Differential regulation of glucokinase activity in pancreatic islets and liver of the rat

Monitoring and modelling the dynamics of the cellular glycolysis pathway: A review and future perspectives

BACKGROUND

The dynamics of the cellular glycolysis pathway underpin cellular function and dysfunction, and therefore ultimately health, disease, diagnostic and therapeutic strategies. Evolving our understanding of this fundamental process and its dynamics remains critical.

SCOPE OF REVIEW

This paper reviews the medical relevance of glycolytic pathway in depth and explores the current state of the art for monitoring and modelling the dynamics of the process. The future perspectives of label free, vibrational microspectroscopic techniques to overcome the limitations of the current approaches are considered.

MAJOR CONCLUSIONS

Vibrational microspectroscopic techniques can potentially operate in the niche area of limitations of other omics technologies for non-destructive, real-time, in vivo label-free monitoring of glycolysis dynamics at a cellular and subcellular level.

Molecular biomarkers for gestational diabetes mellitus and postpartum diabetes

ABSTRACT

Gestational diabetes mellitus (GDM) is a growing public health problem worldwide that threatens both maternal and fetal health. Identifying individuals at high risk for GDM and diabetes after GDM is particularly useful for early intervention and prevention of disease progression. In the last decades, a number of studies have used metabolomics, genomics, and proteomic approaches to investigate associations between biomolecules and GDM progression. These studies clearly demonstrate that various biomarkers reflect pathological changes in GDM. The established markers have potential use as screening and diagnostic tools in GDM and in postpartum diabetes research. In the present review, we summarize recent studies of metabolites, single-nucleotide polymorphisms, microRNAs, and proteins associated with GDM and its transition to postpartum diabetes, with a focus on their predictive value in screening and diagnosis.

Prolonged insulin-induced hypoglycaemia reduces ß-cell activity rather than number in pancreatic islets in non-diabetic rats

Pancreatic β-cells have an extraordinary ability to adapt to acute fluctuations in glucose levels by rapid changing insulin production to meet metabolic needs. Although acute changes have been characterised, effects of prolonged metabolic stress on β-cell dynamics are still unclear. Here, the aim was to investigate pancreatic β-cell dynamics and function during and after prolonged hypoglycaemia. Hypoglycaemia was induced in male and female rats by infusion of human insulin for 8 weeks, followed by a 4-week infusion-free recovery period. Animals were euthanized after 4 or 8 weeks of infusion, and either 2 days and 4 weeks after infusion-stop. Total volumes of pancreatic islets and β-cell nuclei, islet insulin and glucagon content, and plasma c-peptide levels were quantified. Prolonged hypoglycaemia reduced c-peptide levels, islet volume and almost depleted islet insulin. Relative β-cell nuclei: total pancreas volume decreased, while being unchanged relative to islet volume. Glucagon: total pancreas volume decreased during hypoglycaemia, whereas glucagon: islet volume increased. Within two days after infusion-stop, plasma glucose and c-peptide levels normalised and all remaining parameters were fully reversed after 4 weeks. In conclusion, our findings indicate that prolonged hypoglycaemia inactivates β-cells, which can rapidly be reactivated when needed, demonstrating the high plasticity of β-cells even following prolonged suppression.

Telfairia occidentalis stimulates hepatic glycolysis and pyruvate production via insulin-dependent and insulin-independent mechanisms

Background

Telfairia occidentalis (TO), a plant consumed for its nutritional and medicinal values, exhibits hypoglycaemic effect. However, the metabolic fate of the glucose following TO-induced insulin secretion and consequent hypoglycaemia is not clear.

Objective

This study determined the effect of ethyl acetate and n-hexane fractions of TO leaf extracts on some biochemical parameters in the glucose metabolic pathway to explain the possible fate of blood glucose following TO-induced hypoglycaemia.

Methods

Eighteen male Wistar rats (180-200 g) divided into control, n-hexane TO fraction- and ethyl acetate TO fraction-treated groups (n = 6/group) were used. The control animals received normal saline while the treated groups received TO at 100 mg/kg for seven days. After 24 h following the last dose, the animals were anaesthetised using ketamine; blood samples were collected and livers harvested to determine some biochemical parameters.

Results

Ethyl acetate TO fraction significantly increased plasma insulin, liver glucokinase activity and plasma pyruvate concentration, but significantly decreased plasma glucose and liver glycogen, without significant changes in plasma lactate, glucose-6-phosphate, liver glucose-6-phosphatase and lactate dehydrogenase activities when compared with control. N-hexane TO fraction significantly reduced liver glucose-6-phosphatase activity and glycogen but significantly increased plasma pyruvate, without significant changes in plasma glucose, insulin, glucose-6-phosphate and lactate concentrations; and liver glucokinase and lactate dehydrogenase activities.

Conclusion

The present study showed that insulin-mediated TO-induced hypoglycaemia resulted in the stimulation of glycolysis and pyruvate production via insulin-dependent and insulin-independent mechanisms.

Glycometabolic rearrangements--aerobic glycolysis in pancreatic cancer: causes, characteristics and clinical applications

Pancreatic cancer is one of the most malignant tumors worldwide, and pancreatic ductal adenocarcinoma is the most common type. In pancreatic cancer, glycolysis is the primary way energy is produced to maintain the proliferation, invasion, migration, and metastasis of cancer cells, even under normoxia. However, the potential molecular mechanism is still unknown. From this perspective, this review mainly aimed to summarize the current reasonable interpretation of aerobic glycolysis in pancreatic cancer and some of the newest methods for the detection and treatment of pancreatic cancer. More specifically, we reported some biochemical parameters, such as newly developed enzymes and transporters, and further explored their potential as diagnostic biomarkers and therapeutic targets.

Discovery and preclinical development of AR453588 as an anti-diabetic glucokinase activator

Glucose flux through glucokinase (GK) controls insulin release from the pancreas in response to high levels of glucose. Flux through GK is also responsible for reducing hepatic glucose output. Since many individuals with type 2 diabetes appear to have an inadequacy or defect in one or both of these processes, identifying compounds that can activate GK could provide a therapeutic benefit. Herein we report the further structure activity studies of a novel series of glucokinase activators (GKA). These studies led to the identification of pyridine 72 as a potent GKA that lowered post-prandial glucose in normal C57BL/6J mice, and after 14d dosing in ob/ob mice.

