Mitochondrial signals initiate the activation of c-Jun N-terminal kinase (JNK) by hypoxia-reoxygenation

Tumor cell phenotype is sustained by selective MAPK oxidation in mitochondria

Mitochondria are major cellular sources of hydrogen peroxide (H₂O₂), the production of which is modulated by oxygen availability and the mitochondrial energy state. An increase of steady-state cell H₂O₂ concentration is able to control the transition from proliferating to quiescent phenotypes and to signal the end of proliferation; in tumor cells thereby, low H₂O₂ due to defective mitochondrial metabolism can contribute to sustain proliferation. Mitogen-activated protein kinases (MAPKs) orchestrate signal transduction and recent data indicate that are present in mitochondria and regulated by the redox state. On these bases, we investigated the mechanistic connection of tumor mitochondrial dysfunction, H₂O₂ yield, and activation of MAPKs in LP07 murine tumor cells with confocal microscopy, in vivo imaging and directed mutagenesis. Two redox conditions were examined: low 1 µM H₂O₂ increased cell proliferation in ERK1/2-dependent manner whereas high 50 µM H₂O₂ arrested cell cycle by p38 and JNK1/2 activation. Regarding the experimental conditions as a three-compartment model (mitochondria, cytosol, and nuclei), the different responses depended on MAPKs preferential traffic to mitochondria, where a selective activation of either ERK1/2 or p38-JNK1/2 by co-localized upstream kinases (MAPKKs) facilitated their further passage to nuclei. As assessed by mass spectra, MAPKs activation and efficient binding to cognate MAPKKs resulted from oxidation of conserved ERK1/2 or p38-JNK1/2 cysteine domains to sulfinic and sulfonic acids at a definite H₂O₂ level. Like this, high H₂O₂ or directed mutation of redox-sensitive ERK2 Cys²¹⁴ impeded binding to MEK1/2, caused ERK2 retention in mitochondria and restricted shuttle to nuclei. It is surmised that selective cysteine oxidations adjust the electrostatic forces that participate in a particular MAPK-MAPKK interaction. Considering that tumor mitochondria are dysfunctional, their inability to increase H₂O₂ yield should disrupt synchronized MAPK oxidations and the regulation of cell cycle leading cells to remain in a proliferating phenotype.

Modulation of the c-Jun N-terminal kinase activity in the embryonic heart in response to anoxia-reoxygenation: involvement of the Ca2+ and mitoKATP channels

Whether the response of the fetal heart to ischemia-reperfusion is associated with activation of the c-Jun N-terminal kinase (JNK) pathway is not known. In contrast, involvement of the sarcolemmal L-type Ca2+ channel (LCC) and the mitochondrial KATP (mitoKATP) channel has been established. This work aimed at investigating the profile of JNK activity during anoxia-reoxygenation and its modulation by LCC and mitoK(ATP) channel. Hearts isolated from 4-day-old chick embryos were submitted to anoxia (30 min) and reoxygenation (60 min). Using the kinase assay method, the profile of JNK activity in the ventricle was determined every 10 min throughout anoxia-reoxygenation. Effects on JNK activity of the LCC blocker verapamil (10 nM), the mitoK(ATP) channel opener diazoxide (50 microM) and the blocker 5-hydroxydecanoate (5-HD, 500 microM), the mitochondrial Ca2+ uniporter (MCU) inhibitor Ru360 (10 microM), and the antioxidant N-(2-mercaptopropionyl) glycine (MPG, 1 mM) were determined. In untreated hearts, JNK activity was increased by 40% during anoxia and peaked fivefold relative to basal level after 30-40 min reoxygenation. This peak value was reduced by half by diazoxide and was tripled by 5-HD. Furthermore, the 5-HD-mediated stimulation of JNK activity during reoxygenation was abolished by diazoxide, verapamil or Ru360. MPG had no effect on JNK activity, whatever the conditions. None of the tested pharmacological agents altered JNK activity under basal normoxic conditions. Thus, in the embryonic heart, JNK activity exhibits a characteristic pattern during anoxia and reoxygenation and the respective open-state of LCC, MCU and mitoKATP channel can be a major determinant of JNK activity in a ROS-independent manner.

MsrA protects cardiac myocytes against hypoxia/reoxygenation induced cell death

Reactive oxygen species (ROS) are critical in tissue responses to ischemia-reperfusion. The enzyme methionine sulfoxide reductase-A (MsrA) is capable of protecting cells against oxidative damage by reversing damage to proteins caused by methionine oxidation or by decreasing ROS through a scavenger mechanism. The current study employed adenovirus mediated over-expression of MsrA in primary neonatal rat cardiac myocytes to determine the effect of this enzyme in protecting against hypoxia/reoxygenation in this tissue. Cells were transduced with MsrA encoding adenovirus and subjected to hypoxia/reoxygenation. Apoptotic cell death was decreased by greater than 45% in cells over-expressing MsrA relative to cells transduced with a control virus. Likewise total cell death as determined by levels of LDH release was dramatically decreased by MsrA over-expression. These observations indicate that MsrA is protective against hypoxia/reoxygenation stress in cardiac myocytes and point to MsrA as an important therapeutic target for ischemic heart disease.

