Augmentation of Histone Deacetylase 6 Activity Impairs Mitochondrial Respiratory Complex I in Ischemic/Reperfused Diabetic Hearts

BACKGROUND Diabetes augments activity of histone deacetylase 6 (HDAC6) and generation of tumor necrosis factor α (TNFα) and impairs the physiological function of mitochondrial complex I (mCI) which oxidizes reduced nicotinamide adenine dinucleotide (NADH) to nicotinamide adenine dinucleotide to sustain the tricarboxylic acid cycle and β-oxidation. Here we examined how HDAC6 regulates TNFα production, mCI activity, mitochondrial morphology and NADH levels, and cardiac function in ischemic/reperfused diabetic hearts. METHODS HDAC6 knockout, streptozotocin-induced type 1 diabetic, and obese type 2 diabetic db/db mice underwent myocardial ischemia/reperfusion injury in vivo or ex vivo in a Langendorff-perfused system. H9c2 cardiomyocytes with and without HDAC6 knockdown were subjected to hypoxia/reoxygenation injury in the presence of high glucose. We compared the activities of HDAC6 and mCI, TNFα and mitochondrial NADH levels, mitochondrial morphology, myocardial infarct size, and cardiac function between groups. RESULTS Myocardial ischemia/reperfusion injury and diabetes synergistically augmented myocardial HDCA6 activity, myocardial TNFα levels, and mitochondrial fission and inhibited mCI activity. Interestingly, neutralization of TNFα with an anti-TNFα monoclonal antibody augmented myocardial mCI activity. Importantly, genetic disruption or inhibition of HDAC6 with tubastatin A decreased TNFα levels, mitochondrial fission, and myocardial mitochondrial NADH levels in ischemic/reperfused diabetic mice, concomitant with augmented mCI activity, decreased infarct size, and ameliorated cardiac dysfunction. In H9c2 cardiomyocytes cultured in high glucose, hypoxia/reoxygenation augmented HDAC6 activity and TNFα levels and decreased mCI activity. These negative effects were blocked by HDAC6 knockdown. CONCLUSIONS Augmenting HDAC6 activity inhibits mCI activity by increasing TNFα levels in ischemic/reperfused diabetic hearts. The HDAC6 inhibitor, tubastatin A, has high therapeutic potential for acute myocardial infarction in diabetes. What Is Known? 1. Ischemic heart disease (IHS) is a leading cause of death globally, and its presence in diabetic patients is a grievous combination, leading to high mortality and heart failure. 2. Diabetes impairs assembly of mitochondrial complex I (mCI), complex III dimer, and complex IV monomer into the respiratory chain supercomplexes, resulting in electron leak and the formation of reactive oxygen species (ROS). 3. By oxidizing reduced nicotinamide adenine dinucleotide (NADH) and reducing ubiquinone, mCI physiologically regenerates NAD+ to sustain the tricarboxylic acid cycle and β-oxidation. What New Information Does This Article Contribute? 1. Myocardial ischemia/reperfusion injury (MIRI) and diabetes as comorbidities augment myocardial HDCA6 activity and generation of tumor necrosis factor α (TNFα), which inhibit myocardial mCI activity. 2. Genetic disruption of histone deacetylase 6 (HDAC6) decreases mitochondrial NADH levels and augments mCI activity in type 1 diabetic mice undergoing MIRI via decreasing TNFα production, leading to decreases in MIRI. 3. Pretreatment of type 2 diabetic db/db mice with a HDAC6 inhibitor, tubastatin A (TSA), decreases mitochondrial NADH levels and augments mCI activity by decreasing TNFα levels, leading to improvements in cardiac function. What New Information Does This Article Contribute? Patients with diabetes are more susceptible to MIRI than non-diabetics with greater mortality and resultant heart failure. There is an unmet medical need in diabetic patients for the treatment of IHS. Our biochemical studies find that MIRI and diabetes synergistically augment myocardial HDAC6 activity and generation of TNFα, along with cardiac mitochondrial fission and low bioactivity of mCI. Intriguingly, genetic disruption of HDAC6 decreases the MIRI-induced increases in TNFα levels, concomitant with augmented mCI activity, decreased myocardial infarct size, and ameliorated cardiac dysfunction in T1D mice. Importantly, treatment of obese T2D db/db mice with TSA reduces the generation of TNFα and mitochondrial fission and enhances mCI activity during reperfusion after ischemia. Our isolated heart studies revealed that genetic disruption or pharmacological inhibition of HDAC6 reduces mitochondrial NADH release during ischemia and ameliorates dysfunction of diabetic hearts undergoing MIRI. Furthermore, HDAC6 knockdown in cardiomyocytes blocks high glucose- and exogenous TNFα-induced suppression of mCI activity in vitro , implying that HDAC6 knockdown can preserve mCI activity in high glucose and hypoxia/reoxygenation. These results demonstrate that HDAC6 is an important mediator in MIRI and cardiac function in diabetes. Selective inhibition of HDAC6 has high therapeutic potential for acute IHS in diabetes. ### Competing Interest Statement The authors have declared no competing interest. * Cox I : mitochondrial cytochrome c oxidase I Ctrl : control DMEM : Dulbecco’s modified Eagle’s medium DMSO : dimethylsulfoxide +dP/dt : maximum rate of increase of left ventricular developed pressure -dP/dt : maximum rate of decrease of left ventricular developed pressure EDTA : ethylenediaminetetraacetic acid ELISA : enzyme-linked immunosorbent assay FBS : fetal bovine serum GAPDH : glyceraldehyde-3-phosphate dehydrogenase HG : high glucose HRI : hypoxia/reoxygenation injury IHS : ischemic heart disease KDEL : Lys-Asp-Glu-Leu KO : knockout HDAC : histone deacetylase LV : left ventricle LVDP : left ventricular developed pressure LVEDP : left ventricular end-diastolic pressure MIRI : myocardial ischemia/reperfusion injury mCI : mitochondrial complex I