Upregulation of Anti-Angiogenic miR-106b-3p Correlates Negatively with IGF-1 and Vascular Health Parameters in a Model of Subclinical Cardiovascular Disease: Study with Metformin Therapy

Well-controlled type 1 diabetes mellitus (T1DM) is regarded as a model of subclinical cardiovascular disease (CVD), characterized by inflammation and adverse vascular health. However, the underlying mechanisms are not fully understood. We investigated insulin-like growth factor-1 (IGF-1) and IGF-binding protein-3 (IGFBP-3) levels, their correlation to miR-106b-3p expression in a subclinical CVD model, and the cardioprotective effect of metformin. A total of 20 controls and 29 well-controlled T1DM subjects were studied. Plasma IGF-1, IGFBP-3 levels, and miR-106b-3p expression in colony-forming unit-Hills were analyzed and compared with vascular markers. miR-106b-3p was upregulated in T1DM (p < 0.05) and negatively correlated with pro-angiogenic markers CD34+/100-lymphocytes (p < 0.05) and IGF-1 (p < 0.05). IGF-1 was downregulated in T1DM (p < 0.01), which was associated with increased inflammatory markers TNF-α, CRP, and IL-10 and reduced CD34+/100-lymphocytes. IGFBP-3 had no significant results. Metformin had no effect on IGF-1 but significantly reduced miR-106b-3p (p < 0.0001). An Ingenuity Pathway analysis predicted miR-106b-3p to inhibit PDGFA, PIK3CG, GDNF, and ADAMTS13, which activated CVD. Metformin was predicted to be cardioprotective by inhibiting miR-106b-3p. In conclusion: Subclinical CVD is characterized by a cardio-adverse profile of low IGF-1 and upregulated miR-106b-3p. We demonstrated that the cardioprotective effect of metformin may be via downregulation of upregulated miR-106b-3p and its effect on downstream targets.

Pseudoislet Aggregation of Pancreatic β-Cells Improves Glucose Stimulated Insulin Secretion by Altering Glucose Metabolism and Increasing ATP Production

Appropriate glucose-stimulated insulin secretion (GSIS) by pancreatic β-cells is an essential component of blood glucose homeostasis. Configuration of β-cells as 3D pseudoislets (PI) improves the GSIS response compared to 2D monolayer (ML) culture. The aim of this study was to determine the underlying mechanisms. MIN6 β-cells were grown as ML or PI for 5 days. Human islets were isolated from patients without diabetes. Function was assessed by GSIS and metabolic capacity using the Seahorse bioanalyser. Connexin 36 was downregulated using inducible shRNA. Culturing MIN6 as PI improved GSIS. MIN6 PI showed higher glucose-stimulated oxygen consumption (OCR) and extracellular acidification (ECAR) rates. Further analysis showed the higher ECAR was, at least in part, a consequence of increased glycolysis. Intact human islets also showed glucose-stimulated increases in both OCR and ECAR rates, although the latter was smaller in magnitude compared to MIN6 PI. The higher rates of glucose-stimulated ATP production in MIN6 PI were consistent with increased enzyme activity of key glycolytic and TCA cycle enzymes. There was no impact of connexin 36 knockdown on GSIS or ATP production. Configuration of β-cells as PI improves GSIS by increasing the metabolic capacity of the cells, allowing higher ATP production in response to glucose.

Mitochondrial complex I subunit deficiency promotes pancreatic α-cell proliferation

OBJECTIVE

There is strong evidence that mitochondrial DNA mutations and mitochondrial dysfunction play a role in diabetes pathogenesis. The homozygous knock-in mtDNA mutator mouse is a model of premature aging due to the accumulation of mitochondrial DNA mutations. We used this mouse model to investigate the relationship between mitochondrial subunit expression and pancreatic islet cell composition.

METHODS

Quadruple immunofluorescence was used to quantify mitochondrial subunit expression (complex I and IV) and cell composition in pancreatic islets from mitochondrial DNA mutator mice (PolgAmut/mut) and control C57BL/6 mice at 12 and 44 weeks of age.

RESULTS

Mitochondrial complex I subunit expression was decreased in islets from 12 week PolgAmut/mut mice. This complex I deficiency persisted with age and was associated with decreased insulin staining intensity at 44 weeks. Complex I deficiency was greater in α-cells compared with β-cells in islets from 44 week PolgAmut/mut mice. Islet cell composition was normal in 12 week PolgAmut/mut mice, but the β: α cell ratio was decreased in islets from 44 week PolgAmut/mut mice. This was due to an increase in α-cell number linked to an increase in α-cell proliferation.

