Ischemic preconditioning inhibits mitochondrial respiration, increases H2O2 release, and enhances K+ transport

The Ambivalence of Connexin43 Gap Peptides in Cardioprotection of the Isolated Heart against Ischemic Injury

The present study investigates infarct-reducing effects of blocking ischemia-induced opening of connexin43 hemichannels using peptides Gap19, Gap26 or Gap27. Cardioprotection by ischemic preconditioning (IPC) and Gap peptides was compared, and combined treatment was tested in isolated, perfused male rat hearts using function and infarct size after global ischemia, high-resolution respirometry of isolated mitochondrial and peptide binding kinetics as endpoints. The Gap peptides reduced infarct size significantly when given prior to ischemia plus at reperfusion (Gap19 76.2 ± 2.7, Gap26 72.9 ± 5.8 and Gap27 71.9 ± 5.8% of untreated control infarcts, mean ± SEM). Cardioprotection was lost when Gap26, but not Gap27 or Gap19, was combined with triggering IPC (IPC 73.4 ± 5.5, Gap19-IPC 60.9 ± 5.1, Gap26-IPC 109.6 ± 7.8, Gap27-IPC 56.3 ± 8.0% of untreated control infarct). Binding stability of peptide Gap26 to its specific extracellular loop sequence (EL2) of connexin43 was stronger than Gap27 to its corresponding loop EL1 (dissociation rate constant K_d 0.061 ± 0.004 vs. 0.0043 ± 0.0001 s^-1, mean ± SD). Mitochondria from IPC hearts showed slightly but significantly reduced respiratory control ratio (RCR). In vitro addition of Gap peptides did not significantly alter respiration. If transient hemichannel activity is part of the IPC triggering event, inhibition of IPC triggering stimuli might limit the use of cardioprotective Gap peptides.

Newly-synthesized iron-oxide nanoparticles showed synergetic effect with citric acid for alleviating arsenic phytotoxicity in soybean

In the current investigation, we presented the success of the modified hydrothermal method for synthesizing the iron-oxide nanoparticles (Fe2O3-NPs) efficiently. These NPs were further characterized by using different techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM) micrographs, energy-dispersive X-ray spectroscopy (EDAX)/Mapping pattern, Raman Spectroscopy Pattern, ultra violet (UV) and Photoluminescence (PL). All these analyses revealed highly pure nature of Fe2O3-NPs with no internal defects, and suggested its application for plant growth improvement. Therefore, we further investigated the separate as well as combined effects of the Fe2O3-NPs and citric acid (CA) in the alleviation of arsenic (As) toxicity in the soybean (Glycine max L.), by evaluating the different plant growth and metabolic attributes. Results of our study revealed that As-induced growth inhibition, reduction of photosynthesis, water use efficiency (WUE), and reactive oxygen species (ROS) accumulation whereas application of the Fe2O3-NPs and CA significantly reversed all these adverse effects in soybean plants. Moreover, the As-stress induced malondialdehyde (MDA) and hydrogen peroxide (H2O2) production were partially reversed by the Fe2O3-NPs and CA in the As-stressed plants by 16% and 10% (MDA) and 29% and 12% (H2O2). This might have resulted due to the Fe2O3-NPs and CA induced activities of the antioxidant defense in plants. Overall, the Fe2O3-NPs and CA supplementation separately and in combination positively regulated the As tolerance in soybean; however, the effect of the combined application on the As tolerance was more profound relative to the individual application. These results suggested the synergetic effect of the Fe2O3-NPs and CA on the As-tolerance in soybean. However, in-depth mechanism underlying the defense crosstalk between the Fe2O3-NPs and CA needs to be further explored.

Mitochondrial K+ Transport: Modulation and Functional Consequences

The existence of a K⁺ cycle in mitochondria has been predicted since the development of the chemiosmotic theory and has been shown to be crucial for several cellular phenomena, including regulation of mitochondrial volume and redox state. One of the pathways known to participate in K⁺ cycling is the ATP-sensitive K⁺ channel, MitoK_ATP. This channel was vastly studied for promoting protection against ischemia reperfusion when pharmacologically activated, although its molecular identity remained unknown for decades. The recent molecular characterization of MitoK_ATP has opened new possibilities for modulation of this channel as a mechanism to control cellular processes. Here, we discuss different strategies to control MitoK_ATP activity and consider how these could be used as tools to regulate metabolism and cellular events.

Non-invasive in vivo human model of post-ischaemic skin preconditioning by measurement of flow-mediated 460-nm autofluorescence

AIMS

Transient ischaemia and reperfusion (TIAR) induce early ischaemic preconditioning (IPC) in different tissues and organs, including the skin. IPC protects tissues by modifying the mitochondrial function and decreasing the amount of the reduced form of nicotinamide adenine dinucleotide (NADH). Skin 460-nm autofluorescence is proportional to the NADH content and can be non-invasively measured during TIAR. We propose a non-invasive in vivo human model of skin IPC for studying the effects of repeated TIARs on the NADH content.

METHODS

Fifty-one apparently healthy volunteers (36 women) underwent three 100-second forearm ischaemia episodes induced by inflation of brachial pressure cuff to the pressure of 60 mmHg above systolic blood pressure, followed by 500-second long reperfusion episodes. Changes in skin NADH content were measured using 460-nm fluorescence before and during each of the three TIARs.

RESULTS

The first two TIARs caused a significant reduction in the skin NADH content before (P = .0065) and during the third ischaemia (P = .0011) and reperfusion (P = .0003) up to 3.0%. During the third TIAR, the increase in skin NADH was 20% lower than during the first ischaemia (P = .0474).

CONCLUSIONS

The measurement of the 460-nm fluorescence during repeated TIARs allows for a non-invasive in vivo investigation of human skin IPC. Although IPC reduces the overall NADH skin content, the most noticeable NADH reduction appears during ischaemia after earlier TIARs. Studying the skin model of IPC may provide new avenues for in vivo physiological, clinical and pharmacological research on mitochondrial metabolism.

Stress response protein GJA1-20k promotes mitochondrial biogenesis, metabolic quiescence, and cardioprotection against ischemia/reperfusion injury

Connexin 43 (Cx43), a product of the GJA1 gene, is a gap junction protein facilitating intercellular communication between cardiomyocytes. Cx43 protects the heart from ischemic injury by mechanisms that are not well understood. GJA1 mRNA can undergo alternative translation, generating smaller isoforms in the heart, with GJA1-20k being the most abundant. Here, we report that ischemic and ischemia/reperfusion (I/R) injuries upregulate endogenous GJA1-20k protein in the heart, which targets to cardiac mitochondria and associates with the outer mitochondrial membrane. Exploring the functional consequence of increased GJA1-20k, we found that AAV9-mediated gene transfer of GJA1-20k in mouse hearts increases mitochondrial biogenesis while reducing mitochondrial membrane potential, respiration, and ROS production. By doing so, GJA1-20k promotes a protective mitochondrial phenotype, as seen with ischemic preconditioning (IPC), which also increases endogenous GJA1-20k in heart lysates and mitochondrial fractions. As a result, AAV9-GJA1-20k pretreatment reduces myocardial infarct size in mouse hearts subjected to in vivo ischemic injury or ex vivo I/R injury, similar to an IPC-induced cardioprotective effect. In conclusion, GJA1-20k is an endogenous stress response protein that induces mitochondrial biogenesis and metabolic hibernation, preconditioning the heart against I/R insults. Introduction of exogenous GJA1-20k is a putative therapeutic strategy for patients undergoing anticipated ischemic injury.

