Regulated membrane trafficking of the insulin-responsive glucose transporter 4 in adipocytes

Ginsenoside Rg1 inhibits dietary-induced obesity and improves obesity-related glucose metabolic disorders

Obesity and its consequent type 2 diabetes are significant threats to global health. Emerging evidence indicates that ginsenosides from ginseng (Panax ginseng) have anti-diabetic activity. We hypothesized that ginsenosides Rg1 could suppress dietary-induced obesity and improve obesity-related glucose metabolic disorders. Our results showed that ginsenoside Rg1 attenuated dietary-induced body weight gain and fat accumulation in white adipocyte tissue of mice fed a high-fat diet. Furthermore, we found that ginsenosides Rg1 not only decreased fasting glucose concentration and the 2-h postprandial glucose concentration, but also improved insulin resistance and glucose intolerance in those mice. Ginsenoside Rg1 also activated the AMPK pathway in vitro and in vivo and increased plasma membrane translocation of GLUT4 in C2C12 skeletal muscle cells. In conclusion, our observations suggested that ginsenoside Rg1 inhibited dietary-induced obesity and improved obesity-related insulin resistance and glucose intolerance by activation of the AMPK pathway.

Glutamine Enhances the Hypoglycemic Effect of Insulin in L6 Cells via Phosphatidylinositol-3-Kinase (PI3K)/Protein Kinase B (AKT)/Glucose Transporter 4 (GLUT4) Signaling Pathway

Background

Diabetes mellitus (DM) is characterized by a decreased blood level of glutamine (Gln), which may contribute to the disturbance in the effect of insulin on skeletal muscle. Therefore, it is crucial to study how to improve the effect of insulin on skeletal muscle by increasing Gln. In the present study, we investigated the effect of Gln on the hypoglycemic action of insulin in skeletal muscle L6 cells at high glucose levels through the insulin signaling pathway and glycogen synthesis pathway.

Material/Methods

The L6 cells were cultured in and stimulated by Gln and insulin. The glutamine analogue, L-Gamma-Glutamyl-p-nitroanilide (GPNA), was used for verifying the effect of Gln. The expression of insulin signaling molecules, including phosphatidylinositol-3-kinase (PI3K), 3-phosphoinositide-dependent protein kinase-1 (PDK1), protein kinase B (AKT), protein kinase C zeta (PKCζ), and glucose transporter 4 (GLUT4), were detected by real-time PCR and Western blot analysis, GLUT4 translocation was observed by immunofluorescence staining, glycogen synthase kinase (GSK) was analyzed by Western blotting, and glucose uptake was measured by glucose oxidase method (GOD).

Results

The results demonstrated that Gln combined with insulin remarkably up-regulated PI3K and PDK1 and also increased AKT and PKCζ phosphorylation. The present study shows that Gln enhanced the impact of insulin on GLUT4 and its translocation. The results of glucose uptake and GSK phosphorylation further confirmed the hypoglycemic effect of Gln accompanied with insulin. The hypoglycemic effect of Gln was reversed by GPNA.

Conclusions

These findings suggest that Gln enhances the hypoglycemic role of insulin through the PI3K/AKT/GLUT4 signaling pathway and glycogen synthesis pathway.

Mechanism of action of camphoryl-benzene sulfonamide derivative on glucose uptake in adipose tissue

The aim of the present study was to investigate the mechanism of action of a sulfonamide derivative on glucose uptake in adipose tissue, as well as to characterize the effects of this compound on intestinal disaccharidases and advanced glycation end-products (AGEs) formation. Camphoryl-benzene sulfonamide (CS) was able to stimulate glucose uptake in isolated adipocytes, adipose tissue, and in soleus muscle. The stimulatory effect of the compound (10 μM) on glucose uptake on adipose tissue was blocked by diazoxide, wortmannin, U73122, colchicine, and N-ethylmaleimide. On the other hand, the effects of CS were not blocked by glibenclamide, an inhibitor of the K⁺ -ATP channel, or even by the inhibitor of protein p38 MAPK, SB 203580. In vivo, this compound reduced intestinal disaccharidase activity, while, in vitro, CS reduced the formation of AGEs at 7, 14, and 28 days of incubation. The stimulatory effect of CS on glucose uptake requires the activation of the K⁺ -ATP channel, translocation, and fusion of GLUT4 vesicles to the plasma membrane on adipocytes for glucose homeostasis. In addition, the inhibition of disaccharidase activity contributes to the glucose homeostasis in a short-term as well as the remarkable reduction in AGE formation indicates that the CS may prevent of complications of late diabetes.

Magnesium upregulates insulin receptor and glucose transporter-4 in streptozotocin-nicotinamide-induced type-2 diabetic rats

Objective. We investigated the effects of magnesium supplementation on glucose tolerance, insulin sensitivity, oxidative stress as well as the concentration of insulin receptor and glucose transporter-4 in streptozotocin-nicotinamide induced type-2 diabetic (T2D) rats. Methods. Rats were divided into four groups designated as: 1) control (CTR); 2) diabetic untreated (DU); 3) diabetic treated with 1 mg of Mg/kg diet (Mg1-D); and 4) diabetic treated with 2 mg of Mg/kg diet (Mg2-D). T2D was induced with a single intraperitoneal (i.p.) injection of freshly prepared streptozotocin (55 mg/kg) aft er an initial i.p. injection of nicotinamide (120 mg/kg). Glucose tolerance, insulin sensitivity, lipid profile, malondialdehyde (MAD) and glutathione content, insulin receptors (INSR) and glucose transporter-4 (GLUT4), fasting insulin and glucose levels were measured, and insulin resistance index was calculated using the homeostatic model assessment of insulin resistance (HOMA-IR). Results. Magnesium supplementation improved glucose tolerance and lowered blood glucose levels almost to the normal range. We also recorded a noticeable increase in insulin sensitivity in Mg-D groups when compared with DU rats. Lipid perturbations associated T2D were significantly attenuated by magnesium supplementation. Fasting glucose level was comparable to control values in the Mg-D groups while the HOMA-IR index was significantly lower compared with the DU rats. Magnesium reduced MDA but increased glutathione concentrations compared with DU group. Moreover, INSR and GLUT4 levels were elevated following magnesium supplementation in T2D rats. Conclusion. These findings demonstrate that magnesium may mediate effective metabolic control by stimulating the antioxidant defense, and increased levels of INSR and GLUT4 in diabetic rats.

