Inhibition of caspase-3 activity and activation by protein glutathionylation

Drug-induced oxidative stress actively prevents caspase activation and hepatocyte apoptosis

Cell death is a fundamental process in health and disease. Emerging research shows the existence of numerous distinct cell death modalities with similar and intertwined signaling pathways, but resulting in different cellular outcomes, raising the need to understand the decision-making steps during cell death signaling. Paracetamol (Acetaminophen, APAP)-induced hepatocyte death includes several apoptotic processes but eventually is executed by oncotic necrosis without any caspase activation. Here, we studied this paradoxical form of cell death and revealed that APAP not only fails to activate caspases but also strongly impedes their activation upon classical apoptosis induction, thereby shifting apoptosis to necrosis. While APAP intoxication results in massive drop in mitochondrial respiration, low cellular ATP levels could be excluded as an underlying cause of missing apoptosome formation and caspase activation. In contrast, we identified oxidative stress as a key factor in APAP-induced caspase inhibition. Importantly, caspase inhibition and the associated switch from apoptotic to necrotic cell death was reversible through the administration of antioxidants. Thus, exemplified by APAP-induced cell death, our study stresses that cellular redox status is a critical component in the decision-making between apoptotic and necrotic cell death, as it directly affects caspase activity.

Glutathione-Dependent Pathways in Cancer Cells

The most abundant tripeptide-glutathione (GSH)-and the major GSH-related enzymes-glutathione peroxidases (GPxs) and glutathione S-transferases (GSTs)-are highly significant in the regulation of tumor cell viability, initiation of tumor development, its progression, and drug resistance. The high level of GSH synthesis in different cancer types depends not only on the increasing expression of the key enzymes of the γ-glutamyl cycle but also on the changes in transport velocity of its precursor amino acids. The ability of GPxs to reduce hydroperoxides is used for cellular viability, and each member of the GPx family has a different mechanism of action and site for maintaining redox balance. GSTs not only catalyze the conjugation of GSH to electrophilic substances and the reduction of organic hydroperoxides but also take part in the regulation of cellular signaling pathways. By catalyzing the S-glutathionylation of key target proteins, GSTs are involved in the regulation of major cellular processes, including metabolism (e.g., glycolysis and the PPP), signal transduction, transcription regulation, and the development of resistance to anticancer drugs. In this review, recent findings in GSH synthesis, the roles and functions of GPxs, and GST isoforms in cancer development are discussed, along with the search for GST and GPx inhibitors for cancer treatment.

The role of redox-active small molecules and oxidative protein post-translational modifications in seed aging

Seed vigor is a crucial indicator of seed quality. Variations in seed vigor are closely associated with seed properties and storage conditions. The vigor of mature seeds progressively declines during storage, which is called seed deterioration or aging. Seed aging induces a cascade of cellular damage, including impaired subcellular structures and macromolecules, such as lipids, proteins, and DNA. Reactive oxygen species (ROS) act as signaling molecules during seed aging causing oxidative damage and triggering programmed cell death (PCD). Mitochondria are the main site of ROS production and change morphology and function before other organelles during aging. The roles of other small redox-active molecules in regulating cell and seed vigor, such as nitric oxide (NO) and hydrogen sulfide (H2S), were identified later. ROS, NO, and H2S typically regulate protein function through post-translational modifications (PTMs), including carbonylation, S-glutathionylation, S-nitrosylation, and S-sulfhydration. These signaling molecules as well as the PTMs they induce interact to regulate cell fate and seed vigor. This review was conducted to describe the physiological changes and underlying molecular mechanisms that in seed aging and provides a comprehensive view of how ROS, NO, and H2S affect cell death and seed vigor.

Endogenous Hydrogen Sulfide Persulfidates Caspase-3 at Cysteine 163 to Inhibit Doxorubicin-Induced Cardiomyocyte Apoptosis

Doxorubicin (DOX) is an efficient antitumor anthracycline drug, but its cardiotoxicity adversely affects the prognosis of the patients. In this study, we explored whether endogenous gasotransmitter hydrogen sulfide (H₂S) could protect against DOX-induced cardiomyocyte apoptosis and its mechanisms. The results indicated that DOX significantly downregulated endogenous H₂S production and endogenous synthetase cystathionine γ-lyase (CSE) expression and obviously stimulated the apoptosis in H9C2 cells. The supplement of H₂S donor sodium hydrosulfide (NaHS) or overexpression of CSE inhibited DOX-induced H9C2 cell apoptosis. DOX enhanced the activities of caspase family members in cardiomyocytes, while NaHS attenuated DOX-enhanced caspase-3, caspase-2, and caspase-9 activities by 223.1%, 73.94%, and 52.29%, respectively. Therefore, taking caspase-3 as a main target, we demonstrated that NaHS or CSE overexpression alleviated the cleavage of caspase-3, suppressed caspase-3 activity, and inhibited the cleavage of poly ADP-ribose polymerase (PARP). Mechanistically, we found that H₂S persulfidated caspase-3 in H9C2 cells and human recombinant caspase-3 protein, while the thiol-reducing agent dithiothreitol (DTT) abolished H₂S-induced persulfidation of caspase-3 and thereby prevented the antiapoptotic effect of H₂S on caspase-3 in H9C2 cells. The mutation of caspase-3 C148S and C170S failed to block caspase-3 persulfidation by H₂S in H9C2 cells. However, caspase-3 C163S mutation successfully abolished the effect of H₂S on caspase-3 persulfidation and the corresponding protection of H9C2 cells. Collectively, these findings indicate that endogenous H₂S persulfidates caspase-3 at cysteine 163, inhibiting its activity and cardiomyocyte apoptosis. Sufficient endogenous H₂S might be necessary for the protection against myocardial cell apoptosis induced by DOX. The results of the study might open new avenues with respect to the therapy of DOX-stimulated cardiomyopathy.

NO-HDAC dual inhibitors

HDAC inhibitors and NO donors have both demonstrated independently broad therapeutic potential in a variety of diseases. Borretto et al. presented the topic of NO-HDAC dual inhibitors for the first time in 2013 as an attractive new topic. Here we collected the general structure of all synthesized NO-HDAC dual inhibitors, lead compounds, synthesis methods and biological features of the most potent dual NO-HDAC inhibitor in each category with the intention of assisting in the synthesis and optimization of new drug-like compounds for diverse diseases. Based on studies done so far, NO-HDAC dual inhibitors have displayed satisfactory results against wound healing (3), heart hypertrophy (3), inflammatory, cardiovascular, neuromuscular illnesses (11a-11e) and cancer (6a-6o, 9a-9d, 10a-10d, 16 and 17). NO-HDAC dual inhibitors can have high therapeutic potential for various diseases due to their new properties, NO properties, HDAC inhibitor properties and also due to the effects of NO on HDAC enzymes.

