Pre-exposure to glucosamine induces insulin resistance of glucose transport and glycogen synthesis in isolated rat skeletal muscles. Study of mechanisms in muscle and in rat-1 fibroblasts overexpressing the human insulin receptor

Effects of oral glucosamine and chondroitin sulfate alone and in combination on the metabolism of SHR and SD rats

Glucosamine (G), often combined with chondroitin sulfate (CS), is a popular natural supplement used widely to treat osteoarthritis. However, use of glucosamine has been linked to development of insulin resistance. To assess the association between glucosamine and insulin resistance more closely, we challenged two rat strains highly sensitive to sugar-induced insulin resistance-Sprague-Dawley (SD) and Spontaneously Hypertensive (SHR) rats. Since elevations of systolic blood pressure (SBP) have been found to be an early and highly sensitive sign of insulin resistance in these two rat strains, we used this parameter as our primary endpoint. Four groups of both rat strains received either no agent (control), G, CS, or a combination of both for 9 weeks. The intake of each agent was calculated to be approximately 3-7 times comparable to human dose. Throughout the study, SBP of both strains consuming the two ingredients alone and in combination were not elevated. Rather, they were significantly lower than control, contrary to what is found in glucose-induced insulin resistance in rats. Over the study period, body weights of the four groups of SD and SHR did not vary significantly. Furthermore, no consistent trends in circulating glucose concentrations were found among the four different groups in the two strains after oral challenge with glucose. Finally, no significant histological differences were found in hearts, kidneys, and livers among the various groups of SHR and SD. From the above result, we conclude that glucosamine and chondroitin sulfate given alone or together do not produce insulin resistance or other related perturbations in two rat strains highly sensitive to sugar-induced insulin resistance.

Development of glucose-induced insulin resistance in muscle requires protein synthesis

Muscles and fat cells develop insulin resistance when exposed to high concentrations of glucose and insulin. We used an isolated muscle preparation incubated with high levels of glucose and insulin to further evaluate how glucose-induced insulin resistance (GIIR) is mediated. Incubation with 2 milliunits/ml insulin and 36 mm glucose for 5 h resulted in an approximately 50% decrease in insulin-stimulated muscle glucose transport. The decrease in insulin responsiveness of glucose transport induced by glucose was not due to impaired insulin signaling, as insulin-stimulated phosphatidylinositol 3-kinase activity and protein kinase B phosphorylation were not reduced. It has been hypothesized that entry of glucose into the hexosamine biosynthetic pathway with accumulation of UDP-N-acetylhexosamines (UDP-HexNAcs) mediates GIIR. However, inhibition of the rate-limiting enzyme GFAT (glutamine:fructose-6-phosphate amidotransferase) did not protect against GIIR despite a marked reduction of UDP-HexNAcs. The mRNA synthesis inhibitor actinomycin D and the protein synthesis inhibitor cycloheximide both completely protected against GIIR despite the massive increases in UDP-HexNAcs and glycogen that resulted from increased glucose entry. Activation of AMP-activated protein kinase also protected against GIIR. These results provide evidence that GIIR can occur in muscle without increased accumulation of hexosamine pathway end products, that neither high glycogen concentration nor impaired insulin signaling is responsible for GIIR, and that synthesis of a protein with a short half-life mediates GIIR. They also suggest that dephosphorylation of a transcription factor may be involved in the induction of GIIR.

Effects of cellular ATP depletion on glucose transport and insulin signaling in 3T3-L1 adipocytes

Glucosamine induced insulin resistance in 3T3-L1 adipocytes, which was associated with a 15% decrease in cellular ATP content. To study the role of ATP depletion in insulin resistance, we employed sodium azide (NaN3) and dinitrophenol (DNP), which affect mitochondrial oxidative phosphorylation, to achieve a similar 15% ATP depletion. Unlike glucosamine, NaN3 and DNP markedly increased basal glucose transport, and the increased basal glucose transport was associated with increased GLUT-1 content in the plasma membrane without changes in total GLUT-1 content. These agents, like glucosamine, did not affect the early insulin signaling that is implicated in insulin stimulation of glucose transport. In cells with a severe 40% ATP depletion, basal glucose transport was similarly elevated, and insulin-stimulated glucose transport was similar in cells with 15% ATP depletion. In these cells, however, early insulin signaling was severely diminished. These data suggest that cellular ATP depletion by glucosamine, NaN3, and DNP exerts differential effects on basal and insulin-stimulated glucose transport and that ATP depletion per se does not induce insulin resistance in 3T3-L1 adipocytes.

Free fatty acids induce peripheral insulin resistance without increasing muscle hexosamine pathway product levels in rats

To evaluate the role of the hexosamine biosynthesis pathway (HBP) in fat-induced insulin resistance, we examined whether fat-induced insulin resistance is additive to that induced by increased HBP flux via glucosamine infusion and, if so, whether such additive effects correlate with muscle HBP product levels. Prolonged hyperinsulinemic (approximately 550 pmol/l) euglycemic clamps were conducted in conscious overnight-fasted rats. After the initial 150 min to attain steady-state insulin action, rats received an additional infusion of saline, Intralipid, glucosamine, or Intralipid and glucosamine (n = 8 or 9 for each) for 330 min. At the conclusion of clamps, skeletal muscles (soleus, extensor digitorum longus, and tibialis anterior) were taken for the measurement of HBP product levels. Intralipid and glucosamine infusions decreased insulin-stimulated glucose uptake (Rd) by 38 and 28%, respectively. When the infusions were combined, insulin-stimulated Rd decreased 47%, significantly more than with Intralipid or glucosamine alone (P < 0.05). The glucosamine-induced insulin resistance was associated with four- to fivefold increases in muscle HBP product levels. In contrast, the Intralipid-induced insulin resistance was accompanied by absolutely no increase in HBP product levels in all of the muscles examined. Also, when infused with glucosamine, Intralipid decreased insulin action below that with glucosamine alone without changing HBP product levels. In a separate study, short-term (50 and 180 min) Intralipid infusion also failed to increase muscle HBP product levels. In conclusion, increased availability of plasma free fatty acids induces peripheral insulin resistance without increasing HBP product levels in skeletal muscle.

Purification and characterization of glutamine:fructose 6-phosphate amidotransferase from rat liver

The enzyme glutamine:fructose 6-phosphate amidotransferase (L-glutamine:D-fructose-6-phosphate amidotransferase; EC 2.6.1.16, GFAT) catalyzes the formation of glucosamine 6-phosphate from fructose 6-phosphate and glutamine. In view of the important role of GFAT in the hexosamine biosynthetic pathway, we have purified the enzyme from rat liver and characterized its physicochemical properties in comparison to those from the published microbial enzymes. The purified enzyme has a molecular mass of about 75 kDa as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. On a Sephacryl S-200 gel filtration column, the purified enzyme eluted in a single peak corresponding to a molecular mass of about 280 kDa, indicating that the active enzyme may be composed of four subunits. The N-terminal amino acid sequence of the purified enzyme was determined as X-G-I-F-A-Y-L-N-Y-H-X-P-R, where X indicates an unidentified residue. The K(M) values of the purified enzyme for fructose 6-phosphate and glutamine were 0.4 and 0.8 mM, respectively. The purified enzyme was inactivated by 4, 4'-dithiodipyridine, and the activity of the inactivated enzyme was restored by dithiothreitol. The inactivation followed pseudo first-order and saturation kinetics with the K(inact) of 5.0 microM. Kinetic studies also indicated that 4,4'-dithiodipyridine is a competitive inhibitor of the enzyme with respect to glutamine. Isolation and analysis of the cysteine-modified peptide indicated that Cys-1 was the modified site. Cys-1 has been suggested to play an important role in enzymatic activity of the Escherichia coli enzyme (M. N. Isupov, G. Obmolova, S. Butterworth, M. Badet-Denisot, B. Badet, I. Polikarpov, J. A. Littlechild, and A. Teplyakov, 1996, Structure 4, 801-810).

