Ischemic preconditioning decreases mitochondrial proton leak and reactive oxygen species production in the postischemic heart

Mitochondrial function in lungs of rats with different susceptibilities to hyperoxia-induced acute lung injury

Adult rats exposed to hyperoxia (>95% O2) die from respiratory failure in 60-72 h. However, rats preconditioned with >95% O2 for 48 h followed by 24 h in room air acquire tolerance of hyperoxia (H-T), whereas rats preconditioned with 60% O2 for 7 days become more susceptible (H-S). Our objective was to evaluate lung tissue mitochondrial bioenergetics in H-T and H-S rats. Bioenergetics was assessed in mitochondria isolated from lung tissue of H-T, H-S, and control rats. Expressions of complexes involved in oxidative phosphorylation (OxPhos) were measured in lung tissue homogenate. Pulmonary endothelial filtration coefficient (Kf) and tissue mitochondrial membrane potential (Δψm) were evaluated in isolated perfused lungs (IPLs). Results show that ADP-induced state 3 OxPhos capacity (Vmax) decreased in H-S mitochondria but increased in H-T. Δψm repolarization time following ADP-stimulated depolarization increased in H-S mitochondria. Complex I expression decreased in H-T (38%) and H-S (43%) lung homogenate, whereas complex V expression increased (70%) in H-T lung homogenate. Δψm is unchanged in H-S and H-T lungs, but complex II has a larger contribution to Δψm in H-S than H-T lungs. Kf increased in H-S, but not in H-T lungs. For H-T, increased complex V expression and Vmax counter the effect of the decrease in complex I expression on Δψm. A larger complex II contribution to Δψm along with decreased Vmax and increased Kf could make H-S rats more hyperoxia susceptible. Results are clinically relevant since ventilation with ≥60% O2 is often required for extended periods in patients with acute respiratory distress syndrome (ARDS).NEW & NOTEWORTHY We assessed lung tissue mitochondrial bioenergetics in rats with tolerance (H-T) or susceptibility (H-S) to hyperoxia-induced ARDS. Results from studies in isolated mitochondria, tissue homogenate, and isolated perfused lungs show that mitochondrial bioenergetics are differentially altered in H-T and H-S lungs suggesting a potential role for mitochondrial bioenergetics in hyperoxia-induced ARDS. Results are clinically relevant since hyperoxia exposure is a primary therapy for patients with ARDS, and differential sensitivity to hyperoxia surely occurs in humans.

Postnatal growth restriction alters myocardial mitochondrial energetics in mice

Postnatal growth restriction (PGR) can increase the risk of cardiovascular disease (CVD) potentially due to impairments in oxidative phosphorylation (OxPhos) within cardiomyocyte mitochondria. The purpose of this investigation was to determine if PGR impairs cardiac metabolism, specifically OxPhos. FVB (Friend Virus B-type) mice were fed a normal-protein (NP: 20% protein), or low-protein (LP: 8% protein) isocaloric diet 2 weeks before mating. LP dams produce ∼20% less milk, and pups nursed by LP dams experience reduced growth into adulthood as compared to pups nursed by NP dams. At birth (PN1), pups born to dams fed the NP diet were transferred to LP dams (PGR group) or a different NP dam (control group: CON). At weaning (PN21), all mice were fed the NP diet. At PN22 and PN80, mitochondria were isolated for respirometry (oxygen consumption rate,J O 2 ${J_{{{\mathrm{O}}_{\mathrm{2}}}}}$ ) and fluorimetry (reactive oxygen species emission,J H 2 O 2 ${J_{{{\mathrm{H}}_{\mathrm{2}}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) analysis measured as baseline respiration (LEAK) and with saturating ADP (OxPhos). Western blotting at PN22 and PN80 determined protein abundance of uncoupling protein 3, peroxiredoxin-6, voltage-dependent anion channel and adenine nucleotide translocator 1 to provide further insight into mitochondrial function. ANOVAs with the main effects of diet, sex and age with α-level of 0.05 was set a priori. Overall, PGR (7.8 ± 1.1) had significant (P = 0.01) reductions in respiratory control in complex I when compared to CON (8.9 ± 1.0). In general, our results show that PGR led to higher electron leakage in the form of free radical production and reactive oxygen species emission. No significant diet effects were found in protein abundance. The observed reduced respiratory control and increased ROS emission in PGR mice may increase risk for CVD in mice.

