Inhibition of superoxide generation and associated nitrosative damage is involved in metallothionein prevention of diabetic cardiomyopathy

Attenuation by metallothionein of early cardiac cell death via suppression of mitochondrial oxidative stress results in a prevention of diabetic cardiomyopathy

OBJECTIVES

We aimed to test whether attenuation of early-phase cardiac cell death can prevent diabetic cardiomyopathy.

BACKGROUND

Our previous study showed that cardiac apoptosis as a major early cellular response to diabetes is induced by hyperglycemia-derived oxidative stress that activates a mitochondrial cytochrome c-mediated caspase-3 activation pathway. Metallothionein (MT) as a potent antioxidant prevents the development of diabetic cardiomyopathy.

METHODS

Diabetes was induced by a single dose of streptozotocin (STZ) (150 mg/kg) in cardiac-specific, metallothionein-overexpressing transgenic (MT-TG) mice and wild-type (WT) controls. On days 7, 14, and 21 after STZ treatment, cardiac apoptosis was examined by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay and caspase-3 activation. Cardiomyopathy was evaluated by cardiac ultrastructure and fibrosis in the diabetic mice 6 months after STZ treatment.

RESULTS

A significant reduction in diabetes-induced increases in TUNEL-positive cells, caspase-3 activation, and cytochrome c release from mitochondria was observed in the MT-TG mice as compared to WT mice. Cardiac protein nitration (3-nitrotyrosine [3-NT]) and lipid peroxidation were significantly increased, and there was an increase in mitochondrial oxidized glutathione and a decrease in mitochondrial reduced glutathione in the WT, but not in the MT-TG, diabetic mice. Double staining for cardiomyocytes with alpha sarcomeric actin and caspase-3 or 3-NT confirmed the cardiomyocyte-specific effects. A significant prevention of diabetic cardiomyopathy and enhanced animal survival were observed in the MT-TG diabetic mice as compared to WT diabetic mice.

CONCLUSIONS

These results suggest that attenuation of early-phase cardiac cell death by MT results in a significant prevention of the development of diabetic cardiomyopathy. This process is mediated by MT suppression of mitochondrial oxidative stress.

Suppression of nitrative damage by metallothionein in diabetic heart contributes to the prevention of cardiomyopathy

Diabetic cardiomyopathy has become a major contributor to the increased mortality of diabetic patients. Although the development and progression of diabetic cardiomyopathy are considered to be associated with diabetes-derived oxidative stress, the precise mechanisms for and effectively preventive approaches to diabetic cardiomyopathy remain to be explored. Recent studies showed that reactive oxygen or nitrogen species (ROS/RNS) not only play a critical role in the initiation of diabetic cardiomyopathy, but also play an important role in physiological signaling. Therefore, this review will first discuss the dual roles of ROS/RNS in the physiological signaling and pathogenic remodeling leading to cardiomyopathy under diabetic conditions. The significant prevention of diabetic cardiomyopathy by metallothionein (MT) as a potent and nonspecific antioxidant will be also summarized. It is clearly revealed that although dual roles of peroxynitrite-nitrated proteins have been indicated under both physiological and pathogenic conditions, suppression of nitrative damage by MT in the diabetic heart is the major mechanism responsible for its prevention of diabetic cardiomyopathy. Finally the potential for clinical enhancement of the cardiac MT expression to prevent or delay the occurrence of cardiomyopathy in diabetic patients will also be addressed.

Intracerebral hemorrhage elicits aberration in cardiomyocyte contractile function and intracellular Ca2+ transients

BACKGROUND AND PURPOSE

The sequelae of intracerebral hemorrhage involve multiple organ damage including electrocardiographic alteration, although the mechanism(s) behind myocardial dysfunction is unknown. The aim of this study was to examine the impact of intracerebral hemorrhage on cardiomyocyte contractile function, intracellular Ca2+ handling, Ca2+ cycling proteins, I kappa B beta protein (IkappaB) phosphorylation, hypoxia-inducible factor 1alpha (HIF-1alpha), and nitrosative damage within 48 hours of injury.

METHODS

Mechanical and intracellular Ca2+ properties were evaluated including peak shortening (PS), maximal velocity of shortening/relengthening (+/-dL/dt), time-to-PS (TPS), time-to-90% relengthening (TR90), fura-2 fluorescence intensity (FFI), and intracellular Ca2+ decay.

