NADPH oxidase inhibitor apocynin prevents atrial remodeling in alloxan-induced diabetic rabbits

Metabolic remodelling in atrial fibrillation: manifestations, mechanisms and clinical implications

Atrial fibrillation (AF) is a continually growing health-care burden that often presents together with metabolic disorders, including diabetes mellitus and obesity. Current treatments often fall short of preventing AF and its adverse outcomes. Accumulating evidence suggests that metabolic disturbances can promote the development of AF through structural and electrophysiological remodelling, but the underlying mechanisms that predispose an individual to AF are aetiology-dependent, thus emphasizing the need for tailored therapeutic strategies to treat AF that target an individual's metabolic profile. AF itself can induce changes in glucose, lipid and ketone metabolism, mitochondrial function and myofibrillar energetics (as part of a process referred to as 'metabolic remodelling'), which can all contribute to atrial dysfunction. In this Review, we discuss our current understanding of AF in the setting of metabolic disorders, as well as changes in atrial metabolism that are relevant to the development of AF. We also describe the potential of available and emerging treatment strategies to target metabolic remodelling in the setting of AF and highlight key questions and challenges that need to be addressed to improve outcomes in these patients.

IP3R1/GRP75/VDAC1 complex mediates endoplasmic reticulum stress-mitochondrial oxidative stress in diabetic atrial remodeling

Rationale

Endoplasmic reticulum (ER) stress and mitochondrial dysfunction are important mechanisms of atrial remodeling, predisposing to the development of atrial fibrillation (AF) in type 2 diabetes mellitus (T2DM). However, the molecular mechanisms underlying these processes especially their interactions have not been fully elucidated.

Objective

To explore the potential role of ER stress–mitochondrial oxidative stress in atrial remodeling and AF induction in diabetes.

Methods and results

Mouse atrial cardiomyocytes (HL-1 cells) and rats with T2DM were used as study models. Significant ER stress was observed in the diabetic rat atria. After treatment with tunicamycin (TM), an ER stress agonist, mass spectrometry (MS) identified several known ER stress and calmodulin proteins, including heat shock protein family A (HSP70) member [HSPA] 5 [GRP78]) and HSPA9 (GRP75, glucose-regulated protein 75). In situ proximity ligation assay indicated that TM led to increased protein expression of the IP3R1–GRP75–VDAC1 (inositol 1,4,5-trisphosphate receptor 1–glucose-regulated protein 75–voltage-dependent anion channel 1) complex in HL-1 cells. Small interfering RNA silencing of GRP75 in HL-1 cells and GRP75 conditional knockout in a mouse model led to impaired calcium transport from the ER to the mitochondria and alleviated mitochondrial oxidative stress and calcium overload. Moreover, GRP75 deficiency attenuated atrial remodeling and AF progression in Myh6-Cre⁺/Hspa9^flox/flox + TM mice.

Conclusions

The IP3R1–GRP75–VDAC1 complex mediates ER stress–mitochondrial oxidative stress and plays an important role in diabetic atrial remodeling.

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Endoplasmic reticulum stress associated with atrial fibrillation.

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GRP75 contributes to the ER-mitochondria crosstalk.

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Inhibition of GRP75 attenuated diabetic atrial remodeling.

Effective Accentuation of Voltage-Gated Sodium Current Caused by Apocynin (4'-Hydroxy-3'-methoxyacetophenone), a Known NADPH-Oxidase Inhibitor