Pathways for Cardioprotection in Perspective: Focus on Remote Conditioning and Extracellular Vesicles

Despite the development of cutting-edge treatments, coronary artery disease (CAD) morbidity and mortality rates remain present at high levels. Therefore, new cardioprotective approaches are crucial to improve the health of patients. To date, experimental investigations of acute ischemia-reperfusion injury (IRI) have generally demonstrated the efficacy of local ischemic preconditioning and postconditioning cardioprotection techniques as well as of remote conditioning. However, application in clinical settings is still highly controversial and debated. Currently, remote ischemic conditioning (RIC) seems to be the most promising method for heart repair. Protective factors are released into the bloodstream, and protection can be transferred within and across species. For a long time, the cross-function and cross-transmission mechanisms of cardioprotection were largely unknown. Recently, it has been shown that small, anuclear, bilayered lipid membrane particles, known as extracellular vesicles (EVs), are the drivers of signal transduction in cardiac IRI and RIC. EVs are related to the pathophysiological processes of cardiovascular diseases (CVDs), according to compelling evidence. In this review, we will first review the current state of knowledge on myocardial IRI and cardioprotective strategies explored over the past 37 years. Second, we will briefly discuss the role of EVs in CVD and the most recent improvements on EVs as prognostic biomarkers, diagnostic, and therapeutic agents. We will discuss how EVs can be used as a new drug delivery mechanism and how they can be employed in cardiac treatment, also from a perspective of overcoming the impasse that results from neglecting confounding factors.

Ischaemic preconditioning blunts exercise-induced mitochondrial dysfunction, speeds oxygen uptake kinetics but does not alter severe-intensity exercise capacity

NEW FINDINGS

What is the central question of this study? Ischaemic preconditioning is a novel pre-exercise priming strategy. We asked whether ischaemic preconditioning would alter mitochondrial respiratory function and pulmonary oxygen uptake kinetics and improve severe-intensity exercise performance. What is the main finding and its importance? Ischaemic preconditioning expedited overall pulmonary oxygen uptake kinetics and appeared to prevent an increase in leak respiration, proportional to maximal electron transfer system and ADP-stimulated respiration, that was evoked by severe-intensity exercise in sham-control conditions. However, severe-intensity exercise performance was not improved. The results do not support ischaemic preconditioning as a pre-exercise strategy to improve exercise performance in recreationally active participants.

ABSTRACT

We examined the effect of ischaemic preconditioning (IPC) on severe-intensity exercise performance, pulmonary oxygen uptake ( V ̇ O 2 ${\dot V_{{{\rm{O}}_{\rm{2}}}}}$ ) kinetics, skeletal muscle oxygenation (muscle tissue O₂ saturation index) and mitochondrial respiration. Eight men underwent contralateral IPC (4 × 5 min at 220 mmHg) or sham-control (SHAM; 20 mmHg) before performing a cycling time-to-exhaustion test (92% maximum aerobic power). Muscle (vastus lateralis) biopsies were obtained before IPC or SHAM and ∼1.5 min postexercise. The time to exhaustion did not differ between SHAM and IPC (249 ± 37 vs. 240 ± 32 s; P = 0.62). Pre- and postexercise ADP-stimulated (P) and maximal (E) mitochondrial respiration through protein complexes (C) I, II and IV did not differ (P > 0.05). Complex I leak respiration was greater postexercise compared with baseline in SHAM, but not in IPC, when normalized to wet mass (P = 0.01 vs. P = 0.19), mitochondrial content (citrate synthase activity, P = 0.003 vs. P = 0.16; CI+IIP, P = 0.03 vs. P = 0.23) and expressed relative to P (P = 0.006 vs. P = 0.30) and E (P = 0.004 vs. P = 0.26). The V ̇ O 2 ${\dot V_{{{\rm{O}}_{\rm{2}}}}}$ mean response time was faster (51.3 ± 15.5 vs. 63.7 ± 14.5 s; P = 0.003), with a smaller slow component (270 ± 105 vs. 377 ± 188 ml min^-1 ; P = 0.03), in IPC compared with SHAM. The muscle tissue O₂ saturation index did not differ between trials (P > 0.05). Ischaemic preconditioning expedited V ̇ O 2 ${\dot V_{{{\rm{O}}_{\rm{2}}}}}$ kinetics and appeared to prevent an increase in leak respiration through CI, when expressed proportional to E and P evoked by severe-intensity exercise, but did not improve exercise performance.

Hepatic ischemia-reperfusion injury in liver transplant setting: mechanisms and protective strategies

Hepatic ischemia-reperfusion injury is a major cause of liver transplant failure, and is of increasing significance due to increased use of expanded criteria livers for transplantation. This review summarizes the mechanisms and protective strategies for hepatic ischemia-reperfusion injury in the context of liver transplantation. Pharmacological therapies, the use of pre-and post-conditioning and machine perfusion are discussed as protective strategies. The use of machine perfusion offers significant potential in the reconditioning of liver grafts and the prevention of hepatic ischemia-reperfusion injury, and is an exciting and active area of research, which needs more study clinically.