Exogenous Introduction of Initiator and Executioner Caspases Results in Different Apoptotic Outcomes

The balance of pro-apoptotic and pro-survival proteins defines a cell's fate. These processes are controlled through an interdependent and finely tuned protein network that enables survival or leads to apoptotic cell death. The caspase family of proteases is central to this apoptotic network, with initiator and executioner caspases, and their interaction partners, regulating and executing apoptosis. In this work, we interrogate and modulate this network by exogenously introducing specific initiator or executioner caspase proteins. Each caspase is exogenously introduced using redox-responsive polymeric nanogels. Although caspase-3 might be expected to be the most effective due to the centrality of its role in apoptosis and its heightened catalytic efficiency relative to other family members, we observed that caspase-7 and caspase-9 are the most effective at inducing apoptotic cell death. By critically analyzing the introduced activity of the delivered caspase, the pattern of substrate cleavage, as well as the ability to activate endogenous caspases, we conclude that the efficacy of each caspase correlated with the levels of pro-survival factors that both directly and indirectly impact the introduced caspase. These findings lay the groundwork for developing methods for exogenous introduction of caspases as a therapeutic option that can be tuned to the apoptotic balance in a proliferating cell.

Regulation of the Dimerization and Activity of SARS-CoV-2 Main Protease through Reversible Glutathionylation of Cysteine 300

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent for coronavirus disease 2019 (COVID-19), encodes two proteases required for replication. The main protease (M^pro), encoded as part of two polyproteins, pp1a and pp1ab, is responsible for 11 different cleavages of these viral polyproteins to produce mature proteins required for viral replication. M^pro is therefore an attractive target for therapeutic interventions. Certain proteins in cells under oxidative stress undergo modification of reactive cysteines. We show M^pro is susceptible to glutathionylation, leading to inhibition of dimerization and activity. Activity of glutathionylated M^pro could be restored with reducing agents or glutaredoxin. Analytical studies demonstrated that glutathionylated M^pro primarily exists as a monomer and that modification of a single cysteine with glutathione is sufficient to block dimerization and inhibit its activity. Gel filtration studies as well as analytical ultracentrifugation confirmed that glutathionylated M^pro exists as a monomer. Tryptic and chymotryptic digestions of M^pro as well as experiments using a C300S M^pro mutant revealed that Cys300, which is located at the dimer interface, is a primary target of glutathionylation. Moreover, Cys300 is required for inhibition of activity upon M^pro glutathionylation. These findings indicate that M^pro dimerization and activity can be regulated through reversible glutathionylation of a non-active site cysteine, Cys300, which itself is not required for M^pro activity, and provides a novel target for the development of agents to block M^pro dimerization and activity. This feature of M^pro may have relevance to the pathophysiology of SARS-CoV-2 and related bat coronaviruses.

Glutathione in the Brain

Glutathione (GSH) is the most abundant non-protein thiol, and plays crucial roles in the antioxidant defense system and the maintenance of redox homeostasis in neurons. GSH depletion in the brain is a common finding in patients with neurodegenerative diseases, such as Alzheimer’s disease and Parkinson’s disease, and can cause neurodegeneration prior to disease onset. Excitatory amino acid carrier 1 (EAAC1), a sodium-dependent glutamate/cysteine transporter that is selectively present in neurons, plays a central role in the regulation of neuronal GSH production. The expression of EAAC1 is posttranslationally controlled by the glutamate transporter-associated protein 3–18 (GTRAP3-18) or miR-96-5p in neurons. The regulatory mechanism of neuronal GSH production mediated by EAAC1 may be a new target in therapeutic strategies for these neurodegenerative diseases. This review describes the regulatory mechanism of neuronal GSH production and its potential therapeutic application in the treatment of neurodegenerative diseases.

Glutathione S-Transferases in Cancer

In humans, the glutathione S-transferases (GST) protein family is composed of seven members that present remarkable structural similarity and some degree of overlapping functionalities. GST proteins are crucial antioxidant enzymes that regulate stress-induced signaling pathways. Interestingly, overactive GST proteins are a frequent feature of many human cancers. Recent evidence has revealed that the biology of most GST proteins is complex and multifaceted and that these proteins actively participate in tumorigenic processes such as cell survival, cell proliferation, and drug resistance. Structural and pharmacological studies have identified various GST inhibitors, and these molecules have progressed to clinical trials for the treatment of cancer and other diseases. In this review, we discuss recent findings in GST protein biology and their roles in cancer development, their contribution in chemoresistance, and the development of GST inhibitors for cancer treatment.

Role of protein S-Glutathionylation in cancer progression and development of resistance to anti-cancer drugs

The survival, functioning and proliferation of mammalian cells are highly dependent on the cellular response and adaptation to changes in their redox environment. Cancer cells often live in an altered redox environment due to aberrant neo-vasculature, metabolic reprogramming and dysregulated proliferation. Thus, redox adaptations are critical for their survival. Glutathione plays an essential role in maintaining redox homeostasis inside the cells by binding to redox-sensitive cysteine residues in proteins by a process called S-glutathionylation. S-Glutathionylation not only protects the labile cysteine residues from oxidation, but also serves as a sensor of redox status, and acts as a signal for stimulation of downstream processes and adaptive responses to ensure redox equilibrium. The present review aims to provide an updated overview of the role of the unique redox adaptations during carcinogenesis and cancer progression, focusing on their dependence on S-glutathionylation of specific redox-sensitive proteins involved in a wide range of processes including signalling, transcription, structural maintenance, mitochondrial functions, apoptosis and protein recycling. We also provide insights into the role of S-glutathionylation in the development of resistance to chemotherapy. Finally, we provide a strong rationale for the development of redox targeting drugs for treatment of refractory/resistant cancers.

Regulation of the Dimerization and Activity of SARS-CoV-2 Main Protease through Reversible Glutathionylation of Cysteine 300

SARS-CoV-2 encodes main protease (M^pro), an attractive target for therapeutic interventions. We show M^pro is susceptible to glutathionylation leading to inhibition of dimerization and activity. Activity of glutathionylated M^pro could be restored with reducing agents or glutaredoxin. Analytical studies demonstrated that glutathionylated M^pro primarily exists as a monomer and that a single modification with glutathione is sufficient to block dimerization and loss of activity. Proteolytic digestions of M^pro revealed Cys300 as a primary target of glutathionylation, and experiments using a C300S M^pro mutant revealed that Cys300 is required for inhibition of activity upon M^pro glutathionylation. These findings indicate that M^pro dimerization and activity can be regulated through reversible glutathionylation of Cys300 and provides a novel target for the development of agents to block Mpro dimerization and activity. This feature of M^pro may have relevance to human disease and the pathophysiology of SARS-CoV-2 in bats, which develop oxidative stress during flight.

Interactions of zinc- and redox-signaling pathways

Zinc and cellular oxidants such as reactive oxygen species (ROS) each participate in a multitude of physiological functions. There is considerable overlap between the affected events, including signal transduction. While there is no obvious direct connection between zinc and ROS, mainly because the bivalent cation zinc does not change its oxidation state in biological systems, these are linked by their interaction with sulfur, forming the remarkable triad of zinc, ROS, and protein thiols. First, zinc binds to reduced thiols and can be released upon oxidation. Thereby, redox signals are translated into changes in the free zinc concentration, which can act as zinc signals. Second, zinc affects oxidation of thiols in several ways, directly as well as indirectly. A protein incorporating many of these interactions is metallothionein (MT), which is rich in cysteine and capable of binding up to seven zinc ions in its fully reduced state. Zinc binding is diminished after (partial) oxidation, while thiols show increased reactivity in the absence of bound metal ions. Adding still more complexity, the MT promoter is controlled by zinc (via metal regulatory transcription factor 1 (MTF-1)) as well as redox (via nuclear factor erythroid 2-related factor 2 (NRF2)). Many signaling cascades that are important for cell proliferation or apoptosis contain protein thiols, acting as centers for crosstalk between zinc- and redox-signaling. A prominent example for shared molecular targets for zinc and ROS are active site cysteine thiols in protein tyrosine phosphatases (PTP), their activity being downregulated by oxidation as well as zinc binding. Because zinc binding also protects PTP thiols form irreversible oxidation, there is a multi-faceted reciprocal interaction, illustrating that zinc- and redox-signaling are intricately linked on multiple levels.

