Preserved recovery of cardiac function following ischemia-reperfusion in mice lacking SIRT3

Protein lysine acetylation does not contribute to the high rates of fatty acid oxidation seen in the post-ischemic heart

High rates of cardiac fatty acid oxidation during reperfusion of ischemic hearts contribute to contractile dysfunction. This study aimed to investigate whether lysine acetylation affects fatty acid oxidation rates and recovery in post-ischemic hearts. Isolated working hearts from Sprague Dawley rats were perfused with 1.2 mM palmitate and 5 mM glucose and subjected to 30 min of ischemia and 40 min of reperfusion. Cardiac function, fatty acid oxidation, glucose oxidation, and glycolysis rates were compared between pre- and post-ischemic hearts. The acetylation status of enzymes involved in cardiac energy metabolism was assessed in both groups. Reperfusion after ischemia resulted in only a 41% recovery of cardiac work. Fatty acid oxidation and glycolysis rates increased while glucose oxidation rates decreased. The contribution of fatty acid oxidation to ATP production and TCA cycle activity increased from 90 to 93% and from 94.9 to 98.3%, respectively, in post-ischemic hearts. However, the overall acetylation status and acetylation levels of metabolic enzymes did not change in response to ischemia and reperfusion. These findings suggest that acetylation may not contribute to the high rates of fatty acid oxidation and reduced glucose oxidation observed in post-ischemic hearts perfused with high levels of palmitate substrate.

Cellular and molecular biology of sirtuins in cardiovascular disease

Sirtuins (SIRTs) are a nicotinic adenine dinucleotide (+) -dependent histone deacetylase that regulates critical signaling pathways in prokaryotes and eukaryotes. Studies have identified seven mammalian homologs of the yeast SIRT silencing message regulator 2, namely, SIRT1-SIRT7. Recent in vivo and in vitro studies have successfully demonstrated the involvement of SIRTs in key pathways for cell biological function in physiological and pathological processes of the cardiovascular system, including processes including cellular senescence, oxidative stress, apoptosis, DNA damage, and cellular metabolism. Emerging evidence has stimulated a significant evolution in preventing and treating cardiovascular disease (CVD). Here, we review the important roles of SIRTs for the regulatory pathways involved in the pathogenesis of cardiovascular diseases and their molecular targets, including novel protein post-translational modifications of succinylation. In addition, we summarize the agonists and inhibitors currently identified to target novel specific small molecules of SIRTs. A better understanding of the role of SIRTs in the biology of CVD opens new avenues for therapeutic intervention with great potential for preventing and treating CVD.

Fasting increases susceptibility to acute myocardial ischaemia/reperfusion injury through a sirtuin-3 mediated increase in fatty acid oxidation

Fasting increases susceptibility to acute myocardial ischaemia/reperfusion injury (IRI) but the mechanisms are unknown. Here, we investigate the role of the mitochondrial NAD⁺-dependent deacetylase, Sirtuin-3 (SIRT3), which has been shown to influence fatty acid oxidation and cardiac outcomes, as a potential mediator of this effect. Fasting was shown to shift metabolism from glucose towards fatty acid oxidation. This change in metabolic fuel substrate utilisation increased myocardial infarct size in wild-type (WT), but not SIRT3 heterozygous knock-out (KO) mice. Further analysis revealed SIRT3 KO mice were better adapted to starvation through an improved cardiac efficiency, thus protecting them from acute myocardial IRI. Mitochondria from SIRT3 KO mice were hyperacetylated compared to WT mice which may regulate key metabolic processes controlling glucose and fatty acid utilisation in the heart. Fasting and the associated metabolic switch to fatty acid respiration worsens outcomes in WT hearts, whilst hearts from SIRT3 KO mice are better adapted to oxidising fatty acids, thereby protecting them from acute myocardial IRI.

Rescue of Mitochondrial SIRT3 Ameliorates Ischemia-like Injury in Human Endothelial Cells

Structural and functional alterations of vasculature caused by age-related factors is critically involved in the pathogenesis of ischemic stroke. The longevity genes sirtuins (SIRTs) are extensively investigated in aging-associated pathologies, but their distinct roles in ischemic stroke still remain to be clarified. To address this question, we applied oxygen and glucose deprived/reperfusion (OGD/R) to induce ischemic injury in human endothelial cells (ECs), which are the main component of vasculature in the brain. The results showed that OGD/R led to various damages to ECs, including compromised cell viability, increased LDH release, overproduced ROS, enhanced apoptosis and caspase activity. Meanwhile, the expression of mitochondrial SIRT3 was robustly decreased in ECs after OGD/R treatment. Consistently, rescue of SIRT3 by ectopic expression, but not nuclear SIRT1, in ECs reversed the OGD/R-induced cell damage. Interestingly, some front-line drugs for ischemic stroke, including clopidogrel, aspirin and dl-3-n-butylphthalide (NBP), also rescued SIRT3 and reduced OGD/R-induced endothelial injury, suggesting that the recovery of SIRT3 expression was critical for the protection of these drugs. Moreover, our results demonstrated that 10-hydroxy-NBP (OHNBP), a major metabolite of NBP, showed better blood-brain barrier crossing capability than NBP, but still retained the effects on SIRT3 by NBP. Together, our results suggested that SIRT3 may serve as a potential novel target for treatment of ischemic stroke.

Sirtuin 3 and mitochondrial permeability transition pore (mPTP): A systematic review

Mitochondrial permeability transition pore (mPTP) is a channel that opens at the inner mitochondrial membrane under conditions of stress. Sirtuin 3 (Sirt3) is a mitochondrial deacetylase known to play a major role in stress resistance and a regulatory role in cell death. This systematic review aims to elucidate the role of Sirt3 in mPTP inhibition. Electronic databases, including PubMed, EMBASE, Web of Science and Cochrane Library were searched up to May 2020. Original studies that investigated the relationship between Sirt3 and mPTP were included. Two reviewers independently extracted data on study characteristics, methods and outcomes. A total of 194 articles were found. Twenty-nine articles, which met criteria were included in the systematic review. Twenty-three studies provided evidence of the inhibitory effect of Sirt3 on the mPTP aperture. This review summarizes up-to-date evidence of the protective and inhibitory role of Sirt3 through deacetylating Cyclophilin D (CypD) on the mPTP aperture. Furthermore, we discuss the implications of this effect in disease.