Apocynin (aPO, 4'-Hydroxy-3'-methoxyacetophenone) is a cell-permeable, anti-inflammatory phenolic compound that acts as an inhibitor of NADPH-dependent oxidase (NOX). However, the mechanisms through which aPO can interact directly with plasmalemmal ionic channels to perturb the amplitude or gating of ionic currents in excitable cells remain incompletely understood. Herein, we aimed to investigate any modifications of aPO on ionic currents in pituitary GH₃ cells or murine HL-1 cardiomyocytes. In whole-cell current recordings, GH₃-cell exposure to aPO effectively stimulated the peak and late components of voltage-gated Na+ current (I_Na) with different potencies. The EC₅₀ value of aPO required for its differential increase in peak or late I_Na in GH₃ cells was estimated to be 13.2 or 2.8 μM, respectively, whereas the K_D value required for its retardation in the slow component of current inactivation was 3.4 μM. The current-voltage relation of I_Na was shifted slightly to more negative potential during cell exposure to aPO (10 μM); however, the steady-state inactivation curve of the current was shifted in a rightward direction in its presence. Recovery of peak I_Na inactivation was increased in the presence of 10 μM aPO. In continued presence of aPO, further application of rufinamide or ranolazine attenuated aPO-stimulated I_Na. In methylglyoxal- or superoxide dismutase-treated cells, the stimulatory effect of aPO on peak I_Na remained effective. By using upright isosceles-triangular ramp pulse of varying duration, the amplitude of persistent I_Na measured at low or high threshold was enhanced by the aPO presence, along with increased hysteretic strength appearing at low or high threshold. The addition of aPO (10 μM) mildly inhibited the amplitude of erg-mediated K+ current. Likewise, in HL-1 murine cardiomyocytes, the aPO presence increased the peak amplitude of I_Na as well as decreased the inactivation or deactivation rate of the current, and further addition of ranolazine or esaxerenone attenuated aPO-accentuated I_Na. Altogether, this study provides a distinctive yet unidentified finding that, despite its effectiveness in suppressing NOX activity, aPO may directly and concertedly perturb the amplitude, gating and voltage-dependent hysteresis of I_Na in electrically excitable cells. The interaction of aPO with ionic currents may, at least in part, contribute to the underlying mechanisms through which it affects neuroendocrine, endocrine or cardiac function.

Pharmacology of apocynin: a natural acetophenone

Apocynin is a naturally occurring acetophenone, found in the roots of Apocynum cannabinum and Picrorhiza kurroa. Various chemical and pharmaceutical modifications have been carried out to enhance the absorption and duration of action of apocynin, like, formulation of chitosan-based apocynin-loaded solid lipid nanoparticles, chitosan-oligosaccharide based nanoparticles, and biodegradable polyanhydride nanoparticles. Apocynin has been subjected to a wide range of experimental screening and has proved to be useful for amelioration of a variety of disorders, like diabetic complications, neurodegeneration, cardiovascular disorders, lung cancer, hepatocellular cancer, pancreatic cancer, and pheochromocytoma. Apocynin has been primarily reported as an NADPH oxidase (NOX) inhibitor and prevents translocation of its p47^phox subunit to the plasma membrane, observed in neurodegeneration and hypertension. However, recent studies highlight its off-target effects that it is able to function as a scavenger of non-radical oxidant species, which is relevant for its activity against NOX 4 mediated production of hydrogen peroxide. Additionally, apocynin has shown inhibition of eNOS-dependent superoxide production in diabetic cardiomyopathy, reduction of NLRP3 activation and TGFβ/Smad signaling in diabetic nephropathy, diminished VEGF expression and decreased retinal NF-κB activation in diabetic retinopathy, inhibition of P38/MAPK/Caspase3 pathway in pheochromocytoma, inhibition of AKT-GSK3β and ERK1/2 pathways in pancreatic cancer, and decreased FAK/PI3K/Akt signaling in hepatocellular cancer. This review aims to discuss the pharmacokinetics and mechanisms of the pharmacological actions of apocynin.

Effects of Apocynin on Heart Muscle Oxidative Stress of Rats with Experimental Diabetes: Implications for Mitochondria

Diabetes mellitus (DM) constitutes one of the public health problems today. It is characterized by hyperglycemia through a defect in the β-cells function and/or decreased insulin sensitivity. Apocynin has been tasted acting directly as an NADPH oxidase inhibitor and reactive oxygen species (ROS) scavenger, exhibiting beneficial effects against diabetic complications. Hence, the present study’s goal was to dissect the possible mechanisms by which apocynin could mediate its cardioprotective effect against DM-induced oxidative stress. Male Wistar rats were assigned into 4 groups: Control (C), control + apocynin (C+A), diabetes (D), diabetes + apocynin (D+A). DM was induced with streptozotocin. Apocynin treatment (3 mg/kg/day) was applied for 5 weeks. Treatment significantly decreased blood glucose levels and insulin resistance in diabetic rats. In cardiac tissue, ROS levels were higher, and catalase enzyme activity was reduced in the D group compared to the C group; the apocynin treatment significantly attenuated these responses. In heart mitochondria, Complexes I and II of the electron transport chain (ETC) were significantly enhanced in the D+A group. Total glutathione, the level of reduced glutathione (GSH) and the GSH/ oxidized glutathione (GSSG) ratio were increased in the D+A group. Superoxide dismutase (SOD) and the glutathione peroxidase (GSH-Px) activities were without change. Apocynin enhances glucose uptake and insulin sensitivity, preserving the antioxidant defense and mitochondrial function.