Expression of inducible NOS is indispensable for the antiproliferative and proapoptotic effect of imatinib in BCR-ABL positive cells

Chronic myeloid leukemia (CML) is characterized by constitutive BCR-ABL kinase activity, an aggressive proliferation of immature cells, and reduced differentiation. Targeting tyrosine kinase activity of BCR-ABL with imatinib is an effective therapy for the newly diagnosed CML patients; however, 20%-30% of the patients initially treated with imatinib eventually experience treatment failure. Therefore, early identification of these patients is of high clinical relevance. In the present study, we by undertaking a direct comparison of inducible NOS (iNOS) status in neutrophils from healthy volunteers, newly diagnosed, imatinib responder, and resistant CML patients as well as by conducting in vitro studies in K562 cells demonstrated that inhibition of BCR-ABL by imatinib or siRNA significantly enhanced NO generation and iNOS expression. Indeed, patients exhibiting treatment failure or imatinib resistance were less likely to induce NO generation/iNOS expression. Our findings further demonstrated that imatinib mediated antiproliferative and proapoptotic effect in BCR-ABL⁺ cells associated with enhanced iNOS expression, and it was significantly prevented in the presence of L-NAME, 1400W, or iNOS siRNA. Overexpression of iNOS in K562 cells expectedly enhanced imatinib sensitivity on cytostasis and apoptosis, even at lower concentration (0.1 μM) of imatinib. Mechanistically, imatinib or BCR-ABL siRNA following deglutathionylation of NF-κB, enhanced its binding to iNOS promoter and induced iNOS transcription. Deglutathionylation of procaspase-3 however associated with increased caspase-3 activity and cell apoptosis. Taken together, results obtained suggest that monitoring NO/iNOS level could be useful to identify patients likely to be responsive or resistant to imatinib and can be used to personalized alternative therapy.

The Writers, Readers, and Erasers in Redox Regulation of GAPDH

Glyceraldehyde 3–phosphate dehydrogenase (GAPDH) is a key glycolytic enzyme, which is crucial for the breakdown of glucose to provide cellular energy. Over the past decade, GAPDH has been reported to be one of the most prominent cellular targets of post-translational modifications (PTMs), which divert GAPDH toward different non-glycolytic functions. Hence, it is termed a moonlighting protein. During metabolic and oxidative stress, GAPDH is a target of different oxidative PTMs (oxPTM), e.g., sulfenylation, S-thiolation, nitrosylation, and sulfhydration. These modifications alter the enzyme’s conformation, subcellular localization, and regulatory interactions with downstream partners, which impact its glycolytic and non-glycolytic functions. In this review, we discuss the redox regulation of GAPDH by different redox writers, which introduce the oxPTM code on GAPDH to instruct a redox response; the GAPDH readers, which decipher the oxPTM code through regulatory interactions and coordinate cellular response via the formation of multi-enzyme signaling complexes; and the redox erasers, which are the reducing systems that regenerate the GAPDH catalytic activity. Human pathologies associated with the oxidation-induced dysregulation of GAPDH are also discussed, featuring the importance of the redox regulation of GAPDH in neurodegeneration and metabolic disorders.

Redox Regulation by Protein S-Glutathionylation: From Molecular Mechanisms to Implications in Health and Disease

S-glutathionylation, the post-translational modification forming mixed disulfides between protein reactive thiols and glutathione, regulates redox-based signaling events in the cell and serves as a protective mechanism against oxidative damage. S-glutathionylation alters protein function, interactions, and localization across physiological processes, and its aberrant function is implicated in various human diseases. In this review, we discuss the current understanding of the molecular mechanisms of S-glutathionylation and describe the changing levels of expression of S-glutathionylation in the context of aging, cancer, cardiovascular, and liver diseases.

Quercetin Triggers Induction of Apoptotic and Lysosomal Death of Sensitive and Multidrug Resistant Leukaemia HL60 Cells

Multidrug resistance (MDR) constitutes the major cause of the failure in anticancer therapy. One of the most important mechanisms leading to the occurrence of MDR is related to the modulation of cellular death pathways. The aim of this study was to determine the effect of quercetin (Q) on triggering the programed death of human promyelocytic leukemia sensitive cells HL60 as well as multidrug resistant HL60/VINC cells overexpressing P-glycoprotein and HL60/MX2 cells characterized by the presence of mutated α isoform of topoisomerase II and the absence of β isoform of this enzyme. Q exerted comparable cytotoxic activities toward sensitive HL60 cells and their MDR counterparts. It was also found that this compound modulated the cellular level of reactive oxygen species (ROS) and led to the marked decrease in cellular GSH level. Furthermore, it was demonstrated that Q used at IC₅₀ and IC₉₀ significantly increased the percentage of sub-G1 subpopulation of all studied leukemia cells causing oligonucleosomal DNA fragmentation. The present study also indicated that Q used at IC₉₀ triggers predominantly programed cell death of sensitive HL60 cells and their MDR counterparts by induction of apoptosis occurring with the involvement of caspase-3 and caspase-8 as well as by lysosome membrane permeabilization-dependent mechanisms.

Redox-Mediated Post-Translational Modifications of Proteolytic Enzymes and Their Role in Protease Functioning

Proteolytic enzymes play a crucial role in metabolic processes, providing the cell with amino acids through the hydrolysis of multiple endogenous and exogenous proteins. In addition to this function, proteases are involved in numerous protein cascades to maintain cellular and extracellular homeostasis. The redox regulation of proteolysis provides a flexible dose-dependent mechanism for proteolytic activity control. The excessive reactive oxygen species (ROS) and reactive nitrogen species (RNS) in living organisms indicate pathological conditions, so redox-sensitive proteases can swiftly induce pro-survival responses or regulated cell death (RCD). At the same time, severe protein oxidation can lead to the dysregulation of proteolysis, which induces either protein aggregation or superfluous protein hydrolysis. Therefore, oxidative stress contributes to the onset of age-related dysfunction. In the present review, we consider the post-translational modifications (PTMs) of proteolytic enzymes and their impact on homeostasis.

