Allosteric regulation of glycogen synthase and hexokinase by glucosamine-6-phosphate during glucosamine-induced insulin resistance in skeletal muscleand heart

Glucosamine infusion induces insulin resistance in vivo, but the effect of glucosamine on intracellular metabolites of the hexosamine pathway, especia lly glucosamine-6-phosphate (GlcN6P) is unknown. Because of the structural similarity of glucose-6-phosphate (G-6-P) and GlcN6P, we hypothesized that accumulation of this metabolite might alter the activities of enzymes such as glycogen synthase and hexokinase. We infused glucosamine (30 mu mol . kg (-1) . min(-1)) to induce insulin resistance in rats during a euglycemic-hy perinsulinemic clamp. Glucosamine induced whole-body insulin resistance, wh ich was apparent after 90 min and continued progressively for 360 min. Desp ite inducing severe whole-body insulin resistance and decrease in glycogen synthase fractional activity in rectus abdominis muscle (69 +/- 3 vs. 83 +/ - 1%, P < 0.01) and heart (7 +/- 1 vs. 32 +/- 4%, P < 0.001), glucosamine d id not change the glycogen content in rectus and even increased it in the h eart (209 +/- 13 vs. 117 +/- 9 mmol/kg dry wt, P < 0.001). Glucosamine incr eased tissue concentrations of UDP-GlcNAc 4.4- and 4.6-fold in rectus abdom inis and heart, respectively. However, GlcN6P concentrations increased 500- and 700-fold in glucosamine-infused animals in rectus abdominis (590 +/- 8 0 vs. 1.2 +/- 0.1 mu mol/kg wet wt, P < 0.001) and heart (7,703 +/- 993 vs. 11.2 +/- 2.3 mu mol/kg wet wt, P < 0.001). To assess the possible signific ance of GlcN6P accumulation, we measured the effect of GlcN6P on glycogen s ynthase and hexokinase activity in vitro. At the GlcN6P concentrations meas ured in rectus abdominis and heart in vivo, glycogen synthase was activated by 21 and 542%, while similar concentrations inhibited hexokinase activity by 5 and 46%, respectively. This study demonstrates that infusion of gluco samine during a euglycemic-hyperinsulinemic clamp results in marked accumul ation of intracellular GlcN6P. The GlcN6P concentrations in the heart and r ectus abdominis muscle reach levels sufficient to cause allosteric activati on of glycogen synthase and inhibition of hexokinase.