Initiation of Apoptotic Signal by the Peroxidation of Cardiolipin of Mitochondria

Potential role and therapeutic implications of glutathione peroxidase 4 in the treatment of Alzheimer's disease

Alzheimer's disease is an age-related neurodegenerative disorder with a complex and incompletely understood pathogenesis. Despite extensive research, a cure for Alzheimer's disease has not yet been found. Oxidative stress mediates excessive oxidative responses, and its involvement in Alzheimer's disease pathogenesis as a primary or secondary pathological event is widely accepted. As a member of the selenium-containing antioxidant enzyme family, glutathione peroxidase 4 reduces esterified phospholipid hydroperoxides to maintain cellular redox homeostasis. With the discovery of ferroptosis, the central role of glutathione peroxidase 4 in anti-lipid peroxidation in several diseases, including Alzheimer's disease, has received widespread attention. Increasing evidence suggests that glutathione peroxidase 4 expression is inhibited in the Alzheimer's disease brain, resulting in oxidative stress, inflammation, ferroptosis, and apoptosis, which are closely associated with pathological damage in Alzheimer's disease. Several therapeutic approaches, such as small molecule drugs, natural plant products, and non-pharmacological treatments, ameliorate pathological damage and cognitive function in Alzheimer's disease by promoting glutathione peroxidase 4 expression and enhancing glutathione peroxidase 4 activity. Therefore, glutathione peroxidase 4 upregulation may be a promising strategy for the treatment of Alzheimer's disease. This review provides an overview of the gene structure, biological functions, and regulatory mechanisms of glutathione peroxidase 4, a discussion on the important role of glutathione peroxidase 4 in pathological events closely related to Alzheimer's disease, and a summary of the advances in small-molecule drugs, natural plant products, and non-pharmacological therapies targeting glutathione peroxidase 4 for the treatment of Alzheimer's disease. Most prior studies on this subject used animal models, and relevant clinical studies are lacking. Future clinical trials are required to validate the therapeutic effects of strategies targeting glutathione peroxidase 4 in the treatment of Alzheimer's disease.

Synergistic Toxicity of Pollutant and Ultraviolet Exposure from a Mitochondrial Perspective

Ultraviolet (UV) exposure and atmospheric pollution are both independently implicated in skin diseases such as cancer and premature aging. UVA wavelengths, which penetrate in the deep layers of the skin dermis, exert their toxicity mainly through chromophore photosensitization reactions. Benzo[a]pyrene (BaP), the most abundant polycyclic aromatic hydrocarbon originating from the incomplete combustion of organic matter, could act as a chromophore and absorb UVA. We and other groups have previously shown that BaP and UVA synergize their toxicity in skin cells, which leads to important oxidation. Even if mitochondria alterations have been related to premature skin aging and other skin disorders, no studies have focused on the synergy between UV exposure and pollution on mitochondria. Our study aims to investigate the combined effect of UVA and BaP specifically on mitochondria in order to assess the effect on mitochondrial membranes and the consequences on mitochondrial activity. We show that BaP has a strong affinity for mitochondria and that this affinity leads to an important induction of lipid peroxidation and membrane disruption when exposed to UVA. Co-exposure to UVA and BaP synergizes their toxicity to negatively impact mitochondrial membrane potential, mitochondrial metabolism and the mitochondrial network. Altogether, our results highlight the implication of mitochondria in the synergistic toxicity of pollution and UV exposure and the potential of this toxicity on skin integrity.

GPX4: The hub of lipid oxidation, ferroptosis, disease and treatment

Glutathione peroxidase 4 (GPx4) moonlights as structural protein and antioxidase that powerfully inhibits lipid oxidation. In the past years, it is considered as a key regulator of ferroptosis, which takes role in the lipid and amine acid metabolism and influences the cell aging, oncogenesis, and cell death. More and more evidences show that targeting GPX4-induced ferroptosis is a promising strategy for disease therapy, especially cancer treatment. In view of these, we generalize the function of GPX4 and regulatory mechanism between GPX4 and ferroptosis, discuss its roles in the disease pathology, and focus on the recent advances of disease therapeutic potential.

Oxygen toxicity: cellular mechanisms in normobaric hyperoxia

In clinical settings, oxygen therapy is administered to preterm neonates and to adults with acute and chronic conditions such as COVID-19, pulmonary fibrosis, sepsis, cardiac arrest, carbon monoxide poisoning, and acute heart failure. In non-clinical settings, divers and astronauts may also receive supplemental oxygen. In addition, under current standard cell culture practices, cells are maintained in atmospheric oxygen, which is several times higher than what most cells experience in vivo. In all the above scenarios, the elevated oxygen levels (hyperoxia) can lead to increased production of reactive oxygen species from mitochondria, NADPH oxidases, and other sources. This can cause cell dysfunction or death. Acute hyperoxia injury impairs various cellular functions, manifesting ultimately as physiological deficits. Chronic hyperoxia, particularly in the neonate, can disrupt development, leading to permanent deficiencies. In this review, we discuss the cellular activities and pathways affected by hyperoxia, as well as strategies that have been developed to ameliorate injury.

Redox Activation of Mitochondrial DAMPs and the Metabolic Consequences for Development of Autoimmunity

Significance: Reactive oxygen species (ROS) are well known to promote innate immune responses during and in the absence of microbial infections. However, excessive or prolonged exposure to ROS provokes innate immune signaling dysfunction and contributes to the pathogenesis of many autoimmune diseases. The relatively high basal expression of pattern recognition receptors (PRRs) in innate immune cells renders them prone to activation in response to minor intrinsic or extrinsic ROS misbalances in the absence of pathogens. Critical Issues: A prominent source of ROS are mitochondria, which are also major inter-organelle hubs for innate immunity activation, since most PRRs and downstream receptor molecules are directly located either at mitochondria or at mitochondria-associated membranes. Due to their ancestral bacterial origin, mitochondria can also act as quasi-intrinsic self-microbes that mimic a pathogen invasion and become a source of danger-associated molecular patterns (DAMPs) that triggers innate immunity from within. Recent Advances: The release of mitochondrial DAMPs correlates with mitochondrial metabolism changes and increased generation of ROS, which can lead to the oxidative modification of DAMPs. Recent studies suggest that ROS-modified mitochondrial DAMPs possess increased, persistent immunogenicity. Future Directions: Herein, we discuss how mitochondrial DAMP release and oxidation activates PRRs, changes cellular metabolism, and causes innate immune response dysfunction by promoting systemic inflammation, thereby contributing to the onset or progression of autoimmune diseases. The future goal is to understand what the tipping point for DAMPs is to become oxidized, and whether this is a road without return. Antioxid. Redox Signal. 36, 441-461.