Hypoxia activates c-Jun N-terminal kinase via Rac1-dependent reactive oxygen species production in hepatocytes

The earliest events after the induction of hemorrhagic shock (HS) are complex and poorly understood. We have recently demonstrated that decreased tissue perfusion and hypoxia during HS lead to an increased phosphorylation of c-Jun N-terminal kinase (JNK) in vivo. The purpose of these investigations was to test the hypothesis that hypoxia activates JNK via Rac1-dependent reactive oxygen species (ROS) signaling. Mice subjected to HS and resuscitated with Ringer's ethyl pyruvate solution (REPS) or N-acetylcysteine (NAC), two scavengers of ROS, demonstrated decreased levels of phosphorylated JNK. Exposure of primary mouse hepatocytes in culture to 1% oxygen led to increased production of ROS and phosphorylation of JNK. The duration of hypoxia correlated with the level of generation of ROS and JNK activation. The phosphorylation of JNK was attenuated in the presence of ROS scavengers or the nicotinamide adenosine dinucleotide phosphate [NDA(P)H] oxidase inhibitor, diphenyleneiodonium (DPI). In addition, hypoxia increased activation of Rac1. Inhibition of Rac1 activation by adenoviral gene transfer of dominant-negative Rac1 (AdRac1) attenuated both ROS formation and JNK activation. Together, these data suggest that ROS generation during hypoxia in the liver directly leads to JNK activation in a Rac1-dependent process.

Vaccinia-related kinase 2 modulates the stress response to hypoxia mediated by TAK1

Hypoxia represents a major stress that requires an immediate cellular response in which different signaling pathways participate. Hypoxia induces an increase in the activity of TAK1, an atypical mitogen-activated protein kinase kinase kinase (MAPKKK), which responds to oxidative stress by triggering cascades leading to the activation of c-Jun N-terminal kinase (JNK). JNK activation by hypoxia requires assembly with the JIP1 scaffold protein, which might also interact with other intracellular proteins that are less well known but that might modulate MAPK signaling. We report that TAK1 is able to form a stable complex with JIP1 and thus regulate the activation of JNK, which in turn determines the cellular stress response to hypoxia. This activation of TAK1-JIP1-JNK is suppressed by vaccinia-related kinase 2 (VRK2). VRK2A is able to interact with TAK1 by its C-terminal region, forming stable complexes. The kinase activity of VRK2 is not necessary for this interaction or the downregulation of AP1-dependent transcription. Furthermore, reduction of the endogenous VRK2 level with short hairpin RNA can increase the response induced by hypoxia, suggesting that the intracellular levels of VRK2 can determine the magnitude of this stress response.

Oxidative stress enhances phosphorylation of p53 in neonatal rat cardiomyocytes

p53 is an important regulator of cell growth and apoptosis and its activity is regulated by phosphorylation. Accordingly, in neonatal rat cardiomyocytes we examined the involvement of p53 in H(2)O(2)-induced apoptosis. Treatment with 50-100 microM H(2)O(2) markedly induced apoptosis in cardiomyocytes, as assessed by gel electrophoresis of genomic DNA. To examine whether H(2)O(2) increases p53 phosphorylation in cardiomyocytes, we utilized an antibody that specifically recognizes phosphorylated p53 at serine-15. The level of phosphorylated p53 was markedly increased by 100 microM H(2)O(2) at 30 and 60 min. Using specific protein kinase inhibitors we examined the involvement of protein kinases in p53 phosphorylation in response to H(2)O(2) treatment. However, staurosporine, a broad spectrum inhibitor of protein kinases, SB202190, a specific p38 kinase inhibitor, PD98059, a MAP kinase inhibitor, wortmannin, an inhibitor of DNA-PK and PI3 kinase, SP600125, a JNK inhibitor and caffeine,an inhibitor of ATM and ATR, failed to prevent the H(2)O(2)-induced phosphorylation of p53. cDNA microarray revealed that H(2)O(2) markedly increased expression of several p53 upstream modifiers such as the p300 coactivator protein and several downstream effectors such as gadd45, but decreased the expression of MDM2, a negative regulator of p53. Our results suggest that phosphorylation of p53 at serine-15 may be an important signaling event in the H(2)O(2)-mediated apoptotic process.

ROS generation in endothelial hypoxia and reoxygenation stimulates MAP kinase signaling and kinase-dependent neutrophil recruitment

Reactive oxygen species (ROS)-induced injury has been shown to occur during the reperfusion phase of ischemia-reperfusion and ROS are known to induce signaling events. We hypothesized that oxygen sensing in endothelial cells is also dependent on internal redox changes during hypoxia and that endothelial cells respond to changing oxygen environments via signaling, switching to an inflammatory phenotype. Endothelial cells exposed to relative hypoxia or the mitochondrial inhibitors rotenone, antimycin A, or FCCP show loss of mitochondrial membrane potential. During hypoxia, an increase in cytoplasmic ROS and glutathione S-transferase activity occurred, suggesting changes in intracellular redox state, mimicked with rotenone or FCCP but inhibited by antimycin A. Phosphorylation of stress-responsive mitogen-activated protein kinases occurred in hypoxia and was rapid and prolonged. Phosphorylation was inhibited by vitamin C, N-acetyl cysteine, or antimycin A. Chelation of intracellular calcium inhibits phosphorylation but the mitochondrial transition pore inhibitor cyclosporin A had no effect. Reoxygenation caused a further round of signaling, which was rapid but transient. Functionally, adhesion of neutrophils after hypoxia-reoxygenation under flow is ROS, P-selectin, and MAPK dependent. Therefore, changes in cellular signaling and phenotype are abrogated by ROS scavengers and suggest their use as therapeutic agents in ischemia-reperfusion.

