Ncor2/PPARα-Dependent Upregulation of MCUb in the Type 2 Diabetic Heart Impacts Cardiac Metabolic Flexibility and Function.

The contribution of altered mitochondrial Ca2+ handling to metabolic and functional defects in type 2 diabetic (T2D) mouse hearts is not well understood. In this study, we show that the T2D heart is metabolically inflexible and almost exclusively dependent on mitochondrial fatty acid oxidation as a consequence of mitochondrial calcium uniporter complex (MCUC) inhibitory subunit MCUb overexpression. Using a recombinant endonuclease-deficient Cas9-based gene promoter pulldown approach coupled with mass spectrometry, we found that MCUb is upregulated in the T2D heart due to loss of glucose homeostasis regulator nuclear receptor corepressor 2 repression, and chromatin immunoprecipitation assays identified peroxisome proliferator-activated receptor α as a mediator of MCUb gene expression in T2D cardiomyocytes. Upregulation of MCUb limits mitochondrial matrix Ca2+ uptake and impairs mitochondrial energy production via glucose oxidation by depressing pyruvate dehydrogenase complex activity. Gene therapy displacement of endogenous MCUb with a dominant-negative MCUb transgene (MCUbW246R/V251E) in vivo rescued T2D cardiomyocytes from metabolic inflexibility and stimulated cardiac contractile function and adrenergic responsiveness by enhancing phospholamban phosphorylation via protein kinase A. We conclude that MCUb represents one newly discovered molecular effector at the interface of metabolism and cardiac function, and its repression improves the outcome of the chronically stressed diabetic heart.

Overexpression of p53 due to excess protein O-GlcNAcylation is associated with coronary microvascular disease in type 2 diabetes.

AIMS: We previously reported that increased protein O-GlcNAcylation in diabetic mice led to vascular rarefaction in the heart. In this study, we aimed to investigate whether and how coronary endothelial cell (EC) apoptosis is enhanced by protein O-GlcNAcylation and thus induces coronary microvascular disease (CMD) and subsequent cardiac dysfunction in diabetes. We hypothesize that excessive protein O-GlcNAcylation increases p53 that leads to CMD and reduced cardiac contractility. METHODS AND RESULTS: We conducted in vivo functional experiments in control mice, TALLYHO/Jng (TH) mice, a polygenic type 2 diabetic (T2D) model, and EC-specific O-GlcNAcase (OGA, an enzyme that catalyzes the removal of O-GlcNAc from proteins)-overexpressing TH mice, as well as in vitro experiments in isolated ECs from these mice. TH mice exhibited a significant increase in coronary EC apoptosis and reduction of coronary flow velocity reserve (CFVR), an assessment of coronary microvascular function, in comparison to wild-type mice. The decreased CFVR, due at least partially to EC apoptosis, was associated with decreased cardiac contractility in TH mice. Western blot experiments showed that p53 protein level was significantly higher in coronary ECs from TH mice and T2D patients than in control ECs. High glucose treatment also increased p53 protein level in control ECs. Furthermore, overexpression of OGA decreased protein O-GlcNAcylation and down-regulated p53 in coronary ECs, and conferred a protective effect on cardiac function in TH mice. Inhibition of p53 with pifithrin-α attenuated coronary EC apoptosis and restored CFVR and cardiac contractility in TH mice. CONCLUSIONS: The data from this study indicate that inhibition of p53 or down-regulation of p53 by OGA overexpression attenuates coronary EC apoptosis and improves CFVR and cardiac function in diabetes. Lowering coronary endothelial p53 levels via OGA overexpression could be a potential therapeutic approach for CMD in diabetes.

Restoring mitochondrial calcium uniporter expression in diabetic mouse heart improves mitochondrial calcium handling and cardiac function.

Diabetes mellitus is a growing health care problem, resulting in significant cardiovascular morbidity and mortality. Diabetes also increases the risk for heart failure (HF) and decreased cardiac myocyte function, which are linked to changes in cardiac mitochondrial energy metabolism. The free mitochondrial calcium level ([Ca2+] m ) is fundamental in activating the mitochondrial respiratory chain complexes and ATP production and is also known to regulate pyruvate dehydrogenase complex (PDC) activity. The mitochondrial calcium uniporter (MCU) complex (MCUC) plays a major role in mediating mitochondrial Ca2+ import, and its expression and function therefore have a marked impact on cardiac myocyte metabolism and function. Here, we investigated MCU's role in mitochondrial Ca2+ handling, mitochondrial function, glucose oxidation, and cardiac function in the heart of diabetic mice. We found that diabetic mouse hearts exhibit altered expression of MCU and MCUC members and a resulting decrease in [Ca2+] m , mitochondrial Ca2+ uptake, mitochondrial energetic function, and cardiac function. Adeno-associated virus-based normalization of MCU levels in these hearts restored mitochondrial Ca2+ handling, reduced PDC phosphorylation levels, and increased PDC activity. These changes were associated with cardiac metabolic reprogramming toward normal physiological glucose oxidation. This reprogramming likely contributed to the restoration of both cardiac myocyte and heart function to nondiabetic levels without any observed detrimental effects. These findings support the hypothesis that abnormal mitochondrial Ca2+ handling and its negative consequences can be ameliorated in diabetes by restoring MCU levels via adeno-associated virus-based MCU transgene expression.