Oxidants, Antioxidants and Thiol Redox Switches in the Control of Regulated Cell Death Pathways

It is well appreciated that biological reactive oxygen and nitrogen species such as hydrogen peroxide, superoxide and nitric oxide, as well as endogenous antioxidant systems, are important modulators of cell survival and death in diverse organisms and cell types. In addition, oxidative stress, nitrosative stress and dysregulated cell death are implicated in a wide variety of pathological conditions, including cancer, cardiovascular and neurological diseases. Therefore, much effort is devoted to elucidate the molecular mechanisms linking oxidant/antioxidant systems and cell death pathways. This review is focused on thiol redox modifications as a major mechanism by which oxidants and antioxidants influence specific regulated cell death pathways in mammalian cells. Growing evidence indicates that redox modifications of cysteine residues in proteins are involved in the regulation of multiple cell death modalities, including apoptosis, necroptosis and pyroptosis. In addition, recent research suggests that thiol redox switches play a role in the crosstalk between apoptotic and necrotic forms of regulated cell death. Thus, thiol-based redox circuits provide an additional layer of control that determines when and how cells die.

Electrophiles Against (Skin) Diseases: More Than Nrf2

The skin represents an indispensable barrier between the organism and the environment and is the first line of defense against exogenous insults. The transcription factor NRF2 is a central regulator of cytoprotection and stress resistance. NRF2 is activated in response to oxidative stress by reactive oxygen species (ROS) and electrophiles. These electrophiles oxidize specific cysteine residues of the NRF2 inhibitor KEAP1, leading to KEAP1 inactivation and, subsequently, NRF2 activation. As oxidative stress is associated with inflammation, the NRF2 pathway plays important roles in the pathogenesis of common inflammatory diseases and cancer in many tissues and organs, including the skin. The electrophile and NRF2 activator dimethyl fumarate (DMF) is an established and efficient drug for patients suffering from the common inflammatory skin disease psoriasis and the neuro-inflammatory disease multiple sclerosis (MS). In this review, we discuss possible molecular mechanisms underlying the therapeutic activity of DMF and other NRF2 activators. Recent evidence suggests that electrophiles not only activate NRF2, but also target other inflammation-associated pathways including the transcription factor NF-κB and the multi-protein complexes termed inflammasomes. Inflammasomes are central regulators of inflammation and are involved in many inflammatory conditions. Most importantly, the NRF2 and inflammasome pathways are connected at different levels, mainly antagonistically.

Pyroptotic and apoptotic cell death in iNOS and nNOS overexpressing K562 cells: A mechanistic insight

Previous studies from this lab and others have demonstrated that nitric oxide (NO) in a concentration dependent manner, modulated neutrophil and leukemic cell survival. Subsequent studies delineated importance of iNOS in neutrophil differentiation and leukemic cell death. On the contrary, role of nNOS in survival of these cells remains least understood. Present study was therefore undertaken to assess and compare the role of iNOS and nNOS in the survival of NOS overexpressing myelocytic K562 cells. Cells with almost similar iNOS and nNOS activities displayed comparable cell cycle perturbation, Annexin V positivity, mitochondrial dysfunction, augmented DCF fluorescence, and also attenuated expression of antioxidants. Moreover, induction in cell death was also accompanied by the activation of pJNK/p38MAPK/Erk1/2 and reduction in PI3K/Akt/mTOR signaling. Treatment of NOS isoform overexpressing K562 cells with NAC, a potent free radical scavenger prevented cell death and also the modulations in the signaling proteins. In addition, enhanced expression of CASP1 and CASP4 genes, along with increased Caspase-1 cleavage and increased IL-1β release were significantly more in K562^iNOS cells, which indicate priming of these cells for pyroptotic cell death. On the other hand, K562^nNOS cells, displayed much enhanced CASP3 gene expression, Caspase-3 cleavage and Caspase-3 activity. Results obtained indicate that similar level of iNOS or nNOS activation in K562 cells, preferred pyroptotic and apoptotic cell death respectively.

Procaspase-3 Overexpression in Cancer: A Paradoxical Observation with Therapeutic Potential

Many anticancer strategies rely on the promotion of apoptosis in cancer cells as a means to shrink tumors. Crucial for apoptotic function are executioner caspases, most notably caspase-3, that proteolyze a variety of proteins, inducing cell death. Paradoxically, overexpression of procaspase-3 (PC-3), the low-activity zymogen precursor to caspase-3, has been reported in a variety of cancer types. Until recently, this counterintuitive overexpression of a pro-apoptotic protein in cancer has been puzzling. Recent studies suggest subapoptotic caspase-3 activity may promote oncogenic transformation, a possible explanation for the enigmatic overexpression of PC-3. Herein, the overexpression of PC-3 in cancer and its mechanistic basis is reviewed; collectively, the data suggest the potential for exploitation of PC-3 overexpression with PC-3 activators as a targeted anticancer strategy.

Consequences of trace metal variability and supplementation on Chinese hamster ovary (CHO) cell culture performance: A review of key mechanisms and considerations

Trace metals are supplied to chemically-defined media (CDM) for optimal Chinese hamster ovary (CHO) cell culture performance during the production of monoclonal antibodies and other therapeutic proteins. However, lot-to-lot and vendor-to-vendor variability in raw materials consequently leads to an imbalance of trace metals that are supplied to CDM. This imbalance can yield detrimental effects rooted in several primary mechanisms and pathways including oxidative stress, apoptosis, lactate accumulation, and unfavorable glycan synthesis. Recent research endeavors involve supplying zinc, copper, and manganese to CDM in excess to further maximize culture productivity and product quality. These treatments significantly impact critical quality attributes and furthermore highlight the degree to which trace metal availability can affect CHO cell culture performance. This review highlights the role of trace metal variability, supplementation, and interplay on key cellular mechanisms responsible for overall culture performance and the production and quality of therapeutic proteins.

Cysteine Glutathionylation Acts as a Redox Switch in Endothelial Cells

Oxidative post-translational modifications (oxPTM) of receptors, enzymes, ion channels and transcription factors play an important role in cell signaling. oxPTMs are a key way in which oxidative stress can influence cell behavior during diverse pathological settings such as cardiovascular diseases (CVD), cancer, neurodegeneration and inflammatory response. In addition, changes in oxPTM are likely to be ways in which low level reactive oxygen and nitrogen species (RONS) may contribute to redox signaling, exerting changes in physiological responses including angiogenesis, cardiac remodeling and embryogenesis. Among oxPTM, S-glutathionylation of reactive cysteines emerges as an important regulator of vascular homeostasis by modulating endothelial cell (EC) responses to their local redox environment. This review summarizes the latest findings of S-glutathionylated proteins in major EC pathways, and the functional consequences on vascular pathophysiology. This review highlights the diversity of molecules affected by S-glutathionylation, and the complex consequences on EC function, thereby demonstrating an intricate dual role of RONS-induced S-glutathionylation in maintaining vascular homeostasis and participating in various pathological processes.

Chemical Probes for Redox Signaling and Oxidative Stress

SIGNIFICANCE

Cellular reactive oxygen species (ROS) mediate redox signaling cascades that are critical to numerous physiological and pathological processes. Analytical methods to monitor cellular ROS levels and proteomic platforms to identify oxidative post-translational modifications (PTMs) of proteins are critical to understanding the triggers and consequences of redox signaling. Recent Advances: The prevalence and significance of redox signaling has recently been illuminated through the use of chemical probes that allow for sensitive detection of cellular ROS levels and proteomic dissection of oxidative PTMs directly in living cells.

CRITICAL ISSUES

In this review, we provide a comprehensive overview of chemical probes that are available for monitoring ROS and oxidative PTMs, and we highlight the advantages and limitations of these methods.

