The Cellular Fate of Glucose and Its Relevance in Type 2 Diabetes (original) (raw)
Related papers
Metabolic Fate of Glucose in Purified Islet Cells
Journal of Biological Chemistry, 1997
Previous studies in rat islets have suggested that anaplerosis plays an important role in the regulation of pancreatic  cell function and growth. However, the relative contribution of islet  cells versus non- cells to glucose-regulated anaplerosis is not known. Furthermore, the fate of glucose carbon entering the Krebs cycle of islet cells remains to be determined. The present study has examined the anaplerosis of glucose carbon in purified rat  cells using specific 14 C-labeled glucose tracers. Between 5 and 20 mM glucose, the oxidative production of CO 2 from [3,4-14 C]glucose represented close to 100% of the total glucose utilization by the cells. Anaplerosis, quantified as the difference between 14 CO 2 production from [3,4-14 C]glucose and [6-14 C]glucose, was strongly influenced by glucose, particularly between 5 and 10 mM. The dose dependence of glucose-induced insulin secretion correlated with the accumulation of citrate and malate in (INS-1) cells. All glucose carbon that was not oxidized to CO 2 was recovered from the cells after extraction in trichloroacetic acid. This indirectly indicates that lactate output is minimal in  cells. From the effect of cycloheximide upon the incorporation of 14 C-glucose into the acid-precipitable fraction, it could be calculated that 25% of glucose carbon entering the Krebs cycle via anaplerosis is channeled into protein synthesis. In contrast, non- cells (approximately 80% glucagon-producing ␣ cells) exhibited rates of glucose oxidation that were 1 ⁄3 to 1 ⁄6 those of the total glucose utilization and no detectable anaplerosis from glucose carbon. This difference between the two cell types was associated with a 7-fold higher expression of the anaplerotic enzyme pyruvate carboxylase in  cells, as well as a 4-fold lower ratio of lactate dehydrogenase to FADlinked glycerol phosphate dehydrogenase in  cells versus ␣ cells. Finally, glucose caused a dose-dependent suppression of the activity of the pentose phosphate pathway in  cells. In conclusion, rat  cells metabolize glucose essentially via aerobic glycolysis, whereas glycolysis in ␣ cells is largely anaerobic. The results support the view that anaplerosis is an essential pathway implicated in  cell activation by glucose.
Diabetologia, 1997
The hexose transport system in skeletal muscle is subject to complex regulation. We have previously shown that hexose uptake and utilisation in cultured rat myocytes and myotubes and in isolated rat soleus muscles in vitro are dependent on the D-glucose concentration to which the cells and muscles are pre-exposed . Similar effects of glucose withdrawal and refeeding have been reported in cultured L6 and BC3H1 skeletal muscle cells [7] as well as in human skeletal muscle . The maximal velocity of hexose transport (V max ) was reduced in L8 skeletal muscle cells exposed to increasing glucose concentrations (0.5-20.0 mmol/l) in a concentration-dependent manner, while the affinity (K m ) was unaffected [2]. Cytochalasin B (CB) binding and GLUT1 Western blotting of enriched plasma-and microsomal-membrane fractions revealed that high glucose concentrations modulated the subcellular distribution of GLUT1, reducing their number at the plasma membrane of the cell [1]. Similar autoregulatory effects of glucose Summary Exposure of rat skeletal muscle and skeletal muscle cell lines to high glucose levels results in a time-and dose-dependent reduction of the rate of hexose uptake, paralleled by a reduction in the plasma membrane density of glucose transporters. The mechanism of this process was investigated in cultured L8 myocytes. Low concentrations (0.5-2.0 mmol/l) of deoxyglucose mimicked the downregulatory action of 20 mmol/l glucose both regarding the time-course and magnitude of the effect, but in an irreversible manner. A dose-dependent relationship between intracellular accumulation of deoxyglucose 6-phosphate and the magnitude of the downregulatory response was observed. Depletion of intracellular deoxyglucose 6-phosphate restored the rate of hexose transport to the control level. The reduction of hexose transport activity by deoxyglucose occurred independently of ATP depletion which by itself produced the opposite effect. The effects of deoxyglucose and high glucose on hexose transport were associated with reduced transport maximal velocity and GLUT1 transporter abundance in the plasma membranes of myocytes, as assessed by cell surface biotinylation. The reduction of myocyte GLUT1 mRNA content, observed after exposure to high glucose, did not accompany the transport downregulatory action of deoxyglucose. We suggest that hexose 6-phosphate is the mediator of the downregulatory signal for subcellular redistribution of GLUT1 in L8 myocytes. The signal responsible for reducing the GLUT1 mRNA level may be related to glucose metabolites downstream of the hexokinase reaction.