CONCLUSION

Complex I deficiency promotes α-cell proliferation and alters islet cell composition.

Exendin-4 stimulates autophagy in pancreatic β-cells via the RAPGEF/EPAC-Ca2+-PPP3/calcineurin-TFEB axis

Macroautophagy/autophagy is critical for the regulation of pancreatic β-cell mass and its deregulation has been implicated in the pathogenesis of type 2 diabetes (T2D). We have previously shown that treatment of pancreatic β-cells with the GLP1R (glucagon like peptide 1 receptor) agonist exendin-4 stimulates autophagic flux in a setting of chronic nutrient excess. The aim of this study was to identify the underlying pathways contributing to enhanced autophagic flux.

Pancreatic β-cells (INS-1E),mouse and human islets were treated with glucolipotoxic stress (0.5 mM palmitate and 25 mM glucose) in the presence of exendin-4. Consistent with our previous work, exendin-4 stimulated autophagic flux. Using chemical inhibitors and siRNA knockdown, we identified RAPGEF4/EPAC2 (Rap guanine nucleotide exchange factor 4) and downstream calcium signaling to be essential for regulation of autophagic flux by exendin-4. This pathway was independent of AMPK and MTOR signaling. Further analysis identified PPP3/calcineurin and its downstream regulator TFEB (transcription factor EB) as key proteins mediating exendin-4 induced autophagy. Importantly, inhibition of this pathway prevented exendin-4-mediated cell survival and overexpression of TFEB mimicked the cell protective effects of exendin-4 in INS-1E and human islets. Moreover, treatment of db/db mice with exendin-4 for 21 days increased the expression of lysosomal markers within the pancreatic islets. Collectively our data identify the RAPGEF4/EPAC2-calcium-PPP3/calcineurin-TFEB axis as a key mediator of autophagic flux, lysosomal function and cell survival in pancreatic β-cells. Pharmacological modulation of this axis may offer a novel therapeutic target for the treatment of T2D.

Abbreviations: AKT1/protein kinase B: AKT serine/threonine kinase 1; AMPK: 5’ AMP-activated protein kinase; CAMKK: calcium/calmodulin-dependent protein kinase kinase; cAMP: cyclic adenosine monophosphate; CASP3: caspase 3; CREB: cAMP response element-binding protein; CTSD: cathepsin D; Ex4: exendin-4(1-39); GLP-1: glucagon like peptide 1; GLP1R: glucagon like peptide 1 receptor; GLT: glucolipotoxicity; INS: insulin; MTOR: mechanistic target of rapamycin kinase; NFAT: nuclear factor of activated T-cells; PPP3/calcineurin: protein phosphatase 3; PRKA/PKA: protein kinase cAMP activated; RAPGEF3/EPAC1: Rap guanine nucleotide exchange factor 3; RAPGEF4/EPAC2: Rap guanine nucleotide exchange factor 4; SQSTM1/p62: sequestosome 1; T2D: type 2 diabetes; TFEB: transcription factor EB

Metformin lowers glucose 6-phosphate in hepatocytes by activation of glycolysis downstream of glucose phosphorylation

The chronic effects of metformin on liver gluconeogenesis involve repression of the G6pc gene, which is regulated by the carbohydrate-response element-binding protein through raised cellular intermediates of glucose metabolism. In this study we determined the candidate mechanisms by which metformin lowers glucose 6-phosphate (G6P) in mouse and rat hepatocytes challenged with high glucose or gluconeogenic precursors. Cell metformin loads in the therapeutic range lowered cell G6P but not ATP and decreased G6pc mRNA at high glucose. The G6P lowering by metformin was mimicked by a complex 1 inhibitor (rotenone) and an uncoupler (dinitrophenol) and by overexpression of mGPDH, which lowers glycerol 3-phosphate and G6P and also mimics the G6pc repression by metformin. In contrast, direct allosteric activators of AMPK (A-769662, 991, and C-13) had opposite effects from metformin on glycolysis, gluconeogenesis, and cell G6P. The G6P lowering by metformin, which also occurs in hepatocytes from AMPK knockout mice, is best explained by allosteric regulation of phosphofructokinase-1 and/or fructose bisphosphatase-1, as supported by increased metabolism of [3-³H]glucose relative to [2-³H]glucose; by an increase in the lactate m2/m1 isotopolog ratio from [1,2-¹³C₂]glucose; by lowering of glycerol 3-phosphate an allosteric inhibitor of phosphofructokinase-1; and by marked G6P elevation by selective inhibition of phosphofructokinase-1; but not by a more reduced cytoplasmic NADH/NAD redox state. We conclude that therapeutically relevant doses of metformin lower G6P in hepatocytes challenged with high glucose by stimulation of glycolysis by an AMP-activated protein kinase-independent mechanism through changes in allosteric effectors of phosphofructokinase-1 and fructose bisphosphatase-1, including AMP, P_i, and glycerol 3-phosphate.

