Quantitative measurement of GLUT4 translocation to the plasma membrane by flow cytometry

Klotho induces insulin resistance possibly through interference with GLUT4 translocation and activation of Akt, GSK3β, and PFKfβ3 in 3T3-L1 adipocyte cells

Klotho is considered as an anti-aging factor inducing insulin resistance and involved in type 2 diabetes. However, mechanisms by which klotho induces insulin resistance remain to be understood. Thus, in this study, we aimed to evaluate possible interference points of klotho with insulin signaling pathways in 3T3-L1 adipocyte cells by focusing on phosphorylation levels of Akt, GSK3β, PFK-fβ3, and GLUT4 translocation. Differentiation of 3T3-L1 cells to the adipocyte-like cells were performed using specific differentiation kit and confirmed by mRNA expression assay of PPARγ using qRT-PCR, and Sudan black staining of lipid droplets. Then cells were co-treated with klotho and insulin. Expression and translocation of GLUT4 mRNA were evaluated using qRT-PCR and Alexa flour 488 conjugated GLUT4 antibody, respectively. P-Akt/Akt, p-GSK3β/GSK3β, and p-PFKfβ3/PFKfβ3 ratios were determined in insulin and klotho/insulin treated cells using western blot. Our result indicated that GLUT4 expression were decreased to 0.72 ± 0.16 fold in insulin treated cells, however it was calculated 1.12 ± 0.25 fold in klotho/insulin treated cells. In addition, klotho prevented GLUT4 membrane translocation by 27.2% in comparison with insulin-treated cells (P < 0.05). Interestingly, in insulin/klotho co-treated cells, phospho-levels of Akt, GSK3β, and PFKfβ3 proteins was decreased to 2.34 ± 0.14, 2.29 ± 0.63, and 1.95 ± 0.37 fold in comparison with the insulin cells, (P < 0.05). In conclusion, our study indicated that klotho induces insulin resistance in adipocytes possibly through prevention of GLUT4 translocation, and interfere with phosphorylation of Akt, GSK3β, and PFKf3β intracellular signaling mediators by insulin.

Skeletal Muscle-Specific Activation of Gq Signaling Maintains Glucose Homeostasis

Skeletal muscle (SKM) insulin resistance plays a central role in the pathogenesis of type 2 diabetes. Because G-protein-coupled receptors (GPCRs) represent excellent drug targets, we hypothesized that activation of specific functional classes of SKM GPCRs might lead to improved glucose homeostasis in type 2 diabetes. At present, little is known about the in vivo metabolic roles of the various distinct GPCR signaling pathways operative in SKM. In this study, we tested the hypothesis that selective activation of SKM G_q signaling can improve SKM glucose uptake and whole-body glucose homeostasis under physiological and pathophysiological conditions. Studies with transgenic mice expressing a G_q-linked designer GPCR selectively in SKM cells demonstrated that receptor-mediated activation of SKM G_q signaling greatly promoted glucose uptake into SKM and significantly improved glucose homeostasis in obese, glucose-intolerant mice. These beneficial metabolic effects required the activity of SKM AMPK. In contrast, obese mutant mice that lacked both Gα_q and Gα₁₁ selectively in SKM showed severe deficits in glucose homeostasis. Moreover, GPCR-mediated activation of G_q signaling also stimulated glucose uptake in primary human SKM cells. Taken together, these findings strongly suggest that agents capable of enhancing SKM G_q signaling may prove useful as novel antidiabetic drugs.

Annona stenophylla aqueous extract stimulate glucose uptake in established C2Cl2 muscle cell lines

BACKGROUND

Annona stenophylla is a folk medicine popularly used in Zimbabwe for the treatment of many ailments. This study was carried out to determine some of the possible anti diabetic mechanisms of its action using in vitro cell culturing methods.

METHODS

A. stenophylla's effects on glucose uptake were tested using muscle cells (C2Cl2). Expression of glucose 4 transporters was determined by treating cell lines with plant extract. Total RNA was isolated and using RT-PCR, GLUT 4 expression levels were quantified. Translocation of GLUT 4 was assessed using FITC fluorescence measured by flow cytometry.

RESULTS

Treatment of cells with plant extract significantly increased glucose uptake in a concentration dependent manner, with the highest concentration (250 µg/ml) giving 28% increased uptake compared to the negative control. The increase in glucose uptake (2.5 times more than control) was coupled to increase in GLUT 4 mRNA and subsequently GLUT 4 translocation. Wortmannin expunged the A. stenophylla induced increase in GLUT 4 mRNA and glucose uptake.

