Metformin ameliorates hypoxia/reoxygenation-induced cardiomyocyte apoptosis based on the SIRT3 signaling pathway

Metformin protects against retinal ischemia/reperfusion injury through AMPK-mediated mitochondrial fusion

Retinal ischemia/reperfusion (I/R) injury is a common pathological process responsible for cellular damage in glaucoma, diabetic retinopathy and hypertensive retinopathy. Metformin is a biguanide drug that exerts strong effects on multiple diseases. This study aims to evaluate the protective effect of metformin against retinal I/R injury and its underlying mechanism. I/R induced reduction in retina thickness and cell number in ganglion cell layer, and metformin alleviated I/R-induced retinal injury. Both retinal I/R and simulated ischemia/reperfusion (SIR) in R28 cells down-regulated expression of mitochondrial fusion protein Mfn2 and OPA1, which led to mitochondrial fission. Metformin also alleviated damage in R28 cells, and reversed the alteration in Mfn2 and OPA1, mitochondrial fission and mitochondrial membrane potential (MMP) disruption-induced by I/R or SIR as well. Intriguingly, inhibition of AMPK by compound C or siRNA prevented metformin-mediated up-regulation of Mfn2 and OPA1. Compound C and knockdown of Mfn2 or OPA1 dramatically alleviated the protective effect of metformin against intracellular ROS generation, MMP disruption, mitochondrial fission and loss of RGCs in ganglion cell layer induced by SIR or I/R. Moreover, scavenging mitochondrial ROS (mito-ROS) by mito-TEMPO exerted the similar protection against I/R-induced retinal injury or SIR-induced damage in R28 cells as metformin. Our data show for the first time that metformin protects against retinal I/R injury through AMPK-mediated mitochondrial fusion and the decreased mito-ROS generation. These findings might also repurpose metformin as a therapeutic agent for retinal I/R injury.

A Promising Strategy to Treat Neurodegenerative Diseases by SIRT3 Activation

SIRT3, the primary mitochondrial deacetylase, regulates the functions of mitochondrial proteins including metabolic enzymes and respiratory chain components. Although SIRT3's functions in peripheral tissues are well established, the significance of its downregulation in neurodegenerative diseases is beginning to emerge. SIRT3 plays a key role in brain energy metabolism and provides substrate flexibility to neurons. It also facilitates metabolic coupling between fuel substrate-producing tissues and fuel-consuming tissues. SIRT3 mediates the health benefits of lifestyle-based modifications such as calorie restriction and exercise. SIRT3 deficiency is associated with metabolic syndrome (MetS), a precondition for diseases including obesity, diabetes, and cardiovascular disease. The pure form of Alzheimer's disease (AD) is rare, and it has been reported to coexist with these diseases in aging populations. SIRT3 downregulation leads to mitochondrial dysfunction, neuroinflammation, and inflammation, potentially triggering factors of AD pathogenesis. Recent studies have also suggested that SIRT3 may act through multiple pathways to reduce plaque formation in the AD brain. In this review, we give an overview of SIRT3's roles in brain physiology and pathology and discuss several activators of SIRT3 that can be considered potential therapeutic agents for the treatment of dementia.

Involvement of Sirtuins and Klotho in Cardioprotective Effects of Exercise Training Against Waterpipe Tobacco Smoking-Induced Heart Dysfunction