Development and comparison of two 3T3-L1 adipocyte models of insulin resistance: increased glucose flux vs glucosamine treatment

Insulin resistance can be induced in vivo by intravenous infusion of glucosamine or in cells by incubation with glucosamine. However, a publication (Hresko, R. C., et al. (1998) J. Biol. Chem. 273, 20658-20668) suggests a trivial explanation of glucosamine-induced insulin resistance whereby intracellular ATP pools are depleted presumably due to the phosphorylation of glucosamine to glucosamine 6-phosphate, a hexosamine pathway intermediate. The reduced ATP level impaired insulin receptor (IR) autophosphorylation and tyrosine kinase activity toward substrates. The present work describes the development and comparison of two methods for inducing insulin resistance, by treating 3T3-L1 adipocytes overnight using either 25 mM glucose/5 nM insulin or 2 mM glucosamine. Under these conditions basal glucose transport rates were comparable with controls. Insulin-stimulated 2-deoxyglucose uptake, however, was reduced by approximately 45% in response to both high glucose/insulin and glucosamine treatment, relative to control cells. The total relative amounts of the insulin-responsive glucose transporter, Glut4, remained constant under both treatment conditions. The relative phosphotyrosine (Tyr(P)) contents of the insulin receptor and its substrate 1 (IRS-1) were assessed in whole cell homogenates. With both methods to induce insulin resistance, IR/IRS-1 Tyr(P) levels were virtually indistinguishable from those in control cells. Insulin-stimulated phosphorylation of Akt on Ser(473) was not impaired in insulin-resistant cells. Furthermore, the relative Tyr(P) content of the PDGF receptor was comparable in high glucose/insulin- or glucosamine-treated 3T3-L1 adipocytes upon subsequent challenge with PDGF. Finally, the relative amounts of glutamine:fructose-6-phosphate amidotransferase and O-linked N-acetylglucosamine transferase, two important hexosamine pathway enzymes, were similar in both treatments when compared with controls. Thus, 3T3-L1 adipocytes can be used as a model system for studying insulin resistance induced by increased influx of glucose. Under appropriate experimental conditions, glucosamine treatment can mimic the effects of increased glucose flux without impairment of tyrosine phosphorylation-based signaling.

Overexpression of glutamine:fructose-6-phosphate amidotransferase in rat-1 fibroblasts enhances glucose-mediated glycogen accumulation via suppression of glycogen phosphorylase activity

The hexosamine biosynthesis pathway (HBP) mediates many of the adverse effects of excess glucose. We have shown previously that glucose down-regulates basal and insulin-stimulated glycogen synthase (GS) activity. Overexpression of the rate-limiting enzyme in the HBP, glutamine:fructose-6-phosphate amidotransferase (GFA), mimics these effects of high glucose and renders the cells more sensitive to glucose. Here we examine the role of the HBP in regulating cellular glycogen content. Glycogen content and glycogen phosphorylase (GP) activity were determined in Rat-1 fibroblasts that overexpress GFA. In both GFA and controls there was a dose-dependent increase in glycogen content (approximately 8-fold) in cells cultured in increasing glucose concentrations (1-20 mM). There was a shift to the left in the glucose dose-response curve for glycogen content in GFA cells (ED50 for glycogen content = 5.80+/-1.05 vs. 8.84+/-0.87 mM glucose, GFA vs. control). Inhibition of GFA reduced glycogen content by 28.4% in controls cultured in 20 mM glucose. In a dose-dependent manner, glucose resulted in a more than 35% decrease in GP activity in controls. GP activity in GFA cells was suppressed compared with that in controls, and there was no glucose-induced down-regulation of GP activity. Glucosamine and uridine mimicked the effects of glucose on glycogen content and GP activity. However, chronic overexpression of GFA is a unique model of hexosamine excess, as culturing control cells in low dose glucosamine (0.1-0.25 mM) did not suppress GP activity and did not eliminate the glucose-mediated down-regulation of GP activity. We conclude that increased flux through the HBP results in enhanced glycogen accumulation due to suppression of GP activity. These results demonstrate that the HBP is an important regulator of cellular glucose metabolism and supports its role as a cellular glucose/satiety sensor.

Lower calorie intake enhances muscle insulin action and reduces hexosamine levels

Previous studies have demonstrated enhanced insulin sensitivity in calorie-restricted [CR, fed 60% ad libitum (AL) one time daily] compared with AL-fed rats. To evaluate the effects of reduced food intake, independent of temporal differences in consumption, we studied AL (unlimited food access)-fed and CR (fed one time daily) rats along with groups temporally matched for feeding [fed 3 meals (M) daily]: MAL and MCR, eating 100 and 60% of AL intake, respectively. Insulin-stimulated glucose transport by isolated muscle was increased in MCR and CR vs. AL and MAL; there was no significant difference for MCR vs. CR or MAL vs. AL. Intramuscular triglyceride concentration, which is inversely related to insulin sensitivity in some conditions, did not differ among groups. Muscle concentration of UDP-N-acetylhexosamines [end products of the hexosamine biosynthetic pathway (HBP)] was lower in MCR vs. MAL despite unaltered glutamine-fructose-6-phosphate aminotransferase activity (rate-limiting enzyme for HBP). These results indicate that the CR-induced increase in insulin-stimulated glucose transport in muscle is attributable to an altered amount, not timing, of food intake and is independent of lower triglyceride concentration. They further suggest that enhanced insulin action might involve changes in HBP.

Glucosamine-induced insulin resistance in 3T3-L1 adipocytes

To study molecular mechanisms for glucosamine-induced insulin resistance, we induced complete and reversible insulin resistance in 3T3-L1 adipocytes with glucosamine in a dose- and time-dependent manner (maximal effects at 50 mM glucosamine after 6 h). In these cells, glucosamine impaired insulin-stimulated GLUT-4 translocation. Glucosamine (6 h) did not affect insulin-stimulated tyrosine phosphorylation of the insulin receptor and insulin receptor substrate-1 and -2 and weakly, if at all, impaired insulin stimulation of phosphatidylinositol 3-kinase. Glucosamine, however, severely impaired insulin stimulation of Akt. Inhibition of insulin-stimulated glucose transport was correlated with that of Akt activity. In these cells, glucosamine also inhibited insulin stimulation of p70 S6 kinase. Glucosamine did not alter basal glucose transport and insulin stimulation of GLUT-1 translocation and mitogen-activated protein kinase. In summary, glucosamine induced complete and reversible insulin resistance in 3T3-L1 adipocytes. This insulin resistance was accompanied by impaired insulin stimulation of GLUT-4 translocation and Akt activity, without significant impairment of upstream molecules in insulin-signaling pathway.