Action of Phytochemicals on Insulin Signaling Pathways Accelerating Glucose Transporter (GLUT4) Protein Translocation

Diabetes is associated with obesity, generally accompanied by a chronic state of oxidative stress and redox imbalances which are implicated in the progression of micro- and macro-complications like heart disease, stroke, dementia, cancer, kidney failure and blindness. All these complications rise primarily due to consistent high blood glucose levels. Insulin and glucagon help to maintain the homeostasis of glucose and lipids through signaling cascades. Pancreatic hormones stimulate translocation of the glucose transporter isoform 4 (GLUT4) from an intracellular location to the cell surface and facilitate the rapid insulin-dependent storage of glucose in muscle and fat cells. Malfunction in glucose uptake mechanisms, primarily contribute to insulin resistance in type 2 diabetes. Plant secondary metabolites, commonly known as phytochemicals, are reported to have great benefits in the management of type 2 diabetes. The role of phytochemicals and their action on insulin signaling pathways through stimulation of GLUT4 translocation is crucial to understand the pathogenesis of this disease in the management process. This review will summarize the effects of phytochemicals and their action on insulin signaling pathways accelerating GLUT4 translocation based on the current literature.

Glucose Transporters in Diabetic Kidney Disease-Friends or Foes?

Diabetic kidney disease (DKD) is a major microvascular complication of diabetes and a common cause of end-stage renal disease worldwide. DKD manifests as an increased urinary protein excretion (albuminuria). Multiple studies have shown that insulin resistance correlates with the development of albuminuria in non-diabetic and diabetic patients. There is also accumulating evidence that glomerular epithelial cells or podocytes are insulin sensitive and that insulin signaling in podocytes is essential for maintaining normal kidney function. At the cellular level, the mechanisms leading to the development of insulin resistance include mutations in the insulin receptor gene, impairments in the phosphoinositide 3-kinase (PI3K)/AKT signaling pathway, or perturbations in the trafficking of glucose transporters (GLUTs), which mediate the uptake of glucose into cells. Podocytes express several GLUTs, including GLUT1, GLUT2, GLUT3, GLUT4, and GLUT8. Of these, the most studied ones are GLUT1 and GLUT4, both shown to be insulin responsive in podocytes. In the basal state, GLUT4 is preferentially located in perinuclear and cytosolic vesicular structures and to a lesser extent at the plasma membrane. After insulin stimulation, GLUT4 is sorted into GLUT4-containing vesicles (GCVs) that translocate to the plasma membrane. GCV trafficking consists of several steps, including approaching of the GCVs to the plasma membrane, tethering, and docking, after which the lipid bilayers of the GCVs and the plasma membrane fuse, delivering GLUT4 to the cell surface for glucose uptake into the cell. Studies have revealed novel molecular regulators of the GLUT trafficking in podocytes and unraveled unexpected roles for GLUT1 and GLUT4 in the development of DKD, summarized in this review. These findings pave the way for better understanding of the mechanistic pathways associated with the development and progression of DKD and aid in the development of new treatments for this devastating disease.

Testosterone insulin-like effects: an in vitro study on the short-term metabolic effects of testosterone in human skeletal muscle cells

PURPOSE

Testosterone by promoting different metabolic pathways contributes to short-term homeostasis of skeletal muscle, the largest insulin-sensitive tissue and the primary site for insulin-stimulated glucose utilization. Despite evidences indicate a close relationship between testosterone and glucose metabolism, the molecular mechanisms responsible for a possible testosterone-mediated insulin-like effects on skeletal muscle are still unknown.

METHODS

Here we used undifferentiated proliferating or differentiated human fetal skeletal muscle cells (Hfsmc) to investigate the short-term effects of testosterone on the insulin-mediated biomolecular metabolic machinery. GLUT4 cell expression, localization and the phosphorylation/activation of AKT, ERK, mTOR and GSK3β insulin-related pathways at different time points after treatment with testosterone were analyzed.

RESULTS

Independently from cells differentiation status, testosterone, with an insulin-like effect, induced Glut4-mRNA expression, GLUT4 protein translocation to the cytoplasmic membrane, while no effect was observed on GLUT4 protein expression levels. Furthermore, testosterone treatment modulated the insulin-dependent signal transduction pathways inducing a rapid and persistent activation of AKT, ERK and mTOR, and a transient inhibition of GSK3β. T-related effects were shown to be androgen receptor dependent.

CONCLUSION

All together our data indicate that testosterone through the activation of non-genomic pathways, participates in skeletal muscle glucose metabolism by inducing insulin-related effects.

Mulberry branch bark powder significantly improves hyperglycemia and regulates insulin secretion in type II diabetic mice

This experiment, based on the previous study on R. mori, introduces whole mulberry branch powder into the diet to treat diabetic mice. Mulberry branch bark powder (MBBP) was administered orally to streptozotocin (STZ)-induced type II diabetic (T2D) mice to investigate hypoglycemic effects. After a 4-week period of diet consumption containing 5%, 10% and 20% MBBP, the fasting blood glucose, body weight and the related western blotting were measured, pathologic and immunohistochemical were observed. The 20% MBBP group showed a significant reduction in hyperglycemia and hyperinsulinemia; fasting blood glucose and insulin decreased from 25.0 to 14.8 mmol/L and 26.5 to 16.0 mU/L, respectively. Pathologic and immunohistochemical observation showed that MBBP administration lead to the repair of pancreas cells and restoration of insulin secretion. Dietary MBBP was associated with the decrease in the contents of 3, 4-methylenedioxeamphetamine, 8-OHdG, aspartate aminotransferase, and alanine aminotransferase, and the increase in antioxidative ability and glucose tolerance. Western blotting (WB) analysis suggested that MBBP decreased the TNF-α levels, thus relieving inflammation and improving liver function. It also led to the downregulation of apoptosis factor expression. WB also confirmed that MBBP enhanced the gene expression of the key enzymes: insulin receptor, insulin receptor substrate, p-AKT, GSK3β, glycogen synthase, G6Pase and phosphoenolpyruvate carboxykinase, which are related to glucose metabolism in the liver, and increase the expression of the genes PDX-1, GLUT2, MafA, and glucokinase, related to insulin secretion. Thus, oral administration of MBBP regulated insulin secretion and effectively maintained normal levels of glucose metabolism in mice, which may be done by improving the antioxidant capacity and activating insulin signaling with T2D..

Enhancement of Glucose Uptake by Meso-Dihydroguaiaretic Acid through GLUT4 Up-Regulation in 3T3-L1 Adipocytes

Type 2 diabetes is characterized by insulin resistance, which leads to increased blood glucose levels. Adipocytes are involved in the development of insulin resistance, resulting from the dysfunction of the insulin signaling pathway. In this study, we investigated whether meso-dihydroguaiaretic acid (MDGA) may modulate glucose uptake in adipocytes, and examined its mechanism of action. MDGA enhanced adipogenesis through up-regulation of peroxisome proliferator-activated receptor γ and CCAAT/enhancer-binding protein α in 3T3-L1 adipocytes partially differentiated with sub-optimal concentrations of insulin. MDGA also increased glucose uptake by stimulating expression and translocation of glucose transporter 4 (GLUT4) in adipocytes. These results suggest that MDGA may increase GLUT4 expression and its translocation by promoting insulin sensitivity, leading to enhanced glucose uptake.

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.

