Glucose Uptake In Cardiomyocytes Research Articles

Foetal recapitulation of nutrient surplus signalling by O-GlcNAcylation and the failing heart.

The development of the foetal heart is driven by increased glucose uptake and activation of mammalian target of rapamycin (mTOR) and hypoxia-inducible factor-1α (HIF-1α), which drives glycolysis. In contrast, the healthy adult heart is governed by sirtuin-1 (SIRT1) and adenosine monophosphate-activated protein kinase (AMPK), which promote fatty-acid oxidation and the substantial mitochondrial ATP production required for survival in a high-workload normoxic environment. During cardiac injury, the heart recapitulates the foetal signalling programme, which (although adaptive in the short term) is highly deleterious if sustained for long periods of time. Prolonged increases in glucose uptake in cardiomyocytes under stress leads to increased flux through the hexosamine biosynthesis pathway; its endproduct-uridine diphosphate N-acetylglucosamine (UDP-GlcNAc)-functions as a critical nutrient surplus sensor. UDP-GlcNAc drives the post-translational protein modification known as O-GlcNAcylation, which rapidly and reversibly modifies thousands of intracellular proteins. Both O-GlcNAcylation and phosphorylation act at serine/threonine residues, but whereas phosphorylation is regulated by hundreds of specific kinases and phosphatases, O-GlcNAcylation is regulated by only two enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), which adds or removes GlcNAc (N-acetylglucosamine), respectively, from target proteins. Recapitulation of foetal programming in heart failure (regardless of diabetes) is accompanied by marked increases in O-GlcNAcylation, both experimentally and clinically. Heightened O-GlcNAcylation in the heart leads to impaired calcium kinetics and contractile derangements, arrhythmias related to activation of voltage-gated sodium channels and Ca2+ /calmodulin-dependent protein kinase II, mitochondrial dysfunction, and maladaptive hypertrophy, microvascular dysfunction, fibrosis and cardiomyopathy. These deleterious effects can be prevented by suppression of O-GlcNAcylation, which can be achieved experimentally by upregulation of AMPK and SIRT1 or by pharmacological inhibition of OGT or stimulation of OGA. The effects of sodium-glucose cotransporter 2 (SGLT2) inhibitors on the heart are accompanied by reduced O-GlcNAcylation, and their cytoprotective effects are reportedly abrogated if their action to suppress O-GlcNAcylation is blocked. Such an action may represent one of the many mechanisms by which enhanced AMPK and SIRT1 signalling following SGLT2 inhibition leads to cardiovascular benefits. These observations, taken collectively, suggest that UDP-GlcNAc functions as a critical nutrient surplus sensor (which acting in concert with mTOR and HIF-1α) can promote the development of cardiomyopathy.

Interaction of ARRDC4 With GLUT1 Mediates Metabolic Stress in the Ischemic Heart.

An ancient family of arrestin-fold proteins, termed alpha-arrestins, may have conserved roles in regulating nutrient transporter trafficking and cellular metabolism as adaptor proteins. One alpha-arrestin, TXNIP (thioredoxin-interacting protein), is known to regulate myocardial glucose uptake. However, the in vivo role of the related alpha-arrestin, ARRDC4 (arrestin domain-containing protein 4), is unknown. We first tested whether interaction with GLUTs (glucose transporters) is a conserved function of the mammalian alpha-arrestins. To define the in vivo function of ARRDC4, we generated and characterized a novel Arrdc4 knockout (KO) mouse model. We then analyzed the molecular interaction between arrestin domains and the basal GLUT1. ARRDC4 binds to GLUT1, induces its endocytosis, and blocks cellular glucose uptake in cardiomyocytes. Despite the closely shared protein structure, ARRDC4 and its homologue TXNIP operate by distinct molecular pathways. Unlike TXNIP, ARRDC4 does not increase oxidative stress. Instead, ARRDC4 uniquely mediates cardiomyocyte death through its effects on glucose deprivation and endoplasmic reticulum stress. At baseline, Arrdc4-KO mice have mild fasting hypoglycemia. Arrdc4-KO hearts exhibit a robust increase in myocardial glucose uptake and glycogen storage. Accordingly, deletion of Arrdc4 improves energy homeostasis during ischemia and protects cardiomyocytes against myocardial infarction. Furthermore, structure-function analyses of the interaction of ARRDC4 with GLUT1 using both scanning mutagenesis and deep-learning Artificial Intelligence identify specific residues in the C-terminal arrestin-fold domain as the interaction interface that regulates GLUT1 function, revealing a new molecular target for potential therapeutic intervention against myocardial ischemia. These results uncover a new mechanism of ischemic injury in which ARRDC4 drives glucose deprivation-induced endoplasmic reticulum stress leading to cardiomyocyte death. Our findings establish ARRDC4 as a new scaffold protein for GLUT1 that regulates cardiac metabolism in response to ischemia and provide insight into the therapeutic strategy for ischemic heart disease.

