis of Bcl-2 family protein overexpression in CAR T cells alone and in combination with BH3 mimetics

Approximately 50% of patients with hematologic malignancies relapse after chimeric antigen receptor (CAR) T cell treatment; mechanisms of failure include loss of CAR T persistence and tumor resistance to apoptosis. We hypothesized that both of these challenges could potentially be overcome by overexpressing one or more of the Bcl-2 family proteins in CAR T cells to reduce their susceptibility to apoptosis, both alone and in the presence of BH3 mimetics, which can be used to activate apoptotic machinery in malignant cells. We comprehensively investigated overexpression of different Bcl-2 family proteins in CAR T cells with different signaling domains as well as in different tumor types. We found that Bcl-xL and Bcl-2 overexpression in CAR T cells bearing a 4-1BB costimulatory domain resulted in increased expansion and antitumor activity, reduced exhaustion, and decreased apoptotic priming. In addition, CAR T cells expressing either Bcl-xL or a venetoclax-resistant Bcl-2 variant led to enhanced antitumor efficacy and survival in murine xenograft models of lymphoma and leukemia in the presence or absence of the BH3 mimetic venetoclax, a clinically approved BH3 mimetic. In this setting, Bcl-xL overexpression had stronger effects than overexpression of Bcl-2 or the Bcl-2(G101V) variant. These findings suggest that CAR T cells could be optimally engineered by overexpressing Bcl-xL to enhance their persistence while opening a therapeutic window for combination with BH3 mimetics to prime tumors for apoptosis.

BCL-2 inhibition in haematological malignancies: Clinical application and complications

B-cell lymphoma-2 (BCL-2) family proteins are fundamental regulators of the intrinsic apoptotic pathway which modulate cellular fate. In many haematological malignancies, overexpression of anti-apoptotic factors (BCL-2, BCL-XL and MCL-1) circumvent apoptosis. To address this cancer hallmark, a concerted effort has been made to induce apoptosis by inhibiting BCL-2 family proteins. A series of highly selective BCL-2 homology 3 (BH3) domain mimetics are in clinical use and in ongoing clinical trials for acute myeloid leukaemia (AML), chronic myeloid leukaemia (CML), chronic lymphocytic leukaemia (CLL), and multiple myeloma (MM). These inhibitors serve as promising candidates, both as single agents or in combination therapy to improve patient outcomes. In other diseases such as follicular lymphoma, efficacy has been notably limited. There are also clinical problems with BCL-2 family inhibition, including drug resistance, disease relapse, tumour lysis syndrome, and clinically relevant cytopenias. Here, we provide a balanced view on both the clinical benefits of BCL-2 inhibition as well as the associated challenges.

Recent trends and advances in novel formulations as an armament in Bcl-2/Bax targeted breast cancer

Breast cancer (BC) remains a significant health burden worldwide, necessitating the development of innovative therapeutic strategies. The B-cell lymphoma 2 (Bcl-2) family proteins, Bcl-2 and Bax, play a crucial role in regulating apoptosis and thus are promising targets for BC therapy. We focus on the recent advancements in novel formulations that specifically target Bcl-2/Bax pathway to combat BC. It provides an overview on biological functions of Bcl-2/Bax in apoptosis regulation, emphasizing their significance in pathogenesis and progression of the disease while covering the numerous therapeutic approaches aimed at modulating the Bcl-2/Bax pathway, including small-molecule inhibitors, peptides, gene-based therapies and other repurposed drugs harboured onto cutting-edge technologies and nanocarrier systems employed to enhance the targeted delivery of Bcl-2/Bax inhibitors tumor cells. These advanced formulations aim to improve therapeutic efficacy, minimize off-target effects, and overcome drug resistance, offering promising prospects in its treatment. In conclusion, it illuminates the diverse and evolving landscape of novel formulations as an essential armament in targeting these proteins while bridging and unravelling the obscurity of Bcl-2/Bax pathway-targeted drug delivery systems which are presently in their nascent stages of exploration for BC therapy which can benefit researchers, clinicians, and pharmaceutical scientists.

B-cell lymphoma-2 family proteins in the crosshairs: Small molecule inhibitors and activators for cancer therapy

The B-cell lymphoma-2 (BCL-2) family of proteins plays a crucial role in the regulation of apoptosis, offering a dual mechanism for its control. Numerous studies have established a strong association between gene disorders of these proteins and the proliferation of diverse cancer cell types. Consequently, the identification and development of drugs targeting BCL-2 family proteins have emerged as a prominent area in antitumor therapy. Over the last two decades, several small-molecules have been designed to modulate the protein-protein interactions between anti- and proapoptotic BCL-2 proteins, effectively suppressing tumor growth and metastasis in vivo. The primary focus of research has been on developing BCL-2 homology 3 (BH3) mimetics to target antiapoptotic BCL-2 proteins, thereby competitively releasing proapoptotic BCL-2 proteins and restoring the blocked intrinsic apoptotic program. Additionally, for proapoptotic BCL-2 proteins, exogenous small molecules have been explored to activate cell apoptosis by directly interacting with executioner proteins such as BCL-2-associated X protein (BAX) or BCL-2 homologous antagonist/killer protein (BAK). In this comprehensive review, we summarize the inhibitors and activators (sensitizers) of BCL-2 family proteins developed over the past decades, highlighting their discovery, optimization, preclinical and clinical status, and providing an overall landscape of drug development targeting these proteins for therapeutic purposes.

Anoikis and cancer cell differentiation: novel modes of shikonin derivatives anticancer action in vitro

BACKGROUND

Shikonin is a naturally occurring naphthoquinone found in the roots of several genera of the Boraginaceae family, widely known for its numerous biological activities, such as antiinflammatory, antioxidant, antimicrobial and anticancer. In this study, the antitumor effect of six naphthoquinones isolated from the roots of Onosma visianii was evaluated using two cell lines, mouse melanoma B16 and highly aggressive rat glioma cell line C6.

METHODS AND RESULTS

All examined shikonins dose-dependently decreased the viability of tested cells, with compounds 5 and 6 being the most potent ones and hence subjected to further analysis. The diminished viability of B16 melanoma cells was in correlation with detected caspase-mediated apoptosis. Importantly, observed altered cell morphology along with the loss of dividing potential upon exposure to both shikonins implied reprogram of B16 cell phenotype. Elevated expression of myelin basic protein indicated the acquirement of Schwann-like cell phenotype, while detected autophagy might be connected to this phenomenon. On the contrary, upon exposure to both agents, C6 cells underwent specific cell death-anoikis, provoked by detachment from the extracellular matrix and compromised integrin signaling. Oppositely to compound 5, compound 6 realized anoikis in a caspase-independent manner and under sustained ERK1/2 activation, indicating the deviation from standard proanoikis signaling.

CONCLUSIONS

Herein, we have pointed out the diversity and novelty in the mode of action of shikonin derivatives depending on the tumor cell features, which represents a good platform for new investigations of these promising natural compounds.

Long-term culture induces Bax-dependent apoptosis in rat preimplantation embryos

Although rat preimplantation embryos are necessary for producing genetically modified rats, their in vitro culture remains a challenge. Rat zygotes can develop from the one-cell stage to the blastocyst stage in vitro; however, long-term culture reduces their developmental competence via an unknown mechanism. In this study, we examined how in vitro conditions affect rat preimplantation embryos, which may explain this reduced competence. Comprehensive gene expression analysis showed that genes related to apoptosis and energy metabolism were differentially expressed in rat embryos cultured long-term in vitro compared with those developed in vivo. Furthermore, we found that the expression of Bak1 and Bax, which are responsible for mitochondrial outer membrane permeabilization, were more upregulated in embryos cultured in vitro than those developed in vivo. Similarly, apoptosis-dependent DNA fragmentation was also exacerbated in in vitro culture conditions. Finally, gene disruption using CRISPR/Cas9 showed that Bax, but not Bak1, was responsible for these effects. These findings suggest that long-term in vitro culture induces Bax-dependent apoptosis through the mitochondrial pathway and may provide clues to improve the long-term culture of rat preimplantation embryos for genetic engineering research.

