Oxidative stress plays a pivotal role in chronic heart failure. SIRT1, an NAD ؉ -dependent histone/protein deacetylase, promotes cell survival under oxidative stress when it is expressed in the nucleus. However, adult cardiomyocytes predominantly express SIRT1 in the cytoplasm, and its function has not been elucidated. The purpose of this study was to investigate the functional role of SIRT1 in the heart and the potential use of SIRT1 in therapy for heart failure. We investigated the subcellular localization of SIRT1 in cardiomyocytes and its impact on cell survival. SIRT1 accumulated in the nucleus of cardiomyocytes in the failing hearts of TO-2 hamsters, postmyocardial infarction rats, and a dilated cardiomyopathy patient but not in control healthy hearts. Nuclear but not cytoplasmic SIRT1-induced manganese superoxide dismutase (Mn-SOD), which was further enhanced by resveratrol, and increased the resistance of C2C12 myoblasts to oxidative stress. Resveratrol's enhancement of Mn-SOD levels depended on the level of nuclear SIRT1, and it suppressed the cell death induced by antimycin A or angiotensin II. The cell-protective effects of nuclear SIRT1 or resveratrol were canceled by the Mn-SOD small interfering RNA or SIRT1 small interfering RNA. The oral administration of resveratrol to TO-2 hamsters increased Mn-SOD levels in cardiomyocytes, suppressed fibrosis, preserved cardiac function, and significantly improved survival. Thus, Mn-SOD induced by resveratrol via nuclear SIRT1 reduced oxidative stress and participated in cardiomyocyte protection. SIRT1 activators such as resveratrol could be novel therapeutic tools for the treatment of chronic heart failure.Heart failure arises as a consequence of various heart diseases, including myocardial infarction, hypertension, and idiopathic dilated cardiomyopathy (DCM).2 The death of cardiomyocytes and the consequent maladaptive changes in the remaining myocytes and extracellular matrix induce the clinical manifestation of heart failure (1). Over the past 20 years, the arsenal of treatments available for heart failure has increased considerably, with the introduction of ␤ blockers, angiotensin-converting enzyme inhibitors, angiotensin II type 1 receptor blockers, aldosterone antagonists, and nonpharmacological therapies including cardiac resynchronization therapy (2). However, even with the very best current therapy, the annual mortality rate among patients with heart failure is still ϳ10% (3).The mitochondrial electron transport chain is the main source of reactive oxygen species (ROS) in most cells (4). Hearts consume large amounts of O 2 and yield high levels of ROS (5). Various factors, including angiotensin II and tumor necrosis factor-␣, also induce ROS formation, leading to cardiomyocyte death and heart failure (5). Superoxide dismutase (SOD) has a pivotal role in the detoxification of ROS. SOD catalyzes the dismutation of superoxide into oxygen and hydrogen peroxide, which in turn is reduced to water by catalase and glutathione peroxidase (5). Three isoforms of ...

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OBJECTIVEAlteration in endoplasmic reticulum (ER) stress in diabetic hearts and its effect on cytoprotective signaling are unclear. Here, we examine the hypothesis that ER stress in diabetic hearts impairs phospho–glycogen synthase kinase (GSK)-3β–mediated suppression of mitochondrial permeability transition pore (mPTP) opening, compromising myocardial response to cytoprotective signaling.RESEARCH DESIGN AND METHODSA rat model of type 2 diabetes (OLETF) and its control (LETO) were treated with tauroursodeoxycholic acid (TUDCA) (100 mg · kg−1 · day−1 for 7 days), an ER stress modulator. Infarction was induced by 20-min coronary occlusion and 2-h reperfusion.RESULTSLevels of ER chaperones (GRP78 and GRP94) in the myocardium and level of nonphoshopho–GSK-3β in the mitochondria were significantly higher in OLETF than in LETO rats. TUDCA normalized levels of GRP78 and GRP94 and mitochondrial GSK-3β in OLETF rats. Administration of erythropoietin (EPO) induced phosphorylation of Akt and GSK-3β and reduced infarct size (% risk area) from 47.4 ± 5.2% to 23.9 ± 3.5% in LETO hearts. However, neither phosphorylation of Akt and GSK-3β nor infarct size limitation was induced by EPO in OLETF rats. The threshold for mPTP opening was significantly lower in mitochondria from EPO-treated OLETF rats than in those from EPO-treated LETO rats. TUDCA restored responses of GSK-3β, mPTP opening threshold, and infarct size to EPO receptor activation in OLETF rats. There was a significant correlation between mPTP opening threshold and phospho–GSK-3β–to–total GSK-3β ratio in the mitochondrial fraction.CONCLUSIONSDisruption of protective signals leading to GSK-3β phosphorylation and increase in mitochondrial GSK-3β are dual mechanisms by which increased ER stress inhibits EPO-induced suppression of mPTP opening and cardioprotection in diabetic hearts.

