Reversal of hyperglycemic preconditioning by angiotensin II: role of calcium transport

Targeted Modification of Mitochondrial ROS Production Converts High Glucose-Induced Cytotoxicity to Cytoprotection: Effects on Anesthetic Preconditioning

Contradictory reports on the effects of diabetes and hyperglycemia on myocardial infarction range from cytotoxicity to cytoprotection. The study was designed to investigate acute effects of high glucose-driven changes in mitochondrial metabolism and osmolarity on adaptive mechanisms and resistance to oxidative stress of isolated rat cardiomyocytes. We examined the effects of high glucose on several parameters of mitochondrial bioenergetics, including changes in oxygen consumption, mitochondrial membrane potential, and NAD(P)H fluorometry. Effects of high glucose on the endogenous cytoprotective mechanisms elicited by anesthetic preconditioning (APC) and the mediators of cell injury were also tested. These experiments included real-time measurements of reactive oxygen species (ROS) production and mitochondrial permeability transition pore (mPTP) opening in single cells by laser scanning fluorescence confocal microscopy, and cell survival assay. High glucose rapidly enhanced mitochondrial energy metabolism, observed by increase in NAD(P)H fluorescence intensity, oxygen consumption, and mitochondrial membrane potential. This substantially elevated production of ROS, accelerated opening of the mPTP, and decreased survival of cells exposed to oxidative stress. Abrogation of high glucose-induced mitochondrial hyperpolarization with 2,4 dinitrophenol (DNP) significantly, but not completely, attenuated ROS production to a level similar to hyperosmotic mannitol control. DNP treatment reversed high glucose-induced cytotoxicity to cytoprotection. Hyperosmotic mannitol treatment also induced cytoprotection. High glucose abrogated APC-induced mitochondrial depolarization, delay in mPTP opening and cytoprotection. In conclusion, high glucose-induced mitochondrial hyperpolarization abolishes APC and augments cell injury. Attenuation of high glucose-induced ROS production by eliminating mitochondrial hyperpolarization protects cardiomyocytes. J. Cell. Physiol. 232: 216-224, 2017. © 2016 Wiley Periodicals, Inc.

Metabolic dysfunction in diabetic cardiomyopathy

Diabetic cardiomyopathy (DCM) is defined as cardiac disease independent of vascular complications during diabetes. The number of new cases of DCM is rising at epidemic rates in proportion to newly diagnosed cases of diabetes mellitus (DM) throughout the world. DCM is a heart failure syndrome found in diabetic patients that is characterized by left ventricular hypertrophy and reduced diastolic function, with or without concurrent systolic dysfunction, occurring in the absence of hypertension and coronary artery disease. DCM and other diabetic complications are caused in part by elevations in blood glucose and lipids, characteristic of DM. Although there are pathological consequences to hyperglycemia and hyperlipidemia, the combination of the two metabolic abnormalities potentiates the severity of diabetic complications. A natural competition exists between glucose and fatty acid metabolism in the heart that is regulated by allosteric and feedback control and transcriptional modulation of key limiting enzymes. Inhibition of these glycolytic enzymes not only controls flux of substrate through the glycolytic pathway, but also leads to the diversion of glycolytic intermediate substrate through pathological pathways, which mediate the onset of diabetic complications. The present review describes the limiting steps involved in the development of these pathological pathways and the factors involved in the regulation of these limiting steps. Additionally, therapeutic options with demonstrated or postulated effects on DCM are described.

Diabetes-induced increased oxidative stress in cardiomyocytes is sustained by a positive feedback loop involving Rho kinase and PKCβ2

We previously reported that acute inhibition of the RhoA/Rho kinase (ROCK) pathway normalized contractile function of diabetic rat hearts, but the underlying mechanism is unclear. Protein kinase C (PKC) β(2) has been proposed to play a major role in diabetic cardiomyopathy at least in part by increasing oxidative stress. Further evidence suggests that PKC positively regulates RhoA expression through induction of inducible nitric oxide synthase (iNOS) in diabetes. However, in preliminary studies, we found that inhibition of ROCK itself reduced RhoA expression in diabetic hearts. We hypothesized that there is an interaction between RhoA/ROCK and PKCβ(2) in the form of a positive feedback loop that sustains their activation and the production of reactive oxygen species (ROS). This was investigated in cardiomyocytes isolated from diabetic and control rat hearts, incubated with or without cytochalasin D or inhibitors of ROCK, RhoA, PKCβ(2), or iNOS. Inhibition of RhoA and ROCK markedly attenuated the diabetes-induced increases in PKCβ(2) activity and iNOS and RhoA expression in diabetic cardiomyocytes, while having no effect in control cells. Inhibition of PKCβ(2) and iNOS also normalized RhoA expression and ROCK overactivation, whereas iNOS inhibition reversed the increase in PKCβ(2) activity. Each of these treatments also normalized the diabetes-induced increase in production of ROS. Actin cytoskeleton disruption attenuated the increased expression and/or activity of all of these targets in diabetic cardiomyocytes. These data suggest that, in the diabetic heart, the RhoA/ROCK pathway contributes to contractile dysfunction at least in part by sustaining PKCβ(2) activation and ROS production via a positive feedback loop that requires an intact cytoskeleton.

