Rosiglitazone increases cell surface GLUT4 levels in 3T3-L1 adipocytes through an enhancement of endosomal recycling

PPARγ activation inhibits endocytosis of claudin-4 and protects against deoxynivalenol-induced intestinal barrier dysfunction in IPEC-J2 cells and weaned piglets

The role of peroxisome proliferator activated receptor gamma (PPARγ) in the regulation of adipocyte differentiation has been well characterized. Besides adipose tissue, PPARγ is also highly expressed in the intestine. However, the functional role of PPARγ in the regulation of intestinal function still remains poorly understood. In the present study, we sought to understand the role of PPARγ activation on regulation of intestinal barrier function in intestinal porcine epithelial cells (IPEC-J2) and weaned piglets exposed to the mycotoxin, deoxynivalenol (DON). PPARγ activation by rosiglitazone and troglitazone, two pharmacological PPARγ ligands, increased the protein expression of tight junction proteins (TJP), claudin-3 and 4. PPARγ inhibition increased endocytosis of claudin-4 which was reversed by its activation with troglitazone. DON exposure decreased the protein expression of TJP, and also significantly suppressed PPARγ transcriptional activity. Interestingly, PPARγ activation reversed the reduction of claudin-3 and 4 caused by DON in vitro and in vivo. PPARγ activation also partially restored the transepithelial electrical resistance (TEER) and reduced the permeability of fluorescein isothiocyanate-dextran (FITC-dextran) that have been negatively impacted by DON. These effects were lost in the presence of a specific PPARγ antagonist or in PPARγ knockout cells, confirming the importance of PPARγ in the regulation of intestinal barrier function and integrity. Likewise, in weaned pigs exposed to DON, the PPARγ agonist pioglitazone mitigated the impaired villus-crypt morphology caused by DON. Therefore, pharmacological and natural bioactive compounds with PPARγ stimulatory activities could be effective in preventing DON-induced gut barrier dysfunction.

Cellular responses to 8-methyl nonanoic acid, a degradation by-product of dihydrocapsaicin, in 3T3-L1 adipocytes

BACKGROUND

Capsaicinoids, such as dihydrocapsaicin (DHC), exert the health-promoting effects of chili peppers on energy metabolism. The metabolic responses to capsaicinoids are primarily mediated through transient receptor potential cation channel subfamily V member 1 (TRPV1). However, the varying contributions of their metabolites to beneficial health outcomes remain unclear. 8-methyl nonanoic acid (8-MNA), a methyl-branched medium chain fatty acid (MCFA), is an in vivo degradation by-product of DHC. Since MCFAs have emerged as metabolic modulators in adipocytes, here we examined various cellular responses to 8-MNA in 3T3-L1 adipocytes.

METHODS

The viability of 3T3-L1 adipocytes exposed to various concentrations of 8-MNA was assessed by the Calcein AM assay. Biochemical assays for lipid accumulation, AMP-activated protein kinase (AMPK) activity, lipolysis and glucose uptake were performed in 3T3-L1 adipocytes treated with 8-MNA during 48-h nutrient starvation or 5-day maturation.

RESULTS

8-MNA caused no impact on cell viability. During nutrient starvation, 8-MNA decreased lipid amounts in association with AMPK activation, a molecular event that suppresses lipogenic processes. Moreover, 3T3-L1 adipocytes that were treated with 8-MNA during 5-day maturation exhibited a reduced lipolytic response to isoproterenol and an increased glucose uptake when stimulated with insulin.

CONCLUSIONS

These results suggest that 8-MNA derived from DHC modulates energy metabolism in adipocytes and also support the idea that the metabolic benefits of chili consumption are partly attributable to 8-MNA.

Heparan sulfate promotes differentiation of white adipocytes to maintain insulin sensitivity and glucose homeostasis

Heparan sulfate (HS), a highly sulfated linear polysaccharide, is involved in diverse biological functions in various tissues. Although previous studies have suggested a possible contribution of HS to the differentiation of white adipocytes, there has been no direct evidence supporting this. Here, we inhibited the synthesis of HS chains in 3T3-L1 cells using CRISPR–Cas9 technology, resulting in impaired differentiation of adipocytes with attenuated bone morphogenetic protein 4 (BMP4)–fibroblast growth factor 1 (FGF1) signaling pathways. HS reduction resulted in reduced glucose uptake and decreased insulin-dependent intracellular signaling. We then made heterozygous mutant mice for the Ext1 gene, which encodes an enzyme essential for the HS biosynthesis, specifically in the visceral white adipose tissue (Fabp4-Cre⁺::Ext1^flox/WT mice, hereafter called Ext1^Δ/WT) to confirm the importance of HS in vivo. The expression levels of transcription factors that control adipocyte differentiation, such as peroxisome proliferator–activated receptor gamma, were reduced in Ext1^Δ/WT adipocytes, which contained smaller, unilocular lipid droplets, reduced levels of enzymes involved in lipid synthesis, and altered expression of BMP4–FGF1 signaling molecules. Furthermore, we examined the impact of HS reduction in visceral white adipose tissue on systemic glucose homeostasis. We observed that Ext1^Δ/WT mice showed glucose intolerance because of insulin resistance. Our results demonstrate that HS plays a crucial role in the differentiation of white adipocytes through BMP4–FGF1 signaling pathways, thereby contributing to insulin sensitivity and glucose homeostasis.