Mitochondrial Proton Leak Plays a Critical Role in Pathogenesis of Cardiovascular Diseases

Mitochondrial proton leak is the principal mechanism that incompletely couples substrate oxygen to ATP generation. This chapter briefly addresses the recent progress made in understanding the role of proton leak in the pathogenesis of cardiovascular diseases. Majority of the proton conductance is mediated by uncoupling proteins (UCPs) located in the mitochondrial inner membrane. It is evident that the proton leak and reactive oxygen species (ROS) generated from electron transport chain (ETC) in mitochondria are linked to each other. Increased ROS production has been shown to induce proton conductance, and in return, increased proton conductance suppresses ROS production, suggesting the existence of a positive feedback loop that protects the biological systems from detrimental effects of augmented oxidative stress. There is mounting evidence attributing to proton leak and uncoupling proteins a crucial role in the pathogenesis of cardiovascular disease. We can surmise the role of "uncoupling" in cardiovascular disorders as follows; First, the magnitude of the proton leak and the mechanism involved in mediating the proton leak determine whether there is a protective effect against ischemia-reperfusion (IR) injury. Second, uncoupling by UCP2 preserves vascular function in diet-induced obese mice as well as in diabetes. Third, etiology determines whether the proton conductance is altered or not during hypertension. And fourth, proton leak regulates ATP synthesis-uncoupled mitochondrial ROS generation, which determines pathological activation of endothelial cells for recruitment of inflammatory cells. Continue effort in improving our understanding in the role of proton leak in the pathogenesis of cardiovascular and metabolic diseases would lead to identification of novel therapeutic targets for treatment.

Humanin directly protects cardiac mitochondria against dysfunction initiated by oxidative stress by decreasing complex I activity

Humanin (HN) is an endogenous peptide that exerts cytoprotection against oxidative stress and apoptosis. We recently reported that Humanin analogue (HNG) pretreatment can reduce reactive oxygen species production in the heart subjected to ischemia/reperfusion (I/R) injury via attenuating mitochondrial dysfunction. However, it is unclear if HNG has direct effects on mitochondrial function against oxidative stress. Thus, we sought to determine the effects of HNG on mitochondrial function under hydrogen peroxide (H₂O₂) induced oxidative stress in isolated cardiac mitochondria. We found that HNG has direct protective effects on cardiac mitochondrial function against H₂O₂ induced oxidative stress through decreasing complex I activity.

Grape seed proanthocyanidins prevent irradiation-induced differentiation of human lung fibroblasts by ameliorating mitochondrial dysfunction

Radiation-induced lung fibrosis (RILF) is a long-term adverse effect of curative radiotherapy. The accumulation of myofibroblasts in fibroblastic foci is a pivotal feature of RILF. In the study, we found the inhibitory effect of grape seed proanthocyanidins (GSPs) on irradiation-induced differentiation of human fetal lung fibroblasts (HFL1). To explore the mechanism by which GSPs inhibit fibroblast differentiation, we measured the reactive oxygen species (ROS) levels, mitochondrial function, mitochondrial dynamics, glycolysis and the signaling molecules involved in fibroblast transdifferentiation. GSPs significantly reduced the production of cellular and mitochondrial ROS after radiation. The increases in mitochondrial respiration, proton leak, mitochondrial ATP production, lactate release and glucose consumption that occurred in response to irradiation were ameliorated by GSPs. Furthermore, GSPs increased the activity of complex I and improved the mitochondrial dynamics, which were disturbed by irradiation. In addition, the elevation of phosphorylation of p38MAPK and Akt, and Nox4 expression induced by irradiation were attenuated by GSPs. Blocking Nox4 attenuated irradiation-mediated fibroblast differentiation. Taken together, these results indicate that GSPs have the ability to inhibit irradiation-induced fibroblast-to-myofibroblast differentiation by ameliorating mitochondrial dynamics and mitochondrial complex I activity, regulating mitochondrial ROS production, ATP production, lactate release, glucose consumption and thereby inhibiting p38MAPK-Akt-Nox4 pathway.

