Cardiac Dysfunction and Oxidative Stress in the Metabolic Syndrome: an Update on Antioxidant Therapies

Abstract

The metabolic syndrome (MetS) is a cluster of risk factors including obesity, insulin resistance, dyslipidemia, elevated blood pressure and glucose intolerance. The MetS increases the risk for cardiovascular disease (CVD) and type 2 diabetes. Each component of the MetS causes cardiac dysfunction and their combination carries additional risk. The mechanisms underlying cardiac dysfunction in the MetS are complex and might include lipid accumulation, increased fibrosis and stiffness, altered calcium homeostasis, abnormal autophagy, altered substrate utilization, mitochondrial dysfunction and increased oxidative stress. Mitochondrial and extra-mitochondrial sources of reactive oxygen species (ROS) and reduced antioxidant defense mechanisms characterize the myocardium of humans and animals with the MetS. The mechanisms for increased cardiac oxidative stress in the MetS are not fully understood but include increased fatty acid oxidation, mitochondrial dysfunction and enhanced NADPH oxidase activity. Therapies aimed to reduce oxidative stress and enhance antioxidant defense have been employed to reduce cardiac dysfunction in the MetS in animals. In contrast, large scale clinical trials using antioxidants therapies for the treatment of CVD have been disappointing because of the lack of efficacy and undesired side effects. The focus of this review is to summarize the current knowledge about the mechanisms underlying cardiac dysfunction in the MetS with a special interest in the role of oxidative stress. Finally, we will update the reader on the results obtained with natural antioxidant and mitochondria-targeted antioxidant therapies for the treatment of CVD in the MetS.

INTRODUCTION

The metabolic syndrome (MetS) represents a cluster of cardiovascular risk factors that includes abdominal obesity, dyslipidimia, hypertension, and impaired glucose tolerance. The MetS increases the risk for type 2 diabetes (T2D) and cardiovascular disease (CVD). Thus, people with the MetS have a five-fold higher risk of T2D and a two to three-fold higher risk of atherosclerotic CVD than those without [1–2]. The etiology of CVD in patients with MetS may involve: coronary atherosclerotic disease, arterial hypertension, left ventricular (LV) hypertrophy, diastolic dysfunction, endothelial dysfunction, coronary micro-vascular disease and autonomic dysfunction. The pathogenesis of CVD in the MetS is multifactorial as it can be caused by one or more factors associated with this condition such as the systemic abnormalities, insulin resistance, diabetes and/or inflammation. One common characteristic of CVD in the MetS and the insulin resistant state is increased oxidative stress in the heart [3–4]. Indeed, patients with the MetS have elevated systemic oxidative damage as a result of overproduction of ROS and decreased antioxidant protection [5–6]. In this review, we will first summarize the contribution of each component of the MetS to cardiac dysfunction and then highlight the underlying mechanisms with a special focus on the contribution of oxidative stress. Finally, we will summarize and discuss past and current studies using antioxidant therapies to treat CVD in the MetS.

CARDIAC DYSFUNCTION IN THE METABOLIC SYNDROME

Each component of the MetS is known to independently affect cardiac structure and function, but their combination under this syndrome seems to carry additional risk [7–8]. Thus, cardiac dysfunction can occur in patients with normal coronary artery disease or other etiologies, suggesting the existence of specific cardiomyopathies such as obesity-related cardiomyopathy, diabetic cardiomyopathy and insulin resistance-related cardiac dysfunction. As summarized in (Fig. 1), common mechanisms responsible for cardiac dysfunction are shared between obesity, diabetes and insulin resistance, however unique mechanisms characterize each component of the MetS.

Fig. 1. Mechanisms for altered cardiac function in the metabolic syndrome.

Common and distinct mechanisms responsible for cardiac dysfunction are highlighted for three important components of the metabolic syndrome; Obesity and dyslipidemia, diabetes and hyperglycemia and insulin resistance.

