Mikihiko Nakaoka

Kyoto Prefectural University of Medicine, Kioto, Kyōto, Japan

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Publications (4)18.13 Total impact

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    ABSTRACT: Alpha-adrenergic agonists is known to be protective in cardiac myocytes from apoptosis induced by beta-adrenergic stimulation. Although there has been a recent focus on the role of cardiac autophagy in heart failure, its role in heart failure with adrenergic overload has not yet been elucidated. In the present study, we investigated the contribution of autophagy to cardiac failure during adrenergic overload both in vitro and in vivo. Neonatal rat cardiac myocytes overexpressing GFP-tagged LC3 were prepared and stimulated with the alpha1-adrenergic agonist, phenylephrine (PE), the beta-adrenergic agonist, isoproterenol (ISO), or norepinephrine (NE) in order to track changes in the formation of autophagosomes in vitro. All adrenergic stimulators increased cardiac autophagy by stimulating autophagic flux. Blocking autophagy by the knockdown of autophagy-related 5 (ATG5) exacerbated ISO-induced apoptosis and negated the anti-apoptotic effects of PE, which indicated the cardioprotective role of autophagy during adrenergic overload. PE-induced cardiac autophagy was mediated by the PI3-kinase/Akt pathway, but not by MEK/ERK, whereas both pathways mediated the anti-apoptotic effects of PE. Knock down of Akt1 was the most essential among the three Akt family members examined for the induction of cardiac autophagy. The four-week administration of PE kept the high level of cardiac autophagy without heart failure in vivo, whereas autophagy levels in a myocardium impaired by four-week persistent administration of ISO or NE were the same with the control state. These present study indicated that cardiac autophagy played a protective role during adrenergic overload and also that the Akt pathway could mediate cardiac autophagy for the anti-apoptotic effects of the alpha-adrenergic pathway. Copyright © 2014. Published by Elsevier Inc.
    Biochemical and Biophysical Research Communications 11/2014; · 2.28 Impact Factor
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    ABSTRACT: Diabetic cardiomyopathy is characterized by energetic dysregulation caused by glucotoxicity, lipotoxicity, and mitochondrial alterations. p53 and its downstream mitochondrial assembly protein, synthesis of cytochrome c oxidase 2 (SCO2), are important regulators of mitochondrial respiration, whereas the involvement in diabetic cardiomyopathy remains to be determined. The role of p53 and SCO2 in energy metabolism was examined in both type I (streptozotocin [STZ] administration) and type II diabetic (db/db) mice. Cardiac expressions of p53 and SCO2 in 4-week STZ diabetic mice were upregulated (185% and 152% versus controls, respectively, P<0.01), with a marked decrease in cardiac performance. Mitochondrial oxygen consumption was increased (136% versus control, P<0.01) in parallel with augmentation of mitochondrial cytochrome c oxidase (complex IV) activity. Reactive oxygen species (ROS)-damaged myocytes and lipid accumulation were increased in association with membrane-localization of fatty acid translocase protein FAT/CD36. Antioxidant tempol reduced the increased expressions of p53 and SCO2 in STZ-diabetic hearts and normalized alterations in mitochondrial oxygen consumption, lipid accumulation, and cardiac dysfunction. Similar results were observed in db/db mice, whereas in p53-deficient or SCO2-deficient diabetic mice, the cardiac and metabolic abnormalities were prevented. Overexpression of SCO2 in cardiac myocytes increased mitochondrial ROS and fatty acid accumulation, whereas knockdown of SCO2 ameliorated them. Myocardial p53/SCO2 signal is activated by diabetes-mediated ROS generation to increase mitochondrial oxygen consumption, resulting in excessive generation of mitochondria-derived ROS and lipid accumulation in association with cardiac dysfunction.
    Circulation Heart Failure 11/2011; 5(1):106-15. · 6.68 Impact Factor
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    ABSTRACT: Inhibition of tumor suppressor p53 is cardioprotective against ischemic injury and provides resistance to subsequent cardiac remodeling. We investigated p53-mediated expansion of ischemic damage with a focus on mitochondrial integrity in association with autophagy and apoptosis. p53(-/-) heart showed that autophagic flux was promoted under ischemia without a change in cardiac tissue ATP content. Electron micrographs revealed that ischemic border zone in p53(-/-) mice had 5-fold greater numbers of autophagic vacuoles containing mitochondria, indicating the occurrence of mitophagy, with an apparent reduction of abnormal mitochondria compared with those in WT mice. Analysis of autophagic mediators acting downstream of p53 revealed that TIGAR (TP53-induced glycolysis and apoptosis regulator) was exclusively up-regulated in ischemic myocardium. TIGAR(-/-) mice exhibited the promotion of mitophagy followed by decrease of abnormal mitochondria and resistance to ischemic injury, consistent with the phenotype of p53(-/-) mice. In p53(-/-) and TIGAR(-/-) ischemic myocardium, ROS production was elevated and followed by Bnip3 activation which is an initiator of mitophagy. Furthermore, the activation of Bnip3 and mitophagy due to p53/TIGAR inhibition were reversed with antioxidant N-acetyl-cysteine, indicating that this adaptive response requires ROS signal. Inhibition of mitophagy using chloroquine in p53(-/-) or TIGAR(-/-) mice exacerbated accumulation of damaged mitochondria to the level of wild-type mice and attenuated cardioprotective action. These findings indicate that p53/TIGAR-mediated inhibition of myocyte mitophagy is responsible for impairment of mitochondrial integrity and subsequent apoptosis, the process of which is closely involved in p53-mediated ventricular remodeling after myocardial infarction.
    Journal of Molecular and Cellular Cardiology 10/2011; 52(1):175-84. · 5.15 Impact Factor
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    ABSTRACT: Bioenergetic homeostasis is altered in heart failure and may play an important role in pathogenesis. p53 has been implicated in heart failure, and although its role in regulating tumorigenesis is well characterized, its activities on cellular metabolism are just beginning to be understood. We investigated the role of p53 and its transcriptional target gene TP53-induced glycolysis and apoptosis regulator (TIGAR) in myocardial energy metabolism under conditions simulating ischemia that can lead to heart failure. Expression of p53 and TIGAR was markedly upregulated after myocardial infarction, and apoptotic myocytes were decreased by 42% in p53-deficient mouse hearts compared with those in wild-type mice. To examine the effect of p53 on energy metabolism, cardiac myocytes were exposed to hypoxia. Hypoxia induced p53 and TIGAR expression in a p53-dependent manner. Knockdown of p53 or TIGAR increased glycolysis with elevated fructose-2,6-bisphosphate levels and reduced myocyte apoptosis. Hypoxic stress decreased phosphocreatine content and the mitochondrial membrane potential of myocytes without changes in ATP content, the effects of which were prevented by the knockdown of TIGAR. Inhibition of glycolysis by 2-deoxyglucose blocked these bioenergetic effects and TIGAR siRNA-mediated prevention of apoptosis, and, in contrast, overexpression of TIGAR reduced glucose utilization and increased apoptosis. Our data demonstrate that p53 and TIGAR inhibit glycolysis in hypoxic myocytes and that inhibition of glycolysis is closely involved in apoptosis, suggesting that p53 and TIGAR are significant mediators of cellular energy homeostasis and cell death under ischemic stress.
    AJP Heart and Circulatory Physiology 10/2010; 299(6):H1908-16. · 4.01 Impact Factor