E. Dale Abel

Diabetic Cardiomyopathy Revisited

Diabetes mellitus increases the risk of heart failure independently of underlying coronary artery disease, and many believe that diabetes leads to cardiomyopathy. The underlying pathogenesis is partially understood. Several factors may contribute to the development of cardiac dysfunction in the absence of coronary artery disease in diabetes mellitus. This review discusses the latest findings in diabetic humans and in animal models and reviews emerging new mechanisms that may be involved in the development and progression of cardiac dysfunction in diabetes.

Adipose-selective targeting of the GLUT4 gene impairs insulin action in muscle and liver

The earliest defect in developing type 2 diabetes is insulin resistance, characterized by decreased glucose transport and metabolism in muscle and adipocytes. The glucose transporter GLUT4 mediates insulin-stimulated glucose uptake in adipocytes and muscle by rapidly moving from intracellular storage sites to the plasma membrane. In insulin-resistant states such as obesity and type 2 diabetes, GLUT4 expression is decreased in adipose tissue but preserved in muscle. Because skeletal muscle is the main site of insulin-stimulated glucose uptake, the role of adipose tissue GLUT4 downregulation in the pathogenesis of insulin resistance and diabetes is unclear. To determine the role of adipose GLUT4 in glucose homeostasis, we used Cre/loxP DNA recombination to generate mice with adipose-selective reduction of GLUT4 (G4A-/-). Here we show that these mice have normal growth and adipose mass despite markedly impaired insulin-stimulated glucose uptake in adipocytes. Although GLUT4 expression is preserved in muscle, these mice develop insulin resistance in muscle and liver, manifested by decreased biological responses and impaired activation of phosphoinositide-3-OH kinase. G4A-/- mice develop glucose intolerance and hyperinsulinaemia. Thus, downregulation of GLUT4 and glucose transport selectively in adipose tissue can cause insulin resistance and thereby increase the risk of developing diabetes.

PGC-1α deficiency causes multi-system energy metabolic derangements: Muscle dysfunction, abnormal weight control and hepatic steatosis

The gene encoding the transcriptional coactivator peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) was targeted in mice. PGC-1α null (PGC-1α−/−) mice were viable. However, extensive phenotyping revealed multi-system abnormalities indicative of an abnormal energy metabolic phenotype. The postnatal growth of heart and slow-twitch skeletal muscle, organs with high mitochondrial energy demands, is blunted in PGC-1α−/− mice. With age, the PGC-1α−/− mice develop abnormally increased body fat, a phenotype that is more severe in females. Mitochondrial number and respiratory capacity is diminished in slow-twitch skeletal muscle of PGC-1α−/− mice, leading to reduced muscle performance and exercise capacity. PGC-1α−/− mice exhibit a modest diminution in cardiac function related largely to abnormal control of heart rate. The PGC-1α−/− mice were unable to maintain core body temperature following exposure to cold, consistent with an altered thermogenic response. Following short-term starvation, PGC-1α−/− mice develop hepatic steatosis due to a combination of reduced mitochondrial respiratory capacity and an increased expression of lipogenic genes. Surprisingly, PGC-1α−/− mice were less susceptible to diet-induced insulin resistance than wild-type controls. Lastly, vacuolar lesions were detected in the central nervous system of PGC-1α−/− mice. These results demonstrate that PGC-1α is necessary for appropriate adaptation to the metabolic and physiologic stressors of postnatal life.

Targeted disruption of the glucose transporter 4 selectively in muscle causes insulin resistance and glucose intolerance

The prevalence of type 2 diabetes mellitus is growing worldwide. By the year 2020, 250 million people will be afflicted. Most forms of type 2 diabetes are polygenic with complex inheritance patterns, and penetrance is strongly influenced by environmental factors. The specific genes involved are not yet known, but impaired glucose uptake in skeletal muscle is an early, genetically determined defect that is present in non-diabetic relatives of diabetic subjects. The rate-limiting step in muscle glucose use is the transmembrane transport of glucose mediated by glucose transporter (GLUT) 4 (ref. 4), which is expressed mainly in skeletal muscle, heart and adipose tissue. GLUT4 mediates glucose transport stimulated by insulin and contraction/exercise. The importance of GLUT4 and glucose uptake in muscle, however, was challenged by two recent observations. Whereas heterozygous GLUT4 knockout mice show moderate glucose intolerance, homozygous whole-body GLUT4 knockout (GLUT4-null) mice have only mild perturbations in glucose homeostasis and have growth retardation, depletion of fat stores, cardiac hypertrophy and failure, and a shortened life span. Moreover, muscle-specific inactivation of the insulin receptor results in minimal, if any, change in glucose tolerance. To determine the importance of glucose uptake into muscle for glucose homeostasis, we disrupted GLUT4 selectively in mouse muscles. A profound reduction in basal glucose transport and near-absence of stimulation by insulin or contraction resulted. These mice showed severe insulin resistance and glucose intolerance from an early age. Thus, GLUT4-mediated glucose transport in muscle is essential to the maintenance of normal glucose homeostasis.

