Differential effects of GLUT1 or GLUT4 overexpression on hexosamine biosynthesis by muscles of transgenic mice

Transgenic mice that overexpress GLUT1 or GLUT4 in skeletal muscle were studied; the former but not the latter develop insulin resistance. Because increased glucose flux via the hexosamine biosynthesis pathway has been implicated in glucose-induced insulin resistance, we measured the activity of glutamine:fructose-6-phosphate amidotransferase (GFAT; rate-limiting enzyme) and the concentrations of UDP-N-acetyl hexosamines (major products of the pathway) as well as UDP-hexoses and GDP-mannose in hind limb muscles and liver in both transgenic models and controls. GFAT activity was increased 60-70% in muscles of GLUT1 but not in GLUT4 transgenics. GFAT mRNA abundance was unchanged. The concentrations of all nucleotide-linked sugars were increased 2-3-fold in GLUT1 and were unchanged in GLUT4-overexpressing muscles. Similar results were obtained in fed and fasted mice. GFAT and nucleotide sugars were unchanged in liver, where the transgene is not expressed. We concluded that 1) glucose transport appears to be rate limiting for synthesis of nucleotide sugars; 2) chronically increased glucose flux increases muscle GFAT activity posttranscriptionally; 3) increased UDP-glucose likely accounts for the marked glycogen accumulation in muscles of GLUT1-overexpressing mice; and 4) glucose flux via the hexosamine biosynthetic pathway is increased in muscles of GLUT1-overexpressing but not GLUT4-overexpressing mice; products of the pathway may contribute to insulin resistance in GLUT1 transgenics.