Sirtuin 3: Emerging therapeutic target for cardiovascular diseases

Acetylation is one of the most important methods of modification that lead to a change in the function of proteins. In humans, metabolic enzymes commonly undergo acetylation, which regulates the activities of metabolic enzymes and metabolic pathways. Sirtuin 3 (SIRT3) is a prominent deacetylase that participates in mitochondrial metabolism, redox balance, and mitochondrial dynamics by regulating mitochondrial protein acetylation, thereby protecting mitochondria from damage. Normal mitochondrial function is essential for maintaining the metabolism and function of the heart. Therefore, mitochondrial dysfunction caused by SIRT3 consumption and defects leads to the development of a variety of cardiovascular diseases. A comprehensive understanding of the role of SIRT3 in cardiovascular disease is critical for developing new therapeutic strategies. Herein, we summarize the function of SIRT3 in mitochondria, the complex mechanisms mediating cardiovascular diseases, and the potential value of SIRT3 small-molecule agonists in future clinical treatments.

Molecular Signaling to Preserve Mitochondrial Integrity against Ischemic Stress in the Heart: Rescue or Remove Mitochondria in Danger

Cardiovascular diseases are one of the leading causes of death and global health problems worldwide, and ischemic heart disease is the most common cause of heart failure (HF). The heart is a high-energy demanding organ, and myocardial energy reserves are limited. Mitochondria are the powerhouses of the cell, but under stress conditions, they become damaged, release necrotic and apoptotic factors, and contribute to cell death. Loss of cardiomyocytes plays a significant role in ischemic heart disease. In response to stress, protective signaling pathways are activated to limit mitochondrial deterioration and protect the heart. To prevent mitochondrial death pathways, damaged mitochondria are removed by mitochondrial autophagy (mitophagy). Mitochondrial quality control mediated by mitophagy is functionally linked to mitochondrial dynamics. This review provides a current understanding of the signaling mechanisms by which the integrity of mitochondria is preserved in the heart against ischemic stress.

Hypertrophic preconditioning attenuates myocardial ischemia/reperfusion injury through the deacetylation of isocitrate dehydrogenase 2

To test the hypothesis that transient nonischemic stimulation of hypertrophy would render the heart resistant to subsequent ischemic stress, short-term transverse aortic constriction (TAC) was performed in mice and then withdrawn for several days by aortic debanding, followed by subsequent myocardial exposure to ischemia/reperfusion (I/R). Following I/R injury, the myocardial infarct size and apoptosis were markedly reduced, and contractile function was significantly improved in the TAC preconditioning group compared with the control group. Mechanistically, hypertrophic preconditioning remarkably alleviated I/R-induced oxidative stress, as evidenced by the increased reduced nicotinamide adenine dinucleotide phosphate (NADPH)/nicotinamide adenine dinucleotide phosphate (NADP) ratio, increase in the reduced glutathione (GSH)/oxidized glutathione (GSSH) ratio, and reduced mitochondrial reactive oxygen species (ROS) production. Moreover, TAC preconditioning inhibited caspase-3 activation and mitigated the mitochondrial impairment by deacetylating isocitrate dehydrogenase 2 (IDH2) via a sirtuin 3 (SIRT3)-dependent mechanism. In addition, the expression of a genetic deacetylation mimetic IDH2 mutant (IDH2 K413R) in cardiomyocytes, which increased IDH2 enzymatic activity and decreased mitochondrial ROS production, and ameliorated I/R injury, whereas the expression of a genetic acetylation mimetic (IDH2 K413Q) in cardiomyocytes abolished these protective effects of hypertrophic preconditioning. Furthermore, both the activity and expression of the SIRT3 protein were markedly increased in preconditioned mice exposed to I/R. Treatment with an adenovirus encoding SIRT3 partially emulated the actions of hypertrophic preconditioning, whereas genetic ablation of SIRT3 in mice blocked the cardioprotective effects of hypertrophic preconditioning. The present study identifies hypertrophic preconditioning as a novel endogenous self-defensive and cardioprotective strategy for cardiac I/R injury that induces IDH2 deacetylation through a SIRT3-dependent mechanism. A therapeutic strategy targeting IDH2 may be a promising treatment for cardiac ischemic injury.

LncDACH1 promotes mitochondrial oxidative stress of cardiomyocytes by interacting with sirtuin3 and aggravates diabetic cardiomyopathy

Diabetic cardiomyopathy (DCM) is a common complication in diabetic patients. The molecular mechanisms of DCM remain to be fully elucidated. The intronic long noncoding RNA of DACH1 (lncDACH1) has been demonstrated to be closely associated with heart failure and cardiac regeneration. In this study, we investigated the role of lncDACH1 in DCM and the underlying molecular mechanisms. The expression of lncDACH1 was increased in DCM hearts and in high glucose-treated cardiomyocytes. Knockout of lncDACH1 reduced mitochondrial oxidative stress, cell apoptosis, cardiac fibrosis and hypertrophy, and improved cardiac function in DCM mice. Overexpression of lncDACH1 exacerbated mitochondria-derived reactive oxygen species (ROS) level and apoptosis, decreased activity of manganese superoxide dismutase (Mn-SOD); while silencing of lncDACH1 attenuated ROS production, mitochondrial dysfunction, cell apoptosis, and increased the activity of Mn-SOD in cardiomyocytes treated with high glucose. LncDACH1 directly bound to sirtuin3 (SIRT3) and facilitated its degradation by ubiquitination, therefore promoting mitochondrial oxidative injury and cell apoptosis in mouse hearts. In addition, SIRT3 silencing abrogated the protective effects of lncDACH1 deficiency in cardiomyocytes. In summary, lncDACH1 aggravates DCM by promoting mitochondrial oxidative stress and cell apoptosis via increasing ubiquitination-mediated SIRT3 degradation in mouse hearts. Inhibition of lncDACH1 represents a novel therapeutic strategy for the intervention of diabetic cardiomyopathy.