Peroxisome proliferator-activated receptor gamma: a novel therapeutic target for cognitive impairment and mood disorders that functions via the regulation of adult neurogenesis

The proliferation, differentiation, and migration of neural precursor cells occur not only during embryonic development but also within distinct regions of the adult brain through the process of adult neurogenesis. As neurogenesis can potentially regulate brain cognition and neuronal plasticity, the factors that enhance neurogenesis can be attractive therapeutic targets for improving cognitive function and regulating neurodegenerative and neuropsychiatric disorders, including affective and mood disorders. Peroxisome proliferator-activated receptors (PPARs) are a class of ligand-activated transcription factors belonging to the nuclear receptor superfamily. PPARγ is a target for insulin sensitizers and plays an essential role in regulating various metabolic processes, including adipogenesis and glucose homeostasis. Interestingly, evidence demonstrates the role of PPARγ activation in regulating neurogenesis. The pharmacological activation of PPARγ using specific ligands increases the proliferation and differentiation of neural stem cells in specific brain regions, including the hippocampus, and prevents neurodegeneration and improves cognition and anxiety/depression-like behaviors in animal models. We summarize here recent reports on the role of PPARγ in adult neurogenesis, as well as the mechanisms involved, and suggest that PPARγ can serve as a potential therapeutic target for neurological and/or neurodegenerative diseases.

Natural Compound 3β,7β,25-trihydroxycucurbita-5,23(E)-dien-19-al from Momordica charantia Acts as PPARγ Ligand

Momordica charantia is a popular vegetable associated with effective complementary and alternative diabetes management in some parts of the world. However, the molecular mechanism is less commonly investigated. In this study, we investigated the association between a major cucurbitane triterpenoid isolated from M. charantia, 3β,7β,25-trihydroxycucurbita-5,23(E)-dien-19-al (THCB) and peroxisome proliferator activated receptor gamma (PPARγ) activation and its related activities using cell culture and molecular biology techniques. In this study, we report on both M. charantia fruit crude extract and THCB in driving the luciferase activity of Peroxisome Proliferator Response Element, associated with PPARγ activation. Other than that, THCB also induced adipocyte differentiation at far less intensity as compared to the full agonist rosiglitazone. In conjunction, THCB treatment on adipocytes also resulted in upregulation of PPAR gamma target genes expression; AP2, adiponectin, LPL and CD34 at a lower magnitude compared to rosiglitazone's induction. THCB also induced glucose uptake into muscle cells and the mechanism is via Glut4 translocation to the cell membrane. In conclusion, THCB acts as one of the many components in M. charantia to induce hypoglycaemic effect by acting as PPARγ ligand and inducing glucose uptake activity in the muscles by means of Glut4 translocation.

A Citrullus colocynthis fruit extract acutely enhances insulin-induced GLUT4 translocation and glucose uptake in adipocytes by increasing PKB phosphorylation

ETHNOPHARMACOLOGICAL RELEVANCE

Citrullus colocynthis (L.) Schrad is a common fruit in traditional medicine and used as remedy against various diseases, especially diabetes. Up to now, its anti-diabetic effects have been fully attributed to its enhancement of pancreatic insulin secretion. Whether C. colocynthis also ameliorates insulin action in peripheral tissues has not been investigated.

AIM OF THE STUDY

In the present study, using 3T3-L1 adipocytes as cell model, we have investigated whether colocynth fruit extracts affect insulin action.

MATERIALS AND METHODS

Various extracts were prepared from the C. colocynthis fruit and screened using a cell-based 96 well plate GLUT4 translocation assay. Promising extracts were further studied for their effects on glucose uptake and cell viability. The effect on insulin signal transduction was determined by Western blot and the molecular composition was established by LC-MS.

RESULTS

The ethyl acetate fractions of aqueous non-defatted extracts of seed and pulp, designated Sna1 and Pna1, acutely enhanced insulin-induced GLUT4 translocation. In accordance, both extracts increased insulin-stimulated cellular glucose uptake. Pna1, which displayed greater effects on GLUT4 and glucose uptake than Sna1, was further investigated and was demonstrated to increase GLUT4 translocation without changing the half-maximum dose (ED50) of insulin, nor changing GLUT4 translocation kinetics. At the molecular level, Pna1 was found to enhance insulin-induced PKB phosphorylation without changing phosphorylation of the insulin receptor. Pna1 appeared not to be toxic to cells and, like insulin, restored cell viability during serum starvation. By investigating the molecular composition of Pna1, nine compounds were identified that made up 87% of the mass of the extract, one of which is likely to be responsible for the insulin-enhancing effects of Pna1.

CONCLUSIONS

The C. colocynthis fruit possesses insulin-enhancing activity. This activity may explain in part its anti-diabetic effects in traditional medicine. It also identifies the C. colocynthis as a source of a potential novel insulin enhancer that may prove to be useful to reduce hyperglycemia in type 2 diabetes.

A Review on Cellular and Molecular Mechanisms Linked to the Development of Diabetes Complications