Overexpression of hexokinase 2 reduces mitochondrial calcium overload in coronary endothelial cells of type 2 diabetic mice.

Coronary microvascular rarefaction, due to endothelial cell (EC) dysfunction, is one of the causes of increased morbidity and mortality in diabetes. Coronary ECs in diabetes are more apoptotic due partly to mitochondrial calcium overload. This study was designed to investigate the role of hexokinase 2 (HK2, an endogenous inhibitor of voltage-dependent anion channel) in coronary endothelial dysfunction in type 2 diabetes. We used mouse coronary ECs (MCECs) isolated from type 2 diabetic mice and human coronary ECs (HCECs) from type 2 diabetic patients to examine protein levels and mitochondrial function. ECs were more apoptotic and capillary density was lower in the left ventricle of diabetic mice than the control. MCECs from diabetic mice exhibited significant increase in mitochondrial Ca2+ concentration ([Ca2+]mito) compared with the control. Among several regulatory proteins for [Ca2+]mito, hexokinase 1 (HK1) and HK2 were significantly lower in MCECs from diabetic mice than control MCECs. We also found that the level of HK2 ubiquitination was higher in MCECs from diabetic mice than in control MCECs. In line with the data from MCECs, HCECs from diabetic patients showed lower HK2 protein levels than HCECs from nondiabetic patients. High-glucose treatment, but not high-fat treatment, significantly decreased HK2 protein levels in MCECs. HK2 overexpression in MCECs of diabetic mice not only lowered the level of [Ca2+]mito, but also reduced mitochondrial reactive oxygen species production toward the level seen in control MCECs. These data suggest that HK2 is a potential therapeutic target for coronary microvascular disease in diabetes by restoring mitochondrial function in coronary ECs.

O-GlcNAcylation of 8-Oxoguanine DNA Glycosylase (Ogg1) Impairs Oxidative Mitochondrial DNA Lesion Repair in Diabetic Hearts.

mtDNA damage in cardiac myocytes resulting from increased oxidative stress is emerging as an important factor in the pathogenesis of diabetic cardiomyopathy. A prevalent lesion that occurs in mtDNA damage is the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG), which can cause mutations when not repaired properly by 8-oxoguanine DNA glycosylase (Ogg1). Although the mtDNA repair machinery has been described in cardiac myocytes, the regulation of this repair has been incompletely investigated. Here we report that the hearts of type 1 diabetic mice, despite having increased Ogg1 protein levels, had significantly lower Ogg1 activity than the hearts of control, non-type 1 diabetic mice. In diabetic hearts, we further observed increased levels of 8-OHdG and an increased amount of mtDNA damage. Interestingly, Ogg1 was found to be highly O-GlcNAcylated in diabetic mice compared with controls. In vitro experiments demonstrated that O-GlcNAcylation inhibits Ogg1 activity, which could explain the mtDNA lesion accumulation observed in vivo Reducing Ogg1 O-GlcNAcylation in vivo by introducing a dominant negative O-GlcNAc transferase mutant (F460A) restored Ogg1 enzymatic activity and, consequently, reduced 8-OHdG and mtDNA damage despite the adverse hyperglycemic milieu. Taken together, our results implicate hyperglycemia-induced O-GlcNAcylation of Ogg1 in increased mtDNA damage and, therefore, provide a new plausible biochemical mechanism for diabetic cardiomyopathy.

Expression of the mitochondrial calcium uniporter in cardiac myocytes improves impaired mitochondrial calcium handling and metabolism in simulated hyperglycemia.

