BH3 death domain peptide induces cell type-selective mitochondrial outer membrane permeability

Unveiling drug resistance pathways in high-grade serous ovarian cancer(HGSOC): recent advances and future perspectives

High-Grade Serous Ovarian Carcinoma (HGSOC) represents the most prevalent and lethal subtype of ovarian cancer, with approximately 225,000 new cases reported globally each year and a five-year survival rate of merely 49.1%. The clinical management of HGSOC encounters substantial challenges, primarily attributable to its intricate drug resistance mechanisms, which involve multiple biological processes, including tumor cell heterogeneity, microenvironment remodeling, gene mutations, and drug efflux. This study systematically reviews the most recent advancements in HGSOC drug resistance research, concentrating on the molecular biological foundations of resistance mechanisms, innovative detection strategies, and potential therapeutic approaches. The research indicates that HGSOC drug resistance constitutes a complex process characterized by multifactorial interactions, involving aberrant cell signaling pathways, dynamic alterations in the tumor microenvironment, and specific expressions of molecular markers. In this review, we systematically analyzed and investigated the intricate biological behaviors associated with HGSOC drug resistance, which not only enhances the understanding of disease progression but also provides essential theoretical foundations for the development of more precise and effective targeted therapies. This review firstly illustrated the detailed drug resistance cellular and molecular mechanisms underlying HGSOC chemotherapy, which can pave the way for future studies in HGSOC drug resistance practices.

Post-Traumatic Expressions of Aromatase B, Glutamine Synthetase, and Cystathionine-Beta-Synthase in the Cerebellum of Juvenile Chum Salmon, Oncorhynchus keta

In adult fish, neurogenesis occurs in many areas of the brain, including the cerebellum, with the ratio of newly formed cells relative to the total number of brain cells being several orders of magnitude greater than in mammals. Our study aimed to compare the expressions of aromatase B (AroB), glutamine synthetase (GS), and cystathionine-beta-synthase (CBS) in the cerebellum of intact juvenile chum salmon, Oncorhynchus keta. To identify the dynamics that determine the involvement of AroB, GS, and CBS in the cellular mechanisms of regeneration, we performed a comprehensive assessment of the expressions of these molecular markers during a long-term primary traumatic brain injury (TBI) and after a repeated acute TBI to the cerebellum of O. keta juveniles. As a result, in intact juveniles, weak or moderate expressions of AroB, GS, and CBS were detected in four cell types, including cells of the neuroepithelial type, migrating, and differentiated cells (graphic abstract, A). At 90 days post injury, local hypercellular areas were found in the molecular layer containing moderately labeled AroB+, GS+, and CBS+ cells of the neuroepithelial type and larger AroB+, GS+, and CBS+ cells (possibly analogous to the reactive glia of mammals); patterns of cells migration and neovascularization were also observed. A repeated TBI caused the number of AroB+, GS+, and CBS+ cells to further increase; an increased intensity of immunolabeling was recorded from all cell types (graphic abstract, C). Thus, the results of this study provide a better understanding of adult neurogenesis in teleost fishes, which is expected to clarify the issue of the reactivation of adult neurogenesis in mammalian species.

Computational design of an apoptogenic protein that binds BCL-xL and MCL-1 simultaneously and potently

One of the hallmarks of cancer cells is their ability to evade apoptosis, which confers survival advantages and resistance to anti-cancer drugs. Cancers often exhibit overexpression of anti-apoptotic BCL-2 proteins, the loss of which triggers apoptosis. In particular, the inhibition of both BCL-xL and MCL-1, but neither one individually, synergistically enhances apoptotic cell death. Here, we report computational design to produce a protein that inhibits both BCL-xL and MCL-1 simultaneously. To a reported artificial three-helix bundle whose second helix was designed to bind MCL-1, we added a fourth helix and designed it to bind BCL-xL. After structural validation of the design and further structure-based sequence design, we produced a dual-binding protein that interacts with both BCL-xL and MCL-1 with apparent dissociation constants of 820 pM and 196 pM, respectively. Expression of this dual binder in a subset of cancer cells induced apoptotic cell death at levels significantly higher than those induced by the pro-apoptotic BIM protein. With a genetic fusion of a mitochondria-targeting sequence or the BH3 sequence of BIM, the activity of the dual binder was enhanced even further. These data suggest that targeted delivery of this dual binder alone or as a part of a modular protein to cancers in the form of protein, mRNA, or DNA may be an effective way to induce cancer cell apoptosis.

Emerging self-assembling peptide nanomaterial for anti-cancer therapy

Recently it is mainly focused on anti-tumor comprehensive treatments like finding target tumor cells or activating immune cells to inhibit tumor recurrence and metastasis. At present, chemotherapy and molecular-targeted drugs can inhibit tumor cell growth to a certain extent. However, multi-drug resistance and immune escape often make it difficult for new drugs to achieve expected effects. Peptide hydrogel nanoparticles is a new type of biological material with functional peptide chains as the core and self-assembling peptide (SAP) as the framework. It has a variety of significant biological functions, including effective local inflammation suppression and non-drug-resistant cell killing. Besides, it can induce immune activation more persistently in an adjuvant independent manner when compared with simple peptides. Thus, SAP nanomaterial has great potential in regulating cell physiological functions, drug delivery and sensitization, vaccine design and immunotherapy. Not only that, it is also a potential way to focus on some specific proteins and cells through peptides, which has already been examined in previous research. A full understanding of the function and application of SAP nanoparticles can provide a simple and practical strategy for the development of anti-tumor drugs and vaccine design, which contributes to the historical transition of peptide nanohydrogels from bench to bedside and brings as much survival benefits as possible to cancer patients.

Biologic Effect of Hydrogen Sulfide and Its Role in Traumatic Brain Injury

Ever since endogenous hydrogen sulfide (H₂S) was found in mammals in 1989, accumulated evidence has demonstrated that H₂S functions as a novel neurological gasotransmitter in brain tissues and may play a key role in traumatic brain injury. It has been proved that H₂S has an antioxidant, anti-inflammatory, and antiapoptosis function in the neuron system and functions as a neuroprotective factor against secondary brain injury. In addition, H₂S has other biologic effects such as regulating the intracellular concentration of Ca²⁺, facilitating hippocampal long-term potentiation (LTP), and activating ATP-sensitive K channels. Due to the toxic nature of H₂S when exceeding the physiological dose in the human body, only a small amount of H₂S-related therapies was applied to clinical treatment. Therefore, it has huge therapeutic potential and has great hope for recovering patients with traumatic brain injury.

The potential of mitochondrial modulation by neuroglobin in treatment of neurological disorders

Neuroglobin is the third member of the globin family to be identified in 2000 in neurons of both human and mouse nervous systems. Neuroglobin is an oxygen-binding globin found in neurons within the central nervous system as well as in peripheral neurons, that produces a protective effect against hypoxic/ischemic damage induced by promoting oxygen availability within the mitochondria. Numerous investigations have demonstrated that impaired neuroglobin functioning is implicated in the pathogenesis of multiple neurodegenerative disorders. Several in vitro and animal studies have reported the potential of neuroglobin upregulation in improving the neuroprotection through modulation of mitochondrial functions, such as ATP production, clearing reactive oxygen species (ROS), promoting the dynamics of mitochondria, and controlling apoptosis. Neuroglobin acts as a stress-inducible globin, which has been associated hypoxic/ischemic insults where it acts to protect the heart and brain, providing a wide range of applicability in the treatment of human disorders. This review article discusses normal physiological functions of neuroglobin in mitochondria-associated pathways, as well as outlining how dysregulation of neuroglobin is associated with the pathogenesis of neurodegenerative disorders.

