Innumerable studies apply dietary manipulations to induce obesity, insulin resistance, and T2DM in rodents and large animal models [14,17,19]. Insulin signaling in the heart is preserved in T2DM rodent models following short-term high-fat diet (HFD) feeding [22,23]. However, prolonged HFD feeding in animal models impairs its downstream targets of the serine/threonine kinase Akt and forkhead box O-1 (FOXO1) transcription factor phosphorylation [24], which results in persistent FOXO1 nuclear localization and activation. Mice with cardiac-specific deletion of glucose transporter type 4 (GLUT4) showed normal cardiac function in the unstressed state but developed maladaptive hypertrophy and severe contractile dysfunction in response to left ventricular (LV) pressure overload [13,25]. Therefore, GLUT4 is required for the maintenance of cardiac function and structure in response to pathological processes that increase energy demand, in part through secondary changes in mitochondrial metabolism and cellular stress survival signaling, such as the phosphoinositide 3-kinase (PI3K)–Akt pathway [13,25].

In addition to stimulating glucose uptake, both insulin signaling [13,26] and cardiomyocyte contraction [13,27] can promote fatty acid uptake into cardiomyocytes via induction of cluster of differentiation 36 (CD36) translocation to sarcolemma membranes [26,28]. The long-lasting presence of CD36 at the sarcolemma membrane leads to an increased rate of long-chain fatty acid uptake and accumulation of triglycerides in cardiomyocytes, which results in lipotoxic DiaCM [28,29].

The transcription factor peroxisome proliferator-activated receptor-α (PPARα) is a major regulator of lipid metabolism and can increase the expression of genes encoding CD36, fatty acid-binding proteins and proteins involved in β-oxidation in the mitochondria and peroxisome [13,30]. Tribbles-related protein 3 (TRB3) can directly bind to Akt and inhibit Akt phosphorylation [13,31,32]. The expression of TRB3 is upregulated in the heart in T1DM and T2DM rodent models [33,34] and in skeletal muscle in patients with T2DM [32]. Furthermore, a rat model of T2DM induced by a HFD and low-dose streptozotocin (STZ) demonstrated severe insulin resistance and properties of DiaCM, including myocardial fibrosis, cardiac inflammation and LV dysfunction, in addition to increased expression of TRB3, compared with control rats [34].

The hearts from rats with T2DM infused ex vivo with the CD36 inhibitor sulfo-N-succinimidyl oleate (SSO) before inducing hypoxia, which resulted in a 29% reduction in the rate of fatty acid oxidation and an approximately 50% reduction in triglyceride concentration compared with vehicle treatment, showed a restoration of fatty acid metabolism to control levels following hypoxia–reoxygenation [13,35]. SSO infusion into diabetic rat hearts ex vivo before hypoxia also prevented cardiac dysfunction [35]. Fenofibrate treatment prevented fibrosis and diastolic dysfunction in diabetic rats, probably through improvements in cardiac and systemic lipid metabolism [36,37]. Fenofibrate treatment was also associated with reductions in markers of apoptosis and cardiac hypertrophy in rats with STZ-induced T1DM [38]. The glucagon-like peptide-1 (GLP1) analog liraglutide protected against the development of DiaCM in a rat model of STZ-induced T1DM by inhibiting the endoplasmic reticulum (ER) stress pathway [39]. Similarly, the GLP1 analog exendin-4 prevented the development of DiaCM via the amelioration of lipotoxicity in a mouse model of T2DM [40]. The dipeptidyl peptidase-4 (DPP4) inhibitor sitagliptin reduced blood glucose levels, increased GLP1 levels and prevented T2DM-induced DiaCM in mice by shifting the energy substrate utilization in the heart from fatty acids towards glucose [41,42]. Recently, sodium-glucose cotransporter type 2 (SGLT-2) inhibitors, novel hypoglycemic agents that increase urinary Na⁺ and glucose excretion, were introduced to DM and DiaCM research and have come into the spotlight. In addition to the beneficial effects of SGLT-2 inhibitors on glucose-lowering or natriuretic action, several potential cardioprotective mechanisms of SGLT-2 inhibitors have been reported [5,13,43]. A number of studies have shown the multiple effects of SGLT-2 inhibitors on cardiac iron homeostasis, antioxidative stress, anti-inflammation, RAAS activity, antifibrosis, and GlcNAcylation, as well as mitochondrial function in the heart [43-47]. Excessive O-GlcNAcylation following chronic activation of the hexosamine biosynthetic pathway is associated with posttranslational modifications in the diabetic heart. O-GlcNAcylation impairs cardiac mitochondrial function, Ca²⁺ homeostasis, and ER stress in DM. A previous study showed that dapagliflozin prevented DiaCM by reducing the levels of O-GlcNAcylated protein in diabetic mice. These results demonstrated that O-GlcNAcylated levels of FOXO1 reduced by SGLT-2 inhibitors contributed to attenuation of DiaCM and improvement in heart function [43,46].

