Anne D. Hafstad

Novel aspects of ROS signalling in heart failure

Heart failure and many of the conditions that predispose to heart failure are associated with oxidative stress. This is considered to be important in the pathophysiology of the condition but clinical trials of antioxidant approaches to prevent cardiovascular morbidity and mortality have been unsuccessful. Part of the reason for this may be the failure to appreciate the complexity of the effects of reactive oxygen species. At one extreme, excessive oxidative stress damages membranes, proteins and DNA but lower levels of reactive oxygen species may exert much more subtle and specific regulatory effects (termed redox signalling), even on physiological signalling pathways. In this article, we review our current understanding of the roles of such redox signalling pathways in the pathophysiology of heart failure, including effects on cardiomyocyte hypertrophy signalling, excitation–contraction coupling, arrhythmia, cell viability and energetics. Reactive oxygen species generated by NADPH oxidase proteins appear to be especially important in redox signalling. The delineation of specific redox-sensitive pathways and mechanisms that contribute to different components of the failing heart phenotype may facilitate the development of newer targeted therapies as opposed to the failed general antioxidant approaches of the past.

High intensity interval training alters substrate utilization and reduces oxygen consumption in the heart

AIMS although exercise training induces hypertrophy with improved contractile function, the effect of exercise on myocardial substrate metabolism and cardiac efficiency is less clear. High intensity training has been shown to produce more profound effects on cardiovascular function and aerobic capacity than isocaloric low and moderate intensity training. The aim of the present study was to explore metabolic and mechanoenergetic changes in the heart following endurance exercise training of both high and moderate intensity. METHODS AND RESULTS C57BL/6J mice were subjected to 10 wk treadmill running, either high intensity interval training (HIT) or distance-matched moderate intensity training (MIT), where HIT led to a pronounced increase in maximal oxygen uptake. Although both modes of exercise were associated with a 10% increase in heart weight-to-body weight ratio, only HIT altered cardiac substrate utilization, as revealed by a 36% increase in glucose oxidation and a concomitant reduction in fatty acid oxidation. HIT also improved cardiac efficiency by decreasing work-independent myocardial oxygen consumption. In addition, it increased cardiac maximal mitochondrial respiratory capacity. CONCLUSION This study shows that high intensity training is required for induction of changes in cardiac substrate utilization and energetics, which may contribute to the superior effects of high compared with moderate intensity training in terms of increasing aerobic capacity.

Rosiglitazone treatment improves cardiac efficiency in hearts from diabetic mice

Abstract Isolated perfused hearts from type 2 diabetic (db/db) mice show impaired ventricular function, as well as altered cardiac metabolism. Assessment of the relationship between myocardial oxygen consumption (MVO2) and ventricular pressure-volume area (PVA) has also demonstrated reduced cardiac efficiency in db/db hearts. We hypothesized that lowering the plasma fatty acid supply and subsequent normalization of altered cardiac metabolism by chronic treatment with a peroxisome proliferator-activated receptor-γ (PPARγ) agonist will improve cardiac efficiency in db/db hearts. Rosiglitazone (23 mg/kg body weight/day) was administered as a food admixture to db/db mice for five weeks. Ventricular function and PVA were assessed using a miniaturized (1.4 Fr) pressure-volume catheter; MVO2 was measured using a fibre-optic oxygen sensor. Chronic rosiglitazone treatment of db/db mice normalized plasma glucose and lipid concentrations, restored rates of cardiac glucose and fatty acid oxidation, and improved cardiac efficiency. The improved cardiac efficiency was due to a significant decrease in unloaded MVO2, while contractile efficiency was unchanged. Rosiglitazone treatment also improved functional recovery after low-flow ischemia. In conclusion, the present study demonstrates that in vivo PPARγ-treatment restores cardiac efficiency and improves ventricular function in perfused hearts from type 2 diabetic mice.

Cardiac peroxisome proliferator-activated receptor-α activation causes increased fatty acid oxidation, reducing efficiency and post-ischaemic functional loss

AIMS Myocardial fatty acid (FA) oxidation is regulated acutely by the FA supply and chronically at the transcriptional level owing to FA activation of peroxisome proliferator-activated receptor-alpha (PPARalpha). However, in vivo administration of PPARalpha ligands has not been shown to increase cardiac FA oxidation. In this study we have examined the cardiac response to in vivo administration of tetradecylthioacetic acid (TTA, 0.5% w/w added to the diet for 8 days), a PPAR agonist with primarily PPARalpha activity. METHODS AND RESULTS Despite the fact that TTA treatment decreased plasma concentrations of lipids [FA and triacylglycerols (TG)], hearts from TTA-treated mice showed increased mRNA expression of PPARalpha target genes. Cardiac substrate utilization, ventricular function, cardiac efficiency, and susceptibility to ischaemia-reperfusion were examined in isolated perfused hearts. In accordance with the mRNA changes, myocardial FA oxidation was increased 2.5-fold with a concomitant reduction in glucose oxidation. This increase in FA oxidation was abolished in PPARalpha-null mice. Thus, it appears that the metabolic effects of TTA on the heart must be owing to a direct stimulatory effect on cardiac PPARalpha. Hearts from TTA-treated mice also showed a marked reduction in cardiac efficiency (because of a two-fold increase in unloaded myocardial oxygen consumption) and decreased recovery of ventricular contractile function following low-flow ischaemia. CONCLUSION This study for the first time observed that in vivo administration of a synthetic PPARalpha ligand elevated FA oxidation, an effect that was also associated with decreased cardiac efficiency and reduced post-ischaemic functional recovery.

