Distinct functional roles of the two terminal halves of eukaryotic phosphofructokinase.

Eukaryotic PFK (phosphofructokinase), a key regulatory enzyme in glycolysis, has homologous N- and C-terminal domains thought to result from duplication, fusion and divergence of an ancestral prokaryotic gene. It has been suggested that both the active site and the Fru-2,6-P2 (fructose 2,6-bisphosphate) allosteric site are formed by opposing N- and C-termini of subunits orientated antiparallel in a dimer. In contrast, we show in the present study that in fact the N-terminal halves form the active site, since expression of the N-terminal half of the enzymes from Dictyostelium discoideum and human muscle in PFK-deficient yeast restored growth on glucose.

Isolation of novel animal cell lines defective in glycerolipid biosynthesis reveals mutations in glucose-6-phosphate isomerase.

Glycerolipids are structural components for membranes and serve in energy storage. We describe here the use of a photodynamic selection technique to generate a population of Chinese hamster ovary cells that display a global deficiency in glycerolipid biosynthesis. One isolate from this population, GroD1, displayed a profound reduction in the synthesis of phosphatidylcholine, phosphatidylethanolamine, and triglycerides but presented high levels of phosphatidic acid and normal levels of phosphatidylinositol synthesis.

Mice deficient in phosphofructokinase-M have greatly decreased fat stores.

Synthesis of triacylglycerol requires the glucose-derived glycerol component, and glucose uptake has been viewed as the rate-limiting step in glucose metabolism in adipocytes. Furthermore, adipose tissue contains all three isoforms of the glycolytic enzyme phosphofructokinase (PFK). We here report that mice deficient in the muscle isoform PFK-M have greatly reduced fat stores.

Tissue-dependent loss of phosphofructokinase-M in mice with interrupted activity of the distal promoter: impairment in insulin secretion.

Phosphofructokinase is a key enzyme of glycolysis that exists as homo- and heterotetramers of three subunit isoforms: muscle, liver, and C type. Mice with a disrupting tag inserted near the distal promoter of the phosphofructokinase-M gene showed tissue-dependent differences in loss of that isoform: 99% in brain and 95-98% in islets, but only 50-75% in skeletal muscle and little if any loss in heart. This correlated with the continued presence of proximal transcripts specifically in muscle tissues.

Ca2+, NAD(P)H and membrane potential changes in pancreatic beta-cells by methyl succinate: comparison with glucose.

The present study was undertaken to determine the main metabolic secretory signals generated by the mitochondrial substrate MeS (methyl succinate) compared with glucose in mouse and rat islets and to understand the differences. Glycolysis and mitochondrial metabolism both have key roles in the stimulation of insulin secretion by glucose. Both fuels elicited comparable oscillatory patterns of Ca2+ and changes in plasma and mitochondrial membrane potential in rat islet cells and clonal pancreatic beta-cells (INS-1).

Mechanisms for increased glycolysis in the hypertrophied rat heart.

Glycolysis increases in hypertrophied hearts but the mechanisms are unknown. We studied the regulation of glycolysis in hearts with pressure-overload LV hypertrophy (LVH), a model that showed marked increases in the rates of glycolysis (by 2-fold) and insulin-independent glucose uptake (by 3-fold). Although the V(max) of the key glycolytic enzymes was unchanged in this model, concentrations of free ADP, free AMP, inorganic phosphate (P(i)), and fructose-2,6-bisphosphate (F-2,6-P2), all activators of the rate-limiting enzyme phosphofructokinase (PFK), were increased (up to 10-fold).

Dihydroxyacetone-induced oscillations in cytoplasmic free Ca2+ and the ATP/ADP ratio in pancreatic beta-cells at substimulatory glucose.

