Infusion of a biotinylated bis-glucose photolabel: a new method to quantify cell surface GLUT4 in the intact mouse heart

Insulin stimulated GLUT4 translocation - Size is not everything!

Insulin-regulated trafficking of the facilitative glucose transporter GLUT4 has been studied in many cell types. The translocation of GLUT4 from intracellular membranes to the cell surface is often described as a highly specialised form of membrane traffic restricted to certain cell types such as fat and muscle, which are the major storage depots for insulin-stimulated glucose uptake. Here, we discuss evidence that favours the argument that rather than being restricted to specialised cell types, the machinery through which insulin regulates GLUT4 traffic is present in all cell types. This is an important point as it provides confidence in the use of experimentally tractable model systems to interrogate the trafficking itinerary of GLUT4.

Cardiomyocyte-specific deletion of Sirt1 gene sensitizes myocardium to ischaemia and reperfusion injury

Aims

A longevity gene, Sirtuin 1 (SIRT1) and energy sensor AMP-activated protein kinase (AMPK) have common activators such as caloric restriction, oxidative stress, and exercise. The objective of this study is to characterize the role of cardiomyocyte SIRT1 in age-related impaired ischemic AMPK activation and increased susceptibility to ischemic insults.

Methods and results

Mice were subjected to ligation of left anterior descending coronary artery for in vivo ischemic models. The glucose and fatty acid oxidation were measured in a working heart perfusion system. The cardiac functions by echocardiography show no difference in young wild-type C57BL/6 J (WT, 4-6 months), aged WT C57BL/6 J (24-26 months), and young inducible cardiomyocyte-specific SIRT1 knockout (icSIRT1 KO) (4-6 months) mice under physiological conditions. However, after 45 mins ischaemia and 24-h reperfusion, the ejection fraction of aged WT and icSIRT1 KO mice was impaired. The aged WT and icSIRT1 KO hearts vs. young WT hearts also show an impaired post-ischemic contractile function in a Langendorff perfusion system. The infarct size of aged WT and icSIRT1 KO hearts was larger than that of young WT hearts. The immunoblotting data demonstrated that aged WT and icSIRT1 KO hearts vs. young WT hearts had impaired phosphorylation of AMPK and downstream acetyl-CoA carboxylase during ischaemia. Intriguingly, AMPK upstream LKB1 is hyper-acetylated in both aged WT and icSIRT1 KO hearts; this could blunt activation of LKB1, leading to an impaired AMPK activation. The working heart perfusion results demonstrated that SIRT1 deficiency significantly impaired substrate metabolism in the hearts; fatty acid oxidation is augmented and glucose oxidation is blunted during ischaemia and reperfusion. Adeno-associated virus (AAV9)-Sirt1 was delivered into the aged hearts via a coronary delivery approach, which significantly rescued the protein level of SIRT1 and the ischemic tolerance of aged hearts. Furthermore, AMPK agonist can rescue the tolerance of aged heart and icSIRT1 KO heart to ischemic insults.

Conclusions

Cardiac SIRT1 mediates AMPK activation via LKB1 deacetylation, and AMPK modulates SIRT1 activity via regulation of NAD+ level during ischaemia. SIRT1 and AMPK agonists have therapeutic potential for treatment of aging-related ischemic heart disease.

Quantification of Cell-Surface Glucose Transporters in the Heart Using a Biotinylated Photolabeling Assay

The biotinylated photolabeling assay enables quantification of cell-surface glucose transporters (GLUTs). This technique has been successfully applied to quantify the cell-surface GLUT protein content in striated muscles and adipose tissue, as a means to evaluate GLUT trafficking. Here, we describe the detailed method of quantifying the cell-surface content of several GLUT isoforms (1, 4, 8, and 12) in isolated cardiac myocytes, as well as in the intact perfused atria and ventricle.

