The role of GLUT4 activation in glucose uptake: Potential implication for insulin resistance

Insulin stimulates glucose uptake into skeletal muscle via translocation of the facilitative glucose transporter GLUT4 from intracellular compartments to the plasma membrane. In addition, there is growing evidence that insulin may increase the intrinsic activity of these transporters (GLUT4 activation). The objective of this Thesis was to further examine the potential contribution of GLUT4 activation to glucose uptake.; First, the potential of two antidiabetic agents, troglitazone and α-lipoic acid, to activate GLUT4 was examined in L6 muscle cells expressing GLUT4 harbouring an exofacial myc epitope tag (GLUT4myc). In these cells, cell surface GLUT4myc can be detected by a colourimetric ELISA-like assay. Both compounds acutely increased glucose uptake. Evidence is provided that both stimuli do so by enhancing, at least in part, the intrinsic activity of GLUT4. Moreover, different pathways appear to be involved in this process: whereas α-lipoic acid-mediated GLUT4 activation seems to be mediated by the p38 mitogen-activated protein kinase, troglitazone-stimulated activation of GLUT4 is not. However, the latter leads to an acute activation of 5′-AMP-activated protein kinase, which might be responsible for the increase in GLUT4 activation.; Since currently available methods do not allow for accurate quantification of cell surface GLUT4 content in intact primary cells and tissues, the physiological role of GLUT4 activation in insulin-stimulated glucose uptake remains to be clarified. In order to address this question, a transgenic animal was created expressing modest levels of GLUT4myc in insulin-sensitive tissues under the control of the human GLUT4 promoter. In isolated brown adipocytes, the fold increase in insulin-stimulated glucose uptake exceeded greatly the fold increase in insulin-mediated GLUT4myc translocation (determined by fluorescent or enzyme-linked myc immunodetection in intact adipocytes). Since insulin-stimulated glucose uptake is almost entirely mediated by GLUT4 (as functionally assessed using the HIV protease inhibitor indinavir), the observed discrepancy between glucose uptake and GLUT4myc translocation supports a role for GLUT4 activation in achieving the full stimulatory effect of insulin on glucose uptake in brown adipocytes. In conclusion, this Thesis provides further support for a contribution of GLUT4 activation to increased glucose uptake in response to various stimuli. Moreover, different signalling pathways may mediate this activation step.