Trimetazidine Attenuates Heart Failure by Improving Myocardial Metabolism via AMPK.

Energic deficiency of cardiomyocytes is a dominant cause of heart failure. An antianginal agent, trimetazidine improves the myocardial energetic supply. We presumed that trimetazidine protects the cardiomyocytes from the pressure overload-induced heart failure through improving the myocardial metabolism. C57BL/6 mice were subjected to transverse aortic constriction (TAC). After 4 weeks of TAC, heart failure was observed in mice manifested by an increased left ventricular (LV) chamber dimension, an impaired LV ejection fraction evaluated by echocardiography analysis, which were significantly restrained by the treatment of trimetazidine. Trimetazidine restored the mitochondrial morphology and function tested by cardiac transmission electron microscope and mitochondrial dynamic proteins analysis. Positron emission tomography showed that trimetazidine significantly elevated the glucose uptake in TAC mouse heart. Trimetazidine restrained the impairments of the insulin signaling in TAC mice and promoted the translocation of glucose transporter type IV (GLUT4) from the storage vesicle to membrane. However, these cardioprotective effects of trimetazidine in TAC mice were notably abolished by compound C (C.C), a specific AMPK inhibitor. The enlargement of neonatal rat cardiomyocyte induced by mechanical stretch, together with the increased expression of hypertrophy-associated proteins, mitochondria deformation and dysfunction were significantly ameliorated by trimetazidine. Trimetazidine enhanced the isolated cardiomyocyte glucose uptake in vitro. These benefits brought by trimetazidine were also removed with the presence of C.C. In conclusion, trimetazidine attenuated pressure overload-induced heart failure through improving myocardial mitochondrial function and glucose uptake via AMPK.

Phosphatidylinositol 4-kinase III\u03b2 mediates contraction-induced GLUT4 translocation and shows its anti-diabetic action in cardiomyocytes

In the diabetic heart, long-chain fatty acid (LCFA) uptake is increased at the expense of glucose uptake. This metabolic shift ultimately leads to insulin resistance and a reduced cardiac function. Therefore, signaling kinases that mediate glucose uptake without simultaneously stimulating LCFA uptake could be considered attractive anti-diabetic targets. Phosphatidylinositol-4-kinase-IIIβ (PI4KIIIβ) is a lipid kinase downstream of protein kinase D1 (PKD1) that mediates Golgi-to-plasma membrane vesicular trafficking in HeLa-cells. In this study, we evaluated whether PI4KIIIβ is involved in myocellular GLUT4 translocation induced by contraction or oligomycin (an F1F0-ATP synthase inhibitor that activates contraction-like signaling). Pharmacological targeting, with compound MI14, or genetic silencing of PI4KIIIβ inhibited contraction/oligomycin-stimulated GLUT4 translocation and glucose uptake in cardiomyocytes but did not affect CD36 translocation nor LCFA uptake. Addition of the PI4KIIIβ enzymatic reaction product phosphatidylinositol-4-phosphate restored oligomycin-stimulated glucose uptake in the presence of MI14. PI4KIIIβ activation by PKD1 involves Ser294 phosphorylation and altered its localization with unchanged enzymatic activity. Adenoviral PI4KIIIβ overexpression stimulated glucose uptake, but did not activate hypertrophic signaling, indicating that unlike PKD1, PI4KIIIβ is selectively involved in GLUT4 translocation. Finally, PI4KIIIβ overexpression prevented insulin resistance and contractile dysfunction in lipid-overexposed cardiomyocytes. Together, our studies identify PI4KIIIβ as positive and selective regulator of GLUT4 translocation in response to contraction-like signaling, suggesting PI4KIIIβ as a promising target to rescue defective glucose uptake in diabetics.