Role of intrinsic apoptosis in environmental exposure health outcomes

Environmental exposures are linked to diseases of high public health concern, including cancer, neurodegenerative disorders, and autoimmunity. These diseases are caused by excessive or insufficient cell death, prompting investigation of mechanistic links between environmental toxicants and dysregulation of cell death pathways, including apoptosis. This review describes how legacy and emerging environmental exposures target the intrinsic apoptosis pathway to potentially drive pathogenesis. Recent discoveries reveal that dynamic regulation of apoptosis may heighten the vulnerability of healthy tissues to exposures in children, and that apoptotic signaling can guide immune responses, tissue repair, and tumorigenesis. Understanding how environmental toxicants dysregulate apoptosis will uncover opportunities to deploy apoptosis-modulating agents for the treatment or prevention of exposure-linked diseases.

An optimized protocol for expression and purification of monomeric full-length BAX protein for functional interrogations

Diverse developmental signals and pro-death stresses converge on the regulation of the mitochondrial pathway of apoptosis. BAX, a proapoptotic BCL-2 effector, directly forms proteolipid pores in the outer mitochondrial membrane to activate the mitochondrial pathway of apoptosis. BAX is a viable pharmacological target for various human diseases, and increasing efforts have been made to study the molecular regulation of BAX while identifying small molecules selectively targeting BAX. However, generating large quantities of monomeric and functionally competent BAX has been challenging due to its aggregation-prone nature. Additionally, there is a lack of detailed and instructional protocols available for investigators who are not already familiar with recombinant BAX production. Here, we present a comprehensive protocol for expressing, purifying, and storing functional monomeric recombinant BAX protein. We use an intein-chitin binding domain-tagged BAX-expressing construct and employ a two-step chromatography strategy to capture and purify BAX. We also provide examples of standard assays to observe BAX activation, and highlight the best practices for handling and storing BAX to effectively preserve its quality, shelf life, and function.

An optimized high-yield protocol for expression and purification of monomeric full-length BAX protein

Diverse developmental signals and pro-death stresses converge on regulation of the mitochondrial pathway of apoptosis. BAX, a pro-apoptotic BCL-2 effector, directly forms proteolipid pores in the outer mitochondrial member to activate the mitochondrial pathway of apoptosis. BAX is a viable pharmacological target for various human diseases, and increasing efforts have been made to study the molecular regulation of BAX and identify small molecules selectively targeting BAX. However, generating large quantities of monomeric and functionally-competent BAX has been challenging due to its aggregation-prone nature. Additionally, there is a lack of detailed and instructional protocols available for investigators who are not already familiar with recombinant BAX production. Here, we present a comprehensive high-yield protocol for expressing, purifying, and storing functional recombinant BAX protein. We utilize an intein-tagged BAX construct and employ a two-step chromatography strategy to capture and purify BAX, and provide example standard assays to observe BAX activation. We also highlight best practices for handling and storing BAX to effectively preserve its quality, shelf-life, and function.

Radiotherapy-Induced Neurocognitive Impairment Is Driven by Heightened Apoptotic Priming in Early Life and Prevented by Blocking BAX

Although external beam radiotherapy (xRT) is commonly used to treat central nervous system (CNS) tumors in patients of all ages, young children treated with xRT frequently experience life-altering and dose-limiting neurocognitive impairment (NI) while adults do not. The lack of understanding of mechanisms responsible for these differences has impeded the development of neuroprotective treatments. Using a newly developed mouse model of xRT-induced NI, we found that neurocognitive function is impaired by ionizing radiation in a dose- and age-dependent manner, with the youngest animals being most affected. Histologic analysis revealed xRT-driven neuronal degeneration and cell death in neurogenic brain regions in young animals but not adults. BH3 profiling showed that neural stem and progenitor cells, neurons, and astrocytes in young mice are highly primed for apoptosis, rendering them hypersensitive to genotoxic damage. Analysis of single-cell RNA sequencing data revealed that neural cell vulnerability stems from heightened expression of proapoptotic genes including BAX, which is associated with developmental and mitogenic signaling by MYC. xRT induced apoptosis in primed neural cells by triggering a p53- and PUMA-initiated, proapoptotic feedback loop requiring cleavage of BID and culminating in BAX oligomerization and caspase activation. Notably, loss of BAX protected against apoptosis induced by proapoptotic signaling in vitro and prevented xRT-induced apoptosis in neural cells in vivo as well as neurocognitive sequelae. On the basis of these findings, preventing xRT-induced apoptosis specifically in immature neural cells by blocking BAX, BIM, or BID via direct or upstream mechanisms is expected to ameliorate NI in pediatric patients with CNS tumor.

SIGNIFICANCE

Age- and differentiation-dependent apoptotic priming plays a pivotal role in driving radiotherapy-induced neurocognitive impairment and can be targeted for neuroprotection in pediatric patients.

Mechanisms of BCL-2 family proteins in mitochondrial apoptosis

The proteins of the BCL-2 family are key regulators of mitochondrial apoptosis, acting as either promoters or inhibitors of cell death. The functional interplay and balance between the opposing BCL-2 family members control permeabilization of the outer mitochondrial membrane, leading to the release of activators of the caspase cascade into the cytosol and ultimately resulting in cell death. Despite considerable research, our knowledge about the mechanisms of the BCL-2 family of proteins remains insufficient, which complicates cell fate predictions and does not allow us to fully exploit these proteins as targets for drug discovery. Detailed understanding of the formation and molecular architecture of the apoptotic pore in the outer mitochondrial membrane remains a holy grail in the field, but new studies allow us to begin constructing a structural model of its arrangement. Recent literature has also revealed unexpected activities for several BCL-2 family members that challenge established concepts of how they regulate mitochondrial permeabilization. In this Review, we revisit the most important advances in the field and integrate them into a new structure-function-based classification of the BCL-2 family members that intends to provide a comprehensive model for BCL-2 action in apoptosis. We close this Review by discussing the potential of drugging the BCL-2 family in diseases characterized by aberrant apoptosis.

Mitochondrial outer membrane permeabilization and inner membrane permeabilization in regulating apoptosis and inflammation

Recent studies delineate an intimate crosstalk between apoptosis and inflammation. However, the dynamic mechanism linking them by mitochondrial membrane permeabilization remains elusive. Here, we construct a mathematical model consisting of four functional modules. Bifurcation analysis reveals that bistability stems from Bcl-2 family member interaction and time series shows that the time difference between Cyt c and mtDNA release is around 30 min, which are consistent with previous works. The model predicts that Bax aggregation kinetic determines cells to undergo apoptosis or inflammation, and that modulating the inhibitory effect of caspase 3 on IFN-β production allows the concurrent occurrence of apoptosis and inflammation. This work provides a theoretical framework for exploring the mechanism of mitochondrial membrane permeabilization in controlling cell fate.

Peptides from human BNIP5 and PXT1 and non-native binders of pro-apoptotic BAK can directly activate or inhibit BAK-mediated membrane permeabilization

Apoptosis is important for development and tissue homeostasis, and its dysregulation can lead to diseases, including cancer. As an apoptotic effector, BAK undergoes conformational changes that promote mitochondrial outer membrane disruption, leading to cell death. This is termed "activation" and can be induced by peptides from the human proteins BID, BIM, and PUMA. To identify additional peptides that can regulate BAK, we used computational protein design, yeast surface display screening, and structure-based energy scoring to identify 10 diverse new binders. We discovered peptides from the human proteins BNIP5 and PXT1 and three non-native peptides that activate BAK in liposome assays and induce cytochrome c release from mitochondria. Crystal structures and binding studies reveal a high degree of similarity among peptide activators and inhibitors, ruling out a simple function-determining property. Our results shed light on the vast peptide sequence space that can regulate BAK function and will guide the design of BAK-modulating tools and therapeutics.