Sestrin2 prevents age-related intolerance to ischemia and reperfusion injury by modulating substrate metabolism

A novel stress-inducible protein, Sestrin2 (Sesn2), declines in the heart with aging. AMPK has emerged as a pertinent stress-activated kinase that has been shown to have cardioprotective capabilities against myocardial ischemic injury. We identified the interaction between Sesn2 and AMPK in the ischemic heart. To determine whether ischemic AMPK activation-modulated by the Sesn2-AMPK complex in the heart-is impaired in aging that sensitizes the heart to ischemic insults, young C57BL/6 mice (age 3-4 mo), middle-aged mice (age 10-12 mo), and aged mice (age 24-26 mo) were subjected to left anterior descending coronary artery occlusion for in vivo regional ischemia. The ex vivo working heart system was used for measuring substrate metabolism. The protein level of Sesn2 in hearts was gradually decreased with aging. Of interest, ischemic AMPK activation was blunted in aged hearts compared with young hearts (P < 0.05); the AMPK downstream glucose uptake and the rate of glucose oxidation were significantly impaired in aged hearts during ischemia and reperfusion (P < 0.05 vs. young hearts). Myocardial infarction size was larger in aged hearts (P < 0.05 vs. young hearts). Immunoprecipitation with Sesn2 Ab revealed that cardiac Sesn2 forms a complex with AMPK and upstream liver kinase B1 (LKB1) during ischemia. Of interest, the binding affinity between Sesn2 and AMPK upstream LKB1 is impaired in aged hearts during ischemia (P < 0.05 vs. young hearts). Furthermore, Sesn2-knockout hearts demonstrate a cardiac phenotype and response to ischemic stress that is similar to wild-type aged hearts (i.e., impaired ischemic AMPK activation and higher sensitivity to ischemia- and reperfusion- induced injury). Adeno-associated virus-Sesn2 was delivered to aged hearts via a coronary delivery approach and significantly rescued the protein level of Sesn2 and the ischemic tolerance of aged hearts; therefore, Sesn2 is a scaffold protein that mediates AMPK activation in the ischemic myocardium via an interaction with AMPK upstream LKB1. Decreased Sesn2 levels in aging lead to a blunted ischemic AMPK activation, alterations in substrate metabolism, and an increased sensitivity to ischemic insults-Quan, N., Sun, W., Wang, L., Chen, X., Bogan, J. S., Zhou, X., Cates, C., Liu, Q., Zheng, Y., Li J. Sestrin2 prevents age-related intolerance to ischemia and reperfusion injury by modulating substrate metabolism.

Molecular Tools for Facilitative Carbohydrate Transporters (Gluts)

Facilitative carbohydrate transporters-Gluts-have received wide attention over decades due to their essential role in nutrient uptake and links with various metabolic disorders, including diabetes, obesity, and cancer. Endeavors directed towards understanding the mechanisms of Glut-mediated nutrient uptake have resulted in a multidisciplinary research field spanning protein chemistry, chemical biology, organic synthesis, crystallography, and biomolecular modeling. Gluts became attractive targets for cancer research and medicinal chemistry, leading to the development of new approaches to cancer diagnostics and providing avenues for cancer-targeting therapeutics. In this review, the current state of knowledge of the molecular interactions behind Glut-mediated sugar uptake, Glut-targeting probes, therapeutics, and inhibitors are discussed.

Cardiac-Specific Deletion of the Pdha1 Gene Sensitizes Heart to Toxicological Actions of Ischemic Stress

Pyruvate dehydrogenase (PDH) plays a key role in aerobic energy metabolism and occupies a central crossroad between glycolysis and the tricarboxylic acid cycle. We generated inducible cardiac-specific PDH E1α knockout (CreER(T2)-PDH(flox/flox)) mice that demonstrated a high mortality rate. It was hypothesized that PDH modulating cardiac glucose metabolism is crucial for heart functions under normal physiological and/or stress conditions. The myocardial infarction was conducted by a ligation of the left anterior descending coronary arteries. Cardiac PDH E1α deficiency caused large myocardial infarcts size and macrophage infiltration in the hearts (P < .01 vs wild-type [WT]). Wheat germ agglutinin and Masson trichrome staining revealed significantly increased hypertrophy and fibrosis in PDH E1α-deficient hearts (P < .05 vs WT). Measurements of heart substrate metabolism in an ex vivo working heart perfusion system demonstrated a significant impairment of glucose oxidation in PDH E1α-deficient hearts during ischemia/reperfusion (P < .05 vs WT). Dichloroacetate, a PDH activator, increased glucose oxidation in WT hearts during ischemia/reperfusion and reduced myocardial infarct size in WT, but not in PDH E1α-deficient hearts. Immunoblotting results demonstrated that cardiac PDH E1α deficiency leads to an impaired ischemic AMP-activated protein kinase activation through Sestrin2-liver kinase B1 interaction which is responsible for an increased susceptibility of PDH E1α-deficient heart to ischemic insults. Thus, cardiac PDH E1α deficiency impairs ischemic AMP-activated protein kinase signaling and sensitizes hearts to the toxicological actions of ischemic stress.