Modern lifestyle, changing eating habits and reduced physical work have been known to culminate into making diabetes a global pandemic. Hyperglycemia during the course of diabetes is an important causative factor for the development of both microvascular (retinopathy, nephropathy and neuropathy) and macrovascular (coronary artery disease, stroke and peripheral artery disease) complications. In this article, we summarize several mechanisms accountable for the development of both microvascular and macrovascular complications of diabetes. Several metabolic and cellular events are linked to the augmentation of oxidative stress like the activation of advanced glycation end products (AGE) pathway, polyol pathway, Protein Kinase C (PKC) pathway, Poly-ADP Ribose Polymerase (PARP) and hexosamine pathway. Oxidative stress also leads to the production of reactive oxygen species (ROS) like hydroxyl radical, superoxide anion and peroxides. Enhanced levels of ROS rescind the anti-oxidant defence mechanisms associated with superoxide dismutase, glutathione and ascorbic acid. Moreover, ROS triggers oxidative damages at the level of DNA, protein and lipids, which eventually cause cell necrosis or apoptosis. These physiological insults may be related to the microvascular complications of diabetes by negatively impacting the eyes, kidneys and the brain. While underlying pathomechanism of the macrovascular complications is quite complex, hyperglycemia associated atherosclerotic abnormalities like changes in the coagulation system, thrombin formation, fibrinolysis, platelet and endothelial function and vascular smooth muscle are well proven. Since hyperglycemia also modulates the vascular inflammation, cytokines, macrophage activation and gene expression of growth factors, elevated blood glucose level may play a central role in the development of macrovascular complications of diabetes. Taken collectively, chronic hyperglycemia and increased production of ROS are the miscreants for the development of microvascular and macrovascular complications of diabetes.

Towards an Insulin Resistant Adipose Model on a Chip

Introduction

Adipose tissue and adipocytes are primary regulators of insulin sensitivity and energy homeostasis. Defects in insulin sensitivity of the adipocytes predispose the body to insulin resistance (IR) that could lead to diabetes. However, the mechanisms mediating adipocyte IR remain elusive, which emphasizes the need to develop experimental models that can validate the insulin signaling pathways and discover new mechanisms in the search for novel therapeutics. Currently in vitro adipose organ-chip devices show superior cell function over conventional cell culture. However, none of these models represent disease states. Only when these in vitro models can represent both healthy and disease states, they can be useful for developing therapeutics. Here, we establish an organ-on-chip model of insulin-resistant adipocytes, as well as characterization in terms of insulin signaling pathway and lipid metabolism.

Methods

We differentiated, maintained, and induced insulin resistance into primary adipocytes in a microfluidic organ-on-chip. We then characterized IR by looking at the insulin signaling pathway and lipid metabolism, and validated by studying a diabetic drug, rosiglitazone.

Results

We confirmed the presence of insulin resistance through reduction of Akt phosphorylation, Glut4 expression, Glut4 translocation and glucose uptake. We also confirmed defects of disrupted insulin signaling through reduction of lipid accumulation from fatty acid uptake and elevation of glycerol secretion. Testing with rosiglitazone showed a significant improvement in insulin sensitivity and fatty acid metabolism as suggested by previous reports.

Conclusions

The adipose-chip exhibited key characteristics of IR and can serve as model to study diabetes and facilitate discovery of novel therapeutics.

Identification and Characterization of Cannabimovone, a Cannabinoid from Cannabis sativa, as a Novel PPARγ Agonist via a Combined Computational and Functional Study

Phytocannabinoids (pCBs) are a large family of meroterpenoids isolated from the plant Cannabis sativa. Δ⁹-Tetrahydrocannabinol (THC) and cannabidiol (CBD) are the best investigated phytocannabinoids due to their relative abundance and interesting bioactivity profiles. In addition to various targets, THC and CBD are also well-known agonists of peroxisome proliferator-activated receptor gamma (PPARγ), a nuclear receptor involved in energy homeostasis and lipid metabolism. In the search of new pCBs potentially acting as PPARγ agonists, we identified cannabimovone (CBM), a structurally unique abeo-menthane pCB, as a novel PPARγ modulator via a combined computational and experimental approach. The ability of CBM to act as dual PPARγ/α agonist was also evaluated. Computational studies suggested a different binding mode toward the two isoforms, with the compound able to recapitulate the pattern of H-bonds of a canonical agonist only in the case of PPARγ. Luciferase assays confirmed the computational results, showing a selective activation of PPARγ by CBM in the low micromolar range. CBM promoted the expression of PPARγ target genes regulating the adipocyte differentiation and prevented palmitate-induced insulin signaling impairment. Altogether, these results candidate CBM as a novel bioactive compound potentially useful for the treatment of insulin resistance-related disorders.

Gender-specific differences in PPARγ regulation of follicular helper T cell responses with estrogen

Peroxisome proliferator-activated receptor gamma (PPARγ), a master regulator of adipocyte differentiation, has recently been connected with effector T cells, though its role is still not clear. Here, we investigated the roles of PPARγ in follicular helper T (T_FH) cell responses regarding gender specificity. NP-OVA immunization in female but not male CD4-PPARγ^KO mice induced higher proportions of T_FH cells and germinal center (GC) B cells following immunization than were seen in wild type mice. Treatment with the PPARγ agonist pioglitazone significantly reduced T_FH cell responses in female mice while pioglitazone and estradiol (E2) co-treatment ameliorated T_FH cells and GC responses in male mice. E2 treatment significantly enhanced PPARγ expression in male T cells, while T cell activation in the estrus but not in the diestrus stage of the menstrual cycle of females was inhibited by pioglitazone, suggesting that an estrogen-sufficient environment is important for PPARγ-mediated T cell regulation. These results demonstrate gender-based differences in sensitivities of PPARγ in T_FH responses. These findings suggest that appropriate function of PPARγ is required in the regulation of female GC responses and that therapeutic strategies for autoimmune diseases using PPARγ agonists need to be tailored accordingly.