Despite its negative effect on the cardiovascular system, waterpipe smoking (WPS) is currently popular worldwide, especially among youth. This study investigated the effects of moderate endurance exercise on heart function of rats exposed to WPS and its possible mechanism. The animals were randomly divided into four groups: control group (CTL), the exercise group (Ex) which trained for 8 weeks, the waterpipe tobacco smoking group (S) exposed to smoke inhalation (30 min per day, 5 days each week, for 8 weeks), and the group that did exercise training and received waterpipe tobacco smoke inhalation together (Ex + S). One day after the last session of Ex and WPS, cardiac pressures and functional indices were recorded and calculated. The levels of SIRT1, SIRT3, Klotho, Bax, and Bcl-2 in the serum and heart, the expression of phosphorylated GSK3β of heart tissue, and cardiac histopathological changes were assessed. WPS reduced systolic pressure, +dP/dt max, -dP/dt max, and heart contractility indices (P < 0.001 vs. CTL) and increased cardiac tissue lesions (P < 0.05 vs. CTL) and end diastolic pressure and Tau index (P < 0.001 vs. CTL) of the left ventricle. Exercise training normalized the left ventricular end diastolic pressure, +dP/dt max, and contractility index. Also, exercise improved the levels of SIRT1, SIRT3, Klotho, and Bcl-2 and reduced Bax level in the heart. The findings showed that WPS causes left ventricular dysfunction. Moderate exercise prevented WPS-induced heart dysfunction partly through its anti-apoptotic features and activation of the sirtuins and Klotho pathways.

The Hormetic Effect of Metformin: "Less Is More"?

Metformin (MTF) is the first-line therapy for type 2 diabetes (T2DM). The euglycemic effect of MTF is due to the inhibition of hepatic glucose production. Literature reports that the principal molecular mechanism of MTF is the activation of 5′-AMP-activated protein kinase (AMPK) due to the decrement of ATP intracellular content consequent to the inhibition of Complex I, although this effect is obtained only at millimolar concentrations. Conversely, micromolar MTF seems to activate the mitochondrial electron transport chain, increasing ATP production and limiting oxidative stress. This evidence sustains the idea that MTF exerts a hormetic effect based on its concentration in the target tissue. Therefore, in this review we describe the effects of MTF on T2DM on the principal target organs, such as liver, gut, adipose tissue, endothelium, heart, and skeletal muscle. In particular, data indicate that all organs, except the gut, accumulate MTF in the micromolar range when administered in therapeutic doses, unmasking molecular mechanisms that do not depend on Complex I inhibition.

Metformin attenuates ischemia/reperfusion-induced apoptosis of cardiac cells by downregulation of p53/microRNA-34a via activation of SIRT1

Metformin has been demonstrated to be beneficial for the treatment of an impaired myocardium as a result of ischemia/reperfusion (I/R) injury, and miR-34a may be involved in this process. The aim of the present study was to determine the mechanisms by which metformin attenuated myocardial I/R injury-induced apoptosis. In the in vivo I/R model using Sprague-Dawley rats, metformin reduced the area of damaged myocardium and serum creatine MB isoform (CKMB) activity resulting in protection of the myocardium. Metformin also reduced apoptosis and the expression of apoptosis associated proteins, including caspase 3 and cleaved caspase, and decreased the expression of miR-34a, which is upregulated during I/R injury, which in turn resulted in corresponding changes in expression of Bcl-2, a direct target of miR-34a both in vitro and in vivo. To further examine the role of miR-34a in this process, H9C2 cells were transfected by a miR-34a mimic and inhibitor. Overexpression of miR-34a increased apoptosis in H9C2 cells induced by oxygen-glucose deprivation/recovery and knockdown of miR-34a expression-reduced apoptosis under the same conditions. Therefore, the effect of metformin on miR-34a in vitro were assessed. Metformin decreased the deacetylation activity of silent information regulator 1 resulting in reduced Ac-p53 levels, which reduced the levels of pri-miR-34a, and thus in turn reduced miR-34a levels. To confirm these results clinically, 90 patients with ST-segment elevation myocardial infarction following percutaneous coronary intervention were recruited. Patients who took metformin regularly before infarction had lower miR-34a levels and lower serum CKMB activity. Metformin also improved the sum ST-segment recovery following I/R injury. In conclusion, metformin may be helpful in the treatment of myocardial I/R.