MECHANISMS FOR OBESITY-RELATED CARDIOMYOPATHY

Obesity itself or in association with dyslipidimia promotes hearts failure in humans [9–10]. Several mechanisms have been proposed to explain cardiac dysfunction in obesity including increased hemodynamic load, cardiac hypertrophy, increased lipid accumulation and altered substrate metabolism. For example, an association between myocardial triacylglycerol (TG) content and concentric LV hypertrophy with subtle reduction in systolic function was reported in humans [11]. Similarly, a higher cardiac TG content was observed in heart of obese or T2D patients [12], suggesting the involvement of cardiac lipid accumulation in the pathogenesis of cardiac dysfunction in the MetS (See Review by Kusminski et al. [13]). In addition to the above mentioned mechanisms, it was recently suggested that adipose-derived factors and adipokines such as leptin, adiponectin, resistin and fatty acid binding protein 4 (FABP4) can directly affect cardiac structure and function. Indeed, elevated circulating leptin levels are predictors of worse outcome in patients with CVD and heart failure [14]. Furthermore, leptin treatment of neonatal ventricular myocytes promotes cardiac hypertrophy through the regulation of actin dynamics [15]. In contrast, leptin treatment of the leptin-deficient (ob/ob) or the leptin receptor-deficient (db/db) mice completely normalized cardiac hypertrophy [16], suggesting rather an antihypertrophic role for leptin. Depressed plasma adiponectin levels correlated inversely with the MetS and T2D and increased the risk of myocardial infarction and heart failure [17–19]. Similar to findings with leptin, adiponectin also has antihypertrophic properties as its reduction in mice promotes the development of LV hypertrophy [20]. Moreover, serum resistin levels are usually high in mouse models of the MetS and in humans with heart failure [21–24]. Resistin affects both the structure and the function of the heart. Indeed, in-vitro and in-vivo studies have suggested a role for resistin in promoting cardiac hypertrophy and reducing contractility [21, 25]. Finally, adipocytes-derived fatty acid binding protein 4 (FABP4) levels were found to correlate positively with the MetS in a cross-sectional study [26] and this fat-specific factor reduced cardiac function through modulation of intracellular calcium [27–28].

MECHANISMS FOR DIABETIC CARDIOMYOPATHY

Since its first introduction by Rubier et al. [29] forty years ago, the existence of a unique diabetic cardiomyopathy has been confirmed in numerous studies (see reviews by Boudina and Abel [30–31]). Indeed, diabetes increased the risk for heart failure even after adjusting for age, blood pressure, weight, cholesterol and coronary artery disease. Thus, CVD is 2–3 times more common, and survival is worse in subjects with diabetes in comparison with age-matched and sex-matched counterparts [32–34]. According to the molecular theory of diabetic cardiomyopathy, hyperglycemia is the main pathogenetic factor, which causes abnormalities at the cardiac myocyte level, eventually leading to structural and functional abnormalities [35]. Diabetic cardiomyopathy is characterized by an initial diastolic dysfunction that occurs before altered systolic function [36–37]. One proposed mechanism for altered diastolic function in diabetic myocardium is enhanced deposition of glycosylated glycogen, which is known to promote cardiac stiffness via increased fibrosis [38–40]. In parallel, hyperglycemia was shown to directly alter components of calcium homeostasis, leading to diastolic dysfunction (See review by Dobrin and Lebeche [41]). Indeed, the activity and the content of the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) is decreased in diabetes [42–43] and the hemodynamic dysfunction is prevented by up-regulation of SERCA in a rat model of the MetS [44]. In addition to reducing left ventricular relaxation time, SERCA2a gene transfer therapy reduced oxygen cost for contraction in a mouse model of type 2 diabetes [45], highlighting the importance of calcium in metabolic regulation. The mechanisms by which hyperglycemia affects SERCA activity are through (1) oxidative stress-mediated oxidation of its cysteine thiols which interferes with the ATP binding site, making it unable to hydrolyze ATP [46] and (2) through direct cross-linking of collagen with SERCA, which inhibits its activity. More recently, a role of micro RNAs in cardiac dysfunction caused by diabetes has emerged [47]. Indeed, the expression of miR133, the most abundant micro RNA in the heart, was reduced by diabetes, and hyperglycemia-induced cardiac hypertrophy was prevented by miR133 over-expression in cardiac myocytes in-vitro [48].

MECHANISMS FOR INSULIN RESISTANCE-RELATED CARDIAC DYSFUNCTION

The MetS and insulin resistance are associated with abnormal LV diastolic function and structure independently of age, gender, blood pressure and fasting plasma glucose [49]. Furthermore, insulin resistance predicts the incidence of heart failure independently of other established risk factors, including diabetes and obesity [50]. Moreover, the contribution of insulin resistance to cardiac dysfunction without the systemic abnormalities associated with the MetS has recently been studied using a mouse model of cardiac insulin resistance obtained by deletion of insulin receptors specifically in cardiomyocytes (CIRKO mice) [51]. Whereas at baseline, these mice exhibit mild alterations of cardiac performance, their response to pressure overload [52], isoproterenol treatment [53] or myocardial infarction [54] is altered, suggesting that insulin resistance increased the susceptibility for the development of cardiac dysfunction independently of obesity or diabetes. The mechanisms involved in insulin resistance-mediated cardiac dysfunction include altered substrate metabolism, persistent expression of the fetal beta-myosin heavy chain isoform, reduced angiogenesis and mitochondrial dysfunction.

