The role of the regulatory protein of glucokinase in the glucose sensory mechanism of the hepatocyte

Effects of 2-ethylhexyl diphenyl phosphate (EHDPP) on glycolipid metabolism in male adult zebrafish revealed by targeted lipidomic analyses

The present study aims to evaluate the effects of 2-ethylhexyldiphenyl phosphate (EHDPP) on glycolipid metabolism in vivo. Adult male zebrafish were exposed to various concentrations (0, 1, 10, 100 and 250 μg/L) of EHDPP for 28 days, and changes in lipid and glucose levels were measured. Results indicated significant liver damages in the 100 and 250 μg/L EHDPP groups, which both exhibited significant decreases in hepatic somatic index (HSI), elevated activities of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) in serum and liver, as well as hepatocyte vacuolation and nuclear pyknosis. Exposure to 100 and 250 μg/L EHDPP led to significant reductions in serum and liver cholesterol (TC), triglycerides (TGs), and lipid droplet deposition, indicating a significant inhibition of EHDPP on hepatic lipid accumulation. Lipidomic analyses manifested that 250 μg/L EHDPP reduced the levels of 103 lipid metabolites which belong to glycerides (TGs, diglycerides, and monoglycerides), fatty acyles (fatty acids), sterol lipids (cholesterol, bile acids), sphingolipids, and glycerophospholipids, and downregulated genes involved in de novo synthesis of fatty acids (fas, acc, srebp1, and dagt2), while upregulated genes involved in fatty acid β-oxidation (pparα and cpt1). KEGG analyses revealed that EHDPP significantly disrupted glycerolipid metabolism, steroid biosynthesis and fatty acid biosynthesis pathways. Collectively, the results showed that EHDPP induced lipid reduction in zebrafish liver, possibly through inhibiting lipid synthesis and disrupting glycerolipid metabolism. Our findings provide a theoretical basis for evaluating the ecological hazards and health effects of EHDPP on glycolipid metabolism.

Integration of metabolic flux with hepatic glucagon signaling and gene expression profiles in the conscious dog

Glucagon rapidly and profoundly stimulates hepatic glucose production (HGP), but for reasons that are unclear, this effect normally wanes after a few hours, despite sustained plasma glucagon levels. This study characterized the time course of glucagon-mediated molecular events and their relevance to metabolic flux in the livers of conscious dogs. Glucagon was either infused into the hepato-portal vein at a sixfold basal rate in the presence of somatostatin and basal insulin, or it was maintained at a basal level in control studies. In one control group, glucose remained at basal, whereas in the other, glucose was infused to match the hyperglycemia that occurred in the hyperglucagonemic group. Elevated glucagon caused a rapid (30 min) and largely sustained increase in hepatic cAMP over 4 h, a continued elevation in glucose-6-phosphate (G6P), and activation and deactivation of glycogen phosphorylase and synthase activities, respectively. Net hepatic glycogenolysis increased rapidly, peaking at 15 min due to activation of the cAMP/PKA pathway, then slowly returned to baseline over the next 3 h in line with allosteric inhibition by glucose and G6P. Glucagon's stimulatory effect on HGP was sustained relative to the hyperglycemic control group due to continued PKA activation. Hepatic gluconeogenic flux did not increase due to the lack of glucagon's effect on substrate supply to the liver. Global gene expression profiling highlighted glucagon-regulated activation of genes involved in cellular respiration, metabolic processes, and signaling, as well as downregulation of genes involved in extracellular matrix assembly and development.NEW & NOTEWORTHY Glucagon rapidly stimulates hepatic glucose production, but these effects are transient. This study links the molecular and metabolic flux changes that occur in the liver over time in response to a rise in glucagon, demonstrating the strength of the dog as a translational model to couple findings in small animals and humans. In addition, this study clarifies why the rapid effects of glucagon on liver glycogen metabolism are not sustained.

Coffee consumption affects kidney function based on GCKR polymorphism in a Korean population

Kidney function can be preserved through pharmacological interventions and nonpharmacological strategies, such as lifestyle and dietary adjustments. Among these, coffee has been linked to protective effects on kidney function. However, few studies have investigated the effect of coffee consumption on kidney function according to specific genes. We hypothesized that the impact of coffee consumption on kidney function might vary depending on GCKR polymorphism. GCKR rs1260326 polymorphism was examined using the Korean genome and epidemiology data from 656 chronic kidney disease (CKD) cases and 38,540 individuals without CKD (non-CKD). GCKR polymorphism has been previously associated with both coffee consumption and kidney function in Europeans. We replicated the associations between GCKR rs1260326 and coffee consumption and kidney function in Korean individuals. We also explored the effect of coffee consumption on kidney function by multivariate logistic regression analysis. Individuals with the rs1260326 (TC/CC) genotype did not experience significant changes in CKD risk based on their coffee consumption habits. In contrast, individuals with the TT genotype exhibited a significantly lower risk of CKD based on coffee consumption. Interestingly, in the non-CKD group, a beneficial effect on estimated glomerular filtration rate was observed in individuals with the T allele as coffee consumption increased. Our findings supported the hypothesis and revealed that the impact of coffee consumption habits on kidney function may vary based on the GCKR rs1260326 genotype of Korean individuals.

Current Insight on the Role of Glucokinase and Glucokinase Regulatory Protein in Diabetes

The glucokinase regulator (GCKR) gene encodes an inhibitor of the glucokinase enzyme (GCK), found only in hepatocytes and responsible for glucose metabolism. A common GCKR coding variation has been linked to various metabolic traits in genome-wide association studies. Rare GCKR polymorphisms influence GKRP activity, expression, and localization. Despite not being the cause, these variations are linked to hypertriglyceridemia. Because of their crystal structures, we now better understand the molecular interactions between GKRP and the GCK. Finally, small molecules that specifically bind to GKRP and decrease blood sugar levels in diabetic models have been identified. GCKR allelic spectrum changes affect lipid and glucose homeostasis. GKRP dysfunction has been linked to a variety of molecular causes, according to functional analysis. Numerous studies have shown that GKRP dysfunction is not the only cause of hypertriglyceridemia, implying that type 2 diabetes could be treated by activating liver-specific GCK via small molecule GKRP inhibition. The review emphasizes current discoveries concerning the characteristic roles of glucokinase and GKRP in hepatic glucose metabolism and diabetes. This information has influenced the growth of directed molecular therapies for diabetes, which has improved our understanding of lipid and glucose physiology.

Unraveling the pathogenesis of non-alcoholic fatty liver diseases through genome-wide association studies

Non-alcoholic fatty liver disease (NAFLD) is a significant health burden around the world, affecting approximately 25% of the population. Recent advances in human genetic databases have allowed for the identification of various single nucleotide polymorphisms associated with NAFLD-related traits. Investigating the functions of these genetic factors provides insight into the pathogenesis of NAFLD and potentially identifies novel therapeutic targets for NAFLD. In this review, we summarized current research on genes with NAFLD-associated mutations, highlighting phospholipid remodeling and spatially clustered loci as common pathological and genetic features of these mutations. These features suggest a complex yet intriguing mechanism of dissociated steatosis and insulin resistance, which is observed in a subset of patients and may lead to more precise therapy against NAFLD in the future.

Glucokinase regulatory protein: a balancing act between glucose and lipid metabolism in NAFLD

Non-alcoholic fatty liver disease (NAFLD) is a common liver disease worldwide, affected by both genetics and environment. Type 2 diabetes (T2D) stands as an independent environmental risk factor that precipitates the onset of hepatic steatosis and accelerates its progression to severe stages of liver damage. Furthermore, the coexistence of T2D and NAFLD magnifies the risk of cardiovascular disease synergistically. However, the association between genetic susceptibility and metabolic risk factors in NAFLD remains incompletely understood. The glucokinase regulator gene (GCKR), responsible for encoding the glucokinase regulatory protein (GKRP), acts as a regulator and protector of the glucose-metabolizing enzyme glucokinase (GK) in the liver. Two common variants (rs1260326 and rs780094) within the GCKR gene have been associated with a lower risk for T2D but a higher risk for NAFLD. Recent studies underscore that T2D presence significantly amplifies the effect of the GCKR gene, thereby increasing the risk of NASH and fibrosis in NAFLD patients. In this review, we focus on the critical roles of GKRP in T2D and NAFLD, drawing upon insights from genetic and biological studies. Notably, prior attempts at drug development targeting GK with glucokinase activators (GKAs) have shown potential risks of augmented plasma triglycerides or NAFLD. Conversely, overexpression of GKRP in diabetic rats improved glucose tolerance without causing NAFLD, suggesting the crucial regulatory role of GKRP in maintaining hepatic glucose and lipid metabolism balance. Collectively, this review sheds new light on the complex interaction between genes and environment in NAFLD, focusing on the GCKR gene. By integrating evidence from genetics, biology, and drug development, we reassess the therapeutic potential of targeting GK or GKRP for metabolic disease treatment. Emerging evidence suggests that selectively activating GK or enhancing GK-GKRP binding may represent a holistic strategy for restoring glucose and lipid metabolic balance.