CONCLUSION

The results suggest that A. stenophylla aqueous extract increases glucose uptake partly through increasing the GLUT 4 mRNA and translocation potentially acting via the PI-3-K pathway. This study confirms the ethnopharmacological uses of A. stenophylla indicating potential for anti-diabetic products formulation.

Apolipoprotein A-I enhances insulin-dependent and insulin-independent glucose uptake by skeletal muscle

Therapeutic interventions that increase plasma high density lipoprotein (HDL) and apolipoprotein (apo) A-I levels have been reported to reduce plasma glucose levels and attenuate insulin resistance. The present study asks if this is a direct effect of increased glucose uptake by skeletal muscle. Incubation of primary human skeletal muscle cells (HSKMCs) with apoA-I increased insulin-dependent and insulin–independent glucose uptake in a time- and concentration-dependent manner. The increased glucose uptake was accompanied by enhanced phosphorylation of the insulin receptor (IR), insulin receptor substrate-1 (IRS-1), the serine/threonine kinase Akt and Akt substrate of 160 kDa (AS160). Cell surface levels of the glucose transporter type 4, GLUT4, were also increased. The apoA-I-mediated increase in glucose uptake by HSKMCs was dependent on phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/Akt, the ATP binding cassette transporter A1 (ABCA1) and scavenger receptor class B type I (SR-B1). Taken together, these results establish that apoA-I increases glucose disposal in skeletal muscle by activating the IR/IRS-1/PI3K/Akt/AS160 signal transduction pathway. The findings suggest that therapeutic agents that increase apoA-I levels may improve glycemic control in people with type 2 diabetes.

Subcutaneous adipose tissue imaging of human obesity reveals two types of adipocyte membranes: Insulin-responsive and -nonresponsive

In adipose tissue, resistance to insulin's ability to increase glucose uptake can be induced by multiple factors, including obesity. Impaired insulin action may take place at different spatial loci at the cellular or subcellular level. To begin to understand the spatial response to insulin in human subcutaneous adipose tissue (hSAT), we developed a quantitative imaging method for activation of a major signaling node in the glucoregulatory insulin signaling pathway. After treatment with insulin or control media, biopsied tissues were immunostained for Akt phosphorylation at Thr-308/9 (pAkt) and then imaged by confocal fluorescence microscopy automated to collect a large grid of high resolution fields. In hSAT from 40 men and women with obesity, substantial heterogeneity of pAkt densities in adipocyte membranes were quantified in each image mosaic using a spatial unit of at least twice the size of the point spread function. Statistical analysis of the distribution of pAkt spatial units was best fit as the weighted sum of two separate distributions, corresponding to either a low or high pAkt density. A "high pAkt fraction" metric was calculated from the fraction of high pAkt distributed units over the total units. Importantly, upon insulin stimulation, tissues from the same biopsy showed either a minimal or a substantial change in the high pAkt fraction. Further supporting a two-state response to insulin stimulation, subjects with similar insulin sensitivity indices are also segregated into either of two clusters identified by the amount of membrane-localized pAkt.

Atorvastatin impaired glucose metabolism in C2C12 cells partly via inhibiting cholesterol-dependent glucose transporter 4 translocation

Skeletal muscle accounts for approximately 75% of glucose disposal in body and statins impair glucose metabolism. We aimed to investigate the effect of atorvastatin on glucose metabolism in C2C12 cells. Glucose metabolism and expression of glucose transporter 4 (GLUT4) and hexokinase II (HXKII) were measured following incubation with atorvastatin or pravastatin. Roles of cholesterol in atorvastatin-induced glucose metabolism impairment were investigated via adding cholesterol or mevalonic acid and confirmed by cholesterol depletion with methyl-β-cyclodextrin. Hypercholesterolemia mice induced by high fat diet (HFD) feeding, orally received atorvastatin (6 and 12 mg/kg) or pravastatin (12 mg/kg) for 22 days. Results showed that atorvastatin not pravastatin concentration-dependently impaired glucose consumption, glucose uptake and GLUT4 membrane translocation in C2C12 cells without affecting expression of HXKII or total GLUT4 protein. The atorvastatin-induced alterations were reversed by cholesterol or mevalonic acid. Cholesterol depletion exerted similar impact to atorvastatin, which could be alleviated by cholesterol supplement. Glucose consumption or GLUT4 translocation was positively associated with cellular cholesterol levels. In HFD mice, atorvastatin not pravastatin significantly increased blood glucose levels following glucose or insulin dose and decreased expression of membrane not total GLUT4 protein in muscle. Glucose exposure following glucose or insulin dose was negatively correlated to muscular free cholesterol concentration. Expression of membrane GLUT4 protein was positively related to free cholesterol in muscle. In conclusion, atorvastatin impaired glucose utilization in muscle cells partly via inhibiting GLUT4 membrane translocation due to inhibition of cholesterol synthesis by atorvastatin, at least, partly contributing to glucose intolerance in HFD mice.