Excess generation of reactive oxygen species (ROS) or reactive nitrogen species (RNS) is considered to be a central mechanism for diabetes-associated inflammation and remodeling in the heart [13,48,49] and contributes to oxidative stress during both the early and late stages of DiaCM [50,51]. Defects in the antioxidant defense system further increase oxidative stress during the later stages of DiaCM [50,51]. Superoxide dismutase (SOD) has an important role in preventing cardiac damage in the setting of DM. Injection of the SOD mimic mitochondria-targeted mitochondrial triphenylphosphonium chloride (mito-TEMPO) prevented the hyperglycemia-induced increase in superoxide generation, reduced myocardial hypertrophy and improved myocardial function in STZ-induced T1DM mice and db/db T2DM mice compared with vehicle treatment [52].

The transcription factor nuclear factor erythroid 2-related factor 2 (NRF2) is an essential regulator of the antioxidant response with an important role in preventing diabetes-induced oxidative stress and cell death. Isolated cardiomyocytes from Nrf2 knockout (KO) mice were more susceptible to high glucose-induced cell death than wild-type (WT) cells [13,53]. Furthermore, NRF2-deficient mice were more susceptible to diabetes-induced or angiotensin (Ang) II-induced cardiomyopathy than WT mice, whereas cardiomyocyte-specific overexpression of Nrf2 conferred resistance to Ang II-induced cardiomyopathy [54,55]. Naturally occurring activators of NRF2 have been shown to ameliorate diabetes-induced cardiac complications. Sulforaphane is an organosulfur compound derived from cruciferous vegetables such as cabbage, Brussels sprouts, and broccoli that has been shown to upregulate the expression of numerous genes encoding antioxidant proteins by activating NRF2 signaling [13,56]. The cardioprotective benefits of sulforaphane in attenuating fibrosis, oxidative damage, inflammation, hypertrophy, and cardiac dysfunction have been demonstrated in both T1DM and T2DM mouse models and in mice exposed to Ang II [54,55,57,58]. Administration of the antioxidant N-acetylcysteine (NAC) for 5 weeks to rat and mouse models of STZ-induced T1DM normalized the levels of oxidative stress and subsequently prevented the development of DiaCM [59,60]. Interestingly, the earlier the NAC treatment protocol was initiated after induction of diabetes with STZ during the 12-week experiment, the greater the protection against DiaCM [60], suggesting that early damage mediated by increased oxidative stress has a more important role in the development of DiaCM. In diabetic rats, NAC treatment attenuated cardiac dysfunction and damage after myocardial ischemia–reperfusion injury [61,62].

Systemic inflammation, hyperglycemia, and dyslipidemia associated with DM lead to the development of cardiac fibrosis and hypertrophy, which increase myocardial stiffness and result in LV diastolic and systolic dysfunction [13].