Differential effects of saturated and unsaturated fatty acids on autophagy in pancreatic β-cells

Long-chain saturated fatty acids are lipotoxic to pancreatic β-cells, whereas most unsaturates are better tolerated and some may even be cytoprotective. Fatty acids alter autophagy in β-cells and there is increasing evidence that such alterations can impact directly on the regulation of viability. Accordingly, we have compared the effects of palmitate (C16:0) and palmitoleate (C16:1) on autophagy in cultured β-cells and human islets. Treatment of BRIN-BD11 β-cells with palmitate led to enhanced autophagic activity, as judged by cleavage of microtubule-associated protein 1 light chain 3-I (LC3-I) and this correlated with a marked loss of cell viability in the cells. In addition, transfection of these cells with an mCherry-YFP-LC3 reporter construct revealed the accumulation of autophagosomes in palmitate-treated cells, indicating an impairment of autophagosome-lysosome fusion. This was also seen upon addition of the vacuolar ATPase inhibitor, bafilomycin A1. Exposure of BRIN-BD11 cells to palmitoleate (C16:1) did not lead directly to changes in autophagic activity or flux, but it antagonised the actions of palmitate. In parallel, palmitoleate also improved the viability of palmitate-treated BRIN-BD11 cells. Equivalent responses were observed in INS-1E cells and in isolated human islets. Taken together, these data suggest that palmitate may cause an impairment of autophagosome-lysosome fusion. These effects were not reproduced by palmitoleate which, instead, antagonised the responses mediated by palmitate suggesting that attenuation of β-cell stress may contribute to the improvement in cell viability caused by the mono-unsaturated fatty acid.

A role for Glucagon-Like Peptide-1 in the regulation of β-cell autophagy

Autophagy is a highly conserved intracellular recycling pathway that serves to recycle damaged organelles/proteins or superfluous nutrients during times of nutritional stress to provide energy to maintain intracellular homeostasis and sustain core metabolic functions. Under these conditions, autophagy functions as a cell survival mechanism but impairment of this pathway can lead to pro-death stimuli. Due to their role in synthesising and secreting insulin, pancreatic β-cells have a high requirement for robust degradation pathways. Recent research suggests that functional autophagy is required to maintain β-cell survival and function in response to high fat diet suggesting a pro-survival role. However, a role for autophagy has also been implicated in the pathogenesis of type 2 diabetes. Thus, the pro-survival vs pro-death role of autophagy in regulating β-cell mass requires discussion. Emerging evidence suggests that Glucagon-Like Peptide-1 (GLP-1) may exert beneficial effects on glucose homeostasis via autophagy-dependent pathways both in pancreatic β-cells and in other cell types. The aim of the current review is to: i) summarise the literature surrounding β-cell autophagy and its pro-death vs pro-survival role in regulating β-cell mass; ii) review the literature describing the impact of GLP-1 on β-cell autophagy and in other cell types; iii) discuss the potential underlying mechanisms.

Opposite effects of a glucokinase activator and metformin on glucose-regulated gene expression in hepatocytes

AIM

Small molecule activators of glucokinase (GKAs) have been explored extensively as potential anti-hyperglycaemic drugs for type 2 diabetes (T2D). Several GKAs were remarkably effective in lowering blood glucose during early therapy but then lost their glycaemic efficacy chronically during clinical trials.

MATERIALS AND METHODS

We used rat hepatocytes to test the hypothesis that GKAs raise hepatocyte glucose 6-phosphate (G6P, the glucokinase product) and down-stream metabolites with consequent repression of the liver glucokinase gene ( Gck). We compared a GKA with metformin, the most widely prescribed drug for T2D.

