Effects of glucose and glucagon on the fructose 2,6-bisphosphate content of pancreatic islets and purified pancreatic B-cells. A comparison with isolated hepatocytes

A pathway model of glucose-stimulated insulin secretion in the pancreatic β-cell

The pancreas plays a critical role in maintaining glucose homeostasis through the secretion of hormones from the islets of Langerhans. Glucose-stimulated insulin secretion (GSIS) by the pancreatic β-cell is the main mechanism for reducing elevated plasma glucose. Here we present a systematic modeling workflow for the development of kinetic pathway models using the Systems Biology Markup Language (SBML). Steps include retrieval of information from databases, curation of experimental and clinical data for model calibration and validation, integration of heterogeneous data including absolute and relative measurements, unit normalization, data normalization, and model annotation. An important factor was the reproducibility and exchangeability of the model, which allowed the use of various existing tools. The workflow was applied to construct a novel data-driven kinetic model of GSIS in the pancreatic β-cell based on experimental and clinical data from 39 studies spanning 50 years of pancreatic, islet, and β-cell research in humans, rats, mice, and cell lines. The model consists of detailed glycolysis and phenomenological equations for insulin secretion coupled to cellular energy state, ATP dynamics and (ATP/ADP ratio). Key findings of our work are that in GSIS there is a glucose-dependent increase in almost all intermediates of glycolysis. This increase in glycolytic metabolites is accompanied by an increase in energy metabolites, especially ATP and NADH. One of the few decreasing metabolites is ADP, which, in combination with the increase in ATP, results in a large increase in ATP/ADP ratios in the β-cell with increasing glucose. Insulin secretion is dependent on ATP/ADP, resulting in glucose-stimulated insulin secretion. The observed glucose-dependent increase in glycolytic intermediates and the resulting change in ATP/ADP ratios and insulin secretion is a robust phenomenon observed across data sets, experimental systems and species. Model predictions of the glucose-dependent response of glycolytic intermediates and biphasic insulin secretion are in good agreement with experimental measurements. Our model predicts that factors affecting ATP consumption, ATP formation, hexokinase, phosphofructokinase, and ATP/ADP-dependent insulin secretion have a major effect on GSIS. In conclusion, we have developed and applied a systematic modeling workflow for pathway models that allowed us to gain insight into key mechanisms in GSIS in the pancreatic β-cell.

A Consensus Model of Glucose-Stimulated Insulin Secretion in the Pancreatic β -Cell

The pancreas plays a critical role in maintaining glucose homeostasis through the secretion of hormones from the islets of Langerhans. Glucose-stimulated insulin secretion (GSIS) by the pancreatic β -cell is the main mechanism for reducing elevated plasma glucose. Here we present a systematic modeling workflow for the development of kinetic pathway models using the Systems Biology Markup Language (SBML). Steps include retrieval of information from databases, curation of experimental and clinical data for model calibration and validation, integration of heterogeneous data including absolute and relative measurements, unit normalization, data normalization, and model annotation. An important factor was the reproducibility and exchangeability of the model, which allowed the use of various existing tools. The workflow was applied to construct the first consensus model of GSIS in the pancreatic β -cell based on experimental and clinical data from 39 studies spanning 50 years of pancreatic, islet, and β -cell research in humans, rats, mice, and cell lines. The model consists of detailed glycolysis and equations for insulin secretion coupled to cellular energy state (ATP/ADP ratio). Key findings of our work are that in GSIS there is a glucose-dependent increase in almost all intermediates of glycolysis. This increase in glycolytic metabolites is accompanied by an increase in energy metabolites, especially ATP and NADH. One of the few decreasing metabolites is ADP, which, in combination with the increase in ATP, results in a large increase in ATP/ADP ratios in the β -cell with increasing glucose. Insulin secretion is dependent on ATP/ADP, resulting in glucose-stimulated insulin secretion. The observed glucose-dependent increase in glycolytic intermediates and the resulting change in ATP/ADP ratios and insulin secretion is a robust phenomenon observed across data sets, experimental systems and species. Model predictions of the glucose-dependent response of glycolytic intermediates and insulin secretion are in good agreement with experimental measurements. Our model predicts that factors affecting ATP consumption, ATP formation, hexokinase, phosphofructokinase, and ATP/ADP-dependent insulin secretion have a major effect on GSIS. In conclusion, we have developed and applied a systematic modeling workflow for pathway models that allowed us to gain insight into key mechanisms in GSIS in the pancreatic β -cell.

