Intracellular targeting of the insulin-regulatable glucose transporter (GLUT4) is isoform specific and independent of cell type.

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Abstract
Insulin stimulates glucose transport in adipocytes via the rapid redistribution of the GLUT1 and GLUT4 glucose transporters from intracellular membrane compartments to the cell surface. Insulin sensitivity is dependent on the proper intracellular trafficking of the glucose transporters in the basal state. The bulk of insulin-sensitive transport in adipocytes appears to be due to the translocation of GLUT4, which is more efficiently sequestered inside the cell and is present in much greater abundance than GLUT1. The cell type and isoform specificity of GLUT4 intracellular targeting were investigated by examining the subcellular distribution of GLUT1 and GLUT4 in cell types that are refractory to the effect of insulin on glucose transport. Rat GLUT4 was expressed in 3T3-L1 fibroblasts and HepG2 hepatoma cells by DNA-mediated transfection. Transfected 3T3-L1 fibroblasts over-expressing human GLUT1 exhibited increased glucose transport, and laser confocal immunofluorescent imaging of GLUT1 in these cells indicated that the protein was concentrated in the plasma membrane. In contrast, 3T3-L1 fibroblasts expressing GLUT4 exhibited no increase in transport activity, and confocal imaging demonstrated that this protein was targeted almost exclusively to cytoplasmic compartments. 3T3-L1 fibroblasts expressing GLUT4 were unresponsive to insulin with respect to transport activity, and no change was observed in the subcellular distribution of the protein after insulin administration. Immunogold labeling of frozen ultrathin sections revealed that GLUT4 was concentrated in tubulo-vesicular elements of the trans-Golgi reticulum in these cells. Sucrose density gradient analysis of 3T3-L1 homogenates was consistent with the presence of GLUT1 and GLUT4 in discrete cytoplasmic compartments. Immunogold labeling of frozen thin sections of HepG2 cells indicated that endogenous GLUT1 was heavily concentrated in the plasma membrane. Sucrose density gradient analysis of homogenates of HepG2 cells expressing rat GLUT4 suggested that GLUT4 is targeted to an intracellular location in these cells. The density of the putative GLUT4-containing cytoplasmic membrane vesicles was very similar in HepG2 cells, 3T3-L1 fibroblasts, 3T3-L1 adipocytes, and rat adipocytes. These data indicate that the intracellular trafficking of GLUT4 is isoform specific. Additionally, these observations support the notion that GLUT4 is targeted to its proper intracellular locale even in cell types that do not exhibit insulin-responsive glucose transport, and suggest that the machinery that regulates the intracellular targeting of GLUT4 is distinct from the factors that regulate insulin-dependent recruitment to the cell surface.