Redox signalling in anchorage-dependent cell growth

Current data have provided new perspectives concerning the regulation of non-transformed cell proliferation in response to both soluble growth factors and to adhesive cues. Non-transformed cells are anchorage dependent for the execution of the mitotic program and cannot avoid the concomitant signals starting from mitogenic molecules, as growth factors, and adhesive agents belonging to extracellular matrix. Reactive oxygen species play a key role during both growth factor and integrin receptor signalling and these second messengers are recognised to have a synergistic function for anchorage-dependent growth signalling. Redox regulated proteins include protein tyrosine phosphatases and protein tyrosine kinases, although with opposite regulation of their enzymatic activity, and cytoskeletal proteins as beta-actin. In this review we support a role of ROS as key second messengers granting a proper executed mitosis for anchorage-dependent cells, through redox regulation of several downstream targets. Deregulation of these redox pathways may help to guide transformed cells to elude the native apoptotic response to abolishment of signals started by cell/ECM contact, sustaining ectopic anchorage-independent cancer cell growth.

PKC-α and TAK-1 are intermediates in the activation ofc-Jun NH2-terminal kinase by hypoxia-reoxygenation

c-Jun NH2-terminal kinase (JNK), a member of the MAPK family of protein kinases, is a stress-response kinase that is activated by proinflammatory cytokines and growth factors coupled to membrane receptors or through nonreceptor pathways by stimuli such as heat shock, UV irradiation, protein synthesis inhibitors, and conditions that elevate the levels of reactive oxygen intermediates (ROI). Ischemia followed by reperfusion or hypoxia with reoxygenation represents a condition of high oxidative stress where JNK activation is associated with elevated ROI. We recently demonstrated that the activation of JNK by this condition is initiated by ROI generated by mitochondrial electron transport and involves sequential activation of the proline-rich kinase 2 and the small GTP-binding factors Rac-1 and Cdc42. Here we present evidence that protein kinase C (PKC) and transforming growth factor-β-activated kinase-1 (TAK-1) are also components of this pathway. Inhibition of PKC with the broad-range inhibitor calphostin C, the PKC-α/β-selective inhibitor Go9367, or adenovirus-expressing dominant-negative PKC-α blocked the phosphorylation of proline-rich kinase 2 and JNK. Reoxygenation activated the mitogen-activated protein kinase kinase kinase, TAK-1, and promoted the formation of a complex containing Rac-1, TAK-1, and JNK but not apoptosis-stimulating kinase-1 or p21-activated kinase-1, which was detected within the first 10 min of reoxygenation. These results identify two new components, PKC and TAK-1, that have not been previously described in this signaling pathway.

A peptide inhibitor of c-Jun NH2-terminal kinase reduces myocardial ischemia-reperfusion injury and infarct size in vivo

The c-Jun NH2-terminal kinase (JNK) pathway of the mitogen-activated protein kinase (MAPK) signaling cascade regulates cell function and survival after stress stimulation. Equally robust studies reported dichotomous results suggesting both protective and detrimental effects of JNK during myocardial ischemia-reperfusion (I/R). The lack of a highly specific JNK inhibitor contributed to this controversy. We recently developed a cell-penetrating, protease-resistant peptide inhibitor of JNK, d-JNKI-1. Here we report on the effects of d-JNKI-1 in myocardial I/R. d-JNKI-1 was tested in isolated-perfused adult rat hearts. Increased activation of JNK, p38-MAPK, and extracellular signal-regulated kinase-1/2 (ERK1/2), as assessed by kinase assays and Western blotting, occurred during I/R. d-JNKI-1 delivered before onset of ischemia prevented the increase in JNK activity while not affecting ERK1/2 and p38-MAPK activation. JNK inhibition reduced ischemic injury, as manifested by increased time to contracture ( P < 0.05) and decreased left ventricular end-diastolic pressure during ischemia ( P < 0.01), and enhanced posthypoxic recovery of systolic and diastolic function ( P < 0.01). d-JNKI-1 reduced mitochondrial cytochrome- c release, caspase-3 activation, and the number of apoptotic cells determined by terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling ( P < 0.05), indicating suppression of the mitochondrial machinery of apoptosis. d-JNKI-1 delivered at the time of reperfusion did not improve functional recovery but still prevented apoptosis. In vivo, d-JNKI-1 reduced infarct size after coronary artery occlusion and reperfusion by ∼50% ( P < 0.01). In conclusion, d-JNKI-1 is an important compound that can be used in preclinical models to investigate the role of JNK signaling in vivo. Inhibition of JNK during I/R is cardioprotective in anesthetized rats in vivo.