Nutrient modulation of cellular insulin action

Abundant evidence supports a crucial role for dietary factors in the induction and maintenance of insulin resistance. At the cellular and tissue level, the availability of substrates for cellular energy production may play an important role in metabolic regulation and, in particular, in determining the response to insulin stimulation. The infusion of amino acids or fatty acids decreases insulin-stimulated glucose disposal in vivo; sustained hyperglycemia also induces insulin resistance. To determine whether nutrients directly affect insulin signaling, we have evaluated the impact of fatty acids, amino acids, and activation of the hexosamine pathway on insulin signaling in both cultured cells and animal models. We demonstrate that fatty acids and amino acids inhibit early post-receptor steps in insulin action, including tyrosine phosphorylation of insulin receptor substrate (IRS) proteins and activation of phosphatidylinositol 3-kinase (PI3-kinase), both in vitro and in several in vivo models. Similarly, activation of the hexosamine pathway by infusion of glucosamine also reduces insulin-stimulated phosphorylation of IRS proteins, activation of PI3-kinase, and activation of glycogen synthase. These data suggest that nutrients directly modulate insulin signaling, perhaps via common pathways, and thus contribute to cellular insulin resistance.

Discordant effects of glucosamine on insulin-stimulated glucose metabolism and phosphatidylinositol 3-kinase activity

The impact of increased GlcN availability on insulin-stimulated 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 micromol/kg bolus; 30 micromol/kg.min) 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/kg.min) 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/kg.min, 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 role of glucose metabolites in the activation and translocation of glycogen synthase by insulin in 3T3-L1 adipocytes

The role of increased glucose transport in the hormonal regulation of glycogen synthase by insulin was investigated in 3T3-L1 adipocytes. Insulin treatment stimulated glycogen synthase activity 4-5-fold in these cells. Cytosolic glycogen synthase levels decreased by 75% in response to insulin, whereas, conversely, the glycogenolytic agent isoproterenol increased cytosolic enzyme levels by 200%. Removal of extracellular glucose reduced glycogen synthase activation by 40% and completely blocked enzymatic translocation. Addition of 5 mM 2-deoxyglucose did not restore glycogen synthase translocation but did augment dephosphorylation of the protein by insulin. The translocation event could be reconstituted in vitro only by the addition of UDP-glucose to basal cell lysates. Amylase pretreatment of the extracts suppressed glycogen synthase translocation, indicating that the enzyme was binding to glycogen. Incubation of 3T3-L1 adipocytes with 10 mM glucosamine induced a state of insulin resistance, blocked the translocation of glycogen synthase, and inhibited insulin-stimulated glycogen synthesis by 50%. Surprisingly, glycogen synthase activation by insulin was enhanced 4-fold, in part due to allosteric activation by a glucosamine metabolite. In vitro, glucosamine 6-phosphate and glucose 6-phosphate stimulated glycogen synthase activity with similar concentration curves. These results indicate that glucose metabolites have an impact on the regulation of glycogen synthase activation and localization by insulin.

Glucosamine regulation of glucose metabolism in cultured human skeletal muscle cells: divergent effects on glucose transport/phosphorylation and glycogen synthase in non-diabetic and type 2 diabetic subjects

Chronic exposure (48 h) to glucosamine resulted in a dose-dependent reduction of basal and insulin-stimulated glucose uptake activities in human skeletal muscle cell cultures from nondiabetic and type 2 diabetic subjects. Insulin responsiveness of uptake was also reduced. There was no change in total membrane expression of either GLUT1, GLUT3, or GLUT4 proteins. While glucosamine treatment had no significant effects on hexokinase activity measured in cell extracts, glucose phosphorylation in intact cells was impaired after treatment. Under conditions where glucose transport and phosphorylation were down regulated, the fractional velocity (FV) of glycogen synthase was increased by glucosamine treatment. Neither the total activity nor protein expression of glycogen synthase were influenced by glucosamine treatment. The stimulation of glycogen synthase by glucosamine was not due totally to soluble mediators. Reflective of the effects on transport/phosphorylation, total glycogen content and net glycogen synthesis were reduced after glucosamine treatment. These effects were similar in nondiabetic and type 2 cells. In summary: 1) Chronic treatment with glucosamine reduces glucose transport/phosphorylation and storage into glycogen in skeletal muscle cells in culture and impairs insulin responsiveness as well. 2) Down-regulation of glucose transport/phosphorylation occurs at a posttranslational level of GLUTs. 3) Glycogen synthase activity increases with glucosamine treatment. 4) Nondiabetic and type 2 muscle cells display equal sensitivity and responsiveness to glucosamine. Increased exposure of skeletal muscle to glucosamine, a substrate/precursor of the hexosamine pathway, alters intracellular glucose metabolism at multiple sites and can contribute to insulin resistance in this tissue.

Decreased insulin-stimulated GLUT-4 translocation in glycogen-supercompensated muscles of exercised rats

It was recently found that the effect of an exercise-induced increase in muscle GLUT-4 on insulin-stimulated glucose transport is masked by a decreased responsiveness to insulin in glycogen-supercompensated muscle. We evaluated the role of hexosamines in this decrease in insulin responsiveness and found that UDP-N-acetyl hexosamine concentrations were not higher in glycogen-supercompensated muscles than in control muscles with a low glycogen content. We determined whether the smaller increase in glucose transport is due to translocation of fewer GLUT-4 to the cell surface with the 2-N-4-(1-azi-2,2,2-trifluroethyl)-benzoyl-1, 3-bis(D-mannose-4-yloxy)-2-propylamine (ATB-[2-3H]BMPA) photolabeling technique. The insulin-induced increase in GLUT-4 at the cell surface was no greater in glycogen-supercompensated exercised muscle than in muscles of sedentary controls and only 50% as great as in exercised muscles with a low glycogen content. We conclude that the decreased insulin responsiveness of glucose transport in glycogen-supercompensated muscle is not due to increased accumulation of hexosamine biosynthetic pathway end products and that the smaller increase in glucose transport is mediated by translocation of fewer GLUT-4 to the cell surface.

cDNA cloning and mapping of a novel subtype of glutamine:fructose-6-phosphate amidotransferase (GFAT2) in human and mouse

We subcloned human and mouse full-length cDNAs of a novel subtype of glutamine:fructose-6-phosphate amidotransferase (GFAT), which was designated GFAT2 (the previously reported GFAT was named GFAT1). Both the human and the mouse GFAT2 proteins deduced from their open reading frame sequences are composed of 682 amino acids of approximately 77.0 kDa. At the amino acid level, homologies between the human GFAT1 and GFAT2, between the mouse GFAT1 and GFAT2, and between the human GFAT2 and the mouse GFAT2 were 75.6, 74.7, and 97. 2%, respectively. Northern blot analysis using probe specific to human GFAT1 or GFAT2 showed that major transcripts were approximately 3.0 kb in both the human GFAT subtypes. The analysis also revealed different tissue distribution between GFAT1 and GFAT2: GFAT1 was more highly expressed in the placenta, pancreas, and testis than GFAT2; GFAT2 was expressed throughout the central nervous system, especially in the spinal cord, but GFAT1 expression was weak. The locus was mapped to human chromosome 5q and mouse chromosome 11, where a synteny between the two species has been known. GFAT2 can provide insights into understanding the roles of the hexosamine pathway in various tissues, particularly with the development of glucose toxicity and diabetes complications.