Breast Cancer Subtypes Underlying EMT-Mediated Catabolic Metabolism

Efficient catabolic metabolism of adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH) is essentially required for cancer cell survival, especially in metastatic cancer progression. Epithelial–mesenchymal transition (EMT) plays an important role in metabolic rewiring of cancer cells as well as in phenotypic conversion and therapeutic resistance. Snail (SNAI1), a well-known inducer of cancer EMT, is critical in providing ATP and NADPH via suppression of several gatekeeper genes involving catabolic metabolism, such as phosphofructokinase 1 (PFK1), fructose-1,6-bisphosphatase 1 (FBP1), and acetyl-CoA carboxylase 2 (ACC2). Paradoxically, PFK1 and FBP1 are counter-opposing and rate-limiting reaction enzymes of glycolysis and gluconeogenesis, respectively. In this study, we report a distinct metabolic circuit of catabolic metabolism in breast cancer subtypes. Interestingly, PFKP and FBP1 are inversely correlated in clinical samples, indicating different metabolic subsets of breast cancer. The luminal types of breast cancer consist of the pentose phosphate pathway (PPP) subset by suppression of PFKP while the basal-like subtype (also known as triple negative breast cancer, TNBC) mainly utilizes glycolysis and mitochondrial fatty acid oxidation (FAO) by loss of FBP1 and ACC2. Notably, PPP remains active via upregulation of TIGAR in the FBP1-loss basal-like subset, indicating the importance of PPP in catabolic cancer metabolism. These results indicate different catabolic metabolic circuits and thus therapeutic strategies in breast cancer subsets.

Phosphofructo-2-kinase/fructose-2,6-bisphosphatase modulates oscillations of pancreatic islet metabolism

Pulses of insulin from pancreatic beta-cells help maintain blood glucose in a narrow range, although the source of these pulses is unclear. It has been proposed that a positive feedback circuit exists within the glycolytic pathway, the autocatalytic activation of phosphofructokinase-1 (PFK1), which endows pancreatic beta-cells with the ability to generate oscillations in metabolism. Flux through PFK1 is controlled by the bifunctional enzyme PFK2/FBPase2 (6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase) in two ways: via (1) production/degradation of fructose-2,6-bisphosphate (Fru2,6-BP), a potent allosteric activator of PFK1, as well as (2) direct activation of glucokinase due to a protein-protein interaction. In this study, we used a combination of live-cell imaging and mathematical modeling to examine the effects of inducibly-expressed PFK2/FBPase2 mutants on glucose-induced Ca²⁺ pulsatility in mouse islets. Irrespective of the ability to bind glucokinase, mutants of PFK2/FBPase2 that increased the kinase:phosphatase ratio reduced the period and amplitude of Ca²⁺ oscillations. Mutants which reduced the kinase:phosphatase ratio had the opposite effect. These results indicate that the main effect of the bifunctional enzyme on islet pulsatility is due to Fru2,6-BP alteration of the threshold for autocatalytic activation of PFK1 by Fru1,6-BP. Using computational models based on PFK1-generated islet oscillations, we then illustrated how moderate elevation of Fru-2,6-BP can increase the frequency of glycolytic oscillations while reducing their amplitude, with sufficiently high activation resulting in termination of slow oscillations. The concordance we observed between PFK2/FBPase2-induced modulation of islet oscillations and the models of PFK1-driven oscillations furthermore suggests that metabolic oscillations, like those found in yeast and skeletal muscle, are shaped early in glycolysis.