Diabetes Alters the Expression and Translocation of the Insulin-Sensitive Glucose Transporters 4 and 8 in the Atria

Although diabetes has been identified as a major risk factor for atrial fibrillation, little is known about glucose metabolism in the healthy and diabetic atria. Glucose transport into the cell, the rate-limiting step of glucose utilization, is regulated by the Glucose Transporters (GLUTs). Although GLUT4 is the major isoform in the heart, GLUT8 has recently emerged as a novel cardiac isoform. We hypothesized that GLUT-4 and -8 translocation to the atrial cell surface will be regulated by insulin and impaired during insulin-dependent diabetes. GLUT protein content was measured by Western blotting in healthy cardiac myocytes and type 1 (streptozotocin-induced, T1Dx) diabetic rodents. Active cell surface GLUT content was measured using a biotinylated photolabeled assay in the perfused heart. In the healthy atria, insulin stimulation increased both GLUT-4 and -8 translocation to the cell surface (by 100% and 240%, respectively, P<0.05). Upon insulin stimulation, we reported an increase in Akt (Th308 and s473 sites) and AS160 phosphorylation, which was positively (P<0.05) correlated with GLUT4 protein content in the healthy atria. During diabetes, active cell surface GLUT-4 and -8 content was downregulated in the atria (by 70% and 90%, respectively, P<0.05). Akt and AS160 phosphorylation was not impaired in the diabetic atria, suggesting the presence of an intact insulin signaling pathway. This was confirmed by the rescued translocation of GLUT-4 and -8 to the atrial cell surface upon insulin stimulation in the atria of type 1 diabetic subjects. In conclusion, our data suggest that: 1) both GLUT-4 and -8 are insulin-sensitive in the healthy atria through an Akt/AS160 dependent pathway; 2) GLUT-4 and -8 trafficking is impaired in the diabetic atria and rescued by insulin treatment. Alterations in atrial glucose transport may induce perturbations in energy production, which may provide a metabolic substrate for atrial fibrillation during diabetes.

Sarcoplasmic reticulum Ca2+ ATPase pump is a major regulator of glucose transport in the healthy and diabetic heart

Despite intensive research, the pathways that mediate calcium (Ca(2+))-stimulated glucose transport in striated muscle remain elusive. Since the sarcoplasmic reticulum calcium ATPase (SERCA) pump tightly regulates cytosolic [Ca(2+)], we investigated whether the SERCA pump is a major regulator of cardiac glucose transport. We used healthy and insulin-deficient diabetic transgenic (TG) mice expressing SERCA1a in the heart. Active cell surface glucose transporter (GLUT)-4 was measured by a biotinylated photolabeled assay in the intact perfused myocardium and isolated myocytes. In healthy TG mice, cardiac-specific SERCA1a expression increased active cell-surface GLUT4 and glucose uptake in the myocardium, as well as whole body glucose tolerance. Diabetes reduced active cell-surface GLUT4 content and glucose uptake in the heart of wild type mice, all of which were preserved in diabetic TG mice. Decreased basal AS160 and increased proportion of calmodulin-bound AS160 paralleled the increase in cell surface GLUT4 content in the heart of TG mice, suggesting that AS160 regulates GLUT trafficking by a Ca(2+)/calmodulin dependent pathway. In addition, cardiac-specific SERCA1a expression partially rescues hyperglycemia during diabetes. Collectively, these data suggested that the SERCA pump is a major regulator of cardiac glucose transport by an AS160 dependent mechanism during healthy and insulin-deficient state. Our data further indicated that cardiac-specific SERCA overexpression rescues diabetes induced-alterations in cardiac glucose transport and improves whole body glucose homeostasis. Therefore, findings from this study provide novel mechanistic insights linking upregulation of the SERCA pump in the heart as a potential therapeutic target to improve glucose metabolism during diabetes.

