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

What Is the Metabolic Amplification of Insulin Secretion and Is It (Still) Relevant?

The pancreatic beta-cell transduces the availability of nutrients into the secretion of insulin. While this process is extensively modified by hormones and neurotransmitters, it is the availability of nutrients, above all glucose, which sets the process of insulin synthesis and secretion in motion. The central role of the mitochondria in this process was identified decades ago, but how changes in mitochondrial activity are coupled to the exocytosis of insulin granules is still incompletely understood. The identification of ATP-sensitive K⁺-channels provided the link between the level of adenine nucleotides and the electrical activity of the beta cell, but the depolarization-induced Ca²⁺-influx into the beta cells, although necessary for stimulated secretion, is not sufficient to generate the secretion pattern as produced by glucose and other nutrient secretagogues. The metabolic amplification of insulin secretion is thus the sequence of events that enables the secretory response to a nutrient secretagogue to exceed the secretory response to a purely depolarizing stimulus and is thus of prime importance. Since the cataplerotic export of mitochondrial metabolites is involved in this signaling, an orienting overview on the topic of nutrient secretagogues beyond glucose is included. Their judicious use may help to define better the nature of the signals and their mechanism of action.

6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase and small chemical activators affect enzyme activity of activating glucokinase mutants by distinct mechanisms

Glucokinase (GK), a monomeric glucose-phosphorylating enzyme characterised by high structural flexibility, acts as a glucose sensor in pancreatic beta cells and liver. Pharmaceutical efforts to control the enzyme are hampered by an incomplete understanding of GK regulation. We investigated GK characteristics of wild-type and activating S64Y and G68V mutant proteins in the presence of various combinations of the synthetic activators RO-28-1675 and compound A, the endogenous activator fructose-2,6-bisphosphatase (FBPase-2), and the inhibitor mannoheptulose. S64Y impedes formation of a turn structure that is characteristic for the inactive enzyme conformation, and complex formation with compound A induces collision with the large domain. G68V evokes close contact of connecting region I and helix α13 with RO-28-1675 and compound A. Both mutants showed higher activity than the wild-type at low glucose and were susceptible to further activation by FBPase-2 and RO-28-1675, alone and additively. G68V was less active than S64Y, but was activatable by compound A. In contrast, compound A inhibited S64Y, and this effect was even more pronounced in combination with mannoheptulose. Mutant and wild-type GK showed comparable thermal stability and intracellular lifetimes. A GK-6-phosphofructo-2-kinase (PFK-2)/FBPase-2 complex predicted by in silico protein-protein docking demonstrated possible binding of the FBPase-2 domain near the active site of GK. In summary, activating mutations within the allosteric site of GK do not preclude binding of chemical activators (GKAs), but can alter their action into inhibition. Our postulated GK-PFK-2/FBPase-2 complex represents the endogenous principle of activation by substrate channelling which permits binding of other small molecules and proteins.

Molecular and cellular regulation of human glucokinase

Glucose metabolism in humans is tightly controlled by the activity of glucokinase (GCK). GCK is predominantly produced in the pancreas, where it catalyzes the rate-limiting step of insulin secretion, and in the liver, where it participates in glycogen synthesis. A multitude of disease-causing mutations within the gck gene have been identified. Activating mutations manifest themselves in the clinic as congenital hyperinsulinism, while loss-of-function mutations produce several diabetic conditions. Indeed, pharmaceutical companies have shown great interest in developing GCK-associated treatments for diabetic patients. Due to its essential role in maintaining whole-body glucose homeostasis, GCK activity is extensively regulated at multiple levels. GCK possesses a unique ability to self-regulate its own activity via slow conformational dynamics, which allows for a cooperative response to glucose. GCK is also subject to a number of protein-protein interactions and post-translational modification events that produce a broad range of physiological consequences. While significant advances in our understanding of these individual regulatory mechanisms have been recently achieved, how these strategies are integrated and coordinated within the cell is less clear. This review serves to synthesize the relevant findings and offer insights into the connections between molecular and cellular control of GCK.