Mapping mitochondrial respiratory chain deficiencies by respirometry: Beyond the Mito Stress Test

Cell-based respirometers, such as the Seahorse Extracellular Flux Analyzer, are valuable tools to assess the functionality of mitochondria within adherent neurons, as well as other cell types. The Mito Stress Test is the most frequently employed protocol of drug additions to evaluate mitochondrial bioenergetic function. Sequential exposure of cells to an ATP synthase inhibitor such as oligomycin and an uncoupler such as FCCP cause changes in oxygen consumption rate that allow estimation of the cellular efficiency and capacity for mitochondrial ATP synthesis. While a useful first step in assessing whether an experimental treatment or genetic manipulation affects mitochondrial energetics, the Mito Stress Test does not identify specific sites of altered respiratory chain function. This article discusses limitations of the Mito Stress Test, proposes a refined protocol for comparing cell populations that requires independent drug titrations at multiple cell densities, and describes a stepwise series of respirometry-based assays that "map" locations of electron transport deficiency. These include strategies to test for cytochrome c release, to probe the functionality of specific electron transport chain complexes within intact or permeabilized cells, and to measure NADH oxidation by the linked activity of Complexes I, III, and IV. To illustrate utility, we show that although UK5099 and ABT-737 each decrease the spare respiratory capacity of cortical neurons, the stepwise assays reveal different underlying mechanisms consistent with their established drug targets: deficient Complex I substrate supply induced by the mitochondrial pyruvate carrier inhibitor UK5099 and cytochrome c release induced by the anti-apoptotic BCL-2 family protein inhibitor ABT-737.

Calcium uptake and cytochrome c release from normal and ischemic brain mitochondria

At abnormally elevated levels of intracellular Ca²⁺, mitochondrial Ca²⁺ uptake may compromise mitochondrial electron transport activities and trigger membrane permeability changes that allow for release of cytochrome c and other mitochondrial apoptotic proteins into the cytosol. In this study, a clinically relevant canine cardiac arrest model was used to assess the effects of global cerebral ischemia and reperfusion on mitochondrial Ca²⁺ uptake capacity, Ca²⁺ uptake-mediated inhibition of respiration, and Ca²⁺-induced cytochrome c release, as measured in vitro in a K⁺-based medium in the presence of Mg²⁺, ATP, and NADH-linked oxidizable substrates. Maximum Ca²⁺ uptake by frontal cortex mitochondria was significantly lower following 10 min cardiac arrest compared to non-ischemic controls. Mitochondria from ischemic brains were also more sensitive to the respiratory inhibition associated with accumulation of large levels of Ca²⁺. Cytochrome c was released from brain mitochondria in vitro in a Ca²⁺-dose-dependent manner and was more pronounced following both 10 min of ischemia alone and following 24 h reperfusion, in comparison to mitochondria from non-ischemic Shams. These effects of ischemia and reperfusion on brain mitochondria could compromise intracellular Ca²⁺ homeostasis, decrease aerobic and increase anaerobic cerebral energy metabolism, and potentiate the cytochrome c-dependent induction of apoptosis, when re-oxygenated mitochondria are exposed to abnormally high levels of intracellular Ca²⁺.

Mitochondrial involvement in myocyte death and heart failure

As the heart is an energy-demanding organ, impaired cardiac energy metabolism and mitochondrial function have been inexorably linked to cardiac dysfunction. There is a growing recognition that mitochondrial dysfunction contributes to impaired myocardial energetics and increased oxidative stress in cardiomyopathies, cardiac ischemic damage and heart failure (HF), and mitochondrial permeability transition pore opening has been reported a critical trigger of myocyte death and myocardial remodeling. It is well established that mitochondria play pivotal roles in intracellular signaling in both cell death as well as in cardioprotective pathways. Moreover, recent studies have shown that defects in mitochondrial dynamics affecting biogenesis and turnover are linked to cardiac senescence and HF. Accordingly, there has been an increasing interest in targeting mitochondria for HF therapy. This article reviews the background and recent evidence of mitochondrial involvement in several types of cell death (apoptosis, necrosis and autophagy) occurring in HF. In addition, potential strategies for targeting mitochondria are examined, and their utility in HF therapy considered.

Naoxintong Protects Primary Neurons from Oxygen-Glucose Deprivation/Reoxygenation Induced Injury through PI3K-Akt Signaling Pathway

Naoxintong capsule (NXT), developed from Buyang Huanwu Decoction, has shown the neuroprotective effects in cerebrovascular diseases, but the neuroprotection mechanisms of NXT on ischemia/reperfusion injured neurons have not yet been well known. In this study, we established the oxygen-glucose deprivation/reoxygenation (OGD/R) induced neurons injury model and treat the neurons with cerebrospinal fluid containing NXT (BNC) to investigate the effects of NXT on OGD/R induced neurons injury and potential mechanisms. BNC improved neuron viability and decreased apoptotic rate induced by OGD/R. BNC attenuated OGD/R induced cytosolic and mitochondrial Ca(2+) overload, ROS generation, intracellular NO levels and nNOS mRNA increase, and cytochrome-c release when compared with OGD/R group. BNC significantly inhibited both mPTP opening and ΔΨm depolarization. BNC increased Bcl-2 expression and decreased Bax expression, upregulated the Bcl-2/Bax ratio, downregulated caspase-3 mRNA and caspase-9 mRNA expression, and decreased cleaved caspase-3 expression and caspase-3 activity. BNC increased phosphorylation of Akt following OGD/R, while LY294002 attenuated BNC induced increase of phosphorylated Akt expression. Our study demonstrated that NXT protected primary neurons from OGD/R induced injury by inhibiting calcium overload and ROS generation, protecting mitochondria, and inhibiting mitochondrial apoptotic pathway which was mediated partially by PI3K-Akt signaling pathway activation.

Low micromolar concentrations of the superoxide probe MitoSOX uncouple neural mitochondria and inhibit complex IV

MitoSOX Red is a fluorescent probe used for the detection of mitochondrial reactive oxygen species by live cell imaging. The lipophilic, positively charged triphenylphosphonium moiety within MitoSOX concentrates the superoxide-sensitive dihydroethidium conjugate within the mitochondrial matrix. Here we investigated whether common MitoSOX imaging protocols influence mitochondrial bioenergetic function in primary rat cortical neurons and microglial cell lines. MitoSOX dose-dependently uncoupled neuronal respiration, whether present continuously in the assay medium or washed following a ten minute loading protocol. Concentrations of 5-10μM MitoSOX caused severe loss of ATP synthesis-linked respiration. Redistribution of MitoSOX to the cytoplasm and nucleus occurred concomitant to mitochondrial uncoupling. MitoSOX also dose-dependently decreased the maximal respiration rate and this impairment could not be rescued by delivery of a complex IV specific substrate, revealing complex IV inhibition. As in neurons, loading microglial cells with MitoSOX at low micromolar concentrations resulted in uncoupled mitochondria with reduced respiratory capacity whereas submicromolar MitoSOX had no adverse effects. The MitoSOX parent compound dihydroethidium also caused mitochondrial uncoupling and respiratory inhibition at low micromolar concentrations. However, these effects were abrogated by pre-incubating dihydroethidium with cation exchange beads to remove positively charged oxidation products, which would otherwise by sequestered by polarized mitochondria. Collectively, our results suggest that the matrix accumulation of MitoSOX or dihydroethidium oxidation products causes mitochondrial uncoupling and inhibition of complex IV. Because MitoSOX is inherently capable of causing severe mitochondrial dysfunction with the potential to alter superoxide production, its use therefore requires careful optimization in imaging protocols.

Different apoptotic pathways activated by oxaliplatin in primary astrocytes vs. colo-rectal cancer cells

Oxaliplatin-based chemotherapy improves the outcomes of metastatic colorectal cancer patients. Its most significant and dose-limiting side effect is the development of a neuropathic syndrome. The mechanism of the neurotoxicity is unclear. The limited knowledge about differences existing between neurotoxic and antitumor effects hinders the discovery of effective and safe adjuvant therapies. In vitro, we suggested cell-specific activation apoptotic pathways in normal nervous cells (astrocytes) vs. colon-cancer cells (HT-29). In the present research we compared the apoptotic signals evoked by oxaliplatin in astrocytes and HT-29 analyzing the intrinsic and extrinsic apoptotic pathways. In astrocytes, oxaliplatin induced a mitochondrial derangement measured as cytosolic release of cytochrome C, increase in superoxide anion levels and decreased expression of the antiapoptotic protein Bcl-2. Caspase-8, a main initiator of the extrinsic process remained unaltered. On the contrary, in HT-29 oxaliplatin increased caspase-8 activity and Bid expression, thus activating the extrinsic apoptosis, while the Bcl-2 increased expression blocked the mitochondrial damage. Data suggest the preferred activation of the intrinsic apoptosis as oxaliplatin damage signaling in normal nervous cells. The extrinsic pathway prevails in tumor cells indicating a possible strategy for planning new molecules to treat oxaliplatin-dependent neurotoxicity without negatively influence chemotherapy.