Post-translational Acetylation Control of Cardiac Energy Metabolism

Perturbations in myocardial energy substrate metabolism are key contributors to the pathogenesis of heart diseases. However, the underlying causes of these metabolic alterations remain poorly understood. Recently, post-translational acetylation-mediated modification of metabolic enzymes has emerged as one of the important regulatory mechanisms for these metabolic changes. Nevertheless, despite the growing reports of a large number of acetylated cardiac mitochondrial proteins involved in energy metabolism, the functional consequences of these acetylation changes and how they correlate to metabolic alterations and myocardial dysfunction are not clearly defined. This review summarizes the evidence for a role of cardiac mitochondrial protein acetylation in altering the function of major metabolic enzymes and myocardial energy metabolism in various cardiovascular disease conditions.

Role of epigenetic regulation in myocardial ischemia/reperfusion injury

Nowadays acute myocardial infarction (AMI) is a serious cardiovascular disease threatening the human life and health worldwide. The most effective treatment is to quickly restore coronary blood flow through revascularization. However, timely revascularization may lead to reperfusion injury, thereby reducing the clinical benefits of revascularization. At present, no effective treatment is available for myocardial ischemia/reperfusion injury. Emerging evidence indicates that epigenetic regulation is closely related to the pathogenesis of myocardial ischemia/reperfusion injury, indicating that epigenetics may serve as a novel therapeutic target to ameliorate or prevent ischemia/reperfusion injury. This review aimed to briefly summarize the role of histone modification, DNA methylation, noncoding RNAs, and N⁶-methyladenosine (m⁶A) methylation in myocardial ischemia/reperfusion injury, with a view to providing new methods and ideas for the research and treatment of myocardial ischemia/reperfusion injury.

Hypertrophic Preconditioning Attenuates Myocardial Ischaemia-Reperfusion Injury by Modulating SIRT3-SOD2-mROS-Dependent Autophagy

Background

Ischaemic preconditioning elicited by brief periods of coronary occlusion and reperfusion protects the heart from a subsequent prolonged ischaemic insult. Here, we test the hypothesis that short‐term non‐ischaemic stimulation of hypertrophy renders the heart resistant to subsequent ischaemic injury.

Methods and Results

Transient transverse aortic constriction (TAC) was performed for 3 days in mice and then withdrawn for 4 days by aortic debanding, followed by subsequent exposure to myocardial ischaemia‐reperfusion (I/R) injury. Following I/R injury, myocardial infarct size and apoptosis were significantly decreased, and cardiac dysfunction was markedly improved in the TAC preconditioning group compared with the control group. Mechanistically, TAC preconditioning markedly suppressed I/R‐induced autophagy and preserved autophagic flux by deacetylating SOD2 via a SIRT3‐dependent mechanism. Moreover, treatment with an adenovirus encoding SIRT3 partially mimicked the effects of hypertrophic preconditioning, whereas genetic ablation of SIRT3 in mice blocked the cardioprotective effects of hypertrophic preconditioning. Furthermore, in vivo lentiviral‐mediated knockdown of Beclin 1 in the myocardium ameliorated the I/R‐induced impairment of autophagic flux and was associated with a reduction in cell death, whereas treatment with a lentivirus encoding Beclin 1 abolished the cardioprotective effect of TAC preconditioning.

Conclusions

The present study identifies TAC preconditioning as a novel strategy for induction of an endogenous self‐defensive and cardioprotective mechanism against cardiac injury. Specifically, TAC preconditioning reduced myocardial autophagic cell death in a SIRT3/SOD2 pathway‐dependent manner.

Predictive Value of Sirtuins in Acute Myocardial Infarction - Bridging the Bench to the Clinical Practice

Acute myocardial infarction (AMI) is a non-transmissible condition with high prevalence, morbidity, and mortality. Different strategies for the management of AMI are employed worldwide, but its early diagnosis remains a major challenge. Many molecules have been proposed in recent years as predictive agents in the early detection of AMI, including troponin (C, T, and I), creatine kinase MB isoenzyme, myoglobin, heart-type fatty acid-binding protein, and a family of histone deacetylases with enzymatic activities named sirtuins. Sirtuins may be used as predictive or complementary treatment strategies and the results of recent preclinical studies are promising. However, human clinical trials and data are scarce, and many issues have been raised regarding the predictive values of sirtuins. The present review summarizes research on the predictive value of sirtuins in AMI. We also briefly summarize relevant clinical trials and discuss future perspectives and possible clinical applications.

Sirtuins and the circadian clock interplay in cardioprotection: focus on sirtuin 1

Chronic disruption of circadian rhythms which include intricate molecular transcription-translation feedback loops of evolutionarily conserved clock genes has serious health consequences and negatively affects cardiovascular physiology. Sirtuins (SIRTs) are nuclear, cytoplasmic and mitochondrial histone deacetylases that influence the circadian clock with clock-controlled oscillatory protein, NAMPT, and its metabolite NAD+. Sirtuins are linked to the multi-organ protective role of melatonin, particularly in acute kidney injury and in cardiovascular diseases, where melatonin, via upregulation of SIRT1 expression, inhibits the apoptotic pathway. This review focuses on SIRT1, an NAD+-dependent class III histone deacetylase which counterbalances the intrinsic histone acetyltransferase activity of one of the clock genes, CLOCK. SIRT1 is involved in the development of cardiomyocytes, regulation of voltage-gated cardiac sodium ion channels via deacetylation, prevention of atherosclerotic plaque formation in the cardiovascular system, protection against oxidative damage and anti-thrombotic actions. Overall, SIRT1 has a see-saw effect on cardioprotection, with low levels being cardioprotective and higher levels leading to cardiac hypertrophy.

