A role for caspases in lens fiber differentiation

The concealed side of caspases: beyond a killer of cells

Since the late 20th century, researchers have known that caspases are a pillar of cell death, particularly apoptosis. However, recent advances in cell biology have unraveled the multiple roles of caspases. These enzymes have an unconventional role in cell proliferation, differentiation, and invasion. As a result, caspase deregulation can fuel the fire of cancer, incite flames of inflammation, flare neurodegenerative disorders, and exacerbate skin pathologies. Several therapeutic approaches toward caspase inhibition have been investigated, but can caspase inhibitors harness the maladaptive effect of these proteases without causing significant side effects? A few studies have exploited caspase induction for cancer or adoptive cell therapies. Here, we provide a compelling picture of caspases, starting with their evolution, their polytomous roles beyond cell death, the flaws of their deregulation, and the merits of targeting them for therapeutic implications. Furthermore, we provide a deeper understanding of the evolution of caspase-related research up to the current era, pinpointing the role of caspases in cell survival and aiding in the development of effective caspase-targeted therapies.

Toward a predictive understanding of epithelial cell death

Epithelial cell death is highly prevalent during development and tissue homeostasis. While we have a rather good understanding of the molecular regulators of programmed cell death, especially for apoptosis, we still fail to predict when, where, how many and which specific cells will die in a tissue. This likely relies on the much more complex picture of apoptosis regulation in a tissular and epithelial context, which entails cell autonomous but also non-cell autonomous factors, diverse feedback and multiple layers of regulation of the commitment to apoptosis. In this review, we illustrate this complexity of epithelial apoptosis regulation by describing these different layers of control, all demonstrating that local cell death probability is a complex emerging feature. We first focus on non-cell autonomous factors that can locally modulate the rate of cell death, including cell competition, mechanical input and geometry as well as systemic effects. We then describe the multiple feedback mechanisms generated by cell death itself. We also outline the multiple layers of regulation of epithelial cell death, including the coordination of extrusion and regulation occurring downstream of effector caspases. Eventually, we propose a roadmap to reach a more predictive understanding of cell death regulation in an epithelial context.

The involvement of caspases in the process of nuclear removal during lens fiber cell differentiation

The terminal differentiation of lens fiber cells involves elimination of their organelles, which must occur while still maintaining their functionality throughout a lifetime. Removal of non-nuclear organelles is accomplished through induction of autophagy following the spatiotemporal suppression of the PI3K/Akt signaling axis. However, blocking this pathway is not alone sufficient to induce removal of fiber cell nuclei. While the final steps in fiber cell nuclear elimination are highlighted by the appearance of TUNEL-positive nuclei, which are associated with activation of the lens-specific DNaseIIβ, there are many steps in the process that precede the appearance of double stranded DNA breaks. We showed that this carefully regulated process, including the early changes in nuclear morphology resulting in nuclear condensation, cleavage of lamin B, and labeling by pH2AX, is reminiscent of the apoptotic process associated with caspase activation. Multiple caspases are known to be expressed and activated during lens cell differentiation. In this study, we investigated the link between two caspase downstream targets associated with apoptosis, ICAD, whose cleavage by caspase-3 leads to activation of CAD, a DNase that can create both single- and double-stranded DNA cleavages, and lamin B, a primary component of the nuclear lamina. We discovered that the specific inhibition of caspase-3 activation prevents both lamin B and DNA cleavage. Inhibiting caspase-3 did not prevent nuclear condensation or removal of the nuclear membrane. In contrast, a pan-caspase inhibitor effectively suppressed condensation of fiber cell nuclei during differentiation. These studies provide evidence that caspases play an important role in the process of removing fiber cell nuclei during lens differentiation.

Targeting LGSN restores sensitivity to chemotherapy in gastric cancer stem cells by triggering pyroptosis

Gastric cancer (GC) is notoriously resistant to current therapies due to tumor heterogeneity. Cancer stem cells (CSCs) possess infinite self-renewal potential and contribute to the inherent heterogeneity of GC. Despite its crucial role in chemoresistance, the mechanism of stemness maintenance of gastric cancer stem cells (GCSCs) remains largely unknown. Here, we present evidence that lengsin, lens protein with glutamine synthetase domain (LGSN), a vital cell fate determinant, is overexpressed in GCSCs and is highly correlated with malignant progression and poor survival in GC patients. Ectopic overexpression of LGSN in GCSC-derived differentiated cells facilitated their dedifferentiation and treatment resistance by interacting with vimentin and inducing an epithelial-to-mesenchymal transition. Notably, genetic interference of LGSN effectively suppressed tumor formation by inhibiting GCSC stemness maintenance and provoking gasdermin-D-mediated pyroptosis through vimentin degradation/NLRP3 signaling. Depletion of LGSN combined with the chemo-drugs 5-fluorouracil and oxaliplatin could offer a unique and promising approach to synergistically rendering this deadly cancer eradicable in vivo. Our data place focus on the role of LGSN in GCSC regeneration and emphasize the critical importance of pyroptosis in battling GCSC.

Mitochondrial Bcl-xL promotes brain synaptogenesis by controlling non-lethal caspase activation

Non-lethal caspase activation (NLCA) has been linked to neurodevelopmental processes. However, how neurons control NLCA remains elusive. Here, we focused on Bcl-xL, a Bcl-2 homolog regulating caspase activation through the mitochondria. We generated a mouse model, referred to as ER-xL, in which Bcl-xL is absent in the mitochondria, yet present in the endoplasmic reticulum. Unlike bclx knockout mice that died at E13.5, ER-xL mice survived embryonic development but died post-partum because of altered feeding behavior. Enhanced caspase-3 activity was observed in the brain and the spinal cord white matter, but not the gray matter. No increase in cell death was observed in ER-xL cortical neurons, suggesting that the observed caspase-3 activation was apoptosis-independent. ER-xL neurons displayed increased caspase-3 activity in the neurites, resulting in impaired axon arborescence and synaptogenesis. Together, our findings suggest that mitochondrial Bcl-xL finely tunes caspase-3 through Drp-1-dependent mitochondrial fission, which is critical to neural network design.