Activation of NADPH oxidase mediates mitochondrial oxidative stress and atrial remodeling in diabetic rabbits

AIMS

The mechanisms of atrial fibrillation (AF) in diabetes mellitus (DM) involve a complex interplay between increased oxidative stress, mitochondrial dysfunction and atrial remodeling. In this study, we examined the effects of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activation on mitochondrial oxidative stress and atrial remodeling in a rabbit model of diabetes mellitus (DM).

MAIN METHODS

Healthy rabbits were selected and randomly divided into control, diabetic and apocynin administration group. Parameters of echocardiography, atrial electrophysiology, oxidative stress and mitochondrial function were compared between the different groups.

KEY FINDINGS

Compared to the control group, the DM group showed higher activity of NADPH oxidase, increased oxidative stress, larger left atrial diameter, a reduction in atrial mean conduction velocity. These findings were associated with increased interstitial fibrosis of the atria and higher atrial fibrillation (AF) inducibility. Moreover, atrial ultrastructure and mitochondrial function such as the mitochondrial respiratory control rate (RCR) were impaired. NADPH oxidase inhibition using the pharmacological agent apocynin improved these changes.

SIGNIFICANCE

NADPH oxidase activity plays an important role in mitochondrial oxidative stress, which is associated with AF inducibility by promoting adverse atrial remodeling. The NADPH oxidase inhibitor apocynin can prevent these pathological changes and may be a potential drug for AF treatment.

Galectin-3 mediates high-glucose-induced cardiomyocyte injury by the NADPH oxidase/reactive oxygen species pathway

Galectin-3 is a member of the β-galactoside-binding lectin family taking part in the regulation of inflammation, angiogenesis, and fibrosis. This study was designed to study the improved effect of galectin-3 inhibition on diabetic cardiomyopathy (DCM). Sprague-Dawley rats were randomized into the control, DCM, and DCM + modified citrus pectin (MCP) (a galectin-3 pharmacological inhibitor) groups. After 8 weeks, streptozotocin-induced DCM led to high blood glucose level, oxidative stress, cardiac injury, and dysfunction accompanied by suppressed body mass. On the contrary, MCP (100 mg·kg^-1·day^-1) administration improved body mass and blood glucose level and attenuated cardiac injury and dysfunction in DCM rats. Additionally, MCP attenuated pathological changes in plasma and myocardial tissue markers of oxidative stress, such as hydrogen peroxide and malonyldialdehyde, although it did not change superoxide dismutase activities, which were decreased in the DCM group. The levels of oxidative stress associated proteins evaluated by Western blot, such as p67^phox and NADPH oxidase 4, were obviously increased in the DCM group, while they were reversed by MCP treatment. Therefore, galectin-3-mediated high-glucose-induced cardiomyocyte injury and galectin-3 inhibition attenuated DCM by suppressing NADPH oxidase. These findings suggested that galectin-3 could be a potential target for treatment of patients with DCM.

Wenxin Keli Regulates Mitochondrial Oxidative Stress and Homeostasis and Improves Atrial Remodeling in Diabetic Rats

