Differential expression of the GLUT1 and GLUT4 glucose transporters during differentiation of L6 muscle cells

Pancreastatin inhibitor PSTi8 ameliorates streptozotocin-induced diabetes by suppressing hepatic glucose production

Elevated plasma glucose concentration, as a consequence of excessive hepatic glucose production, plays a pivotal role in the development of diabetes. A chromogranin A-derived diabetogenic peptide Pancreastatin (PST) enhances hepatic glucose output leading to diabetes. Therefore, here we probed the role of PSTi8, a PST inhibitor in ameliorating diabetes by investigating the effect of high glucose (HG) or PST on glucose metabolism. Further, we also explored the action mechanism of the underlying anti-hyperglycemic effect of PSTi8. PSTi8 treatment rescue cultured L6 and HepG2 cells from HG and PST-induced insulin resistance, respectively. It also enhances insulin receptor kinase activity by interacting with the insulin receptor and enhancing GLUT4 translocation and glucose uptake. Thus, our in-silico and in-vitro data support the PST-dependent and independent activity of PSTi8. Additionally, PSTi8 treatment in streptozotocin-induced diabetic rats improved glucose tolerance by lowering blood glucose and plasma PST levels. Concomitantly, the treated animals exhibited reduced hepatic glucose production accompanied by downregulation of hepatic gluconeogenic genes PEPCK and G6Pase. PSTi8-treated rats also exhibited enhanced hepatic glycogen in line with reduced plasma glucagon concentrations. Consistently, improved plasma insulin levels in PSTi8-treated rats enhanced skeletal muscle glucose disposal via enhanced P-Akt expression. In summary, these findings suggest PSTi8 has anti-hyperglycemic properties with enhanced skeletal muscle glucose disposal and reduced hepatic gluconeogenesis both PST dependent as well as independent.

Rare Sugar Metabolism and Impact on Insulin Sensitivity along the Gut-Liver-Muscle Axis In Vitro

Rare sugars have recently attracted attention as potential sugar replacers. Understanding the biochemical and biological behavior of these sugars is of importance in (novel) food formulations and prevention of type 2 diabetes. In this study, we investigated whether rare sugars may positively affect intestinal and liver metabolism, as well as muscle insulin sensitivity, compared to conventional sugars. Rare disaccharide digestibility, hepatic metabolism of monosaccharides (respirometry) and the effects of sugars on skeletal muscle insulin sensitivity (impaired glucose uptake) were investigated in, respectively, Caco-2, HepG2 and L6 cells or a triple coculture model with these cells. Glucose and fructose, but not l-arabinose, acutely increased extracellular acidification rate (ECAR) responses in HepG2 cells and impaired glucose uptake in L6 cells following a 24 h exposure at 28 mM. Cellular bioenergetics and digestion experiments with Caco-2 cells indicate that especially trehalose (α1-1α), D-Glc-α1,2-D-Gal, D-Glc-α1,2-D-Rib and D-Glc-α1,3-L-Ara experience delayed digestion and reduced cellular impact compared to maltose (α1-4), without differences on insulin-stimulated glucose uptake in a short-term setup with a Caco-2/HepG2/L6 triple coculture. These results suggest a potential for l-arabinose and specific rare disaccharides to improve metabolic health; however, additional in vivo research with longer sugar exposures should confirm their beneficial impact on insulin sensitivity in humans.

Synthesis of vildagliptin loaded acrylamide-g-psyllium/alginate-based core-shell nanoparticles for diabetes treatment

Diabetes mellitus has become a major public health concern all over the world. Vildagliptin is one of the antidiabeticdrug that can overcome the existing problem of this prevalent disease. Present study aims to synthesize and investigate the role of vildagliptin-loaded core-shell nanoparticle of grafted psyllium and alginate (VG@P/A-NPs) in anti-diabetes application. FTIR, SEM, XRD, ¹³CNMR and zeta analyzer were used for characterization of the core-shell nanoparticles (VG@P/A-NPs). The synthesized acrylamide-grafted-psyllium was also optimized through varying grafting parameters such as acrylamide and ceric ammonium nitrate (CAN) concentration, time and temperature to obtain the maximum yield of acrylamide-grafted-psyllium. Rheological analysis of pure psyllium, grafted psyllium and alginate were also performed. For biological studies, the first cytotoxicity of grafted psyllium and VG@P/A-NPs were examined on human lung adenocarcinoma cell line A549 in which it was observed that VG@P/A-NPs did not exhibited any toxicity. The antidiabetic potential of VG@P/A-NPs was investigated by glucose uptake assay, using TNF-α induced insulin resistance skeletal cell model using mouse muscle L6 cell line. The insulin signaling impaired cell line displayed a highly significant (p < 0.0001) dose-dependent increase in glucose uptake after treatment with increasing doses of VG@P/A-NPs.The drug release behavior of VG@P/A-NPs was examined at various pH and the highest drug release (98 %) was obtained at pH (7.4). The drug release kinetic data was following the Higuchi (R² = 0.9848) kinetic model, suggesting the release of drug from vildagliptin-loaded grafted psyllium-alginate core-shell nanoparticles (VG@P/A-NPs) as a square root of time-dependent process and diffusion controlled. This study provides an economical and environment-friendly approach towards the synthesis of VG@P/A-NPs with antidiabetes applications.

Skeletal muscle RUNX1 is related to insulin sensitivity through its effect on myogenic potential

OBJECTIVE

Skeletal muscle is the major site of insulin action. There are limited data on the relationship between insulin action and skeletal muscle myogenic/regenerative potential. RUNX1 is a transcription factor which plays a role in muscle development and regeneration. The aim of our study was to assess the role of skeletal muscle myogenic/regenerative potential in the development of insulin resistance through the studies on RUNX1 transcription factor.

DESIGN

This study is a cross-sectional study. Experimental part with myoblast cell line culture.

METHODS

We examined 41 young healthy volunteers, 21 normal weight and 20 with overweight or obesity. Hyperinsulinemic-euglycemic clamp and vastus lateralis muscle biopsy were performed. In L6 myoblast and human skeletal muscle myoblasts (hSkMM) cell cultures, RUNX1 was silenced at two stages of development. Cell growth, the expression of markers of myogenesis, nuclei fusion index, Akt phosphorylation and glucose uptake were measured.

RESULTS

Skeletal muscle RUNX1 expression was decreased in overweight/obese individuals in comparison with normal-weight individuals and was positively related to insulin sensitivity, independently of BMI. Runx1 loss-of-function at the stage of myoblast inhibited myoblast proliferation and differentiation and reduced insulin-stimulated Akt phosphorylation and insulin-stimulated glucose uptake. In contrast, Runx1 knockdown in myotubes did not affect Akt phosphorylation, glucose uptake and other parameters studied.

CONCLUSIONS

Myogenic/regenerative potential of adult skeletal muscle may be an important determinant of insulin action. Our data suggest that muscle RUNX1 may play a role in the modulation of insulin action through its effect on myogenesis.

Expression Patterns of Energy-Related Genes in Single Cells Uncover Key Isoforms and Enzymes That Gain Priority Under Nanoparticle-Induced Stress

Cellular responses to nanoparticles (NPs) have been largely studied in cell populations, providing averaged values that often misrepresent the true molecular processes that occur in the individual cell. To understand how a cell redistributes limited molecular resources to achieve optimal response and survival requires single-cell analysis. Here we applied multiplex single molecule-based fluorescence in situ hybridization (fliFISH) to quantify the expression of 10 genes simultaneously in individual intact cells, including glycolysis and glucose transporter genes, which are critical for restoring and maintaining energy balance. We focused on individual gill epithelial cell responses to lithium cobalt oxide (LCO) NPs, which are actively pursued as cathode materials in lithium-ion batteries, raising concerns about their impact on the environment and human health. We found large variabilities in the expression levels of all genes between neighboring cells under the same exposure conditions, from only a few transcripts to over 100 copies in individual cells. Gene expression ratios among the 10 genes in each cell uncovered shifts in favor of genes that play key roles in restoring and maintaining energy balance. Among these genes are isoforms that can secure and increase glycolysis rates more efficiently, as well as genes with multiple cellular functions, in addition to glycolysis, including DNA repair, regulation of gene expression, cell cycle progression, and proliferation. Our study uncovered prioritization of gene expression in individual cells for restoring energy balance under LCO NP exposures. Broadly, our study gained insight into single-cell strategies for redistributing limited resources to achieve optimal response and survival under stress.