Nuclear import of glucokinase in pancreatic beta-cells is mediated by a nuclear localization signal and modulated by SUMOylation

The localization of glucokinase in pancreatic beta-cell nuclei is a controversial issue. Although previous reports suggest such a localization, the mechanism for its import has so far not been identified. Using immunofluorescence, subcellular fractionation and mass spectrometry, we present evidence in support of glucokinase localization in beta-cell nuclei of human and mouse pancreatic sections, as well as in human and mouse isolated islets, and murine MIN6 cells. We have identified a conserved, seven-residue nuclear localization signal (³⁰LKKVMRR³⁶) in the human enzyme. Substituting the residues KK^31,32 and RR^35,36 with AA led to a loss of its nuclear localization in transfected cells. Furthermore, our data indicates that SUMOylation of glucokinase modulates its nuclear import, while high glucose concentrations do not significantly alter the enzyme nuclear/cytosolic ratio. Thus, for the first time, we provide data in support of a nuclear import of glucokinase mediated by a redundant mechanism, involving a nuclear localization signal, and which is modulated by its SUMOylation. These findings add new knowledge to the functional role of glucokinase in the pancreatic beta-cell.

Insights into the Hexose Liver Metabolism-Glucose versus Fructose

High-fructose intake in healthy men is associated with characteristics of metabolic syndrome. Extensive knowledge exists about the differences between hepatic fructose and glucose metabolism and fructose-specific mechanisms favoring the development of metabolic disturbances. Nevertheless, the causal relationship between fructose consumption and metabolic alterations is still debated. Multiple effects of fructose on hepatic metabolism are attributed to the fact that the liver represents the major sink of fructose. Fructose, as a lipogenic substrate and potent inducer of lipogenic enzyme expression, enhances fatty acid synthesis. Consequently, increased hepatic diacylglycerols (DAG) are thought to directly interfere with insulin signaling. However, independently of this effect, fructose may also counteract insulin-mediated effects on liver metabolism by a range of mechanisms. It may drive gluconeogenesis not only as a gluconeogenic substrate, but also as a potent inducer of carbohydrate responsive element binding protein (ChREBP), which induces the expression of lipogenic enzymes as well as gluconeogenic enzymes. It remains a challenge to determine the relative contributions of the impact of fructose on hepatic transcriptome, proteome and allosterome changes and consequently on the regulation of plasma glucose metabolism/homeostasis. Mathematical models exist modeling hepatic glucose metabolism. Future models should not only consider the hepatic adjustments of enzyme abundances and activities in response to changing plasma glucose and insulin/glucagon concentrations, but also to varying fructose concentrations for defining the role of fructose in the hepatic control of plasma glucose homeostasis.

MiR-206 is expressed in pancreatic islets and regulates glucokinase activity

Glucose homeostasis is a complex indispensable process, and its dysregulation causes hyperglycemia and type 2 diabetes mellitus. Glucokinase (GK) takes a central role in these pathways and is thus rate limiting for glucose-stimulated insulin secretion (GSIS) from pancreatic islets. Several reports have described the transcriptional regulation of Gck mRNA, whereas its posttranscriptional mechanisms of regulation, especially those involving microRNAs (miR), are poorly understood. In this study, we investigated the role of miR-206 as a posttranscriptional regulator of Gck In addition, we examined the effects of miR-206 on glucose tolerance, GSIS, and gene expression in control and germ line miR-206 knockout (KO) mice fed either with chow or high-fat diet (HFD). MiR-206 was found in Gck-expressing tissues and was differentially altered in response to HFD feeding. Pancreatic islets showed the most profound induction in the expression of miR-206 in response to HFD. Chow- and HFD-fed miR-206KO mice have improved glucose tolerance and GSIS but unaltered insulin sensitivity. In silico analysis of Gck mRNA revealed a conserved 8-mer miR-206 binding site. Hence, the predicted regulation of Gck by miR-206 was confirmed in reporter and GK activity assays. Concomitant with increased GK activity, miR-206KO mice had elevated liver glycogen content and plasma lactate concentrations. Our findings revealed a novel mechanism of posttranscriptional regulation of Gck by miR-206 and underline the crucial role of pancreatic islet miR-206 in the regulation of whole body glucose homeostasis in a murine model that mimics the metabolic syndrome.

Factors affecting insulin-regulated hepatic gene expression

Obesity has become a major concern of public health. A common feature of obesity and related metabolic disorders such as noninsulin-dependent diabetes mellitus is insulin resistance, wherein a given amount of insulin produces less than normal physiological responses. Insulin controls hepatic glucose and fatty acid metabolism, at least in part, via the regulation of gene expression. When the liver is insulin-sensitive, insulin can stimulate the expression of genes for fatty acid synthesis and suppress those for gluconeogenesis. When the liver becomes insulin-resistant, the insulin-mediated suppression of gluconeogenic gene expression is lost, whereas the induction of fatty acid synthetic gene expression remains intact. In the past two decades, the mechanisms of insulin-regulated hepatic gene expression have been studied extensively and many components of insulin signal transduction pathways have been identified. Factors that alter these pathways, and the insulin-regulated hepatic gene expression, have been revealed and the underlying mechanisms have been proposed. This chapter summarizes the recent progresses in our understanding of the effects of dietary factors, drugs, bioactive compounds, hormones, and cytokines on insulin-regulated hepatic gene expression. Given the large amount of information and progresses regarding the roles of insulin, this chapter focuses on findings in the liver and hepatocytes and not those described for other tissues and cells. Typical insulin-regulated hepatic genes, such as insulin-induced glucokinase and sterol regulatory element-binding protein-1c and insulin-suppressed cytosolic phosphoenolpyruvate carboxyl kinase and insulin-like growth factor-binding protein 1, are used as examples to discuss the mechanisms such as insulin regulatory element-mediated transcriptional regulation. We also propose the potential mechanisms by which these factors affect insulin-regulated hepatic gene expression and discuss potential future directions of the area of research.

Insulin-receptor substrate-2 (irs-2) is required for maintaining glucokinase and glucokinase regulatory protein expression in mouse liver

Insulin receptor substrate (IRS) proteins play important roles in hepatic nutrient homeostasis. Since glucokinase (GK) and glucokinase regulatory protein (GKRP) function as key glucose sensors, we have investigated the expression of GK and GKRP in liver of Irs-2 deficient mice and Irs2(−/−) mice where Irs2 was reintroduced specifically into pancreatic β-cells [RIP-Irs-2/IRS-2(−/−)]. We observed that liver GK activity was significantly lower (p<0.0001) in IRS-2(−/−) mice. However, in RIP-Irs-2/IRS-2(−/−) mice, GK activity was similar to the values observed in wild-type animals. GK activity in hypothalamus was not altered in IRS-2(−/−) mice. GK and GKRP mRNA levels in liver of IRS-2(−/−) were significantly lower, whereas in RIP-Irs-2/IRS-2(−/−) mice, both GK and GKRP mRNAs levels were comparable to wild-type animals. At the protein level, the liver content of GK was reduced in IRS-2(−/−) mice as compared with controls, although GKRP levels were similar between these experimental models. Both GK and GKRP levels were lower in RIP-Irs-2/IRS-2(−/−) mice. These results suggest that IRS-2 signalling is important for maintaining the activity of liver GK. Moreover, the differences between liver and brain GK may be explained by the fact that expression of hepatic, but not brain, GK is controlled by insulin. GK activity was restored by the β-cell compensation in the RIP-Irs-2/IRS-2 mice. Interestingly, GK and GKRP protein expression remained low in RIP-Irs-2/IRS-2(−/−) mice, perhaps reflecting different mRNA half-lives or alterations in the process of translation and post-translational regulation.