A Walk in the Memory, from the First Functional Approach up to Its Regulatory Role of Mitochondrial Bioenergetic Flow in Health and Disease: Focus on the Adenine Nucleotide Translocator

The mitochondrial adenine nucleotide translocator (ANT) plays the fundamental role of gatekeeper of cellular energy flow, carrying out the reversible exchange of ADP for ATP across the inner mitochondrial membrane. ADP enters the mitochondria where, through the oxidative phosphorylation process, it is the substrate of Fo-F1 ATP synthase, producing ATP that is dispatched from the mitochondrion to the cytoplasm of the host cell, where it can be used as energy currency for the metabolic needs of the cell that require energy. Long ago, we performed a method that allowed us to monitor the activity of ANT by continuously detecting the ATP gradually produced inside the mitochondria and exported in the extramitochondrial phase in exchange with externally added ADP, under conditions quite close to a physiological state, i.e., when oxidative phosphorylation takes place. More than 30 years after the development of the method, here we aim to put the spotlight on it and to emphasize its versatile applicability in the most varied pathophysiological conditions, reviewing all the studies, in which we were able to observe what really happened in the cell thanks to the use of the "ATP detecting system" allowing the functional activity of the ANT-mediated ADP/ATP exchange to be measured.

Mitochondrial dysfunction, AMPK activation and peroxisomal metabolism: A coherent scenario for non-canonical 3-methylglutaconic acidurias

The occurrence of 3-methylglutaconic aciduria (3-MGA) is a well understood phenomenon in leucine oxidation and ketogenesis disorders (primary 3-MGAs). In contrast, its genesis in non-canonical (secondary) 3-MGAs, a growing-up group of disorders encompassing more than a dozen of inherited metabolic diseases, is a mystery still remaining unresolved for three decades. To puzzle out this anthologic problem of metabolism, three clues were considered: (i) the variety of disorders suggests a common cellular target at the cross-road of metabolic and signaling pathways, (ii) the response to leucine loading test only discriminative for primary but not secondary 3-MGAs suggests these latter are disorders of extramitochondrial HMG-CoA metabolism as also attested by their failure to increase 3-hydroxyisovalerate, a mitochondrial metabolite accumulating only in primary 3-MGAs, (iii) the peroxisome is an extramitochondrial site possessing its own pool and displaying metabolism of HMG-CoA, suggesting its possible involvement in producing extramitochondrial 3-methylglutaconate (3-MG). Following these clues provides a unifying common basis to non-canonical 3-MGAs: constitutive mitochondrial dysfunction induces AMPK activation which, by inhibiting early steps in cholesterol and fatty acid syntheses, pipelines cytoplasmic acetyl-CoA to peroxisomes where a rise in HMG-CoA followed by local dehydration and hydrolysis may lead to 3-MGA yield. Additional contributors are considered, notably for 3-MGAs associated with hyperammonemia, and to a lesser extent in CLPB deficiency. Metabolic and signaling itineraries followed by the proposed scenario are essentially sketched, being provided with compelling evidence from the literature coming in their support.

Genetic re-engineering of polyunsaturated phospholipid profile of Saccharomyces cerevisiae identifies a novel role for Cld1 in mitigating the effects of cardiolipin peroxidation

Cardiolipin (CL) is a unique phospholipid localized almost exclusively within the mitochondrial membranes where it is synthesized. Newly synthesized CL undergoes acyl remodeling to produce CL species enriched with unsaturated acyl groups. Cld1 is the only identified CL-specific phospholipase in yeast and is required to initiate the CL remodeling pathway. In higher eukaryotes, peroxidation of CL, yielding CLOX, has been implicated in the cellular signaling events that initiate apoptosis. CLOX can undergo enzymatic hydrolysis, resulting in the release of lipid mediators with signaling properties. Our previous findings suggested that CLD1 expression is upregulated in response to oxidative stress, and that one of the physiological roles of CL remodeling is to remove peroxidized CL. To exploit the powerful yeast model to study functions of CLD1 in CL peroxidation, we expressed the H. brasiliensis Δ12-desaturase gene in yeast, which then synthesized poly unsaturated fatty acids(PUFAs) that are incorporated into CL species. Using LC-MS based redox phospholipidomics, we identified and quantified the molecular species of CL and other phospholipids in cld1Δ vs. WT cells. Loss of CLD1 led to a dramatic decrease in chronological lifespan, mitochondrial membrane potential, and respiratory capacity; it also resulted in increased levels of mono-hydroperoxy-CLs, particularly among the highly unsaturated CL species, including tetralinoleoyl-CL. In addition, purified Cld1 exhibited a higher affinity for CLOX, and treatment of cells with H2O2 increased CLD1 expression in the logarithmic growth phase. These data suggest that CLD1 expression is required to mitigate oxidative stress. The findings from this study contribute to our overall understanding of CL remodeling and its role in mitigating oxidative stress.

Effect of tanshinone IIA on oxidative stress and apoptosis in a rat model of fatty liver

Oxidative stress is a crucial factor associated with fatty liver disease, which raises the possibility of using antioxidants to improve liver steatosis. Tanshinone IIA (TSIIA) is a traditional Chinese medicine that has been reported to have antioxidant effects in vitro. The present study aimed to investigate whether TSIIA possesses antioxidant effects in vivo and whether TSIIA was able to improve liver steatosis. Hence, the ability of TSIIA to protect rats from liver disease was explored, particularly in regard to antioxidant activity. Rats were fed a high-lipid diet for 90 days, causing severe liver steatosis, both morphologically and biochemically. An increase in reactive oxygen species (ROS) in the liver was exhibited in addition to significantly elevated serum lipids and malondialdehyde (MDA). Furthermore, hepatocyte apoptosis was measured by Hoechst staining, reverse transcription-quantitative polymerase chain reaction and western blot analysis and an increase in hepatocyte apoptosis rate was indicated in mice on a high-fat diet. Following intraperitoneal injection of TSIIA (10 mg/kg/day), liver steatosis was significantly inhibited. In rats receiving TSIIA treatment, less ROS were indicated in the liver and significantly decreased levels of MDA (P<0.05) in serum were exhibited, whereas significantly increased activities of total superoxide dismutase (T-SOD) and glutathione peroxidase (GSH-PX) were observed (P<0.05 and P<0.01, respectively). In addition, the rate of hepatocyte apoptosis was significantly decreased in the TSIIA group (P<0.01). However, TSIIA elicited no effect on serum lipid profiles. These results suggest that TSIIA attenuates oxidative stress by decreasing ROS and MDA production and enhancing the activity of T-SOD and GSH-PX, which may contribute to the inhibition of apoptosis and amelioration of liver steatosis.

The mitochondria-targeted imidazole substituted oleic acid 'TPP-IOA' affects mitochondrial bioenergetics and its protective efficacy in cells is influenced by cellular dependence on aerobic metabolism

A variety of mitochondria-targeted small molecules have been invented to manipulate mitochondrial redox activities and improve function in certain disease states. 3-Hydroxypropyl-triphenylphosphonium-conjugated imidazole-substituted oleic acid (TPP-IOA) was developed as a specific inhibitor of cytochrome c peroxidase activity that inhibits apoptosis by preventing cardiolipin oxidation and cytochrome c release to the cytosol. Here we evaluate the effects of TPP-IOA on oxidative phosphorylation in isolated mitochondria and on mitochondrial function in live cells. We demonstrate that, at concentrations similar to those required to achieve inhibition of cytochrome c peroxidase activity, TPP-IOA perturbs oxidative phosphorylation in isolated mitochondria. In live SH-SY5Y cells, TPP-IOA partially collapsed mitochondrial membrane potential, caused extensive fragmentation of the mitochondrial network, and decreased apparent mitochondrial abundance within 3h of exposure. Many cultured cell lines rely primarily on aerobic glycolysis, potentially making them less sensitive to small molecules disrupting oxidative phosphorylation. We therefore determined the anti-apoptotic efficacy of TPP-IOA in SH-SY5Y cells growing in glucose or in galactose, the latter of which increases reliance on oxidative phosphorylation for ATP supply. The anti-apoptotic activity of TPP-IOA that was observed in glucose media was not seen in galactose media. It therefore appears that, at concentrations required to inhibit cytochrome c peroxidase activity, TPP-IOA perturbs oxidative phosphorylation. In light of these data it is predicted that potential future therapeutic applications of TPP-IOA will be restricted to highly glycolytic cell types with limited reliance on oxidative phosphorylation.