A role for PFK-2/FBPase-2, as distinct from fructose 2,6-bisphosphate, in regulation of insulin secretion in pancreatic β-cells

PFK-2/FBPase-2 (6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase) catalyses the formation and degradation of fructose 2,6-P2 (fructose 2,6-bisphosphate) and is also a glucokinase-binding protein. The role of fructose 2,6-P2 in regulating glucose metabolism and insulin secretion in pancreatic β-cells is unresolved. We down-regulated the endogenous isoforms of PFK-2/FBPase-2 with siRNA (small interfering RNA) and expressed KA (kinase active) and KD (kinase deficient) variants to distinguish between the role of PFK-2/FBPase-2 protein and the role of its product, fructose 2,6-P2, in regulating β-cell function. Human islets expressed the PFKFB2 (the gene encoding isoform 2 of the PFK2/FBPase2 protein) and PFKFB3 (the gene encoding isoform 3 of the PFK2/FBPase2 protein) isoforms and mouse islets expressed PFKFB2 at the mRNA level [RT–PCR (reverse transcription–PCR)]. Rat islets expressed PFKFB2 lacking the C-terminal phosphorylation sites. The glucose-responsive MIN6 and INS1E cell lines expressed PFKFB2 and PFKFB3. PFK-2 activity and the cell content of fructose 2,6-P2 were increased by elevated glucose concentration and during pharmacological activation of AMPK (AMP-activated protein kinase), which also increased insulin secretion. Partial down-regulation of endogenous PFKFB2 and PFKFB3 in INS1E by siRNA decreased PFK-2/FBPase-2 protein, fructose 2,6-P2 content, glucokinase activity and glucoseinduced insulin secretion. Selective down-regulation of glucose-induced fructose 2,6-P2 in the absence of down-regulation of PFK-2/FBPase-2 protein, using a KD PFK-2/FBPase-2 variant, resulted in sustained glycolysis and elevated glucose-induced insulin secretion, indicating an over-riding role of PFK-2/FBPase-2 protein, as distinct from its product fructose 2,6-P2, in potentiating glucose-induced insulin secretion. Whereas down-regulation of PFK-2/FBPase-2 decreased glucokinase activity, overexpression of PFK-2/FBPase-2 only affected glucokinase distribution. It is concluded that PFK-2/FBPase-2 protein rather than its product fructose 2,6-P2 is the over-riding determinant of glucose-induced insulin secretion through regulation of glucokinase activity or subcellular targeting.

Improved metabolic stimulus for glucose-induced insulin secretion through GK and PFK-2/FBPase-2 coexpression in insulin-producing RINm5F cells

The glucose sensor enzyme glucokinase plays a pivotal role in the regulation of glucose-induced insulin secretion in pancreatic beta-cells. Activation of glucokinase represents a promising concept for the treatment of type 2 diabetes. Therefore, we analyzed the glucokinase activation through its physiological interaction partner, the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) and the resulting effect on glucose metabolism in insulin-producing cells. In RINm5F-GK-PFK-2/FBPase-2 cells stably overexpressing glucokinase plus islet PFK-2/FBPase-2, colocalization between both enzymes as well as elevation of glucokinase activity were significantly increased at a stimulatory glucose concentration of 10 mmol/liter. RINm5F-GK-PFK-2/FBPase-2 cells showed under this culture condition a significant increase in glucose utilization and in the ATP/ADP ratio compared with RINm5F-GK cells, which only overexpress glucokinase. Also glucose-induced insulin secretion was elevated in RINm5F-GK-PFK-2/FBPase-2 cells in comparison to RINm5F-GK cells. Furthermore, pyruvate accumulation and lactate production in RINm5F-GK-PFK-2/FBPase-2 cells were significantly lower at both 10 and 30 mmol/liter glucose than in RINm5F-GK and RINm5F cells. The significant improvement of glucose metabolism after PFK-2/FBPase-2 overexpression is apparently not exclusively the result of high glucokinase enzyme activity. Stabilization of the closed glucokinase conformation by PFK-2/FBPase-2 may not only activate the enzyme but also improve metabolic channeling in beta-cells.

