Mitochondria are double-membraned organelles with their own circular genome, mitochondrial DNA (mtDNA), which is replicated independently of the host genome [
19]. Mitochondria are involved in diverse yet interconnected functions, including the production of adenosyl triphosphate (ATP), and the regulation of nutritional metabolism, calcium homeostasis and programmed cell death [
20,
21,
22]. They are found in the cytoplasm of nearly all eukaryotic cells as highly dynamic networks, undergoing coordinated cycles of biogenesis, fusion, fission, and degradation (mitophagy) to sustain their homeostasis and to adapt energy production based on the cellâs needs [
23]. Proper mitochondrial function and dynamics are particularly necessary in tissues and cells with high energy demand such as the heart, and particularly in cardiomyocytes, which continuously require ATP to sustain cardiac activity. In adult cardiomyocytes, mitochondria occupy nearly one-third of the total intracellular volume [
24] and provide approximately 95% of the ATP consumed by the heart [
25]. It is therefore unsurprising that functional abnormalities of cardiac mitochondria have emerged as a key factor in cardiovascular disease (CVD) leading to decreased ATP production and energy supply, increased reactive oxygen species (ROS) production, cell apoptosis, and mitochondrial dynamic imbalance [
26].
2.1. Energy Starvation and Oxidative Stress
Decreased energy supply is considered a leading consequence of mitochondrial dysfunction. The heartâs voracious requirement for energy, in the form of ATP, mainly relies on oxidative phosphorylation (OXPHOS) or ÎČ-oxidation of fatty acids and the tricarboxylic acid (TCA) cycle in the mitochondria. During pathological myocardial remodelling, there is a reduction in the levels of carnitine in the heart [
27,
28], an essential cofactor for mediating the entry of fatty acids into the mitochondria to the site of ÎČ-oxidation [
29]. Due to this reduction of fatty acids availability within the mitochondria, cardiac metabolism is reprogrammed towards increased reliance on glucose as the energy resource with a significant increase of glycolysis, to maintain ATP production [
24]. However, ATP generated from glycolysis contributes less than 5% of the total ATP consumed [
28], which is not enough to compensate the reduction of fatty acid oxidation, and therefore, cardiac ATP is progressively depleted. The role of energy deprivation in the induction and pathogenesis of heart failure (HF) is well supported by clinical evidence, in which therapeutic measures to reduce energy consumption have been demonstrated to improve survival while treatment increasing energy demand is detrimental [
24]. In the mitochondria, the synthesis of ATP takes place in the electron transport chain (ETC) [
30]. Reduced nicotinamide adenine dinucleotide (NADH) and reduced flavin adenine dinucleotide (FADH
2) generated from the Krebs cycle, and from ÎČ-oxidation, transfer protons and electrons through the ETC, creating an electrochemical gradient that is then used to activate ATP-synthase and produces ATP. Alterations in mtDNA genes such as NADH-dehydrogenase genes (
MT-ND1,
MT-ND5 and
MT-ND6), cytochrome b (
MT-CYB), cytochrome c oxidase I and II (
MT-CO1 and
MT-CO2) and ATP synthase 6 (
MT-ATP6), have been described in dilated cardiomyopathies [
31]. Reduced activities of complexes I and IV, as well as of the NADH phosphate (NADPH)-transhydrogenase and the Krebs cycle enzymes have been also observed in patients with HF [
26]. Interestingly, some studies suggest that mtDNA mutations induce cardiovascular senescence and CVD, as demonstrated by the observation that
Polgm/m mice, which are prone to accumulation of mitochondrial DNA mutations, have increased expression of senescent markers
p16ink4a and display early onset cardiomyopathy [
32,
33]. It remains unclear what mechanism mediates this induction, but an increase in mitochondrial ROS (mtROS) has been proposed as causal.