Global-run on sequencing identifies Gm11967 as an Akt-dependent long noncoding RNA involved in insulin sensitivity

The insulin responsive Akt and FoxO1 signaling axis is a key regulator of the hepatic transcriptional response to nutrient intake. Here, we used global run-on sequencing (GRO-seq) to measure the nascent transcriptional response to fasting and refeeding as well as define the specific role of hepatic Akt and FoxO1 signaling in mediating this response. We identified 599 feeding-regulated transcripts, as well as over 6,000 eRNAs, and mapped their dependency on Akt and FoxO1 signaling. Further, we identified several feeding-regulated lncRNAs, including the lncRNA Gm11967, whose expression was dependent upon the liver Akt-FoxO1 axis. Restoring Gm11967 expression in mice lacking liver Akt improved insulin sensitivity and induced glucokinase protein expression, indicating that Akt-dependent control of Gm11967 contributes to the translational control of glucokinase. More broadly, we have generated a unique genome-wide dataset that defines the feeding and Akt/FoxO1-dependent transcriptional changes in response to nutrient availability.

Glucokinase as an emerging anti-diabetes target and recent progress in the development of its agonists

Type 2 diabetes mellitus is a metabolic disorder with complicated pathogenesis, and mono-target therapy often fails to effectively manage the levels of blood glucose. In recent years, the anti-diabetes target glucokinase (GK) has attracted the attention of researchers. It acts as a glucose sensor, triggering counter regulatory responses following a change in glucose levels to aid restoration of normoglycemia. Activation of GK induces glucose metabolism and reduces glucose levels for the treatment of type 2 diabetes. GK agonists (GKA) are a new class of antidiabetic drugs. Among these agents, dorzagliatin is currently being investigated in phase III clinical trials, while PB-201 and AZD-1656 have reached phase II clinical trials. This article describes the mechanism of action of GK in diabetes and of action of GKA at the protein level, and provides a review of the research, trends, and prospects regarding the use of GKA in this setting.

Kinetic modeling of glucose central metabolism in hepatocytes and hepatoma cells

BACKGROUND

Kinetic modeling and control analysis of a metabolic pathway may identify the steps with the highest control in tumor cells, and low control in normal cells, which can be proposed as the best therapeutic targets.

METHODS

Enzyme kinetic characterization, pathway kinetic modeling and control analysis of the glucose central metabolism were carried out in rat (hepatoma AS-30D) and human (cervix HeLa) cancer cells and normal rat hepatocytes.

RESULTS

The glycogen metabolism enzymes in AS-30D, HeLa cells and hepatocytes showed similar kinetic properties, except for higher AS-30D glycogen phosphorylase (GP) sensitivity to AMP. Pathway modeling indicated that fluxes of glycogen degradation and PPP were mainly controlled by GP and NADPH consumption, respectively, in both hepatocytes and cancer cells. Likewise, hexose-6-phosphate isomerase (HPI) and phosphoglucomutase (PGM) exerted significant control on glycolysis and glycogen synthesis fluxes in cancer cells but not in hepatocytes. Modeling also indicated that glycolytic and glycogen synthesis fluxes could be strongly decreased when HPI and PGM were simultaneously inhibited in AS-30D cells but not in hepatocytes. Experimental assessment of these predictions showed that both the glycolytic and glycogen synthesis fluxes of AS-30D cells, but not of hepatocytes, were inhibited by oxamate, by inducing increased Fru1,6BP levels, a competitive inhibitor of HPI and PGM.

CONCLUSION

HPI and PGM seem suitable targets for decreasing glycolytic and glycogen synthesis fluxes in AS-30D cells but not in hepatocytes.

GENERAL SIGNIFICANCE

The present study identified new therapeutic targets within glucose central metabolism in the analyzed cancer cells, with no effects on non-cancer cells.

Metabolic control analysis of hepatic glycogen synthesis in vivo

Hepatic glycogen synthesis plays a critical role in maintaining normal glucose homeostasis; however, the rate-controlling step regulating this process is unknown. Applying metabolic control analysis in vivo, we show that the regulation of insulin-stimulated hepatic glycogen synthesis under both normal and pathophysiological conditions of fatty liver-associated hepatic insulin resistance is controlled at the glucokinase (GCK) step through GCK translocation.

Multiple insulin-regulated enzymes participate in hepatic glycogen synthesis, and the rate-controlling step responsible for insulin stimulation of glycogen synthesis is unknown. We demonstrate that glucokinase (GCK)-mediated glucose phosphorylation is the rate-controlling step in insulin-stimulated hepatic glycogen synthesis in vivo, by use of the somatostatin pancreatic clamp technique using [¹³C₆]glucose with metabolic control analysis (MCA) in three rat models: 1) regular chow (RC)-fed male rats (control), 2) high fat diet (HFD)-fed rats, and 3) RC-fed rats with portal vein glucose delivery at a glucose infusion rate matched to the control. During hyperinsulinemia, hyperglycemia dose-dependently increased hepatic glycogen synthesis. At similar levels of hyperinsulinemia and hyperglycemia, HFD-fed rats exhibited a decrease and portal delivery rats exhibited an increase in hepatic glycogen synthesis via the direct pathway compared with controls. However, the strong correlation between liver glucose-6-phosphate concentration and net hepatic glycogen synthetic rate was nearly identical in these three groups, suggesting that the main difference between models is the activation of GCK. MCA yielded a high control coefficient for GCK in all three groups. We confirmed these findings in studies of hepatic GCK knockdown using an antisense oligonucleotide. Reduced liver glycogen synthesis in lipid-induced hepatic insulin resistance and increased glycogen synthesis during portal glucose infusion were explained by concordant changes in translocation of GCK. Taken together, these data indicate that the rate of insulin-stimulated hepatic glycogen synthesis is controlled chiefly through GCK translocation.

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.

Exercise-Induced Improvements to Whole Body Glucose Metabolism in Type 2 Diabetes: The Essential Role of the Liver

Type 2 diabetes (T2D) is a metabolic disease characterized by obesity, insulin resistance, and the dysfunction of several key glucoregulatory organs. Among these organs, impaired liver function is recognized as one of the earliest contributors to impaired whole-body glucose homeostasis, with well-characterized hepatic insulin resistance resulting in elevated rates of hepatic glucose production (HGP) and fasting hyperglycemia. One portion of this review will provide an overview of how HGP is regulated during the fasted state in healthy humans and how this process becomes dysregulated in patients with T2D. Less well-appreciated is the liver's role in post-prandial glucose metabolism, where it takes up and metabolizes one-third of orally ingested glucose. An abundance of literature has shown that the process of hepatic glucose uptake is impaired in patients with T2D, thereby contributing to glucose intolerance. A second portion of this review will outline how hepatic glucose uptake is regulated during the post-prandial state, and how it becomes dysfunctional in patients with T2D. Finally, it is well-known that exercise training has an insulin-sensitizing effect on the liver, which contributes to improved whole-body glucose metabolism in patients with T2D, thereby making it a cornerstone in the management of the disease. To this end, the impact of exercise on hepatic glucose metabolism will be thoroughly discussed, referencing key findings in the literature. At the same time, sources of heterogeneity that contribute to inconsistent findings in the field will be pointed out, as will important topics for future investigation.