Structure and activity study of tripeptide IRW in TNF-α induced insulin resistant skeletal muscle cells

Egg white protein ovotransferrin derived peptide IRW (Ile-Arg-Trp) was found to improve tumor necrosis factor alpha (TNF-α) or angiotensin II induced insulin resistance in L6 cells. Our recent study further showed that this peptide can improve glucose tolerance in high fat diet fed C57BL/6 mice. However, the structural requirements of IRW, especially the significance of each amino acid residue of IRW, is unknown. The study was aimed to investigate the structure and activity relationships of IRW in TNF-α induced insulin resistance L6 cells. The peptides were designed to determine the significance of individual amino acids in IRW using alanine scanning (replacing one amino acid at one time), the order of the peptide sequence and the constituting elements of IRW. Among the tested peptides and amino acids, only IRA and IR showed the same effects as that of IRW: enhanced glucose uptake, improvement in the impaired insulin signaling pathway and increased glucose transporter protein 4 (GLUT4) translocation in TNF-α treated L6 myotubes. This study demonstrated that C-terminal W is not essential to the activity of IRW. Further study is necessary to establish if IR and IRA show similar effects to that of IRW in vivo.

AMPK-mediated phosphorylation enhances the auto-inhibition of TBC1D17 to promote Rab5-dependent glucose uptake

Dysregulation of glucose homeostasis contributes to insulin resistance and type 2 diabetes. Whilst exercise stimulated activation of AMP-activated protein kinase (AMPK), an important energy sensor, has been highlighted for its potential to promote insulin-stimulated glucose uptake, the underlying mechanisms for this remain largely unknown. Here we found that AMPK positively regulates the activation of Rab5, a small GTPase which is involved in regulating Glut4 translocation, in both myoblasts and skeletal muscles. We further verified that TBC1D17, identified as a potential interacting partner of Rab5 in our recent study, is a novel GTPase activating protein (GAP) of Rab5. TBC1D17-Rab5 axis regulates transport of Glut1, Glut4, and transferrin receptor. TBC1D17 interacts with Rab5 or AMPK via its TBC domain or N-terminal 1-306 region (N-Ter), respectively. Moreover, AMPK phosphorylates the Ser 168 residue of TBC1D17 which matches the predicted AMPK consensus motif. N-Ter of TBC1D17 acts as an inhibitory region by directly interacting with the TBC domain. Ser168 phosphorylation promotes intra-molecular interaction and therefore enhances the auto-inhibition of TBC1D17. Our findings reveal that TBC1D17 acts as a molecular bridge that links AMPK and Rab5 and delineate a previously unappreciated mechanism by which the activation of TBC/RabGAP is regulated.

TRAIL/DR5 pathway promotes AKT phosphorylation, skeletal muscle differentiation, and glucose uptake

TNF-related apoptosis-inducing ligand (TRAIL) is a protein that induces apoptosis in cancer cells but not in normal ones, where its effects remain to be fully understood. Previous studies have shown that in high-fat diet (HFD)-fed mice, TRAIL treatment reduced body weight gain, insulin resistance, and inflammation. TRAIL was also able to increase skeletal muscle free fatty acid oxidation. The aim of the present work was to evaluate TRAIL actions on skeletal muscle. Our in vitro data on C2C12 cells showed that TRAIL treatment significantly increased myogenin and MyHC and other hallmarks of myogenic differentiation, which were reduced by Dr5 (TRAIL receptor) silencing. In addition, TRAIL treatment significantly increased AKT phosphorylation, which was reduced by Dr5 silencing, as well as glucose uptake (alone and in combination with insulin). Our in vivo data showed that TRAIL increased myofiber size in HFD-fed mice as well as in db/db mice. This was associated with increased myogenin and PCG1α expression. In conclusion, TRAIL/DR5 pathway promotes AKT phosphorylation, skeletal muscle differentiation, and glucose uptake. These data shed light onto a pathway that might hold therapeutic potential not only for the metabolic disturbances but also for the muscle mass loss that are associated with diabetes.

Whole-life body composition trajectory and longevity: role of insulin

The present study assessed the body composition trajectory of rats (N = 96) placed into 5 groups according to lifespan, using dual-energy x-ray absorptiometry every 6 months until end-of-life. A striking linearity between lifespan and bone mass percentage (not absolute bone mass) was observed. Long-lived rats show a higher bone mass percentage with a delayed insulin rise to a similar peak level as short-lived counterparts, followed by insulin declines and bone mass loss. Decreasing insulin after streptozotocin (STZ) injection caused a rapid bone mass loss (-10.5%) with a decreased 5-day survival rate to 35% in old rats (20 months). Insulin replacement to STZ-injected rats completely blocked bone mass loss and increased the survival rate to 71%. Normal old rats (20 months) had faster lean mass loss despite greater myofiber regeneration (centronucleation) compared with the young rats (4 months). Increased CD68⁺ and CD163⁺ cell infiltration into insulin-depleted muscle suggests a bone marrow cell exhaustion by aging muscle. Bone produces stem cells and phagocytes to continuously rejuvenate peripheral tissues. Our data suggests that aging and unsustainable life is associated with development of disproportionality between bone and the growing body size, partly due to insulin reversal from hyperinsulinemia during late life.

Plant Extracts for Type 2 Diabetes: From Traditional Medicine to Modern Drug Discovery

Type 2 diabetes mellitus (T2DM) is one of the largest public health problems worldwide. Insulin resistance-related metabolic dysfunction and chronic hyperglycemia result in devastating complications and poor prognosis. Even though there are many conventional drugs such as metformin (MET), Thiazolidinediones (TZDs), sulfonylureas (SUF), dipeptidyl peptidase 4 (DPP-4) inhibitors, glucagon like peptide 1 (GLP-1) and sodium-glucose cotransporter-2 (SGLT-2) inhibitors, side effects still exist. As numerous plant extracts with antidiabetic effects have been widely reported, they have the potential to be a great therapeutic agent for type 2 diabetes with less side effects. In this study, sixty-five recent studies regarding plant extracts that alleviate type 2 diabetes were reviewed. Plant extracts regulated blood glucose through the phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) pathway. The anti-inflammatory and antioxidant properties of plant extracts suppressed c-Jun amino terminal kinase (JNK) and nuclear factor kappa B (NF-κB) pathways, which induce insulin resistance. Lipogenesis and fatty acid oxidation, which are also associated with insulin resistance, are regulated by AMP-activated protein kinase (AMPK) activation. This review focuses on discovering plant extracts that alleviate type 2 diabetes and exploring its therapeutic mechanisms.

α-Glucosidase Inhibitory and Glucose Uptake Stimulatory Effects of Phenolic Compounds FromDendrobium christyanum

A methanolic extract from the dried root of Dendrobium christyanum Rchb.f. (Orchidaceae) exhibited α-glucosidase inhibitory activity and glucose uptake stimulatory effect. Chromatographic separation of the extract led to the isolation of 13 phenolic compounds (1-13). Their structures were determined by spectroscopic analysis. The isolates were then evaluated for in vitro α-glucosidase inhibitory and glucose uptake stimulatory activities. Methyl haematommate (1), methyl 2,4-dihydroxy-3,6-dimethylbenzoate (3), n-docosyl 4-hydroxy- trans-cinnamate (4), coniferyl aldehyde (6), 4,5-dihydroxy-2-methoxy-9,10-dihydrophenanthrene (7), gigantol (10), and diorcinolic acid (13) showed higher α-glucosidase inhibitory activity than the drug acarbose. Moreover, n-docosyl 4-hydroxyl- trans-cinnamate (4), vanillin (5), and coniferyl aldehyde (6) could enhance glucose uptake by L6 myotubes. Compounds 4 and 6 appear to be potential hypoglycemic agents since they possess both α-glucosidase inhibitory and glucose uptake stimulatory activities. This study is the first report on the chemical constituents and antidiabetic activity of D. christyanum.

Acacetin enhances glucose uptake through insulin-independent GLUT4 translocation in L6 myotubes

BACKGROUND

Lowering blood glucose levels by increasing glucose uptake in insulin target tissues, such as skeletal muscle and adipose tissue, is one strategy to discover and develop antidiabetic drugs from natural products used as traditional medicines.

PURPOSE

Our goal was to reveal the mechanism and activity of acacetin (5,7-dihydroxy-4'-methoxyflavone), one of the major compounds in Agastache rugose, in stimulating glucose uptake in muscle cells.

METHODS

To determine whether acacetin promotes GLUT4-dependent glucose uptake in cultured L6 skeletal muscle cells, we performed a [¹⁴C] 2-deoxy-_D-glucose (2-DG) uptake assay after treating differentiated L6-GLUT4myc cells with acacetin.

RESULTS

Acacetin dose-dependently increased 2-DG uptake by enhancing GLUT4 translocation to the plasma membrane. Our results revealed that acacetin activated the CaMKII-AMPK pathway by increasing intracellular calcium concentrations. We also found that aPKCλ/ζ phosphorylation and intracellular reactive oxygen species (ROS) production were involved in acacetin-induced GLUT4 translocation. Moreover, acacetin-activated AMPK inhibited intracellular lipid accumulation and increased 2-DG uptake in HepG2 cells.

