Hexose metabolism in pancreatic islets: compartmentation of hexokinase in islet cells

Glucose Response by Stem Cell-Derived β Cells In Vitro Is Inhibited by a Bottleneck in Glycolysis

Stem cell-derived β (SC-β) cells could provide unlimited human β cells toward a curative diabetes treatment. Differentiation of SC-β cells yields transplantable islets that secrete insulin in response to glucose challenges. Following transplantation into mice, SC-β cell function is comparable to human islets, but the magnitude and consistency of response in vitro are less robust than observed in cadaveric islets. Here, we profile metabolism of SC-β cells and islets to quantify their capacity to sense glucose and identify reduced anaplerotic cycling in the mitochondria as the cause of reduced glucose-stimulated insulin secretion in SC-β cells. This activity can be rescued by challenging SC-β cells with intermediate metabolites from the TCA cycle and late but not early glycolysis, downstream of the enzymes glyceraldehyde 3-phosphate dehydrogenase and phosphoglycerate kinase. Bypassing this metabolic bottleneck results in a robust, bi-phasic insulin release in vitro that is identical in magnitude to functionally mature human islets.

Glucose-stimulated insulin secretion is deficient in stem cell-derived β (SC-β) cells in vitro. Davis et al. use metabolomic analysis to define a glycolytic bottleneck inhibiting glucose metabolism and sensing in SC-β cells. Cell-permeable intermediates bypass this bottleneck, as does transplantation in vivo, producing insulin secretion indistinguishable from human islets.

Exposure to glibenclamide increases rat beta cells sensitivity to glucose

An increased sensitivity to glucose was observed in islets pre-exposed for 1 h to glibenclamide (0.1 micromol 1(-1)), but not to tolbutamide (100 micromol l(-1)), as indicated by a shift to the left of the dose-response curve (EC(50) at 5.8+/-0.3 mmol l(-1) glucose vs 10.6+/-0.8 in control islets; n=11, P<0.005). According to this secretory pattern also glucose utilization at 2.5 and 5.0 mmol l(-1) glucose was higher in islets exposed to glibenclamide. Since binding to mitochondria results in an increased enzyme activity, we measured hexokinase (HK) and glucokinase (GK) activity both in a cytosolic and in a mitochondrion-enriched fractions. Cytosolic hexokinase activity was similar in islets exposed to glibenclamide and in control islets but mitochondrial hexokinase activity was significantly increased after exposure to glibenclamide (124+/-7 vs 51+/-9 nmol microgram prot(-1) 90 min(-1), P<0.01), with no change in the enzyme protein content. In contrast, glucokinase activity in the two groups of islets was similar. When in islets < exposed to glibenclamide hexokinase binding to mitochondria was inhibited by the addition of 20 nmol l(-1) dicyclohexylcarbodiimide (DCC), no increase of glucose sensitivity was observed (EC(50) 10.9+/-1.3 mmol l(-1) glucose, n=3, similar to that of control islets). These data indicate that a 1 h exposure to glibenclamide causes the beta cell to become more sensitive to glucose. This increased sensitivity is associated with (and may be due to) an increased hexokinase activity, in particular the mitochondrial-bound, more active, form. This mechanism may contribute to the hypoglycemic action of this drug.

Anomeric specificity of human liver and B-cell glucokinase: modulation by the glucokinase regulatory protein

The anomeric specificity of the wild-type recombinant forms of human liver and B-cell glucokinase was investigated using radioactive anomers of d-glucose as tracers. With d-glucose at anomeric equilibrium and at 30 degrees C, the maximal velocity, Hill number, and K(s) amounted, respectively, to 16 micromol min(-1) mg(-1), 1.8 and 6.9 mM in the case of liver glucokinase, and 7.3 micromol min(-1) mg(-1), 2.0 and 7.1 mM in the case of B-cell glucokinase. Whether at 20-22 or 30 degrees C, the maximal velocity, Hill number, and K(m) were significantly lower with alpha-d-glucose than with beta-d-glucose in both liver and B-cell glucokinase. As a result of these differences, the reaction velocity was higher with alpha-d-glucose at low hexose concentrations, while the opposite situation prevailed at high hexose concentrations. In the presence of 0.2 mM d-fructose 6-phosphate, the glucokinase regulatory protein caused a concentration-related inhibition of d-glucose phosphorylation, such an effect fading out at high concentrations of either d-glucose or glucokinase relative to that of its regulatory protein. The phosphorylation of alpha-d-glucose by liver glucokinase appeared more resistant than that of beta-d-glucose to the inhibitory action of d-fructose 6-phosphate, as mediated by the glucokinase regulatory protein. Such a phenomenon failed to achieve statistical significance in the case of the B-cell glucokinase. It is proposed that this information, especially the novel findings concerning the anomeric difference in both Hill number and sensitivity to the glucokinase regulatory protein, should be taken into account when considering the respective contributions of alpha- and beta-d-glucose to the overall phosphorylation of equilibrated d-glucose by glucokinase.