RESULTS

Treatment of hepatocytes with 25 mM glucose raised cell G6P, concomitantly with Gck repression and induction of G6pc (glucose 6-phosphatase) and Pklr (pyruvate kinase). A GKA mimicked high glucose by raising G6P and fructose-2,6-bisphosphate, a regulatory metabolite, causing a left-shift in glucose responsiveness on gene regulation. Fructose, like the GKA, repressed Gck but modestly induced G6pc. 2-Deoxyglucose, which is phosphorylated by glucokinase but not further metabolized caused Gck repression but not G6pc induction, implicating the glucokinase product in Gck repression. Metformin counteracted the effect of high glucose on the elevated G6P and fructose 2,6-bisphosphate and on Gck repression, recruitment of Mlx-ChREBP to the G6pc and Pklr promoters and induction of these genes.

CONCLUSIONS

Elevation in hepatocyte G6P and downstream metabolites, with consequent liver Gck repression, is a potential contributing mechanism to the loss of GKA efficacy during chronic therapy. Cell metformin loads within the therapeutic range attenuate the effect of high glucose on G6P and on glucose-regulated gene expression.

Glucagon-Like Peptide 1 Protects Pancreatic β-Cells From Death by Increasing Autophagic Flux and Restoring Lysosomal Function

Studies in animal models of type 2 diabetes have shown that glucagon-like peptide 1 (GLP-1) receptor agonists prevent β-cell loss. Whether GLP-1 mediates β-cell survival via the key lysosomal-mediated process of autophagy is unknown. In this study, we report that treatment of INS-1E β-cells and primary islets with glucolipotoxicity (0.5 mmol/L palmitate and 25 mmol/L glucose) increases LC3 II, a marker of autophagy. Further analysis indicates a blockage in autophagic flux associated with lysosomal dysfunction. Accumulation of defective lysosomes leads to lysosomal membrane permeabilization and release of cathepsin D, which contributes to cell death. Our data further demonstrated defects in autophagic flux and lysosomal staining in human samples of type 2 diabetes. Cotreatment with the GLP-1 receptor agonist exendin-4 reversed the lysosomal dysfunction, relieving the impairment in autophagic flux and further stimulated autophagy. Small interfering RNA knockdown showed the restoration of autophagic flux is also essential for the protective effects of exendin-4. Collectively, our data highlight lysosomal dysfunction as a critical mediator of β-cell loss and shows that exendin-4 improves cell survival via restoration of lysosomal function and autophagic flux. Modulation of autophagy/lysosomal homeostasis may thus define a novel therapeutic strategy for type 2 diabetes, with the GLP-1 signaling pathway as a potential focus.

Glucagon induces translocation of glucokinase from the cytoplasm to the nucleus of hepatocytes by transfer between 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase-2 and the glucokinase regulatory protein

Glucokinase activity is a major determinant of hepatic glucose metabolism and blood glucose homeostasis. Liver glucokinase activity is regulated acutely by adaptive translocation between the nucleus and the cytoplasm through binding and dissociation from its regulatory protein (GKRP) in the nucleus. Whilst the effect of glucose on this mechanism is well established, the role of hormones in regulating glucokinase location and its interaction with binding proteins remains unsettled. Here we show that treatment of rat hepatocytes with 25mM glucose caused decreased binding of glucokinase to GKRP, translocation from the nucleus and increased binding to 6-phosphofructo 2-kinase/fructose 2,6 bisphosphatase-2 (PFK2/FBPase2) in the cytoplasm. Glucagon caused dissociation of glucokinase from PFK2/FBPase2, concomitant with phosphorylation of PFK2/FBPase2 on Ser-32, uptake of glucokinase into the nucleus and increased interaction with GKRP. Two novel glucagon receptor antagonists attenuated the action of glucagon. This establishes an unequivocal role for hormonal control of glucokinase translocation. Given that glucagon excess contributes to the pathogenesis of diabetes, glucagon may play a role in the defect in glucokinase translocation and activity evident in animal models and human diabetes.

A novel mechanism for regulating hepatic glycogen synthesis involving serotonin and cyclin-dependent kinase-5

Hepatic autonomic nerves regulate postprandial hepatic glucose uptake, but the signaling pathways remain unknown. We tested the hypothesis that serotonin (5-hydroxytryptamine [5-HT]) exerts stimulatory and inhibitory effects on hepatic glucose disposal. Ligands of diverse 5-HT receptors were used to identify signaling pathway(s) regulating glucose metabolism in hepatocytes. 5-HT had stimulatory and inhibitory effects on glycogen synthesis in hepatocytes mediated by 5-HT1/2A and 5-HT2B receptors, respectively. Agonists of 5-HT1/2A receptors lowered blood glucose and increased hepatic glycogen after oral glucose loading and also stimulated glycogen synthesis in freshly isolated hepatocytes with greater efficacy than 5-HT. This effect was blocked by olanzapine, an antagonist of 5-HT1/2A receptors. It was mediated by activation of phosphorylase phosphatase, inactivation of glycogen phosphorylase, and activation of glycogen synthase. Unlike insulin action, it was not associated with stimulation of glycolysis and was counteracted by cyclin-dependent kinase (cdk) inhibitors. A role for cdk5 was supported by adaptive changes in the coactivator protein p35 and by elevated glycogen synthesis during overexpression of p35/cdk5. These results support a novel mechanism for serotonin stimulation of hepatic glycogenesis involving cdk5. The opposing effects of serotonin, mediated by distinct 5-HT receptors, could explain why drugs targeting serotonin function can cause either diabetes or hypoglycemia in humans.

