Role of plasma membrane transporters in muscle metabolism

The effect of sport and physical activity on transport proteins: implications for cancer prevention and control

The present contribution briefly overviews the major biological functions of the plasma membrane and of the transport proteins (transporters), which enable the movement of different molecules and substrates (either charged or uncharged) by passive (facilitated diffusion) or active transport. In particular, transporters are overviewed at the level of the skeletal muscles, which represent a highly complex, heterogeneous, plastic and dynamic tissue and are one of the most abundant tissues in humans, accounting for up to 40% of their total weight and containing up to 50%-75% of all body proteins. Moreover, it is shown how sport and physical activity finely tune and modulate human proteome, especially in terms of structural and functional improvements concerning the density of the transport proteins. These changes are among the factors responsible for the positive outcomes of training, which involve mainly the cardiovascular and the endocrine/metabolic systems. Different kinds of training (strength and endurance) enable to achieve such improvements, even though there seems to exist a dose-relationship intensity-dependent effect, with responses after 6-8 weeks of exercise and disappearing in the chronic period (years of training). Finally, exercise-induced changes at the level of transporters can play a role in terms of cancer prevention and management. Regular physical activity and exercise can, indeed, counteract the side-effects of chemotherapy drugs, including doxorubicin and other anthracycline derivatives, which may impair the functions of cardiac and skeletal muscles, probably modulating the expression of multidrug resistance proteins.

SSO and other putative inhibitors of FA transport across membranes by CD36 disrupt intracellular metabolism, but do not affect FA translocation

Membrane-bound proteins have been proposed to mediate the transport of long-chain FA (LCFA) transport through the plasma membrane (PM). These proposals are based largely on reports that PM transport of LCFAs can be blocked by a number of enzymes and purported inhibitors of LCFA transport. Here, using the ratiometric pH indicator (2',7'-bis-(2-carboxyethyl)-5-(and-6-)-carboxyfluorescein and acrylodated intestinal FA-binding protein-based dual fluorescence assays, we investigated the effects of nine inhibitors of the putative FA transporter protein CD36 on the binding and transmembrane movement of LCFAs. We particularly focused on sulfosuccinimidyl oleate (SSO), reported to be a competitive inhibitor of CD36-mediated LCFA transport. Using these assays in adipocytes and inhibitor-treated protein-free lipid vesicles, we demonstrate that rapid LCFA transport across model and biological membranes remains unchanged in the presence of these purported inhibitors. We have previously shown in live cells that CD36 does not accelerate the transport of unesterified LCFAs across the PM. Our present experiments indicated disruption of LCFA metabolism inside the cell within minutes upon treatment with many of the "inhibitors" previously assumed to inhibit LCFA transport across the PM. Furthermore, using confocal microscopy and a specific anti-SSO antibody, we found that numerous intracellular and PM-bound proteins are SSO-modified in addition to CD36. Our results support the hypothesis that LCFAs diffuse rapidly across biological membranes and do not require an active protein transporter for their transmembrane movement.

Antidiabetic activities of entagenic acid in type 2 diabetic db/db mice and L6 myotubes via AMPK/GLUT4 pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Entada phaseoloides (L.) Merr., a traditional Chinese folk medicine, has been used in treating diabetes and other inflammatory disorders. Our previous study revealed that the triterpene saponins in E.Phaseoloides possessed an antidiabetic effect in type 2 diabetic rats by activating AMP-activated protein kinase (AMPK). Entagenic acid, the principal aglycon, isolated from the seed kernels of E. phaseoloides, has been proposed to possess a significant role in the antidiabetic effect, however, its actual effect and pertinent mechanisms are still unknown.

AIM OF THE STUDY

The aim of the present study was to investigate the antidiabetic effect of entagenic acid in a type 2 diabetic animal model (C57BIKsj db/db mice) and its role in the regulation of glucose uptake in L6 myotubes, and to explore the possible molecular mechanisms.

MATERIALS AND METHODS

In vivo, average weekly body weight, daily water, food intake and postprandial blood glucose levels, the intraperitoneal insulin tolerance test, glucose tolerance test, serum lipid profiles and pancreatic histopathological changes in db/db mice treated with entagenic acid orally at different doses (5, 10 and 20mg/kg) were assessed and compared with wild-type littermates or vehicle- and metformin-treated db/db mice. In vitro, effects of entagenic acid on the glucose consumption and the phosphorylation of protein kinase B (AKT) and AMPK in L6 myotubes were evaluated.

RESULTS

In vivo, entagenic acid significantly lowered postprandial blood glucose levels but not the body weight, normalized the serum lipid imbalance, improved the impaired glucose tolerance, insulin resistance, as well as the pathological changes in pancreatic islets. In vitro, entagenic acid dose-dependently promoted glucose utilization and enhanced the translocation and expression of glucose transporter 4 (GLUT4), and phosphorylation of AMPK but not AKT.

CONCLUSIONS

The present study demonstrated that entagenic acid can markedly maintain the glucose homeostasis, improve insulin resistance and ameliorate dyslipidemia. Its antihyperglycemic effect could be caused by promoting AMPK mediated cellular signaling and GLUT4 translocation in muscles.

Amino acid homeostasis and signalling in mammalian cells and organisms

Cells have a constant turnover of proteins that recycle most amino acids over time. Net loss is mainly due to amino acid oxidation. Homeostasis is achieved through exchange of essential amino acids with non-essential amino acids and the transfer of amino groups from oxidised amino acids to amino acid biosynthesis. This homeostatic condition is maintained through an active mTORC1 complex. Under amino acid depletion, mTORC1 is inactivated. This increases the breakdown of cellular proteins through autophagy and reduces protein biosynthesis. The general control non-derepressable 2/ATF4 pathway may be activated in addition, resulting in transcription of genes involved in amino acid transport and biosynthesis of non-essential amino acids. Metabolism is autoregulated to minimise oxidation of amino acids. Systemic amino acid levels are also tightly regulated. Food intake briefly increases plasma amino acid levels, which stimulates insulin release and mTOR-dependent protein synthesis in muscle. Excess amino acids are oxidised, resulting in increased urea production. Short-term fasting does not result in depletion of plasma amino acids due to reduced protein synthesis and the onset of autophagy. Owing to the fact that half of all amino acids are essential, reduction in protein synthesis and amino acid oxidation are the only two measures to reduce amino acid demand. Long-term malnutrition causes depletion of plasma amino acids. The CNS appears to generate a protein-specific response upon amino acid depletion, resulting in avoidance of an inadequate diet. High protein levels, in contrast, contribute together with other nutrients to a reduction in food intake.

Spatholobus suberectus Exhibits Antidiabetic Activity In Vitro and In Vivo through Activation of AKT-AMPK Pathway

Glucose deposition in peripheral tissue is an important parameter for the treatment of type 2 diabetes mellitus. The aim of this study was to investigate the effects of Spatholobus suberectus (Ss) on glucose disposal in skeletal muscle cells and additionally explore its in vivo antidiabetic potential. Treatment of ethanolic extract of S. suberectus (EeSs) significantly enhanced the glucose uptake, mediated through the enhanced expression of GLUT4 in skeletal muscle via the stimulation of AKT and AMPK pathways in C2C12 cells. Moreover, EeSs have potential inhibitory action on α-glucosidase activity and significantly lowered the postprandial blood glucose levels in STZ-induced diabetic mice, associated with increased expression of GLUT4 and AKT and/or AMPK-mediated signaling cascade in skeletal muscle. Furthermore, administration of EeSs significantly boosted up the antioxidant enzyme expression and also mitigated the gluconeogenesis enzyme such as PEPCK and G-6-Pase enzyme expression in liver tissue of STZ-induced diabetic mice model. Collectively, these findings suggest that EeSs have a high potentiality to mitigate diabetic symptoms through stimulating glucose uptake in peripheral tissue via the activation of AKT and AMPK signaling cascade and augmenting antioxidant potentiality as well as blocking the gluconeogenesis process in diabetic mice.