CONCLUSION

Taken together, these results suggest that acacetin might be useful as an antidiabetic functional ingredient. Subsequent experiments using disease model animals are needed to verify our results.

Egg white hydrolysate and peptide reverse insulin resistance associated with tumor necrosis factor-α (TNF-α) stimulated mitogen-activated protein kinase (MAPK) pathway in skeletal muscle cells

Purpose

Excessive formation of tumor necrosis factor-α (TNF-α), a pro-inflammatory cytokine, has been implicated in the development of insulin resistance in obesity and type-2 diabetes. In skeletal muscle, chronic exposure to TNF-α impairs insulin-stimulated glucose uptake and insulin signaling. The aim of this study is to investigate the effects of enzymatic egg white hydrolysate (EWH) and its responsible peptide, IRW, on TNF-α-induced insulin resistance and the underlying molecular mechanisms using rat skeletal muscle cells (L6 cells).

Methods

Insulin resistance was induced by treating L6 cells with 5 ng/ml TNF-α for 24 h. Effects of EWH and IRW on glucose uptake were detected by glucose uptake assay, glucose transporter 4 (GLUT4) translocation by immunofluorescence, and western blot, while insulin-signaling pathway and mitogen-activated protein kinase (MAPK) pathway were investigated using western blot.

Results

Adding both EWH and IRW significantly improved glucose uptake in TNF-α-treated cells, increased activation of insulin receptor substrate (IRS-1) tyrosine residue and protein kinase B (Akt), whereas decreased activation of IRS-1 serine residue. In addition, TNF-α-induced activation of p38-mitogen-activated protein kinase (p38) and c-Jun N-terminal kinases (JNK) 1/2 were decreased by either EWH or IRW treatment.

Conclusion

EWH and IRW improve impaired insulin sensitivity by down-regulating the activation of p38 and JNK1/2 in TNF-α-treated skeletal muscle cells.

Egg White Ovotransferrin-Derived ACE Inhibitory Peptide Ameliorates Angiotensin II-Stimulated Insulin Resistance in Skeletal Muscle Cells

SCOPE

The renin-angiotensin system (RAS) is a major contributor to the development of insulin resistance and its related complications. Egg white ovotransferrin-derived tripeptides, IRW (Ile-Arg-Trp), IQW (Ile-Gln-Trp), or LKP (Leu-Lys-Pro) are previously identified as the inhibitors of angiotensin-converting enzyme (ACE), a key enzyme in the RAS. This study aims at determining whether these peptides are effective in improving insulin resistance, and their mechanisms of action, in a rat derived skeletal muscle cell line (L6 cells).

METHODS AND RESULTS

Insulin resistance is induced by treating L6 cells with 1 μm angiotensin II (Ang II) for 24 h. Effects of peptides on glucose uptake are determined using glucose uptake assay, glucose transporter 4 (GLUT4) translocation by immunofluorescence, reactive oxygen species (ROS) by dihydroethidium (DHE) staining, while insulin signaling pathway, Ang II receptor (AT1R or AT2R) levels, and NADPH oxidase activation are measured using Western Blot. Only IRW treatment significantly improves insulin resistance in L6 cells via stimulation of insulin signaling. IRW decreases Ang II-stimulated AT1R expression, ROS formation, and NADPH oxidase activation.

CONCLUSIONS

Of three ACE inhibitory peptides studied, only IRW improves insulin resistance in L6 cells, at least partially via reduced AT1R expression and its anti-oxidative activity.

[Molecular mechanism for ET-1-induced insulin resistance in skeletal muscle cells]

Insulin resistance is a condition where the sensitivity to insulin of the tissues expressing insulin receptor (InsR) is decreased due to a functional disturbance of InsR-mediated intracellular signaling. Insulin promotes the entry of glucose into the tissues and skeletal muscle is the most important tissue responsible for the insulin's action of decreasing blood glucose levels. Endothelin-1 (ET-1), a potent vasoconstrictor and pro-inflammatory peptide, induces insulin resistance through a direct action on skeletal muscle. However, the signaling pathways of ET-1-induced insulin resistance in skeletal muscle remain unclear. Here we show molecular mechanism underlying the inhibitory effect of ET-1 on insulin-stimulated Akt phosphorylation and glucose uptake in myotubes of rat L6 skeletal muscle cell line. mRNA expression levels of differentiation marker genes, MyoD and myogenin, were increased during L6 myoblasts differentiation into myotubes. Some of myotubes possessed the ability to spontaneously contract. In myotubes, insulin promoted Akt phosphorylation at Thr³⁰⁸ and Ser⁴⁷³, and [³H]-labelled 2-deoxy-D-glucose ([³H]2-DG) uptake. The insulin-facilitated Akt phosphorylation and [³H]2-DG uptake were inhibited by ET-1. The inhibitory effect of ET-1 was counteracted by blockade of ET type A receptor (ET_AR), inhibition of G_q/11 protein, and siRNA knockdown of G protein-coupled receptor kinase 2 (GRK2). The exogenously overexpressed GRK2 directly bound to endogenous Akt and their association was facilitated by ET-1. In summary, activation of ET_AR with ET-1 inhibits insulin-induced Akt phosphorylation and [³H]2-DG uptake in a G_q/11 protein- and GRK2-dependent manner in skeletal muscle. These findings indicate that ET_AR and GRK2 are potential targets for insulin resistance.

GLUT4 Translocation in Single Muscle Cells in Culture: Epitope Detection by Immunofluorescence

GLUT4 is the major glucose transporter in skeletal muscle. GLUT4 cycles to and from the plasma membrane and its exocytic rate is accelerated by insulin and muscle contraction to achieve a new steady state with more GLUT4 proteins at the muscle cell surface. To gain a better understanding of the molecular and cellular mechanisms that govern GLUT4 protein recycling, we developed an in vitro model in which myc-epitope-tagged GLUT4 or GLUT4-GFP is expressed in L6 skeletal muscle cells. The myc-epitope is inserted into an exofacial domain that is accessible to anti-myc antibodies from the outside of non-permeabilized cells, allowing one to count the number of transporters at the cell surface. This enables one to perform single-cell analysis using confocal fluorescence microscopy to quantify cell surface GLUT4myc or GLUT4myc-GFP in cells co-transfected with diverse cDNA constructs, treated with siRNAs, or co-stained with antibodies for other proteins of interest. Herein, we describe the methodology to perform these experimental approaches in insulin-stimulated L6 muscle cells.

Distinct Akt phosphorylation states are required for insulin regulated Glut4 and Glut1-mediated glucose uptake

Insulin, downstream of Akt activation, promotes glucose uptake into fat and muscle cells to lower postprandial blood glucose, an enforced change in cellular metabolism to maintain glucose homeostasis. This effect is mediated by the Glut4 glucose transporter. Growth factors also enhance glucose uptake to fuel an anabolic metabolism required for tissue growth and repair. This activity is predominantly mediated by the Glut1. Akt is activated by phosphorylation of its kinase and hydrophobic motif (HM) domains. We show that insulin-stimulated Glut4-mediated glucose uptake requires PDPK1 phosphorylation of the kinase domain but not mTORC2 phosphorylation of the HM domain. Nonetheless, an intact HM domain is required for Glut4-mediated glucose uptake. Whereas, Glut1-mediated glucose uptake also requires mTORC2 phosphorylation of the HM domain, demonstrating both phosphorylation-dependent and independent roles of the HM domain in regulating glucose uptake. Thus, mTORC2 links Akt to the distinct physiologic programs related to Glut4 and Glut1-mediated glucose uptake.

DOI:http://dx.doi.org/10.7554/eLife.26896.001

Endothelin-1 suppresses insulin-stimulated Akt phosphorylation and glucose uptake via GPCR kinase 2 in skeletal muscle cells

BACKGROUND AND PURPOSE

Endothelin-1 (ET-1) reduces insulin-stimulated glucose uptake in skeletal muscle, inducing insulin resistance. Here, we have determined the molecular mechanisms underlying negative regulation by ET-1 of insulin signalling.

EXPERIMENTAL APPROACH

We used the rat L6 skeletal muscle cells fully differentiated into myotubes. Changes in the phosphorylation of Akt was assessed by Western blotting. Effects of ET-1 on insulin-stimulated glucose uptake was assessed with [(3) H]-2-deoxy-d-glucose ([(3) H]2-DG). The C-terminus region of GPCR kinase 2 (GRK2-ct), a dominant negative GRK2, was overexpressed in L6 cells using adenovirus-mediated gene transfer. GRK2 expression was suppressed by transfection of the corresponding short-interfering RNA (siRNA).