Nano-TRAIL: a promising path to cancer therapy

Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand, also called apo-2 ligand (TRAIL/Apo-2L), is a cytokine that triggers apoptosis by binding to TRAIL-R1 (DR4) and TRAIL-R2 (DR5) death receptors. Apoptosis occurs through either the extrinsic or intrinsic pathway. The administration of recombinant human TRAIL (rhTRAIL) or TRAIL-receptor (TRAIL-R) agonists promotes apoptosis preferentially in cancerous cells over normal cells in vitro; this phenomenon has also been observed in clinical studies. The limited efficacy of rhTRAIL in clinical trials could be attributed to drug resistance, short half-life, targeted delivery issues, and off-target toxicities. Nanoparticles are excellent drug and gene delivery systems characterized by improved permeability and retention, increased stability and biocompatibility, and precision targeting. In this review, we discuss resistance mechanisms to TRAIL and methods to overcome TRAIL resistance by using nanoparticle-based formulations developed for the delivery of TRAIL peptides, TRAIL-R agonists, and TRAIL genes to cancer cells. We also discuss combinatorial approaches of chemotherapeutic drugs with TRAIL. These studies demonstrate TRAIL's potential as an anticancer agent.

Endoplasmic reticulum protein BIK binds to and inhibits mitochondria-localized antiapoptotic proteins

The proapoptotic BCL-2 homology (BH3)-only endoplasmic reticulum (ER)-resident protein BCL-2 interacting killer (BIK) positively regulates mitochondrial outer membrane permeabilization, the point of no return in apoptosis. It is generally accepted that BIK functions at a distance from mitochondria by binding and sequestering antiapoptotic proteins at the ER, thereby promoting ER calcium release. Although BIK is predominantly localized to the ER, we detect by fluorescence lifetime imaging microscopy-FRET microscopy, BH3 region-dependent direct binding between BIK and mitochondria-localized chimeric mutants of the antiapoptotic proteins BCL-XL and BCL-2 in both baby mouse kidney (BMK) and MCF-7 cells. Direct binding was accompanied by cell type-specific differential relocalization in response to coexpression of either BIK or one of its target binding partners, BCL-XL, when coexpressed in cells. In BMK cells with genetic deletion of both BAX and BAK (BMK-double KO), our data suggest that a fraction of BIK protein moves toward mitochondria in response to the expression of a mitochondria-localized BCL-XL mutant. In contrast, in MCF-7 cells, our data suggest that BIK is localized at both ER and mitochondria-associated ER membranes and binds to the mitochondria-localized BCL-XL mutant via relocalization of BCL-XL to ER and mitochondria-associated ER membrane. Rather than functioning at a distance, our data suggest that BIK initiates mitochondrial outer membrane permeabilization via direct interactions with ER and mitochondria-localized antiapoptotic proteins, which occur via ER-mitochondria contact sites, and/or by relocalization of either BIK or antiapoptotic proteins in cells.

Endogenous and imposed determinants of apoptotic vulnerabilities in cancer

The intrinsic apoptosis pathway is controlled by the BCL-2 family of proteins. Although the pro-survival members of this family can help cancer cells evade apoptosis, they may also produce apoptotic vulnerabilities that can potentially be exploited therapeutically. Apoptotic vulnerabilities can be driven by endogenous factors including altered genetics, signaling, metabolism, structure and lineage or differentiation state as well as imposed factors, the most prominent being exposure to anti-cancer agents. The recent development of BH3 mimetics that inhibit pro-survival BCL-2 family proteins has allowed these apoptotic vulnerabilities to be targeted with demonstrable clinical success. Here, we review the key concepts that are vital for understanding, uncovering, and exploiting apoptotic vulnerabilities in cancer for the potential improvement of patient outcomes.

Heightened apoptotic priming of vascular cells across tissues and life span predisposes them to cancer therapy-induced toxicities

Although major organ toxicities frequently arise in patients treated with cytotoxic or targeted cancer therapies, the mechanisms that drive them are poorly understood. Here, we report that vascular endothelial cells (ECs) are more highly primed for apoptosis than parenchymal cells across many adult tissues. Consequently, ECs readily undergo apoptosis in response to many commonly used anticancer agents including cytotoxic and targeted drugs and are more sensitive to ionizing radiation and BH3 mimetics than parenchymal cells in vivo. Further, using differentiated isogenic human induced pluripotent stem cell models of ECs and vascular smooth muscle cells (VSMCs), we find that these vascular cells exhibit distinct drug toxicity patterns, which are linked to divergent therapy-induced vascular toxicities in patients. Collectively, our results demonstrate that vascular cells are highly sensitive to apoptosis-inducing stress across life span and may represent a "weakest link" vulnerability in multiple tissues for development of toxicities.

Preclinical models of radiation-induced cardiac toxicity: Potential mechanisms and biomarkers

Radiation therapy (RT) is an important modality in cancer treatment with >50% of cancer patients undergoing RT for curative or palliative intent. In patients with breast, lung, and esophageal cancer, as well as mediastinal malignancies, incidental RT dose to heart or vascular structures has been linked to the development of Radiation-Induced Heart Disease (RIHD) which manifests as ischemic heart disease, cardiomyopathy, cardiac dysfunction, and heart failure. Despite the remarkable progress in the delivery of radiotherapy treatment, off-target cardiac toxicities are unavoidable. One of the best-studied pathological consequences of incidental exposure of the heart to RT is collagen deposition and fibrosis, leading to the development of radiation-induced myocardial fibrosis (RIMF). However, the pathogenesis of RIMF is still largely unknown. Moreover, there are no available clinical approaches to reverse RIMF once it occurs and it continues to impair the quality of life of long-term cancer survivors. Hence, there is an increasing need for more clinically relevant preclinical models to elucidate the molecular and cellular mechanisms involved in the development of RIMF. This review offers an insight into the existing preclinical models to study RIHD and the suggested mechanisms of RIMF, as well as available multi-modality treatments and outcomes. Moreover, we summarize the valuable detection methods of RIHD/RIMF, and the clinical use of sensitive radiographic and circulating biomarkers.

Exploiting endogenous and therapy-induced apoptotic vulnerabilities in immunoglobulin light chain amyloidosis with BH3 mimetics

Immunoglobulin light chain (AL) amyloidosis is an incurable hematologic disorder typically characterized by the production of amyloidogenic light chains by clonal plasma cells. These light chains misfold and aggregate in healthy tissues as amyloid fibrils, leading to life-threatening multi-organ dysfunction. Here we show that the clonal plasma cells in AL amyloidosis are highly primed to undergo apoptosis and dependent on pro-survival proteins MCL-1 and BCL-2. Notably, this MCL-1 dependency is indirectly targeted by the proteasome inhibitor bortezomib, currently the standard of care for this disease and the related plasma cell disorder multiple myeloma, due to upregulation of pro-apoptotic Noxa and its inhibitory binding to MCL-1. BCL-2 inhibitors sensitize clonal plasma cells to multiple front-line therapies including bortezomib, dexamethasone and lenalidomide. Strikingly, in mice bearing AL amyloidosis cell line xenografts, single agent treatment with the BCL-2 inhibitor ABT-199 (venetoclax) produces deeper remissions than bortezomib and triples median survival. Mass spectrometry-based proteomic analysis reveals rewiring of signaling pathways regulating apoptosis, proliferation and mitochondrial metabolism between isogenic AL amyloidosis and multiple myeloma cells that divergently alter their sensitivity to therapies. These findings provide a roadmap for the use of BH3 mimetics to exploit endogenous and induced apoptotic vulnerabilities in AL amyloidosis.