Double-edged sword in cells: chemical biology studies of the vital role of cytochrome c in the intrinsic pre-apoptotic mitochondria leakage pathway

Besides functioning as an electron transporter in the mitochondrial electron transport chain, cytochrome c (cyt c) is also one of the determinants in the execution of cell death.

Mitochondrial reactive oxygen species (ROS) in animal cells: relevance to aging and normal physiology

In animal mitochondria, the four electron reduction of molecular oxygen to produce water at respiratory complex IV is the terminal step in substrate oxidation. However, respiratory complexes I, II, and III can participate in the single electron reduction of oxygen to produce the radical species superoxide. This progenitor reactive oxygen species (ROS) participates in a number of reactions that generate other ROS. These molecules may react with, and damage, intracellular macromolecules, leading to cellular dysfunction. Mitochondrial ROS production is often considered from this perspective of macromolecular damage and is central to the “oxidative damage theory of aging”, which suggests the accumulation of oxidative damage in animal cells underlies the aging phenotype and limits lifespan. In this review, we discuss some experimental results accumulated over the past decade that are inconsistent with this theory. A limitation of the theory is that it presupposes mitochondrial ROS are inherently harmful. However, it is increasingly apparent that some basic cellular functions are physiologically regulated by normal levels of mitochondrial ROS. For example, cell growth and division, the apoptotic pathway, and mitochondrial fusion–fission dynamics all appear to be redox-regulated by mitochondrial ROS and perhaps the matrix manganese superoxide dismutase (MnSOD). Therefore, it is less clear how the balance between ROS regulation of normal cellular activities and ROS-mediated macromolecular damage is maintained and how this relates to aging and longevity in animals.

Oxidized but not native cardiolipin has pro-inflammatory effects, which are inhibited by Annexin A5

OBJECTIVE

Cardiolipin (CL) is a phospholipid with an unusual dimeric structure containing four double-bonds and is easily oxidized. CL is present in mitochondria. Here we explored potential pro-inflammatory properties implicated in cardiovascular disease (CVD): activation of endothelial cells, 5-lipoxygenase (5-LOX) and leukotriene B4 (LTB4), by oxidized CL (oxCL) and inhibitory effects of Annexin A5, an antithrombotic and antiinflammatory plasma protein.

METHODS

In monocytes/macrophages and neutrophils, calcium mobilization was monitored spectrophotometrically with Fura-2 and synthesis of LTB4 was analyzed by EIA. Expression of adhesion molecules on endothelial cells was studied by FACScan. Binding of Annexin A5 were analyzed by ELISA. The mRNA expression of 5-LOX and cyclooxygenase-2 was assessed by Real-Time PCR.

RESULTS

We demonstrate that oxCL but not its non-oxidized counterpart CL induces biosynthesis of LTB4 and increases intracellular concentrations of calcium in monocytes/macrophages and neutrophils. oxCL rather than CL selectively elevates gene expression of 5-LOX but not COX-2 in human macrophages. Furthermore, oxCL but not CL raises levels of adhesion molecules ICAM-1 and VCAM-1 in endothelial cells. Annexin A5 can bind oxCL to abolish all these oxCL-induced effects.

CONCLUSIONS

oxCL may promote inflammation and related diseases especially in conditions involving unresolved apoptosis and necrosis, such as atherosclerosis, where free oxCL is likely to be released from liberated mitochondria. Increased intracellular calcium could activate 5-LOX to produce Leukotriene B4 (LTB4). Annexin A5 inhibits the pro-inflammatory effects of oxCL and its potential therapeutic use when oxCL is implicated in inflammation could be of interest.

Novel therapeutic strategies for traumatic brain injury: acute antioxidant reinforcement

Traumatic brain injury (TBI) is the most important cause of disability in individuals under the age of 45 years and thus represents a significant social and economic burden. Evidence strongly suggests that oxidative stress is a cornerstone event leading to and propagating secondary injury mechanisms such as excitotoxicity, mitochondrial dysfunction, apoptosis, autophagy, brain edema, and inflammation. TBI has defied conventional approaches to diagnosis and therapy development because of its heterogeneity and complexity. Therefore, it is necessary to explore alternative approaches to therapy development for TBI. The aim of this review is to present a therapeutic approach for TBI, taking into account the evidence supporting the role for oxidative stress in the pathophysiological processes of secondary brain injury. The role of agents such as mitochondria-targeted antioxidants (melatonin and new mitochondria-targeted antioxidants), nicotinamide adenine dinucleotide phosphate (NADPH) inhibitors (antioxidant vitamins and apocynin), and other compounds having mainly antioxidant properties (hydrogen-rich saline, sulforaphane, U-83836E, omega-3, and polyphenols) is covered. The rationale for innovative antioxidant therapies based on current knowledge and particularly the most recent studies regarding this field is discussed. Particular considerations and translational potential of new TBI treatments are examined and a novel therapeutic proposal for TBI is presented.

Reactive oxygen species in inflammation and tissue injury

Abstract Reactive oxygen species (ROS) are key signaling molecules that play an important role in the progression of inflammatory disorders. An enhanced ROS generation by polymorphonuclear neutrophils (PMNs) at the site of inflammation causes endothelial dysfunction and tissue injury. The vascular endothelium plays an important role in passage of macromolecules and inflammatory cells from the blood to tissue. Under the inflammatory conditions, oxidative stress produced by PMNs leads to the opening of inter-endothelial junctions and promotes the migration of inflammatory cells across the endothelial barrier. The migrated inflammatory cells not only help in the clearance of pathogens and foreign particles but also lead to tissue injury. The current review compiles the past and current research in the area of inflammation with particular emphasis on oxidative stress-mediated signaling mechanisms that are involved in inflammation and tissue injury.

Elucidating the mechanism of ferrocytochrome c heme disruption by peroxidized cardiolipin

The interaction of peroxidized cardiolipin with ferrocytochrome c induces two kinetically and chemically distinct processes. The first is a rapid oxidation of ferrocytochrome c, followed by a slower, irreversible disruption of heme c. The oxidation of ferrocytochrome c by peroxidized cardiolipin is explained by a Fenton-type reaction. Heme scission is a consequence of the radical-mediated reactions initiated by the interaction of ferric heme iron with peroxidized cardiolipin. Simultaneously with the heme c disruption, generation of hydroxyl radical is detected by EPR spectroscopy using the spin trapping technique. The resulting apocytochrome c sediments as a heterogeneous mixture of high aggregates, as judged by sedimentation analysis. Both the oxidative process and the destructive process were suppressed by nonionic detergents and/or high ionic strength. The mechanism for generating radicals and heme rupture is presented.

