Mitochondrially targeted nitro-linoleate: a new tool for the study of cardioprotection

S-nitroso-glutathione (GSNO) inhibits hydrogen peroxide production by alpha-ketoglutarate dehydrogenase: An investigation into sex and diet effects

Pyruvate dehydrogenase (PDH) and α-ketoglutarate dehydrogenase (KGDH) are vital sources of hydrogen peroxide (H₂O₂) and key sites for redox regulation. Here, we report KGDH is more sensitive to inhibition by S-nitroso-glutathione (GSNO) when compared to PDH and deactivation of both enzymes by nitro modification is affected by sex and diet. Liver mitochondria from male C57BL/6N mice displayed a robust inhibition of H₂O₂ production after exposure to 500-2000 μM GSNO. H₂O₂ genesis by PDH was not significantly affected by GSNO. Purified KGDH of porcine heart origin displayed a ∼82% decrease in H₂O₂ generating activity at 500 μM GSNO, which was mirrored by a decrease in NADH production. By contrast, H₂O₂- and NADH-producing activity of purified PDH was only minimally affected by an incubation in 500 μM GSNO. Incubations in GSNO had no significant effect on the H₂O₂-generating activity of KGDH and PDH in female liver mitochondria when compared to samples collected from males, which was attributed to higher GSNO reductase (GSNOR) activity. High fat feeding augmented the GSNO-mediated inhibition of KGDH in liver mitochondria from male mice. Exposure of male mice to a HFD also resulted in a significant decrease in the GSNO-mediated inhibition of H₂O₂ genesis by PDH, an effect not observed in mice fed a control-matched diet (CD). Female mice displayed higher resistance to the GSNO-induced inhibition of H₂O₂ production, regardless of being fed a CD or HFD. However, exposure to a HFD did result in a small but significant decrease in H₂O₂ production by KGDH and PDH when female liver mitochondria were treated with GSNO. Although, the effect was less when compared to their male counterparts. Collectively, we show for the first time GSNO deactivates H₂O₂ production by α-keto acid dehydrogenases and we demonstrate that sex and diet are determinants for the nitro-inhibition of both KGDH and PDH.

Distinct effects of intracellular vs. extracellular acidic pH on the cardiac metabolome during ischemia and reperfusion

Tissue ischemia results in intracellular pH (pHIN) acidification, and while metabolism is a known driver of acidic pHIN, less is known about how acidic pHIN regulates metabolism. Furthermore, acidic extracellular (pHEX) during early reperfusion confers cardioprotection, but how this impacts metabolism is unclear. Herein we employed LCMS based targeted metabolomics to analyze perfused mouse hearts exposed to: (i) control perfusion, (ii) hypoxia, (iii) ischemia, (iv) enforced acidic pHIN, (v) control reperfusion, and (vi) acidic pHEX (6.8) reperfusion. Surprisingly little overlap was seen between metabolic changes induced by hypoxia, ischemia, and acidic pHIN. Acidic pHIN elevated metabolites in the top half of glycolysis, and enhanced glutathione redox state. Meanwhile, acidic pHEX reperfusion induced substantial metabolic changes in addition to those seen in control reperfusion. This included elevated metabolites in the top half of glycolysis, prevention of purine nucleotide loss, and an enhancement in glutathione redox state. These data led to hypotheses regarding potential roles for methylglyoxal inhibiting the mitochondrial permeability transition pore, and for acidic inhibition of ecto-5'-nucleotidase, as potential mediators of cardioprotection by acidic pHEX reperfusion. However, neither hypothesis was supported by subsequent experiments. In contrast, analysis of cardiac effluents revealed complex effects of pHEX on metabolite transport, suggesting that mildly acidic pHEX may enhance succinate release during reperfusion. Overall, each intervention had distinct and overlapping metabolic effects, suggesting acidic pH is an independent metabolic regulator regardless which side of the cell membrane it is imposed.

Lipid nitroalkene nanoparticles for the focal treatment of ischemia reperfusion

Rationale: The treatment of microvascular obstruction (MVO) using ultrasound-targeted LNP cavitation (UTC) therapy mechanically relieves the physical obstruction in the microcirculation but does not specifically target the associated inflammatory milieu. Electrophilic fatty acid nitroalkene derivatives (nitro-fatty acids), that display pleiotropic anti-inflammatory signaling and transcriptional regulatory actions, offer strong therapeutic potential but lack a means of rapid targeted delivery. The objective of this study was to develop nitro-fatty acid-containing lipid nanoparticles (LNP) that retain the mechanical efficacy of standard LNP and can rapidly target delivery of a tissue-protective payload that reduces inflammation and improves vascular function following ischemia-reperfusion. Methods: The stability and acoustic behavior of nitro-fatty acid LNP (NO₂-FA-LNP) were characterized by HPLC-MS/MS and ultra-high-speed microscopy. The LNP were then used in a rat hindlimb model of ischemia-reperfusion injury with ultrasound-targeted cavitation. Results: Intravenous administration of NO₂-FA-LNP followed by ultrasound-targeted LNP cavitation (UTC) in both healthy rat hindlimb and following ischemia-reperfusion injury showed enhanced NO₂-FA tissue delivery and microvascular perfusion. In addition, vascular inflammatory mediator expression and lipid peroxidation were decreased in tissues following ischemia-reperfusion revealed NO₂-FA-LNP protected against inflammatory injury. Conclusions: Vascular targeting of NO₂-FA-LNP with UTC offers a rapid method of focal anti-inflammatory therapy at sites of ischemia-reperfusion injury.