"Nutraceuticals" in relation to human skeletal muscle and exercise

Skeletal muscles have a fundamental role in locomotion and whole body metabolism, with muscle mass and quality being linked to improved health and even lifespan. Optimizing nutrition in combination with exercise is considered an established, effective ergogenic practice for athletic performance. Importantly, exercise and nutritional approaches also remain arguably the most effective countermeasure for muscle dysfunction associated with aging and numerous clinical conditions, e.g., cancer cachexia, COPD, and organ failure, via engendering favorable adaptations such as increased muscle mass and oxidative capacity. Therefore, it is important to consider the effects of established and novel effectors of muscle mass, function, and metabolism in relation to nutrition and exercise. To address this gap, in this review, we detail existing evidence surrounding the efficacy of a nonexhaustive list of macronutrient, micronutrient, and "nutraceutical" compounds alone and in combination with exercise in relation to skeletal muscle mass, metabolism (protein and fuel), and exercise performance (i.e., strength and endurance capacity). It has long been established that macronutrients have specific roles and impact upon protein metabolism and exercise performance, (i.e., protein positively influences muscle mass and protein metabolism), whereas carbohydrate and fat intakes can influence fuel metabolism and exercise performance. Regarding novel nutraceuticals, we show that the following ones in particular may have effects in relation to 1) muscle mass/protein metabolism: leucine, hydroxyl β-methylbutyrate, creatine, vitamin-D, ursolic acid, and phosphatidic acid; and 2) exercise performance: (i.e., strength or endurance capacity): hydroxyl β-methylbutyrate, carnitine, creatine, nitrates, and β-alanine.

ST2 from rainbow trout quenches TLR signalling, localises at the nuclear membrane and allows the nuclear translocation of MYD88

The mammalian interleukin 1 receptor-like 1 receptor (IL1RL1), commonly known as ST2, is thought to downregulate TLR signalling by sequestering the signalling adapter MYD88 (myeloid differentiation primary response protein 88). ST2 sequences are known in several fish species, but none of them have functionally been examined. We characterised ST2 from rainbow trout (Oncorhynchus mykiss) and the structure of its encoding gene. The primary sequence of ST2 is only weakly conserved from fish to human. However, the amino acid sequences forming the interfaces for ST2 and MYD88 interaction are well conserved throughout evolution. High similarity of the gene segmentation unambiguously proves the common ancestry of fish and mammalian ST2. Trout ST2 and trout MYD88 genes were constitutively expressed in embryonic, larval and adult trout. In vivo infection with Aeromonas salmonicida did not modulate the mRNA levels of both factors. Overexpressing trout ST2 in the mammalian HEK-293 reconstitution system of TLR2 signalling quenched the Escherichia coli-induced activation of NF-κB and SAA promoters in a dose-dependent fashion. The expression of GFP-tagged trout ST2 in human HEK-293 or trout CHSE-214 cells surprisingly revealed that (i) ST2 localised abundantly at the nuclear membrane rather than at the cell membrane and (ii) the coexpression of both ST2 and MYD88 allowed the translocation of trout MYD88 from cytoplasm to nucleus, as assessed using confocal microscopy and Western blotting. Hence, we validated that trout ST2 is a dampener of TLR signalling and interacts with MYD88. The spatial distribution of these factors raises questions about how this repressive mechanism functions.

Targeting Cancer Cells using 3-bromopyruvate for Selective Cancer Treatment

Cancer treatment deserves more research efforts despite intensive conventional treatment modalities for many types of malignancies. Metastasis and resistance to chemotherapy and radiotherapy receive a lot of global research efforts. The current advances in cancer biology may improve targeting the critical metabolic differences that distinguish cancer cells from normal cells. Cancer cells are highly glycolytic for energy production, exhibit the Warburg effect, establish aggressive acidic microenvironment, maintain cancer stem cells, exhibit resistance to chemotherapy, have low antioxidant systems but different ΔΨm (delta psi, mitochondrial transmembrane potential), express P-glycoprotein for multidrug resistance, upregulate glucose transporters and monocarboxylate transporters and are under high steady-state reactive oxygen species conditions. Normal cells differ in all these aspects. Lactate produced through the Warburg effect helps cancer metastasis. Targeting glycolysis reactions for energy production in cancer cells seems promising in decreasing the proliferation and metastasis of cancer cells. 3-bromopyruvate makes use of cancer biology in treating cancer cells, cancer stem cells and preventing metastasis in human cancer as discussed in this review. Updated advances are analyzed here, which include research analysis of background, experience, readings in the field of cancer biology, oncology and biochemistry.

In vivo tissue sampling using solid-phase microextraction for non-lethal exposome-wide association study of CYP1A1 induction in Catostomus commersonii

Fish are widely used for monitoring aquatic ecosystem health and water contamination by organic toxicants from natural and anthropogenic sources. However, most of these studies only focused on the measurement of specific toxicants and did not examine the impact of chemical mixtures. In this study, we examined whether the tissue exposome captured in vivo with solid-phase microextraction (SPME) without lethal sampling and analyzed by liquid chromatography-high resolution mass spectrometry can detect differences between Catostomus commersonii exhibiting a significant induction of CYP1A1, through case/control comparisons, controlling for false discovery rates. We observed the presence of environmental toxicants in induced case fish known as potential inducers of CYP1A1. We also found significant changes in the levels of anti-oxidants, short-lived oxysterols and other lipids associated with CYP1A1 induction, possibly due to oxidative stress, lipid peroxidation and free fatty acids mobilization to maintain homeostatic state. In vivo SPME opens the way to perform repeated sampling on the same animal over the time and explore the individual internal exposome trajectory for better characterization of the links between toxicant load and health effects, at the individual scale.

Glucose uptake stimulatory potential and antidiabetic activity of the Arnebin-1 from Arnabia nobelis

The enhanced disposal of glucose by the peripheral tissue is an important mechanism to regulate hyperglycemia. Here, we investigated the effect of Arnebin-1 from Arnebia nobilis, on glucose disposal in skeletal muscle cells and explored its in vivo antihyperglycemic potential. In L6 myotubes, Arnebin-1 stimulated glucose uptake, mediated through the enhanced translocation of the glucose transporter-4 (GLUT4) to plasma membrane, without changing the amount of GLUT4 or GLUT1. These effects of Arnebin-1 were synergistic with that of insulin. The effect of Arnebin-1 on glucose uptake was abolished in presence of wortmannin, and Arnebin-1 significantly stimulated the phosphorylation of Akt and downstream marker GSK-3β. Moreover, treatment with Arnebin-1 lowered postprandial blood glucose levels in streptozotocin-induced diabetic rats, and improved glucose tolerance and suppressed the rises in the fasting blood glucose, serum insulin, triglycerides, and total cholesterol in db/db mice, associated with enhanced expression of the major marker of the PI-3-Kinase-mediated signaling cascade in skeletal muscle. These findings suggest that Arnebin-1 exert antihyperglycemic activity through stimulating glucose disposal in peripheral tissues via PI-3-Kinase-dependent pathway.

Na,K-ATPase regulation in skeletal muscle

Skeletal muscle contains one of the largest and the most dynamic pools of Na,K-ATPase (NKA) in the body. Under resting conditions, NKA in skeletal muscle operates at only a fraction of maximal pumping capacity, but it can be markedly activated when demands for ion transport increase, such as during exercise or following food intake. Given the size, capacity, and dynamic range of the NKA pool in skeletal muscle, its tight regulation is essential to maintain whole body homeostasis as well as muscle function. To reconcile functional needs of systemic homeostasis with those of skeletal muscle, NKA is regulated in a coordinated manner by extrinsic stimuli, such as hormones and nerve-derived factors, as well as by local stimuli arising in skeletal muscle fibers, such as contractions and muscle energy status. These stimuli regulate NKA acutely by controlling its enzymatic activity and/or its distribution between the plasma membrane and the intracellular storage compartment. They also regulate NKA chronically by controlling NKA gene expression, thus determining total NKA content in skeletal muscle and its maximal pumping capacity. This review focuses on molecular mechanisms that underlie regulation of NKA in skeletal muscle by major extrinsic and local stimuli. Special emphasis is given to stimuli and mechanisms linking regulation of NKA and energy metabolism in skeletal muscle, such as insulin and the energy-sensing AMP-activated protein kinase. Finally, the recently uncovered roles for glutathionylation, nitric oxide, and extracellular K(+) in the regulation of NKA in skeletal muscle are highlighted.

