The heart’s continuous beating depends on a substantial supply of high-energy phosphate compounds, with ATP serving as the main energy source. The myocardium’s ATP consumption constitutes approximately 8% of the total ATP consumed by the human body, with around 95% produced by mitochondria through OXPHOS (11). Mitochondria comprise 30% to 40% of the volume of myocardial cells, reflecting the heart’s requirement for efficient energy metabolism (2, 12).

Under normal conditions, FA oxidation provides approximately 70% of cardiac energy, with the remainder derived from the oxidation of other nutrients such as glucose, ketones, lactate, and amino acids (13–15).It is important to note that FA oxidation consumes approximately 12% more oxygen than glucose oxidation to generate an equivalent amount of ATP (14).

Mitochondria are essential for maintaining Ca²⁺ homeostasis in myocardial cells. The dynamic equilibrium of Ca²⁺ between the sarcoplasmic reticulum (SR) and mitochondria significantly affects myocardial excitation-contraction coupling (ECC) (14, 16). During myocardial depolarization, Ca²⁺ first moves into the cytoplasm via L-type voltage-dependent calcium channels, subsequently triggering additional Ca²⁺ release from the SR, causing myocardial contraction. In the relaxation phase, excess Ca²⁺ is primarily transported back into the SR via SR Ca²⁺-ATPase, while some Ca²⁺ is expelled from cells through Na⁺/Ca²⁺ exchangers (14, 17).

OS is typified by a relative imbalance between excessive ROS production and inadequate antioxidant defense within the organism. Under standard physiological conditions, mitochondria generate ATP through OXPHOS, where the electron reduction process in the respiratory chain turns a minor portion of oxygen into ROS (18). Cells typically depend on the antioxidant system to maintain a dynamic balance of ROS and prevent oxidative damage (19).

The phenomenon of glucose toxicity from hyperglycemia is intimately linked with various cardiac conditions in diabetes, including AGE formation, fibrosis, aberrant inflammatory signaling, O-GlcNAc modification, and impaired Ca²⁺ processing. Hyperglycemia can trigger excessive ROS production in the diabetic heart via several mechanisms, including the activation of enzymes such as NOX, XO, and NOS (38–40). Elevated glucose levels also intensify the production of O₂ ^-, NO, ONOO⁻, and AGEs by boosting flux through the polyol and hexosamine pathways and by activating protein kinases (19). This rampant production of mitochondria-derived ROS, exacerbated by these pathways, promotes OS and contributes to the progression of cardiac disease in diabetes, highlighting the critical pathogenic role of ROS accumulation induced by hyperglycemia (19). ( Figure 1 )

Myocardial IR is a key factor in the structural and functional alterations of the heart, associated with systemic metabolic disorders such as hyperinsulinemia, hyperglycemia, and hyperlipidemia, and anomalies in myocardial insulin signaling (41). Initial changes in myocardial glucose uptake, marked by reduced GLUT4 expression and impaired translocation, often precede dysfunction in insulin’s activation of the PI3K-Akt pathway. This correlation has been substantiated in both animal models and clinical observations of patients with T2DM (42, 43). In scenarios of IR, obesity, and T2DM, pinpointing the most impactful factor on cardiac health is challenging due to the associated metabolic disturbances. Nonetheless, evidence from mouse models devoid of myocardial insulin receptors strongly indicates a direct association between impaired glucose uptake, mitochondrial dysfunction, and contractile issues with disruptions in myocardial insulin signaling (19).

It is noteworthy that these mice also exhibited elevated mitochondrial ROS levels, potentially related to the inhibition of mitochondrial ETC proteins (44). Furthermore, some studies suggest that superimposed diabetes may enhance the production of mitochondrial ROS, possibly through an increased capacity to induce FAO, thus heightening the electronic load on the ETC, leading to electronic leakage and an upsurge in ROS production (45). Therefore, the adaptive regulation of compromised insulin signaling by myocardial metabolism may result in increased mitochondrial ROS levels in the heart, further exacerbating myocardial damage.

In diabetic hearts, reduced glucose utilization by myocardial cells due to impaired insulin signaling or diminished glucose uptake (46) shifts the primary energy substrate to FAs, leading to an increase in FAO. This metabolic shift causes an excessive accumulation of acyl CoA, which not only boosts ROS production in both the cytoplasm and mitochondria but may also trigger various metabolic disorders (20). Prolonged exposure to the FA palmitate has been demonstrated to promote ROS generation and increase mitochondrial fission, further exacerbating mitochondrial dysfunction (47). Notably, accumulated acyl CoA in diabetic hearts can be repurposed for triglyceride synthesis or transformed into ceramide and other derivatives. Ceramide, known to directly inhibit ETC complex III, increases ROS production and may also initiate inflammatory responses, thus aggravating the progression of diabetic heart disease (19).