FUTURE DIRECTIONS

Despite significant advances in chemical probes, the low levels of cellular ROS and low stoichiometry of oxidative PTMs present challenges for accurately measuring the extent and dynamics of ROS generation and redox signaling. Further improvements in sensitivity and ability to spatially and temporally control readouts are essential to fully illuminate cellular redox signaling.

Critical Roles of Glutaredoxin in Brain Cells-Implications for Parkinson's Disease

SIGNIFICANCE

Glutaredoxin (Grx)1, an evolutionarily conserved and ubiquitous enzyme, regulates redox signal transduction and protein redox homeostasis by catalyzing reversible S-glutathionylation. Grx1 plays different roles in different cell types. In Parkinson's disease (PD), Grx1 regulates apoptosis signaling in dopaminergic neurons, so that loss of Grx1 leads to increased cell death; in microglial cells, Grx1 regulates proinflammatory signaling, so that upregulation of Grx1 promotes cytokine production. Here we examine the regulatory roles of Grx1 in PD with a view toward therapeutic innovation. Recent Advances: In postmortem midbrain PD samples, Grx1 was decreased relative to controls, specifically within dopaminergic neurons. In Caenorhabditis elegans models of PD, loss of the Grx1 homologue led to exacerbation of the neurodegenerative phenotype. This effect was partially relieved by overexpression of neuroprotective DJ-1, consistent with regulation of DJ-1 content by Grx1. Increased GLRX copy number in PD patients was associated with earlier PD onset; and Grx1 levels correlated with levels of proinflammatory tumor necrosis factor-α in mouse and human brain samples. In vitro studies showed Grx1 to be upregulated on proinflammatory activation of microglia. Direct overexpression of Grx1 increased microglial activation; silencing Grx1 diminished activation. Grx1 upregulation in microglia corresponded to increased neuronal cell death in coculture. Overall, these studies identify competing roles of Grx1 in PD etiology.

CRITICAL ISSUES

The dilemma regarding Grx1 as a PD therapeutic target is whether to stimulate its upregulation for neuroprotection or inhibit its proinflammatory activity.

FUTURE DIRECTIONS

Further investigation is needed to understand the preponderant role of Grx1 regarding dopaminergic neuronal survival.

Thioredoxin system as a gatekeeper in caspase-6 activation and nuclear lamina integrity: Implications for Alzheimer's disease

Recent reports in pathophysiology of neurodegenerative diseases (ND) have linked nuclear lamina degradation/deficits to neuronal cell death. Lamin-B1 damage is specifically involved in this process leading to nuclear envelope invagination and heterochromatin rearrangement. The underlying mechanisms involved in these events are not yet defined. In this study, while examining the effect of Thioredoxin-1(Trx1) inhibition on cell death in a model of oxidative stress, we noted robust nuclear invagination in SH-SY5Y cells. Evaluation of nuclear lamina proteins revealed lamin-B1 cleavage that was prevented by caspase-6 (CASP6) inhibitor and exacerbated after pharmacologic/genetic inhibition of Trx1 system, but not after glutathione depletion. Activation of CASP6 was upstream of CASP3/7 activation and its inhibition was sufficient to prevent cell death in our system. The effect of Trx1 redox status on CASP6 activation was assessed by administration of reduced/oxidized forms in cell-free nuclei preparation and purified enzymatic assays. Although reduced Trx1 decreased CASP6 enzymatic activity and lamin-B1 cleavage, the fully oxidized Trx1 showed opposite effects. The enhanced CASP6 activation was also associated with lower levels of DJ-1, a neuroprotective and master regulator of cellular antioxidants. The implication of our findings in ND pathophysiology was strengthened with detection of lower Trx1 levels in the hippocampi tissue of a mouse model of Alzheimer's disease. This coincided with higher CASP6 activation resulting in increased lamin-B1 and DJ-1 depletion. This study provides a first mechanistic explanation for the key regulatory role of Trx1 as a gatekeeper in activation of CASP6 and induction of nuclear invagination, an important player in ND pathophysiology.

Glycolysis Induces MCJ Expression That Links T Cell Proliferation With Caspase-3 Activity and Death

An effective adaptive immune response requires rapid T cell proliferation, followed by equally robust cell death. These two processes are coordinately regulated to allow sufficient magnitude of response followed by its rapid resolution, while also providing the maintenance of T cell memory. Both aspects of this T cell response are characterized by profound changes in metabolism; glycolysis drives proliferation whereas oxidative phosphorylation supports the survival of memory T cells. While much is known about the separate aspects of T cell expansion and contraction, considerably less is understood regarding how these processes might be connected. We report a link between the induction of glycolysis in CD8⁺ T cells and upregulation of the inhibitor of complex I and oxidative phosphorylation, methylation-controlled J protein (MCJ). MCJ acts synergistically with glycolysis to promote caspase-3 activity. Effector CD8⁺ T cells from MCJ-deficient mice manifest reduced glycolysis and considerably less active caspase-3 compared to wild-type cells. Consistent with these observations, in non-glycolytic CD8⁺ T cells cultured in the presence of IL-15, MCJ expression is repressed by methylation, which parallels their reduced active caspase-3 and increased survival compared to glycolytic IL-2-cultured T cells. Elevated levels of MCJ are also observed in vivo in the highly proliferative and glycolytic subset of CD4^-CD8^- T cells in Fas-deficient lpr mice. This subset also manifests elevated levels of activated caspase-3 and rapid cell death. Collectively, these data demonstrate tight linkage of glycolysis, MCJ expression, and active caspase-3 that serves to prevent the accumulation and promote the timely death of highly proliferative CD8⁺ T cells.

A new pyridazinone exhibits potent cytotoxicity on human cancer cells via apoptosis and poly-ubiquitinated protein accumulation

In the last 15 years, pyridazinone derivatives have acquired extensive attention due to their widespread biological activities and pharmacological applications. Pyridazinones are well known for their anti-microbial, anti-viral, anti-inflammatory, anti-cancer, and cardiovascular activities, among others. In this study, we evaluated the anti-cancer activity of a new pyridazinone derivative and propose it as a potential anti-neoplastic agent in acute promyelocytic leukemia cells. Pyr-1 cytotoxicity was assessed on several human cancer and two non-cancerous cell lines by the DNS assay. Pyr-1 demonstrated potent cytotoxicity against 22 human cancer cell lines, exhibiting the most favorable selective cytotoxicity on leukemia (CEM and HL-60), breast (MDA-MB-231 and MDA-MB-468), and lung (A-549) cancer cell lines, when compared with non-cancerous breast epithelial MCF-10A cells. Analyses of apoptosis/necrosis pathways, reactive oxygen species (ROS) production, mitochondria health, caspase-3 activation, and cell cycle profile were performed via flow cytometry. Both hmox-1 RNA and protein expression levels were evaluated by quantitative real-time PCR and Western blotting assays, respectively. Pyr-1 induced apoptosis in acute promyelocytic leukemia cells as confirmed by phosphatidylserine externalization, mitochondrial depolarization, caspase-3 activation, DNA fragmentation, and disrupted cell cycle progression. Additionally, it was determined that Pyr-1 generates oxidative and proteotoxic stress by provoking the accumulation of ROS, resulting in the overexpression of the stress-related hmox-1 mRNA transcripts and protein and a marked increase in poly-ubiquitinated proteins. Our data demonstrate that Pyr-1 induces cell death via the intrinsic apoptosis pathway by accumulating ROS and by impairing proteasome activity.