The Central Role of Glucokinase in Glucose Homeostasis: A Perspective 50 Years After Demonstrating the Presence of the Enzyme in Islets of Langerhans

It is hypothesized that glucokinase (GCK) is the glucose sensor not only for regulation of insulin release by pancreatic β-cells, but also for the rest of the cells that contribute to glucose homeostasis in mammals. This includes other cells in endocrine pancreas (α- and δ-cells), adrenal gland, glucose sensitive neurons, entero-endocrine cells, and cells in the anterior pituitary. Glucose transport is by facilitated diffusion and is not rate limiting. Once inside, glucose is phosphorylated to glucose-6-phosphate by GCK in a reaction that is dependent on glucose throughout the physiological range of concentrations, is irreversible, and not product inhibited. High glycerol phosphate shuttle, pyruvate dehydrogenase, and pyruvate carboxylase activities, combined with low pentose-P shunt, lactate dehydrogenase, plasma membrane monocarboxylate transport, and glycogen synthase activities constrain glucose-6-phosphate to being metabolized through glycolysis. Under these conditions, glycolysis produces mostly pyruvate and little lactate. Pyruvate either enters the citric acid cycle through pyruvate dehydrogenase or is carboxylated by pyruvate carboxylase. Reducing equivalents from glycolysis enter oxidative phosphorylation through both the glycerol phosphate shuttle and citric acid cycle. Raising glucose concentration increases intramitochondrial [NADH]/[NAD⁺] and thereby the energy state ([ATP]/[ADP][Pi]), decreasing [Mg²⁺ADP] and [AMP]. [Mg²⁺ADP] acts through control of K_ATP channel conductance, whereas [AMP] acts through regulation of AMP-dependent protein kinase. Specific roles of different cell types are determined by the diverse molecular mechanisms used to couple energy state to cell specific responses. Having a common glucose sensor couples complementary regulatory mechanisms into a tightly regulated and stable glucose homeostatic network.

A Rare Nonsense Mutation in the Glucokinase Regulator Gene Is Associated With a Rapidly Progressive Clinical Form of Nonalcoholic Steatohepatitis

We report on the presence of a rare nonsense mutation (rs149847328, p.Arg227Ter) in the glucokinase regulator (GCKR) gene in an adult patient with nonalcoholic fatty liver disease (NAFLD), morbid obesity, and type 2 diabetes; this patient developed a progressive histological form of the disease. Analysis of paired (5 years apart) liver biopsies (at baseline and follow‐up) showed progression of simple steatosis to severe nonalcoholic steatohepatitis and cirrhosis. Study design involved an initial exploration that consisted of deep sequencing of 14 chromosomal regions in 96 individuals (64 of whom were patients with NAFLD who were diagnosed by liver biopsy that showed the full spectrum of histological severity). We further performed a replication study to explore the presence of rs149847328 that included a sample of 517 unrelated individuals in a case‐control study (n = 390), including patients who were morbidly obese (n = 127). Exploration of sequence variation by next‐generation sequencing of exons, exon–intron boundaries, and 5′ and 3′ untranslated regions of 14 genomic loci that encode metabolic enzymes of the tricarboxylic acid cycle revealed the presence of heterozygosity for the p.Arg227Ter mutation, the frequency of which is 0.0003963 (4:10,000; Exome Aggregation Consortium database). GCKR protein expression was markedly decreased in the liver of the affected patient compared with patients with NAFLD who carry the wild‐type allele. Sequencing of the same 14 genomic loci in 95 individuals failed to reveal the rare mutation. The rarity of p.Arg227Ter was confirmed in a more extensive screening. Conclusion: While rare variants/mutations are difficult to detect in even reasonably large samples (frequency of the mutant allele of p.Arg227Ter was ~1:1,000 in our data set), the presence of this mutation should be suspected as potentially associated with NAFLD, particularly in young adults at the extreme of histological phenotypes. Hepatology Communications 2018;0:0‐0)

Kaempferol ameliorates hyperglycemia through suppressing hepatic gluconeogenesis and enhancing hepatic insulin sensitivity in diet-induced obese mice

Obesity-associated insulin resistance (IR) is a major risk factor for developing type 2 diabetes and an array of other metabolic disorders. In particular, hepatic IR contributes to the increase in hepatic glucose production and consequently the development of fasting hyperglycemia. In this study, we explored whether kaempferol, a flavonoid isolated from Gink go biloba, is able to regulate hepatic gluconeogenesis and blood glucose homeostasis in high-fat diet-fed obese mice and further explored the underlying mechanism by which it elicits such effects. Oral administration of kaempferol (50 mg/kg/day), which is the human equivalent dose of 240 mg/day for an average 60 kg human, significantly improved blood glucose control in obese mice, which was associated with reduced hepatic glucose production and improved whole-body insulin sensitivity without altering body weight gain, food consumption or adiposity. In addition, kaempferol treatment increased Akt and hexokinase activity, but decreased pyruvate carboxylase (PC) and glucose-6 phosphatase activity in the liver without altering their protein expression. Consistently, kaempferol decreased PC activity and suppressed gluconeogenesis in HepG2 cells as well as primary hepatocytes isolated from the livers of obese mice. Furthermore, we found that kaempferol is a direct inhibitor of PC. These findings suggest that kaempferol may be a naturally occurring antidiabetic compound that acts by suppressing glucose production and improving insulin sensitivity. Kaempferol suppression of hepatic gluconeogenesis is due to its direct inhibitory action on the enzymatic activity of PC.

Identification and characterization of a FOXA2-regulated transcriptional enhancer at a type 2 diabetes intronic locus that controls GCKR expression in liver cells

BACKGROUND

Genome-wide association studies (GWAS) have identified more than 100 genetic loci associated with type 2 diabetes (T2D). However, the underlying biological mechanisms for many of these associations remain unknown. GWAS signals close to the glucokinase regulatory protein gene (GCKR) have been reported for lipid and glucose metabolism traits and the risk of T2D. We investigated the regulatory function of an intronic locus at GCKR represented by the lead single nucleotide polymorphism (SNP) rs780094.

METHODS

We used ENCODE project histone modification and transcription factor binding data to determine the regulatory features of a GCKR intronic locus formed by the high linkage disequilibrium rs780094(C/T), rs780095(G/A), and rs780096(G/C) SNPs. Characterization of the transcriptional activity of this region was assessed by luciferase reporter assays in HepG2 cells and mouse primary hepatocytes. ChIP-qPCR was used to determine the levels of haplotype specific transcription factor binding and histone marks. A CRISPR-dCas9 transcriptional activator system and qPCR were used to activate the locus and measure GCKR expression, respectively. Differential haplotype expression was measured from human liver biopsies.

RESULTS

The ENCODE data suggest the existence of a liver-specific intragenic enhancer at the locus represented by s780094. We observed that FOXA2 increased the transcriptional activity of this region in a haplotype specific way (CGG > TAC; rs780094, rs780095, and rs780096). In addition, the CGG haplotype showed higher binding to FOXA2 and higher levels of the H3K27Ac histone mark. The epigenetic activation of this locus increased the expression of endogenous GCKR in HepG2 cells, confirming that GCKR is the direct target gene of the enhancer. Finally, we confirmed that the CGG haplotype exhibits higher levels of transcription in human liver.

CONCLUSIONS

Our results demonstrate the existence of a liver-specific FOXA2-regulated transcriptional enhancer at an intronic T2D locus represented by rs780094, rs780095, and rs780096 SNPs that increases GCKR expression. Differential haplotype regulation suggests the existence of cis regulatory effects that may contribute to the associated traits at this locus.

Adenovirus-mediated suppression of hypothalamic glucokinase affects feeding behavior

Glucokinase (GK), the hexokinase involved in glucosensing in pancreatic β-cells, is also expressed in arcuate nucleus (AN) neurons and hypothalamic tanycytes, the cells that surround the basal third ventricle (3V). Several lines of evidence suggest that tanycytes may be involved in the regulation of energy homeostasis. Tanycytes have extended cell processes that contact the feeding-regulating neurons in the AN, particularly, agouti-related protein (AgRP), neuropeptide Y (NPY), cocaine- and amphetamine-regulated transcript (CART) and proopiomelanocortin (POMC) neurons. In this study, we developed an adenovirus expressing GK shRNA to inhibit GK expression in vivo. When injected into the 3V of rats, this adenovirus preferentially transduced tanycytes. qRT-PCR and Western blot assays confirmed GK mRNA and protein levels were lower in GK knockdown animals compared to the controls. In response to an intracerebroventricular glucose injection, the mRNA levels of anorexigenic POMC and CART and orexigenic AgRP and NPY neuropeptides were altered in GK knockdown animals. Similarly, food intake, meal duration, frequency of eating events and the cumulative eating time were increased, whereas the intervals between meals were decreased in GK knockdown rats, suggesting a decrease in satiety. Thus, GK expression in the ventricular cells appears to play an important role in feeding behavior.

