Selective chronic regulation of GLUT1 and GLUT4 content by insulin, glucose, and lipid in rat cardiac muscle in vivo

Pathological Significance of GLUT-1 Expression in Breast Cancer Cells in Diabetic and Obese Patients: The French Guiana Study

Simple Summary

This study describes the clinical, histological, and molecular features of breast cancer in French Guiana, and characterizes the expression of the tumor metabolic marker GLUT-1 in breast cancers cells in diabetic and obese patients compared to a control group. This study reveals an overall overexpression of GLUT-1 in 60% of invasive breast carcinomas and in all medullary pattern and carcinoma in situ lesions. Our results highlight the potential role of GLUT-1 as a tumor metabolic prognostic marker and also as an interesting target therapy, independently of patient metabolic disorder.

Abstract

The prevalence of obesity and type 2 diabetes is higher in French Guiana compared to mainland France. These metabolic disorders are associated with an increased risk of cancer. One of the factors involved is hyperinsulinemia that promotes the action of glucose transporter 1 (GLUT-1). The objective of this study is to characterize the expression of GLUT-1 in breast cancers cells in diabetic and obese patients compared to those who are not and to describe the clinical and histological prognostic factors of breast cancer in this population. We conducted a monocentric study including patients with breast cancer diagnosed between 2014 and 2020. Patients were classified into three groups: diabetes, obesity, and control group. The GLUT-1 expression was assessed by immunohistochemistry. In total, 199 patients were included in this study. The median age was 53.5 years, and the median tumor size was 2.8 cm. Luminal A was the most frequent molecular type (58.1%), followed by the triple-negative type (19.9%). The breast cancer in our population was characterized by a younger age at diagnosis, more aggressive molecular types, and larger tumor size. Thus, we suggest the advancement of the age of breast cancer screening in this territory. A total of 144 patients (31 diabetes, 22 obese, and 91 control group) were included for the study of GLUT-1 expression. Overexpression of GLUT-1 was observed in 60.4% of cases and in all carcinoma in situ lesions. GLUT-1 overexpression was associated with more aggressive cancers. This overexpression is correlated with high histological grade, high proliferation index, and aggressive molecular types. Our study found no difference in GLUT-1 expression between the diabetic or obese patients and the control group. These results highlight the potential role of GLUT-1 as a tumor metabolic prognostic marker and also as an interesting target therapy, independently of patient metabolic disorder.

Characteristics of Physiological 18F-Fluoro-2-Deoxy-D-Glucose Uptake and Comparison Between Cats and Dogs With Positron Emission Tomography

This study aimed to identify the physiological 18F-fluoro-2-deoxy-D-glucose (FDG) uptake in cats using positron emission tomography/computed tomography (PET/CT) and determine its characteristics by comparing physiological differences with dogs. Seven healthy cats and six healthy beagle dogs were examined using FDG-PET/CT. Regions of interest (ROIs) were manually drawn over 41 detailed structures of 5 gross structures (brain, head and neck, musculoskeleton, thorax, and abdomen). The mean and maximum standard uptake values (SUVmean and SUVmax) were calculated for each ROI. Physiological variation was classified as having increased radiopharmaceutical activity with no evidence of abnormal clinical or radiological findings. The brain had the highest SUV, which was observed in the cerebellum of both cats (SUVmean: 4.90 ± 1.04, SUVmax: 6.04 ± 1.24) and dogs (SUVmean: 3.15 ± 0.57, SUVmax: 3.90 ± 0.74). Cats had a significantly higher intracranial uptake than dogs did (P < 0.01). In the digestive system, the SUVs of the duodenum and jejunum were significantly higher in dogs than in cats (P < 0.05). FDG uptake of the submandibular tip, tonsils, neck of the gallbladder, and caudal colliculus were physiologically increased in cats. This study demonstrates physiological FDG uptake in normal tissues, and the differences between cats and dogs were interpreted based on species-specificity. This information contributes to improving the accurate diagnosis of cancer in cats and will aid in understanding glucose metabolism in both cats and dogs.