COMMON MECHANISMS BETWEEN OBESITY, DIABETES AND INSULIN RESISTANCE

1- Altered Cardiac Substrate Metabolism

The mammalian heart possesses the capacity of oxidizing any available substrate to maintain a steady level of ATP required for contraction. Although, oxidation of fatty acids (FA) is predominant in the adult heart, the use of glucose, lactate and ketones can be enhanced in certain pathological condition (See review by Duncan JG [55]. This flexibility in substrate use is important for normal cardiac function and its alteration by the MetS contribute to cardiac dysfunction. Thus, obesity, diabetes and/or insulin resistance independently or in combination affect this flexibility due to alteration in substrate availability or to impairment in transcriptional regulation of oxidation pathways. For example, obesity in mice enhances cardiac FA oxidation and reduces glucose oxidation independently of diabetes [56]. The mechanisms for obesity-related alteration in cardiac substrate utilization are not completely understood but involve enhanced FA and reduced glucose availability and leptin resistance [57]. Similarly, type 1 and type 2 diabetes enhanced FA oxidation and uptake whereas glucose utilization was reduced [58–60]. The mechanisms for increased cardiac FA uptake and oxidation in the MetS include impaired glucose transport, enhanced long-chain FA uptake through relocation of the FA transporter CD36 in the sarcolema [61] and increased mitochondrial CPT-1 activity [62]. Whereas, altered cardiac substrate metabolism is evident in the MetS, there was no correlation between impaired substrate use and LV diastolic dysfunction in type 2 diabetic patients, thus excluding a causal role in the development of cardiac dysfunction during the MetS [60]. Finally, insulin resistance without confounding systemic abnormalities was shown to reduce both glucose and FA oxidation in the heart possibly via a reduction in the expression of genes involved in FA oxidation and by impairing mitochondrial oxidative capacity [51, 63].

2- Altered Cardiac Mitochondrial Function and Biogenesis

Mitochondrial dysfunction plays a crucial role in the pathogenesis of cardiac dysfunction in the MetS. Indeed, each component of the MetS is known to independently modulate mitochondrial function, proteome and biogenesis. Whereas most studies examining changes in mitochondrial function in the MetS in humans have been performed in skeletal muscle, the results cannot be extrapolated to the heart due to its higher mitochondrial oxidative capacity and content. Thus, most of what we currently know about mitochondrial function in the heart comes from animal models of the MetS with the exception of few indirect studies looking at cardiac oxygen consumption, phosphocreatine (Pcr)/ATP ratios or atrium mitochondrial oxygen consumption in obese or T2D patients. These studies associated mitochondrial uncoupling and/or dysfunction with increased cardiac oxygen cost for contraction in obese individuals [64], decreased high energy phosphate metabolism in the diabetic heart [65–66] and reduced mitochondrial maximal capacity to oxidize FA and glutamate in T2D patients [67]. In contrast to human studies, mitochondrial dysfunction in the heart of genetically obese db/db mice was first reported in the 80s by Kuo et al. [68] and then confirmed by a recent study [69]. Furthermore, impaired mitochondrial function and biogenesis was identified in other mouse models of obesity and insulin resistance such as the leptin-deficient ob/ob mice [70] and UCP-DTA mice [71]. The mechanisms for impaired cardiac mitochondrial function and biogenesis in the MetS and the insulin resistant state have been extensively reviewed [3, 72–74] and include enhanced FA-induced mitochondrial uncoupling [69], increased mitochondrial oxidative stress [67, 75–76], impaired mitochondrial calcium handling [77–78], enhanced mitochondrial DNA damage [79], altered mitochondrial proteome [80–84] and deregulation of mitochondrial biogenesis [72–73, 85].