Phosphorylation of the exocyst protein Exo84 by TBK1 promotes insulin-stimulated GLUT4 trafficking

Insulin stimulates glucose uptake through the translocation of the glucose transporter GLUT4 to the plasma membrane. The exocyst complex tethers GLUT4-containing vesicles to the plasma membrane, a process that requires the binding of the G protein (heterotrimeric guanine nucleotide-binding protein) RalA to the exocyst complex. We report that upon activation of RalA, the protein kinase TBK1 phosphorylated the exocyst subunit Exo84. Knockdown of TBK1 blocked insulin-stimulated glucose uptake and GLUT4 translocation; knockout of TBK1 in adipocytes blocked insulin-stimulated glucose uptake; and ectopic overexpression of a kinase-inactive mutant of TBK1 reduced insulin-stimulated glucose uptake in 3T3-L1 adipocytes. The phosphorylation of Exo84 by TBK1 reduced its affinity for RalA and enabled its release from the exocyst. Overexpression of a kinase-inactive mutant of TBK1 blocked the dissociation of the TBK1/RalA/exocyst complex, and treatment of 3T3-L1 adipocytes with specific inhibitors of TBK1 reduced the rate of complex dissociation. Introduction of phosphorylation-mimicking or nonphosphorylatable mutant forms of Exo84 blocked insulin-stimulated GLUT4 translocation. Thus, these data indicate that TBK1 controls GLUT4 vesicle engagement and disengagement from the exocyst, suggesting that exocyst components not only constitute a tethering complex for the GLUT4 vesicle but also act as "gatekeepers" controlling vesicle fusion at the plasma membrane.

A multiscale study of the role of dynamin in the regulation of glucose uptake

Cells- and organisms-on-a-chip strategies were used to highlight the role of the molecular motor dynamin in regulating the translocation of specific glucose transporters.

Dodeca-2(E),4(E)-dienoic acid isobutylamide enhances glucose uptake in 3T3-L1 cells via activation of Akt signaling

Dodeca-2(E),4(E)-dienoic acid isobutylamide (DDI), an alkamide derived from the plant Echinacea purpurea, promotes adipocyte differentiation and activates peroxisome proliferator-activated receptor γ, which is associated with enhanced insulin sensitivity. In the present study, we investigated whether DDI may increase glucose uptake through activation of the insulin signaling pathway in 3T3-L1 adipocytes. DDI increased insulin-stimulated glucose uptake, and expression and translocation of glucose transporter 4 in adipocytes treated with sub-optimal levels of insulin. Additionally, DDI enhanced Akt phosphorylation, whereas phosphoinositide 3-kinase/Akt inhibitors suppressed DDI-induced glucose uptake. These results suggest that DDI may improve insulin sensitivity through the activation of Akt signaling, which leads to enhanced glucose uptake.

Hypoglycemic effect of D-chiro-inositol in type 2 diabetes mellitus rats through the PI3K/Akt signaling pathway

In this investigation, a model of type 2 diabetes mellitus (T2DM) was used on Sprague-Dawley (SD) rats to clarify more details of the mechanism in the therapy of T2DM. D-chiro-inositol (DCI) was administrated to the diabetic rats as two doses [30, 60 mg/(kg·body weight·day)]. The biochemical indices revealed that DCI had a positive effect on hypoglycemic activity and promoted the glycogen synthesis. The rats in DCI high-dosage group had a blood glucose reduction rate of 21.5% after 5 weeks of treatment, and had insulin content in serum about 15.3 ± 2.37 mIU/L which was significantly decreased than diabetes control group. Real-time polymerase chain reaction (RT-PCR) results revealed that DCI gave a positive regulation on glycogen synthase (GS) and protein glucose transporter-4 (Glut4). Western blotting suggested that DCI could up-regulated the expression of the phosphatidylinositol-3-kinase (PI3K) p85, PI3Kp110, GS as well as the phosphorylation of protein kinase B (Akt) both in the liver and the skeletal muscle. The results also revealed that DCI enhanced the Glut4 expression on skeletal muscle. Above all, DCI played a positive role in regulating insulin-mediated glucose uptake through the PI3K/Akt signaling pathway in T2DM rats.

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.

Signaling pathways involved in cardiac energy metabolism

Various signaling pathways coordinate energy metabolism and contractile function in the heart. Myocardial uptake of long-chain fatty acids largely occurs by facilitated diffusion, involving the membrane-associated protein, CD36. Glucose uptake, the rate-limiting step in glucose utilization, is mediated predominantly by the glucose transporter protein, GLUT4. Insulin and contraction-mediated AMPK signaling each are implicated in tightly regulating these myocardial 'gate-keepers' of energy balance, that is, CD36 and GLUT4. The insulin and AMPK signaling cascades are complex and their cross-talk is only beginning to be understood. Moreover, transcriptional regulation of the CD36 and GLUT4 is significantly understudied. This review focuses on recent advances on the role of these signaling pathways and transcription factors involved in the regulation of CD36 and GLUT4.

Hypoglycaemic effects of functional tri-peptides from silk in differentiated adipocytes and streptozotocin-induced diabetic mice

BACKGROUND

In this study, the tri-peptides Gly-Glu-Tyr (GEY) and Gly-Tyr-Gly (GYG), identified previously as active compounds from the silk peptide E5K6, significantly stimulated basal and insulin-mediated glucose uptake by 3T3-L1 fibroblasts in a dose-dependent manner.

RESULTS

Synthetic GEY and GYG peptides at a concentration of 500 µmol L(-1) significantly increased glucose transporter type 4 expression by 157% and 239%, respectively. Differentiation of 3T3-L1 cells into adipocytes leads to accumulation of intracellular fat droplets, and GEY and GYG at a concentration of 250 µmol L(-1) suppressed this effect by 72% and 75%, respectively. GYG improved glucose tolerance in steptozotocin (STZ)-induced diabetic mice in a dose-dependent manner.

CONCLUSION

These results suggest that GYG isolated from E5K6 has anti-diabetic potential and silk waste products containing bioactive peptides could be used to the developments of treatments to lower blood glucose.

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

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

Bifidobacterium species lower serum glucose, increase expressions of insulin signaling proteins, and improve adipokine profile in diabetic mice

This study, using C57BL/6J mice with streptozotocin (STZ)-induced diabetes, aimed to determine whether Bifidobacterium species (spp.) both induces the expressions of proteins in the insulin signaling pathway and enhances the expressions of certain adipocytokines. The protein expressions of IκB kinase alpha (IKKα), IκB kinase beta (IKKβ), nuclear factor-kappaB inhibitor alpha (IκBα), and the mitogen-activated protein kinase (MAPK) pathway were also investigated. Oral administration of Bifidobacterium spp. reduced blood glucose levels significantly and increased the protein expressions of insulin receptor beta, insulin receptor substrate 1, protein kinase B (Akt/PKB), IKKα, and IκBα. Extracellular-signal-regulated kinase 2 (ERK2) showed increased expression. Bifidobacterium spp. also induced the adiponectin expression and decreased both macrophage chemoattractant protein-1 (MCP-1) and interleukin-6 (IL-6) expression. In addition, IKKβ, c-Jun NH2-terminal kinase (JNK) and p38 MAP kinase expressions showed no significant changes in both groups. In conclusion, Bifidobacterium spp. may be the promising bacteria for treating diabetes.