Mutational analysis of allosteric activation and inhibition of glucokinase

GK (glucokinase) is activated by glucose binding to its substrate site, is inhibited by GKRP (GK regulatory protein) and stimulated by GKAs (GK activator drugs). To explore further the mechanisms of these processes we studied pure recombinant human GK (normal enzyme and a selection of 31 mutants) using steady-state kinetics of the enzyme and TF (tryptophan fluorescence). TF studies of the normal binary GK-glucose complex corroborate recent crystallography studies showing that it exists in a closed conformation greatly different from the open conformation of the ligand-free structure, but indistinguishable from the ternary GK-glucose-GKA complex. GKAs did activate and GKRP did inhibit normal GK, whereas its TF was doubled by glucose saturation. However, the enzyme kinetics, GKRP inhibition, TF enhancement by glucose and responsiveness to GKA of the selected mutants varied greatly. Two predominant response patterns were identified accounting for nearly all mutants: (i) GK mutants with a normal or close to normal response to GKA, normally low basal TF (indicating an open conformation), some variability of kinetic parameters (k(cat), glucose S(0.5), h and ATP K(m)), but usually strong GKRP inhibition (13/31); and (ii) GK mutants that are refractory to GKAs, exhibit relatively high basal TF (indicating structural compaction and partial closure), usually show strongly enhanced catalytic activity primarily due to lowering of the glucose S(0.5), but with reduced or no GKRP inhibition in most cases (14/31). These results and those of previous studies are best explained by envisioning a common allosteric regulator region with spatially non-overlapping GKRP- and GKA-binding sites.

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.

Generation of N-ethyl-N-nitrosourea (ENU) diabetes models in mice demonstrates genotype-specific action of glucokinase activators

We performed genome-wide mutagenesis in C57BL/6J mice using N-ethyl-N-nitrosourea to identify mutations causing high blood glucose early in life and to produce new animal models of diabetes. Of a total of 13 new lines confirmed by heritability testing, we identified two semi-dominant pedigrees with novel missense mutations (Gck(K140E) and Gck(P417R)) in the gene encoding glucokinase (Gck), the mammalian glucose sensor that is mutated in human maturity onset diabetes of the young type 2 and the target of emerging anti-hyperglycemic agents that function as glucokinase activators (GKAs). Diabetes phenotype corresponded with genotype (mild-to-severe: Gck(+/+) < Gck(P417R/+), Gck(K140E)(/+) < Gck(P417R/P417R), Gck(P417R/K140E), and Gck(K140E/K140E)) and with the level of expression of GCK in liver. Each mutant was produced as the recombinant enzyme in Escherichia coli, and analysis of k(cat) and tryptophan fluorescence (I(320/360)) during thermal shift unfolding revealed a correlation between thermostability and the severity of hyperglycemia in the whole animal. Disruption of the glucokinase regulatory protein-binding site (GCK(K140E)), but not the ATP binding cassette (GCK(P417R)), prevented inhibition of enzyme activity by glucokinase regulatory protein and corresponded with reduced responsiveness to the GKA drug. Surprisingly, extracts from liver of diabetic GCK mutants inhibited activity of the recombinant enzyme, a property that was also observed in liver extracts from mice with streptozotocin-induced diabetes. These results indicate a relationship between genotype, phenotype, and GKA efficacy. The integration of forward genetic screening and biochemical profiling opens a pathway for preclinical development of mechanism-based diabetes therapies.

New oral antidiabetic agents

The pathophysiology of hyperglycemia in type 2 diabetes (T2DM) involves 3 main defects: insulin deficiency, excess hepatic glucose output, and insulin resistance. Oral anti-diabetic agents act in a variety of ways. These include agents that stimulate insulin secretion, reduce hepatic glucose production, delay digestion and absorption of intestinal carbohydrate or improve insulin action. Because of improved knowledge of pathophysiology, new drugs with mechanisms of action focussed on specific pathophysiological alterations have appeared, in order to utilize all the possibilities of treating this condition. Here, we focus on the new agents used in the latest years and the overcoming ones in future, in particular incretin-based therapies, drugs inhibiting kidney glucose reabsorption (SGLT2 inhibitors), and glucokinase activators. The strategy for new drug development advocated here is to establish a broad range of anti-diabetic medicines with different mechanisms of action and potential opportunities for effective combination therapies.

Research and development of glucokinase activators for diabetes therapy: theoretical and practical aspects

Glucokinase Glucokinase (GK GK ; EC 2.7.1.1.) phosphorylates and regulates glucose metabolism in insulin-producing pancreatic beta-cells, hepatocytes, and certain cells of the endocrine and nervous systems allowing it to play a central role in glucose homeostasis glucose homeostasis . Most importantly, it serves as glucose sensor glucose sensor in pancreatic beta-cells mediating glucose-stimulated insulin biosynthesis and release and it governs the capacity of the liver to convert glucose to glycogen. Activating and inactivating mutations of the glucokinase gene cause autosomal dominant hyperinsulinemic hypoglycemia and hypoinsulinemic hyperglycemia in humans, respectively, illustrating the preeminent role of glucokinase in the regulation of blood glucose and also identifying the enzyme as a potential target for developing antidiabetic drugs antidiabetic drugs . Small molecules called glucokinase activators (GKAs) glucokinase activators (GKAs) which bind to an allosteric activator allosteric activator site of the enzyme have indeed been discovered and hold great promise as new antidiabetic agents. GKAs increase the enzyme's affinity for glucose and also its maximal catalytic rate. Consequently, they stimulate insulin biosynthesis and secretion, enhance hepatic glucose uptake, and augment glucose metabolism and related processes in other glucokinase-expressing cells. Manifestations of these effects, most prominently a lowering of blood glucose, are observed in normal laboratory animals and man but also in animal models of diabetes and patients with type 2 diabetes mellitus (T2DM T2DM ) type 2 diabetes mellitus (T2DM) . These compelling concepts and results sustain a strong R&D effort by many pharmaceutical companies to generate GKAs with characteristics allowing for a novel drug treatment of T2DM.