Cardiac Remodeling in Obesity

The dramatic increase in the prevalence of obesity and its strong association with cardiovascular disease have resulted in unprecedented interest in understanding the effects of obesity on the cardiovascular system. A consistent, but puzzling clinical observation is that obesity confers an increased susceptibility to the development of cardiac disease, while at the same time affording protection against subsequent mortality (termed the obesity paradox). In this review we focus on evidence available from human and animal model studies and summarize the ways in which obesity can influence structure and function of the heart. We also review current hypotheses regarding mechanisms linking obesity and various aspects of cardiac remodeling. There is currently great interest in the role of adipokines, factors secreted from adipose tissue, and their role in the numerous cardiovascular complications of obesity. Here we focus on the role of leptin and the emerging promise of adiponectin as a cardioprotective agent. The challenge of understanding the association between obesity and heart failure is complicated by the multifaceted interplay between various hemodynamic, metabolic, and other physiological factors that ultimately impact the myocardium. Furthermore, the end result of obesity-associated changes in the myocardial structure and function may vary at distinct stages in the progression of remodeling, may depend on the individual pathophysiology of heart failure, and may even remain undetected for decades before clinical manifestation. Here we summarize our current knowledge of this complex yet intriguing topic.

Mitochondrial Energetics in the Heart in Obesity-Related Diabetes: Direct Evidence for Increased Uncoupled Respiration and Activation of Uncoupling Proteins

OBJECTIVE—In obesity and diabetes, myocardial fatty acid utilization and myocardial oxygen consumption (MVo2) are increased, and cardiac efficiency is reduced. Mitochondrial uncoupling has been proposed to contribute to these metabolic abnormalities but has not been directly demonstrated. RESEARCH DESIGN AND METHODS—Oxygen consumption and cardiac function were determined in db/db hearts perfused with glucose or glucose and palmitate. Mitochondrial function was determined in saponin-permeabilized fibers and proton leak kinetics and H2O2 generation determined in isolated mitochondria. RESULTS—db/db hearts exhibited reduced cardiac function and increased MVo2. Mitochondrial reactive oxygen species (ROS) generation and lipid and protein peroxidation products were increased. Mitochondrial proliferation was increased in db/db hearts, oxidative phosphorylation capacity was impaired, but H2O2 production was increased. Mitochondria from db/db mice exhibited fatty acid–induced mitochondrial uncoupling that is inhibitable by GDP, suggesting that these changes are mediated by uncoupling proteins (UCPs). Mitochondrial uncoupling was not associated with an increase in UCP content, but fatty acid oxidation genes and expression of electron transfer flavoproteins were increased, whereas the content of the F1 α-subunit of ATP synthase was reduced. CONCLUSIONS—These data demonstrate that mitochondrial uncoupling in the heart in obesity and diabetes is mediated by activation of UCPs independently of changes in expression levels. This likely occurs on the basis of increased delivery of reducing equivalents from β-oxidation to the electron transport chain, which coupled with decreased oxidative phosphorylation capacity increases ROS production and lipid peroxidation.

Reduced Mitochondrial Oxidative Capacity and Increased Mitochondrial Uncoupling Impair Myocardial Energetics in Obesity

Background— Obesity is a risk factor for cardiovascular disease and is strongly associated with insulin resistance and type 2 diabetes. Recent studies in obese humans and animals demonstrated increased myocardial oxygen consumption (M&OV0312;o2) and reduced cardiac efficiency (CE); however, the underlying mechanisms remain unclear. The present study was performed to determine whether mitochondrial dysfunction and uncoupling are responsible for reduced cardiac performance and efficiency in ob/ob mice. Methods and Results— Cardiac function, M&OV0312;o2, mitochondrial respiration, and ATP synthesis were measured in 9-week-old ob/ob and control mouse hearts. Contractile function and M&OV0312;o2 in glucose-perfused ob/ob hearts were similar to controls under basal conditions but were reduced under high workload. Perfusion of ob/ob hearts with glucose and palmitate increased M&OV0312;o2 and reduced CE by 23% under basal conditions, and CE remained impaired at high workload. In glucose-perfused ob/ob hearts, mitochondrial state 3 respirations were reduced but ATP/O ratios were unchanged. In contrast, state 3 respiration rates were similar in ob/ob and control mitochondria from hearts perfused with palmitate and glucose, but ATP synthesis rates and ATP/O ratios were significantly reduced in ob/ob, which suggests increased mitochondrial uncoupling. Pyruvate dehydrogenase activity and protein levels of complexes I, III, and V were reduced in obese mice. Conclusions— These data indicate that reduced mitochondrial oxidative capacity may contribute to cardiac dysfunction in ob/ob mice. Moreover, fatty acid but not glucose-induced mitochondrial uncoupling reduces CE in obese mice by limiting ATP production and increasing M&OV0312;o2.