Energy-dependent intracellular translocation of glucokinase in rat pancreatic islets

It was recently reported that hyperglycemia provokes a rapid and sustained translocation of glucokinase in rat pancreatic B-cells, and it was speculated that this may be associated with enhancement of its catalytic activity, as possibly attributable to the mitochondrial binding of the enzyme. In the present work, the activities of both hexokinase and glucokinase were measured in particulated and cytosolic subcellular fractions prepared from islets first incubated for 60 min either in the absence of exogenous nutrient or in the presence of D-glucose, tested at both low (2.8 mmol/L) and high (16.7 mmol/L) concentrations. The relative contribution of the cytosolic domain to the total activity of glucokinase recovered in the two subcellular fractions was higher in islets deprived of exogenous nutrient than in islets first exposed to 2.8 or 16.7 mmol/L D-glucose, the results obtained at each of the latter two hexose concentrations being comparable to one another. The subcellular distribution of hexokinase, however, was not significantly different in islets deprived of D-glucose or exposed to the hexose. These findings are interpreted as indicative of an energy-dependent translocation of glucokinase in the B-cell, distinct from the redistribution of the enzyme occurring in response to a rise in D-glucose concentration above its physiological value.

Subcellular distribution of hexokinase isoenzymes in pancreatic islet cells exposed to digitonin after incubation at a low or high concentration of D-glucose

It was recently proposed that stimulation of pancreatic islet by D-glucose results in the translocation of glucokinase from the perinuclear area to the cell periphery, where the enzyme might conceivably interact with either the glucose transporter GLUT-2 or some other proteins and, by doing so, become better able to express its full catalytic activity. To explore the possible interaction between glucokinase and the cell boundary, dispersed rat pancreatic islet cells were preincubated for 60 min at a low (2.8 mM) or high (16.7 mM) concentration of D-glucose, then exposed for 1 min to digitonin (0.5 mg/ml) and eventually centrifuged through a layer of oil for separation of the cell pellet from the supernatant fraction containing the material released by digitonin. Under these conditions, the bulk of lactate dehydrogenase and glutamate dehydrogenase activities were recovered in the supernatant fraction and cell pellet, respectively. The measurement of hexokinase isoenzyme activities in the two subcellular fractions, as conducted at low or high hexose concentrations and in either the absence or presence of exogenous hexose phosphates (3.0 mM glucose 6-phosphate and 1.0 mM fructose 1-phosphate) indicated a preferential location of the low-Km hexokinase in the cell pellet and of the high-Km glucokinase in the cytosolic fraction. Such a distribution pattern failed to be significantly affected by the concentration of D-glucose used during the initial incubation of the dispersed islet cells. These findings argue against the view that the glucose-induced translocation of glucokinase would result in any sizeable binding of the enzyme to a plasma membrane-associated protein.

Rat C6 glioma cell growth is related to glucose transport and metabolism

In order to establish whether growth of glioma cells is associated with glucose transport and metabolism, we investigated expression of the glucose transporter and hexokinase, as well as glucose transport and glucose phosphorylation in rat C6 glioma cells growing at different rates. Rat C6 glioma cells were subcloned to produce four different cell lines (CL1, CL2, CL3 and CL4) differing in growth, differentiation and morphology: CL1 cells were slow-growing with an astrocytic appearance whereas CL4 cells grew rapidly and were small and spindle-shaped. Immunocytochemical analysis using glial fibrillary acidic protein and galactocerebroside antibodies revealed that CL1 and CL4 cells differentiate to astrocytes and oligodendrocytes respectively. Both of these cell lines expressed GLUT1 mRNA predominantly, whereas little GLUT3 mRNA was evident by Northern-blot analysis. The GLUT1 mRNA level was much higher in CL4 than in CL1 cells, and the uptake of 2-deoxy-D-glucose and 3-O-methyl-D-glucose by CL4 cells was markedly higher than that by CL1 cells, indicating a correlation between the growth rate, glucose transporter (GLUT1) level and glucose-transport rate of C6 glioma cells. We then studied glucose metabolism by CL1 and CL4 cells by measuring their hexokinase activities and intracellular concentrations of glucose and ATP. The mitochondrial hexokinase activity of CL4 cells was about three times higher than that of CL1 cells, whereas the cytosolic hexokinase activity of CL4 cells was only about half that of CL1 cells. As the total amount of cellular hexokinase protein in CL4 cells was only slightly higher (about 20%) than that in CL1 cells, the hexokinase protein of CL4 cells was considered to have moved from the cytosol to the mitochondrial membranes. Consistent with the increased mitochondrial hexokinase activity of CL4 cells, the intracellular glucose concentration was undetectable, and the ATP concentration was higher than that of CL1 cells, suggesting that glucose transport is the rate-limiting factor for overall glucose metabolism is rapidly growing C6 cells. Therefore the present data demonstrate that glioma cell growth is related to glucose transport, which is closely associated with glucose metabolism.

