Andrey Kazakov

Effects of Physical Exercise on Myocardial Telomere- Regulating Proteins, Survival Pathways, and Apoptosis

OBJECTIVES The purpose of this study was to study the underlying molecular mechanisms of the protective cardiac effects of physical exercise. BACKGROUND Telomere-regulating proteins affect cellular senescence, survival, and regeneration. METHODS C57/Bl6 wild-type, endothelial nitric oxide synthase (eNOS)-deficient and telomerase reverse transcriptase (TERT)-deficient mice were randomized to voluntary running or no running wheel conditions (n = 8 to 12 per group). RESULTS Short-term running (21 days) up-regulated cardiac telomerase activity to >2-fold of sedentary controls, increased protein expression of TERT and telomere repeat binding factor (TRF) 2, and reduced expression of the proapoptotic mediators cell-cycle-checkpoint kinase 2 (Chk2), p53, and p16. Myocardial and leukocyte telomere length did not differ between 3-week- and 6-month-old sedentary or running mice, but telomerase activity, TRF2 and TERT expression were persistently increased after 6 months and the expression of Chk2, p53, and p16 remained down-regulated. The exercise-induced changes were absent in both TERT(-/-) and eNOS(-/-) mice. Running increased cardiac expression of insulin-like growth factor (IGF)-1. Treatment with IGF-1 up-regulated myocardial telomerase activity >14-fold and increased the expression of phosphorylated Akt protein kinase and phosphorylated eNOS. To test the physiologic relevance of these exercise-mediated prosurvival pathways, apoptotic cardiomyopathy was induced by treatment with doxorubicin. Up-regulation of telomere-stabilizing proteins by physical exercise in mice reduced doxorubicin-induced p53 expression and potently prevented cardiomyocyte apoptosis in wild-type, but not in TERT(-/-) mice. CONCLUSIONS Long- and short-term voluntary physical exercise up-regulates cardiac telomere-stabilizing proteins and thereby induces antisenescent and protective effects, for example, to prevent doxorubicin-induced cardiomyopathy. These beneficial cardiac effects are mediated by TERT, eNOS, and IGF-1.

Reversal of Mitochondrial Transhydrogenase Causes Oxidative Stress in Heart Failure

Mitochondrial reactive oxygen species (ROS) play a central role in most aging-related diseases. ROS are produced at the respiratory chain that demands NADH for electron transport and are eliminated by enzymes that require NADPH. The nicotinamide nucleotide transhydrogenase (Nnt) is considered a key antioxidative enzyme based on its ability to regenerate NADPH from NADH. Here, we show that pathological metabolic demand reverses the direction of the Nnt, consuming NADPH to support NADH and ATP production, but at the cost of NADPH-linked antioxidative capacity. In heart, reverse-mode Nnt is the dominant source for ROS during pressure overload. Due to a mutation of the Nnt gene, the inbred mouse strain C57BL/6J is protected from oxidative stress, heart failure, and death, making its use in cardiovascular research problematic. Targeting Nnt-mediated ROS with the tetrapeptide SS-31 rescued mortality in pressure overload-induced heart failure and could therefore have therapeutic potential in patients with this syndrome.

Heart rate reduction by If-inhibition improves vascular stiffness and left ventricular systolic and diastolic function in a mouse model of heart failure with preserved ejection fraction

AIMS In diabetes mellitus, heart failure with preserved ejection fraction (HFPEF) is a significant comorbidity. No therapy is available that improves cardiovascular outcomes. The aim of this study was to characterize myocardial function and ventricular-arterial coupling in a mouse model of diabetes and to analyse the effect of selective heart rate (HR) reduction by If-inhibition in this HFPEF-model. METHODS AND RESULTS Control mice, diabetic mice (db/db), and db/db mice treated for 4 weeks with the If-inhibitor ivabradine (db/db-Iva) were compared. Aortic distensibility was measured by magnetic resonance imaging. Left ventricular (LV) pressure-volume analysis was performed in isolated working hearts, with biochemical and histological characterization of the cardiac and aortic phenotype. In db/db aortic stiffness and fibrosis were significantly enhanced compared with controls and were prevented by HR reduction in db/db-Iva. Left ventricular end-systolic elastance (Ees) was increased in db/db compared with controls (6.0 ± 1.3 vs. 3.4 ± 1.2 mmHg/µL, P < 0.01), whereas other contractility markers were reduced. Heart rate reduction in db/db-Iva lowered Ees (4.0 ± 1.1 mmHg/µL, P < 0.01), and improved the other contractility parameters. In db/db active relaxation was prolonged and end-diastolic capacitance was lower compared with controls (28 ± 3 vs. 48 ± 8 μL, P < 0.01). These parameters were ameliorated by HR reduction. Neither myocardial fibrosis nor hypertrophy were detected in db/db, whereas titin N2B expression was increased and phosphorylation of phospholamban was reduced both being prevented by HR reduction in db/db-Iva. CONCLUSION In db/db, a model of HFPEF, selective HR reduction by If-inhibition improved vascular stiffness, LV contractility, and diastolic function. Therefore, If-inhibition might be a therapeutic concept for HFPEF, if confirmed in humans.