Metformin protects cardiomyocytes against oxygen-glucose deprivation injury by promoting autophagic flux through AMPK pathway

Metformin has been shown to protect myocardial ischaemia/reperfusion or hypoxia/reoxygenation injury. In our current study, we investigated the effects of metformin on autophagy and its possible underlying mechanisms in in vivo myocardial infarction (MI) model and in vitro oxygen-glucose deprivation (OGD) model. A rat model of MI was made by ligating coronary artery in vivo study. Metformin (200 mg/kg/day) could improve cardiac function, prevent rats from MI-induced injury by reducing myocardial infarct size and apoptosis. Moreover, metformin furtherly promoted autophagy by increasing the protein expression of LC3-II, ATG5, ATG7 and Beclin1, and by involving AMPK pathway during MI. H9c2 cells were treated with metformin (4 mM) in vitro study to assess its effects after exposure to OGD. Metformin increased cell viability and inhibited OGD-induced LDH synthesis and cell apoptosis. Furthermore, metformin increased autophagosome formations as well as expression of autophagy-related proteins, promoted autophagic flux. In addition, metformin augmented the protein level of Bcl-2 and diminished the protein levels of Bax and cleaved caspase-3. Metformin also upregulated p-AMPK expression. Nevertheless, the above-mentioned effects of metformin on H9c2 cells were remarkably eliminated by compound C (an AMPK inhibitor). In summary, we displayed that metformin protected cardiomyocytes against OGD-induced injury and apoptosis by promoting autophagic flux through the AMPK pathway.

Cancer-induced Cardiac Atrophy Adversely Affects Myocardial Redox State and Mitochondrial Oxidative Characteristics

Cachexia presents in 80% of advanced cancer patients; however, cardiac atrophy in cachectic patients receives little attention. This cardiomyopathy contributes to increased occurrence of adverse cardiac events compared to age-matched population norms. Research on cardiac atrophy has focused on remodeling; however, alterations in metabolic properties may be a primary contributor.

PURPOSE

Determine how cancer-induced cardiac atrophy alters mitochondrial turnover, mitochondrial mRNA translation machinery and in-vitro oxidative characteristics.

METHODS

Lewis lung carcinoma (LLC) tumors were implanted in C57BL6/J mice and grown for 28days to induce cardiac atrophy. Endogenous metabolic species, and markers of mitochondrial function were assessed. H9c2 cardiomyocytes were cultured in LLC-conditioned media with(out) the antioxidant MitoTempo. Cells were analyzed for ROS, oxidative capacity, and hypoxic resistance.

RESULTS

LLC heart weights were ~10% lower than controls. LLC hearts demonstrated ~15% lower optical redox ratio (FAD/FAD+NADH) compared to PBS controls. When compared to PBS, LLC hearts showed ~50% greater COX-IV and VDAC, attributed to ~50% lower mitophagy markers. mt-mRNA translation machinery was elevated similarly to markers of mitochondrial content. mitochondrial DNA-encoded Cytb was ~30% lower in LLC hearts. ROS scavengers GPx-3 and GPx-7 were ~50% lower in LLC hearts. Treatment of cardiomyocytes with LLC-conditioned media resulted in higher ROS (25%), lower oxygen consumption rates (10% at basal, 75% at maximal), and greater susceptibility to hypoxia (~25%) -- which was reversed by MitoTempo.

CONCLUSION

These results substantiate metabolic cardiotoxic effects attributable to tumor-associated factors and provide insight into interactions between mitochondrial mRNA translation, ROS mitigation, oxidative capacity and hypoxia resistance.

Acute metformin treatment provides cardioprotection via improved mitochondrial function in cardiac ischemia / reperfusion injury