Glucose transporter expression and functional role of hexokinase in insulin biosynthesis in mouse beta TC3 cells

It was previously reported that insulin biosynthesis in mouse beta TC3 cells was regulated by glucose (Nagamatsu, S., and D. F. Steiner. Endocrinology 130: 748-754, 1992). In the present study, we examined the effect of glucose on the glucose transporter expression and hexokinase activities and determined the relationship between them and glucose-stimulated insulin biosynthesis in beta TC3 cells. Reverse transcriptase-polymerase chain reaction and Northern blot analysis revealed that beta TC3 cells expressed GLUT-1 and GLUT-3 glucose transporter mRNAs, but not GLUT-2. The levels of GLUT-1 and GLUT-3 mRNAs were not affected by glucose (0 or 11 mM glucose) over a period of 48 h. Immunoprecipitation of metabolically labeled beta TC3 cells with specific antibodies against GLUT-1 or GLUT-3 proteins revealed no effect of glucose on the biosynthesis of glucose transporters. Hexokinase [low Michaelis constant (Km) hexokinase] activity from cells incubated in 11 mM glucose for 48 h increased nearly twofold compared with cells maintained in 0 mM glucose, although the amount of cellular hexokinase protein detected by immunoblot analysis was unchanged between 0 and 11 mM glucose conditions. Glucokinase (high Km hexokinase) activity, in contrast, was not affected by glucose. Preincubation of beta TC3 cells with 2-deoxyglucose to inhibit hexokinase, thereby inhibiting all glycolysis, resulted in the decrease of glucose-stimulated insulin biosynthesis. Thus, in mouse beta TC3 cells that do not express GLUT-2, there is a close relationship between hexokinase activity and glucose-stimulated insulin biosynthesis, but not between the glucose transporter and glucose-stimulated insulin biosynthesis.

A computer model of pancreatic islet glycolysis

We have modeled an experiment with perifused pancreatic islet cells using our BIOSSIM language. The experiment and the resulting model are concerned with glucose uptake and glycolysis by the beta-cells of pancreatic islets. Although glycolysis appears to be involved in insulin release, we do not have enough information to represent insulin release in detail. The rapid entry of glucose into the beta-cell is promoted by a carrier having a very high tissue capacity. Phosphorylation of glucose by the low affinity enzyme glucokinase appears to be limiting for glycolysis. The effects of several hexose diphosphate activators of phosphofructokinase are modeled. Model behavior is described. The kinetic parameters of the enzyme submodels are given. Because of the difficulties of preparing large amounts of experimental material, information on pancreatic islet metabolism is limited. This model is a plausible explanation of the experimental results. Recent work on the genetically engineered glucose transporter and glucokinase is discussed.

Modulation of hexokinase association with mitochondria analyzed with quantitative three-dimensional confocal microscopy