Insulin-sensitive regulation of glucose transport and GLUT4 translocation in skeletal muscle of GLUT1 transgenic mice

Skeletal muscle glucose transport was examined in transgenic mice overexpressing the glucose transporter GLUT1 using both the isolated incubated-muscle preparation and the hind-limb perfusion technique. In the absence of insulin, 2-deoxy-d-glucose uptake was increased approximately 3-8-fold in isolated fast-twitch muscles of GLUT1 transgenic mice compared with non-transgenic siblings. Similarly, basal glucose transport activity was increased approximately 4-14-fold in perfused fast-twitch muscles of transgenic mice. In non-transgenic mice insulin accelerated glucose transport activity approximately 2-3-fold in isolated muscles and to a much greater extent ( approximately 7-20-fold) in perfused hind-limb preparations. The observed effect of insulin on glucose transport in transgenic muscle was similarly dependent upon the technique used for measurement, as insulin had no effect on isolated fast-twitch muscle from transgenic mice, but significantly enhanced glucose transport in perfused fast-twitch muscle from transgenic mice to approximately 50-75% of the magnitude of the increase observed in non-transgenic mice. Cell-surface glucose transporter content was assessed via 2-N-4-(l-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(d -mannos-4-yloxy)-2-propylamine photolabelling methodology in both isolated and perfused extensor digitorum longus (EDL). Cell-surface GLUT1 was enhanced by as much as 70-fold in both isolated and perfused EDL of transgenic mice. Insulin did not alter cell-surface GLUT1 in either transgenic or non-transgenic mice. Basal levels of cell-surface GLUT4, measured in either isolated or perfused EDL, were similar in transgenic and non-transgenic mice. Interestingly, insulin enhanced cell-surface GLUT4 approximately 2-fold in isolated EDL and approximately 6-fold in perfused EDL of both transgenic and non-transgenic mice. In summary, these results reveal differences between isolated muscle and perfused hind-limb techniques, with the latter method showing a more robust responsiveness to insulin. Furthermore, the results demonstrate that muscle overexpressing GLUT1 has normal insulin-induced GLUT4 translocation and the ability to augment glucose-transport activity above the elevated basal rates.

Decreased hexosamine biosynthesis in GH-deficient dwarf rat muscle. reversal with GH, but not IGF-I, therapy

Enhanced glucose flux via the hexosamine biosynthesis pathway (HNSP) has been implicated in insulin resistance. We measured L-glutamine:D-fructose-6-phosphate amidotransferase activity (GFAT, a rate-limiting enzyme) and concentrations of UDP-N-acetyl hexosamines (UDP-HexNAc, major products of HNSP) in muscle and liver of growth hormone (GH)-deficient male dwarf (dw) rats. All parameters measured, except body weight, were similar in 5-wk-old control and dw rats. Muscle GFAT activity declined progressively with age in controls and dw rats but was consistently 30-60% lower in 8- to 14-wk-old dw rats vs. age-matched controls; UDP-HexNAc concentrations in muscle were concomitantly 30% lower in dw rats vs. controls (P < 0.01). Concentrations of UDP-hexoses, GDP-mannose, and UDP in muscle were similar in control and dw rats. Muscle HNSP activity was similarly diminished in fed and fasted dw rats. In liver, only a small difference in GFAT activity was evident between controls and dw rats, and no differences in UDP-HexNAc concentrations were observed. Treatment with recombinant human GH (rhGH) for 5 days restored UDP-HexNAc to control levels in dw muscles (P < 0.01) and partially restored GFAT activity. Insulin-like growth factor I treatment was ineffective. We conclude that GH participates in HNSP regulation in muscle.

GLUT-1 or GLUT-4 transgenes in obese mice improve glucose tolerance but do not prevent insulin resistance

Insulin-stimulated glucose uptake is defective in patients with type 2 diabetes. To determine whether transgenic glucose transporter overexpression in muscle can prevent diabetes induced by a high-fat, high-sugar diet, singly (GLUT-1, GLUT-4) and doubly (GLUT-1 and -4) transgenic mice were placed on a high-fat, high-sugar diet or a standard chow diet. On the high-fat, high-sugar diet, wild-type but not transgenic mice developed fasting hyperglycemia and glucose intolerance (peak glucose of 337 +/- 19 vs. 185-209 mg/dl in the same groups on the high-fat, high-sugar diet and 293 +/- 13 vs. 166-194 mg/dl on standard chow). Hyperinsulinemic clamps showed that transporter overexpression elevated insulin-stimulated glucose utilization on standard chow (49 +/- 4 mg. kg-1. min-1 in wild-type vs. 61 +/- 4, 67 +/- 5, and 63 +/- 6 mg. kg-1. min-1 in GLUT-1, GLUT-4, and GLUT-1 and -4 transgenic mice given 20 mU. kg-1. min-1 insulin, and 54 +/- 7, 85 +/- 4, and 98 +/- 11 in wild-type, GLUT-1, and GLUT-4 mice given 60-80 mU. kg-1. min-1 insulin). On the high-fat, high-sugar diet, wild-type and GLUT-1 mice developed marked insulin resistance, but GLUT-4 and GLUT-1 and -4 mice were somewhat protected (glucose utilization during hyperinsulinemic clamp of 28.5 +/- 3.4 vs. 42.4 +/- 5.9, 51.2 +/- 8.1, and 55.9 +/- 4. 9 mg. kg-1. min-1 in wild type, GLUT-1, GLUT-4, GLUT-1 and -4 mice). These data demonstrate that overexpression of GLUT-1 and/or GLUT-4 enhances whole body glucose utilization and prevents the development of fasting hyperglycemia and glucose intolerance induced by a high-fat, high-sugar diet. GLUT-4 overexpression improves the insulin resistance induced by the diet. We conclude that upregulation of glucose transporters in skeletal muscle may be an effective therapeutic approach to the treatment of human type 2 diabetes.

Interrelationship between the Na+/glucose cotransporter and CFTR in Caco-2 cells: relevance to cystic fibrosis

Both the Na+-dependent glucose cotransporter (SGLT1) and the cystic fibrosis transmembrane conductance regulator (CFTR) modulate Na+ and fluid movement, although in opposite directions. Yet few studies have investigated a possible interrelationship between these two transporters. By using the Caco-2 human colon carcinoma cell line, we confirmed that the activities of these transporters increased with spontaneous differentiation to the enterocytic phenotype. We showed that SGLT1 was positively regulated by Cl- and that optimal activity of CFTR was dependent on the presence of glucose. We also demonstrated that inhibition of CFTR by glibenclamide or diphenylamine-2-carboxylate did not modify the activity of SGLT1 and inhibition of SGLT1 by phlorizin did not modify the activity of CFTR, although it resulted in inhibition of glycoconjugate synthesis. These results point to positive substrate-cross regulation of SGLT1 and CFTR and suggest that NaCl and glucose are important for not only Na+ absorption and fluid movement, but also for cAMP-dependent Cl- efflux, and glycoconjugate synthesis, functions that are known to be anomalous in cystic fibrosis.

