Glucose transporters and in vivo glucose uptake in skeletal and cardiac muscle: fasting, insulin stimulation and immunoisolation studies of GLUT1 and GLUT4

The paradox of fatty-acid β-oxidation in muscle insulin resistance: Metabolic control and muscle heterogeneity

The skeletal muscle is a metabolically heterogeneous tissue that plays a key role in maintaining whole-body glucose homeostasis. It is well known that muscle insulin resistance (IR) precedes the development of type 2 diabetes. There is a consensus that the accumulation of specific lipid species in the tissue can drive IR. However, the role of the mitochondrial fatty-acid β-oxidation in IR and, consequently, in the control of glucose uptake remains paradoxical: interventions that either inhibit or activate fatty-acid β-oxidation have been shown to prevent IR. We here discuss the current theories and evidence for the interplay between β-oxidation and glucose uptake in IR. To address the underlying intricacies, we (1) dive into the control of glucose uptake fluxes into muscle tissues using the framework of Metabolic Control Analysis, and (2) disentangle concepts of flux and catalytic capacities taking into account skeletal muscle heterogeneity. Finally, we speculate about hitherto unexplored mechanisms that could bring contrasting evidence together. Elucidating how β-oxidation is connected to muscle IR and the underlying role of muscle heterogeneity enhances disease understanding and paves the way for new treatments for type 2 diabetes.

Protein kinase-D1 and downstream signaling mechanisms involved in GLUT4 translocation in cardiac muscle

The Ser/Thr kinase protein kinase-D1 (PKD1) is involved in induction of various cell physiological processes in the heart such as myocellular hypertrophy and inflammation, which may turn maladaptive during long-term stimulation. Of special interest is a key role of PKD1 in the regulation of cardiac substrate metabolism. Glucose and fatty acids are the most important substrates for cardiac energy provision, and the ratio at which they are utilized determines the health status of the heart. Cardiac glucose uptake is mainly regulated by translocation of the glucose transporter GLUT4 from intracellular stores (endosomes) to the sarcolemma, and fatty acid uptake via a parallel translocation of fatty acid transporter CD36 from endosomes to the sarcolemma. PKD1 is involved in the regulation of GLUT4 translocation, but not CD36 translocation, giving it the ability to modulate glucose uptake without affecting fatty acid uptake, thereby altering the cardiac substrate balance. PKD1 would therefore serve as an attractive target to combat cardiac metabolic diseases with a tilted substrate balance, such as diabetic cardiomyopathy. However, PKD1 activation also elicits cardiac hypertrophy and inflammation. Therefore, identification of the events upstream and downstream of PKD1 may provide superior therapeutic targets to alter the cardiac substrate balance. Recent studies have identified the lipid kinase phosphatidylinositol 4-kinase IIIβ (PI4KIIIβ) as signaling hub downstream of PKD1 to selectively stimulate GLUT4-mediated myocardial glucose uptake without inducing hypertrophy. Taken together, the PKD1 signaling pathway serves a pivotal role in cardiac glucose metabolism and is a promising target to selectively modulate glucose uptake in cardiac disease.

Enhanced protein acetylation initiates fatty acid-mediated inhibition of cardiac glucose transport

Fatty acids (FAs) rapidly and efficiently reduce cardiac glucose uptake in the Randle cycle or glucose-FA cycle. This fine-tuned physiological regulation is critical to allow optimal substrate allocation during fasted and fed states. However, the mechanisms involved in the direct FA-mediated control of glucose transport have not been totally elucidated yet. We previously reported that leucine and ketone bodies, other cardiac substrates, impair glucose uptake by increasing global protein acetylation from acetyl-CoA. As FAs generate acetyl-CoA as well, we postulated that protein acetylation is enhanced by FAs and participates in their inhibitory action on cardiac glucose uptake. Here, we demonstrated that both palmitate and oleate promoted a rapid increase in protein acetylation in primary cultured adult rat cardiomyocytes, which correlated with an inhibition of insulin-stimulated glucose uptake. This glucose absorption deficit was caused by an impairment in the translocation of vesicles containing the glucose transporter GLUT4 to the plasma membrane, although insulin signaling remained unaffected. Interestingly, pharmacological inhibition of lysine acetyltransferases (KATs) prevented this increase in protein acetylation and glucose uptake inhibition induced by FAs. Similarly, FA-mediated inhibition of insulin-stimulated glucose uptake could be prevented by KAT inhibitors in perfused hearts. To summarize, enhanced protein acetylation can be considered as an early event in the FA-induced inhibition of glucose transport in the heart, explaining part of the Randle cycle.NEW & NOTEWORTHY Our results show that cardiac metabolic overload by oleate or palmitate leads to increased protein acetylation inhibiting GLUT4 translocation to the plasma membrane and glucose uptake. This observation suggests an additional regulation mechanism in the physiological glucose-FA cycle originally discovered by Randle.

Cardiomyocyte infection by Trypanosoma cruzi promotes innate immune response and glycolysis activation

Introduction

Chagas cardiomyopathy, a disease caused by Trypanosoma cruzi (T. cruzi) infection, is a major contributor to heart failure in Latin America. There are significant gaps in our understanding of the mechanism for infection of human cardiomyocytes, the pathways activated during the acute phase of the disease, and the molecular changes that lead to the progression of cardiomyopathy.

Methods

To investigate the effects of T. cruzi on human cardiomyocytes during infection, we infected induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM) with the parasite and analyzed cellular, molecular, and metabolic responses at 3 hours, 24 hours, and 48 hours post infection (hpi) using transcriptomics (RNAseq), proteomics (LC-MS), and metabolomics (GC-MS and Seahorse) analyses.

Results

Analyses of multiomic data revealed that cardiomyocyte infection caused a rapid increase in genes and proteins related to activation innate and adaptive immune systems and pathways, including alpha and gamma interferons, HIF-1α signaling, and glycolysis. These responses resemble prototypic responses observed in pathogen-activated immune cells. Infection also caused an activation of glycolysis that was dependent on HIF-1α signaling. Using gene editing and pharmacological inhibitors, we found that T. cruzi uptake was mediated in part by the glucose-facilitated transporter GLUT4 and that the attenuation of glycolysis, HIF-1α activation, or GLUT4 expression decreased T. cruzi infection. In contrast, pre-activation of pro-inflammatory immune responses with LPS resulted in increased infection rates.

Conclusion

These findings suggest that T. cruzi exploits a HIF-1α-dependent, cardiomyocyte-intrinsic stress-response activation of glycolysis to promote intracellular infection and replication. These chronic immuno-metabolic responses by cardiomyocytes promote dysfunction, cell death, and the emergence of cardiomyopathy.

Hypoxia Resistance Is an Inherent Phenotype of the Mouse Flexor Digitorum Brevis Skeletal Muscle

The various functions of skeletal muscle (movement, respiration, thermogenesis, etc.) require the presence of oxygen (O₂). Inadequate O₂ bioavailability (ie, hypoxia) is detrimental to muscle function and, in chronic cases, can result in muscle wasting. Current therapeutic interventions have proven largely ineffective to rescue skeletal muscle from hypoxic damage. However, our lab has identified a mammalian skeletal muscle that maintains proper physiological function in an environment depleted of O₂. Using mouse models of in vivo hindlimb ischemia and ex vivo anoxia exposure, we observed the preservation of force production in the flexor digitorum brevis (FDB), while in contrast the extensor digitorum longus (EDL) and soleus muscles suffered loss of force output. Unlike other muscles, we found that the FDB phenotype is not dependent on mitochondria, which partially explains the hypoxia resistance. Muscle proteomes were interrogated using a discovery-based approach, which identified significantly greater expression of the transmembrane glucose transporter GLUT1 in the FDB as compared to the EDL and soleus. Through loss-and-gain-of-function approaches, we determined that GLUT1 is necessary for the FDB to survive hypoxia, but overexpression of GLUT1 was insufficient to rescue other skeletal muscles from hypoxic damage. Collectively, the data demonstrate that the FDB is uniquely resistant to hypoxic insults. Defining the mechanisms that explain the phenotype may provide insight towards developing approaches for preventing hypoxia-induced tissue damage.