Sustained Activation of AMPK Enhances Differentiation of Human iPSC-Derived Cardiomyocytes via Sirtuin Activation

Recent studies suggest that metabolic regulation may improve differentiation of cardiomyocytes derived from induced pluripotent stem cells (iPSCs). AMP-activated protein kinase (AMPK) is a master regulator of metabolic activities. We investigated whether AMPK participates in iPSC-derived cardiomyocyte differentiation. We observed that AMPK phosphorylation at Thr172 increased at day 9 but then decreased after day 11 of differentiation to cardiomyocytes. Inhibition of AMPK with compound C significantly reduced mRNA and protein expression of cardiac troponins TNNT2 and TNNI3. Moreover, sustained AMPK activation using AICAR from days 9 to 14 of differentiation increased mRNA and protein expression of both TNNT2 and TNNI3. AICAR decreased acetylation of histone 3 at Lys9 and 56 and histone 4 at Lys16 (known target sites for nuclear-localized sirtuins [SIRT1, SIRT6]), suggesting that AMPK activation enhances sirtuin activity. Sustained AMPK activation during days 9–14 of differentiation induces sirtuin-mediated histone deacetylation and may enhance cardiomyocyte differentiation from iPSCs.

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iPSC-derived cardiomyocytes transiently increased AMPK phosphorylation at Thr172

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Chemical inhibition of AMPK with compound C decreased TNNI3 and TNNT2 expression

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Sustained activation of AMPK using AICAR increased expression of TNNT2 and TNNI3

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AICAR decreased acetylation of histones H3 (at Lys9 and Lys56) and H4 (at Lys16).

Impact of hyperinsulinemia and hyperglycemia on valvular interstitial cells - A link between aortic heart valve degeneration and type 2 diabetes

Type 2 diabetes is a known risk factor for cardiovascular diseases and is associated with an increased risk to develop aortic heart valve degeneration. Nevertheless, molecular mechanisms leading to the pathogenesis of valve degeneration in the context of diabetes are still not clear. Hence, we hypothesized that classical key factors of type 2 diabetes, hyperinsulinemia and hyperglycemia, may affect signaling, metabolism and degenerative processes of valvular interstitial cells (VIC), the main cell type of heart valves. Therefore, VIC were derived from sheep and were treated with hyperinsulinemia, hyperglycemia and the combination of both. The presence of insulin receptors was shown and insulin led to increased proliferation of the cells, whereas hyperglycemia alone showed no effect. Disturbed insulin response was shown by impaired insulin signaling, i.e. by decreased phosphorylation of Akt/GSK-3α/β pathway. Analysis of glucose transporter expression revealed absence of glucose transporter 4 with glucose transporter 1 being the predominantly expressed transporter. Glucose uptake was not impaired by disturbed insulin response, but was increased by hyperinsulinemia and was decreased by hyperglycemia. Analyses of glycolysis and mitochondrial respiration revealed that VIC react with increased activity to hyperinsulinemia or hyperglycemia, but not to the combination of both. VIC do not show morphological changes and do not acquire an osteogenic phenotype by hyperinsulinemia or hyperglycemia. However, the treatment leads to increased collagen type 1 and decreased α-smooth muscle actin expression. This work implicates a possible role of diabetes in early phases of the degeneration of aortic heart valves.

Glucose Transporters in Cardiac Metabolism and Hypertrophy

The heart is adapted to utilize all classes of substrates to meet the high-energy demand, and it tightly regulates its substrate utilization in response to environmental changes. Although fatty acids are known as the predominant fuel for the adult heart at resting stage, the heart switches its substrate preference toward glucose during stress conditions such as ischemia and pathological hypertrophy. Notably, increasing evidence suggests that the loss of metabolic flexibility associated with increased reliance on glucose utilization contribute to the development of cardiac dysfunction. The changes in glucose metabolism in hypertrophied hearts include altered glucose transport and increased glycolysis. Despite the role of glucose as an energy source, changes in other nonenergy producing pathways related to glucose metabolism, such as hexosamine biosynthetic pathway and pentose phosphate pathway, are also observed in the diseased hearts. This article summarizes the current knowledge regarding the regulation of glucose transporter expression and translocation in the heart during physiological and pathological conditions. It also discusses the signaling mechanisms governing glucose uptake in cardiomyocytes, as well as the changes of cardiac glucose metabolism under disease conditions.