H2O2 Signaling-Triggered PI3K Mediates Mitochondrial Protection to Participate in Early Cardioprotection by Exercise Preconditioning

Previous studies have shown that early exercise preconditioning (EEP) imparts a protective effect on acute cardiovascular stress. However, how mitophagy participates in exercise preconditioning- (EP-) induced cardioprotection remains unclear. EEP may involve mitochondrial protection, which presumably crosstalks with predominant H₂O₂ oxidative stress. Our EEP protocol involves four periods of 10 min running with 10 min recovery intervals. We added a period of exhaustive running and a pretreatment using phosphoinositide 3-kinase (PI3K)/autophagy inhibitor wortmannin to test this protective effect. By using transmission electron microscopy (TEM), laser scanning confocal microscopy, and other molecular biotechnology methods, we detected related markers and specifically analyzed the relationship between mitophagic proteins and mitochondrial translocation. We determined that exhaustive exercise associated with various elevated injuries targeted the myocardium, oxidative stress, hypoxia-ischemia, and mitochondrial ultrastructure. However, exhaustion induced limited mitochondrial protection through a H₂O₂-independent manner to inhibit voltage-dependent anion channel isoform 1 (VDAC1) instead of mitophagy. EEP was apparently safe to the heart. In EEP-induced cardioprotection, EEP provided suppression to exhaustive exercise (EE) injuries by translocating Bnip3 to the mitochondria by recruiting the autophagosome protein LC3 to induce mitophagy, which is potentially triggered by H₂O₂ and influenced by Beclin1-dependent autophagy. Pretreatment with the wortmannin further attenuated these effects induced by EEP and resulted in the expression of proapoptotic phenotypes such as oxidative injury, elevated Beclin1/Bcl-2 ratio, cytochrome c leakage, mitochondrial dynamin-1-like protein (Drp-1) expression, and VDAC1 dephosphorylation. These observations suggest that H₂O₂ generation regulates mitochondrial protection in EEP-induced cardioprotection.

The Slo(w) path to identifying the mitochondrial channels responsible for ischemic protection

Mitochondria play an important role in tissue ischemia and reperfusion (IR) injury, with energetic failure and the opening of the mitochondrial permeability transition pore being the major causes of IR-induced cell death. Thus, mitochondria are an appropriate focus for strategies to protect against IR injury. Two widely studied paradigms of IR protection, particularly in the field of cardiac IR, are ischemic preconditioning (IPC) and volatile anesthetic preconditioning (APC). While the molecular mechanisms recruited by these protective paradigms are not fully elucidated, a commonality is the involvement of mitochondrial K⁺ channel opening. In the case of IPC, research has focused on a mitochondrial ATP-sensitive K⁺ channel (mitoK_ATP), but, despite recent progress, the molecular identity of this channel remains a subject of contention. In the case of APC, early research suggested the existence of a mitochondrial large-conductance K⁺ (BK, big conductance of potassium) channel encoded by the Kcnma1 gene, although more recent work has shown that the channel that underlies APC is in fact encoded by Kcnt2 In this review, we discuss both the pharmacologic and genetic evidence for the existence and identity of mitochondrial K⁺ channels, and the role of these channels both in IR protection and in regulating normal mitochondrial function.

Cardioprotection induced by remote ischemic preconditioning preserves the mitochondrial respiratory function in acute diabetic myocardium

A 2×2 factorial design was used to evaluate possible preservation of mitochondrial functions in two cardioprotective experimental models, remote ischemic preconditioning and streptozotocin-induced diabetes mellitus, and their interaction during ischemia/reperfusion injury (I/R) of the heart. Male Wistar rats were randomly allocated into four groups: control (C), streptozotocin-induced diabetic (DM), preconditioned (RPC) and preconditioned streptozotocin-induced diabetic (DM+RPC). RPC was conducted by 3 cycles of 5-min hind-limb ischemia and 5-min reperfusion. DM was induced by a single dose of 65 mg/kg streptozotocin. Isolated hearts were exposed to ischemia/reperfusion test according to Langendorff. Thereafter mitochondria were isolated and the mitochondrial respiration was measured. Additionally, the ATP synthase activity measurements on the same preparations were done. Animals of all groups subjected to I/R exhibited a decreased state 3 respiration with the least change noted in DM+RPC group associated with no significant changes in state 2 respiration. In RPC, DM and DM+RPC group, no significant changes in the activity of ATP synthase were observed after I/R injury. These results suggest that the endogenous protective mechanisms of RPC and DM do preserve the mitochondrial function in heart when they act in combination.

The effect of atp-dependent potassium uptake on mitochondrial functions under acute hypoxia

The opening of mitochondrial K(+) АТР-channel (mtK(+) АТР-channel) is supposed to be important in the modulation of mitochondrial functions under hypoxia, but the underlying mechanisms have not been clarified yet. The aim of this work was to study the effect of acute hypoxia on mtK(+) АТР-channel activity and to estimate the contribution of the channel in the modulation of mitochondrial functions. MtK(+) АТР-channel activity was assessed polarographically from the rate of State 4 respiration and by potentiometric monitoring of potassium efflux from deenergized mitochondria. It was shown that hypoxia reliably increased mtK(+) АТР-channel activity, which resulted in the changes of respiration rates (increase of State 4 and suppression of State 3 respiration), uncoupling (the decrease of respiratory control ratio) and suppression of phosphorylation. These effects were well mimicked by mtK(+) АТР-channel opener diazoxide (DZ) in isolated rat liver mitochondria. MtK(+) АТР-channel opening in vitro suppressed phosphorylation too, but increased phosphorylation efficiency, while mtK(+) АТР-channel blockers reduced it dramatically. The correlation was established between mtK(+) АТР-channel activity and the endurance of the rats to physical training under hypoxia. Hypoxia improved physical endurance, but treatment by mtK(+) АТР-channel blockers glibenklamide and 5-hydroxydecanoate (5-HD) prior to hypoxia strongly reduced both the channel activity and the endurance limits. This was in accord with the observation that under glibenklamide and 5-HD administration hypoxia failed to restore mtK(+) АТР-channel activity. Based on the experiments, we came to the conclusion that mtK(+) АТР-channel opening played a decisive role in the regulation of energy metabolism under acute hypoxia via the modulation of phosphorylation system in mitochondria.

Mitochondria-controlled signaling mechanisms of brain protection in hypoxia

The article is focused on the role of the cell bioenergetic apparatus, mitochondria, involved in development of immediate and delayed molecular mechanisms for adaptation to hypoxic stress in brain cortex. Hypoxia induces reprogramming of respiratory chain function and switching from oxidation of NAD-related substrates (complex I) to succinate oxidation (complex II). Transient, reversible, compensatory activation of respiratory chain complex II is a major mechanism of immediate adaptation to hypoxia necessary for (1) succinate-related energy synthesis in the conditions of oxygen deficiency and formation of urgent resistance in the body; (2) succinate-related stabilization of HIF-1α and initiation of its transcriptional activity related with formation of long-term adaptation; (3) succinate-related activation of the succinate-specific receptor, GPR91. This mechanism participates in at least four critical regulatory functions: (1) sensor function related with changes in kinetic properties of complex I and complex II in response to a gradual decrease in ambient oxygen concentration; this function is designed for selection of the most efficient pathway for energy substrate oxidation in hypoxia; (2) compensatory function focused on formation of immediate adaptive responses to hypoxia and hypoxic resistance of the body; (3) transcriptional function focused on activated synthesis of HIF-1 and the genes providing long-term adaptation to low pO2; (4) receptor function, which reflects participation of mitochondria in the intercellular signaling system via the succinate-dependent receptor, GPR91. In all cases, the desired result is achieved by activation of the succinate-dependent oxidation pathway, which allows considering succinate as a signaling molecule. Patterns of mitochondria-controlled activation of GPR-91- and HIF-1-dependent reaction were considered, and a possibility of their participation in cellular-intercellular-systemic interactions in hypoxia and adaptation was proved.

Combined exposure to hypercapnia and hypoxia provides its maximum neuroprotective effect during focal ischemic injury in the brain

BACKGROUND

In the present research, we compared the neuroprotective efficiency of combined and isolated exposure to hypoxia and hypercapnia preceding focal cerebral ischemic injury in rats. The study was conducted to verify the hypothesis of a possible increase in normobaric hypoxia (NbH; 90 mm Hg) efficiency when combined with permissive hypercapnia (PH; 50 mm Hg).