The GCKR Gene Polymorphism rs780094 is a Risk Factor for Gestational Diabetes in a Brazilian Population

BACKGROUND

The glucokinase regulatory protein (GCKR) regulates the activity of the glucokinase (GCK), which plays a key role in glucose homeostasis. Genetic variants in GCK have been associated with diabetes and gestational diabetes (GDM). Due to the relationship between GCKRP and GCK, polymorphisms in GCKR are also candidates for genetic association with GDM. The aim of this study was to evaluate the association between the GCKR rs780094 polymorphism and GDM in a Brazilian population.

METHODS

252 unrelated Euro-Brazilian pregnant women were classified as control (healthy pregnant women, n = 125) and GDM (pregnant women with GDM, n = 127) age-matched groups. Clinical and anthropometric data were obtained from all subjects. The GCKR rs780094 polymorphism was genotyped using fluorescent probes (TaqMan® , code C_2862873_10).

RESULTS

Both groups were in Hardy-Weinberg equilibrium. The GCKR rs780094 polymorphism was associated with GDM in codominant and dominant models (P = 0.022 and P = 0.010, respectively). The minor allele (T) frequency for the control group in the study was 38.4% (95% CI: 32-44%), similar to frequencies reported for other Caucasian populations.

CONCLUSION

Carriers of the C allele of rs780094 were 1.41 (odds ratio, 95% CI, 0.97-2.03) times more likely to develop GDM.

Multiple Conformations of Gal3 Protein Drive the Galactose-Induced Allosteric Activation of the GAL Genetic Switch of Saccharomyces cerevisiae

Gal3p is an allosteric monomeric protein that activates the GAL genetic switch of Saccharomyces cerevisiae in response to galactose. Expression of constitutive mutant of Gal3p or overexpression of wild-type Gal3p activates the GAL switch in the absence of galactose. These data suggest that Gal3p exists as an ensemble of active and inactive conformations. Structural data have indicated that Gal3p exists in open (inactive) and closed (active) conformations. However, a mutant of Gal3p that predominantly exists in inactive conformation and is yet capable of responding to galactose has not been isolated. To understand the mechanism of allosteric transition, we have isolated a triple mutant of Gal3p with V273I, T404A, and N450D substitutions, which, upon overexpression, fails to activate the GAL switch on its own but activates the switch in response to galactose. Overexpression of Gal3p mutants with single or double mutations in any of the three combinations failed to exhibit the behavior of the triple mutant. Molecular dynamics analysis of the wild-type and the triple mutant along with two previously reported constitutive mutants suggests that the wild-type Gal3p may also exist in super-open conformation. Furthermore, our results suggest that the dynamics of residue F237 situated in the hydrophobic pocket located in the hinge region drives the transition between different conformations. Based on this study, we suggest that conformational selection mechanism is the driving force in the allosteric transition of Gal3p, which may have implications in other signaling pathways involving monomeric proteins.

Genome-wide meta-analysis uncovers novel loci influencing circulating leptin levels

Leptin is an adipocyte-secreted hormone, the circulating levels of which correlate closely with overall adiposity. Although rare mutations in the leptin (LEP) gene are well known to cause leptin deficiency and severe obesity, no common loci regulating circulating leptin levels have been uncovered. Therefore, we performed a genome-wide association study (GWAS) of circulating leptin levels from 32,161 individuals and followed up loci reaching P<10(-6) in 19,979 additional individuals. We identify five loci robustly associated (P<5 × 10(-8)) with leptin levels in/near LEP, SLC32A1, GCKR, CCNL1 and FTO. Although the association of the FTO obesity locus with leptin levels is abolished by adjustment for BMI, associations of the four other loci are independent of adiposity. The GCKR locus was found associated with multiple metabolic traits in previous GWAS and the CCNL1 locus with birth weight. Knockdown experiments in mouse adipose tissue explants show convincing evidence for adipogenin, a regulator of adipocyte differentiation, as the novel causal gene in the SLC32A1 locus influencing leptin levels. Our findings provide novel insights into the regulation of leptin production by adipose tissue and open new avenues for examining the influence of variation in leptin levels on adiposity and metabolic health.

Theoretical investigations on the interactions of glucokinase regulatory protein with fructose phosphates

Glucokinase (GK) plays a critical role in maintaining glucose homeostasis in the human liver and pancreas. In the liver, the activity of GK is modulated by the glucokinase regulatory protein (GKRP) which functions as a competitive inhibitor of glucose to bind to GK. Moreover, the inhibitory intensity of GKRP-GK is suppressed by fructose 1-phosphate (F1P), and reinforced by fructose 6-phosphate (F6P). Here, we employed a series of computational techniques to explore the interactions of fructose phosphates with GKRP. Calculation results reveal that F1P and F6P can bind to the same active site of GKRP with different binding modes, and electrostatic interaction provides a major driving force for the ligand binding. The presence of fructose phosphate severely influences the motions of protein and the conformational space, and the structural change of sugar phosphate influences its interactions with GKRP, leading to a large conformational rearrangement of loop2 in the SIS2 domain. In particular, the binding of F6P to GKRP facilitates the protruding loop2 contacting with GK to form the stable GK-GKRP complex. The conserved residues 179-184 of GKRP play a major role in the binding of phosphate group and maintaining the stability of GKRP. These results may provide deep insight into the regulatory mechanism of GKRP to the activity of GK.

Lifestyle Intervention for Weight Loss and Cardiometabolic Changes in the Setting of Glucokinase Regulatory Protein Inhibition: Glucokinase Regulatory Protein-Leu446Pro Variant in Look AHEAD

BACKGROUND

Glucokinase regulatory protein (GCKR) inhibitors offer a novel treatment approach for glucose control in diabetes mellitus; however, their cardiometabolic effects, particularly in relation to increased triglycerides and C-reactive protein (CRP) levels, are of concern. GCKR Leu446Pro is a common variant associated with reduced GCKR function, increased triglycerides, and CRP.

METHODS AND RESULTS

We investigated whether a 1-year intensive lifestyle intervention (ILI) for weight loss would avert the unfavorable cardiometabolic effects associated with GCKR Leu446Pro when compared with a diabetes mellitus support and education arm in overweight/obese individuals with type 2 diabetes mellitus with triglyceride (n=3214) and CRP (n=1411) data participating in a randomized lifestyle intervention study for weight loss, Action for Health in Diabetes Mellitus (Look AHEAD). Once demographics, medication use and baseline adiposity, and fitness were accounted for, ILI did not modify the baseline association of GCKR-Leu446Pro with elevated triglycerides (β±SE=0.067±0.013, P=1.5×10(-7) and β±SE=0.052±0.015, P=5×10(-4)) or with elevated CRP (β±SE=0.136±0.034, P=5.1×10(-5)and β±SE=0.903±0.038, P=0.015) in the overall sample and Non-Hispanic Whites, respectively. The lack of a protective effect from ILI at 1 year when compared with diabetes mellitus support and education (ILI versus diabetes mellitus support and education interaction for triglyceride and CRP change, respectively: P=0.64 and 0.37 in the overall sample; P=0.27 and 0.05 in Non-Hispanic Whites) persisted after additional adjustment for changes in adiposity and fitness.

CONCLUSIONS

Moderate improvements in adiposity and fitness with ILI did not mitigate the adverse cardiometabolic effects of GCKR inhibition in overweight/obese individuals with diabetes mellitus.