High- and Moderate-Intensity Training Normalizes Ventricular Function and Mechanoenergetics in Mice With Diet-Induced Obesity

Although exercise reduces several cardiovascular risk factors associated with obesity/diabetes, the metabolic effects of exercise on the heart are not well-known. This study was designed to investigate whether high-intensity interval training (HIT) is superior to moderate-intensity training (MIT) in counteracting obesity-induced impairment of left ventricular (LV) mechanoenergetics and function. C57BL/6J mice with diet-induced obesity (DIO mice) displaying a cardiac phenotype with altered substrate utilization and impaired mechanoenergetics were subjected to a sedentary lifestyle or 8–10 weeks of isocaloric HIT or MIT. Although both modes of exercise equally improved aerobic capacity and reduced obesity, only HIT improved glucose tolerance. Hearts from sedentary DIO mice developed concentric LV remodeling with diastolic and systolic dysfunction, which was prevented by both HIT and MIT. Both modes of exercise also normalized LV mechanical efficiency and mechanoenergetics. These changes were associated with altered myocardial substrate utilization and improved mitochondrial capacity and efficiency, as well as reduced oxidative stress, fibrosis, and intracellular matrix metalloproteinase 2 content. As both modes of exercise equally ameliorated the development of diabetic cardiomyopathy by preventing LV remodeling and mechanoenergetic impairment, this study advocates the therapeutic potential of physical activity in obesity-related cardiac disorders.

Increased O2 cost of basal metabolism and excitation-contraction coupling in hearts from type 2 diabetic mice

We have reported previously that hearts from type 2 diabetic (db/db) mice show decreased cardiac efficiency due to increased work-independent myocardial O(2) consumption (unloaded MVo(2)), indicating higher O(2) use for nonmechanical processes such as basal metabolism (MVo(2)(BM)) and excitation-contraction coupling (MVo(2)(ECC)). Although alterations in cardiac metabolism and/or Ca(2+) handling may contribute to increased energy expenditure in diabetic hearts, direct measurements of the O(2) cost for these individual processes have not been determined. In this study, we 1) validate a procedure for measuring unloaded MVo(2) directly (MVo(2)(unloaded)) and for determining MVo(2)(BM) and MVo(2)(ECC) separately in isolated perfused mouse hearts and 2) determine O(2) cost for these processes in hearts from db/db mice. Unloaded MVo(2), extrapolated from the relationship between cardiac work (measured as pressure-volume area, PVA) and MVo(2), was found to correspond with MVo(2) measured directly in unloaded retrograde perfused hearts (MVo(2)(unloaded)). MVo(2) in K(+)-arrested hearts was defined as MVo(2)(BM); the difference between MVo(2)(unloaded) and MVo(2)(BM) represented MVo(2)(ECC). This procedure was validated by demonstrating that elevations in perfusate fatty acid (FA) and/or Ca(2+) concentrations resulted in changes in either MVo(2)(BM) and/or MVo(2)(ECC). The higher MVo(2)(unloaded) in db/db mice was due to both a higher MVo(2)(BM) and MVo(2)(ECC). Elevation of glucose and insulin decreased FA oxidation and reduced both MVo(2)(unloaded) and MVo(2)(BM). In conclusion, this study provides direct evidence that MVo(2)(BM) and MVo(2)(ECC) are elevated in diabetes and that acute metabolic interventions can have a therapeutic benefit in diabetic hearts due to a MVo(2)-lowering effect.

Targeted redox inhibition of protein phosphatase 1 by Nox4 regulates eIF2α‐mediated stress signaling

Phosphorylation of translation initiation factor 2α (eIF2α) attenuates global protein synthesis but enhances translation of activating transcription factor 4 (ATF4) and is a crucial evolutionarily conserved adaptive pathway during cellular stresses. The serine–threonine protein phosphatase 1 (PP1) deactivates this pathway whereas prolonging eIF2α phosphorylation enhances cell survival. Here, we show that the reactive oxygen species‐generating NADPH oxidase‐4 (Nox4) is induced downstream of ATF4, binds to a PP1‐targeting subunit GADD34 at the endoplasmic reticulum, and inhibits PP1 activity to increase eIF2α phosphorylation and ATF4 levels. Other PP1 targets distant from the endoplasmic reticulum are unaffected, indicating a spatially confined inhibition of the phosphatase. PP1 inhibition involves metal center oxidation rather than the thiol oxidation that underlies redox inhibition of protein tyrosine phosphatases. We show that this Nox4‐regulated pathway robustly enhances cell survival and has a physiologic role in heart ischemia–reperfusion and acute kidney injury. This work uncovers a novel redox signaling pathway, involving Nox4–GADD34 interaction and a targeted oxidative inactivation of the PP1 metal center, that sustains eIF2α phosphorylation to protect tissues under stress.