Glucosamine-induced insulin resistance in 3T3-L1 adipocytes is caused by depletion of intracellular ATP

Glucosamine, which enters the hexosamine pathway downstream of the rate-limiting step, has been routinely used to mimic the insulin resistance caused by high glucose and insulin. We investigated the effect of glucosamine on insulin-stimulated glucose transport in 3T3-L1 adipocytes. The Delta-insulin (insulin-stimulated minus basal) value for 2-deoxyglucose uptake was dramatically inhibited with increasing concentrations of glucosamine with an ED50 of 1.95 mM. Subcellular fractionation experiments demonstrated that reduction in insulin-stimulated 2-deoxyglucose uptake by glucosamine was due to an inhibition of translocation of both Glut 1 and Glut 4 from the low density microsomes (LDM) to the plasma membrane. Analysis of the insulin signaling cascade revealed that glucosamine impaired insulin receptor autophosphorylation, insulin receptor substrate (IRS-1) phosphorylation, IRS-1-associated PI 3-kinase activity in the LDM, and AKT-1 activation by insulin. Measurement of intracellular ATP demonstrated that the effects of glucosamine were highly correlated with its ability to reduce ATP levels. Reduction of intracellular ATP using azide inhibited Glut 1 and Glut 4 translocation from the LDM to the plasma membrane, insulin receptor autophosphorylation, and IRS-1 tyrosine phosphorylation. Additionally, both the reduction in intracellular ATP and the effects on insulin action caused by glucosamine could be prevented by the addition of inosine, which served as an alternative energy source in the medium. We conclude that direct administration of glucosamine can rapidly lower cellular ATP levels and affect insulin action in fat cells by mechanisms independent of increased intracellular UDP-N-acetylhexosamines and that increased metabolism of glucose via the hexosamine pathway may not represent the mechanism of glucose toxicity in fat cells.

A high-capacity assay for activators of glucose incorporation into glycogen in L6 muscle cells

Skeletal muscle is the major tissue responsible for insulin-stimulated glucose uptake and incorporation into glycogen. Skeletal muscle insulin resistance and the resulting postprandial hyperglycemia are hallmarks of non-insulin-dependent diabetes. Therefore, compounds that serve as insulin mimetics or insulin sensitizers in skeletal muscle may be effective in the treatment of diabetes. In order to identify such hypoglycemic agents, a novel assay for activators of glucose incorporation into glycogen has been developed utilizing differentiated L6 muscle cells in 96-well plates. We found that glucose incorporation occurs in a time- and concentration-dependent manner. It is inhibitable by phloretin, an inhibitor of glucose transport. Both insulin and the insulin-mimetic compound pervanadate activate L6 cell glucose incorporation in dose-responsive manners. We conclude that this assay should serve as a high-capacity screen to identify novel compounds that upregulate glucose anabolic metabolism in skeletal muscle. Such chemical entities may prove useful as antidiabetic agents.

A nutrient-sensing pathway regulates leptin gene expression in muscle and fat

Leptin, the protein encoded by the obese (ob) gene, is synthesized and released in response to increased energy storage in adipose tissue. However, it is still not known how incoming energy is sensed and transduced into increased expression of the ob gene. The hexosamine biosynthetic pathway is a cellular 'sensor' of energy availability and mediates the effects of glucose on the expression of several gene products. Here we provide evidence for rapid activation of ob gene expression in skeletal muscle by glucosamine. Increased tissue concentrations of the end product of the hexosamine biosynthetic pathway, UDP-N-acetylglucosamine (UDP-GlcNAc), result in rapid and marked increases in leptin messenger RNA and protein levels (although these levels were much lower than those in fat). Plasma leptin levels and leptin mRNA and protein levels in adipose tissue also increase. Most important, stimulation of leptin synthesis is reproduced by either hyperglycaemia or hyperlipidaemia, which also increase tissue levels of UDP-N-acetylglucosamine in conscious rodents. Finally, incubation of 3T3-L1 pre-adipocytes and L6 myocytes with glucosamine rapidly induces ob gene expression. Our findings are the first evidence of inducible leptin expression in skeletal muscle and unveil an important biochemical link between increased availability of nutrients and leptin expression.

Regulation of GLUT4 protein and glycogen synthase during muscle glycogen synthesis after exercise

The pattern of muscle glycogen synthesis following its depletion by exercise is biphasic. Initially, there is a rapid, insulin independent increase in the muscle glycogen stores. This is then followed by a slower insulin dependent rate of synthesis. Contributing to the rapid phase of glycogen synthesis is an increase in muscle cell membrane permeability to glucose, which serves to increase the intracellular concentration of glucose-6-phosphate (G6P) and activate glycogen synthase. Stimulation of glucose transport by muscle contraction as well as insulin is largely mediated by translocation of the glucose transporter isoform GLUT4 from intracellular sites to the plasma membrane. Thus, the increase in membrane permeability to glucose following exercise most likely reflects an increase in GLUT4 protein associated with the plasma membrane. This insulin-like effect on muscle glucose transport induced by muscle contraction, however, reverses rapidly after exercise is stopped. As this direct effect on transport is lost, it is replaced by a marked increase in the sensitivity of muscle glucose transport and glycogen synthesis to insulin. Thus, the second phase of glycogen synthesis appears to be related to an increased muscle insulin sensitivity. Although the cellular modifications responsible for the increase in insulin sensitivity are unknown, it apparently helps maintain an increased number of GLUT4 transporters associated with the plasma membrane once the contraction-stimulated effect on translocation has reversed. It is also possible that an increase in GLUT4 protein expression plays a role during the insulin dependent phase.

Possible role for gp160 in constitutive but not insulin-stimulated GLUT4 trafficking: dissociation of gp160 and GLUT4 localization

GLUT4-containing vesicles are constantly cycling in both basal and insulin-stimulated states. Our previous studies have shown that basal cycling of GLUT4 is impaired under conditions of high glucose or glucosamine and, as a consequence, GLUT4 is retained intracellularly in low-density microsomes [Filippis A., Clark, S., and Proietto, J. (1997) Biochem. J. 324, 981-985]. In addition to GLUT4 itself, a major protein component of GLUT4-containing vesicles is a glycoprotein of Mr 160000 (gp160). In all studies so far published gp160 has been co-localized with GLUT4 under all conditions. In this study, we show that retention of GLUT4 in low-density microsomes (enriched in Golgi apparatus) is associated with a decrease in gp160 levels in this compartment. A concomitant increase of gp160 in high-density microsomes (enriched in endoplasmic reticulum), demonstrates for the first time a dissociation in the localization of gp160 and GLUT4. Despite the marked decrease in gp160 levels in the GLUT4-containing compartment, insulin-stimulated translocation was normal, while little gp160 appeared in the plasma membrane in response to insulin. The retention of gp160 in the high-density microsomes is apparently not due to a change in the glycosylation state of gp160 as measured by [3H]mannose incorporation. It is concluded that, in rat adipocytes, gp160 is not required for insulin-stimulated translocation, but may be necessary for constitutive trafficking of the GLUT4-containing vesicle.