Mitochondrial dysfunction and oxidative stress play an important role in the pathogenesis of both atrial fibrillation (AF) and diabetes mellitus (DM). Wenxin Keli (WXKL), an antiarrhythmic traditional Chinese medicine, has been shown to prevent cardiac arrhythmias through modulation of cardiac ion channels. This study tested the hypothesis that WXKL can improve atrial remodeling in diabetic rats by restoring mitochondrial function. Primary atrial fibroblasts of neonatal SD rats were divided into four groups: control, hydrogen peroxide (H₂O₂), H₂O₂+WXKL 1 g/L, and H₂O₂+WXKL 3 g/L groups. Intracellular mitochondrial membrane potential (MMP), reactive oxygen species (ROS), and mitochondrial oxygen consumption were measured. SD male rats were randomly divided into three groups: control, DM, and DM+WXKL groups. Rats in the DM+WXKL group were treated with daily gavage of WXKL at 3 g/kg. After eight weeks, echocardiography, hemodynamic examination, histology, electrophysiology study, mitochondrial respiratory function, and western blots were assessed. H₂O₂ treatment led to increased ROS and decreased intracellular MMP and mitochondrial oxygen consumption in primary atrial fibroblasts. WXKL improved the above changes. DM rats showed increased atrial fibrosis, greater left atrial diameter, lower atrial conduction velocity, higher conduction heterogeneity, higher AF inducibility, and lower mitochondrial protein expression, and all these abnormal changes except for left atrial diameter were improved in the DM+WXKL group. WXKL improves atrial remodeling by regulating mitochondrial function and homeostasis and reducing mitochondrial ROS in diabetic rats.

Role of NLRP3-Inflammasome/Caspase-1/Galectin-3 Pathway on Atrial Remodeling in Diabetic Rabbits

Both diabetes mellitus (DM) and atrial fibrillation (AF) are usually associated with enhanced inflammatory response. The effect of the "NACHT, LRR and PYD domain containing protein 3" (NLRP3)-inflammasome/caspase-1/galectin-3 pathway and the potential benefits of NLRP3-inflammasome inhibitor glibenclamide (GLB) on atrial remodeling in the DM state are still unknown. Here, we demonstrated that higher AF inducibility and conduction inhomogeneity, slower epicardial conduction velocity, and increased amount of fibrosis in diabetic rabbits as against normal ones were markedly reduced by GLB. Atrial caspase-1 activity as well as serum IL-1β and IL-18 levels were elevated in diabetic animals but suppressed by GLB. Moreover, GLB decreased the DM-induced protein expression enhancement of NLRP3, Gal-3, TGF-β1, and CaV1.2 according to western blot analysis. Summarily, our findings indicate that the NLRP3-inflammasome/caspase-1/Gal-3 signaling pathway is related to the pathogenesis of AF in the diabetic state. NLRP3-inflammasome inhibitor GLB prevents AF inducibility and moderates atrial structural remodeling in DM.

Empagliflozin, a sodium glucose co-transporter-2 inhibitor, alleviates atrial remodeling and improves mitochondrial function in high-fat diet/streptozotocin-induced diabetic rats

Background

Diabetes mellitus is an important risk factor for atrial fibrillation (AF) development. Sodium–glucose co-transporter-2 (SGLT-2) inhibitors are used for the treatment of type 2 diabetes mellitus (T2DM). Their cardioprotective effects have been reported but whether they prevent AF in T2DM patients are less well-explored. We tested the hypothesis that the SGLT-2 inhibitor, empagliflozin, can prevent atrial remodeling in a diabetic rat model.

Methods

High-fat diet and low-dose streptozotocin (STZ) treatment were used to induce T2DM. A total of 96 rats were randomized into the following four groups: (i) control (ii) T2DM, (iii) low-dose empagliflozin (10 mg/kg/day)/T2DM; and (iv) high-dose empagliflozin (30 mg/kg/day)/T2DM by the intragastric route for 8 weeks.

Results

Compared with the control group, left atrial diameter, interstitial fibrosis and the incidence of AF inducibility were significantly increased in the DM group. Moreover, atrial mitochondrial respiratory function, mitochondrial membrane potential, and mitochondrial biogenesis were impaired. Empagliflozin treatment significantly prevented the development of these abnormalities in DM rats, likely via the peroxisome proliferator-activated receptor-c coactivator 1α (PGC-1α)/nuclear respiratory factor-1 (NRF-1)/mitochondrial transcription factor A (Tfam) signaling pathway.

Conclusions

Empagliflozin can ameliorate atrial structural and electrical remodeling as well as improve mitochondrial function and mitochondrial biogenesis in T2DM, hence may be potentially used in the prevention of T2DM-related atrial fibrillation.