Dietary carbohydrate and control of hepatic gene expression: mechanistic links from ATP and phosphate ester homeostasis to the carbohydrate-response element-binding protein

Type 2 diabetes and non-alcoholic fatty liver disease (NAFLD) are associated with elevated hepatic glucose production and fatty acid synthesis (de novo lipogenesis (DNL)). High carbohydrate diets also increase hepatic glucose production and lipogenesis. The carbohydrate-response element-binding protein (ChREBP, encoded by MLXIPL) is a transcription factor with a major role in the hepatic response to excess dietary carbohydrate. Because its target genes include pyruvate kinase (PKLR) and enzymes of lipogenesis, it is regarded as a key regulator for conversion of dietary carbohydrate to lipid for energy storage. An alternative hypothesis for ChREBP function is to maintain hepatic ATP homeostasis by restraining the elevation of phosphate ester intermediates in response to elevated glucose. This is supported by the following evidence: (i) A key stimulus for ChREBP activation and induction of its target genes is elevation of phosphate esters; (ii) target genes of ChREBP include key negative regulators of the hexose phosphate ester pool (GCKR, G6PC, SLC37A4) and triose phosphate pool (PKLR); (iii) ChREBP knock-down models have elevated hepatic hexose phosphates and triose phosphates and compromised ATP phosphorylation potential; (iv) gene defects in G6PC and SLC37A4 and common variants of MLXIPL, GCKR and PKLR in man are associated with elevated hepatic uric acid production (a marker of ATP depletion) or raised plasma uric acid levels. It is proposed that compromised hepatic phosphate homeostasis is a contributing factor to the elevated hepatic glucose production and lipogenesis that associate with type 2 diabetes, NAFLD and excess carbohydrate in the diet.

Hepatic De Novo Lipogenesis in Obese Youth Is Modulated by a Common Variant in the GCKR Gene

OBJECTIVE

This study's aim was to evaluate whether the GCKR rs1260326 variant increases hepatic de novo lipogenesis (DNL).

SETTING AND DESIGN

To test this hypothesis, 14 adolescents, seven homozygous for the common allele (CC) and seven homozygous for the risk allele (TT), underwent measurement of hepatic DNL during the fasting state and after consumption of a carbohydrate (CHO) drink (75 g glucose and 25 g fructose). DNL was assessed through incorporation of deuterium in the palmitate contained in the very low-density lipoprotein.

RESULTS

Subjects with TT demonstrated higher fasting fractional DNL (P = .036) and a lower increase in fractional DNL after the CHO challenge (P = .016). With regard to absolute lipogenesis, TT subjects had both higher fasting rates (P = .015) and 44% greater area under the curve of absolute lipogenesis during the study (P = .016), compared to CC subjects. Furthermore, subjects carrying the TT genotype showed higher basal rates of glucose oxidation (P = .0028) and a lower ability than CC subjects to increase the rates of glucose oxidation after the CHO load (P = .054).

CONCLUSIONS

This study reports for the first time rates of DNL in obese adolescents and suggests that the GCKR rs1260326 gene variant, which is associated with greater glycolysis, increases hepatic DNL. These data highlight the role of glycolytic carbon flux in liver lipid synthesis and hypertriglyceridemia in these youngsters.

The role of tanycytes in hypothalamic glucosensing

Tanycytes are elongated hypothalamic glial cells that cover the basal walls of the third ventricle; their apical regions contact the cerebrospinal fluid (CSF), and their processes reach hypothalamic neuronal nuclei that control the energy status of an organism. These nuclei maintain the balance between energy expenditure and intake, integrating several peripheral signals and triggering cellular responses that modify the feeding behaviour and peripheral glucose homeostasis. One of the most important and well-studied signals that control this process is glucose; however, the mechanism by which this molecule is sensed remains unknown. We along with others have proposed that tanycytes play a key role in this process, transducing changes in CSF glucose concentration to the neurons that control energy status. Recent studies have demonstrated the expression and function of monocarboxylate transporters and canonical pancreatic β cell glucose sensing molecules, including glucose transporter 2 and glucokinase, in tanycytes. These and other data, which will be discussed in this review, suggest that hypothalamic glucosensing is mediated through a metabolic interaction between tanycytes and neurons through lactate. This article will summarize the recent evidence that supports the importance of tanycytes in hypothalamic glucosensing, and discuss the possible mechanisms involved in this process. Finally, it is important to highlight that a detailed analysis of this mechanism could represent an opportunity to understand the evolution of associated pathologies, including diabetes and obesity, and identify new candidates for therapeutic intervention.

Preserving Mafa expression in diabetic islet β-cells improves glycemic control in vivo

The murine Mafa transcription factor is a key regulator of postnatal islet β-cell activity, affecting insulin transcription, insulin secretion, and β-cell mass. Human MAFA expression is also markedly decreased in islet β-cells of type 2 diabetes mellitus (T2DM) patients. Moreover, levels are profoundly reduced in db/db islet β-cells, a mouse model of T2DM. To examine the significance of this key islet β-cell-enriched protein to glycemic control under diabetic conditions, we generated transgenic mice that conditionally and specifically produced Mafa in db/db islet β-cells. Sustained expression of Mafa resulted in significantly lower plasma glucose levels, higher plasma insulin, and augmented islet β-cell mass. In addition, there was increased expression of insulin, Slc2a2, and newly identified Mafa-regulated genes involved in reducing β-cell stress, like Gsta1 and Gckr. Importantly, the levels of human GSTA1 were also compromised in T2DM islets. Collectively, these results illustrate how consequential the reduction in Mafa activity is to islet β-cell function under pathophysiological conditions.

Molecular pathways in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a clinicopathological change characterized by the accumulation of triglycerides in hepatocytes and has frequently been associated with obesity, type 2 diabetes mellitus, hyperlipidemia, and insulin resistance. It is an increasingly recognized condition that has become the most common liver disorder in developed countries, affecting over one-third of the population and is associated with increased cardiovascular- and liver-related mortality. NAFLD is a spectrum of disorders, beginning as simple steatosis. In about 15% of all NAFLD cases, simple steatosis can evolve into non-alcoholic steatohepatitis, a medley of inflammation, hepatocellular injury, and fibrosis, often resulting in cirrhosis and even hepatocellular cancer. However, the molecular mechanism underlying NAFLD progression is not completely understood. Its pathogenesis has often been interpreted by the “double-hit” hypothesis. The primary insult or the “first hit” includes lipid accumulation in the liver, followed by a “second hit” in which proinflammatory mediators induce inflammation, hepatocellular injury, and fibrosis. Nowadays, a more complex model suggests that fatty acids (FAs) and their metabolites may be the true lipotoxic agents that contribute to NAFLD progression; a multiple parallel hits hypothesis has also been suggested. In NAFLD patients, insulin resistance leads to hepatic steatosis via multiple mechanisms. Despite the excess hepatic accumulation of FAs in NAFLD, it has been described that not only de novo FA synthesis is increased, but FAs are also taken up from the serum. Furthermore, a decrease in mitochondrial FA oxidation and secretion of very-low-density lipoproteins has been reported. This review discusses the molecular mechanisms that underlie the pathophysiological changes of hepatic lipid metabolism that contribute to NAFLD.

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.

Dynamic localization of glucokinase and its regulatory protein in hypothalamic tanycytes

Glucokinase (GK), the hexokinase involved in glucose sensing in pancreatic β cells, is also expressed in hypothalamic tanycytes, which cover the ventricular walls of the basal hypothalamus and are implicated in an indirect control of neuronal activity by glucose. Previously, we demonstrated that GK was preferentially localized in tanycyte nuclei in euglycemic rats, which has been reported in hepatocytes and is suggestive of the presence of the GK regulatory protein, GKRP. In the present study, GK intracellular localization in hypothalamic and hepatic tissues of the same rats under several glycemic conditions was compared using confocal microscopy and Western blot analysis. In the hypothalamus, increased GK nuclear localization was observed in hyperglycemic conditions; however, it was primarily localized in the cytoplasm in hepatic tissue under the same conditions. Both GK and GKRP were next cloned from primary cultures of tanycytes. Expression of GK by Escherichia coli revealed a functional cooperative protein with a S_0.5 of 10 mM. GKRP, expressed in Saccharomyces cerevisiae, inhibited GK activity in vitro with a K_i 0.2 µM. We also demonstrated increased nuclear reactivity of both GK and GKRP in response to high glucose concentrations in tanycyte cultures. These data were confirmed using Western blot analysis of nuclear extracts. Results indicate that GK undergoes short-term regulation by nuclear compartmentalization. Thus, in tanycytes, GK can act as a molecular switch to arrest cellular responses to increased glucose.