Targeting redox regulation to treat substance use disorder using N‐acetylcysteine

Substance use disorder (SUD) is a chronic relapsing disorder characterized by transitioning from acute drug reward to compulsive drug use. Despite the heavy personal and societal burden of SUDs, current treatments are limited and unsatisfactory. For this reason, a deeper understanding of the mechanisms underlying addiction is required. Altered redox status, primarily due to drug-induced increases in dopamine metabolism, is a unifying feature of abused substances. In recent years, knowledge of the effects of oxidative stress in the nervous system has evolved from strictly neurotoxic to include a more nuanced role in redox-sensitive signaling. More specifically, S-glutathionylation, a redox-sensitive post-translational modification, has been suggested to influence the response to drugs of abuse. In this review we will examine the evidence for redox-mediating drugs as therapeutic tools focusing on N-acetylcysteine as a treatment for cocaine addiction. We will conclude by suggesting future research directions that may further advance this field.

Effects of exposure to 5-MeO-DIPT during adolescence on brain neurotransmission and neurotoxicity in adult rats

PURPOSE

Tryptamine hallucinogen 5-methoxy-N,N-diisopropyltryptamine (5-MeO-DIPT) is a serotonin transporter inhibitor with high affinity for serotonin 5-HT1A and 5-HT2A/C receptors. We showed previously that 5-MeO-DIPT in a single dose increased neurotransmitter release in brain regions of rats and elicited single- and double-strand DNA breaks. Herein we investigated the effects of repeated-intermittent 5-MeO-DIPT administration in adolescence on dopamine (DA), serotonin (5-HT) and glutamate release in brain regions of adult rats. Furthermore, we examined caspase-3 activity, oxidative DNA damage, the Gpx3, Sod1, Ht1a and Ht2a mRNA expression levels, and cell viability.

METHODS

Neurotransmitter release was measured by microdialysis in freely moving animals. Caspase-3 activity was assessed colorimetrically, and oxidative DNA damage with the comet assay, while the Gpx3, Sod1, Ht1a and Ht2a mRNA expression levels were assessed by real-time polymerase chain reaction. Cell viability was studied in SH-SY5Y and Hep G2 cells by the MTT test.

RESULTS

We observed changed responses of DA, 5-HT and glutamate neurons to a challenge dose of 5-MeO-DIPT when animals were treated repeatedly in adolescence with this hallucinogen. The basal extracellular levels of DA and 5-HT were decreased in the striatum and nucleus accumbens, while glutamate level was increased in the nucleus accumbens and frontal cortex. The damage of cortical DNA, increased Gpx3 and Sod1 mRNA expression and affected caspase-3 activity were also observed. Furthermore, decreased Ht1a and Ht2a mRNA expression in the frontal cortex and marked cytotoxicity of 5-MeO-DIPT were found.

CONCLUSIONS

These results suggest that 5-MeO-DIPT given repeatedly during adolescence affects brain neurotransmission and shows neurotoxic potential observed in adult animals.

Glutathione: Antioxidant Properties Dedicated to Nanotechnologies

Which scientist has never heard of glutathione (GSH)? This well-known low-molecular-weight tripeptide is perhaps the most famous natural antioxidant. However, the interest in GSH should not be restricted to its redox properties. This multidisciplinary review aims to bring out some lesser-known aspects of GSH, for example, as an emerging tool in nanotechnologies to achieve targeted drug delivery. After recalling the biochemistry of GSH, including its metabolism pathways and redox properties, its involvement in cellular redox homeostasis and signaling is described. Analytical methods for the dosage and localization of GSH or glutathiolated proteins are also covered. Finally, the various therapeutic strategies to replenish GSH stocks are discussed, in parallel with its use as an addressing molecule in drug delivery.

Modifications to a common phosphorylation network provide individualized control in caspases

Caspase-3 activation and function have been well-defined during programmed cell death, but caspase activity, at low levels, is also required for developmental processes such as lymphoid proliferation and erythroid differentiation. Post-translational modification of caspase-3 is one method used by cells to fine-tune activity below the threshold required for apoptosis, but the allosteric mechanism that reduces activity is unknown. Phosphorylation of caspase-3 at a conserved allosteric site by p38-MAPK (mitogen-activated protein kinase) promotes survival in human neutrophils, and the modification of the loop is thought to be a key regulator in many developmental processes. We utilized phylogenetic, structural, and biophysical studies to define the interaction networks that facilitate the allosteric mechanism in caspase-3. We show that, within the modified loop, Ser¹⁵⁰ evolved with the apoptotic caspases, whereas Thr¹⁵² is a more recent evolutionary event in mammalian caspase-3. Substitutions at Ser¹⁵⁰ result in a pH-dependent decrease in dimer stability, and localized changes in the modified loop propagate to the active site of the same protomer through a connecting surface helix. Likewise, a cluster of hydrophobic amino acids connects the conserved loop to the active site of the second protomer. The presence of Thr¹⁵² in the conserved loop introduces a "kill switch" in mammalian caspase-3, whereas the more ancient Ser¹⁵⁰ reduces without abolishing enzyme activity. These data reveal how evolutionary changes in a conserved allosteric site result in a common pathway for lowering activity during development or a more recent cluster-specific switch to abolish activity.

Glycolysis promotes caspase-3 activation in lipid rafts in T cells

Resting T cells undergo a rapid metabolic shift to glycolysis upon activation in the presence of interleukin (IL)-2, in contrast to oxidative mitochondrial respiration with IL-15. Paralleling these different metabolic states are striking differences in susceptibility to restimulation-induced cell death (RICD); glycolytic effector T cells are highly sensitive to RICD, whereas non-glycolytic T cells are resistant. It is unclear whether the metabolic state of a T cell is linked to its susceptibility to RICD. Our findings reveal that IL-2-driven glycolysis promotes caspase-3 activity and increases sensitivity to RICD. Neither caspase-7, caspase-8, nor caspase-9 activity is affected by these metabolic differences. Inhibition of glycolysis with 2-deoxyglucose reduces caspase-3 activity as well as sensitivity to RICD. By contrast, IL-15-driven oxidative phosphorylation actively inhibits caspase-3 activity through its glutathionylation. We further observe active caspase-3 in the lipid rafts of glycolytic but not non-glycolytic T cells, suggesting a proximity-induced model of self-activation. Finally, we observe that effector T cells during influenza infection manifest higher levels of active caspase-3 than naive T cells. Collectively, our findings demonstrate that glycolysis drives caspase-3 activity and susceptibility to cell death in effector T cells independently of upstream caspases. Linking metabolism, caspase-3 activity, and cell death provides an intrinsic mechanism for T cells to limit the duration of effector function.

