Identification of fructose 6-phosphate- and fructose 1-phosphate-binding residues in the regulatory protein of glucokinase

Knockdown of ketohexokinase versus inhibition of its kinase activity exert divergent effects on fructose metabolism

Excessive fructose intake is a risk factor for the development of obesity and its complications. Targeting ketohexokinase (KHK), the first enzyme of fructose metabolism, has been investigated for the management of metabolic dysfunction-associated steatotic liver disease (MASLD). We compared the effects of systemic, small molecule inhibitor of KHK enzymatic activity with hepatocyte-specific, N-acetylgalactosamine siRNA-mediated knockdown of KHK in mice on an HFD. We measured KHK enzymatic activity, extensively quantified glycogen accumulation, performed RNA-Seq analysis, and enumerated hepatic metabolites using mass spectrometry. Both KHK siRNA and KHK inhibitor led to an improvement in liver steatosis; however, via substantially different mechanisms, KHK knockdown decreased the de novo lipogenesis pathway, whereas the inhibitor increased the fatty acid oxidation pathway. Moreover, KHK knockdown completely prevented hepatic fructolysis and improved glucose tolerance. Conversely, the KHK inhibitor only partially reduced fructolysis, but it also targeted triokinase, mediating the third step of fructolysis. This led to the accumulation of fructose-1 phosphate, resulting in glycogen accumulation, hepatomegaly, and impaired glucose tolerance. Overexpression of wild-type, but not kinase-dead, KHK in cultured hepatocytes increased hepatocyte injury and glycogen accumulation after treatment with fructose. The differences between KHK inhibition and knockdown are, in part, explained by the kinase-dependent and -independent effects of KHK on hepatic metabolism.

Hepatic ChREBP orchestrates intrahepatic carbohydrate metabolism to limit hepatic glucose 6-phosphate and glycogen accumulation in a mouse model for acute Glycogen Storage Disease type Ib

OBJECTIVE

Carbohydrate Response Element Binding Protein (ChREBP) is a glucose 6-phosphate (G6P)-sensitive transcription factor that acts as a metabolic switch to maintain intracellular glucose and phosphate homeostasis. Hepatic ChREBP is well-known for its regulatory role in glycolysis, the pentose phosphate pathway, and de novo lipogenesis. The physiological role of ChREBP in hepatic glycogen metabolism and blood glucose regulation has not been assessed in detail, and ChREBP's contribution to carbohydrate flux adaptations in hepatic Glycogen Storage Disease type 1 (GSD I) requires further investigation.

METHODS

The current study aimed to investigate the role of ChREBP as a regulator of glycogen metabolism in response to hepatic G6P accumulation, using a model for acute hepatic GSD type Ib. The immediate biochemical and regulatory responses to hepatic G6P accumulation were evaluated upon G6P transporter inhibition by the chlorogenic acid S4048 in mice that were either treated with a short hairpin RNA (shRNA) directed against ChREBP (shChREBP) or a scrambled shRNA (shSCR). Complementary stable isotope experiments were performed to quantify hepatic carbohydrate fluxes in vivo.

RESULTS

ShChREBP treatment normalized the S4048-mediated induction of hepatic ChREBP target genes to levels observed in vehicle- and shSCR-treated controls. In parallel, hepatic shChREBP treatment in S4048-infused mice resulted in a more pronounced accumulation of hepatic glycogen and further reduction of blood glucose levels compared to shSCR treatment. Hepatic ChREBP knockdown modestly increased glucokinase (GCK) flux in S4048-treated mice while it enhanced UDP-glucose turnover as well as glycogen synthase and phosphorylase fluxes. Hepatic GCK mRNA and protein levels were induced by shChREBP treatment in both vehicle- and S4048-treated mice, while glycogen synthase 2 (GYS2) and glycogen phosphorylase (PYGL) mRNA and protein levels were reduced. Finally, knockdown of hepatic ChREBP expression reduced starch domain binding protein 1 (STBD1) mRNA and protein levels while it inhibited acid alpha-glucosidase (GAA) activity, suggesting reduced capacity for lysosomal glycogen breakdown.