Effects of DPP-4 inhibition on cardiac metabolism and function in mice

Type 2 diabetes is associated with an increased risk of cardiac complications. Inhibitors of dipeptidylpeptidase 4 (DPP-4) are novel drugs for the treatment of patients with type 2 diabetes. The effect of DPP-4 inhibitors on myocardial metabolism has not been studied in detail. In wild-type C57Bl6-mice, 3weeks of treatment with sitagliptin had no effect on body weight and glucose tolerance nor on phosphorylation of AMP-activated protein kinase (AMPK) and acetyl-CoAcarboxylase (ACC), phosphofructokinase-2 (PFK2) or tuberin-2 (TSC2) in the left ventricular myocardium. However, in 10week old db/db-/- mice, a model of diabetes and obesity, sitagliptin potently reduced plasma glucose rise in peritoneal glucose tolerance tests and reduced weight increase. The myocardium of untreated db/db-/- mice exhibited a marked increase of the phosphorylation of AMPK, ACC, TSC2, expression of p53 and fatty acid translocase (FAT/CD36) membrane expression. These changes were reduced by DPP-4 inhibition. Sitagliptin showed no effect on cardiomyocyte size but prevented myocardial fibrosis in the 10week old db/db-/- mice and reduced expression of TGF-β1, markers of oxidative stress and the accumulation of advanced glycation end products in cardiomyocytes. Working heart analyses did not show an effect of sitagliptin on parameters of systolic cardiac function. In animals with diabetes and obesity, sitagliptin improved glucose tolerance, reduced weight gain, myocardial fibrosis and oxidative stress. Furthermore the study provides evidence that treatment with sitagliptin decreases elevated myocardial fatty acid uptake and oxidation in the diabetic heart. These observations show beneficial myocardial metabolic effect of DPP-4 inhibition in this mouse model of diabetes and obesity.

Cardiac Rac1 overexpression in mice creates a substrate for atrial arrhythmias characterized by structural remodelling

AIMS The small GTPase Rac1 seems to play a role in the pathogenesis of atrial fibrillation (AF). The aim of the present study was to characterize the effects of Rac1 overexpression on atrial electrophysiology. METHODS AND RESULTS In mice with cardiac overexpression of constitutively active Rac1 (RacET), statin-treated RacET, and wild-type controls (age 6 months), conduction in the right and left atrium (RA and LA) was mapped epicardially. The atrial effective refractory period (AERP) was determined and inducibility of atrial arrhythmias was tested. Action potentials were recorded in isolated cells. Left ventricular function was measured by pressure-volume analysis. Five of 11 RacET hearts showed spontaneous or inducible atrial tachyarrhythmias vs. 0 of 9 controls (P < 0.05). In RacET, the P-wave duration was significantly longer (26.8 +/- 2.1 vs. 16.7 +/- 1.1 ms, P = 0.001) as was total atrial activation time (RA: 13.6 +/- 4.4 vs. 3.2 +/- 0.5 ms; LA: 7.1 +/- 1.2 vs. 2.2 +/- 0.3 ms, P < 0.01). Prolonged local conduction times occurred more often in RacET (RA: 24.4 +/- 3.8 vs. 2.7 +/- 2.1%; LA: 19.1 +/- 6.3 vs. 1.2 +/- 0.7%, P < 0.01). The AERP and action potential duration did not differ significantly between both groups. RacET demonstrated significant atrial fibrosis but only moderate systolic heart failure. RacET and statin-treated RacET were not significantly different regarding atrial electrophysiology. CONCLUSION The substrate for atrial arrhythmias in mice with Rac1 overexpression is characterized by conduction disturbances and atrial fibrosis. Electrical remodelling (i.e. a shortening of AERP) does not play a role. Statin treatment cannot prevent the structural and electrophysiological effects of pronounced Rac1 overexpression in this model.