Drp1 is dispensable for apoptotic cytochrome c release in primed MCF10A and fibroblast cells but affects Bcl-2 antagonist-induced respiratory changes

BACKGROUND AND PURPOSE

Dynamin-related protein 1 (Drp1) mediates mitochondrial fission and is thought to promote Bax/Bak-induced cytochrome c release during apoptosis. Conformationally active Bax, Bak and Bax/Bak-activating BH3-only proteins, such as Bim, are restrained by anti-apoptotic Bcl-2 proteins in cells that are 'primed for death'. Inhibition of Bcl-2/Bcl-xL/Bcl-w by the antagonist ABT-737 causes rapid apoptosis of primed cells. Hence, we determined whether Drp1 is required for cytochrome c release, respiratory alterations and apoptosis of cells that are already primed for death.

EXPERIMENTAL APPROACH

We tested the Drp1 inhibitor mdivi-1 for inhibition of cytochrome c release in MCF10A cells primed by Bcl-2 overexpression. We measured ATP synthesis-dependent, -independent and cytochrome c-limited maximal oxygen consumption rates (OCRs) and cell death of immortalized wild-type (WT) and Drp1 knockout (KO) mouse embryonic fibroblasts (MEFs) treated with ABT-737.

KEY RESULTS

Mdivi-1 failed to attenuate ABT-737-induced cytochrome c release. ABT-737 decreased maximal OCR measured in the presence of uncoupler in both WT and Drp1 KO MEF, consistent with respiratory impairment due to release of cytochrome c. However, Drp1 KO MEF were slightly less sensitive to this ABT-737-induced respiratory inhibition compared with WT, and were resistant to an initial ABT-737-induced increase in ATP synthesis-independent O2 consumption. Nevertheless, caspase-dependent cell death was not reduced. Pro-apoptotic Bax was unaltered, whereas Bak was up-regulated in Drp1 KO MEF.

CONCLUSIONS AND IMPLICATIONS

The findings indicate that once fibroblast cells are primed for death, Drp1 is not required for apoptosis. However, Drp1 may contribute to ABT-737-induced respiratory changes and the kinetics of cytochrome c release.

Role of hydrogen sulfide in secondary neuronal injury

In acute neuronal insult events, such as stroke, traumatic brain injury, and spinal cord injury, pathological processes of secondary neuronal injury play a key role in the severity of insult and clinical prognosis. Along with nitric oxide (NO) and carbon monoxide (CO), hydrogen sulfide (H2S) is regarded as the third gasotransmitter and endogenous neuromodulator and plays multiple roles in the central nervous system under physiological and pathological states, especially in secondary neuronal injury. The endogenous level of H2S in the brain is significantly higher than that in peripheral tissues, and is mainly formed by cystathionine β-synthase (CBS) in astrocytes and released in response to neuronal excitation. The mechanism of secondary neuronal injury exacerbating the damage caused by the initial insult includes microcirculation failure, glutamate-mediated excitotoxicity, oxidative stress, inflammatory responses, neuronal apoptosis and calcium overload. H2S dilates cerebral vessels by activating smooth muscle cell plasma membrane ATP-sensitive K channels (KATP channels). This modification occurs on specific cysteine residues of the KATP channel proteins which are S-sulfhydrated. H2S counteracts glutamate-mediated excitotoxicity by inducing astrocytes to intake more glutamate from the extracellular space and thus increasing glutathione in neurons. In addition, H2S protects neurons from secondary neuronal injury by functioning as an anti-oxidant, anti-inflammatory and anti-apoptotic mediator. However, there are still some reports suggest that H2S elevates neuronal Ca(2+) concentration and may contribute to the formation of calcium overload in secondary neuronal injury. H2S also elicits calcium waves in primary cultures of astrocytes and may mediate signals between neurons and glia. Consequently, further exploration of the molecular mechanisms of H2S in secondary neuronal injury will provide important insights into its potential therapeutic uses for the treatment of acute neuronal insult events.

Lanthanum induced primary neuronal apoptosis through mitochondrial dysfunction modulated by Ca²⁺ and Bcl-2 family

As a representative element of lanthanide, lanthanum has been widely used in various fields and eventually entered environment and accumulated in human body. Epidemiological and experimental evidences indicated that lanthanum has neurotoxicity; however, the detailed mechanism is still elusive. Here, we chose primary cerebral cortical neurons as model in vitro to investigate the mechanism underlying the toxic effects of lanthanum chloride (LaCl3). This study revealed the following findings: (1) LaCl3 treatment (0.01, 0.1, and 1.0 mM for 24 h) reduced the viability of cortical neurons and elevated apoptotic rate significantly in a dose-dependent manner. (2) LaCl3 triggered mitochondrial apoptotic pathway in cortical neurons, characterized with collapsed mitochondrial membrane potential, release of cytochrome c into cytosol, and increasing expression of activated caspase-3. (3) LaCl3 elevated intracellular Ca(2+) concentration, promoted reactive oxygen species generation, and upregulated pro-apoptotic Bax, whereas it downregulated anti-apoptotic Bcl-2 expression and consequently altered Bax/Bcl-2 ratio, which ultimately lead to neuronal mitochondrial apoptosis. Our results demonstrated that toxicity of lanthanum in cortical neurons perhaps partly attributed to enhanced mitochondrial apoptosis due to mitochondrial dysfunction modulated by Ca(2+) and Bcl-2 family.

Mitochondrial mechanisms of neuroglobin's neuroprotection

Neuroglobin (Ngb) is an oxygen-binding globin protein that has been demonstrated to be neuroprotective against stroke and related neurological disorders. However, the underlying mechanisms of Ngb's neuroprotection remain largely undefined. Mitochondria play critical roles in multiple physiological pathways including cell respiration, energy production, free radical generation, and cellular homeostasis and apoptosis. Mitochondrial dysfunction is widely involved in the pathogenesis of stroke and neurodegenerative diseases including Alzheimer's, Parkinson's, and Huntington's diseases. Accumulating evidence showed that elevated Ngb level is associated with preserved mitochondrial function, suggesting that Ngb may play neuroprotective roles through mitochondria-mediated pathways. In this paper we briefly discuss the mitochondria-related mechanisms in Ngb's neuroprotection, especially those involved in ATP production, ROS generation and scavenging, and mitochondria-mediated cell death signaling pathways.

Bax activation initiates the assembly of a multimeric catalyst that facilitates Bax pore formation in mitochondrial outer membranes

Bax/Bak-mediated mitochondrial outer membrane permeabilization (MOMP) is essential for "intrinsic" apoptotic cell death. Published studies used synthetic liposomes to reveal an intrinsic pore-forming activity of Bax, but it is unclear how other mitochondrial outer membrane (MOM) proteins might facilitate this function. We carefully analyzed the kinetics of Bax-mediated pore formation in isolated MOMs, with some unexpected results. Native MOMs were more sensitive than liposomes to added Bax, and MOMs displayed a lag phase not observed with liposomes. Heat-labile MOM proteins were required for this enhanced response. A two-tiered mathematical model closely fit the kinetic data: first, Bax activation promotes the assembly of a multimeric complex, which then catalyzes the second reaction, Bax-dependent pore formation. Bax insertion occurred immediately upon Bax addition, prior to the end of the lag phase. Permeabilization kinetics were affected in a reciprocal manner by [cBid] and [Bax], confirming the "hit-and-run" hypothesis of cBid-induced direct Bax activation. Surprisingly, MOMP rate constants were linearly related to [Bax], implying that Bax acts non-cooperatively. Thus, the oligomeric catalyst is distinct from Bax. Moreover, contrary to common assumption, pore formation kinetics depend on Bax monomers, not oligomers. Catalyst formation exhibited a sharp transition in activation energy at ∼28°C, suggesting a role for membrane lipid packing. Furthermore, catalyst formation was strongly inhibited by chemical antagonists of the yeast mitochondrial fission protein, Dnm1. However, the mammalian ortholog, Drp1, was undetectable in mitochondrial outer membranes. Moreover, ATP and GTP were dispensable for MOMP. Thus, the data argue that oligomerization of a catalyst protein, distinct from Bax and Drp1, facilitates MOMP, possibly through a membrane-remodeling event.