Deoxyandrographolide promotes glucose uptake through glucose transporter-4 translocation to plasma membrane in L6 myotubes and exerts antihyperglycemic effect in vivo

Skeletal muscle is the principal site for postprandial glucose utilization and augmenting the rate of glucose utilization in this tissue may help to control hyperglycemia associated with diabetes mellitus. Here, we explored the effect of Deoxyandrographolide (DeoAn) isolated from the Andrographis paniculata Nees on glucose utilization in skeletal muscle and investigated its antihyperglycemic effect in vivo in streptozotocin-induced diabetic rats and genetically diabetic db/db mice. In L6 myotubes, DeoAn dose-dependently stimulated glucose uptake by enhancing the translocation of glucose transporter 4 (GLUT4) to cell surface, without affecting the total cellular GLUT4 and GLUT1 content. These effects of DeoAn were additive to insulin. Further analysis revealed that DeoAn activated PI-3-K- and AMPK-dependent signaling pathways, account for the augmented glucose transport in L6 myotubes. Furthermore, DeoAn lowered postprandial blood glucose levels in streptozotocin-induced diabetic rats and also suppressed the rises in the fasting blood glucose, serum insulin, triglycerides and LDL-Cholesterol levels of db/db mice. These findings suggest the therapeutic efficacy of the DeoAn for type 2 diabetes mellitus and can be potential phytochemical for its management.

Fructose-induced ROS generation impairs glucose utilization in L6 skeletal muscle cells

High fructose consumption has implicated in insulin resistance and metabolic syndrome. Fructose is a highly lipogenic sugar that has intense metabolic effects in liver. Recent evidences suggest that fructose exposure to other tissues has substantial and profound metabolic consequences predisposing toward chronic conditions such as type 2 diabetes. Since skeletal muscle is the major site for glucose utilization, in the present study we define the effects of fructose exposure on glucose utilization in skeletal muscle cells. Upon fructose exposure, the L6 skeletal muscle cells displayed diminished glucose uptake, glucose transporter type 4 (GLUT4) translocation, and impaired insulin signaling. The exposure to fructose elevated reactive oxygen species (ROS) production in L6 myotubes, accompanied by activation of the stress/inflammation markers c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase 1/2 (ERK1/2), and degradation of inhibitor of NF-κB (IκBα). We found that fructose caused impairment of glucose utilization and insulin signaling through ROS-mediated activation of JNK and ERK1/2 pathways, which was prevented in the presence of antioxidants. In conclusion, our data demonstrate that exposure to fructose induces cell-autonomous oxidative response through ROS production leading to impaired insulin signaling and attenuated glucose utilization in skeletal muscle cells.

Suppression of phosphatase and tensin homolog protects insulin-resistant cells from apoptosis

In the present study, a glucosamine-induced model of insulin-resistant skeletal muscle cells was established in order to investigate the effect of inhibition of phosphatase and tensin homolog (PTEN)/5'-adenosine monophosphate-activated protein kinase (AMPK) on these cells. The glucosamine-induced insulin-resistant skeletal muscle cells were produced and the rate of glucose uptake was measured using the glucose oxidase-peroxidase method. The expression levels of PTEN and phosphorylated PTEN (p-PTEN) were assessed using western blotting. Glucose transporter 4 (GLUT4) translocation was detected by immunofluorescence. Cell apoptosis was evaluated using flow cytometry. Following insulin stimulation, the rate of glucose uptake was significantly reduced in the cells with glucosamine-induced insulin-resistance in comparison with those in the control group. The expression and translocation of GLUT4 were reduced in the insulin-resistant muscle cells. By contrast, the expression of PTEN and p-PTEN as well as apoptosis were significantly increased. Following treatment with bisperoxopicolinatooxovanadate (BPV) or metformin in the insulin-resistant skeletal muscle cells, there was an increase in the rate of glucose uptake, an increase in GLUT4 expression and its translocation, a reduction in the expression of PTEN and p-PTEN, and a decrease in cell apoptosis compared with untreated insulin-resistant cells. Glucosamine may be used to produce an effective model of insulin-resistant skeletal muscle cells. Cells with glucosamine-induced insulin-resistance exhibited a reduced expression of GLUT4 and dysfunction in GLUT4 translocation, as well as increased activation of PTEN and increased cell apoptosis. Inhibition of PTEN or its upstream regulator, AMPK, protects glucosamine-induced insulin-resistant skeletal muscle cells from apoptosis.

Age-related obesity is a heritage of the evolutionary past

In the process of human aging, an increase in the total amount of fat is observed mainly due to accumulation of lipids in non-adipose tissues. Insulin resistance, provoked by the intracellular accumulation of triglycerides, is often associated with development of such age-related diseases as atherosclerosis, type 2 diabetes, cancer, osteoporosis, and also with systemic inflammation and lipo- and glucose toxicity. Accumulation of lipids and lipophilic compounds is a biological phenomenon common for both prokaryotes and eukaryotes. Initially, it arose as an adaptation to starvation and shortage of nitrogen-containing nutrients, but later it converted into a depot of membrane material, needed on recommencement of cell division. In rodents and humans, the accumulation of non-metabolized fat in non-adipose tissues can be regarded as an adaptation to changes in the internal medium on a certain stage of ontogenesis as a result of age-related dysfunction of adipose tissue.

Different protein expression patterns associated with polycystic ovary syndrome in human follicular fluid during controlled ovarian hyperstimulation

Polycystic ovary syndrome (PCOS) is one of the most common causes of anovulatory infertility, affecting 5-10% of females during their reproductive life. Currently the pathology of PCOS is largely unknown. To identify the differential protein expression in follicular fluids from PCOS and normal subjects during controlled ovarian hyperstimulation, we performed an initial proteomic study including two-dimensional gel electrophoresis (2DE) analysis and mass spectroscopy, and confirmed results by western blot. Thirty-two protein spots were shown to be significantly differentially expressed between PCOS and normal follicular fluids, of which 20 unique proteins were identified to be associated with cellular metabolism and physiological processes; 13 of these proteins were upregulated while seven were downregulated in PCOS follicular fluids. Western blotting analyses confirmed the differential expressions for three randomly selected proteins, i.e. upregulated α1-antitrypsin, apolipoprotein A-I and transferrin in follicular fluid from PCOS patients than normal controls. Furthermore, semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) analyses revealed that mRNA levels of serine palmitoyltransferase 2, serine/threonine-protein kinase male germ cell-associated kinase (MAK) and DNA damage-regulated autophagy modulator protein 2 decreased significantly in granulosa cells of PCOS patients compared with normal samples. These results increase our understanding of PCOS and the identified genes may serve as candidate biomarkers to develop diagnostic and therapeutic tools.