Preconditioning methods in the management of hepatic ischemia reperfusion- induced injury: Update on molecular and future perspectives

Hepatic IR (ischemia reperfusion) injury is a commonly encountered obstacle in the post-operative management of hepatic surgery. Hepatic IR occurs during 'Pringle maneuver' for reduction of blood loss or during a brief period of cold storage followed by reperfusion of liver grafts. The stress induced during hepatic IR, triggers a spectrum of cellular responses leading to the varying degrees of hepatic complications which in turn affect the post operative care. Different preconditioning methods either activate or subdue different sets of molecular signals, resulting in varied levels of protection against hepatic IR injury. Yet, there is a serious lacuna in the knowledge regarding the choice of preconditioning methods and the resulting molecular changes in order to assess the efficiency and choice of these methods correctly. This review provides an update on the various preconditioning approaches such as surgical/ischemic, antioxidant, pharmaceutical and genetic preconditioning strategies published during last six years (2009-2015). Further, we discuss the attenuation or inhibition of specific inflammatory, apoptotic and necrotic markers in the various experimental models of liver IR subjected to different preconditioning strategies. While enlisting the controversies in the ischemic preconditioning strategy, we bring out the uncertainties in the existing molecular targets and their reliability in the attenuation of hepatic IR injury. Future research studies would include the novel preconditioning strategies employ i) the targeted gene silencing of key molecular targets inducing IR, ii) hyper expression of beneficial molecular signals against IR via gene transfer techniques. The above studies would see the combination of these latest techniques with the established preconditioning strategies for better post-operative hepatic management.

Zofenopril Protects Against Myocardial Ischemia-Reperfusion Injury by Increasing Nitric Oxide and Hydrogen Sulfide Bioavailability

Background

Zofenopril, a sulfhydrylated angiotensin‐converting enzyme inhibitor (ACEI), reduces mortality and morbidity in infarcted patients to a greater extent than do other ACEIs. Zofenopril is a unique ACEI that has been shown to increase hydrogen sulfide (H₂S) bioavailability and nitric oxide (NO) levels via bradykinin‐dependent signaling. Both H₂S and NO exert cytoprotective and antioxidant effects. We examined zofenopril effects on H₂S and NO bioavailability and cardiac damage in murine and swine models of myocardial ischemia/reperfusion (I/R) injury.

Methods and Results

Zofenopril (10 mg/kg PO) was administered for 1, 8, and 24 hours to establish optimal dosing in mice. Myocardial and plasma H₂S and NO levels were measured along with the levels of H₂S and NO enzymes (cystathionine β‐synthase, cystathionine γ‐lyase, 3‐mercaptopyruvate sulfur transferase, and endothelial nitric oxide synthase). Mice received 8 hours of zofenopril or vehicle pretreatment followed by 45 minutes of ischemia and 24 hours of reperfusion. Pigs received placebo or zofenopril (30 mg/daily orally) 7 days before 75 minutes of ischemia and 48 hours of reperfusion. Zofenopril significantly augmented both plasma and myocardial H₂S and NO levels in mice and plasma H₂S (sulfane sulfur) in pigs. Cystathionine β‐synthase, cystathionine γ‐lyase, 3‐mercaptopyruvate sulfur transferase, and total endothelial nitric oxide synthase levels were unaltered, while phospho‐endothelial nitric oxide synthase¹¹⁷⁷ was significantly increased in mice. Pretreatment with zofenopril significantly reduced myocardial infarct size and cardiac troponin I levels after I/R injury in both mice and swine. Zofenopril also significantly preserved ischemic zone endocardial blood flow at reperfusion in pigs after I/R.

Conclusions

Zofenopril‐mediated cardioprotection during I/R is associated with an increase in H₂S and NO signaling.

Succinate metabolism: a new therapeutic target for myocardial reperfusion injury

Myocardial ischaemia/reperfusion (IR) injury is a major cause of death worldwide and remains a disease for which current clinical therapies are strikingly deficient. While the production of mitochondrial reactive oxygen species (ROS) is a critical driver of tissue damage upon reperfusion, the precise mechanisms underlying ROS production have remained elusive. More recently, it has been demonstrated that a specific metabolic mechanism occurs during ischaemia that underlies elevated ROS at reperfusion, suggesting a unifying model as to why so many different compounds have been found to be cardioprotective against IR injury. This review will discuss the role of the citric acid cycle intermediate succinate in IR pathology focusing on the mechanism by which this metabolite accumulates during ischaemia and how it can drive ROS production at Complex I via reverse electron transport. We will then examine the potential for manipulating succinate accumulation and metabolism during IR injury in order to protect the heart against IR damage and discuss targets for novel therapeutics designed to reduce reperfusion injury in patients.