Diabetic cardiomyopathy is associated with metabolic changes, including decreased glucose oxidation (Gox) and increased fatty acid oxidation (FAox), which result in cardiac energetic deficiency. Diabetic hyperglycemia is a pathophysiological mechanism that triggers multiple maladaptive phenomena. The mitochondrial Ca2+ uniporter (MCU) is the channel responsible for Ca2+ uptake in mitochondria, and free mitochondrial Ca2+ concentration ([Ca2+]m) regulates mitochondrial metabolism. Experiments with cardiac myocytes (CM) exposed to simulated hyperglycemia revealed reduced [Ca2+]m and MCU protein levels. Therefore, we investigated whether returning [Ca2+]m to normal levels in CM by MCU expression could lead to normalization of Gox and FAox with no detrimental effects. Mouse neonatal CM were exposed for 72 h to normal glucose [5.5 mM glucose + 19.5 mM mannitol (NG)], high glucose [25 mM glucose (HG)], or HG + adenoviral MCU expression. Gox and FAox, [Ca2+]m, MCU levels, pyruvate dehydrogenase (PDH) activity, oxidative stress, mitochondrial membrane potential, and apoptosis were assessed. [Ca2+]m and MCU protein levels were reduced after 72 h of HG. Gox was decreased and FAox was increased in HG, PDH activity was decreased, phosphorylated PDH levels were increased, and mitochondrial membrane potential was reduced. MCU expression returned these parameters toward NG levels. Moreover, increased oxidative stress and apoptosis were reduced in HG by MCU expression. We also observed reduced MCU protein levels and [Ca2+]m in hearts from type 1 diabetic mice. Thus we conclude that HG-induced metabolic alterations can be reversed by restoration of MCU levels, resulting in return of [Ca2+]m to normal levels.

SGLT inhibitors attenuate NO-dependent vascular relaxation in the pulmonary artery but not in the coronary artery.

Inhibitors of sodium-glucose cotransporter (SGLT)2 are a new class of oral drugs for type 2 diabetic patients that reduce plasma glucose levels by inhibiting renal glucose reabsorption. There is increasing evidence showing the beneficial effect of SGLT2 inhibitors on glucose control; however, less information is available regarding the impact of SGLT2 inhibitors on cardiovascular outcomes. The present study was designed to determine whether SGLT inhibitors regulate vascular relaxation in mouse pulmonary and coronary arteries. Phlorizin (a nonspecific SGLT inhibitor) and canagliflozin (a SGLT2-specific inhibitor) relaxed pulmonary arteries in a dose-dependent manner, but they had little or no effect on coronary arteries. Pretreatment with phlorizin or canagliflozin significantly inhibited sodium nitroprusside (SNP; a nitric oxide donor)-induced vascular relaxation in pulmonary arteries but not in coronary arteries. Phlorizin had no effect on cGMP-dependent relaxation in pulmonary arteries. SNP induced membrane hyperpolarization in human pulmonary artery smooth muscle cells, and pretreatment of cells with phlorizin and canagliflozin attenuated SNP-induced membrane hyperpolarization by decreasing K(+) activities induced by SNP. Contrary to the result observed in ex vivo experiments with SGLT inhibitors, SNP-dependent relaxation in pulmonary arteries was not altered by chronic administration of canagliflozin. On the other hand, canagliflozin administration significantly enhanced SNP-dependent relaxation in coronary arteries in diabetic mice. These data suggest that SGLT inhibitors differentially regulate vascular relaxation depending on the type of arteries, duration of the treatment, and health condition, such as diabetes.

O-GlcNAcase overexpression reverses coronary endothelial cell dysfunction in type 1 diabetic mice.

Cardiovascular disease is the primary cause of morbidity and mortality in diabetes, and endothelial dysfunction is commonly seen in these patients. Increased O-linked N-acetylglucosamine (O-GlcNAc) protein modification is one of the central pathogenic features of diabetes. Modification of proteins by O-GlcNAc (O-GlcNAcylation) is regulated by two key enzymes: ß-N-acetylglucosaminidase [O-GlcNAcase (OGA)], which catalyzes the reduction of protein O-GlcNAcylation, and O-GlcNAc transferase (OGT), which induces O-GlcNAcylation. However, it is not known whether reducing O-GlcNAcylation can improve endothelial dysfunction in diabetes. To examine the effect of endothelium-specific OGA overexpression on protein O-GlcNAcylation and coronary endothelial function in diabetic mice, we generated tetracycline-inducible, endothelium-specific OGA transgenic mice, and induced OGA by doxycycline administration in streptozotocin-induced type 1 diabetic mice. OGA protein expression was significantly decreased in mouse coronary endothelial cells (MCECs) isolated from diabetic mice compared with control MCECs, whereas OGT protein level was markedly increased. The level of protein O-GlcNAcylation was increased in diabetic compared with control mice, and OGA overexpression significantly decreased the level of protein O-GlcNAcylation in MCECs from diabetic mice. Capillary density in the left ventricle and endothelium-dependent relaxation in coronary arteries were significantly decreased in diabetes, while OGA overexpression increased capillary density to the control level and restored endothelium-dependent relaxation without changing endothelium-independent relaxation. We found that connexin 40 could be the potential target of O-GlcNAcylation that regulates the endothelial functions in diabetes. These data suggest that OGA overexpression in endothelial cells improves endothelial function and may have a beneficial effect on coronary vascular complications in diabetes.