How Exercise May Amend Metabolic Disturbances in Diabetic Cardiomyopathy

Abstract Significance: Over-nutrition and sedentary lifestyle has led to a worldwide increase in obesity, insulin resistance, and type 2 diabetes (T2D) associated with an increased risk of development of cardiovascular disorders. Diabetic cardiomyopathy, independent of hypertension or coronary disease, is induced by a range of systemic changes and may through multiple processes result in functional and structural cardiac derangements. The pathogenesis of this cardiomyopathy is complex and multifactorial, and it will eventually lead to reduced cardiac working capacity and increased susceptibility to ischemic injury. Recent Advances: Metabolic disturbances such as altered lipid handling and substrate utilization, decreased mechanical efficiency, mitochondrial dysfunction, disturbances in nonoxidative glucose pathways, and increased oxidative stress are hallmarks of diabetic cardiomyopathy. Interestingly, several of these disturbances are found to precede the development of cardiac dysfunction. Critical Issues: Exercise training is effective in the prevention and treatment of obesity and T2D. In addition to its beneficial influence on diabetes/obesity-related systemic changes, it may also amend many of the metabolic disturbances characterizing the diabetic myocardium. These changes are due to both indirect effects, exercise-mediated systemic changes, and direct effects originating from the high contractile activity of the heart during physical training. Future Directions: Revealing the molecular mechanisms behind the beneficial effects of exercise training is of considerable scientific value to generate evidence-based therapy and in the development of new treatment strategies. Antioxid. Redox Signal. 22, 1587–1605.

Changes in substrate metabolism in isolated mouse hearts following ischemia-reperfusion

Several genetic and transgenic mouse models are currently being used for studying the regulation of myocardial contractility under normal conditions and in disease states. Little information has been provided, however, about myocardial energy metabolism in mouse hearts. We measured glycolysis, glucose oxidation and palmitate oxidation (using 3H-glucose, 14C-glucose and3H-palmitate) in isolated working mouse hearts during normoxic conditions (control group) and following a 15 min global no-flow ischemic period (reperfusion group). Fifty min following reperfusion (10 min Langendorff perfusion + 40 min working heart perfusion) aortic flow, coronary flow, cardiac output, peak systolic pressure and heart rate were 44 ± 4, 88 ± 4, 57 ± 4, 94 ± 2 and 81 ± 4% of pre-ischemic values). Rates of glycolysis and glucose oxidation in the reperfusion group (13.6 ± 0.8 and 2.8 ± 0.2 μmol/min/g dry wt) were not different from the control group (12.3 ± 0.6 and 2.5 ± 0.2 umol/min/g dry wt). Palmitate oxidation, however, was markedly elevated in the reperfusion group as compared to the control group (576 ± 37 vs. 357 ±21 nmol/min/g dry wt, p < 0.05). This change in myocardial substrate utilization was accompanied by a marked fall in cardiac efficiency measured as cardiac output/oxidative ATP production (136 ± 10 vs. 54 ± 5 ml/μmol ATP, p < 0.05, control and reperfusion group, respectively). We conclude that ischemia-reperfusion in isolated working mouse hearts is associated with a shift in myocardial substrate utilization in favour of fatty acids, in line with previous observations in rat. (Mol Cell Biochem 249: 97–103, 2003)

Exercise training promotes cardioprotection through oxygen-sparing action in high fat-fed mice

Although exercise training has been demonstrated to have beneficial cardiovascular effects in diabetes, the effect of exercise training on hearts from obese/diabetic models is unclear. In the present study, mice were fed a high-fat diet, which led to obesity, reduced aerobic capacity, development of mild diastolic dysfunction, and impaired glucose tolerance. Following 8 wk on high-fat diet, mice were assigned to 5 weekly high-intensity interval training (HIT) sessions (10 × 4 min at 85-90% of maximum oxygen uptake) or remained sedentary for the next 10 constitutive weeks. HIT increased maximum oxygen uptake by 13%, reduced body weight by 16%, and improved systemic glucose homeostasis. Exercise training was found to normalize diastolic function, attenuate diet-induced changes in myocardial substrate utilization, and dampen cardiac reactive oxygen species content and fibrosis. These changes were accompanied by normalization of obesity-related impairment of mechanical efficiency due to a decrease in work-independent myocardial oxygen consumption. Finally, we found HIT to reduce infarct size by 47% in ex vivo hearts subjected to ischemia-reperfusion. This study therefore demonstrated for the first time that exercise training mediates cardioprotection following ischemia in diet-induced obese mice and that this was associated with oxygen-sparing effects. These findings highlight the importance of optimal myocardial energetics during ischemic stress.