5-Aminolevulinic acid combined with ferrous iron improves glucose tolerance in high-fat diet-fed mice via upregulation of glucose transporter 1

Decreased mitochondrial metabolism suppresses glucose metabolism, resulting in obesity and diabetes. The present study aimed to investigate mechanisms underlying the 5-aminolevulinic acid (5-ALA) hydrochloride-mediated increase in glucose uptake in high-fat diet (HFD)-fed mice in vivo and C2C12 myotube cells in vitro. C57BL/6N male mice (20 weeks old) were fed either HFD or normal diet (ND) for 4 weeks. A total of five HFD-fed mice were orally administered with 300 mg/kg 5-ALA hydrochloride and 47.1 mg/kg sodium ferrous citrate (SFC; HFD + 5-ALA/SFC), whereas ND and other HFD-fed mice were orally administered with saline. After 4 weeks, these mice were intraperitoneally administered with 2 g/kg glucose and 3.2 mg/kg 2-deoxyglucose (2DG) for intraperitoneal glucose tolerance test (IPGTT) and glucose uptake test. Body weights, plasma glucose levels and the area under the curve of IPGTT were lower in mice treated with HFD + 5-ALA/SFC compared with in those treated with HFD alone. 2DG uptake in the gastrocnemius muscle and heart were more significantly improved in the HFD + 5-ALA/SFC mice compared with the HFD-fed mice. Furthermore, 5-ALA/SFC increased 2DG uptake in C2C12 cells to a similar level to the insulin-treated group. Moreover, it increased glucose transport (GLUT)1 translocation in the plasma membrane by 2.5-fold relative to the controls without affecting GLUT1 expression; however, it had no effect on GLUT4 translocation. Therefore, 5-ALA/SFC enhanced gastrocnemius and cardiac glucose uptake in HFD-fed mice, and upregulated GLUT1 translocation to the plasma membrane, but not GLUT4 in C2C12 myotube cells. Therefore, it could potentially be used as a novel drug for the treatment of diabetes.

Placental insufficiency induces a sexually dimorphic response in the expression of cardiac growth and metabolic signalling molecules upon exposure to a postnatal western diet in guinea pigs

There is a strong relationship between low birth weight (LBW) and an increased risk of developing cardiovascular disease (CVD). In postnatal life, LBW offspring are becoming more commonly exposed to the additional independent CVD risk factors, such as an obesogenic diet. However, how an already detrimentally programmed LBW myocardium responds to a secondary insult, such as an obesogenic diet (western diet; WD), during postnatal life is ill defined. Herein, we aimed to determine in a pre-clinical guinea pig model of CVD, both the independent and interactive effects of LBW and a postnatal WD on the molecular pathways that regulate cardiac growth and metabolism. Uterine artery ablation was used to induce placental insufficiency (PI) in pregnant guinea pigs to generate LBW offspring. Normal birth weight (NBW) and LBW offspring were weaned onto either a Control diet or WD. At ˜145 days after birth (young adulthood), male and female offspring were humanely killed, the heart weighed and left ventricle tissue collected. The mRNA expression of signalling molecules involved in a pathological hypertrophic and fibrotic response was increased in the myocardium of LBW male, but not female offspring, fed a WD as was the mRNA expression of transcription factors involved in fatty acid oxidation. The mRNA expression of glucose transporters was downregulated by LBW and WD in male, but not female hearts. This study has highlighted a sexually dimorphic cardiac pathological hypertrophic and fibrotic response to the secondary insult of postnatal WD consumption in LBW offspring.

Cardiac Glucose and Fatty Acid Transport After Experimental Mono- and Polytrauma

OBJECTIVE

The aim of this study was to define the influence of trauma on cardiac glucose and fatty acid transport. The effects were investigated in vivo in a porcine mono- and polytrauma model and in vitro in human cardiomyocytes, which were treated simultaneously with different inflammatory substances, mimicking posttraumatic inflammatory conditions.

METHODS AND RESULTS

In the porcine fracture- and polytrauma model, blood glucose concentrations were measured by blood gas analysis during an observation period of 72 h. The expression of cardiac glucose and fatty acid transporters in the left ventricle was determined by RT-qPCR and immunofluorescence. Cardiac and hepatic glycogen storage was examined. Furthermore, human cardiomyocytes were exposed to a defined trauma-cocktail and the expression levels of glucose- and fatty acid transporters were determined. Early after polytrauma, hyperglycemia was observed. After 48 and 72 h, pigs with fracture- and polytrauma developed hypoglycemia. The propofol demand significantly increased posttrauma. The hepatic glycogen concentration was reduced 72 h after trauma. Cardiac glucose and fatty acid transporters changed in both trauma models in vivo as well as in vitro in human cardiomyocytes in presence of proinflammatory mediators.

CONCLUSIONS

Monotrauma as well as polytrauma changed the cardiac energy transport by altering the expression of glucose and fatty acid transporters. In vitro data suggest that human cardiomyocytes shift to a state alike myocardial hibernation preferring glucose as primary energy source to maintain cardiac function.

Cardiomyocyte-specific Txnip C247S mutation improves left ventricular functional reserve in streptozotocin-induced diabetic mice

Underlying molecular mechanisms for the development of diabetic cardiomyopathy remain to be determined. Long-term exposure to hyperglycemia causes oxidative stress, which leads to cardiomyocyte dysfunction. Previous studies established the importance of thioredoxin-interacting protein (Txnip) in cellular redox homeostasis and glucose metabolism. Txnip is a highly glucose-responsive molecule that interacts with the catalytic center of reduced thioredoxin and inhibits the antioxidant function of thioredoxin. Here, we show that the molecular interaction between Txnip and thioredoxin plays a pivotal role in the regulation of redox balance in the diabetic myocardium. High glucose increased Txnip expression, decreased thioredoxin activities, and caused oxidative stress in cells. The Txnip-thioredoxin complex was detected in cells with overexpressing wild-type Txnip but not Txnip cysteine 247 to serine (C247S) mutant that disrupts the intermolecular disulfide bridge. Then, diabetes was induced in cardiomyocyte-specific Txnip C247S knock-in mice and their littermate control animals by injections of streptozotocin (STZ). Prolonged hyperglycemia upregulated myocardial Txnip expression in both genotypes. The absence of Txnip's inhibition of thioredoxin in Txnip C247S mutant hearts promoted mitochondrial antioxidative capacities in cardiomyocytes, thereby protecting the heart from oxidative damage by diabetes. Stress hemodynamic analysis uncovered that Txnip C247S knock-in hearts have a greater left ventricular contractile reserve than wild-type hearts under STZ-induced diabetic conditions. These results provide novel evidence that Txnip serves as a regulator of hyperglycemia-induced cardiomyocyte toxicities through direct inhibition of thioredoxin and identify the single cysteine residue in Txnip as a therapeutic target for diabetic injuries.NEW & NORTEWORTHY Thioredoxin-interacting protein (Txnip) has been of great interest as a molecular mechanism to mediate diabetic organ damage. Here, we provide novel evidence that a single mutation of Txnip confers a defense mechanism against myocardial oxidative stress in streptozotocin-induced diabetic mice. The results demonstrate the importance of Txnip as a cysteine-containing redox protein that regulates antioxidant thioredoxin via disulfide bond-switching mechanism and identify the cysteine in Txnip as a therapeutic target for diabetic cardiomyopathy.

Run for your life: can exercise be used to effectively target GLUT4 in diabetic cardiac disease?

The global incidence, associated mortality rates and economic burden of diabetes are now such that it is considered one of the most pressing worldwide public health challenges. Considerable research is now devoted to better understanding the mechanisms underlying the onset and progression of this disease, with an ultimate aim of improving the array of available preventive and therapeutic interventions. One area of particular unmet clinical need is the significantly elevated rate of cardiomyopathy in diabetic patients, which in part contributes to cardiovascular disease being the primary cause of premature death in this population. This review will first consider the role of metabolism and more specifically the insulin sensitive glucose transporter GLUT4 in diabetic cardiac disease, before addressing how we may use exercise to intervene in order to beneficially impact key functional clinical outcomes.