Mitochondrial ATP production is accompanied by the generation of ROS, a generic term for an array of short-lived and unstable free radicals that contain oxygen with vastly different properties and biological functions that range from signalling (when strictly regulated) to causing cell damage [
34]. Physiologically, ROS-mediated signalling pathways are associated with cell survival and proliferation, combatting infectious agents, and have mitogenic effects on cells [
35,
36]. However, excessive ROS production drives oxidative stress, a deleterious process that potentially causes irreversible damage to various molecules and structures within the cell [
37], leading to further mitochondrial dysfunction, oxidative stress, and cell death [
21,
38]. Increased ROS appears to be capable of inducing senescence through several mechanisms. Telomeres are particularly sensitive to ROS-induced damage, possibly due to their guanine-rich regions, which increase susceptibility to oxidation [
39] and increased ROS can accelerate telomere attrition contributing to telomere dysfunction, premature senescence, and accelerated ageing [
40]. ROS also generates DNA lesions in the form of single-stranded DNA and/or double-stranded breaks (DSBs) within the genomic or telomeric DNA. Eventually, as a result of telomere shortening or DNA damage, activation of the DNA damage response (DDR) occurs [
41]. The DDR is an evolutionarily conserved signal transduction pathway required for genome integrity preservation. It coordinates cellular efforts to repair DNA damage, which if unsuccessful directs cell fate towards apoptosis or senescence thereby impeding the propagation of corrupted genetic information [
42,
43]. The DDR is characterised by the recruitment and activation of two large protein sensor kinases at the site of the lesion: ataxia telangiectasia and Rad3-related (ATR) when single-stranded DNA is exposed, and ataxia-telangiectasia mutated (ATM) at DSBs. The recruitment of ATR or ATM to the lesion causes the local formation of DNA damage foci containing the phosphorylated form of the histone H2AX (ÎłH2AX) and ultimately induces cell-cycle arrest through the activation of checkpoint proteins, including p53 [
41]. Furthermore, once senescent, cells exhibit a decreased mitochondrial membrane potential, increased proton leak and enhanced production of mtROS [
44]. As such, the elevated ROS observed in senescent cells may drive mtDNA damage creating a positive feedback loop leading to further increases in ROS and DNA damage highlighting the cyclical interactions between mitochondrial dysfunction, oxidative stress, and senescence, and illustrates how the initiation of any of these processes could lead to a downward spiral in tissue function.
Perhaps unsurprisingly, given the high-volume density of mitochondria required to fulfil the heartâs energy demand, the heart has both high mtROS production and elevated mtROS, which has been shown to contribute to the pathophysiology of a variety of CVDs, including atherosclerosis, cardiac ischemia/reperfusion (IR) injury, HF, cardiac hypertrophy, and degenerative aortic valve disease [
21,
25,
38,
45]. ROS has also been shown to be a powerful inducer of senescence in multiple tissues and cell types, including the heart. Monoamine oxidase A (MAO-A) is a protein linked with driving oxidative stress - it is located at the outer mitochondrial membrane, involved in catalysing the oxidative deamination of monoamines, and produces hydrogen peroxide as one of its by-products [
46]. Interestingly, cardiomyocyte-specific overexpression of MAO-A results in elevated ROS, increased senescence and mice display a dilated cardiomyopathy and myocardial dysfunction [
47]. All of these can be rescued by treatment with antioxidants [
4]. Similarly, the accelerated ageing mouse model
nfkb1â/â showed increased ROS, telomere dysfunction and cardiomyocyte hypertrophy [
48,
49]. Similar observations have been reported in more clinically relevant models: aged mice treated with the mitochondrial-targeted peptide SS-31 elamipretide had reduced myocardial ROS and improved cardiac function, which was associated with reduced senescence [
50]. Furthermore, aged, senescent cardiomyocytes demonstrated an overall decline in expression of most mitochondrial genesâparticularly those genes involved in the ETC, and mitochondrial ultrastructural defects by transmission electron microscopy [
50]. Myocardial infarction (MI) results in alterations to mitochondrial dynamics, culminating in increased ROS production and increased oxidative stress - particularly when followed by the clinical gold-standard treatment of reperfusion [
51]. Several studies have shown that even in this acute setting of increased oxidative stress, senescence is induced in multiple cell populations, including cardiomyocytes, and that these cells are active participants in post-MI myocardial remodelling since their elimination attenuates, inflammation remodelling and improves functional outcomes [
52,
53].