Glucose stimulation of pancreatic beta-cells causes oscillatory influx of Ca2+, leading to pulsatile insulin secretion. We have proposed that this is due to oscillations of glycolysis and the ATP/ADP ratio, which modulate the activity of ATP-sensitive K+ channels. We show here that dihydroxyacetone, a secretagogue that feeds into glycolysis below the putative oscillator phosphofructokinase, could cause a single initial peak in cytoplasmic free Ca2+ ([Ca2+]i) but did not by itself cause repeated oscillations in [Ca2+]i in mouse pancreatic beta-cells.

Glucose autoregulates its uptake in skeletal muscle: involvement of AMP-activated protein kinase.

Preexposure to a low concentration of glucose upregulates glucose transport into skeletal muscle, whereas exposure to a high concentration of glucose has the opposite effect. This autoregulatory process occurs independently of insulin, and the mechanism by which it operates is incompletely understood. Activation of the energy-sensing enzyme AMP-activated protein kinase (AMPK) has been shown to increase insulin-independent glucose transport into skeletal muscle in response to such stimuli as exercise and hypoxia.

Potentiation of insulin secretion by phorbol esters is mediated by PKC-alpha and nPKC isoforms.

Culturing clonal beta-cells (HIT-T15) overnight in the presence of phorbol ester [phorbol myristate acetate (PMA)] enhanced insulin secretion while causing downregulation of some protein kinase C (PKC) isoforms and most PKC activity. We show here that this enhanced secretion required the retention of PMA in the cell. Hence, it could not be because of long-lived phosphorylation of cellular substrates by the isoforms that were downregulated, namely PKC-alpha, -betaII, and -epsilon, but could be because of the continued activation of the two remaining diacylglycerol-sensitive isoforms delta and mu.

Glucagon-like peptide 1 and fatty acids amplify pulsatile insulin secretion from perifused rat islets.

Glucose-induced insulin secretion from isolated, perifused rat islets is pulsatile with a period of about 5-10 min, similar to the insulin oscillations that are seen in healthy humans but which are impaired in Type II diabetes. We evaluated the pattern of enhancement by the potent incretin, glucagon-like peptide 1 (GLP-1). GLP-1 increased the amplitude of pulses and the magnitude of insulin secretion from the perifused islets, without affecting the average time interval between pulses.

Hyperglycemia increases endothelial superoxide that impairs smooth muscle cell Na+-K+-ATPase activity.

Nitric oxide (NO) plays an important role in the control of numerous vascular functions including basal Na+-K+-ATPase activity in arterial tissue. Hyperglycemia inhibits Na+-K+-ATPase activity in rabbit aorta, in part, through diminished bioactivity of NO. The precise mechanism(s) for such observations, however, are not yet clear.

Glucose-induced metabolic oscillations parallel those of Ca(2+) and insulin release in clonal insulin-secreting cells. A multiwell approach to oscillatory cell behavior.

Insulin secretion from glucose-stimulated pancreatic beta-cells is oscillatory, and this is thought to result from oscillations in glucose metabolism. One of the primary metabolic stimulus-secretion coupling factors is the ATP/ADP ratio, which can oscillate as a result of oscillations in glycolysis. Using a novel multiwell culture plate system, we examined oscillations in insulin release and the ATP/ADP ratio in the clonal insulin-secreting cell lines HIT T-15 and INS-1.

Glucose-induced toxicity in insulin-producing pituitary cells that coexpress GLUT2 and glucokinase. Implications for metabolic engineering.

We have shown that intermediate lobe (IL) pituitary cells can be engineered to produce sufficient amounts of insulin (ins) to cure diabetes in nonobese diabetic mice but, unlike transplanted islets, ILins cells evade immune attack. To confer glucose-sensing capabilities into these cells, they were further modified with recombinant adenoviruses to express high levels of GLUT2 and the beta-cell isoform of glucokinase (GK). Although expression of GLUT2 alone had negligible effects on glucose usage and lactate production, expression of GK alone resulted in approximately 2-fold increase in glycolytic flux within the physiological (3-20 mm) glucose range.

Acute regulation of fatty acid oxidation and amp-activated protein kinase in human umbilical vein endothelial cells.

