Glucosamine induces insulin resistance in vivo by affecting GLUT 4 translocation in skeletal muscle - Implications for glucose toxicity

Glucosamine (Glmn), a product of glucose metabolism via the hexosamine pathway, causes insulin resistance in isolated adipocytes by impairing insulin-induced GLUT 4 glucose transporter translocation to the plasma membrane, We hypothesized that Glmn causes insulin resistance in vivo by a similar mechanism in skeletal muscle, We performed euglycemic hyperinsulinemic clamps (12 mu/kg/min + H-3-3-glucose) in awake male Sprague-Dawley rats with and without Glmn infusion at rates ranging from 0.1 to 6.5 mg/kg/min, After 4 h of euglycemic clamping, hindquarter muscles were quick-frozen and homogenized, and membranes were subfractionated by differential centrifugation and separated on a discontinuous sucrose gradient (25, 30, and 35% sucrose), Membrane proteins were solubilized and immunoblotted for GLUT 4, With Glmn, glucose uptake (GU) was maximally reduced by 33 +/- 1%, P < 0.001, The apparent Glmn dose to reduce maximal GU by 50% was 0.1 mg/kg/min or 1/70th the rate of GU on a molar basis, Control galactosamine and mannosamine infusions had no effect on GU, Relative to baseline, insulin caused a 2.6-fold increase in GLUT 4 in the 25% membrane fraction (f), P < 0.01, and a 40% reduction in the 35%f, P < 0.05, but had no effect on GLUT 4 in the 30%f, P = NS, Addition of Glmn to insulin caused a 41% reduction of GLUT 4 in the 25%f, P < 0.05, a 29% fall in the 30%f, and prevented the reduction of GLUT 4 in the 35%f, The 30%f membranes were subjected to a second separation,vith a 27 and 30% sucrose gradient, Insulin mobilized GLUT 4 away from the 30%f, P < 0.05, but not the 27%f, In contrast, Glmn reduced GLUT 4 in the 27%f, P < 0.05, but not the 30%f, Thus, Glmn appears to alter translocation of an insulin-insensitive GLUT 4 pool, Coinfusion of Glmn did not alter enrichment of the sarcolemmal markers 5'-nucleotidase, Na+/K(+)ATPase, and phospholemman in either 25, 30, or 35%f, Thus, Glmn completely blocked movement of GLUT 4 induced by insulin, Glmn is a potent inducer of insulin resistance in vivo by causing (at least in part) a defect intrinsic to GLUT 4 translocation and/or trafficking, These data support a potential role for Glmn to cause glucose-induced insulin resistance (glucose toxicity).