Glucose and fatty acid metabolism in infarcted heart from streptozotocin-induced diabetic rats after 2 weeks of tissue remodeling

Background

The effects of streptozotocin (STZ)-induced diabetes on heart metabolism and function after myocardial infarction (MI) remodelling were investigated in rats.

Methods

Fifteen days after STZ (50 mg/kg b.w. i.v.) injection, MI was induced by surgical occlusion of the left coronary artery. Two weeks after MI induction, contents of glycogen, ATP, free fatty acids and triacylglycerols (TG) and enzyme activities of glycolysis and Krebs cycle (hexokinase, glucose-6-phosphate dehydrogenase, phosphofructokinase, citrate synthase) and expression of carnitine palmitoyl-CoA transferase I (a key enzyme of mitochondrial fatty acid oxidation) were measured in the left ventricle (LV). Plasma glucose, free fatty acids and triacylglycerol levels were determined. Ejection fraction (EF) and shortening fraction (SF) were also measured by echocardiography.

Results

Glycogen and TG contents were increased (p < 0.05) whereas ATP content was decreased in the LV of the non-infarcted diabetic group when compared to the control group (p < 0.05). When compared to infarcted control rats (MI), the diabetic infarcted rats (DI) showed (p < 0.05): increased plasma glucose and TG levels, elevated free fatty acid levels and increased activity of, citrate synthase and decreased ATP levels in the LV. Infarct size was smaller in the DI group when compared to MI rats (p < 0.05), and this was associated with higher EF and SF (p < 0.05).

Conclusions

Systolic function was preserved or recovered more efficiently in the heart from diabetic rats two weeks after MI, possibly due to the high provision of glucose and free fatty acids from both plasma and heart glycogen and triacylglycerol stores.

Glucose-insulin-potassium solution protects ventricular myocytes of neonatal rat in an in vitro coverslip ischemia/reperfusion model

Background and Objectives

The benefit of high glucose-insulin-potassium (GIK) solution in clinical applications is controversial. We established a neonatal rat ventricular myocyte (NRVM) in vitro coverslip ischemia/reperfusion (I/R) model and investigated the effects of GIK solution on suppressing reactive oxygen species (ROS) and upregulating O-GlcNacylation, which protects cells from ischemic injury.

Materials and Methods

NRVMs were isolated from postnatal day 3-4 Sprague-Dawley rat pups and grown in Dulbecco's modified Eagle's medium containing high glucose (4.5 g/L), fetal bovine serum, and penicillin/streptomycin. The effects of the GIK solution on ROS production, apoptosis, and expression of O-GlcNAc and O-GlcNAc transferase (OGT) were investigated in the coverslip I/R model.

Results

Covering the 24-well culture plates for 3 hr with 12 mm diameter coverslips resulted in the appropriate ischemic shock. Glucose and insulin synergistically reduced ROS production, protected NRVM dose-dependently from apoptosis, and altered O-GlcNAc and OGT expression.

Conclusion

The high GIK solution protected NRVM from I/R injury in vitro by reducing ROS and altering O-GlcNacylation.

High dose of aspirin moderates diabetes-induced changes of heart glycogen/glucose metabolism in rats

Aspirin (ASA), as a multifunctional drug has been used as a hypoglycaemic agent in the treatment of diabetes and severe hyperglycaemia and has been established as a secondary strategy which may prevent many cardiovascular events. In this study we investigated high dose (100 mg/kg b.w./i.p) and time-dependent (2, 7 and 14 days) effects of ASA on the heart key enzymes and substrates from glycogen/glucose metabolism in control and diabetic rats. The results accomplished demonstrated that ASA significantly potentiates glycogen accumulation, as well as decreased blood glucose level and heart glycolytic potential in control rats. The treatment of diabetic rats with ASA caused moderation of the diabetic complication-significant inhibition of glycogen accumulation, lowering of blood glucose, as well as elevation of glycolytic potential. In conclusion, we propose that use of high-dose of ASA has anabolic effects in control rats and reduces heart glycogen glucose complications in diabetic rats. The moderation of diabetes-induced changes is time-dependent and involves reduction of glycogenogenesis and inhibited depression of glycolysis, with a tendency to maintenance control values.