Lipidomics identifies cardiolipin oxidation as a mitochondrial target for redox therapy of brain injury

The brain contains a highly diversified complement of molecular species of a mitochondria-specific phospholipid, cardiolipin, which, because of its polyunsaturation, can readily undergo oxygenation. Using global lipidomics analysis in experimental traumatic brain injury (TBI), we found that TBI was accompanied by oxidative consumption of polyunsaturated cardiolipin and the accumulation of more than 150 new oxygenated molecular species of cardiolipin. RNAi-based manipulations of cardiolipin synthase and cardiolipin levels conferred resistance to mechanical stretch, an in vitro model of traumatic neuronal injury, in primary rat cortical neurons. By applying a brain-permeable mitochondria-targeted electron scavenger, we prevented cardiolipin oxidation in the brain, achieved a substantial reduction in neuronal death both in vitro and in vivo, and markedly reduced behavioral deficits and cortical lesion volume. We conclude that cardiolipin oxygenation generates neuronal death signals and that prevention of it by mitochondria-targeted small molecule inhibitors represents a new target for neuro-drug discovery.

Spatiotemporal autophagic degradation of oxidatively damaged organelles after photodynamic stress is amplified by mitochondrial reactive oxygen species

Although reactive oxygen species (ROS) have been reported to evoke different autophagic pathways, how ROS or their secondary products modulate the selective clearance of oxidatively damaged organelles is less explored. To investigate the signaling role of ROS and the impact of their compartmentalization in autophagy pathways, we used murine fibrosarcoma L929 cells overexpressing different antioxidant enzymes targeted to the cytosol or mitochondria and subjected them to photodynamic (PD) stress with the endoplasmic reticulum (ER)-associated photosensitizer hypericin. We show that following apical ROS-mediated damage to the ER, predominantly cells overexpressing mitochondria-associated glutathione peroxidase 4 (GPX4) and manganese superoxide dismutase (SOD2) displayed attenuated kinetics of autophagosome formation and overall cell death, as detected by computerized time-lapse microscopy. Consistent with a primary ER photodamage, kinetics and colocalization studies revealed that photogenerated ROS induced an initial reticulophagy, followed by morphological changes in the mitochondrial network that preceded clearance of mitochondria by mitophagy. Overexpression of cytosolic and mitochondria-associated GPX4 retained the tubular mitochondrial network in response to PD stress and concomitantly blocked the progression toward mitophagy. Preventing the formation of phospholipid hydroperoxides and H(2)O(2) in the cytosol as well as in the mitochondria significantly reduced cardiolipin peroxidation and apoptosis. All together, these results show that in response to apical ER photodamage ROS propagate to mitochondria, which in turn amplify ROS production, thereby contributing to two antagonizing processes, mitophagy and apoptosis.

Redox regulation of mitochondrial function

Redox-dependent processes influence most cellular functions, such as differentiation, proliferation, and apoptosis. Mitochondria are at the center of these processes, as mitochondria both generate reactive oxygen species (ROS) that drive redox-sensitive events and respond to ROS-mediated changes in the cellular redox state. In this review, we examine the regulation of cellular ROS, their modes of production and removal, and the redox-sensitive targets that are modified by their flux. In particular, we focus on the actions of redox-sensitive targets that alter mitochondrial function and the role of these redox modifications on metabolism, mitochondrial biogenesis, receptor-mediated signaling, and apoptotic pathways. We also consider the role of mitochondria in modulating these pathways, and discuss how redox-dependent events may contribute to pathobiology by altering mitochondrial function.

Clair DK. Manganese superoxide dismutase: guardian of the powerhouse

The mitochondrion is vital for many metabolic pathways in the cell, contributing all or important constituent enzymes for diverse functions such as β-oxidation of fatty acids, the urea cycle, the citric acid cycle, and ATP synthesis. The mitochondrion is also a major site of reactive oxygen species (ROS) production in the cell. Aberrant production of mitochondrial ROS can have dramatic effects on cellular function, in part, due to oxidative modification of key metabolic proteins localized in the mitochondrion. The cell is equipped with myriad antioxidant enzyme systems to combat deleterious ROS production in mitochondria, with the mitochondrial antioxidant enzyme manganese superoxide dismutase (MnSOD) acting as the chief ROS scavenging enzyme in the cell. Factors that affect the expression and/or the activity of MnSOD, resulting in diminished antioxidant capacity of the cell, can have extraordinary consequences on the overall health of the cell by altering mitochondrial metabolic function, leading to the development and progression of numerous diseases. A better understanding of the mechanisms by which MnSOD protects cells from the harmful effects of overproduction of ROS, in particular, the effects of ROS on mitochondrial metabolic enzymes, may contribute to the development of novel treatments for various diseases in which ROS are an important component.

The multiple functions of cytochrome c and their regulation in life and death decisions of the mammalian cell: From respiration to apoptosis

Cytochrome c (Cytc) is essential in mitochondrial electron transport and intrinsic type II apoptosis. Mammalian Cytc also scavenges reactive oxygen species (ROS) under healthy conditions, produces ROS with the co-factor p66(Shc), and oxidizes cardiolipin during apoptosis. The recent finding that Cytc is phosphorylated in vivo underpins a model for the pivotal role of Cytc regulation in making life and death decisions. An apoptotic sequence of events is proposed involving changes in Cytc phosphorylation, increased ROS via increased mitochondrial membrane potentials or the p66(Shc) pathway, and oxidation of cardiolipin by Cytc followed by its release from the mitochondria. Cytc regulation in respiration and cell death is discussed in a human disease context including neurodegenerative and cardiovascular diseases, cancer, and sepsis.

Formation of 4-hydroxynonenal from cardiolipin oxidation: Intramolecular peroxyl radical addition and decomposition

We report herein that oxidation of a mitochondria-specific phospholipid tetralinoleoyl cardiolipin (L(4)CL) by cytochrome c and H(2)O(2) leads to the formation of 4-hydroxy-2-nonenal (4-HNE) via a novel chemical mechanism that involves cross-chain peroxyl radical addition and decomposition. As one of the most bioactive lipid electrophiles, 4-HNE possesses diverse biological activities ranging from modulation of multiple signal transduction pathways to the induction of intrinsic apoptosis. However, where and how 4-HNE is formed in vivo are much less understood. Recently a novel chemical mechanism has been proposed that involves intermolecular dimerization of fatty acids by peroxyl bond formation; but the biological relevance of this mechanism is unknown because a majority of the fatty acids are esterified in phospholipids in the cellular membrane. We hypothesize that oxidation of cardiolipins, especially L(4)CL, may lead to the formation of 4-HNE via this novel mechanism. We employed L(4)CL and dilinoleoylphosphatidylcholine (DLPC) as model compounds to test this hypothesis. Indeed, in experiments designed to assess the intramolecular mechanism, more 4-HNE is formed from L(4)CL and DLPC oxidation than 1-palmitoyl-2-linoleoylphosphatydylcholine. The key products and intermediates that are consistent with this proposed mechanism of 4-HNE formation have been identified using liquid chromatography-mass spectrometry. Identical products from cardiolipin oxidation were identified in vivo in rat liver tissue after carbon tetrachloride treatment. Our studies provide the first evidence in vitro and in vivo for the formation 4-HNE from cardiolipin oxidation via cross-chain peroxyl radical addition and decomposition, which may have implications in apoptosis and other biological activities of 4-HNE.