Optogenetic activation reveals distinct roles of PIP3 and Akt in adipocyte insulin action

Glucose transporter 4 (GLUT4; also known as SLC2A4) resides on intracellular vesicles in muscle and adipose cells, and translocates to the plasma membrane in response to insulin. The phosphoinositide 3-kinase (PI3K)-Akt signaling pathway plays a major role in GLUT4 translocation; however, a challenge has been to unravel the potentially distinct contributions of PI3K and Akt (of which there are three isoforms, Akt1-Akt3) to overall insulin action. Here, we describe new optogenetic tools based on CRY2 and the N-terminus of CIB1 (CIBN). We used these 'Opto' modules to activate PI3K and Akt selectively in time and space in 3T3-L1 adipocytes. We validated these tools using biochemical assays and performed live-cell kinetic analyses of IRAP-pHluorin translocation (IRAP is also known as LNPEP and acts as a surrogate marker for GLUT4 here). Strikingly, Opto-PIP3 largely mimicked the maximal effects of insulin stimulation, whereas Opto-Akt only partially triggered translocation. Conversely, drug-mediated inhibition of Akt only partially dampened the translocation response of Opto-PIP3 In spatial optogenetic studies, focal targeting of Akt to a region of the cell marked the sites where IRAP-pHluorin vesicles fused, supporting the idea that local Akt-mediated signaling regulates exocytosis. Taken together, these results indicate that PI3K and Akt play distinct roles, and that PI3K stimulates Akt-independent pathways that are important for GLUT4 translocation.

The first intracellular loop of GLUT4 contains a retention motif

Glucose transporter GLUT4 plays a major role in glucose homeostasis and is efficiently retained intracellularly in adipocytes and myocytes. To simplify the analysis of its retention, various intracellular GLUT4 domains were fused individually to reporter molecules. Of the four short cytoplasmic loops of GLUT4, only the first nine-residue-long loop conferred intracellular retention of truncated forms of the transferrin receptor and CD4 in adipocytes. In contrast, the same loop of GLUT1 was without effect. The reporter molecules to which the first loop of GLUT4 was fused localized, unlike GLUT4, to the TGN, possibly explaining why these molecules did not respond to insulin. The retention induced by the GLUT4 loop was specific to adipocytes as it did not induce retention in preadipocytes. Of the SQWLGRKRA sequence that constitutes this loop, mutation of either the tryptophan or lysine residue abrogated reporter retention. Mutation of these residues individually into alanines in the full-length GLUT4 molecule resulted in a decreased retention for GLUT4-W105A. We conclude that the first intracellular loop of GLUT4 contains retention motif WLGRK, in which Trp105 plays a prominent role.

(-)-Epicatechin-3-O-β-D-allopyranoside from Davallia formosana, Prevents Diabetes and Hyperlipidemia by Regulation of Glucose Transporter 4 and AMP-Activated Protein Kinase Phosphorylation in High-Fat-Fed Mice

The purpose of this experiment was to determine the antidiabetic and lipid-lowering effects of (−)-epicatechin-3-O-β-d-allopyranoside (BB) from the roots and stems of Davallia formosana in mice. Animal treatment was induced by high-fat diet (HFD) or low-fat diet (control diet, CD). After eight weeks of HFD or CD exposure, the HFD mice were treating with BB or rosiglitazone (Rosi) or fenofibrate (Feno) or water through gavage for another four weeks. However, at 12 weeks, the HFD-fed group had enhanced blood levels of glucose, triglyceride (TG), and insulin. BB treatment significantly decreased blood glucose, TG, and insulin levels. Moreover, visceral fat weights were enhanced in HFD-fed mice, accompanied by increased blood leptin concentrations and decreased adiponectin levels, which were reversed by treatment with BB. Muscular membrane protein levels of glucose transporter 4 (GLUT4) were reduced in HFD-fed mice and significantly enhanced upon administration of BB, Rosi, and Feno. Moreover, BB treatment markedly increased hepatic and skeletal muscular expression levels of phosphorylation of AMP-activated (adenosine monophosphate) protein kinase (phospho-AMPK). BB also decreased hepatic mRNA levels of phosphenolpyruvate carboxykinase (PEPCK), which are associated with a decrease in hepatic glucose production. BB-exerted hypotriglyceridemic activity may be partly associated with increased mRNA levels of peroxisome proliferator activated receptor α (PPARα), and with reduced hepatic glycerol-3-phosphate acyltransferase (GPAT) mRNA levels in the liver, which decreased triacylglycerol synthesis. Nevertheless, we demonstrated BB was a useful approach for the management of type 2 diabetes and dyslipidemia in this animal model.

Proteomic Analysis of GLUT4 Storage Vesicles Reveals Tumor Suppressor Candidate 5 (TUSC5) as a Novel Regulator of Insulin Action in Adipocytes

Insulin signaling augments glucose transport by regulating glucose transporter 4 (GLUT4) trafficking from specialized intracellular compartments, termed GLUT4 storage vesicles (GSVs), to the plasma membrane. Proteomic analysis of GSVs by mass spectrometry revealed enrichment of 59 proteins in these vesicles. We measured reduced abundance of 23 of these proteins following insulin stimulation and assigned these as high confidence GSV proteins. These included established GSV proteins such as GLUT4 and insulin-responsive aminopeptidase, as well as six proteins not previously reported to be localized to GSVs. Tumor suppressor candidate 5 (TUSC5) was shown to be a novel GSV protein that underwent a 3.7-fold increase in abundance at the plasma membrane in response to insulin. siRNA-mediated knockdown of TUSC5 decreased insulin-stimulated glucose uptake, although overexpression of TUSC5 had the opposite effect, implicating TUSC5 as a positive regulator of insulin-stimulated glucose transport in adipocytes. Incubation of adipocytes with TNFα caused insulin resistance and a concomitant reduction in TUSC5. Consistent with previous studies, peroxisome proliferator-activated receptor (PPAR) γ agonism reversed TNFα-induced insulin resistance. TUSC5 expression was necessary but insufficient for PPARγ-mediated reversal of insulin resistance. These findings functionally link TUSC5 to GLUT4 trafficking, insulin action, insulin resistance, and PPARγ action in the adipocyte. Further studies are required to establish the exact role of TUSC5 in adipocytes.