CXCL6 regulates cell permeability, proliferation, and apoptosis after ischemia-reperfusion injury by modulating Sirt3 expression via AKT/FOXO3a activation

Chemokine (C-X-C motif) ligand 6 (CXCL6), a member of the CXC chemokine family, reportedly mediates several processes such as inflammation, immunoreaction, cell growth, and metastasis through interaction with the chemokine receptors CXCR1 and CXCR2 in humans; further, CXCR1 and CXCR2 can promote repair and regeneration of organs or tissues after ischemia-reperfusion injury (IRI). In this study, we found that HIF-1α, CXCL6, and CXCR2 expression levels were elevated in human brain microvascular endothelial cells (HBMECs) after IRI, whereas silent information regulator of transcription (Sirt) 3 expression level had reduced. HIF-1α inhibition in an IRI model potently promoted HBMEC proliferation, accompanied by increased Sirt3 and decreased CXCL6/CXCR2 expression levels. CXCL6 knockdown in the IRI model significantly decreased HBMEC permeability and promoted HBMEC proliferation, concurrent with a decrease in apoptosis; it also increased Sirt3 expression levels and decreased CXCL6/CXCR2 protein and phosphorylated AKT (p-AKT) and class O of forkhead box (FOXO) 3a (p-FOXO3a) levels. In addition, CXCL6-induced HBMEC permeability and inhibition of HBMEC proliferation were counteracted by Sirt3 overexpression, and the AKT inhibitor LY294002 counteracted the effect of CXCL6 recombinant proteins on Sirt3, p-AKT, and p-FOXO3a expressions. These results suggest that CXCL6 and Sirt3 are downstream of HIF-1α and that CXCL6 regulatesHBMEC permeability, proliferation, and apoptosis after IRI by modulating Sirt3 expression via AKT/FOXO3a activation.

Regulation of SIRT3 on mitochondrial functions and oxidative stress in Parkinson's disease

Sirtuin-3 (SIRT3) is a NAD⁺-dependent protein deacetylase that is located in mitochondria, regulating mitochondrial proteins and maintaining cellular antioxidant status. Increasing evidence demonstrates that SIRT3 plays a role in degenerative disorders including Parkinson's disease (PD), which is a devastating nervous system disease currently with no effective treatments available. Although the etiology of PD is still largely ambiguous, substantial evidence indicates that mitochondrial dysfunction and oxidative stress play major roles in the pathogenesis of PD. The imbalance of reactive oxygen species (ROS) production and detoxification leads to oxidative stress that can accelerate the progression of PD. By causing conformational changes in the deacetylated proteins SIRT3 modulates the activities and biological functions of a variety of proteins involved in mitochondrial antioxidant defense and various mitochondrial functions. Increasingly more studies have suggested that upregulation of SIRT3 confers beneficial effect on neuroprotection in various PD models. This review discusses the mechanism by which SIRT3 regulates intracellular oxidative status and mitochondrial function with an emphasis in discussing in detail the regulation of SIRT3 on each component of the five complexes of the mitochondrial respiratory chain and mitochondrial antioxidant defense, as well as the pharmacological regulation of SIRT3 in light of therapeutic strategies for PD.

A machine learning-driven study indicates emodin improves cardiac hypertrophy by modulation of mitochondrial SIRT3 signaling

Cardiac hypertrophy (CH) is an enormous risk factor in the process of heart failure development, however, there is still lack of effective treatment for CH. Mitochondrial protection is an effective way against CH. Rheum palmatum L. (rhubarb) has been used to treat chronic heart diseases such as heart failure, especially to inhibit cardiac compensatory enlargement. The aim of this study was to explore the pharmacodynamic component of rhubarb and reveal its pharmacological effects and targets in the treatment of CH. Based on network pharmacology and machine learning approach, ingredients of rhubarb and targets for CH were extracted and surflex docking was conducted for obtaining the optimal ingredient-target combination(s) and emodin-SIRT3 was identified for further functional analysis. Transverse aortic constriction or isoproterenol induced CH mice and phenylephrine injured cardiomyocytes were used to verify the mitochondria protection effect and CH improvement of emodin in vivo and in vitro by modulation of mitochondrial SIRT3 signaling. The results showed that emodin could block agonist-induced and pressure overload-mediated CH. Emodin prevented mitochondrial dysfunction and its underlying mechanism was attributed to the activation of SIRT3, but the effect was not obvious with the presence of SIRT3 inhibitors (3-TYP)/SIRT3 siRNA. Furthermore, PGC-1ɑ was involved in the process of emodin regulating SIRT3 signaling pathway as an upstream target. Our findings clarified the main material basis and mechanism of rhubarb in the treatment of CH. Emodin, as the major ingredient of rhubarb, has therapeutic potential for CH through mitochondrial protection due to the modulation of SIRT3 signaling.

SIRT3-mediated inhibition of FOS through histone H3 deacetylation prevents cardiac fibrosis and inflammation

Sirtuin 3 (SIRT3) is a deacetylase that modulates proteins that control metabolism and protects against oxidative stress. Modulation of SIRT3 activity has been proposed as a promising therapeutic target for ameliorating metabolic diseases and associated cardiac disturbances. In this study, we investigated the role of SIRT3 in inflammation and fibrosis in the heart using male mice with constitutive and systemic deletion of SIRT3 and human cardiac AC16 cells. SIRT3 knockout mice showed cardiac fibrosis and inflammation that was characterized by augmented transcriptional activity of AP-1. Consistent with this, SIRT3 overexpression in human and neonatal rat cardiomyocytes partially prevented the inflammatory and profibrotic response induced by TNF-α. Notably, these effects were associated with a decrease in the mRNA and protein levels of FOS and the DNA-binding activity of AP-1. Finally, we demonstrated that SIRT3 inhibits FOS transcription through specific histone H3 lysine K27 deacetylation at its promoter. These findings highlight an important function of SIRT3 in mediating the often intricate profibrotic and proinflammatory responses of cardiac cells through the modulation of the FOS/AP-1 pathway. Since fibrosis and inflammation are crucial in the progression of cardiac hypertrophy, heart failure, and diabetic cardiomyopathy, our results point to SIRT3 as a potential target for treating these diseases.

