Overactivity of the hexosamine pathway mediates glucose-induced insulin resistance in rat adipocytes. Glutamine:fructose-6-phosphate amidotransferase (GFA) is the rate-limiting enzyme of this pathway. We determined GFA activity in human skeletal muscle biopsies and rates of insulin-stimulated whole- body, oxidative, and nonoxidative glucose disposal using the euglycemic insulin clamp technique combined with indirect calorimetry (insulin infusion rate 1.5 mU · kg-1 · min-1) in 12 male patients with NIDDM (age 54 ±± 2 years, BMI 27.5 ± 0.9 kg/m2, fasting plasma glucose 8.5 ± 0.6 mmol/l) and 9 matched normal men. GFA activity was detectable in human skeletal muscles and completely inhibited by uridine-5'-diphospho-N-acetylglucosamine (UDP-GlcNAc) in all subjects. GFA activity was 46% increased in the NIDDM patients compared with the normal subjects (9.5 ± 1.3 vs. 6.5 ± 1.2 pmol, P < 0.05). Whole-body glucose uptake was 58% decreased in patients with NIDDM (20 ± 3 μmol · kg body wt-1 · min-1) compared with normal subjects (47 ± 4 μmol · kg body wt-1 · min-1, P < 0.001). This decrease was attributable to decreases in both glucose oxidation (9 ± 1 vs. 15 ± 1 μmol · kg-1 · min-1, NIDDM patients vs. control subjects, P < 0.002) and nonoxidative glucose disposal (11 ± 2 vs. 31 ± 4 μmol · kg-1 · min- 1, P < 0.001). In patients with NIDDM, both HbA(1c) (r = -0.51, P < 0.05) and BMI (r = -0.57, P < 0.05) correlated with whole-body glucose uptake. HbA(1c) but not BMI or insulin sensitivity was correlated with basal GFA activity (r = 0.57, P < 0.01) in NIDDM patients and control subjects. We conclude that GFA is found in human skeletal muscle and that all this activity is sensitive to feedback inhibition by UDP-GlcNAc. Chronic hyperglycemia is associated with an increase in skeletal muscle GFA activity, suggesting that increased activity of the hexosamine pathway may contribute to glucose toxicity and insulin resistance in humans.
ASJC Scopus subject areas
- Internal Medicine
- Endocrinology, Diabetes and Metabolism