Regulation of Glucokinase by Intracellular Calcium Levels in Pancreatic β Cells

Glucokinase (GCK) controls the rate of glucose metabolism in pancreatic β cells, and its activity is rate-limiting for insulin secretion. Posttranslational GCK activation can be stimulated through either G protein-coupled receptors or receptor tyrosine kinase signaling pathways, suggesting a common mechanism. Here we show that inhibiting Ca(2+) release from the endoplasmic reticulum (ER) decouples GCK activation from receptor stimulation. Furthermore, pharmacological release of ER Ca(2+) stimulates activation of a GCK optical biosensor and potentiates glucose metabolism, implicating rises in cytoplasmic Ca(2+) as a critical regulatory mechanism. To explore the potential for glucose-stimulated GCK activation, the GCK biosensor was optimized using circularly permuted mCerulean3 proteins. This new sensor sensitively reports activation in response to insulin, glucagon-like peptide 1, and agents that raise cAMP levels. Transient, glucose-stimulated GCK activation was observed in βTC3 and MIN6 cells. An ER-localized channelrhodopsin was used to manipulate the cytoplasmic Ca(2+) concentration in cells expressing the optimized FRET-GCK sensor. This permitted quantification of the relationship between cytoplasmic Ca(2+) concentrations and GCK activation. Half-maximal activation of the FRET-GCK sensor was estimated to occur at ∼400 nm Ca(2+). When expressed in islets, fluctuations in GCK activation were observed in response to glucose, and we estimated that posttranslational activation of GCK enhances glucose metabolism by ∼35%. These results suggest a mechanism for integrative control over GCK activation and, therefore, glucose metabolism and insulin secretion through regulation of cytoplasmic Ca(2+) levels.

A cis-eQTL in PFKFB2 is associated with diabetic nephropathy, adiposity and insulin secretion in American Indians

A prior genome-wide association study (GWAS) in Pima Indians identified a variant within PFKFB2 (rs17258746) associated with body mass index (BMI). PFKFB2 encodes 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase isoform 2, which plays a role in glucose metabolism. To follow-up on the GWAS, tag SNPs across PFKFB2 were genotyped in American Indians (AI) who had longitudinal data on BMI (n = 6839), type 2 diabetes (T2D; n = 7710), diabetic nephropathy (DN; n = 2452), % body fat (n = 555) and insulin secretion (n = 298). Two SNPs were further genotyped in urban AI to assess replication for DN (n = 864). PFKFB2 expression was measured in 201 adipose biopsies using real-time RT-PCR and 61 kidney biopsies using the Affymetrix U133 array. Two SNPs (rs17258746 and rs11120137), which capture the same signal, were associated with maximum BMI in adulthood (β = 1.02 per risk allele, P = 7.3 × 10(-4)), maximum BMI z-score in childhood (β = 0.079, P = 0.03) and % body fat in adulthood (β = 3.4%, P = 3 × 10(-7)). The adiposity-increasing allele correlated with lower PFKFB2 adipose expression (β = 0.81, P = 9.4 × 10(-4)). Lower expression of PFKFB2 further correlated with higher % body fat (r = -0.16, P = 0.02) and BMI (r = -0.17, P = 0.02). This allele was also associated with increased risk for DN in both cohorts of AI [odds ratio = 1.64 (1.32-2.02), P = 5.8 × 10(-6)], and similarly correlated with lower PFKFB2 expression in kidney glomeruli (β = 0.87, P = 0.03). The same allele was also associated with lower insulin secretion assessed by acute insulin response (β = 0.78, P = 0.03) and 30-min plasma insulin concentrations (β = 0.78, P = 1.1 × 10(-4)). Variation in PFKFB2 appears to reduce PFKFB2 expression in adipose and kidney tissues, and thereby increase risk for adiposity and DN.