3- Impaired Cardiac Autophagy

Although autophagy, which is a physiologic process by which a cell clean damaged proteins and organelles, has been known since the 60s, its role in CVD has been recently introduced. Thus, defect in autophagy causes cardiac dysfunction and heart failure particularly under increased cellular stress such as ischemia/reperfusion (I/R) [86]. Furthermore, autophagy plays a central role in cardiac dysfunction during aging and its modulation might represent a promising way to treat cardiac senescence [87–89]. Similarly, long-term caloric restriction enhances autophagy and preserves cardiac function in otherwise healthy mice [90]. Although autophagy has been implicated in various pathologies of the heart, it is until recently that the pathophysiologic role of autophagy in the MetS has been introduced [91–92]. Thus, autophagy is reduced in the hearts of OVE26 mice, a mouse model of severe type 1 diabetes that develop diabetic cardiomyopathy, an effect that was exacerbated by the inhibition of AMPK and alleviated by metformin treatment [93]. More recently, autophagy was found to be deregulated in the heart of high fat-fed mice (a mouse model of the MetS), rendering them more susceptible to I/R injury [94]. The mechanisms underlying the deregulation of autophagy and whether it can be targeted to treat cardiac dysfunction in the MetS require more work.

4- Increased Cardiac Oxidative Stress

Oxidative stress (OS) is defined as an excess formation or insufficient removal of highly reactive molecules such as reactive oxygen species (ROS) and reactive nitrogen species (RNS) [95]. Many aspects of the relationship between OS and endothelial dysfunction in the MetS and diabetes have been previously reviewed [96–97], this review will focus on the role of OS in cardiac dysfunction in the MetS. Increased systemic OS, as evidenced by reduced serum vitamin C and α-tocopherol concentrations and decreased superoxide dismutase activity, has been previously documented in patient with the MetS [5–6, 98]. Furthermore, a positive correlation between systemic OS and the development of insulin resistance and diabetes was found in the Framingham Offspring Study [99]. Thus, hydrogen peroxide (H₂O₂) emission was found to be higher in right atrial appendages obtained from patients with T2D undergoing non-emergent coronary artery bypass graft surgery [67]. In contrast to the fewer human studies, many studies have confirmed the existence of OS in the myocardium of animal models with one or more components of the MetS. Thus, succinate-supported H₂O₂ production as well as lipid and protein oxidation markers were increased in the heart of db/db mice [69]. Furthermore, insulin resistance enhanced cardiac ROS generation independently of hyperglycemia, hyperlipidemia and hyperinsulinemia in mice [63], and superoxide production was elevated in the myocardium of high fat-fed spontaneously hypertensive (SHR) rats [100]. Furthermore reduced GSH/GSSG ratio was shown in ob/ob hearts [101] and decreased cardiac expression of manganese superoxide dismutase (MnSOD), glutathione peroxidase I (GPxI) was observed in high fat-fed and obese Zucker rats [102–103]. Although an association between elevated OS and cardiac dysfunction in the MetS has been established, a causal role for OS in the development of myocardial dysfunction has not been proven yet but one could emphasize that ROS-mediated damage to proteins, DNA and RNA may exacerbate cardiac dysfunction. Furthermore, OS is involved in the pathogenesis of apoptosis as it can directly activate pro-apoptotic signaling pathways such as JNK, p38 and ASK-1 [104–105]. In addition, increased mitochondrial ROS can lead to cytochrome c release and the initiation of apoptosis [106]. The induction of apoptosis by OS plays an important role in cardiac remodeling and fibrosis.

THE SOURCES AND THE MOLECULAR MECHANISMS OF INCREASED OXIDATIVE STRESS IN THE METS

1- Mitochondrial Sources

Mitochondrial function is particularly important in the heart since it provides over 90% of myocardial ATP [107]. In the process of normal respiration, 0.4 to 4% of oxygen consumed by mitochondria is incompletely reduced and form ROS [108]. As illustrated in (Fig. 2), ROS are generated at several sites of the electron transport chain (ETC), where electrons leak to O₂ to generate superoxide [109]. A significant amount of superoxide is produced at the level of complex I and III of the ETC. Complex I produces superoxide in the matrix [110] whereas its production by complex III happens both in the matrix and in the inter-membrane space [111]. In addition to complex I and III, other less important sources of mitochondrial superoxide have been documented and include pyruvate dehydrogenase, 2-oxoglutarate dehydrogenase, the electron transferring flavoprotein ubiquinone oxidoreductase (ETF-QOR) (receiving electrons from the β-oxidation) and the glyceraol 3-phosphate dehydrogenase [112–113]. Because of the susceptibility of mitochondrial membranes and DNA to oxidative damage, detoxifying systems are in place to reduce superoxide accumulation. This detoxification is achieved by the conversion of superoxide to H₂O₂ by MnSOD [114–115] and peroxide reduction by GPx1 and GPx4, thioredoxin reductases (Trx2), glutaredoxin (Grx2) and peroredoxins (Prdx3 and Prdx5), which are all expressed in the mitochondria [116].

