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

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

Disrupted erythropoietin signalling promotes obesity and alters hypothalamus proopiomelanocortin production.

Although erythropoietin (Epo) is the cytokine known to regulate erythropoiesis, erythropoietin receptor (EpoR) expression and associated activity beyond haematopoietic tissue remain uncertain. Here we show that mice with EpoR expression restricted to haematopoietic tissues (Tg) develop obesity and insulin resistance. Tg-mice exhibit a decrease in energy expenditure and an increase in white fat mass and adipocyte number. Conversely, Epo treatment of wild-type (WT)-mice increases energy expenditure and reduces food intake and fat mass accumulation but shows no effect in body weight of Tg-mice. EpoR is expressed at a high level in white adipose tissue and in the proopiomelanocortin (POMC) neurons of the hypothalamus. Although Epo treatment in WT-mice induces the expression of the polypeptide hormone precursor, POMC, mice lacking EpoR show reduced levels of POMC in the hypothalamus. This study provides the first evidence that mice lacking EpoR in non-haematopoietic tissue become obese and insulin resistant with loss of Epo regulation of energy homeostasis.

Identification and characterization of p49/STRAP as a novel GLUT4-binding protein.

To identify novel regulatory components involved in the recycling of the insulin-responsive glucose transporter GLUT4, we have used the yeast two-hybrid system to isolate GLUT4-binding proteins from a rat adipose cell cDNA library. We found a 49-kDa protein (p49/STRAP) that specifically interacts with an acidic amino acid motif (Q7IGSEDG) in the N-terminus of GLUT4. Confocal immunofluorescence microscopy of primary rat adipose cells shows co-localization of myc-p49 with GLUT4 and also with the ER-resident protein calnexin. Insulin stimulation had no effect on GLUT4-binding and subcellular distribution of p49 in adipose cells. However, overexpression of the GLUT4-binding domain of p49 in adipose cells reduces protein synthesis and cell-surface expression of GLUT4, but not of GLUT8. Moreover, cell-surface expression of a p49-binding-deficient GLUT4 mutant (ED/QN) is also reduced. Kinetic analysis of HA-epitope-tagged GLUT4 protein synthesis indicates a possible role of p49 in biosynthesis and/or processing of GLUT4 in adipose cells.

7-hydroxystaurosporine (UCN-01) inhibition of Akt Thr308 but not Ser473 phosphorylation: a basis for decreased insulin-stimulated glucose transport.

7-hydroxystaurosporine (UCN-01) infused for 72 hours by continuous i.v. infusion induced insulin resistance during phase I clinical trials. To understand the mechanism for this observation, we examined the effect of UCN-01 on insulin-stimulated glucose transport activity with 3-O-methylglucose in isolated rat adipose cells. UCN-01 inhibits glucose transport activity in a dose-dependent manner at all insulin concentrations. At the clinically relevant concentration of 0.25 mumol/L UCN-01, glucose transport is inhibited 66, 29, and 26% at insulin concentrations of 10, 50, and 100,000 (100K) microunits/mL respectively, thus shifting the dose-response curve to the right. Increasing concentrations of UCN-01 up to 2.5 mumol/L progressively shift the insulin dose-response curve even further. As Akt is known to mediate in part action initiated at the insulin receptor, we also studied the effect of UCN-01 on Akt activation in whole-cell homogenates of these cells. Decreased glucose transport activity directly parallels decreased Akt Thr308 phosphorylation in both an insulin and UCN-01 dose-dependent manner, whereas Akt Ser473 phosphorylation is inhibited only at the lowest insulin concentration, and then, only modestly. UCN-01 also inhibits insulin-induced Thr308 but not Ser473 phosphorylation of Akt associated with the plasma membranes and low-density microsomes and inhibits translocation of GLUT4 from low-density microsomes to plasma membranes as expected from the glucose transport activity measurements. These data suggest that UCN-01 induces clinical insulin resistance by blocking Akt activation and subsequent GLUT4 translocation in response to insulin, and this effect appears to occur by inhibiting Thr308 phosphorylation even in the face of almost completely unaffected Ser473 phosphorylation.