TNFα-induced cytoprotection requires the production of free radicals within mitochondria in C2C12 myotubes

Characterization of Glutathione Dithiophosphates as Long-Acting H2S Donors

Considering the important cytoprotective and signaling roles but relatively narrow therapeutic index of hydrogen sulfide (H₂S), advanced H₂S donors are required to achieve a therapeutic effect. In this study, we proposed glutathione dithiophosphates as new combination donors of H₂S and glutathione. The kinetics of H₂S formation in dithiophosphate solutions suggested a continuous H₂S release by the donors, which was higher for the dithiophosphate of reduced glutathione than oxidized glutathione. The compounds, unlike NaHS, inhibited the proliferation of C2C12 myoblasts at submillimolar concentrations due to an efficient increase in intracellular H₂S. The H₂S donors more profoundly affected reactive oxygen species and reduced glutathione levels in C2C12 myocytes, in which these parameters were elevated compared to myoblasts. Oxidized glutathione dithiophosphate as well as control donors exerted antioxidant action toward myocytes, whereas the effect of reduced glutathione dithiophosphate at (sub-)micromolar concentrations was rather modulating. This dithiophosphate showed an enhanced negative inotropic effect mediated by H₂S upon contraction of the atrial myocardium, furthermore, its activity was prolonged and reluctant for washing. These findings identify glutathione dithiophosphates as redox-modulating H₂S donors with long-acting profile, which are of interest for further pharmacological investigation.

Carboxyl-terminal modulator protein (CTMP) deficiency mitigates denervation-induced skeletal muscle atrophy

Denervated skeletal muscles show decreased Akt activity and phosphorylation, resulting in atrophy. Akt inhibits downstream transcription of atrophy-associated ubiquitin ligases like muscle ring-finger protein 1 (MuRF-1). In addition, reduced Akt signaling contributes to aberrant protein synthesis in muscles. In ALS mice, we recently found that carboxyl-terminator modulator protein (CTMP) expression is increased and correlated with reduced Akt signaling in atrophic skeletal muscle. CTMP has also been implicated in promoting muscle degeneration and catabolism in an in vitro muscle atrophy model. The present study examined whether sciatic nerve injury (SNI) stimulated CTMP expression in denervated skeletal muscle during muscle atrophy. We hypothesized that CTMP deficiency would reduce neurogenic atrophy and reverse Akt signaling downregulation. Compared to the unaffected contralateral muscle, wild-type (WT) gastrocnemius muscle had a significant increase in CTMP (p < 0.05). Furthermore, denervated CTMP knockout (CTMP-KO) gastrocnemius weighed more than WT muscle (p < 0.05). Denervated CTMP-KO gastrocnemius also showed higher Akt and downstream glycogen synthase kinase 3β (GSK3β) phosphorylation compared to WT muscle (p < 0.05) as well as ribosomal proteins S6 and 4E-BP1 phosphorylation (p < 0.001 and p < 0.05, respectively). Moreover, CTMP-KO mice showed significantly lower levels of E3 ubiquitin ligase MuRF-1 and myostatin than WT muscle (p < 0.05). Our findings suggest that CTMP is essential to muscle atrophy after denervation and it may act by reducing Akt signaling, protein synthesis, and increasing myocellular catabolism.

Nuclear Genome-Encoded Long Noncoding RNAs and Mitochondrial Damage in Diabetic Retinopathy