Normal glucose metabolism in carnivores overlaps with diabetes pathology in non-carnivores

Carnivores, such as the dolphin and the domestic cat, have numerous adaptations that befit consumption of diets with high protein and fat content, with little carbohydrate content. Consequently, nutrient metabolism in carnivorous species differs substantially from that of non-carnivores. Important metabolic pathways known to differ between carnivores and non-carnivores are implicated in the development of diabetes and insulin resistance in non-carnivores: (1) the hepatic glucokinase (GCK) pathway is absent in healthy carnivores yet GCK deficiency may result in diabetes in rodents and humans, (2) healthy dolphins and cats are prone to periods of fasting hyperglycemia and exhibit insulin resistance, both of which are risk factors for diabetes in non-carnivores. Similarly, carnivores develop naturally occurring diseases such as hemochromatosis, fatty liver, obesity, and diabetes that have strong parallels with the same disorders in humans. Understanding how evolution, environment, diet, and domestication may play a role with nutrient metabolism in the dolphin and cat may also be relevant to human diabetes.

Identification of the ubiquitin-like domain of midnolin as a new glucokinase interaction partner

Glucokinase acts as a glucose sensor in pancreatic beta cells. Its posttranslational regulation is important but not yet fully understood. Therefore, a pancreatic islet yeast two-hybrid library was produced and searched for glucokinase-binding proteins. A protein sequence containing a full-length ubiquitin-like domain was identified to interact with glucokinase. Mammalian two-hybrid and fluorescence resonance energy transfer analyses confirmed the interaction between glucokinase and the ubiquitin-like domain in insulin-secreting MIN6 cells and revealed the highest binding affinity at low glucose. Overexpression of parkin, an ubiquitin E3 ligase exhibiting an ubiquitin-like domain with high homology to the identified, diminished insulin secretion in MIN6 cells but had only some effect on glucokinase activity. Overexpression of the elucidated ubiquitin-like domain or midnolin, containing exactly this ubiquitin-like domain, significantly reduced both intrinsic glucokinase activity and glucose-induced insulin secretion. Midnolin has been to date classified as a nucleolar protein regulating mouse development. However, we could not confirm localization of midnolin in nucleoli. Fluorescence microscopy analyses revealed localization of midnolin in nucleus and cytoplasm and co-localization with glucokinase in pancreatic beta cells. In addition we could show that midnolin gene expression in pancreatic islets is up-regulated at low glucose and that the midnolin protein is highly expressed in pancreatic beta cells and also in liver, muscle, and brain of the adult mouse and cell lines of human and rat origin. Thus, the results of our study suggest that midnolin plays a role in cellular signaling of adult tissues and regulates glucokinase enzyme activity in pancreatic beta cells.

Hexokinase and not glycogen synthase controls the flux through the glycogen synthesis pathway in frog oocytes

Here we set out to evaluate the role of hexokinase and glycogen synthase in the control of glycogen synthesis in vivo. We used metabolic control analysis (MCA) to determine the flux control coefficient for each of the enzymes involved in the pathway. Acute microinjection experiments in frog oocytes were specifically designed to change the endogenous activities of the enzymes, either by directly injecting increasing amounts of a given enzyme (HK, PGM and UGPase) or by microinjection of a positive allosteric effector (glc-6P for GS). Values of 0.61 ± 0.07, 0.19 ± 0.03, 0.13 ± 0.03, and -0.06 ± 0.08 were obtained for the flux control coefficients of hexokinase EC 2.7.1.1 (HK), phosphoglucomutase EC 5.4.2.1 (PGM), UDPglucose pyrophosphorylase EC 2.7.7.9 (UGPase) and glycogen synthase EC 2.4.1.11 (GS), respectively. These values satisfy the summation theorem since the sum of the control coefficients for all the enzymes of the pathway is 0.87. The results show that, in frog oocytes, glycogen synthesis through the direct pathway is under the control of hexokinase. Phosphoglucomutase and UDPG-pyrophosphorylase have a modest influence, while the control exerted by glycogen synthase is null.

GKAs for diabetes therapy: why no clinically useful drug after two decades of trying?

Results of basic biochemical and physiological research, strongly endorsed by findings in human pathophysiology and genetics, had characterized the glucose phosphorylating enzyme glucokinase as a critical player in normal glucose homeostasis, diabetes mellitus, and hyperinsulinemic hypoglycemia, and identified the enzyme as a promising new drug target. R&D initiated in the early 1990s and directed at this target discovered glucokinase activators (GKAs) as a new class of potentially antidiabetic drugs. GKAs were characterized as nonessential allosteric activators that increase glucose affinity and V(max) of the enzyme, thus stimulating glucose metabolism in glucokinase expressing tissue, of foremost functional significance in the insulin producing pancreatic beta cells and the liver. The results of preclinical testing of GKAs by many pharmaceutical companies demonstrated uniformly high hypoglycemic efficacy in normal and diabetic animals. GKAs were also highly effective in Phase I trials in patients with type 2 diabetes mellitus (T2DM). However, results of a recent Phase II trial were less encouraging because patients developed hyperlipidemia and vascular hypertension, and the drug lost efficacy within several months. This outcome is prompting a reappraisal of the GKA strategy. In this opinion article, the 'pros and cons' of the strategy to use these compounds in diabetes management are critically reexamined and suggestions are made that might facilitate progress of GKA R&D that could still result in a novel antidiabetic medicine.

High-carbohydrate diets induce hepatic insulin resistance to protect the liver from substrate overload

In population studies hepatic steatosis in subjects with Non-alcoholic fatty liver disease (NAFLD) is strongly associated with insulin resistance. This association has encouraged debate whether hepatic steatosis is the cause or the consequence of hepatic insulin resistance? Although genome-wide studies have identified several gene variants associated with either hepatic steatosis or type 2 diabetes, no variants have been identified associated with both hepatic steatosis and insulin resistance. Here, the hypothesis is proposed that high-carbohydrate diets contribute to the association between hepatic steatosis and insulin resistance through activation of the transcription factor ChREBP (Carbohydrate response element binding protein). Postprandial hyperglycaemia raises the hepatic concentrations of phosphorylated intermediates causing activation of ChREBP and induction of its target genes. These include not only enzymes of glycolysis and lipogenesis that predispose to hepatic steatosis but also glucose 6-phosphatase (G6PC) that catalyses the final reaction in glucose production and GCKR, the inhibitor of hepatic glucokinase that curtails hepatic glucose uptake. Induction of G6PC and GCKR manifests as hepatic glucose intolerance or insulin resistance. Induction of these two genes by high glucose serves to safeguard intrahepatic homeostasis of phosphorylated intermediates. The importance of GCKR in this protective mechanism is supported by "less-active" GCKR variants in association not only with hepatic steatosis and hyperuricaemia but also with lower fasting plasma glucose and decreased insulin resistance. This supports a role for GCKR in restricting hepatic glucose phosphorylation to maintain intrahepatic homeostasis. Pharmacological targeting of the glucokinase-GCKR interaction can favour either glucose clearance by the liver or intrahepatic metabolite homeostasis.

Hyperglycemia and a common variant of GCKR are associated with the levels of eight amino acids in 9,369 Finnish men

We investigated the association of glycemia and 43 genetic risk variants for hyperglycemia/type 2 diabetes with amino acid levels in the population-based Metabolic Syndrome in Men (METSIM) Study, including 9,369 nondiabetic or newly diagnosed type 2 diabetic Finnish men. Plasma levels of eight amino acids were measured with proton nuclear magnetic resonance spectroscopy. Increasing fasting and 2-h plasma glucose levels were associated with increasing levels of several amino acids and decreasing levels of histidine and glutamine. Alanine, leucine, isoleucine, tyrosine, and glutamine predicted incident type 2 diabetes in a 4.7-year follow-up of the METSIM Study, and their effects were largely mediated by insulin resistance (except for glutamine). We also found significant correlations between insulin sensitivity (Matsuda insulin sensitivity index) and mRNA expression of genes regulating amino acid degradation in 200 subcutaneous adipose tissue samples. Only 1 of 43 risk single nucleotide polymorphisms for type 2 diabetes or hyperglycemia, the glucose-increasing major C allele of rs780094 of GCKR, was significantly associated with decreased levels of alanine and isoleucine and elevated levels of glutamine. In conclusion, the levels of branched-chain, aromatic amino acids and alanine increased and the levels of glutamine and histidine decreased with increasing glycemia, reflecting, at least in part, insulin resistance. Only one single nucleotide polymorphism regulating hyperglycemia was significantly associated with amino acid levels.