Metabolic alterations in a rat model of Takotsubo syndrome

AIMS

Cardiac energetic impairment is a major finding in takotsubo patients. We investigate specific metabolic adaptations to direct future therapies.

METHODS AND RESULTS

An isoprenaline-injection female rat model (versus sham) was studied at day-3; recovery assessed at day-7. Substrate uptake, metabolism, inflammation and remodelling were investigated by 18F-FDG-PET, metabolomics, qPCR and WB. Isolated cardiomyocytes were patch-clamped during stress protocols for redox states of NAD(P)H/FAD or [Ca2+]c, [Ca2+]m and sarcomere length. Mitochondrial respiration was assessed by seahorse/Clark electrode (glycolytic and β-oxidation substrates).Cardiac 18F-FDG metabolic rate was increased in takotsubo (p = 0.006), as were expression of GLUT4-RNA/GLUT1/HK2-RNA and HK activity (all p < 0.05), with concomitant accumulation of glucose- and fructose-6-phosphates (p > 0.0001). Both lactate and pyruvate were lower (p < 0.05) despite increases in LDH-RNA and PDH (p < 0.05 both). β-oxidation enzymes CPT1b-RNA and 3KAT were increased (p < 0.01) but malonyl-CoA (CPT-1 regulator) was upregulated (p = 0.01) with decreased fatty acids and acyl-carnitines levels (p = 0.0001-0.02). Krebs cycle intermediates α-ketoglutarate and succinyl-carnitine were reduced (p < 0.05) as was cellular ATP reporter dihydroorotate (p = 0.003). Mitochondrial Ca2+ uptake during high workload was impaired on day-3 (p < 0.0001), inducing oxidation of NAD(P)H and FAD (p = 0.03) but resolved by day-7. There were no differences in mitochondrial respiratory function, sarcomere shortening or [Ca2+] transients of isolated cardiomyocytes, implying preserved integrity of both mitochondria and cardiomyocyte. Inflammation and remodelling were upregulated - increased CD68-RNA, collagen RNA/protein and skeletal actin RNA (all p < 0.05).

CONCLUSION

Dys-regulation of glucose and lipid metabolic pathways with decreases in final glycolytic and β-oxidation metabolites and reduced availability of Krebs intermediates characterises takotsubo myocardium. The energetic deficit accompanies defective Ca2+ handling, inflammation and upregulation of remodelling pathways, with preservation of sarcomeric and mitochondrial integrity.

TRANSLATIONAL PERSPECTIVE

The simultaneous dysregulation in the glycolytic and beta-oxidation pathways which underlies the energetic deficit of the takotsubo heart supports further testing of currently available metabolic modulators as possible candidates for successful therapy, as well as targeting the inflammatory and remodelling pathways.

TMF1 is upregulated by insulin and is required for a sustained glucose homeostasis

Insulin-regulated glucose homeostasis is a critical and intricate physiological process, of which not all regulatory components have been deciphered. One of the key players in modulating glucose uptake by cells is the glucose transporter-GLUT4. In this study, we aimed to explore the regulatory role of the trans-Golgi-associated protein-TATA Element Modulatory Factor (TMF1) in the GLUT4 mediated, insulin-directed glucose uptake. By establishing and using TMF1-/- myoblasts and mice, we examined the effect of TMF1 absence on the insulin driven functioning of GLUT4. We show that TMF1 is upregulated by insulin in myoblasts, and is essential for the formation of insulin responsive, glucose transporter GLUT4-containing vesicles. Absence of TMF1 leads to the retention of GLUT4 in perinuclear compartments, and to severe impairment of insulin-stimulated GLUT4 trafficking throughout the cytoplasm and to the cell plasma membrane. Accordingly, glucose uptake is impaired in TMF1-/- cells, and TMF1-/- mice are hyperglycemic. This is reflected by the mice impaired blood glucose clearance and increased blood glucose level. Correspondingly, TMF1-/- animals are leaner than their normal littermates. Thus, TMF1 is a novel effector of insulin-regulated glucose homeostasis, and dys-functioning of this protein may contribute to the onset of a diabetes-like disorder.

Stress hyperglycemia, cardiac glucotoxicity, and critically ill patient outcomes current clinical and pathophysiological evidence

Stress hyperglycemia is a transient increase in blood glucose during acute physiological stress in the absence of glucose homeostasis dysfunction. Its's presence has been described in critically ill patients who are subject to many physiological insults. In this regard, hyperglycemia and impaired glucose tolerance are also frequent in patients who are admitted to the intensive care unit for heart failure and cardiogenic shock. The hyperglycemia observed at the beginning of these cardiac disorders appears to be related to a variety of stress mechanisms. The release of major stress and steroid hormones, catecholamine overload, and glucagon all participate in generating a state of insulin resistance with increased hepatic glucose output and glycogen breakdown. In fact, the observed pathophysiological response, which appears to regulate a stress situation, is harmful because it induces mitochondrial impairment, oxidative stress-related injury to cells, endothelial damage, and dysfunction of several cellular channels. Paradigms are now being challenged by growing evidence of a phenomenon called glucotoxicity, providing an explanation for the benefits of lowering glucose levels with insulin therapy in these patients. In the present review, the authors present the data published on cardiac glucotoxicity and discuss the benefits of lowering plasma glucose to improve heart function and to positively affect the course of critical illness.

MicroRNA-125b reduces glucose uptake in papillary thyroid carcinoma cells

Previous studies have shown that microRNA (miR)-125b plays important roles in several human cancer types. The aim of the present study was to analyze the potential roles of miR-125b in papillary thyroid carcinoma (PTC). It was found that miR-125b was downregulated in PTC and its expression was affected by clinical stages. Glucose transporter 1 (GLUT1) was upregulated in PTC and was negatively correlated with miR-125b. In PTC cells, overexpression of miR-125b suppressed glucose uptake and downregulated GLUT1. Furthermore, GLUT1 overexpression reduced the effects of miR-125b overexpression on glucose uptake. Moreover, miR-125b overexpression suppressed PTC cell proliferation. GLUT1 overexpression promoted the proliferation of PTC cells and reduced the effects of miR-125b overexpression on cancer cell proliferation. Overall, miR-125b decreased glucose uptake in PTC cells by downregulating GLUT1.

Changes in Myocardial Metabolism Preceding Sudden Cardiac Death

Heart disease is widely recognized as a major cause of death worldwide and is the leading cause of mortality in the United States. Centuries of research have focused on defining mechanistic alterations that drive cardiac pathogenesis, yet sudden cardiac death (SCD) remains a common unpredictable event that claims lives in every age group. The heart supplies blood to all tissues while maintaining a constant electrical and hormonal feedback communication with other parts of the body. As such, recent research has focused on understanding how myocardial electrical and structural properties are altered by cardiac metabolism and the various signaling pathways associated with it. The importance of cardiac metabolism in maintaining myocardial function, or lack thereof, is exemplified by shifts in cardiac substrate preference during normal development and various pathological conditions. For instance, a shift from fatty acid (FA) oxidation to oxygen-sparing glycolytic energy production has been reported in many types of cardiac pathologies. Compounded by an uncoupling of glycolysis and glucose oxidation this leads to accumulation of undesirable levels of intermediate metabolites. The resulting accumulation of intermediary metabolites impacts cardiac mitochondrial function and dysregulates metabolic pathways through several mechanisms, which will be reviewed here. Importantly, reversal of metabolic maladaptation has been shown to elicit positive therapeutic effects, limiting cardiac remodeling and at least partially restoring contractile efficiency. Therein, the underlying metabolic adaptations in an array of pathological conditions as well as recently discovered downstream effects of various substrate utilization provide guidance for future therapeutic targeting. Here, we will review recent data on alterations in substrate utilization in the healthy and diseased heart, metabolic pathways governing cardiac pathogenesis, mitochondrial function in the diseased myocardium, and potential metabolism-based therapeutic interventions in disease.