PREB regulates transcription of pancreatic glucokinase in response to glucose and cAMP

Prolactin regulatory element binding (PREB) is a transcription factor that regulates prolactin promoter activity in rat anterior pituitary. The PREB protein is not only expressed in the anterior pituitary but also in the pancreas. We have recently reported that in pancreatic beta-cells, PREB regulates the transcription of the insulin gene in response to glucose stimulation. In the current study, we have examined the role of PREB in regulating glucokinase (GK) in pancreatic beta-cells. To analyse the effects of PREB on GK gene transcription, we employed a reporter gene assay. In the cells expressing or with knocked down PREB, GK expression was determined. GK expression was regulated by glucose and cAMP, and both glucose and cAMP stimulated the expression of PREB in a dose-dependent manner. Conversely, overexpression of PREB using a PREB-expressing adenovirus increased the expression of the GK protein. GK enzymatic activity was also significantly increased in the cells that stably expressed PREB. In addition, PREB induced GK promoter activity. Chromatin immunoprecipitation (ChIP) analyses showed that PREB mediated its transcriptional effect by binding to the PREB-responsive cis-element of the GK promoter. Finally, we used siRNA to inhibit PREB expression in cells and demonstrated that the knockdown of PREB attenuated the effects of glucose and cAMP on GK expression. Our data show that in pancreatic -cells, PREB regulates the transcription of the GK gene in response to glucose and cAMP.

Glucokinase and glucokinase regulatory proteins are functionally coexpressed before birth in the rat brain

Our previous description of functional glucokinase (GK) isoforms and their interactions with glucokinase regulatory protein (GKRP) in adult rat and human brains suggested that both participate in glucose sensing in the central nervous system. To determine whether both proteins are coexpressed and active before birth or during early post-natal life, we characterised these molecules in the brains of foetal and post-natal pup rats. We found GK and GKRP mRNAs that were similar to those previously reported in the adult rat brain. Likewise, GK and GKRP gene expression gave rise to proteins of 52 and 69 kDa, respectively. Immunohistochemistry experiments showed the colocalisation of both GK and GKRP proteins in the same brain cells of 21-day-old rat foetuses. Furthermore, coprecipitation of GK and GKRP in the presence of fructose 6-phosphate suggests interactions between both proteins. The presence of GK phosphorylating activity was detected in different brain areas of 21-day-old foetuses with a contribution to the total glucose-phosphorylating activity of between 17.2 +/- 1.7% and 12.4 +/- 3.7%, with the hypothalamus being the region of maximum activity. The hypothalamic GK activity in 21-day-old foetuses has a high apparent K(m) for glucose and no product inhibition by glucose 6-phosphate. Our findings indicate that both proteins may be functionally active before birth and that they can act within a glucose sensor system involved in controlling food intake.

Exendin-4 regulates glucokinase expression by CaMKK/CaMKIV pathway in pancreatic beta-cell line

AIM

Glucokinase (GK) in pancreatic beta cells is thought to be involved in insulin secretion and glucose homeostasis. This study investigates whether the long-acting agonist of the glucagon-like peptide 1, namely exendin-4, mediates stimulatory effects on GK gene expression through the Ca(2+)/calmodulin (CaM)-dependent protein kinase (CaMK) cascade.

METHODS

GK expression was examined by real-time PCR, western blot analysis and reporter gene assay in rat insulin-secreting INS-1 cells incubated with exendin-4. CaMKIV activity was assessed by detection of activation loop phosphorylation (Thr(196)) of CaMKIV. We investigated the effect of the constitutively active form (CaMKIVc) of CaMKIV on GK promoter activity.

RESULTS

Increased expression level of GK protein was noted in response to rising concentrations of exendin-4 with maximum induction at 10 nM. Real-time PCR analysis showed a significant increase in the amount of GK mRNA in response to rising concentrations of exendin-4. Exendin-4 also stimulated GK promoter activity but failed to do so in the presence of STO-609, a CaMKK inhibitor. This result is consistent with the observations that the upregulation of CaMKIV phosphorylation (at Thr(196)) peaked after 15 min of exposure to exendin-4 and that CaMKIVc enhanced or upregulated GK promoter activity in INS-1 cells. Furthermore, STO-609 significantly suppressed the exendin-4 - upregulated the expression of the GK protein.

CONCLUSION

Activation of the CaMKK/CaMKIV cascade might be required for exendin-4-induced GK gene transcription, indicating that exendin-4 plays an important role in insulin secretion in pancreatic beta cells.

Assessing the potential of glucokinase activators in diabetes therapy. Nat Rev Drug Discov. 2009;8:399-416. [PMID: 19373249 DOI: 10.1038/nrd2850]

Glucokinase, a unique isoform of the hexokinase enzymes, which are known to phosphorylate D-glucose and other hexoses, was identified during the past three to four decades as a new, promising drug target for type 2 diabetes. Glucokinase serves as a glucose sensor of the insulin-producing pancreatic islet beta-cells, controls the conversion of glucose to glycogen in the liver and regulates hepatic glucose production. Guided by this fundamental knowledge, several glucokinase activators are now being developed, and have so far been shown to lower blood glucose in several animal models of type 2 diabetes and in initial trials in humans with the disease. Here, the scientific basis and current status of this new approach to diabetes therapy are discussed.

Glucokinase Thermolability and Hepatic Regulatory Protein Binding Are Essential Factors for Predicting the Blood Glucose Phenotype of Missense Mutations

To better understand how glucokinase (GK) missense mutations associated with human glycemic diseases perturb glucose homeostasis, we generated and characterized mice with either an activating (A456V) or inactivating (K414E) mutation in the gk gene. Animals with these mutations exhibited alterations in their blood glucose concentration that were inversely related to the relative activity index of GK. Moreover, the threshold for glucose-stimulated insulin secretion from islets with either the activating or inactivating mutation were left- or right-shifted, respectively. However, we were surprised to find that mice with the activating mutation had markedly reduced amounts of hepatic GK activity. Further studies of bacterially expressed mutant enzymes revealed that GK(A456V) is as stable as the wild type enzyme, whereas GK(K414E) is thermolabile. However, the ability of GK regulatory protein to inhibit GK(A456V) was found to be less than that of the wild type enzyme, a finding consistent with impaired hepatic nuclear localization. Taken together, this study indicates that it is necessary to have knowledge of both thermolability and the interactions of mutant GK enzymes with GK regulatory protein when attempting to predict in vivo glycemic phenotypes based on the measurement of enzyme kinetics.