Metformin Regulates the Expression of SK2 and SK3 in the Atria of Rats With Type 2 Diabetes Mellitus Through the NOX4/p38MAPK Signaling Pathway

We previously found that metformin regulates the ion current conducted by the small conductance calcium-activated potassium channels (SK channels) in the atria of rats with type 2 diabetes mellitus (T2DM) as well as the mRNA and protein expression of the SK2 and SK3 subtypes of SK channels. In this study, we hypothesized that the nicotinamide adenine dinucleotide phosphate oxidase 4 (NOX4)/p38 mitogen-activated protein kinase (p38MAPK) signaling pathway was involved in the metformin-mediated regulation of SK2 and SK3 expression in the atria of rats with T2DM. We randomly divided Wistar rats into the control group, the untreated T2DM group, the metformin-treated group, the group receiving subcutaneous injections of the nicotinamide adenine dinucleotide phosphate oxidase (NOX) inhibitor diphenyleneiodonium (DPI), and the group receiving tail vein injections of the p38MAPK agonist anisomycin. Real-time polymerase chain reaction, Western blot, and immunohistochemistry were applied to examine the expression levels of SK2, SK3, NOX4, and phospho-p38MAPK (p-p38MAPK) mRNAs and proteins in the atrial tissue of relevant groups. We observed that the expression levels of NOX4 mRNA and protein and p-p38MAPK protein were significantly elevated in the atria of rats with T2DM compared with the control group. In addition, SK2 protein expression was reduced, whereas SK3 protein expression was increased. The 8-week treatment with metformin markedly reduced the expression levels of NOX4 mRNA and protein and p-p38MAPK protein, upregulated the SK2 expression, and downregulated the SK3 expression. Tail vein injection with anisomycin significantly increased the p-p38MAPK expression while further inhibiting the expression of SK2 and enhancing the expression of SK3. Subcutaneous injection with DPI considerably inhibited the expression of NOX4, further enhanced the expression of SK2 and suppressed the expression of SK3. In addition, subcutaneous injection with DPI significantly suppressed the phosphorylation of p38MAPK. In conclusion, the NOX4/p38MAPK signaling pathway mediates the downregulation of SK2 and the upregulation of SK3 in the atria of rats with T2DM. Long-term metformin treatment upregulates SK2 protein expression and downregulates SK3 protein expression by inhibiting the NOX4/p38MAPK signaling pathway.

Redox State in Atrial Fibrillation Pathogenesis and Relevant Therapeutic Approaches

BACKGROUND

Myocardial redox state is a critical determinant of atrial biology, regulating cardiomyocyte apoptosis, ion channel function, and cardiac hypertrophy/fibrosis and function. Nevertheless, it remains unclear whether the targeting of atrial redox state is a rational therapeutic strategy for atrial fibrillation prevention.

OBJECTIVE

To review the role of atrial redox state and anti-oxidant therapies in atrial fibrillation.

METHOD

Published literature in Medline was searched for experimental and clinical evidence linking myocardial redox state with atrial fibrillation pathogenesis as well as studies looking into the role of redoxtargeting therapies in the prevention of atrial fibrillation.

RESULTS

Data from animal models have shown that altered myocardial nitroso-redox balance and NADPH oxidases activity are causally involved in the pathogenesis of atrial fibrillation. Similarly experimental animal data supports that increased reactive oxygen / nitrogen species formation in the atrial tissue is associated with altered electrophysiological properties of atrial myocytes and electrical remodeling, favoring atrial fibrillation development. In humans, randomized clinical studies using redox-related therapeutic approaches (e.g. statins or antioxidant agents) have not documented any benefits in the prevention of atrial fibrillation development (mainly post-operative atrial fibrillation risk).

CONCLUSION

Despite strong experimental and translational data supporting the role of atrial redox state in atrial fibrillation pathogenesis, such mechanistic evidence has not been translated to clinical benefits in atrial fibrillation risk in randomized clinical studies using redox-related therapies.

Metformin regulates atrial SK2 and SK3 expression through inhibiting the PKC/ERK signaling pathway in type 2 diabetic rats

Background

Our previous study showed that metformin regulates the mRNA and protein levels of type 2 small conductance calcium-activated potassium channel (SK2) and type 3 small conductance calcium-activated potassium channels (SK3) in atrial tissue as well as the ion current of atrial myocytes in rats with type 2 diabetes mellitus (T2DM), but the underlying signaling mechanism is unknown. This study aimed to investigate whether metformin regulates atrial SK2 and SK3 protein expression in T2DM rats though the protein kinase C (PKC)/extracellular signal-regulated kinase (ERK) signaling pathway.