Retinal mitochondria are damaged in diabetes-accelerating apoptosis of capillary cells, and ultimately, leading to degenerative capillaries. Diabetes also upregulates many long noncoding RNAs (LncRNAs), including LncMALAT1 and LncNEAT1. These RNAs have more than 200 nucleotides and no open reading frame for translation. LncMALAT1 and LncNEAT1 are encoded by nuclear genome, but nuclear-encoded LncRNAs can also translocate in the mitochondria. Our aim was to investigate the role of LncMALAT1 and LncNEAT1 in mitochondrial homeostasis. Using human retinal endothelial cells, the effect of high glucose on LncMALAT1 and LncNEAT1 mitochondrial localization was examined by RNA fluorescence in situ hybridization. The role of these LncRNAs in mitochondrial membrane potential (by JC-I staining), mtDNA integrity (by extended length PCR) and in protective mtDNA nucleoids (by SYBR green staining) was examined in MALAT1- or NEAT1-siRNA transfected cells. High glucose increased LncMALAT1 and LncNEAT1 mitochondrial expression, and MALAT1-siRNA or NEAT1-siRNA ameliorated glucose-induced damage to mitochondrial membrane potential and mtDNA, and prevented decrease in mtDNA nucleoids. Thus, increased mitochondrial translocation of LncMALAT1 or LncNEAT1 in a hyperglycemic milieu plays a major role in damaging the mitochondrial structural and genomic integrity. Regulation of these LncRNAs can protect mitochondrial homeostasis, and ameliorate formation of degenerative capillaries in diabetic retinopathy.

Sex Differences of Human Cardiac Progenitor Cells in the Biological Response to TNF-α Treatment

Adult cardiac progenitor cells (CPCs), isolated as cardiosphere-derived cells (CDCs), represent promising candidates for cardiac regenerative therapy. CDCs can be expanded in vitro manyfolds without losing their differentiation potential, reaching numbers that are appropriate for clinical applications. Since mechanisms of successful CDC survival and engraftment in the damaged myocardium are still critical and unresolved issues, we aimed at deciphering possible key factors capable of bolstering CDC function. In particular, the response and the phenotype of CDCs exposed to low concentrations of the multifunctional cytokine tumor necrosis factor α (TNF-α), known to be capable of activating cell survival pathways, have been investigated. Furthermore, differential biological responses of CDCs from male and female donors, in terms of cell cycle progression and cell spreading, have also been assessed. The results obtained indicate that (i) the intracellular signaling activated in our experimental conditions is most likely due to the prosurvival and proliferative signaling of TNF-α receptor 2 and that (ii) cells from female patients appear more responsive to TNF-α treatment in terms of cell cycle progression and migration ability. In conclusion, the present report highlights the hypothesis that TNF-stimulated CDCs isolated from females may represent a promising candidate for cardiac regenerative therapy applications.

SS-31 attenuates TNF-α induced cytokine release from C2C12 myotubes

TNF-α is a key inflammatory mediator and is proposed to induce transcriptional responses via the mitochondrial generation of Reactive Oxygen Species (ROS). The aim of this study was to determine the effect of TNF-α on the production of myokines by skeletal muscle. Significant increases were seen in the release of IL-6, MCP-1/CCL2, RANTES/CCL5 and KC/CXCL1 and this release was inhibited by treatment with Brefeldin A, suggesting a golgi-mediated release of cytokines by muscle cells. An increase was also seen in superoxide in response to treatment with TNF-α, which was localised to the mitochondria and this was also associated with activation of NF-κB. The changes in superoxide, activation of NF-kB and release of myokines were attenuated following pre-treatment with SS-31 peptide indicating that the ability of TNF-α to induce myokine release may be mediated through mitochondrial superoxide, which is, at least in part, associated with activation of the redox sensitive transcription factor NF-kB.

The protective effects of 17beta-estradiol against ischemia-reperfusion injury and its effect on pacing postconditioning protection to the heart