Hexokinase isozyme I is proposed to be associated with mitochondria in vivo. Moreover, it has been suggested that this association is modulated in coordination with changes in cell metabolic state. To test these hypotheses, we analyzed the subcellular distribution of hexokinase relative to mitochondria in paraformaldehyde-fixed astrocytes using immunocytochemistry and quantitative three-dimensional confocal microscopy. Analysis of the extent of colocalization between hexokinase and mitochondria revealed that approximately 70% of cellular hexokinase is associated with mitochondria under basal metabolic conditions. In contrast to the immunocytochemical studies, between 15 to 40% of cellular hexokinase was found to be associated with mitochondria after fractionation of astrocyte cultures depending on the exact fractionation conditions. The discrepancy between fractionation studies and those based on imaging of distributions in fixed cells indicates the usefulness of using techniques that can evaluate the distributions of "cytosolic" enzymes in cells whose subcellular ultrastructure is not severely disrupted. To determine if hexokinase distribution is modulated in concert with changes in cell metabolism, the localization of hexokinase with mitochondria was evaluated after inhibition of glucose metabolism with 2-deoxyglucose. After incubation with 2-deoxyglucose there was an approximate 35% decrease in the amount of hexokinase associated with mitochondria. These findings support the hypothesis that hexokinase is bound to mitochondria in rat brain astrocytes in vivo, and that this association is sensitive to cell metabolic state.

Hexose metabolism in pancreatic islets. Regulation of aerobic glycolysis and pyruvate decarboxylation

1. D-Glucose (0.5-16.7 mM) preferentially stimulates aerobic glycolysis and D-[3,4-14C]glucose oxidation, relative to D-[5-3H]glucose utilization in rat pancreatic islets, the concentration dependency of such a preferential effect displaying a sigmoidal pattern. 2. Inorganic and organic calcium antagonists, as well as Ca2+ deprivation, only cause a minor decrease in the ratio between D-[3,4-14C]glucose oxidation and D-[5-3H]glucose utilization in islets exposed to a high concentration of the hexose (16.7 mM). 3. Non-glucidic nutrient secretagogues such as 2-aminobicyclo[2,2,1]heptane-2-carboxylate (BCH), 2-ketoisocaproate and 3-phenylpyruvate fail to stimulate aerobic glycolysis and D-[3,4-14C]glucose oxidation in islets exposed to 6.0 mM D-glucose. Nevertheless, BCH augments [1-14C]pyruvate and [2-14C]pyruvate oxidation. 4. The glucose-induced increment in the paired ratio between D-[3,4-14C]glucose oxidation and D-[5-3H]glucose utilization is impaired in the presence of either cycloheximide or ouabain. 5. These findings suggest that the preferential effect of D-glucose upon aerobic glycolysis and pyruvate decarboxylation is not attributable solely to a Ca(2+)-induced activation of FAD-linked glycerophosphate dehydrogenase and/or pyruvate dehydrogenase, but may also involve an ATP-modulated regulatory process.

Impairment of the mitochondrial oxidative response to D-glucose in pancreatic islets from adult rats injected with streptozotocin during the neonatal period

Pancreatic islets removed from adult rats injected with streptozotocin during the neonatal period display an impaired secretory response to D-glucose and, to a lesser extent, to L-leucine. Despite normal to elevated hexokinase and glucokinase activities in the islets of these glucose-intolerant animals and despite normal mitochondrial binding of the hexokinase isoenzymes, the metabolic response to a high concentration of D-glucose is severely affected, especially in terms of D-[6-14C]glucose oxidation. Thus, the ratio in D-[6-14C]glucose oxidation/D-[5-3H]glucose utilization is much less markedly increased in response to a rise in hexose concentration and, at a high concentration of D-glucose (16.7 mmol/l), less markedly decreased by the absence of Ca2+ and presence of cycloheximide in diabetic than control rats. This metabolic defect contrasts with (1) a close-to-normal or even increased capacity of the islets of diabetic rats to oxidize D-[6-14C]glucose, [2-14C]pyruvate, L-[U-14C]glutamine and L-[U-14C]leucine at low, non-insulinotropic, concentrations of these substrates; (2) a lesser impairment of the oxidation of L-[U-14C]leucine tested in high concentration (20 mmol/l), the effect of Ca2+ deprivation upon the latter variable being comparable in diabetic and control rats; (3) an unaltered transamination of either [2-14C]pyruvate or L-[U-14C]leucine; and (4) a modest perturbation of glycolysis. The most obvious alteration in glycolysis consists in a lesser increase of the glycolytic flux in response to a rise of D-glucose concentration in diabetic than control rats, this coinciding with an apparent decrease in affinity of glucokinase for the hexose.(ABSTRACT TRUNCATED AT 250 WORDS)

Hexose metabolism in pancreatic islet cells: the coupling between hexose phosphorylation and mitochondrial respiration