The effect of pulp and seed extract of Citrullus Colocynthis, as an antidaibetic medicinal herb, on hepatocytes glycogen stores in diabetic rabbits

Background:

Medicinal herbs such as Citrullus Colocynthis (C.C) have been used traditionally in the treatment of diabetes mellitus. However therapeutic applications and adverse effects of C.C and its natural variants are not determined well. The current work investigates the effects of pulp and seed extract of C.C on hepatocyte's glycogen stores.

Materials and Methods:

Thirty six male rabbits were divided into six groups (control and diabetic) randomly. Alloxan was used in order to induce diabetes mellitus in animals. Among 5 diabetic groups, one remained as control and the rest received 100 and 200 mg/kg/day of either pulp or seed extract. One month later, animals were sacrificed and their liver specimen fixed in 10% Formalin was stained with periodic acid schiff (PAS) for light microscopic scanning.

Results:

PAS staining of hepatocytes revealed large amounts of glycogen stores in diabetic animals treated with pulp and seed extracts of C.C, contrary with non-treated diabetic rabbits. Sites of glycogen deposition were also different in animals treated with seed extract (P < 0.0001). No hepatic congestion was seen in treated animals. Dose escalation has no effect on the obtained results.

Conclusions:

The anti-diabetic effects of C.C can be explained by its effects on accumulation of glycogen stores in hepatocytes. The importance of varied sites of glycogen deposition by the application of C.C needs to be determined.

The ubiquitin-proteasome system regulates the stability and activity of the glucose sensor glucokinase in pancreatic β-cells

The ubiquitin-proteasome system is important to maintain pancreatic β-cell function. Inhibition of the proteasome significantly reduced glucose-induced insulin secretion. Key regulators of the stimulus/secretion cascade seem to be affected by protein misfolding if the proteasome is down-regulated as recently reported in humans with Type 2 diabetes. It remains unknown, however, whether the glucose sensor enzyme glucokinase is involved in this process. A direct interaction between glucokinase and ubiquitin could be shown in vivo by FRET, suggesting regulation of glucokinase by the proteasome. After proteasome inhibition glucokinase activity was significantly reduced in MIN6 cells, whereas the protein content was increased, indicating protein misfolding. Enhancing the availability of chaperones by cyclohexamide could induce refolding and restored glucokinase activity. Glucokinase aggregation due to proteasome blocking with MG132, bortezomib, epoxomicin or lactacystin could be detected in MIN6 cells, primary β-cells and hepatocytes using fluorescence-based assays. Glucokinase aggresome formation proceeded microtubule-assisted and was avoided by cyclohexamide. Thus the results of the present study provide support for glucokinase misfolding and aggregation in case of a diminished capacity of the ubiquitin-proteasome system in pancreatic β-cells. In the Type 2 diabetic situation this could contribute to reduced glucose-induced insulin secretion.

Mitochondrial localization of TIGAR under hypoxia stimulates HK2 and lowers ROS and cell death

The p53-inducible protein TIGAR (Tp53-induced Glycolysis and Apoptosis Regulator) functions as a fructose-2,6-bisphosphatase (Fru-2,6-BPase), and through promotion of the pentose phosphate pathway, increases NADPH production to help limit reactive oxygen species (ROS). Here, we show that under hypoxia, a fraction of TIGAR protein relocalized to mitochondria and formed a complex with hexokinase 2 (HK2), resulting in an increase in HK2 activity. Mitochondrial localization of TIGAR depended on mitochondrial HK2 and hypoxia-inducible factor 1 (HIF1α) activity. The ability of TIGAR to function as a Fru-2,6-BPase was independent of HK2 binding and mitochondrial localization, although both of these activities can contribute to the full activity of TIGAR in limiting mitochondrial ROS levels and protecting from cell death.