METHODS

The rats from the test groups were subjected to a 15-fold exposure to NbH (90 mm Hg) and/or PH (50 mm Hg). After the 15th exposure, cerebral ischemic injury was induced by photochemical thrombosis. Seventy-two hours later, neurologic deficit was determined on the Neurological Severity Score scale and by the rotarod test, and the volume of cerebral infarction was measured after focal photochemical thrombosis.

RESULTS

The neurologic deficit decreased most efficiently in rats that underwent PH and hypercapnic hypoxia (HH) exposure, whereas NbH had no impact on the neurologic status of the animals. On the contrary, motor coordination disturbances were minimal during exposure to hypoxia and HH. All respiratory interventions reduced the cerebral ischemic infarction volume in rats. The smallest infarction volumes were registered in the area of photochemical thrombosis in rats from the hypercapnic-hypoxic impact group, whereas exposure to NbH or PH did not show any cross difference.

CONCLUSIONS

The impact of PH has greater neuroprotective potential compared with NbH. Thus, we can assume that hypercapnia is a predominant factor in providing neuroprotection in combination with hypoxia.

Manganese superoxide dismutase in breast cancer: from molecular mechanisms of gene regulation to biological and clinical significance

Breast cancer is one of the most common malignancies of all cancers in women worldwide. Many difficulties reside in the prediction of tumor metastatic progression because of the lack of sufficiently reliable predictive biological markers, and this is a permanent preoccupation for clinicians. Manganese superoxide dismutase (MnSOD) may represent a rational candidate as a predictive biomarker of breast tumor metastatic progression, because its gene expression is profoundly altered between early and advanced breast cancer, in contrast to expression in the normal mammary gland. In this review, we report the characterization of some gene polymorphisms and molecular mechanisms of SOD2 gene regulation, which allows a better understanding of how MnSOD is decreased in early breast cancer and increased in advanced breast cancer. Several studies display the biological significance of MnSOD level in proliferation as well as in invasive and angiogenic abilities of breast tumor cells by controlling superoxide anion radical (O2(•-)) and hydrogen peroxide (H2O2). Particularly, they report how these reactive oxygen species may activate some signaling pathways involved in breast tumor growth. Emerging understanding of these findings provides an interesting framework for guiding translational research and suggests a way to define precisely the clinical interest of MnSOD as a prognostic and/or predicting marker in breast cancer, by associating with some regulators involved in SOD2 gene regulation and other well-known biomarkers, in addition to the typical clinical parameters.

Intermittent hypoxia protects cerebral mitochondrial function from calcium overload

Hypoxia leads to Ca(2+) overload and results in mitochondrial uncoupling, decreased ATP synthesis, and neuronal death. Inhibition of mitochondrial Ca(2+) overload protects mitochondrial function after hypoxia. The present study was aimed to investigate the effect of intermittent hypoxia on mitochondrial function and mitochondrial tolerance to Ca(2+) overload. Wistar rats were divided into control and intermittent hypoxia (IH) groups. The IH group was subject to hypoxia for 4 h daily in a hypobaric cabin (5,000 m) for 7 days. Brain mitochondria were isolated on day 7 following hypoxia. The baseline mitochondrial functions, such as ST3, ST4, and respiratory control ratio (RCR = ST3/ST4), were measured using a Clark-type oxygen electrode. Mitochondrial adenine nucleotide concentrations were measured by HPLC. Mitochondrial membrane potential was determined by measuring rhodamine 123 (Rh-123) fluorescence in the absence and presence of high Ca(2+) concentration (0.1 M), which simulates Ca(2+) overload. Our results revealed that IH did not affect mitochondrial respiratory functions, but led to a reduction in AMP and an increase in ADP concentrations in mitochondria. Both control and IH groups demonstrated decreased mitochondrial membrane potential in the presence of high Ca(2+) (0.1 M), while the IH group showed a relative higher mitochondrial membrane potential. These results indicated that the neuroprotective effect of intermittent hypoxia was resulted partly from preserving mitochondrial membrane potential, and increasing mitochondrial tolerance to high calcium levels. The increased ADP and decreased AMP in mitochondria following intermittent hypoxia may be a mechanism underlying this protection.

The extent of neurodegeneration and neuroprotection in two chemical in vitro models related to Parkinson's disease is critically dependent on cell culture conditions

The proteasome inhibition and mitochondrial dysfunction are involved in pathomechanism of Parkinson's disease. The main aim of this study was to assess how particular culture conditions of human dopaminergic neuroblastoma SH-SY5Y cells could affect the extent of neurodegeneration induced by proteasome inhibitor-lactacystin (LC) and mitochondrial toxin-rotenone (Rot). This study revealed that induction of neuronal differentiation of SH-SY5Y cells with retinoic acid (RA-SH-SY5Y) caused a higher resistance of these cells to LC-evoked cell death when compared to undifferentiated cells (UN-SH-SY5Y). In contrast, RA-SH-SY5Y cells were more vulnerable than the UN-SH-SY5Y to Rot-induced cell damage. Furthermore, we found that a prolonged incubation of the cells under low serum condition (PLSC) significantly increased the LC toxicity in both differentiated and undifferentiated cells. Next, the effects of combined treatment with LC and Rot on cell viability were studied in RA-SH-SY5Y cells under PLSC and normal low serum condition (NLSC). At a low concentration, Rot (0.001-1 μM) attenuated the LC-evoked cell death in RA-SH-SY5Y cells exposed to NLSC. In contrast, under PLSC low concentrations of Rot lacked neuroprotective action while its higher levels (10 μM) enhanced the LC toxicity. Further, we showed that low concentrations of celastrol (Cel; 0.001 μM), a putative neuroprotective agent with antioxidant and anti-inflammatory properties, were able to partially attenuate the Rot-evoked toxicity under both PLSC and NLSC. On the other hand, Cel (0.001 and 0.01 μM) attenuated the LC-induced cell damage only under PLSC. Interestingly, higher concentrations of Cel (>1 μM) reduced cell viability in both UN- and RA-SH-SY5Y but only in UN-SH-SY5Y cells the effect was enhanced under PLSC. The obtained data indicate that toxicity of LC and Rot in SH-SY5Y cell line depends on the stage of cell differentiation and is enhanced in cells cultured for a longer time in low serum medium. Moreover, the neuroprotective properties of Rot and Cel against the LC-induced cell damage can be observed only under particular low serum conditions.

Physiological consequences of complex II inhibition for aging, disease, and the mKATP channel

In recent years, it has become apparent that there exist several roles for respiratory complex II beyond metabolism. These include: (i) succinate signaling, (ii) reactive oxygen species (ROS) generation, (iii) ischemic preconditioning, (iv) various disease states and aging, and (v) a role in the function of the mitochondrial ATP-sensitive K(+) (mKATP) channel. This review will address the involvement of complex II in each of these areas, with a focus on how complex II regulates or may be involved in the assembly of the mKATP. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Impact of O-GlcNAc on cardioprotection by remote ischaemic preconditioning in non-diabetic and diabetic patients

AIMS

Post-translational modification of proteins by O-linked β-N-acetylglucosamine (O-GlcNAc) is cardioprotective but its role in cardioprotection by remote ischaemic preconditioning (rIPC) and the reduced efficacy of rIPC in type 2 diabetes mellitus is unknown. In this study we achieved mechanistic insight into the remote stimulus mediating and the target organ response eliciting the cardioprotective effect by rIPC in non-diabetic and diabetic myocardium and the influence of O-GlcNAcylation.