Reduced glucose clearance as the major determinant of postabsorptive hyperglycemia in diabetic rats

The relationships between postabsorptive glucose concentration and hepatic glucose output (HGO) and glucose clearance were studied in rats one day after treatment with various doses of streptozotocin (STZ; 0, 15, 30, 40, 50, or 75 mg/kg; n = 6 per dose; study 1). Glucose fluxes were estimated using a prolonged (6-h) infusion of [3-3H]glucose to ensure complete tracer equilibration at hyperglycemia. Postabsorptive glucose was significantly increased at the high doses of STZ (50 and 75 mg/kg; P < 0.01) and was strongly correlated with glucose clearance across all doses (r = -0.85, P < 0.001) but less strongly with HGO (r = 0.46, P < 0.01). In the group treated with 50 mg/kg STZ, postabsorptive glucose was increased twofold compared with the control (i.e., zero dose) group, with no change in HGO and a 45% decrease in glucose clearance, indicating that the hyperglycemia was due to a decrease in glucose clearance. To understand the cellular mechanisms of decreased glucose clearance in STZ diabetic rats, skeletal muscle glucose clearance and intracellular glucose and glucose 6-phosphate (G-6-P) concentrations were determined in normal and STZ (50 mg/kg) diabetic rats at their postabsorptive glucose levels as well as at matched hyperglycemia (12 mM; study 2). Glucose clearance was significantly decreased in soleus (P < 0.05) muscles of the diabetic rats, and this was associated with significantly decreased intracellular glucose and G-6-P levels at matched hyperglycemia (P < 0.05), suggestive of decreased glucose transport. In conclusion, postabsorptive hyperglycemia in STZ diabetic rats was largely due to decreased glucose clearance, although increased HGO may also have been a contributing factor at the highest STZ dose. The decrease in postabsorptive glucose clearance in STZ diabetic rats appeared to be associated with an impairment of glucose transport in soleus (type I) muscles.

Weaning marginally affects glucose transporter (GLUT4) expression in calf muscles and adipose tissues

The nutritional regulation of glucose transporter GLUT4 was studied in eight muscles and four adipose tissues from two groups of preruminant (PR) or ruminant (R) calves of similar age (170 d), empty body weight (194 kg) at slaughter, and level of net energy intake from birth onwards. Isocitrate dehydrogenase (EC 1.1.1.41) activity in muscles was not different between PR and R except in masseter muscle from the cheek (+71% in R; P < 0.003), which becomes almost constantly active at weaning for food chewing. Basal and maximally-insulin-stimulated glucose transport rate (GTR) per g tissue wet weight in rectus abdominis muscle were significantly higher in R calves (+31 and 41% respectively; P < 0.05). GLUT4 protein contents did not differ in muscles from PR and R except in masseter (+74% in R; P < 0.05) indicating that the increased GTR in rectus abdominis cannot be accounted for by an enhanced GLUT4 expression. GLUT4 mRNA levels did not differ between the two groups of animals in all muscles suggesting a regulation of GLUT4 at the protein level in masseter. GLUT4 number expressed on a per cell basis was lower in adipose tissue from R calves (-39%; P < 0.05) and higher in internal than in peripheral adipose tissues. In summary, the regulation of GLUT4 in calves at weaning differs markedly from that previously described in rodents (for review, see Girard et al. 1992). Furthermore, significant inter-individual variations were shown for metabolic activities in muscle and for biochemical variables in adipose tissue.

Dolichol-mediated enhanced protein N-glycosylation in experimental diabetes--a possible additional deleterious effect of hyperglycemia

In liver cells from diabetic rats, an increased incorporation of labeled glucosamine into cellular and secretory proteins was found, when related to the incorporation of labeled leucine. This increased N-glycosylation was present in the face of decreased synthesis of hepatic cellular and secretory proteins evident from reduced leucine incorporation and diminished glycosyltransferase activity. To elucidate the mechanisms involved we incubated isolated hepatocytes with two N-glycosylation inhibitors: tunicamycin and 2-deoxyglucose. Tunicamycin exerted a marked inhibitory effect on the incorporation rate of labeled glucosamine into proteins in liver cells from diabetic rats, while 2-deoxyglucose had a negligible effect on this process in these cells. These diverse effects might be explained by the fact that tunicamycin acts through strong association with the enzyme catalyzing the first step in glycoprotein synthesis, namely, the transfer of UDP-GlcNAc to dolichol-P (indicating noncompetitive inhibition). This enzyme is reduced in liver cells from diabetic animals. On the other hand, 2-deoxyglucose exerts its effect by being attached to dolichol-P, preventing further elongation of oligosaccharide chain on the protein backbone. This latter effect might be eliminated by excess dolichol-P (indicating competitive inhibition). The dolichol content in liver extract from diabetic rats was about 2.5-fold higher compared with nondiabetic rats (51.6 micrograms/g versus 20.6 micrograms/g wet liver weight). These two lines of evidence confirm the notion that the enhanced enzymatic glycosylation in liver from diabetic animals is maintained by an increased hepatic dolichol concentration, which is most probably related to the hyperglycemia. Thus, the dolichol-N-glycosylation pathway may represent another detrimental aspect of hyperglycemia and may operate by dolichol mass action rather than through glycosylating enzyme activity.

Increased flux through the hexosamine biosynthesis pathway inhibits glucose transport acutely by activation of protein kinase C

The hexosamine biosynthesis pathway and protein kinase C (PKC) activation mediate hyperglycaemia-induced impaired glucose transport, but the relative role of each pathway is unknown. Following a 2 h preincubation of rat adipocytes in the presence of either high glucose (30 mM) plus insulin (0.7 nM) or glucosamine (3 mM), both high glucose and glucosamine inhibited subsequent basal and insulin-stimulated glucose transport, measured at 5.0 mM glucose. Azaserine, an inhibitor of the enzyme glutamine:fructose-6-phosphate aminotransferase, abolished the effect of high glucose, but not that of glucosamine. Ro-31-8220, an inhibitor of PKC, reversed the effects of both high glucose and glucosamine, suggesting that flux through the hexosamine biosynthesis pathway impaired glucose transport acutely by activating PKC. Both high glucose and glucosamine caused a 3-fold increase in PKC activity; this effect of high glucose, but not that of glucosamine, was partially decreased by azaserine. Neither high glucose nor glucosamine altered basal or insulin-stimulated plasma membrane GLUT1 levels, whereas both treatments decreased basal, but not insulin-stimulated, GLUT4 levels. Azaserine abolished the effect of high glucose, but not that of glucosamine, on basal plasma membrane GLUT4 levels. Ro-31-8220, which returned glucose transport to control values, caused a further decrease in plasma membrane GLUT4 levels. It is concluded that, in rat adipocytes, an acute increase in flux through the hexosamine biosynthesis pathway inhibits glucose transport by activation of PKC.

Role of the glucosamine pathway in fat-induced insulin resistance

To examine whether the hexosamine biosynthetic pathway might play a role in fat-induced insulin resistance, we monitored the effects of prolonged elevations in FFA availability both on skeletal muscle levels of UDP-N-acetyl-hexosamines and on peripheral glucose disposal during 7-h euglycemic-hyperinsulinemic (approximately 500 microU/ml) clamp studies. When the insulin-induced decrease in the plasma FFA levels (to approximately 0.3 mM) was prevented by infusion of a lipid emulsion in 15 conscious rats (plasma FFA approximately 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 synthesis (P < 0.01) were markedly decreased. During lipid infusion, muscle 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 nmol/gram at 0 h, P < 0.01). To discern whether such an increase in the skeletal muscle UDP-GlcNAc concentration could account for the development of insulin resistance, we generated similar increases in muscle UDP-GlcNAc using three alternate experimental approaches. Euglycemic clamps were performed after prolonged hyperglycemia (18 mM, n = 10), or increased availability of either glucosamine (3 micromol/kg per min; n = 10) or uridine (30 micromol/kg per min; n = 4). These conditions all resulted in very similar increases in the skeletal muscle UDP-GlcNAc (to approximately 55 nmol/gram) and markedly impaired glucose uptake and glycogen synthesis. Thus, fat-induced insulin resistance is associated with: (a) decreased skeletal muscle Glc-6-P levels indicating defective transport/phosphorylation of glucose; (b) marked accumulation of the endproducts of the hexosamine biosynthetic pathway preceding the onset of insulin resistance. Most important, the same degree of insulin resistance 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 availability induces skeletal muscle insulin resistance by increasing the flux of fructose-6-phosphate into the hexosamine pathway.