Modulation of Ca2+ signaling by antiapoptotic Bcl-2 versus Bcl-xL: From molecular mechanisms to relevance for cancer cell survival

Members of the Bcl-2-protein family are key controllers of apoptotic cell death. The family is divided into antiapoptotic (including Bcl-2 itself, Bcl-xL, Mcl-1, etc.) and proapoptotic members (Bax, Bak, Bim, Bim, Puma, Noxa, Bad, etc.). These proteins are well known for their canonical role in the mitochondria, where they control mitochondrial outer membrane permeabilization and subsequent apoptosis. However, several proteins are recognized as modulators of intracellular Ca²⁺ signals that originate from the endoplasmic reticulum (ER), the major intracellular Ca²⁺-storage organelle. More than 25 years ago, Bcl-2, the founding member of the family, was reported to control apoptosis through Ca²⁺ signaling. Further work elucidated that Bcl-2 directly targets and inhibits inositol 1,4,5-trisphosphate receptors (IP₃Rs), thereby suppressing proapoptotic Ca²⁺ signaling. In addition to Bcl-2, Bcl-xL was also shown to impact cell survival by sensitizing IP₃R function, thereby promoting prosurvival oscillatory Ca²⁺ release. However, new work challenges this model and demonstrates that Bcl-2 and Bcl-xL can both function as inhibitors of IP₃Rs. This suggests that, depending on the cell context, Bcl-xL could support very distinct Ca²⁺ patterns. This not only raises several questions but also opens new possibilities for the treatment of Bcl-xL-dependent cancers. In this review, we will discuss the similarities and divergences between Bcl-2 and Bcl-xL regarding Ca²⁺ homeostasis and IP₃R modulation from both a molecular and a functional point of view, with particular emphasis on cancer cell death resistance mechanisms.

DPCPX induces Bim-dependent apoptosis in nasopharyngeal carcinoma cells

ADORA1 promotes tumor growth and development in multiple cancers. DPCPX (a selective adenosine A1 receptor antagonist), a specific ADORA1 antagonist, has shown antitumor effects in many cancer types. Nevertheless, the function of DPCPX in nasopharyngeal carcinoma (NPC) still remains to be unraveled. In this study, we investigated the functional role of DPCPX on NPC cells. We found that DPCPX promotes NPC cells growth inhibition. DPCPX induced Bim-dependent apoptosis in NPC cells irrespective of p53 status via the FoxO3a pathway following PI3K/AKT inhibition. Furthermore, DPCPX enhanced the antitumor effect of cisplatin, 5-FU and Paclitaxel in NPC. Xenograft experiment revealed that deficiency of Bim in vivo stalls apoptosis and antitumor activity of DPCPX. In conclusion, the PI3K/AKT/FoxO3a/Bim axis plays a critical role in the anticancer effects of DPCPX in NPC.

ELK3 modulates the antitumor efficacy of natural killer cells against triple negative breast cancer by regulating mitochondrial dynamics

Background

Triple negative breast cancer (TNBC) is the most lethal subtype of breast cancer due to its aggressive behavior and frequent development of resistance to chemotherapy. Although natural killer (NK) cell-based immunotherapy is a promising strategy for overcoming barriers to cancer treatment, the therapeutic efficacy of NK cells against TNBC is below expectations. E26 transformation-specific transcription factor ELK3 (ELK3) is highly expressed in TNBCs and functions as a master regulator of the epithelial-mesenchymal transition.

Methods

Two representative human TNBC cell lines, MDA-MB231 and Hs578T, were exposed to ELK3-targeting shRNA or an ELK3-expressing plasmid to modulate ELK3 expression. The downstream target genes of ELK3 were identified using a combined approach comprising gene expression profiling and molecular analysis. The role of ELK3 in determining the immunosensitivity of TNBC to NK cells was investigated in terms of mitochondrial fission–fusion transition and reactive oxygen species concentration both in vitro and in vivo.

Results

ELK3-dependent mitochondrial fission–fusion status was linked to the mitochondrial superoxide concentration in TNBCs and was a main determinant of NK cell-mediated immune responses. We identified mitochondrial dynamics proteins of 51 (Mid51), a major mediator of mitochondrial fission, as a direct downstream target of ELK3 in TNBCs. Also, we demonstrated that expression of ELK3 correlated inversely with that of Mid51, and that the ELK3-Mid51 axis is associated directly with the status of mitochondrial dynamics. METABRIC analysis revealed that the ELK3-Mid51 axis has a direct effect on the immune score and survival of patients with TNBC.

Conclusions

Taken together, the data suggest that NK cell responses to TNBC are linked directly to ELK3 expression levels, shedding new light on strategies to improve the efficacy of NK cell-based immunotherapy of TNBC.

FLAMBE: A kinetic fluorescence polarization assay to study activation of monomeric BAX

BAX activation techniques are crucial to studying the intrinsic pathway of apoptosis- thousands of pro-apoptotic signals converge on BAX activation. Current methodologies are predominantly limited to membrane permeabilization studies, which assess endpoint functionality of oligomeric BAX, but overlook early activation steps of cytosolic BAX. Here we detail FLAMBE: a fluorescence polarization ligand assay for monitoring BAX early-activation in solution. We also describe a dual-metric parameterization strategy for distillation of kinetic data and comparative analyses when studying candidate ligands. For complete details on the use and execution of this protocol, please refer to Gelles et al. (2022).

Adapted to Survive: Targeting Cancer Cells with BH3 Mimetics

A hallmark of cancer is cell death evasion, underlying suboptimal responses to chemotherapy, targeted agents, and immunotherapies. The approval of the antiapoptotic BCL2 antagonist venetoclax has finally validated the potential of targeting apoptotic pathways in patients with cancer. Nevertheless, pharmacologic modulators of cell death have shown markedly varied responses in preclinical and clinical studies. Here, we review emerging concepts in the use of this class of therapies. Building on these observations, we propose that treatment-induced changes in apoptotic dependency, rather than pretreatment dependencies, will need to be recognized and targeted to realize the precise deployment of these new pharmacologic agents.

SIGNIFICANCE

Targeting antiapoptotic family members has proven efficacious and tolerable in some cancers, but responses are infrequent, particularly for patients with solid tumors. Biomarkers to aid patient selection have been lacking. Precision functional approaches that overcome adaptive resistance to these compounds could drive durable responses to chemotherapy, targeted therapy, and immunotherapies.

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.

Functional Genomic Identification of Predictors of Sensitivity and Mechanisms of Resistance to Multivalent Second-Generation TRAIL-R2 Agonists

Multivalent second-generation TRAIL-R2 agonists are currently in late preclinical development and early clinical trials. Herein, we use a representative second-generation agent, MEDI3039, to address two major clinical challenges facing these agents: lack of predictive biomarkers to enable patient selection and emergence of resistance. Genome-wide CRISPR knockout screens were notable for the lack of resistance mechanisms beyond the canonical TRAIL-R2 pathway (caspase-8, FADD, BID) as well as p53 and BAX in TP53 wild-type models, whereas a CRISPR activatory screen identified cell death inhibitors MCL-1 and BCL-XL as mechanisms to suppress MEDI3039-induced cell death. High-throughput drug screening failed to identify genomic alterations associated with response to MEDI3039; however, transcriptomics analysis revealed striking association between MEDI3039 sensitivity and expression of core components of the extrinsic apoptotic pathway, most notably its main apoptotic effector caspase-8 in solid tumor cell lines. Further analyses of colorectal cell lines and patient-derived xenografts identified caspase-8 expression ratio to its endogenous regulator FLIP(L) as predictive of sensitivity to MEDI3039 in several major solid tumor types and a further subset indicated by caspase-8:MCL-1 ratio. Subsequent MEDI3039 combination screening of TRAIL-R2, caspase-8, FADD, and BID knockout models with 60 compounds with varying mechanisms of action identified two inhibitor of apoptosis proteins (IAP) that exhibited strong synergy with MEDI3039 that could reverse resistance only in BID-deleted models. In summary, we identify the ratios of caspase-8:FLIP(L) and caspase-8:MCL-1 as potential predictive biomarkers for second-generation TRAIL-R2 agonists and loss of key effectors such as FADD and caspase-8 as likely drivers of clinical resistance in solid tumors.