Cellular effects and clinical implications of SLC2A3 copy number variation

SLC2A3 encodes the predominantly neuronal glucose transporter 3 (GLUT3), which facilitates diffusion of glucose across plasma membranes. The human brain depends on a steady glucose supply for ATP generation, which consequently fuels critical biochemical processes, such as axonal transport and neurotransmitter release. Besides its role in the central nervous system, GLUT3 is also expressed in nonneural organs, such as the heart and white blood cells, where it is equally involved in energy metabolism. In cancer cells, GLUT3 overexpression contributes to the Warburg effect by answering the cell's increased glycolytic demands. The SLC2A3 gene locus at chromosome 12p13.31 is unstable and prone to non-allelic homologous recombination events, generating multiple copy number variants (CNVs) of SLC2A3 which account for alterations in SLC2A3 expression. Recent associations of SLC2A3 CNVs with different clinical phenotypes warrant investigation of the potential influence of these structural variants on pathomechanisms of neuropsychiatric, cardiovascular, and immune diseases. In this review, we accumulate and discuss the evidence how SLC2A3 gene dosage may exert diverse protective or detrimental effects depending on the pathological condition. Cellular states which lead to increased energetic demand, such as organ development, proliferation, and cellular degeneration, appear particularly susceptible to alterations in SLC2A3 copy number. We conclude that better understanding of the impact of SLC2A3 variation on disease etiology may potentially provide novel therapeutic approaches specifically targeting this GLUT.

High-throughput proteomics fiber typing (ProFiT) for comprehensive characterization of single skeletal muscle fibers

BACKGROUND

Skeletal muscles are composed of a heterogeneous collection of fiber types with different physiological adaption in response to a stimulus and disease-related conditions. Each fiber has a specific molecular expression of myosin heavy chain molecules (MyHC). So far, MyHCs are currently the best marker proteins for characterization of individual fiber types, and several proteome profiling studies have helped to dissect the molecular signature of whole muscles and individual fibers.

METHODS

Herein, we describe a mass spectrometric workflow to measure skeletal muscle fiber type-specific proteomes. To bypass the limited quantities of protein in single fibers, we developed a Proteomics high-throughput fiber typing (ProFiT) approach enabling profiling of MyHC in single fibers. Aliquots of protein extracts from separated muscle fibers were subjected to capillary LC-MS gradients to profile MyHC isoforms in a 96-well format. Muscle fibers with the same MyHC protein expression were pooled and subjected to proteomic, pulsed-SILAC, and phosphoproteomic analysis.

RESULTS

Our fiber type-specific quantitative proteome analysis confirmed the distribution of fiber types in the soleus muscle, substantiates metabolic adaptions in oxidative and glycolytic fibers, and highlighted significant differences between the proteomes of type IIb fibers from different muscle groups, including a differential expression of desmin and actinin-3. A detailed map of the Lys-6 incorporation rates in muscle fibers showed an increased turnover of slow fibers compared to fast fibers. In addition, labeling of mitochondrial respiratory chain complexes revealed a broad range of Lys-6 incorporation rates, depending on the localization of the subunits within distinct complexes.

CONCLUSION

Overall, the ProFiT approach provides a versatile tool to rapidly characterize muscle fibers and obtain fiber-specific proteomes for different muscle groups.

Insulin acutely increases glucose transporter 1 on plasma membranes and glucose uptake in an AKT-dependent manner in chicken adipocytes

Avian glucose transporters (GLUT) responsible for insulin-responsive glucose uptake into adipocytes remain poorly characterized. We aimed to identify the insulin-responsive GLUT using primary culture of chicken adipocytes. Acute stimulation with 1 μM insulin for 20 min increased 2-deoxyglucose uptake, AKT protein phosphorylation, and GLUT1 protein levels on the plasma membrane of the chicken adipocytes, whereas pretreatment with 10 μM triciribine, an AKT inhibitor, canceled these effects. Furthermore, the insulin stimulation did not affect GLUT12 protein levels on the plasma membrane of the chicken adipocytes. Our results suggest that GLUT1 is an insulin-responsive GLUT in chicken adipocytes.

Regulation of Skeletal Muscle Glucose Transport and Glucose Metabolism by Exercise Training

Aerobic exercise training and resistance exercise training are both well-known for their ability to improve human health; especially in individuals with type 2 diabetes. However, there are critical differences between these two main forms of exercise training and the adaptations that they induce in the body that may account for their beneficial effects. This article reviews the literature and highlights key gaps in our current understanding of the effects of aerobic and resistance exercise training on the regulation of systemic glucose homeostasis, skeletal muscle glucose transport and skeletal muscle glucose metabolism.

Restoration of myocardial glucose uptake with facilitated myocardial glucose transporter 4 translocation contributes to alleviation of diabetic cardiomyopathy in rats after duodenal-jejunal bypass

AIMS/INTRODUCTION

Duodenal-jejunal bypass (DJB) surgery has been reported to effectively relieve diabetic cardiomyopathy (DCM). However, the specific mechanisms remain largely unknown. The present study was designed to determine the alterations of myocardial glucose uptake (MGU) after DJB and their effects on DCM.

MATERIALS AND METHODS

Duodenal-jejunal bypass and sham surgeries were carried out in diabetic rats induced by a high-fat diet and a low dose of streptozotocin, with chow-diet fed rats as controls. Bodyweight, food intake, glucose homeostasis and lipid profiles were measured at indicated time-points. Cardiac function was evaluated by transthoracic echocardiography and hemodynamic measurement. Cardiac remodeling was assessed by a series of morphometric analyses along with transmission electron microscopy. Positron-emission tomography with fluorine-18 labeled fluorodeoxyglucose was carried out to evaluate the MGU in vivo. Furthermore, myocardial glucose transporters (GLUT; GLUT1 and GLUT4), myocardial insulin signaling and GLUT-4 translocation-related proteins were investigated to elucidate the underlying mechanisms.

RESULTS

The DJB group showed restored systolic and diastolic cardiac function, along with significant remission in cardiac hypertrophy, cardiac fibrosis, lipid deposit and ultrastructural disorder independent of weight loss compared with the sham group. Furthermore, the DJB group showed upregulated myocardial insulin signaling, hyperphosphorylation of AKT substrate of 160 kDa (AS160) and TBC1D1, along with preserved soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins, facilitating the GLUT-4 translocation to the myocardial cell surface and restoration of MGU.

CONCLUSIONS

The present findings provide evidence that restoration of MGU is implicated in the alleviation of DCM after DJB through facilitating GLUT-4 translocation, suggesting a potential choice for treatment of human DCM if properly implemented.

Overview of the Components of Cardiac Metabolism

Metabolism in organs other than the liver and kidneys may play a significant role in how a specific organ responds to chemicals. The heart has metabolic capability for energy production and homeostasis. This homeostatic machinery can also process xenobiotics. Cardiac metabolism includes the expression of numerous organic anion transporters, organic cation transporters, organic carnitine (zwitterion) transporters, and ATP-binding cassette transporters. Expression and distribution of the transporters within the heart may vary, depending on the patient’s age, disease, endocrine status, and various other factors. Several cytochrome P450 (P450) enzyme classes have been identified within the heart. The P450 hydroxylases and epoxygenases within the heart produce hydroxyeicosatetraneoic acids and epoxyeicosatrienoic acids, metabolites of arachidonic acid, which are critical in regulating homeostatic processes of the heart. The susceptibility of the cardiac P450 system to induction and inhibition from exogenous materials is an area of expanding knowledge, as are the metabolic processes of glucuronidation and sulfation in the heart. The susceptibility of various transcription factors and signaling pathways of the heart to disruption by xenobiotics is not fully characterized but is an area with implications for disruption of normal postnatal development, as well as modulation of adult cardiac health. There are knowledge gaps in the timelines of physiologic maturation and deterioration of cardiac metabolism. Cross-species characterization of cardiac-specific metabolism is needed for nonclinical work of optimum translational value to predict possible adverse effects, identify sensitive developmental windows for the design and conduct of informative nonclinical and clinical studies, and explore the possibilities of organ-specific therapeutics.