Regulation of metabolism by mitochondrial enzyme acetylation in cardiac ischemia-reperfusion injury

Ischemia reperfusion injury (I/R injury) contributes significantly to morbidity and mortality following myocardial infarction (MI). Although rapid reperfusion of the ischemic myocardium was established decades ago as a highly beneficial therapy for MI, significant cell death still occurs after the onset of reperfusion. Mitochondrial dysfunction is closely associated with I/R injury, resulting in the uncontrolled production of reactive oxygen species (ROS). Considerable efforts have gone into understanding the metabolic perturbations elicited by I/R injury. Recent work has identified the critical role of reversible protein acetylation in maintaining normal mitochondrial biologic function and energy metabolism both in the normal heart and during I/R injury. Several studies have shown that modification of class I HDAC and/or Sirtuin (Sirt) activity is cardioprotective in the setting of I/R injury. A better understanding of the role of these metabolic pathways in reperfusion injury and their regulation by reversible protein acetylation presents a promising way forward in improving the treatment of cardiac reperfusion injury. Here we briefly review some of what is known about how acetylation regulates mitochondrial metabolism and how it relates to I/R injury.

Androgen-Regulated Cardiac Metabolism in Aging Men

The prevalence of cardiovascular mortality is higher in men than in age-matched premenopausal women. Gender differences are linked to circulating sex-related steroid hormone levels and their cardio-specific actions, which are critical factors involved in the prevalence and features of age-associated cardiovascular disease. In women, estrogens have been described as cardioprotective agents, while in men, testosterone is the main sex steroid hormone. The effects of testosterone as a metabolic regulator and cardioprotective agent in aging men are poorly understood. With advancing age, testosterone levels gradually decrease in men, an effect associated with increasing fat mass, decrease in lean body mass, dyslipidemia, insulin resistance and adjustment in energy substrate metabolism. Aging is associated with a decline in metabolism, characterized by modifications in cardiac function, excitation-contraction coupling, and lower efficacy to generate energy. Testosterone deficiency -as found in elderly men- rapidly becomes an epidemic condition, associated with prominent cardiometabolic disorders. Therefore, it is highly probable that senior men showing low testosterone levels will display symptoms of androgen deficiency, presenting an unfavorable metabolic profile and increased cardiovascular risk. Moreover, recent reports establish that testosterone replacement improves cardiomyocyte bioenergetics, increases glucose metabolism and reduces insulin resistance in elderly men. Thus, testosterone-related metabolic signaling and gene expression may constitute relevant therapeutic target for preventing, or treating, age- and gender-related cardiometabolic diseases in men. Here, we will discuss the impact of current evidence showing how cardiac metabolism is regulated by androgen levels in aging men.

The yin and yang faces of the mitochondrial deacetylase sirtuin 3 in age-related disorders

Aging, the most important risk factor for many of the chronic diseases affecting Western society, is associated with a decline in mitochondrial function and dynamics. Sirtuin 3 (SIRT3) is a mitochondrial deacetylase that has emerged as a key regulator of fundamental processes which are frequently dysregulated in aging and related disorders. This review highlights recent advances and controversies regarding the yin and yang functions of SIRT3 in metabolic, cardiovascular and neurodegenerative diseases, as well as the use of SIRT3 modulators as a therapeutic strategy against those disorders. Although most studies point to a protective role upon SIRT3 activation, there are conflicting findings that need a better elucidation. The discovery of novel SIRT3 modulators with higher selectivity together with the assessment of the relative importance of different SIRT3 enzymatic activities and the relevance of crosstalk between distinct sirtuin isoforms will be pivotal to validate SIRT3 as a useful drug target for the prevention and treatment of age-related diseases.

SIRT4 and Its Roles in Energy and Redox Metabolism in Health, Disease and During Exercise

NAD⁺-dependent SIRT4 has been reported to be a key regulator of metabolic enzymes and antioxidant defense mechanisms in mitochondria. It also plays an important role in regulation of mitochondrial metabolism in response to exercise. Recent studies have shown that SIRT4 is involved in a wide range of mitochondrial metabolic processes, including depressing insulin secretion in pancreatic beta cells, promoting lipid synthesis, regulating mitochondrial adenosine triphosphate (ATP) homeostasis, controlling apoptosis and regulating redox. SIRT4 also appears to have enzymatic functions involved in posttranslational modifications such as ADP-ribosylation, lysine deacetylation and lipoamidation. However, the effects on SIRT4 by metabolic diseases and changes in metabolic homeostasis such as during exercise, along with the roles of SIRT4 in the regulation of metabolism during disease, are not well understood. The main goal of this review is to critically analyse and summarise the current research evidence on the significance of the SIRT4 as a metabolic regulator and in mitochondrial function and its putative roles in relation to metabolic diseases and exercise.

Impaired SIRT3 activity mediates cardiac dysfunction in endotoxemia by calpain-dependent disruption of ATP synthesis

BACKGROUND

Sepsis-induced cardiomyopathy contributes to the high mortality of septic shock in critically ill patients. Since the underlying mechanisms are incompletely understood, we hypothesized that sepsis-induced impairment of sirtuin 3 (SIRT3) activity contributes to the development of septic cardiomyopathy.