METHODS AND RESULTS

The cardioprotective capacity and the influence on myocardial O-GlcNAc levels of plasma dialysate from eight healthy volunteers and eight type 2 diabetic patients drawn before and after subjection to an rIPC stimulus were tested on human isolated atrial trabeculae subjected to ischaemia/reperfusion injury. Dialysate from healthy volunteers exposed to rIPC improved post-ischaemic haemodynamic recovery (40 ± 6 vs. 16 ± 2%; P < 0.01) and increased myocardial O-GlcNAc levels. Similar observations were made with dialysate from diabetic patients before exposure to rIPC (43 ± 3 vs. 16 ± 2%; P < 0.001) but no additional cardioprotection or further increase in O-GlcNAc levels was achieved by perfusion with dialysate after exposure to rIPC (44 ± 4 and 42 ± 5 vs. 43 ± 3%; P = 0.7). The glutamine:fructose-6-phosphate amidotransferase (GFAT) inhibitor azaserine abolished the cardioprotective effects and the increment in myocardial O-GlcNAc levels afforded by plasma from diabetic patients and healthy volunteers treated with rIPC.

CONCLUSIONS

rIPC and diabetes mellitus per se influence myocardial O-GlcNAc levels through circulating humoral factors. O-GlcNAc signalling participates in mediating rIPC-induced cardioprotection and maintaining a state of inherent chronic activation of cardioprotection in diabetic myocardium, restricting it from further protection by rIPC.

Effect of hypoxic preconditioning on free radical processes in tissues of rats with different resistance to hypoxia

We studied the effect of single hypoxic preconditioning exposure (hypobaric hypoxia, 5000 m, 60 min) on free radical processes, glutathione system, and antioxidant defense enzymes in tissues of rats with different resistance to acute hypoxia. The intensity of free radical processes was shown to increase or decrease on day 1 after hypoxic preconditioning. These changes were tissue-specific and opposite in animals with genetically determined differences in the resistance to hypoxia. Hypoxic preconditioning contributes to the immediate resistance. The effect was more pronounced in low resistant animals, who did not exhibit signs of oxidative stress in tissues during the early posthypoxic period. By contrast, hypoxic preconditioning was followed by activation of free radical processes in tissues of highly resistant animals. These rats were characterized by low ability for the development of immediate resistance. Activation of free radical processes in the early period of adaptation (first hours after hypoxic preconditioning) does not play a role in the induction of immediate adaptive mechanisms for hypoxia.

Complex I and ATP synthase mediate membrane depolarization and matrix acidification by isoflurane in mitochondria

Short application of the volatile anesthetic isoflurane at reperfusion after ischemia exerts strong protection of the heart against injury. Mild depolarization and acidification of the mitochondrial matrix are involved in the protective mechanisms of isoflurane, but the molecular basis for these changes is not clear. In this study, mitochondrial respiration, membrane potential, matrix pH, matrix swelling, ATP synthesis and -hydrolysis, and H(2)O(2) release were assessed in isolated mitochondria. We hypothesized that isoflurane induces mitochondrial depolarization and matrix acidification through direct action on both complex I and ATP synthase. With complex I-linked substrates, isoflurane (0.5mM) inhibited mitochondrial respiration by 28 ± 10%, and slightly, but significantly depolarized membrane potential and decreased matrix pH. With complex II- and complex IV-linked substrates, respiration was not changed, but isoflurane still decreased matrix pH and depolarized mitochondrial membrane potential. Depolarization and matrix acidification were attenuated by inhibition of ATP synthase with oligomycin, but not by inhibition of mitochondrial ATP- and Ca(2+)-sensitive K(+) channels or uncoupling proteins. Isoflurane did not induce matrix swelling and did not affect ATP synthesis and hydrolysis, but decreased H(2)O(2) release in the presence of succinate in an oligomycin- and matrix pH-sensitive manner. Isoflurane modulated H(+) flux through ATP synthase in an oligomycin-sensitive manner. Our results indicate that isoflurane-induced mitochondrial depolarization and acidification occur due to inhibition of the electron transport chain at the site of complex I and increased proton flux through ATP synthase. K(+) channels and uncoupling proteins appear not to be involved in the direct effects of isoflurane on mitochondria.

Mitochondrial therapeutics for cardioprotection

Mitochondria represent approximately one-third of the mass of the heart and play a critical role in maintaining cellular function-however, they are also a potent source of free radicals and pro-apoptotic factors. As such, maintaining mitochondrial homeostasis is essential to cell survival. As the dominant source of ATP, continuous quality control is mandatory to ensure their ongoing optimal function. Mitochondrial quality control is accomplished by the dynamic interplay of fusion, fission, autophagy, and mitochondrial biogenesis. This review examines these processes in the heart and considers their role in the context of ischemia-reperfusion injury. Interventions that modulate mitochondrial turnover, including pharmacologic agents, exercise, and caloric restriction are discussed as a means to improve mitochondrial quality control, ameliorate cardiovascular dysfunction, and enhance longevity.

Mitochondrial metabolic suppression and reactive oxygen species production in liver and skeletal muscle of hibernating thirteen-lined ground squirrels

During hibernation, animals cycle between periods of torpor, during which body temperature (Tb) and metabolic rate (MR) are suppressed for days, and interbout euthermia (IBE), during which Tb and MR return to resting levels for several hours. In this study, we measured respiration rates, membrane potentials, and reactive oxygen species (ROS) production of liver and skeletal muscle mitochondria isolated from ground squirrels ( Ictidomys tridecemlineatus ) during torpor and IBE to determine how mitochondrial metabolism is suppressed during torpor and how this suppression affects oxidative stress. In liver and skeletal muscle, state 3 respiration measured at 37°C with succinate was 70% and 30% lower, respectively, during torpor. In liver, this suppression was achieved largely via inhibition of substrate oxidation, likely at succinate dehydrogenase. In both tissues, respiration by torpid mitochondria further declined up to 88% when mitochondria were cooled to 10°C, close to torpid Tb. In liver, this passive thermal effect on respiration rate reflected reduced activity of all components of oxidative phosphorylation (substrate oxidation, phosphorylation, and proton leak). With glutamate + malate and succinate, mitochondrial free radical leak (FRL; proportion of electrons leading to ROS production) was higher in torpor than IBE, but only in liver. With succinate, higher FRL likely resulted from increased reduction state of complex III during torpor. With glutamate + malate, higher FRL resulted from active suppression of complex I ROS production during IBE, which may limit ROS production during arousal. In both tissues, ROS production and FRL declined with temperature, suggesting ROS production is also reduced during torpor by passive thermal effects.

α-Ketoglutarate dehydrogenase: a mitochondrial redox sensor

α-Ketoglutarate dehydrogenase (KGDH), a key regulatory enzyme within the Krebs cycle, is sensitive to mitochondrial redox status. Treatment of mitochondria with H₂O₂ results in reversible inhibition of KGDH due to glutathionylation of the cofactor, lipoic acid. Upon consumption of H₂O₂, glutathione is removed by glutaredoxin restoring KGDH activity. Glutathionylation appears to be enzymatically catalysed or require a unique microenvironment. This may represent an antioxidant response, diminishing the flow of electrons to the respiratory chain and protecting sulphydryl residues from oxidative damage. KGDH is, however, also susceptible to oxidative damage. 4-Hydroxy-2-nonenal (HNE), a lipid peroxidation product, reacts with lipoic acid resulting in enzyme inactivation. Evidence indicates that HNE modified lipoic acid is cleaved from KGDH, potentially the first step of a repair process. KGDH is therefore a likely redox sensor, reversibly altering metabolism to reduce oxidative damage and, under severe oxidative stress, acting as a sentinel of mitochondrial viability.

Potential therapeutic benefits of strategies directed to mitochondria

The mitochondrion is the most important organelle in determining continued cell survival and cell death. Mitochondrial dysfunction leads to many human maladies, including cardiovascular diseases, neurodegenerative disease, and cancer. These mitochondria-related pathologies range from early infancy to senescence. The central premise of this review is that if mitochondrial abnormalities contribute to the pathological state, alleviating the mitochondrial dysfunction would contribute to attenuating the severity or progression of the disease. Therefore, this review will examine the role of mitochondria in the etiology and progression of several diseases and explore potential therapeutic benefits of targeting mitochondria in mitigating the disease processes. Indeed, recent advances in mitochondrial biology have led to selective targeting of drugs designed to modulate and manipulate mitochondrial function and genomics for therapeutic benefit. These approaches to treat mitochondrial dysfunction rationally could lead to selective protection of cells in different tissues and various disease states. However, most of these approaches are in their infancy.