Improved adipogenic in vitro differentiation: comparison of different adipogenic cell culture media on human fat and bone stroma cells for fat tissue engineering

To date there is no sufficient in vitro fat tissue engineering and a protocol has not been well established for this purpose. Therefore, we evaluated the in vitro influence of two different adipogenic growth media for their stimulation potential on different cell lineages to clearly define the most potent adipogenic growth media for future in vitro tissue engineering approaches. The samples for differentiation were composed of human adipogenic-derived stroma cells (hADSCs) and human bone marrow mesenchymal stroma cells (hMSCs). A normal adipogenic medium (NAM) and a specific adipogenic medium (SAM) were tested for their adipogenic stimulation potential. After 10 days and 21 days the relative gene expression was measured for the adipogenic marker genes PPARγ2, C/EBPα, FABP4, LPL, and GLUT4 detected through real time reverse transcriptase polymease chain reaction (RT-PCR). Other study variables were the comparison between NAM and SAM and between the used cells hADSCs and hMSCs. Additionally an Oil-Red staining was performed after 21 days. Our results revealed that only SAM was significantly (P<0.05) superior in the differentiation process in contrast to NAM for 10 days and 21 days. As well was SAM superior to differentiate the used cell lineages. This was evaluated by the detected marker genes PPARγ2, C/EBPα, FABP4, LPL, and GLUT4 through real time RT-PCR and by Oil-Red staining. In addition, the hMSCs proofed to be equal donor cells for adipogenic differentiation especially when stimulated by SAM. The results suggest that the SAM should be established as a new standard medium for a more promising in vitro adipogenic differentiation.

Ergostatrien-3β-ol from Antrodia camphorata inhibits diabetes and hyperlipidemia in high-fat-diet treated mice via regulation of hepatic related genes, glucose transporter 4, and AMP-activated protein kinase phosphorylation

This study was designed to explore the effects and mechanism of ergostatrien-3β-ol (EK100) from the submerged whole broth of Antrodia camphorata on diabetes and dyslipidemia in high fat diet (HFD)-fed mice for 12 weeks. The C57BL/6J mouse fed with a high fat diet (HFD) could induce insulin resistance and hyperlipidemia. After 8 week of induction, mice were receiving EK100 (at three dosages) or fenofibrate (Feno) or rosiglitazone (Rosi) or vehicle by oral gavage 4 weeks afterward. HFD-fed mice display increased blood glucose, glycated hemoglobin (HbA1c), total cholesterol (TC), triglyceride (TG), insulin, and leptin levels. These blood markers were significantly lower in EK100-treated mice, and finally ameliorated insulin resistance. EK100 treatment exhibited reduced hepatic ballooning degeneration and size of visceral adipocytes. Glucose transporter 4 (GLUT4) proteins and phosphorylation of Akt in skeletal muscle were significantly increased in EK100- and Rosi-treated mice. EK100, Feno, and Rosi treatment led to significant increases in phosphorylation of AMP-activated protein kinase (phospho-AMPK) protein in both skeletal muscle and liver. Moreover, EK100 caused a decrease in hepatic expressions of phosphenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6 Pase), and decreased glucose production. EK100 lowered blood TG level by inhibition of hepatic fatty acid synthesis by dampening sterol response element binding protein-1c (SREBP-1c) but increasing expression of peroxisome proliferator activated receptor α (PPARα). Moreover, EK100-treated mice reduced blood TC levels by decreased hepatic expressions of SREBP2, which plays a major role in the regulation of cholesterol synthesis. EK100 increased high-density lipoprotein cholesterol (HDL-C) concentrations by increasing expressions of apolipoprotein A-I (apo A-I) in liver tissue. Our findings manifest that EK100 may have therapeutic potential in treating type 2 diabetes associated with hyperlipidemia in HFD-fed mice by regulation of GLUT4, PEPCK, G6 Pase, SREBP1c, SREBP2, apo A-I, and AMPK phosphorylation.

Effect of glucose tolerance factor (GTF) from high chromium yeast on glucose metabolism in insulin-resistant 3T3-L1 adipocytes

Relative expression of IR, IRS-1, IRS-2 and GLUT4 mRNA was determined in insulin-resistant 3T3-L1 adipocytes with different treatments.

Tormentic acid, a major component of suspension cells of Eriobotrya japonica, suppresses high-fat diet-induced diabetes and hyperlipidemia by glucose transporter 4 and AMP-activated protein kinase phosphorylation

This study was designed to evaluate the effects and mechanism of tormentic acid (PTA) on diabetes and dyslipidemia in high-fat (HF)-fed mice. Feeding C57BL/6J mice with a HF diet for 12 weeks induced type 2 diabetes and hyperlipidemia. During the last 4 weeks, the mice were given orally PTA (at two dosages) or rosiglitazone (Rosi) or water. In this study, the HF diet increased glucose, triglyceride, insulin, and leptin levels, whereas PTA effectively prevented these phenomena and ameliorated insulin resistance. PTA reduced visceral fat mass and hepatic triacylglycerol contents; moreover, PTA significantly decreased both the area of adipocytes and ballooning degeneration of hepatocytes. PTA caused increased skeletal muscular AMP-activated protein kinase (AMPK) phosphorylation and Akt phosphorylation and glucose transporter 4 (GLUT4) proteins, but reduced the hepatic expressions of phosphenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6 Pase) genes. PTA enhanced skeletal muscular Akt phosphorylation and increased insulin sensitivity. PTA also enhanced phospho-AMPK in the liver. Therefore, it is possible that the activation of AMPK by PTA results in decreasing hepatic glucose production while increasing skeletal muscular GLUT4 contents, thus contributing to attenuating the diabetic state. Moreover, PTA exhibits an antihyperlipidemic effect by down-regulations of the hepatic sterol regulatory element binding protein-1c (SREBP-1c) and apolipoprotein C-III (apo C-III) and an increased peroxisome proliferator activated receptor (PPAR)-α expression, thus resulting in decreases in blood triglycerides. These findings demonstrated that PTA was effective for the treatment of diabetes and hyperlipidemia in HF-fed mice.