The mitochondrial Atp8 mutation induces mitochondrial ROS generation, secretory dysfunction, and β-cell mass adaptation in conplastic B6-mtFVB mice

Mutations in mitochondrial DNA (mtDNA) cause a variety of pathologic phenotypes. In this study, we used conplastic mouse strains to characterize the impact of a mtDNA mutation in the Atp8 gene on β-cell function, reactive oxygen species (ROS) generation, β-cell mass, and glucose metabolism in response to high-fat diet (HFD). In comparison with B6-mt(AKR) controls, the B6-mt(FVB) strain carries a point mutation of the mtDNA-coded Atp8 gene (ATP synthase), leading to a fragmentated mitochondrial phenotype. Isolated pancreatic islets from 3-month-old B6-mt(FVB) mice showed increased mitochondrial generation of ROS, reduced cellular ATP levels, reduced glucose-induced insulin secretion, higher susceptibility to palmitate stress, and pathological morphology of mitochondria. ROS generation in β-cells was not affected by changes of the ambient glucose concentrations. Feeding a HFD for 3 months resulted in impaired glucose tolerance in B6-mt(FVB) mice but not in B6-mt(AKR) controls. In B6-mt(FVB) animals, glucose intolerance positively correlated with gain of body weight. Serum insulin levels and β-cell mass significantly increased in B6-mt(FVB) mice after a 3-month HFD. The data indicate that the mutation in the Atp8 gene induces mitochondrial dysfunction in β-cells with concomitant impairment of secretory responsiveness. This mitochondrial dysfunction induced a higher susceptibility to metabolic stressors, although this effect appeared not strictly linked to nutrient-induced ROS generation. The Atp8 gene mutation caused mitochondrial dysfunction, apparently stimulating an adaptive increase of β-cell mass in response to HFD, whereas mitochondrial ROS might have had an supportive role.

Effects of the novel mitochondrial protein mimitin in insulin-secreting cells

Mimitin, a novel mitochondrial protein, has been shown to act as a molecular chaperone for the mitochondrial complex I and to regulate ATP synthesis. During Type 1 diabetes development, pro-inflammatory cytokines induce mitochondrial damage in pancreatic β-cells, inhibit ATP synthesis and reduce glucose-induced insulin secretion. Mimitin was expressed in rat pancreatic islets including β-cells and decreased by cytokines. In the ob/ob mouse, a model of insulin resistance and obesity, mimitin expression was down-regulated in liver and brain, up-regulated in heart and kidney, but not affected in islets. To further analyse the impact of mimitin on β-cell function, two β-cell lines, one with a low (INS1E) and another with a higher (MIN6) mimitin expression were studied. Mimitin overexpression protected INS1E cells against cytokine-induced caspase 3 activation, mitochondrial membrane potential reduction and ATP production inhibition, independently from the NF-κB (nuclear factor κB)-iNOS (inducible NO synthase) pathway. Mimitin overexpression increased basal and glucose-induced insulin secretion and prevented cytokine-mediated suppression of insulin secretion. Mimitin knockdown in MIN6 cells had opposite effects to those observed after overexpression. Thus mimitin has the capacity to modulate pancreatic islet function and to reduce cytokine toxicity.

Association with nitric oxide synthase on insulin secretory granules regulates glucokinase protein levels