Impacts of autophagy on the formation of organelle-free zone during the lens development

The thorough degeneration of organelles in the core of the lens is certainly a hallmark event during the lens development. Organelles degradation in the terminal differentiation process of lens fiber cells to form an organelle-free zone is critical for lens maturation and transparency. Several mechanisms have been proposed to expand our understanding of lens organelles degradation, including apoptotic pathways, the participation of ribozyme, proteolytic enzyme and phospholipase A and acyltransferase, and the newly discovered roles for autophagy. Autophagy is a lysosome-dependent degradation reaction during which the "useless" cellular components are degraded and recycled. These cellular components, such as incorrectly folded proteins, damaged organelles and other macromolecules, are first engulfed by the autophagosome before being further delivered to lysosomes for degradation. Although autophagy has been recognized involving in organelle degradation of the lens, the detailed functions remain to be discovered. Recent advances have revealed that autophagy not only plays a vital role in the intracellular quality control of the lens but is also involved in the degradation of nonnuclear organelles in the process of lens fiber cell differentiation. Herein, we first review the potential mechanisms of organelle-free zone formation, then discuss the roles of autophagy in intracellular quality control and cataract formation, and finally substantially summarize the potential involvement of autophagy in the development of organelle-free zone formation.

Recent updates of stem cell-based erythropoiesis

Blood transfusions are now an essential part of modern medicine. Transfusable red blood cells (RBCs) are employed in various therapeutic strategies; however, the processes of blood donation, collection, and administration still involve many limitations. Notably, a lack of donors, the risk of transfusion-transmitted disease, and recent pandemics such as COVID-19 have prompted us to search for alternative therapeutics to replace this resource. Originally, RBC production was attempted via the ex vivo differentiation of stem cells. However, a more approachable and effective cell source is now required for broader applications. As a viable alternative, pluripotent stem cells have been actively used in recent research. In this review, we discuss the basic concepts related to erythropoiesis, as well as early research using hematopoietic stem cells ex vivo, and discuss the current trend of in vitro erythropoiesis using human-induced pluripotent stem cells.

Making the head: Caspases in life and death

The term apoptosis, as a way of programmed cell death, was coined a half century ago and since its discovery the process has been extensively investigated. The anatomy and physiology of the head are complex and thus apoptosis has mostly been followed in separate structures, tissues or cell types. This review aims to provide a comprehensive overview of recent knowledge concerning apoptosis-related molecules involved in the development of structures of head with a particular focus on caspases, cysteine proteases having a key position in apoptotic pathways. Since many classical apoptosis-related molecules, including caspases, are emerging in several non-apoptotic processes, these were also considered. The largest organ of the head region is the brain and its development has been extensively investigated, including the roles of apoptosis and related molecules. Neurogenesis research also includes sensory organs such as the eye and ear, efferent nervous system and associated muscles and glands. Caspases have been also associated with normal function of the skin and hair follicles. Regarding mineralised tissues within craniofacial morphogenesis, apoptosis in bones has been of interest along with palate fusion and tooth development. Finally, the role of apoptosis and caspases in angiogenesis, necessary for any tissue/organ development and maintenance/homeostasis, are discussed. Additionally, this review points to abnormalities of development resulting from improper expression/activation of apoptosis-related molecules.

Effects of elevated temperature on 8-OHdG expression in the American oyster (Crassostrea virginica): Induction of oxidative stress biomarkers, cellular apoptosis, DNA damage and γH2AX signaling pathways

Global temperature is increasing due to anthropogenic activities and the effects of elevated temperature on DNA lesions are not well documented in marine organisms. The American oyster (Crassostrea virginica, an edible and commercially important marine mollusk) is an ideal shellfish species to study oxidative DNA lesions during heat stress. In this study, we examined the effects of elevated temperatures (24, 28, and 32 °C for one-week exposure) on heat shock protein-70 (HSP70, a biomarker of heat stress), 8‑hydroxy-2'-deoxyguanosine (8-OHdG, a biomarker of pro-mutagenic DNA lesion), double-stranded DNA (dsDNA), γ-histone family member X (γH2AX, a molecular biomarker of DNA damage), caspase-3 (CAS-3, a key enzyme of apoptotic pathway) and Bcl-2-associated X (BAX, an apoptosis regulator) protein and/or mRNA expressions in the gills of American oysters. Immunohistochemical and qRT-PCR results showed that HSP70, 8-OHdG, dsDNA, and γH2AX expressions in gills were significantly increased at high temperatures (28 and 32 °C) compared with control (24°C). In situ TUNEL analysis showed that the apoptotic cells in gill tissues were increased in heat-exposed oysters. Interestingly, the enhanced apoptotic cells were associated with increased CAS-3 and BAX mRNA and/or protein expressions, along with 8-OHdG levels in gills after heat exposure. Moreover, the extrapallial (EP) fluid (i.e., extracellular body fluid) protein concentrations were lower; however, the EP glucose levels were higher in heat-exposed oysters. Taken together, these results suggest that heat shock-driven oxidative stress alters extracellular body fluid conditions and induces cellular apoptosis and DNA damage, which may lead to increased 8-OHdG levels in cells/tissues in oysters.

Deep conservation and co-option of programmed cell death facilitates evolution of alternative phenotypes at multiple biological levels

Alternative phenotypes, such as polyphenisms and sexual dimorphisms, are widespread in nature and appear at all levels of biological organization, from genes and cells to morphology and behavior. Yet, our understanding of the mechanisms through which alternative phenotypes develop and how they evolve remains understudied. In this review, we explore the association between alternative phenotypes and programmed cell death, a mechanism responsible for the elimination of superfluous cells during development. We discuss the ancient origins and deep conservation of programmed cell death (its function, forms and underlying core regulatory gene networks), and propose that it was co-opted repeatedly to generate alternative phenotypes at the level of cells, tissues, organs, external morphology, and even individuals. We review several examples from across the tree of life to explore the conditions under which programmed cell death is likely to facilitate the evolution of alternative phenotypes.

Suppression of PI3K signaling is linked to autophagy activation and the spatiotemporal induction of the lens organelle free zone

The terminal steps of lens cell differentiation require elimination of all organelles to create a central Organelle Free Zone (OFZ) that is required for lens function of focusing images on the retina. Previous studies show that the spatiotemporal elimination of these organelles during development is autophagy-dependent. We now show that the inhibition of PI3K signaling in lens organ culture results in the premature induction of autophagy within 24 h, including a significant increase in LAMP1+ lysosomes, and the removal of lens organelles from the center of the lens. Specific inhibition of just the PI3K/Akt signaling axis was directly linked to the elimination of mitochondria and ER, while pan-PI3K inhibitors that block all PI3K downstream signaling removed all organelles, including nuclei. Therefore, blocking the PI3K/Akt pathway was alone insufficient to remove nuclei. RNAseq analysis revealed increased mRNA levels of the endogenous inhibitor of PI3K activation, PIK3IP1, in differentiating lens fiber cells preceding the induction of OFZ formation. Co-immunoprecipitation confirmed that PIK3IP1 associates with multiple PI3K p110 isoforms just prior to formation of the OFZ, providing a likely endogenous mechanism for blocking all PI3K signaling and activating the autophagy pathway required to form the OFZ during lens development.