In DiaCM, increased collagen accumulation is observed in perivascular loci, intermyofiber spaces, and replacement fibrosis [14]. Thus, cardiac fibrosis increased in some animal models of both T1DM [14,63-66] and T2DM [67,68]. Under diabetic conditions, advanced glycation end products created by the exposure of proteins and lipids to high glucose levels crosslink extracellular matrix (ECM) proteins, impair ECM degradation by matrix metalloproteinases and increase cardiac stiffness, which together manifest as early LV diastolic dysfunction [13,69,70]. Genetically obese mice exhibited severe diastolic dysfunction, as evidenced by decreasing the ratio of the early (E) to late (A) (E/A) velocities in db/db and ob/ob mice [21,71,72]. Contractile properties were still slightly affected in ob/ob mice [75], while db/db mice displayed reduced fractional shortening and velocity of circumferential fiber shortening at 12 weeks of age [21,72].

Epicardial and endothelial cells can also contribute to the development of cardiac fibrosis through epithelial-to-mesenchymal or endothelial-to-mesenchymal transition to myofibroblasts [13,73-75].

The antifibrotic agent cinnamoyl anthranilate reduced collagen production stimulated by transforming growth factor β (TGF-β) signaling in cultured renal mesangial cells [76]. Administration of FT23 and FT011, which are derivatives of cinnamoyl anthranilate, attenuated cardiac structural and functional abnormalities in an animal model of DiaCM [77,78].

In the diabetic heart, chemokines, cytokines, and exosomes secreted by inflammatory cells contribute to the development of cardiomyocyte hypertrophy and ECM remodeling. Several myocardial processes are activated by a number of proinflammatory factors, dyslipidemia, hyperglycemia, and elevated Ang II levels that are upregulated in the setting of DM [13]. Together, these factors promote the infiltration and accumulation of proinflammatory lymphocytes and macrophages into the lesion site. These inflammatory cells secrete cytokines such as TGF-β, interleukin (IL)-1β, tumor necrosis factor (TNF), IL-6, and interferon-γ that can cause or exacerbate myocardial injury, contributing to further adverse cardiac remodeling [79,80].

Mice with STZ-induced T1DM have higher T cell infiltration into the myocardium, which is associated with increased myocardial fibrosis and LV dysfunction, than control mice [81]. Inhibition of T cell trafficking in diabetic mice prevented myocardial fibrosis and cardiac dysfunction [82,83].

Toll-like receptor 4 (TLR4) is expressed in cardiomyocytes, inflammatory cells, and cardiac fibroblasts in both normal and failing hearts [13]. The role of TLR4-mediated inflammatory signaling in the development of DiaCM has been reported in animal models of T1DM and T2DM [84,85]. Inflammatory factors, including nuclear factor-κB and TNF, and protein kinases, such as c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK), can directly lead to cardiomyocyte hypertrophy and can advance myocardial fibrosis [86,87]. Activation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome, a regulator of cell death and inflammation [88], has been associated with cardiac inflammation, fibrosis, and cell death triggered by HFD and STZ administration in a rat model of T2DM [89]. These effects were attenuated by microRNA (miRNA)-mediated Nlrp3 silencing [89] or by pharmacological suppression of NLRP3 inflammasome activation [90].

Suppression of TLR4 signaling with triptolide or matrine improved cardiac LV function and reduced collagen accumulation in rat models of DiaCM [91,92]. Long-term blockade of TLR4 with the TLR4 inhibitor TAK-242 (also known as CLI-095) was associated with a slight improvement in diabetes-induced erectile dysfunction in rats compared with no treatment, mediated by an increase in cyclic guanosine monophosphate levels and the attenuation of oxidative stress in penile tissue [93]. Numerous small-molecule inhibitors of the NLRP3 inflammasome have evolved in the past several years. The orally active NLRP3 inhibitor 16673-34-0 prevented Western diet-induced systolic and diastolic LV dysfunction in obese mice [94].

Apoptosis is an extremely controlled mechanism of programmed cell death and seems to be the principal form of cell death in DiaCM, compared with lower rates due to necrosis [95,96].