Elevated glucose represses liver glucokinase and induces its regulatory protein to safeguard hepatic phosphate homeostasis

OBJECTIVE

The induction of hepatic glucose 6-phosphatase (G6pc) by glucose presents a paradox of glucose-induced glucose intolerance. We tested whether glucose regulation of liver gene expression is geared toward intracellular homeostasis.

RESEARCH DESIGN AND METHODS

The effect of glucose-induced accumulation of phosphorylated intermediates on expression of glucokinase (Gck) and its regulator Gckr was determined in hepatocytes. Cell ATP and uric acid production were measured as indices of cell phosphate homeostasis.

RESULTS

Accumulation of phosphorylated intermediates in hepatocytes incubated at elevated glucose induced rapid and inverse changes in Gck (repression) and Gckr (induction) mRNA concomitantly with induction of G6pc, but had slower effects on the Gckr-to-Gck protein ratio. Dynamic metabolic labeling in mice and liver proteome analysis confirmed that Gckr and Gck are low-turnover proteins. Involvement of Max-like protein X in glucose-mediated Gck-repression was confirmed by chromatin immunoprecipitation analysis. Elevation of the Gck-to-Gckr ratio in hepatocytes was associated with glucose-dependent ATP depletion and elevated urate production confirming compromised phosphate homeostasis.

CONCLUSIONS

The lowering by glucose of the Gck-to-Gckr ratio provides a potential explanation for the impaired hepatic glucose uptake in diabetes. Elevated uric acid production at an elevated Gck-to-Gckr ratio supports a role for glucose regulation of gene expression in hepatic phosphate homeostasis.

Susceptibility of glucokinase-MODY mutants to inactivation by oxidative stress in pancreatic β-cells

OBJECTIVE

The posttranslational regulation of glucokinase (GK) differs in hepatocytes and pancreatic β-cells. We tested the hypothesis that GK mutants that cause maturity-onset diabetes of the young (GK-MODY) show compromised activity and posttranslational regulation in β-cells.

RESEARCH DESIGN AND METHODS

Activity and protein expression of GK-MODY and persistent hyperinsulinemic hypoglycemia of infancy (PHHI) mutants were studied in β-cell (MIN6) and non-β-cell (H4IIE) models. Binding of GK to phosphofructo-2-kinase, fructose-2,6-bisphosphatase (PFK2/FBPase2) was studied by bimolecular fluorescence complementation in cell-based models.

RESULTS

Nine of 11 GK-MODY mutants that have minimal effect on enzyme kinetics in vitro showed decreased specific activity relative to wild type when expressed in β-cells. A subset of these were stable in non-β-cells but showed increased inactivation in conditions of oxidative stress and partial reversal of inactivation by dithiothreitol. Unlike the GK-MODY mutants, four of five GK-PHHI mutants had similar specific activity to wild type and Y214C had higher activity than wild type. The GK-binding protein PFK2/FBPase2 protected wild-type GK from oxidative inactivation and the decreased stability of GK-MODY mutants correlated with decreased interaction with PFK2/FBPase2.

CONCLUSIONS

Several GK-MODY mutants show posttranslational defects in β-cells characterized by increased susceptibility to oxidative stress and/or protein instability. Regulation of GK activity through modulation of thiol status may be a physiological regulatory mechanism for the control of GK activity in β-cells.