Fig. 2. Major sites for mitochondrial superoxide (O₂⁻) generation and its detoxification.

Black arrows indicate electron flow, blue hatched arrows indicate proton flow and red arrows indicate superoxide production. SOD1 and SOD2: superoxide dismutase 1 and 2; GPx: glutathione peroxidase; Trx: thioredoxin and Grx: glutaredoxin. (The color version of the figure is available in the electronic copy of the article).

Mitochondrial-generated H₂O₂ was documented in the hearts of genetically obese and diabetic db/db mice and in CIRKO mice lacking insulin receptors in cardiac cells [63, 69]. One common finding between these mice is enhanced FA oxidation, which can promote ROS formation. Indeed, a study by St-Pierre et al. [111] demonstrated that mitochondrial H₂O₂ production in the heart is enhanced when mitochondria are respiring on the FA substrate palmitoylcarnitine compared to other substrate such as glutamate or pyruvate. This is possibly due to enhanced superoxide generation from the ETF-QOR, as a result of accelerated electrons flux through the β-oxidation. This is further confirmed by the association of enhanced activity of enzymes involved in β-oxidation with mitochondrial generation of H₂O₂ in db/db and CIRKO mice [63, 69], whereas no changes in β-oxidation enzymes activity and no mitochondrial H₂O₂ generation was detected in ob/ob hearts despite increased FA oxidation [69]. Another possible mechanism for increased mitochondrial ROS is inhibition of the ETC, which can trigger superoxide generation through the reverse electron transfer [111]. This inhibition can be caused in part by glucose or hyperglycemia-mediated O-linked β-N-acetylglucosamine glycosylation (O-GlcNAcylation) of mitochondrial complex I, which can lead to ROS generation especially in the presence of excess reducing equivalents [81]. Furthermore, hyperglycemia increased H₂O₂ formation in neonatal cardiomyocyte cell line that was dependent on mitochondrial fission [117]. Whether increased mitochondrial ROS formation is indeed responsible for cardiac dysfunction in animals with the MetS is possible but has not been fully investigated (see antioxidant therapy section bellow).

2- Extra-mitochondrial Sources

While mitochondria are considered the major source of cell-damaging ROS in the heart, there are other cellular sources. The three predominant extra-mitochondrial systems that produce ROS mammalian cells are NADPH oxidase (NOX), xanthine oxidase (a form of xanthine oxidoreductase) and uncoupled nitric oxide [118]. Thus, the activity of NOX is enhanced in the hearts of obese Zucker rats, leptin-deficient ob/ob mice and high fat-fed rats [103, 119–121]. Interestingly, and confirming the involvement of NOX in ROS generation, NOX inhibition abolished superoxide production in the hearts of these animals [103]. In addition to NOX, xanthine oxidoreductase activity was shown to be elevated in ob/ob hearts whereas nitric oxide synthase level was decreased [122]. All these studies highlight a role for NOX and xanthine oxidoreductase as potential extra-mitochondrial source of ROS but the consequences of inhibiting these enzymes on cardiac function in the MetS have not been extensively investigated. Thus, inhibition of NOX alleviated contractile defects in ob/ob mice, in high fat-fed mice subjected to myocardial infarction and in mice with experimental diabetes [120, 123–124]. One additional cardiac ROS-generating system that is relevant in the MetS is the rennin-angiotensin system (RAS). Indeed, RAS is up-regulated by various components of the MetS such as glucose, circulating lipids, obesity and blood pressure [125], and its activation promotes ROS generation by NADPH oxidase and mitochondria [126].

ANTIOXIDANT THERAPIES AND CARDIAC DYSFUNCTION IN THE METS

I- Non Mitochondria-targeted Antioxidants

Systemic therapeutics for the treatment of CVD in the context of the MetS need to address one or several underlying conditions, including metabolic abnormalities (dyslipidemia and hyperglycemia), hypertension, arterosclerosis, and sleep apnea. There has been a substantial interest in using natural antioxidant compounds for the treatment of CVD such as vitamins, flavonoids and polyphenols. More recently, synthetic antioxidants with selective mitochondrial targeting property have been discovered and used to treat abnormalities associated with the MetS (See Table 1).