Cell Death Related Proteins Beyond Apoptosis in the CNS

Cell death related (CDR) proteins are a diverse group of proteins whose original function was ascribed to apoptotic cell death signaling. Recently, descriptions of non-apoptotic functions for CDR proteins have increased. In this minireview, we comment on recent studies of CDR proteins outside the field of apoptosis in the CNS, encompassing areas such as the inflammasome and non-apoptotic cell death, cytoskeleton reorganization, synaptic plasticity, mitophagy, neurodegeneration and calcium signaling among others. Furthermore, we discuss the evolution of proteomic techniques used to predict caspase substrates that could potentially explain their non-apoptotic roles. Finally, we address new concepts in the field of non-apoptotic functions of CDR proteins that require further research such the effect of sexual dimorphism on non-apoptotic CDR protein function and the emergence of zymogen-specific caspase functions.

Comparative transcriptomic analysis reveals gene regulation mediated by caspase activity in a chordate organism

BACKGROUND

Apoptosis is a caspase regulated cell death present in all metazoans defined by a conserved set of morphological features. A well-described function of apoptosis is the removal of excessive cells during development and homeostasis. Recent studies have shown an unexpected signalling property of apoptotic cells, affecting cell fate and/or behaviour of neighbouring cells. In contrast to the apoptotic function of cell elimination, this new role of apoptosis is not well understood but seems caspase-dependent. To deepen our understanding of apoptotic functions, it is necessary to work on a biological model with a predictable apoptosis pattern affecting cell fate and/or behaviour. The tunicate Ciona intestinalis has a bi-phasic life cycle with swimming larvae which undergo metamorphosis after settlement. Previously, we have shown that the tail regression step during metamorphosis, characterized by a predictable polarized apoptotic wave, ensures elimination of most tail cells and controls primordial germ cells survival and migration.

RESULTS

We performed differential transcriptomic analysis between control metamorphosing larvae and larvae treated with the pan-caspase inhibitor Z-VAD-fmk in order to explore the transcriptional control of apoptotic cells on neighbouring cells that survive and migrate. When caspase activity was impaired, genes known to be involved in metamorphosis were downregulated along with other implicated in cell migration and survival molecular pathways.

CONCLUSION

We propose these results as a confirmation that apoptotic cells can control surrounding cells fate and as a reference database to explore novel apoptotic functions in animals, including those related to migration and differentiation.

Caspase-3: Structure, function, and biotechnological aspects

Caspase-3, a cysteine-aspartic acid protease, has recently attracted much attention because of its incredible roles in tissue differentiation, regeneration, and neural development. This enzyme is a key zymogen in cell apoptosis and is not activated until it is cleaved by initiator caspases during apoptotic flux. Since caspase-3 has represented valuable capabilities in the field of medical research, biotechnological aspects of this enzyme, including the production of recombinant type, protein engineering, and designing delivery systems, have been considered as emerging therapeutic strategies in treating the apoptosis-related disorders. To date, several advances have been made in the therapeutic use of caspase-3 in the management of some diseases such as cancers, heart failure, and neurodegenerative disorders. In the current review, we intend to discuss the caspase-3's structure, functions, therapeutic applications, as well as its molecular cloning, protein engineering, and relevant delivery systems.

Non-lethal message from the Holy Land: The first international conference on nonapoptotic roles of apoptotic proteins

Apoptosis is a major form of programmed cell death (PCD) that eliminates unnecessary and potentially dangerous cells in all metazoan organisms, thus ensuring tissue homeostasis and many developmental processes. Accordingly, defects in the activation of the apoptotic pathway often pave the way to disease. After several decades of intensive research, the molecular details controlling the apoptosis program have largely been unraveled, as well as the regulatory mechanisms of caspase activation during apoptosis. Nevertheless, an ever-growing list of studies is suggesting the essential role of caspases and other apoptotic proteins in ensuring nonlethal cellular functions during normal development, tissue repair, and regeneration. Moreover, if deregulated, these novel nonapoptotic functions can also instigate diseases. The difficulty of identifying and manipulating the caspase-dependent nonlethal cellular processes (CDPs), as well as the nonlethal functions of other cell death proteins (NLF-CDPs), meant that CDPs and NLF-CDPs have been only curiosities within the apoptotic field; however, the recent technical advancements and the latest biological findings are assigning an unanticipated biological significance to these nonapoptotic functions. Here, we summarize the various talks presented in the first international conference fully dedicated to discuss CDPs and NFL-CDPs and named 'The Batsheva de Rothschild Seminar on Non-Apoptotic Roles of Apoptotic Proteins'. The conference was organized between September 22, 2019, and 25, 2019, by Eli Arama (Weizmann Institute of Science), Luis Alberto Baena-Lopez (University of Oxford), and Howard O. Fearnhead (NUI Galway) at the Weizmann Institute of Science in Israel, and hosted a large international group of researchers.

Analysis of the lamprey genotype provides insights into caspase evolution and functional divergence

The cysteine-containing aspartate specific proteinase (caspase) family plays important roles in apoptosis and the maintenance of homeostasis in lampreys. We conducted genomic and functional comparisons of six distinct lamprey caspase groups with human counterparts to determine how these expanded molecules evolved to adapt to the changing caspase-mediated signaling pathways. Our results showed that lineage-specific duplication and rearrangement were responsible for expanding lamprey caspases 3 and 7, whereas caspases 1, 6, 8, and 9 maintained a relatively stable genome and protein structure. Lamprey caspase family molecules displayed various expression patterns and were involved in the innate immune response. Caspase 1 and 7 functioned as a pattern recognition receptor with a broad-spectrum of microbial recognition and bactericidal effect. Additionally, caspases 1 and 7 may induce cell apoptosis in a time- and dose-dependent manner; however, apoptosis was inhibited by caspase inhibitors. Thus, these molecules may reflect the original state of the vertebrates caspase family. Our phylogenetic and functional data provide insights into the evolutionary history of caspases and illustrate their functional characteristics in primitive vertebrates.

Initiator and executioner caspases in salivary gland apoptosis of Rhipicephalus haemaphysaloides

BACKGROUND

Apoptosis is fundamental in maintaining cell balance in multicellular organisms, and caspases play a crucial role in apoptosis pathways. It is reported that apoptosis plays an important role in tick salivary gland degeneration. Several different caspases have been found in ticks, but the interactions between them are currently unknown. Here, we report three new caspases, isolated from the salivary glands of the tick Rhipicephalus haemaphysaloides.