GLUT12 functions as a basal and insulin-independent glucose transporter in the heart

Glucose uptake from the bloodstream is the rate-limiting step in whole body glucose utilization, and is regulated by a family of membrane proteins called glucose transporters (GLUTs). Although GLUT4 is the predominant isoform in insulin-sensitive tissues, there is recent evidence that GLUT12 could be a novel second insulin-sensitive GLUT. However, its physiological role in the heart is not elucidated and the regulation of insulin-stimulated myocardial GLUT12 translocation is unknown. In addition, the role of GLUT12 has not been investigated in the diabetic myocardium. Thus, we hypothesized that, as for GLUT4, insulin regulates GLUT12 translocation to the myocardial cell surface, which is impaired during diabetes. Active cell surface GLUT (-4 and -12) content was quantified (before and after insulin stimulation) by a biotinylated photolabeled assay in both intact perfused myocardium and isolated cardiac myocytes of healthy and type 1 diabetic rodents. GLUT localization was confirmed by immunofluorescent confocal microscopy, and total GLUT protein expression was measured by Western blotting. Insulin stimulation increased translocation of GLUT-4, but not -12, in the healthy myocardium. Total GLUT4 content of the heart was decreased during diabetes, while there was no difference in total GLUT12. Active cell surface GLUT12 content was increased in the diabetic myocardium, potentially as a compensatory mechanism for the observed downregulation of GLUT4. Collectively, our data suggest that, in contrast to GLUT4, insulin does not mediate GLUT12 translocation, which may function as a basal GLUT located primarily at the cell surface in the myocardium.

Activated protein C modulates cardiac metabolism and augments autophagy in the ischemic heart

BACKGROUND

Modulation of energy substrate metabolism may constitute a novel therapeutic intervention against ischemia/reperfusion (I/R) injury. AMP-activated protein kinase (AMPK) has emerged as a key regulator of favorable metabolic signaling pathways in response to myocardial ischemia. Recently, we demonstrated that activated protein C (APC) is cardioprotective against ischemia/reperfusion (I/R) injury by augmenting AMPK signaling.

OBJECTIVES

The objective of this study was to determine whether the APC modulation of substrate metabolism contributes to its cardioprotective effect against I/R injury.

METHODS

An ex vivo working mouse heart perfusion system was used to characterize the effect of wild-type APC and its signaling-proficient mutant, APC-2Cys (which has dramatically reduced anticoagulant activity), on glucose transport in the ischemic heart.

RESULTS

Both APC and APC-2Cys (0.2 μg g(-1)) augment the ischemic stress-induced translocation of the glucose transporter (GLUT4) to the myocardial cell membrane, leading to increased glucose uptake and glucose oxidation in the ischemic heart (P < 0.05 vs. vehicle). Both APC derivatives increased the autophagic flux in the heart following I/R. The activity of APC-2Cys in modulating these metabolic pathways was significantly higher than APC during I/R (P < 0.05). Intriguingly, APC-2Cys, but not wild-type APC, attenuated the I/R-initiated fatty acid oxidation by 80% (P < 0.01 vs. vehicle).

CONCLUSIONS

APC exerts a cardioprotective effect against I/R injury by preferentially enhancing the oxidation of glucose over fatty acids as energy substrates in the ischemic heart. Given its significantly higher beneficial metabolic modulatory effect, APC-2Cys may be developed as a potential therapeutic drug for treating ischemic heart disease without risk of bleeding.