In vivo PET imaging with [(18)F]FDG to explain improved glucose uptake in an apolipoprotein A-I treated mouse model of diabetes

AIMS/HYPOTHESIS

Type 2 diabetes is characterised by decreased HDL levels, as well as the level of apolipoprotein A-I (apoA-I), the main apolipoprotein of HDLs. Pharmacological elevation of HDL and apoA-I levels is associated with improved glycaemic control in patients with type 2 diabetes. This is partly due to improved glucose uptake in skeletal muscle.

METHODS

This study used kinetic modelling to investigate the impact of increasing plasma apoA-I levels on the metabolism of glucose in the db/db mouse model.

RESULTS

Treatment of db/db mice with apoA-I for 2 h significantly improved both glucose tolerance (AUC 2574 ± 70 mmol/l × min vs 2927 ± 137 mmol/l × min, for apoA-I and PBS, respectively; p < 0.05) and insulin sensitivity (AUC 388.8 ± 23.8 mmol/l × min vs 194.1 ± 19.6 mmol/l × min, for apoA-I and PBS, respectively; p < 0.001). ApoA-I treatment also increased glucose uptake by skeletal muscle in both an insulin-dependent and insulin-independent manner as evidenced by increased uptake of fludeoxyglucose ([(18)F]FDG) from plasma into gastrocnemius muscle in apoA-I treated mice, both in the absence and presence of insulin. Kinetic modelling revealed an enhanced rate of insulin-mediated glucose phosphorylation (k 3) in apoA-I treated mice (3.5 ± 1.1 × 10(-2) min(-1) vs 2.3 ± 0.7 × 10(-2) min(-1), for apoA-I and PBS, respectively; p < 0.05) and an increased influx constant (3.7 ± 0.6 × 10(-3) ml min(-1) g(-1) vs 2.0 ± 0.3 × 10(-3) ml min(-1) g(-1), for apoA-I and PBS, respectively; p < 0.05). Treatment of L6 rat skeletal muscle cells with apoA-I for 2 h indicated that increased hexokinase activity mediated the increased rate of glucose phosphorylation.

CONCLUSIONS/INTERPRETATION

These findings indicate that apoA-I improves glucose disposal in db/db mice by improving insulin sensitivity and enhancing glucose phosphorylation.

Curcumin regulates cell fate and metabolism by inhibiting hedgehog signaling in hepatic stellate cells

Accumulating evidence indicates that Hedgehog (Hh) signaling becomes activated in chronic liver injury and plays a role in the pathogenesis of hepatic fibrosis. Hepatic stellate cells (HSCs) are Hh-responsive cells and activation of the Hh pathway promotes transdifferentiation of HSCs into myofibroblasts. Targeting Hh signaling may be a novel therapeutic strategy for treatment of liver fibrosis. We previously reported that curcumin has potent antifibrotic effects in vivo and in vitro, but the underlying mechanisms are not fully elucidated. This study shows that curcumin downregulated Patched and Smoothened, two key elements in Hh signaling, but restored Hhip expression in rat liver with carbon tetrachloride-induced fibrosis and in cultured HSCs. Curcumin also halted the nuclear translocation, DNA binding, and transcription activity of Gli1. Moreover, the Hh signaling inhibitor cyclopamine, like curcumin, arrested the cell cycle, induced mitochondrial apoptosis, reduced fibrotic gene expression, restored lipid accumulation, and inhibited invasion and migration in HSCs. However, curcumin's effects on cell fate and fibrogenic properties of HSCs were abolished by the Hh pathway agonist SAG. Furthermore, curcumin and cyclopamine decreased intracellular levels of adenosine triphosphate and lactate, and inhibited the expression and/or function of several key molecules controlling glycolysis. However, SAG abrogated the curcumin effects on these parameters of glycolysis. Animal data also showed that curcumin downregulated glycolysis-regulatory proteins in rat fibrotic liver. These aggregated data therefore indicate that curcumin modulated cell fate and metabolism by disrupting the Hh pathway in HSCs, providing novel molecular insights into curcumin reduction of HSC activation.