CONCLUSIONS

Our data show that ChREBP activation controls hepatic glycogen and blood glucose levels in acute hepatic GSD Ib through concomitant regulation of glucose phosphorylation, glycogenesis, and glycogenolysis. ChREBP-mediated control of GCK enzyme levels aligns with corresponding adaptations in GCK flux. In contrast, ChREBP activation in response to acute hepatic GSD Ib exerts opposite effects on GYS2/PYGL enzyme levels and their corresponding fluxes, indicating that GYS2/PYGL expression levels are not limiting to their respective fluxes under these conditions.

Insulin Regulation of Hepatic Lipid Homeostasis

The incidence of obesity, insulin resistance, and type II diabetes (T2DM) continues to rise worldwide. The liver is a central insulin-responsive metabolic organ that governs whole-body metabolic homeostasis. Therefore, defining the mechanisms underlying insulin action in the liver is essential to our understanding of the pathogenesis of insulin resistance. During periods of fasting, the liver catabolizes fatty acids and stored glycogen to meet the metabolic demands of the body. In postprandial conditions, insulin signals to the liver to store excess nutrients into triglycerides, cholesterol, and glycogen. In insulin-resistant states, such as T2DM, hepatic insulin signaling continues to promote lipid synthesis but fails to suppress glucose production, leading to hypertriglyceridemia and hyperglycemia. Insulin resistance is associated with the development of metabolic disorders such as cardiovascular and kidney disease, atherosclerosis, stroke, and cancer. Of note, nonalcoholic fatty liver disease (NAFLD), a spectrum of diseases encompassing fatty liver, inflammation, fibrosis, and cirrhosis, is linked to abnormalities in insulin-mediated lipid metabolism. Therefore, understanding the role of insulin signaling under normal and pathologic states may provide insights into preventative and therapeutic opportunities for the treatment of metabolic diseases. Here, we provide a review of the field of hepatic insulin signaling and lipid regulation, including providing historical context, detailed molecular mechanisms, and address gaps in our understanding of hepatic lipid regulation and the derangements under insulin-resistant conditions. © 2023 American Physiological Society. Compr Physiol 13:4785-4809, 2023.

The GCKR-P446L gene variant predisposes to raised blood cholesterol and lower blood glucose in the P446L mouse-a model for GCKR rs1260326

OBJECTIVES

The Glucokinase Regulatory Protein GKRP, encoded by GCKR, enables acute regulation of liver glucokinase to support metabolic demand. The common human GCKR rs1260326:Pro446 > Leu variant within a large linkage disequilibrium region associates with pleiotropic traits including lower Type 2 diabetes risk and raised blood triglycerides and cholesterol. Whether the GCKR-P446 > L substitution is causal to the raised lipids is unknown. We determined whether mouse GKRP phenocopies the human GKRP:P446 > L substitution and studied a GKRP:P446L knockin mouse to identify physiological consequences to P446 > L.

METHODS

GKRP-deficient hepatocytes were transfected with adenoviral vectors for human or mouse GKRP:446 P or 446 L for cellular comprehensive analysis including transcriptomics consequent to P446 > L. Physiological traits in the diet-challenged P446L mouse were compared with pleiotropic associations at the human rs1260326 locus. Transcriptomics was compared in P446L mouse liver with hepatocytes overexpressing glucokinase or GKRP:446 P/L.

RESULTS

1. P446 > L substitution in mouse or human GKRP similarly compromises protein expressivity of GKRP:446 L, nuclear sequestration of glucokinase and counter-regulation of gene expression. 2. The P446L knockin mouse has lower liver glucokinase and GKRP protein similar to human liver homozygous for rs1260326-446 L. 3. The diet-challenged P446L mouse has lower blood glucose, raised blood cholesterol and altered hepatic cholesterol homeostasis consistent with relative glucokinase-to-GKRP excess, but not raised blood triglycerides.

CONCLUSIONS

Mouse GKRP phenocopies the human GKRP:P446 > L substitution despite the higher affinity for glucokinase of human GKRP. The diet-challenged P446L mouse replicates several traits found in association with the rs1260326 locus on chromosome 2 including raised blood cholesterol, lower blood glucose and lower liver glucokinase and GKRP protein but not raised blood triglycerides.