Effect of dichlorvos on hepatic and pancreatic glucokinase activity and gene expression, and on insulin mRNA levels

Several studies have shown that organophosphate pesticides affect carbohydrate metabolism and produce hyperglycemia. It has been reported that exposure to the organophosphate pesticide dichlorvos affects glucose homeostasis and decreases liver glycogen content. Glucokinase (EC 2.7.1.1) is a tissue-specific enzyme expressed in liver and in pancreatic beta cells that plays a crucial role in glycogen synthesis and glucose homeostasis. In the present study we analyzed the effect of one or three days of dichlorvos administration [20 mg/kg body weight] on the activity and mRNA levels of hepatic and pancreatic glucokinase as well as on insulin mRNA abundance in the rat. We found that the pesticide affects pancreatic and hepatic glucokinase activity and expression differently. In the liver the pesticide decreased the enzyme activity; on the contrary glucokinase mRNA levels were increased. In contrast, pancreatic glucokinase activity as well as mRNA levels were not affected by the treatment. Insulin mRNA levels were not modified by dichlorvos administration. Our results suggest that the decreased activity of hepatic glucokinase may account for the adverse effects of dichlorvos on glucose metabolism.

A novel glucokinase activator modulates pancreatic islet and hepatocyte function

The glucose-sensing enzyme glucokinase (GK) plays a key role in glucose metabolism. We report here the effects of a novel glucokinase activator, LY2121260. The activator enhanced GK activity via binding to the allosteric site located in the hinge region of the enzyme. LY2121260 stimulated insulin secretion in a glucose-dependent manner in pancreatic beta-cells and increased glucose use in rat hepatocytes. In addition, incubation of beta-cells with the GK activator resulted in increased GK protein levels, suggesting that enhanced insulin secretion on chronic treatment with a GK activator may be due to not only changed enzyme kinetics but also elevated enzyme levels. Animals treated with LY2121260 showed an improved glucose tolerance after oral glucose challenge. These results support the concept that GK activators represent a new class of compounds that increase both insulin secretion and hepatic glucose use and in doing so may prove to be effective agents for the control of blood glucose levels in patients with type 2 diabetes.

Glucokinase and glucose homeostasis: proven concepts and new ideas

The enzyme GK (glucokinase), which phosphorylates glucose to form glucose 6-phosphate, serves as the glucose sensor of insulin-producing beta-cells. GK has thermodynamic, kinetic, regulatory and molecular genetic characteristics that are ideal for its glucose sensor function and allow it to control glycolytic flux of the beta-cells as indicated by control-, elasticity- and response-coefficients close to or larger than 1.0. GK operates in tandem with the K(+) and Ca(2+) channels of the beta-cell membrane, resulting in a threshold for glucose-stimulated insulin release of approx. 5 mM, which is the set point of glucose homoeostasis for most laboratory animals and humans. Point mutations of GK cause 'glucokinase disease' in humans, which includes hypo- and hyper-glycaemia syndromes resulting from activating or inactivating mutations respectively. GK is allosterically activated by pharmacological agents (called GK activators), which lower blood glucose in normal animals and animal models of T2DM. On the basis of crystallographic studies that identified a ligand-free 'super-open' and a liganded closed structure of GK, on thermostability studies using glucose or mannoheptulose as ligands and studies showing that mannoheptulose alone or combined with GK activators induces expression of GK in pancreatic islets and partially preserves insulin secretory competency, a new hypothesis was developed that GK may function as a metabolic switch per se without involvement of enhanced glucose metabolism. Current research has the goal to find molecular targets of this putative 'GK-switch'. The case of GK research illustrates how basic science may culminate in therapeutic advances of human medicine.

Glucokinase in chicken (Gallus gallus). Partial cDNA cloning, immunodetection and activity determination

Chickens are more hyperglycaemic and insulin-resistant than mammals, and in efforts to understand their glucose metabolism we investigated whether glucokinase (GK) is present in chicken liver or pancreas. This enzyme plays a major role in glucose-sensing in mammals and we have examined whether it also contributes to glucose homeostasis in chickens. Using RT-PCR, we cloned and sequenced a partial cDNA fragment (750 bp) from liver and pancreas that showed a high degree of identity with mammalian GK. Using antibodies directed towards human GK, we immunodetected a 50 kDa band in chicken liver and pancreas. The molecular mass of the band and its specific interaction with the antibody suggest that this protein corresponds to a chicken homologue of human GK. We also determined by spectrophotometry a glucokinase-like activity in crude liver homogenates with an apparent half saturating concentration for glucose of 8.6 mM. GK gene and protein expression did not differ between fed and 24 h fasted states but GK-like activity was significantly increased in fed chickens. In conclusion, our study provides evidence for the presence of GK gene and protein in chicken liver and pancreas and shows that the liver enzyme is active.

Glucokinase, glucose homeostasis, and diabetes mellitus

The enzyme glucokinase (GK) regulates the rate of glucose metabolism in many tissues, including liver, the pancreatic b cells, certain neurons, enteroendocrine cells, and the pituitary, serving as a glucose sensor in many of these. Thus, GK plays a critical role in glucose homeostasis. Spontaneous mutants of GK in humans result in autosomal-dominant hypo- and hyperglycemia syndromes described as "GK disease." GK activator drugs have been discovered that lower blood glucose in normal and diabetic animals and promise to be useful in the treatment of type 2 diabetes mellitus. There is no question that the GK molecule and related issues will continue to be a fruitful topic for future research.

The cellular fate of glucose and its relevance in type 2 diabetes

Type 2 diabetes is a complex disorder with diminished insulin secretion and insulin action contributing to the hyperglycemia and wide range of metabolic defects that underlie the disease. The contribution of glucose metabolic pathways per se in the pathogenesis of the disease remains unclear. The cellular fate of glucose begins with glucose transport and phosphorylation. Subsequent pathways of glucose utilization include aerobic and anaerobic glycolysis, glycogen formation, and conversion to other intermediates in the hexose phosphate or hexosamine biosynthesis pathways. Abnormalities in each pathway may occur in diabetic subjects; however, it is unclear whether perturbations in these may lead to diabetes or are a consequence of the multiple metabolic abnormalities found in the disease. This review is focused on the cellular fate of glucose and relevance to human type 2 diabetes.

Generating new pancreas from old

Pancreas regeneration after tissue damage is a key response to pancreatic injury, involving pancreatic duct progenitor cells and intra-islet precursor cells. Surgical removal of the pancreas, duct obstruction by cellophane wrapping and bone marrow-derived stem cell transplantation act as inductive stimuli, leading to pancreas regeneration. The exact role of growth and differentiation factors regulating pancreatic beta-cell mass remains unknown. Here, I will attempt to integrate recent findings and speculate on the factors that trigger this fascinating response, wherein the pancreas responds to a deficit in cell mass and undergoes new islet formation, leading to restoration of normal beta-cell mass. I will also discuss recent advances in regenerating endocrine pancreatic cells, which could affect stem cell-based approaches to treating diabetes mellitus.