In T1DM animals, both increased death receptor signaling and mitochondria-dependent proapoptotic signaling led to elevated apoptosis in DiaCM, and antioxidant treatment diminished both of these signaling pathways and apoptosis, suggesting an essential role of increased ROS in apoptosis induction in DiaCM [95,97]. A recent study also proposed that dissociation of B-cell lymphoma 2 (Bcl-2) protein from beclin-1 by restoration of impaired AMP-dependent protein kinase (AMPK) activity may decrease apoptosis in DiaCM by restoring autophagy, supporting the suggestion that an interplay between apoptosis and autophagy may be important in DiaCM [98]. Furthermore, ER stress may encourage apoptosis in DiaCM by activating JNK signaling and apoptosis via the extrinsic and intrinsic pathways or by increasing protein kinase RNA-like ER kinase (PERK)-C/EBP homologous protein (CHOP) signaling, which may provoke apoptosis by switching expression towards proapoptotic Bcl-2 proteins [99].

In DM, the process of cardiac calcium cycling (Ca²⁺ entry, intracellular Ca²⁺ concentration, and Ca²⁺ efflux) is modified in both humans and animal models, contributing to impaired cardiac contraction and relaxation. Decreased Ca²⁺ entry is the consequence of both altered voltage dependence of the L-type calcium channel (LTCC) and reduced expression [95]. Impaired intracellular Ca²⁺ cycling consists of reductions in the amplitude of Ca²⁺ and in the systolic rate of the Ca²⁺ rise and fall [100,101]. Prolonged rates of Ca²⁺ decay may arise from impaired sarco/endoplasmic reticulum Ca²⁺-ATPase 2a (SER-CA2a) activity during the diastolic period, which may cause a decrease in sarcoplasmic reticulum (SR) Ca²⁺ storage of up to 50% and, thus, can lead to diastolic dysfunction and impaired relaxation [102].

In models of T2DM, contractile dysfunction may be driven by a significant decrease in the Ca²⁺ transient due to reduced Ca²⁺ influx as a consequence of decreased LTCC expression, by decreased SR Ca²⁺ content due to increased phospholamban expression and decreased SERCA2a expression, and by the diminished activity and content of ryanodine receptor (RyR) [95,103]. In addition, hyperglycemia may lead to the O-GlcNAcylation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), which may accelerate diastolic SR Ca²⁺ leakage via RyRs, leading to SR Ca²⁺ depletion [104].

A potent late Na⁺ current inhibitor, ranolazine, might normalize altered intracellular Ca²⁺ levels in cardiomyocytes due to the close relationship between Ca²⁺ and Na⁺ coupling handled by the Na⁺/Ca²⁺ exchanger [5,105]. Ranolazine improved several hemodynamic parameters but not cardiac relaxation variables. This result showed that a single treatment using ranolazine is probably not sufficient to influence myocardial structure and cardiac function [5,105].

Current evidence from animal experiments and human patients has identified a critical role for RAAS in DiaCM [5]. Cytoplasmic Ang II enhances cell growth in animal models. Ang II has a definite influence on cell signaling, resulting in cardiomyocyte hypertrophy and proliferation of cardiac fibroblasts [106]. Other factors, such as inflammation, oxidative stress, and aldosterone, may potentiate the harmful effects of Ang II on the heart that lead to myocardial damage in DM [107]. Moreover, the enhanced activation of Ang II and mineralocorticoid receptor signaling might promote insulin resistance by initiating the mammalian target of rapamycin (mTOR)-S6 kinase 1 signal transduction pathway [5,108].

Recently, renin inhibitors (aliskiren), angiotensin II receptor blockers (ARBs), and angiotensin converting enzyme inhibitors (ACEis) were shown to be protective medications against DiaCM in rat models [5,109]. ACEis and ARBs were also useful agents in both human and animal models of DiaCM [110,111]. The favorable effect of β-adrenoreceptor blockers was also demonstrated in experimental models of DiaCM [112].