KEY RESULTS

In L6 myotubes, insulin elicited sustained Akt phosphorylation at Thr(308) and Ser(473) , which was suppressed by ET-1. The inhibitory effects of ET-1 were prevented by treatment with a selective ETA receptor antagonist and a Gq protein inhibitor, overexpression of GRK2-ct and knockdown of GRK2. Insulin increased [(3) H]2-DG uptake rate in a concentration-dependent manner. ET-1 noncompetitively antagonized insulin-stimulated [(3) H]2-DG uptake. Blockade of ETA receptors, overexpression of GRK2-ct and knockdown of GRK2 prevented the ET-1-induced suppression of insulin-stimulated [(3) H]2-DG uptake. In L6 myotubes overexpressing FLAG-tagged GRK2, ET-1 facilitated the interaction of endogenous Akt with FLAG-GRK2.

CONCLUSIONS AND IMPLICATIONS

Activation of ETA receptors with ET-1 suppressed insulin-induced Akt phosphorylation at Thr(308) and Ser(473) and [(3) H]2-DG uptake in a GRK2-dependent manner in skeletal muscle cells. These findings suggest that ETA receptors and GRK2 are potential targets for overcoming insulin resistance.

Syntaxin 4 regulates the surface localization of a promyogenic receptor Cdo thereby promoting myogenic differentiation

Background

Syntaxins are a family of membrane proteins involved in vesicle trafficking, such as synaptic vesicle exocytosis. Syntaxin 4 (Stx4) is expressed highly in skeletal muscle and plays a critical role in insulin-stimulated glucose uptake by promoting translocation of glucose transporter 4 (GLUT4) to the cell surface. A cell surface receptor cell adhesion molecule-related, down-regulated by oncogenes (Cdo) is a component of cell adhesion complexes and promotes myoblast differentiation via activation of key signalings, including p38MAPK and AKT. In this study, we investigate the function of Stx4 in myoblast differentiation and the crosstalk between Stx4 and Cdo in myoblast differentiation.

Methods

The effects of overexpression or shRNA-based depletion of Stx4 and Cdo genes on C2C12 myoblast differentiation are assessed by Western blotting and immunofluorescence approaches. The interaction between Cdo and Stx4 and the responsible domain mapping are assessed by coimmunoprecipitation or pulldown assays. The effect of Stx4 depletion on cell surface localization of Cdo and GLUT4 in C2C12 myoblasts is assessed by surface biotinylation and Western blotting.

Results

Overexpression or knockdown of Stx4 enhances or inhibits myogenic differentiation, respectively. Stx4 binds to the cytoplasmic tail of Cdo, and this interaction seems to be critical for induction of p38MAPK activation and myotube formation. Stx4 depletion decreases specifically the cell surface localization of Cdo without changes in surface N-Cadherin levels. Interestingly, Cdo depletion reduces the level of GLUT4 and Stx4 at cell surface. Consistently, overexpression of Cdo in C2C12 myoblasts generally increases glucose uptake, while Cdo depletion reduces it.

Conclusions

Stx4 promotes myoblast differentiation through interaction with Cdo and stimulation of its surface translocation. Both Cdo and Stx4 are required for GLUT4 translocation to cell surface and glucose uptake in myoblast differentiation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0052-8) contains supplementary material, which is available to authorized users.

Intestinal Sodium Glucose Cotransporter 1 Inhibition Enhances Glucagon-Like Peptide-1 Secretion in Normal and Diabetic Rodents

The sodium glucose cotransporter (SGLT) 1 plays a major role in glucose absorption and incretin hormone release in the gastrointestinal tract; however, the impact of SGLT1 inhibition on plasma glucagon-like peptide-1 (GLP-1) levels in vivo is controversial. We analyzed the effects of SGLT1 inhibitors on GLP-1 secretion in normoglycemic and hyperglycemic rodents using phloridzin, CGMI [3-(4-cyclopropylphenylmethyl)-1-(β-d-glucopyranosyl)-4-methylindole], and canagliflozin. These compounds are SGLT2 inhibitors with moderate SGLT1 inhibitory activity, and their IC50 values against rat SGLT1 and mouse SGLT1 were 609 and 760 nM for phloridzin, 39.4 and 41.5 nM for CGMI, and 555 and 613 nM for canagliflozin, respectively. Oral administration of these inhibitors markedly enhanced and prolonged the glucose-induced plasma active GLP-1 (aGLP-1) increase in combination treatment with sitagliptin, a dipeptidyl peptidase-4 (DPP4) inhibitor, in normoglycemic mice and rats. CGMI, the most potent SGLT1 inhibitor among them, enhanced glucose-induced, but not fat-induced, plasma aGLP-1 increase at a lower dose compared with canagliflozin. Both CGMI and canagliflozin delayed intestinal glucose absorption after oral administration in normoglycemic rats. The combined treatment of canagliflozin and a DPP4 inhibitor increased plasma aGLP-1 levels and improved glucose tolerance compared with single treatment in both 8- and 13-week-old Zucker diabetic fatty rats. These results suggest that transient inhibition of intestinal SGLT1 promotes GLP-1 secretion by delaying glucose absorption and that concomitant inhibition of intestinal SGLT1 and DPP4 is a novel therapeutic option for glycemic control in type 2 diabetes mellitus.

Advanced glycation end-products and insulin signaling in granulosa cells

Advanced glycation end-products (AGEs) may interfere with insulin intracellular signaling and glucose transport in human granulosa cells, potentially affecting ovarian function, follicular growth, linked with diminished fertility. The potential interaction of AGEs with insulin signaling pathways and glucose transport was investigated in human granulosa KGN cells. KGN cells were cultured with variable concentrations of human glycated albumin (HGA, 50-200 µg/mL) or insulin (100 ng/mL). Combined treatments of KGN cells with insulin (100 ng/mL) and HGA (200 µg/mL) were also performed. p-AKT levels and glucose transporter type 4 (Glut-4) translocation analysis were performed by Western blot. Phosphatidylinositol-3-kinase (PI3K)-specific signaling was checked by using the PI3K-inhibitor, LY294002. p-AKT levels were significantly increased following insulin treatment compared to basal levels or HGA exposure. This insulin-mediated AKT-phosphorylation was PI3K-specific and it was inhibited after combined treatment of insulin and HGA. Furthermore, Glut-4 translocation from the cytoplasm to the membrane compartments of KGN cells was remarkably reduced after the combined treatment of insulin and HGA. The present findings support that AGEs interfere with insulin signaling in granulosa cells and prevent Glut-4 membrane translocation suggesting that intra ovarian AGEs accumulation, from endogenous or exogenous sources, may contribute to the pathophysiology of states characterized with anovulation and insulin resistance such as polycystic ovary syndrome.

Myricetin suppresses differentiation of 3 T3-L1 preadipocytes and enhances lipolysis in adipocytes

Myricetin (MyR), a naturally occurring flavonol widely distributed in fruits, vegetables, and medicinal plants, has anticancer, anti-inflammatory, antihyperlipidaemic, and antiobesity activities. In the present study, we hypothesized that the antiobesity property of MyR is mediated via suppression of differentiation of preadipocytes into adipocytes and promotion of lipolysis of mature adipocytes, which effectively decrease the intracellular triglyceride concentration of adipocytes. Accordingly, the aim of this work was to investigate the effects of MyR on adipocyte differentiation and lipolysis in differentiated 3 T3-L1 adipocytes. Our results showed that MyR inhibited differentiation of 3 T3-L1 preadipocytes in a concentration-dependent manner. Myricetin downregulated the mRNA and protein levels of CCAAT/enhancer-binding protein α and peroxisome proliferator-activated receptor γ, both of which are major adipogenic transcription factors. Furthermore, the mRNA levels of other adipogenesis-related transcription factors, namely, CCAAT/enhancer-binding protein β, sterin regulatory element binding protein 1-c, peroxisome proliferator-activated receptor γ coactivator-1, adipocyte protein 2, lipoprotein lipase and glucose transporter 4, were also reduced by MyR treatment. Moreover, MyR significantly inhibited the phosphorylation of extracellular signal-regulated kinase, Jun N-terminal kinase, and p38 during the differentiation process. On the other hand, MyR induced a dose-dependent increase in glycerol release in fully differentiated adipocytes, indicating its stimulatory effect on adipocyte lipolysis. Furthermore, MyR downregulated mRNA level of perilipin A and enhanced the phosphorylation level of extracellular signal-regulated kinase, Jun N-terminal kinase, and p38 during lipolysis. Taken together, these findings indicate that MyR exerts antiobesity activity in adipocytes.

Novel Indole-N-glucoside, TA-1887 As a Sodium Glucose Cotransporter 2 Inhibitor for Treatment of Type 2 Diabetes

Inhibition of the renal sodium glucose cotransporter (SGLT) increases urinary glucose excretion (UGE) and thus reduces blood glucose levels during hyperglycemia. To explore the potential of new antihyperglycemic agents, we synthesized and determined the human SGLT2 (hSGLT2) inhibitory potential of novel substituted 3-benzylindole-N-glucosides 6. Optimization of 6 resulted in the discovery of 3-(4-cyclopropylbenzyl)-4-fluoroindole-N-glucoside 6a-4 (TA-1887), a highly potent and selective hSGLT2 inhibitor, with pronounced antihyperglycemic effects in high-fat diet-fed KK (HF-KK) mice. Our results suggest the potential of indole-N-glucosides as novel antihyperglycemic agents through inhibition of renal SGLT2.