D-Amino acid oxidase-induced oxidative stress, 3-bromopyruvate and citrate inhibit angiogenesis, exhibiting potent anticancer effects

Angiogenesis is critical for cancer growth and metastasis. Steps of angiogenesis are energy consuming, while vascular endothelial cells are highly glycolytic. Glioblastoma multiforme (GBM) is a highly vascular tumor and this enhances its aggressiveness. D-amino acid oxidase (DAO) is a promising therapeutic protein that induces oxidative stress upon acting on its substrates. Oxidative stress-energy depletion (OSED) therapy was recently reported (El Sayed et al., Cancer Gene Ther, 19, 1-18, 2012). OSED combines DAO-induced oxidative stress with energy depletion caused by glycolytic inhibitors such as 3-bromopyruvate (3BP), a hexokinase II inhibitor that depleted ATP in cancer cells and induced production of hydrogen peroxide. 3BP disturbs the Warburg effect and antagonizes effects of lactate and pyruvate (El Sayed et al., J Bioenerg Biomembr, 44, 61-79, 2012). Citrate is a natural organic acid capable of inhibiting glycolysis by targeting phosphofructokinase. Here, we report that DAO, 3BP and citrate significantly inhibited angiogenesis, decreased the number of vascular branching points and shortened the length of vascular tubules. OSED delayed the growth of C6/DAO glioma cells. 3BP combined with citrate delayed the growth of C6 glioma cells and decreased significantly the number and size of C6 glioma colonies in soft agar. Human GBM cells (U373MG) were resistant to chemotherapy e.g. cisplatin and cytosine arabinoside, while 3BP was effective in decreasing the viability and disturbing the morphology of U373MG cells.

Metabolic Effects of Insulin and IGFs on Gilthead Sea Bream (Sparus aurata) Muscle Cells

Primary cultures of gilthead sea bream myocytes were performed in order to examine the relative metabolic function of insulin compared with IGF-I and IGF-II (insulin-like growth factors, IGFs) at different stages in the cell culture. In these cells, the in vitro effects of insulin and IGFs on 2-deoxyglucose (2-DG) and l-alanine uptake were studied in both myocytes (day 4) and small myotubes (day 9). 2-DG uptake in gilthead sea bream muscle cells was increased in the presence of insulin and IGFs in a time dependent manner and along with muscle cell differentiation. On the contrary, l-alanine uptake was also stimulated by insulin and IGFs but showed an inverse pattern, being the uptake higher in small myocytes than in large myotubes. The results of preincubation with inhibitors (PD-98059, wortmannin, and cytochalasin B) on 2-DG uptake indicated that insulin and IGFs stimulate glucose uptake through the same mechanisms, and evidenced that mitogenesis activator protein kinase (MAPK) and PI3K-Akt transduction pathways mediate the metabolic function of these peptides. In the same way, we observed that GLUT4 protein synthesis was stimulated in the presence of insulin and IGFs in gilthead sea bream muscle cells in a different manner at days 4 or 9 of the culture. In summary we describe here, for the first time, the effects of insulin and IGFs on 2-DG and l-alanine uptake in primary culture of gilthead sea bream muscle cells. We show that both MAPK and PI3K-Akt transduction pathways are needed in order to control insulin and IGFs actions in these cells. Moreover, changes in glucose uptake can be explained by the action of the GLUT4 transporter, which is stimulated in the presence of insulin and IGFs throughout the cell culture.

Contractile activity per se induces transcriptional activation of SLC2A4 gene in soleus muscle: involvement of MEF2D, HIF-1a, and TRalpha transcriptional factors

Skeletal muscle is a target tissue for approaches that can improve insulin sensitivity in insulin-resistant states. In muscles, glucose uptake is performed by the GLUT-4 protein, which is encoded by the SLC2A4 gene. SLC2A4 gene expression increases in response to conditions that improve insulin sensitivity, including chronic exercise. However, since chronic exercise improves insulin sensitivity, the increased SLC2A4 gene expression could not be clearly attributed to the muscle contractile activity per se and/or to the improved insulin sensitivity. The present study was designed to investigate the role of contractile activity per se in the regulation of SLC2A4 gene expression as well as in the participation of the transcriptional factors myocyte enhancer factor 2D (MEF2D), hypoxia inducible factor 1a (HIF-1a), and thyroid hormone receptor-alpha (TRalpha). The performed in vitro protocol excluded the interference of metabolic, hormonal, and neural effects. The results showed that, in response to 10 min of electrically induced contraction of soleus muscle, an early 40% increase in GLUT-4 mRNA (30 min) occurred, with a subsequent 65% increase (120 min) in GLUT-4 protein content. EMSA and supershift assays revealed that the stimulus rapidly increased the binding activity of MEF2D, HIF-1a, and TRalpha into the SLC2A4 gene promoter. Furthermore, chromatin immunoprecipitation assay confirmed, in native nucleosome, that contraction induced an approximate fourfold (P < 0.01) increase in MEF2D and HIF-1a-binding activity. In conclusion, muscle contraction per se enhances SLC2A4 gene expression and that involves MEF2D, HIF-1a, and TRalpha transcription factor activation. This finding reinforces the importance of physical activity to improve glycemic homeostasis independently of other additional insulin sensitizer approaches.

Role of NADH/NAD+ transport activity and glycogen store on skeletal muscle energy metabolism during exercise: in silico studies

Skeletal muscle can maintain ATP concentration constant during the transition from rest to exercise, whereas metabolic reaction rates may increase substantially. Among the key regulatory factors of skeletal muscle energy metabolism during exercise, the dynamics of cytosolic and mitochondrial NADH and NAD+ have not been characterized. To quantify these regulatory factors, we have developed a physiologically based computational model of skeletal muscle energy metabolism. This model integrates transport and reaction fluxes in distinct capillary, cytosolic, and mitochondrial domains and investigates the roles of mitochondrial NADH/NAD+ transport (shuttling) activity and muscle glycogen concentration (stores) during moderate intensity exercise (60% maximal O2 consumption). The underlying hypothesis is that the cytosolic redox state (NADH/NAD+) is much more sensitive to a metabolic disturbance in contracting skeletal muscle than the mitochondrial redox state. This hypothesis was tested by simulating the dynamic metabolic responses of skeletal muscle to exercise while altering the transport rate of reducing equivalents (NADH and NAD+) between cytosol and mitochondria and muscle glycogen stores. Simulations with optimal parameter estimates showed good agreement with the available experimental data from muscle biopsies in human subjects. Compared with these simulations, a 20% increase (or approximately 20% decrease) in mitochondrial NADH/NAD+ shuttling activity led to an approximately 70% decrease (or approximately 3-fold increase) in cytosolic redox state and an approximately 35% decrease (or approximately 25% increase) in muscle lactate level. Doubling (or halving) muscle glycogen concentration resulted in an approximately 50% increase (or approximately 35% decrease) in cytosolic redox state and an approximately 30% increase (or approximately 25% decrease) in muscle lactate concentration. In both cases, changes in mitochondrial redox state were minimal. In conclusion, the model simulations of exercise response are consistent with the hypothesis that mitochondrial NADH/NAD+ shuttling activity and muscle glycogen stores affect primarily the cytosolic redox state. Furthermore, muscle lactate production is regulated primarily by the cytosolic redox state.

Size dependence of protein diffusion very close to membrane surfaces: measurement by total internal reflection with fluorescence correlation spectroscopy

The diffusion coefficients of nine fluorescently labeled antibodies, antibody fragments, and antibody complexes have been measured in solution very close to supported planar membranes by using total internal reflection with fluorescence correlation spectroscopy (TIR-FCS). The hydrodynamic radii (3-24 nm) of the nine antibody types were determined by comparing literature values with bulk diffusion coefficients measured by spot FCS. The diffusion coefficients very near membranes decreased significantly with molecular size, and the size dependence was greater than that predicted to occur in bulk solution. The observation that membrane surfaces slow the local diffusion coefficient of proteins in a size-dependent manner suggests that the primary effect is hydrodynamic as predicted for simple spheres diffusing close to planar walls. The TIR-FCS data are consistent with predictions derived from hydrodynamic theory. This work illustrates one factor that could contribute to previously observed nonideal ligand-receptor kinetics at model and natural cell membranes.