Methods

A T2DM rat model was established using a high-fat and high-sugar diet combined with a low-dose intraperitoneal injection of streptozotocin (STZ). The rats were randomly divided into the following five groups: the control group, the untreated T2DM group, the metformin-treated only group, the phorbol 12-myristate 13-acetate (PMA; a PKC agonist administered by intraperitoneal injection) treatment group, and the recombinant human epidermal growth factor (rh-EGF; an ERK agonist administered by tail vein injection) treatment group. The activity of PKC in atrial tissues was assayed by a PKC kinase activity assay kit. The protein expression of SK2, SK3, and phosphorylated ERK (pERK) were determined by western blotting and immunohistochemistry.

Results

Compared with the Control group, atrial PKC activity and pERK and SK3 protein expression were increased, while SK2 protein expression was decreased in atrial tissues of T2DM rats. Eight weeks of metformin treatment inhibited the PKC activity and pERK and SK3 expression, and elevated SK2 expression compared with the T2DM group. Compared with the metformin-treated only group, the injection of rh-EGF increased pERK and SK3 expression, and decreased SK2 expression; the injection of PMA increased PKC activity and SK3 expression, and decreased SK2 expression. In addition, the injection with PMA significantly elevated the expression of pERK.

Conclusions

The PKC/ERK signaling pathway is involved in the downregulation of SK2 expression and the upregulation of SK3 expression in the atrium of T2DM rats. Long-term metformin treatment prevents the SK2 downregulation and the SK3 upregulation through inhibiting the PKC/ERK signaling pathway.

Electronic supplementary material

The online version of this article (10.1186/s12872-018-0950-x) contains supplementary material, which is available to authorized users.

Ursolic acid ameliorates CCl4-induced liver fibrosis through the NOXs/ROS pathway

Liver fibrosis is a reversible wound-healing response that occurs after liver injury. NADPH oxidases (NOXs) and reactive oxygen species (ROS) which are expressed in hepatocytes (HCs), hepatic stellate cells (HSCs), and Kupffer cells (KCs) play an important role in the development of hepatic fibrosis. In in vitro studies, we had shown that ursolic acid (UA) could reverse liver fibrosis by inhibiting the activation of NOX-mediated fibrotic signaling networks in HSCs. In this study, we verified that UA could alleviate CCl4-induced liver fibrosis by reducing the expression of NOXs/ROS in HCs, HSCs, KCs. Meanwhile, the phagocytic index α and clearance index K which represent phagocytosis of KCs were unchanged. Together, all our data demonstrated that UA induced the proliferation of HCs, promoted apoptosis in HSCs, and prevented activation of KCs in vivo by reducing the expression of NOXs/ROS in HCs, HSCs, KCs. Besides, UA had no effect on the host defense function.

Oxidative stress and atrial fibrillation: an update

Atrial remodelling involves electrophysiological and structural abnormalities that promote the development and perpetuation of atrial fibrillation. Experimental and clinical data indicate that oxidative stress is implicated in the pathophysiology of atrial remodelling. The mechanistic links between atrial remodelling and oxidative stress are complex with several underlying diseases and conditions may affect these pathways. Therefore, the development of antioxidant interventions in this setting remains difficult. Besides classical antioxidant compounds, several agents with pleiotropic effects, including anti-inflammatory and antioxidant, have been tested in experimental and clinical settings with variable results. Strategies applying conventional antioxidants in specific situations such as postoperative atrial fibrillation show beneficial effects, especially the two-step regimen of antioxidants combination. Of note, there are limited data on the development of strategies that target specific sources of reactive oxygen species implicated in atrial remodelling. Lifestyle, diet, and risk factors modification is a complementary promising approach. This updated review provides a concise and critical overview of all available data regarding oxidative stress and its modulation in atrial fibrillation. Future directions on this exciting field are also discussed.

Xanthine Oxidase Inhibitor Allopurinol Prevents Oxidative Stress-Mediated Atrial Remodeling in Alloxan-Induced Diabetes Mellitus Rabbits

Background

There are several mechanisms, including inflammation, oxidative stress and abnormal calcium homeostasis, involved in the pathogenesis of atrial fibrillation. In diabetes mellitus (DM), increased oxidative stress may be attributable to higher xanthine oxidase activity. In this study, we examined the relationship between oxidative stress and atrial electrical and structural remodeling, and calcium handling abnormalities, and the potential beneficial effects of the xanthine oxidase inhibitor allopurinol upon these pathological changes.