Mitochondrial dysfunction and reduced prostaglandin synthesis in skeletal muscle of Group VIB Ca2+-independent phospholipase A2gamma-deficient mice

Group VIB Ca(2+)-independent phospholipase A(2)γ (iPLA(2)γ) is a membrane-bound iPLA(2) enzyme with unique features, such as the utilization of distinct translation initiation sites and the presence of mitochondrial and peroxisomal localization signals. Here we investigated the physiological functions of iPLA(2)γ by disrupting its gene in mice. iPLA(2)γ-knockout (KO) mice were born with an expected Mendelian ratio and appeared normal and healthy at the age of one month but began to show growth retardation from the age of two months as well as kyphosis and significant muscle weakness at the age of four months. Electron microscopy revealed swelling and reduced numbers of mitochondria and atrophy of myofilaments in iPLA(2)γ-KO skeletal muscles. Increased lipid peroxidation and the induction of several oxidative stress-related genes were also found in the iPLA(2)γ-KO muscles. These results provide evidence that impairment of iPLA(2)γ causes mitochondrial dysfunction and increased oxidative stress, leading to the loss of skeletal muscle structure and function. We further found that the compositions of cardiolipin and other phospholipid subclasses were altered and that the levels of myoprotective prostanoids were reduced in iPLA(2)γ-KO skeletal muscle. Thus, in addition to maintenance of homeostasis of the mitochondrial membrane, iPLA(2)γ may contribute to modulation of lipid mediator production in vivo.

Degradation of phospholipids by oxidative stress--exceptional significance of cardiolipin

The aim of this study was to investigate the effect of oxidative stress on mitochondrial phospholipids. In this context, this study investigated (i) the content of phosphatidylethanolamine (PE), phosphatidylcholine (PC) and cardiolipin (CL), (ii) the correlation of CL degradation with mitochondrial function and (iii) the correlation of CL degradation and CL oxidation. Oxidative stress induced by iron/ascorbate caused a dramatic decrease of these phospholipids, in which CL was the most sensitive phospholipid. Even moderate oxidative stress by hypoxia/reoxygenation caused a decrease in CL that was parallelled by a decrease in active respiration of isolated rat heart mitochondria. The relation between oxidative stress, CL degradation and CL oxidation was studied by in vitro treatment of commercially available CL with superoxide anion radicals and H2O2. The degradation of CL was mediated by H2O2 and required the presence of cytochrome c. Other peroxidases such as horse radish peroxidase and glutathione peroxidase had no effect. Cytochrome c in the presence of H2O2 caused CL oxidation. The data demonstrate that oxidative stress may cause degradation of phospholipids by oxidation, in particular CL; resulting in mitochondrial dysfunction.

Fenretinide induces mitochondrial ROS and inhibits the mitochondrial respiratory chain in neuroblastoma

Fenretinide induces apoptosis in neuroblastoma by induction of reactive oxygen species (ROS). In this study, we investigated the role of mitochondria in fenretinide-induced cytotoxicity and ROS production in six neuroblastoma cell lines. ROS induction by fenretinide was of mitochondrial origin, demonstrated by detection of superoxide with MitoSOX, the scavenging effect of the mitochondrial antioxidant MitoQ and reduced ROS production in cells without a functional mitochondrial respiratory chain (Rho zero cells). In digitonin-permeabilized cells, a fenretinide concentration-dependent decrease in ATP synthesis and substrate oxidation was observed, reflecting inhibition of the mitochondrial respiratory chain. However, inhibition of the mitochondrial respiratory chain was not required for ROS production. Co-incubation of fenretinide with inhibitors of different complexes of the respiratory chain suggested that fenretinide-induced ROS production occurred via complex II. The cytotoxicity of fenretinide was exerted through the generation of mitochondrial ROS and, at higher concentrations, also through inhibition of the mitochondrial respiratory chain.

Oxidative lipidomics of apoptosis: quantitative assessment of phospholipid hydroperoxides in cells and tissues

Oxidized phospholipids play essential roles in execution of mitochondrial stage of apoptosis and clearance of apoptotic cells. The identification and quantification of oxidized phospholipids generated during apoptosis can be successfully achieved by oxidative lipidomics. With this approach, diverse molecular species of phospholipids and their hydroperoxides are identified and characterized by soft-ionization mass-spectrometry techniques such as electrospray ionization (ESI). Quantitative assessment of lipid hydroperoxides is performed by fluorescence HPLC-based protocol. The protocol is based on separation of phospholipids using two-dimensional-high-performance thin-layer chromatography (2-D-HPTLC). Phospholipids are hydrolyzed using phospholipase A(2). The fatty acid hydroperoxides (FA-OOH) released is quantified by a fluorometric assay using Amplex red reagent and microperoxidase-11 (MP-11). Detection limit of this protocol is 1-2 pmol of lipid hydroperoxides. Lipid arrays vs. oxidized lipid arrays can be performed by comparing the abundance of phospholipids with the abundance of oxidized phospholipids. Using oxidative lipidomics approach we show that the pattern of phospholipid oxidation during apoptosis is nonrandom and does not follow their abundance in several types of cells undergoing apoptosis and a variety of disease states. This has important implications for evaluation of apoptosis in vivo. The anionic phospholipids, cardiolipin (CL) and phosphatidylserine (PS), are the preferred peroxidation substrates.

Malarial infection develops mitochondrial pathology and mitochondrial oxidative stress to promote hepatocyte apoptosis

Activation of the mitochondrial apoptosis pathway by oxidative stress has been implicated in hepatocyte apoptosis during malaria. Because mitochondria are the source and target of reactive oxygen species (ROS), we have investigated whether hepatocyte apoptosis is linked to mitochondrial pathology and mitochondrial ROS generation during malaria. Malarial infection induces mitochondrial pathology by inhibiting mitochondrial respiration, dehydrogenases, and transmembrane potential and damaging the ultrastructure as evident from transmission electron microscopic studies. Mitochondrial GSH depletion and formation of protein carbonyl indicate that mitochondrial pathology is associated with mitochondrial oxidative stress. Fluorescence imaging of hepatocytes documents intramitochondrial superoxide anion (O(2)(-)) generation during malaria. O(2)(-) inactivates mitochondrial aconitase to release iron from iron-sulfur clusters, which forms the hydroxyl radical ((.)OH) interacting with H(2)O(2) produced concurrently. Malarial infection inactivates mitochondrial aconitase, and carbonylation of aconitase is evident from Western immunoblotting. The release of iron has been documented by fluorescence imaging of hepatocytes using Phen Green SK, and mitochondrial (.)OH generation has been confirmed. During malaria, the depletion of cardiolipin and formation of the mitochondrial permeability transition pore favor cytochrome c release to activate caspase-9. Interestingly, mitochondrial (.)OH generation correlates with the activation of both caspase-9 and caspase-3 with the progress of malarial infection, indicating the critical role of (.)OH.