RESULTS

Myocytes from intracerebral hemorrhage rats exhibited depressed PS, +/-dL/dt, prolonged TPS and TR90, as well as declined baseline FFI and slowed intracellular Ca2+ decay between 12 and 24 hours after injury. Most of these aberrations returned to normal levels 48 hours after hemorrhage with the exception of -dL/dt and TR90. Myocytes from 24-hour posthemorrhage rats exhibited a stepper negative staircase in PS with increased stimulus frequency. Cardiac expression of sarco(endo)plasmic reticulum Ca2+-ATPase 2a and phospholamban was enhanced, whereas that of Na+-Ca2+ exchanger and voltage-dependent K+ channel was decreased. IkappaB phosphorylation, HIF-1alpha, inducible NO synthase, and 3-nitrotyrosine were enhanced 12 hours after injury.

CONCLUSIONS

These data demonstrated that intracerebral hemorrhage initiates cardiomyocyte contractile and intracellular Ca2+ dysregulation possibly related to altered expression of Ca2+ cycling proteins, nitrosative damage, and myocardial phosphorylation of IkappaB.

Metallothionein alleviates cardiac dysfunction in streptozotocin-induced diabetes: role of Ca2+ cycling proteins, NADPH oxidase, poly(ADP-Ribose) polymerase and myosin heavy chain isozyme

Diabetic cardiomyopathy contributes to high morbidity and mortality in diabetic populations. It is manifested by compromised ventricular contraction and prolonged relaxation attributable to multiple causative factors including oxidative stress. This study was designed to examine the effect of cardiac overexpression of the heavy metal scavenger metallothionein (MT) on cardiac contractile function, intracellular Ca(2+) cycling proteins, stress-activated signaling molecules and the myosin heavy chain (MHC) isozyme in diabetes. Adult male wild-type (FVB) and MT transgenic mice were made diabetic by a single injection of streptozotocin (STZ). Contractile properties were evaluated in cardiomyocytes including peak shortening (PS), time-to-PS (TPS), time-to-relengthening (TR(90)), maximal velocity of shortening/relengthening (+/-dL/dt) and intracellular Ca(2+) fluorescence. Diabetes significantly depressed PS, +/-dL/dt, prolonged TPS, TR(90) and intracellular Ca(2+) clearing, elevated resting intracellular Ca(2+), reduced caffeine-induced sarcoplasmic reticulum Ca(2+) release and dampened stress tolerance at high stimulus frequencies. MT itself exhibited little effect on myocyte mechanics but it significantly alleviated STZ-induced myocyte contractile dysfunctions. Diabetes enhanced expression of the AT(1) receptor, phospholamban, the p47(phox) NADPH oxidase subunit and poly(ADP-ribose) polymerase (PARP), depressed the level of SERCA2a, Na(+)-Ca(2+) exchanger and triggered a beta-MHC isozyme switch. All of these STZ-induced alterations with the exception of depressed SERCA2a and enhanced phospholamban were reconciled by MT. Collectively, these data suggest a beneficial effect of MT in the therapeutics of diabetic cardiomyopathy, possibly through a mechanism related to NADPH oxidase, PARP and MHC isozyme switch.

Metallothionein prolongs survival and antagonizes senescence‐associated cardiomyocyte diastolic dysfunction: role of oxidative stress

Senescence is accompanied by oxidative stress and cardiac dysfunction, although the link between the two remains unclear. This study examined the role of antioxidant metallothionein on cardiomyocyte function, superoxide generation, the oxidative stress biomarker aconitase activity, cytochrome c release, and expression of oxidative stress-related proteins, such as the GTPase RhoA and NADPH oxidase protein p47phox in young (5-6 mo) and aged (26-28 mo) FVB wild-type (WT) and cardiac-specific metallothionein transgenic mice. Metallothionein mice showed a longer life span (by approximately 4 mo) than FVB mice evaluated by the Kaplan-Meier survival curve. Compared with young cardiomyocytes, aged myocytes displayed prolonged TR(90), reduced tolerance to high stimulus frequency, and slowed intracellular Ca2+ decay, all of which were nullified by metallothionein. Aging increased superoxide generation, active RhoA abundance, cytochrome c release, and p47phox expression and suppressed aconitase activity without affecting protein nitrotyrosine formation in the hearts. These aging-induced changes in oxidative stress and related protein biomarkers were attenuated by metallothionein. Aged metallothionein mouse myocytes were more resistant to the superoxide donor pyrogallol-induced superoxide generation and apoptosis. In addition, aging-associated prolongation in TR90 was blunted by the Rho kinase inhibitor Y-27632. Collectively, our data demonstrated that metallothionein may alleviate aging-induced cardiac contractile defects and oxidative stress, which may contribute to prolonged life span in metallothionein transgenic mice.