Pseudoislets as primary islet replacements for research: report on a symposium at King's College London, London UK

Laboratory-based research aimed at understanding processes regulating insulin secretion and mechanisms underlying β-cell dysfunction and loss in diabetes often makes use of rodents, as these processes are in many respects similar between rats/mice and humans. Indeed, a rough calculation suggests that islets have been isolated from as many as 150,000 rodents to generate the data contained within papers published in 2009 and the first four months of 2010. Rodent use for islet isolation has been mitigated, to a certain extent, by the availability of a variety of insulin-secreting cell lines that are used by researchers world-wide. However, when maintained as monolayers the cell lines do not replicate the robust, sustained secretory responses of primary islets which limits their usefulness as islet surrogates. On the other hand, there have been several reports that configuration of MIN6 β-cells, derived from a mouse insulinoma, as three-dimensional cell clusters termed ‘pseudoislets’ largely recapitulates the function of primary islet β-cells. The Diabetes Research Group at King’s College London has been using the MIN6 pseudoislet model for over a decade and they hosted a symposium on “Pseudoislets as primary islet replacements for research”, which was funded by the UK National Centre for the Replacement, Refinement and Reduction of Animals in Research (NC3Rs), in London on 15th and 16th April 2010. This small, focused meeting was conceived as an opportunity to consolidate information on experiences of working with pseudoislets between different UK labs, and to introduce the theory and practice of pseudoislet culture to laboratories working with islets and/or β-cell lines but who do not currently use pseudoislets. This short review summarizes the background to the development of the cell line-derived pseudoislet model, the key messages arising from the symposium and emerging themes for future pseudoislet research.

Inhibition of glucokinase translocation by AMP-activated protein kinase is associated with phosphorylation of both GKRP and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

The rate of glucose phosphorylation in hepatocytes is determined by the subcellular location of glucokinase and by its association with its regulatory protein (GKRP) in the nucleus. Elevated glucose concentrations and precursors of fructose 1-phosphate (e.g., sorbitol) cause dissociation of glucokinase from GKRP and translocation to the cytoplasm. In this study, we investigated the counter-regulation of substrate-induced translocation by AICAR (5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside), which is metabolized by hepatocytes to an AMP analog, and causes activation of AMP-activated protein kinase (AMPK) and depletion of ATP. During incubation of hepatocytes with 25 mM glucose, AICAR concentrations below 200 microM activated AMPK without depleting ATP and inhibited glucose phosphorylation and glucokinase translocation with half-maximal effect at 100-140 microM. Glucose phosphorylation and glucokinase translocation correlated inversely with AMPK activity. AICAR also counteracted translocation induced by a glucokinase activator and partially counteracted translocation by sorbitol. However, AICAR did not block the reversal of translocation (from cytoplasm to nucleus) after substrate withdrawal. Inhibition of glucose-induced translocation by AICAR was greater than inhibition by glucagon and was associated with phosphorylation of both GKRP and the cytoplasmic glucokinase binding protein, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2) on ser-32. Expression of a kinase-active PFK2 variant lacking ser-32 partially reversed the inhibition of translocation by AICAR. Phosphorylation of GKRP by AMPK partially counteracted its inhibitory effect on glucokinase activity, suggesting altered interaction of glucokinase and GKRP. In summary, mechanisms downstream of AMPK activation, involving phosphorylation of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase and GKRP are involved in the ATP-independent inhibition of glucose-induced glucokinase translocation by AICAR in hepatocytes.

Contributions of glucokinase and phosphofructokinase-2/fructose bisphosphatase-2 to the elevated glycolysis in hepatocytes from Zucker fa/fa rats

The insulin-resistant Zucker fa/fa rat has elevated hepatic glycolysis and activities of glucokinase and phosphofructokinase-2/fructose bisphosphatase-2 (PFK2). The latter catalyzes the formation and degradation of fructose-2,6-bisphosphate (fructose-2,6-P2) and is a glucokinase-binding protein. The contributions of glucokinase and PFK2 to the elevated glycolysis in fa/fa hepatocytes were determined by overexpressing these enzymes individually or in combination. Metabolic control analysis was used to determine enzyme coefficients on glycolysis and metabolite concentrations. Glucokinase had a high control coefficient on glycolysis in all hormonal conditions tested, whereas PFK2 had significant control only in the presence of glucagon, which phosphorylates PFK2 and suppresses glycolysis. Despite the high control strength of glucokinase, the elevated glycolysis in fa/fa hepatocytes could not be explained by the elevated glucokinase activity alone. In hepatocytes from fa/fa rats, glucokinase translocation between the nucleus and the cytoplasm was refractory to glucose but responsive to glucagon. Expression of a kinase-active PFK2 variant reversed the glucagon effect on glucokinase translocation and glucose phosphorylation, confirming the role for PFK2 in sequestering glucokinase in the cytoplasm. Glucokinase had a high control on glucose-6-phosphate content; however, like PFK2, it had a relative modest effect on the fructose-2,6-P2 content. However, combined overexpression of glucokinase and PFK2 had a synergistic effect on fructose-2,6-P2 levels, suggesting that interaction of these enzymes may be a prerequisite for formation of fructose-2,6-P2. Cumulatively, this study provides support for coordinate roles for glucokinase and PFK2 in the elevated hepatic glycolysis in fa/fa rats.