Regulated production of free radicals by the mitochondrial electron transport chain: Cardiac ischemic preconditioning

Excessive production of free radicals by mitochondria is associated with, and likely contributes to, the progression of numerous pathological conditions. Nevertheless, the production of free radicals by the mitochondria may have important biological functions under normal or stressed conditions by activating or modulating redox-sensitive cellular signaling pathways. This raises the intriguing possibility that regulated mitochondrial free radical production occurs via mechanisms that are distinct from pathologies associated with oxidative damage. Indeed, the capacity of mitochondria to produce free radicals in a limited manner may play a role in ischemic preconditioning, the phenomenon whereby short bouts of ischemia protect from subsequent prolonged ischemia and reperfusion. Ischemic preconditioning can thus serve as an important model system for defining regulatory mechanisms that allow for transient, signal-inducing, production of free radicals by mitochondria. Defining how these mechanism(s) occur will provide insight into therapeutic approaches that minimize oxidative damage without altering normal cellular redox biology. The aim of this review is to present and discuss evidence for the regulated production of superoxide by the electron transport chain within the ischemic preconditioning paradigm of redox regulation.

Mitochondria and reactive oxygen species

Mitochondria are a quantitatively relevant source of reactive oxygen species (ROS) in the majority of cell types. Here we review the sources and metabolism of ROS in this organelle, including the conditions that regulate the production of these species, such as mild uncoupling, oxygen tension, respiratory inhibition, Ca2+ and K+ transport, and mitochondrial content and morphology. We discuss substrate-, tissue-, and organism-specific characteristics of mitochondrial oxidant generation. Several aspects of the physiological and pathological roles of mitochondrial ROS production are also addressed.

Mitochondrial reactive oxygen species production in excitable cells: modulators of mitochondrial and cell function

The mitochondrion is a major source of reactive oxygen species (ROS). Superoxide (O(2)(*-)) is generated under specific bioenergetic conditions at several sites within the electron-transport system; most is converted to H(2)O(2) inside and outside the mitochondrial matrix by superoxide dismutases. H(2)O(2) is a major chemical messenger that, in low amounts and with its products, physiologically modulates cell function. The redox state and ROS scavengers largely control the emission (generation scavenging) of O(2)(*-). Cell ischemia, hypoxia, or toxins can result in excess O(2)(*-) production when the redox state is altered and the ROS scavenger systems are overwhelmed. Too much H(2)O(2) can combine with Fe(2+) complexes to form reactive ferryl species (e.g., Fe(IV) = O(*)). In the presence of nitric oxide (NO(*)), O(2)(*-) forms the reactant peroxynitrite (ONOO(-)), and ONOOH-induced nitrosylation of proteins, DNA, and lipids can modify their structure and function. An initial increase in ROS can cause an even greater increase in ROS and allow excess mitochondrial Ca(2+) entry, both of which are factors that induce cell apoptosis and necrosis. Approaches to reduce excess O(2)(*-) emission include selectively boosting the antioxidant capacity, uncoupling of oxidative phosphorylation to reduce generation of O(2)(*-) by inducing proton leak, and reversibly inhibiting electron transport. Mitochondrial cation channels and exchangers function to maintain matrix homeostasis and likely play a role in modulating mitochondrial function, in part by regulating O(2)(*-) generation. Cell-signaling pathways induced physiologically by ROS include effects on thiol groups and disulfide linkages to modify posttranslationally protein structure to activate/inactivate specific kinase/phosphatase pathways. Hypoxia-inducible factors that stimulate a cascade of gene transcription may be mediated physiologically by ROS. Our knowledge of the role played by ROS and their scavenging systems in modulation of cell function and cell death has grown exponentially over the past few years, but we are still limited in how to apply this knowledge to develop its full therapeutic potential.

Mitochondrial superoxide production and respiratory activity: biphasic response to ischemic duration

Long bouts of ischemia are associated with electron transport chain deficits and increases in free radical production. In contrast, little is known regarding the effect of brief ischemia on mitochondrial function and free radical production. This study was undertaken to examine the relationship between the duration of ischemia, effects upon electron transport chain activities, and the mitochondrial production of free radicals. Rat hearts were subjected to increasing ischemic durations, mitochondria were isolated, and superoxide production and electron transport chain activities were measured. Results indicate that even brief ischemic durations induced a significant increase in superoxide production. This rate was maintained with ischemic durations less than 15 min, and then increased further with longer ischemic times. Mechanistically, brief ischemia was accompanied by an increase in NADH oxidase activity, reflected by a specific increase in complex IV activity. In contrast, longer ischemic durations were accompanied by a decrease in NADH oxidase activity, reflected by deficits in complexes I and IV activities.

Transient opening of mitochondrial permeability transition pore by reactive oxygen species protects myocardium from ischemia-reperfusion injury

Reactive oxygen species (ROS) production during ischemia-reperfusion (I/R) is thought to be a critical factor for myocardial injury. However, a small amount of ROS during the ischemic preconditioning (IPC) may provide a signal for cardioprotection. We have previously reported that the repetitive pretreatment of a small amount of ROS [hydrogen peroxide (H(2)O(2)), 2 microM] mimicked the IPC-induced cardioprotection in the Langendorff-perfused rat hearts. We further investigated the mechanisms of the ROS-induced cardioprotection against I/R injury and tested the hypothesis whether it could mediate the mitochondrial permeability transition pore (mPTP) opening. The Langendorff-perfused rat hearts were subjected to 35 min ischemia and 40 min reperfusion, and the pretreatment of H(2)O(2) (2 microM) significantly improved the postischemic recoveries in left ventricular developed pressure, intracellular phosphocreatine, and ATP levels. A specific mPTP inhibitor cyclosporin A (CsA; 0.2 microM) canceled these H(2)O(2)-induced effects. In isolated permeabilized myocytes, H(2)O(2) (1 microM) accelerated the calcein leakage from mitochondria in a CsA-sensitive manner, indicating the opening of mPTP by H(2)O(2). However, H(2)O(2) did not depolarize mitochondrial membrane potential (DeltaPsi(m)) even in the presence of oligomycin (F(1)/F(0) ATPase inhibitor; 1 microM) and decreased mitochondrial Ca(2+) concentration ([Ca(2+)](m)) by accelerating the mitochondrial Ca(2+) extrusion via an mPTP. We conclude that the transient mPTP opening could be involved in the H(2)O(2)-induced cardioprotection against reperfusion injury, and the reduction of [Ca(2+)](m) without the change in DeltaPsi(m) might be a possible mechanism for the protection.

Myocardial protection by nitrite: evidence that this reperfusion therapeutic will not be lost in translation

The circulating anion nitrite (NO(2)(-)), previously thought to be an inert product of nitric oxide (NO) oxidation, has now been identified as an important storage reservoir of bioavailable NO in the blood and tissues. Reduction of NO(2)(-) to NO over the physiologic pH and oxygen gradient by deoxyhemoglobin, myoglobin, xanthine oxidoreductase, and by nonenzymatic acidic disproportionation has been demonstrated to confer cytoprotection against ischemia-reperfusion injury in the heart, liver, brain, and kidney. Here, we review the mechanisms that have been established to regulate hypoxic NO(2)(-) reduction to NO, analyze the preclinical and clinical evidence supporting NO(2)(-)-mediated cytoprotection after ischemia-reperfusion injury, and examine the therapeutic potential of NO(2)(-) for cardiovascular disease. Evidence is accumulating that suggests NO(2)(-) has surmounted many of the direct challenges to reperfusion therapeutics summarized by the National Heart, Lung, and Blood Institute Working Group in "Myocardial protection at a crossroads: the need for translation into clinical therapy." In this context, we discuss important considerations in designing human clinical trials to test the efficacy of NO(2)(-) in the setting of ischemia-reperfusion injury, with particular attention to the study of ST-segment elevation myocardial infarction.