Distinct sensor pathways in the hierarchical control of SNAT2, a putative amino acid transceptor, by amino acid availability

Mammalian nutrient sensors are novel targets for therapeutic intervention in disease states such as insulin resistance and muscle wasting; however, the proteins responsible for this important task are largely uncharacterized. To address this issue we have dissected an amino acid (AA) sensor/effector regulon that controls the expression of the System A amino acid transporter SNAT2 in mammalian cells, a paradigm nutrient-responsive process, and found evidence for the convergence of at least two sensor/effector pathways. During AA withdrawal, JNK is activated and induces the expression of SNAT2 in L6 myotubes by stimulating an intronic nutrient-sensitive domain. A sensor for large neutral AA (e.g. Tyr, Gln) inhibits JNK activation and SNAT2 up-regulation. Additionally, shRNA and transporter chimeras demonstrate that SNAT2 provides a repressive signal for gene transcription during AA sufficiency, thus echoing AA sensing by transceptor (transporter-receptor) orthologues in yeast (Gap1/Ssy1) and Drosophila (PATH). Furthermore, the SNAT2 protein is stabilized during AA withdrawal.

Prenatal and postnatal pathways to obesity: different underlying mechanisms, different metabolic outcomes

Obesity and type 2 diabetes are worldwide health issues. The present paper investigates prenatal and postnatal pathways to obesity, identifying different metabolic outcomes with different effects on insulin sensitivity and different underlying mechanisms involving key components of insulin receptor signaling pathways. Pregnant Wistar rats either were fed chow ad libitum or were undernourished throughout pregnancy, generating either control or intrauterine growth restricted (IUGR) offspring. Male offspring were fed either standard chow or a high-fat diet from weaning. At 260 d of age, whole-body insulin sensitivity was assessed by hyperinsulinemic-euglycemic clamp, and other metabolic parameters were measured. As expected, high-fat feeding caused diet-induced obesity (DIO) and insulin resistance. Importantly, the insulin sensitivity of IUGR offspring was similar to that of control offspring, despite fasting insulin hypersecretion and increased adiposity, irrespective of postnatal nutrition. Real-time PCR and Western blot analyses of key markers of insulin sensitivity and metabolic regulation showed that IUGR offspring had increased hepatic levels of atypical protein kinase C zeta (PKC zeta) and increased expression of fatty acid synthase mRNA. In contrast, DIO led to decreased expression of fatty acid synthase mRNA and hepatic steatosis. The decrease in hepatic PKC zeta with DIO may explain, at least in part, the insulin resistance. Our data suggest that the mechanisms of obesity induced by prenatal events are fundamentally different from those of obesity induced by postnatal high-fat nutrition. The origin of insulin hypersecretion in IUGR offspring may be independent of the mechanistic events that trigger the insulin resistance commonly observed in DIO.

Signalling components involved in contraction-inducible substrate uptake into cardiac myocytes

Glucose and long-chain fatty acids (LCFA) are two major substrates used by heart and skeletal muscle to support contractile activity. In quiescent cardiac myocytes a substantial portion of the glucose transporter GLUT4 and the putative LCFA transporter fatty acid translocase (FAT)/CD36 are stored in intracellular compartments. Induction of cellular contraction by electrical stimulation results in enhanced uptake of both glucose and LCFA through translocation of GLUT4 and FAT/CD36 respectively to the sarcolemma. The involvement of protein kinase A, AMP-activated protein kinase (AMPK), protein kinase C (PKC) isoforms and the extracellular signal-regulated kinases was evaluated in cardiac myocytes as candidate signalling enzymes involved in recruiting these transporters in response to contraction. The collected evidence excluded the involvement of PKA and implicated an important role for AMPK and for one (or more) PKC isoform(s) in contraction-induced translocation of both GLUT4 and FAT/CD36. The unravelling of further components along this contraction pathway can provide valuable information on the coordinated regulation of the uptake of glucose and of LCFA by an increase in mechanical activity of heart and skeletal muscle.

New insights concerning the role of carnitine in the regulation of fuel metabolism in skeletal muscle

In skeletal muscle, carnitine plays an essential role in the translocation of long-chain fatty-acids into the mitochondrial matrix for subsequent beta-oxidation, and in the regulation of the mitochondrial acetyl-CoA/CoASH ratio. Interest in these vital metabolic roles of carnitine in skeletal muscle appears to have waned over the past 25 years. However, recent research has shed new light on the importance of carnitine as a regulator of muscle fuel selection. It has been established that muscle free carnitine availability may be limiting to fat oxidation during high intensity submaximal exercise. Furthermore, increasing muscle total carnitine content in resting healthy humans (via insulin-mediated stimulation of muscle carnitine transport) reduces muscle glycolysis, increases glycogen storage and is accompanied by an apparent increase in fat oxidation. By increasing muscle pyruvate dehydrogenase complex (PDC) activity and acetylcarnitine content at rest, it has also been established that PDC flux and acetyl group availability limits aerobic ATP re-synthesis at the onset of exercise (the acetyl group deficit). Thus, carnitine plays a vital role in the regulation of muscle fuel metabolism. The demonstration that its availability can be readily manipulated in humans, and impacts on physiological function, will result in renewed business and scientific interest in this compound.

Melatonin protects against oxidative damage and restores expression of GLUT4 gene in the hyperthyroid rat heart

To understand the mechanism of cardiovascular dysfunction in the hyperthyroid condition, the role of oxidative stress was examined in rats treated with 3,5,3'-triiodo-l-thyronine (T3). Treatment of rats daily with T3 (8 microg/100 g BW) for 15 days resulted in an increase in heart weight to body weight ratio, which was ameliorated by antioxidants, melatonin (2 mg/100 g BW) or vitamin E (4 mg/100 g BW). Both melatonin and vitamin E also inhibited rises of lipid peroxidation and hydroxyl radical generation and prevented the inhibition of Cu,Zn-superoxide dismutase in the hypertrophic heart. The expression of the glucose transporter, GLUT4, was reduced in response to T3, which was completely restored by melatonin and partially by vitamin E. However, neither antioxidant prevented down regulation of peroxisome proliferator-activated receptor-alpha in the hyperthyroid heart. Furthermore, the reduced level of myocyte enhancer factor-2, a regulator of GLUT4 transcription was restored completely by melatonin and partially by vitamin E treatment. Glucose uptake in hypertrophic left ventricular cells was also restored by these antioxidants. The expression of B-type natriuretic peptide, a marker of heart failure, was significantly increased by T3 and ameliorated by melatonin or vitamin E treatments. In general, the beneficial effects of melatonin given as a co-treatment with T3 were better than those induced by vitamin E. These data show that melatonin ameliorates hypertrophic growth of the myocardium induced by hyperthyroidism and provide an insight into the mechanism of reactive oxygen species-mediated down regulation of metabolically important genes such as GLUT4 in the heart.

Electrolysis stimulates creatine transport and transporter cell surface expression in incubated mouse skeletal muscle: potential role of ROS

Electrical field stimulation of isolated, incubated rodent skeletal muscles is a frequently used model to study the effects of contractions on muscle metabolism. In this study, this model was used to investigate the effects of electrically stimulated contractions on creatine transport. Soleus and extensor digitorum longus muscles of male NMRI mice (35-50 g) were incubated in an oxygenated Krebs buffer between platinum electrodes. Muscles were exposed to [(14)C]creatine for 30 min after either 12 min of repeated tetanic isometric contractions (contractions) or electrical stimulation of only the buffer before incubation of the muscle (electrolysis). Electrolysis was also investigated in the presence of the reactive oxygen species (ROS) scavenging enzymes superoxide dismutase (SOD) and catalase. Both contractions and (to a lesser degree) electrolysis stimulated creatine transport severalfold over basal. The amount of electrolysis, but not contractile activity, induced (determined) creatine transport stimulation. Incubation with SOD and catalase at 100 and 200 U/ml decreased electrolysis-induced creatine transport by approximately 50 and approximately 100%, respectively. The electrolysis effects on creatine uptake were completely inhibited by beta-guanidino propionic acid, a competitive inhibitor of (creatine for) the creatine transporter (CRT), and were accompanied by increased cell surface expression of CRT. Muscle glucose transport was not affected by electrolysis. The present results indicate that electrical field stimulation of incubated mouse muscles, independently of contractions per se, stimulates creatine transport by a mechanism that depends on electrolysis-induced formation of ROS in the incubation buffer. The increased creatine uptake is paralleled by an increased cell surface expression of the creatine transporter.