It is generally accepted that endothelial cells generate most of their ATP by anaerobic glycolysis and that very little ATP is derived from the oxidation of fatty acids or glucose. Previously, we have reported that, in cultured human umbilical vein endothelial cells (HUVECs), activation of AMP-activated protein kinase (AMPK) by the cell-permeable activator 5-aminoimidazole-4-carboximide riboside (AICAR) is associated with an increase in the oxidation of (3)H-palmitate. In the present study, experiments carried out with cultured HUVECs revealed the following: (1) AICAR-induced increases in palmitate oxidation during a 2-hour incubation are associated with a decrease in the concentration of malonyl coenzyme A (CoA) (an inhibitor of carnitine palmitoyl transferase 1), which temporally parallels the increase in AMPK activity and a decrease in the activity of acetyl CoA carboxylase (ACC).

Glucagon-like peptide 1 stimulates lipolysis in clonal pancreatic beta-cells (HIT).

Glucagon-like peptide 1 (GLP-1) is the most potent physiological incretin for insulin secretion from the pancreatic beta-cell, but its mechanism of action has not been established. It interacts with specific cell-surface receptors, generates cAMP, and thereby activates protein kinase A (PKA). Many changes in pancreatic beta-cell function have been attributed to PKA activation, but the contribution of each one to the secretory response is unknown.

Oxygen consumption oscillates in single clonal pancreatic beta-cells (HIT).

Based on population studies, we have hypothesized that changes in metabolism in pancreatic beta-cells precede changes in Ca2+. It is well known from single-cell Ca2+ studies that variable oscillatory patterns in Ca2+ occur in response to glucose stimulation. The present studies, using the clonal beta-cell line HIT-T-15, were undertaken to evaluate the relationship between glucose concentration, insulin secretion, and O2 consumption and to determine the Ca2+ dependency of glucose-induced changes in O2 consumption.

Activation of malonyl-CoA decarboxylase in rat skeletal muscle by contraction and the AMP-activated protein kinase activator 5-aminoimidazole-4-carboxamide-1-beta -D-ribofuranoside.

Alterations in the concentration of malonyl-CoA, an inhibitor of carnitine palmitoyltransferase I, have been linked to the regulation of fatty acid oxidation in skeletal muscle. During contraction decreases in muscle malonyl-CoA concentration have been related to activation of AMP-activated protein kinase (AMPK), which phosphorylates and inhibits acetyl-CoA carboxylase (ACC), the rate-limiting enzyme in malonyl-CoA formation. We report here that the activity of malonyl-CoA decarboxylase (MCD) is increased in contracting muscle.

The role of long-chain fatty acyl-CoA esters in beta-cell signal transduction.

Glucose-induced insulin secretion is associated with inhibition of free fatty acid (FFA) oxidation, increased esterification and complex lipid formation by pancreatic beta-cells. Abundant evidence favors a role for cytosolic long-chain acyl-CoA (LC-CoA), including the rapid rise in malonyl CoA, the inhibitory effect of hydroxycitrate or acetyl CoA carboxylase knockout, both of which prevent malonyl CoA formation, and the stimulatory effect of exogenous FFA. On the other hand, some evidence opposes the concept, including the fall in total LC-CoA levels in response to glucose, the stimulatory effect of LC-CoA on K(ATP) channels and the lack of inhibition of glucose-stimulated secretion either by overexpression of malonyl CoA decarboxylase, which markedly lowers malonyl CoA levels, or by triacsin C, which blocks FFA conversion to LC-CoA.

The effect of AMP-activated protein kinase and its activator AICAR on the metabolism of human umbilical vein endothelial cells.

In several non-vascular tissues in which it has been studied, AMP-activated protein kinase (AMPK) appears to modulate the cellular response to stresses such as ischemia. In liver and muscle, it phosphorylates and inhibits acetyl CoA carboxylase (ACC), leading to an increase in fatty acid oxidation; and in muscle, its activation is associated with an increase in glucose transport. Here we report the presence of both AMPK and ACC in human umbilical vein endothelial cells (HUVEC).