Glucokinase (GCK) association with insulin-secretory granules is controlled by interaction with nitric oxide synthase (NOS) and is reversed by GCK S-nitrosylation. Nonetheless, the function of GCK sequestration on secretory granules is unknown. Here we report that the S-nitrosylation blocking V367M mutation prevents GCK accumulation on secretory granules by inhibiting association with NOS. Expression of this mutant is reduced compared with a second S-nitrosylation blocking GCK mutant (C371S) that accumulates to secretory granules and is expressed at levels greater than wild type. Even so, the rate of degradation for wild type and mutant GCK proteins were not significantly different from one another, and neither mutation disrupted the ability of GCK to be ubiquitinated. Furthermore, gene silencing of NOS reduced endogenous GCK content but did not affect β-actin content. Treatment of GCK(C371S) expressing cells with short interfering RNA specific for NOS also blocked accumulation of this protein to secretory granules and reduced expression levels to that of GCK(V367M). Conversely, cotransfection of catalytically inactive NOS increased GCK-mCherry levels. Expression of GCK(C371S) in βTC3 cells enhanced glucose metabolism compared with untransfected cells and cells expressing wild type GCK, even though this mutant has slightly reduced enzymatic activity in vitro. Finally, molecular dynamics simulations revealed that V367M induces conformational changes in GCK that are similar to S-nitrosylated GCK, thereby suggesting a mechanism for V367M-inhibition of NOS association. Our findings suggest that sequestration of GCK on secretory granules regulates cellular GCK protein content, and thus cellular GCK activity, by acting as a storage pool for GCK proteins.

Dimer interface rearrangement of the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase rat liver isoenzyme by cAMP-dependent Ser-32 phosphorylation

The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) is a key regulator of carbohydrate metabolism in liver. The goal of this study was to elucidate the regulatory role of Ser-32 phosphorylation on the kinase domain mediated dimerization of PFK-2/FBPase-2. Fluorescence-based mammalian two-hybrid and sensitized emission fluorescence resonance energy transfer analyses in cells revealed preferential binding within homodimers in contrast to heterodimers. Using isolated proteins a close proximity of two PFK-2/FBPase-2 monomers was only detectable in the phosphorylated enzyme dimer. Thus, a flexible kinase interaction mode exists, suggesting dimer conformation mediated coupling of hormonal and posttranslational enzyme regulation to the metabolic response in liver.

Real-time analysis of intracellular glucose and calcium in pancreatic beta cells by fluorescence microscopy

Glucose is the physiological stimulus for insulin secretion in pancreatic beta cells. The uptake and phosphorylation of glucose initiate and control downstream pathways, resulting in insulin secretion. However, the temporal coordination of these events in beta cells is not fully understood. The recent development of the FLII(12)Pglu-700μ-δ6 glucose nanosensor facilitates real-time analysis of intracellular glucose within a broad concentration range. Using this fluorescence-based technique, we show the shift in intracellular glucose concentration upon external supply and removal in primary mouse beta cells with high resolution. Glucose influx, efflux, and metabolism rates were calculated from the time-dependent plots. Comparison of insulin-producing cells with different expression levels of glucose transporters and phosphorylating enzymes showed that a high glucose influx rate correlated with GLUT2 expression, but was largely also sustainable by high GLUT1 expression. In contrast, in cells not expressing the glucose sensor enzyme glucokinase glucose metabolism was slow. We found no evidence of oscillations of the intracellular glucose concentration in beta cells. Concomitant real-time analysis of glucose and calcium dynamics using FLII(12)Pglu-700μ-δ6 and fura-2-acetoxymethyl-ester determined a glucose threshold of 4mM for the [Ca(2+)](i) increase in beta cells. Indeed, a glucose concentration of 7mM had to be reached to evoke large amplitude [Ca(2+)](i) oscillations. The K(ATP) channel closing agent glibenclamide was not able to induce large amplitude [Ca(2+)](i) oscillations in the absence of glucose. Our findings suggest that glucose has to reach a threshold to evoke the [Ca(2+)](i) increase and subsequently initiate [Ca(2+)](i) oscillations in a K(ATP) channel independent manner.