Rapid detection of an ABT-737-sensitive primed for death state in cells using microplate-based respirometry

Cells that exhibit an absolute dependence on the anti-apoptotic BCL-2 protein for survival are termed “primed for death” and are killed by the BCL-2 antagonist ABT-737. Many cancers exhibit a primed phenotype, including some that are resistant to conventional chemotherapy due to high BCL-2 expression. We show here that 1) stable BCL-2 overexpression alone can induce a primed for death state and 2) that an ABT-737-induced loss of functional cytochrome c from the electron transport chain causes a reduction in maximal respiration that is readily detectable by microplate-based respirometry. Stable BCL-2 overexpression sensitized non-tumorigenic MCF10A mammary epithelial cells to ABT-737-induced caspase-dependent apoptosis. Mitochondria within permeabilized BCL-2 overexpressing cells were selectively vulnerable to ABT-737-induced cytochrome c release compared to those from control-transfected cells, consistent with a primed state. ABT-737 treatment caused a dose-dependent impairment of maximal O₂ consumption in MCF10A BCL-2 overexpressing cells but not in control-transfected cells or in immortalized mouse embryonic fibroblasts lacking both BAX and BAK. This impairment was rescued by delivering exogenous cytochrome c to mitochondria via saponin-mediated plasma membrane permeabilization. An ABT-737-induced reduction in maximal O₂ consumption was also detectable in SP53, JeKo-1, and WEHI-231 B-cell lymphoma cell lines, with sensitivity correlating with BCL-2:MCL-1 ratio and with susceptibility (SP53 and JeKo-1) or resistance (WEHI-231) to ABT-737-induced apoptosis. Multiplexing respirometry assays to ELISA-based determination of cytochrome c redistribution confirmed that respiratory inhibition was associated with cytochrome c release. In summary, cell-based respiration assays were able to rapidly identify a primed for death state in cells with either artificially overexpressed or high endogenous BCL-2. Rapid detection of a primed for death state in individual cancers by “bioenergetics-based profiling” may eventually help identify the subset of patients with chemoresistant but primed tumors who can benefit from treatment that incorporates a BCL-2 antagonist.

Peptides for cancer therapy: a drug-development opportunity and a drug-delivery challenge

Therapeutic peptides (TPs) are a class of peptide-based agents capable of eliciting a therapeutic response by modulation of targets within or on the surface of cells. TPs are advantageous because they are amenable to rational design, they have high specificity for their targets and can be made to target almost any protein of interest, including proteins for which we have no small-molecule drugs. Owing to this versatility, TPs have a great potential for cancer therapy in an age of personalized medicine, in which we need novel drugs to target the many novel pathways being discovered as tumor drivers. However, in order to utilize TPs as drugs, many obstacles must be overcome. TPs have short half-lives in systemic circulation, are easily degraded by proteases in plasma and target cells, are often cleared by the reticuloendothelial system and can be immunogenic. This article will discuss ways of overcoming many of these hurdles by utilizing macromolecular peptide delivery systems and tumor-targeting agents.

The MUC1-C oncoprotein binds to the BH3 domain of the pro-apoptotic BAX protein and blocks BAX function

The pro-apoptotic BAX protein contains a BH3 domain that is necessary for its dimerization and for activation of the intrinsic apoptotic pathway. The MUC1 (mucin 1) heterodimeric protein is overexpressed in diverse human carcinomas and blocks apoptosis in the response to stress. In this study, we demonstrate that the oncogenic MUC1-C subunit associates with BAX in human cancer cells. MUC1-C·BAX complexes are detectable in the cytoplasm and mitochondria and are induced by genotoxic and oxidative stress. The association between MUC1-C and BAX is supported by the demonstration that the MUC1-C cytoplasmic domain is sufficient for the interaction with BAX. The results further show that the MUC1-C cytoplasmic domain CQC motif binds directly to the BAX BH3 domain at Cys-62. Consistent with binding to the BAX BH3 domain, MUC1-C blocked BAX dimerization in response to (i) truncated BID in vitro and (ii) treatment of cancer cells with DNA-damaging agents. In concert with these results, MUC1-C attenuated localization of BAX to mitochondria and the release of cytochrome c. These findings indicate that the MUC1-C oncoprotein binds directly to the BAX BH3 domain and thereby blocks BAX function in activating the mitochondrial death pathway.

Investigation of mitochondrial dysfunction by sequential microplate-based respiration measurements from intact and permeabilized neurons

Mitochondrial dysfunction is a component of many neurodegenerative conditions. Measurement of oxygen consumption from intact neurons enables evaluation of mitochondrial bioenergetics under conditions that are more physiologically realistic compared to isolated mitochondria. However, mechanistic analysis of mitochondrial function in cells is complicated by changing energy demands and lack of substrate control. Here we describe a technique for sequentially measuring respiration from intact and saponin-permeabilized cortical neurons on single microplates. This technique allows control of substrates to individual electron transport chain complexes following permeabilization, as well as side-by-side comparisons to intact cells. To illustrate the utility of the technique, we demonstrate that inhibition of respiration by the drug KB-R7943 in intact neurons is relieved by delivery of the complex II substrate succinate, but not by complex I substrates, via acute saponin permeabilization. In contrast, methyl succinate, a putative cell permeable complex II substrate, failed to rescue respiration in intact neurons and was a poor complex II substrate in permeabilized cells. Sequential measurements of intact and permeabilized cell respiration should be particularly useful for evaluating indirect mitochondrial toxicity due to drugs or cellular signaling events which cannot be readily studied using isolated mitochondria.

Poly(ADP-ribose) (PAR) binding to apoptosis-inducing factor is critical for PAR polymerase-1-dependent cell death (parthanatos)

The mitochondrial protein apoptosis-inducing factor (AIF) plays a pivotal role in poly(ADP-ribose) polymerase-1 (PARP-1)-mediated cell death (parthanatos), during which it is released from the mitochondria and translocates to the nucleus. We show that AIF is a high-affinity poly(ADP-ribose) (PAR)-binding protein and that PAR binding to AIF is required for parthanatos both in vitro and in vivo. AIF bound PAR at a site distinct from AIF's DNA binding site, and this interaction triggered AIF release from the cytosolic side of the mitochondrial outer membrane. Mutation of the PAR binding site in AIF did not affect its NADH (reduced form of nicotinamide adenine dinucleotide) oxidase activity, its ability to bind FAD (flavin adenine dinucleotide) or DNA, or its ability to induce nuclear condensation. However, this AIF mutant was not released from mitochondria and did not translocate to the nucleus or mediate cell death after PARP-1 activation. These results suggest a mechanism for PARP-1 to initiate AIF-mediated cell death and indicate that AIF's bioenergetic cell survival-promoting functions are separate from its effects as a mitochondrially derived death effector. Interference with the PAR-AIF interaction or PAR signaling may provide notable opportunities for preventing cell death after activation of PARP-1.