METHODS

The full-length cDNA of the RhCaspases 7, 8 and 9 genes were obtained by transcriptome, and RhCaspases 7, 8 and 9 were expressed in E. coli; after protein purification and immunization in mice, specific polyclonal antibodies (PcAb) were created in response to the recombinant protein. Reverse-transcription quantitative PCR (RT-qPCR) and western blot were used to detect the existence of RhCaspases 7, 8 and 9 in ticks. TUNEL assays were used to determine the apoptosis level in salivary glands at different feeding times after gene silencing. The interaction between RhCaspases 7, 8 and 9 were identified by co-transfection assays.

RESULTS

The transcription of apoptosis-related genes in R. haemaphysaloides salivary glands increased significantly after tick engorgement. Three caspase-like molecules containing conserved caspase domains were identified and named RhCaspases 7, 8 and 9. RhCaspase8 and RhCaspase9 contain a long pro-domain at their N-terminals. An RT-qPCR assay demonstrated that the transcription of these three caspase genes increased significantly during the engorged periods of the tick developmental stages (engorged larval, nymph, and adult female ticks). Transcriptional levels of RhCaspases 7, 8 and 9 in salivary glands increased more significantly than other tissues post-engorgement. RhCaspase9-RNAi treatment significantly inhibited tick feeding. In contrast, knockdown of RhCaspase7 and RhCaspase8 had no influence on tick feeding. Compared to the control group, apoptosis levels were significantly reduced after interfering with RhCaspase 7, 8 and 9 expressions. Co-transfection assays showed RhCaspase7 was cleaved by RhCaspases 8 and 9, demonstrating that RhCaspases 8 and 9 are initiator caspases and RhCaspase7 is an executioner caspase.

CONCLUSIONS

To the best of our knowledge, this is the first study to identify initiator and executioner caspases in ticks, confirm the interaction among them, and associate caspase activation with tick salivary gland degeneration.

Effects of Lysosomal-Mitochondrial Apoptotic Pathway on Tenderness in Post-Mortem Bovine Longissimus Muscle

The objective of this study was to investigate the mechanism underlying lysosome-mediated apoptosis, the cross-talk between the lysosomes and mitochondria, and the effect of the pathway on bovine longissimus muscle tenderness during 7 d post-mortem aging through the observation and analysis of longissimus dorsi (LD) muscles of six crossbred cattle. Results showed that an elevated reactive oxygen species level ( P < 0.05) can damage lysosomal membrane stability ( P < 0.05) through accumulating redox-active iron of bovine muscle during post-mortem aging. In addition, the activities of cathepsins B and D increased with post-mortem aging ( P < 0.05). Moreover, cathepsin B and D activated Bid and Bax in the mitochondria ( P < 0.05). Activated Bid and Bax triggered mitochondrial membrane permeability ( P < 0.05) and further activated caspase-9 and caspase-3 ( P < 0.05), leading to apoptosis. Ultimately, the tenderness of bovine muscle was improved during post-mortem aging ( P < 0.05). Importantly, cathepsin D plays a crucial role in the lysosomal-mitochondrial apoptotic pathway and tenderness in post-mortem muscle. These findings provide new insights into the apoptotic pathway of bovine muscle during post-mortem aging.

Genome Damage Sensing Leads to Tissue Homeostasis in Drosophila

DNA repair is a critical cellular process required for the maintenance of genomic integrity. It is now well appreciated that cells employ several DNA repair pathways to take care of distinct types of DNA damage. It is also well known that a cascade of signals namely DNA damage response or DDR is activated in response to DNA damage which comprise cellular responses, such as cell cycle arrest, DNA repair and cell death, if the damage is irreparable. There is also emerging literature suggesting a cross-talk between DNA damage signaling and several signaling networks within a cell. Moreover, cell death players themselves are also well known to engage in processes outside their canonical function of apoptosis. This chapter attempts to build a link between DNA damage, DDR and signaling from the studies mainly conducted in mammals and Drosophila model systems, with a special emphasis on their relevance in overall tissue homeostasis and development.

Development of a quantitative pharmacodynamic assay for apoptosis in fixed tumor tissue and its application in distinguishing cytotoxic drug-induced DNA double strand breaks from DNA double strand breaks associated with apoptosis

DNA double strand breaks (DSBs) induced by cancer therapeutic agents can lead to DNA damage repair or persistent DNA damage, which can induce apoptotic cell death; however, apoptosis also induces DSBs independent of genotoxic insult. γH2AX is an established biomarker for DSBs but cannot distinguish between these mechanisms. Activated cleaved caspase-3 (CC3) promotes apoptosis by enhancing nuclear condensation, DNA fragmentation, and plasma membrane blebbing. Here, we describe an immunofluorescence assay that distinguishes between apoptosis and drug-induced DSBs by measuring coexpression of γH2AX and membrane blebbing−associated CC3 to indicate apoptosis, and γH2AX in the absence of CC3 blebbing to indicate drug-induced DNA damage. These markers were examined in xenograft models following treatment with topotecan, cisplatin, or birinapant. A topotecan regimen conferring tumor regression induced tumor cell DSBs resulting from both apoptosis and direct DNA damage. In contrast, a cisplatin regimen yielding tumor growth delay, but not regression, resulted in tumor cell DSBs due solely to direct DNA damage. MDA-MB-231 xenografts exposed to birinapant, which promotes apoptosis but does not directly induce DSBs, exhibited dose-dependent increases in colocalized γH2AX/CC3 blebbing in tumor cells. Clinical feasibility was established using formalin-fixed, paraffin-embedded biopsies from a canine cancer clinical trial; γH2AX/CC3 colocalization analysis revealed apoptosis induction by two novel indenoisoquinoline topoisomerase I inhibitors, which was consistent with pathologist-assessed apoptosis and reduction of tumor volume. This assay is ready for use in clinical trials to elucidate the mechanism of action of investigational agents and combination regimens intended to inflict DNA damage, apoptotic cell death, or both.

Apoptotic Caspases: Multiple or Mistaken Identities?

The mitochondrial caspase cascade was originally thought to be required for apoptotic death driven by Bak/Bax-mediated intrinsic apoptosis. It has also been ascribed several 'non-apoptotic' functions, including differentiation, proliferation, and cellular reprogramming. Recent work has demonstrated that, during apoptosis, the caspase cascade suppresses damage-associated molecular pattern (DAMP)-initiated production of cytokines such as type I interferon by the dying cell. The caspase cascade is not required for death to occur; instead, it shapes the immunogenic properties of the apoptotic cell. This raises questions about the role of apoptotic caspases in regulating DAMP signaling more generally, puts a new perspective on their non-apoptotic functions, and suggests that pharmacological caspase inhibitors might find new applications as antiviral or anticancer agents.

Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018

Over the past decade, the Nomenclature Committee on Cell Death (NCCD) has formulated guidelines for the definition and interpretation of cell death from morphological, biochemical, and functional perspectives. Since the field continues to expand and novel mechanisms that orchestrate multiple cell death pathways are unveiled, we propose an updated classification of cell death subroutines focusing on mechanistic and essential (as opposed to correlative and dispensable) aspects of the process. As we provide molecularly oriented definitions of terms including intrinsic apoptosis, extrinsic apoptosis, mitochondrial permeability transition (MPT)-driven necrosis, necroptosis, ferroptosis, pyroptosis, parthanatos, entotic cell death, NETotic cell death, lysosome-dependent cell death, autophagy-dependent cell death, immunogenic cell death, cellular senescence, and mitotic catastrophe, we discuss the utility of neologisms that refer to highly specialized instances of these processes. The mission of the NCCD is to provide a widely accepted nomenclature on cell death in support of the continued development of the field.

Revisiting the Road Map of Medullary Thymic Epithelial Cell Differentiation

The basic two-step terminal differentiation model of the medullary thymic epithelial cell (mTEC) lineage from immature MHC class II (MHCII)lo to mature MHCIIhi mTECs has recently been extended to include a third stage, namely the post-Aire MHCIIlo subset as identified by lineage-tracing models. However, a suitable surface marker distinguishing the phenotypically overlapping pre- from the post-Aire MHCIIlo stage has been lacking. In this study, we introduce the lectin Tetragonolobus purpureas agglutinin (TPA) as a novel cell surface marker that allows for such delineation. Based on our data, we derived the following sequence of mTEC differentiation: TPAloMHCIIlo → TPAloMHCIIhi → TPAhiMHCIIhi → TPAhiMHCIIlo Surprisingly, in the steady-state postnatal thymus TPAloMHCIIlo pre-Aire rather than terminally differentiated post-Aire TPAhiMHCIIlo mTECs were marked for apoptosis at an exceptionally high rate of ∼70%. Hence, only the minor cycling fraction of the MHCIIlo subset (<20%) potentially qualified as mTEC precursors. FoxN1 expression inversely correlated with the fraction of slow cycling and apoptotic cells within the four TPA subsets. TPA also further subdivided human mTECs, although with different subset distribution. Our revised road map emphazises close parallels of terminal mTEC development with that of skin, undergoing an alternative route of cell death, namely cornification rather than apoptosis. The high rate of apoptosis in pre-Aire MHCIIlo mTECs points to a "quality control" step during early mTEC differentiation.

Caspase signaling, a conserved inductive cue for metazoan cell differentiation

Caspase signaling pathways were originally discovered as conveyors of programmed cell death, yet a compendium of research over the past two decades have demonstrated that these same conduits have a plethora of physiologic functions. Arguably the most extensive non-death activity that has been attributed to this protease clade is the capacity to induce cell differentiation. Caspase control of differentiation is conserved across diverse metazoan organisms from flies to humans, suggesting an ancient origin for this form of cell fate control. Here we discuss the mechanisms by which caspase enzymes manage differentiation, the targeted substrates that may be common across cell lineages, and the countervailing signals that may be essential for these proteases to 'execute' this non-death cell fate.

HSF4 regulates lens fiber cell differentiation by activating p53 and its downstream regulators

Cataract refers to opacities of the lens that impede the passage of light. Mutations in heat shock transcription factor 4 (HSF4) have been associated with cataract; however, the mechanisms regarding how mutations in HSF4 cause cataract are still obscure. In this study, we generated an hsf4 knockout zebrafish model using TALEN technology. The mutant zebrafish developed an early-onset cataract with multiple developmental defects in lens. The epithelial cells of the lens were overproliferated, resulting in the overabundance of lens fiber cells in hsf4^null zebrafish lens. Consequently, the arrangement of the lens fiber cells became more disordered and irregular with age. More importantly, the terminal differentiation of the lens fiber cell was interrupted as the organelles cannot be cleaved in due time. In the cultured human lens epithelial cells, HSF4 could stabilize and retain p53 in the nucleus to activate its target genes such as fas cell surface death receptor (Fas) and Bcl-2-associated X apoptosis regulator (Bax). In the hsf4^null fish, both p53 and activated-caspase3 were significantly decreased. Combined with the finding that the denucleation defect could be partially rescued through microinjection of p53, fas and bax mRNA into the mutant embryos, we directly proved that HSF4 promotes lens fiber cell differentiation by activating p53 and its downstream regulators. The data we presented suggest that apoptosis-related genes are involved in the lens fiber cell differentiation. Our finding that HSF4 functions in the upstream to activate these genes highlighted the new regulatory modes of HSF4 in the terminal differentiation of lens fiber cell.

A Novel Non-Apoptotic Role of Procaspase-3 in the Regulation of Mitochondrial Biogenesis Activators

The executioner caspase-3 has been proposed as a pharmacological intervention target to preserve degenerating dopaminergic (DA) neurons because apoptotic mechanisms involving caspase-3 contribute, at least in part, to the loss of DA neurons in patients and experimental models of Parkinson's disease (PD). Here, we determined that genetic intervention of caspase-3 was sufficient to prevent cell death against oxidative stress (OS), accompanied by unexpected severe mitochondrial dysfunction. Specifically, as we expected, caspase-3-deficient DA neuronal cells were very significantly resistant to OS-induced cell death, while the activation of the initiator caspase-9 by OS was preserved. Moreover, detailed phenotypic characterization of caspase-3-deficient DA cells revealed severe mitochondrial dysfunction, including an accumulation of damaged mitochondria with a characteristic swollen structure and broken cristae, reduced membrane potential, increased levels of reactive oxygen species (ROS), and deficits in mitochondrial oxidative phosphorylation (OXPHOS) enzymes. Of great interest, we found that mitochondrial biogenesis was dramatically decreased in caspase-3-deficient DA cells, whereas their capability of mitophagy was normal. In accordance with this observation, caspase-3 gene knock down (KD) resulted in dramatically decreased expression of the key transcriptional activators of mitochondrial biogenesis, such as Tfam and Nrf-1, implicating a non-apoptotic role of procaspase-3 in mitochondrial biogenesis. Therefore, a prolonged anti-apoptotic intervention targeting caspase-3 should be considered with caution due to the potential adverse effects in mitochondria dynamics resulting from a novel potential functional role of procaspase-3 in mitochondrial biogenesis via regulating the expression of mitochondrial biogenesis activators. J. Cell. Biochem. 119: 347-357, 2018. © 2017 Wiley Periodicals, Inc.