METHODS AND RESULTS

Treatment of mice with lipopolysaccharide (LPS) for 6 h resulted in myocardial NAD⁺ depletion and increased mitochondrial protein acetylation, indicating impaired myocardial SIRT3 activity due to NAD⁺ depletion. LPS treatment also resulted in impaired cardiac output in isolated working hearts, indicating endotoxemia-induced cardiomyopathy. Maintaining normal myocardial NAD⁺ levels in LPS-treated mice by Poly(ADP-ribose)polymerase 1 (PARP1) deletion prevented cardiac dysfunction, whereas additional SIRT3 deficiency blunted this beneficial effect, indicating that impaired SIRT3 activity contributes to cardiac dysfunction in endotoxemia. Measurements of mitochondrial ATP synthesis suggest that LPS-induced contractile dysfunction may result from cardiac energy depletion due to impaired SIRT3 activity. Pharmacological inhibition of mitochondrial calpains using MDL28170 normalized LPS-induced cleavage of the ATP5A1 subunit of ATP synthase and normalized contractile dysfunction, suggesting that cardiac energy depletion may result from calpain-mediated cleavage of ATP5A1. These beneficial effects were completely blunted by SIRT3 deficiency. Finally, a gene set enrichment analysis of hearts of patients with septic, ischemic or dilated cardiomyopathy revealed a sepsis-specific suppression of SIRT3 deacetylation targets, including ATP5A1, indicating a functional relevance of SIRT3-dependent pathways in human sepsis.

CONCLUSIONS

Impaired SIRT3 activity may mediate cardiac dysfunction in endotoxemia by facilitating calpain-mediated disruption of ATP synthesis, suggesting SIRT3 activation as a potential therapeutic strategy to treat septic cardiomyopathy.

Emerging role of SIRT3 in mitochondrial dysfunction and cardiovascular diseases

As a nicotinamide adenine dinucleotide (NAD)⁺-dependent protein deacetylase, SIRT3 is highly expressed in tissues with high metabolic turnover and mitochondrial content. It has been demonstrated that SIRT3 plays a critical role in maintaining normal mitochondrial biological function through reversible protein lysine deacetylation. SIRT3 has a variety of substrates that are involved in mitochondrial biological processes such as energy metabolism, reactive oxygen species production and clearance, electron transport chain flux, mitochondrial membrane potential maintenance, and mitochondrial dynamics. In the suppression of SIRT3, functional deficiencies of mitochondria contribute to the development of various cardiovascular disorders. Activation of SIRT3 may represent a promising therapeutic strategy for the improvement of mitochondrial function and the treatment of relevant cardiovascular disorders. In the current review, we discuss the emerging roles of SIRT3 in mitochondrial derangements and subsequent cardiovascular malfunctions, including cardiac hypertrophy and heart failure, ischemia-reperfusion injury, and endothelial dysfunction in hypertension and atherosclerosis.

Sirt3 enhances glioma cell viability by stabilizing Ku70-BAX interaction

Background

As one of the most prevalent malignancies, glioma is characterized by poor prognosis and high mortality rate. Glioma patients may show completely distinct clinical outcomes due to their different molecular patterns. Sirtuin 3 (Sirt3) participates in aging, stress resistance, and metabolic regulation. Here we aimed to test the expression and function of Sirt3 in glioma.

Methods

We enrolled 114 patients and tested the protein level of Sirt3 in their glioma tissues. The correlation between prognosis and Sirt3 was evaluated by univariate and multivariate analyses. We also conducted cellular experiments in U87 and U251 glioma cells, including overexpression and knockdown assays.

Results

Sirt3 expression was lower in glioma tissues than normal brain tissues. Higher Sirt3 is significantly correlated to advanced tumor grade (P=0.004), showing its potential role in cancer progression. Consistently, univariate and multivariate analyses identified Sirt3 as an independent prognostic factor (P=0.017). Patients with higher Sirt3 expression showed significantly shorter overall survival time. Moreover, overexpression of Sirt3 in either cell line enhanced cell viability, while silencing Sirt3 attenuated cell growth. Molecular assays showed Sirt3 can deacetylate Ku70 protein, therefore stabilizing the Ku70-BAX interaction. Since Ku70 can help prevent BAX transporting into mitochondria and decrease cell apoptosis, Sirt3 protein may play roles in maintaining cell viability. In addition, silencing Ku70 inhibited the pro-proliferative effect by Sirt3.

Conclusion

Taken together, our results not only identified the prognostic role of Sirt3 in glioma patients but also provided signaling pathway evidence for its functioning mechanisms.

Emerging role of SIRT3 in endothelial metabolism, angiogenesis, and cardiovascular disease

Sirtuin 3 (SIRT3) a mitochondrial enzyme that plays an important role in energy homeostasis, cardiac remodeling, and heart failure (HF). The expression of SIRT3 declines with advanced age, cardiovascular, and metabolic diseases. Accumulating evidence suggests that SIRT3 plays a critical role in protecting the heart from cardiac hypertrophy, cardiac dysfunction associated with HF, and in the protection of cardiac cells from stress-mediated cell death. Clinical studies have demonstrated that HF with preserved ejection fraction (HFpEF) in patients present with abnormalities in coronary microcirculation related to endothelial dysfunction and coronary microvascular rarefaction. Although SIRT3-mediated regulation of mitochondrial homeostasis and heart function has been intensively investigated, the effect of SIRT3 on endothelial cell (EC) glycolytic metabolism and microvascular function has not been well studied. ECs utilize glycolysis for generating ATP rather than oxidative phosphorylation to maintain their normal functions and promote angiogenesis and EC-cardiomyocyte interactions. Emerging evidence indicates that SIRT3 is involved in the regulation of endothelial metabolism and angiogenesis and thus affects the development of cardiovascular diseases associated with aging. This review will discuss the current knowledge of SIRT3 and its functional role on endothelial metabolism, cardiac function, and cardiovascular diseases.