The C. elegans mitochondrial K+(ATP) channel: a potential target for preconditioning

Ischemic preconditioning (IPC) is an evolutionarily conserved endogenous mechanism whereby short periods of non-lethal exposure to hypoxia alleviate damage caused by subsequent ischemia reperfusion (IR). Pharmacologic targeting has suggested that the mitochondrial ATP-sensitive potassium channel (mK(ATP)) is central to IPC signaling, despite its lack of molecular identity. Here, we report that isolated Caenorhabditis elegans mitochondria have a K(ATP) channel with the same physiologic and pharmacologic characteristics as the vertebrate channel. Since C. elegans also exhibit IPC, our observations provide a framework to study the role of mK(ATP) in IR injury in a genetic model organism.

The endogenous mitochondrial complex II inhibitor malonate regulates mitochondrial ATP-sensitive potassium channels: implications for ischemic preconditioning

Ischemic preconditioning (IPC) affords cardioprotection against ischemia-reperfusion (IR) injury, and while the molecular mechanisms of IPC are debated, the mitochondrial ATP-sensitive K(+) channel (mK(ATP)) has emerged as a candidate effector for several IPC signaling pathways. The molecular identity of this channel is unknown, but significant pharmacologic overlap exists between mK(ATP) and mitochondrial respiratory complex II (succinate dehydrogenase). In this investigation, we utilized isolated cardiac mitochondria, Langendorff perfused hearts, and a variety of biochemical methods, to make the following observations: (i) The competitive complex II inhibitor malonate is formed in mitochondria under conditions resembling IPC. (ii) IPC leads to a reversible inhibition of complex II that has likely been missed in previous investigations due to the use of saturating concentrations of succinate. (iii) Malonate opens mK(ATP) channels even when mitochondria are respiring on complex I-linked substrates, suggesting an effect of this inhibitor on the mK(ATP) channel independent of complex II inhibition. Together, these observations suggest that complex II inhibition by endogenously formed malonate may represent an important activation pathway for mK(ATP) channels during IPC.

Ischemic preconditioning enhances fatty acid-dependent mitochondrial uncoupling

This study tests the hypothesis that ischemic preconditioning (IP) changes fatty acid (FA)-dependent uncoupling between mitochondrial respiration and oxidative phosphorylation. We found that IP does not alter mitochondrial membrane integrity or FA levels, but enhances membrane potential decreases when FA are present, in an ATP-sensitive manner. FA hydroperoxides had equal effects in control and preconditioned mitochondria, and GTP did not abrogate the IP effect, suggesting uncoupling proteins were not involved. Conversely, thiol reductants and atractyloside, which inhibits the adenine nucleotide translocator, eliminated the differences in responses to FA. Together, our results suggest that IP leads to thiol oxidation and activation of the adenine nucleotide translocator, resulting in enhanced FA transport and mild mitochondrial uncoupling.

Nitrite augments tolerance to ischemia/reperfusion injury via the modulation of mitochondrial electron transfer

Nitrite (NO(2)(-)) is an intrinsic signaling molecule that is reduced to NO during ischemia and limits apoptosis and cytotoxicity at reperfusion in the mammalian heart, liver, and brain. Although the mechanism of nitrite-mediated cytoprotection is unknown, NO is a mediator of the ischemic preconditioning cell-survival program. Analogous to the temporally distinct acute and delayed ischemic preconditioning cytoprotective phenotypes, we report that both acute and delayed (24 h before ischemia) exposure to physiological concentrations of nitrite, given both systemically or orally, potently limits cardiac and hepatic reperfusion injury. This cytoprotection is associated with increases in mitochondrial oxidative phosphorylation. Remarkably, isolated mitochondria subjected to 30 min of anoxia followed by reoxygenation were directly protected by nitrite administered both in vitro during anoxia or in vivo 24 h before mitochondrial isolation. Mechanistically, nitrite dose-dependently modifies and inhibits complex I by posttranslational S-nitrosation; this dampens electron transfer and effectively reduces reperfusion reactive oxygen species generation and ameliorates oxidative inactivation of complexes II-IV and aconitase, thus preventing mitochondrial permeability transition pore opening and cytochrome c release. These data suggest that nitrite dynamically modulates mitochondrial resilience to reperfusion injury and may represent an effector of the cell-survival program of ischemic preconditioning and the Mediterranean diet.

Ischemic preconditioning does not protect via blockade of electron transport

Ischemic preconditioning (IPC) before sustained ischemia decreases myocardial infarct size mediated in part via protection of cardiac mitochondria. Reversible blockade of electron transport at complex I immediately before sustained ischemia also preserves mitochondrial respiration and decreases infarct size. We proposed that IPC would attenuate electron transport from complex I as a potential effector mechanism of cardioprotection. Isolated, Langendorff-perfused rat hearts underwent IPC (3 cycles of 5-min 37°C global ischemia and 5-min reperfusion) or were perfused for 40 min without ischemia as controls. Subsarcolemmal (SSM) and interfibrillar (IFM) populations of mitochondria were isolated. IPC did not decrease ADP-stimulated respiration measured in intact mitochondria using substrates that donate reducing equivalents to complex I. Maximally expressed complex I activity measured as rotenone-sensitive NADH:ubiquinone oxidoreductase in detergent-solubilized mitochondria was also unaffected by IPC. Thus the protection of IPC does not occur as a consequence of a partial decrease in complex I activity leading to a decrease in integrated respiration through complex I. IPC and blockade of electron transport both converge on mitochondria as effectors of cardioprotection; however, each modulates mitochondrial metabolism during ischemia by different mechanisms to achieve cardioprotection.

Ischemic preconditioning preserves proton leakage from mitochondrial membranes but not oxidative phosphorylation during heart reperfusion

The aim of this study was to evaluate the role of mitochondria in the recovery of cardiac energetics induced by ischaemic preconditioning at reperfusion. Isolated rat hearts were aerobically perfused (control), subjected to global ischaemia and reperfusion (reperfusion), or subjected to 3 brief cycles of ischaemia/reperfusion and then to the protocol of reperfusion (preconditioning). At the end of the perfusion, antimycin A was delivered to the heart for 25 min, to inhibit mitochondrial respiration and stimulate glycolysis. The increased amount of lactate released in the coronary effluent was correlated with the number of viable cells producing this end-product of glycolysis. Preconditioned hearts released 18% more lactate than reperfused hearts (p < 0.05). This result indicates that preconditioning partially preserved cell viability, as was also evidenced by the MTT assay performed on cardiac biopsies. The difference between antimycin A-stimulated and basal lactate concentration, representing the contribution of mitochondria to the overall energetics of cardiac tissue, was also 18% more elevated in the preconditioned hearts than in the reperfused hearts (p < 0.01). The study of the respiratory function of mitochondria isolated at the end of perfusion, showed that preconditioning did not improve the oxygen-dependent production of ATP (state 3 respiration, ADP/O). On the contrary, state 4 respiration, which is related to proton leakage, was 35.0% lower in the preconditioned group than reperfusion group (p < 0.05). Thus, preconditioning ameliorates cardiac energetics by preserving cell death, but without affecting mitochondrial oxidative phosphorylation. Mitochondria can contribute to cell survival by the attenuation of proton leak from inner membrane.