Multifaceted Roles of Glutathione and Glutathione-Based Systems in Carcinogenesis and Anticancer Drug Resistance

SIGNIFICANCE

Glutathione is the most abundant antioxidant molecule in living organisms and has multiple functions. Intracellular glutathione homeostasis, through its synthesis, consumption, and degradation, is an intricately balanced process. Glutathione levels are often high in tumor cells before treatment, and there is a strong correlation between elevated levels of intracellular glutathione/sustained glutathione-mediated redox activity and resistance to pro-oxidant anticancer therapy. Recent Advances: Ample evidence demonstrates that glutathione and glutathione-based systems are particularly relevant in cancer initiation, progression, and the development of anticancer drug resistance.

CRITICAL ISSUES

This review highlights the multifaceted roles of glutathione and glutathione-based systems in carcinogenesis, anticancer drug resistance, and clinical applications.

FUTURE DIRECTIONS

The evidence summarized here underscores the important role played by glutathione and the glutathione-based systems in carcinogenesis and anticancer drug resistance. Future studies should address mechanistic questions regarding the distinct roles of glutathione in different stages of cancer development and cancer cell death. It will be important to study how metabolic alterations in cancer cells can influence glutathione homeostasis. Sensitive approaches to monitor glutathione dynamics in subcellular compartments will be an indispensible step. Therapeutic perspectives should focus on mechanism-based rational drug combinations that are directed against multiple redox targets using effective, specific, and clinically safe inhibitors. This new strategy is expected to produce a synergistic effect, prevent drug resistance, and diminish doses of single drugs. Antioxid. Redox Signal. 27, 1217-1234.

S-glutathionylation of glyceraldehyde-3-phosphate dehydrogenase induces formation of C150-C154 intrasubunit disulfide bond in the active site of the enzyme

BACKGROUND

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a glycolytic protein involved in numerous non-glycolytic functions. S-glutathionylated GAPDH was revealed in plant and animal tissues. The role of GAPDH S-glutathionylation is not fully understood.

METHODS

Rabbit muscle GAPDH was S-glutathionylated in the presence of H₂O₂ and reduced glutathione (GSH). The modified protein was assayed by MALDI-MS analysis, differential scanning calorimetry, dynamic light scattering, and ultracentrifugation.

RESULTS

Incubation of GAPDH in the presence of H₂O₂ together with GSH resulted in the complete inactivation of the enzyme. In contrast to irreversible oxidation of GAPDH by H₂O₂, this modification could be reversed in the excess of GSH or dithiothreitol. By data of MALDI-MS analysis, the modified protein contained both mixed disulfide between Cys150 and GSH and the intrasubunit disulfide bond between Cys150 and Cys154 (different subunits of tetrameric GAPDH may contain different products). S-glutathionylation results in loosening of the tertiary structure of GAPDH, decreases its affinity to NAD⁺ and thermal stability.

CONCLUSIONS

The mixed disulfide between Cys150 and GSH is an intermediate product of S-glutathionylation: its subsequent reaction with Cys154 results in the intrasubunit disulfide bond in the active site of GAPDH. The mixed disulfide and the C150-C154 disulfide bond protect GAPDH from irreversible oxidation and can be reduced in the excess of thiols. Conformational changes that were observed in S-glutathionylated GAPDH may affect interactions between GAPDH and other proteins (ligands), suggesting the role of S-glutathionylation in the redox signaling.

GENERAL SIGNIFICANCE

The manuscript considers one of the possible mechanisms of redox regulation of cell functions.

Friction-Induced Mitochondrial Dysregulation Contributes to Joint Deterioration in Prg4 Knockout Mice

Deficiency of PRG4 (lubricin), the boundary lubricant in mammalian joints, contributes to increased joint friction accompanied by superficial and upper intermediate zone chondrocyte caspase-3 activation, as shown in lubricin-null (Prg4^-/-) mice. Caspase-3 activity appears to be reversible upon the restitution of Prg4 either endogenously in vivo, in a gene trap mouse, or as an applied lubricant in vitro. In this study we show that intra-articular injection of human PRG4 in vivo in Prg4^-/- mice prevented caspase-3 activation in superficial zone chondrocytes and was associated with a modest decrease in whole joint friction measured ex vivo using a joint pendulum method. Non-lubricated Prg4^-/- mouse cartilage shows caspase cascade activation caused by mitochondrial dysregulation, and significantly higher levels of peroxynitrite (ONOO^- and ^-OH) and superoxide (O^-₂) compared to Prg4^+/+ and Prg4^+/- cartilage. Enzymatic activity levels of caspase 8 across Prg4 mutant mice were not significantly different, indicating no extrinsic apoptosis pathway activation. Western blots showed caspase-3 and 9 activation in Prg4^-/- tissue extracts, and the appearance of nitrosylated Cys163 in the active cleft of caspase-3 which inhibits its enzymatic activity. These findings are relevant to patients at risk for arthrosis, from camptodactyl-arthropathy-coxa vara-pericarditis (CACP) syndrome and transient lubricin insufficiency due to trauma and inflammation.

ROS in Cancer: The Burning Question

An unanswered question in human health is whether antioxidation prevents or promotes cancer. Antioxidation has historically been viewed as chemopreventive, but emerging evidence suggests that antioxidants may be supportive of neoplasia. We posit this contention to be rooted in the fact that ROS do not operate as one single biochemical entity, but as diverse secondary messengers in cancer cells. This cautions against therapeutic strategies to increase ROS at a global level. To leverage redox alterations towards the development of effective therapies necessitates the application of biophysical and biochemical approaches to define redox dynamics and to functionally elucidate specific oxidative modifications in cancer versus normal cells. An improved understanding of the sophisticated workings of redox biology is imperative to defeating cancer.

In Vitro and In Vivo Tumor Growth Inhibition by Glutathione Disulfide Liposomes

Glutathione disulfide (GSSG) is an endogenous peptide and the oxidized form of glutathione. The impacts of GSSG on cell function/dysfunction remain largely unexplored due to a lack of method to specifically increase intracellular GSSG. We recently developed GSSG liposomes that can specifically increase intracellular GSSG. The increase affected 3 of the 4 essential steps (cell detachment, migration, invasion, and adhesion) of cancer metastasis in vitro and, accordingly, produced a significant inhibition of cancer metastasis in vivo. In this investigation, the effect of GSSG liposomes on cancer growth was investigated with B16-F10 and NCI-H226 cells in vitro and with B16-F10 cells in C57BL/6 mice in vivo. Experiments were conducted to elucidate the effect on cell death through promotion of apoptosis and the effect on the cell cycle. The in vivo results with C57BL/6 mice implanted subcutaneously with B16-F10 cells showed that GSSG liposomes retarded tumor proliferation more effectively than that of dacarbazine, a chemotherapeutic drug for the treatment of melanoma. The GSSG liposomes by intravenous injection (GLS IV) and GSSG liposomes by intratumoral injection (GLS IT) showed a tumor proliferation retardation of 85% ± 5.7% and 90% ± 3.9%, respectively, compared with the phosphate-buffered saline (PBS) control group. The median survival rates for mice treated with PBS, blank liposomes, aqueous GSSG, dacarbazine, GLS IV, and GLS IT were 7, 7, 7.5, 7.75, 11.5, and 16.5 days, respectively. The effective antimetastatic and antigrowth activities warrant further investigation of the GSSG liposomes as a potentially effective therapeutic treatment for cancer.

