ROLE OF THE GLUCOSAMINE PATHWAY IN FAT-INDUCED INSULIN-RESISTANCE

To examine whether the hexosamine biosynthetic pathway might play a ro le in fat-induced insulin resistance, we monitored the effects of prol onged elevations in FFA availability both on skeletal muscle levels of UDP-N-acetyl-hexosamines and on peripheral glucose disposal during 7- h euglycemic-hyperinsulinemic (similar to 500 mu U/ml) clamp studies. When the insulin-induced decrease in the plasma FFA levels (to similar to 0.3 mM) was prevented by infusion of a lipid emulsion in 15 consci ous rats (plasma FFA similar to 1.4 mM), glucose uptake (5-7 h = 32.5/-1.7 vs 0-2 h = 45.2+/-2.8 mg/kg per min; P < 0.01) and glycogen synt hesis (P < 0.01) were markedly decreased. During lipid infusion, muscl e UDP-N-acetyl-glucosamine (UDP-GlcNAc) increased by twofold (to 53.4/-1.1 at 3 h and to 55.5+/-1.1 nmol/gram at 7 h vs 20.4+/-1.7 at 0 h, P < 0.01) while glucose-6-phosphate (Glc-6-P) levels were increased at 3 h (475+/-49 nmol/gram) and decreased at 7 h (133+/-7 vs 337+/-28 nm ol/gram at 0 h, P < 0.01). To discern whether such an increase in the skeletal muscle UDP-GlcNAc concentration could account for the develop ment of insulin resistance, we generated similar increases in muscle U DP-GlcNAc using three alternate experimental approaches. Euglycemic cl amps were performed after prolonged hyperglycemia (18 mM, n = 10), or increased availability of either glucosamine (3 mu mol/kg per min; n = 10) or uridine (30 mu mol/kg per min; n = 4). These conditions all re sulted in very similar increases in the skeletal muscle UDP-GlcNAc (to similar to 55 nmol/gram) and markedly impaired glucose uptake and gly cogen synthesis. Thus, fat-induced insulin resistance is associated wi th: (a) decreased skeletal muscle Glc-6-P levels indicating defective transport/phosphorylation of glucose; (b) marked accumulation of the e ndproducts of the hexosamine biosynthetic pathway preceding the onset of insulin resistance. Most important, the same degree of insulin resi stance can be reproduced in the absence of increased FFA availability by a similar increase in skeletal muscle UDP-N-acetyl-hexosamines. In conclusion, our results support the hypothesis that increased FFA avai lability induces skeletal muscle insulin resistance by increasing the flux of fructose-6-phosphate into the hexosamine pathway.