C-Glucosides with heteroaryl thiophene as novel sodium-dependent glucose cotransporter 2 inhibitors

Canagliflozin (1), a novel inhibitor for sodium-dependent glucose cotransporter 2 (SGLT2), has been developed for the treatment of type 2 diabetes. To investigate the effect of replacement of the phenyl ring in 1 with heteroaromatics, C-glucosides 2 were designed, synthesized, and evaluated for their inhibitory activities against SGLT2. Of these, 3-pyridyl, 2-pyrimidyl or 5-membered heteroaryl substituted derivatives showed highly potent inhibitory activity against SGLT2, while 5-pyrimidyl substitution was associated with slightly reduced activity. In particular, 2g (TA-3404) had remarkable anti-hyperglycemic effects in high-fat diet fed KK (HF-KK) mice.

Possible hypoglycemic action of SX-fraction targeting insulin signal transduction pathway

BACKGROUND

SX-fraction (SXF) is a bioactive glycoprotein with hypoglycemic activity that has been demonstrated in our pilot clinical study. However, how it would actually work in diabetic patients remains unclear. To explore such a mechanism, the effects of SXF on the insulin signal transduction pathway were investigated using skeletal muscle L6 cells in vitro.

METHODS

L6 cells were first differentiated to myotubes expressing several biochemical parameters that were examined in this study. Myotubes were exposed to a high concentration (35 mM) of glucose (Glc) alone or in combination with SXF or insulin for 24 hours. Possible effects of these agents on activities of insulin receptor (IR), IR substrate 1 (IRS-1), and Akt, which are key elements involved in the signal pathway, were assessed using enzyme-linked immunosorbent assay (ELISA). Any changes in Glc uptake were also determined.

RESULTS

High Glc indeed led to inactivation of IR, IRS-1, and subsequent Akt in myotubes, indicating an interruption of the signal pathway. However, such inactivation was reversed or reactivated by SXF, presumably aiding the occurrence of successive signaling events. Measurement of Glc uptake to assess the outcome of this signaling cascade showed that high Glc decreased Glc uptake (interfering with the signal pathway), but SXF was capable of overcoming such a suppressive effect, resulting in the increased Glc uptake. Insulin was used as a positive control in this study and all results were nearly compatible to those obtained from SXF.

CONCLUSION

The present study suggests that SXF may specifically target the insulin signal pathway, and, in particular, the IR and IRS-1 therein that trigger the subsequent signaling events. As a result, SXF could activate such an impaired signal pathway through high Glc or under a hyperglycemic milieu, thereby ultimately facilitating Glc uptake. This may then account for possible hypoglycemic action of SXF.

Honokiol and magnolol stimulate glucose uptake by activating PI3K-dependent Akt in L6 myotubes

Honokiol and magnolol, ingredients of Magnolia officinalis, which is used in traditional Chinese and Japanese medicines, have been reported to have antioxidant, anticancer, and antiangiogenic effects. Effects of these compounds on glucose metabolism in adipocytes have also been reported. However, their effects on skeletal muscle glucose uptake and the underlying molecular mechanisms are still unknown. Here, we investigated the direct effects and signaling pathways activated by honokiol and magnolol in skeletal muscle cells using L6 myotubes. We found that honokiol and magnolol dose-dependently acutely stimulated glucose uptake without synergistic effects of combined administration in L6 myotubes. Treatment with honokiol and magnolol also stimulated glucose transporter-4 translocation to the cell surface. Honokiol- and magnolol-stimulated glucose uptake was blocked by the phosphatidylinositol-3 kinase inhibitor, wortmannin. Both honokiol and magnolol stimulated Akt phosphorylation, a key element in the insulin signaling pathway, which was completely inhibited by wortmannin. These results suggest that honokiol and magnolol might have beneficial effects on glucose metabolism by activating the insulin signaling pathway.

Fargesin, a component of Flos Magnoliae, stimulates glucose uptake in L6 myotubes

Flos Magnoliae (FM) is a commonly used Chinese medicinal herb for symptomatic relief of allergic rhinitis, sinusitis and headache. Although several FM species have been used as substitutes or adulterants for clinical use, possible differences in their pharmacological actions have not been reported. To confirm the effects of FM on skeletal muscle glucose metabolism, we tested the effects of several compounds isolated from FM on glucose uptake by L6 myotubes. We found that fargesin, a component of FM, dose-dependently stimulated glucose consumption in L6 myotubes, which was accompanied by enhanced glucose transporter (GLUT)-4 translocation to the cell surface. Fargesin-stimulated glucose uptake was blocked by wortmannin, a phosphatidylinositol-3 kinase (PI3 K) inhibitor. Fargesin stimulated Akt phosphorylation, a key component in the insulin signaling pathway, which was completely inhibited by wortmannin. Here, we demonstrated that fargesin, a bioactive component of Flos Magnoliae, increases basal glucose uptake and GLUT4 translocation in L6 myotubes by activating the PI3 K-Akt pathway.

Insulin stimulates fusion, but not tethering, of GLUT4 vesicles in skeletal muscle of HA-GLUT4-GFP transgenic mice

Insulin regulates glucose uptake into fat and muscle by modulating the subcellular distribution of GLUT4 between the cell surface and intracellular compartments. However, quantification of these translocation processes in muscle by classical subcellular fractionation techniques is confounded by contaminating microfibrillar protein; dynamic studies at the molecular level are almost impossible. In this study, we introduce a muscle-specific transgenic mouse model in which HA-GLUT4-GFP is expressed under the control of the MCK promoter. HA-GLUT4-GFP was found to translocate to the plasma membrane and T-tubules after insulin stimulation, thus mimicking endogenous GLUT4. To investigate the dynamics of GLUT4 trafficking in skeletal muscle, we quantified vesicles containing HA-GLUT4-GFP near the sarcolemma and T-tubules and analyzed insulin-stimulated exocytosis at the single vesicle level by total internal reflection fluorescence and confocal microscopy. We found that only 10% of the intracellular GLUT4 pool comprised mobile vesicles, whereas most of the GLUT4 structures remained stationary or tethered at the sarcolemma or T-tubules. In fact, most of the insulin-stimulated exocytosis emanated from pretethered vesicles, whereas the small pool of mobile GLUT4 vesicles was not significantly affected by insulin. Our data strongly suggest that the mobile pool of GLUT4 vesicles is not a major site of insulin action but rather locally distributed. Most likely, pretethered GLUT4 structures are responsible for the initial phase of insulin-stimulated exocytosis.

Regulation of glucose transport by ROCK1 differs from that of ROCK2 and is controlled by actin polymerization

A role of Rho-associated coiled-coil-containing protein kinase (ROCK)1 in regulating whole-body glucose homeostasis has been reported. However, cell-autonomous effects of ROCK1 on insulin-dependent glucose transport in adipocytes and muscle cells have not been elucidated. To determine the specific role of ROCK1 in glucose transport directly, ROCK1 expression in 3T3-L1 adipocytes and L6 myoblasts was biologically modulated. Here, we show that small interfering RNA-mediated ROCK1 depletion decreased insulin-induced glucose transport in adipocytes and myoblasts, whereas adenovirus-mediated ROCK1 expression increased this in a dose-dependent manner, indicating that ROCK1 is permissive for glucose transport. Inhibition of ROCK1 also impaired glucose transporter 4 translocation in 3T3-L1 adipocytes. Importantly, the ED₅₀ of insulin for adipocyte glucose transport was reduced when ROCK1 was expressed, leading to hypersensitivity to insulin. These effects are dependent on actin cytoskeleton remodeling, because inhibitors of actin polymerization significantly decreased ROCK1's effect to promote insulin-stimulated glucose transport. Unlike ROCK2, ROCK1 binding to insulin receptor substrate (IRS)-1 was not detected by immunoprecipitation, although cell fractionation demonstrated both ROCK isoforms localize with IRS-1 in low-density microsomes. Moreover, insulin's ability to increase IRS-1 tyrosine 612 and serine 632/635 phosphorylation was attenuated by ROCK1 suppression. Replacing IRS-1 serine 632/635 with alanine reduced insulin-stimulated phosphatidylinositol 3-kinase activation and glucose transport in 3T3-L1 adipocytes, indicating that phosphorylation of these serine residues of IRS-1, which are substrates of the ROCK2 isoform in vitro, are crucial for maximal stimulation of glucose transport by insulin. Our studies identify ROCK1 as an important positive regulator of insulin action on glucose transport in adipocytes and muscle cells.