Small-molecule targeted therapies induce dependence on DNA double-strand break repair in residual tumor cells

Residual cancer cells that survive drug treatments with targeted therapies act as a reservoir from which eventual resistant disease emerges. Although there is great interest in therapeutically targeting residual cells, efforts are hampered by our limited knowledge of the vulnerabilities existing in this cell state. Here, we report that diverse oncogene-targeted therapies, including inhibitors of epidermal growth factor receptor (EGFR), anaplastic lymphoma kinase (ALK), KRAS, and BRAF, induce DNA double-strand breaks and, consequently, ataxia-telangiectasia mutated (ATM)-dependent DNA repair in oncogene-matched residual tumor cells. This DNA damage response, observed in cell lines, mouse xenograft models, and human patients, is driven by a pathway involving the activation of caspases 3 and 7 and the downstream caspase-activated deoxyribonuclease (CAD). CAD is, in turn, activated through caspase-mediated degradation of its endogenous inhibitor, ICAD. In models of EGFR mutant non-small cell lung cancer (NSCLC), tumor cells that survive treatment with small-molecule EGFR-targeted therapies are thus synthetically dependent on ATM, and combined treatment with an ATM kinase inhibitor eradicates these cells in vivo. This led to more penetrant and durable responses in EGFR mutant NSCLC mouse xenograft models, including those derived from both established cell lines and patient tumors. Last, we found that rare patients with EGFR mutant NSCLC harboring co-occurring, loss-of-function mutations in ATM exhibit extended progression-free survival on first generation EGFR inhibitor therapy relative to patients with EGFR mutant NSCLC lacking deleterious ATM mutations. Together, these findings establish a rationale for the mechanism-based integration of ATM inhibitors alongside existing targeted therapies.

A kinetic fluorescence polarization ligand assay for monitoring BAX early activation

Developmental, homeostatic, and pharmacological pro-apoptotic signals converge by activating the BCL-2 family member BAX. Studies investigating molecular regulation of BAX are commonly limited to methodologies measuring endpoint phenotypes and do not assess activation of monomeric BAX. Here, we present FLAMBE, a fluorescence polarization ligand assay for monitoring BAX early activation, that measures activation-induced release of a peptide probe in real time. Using complementary parallel and tandem biochemical techniques, we validate, corroborate, and apply FLAMBE to a contemporary repertoire of BAX modulators, characterizing their contributions within the early steps of BAX activation. Additionally, we use FLAMBE to reveal that historically "dead" BAX mutants remain responsive to activation as quasi-functional monomers. We also identify data metrics for comparative analyses and demonstrate that FLAMBE data align with downstream functional observations. Collectively, FLAMBE advances our understanding of BAX activation and fills a methodological void for studying BAX with broad applications in cell biology and therapeutic development. MOTIVATION In vitro BAX activation studies are invaluable platforms for studying cellular and pharmacological modulators of apoptosis. The gold standard for studying BAX function relies on membrane permeabilization assays, which assess the pore-forming activity of oligomeric BAX. However, there are currently no rapid or kinetic assays to interrogate real-time activation of monomeric BAX in solution, thereby limiting any molecular insights that occur upstream of mitochondrial permeabilization. Furthermore, available methods to observe the activation of monomeric BAX suffer from low throughput and static observations. To address this methodological gap, we developed FLAMBE, a kinetic fluorescence polarization-based assay to measure monomeric BAX activation in solution via concomitant displacement of a labeled peptide. This approach maintains the benefits of rapid kinetic data generation in a low-cost microplate format without requiring specialized equipment or large quantities of protein. FLAMBE compliments available experimental strategies and expands the accessibility of investigators to monitor early steps within the BAX activation continuum.

How Do Hexokinases Inhibit Receptor-Mediated Apoptosis?

Simple Summary

In multicellular animals, cells autonomously respond to lethal stress by inducing cell death programs. The most common regulated cell death is apoptosis. Cells protect their neighbors from damage by their cell contents or infection through this process. Apoptosis can occur as a result of intrinsic stress or induced by surface receptors, for example, by immune cells. In most cases, receptor-mediated apoptosis also requires the intrinsic signaling pathway. Intrinsic apoptosis is controlled by proteins of the B-cell lymphoma 2 (BCL-2) family. Pro-apoptotic BCL-2 proteins are inhibited by retrotranslocation from the mitochondria into the cytosol until the cell commits to apoptosis. Increasingly, discoveries show that BCL-2 proteins are regulated by proteins that are not themselves members of the BCL-2 family. Here, we discuss the selective inhibition of the link between death receptors activation and intrinsic apoptosis by hexokinases. These enzymes funnel glucose into the cellular metabolism. Independently, hexokinases retrotranslocate BCL-2 proteins and thereby protect cells from receptor-mediated apoptosis.

Abstract

The regulated cell death apoptosis enables redundant or compromised cells in ontogeny and homeostasis to remove themselves receptor-dependent after extrinsic signaling or after internal stress by BCL-2 proteins on the outer mitochondrial membrane (OMM). Mitochondrial BCL-2 proteins are also often needed for receptor-mediated signaling in apoptosis. Then, the truncated BH3-only protein BID (tBID) blocks retrotranslocation of the pro-apoptotic BCL-2 proteins BAX and BAK from the mitochondria into the cytosol. BAX and BAK in turn permeabilize the OMM. Although the BCL-2 proteins are controlled by a complex regulatory network, a specific mechanism for the inhibition of tBID remained unknown. Curiously, it was suggested that hexokinases, which channel glucose into the metabolism, have an intriguing function in the regulation of apoptosis. Recent analysis of transient hexokinase interactions with BAX revealed its participation in the inhibition of BAX and also BAK by retrotranslocation from mitochondria to the cytosol. In contrast to general apoptosis inhibition by anti-apoptotic BCL-2 proteins, hexokinase I and hexokinase 2 specifically inhibit tBID and thus the mitochondrial apoptosis pathway in response to death receptor signaling. Mitochondrial hexokinase localization and BH3 binding of cytosolic hexokinase domains are prerequisites for protection against receptor-mediated cell death, whereas glucose metabolism is not. This mechanism protects cells from apoptosis induced by cytotoxic T cells.

Physiological and pharmacological modulation of BAX

Bcl-2-associated X protein (BAX) is a critical executioner of mitochondrial regulated cell death through its lethal activity of permeabilizing the mitochondrial outer membrane (MOM). While the physiological function of BAX ensures tissue homeostasis, dysregulation of BAX leads to aberrant cell death. Despite BAX being a promising therapeutic target for human diseases, historically the development of drugs has focused on antiapoptotic BCL-2 proteins, due to challenges in elucidating the mechanism of BAX activation and identifying druggable surfaces of BAX. Here, we discuss recent studies that have provided structure-function insights and identified regulatory surfaces that control BAX activation. Moreover, we emphasize the development of small molecule orthosteric, allosteric, and oligomerization modulators that provide novel opportunities for biological investigation and progress towards drugging BAX.

Reduced ER-mitochondria connectivity promotes neuroblastoma multidrug resistance

Most cancer deaths result from progression of therapy resistant disease, yet our understanding of this phenotype is limited. Cancer therapies generate stress signals that act upon mitochondria to initiate apoptosis. Mitochondria isolated from neuroblastoma cells were exposed to tBid or Bim, death effectors activated by therapeutic stress. Multidrug‐resistant tumor cells obtained from children at relapse had markedly attenuated Bak and Bax oligomerization and cytochrome c release (surrogates for apoptotic commitment) in comparison with patient‐matched tumor cells obtained at diagnosis. Electron microscopy identified reduced ER–mitochondria‐associated membranes (MAMs; ER–mitochondria contacts, ERMCs) in therapy‐resistant cells, and genetically or biochemically reducing MAMs in therapy‐sensitive tumors phenocopied resistance. MAMs serve as platforms to transfer Ca²⁺ and bioactive lipids to mitochondria. Reduced Ca²⁺ transfer was found in some but not all resistant cells, and inhibiting transfer did not attenuate apoptotic signaling. In contrast, reduced ceramide synthesis and transfer was common to resistant cells and its inhibition induced stress resistance. We identify ER–mitochondria‐associated membranes as physiologic regulators of apoptosis via ceramide transfer and uncover a previously unrecognized mechanism for cancer multidrug resistance.