Potential mechanisms of muscle mitochondrial dysfunction in aging and obesity and cellular consequences

Mitochondria play a key role in the energy metabolism in skeletal muscle. A new concept has emerged suggesting that impaired mitochondrial oxidative capacity in skeletal muscle may be the underlying defect that causes insulin resistance. According to current knowledge, the causes and the underlying molecular mechanisms at the origin of decreased mitochondrial oxidative capacity in skeletal muscle still remain to be elucidated. The present review focuses on recent data investigating these issues in the area of metabolic disorders and describes the potential causes, mechanisms and consequences of mitochondrial dysfunction in the skeletal muscle.

Inhibition of mitochondrial metabolism by methyl-2-cyano-3,12-dioxooleana-1,9-diene-28-oate induces apoptotic or autophagic cell death in chronic myeloid leukemia cells

The initial success of the first synthetic bcr-abl kinase inhibitor imatinib has been dampened by the emergence of imatinib-resistant disease in blast crisis chronic myeloid leukemia. Here, we report that the novel triterpenoid methyl-2-cyano-3,12-dioxooleana-1,9-diene-28-oate (CDDO-Me) potently induced cytotoxicity in imatinib-resistant KBM5 cells expressing the T315I mutation of bcr-abl (24-h EC50, 540 nmol/L). In long-term culture, CDDO-Me abrogated the growth of human parental KBM5 and KBM5-STI cells with 96-h IC50 of 205 and 221 nmol/L, respectively. In addition, CDDO-Me rapidly decreased the viability of murine lymphoid Ba/F3 cells expressing wild-type p210 as well as the imatinib-resistant E255K and T315I mutations of bcr-abl. The low-dose effects of CDDO-Me are associated with inhibition of mitochondrial oxygen consumption, whereas the cytotoxic effects appear to be mediated by a rapid and selective depletion of mitochondrial glutathione that accompanies the increased generation of reactive oxygen species and mitochondrial dysfunction. Interestingly, the mitochondriotoxic effects of CDDO-Me are followed by the rapid autophagocytosis of intracellular organelles or the externalization of phosphatidylserine in different cell types. We conclude that alterations in mitochondrial function by CDDO-Me can result in autophagy or apoptosis of chronic myeloid leukemia cells regardless of the mutational status of bcr-abl. CDDO-Me is in clinical trials and shows signs of clinical activity, with minimal side effects and complete lack of cardiotoxicity. Studies in leukemias are in preparation.

Adenine nucleotide translocase in the focus of cardiovascular diseases

Adenine nucleotide translocase (ANT) facilitates the exchange of extramitochondrial adenosine diphosphate and intramitochondrial adenosine triphosphate across the inner mitochondrial membrane and appears to be a member of the mitochondrial permeability transition pore whose opening induces apoptosis. Genetically or physiologically restricted ANT function associated with insufficient energy supply and induced apoptosis leads to severe cardiac disturbance. In contrast, to counter myocardial stress, heart tissue developed cell protecting gene programs including ANT1 up-regulation to stabilize energy supply and concurrently suppress apoptotic processes. This review describes characteristics of ANT function and expression in cardiovascular diseases and ANT's role in cardioprotection.

4-HPR-mediated leukemia cell cytotoxicity is triggered by ceramide-induced mitochondrial oxidative stress and is regulated downstream by Bcl-2

We have previously reported that, in leukemia cells, the cytotoxicity of the anticancer agent N-(4-hydroxyphenyl)retinamide (4-HPR) is mediated by mitochondria-derived reactive oxygen species (ROS) and cardiolipin peroxidation. Here, we have analyzed at greater depth the 4-HPR-triggered molecular events, demonstrating that 4-HPR induces an early (15 min) increase in ceramide levels by sphingomyelin hydrolysis and later (from 1 h) by de novo synthesis. Using specific inhibitors of both pathways, we demonstrate that ceramide accumulation is responsible for early ROS generation, which act as apoptotic signalling intermediates leading to conformational activation of Bak and Bax, loss of mitochondrial membrane potential (DeltaPsim), mitochondrial membrane permeabilization (MMP) and cell death. Enforced expression of Bcl-2 has no effect on 4-HPR-induced oxidative stress, but notably prevents mitochondrial alterations and apoptosis, indicating that Bcl-2 functions by regulating events downstream of ROS generation. In conclusion, our study delineates for the fist time the sequence and timing of the principal events induced by 4-HPR in leukemia cells and points to the potential use of modulators of ceramide metabolism as enhancers in 4-HPR-based therapies.

In the early phase of programmed cell death in Tobacco Bright Yellow 2 cells the mitochondrial adenine nucleotide translocator, adenylate kinase and nucleoside diphosphate kinase are impaired in a reactive oxygen species-dependent manner

To investigate whether and how mitochondria can change in plant programmed cell death (PCD), we used the non-photosynthetic Tobacco Bright Yellow 2 (TBY-2) cells. These can be synchronized to high levels, stand out in terms of growth rate and homogeneity and undergo PCD as a result of heat shock. Using these cells we investigated the activity of certain mitochondrial proteins that have a role in providing ATP and/or other nucleoside triphosphates (NTPs). We show that, already after 2 h from the heat shock, when cell viability remains unaffected, the rate of ADP/ATP exchange due to adenine nucleotide translocator (ANT) activity, and the rate of the reactions catalysed by adenylate kinase (ADK; EC 2.7.4.3) and nucleoside diphosphate kinase (NDPK; EC 2.7.4.6) are inhibited in a non-competitive-like manner. In all cases, externally added ascorbate partially prevented the inhibition. These effects occurred in spite of minor (for ANT) or no changes in the mitochondrial protein levels as immunologically investigated. Interestingly, a decrease of both the steady state level of the ascorbate pool and of the activity of l-galactono-gamma-lactone dehydrogenase (GLDH) (EC 1.3.2.3), the mitochondrial enzyme catalysing the last step of ascorbate biosynthesis, were also found.

Mitochondrial regulation of apoptotic cell death

Mitochondria play a decisive role in the regulation of both apoptotic and necrotic cell death. Permeabilization of the outer mitochondrial membrane and subsequent release of intermembrane space proteins are important features of both models of cell death. The mechanisms by which these proteins are released depend presumably on cell type and the nature of stimuli. Of the mechanisms involved, mitochondrial permeability transition appears to be associated mainly with necrosis, whereas the release of caspase activating proteins during early apoptosis is regulated primarily by the Bcl-2 family of proteins. However, there is increasing evidence for interaction and co-operation between these two mechanisms. The multiple mechanisms of mitochondrial permeabilization may explain diversities in the response of mitochondria to numerous apoptotic stimuli in different types of cells.