Shikonin increases glucose uptake in skeletal muscle cells and improves plasma glucose levels in diabetic Goto-Kakizaki rats

BACKGROUND

There is considerable interest in identifying compounds that can improve glucose homeostasis. Skeletal muscle, due to its large mass, is the principal organ for glucose disposal in the body and we have investigated here if shikonin, a naphthoquinone derived from the Chinese plant Lithospermum erythrorhizon, increases glucose uptake in skeletal muscle cells.

METHODOLOGY/PRINCIPAL FINDINGS

Shikonin increases glucose uptake in L6 skeletal muscle myotubes, but does not phosphorylate Akt, indicating that in skeletal muscle cells its effect is medaited via a pathway distinct from that used for insulin-stimulated uptake. Furthermore we find no evidence for the involvement of AMP-activated protein kinase in shikonin induced glucose uptake. Shikonin increases the intracellular levels of calcium in these cells and this increase is necessary for shikonin-mediated glucose uptake. Furthermore, we found that shikonin stimulated the translocation of GLUT4 from intracellular vesicles to the cell surface in L6 myoblasts. The beneficial effect of shikonin on glucose uptake was investigated in vivo by measuring plasma glucose levels and insulin sensitivity in spontaneously diabetic Goto-Kakizaki rats. Treatment with shikonin (10 mg/kg intraperitoneally) once daily for 4 days significantly decreased plasma glucose levels. In an insulin sensitivity test (s.c. injection of 0.5 U/kg insulin), plasma glucose levels were significantly lower in the shikonin-treated rats. In conclusion, shikonin increases glucose uptake in muscle cells via an insulin-independent pathway dependent on calcium.

CONCLUSIONS/SIGNIFICANCE

Shikonin increases glucose uptake in skeletal muscle cells via an insulin-independent pathway dependent on calcium. The beneficial effects of shikonin on glucose metabolism, both in vitro and in vivo, show that the compound possesses properties that make it of considerable interest for developing novel treatment of type 2 diabetes.

Trans-10, cis-12 conjugated linoleic acid inhibits skeletal muscle differentiation and GLUT4 expression independently from NF-κB activation

SCOPE

The capacity of skeletal muscle to contribute to glucose homeostasis depends on muscular insulin sensitivity. The expression of glucose transporter (GLUT)-4 is increased during myoblast differentiation, a process essential in maintenance of adult muscle. Therefore, processes that affect muscle differentiation may influence insulin dependent glucose homeostasis. Conjugated linoleic acids, and in particular trans-10, cis-12 CLA (t10, c12-CLA), are potent inducers of NF-kB in cultured skeletal myotubes, and NF-kB activation inhibits muscle differentiation. The aims of this study were to evaluate whether CLAs inhibit myogenic differentiation and lower GLUT4 mRNA expression and to address the involvement of NF-kB activation in potential effects of CLA on these processes.

METHODS AND RESULTS

Incubation of C2C12 cells with t10, c12-CLA blocked the formation of myotubes, which was accompanied by reduced expression of the muscle specific genes creatine kinase, myogenin, myosin heavy chain perinatal and myosin heavy chain IIB, as well as decreased GLUT4 mRNA levels. However, genetic blockade of NF-kB was not sufficient to restore reduced myosin heavy chain protein expression following t10, c12-CLA treatment. Surprisingly, in contrast to myotubes, t10, c12-CLA was not able to activate NF-kB transcriptional activity in myoblasts.

CONCLUSION

In conclusion, t10, c12-CLA inhibits myogenic differentiation and GLUT4 expression, independently from NF-kB activation.

Discovery of canagliflozin, a novel C-glucoside with thiophene ring, as sodium-dependent glucose cotransporter 2 inhibitor for the treatment of type 2 diabetes mellitus

We discovered that C-glucosides 4 bearing a heteroaromatic ring formed metabolically more stable inhibitors for sodium-dependent glucose cotransporter 2 (SGLT2) than the O-glucoside, 2 (T-1095). A novel thiophene derivative 4b-3 (canagliflozin) was a highly potent and selective SGLT2 inhibitor and showed pronounced anti-hyperglycemic effects in high-fat diet fed KK (HF-KK) mice.

Naringenin, a citrus flavonoid, increases muscle cell glucose uptake via AMPK

Naringenin, a flavonoid found in high concentrations in grapefruit, has been reported to have antioxidant, antiatherogenic, and anticancer effects. Effects on lipid and glucose metabolism have also been reported. Naringenin is structurally similar to the polyphenol resveratrol, that has been reported to activate the SIRT1 protein deacetylase and to have antidiabetic properties. In the present study we examined the direct effects of naringenin on skeletal muscle glucose uptake and investigated the mechanism involved. Naringenin stimulated glucose uptake in L6 myotubes in a dose- and time-dependent manner. Maximum stimulation was seen with 75 microM naringenin for 2 h (192.8+/-24%, p<0.01), a response comparable to maximum insulin response (190.1+/-13%, p<0.001). Similar to insulin, naringenin did not increase glucose uptake in myoblasts indicating that GLUT4 glucose transporters may be involved in the naringenin-stimulated glucose uptake. In addition, naringenin did not have a significant effect on basal or insulin-stimulated Akt phosphorylation while significantly increased AMPK phosphorylation/activation. Furthermore, silencing of AMPK, using siRNA approach, abolished the naringenin-stimulated glucose uptake. The SIRT1 inhibitors nicotinamide and EX527 did not have an effect on naringenin-stimulated AMPK phosphorylation and glucose uptake. Our data show that naringenin increases glucose uptake by skeletal muscle cells in an AMPK-dependent manner.

Smad5 regulates Akt2 expression and insulin-induced glucose uptake in L6 myotubes

Insulin-induced glucose uptake by skeletal muscle results from Akt2 activation and is severely impaired during insulin resistance. Recently, we and others have demonstrated that BMP9 improves glucose homeostasis in diabetic and non-diabetic rodents. However, the mechanism by which BMP9 modulates insulin action remains unknown. Here we demonstrate that Smad5, a transcription factor activated by BMP9, and Akt2, are upregulated in differentiated L6 myotubes. Smad5, rather than Smad1/8, is downregulated "in vivo" and "in vitro" by dexamethasone. Smad5 knockdown decreased Akt2 expression and serine phosphorylation and insulin-induced glucose uptake, and increased the expression of the lipid phosphatase Ship2. Additionally, binding of Smad5 to Akt2 gene is decreased in dexamethasone-treated rats and increased in L6 myotubes compared to myoblasts. The present study indicates that Smad5 regulates glucose uptake in skeletal muscle by controlling Akt2 expression and phosphorylation. These finding reveals Smad5 as a potential target for the therapeutic of type 2 diabetes.

Potential role of sugar transporters in cancer and their relationship with anticancer therapy

Sugars, primarily glucose and fructose, are the main energy source of cells. Because of their hydrophilic nature, cells use a number of transporter proteins to introduce sugars through their plasma membrane. Cancer cells are well known to display an enhanced sugar uptake and consumption. In fact, sugar transporters are deregulated in cancer cells so they incorporate higher amounts of sugar than normal cells. In this paper, we compile the most significant data available about biochemical and biological properties of sugar transporters in normal tissues and we review the available information about sugar carrier expression in different types of cancer. Moreover, we describe the possible pharmacological interactions between drugs currently used in anticancer therapy and the expression or function of facilitative sugar transporters. Finally, we also go into the insights about the future design of drugs targeted against sugar utilization in cancer cells.

Participation of ERalpha and ERbeta in glucose homeostasis in skeletal muscle and white adipose tissue

Glucose uptake and homeostasis are regulated mainly by skeletal muscle (SM), white adipose tissue (WAT), pancreas, and the liver. Participation of estradiol in this regulation is still under intense investigation. We have demonstrated that, in SM of male mice, expression of the insulin-regulated glucose transporter (GLUT)4 is reduced by estrogen receptor (ER)beta agonists. In the present study, to investigate the relative contributions of ERalpha and ERbeta in glucose homeostasis, we examined the effects of tamoxifen (Tam) on GLUT4 expression in SM and WAT in wild-type (WT) and ER-/- mice. ERbeta-/- mice were characterized by fasting hypoglycemia, increased levels of SM GLUT4, pancreatic islet hypertrophy, and a belated rise in plasma insulin in response to a glucose challenge. ERalpha-/- mice, on the contrary, were hyperglycemic and glucose intolerant, and expression of SM GLUT4 was markedly lower than in WT mice. Tam had no effect on glucose tolerance or insulin sensitivity in WT mice. In ERalpha-/- mice, Tam increased GLUT4 and improved insulin sensitivity. i.e., it behaved as an ERbeta antagonist in SM but had no effect on WAT. In ERbeta-/- mice, Tam did not affect GLUT4 in SM but acted as an ERalpha antagonist in WAT, decreasing GLUT4. Thus, in the interplay between ERalpha and ERbeta, ERbeta-mediated repression of GLUT4 predominates in SM but ERalpha-mediated induction of GLUT4 predominates in WAT. This tissue-specific difference in dominance of one ER over the other is reflected in the ratio of the expression of the two receptors. ERalpha predominates in WAT and ERbeta in SM.