Being so closely associated with cardiomyocyte dysfunction, mitochondrial damage is of major interest when exploring the mechanisms underpinning the cardiotoxicity of many otherwise beneficial therapeutics. This is relevant to both preclinical drug development, where cardiac liabilities remain a leading cause of drug attrition [
54,
55], but also to therapies approved for clinical use today which risk future withdrawal from the market due to cardiac adverse drug reactions (ADRs) [
56]. Both traditional and new-generation oncology treatments are plagued by off-target cardiotoxic effects [
57]. With cardiovascular disease being a leading noncancer cause of death in an ever-growing population of cancer survivors, understanding the mechanisms behind these cardiotoxicities is increasingly important [
58]. As a case in point, anthracycline chemotherapies are notoriously chronically cardiotoxic and have been shown to deleteriously affect mitochondrial function in many ways.
Doxorubicin (DOX), an anthracycline commonly used in the clinic, was historically shown to redox cycle via interactions with mitochondrial complex I, generating excessive ROS as a result [
59,
60]. Studies have subsequently demonstrated that DOX has a high affinity for cardiolipin, a lipid housed in the inner mitochondrial membrane which is essential for effective energy metabolism and proper mitochondrial function [
61,
62,
63]. Notably, DOX becomes concentrated in the mitochondria of isolated neonatal rat cardiomyocytes, supporting the notion that this organelle is particularly vulnerable to off-target anthracycline toxicity [
64]. Zhang and colleagues showed that mitochondrial function and oxidative phosphorylation pathways were disturbed in cardiomyocytes isolated from DOX-dosed mice, and that this was dependent on the topoisomerase IIÎČ (TopIIÎČ) enzyme which is thought to be crucial in the cardiotoxicity of this drug [
65]. It has since been shown that DOX intercalates into mtDNA, which aids its accumulation in cardiomyocyte mitochondria in the same model [
66]. Furthermore, Ichikawa and colleagues showed that DOX causes iron accumulation in cardiomyocyte mitochondria, leading to downstream toxicity. DOX treatment is associated with a depletion and mutation of mtDNA, as identified in hearts of cancer patients [
67]. The interplay between DOX-induced mitochondrial damage and cardiomyocyte senescence within this cardiotoxicity is less well-understood, but it has been shown that the two phenomena go hand-in-hand using in vitro and in vivo studies, as evidenced by Mitry et al., amongst others [
40,
68,
69]. As a long-established therapy, the impact of DOX upon cardiomyocyte mitochondria has been well reviewed [
70], but newer oncology therapies are far less well-understood. For example, though tyrosine kinase inhibitors (TKIs) provide more targeted anticancer actions, the TKI sorafenib has historically been shown to impair cardiomyocyte mitochondrial function at clinically relevant doses
in vitro, and more recent reports highlight that sunitinib may also induce cardiomyocyte mitochondrial damage via ROS accumulation [
71,
72]. Several other oncology therapies display off-target cardiovascular effects and mitochondrial toxicities [
73] and it is clear the changing landscape in cancer survivorship necessitates thorough investigations into the long-term cardiac effects of both established and emerging antineoplastic therapies, and mitochondrial toxicity remains an attractive avenue of research.
2.2. Mitochondria Dynamics Imbalance
In physiological conditions, mitochondria constantly undergo co-ordinated cycles of fusion and fission, also referred to as mitochondrial dynamics [
74]. Mitochondrial fusion is characterised by the union of two mitochondria resulting in one elongated mitochondrion, which allows a dynamic repair of reversibly damaged mitochondria. Conversely, mitochondrial fission is characterised by the fragmentation of one irreversibly damaged and potentially harmful mitochondrion into small and spherical mitochondria that can be isolated and removed by mitophagy [
75,
76]. The coordination of these events is essential for the maintenance of mitochondrial quantity and quality and therefore, the balance between them plays a vital role in the normal function of the cardiovascular system. Indeed, accumulating evidence has confirmed the influence of mitochondrial dynamics on the pathogenesis of CVD [
74].