The Role of Mitochondrial Reactive Oxygen Species in Cardiovascular Injury and Protective Strategies

Ischaemia/reperfusion (I/R) injury of the heart represents a major health burden mainly associated with acute coronary syndromes. While timely coronary reperfusion has become the established routine therapy in patients with ST-elevation myocardial infarction, the restoration of blood flow into the previously ischaemic area is always accompanied by myocardial injury. The central mechanism involved in this phenomenon is represented by the excessive generation of reactive oxygen species (ROS). Besides their harmful role when highly generated during early reperfusion, minimal ROS formation during ischaemia and/or at reperfusion is critical for the redox signaling of cardioprotection. In the past decades, mitochondria have emerged as the major source of ROS as well as a critical target for cardioprotective strategies at reperfusion. Mitochondria dysfunction associated with I/R myocardial injury is further described and ultimately analyzed with respect to its role as source of both deleterious and beneficial ROS. Furthermore, the contribution of ROS in the highly investigated field of conditioning strategies is analyzed. In the end, the vascular sources of mitochondria-derived ROS are briefly reviewed.

Moving Forwards by Blocking Back-Flow: The Yin and Yang of MI Therapy

Mitochondrial reactive oxygen species production has emerged as an important pathological mechanism in myocardial ischemia/reperfusion injury. Attempts at targeting reactive oxygen species by scavenging using antioxidants have, however, been clinically disappointing. This review will provide an overview of the current understanding of mitochondrial reactive oxygen species in ischemia/reperfusion injury. We will outline novel therapeutic approaches designed to directly target the mitochondrial respiratory chain and prevent excessive reactive oxygen species production and its associated pathology. This approach could lead to more effective interventions in an area where there is an urgent need for new treatments.

Mitochondrial uncoupling does not decrease reactive oxygen species production after ischemia-reperfusion

Cardiac ischemia-reperfusion (IR) leads to myocardial dysfunction by increasing production of reactive oxygen species (ROS). Mitochondrial H(+) leak decreases ROS formation; it has been postulated that increasing H(+) leak may be a mechanism of decreasing ROS production after IR. Ischemic preconditioning (IPC) decreases ROS formation after IR, but the mechanism is unknown. We hypothesize that pharmacologically increasing mitochondrial H(+) leak would decrease ROS production after IR. We further hypothesize that IPC would be associated with an increase in the rate of H(+) leak. Isolated male Sprague-Dawley rat hearts were subjected to either control or IPC. Mitochondria were isolated at end equilibration, end ischemia, and end reperfusion. Mitochondrial membrane potential (mΔΨ) was measured using a tetraphenylphosphonium electrode. Mitochondrial uncoupling was achieved by adding increasing concentrations of FCCP. Mitochondrial ROS production was measured by fluorometry using Amplex-Red. Pyridine dinucleotide levels were measured using HPLC. Before IR, increasing H(+) leak decreased mitochondrial ROS production. After IR, ROS production was not affected by increasing H(+) leak. H(+) leak increased at end ischemia in control mitochondria. IPC mitochondria showed no change in the rate of H(+) leak throughout IR. NADPH levels decreased after IR in both IPC and control mitochondria while NADH increased. Pharmacologically, increasing H(+) leak is not a method of decreasing ROS production after IR. Replenishing the NADPH pool may be a means of scavenging the excess ROS thereby attenuating oxidative damage after IR.

Beating oxygen: chronic anoxia exposure reduces mitochondrial F1FO-ATPase activity in turtle (Trachemys scripta) heart

The freshwater turtle Trachemys scripta can survive in the complete absence of O2 (anoxia) for periods lasting several months. In mammals, anoxia leads to mitochondrial dysfunction, which culminates in cellular necrosis and apoptosis. Despite the obvious clinical benefits of understanding anoxia tolerance, little is known about the effects of chronic oxygen deprivation on the function of turtle mitochondria. In this study, we compared mitochondrial function in hearts of T. scripta exposed to either normoxia or 2 weeks of complete anoxia at 5°C and during simulated acute anoxia/reoxygenation. Mitochondrial respiration, electron transport chain activities, enzyme activities, proton conductance and membrane potential were measured in permeabilised cardiac fibres and isolated mitochondria. Two weeks of anoxia exposure at 5°C resulted in an increase in lactate, and decreases in ATP, glycogen, pH and phosphocreatine in the heart. Mitochondrial proton conductance and membrane potential were similar between experimental groups, while aerobic capacity was dramatically reduced. The reduced aerobic capacity was the result of a severe downregulation of the F1FO-ATPase (Complex V), which we assessed as a decrease in enzyme activity. Furthermore, in stark contrast to mammalian paradigms, isolated turtle heart mitochondria endured 20 min of anoxia followed by reoxygenation without any impact on subsequent ADP-stimulated O2 consumption (State III respiration) or State IV respiration. Results from this study demonstrate that turtle mitochondria remodel in response to chronic anoxia exposure and a reduction in Complex V activity is a fundamental component of mitochondrial and cellular anoxia survival.