Ischemia/Reperfusion Injury Revisited: An Overview of the Latest Pharmacological Strategies

Ischemia/reperfusion injury (IRI) permeates a variety of diseases and is a ubiquitous concern in every transplantation proceeding, from whole organs to modest grafts. Given its significance, efforts to evade the damaging effects of both ischemia and reperfusion are abundant in the literature and they consist of several strategies, such as applying pre-ischemic conditioning protocols, improving protection from preservation solutions, thus providing extended cold ischemia time and so on. In this review, we describe many of the latest pharmacological approaches that have been proven effective against IRI, while also revisiting well-established concepts and presenting recent pathophysiological findings in this ever-expanding field. A plethora of promising protocols has emerged in the last few years. They have been showing exciting results regarding protection against IRI by employing drugs that engage several strategies, such as modulating cell-surviving pathways, evading oxidative damage, physically protecting cell membrane integrity, and enhancing cell energetics.

Cardioprotection by nicotinamide mononucleotide (NMN): Involvement of glycolysis and acidic pH

Stimulation of the cytosolic NAD⁺ dependent deacetylase SIRT1 is cardioprotective against ischemia-reperfusion (IR) injury. NAD⁺ precursors including nicotinamide mononucleotide (NMN) are thought to induce cardioprotection via SIRT1. Herein, while NMN protected perfused hearts against IR (functional recovery: NMN 42 ± 7% vs. vehicle 11 ± 3%), this protection was insensitive to the SIRT1 inhibitor splitomicin (recovery 47 ± 8%). Although NMN-induced cardioprotection was absent in Sirt3^-/- hearts (recovery 9 ± 5%), this was likely due to enhanced baseline injury in Sirt3^-/- (recovery 6 ± 2%), since similar injury levels in WT hearts also blunted the protective efficacy of NMN. Considering alternative cardiac effects of NMN, and the requirement of glycolysis for NAD⁺, we hypothesized NMN may confer protection in part via direct stimulation of cardiac glycolysis. In primary cardiomyocytes, NMN induced cytosolic and extracellular acidification and elevated lactate. In addition, [U-¹³C]glucose tracing in intact hearts revealed that NMN stimulated glycolytic flux. Consistent with a role for glycolysis in NMN-induced protection, hearts perfused without glucose (palmitate as fuel source), or hearts perfused with galactose (no ATP from glycolysis) exhibited no benefit from NMN (recovery 11 ± 4% and 15 ± 2% respectively). Acidosis during early reperfusion is known to be cardioprotective (i.e., acid post-conditioning), and we also found that NMN was cardioprotective when delivered acutely at reperfusion (recovery 39 ± 8%). This effect of NMN was not additive with acidosis, suggesting overlapping mechanisms. We conclude that the acute cardioprotective benefits of NMN are mediated in part via glycolytic stimulation, with the downstream protective mechanism involving enhanced ATP synthesis during ischemia and/or enhanced acidosis during reperfusion.

Potential mechanisms linking SIRT activity and hypoxic 2-hydroxyglutarate generation: no role for direct enzyme (de)acetylation

2-Hydroxyglutarate (2-HG) is a hypoxic metabolite with potentially important epigenetic signaling roles. The mechanisms underlying 2-HG generation are poorly understood, but evidence suggests a potential regulatory role for the sirtuin family of lysine deacetylases. Thus, we hypothesized that the acetylation status of the major 2-HG-generating enzymes [lactate dehydrogenase (LDH), isocitrate dehydrogenase (IDH) and malate dehydrogenase (MDH)] may govern their 2-HG-generating activity. In vitro acetylation of these enzymes, with confirmation by western blotting, mass spectrometry, reversibility by recombinant sirtuins and an assay for global lysine occupancy, yielded no effect on 2-HG-generating activity. In addition, while elevated 2-HG in hypoxia is associated with the activation of lysine deacetylases, we found that mice lacking mitochondrial SIRT3 exhibited hyperacetylation and elevated 2-HG. These data suggest that there is no direct link between enzyme acetylation and 2-HG production. Furthermore, our observed effects of in vitro acetylation on the canonical activities of IDH, MDH and LDH appeared to contrast with previous findings wherein acetyl-mimetic lysine mutations resulted in the inhibition of these enzymes. Overall, these data suggest that a causal relationship should not be assumed between acetylation of metabolic enzymes and their activities, canonical or otherwise.