Upregulation of miR-497 induces hepatic insulin resistance in E3 rats with HFD-MetS by targeting insulin receptor

OBJECTIVE

The study aims to find regulatory microRNA(s) responsible for down-regulated insulin receptor (InsR) in the liver of HFD-MetS E3 rats with insulin resistance.

METHODS

Firstly, hepatic insulin resistance in HFD-MetS E3 rats was evaluated by RT-qPCR, western blotting, immunohistochemistry and PAS staining. Secondly, the candidate miRNAs targeting rat InsR were predicted through online softwares and detected in the liver of HFD-MetS E3 rats with insulin resistance. Then, the expression of InsR, phosphorylated IRS-1 (pIRS-1) at Tyr632, phosphorylated AKTs (pAKTs) at Ser473 and Thr308, phosphorylated GSK-3β (p GSK-3β) at Ser9, phosphorylated GS (pGS) at Ser641 and the glycogen content were detected in CBRH-7919 cells treated with 100 nM insulin for different time periods by western blotting or PAS staining respectively, after transient transfection with miR-497 mimics or inhibitors for 24 h. Lastly, the relation between miR-497 and InsR was further determined using dual luciferase reporter assay.

RESULTS

Elevated miR-497 was negatively related with down-regulated InsR in the liver of HFD-MetS E3 rats with insulin resistance. Comparing with the mNC group, glycogen content and the expression of InsR, pIRS-1 (Tyr632), pAKTs (Ser473 and Thr308) and pGSK-3β (Ser9) decreased significantly in CBRH-7919 cells, while pGS (Ser641) increased significantly, after transient transfection with miR-497 mimics for 24 h and treatment with 100 nM insulin for corresponding time periods, counter to those results in CBRH-7919 cells after similar procedures with miR-497 inhibitors and insulin. In addition, dual luciferase reporter assay further confirmed that miR-497 can bind to the 3'UTR of rat InsR.

CONCLUSION

Insulin receptor is the target gene of miR-497, and elevated miR-497 might induce hepatic insulin resistance in HFD-MetS E3 Rats through inhibiting the expression of insulin receptor and confining the activation of IRS-1/PI3K/Akt/GSK-3β/GS pathway to insulin.

Prolactin-induced Subcellular Targeting of GLUT1 Glucose Transporter in Living Mammary Epithelial Cells

BACKGROUND

Studying the biological pathways involved in mammalian milk production during lactation could have many clinical implications. The mammary gland is unique in its requirement for transport of free glucose into the cell for the synthesis of lactose, the primary carbohydrate in milk.

OBJECTIVE

To study GLUT1 trafficking and subcellular targeting in living mammary epithelial cells (MEC) in culture.

METHODS

Immunocytochemistry was used to study GLUT1 hormonally regulated subcellular targeting in human MEC (HMEC). To study GLUT1 targeting and recycling in living mouse MEC (MMEC) in culture, we constructed fusion proteins of GLUT1 and green fluorescent protein (GFP) and expressed them in CIT3 MMEC. Cells were maintained in growth medium (GM), or exposed to secretion medium (SM), containing prolactin.

RESULTS

GLUT1 in HMEC localized primarily to the plasma membrane in GM. After exposure to prolactin for 4 days, GLUT1 was targeted intracellularly and demonstrated a perinuclear distribution, co-localizing with lactose synthetase. The dynamic trafficking of GFP-GLUT1 fusion proteins in CIT3 MMEC suggested a basal constitutive GLUT1 recycling pathway between an intracellular pool and the cell surface that targets most GLUT1 to the plasma membrane in GM. Upon exposure to prolactin in SM, GLUT1 was specifically targeted intracellularly within 90-110 minutes.

CONCLUSIONS

Our studies suggest intracellular targeting of GLUT1 to the central vesicular transport system upon exposure to prolactin. The existence of a dynamic prolactin-induced sorting machinery for GLUT1 could be important for transport of free glucose into the Golgi for lactose synthesis during lactation.

Recycling of the insulin-responsive glucose transporter Glut4 regulated by the small GTPase RalA and the exocyst complex

Endocytic recycling represents a major mechanism for continuous supply of molecules to the plasma membrane. Particularly, outbound trafficking of the recycling endosome (RE) or RE-derived vesicles can be upregulated by cellular signaling, through mobilization of specialized protein complexes acting as transport machineries. Therefore, biochemical and functional characterization of cell signaling molecules that operate multimeric protein complexes in membrane transport provides important insights to signaling-regulated trafficking events. In this chapter, we described biochemical approaches and reporter assays in differentiated adipocytes to determine the activity and function of the small GTPase RalA, which relays upstream insulin signaling to the exocyst complex that targets intracellular vesicles bearing the Glut4 transporter to the plasma membrane. The experimental design outlined in this chapter can be applied to other regulated transport events facilitated by the exocyst complex, as well as other GTPases that operate distinct transport complexes in specific physiological settings.

Canavanine increases glucose uptake in C2 C12 cells through the activation of imidazoline I-2B receptors

Canavanine is a guanidinium derivative that contains the basic structure of the ligand(s) of imidazoline receptor (I-R). Canavanine has been reported to activate the imidazoline I-3 receptor (I-3R) both in vivo and in vitro. Additionally, the activation of the imidazoline I-2B receptor (I-2BR) by guanidinium derivatives may increase glucose uptake. Therefore, the effect of canavanine on the I-2BR was investigated in the present study. Glucose uptake into cultured C2 C12 cells was determined using the radio-ligated tracer 2-[(14) C]-deoxy-glucose. The changes in 5' AMP-activated protein kinase (AMPK) expression were also identified using Western blotting analysis. The canavanine-induced glucose uptake was inhibited in a dose-dependent manner by BU224 (0.01-1 μmol/L), which is a specific I-2BR antagonist, in the C2 C12 cells. Additionally, the canavanine-stimulated AMPK phosphorylation and glucose transporter (GLUT4) expression were also sensitive to BU224 inhibition in the C2 C12 cells. Moreover, both canavanine-stimulated glucose uptake and AMPK phosphorylation were attenuated by high concentrations of amiloride (1-2 μmol/L), which is another established I-2BR inhibitor, in a dose-dependent manner in C2 C12 cells. Additionally, compound C abolished the canavanine-induced glucose uptake and AMPK phosphorylation at a concentration (0.1 μmol/L) sufficient to inhibit AMPK. In conclusion, these data demonstrated that canavanine has an ability to activate I-2BR through the AMPK pathway to increase glucose uptake, which indicates I-2BR as a new target for diabetic therapy.