Regulation of renal amino acid transporters during metabolic acidosis

The kidney plays a major role in acid-base homeostasis by adapting the excretion of acid equivalents to dietary intake and metabolism. Urinary acid excretion is mediated by the secretion of protons and titratable acids, particularly ammonia. NH(3) is synthesized in proximal tubule cells from glutamine taken up via specific amino acid transporters. We tested whether kidney amino acid transporters are regulated in mice in which metabolic acidosis was induced with NH(4)Cl. Blood gas and urine analysis confirmed metabolic acidosis. Real-time RT-PCR was performed to quantify the mRNAs of 16 amino acid transporters. The mRNA of phosphoenolpyruvate carboxykinase (PEPCK) was quantified as positive control for the regulation and that of GAPDH, as internal standard. In acidosis, the mRNA of kidney system N amino acid transporter SNAT3 (SLC38A3/SN1) showed a strong induction similar to that of PEPCK, whereas all other tested mRNAs encoding glutamine or glutamate transporters were unchanged or reduced in abundance. At the protein level, Western blotting and immunohistochemistry demonstrated an increased abundance of SNAT3 and reduced expression of the basolateral cationic amino acid/neutral amino acid exchanger subunit y(+)-LAT1 (SLC7A7). SNAT3 was localized to the basolateral membrane of the late proximal tubule S3 segment in control animals, whereas its expression was extended to the earlier S2 segment of the proximal tubule during acidosis. Our results suggest that the selective regulation of SNAT3 and y(+)LAT1 expression may serve a major role in the renal adaptation to acid secretion and thus for systemic acid-base balance.

Creatine as a compatible osmolyte in muscle cells exposed to hypertonic stress

Exposure of C2C12 muscle cells to hypertonic stress induced an increase in cell content of creatine transporter mRNA and of creatine transport activity, which peaked after about 24 h incubation at 0.45 osmol (kg H(2)O)(-1). This induction of transport activity was prevented by addition of either cycloheximide, to inhibit protein synthesis, or of actinomycin D, to inhibit RNA synthesis. Creatine uptake by these cells is largely Na(+) dependent and kinetic analysis revealed that its increase under hypertonic conditions resulted from an increase in V(max) of the Na(+)-dependent component, with no significant change in the K(m) value of about 75 mumol l(-1). Quantitative real-time PCR revealed a more than threefold increase in the expression of creatine transporter mRNA in cells exposed to hypertonicity. Creatine supplementation significantly enhanced survival of C2C12 cells incubated under hypertonic conditions and its effect was similar to that obtained with the well known compatible osmolytes, betaine, taurine and myo-inositol. This effect seemed not to be linked to the energy status of the C2C12 cells because hypertonic incubation caused a decrease in their ATP content, with or without the addition of creatine at 20 mmol l(-1) to the medium. This induction of creatine transport activity by hypertonicity is not confined to muscle cells: a similar induction was shown in porcine endothelial cells.

Insulin stimulates L-carnitine accumulation in human skeletal muscle

Increasing skeletal muscle carnitine content may alleviate the decline in muscle fat oxidation seen during intense exercise. Studies to date, however, have failed to increase muscle carnitine content, in healthy humans, by dietary or intravenous L-carnitine administration. We hypothesized that insulin could augment Na+-dependent skeletal muscle carnitine transport. On two randomized visits, eight healthy men underwent 5 h of intravenous L-carnitine infusion with serum insulin maintained at fasting (7.4+/-0.4 mIU*l(-1)) or physiologically high (149.2+/-6.9 mIU*l(-1)) concentrations. The combination of hypercarnitinemia (approximately 500 micromol*l(-1)) and hyperinsulinemia increased muscle total carnitine (TC) content from 22.0 +/- 0.9 to 24.7 +/- 1.4 mmol*(kg dm)(-1) (P<0.05) and was associated with a 2.3 +/- 0.3-fold increase in carnitine transporter protein (OCTN2) mRNA expression (P<0.05). Hypercarnitinemia in the presence of a fasting insulin concentration had no effect on either of these parameters. This study demonstrates that insulin can acutely increase muscle TC content in humans during hypercarnitinemia, which is associated with an increase in OCTN2 transcription. These novel findings may be of importance to the regulation of muscle fat oxidation during exercise, particularly in obesity and type 2 diabetes where it is known to be impaired.

Type I diabetes affects skeletal muscle glutamine uptake in a fiber-specific manner

Skeletal muscle serves as the body's major glutamine repository, and releases glutamine at enhanced rates during diabetes, but whether all muscles are equally affected is unknown. System N(m) activity mediates most trans-sarcolemmal glutamine movement, and although two System N (SN) isoforms have been identified (SN1/sodium-coupled neutral amino acid transporter or System N and A transporters [SNAT]-3; and SN2/SNAT5), their expression in skeletal muscle remains controversial. Here, the impact of Type I diabetes on glutamine uptake and System N transporter expression were examined in fast- and slow-twitch skeletal muscle from spontaneously diabetic (BB/Wor-DP) rats. Net glutamine uptake in fast-twitch fibers was decreased 75%-95%, but enhanced more than 2-fold in slow-twitch muscle from diabetic animals relative to nondiabetic controls. Both SNAT3 and SNAT5 mRNA were expressed in both muscle fiber types and their abundance was unaffected by diabetes. This represents the first report of differential fiber-specific effects of diabetes on skeletal muscle glutamine transport and the co-expression of distinct System N transporters in skeletal muscle.

NF-kappaB, MEF2A, MEF2D and HIF1-a involvement on insulin- and contraction-induced regulation of GLUT4 gene expression in soleus muscle

The GLUT4 gene transcriptional activity has a profound impact on the insulin-mediated glucose disposal and it is, therefore, important to understand the mechanisms underlying it. Insulin and exercise modulate GLUT4 expression in vivo, but the net control and involved mechanisms of each one have not been established yet. This paper sought to discriminate, in soleus muscle, the effects of insulin and muscle contraction on GLUT4 gene expression, and the involvement of transcriptional factors: myocite enhancer factor 2 (MEF2 A/C/D), hypoxia inducible factor 1-a (HIF1-a) and nuclear factor-kappa B (NF-kappaB). The GLUT4 mRNA was reduced by fasting (40%), and increased by in vitro incubation with insulin (25%) or insulin plus glucose (40%), which was accompanied by opposite regulations of NF-kappaB mRNA. Differently, in vitro, muscle contraction led to a rapid increase (35-80%) in GLUT4, MEF2A, MEF2D and HIF1-a mRNAs. Additionally, electrophoretic mobility shift assay confirmed changes in the binding activity of nuclear proteins to consensus NF-kappaB, GLUT4-Ebox and GLUT4-AT-rich element probes, parallel to the mRNA changes of their respective transcriptional factors NF-kappaB, HIF1-a and MEF2s. Concluding, insulin- and contraction-induced regulation of GLUT4 expression involves distinct transcriptional factors.