Metallothionein antagonizes aging-induced cardiac contractile dysfunction: role of PTP1B, insulin receptor tyrosine phosphorylation and Akt

Aging is often accompanied by reduced insulin sensitivity and cardiac dysfunction. However, the causal relationship between the two remains poorly understood. This study was designed to determine the impact of cardiac-specific overexpression of antioxidant metallothionein (MT) on aging-associated cardiac dysfunction and impaired insulin signaling. Contractile and intracellular Ca(2+) properties were evaluated in left ventricular myocytes including peak shortening (PS), maximal velocity of shortening/relengthening (+/- dL/dt), time-to-PS (TPS), time-to-90% relengthening (TR(90)), fura-2 fluorescence intensity change (DeltaFFI) and intracellular Ca(2+) decay rate. Expression of insulin receptor, protein-tyrosine phosphatase 1B (PTP1B), phosphorylation of insulin receptor (Tyr1146) and Akt were evaluated by Western blot analysis. Aged wild-type FVB and MT transgenic mice (26-28 months old) displayed glucose intolerance and hyperinsulinemia. Cardiomyocytes from aged FVB mice exhibited prolonged TR(90) and intracellular Ca(2+) decay associated with normal PS, +/- dL/dt, TPS and DeltaFFI compared with those from young (2-3 months old) mice. Western blot analysis revealed reduced Akt expression and insulin (5 mU g(-1))-stimulated Akt phosphorylation, elevated PTP1B expression and diminished basal insulin receptor tyrosine phosphorylation associated with comparable insulin receptor expression in aged FVB mouse hearts. All of these aging-related defects in cardiac contractile function and insulin signaling (although not hyperinsulinemia and glucose intolerance) were significantly attenuated or ablated by MT transgene. These data indicate that enhanced antioxidant defense is beneficial for aging-induced cardiac contractile dysfunction and alteration in insulin signaling.

Role of peroxynitrite in the pathogenesis of cardiovascular complications of diabetes

Hyperglycemic episodes, which complicate even well-controlled cases of diabetes, lead to increased polyol pathway flux, activation of protein kinase C and accelerated non-enzymatic formation of advanced glycation end products. Many of these pathways become activated in response to the production of superoxide anion. Superoxide can interact with nitric oxide, forming the potent cytotoxin peroxynitrite. Peroxynitrite attacks various biomolecules in the vascular endothelium, vascular smooth muscle and myocardium, eventually leading to cardiovascular dysfunction via multiple mechanisms. This review focuses on emerging evidence suggesting that peroxynitrite plays a key role in the pathogenesis of the cardiovascular complications of diabetes, which underlie the development and progression of diabetic retinopathy, neuropathy and nephropathy.

Selenium prevents diabetes-induced alterations in [Zn2+]iand metallothionein level of rat heart via restoration of cell redox cycle

Intracellular free zinc concentration ([Zn2+]i) is very important for cell functions, and its excessive accumulation is cytotoxic. [Zn2+]ican increase rapidly in cardiomyocytes because of mobilization of Zn2+from intracellular stores by reactive oxygen species (ROS). Moreover, ROS have been proposed to contribute to direct and/or indirect damage to cardiomyocytes in diabetes. To address these hypotheses, we investigated how elevated [Zn2+]iin cardiomyocytes could contribute to diabetes-induced alterations in intracellular free calcium concentration ([Ca2+]i). We also investigated its relationship to the changes of metallothionein (MT) level of the heart. Cardiomyocytes from normal rats loaded with fura-2 were used to fluorometrically measure resting [Zn2+]i(0.52 ± 0.06 nM) and [Ca2+]i(26.53 ± 3.67 nM). Fluorescence quenching by the heavy metal chelator N, N, N′, N′-tetrakis(2-pyridylmethyl)ethylenediamine was used to quantify [Zn2+]i. Our data showed that diabetic cardiomyocytes exhibited significantly increased [Zn2+]i(0.87 ± 0.05 nM ) and [Ca2+]i(49.66 ± 9.03 nM), decreased levels of MT and reduced glutathione, increased levels of lipid peroxidation and nitric oxide products, and decreased activities of superoxide dismutase, glutathione reductase, and glutathione peroxidase. Treatment (4 wk) of diabetic rats with sodium selenite (5 μmol·kg body wt−1·day−1) prevented these defects induced by diabetes. A comparison of present data with previously observed beneficial effects of selenium treatment on diabetes-induced contractile dysfunction of the heart can suggest that an increase in [Zn2+]imay contribute to oxidant-induced alterations of excitation-contraction coupling in diabetes. In addition, we showed that oxidative stress is involved in the etiology of diabetes-induced downregulation of heart function via depressed endogenous antioxidant defense mechanisms.