Energy sensing and regulation of gene expression in skeletal muscle

Major modifications in energy homeostasis occur in skeletal muscle during exercise. Emerging evidence suggests that changes in energy homeostasis take part in the regulation of gene expression and contribute to muscle plasticity. A number of energy-sensing molecules have been shown to sense variations in energy homeostasis and trigger regulation of gene expression. The AMP-activated protein kinase, hypoxia-inducible factor 1, peroxisome proliferator-activated receptors, and Sirt1 proteins all contribute to altering skeletal muscle gene expression by sensing changes in the concentrations of AMP, molecular oxygen, intracellular free fatty acids, and NAD+, respectively. These molecules may therefore sense information relating to the intensity, duration, and frequency of muscle exercise. Mitochondria also contribute to the overall response, both by modulating the response of energy-sensing molecules and by generating their own signals. This review seeks to examine our current understanding of the roles that energy-sensing molecules and mitochondria can play in the regulation of gene expression in skeletal muscle.

Mitochondrial c-Jun NH2-terminal kinase prevents the accumulation of reactive oxygen species and reduces necrotic damage in neural tumor cells that lack trophic support

In response to different stress signals, the c-Jun NH(2)-terminal kinase (JNK) can trigger cell death. However, JNK also facilitates the survival and cell cycle progression of tumor cells by mechanisms that are poorly defined. Here, we show that schwannoma RN22 cells can survive and proliferate under serum-free conditions although serum withdrawal rapidly induces mitochondrial fission and swelling. Although the morphologic changes observed in the mitochondria did not trigger cytochrome c release, they were accompanied by an increase in the mitochondrial membrane potential (DeltaPsi(M)) and of immunoreactivity for active JNK in these organelles. Pharmacologic inhibition of JNK provoked a further increase of the DeltaPsi(M), an increase in reactive oxygen species (ROS) production, and a sustained decrease in cell viability due to necrosis. This increase in necrosis was prevented by the presence of ROS scavengers. Immunoreactivity for active JNK was also observed in the mitochondria of neuroblastoma 1E-115 and neuroblastoma 2a neuroblastoma cell lines on serum withdrawal, whereas active JNK was barely detected in serum-deprived fibroblasts. Accordingly, the reduction in neural tumor cell viability induced by JNK inhibition was largely attenuated in serum-deprived fibroblasts. These data indicate that local activation of JNK in the mitochondria can protect against necrotic cell death associated with ROS production, facilitating the growth of neural tumor cells subjected to serum deprivation.

Cellular mechanisms of neuronal damage from hyperthermia

Hyperthermia can cause brain damage and also exacerbate the brain damage produced by stroke and amphetamines. The developing brain is especially sensitive to hyperthermia. The severity of, and mechanisms underlying, hyperthermia-induced neuronal death depend on both temperature and duration of exposure. Severe hyperthermia can produce necrotic neuronal death. For a window of less severe heat stresses, cultured neurons exhibit a delayed death with apoptotic characteristics including cytochrome c release and caspase activation. Little is known about mechanisms of hyperthermia-induced damage upstream of these late apoptotic effects. This chapter considers several possible upstream mechanisms, drawing on both in vivo and in vitro studies of the nervous system and other tissues. Hyperthermia-induced damage in some non-neuronal cells includes endoplasmic reticular stress due to denaturing of nascent polypeptide chains, as well as nuclear and cytoskeletal damage. Evidence is presented that hyperthermia produces mitochondrial damage, including depolarization, in cultured mammalian neurons.

Ischemia-reperfusion and cardioprotection: a delicate balance between reactive oxygen species generation and redox homeostasis

Ischemia-reperfusion injury of the myocardium has long been a subject of intense research. Cardiac preconditioning, an associated phenomenon, has also been critically investigated over the past two decades. Although the biochemistry of ischemia-reperfusion and its association with oxidative metabolism has long been established, recent studies have further revealed a more intricate role of a number of reactive oxygen-nitrogen species in those processes. Emerging evidence suggests that an elaborate network of enzymes (and other biomolecules) dedicated to the generation, utilization, and diminution of reactive oxygen-nitrogen species maintains the redox homeostasis in the myocardium, and any perturbation of its status has distinctive effects. It thus appears that while excessive generation of reactive species leads to cellular injury, their regulated generation may cause transient and reversible modifications of cellular proteins leading the transmission of intracellular signals with specific effects. Taken together, generation of reactive oxygen-nitrogen species in the myocardium plays a nodal role in mediating both ischemic injury and cardioprotection.

Protein tyrosine phosphorylation and reversible oxidation: two cross-talking posttranslation modifications

In addition to protein phosphorylation, redox-dependent posttranslational modification of proteins is emerging as a key signaling system, conserved throughout evolution, and influencing many aspects of cellular homeostasis. Recent data have provided new insight about the interplay between phosphorylation- and redox-dependent signaling, and reactive oxygen species have been included among intracellular signal transducers of growth factor and extracellular matrix receptors. Both tyrosine phosphorylation and thiol oxidation are reversible and dynamic, and this review will particularly focus on the cross-talk between these posttranslational protein regulatory means. Although these modifications share their reversibility, their effects on enzymatic activity of protein tyrosine phosphatases (PTPs) and protein tyrosine kinases (PTKs) may be even opposite. Indeed, while tyrosine phosphorylation is frequently correlated to enzyme activation, thiol oxidation leads to inactivation of PTPs and to superactivation of PTKs. Several papers describe that both these modifications occur during the same input, (i.e., cell proliferation and motility induced by numerous growth factors and cytokines). The review will discuss several aspects of phosphorylation\oxidation interplay, describing both convergent and divergent features of the integrated and coordinated function of PTPs and PTKs during signaling.