Maternal food restriction enhances insulin-induced GLUT-4 translocation and insulin signaling pathway in skeletal muscle from suckling rats

Restriction of protein calories during stages of immaturity has a major influence on glucose metabolism and increases the risk of type 2 diabetes in adulthood. However, it is known that reduction of food intake alleviates insulin resistance. We previously demonstrated an improved insulin-induced glucose uptake in skeletal muscle of chronically undernourished adult rats. The purpose of this work was to investigate whether this condition is present during suckling, a period characterized by physiological insulin resistance as well as elucidate some of the underlying mechanisms. With this aim, 10-d-old pups from food-restricted dams were studied. We showed that undernourished suckling rats are glucose normotolerants, despite their depressed insulin secretion capacity. The content of the main glucose transporters in muscle, GLUT-4 and GLUT-1, was not affected by undernutrition, but fractionation studies showed an improved insulin-stimulated GLUT-4 translocation. p38MAPK protein, implicated in up-regulation of intrinsic activity of translocated GLUT-4, was increased. These changes suggest an improved insulin-induced glucose uptake associated with undernutrition. Insulin receptor content as well as that of both regulatory and catalytic phosphoinositol 3-kinase subunits was increased by food restriction. Insulin receptor substrate-1-associated phosphoinositol 3-kinase activity after insulin was enhanced in undernourished rats, as was phospho-glycogen synthase kinase-3, in line with insulin hypersensitivity. Surprisingly, protein tyrosine phosphatase-1B association with insulin receptor was also increased by undernutrition. These adaptations to a condition of severely limited nutritional resources might result in changes in the development of key tissues and be detrimental later in life, when a correct amount of nutrients is available, as the thrifty phenotype hypothesis predicts.

Differential Regulation of Amino Acid Transporter SNAT3 by Insulin in Hepatocytes

The liver is a metabolism and transfer center of amino acids as well as the prime target organ of insulin. In this report, we characterized the regulation of system N/A transporter 3 (SNAT3) in the liver of dietary-restricted mice and in hepatocytes treated with serum starvation and insulin. The expression of SNAT3 was up-regulated in dietary-restricted mice. The expression of SNAT3 protein was detected on the plasma membrane of hepatocyte-like H2.35 cells with a half-life of 6-8 h. When H2.35 cells were depleted of serum, the expression of SNAT3 was increased. An increased concentration of insulin, however, suppressed SNAT3 expression. Interestingly, the down-regulation of SNAT3 expression by insulin was blocked by the specific phosphoinositide 3-kinase inhibitor LY294002 and mammalian target of rapamycin inhibitor, but not by MAPK inhibitor PD98059, suggesting that insulin exerts its effect on SNAT3 through phosphoinositide 3-kinase-mammalian target of rapamycin signaling. Surface biotinylation assay showed an increased level of SNAT3 on the cell surface after 0.5 h of insulin treatment, although no effect was observed after 24 h of treatment. Consistently, the transport of the substrate l-histidine was increased with short, but not long, treatment by insulin in both H2.35- and SNAT3-transfected COS-7 cells. The L-histidine uptake was inhibited significantly by L-histidine followed by 2-endoamino-bicycloheptane-2-carboxylic acid and L-cysteine and to a lesser extent by L-alanine and aminoisobutyric acid, but was not inhibited by alpha-(methylamino)isobutyric acid, implying that uptake of L-histidine in H2.35 cells is primarily mediated by system N transporters. In conclusion, differential regulation of SNAT3 by insulin and serum starvation reinforces the functional significance of this transporter in liver physiology.

C2C12 Skeletal Muscle Cells Exposure to Phosphatidylcholine Triggers IGF-1 Like-Responses

Glucose uptake by cells in response to stimulation with either IGF-1 or insulin is associated with the translocation of GLUT (glucose transporter) proteins from intracellular cytoplasmic compartments to the plasma membrane. In response to such stimulation, GLUT4 and GLUT1 translocation to the plasma membrane is triggered through an increase in their exocytosis involving phospholipase D (PLD) activation, disrupting the recycling of intracellular GLUT-containing vesicles between the plasma membrane and internal compartments. In skeletal muscle, insulin resistance is observed in association with an increase of dipalmitoyl-phosphatidylcholine, which is also known to interact with PLD. Based on evidence that the recycling process is important for GLUT translocation, we decided to address whether dipalmitoyl-phosphatidylcholine, a non-translocatable phospholipid known to alter the recycling of intracellular vesicles and to interact with PLD, can be involved in glucose metabolism. We show that an acute change in phospholipid composition, by addition of dipalmitoyl-phophatidylcholine, leads to GLUT1 translocation to the plasma membrane in conjunction to an increase of Akt and GSK3beta phosphorylation, which are sensitive to PI3K and PLD inhibitors. Moreover, we also show that long-term change in phospholipid composition disrupts both the IGF-1 signalling pathway and GLUT1 partitioning within the cells.

Differential utilization of saturated palmitate and unsaturated oleate: evidence from cultured myotubes

We recently described a primarily reduced palmitate oxidation in myotubes established from type 2 diabetic subjects, whereas triacylglycerol (TAG) accumulation seemed to be adaptive. However, it is still uncertain whether these changes are similar for saturated and unsaturated fatty acids and whether high concentrations of glucose and/or insulin may change this picture. Studies of palmitic acid and oleic acid metabolism in human myotubes established from control and type 2 diabetic subjects under conditions of acute high concentrations of insulin and/or glucose may solve these questions. Total oleic acid and palmitic acid uptake in myotubes was increased during acute insulin stimulation (P < 0.01) but not under acute, high-glucose concentrations, and no differences were found between the groups. Type 2 diabetic myotubes expressed a reduced palmitic acid oxidation to carbon dioxide (P </= 0.04), whereas oleic acid oxidation showed no differences between myotubes from both groups. High glucose concentrations decreased oleic acid oxidation (P </= 0.03). Lipid distribution was not different in diabetic and control myotubes when palmitic acid and oleic acid incorporation into cellular lipids was compared. Myotubes that were exposed to palmitic acid showed an increased palmitic acid incorporation into diacylglycerol (DAG) and TAG compared with myotubes that were exposed to oleic acid (P < 0.05) expressing an increased intracellular free fatty acid (FFA) level (P < 0.05). Lipid distribution was not affected by high glucose, whereas insulin increased FFAs, DAG, and TAG (P < 0.05). De novo lipid synthesis from glucose in both diabetic and control myotubes was of the same magnitude independent of glucose and insulin concentrations. These results indicate that palmitic acid and oleic acid are utilized in the same pattern in diabetic and control myotubes even though palmitic acid oxidation is primarily reduced in diabetic cells. Palmitic acid and oleic acid are handled differently by myotubes: Palmitic acid seems to accumulate as DAG and TAG, whereas oleic acid accumulates as intracellular FFAs. These observations indicate that oleic acid is preferable as fatty acid as it accumulates to a lesser extent as DAG and TAG than palmitic acid. Neither acute hyperglycemia nor de novo lipid synthesis from glucose seems central to the TAG accumulation in obesity or type 2 diabetes.

Ceramide down-regulates System A amino acid transport and protein synthesis in rat skeletal muscle cells

Skeletal muscle is a major insulin target tissue and has a prominent role in the control of body amino acid economy, being the principal store of free and protein-bound amino acids and a dominant locus for amino acid metabolism. Interplay between diverse stimuli (e.g., hormonal/nutritional/mechanical) modulates muscle insulin action to serve physiological need through the action of factors such as intramuscular signaling molecules. Ceramide, a product of sphingolipid metabolism and cytokine signaling, has a potent contra-insulin action with respect to the transport and deposition of glucose in skeletal muscle, although ceramide effects on muscle amino acid turnover have not previously been documented. Here, membrane permeant C2-ceramide is shown to attenuate the basal and insulin-stimulated activity of the Na+-dependent System A amino acid transporter in rat muscle cells (L6 myotubes) by depletion of the plasma membrane abundance of SNAT2 (a System A isoform). Concomitant with transporter down-regulation, ceramide diminished both intramyocellular amino acid abundance and the phosphorylation of translation regulators lying downstream of mTOR. The physiological outcome of ceramide signaling in this instance is a marked reduction in cellular protein synthesis, a result that is likely to represent an important component of the processes leading to muscle wasting in catabolic conditions.