Unsaturated FAs are vulnerable targets for ROS, and lipid peroxidation (LPO) is a primary damage process induced by ROS. Malondialdehyde (MDA), a lipid peroxidation product, is an important biomarker of OS. Studies have indicated that levels of lipid peroxides in the hearts and serum of diabetic patients and animal models are significantly increased (48, 49). Furthermore, hyperglycemia and hyperlipidemia exacerbate lipid peroxidation reactions in the heart or myocardial cells (19, 50). It is noteworthy that enzymes inhibiting ROS generation (39), activating antioxidant systems, or enhancing ROS clearance mechanisms can effectively reduce MDA levels (51), thereby mitigating the effects of lipid peroxidation. Specifically, increased myocardial lipid peroxidation may be closely associated with lipid overload. Under sustained lipid peroxidation, proteins also undergo oxidative modification (52), particularly the FAD-containing subunit of succinate dehydrogenase, which may lead to mitochondrial dysfunction and further result in myocardial contractile damage in diabetes (53).

Unbalanced Ca²⁺ regulation is a significant characteristic of DCM. Although the mechanisms involved remain unclear, ROS oxidatively modify calcium ion channels on the myocardial cell membrane, including L-type calcium channels (LTCC) and calcium pumps, altering their function and disrupting calcium entry and release dynamics (54). ROS also activates calcium release channels, such as RyR2 in the SR, through oxidative reactions, promoting calcium release and thereby impairing the myocardium’s contractile and diastolic functions (54, 55). Furthermore, myocardial cells maintain calcium ion balance through calcium pumps, such as SERCA2a, and calcium-sodium exchange proteins (NCX). ROS oxidation of these proteins may decrease the activity of calcium pumps or alter the direction of calcium-sodium exchange, leading to obstructed calcium removal, potentially causing calcium overload or loss, and affecting myocardial contractility (54).

Under normal conditions, the mitochondrial membrane potential maintains a high electrochemical gradient essential for ATP synthesis (56) and plays a critical role in intracellular calcium ion homeostasis (54). Excessive ROS damages mitochondrial proteins and lipids through oxidation, disrupting mitochondrial membrane integrity and leading to loss of membrane potential. This impairment directly affects ATP synthesis, resulting in inadequate cellular energy supply and potentially triggering cellular apoptosis or necrosis (20). Changes in membrane potential also influence calcium ion balance within mitochondria, promoting intracellular calcium ion accumulation, increasing OS responses, forming a vicious cycle, and exacerbating myocardial function decline (54). Thus, changes in mitochondrial membrane potential are central to ROS-induced myocardial dysfunction.

Mitochondrial OS impairs myocardial cell function by activating apoptosis or fibrosis-related pathways. ROS generated by mitochondrial OS react with NO to form hydroxyl radicals, damaging proteins, lipids, and DNA, altering the overall gene expression program of diabetic cardiomyocytes, and reducing myocardial contractility (57). Mitochondrial OS has been shown to activate multiple pathways ( Figure 2 ).

Excessive ROS production occurs due to hyperglycemia, hyperlipidemia, and, compounded by inadequate endogenous antioxidant defense mechanisms. This imbalance results in mitochondrial dysfunction and persistent ROS accumulation. Consequently, key inflammatory pathways, such as NF-κB and the NLRP3 inflammasome, are activated, prompting immune cells such as macrophages and neutrophils to secrete inflammatory cytokines (IL-1β, TNF-α, IL-6, TGF-β1), thus perpetuating chronic inflammation (9, 58).