Cardiac ischemia/reperfusion (I/R) injury following reperfusion therapy in acute myocardial infarction results in mitochondrial dynamic imbalance and cardiomyocyte apoptosis. Although diabetic patients taking metformin have been shown to have a lower risk of myocardial infarction, the efficacy of the cardioprotection conferred by metformin regarding the mitochondrial function and dynamic in cardiac I/R injury are still inconclusive. In addition, the comparative effects between different doses of metformin given acutely prior to cardiac I/R injury have never been investigated. Fifty 8-week-old male Wistar rats weighing 300-350 g were divided into sham-operated (n = 10) and cardiac I/R-operated (n = 40) groups. In the cardiac I/R group, rats underwent 30-min ischemia followed by 120-min reperfusion and were randomly divided into four subgroups (n = 10/group): control (received normal saline), metformin (100, 200, and 400 mg/kg). The arrhythmia score, cardiac function, infarct size, mortality rate, mitochondrial function and apoptosis, were determined. Metformin (200 mg/kg) exerted the highest level of cardioprotection through reduction in arrhythmia, infarct size, mitochondrial fission, and apoptosis, in addition to preservation of mitochondrial function, leading to the attenuation of cardiac dysfunction. Doses of metformin (100 and 400 mg/kg) also improved mitochondrial and cardiac function, but to a lesser extent than metformin (200 mg/kg). In conclusion, metformin exerts cardioprotection by attenuating mitochondrial dysfunction, mitochondrial dynamic imbalance, and apoptosis. These led to decreased infarct size and eventual improvement in cardiac function in rats with acute cardiac I/R injury. These findings indicate the potential clinical benefits of acute metformin treatment in acute myocardial infarction.

Background and Proposed Design for a Metformin Abdominal Aortic Aneurysm Suppression Trial

Abdominal aortic aneurysm (AAA) may lead to rupture and death if left untreated. While endovascular or surgical repair is generally recommended for AAA greater than 5–5.5 cm, the vast majority of aneurysms detected by screening modalities are smaller than this threshold. Once discovered, there would be a significant potential benefit in suppressing the growth of these small aneurysms in order to obviate the need for repair and mitigate rupture risk. Patients with diabetes, in particular those taking the oral hypoglycaemic medication metformin, have been shown to have lower incidence, growth rate, and rupture risk of AAA. Metformin therefore represents a widely available, non-toxic, potential inhibitor of AAA growth, but thus far no prospective clinical studies have evaluated this. Here, we present the background, rationale, and design for a randomised, double-blind, placebo-controlled clinical trial of metformin for growth suppression in patients with small AAA.

Treatment with metformin prevents myocardial ischemia-reperfusion injury via STEAP4 signaling pathway

Objective:

The aim of the present study was to investigate the underlying mechanism of metformin in reducing myocardial apoptosis and improving mitochondrial function in rats and H9c2 cells subjected to myocardial ischemia–reperfusion (I/R) or hypoxia–reoxygenation (H/R) injuries, respectively.

Methods:

Following pretreatment with metformin, male Sprague–Dawley rats were used to establish an I/R model in vivo. Serum creatinine kinase-MB and cardiac troponin T levels were examined by enzyme-linked immunosorbent assay. Infarct size and apoptosis were measured by triphenyl tetrazolium chloride staining and terminal deoxynucleotidyl transferase dUTP nick end labeling assay. Pathological changes were evaluated by hematoxylin and eosin staining. H9c2 cells were used to establish an H/R model in vitro. Cell apoptosis and mitochondrial membrane potential (MMP) were examined by flow cytometry and Rhodamine 123. The expression levels of six-transmembrane epithelial antigen of prostate 4 (STEAP4), B-cell lymphoma 2, Bcl-2-associated X protein, and glyceraldehyde 3-phosphate dehydrogenase in both myocardial tissues and H9c2 cells were determined by western blotting.

Results:

We found that metformin decreased infarct size, increased STEAP4 expression, mitigated myocardial apoptosis, and increased MMP when the models were subjected to H/R or I/R injuries. However, STEAP4 knockdown significantly abrogated the beneficial effect of metformin.

Conclusion:

We further demonstrated the protective effect of metformin on cardiomyocytes, which might be at least partly attributable to the upregulation of STEAP4. Therefore, STEAP4 might be a new target to decrease apoptosis and rescue mitochondrial function in myocardial I/R injury.