Acidic pH Is a Metabolic Switch for 2-Hydroxyglutarate Generation and Signaling

2-Hydroxyglutarate (2-HG) is an important epigenetic regulator, with potential roles in cancer and stem cell biology. The d-(R)-enantiomer (d-2-HG) is an oncometabolite generated from α-ketoglutarate (α-KG) by mutant isocitrate dehydrogenase, whereas l-(S)-2-HG is generated by lactate dehydrogenase and malate dehydrogenase in response to hypoxia. Because acidic pH is a common feature of hypoxia, as well as tumor and stem cell microenvironments, we hypothesized that pH may regulate cellular 2-HG levels. Herein we report that cytosolic acidification under normoxia moderately elevated 2-HG in cells, and boosting endogenous substrate α-KG levels further stimulated this elevation. Studies with isolated lactate dehydrogenase-1 and malate dehydrogenase-2 revealed that generation of 2-HG by both enzymes was stimulated severalfold at acidic pH, relative to normal physiologic pH. In addition, acidic pH was found to inhibit the activity of the mitochondrial l-2-HG removal enzyme l-2-HG dehydrogenase and to stimulate the reverse reaction of isocitrate dehydrogenase (carboxylation of α-KG to isocitrate). Furthermore, because acidic pH is known to stabilize hypoxia-inducible factor (HIF) and 2-HG is a known inhibitor of HIF prolyl hydroxylases, we hypothesized that 2-HG may be required for acid-induced HIF stabilization. Accordingly, cells stably overexpressing l-2-HG dehydrogenase exhibited a blunted HIF response to acid. Together, these results suggest that acidosis is an important and previously overlooked regulator of 2-HG accumulation and other oncometabolic events, with implications for HIF signaling.

Nitro-fatty acids in cardiovascular regulation and diseases: characteristics and molecular mechanisms

Electrophilic nitro-fatty acids (NO2-FAs) are endogenously formed by redox reactions of nitric oxide ((.)NO)- and nitrite ((.)NO2)- derived nitrogen dioxide with unsaturated fatty acids. Nitration preferentially occurs on polyunsaturated fatty acids with conjugated dienes under physiological or pathophysiological conditions such as during digestion, metabolism and as adaptive inflammatory processes. Nitro-fatty acids are present in free and esterified forms achieving broad biodistribution in humans and experimental models. Structural, functional and biological characterization of NO2-FAs has revealed clinically relevant protection from inflammatory injury in a number of cardiovascular, renal and metabolic experimental models. NO2-FAs are engaged in posttranslational modifications (PTMs) of a selective redox sensitive pool of proteins and regulate key adaptive signaling pathways involved in cellular homeostasis and inflammatory response. Here, we review and update the biosynthesis, metabolism and signaling actions of NO2-FAs, highlighting their diverse protective roles relevant to the cardiovascular system.

Metabolomic profiling of the heart during acute ischemic preconditioning reveals a role for SIRT1 in rapid cardioprotective metabolic adaptation

Ischemic preconditioning (IPC) protects tissues such as the heart from prolonged ischemia-reperfusion (IR) injury. We previously showed that the lysine deacetylase SIRT1 is required for acute IPC, and has numerous metabolic targets. While it is known that metabolism is altered during IPC, the underlying metabolic regulatory mechanisms are unknown, including the relative importance of SIRT1. Thus, we sought to test the hypothesis that some of the metabolic adaptations that occur in IPC may require SIRT1 as a regulatory mediator. Using both ex-vivo-perfused and in-vivo mouse hearts, LC-MS/MS based metabolomics and (13)C-labeled substrate tracing, we found that acute IPC altered several metabolic pathways including: (i) stimulation of glycolysis, (ii) increased synthesis of glycogen and several amino acids, (iii) increased reduced glutathione levels, (iv) elevation in the oncometabolite 2-hydroxyglutarate, and (v) inhibition of fatty-acid dependent respiration. The majority (83%) of metabolic alterations induced by IPC were ablated when SIRT1 was acutely inhibited with splitomicin, and a principal component analysis revealed that metabolic changes in response to IPC were fundamentally different in nature when SIRT1 was inhibited. Furthermore, the protective benefit of IPC was abrogated by eliminating glucose from perfusion media while sustaining normal cardiac function by burning fat, thus indicating that glucose dependency is required for acute IPC. Together, these data suggest that SIRT1 signaling is required for rapid cardioprotective metabolic adaptation in acute IPC.

Mitochondrial pharmacology: energy, injury and beyond

While the mitochondrion has long fascinated biologists and the sheer diversity of druggable targets has made it attractive for potential drug development, there has been little success translatable to the clinic. Given the diversity of inborn errors of metabolism and mitochondrial diseases, mitochondrially mediated oxidative stress (myopathies, reperfusion injury, Parkinson's disease, ageing) and the consequences of disturbed energetics (circulatory shock, diabetes, cancer), the potential for meaningful gain with novel drugs targeting mitochondrial mechanisms is huge both in terms of patient quality of life and health care costs. In this themed issue of the British Journal of Pharmacology, we highlight the key directions of the contemporary advances in the field of mitochondrial biology, emerging drug targets and new molecules which are close to clinical application. Authors' contributions are diverse both in terms of species and organs in which the mitochondrially related studies are performed, and from the perspectives of mechanisms under study. Defined roles of mitochondria in disease are updated and previously unknown contributions to disease are described in terms of the interface between basic science and pathological relevance.