Interaction of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) with glucokinase activates glucose phosphorylation and glucose metabolism in insulin-producing cells

The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) was recently identified as a new intracellular binding partner for glucokinase (GK). Therefore, we studied the importance of this interaction for the activity status of GK and glucose metabolism in insulin-producing cells by overexpression of the rat liver and pancreatic islet isoforms of PFK-2/FBPase-2. PFK-2/FBPase-2 overexpression in RINm5F-GK cells significantly increased the GK activity by 78% in cells expressing the islet isoform, by 130% in cells expressing the liver isoform, and by 116% in cells expressing a cAMP-insensitive liver S32A/H258A double mutant isoform. Only in cells overexpressing the wild-type liver PFK-2/FBPase-2 isoform was the increase of GK activity abolished by forskolin, apparently due to the regulatory site for phosphorylation by a cAMP-dependent protein kinase. In cells overexpressing any isoform of the PFK-2/FBPase-2, the increase of the GK enzyme activity was antagonized by treatment with anti-FBPase-2 antibody. Increasing the glucose concentration from 2 to 10 mmol/l had a significant stimulatory effect on the GK activity in RINm5F-GK cells overexpressing any isoform of PFK-2/FBPase-2. The interaction of GK with PFK-2/FBPase-2 takes place at glucose concentrations that are physiologically relevant for the activation of GK and the regulation of glucose-induced insulin secretion. This new mechanism of posttranslational GK regulation may also represent a new site for pharmacotherapeutic intervention in type 2 diabetes treatment.

Characterization of glucokinase-binding protein epitopes by a phage-displayed peptide library. Identification of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase as a novel interaction partner

The low affinity glucose-phosphorylating enzyme glucokinase shows the phenomenon of intracellular translocation in beta cells of the pancreas and the liver. To identify potential binding partners of glucokinase by a systematic strategy, human beta cell glucokinase was screened by a 12-mer random peptide library displayed by the M13 phage. This panning procedure revealed two consensus motifs with a high binding affinity for glucokinase. The first consensus motif, LSAXXVAG, corresponded to the glucokinase regulatory protein of the liver. The second consensus motif, SLKVWT, showed a complete homology to the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2), which acts as a key regulator of glucose metabolism. Through yeast two-hybrid analysis it became evident that the binding of glucokinase to PFK-2/FBPase-2 is conferred by the bisphosphatase domain, whereas the kinase domain is responsible for dimerization. 5'-Rapid amplification of cDNA ends analysis and Northern blot analysis revealed that rat pancreatic islets express the brain isoform of PFK-2/FBPase-2. A minor portion of the islet PFK-2/FBPase-2 cDNA clones comprised a novel splice variant with 8 additional amino acids in the kinase domain. The binding of the islet/brain PFK-2/ FBPase-2 isoform to glucokinase was comparable with that of the liver isoform. The interaction between glucokinase and PFK-2/FBPase-2 may provide the rationale for recent observations of a fructose-2,6-bisphosphate level-dependent partial channeling of glycolytic intermediates between glucokinase and glycolytic enzymes. In pancreatic beta cells this interaction may have a regulatory function for the metabolic stimulus-secretion coupling. Changes in fructose-2,6-bisphosphate levels and modulation of PFK-2/FBPase-2 activities may participate in the physiological regulation of glucokinase-mediated glucose-induced insulin secretion.

Glycolysis revisited

Glycolysis is usually considered as a paradigm metabolic pathway, due to the fact that it is present in most organisms, and also because it is the pathway by which an important nutrient, glucose, is consumed. Far from being completely understood, the regulation of this pathway witnessed several important progresses during the last few years. One of these is the discovery of fructose 2,6-bisphosphate, a potent stimulator of phosphofructokinase and inhibitor of fructose-1,6-bisphosphatase. Originally found in the liver during the course of a study on the mechanism by which glucagon acts on gluconeogenesis, this compound is now recognized as a major element in the control of glycolysis and/or gluconeogenesis in many cell types and in various organisms. The other finding is that of a regulatory protein that modulates the activity of glucokinase, the enzyme that phosphorylates glucose in the liver and in the beta cells of pancreatic islets.