Mechanism of prostacyclin-induced potentiation of glucose-induced insulin secretion

Arachidonic acid metabolites are crucial mediators of inflammation in diabetes. Although eicosanoids are established modulators of pancreatic β-cell function, the role of prostacyclin (prostaglandin I2) is unknown. Therefore, this study aimed to analyze the role of prostacyclin in β-cell function. Prostacyclin synthase (PGIS) was weakly expressed in rat islet cells but nevertheless significantly increased by incubation with 30 mM glucose, especially in non-β-cells. PGIS was overexpressed in INS1E cells, and the regulation of insulin secretion was analyzed. PGIS overexpression strongly potentiated glucose-induced insulin secretion along with increased insulin content and ATP production. Importantly, overexpression of PGIS potentiated only nutrient-induced insulin secretion. The effect of PGIS overexpression was mediated by prostacyclin released from insulin-secreting cells and dependent on prostacyclin receptor (IP receptor) activation, with concomitant cAMP production. The cAMP-mediated potentiation of glucose-induced insulin secretion by prostacyclin was independent of the protein kinase A pathway but strongly attenuated by the knockdown of the exchange protein directly activated by cAMP 2 (Epac2), pointing to a crucial role for Epac2 in this process. Thus, prostacyclin is a powerful potentiator of glucose-induced insulin secretion. It improves the secretory capacity by inducing insulin biosynthesis and probably by stimulating exocytosis. Our findings open a new therapeutical perspective for an improved treatment of type 2 diabetes.

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.

Glucokinase mediates coupling of glycolysis to mitochondrial metabolism but not to beta cell damage at high glucose exposure levels

AIMS/HYPOTHESIS

Glucose is the main stimulus of insulin secretion in pancreatic beta cells. However, high glucose has also been considered to damage beta cells. In this study we examined, with special emphasis on the role of the glucose sensor enzyme glucokinase, whether elevated glucose metabolism evokes toxicity to beta cells.

METHODS

RINm5F-R-EYFP-GK cells, producing glucokinase in response to a synthetic inducer, and rat beta cells were incubated at different glucose concentrations. Glucokinase enzyme activity, insulin secretion, cell viability and mitochondrial metabolism were analysed.

RESULTS

Glucokinase production evoked a concentration-dependent increase in glucose-induced insulin secretion from RINm5F-R-EYFP-GK cells without reducing cell viability. Pre-culture at high glucose (30 mmol/l) in the absence of high concentrations of NEFA neither reduced viability nor significantly increased apoptosis in RINm5F-R-EYFP-GK cells and rat beta cells. The integrity of the mitochondrial respiratory chain and mitochondrial dynamics, namely fusion and fission, were not impaired by high glucose pre-culture. As previously demonstrated in mouse beta cells, pre-culture at high glucose significantly decreased the mitochondrial membrane potential heterogeneity in RINm5F-R-EYFP-GK cells. Indeed, after starvation, in response to glucose, rat beta cells and RINm5F-R-EYFP-GK cells with glucokinase production pre-cultured for 48 h at high glucose showed the fastest increase in the mitochondrial membrane potential.

CONCLUSIONS/INTERPRETATION

Our experiments do not support the hypothesis that glucokinase and the glucose metabolism on its own act as a mediator of beta cell toxicity. By contrast, rather a beneficial effect on glucose-induced insulin secretion after glucokinase production was observed, based on an improved coupling of the glucose stimulus to the mitochondrial metabolism.

Endogenous activation of glucokinase by 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase is glucose dependent

Glucokinase (GK) plays a crucial role as glucose sensor in glucose-induced insulin secretion in pancreatic β-cells. The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) acts as an endogenous GK activator. Therefore, the goal of this study was the analysis of GK-PFK-2/FBPase-2 complex formation and its effect on metabolic stimulus-secretion coupling in β-cells in dependence upon glucose. The interaction between GK and PFK-2/FBPase-2 was analyzed in insulin-secreting MIN6 cells with a new fluorescence-based mammalian two-hybrid system. In contrast to the commonly used mammalian two-hybrid systems that require sampling before detection, the system used allows monitoring of the effects of environmental changes on protein-protein interactions on the single-cell level. Increasing the glucose concentration in the cell culture medium from 3 to 10 and 25 mmol/liter amplified the interaction between the enzymes stepwise. Importantly, in line with these results, overexpression of PFK-2/FBPase-2 in MIN6 cells evoked only at 10 and 25 mmol/liter, an increase in insulin secretion. Furthermore, a PFK-2/FBPase-2 mutant with an abolished GK-binding motif neither showed a glucose-dependent GK binding nor was able to increase insulin secretion. The results obtained with the mammalian two-hybrid system could be confirmed by fluorescence resonance energy transfer experiments in COS cells. Furthermore, the established interaction between GK and the liver GRP served in all experiments as a control. Thus, this study clearly showed that binding and activation of GK by PFK-2/FBPase-2 in β-cells is promoted by glucose, resulting in an enhancement of insulin secretion at stimulatory glucose concentrations, without affecting basal insulin secretion.