Caspase-dependent non-apoptotic processes in development

Caspases are at the core of executing apoptosis by orchestrating cellular destruction with proteolytic cascades. Caspase-mediated proteolysis also controls diverse nonlethal cellular activities such as proliferation, differentiation, cell fate decision, and cytoskeletal reorganization. During the last decade or so, genetic studies of Drosophila have contributed to our understanding of the in vivo mechanism of the non-apoptotic cellular responses in developmental contexts. Furthermore, recent studies using C. elegans suggest that apoptotic signaling may play unexpected roles, which influence ageing and normal development at the organism level. In this review, we describe how the caspase activity is elaborately controlled during vital cellular processes at the level of subcellular localization, the duration and timing to avoid full apoptotic consequences, and also discuss the novel roles of non-apoptotic caspase signaling in adult homeostasis and physiology.

Evolution of caspase-mediated cell death and differentiation: twins separated at birth

The phenotypic and biochemical similarities between caspase-mediated apoptosis and cellular differentiation are striking. They include such diverse phenomenon as mitochondrial membrane perturbations, cytoskeletal rearrangements and DNA fragmentation. The parallels between the two disparate processes suggest some common ancestry and highlight the paradoxical nature of the death-centric view of caspases. That is, what is the driving selective pressure that sustains death-inducing proteins throughout eukaryotic evolution? Plausibly, caspase function may be rooted in a primordial non-death function, such as cell differentiation, and was co-opted for its role in programmed cell death. This review will delve into the links between caspase-mediated apoptosis and cell differentiation and examine the distinguishing features of these events. More critically, we chronicle the evolutionary origins of caspases and propose that caspases may have held an ancient role in mediating the fidelity of cell division/differentiation through its effects on proteostasis and protein quality control.

Metacaspases versus caspases in development and cell fate regulation

Initially found to be critically involved in inflammation and apoptosis, caspases have since then been implicated in the regulation of various signaling pathways in animals. How caspases and caspase-mediated processes evolved is a topic of great interest and hot debate. In fact, caspases are just the tip of the iceberg, representing a relatively small group of mostly animal-specific enzymes within a broad family of structurally related cysteine proteases (family C14 of CD clan) found in all kingdoms of life. Apart from caspases, this family encompasses para- and metacaspases, and all three groups of proteases exhibit significant variation in biochemistry and function in vivo. Notably, metacaspases are present in all eukaryotic lineages with a remarkable absence in animals. Thus, metacaspases and caspases must have adapted to operate under distinct cellular and physiological settings. Here we discuss biochemical properties and biological functions of metacaspases in comparison to caspases, with a major focus on the regulation of developmental aspects in plants versus animals.

Axonal Cleaved Caspase-3 Regulates Axon Targeting and Morphogenesis in the Developing Auditory Brainstem

Caspase-3 is a cysteine protease that is most commonly associated with cell death. Recent studies have shown additional roles in mediating cell differentiation, cell proliferation and development of cell morphology. We investigated the role of caspase-3 in the development of chick auditory brainstem nuclei during embryogenesis. Immunofluorescence from embryonic days E6–13 revealed that the temporal expression of cleaved caspase-3 follows the ascending anatomical pathway. The expression is first seen in the auditory portion of VIIIth nerve including central axonal regions projecting to nucleus magnocellularis (NM), then later in NM axons projecting to nucleus laminaris (NL), and subsequently in NL dendrites. To examine the function of cleaved caspase-3 in chick auditory brainstem development, we blocked caspase-3 cleavage in developing chick embryos with the caspase-3 inhibitor Z-DEVD-FMK from E6 to E9, then examined NM and NL morphology and NM axonal targeting on E10. NL lamination in treated embryos was disorganized and the neuropil around NL contained a significant number of glial cells normally excluded from this region. Additionally, NM axons projected into inappropriate portions of NL in Z-DEVD-FMK treated embyros. We found that the presence of misrouted axons was associated with more severe NL disorganization. The effects of axonal caspase-3 inhibition on both NL morphogenesis and NM axon targeting suggest that these developmental processes are coordinated, likely through communication between axons and their targets.

Apoptosis in the Trophoblast—Role of Apoptosis in Placental Morphogenesis

Villous trophoblast is the epithelial cover of the placental villous tree and comes in direct contact with maternal blood. The turnover of villous trophoblast includes proliferation and differentiation of cytotrophoblast, syncytial fusion of cytotrophoblast with the overlying syncytiotrophoblast, differentiation in the syncytiotrophoblast, and finally extrusion of apoptotic material into the maternal circulation. In recent years, it has become clear that apoptosis is a normal constituent of trophoblast turnover and the release of apoptotic material does not lead to an inflammatory response of the mother. During preeclampsia there seems to be an altered balance between proliferation and apoptosis of villous trophoblast leading to a dysregulation of the release from the syncytiotrophoblast. The normal apoptotic release may be reduced in favor of a necrotic release. Since apoptosis is still ongoing in the syncytiotrophoblast, a necrotic release of intrasyncytial and partly apoptotic material lead us to call this type of release "aponecrotic shedding." In this situation, cell-free components such as G-actin and DNA freely floating in maternal blood may trigger damage to the maternal endothelium, thereby triggering preeclampsia. This review highlights the importance of the apoptosis cascade in permitting normal physiologic turnover of villous trophoblast. It will demonstrate the participation of initial stages of this cascade within the cytotrophoblast and of the execution stages within the syncytiotrophoblast. Moreover, this review presents hypotheses of how dysregulation of the apoptosis cascade may be linked to endothelial dysfunction of the maternal vasculature in preeclampsia.

Is the natural PCDD/PCDF mixture toxic for human placental JEG-3 cell line? The action of the toxicants on hormonal profile, CYP1A1 activity, DNA damage and cell apoptosis

The present study was conducted to define the action of a mixture obtained by the extraction and purification of real fly ash, on specific toxicity endpoints, such as hormonal secretion, CYP1A1 expression, DNA damage and cell apoptosis. JEG-3 cell line was exposed in vitro to different doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or Polychlorinated dibenzo-p-dioxin/Polychlorinated dibenzo-P-furan (PCDD/PCDF) mixture. Both TCDD and the mixture decreased hCG secretion, while inhibition of progesterone levels was noted only under the influence of TCDD. The changes in hormone production were not due to the action on cell viability. There were time-dependent differences in CYP1A1 expression in cells exposed to TCDD and PCDD/PCDF mixture. Both TCDD and PCDD/PCDF mixture did not induce the DNA damage, as evaluated by the comet assay. Significantly lower DNA migration from the head of comet into the comet tail was noted after the removal of reagents. The highest efficiency of this process was noted 4 h after the TCDD and 24 h after the PCDD/PCDF mixture removal. These results suggest that the DNA adducts and/or DNA—DNA cross-links were formed. Neither TCDD nor PCDD/PCDF mixture had any effect on cell apoptosis assessed by caspase-3 activity and Hoechst 33258. Taken together, these findings clearly indicate a weaker action of the mixture when compared with TCDD. However, in both cases, their action was not due to the induction of the DNA damage and subsequent cell apoptosis but due to a direct influence of these toxicants on placental hormone production. Human & Experimental Toxicology ( 2007) 26, 407—417

