Diabetes mellitus and hypothyroidism induce changes in myosin isoenzyme distribution in the rat heart--do alterations in fuel flux mediate these changes?

Insights into the role of maladaptive hexosamine biosynthesis and O-GlcNAcylation in development of diabetic cardiac complications

Diabetes mellitus significantly increases the risk of heart failure, independent of coronary artery disease. The mechanisms implicated in the development of diabetic heart disease, commonly termed diabetic cardiomyopathy, are complex, but much of the impact of diabetes on the heart can be attributed to impaired glucose handling. It has been shown that the maladaptive nutrient-sensing hexosamine biosynthesis pathway (HBP) contributes to diabetic complications in many non-cardiac tissues. Glucose metabolism by the HBP leads to enzymatically-regulated, O-linked attachment of a sugar moiety molecule, β-N-acetylglucosamine (O-GlcNAc), to proteins, affecting their biological activity (similar to phosphorylation). In normal physiology, transient activation of HBP/O-GlcNAc mechanisms is an adaptive, protective means to enhance cell survival; interventions that acutely suppress this pathway decrease tolerance to stress. Conversely, chronic dysregulation of HBP/O-GlcNAc mechanisms has been shown to be detrimental in certain pathological settings, including diabetes and cancer. Most of our understanding of the impact of sustained maladaptive HBP and O-GlcNAc protein modifications has been derived from adipose tissue, skeletal muscle and other non-cardiac tissues, as a contributing mechanism to insulin resistance and progression of diabetic complications. However, the long-term consequences of persistent activation of cardiac HBP and O-GlcNAc are not well-understood; therefore, the goal of this timely review is to highlight current understanding of the role of the HBP pathway in development of diabetic cardiomyopathy.

Mammalian target of rapamycin (mTOR) inhibition with rapamycin improves cardiac function in type 2 diabetic mice: potential role of attenuated oxidative stress and altered contractile protein expression

Elevated mammalian target of rapamycin (mTOR) signaling contributes to the pathogenesis of diabetes, with increased morbidity and mortality, mainly because of cardiovascular complications. Because mTOR inhibition with rapamycin protects against ischemia/reperfusion injury, we hypothesized that rapamycin would prevent cardiac dysfunction associated with type 2 diabetes (T2D). We also investigated the possible mechanisms and novel protein targets involved in rapamycin-induced preservation of cardiac function in T2D mice. Adult male leptin receptor null, homozygous db/db, or wild type mice were treated daily for 28 days with vehicle (5% DMSO) or rapamycin (0.25 mg/kg, intraperitoneally). Cardiac function was monitored by echocardiography, and protein targets were identified by proteomics analysis. Rapamycin treatment significantly reduced body weight, heart weight, plasma glucose, triglyceride, and insulin levels in db/db mice. Fractional shortening was improved by rapamycin treatment in db/db mice. Oxidative stress as measured by glutathione levels and lipid peroxidation was significantly reduced in rapamycin-treated db/db hearts. Rapamycin blocked the enhanced phosphorylation of mTOR and S6, but not AKT in db/db hearts. Proteomic (by two-dimensional gel and mass spectrometry) and Western blot analyses identified significant changes in several cytoskeletal/contractile proteins (myosin light chain MLY2, myosin heavy chain 6, myosin-binding protein C), glucose metabolism proteins (pyruvate dehydrogenase E1, PYGB, Pgm2), and antioxidant proteins (peroxiredoxin 5, ferritin heavy chain 1) following rapamycin treatment in db/db heart. These results show that chronic rapamycin treatment prevents cardiac dysfunction in T2D mice, possibly through attenuation of oxidative stress and alteration of antioxidants and contractile as well as glucose metabolic protein expression.

Protein O-GlcNAcylation: a new signaling paradigm for the cardiovascular system

The posttranslational modification of serine and threonine residues of nuclear and cytoplasmic proteins by the O-linked attachment of the monosaccharide beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic and ubiquitous protein modification. Protein O-GlcNAcylation is rapidly emerging as a key regulator of critical biological processes including nuclear transport, translation and transcription, signal transduction, cytoskeletal reorganization, proteasomal degradation, and apoptosis. Increased levels of O-GlcNAc have been implicated as a pathogenic contributor to glucose toxicity and insulin resistance, which are both major hallmarks of diabetes mellitus and diabetes-related cardiovascular complications. Conversely, there is a growing body of data demonstrating that the acute activation of O-GlcNAc levels is an endogenous stress response designed to enhance cell survival. Reports on the effect of altered O-GlcNAc levels on the heart and cardiovascular system have been growing rapidly over the past few years and have implicated a role for O-GlcNAc in contributing to the adverse effects of diabetes on cardiovascular function as well as mediating the response to ischemic injury. Here, we summarize our present understanding of protein O-GlcNAcylation and its effect on the regulation of cardiovascular function. We examine the pathways regulating protein O-GlcNAcylation and discuss, in more detail, our understanding of the role of O-GlcNAc in both mediating the adverse effects of diabetes as well as its role in mediating cellular protective mechanisms in the cardiovascular system. In addition, we also explore the parallels between O-GlcNAc signaling and redox signaling, as an alternative paradigm for understanding the role of O-GlcNAcylation in regulating cell function.