Cardiac metallothionein induction plays the major role in the prevention of diabetic cardiomyopathy by zinc supplementation

BACKGROUND

Our previous studies showed that transgenic mice that overexpress cardiac-specific metallothionein (MT) are highly resistant to diabetes-induced cardiomyopathy. Zinc is the major metal that binds to MT under physiological conditions and is a potent inducer of MT. The present study therefore explored whether zinc supplementation can protect against diabetic cardiomyopathy through cardiac MT induction.

METHODS AND RESULTS

Diabetes was induced in mice (C57BL/6J strain) by a single injection of streptozotocin. Half were supplemented intraperitoneally with zinc sulfate (5 mg/kg) every other day for 3 months. After zinc supplementation, mice were maintained for 3 more months and then examined for cardiomyopathy by functional and morphological analysis. Significant increases in cardiac morphological impairment, fibrosis, and dysfunction were observed in diabetic mice but not in diabetic mice supplemented with zinc. Zinc supplementation also induced a significant increase in cardiac MT expression. The role of MT in cardiac protection by zinc supplementation was examined in cultured cardiac cells that were directly exposed to high levels of glucose (HG) and free fatty acid (FFA) (palmitate), treatment that mimics diabetic conditions. Cell survival rate was significantly decreased for cells exposed to HG/FFA but did not change for cells exposed to HG/FFA and pretreated with zinc or low-dose cadmium, each of which induces significant MT synthesis. When MT expression was silenced with the use of MT small-interfering RNA, the preventive effect of pretreatment with zinc or low-dose cadmium was abolished.

CONCLUSIONS

These results suggest that the prevention of diabetic cardiomyopathy by zinc supplementation is predominantly mediated by an increase in cardiac MT.

Metallothionein alleviates cardiac contractile dysfunction induced by insulin resistance: role of Akt phosphorylation, PTB1B, PPARgamma and c-Jun

AIMS/HYPOTHESIS

Insulin resistance is concomitant with metabolic syndrome, oxidative stress and cardiac contractile dysfunction. However, the causal relationship between oxidative stress and cardiac dysfunction is unknown. This study was designed to determine the impact of overexpression of the cardiac antioxidant metallothionein on cardiac dysfunction induced by insulin resistance in mice.

METHODS

Whole-body insulin resistance was generated in wild-type FVB and metallothionein transgenic mice by feeding them with sucrose for 12 weeks. Contractile and intracellular Ca(2+) properties were evaluated in ventricular myocytes using an IonOptix system. The contractile indices analysed included: peak shortening (PS), time to 90% PS (TPS(90)), time to 90% relengthening (TR(90)), half-width duration, maximal velocity of shortening (+dL/dt) and relengthening (-dL/dt), fura-fluorescence intensity change (DeltaFFI) and decay rate (tau).

RESULTS

The sucrose-fed mice displayed glucose intolerance, enhanced oxidative stress, hyperinsulinaemia, hypertriglyceridaemia and normal body weight. Compared with myocytes in starch-fed mice, those from sucrose-fed mice exhibited depressed PS, +dL/dt, -dL/dt, prolonged TR(90) and decay rate, and reduced DeltaFFI associated with normal TPS(90) and half-width duration. Western blot analysis revealed enhanced basal, but blunted insulin (15 mU/g)-stimulated Akt phosphorylation. It also showed elevated expression of insulin receptor beta, insulin receptor tyrosine phosphorylation, peroxisome proliferator-activated receptor gamma, protein tyrosine phosphatase 1B and phosphorylation of the transcription factor c-Jun, associated with a reduced fold increase of insulin-stimulated insulin receptor tyrosine phosphorylation in sucrose-fed mice. All western blot findings may be attenuated or ablated by metallothionein.

CONCLUSIONS/INTERPRETATION

These data indicate that oxidative stress may play an important role in cardiac contractile dysfunction associated with glucose intolerance and possibly related to alteration in insulin signalling at the receptor and post-receptor levels.