Table 1.

Human and animals studies using non mitochondria-targeted or mitochondria-targeted antioxidants for the treatment of cardiovascular disease in the metabolic syndrome Table 1. Human and animals studies using non mitochondria-targeted or mitochondria-targeted antioxidants for the treatment of cardiovascular disease in the metabolic syndrome

Class of Antioxidants	Name of Antioxidants	Mode of Action	Effect	Animal/In Vitro Studies	Human Studies
Non-mitochondria-targeted antioxidants
Vitamins	Vitamin C (L-ascorbic acid)	Suppression of adrenergic activity	Reduction of blood pressure		Reduction of blood pressure in non-MetS patients [169]
	Vitamin E		No effect		CHAOS trial [128]
Natural compounds	CoenzymeQ10	Vitamin-like lipid-soluble component of mitochondrial electron transport chain	Slight reduction of LV mass and systolic blood pressure	Female db/db mice [152]
			Reduction of diastolic dysfunction		Children with cardiomyopathy [153]
			No effect on hypertension		Patients with the MetS [154–155]
	Resveratrol	Increases expression of SIRT1 [138]. Increases GLUT1, IRS1 expression, AMPK and mTOR phosphorylation, reduces RBP4 and PPAR gamma expression [135].	Prevention of LV hypertrophy, diastolic dysfunction, interstitial fibrosis	HFHS-fed C57B16 mice [134].
			Improvement of blood pressure, and cardiac function	Yorkshire miniswine [135].
			Moderate reduction of blood pressure		Obese humans [138]
	Anthocyanin	Nonspecific/Unknown	Reduces oxidative stress	In vitro [141]
			No reduction of blood pressure		Dyslipedemic patients [142]
	Curcumin	p300 blocker	Regulation of cardiac hypertrophy	Rat neonatal cardiomyocytes cultured in high-glucose medium and rats with STZ-induced diabetes [170]
	Melatonin	Scavenges HO−, O2−, and NO) capable of crossing cell membranes and the blood–brain barrier [171–175]	Reduction of infarct size after IR diet and whole-body metabolic abnormalities	Wistar rats on a high-calorie diet
	Allopurinol	Xantine oxidase inhibitor
	N-acetylcysteine		Reduction of oxidative stress but compromised cardioprotection	Rats [176].
Synthetic compounds	Edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one)	Non-specific ROS scavenger	Significant improvement of clinical outcomes		Patients with AMI given during, given during stenting of stenotic arteries [177–178]
Derivatives of natural products	SkQBerb, SkQ-Palm	Mitochondria-targeted derivatives of natural products	Potent ROS-scavenging	Isolated mitochondria and human cells [165]
	S17834	Synthetic analog of resveratrol
Mitochondria targeted antioxidants
	Mn (III) tetrakis 4-benzoic acid porphyrin (MnTBAP)		Improvement of glucose tolerance	Ob/ob mice [148]
			Prevention of cardiac hypertrophy	UCP-DTA mice (unpublished data)
	MitoTempo and MitoQuinone		Improvement of mitochondrial function and coronary collateral growth after ischemia injury	Zucher obese fatty rats [160]
Mitochondria-targeted synthetic antioxidant peptide	SS-31	Targeting to inner mitochondrial membrane	Prevention of cardiac ischemia/reperfusion injury	HF-fed rats [179].
			Prevention of mitochondrial depolarization	Immortalized cell lines [180]
	Bendavia	Analog of SS-31	Moderate reduction of infarct size and improved cardiomyocyte survival	Sheep, rabbit, and guinea pig models of ischemia/reperfusion injury [162]
			Scavenging of H2O2	Immortalized cell lines [163]