Redox implications in adipose tissue (dys)function--A new look at old acquaintances

Obesity is an energy balance disorder associated with dyslipidemia, insulin resistance and diabetes type 2, also summarized with the term metabolic syndrome or syndrome X. Increasing evidence points to “adipocyte dysfunction”, rather than fat mass accretion per se, as the key pathophysiological factor for metabolic complications in obesity. The dysfunctional fat tissue in obesity characterizes a failure to safely store metabolic substrates into existing hypertrophied adipocytes and/or into new preadipocytes recruited for differentiation. In this review we briefly summarize the potential of redox imbalance in fat tissue as an instigator of adipocyte dysfunction in obesity. We reveal the challenge of the adipose redox changes, insights in the regulation of healthy expansion of adipose tissue and its reduction, leading to glucose and lipids overflow.

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Adipose tissue (AT) buffers nutrient excess determining overall metabolic health.

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Redox insight in lipid storage and adipogenesis of AT is reviewed.

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Redox modulation of AT as therapeutic target in obesity/syndrome X is considered.

L-Citrulline increases hepatic sensitivity to insulin by reducing the phosphorylation of serine 1101 in insulin receptor substrate-1

BACKGROUND

Insulin resistance is characterized by deficient responses to insulin in its target tissues. In the present study, we examined the effects of L-Citrulline (L-Cit) on insulin sensitivity and signaling cascades in rat hepatoma H4IIE cells and SHRSP.Z-Leprfa/IzmDmcr rats.

METHODS

H4IIE cells were pretreated in the presence or absence of 250 μM L-Cit in serum-free medium and then incubated in the presence or absence of 0.1 nM insulin. Rats were allocated into 2 groups; a control group (not treated) and L-Cit group (2 g/kg/day, L-Cit) and treated for 8 weeks.

RESULTS

L-Cit enhanced the insulin-induced phosphorylation of Akt in H4IIE cells. Moreover, the inhibited expression of Dex/cAMP-induced PEPCK mRNA by insulin was enhanced by the L-Cit treatment. The phosphorylation of tyrosine, which is upstream of Akt, in insulin receptor substrate-1 (IRS-1) was increased by the L-Cit treatment. The L-Cit-induced enhancement in insulin signaling was not related to the binding affinity of insulin to the insulin receptor or to the expression of the insulin receptor, but to a decrease in the phosphorylation of serine 1101 in IRS-1. These results were also confirmed in animal experiments. In the livers of L-Cit-treated rats, PI3K/Akt signaling was improved by decreases in the phosphorylation of serine 1101.

CONCLUSIONS

We herein demonstrated for the first time the beneficial effects of L-Cit on improved insulin resistance associated with enhanced insulin sensitivity. These results may have clinical applications for insulin resistance and the treatment of type-2 diabetes.

High levels of chorionic gonadotrophin attenuate insulin sensitivity and promote inflammation in adipocytes

Gestational diabetes mellitus (GDM) presents with moderate inflammation, insulin resistance and impaired glucose uptake, which may result from increased maternal fat mass and increased circulation of placental hormones and adipokines. In this study, we set out to test whether the surge in chorionic gonadotrophin (CG) secretion is a cause of inflammation and impaired insulin sensitivity in GDM. We first found that LH/chorionic gonadotrophin receptors (CG/LHR) were expressed at low levels in insulin-sensitive murine 3T3-L1 adipocytes and murine C2C12 myocytes. CG treatment not only directly reduced insulin-responsive gene expression, including that of glucose transporter 4 (GLUT4), but also impaired insulin-stimulated glucose uptake in 3T3-L1 cells. Moreover, CG treatment increased the expression of the proinflammatory cytokine monocyte chemotactic protein 1 (MCP1) and upregulated nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) activity in 3T3-L1 cells. Clinically, pregnant women who had higher CG levels and elevated MCP1 developed GDM. Above all, apart from prepregnancy BMI and MCP1 level, CG level was associated with abnormal glucose tolerance. In summary, our findings confirmed that higher CG levels in pregnancy possibly played a role in GDM development partly by impairing the functions of insulin, such those involved in as glucose uptake, while promoting inflammation in adipocyte.

Promotion of insulin-induced glucose uptake in C2C12 myotubes by osteocalcin

A close relationship between the bone and systemic glucose metabolism has recently been the center of attention, since the uncarboxylated form of osteocalcin (GluOC), a bone-derived protein, but not the γ-carboxylated form, is involved in glucose metabolism. However, the analysis of GluOC effect using isolated organs and related cell lines are required to understand its roles in a whole systemic metabolic status. In the present study, we examined the effect of GluOC on cell lines derived from skeletal muscle to explore the mechanisms by which GluOC regulates glucose uptake. In the differentiated C2C12 myotubes, GluOC dose-dependently induced the phosphorylation of ERK without affecting intracellular cAMP and Ca(2+) levels. This effect was inhibited by U0126, an inhibitor of ERK kinase (MEK). Additionally, U73122, an inhibitor of phospholipase C tended to inhibit it as well. Furthermore, cell treatment with GluOC for a long period promoted insulin-induced Akt phosphorylation and glucose uptake in the myotubes, which was abolished by ERK signaling inhibition. These results indicate that GluOC does not triggered Akt phosphorylation and glucose uptake by itself but promotes insulin-induced glucose uptake in myotubes, probably by up-regulating Akt signaling through ERK activation.

Tomosyn is a novel Akt substrate mediating insulin-dependent GLUT4 exocytosis

Insulin triggers glucose uptake into skeletal muscle and adipose tissues by gaining the available number of glucose transporter 4 (GLUT4) on the cell surface. GLUT4-loaded vesicles are targeted to plasma membrane from the intracellular reservoir through multiple trafficking and fusion processes that are mainly regulated by Akt. However, it is still largely unknown how GLUT4 expression in the cell surface is promoted by insulin. In the present study, we identified tomosyn at Ser-783 as a possible Akt-substrate motif and examined whether the phosphorylation at Ser-783 is involved in the regulation of GLUT4 expression. Both Akt1 and Akt2 phosphorylated the wild-type tomosyn, but not the mutant tomosyn in which Ser-783 was replaced with Ala. Phosphorylation of tomosyn at Ser-783 was also observed in the intact cells by insulin stimulation, which was blocked by PI3K inhibitor, LY294002. In vitro pull-down assay showed that phosphorylation of tomosyn at Ser-783 by Akt inhibited the interaction with syntaxin 4. Insulin stimulation increased GLUT4 in the cell surface of CHO-K1 cells to promote glucose uptake, however exogenous expression of the mutant tomosyn attenuated the increase by insulin. These results suggest that Ser-783 of tomosyn is a target of Akt and is implicated in the interaction with syntaxin 4.