Liver Glucokinase(A456V) Induces Potent Hypoglycemia without Dyslipidemia through a Paradoxical Induction of the Catalytic Subunit of Glucose-6-Phosphatase

Recent reports point out the importance of the complex GK-GKRP in controlling glucose and lipid homeostasis. Several GK mutations affect GKRP binding, resulting in permanent activation of the enzyme. We hypothesize that hepatic overexpression of a mutated form of GK, GK(A456V), described in a patient with persistent hyperinsulinemic hypoglycemia of infancy (PHHI) and could provide a model to study the consequences of GK-GKRP deregulation in vivo. GK(A456V) was overexpressed in the liver of streptozotocin diabetic mice. Metabolite profiling in serum and liver extracts, together with changes in key components of glucose and lipid homeostasis, were analyzed and compared to GK wild-type transfected livers. Cell compartmentalization of the mutant but not the wild-type GK was clearly affected in vivo, demonstrating impaired GKRP regulation. GK(A456V) overexpression markedly reduced blood glucose in the absence of dyslipidemia, in contrast to wild-type GK-overexpressing mice. Evidence in glucose utilization did not correlate with increased glycogen nor lactate levels in the liver. PEPCK mRNA was not affected, whereas the mRNA for the catalytic subunit of glucose-6-phosphatase was upregulated ~4 folds in the liver of GK(A456V)-treated animals, suggesting that glucose cycling was stimulated. Our results provide new insights into the complex GK regulatory network and validate liver-specific GK activation as a strategy for diabetes therapy.

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.

GWAS of butyrylcholinesterase activity identifies four novel loci, independent effects within BCHE and secondary associations with metabolic risk factors

Serum butyrylcholinesterase (BCHE) activity is associated with obesity, blood pressure and biomarkers of cardiovascular and diabetes risk. We have conducted a genome-wide association scan to discover genetic variants affecting BCHE activity, and to clarify whether the associations between BCHE activity and cardiometabolic risk factors are caused by variation in BCHE or whether BCHE variation is secondary to the metabolic abnormalities. We measured serum BCHE in adolescents and adults from three cohorts of Australian twin and family studies. The genotypes from ∼2.4 million single-nucleotide polymorphisms (SNPs) were available in 8791 participants with BCHE measurements. We detected significant associations with BCHE activity at three independent groups of SNPs at the BCHE locus (P = 5.8 × 10(-262), 7.8 × 10(-47), 2.9 × 10(-12)) and at four other loci: RNPEP (P = 9.4 × 10(-16)), RAPH1-ABI2 (P = 4.1 × 10(-18)), UGT1A1 (P = 4.0 × 10(-8)) and an intergenic region on chromosome 8 (P = 1.4 × 10(-8)). These loci affecting BCHE activity were not associated with metabolic risk factors. On the other hand, SNPs in genes previously associated with metabolic risk had effects on BCHE activity more often than can be explained by chance. In particular, SNPs within FTO and GCKR were associated with BCHE activity, but their effects were partly mediated by body mass index and triglycerides, respectively. We conclude that variation in BCHE activity is due to multiple variants across the spectrum from uncommon/large effect to common/small effect, and partly results from (rather than causes) metabolic abnormalities.

Effects of 34 risk loci for type 2 diabetes or hyperglycemia on lipoprotein subclasses and their composition in 6,580 nondiabetic Finnish men

OBJECTIVE

We investigated the effects of 34 genetic risk variants for hyperglycemia/type 2 diabetes on lipoprotein subclasses and particle composition in a large population-based cohort.

RESEARCH DESIGN AND METHODS

The study included 6,580 nondiabetic Finnish men from the population-based Metabolic Syndrome in Men (METSIM) study (aged 57 ± 7 years; BMI 26.8 ± 3.7 kg/m(2)). Genotyping of 34 single nucleotide polymorphism (SNPs) for hyperglycemia/type 2 diabetes was performed. Proton nuclear magnetic resonance spectroscopy was used to measure particle concentrations of 14 lipoprotein subclasses and their composition in native serum samples.

RESULTS

The glucose-increasing allele of rs780094 in GCKR was significantly associated with low concentrations of VLDL particles (independently of their size) and small LDL and was nominally associated with low concentrations of intermediate-density lipoprotein, all LDL subclasses, and high concentrations of very large and large HDL particles. The glucose-increasing allele of rs174550 in FADS1 was significantly associated with high concentrations of very large and large HDL particles and nominally associated with low concentrations of all VLDL particles. SNPs rs10923931 in NOTCH2 and rs757210 in HNF1B genes showed nominal or significant associations with several lipoprotein traits. The genetic risk score of 34 SNPs was not associated with any of the lipoprotein subclasses.

CONCLUSIONS

Four of the 34 risk loci for type 2 diabetes or hyperglycemia (GCKR, FADS1, NOTCH2, and HNF1B) were significantly associated with lipoprotein traits. A GCKR variant predominantly affected the concentration of VLDL, and the FADS1 variant affected very large and large HDL particles. Only a limited number of risk loci for hyperglycemia/type 2 diabetes significantly affect lipoprotein metabolism.

Variants of GCKR affect both β-cell and kidney function in patients with newly diagnosed type 2 diabetes: the Verona newly diagnosed type 2 diabetes study 2

OBJECTIVE

In genome-wide association studies, performed mostly in nondiabetic individuals, genetic variability of glucokinase regulatory protein (GCKR) affects type 2 diabetes-related phenotypes, kidney function, and risk of chronic kidney disease (CKD). We tested whether GCKR variability affects type 2 diabetes or kidney-related phenotypes in newly diagnosed type 2 diabetes.

RESEARCH DESIGN AND METHODS

In 509 GAD-negative patients with newly diagnosed type 2 diabetes, we 1) genotyped six single nucleotide polymorphisms in GCKR genomic region: rs6717980, rs1049817, rs6547626, rs780094, rs2384628, and rs8731; 2) assessed clinical phenotypes, insulin sensitivity by the euglycemic insulin clamp, and β-cell function by state-of-the-art modeling of glucose/C-peptide curves during an oral glucose tolerance test; and 3) estimated glomerular filtration rate (eGFR) by the Modification of Diet in Renal Disease formula.

RESULTS

The major alleles of rs6717980 and rs2384628 were associated with reduced β-cell function (P < 0.05), with mutual additive effects of each variant (P < 0.01). The minor alleles of rs1049817 and rs6547626 and the major allele of rs780094 were associated with reduced eGFR according to a recessive model (P < 0.03), but with no mutual additive effects of the variants. Additional associations were found between rs780094 and 2-h plasma glucose (P < 0.05) and rs8731 and insulin sensitivity (P < 0.05) and triglycerides (P < 0.05).

CONCLUSIONS

Our findings are compatible with the idea that GCKR variability may play a pathogenetic role in both type 2 diabetes and CKD. Genotyping GCKR in patients with newly diagnosed type 2 diabetes might help in identifying patients at high risk for metabolic derangements or CKD.

Fructose induced lipogenesis: from sugar to fat to insulin resistance

Increasing consumption of sugars is one of the contributing factors to the obesity epidemic. Both cane sugar and high-fructose corn syrup contain glucose and fructose. Fructose, in contrast to glucose, is known to potently stimulate lipogenesis, but the mechanisms responsible are not yet fully known. This paper reviews several possible pathways that might be involved, such as activation of pyruvate dehydrogenase, and transcriptional activation of sterol regulatory element binding protein 1c by key regulators such as peroxisome proliferator activated receptor-γ co-activator 1β and the splice variant of X-box binding protein 1. Together, these pathways might establish a feed forward cycle that can rapidly increase hepatic lipogenesis. As a result, dietary fructose might promote the development of nonalcoholic fatty liver disease, which in and of itself, can result in hepatic insulin resistance, a key feature of type 2 diabetes mellitus.