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.

Bioactivity of glycogen phosphorylase inhibitors that bind to the purine nucleoside site

The bioactivity in hepatocytes of glycogen phosphorylase inhibitors that bind to the active site, the allosteric activator site and the indole carboxamide site has been described. However, the pharmacological potential of the purine nucleoside inhibitor site has remained unexplored. We report the chemical synthesis and bioactivity in hepatocytes of four new olefin derivatives of flavopiridol (1-4) that bind to the purine site. Flavopiridol and 1-4 counteracted the activation of phosphorylase in hepatocytes caused by AICAR (5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside), which is metabolised to an AMP analogue. Unlike an indole carboxamide inhibitor, the analogues 1 and 4 suppressed the basal rate of glycogenolysis in hepatocytes by allosteric inhibition rather than by inactivation of phosphorylase, and accordingly caused negligible stimulation of glycogen synthesis. However, they counteracted the stimulation of glycogenolysis by dibutyryl cAMP by both allosteric inhibition and inactivation of phosphorylase. Cumulatively, the results show key differences between purine site and indole carboxamide site inhibitors in terms of (i) relative roles of dephosphorylation of phosphorylase-a as compared with allosteric inhibition, (ii) counteraction of the efficacy of the inhibitors on glycogenolysis by dibutyryl-cAMP and (iii) stimulation of glycogen synthesis.

Increased sensitivity of glycogen synthesis to phosphorylase-a and impaired expression of the glycogen-targeting protein R6 in hepatocytes from insulin-resistant Zucker fa/fa rats

Hepatic insulin resistance in the leptin-receptor defective Zucker fa/fa rat is associated with impaired glycogen synthesis and increased activity of phosphorylase-a. We investigated the coupling between phosphorylase-a and glycogen synthesis in hepatocytes from fa/fa rats by modulating the concentration of phosphorylase-a. Treatment of hepatocytes from fa/fa rats and Fa/? controls with a selective phosphorylase inhibitor caused depletion of phosphorylase-a, activation of glycogen synthase and stimulation of glycogen synthesis. The flux-control coefficient of phosphorylase on glycogen synthesis was glucose dependent and at 10 mm glucose was higher in fa/fa than Fa/? hepatocytes. There was an inverse correlation between the activities of glycogen synthase and phosphorylase-a in both fa/fa and Fa/? hepatocytes. However, fa/fa hepatocytes had a higher activity of phosphorylase-a, for a corresponding activity of glycogen synthase. This defect was, in part, normalized by expression of the glycogen-targeting protein, PTG. Hepatocytes from fa/fa rats had normal expression of the glycogen-targeting proteins G(L) and PTG but markedly reduced expression of R6. Expression of R6 protein was increased in hepatocytes from Wistar rats after incubation with leptin and insulin. Diminished hepatic R6 expression in the leptin-receptor defective fa/fa rat may be a contributing factor to the elevated phosphorylase activity and/or its high control strength on glycogen synthesis.

Glucokinase regulatory protein is associated with mitochondria in hepatocytes

The association of glucokinase with liver mitochondria has been reported [Danial et al. (2003) BAD and glucokinase reside in a mitochondrial complex that integrates glycolysis and apoptosis. Nature 424, 952-956]. We confirmed association of glucokinase immunoreactivity with rat liver mitochondria using Percoll gradient centrifugation and demonstrated its association with the 68 kDa regulatory protein (GKRP) but not with the binding protein phosphofructokinase-2/fructose bisphosphatase-2. Substrates and glucagon induced adaptive changes in the mitochondrial glucokinase/GKRP ratio suggesting a regulatory role for GKRP. Combined with previous observations that GKRP overexpression partially inhibits glycolysis [de la Iglesia et al. (2000) The role of the regulatory protein of glucokinase in the glucose sensory mechanism of the hepatocyte. J. Biol. Chem. 275, 10597-10603] these findings suggest that there may be distinct glycolytic pools of glucokinase.

The role of protein kinase B/Akt in insulin-induced inactivation of phosphorylase in rat hepatocytes

AIMS/HYPOTHESIS

An insulin signalling pathway leading from activation of protein kinase B (PKB, also known as Akt) to phosphorylation (inactivation) of glycogen synthase kinase-3 (GSK-3) and activation of glycogen synthase is well characterised. However, in hepatocytes, inactivation of GSK-3 is not the main mechanism by which insulin stimulates glycogen synthesis. We therefore tested whether activation of PKB causes inactivation of glycogen phosphorylase.