Heart Rate Contributes to the Vascular Effects of Chronic Mental Stress: Effects on Endothelial Function and Ischemic Brain Injury in Mice

Background and Purpose— Vascular effects of mental stress are only partially understood. Therefore, we studied effects of chronic stress and heart rate (HR) on endothelial function and cerebral ischemia. Methods— 129S6/SvEv mice were randomized to the I(f)-channel inhibitor ivabradine (10 mg/kg per day) or vehicle and underwent a chronic stress protocol for 28 days. Results— Stress increased HR from 514±10 bpm to 570±14 bpm, this was prevented by ivabradine (485±7 bpm). Endothelium-dependent relaxation of aortic rings was impaired in mice exposed to stress. HR reduction restored endothelial function to the level of naive controls. Vascular lipid hydroperoxides were increased to 333%±24% and vascular NADPH oxidase activity was upregulated to 223±38% in stressed mice, which was prevented by ivabradine. Stress reduced aortic endothelial nitric oxide synthase mRNA expression to 84%±3% and increased AT1 receptor mRNA to 168%±18%. Both effects were attenuated by HR reduction. In brain tissue, stress resulted in an upregulation of lipid hydroperoxides to 140%±11%, which was attenuated by HR reduction. Ivabradine increased brain capillary density in naive and in stressed mice. Mice exposed to chronic stress before induction of ischemic stroke by transient middle cerebral artery occlusion exhibited increased lesion size (33.7±2.3 mm3 versus 23.9±2.4 mm3). HR reduction led to a marked reduction of the infarct volume to 12.9±3.3 mm3. Conclusions— Chronic stress impairs endothelial function and aggravates ischemic brain injury. HR reduction protects from cerebral ischemia via improvement of endothelial function and reduction of oxidative stress. These results identify heart rate as a mediator of vascular effects induced by chronic stress.

ACE inhibition promotes upregulation of endothelial progenitor cells and neoangiogenesis in cardiac pressure overload

AIMS Inhibition of the angiotensin-converting enzyme (ACE) prevents maladaptive cardiac remodelling. Endothelial progenitor cells (EPC) from the bone marrow contribute to endothelial repair and neovascularization, effects that are potentially important during cardiac remodelling. We hypothesized that ACE inhibitors may exert beneficial effects during pressure-induced myocardial hypertrophy by regulating progenitor cell function. METHODS AND RESULTS In C57/Bl6 mice, development of cardiac hypertrophy induced by transaortic constriction (TAC) for 5 weeks was reduced by ramipril, 5 mg/kg p.o., independent of blood pressure lowering. Ramipril prevented TAC-induced apoptosis of cardiac myocytes and endothelial cells. On day 1 after TAC, upregulation of Sca-1(pos)/KDR(pos) EPC was observed, which was further increased by ramipril. EPC were persistently elevated in the TAC mice receiving vehicle treatment but not in the ramipril group after 5 weeks. These effects were independent of hypoxia-inducible factor-1alpha mRNA and protein expression. The ACE inhibitor but not TAC improved the migratory capacity of DiLDL(pos) EPC. Increased cardiac afterload induced upregulation of extracardiac neoangiogenesis. This effect was enhanced by ACE inhibition. Ramipril but not TAC markedly increased cardiac capillary density determined by the ratio of CD31(pos) cells to cardiomyocytes. Bone marrow transplantation studies revealed that TAC increased the percentage of bone marrow-derived GFP(pos) endothelial cells in the myocardium, and ramipril made this effect more pronounced. CONCLUSIONS ACE inhibition prevents pressure-induced maladaptive cardiac hypertrophy and increases intra- and extracardiac neoangiogenesis associated with the upregulation of EPC and amelioration of EPC migration. The regulation of progenitor cells from the bone marrow identifies a novel effect of ACE inhibitors during cardiac remodelling.