Survival response-linked Pyk2 activation during potassium depletion-induced apoptosis of cerebellar granule neurons

Numerous extracellular stimuli trigger trans-autophosphorylation at Tyr402 of Pyk2, inducing its activation. Pyk2 is a key mediator of several signaling pathways and has been implicated in apoptosis induced by specific stress signals. We investigated whether Pyk2 participates in cerebellar granule neuron (CGN) apoptosis induced by the suppression of membrane depolarization. We demonstrate that shifting CGN cultures from 25 mM to 5 mM KCl-containing medium induces an early, transient 70% increase in phosphorylated Tyr402 and Tyr580 Pyk2 levels that is triggered by Ca(2+) released from intracellular stores and mediated by calmodulin (CaM). Overexpression of Pyk2 increases CGN survival after 24 h by 70% compared to the control, thus suggesting that Pyk2 is involved in an anti-apoptotic response to K+ lowering. Furthermore, we show that CGN grown in K25 medium exhibit detectable CaM-dependent Pyk2 activity. When silencing Pyk2 activity by expressing a dominant-negative form, only 40% of the transfected neurons were alive 24 h after transfection when compared to the control. Overall, the present findings demonstrate for the first time that Pyk2 is a critical mediator of CGN survival.

Beyond anoxia: the physiology of metabolic downregulation and recovery in the anoxia-tolerant turtle

The freshwater turtle Trachemys scripta is among the most anoxia-tolerant of vertebrates, a true facultative anaerobe able to survive without oxygen for days at room temperature to weeks or months during winter hibernation. Our good friend and colleague Peter Lutz devoted nearly 25 years to the study of the physiology of anoxia tolerance in these and other model organisms, promoting not just the basic science but also the idea that understanding the physiology and molecular mechanisms behind anoxia tolerance provides insights into critical survival pathways that may be applicable to the hypoxic/ischemic mammalian brain. Work by Peter and his colleagues focused on the factors which enable the turtle to enter a deep hypometabolic state, including decreases in ion flux ("channel arrest"), increases in inhibitory neuromodulators like adenosine and GABA, and the maintenance of low extracellular levels of excitatory compounds such as dopamine and glutamate. Our attention has recently turned to molecular mechanisms of anoxia tolerance, including the upregulation of such protective factors as heat shock proteins (Hsp72, Hsc73), the reversible downregulation of voltage gated potassium channels, and the modulation of MAP kinase pathways. In this review we discuss three phases of anoxia tolerance, including the initial metabolic downregulation over the first several hours, the long-term maintenance of neuronal function over days to weeks of anoxia, and finally recovery upon reoxygenation, with necessary defenses against reactive oxygen stress.

Phosphoregulation of Signal Transduction Pathways in Ischemia and Reperfusion

Ischemia/reperfusion (I/R) injury triggered by pathogenic processes, such as organ transplant dysfunction, stroke, myocardial infarction, and shock, stimulate both immune and inflammatory pathways. Inflammatory cell activation and cytotoxic cytokine expression are associated with reperfusion injury. The activation of these inflammatory mediators initiates several interconnected downstream cascades regulated by phosphorylation and dephosphorylation reactions. These complex phosphorylation-dependent signal transduction pathways ultimately initiate nuclear transcription of inflammatory as well as anti-inflammatory genes to repair and assist in the recovery of damaged cells. Radical oxygen species (ROS) production, under ischemic conditions, initiates a cascade of events regulated by phosphorylation/dephosphorylation reactions and inflammatory gene expression. This is a review of the current understanding of the phosphoregulatory mechanisms that mediate the complex processes of signal transduction secondary to I/R injury. The rationale for inhibiting or activating signaling pathways as a promising molecular target for ameliorating reperfusion injury in I/R-related diseases, such as stroke, myocardial infarction, and storage for transplantation, is discussed on the basis of a new understanding of the mechanisms modulating phosphoregulatory pathways. In addition, we present part of our ongoing research in this field with phosphoregulatory signal transduction and its potential application.

S-nitrosothiol inhibition of mitochondrial complex I causes a reversible increase in mitochondrial hydrogen peroxide production

We found that reversible inactivation of mitochondrial complex I by S-nitroso-N-acetyl-D,L-penicillamine (SNAP) in isolated rat heart mitochondria resulted in a three-fold increase in H2O2 production, when mitochondria were respiring on pyruvate and malate, (but not when respiring on succinate or in the absence of added respiratory substrate). The inactivation of complex I and the increased H2O2 production were present in mitochondria washed free of SNAP or NO, but were partially reversed by light or dithiothreitol, treatments known to reverse S-nitrosation. Specific inhibition of complex I with rotenone increased H2O2 production to a similar extent as that caused by SNAP. The results suggest that S-nitrosation of complex I can reversibly increase oxidant production by mitochondria, which is potentially important in cell signalling and/or pathology.