The role of pacing postconditioning (PPC) in the heart protection against ischemia-reperfusion injury is not completely understood. The aim of this study was to investigated if 17-β-estradiol (estrogen, E2), endogenous atrial natriuretic peptide (ANP), endogenous brain natriuretic peptide (BNP), and tumor necrosis factor-alpha (TNF-α) are involved in PPC-mediated protection. Langendorff perfused female Wistar rat hearts were used for this study. Hearts challenged with regional ischemia for 30 min subjected to no further treatment served as a control. The PPC protocol was 3 cycles of 30 s pacing alternated between the right atrium and left ventricle (LV). Protection was assessed by recovery of LV contractility and coronary vascular-hemodynamics. Ischemia induced a significant (P < 0.05) deterioration in the heart function compared with baseline data. PPC alone or in combination with short-term E2 treatment (E2 infusion at the beginning of reperfusion) significantly (P < 0.05) improved the heart functions. Short-term E2 treatment post-ischemically afforded protection similar to that of PPC. However, long-term E2 substitution for 6 weeks completely attenuated the protective effects of PPC. Although no changes were noted in endogenous ANP levels, PPC significantly increased BNP expression level and decreased TNF-α in the cardiomyocyte lysate and coronary effluent compared to ischemia and controls. Our data suggested a protective role for short-term E2 treatment similar to that of PPC mediated by a pathway recruiting BNP and downregulating TNF-α. Our study further suggested a bad influence for long-term E2 substitution on the heart as it completely abrogated the protective effects of PPC.

Development of an in vitro model of myotube ischemia

Critical limb ischemia causes severe damage to the skeletal muscle. This study develops a reproducible model of myotube ischemia by simulating, in vitro, the critical parameters that occur in skeletal muscle ischemia. Monolayers of C2C12 myoblasts were differentiated into mature myotubes and exposed to nutrition depletion, hypoxia and hypercapnia for variable time periods. A range of culture media and gas mixture combinations were used to obtain an optimum ischemic environment. Nuclear staining, cleaved caspase-3 and lactate dehydrogenase (LDH) release assay were used to assess apoptosis and myotube survival. HIF-1α concentration of cell lysates, pH of conditioned media as well as partial pressures of oxygen (PO₂) and carbon dioxide (PCO₂) in the media were used to confirm ischemic simulation. Culturing myotubes in depleted media, in a gas mixture containing 20% CO+80% N₂ for 6-12 h increased the PCO₂ and decreased the pH and PO₂ of culture media. This attempts to mimic the in vivo ischemic state of skeletal muscle. These conditions were used to study the potential tissue-protective effects of erythropoietin (EPO) in C2C12 myotubes exposed to ischemia. EPO (60 ng/ml) suppressed LDH release, decreased cleaved caspase-3 and reduced the number of apoptotic nuclei, suggesting significantly decreased ischemia-induced apoptosis in myotubes (P<0.01) and a potential role in tissue protection. Additional therapeutic agents designed for tissue protection can also be evaluated using this model.

Reperfusion injury salvage kinase and survivor activating factor enhancement prosurvival signaling pathways in ischemic postconditioning: two sides of the same coin

The discovery of ischemic postconditioning (IPost) has rejuvenated the field of cardioprotection. As an interventional strategy to be applied at the onset of myocardial reperfusion, the transition of IPost from a bench-side curiosity to potential clinical therapy has been impressively rapid. Its existence also confirms the existence of lethal myocardial reperfusion injury in man, suggesting that 40%-50% of the final reperfused myocardial infarct may actually be due to myocardial reperfusion injury. Intensive analysis of the signal transduction pathways underlying IPost has identified similarities with the signaling pathways underlying its preischemic counterpart, ischemic preconditioning. In this article, the reperfusion injury salvage kinase pathway and the more recently described survivor activating factor enhancement pathway, two apparently distinct signaling pathways that actually interact to convey the IPost stimulus from the cell surface to the mitochondria, where many of the prosurvival and death signals appear to converge. The elucidation of the reperfusion signaling pathways underlying IPost may result in the identification of novel pharmacological targets for cardioprotection.

