The hexosamine biosynthetic pathway has been hypothesized to be involved in mediating some of the toxic effects of hyperglycemia. Glutamine:fructose-6-phosphate amidotransferase (GFA), the first and rate limiting enzyme of the hexosamine biosynthetic pathway, was overexpressed in skeletal muscle and adipose tissue of transgenic mice. A 2.4-fold increase of GFA activity in muscle of the transgenic mice led to weight-dependent hyperinsulinemia in random-fed mice. The hyperinsulinemic-euglycemic clamp technique confirmed that transgenic mice develop insulin resistance, with a glucose disposal rate of 68.5 Ϯ 3.5 compared with 129.4 Ϯ 9.4 mg/kg per min ( P Ͻ 0.001) for littermate controls. The decrease in the glucose disposal rate of the transgenic mice is accompanied by decreased protein but not mRNA levels of the insulin-stimulated glucose transporter (GLUT4). These data support the hypothesis that excessive flux through the hexosamine biosynthesis pathway mediates adverse regulatory and metabolic effects of hyperglycemia, specifically insulin resistance of glucose disposal. These mice can serve as a model system to study the mechanism for the regulation of glucose homeostasis by hexosamines.

Glucose toxicity (i.e., glucose-induced reduction in insulin secretion and action) may be mediated by an increased flux through the hexosamine-phosphate p a t hw a y. Glucosamine (GlcN) is widely used to accelerate the hexosamine pathway flux, independently of glucose. We tested the hypothesis that GlcN can aff e c t insulin secretion and/or action in humans. In 10 healthy subjects, we sequentially performed an intravenous g l ucose (

To examine the effect of increased hexosamine flux in liver, the rate-limiting enzyme in hexosamine biosynthesis (glutamine:fructose-6-phosphate amidotransferase [GFA]) was overexpressed in transgenic mice using the PEPCK promoter. Liver from random-fed transgenic mice had 1.6-fold higher GFA activity compared with nontransgenic control littermates (276 ± 24 pmol · mg -1 · min -1 in transgenic mice vs. 176 ± 18 pmol · mg -1 · min -1 in controls, P < 0.05) and higher levels of the hexosamine end product UDP-N-acetyl glucosamine (288 ± 11 pmol/g in transgenic mice vs. 233 ± 10 pmol/g in controls, P < 0.001). Younger transgenic mice compared with control mice had lower fasting serum glucose (4.8 ± 0.5 mmol/l in transgenic mice vs. 6.5 ± 0.8 mmol/l in controls, P < 0.05) without higher insulin levels (48.0 ± 7.8 pmol/l in transgenic mice vs. 56.4 ± 5.4 pmol/l in controls, P = NS); insulin levels were significantly lower in transgenic males (P < 0.05). At 6 months of age, transgenic animals had normal insulin sensitivity by the hyperinsulinemic clamp technique. Hepatic glycogen content was higher in the transgenic mice (108.6 ± 5.2 µmol/g in transgenic mice vs. 32.8 ± 1.3 µmol/g in controls, P < 0.01), associated with an inappropriate activation of glycogen synthase. Serum levels of free fatty acids (FFAs) and triglycerides were also elevated (FFAs, 0.67 ± 0.03 mmol/l in transgenic mice vs. 0.14 ± 0.01 in controls; triglycerides, 1.34 ± 0.15 mmol/l in transgenic mice vs. 0.38 ± 0.01 in controls, P < 0.01). Older transgenic mice became heavier than control mice and exhibited relative glucose intolerance and insulin resistance. The glucose disposal rate at 8 months of age was 154 ± 5 mg · kg -1 · min -1 in transgenic mice vs. 191 ± 6 mg · kg -1 · min -1 in controls (P < 0.05). We conclude that hexosamines are mediators of glucose sensing for the regulation of hepatic glycogen and lipid metabolism. Increased hexosamine flux in the liver signals a shift toward fuel storage, resulting ultimately in obesity and insulin resistance.

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