MiR-150 regulates glucose utilization through targeting GLUT4 in insulin-resistant cardiomyocytes.

MicroRNAs (miRNAs) play an important role in cardiac function and metabolism. However, whether they regulate insulin resistance (IR) of cardiomyocytes remains unclear. The aim of the present study was to shed light on this issue with a focus on miR-150. We found here that miR-150 level was elevated in myocardium of type 2 diabetes mellitus (T2DM) rat model and in insulin-resistant cardiomyocytes induced by high glucose (25 mM) and high insulin (1 μM). Deregulation of miR-150 downregulated the protein and mRNA levels of glucose transporter 4 (GLUT4) as assessed by western blot, real-time polymerase chain reaction (qPCR), and immunofluorescence assays. Overexpression of miR-150 inhibited glucose utilization in cardiomyocytes as detected by 2-deoxyglucose transport and glucose consumption assays. In contrast, knockdown of miR-150 significantly increased glucose uptake in cardiomyocytes. Moreover, GLUT4 translocation was increased after transfection of miR-150 inhibitor (AMO-150). Collectively, miR-150 reduced glucose utilization by directly decreasing the expression and translocation of GLUT4 in the cardiomyocytes with IR and therefore might be a new therapeutic target for metabolic diseases such as T2DM.

Maintaining Myocardial Glucose Utilization in Diabetic Cardiomyopathy Accelerates Mitochondrial Dysfunction.

Cardiac glucose uptake and oxidation are reduced in diabetes despite hyperglycemia. Mitochondrial dysfunction contributes to heart failure in diabetes. It is unclear whether these changes are adaptive or maladaptive. To directly evaluate the relationship between glucose delivery and mitochondrial dysfunction in diabetic cardiomyopathy, we generated transgenic mice with inducible cardiomyocyte-specific expression of the GLUT4. We examined mice rendered hyperglycemic following low-dose streptozotocin prior to increasing cardiomyocyte glucose uptake by transgene induction. Enhanced myocardial glucose in nondiabetic mice decreased mitochondrial ATP generation and was associated with echocardiographic evidence of diastolic dysfunction. Increasing myocardial glucose delivery after short-term diabetes onset exacerbated mitochondrial oxidative dysfunction. Transcriptomic analysis revealed that the largest changes, driven by glucose and diabetes, were in genes involved in mitochondrial function. This glucose-dependent transcriptional repression was in part mediated by O-GlcNAcylation of the transcription factor Sp1. Increased glucose uptake induced direct O-GlcNAcylation of many electron transport chain subunits and other mitochondrial proteins. These findings identify mitochondria as a major target of glucotoxicity. They also suggest that reduced glucose utilization in diabetic cardiomyopathy might defend against glucotoxicity and caution that restoring glucose delivery to the heart in the context of diabetes could accelerate mitochondrial dysfunction by disrupting protective metabolic adaptations.

Multimodality Imaging of Inflammation and Ventricular Remodeling in Pressure-Overload Heart Failure.