The Rac and Rho hall of fame: a decade of hypertrophic signaling hits

Over the last decade, the Rho family GTPases have gained considerable recognition as powerful regulators of actin cytoskeletal organization. As with many high profile signal transducers, these molecules soon attracted the attention of the cardiovascular research community. Shortly thereafter, two prominent members known as RhoA and Rac1 were linked to agonist-induced gene expression and myofilament organization using the isolated cardiomyocyte cell model. Subsequent creation of transgenic mouse lines provided evidence for more complex roles of RhoA and Rac1 signaling. Clues from in vitro and in vivo studies suggest the involvement of numerous downstream targets of RhoA and Rac1 signaling including serum response factor, NF-kappaB, and other transcription factors, myofilament proteins, ion channels, and reactive oxygen species generation. Which of these contribute to the observed phenotypic effects of enhanced RhoA and Rac activation in vivo remain to be determined. Current research efforts with a more translational focus have used statins or Rho kinase blockers to assess RhoA and Rac1 as targets for interventional approaches to blunt hypertrophy or heart failure. Generally, salutary effects on remodeling and ischemic damage are observed, but the broad specificity and multiple cellular targets for these drugs within the myocardium demands caution in interpretation. In this review, we assess the evolution of knowledge related to Rac1 and RhoA in the context of hypertrophy and heart failure and highlight the direction that future exploration will lead.

Cell signalling by oxidized lipids and the role of reactive oxygen species in the endothelium

The controlled formation of ROS (reactive oxygen species) and RNS (reactive nitrogen species) is now known to be critical in cellular redox signalling. As with the more familiar phosphorylation-dependent signal transduction pathways, control of protein function is mediated by the post-translational modification at specific amino acid residues, notably thiols. Two important classes of oxidant-derived signalling molecules are the lipid oxidation products, including those with electrophilic reactive centres, and decomposition products such as lysoPC (lysophosphatidylcholine). The mechanisms can be direct in the case of electrophiles, as they can modify signalling proteins by post-translational modification of thiols. In the case of lysoPC, it appears that secondary generation of ROS/RNS, dependent on intracellular calcium fluxes, can cause the secondary induction of H2O2 in the cell. In either case, the intracellular source of ROS/RNS has not been defined. In this respect, the mitochondrion is particularly interesting since it is now becoming apparent that the formation of superoxide from the respiratory chain can play an important role in cell signalling, and oxidized lipids can stimulate ROS formation from an undefined source. In this short overview, we describe recent experiments that suggest that the cell signalling mediated by lipid oxidation products involves their interaction with mitochondria. The implications of these results for our understanding of adaptation and the response to stress in cardiovascular disease are discussed.

c-Jun N-Terminal Kinases Mediate Reactivation of Akt and Cardiomyocyte Survival After Hypoxic Injury In Vitro and In Vivo

Akt is a central regulator of cardiomyocyte survival after ischemic injury in vitro and in vivo, but the mechanisms regulating Akt activity in the postischemic cardiomyocyte are not known. Furthermore, although much is known about the detrimental role that the c-Jun N-terminal kinases (JNKs) play in promoting death of cells exposed to various stresses, little is known of the molecular mechanisms by which JNK activation can be protective. We report that JNKs are necessary for the reactivation of Akt after ischemic injury. We identified Thr450 of Akt as a residue that is phosphorylated by JNKs, and the phosphorylation status of Thr450 regulates reactivation of Akt after hypoxia, apparently by priming Akt for subsequent phosphorylation by 3-phosphoinositide–dependent protein kinase. The reduction in Akt activity that is induced by JNK inhibition may have significant biological consequences, as we find that JNKs, acting via Akt, are critical determinants of survival in posthypoxic cardiomyocytes in culture. Furthermore, in contrast to selective p38–mitogen-activated protein kinase inhibition, which was cardioprotective in vivo, concurrent inhibition of both JNKs and p38–mitogen-activated protein kinases increased ischemia/reperfusion injury in the heart of the intact rat. These studies demonstrate that reactivation of Akt after resolution of hypoxia and ischemia is regulated by JNKs and suggest that this is likely a central mechanism of the myocyte protective effect of JNKs.

Subthalamic nucleus lesioning inhibits expression and phosphorylation of c-Jun in nigral neurons in the rat's 6-OHDA model of Parkinson's disease