Cardiolipin deficiency releases cytochrome c from the inner mitochondrial membrane and accelerates stimuli-elicited apoptosis

Cardiolipin (CL) is a mitochondria-specific phospholipid synthesized by CL synthase (CLS). We describe here a human gene for CLS and its analysis via RNAi knockdown on apoptotic progression. Although mitochondrial membrane potential is unchanged in cells containing only 25% of the normal amount of CL, free cytochrome c (cyt. c) is detected in the intermembrane space and the mitochondria exhibit signs of reorganized cristae. However, the release of cyt. c from the mitochondria still requires apoptotic stimulation. Increased sensitivity to apoptotic signals and accelerated rates of apoptosis are observed in CL-deficient cells, followed by elevated levels of secondary necrosis. Apoptosis is thought to progress via binding of truncated Bid (tBid) to mitochondrial CL, followed by CL oxidation which results in cyt. c release. The exaggerated and accelerated apoptosis observed in CL-deficient cells is matched by an accelerated reduction in membrane potential and increased cyt. c release, but not by decreased tBid binding. This study suggests that the CL/cyt. c relationship is important in apoptotic progression and that regulating CL oxidation or/and deacylation could represent a possible therapeutic target.

Calcium-independent phospholipase A2 localizes in and protects mitochondria during apoptotic induction by staurosporine

Mitochondria-mediated production of reactive oxygen species (ROS) plays a key role in apoptosis. Mitochondrial phospholipid cardiolipin molecules are likely the main target of ROS because they are particularly rich in polyunsaturated fatty acids. They are also located in the inner mitochondrial membrane near the ROS-producing sites. Under physiological conditions mitochondria can repair peroxidative damage in part through a remodeling mechanism via the deacylation-reacylation cycle mediated by phospholipase A2 (PLA2) and acyl-coenzyme A-dependent monolysocardiolipin acyltransferase. Here we investigate whether group VIA Ca2+-independent PLA2 (iPLA2) plays a role in the protection of mitochondrial function from damage caused by mitochondrially generated ROS during apoptotic induction by staurosporine (STS). We show that iPLA2-expressing cells were relatively resistant to STS-induced apoptosis. iPLA2 localized to mitochondria even before apoptotic induction, and most iPLA2-associated mitochondria were intact in apoptotic resistant cells. Expression of iPLA2 in INS-1 cells prevented the loss of mitochondrial membrane potential, attenuated the release of cytochrome c, Smac/DIABLO, and apoptosis inducing factor from mitochondria, and reduced mitochondrial reactive oxygen species production. Inhibition of caspase 8 has little effect on STS-induced apoptosis in INS-1 cells. Finally, we found that STS down-regulated endogenous iPLA2 transcription in both INS-1 and iPLA2-expressing INS-1 cells without affecting the expression of group IV Ca2+-dependent PLA2. Together, our data indicate that iPLA2 is important for the protection of mitochondrial function from oxidative damage during apoptotic induction. Down-regulation of endogenous iPLA2 by STS may result in the loss of mitochondrial membrane repair functions and lead to mitochondrial failure and apoptosis.

Regulation of cardiolipin synthase levels in Saccharomyces cerevisiae

The Saccharomyces cerevisiae cardiolipin (CL) synthase encoded by the CRD1 gene catalyses the synthesis of CL, which is localized to the inner mitochondrial membrane and plays an important role in mitochondrial function. To investigate how CRD1 expression is regulated, a lacZ reporter gene was placed under control of the CRD1 promoter and the 5'-untranslated region of its mRNA (P(CRD1)-lacZ). P(CRD1)-lacZ expression was 2.5 times higher in early stationary phase than in logarithmic phase for glucose grown cells. Non-fermentable growth resulted in a two-fold elevation in expression relative to glucose grown cells. A shift from glycerol to glucose rapidly repressed expression, whereas a shift from glucose to glycerol had the opposite effect. The derepression of P(CRD1)-lacZ expression by non-fermentable carbon sources was dependent on mitochondrial respiration. These results support a tight coordination between translation and transcription of the CRD1 gene, since similar effects by the above factors on CRD1 mRNA levels have been reported. In glucose-grown cells, P(CRD1)-lacZ expression was repressed 70% in a pgs1delta strain (lacks phosphatidylglycerol and CL) compared with wild-type and rho- cells and elevated 2.5-fold in crd1delta cells, which have increased phosphatidylglycerol levels, suggesting a role for phosphatidylglycerol in regulating CRD1 expression. Addition of inositol to the growth medium had no effect on expression. However, expression was elevated in an ino4delta mutant but not in ino2delta cells, suggesting multiple and separate functions for the inositol-responsive INO2/INO4 gene products, which normally function as a dimer in regulating gene function.

Mechanisms of cytochrome c release from mitochondria

In healthy cells, cytochrome c (Cyt c) is located in the mitochondrial intermembrane/intercristae spaces, where it functions as an electron shuttle in the respiratory chain and interacts with cardiolipin (CL). Several proapoptotic stimuli induce the permeabilization of the outer membrane, facilitate the communication between intermembrane and intercristae spaces and promote the mobilization of Cyt c from CL, allowing for Cyt c release. In the cytosol, Cyt c mediates the allosteric activation of apoptosis-protease activating factor 1, which is required for the proteolytic maturation of caspase-9 and caspase-3. Activated caspases ultimately lead to apoptotic cell dismantling. Nevertheless, cytosolic Cyt c has been associated also to vital cell functions (i.e. differentiation), suggesting that its release not always occurs in an all-or-nothing fashion and that mitochondrial outer membrane permeabilization may not invariably lead to cell death. This review deals with the events involved in Cyt c release from mitochondria, with special attention to its regulation and final consequences.

Caspase-dependent alteration of the ADP/ATP translocator triggers the mitochondrial permeability transition which is not required for the low-potassium-dependent apoptosis of cerebellar granule cells

We investigated ADP/ATP exchange mediated by the adenine nucleotide translocator and opening of the mitochondrial permeability transition pore in homogenates from cerebellar granule cells en route to apoptosis induced by low potassium. We showed that, in the first 3 h of apoptosis, when maximum cytochrome c release had already occurred, adenine nucleotide translocator function was impaired owing to the action of reactive oxygen species, but no permeability transition pore opening occurred. Over 3-8 h of apoptosis, the permeability transition pore progressively opened, owing to caspase action, and further ADP/ATP translocator impairment occurred. The kinetics of transport and permeability transition pore opening were inversely correlated, both in the absence and presence of inhibitors of antioxidant and proteolytic systems. We conclude that, en route to apoptosis, alteration of the adenine nucleotide translocator occurs, resulting in permeability transition pore opening. This process depends on the action of caspase on pore component(s) other than the ADP/ATP translocator, because no change in either amount or molecular weight of the latter protein was noted during apoptosis, as measured by western blotting. Cell death occurs via apoptosis in the presence of cyclosporin A, the permeability transition pore inhibitor, thus showing that permeability transition pore opening, not needed for cytochrome c release, is also unnecessary for apoptosis to occur.