Mitochondrial dysfunction is a well-known feature of DiaCM in both animal and human DM. Mitochondrial dysfunction refers to abnormal mitochondrial ultrastructure, increased mitochondrial oxidative stress, impaired activity of Ca²⁺-sensitive dehydrogenases and F0F1-ATPase, increased sensitivity for Ca²⁺-induced opening of the mitochondrial permeability transition pore, transcriptional and translational downregulation of oxidative phosphorylation (OXPHOS) subunits, and impaired mitochondrial respiratory capacity and coupling [95,113].

In humans, several studies have demonstrated mitochondrial dysfunction in the atrium and atrial appendages of DM patients [114-116], with impaired respiration rates and electron transport chain complex activities in patients with DM. In a diabetic mouse model, as early as 1985, an impairment in state 3 respiration of isolated cardiac mitochondria was observed [117]. Since then, mitochondrial dysfunctions have been reported in numerous diabetic rodent models [118]. In terms of T1DM models, STZ-treated rats showed reduced antioxidant glutathione, increased ROS production, and ultimately loss of mitochondrial membrane potential [119]. OVE26 mice also displayed a reduction in glutathione, altered mitochondrial function, and an increase in mitochondrial biogenesis [120]. Akita mice revealed an increased volume of mitochondria with reduced crista densities and respiratory defects [121]. In T2DM models, db/db mice displayed increases in O² consumption, lipid peroxidation, and mitochondrial ROS generation [122]. Otsuka Long-Evans Tokushima fatty (OLETF) and ob/ob mouse models maintained unchanged levels of uncoupled proteins despite mitochondrial dysfunction [123,124]. Zucker diabetic fatty (ZDF) rats showed increased lipid peroxidation and mitochondrial ROS production rates with elevated antioxidant levels [125,126]. Goto-Kakizaki (GK) and OLETF rats also revealed higher lipid peroxidation and mitochondrial ROS production [125,127,128]. The activity of Sirtuin 3 (SIRT3), a major regulator of intramitochondrial protein acetylation and NAD+-dependent mitochondrial deacetylase, may be reduced in the diabetic heart, causing ROS deposition due to increased acetylation and, thus, suppression of manganese superoxide dismutase (MnSOD) [95,129]. Furthermore, SIRT3 deficiency seems to exacerbate suppression of mitophagy and autophagy in the diabetic heart, whereas SIRT3 overexpression promoted mitophagy and autophagy, attenuated cardiomyocyte apoptosis and diminished mitochondrial defects [130].

In DiaCM, dysregulations of 316 out of 1,008 total miRNAs were discovered, and pathway analysis demonstrated that several miRNAs are involved in cardiac hypertrophy, oxidative stress, apoptosis, and autophagy [95,131].

Adenovirus-mediated rescue of the myocardial proviral integration site for Moloney murine leukemia virus-1 (Pim-1) expression in vivo improved systolic and diastolic function, attenuated apoptosis and fibrosis, attenuated ventricular dilation, and restored SERCA2a content in DiaCM [132,133]. The expression of miR-133 is decreased in the STZ-induced diabetic mice, and miR-133 has direct inhibitory effects on collagen deposition by deteriorating connective tissue growth factor expression, indicating that increased miR-133 concentrations may attenuate myocardial fibrosis in DiaCM [134]. Myocardial expression of miR-451 is distinctly increased in mice fed a HFD, and cardiomyocyte-specific deletion of miR-451 decreases ceramide deposition, cardiac hypertrophy, and myocardial fibrosis in this mouse model. Diminution of hypertrophy may come from restoration of attenuated AMPK activity, which may normalize increased mTOR phosphorylation and thus restrict HFD-induced cardiomyocyte growth [135]. Upregulation of miR-30d in DiaCM was suggested to reduce FoxO3a signaling, causing caspase 1 activation and increasing inflammatory signaling, thus resulting in pyroptosis [136]. Based on the various characteristics and mechanisms of DiaCM that can be controlled by miRNAs, a significant contribution of miRNAs to the development of DiaCM was suggested [137].