Diazoxide-induced respiratory inhibition - a putative mitochondrial K(ATP) channel independent mechanism of pharmacological preconditioning

The ischemic preconditioning biological phenomenon has been explored to identify putative pharmacologic agents to mimic this cytoprotective program against cellular ischemic injury. Diazoxide administration confers this cytoprotection, however, whether this is via direct activation of the putative mitochondrial K(ATP) (mK(ATP)) channel which was originally proposed has been questioned. Here, we present data supporting an alternate hypothesis evoking mitochondrial respiratory inhibition rather than mK(ATP) channel activation, as a mediating event in the diazoxide-activated cytoprotective program. Mitochondrial respiration and reactive oxygen species (ROS) production was measured in digitonin-permeabilized C2C12 myotubes, allowing for the modulation of mK(ATP) conductance by changing the potassium concentration of the medium (0-130 mM). Diazoxide dose-dependently attenuated succinate-supported respiration, an effect that was independent of mK(ATP) channel conductance. Similarly, 5-hydroxydecanoate (5-HD), a putative mK(ATP) channel blocker, released diazoxide-induced respiratory inhibition independently of potassium concentration. Since diazoxide-induced cytoprotection and respiratory inhibition are both integrally linked to ROS generation we repeated above experiments following ROS generation using DCF fluorescence. Cytoprotective doses of diazoxide increased ROS generation independently of potassium concentration and 5-HD inhibited ROS production under the same conditions. Collectively these data support the hypothesis that diazoxide-mediated cytoprotection is independent of the conductance of the mK(ATP) channel and rather implicate mitochondrial respiratory inhibition-triggered ROS signaling.

MitoK(ATP)-dependent changes in mitochondrial volume and in complex II activity during ischemic and pharmacological preconditioning of Langendorff-perfused rat heart

It has been proposed that activation of the mitochondrial ATP-sensitive potassium channel (mitoK(ATP)) is part of signaling pathways triggering the cardioprotection afforded by ischemic preconditioning of the heart. This work was to analyze the mitochondrial function profile of Langendorff-perfused rat hearts during the different phases of various ischemia-reperfusion protocols. Specifically, skinned fibers of ischemic preconditioned hearts exhibit a decline in the succinate-supported respiration and complex II activity during ischemia, followed by a recovery during reperfusion. Meanwhile, the apparent affinity of respiration for ADP (which reflects the matrix volume expansion) is increased during preconditioning stimulus and, to a larger extent, during prolonged ischemia. This evolution pattern is mimicked by diazoxide and abolished by 5-hydroxydecanoate. It is concluded that opening the mitoK(ATP) channel mediates the preservation of mitochondrial structure-function via a mitochondrial matrix shrinkage and a reversible inactivation of complex II during prolonged ischemic insult.

Modulation of electron transport protects cardiac mitochondria and decreases myocardial injury during ischemia and reperfusion

Mitochondria are increasingly recognized as lynchpins in the evolution of cardiac injury during ischemia and reperfusion. This review addresses the emerging concept that modulation of mitochondrial respiration during and immediately following an episode of ischemia can attenuate the extent of myocardial injury. The blockade of electron transport and the partial uncoupling of respiration are two mechanisms whereby manipulation of mitochondrial metabolism during ischemia decreases cardiac injury. Although protection by inhibition of electron transport or uncoupling of respiration initially appears to be counterintuitive, the continuation of mitochondrial oxidative phosphorylation in the pathological milieu of ischemia generates reactive oxygen species, mitochondrial calcium overload, and the release of cytochrome c. The initial target of these deleterious mitochondrial-driven processes is the mitochondria themselves. Consequences to the cardiomyocyte, in turn, include oxidative damage, the onset of mitochondrial permeability transition, and activation of apoptotic cascades, all favoring cardiomyocyte death. Ischemia-induced mitochondrial damage carried forward into reperfusion further amplifies these mechanisms of mitochondrial-driven myocyte injury. Interruption of mitochondrial respiration during early reperfusion by pharmacologic blockade of electron transport or even recurrent hypoxia or brief ischemia paradoxically decreases cardiac injury. It increasingly appears that the cardioprotective paradigms of ischemic preconditioning and postconditioning utilize modulation of mitochondrial oxidative metabolism as a key effector mechanism. The initially counterintuitive approach to inhibit mitochondrial respiration provides a new cardioprotective paradigm to decrease cellular injury during both ischemia and reperfusion.

Different mechanisms of mitochondrial proton leak in ischaemia/reperfusion injury and preconditioning: implications for pathology and cardioprotection

The mechanisms of mitochondrial proton (H+) leak under various pathophysiological conditions are poorly understood. In the present study it was hypothesized that different mechanisms underlie H+ leak in cardiac IR (ischaemia/reperfusion) injury and IPC (ischaemic preconditioning). Potential H(+) leak mechanisms examined were UCPs (uncoupling proteins), allosteric activation of the ANT (adenine nucleotide translocase) by AMP, or the PT (permeability transition) pore. Mitochondria isolated from perfused rat hearts that were subjected to IPC exhibited a greater H+ leak than did controls (202+/-27%, P<0.005), and this increased leakage was completely abolished by the UCP inhibitor, GDP, or the ANT inhibitor, CAT (carboxyattractyloside). Mitochondria from hearts subjected to IR injury exhibited a much greater amount of H+ leak than did controls (411+/-28%, P<0.001). The increased leakage after IR was weakly inhibited by GDP, but was inhibited, >50%, by carboxyattractyloside. In addition, it was inhibited by cardioprotective treatment strategies including pre-IR perfusion with the PT pore inhibitors cyclosporin A or sanglifehrin A, the adenylate kinase inhibitor, AP5A (diadenosine pentaphosphate), or IPC. Together these data suggest that the small increase in H+ leak in IPC is mediated by UCPs, while the large increase in H+ leak in IR is mediated by the ANT. Furthermore, under all conditions studied, in situ myocardial O2 efficiency was correlated with isolated mitochondrial H+ leak (r2=0.71). In conclusion, these data suggest that the modulation of H+ leak may have important implications for the outcome of IR injury.

A biophysical model of the mitochondrial respiratory system and oxidative phosphorylation

A computational model for the mitochondrial respiratory chain that appropriately balances mass, charge, and free energy transduction is introduced and analyzed based on a previously published set of data measured on isolated cardiac mitochondria. The basic components included in the model are the reactions at complexes I, III, and IV of the electron transport system, ATP synthesis at F₁F₀ ATPase, substrate transporters including adenine nucleotide translocase and the phosphate–hydrogen co-transporter, and cation fluxes across the inner membrane including fluxes through the K⁺/H⁺ antiporter and passive H⁺ and K⁺ permeation. Estimation of 16 adjustable parameter values is based on fitting model simulations to nine independent data curves. The identified model is further validated by comparison to additional datasets measured from mitochondria isolated from rat heart and liver and observed at low oxygen concentration. To obtain reasonable fits to the available data, it is necessary to incorporate inorganic-phosphate-dependent activation of the dehydrogenase activity and the electron transport system. Specifically, it is shown that a model incorporating phosphate-dependent activation of complex III is able to reasonably reproduce the observed data. The resulting validated and verified model provides a foundation for building larger and more complex systems models and investigating complex physiological and pathophysiological interactions in cardiac energetics.

Cells are able to perform tasks that consume energy (such as producing mechanical force in muscle contraction) by using chemical energy delivered in the form of a chemical compound called adenosine triphosphate, or ATP. Two Nobel Prizes were awarded (in 1978 to Peter D. Mitchell and in 1997 to Paul D. Boyer and John E. Walker) for the determination of how ATP is synthesized from the components adenosine diphosphate (ADP) and inorganic phosphate in a subcellular body called the mitochondrion. The operating theory, called the chemiosmotic theory, describes how a driving force called the proton motive force, which arises from the sum of contributions from the electrical potential and the hydrogen ion concentration difference across the mitochondrial inner membrane, is developed by reactions catalyzed by certain enzymes and consumed in generating ATP. Yet, to date, no computer model has successfully described the development and consumption of both the chemical and electrical components of the proton motive force in a thermodynamically balanced simulation. Beard introduces such a model, which is extensively validated based on previously published sets of data obtained on isolated mitochondria. The model is used to test hypotheses about how intracellular respiration is regulated; this model could serve as a foundation for investigating the control of mitochondrial function and for developing larger integrated simulations of cellular metabolism.