A glucokinase-like enzyme induced in Mule duck livers by overfeeding

The Mule duck develops a fatty liver in response to overfeeding, which results from a dramatic increase in de novo liver lipogenesis, and thus raises questions regarding the role of glucokinase (GK), a key enzyme regulating carbohydrate metabolism in mammals. However, the presence of GK in avian species is still a matter of debate. The aim of the present study was to characterize a GK-like protein (using an immunological technique) and a GK-like activity (using an enzymatic assay) in duck liver and to measure their respective variations during various stages of overfeeding. Duck liver protein cross-reacted with antibodies directed against mammalian GK yielding a band at 50 kDa, i.e., the same molecular weight as mammalian GK. The intensity of the signal varied significantly between overfed and control ducks but in opposing ways according to the GK antibodies used, which suggests the presence of 2 isoforms of GK in the duck liver as in mammals. Enzymatic analysis demonstrated the presence of glucose phosphorylation activity sensitive to high and low glucose concentrations (high/low ratio between 1.7 and 3.7) in the soluble and particulate fractions of liver homogenates. Glucokinase-like activity per milligram protein was strongly induced by overfeeding, and plasma insulin levels increased concomitantly. More than 80% of total GK-like activity was concentrated in the soluble component from 1 to 13 d of overfeeding. These results suggest that a GK-like enzyme may actively contribute to glucose disposal throughout the overfeeding period in Mule ducks fed a carbohydrate-rich diet.

Insulin gene transfer enhances the function of human islet grafts

AIMS/HYPOTHESIS

Recent success in islet transplantation renews the hope for the complete cure of patients afflicted with Type 1 diabetes. However, in the Edmonton series, two to four pancreas donors were required to obtain a sufficient islet mass to reverse the diabetes of each patient. In view of the donor shortage, this represents a major obstacle preventing greater application of islet transplantation to diabetic patients. We hypothesised that increasing the expression of the insulin gene in transplanted islets would augment their capacity for insulin production, thereby allowing reversal of diabetes with a reduced islet mass.

METHODS

We used a replication defective adenovirus to deliver the human proinsulin gene (Ad-Ins) to isolated human islets. The function of Ad-Ins-transduced human islets was compared to islets transduced with a control vector (Ad-lacz).

RESULTS

Ad-Ins-transduced islets produced two to three times more insulin than normal islets or those infected with Ad-lacz, as assessed by in vitro perifusion tests of glucose stimulated insulin release. When transplanted, Ad-Ins-transduced islets normalised the blood glucose of diabetic immunodeficient NOD-Scid mice, and less than half as many Ad-Ins islets were required for reversal of diabetes than when normal islets were transplanted.

CONCLUSION/INTERPRETATION

Our results suggest a simple and effective approach that could enhance the efficiency of islet transplantation for treatment of diabetes in humans.

Models of pancreatic regeneration in diabetes

The number of functionally intact beta cells in the islet organ is of decisive importance for the development, course and outcome of diabetes mellitus. Generally speaking, the total beta-cell mass reflects the balance between the renewal and loss of these cells. Assuming that virtually all forms of diabetes mellitus are characterized by an insufficient extent of beta cell replication needed to compensate for the loss or dysfunction of beta cells occurring in diabetes, elucidation of the regenerating potential in experimentally induced diabetic animal would be of interest as alternative therapy for diabetes. Here we have attempted to take a stock of different models developed in the last few years, which permit investigation of regenerative process from various angles. The review focuses on factors responsible for induction of islet neogenesis in the diabetic pancreas, ultimately leading to pancreatic regeneration and possible reversal of diabetes. On the whole the study of these models will enhance our understanding of regenerative potential of diabetic pancreas and factors necessary to trigger stem cells' population within the pancreas so as to suggest an alternative therapeutic approach for the control and/or cure of diabetes.

Regulation of pancreatic beta-cell glucokinase: from basics to therapeutics

Glucokinase (GK) serves as glucose sensor in pancreatic beta-cells and in other glucose sensor cells in the body. Biochemical genetic studies have characterized many activating and inactivating GK mutants that have been discovered in patients with hyperinsulinemic hypoglycemia or diabetes, all inherited as autosomal dominant traits. Mathematical modeling of the kinetic data of recombinant human wild-type and mutant GK accurately predicts the effects of GK mutations on the threshold of glucose-stimulated insulin release and glucose homeostasis. Structure/function studies of the enzyme suggest the existence of a hitherto unknown allosteric activator site of the enzyme that has significant implications for the physiological chemistry of GK-containing cells, particularly the pancreatic beta-cells. Glucose is the preeminent positive regulator of beta-cell GK expression and involves molecular mechanisms that are still to be elucidated in detail, but seem to have a specific requirement for increased glucose metabolism. Pharmaceutical chemists, motivated by the clear tenets of the GK glucose-sensor paradigm, have searched for and have discovered a novel class of GK activator molecules. The therapeutic application of this basic discovery offers a new principle for drug therapy of diabetes.

Cyclic adenosine 3',5'-monophosphate increases pancreatic glucokinase activity and gene expression

Comparison of the pancreatic and hepatic glucokinase gene transcripts reveals tissue-specific control of expression and the existence of two distinct promoters in a single glucokinase gene. The existence of alternate promoters suggests that separate factors regulate glucokinase transcription in the two tissues. Hepatic glucokinase expression has been shown to be repressed by cAMP; however, in the pancreatic beta-cell it is unlikely that cAMP represses glucokinase activity, as cAMP is known to positively affect glucose-induced insulin secretion, a process that in mature islets requires pancreatic glucokinase activity. In this work we demonstrate that cAMP indeed has a stimulatory effect on pancreatic glucokinase. The cyclic nucleotide stimulates pancreatic glucokinase activity after 3-h incubation, and maximal effects are observed after 6 and 12 h of treatment. Using the bDNA assay, a sensitive signal amplification technique, we detected relative increases in glucokinase messenger RNA levels of 40.5 +/- 7.5% after 3-h incubation with cAMP. This stimulatory effect was increased to 106.3 +/- 22% after 6-h incubation and sustained up to 12 h of incubation. Inhibition of gene transcription by actinomycin D abolishes cAMP-induced glucokinase activity. In transfected fetal islets, cAMP increased the activity of the -1000 bp rat glucokinase promoter by 60 +/- 6%. These data demonstrate that cAMP has a stimulatory effect on pancreatic glucokinase gene expression and that the nucleotide has opposite effects on pancreatic and hepatic glucokinase, supporting the concept that glucokinase transcription in the liver and that in the beta-cell differ.