Modulatory role of leptin on ovarian functions in water buffalo (Bubalus bubalis)

The aim of the present study was to demonstrate the modulatory role of leptin on bubaline granulosa cells (GCs) and luteal cells (LCs) functions using an in vitro cell culture system and to establish a cross talk between leptin and insulin-like growth factor-1 (IGF-1). GCs were collected from group IV follicles (>13 mm size) and LCs from mid-luteal phase corpus luteum and were grown in serum-containing media supplemented with leptin at three different dose rates (0.1, 1, and 10 ng/mL) and time durations (24, 48, and 72 hours). We evaluated the production and secretion of estradiol (E2) and progesterone (P4) using RIA and the mRNA expression of steroidogenic acute regulatory protein (STARD1), cytochrome P450 cholesterol side-chain cleavage (CYP11A1), 3β-hydroxysteroid dehydrogenase (3β-HSD), cytochrome P450 aromatase (CYP19A1), sterol regulatory element-binding protein 1 (SREBP1), steroidogenic factor-1 (SF1), anti-apoptotic gene PCNA, pro-apoptotic gene caspase 3 and endothelial cell marker, Von Willebrand factor (vWF), using quantitative real-time polymerase chain reaction. The results depicted a direct inhibitory action of leptin on GCs steroidogenesis in a time-dependent manner (P < 0.05), whereas in the presence of IGF-1 the inhibitory effect was reverted. Furthermore, leptin augmented both cellular proliferation (PCNA) and apoptosis (caspase 3). On the other hand, in LCs, leptin alone showed an apparent stimulatory effect on steroidogenesis (P < 0.05); however, in the presence of IGF-1, an antagonistic effect was witnessed. Moreover, leptin had an inhibitory effect on apoptosis while promoted cellular proliferation and angiogenesis. These findings were further strengthened by immunocytochemistry. To conclude, these observations for the first time reported that in buffaloes leptin has a direct dose-, time-, and tissue-dependent effect on ovarian steroidogenesis, angiogenesis, and cytoprotection, and furthermore, it can regulate the effect of systemic factors like IGF-1. Hence, this in vitro study provides an insight into the putative roles of leptin alone and its interactions in vivo.

A Krebs Cycle Component Limits Caspase Activation Rate through Mitochondrial Surface Restriction of CRL Activation

How cells avoid excessive caspase activity and unwanted cell death during apoptotic caspase-mediated removal of large cellular structures is poorly understood. We investigate caspase-mediated extrusion of spermatid cytoplasmic contents in Drosophila during spermatid individualization. We show that a Krebs cycle component, the ATP-specific form of the succinyl-CoA synthetase β subunit (A-Sβ), binds to and activates the Cullin-3-based ubiquitin ligase (CRL3) complex required for caspase activation in spermatids. In vitro and in vivo evidence suggests that this interaction occurs on the mitochondrial surface, thereby limiting the source of CRL3 complex activation to the vicinity of this organelle and reducing the potential rate of caspase activation by at least 60%. Domain swapping between A-Sβ and the GTP-specific SCSβ (G-Sβ), which functions redundantly in the Krebs cycle, show that the metabolic and structural roles of A-Sβ in spermatids can be uncoupled, highlighting a moonlighting function of this Krebs cycle component in CRL activation.

New Views on the Misconstrued: Executioner Caspases and Their Diverse Non-apoptotic Roles

Initially characterized for their roles in apoptosis, executioner caspases have emerged as important regulators of an array of cellular activities. This is especially true in the nervous system, where sublethal caspase activity has been implicated in axonal pathfinding and branching, axonal degeneration, dendrite pruning, regeneration, long-term depression, and metaplasticity. Here we examine the roles of sublethal executioner caspase activity in nervous system development and maintenance, consider the mechanisms that locally activate and restrain these potential killers, and discuss how their activity be subverted in neurodegenerative disease.

Vasculogenic mimicry: Possible role of effector caspase-3, caspase-6 and caspase-7

Vasculogenic mimicry (VM) describes the process by which aggressive cancer cells form extracellular matrix-rich, vessel-like mesh works, which supply nutrients and oxygen. Furthermore, it offers a new route for tumor cell invasion and metastasis and thus a correspondingly poor prognosis and survival rate for affected patients. Effector caspases are well known for their apoptotic function, whereas a non-apoptotic function in tumor progression is highly disputed. Caspase-3, -6 and -7 are expressed in aggressive tumor cells in a non-mutated form, indicating an active function independent of apoptosis. This review summarizes the possible functions of the above-mentioned caspases in VM. We also discuss the possible involvement of caspases in potential mechanisms towards the formation of vessel-like structures. Furthermore, this review illustrates the importance of new studies in the ongoing investigation into the role of effector caspases in VM, invasion, and migration of aggressive tumor cells.

The Cellular Apoptosis Susceptibility Protein (CAS) Promotes Tumor Necrosis Factor-related Apoptosis-inducing Ligand (TRAIL)-induced Apoptosis and Cell Proliferation

A signature event during the cell intrinsic apoptotic pathway is mitochondrial outer membrane permeabilization, leading to formation of the apoptosome, a caspase activation complex. The cellular apoptosis susceptibility protein (CAS) can facilitate apoptosome assembly by stimulating nucleotide exchange on Apaf-1 following binding of cytochrome c. We report here that CAS expression itself is up-regulated during tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis, and knockdown of CAS renders cells resistant to TRAIL. We find that TRAIL induces up-regulation of CAS in a posttranscriptional, caspase-8-dependent manner through degradation of cIAP1, an E3 ligase that targets CAS for ubiquitin-dependent proteasomal degradation. We identified a novel signaling pathway whereby caspase-8 engages a feedforward cascade that leads to CAS up-regulation and amplifies the apoptotic signal. Furthermore, in silico analysis revealed that expression of CAS is up-regulated at both the mRNA and DNA levels in human breast tumors, consistent with its role in promoting cell proliferation. Overexpression of various oncogenes led to CAS up-regulation in non-transformed cells. Intriguingly, oncogene-induced CAS up-regulation also resulted in greater susceptibility to TRAIL-induced cell death, consistent with its proapoptotic function. These findings suggest that CAS plays contrasting roles in proliferation and apoptosis and that overexpression of CAS in tumors could serve as a potential biomarker to guide therapeutic choices.