Mitochondrial fusion is first mediated by the transmembrane guanine triphosphatase (GTPase) proteins, mitofusin 1 (MFN1) and MFN2 in the outer mitochondrial membrane, and then by the optic atrophy protein 1 (OPA1) in the inner membrane [
77]. Decreased levels of MFN1 and MFN2 have been found in animal models of atherosclerosis [
78], and a decreased expression of OPA1 has been observed in post-MI hearts, which correlated with the downregulation of mtDNA and antioxidant genes [
79]. Suggesting a causal role of fusion in CVD, ablation of the murine
Mfn1 and
Mfn2 genes in adult hearts induced mitochondrial fragmentation and dysfunction, and rapidly progressive and lethal dilated cardiomyopathy [
80,
81]. Different cardiac pathologies have also been associated with the formation of giant mitochondria or megamitochondria [
26], which evolve by fusion of the membranes of numerous large individual organelles due to overexpression of protein fusion [
82]. The opposing process, mitochondrial fission, is controlled by Mitochondrial fission protein 1 (Fis1) and Dynamin-related protein 1 (Drp1). It has been reported that
Drp1 activation during cardiac IR results in left ventricular dysfunction and that
Drp1 inhibition reduces cell death, preserves mitochondrial morphology, and inhibits mitochondrial permeability transition pore [
83,
84]. While the relationship between mitochondrial fusion and myocardial senescence has yet to be investigated, elongated mitochondria have been observed in senescent cells in vitro [
85,
86] and depletion of
Fis1 mRNA levels leads to mitochondrial elongation, induces senescence, and increases ROS production [
87].
Fusion and fission events control mitochondria biogenesis [
88], a process that increases the number of mitochondria, improves the replication and repair of mtDNA, and induces the synthesis of mitochondrial enzymes and proteins [
21]. The co-transcriptional regulator factor peroxisome-proliferator-activated receptor γ co-activator-1α (PGC-1α) induces mitochondrial biogenesis by activating the mitochondrial transcription factor A (TFAM), which drives transcription and replication of mtDNA [
89]. Reduced gene expression of PGC-1α has been associated with failing human hearts [
90] and there is evidence that sirtuin-1 (SIRT1), a protein involved in metabolic regulation, delays molecular characteristics of myocardial ageing by mediating deacetylation of PGC-1α and activation of mitochondrial biogenesis [
91]. The
PGC-1α+/-/ApoE-/- mouse model has shown that
PGC-1α deficiency promotes vascular senescence, which is associated with increased oxidative stress, mitochondrial abnormalities, and reduced telomerase activity [
92]. Mitochondrial biogenesis is also accompanied by variations in mitochondrial morphology [
89]. Generally, various aspects of cardiovascular biology, including cardiac development, the response to cardiac IR injury and HF, are related to morphological and structural changes in mitochondria [
93]. Dramatic changes in mitochondrial morphology have been also found in senescent cells, where mitochondria exist in a state of hyper-fusion as a response to reduced expression of mediators of the fission process and an overall reduction in the frequency of the fission and fusion events [
40].
2.3. Cell Apoptosis and Mitophagy
Mitochondria are pivotal in controlling apoptosis, including the release of caspase activators and participation of B-cell lymphoma-2 (BCL-2) family proteins [
94]. Cardiomyocyte apoptosis plays a critical role in the pathogenesis and progression of all types of heart disease, particularly in ischemic heart disease and HF of various aetiologies [
95]. For example, cardiac IR injury is related to the apoptotic death of cardiac muscle cells by activating the pro-apoptotic BCL-2 regulators BAX and BAK to change the integrity of the mitochondrial membrane and the cytosolic release of pro-apoptotic factors, which triggers caspase-dependent cell death [
96]. In hypertension, the hormone angiotensin II, which plays an important role in volume and blood pressure control, has been linked to cardiomyocyte apoptosis in rats, and treatment with losartan has been associated with a reduction of cardiomyocyte apoptosis in both spontaneous hypertensive rats and hypertensive patients [
97]. The subfamily of pro-apoptotic BCL-2 homology (BH) BH3-only proteins, BCL2/adenovirus E1B 19 kDa protein-interacting protein 3 (BNIP3) and its homologue BNIP3-like (BNIP3L or Nix), also induce apoptosis [
98] and the forced expression of these genes is sufficient to induce cardiomyopathy in murine models [
26,
99,
100].