MSC attenuate diabetes-induced functional impairment in adipocytes via secretion of insulin-like growth factor-1

The function of subcutaneous adipocytes in promoting wound healing is significantly suppressed in diabetic wounds. Recent studies have demonstrated the ability of mesenchymal stem cell (MSC) to ameliorate impaired diabetic wound healing. We hypothesized that MSC function may involve subcutaneous adipocytes. The abnormal function of subcutaneous adipocytes from STZ induced diabetic mice including glucose uptake and free fatty acid (FFA) secretion level were assessed. Then these cells were co-cultured with MSC via a transwell system to observe the changes of metabolic index and glucose transporter four (GLUT4) as well as phosphoinositide 3-kinase/protein kinase (PI3K/AKT) signaling pathway expression. The results of metabolic index suggest that MSC obviously attenuated the diabetes-induced functional impairment. Both mRNA and protein expression analyses showed that PI3K/AKT insulin signaling pathway and GLUT4 expression were up-regulated. These changes were substantially associated with a increased level of insulin-like growth factor-1 (IGF-1) secretion from MSC. These findings suggest that MSC could attenuate abnormal function of diabetic adipocytes by IGF-1secretion, which was more or less associated with the beneficial effects of MSC on improving diabetic wound healing.

Urtica dioica extract attenuates depressive like behavior and associative memory dysfunction in dexamethasone induced diabetic mice

Evidences suggest that glucocorticoids results in depression and is a risk factor for type 2 diabetes. Further diabetes induces oxidative stress and hippocampal dysfunction resulting in cognitive decline. Traditionally Urtica dioica has been used for diabetes mellitus and cognitive dysfunction. The present study investigated the effect of the hydroalcoholic extract of Urtica dioica leaves (50 and 100 mg/kg, p.o.) in dexamethasone (1 mg/kg, i.m.) induced diabetes and its associated complications such as depressive like behavior and cognitive dysfunction. We observed that mice administered with chronic dexamethasone resulted in hypercortisolemia, oxidative stress, depressive like behavior, cognitive impairment, hyperglycemia with reduced body weight, increased water intake and decreased hippocampal glucose transporter-4 (GLUT4) mRNA expression. Urtica dioica significantly reduced hyperglycemia, plasma corticosterone, oxidative stress and depressive like behavior as well as improved associative memory and hippocampal GLUT4 mRNA expression comparable to rosiglitazone (5 mg/kg, p.o.). Further, Urtica dioica insignificantly improved spatial memory and serum insulin. In conclusion, Urtica dioica reversed dexamethasone induced hyperglycemia and its associated complications such as depressive like behavior and cognitive dysfunction.

Rosiglitazone enhances apolipoprotein M (Apom) expression in rat's liver

Apolipoprotein M (APOM) has been suggested as a vasculoprotective constituent of high density lipoprotein (HDL), which plays a crucial role behind the mechanism of HDL-mediated anti-atherosclerosis. Previous studies demonstrated that insulin resistance could associate with decreased APOM expressions. In agreement with our previous reports, here, we further confirmed that the insulin sensitivity was also reduced in rats treated with high concentrations of glucose; such effect could be reversed by administration of rosiglitazone, a peroxisome proliferator-activated receptor-γ (PPARγ). The present study shows that Apom expression is significantly affected by either rosiglitazone or hyperglycemia alone without cross interaction with each other, which indicates that the pathway of Apom expression regulating by hyperglycemia might be differed from that by rosiglitazone. Further study indicated that hyperglycemia could significantly inhibit mRNA levels of Lxrb (P=0.0002), small heterodimer partner 1 (Shp1) (P<0.0001), liver receptor homologue-1 (Lrh1) (P=0.0012), ATP-binding cassette transporter 1 (Abca1) (P=0.0012) and Pparb/d (P=0.0043). Two-way ANOVA analysis demonstrated that the interactions between rosiglitazone and infusion of 25% glucose solution on Shp1 (P=0.0054) and Abca1 (4E, P=0.0004) mRNA expression was statistically significant. It is concluded that rosiglitazone could increase Apom expression, of which the detailed mechanism needs to be further investigated. The downregulation of Apom by hyperglycemia might be mainly through decreasing expression of Pparg and followed by inhibiting Lxrb in rats.

Cellular regulation of glucose uptake by glucose transporter GLUT4

GLUT4 is regulated by its intracellular localization. In the absence of insulin, GLUT4 is efficiently retained intracellularly within storage compartments in muscle and fat cells. Upon insulin stimulation (and contraction in muscle), GLUT4 translocates from these compartments to the cell surface where it transports glucose from the extracellular milieu into the cell. Its implication in insulin-regulated glucose uptake makes GLUT4 not only a key player in normal glucose homeostasis but also an important element in insulin resistance and type 2 diabetes. Nevertheless, how GLUT4 is retained intracellularly and how insulin acts on this retention mechanism is largely unclear. In this review, the current knowledge regarding the various molecular processes that govern GLUT4 physiology is discussed as well as the questions that remain.