Measuring GLUT4 translocation in mature muscle fibers

Skeletal muscle is the major tissue for postprandial glucose disposal. Facilitated glucose uptake into muscle fibers is mediated by increases in surface membrane levels of the glucose transporter GLUT4 via insulin- and/or muscle contraction-mediated GLUT4 translocation. However, the regulatory mechanisms controlling GLUT4 translocation in skeletal muscle have been difficult to characterize at the cell biology level due to muscle tissue complexity. Muscle cell culture models have improved our understanding of GLUT4 translocation and glucose transport regulation, but in vitro muscle models lack many of the characteristics of mature muscle fibers. Thus, the molecular and cellular details of GLUT4 translocation in mature skeletal muscle are deficient. The objective of this review is to highlight how advances in recent experimental approaches translate into an enhanced understanding of the regulation of GLUT4 translocation and glucose transport in mature skeletal muscle.

The many ways to regulate glucose transporter 4

Glucose uptake into skeletal muscle is primarily mediated by glucose transporter 4 (GLUT4). The number of GLUT4 polypeptides at the surface of muscle cells rises rapidly in response to insulin, contraction, depolarization, or energy deprivation. However, distinct mechanisms underlie the gain in surface GLUT4 in each case. Insulin promotes its exocytosis to the membrane, regulating vesicle movement, tethering, docking, and fusion. In contrast, muscle contraction, depolarization, and energy demand reduce GLUT4 endocytosis. The signals involved in each case also differ. Insulin utilizes Akt, Rabs, and selective actin remodelling, whereas depolarization and energy deprivation engage AMP-activated protein kinase and Ca2+-dependent signals. GLUT4 internalizes via 2 major routes that involve dynamin, but only one requires clathrin. The clathrin-independent route is slowed down by energy deprivation, and is regulated by AMP-activated protein kinase. In addition to regulation of the exocytic and endocytic movement of GLUT4, glucose uptake is also modulated through changes in the transporter's intrinsic activity. The glycolytic enzymes glyceraldehyde-3-dehydrogenase and hexokinase II contribute to such regulation, through differential binding to GLUT4.

A transgenic mouse model to study glucose transporter 4myc regulation in skeletal muscle

Skeletal muscle is the major site for dietary glucose disposal, taking up glucose via glucose transporter 4 (GLUT4). Although subcellular fractionation studies demonstrate that insulin increases GLUT4 density in sarcolemma and transverse tubules, fractionation cannot discern GLUT4 vesicle-membrane association from insertion and exofacial exposure. Clonal muscle cultures expressing exofacially tagged GLUT4 have allowed quantification of GLUT4 exposure at the cell surface, its exocytosis, endocytosis, and partner proteins. We hypothesized that transgenic expression of GLUT4myc in skeletal muscles would provide a useful model to investigate GLUT4 biology in vivo. A homozygous mouse colony was generated expressing GLUT4myc driven by the muscle creatine kinase (MCK) promoter. GLUT4 protein levels were about 3-fold higher in hindlimb muscles of MCK-GLUT4myc transgenic mice compared with littermates (P < 0.05). Insulin (12 nm, 30 min) induced a 2.1-fold increase in surface GLUT4myc detected by immunofluorescence of the exofacial myc epitope in nonpermeabilized muscle fiber bundles (P < 0.05). Glucose uptake and surface GLUT4myc levels were 3.5- and 3-fold higher, respectively, in giant membrane vesicles blebbed from hindlimb muscles of insulin-stimulated transgenic mice compared with unstimulated counterparts (P < 0.05). Muscle contraction also elevated both parameters, an effect partially additive to insulin's. GLUT4myc immunoprecipitation with anti-myc antibodies avoids interfering with associated intracellular binding proteins. Tether, containing a UBX domain, for GLUT4 coimmunoprecipitated with GLUT4myc and insulin stimulation significantly decreased such association (P < 0.05). MCK-GLUT4myc transgenic mice are thus useful to quantify exofacial GLUT4 exposure at the sarcolemma and GLUT4 binding partners in skeletal muscle, essential elements in the investigation of muscle GLUT4 regulation in physiological and pathological states in vivo.