Primary alterations in mitochondrial biogenesis may be associated with the progression of cardiac pathologies. Mitochondrial DNA (mtDNA) copy number and mitochondrial content are significantly reduced in both human and rat models of failing myocardium, which can be attributed to downregulation of the mitochondrial biogenesis signaling pathways [9-12]. It is suggested that a disturbance in mitochondrial biogenesis occurs at early onset of HF, which is cardioprotective upon reversal. Recently, peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α) has been studied for its central role in the regulation of transcription factors in the mitochondria, such as nuclear respiratory factor 1/2 (NRF 1/2) and mitochondrial transcription factor A (mtTFA). PGC-1α is a protein encoded in the nucleus and is activated during periods of high energy demand, such as those during increased cardiac workload, physical training or exercise, or starvation. PGC-1α is responsible for the activation of mitochondrial proliferation through its intercommunication with different transcription factors. PGC-1α was evidently decreased in various HF experimental models, and this affected essential, related transcription factors, as evidenced by decreased NRF 1/2 and mtTFA expression [13,14]. Targeted mtTFA disruption in cardiac tissue affected electron transport chain (ETC) capacity, eventually resulting in spontaneous cardiomyopathy and HF [15,16].

Increased mtDNA and mitochondrial proliferation with myofibril displacement is associated with primary mitochondrial defects and cardiomyopathy, and commonly related with increased mitochondrial biogenesis-related gene expression [17]. It was previously observed that mitochondrial proliferation was higher in cardiomyopathic murine models, and associated with the removal of adenine nucleotide translocase 1 (ANT1) [18], manganese superoxide dismutase (Mn-SOD) [19], and mtTFA [20]. Furthermore, there was increased mitochondrial mass in mtTFA-knocked out mice; in addition to decreased levels of mtDNA and cytochrome c oxidase subunit I, peroxisome proliferator-activated receptors α (PPARα), and PPARα-dependent transcripts; and dysfunctional ETC [21]. The significant decrease in ATP revealed that increase in mitochondrial number is not enough to compensate for damage in the mitochondria. These examples demonstrate the importance of improved mitochondrial biogenesis and mtDNA copy number in cardioprotection against HF.

Recent studies have focused on targeting the mitochondria and improving mitochondrial biogenesis through various methods, including those involving pharmacological agents. Currently, there are no pharmacologic agents that specifically target mitochondrial biogenesis in HF. However, stimulation of mitochondrial biogenesis, leading to its beneficial effects, is accelerated by targeting other components such as endothelial nitric oxide synthase, adenosine monophosphate-activated kinase, and other mitochondrial biogenesis pathways, which all hold promise in addressing the disease [22,23]. A summary of some common and recent experimental and clinical interventions are presented in Table 1. One of the earlier drugs to be used, an angiotensin-converting enzyme (ACE) inhibitor named captopril (now marketed as Capoten), increased mitochondrial content in canine hearts after coronary ligation [24]. This indicates that the favorable effects may, in part, be related to mitochondrial invigoration. Current studies have examined the effects of various ACE inhibitors in human patients, including that of captopril. Although captopril is widely used, patients treated with the drug presented higher incidence of coughing, and it was suggested that enalapril is more potent, owing to its effect on ejection fraction, stroke volume, and mean arterial pressure [25].

Mitochondrial oxidative stress, caused by either ROS overproduction or decreased endogenous antioxidant defenses, has been implicated in the structural and functional alterations during myocardial failure [39]. Sources of ROS in the failing heart include mitochondrial ETC, nicotinamide adenine dinucleotide phosphate oxidases, nitric oxide synthases, and xanthine oxidase [40]. The change in oxidative stress levels may contribute to changes in the abundance and copy number of mitochondria, and in the integrity of mtDNA in human cells during pathological conditions [41]. ROS may act within a threshold to generate stress responses through various cellular processes, such as modifications in specific nuclear genes, in order to maintain energy metabolism and production [42]. Decreased mitochondrial biogenesis occurs during insignificant increases in oxidative stress, in addition to increased mitochondrial DNA mutations, which eventually lead to impaired OXPHOS and cardiomyopathy. On the other hand, induction of PGC-1α, mitochondrial biogenesis, and enzymes related to metabolism were observed during marked elevation in oxidative stress [43].

The complex role of ROS in the progression of HF remains to be clarified. A suggested mechanism involves impairment of cellular and mitochondrial structures, such as excitation-contraction coupling proteins, which affect the mechanical function of the heart [44]. Considering that majority of the ROS originates in the mitochondria during HF, it is not surprising that these organelles are the most susceptible to oxidative damage. In fact, mtDNA is more prone to ROS damage since it lacks histones, which can serve as protection from ROS attack. This renders the mtDNA to a less efficient DNA repair system, which results in mtDNA gene mutations. Furthermore, decrease in PGC-1α in affected hearts aggravates oxidative stress and mitochondrial damage, owing to the protein’s role in maintaining mitochondrial antioxidant defenses [43]. ROS also regulates signaling cascades such as those involving protein kinase C (PKC), mitogen-activated protein kinase (MAPK), Jun N-terminal kinase, and Ras—all implicated in hypertrophy [45]. Further, ROS facilitates extracellular matrix remodeling by promoting matrix metalloproteinases either directly via posttranslational modifications (PTMs) or indirectly via nuclear factor κB (NFκB) pathway [46]. The aforementioned roles of ROS highlight its importance and novelty as a therapeutic target for treating HF. As mentioned in the previous section, therapies involving ACE inhibitors and angiotensin II receptor blockers have been used experimentally, owing to the agents’ antioxidant properties. However, it is unclear whether these agents target mitochondrial ROS directly or indirectly [47,48].