Inflammation contributes to ventricular remodeling after myocardial ischemia, but its role in nonischemic heart failure is poorly understood. Local tissue inflammation is difficult to assess serially during pathogenesis. Although 18F-FDG accumulates in inflammatory leukocytes and thus may identify inflammation in the myocardial microenvironment, it remains unclear whether this imaging technique can isolate diffuse leukocytes in pressure-overload heart failure. We aimed to evaluate whether inflammation with 18F-FDG can be serially imaged in the early stages of pressure-overload-induced heart failure and to compare the time course with functional impairment assessed by cardiac MRI. Methods: C57Bl6/N mice underwent transverse aortic constriction (TAC) (n = 22), sham surgery (n = 12), or coronary ligation as an inflammation-positive control (n = 5). MRI assessed ventricular geometry and contractile function at 2 and 8 d after TAC. Immunostaining identified the extent of inflammatory leukocyte infiltration early in pressure overload. 18F-FDG PET scans were acquired at 3 and 7 d after TAC, under ketamine-xylazine anesthesia to suppress cardiomyocyte glucose uptake. Results: Pressure overload evoked rapid left ventricular dilation compared with sham (end-systolic volume, day 2: 40.6 ± 10.2 μL vs. 23.8 ± 1.7 μL, P < 0.001). Contractile function was similarly impaired (ejection fraction, day 2: 40.9% ± 9.7% vs. 59.2% ± 4.4%, P < 0.001). The severity of contractile impairment was proportional to histology-defined myocardial macrophage density on day 8 (r = -0.669, P = 0.010). PET imaging identified significantly higher left ventricular 18F-FDG accumulation in TAC mice than in sham mice on day 3 (10.5 ± 4.1 percentage injected dose [%ID]/g vs. 3.8 ± 0.9 %ID/g, P < 0.001) and on day 7 (7.8 ± 3.7 %ID/g vs. 3.0 ± 0.8 %ID/g, P = 0.006), though the efficiency of cardiomyocyte suppression was variable among TAC mice. The 18F-FDG signal correlated with ejection fraction (r = -0.75, P = 0.01) and ventricular volume (r = 0.75, P < 0.01). Western immunoblotting demonstrated a 60% elevation of myocardial glucose transporter 4 expression in the left ventricle at 8 d after TAC, indicating altered glucose metabolism. Conclusion: TAC induces rapid changes in left ventricular geometry and contractile function, with a parallel modest infiltration of inflammatory macrophages. Metabolic remodeling overshadows inflammatory leukocyte signal using 18F-FDG PET imaging. More selective inflammatory tracers are requisite to identify the diffuse local inflammation in pressure overload.

Nicotine induces insulin resistance via downregulation of Nrf2 in cardiomyocyte

Clinical studies have demonstrated that cigarette smoking is strongly associated with insulin resistance and heart disease. Nicotine is considered the primary toxin constituent associated with smoking. However, the distinct molecular mechanism of nicotine-induced cardiac dysfunction remains unclear. Cardiomyocytes with nicotine-induced insulin resistance are characterized by decreased glucose uptake, as measured by 2-[N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-d-glucose (2-NBDG), a fluorescent derivative of glucose, and reactive oxygen species (ROS) generation. Immunoblotting was used to evaluate the expression of nuclear factor erythroid 2-related factor 2 (Nrf2), extracellular signal-related kinase (ERK) and phosphoinositide 3-kinase (PI3K, p85, Y607). We determined the impact of nicotine on insulin resistance and Nrf2, phospho-ERK and phospho-PI3K expression in the myocardial tissue of a mouse model. Nicotine increased ROS production and depressed insulin-induced glucose uptake in cardiomyocytes. Pretreatment with N-acetyl-L-cysteine (NAC), an antioxidant, reversed nicotine-inhibited glucose uptake induced by insulin. Nicotine exposure directly inhibited Nrf2 and increased ERK phosphorylation in cardiomyocytes, which were obstructed by NAC. Further exploration of signaling cascades revealed nicotine-induced ROS involved in inhibiting PI3K/Nrf2 and activating ERK in cardiomyocytes. Moreover, the mouse model treated with nicotine showed glucose intolerance and impaired insulin tolerance accompanied by inhibited PI3K/Nrf2 and increased ERK in myocardial tissues. Thus, nicotine induces insulin resistance via the downregulation of Nrf2 activity in cardiomyocytes, which is a potential mechanism of the pharmacological effects of nicotine. This study identified potential therapeutic targets against nicotine-related cardiovascular diseases.

Glucose Metabolism in Cardiac Hypertrophy and Heart Failure.

Glucose Metabolism in Cardiac Hypertrophy and Heart Failure.

