Study of Glucose Uptake in Adipose Cells
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It is well established that insulin stimulation of glucose uptake in skeletal muscle cells is mediated through translocation of GLUT4 from intracellular storage sites to the cell surface. However, the established skeletal muscle cell lines, with the exception of L6 myocytes, reportedly show minimal insulin-dependent glucose uptake and GLUT4 translocation. Using C(2)C(12) myocytes expressing exofacial-Myc-GLUT4-enhanced cyan fluorescent protein, we herein show that differentiated C(2)C(12) myotubes are equipped with basic GLUT4 translocation machinery that can be activated by insulin stimulation ( approximately 3-fold increase as assessed by anti-Myc antibody uptake and immunostaining assay). However, this insulin stimulation of GLUT4 translocation was difficult to demonstrate with a conventional 2-deoxyglucose uptake assay because of markedly elevated basal glucose uptake via other glucose transporter(s). Intriguingly, the basal glucose transport activity in C(2)C(12) myotubes appeared to be acutely suppressed within 5 min by preincubation with a pathophysiologically high level of extracellular glucose (25 mM). In contrast, this activity was augmented by acute glucose deprivation via an unidentified mechanism that is independent of GLUT4 translocation but is dependent on phosphatidylinositol 3-kinase activity. Taken together, these findings indicate that regulation of the facilitative glucose transport system in differentiated C(2)C(12) myotubes can be achieved through surprisingly acute glucose-dependent modulation of the activity of glucose transporter(s), which apparently contributes to obscuring the insulin augmentation of glucose uptake elicited by GLUT4 translocation. We herein also describe several methods of monitoring insulin-dependent glucose uptake in C(2)C(12) myotubes and propose this cell line to be a useful model for analyzing GLUT4 translocation in skeletal muscle.
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To explore the effect of high glucose on glucose transport activity, protein kinase B (PKB) activity and glucose transporter 4 (GLUT4) in primary cultured rat adipocytes.Isolated rat adipocytes were cultured for 24 h at different glucose concentrations (5, 10, 15 and 25 mmol.L-1). The glucose uptake, cellular and membrane GLUT4 expression, PKB protein expression, and PKB serine phosphorylation and activity were measured.These adipocytes treated with glucose of different concentrations showed that high glucose impaired glucose uptake, PKB phosphorylation and activity, and up-regulated GLUT4 translocation, but didn't affect protein expression of PKB and GLUT4.High glucose can induce insulin resistance; the mechanism may be involved in the effect of high glucose on PKB serine phosphorylation and activity as well as GLUT4 function.
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Skeletal muscle glucose transport and metabolism were studied in a line of transgenic mice overexpressing the human Glut4 facilitative glucose transporter. Skeletal muscle Glut4 protein levels were increased 2-4-fold in transgenic animals relative to their nontransgenic litter mates. Glut4 overexpression increased total transport activity (measured with 1 mm 2-deoxy-d-glucose) in the isolated extensor digitorum brevis muscle in the presence of insulin; this increase was due to 1) an increase in basal glucose transport (0.8 ± 0.1 versus 0.5 ± 0.1 μmol.ml−1.20 min−1 in transgenic and control mice, respectively) and 2) an increase in insulin-stimulated transport (1.5 ± 0.1 versus 0.8 ± 0.1 μmol.ml−1.20 min−1 above basal transport in transgenic and control mice, respectively). Glut4 overexpression also increased glucose transport stimulated by muscle contractions. In addition, glycolysis and glucose incorporation into glycogen were enhanced in muscle isolated from transgenic mice compared to controls. These data demonstrate that Glut4 overexpression in skeletal muscle increases insulin- and contraction-stimulated glucose transport activity and glucose metabolism. These findings are consistent with the role of Glut4 as the primary mediator of transport stimulated by insulin or contractions. Skeletal muscle glucose transport and metabolism were studied in a line of transgenic mice overexpressing the human Glut4 facilitative glucose transporter. Skeletal muscle Glut4 protein levels were increased 2-4-fold in transgenic animals relative to their nontransgenic litter mates. Glut4 overexpression increased total transport activity (measured with 1 mm 2-deoxy-d-glucose) in the isolated extensor digitorum brevis muscle in the presence of insulin; this increase was due to 1) an increase in basal glucose transport (0.8 ± 0.1 versus 0.5 ± 0.1 μmol.ml−1.20 min−1 in transgenic and control mice, respectively) and 2) an increase in insulin-stimulated transport (1.5 ± 0.1 versus 0.8 ± 0.1 μmol.ml−1.20 min−1 above basal transport in transgenic and control mice, respectively). Glut4 overexpression also increased glucose transport stimulated by muscle contractions. In addition, glycolysis and glucose incorporation into glycogen were enhanced in muscle isolated from transgenic mice compared to controls. These data demonstrate that Glut4 overexpression in skeletal muscle increases insulin- and contraction-stimulated glucose transport activity and glucose metabolism. These findings are consistent with the role of Glut4 as the primary mediator of transport stimulated by insulin or contractions. we thank Xiang-Jing Wang, Connie Skillington, Guofeng Zhou, and Dan Johnson for their excellent technical assitance.
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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.
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Glucose transporter‐4 (GLUT4) is a transmembrane protein that plays a major role in insulin‐mediated glucose transport in muscle and adipocytes. For glucose transport to occur, the GLUT4 protein needs to be translocated from the intracellular pool to the plasma membrane, and certain compounds may enhance this process. The present study investigated the promotion of glucose uptake in differentiated L6 myotubes by cardamonin, isolated from Alpinia katsumadai . Cardamonin increased translocation of GLUT4 to the plasma membrane in L6 cells, but did not activate protein kinase C ζ/ λ , Akt, or AMP‐activated protein‐kinase, all of which are known to regulate GLUT4 translocation. The glucose‐uptake‐promoting activity of cardamonin was not lowered by treatment with a phosphatidylinositol 3′‐kinase inhibitor. These results suggest that cardamonin is a promising active compound for maintaining glucose homeostasis, and that it acts via an unknown mechanism that does not involve activation of the downstream insulin signal and AMP‐activated protein kinase. Copyright © 2011 John Wiley & Sons, Ltd.
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