Absence of glucose transporter 4 diminishes electrical activity of mouse hearts during hypoxia

Insulin resistance, which characterizes type 2 diabetes, is associated with reduced translocation of glucose transporter 4 (GLUT4) to the plasma membrane following insulin stimulation, and diabetic patients with insulin resistance show a higher incidence of ischaemia, arrhythmias and sudden cardiac death. The aim of this study was to examine whether GLUT4 deficiency leads to more severe alterations in cardiac electrical activity during cardiac stress due to hypoxia. To fulfil this aim, we compared cardiac electrical activity from cardiac-selective GLUT4-ablated (G4H-/-) mouse hearts and corresponding control (CTL) littermates. A custom-made cylindrical 'cage' electrode array measured potentials (Ves) from the epicardium of isolated, perfused mouse hearts. The normalized average of the maximal downstroke of Ves ( (|d Ves/dt(min)|na), which we previously introduced as an index of electrical activity in normal, ischaemic and hypoxic hearts, was used to assess the effects of GLUT4 deficiency on electrical activity. The |d Ves/dt(min)|na of G4H −/− and CTL hearts decreased by 75 and 47%, respectively (P < 0.05), 30 min after the onset of hypoxia. Administration of insulin attenuated decreases in values of |d Ves/dt(min)|na in G4H −/− hearts as well as in CTL hearts, during hypoxia. In general, however, G4H −/− hearts showed a severe alteration of the propagation sequence and a prolonged total activation time. Results of this study demonstrate that reduced glucose availability associated with insulin resistance and a reduction in GLUT4-mediated glucose transport impairs electrical activity during hypoxia, and may contribute to cardiac vulnerability to arrhythmias in diabetic patients.

SIRT6 deficiency results in severe hypoglycemia by enhancing both basal and insulin-stimulated glucose uptake in mice

Glucose homeostasis in mammals is mainly regulated by insulin signaling. It was previously shown that SIRT6 mutant mice die before 4 weeks of age, displaying profound abnormalities, including low insulin, hypoglycemia, and premature aging. To investigate mechanisms underlying the pleiotropic phenotypes associated with SIRT6 deficiency, we generated mice carrying targeted disruption of SIRT6. We found that 60% of SIRT6(-/-) animals had very low levels of blood glucose and died shortly after weaning. The remaining animals, which have relatively higher concentrations of glucose, survived the early post-weaning lethality, but most died within one year of age. Significantly, feeding the mice with glucose-containing water increased blood glucose and rescued 83% of mutant mice, suggesting that the hypoglycemia is a major cause for the lethality. We showed that SIRT6 deficiency results in more abundant membrane association of glucose transporters 1 and 4, which enhances glucose uptake. We further demonstrated that SIRT6 negatively regulates AKT phosphorylation at Ser-473 and Thr-308 through inhibition of multiple upstream molecules, including insulin receptor, IRS1, and IRS2. The absence of SIRT6, consequently, enhances insulin signaling and activation of AKT, leading to hypoglycemia. These data uncover an essential role of SIRT6 in modulating glucose metabolism through mediating insulin sensitivity.

Immunogold labeling study of the distribution of GLUT-1 and GLUT-4 in cardiac tissue following stimulation by insulin or ischemia