Effects of exercise and feeding on the hexosamine biosynthetic pathway in rat skeletal muscle

Products of the hexosamine biosynthesis pathway (HSNP) have been implicated in glucose-induced insulin resistance. We measured the major products of HSNP, UDP-N-acetyl hexosamines (UDP-HexNAc), and the activity of L-glutamine: D-fructose-6-phosphate amidotransferase (GFAT, rate-limiting enzyme) in rat hindlimb muscles immediately after exercise and 1, 3, and 16 h postexercise (swimming) in fed and fasted rats and sedentary controls. Muscle glycogen decreased by 50-75% postexercise. In sedentary rats, muscle GFAT activity decreased by approximately 30% (P < 0.002) after an 18-h fast. GFAT activity was not affected by exercise under any condition. Muscle UDP-HexNAc increased approximately 30% postexercise (P < 0.01) in ad libitum-fed but not in fasted rats. UDP-HexNAc remained elevated (approximately 30%, P < 0.002) for 16 h if animals were fed postexercise. Concentrations of UDP-hexoses, GDP-mannose, and UDP were unchanged postexercise. Conclusions are that muscle GFAT activity is regulated by the nutritional state but not by acute exercise. Glucose flux via HNSP may be increased postexercise in muscles of ad libitum-fed rats. Increased HSNP products may serve as negative feedback regulators to limit excessive muscle glycogen deposition postexercise.

The tissue concentration of UDP-N-acetylglucosamine modulates the stimulatory effect of insulin on skeletal muscle glucose uptake

To delineate the biochemical mechanism by which increased availability of GlcN impairs insulin action on skeletal muscle glucose uptake, we replenished the uridine pool during GlcN administration. Co-infusion of uridine with GlcN prevented the GlcN-induced fall in skeletal muscle UDP-glucose levels (24.9 +/- 5. 3 versus 10.1 +/- 2.9 nmol/g; p < 0.01) and further increased the skeletal muscle UDP-GlcNAc levels (198.4 +/- 26.3 versus 96.0 +/- 8. 4 nmol/g; p < 0.01). Greater reductions in the rates of glucose infusion ( approximately 53%), glucose uptake ( approximately 43%), and glycogen synthesis ( approximately 60%) were observed with the addition of uridine. Similarly, the infusion of uridine alone markedly increased the skeletal muscle levels of both UDP-glucose (55.2 +/- 14.2 versus 17.8 +/- 6.1 nmol/g; p < 0.01) and UDP-GlcNAc (86.8 +/- 8.8 versus 35.9 +/- 8.4 nmol/g; p < 0.05) and induced marked insulin resistance. The decrease in insulin action on peripheral glucose uptake was highly correlated with the increase in skeletal muscle UDP-GlcNAc levels. Finally, immunoisolation of GLUT4-containing vesicles revealed that the rate of labeled GlcN incorporation was approximately 100-fold greater following GlcN compared with saline infusions (p < 0.01). We suggest that the marked reduction in insulin action induced by GlcN and uridine is mediated by increased accumulation of muscle UDP-N-acetylhexosamines, perhaps via altered glycosylation of protein(s) in GLUT4-containing vesicles.

Insulin action on glucose transport and plasma membrane GLUT4 content in skeletal muscle from patients with NIDDM

We investigated the response of the glucose transport system to insulin, in the presence of ambient glucose concentrations, in isolated skeletal muscle from seven patients with non-insulin-dependent diabetes mellitus (NIDDM) (age, 55 +/- 3 years, BMI 27.4 +/- 1.8 kg/m2) and seven healthy control subjects (age, 54 +/- 3 years, BMI 26.5 +/- 1.1 kg/m2). Insulin-mediated whole body glucose utilization was similar between the groups when studied in the presence of ambient glucose concentrations (approximately 10 mmol/l for the NIDDM patients and 5 mmol/l for the control subjects). Samples were obtained from the vastus lateralis muscle, by means of an open muscle biopsy procedure, before and after a 40-min insulin infusion. An increase in serum insulin levels from 54 +/- 12 to 588 +/- 42 pmol/l, induced a 1.6 +/- 0.2-fold increase in glucose transporter protein (GLUT4) in skeletal muscle plasma membranes obtained from the control subjects (p < 0.05), whereas no significant increase was noted in plasma membrane fractions prepared from NIDDM muscles, despite a similar increase in serum insulin levels. At concentrations of 5 mmol/l 3-O-methylglucose in vitro, insulin (600 pmol/l) induced a 2.2-fold (p < 0.05) increase in glucose transport in NIDDM muscles and a 3.4-fold (p < 0.001) increase in the control muscles. Insulin-stimulated 3-O-methylglucose transport was positively correlated with whole body insulin-mediated glucose uptake in all participants (r = 0.78, p < 0.001) and negatively correlated with fasting plasma glucose levels in the NIDDM subjects (r = 0.93, p < 0.001). Muscle fibre type distribution and capillarization were similar between the groups. Our results suggest that insulin-stimulated glucose transport in skeletal muscle from patients with NIDDM is down-regulated in the presence of hyperglycaemia. The increased flux of glucose as a consequence of hyperglycaemia may result in resistance to any further insulin-induced gain of GLUT4 at the level of the plasma membrane.

Differential effects of GLUT1 or GLUT4 overexpression on hexosamine biosynthesis by muscles of transgenic mice

Transgenic mice that overexpress GLUT1 or GLUT4 in skeletal muscle were studied; the former but not the latter develop insulin resistance. Because increased glucose flux via the hexosamine biosynthesis pathway has been implicated in glucose-induced insulin resistance, we measured the activity of glutamine:fructose-6-phosphate amidotransferase (GFAT; rate-limiting enzyme) and the concentrations of UDP-N-acetyl hexosamines (major products of the pathway) as well as UDP-hexoses and GDP-mannose in hind limb muscles and liver in both transgenic models and controls. GFAT activity was increased 60-70% in muscles of GLUT1 but not in GLUT4 transgenics. GFAT mRNA abundance was unchanged. The concentrations of all nucleotide-linked sugars were increased 2-3-fold in GLUT1 and were unchanged in GLUT4-overexpressing muscles. Similar results were obtained in fed and fasted mice. GFAT and nucleotide sugars were unchanged in liver, where the transgene is not expressed. We concluded that 1) glucose transport appears to be rate limiting for synthesis of nucleotide sugars; 2) chronically increased glucose flux increases muscle GFAT activity posttranscriptionally; 3) increased UDP-glucose likely accounts for the marked glycogen accumulation in muscles of GLUT1-overexpressing mice; and 4) glucose flux via the hexosamine biosynthetic pathway is increased in muscles of GLUT1-overexpressing but not GLUT4-overexpressing mice; products of the pathway may contribute to insulin resistance in GLUT1 transgenics.