IL‐15 Activates Akt Independent of AMPK Signaling in Cardiomyocytes

Obesity is a major contributor to the onset and progression of many diseases, such as cardiovascular disease (CVD). Diet and exercise are known to be successful treatments for obesity and ultimately CVD. However, implementation of lifestyle interventions is difficult to maintain. Thus, focus has turned to examine the role of molecular mediators of physical activity to prevent obesity and its related disorders. Following exercise, many factors are secreted into circulation and act to regulate various metabolic processes. Interleukin‐IL‐15 (IL‐15) is a myokine, secreted from skeletal muscle (SKM), and has been shown to increase in circulation as a result of exercise. Further, IL‐15 acts to reduce fat mass while increasing lean mass. On a cellular level, IL‐15 induces mitochondrial activity and glucose metabolism in various cell types. However, it is not known if IL‐15 recapitulates its effects in cardiomyocytes (CM). The objective of the current study was to determine if IL‐15 regulates metabolic processes in CM. Specifically, we aimed to examine the activation of enzymes associated with glucose metabolism following treatment with IL‐15 in CM. AMPK and Akt have been verified to be involved in regulating glucose metabolism and are suggested to be downstream of IL‐15 signaling. H9c2 CM were cultured and differentiated into mature myocytes. Following differentiation, CM were treated for 24 hrs with 100 ng/ml of IL‐15. Total protein lysates were harvested from the CM and a Bradford assay was used to quantify the protein concentrations. Western blotting procedures were carried out to asses both activation and total content of the proteins. Thirty‐five micrograms of total protein was loaded onto 4–20% polyacrylamide gels for electrophoresis. Proteins were then transferred from the gel onto a PVDF membrane. Membranes were blocked with 5% BSA for 1 hour and primary antibodies (phospho‐AMPK, total AMPK, phospho‐Akt and phospho‐Akt substrates) were applied (1:1000). Subsequently, membranes were probed with secondary antibodies (1:2000). The membranes were then imaged using enhanced chemiluminescence. Image J was used to quantify the density of the visualized protein bands. An unpaired t‐test (P&lt;0.05) was used to determine if IL‐15 had significant effects on protein levels. IL‐15 did not alter either phospho‐AMPK or total AMPK levels (P&gt;0.05). Phosphorylation of Akt and its substrates were induced (nearly 10‐fold) with IL‐15 treatment. Our results indicate that IL‐15 acts to increase phosphorylation of Akt and its downstream substrates without activating AMPK. Since Akt is on the insulin signaling pathway, it is feasible that IL‐15 acts to induce glucose metabolism in CM. However, it is not clear if IL‐15 directly mediates glucose uptake in CM. Nevertheless, it appears that IL‐15 may play a role in regulating CM glucose metabolism in a similar manner as in SKM cells. Further studies are needed to determine if our results translate into a whole‐body in vivo situation. If IL‐15 is involved in substrate metabolism in CM, it is necessary to determine if its effects alter heart function, especially in models of obesity.Support or Funding InformationAmerican Heart Association 16SDG30680003This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Contribution of SGLT1 in cardiac glucose transport

Sodium-glucose transporter 1 (SGLT1) is the major SGLT isoform expressed in the heart. Under baseline condition, its absence does not affect heart function and structure. Using the pharmacological SGLT1 inhibitor, phlorizin, several data suggested that SGLT1 could contribute to overall glucose transport under physiological as well as under pathological conditions, i.e. in the ischemic heart. However, SGL1 role in glucose transport has never been formerly demonstrated. This study aimed to evaluate, in physiological and pathological conditions, the consequence of SGLT1 transporter invalidation on cardiomyocyte glucose uptake. Glucose uptake was measured in isolated cardiomyocytes using 2,3[H]-glucose and 3[H]-2-deoxyglucose. SGLT1 contribution was evaluated by pharmacologic (phlorizin) and genetic means (KO mice). GLUT transporters expression was examined by RT-qPCR and Western Blot. Under basal and hyperglycemic conditions as well as under insulin or oligomycin stimulation, glucose uptake was similar in SGLT1 + / + and SGLT1 − / − cardiomyocytes. We verified that SGLT1 absence was not compensated by an increase in GLUT1 or GLUT4 expression. Phlorizin inhibited insulin-stimulated glucose transport in a dose dependent manner, leading to its complete inhibition at high concentration (1 mM). However, this effect was independent of SGLT1 as phlorizin impact on glucose transport was similar in SGLT1 + / + and SGLT1 − / − cardiomyocytes. Furthermore, the phlorizin-induced glucose uptake inhibition was not explained by any change on insulin signaling or GLUT4 translocation, suggesting that phlorizin directly affected the GLUTs facilitated glucose transporters. Phlorizin effect on glucose transport does not involve SGLT1 but rather a direct effect on GLUTs. We also demonstrated that SGLT1 does not significantly contribute to glucose uptake in the cardiomyocyte in physiological and pathological conditions.