Involvement of the p66Shc protein in glucose transport regulation in skeletal muscle myoblasts

The p66(Shc) protein isoform regulates MAP kinase activity and the actin cytoskeleton turnover, which are both required for normal glucose transport responses. To investigate the role of p66(Shc) in glucose transport regulation in skeletal muscle cells, L6 myoblasts with antisense-mediated reduction (L6/p66(Shc)as) or adenovirus-mediated overexpression (L6/p66(Shc)adv) of the p66(Shc) protein were examined. L6/(Shc)as myoblasts showed constitutive activation of ERK-1/2 and disruption of the actin network, associated with an 11-fold increase in basal glucose transport. GLUT1 and GLUT3 transporter proteins were sevenfold and fourfold more abundant, respectively, and were localized throughout the cytoplasm. Conversely, in L6 myoblasts overexpressing p66(Shc), basal glucose uptake rates were reduced by 30% in parallel with a approximately 50% reduction in total GLUT1 and GLUT3 transporter levels. Inhibition of the increased ERK-1/2 activity with PD98059 in L6/(Shc)as cells had a minimal effect on increased GLUT1 and GLUT3 protein levels, but restored the actin cytoskeleton, and reduced the abnormally high basal glucose uptake by 70%. In conclusion, p66(Shc) appears to regulate the glucose transport system in skeletal muscle myoblasts by controlling, via MAP kinase, the integrity of the actin cytoskeleton and by modulating cellular expression of GLUT1 and GLUT3 transporter proteins via ERK-independent pathways.

The Rab GTPase-activating protein AS160 as a common regulator of insulin- and Galphaq-mediated intracellular GLUT4 vesicle distribution

Akt substrate of 160kDa (AS160) is a Rab GTPase activating protein (GAP) and was recently identified as a component of the insulin signaling pathway of glucose transporter type 4 (GLUT4) translocation. We and others, previously reported that the activation of Galphaq protein-coupled receptors (GalphaqPCRs) also stimulated GLUT4 translocation and glucose uptake in several cell lines. Here, we report that the activation of GalphaqPCRs also promoted phosphorylation of AS160 by the 5'-AMP activated protein kinase (AMPK). The suppression of AS160 phosphorylation by the siRNA mediated AMPKalpha1 subunit knockdown promoted GLUT4 vesicle retention in intracellular compartments. This suppression did not affect the ratio of non-induced cell surface GLUT4 to Galphaq-induced it. Rat 3Y1 cells lacking AS160 did not show insulin-induced GLUT4 translocation. The cells stably expressing GLUT4 revealed GLUT4 vesicles that were mainly localized in the perinuclear region and less frequently on the cell surface. After expression of exogenous AS160, GLUT4 on the cell surface decreased and GLUT4 vesicles were redistributed throughout the cytoplasm. Although PMA-induced or sodium fluoride-induced GLUT4 translocation was significantly increased in these cells, insulin did not affect GLUT4 translocation. These results suggest that AS160 is a common regulator of insulin- and GalphaqPCR activation-mediated GLUT4 distribution in the cells.

Expression of rainbow trout glucose transporters GLUT1 and GLUT4 during in vitro muscle cell differentiation and regulation by insulin and IGF-I

Insulin is an important factor for the maintenance of glucose homeostasis, enhancing glucose uptake in its target tissues in a process that has been conserved between fish and mammals. In fish skeletal muscle cells, like in mammals, insulin promotes GLUT4 translocation to the plasma membrane and, consequently, glucose uptake, but its role regulating the expression of glucose transporters in vitro has not been demonstrated to date. Thus, we investigated the expression of GLUT4 and GLUT1 throughout skeletal muscle cell differentiation and their regulation by insulin and IGF-I using a primary culture of trout muscle cells. GLUT4 expression gradually increased during the muscle cell differentiation process, whereas GLUT1 expression remained fairly constant. Insulin and IGF-I similarly increased the mRNA levels of GLUT4 in myoblasts and myotubes. On the other hand, IGF-I appeared to be more potent than insulin in stimulating GLUT1 expression, particularly at the myoblast stage. Therefore, this work provides the first demonstration in nonmammalian vertebrates that insulin and IGF-I may act directly on trout muscle cells to regulate the expression of GLUT4 and GLUT1.

Maternal protein restriction during early lactation induces GLUT4 translocation and mTOR/Akt activation in adipocytes of adult rats

Epidemiological and experimental studies have demonstrated that early postnatal nutrition has been associated with long-term effects on glucose homeostasis in adulthood. Recently, our group demonstrated that undernutrition during early lactation affects the expression and activation of key proteins of the insulin signaling cascade in rat skeletal muscle during postnatal development. To elucidate the molecular mechanisms by which undernutrition during early life leads to changes in insulin sensitivity in peripheral tissues, we investigated the insulin signaling in adipose tissue. Adipocytes were isolated from epididymal fat pads of adult male rats that were the offspring of dams fed either a normal or a protein-free diet during the first 10 days of lactation. The cells were incubated with 100 nM insulin before the assays for immunoblotting analysis, 2-deoxyglucose uptake, immunocytochemistry for GLUT4, and/or actin filaments. Following insulin stimulation, adipocytes isolated from undernourished rats presented reduced tyrosine phosphorylation of IR and IRS-1 and increased basal phosphorylation of IRS-2, Akt, and mTOR compared with controls. Basal glucose uptake was increased in adipocytes from the undernourished group, and the treatment with LY294002 induced only a partial inhibition both in basal and in insulin-stimulated glucose uptake, suggesting an involvement of phosphoinositide 3-kinase activity. These alterations were accompanied by higher GLUT4 content in the plasma membrane and alterations in the actin cytoskeleton dynamics. These data suggest that early postnatal undernutrition impairs insulin sensitivity in adulthood by promoting changes in critical steps of insulin signaling in adipose tissue, which may contribute to permanent changes in glucose homeostasis.

Histone code modifications repress glucose transporter 4 expression in the intrauterine growth-restricted offspring

We examined transcriptional and epigenetic mechanism(s) behind diminished skeletal muscle GLUT4 mRNA in intrauterine growth-restricted (IUGR) female rat offspring. An increase in MEF2D (inhibitor) with a decline in MEF2A (activator) and MyoD (co-activator) binding to the glut4 promoter in IUGR versus control was observed. The functional role of MEF2/MyoD-binding sites and neighboring three CpG clusters in glut4 gene transcription was confirmed in C2C12 muscle cells. No differential methylation of these three and other CpG clusters in the glut4 promoter occurred. DNA methyltransferase 1 (DNMT1) in postnatal, DNMT3a, and DNMT3b in adult was differentially recruited with increased MeCP2 (methyl CpG-binding protein) concentrations to bind the IUGR glut4 gene. Covalent modifications of the histone (H) code consisted of H3.K14 de-acetylation by recruitment of histone deacetylase (HDAC) 1 and enhanced association of HDAC4 enzymes. This set the stage for Suv39H1 methylase-mediated di-methylation of H3.K9 and increased recruitment of heterochromatin protein 1alpha, which partially inactivates postnatal and adult IUGR glut4 gene transcription. Further increased interactions in the adult IUGR between DNMT3a/DNMT3b and HDAC1 and MEF2D and HDAC1/HDAC4 and decreased association between MyoD and MEF2A existed. We conclude that epigenetic mechanisms consisting of histone code modifications repress skeletal muscle glut4 transcription in the postnatal period and persist in the adult female IUGR offspring.

Functional role of sortilin in myogenesis and development of insulin-responsive glucose transport system in C2C12 myocytes

Sortilin has been implicated in the formation of insulin-responsive GLUT4 storage vesicles in adipocytes by regulating sorting events between the trans-Golgi-network and endosomes. We herein show that sortilin serves as a potent myogenic differentiation stimulator for C2C12 myocytes by cooperatively functioning with p75NTR, which subsequently further contributes to development of the insulin-responsive glucose transport system in C2C12 myotubes. Sortilin expression was up-regulated upon C2C12 differentiation, and overexpression of sortilin in C2C12 cells significantly stimulated myogenic differentiation, a response that was completely abolished by either anti-p75NTR- or anti-nerve growth factor (NGF)-neutralizing antibodies. Importantly, small interference RNA-mediated suppression of endogenous sortilin significantly inhibited C2C12 differentiation, indicating the physiological significance of sortilin expression in the process of myogenesis. Although sortilin overexpression in C2C12 myotubes improved insulin-induced 2-deoxyglucose uptake, as previously reported, this effect apparently resulted from a decrease in the cellular content of GLUT1 and an increase in GLUT4 via differentiation-dependent alterations at both the gene transcriptional and the post-translational level. In addition, cellular contents of Ubc9 and SUMO-modified proteins appeared to be increased by sortilin overexpression. Taken together, these data demonstrate that sortilin is involved not only in development of the insulin-responsive glucose transport system in myocytes, but is also directly involved in muscle differentiation via modulation of proNGF-p75NTR.