Molecular Mechanisms Responsible for Diastolic Dysfunction in Diabetes Mellitus Patients

In diabetic patients, cardiomyopathy is an important cause of heart failure, but its pathophysiology has not been completely understood thus far. Myocardial hypertrophy and diastolic dysfunction have been considered the hallmarks of diabetic cardiomyopathy (DCM), while systolic function is affected in the latter stages of the disease. In this article we propose the potential pathophysiological mechanisms responsible for myocardial hypertrophy and increased myocardial stiffness leading to diastolic dysfunction in this specific entity. According to our model, increased myocardial stiffness results from both cellular and extracellular matrix stiffness as well as cell–matrix interactions. Increased intrinsic cardiomyocyte stiffness is probably the most important contributor to myocardial stiffness. It results from the impairment in cardiomyocyte cytoskeleton. Several other mechanisms, specifically affected by diabetes, seem to also be significantly involved in myocardial stiffening, i.e., impairment in the myocardial nitric oxide (NO) pathway, coronary microvascular dysfunction, increased inflammation and oxidative stress, and myocardial sodium glucose cotransporter-2 (SGLT-2)-mediated effects. Better understanding of the complex pathophysiology of DCM suggests the possible value of drugs targeting the listed mechanisms. Antidiabetic drugs, NO-stimulating agents, anti-inflammatory agents, and SGLT-2 inhibitors are emerging as potential treatment options for DCM.

Cardiac SNARE Expression in Health and Disease

SNARE proteins are integral to intracellular vesicular trafficking, which in turn is the process underlying the regulated expression of substrate transporters such as the glucose transporter GLUT4 at the cell surface of insulin target tissues. Impaired insulin stimulated GLUT4 trafficking is associated with reduced cardiac function in many disease states, most notably diabetes. Despite this, our understanding of the expression and regulation of SNARE proteins in cardiac tissue and how these may change in diabetes is limited. Here we characterize the array of SNARE proteins expressed in cardiac tissue, and quantify the levels of expression of VAMP2, SNAP23, and Syntaxin4-key proteins involved in insulin-stimulated GLUT4 translocation. We examined SNARE protein levels in cardiac tissue from two rodent models of insulin resistance, db/db mice and high-fat fed mice, and show alterations in patterns of expression are evident. Such changes may have implications for cardiac function.

Glucose-Mediated Glucose Disposal at Baseline Insulin Is Impaired in IFG

OBJECTIVE

To quantify glucose-mediated glucose disposal with and without basal insulin replacement and insulin-mediated glucose disposal in subjects with impaired fasting glucose (IFG).

RESEARCH DESIGN AND METHODS

We used the hyperglycemic/pancreatic clamp and stepped euglycemic clamp techniques to quantify glucose disposal and suppression of endogenous glucose production (EGP) in those with normal glucose tolerance (NGT; n = 14) and those with IFG (n = 14).

RESULTS

Total body glucose-mediated glucose uptake, measured with the hyperglycemic/pancreatic clamp, was not significantly affected by the basal plasma insulin levels in subjects with IFG and those with NGT. Compared with subjects with NGT, those with IFG had significantly lower glucose-mediated glucose uptake (by 15%) during the hyperglycemic clamp performed with and without basal insulin replacement. In contrast, insulin-mediated glucose disposal was comparable in both groups. The suppression of EGP by hyperglycemia was similar in both groups. However, the suppression of EGP by insulin was attenuated in those with IFG compared with those with NGT.

CONCLUSIONS

The results of the present study have demonstrated that (i) glucose-mediated glucose disposal is impaired in those with IFG; (ii) insulin-mediated glucose uptake in IFG is normal; and (iii) insulin action to suppress EGP is impaired.

The synergism of high-intensity intermittent exercise and every-other-day intermittent fasting regimen on energy metabolism adaptations includes hexokinase activity and mitochondrial efficiency

Visceral lipid accumulation, organ hypertrophy and a reduction in skeletal muscle strength are all signs associated with the severity of obesity-related disease. Intermittent fasting (IF) and high-intensity intermittent exercise (HIIE) are natural strategies that, individually, can prevent and help treat obesity along with metabolic syndrome and its associated diseases. However, the combinatorial effect of IF and HIIE on energetic metabolism is currently not well understood. We hypothesized that their combination could have a potential for more than strictly additive benefits. Here, we show that two months of every-other-day intermittent fasting regimen combined with a high-intensity intermittent exercise protocol (IF/HIIE) produced a synergistic effect, enhancing physical endurance (vs. control, HIIE and IF) and optimizing metabolic pathways of energy production in male Wistar rats. The IF/HIIE group presented enhanced glucose tolerance (vs. control, HIIE and IF), lower levels of plasma insulin (vs. control and HIIE), and a global activation of low Km hexokinases in liver (vs. control, HIIE and IF), heart (vs. control and HIIE) and skeletal muscle (vs. control, HIIE and IF). The IF/HIIE synergism, rather than a simply additive effect, is evidenced by increase in muscle mass and cross-section area, activation of the FoF1 ATP synthase, and the gain of characteristics suggestive of augmented mitochondrial mass and efficiency observed in this group. Finally, important reductions in plasma oxidative stress markers were present preferentially in IF/HIIE group. These findings provide new insights for the implementation of non-pharmaceutical strategies to prevent/treat metabolic syndrome and associated diseases.

Cardiac applications of hyperpolarised magnetic resonance

Cardiovascular disease is the leading cause of death world-wide. It is increasingly recognised that cardiac pathologies show, or may even be caused by, changes in metabolism, leading to impaired cardiac energetics. The heart turns over 15 times its own weight in ATP every day and thus relies heavily on the availability of substrates and on efficient oxidation to generate this ATP. A number of old and emerging drugs that target different aspects of metabolism are showing promising results with regard to improved cardiac outcomes in patients. A non-invasive imaging technique that could assess the role of different aspects of metabolism in heart disease, as well as measure changes in cardiac energetics due to treatment, would be valuable in the routine clinical care of cardiac patients. Hyperpolarised magnetic resonance spectroscopy and imaging have revolutionised metabolic imaging, allowing real-time metabolic flux assessment in vivo for the first time. In this review we summarise metabolism in the healthy and diseased heart, give an introduction to the hyperpolarisation technique, 'dynamic nuclear polarisation' (DNP), and review the preclinical studies that have thus far explored healthy cardiac metabolism and different models of human heart disease. We furthermore show what advances have been made to translate this technique into the clinic, what technical challenges still remain and what unmet clinical needs and unexplored metabolic substrates still need to be assessed by researchers in this exciting and fast-moving field.

Preliminary Evidence for Adipocytokine Signals in Skeletal Muscle Glucose Uptake

The cross talk between the adipose tissue and insulin target tissues is a key mechanism for obesity-associated insulin resistance. However, the precise role of the interaction between the skeletal muscle and adipose tissue for insulin signaling and glucose uptake is questionable. L6 myocytes were co-cultured with or without 3T3-L1 adipocytes (~5 × 10³ cells/cm²) up to 24 h. Glucose uptake was evaluated by 2-[³H] deoxyglucose uptake assay. Levels of mRNA expression of Glut1 and Glut4 and mitochondrial enzymes were analyzed by quantitative real-time reverse transcription polymerase chain reaction. Levels of Glut1 and Glut4 protein and phosphorylation of Akt (Ser473 and Thr308) were analyzed by immunoblotting. Study 1: co-culture with 3T3-L1 adipocytes increased glucose uptake in dose- and time-dependent manner in L6 myocytes under insulin-untreated conditions. When co-cultured with 3T3-L1 cells, reactive oxygen species production and levels of Glut1 mRNA and protein were increased in L6 cells, while these changes were abrogated and the glucose uptake partially inhibited by antioxidant treatment. Study 2: co-culture with 3T3-L1 adipocytes suppressed insulin-stimulated glucose uptake in L6 myocytes. Insulin-induced Akt phosphorylation at Ser473 decreased, which was proportional to 3T3-L1 density. Antioxidant treatment partially reversed this effect. Interactions between skeletal muscle and adipose tissues are important for glucose uptake under insulin-untreated or -treated condition through oxygen stress mechanism.