Empagliflozin enhances human and murine cardiomyocyte glucose uptake by increased expression of GLUT1

Empagliflozin enhances human and murine cardiomyocyte glucose uptake by increased expression of GLUT1

Revisiting the Cardiotoxic Effect of Chloroquine.

Cardiotoxicity is a well-known side effect of chloroquine. Several studies have proposed chloroquine as a potential anti-diabetic treatment but do not address this problem. The current study investigated the effect of ex vivo chloroquine treatment on (1) heart function and glucose uptake, (2) mitochondrial function and (3) in vivo treatment on heart function. Control or obese male Wistar rats were used throughout. Dose responses of increasing chloroquine concentrations versus vehicle on cardiac function were measured using isolated, Langendorff-perfused hearts whilst glucose uptake and cell viability were determined in ventricular cardiomyocytes. Mitochondrial function was assessed with a Clark-type oxygraph (Hansatech) after ex vivo perfusion with 30μM chloroquine versus vehicle. Animals were treated orally with 5mg/kg/day chloroquine for 6weeks. Acute chloroquine treatment of 10μM was sufficient to significantly decrease heart function (p < 0.05) whilst 30μM significantly reduced heart rate (p < 0.05). Chloroquine became toxic to isolated cardiomyocytes at high concentrations (100μM), and had no effect on cardiomyocyte glucose uptake. Ex vivo treatment did not affect mitochondrial function, but chronic low-dose in vivo chloroquine treatment significantly decreased aortic output and total work in hearts (p < 0.005). Low and intermediate chloroquine doses administered either chronically or acutely are sufficient to result in myocardial dysfunction.

Contribution of SGLT1 in cardiac glucose uptake

The sodium-glucose transporter 1 (SGLT1) has been identified as the major SGLT isoform expressed in the heart. The exact role of this transporter in the heart is still unknown. Several authors have postulated a role on glucose transport using phlorizin as a specific inhibitor of SGLT transporters. However, high concentrations of phlorizin have been used in these studies and questions remain towards its potential effects on facilitated glucose transporters (GLUTs). Indeed, phlorizin has a similar structure to phloretin, GLUT-specific inhibitor and it could be hydrolyzed into phloretin acting on glucose transport independently of SGLT transporters. The aim of this study is to determine the contribution of SGLT1 transporter on glucose uptake in hearts and the impact of phlorizin on this uptake. We studied glucose uptake in vitro (cardiomyocytes), ex vivo (whole working hearts) and in vivo with SGLT1 WT and KO mice using 2- 3 [H]-glucose. Different phlorizin concentrations were tested to determine its impact on glucose uptake. We showed that glucose uptake was similar in SGLT1 WT and KO mice in basal, hyperglycemic and insulin-stimulated conditions. Moreover, we observed that high concentrations of phlorizin (10 −3 M) completely inhibit glucose uptake in cardiomyocytes subjected to insulin stimulation and that this inhibition was similar in WT and KO SGLT1 mice. An inhibition was also found at lower concentration of phlorizin (10 −4 M) to a lesser extent. The contribution of SGLT1 in glucose uptake in the heart is marginal compared to facilitate glucose uptake. At least at high concentrations, phlorizin inhibits glucose uptake independently of SGLT1 questioning the use of this compound to study SGLT-dependent glucose uptake.