Impact of Type 2 diabetes and aging on cardiomyocyte function and O-linked N-acetylglucosamine levels in the heart

Increased levels of O-linked attachment of N-acetylglucosamine (O-GlcNAc) on nucleocytoplasmic proteins are implicated in the development of diabetic cardiomyopathy and are regulated by O-GlcNAc transferase (OGT) expression and its substrate UDP-GlcNAc. Therefore, the goal of this study was to determine whether the development of diabetes in the Zucker diabetic fatty (ZDF) rat, a model of Type 2 diabetes, results in defects in cardiomyocyte mechanical function and, if so, whether this is associated with increased levels of O-GlcNAc and increased OGT expression. Six-week-old ZDF rats were hyperinsulinemic but normoglycemic, and there were no differences in cardiomyocyte mechanical function, UDP-GlcNAc, O-GlcNAc, or OGT compared with age-matched lean control rats. Cardiomyocytes isolated from 22-wk-old hyperglycemic ZDF rats exhibited significantly impaired relaxation, compared with both age-matched lean control and 6-wk-old ZDF groups. There was also a significant increase in O-GlcNAc levels in high-molecular-mass proteins in the 22-wk-old ZDF group compared with age-matched lean control and 6-wk-old ZDF groups; this was associated with increased UDP-GlcNAc levels but not increased OGT expression. Surprisingly, there was a significant decrease in overall O-GlcNAc levels between 6 and 22 wk of age in lean, ZDF, and Sprague-Dawley rats that was associated with decreased OGT expression. These results support the notion that an increase in O-GlcNAc on specific proteins may contribute to impaired cardiomyocyte function in diabetes. However, this study also indicates that in the heart the level of O-GlcNAc on proteins appears to be differentially regulated by age and diabetes.

Heart failure: the frequent, forgotten, and often fatal complication of diabetes

There is a high frequency of heart failure (HF) accompanied by an increased mortality risk for patients with diabetes. The poor prognosis of these patients has been explained by an underlying diabetic cardiomyopathy exacerbated by hypertension and ischemic heart disease. In these patients, activation of the sympathetic nervous system results in increased myocardial utilization of fatty acids and induction of fetal gene programs, decreasing myocardial function. Activation of the renin-angiotensin system results in myocardial remodeling. It is imperative for physicians to intercede early to stop the progression of HF, yet at least half of patients with left ventricular dysfunction remain undiagnosed and untreated until advanced disease causes disability. This delay is largely because of the asymptomatic nature of early HF, which necessitates more aggressive assessment of HF risk factors and early clinical signs. Utilization of beta-blockade, ACE inhibitors, or possibly angiotensin receptor blockers is essential in preventing remodeling with its associated decline in ventricular function. beta-Blockers not only prevent, but may also reverse, cardiac remodeling. Glycemic control may also play an important role in the therapy of diabetic HF. The adverse metabolic side effects that have been associated with beta-adrenergic inhibitors in the diabetic patient may be circumvented by use of a third-generation beta-blocker. Prophylactic utilization of ACE inhibitors and beta-blockers to avoid, rather than await, the need to treat HF should be considered in high-risk diabetic patients.

Therapeutic potential of CPT I inhibitors: cardiac gene transcription as a target

Inhibitors of carnitine palmitoyl-transferase I (CPT I), the key enzyme for the transport of long-chain acyl-coenzyme A (acyl-CoA) compounds into mitochondria, have been developed as agents for treating diabetes mellitus Type 2. Findings that the CPT I inhibitor, etomoxir, has effects on overloaded heart muscle, which are associated with an improved function, were unexpected and can be attributed to selective changes in the dysregulated gene expression of hypertrophied cardiomyocytes. Also, the first clinical trial with etomoxir in patients with heart failure showed that etomoxir improved the clinical status and several parameters of heart function. In view of the action of etomoxir on gene expression, putative molecular mechanisms involved in an increased expression of SERCA2, the Ca(2+) pump of sarcoplasmic reticulum (SR) and alpha-myosin heavy chain (MHC) of failing overloaded heart muscle are described. The first 225 bp of human, rabbit, rat and mouse SERCA2 promoter sequence have high identity. Various cis-regularory elements are also given for the promoter of the rat cardiac alpha-MHC gene. It is hypothesised that etomoxir increases glucose-phosphate intermediates resulting in activation of signalling pathway(s) mediated by phosphatases. Regarding the possible direct action of etomoxir on peroxisome proliferator activated receptor alpha (PPAR-alpha) activation, it could upregulate the expression of various enzymes that participate in beta-oxidation, thereby modulating some effects of CPT 1 inhibition. Any development of alternative drugs requires a better understanding of the signal pathways involved in the altered gene expression. In particular, signals need to be identified which are altered in overloaded hearts and can selectively be re-activated by etomoxir.