Whereas glucose transporter 1 (GLUT-1) is thought to be responsible for basal glucose uptake in cardiac myocytes, little is known about its relative distribution between the different plasma membranes and cell types in the heart. GLUT-4 translocates to the myocyte surface to increase glucose uptake in response to a number of stimuli. The mechanisms underlying ischemia- and insulin-mediated GLUT-4 translocation are known to be different, raising the possibility that the intracellular destinations of GLUT-4 following these stimuli also differ. Using immunogold labeling, we describe the cellular localization of these two transporters and investigate whether insulin and ischemia induce differential translocation of GLUT-4 to different cardiac membranes. Immunogold labeling of GLUT-1 and GLUT-4 was performed on left ventricular sections from isolated hearts following 30 min of either insulin, ischemia, or control perfusion. In control tissue, GLUT-1 was predominantly (76%) localized in the capillary endothelial cells, with only 24% of total cardiac GLUT-1 present in myocytes. GLUT-4 was found predominantly in myocytes, distributed between sarcolemmal and T tubule membranes (1.84 +/- 0.49 and 1.54 +/- 0.33 golds/microm, respectively) and intracellular vesicles (127 +/- 18 golds/microm(2)). Insulin increased T tubule membrane GLUT-4 content (2.8 +/- 0.4 golds/microm, P < 0.05) but had less effect on sarcolemmal GLUT-4 (1.72 +/- 0.53 golds/microm). Ischemia induced greater GLUT-4 translocation to both membrane types (4.25 +/- 0.84 and 4.01 +/- 0.27 golds/microm, respectively P < 0.05). The localization of GLUT-1 suggests a significant role in transporting glucose across the capillary wall before myocyte uptake via GLUT-1 and GLUT-4. We demonstrate independent spatial translocation of GLUT-4 under insulin or ischemic stimulation and propose independent roles for T-tubular and sarcolemmal GLUT-4.

Role of changes in cardiac metabolism in development of diabetic cardiomyopathy

In patients with diabetes, an increased risk of symptomatic heart failure usually develops in the presence of hypertension or ischemic heart disease. However, a predisposition to heart failure might also reflect the effects of underlying abnormalities in diastolic function that can occur in asymptomatic patients with diabetes alone (termed diabetic cardiomyopathy). Evidence of cardiomyopathy has also been demonstrated in animal models of both Type 1 (streptozotocin-induced diabetes) and Type 2 diabetes (Zucker diabetic fatty rats and ob/ob or db/db mice). During insulin resistance or diabetes, the heart rapidly modifies its energy metabolism, resulting in augmented fatty acid and decreased glucose consumption. Accumulating evidence suggests that this alteration of cardiac metabolism plays an important role in the development of cardiomyopathy. Hence, a better understanding of this dysregulation in cardiac substrate utilization during insulin resistance and diabetes could provide information as to potential targets for the treatment of cardiomyopathy. This review is focused on evaluating the acute and chronic regulation and dysregulation of cardiac metabolism in normal and insulin-resistant/diabetic hearts and how these changes could contribute toward the development of cardiomyopathy.

Effects of chronic Akt activation on glucose uptake in the heart

Acute activation of the serine-threonine kinase Akt is cardioprotective and increases glucose uptake, at least in part, through enhanced expression of GLUT4 on the sarcolemma. The effects of chronic Akt activation on glucose uptake in the heart remain unclear. To address this issue, we examined the effects of chronic Akt activation on glucose uptake, glycogen storage, and relevant glucose transporters in the hearts of transgenic mice. We found that chronic cardiac activation of Akt led to a substantial increase in the rate of basal glucose uptake (P < 0.05) but blunted the response to insulin (1.9 vs. 18.1-fold increase compared with baseline) using NMR in ex vivo perfused heart. Basal glucose uptake was also increased in Akt transgenic mice in vivo (P < 0.005). These changes were associated with an increase on glycogen deposition, examined with histochemical staining, biochemical (>6-fold, P < 0.001) and in vivo radioactive (5-fold, P < 0.01) assays. Studies in chimeric hearts of female X-linked transgenic Akt mice suggested that increased glycogen deposition occurred as a cell autonomous effect of transgene expression. Interestingly, although sarcolemmal GLUT1 was not significantly altered, chronic Akt activation actually decreased plasma membrane GLUT4. Moreover, intracellular pools of GLUT1 were modestly reduced, whereas intracellular GLUT4 was substantially reduced. It seems likely that neither GLUT1 nor GLUT4 explains the increase in basal glucose uptake but that these reductions contribute to the loss of insulin responsiveness that we observed. These data demonstrate that chronic Akt activation increases basal glucose uptake and glycogen deposition while inhibiting the response to insulin.

