Targeting Succinate Metabolism in Ischemia/Reperfusion Injury

HomeCirculationVol. 140, No. 24Targeting Succinate Metabolism in Ischemia/Reperfusion Injury Free AccessResearch ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessResearch ArticlePDF/EPUBTargeting Succinate Metabolism in Ischemia/Reperfusion Injury Duvaraka Kula-Alwar, PhD, Hiran A. Prag, PhD and Thomas Krieg, MD Duvaraka Kula-AlwarDuvaraka Kula-Alwar Department of Medicine (D.K.-A., T.K.), University of Cambridge, United Kingdom. *Drs Kula-Alwar and Prag contributed equally. Search for more papers by this author , Hiran A. PragHiran A. Prag MRC Mitochondrial Biology Unit (H.A.P.), University of Cambridge, United Kingdom. *Drs Kula-Alwar and Prag contributed equally. Search for more papers by this author and Thomas KriegThomas Krieg Thomas Krieg, MD, Department of Medicine, University of Cambridge, CB2 0QQ, United Kingdom. Email E-mail Address: [email protected] Department of Medicine (D.K.-A., T.K.), University of Cambridge, United Kingdom. Search for more papers by this author Originally published9 Dec 2019https://doi.org/10.1161/CIRCULATIONAHA.119.042791Circulation. 2019;140:1968–1970Timely reperfusion is critical for salvaging ischemic tissue in myocardial infarction, in stroke, and during resuscitation. Paradoxically, the reperfusion of blood into the ischemic organ is damaging in itself, leading to ischemia/reperfusion (IR) injury. Best clinical practice is to reperfuse rapidly to limit ischemic time. Despite this, extensive IR injury still occurs, which is a major driver of pathology, making the prevention of this damage a clear unmet clinical need.1 Recent work has shown a role for mitochondrial succinate metabolism in IR injury that opens up exciting new therapeutic approaches.Succinate Metabolism in IR InjuryIR injury was long thought of as a random and chaotic series of damaging events resulting in reactive oxygen species production from reperfusing ischemic tissue. A different picture is now emerging, suggesting that metabolic mechanisms within mitochondria are central to IR injury, providing a rational basis for therapies. During ischemia, the citric acid cycle metabolite succinate builds up.2 Upon reperfusion, this succinate is oxidized rapidly by succinate dehydrogenase (SDH), driving a burst of reactive oxygen species production by mitochondrial complex I.2 This reactive oxygen species pulse, together with calcium dysregulation and ATP depletion, initiates a cascade of damaging events that culminate in cardiomyocyte death, termed reperfusion injury.1 Whereas cell death from ischemia contributes to the infarct, reperfusion leads to damage additional to that from ischemia alone, thus reperfusion injury provides a therapeutic window to reduce organ damage.Many clinical trials have targeted different facets of IR injury after heart attack but translation to the clinic has been unsuccessful. We highlight the emerging therapeutic strategy of targeting succinate metabolism.SDH Inhibitors for CardioprotectionSDH is a key enzyme in succinate formation during ischemia and its oxidation upon reperfusion.1 Malonate, a competitive inhibitor of SDH, has emerged as a candidate therapy for selective SDH inhibition to diminish reperfusion injury. This was first shown using the malonate prodrug dimethyl malonate, where it was protective when administered before and throughout ischemia.2 Disodium malonate also had a cardioprotective effect when administered intracoronary at reperfusion in a pig model of IR injury.3 Both approaches alter succinate metabolism, either preventing its accumulation during ischemia (thus less succinate is available to be oxidized during reperfusion) or directly blocking its oxidation during reperfusion.Not all the succinate that accumulates in the heart during ischemia is oxidized by mitochondria upon reperfusion. A significant proportion is released selectively into the circulation, as has been demonstrated in patients with an ST-segment–elevation myocardial infarction. In this situation, succinate was released into the bloodstream after reperfusion by primary percutaneous coronary intervention.4 The mechanism of succinate release from the cardiomyocytes is unclear, but once released it may be detected by the succinate receptor (SUCNR1), which is involved in inflammation. Thus by targeting succinate accumulation during ischemia, subsequent release, and action in the circulation, malonate may provide additional therapeutic strategies for the heart damage underlying chronic heart failure (Figure).SDH Inhibition for Other IR Injury PathologiesIn addition to heart attack, ischemic stroke and kidney IR injury may benefit from malonate therapy.2 Furthermore, disrupting succinate metabolism during predictable periods of ischemia, such as that occurring in elective surgery or organ transplantation, is another potential therapeutic target. Cessation of circulation in these scenarios leads to ischemia and a rise in succinate. Upon reperfusion, the succinate will be oxidized rapidly, driving IR injury, tissue damage, and inflammation. Treatment with malonate to blunt succinate accumulation before elective surgery or transplantation thus provides a promising therapeutic opportunity. Dimethyl malonate has also been shown to decrease the brain damage associated with resuscitation after cardiac arrest in rats.5 Infusing dimethyl malonate intravenously before cardiopulmonary resuscitation improved neurologic performance after cardiac arrest, suggesting SDH inhibition by malonate in the brain decreased the production of mitochondrial reactive oxygen species and downstream damage, although whether the action of dimethyl malonate is during ischemia or at reperfusion is unclear. IR injury to the brain, either during cardiopulmonary resuscitation