Parkinson's Disease (PD) is characterized by a loss of nigral dopamine (DA) neurons, followed by a striatal DA deficit. Inhibition of the subthalamic nucleus (STN) reverses L-DOPA sensitive motor symptoms and improves efficacy of pharmacotherapy in PD-patients. The underlying mechanism of these effects, however, remains largely unknown. Previously, we could show in the rat's 6-hydroxyDA (6-OHDA) model of PD that ablative STN-lesioning exerts functionally neuroprotective effects on the DAergic nigrostriatal pathway against 6-OHDA toxicity, in terms of elevating the number of tyrosine hydroxylase (TH)-expressing neurons rather than enhancing the total number of cells surviving 2 and 6 weeks post lesioning, as assessed via fluorogold staining. These data were correlated with increased functional recovery of 6-OHDA-lesioned rats with preceding STN-lesioning. Here, we extend the previous study design to observation periods of up to 12 weeks to assess long-term effects. Furthermore, to elucidate cellular mechanisms underlying potential neuroprotective effects, we explore the regulation of cellular markers involved in neurodegenerative cascades via immunocytochemistry. We show that preceding STN-lesioning significantly inhibits 6-OHDA induced expression/phosphorylation of the transcription factor c-Jun in surviving nigral neurons in comparison with controls. However, we also demonstrate that functionally neuroprotective effects of preceding STN-lesioning subside after 12 weeks, as assessed with TH immunostaining. We therefore conclude that c-Jun induction/phosphorylation is involved in 6-OHDA toxicity and that STN-lesioning transiently preserves of dopaminergic phenotype of nigral neurons partially via delaying the induction and attenuating the expression and phosphorylation of c-Jun.

Salvage of ischemic myocardium: a focus on JNK

Myocardial infarction is a problem of utmost clinical significance, associated with an important morbidity and mortality. Actual treatment of this affection is focusing on the reperfusion of the occluded coronary-artery. A complementary approach would be to prevent the death of the ischemic myocardium by interacting with detrimental intracellular pathways. Several strategies have been successfully used to reduce the size of myocardial infarction in animal models. In this article, we will focus on the c-Jun N-terminal kinase (JNK), a member of the mitogen-activated (MAPK) protein kinase family and an important determinant of cell survival/death. We will review the role of JNK in cardiac ischemia/reperfusion and summarize recent advances in the use of JNK inhibitors to protect the myocardium.

Hypothyroid phenotype is contributed by mitochondrial complex I inactivation due to translocated neuronal nitric-oxide synthase

Although transcriptional effects of thyroid hormones have substantial influence on oxidative metabolism, how thyroid sets basal metabolic rate remains obscure. Compartmental localization of nitric-oxide synthases is important for nitric oxide signaling. We therefore examined liver neuronal nitric-oxide synthase-alpha (nNOS) subcellular distribution as a putative mechanism for thyroid effects on rat metabolic rate. At low 3,3',5-triiodo-L-thyronine levels, nNOS mRNA increased by 3-fold, protein expression by one-fold, and nNOS was selectively translocated to mitochondria without changes in other isoforms. In contrast, under thyroid hormone administration, mRNA level did not change and nNOS remained predominantly localized in cytosol. In hypothyroidism, nNOS translocation resulted in enhanced mitochondrial nitric-oxide synthase activity with low O2 uptake. In this context, NO utilization increased active O2 species and peroxynitrite yields and tyrosine nitration of complex I proteins that reduced complex activity. Hypothyroidism was also associated to high phospho-p38 mitogen-activated protein kinase and decreased phospho-extracellular signal-regulated kinase 1/2 and cyclin D1 levels. Similarly to thyroid hormones, but without changing thyroid status, nitric-oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester increased basal metabolic rate, prevented mitochondrial nitration and complex I derangement, and turned mitogen-activated protein kinase signaling and cyclin D1 expression back to control pattern. We surmise that nNOS spatial confinement in mitochondria is a significant downstream effector of thyroid hormone and hypothyroid phenotype.

Cell signalling by oxidized lipids and the role of reactive oxygen species in the endothelium

The controlled formation of ROS (reactive oxygen species) and RNS (reactive nitrogen species) is now known to be critical in cellular redox signalling. As with the more familiar phosphorylation-dependent signal transduction pathways, control of protein function is mediated by the post-translational modification at specific amino acid residues, notably thiols. Two important classes of oxidant-derived signalling molecules are the lipid oxidation products, including those with electrophilic reactive centres, and decomposition products such as lysoPC (lysophosphatidylcholine). The mechanisms can be direct in the case of electrophiles, as they can modify signalling proteins by post-translational modification of thiols. In the case of lysoPC, it appears that secondary generation of ROS/RNS, dependent on intracellular calcium fluxes, can cause the secondary induction of H2O2 in the cell. In either case, the intracellular source of ROS/RNS has not been defined. In this respect, the mitochondrion is particularly interesting since it is now becoming apparent that the formation of superoxide from the respiratory chain can play an important role in cell signalling, and oxidized lipids can stimulate ROS formation from an undefined source. In this short overview, we describe recent experiments that suggest that the cell signalling mediated by lipid oxidation products involves their interaction with mitochondria. The implications of these results for our understanding of adaptation and the response to stress in cardiovascular disease are discussed.