Journal of Clinical Investigation, 1993
Seven non-insulin-dependent diabetes mellitus (NIDDM) patients participated in three clamp studies performed with 13-3HIand [U-14CIglucose and indirect calorimetry: study I, euglycemic (5.2±0.1 mM) insulin (269±39 pM) clamp; study II, hyperglycemic (14.9±1.2 mM) insulin (259±19 pM) clamp; study III, euglycemic (5.5±0.3 mM) hyperinsulinemic (1650±529 pM) clamp. Seven control subjects received a euglycemic (5.1±0.2 mM) insulin (258±24 pM) clamp. Glycolysis and glucose oxidation were quantitated from the rate of appearance of 3H20 and '4CO2; glycogen synthesis was calculated as the difference between body glucose disposal and glycolysis. In study I, glucose uptake was decreased by 54% in NIDDM vs. controls. Glycolysis, glycogen synthesis, and glucose oxidation were reduced in NIDDM patients (P < 0.05-0.001). Nonoxidative glycolysis and lipid oxidation were higher. In studies II and III, glucose uptake in NIDDM was equal to controls (40.7±2.1 and 40.7±1.7 gmol/min * kg fat-free mass, respectively). In study II, glycolysis, but not glucose oxidation, was normal (P < 0.01 vs. controls). Nonoxidative glycolysis remained higher (P < 0.05). Glycogen deposition increased (P < 0.05 vs. study I), and lipid oxidation remained higher (P < 0.01). In study III, hyperinsulinemia normalized glycogen formation, glycolysis, and lipid oxidation but did not normalize the elevated nonoxidative glycolysis or the decreased glucose oxidation. Lipid oxidation and glycolysis (r = -0.65; P < 0.01), and glucose oxidation (r = -0.75; P < 0.01) were inversely correlated. In conclusion, in NIDDM: (a) insulin resistance involves glycolysis, glycogen synthesis, and glucose oxidation; (b) hyperglycemia and hyperinsulinemia can normalize total body glucose uptake; (c) marked hyperinsulinemia normalizes glycogen synthesis and total flux through glycolysis, but does not restore a normal distribution between oxidation and nonoxidative glycolysis; (d) hyperglycemia cannot overcome the defects in glucose oxidation and nonoxidative glycolysis; (e) lipid oxidation is elevated and is suppressed only with hyperinsulinemia. (J. Clin. Invest. 1993. 91:484-494.)
Biochimica Et Biophysica Acta-molecular Cell Research, 1999
Glucose uptake is autoregulated in a variety of cell types and it is thought that glucose transport is the major step that is subjected to control by sugar availability. Here, we examined the effect of high glucose concentrations on the rate of glucose uptake by human ECV-304 umbilical vein-derived endothelial cells. A rise in the glucose concentration in the medium led a dose-dependent decrease in the rate of 2-deoxyglucose uptake. The effect of high glucose was independent of protein synthesis and the time-course analysis indicated that it was relatively slow. The effect was not due to inhibition of glucose transport since neither the expression nor the subcellular distribution of the major glucose transporter GLUT1, nor the rate of 3-O-methylglucose uptake was affected. The total in vitro assayed hexokinase activity and the expression of hexokinase-I were similar in cells treated or not with high concentrations of glucose. In contrast, exposure of cells to a high glucose concentration caused a marked decrease in phosphorylated 2-deoxyglucose/free 2-deoxyglucose ratio. This suggests the existence of alterations in the rate of in vivo glucose phosphorylation in response to high glucose. In summary, we conclude that ECV304 human endothelial cells reduce glucose utilization in response to enhanced levels of glucose in the medium by inhibiting the rate of glucose phosphorylation, rather than by blocking glucose transport. This suggests a novel metabolic effect of high glucose on cellular glucose utilization. ß
Diabetologia, 1995