The interplay between BAX and BAK tunes apoptotic pore growth to control mitochondrial-DNA-mediated inflammation

BAX and BAK are key apoptosis regulators that mediate the decisive step of mitochondrial outer membrane permeabilization. However, the mechanism by which they assemble the apoptotic pore remains obscure. Here, we report that BAX and BAK present distinct oligomerization properties, with BAK organizing into smaller structures with faster kinetics than BAX. BAK recruits and accelerates BAX assembly into oligomers that continue to grow during apoptosis. As a result, BAX and BAK regulate each other as they co-assemble into the same apoptotic pores, which we visualize. The relative availability of BAX and BAK molecules thereby determines the growth rate of the apoptotic pore and the relative kinetics by which mitochondrial contents, most notably mtDNA, are released. This feature of BAX and BAK results in distinct activation kinetics of the cGAS/STING pathway with implications for mtDNA-mediated paracrine inflammatory signaling.

African Swine Fever Virus pE199L Induces Mitochondrial-Dependent Apoptosis

African swine fever (ASF) is a severe hemorrhagic disease in swine characterized by massive lymphocyte depletion and cell death, with apoptosis and necrosis in infected lymphoid tissues. However, the molecular mechanism regarding ASFV-induced cell death remains largely unknown. In this study, 94 ASFV-encoded proteins were screened to determine the viral proteins involved in cell death in vitro, and pE199L showed the most significant effect. Ectopic expression of pE199L in porcine cells (CRL-2843) and human cells (HEK293T and HeLa cells) induced cell death remarkably, showing obvious shrinking, blistering, apoptotic bodies, and nuclear DNA fragments. Meanwhile, cell death was markedly alleviated when the expression of pE199L was knocked down during ASFV infection. Additionally, the expression of pE199L caused a loss of mitochondrial membrane potential, release of cytochrome C, and caspase-9 and -3/7 activation, indicating that the mitochondrial apoptotic pathway was involved in pE199L-induced apoptosis. Further investigations showed that pE199L interacted with several anti-apoptotic BCL-2 subfamily members (such as BCL-XL, MCL-1, BCL-W, and BCL-2A1) and competed with BAK for BCL-XL, which promoted BAK and BAX activation. Taken together, ASFV pE199L induces the mitochondrial-dependent apoptosis, which may provide clues for a comprehensive understanding of ASFV pathogenesis.

The Stress-Inducible BCL2A1 Is Required for Ovarian Cancer Metastatic Progression in the Peritoneal Microenvironment

Emerging evidence indicates that hypoxia plays a critical role in governing the transcoelomic metastasis of ovarian cancer. Hence, targeting hypoxia may be a promising approach to prevent the metastasis of ovarian cancer. Here, we report that BCL2A1, a BCL2 family member, acts as a hypoxia-inducible gene for promoting tumor progression in ovarian cancer peritoneal metastases. We demonstrated that BCL2A1 was induced not only by hypoxia but also other physiological stresses through NF-κB signaling and then was gradually reduced by the ubiquitin-proteasome pathway in ascites-derived ovarian cancer cells. The upregulated BCL2A1 was frequently found in advanced metastatic ovarian cancer cells, suggesting its clinical relevance in ovarian cancer metastatic progression. Functionally, BCL2A1 enhanced the foci formation ability of ovarian cancer cells in a stress-conditioned medium, colony formation in an ex vivo omental tumor model, and tumor dissemination in vivo. Under stress conditions, BCL2A1 accumulated and colocalized with mitochondria to suppress intrinsic cell apoptosis by interacting with the BH3-only subfamily BCL2 members HRK/BAD/BID in ovarian cancer cells. These findings indicate that BCL2A1 is an early response factor that maintains the survival of ovarian cancer cells in the harsh tumor microenvironment.

Hexokinases inhibit death receptor-dependent apoptosis on the mitochondria

Receptor–ligand interactions on the cell surface or intrinsic stress signals can commit mammalian cells to apoptosis. In this study, we discover how hexokinases confer resistance to receptor-mediated apoptosis through specific inhibition of B-cell lymphoma 2 (BCL-2) proteins. Hexokinases retrotranslocate activator and effector BCL-2 proteins from the mitochondria into the cytosol. Hexokinase-dependent BCL-2 protein retrotranslocation can protect cells from apoptosis despite death receptor signaling.

Death receptor–mediated apoptosis requires the mitochondrial apoptosis pathway in many mammalian cells. In response to death receptor signaling, the truncated BH3-only protein BID can activate the proapoptotic BCL-2 proteins BAX and BAK and trigger the permeabilization of the mitochondria. BAX and BAK are inhibited by prosurvival BCL-2 proteins through retrotranslocation from the mitochondria into the cytosol, but a specific resistance mechanism to truncated BID-dependent apoptosis is unknown. Here, we report that hexokinase 1 and hexokinase 2 inhibit the apoptosis activator truncated BID as well as the effectors BAX and BAK by retrotranslocation from the mitochondria into the cytosol. BCL-2 protein shuttling and protection from TRAIL- and FasL-induced cell death requires mitochondrial hexokinase localization and interactions with the BH3 motifs of BCL-2 proteins but not glucose phosphorylation. Together, our work establishes hexokinase-dependent retrotranslocation of truncated BID as a selective protective mechanism against death receptor–induced apoptosis on the mitochondria.

Methods for Studying Myofibroblast Apoptotic Pathways

Evasion of apoptosis by myofibroblasts is a hallmark of fibrotic diseases, ultimately leading to persistent myofibroblast activation, extracellular matrix (ECM) deposition, and remodeling. Targeting myofibroblast apoptosis is emerging as a novel therapeutic strategy to reverse established fibrosis. We have recently discovered that in the process of fibroblast-to-myofibroblast transdifferentiation driven by matrix stiffness, the "mitochondrial priming" (readiness to undergo apoptosis) is dramatically increased in stiffness-activated myofibroblasts. Thus, myofibroblasts, traditionally viewed as apoptosis-resistant cells, appear poised to die when survival pathways are blocked, a cellular state we call "primed for death." This apoptosis-prone phenotype is driven by high levels of pro-apoptotic proteins loaded in myofibroblast's mitochondria, which require concomitant upregulation of pro-survival BCL-2 proteins to suppress mitochondrial apoptosis and ensure survival. Here, we describe a method called BH3 profiling which measures myo/fibroblast apoptotic priming as well as their antiapoptotic dependencies for survival. In addition, we describe how BH3 profiling can be used to predict myofibroblast responses to therapeutic agents targeting pro-survival BCL-2 proteins, also known as BH3 mimetic drugs. Finally, we describe methods to assess myofibroblast sensitivity to extrinsic apoptosis via Annexin V staining.

Identification of 22 susceptibility loci associated with testicular germ cell tumors

Testicular germ cell tumors (TGCT) are the most common tumor in young white men and have a high heritability. In this study, the international Testicular Cancer Consortium assemble 10,156 and 179,683 men with and without TGCT, respectively, for a genome-wide association study. This meta-analysis identifies 22 TGCT susceptibility loci, bringing the total to 78, which account for 44% of disease heritability. Men with a polygenic risk score (PRS) in the 95th percentile have a 6.8-fold increased risk of TGCT compared to men with median scores. Among men with independent TGCT risk factors such as cryptorchidism, the PRS may guide screening decisions with the goal of reducing treatment-related complications causing long-term morbidity in survivors. These findings emphasize the interconnected nature of two known pathways that promote TGCT susceptibility: male germ cell development within its somatic niche and regulation of chromosomal division and structure, and implicate an additional biological pathway, mRNA translation.

Pharmacological Targeting of Executioner Proteins: Controlling Life and Death

Small-molecule mediated modulation of protein interactions of Bcl-2 (B-cell lymphoma-2) family proteins was clinically validated in 2015 when Venetoclax, a selective inhibitor of the antiapoptotic protein BCL-2, achieved breakthrough status designation by the FDA for treatment of lymphoid malignancies. Since then, substantial progress has been made in identifying inhibitors of other interactions of antiapoptosis proteins. However, targeting their pro-apoptotic counterparts, the "executioners" BAX, BAK, and BOK that both initiate and commit the cell to dying, has lagged behind. However, recent publications demonstrate that these proteins can be positively or negatively regulated using small molecule tool compounds. The results obtained with these molecules suggest that pharmaceutical regulation of apoptosis will have broad implications that extend beyond activating cell death in cancer. We review recent advances in identifying compounds and their utility in the exogenous control of life and death by regulating executioner proteins, with emphasis on the prototype BAX.