Acetylation of Mitochondrial Proteins in the Heart: The Role of SIRT3

A growing number of studies have demonstrated the role of post-translational modifications of proteins, particularly acetylation, in human diseases including neurodegenerative and cardiovascular diseases, diabetes, cancer, and in aging. Acetylation of mitochondrial proteins has been shown to be involved in the pathogenesis of cardiac diseases such as myocardial infarction (ischemia-reperfusion) and heart failure. Indeed, over 60% of mitochondrial proteins contain acetylation sites, and most of these proteins are involved in mitochondrial bioenergetics. Mitochondrial non-enzymatic acetylation is enabled by acetyl-coenzyme A abundance and serves as the primary pathway of acetylation in mitochondria. Hence, regulation of enzymatic deacetylation becomes the most important mechanism to control acetylation/deacetylation of mitochondrial proteins. Acetylation/deacetylation of mitochondrial proteins has been regarded as a key regulator of mitochondrial metabolism and function. Proteins are deacetylated by NAD⁺-dependent deacetylases known as sirtuins (SIRTs). Among seven sirtuin isoforms, only SIRT3, SIRT4, and SIRT5 are localized in the mitochondria. SIRT3 is the main mitochondrial sirtuin which plays a key role in maintaining metabolic and redox balance in the mitochondria under physiological and pathological conditions. SIRT3 regulates the enzymatic activity of proteins involved in fatty acid oxidation, tricarboxylic acid cycle, electron transport chain, and oxidative phosphorylation. Although many enzymes have been identified as targets for SIRT3, cardiac-specific SIRT3 effects and regulations could differ from those in non-cardiac tissues. Therefore, it is important to elucidate the contribution of SIRT3 and mitochondrial protein acetylation/deacetylation in mitochondrial metabolism and cardiac dysfunction. Here, we summarize previous studies and provide a comprehensive analysis of the role of SIRT3 in mitochondria metabolism and bioenergetics under physiological conditions and in cardiac diseases. In addition, the review discusses mitochondrial protein acetylation as a potential target for cardioprotection.

The effect of oxygen in Sirt3-mediated myocardial protection: a proof-of-concept study in cultured cardiomyoblasts

Sirtuin 3 is a nicotinamide adenine dinucleotide dependent mitochondrial deacetylase that governs mitochondrial metabolism and oxidative defense. The demise in myocardial function following myocardial ischemia has been associated with mitochondrial dysfunction. Sirt3 maintains myocardial contractile function and protects from cardiac hypertrophy. The role of Sirt3 in ischemia is controversial. Our objective was to understand, under what circumstances Sirt3 is protective in different facets of ischemia, using an in vitro proof-of-concept approach based on simulated ischemia in cultured cardiomyoblasts. Cultured H9c2 cardiomyoblasts were subjected to hypoxia and/or serum deprivation, the combination of which we refer to as simulated ischemia. Apoptosis, as assessed by Annexin V staining in life-cell imaging and propidium-iodide inclusion in flow cytometry, was enhanced following simulated ischemia. Interestingly, serum deprivation was a stronger trigger of apoptosis than hypoxia. Knockdown of Sirt3 further increased apoptosis upon serum deprivation, whereas no such effect occurred upon additional hypoxia. Similarly, only upon serum deprivation but not upon simulated ischemia, silencing of Sirt3 led to a deterioration of mitochondrial function in extracellular flux analysis. In the absence of oxygen these Sirt3-dependent effects were abolished. These data indicate, that Sirt3-mediated myocardial protection is oxygen-dependent. Thus, mitochondrial respiration takes center-stage in Sirt3-dependent prevention of stress-induced myocardial damage.

Mitochondrial sirtuins in the heart

Sirtuins (SIRTs) are NAD(+)-dependent enzymes that catalyze deacylation of protein lysine residues. In mammals, seven sirtuins have been identified, SIRT1-7. SIRT3-5 are mainly or exclusively localized within mitochondria and mainly participate in the regulation of energy metabolic pathways. Since mitochondrial ATP regeneration is inevitably linked to the maintenance of cardiac pump function, it is not surprising that recent studies revealed a role for mitochondrial sirtuins in the regulation of myocardial energetics and function. In addition, mitochondrial sirtuins modulate the extent of myocardial ischemia reperfusion injury and the development of cardiac hypertrophy and failure. Thus, targeting mitochondrial sirtuins has been proposed as a novel approach to improve myocardial mitochondrial energetics, which is frequently impaired in cardiac disease and considered an important underlying cause contributing to several cardiac pathologies, including myocardial ischemia reperfusion injury and heart failure. In the current review, we present and discuss the available literature on mitochondrial sirtuins and their potential roles in cardiac physiology and disease.

SIRT3: A New Regulator of Cardiovascular Diseases

Cardiovascular diseases (CVDs) are the leading causes of death worldwide, and defects in mitochondrial function contribute largely to the occurrence of CVDs. Recent studies suggest that sirtuin 3 (SIRT3), the mitochondrial NAD⁺-dependent deacetylase, may regulate mitochondrial function and biosynthetic pathways such as glucose and fatty acid metabolism and the tricarboxylic acid (TCA) cycle, oxidative stress, and apoptosis by reversible protein lysine deacetylation. SIRT3 regulates glucose and lipid metabolism and maintains myocardial ATP levels, which protects the heart from metabolic disturbances. SIRT3 can also protect cardiomyocytes from oxidative stress-mediated cell damage and block the development of cardiac hypertrophy. Recent reports show that SIRT3 is involved in the protection of several heart diseases. This review discusses the progress in SIRT3-related research and the role of SIRT3 in the prevention and treatment of CVDs.