Integrative Analysis of PRKAG2 Cardiomyopathy iPS and Microtissue Models Identifies AMPK as a Regulator of Metabolism, Survival, and Fibrosis

AMP-activated protein kinase (AMPK) is a metabolic enzyme that can be activated by nutrient stress or genetic mutations. Missense mutations in the regulatory subunit, PRKAG2, activate AMPK and cause left ventricular hypertrophy, glycogen accumulation, and ventricular pre-excitation. Using human iPS cell models combined with three-dimensional cardiac microtissues, we show that activating PRKAG2 mutations increase microtissue twitch force by enhancing myocyte survival. Integrating RNA sequencing with metabolomics, PRKAG2 mutations that activate AMPK remodeled global metabolism by regulating RNA transcripts to favor glycogen storage and oxidative metabolism instead of glycolysis. As in patients with PRKAG2 cardiomyopathy, iPS cell and mouse models are protected from cardiac fibrosis, and we define a crosstalk between AMPK and post-transcriptional regulation of TGFβ isoform signaling that has implications in fibrotic forms of cardiomyopathy. Our results establish critical connections among metabolic sensing, myocyte survival, and TGFβ signaling.

Development of GLUT4-selective antagonists for multiple myeloma therapy

Cancer cells consume more glucose to fuel metabolic programs fundamental to sustaining their survival, growth and proliferation. Among the fourteen SLC2A family members, GLUTs 1 and 4 are high-affinity glucose transporters. GLUT4 (SLC2A4) is highly expressed in muscle and adipose tissue. Basally retained within the cell, GLUT4 traffics to the plasma membrane (PM) in response to insulin and exercise-stimulation. The plasma cell malignancy multiple myeloma (MM) exhibits increased constitutive expression of GLUT4 on the PM, co-opting use of GLUT4 for survival and proliferation. GLUT4 inhibition by knockdown or treatment with the FDA-approved HIV protease inhibitor ritonavir leads to cytostatic and/or cytotoxic and chemosensitizing effects in tumor cells both in vitro and in vivo. We recently reported our generation of GLUT4 homology models and virtual high-throughput screening (vHTS) to identify multiple series of novel GLUT4 antagonists. In this report, we describe our initial hit-to-lead optimization to synthesize new analogs with improved potency and selectivity for GLUT4, and the biological characterization of these compounds in a variety of assays. We show that our lead compound (compound 20) decreases glucose uptake and cell proliferation as well as inhibits the expression of pro-survival MCL-1 in MM similar to the effect observed via knockdown of GLUT4 expression. Compound 20 is also effective at chemosensitizing multiple myeloma cell lines and patient samples to venetoclax, dexamethasone and melphalan. In sum, we report development of selective GLUT4 inhibitors lacking inhibitory activity against GLUT1 and GLUT8. We show that selective pharmacological inhibition of GLUT4 is feasible and this may represent a novel strategy for the treatment and chemosensitization of multiple myeloma to standard therapeutics.

Bone Marrow Mesenchymal Stem Cells (BM-MSCs) Improve Heart Function in Swine Myocardial Infarction Model through Paracrine Effects

Stem cells are promising for the treatment of myocardial infarction (MI) and large animal models should be used to better understand the full spectrum of stem cell actions and preclinical evidences. In this study, bone marrow mesenchymal stem cells (BM-MSCs) were transplanted into swine heart ischemia model. To detect glucose metabolism in global left ventricular myocardium and regional myocardium, combined with assessment of cardiac function, positron emission tomography-computer tomography (PET-CT) and magnetic resonance imaging (MRI) were performed. To study the changes of glucose transporters and glucose metabolism-related enzymes and the signal transduction pathway, RT-PCR, Western-blot, and immunohistochemistry were carried out. Myocardium metabolic evaluation by PET-CT showed that mean signal intensity (MSI) increased in these segments at week 4 compared with that at week 1 after BM-MSCs transplantation. Moreover, MRI demonstrated significant function enhancement in BM-MSCs group. The gene expressions of glucose transporters (GLUT1, GLUT4), glucose metabolism-related enzymes phosphofructokinase (PFK), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH)) and 70-kDa ribosomal protein S6 kinase (p70s6k) in BM-MSCs injected areas were up-regulated at week 4 after BM-MSCs transplantation and this was confirmed by Western-blot and immunohistochemistry. In conclusions, BM-MSCs transplantation could improve cardiac function in swine MI model by activation of mTOR signal transduction pathway.

Gene expression pattern of glucose transporters in the skeletal muscles of newly hatched chicks

The gene expression pattern of the glucose transporters (GLUT1, GLUT3, GLUT8, and GLUT12) among pectoralis major and minor, biceps femoris, and sartorius muscles from newly hatched chicks was examined. GLUT1 mRNA level was higher in pectoralis major muscle than in the other muscles. Phosphorylated AKT level was also high in the same muscle, suggesting a relationship between AKT and GLUT1 expression.

Cardiomyocyte-endothelial cell control of lipoprotein lipase

In people with diabetes, inadequate pharmaceutical management predisposes the patient to heart failure, which is the leading cause of diabetes related death. One instigator for this cardiac dysfunction is change in fuel utilization by the heart. Thus, following diabetes, when cardiac glucose utilization is impaired, the heart undergoes metabolic transformation wherein it switches to using fats as an exclusive source of energy. Although this switching is geared to help the heart initially, in the long term, this has detrimental effects on cardiac function. These include the generation of noxious byproducts, which damage the cardiomyocytes, and ultimately result in increased morbidity and mortality. A key perpetrator that may be responsible for organizing this metabolic disequilibrium is lipoprotein lipase (LPL), the enzyme responsible for providing fat to the hearts. Either exaggeration or reduction in its activity following diabetes could lead to heart dysfunction. Given the disturbing news that diabetes is rampant across the globe, gaining more insight into the mechanism(s) by which cardiac LPL is regulated may assist other researchers in devising new therapeutic strategies to restore metabolic equilibrium, to help prevent or delay heart disease seen during diabetes. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

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.

α1A-Adrenergic receptor prevents cardiac ischemic damage through PKCδ/GLUT1/4-mediated glucose uptake

While α(1)-adrenergic receptors (ARs) have been previously shown to limit ischemic cardiac damage, the mechanisms remain unclear. Most previous studies utilized low oxygen conditions in addition to ischemic buffers with glucose deficiencies, but we discovered profound differences if the two conditions are separated. We assessed both mouse neonatal and adult myocytes and HL-1 cells in a series of assays assessing ischemic damage under hypoxic or low glucose conditions. We found that α(1)-AR stimulation protected against increased lactate dehydrogenase release or Annexin V(+) apoptosis under conditions that were due to low glucose concentration not to hypoxia. The α(1)-AR antagonist prazosin or nonselective protein kinase C (PKC) inhibitors blocked the protective effect. α(1)-AR stimulation increased (3)H-deoxyglucose uptake that was blocked with either an inhibitor to glucose transporter 1 or 4 (GLUT1 or GLUT4) or small interfering RNA (siRNA) against PKCδ. GLUT1/4 inhibition also blocked α(1)-AR-mediated protection from apoptosis. The PKC inhibitor rottlerin or siRNA against PKCδ blocked α(1)-AR stimulated GLUT1 or GLUT4 plasma membrane translocation. α(1)-AR stimulation increased plasma membrane concentration of either GLUT1 or GLUT4 in a time-dependent fashion. Transgenic mice overexpressing the α(1A)-AR but not α(1B)-AR mice displayed increased glucose uptake and increased GLUT1 and GLUT4 plasma membrane translocation in the adult heart while α(1A)-AR but not α(1B)-AR knockout mice displayed lowered glucose uptake and GLUT translocation. Our results suggest that α(1)-AR activation is anti-apoptotic and protective during cardiac ischemia due to glucose deprivation and not hypoxia by enhancing glucose uptake into the heart via PKCδ-mediated GLUT translocation that may be specific to the α(1A)-AR subtype.