Numerous studies have focused on oxidative-stress targets or energetics in HF models. Large randomized trials of antioxidant vitamins such as vitamin E have so far been futile, which might be due to the nonspecific nature of the vitamins, possibly inhibiting both the beneficial and negative effects of ROS. Nevertheless, it has been shown that mitochondrial targeting of ROS-scavenging molecules is protective. Modulation of substrate utilization with drugs such as perhexiline to increase myocardial energy production has only been performed in small-scale clinical studies [49]. A two-pronged approach of addressing oxidative stress and mitochondrial dysfunction together has been deemed a more efficient approach. The use of antioxidants such as triphenylphosphonium conjugation (MitoQ) and novel Mn-SOD, or catalase analogues could be considered in future HF studies.

Decreased energy metabolism due to dysfunction in mitochondria is further aggravated by ROS generation, which, when taken together with Ca²⁺ mishandling, favors mPTP opening. mPTP is a nonselective pore in the inner mitochondrial membrane, activated under high Ca²⁺ and ROS levels, resulting in mitochondrial dysfunction and inhibition of OXPHOS [50,51]. It was earlier thought that the inner-membrane ANT [52], outer-membrane voltage-dependent anion channel [53], and inner phosphate carrier [54] all play essential roles in mPTP induction. However, recent studies have pointed out that these components only play a supporting, regulatory role, while more compelling evidence points at the importance of matrix protein cyclophilin D (CyP-D) and mitochondrial ATPase in mPTP structure and regulation [55]. Determining the structure of mPTP would aid the development of better methods to inhibit mPTP opening, helping in the maintenance of inner-membrane integrity and preservation of ATP production, which would prevent cell death during the onset of HF.

mPTP is widely studied in cardiac pathology, including ischemia/reperfusion (I/R) and HF. It was earlier observed that inhibition of mPTP opening by cyclosporine A (CsA) offers possible cardioprotection. Subsequent research in a myocyte model of hypoxia/reoxygenation found that CsA is a potent inhibitor of mPTP pore in isolated mitochondria. Earlier studies involving CsA also revealed cardioprotection from reoxygenation injury in isolated cardiac myocytes [56]. This was supported by another study where CsA exhibited cardioprotection from reperfusion injury in the Langendorff perfused heart [57]. Cardioprotective effects of CsA were also observed in CyP-D-knocked out mice, exhibiting a significant reduction in infarct size in an I/R model [58]. However, owing to its affinity to cytosolic cyclophilin-A, CsA poses a problem regarding cardioprotection, wherein it inhibits calcineurin, which directly affects heart function [59] and is reported to possess immunosuppressive activity [60]. In order to address such problems exhibited by CsA, analogues have been developed that are inactive against calcineurin, but still bind strongly to CyP-D. The cardioprotective properties of these analogues are similar to that of CsA, and are potent even when administered only during early reperfusion [61-64].

Several other indirect inhibition methods for mPTP have been suggested, mostly focused on reduction of mitochondrial build-up of pore-formation inducers such as ROS and Ca²⁺. Considering that oxidative stress is a robust mPTP-opening activator, clinical use of ROS scavengers might be effective [65]. Propofol, which is used during cardiac surgeries, inhibited mPTP opening in an I/R model of isolated rat hearts, exhibiting significant post-ischemic recovery of heart function. This cardioprotective effect could be attributed to propofol’s role in inhibiting the formation of ROS–cardiolipin complexes in the respiratory chain by maintaining the integrity of cardiolipin under ROS attack [66]. In another case, 3-Methyl-1-phenyl-2-pyrazolin-5-one (MCI-186) significantly decreased myocardial infarction size and inhibited mPTP opening in an I/R rat model. This effect could be attributed to the action of MCI-186 in inhibiting cellular Ca²⁺ overload brought about by oxidative stress, in addition to the inhibitory effect on mitochondrial swelling and cytochrome c release [67].

Ca²⁺ channel blockers have been well studied in cardiac disease models, but the role of mPTP in relation to these blockers remains unclear. Ruthenium 360 is a specific Ca²⁺ uniport inhibitor, which reduces mitochondrial Ca²⁺ by blocking mPTP while not affecting Ca²⁺ movement. This was associated with cardiac function recovery after ischemia [68]. Hearts administered with Na⁺/H⁺ exchanger isoform 1 (NHE-1) inhibitors showed decreased muscle cell injury as evidenced by low lactate dehydrogenase release and improved cardiac function during reperfusion [69]. The NHE-1 inhibitor cariporide was also able to mitigate oxidative stress-induced mitochondrial Ca²⁺ overload and mitochondrial membrane potential (ΔΨm) loss in neonatal cardiomyocytes [70]. Thus, there is a need to develop drugs that are more potent in targeting other aspects of mPTP, without disrupting normal mitochondrial function.