The rab11 effector protein FIP1 regulates adiponectin trafficking and secretion

Adiponectin is an adipokine secreted by white adipocytes involved in regulating insulin sensitivity in peripheral tissues. Secretion of adiponectin in adipocytes relies on the endosomal system, however, the intracellular machinery involved in mediating adiponectin release is unknown. We have previously reported that intracellular adiponectin partially compartmentalizes with rab 5 and rab11, markers for the early/sorting and recycling compartments respectively. Here we have examined the role of several rab11 downstream effector proteins (rab11 FIPs) in regulating adiponectin trafficking and secretion. Overexpression of wild type rab11 FIP1, FIP3 and FIP5 decreased the amount of secreted adiponectin expressed in HEK293 cells, whereas overexpression of rab11 FIP2 or FIP4 had no effect. Furthermore shRNA-mediated depletion of FIP1 enhanced adiponectin release whereas knock down of FIP5 decreased adiponectin secretion. Knock down of FIP3 had no effect. In 3T3L1 adipocytes, endogenous FIP1 co-distributed intracellularly with endogenous adiponectin and FIP1 depletion enhanced adiponectin release without altering insulin-mediated trafficking of the glucose transporter Glut4. While adiponectin receptors internalized with transferrin receptors, there were no differences in transferrin receptor recycling between wild type and FIP1 depleted adipocytes. Consistent with its inhibitory role, FIP1 expression was decreased during adipocyte differentiation, by treatment with thiazolidinediones, and with increased BMI in humans. In contrast, FIP1 expression increased upon exposure of adipocytes to TNFα. In all, our findings identify FIP1 as a novel protein involved in the regulation of adiponectin trafficking and release.

Labisia pumila Upregulates Peroxisome Proliferator-Activated Receptor Gamma Expression in Rat Adipose Tissues and 3T3-L1 Adipocytes

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-activated transcription factor that regulates lipid and glucose metabolism. We investigated the effects of Labisia pumila (LP) standardized water extract on PPARgamma transcriptional activity in adipocytes in vitro and in vivo. We used a rat model of dihydrotestosterone- (DHT-) induced polycystic ovary syndrome (PCOS), a condition characterized by insulin resistance. At 9 weeks of age, the PCOS rats were randomly subdivided into two groups: PCOS-LP (50 mg/kg/day of LP) and PCOS-control (1 mL of deionised water) for 4-5 weeks on the same schedule. Real-time RT-PCR was performed to determine the PPARgamma mRNA levels. LP upregulated PPARgamma mRNA level by 40% in the PCOS rats. Western blot analysis further demonstrated the increased PPARgamma protein levels in parallel with upregulation in mRNA. These observations were further proven by adipocytes culture. Differentiated 3T3-L1 adipocytes were treated with final concentration of 100  μ g/mL LP and compared to untreated control and 10  μ M of rosiglitazone (in type of thiazolidinediones). LP increased PPARgamma expressions at both mRNA and protein levels and enhanced the effect of glucose uptake in the insulin-resistant cells. The data suggest that LP may ameliorate insulin resistance in adipocytes via the upregulation of PPARgamma pathway.

Single point mutations result in the miss-sorting of Glut4 to a novel membrane compartment associated with stress granule proteins

Insulin increases cellular glucose uptake and metabolism in the postprandial state by acutely stimulating the translocation of the Glut4 glucose transporter from intracellular membrane compartments to the cell surface in muscle and fat cells. The intracellular targeting of Glut4 is dictated by specific structural motifs within cytoplasmic domains of the transporter. We demonstrate that two leucine residues at the extreme C-terminus of Glut4 are critical components of a motif (IRM, insulin responsive motif) involved in the sorting of the transporter to insulin responsive vesicles in 3T3L1 adipocytes. Light microscopy, immunogold electron microscopy, subcellular fractionation, and sedimentation analysis indicate that mutations in the IRM cause the aberrant targeting of Glut4 to large dispersed membrane vesicles that are not insulin responsive. Proteomic characterization of rapidly and slowly sedimenting membrane vesicles (RSVs and SSVs) that were highly enriched by immunoadsorption for either wild-type Glut4 or an IRM mutant revealed that the major vesicle fraction containing the mutant transporter (IRM-RSVs) possessed a relatively small and highly distinct protein population that was enriched for proteins associated with stress granules. We suggest that the IRM is critical for an early step in the sorting of Glut4 to insulin-responsive subcellular membrane compartments and that IRM mutants are miss-targeted to relatively large, amorphous membrane vesicles that may be involved in a degradation pathway for miss-targeted or miss-folded proteins or represent a transitional membrane compartment that Glut4 traverses en route to insulin responsive storage compartments.

A nonradioisotope chemiluminescent assay for evaluation of 2-deoxyglucose uptake in 3T3-L1 adipocytes. Effect of various carbonyls species on insulin action

We have developed a rapid nonradioisotope chemiluminescent assay adapted to high-throughput screening experiments, to evaluate glucose uptake activity in cultured cells. For chemiluminescence quantification of 2-deoxyglucose, we used a luminol oxidation reaction after an enzymatic dephosphorylation of 2-deoxyglucose-6-phosphate. All reactions were performed at 37 °C by consecutive addition of reagents, and the assay is able to quantify 2DG in picomole per well. To confirm the reliability of this method, we have evaluated the dose-effect of insulin, GLUT4 inhibitors and insulin-sensitizing agent on 2DG uptake into 3T3-L1 cells. The results obtained with the assay for 2DG uptake in vitro in the absence or presence of insulin stimulation, were similar to those obtained by the previous radioisotopic and enzymatic methods. We have also used this assay to evaluate the effect of various reactive carbonyl and oxygen species on insulin-stimulated 2DG-uptake into adipocytes. All reactive carbonyl species tested decreased insulin-stimulated glucose uptake in a time- and dose-dependent manner without affecting basal glucose uptake in 3T3-L1 cells. 4-hydroxynonenal was found to be the most potent in the impairment of glucose uptake. This new enzymatic chemiluminescent assay is rapid and useful for measurement of 2DG uptake in insulin-responsive in cultured cells.