Amphipathic tail-anchoring peptide and Bcl-2 homology domain-3 (BH3) peptides from Bcl-2 family proteins induce apoptosis through different mechanisms

Bcl-2 homology domain-3 (BH3) peptides are potent cancer therapeutic reagents that target regulators of apoptotic cell death in cancer cells. However, their cytotoxic effects are affected by different expression levels of Bcl-2 family proteins. We recently found that the amphipathic tail-anchoring peptide (ATAP) from Bfl-1, a bifunctional Bcl-2 family member, produced strong pro-apoptotic activity by permeabilizing the mitochondrial outer membrane. Here, we test whether the activity of ATAP requires other cellular factors and whether ATAP has an advantage over the BH3 peptides in targeting cancer cells. Confocal microscopic imaging illustrates specific targeting of ATAP to mitochondria, whereas BH3 peptides show diffuse patterns of cytosolic distribution. Although the pro-apoptotic activities of BH3 peptides are largely inhibited by either overexpression of anti-apoptotic Bcl-2 or Bcl-xL or nullification of pro-apoptotic Bax and Bak in cells, the pro-apoptotic function of ATAP is not affected by these cellular factors. Reconstitution of synthetic ATAP into liposomal membranes results in release of fluorescent molecules of the size of cytochrome c from the liposomes, suggesting that the membrane permeabilizing activity of ATAP does not require additional protein factors. Because ATAP can target to the mitochondrial membrane and its pro-apoptotic activity does not depend on the content of Bcl-2 family proteins, it represents a promising candidate for anti-cancer drugs that can potentially overcome the intrinsic apoptosis-resistant nature of cancer cells.

Synthetic and natural polyanions induce cytochrome c release from mitochondria in vitro and in situ

A synthetic polyanion composed of styrene, maleic anhydride, and methacrylic acid (molar ratio 56:37:7) significantly inhibited the respiration of isolated rat liver mitochondria in a time-dependent fashion that correlated with 1) collapse of the mitochondrial membrane potential and 2) high amplitude mitochondrial swelling. The process is apparently Ca(2+) dependent. Since it is blocked by cyclosporin A, the process is ascribed to induction of the mitochondrial permeability transition. In mitoplasts, i.e., mitochondria lacking their outer membranes, the polyanion rapidly blocked respiration. After incubation of rat liver mitochondria with the polyanion, cytochrome c was released into the incubation medium. In solution, the polyanion modified by conjugation with fluorescein formed a complex with cytochrome c. Addition of the polyanion to cytochrome c-loaded phosphatidylcholine/cardiolipin liposomes induced the release of the protein from liposomal membrane evidently due to coordinated interplay of Coulomb and hydrophobic interactions of the polymer with cytochrome c. We conclude that binding of the polyanion to cytochrome c renders it inactive in the respiratory chain due to exclusion from its native binding sites. Apparently, the polyanion interacts with cytochrome c in mitochondria and releases it to the medium through breakage of the outer membrane as a result of severe swelling. Similar properties were demonstrated for the natural polyanion, tobacco mosaic virus RNA. An electron microscopy study confirmed that both polyanions caused mitochondrial swelling. Exposure of cerebellar astroglial cells in culture to the synthetic polyanion resulted in cell death, which was associated with nuclear fragmentation.

Bioenergetics and cell death

Mitochondrial bioenergetic function is a key to cell life and death. Cells need energy not only to support their vital functions but also to die gracefully. Execution of an apoptotic program includes energy-dependent steps, including kinase signaling, formation of the apoptosome, and effector caspase activation. Under conditions of bioenergetic collapse, cells are diverted toward necrotic demise. Mitochondrial outer membrane permeabilization (MOMP) is a decisive event in the execution of apoptosis. It is also causally linked to a decline in bioenergetic function via different mechanisms, not merely due to cytochrome c dispersion. MOMP-induced bioenergetic deficiency is usually irreversible and commits cells to die, even when caspases are inactive. Here, we discuss the mechanisms by which MOMP impacts bioenergetics in different cell death paradigms.

Mitochondria as decision-makers in cell death

Mitochondria play an essential role in both cell health and death. Increasing experimental evidence suggests that mitochondria are involved in active control of cell death processes at several levels including (1) mitochondrial membrane permeabilization and release of proapoptotic proteins, (2) post-cytochrome c regulation of caspase activation, and (3) supply of energy for execution of death program. The purpose of this review is to discuss the main mechanisms by which alterations in mitochondrial outer membrane permit the translocation of proapoptotic proteins into cytosol, how mitochondria "make decisions" on the mode of cell death, and how they regulate caspase activation by changing the redox state of cytosolic cytochrome c. The interventions into these processes may constitute an important strategy for the pharmacological prevention of unwanted cell death in various pathologies or, conversely, for facilitation of anticancer therapy.

Use of human cancer cell lines mitochondria to explore the mechanisms of BH3 peptides and ABT-737-induced mitochondrial membrane permeabilization

Current limitations of chemotherapy include toxicity on healthy tissues and multidrug resistance of malignant cells. A number of recent anti-cancer strategies aim at targeting the mitochondrial apoptotic machinery to induce tumor cell death. In this study, we set up protocols to purify functional mitochondria from various human cell lines to analyze the effect of peptidic and xenobiotic compounds described to harbour either Bcl-2 inhibition properties or toxic effects related to mitochondria. Mitochondrial inner and outer membrane permeabilization were systematically investigated in cancer cell mitochondria versus non-cancerous mitochondria. The truncated (t-) Bid protein, synthetic BH3 peptides from Bim and Bak, and the small molecule ABT-737 induced a tumor-specific and OMP-restricted mitochondrio-toxicity, while compounds like HA-14.1, YC-137, Chelerythrine, Gossypol, TW-37 or EM20-25 did not. We found that ABT-737 can induce the Bax-dependent release of apoptotic proteins (cytochrome c, Smac/Diablo and Omi/HtrA2 but not AIF) from various but not all cancer cell mitochondria. Furthermore, ABT-737 addition to isolated cancer cell mitochondria induced oligomerization of Bax and/or Bak monomers already inserted in the mitochondrial membrane. Finally immunoprecipatations indicated that ABT-737 induces Bax, Bak and Bim desequestration from Bcl-2 and Bcl-xL but not from Mcl-1L. This study investigates for the first time the mechanism of action of ABT-737 as a single agent on isolated cancer cell mitochondria. Hence, this method based on MOMP (mitochondrial outer membrane permeabilization) is an interesting screening tool, tailored for identifying Bcl-2 antagonists with selective toxicity profile against cancer cell mitochondria but devoid of toxicity against healthy mitochondria.

Therapeutic peptides for cancer therapy. Part II - cell cycle inhibitory peptides and apoptosis-inducing peptides

BACKGROUND

Therapeutic peptides have great potential as anticancer agents owing to their ease of rational design and target specificity. However, their utility in vivo is limited by low stability and poor tumor penetration.

OBJECTIVE

The authors review the development of peptide inhibitors with potential for cancer therapy. Peptides that arrest the cell cycle by mimicking CDK inhibitors or induce apoptosis directly are discussed.

METHODS

The authors searched Medline for articles concerning the development of therapeutic peptides and their delivery.

RESULTS/CONCLUSION

Inhibition of cancer cell proliferation directly using peptides that arrest the cell cycle or induce apoptosis is a promising strategy. Peptides can be designed that interact very specifically with cyclins and/or cyclin-dependent kinases and with members of apoptotic cascades. Use of these peptides is not limited by their design, as a rational approach to peptide design is much less challenging than the design of small molecule inhibitors of specific protein-protein interactions. However, the limitations of peptide therapy lie in the poor pharmacokinetic properties of these large, often charged molecules. Therefore, overcoming the drug delivery hurdles could open the door for effective peptide therapy, thus making an entirely new class of molecules useful as anticancer drugs.

Outer mitochondrial membrane localization of apoptosis-inducing factor: mechanistic implications for release

Poly(ADP-ribose) polymerase-1-dependent cell death (known as parthanatos) plays a pivotal role in many clinically important events including ischaemia/reperfusion injury and glutamate excitotoxicity. A recent study by us has shown that uncleaved AIF (apoptosis-inducing factor), but not calpain-hydrolysed truncated-AIF, was rapidly released from the mitochondria during parthanatos, implicating a second pool of AIF that might be present in brain mitochondria contributing to the rapid release. In the present study, a novel AIF pool is revealed in brain mitochondria by multiple biochemical analyses. Approx. 30% of AIF loosely associates with the outer mitochondrial membrane on the cytosolic side, in addition to its main localization in the mitochondrial intermembrane space attached to the inner membrane. Immunogold electron microscopic analysis of mouse brain further supports AIF association with the outer, as well as the inner, mitochondrial membrane in vivo. In line with these observations, approx. 20% of uncleaved AIF rapidly translocates to the nucleus and functionally causes neuronal death upon NMDA (N-methyl-d-aspartate) treatment. In the present study we show for the first time a second pool of AIF in brain mitochondria and demonstrate that this pool does not require cleavage and that it contributes to the rapid release of AIF. Moreover, these results suggest that this outer mitochondrial pool of AIF is sufficient to cause cell death during parthanatos. Interfering with the release of this outer mitochondrial pool of AIF during cell injury paradigms that use parthanatos hold particular promise for novel therapies to treat neurological disorders.