α1A-Adrenoceptors activate mTOR signalling and glucose uptake in cardiomyocytes

The capacity of G protein-coupled receptors to modulate mechanistic target of rapamycin (mTOR) activity is a newly emerging paradigm with the potential to link cell surface receptors with cell survival. Cardiomyocyte viability is linked to signalling pathways involving Akt and mTOR, as well as increased glucose uptake and utilization. Our aim was to determine whether the α1A-adrenoceptor (AR) couples to these protective pathways, and increased glucose uptake. We characterised α1A-AR signalling in CHO-K1 cells co-expressing the human α1A-AR and GLUT4 (CHOα1AGLUT4myc) and in neonatal rat ventricular cardiomyocytes (NRVM), and measured glucose uptake, intracellular Ca2+ mobilization, and phosphorylation of mTOR, Akt, 5′ adenosine monophosphate-activated kinase (AMPK) and S6 ribosomal protein (S6rp). In both systems, noradrenaline and the α1A-AR selective agonist A61603 stimulated glucose uptake by parallel pathways involving mTOR and AMPK, whereas another α1-AR agonist oxymetazoline increased glucose uptake predominantly by mTOR. All agonists promoted phosphorylation of mTOR at Ser2448 and Ser2481, indicating activation of both mTORC1 and mTORC2, but did not increase Akt phosphorylation. In CHOα1AGLUT4myc cells, siRNA directed against rictor but not raptor suppressed α1A-AR mediated glucose uptake. We have thus identified mTORC2 as a key component in glucose uptake stimulated by α1A-AR agonists. Our findings identify a novel link between the α1A-AR, mTORC2 and glucose uptake, that have been implicated separately in cardiomyocyte survival. Our studies provide an improved framework for examining the utility of α1A-AR selective agonists as tools in the treatment of cardiac dysfunction.

High-density lipoprotein delivered after myocardial infarction increases cardiac glucose uptake and function in mice.

Protecting the heart after an acute coronary syndrome is a key therapeutic goal to support cardiac recovery and prevent progression to heart failure. A potential strategy is to target cardiac glucose metabolism at the early stages after ischemia when glycolysis is critical for myocyte survival. Building on our discovery that high-density lipoprotein (HDL) modulates skeletal muscle glucose metabolism, we now demonstrate that a single dose of reconstituted HDL (rHDL) delivered after myocardial ischemia increases cardiac glucose uptake, reduces infarct size, and improves cardiac remodeling in association with enhanced functional recovery in mice. These findings applied equally to metabolically normal and insulin-resistant mice. We further establish direct effects of HDL on cardiomyocyte glucose uptake, glycolysis, and glucose oxidation via the Akt signaling pathway within 15 min of reperfusion. These data support the use of infusible HDL preparations for management of acute coronary syndromes in the setting of primary percutaneous interventions.

Impact of Patient Preparation on the Diagnostic Performance of 18F-FDG PET in Cardiac Sarcoidosis

F-FDG PET/CT is a valuable diagnostic tool in the evaluation of cardiac sarcoidosis. Appropriate patient preparation is important because the diagnostic accuracy of this procedure depends on adequate suppression of physiologic glucose uptake. This systematic review and meta-analysis aims to assess the effect of different patient preparations on the diagnostic accuracy of F-FDG PET/CT in cardiac sarcoidosis. We searched the PubMed/MEDLINE, Embase, and Cinicaltrials.gov databases. Sixteen studies (n = 559) were identified to be suitable for this systemic review. Studies were stratified according to fasting duration and means of physiologic suppression of F-FDG by cardiac tissue, which involves the use of heparin infusion or high-fat, low-carbohydrate diet before imaging. Study quality was assessed using the QUADAS-2 tool. Forest plots of sensitivity and specificity were calculated in Review Manager 5.3, and a random-effects hierarchical summary receiver operating characteristic model was created using MetaDiSc. Meta-regression was performed to investigate sources of heterogeneity. PET/CT had an overall Specificity of 0.75 (95% confidence interval [CI], 0.69-0.80) and specificity of 0.81 (95% CI, 0.76-0.85) for the diagnosis of cardiac sarcoidosis. This modest diagnostic accuracy was attributed to the inclusion of large single study in which a short fasting duration before scanning likely influenced its sensitivity. Its exclusion resulted in an overall sensitivity of 0.81 (95% CI, 0.76-0.86) and specificity of 0.82 (95% CI, 0.77-0.86). Meta-regression showed that the diagnostic odds ratio was significantly affected by fasting time and heparin administration before scanning (P = 0.01, 0.02) but not with high-fat, low-carbohydrate diet (P = 0.17). F-FDG PET/CT plays an integral role in the diagnosis of cardiac sarcoidosis. Diagnostic accuracy is affected by fasting duration and means of cardiomyocyte glucose uptake suppression before scanning.