Insulin regulates the expression of the GLUT5 transporter in L6 skeletal muscle cells

Skeletal muscle, a primary insulin target tissue, expresses the GLUT5 fructose transporter. Although insulin has no acute effect on GLUT5 expression and function in muscle, we show here that long-term (24 h) insulin treatment of L6 muscle cells induces a dose-dependent increase in GLUT5 protein (by up to two-fold), leading to a concomitant increase in fructose uptake. The increase in GLUT5 expression and function was suppressed by inhibitors of gene transcription and protein synthesis, suggesting that insulin promotes de novo carrier synthesis. Transfection of the GLUT5 gene promoter fused to luciferase into L6 cells revealed that insulin induced a 1.8-fold increase in GLUT5 promoter activity. Our findings indicate that insulin is capable of increasing the abundance and functional activity of GLUT5 in skeletal muscle cells and that this is most likely mediated via activation of the GLUT5 promoter.

Muscle glycogenosis with low phosphorylase kinase activity: mutations in PHKA1, PHKG1 or six other candidate genes explain only a minority of cases

Muscle-specific deficiency of phosphorylase kinase (Phk) causes glycogen storage disease, clinically manifesting in exercise intolerance with early fatiguability, pain, cramps and occasionally myoglobinuria. In two patients and in a mouse mutant with muscle Phk deficiency, mutations were previously found in the muscle isoform of the Phk alpha subunit, encoded by the X-chromosomal PHKA1 gene (MIM # 311870). No mutations have been identified in the muscle isoform of the Phk gamma subunit (PHKG1). In the present study, we determined Q1the structure of the PHKG1 gene and characterized its relationship to several pseudogenes. In six patients with adult- or juvenile-onset muscle glycogenosis and low Phk activity, we then searched for mutations in eight candidate genes. The coding sequences of all six genes that contribute to Phk in muscle were analysed: PHKA1, PHKB, PHKG1, CALM1, CALM2 and CALM3. We also analysed the genes of the muscle isoform of glycogen phosphorylase (PYGM), of a muscle-specific regulatory subunit of the AMP-dependent protein kinase (PRKAG3), and the promoter regions of PHKA1, PHKB and PHKG1. Only in one male patient did we find a PHKA1 missense mutation (D299V) that explains the enzyme deficiency. Two patients were heterozygous for single amino-acid replacements in PHKB that are of unclear significance (Q657K and Y770C). No sequence abnormalities were found in the other three patients. If these results can be generalized, only a fraction of cases with muscle glycogenosis and a biochemical diagnosis of low Phk activity are caused by coding, splice-site or promoter mutations in PHKA1, PHKG1 or other Phk subunit genes. Most patients with this diagnosis probably are affected either by elusive mutations of Phk subunit genes or by defects in other, unidentified genes.

Mouse system-N amino acid transporter, mNAT3, expressed in hepatocytes and regulated by insulin-activated and phosphoinositide 3-kinase-dependent signalling

Amino acid transporters are essential for normal cell function and physiology. In the present study, we report the identification and functional and regulatory characterization of a mouse system-N amino acid transporter, mNAT3. Expression of mNAT3 in Xenopus oocytes revealed that the strongest transport activities were preferred for L-alanine. In addition, mNAT3 is an Na(+)- and pH-dependent low-affinity transporter and it partially tolerates substitution of Na(+) by Li(+). mNAT3 has been found to be expressed predominantly in the liver, where it is localized to the plasma membrane of hepatocytes, with the strongest expression in those cells adjacent to the central vein, decreasing gradually towards the portal tract. Treatment of mouse hepatocyte-like H2.35 cells with insulin led to a significant increase in the expression of mNAT3, and this stimulation was associated closely with an increase in the uptake of L-alanine. Interestingly, this insulin-induced stimulatory effect on mNAT3 expression was attenuated by the phosphoinositide 3-kinase inhibitor LY294002, but not by the mitogen-activated protein kinase inhibitor PD98059, although both kinases were fully activated by insulin. The results suggest that insulin-mediated regulation of mNAT3 is likely to be mediated through a phosphoinositide 3-kinase-dependent signalling pathway. The unique expression pattern and insulin-mediated regulatory properties of mNAT3 suggest that this transporter may play an important role in liver physiology.

Ketone bodies disturb fatty acid handling in isolated cardiomyocytes derived from control and diabetic rats

According to the current paradigm, fatty acid (FA) utilization is increased in the diabetic heart. Since plasma levels of competing substrates such as ketone bodies are increased during diabetes, the effect of those substrates on cardiac FA handling was explored. Cardiomyocytes were isolated from control and streptozotocin-treated diabetic rats and incubated with normal (80 microM) and elevated (160 microM) palmitate concentrations in the absence or presence of ketone bodies, including acetoacetate (AcAc). Comparing control cardiomyocytes under normal conditions (80 microM, no AcAc) with diabetic cardiomyocytes (160 microM, 3 mM AcAc) showed that palmitate uptake was increased from 35.2 +/- 4.8 to 60.2 +/- 14.0 nmol x 3 min(-1) x g wet weight(-1) respectively. Under these conditions, palmitate oxidation rates were comparable (58.9 +/- 23.6 versus 53.2 +/- 18.5 nmol x 30 min(-1) x g wet weight(-1)). However, in the absence of AcAc, palmitate oxidation was significantly enhanced in diabetic cardiomyocytes, indicating that ketone bodies are able to suppress cardiac FA oxidation in diabetes. The concomitantly increased FA uptake in diabetic cells, mainly due to the elevated extracellular FA levels, may be responsible for the accumulation of FA and triacylglycerol, as observed in the diabetic heart in situ.

Oral creatine supplementation and skeletal muscle metabolism in physical exercise

Creatine is the object of growing interest in the scientific literature. This is because of the widespread use of creatine by athletes, on the one hand, and to some promising results regarding its therapeutic potential in neuromuscular disease on the other. In fact, since the late 1900s, many studies have examined the effects of creatine supplementation on exercise performance. This article reviews the literature on creatine supplementation as an ergogenic aid, including some basic aspects relating to its metabolism, pharmacokinetics and side effects. The use of creatine supplements to increase muscle creatine content above approximately 20 mmol/kg dry muscle mass leads to improvements in high-intensity, intermittent high-intensity and even endurance exercise (mainly in nonweightbearing endurance activities). An effective supplementation scheme is a dosage of 20 g/day for 4-6 days, and 5 g/day thereafter. Based on recent pharmacokinetic data, new regimens of creatine supplementation could be used. Although there are opinion statements suggesting that creatine supplementation may be implicated in carcinogenesis, data to prove this effect are lacking, and indeed, several studies showing anticarcinogenic effects of creatine and its analogues have been published. There is a shortage of scientific evidence concerning the adverse effects following creatine supplementation in healthy individuals even with long-term dosage. Therefore, creatine may be considered as a widespread, effective and safe ergogenic aid.

Effects of chronic undernutrition on glucose uptake and glucose transporter proteins in rat heart

The high energy demands of myocardium are met through the metabolism of lipids and glucose. Importantly, enhanced glucose utilization rates are crucial adaptations of the cardiac cell to some pathological conditions, such as hypertrophy and ischemia, but the effects of undernutrition on heart glucose metabolism are unknown. Our previous studies have shown that undernutrition increases insulin-induced glucose uptake by skeletal muscle. Consequently, we considered the possibility of a similar adaptation in the heart. With this aim, undernourished rats both in the basal state and after euglycemic hyperinsulinemic clamps were used to determine the following parameters in myocardium: glucose uptake, glucose transporter (GLUT) content, and some key components of the insulin signaling cascade. Heart membranes were prepared by subcellular fractionation in sucrose gradients. Although GLUT-4, GLUT-1, and GLUT-3 proteins and GLUT-4/1 mRNAs were reduced by undernutrition, basal and insulin-stimulated 2-deoxyglucose uptake were significantly enhanced. Phosphoinositol 3-kinase activity remained greater than control values in both conditions. The abundance of p85alpha and p85beta regulatory subunits of phosphoinositol 3-kinase was increased as was phospho-Akt during hyperinsulinemia. These changes seem to improve the insulin stimulus of GLUT-1 translocation, as its content was increased at the surface membrane. Such adaptations associated with undernutrition must be crucial to improvement of cardiac glucose uptake.