Redox signaling in cardiovascular health and disease

Spatiotemporal regulation of the activity of a vast array of intracellular proteins and signaling pathways by reactive oxygen species (ROS) governs normal cardiovascular function. However, data from experimental and animal studies strongly support that dysregulated redox signaling, resulting from hyperactivation of various cellular oxidases or mitochondrial dysfunction, is integral to the pathogenesis and progression of cardiovascular disease (CVD). In this review, we address how redox signaling modulates the protein function, the various sources of increased oxidative stress in CVD, and the labyrinth of redox-sensitive molecular mechanisms involved in the development of atherosclerosis, hypertension, cardiac hypertrophy and heart failure, and ischemia-reperfusion injury. Advances in redox biology and pharmacology for inhibiting ROS production in specific cell types and subcellular organelles combined with the development of nanotechnology-based new in vivo imaging systems and targeted drug delivery mechanisms may enable fine-tuning of redox signaling for the treatment and prevention of CVD.

Ischemia preconditioning is neuroprotective in a rat cerebral ischemic injury model through autophagy activation and apoptosis inhibition

Sublethal ischemic preconditioning (IPC) is a powerful inducer of ischemic brain tolerance. However, its underlying mechanisms are still not well understood. In this study, we chose four different IPC paradigms, namely 5 min (5 min duration), 5×5 min (5 min duration, 2 episodes, 15-min interval), 5×5×5 min (5 min duration, 3 episodes, 15-min intervals), and 15 min (15 min duration), and demonstrated that three episodes of 5 min IPC activated autophagy to the greatest extent 24 h after IPC, as evidenced by Beclin expression and LC3-I/II conversion. Autophagic activation was mediated by the tuberous sclerosis type 1 (TSC1)-mTor signal pathway as IPC increased TSC1 but decreased mTor phosphorylation. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and hematoxylin and eosin staining confirmed that IPC protected against cerebral ischemic/reperfusion (I/R) injury. Critically, 3-methyladenine, an inhibitor of autophagy, abolished the neuroprotection of IPC and, by contrast, rapamycin, an autophagy inducer, potentiated it. Cleaved caspase-3 expression, neurological scores, and infarct volume in different groups further confirmed the protection of IPC against I/R injury. Taken together, our data indicate that autophagy activation might underlie the protection of IPC against ischemic injury by inhibiting apoptosis.

Radiation protection following nuclear power accidents: a survey of putative mechanisms involved in the radioprotective actions of taurine during and after radiation exposure

There are several animal experiments showing that high doses of ionizing radiation lead to strongly enhanced leakage of taurine from damaged cells into the extracellular fluid, followed by enhanced urinary excretion. This radiation-induced taurine depletion can itself have various harmful effects (as will also be the case when taurine depletion is due to other causes, such as alcohol abuse or cancer therapy with cytotoxic drugs), but taurine supplementation has been shown to have radioprotective effects apparently going beyond what might be expected just as a consequence of correcting the harmful consequences of taurine deficiency per se. The mechanisms accounting for the radioprotective effects of taurine are, however, very incompletely understood. In this article an attempt is made to survey various mechanisms that potentially might be involved as parts of the explanation for the overall beneficial effect of high levels of taurine that has been found in experiments with animals or isolated cells exposed to high doses of ionizing radiation. It is proposed that taurine may have radioprotective effects by a combination of several mechanisms: (1) during the exposure to ionizing radiation by functioning as an antioxidant, but perhaps more because it counteracts the prooxidant catalytic effect of iron rather than functioning as an important scavenger of harmful molecules itself, (2) after the ionizing radiation exposure by helping to reduce the intensity of the post-traumatic inflammatory response, and thus reducing the extent of tissue damage that develops because of severe inflammation rather than as a direct effect of the ionizing radiation per se, (3) by functioning as a growth factor helping to enhance the growth rate of leukocytes and leukocyte progenitor cells and perhaps also of other rapidly proliferating cell types, such as enterocyte progenitor cells, which may be important for immunological recovery and perhaps also for rapid repair of various damaged tissues, especially in the intestines, and (4) by functioning as an antifibrogenic agent. A detailed discussion is given of possible mechanisms involved both in the antioxidant effects of taurine, in its anti-inflammatory effects and in its role as a growth factor for leukocytes and nerve cells, which might be closely related to its role as an osmolyte important for cellular volume regulation because of the close connection between cell volume regulation and the regulation of protein synthesis as well as cellular protein degradation. While taurine supplementation alone would be expected to exert a therapeutic effect far better than negligible in patients that have been exposed to high doses of ionizing radiation, it may on theoretical grounds be expected that much better results may be obtained by using taurine as part of a multifactorial treatment strategy, where it may interact synergistically with several other nutrients, hormones or other drugs for optimizing antioxidant protection and minimizing harmful posttraumatic inflammatory reactions, while using other nutrients to optimize DNA and tissue repair processes, and using a combination of good diet, immunostimulatory hormones and perhaps other nontoxic immunostimulants (such as beta-glucans) for optimizing the recovery of antiviral and antibacterial immune functions. Similar multifactorial treatment strategies may presumably be helpful in several other disease situations (including severe infectious diseases and severe asthma) as well as for treatment of acute intoxications or acute injuries (both mechanical ones and severe burns) where severely enhanced oxidative and/or nitrative stress and/or too much secretion of vasodilatory neuropeptides from C-fibres are important parts of the pathogenetic mechanisms that may lead to the death of the patient. Some case histories (with discussion of some of those mechanisms that may have been responsible for the observed therapeutic outcome) are given for illustration of the likely validity of these concepts and their relevance both for treatment of severe infections and non-infectious inflammatory diseases such as asthma and rheumatoid arthritis.