Organelle degradation in the lens by PLAAT phospholipases

The eye lens of vertebrates is composed of fibre cells in which all membrane-bound organelles undergo degradation during terminal differentiation to form an organelle-free zone¹. The mechanism that underlies this large-scale organelle degradation remains largely unknown, although it has previously been shown to be independent of macroautophagy^2,3. Here we report that phospholipases in the PLAAT (phospholipase A/acyltransferase, also known as HRASLS) family-Plaat1 (also known as Hrasls) in zebrafish and PLAAT3 (also known as HRASLS3, PLA2G16, H-rev107 or AdPLA) in mice^4-6-are essential for the degradation of lens organelles such as mitochondria, the endoplasmic reticulum and lysosomes. Plaat1 and PLAAT3 translocate from the cytosol to various organelles immediately before organelle degradation, in a process that requires their C-terminal transmembrane domain. The translocation of Plaat1 to organelles depends on the differentiation of fibre cells and damage to organelle membranes, both of which are mediated by Hsf4. After the translocation of Plaat1 or PLAAT3 to membranes, the phospholipase induces extensive organelle rupture that is followed by complete degradation. Organelle degradation by PLAAT-family phospholipases is essential for achieving an optimal transparency and refractive function of the lens. These findings expand our understanding of intracellular organelle degradation and provide insights into the mechanism by which vertebrates acquired transparent lenses.

Evaluation of the Anticancer Activity of pH-Sensitive Polyketal Nanoparticles for Acute Myeloid Leukemia

Polyketals are a class of acid-responsive polymers that have been relatively less explored for drug delivery applications compared to polyesters. The degradation of these polymers is accelerated in an acidic medium and does not result in acidic byproducts. Their biocompatibility depends on the diol used for the synthesis. The present work aims to synthesize, characterize, and fabricate nanospheres of an aliphatic polyketal for delivery of the nucleotide analogue cytarabine toward the treatment of acute myeloid leukemia (AML). The internalization mechanism of the nanospheres was probed, and its implication on the nuclear localization and escape from the endo-lysosomal compartments were studied. The drug-loaded polyketal nanoparticles reduced the cell viability to a greater extent compared with the free drug. The effect of the drug-loaded polyketal nanoparticles on the differential gene expression of leukemic cells was investigated for the first time to understand their therapeutic implications. It was found that treatment with drug-loaded polyketal nanoparticles downregulated AML-specific genes involved in cell proliferation and recurrence compared to the free drug. The protein expression studies were performed for selected genes obtained from gene expression analysis. Biodistribution studies showed that the poly(cyclohexane-1,4-diyl acetone dimethylene ketal) (PCADK) nanoparticles exhibit prolonged circulation time. Overall, our results suggest that polyketal-based delivery of cytarabine represents a more effective alternative strategy for AML therapy.

Zika virus infection induced apoptosis by modulating the recruitment and activation of pro-apoptotic protein Bax

Zika virus (ZIKV) infection is associated with microcephaly in newborns and serious neurological complications in adults. Apoptosis of neural progenitor cells induced by ZIKV infection is believed to be a main reason for ZIKV infection-related microcephaly. However, the detailed mechanism of ZIKV infection-induced apoptosis remains to be elucidated. In this report, ZIKV infection induced the conformational activation of the pro-apoptotic protein Bax, with subsequent formation of oligomers of Bax in the mitochondria. Cell apoptosis was reduced significantly in SY5Y cells subjected to Bax knockdown. Additionally, while decreasing Bax expression inhibited the release of Cyt c from the mitochondria and reduced the rate of loss of mitochondrial membrane potential induced by ZIKV infection, silencing Bak, caspase-8, and/or caspase-10 expression did not. Mitochondria isolated from the untreated ZIKV-infected cells displayed Bax-binding ability and the subsequent release of Cyt c. This study also indicated that the NS4B protein of ZIKV recruited Bax to the mitochondria and induced Bax conformational activation. The overexpressed NS4B was localized to the mitochondria and induced cell apoptosis by activating the pro-apoptotic protein Bax. All the above results indicated that ZIKV infection directly impacted the mitochondrial apoptotic pathway by modulating the recruitment and activation of Bax.Importance: Since the large outbreaks that occurred in the Pacific Islands and Latin America in 2013, Zika virus has been confirmed a neuroteratogenic pathogen and causative agent of microcephaly and other developmental anomalies of the central nervous system in children born to infected mothers. As the widespread apoptosis throughout the whole brain, studies in animal models have reinforced the link between microcephaly caused by ZIKV infection and NPC apoptosis. Currently, the detailed mechanism of ZIKV infection-induced apoptosis still remains to be elucidated. Here, we firstly demonstrate that ZIKV infection activated the classic signs of mitochondrial apoptotic pathway by modulating the recruitment and activation of Bax. ZIKV NS4B represents a novel viral apoptotic protein that can modulate the recruitment and activation of Bax and trigger the apoptotic program. This is a new insight into understanding the interplay between apoptosis and ZIKV infection.

The role of G-CSF neuroprotective effects in neonatal hypoxic-ischemic encephalopathy (HIE): current status

Hypoxic-ischemic encephalopathy (HIE) is an important cause of permanent damage to central nervous system (CNS) that may result in neonatal death or manifest later as mental retardation, epilepsy, cerebral palsy, or developmental delay. The primary cause of this condition is systemic hypoxemia and/or reduced cerebral blood flow with long-lasting neurological disabilities and neurodevelopmental impairment in neonates. About 20 to 25% of infants with HIE die in the neonatal period, and 25-30% of survivors are left with permanent neurodevelopmental abnormalities. The mechanisms of hypoxia-ischemia (HI) include activation and/or stimulation of myriad of cascades such as increased excitotoxicity, oxidative stress, N-methyl-D-aspartic acid (NMDA) receptor hyperexcitability, mitochondrial collapse, inflammation, cell swelling, impaired maturation, and loss of trophic support. Different therapeutic modalities have been implicated in managing neonatal HIE, though translation of most of these regimens into clinical practices is still limited. Therapeutic hypothermia, for instance, is the most widely used standard treatment in neonates with HIE as studies have shown that it can inhibit many steps in the excito-oxidative cascade including secondary energy failure, increases in brain lactic acid, glutamate, and nitric oxide concentration. Granulocyte-colony stimulating factor (G-CSF) is a glycoprotein that has been implicated in stimulation of cell survival, proliferation, and function of neutrophil precursors and mature neutrophils. Extensive studies both in vivo and ex vivo have shown the neuroprotective effect of G-CSF in neurodegenerative diseases and neonatal brain damage via inhibition of apoptosis and inflammation. Yet, there are still few experimentation models of neonatal HIE and G-CSF's effectiveness, and extrapolation of adult stroke models is challenging because of the evolving brain. Here, we review current studies and/or researches of G-CSF's crucial role in regulating these cytokines and apoptotic mediators triggered following neonatal brain injury, as well as driving neurogenesis and angiogenesis post-HI insults.

In Vitro Apoptotic Cell Death Assessment

Chemical compounds induce cytotoxicity by various mechanisms, including interference in membrane integrity, metabolism, cellular component degradation or release, and cell division. Between the classic death pathways, namely, autophagy, apoptosis, and necrosis, apoptosis have been in the focus for the last several years as an important pathway for the toxicity of different types of xenobiotics. Because of that, having the tools to evaluate it is key for understanding and explaining the toxicodynamics of different classes of substances. Here, we describe a wide array of classic assays that can be easily implemented to evaluate apoptosis induction.