The aim of the present study was to examine the hypothesis that acceleration of gap junction (GJ) closure during ischemia contributes to anti-infarct tolerance afforded by preconditioning (PC). First, the effects of PC on GJ communication during ischemia were assessed. Isolated buffer-perfused rabbit hearts were subjected to 5-min global ischemia with or without PC with two cycles of 5-min ischemia/5-min reperfusion or a GJ blocker (2 mM heptanol), and then the tissue excised from the ischemic region was incubated in anoxic buffer containing lucifer yellow (LY; 2.5 mg/ml), a tracer of GJ permeability, for 20 min at 37 degrees C. PC and heptanol significantly reduced the area to which LY was transported in the ischemic myocardium by 39% and by 54%, respectively. In the second series of experiments, three GJ blockers (heptanol, 18beta-glycyrrhetinic acid, and 2,3-butanedione monoxime) infused after the onset of ischemia reduced infarct size after 30-min ischemia/2-h reperfusion to an extent equivalent to that in the case of PC. In the third series of experiments, Western blotting for connexin43 (Cx43) showed that PC shortened the time to the onset of ischemia-induced Cx43 dephosphorylation but reduced the extent of Cx43 dephosphorylation during a 30-min period of ischemia. Calphostin C, a protein kinase C (PKC) inhibitor, abolished preservation of phosphorylated Cx43 but not the early onset of Cx43 dephosphorylation after ischemia in the preconditioned myocardium. These results suggest that PC-induced reduction of GJ permeability during ischemia, presumably by PKC-mediated Cx43 phosphorylation, contributes to infarct size limitation.

To explore mechanisms by which SGLT2 inhibitors protect diabetic hearts from heart failure, we examined the effect of empagliflozin (Empa) on the ultrastructure of cardiomyocytes in the noninfarcted region of the diabetic heart after myocardial infarction (MI). OLETF, a rat model of type 2 diabetes, and its nondiabetic control, LETO, received a sham operation or left coronary artery ligation 12 h before tissue sampling. Tissues were sampled from the posterior ventricle (i.e., the remote noninfarcted region in rats with MI). The number of mitochondria was larger and small mitochondria were more prevalent in OLETF than in LETO. Fis1 expression level was higher in OLETF than in LETO, while phospho‐Ser637‐Drp1, total Drp1, Mfn1/2, and OPA1 levels were comparable. MI further reduced the size of mitochondria with increased Drp1‐Ser616 phosphorylation in OLETF. The number of autophagic vacuoles was unchanged after MI in LETO but was decreased in OLETF. Lipid droplets in cardiomyocytes and tissue triglycerides were increased in OLETF. Empa administration (10 mg/kg per day) reduced blood glucose and triglycerides and paradoxically increased lipid droplets in cardiomyocytes in OLETF. Empa suppressed Fis1 upregulation, increased Bnip3 expression, and prevented reduction in both mitochondrial size and autophagic vacuole number after MI in OLETF. Together with the results of our parallel study showing upregulation of SOD2 and catalase by Empa, the results indicate that Empa normalizes the size and number of mitochondria in diabetic hearts and that diabetes‐induced excessive reduction in mitochondrial size after MI was prevented by Empa via suppression of ROS and restoration of autophagy.