Functional glucokinase isoforms are expressed in rat brain

Recently, the description of glucokinase mRNA in certain neuroendocrine cells has opened new ways to characterize this enzyme in the rat brain. In this study, we found glucokinase mRNA and a similar RNA splicing pattern of the glucokinase gene product in rat hypothalamus and pancreatic islets; the mRNA that codes for B1 isoform was the most abundant, with minor amounts of those coding for the B2, P1, P2, P1/B2, and P2/B2 isoforms. Glucokinase gene expression in rat brain gave rise to a protein of 52 kDa with a high apparent Km for glucose and no product inhibition by glucose 6-phosphate, with a contribution to the total glucose phosphorylating activity of between 40 and 14%; the hypothalamus and cerebral cortex were the regions of maximal activity. Low and high Km hexokinases were characterized by several criteria. Also, using RT-PCR analysis we found a glucokinase regulatory protein mRNA similar to that previously reported in liver. These findings indicate that the glucokinase present in rat brain should facilitate the adaptation of this organ to fluctuations in blood glucose concentrations, and the expression of glucokinase and GLUT-2 in the same hypothalamic neurons suggests a role in glucose sensing.

Nuclear import of hepatic glucokinase depends upon glucokinase regulatory protein, whereas export is due to a nuclear export signal sequence in glucokinase

Hepatic glucokinase (GK) moves between the nucleus and cytoplasm in response to metabolic alterations. Here, using heterologous cell systems, we have found that at least two different mechanisms are involved in the intracellular movement of GK. In the absence of the GK regulatory protein (GKRP) GK resides only in the cytoplasm. However, in the presence of GKRP, GK moves to the nucleus and resides there in association with this protein until changes in the metabolic milieu prompt its release. GK does not contain a nuclear localization signal sequence and does not enter the nucleus in a GKRP-independent manner because cells treated with leptomycin B, a specific inhibitor of leucine-rich NES-dependent nuclear export, do not accumulate GK in the nucleus. Instead, entry of GK into the nucleus appears to occur via a piggy-back mechanism that involves binding to GKRP. Nuclear export of GK, which occurs after its release from GKRP, is due to a leucine-rich nuclear export signal within the protein ((300)ELVRLVLLKLV(310)). Thus, GKRP appears to function as both a nuclear chaperone and metabolic sensor and is a critical component of a hepatic GK translocation cycle for regulating the activity of this enzyme in response to metabolic alterations.

GLUT2 and glucokinase expression is coordinately regulated by sulfonylurea

In the present study we examined the effect of sulfonylurea on the expression of the glucose transporter GLUT2 and the glucose phosphorylating enzyme Glucokinase (GK) in betaTC6-F7 cells; furthermore, we studied the modifications induced by sulfonylurea on glucose-responsiveness and -sensitivity. Results demonstrate that sulfonylurea increases GLUT2 and GK mRNA expression after 24 h in a dose-dependent manner. On the contrary, after 48 and 72 h a time-dependent reduction of both GLUT2 and GK mRNA occurs. GLUT2 and GK protein expression follow the same modifications. Therefore, GLUT2 and GK are coordinately regulated by sulfonylurea, probably by a common mechanism. Glucose-induced insulin release is increased by sulfonylurea as well as glucose sensitivity. Our study suggests that short-term effect of sulfonylurea increases while long-term effect reduces the expression of glucose sensing elements. The long-term inhibitory effect on glucose sensing elements would explain the reduced insulin secretion occurring after chronic sulfonylurea treatment.

Effect of retinoic acid on glucokinase activity and gene expression and on insulin secretion in primary cultures of pancreatic islets

Retinoic acid has manifold effects on pancreatic beta-cells. Previously we reported that retinoic acid increases glucokinase activity and messenger RNA (mRNA) levels in the insulinoma cell line RIN-m5F; however, we could not rule out the possibility that the effect of retinoic acid on RIN-m5F glucokinase was inherent to the cell line or related to its differentiating capacity. In this report, we demonstrate that physiologic concentrations of retinoic acid stimulate glucokinase activity in both fetal islets and differentiated adult islets in culture. In the adult tissue, the response to the retinoid was less pronounced, achieving about half of the maximal effect produced on the fetal tissue. Using the branched DNA (bDNA) assay, a sensitive signal amplification technique, we detected relative increases in glucokinase mRNA levels of 51.8+/-13.3% and 62.8+/-16.1% at 12 and 24 h, respectively, in adult islets treated with] 10(-6) M retinoic acid. In fetal islets, increases of 55+/-14.9% and 107+/-30.5% at 12 and 24 h, respectively, were observed. In transfected fetal islets, retinoic acid increased the activity of the -1000 kb rat glucokinase promoter by 51.3%. Because glucokinase activity controls insulin secretion, we also investigated the effect of retinoic acid on insulin secretion. Treatment with 10(-6) M retinoic acid for 24 h increased insulin secretion in both fetal and adult islets; however, the increases on insulin secretion were more pronounced in the mature islets; in contrast, retinoic acid produced higher levels of insulin mRNA in the fetal islets. These data show that retinoic acid increases pancreatic glucokinase in cultured islets and that the mechanism may involve a stimulatory effect on the glucokinase promoter.

Effects of increased glucokinase gene copy number on glucose homeostasis and hepatic glucose metabolism

The relationship between glucokinase (GK) gene copy number and glucose homeostasis was studied in transgenic mice with additional copies of the entire GK gene locus (Niswender, K. D., Postic, C., Jetton, T. L., Bennett, B. D., Piston, D. W., Efrat, S., and Magnuson, M. A. (1997) J. Biol. Chem. 272, 22564-22569). The plasma glucose concentration was reduced by 25 +/- 3% and 37 +/- 4% in mice with one or two extra copies of the gene locus, respectively. The basis for the hypoglycemic phenotype was determined using metabolic tracer techniques in chronically cannulated, conscious mice with one extra GK gene copy. Under basal conditions (6-h fasted) transgenic mice had a lower blood glucose concentration (-12 +/- 1%) and a slightly higher glucose turnover rate (+8 +/- 3%), resulting in a significantly higher glucose clearance rate (+21 +/- 2%). Plasma insulin levels were not different, suggesting that increased glucose clearance was due to augmented hepatic, not islet, GK gene expression. Under hyperglycemic clamp conditions the transgenic mice had glucose turnover and clearance rates similar to the controls, but showed a lower plasma insulin response (-48 +/- 5%). Net hepatic glycogen synthesis was markedly elevated (+360%), whereas skeletal muscle glycogen synthesis was decreased (-40%). These results indicate that increased GK gene dosage leads to increased hepatic glucose metabolism and, consequently, a lower plasma glucose concentration. Increased insulin secretion was not observed, even though the transgene is expressed in islets, because hypoglycemia causes a down-regulation in islet GK content (Niswender, K. D., Postic, C., Jetton, T. L., Bennett, B. D., Piston, D. W., Efrat, S., and Magnuson, M. A. (1997) J. Biol. Chem. 272, in press).