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.

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.

Pancreatic islet discrimination of hexose anomers. I. Steady-state computer simulation

Pancreatic islets detect glucose level by phosphorylating it and converting the glycolytic rate to a signal to secrete insulin. Insulin secretion is greater from the alpha- than from the beta-anomer when the D-glucose level is below 22 mM. D-mannose behaves similarly but at nearly twofold higher concentrations. Two explanations have been proposed: 1) glucokinase, which has the same anomeric preference, is the principal hexose phosphorylating enzyme and limits glycolytic rate. 2) Phosphofructokinase limits glycolysis and hexokinase is the principal enzyme phosphorylating hexose; hexosediphosphate activators of phosphofructokinase are more readily synthesized from alpha-anomers of hexose phosphates. We have simulated both alternatives with a detailed anomerically specific model of the hexose-metabolizing glycolytic enzymes. The pathway preference for alpha-anomer of both hexoses was adequately reproduced with anomerically active limiting glucokinase. The other mechanism did not reproduce the observed pathway preference.

Fructose metabolism via the pentose cycle in tumoral islet cells

In tumoral islet cells (RINm5F line) the phosphorylation of D-fructose is catalyzed by hexokinase rather than fructokinase. Fructose 6-phosphate appears to be preferentially channelled into the pentose cycle, as suggested by a ratio of D-[1-14C]fructose/D-[U-14C]fructose oxidation close to 2.7, the failure to generate 14C-labelled lactate from D-[1-14C]fructose and a poor metabolic response to menadione. When the islet cells are exposed to both D-fructose and D-glucose, however, the metabolism of the former hexose is dramatically modified, fructose 6-phosphate being now formed at a lower rate and preferentially channelled into the glycolytic pathway. These findings illustrate the existence of regulatory steps in fructose catabolism located distally to its site of phosphorylation.

Mechanism of thrombin-induced rise in platelet fructose 2,6-bisphosphate content. Studies using phorbol myristate acetate, dioctanoylglycerol and ionophore A23187

The mechanism by which thrombin increases platelet fructose 2,6-bisphosphate content was investigated. The action of thrombin was mimicked by phorbol 12 myristate 13-acetate and 1,2-dioctanoylglycerol. Ca2+ with A23187 potentiated the action of both these compounds. The action of thrombin required mobilization of intracellular and extracellular Ca2+ and was not decreased by indomethacin. This study suggests that protein kinase C activation and Ca2+ mobilization are both involved in the activation of glycolysis by thrombin.

Fructose 2,6-bisphosphate in Dictyostelium discoideum. Independence of cyclic AMP production and inhibition of fructose-1,6-bisphosphatase

The occurrence of fructose 2,6-bisphosphate was detected in Dictyostelium discoideum. The levels of this compound were compared with those of cyclic AMP and several glycolytic intermediates during the early stages of development. Removal of the growth medium and resuspension of the organism in the differentiation medium decreased the content of fructose 2,6-bisphosphate to about 20% within 1 h, remaining low when starvation-induced development was followed for 8 h. The content of cyclic AMP exhibited a transient increase that did not correlate with the change in fructose 2,6-bisphosphate. If after 1 h of development 2% glucose was added to the differentiation medium, fructose 2,6-bisphosphate rapidly rose to similar levels to those found in the vegetative state, while the increase in cyclic AMP was prevented. The contents of hexose 6-phosphates, fructose 1,6-bisphosphate and triose phosphates changed in a way that was parallel to that of fructose 2,6-bisphosphate, and addition of sugar resulted in a large increase in the levels of these metabolites. The content of fructose 2,6-bisphosphate was not significantly modified by the addition of the 8-bromo or dibutyryl derivatives of cyclic AMP to the differentiation medium. These results provide evidence that the changes in fructose 2,6-bisphosphate levels in D. discoideum development are not related to a cyclic-AMP-dependent mechanism but to the availability of substrate. Fructose 2,6-bisphosphate was found to inhibit fructose-1,6-bisphosphatase activity of this organism at nanomolar concentrations, while it does not affect the activity of phosphofructokinase in the micromolar range. The possible physiological implications of these phenomena are discussed.