To prevent cardiomyocytes containing damaged mitochondria from undergoing apoptosis, mitophagy, a cargo-specific form of autophagy selectively targets the degradation of dysfunctional and damaged, and hence potentially cytotoxic, mitochondria within a cell [
21]. There are two described mechanisms for mitophagy: adaptor-mediated and receptor-mediated. The former pathway functions via Phosphatase and Tensin Homolog (PTEN)-induced putative kinase 1 (PINK1) and Parkin-mediated mitophagy [
101]. PINK1 is a serine/threonine kinase that continuously monitors mitochondrial health and provides a rapid response when mitochondrial function collapses [
102]. PINK1 phosphorylates MFN2, which, in turn, interacts with the E3 ubiquitin ligase Parkin [
103]. Parkin conjugates ubiquitin onto key mitochondria-associated proteins, amplifying the signalling cascade involved in the recruitment of autophagosomes to target the damaged mitochondria. The mitochondria-containing autophagosome is trafficked to, and fused with, a lysosome and degraded [
104]. In healthy young hearts, there is an underlying level of baseline mitophagy essential for maintaining the cellular homeostasis in an energy-efficient heart, and for responding and adapting to stress [
105]. However, decreased mitophagy is associated with CVD, as an accumulation of âoldâ defective mitochondria may reduce the heartâs potential to adapt to stress. Indeed, multiple animal studies have linked the deletion of mitophagy-related genes at the whole-body level or cardiomyocytes with the spontaneous development of cardiovascular disorders [
106]. For example, mice bearing a cardiomyocyte-specific deletion of
Mnf2 prematurely succumbed to progressive cardiomyopathy, which could be partially reversed by restoring mitophagy in cardiomyocytes via the expression of the antioxidant enzyme catalase [
107]. The whole-body
Pink1-/- mice caused left ventricular dysfunction and pathological cardiac hypertrophy by 2 months of age [
108]. Mitophagy is also essential for reducing cardiac injury following MI. Under baseline conditions,
Parkin-deficient mice hearts shown smaller and disorganised mitochondria as revealed by ultrastructural analysis, but mitochondrial and cardiac function were unaffected [
109]. However, after MI, these mice had reduced survival and developed larger infarcts when compared to control mice, which was associated with rapid accumulation of dysfunctional mitochondria in the infarct border zone [
110]. In patients with late-stage heart disease, a low number of autophagosomes in cardiomyocytes is associated with a poor prognosis [
111]. Damaging events (e.g., acute cardiac IR injury) lead to the reduction of the autophagy flux, and in consequence, damaged dysfunctional mitochondria accumulate in cardiomyocytes, leading to severe oxidative stress and apoptosis [
112]. The destabilisation of atherosclerotic plaques has also been associated with deficient mitophagy [
113,
114]. Furthermore, a reduced expression of autophagic markers p62 and microtubule-associated protein light chain (LC3)-II has been detected within atherosclerotic plaques from human samples and mouse models [
115,
116,
117]. Activation of mitophagy through antioxidant therapeutic strategies has been explored to stabilise atherosclerotic plaques [
118].
Interestingly, despite and perhaps because of mitochondrial dysfunction, senescent cells express pro-survival pathways, enhancing survival and increasing resistance to apoptosis. Senescent cells are more resistant to apoptosis in response to stimuli, including serum withdrawal, ultraviolet damage, oxidative stress and treatment with cytotoxic drugs [
119]. While there is heterogeneity between cell types and senescence stimuli, enhanced activation of several pathways including BCL-2 family members, p53/p21Cip, ephrins (EFNB1 or 3), the phosphatidylinositol-4,5-bisphosphate 3-kinase delta catalytic subunit (PI3KCD), plasminogen-activated inhibitor-1 and 2 (PAI1 and 2) and hypoxia-inducible factor-1α (HIF1α) can be involved [
120,
121,
122] and are referred to as senescent cell anti-apoptotic pathways (SCAPs). As discussed below, activation of these pathways may contribute to the proinflammatory nature of senescent cells.