Accumulated mitochondrial ROS activate uncoupling proteins (UCP) in the mitochondrial inner membrane (74), promoting proton reflux from the mitochondrial intermembrane space to the matrix, diminishing the proton gradient required for OXPHOS, reducing ATP synthesis, and releasing metabolic energy as heat (75). Mild mitochondrial uncoupling mediated by UCPs moderately reduces mitochondrial membrane potential, thus lowering electron leakage at complexes I and III and subsequently reducing ROS generation, exerting antioxidant effects on myocardial cells and protecting them from oxidative damage (74). However, under pathological conditions such as HF or diabetes, excessive uncoupling leads to ATP synthesis reduction, resulting in energy loss primarily as heat, causing insufficient energy supply to myocardial cells, metabolic disorder, and mitochondrial dysfunction. Continuous ROS accumulation not only aggravates OS but also affects myocardial contraction and relaxation, escalating cardiac strain and potentially promoting persistent MF (76). Modulating the expression of uncoupling proteins like UCP2 and UCP3 can mitigate MF occurrence while preserving cellular antioxidant capabilities (77).

Mitochondrial OS results in an excessive generation of ROS, inflicting oxidative harm to mitochondrial DNA (mtDNA) and its encoded proteins, causing cellular dysfunction and inner mitochondrial membrane damage (20). This damage reduces the mitochondrial membrane potential, decreases ATP synthesis capability, and increases ROS production, creating a vicious cycle that further exacerbates mtDNA damage (20). Mitochondrial DNA, lacking histone protection and possessing weaker DNA repair mechanisms compared to nuclear DNA, is more vulnerable to OS. Damage to mtDNA severely affects the function of the mitochondrial respiratory chain, which relies heavily on mtDNA-encoded proteins. Once mtDNA repair is compromised, damaged DNA cannot be effectively repaired (78). Damaged mtDNA activates apoptotic signaling pathways in cells. Dysfunctional mitochondria and damaged mtDNA release cytokines into the cytoplasm, inducing caspase-dependent apoptosis, and leading to cardiomyocyte apoptosis. Apoptotic cardiomyocytes release damaging factors, exacerbating damage to surrounding tissues and promoting pathological changes in the heart (79). Long-term mtDNA damage not only leads to mitochondrial dysfunction but also has irreversible effects on the physiological functions of myocardial cells, ultimately inducing a series of CADs, especially MF.

Moreover, mitochondrial OS induces oxidative damage to mitochondrial membranes and lipids, compromising the structure of the mitochondrial inner membrane, thereby diminishing its fluidity and stability and affecting its permeability. This damage not only leads to endometrial dysfunction but also alters membrane permeability, impacting cellular function (80, 81). Crucially, ROS regulate mitochondrial permeability transition pore (mPTP) opening. Excessive ROS production, triggered by hyperglycemia, prolongs mPTP opening duration (82), causing mitochondrial membrane potential depolarization, reversal of ATP synthase transport, and release of pro-apoptotic factors such as cytochrome c and TNF, thus initiating caspase cascades. Caspases, involved in cell growth, differentiation, and apoptosis regulation, accelerate myocardial apoptosis (83). Additionally, mPTP opening allows water into mitochondria, leading to swelling or rupture, reducing energy supply, and further promoting cardiomyocyte apoptosis (84). The buildup of mitochondrial ROS also heightens mitochondrial membrane permeability, mediates lipid peroxidation and protein carbonylation, intensifies OS, creates a destructive cycle, and ultimately leads to cell apoptosis or necrosis, thus fostering MF (85).

Mitochondrial OS reduces mitochondrial autophagy levels, promoting cell apoptosis and exacerbating MI. Mitochondrial autophagy is crucial for repairing mitochondrial function and maintaining cellular homeostasis by transporting damaged or dysfunctional mitochondria to lysosomes for degradation (86). Typically activated by nutrient deficiency or decreased mitochondrial membrane potential, mitophagy involves PINK1 and its substrate Parkin as key components (20). Effective in removing damaged or aging mitochondria, mitochondrial autophagy blocks harmful mitochondrial signals and recovers small molecules such as glucose, amino acids, nucleotides, phospholipids, and FAs, maintaining cellular energy balance (20). In diabetes patients and their animal models, significant changes in the expression of mitochondrial autophagy-related genes and proteins have been observed. In the hearts of diabetic mice, mitochondrial autophagy activity is notably reduced (87) and the PINK1/Parkin pathway is inhibited (88). High glucose conditions further reduce mitochondrial autophagy levels due to ROS accumulation, leading to increased apoptosis, inhibited cell proliferation, and downregulation of autophagy-related proteins such as PINK1 and Parkin. The downregulation can be reversed by ROS scavengers such as N-acetyl-L-cysteine (NAC) (89), highlighting ROS’s critical role in impairing mitophagy. Impaired mitophagy contributes to mitochondrial dysfunction, lipid accumulation, and promotes MF. Conversely, activating mitophagy alleviates MF, underscoring its potential therapeutic value in preventing and treating CADs.