Hormonal control of fructose 2,6-bisphosphate concentration in the HT29 human colon adenocarcinoma cell line. Alpha 2-adrenergic agonists counteract effect of vasoactive intestinal peptide

Vasoactive intestinal peptide (VIP) was found to cause a dose-dependent decrease in fructose 2,6-bisphosphatase concomitant with an increase in cyclic AMP in cultured HT29 cancer cells from human colon. The maximum effect was a 41% decrease obtained with 10 nM-VIP, and half-maximum effect was obtained with 0.75 nM-VIP. The effect of 2.5 nM-VIP was almost totally counteracted (i.e. fructose 2,6-bisphosphate concentration was restored) by either adrenaline (1 microM) or the alpha 2-adrenergic agonist UK-14304 (1 microM); the alpha 2-agonist clonidine (1 microM) was less efficient, since the VIP effect was decreased by 72% only. The adrenaline effect was totally antagonized by 1 microM-yohimbine. It is concluded that, in the HT29 cancer cells, the fructose 2,6-bisphosphate-producing system is sensitive to variations of cyclic AMP concentration and is under the dual control of VIP and alpha 2-adrenergic receptors.

Purification and properties of phosphofructokinase 2/fructose 2,6-bisphosphatase from chicken liver and from pigeon muscle

Phosphofructokinase 2 and fructose 2,6-bisphosphatase extracted from either chicken liver or pigeon muscle co-purified up to homogeneity. The two homogeneous proteins were found to be dimers of relative molecular mass (Mr) close to 110,000 with subunits of Mr 54,000 for the chicken liver enzyme and 53,000 for the pigeon muscle enzyme. The latter also contained a minor constituent of Mr 54,000. Incubation of the chicken liver enzyme with the catalytic subunit of cyclic-AMP-dependent protein kinase in the presence of [gamma-32P]ATP resulted in the incorporation of about 0.8 mol phosphate/mol enzyme. Under similar conditions, the pigeon muscle enzyme was phosphorylated to an extent of only 0.05 mol phosphate/mol enzyme and all the incorporated phosphate was found in the minor 54,000-Mr constituent. The maximal activity of the native avian liver phosphofructokinase 2 was little affected by changes of pH between 6 and 10. Its phosphorylation by cyclic-AMP-dependent protein kinase resulted in a more than 90% inactivation at pH values below 7.5 and in no or little change in activity at pH 10. Intermediary values of inactivation were observed at pH values between 8 and 10. Muscle phosphofructokinase 2 had little activity at pH below 7 and was maximally active at pH 10. Its partial phosphorylation resulted in a further 25% decrease of its already low activity measured at pH 7.1 and in a negligible inactivation at pH 8.5. Phosphoenolpyruvate and citrate inhibited phosphofructokinase 2 from both origins non-competitively. The muscle enzyme and the phosphorylated liver enzyme displayed much more affinity for these inhibitors than the native liver enzyme. Fructose 2,6-bisphosphatase from both sources had about the same specific activity but only the chicken liver enzyme was activated about twofold upon incubation with ATP and cyclic-AMP-dependent protein kinase. All enzyme forms were inhibited by fructose 6-phosphate and this inhibition was released by inorganic phosphate and by glycerol 3-phosphate. Both liver and muscle fructose 2,6-bisphosphatases formed a 32P-labeled enzyme intermediate when incubated in the presence of fructose 2,6-[2-32P]bisphosphate.

Fructose 2,6-bisphosphate in human platelets: its possible role in the control of basal and thrombin-stimulated glycolysis

Human platelets contain fructose 2,6-bisphosphate, 6-phosphofructo-l-kinase (ATP: D-Fructose-6-phosphate-1-phosphotransferase, E.C.2.7. 1.11), the rate-limiting enzyme in platelet glycolysis appear to be significantly activated by physiological concentration of the compound, suggesting for fructose 2,6-bisphosphate a key regulatory role in the control of the glycolytic flux. Incubation of human platelets with thrombin results in a parallel and rapid increase of fructose-2,6-bisphosphate levels and glycolytic flux, suggesting that the compound may also be involved in the enhancement of glycolysis elicited by the stimulating agent.