Glucose-fatty acid cycle to inhibit glucose utilization and oxidation is not operative in fatty acid-cultured islets.

The glucose-fatty acid cycle of Randle entails two elements: decreased pyruvate dehydrogenase (PDH) activity, which inhibits glucose oxidation, and inhibition of phosphofructokinase (PFK) by a rise in citrate so that glucose-6-phosphate (G-6-P) levels increase, thereby inhibiting hexokinase activity and hence glucose utilization. Chronic exposure of islets to long-chain fatty acids (FA) is reported to lower PDH activity, but the effect on glucose oxidation and glucose-induced insulin secretion is uncertain. We investigated rat islets that were cultured for 4 days with 0.

Synthesis of potent and selective inhibitors of human plasma kallikrein.

The synthesis and in vitro enzyme inhibition profile of a series of novel trifluoromethylketone (TFMK) inhibitors of human plasma kallikrein (PK) are described. We have developed an efficient method for the construction of peptide TFMKs that provides the final product devoid of compromised stereochemical integrity. Many of these compounds are potent inhibitors of PK and exhibit reduced inhibition of tissue kallikrein (TK) and plasmin (HP).

Shared biochemical properties of glucotoxicity and lipotoxicity in islets decrease citrate synthase activity and increase phosphofructokinase activity.

Diabetic states are characterized by a raised serum/islet level of triglycerides and a lowered EC50 (concentration at half-maximal stimulation) for glucose-induced insulin secretion. Culturing islets with long-chain fatty acids (FAs) replicates the basal insulin hypersecretion. In a previous study, we showed that the mechanism involved deinhibition of hexokinase by a 60% decrease in glucose-6-phosphate (G-6-P).

Peptide aldehyde inhibitors of the kallikreins: an investigation of subsite interactions with tripeptides containing structural variations at the amino terminus.

A series of tripeptide aldehyde derivatives containing variations at the P3 subsite and the amino terminus has been prepared and evaluated for trypsin-like serine protease inhibition. These compounds exhibit strong in vitro inhibition of human plasma kallikrein (HPK), porcine pancreatic kallikrein (PPK) and human plasmin (HP). As suspected from an examination of a related crystal structure, the presence of a hydrophobic residue (adamantyl) at the amino terminus dramatically improves the binding to PPK.

Fatty acid-induced beta cell hypersensitivity to glucose. Increased phosphofructokinase activity and lowered glucose-6-phosphate content.

Diabetic states are characterized by a raised serum/islet level of long chain fatty acids and a lowered ED50 for glucose-induced insulin secretion. Prolonged culture (> 6 h) of islets with long chain fatty acids replicates the basal insulin hypersecretion. We examined this effect in rat islets cultured for 24 h with 0.

Are metabolic oscillations responsible for normal oscillatory insulin secretion?

Normal insulin secretion is oscillatory in vivo and in vitro, with a period of approximately 5-10 min. The mechanism of generating these oscillations is not yet established, but a metabolic basis seems most likely for glucose-stimulated secretion. The rationale is that 1) spontaneous oscillatory operation of glycolysis is a well-established phenomenon; 2) oscillatory behavior of glycolysis involves oscillations in the ATP/ADP ratio, which can cause alternating opening and closing of ATP-sensitive K+ channels, leading to the observed oscillations in membrane potential and Ca2+ influx in pancreatic beta-cells, and may also have downstream effects on exocytosis; 3) spontaneous Ca2+ oscillations are an unlikely basis in this case, since intracellular stores are not of primary importance in the stimulus-secretion coupling, and furthermore, insulin oscillations occur under conditions when intracellular Ca2+ levels are not changing; 4) a neural basis cannot account for insulin oscillations from perifused islets and clonal beta-cells or from transplanted islets or pancreas in vivo; 5) observed oscillations in metabolite levels and fluxes further support a metabolic basis, as does the presence in beta-cells of the oscillatory isoform of phosphofructokinase (PFK-M).