Inhibition of mitochondrial respiration elevates oxygen concentration but leaves regulation of hypoxia-inducible factor (HIF) intact

The transcription factor hypoxia-inducible factor-1 (HIF-1) is critical for erythropoietin and other factors involved in the adaptation of the organism to hypoxic stress. Conflicting results have been published regarding the role of the mitochondrial electron transport chain (ETC) in the regulation of HIF-1α. We assessed cellular hypoxia by pimonidazole staining and blotting of the O2-labile HIF-1 α-subunit in human osteosarcoma cell cultures (U2OS and 143B). In conventional, gas-impermeable cell culture dishes, ETC inhibitors had no effect on pimonidazole staining or HIF-1α abundance in a 20% O2 atmosphere; both parameters were undetectable. Pimonidazole staining and HIF activity were substantial in 0.1% O2 irrespective of ETC inhibition. At an intermediate oxygen concentration (3% O2) pimonidazole staining and HIF-α expression were detectable but strongly reduced after ETC inhibition in conventional cell cultures. All effects of ETC inhibition on HIF-1α regulation were eliminated in gas-permeable dishes. As shown in a 143B subclone deficient in mitochondrial DNA (206ρ0), genetic inactivation of the ETC led to similar responses with respect to HIF-1α regulation as ETC inhibitors. Our data demonstrate that reduction of oxygen consumption reduces the O2 gradient in conventional cell cultures, causing elevation of the cellular O2 concentration, which leads to degradation of HIF-α.

Estrogen-induced mitochondrial reactive oxygen species as signal-transducing messengers

We report here evidence in support of the role of 17beta-estradiol- (E2-) induced mitochondrial (mt) reactive oxygen species (ROS) as signal-transducing messengers. On the basis of monitoring the oxidation of 2',7'-dichlorofluorescin by spectrofluorometry, flow cytometry, and confocal microscopy, we have identified that exposure of E2 triggers the immediate rapid production of intracellular ROS ranging from a 1- to severalfold increase in a variety of cells. E2-stimulated ROS production does not correlate with the activity of the estrogen receptor (ER) in the cells. The ROS is most likely hydrogen peroxide based on its inhibition by antioxidants and catalase and lack of any effects of E2 on O(2)(*)(-) or NO(*) formation. Confocal microscopy showed that ROS is localized in the perinuclear mitochondria. E2 through anchorage- and integrin-dependent signaling to mitochondria increased ROS generation. Increased intracellular ROS formation identified here for the first time may explain the mechanism of previously reported oxidative damage and subsequent genetic alterations including mutations produced by elevated concentrations of estrogens. The functional consequences of E2-induced ROS formation included the enhanced cell motility as shown by the increase in cdc42 and activation of Pyk2 and the increased phosphorylation of signaling proteins c-jun and CREB. E2-induced ROS activated the binding of three oxidant-sensitive transcription factors: AP-1, CREB, and nuclear respiratory factor 1. In addition to ERs as signaling molecules, our findings further revealed that E2-induced mt ROS also act as signal transducing messengers and suggest new targets for the development of antioxidant-based drugs or antioxidant gene therapy for the prevention and treatment of estrogen-dependent cancer.

Mechanism of redox-mediated guanine nucleotide exchange on redox-active Rho GTPases

Rho GTPases regulate multiple cellular processes including actin cytoskeletal rearrangements, transcriptional regulation, and oxidant production. The studies described herein demonstrate that small molecule redox agents, in addition to protein regulatory factors, can regulate the activity of redox-active Rho GTPases. A novel (GXXXXGK(S/T)C) motif, conserved in a number of Rho GTPases, appears critical for redox-mediated guanine nucleotide dissociation in vitro. A detailed molecular mechanism for redox regulation of GXXXXGK(S/T)C motif-containing Rho GTPases is proposed.

A unique pathway of cardiac myocyte death caused by hypoxia-acidosis

Chronic hypoxia in the presence of high glucose leads to progressive acidosis of cardiac myocytes in culture. The condition parallels myocardial ischemia in vivo, where ischemic tissue becomes rapidly hypoxic and acidotic. Cardiac myocytes are resistant to chronic hypoxia at neutral pH but undergo extensive death when the extracellular pH (pH[o]) drops below 6.5. A microarray analysis of 20 000 genes (cDNAs and expressed sequence tags) screened with cDNAs from aerobic and hypoxic cardiac myocytes identified >100 genes that were induced by >2-fold and approximately 20 genes that were induced by >5-fold. One of the most strongly induced transcripts was identified as the gene encoding the pro-apoptotic Bcl-2 family member BNIP3. Northern and western blot analyses confirmed that BNIP3 was induced by 12-fold (mRNA) and 6-fold (protein) during 24 h of hypoxia. BNIP3 protein, but not the mRNA, accumulated 3.5-fold more rapidly under hypoxia-acidosis. Cell fractionation experiments indicated that BNIP3 was loosely bound to mitochondria under conditions of neutral hypoxia but was translocated into the membrane when the myocytes were acidotic. Translocation of BNIP3 coincided with opening of the mitochondrial permeability pore (MPTP). Paradoxically, mitochondrial pore opening did not promote caspase activation, and broad-range caspase inhibitors do not block this cell death pathway. The pathway was blocked by antisense BNIP3 oligonucleotides and MPTP inhibitors. Therefore, cardiac myocyte death during hypoxia-acidosis involves two distinct steps: (1) hypoxia activates transcription of the death-promoting BNIP3 gene through a hypoxia-inducible factor-1 (HIF-1) site in the promoter and (2) acidosis activates BNIP3 by promoting membrane translocation. This is an atypical programmed death pathway involving a combination of the features of apoptosis and necrosis. In this article, we will review the evidence for this unique pathway of cell death and discuss its relevance to ischemic heart disease. The article also contains new evidence that chronic hypoxia at neutral pH does not promote apoptosis or activate caspases in neonatal cardiac myocytes.