Proteomic analysis of enriched lysosomes at early phase of camptothecin-induced apoptosis in human U-937 cells

A lysosomal pathway, characterized by partial rupture or labilization of lysosomal membranes and cathepsin activation, is evoked during camptothecin-induced apoptosis in human cancer cells, including human histiocytic lymphoma U-937 cells. These lysosomal events begin rapidly and simultaneously with mitochondrial permeabilization and caspase activation within 3 h after drug treatment. In this study, comparative and quantitative proteome analyses were performed to identify early changes in lysosomal protein expression/localization from U-937 cells undergoing apoptosis. In 2 independent experiments, among a total of more than 538 proteins putatively identified and quantitated by iTRAQ isobaric labeling and LC-ESI-MS/MS, 18 proteins were found to be upregulated and 9 downregulated in lysosomes purified from early apoptotic compared to control cells. Protein expression was validated by Western blotting on enriched lysosome fractions, and protein localization confirmed by fluorescence confocal microscopy of representative protein candidates, whose functions are associated with lysosomal membrane fluidity and dynamics. These include sterol-4-alpha-carboxylate 3-dehydrogenase (NSDHL), prosaposin (PSAP) and protein kinase C delta (PKC-delta). This comparative proteome analysis provides the basis for novel hypothesis and rationale functional experimentation, where the 3 validated candidate proteins are associated with lysosomal membrane fluidity and dynamics, particularly cholesterol, sphingolipid and glycosphingolipid metabolism.

Parkin selectively alters the intrinsic threshold for mitochondrial cytochrome c release

Autosomal-recessive mutations in the Parkin gene are the second most common cause of familial Parkinson's disease (PD). Parkin deficiency leads to the premature demise of the catecholaminergic neurons of the ventral midbrain in familial PD. Thus, a better understanding of parkin function may elucidate molecular aspects of their selective vulnerability in idiopathic PD. Numerous lines of evidence suggest a mitochondrial function for parkin and a protective effect of ectopic parkin expression. Since mitochondria play a critical role in cell survival/cell death through regulated cytochrome c release and control of apoptosis, we sought direct evidence of parkin function in this pathway. Mitochondria were isolated from cells expressing either excess levels of human parkin or shRNA directed against endogenous parkin and then treated with peptides corresponding to the active Bcl-2 homology 3 (BH3) domains of pro-apoptotic proteins and the threshold for cytochrome c release was analyzed. Data obtained from both rodent and human neuroblastoma cell lines showed that the expression levels of parkin were inversely correlated with cytochrome c release. Parkin was found associated with isolated mitochondria, but its binding per se was not sufficient to inhibit cytochrome c release. In addition, pathogenic parkin mutants failed to influence cytochrome c release. Furthermore, PINK1 expression had no effect on cytochrome c release, suggesting a divergent function for this autosomal recessive PD-linked gene. In summary, these data demonstrate a specific autonomous effect of parkin on mitochondrial mechanisms governing cytochrome c release and apoptosis, which may be relevant to the selective vulnerability of certain neuronal populations in PD.

Molecular participants in mitochondrial cell death channel formation during neuronal ischemia

Mitochondrial ion channels are involved in numerous cellular processes. Membrane pores and transporters regulate the influx and efflux of calcium, sodium, potassium, zinc and determine the membrane compartmentalization of numerous cytosolic metabolites. The permeability of the inner membrane to ions and solutes helps determine the membrane potential of the inner membrane, but the permeability of the outer membrane, controlled in part by VDAC and the BCL-2 family proteins, regulates the release of important signaling molecules that determine the onset of programmed cell death. BCL-2 family proteins have properties of ion channels and perform specialized physiological functions, for example, regulating the strength and pattern of synaptic transmission, in addition to their well known role in cell death. The ion channels of the inner and outer membranes may come together in a complex of proteins during programmed cell death, particularly during neuronal ischemia, where elevated levels of the divalents calcium and zinc activate inner membrane ion channel conductances. The variety of possible molecular participants within the ion channel complex may be matched only by the variety of different types of programmed cell death.

Mitochondrial mechanisms of cell death and neuroprotection in pediatric ischemic and traumatic brain injury

There are several forms of acute pediatric brain injury, including neonatal asphyxia, pediatric cardiac arrest with global ischemia, and head trauma, that result in devastating, lifelong neurologic impairment. The only clinical intervention that appears neuroprotective is hypothermia initiated soon after the initial injury. Evidence indicates that oxidative stress, mitochondrial dysfunction, and impaired cerebral energy metabolism contribute to the brain cell death that is responsible for much of the poor neurologic outcome from these events. Recent results obtained from both in vitro and animal models of neuronal death in the immature brain point toward several molecular mechanisms that are either induced or promoted by oxidative modification of macromolecules, including consumption of cytosolic and mitochondrial NAD(+) by poly-ADP ribose polymerase, opening of the mitochondrial inner membrane permeability transition pore, and inactivation of key, rate-limiting metabolic enzymes, e.g., the pyruvate dehydrogenase complex. In addition, the relative abundance of pro-apoptotic proteins in immature brains and neurons, and particularly within their mitochondria, predisposes these cells to the intrinsic, mitochondrial pathway of apoptosis, mediated by Bax- or Bak-triggered release of proteins into the cytosol through the mitochondrial outer membrane. Based on these pathways of cell dysfunction and death, several approaches toward neuroprotection are being investigated that show promise toward clinical translation. These strategies include minimizing oxidative stress by avoiding unnecessary hyperoxia, promoting aerobic energy metabolism by repletion of NAD(+) and by providing alternative oxidative fuels, e.g., ketone bodies, directly interfering with apoptotic pathways at the mitochondrial level, and pharmacologic induction of antioxidant and anti-inflammatory gene expression.

Assembly of the mitochondrial apoptosis-induced channel, MAC

Although Bcl-2 family proteins control intrinsic apoptosis, the mechanisms underlying this regulation are incompletely understood. Patch clamp studies of mitochondria isolated from cells deficient in one or both of the pro-apoptotic proteins Bax and Bak show that at least one of the proteins must be present for formation of the cytochrome c-translocating channel, mitochondrial apoptosis-induced channel (MAC), and that the single channel behaviors of MACs containing exclusively Bax or Bak are similar. Truncated Bid catalyzes MAC formation in isolated mitochondria containing Bax and/or Bak with a time course of minutes and does not require VDAC1 or VDAC3. Mathematical analysis of the stepwise changes in conductance associated with MAC formation is consistent with pore assembly by a barrel-stave model. Assuming the staves are two transmembrane alpha-helices in Bax and Bak, mature MAC pores would typically contain approximately 9 monomers and have diameters of 5.5-6 nm. The mitochondrial permeability data are inconsistent with formation of lipidic pores capable of transporting megadalton-sized macromolecules as observed with recombinant Bax in liposomes.

Postischemic oxidative stress promotes mitochondrial metabolic failure in neurons and astrocytes

Oxidative stress and mitochondrial dysfunction have been closely associated in many subcellular, cellular, animal, and human studies of both acute brain injury and neurodegenerative diseases. Our animal models of brain injury caused by cardiac arrest illustrate this relationship and demonstrate that both oxidative molecular modifications and mitochondrial metabolic impairment are exacerbated by reoxygenation of the brain using 100% ventilatory O(2) compared to lower levels that maintain normoxemia. Numerous molecular mechanisms may be responsible for mitochondrial dysfunction caused by oxidative stress, including oxidation and inactivation of mitochondrial proteins, promotion of the mitochondrial membrane permeability transition, and consumption of metabolic cofactors and intermediates, for example, NAD(H). Moreover, the relative contribution of these mechanisms to cell injury and death is likely different among different types of brain cells, for example, neurons and astrocytes. In order to better understand these oxidative stress mechanisms and their relevance to neurologic disorders, we have undertaken studies with primary cultures of astrocytes and neurons exposed to O(2) and glucose deprivation and reoxygenation and compared the results of these studies to those using a rat model of neonatal asphyxic brain injury. These results support the hypothesis that release and or consumption of mitochondrial NAD(H) is at least partially responsible for respiratory inhibition, particularly in neurons.