Retinol-Binding Protein 4 Induces Cardiomyocyte Hypertrophy by Activating TLR4/MyD88 Pathway.

Insulin resistance plays a major role in the development and progression of cardiac hypertrophy and heart failure. Heart failure in turn promotes insulin resistance and increases the risk for diabetes. The vicious cycle determines significant mortality in patients with heart failure and diabetes. However, the underlying mechanisms for the vicious cycle are not fully elucidated. Here we show that circulating levels and adipose expression of retinol-binding protein 4 (RBP4), an adipokine that contributes to systemic insulin resistance, were elevated in cardiac hypertrophy induced by transverse aortic constriction and angiotensin-II (Ang-II) infusion. Ang-II increased RBP4 expression in adipocytes, which was abolished by losartan, an Ang-II receptor blocker. The elevated RBP4 in cardiac hypertrophy may have pathophysiological consequences because RBP4 increased cell size, enhanced protein synthesis, and elevated the expression of hypertrophic markers including Anp, Bnp, and Myh7 in primary cardiomyocytes. Mechanistically, RBP4 induced the expression and activity of toll-like receptor 4 (TLR4) and myeloid differentiation primary response gene 88 (MyD88) in cardiomyocytes, resulting in enhanced inflammation and reactive oxygen species production. Inhibition or knockdown of the TLR4/MyD88 pathway attenuated inflammatory and hypertrophic responses to RBP4 stimulation. Importantly, RBP4 also reduced the expression of glucose transporter-4 and impaired insulin-stimulated glucose uptake in cardiomyocytes. This impairment was ameliorated in cardiomyocytes from TLR4 knockout mice. Therefore, RBP4 may be a critical modulator promoting the vicious cycle of insulin resistance and heart failure by activating TLR4/MyD88-mediated inflammatory pathways. Potentially, lowering RBP4 might break the vicious cycle and improve both insulin resistance and cardiac hypertrophy.

Neuregulin-1β promotes glucose uptake via PI3K/Akt in neonatal rat cardiomyocytes.

Nrg1β is critically involved in cardiac development and also maintains function of the adult heart. Studies conducted in animal models showed that it improves cardiac performance under a range of pathological conditions, which led to its introduction in clinical trials to treat heart failure. Recent work also implicated Nrg1β in the regenerative potential of neonatal and adult hearts. The molecular mechanisms whereby Nrg1β acts in cardiac cells are still poorly understood. In the present study, we analyzed the effects of Nrg1β on glucose uptake in neonatal rat ventricular myocytes and investigated to what extent mTOR/Akt signaling pathways are implicated. We show that Nrg1β enhances glucose uptake in cardiomyocytes as efficiently as IGF-I and insulin. Nrg1β causes phosphorylation of ErbB2 and ErbB4 and rapidly induces the phosphorylation of FAK (Tyr(861)), Akt (Thr(308) and Ser(473)), and its effector AS160 (Thr(642)). Knockdown of ErbB2 or ErbB4 reduces Akt phosphorylation and blocks the glucose uptake. The Akt inhibitor VIII and the PI3K inhibitors LY-294002 and Byl-719 abolish Nrg1β-induced phosphorylation and glucose uptake. Finally, specific mTORC2 inactivation after knockdown of rictor blocks the Nrg1β-induced increases in Akt-p-Ser(473) but does not modify AS160-p-Thr(642) or the glucose uptake responses to Nrg1β. In conclusion, our study demonstrates that Nrg1β enhances glucose uptake in cardiomyocytes via ErbB2/ErbB4 heterodimers, PI3Kα, and Akt. Furthermore, although Nrg1β activates mTORC2, the resulting Akt-Ser(473) phosphorylation is not essential for glucose uptake induction. These new insights into pathways whereby Nrg1β regulates glucose uptake in cardiomyocytes may contribute to the understanding of its regenerative capacity and protective function in heart failure.

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.