Interleukin-1 reduces the glycolytic utilization of glucose by pancreatic islets and reduces glucokinase mRNA content and protein synthesis by a nitric oxide-dependent mechanism

Culture of rat pancreatic islets with interleukin-1 (IL-1) results in up-regulation of the inducible isoform of nitric oxide synthase and overproduction of nitric oxide (NO). This is associated with reversible inhibition of both glucose-induced insulin secretion and islet glucose oxidation, and these effects are prevented by the inducible nitric oxide synthase inhibitor NG-monomethylarginine. IL-1 also induces accumulation of nonesterified arachidonic acid in islets by an NO-dependent mechanism, and one potential explanation for that effect would involve an IL-1-induced enhancement of islet glycolytic flux. We have therefore examined effects of IL-1 on islet glycolytic utilization of glucose and find that culture of islets with IL-1 in medium containing 5.5 mM glucose results in suppression of islet glucose utilization subsequently measured at glucose concentrations between 6 and 18 mM. The IL-1-induced suppression of islet glucose utilization is associated with a decline in islet glucokinase mRNA content, as determined by competitive reverse transcriptase-polymerase chain reaction, and in glucokinase protein synthesis, as determined by immuoprecipitation experiments, and all of these effects are prevented by NG-monomethylarginine. These findings suggest that IL-1 can down-regulate islet glucokinase, which is the primary component of the islet glucose-sensor apparatus, by an NO-dependent mechanism. Because reductions in islet glucokinase levels are known to cause a form of type II diabetes mellitus, these observations raise the possibility that factors which increase islet NO levels might contribute to development of glucose intolerance.

Effect of biotin on glucokinase activity, mRNA expression and insulin release in cultured beta-cells

Biotin is known to influence hepatic glucokinase (GK) expression both at a transcriptional and at a translational level. The aim of the present paper was to investigate the effect of biotin on pancreatic GK. For this purpose, RIN1046-38 cells were cultured in the presence of different biotin concentrations for different times; there-after, GK mRNA expression, GK activity and insulin release were studied. Results demonstrated that biotin has a biphasic effect on GK mRNA expression, being stimulatory after short-term treatment and inhibitory after longterm treatment. GK activity was increased after long-term treatment. Insulin release was not affected by biotin treatment. These data suggest that biotin may influence glucose metabolism also by acting directly at the level of beta-cells.

Quantitative analysis of pancreatic glucokinase gene expression in cultured beta cells by competitive polymerase chain reaction

Regulation of glucokinase (GK) gene expression in pancreatic beta cells has been poorly investigated, both due to low abundance of the gene and to difficulties in cells isolation. The present study describes the establishment of a competitive RT-PCR method for quantitative analysis of GK gene. The method has been applied to the analysis of GK mRNA expression RIN 1046-38 cells. We have monitored modifications of GK mRNA expression after different periods of time in culture and we have studied the effect induced by dexamethasone (DEX) treatment. We show that the method is very sensitive and requires very low amount of RNA. Data demonstrate that GK mRNA expression in RIN cells is reduced as a function of passages in culture and that the reduction is positively correlated with the decrease of insulin responsiveness observed in high passages cells. DEX treatment inhibits GK mRNA expression in RIN cells in a dose-dependent and time-dependent manner.

Effect of adrenalectomy on the development of a pancreatic islet lesion in fa/fa rats

Adrenalectomy prevents development of obesity and hyperinsulinaemia in obese (fa/fa) Zucker rats, thereby implicating the hypothalamo- pituitary-adrenal axis in the pathogenesis of obesity. In this study glucose-induced insulin secretion and glucokinase activity were investigated in isolated islets from adrenalectomized and control obese and lean female rats. Islets from control fa/fa rats were more sensitive to glucose with a half-maximal effective concentration (EC50) of 6.1 +/- 2.0 mmol. 1(-1) compared with 10.6 +/- 2.7 mmol. 1(-1) for adrenalectomized fa/fa rat islets. Adrenalectomy did not alter the islet sensitivity to glucose in the lean rats (EC50 of 9.4 +/- 1.5 mmol.1(-1) and 9.3 +/- 2.0 mmol. 1(-1) for adrenalectomized and control lean rats respectively). Mannoheptulose did not inhibit insulin secretion from control obese rats; however at concentrations of 1.0 mmol. 1(-1) or more it significantly inhibited glucose-induced insulin secretion in adrenalectomized obese and lean, and control lean rat islets (P < 0.05). In adrenalectomized fa/fa islets the glucokinase Km was increased twofold compared with the control fa/fa rats (9.5 +/- 1.5 mmol. 1(-1) vs 5.0 +/- 1.5 mmol. 1(-1), respectively), but there was no significant change in glucokinase Km in the lean rat islets after adrenalectomy. Mannoheptulose (10 mmol.1(-1) caused a significant reduction in glucose phosphorylation in disrupted islets of adrenalectomized fa/fa and lean, and of control lean rats, but not of control fa/fa rats. These data demonstrate that development of abnormal regulation of glycolysis in pancreatic islet beta cells of fa/fa rats, as indicated by the insulin response to manno-heptulose and glucokinase activity, is dependent on an intact hypothalamo-pituitary-adrenal axis.

Transgenic knockouts reveal a critical requirement for pancreatic beta cell glucokinase in maintaining glucose homeostasis

The secretion of insulin is controlled by the rate of glucose metabolism in the pancreatic beta cells. As phosphorylation by glucokinase (GLK) appears to be the rate-limiting step for glucose catabolism in beta cells, this enzyme may be the glucose sensor. To test this possibility and to resolve the relative roles of liver and beta cell GLK in maintaining glucose levels, we have generated mice completely deficient in GLK and transgenic mice in which GLK is expressed only in beta cells. In mice with only one GLK allele, blood glucose levels are elevated and insulin secretion is reduced. GLK-deficient mice die perinatally with severe hyperglycemia. Expression of GLK in beta cells in the absence of expression in the liver is sufficient for survival. These mice demonstrate the critical need for beta cell GLK in maintaining normal glucose levels and provide a novel model for one form of noninsulin-dependent diabetes.