Inhibition of endothelial nitric oxide synthase induces and enhances myocardial fibrosis

AIMS The endothelial nitric oxide synthase (eNOS) contributes to cardiac remodelling. We studied the role of eNOS in the development of myocardial fibrosis during cardiac overload. METHODS AND RESULTS Ten-week-old male C57/Bl6 wildtype (WT) and eNOS mice (eNOS(-/-)) were subjected to transverse aortic constriction (TAC, 360 μm) and WT were treated with L-N(G)-nitroarginine methyl ester (L-NAME, 100 mg/kg/day) for 35 days. Inhibition of eNOS by L-NAME induced interstitial fibrosis, augmented replacement fibrosis, and induced apoptosis of cardiac fibroblasts and cardiomyocytes. L-NAME and eNOS(-/-) markedly increased the fibrosis induced by TAC and enhanced the myocardial prevalence of CXCR4(pos) fibroblasts. Myocardial stromal-derived factor-1 (SDF-1) expression was up-regulated by l-NAME and down-regulated after TAC. Blood pressure lowering by co-treatment with hydralazine (250 mg/L/day) did not reverse the L-NAME effects. In mice transplanted with green fluorescent protein (GFP)(pos) bone marrow, L-NAME increased the percentage of GFP(pos) fibroblasts in the myocardium to 45-70%. Strain-mismatched BMT of eNOS(-/-)-BM increased and of WT-BM decreased the percentage of CXCR4(pos) fibroblasts in all groups. The number of fibrocytes (CD45(pos) collagen I(pos) cells) in the peripheral blood and in the bone marrow was increased both by TAC and L-NAME. L-NAME but not the inhibitor of inducible NOS 1400 W and of neuronal NOS 7-nitroindazole increased hydroxyproline and collagen Iα1. L-NAME up-regulated SDF-1 mRNA in cultured neonatal rat cardiac fibroblasts as well as their migratory capacity. CONCLUSION eNOS inhibition induces and enhances cardiac fibrosis independently of blood pressure by activating SDF-1/CXCR4, extracellular matrix production in cardiac fibroblasts and by increasing recruitment of fibrocytes from the bone marrow.

Endothelial nitric oxide synthase of the bone marrow regulates myocardial hypertrophy, fibrosis, and angiogenesis

AIMS The endothelial nitric oxide synthase (eNOS) regulates the mobilization and function of endothelial progenitor cells (EPC). We hypothesized that eNOS of the bone marrow (BM) affects cardiac remodelling during myocardial hypertrophy via the regulation of BM-derived vascular progenitor cells. METHODS AND RESULTS Ten-week-old male C57/Bl6 wild-type (WT) and eNOS mice (eNOS(-/-)) were subjected to transverse aortic constriction (TAC, 360 μm, 35 days) or sham operation inducing cardiac hypertrophy and increasing the numbers of Ki67+ cardiomyocytes in both strains. Myocardial fibrosis was more pronounced in eNOS(-/-) TAC (3.4 ± 0.4 vs. 2.1 ± 0.2% in WT-TAC, P < 0.05). TAC up-regulated the number of EPCs in the peripheral blood and in the BM in WT but not in eNOS(-/-). Baseline migratory capacity of EPCs was lower in eNOS(-/-) and was not raised by TAC in either strain. To test the role of eNOS in the BM during pressure overload, strain-mismatched (WT/eNOS(-/-); eNOS(-/-)/WT) and strain-matched (WT/WT; eNOS(-/-)/eNOS(-/-)) BM transplantations (BMTs) were performed. Cardiac hypertrophy was most pronounced in WT/eNOS(-/-) TAC. Strain-mismatched BMT of eNOS(-/-) BM deteriorated and of WT BM ameliorated cardiac fibrosis, capillary density, the numbers of EPCs in the peripheral blood and in the BM, and their migratory capacity in pressure overload. Following transplantation of green fluorescent protein (GFP)-positive BM, TAC increased the number of BM-derived podocalyxin(pos)GFP(pos) endothelial cells in both strains. CONCLUSION eNOS of the BM plays a key role for amelioration of cardiac hypertrophy, capillary density, and fibrosis during increased afterload.

Adenosine A1 receptor activation attenuates cardiac hypertrophy and fibrosis in response to α1‐adrenoceptor stimulation in vivo

Adenosine has been proposed to exert anti‐hypertrophic effects. However, the precise regulation and the role of the different adenosine receptor subtypes in the heart and their effects on hypertrophic signalling are largely unknown. We aimed to characterize expression and function of adenosine A1 receptors following hypertrophic stimulation in vitro and in vivo.