Expression of genes participating in regulation of fatty acid and glucose utilization and energy metabolism in developing rat hearts

The heart is a unique organ that can use several fuels for energy production. During development, the heart undergoes changes in fuel supply, and it must be able to respond to these changes. We have examined changes in the expression of several genes that regulate fuel transport and metabolism in rat hearts during early development. At birth, there was increased expression of fatty acid transporters and enzymes of fatty acid metabolism that allow fatty acids to become the major source of energy for cardiac muscle during the first 2 wk of life. At the same time, expression of genes that control glucose transport and oxidation was downregulated. After 2 wk, expression of genes for glucose uptake and oxidation was increased, and expression of genes for fatty acid uptake and utilization was decreased. Expression of carnitine palmitoyltransferase I (CPT I) isoforms during development was different from published data obtained from rabbit hearts. CPT Iα and Iβ isoforms were both highly expressed in hearts before birth, and both increased further at birth. Only after the second week did CPT Iα expression decrease appreciably below the level of CPT Iβ expression. These results represent another example of different expression patterns of CPT I isoforms among various mammalian species. In rats, changes in gene expression followed nutrient availability during development and may render cardiac fatty acid oxidation less sensitive to factors that influence malonyl-CoA content (e.g., fluctuations in glucose concentration) and thereby favor fatty acid oxidation as an energy source for cardiomyocytes in early development.

Glycogen metabolism in rat heart muscle cultures after hypoxia

Elevated glycogen levels in heart have been shown to have cardioprotective effects against ischemic injury. We have therefore established a model for elevating glycogen content in primary rat cardiac cells grown in culture and examined potential mechanisms for the elevation (glycogen supercompensation). Glycogen was depleted by exposing the cells to hypoxia for 2 h in the absence of glucose in the medium. This was followed by incubating the cells with 28 mM glucose in normoxia for up to 120 h. Hypoxia decreased glycogen content to about 15% of control, oxygenated cells. This was followed by a continuous increase in glycogen in the hypoxia treated cells during the 120 h recovery period in normoxia. By 48 h after termination of hypoxia, the glycogen content had returned to baseline levels and by 120 h glycogen was about 150% of control. The increase in glycogen at 120 h was associated with comparable relative increases in glucose uptake (approximately 180% of control) and the protein level of the glut-1 transporter (approximately 170% of control), whereas the protein level of the glut-4 transporter was decreased to < 10% of control. By 120 h, the hypoxia-treated cells also exhibited marked increases in the total (approximately 170% of control) and fractional activity of glycogen synthase (control, approximately 15%; hypoxia-treated, approximately 30%). Concomitantly, the hypoxia-treated cells also exhibited marked decreases in the total (approximately 50% of control) and fractional activity of glycogen phosphorylase (control, approximately 50%; hypoxia-treated, approximately 25%). Thus, we have established a model of glycogen supercompensation in cultures of cardiac cells that is explained by concerted increases in glucose uptake and glycogen synthase activity and decreases in phosphorylase activity. This model should prove useful in studying the cardioprotective effects of glycogen.

Increased glucose uptake promotes oxidative stress and PKC-delta activation in adipocytes of obese, insulin-resistant mice

Increased oxidative stress is believed to be one of the mechanisms responsible for hyperglycemia-induced tissue damage and diabetic complications. In these studies, we undertook to characterize glucose uptake and oxidative stress in adipocytes of type 2 diabetic animals and to determine whether these promote the activation of PKC-delta. The adipocytes used were isolated either from C57Bl/6J mice that were raised on a high-fat diet (HF) and developed obesity and insulin resistance or from control animals. Basal glucose uptake significantly increased (8-fold) in HF adipocytes, and this was accompanied with upregulation of GLUT1 expression levels. Insulin-induced glucose uptake was inhibited in HF adipocytes and GLUT4 content reduced by 20% in these adipocytes. Reactive oxygen species (ROS) increased twofold in HF adipocytes compared with control adipocytes and were largely reduced with decreased glucose concentrations. At zero glucose, ROS levels were reduced to the normal levels seen in control adipocytes. The activity of PKC-delta increased twofold in HF adipocytes compared with control adipocytes and was further activated by H2O2. Moreover, PKC-delta activity was inhibited in HF adipocytes either by glucose deprivation or by treatment with the antioxidant N-acetyl-l-cysteine. In summary, we propose that increased glucose intake in HF adipocytes increases oxidative stress, which in turn promotes the activation of PKC-delta. These consequential events may be responsible, at least in part, for development of HF diet-induced insulin resistance in the fat tissue.

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.

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.