Hepatocyte growth factor is a novel stimulator of glucose uptake and metabolism in skeletal muscle cells

Skeletal muscle plays a major role in glucose and lipid metabolism. Active hepatocyte growth factor (HGF) is present in the extracellular matrix in skeletal muscle. However, the effects of HGF on glucose and lipid metabolism in skeletal muscle are completely unknown. We therefore examined the effects of HGF on deoxyglucose uptake (DOGU), glucose utilization, and fatty acid oxidation (FAO) in skeletal muscle cells. HGF significantly enhanced DOGU in mouse soleus muscles in vitro. Furthermore, HGF significantly increased: (i) DOGU in a time- and dose-dependent manner; (ii) glucose utilization; and (iii) plasma membrane expression of Glut-1 and Glut-4 in the rat skeletal muscle model of L6 myotubes. HGF-mediated effect on DOGU was dependent on the activation of phosphatidylinositol 3-kinase signaling pathway. On the other hand, HGF markedly and significantly decreased FAO in L6 myotubes without affecting the activities of carnitine palmitoyltransferase I and II. Collectively, these results indicate that HGF is a potent activator of glucose transport and metabolism and also a strong inhibitor of FAO in rodent myotubes. HGF, through its ability to stimulate glucose transport and metabolism and to impair FAO, may participate in the regulation of glucose disposal in skeletal muscle.

Angiotensin II-induced skeletal muscle insulin resistance mediated by NF-kappaB activation via NADPH oxidase

Reduced insulin sensitivity is a key factor in the pathogenesis of type 2 diabetes and hypertension. Skeletal muscle insulin resistance is particularly important for its major role in insulin-mediated glucose disposal. Angiotensin II (ANG II) is integral in regulating blood pressure and plays a role in the pathogenesis of hypertension. In addition, we have documented that ANG II-induced skeletal muscle insulin resistance is associated with generation of reactive oxygen species (ROS). However, the linkage between ROS and insulin resistance in skeletal muscle remains unclear. To explore potential mechanisms, we employed the transgenic TG(mRen2)27 (Ren-2) hypertensive rat, which harbors the mouse renin transgene and exhibits elevated tissue ANG II levels, and skeletal muscle cell culture. Compared with Sprague-Dawley normotensive control rats, Ren-2 skeletal muscle exhibited significantly increased oxidative stress, NF-kappaB activation, and TNF-alpha expression, which were attenuated by in vivo treatment with an angiotensin type 1 receptor blocker (valsartan) or SOD/catalase mimetic (tempol). Moreover, ANG II treatment of L6 myotubes induced NF-kappaB activation and TNF-alpha production and decreased insulin-stimulated Akt activation and GLUT-4 glucose transporter translocation to plasma membranes. These effects were markedly diminished by treatment of myotubes with valsartan, the antioxidant N-acetylcysteine, NADPH oxidase-inhibiting peptide (gp91 ds-tat), or NF-kappaB inhibitor (MG-132). Similarly, NF-kappaB p65 small interfering RNA reduced NF-kappaB p65 subunit expression and nuclear translocation and TNF-alpha production but improved insulin-stimulated phosphorylation (Ser(473)) of Akt and translocation of GLUT-4. These findings suggest that NF-kappaB plays an important role in ANG II/ROS-induced skeletal muscle insulin resistance.

Insulin sensitivity and inhibition by forskolin, dipyridamole and pentobarbital of glucose transport in three L6 muscle cell lines

L6 skeletal muscle myoblasts stably overexpressing glucose transporter GLUT1 or GLUT4 with exofacial myc-epitope tags were characterized for their response to insulin. In clonally selected cultures, 2-deoxyglucose uptake into L6-GLUT1myc myoblasts and myotubes was linear within the time of study. In L6-GLUT1myc and L6-GLUT4myc myoblasts, 100 nmol/L insulin treatment increased the GLUT1 content of the plasma membrane by 1.58+/-0.01 fold and the GLUT4 content 1.96+/-0.11 fold, as well as the 2-deoxyglucose uptake 1.53+/-0.09 and 1.86+/-0.17 fold respectively, all by a wortmannin-inhibitable manner. The phosphorylation of Akt in these two cell lines was increased by insulin. L6-GLUT1myc myoblasts showed a dose-dependent stimulation of glucose uptake by insulin, with unaltered sensitivity and maximal responsiveness compared with wild type cells. By contrast, the improved insulin responsiveness and sensitivity of glucose uptake were observed in L6-GLUT4myc myoblasts. Earlier studies indicated that forskolin might affect insulin-stimulated GLUT4 translocation. A 65% decrease of insulin-stimulated 2-deoxyglucose uptake in GLUT4myc cells was not due to an effect on GLUT4 mobilization to the plasma membrane, but instead on direct inhibition of GLUT4. Forskolin and dipyridamole are more potent inhibitors of GLUT4 than GLUT1. Alternatively, pentobarbital inhibits GLUT1 more than GLUT4. The use of these inhibitors confirmed that the overexpressed GLUT1 or GLUT4 are the major functional glucose transporters in unstimulated and insulin-stimulated L6 myoblasts. Therefore, L6-GLUT1myc and L6-GLUT4myc cells provide a platform to screen compounds that may have differential effects on GLUT isoform activity or may influence GLUT isoform mobilization to the cell surface of muscle cells.

The SUMO conjugating enzyme Ubc9 is a regulator of GLUT4 turnover and targeting to the insulin-responsive storage compartment in 3T3-L1 adipocytes

The small ubiquitin-related modifier (SUMO) conjugating enzyme Ubc9 has been shown to upregulate GLUT4 in L6 myoblast cells, although the mechanism of action has remained undefined. Here we investigated the physiological significance of Ubc9 in GLUT4 turnover and subcellular targeting by adenovirus vector-mediated overexpression and by small interfering RNA (siRNA)-mediated gene silencing of Ubc9 in 3T3-L1 adipocytes. Overexpression of Ubc9 resulted in an inhibition of GLUT4 degradation and promoted its targeting to the unique insulin-responsive GLUT4 storage compartment (GSC), leading to an increase in GLUT4 amount and insulin-responsive glucose transport in 3T3-L1 adipocytes. Overexpression of Ubc9 also antagonized GLUT4 downregulation and its selective loss in GSC induced by long-term insulin stimulation. By contrast, siRNA-mediated depletion of Ubc9 accelerated GLUT4 degradation and decreased the amount of the transporter, concurrent with its selective loss in GSC, which resulted in attenuated insulin-responsive glucose transport. Intriguingly, overexpression of the catalytically inactive mutant Ubc9-C93A produced effects indistinguishable from those with wild-type Ubc9, suggesting that Ubc9 regulates GLUT4 turnover and targeting to GSC by a mechanism independent of its catalytic activity. Thus, Ubc9 is a pivotal regulator of the insulin sensitivity of glucose transport in adipocytes.

Glucose, dexamethasone, and the unfolded protein response regulate TRB3 mRNA expression in 3T3-L1 adipocytes and L6 myotubes

Tribbles 3 (TRB3) is a recently recognized atypical inactive kinase that negatively regulates Akt activity in hepatocytes, resulting in insulin resistance. Recent reports link TRB3 to nutrient sensing and regulation of cell survival under stressful conditions. We studied the regulation of TRB3 by glucose, insulin, dexamethasone (Dex), and the unfolded protein response (UPR) in 3T3-L1 adipocytes and in L6 myotubes. In 3T3-L1 adipocytes, incubation in high glucose with insulin did not increase TRB3 mRNA expression. Rather, TRB3 mRNA increased fourfold with glucose deprivation and two- to threefold after incubation with tunicamcyin (an inducer of the UPR). Incubation of cells in no glucose or in tunicamcyin stimulated the expression of CCAAT/enhancer-binding protein homologous protein. In L6 myotubes, absent or low glucose induced TRB3 mRNA expression by six- and twofold, respectively. The addition of Dex to 5 mM glucose increased TRB3 mRNA expression twofold in 3T3-L1 adipocytes but decreased it 16% in L6 cells. In conclusion, TRB3 is not the mediator of high glucose or glucocorticoid-induced insulin resistance in 3T3-L1 adipocytes or L6 myotubes. TRB3 is induced by glucose deprivation in both cell types as a part of the UPR, where it may be involved in regulation of cell survival in response to glucose depletion.