Phenotypic and pathomorphological characteristics of a novel mutant mouse model for maturity-onset diabetes of the young type 2 (MODY 2)

Several mutant mouse models for human diseases such as diabetes mellitus have been generated in the large-scale Munich ENU (N-ethyl-N-nitrosourea) mouse mutagenesis project. The aim of this study was to identify the causal mutation of one of these strains and to characterize the resulting diabetic phenotype. Mutants exhibit a T to G transversion mutation at nt 629 in the glucokinase (Gck) gene, leading to an amino acid exchange from methionine to arginine at position 210. Adult Munich Gck(M210R) mutant mice demonstrated a significant reduction of hepatic glucokinase enzyme activity but equal glucokinase mRNA and protein abundances. While homozygous mutant mice exhibited growth retardation and died soon after birth in consequence of severe hyperglycemia, heterozygous mutant mice displayed only slightly elevated blood glucose levels, present from birth, with development of disturbed glucose tolerance and glucose-induced insulin secretion. Additionally, insulin sensitivity and fasting serum insulin levels were slightly reduced in male mutant mice from an age of 90 days onward. While beta-cell mass was unaltered in neonate heterozygous and homozygous mutant mice, the total islet and beta-cell volumes and the total volume of isolated beta-cells were significantly decreased in 210-day-old male, but not female heterozygous mutant mice despite undetectable apoptosis. These findings indicate that reduced total islet and beta-cell volumes of male mutants might emerge from disturbed postnatal islet neogenesis. Considering the lack of knowledge about the pathomorphology of maturity-onset diabetes of the young type 2 (MODY 2), this glucokinase mutant model of reduced total islet and total beta-cell volume provides the opportunity to elucidate the impact of a defective glucokinase on development and maintenance of beta-cell mass and its relevance in MODY 2 patients.

Naturally occurring glucokinase mutations are associated with defects in posttranslational S-nitrosylation

Posttranslational activation of glucokinase (GCK) through S-nitrosylation has been recently observed in the insulin-secreting pancreatic beta-cell; however, the function of this molecular mechanism in regulating the physiology of insulin secretion is not well understood. To more fully understand the function of posttranslational regulation of GCK, we examined two naturally occurring GCK mutations that map to residues proximal to the S-nitrosylated cysteine and cause mild fasting hyperglycemia (maturity-onset diabetes of the young; subtype glucokinase). The kinetics of recombinantly generated GCK-R369P and GCK-V367M were assessed in vitro. The GCK-R369P protein has greatly reduced catalytic activity (relative activity index 0.05 vs. 1.00 for wild type), whereas the GCK-V367M has near normal kinetics (relative activity index 1.26 vs. 1.00 for wild type). Quantitative imaging and biochemical assays were used to assess the effect of these mutants on the metabolic response to glucose, GCK activation, and S-nitrosylation of GCK in betaTC3 insulinoma cells. Expression of either mutant in betaTC3 cells did not affect the metabolic response to 5 mM glucose. However, expression of either mutant blocked the effects of insulin on glucose-stimulated nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate reduction, suggesting defects in posttranslational regulation of GCK. Each of these mutations blocked GCK activation, and prevented posttranslational cysteine S-nitrosylation. Our findings link defects in hormone-regulated GCK S-nitrosylation to hyperglycemia and support a role for posttranslational regulation of GCK S-nitrosylation as a vital regulatory mechanism for glucose-stimulated insulin secretion.