Fructose 2,6-bisphosphate and the control of glycolysis in bovine spermatozoa

Epididymal bovine sperm contain fructose-1,6-bisphosphatase activity which is inhibited by AMP and by fructose 2,6-bisphosphate. Sperm phosphofructokinase displays kinetic characteristics that are typical of the F-type and it is stimulated by fructose 2,6-bisphosphate. The concentration of sperm fructose 2,6-bisphosphate remained unaffected at 1-2 microM when the glycolytic rate was either increased by glucose, caffeine or antimycin, or decreased by alpha-chlorohydrin or 6-chloro-6-deoxyglucose.

Comparison of purified bovine heart and rat liver 6-phosphofructo-2-kinase. Evidence for distinct isoenzymes

Rat liver and bovine heart 6-phosphofructo-2-kinase were purified by the same procedure. Compared with the liver enzyme, the heart enzyme had a smaller apparent Mr, different kinetic properties, was not inactivated by cyclic AMP-dependent protein kinase, and contained less fructose-2,6-bisphosphatase activity. These differences suggest that heart and liver 6-phosphofructo-2-kinase are distinct isoenzymes. Likewise, 6-phosphofructo-2-kinase from rat heart and skeletal muscle was not inactivated on treatment with cyclic AMP-dependent protein kinase.

Phorbol 12-myristate 13-acetate and insulin increase the concentration of fructose 2,6-bisphosphate and stimulate glycolysis in chicken embryo fibroblasts

Incubation of chicken embryo fibroblasts with mitogenic concentrations of insulin for 24 hr or with the tumor promoter phorbol 12-myristate 13-acetate for 6 hr stimulated lactate release and 3-O-methylglucose uptake. Insulin also increased the Vmax of 6-phosphofructo-1-kinase (ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11). Both agents increased the concentration of fructose 2,6-bisphosphate and the activity of 6-phosphofructo-2-kinase (EC 2.7.1.-), the enzyme that catalyzes the synthesis of this stimulator of 6-phosphofructo-1-kinase. These changes provide an explanation for the stimulation of glycolysis by insulin and phorbol esters. In contrast to the situation in rat liver, fructose 2,6-bisphosphate concentration did not decrease after cyclic AMP treatment. Incubation of cells with phorbol ester analogues or with glycerol derivatives that are known to stimulate, or to bind to, protein kinase C did increase the concentration of fructose 2,6-bisphosphate, suggesting that the stimulation of 6-phosphofructo-2-kinase by phorbol 12-myristate 13-acetate is mediated by protein kinase C.

Glucokinase is not the pancreatic B-cell glucoreceptor

A series of recent experimental findings are reviewed to indicate that glucokinase does not represent the pancreatic B-cell glucoreceptor. Whether in liver, pancreatic islet or insulin-producing tumoral cell homogenates, glucokinase fails to yield a higher reaction velocity with alpha-than beta-D-glucose. At a high glucose concentration (40 mmol/l), when the phosphorylation of glucose by glucokinase is indeed higher with beta- than alpha-D-glucose, no preference for beta-D-glucose is observed in intact islets, as judged from the utilization of D-[5-3H]glucose, production of lactic acid, oxidation of D-[U-14C]glucose, net uptake of 45Ca or release of insulin. The glucose 6-phosphate content of intact islets is higher in the presence of beta- than alpha-D-glucose. At a low glucose concentration (3.3 mmol/l), when the participation of glucokinase to hexose phosphorylation is minimal, alpha-D-glucose is still better metabolized and stimulates both 45Ca net uptake and insulin release more efficiently than beta-D-glucose, despite the fact that hexokinase yields a higher reaction velocity with beta- than alpha-D-glucose. In intact islets, beta-D-glucose is used preferentially to alpha-D-glucose in the pentose cycle pathway as judged from the oxidation of alpha- or beta-D-[1-14C]glucose relative to that of alpha- or beta-D-[6-14C]glucose. In islets removed from fasted rats, the rate of glycolysis is more severely decreased than expected from the repression of glucokinase. The metabolism of glucose in tumoral insulin-producing cells differs, in several respects, from that in normal pancreatic islets, although the pattern of hexokinase and glucokinase activities is similar in these two types of cells. All these observations point to the participation of regulatory sites distal to glucose phosphorylation in the control of glucose metabolism in islet cells.