Overexpression of glutamine:fructose-6-phosphate amidotransferase in transgenic mice leads to insulin resistance

The hexosamine biosynthetic pathway has been hypothesized to be involved in mediating some of the toxic effects of hyperglycemia. Glutamine:fructose-6-phosphate amidotransferase (GFA), the first and rate limiting enzyme of the hexosamine biosynthetic pathway, was overexpressed in skeletal muscle and adipose tissue of transgenic mice. A 2.4-fold increase of GFA activity in muscle of the transgenic mice led to weight-dependent hyperinsulinemia in random-fed mice. The hyperinsulinemic-euglycemic clamp technique confirmed that transgenic mice develop insulin resistance, with a glucose disposal rate of 68.5 +/- 3.5 compared with 129.4 +/- 9.4 mg/kg per min (P < 0.001) for littermate controls. The decrease in the glucose disposal rate of the transgenic mice is accompanied by decreased protein but not mRNA levels of the insulin-stimulated glucose transporter (GLUT4). These data support the hypothesis that excessive flux through the hexosamine biosynthesis pathway mediates adverse regulatory and metabolic effects of hyperglycemia, specifically insulin resistance of glucose disposal. These mice can serve as a model system to study the mechanism for the regulation of glucose homeostasis by hexosamines.

Glutamine:fructose-6-phosphate amidotransferase activity in cultured human skeletal muscle cells: relationship to glucose disposal rate in control and non-insulin-dependent diabetes mellitus subjects and regulation by glucose and insulin

We examined the activity of the rate-limiting enzyme for hexosamine biosynthesis, glutamine:fructose-6-phosphate amidotransferase (GFA) in human skeletal muscle cultures (HSMC), from 17 nondiabetic control and 13 subjects with non-insulin-dependent diabetes. GFA activity was assayed from HSMC treated with low (5 mM) or high (20 mM) glucose and low (22 pM) or high (30 microM) concentrations of insulin. In control subjects GFA activity decreased with increasing glucose disposal rate (r = -0.68, P < 0.025). In contrast, a positive correlation existed between GFA and glucose disposal in the diabetics (r = 0.86, P < 0.005). Increased GFA activity was also correlated with body mass index in controls but not diabetics. GFA activity was significantly stimulated by high glucose (22%), high insulin (43%), and their combination (61%). GFA activity and its regulation by glucose and insulin were not significantly different in diabetic HSMC. We conclude that glucose and insulin regulate GFA activity in skeletal muscle. More importantly, our results are consistent with a regulatory role for the hexosamine pathway in human glucose homeostasis. This relationship between hexosamine biosynthesis and the regulation of glucose metabolism is altered in non-insulin-dependent diabetes.

Synergistic effects of insulin and phorbol ester on mitogen-activated protein kinase in Rat-1 HIR cells

Regulation of the activity of the extracellular signal regulated kinase (ERK) mitogen-activated protein kinases was examined in Rat-1 HIR, a fibroblast cell line overexpressing the human insulin receptor. Insulin or phorbol ester induced partial activations of ERKs, while a combination of insulin and phorbol ester resulted in a synergistic activation. Preincubation with phorbol ester increased the subsequent response to insulin. Phorbol ester did not enhance tyrosine phosphorylation of the insulin receptor. Insulin did not enhance activation of phospholipase D in response to phorbol ester. Lysophosphatidic acid also acted synergistically with insulin to induce ERK activation. Lysophosphatidic acid alone had little effect on ERK, and did not activate phospholipase D. The combination of phorbol ester and insulin maintained tyrosine phosphorylation of focal adhesion kinase, while insulin alone decreased its tyrosine phosphorylation. Phorbol ester induced phosphorylation of She on serine/threonine, while insulin induced tyrosine phosphorylation of She and She-Grb2 binding. These results suggest that full activation of ERKs in fibroblasts can require the cooperation of at least two signaling pathways, one of which may result from a protein kinase C-dependent phosphorylation of effectors regulating ERK activation. In this manner, phorbol esters may enhance mitogenic signals initiated by growth factor receptors.

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 + 3H-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 4h 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 with 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).

In vivo glucosamine infusion induces insulin resistance in normoglycemic but not in hyperglycemic conscious rats

To test the hypothesis that increased flux through the hexosamine biosynthetic pathway can induce insulin resistance in skeletal muscle in vivo, we monitored glucose uptake, glycolysis, and glycogen synthesis during insulin clamp studies in 6-h fasted conscious rats in the presence of a sustained (7-h) increase in glucosamine (GlcN) availability. Euglycemic (approximately 7 mM) insulin (approximately 2,500 pM) clamps with saline or GlcN infusions were performed in control (CON; plasma glucose [PG] = 7.4 +/- 0.2 mM), diabetic (D; PG = 19.7 +/- 1.1), and phlorizin-treated (3-wk) diabetic rats (D + PHL; PG = 7.6 +/- 0.9). 7-h euglycemic hyperinsulinemia with saline did not significantly decrease Rd (360-420 min = 39.2 +/- 3.6 vs. 60-120 min = 42.2 +/- 3.7 mg/kg.min; P = NS). GlcN infusion raised plasma GlcN concentrations to approximately 1.2 mM and increased muscle and liver UDP-GlcNAc levels by 4-5-fold in all groups. GlcN markedly decreased Rd in CON (360-420 min = 30.4 +/- 1.3 vs. 60-120 min = 44.1 +/- 3.5 mg/kg.min; P < 0.01) and D + PHL (360-420 min = 29.4 +/- 2.5 vs. 60-120 min = 43.8 +/- 2.9 mg/kg.min; P < 0.01), but not in D (5-7 h = 21.5 +/- 0.8 vs. 0-2 h = 24.3 +/- 1.1 mg/kg.min; P = NS). Thus, increased GlcN availability induces severe skeletal muscle insulin resistance in normoglycemic but not in chronically hyperglycemic rats. The lack of additive effects of GlcN and chronic hyperglycemia (experimental diabetes) provides support for the hypothesis that increased flux through the GlcN pathway in skeletal muscle may play an important role in glucose-induced insulin resistance in vivo.

In vivo effects of glucosamine on insulin secretion and insulin sensitivity in the rat: possible relevance to the maladaptive responses to chronic hyperglycaemia

We tested the hypothesis that glucosamine, a putative activator of glucose toxicity in vitro through acceleration of the hexosamine pathway, may determine in vivo the two key features of glucose toxicity in diabetes, namely, peripheral insulin resistance and decreased insulin secretion. Two groups of awake rats were studied either with intraarterial administration of glucosamine (5 mumol.kg-1.min-1) or saline. Insulin secretion was determined after arginine, glucose (hyperglycaemic clamp), and arginine/glucose infusions, while insulin-mediated glucose metabolism was assessed by the euglycaemic hyperinsulinaemic clamp in combination with [3-3H]-glucose infusion. Glucosamine had no effects on arginine-induced insulin secretion both at euglycaemia and hyperglycaemia, but significantly (40-50%) impaired glucose-induced insulin secretion (both first and second phases). During euglycaemic hyperinsulinaemic clamp studies, glucosamine decreased glucose uptake by approximately 30%, affecting glycolysis (estimated from 3H2O rate of appearance) and muscle glycogen synthesis (calculated from accumulation of [3H]-glucosyl units in muscle glycogen) to a similar extent. Muscle glucose 6-phosphate concentration was markedly reduced in the glucosamine-infused rats, suggesting an impairment in glucose transport/phosphorylation. Therefore, an increase in hexosamine metabolism in vivo: 1) inhibits glucose-induced insulin secretion, and 2) reduces insulin stimulation of both glycolysis and glycogen synthesis, thereby mimicking in normal rats the major alterations due to glucose toxicity in diabetes.