Molecular physiology of mammalian glucokinase

The glucokinase (GCK) gene was one of the first candidate genes to be identified as a human “diabetes gene". Subsequently, important advances were made in understanding the impact of GCK in the regulation of glucose metabolism. Structure elucidation by crystallography provided insight into the kinetic properties of GCK. Protein interaction partners of GCK were discovered. Gene expression studies revealed new facets of the tissue distribution of GCK, including in the brain, and its regulation by insulin in the liver. Metabolic control analysis coupled to gene overexpression and knockout experiments highlighted the unique impact of GCK as a regulator of glucose metabolism. Human GCK mutants were studied biochemically to understand disease mechanisms. Drug development programs identified small molecule activators of GCK as potential antidiabetics. These advances are summarized here, with the aim of offering an integrated view of the role of GCK in the molecular physiology and medicine of glucose homeostasis.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.

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.

Cell biology assessment of glucokinase mutations V62M and G72R in pancreatic beta-cells: evidence for cellular instability of catalytic activity

Mutations in the glucokinase (GK) gene cause defects in blood glucose homeostasis. In some cases (V62M and G72R), the phenotype cannot be explained by altered enzyme kinetics or protein instability. We used transient and stable expression of green fluorescent protein (GFP) GK chimaeras in MIN6 beta-cells to study the phenotype defect of V62M and G72R. GK activity in lysates of MIN6 cell lines stably expressing wild-type or mutant GFP GK showed the expected affinity for glucose and response to pharmacological activators, indicating the expression of catalytically active enzymes. MIN6 cells stably expressing GFP V62M or GFP G72R had a lower GK activity-to-GK immunoreactivity ratio and GK activity-to-GK mRNA ratio but not GK immunoreactivity-to-GK mRNA ratio than wild-type GFP GK. Heterologous expression of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2/FDP2) in cell lines increased GK activity for wild-type GK and V62M but not for G72R, whereas expression of liver GK regulatory protein (GKRP) increased GK activity for wild type but not V62M or G72R. Lack of interaction of these mutants with GKRP was also evident in hepatocyte transfections from the lack of nuclear accumulation. These results suggest that cellular loss of GK catalytic activity rather than impaired translation or enhanced protein degradation may account for the hyperglycemia in subjects with V62M and G72R mutations.

Novel insights into the regulation of the bound and diffusible glucokinase in MIN6 beta-cells

A stable MIN6 beta-cell clone overexpressing glucokinase as an enhanced cyan fluorescent protein (ECFP) fusion construct was generated for analysis of glucokinase regulation in these glucose-responsive insulin-secreting cells. A higher glucokinase enzyme activity accompanied by an improved glucose-induced insulin secretion indicated the integration of ECFP-glucokinase into the functional pool of glucokinase protein in MIN6-ECFP-glucokinase cells. Fluorescence recovery after photobleaching experiments of MIN6-ECFP-glucokinase cells and photoactivation of a transiently transfected photoswitchable cyan fluorescent protein (PS-CFP)-glucokinase construct in MIN6 cells indicate a higher motility of the diffusible glucokinase fraction at high glucose concentrations. In agreement with previous studies, we observed significant binding of ECFP-glucokinase to insulin secretory granules. Using fluorescence lifetime imaging, we obtained evidence for an association between glucokinase and alpha-tubulin in MIN6-ECFP-glucokinase cells. Furthermore, immunohistochemistry and fluorescence resonance energy transfer analysis by acceptor photobleaching showed distinct association between endogenous glucokinase and alpha-tubulin as well as beta-tubulin in MIN6 cells. Interestingly, glucokinase was also colocalized with kinesin, a motor protein involved in insulin secretory granule movement. Therefore, we suggest a role of a bound glucokinase protein fraction in the regulation of insulin granule movement along tubulin filaments.