Deciphering the role of GLUT4 N-glycosylation in adipocyte and muscle cell models

GLUT4 (glucose transporter 4) is responsible for the insulin-induced uptake of glucose by muscle and fat cells. In non-stimulated (basal) cells, GLUT4 is retained intracellularly, whereas insulin stimulation leads to its translocation from storage compartments towards the cell surface. How GLUT4 is retained intracellularly is largely unknown. Previously, aberrant GLUT4 N-glycosylation has been linked to increased basal cell-surface levels, while N-glycosylation-deficient GLUT4 was found to be quickly degraded. As recycling and degradation of GLUT4 are positively correlated, we hypothesized that incorrect N-glycosylation of GLUT4 might reduce its intracellular retention, resulting in an increased cell-surface recycling, in increased basal cell-surface levels, and in enhanced GLUT4 degradation. In the present study, we have investigated N-glycosylation-deficient GLUT4 in detail in 3T3-L1 preadipocytes, 3T3-L1 adipocytes and L6 myoblasts. We have found no alterations in retention, insulin response, internalization or glucose transport activity. Degradation of the mutant molecule was increased, although once present at the cell surface, its degradation was identical with that of wild-type GLUT4. Our findings indicate that N-glycosylation is important for efficient trafficking of GLUT4 to its proper compartments, but once the transporter has arrived there, N-glycosylation plays no further major role in its intracellular trafficking, nor in its functional activity.

The lignan-rich fractions of Fructus Schisandrae improve insulin sensitivity via the PPAR-γ pathways in in vitro and in vivo studies

ETHNOPHARMACOLOGICAL RELEVANCE

Fructus Schisandrae, the fruit of Schisandra chinensis (Turcz.) Baillon, has been traditionally used as a hypoglycemic agent in Asia and its extracts have been shown to improve insulin-stimulated glucose uptake in cell-based assays in previous studies.

AIM OF THE STUDY

We set out to determine which fractions of Fructus Schisandrae improved peroxisome proliferator-activated receptor (PPAR)-γ activity and glucose-stimulated insulin secretion in cell-based experiments. The fractions that enhance glucose homeostasis were then tested for their hypoglycemic effects and mechanism was examined.

MATERIALS AND METHODS

The fractions (FS-0, FS-20, FS-40, FS-60, FS-80, FS-100) were made by extracting Fructus Schisandrae with 70% ethanol followed by its fractionation with a XDA column with a different ratio of methanol and water. The insulin-stimulated glucose uptake and PPAR-γ agonistic actions of each fraction were investigated in 3T3-L1 adipocytes and glucose-stimulated insulin secretion was determined in Min6 cells. The fraction(s) that were efficacious (200mg/kg bw) were orally given to 90% pancreatectomized (Px) diabetic rats for 8 weeks to evaluate insulin sensitivity in euglycemic hyperinsulinemic clamp and insulin secretion at hyperglycemic clamp.

RESULTS

FS-60 contains schizandrin, gomisin A and angeloylgomisin H while FS-80 contains deoxyschizandrin, γ-schizandrin, and gomisin N. A PPAR-γ agonistic action was greater in the ascending order of the control, FS-80 and FS-60 in 3T3-L1 adipocytes. FS-60 increased the glucose disposal rates of Px rats as much as rosiglitazone during euglycemic hyperinsulinemic clamp while hepatic glucose output at hyperinsulinemic clamped states decreased in the descending order of the control, FS-80, FS-60 with potentiating insulin signaling. At hyperglycemic clamp only FS-60 potentiated first phase insulin secretion in diabetic animals; the second phase was not increased.

CONCLUSIONS

FS-60, a lignan-rich fraction, improves glucose homeostasis by increasing glucose disposal rates and enhancing hepatic insulin sensitivity by working as a PPAR-γ agonist in type-2 diabetic rats.

A serum factor induces insulin-independent translocation of GLUT4 to the cell surface which is maintained in insulin resistance

In response to insulin, glucose transporter GLUT4 translocates from intracellular compartments towards the plasma membrane where it enhances cellular glucose uptake. Here, we show that sera from various species contain a factor that dose-dependently induces GLUT4 translocation and glucose uptake in 3T3-L1 adipocytes, human adipocytes, myoblasts and myotubes. Notably, the effect of this factor on GLUT4 is fully maintained in insulin-resistant cells. Our studies demonstrate that the serum-induced increase in cell surface GLUT4 levels is not due to inhibition of its internalization and is not mediated by insulin, PDGF, IGF-1, or HGF. Similarly to insulin, serum also augments cell surface levels of GLUT1 and TfR. Remarkably, the acute effect of serum on GLUT4 is largely additive to that of insulin, while it also sensitizes the cells to insulin. In accordance with these findings, serum does not appear to activate the same repertoire of downstream signaling molecules that are implicated in insulin-induced GLUT4 translocation. We conclude that in addition to insulin, at least one other biological proteinaceous factor exists that contributes to GLUT4 regulation and still functions in insulin resistance. The challenge now is to identify this factor.