or stroke, is clinically important and there are no pharmacologic interventions available. Thus therapies modulating SDH may help address this unmet clinical need.Translating SDH Inhibitors for IR InjuryDespite clear in vitro and animal model efficacy of many compounds in preventing IR injury, translation has proved difficult.1 High failure rates in human trials are often caused by low drug exposure at the target site or clinical safety problems. Translation failure in IR injury is likely caused by delivery difficulties or insufficient knowledge of the pathologic mechanisms, leading to inappropriate drug targets, such as the broad use of antioxidants. In addition, target-specific drugs such as cyclosporine A may be too far downstream of the damage-initiating mechanism. Furthermore, inappropriate trial design ranging from poor inclusion criteria to insufficient powering has led to disappointing outcomes.Malonate use nullifies a number of barriers to clinical translation. Malonate can enter mitochondria by endogenous transport mechanisms, thus allowing the compound to reach the target site in a timely manner. Furthermore, malonate has limited toxicity, a well-established metabolism, and has been used as an excipient in pharmaceutical development.For the successful translation of malonate therapy, some hurdles remain. First, because malonate is a competitive inhibitor, high concentrations are required for protection. Strategies to improve the cellular delivery of malonate may therefore enable a dose reduction to overcome this issue. Second, the correct timing of malonate administration is critical. For clinical use to reduce reperfusion injury, the drug must be at the correct concentration in the affected tissue at the time of reperfusion. Therefore, understanding the pharmacokinetics of malonate during administration will be important to know the timing of malonate delivery. Third, selectivity to the ischemic tissue by malonate is unlikely. However, strategies to limit off-target malonate delivery are possible and may enable rapid cardioselective malonate delivery to prevent IR injury. Overcoming each hurdle presented here may lead to successful translation of malonate-based therapeutics from bench to bedside in the coming years.Download figureDownload PowerPointFigure. Targeting succinate metabolism to protect against ischemia/reperfusion injury. During ischemia, succinate is accumulated extensively via reversal of succinate dehydrogenase (SDH) to reduce fumarate (left). Fumarate supply is maintained by either (1) the degradation of purine nucleotides by the purine nucleotide cycle or (2) arising from upstream aspartate transamination and translocating to the mitochondria via the dicarboxylate carrier (DIC). With the depletion of the GTP and coenzyme A (CoA) pool in mitochondria, succinate activation to succinyl-CoA is inhibited, thus succinate is a terminal metabolite during ischemia. The canonical tricarboxylic acid (TCA) cycle may also contribute to succinate accumulation through glutaminolysis to α-ketoglutarate (α-KG). Because of the extent of succinate accumulation during ischemia, succinate is exchanged from mitochondria into the cytosol, where it can act to inhibit prolyl hydroxylases (PHD) involved in the degradation of hypoxia-inducible factor 1α (HIF-1α). As HIF escapes degradation, it can translocate to the nucleus and promote the transcription of a number of genes involved in the hypoxic response. Inhibiting SDH with dimethyl malonate prevents the accumulation of succinate during ischemia. (Right) During reperfusion, the accumulated succinate has 2 fates: oxidation by SDH or efflux from the cell. Succinate is rapidly oxidized by SDH, generating a highly reduced coenzyme Q pool and large mitochondrial membrane potential. These conditions drive reverse electron transport through complex I (CxI), generating superoxide, the proximal reactive oxygen species (ROS). Together with calcium dyshomeostasis, these events support the opening of the mitochondrial permeability transition pore (MPTP) and subsequent cell death associated with ischemia/reperfusion injury. By inhibiting SDH with malonate during reperfusion, succinate oxidation is slowed, reducing ROS production. In addition to oxidation, succinate is exported from the cell and able to enter the systemic circulation, and thus has the potential to carry out a signaling role by interacting with the succinate receptor, SUCNR1. ADSL indicates adenylosuccinate lyase; AST, aspartate transaminase; FH, fumarate hydratase; HRE, hypoxia response element; IMP, inosine monophosphate; and MDH, malate dehydrogenase.Sources of FundingDr Krieg is supported by grants from the Medical Research Council UK (MR/P000320/1) and the British Heart Foundation (PG/15/84/31670). Drs Prag and Kula-Alwar are supported by the Medical Research Council UK.DisclosuresDr Krieg is named inventor on pending patents held by the University of Cambridge relating to inhibiting succinate metabolism and has received research funding from Takeda.Footnotes*Drs Kula-Alwar and Prag contributed equally.The opinions expressed in this article are not necessarily those of the editors or of the American Heart Association.https://www.ahajournals.org/journal/circThomas Krieg, MD, Department of Medicine, University of Cambridge, CB2 0QQ, United Kingdom. Email [email protected]cam.ac.ukReferences1. Davidson SM, Ferdinandy P, Andreadou I, Bøtker HE, Heusch G, Ibáñez B, Ovize M, Schulz R, Yellon DM, Hausenloy DJ, et al; CARDIOPROTECTION COST Action (CA16225). Multitarget strategies to reduce myocardial ischemia/reperfusion injury: JACC review topic of the week.J Am Coll Cardiol. 2019; 73:89–99. doi: 10.1016/j.jacc.2018.09.086CrossrefMedlineGoogle Scholar2. 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