Mitochondrial ('mild') uncoupling and ROS production: physiologically relevant or not?

During the last decade, the possibility that 'mild' uncoupling could be protective against oxidative damage by diminishing ROS (reactive oxygen species) production has attracted much interest. In the present paper, we briefly examine the evidence for this possibility. It is only ROS production from succinate under reverse electron-flow conditions that is sensitive to membrane potential fluctuations, and so only this type of ROS production could be affected; however, the conditions under which succinate-supported ROS production is observed include succinate concentrations that are supraphysiological. Any decrease in membrane potential, even 'mild uncoupling', must necessarily lead to large increases in respiration, i.e. it must be markedly thermogenic. Mitochondria within cells are normally ATP-producing and thus already have a diminished membrane potential, and treatment of cells, organs or animals with small amounts of artificial uncoupler does not seem to have beneficial effects that are explainable via reduced ROS production. Although it has been suggested that members of the uncoupling protein family (UCP1, UCP2 and UCP3) may mediate a mild uncoupling, present evidence does not unequivocally support such an effect, e.g. the absence of the truly uncoupling protein UCP1 is not associated with increased oxidative damage. Thus present evidence does not support mild uncoupling as a physiologically relevant alleviator of oxidative damage.

Preconditioning approach in stem cell therapy for the treatment of infarcted heart

Nearly two decades of research in regenerative medicine have been focused on the development of stem cells as a therapeutic option for treatment of the ischemic heart. Given the ability of stem cells to regenerate the damaged tissue, stem-cell-based therapy is an ideal approach for cardiovascular disorders. Preclinical studies in experimental animal models and clinical trials to determine the safety and efficacy of stem cell therapy have produced encouraging results that promise angiomyogenic repair of the ischemically damaged heart. Despite these promising results, stem cell therapy is still confronted with issues ranging from uncertainty about the as-yet-undetermined "ideal" donor cell type to the nonoptimized cell delivery strategies to harness optimal clinical benefits. Moreover, these lacunae have significantly hampered the progress of the heart cell therapy approach from bench to bedside for routine clinical applications. Massive death of donor cells in the infarcted myocardium during acute phase postengraftment is one of the areas of prime concern, which immensely lowers the efficacy of the procedure. An overview of the published data relevant to stem cell therapy is provided here and the various strategies that have been adopted to develop and optimize the protocols to enhance donor stem cell survival posttransplantation are discussed, with special focus on the preconditioning approach.

Nitroalkenes confer acute cardioprotection via adenine nucleotide translocase 1

Electrophilic nitrated lipids (nitroalkenes) are emerging as an important class of protective cardiovascular signaling molecules. Although species such as nitro-linoleate (LNO(2)) and nitro-oleate can confer acute protection against cardiac ischemic injury, their mechanism of action is unclear. Mild uncoupling of mitochondria is known to be cardioprotective, and adenine nucleotide translocase 1 (ANT1) is a key mediator of mitochondrial uncoupling. ANT1 also contains redox-sensitive cysteines that may be targets for modification by nitroalkenes. Therefore, in this study we tested the hypothesis that nitroalkenes directly modify ANT1 and that nitroalkene-mediated cardioprotection requires ANT1. Using biotin-tagged LNO(2) infused into intact perfused hearts, we obtained mass spectrometric (MALDI-TOF-TOF) evidence for direct modification (nitroalkylation) of ANT1 on cysteine 57. Furthermore, in a cell model of ischemia-reperfusion injury, siRNA knockdown of ANT1 inhibited the cardioprotective effect of LNO(2). Although the molecular mechanism linking ANT1-Cys(57) nitroalkylation and uncoupling is not yet known, these data suggest that ANT1-mediated uncoupling may be a mechanism for nitroalkene-induced cardioprotection.