List of non- standard abbreviations and acronyms

CPT-1: Carnitine palmitoyl transferase 1

FA: Fatty acid

AMPK: AMP-activated protein kinase

OS: Oxidative stress

GSH: Reduced glutathione

GSSG: Oxidized glutathione

ETC: Electron transport chain

NADPH: Nicotinamide adenine dinucleotide phosphate-oxidase

SIRT-1: Sirtuin 1

LDL: Low density lipoprotein

JNK: Jun N-terminal kinase

ASK-1: apoptosis signal-regulated kinase 1

Vitamins

Vitamin E supplementation in Chinese women for 4 month improved plasma cholesterol levels and markers of oxidative stress [127]. However, unlike smaller trials, investigation of vitamins administration to larger cohorts of patients did not show positive results. A randomized trial using Vitamin E daily did not have significant effects on cardiovascular outcomes in patients enrolled in the Cambridge Heart Antioxidant Study (CHAOS) [128]. A combination of vitamins C and E did not affect metabolic parameters (body weights, hemoglobin Alc, low density lipoprotein or triglycerides) of patients with the MetS or T2D [129]. Furthermore, co-administration of α-lipoic acid and vitamin E to patients with the MetS, failed to improve their metabolic profile [130]. Overall, vitamin supplementation does not appear sufficient to improve preexisting metabolic or cardiovascular complications in humans as reviewed elsewhere [131–132].

Flavonoids and Polyphenols

In contrast to vitamins, flavonoids and polyphenols supplementation has proven to be efficacious in reducing the metabolic abnormalities as well as cardiac dysfunction in patients or animals with the MetS. Thus, resveratrol, which is an antioxidant found in red wine and grape skin/seed, has protective cardiovascular properties [133]. Resveratrol and S17834 (a synthetic flavonoid derivative) prevented LV hypertrophy, diastolic dysfunction, and interstitial fibrosis and reduced levels of oxidative modifications and hyper-insulinemia in high fat high sucrose-fed C57B16 mice [134]. Similarly, resveratrol improved LDL, plasma glucose and insulin, blood pressure, and cardiac function, in Yorkshire mini swine fed a high cholesterol diet [135]. Furthermore, resveratrol supplementation in rats fed 65% sucrose, improved glucose tolerance, plasma insulin and triglyceride levels and enhanced hepatic catalase and superoxide dismutase enzyme activities [136]. In humans, moderate wine consumption may lower the incidence of the MetS and the associated cardiovascular complications, a finding that has been recently reviewed (See review by Liu et al. [137]). A recent study in obese humans showed that 30 days of resveratrol supplementation increased metabolic rate, moderately lowered blood pressure, and reduced plasma insulin, glucose and triglyceride levels. This was paralleled by an increase in skeletal muscle mitochondrial function as a result of enhanced AMPK activity and SIRT1 expression [138]. These beneficial effects of resveratrol are believed to be mediated by AMPK as mice deficient in this metabolic sensor are resistant to the beneficial metabolic effect of resveratrol in high fat-fed mice [139]. Overall, human and animal studies hold a great promise for resveratrol use to treat cardiovascular complications in the MetS [140].

Similar to resveratrol, anthocyanin was shown to reduce oxidative stress in-vitro [141] and LDL cholesterol but not blood pressure or other metabolic parameters in dyslipidemic patients [142]. Finally, quercetin reduced systolic blood pressure and plasma oxidized LDL concentration in overweight subjects with high-cardiovascular disease risk [143] reduced blood pressure and improved cardiac function in Wistar rats fed high carbohydrate high fat diet [144]. Other natural supplements such as genistein, triterpenoid, naringenin and curcumin have shown some in-vitro activity against the MetS but in-vivo studies are required to confirm their benefits on CVD [145].

II- Mitochondria-Targeted Antioxidants

Since mitochondria is considered a substantial source of ROS in the heart of humans and animals with the MetS and mitochondrial dysfunction is believed to participate in the development of CVD under this condition, antioxidant therapies should focus on novel class of compounds with high mitochondrial affinity as the new way to treat CVD in the MetS, a topic that has been recently reviewed by Subramanian et al [146]. Among mitochondria-targeted compounds that have been used in animal studies are superoxide dismutase (SOD) mimetics, CoenzymeQ₁₀ and its analogues and mitochondria-targeted small peptides.

SOD Mimetics

Pharmacological mimetics of antioxidant enzymes, including MnSOD, were shown to be effective in reducing ROS and restoring mitochondrial function [147]. Treatment of ob/ob mice with the SOD mimetic and peroxynitrite scavenger MnTBAP, improved glucose tolerance but cardiac function was not assessed in this study [148]. Furthermore, treatment of L6 myotubes with MnTBAP was able to restore insulin-stimulated GLUT4 translocation after palmitate treatment and in high fat feeding in mice [149]. Whether MnTBAP treatment improves cardiac dysfunction in the MetS is yet to be determined in future studies (See Table 1).

