HIGH-LEVEL OVEREXPRESSION OF GLUCOSE TRANSPORTER-4 DRIVEN BY AN ADIPOSE-SPECIFIC PROMOTER IS MAINTAINED IN TRANSGENIC MICE ON A HIGH-FAT DIET, BUT DOES NOT PREVENT IMPAIRED GLUCOSE-TOLERANCE

High fat feeding is associated with impaired insulin action, an obese body composition, and down-regulation of glucose transporter-4 (GLUT4) expression in adipocytes. We recently showed that overexpression of GLUT4 selectively in adipocytes of transgenic mice using the aP2 (fatty acid-binding protein) promoter/enhancer results in enhanced glucose tolerance and adipocyte hyperplasia. Here, we fed these GLUT4-overexpressing transgenic mice a high fat (55%) or a low fat (10%) diet for 13-15 weeks to determine the role of alterations in GLUT4 expression in adipocytes in the development of insulin resistance and obesity, which are characteristic of high fat consumption. In nontransgenic mice, high fat feeding results in 45-50% reduction of GLUT4 levels in white and brown adipose tissue, with a parallel decrease in insulin-stimulated glucose transport. In transgenic mice receiving the low fat diet, GLUT4 is overexpressed 20-fold in white and 4-fold in brown adipose tissue. Glucose transport in epididymal adipocytes is increased 20-fold in the basal state and 6-fold in the insulin-stimulated state. Even after transgenic mice are fed a high fat diet, GLUT4 expression and glucose transport in their adipocytes remains 14- to 30-fold greater than that in nontransgenic mice receiving the same diet. Despite these marked effects at the adipose cell level, glucose tolerance is not improved, probably due to insulin resistance in skeletal muscle and liver, where the transgene is not expressed. During the low fat diet, transgenic mice have 80% more body lipid than nontransgenics. High fat feeding increases body lipid 76% and adipocyte size 65% in nontransgenic mice, but has no effect in transgenic mice. Thus, overexpression of GLUT4 selectively in adipocytes protects against a further increase in adiposity. Furthermore, by using a heterologous promoter, high level overexpression of GLUT4 can be maintained even under metabolic conditions where it is normally down-regulated in adipocytes. This overexpression results in markedly increased glucose transport at the cellular level, but adipose-specific GLUT4 overexpression does not prevent the decrease in glucose tolerance associated with high fat feeding.