Effect of pressure overload on cardioprotection via PI3K-Akt: comparison of postconditioning, insulin, and pressure unloading

BACKGROUND

Postconditioning (PC) and insulin exert cardioprotection by activating phosphatidylinositol-3 kinase (PI3K) signaling. Because pressure overload exacerbates ischemia-reperfusion (IR) injury, we tested the hypothesis that (i) pressure overload attenuates PC- and insulin-induced cardioprotection, an effect caused by reduced PI3K-Akt signaling and (ii) pressure unloading confers cardioprotection comparable to either PC or insulin.

METHODS

Infarct size (IS) and levels of relevant proteins (i.e., Akt, glycogen synthase kinase-3beta (GSK-3beta), 3'-phosphoinositide dependent kinase 1 (PDK1), phosphatase and tensin homolog on chromosome ten (PTEN)) were determined in hearts subjected to IR.

RESULTS

Pressure overload increased IS in association with changes in protein levels consistent with reduced PI3K-Akt signaling (i.e., ischemic reperfused vs. normoxic hearts). PC and insulin reduced IS but it was greater in hearts perfused at the higher, than the lower, pressure. Wortmannin (PI3K inhibitor) partially reversed PC-induced cardioprotection, with IS being greater in the high-pressure group. Pressure unloading during reperfusion caused the most marked reduction in IS whereas pressure loading abolished PC-induced cardioprotection. Nonetheless, the phospho-Akt/total Akt ratios and phospho-GSK-3beta levels were unaffected by perfusion pressure in insulin-treated or postconditioned hearts. Moreover, protein levels were similar in pressure-unloaded and pressure-loaded hearts.

CONCLUSIONS

Pressure overload reduces PI3K-Akt signaling following IR. However, a differential in PI3K-Akt signaling was not observed in ischemia-reperfused, insulin-treated, and postconditioned hearts, suggesting involvement of pathways other than PI3K-Akt for the effect of pressure on IS. Importantly, pressure unloading at reperfusion represents a novel and effective cardioprotective maneuver.

Effect of hypernatremia on injury caused by energy deficiency: role of T-type Ca2+ channel

Hypernatremia exerts multiple cellular effects, many of which could influence the outcome of an ischemic event. To further evaluate these effects of hypernatremia, isolated neonatal cardiomyocytes were chronically incubated with medium containing either normal (142 mM) or elevated sodium (167 mM) and then transferred to medium containing deoxyglucose and the electron transport chain inhibitor amobarbital. Chronic hypernatremia diminished the degree of calcium accumulation and reactive oxygen species generation during the period of metabolic inhibition. The improvement in calcium homeostasis was traced in part to the downregulation of the Ca(V)3.1 T-type calcium channel, as deficiency in the Ca(V)3.1 subtype using short hairpin RNA or treatment with an inhibitor of the Ca(V)3.1 variant of the T-type calcium channel (i.e., diphenylhydantoin) attenuated energy deficiency-mediated calcium accumulation and cell death. Although hyperosmotically stressed cells (exposed to 50 mM mannitol) had no effect on T-type calcium channel activity, they were also resistant to death during metabolic inhibition. Both hyperosmotic stress and hypernatremia activated Akt, suggesting that they initiate the phosphatidylinositol 3-kinase/Akt cytoprotective pathway, which protects the cell against calcium overload and oxidative stress. Thus hypernatremia appears to protect the cell against metabolic inhibition by promoting the downregulation of the T-type calcium channel and stimulating cytoprotective protein kinase pathways.

Effects of chromium picolinate on glycemic control and kidney of the obese Zucker rat

Background

Chromium picolinate (Cr(pic)3) is advocated as adjuvant therapy for impaired glycemic control, despite concerns for DNA damage. Potential toxicity of Cr(pic)3 should be greater for the kidney that accumulates chromium. Therefore, we tested the hypothesis that Cr(pic)3 treatment of obese Zucker rats (OZR) exacerbates renal abnormalities associated with dysglycemia.