Cell-autonomous heterogeneity of nutrient uptake in white adipose tissue of rhesus macaques

Phenotypic diversity may play an adaptive role by providing graded biological responses to fluctuations in environmental stimuli. We used single-cell imaging of the metabolizable fluorescent fatty acid analog 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-C12 and fluorescent 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino]-2-deoxy-D-glucose (2-NBDG) to explore cellular heterogeneity in nutrient uptake in white adipose tissue (WAT) explants of rhesus macaques. Surprisingly, WAT displayed a striking cell size-independent mosaic pattern, in that adjacent adipocytes varied with respect to insulin-stimulated BODIPY-C12 and 2-NBDG uptake. Relative free fatty acid (FFA) transport activity correlated with the cellular levels of FFA transporter protein-1 and the scavenger receptor CD36 in individual adipocytes. In vitro incubation of WAT explants for 24 hours caused partial desynchronization of cellular responses, suggesting that adipocytes may slowly alter their differential nutrient uptake activity. In vitro-differentiated human adipocytes also exhibited a mosaic pattern of BODIPY-C12 uptake. WAT from animals containing a homogeneous population of large adipocytes was nonmosaic, in that every adipocyte exhibited a similar level of BODIPY-C12 fluorescence, suggesting that the development of obesity is associated with the loss of heterogeneity in WAT. Hence, for the first time, we demonstrate an intrinsic heterogeneity in FFA and glucose transport activity in WAT.

Targeting steroid receptor coactivator 1 with antisense oligonucleotides increases insulin-stimulated skeletal muscle glucose uptake in chow-fed and high-fat-fed male rats

The steroid receptor coactivator 1 (SRC1) regulates key metabolic pathways, including glucose homeostasis. SRC1(-/-) mice have decreased hepatic expression of gluconeogenic enzymes and a reduction in the rate of endogenous glucose production (EGP). We sought to determine whether decreasing hepatic and adipose SRC1 expression in normal adult rats would alter glucose homeostasis and insulin action. Regular chow-fed and high-fat-fed male Sprage-Dawley rats were treated with an antisense oligonucleotide (ASO) against SRC1 or a control ASO for 4 wk, followed by metabolic assessments. SRC1 ASO did not alter basal EGP or expression of gluconeogenic enzymes. Instead, SRC1 ASO increased insulin-stimulated whole body glucose disposal by ~30%, which was attributable largely to an increase in insulin-stimulated muscle glucose uptake. This was associated with an approximately sevenfold increase in adipose expression of lipocalin-type prostaglandin D2 synthase, a previously reported regulator of insulin sensitivity, and an approximately 70% increase in plasma PGD2 concentration. Muscle insulin signaling, AMPK activation, and tissue perfusion were unchanged. Although GLUT4 content was unchanged, SRC1 ASO increased the cleavage of tether-containing UBX domain for GLUT4, a regulator of GLUT4 translocation. These studies point to a novel role of adipose SRC1 as a regulator of insulin-stimulated muscle glucose uptake.

Phthalate exposure in utero causes epigenetic changes and impairs insulin signalling

Di-(2-ethylhexyl)phthalate (DEHP) is an endocrine-disrupting chemical (EDC), widely used as a plasticiser. Developmental exposure to EDCs could alter epigenetic programming and result in adult-onset disease. We investigated whether DEHP exposure during development affects glucose homoeostasis in the F1 offspring as a result of impaired insulin signal transduction in gastrocnemius muscle. Pregnant Wistar rats were administered DEHP (0, 1, 10 and 100 mg/kg per day) from embryonic days 9-21 orally. DEHP-exposed offspring exhibited elevated blood glucose, impaired serum insulin, glucose tolerance and insulin tolerance, along with reduced insulin receptor, glucose uptake and oxidation in the muscle at postnatal day 60. The levels of insulin signalling molecules and their phosphorylation were down-regulated in DEHP-exposed offspring. However, phosphorylated IRS1(Ser636/639), which impedes binding of downstream effectors and the negative regulator (PTEN) of PIP3, was increased in DEHP-exposed groups. Down-regulation of glucose transporter 4 (Glut4 (Slc2a4)) gene expression and increased GLUT4(Ser488) phosphorylation, which decreases its intrinsic activity and translocation towards the plasma membrane, were recorded. Chromatin immunoprecipitation assays detected decreased MYOD binding and increased histone deacetylase 2 interaction towards Glut4, indicative of the tight chromatin structure at the Glut4 promoter. Increased DNMTs and global DNA methylation levels were also observed. Furthermore, methylation of Glut4 at the MYOD-binding site was increased in DEHP-exposed groups. These findings indicate that, gestational DEHP exposure predisposes F1 offspring to glucometabolic dysfunction at adulthood by down-regulating the expression of critical genes involved in the insulin signalling pathway. Furthermore, DEHP-induced epigenetic alterations in Glut4 appear to play a significant role in disposition towards this metabolic abnormality.

4-Hydroxyisoleucine ameliorates fatty acid-induced insulin resistance and inflammatory response in skeletal muscle cells

The 4-hydroxyisoleucine (4-HIL), an unusual amino acid isolated from the seeds of Trigonella foenum-graecum was investigated for its metabolic effects to ameliorate free fatty acid-induced insulin resistance in skeletal muscle cells. An incubation of L6 myotubes with palmitate inhibited insulin stimulated-glucose uptake and -translocation of glucose transporter 4 (GLUT4) to the cell surface. Addition of 4-HIL strongly prevented this inhibition. We then examined the insulin signaling pathway, where 4-HIL effectively inhibited the ability of palmitate to reduce insulin-stimulated phosphorylation of insulin receptor substrate-1 (IRS-1), protein kinase B (PKB/AKT), AKT substrate of 160 kD (AS160) and glycogen synthase kinase 3β (GSK-3β) in L6 myotubes. Moreover, 4-HIL presented strong inhibition on palmitate-induced production of reactive oxygen species (ROS) and associated inflammation, as the activation of NF-κB, JNK1/2, ERK1/2 and p38 MAPK was greatly reduced. 4-HIL also inhibited inflammation-stimulated IRS-1 serine phosphorylation and restored insulin-stimulated IRS-1 tyrosine phosphorylation in the presence of palmitate, leading to enhanced insulin sensitivity. These findings suggested that 4-HIL could inhibit palmitate-induced, ROS-associated inflammation and restored insulin sensitivity through regulating IRS-1 function.

Ivy gourd (Coccinia grandis L. Voigt) root suppresses adipocyte differentiation in 3T3-L1 cells

Background

Ivy gourd (Coccinia grandis L. Voigt) is a tropical plant widely distributed throughout Asia, Africa, and the Pacific Islands. The anti-obesity property of this plant has been claimed but still remains to be scientifically proven. We therefore investigated the effects of ivy gourd leaf, stem, and root on adipocyte differentiation by employing cell culture model.

Methods

Dried roots, stems, and leaves of ivy gourd were separately extracted with ethanol. Each extract was then applied to 3T3-L1 pre-adipocytes upon induction with a mixture of insulin, 3-isobutyl-1-methylxanthine, and dexamethasone, for anti-adipogenesis assay. The active extract was further fractionated by a sequential solvent partitioning method, and the resulting fractions were examined for their abilities to inhibit adipogenesis in 3T3-L1 cells. Differences in the expression of adipogenesis-related genes between the treated and untreated cells were determined from their mRNA and protein levels.