Comparative study of the cardioprotective effects of local and remote preconditioning in ischemia/reperfusion injury

AIMS

Though the cardioprotective effects of local or remote preconditioning have been estimated, it is still unclear which of them is more reliable and provides more cardioprotection. The present investigation was directed to compare, in one study, the cardioprotective effects of different cycles of local or remote preconditioning in ischemia/reperfusion (I/R)-induced electrophysiological, biochemical and histological changes in rats.

MAIN METHODS

Rats were randomly assigned into 10 groups. Groups 1 and 2 were normal and I/R groups, respectively. Other groups were subjected to 1, 2, 3, 4 cycles of local or remote preconditioning before myocardial I/R (40 min/10 min). Heart rate and ventricular arrhythmias were recorded during I/R progress. At the end of reperfusion, plasma creatine kinase-MB (CK-MB) activity and total nitrate/nitrite (NO(x)) were determined. In addition, lactate, adenine nucleotides, thiobarbituric acid reactive substances (TBARS), reduced glutathione (GSH) and myeloperoxidase (MPO) activity were estimated in the heart left ventricle. Histological examination was also performed to visualize the protective cellular effects of the effective cycle of local or remote preconditioning.

KEY FINDINGS

In general, local preconditioning was more effective than remote preconditioning in reducing ventricular arrhythmias, CK-MB release, lactate accumulation and elevated MPO activity as well as preserving adenine nucleotides. Concerning the most effective group in each therapy, 3 cycles of local preconditioning provided more cardioprotection than that of remote preconditioning in the histological examination.

SIGNIFICANCE

Despite being invasive, local preconditioning provided more effective cardioprotection than remote preconditioning in ameliorating the overall electrophysiological, biochemical and histological changes.

Preconditioning, postconditioning, and remote conditioning in solid organ transplantation: basic mechanisms and translational applications

Ischemia and reperfusion (I/Rp) injury is inherent to solid organ transplantation and can result in primary nonfunction or delayed function of grafts, which is associated with a significant morbidity and mortality posttransplantation. It is also a major obstacle for the use of marginal grafts to increase the donor pool, as these grafts are prone to a higher degree of I/Rp injury. Pre-, post-, and remote conditioning are protective strategies against I/Rp injury, which can be applied in the transplant setting. These strategies hold the potential to reduce graft injury and to safely expand the donor pool. However, despite convincing experimental data, the protective effects of the "conditioning" protocols remain unclear, and only few have translated to clinical practice. This review summarizes pre-, post-, and remote conditioning strategies in clinical use in solid organ transplantation and discusses an overview of the mechanistic pathways involved in each strategy.

Role of calcium-activated potassium channels in the genesis of 3,4-diaminopyridine-induced periodic contractions in isolated canine coronary artery smooth muscles

We found that 3,4-diaminopyridine (3,4-DAP), a voltage-gated potassium channel (K(V)) inhibitor, elicits pH-sensitive periodic contractions (PCs) of coronary smooth muscles. Underlying mechanisms of PCs, however, remained to be elucidated. The present study was performed to examine the roles of ion channels in the genesis of PCs. To determine the electromechanical changes of smooth muscles, isolated coronary arterial rings from beagles were suspended in organ chambers filled with Krebs-Henseleit solution, and 10(-2) M 3,4-DAP was added to elicit PCs. 3,4-DAP caused periodic spike-and-plateau depolarization accompanied by contraction. PCs were not produced when the CaCl(2) concentration in the chamber was ≤ 0.3 × 10(-3) or ≥ 10(-2) M. PCs were eliminated by a CaCl(2) concentration ≥ 5 × 10(-3) M or by lowering pH below 7.20 with HCl and recovered by the addition of iberiotoxin or charybdotoxin, which inhibit large-conductance calcium-activated potassium channels (K(Ca)), or by elevating pH above 7.35 with NaOH. PCs, as well as the spike-and-plateau depolarization, were eliminated by nifedipine, which inhibits L-type voltage-gated calcium channels (Ca(V)). Influx of Ca(2+) through L-type Ca(V), which was opened because closing of K(Ca), secondary to 3,4-DAP-induced closing of K(V), resulted in contraction; the intracellular Ca(2+) increased by this influx opened K(Ca), leading to closure of Ca(V) and consequent cessation of Ca(2+) influx with resultant relaxation. These processes were repeated spontaneously to cause PCs. H(+) and OH(-) were considered to act as the opener and closer of K(Ca), respectively.