Methods

Male OZR were treated with diets lacking or containing 5 and 10 mg/kg of chromium, as Cr(pic)3, for 20 weeks; lean Zucker rats (LZR) served as controls. Glycemic and renal effects of Cr(pic)3 were determined in the context of indices of oxidative stress and inflammation.

Results

The OZR displayed increased fasting plasma glucose and insulin in association with enlarged pancreatic islets exhibiting collagen and periodic acid Schiff-positive deposits compared to LZR; Cr(pic)3 treatment did not affect these parameters. The OZR, irrespective of Cr(pic)3, excreted more albumin than LZR. Also, other indices of renal function or histopathology were not affected by Cr(pic)3 treatment. Urinary excretion of 8-hydroxydeoxyguanosine (8-OHdG), an index of oxidative DNA damage, was greater in the OZR than LZR; dietary Cr(pic)3 treatment attenuated 8-OHdG excretion. However, immunostaining of kidney for 8-OHdG revealed similar staining pattern and intensity, despite significant renal accumulation of chromium in Cr(pic)3-treated groups. Finally, increased renal nitrotyrosine and cyclooxygenase-2 levels and urinary excretion of monocyte chemoattractant protein-1 of OZR were partially reversed by Cr(pic)3 treatment.

Conclusion

Dietary Cr(pic)3 treatment of OZR does not beneficially influence glycemic status or increase the risk for oxidative DNA damage; rather, the treatment attenuates indices of oxidative stress and inflammation.

Modification of intracellular calcium concentration in cardiomyocytes by inhibition of sarcolemmal Na+/H+exchanger

Although the Na+/H+exchanger (NHE) is considered to be involved in regulation of intracellular Ca2+concentration ([Ca2+]i) through the Na+/Ca2+exchanger, the exact mechanisms of its participation in Ca2+handling by cardiomyocytes are not fully understood. Isolated rat cardiomyocytes were treated with or without agents that are known to modify Ca2+movements in cardiomyocytes and exposed to an NHE inhibitor, 5-( N-methyl- N-isobutyl)amiloride (MIA). [Ca2+]iin cardiomyocytes was measured spectrofluorometrically with fura 2-AM in the absence or presence of KCl, a depolarizing agent. MIA increased basal [Ca2+]iand augmented the KCl-induced increase in [Ca2+]iin a concentration-dependent manner. The MIA-induced increase in basal [Ca2+]iwas unaffected by extracellular Ca2+, antagonists of the sarcolemmal (SL) L-type Ca2+channel, and inhibitors of the SL Na+/Ca2+exchanger, SL Ca2+pump ATPase and mitochondrial Ca2+uptake. However, the MIA-induced increase in basal [Ca2+]iwas attenuated by inhibitors of SL Na+-K+-ATPase and sarcoplasmic reticulum (SR) Ca2+transport. On the other hand, the MIA-mediated augmentation of the KCl response was dependent on extracellular Ca2+concentration and attenuated by agents that inhibit SL L-type Ca2+channels, the SL Na+/Ca2+exchanger, SL Na+-K+-ATPase, and SR Ca2+release channels and the SR Ca2+pump. However, the effect of MIA on the KCl-induced increase in [Ca2+]iremained unaffected by treatment with inhibitors of SL Ca2+pump ATPase and mitochondrial Ca2+uptake. MIA and a decrease in extracellular pH lowered intracellular pH and increased basal [Ca2+]i, whereas a decrease in extracellular pH, in contrast to MIA, depressed the KCl-induced increase in [Ca2+]iin cardiomyocytes. These results suggest that NHE may be involved in regulation of [Ca2+]iand that MIA-induced increases in basal [Ca2+]i, as well as augmentation of the KCl-induced increase in [Ca2+]i, in cardiomyocytes are regulated differentially.

N-acetylcysteine attenuates PKCbeta2 overexpression and myocardial hypertrophy in streptozotocin-induced diabetic rats

OBJECTIVE

Oxidative stress-mediated activation of protein kinase C (PKC) beta(2) in the myocardium has been implicated in the development of cardiomyopathy. Overexpression of PKCbeta(2) is associated with increased expression of connective tissue growth factor (CTGF) in myocardium, resulting in myocardial hypertrophy. We hypothesized that chronic treatment with the antioxidant N-acetylcysteine (NAC) would normalize oxidative stress-mediated overexpression of myocardial PKCbeta(2) and CTGF and attenuate the development of myocardial hypertrophy.