Coronary endothelial dysfunction and mitochondrial reactive oxygen species in type 2 diabetic mice.

Endothelial cell (EC) dysfunction is implicated in cardiovascular diseases, including diabetes. The decrease in nitric oxide (NO) bioavailability is the hallmark of endothelial dysfunction, and it leads to attenuated vascular relaxation and atherosclerosis followed by a decrease in blood flow. In the heart, decreased coronary blood flow is responsible for insufficient oxygen supply to cardiomyocytes and, subsequently, increases the incidence of cardiac ischemia. In this study we investigate whether and how reactive oxygen species (ROS) in mitochondria contribute to coronary endothelial dysfunction in type 2 diabetic (T2D) mice. T2D was induced in mice by a high-fat diet combined with a single injection of low-dose streptozotocin. ACh-induced vascular relaxation was significantly attenuated in coronary arteries (CAs) from T2D mice compared with controls. The pharmacological approach reveals that NO-dependent, but not hyperpolarization- or prostacyclin-dependent, relaxation was decreased in CAs from T2D mice. Attenuated ACh-induced relaxation in CAs from T2D mice was restored toward control level by treatment with mitoTempol (a mitochondria-specific O2(-) scavenger). Coronary ECs isolated from T2D mice exhibited a significant increase in mitochondrial ROS concentration and decrease in SOD2 protein expression compared with coronary ECs isolated from control mice. Furthermore, protein ubiquitination of SOD2 was significantly increased in coronary ECs isolated from T2D mice. These results suggest that augmented SOD2 ubiquitination leads to the increase in mitochondrial ROS concentration in coronary ECs from T2D mice and attenuates coronary vascular relaxation in T2D mice.

Synthesis and evaluation of a bifunctional chelate for development of Bi(III)-labeled radioimmunoconjugates.

A new bifunctional ligand C-DEPA was designed and synthesized as a component for antibody-targeted radiation therapy (radioimmunotherapy, RIT) of cancer. C-DEPA was conjugated to a tumor targeting antibody, trastuzumab, and the corresponding C-DEPA-trastuzumab conjugate was evaluated for radiolabeling kinetics with (205/6)Bi. C-DEPA-trastuzumab conjugate rapidly bound (205/6)Bi, and (205/6)Bi-C-DEPA-trastuzumab conjugate was stable in human serum for 72 h. The in vitro radiolabeling kinetics and serum stability data suggest that C-DEPA is a potential chelate for preclinical RIT applications using (212)Bi and (213)Bi.

Efficient bifunctional decadentate ligand 3p-C-DEPA for targeted α-radioimmunotherapy applications.

A new bifunctional ligand 3p-C-DEPA was synthesized and evaluated for use in targeted α-radioimmunotherapy. 3p-C-DEPA was efficiently prepared via regiospecific ring opening of an aziridinium ion and conjugated with trastuzumab. The 3p-C-DEPA-trastuzumab conjugate was extremely rapid in binding (205/6)Bi, and the corresponding (205/6)Bi-3p-C-DEPA-trastuzumab complex was stable in human serum. Biodistribution studies were performed to evaluate in vivo stability and tumor targeting of (205/6)Bi-3p-C-DEPA-trastuzumab conjugate in tumor bearing athymic mice. (205/6)Bi-3p-C-DEPA-trastuzumab conjugate displayed excellent in vivo stability and targeting as evidenced by low organ uptake and high tumor uptake. The results of the in vitro and in vivo studies indicate that 3p-C-DEPA is a promising chelator for radioimmunotherapy of (212)Bi and (213)Bi.

A highly effective bifunctional ligand for radioimmunotherapy applications.

A novel bifunctional ligand (3p-C-NETA) for antibody-targeted radioimmunotherapy (RIT) of ß-emitting radioisotopes (90)Y and (177)Lu was efficiently synthesized via an unexpected regiospecific ring opening of an aziridinium ion. 3p-C-NETA instantly formed a very stable complex with (90)Y or (177)Lu. 3p-C-NETA is an excellent bifunctional ligand for RIT.