Aqueous fraction of Beta vulgaris ameliorates hyperglycemia in diabetic mice due to enhanced glucose stimulated insulin secretion, mediated by acetylcholine and GLP-1, and elevated glucose uptake via increased membrane bound GLUT4 transporters

Background

The study was designed to investigate the probable mechanisms of anti-hyperglycemic activity of B. Vulgaris.

Methodology/Principal Findings

Aqueous fraction of B. Vulgaris extract was the only active fraction (50mg/kg). Plasma insulin level was found to be the highest at 30 mins after B. Vulgaris administration at a dose of 200mg/kg. B. Vulgaris treated mice were also assayed for plasma Acetylcholine, Glucagon Like Peptide-1 (GLP-1), Gastric Inhibitory Peptide (GIP), Vasoactive Intestinal Peptide, Pituitary Adenylate Cyclase-Activating Peptide (PACAP), Insulin Like Growth Factor-1 (IGF-1), Pancreatic Polypeptides (PP), and Somatostatin, along with the corresponding insulin levels. Plasma Acetylcholine and GLP-1 significantly increased in B. Vulgaris treated animals and were further studied. Pharmacological enhancers, inhibitors, and antagonists of Acetylcholine and GLP-1 were also administered to the test animals, and corresponding insulin levels were measured. These studies confirmed the role of acetylcholine and GLP-1 in enhanced insulin secretion (p<0.05). Principal signaling molecules were quantified in isolated mice islets for the respective pathways to elucidate their activities. Elevated concentrations of Acetylcholine and GLP-1 in B. Vulgaris treated mice were found to be sufficient to activate the respective pathways for insulin secretion (p<0.05). The amount of membrane bound GLUT1 and GLUT4 transporters were quantified and the subsequent glucose uptake and glycogen synthesis were assayed. We showed that levels of membrane bound GLUT4 transporters, glucose-6-phosphate in skeletal myocytes, activity of glycogen synthase, and level of glycogen deposited in the skeletal muscles all increased (p<0.05).

Conclusion

Findings of the present study clearly prove the role of Acetylcholine and GLP-1 in the Insulin secreting activity of B. Vulgaris. Increased glucose uptake in the skeletal muscles and subsequent glycogen synthesis may also play a part in the anti-hyperglycemic activity of B. Vulgaris.

Absolute quantitative profiling of the key metabolic pathways in slow and fast skeletal muscle

Slow and fast skeletal muscles are composed of, respectively, mainly oxidative and glycolytic muscle fibers, which are the basic cellular motor units of the motility apparatus. They largely differ in excitability, contraction mechanism, and metabolism. Because of their pivotal role in body motion and homeostasis, the skeletal muscles have been extensively studied using biochemical and molecular biology approaches. Here we describe a simple analytical and computational approach to estimate titers of enzymes of basic metabolic pathways and proteins of the contractile machinery in the skeletal muscles. Proteomic analysis of mouse slow and fast muscles allowed estimation of the titers of enzymes involved in the carbohydrate, lipid, and energy metabolism. Notably, we observed that differences observed between the two muscle types occur simultaneously for all proteins involved in a specific process such as glycolysis, free fatty acid catabolism, Krebs cycle, or oxidative phosphorylation. These differences are in a good agreement with the well-established biochemical picture of the muscle types. We show a correlation between maximal activity and the enzyme titer, suggesting that change in enzyme concentration is a good proxy for its catalytic potential in vivo. As a consequence, proteomic profiling of enzyme titers can be used to monitor metabolic changes in cells. Additionally, quantitative data of structural proteins allowed studying muscle type specific cell architecture and its remodeling. The presented proteomic approach can be applied to study metabolism in any other tissue or cell line.

Advanced glycation end products: role in pathology of diabetic cardiomyopathy

Increasing evidence demonstrates that advanced glycation end products (AGEs) play a pivotal role in the development and progression of diabetic heart failure, although there are numerous other factors that mediate the disease response. AGEs are generated intra- and extracellularly as a result of chronic hyperglycemia. Then, following the interaction with receptors for advanced glycation end products (RAGEs), a series of events leading to vascular and myocardial damage are elicited and sustained, which include oxidative stress, increased inflammation, and enhanced extracellular matrix accumulation resulting in diastolic and systolic dysfunction. Whereas targeting glycemic control and treating additional risk factors, such as obesity, dyslipidemia, and hypertension, are mandatory to reduce chronic complications and prolong life expectancy in diabetic patients, drug therapy tailored to reducing the deleterious effects of the AGE-RAGE interactions is being actively investigated and showing signs of promise in treating diabetic cardiomyopathy and associated heart failure. This review shall discuss the formation of AGEs in diabetic heart tissue, potential targets of glycation in the myocardium, and underlying mechanisms that lead to diabetic cardiomyopathy and heart failure along with the use of AGE inhibitors and breakers in mitigating myocardial injury.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

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.

Anti-diabetic effects of Centratherum anthelminticum seeds methanolic fraction on pancreatic cells, β-TC6 and its alleviating role in type 2 diabetic rats

ETHNOPHARMACOLOGICAL RELEVANCE

Seeds of Centratherum anthelminticum (Asteraceae) have been popularly used in Ayurvedic medicine to treat diabetes and skin disorders. Folk medicine from Rayalaseema (Andhra Pradesh, India) reported wide spread usage in diabetes.

AIM OF THE STUDY

To investigate the hypoglycemic properties and mechanism of the methanolic fraction of C. anthelminticum seeds (CAMFs) on mouse β-TC6 pancreatic cell line and streptozotocin (STZ)-induced diabetic rat models.

MATERIALS AND METHODS

We investigated the crude methanolic fraction of C. anthelminticum seeds (CAMFs) on β-TC6 cell line and confirmed its effects on type 1 and type 2 diabetic rats to understand its mechanism in managing diabetes mellitus. CAMFs were initially tested on β-TC6 cells for cytotoxicity, 2-NBDG glucose uptake, insulin secretion and glucose transporter (GLUT-1, 2 and 4) protein expression. Furthermore, streptozotocin (STZ)-induced type 1 diabetic and STZ-nicotinamide-induced type 2 diabetic rats were intraperitoneally (i.p) injected or administered orally with CAMFs daily for 28 days. The effect of CAMFs on blood glucose and insulin levels was subsequently evaluated.

RESULTS

In cell line studies, CAMFs showed non-cytotoxic effect on β-TC6 cell proliferation compared to untreated control cells at 50 μg/ml. CAMFs increased glucose uptake and insulin secretion dose-dependently by up-regulating GLUT-2 and GLUT-4 expression in these cells. Further in vivo studies on streptozotocin induced diabetic rat models revealed that CAMFs significantly reduced hyperglycemia by augmenting insulin secretion in type 2 diabetic rats. However, CAMFs displayed less significant effects on type 1 diabetic rats.

CONCLUSIONS

CAMFs demonstrated anti-diabetic potential on β-TC6 cells and type 2 diabetic rat model, plausibly through enhancing glucose uptake and insulin secretion.

Cardiac remodeling and myocardial dysfunction in obese spontaneously hypertensive rats

Background

The additive effects of obesity and metabolic syndrome on left ventricular (LV) maladaptive remodeling and function in hypertension are not characterized.

Methods

We compared an obese spontaneously hypertensive rat model (SHR-ob) with lean spontaneously hypertensive rats (SHR-lean) and normotensive controls (Ctr). LV-function was investigated by cardiac magnetic resonance imaging and invasive LV-pressure measurements. LV-interstitial fibrosis was quantified and protein levels of phospholamban (PLB), Serca2a and glucose transporters (GLUT1 and GLUT4) were determined by immunohistochemistry.