METHODS

Control and streptozotocin-induced diabetic rats were treated with NAC in drinking water for 8 weeks. At termination rats were surgically prepared for hemodynamic measurement, subsequent to which their hearts were removed to evaluate cardiac performance and histological and biochemical changes. Further, the role of PKCbeta(2) in hyperglycemia-induced cardiomyocyte hypertrophy was tested in cultured neonatal cardiomyocytes.

RESULTS

Myocardial hypertrophy, characterized by an increased ratio of ventricle weight to body weight and cardiomyocyte cross-sectional area was found to be higher in untreated diabetic rats. Further, in myocardium, increased levels of 15-F(2t)-isoprostane were accompanied by an increased expression of membrane-bound PKCbeta(2) and CTGF. N-acetylcysteine treatment not only attenuated these changes but also prevented hyperglycemia-induced hypertrophy in cultured neonatal rat cardiomyocytes.

CONCLUSIONS

The results suggest that PKCbeta(2) overexpression represents a mechanism causing hyperglycemia-mediated myocardial hypertrophy, which can be prevented by the antioxidant N-acetylcysteine.

Alterations in ion channel physiology in diabetic cardiomyopathy

Diabetes mellitus is one of the most common chronic illnesses worldwide. This article focuses on a subgroup of diabetic patients with a specific cardiac complication of this disease--diabetic cardiomyopathy. This article initially gives some general background on diabetic cardiomyopathy and ion channels. Next the focus is on how diabetic cardiomyopathy alters calcium homeostasis in cardiac myocytes and highlights the specific alterations in ion channel function that are characteristic of this type of cardiomyopathy. Finally, the importance of the renin-angiotensin system in diabetic cardiomyopathy is reviewed.

High glucose protects embryonic cardiac cells against simulated ischemia

In the present study we investigated whether acute glucose administration could be protective against hypoxic stress. H9c2 cells were exposed to either 4.5 mM or 22 mM of glucose for 15,min and then were submitted to simulated ischemia. Cell death was microscopically assessed by combined staining with propidium iodide (PI) and Hoeschst 33358. Intracellular content of glucose was measured by enzymatic analysis. Clucose content of H9c2 cells was 48.24+/- 7.94 micromol/L in the 22 mM vs 23.86+/- 4.8 micromol/L in the 4.5 mM group (p < 0.05). PKCepsilon expression was increased 1.6 fold in the membrane fraction after pretreatment with high glucose (p < 0.05), while was decreased 1.6 fold in the cytosol (p < 0.05). In addition, no difference to PKCdelta translocation was observed after pretreatment with low glucose. After hypoxia, in the 22 mM group, cell death was found to be 17.36+/- 2.66% vs 38.2+/- 5.4% in the 4.5 mM group (p < 0.05). In the presence of iodoacetic acid, a glycolytic inhibitor, cell death was not different between the two groups (23.54+/- 3.2% in 22 mM vs 22.06+/- 5.3% in 4.5 mM). Addition of chelerythrine did not change the protective effect of high glucose (13.4+/- 1.7% cell death in 22 mM vs 27.5+/- 5.5% in 4.5 mM, p < 0.05). In conclusion, short pretreatment with high glucose protects H9c2 cells against hypoxia. Although this protective effect is associated with translocation of PKCepsilon and increased glucose uptake, it was abrogated only by inhibition of glycolysis.

Effects of excess salt and fat intake on myocardial function and infarct size in rat

Important risk factors for cardiovascular disease include excess dietary intake of saturated fat and (or) salt. This study tested the hypothesis that excess intakes of saturated fat (e.g., beef tallow) and salt cause greater myocardial cell death following ischemia-reperfusion injury than each risk factor alone. Male rats were divided into four groups: basal fat diet (4.5% as calories; control), high fat diet (40% as calories; FAT), basal fat diet and high salt (1% NaCl solution; SALT) and high fat diet and high salt (FATSALT). The gain in body weight was significantly higher for FAT and FATSALT groups than those of either the control or the SALT group. Five weeks of exposure to the dietary regimens did not significantly affect the coronary flow rate and except for the salt-fed group, had no effect on the rate-pressure-product of the isolated heart perfused in Langendorff mode. Although infarct size was not affected by the high fat diet, it was reduced by the high salt regimen relative to the high fat diet or the control groups. When rats were fed the FAT and SALT combination, the effect of salt feeding on infarct size was not observed. In addition, the FATSALT group displayed a more marked deterioration in contractile function following ischemia-reperfusion injury than the other groups. In conclusion, short-term intake of a high fat diet, which significantly increases body weight, does not worsen ischemia-reperfusion injury although the treatment prevents the reduction of infarct size associated with high salt feeding.