Flow cytometric analysis of bimolecular fluorescence complementation: a high throughput quantitative method to study protein-protein interaction

Among methods to study protein-protein interaction inside cells, Bimolecular Fluorescence Complementation (BiFC) is relatively simple and sensitive. BiFC is based on the production of fluorescence using two non-fluorescent fragments of a fluorescent protein (Venus, a Yellow Fluorescent Protein variant, is used here). Non-fluorescent Venus fragments (VN and VC) are fused to two interacting proteins (in this case, AKAP-Lbc and PDE4D3), yielding fluorescence due to VN-AKAP-Lbc-VC-PDE4D3 interaction and the formation of a functional fluorescent protein inside cells. BiFC provides information on the subcellular localization of protein complexes and the strength of protein interactions based on fluorescence intensity. However, BiFC analysis using microscopy to quantify the strength of protein-protein interaction is time-consuming and somewhat subjective due to heterogeneity in protein expression and interaction. By coupling flow cytometric analysis with BiFC methodology, the fluorescent BiFC protein-protein interaction signal can be accurately measured for a large quantity of cells in a short time. Here, we demonstrate an application of this methodology to map regions in PDE4D3 that are required for the interaction with AKAP-Lbc. This high throughput methodology can be applied to screening factors that regulate protein-protein interaction.

PIP3 but not PIP2 increases GLUT4 surface expression and glucose metabolism mediated by AKT/PKCζ/λ phosphorylation in 3T3L1 adipocytes

Phosphatidylinositol-3,4,5-triphosphate (PIP3) and phosphatidylinositol-4,5-biphosphate (PIP2) are two well-known membrane bound polyphosphoinositides. Diabetes is associated with impaired glucose metabolism. Using a 3T3L1 adipocyte cell model, this study investigated the role of PIP3 and PIP2 on insulin stimulated glucose metabolism in high glucose (HG) treated cells. Exogenous PIP3 supplementation (1, 5, or 10 nM) increased the phosphorylation of AKT and PKCζ/λ, which in turn upregulated GLUT4 total protein expression as well as its surface expression, glucose uptake, and glucose utilization in cells exposed to HG (25 mM); however, PIP2 had no effect. Comparative signal silencing studies with antisense AKT2 and antisense PKCζ revealed that phosphorylation of PKCζ/λ is more effective in PIP3 mediated GLUT4 activation and glucose utilization than in AKT phosphorylation. Supplementation with PIP3 in combination with insulin enhanced glucose uptake and glucose utilization compared to PIP2 with insulin, or insulin alone, in HG-treated adipocytes. This suggests that a decrease in cellular PIP3 levels may cause impaired insulin sensitivity in diabetes. PIP3 supplementation also prevented HG-induced MCP-1 and resistin secretion and lowered adiponectin levels. This study for the first time demonstrates that PIP3 but not PIP2 plays an important role in GLUT4 upregulation and glucose metabolism mediated by AKT/PKCζ/λ phosphorylation. Whether PIP3 levels in blood can be used as a biomarker of insulin resistance in diabetes needs further investigation.

Vitamin D up-regulates glucose transporter 4 (GLUT4) translocation and glucose utilization mediated by cystathionine-γ-lyase (CSE) activation and H2S formation in 3T3L1 adipocytes

A scientific explanation for the beneficial role of vitamin D supplementation in the lowering of glycemia in diabetes remains to be determined. This study examined the biochemical mechanism by which vitamin D supplementation regulates glucose metabolism in diabetes. 3T3L1 adipocytes were treated with high glucose (HG, 25 mm) in the presence or absence of 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) (25, 50 nm), the active form of vitamin D. 1,25(OH)(2)D(3) treatment caused significant up-regulation of GLUT4 total protein expression and its translocation to cell surface, and an increase in glucose uptake as well as glucose utilization in HG-treated cells. 1,25(OH)(2)D(3) also caused cystathionine-γ-lyase (CSE) activation and H(2)S formation in HG-treated adipocytes. The effect of 1,25(OH)(2)D(3) on GLUT4 translocation, glucose utilization, and H(2)S formation was prevented by propargylglycine, an inhibitor of CSE that catalyzes H(2)S formation. Studies using antisense CSE also demonstrated the inhibition of GLUT4 translocation as well as glucose uptake and utilization in 1,25(OH)(2)D(3)-supplemented CSE-siRNA-transfected adipocytes compared with controls. 1,25(OH)(2)D(3) treatment along with insulin enhanced GLUT4 translocation and glucose utilization compared with either insulin or 1,25(OH)(2)D(3) alone in HG-treated adipocytes. 1,25(OH)(2)D(3) supplementation also inhibited monocyte chemoattractant protein-1 and stimulated adiponectin secretion in HG-treated adipocytes, and this positive effect was prevented in propargylglycine-treated or CSE-knockdown adipocytes. This is the first report to demonstrate that 1,25(OH)(2)D(3) up-regulates GLUT4 translocation and glucose utilization and decreases inflammatory markers, which is mediated by CSE activation and H(2)S formation in adipocytes. This study provides evidence for a novel molecular mechanism by which 1,25(OH)(2)D(3) can up-regulate the GLUT4 translocation essential for maintenance of glucose metabolism.