Sodium pivalate reduces cardiac carnitine content and increases glucose oxidation without affecting cardiac functional capacity

This study determined how selected functional, metabolic, and contractile properties were impacted by sodium pivalate, a compound which creates a secondary carnitine deficiency. Young male rats received either sodium pivalate (20 mM, PIV) or sodium bicarbonate (20 mM, CONTR) in their drinking water. After 11-12 weeks cardiac function and glucose oxidation rates were measured in isolated, perfused working heart preparations. Hearts were also analyzed for carnitine content, activities of hexokinase (HK), citrate synthase (CS), and B-hydroxyacyl CoA dehydrogenase (HOAD), and myosin isoenzyme distribution. Sodium pivalate treatment significantly reduced cardiac carnitine content and increased glucose oxidation but did not alter cardiac functional capacity. HK activity was increased in the PIV group (p < 0.05), and HOAD activity decreased (p < 0.05). CS activity and myosin isoform distribution (VI > 85%) remained unchanged. These results demonstrate that pivalate treatment of this duration and the accompanying carnitine deficiency shift cardiac substrate utilization without compromising cardiac functional capacity.

Effect of positive inotropic agents on myosin isozyme population and mechanical activity of cultured rat heart myocytes

To examine whether catecholamines have a direct effect on myosin heavy chain expression of heart myocytes or whether they act via an altered work load, myocytes from neonatal rat hearts were cultured in thyroid hormone-free media containing various positive inotropic and chronotropic agents. The velocity and frequency of contraction of the myocytes were monitored using an optoelectronic system. After 3-5 days of culture, myosin isozyme populations, cellular cAMP content, and 2-deoxy-D-glucose uptake of the myocytes were determined. Compared with myocytes cultured in the absence of inotropic agents (32.6 +/- 3.5% V1), the proportion of myosin V1 was significantly (p less than 0.05) increased in the case of 1 microM isoproterenol (48.2 +/- 5.9% V1), 1 microM forskolin (57.1 +/- 11.7% V1), and 1 mM dibutyryl cAMP (79.1 +/- 2.0% V1). Dibutyryl cAMP increased V1 to a similar level as 30 nM triiodothyronine did (70.2 +/- 13.0% V1). Only a small increase was observed in myocytes cultured in the presence of 10 microM phenylephrine (40.4 +/- 8.4% V1), 10 microM ouabain (40.6 +/- 11.9% V1), or 10 microM Bay K 8644 (40.7 +/- 11.7% V1). The agents with a marked effect on myosin heavy chain expression resulted in a higher cAMP content; isoproterenol and forskolin also stimulated 2-deoxy-D-glucose uptake. All agents resulted in a higher velocity of contraction; with the exception of ouabain, frequency of contraction was also increased. A change in Ca2+ concentration in the medium from 1.3 to 2.4 mM resulted in a small increase in V1 (40.7 +/- 5.2% V1) but had the same effect on contraction velocity as dibutyryl cAMP did. Furthermore, 10 nM isoproterenol also increased V1 in myocytes that were arrested with 10 microM verapamil. The increase in V1 in the case of dibutyryl cAMP, isoproterenol, and forskolin is thus most probably not a correlate of the increased mechanical activity but of the high cellular cAMP content.

Diabetes-like action of intermittent fasting on sarcoplasmic reticulum Ca2+-pump ATPase and myosin isoenzymes can be prevented by sucrose

Experimental diabetes results in a reduction of the sarcoplasmic reticulum (SR) Ca2+-stimulated ATPase activity and a redirection of myosin isoenzymes from V1 to V3. Similar, but less pronounced, changes were induced by subjecting rats to intermittent fasting for 6 weeks. Low amounts of sucrose (0.8%) in the drinking water prevented the subcellular changes in fasted rats; however, sucrose neither affected the levels of plasma thyroid hormones nor normalized the reduced body weight. Plasma glucose was lowered without any changes in plasma insulin in the fasted rats receiving sucrose; this suggested an enhanced peripheral glucose utilization. Thus, the signals in the diabetic heart leading to changes in SR and myosin can be mimicked by intermittent fasting and seem to be linked to a shift in fuel utilization by the myocytes.