The mechanism of increased hepatic glucose production in obese non-insulin-dependent diabetic (NIDDM) patients is unknown. The New Zealand Obese (NZO) mouse, a polygenic model of obesity and NIDDM shows increased hepatic glucose production. To determine the mechanism of this phenomenon, we measured gluconeogenesis from U-s4Cglycerol and U-14C-alanine and relevant gluconeogenic enzymes. Gluconeogenesis from glycerol (0.07 + 0.01 vs 0.21 + 0.02 ~tmol 9 min-~ 9 body mass index (BMI)-1, p < 0.005) and alanine (0.57 + 0.07 vs 0.99 _+ 0.07 ~tmol 9 min-1 9 BMI-I,p < 0.005) was elevated in control mice NZO vs as was glycerol turnover (0.25 + 0.02 vs 0.63 + 0.09 ~tmol 9 min-1 9 BM1-1, p < 0.05). Fructose 1,6-bisphosphatase activity (44.2 + 1.9 vs 55.7 + 4.1 nmol. min-S, mg proteins , p < 0.05) and protein levels (6.9 + 1.1 vs 16.7 + 2.3 arbitrary units, p < 0.01) were increased in NZO mouse livers, as was the activity of pyruvate carboxylase (0.12 + 0.01 vs 0.17 + 0.02 nmol-min-~ 9 mg proteins , p < 0.05). To ascertain whether elevated lipid supply is responsible for these biochemical changes in NZO mice, we fed lean control mice a 60 % fat diet for 2 weeks. Fat-fed mice were hyperinsulinaemic (76.37 + 4.06 vs 98.00 + 7.07 pmol/1, p = 0.05) and had elevated plasma non-esterified fatty acid levels (0.44 + 0.05 vs 0.59 _+ 0.03 mmol/1, p = 0.05). Fructose 1,6-bisphosphatase activity (43.86+2.54 vs 52.93 + 3.09 nmol-min-1-mg protein-1, p = 0.05) and protein levels (33.03 _+ 0.96 vs 40,04 + 1.26 arbitrary units, p = 0.005) and pyruvate carboxylase activity (0.10 + 0.003 vs 0.14 + 0.01 nmol. min-1-mg protein-1, p < 0.05) were elevated in fat-fed mice. We conclude that in NZO mice increased hepatic glucose production is due to elevated lipolysis resulting from obesity. [Diabetologia (1995) 38: 1389-1396] Key words Gluconeogenesis, glycerol, alanine, fructose 1,6-bisphosphatase, pyruvate carboxylase. Fasting hyperglycaemia is the characteristic feature of Type 2 non-insulin-dependent diabetes mellitus (NIDDM). A major contributing cause of fasting hyperglycaemia is an inappropriately high hepatic glu
Hexose metabolism in pancreatic islets. Feedback control of d-glucose oxidation by functional events
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1988
A rise in extraceUular v-glucose concentration in pancreatic Islet cells causes a greater relative increase in the oxidation of pyruvate and acet~l residues than in 81ycolysis. A possible explanation for such an unusual situation was sought in the present study. The preferential stimulation of mitochondrial oxidative events was found to dis#ay a sigmoidal dependency on hexose concentration, and an exlmnential time course during Igolunged exposure of the islets to a high concentration of v-81ueose. The preferential stimulation of mitochondrinl oxidative events was abolished in islets incubated in the presence of eyeloheximide and absance of Ca 2÷, in which case the oxidation of ~[6-14Clg]~ wits more severely inhibited than that of D-l&4-uClglucose. Likewise, the inhibitor of protein biosynthesis and the absence of Ca 2+ Mfeeted the oxidation of L-[U-14C]leucine preferentially, relative to that of L-[l-UC]leucine, in islets exposed to a high, but not a low, concentration of the amino acid, These results demonstrate that in pancreatic islets it is possible to dissociate bath glyecdysls from mitochondrial oxidative events and the oxidation of acetyi residues from their generation rate. Moreover, the experimental data suggest that nutrient-responsive and ATP-requiring functional precesses exert a feedback control on mitochonddul respiration in this fuel-sensor organ.
Diabetes – A Glucose Metabolism Problem
2008
Diabetes is fast becoming a global pandemic and diabetic complications are moving upwards as the top seven cause of death. Diabetes is a chronic disorder of glucose metabolism. Studies in the UK show that proper change in diet, daily exercise and brisk walking can greatly aid to reduce blood sugar levels that are not too far above the healthy levels. A blood sugar reading of 10.4 can drop to 9.2, a decline by 12% by a twenty minute brisk walk. A change in the fat:protein:bone ratio with reduction in total body fat and a reduction in the girth circumference by 8-12% can reverse the diabetic condition in borderline cases. The primary issue here is the conversion of fat into glucose that is then utilized in cells to yield energy. Unfortunately many diabetics suffer a further problem in glucose metabolism when they are put on drugs that block the conversion of lipids into glucose. Over a short time, their blood lipid profile begins to change, adding a new risk to health.