Mitochondrial ATP-Sensitive K+ Channels Prevent Oxidative Stress, Permeability Transition and Cell Death

Ischemia followed by reperfusion results in impairment of cellular and mitochondrial functionality due to opening of mitochondrial permeability transition pores. On the other hand, activation of mitochondrial ATP-sensitive K(+) channels (mitoK(ATP)) protects the heart against ischemic damage. This study examined the effects of mitoK(ATP) and mitochondrial permeability transition on isolated rat heart mitochondria and cardiac cells submitted to simulated ischemia and reperfusion (cyanide/aglycemia). Both mitoK(ATP) opening, using diazoxide, and the prevention of mitochondrial permeability transition, using cyclosporin A, protected against cellular damage, without additive effects. MitoK(ATP) opening in isolated rat heart mitochondria slightly decreased Ca(2+) uptake and prevented mitochondrial reactive oxygen species production, most notably in the presence of added Ca(2+). In ischemic cells, diazoxide decreased ROS generation during cyanide/aglycemia while cyclosporin A prevented oxidative stress only during simulated reperfusion. Collectively, these studies indicate that opening mitoK(ATP) prevents cellular death under conditions of ischemia/reperfusion by decreasing mitochondrial reactive oxygen species release secondary to Ca(2+) uptake, inhibiting mitochondrial permeability transition.

Manganese superoxide dismutase overexpression inhibits the growth of androgen-independent prostate cancer cells

This study investigates the role of the antioxidant enzyme manganese superoxide dismutase (MnSOD) in androgen-independent human prostate cancer (PC-3) cells' growth rate in vitro and in vivo. MnSOD levels were found to be lower in parental PC-3 cells compared to nonmalignant, immortalized human prostate epithelial cells (P69SV40T). To unravel the role of MnSOD in the prostate cancer phenotype, PC-3 cells were stably transfected with MnSOD cDNA plasmid. The MnSOD protein and activity levels in clones overexpressing MnSOD were increased seven- to eightfold. These cell lines showed elongated cell doubling time, reduced anchorage-independent growth in soft agar compared to parental PC-3 (Wt) cells, and reduced growth rate of PC-3 tumor xenografts in athymic nude mice. Flow cytometric studies showed an increase in membrane potential in the MnSOD-overexpressing clone (Mn32) compared to Wt and Neo cells. Also, production of extracellular H(2)O(2) was increased in the MnSOD-overexpressing clones. As determined by DNA cell cycle analysis, the proportion of cells in G(1) phase was enhanced by MnSOD overexpression. Therefore, MnSOD not only regulates cell survival but also affects PC-3 cell proliferation by retarding G(1) to S transition. Our results are consistent with MnSOD being a tumor suppressor gene in human prostate cancer.

Cellular and molecular events in ischemic preconditioning: potential therapeutic applications in cardioprotection

Cardioprotection is a mechanism of guarding the heart from damage secondary to different insults including ischaemia, ischaemia/reperfusion, chemical, metabolic and physical stressors. Ischemic preconditioning, by single or multiple brief periods of ischaemia, protects the heart against a more prolonged ischemic insult (index ischaemia). Understanding the cellular, molecular and biochemical events occurring in cardioprotection will allow the development of new interventions to improve the outcome of patients with myocardial diseases. Most of the present experience with cardioprotection has been obtained from studies in young and middle-aged animals, and cells. In the future, cardioprotection research should be carried out mainly in the aging or senescent heart since this will be most relevant to humans. With aging, the heart has a decreased capacity to tolerate and respond to various forms of stress, and the likelihood of myocardial ischaemia and cardiac dysfunction increases.

Mitochondria play a critical role in cardioprotection

BACKGROUND

There is increasing evidence documenting the capacity of myocardial cells exposed to a variety of insults to mount a cardioprotective response. Although this cardioprotection has been most well characterized with respect to ischemic preconditioning, other chemical and metabolic stressors have been shown to share features of the ischemic preconditioning model, including the involvement of mitochondria in the triggering, signaling, and mediation of the cardioprotective response.

METHODS

In this article, we review the evidence showing that mitochondria play a critical role in cardioprotection from multiple (often interrelated) standpoints: its primary function in producing the cellular bioenergetic supply, its control over events in apoptosis, its contribution to myocardial signal transducing processes, and its role in producing reactive oxidative species and in providing an appropriate antioxidant response to a variety of cellular insults.

CONCLUSIONS

Although our understanding of cytoprotection has increased substantially within the last few years, the mechanisms mediating mitochondrial resistance to insults leading to cardiac protection remain to be fully delineated, and represents a significant approach in the clinical treatment of heart disease.

Ischemic preconditioning exaggerates cardiac damage in PKC-δ null mice

Ischemic preconditioning confers cardiac protection during subsequent ischemia-reperfusion, in which protein kinase C (PKC) is believed to play an essential role, but controversial data exist concerning the PKC-delta isoform. In an accompanying study (26), we described metabolic changes in PKC-delta knockout mice. We now wanted to explore their effect on early preconditioning. Both PKC-delta(-/-) and PKC-delta(+/+) mice underwent three cycles of 5-min left descending artery occlusion/5-min reperfusion, followed by 30-min occlusion and 2-h reperfusion. Unexpectedly, preconditioning exaggerated ischemia-reperfusion injury in PKC-delta(-/-) mice. Whereas ischemic preconditioning increased superoxide anion production in PKC-delta(+/+) hearts, no increase in reactive oxygen species was observed in PKC-delta(-/-) hearts. Proteomic analysis of preconditioned PKC-delta(+/+) hearts revealed profound changes in enzymes related to energy metabolism, e.g., NADH dehydrogenase and ATP synthase, with partial fragmentation of these mitochondrial enzymes and of the E(2) component of the pyruvate dehydrogenase complex. Interestingly, fragmentation of mitochondrial enzymes was not observed in PKC-delta(-/-) hearts. High-resolution NMR analysis of cardiac metabolites demonstrated a similar rise of phosphocreatine in PKC-delta(+/+) and PKC-delta(-/-) hearts, but the preconditioning-induced increase in phosphocholine, alanine, carnitine, and glycine was restricted to PKC-delta(+/+) hearts, whereas lactate concentrations were higher in PKC-delta(-/-) hearts. Taken together, our results suggest that reactive oxygen species generated during ischemic preconditioning might alter mitochondrial metabolism by oxidizing key mitochondrial enzymes and that metabolic adaptation to preconditioning is impaired in PKC-delta(-/-) hearts.

Doxorubicin Paradoxically Protects Cardiomyocytes against Iron-mediated Toxicity

The cardiotoxicity induced by the anticancer anthracycline doxorubicin (DOX) is attributed to reactions between iron and reactive oxygen species (ROS) that lead to oxidative damage. We found that DOX forms ROS in H9c2 cardiomyocytes, as shown by dichlorodihydrofluorescein oxidation and the expression of stress-responsive genes such as catalase or aldose reductase. DOX also increased ferritin levels in these cells, particularly the H subunit. A considerable increase in ferritin mRNA levels showed that DOX acted at transcriptional level, but an additional potential mechanism was identified as the down-regulation of iron regulatory protein-2, post-transcriptional inhibitor of ferritin synthesis. Pretreatment with DOX protected H9c2 cells against the damage induced by subsequent exposure to ferric ammonium citrate, and experiments with (55)Fe revealed that the protection was due to the deposition of iron in ferritin. Cytoprotection was also observed when DOX was replaced by glucose/glucose oxidase, a source of H(2)O(2), thus suggesting that DOX increases ferritin synthesis through the action of ROS. This concept was supported by three more lines of evidence. (i) DOX-induced ferritin synthesis was blocked by N-acetylcysteine, a scavenger of ROS. (ii) Mitoxantrone, a ROS-forming analogue, similarly induced ferritin expression and protected the cells against iron toxicity. (iii) 5-Iminodaunorubicin, an analogue lacking ROS-forming activity, did not induce ferritin synthesis or protect the cells against iron toxicity. These results characterize a paradoxically beneficial link between anthracycline-derived ROS, increased ferritin synthesis, and resistance to iron-mediated damage. The role of iron and ROS in anthracycline-induced cardiotoxicity may, therefore, be more complex than previously believed.