Post-translational Modification of Caspases: The Other Side of Apoptosis Regulation

Apoptosis is a crucial program of cell death that controls development and homeostasis of multicellular organisms. The main initiators and executors of this process are the Cysteine-dependent ASPartate proteASES - caspases. A number of regulatory circuits tightly control caspase processing and activity. One of the most important, yet, at the same time still poorly understood control mechanisms of activation of caspases involves their post-translational modifications. The addition and/or removal of chemical groups drastically alters the catalytic activity of caspases or stimulates their nonapoptotic functions. In this review, we will describe and discuss the roles of key caspase modifications such as phosphorylation, ubiquitination, nitrosylation, glutathionylation, SUMOylation, and acetylation in the regulation of apoptotic cell death and cell survival.

Tunable allosteric library of caspase-3 identifies coupling between conserved water molecules and conformational selection

The native ensemble of caspases is described globally by a complex energy landscape where the binding of substrate selects for the active conformation, whereas targeting an allosteric site in the dimer interface selects an inactive conformation that contains disordered active-site loops. Mutations and posttranslational modifications stabilize high-energy inactive conformations, with mostly formed, but distorted, active sites. To examine the interconversion of active and inactive states in the ensemble, we used detection of related solvent positions to analyze 4,995 waters in 15 high-resolution (<2.0 Å) structures of wild-type caspase-3, resulting in 450 clusters with the most highly conserved set containing 145 water molecules. The data show that regions of the protein that contact the conserved waters also correspond to sites of posttranslational modifications, suggesting that the conserved waters are an integral part of allosteric mechanisms. To test this hypothesis, we created a library of 19 caspase-3 variants through saturation mutagenesis in a single position of the allosteric site of the dimer interface, and we show that the enzyme activity varies by more than four orders of magnitude. Altogether, our database consists of 37 high-resolution structures of caspase-3 variants, and we demonstrate that the decrease in activity correlates with a loss of conserved water molecules. The data show that the activity of caspase-3 can be fine-tuned through globally desolvating the active conformation within the native ensemble, providing a mechanism for cells to repartition the ensemble and thus fine-tune activity through conformational selection.

Mixed results with mixed disulfides

A period of research with Helmut Sies in the 1980s is recalled. Our experiments aimed at an in-depth understanding of metabolic changes due to oxidative challenges under near-physiological conditions, i.e. perfused organs. A major focus were alterations of the glutathione and the NADPH/NADP(+) system by different kinds of oxidants, in particular formation of glutathione mixed disulfides with proteins. To analyze mixed disulfides, a test was adapted which is widely used until today. The observations in perfused rat livers let us believe that glutathione-6-phosphate dehydrogenase (G6PDH), i.a. might be activated by glutathionylation. Although we did not succeed to verify this hypothesis for the special case of G6PDH, the regulation of enzyme/protein activities by glutathionylation today is an accepted posttranslational mechanism in redox biology in general. Our early experimental approaches are discussed in the context of present knowledge.

Nitric oxide-mediated apoptosis of neutrophils through caspase-8 and caspase-3-dependent mechanism

Neutrophils play an indispensable role in killing of invading pathogens by enhancing reactive oxygen species (ROS) and NO generation, and subsequently undergoing apoptosis. Unlike ROS/NOX2, role of NO/NOS still remains undefined in the apoptosis of neutrophils (PMNs) and the present study attempts to decipher the importance of NO/NOS in the neutrophil apoptosis. Prolonged treatment of human PMNs or mice bone marrow derived neutrophils (BMDN) with NO led to enhanced ROS generation, caspase-8/caspase-3 cleavage, reduced mitochondrial membrane potential and finally cellular apoptosis. NO-induced ROS generation led to caspase-8 deglutathionylation and activation, which subsequently activated mitochondrial death pathway via BID (Bcl-2 family protein) cleavage. NO-mediated augmentation of caspase-8 and BID cleavage was significantly prevented in BMDN from neutrophil cytosolic factor-1 (NCF-1) knockout (KO) mice, implying the involvement of NOX2 in NO-induced apoptosis of PMNs. Furthermore, ROS, NO generation and inducible nitric oxide synthase (iNOS) expression were enhanced in a time-dependent manner in human PMNs and mice BMDN undergoing spontaneous apoptosis. Pharmacological and genetic ablation of iNOS in human PMNs and mice BMDN significantly reduced the levels of apoptosis. Impaired apoptosis of BMDN from iNOS KO mice was due to reduced caspase-8 activity which subsequently prevented caspase-3 and -9 activation. Altogether, our results suggest a crucial role of NO/iNOS in neutrophil apoptosis via enhanced ROS generation and caspase-8 mediated activation of mitochondrial death pathway.

Effects of different light wavelengths from LEDs on oxidative stress and apoptosis in olive flounder (Paralichthys olivaceus) at high water temperatures

We investigated how different light spectra affect thermal stress in olive flounder (Paralichthys olivaceus), using light emitting diodes (LEDs; blue, 450 nm; green, 530 nm; red, 630 nm) at two intensities (0.3 and 0.5 W/m(2)) at relatively high water temperatures (25 and 30 °C, compared to a control condition of 20 °C). We measured the expression and activity of antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), and the levels of plasma hydrogen peroxide (H2O2) and lipid peroxidation (LPO). Furthermore, the levels and mRNA expression of caspase-3 were measured, and terminal transferase dUTP nick end labeling (TUNEL) assays of liver and comet assays were performed. The expression and activity of antioxidant enzymes, as well as plasma H2O2 and LPO levels were significantly higher after exposure to high temperatures, and significantly lower after exposure to green and blue light. Caspase-3 levels and mRNA expression showed a similar pattern. The TUNEL assay showed that apoptosis markedly increased at higher water temperatures, compared with the 20 °C control. In contrast, green light irradiation decreased apoptosis rate. Furthermore, the comet assays showed that nuclear DNA damage was caused by thermal stress, and that green light irradiation played a role in partially preventing this damage. Overall, these results suggest that light with green and blue wavelengths can reduce both high temperature-induced oxidative stress and apoptosis, and that particularly green light is efficient for this. Therefore, green light can play a role in protecting in olive flounder from thermal stress damage.

Caspase Allostery and Conformational Selection

The role of caspase proteases in regulated processes such as apoptosis and inflammation has been studied for more than two decades, and the activation cascades are known in detail. Apoptotic caspases also are utilized in critical developmental processes, although it is not known how cells maintain the exquisite control over caspase activity in order to retain subthreshold levels required for a particular adaptive response while preventing entry into apoptosis. In addition to active site-directed inhibitors, caspase activity is modulated by post-translational modifications or metal binding to allosteric sites on the enzyme, which stabilize inactive states in the conformational ensemble. This review provides a comprehensive global view of the complex conformational landscape of caspases and mechanisms used to select states in the ensemble. The caspase structural database provides considerable detail on the active and inactive conformations in the ensemble, which provide the cell multiple opportunities to fine tune caspase activity. In contrast, the current database on caspase modifications is largely incomplete and thus provides only a low-resolution picture of global allosteric communications and their effects on the conformational landscape. In recent years, allosteric control has been utilized in the design of small drug compounds or other allosteric effectors to modulate caspase activity.