Inhibition of Bcl-2 and Bcl-xL overcomes the resistance to the third-generation EGFR tyrosine kinase inhibitor osimertinib in non-small cell lung cancer

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) have demonstrated significant benefits to patients with non‑small cell lung cancer (NSCLC) harboring EGFR‑activating mutations; however, acquired resistance limits their long‑term efficacy. Therefore, it remains an urgent requirement to discover the underlying mechanisms and investigate novel therapeutic strategies for overcoming the resistance to EGFR TKIs. The present study aimed to determine the mechanism underlying the resistance of NSCLC cells to osimertinib, a third‑generation EGFR tyrosine kinase inhibitor, the osimertinib‑resistant NSCLC cell sub‑line HCC827/OR was established in the present study. It was found that the expression levels of Bcl‑2 and Bcl‑xL were significantly upregulated in resistant cells compared with sensitive cells. Furthermore, the suppression of Bcl‑2 and Bcl‑xL through small interfering RNA‑mediated gene knockdown or using a small molecule specific inhibitor ABT‑263 re‑sensitized HCC827/OR cells to osimertinib treatment. Moreover, the combined treatment of HCC827/OR cells with ABT‑263 and osimertinib enhanced the rate of cell apoptosis through the mitochondrial apoptotic pathway. Finally, ABT‑263 was able to overcome the resistance of osimertinib in xenograft tumor models. In conclusion, these findings may provide an improved concept for the development of a novel combined therapeutic strategy for the treatment of NSCLC resistance to EGFR TKIs.

Reduced Mitochondrial Apoptotic Priming Drives Resistance to BH3 Mimetics in Acute Myeloid Leukemia

Acquired resistance to BH3 mimetic antagonists of BCL-2 and MCL-1 is an important clinical problem. Using acute myelogenous leukemia (AML) patient-derived xenograft (PDX) models of acquired resistance to BCL-2 (venetoclax) and MCL-1 (S63845) antagonists, we identify common principles of resistance and persistent vulnerabilities to overcome resistance. BH3 mimetic resistance is characterized by decreased mitochondrial apoptotic priming as measured by BH3 profiling, both in PDX models and human clinical samples, due to alterations in BCL-2 family proteins that vary among cases, but not to acquired mutations in leukemia genes. BCL-2 inhibition drives sequestered pro-apoptotic proteins to MCL-1 and vice versa, explaining why in vivo combinations of BCL-2 and MCL-1 antagonists are more effective when concurrent rather than sequential. Finally, drug-induced mitochondrial priming measured by dynamic BH3 profiling (DBP) identifies drugs that are persistently active in BH3 mimetic-resistant myeloblasts, including FLT-3 inhibitors and SMAC mimetics.

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.

Bcl‑2 family: Novel insight into individualized therapy for ovarian cancer (Review)

Chemoresistance to platinum‑based chemotherapy for ovarian cancer in the advanced stage remains a formidable concern clinically. Increasing evidence has revealed that apoptosis represents the terminal events of the anti‑tumor mechanisms of a number of chemical drugs and has a close association with chemoresistance in ovarian cancer. The B‑cell lymphoma‑2 (Bcl‑2) family plays a crucial role in apoptosis and has a close association with chemoresistance in ovarian cancer. Some drugs that target Bcl‑2 family members have shown efficacy in overcoming the chemoresistance of ovarian cancer. A BH3 profiling assay was found to be able to predict how primed a cell is when treated with antitumor drugs. The present review summarizes the role of the Bcl‑2 family in mediating cell death in response to antitumor drugs and novel drugs that target Bcl‑2 family members. The application of the new functional assay, BH3 profiling, is also discussed herein. Furthermore, the present review presents the hypothesis that targeting Bcl‑2 family members may prove to be helpful for the individualized therapy of ovarian cancer in clinical practice and in laboratory research.

Sequential combinations of chemotherapeutic agents with BH3 mimetics to treat rhabdomyosarcoma and avoid resistance

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in childhood and adolescence. Refractory/relapsed RMS patients present a bad prognosis that combined with the lack of specific biomarkers impairs the development of new therapies. Here, we utilize dynamic BH3 profiling (DBP), a functional predictive biomarker that measures net changes in mitochondrial apoptotic signaling, to identify anti-apoptotic adaptations upon treatment. We employ this information to guide the use of BH3 mimetics to specifically inhibit BCL-2 pro-survival proteins, defeat resistance and avoid relapse. Indeed, we found that BH3 mimetics that selectively target anti-apoptotic BCL-xL and MCL-1, synergistically enhance the effect of clinically used chemotherapeutic agents vincristine and doxorubicin in RMS cells. We validated this strategy in vivo using a RMS patient-derived xenograft model and observed a reduction in tumor growth with a tendency to stabilization with the sequential combination of vincristine and the MCL-1 inhibitor S63845. We identified the molecular mechanism by which RMS cells acquire resistance to vincristine: an enhanced binding of BID and BAK to MCL-1 after drug exposure, which is suppressed by subsequently adding S63845. Our findings validate the use of DBP as a functional assay to predict treatment effectiveness in RMS and provide a rationale for combining BH3 mimetics with chemotherapeutic agents to avoid tumor resistance, improve treatment efficiency, and decrease undesired secondary effects.

The third model of Bax/Bak activation: a Bcl-2 family feud finally resolved?

Bax and Bak, two functionally similar, pro-apoptotic proteins of the Bcl-2 family, are known as the gateway to apoptosis because of their requisite roles as effectors of mitochondrial outer membrane permeabilization (MOMP), a major step during mitochondria-dependent apoptosis. The mechanism of how cells turn Bax/Bak from inert molecules into fully active and lethal effectors had long been the focal point of a major debate centered around two competing, but not mutually exclusive, models: direct activation and indirect activation. After intensive research efforts for over two decades, it is now widely accepted that to initiate apoptosis, some of the BH3-only proteins, a subclass of the Bcl-2 family, directly engage Bax/Bak to trigger their conformational transformation and activation. However, a series of recent discoveries, using previously unavailable CRISPR-engineered cell systems, challenge the basic premise that undergirds the consensus and provide evidence for a novel and surprisingly simple model of Bax/Bak activation: the membrane (lipids)-mediated spontaneous model. This review will discuss the evidence, rationale, significance, and implications of this new model.

The pseudo-caspase FLIP(L) regulates cell fate following p53 activation

p53 is the most frequently mutated, well-studied tumor-suppressor gene, yet the molecular basis of the switch from p53-induced cell-cycle arrest to apoptosis remains poorly understood. Using a combination of transcriptomics and functional genomics, we unexpectedly identified a nodal role for the caspase-8 paralog and only human pseudo-caspase, FLIP(L), in regulating this switch. Moreover, we identify FLIP(L) as a direct p53 transcriptional target gene that is rapidly up-regulated in response to Nutlin-3A, an MDM2 inhibitor that potently activates p53. Genetically or pharmacologically inhibiting expression of FLIP(L) using siRNA or entinostat (a clinically relevant class-I HDAC inhibitor) efficiently promoted apoptosis in colorectal cancer cells in response to Nutlin-3A, which otherwise predominantly induced cell-cycle arrest. Enhanced apoptosis was also observed when entinostat was combined with clinically relevant, p53-activating chemotherapy in vitro, and this translated into enhanced in vivo efficacy. Mechanistically, FLIP(L) inhibited p53-induced apoptosis by blocking activation of caspase-8 by the TRAIL-R2/DR5 death receptor; notably, this activation was not dependent on receptor engagement by its ligand, TRAIL. In the absence of caspase-8, another of its paralogs, caspase-10 (also transcriptionally up-regulated by p53), induced apoptosis in Nutlin-3A-treated, FLIP(L)-depleted cells, albeit to a lesser extent than in caspase-8-proficient cells. FLIP(L) depletion also modulated transcription of canonical p53 target genes, suppressing p53-induced expression of the cell-cycle regulator p21 and enhancing p53-induced up-regulation of proapoptotic PUMA. Thus, even in the absence of caspase-8/10, FLIP(L) silencing promoted p53-induced apoptosis by enhancing PUMA expression. Thus, we report unexpected, therapeutically relevant roles for FLIP(L) in determining cell fate following p53 activation.