Results

Systolic blood pressure was similar in SHR-lean and SHR-ob (252 ± 7 vs. 242 ± 7 mmHg, p = 0.398) but was higher when compared to Ctr (155 ± 2 mmHg, p < 0.01 for both). Compared to SHR-lean and Ctr, SHR-ob showed impaired glucose tolerance and increased body-weight. In SHR-ob, LV-ejection fraction was impaired vs. Ctr (46.2 ± 1.1 vs. 59.6 ± 1.9%, p = 0.007). LV-enddiastolic pressure was more increased in SHR-ob than in SHR-lean (21.5 ± 4.1 vs. 5.9 ± 0.81 mmHg, p = 0.0002) when compared to Ctr (4.3 ± 1.1 mmHg, p < 0.0001 for both), respectively. Increased LV-fibrosis together with increased myocyte diameters and ANF gene expression in SHR-ob were associated with increased GLUT1-protein levels in SHR-ob suggestive for an upregulation of the GLUT1/ANF-axis. Serca2a-protein levels were decreased in SHR-lean but not altered in SHR-ob compared to Ctr. PLB-phosphorylation was not altered.

Conclusion

In addition to hypertension alone, metabolic syndrome and obesity adds to the myocardial phenotype by aggravating diastolic dysfunction and a progression towards systolic dysfunction. SHR-ob may be a useful model to develop new interventional and pharmacological treatment strategies for hypertensive heart disease and metabolic disorders.

Normal muscle glucose uptake in mice deficient in muscle GLUT4

Skeletal muscle insulin resistance is a major characteristic underpinning type 2 diabetes. Impairments in the insulin responsiveness of the glucose transporter, Glut4 (Slc2a4), have been suggested to be a contributing factor to this disturbance. We have produced muscle-specific Glut4 knockout (KO) mice using Cre/LoxP technology on a C57BL6/J background and shown undetectable levels of GLUT4 in both skeletal muscle and heart. Our aim was to determine whether complete deletion of muscle GLUT4 does in fact lead to perturbations in glucose homoeostasis. Glucose tolerance, glucose turnover and 2-deoxyglucose uptake into muscle and fat under basal and insulin-stimulated conditions were assessed in 12-week-old KO and control mice using the oral glucose tolerance test (OGTT) and hyperinsulinaemic/euglycaemic clamp respectively. KO mice weighed ~17% less and had significantly heavier hearts compared with control mice. Basally, plasma glucose and plasma insulin were significantly lower in the KO compared with control mice, which conferred normal glucose tolerance. Despite the lack of GLUT4 in the KO mouse muscle, glucose uptake was not impaired in skeletal muscle but was reduced in heart under insulin-stimulated conditions. Neither GLUT1 nor GLUT12 protein levels were altered in the skeletal muscle or heart tissue of our KO mice. High-fat feeding did not alter glucose tolerance in the KO mice but led to elevated plasma insulin levels during the glucose tolerance test. Our study demonstrates that deletion of muscle GLUT4 does not adversely affect glucose disposal and glucose tolerance and that compensation from other transporters may contribute to this unaltered homoeostasis of glucose.

Lipoprotein lipase mediated fatty acid delivery and its impact in diabetic cardiomyopathy

Although cardiovascular disease is the leading cause of diabetes-related death, its etiology is still not understood. The immediate change that occurs in the diabetic heart is altered energy metabolism where in the presence of impaired glucose uptake, glycolysis, and pyruvate oxidation, the heart switches to exclusively using fatty acids (FA) for energy supply. It does this by rapidly amplifying its lipoprotein lipase (LPL-a key enzyme, which hydrolyzes circulating lipoprotein-triglyceride to release FA) activity at the coronary lumen. An abnormally high capillary LPL could provide excess fats to the heart, leading to a number of metabolic, morphological, and mechanical changes, and eventually to cardiac disease. Unlike the initial response, chronic severe diabetes "turns off" LPL, this is also detrimental to cardiac function. In this review, we describe a number of post-translational mechanisms that influence LPL vesicle formation, actin cytoskeleton rearrangement, and transfer of LPL from cardiomyocytes to the vascular lumen to hydrolyze lipoprotein-triglyceride following diabetes. Appreciating the mechanism of how the heart regulates its LPL following diabetes should allow the identification of novel targets for therapeutic intervention, to prevent heart failure. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Effects of administration route, dietary condition, and blood glucose level on kinetics and uptake of 18F-FDG in mice

The effects of dietary condition and blood glucose level on the kinetics and uptake of (18)F-FDG in mice were systematically investigated using intraperitoneal and tail-vein injection.

METHODS

Dynamic PET was performed for 60 min on 23 isoflurane-anesthetized male C57BL/6 mice after intravenous (n = 11) or intraperitoneal (n = 12) injection of (18)F-FDG. Five and 6 mice in the intravenous and intraperitoneal groups, respectively, were kept fasting overnight (18 ± 2 h), and the others were fed ad libitum. Serial blood samples were collected from the femoral artery to measure (18)F-FDG and glucose concentrations. Image data were reconstructed using filtered backprojection with CT-based attenuation correction. The standardized uptake value (SUV) was estimated from the 45- to 60-min image. The metabolic rate of glucose (MRGlu) and (18)F-FDG uptake constant (K(i)) were derived by Patlak graphical analysis.

RESULTS

In the brain, SUV and K(i) were significantly higher in fasting mice with intraperitoneal injection, but MRGlu did not differ significantly under different dietary states and administration routes. Cerebral K(i) was inversely related to elevated blood glucose levels, irrespective of administration route or dietary state. In myocardium, SUV, K(i), and MRGlu were significantly lower in fasting than in nonfasting mice for both routes of injection. Myocardial SUV and K(i) were strongly dependent on the dietary state, and K(i) did not correlate with the blood glucose level. Similar results were obtained for skeletal muscle, although the differences were not as pronounced.

CONCLUSION

Intraperitoneal injection is a valid alternative route, providing pharmacokinetic data equivalent to data from tail-vein injection for small-animal (18)F-FDG PET. Cerebral K(i) varies inversely with blood glucose level, but the measured cerebral MRGlu does not correlate with blood glucose level or dietary condition. Conversely, the K(i) values of the myocardium and skeletal muscle are strongly dependent on dietary condition but not on blood glucose level. In tissue in which (18)F-FDG uptake declines with increasing blood glucose, correction for blood glucose level will make SUV a more robust outcome measure of MRGlu.

Cloning and molecular characterization of the glucose transporter 1 in tilapia (Oreochromis niloticus)

Facilitative glucose transporters (GLUTs) are responsible for passively transporting monosaccharides across the plasma membrane. We sequenced and characterized the Nile tilapia (Oreochromis niloticus) GLUT-1 (tGLUT-1) cDNA and genomic DNA. Using rapid amplification of the cDNA ends (RACE), two tGLUT-1 transcripts were detected differing in the length of the 3' untranslated region, 2851 and 4577 bp. Translated tGLUT-1 is a 490 amino acid product, which shares 74% homology with that of humans. Computer analysis of the amino acid sequence predicted 12 transmembrane domains, which are conserved in the GLUT-1 of various species. The tGLUT-1 gene spans more than 11 kb, and similar to the mammalian GLUT-1 genes has a 10 exon, 9 intron organization. Potential promoter regulatory elements have some similarity to those recorded for human, mouse, and rat GLUT-1 genes. Tissue expression studies revealed both GLUT-1 transcripts in liver, Brockmann bodies (BB), heart, small intestine, adipose tissue, white and red muscle, gill, spleen, pituitary gland, and brain. The highest level of expression was detected in tilapia heart, followed by BB, brain, and muscle. Protein based food and glucose had minor or no effects on the level of tGLUT-1 expression in most tissues. The tGLUT-1 mRNA level was significantly induced by glucose and food only in white muscle. Current results suggest that tGLUT-1 is similar to the GLUT-1 of other teleost species and mammals at the genomic, mRNA, and amino acid levels, supporting the concept that tGLUT-1 functions as a ubiquitous basal level glucose transporter.