Cytoprotective effects of melatonin on C6 astroglial cells exposed to glutamate excitotoxicity and oxidative stress

To preserve the central nervous system (CNS) function after a traumatic injury, therapeutic agents must be administered to protect neurons as well as glial cells. Cell death in CNS injuries and diseases are attributed to many factors including glutamate toxicity and oxidative stress. We examined whether melatonin, a potent anti-oxidant and free radical scavenger, would attenuate apoptotic death of rat C6 astroglial cells under glutamate excitotoxicity and oxidative stress. Exposure of C6 cells to 500 microM L-glutamic acid (LGA) and 100 microm hydrogen peroxide (H(2)O(2)) for 24 hr caused significant increases in apoptosis. Apoptosis was evaluated by Wright staining and ApopTag assay. Melatonin receptor 1 appeared to be involved in the protection of these cells from excitotoxic and oxidative damage. Cells undergoing excitotoxic and oxidative stress for 15 min were then treated with 150 nM melatonin, which prevented Ca(2+)influx and cell death. Western blot analyses showed alterations in Bax and Bcl-2 expression resulting in increased Bax:Bcl-2 ratio during apoptosis. Western blot analyses also showed increases in calpain and caspase-3 activities, which cleaved 270 kD alpha-spectrin at specific sites to generate 145 kD spectrin breakdown product (SBDP) and 120 kD SBDP, respectively. However, 15-min post-treatment of C6 cells with melatonin dramatically reduced Bax:Bcl-2 ratio and proteolytic activities, decreasing LGA or H(2)O(2)-induced apoptosis. Our data showed that melatonin prevented proteolysis and apoptosis in C6 astroglial cells. The results suggest that melatonin may be an effective cytoprotective agent against glutamate excitotoxicity and oxidative stress in CNS injuries and diseases.

Gossypol, a BH3 mimetic, induces apoptosis in chronic lymphocytic leukemia cells

Gossypol, a cottonseed extract derivative, acts as a BH3-mimetic, binding to the BH3 pocket of antiapoptotic proteins and displacing pro-death partners to induce apoptosis. However, knowledge on the molecular underpinnings of its downstream effects is limited. Since chronic lymphocytic leukemia (CLL) cells express high levels of antiapoptotic proteins that act as a survival mechanism for these replicationally quiescent lymphocytes, we investigated whether gossypol induces apoptosis in these cells and what mechanism underlies gossypol-mediated cytotoxicity. Gossypol induced cell death in a concentration- and time-dependent manner; 24-hour incubation with 30 microM gossypol resulted in 50% cell death (median; range, 10%-80%; n = 47) that was not abrogated by pan-specific caspase inhibitor. Starting at 4 hours, the mitochondrial outer membrane was significantly permeabilized (median, 77%; range, 54%-93%; n = 15). Mitochondrial outer membrane permeabiliztaion (MOMP) was concurrent with increased production of reactive oxygen species (ROS); however, antioxidants did not abrogate gossypol-induced cell death. Mitochondrial membrane permeabilization was also associated with loss of intracellular adenosine triphosphate (ATP), activation of BAX, and release of cytochrome c and apoptosis-inducing factor (AIF), which was translocated to the nucleus. Blocking AIF translocation resulted in a decreased apoptosis, suggesting that AIF contributes to gossypol-mediated cytotoxicity in CLL lymphocytes.

Mechanisms of impaired mitochondrial energy metabolism in acute and chronic neurodegenerative disorders

Altered mitochondrial energy metabolism contributes to the pathophysiology of acute brain injury caused by ischemia, trauma, and neurotoxins and by chronic neurodegenerative disorders such as Parkinson's and Huntington's diseases. Although much evidence supports that the electron transport chain dysfunction in these metabolic abnormalities has both genetic and intracellular environmental causes, alternative mechanisms are being explored. These include direct, reversible inhibition of cytochrome oxidase by nitric oxide, release of mitochondrial cytochrome c, oxidative inhibition of mitochondrial matrix dehydrogenases and adenine nucleotide transport, the availability of NAD for dehydrogenase reactions, respiratory uncoupling by activities such as that of the permeability transition pore, and altered mitochondrial structure and intracellular trafficking. This review focuses on the catabolism of neuronal NAD and the release of neuronal mitochondrial NAD as important contributors to metabolic dysfunction. In addition, the relationship between apoptotic signaling cascades and disruption of mitochondrial energy metabolism is considered in light of the fine balance between apoptotic and necrotic neural cell death.

beta-Cell mitochondria exhibit membrane potential heterogeneity that can be altered by stimulatory or toxic fuel levels

OBJECTIVE

beta-Cell response to glucose is characterized by mitochondrial membrane potential (Delta Psi) hyperpolarization and the production of metabolites that serve as insulin secretory signals. We have previously shown that glucose-induced mitochondrial hyperpolarization accompanies the concentration-dependent increase in insulin secretion within a wide range of glucose concentrations. This observation represents the integrated response of a large number of mitochondria within each individual cell. However, it is currently unclear whether all mitochondria within a single beta-cell represent a metabolically homogenous population and whether fuel or other stimuli can recruit or silence sizable subpopulations of mitochondria. This study offers insight into the different metabolic states of beta-cell mitochondria.

RESULTS

We show that mitochondria display a wide heterogeneity in Delta Psi and a millivolt range that is considerably larger than the change in millivolts induced by fuel challenge. Increasing glucose concentration recruits mitochondria into higher levels of homogeneity, while an in vitro diabetes model results in increased Delta Psi heterogeneity. Exploration of the mechanism behind heterogeneity revealed that temporary changes in Delta Psi of individual mitochondria, ATP-hydrolyzing mitochondria, and uncoupling protein 2 are not significant contributors to Delta Psi heterogeneity. We identified BAD, a proapoptotic BCL-2 family member previously implicated in mitochondrial recruitment of glucokinase, as a significant factor influencing the level of heterogeneity.

CONCLUSIONS

We suggest that mitochondrial Delta Psi heterogeneity in beta-cells reflects a metabolic reservoir recruited by an increased level of fuels and therefore may serve as a therapeutic target.

DNA-damage response network at the crossroads of cell-cycle checkpoints, cellular senescence and apoptosis

Tissue homeostasis requires a carefully-orchestrated balance between cell proliferation, cellular senescence and cell death. Cells proliferate through a cell cycle that is tightly regulated by cyclin-dependent kinase activities. Cellular senescence is a safeguard program limiting the proliferative competence of cells in living organisms. Apoptosis eliminates unwanted cells by the coordinated activity of gene products that regulate and effect cell death. The intimate link between the cell cycle, cellular senescence, apoptosis regulation, cancer development and tumor responses to cancer treatment has become eminently apparent. Extensive research on tumor suppressor genes, oncogenes, the cell cycle and apoptosis regulatory genes has revealed how the DNA damage-sensing and -signaling pathways, referred to as the DNA-damage response network, are tied to cell proliferation, cell-cycle arrest, cellular senescence and apoptosis. DNA-damage responses are complex, involving "sensor" proteins that sense the damage, and transmit signals to "transducer" proteins, which, in turn, convey the signals to numerous "effector" proteins implicated in specific cellular pathways, including DNA repair mechanisms, cell-cycle checkpoints, cellular senescence and apoptosis. The Bcl-2 family of proteins stands among the most crucial regulators of apoptosis and performs vital functions in deciding whether a cell will live or die after cancer chemotherapy and irradiation. In addition, several studies have now revealed that members of the Bcl-2 family also interface with the cell cycle, DNA repair/recombination and cellular senescence, effects that are generally distinct from their function in apoptosis. In this review, we report progress in understanding the molecular networks that regulate cell-cycle checkpoints, cellular senescence and apoptosis after DNA damage, and discuss the influence of some Bcl-2 family members on cell-cycle checkpoint regulation.