Methods and Results

Ninety rabbits were randomly and equally divided into 3 groups: control, DM, and allopurinol‐treated DM group. Echocardiographic and hemodynamic assessments were performed in vivo. Serum and tissue markers of oxidative stress and atrial fibrosis, including the protein expression were examined. Atrial interstitial fibrosis was evaluated by Masson trichrome staining. I_CaL was measured from isolated left atrial cardiomyocytes using voltage‐clamp techniques. Confocal microscopy was used to detect intracellular calcium transients. The Ca²⁺ handling protein expression was analyzed by Western blotting. Mitochondrial‐related proteins were analyzed as markers of mitochondrial function. Compared with the control group, rabbits with DM showed left ventricular hypertrophy, increased atrial interstitial fibrosis, oxidative stress and fibrosis markers, I_CaL and intracellular calcium transient, and atrial fibrillation inducibility. These abnormalities were alleviated by allopurinol treatment.

Conclusions

Allopurinol, via its antioxidant effects, reduces atrial mechanical, structural, ion channel remodeling and mitochondrial synthesis abnormalities induced by DM‐related increases in oxidative stress.

Apocynin attenuates left ventricular remodeling in diabetic rabbits

INTRODUCTION

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidases are responsible for the generation of reactive oxygen species, producing vascular and myocardial dysfunction in diabetes mellitus. However, the potential benefits of the NADPH oxidase inhibitor, apocynin, on left ventricular (LV) remodeling remain unknown.

RESULTS

In the diabetic group, interventricular septal thickness and left ventricular posterior wall thickness were markedly increased compared to control. These changes were accompanied by increased LV cardiomyocyte cross-sectional area and greater degree of interstitial fibrosis. NO, myeloperoxidase, and malonaldehyde levels in the serum were significantly increased Moreover, protein expression levels of rac1, nuclear factor-κB, transforming growth factor-β, p38, P-p38, and metalloproteinase-9 were also raised. Apocynin treatment prevented all of these structural, histological and biochemical changes and additionally increased superoxide dismutase levels.

METHODS

Thirty Japanese rabbits were randomized into three groups: control, alloxan-induced diabetes with and without apocynin treatment at 15 mg/kg/day for 8 weeks (n = 10 for each group). Echocardiography was performed and hemodynamics were assessed by carotid and LV catheterization. LV cardiomyocyte cross-sectional area and interstitial fibrosis were evaluated by histology. Serum nitric oxide (NO), malonaldehyde, myeloperoxidase, superoxide dismutase (SOD) levels, and activity of LV tissue NADPH oxidases was assessed. Expression of proteins involved in pro-inflammatory and pro-fibrotic signaling were determined by Western blotting.

CONCLUSIONS

Inhibition of NADPH oxidase using apocynin is an effective upstream therapy for preventing diabetes-induced adverse remodeling of the left ventricular myocardium.

Reactive oxygen species mediated oxidative stress links diabetes and atrial fibrillation

Diabetes is an independent risk factor for atrial fibrillation (AF); however, the underlying mechanism linking diabetes and AF remains to be clarified. The present study aimed to explore the molecular mechanism of increased reactive oxygen species (ROS) production in AF and the ROS-mediated downstream events in diabetes. Firstly, the atrial fibroblasts were isolated from the left atrium of rabbits using enzyme digestion and differential adhesion. Then, the isolated cells were identified by morphology analysis under a microscope, collagen distribution using Masson trichrome staining and vimentin by immunofluorescence. Following this, the collected atrial fibroblasts were randomly divided into 7 groups and administered with high glucose (25 mM glucose), H₂O₂ stimulation (100 nmol/l), glucose + apocynin (100 µg/ml), H₂O₂ + apocynin, glucose + H₂O₂, and a combination of glucose, apocynin and H₂O₂, as well as the negative control (NC). An MTS assay was performed to investigate cell proliferation following the different treatments, and western blotting was conducted to explore the expression of several proteins including NAD(P)H oxidative (NOX) subunits, key factors involved in mitogen-activated protein kinase (MAPK) signaling pathways and matrix metalloproteinases (MMPs). The atrial fibroblasts were spindle-shaped with one or more protuberances. Vimentin was positively expressed in collected cells under confocal laser scanning microscopy. This result indicated that the atrial fibroblasts were successfully prepared. High glucose and H₂O₂ stimulation significantly increased the proliferation of atrial fibroblasts and apocynin markedly attenuated the promoting effects on cell proliferation induced by high glucose and H₂O₂ treatment (P<0.05). Additionally, high glucose and H₂O₂ stimulation increased the expression of Rac1, phospho(p)-c-Jun N-terminal kinase 1, p38, p-p38 and MMP9, which was markedly decreased by the addition of apocynin (P<0.05). The mechanism associated with diabetes and AF may be attributed to oxidative stress (ROS production) derived from NOX activity, and then induced activation of the MAPK signaling pathways and MMP9 expression.