CoenzymeQ₁₀ and its Analogs

CoenzymeQ₁₀ is a vitamin-like lipid-soluble component of the mitochondrial ETC. Studies in cells showed that exogenous administration of CoenzymeQ₁₀ leads to its mitochondrial localization in contrast to vitamin E, as its distribution in cells correlates directly with lipid distribution [150]. A recent study demonstrated that the use of CoenzymeQ₁₀ supplementation reduced superoxide generation and ameliorated diastolic dysfunction in db/db mice [151]. Furthermore, CoenzymeQ₁₀ treatment, in female db/db mice, slightly lowered LV mass, systolic blood pressure, and lipid peroxidation [152]. Similarly, addition of CoenzymeQ₁₀ to regular medications, reduced diastolic dysfunction in children with cardiomyopathy [153]. However, supplementation with CoenzymeQ₁₀ was not sufficient to reduce hypertension in patients with the MetS [154–155].

MitoQ, a triphenylphosphonium-conjugated derivative of Co-enzymeQ, is a mitochondria-targeted antioxidant that efficiently reduces oxidative stress [156] but has no adverse effects on wild-type mice [156–157]. When supplemented in drinking water, MitoQ decreased cardiac dysfunction in rats subjected to I/R [158]. Similarly, MitoQ decreased adiposity, hypercholesterolemia and hypertriglyceridemia in high fat-fed ApoE^−/− and ATM^+/−/ApoE^−/− mouse models of the MetS [159]. So far, human studies using this compound have been performed only in the context of Parkinson’s disease and chronic hepatitis C [157]. Finally, administration of MitoTempol (another mitochondria-targeted antioxidant) and MitoQ in drinking water improved mitochondrial function and coronary collateral growth after I/R in Zucher obese fatty rats [160].

Mitochondria-targeted Peptides

A mitochondria-targeted synthetic antioxidant peptide SS-31 protected against cardiac I/R injury when given ex-vivo and ameliorated hypertensive cardiomyopathy and myocardial oxidative stress induced by Angiotensin-II [161]. In sheep, rabbit, and guinea pig models of I/R, SS-31 analogs moderately reduced infarct size and improved cardiomyocyte survival [162]. Administration of another SS-31 analog at the onset of reperfusion reduced infarct size in diabetes [163–164]. Whereas the use of these small mitochondria-targeted peptides is protective against I/R, their use for the treatment of cardiac dysfunction in the MetS needs further investigations.

Other Semi-natural Products

SkQBerb and SkQPalm (derivatives of natural products) are novel mitochondria-targeted antioxidants that showed potent ROS-scavenging properties in isolated mitochondria and in human cells [165]. Their use in the context of CVD and the MetS has not yet been explored.

Ill- Gene Transfer Therapy

Despite disappointing results of various oral antioxidant treatment trials, promising findings have been reported using gene delivery of enzymes to improve NO bioavailability and decrease oxidative stress in animal models for cardiovascular diseases. Increased MnSOD expression in diabetic cardiomyocytes led to improved contractility [166]. Furthermore, over-expression of cardiac specific metallothionein (a heavy metal scavenger) in mice reduced ROS levels and improved cardiac and mitochondrial function after long-term high-fat feeding [167]. Finally, enhanced MnSOD or catalase expression normalized contractility in mouse models of type 1 and type 2 diabetes [166, 168]. Whereas these results suggest a protective role of anti-oxidants gene delivery, more work is needed to investigate the signaling pathways involved.

CONCLUSION

Because of the increasing obesity and T2D rates worldwide, there is an urgent need to develop new therapeutic strategies to prevent the CVD associated with these conditions. Therapeutic strategies aimed to reducing systemic abnormalities associated with these conditions such as reducing circulating glucose, cholesterol and triglyceride levels, were unable to reverse cardiovascular complications (ACCORD study) or were abandoned due to failure to reduce the risk of cardiovascular events. These disappointing results indicate that targeted therapies are indeed required to reduce or prevent the development of CVD in the MetS. The use of antioxidant as a therapy for the treatment of CVD in the MetS is to be considered however, care in their use in hearts exhibiting oxidative stress might be useful. Furthermore, caution has to be taken when the rates of FA oxidation are high, because the use of antioxidants in this case might eliminate the beneficial effect of ROS on facilitating FA-induced mitochondrial uncoupling, a process that is required to reduce further ROS generation. Finally, and based on animal studies, antioxidant therapies have proven to be effective only as treatments but not as prevention strategies potentially because of negative effects associated with excessive antioxidant scavenging in non-stressed hearts.