Cataract: Window for systemic disorders

Cataract is the leading cause of visual handicap throughout the world, and almost all elderly individuals develop lens opacities. Epidemiological studies have shown that nuclear cataracts in young adults are associated with higher mortality. Many cataractogenic stressors induce endoplasmic reticulum (ER) stress, which in turn induces the unfolded protein response (UPR). The UPR can damage or kill a wide range of cell types and may be involved in many human diseases. We hypothesize that a cataract can be considered a window that can indicate the presence of systemic disorders. This is important because cataract is easily detected during a routine ocular examination. The slightest opacity in any region of the lenses, especially in younger patients, may be a sign of systemic disorders. Earlier detection of systemic disorders can save the lives of patients. If our hypothesis is correct, then elimination of known ER/cataractogenic stressors from individuals with cataracts should be the one of the first steps for treatments of the systemic disorders. We discuss the potential risk factors and beneficial effects of removal of such risk factors in patients with early cataracts. All patients with cataract should be referred for comprehensive medical examination.

Up-regulation of Bcl-2 in APP transgenic mice is associated with neuroprotection

Abeta-induced neurodegeneration is limited in APP and APP+PS1 transgenic mice. In middle-aged APP + PS1 transgenic mice, we found significantly increased Bcl-2 expression. The increase in Bcl-2 is restricted to amyloid-containing brain regions and is not found at young ages, suggesting that Abeta deposition is the stimulus for increased Bcl-2. Western blot results were confirmed with immunohistochemistry and qRT-PCR. In addition, we found that APP transgenic mice were protected from neurotoxicity caused by an injection of bak BH3 fusion peptides, known to induce apoptosis by antagonizing bcl protein activity. Nissl and fluorojade-stained slides showed that the active bak BH3 peptide caused substantial neuronal loss in the dentate gyrus and CA3 regions of nontransgenic, but not APP mice. The inactive mutant bak BH3 peptide did not cause degeneration in any mice. These data demonstrate that the increased Bcl-2 expression in brain regions containing Abeta deposits is associated with neuroprotection.

Auto-activation of the apoptosis protein Bax increases mitochondrial membrane permeability and is inhibited by Bcl-2

Interactions among Bcl-2 family proteins mediated by Bcl-2 homology (BH) regions transform apoptosis signals into actions. The interactions between BH3 region-only proteins and multi-BH region proteins such as Bax and Bcl-2 have been proposed to be the dominant interactions required for initiating apoptosis. Experimental evidence also suggests that both homo- and hetero-interactions are mediated primarily by the BH3 regions in all Bcl-2 family proteins and contribute to commitment to or inhibition of apoptosis. We found that a peptide containing the BH3 helix of Bax was not sufficient to activate recombinant Bax to permeabilize mitochondria. However, an extended peptide containing the BH3 helix and additional downstream sequences activated Bax to permeabilize mitochondria and liposomes. Bcl-2 inhibited the membrane-permeabilizing activity of peptide-activated Bax. This activity of Bcl-2 was inhibited by the extended but not the BH3-only peptide despite both peptides binding to Bcl-2 with similar affinity. Further, membrane-bound Bax activation intermediates directly activated soluble Bax further permeabilizing the membrane. Bcl-2 inhibited Bax auto-activation. We therefore propose that Bax auto-activation amplifies the initial death signal produced by BH3-only proteins and that Bcl-2 functions as an inhibitor of Bax auto-activation.

The role of the mitochondrial apoptosis induced channel MAC in cytochrome c release

Permeabilization of the mitochondrial outer membrane is a crucial event during apoptosis. It allows the release of proapoptotic factors, like cytochrome c, from the intermembrane space, and represents the commitment step in apoptosis. The mitochondrial apoptosis-induced channel, MAC, is a high-conductance channel that forms during early apoptosis and is the putative cytochrome c release channel. Unlike activation of the permeability transition pore, MAC formation occurs without loss of outer membrane integrity and depolarization. The single channel behavior and pharmacology of reconstituted MAC has been characterized with patch-clamp techniques. Furthermore, MAC's activity is compared to that detected in mitochondria inside the cells at the time cytochrome c is released. Finally, the regulation of MAC by the Bcl-2 family proteins and insights concerning its molecular composition are also discussed.

Mitochondrial membrane permeabilization: the sine qua non for cell death

Mitochondria are essential for maintaining cell life but they also play a role in regulating cell death, which occurs when their membranes become permeabilized. Mitochondria possess two distinct membrane systems including an outer membrane in close communication with the cytosol and an inner membrane involved in energy transduction. Outer membrane permeabilization is regulated by Bcl-2 family proteins, which control the release of proteins from the mitochondrial intermembrane space; these proteins then activate apoptosis. Inner membrane permeabilization is regulated by the mitochondrial permeability transition (MPT), which is activated by calcium and oxidative stress and leads to bioenergetic failure and necrosis. The purpose of this review is to discuss the biochemical mechanisms regulating mitochondrial membrane permeabilization; this is crucial to our understanding of the role of cell death in diseases such as cancer and the neurodegenerative diseases.

Mutually exclusive subsets of BH3-only proteins are activated by the p53 and c-Jun N-terminal kinase/c-Jun signaling pathways during cortical neuron apoptosis induced by arsenite

The c-Jun N-terminal protein kinase (JNK)/c-Jun and p53 pathways form distinct death-signaling modules in neurons that culminate in Bax-dependent apoptosis. To investigate whether this signaling autonomy is due to recruitment of particular BH3-only proteins, we searched for a toxic signal that would activate both pathways in the same set of neurons. We show that arsenite activates both the JNK/c-Jun and p53 pathways in cortical neurons, which together account for >95% of apoptosis, as determined by using the mixed-lineage kinase (JNK/c-Jun) pathway inhibitor CEP11004 and p53-null mice. Despite the coexistence of both pathways in at least 30% of the population, Bim mRNA and protein expression was increased only by the JNK/c-Jun signaling pathway, whereas Noxa and Puma mRNA and Puma protein expression was entirely JNK/c-Jun independent. About 50% of Puma/Noxa expression was p53 dependent, with the remaining signal being independent of both pathways and possibly facilitated by arsenite-induced reduction in P-Akt. However, functionally, Puma was predominant in mediating Bax-dependent apoptosis, as evidenced by the fact that more than 90% of apoptosis was prevented in Puma-null neurons, although Bim was still upregulated, while Bim- and Noxa-null neurons died similarly to wild-type neurons. Thus, the p53 and JNK/c-Jun pathways can activate mutually exclusive subclasses of BH3-only proteins in the same set of neurons. However, other factors besides expression may determine which BH3-only proteins mediate apoptosis.

Actions of BAX on mitochondrial channel activity and on synaptic transmission

Changes in mitochondrial architecture and permeability facilitate programmed cell death. The BCL-2 family protein BAX is implicated in the formation of large "death channels" in outer mitochondrial membranes. We found that BAX-induced channels on mitochondria may have alternative functions. By patch clamping mitochondrial membranes inside the presynaptic terminal of the living squid giant synapse, we made direct measurements of channel activity produced by BAX application. Only infrequently did BAX application result in large conductance channels similar to those produced by a proapoptotic BCL-xL fragment or by application of a BH3-only peptide. Instead, the majority of outer mitochondrial channels induced by BAX had much smaller conductances than those found previously for the proapoptotic protein. Injection of BAX into the presynaptic terminal did not abolish synaptic transmission, contrary to previous findings with the proapoptotic fragment of BCL-xL. Instead, injection of BAX caused an increase in neurotransmitter release, as has also been found for the full-length antiapoptotic BCL-xL protein. We suggest that BAX can act to enhance synaptic efficacy in a normal physiological setting. Furthermore, the occasional large openings may reflect the function of "activated" BAX either to facilitate cell death or to play a physiological role in decreasing synaptic activity.