Focal cerebral ischemia and mitochondrial dysfunction in the TNFα-transgenic rat

Post-ischemic neurodegeneration may be accelerated by a cytokine-receptor mediated apoptotic pathway, as shown in a transgenic rat overexpressing tumor necrosis factor-alpha (TNFα) in brain. To further investigate the mechanism of ischemic cellular injury in this animal, we tested the hypothesis that increased synthesis of TNFα augments neuronal death by promoting mitochondrial dysfunction, calcium dysregulation, and oxidative stress. Adult male TNFα-transgenic (TNFα-Tg) and non-transgenic (non-Tg) littermates underwent reversible middle cerebral artery occlusion (MCAO) for 1 hour followed by 1 hour of reperfusion. Cortical mitochondria were isolated from injured (ipsilateral) and uninjured (contralateral) hemispheres of ischemic rats or from pooled hemispheres of control animals. ATP synthesis was attenuated in non-ischemic TNFα-Tg rats, demonstrated by reduction of state III and respiratory control ratio, increased production of reactive oxygen species, and earlier formation of the calcium-induced membrane permeability transition pore. After MCAO, mitochondrial dysfunction was augmented more significantly in ischemic TNFα-Tg brain mitochondria than in non-Tg rats. These results show that mitochondrial dysfunction may be caused by increased brain levels of TNFα without physiological stress but will be exacerbated after MCAO. We conclude that ischemic stress and synthesis of inflammatory cytokines synergistically augment mitochondrial dysfunction to promote neuronal death.

TNFα protects cardiac mitochondria independently of its cell surface receptors

Our novel proposal is that TNFα exerts a direct effect on mitochondrial respiratory function in the heart, independently of its cell surface receptors. TNFα-induced cardioprotection is known to involve reactive oxygen species (ROS) and sphingolipids. We therefore further propose that this direct mitochondrial effect is mediated via ROS and sphingolipids. The protective concentration of TNFα (0.5 ng/ml) was added to isolated heart mitochondria from black 6 × 129 mice (WT) and double TNF receptor knockout mice (TNFR1&2(-/-)). Respiratory parameters and inner mitochondrial membrane potential were analyzed in the presence/absence of two antioxidants, N-acetyl-L: -cysteine or N-tert-butyl-α-(2-sulfophenyl)nitrone or two antagonists of the sphingolipid pathway, N-oleoylethanolamine (NOE) or imipramine. In WT, TNFα reduced State 3 respiration from 279.3 ± 3 to 119.3 ± 2 (nmol O₂/mg protein/min), increased proton leak from 15.7 ± 0.6% (control) to 36.6 ± 4.4%, and decreased membrane potential by 20.5 ± 3.1% compared to control groups. In TNFR1&2(-/-) mice, TNFα reduced State 3 respiration from 205.2 ± 4 to 75.7 ± 1 (p < 0.05 vs. respective control). In WT mice, both antioxidants added with TNFα restored State 3 respiration to 269.2 ± 2 and 257.6 ± 2, respectively. Imipramine and NOE also restored State 3 respiration to 248.4 ± 2 and 249.0 ± 2, respectively (p < 0.01 vs. TNFα alone). Similarly, both antioxidant and inhibitors of the sphingolipid pathway restored the proton leak to pre-TNF values. TNFα-treated mitochondria or isolated cardiac muscle fibers showed an increase in respiration after anoxia-reoxygenation, but this effect was lost in the presence of an antioxidant or NOE. Similar data were obtained in TNFR1&2(-/-) mice. TNFα exerts a protective effect on respiratory function in isolated mitochondria subjected to an anoxia-reoxygenation insult. This effect appears to be independent of its cell surface receptors, but is likely to be mediated by ROS and sphingolipids.