Regulation of liver glucokinase activity in rats with fructose-induced insulin resistance and impaired glucose and lipid metabolism

We evaluated the relative role of different regulatory mechanisms, particularly 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase (PFK2/FBPase-2), in liver glucokinase (GK) activity in intact animals with fructose-induced insulin resistance and impaired glucose and lipid metabolism. We measured blood glucose, triglyceride and insulin concentration, glucose tolerance, liver triglyceride content, GK activity, and GK and PFK2 protein and gene expression in fructose-rich diet (FRD) and control rats. After 3 weeks, FRD rats had significantly higher blood glucose, insulin and triglyceride levels, and liver triglyceride content, insulin resistance, and impaired glucose tolerance. FRD rats also had significantly higher GK activity in the cytosolic fraction (18.3 ± 0.35 vs. 11.27 ± 0.34 mU/mg protein). Differences in GK protein concentration (116% and 100%) were not significant, suggesting a potentially impaired GK translocation in FRD rats. Although GK transcription level was similar, PFK2 gene expression and protein concentration were 4- and 5-fold higher in the cytosolic fraction of FRD animals. PFK2 immunological blockage significantly decreased GK activity in control and FRD rats; in the latter, this blockage decreased GK activity to control levels. Results suggest that increased liver GK activity might participate in the adaptative response to fructose overload to maintain glucose/triglyceride homeostasis in intact animals. Under these conditions, PFK2 increase would be the main enhancer of GK activity.

Brought to life: targeted activation of enzyme function with small molecules

Cell-permeable small molecules that are capable of activating particular enzymes would be invaluable tools for studying protein function in complex cell-signaling cascades. But, is it feasible to identify compounds that allow chemical-biology researchers to activate specific enzymes in a cellular context? In this review, we describe some recent advances in achieving targeted enzyme activation with small molecules. In addition to surveying progress in the identification and targeting of enzymes that contain natural allosteric-activation sites, we focus on recently developed protein-engineering strategies that allow researchers to render an enzyme of interest "activatable" by a pre-chosen compound. Three distinct strategies for targeting an engineered enzyme are discussed: direct chemical "rescue" of an intentionally inactivated enzyme, activation of an enzyme by targeting a de novo small-molecule-binding site, and the generation of activatable enzymes via fusion of target enzymes to previously characterized small-molecule-binding domains.

Lack of glucokinase regulatory protein expression may contribute to low glucokinase activity in feline liver

In most mammals, glucokinase (GK) acts as a hepatic "glucose sensor" that permits hepatic metabolism to respond appropriately to changes in plasma glucose concentrations. GK activity is potently regulated by the glucokinase regulatory protein (GKRP), which is encoded by the GCKR gene. GKRP binds GK in the nucleus and inhibits its activity. GK becomes active when it is released from GKRP and translocates to the cytosol. Low glucokinase (GK) activity is reported to be a principal feature of feline hepatic carbohydrate metabolism but the molecular pathways that regulate GK activity are not known. This study examined the hypothesis that species-specific differences in GKRP expression parallel the low GK activity observed in feline liver. Hepatic GKRP expression was examined using RT-PCR, immunoblot, and confocal immunomicroscopy. The results show that the GCKR gene is present in the feline genome but GCKR mRNA and the GKRP protein were absent in feline liver. The lack of GKRP expression in feline liver indicates that the low GK activity cannot be the result of GKRP-mediated inhibition of the GK enzyme. However, the absence of the permissive effects of GCKR expression on GK expression and activity may contribute to reduced GK enzyme activity in feline liver. The study results show that the cat is a natural model for GCKR knockout and may be useful to study regulation of GCKR expression and its role in hepatic glucose-sensing and carbohydrate metabolism.

Glucokinase and molecular aspects of liver glycogen metabolism

Conversion of glucose into glycogen is a major pathway that contributes to the removal of glucose from the portal vein by the liver in the postprandial state. It is regulated in part by the increase in blood-glucose concentration in the portal vein, which activates glucokinase, the first enzyme in the pathway, causing an increase in the concentration of glucose 6-P (glucose 6-phosphate), which modulates the phosphorylation state of downstream enzymes by acting synergistically with other allosteric effectors. Glucokinase is regulated by a hierarchy of transcriptional and post-transcriptional mechanisms that are only partially understood. In the fasted state, glucokinase is in part sequestered in the nucleus in an inactive state, complexed to a specific regulatory protein, GKRP (glucokinase regulatory protein). This reserve pool is rapidly mobilized to the cytoplasm in the postprandial state in response to an elevated concentration of glucose. The translocation of glucokinase between the nucleus and cytoplasm is modulated by various metabolic and hormonal conditions. The elevated glucose 6-P concentration, consequent to glucokinase activation, has a synergistic effect with glucose in promoting dephosphorylation (inactivation) of glycogen phosphorylase and inducing dephosphorylation (activation) of glycogen synthase. The latter involves both a direct ligand-induced conformational change and depletion of the phosphorylated form of glycogen phosphorylase, which is a potent allosteric inhibitor of glycogen synthase phosphatase activity associated with the glycogen-targeting protein, GL [hepatic glycogen-targeting subunit of PP-1 (protein phosphatase-1) encoded by PPP1R3B]. Defects in both the activation of glucokinase and in the dephosphorylation of glycogen phosphorylase are potential contributing factors to the dysregulation of hepatic glucose metabolism in Type 2 diabetes.

The common P446L polymorphism in GCKR inversely modulates fasting glucose and triglyceride levels and reduces type 2 diabetes risk in the DESIR prospective general French population

OBJECTIVE

Hepatic glucokinase (GCK) is a key regulator of glucose storage and disposal in the liver, where its activity is competitively modulated, with respect to glucose, by binding to glucokinase regulatory protein (GCKR) in the presence of fructose 6-phosphate. Genome-wide association studies for type 2 diabetes identified GCKR as a potential locus for modulating triglyceride levels. We evaluated, in a general French population, the contribution of the GCKR rs1260326-P446L polymorphism to quantitative metabolic parameters and to dyslipidemia and hyperglycemia risk.

RESEARCH DESIGN AND METHODS

Genotype effects of rs1260326 were studied in 4,833 participants from the prospective DESIR (Data from an Epidemiological Study on the Insulin Resistance syndrome) cohort both at inclusion and using the measurements at follow-up.

RESULTS

The minor T-allele of rs1260326 was strongly associated with lower fasting glucose (-1.43% per T-allele; P = 8 x 10(-13)) and fasting insulin levels (-4.23%; P = 3 x 10(-7)), lower homeostasis model assessment of insulin resistance index (-5.69%; P = 1 x 10(-8)), and, conversely, higher triglyceride levels (3.41%; P = 1 x 10(-4)) during the 9-year study. These effects relate to a lower risk of hyperglycemia (odds ratio [OR] 0.79 [95% CI 0.70-0.88]; P = 4 x 10(-5)) and of incident cases during the study (hazard ratio [HR] 0.83 [0.74-0.95]; P = 0.005). Moreover, an additive effect of GCKR rs1260326(T) and GCK (-30G) alleles conferred lower fasting glycemia (P = 1 x 10(-13)), insulinemia (P = 5 x 10(-6)), and hyperglycemia risk (P = 1 x 10(-6)).

CONCLUSIONS

GCKR-L446 carriers are protected against type 2 diabetes despite higher triglyceride levels and risk of dyslipidemia, which suggests a potential molecular mechanism by which these two components of the metabolic syndrome can be dissociated.

Genetic and environmental interactions in obesity and type 2 diabetes

Obesity and type 2 diabetes are increasing in prevalence, bringing with them considerable morbidity and mortality. The pathophysiology of each condition is quite complex and incompletely understood. Many hormone-like signaling agents that affect energy metabolism are released from adipocytes (eg, leptin) and from immune cells (eg, tumor necrosis factor-alpha). Type 2 diabetes and obesity are each characterized by the strong interaction of genetics and environment over time. A few single-gene defects have been found that lead to these conditions, but in most cases there is a polygenic contribution. In addition, several environmental factors such as lifestyle, social influences, and fetal surroundings may lead to development of these health problems. Genetic and environmental factors associated with these disorders are explored.