MATERIALS AND METHODS

We used a conditionally active form of PKB, produced using recombinant adenovirus, to test the role of acute PKB activation in the control of glycogen phosphorylase and glycogen synthesis in hepatocytes.

RESULTS

Conditional activation of PKB mimicked the inactivation of phosphorylase, the activation of glycogen synthase, and the stimulation of glycogen synthesis caused by insulin. In contrast, inhibition of GSK-3 caused activation of glycogen synthase but did not mimic the stimulation of glycogen synthesis by insulin. PKB activation and GSK-3 inhibition had additive effects on the activation of glycogen synthase, indicating convergent mechanisms downstream of PKB involving inactivation of either phosphorylase or GSK-3. Glycogen synthesis correlated inversely with the activity of phosphorylase-a, irrespective of whether this was modulated by insulin, by PKB activation or by a selective phosphorylase ligand, supporting an essential role for phosphorylase inactivation in the glycogenic action of insulin in hepatocytes.

CONCLUSIONS/INTERPRETATION

In hepatocytes, the acute activation of PKB, but not the inhibition of GSK-3, mimics the stimulation of glycogen synthesis by insulin. This is explained by a pathway downstream of PKB leading to inactivation of phosphorylase, activation of glycogen synthase, and stimulation of glycogen synthesis, independent of the GSK-3 pathway.

Dual role of phosphofructokinase-2/fructose bisphosphatase-2 in regulating the compartmentation and expression of glucokinase in hepatocytes

Hepatic glucokinase is regulated by a 68-kDa regulatory protein (GKRP) that is both an inhibitor and nuclear receptor for glucokinase. We tested the role of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2) in regulating glucokinase compartmentation in hepatocytes. PFK2 catalyzes formation or degradation of the regulator of glycolysis fructose 2,6-bisphosphate (fructose 2,6-P2), depending on its phosphorylation state (ser-32), and is also a glucokinase-binding protein. Incubation of hepatocytes at 25 mmol/l glucose causes translocation of glucokinase from the nucleus to the cytoplasm and an increase in fructose 2,6-P2. Glucagon caused phosphorylation of PFK2-ser-32, lowered the fructose 2,6-P2 concentration, and inhibited glucose-induced translocation of glucokinase. These effects of glucagon were reversed by expression of a kinase-active PFK2 mutant (S32A/H258A) that overrides the suppression of fructose 2,6-P2 but not by overexpression of wild-type PFK2. Overexpression of PFK2 potentiated glucokinase expression in hepatocytes transduced with an adenoviral vector-encoding glucokinase by a mechanism that does not involve stabilization of glucokinase protein from degradation. It is concluded that PFK2 has a dual role in regulating glucokinase in hepatocytes: it potentiates glucokinase protein expression by posttranscriptional mechanisms and favors its cytoplasmic compartmentation. Thus, it acts in a complementary mechanism to GKRP, which also regulates glucokinase protein expression and compartmentation.

Glucokinase is an integral component of the insulin granules in glucose-responsive insulin secretory cells and does not translocate during glucose stimulation

The association of glucokinase with insulin secretory granules has been shown by cell microscopy techniques. We used MIN6 insulin-secretory cells and organelle fractionation to determine the effects of glucose on the subcellular distribution of glucokinase. After permeabilization with digitonin, 50% of total glucokinase remained bound intracellularly, while 30% was associated with the 13,000g particulate fraction. After density gradient fractionation of the organelles, immunoreactive glucokinase was distributed approximately equally between dense insulin granules and low-density organelles that cofractionate with mitochondria. Although MIN6 cells show glucose-responsive insulin secretion, glucokinase association with the granules and low-density organelles was not affected by glucose. Subfractionation of the insulin granule components by hypotonic lysis followed by sucrose gradient centrifugation showed that glucokinase colocalized with the granule membrane marker phogrin and not with insulin. PFK2 (6-phosphofructo-2-kinase-2/fructose-2,6-bisphosphatase)/FDPase-2, a glucokinase-binding protein, and glyceraldehyde phosphate dehydrogenase, which has been implicated in granule fusion, also colocalized with glucokinase after hypotonic lysis or detergent extaction of the granules. The results suggest that glucokinase is an integral component of the granule and does not translocate during glucose stimulation.