Discordant Effects of Glucosamine on Insulin-stimulated Glucose Metabolism and Phosphatidylinositol 3-Kinase Activity

Abstract

The impact of increased GlcN availability on insulinstimulated p85/p110 phosphatidylinositol 3-kinase (PI3K) activity in skeletal muscle was examined in relation to GlcN-induced defects in peripheral insulin action. Primed continuous GlcN infusion (750 mol/kg bolus; 30 mol/kg⅐min) in conscious rats limited both maximal stimulation of muscle PI3K by acute insulin (I) (1 unit/kg) bolus (I ؉ GlcN ‫؍‬ 1.9-fold versus saline ‫؍‬ 3.3-fold above fasting levels; p < 0.01) and chronic activation of PI3K following 3-h euglycemic, hyperinsulinemic (18 milliunits/kg⅐min) clamp studies (I ؉ GlcN ‫؍‬ 1.2-fold versus saline ‫؍‬ 2.6-fold stimulation; p < 0.01). To determine the time course of GlcN-induced defects in insulin-stimulated PI3K activity and peripheral insulin action, GlcN was administered for 30, 60, 90, or 120 min during 2-h euglycemic, hyperinsulinemic clamp studies. Activation of muscle PI3K by insulin was attenuated following only 30 min of GlcN infusion (GlcN 30 min ‫؍‬ 1.5-fold versus saline ‫؍‬ 2.5-fold stimulation; p < 0.05). In contrast, the first impairment in insulin-mediated glucose uptake (Rd) developed following 110 min of GlcN infusion (110 min ‫؍‬ 39.9 ؎ 1.8 versus 30 min ‫؍‬ 42.8 ؎ 1.4 mg/kg⅐min, p < 0.05). However, the ability of insulin to stimulate phosphatidylinositol 3,4,5-trisphosphate production and to activate glycogen synthase in skeletal muscle was preserved following up to 180 min of GlcN infusion. Thus, increased GlcN availability induced (a) profound and early inhibition of proximal insulin signaling at the level of PI3K and (b) delayed effects on insulin-mediated glucose uptake, yet (c) complete sparing of insulin-mediated glycogen synthase activation. The pattern and time sequence of GlcN-induced defects suggest that the etiology of peripheral insulin resistance may be distinct from the rapid and marked impairment in insulin signaling.The hexosamine biosynthetic pathway in skeletal muscle serves a vital role in the production of the amino sugars that are utilized in multiple glycosylation pathways. Increased biosynthetic activity within the hexosamine pathway is associated with the development of insulin resistance (1-5). In fact, increasing the amount of flux into the GlcN pathway by various means has been shown to induce defects in insulin-stimulated glucose uptake (1, 2, 4 -6), GLUT4 translocation (3), and glycogen synthase activation (5,7,8).Entry into the hexosamine pathway involves the conversion of fructose 6-phosphate to glucosamine 6-phosphate via the rate-limiting enzyme glutamine fructose-6-P-amidotransferase (9). The principal end product of the pathway is UDP-GlcNAc (10), which modifies intracellular proteins by glycosylation. Thus, even modest perturbations of the amount of flux through the hexosamine pathway could have diverse effects on protein functions. The amount of flux into the pathway, estimated by the accumulation of UDP-GlcNAc in muscle, is strongly correlated with the degree of impairment in peripheral insulin action (5).The mechanism(s) of GlcN-induced defect...

Acute glucosamine-induced insulin resistance in muscle in vivo is associated with impaired capillary recruitment

“…Interestingly, when this was measured after only 2 h infusion, glucosamine had no effect [31]. In contrast to this, impaired insulin stimulation of PI3K activity was seen as early as 30 min after glucosamine infusion [33]. Despite this, there was no significant reduction in glucose uptake until 110 min of glucosamine infusion.…”

Evidence for Central Regulation of Glucose Metabolism

“…Of note, the rodent studies discussed above were of longer duration and in some cases involved chronic models. Indeed, although the maximal effects of insulin on tissue signaling are seen shortly after administration (42), an extended duration of about 5 h of hyperinsulinemia is needed to demonstrate its maximal whole body effects on glucose handling (43), highlighting the potential importance of more prolonged central signaling on the regulation of glucose production. Importantly, no studies to date have evaluated the time-dependent effect of CNS insulin on glucose production in rodents, and such studies would add important insights to the field.…”

Dysfunction of Retinal Cell and Optic Nerve by Continuous Cerebroventricular Infusion of Glucosamine

“…These results suggest that horizontal cell may play a role in protecting retina from external environmental or biochemical changes. It has been suggested that increased flux into the glucosamine pathway could quantitatively or qualitatively alter the O-linked glycosylation state of various proteins involved in insulin action and thereby affect their function via numerous potential mechanisms (Hawkins et al, 1999). In addition, O-linked glycosylation of serine or threonine residues can competitively inhibit phosphorylation at the same sites (Griffith et al, 1995a;Hart et al, 1996).…”

“…In addition, O-linked glycosylation of serine or threonine residues can competitively inhibit phosphorylation at the same sites (Griffith et al, 1995a;Hart et al, 1996). The reciprocal glycosylation/phosphorylation state of key serine/threonine kinase could affect the tyrosine phosphorylation state and thus the activation of insulin receptor substrate-1 or other upstream signaling molecules Hawkins et al, 1999). O-Linked glycosylation can also affect the intracellular localization of proteins .…”

“…O-Linked glycosylation can also affect the intracellular localization of proteins . Alternatively, increased glucosamine availability might directly affect glucose transport due to abnormal O-linked glycosylation of the GLUT4 vesicle and high glucosamine induces apoptosis in retinal cells as result of O-glycosylation (Snow and Hart, 1998;Hawkins et al, 1999;Nakamura et al, 2001).…”