Results

Of the three ivy gourd extracts, the root extract exhibited an anti-adipogenic effect. It significantly reduced intracellular fat accumulation during the early stages of adipocyte differentiation. Together with the suppression of differentiation, expression of the genes encoding PPARγ, C/EBPα, adiponectin, and GLUT4 were down-regulated. Hexane-soluble fraction of the root extract also inhibited adipocyte differentiation and decreased the mRNA levels of various adipogenic genes in the differentiating cells.

Conclusions

This is the first study to demonstrate that ivy gourd root may prevent obesity based mainly on the ability of its active constituent(s) to suppress adipocyte differentiation in vitro. Such an inhibitory effect is mediated by at least down-regulating the expression of PPARγ-the key transcription factor of adipogenesis in pre-adipocytes during their early differentiation processes.

Characterization of VAMP-2 in the lung: implication in lung surfactant secretion

Lung surfactant is crucial for reducing the surface tension of alveolar space, thus preventing the alveoli from collapse. Lung surfactant is synthesized in alveolar epithelial type II cells and stored in lamellar bodies before being released via the fusion of lamellar bodies with the apical plasma membrane. SNAREs (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptors) play an essential role in membrane fusion. We have previously demonstrated the requirement of t-SNARE (target SNARE) proteins, syntaxin 2 and SNAP-23 (N-ethylmaleimide-sensitive factor-attachment protein 23), in regulated surfactant secretion. Here, we characterized the distribution of VAMPs (vesicle-associated membrane proteins) in rat lung and alveolar type II cells. VAMP-2, -3 and -8 are shown in type II cells at both mRNA and protein levels. VAMP-2 and -8 were enriched in LB (lamellar body) fraction. Immunochemistry studies indicated that VAMP-2 was co-localized with the LB marker protein, LB-180. Functionally, the cytoplasmic domain of VAMP-2, but not VAMP-8 inhibited surfactant secretion in type II cells. We suggest that VAMP-2 is the v-SNARE (vesicle SNARE) involved in regulated surfactant secretion.

Impaired lipid and glucose homeostasis in hexabromocyclododecane-exposed mice fed a high-fat diet

BACKGROUND

Hexabromocyclododecane (HBCD) is an additive flame retardant used in the textile industry and in polystyrene foam manufacturing. Because of its lipophilicity and persistency, HBCD accumulates in adipose tissue and thus has the potential of causing metabolic disorders through disruption of lipid and glucose homeostasis. However, the association between HBCD and obesity remains unclear.

OBJECTIVES

We investigated whether exposure to HBCD contributes to initiation and progression of obesity and related metabolic dysfunction in mice fed a normal diet (ND) or a high-fat diet (HFD).

METHODS

Male C57BL/6J mice were fed a HFD (62.2 kcal% fat) or a ND and treated orally with HBCD (0, 1.75, 35, or 700 μg/kg body weight) weekly from 6 to 20 weeks of age. We examined body weight, liver weight, blood biochemistry, histopathological changes, and gene expression profiles in the liver and adipose tissue.

RESULTS

In HFD-fed mice, body and liver weight were markedly increased in mice treated with the high (700 μg/kg) and medium (35 μg/kg) doses of HBCD compared with vehicle. This effect was more prominent in the high-dose group. These increases were paralleled by increases in random blood glucose and insulin levels and enhancement of microvesicular steatosis and macrophage accumulation in adipose tissue. HBCD-treated HFD-fed mice also had increased mRNA levels of Pparg (peroxisome proliferator-activated receptor-γ) in the liver and decreased mRNA levels of Glut4 (glucose transporter 4) in adipose tissue compared with vehicle-treated HFD-fed mice.

CONCLUSIONS

Our findings suggest that HBCD may contribute to enhancement of diet-induced body weight gain and metabolic dysfunction through disruption of lipid and glucose homeostasis, resulting in accelerated progression of obesity.

CITATION

Yanagisawa R, Koike E, Win-Shwe TT, Yamamoto M, Takano H. 2014. Impaired lipid and glucose homeostasis in hexabromocyclododecane-exposed mice fed a high-fat diet. Environ Health Perspect 122:277-283; http://dx.doi.org/10.1289/ehp.1307421.

Lowered extracellular pH is involved in the pathogenesis of skeletal muscle insulin resistance

Insulin resistance in the skeletal muscle is manifested by diminished insulin-stimulated glucose uptake and is a core factor in the pathogenesis of type 2 diabetes mellitus (DM), but the mechanism causing insulin resistance is still unknown. Our recent study has shown that pH of interstitial fluids was lowered in early developmental stage of insulin resistance in OLETF rats, a model of type 2 DM. Therefore, in the present study, we confirmed effects of the extracellular pH on the insulin signaling pathway in a rat skeletal muscle-derived cell line, L6 cell. The phosphorylation level (activation) of the insulin receptor was significantly diminished in low pH media. The phosphorylation level of Akt, which is a downstream target of the insulin signaling pathway, also decreased in low pH media. Moreover, the insulin binding to its receptor was reduced by lowering extracellular pH, while the expression of insulin receptors on the plasma membrane was not affected by the extracellular pH. Finally, insulin-stimulated 2-deoxyglucose uptake in L6 cells was diminished in low pH media. Our present study suggests that lowered extracellular pH conditions may produce the pathogenesis of insulin resistance in skeletal muscle cells.

PI3 kinase directly phosphorylates Akt1/2 at Ser473/474 in the insulin signal transduction pathway

Insulin stimulated translocation of the glucose transporter GLUT4 from the cytosol to the plasma membrane in a concentration (1  nM-1  μM)-dependent manner and increased glucose uptake in 3T3-L1 adipocytes. Insulin-induced GLUT4 translocation to the cell surface was prevented by the phosphoinositide 3 kinase (PI3K) inhibitor wortmannin, the 3-phosphoinositide-dependent protein kinase 1 (PDK1) inhibitor BX912 or the Akt1/2 inhibitor MK2206, and by knocking-down PI3K, PDK1 or Akt1/2. Insulin increased phosphorylation of Akt1/2 at Thr308/309 and Ser473/474, to activate Akt1/2, in the adipocytes. Insulin-induced phosphorylation of Akt1/2 was suppressed by wortmannin and knocking-down PI3K, while no significant inhibition of the phosphorylation was obtained with BX912 or knocking-down PDK1. In the cell-free Akt assay, PI3K phosphorylated Akt1 both at Thr308 and Ser473 and Akt2 at Ser474 alone. In contrast, PDK1 phosphorylates Akt1 at Thr308 and Akt2 at Thr309. The results of this study indicate that PI3K activates Akt1, independently of PDK1, and Akt2 by cooperating with PDK1 in the insulin signal transduction pathway linked to GLUT4 translocation.