Apoptotic interactions of cytochrome c: redox flirting with anionic phospholipids within and outside of mitochondria

Since the (re)discovery of cytochrome c (cyt c) in the early 1920s and subsequent detailed characterization of its structure and function in mitochondrial electron transport, it took over 70 years to realize that cyt c plays a different, not less universal role in programmed cell death, apoptosis, by interacting with several proteins and forming apoptosomes. Recently, two additional essential functions of cyt c in apoptosis have been discovered that are carried out via its interactions with anionic phospholipids: a mitochondria specific phospholipid, cardiolipin (CL), and plasma membrane phosphatidylserine (PS). Execution of apoptotic program in cells is accompanied by substantial and early mitochondrial production of reactive oxygen species (ROS). Because antioxidant enhancements protect cells against apoptosis, ROS production was viewed not as a meaningless side effect of mitochondrial disintegration but rather playing some - as yet unidentified - role in apoptosis. This conundrum has been resolved by establishing that mitochondria contain a pool of cyt c, which interacts with CL and acts as a CL oxygenase. The oxygenase is activated during apoptosis, utilizes generated ROS and causes selective oxidation of CL. The oxidized CL is required for the release of pro-apoptotic factors from mitochondria into the cytosol. This redox mechanism of cyt c is realized earlier than its other well-recognized functions in the formation of apoptosomes and caspase activation. In the cytosol, released cyt c interacts with another anionic phospholipid, PS, and catalyzes its oxidation in a similar oxygenase reaction. Peroxidized PS facilitates its externalization essential for the recognition and clearance of apoptotic cells by macrophages. Redox catalysis of plasma membrane PS oxidation constitutes an important redox-dependent function of cyt c in apoptosis and phagocytosis. Thus, cyt c acts as an anionic phospholipid specific oxygenase activated and required for the execution of essential stages of apoptosis. This review is focused on newly discovered redox mechanisms of complexes of cyt c with anionic phospholipids and their role in apoptotic pathways in health and disease.

Mitochondrial targeting of selective electron scavengers: synthesis and biological analysis of hemigramicidin-TEMPO conjugates

Synthetic hemigramicidin S-peptidyl TEMPO conjugates are effectively delivered into cells and mitochondria, where they act as electron scavengers and exert protection against apoptosis. Our delivery approach is based on the use of specific structural signaling features recognizable by cells as mitochondria targeting sequences and offers considerable therapeutic anti-apoptotic potential.

Analysis of cardiolipin in human muscle biopsy

Cardiolipin is a phospholipid that is specific to the inner mitochondrial membrane and essential for numerous mitochondrial functions. Accordingly, a quantitative assay for cardiolipin can be a valuable aspect of assessing mitochondrial content and functional capacity. The current study was undertaken to develop a simple and reliable method for direct analysis of the major molecular species of cardiolipin and with particular application for analysis of human skeletal muscle. The method that is presented is based on derivatization of cardiolipin in a total lipid extract with 1-pyrenyldiazomethane (PDAM), to form stable, fluorescent 1-pyrenylmethyl esters. The derivatization reaction takes 30 min on ice in a two-phase system (chloroform:methanol:H(2)O:H(2)SO(4)) containing 0.5-1.0mM PDAM and detergent. The contents of the major cardiolipin species in the derivatization mixture can be estimated by HPLC separation with fluorescent detection during a 20 min run on a reverse phase column and with HPLC grade ethanol/0.5mM H(3)PO(4) as the mobile phase. The recovery is about 80%. The method is specific and sensitive with quantitation limits of 0.5-1 pmol cardiolipin. The response of the fluorescence detector (peak area) is linear across a range 5-40 pmol. The assay is linear over the range between 0.3 and 3.0mg of tissue (R(2)=0.998). The assay provides good reproducibility and accuracy (within 5-10%).

Regulatory effects of the mitochondrial energetic status on mitochondrial p66Shc

p66(Shc) promotes apoptosis and controls the intracellular redox balance. A fraction of p66(Shc) exists within mitochondria, where it oxidizes cytochrome c to form hydrogen peroxide, which in turn induces mitochondrial permeability and apoptosis. However, cells tolerate p66(Shc) expression and accumulate oxidative damage under normal conditions, implying that the p66(Shc) functions must be tightly regulated. Here we review available knowledge on the regulation of p66(Shc) transcription, protein stabilization and post-translational modifications. In addition, we report novel investigations into the role of the mitochondrial import machinery on p66(Shc) activation, which highlight the energetic status of mitochondria as a crucial determinant of p66(Shc) function.

Inhibition of adenine nucleotide translocator pore function and protection against apoptosis in vivo by an HIV protease inhibitor

Inhibitors of HIV protease have been shown to have antiapoptotic effects in vitro, yet whether these effects are seen in vivo remains controversial. In this study, we have evaluated the impact of the HIV protease inhibitor (PI) nelfinavir, boosted with ritonavir, in models of nonviral disease associated with excessive apoptosis. In mice with Fas-induced fatal hepatitis, Staphylococcal enterotoxin B-induced shock, and middle cerebral artery occlusion-induced stroke, we demonstrate that PIs significantly reduce apoptosis and improve histology, function, and/or behavioral recovery in each of these models. Further, we demonstrate that both in vitro and in vivo, PIs block apoptosis through the preservation of mitochondrial integrity and that in vitro PIs act to prevent pore function of the adenine nucleotide translocator (ANT) subunit of the mitochondrial permeability transition pore complex.

Cytochrome c acts as a cardiolipin oxygenase required for release of proapoptotic factors

Programmed death (apoptosis) is turned on in damaged or unwanted cells to secure their clean and safe self-elimination. The initial apoptotic events are coordinated in mitochondria, whereby several proapoptotic factors, including cytochrome c, are released into the cytosol to trigger caspase cascades. The release mechanisms include interactions of B-cell/lymphoma 2 family proteins with a mitochondria-specific phospholipid, cardiolipin, to cause permeabilization of the outer mitochondrial membrane. Using oxidative lipidomics, we showed that cardiolipin is the only phospholipid in mitochondria that undergoes early oxidation during apoptosis. The oxidation is catalyzed by a cardiolipin-specific peroxidase activity of cardiolipin-bound cytochrome c. In a previously undescribed step in apoptosis, we showed that oxidized cardiolipin is required for the release of proapoptotic factors. These results provide insight into the role of reactive oxygen species in triggering the cell-death pathway and describe an early role for cytochrome c before caspase activation.

Separation and quantitation of phospholipid hydroperoxide families using high-performance liquid chromatography with mercury cathode electrochemical detection

High-performance liquid chromatography with mercury cathode electrochemical detection (HPLC-EC(Hg)) was used to separate and quantify various phospholipid hydroperoxide (PLOOH) families. Under the conditions used, baseline separation of four major biologically relevant PLOOH classes was achieved. Responsiveness was linear up to at least 1 nmol of PLOOH with a detection limit in the subpicomolar range (0.1-0.5 pmol). Applying this method to photodynamically stressed murine leukemia cells and mitochondria isolated from these cells, we identified and quantified PLOOHs derived from phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and cardiolipin. In terms of high sensitivity, specificity, and reliability, HPLC-EC(Hg) has a clear advantage over all other existing techniques for determining PLOOHs in complex biological systems.