Pyrophosphate:fructose 6-phosphate 1-phosphotransferase and glycolysis in non-photosynthetic tissues of higher plants

The activity of pyrophosphate:fructose-6-phosphate 1-phosphotransferase [PFK (PPi); EC 2.7.1.90] in extracts of the storage tissues of leek (Allium porrum), beetroot (Beta vulgaris) and roots of darnel (Lolium temulentum) exceeded 0.15 mumol/min per g fresh wt. As net flux from fructose 1,6-bisphosphate to fructose 6-phosphate in these tissues is unlikely, it is suggested that PFK (PPi) does not contribute to gluconeogenesis or starch synthesis. The maximum catalytic activities of PFK (PPi) in apex, stele and cortex of the root of pea (Pisum sativum) and in the developing and the thermogenic club of the spadix of cuckoo-pint (Arum maculatum) were measured and compared with those of phosphofructokinase, and to estimates of the rates of carbohydrate oxidation. PPi and fructose 2,6-bisphosphate in Arum clubs were measured. The above measurements are consistent with a glycolytic role for PFK (PPi) in tissues where there is marked biosynthesis, but not in the thermogenic club of Arum. The possibility that PFK (PPi) is a means of synthesizing pyrophosphate is discussed.

The mechanism by which ethanol decreases the concentration of fructose 2,6-bisphosphate in the liver

The intragastric administration of ethanol to fed rats caused in their liver, within about 1 h, a 20-fold decrease in the concentration of fructose 2,6-bisphosphate, an activation of fructose 2,6-bisphosphatase, an inactivation of phosphofructo-2-kinase but no change in the concentration of cyclic AMP. Incubation of isolated hepatocytes in the presence of ethanol caused a rapid increase in the concentration of sn-glycerol 3-phosphate and a slower and continuous decrease in the concentration of fructose 2,6-bisphosphate with no change in that of hexose 6-phosphates. There was also a relatively slow activation of fructose 2,6-bisphosphatase and inactivation of phosphofructo-2-kinase. Glycerol and acetaldehyde had effects similar to those of ethanol on the concentration of phosphoric esters in the isolated liver cells. 4-Methylpyrazole cancelled the effect of ethanol but reinforced those of acetaldehyde. High concentrations of glucose or of dihydroxyacetone caused an increase in the concentration of hexose 6-phosphates and counteracted the effect of ethanol to decrease the concentration of fructose 2,6-bisphosphate. As a rule, hexose 6-phosphates had a positive effect and sn-glycerol 3-phosphate had a negative effect on the concentration of fructose 2,6-bisphosphate in the liver, so that, at a given concentration of hexose 6-phosphates, there was an inverse relationship between the concentration of fructose 2,6-bisphosphate and that of sn-glycerol 3-phosphate. These effects could be explained by the ability of sn-glycerol 3-phosphate to inhibit phosphofructo-2-kinase and to counteract the inhibition of fructose 2,6-bisphosphatase by fructose 6-phosphate. sn-Glycerol 3-phosphate had also the property to accelerate the inactivation of phosphofructo-2-kinase by cyclic AMP-dependent protein kinase whereas fructose 2,6-bisphosphate had the opposite effect. The changes in the activity of phosphofructo-2-kinase and fructose 2,6-bisphosphatase appear therefore to be the result rather than the cause of the decrease in the concentration of fructose 2,6-bisphosphate.