Mitochondrial Sirtuins in cardiometabolic diseases

Mitochondria are heterogeneous and essentially contribute to cellular functions and tissue homeostasis. Mitochondrial dysfunction compromises overall cell functioning, tissue damage, and diseases. The advances in mitochondrion biology increase our understanding of mitochondrial dynamics, bioenergetics, and redox homeostasis, and subsequently, their functions in tissue homeostasis and diseases, including cardiometabolic diseases (CMDs). The functions of mitochondria mainly rely on the enzymes in their matrix. Sirtuins are a family of NAD+-dependent deacylases and ADP-ribosyltransferases. Three members of the Sirtuin family (SIRT3, SIRT4, and SIRT5) are located in the mitochondrion. These mitochondrial Sirtuins regulate energy and redox metabolism as well as mitochondrial dynamics in the mitochondrial matrix and are involved in cardiovascular homeostasis and CMDs. In this review, we discuss the advances in our understanding of mitochondrial Sirtuins in mitochondrion biology and CMDs, including cardiac remodeling, pulmonary artery hypertension, and vascular dysfunction. The potential therapeutic strategies by targetting mitochondrial Sirtuins to improve mitochondrial function in CMDs are also addressed.

Hydrogen sulfide pretreatment improves mitochondrial function in myocardial hypertrophy via a SIRT3-dependent manner

BACKGROUND AND PURPOSE

Hydrogen sulfide (H₂ S) is a gaseous signal molecule with antioxidative properties. Sirtuin 3 (SIRT3) is closely associated with mitochondrial function and oxidative stress. The study was to investigate whether and how H₂ S improved myocardial hypertrophy via a SIRT3-dependent manner.

EXPERIMENTAL APPROACH

Neonatal rat cardiomyocytes were pretreated with NaHS (50 μM) for 4 h followed by angiotensin II (Ang II, 100 nM) for 24 h. SIRT3 was silenced with siRNA technology. SIRT3 promoter activity and expression, cell surface, hypertrophic gene mRNA expression, mitochondrial oxygen consumption rate and membrane potential were measured. Male 129S1/SvImJ [wild-type (WT)] and SIRT3 knockout (KO) mice were injected with NaHS (50 μmol·kg^-1 ·day^-1 ; i.p.) followed by transverse aortic constriction (TAC). Echocardiography, heart mass, mitochondrial ultrastructure, volume and number, oxidative stress, mitochondria fusion and fission-related protein expression were measured.

KEY RESULTS

In vitro, NaHS increased SIRT3 promoter activity and SIRT3 expression in Ang II-induced cardiomyocyte hypertrophy. SIRT3 silencing abolished the ability of NaHS to reverse the Ang II-induced cardiomyocyte hypertrophy, mitochondrial function impairment and permeability potential dysfunction, along with the decline in FOXO3a and SOD2 expression. In vivo, after TAC. NaHS attenuated myocardial hypertrophy, inhibited oxidative stress, improved mitochondrial ultrastructure, suppressed mitochondrial volume but increased mitochondrial numbers, enhanced OPA1, MFN1 and MFN2 expression but suppressed DRP1 and FIS1 expression in WT mice but not in SIRT3 KO mice CONCLUSION AND IMPLICATIONS: NaHS improved mitochondrial function and inhibited oxidative stress in myocardial hypertrophy in a SIRT3-dependent manner.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

High Sensitivity of SIRT3 Deficient Hearts to Ischemia-Reperfusion Is Associated with Mitochondrial Abnormalities

Aim: Sirtuins are NAD⁺-dependent deacetylases that regulate cell metabolism through protein acetylation/deacetylation, and SIRT3 is the major deacetylase among mitochondrial isoforms. Here, we elucidated the possible role of acetylation of cyclophilin D, a key regulator of the mitochondrial permeability transition pore (mPTP), in mitochondria-mediated cardiac dysfunction induced by ischemia-reperfusion (IR) in wild type (WT) and SIRT3 knockout (SIRT3^-/-) mice.

Materials and Methods: Isolated and Langendorff-mode perfused hearts of WT and SIRT3^-/- mice were subjected to 25-min global ischemia followed by 60-min of reperfusion in the presence or absence of the mPTP inhibitor, sanglifehrin A (SfA).

Results: Analysis of mitochondrial sirtuins demonstrated that SIRT3 deficiency upregulated SIRT4 with no effect on SIRT5 expression. Hearts of SIRT3^-/- mice exhibited significantly less recovery of cardiac function at the end of IR compared to WT mice. Intact (non-perfused) SIRT3^-/- hearts exhibited an increased rate of Ca²⁺-induced swelling in mitochondria as an indicator of mPTP opening. However, there was no difference in mPTP opening and cyclophilin D acetylation between WT and SIRT3^-/- hearts subjected to IR injury. Ca²⁺-stimulated H₂O₂ production was significantly higher in SIRT3^-/- mitochondria that was prevented by SfA. Superoxide dismutase activity was lower in SIRT3^-/- heart mitochondria subjected to IR which correlated with an increase in protein carbonylation. However, mitochondrial DNA integrity was not affected in SIRT3^-/- hearts after IR.

Conclusion: SIRT3 deficiency exacerbates cardiac dysfunction during post-ischemic recovery, and increases mPTP opening and ROS generation without oxidative damage to mitochondrial proteins and DNA.

SIRT3 in Cardiac Physiology and Disease

Functional defects in mitochondrial biology causally contribute to various human diseases, including cardiovascular disease. Impairment in oxidative phosphorylation, mitochondrial oxidative stress, and increased opening of the mitochondrial permeability transition pore add to the underlying mechanisms of heart failure or myocardial ischemia–reperfusion (IR) injury. Recent evidence demonstrated that the mitochondrial NAD⁺-dependent deacetylase sirtuin 3 (SIRT3) may regulate these mitochondrial functions by reversible protein lysine deacetylation. Loss of function studies demonstrated a role of impaired SIRT3 activity in the pathogenesis of myocardial IR injury as well as in the development of cardiac hypertrophy and the transition into heart failure. Gain of function studies and treatment approaches increasing mitochondrial NAD⁺ availability that ameliorate these cardiac pathologies have led to the proposal that activation of SIRT3 may represent a promising therapeutic strategy to improve mitochondrial derangements in various cardiac pathologies. In the current review, we will present and discuss the available literature on the role of SIRT3 in cardiac physiology and disease.