Regulation of myosin IIA and filamentous actin during insulin-stimulated glucose uptake in 3T3-L1 adipocytes

Insulin stimulated glucose uptake requires the colocalization of myosin IIA (MyoIIA) and the insulin-responsive glucose transporter 4 (GLUT4) at the plasma membrane for proper GLUT4 fusion. MyoIIA facilitates filamentous actin (F-actin) reorganization in various cell types. In adipocytes F-actin reorganization is required for insulin-stimulated glucose uptake. What is not known is whether MyoIIA interacts with F-actin to regulate insulin-induced GLUT4 fusion at the plasma membrane. To elucidate the relationship between MyoIIA and F-actin, we examined the colocalization of MyoIIA and F-actin at the plasma membrane upon insulin stimulation as well as the regulation of this interaction. Our findings demonstrated that MyoIIA and F-actin colocalized at the site of GLUT4 fusion with the plasma membrane upon insulin stimulation. Furthermore, inhibition of MyoII with blebbistatin impaired F-actin localization at the plasma membrane. Next we examined the regulatory role of calcium in MyoIIA-F-actin colocalization. Reduced calcium or calmodulin levels decreased colocalization of MyoIIA and F-actin at the plasma membrane. While calcium alone can translocate MyoIIA it did not stimulate F-actin accumulation at the plasma membrane. Taken together, we established that while MyoIIA activity is required for F-actin localization at the plasma membrane, it alone is insufficient to localize F-actin to the plasma membrane.

Joint conditional Gaussian graphical models with multiple sources of genomic data

It is challenging to identify meaningful gene networks because biological interactions are often condition-specific and confounded with external factors. It is necessary to integrate multiple sources of genomic data to facilitate network inference. For example, one can jointly model expression datasets measured from multiple tissues with molecular marker data in so-called genetical genomic studies. In this paper, we propose a joint conditional Gaussian graphical model (JCGGM) that aims for modeling biological processes based on multiple sources of data. This approach is able to integrate multiple sources of information by adopting conditional models combined with joint sparsity regularization. We apply our approach to a real dataset measuring gene expression in four tissues (kidney, liver, heart, and fat) from recombinant inbred rats. Our approach reveals that the liver tissue has the highest level of tissue-specific gene regulations among genes involved in insulin responsive facilitative sugar transporter mediated glucose transport pathway, followed by heart and fat tissues, and this finding can only be attained from our JCGGM approach.

Phosphatidylinositol 4,5-bisphosphate influences PIN polarization by controlling clathrin-mediated membrane trafficking in Arabidopsis

The functions of the minor phospholipid phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] during vegetative plant growth remain obscure. Here, we targeted two related phosphatidylinositol 4-phosphate 5-kinases (PI4P 5-kinases) PIP5K1 and PIP5K2, which are expressed ubiquitously in Arabidopsis thaliana. A pip5k1 pip5k2 double mutant with reduced PtdIns(4,5)P2 levels showed dwarf stature and phenotypes suggesting defects in auxin distribution. The roots of the pip5k1 pip5k2 double mutant had normal auxin levels but reduced auxin transport and altered distribution. Fluorescence-tagged auxin efflux carriers PIN-FORMED (PIN1)-green fluorescent protein (GFP) and PIN2-GFP displayed abnormal, partially apolar distribution. Furthermore, fewer brefeldin A-induced endosomal bodies decorated by PIN1-GFP or PIN2-GFP formed in pip5k1 pip5k2 mutants. Inducible overexpressor lines for PIP5K1 or PIP5K2 also exhibited phenotypes indicating misregulation of auxin-dependent processes, and immunolocalization showed reduced membrane association of PIN1 and PIN2. PIN cycling and polarization require clathrin-mediated endocytosis and labeled clathrin light chain also displayed altered localization patterns in the pip5k1 pip5k2 double mutant, consistent with a role for PtdIns(4,5)P2 in the regulation of clathrin-mediated endocytosis. Further biochemical tests on subcellular fractions enriched for clathrin-coated vesicles (CCVs) indicated that pip5k1 and pip5k2 mutants have reduced CCV-associated PI4P 5-kinase activity. Together, the data indicate an important role for PtdIns(4,5)P2 in the control of clathrin dynamics and in auxin distribution in Arabidopsis.

Insulin stimulates translocation of human GLUT4 to the membrane in fat bodies of transgenic Drosophila melanogaster

The fruit fly Drosophila melanogaster is an excellent model system for studies of genes controlling development and disease. However, its applicability to physiological systems is less clear because of metabolic differences between insects and mammals. Insulin signaling has been studied in mammals because of relevance to diabetes and other diseases but there are many parallels between mammalian and insect pathways. For example, deletion of Drosophila Insulin-Like Peptides resulted in 'diabetic' flies with elevated circulating sugar levels. Whether this situation reflects failure of sugar uptake into peripheral tissues as seen in mammals is unclear and depends upon whether flies harbor the machinery to mount mammalian-like insulin-dependent sugar uptake responses. Here we asked whether Drosophila fat cells are competent to respond to insulin with mammalian-like regulated trafficking of sugar transporters. Transgenic Drosophila expressing human glucose transporter-4 (GLUT4), the sugar transporter expressed primarily in insulin-responsive tissues, were generated. After expression in fat bodies, GLUT4 intracellular trafficking and localization were monitored by confocal and total internal reflection fluorescence microscopy (TIRFM). We found that fat body cells responded to insulin with increased GLUT4 trafficking and translocation to the plasma membrane. While the amplitude of these responses was relatively weak in animals reared on a standard diet, it was greatly enhanced in animals reared on sugar-restricted diets, suggesting that flies fed standard diets are insulin resistant. Our findings demonstrate that flies are competent to mobilize translocation of sugar transporters to the cell surface in response to insulin. They suggest that Drosophila fat cells are primed for a response to insulin and that these pathways are down-regulated when animals are exposed to constant, high levels of sugar. Finally, these studies are the first to use TIRFM to monitor insulin-signaling pathways in Drosophila, demonstrating the utility of TIRFM of tagged sugar transporters to monitor signaling pathways in insects.

Identification and Association of SNPs in TBC1D1 Gene with Growth Traits in Two Rabbit Breeds

The TBC1D1 plays a key role in body energy homeostasis by regulating the insulin-stimulated glucose uptake in skeletal muscle. The present study aimed to identify the association between genetic polymorphisms of TBC1D1 and body weight (BW) in rabbits. Among the total of 12 SNPs detected in all 20 exons, only one SNP was non-synonymous (c.214G>A. p.G72R) located in exon 1. c.214G>A was subsequently genotyped among 491 individuals from two rabbit breeds by the high-resolution melting method. Allele A was the predominant allele with frequencies of 0.7780 and 0.6678 in European white rabbit (EWR, n = 205) and New Zealand White rabbit (NZW, n = 286), respectively. The moderate polymorphism information content (0.250.05). Our results implied that the c.214G>A of TBC1D1 gene might be one of the candidate loci affecting the trait of 35 d BW in the rabbit.