Insulin signaling coordinately regulates cardiac size, metabolism, and contractile protein isoform expression

To investigate the role of insulin signaling on postnatal cardiac development, physiology, and cardiac metabolism, we generated mice with a cardiomyocyte-selective insulin receptor knockout (CIRKO) using cre/loxP recombination. Hearts of CIRKO mice were reduced in size by 20-30% due to reduced cardiomyocyte size and had persistent expression of the fetal beta-myosin heavy chain isoform. In CIRKO hearts, glucose transporter 1 (GLUT1) expression was reduced by about 50%, but there was a twofold increase in GLUT4 expression as well as increased rates of cardiac glucose uptake in vivo and increased glycolysis in isolated working hearts. Fatty acid oxidation rates were diminished as a result of reduced expression of enzymes that catalyze mitochondrial beta-oxidation. Although basal rates of glucose oxidation were reduced, insulin unexpectedly stimulated glucose oxidation and glycogenolysis in CIRKO hearts. Cardiac performance in vivo and in isolated hearts was mildly impaired. Thus, insulin signaling plays an important developmental role in regulating postnatal cardiac size, myosin isoform expression, and the switching of cardiac substrate utilization from glucose to fatty acids. Insulin may also modulate cardiac myocyte metabolism through paracrine mechanisms by activating insulin receptors in other cell types within the heart.

Phosphoenolpyruvate Is a Metabolic Checkpoint of Anti-tumor T Cell Responses

Activated T cells engage aerobic glycolysis and anabolic metabolism for growth, proliferation, and effector functions. We propose that a glucose-poor tumor microenvironment limits aerobic glycolysis in tumor-infiltrating T cells, which suppresses tumoricidal effector functions. We discovered a new role for the glycolytic metabolite phosphoenolpyruvate (PEP) in sustaining T cell receptor-mediated Ca(2+)-NFAT signaling and effector functions by repressing sarco/ER Ca(2+)-ATPase (SERCA) activity. Tumor-specific CD4 and CD8 T cells could be metabolically reprogrammed by increasing PEP production through overexpression of phosphoenolpyruvate carboxykinase 1 (PCK1), which bolstered effector functions. Moreover, PCK1-overexpressing T cells restricted tumor growth and prolonged the survival of melanoma-bearing mice. This study uncovers new metabolic checkpoints for T cell activity and demonstrates that metabolic reprogramming of tumor-reactive T cells can enhance anti-tumor T cell responses, illuminating new forms of immunotherapy.

Cardiac hypertrophy with preserved contractile function after selective deletion of GLUT4 from the heart

Glucose enters the heart via GLUT1 and GLUT4 glucose transporters. GLUT4-deficient mice develop striking cardiac hypertrophy and die prematurely. Whether their cardiac changes are caused primarily by GLUT4 deficiency in cardiomyocytes or by metabolic changes resulting from the absence of GLUT4 in skeletal muscle and adipose tissue is unclear. To determine the role of GLUT4 in the heart we used cre-loxP recombination to generate G4H(-/-) mice in which GLUT4 expression is abolished in the heart but is present in skeletal muscle and adipose tissue. Life span and serum concentrations of insulin, glucose, FFAs, lactate, and beta-hydroxybutyrate were normal. Basal cardiac glucose transport and GLUT1 expression were both increased approximately 3-fold in G4H(-/-) mice, but insulin-stimulated glucose uptake was abolished. G4H(-/-) mice develop modest cardiac hypertrophy associated with increased myocyte size and induction of atrial natriuretic and brain natriuretic peptide gene expression in the ventricles. Myocardial fibrosis did not occur. Basal and isoproterenol-stimulated isovolumic contractile performance was preserved. Thus, selective ablation of GLUT4 in the heart initiates a series of events that results in compensated cardiac hypertrophy.