The more effective treatment of atrial fibrillation applying the natural compounds; as NADPH oxidase and ion channel inhibitors

Atrial fibrillation (AF) is the most common cardiac arrhythmia that occurs because of several different risk factors, e.g., valvular heart disease, coronary artery disease, age ≥75 years, hypertension and diabetes mellitus. One key risk factor that results in AF, is oxidative stress. Evidence suggests that there is a correlation between oxidative processes and the genesis of AF. Oxidative stress occurs when the generation of reactive oxygen species (ROS) increase due to excessive activity of enzymes including NADPH oxidase (NOX) and xanthine oxidase; or its degradation decrease by dysfunctional antioxidant enzyme systems, such as superoxide dismutase (SOD), catalase and glutathione peroxidase (GPx). Afterwards, elevated ROS may shift ion channel activity to increase AF susceptibility. The outbreak of AF continues to grow. Unfortunately, current treatment strategies may have limited efficacy or adverse effects. On the other hand, the inhibition of ROS formation and alteration of ion channel activity could be important therapeutic targets for prevention or treatments of AF. Additionally, many studies have been shown that several natural compounds have the ability to inhibit NADPH oxidases directly. This review focuses on natural compounds which specially inhibit NOX isoforms and have direct effects on ion channels, suggesting these compounds can be helpful in AF treatment.

Alogliptin, a Dipeptidyl Peptidase-4 Inhibitor, Alleviates Atrial Remodeling and Improves Mitochondrial Function and Biogenesis in Diabetic Rabbits

Background

There is increasing evidence implicating atrial mitochondrial dysfunction in the pathogenesis of atrial fibrillation. In this study, we explored whether alogliptin, a dipeptidyl peptidase‐4 inhibitor, can prevent mitochondrial dysfunction and atrial remodeling in a diabetic rabbit model.

Methods and Results

A total of 90 rabbits were randomized into 3 groups as follows: control group (n=30), alloxan‐induced diabetes mellitus group (n=30), and alogliptin‐treated (12.5 mg/kg per day for 8 weeks) diabetes mellitus group (n=30). Echocardiographic and hemodynamic assessments were performed in vivo. The serum concentrations of glucagon‐like peptide‐1, insulin, and inflammatory and oxidative stress markers were measured. Electrophysiological properties of Langendorff‐perfused rabbit hearts were assessed. Mitochondrial morphology, respiratory function, membrane potential, and reactive oxygen species generation rate were assessed. The protein expression of transforming growth factor β1, nuclear factor κB p65, and mitochondrial biogenesis–related proteins were measured by Western blot analysis. Diabetic rabbits exhibited left ventricular hypertrophy and left atrial dilation without obvious hemodynamic abnormalities, and all of these changes were attenuated by alogliptin. Compared with the control group, higher atrial fibrillation inducibility in the diabetes mellitus group was observed, and markedly reduced by alogliptin. Alogliptin decreased mitochondrial reactive oxygen species production rate, prevented mitochondrial membrane depolarization, and alleviated mitochondrial swelling in diabetic rabbits. It also improved mitochondrial biogenesis by peroxisome proliferator–activated receptor‐γ coactivator 1α/nuclear respiratory factor‐1/mitochondrial transcription factor A signaling regulated by adiponectin/AMP‐activated protein kinase.

Conclusions

Dipeptidyl peptidase‐4 inhibitors can prevent atrial fibrillation by reversing electrophysiological abnormalities, improving mitochondrial function, and promoting mitochondrial biogenesis.