The possible relevance of D-glucose phosphorylation by mitochondria-bound hexokinase to the control of respiration was examined in mitochondria prepared from either tumoral pancreatic islet cells (RINm5F line) or normal rat liver. In both systems, ATP generated by mitochondria exposed to ADP and succinate could serve as a substrate for the phosphorylation of D-glucose. However, after exposure to exogenous ADP in the presence of succinate, only mitochondria isolated from RINm5F cells displayed a sizeable increase in O2 consumption in response to a subsequent administration of D-glucose. In this respect, the discrepancy between mitochondria from islet cells and liver, respectively, was found to be attributable to the much lower hexokinase activity, relative to respiratory rate, in liver than in RINm5F cell mitochondria. It is speculated that the coupling between hexose phosphorylation and respiration in islet cells may prime the mitochondria to generate ATP during the early metabolic and secretory response to a rise in extracellular D-glucose concentration.

Hexose metabolism in pancreatic islets: preferential utilization of mitochondrial ATP for glucose phosphorylation

The respective contribution of exogenous and intramitochondrially formed ATP to D-glucose phosphorylation by mitochondria-bound hexokinase was examined in both rat liver and pancreatic islet mitochondria by comparing the generation of D-glucose 6-[32P]phosphate from exogenous [gamma-32P]ATP to the total rate of D-[U-14C]glucose phosphorylation. In liver mitochondria, the fractional contribution of exogenous ATP to D-glucose phosphorylation ranged from 4 to 74%, depending on the availability of endogenous ATP formed by either oxidative phosphorylation or in the reaction catalyzed by adenylate kinase. Likewise, in islet mitochondria exposed to exogenous ATP but deprived of exogenous nutrient, about 60% of D-glucose phosphorylation was supported by mitochondrial ATP. Such a fractional contribution was further increased in the presence of ADP and succinate, and suppressed by mitochondrial poisons. It is concluded that, in islet like in liver mitochondria, mitochondrial ATP is used preferentially to exogenous ATP as a substrate for D-glucose phosphorylation by mitochondria-bound hexokinase. This may favour the maintenance of a high cytosolic ATP concentration in glucose-stimulated islet cells.

Mitochondrial hexokinase from differentiated and undifferentiated HT29 colon cancer cells: effect of some metabolites on the bound/soluble equilibrium

1. Solubilization of mitochondrial bound hexokinase (HK), which represents 75-80% of the total enzyme activity in the cells, was investigated in freshly isolated mitochondria from undifferentiated (Glc+) or differentiated (Glc-) HT29 adenocarcinoma cells. In both models, the bound HK is almost completely released in vitro by 100 microM glucose 6-P (G 6-P). 2. Free ATP (5 mM) or palmitate (800 microM) produce a partial solubilization of bound HK, more markedly in the case of Glc- mitochondria. 3. Glucose or glucose 1-P are found unable to solubilize bound HK. Glucose 1,6-P2, 2-deoxyglucose 6-P or glucosamine 6-P can solubilize the enzyme but are less efficient than G 6-P. 4. Mg2+ and Pi are found to counteract the glucose 6-P induced solubilization of HK in both types of mitochondria. Taking into account the intracellular concentrations of these ions, this could in part explain why, in HT29 cells, HK is predominantly bound to the mitochondria.

Hexose metabolism in pancreatic islets: regulation of mitochondrial hexokinase binding

A major fraction of hexokinase was found to be bound, presumably to mitochondria, in both normal and tumoral rat pancreatic islet cells examined after either mechanical disruption or digitonin treatment. Spermidine enhanced the binding and glucose 6-phosphate caused the release of hexokinase to and from islet mitochondria, in a manner comparable to that seen in parotid or brain homogenates. In hepatocytes, some hexokinase, but no glucokinase, was found in the bound form. In islet cells, however, the pattern of glucokinase binding was similar to that of hexokinase. It is speculated that the preferential location of both hexokinase and glucokinase on mitochondria may favor the maintenance of a high cytosolic ATP content in islet cells.

Glycerol phosphorylation and oxidation in pancreatic islets

Homogenates of rat pancreatic islets and tumoral islet cells (RINm5F line) were found to display glycerokinase activity. In the islets like in the liver, about one-sixth of the enzyme appears bound to mitochondria. The enzymatic activities in liver and islets differ from one another, however, by their response to increasing concentrations of either glycerol or ATP and sensitivity to inhibition by D-glyceraldehyde. In intact islets, [U-14C]glycerol is efficiently oxidized, albeit at a much lower rate than that found for its phosphorylation by islet homogenates. These findings are relevant to the role played by glycerol liberated from endogenous triglycerides in the basal respiration of islet cells.