In diabetes patients, substantial amounts of metabolic substances such as acyl CoA and acylcarnitine affects mitochondrial ATP/ADP ratios, impairing metabolic function. Elevated glucose promotes lipid and protein oxidation through mitochondrial OS, increasing advanced glycation end products (AGEs), thereby altering myocardial structure (37). AGEs disrupt insulin metabolism signaling, diminish NO production, and augment cardiac fibrosis. They bind to the receptor for glycation end products (RAGE), activate the MAPK pathway, initiate inflammatory responses, promote matrix protein synthesis, and connective tissue growth, leading to MI (37). Further studies indicate that increased ROS in diabetic hearts activate peroxisome proliferator-activated receptor alpha (PPARα), enhance FA uptake and oxidation, result in the accumulation of metabolic by-products such as AGEs, provoke inflammatory responses, and ultimately cause MF (37).

During MF, fibroblasts secrete large quantities of collagen and other ECM components, forming fibrotic tissue. ECM deposition and cardiac fibrosis increase the distance between capillaries and myocardial cells, leading to reduced oxygen diffusion efficiency. Excessive ECM accumulation also causes hardening and structural changes in myocardial tissue, impairing normal blood circulation, especially in microvessels, and putting the myocardium at risk of hypoxia (90, 91). In heart tissues of diabetic animals and humans, the expression of VEGF and its receptors is downregulated, exacerbating hypoxia and leading to severe damage (90). This results in reduced ATP production, impaired mitochondrial function, and activation of ROS generation pathways, including NADPH oxidase. Increased ROS levels further damage myocardial cells and promote fibroblast activation, forming a vicious cycle.

During MF, fibroblasts are stimulated by various growth factors such as TGF-β, Ang II, PDGF, and transform into myofibroblasts. Myofibroblasts, with their enhanced ability to synthesize ECM, secrete more ROS. These ROS not only damage myocardial cells but may also promote further fibroblast proliferation and activation through paracrine effects, thus forming a vicious cycle (92).

Fibrosis often coincides with an inflammatory response, with TGF-β acting as a primary regulator in both in vivo and in vitro fibrosis scenarios, closely linked to MF (93). Macrophages, significant sources of TGF-β1 when activated by interferon-κ (IFN-κ), exhibit enhanced NF-κB activity. These macrophages also manifest pro-inflammatory phenotypes, generating a variety of chemokines and ROS, which precipitate a vicious cycle of tissue damage and fibrosis (94). Macrophage depletion diminishes fibrosis post-injury, whereas their recruitment increases fibrotic lesions. Hence, the infiltration and activation of inflammatory cells likely drive the initiation and progression of fibrotic diseases. Mounting evidence suggests inflammation is intricately connected to fibrosis, playing a substantial role in the pathogenesis of diabetes and heart disease (95). Characteristics of fibrosis include heightened inflammatory response, tissue degradation, and activation of signaling pathways such as NF-κB and JNK, which facilitate the infiltration of inflammatory cells and the release of mediators including TGF-β1, tumor necrosis factor-alpha (TNF-α), monocyte chemoattractant protein-1 (MCP-1), interleukin-6 (IL-6), and IL-8. These factors not only exacerbate OS but also further activate fibroblasts, promoting ECM synthesis and deposition, and accelerate fibrosis progression (92).

Mitochondrial damage is common during MF, significantly impacted by OS, which reduces mitochondrial membrane potential and activate mPTP, exacerbating intracellular calcium ion accumulation and further increasing OS (83). Mitochondrial damage also leads to ROS accumulation within cells, further harming cardiac myocytes. During MF, mitochondrial autophagy may be impaired due to ROS accumulation and cytokine involvement, hindering the timely clearance of damaged mitochondria, leading to increased ROS generation and heightened OS (89).

During MF, ECM deposition and cardiac fibrosis worsen the heart’s hypoxic conditions. Under insufficient oxygen supply, mitochondria resort to anaerobic metabolic pathways, such as anaerobic glycolysis, to produce energy, which is significantly less efficient than aerobic metabolism. Excessive accumulation of lactic acid as a metabolic byproduct may be toxic to cells, impairing cellular function. In this metabolic state, more ROS are produced. Hypoxia and an increase in ROS not only damage mitochondria but also lead to the activation of more fibroblasts and the synthesis of ECM, forming a vicious cycle (96).