Implications for chronic toxicity of benzo[a]pyrene in sea bream cultured hepatocytes: Cytotoxicity, inflammation, and cancerogenesis

Benzo[a]pyrene (B[a]P) is the most studied dangerous polycyclic aromatic hydrocarbon for its hepatotoxic, carcinogenic, mutagenic, teratogenic, and immunosuppressant effects, which can affect both wild and farmed marine fish through the trophic chain. This study investigated, for the first time, the chronic effects induced in vitro by B[a]P prolonged exposure on gilthead sea bream (Sparus aurata L.) hepatocytes, evaluating the cellular and nuclear latent damage. The purpose was to characterize the kind of B[a]P cyto- and genotoxic damage by morphological and immunocytochemical parameters applied in combination with the use of multiple assay endpoints. In light of our results, the short-term effects at higher B[a]P doses were linked to higher cytotoxicities and necrotic lysis, whereas a sustained inflammatory response at medium-low doses was perceived as a mitochondria-mediated apoptosis, both by surface and nuclear morphological changes. The strong immunoreactivity for the cleaved caspase-3 showed that the labeled cells committed suicide by apoptosis. B[a]P involvement on carcinogenesis comes from prolonged exposure at lower doses, establishing the connection between the escape from apoptosis and the selection of a tumoral phenotype. Cells colabeled with proliferating cell nuclear antigen/caspase-3 within the proliferative foci, were proliferating transformed oval stem cells, which escaped the suicide by apoptosis allowing cancer development. Finally, it was established that sea bream cultured hepatocytes are highly sensitive to chronic B[a]P exposure, as serious genotoxic effects were found even at the lowest doses.

Surviving apoptosis: life-death signaling in single cells

Tissue development and homeostasis are regulated by opposing pro-survival and pro-death signals. An interesting feature of the Tumor Necrosis Factor (TNF) family of ligands is that they simultaneously activate opposing signals within a single cell via the same ligand-receptor complex. The magnitude of pro-death events such as caspase activation and pro-survival events such as Nuclear Factor (NF)-κB activation vary not only from one cell type to the next but also among individual cells of the same type due to intrinsic and extrinsic noise. The molecules involved in these pro-survival and/or pro-death pathways, and the different phenotypes that result from their activities, have been recently reviewed. Here we focus on the impact of cell-to-cell variability in the strength of these opposing signals on shaping cell fate decisions.

Analysis of expression patterns of IGF-1, caspase-3 and HSP-70 in developing human tooth germs

AIMS

To analyze expression patterns of IGF-1, caspase-3 and HSP-70 in human incisor and canine tooth germs during the late bud, cap and bell stages of odontogenesis.

MATERIALS AND METHODS

Head areas or parts of jaw containing teeth from 10 human fetuses aged between 9th and 20th developmental weeks were immunohistochemically analyzed using IGF-1, active caspase-3 and HSP-70 markers. Semi-quantitative analysis of each marker's expression pattern was also performed.

RESULTS

During the analyzed period, IGF-1 and HSP-70 were mostly expressed in enamel organ. As development progressed, expression of IGF-1 and HSP-70 became more confined to differentiating tissues in the future cusp tip area, as well as in highly proliferating cervical loops. Few apoptotic bodies highly positive to active caspase-3 were observed in enamel organ and dental papilla from the cap stage onward. However, both enamel epithelia moderately expressed active caspase-3 throughout the investigated period.

CONCLUSIONS

Expression patterns of IGF-1, active caspase-3 and HSP-70 imply importance of these factors for early human tooth development. IGF-1 and HSP-70 have versatile functions in control of proliferation, differentiation and anti-apoptotic protection of epithelial parts of human enamel organ. Active caspase-3 is partially involved in formation and apoptotic removal of primary enamel knot, although present findings might reflect its ability to perform other non-death functions such as differentiation of hard dental tissues secreting cells and guidance of ingrowth of proliferating cervical loops.

Caspase-3 and caspase-6 cleave STAT1 in leukemic cells

Signal Transducer and Activator of Transcription-1 (STAT1) is phosphorylated upon interferon (IFN) stimulation, which can restrict cell proliferation and survival. Nevertheless, in some cancers STAT1 can act in an anti-apoptotic manner. Moreover, certain malignancies are characterized by the overexpression and constitutive activation of STAT1. Here, we demonstrate that the treatment of transformed hematopoietic cells with epigenetic drugs belonging to the class of histone deacetylase inhibitors (HDACi) leads to the cleavage of STAT1 at multiple sites by caspase-3 and caspase-6. This process does not occur in solid tumor cells, normal hematopoietic cells, and leukemic cells that underwent granulocytic or monocytic differentiation. STAT1 cleavage was studied under cell free conditions with purified STAT1 and a set of candidate caspases as well as with mass spectrometry. These assays indicate that unmodified STAT1 is cleaved at multiple sites by caspase-3 and caspase-6. Our study shows that STAT1 is targeted by caspases in malignant undifferentiated hematopoietic cells. This observation may provide an explanation for the selective toxicity of HDACi against rapidly proliferating leukemic cells.

[From dualism to multiplicity: seeing BCL-2 family proteins and cell death with new eyes]

The concept of cell death has many links to the concept of death itself, defined as the opposite of life. Achievements obtained through research on apoptosis have apparently allowed us to transcend this Manichean view. Death is no longer outside, but rather inside living systems, as a constitutive force at work within the living matter. Whereas the death of cells can be positive and breed "creation" (e.g. during morphogenesis), its dysregulation can also cause or contribute to fatal diseases including cancer. It is tempting to apply this biological discourse to illuminate the relations between life and death, taken in general terms, but does this generalization actually hold? Is this discourse not essentially a metaphor? If cell death is considered as a vital aspect of various biological processes, then are we not faced with some vitalistic conception of death? Are there one or more meanings to the word "death"? Does the power to self-destruct act in opposition to other key features of living entities, or rather in juxtaposition to them? In this article, we first describe how the field of cell death has been developed on the basis of perceived and built dichotomies, mirroring the original opposition between life and death. We detail the limitations of the current paradigm of apoptosis regulation by BCL-2 family proteins, which nicely illustrate the problem of binary thinking in biology. Last, we try to show a way out of this dualistic matrix, by drawing on the notions of multiplicity, complexity, diversity, evolution and contingency.