Effects of metformin on the heart with ischaemia-reperfusion injury: Evidence of its benefits from in vitro, in vivo and clinical reports

Ischaemia reperfusion (I/R) injury following myocardial infarction reperfusion therapy is a phenomenon that results in further loss of cardiomyocytes and cardiac contractility. Among the potential therapeutics to counter cardiac I/R injury, the antidiabetic drug metformin has shown promising experimental results. This review encompasses evidence available from studies of metformin's protective effects on the heart following cardiac I/R in vitro, ex vivo and in vivo, alongside clinical trials. Experimental data describes potential mechanisms of metformin, including activation of AMPK, an energy sensing kinase with many downstream effects. Suggested effects include upregulation of superoxide dismutases (SODs), which reduce oxidative stress and improve mitochondrial function. Additionally, metformin demonstrates anti-apoptotic effects, most likely by inhibiting mitochondrial permeability transition pore (mPTP) opening, and anti-inflammatory effects, by JNK inhibition. Recent reports of metformin's role in modulating complex I activity of the electron transport chain following cardiac I/R are also presented and discussed. Furthermore, clinical reports present mixed findings, suggesting that beneficial effects may depend on dosage, timing and condition of patients receiving metformin treatment. Conclusively there is an increased need for prospective, placebo-controlled clinical studies to confirm the mechanisms and to demonstrate that metformin is a suitable and safe drug for treatment of cardiac I/R injury.

Heme oxygenase-1 ameliorates hypoxia/reoxygenation via suppressing apoptosis and enhancing autophagy and cell proliferation though Sirt3 signaling pathway in H9c2 cells

Cardiomyocyte infarction could lead to high morbidity and mortality worldwide. Recent studies demonstrated that Heme oxygenase-1 (HO-1) could exert cardiac protective effect and arouse attention. However, the detailed mechanism is still unclear. Our study provided evidences of the protective effect of HO-1 overexpression on cardiomyocytes against hypoxia/reoxygenation (H/R). We divided the treatment into four groups: the control group, H/R group, H/R+HO-1 group, and H/R+Null group. Immunofluorescent study was utilized to label the BrdU-positive and LC3-positive cells. Flow cytometry and TUNEL assay were used to examine the cell apoptosis. Protein levels of Bax, Bcl-2, Sirt3, beclin-1, LC3-I, and LC3-II were both measured using western blotting. The results indicated that HO-1 overexpression decreased the cell apoptosis and enhanced the cell proliferation. The level of Sirt3 and autophagy were also increased in H/R+HO-1 group compared with H/R group. However, ZnPP, a HO-1 inhibitor, and SiRNA of Sirt3 are both reversed the decrease of cell apoptosis of HO-1 overexpression. Moreover, ZnPP also decreased the expression of Sirt3 in HO-1 overexpression treatment group. In summary, HO-1 overexpression protects cardiomyocytes against H/R injury via ameliorating cell apoptosis and enhancing cell proliferation and autophagy through Sirt3 signaling pathway.

Emerging potential benefits of modulating NAD+ metabolism in cardiovascular disease

Nicotinamide adenine dinucleotide (NAD⁺) and related metabolites are central mediators of fuel oxidation and bioenergetics within cardiomyocytes. Additionally, NAD⁺ is required for the activity of multifunctional enzymes, including sirtuins and poly(ADP-ribose) polymerases that regulate posttranslational modifications, DNA damage responses, and Ca²⁺ signaling. Recent research has indicated that NAD⁺ participates in a multitude of processes dysregulated in cardiovascular diseases. Therefore, supplementation of NAD⁺ precursors, including nicotinamide riboside that boosts or repletes the NAD⁺ metabolome, may be cardioprotective. This review examines the molecular physiology and preclinical data with respect to NAD⁺ precursors in heart failure-related cardiac remodeling, ischemic-reperfusion injury, and arrhythmias. In addition, alternative NAD⁺-boosting strategies and potential systemic effects of NAD⁺ supplementation with implications on cardiovascular health and disease are surveyed.