Ethanolamine is a novel STAT-3 dependent cardioprotective agent

Ethanolamine is a biogenic amine found naturally in the body as part of membrane lipids and as a metabolite of the cardioprotective substances, sphingosine-1-phosphate (S1P) and anandamide. In the brain, ethanolamine, formed from the breakdown of anandamide protects against ischaemic apoptosis. However, the effects of ethanolamine in the heart are unknown. Signal transducer and activator of transcription 3 (STAT-3) is a critical prosurvival factor in ischaemia/reperfusion (I/R) injury. Therefore, we investigated whether ethanolamine protects the heart via activation of STAT-3. Isolated hearts from wildtype or cardiomyocyte specific STAT-3 knockout (K/O) mice were pre-treated with ethanolamine (Etn) (0.3 mmol/L) before I/R insult. In vivo rat hearts were subjected to 30 min ischaemia/2 h reperfusion in the presence or absence of 5 mg/kg S1P and/or the FAAH inhibitor, URB597. Infarct size was measured at the end of each protocol by triphenyltetrazolium chloride staining. Pre-treatment with ethanolamine decreased infarct size in isolated mouse or rat hearts subjected to I/R but this infarct sparing effect was lost in cardiomyocyte specific STAT-3 deficient mice. Pre-treatment with ethanolamine increased nuclear phosphorylated STAT-3 [control 0.75 ± 0.08 vs. Etn 1.50 ± 0.09 arbitrary units; P < 0.05]. Our findings suggest a novel cardioprotective role for ethanolamine against I/R injury via activation of STAT-3.

Activation of the protective Survivor Activating Factor Enhancement (SAFE) pathway against reperfusion injury: Does it go beyond the RISK pathway?

Lethal reperfusion injury is now recognized as a major limitation of current reperfusion therapy by primary percutaneous coronary intervention for acute myocardial infarction. Interestingly, the heart itself is capable of activating an intrinsic protective signaling programme to limit cell death during reperfusion. Tumor necrosis factor alpha (TNFalpha) is a cytokine generally thought to contribute to myocardial dysfunction in ischemia/reperfusion or heart failure. We review evidence that TNFalpha can paradoxically initiate the activation of a novel protective pathway against reperfusion injuries that we have named the Survivor Activating Factor Enhancement (SAFE) pathway. This path requires the activation of the signal transducer and activator of transcription 3 (STAT-3) and it can successfully lessen cardiomyocyte death at the time of reperfusion, independently of the activation of the already well-described Reperfusion Injury Salvage Kinase (RISK) pathway (which includes activation of Akt and Erk 1/2). Emerging knowledge on this novel protective path is presented here with the aim of unravelling its interaction with the RISK pathway and its potential human application to protect against lethal reperfusion injury.

TNFα is required to confer protection in an in vivo model of classical ischaemic preconditioning

Although Tumor Necrosis Factor alpha (TNFalpha) is used as a preconditioning mimetic in vitro, its role in ischaemic preconditioning (IPC) has not been clearly defined. Here, we propose to use an in vivo model (that takes into account the activation of leukocytes which may affect levels of TNFalpha) to demonstrate that i) TNFalpha acts as a trigger in IPC and ii) the dose-dependent nature of this cardioprotective effect of TNFalpha. Male Wistar rats were subjected to 30 min of left coronary artery occlusion (index ischaemia), followed by 24 h reperfusion. In the presence or absence of a soluble TNFalpha receptor (sTNFalpha-R), preconditioning was induced by 3 cycles of ischaemia (3 min)/reperfusion (5 min) (IPC) or various doses (0.05-4 microg/kg) of exogenous TNFalpha. Following 24 h reperfusion, infarct size (IS, expressed as % of the area at risk (AAR)) was assessed. Tissue levels of TNFalpha from the AAR, following IPC and TNFalpha stimulus were determined using Western Blot. IPC caused decrease in IS (4.5+/-1.3% vs 30.8+/-4.3% in ischaemic rats; P<0.001) and increase of TNFalpha levels following the IPC stimulus. The protective effect of IPC was abrogated in the presence of the sTNFalpha-R. In addition, exogenous TNFalpha dose-dependently reduced IS with maximal protection at a dose of 0.1 microg/kg (IS=12.6%, P<0.01 vs ischaemic). In conclusion our data provide strong evidence for a role of TNFalpha during the trigger phase of IPC. In addition, exogenous TNFalpha mimics IPC by providing a dose-dependent cardioprotective effect against ischaemia-reperfusion injury in vivo.