Pleiotropic Effects of Common and Rare GCKR Exonic Mutations on Cardiometabolic Traits

Background: The common non-synonymous mutation of the glucokinase regulator (GCKR) gene, namely rs1260326, is widely reported to have pleiotropic effects on cardio-metabolic traits and hematological parameters. Objective: This study aimed to identify whether other GCKR variants may have pleiotropic effects independent of the rs1260326 genotypes. Methods: In total, 81,097 Taiwan Biobank participants were enrolled for the regional plot association studies and candidate variant analysis of the region around the GCKR gene. Results: The initial candidate variant approach showed the significant association of the rs1260326 genotypes with multiple phenotypes. Regional plot association analysis of the GCKR gene region further revealed genome-wide significant associations between GCKR variants and serum total and low-density lipoprotein cholesterol; triglyceride, uric acid, creatinine, aspartate aminotransferase, γ-Glutamyl transferase, albumin, and fasting plasma glucose levels; estimated glomerular filtration rate; leukocyte and platelet counts; microalbuminuria, and metabolic syndrome, with rs1260326 being the most common lead polymorphism. Serial conditional analysis identified genome-wide significant associations of two low-frequency exonic mutations, rs143881585 and rs8179206, with high serum triglyceride and albumin levels. In five rare GCKR exonic non-synonymous or nonsense mutations available for analysis, GCKR rs146175795 showed an independent association with serum triglyceride and albumin levels and rs150673460 showed an independent association with serum triglyceride levels. Weighted genetic risk scores from the combination of GCKR rs143881585 and rs146175795 revealed a significant association with metabolic syndrome. Conclusion: In addition to the rs1260326 variant, low-frequency and rare GCKR exonic mutations exhibit pleiotropic effects on serum triglyceride and albumin levels and the risk of metabolic syndrome. These results provide evidence that both common and rare GCKR variants may play a critical role in predicting the risk of cardiometabolic disorders.

Triose Kinase Controls the Lipogenic Potential of Fructose and Dietary Tolerance

The surge in fructose consumption is a major factor behind the rapid rise of nonalcoholic fatty liver disease in modern society. Through flux and genetic analyses, we demonstrate that fructose is catabolized at a much higher rate than glucose, and triose kinase (TK) couples fructolysis with lipogenesis metabolically and transcriptionally. In the absence of TK, fructose oxidation is accelerated through the activation of aldehyde dehydrogenase (ALDH) and serine biosynthesis, accompanied by increased oxidative stress and fructose aversion. TK is also required by the endogenous fructolysis pathway to drive lipogenesis and hepatic triglyceride accumulation under high-fat diet and leptin-deficient conditions. Intriguingly, a nonsynonymous TK allele (rs2260655_A) segregated during human migration out of Africa behaves as TK null for its inability to rescue fructose toxicity and increase hepatic triglyceride accumulation. Therefore, we posit TK as a metabolic switch controlling the lipogenic potential of fructose and its dietary tolerance.

Genetic Susceptibility to Chronic Liver Disease in Individuals from Pakistan

Chronic liver disease, with viral or non-viral etiology, is endemic in many countries and is a growing burden in Asia. Among the Asian countries, Pakistan has the highest prevalence of chronic liver disease. Despite this, the genetic susceptibility to chronic liver disease in this country has not been investigated. We performed a comprehensive analysis of the most robustly associated common genetic variants influencing chronic liver disease in a cohort of individuals from Pakistan. A total of 587 subjects with chronic liver disease and 68 healthy control individuals were genotyped for the HSD17B13 rs7261356, MBOAT7 rs641738, GCKR rs1260326, PNPLA3 rs738409, TM6SF2 rs58542926 and PPP1R3B rs4841132 variants. The variants distribution between case and control group and their association with chronic liver disease were tested by chi-square and binary logistic analysis, respectively. We report for the first time that HSD17B13 variant results in a 50% reduced risk for chronic liver disease; while MBOAT7; GCKR and PNPLA3 variants increase this risk by more than 35% in Pakistani individuals. Our genetic analysis extends the protective role of the HSD17B13 variant against chronic liver disease and disease risk conferred by the MBOAT7; GCKR and PNPLA3 variants in the Pakistani population.

The two major glucokinase isoforms show conserved functionality in β-cells despite different subcellular distribution

Glucokinase (GCK) is crucial to regulating glucose metabolism in the liver and in pancreatic β-cells. There are two major GCK isoforms, hepatic and pancreatic GCKs, which differ only in exon 1. However, the functional differences between the two GCK isoforms remain poorly understood. Here, we used a β-cell-targeted gene transfer vector to determine the impact of isoform-specific GCK overexpression on β-cells in vitro and in vivo. We showed that pancreatic GCK had a nuclear localization signal unique to the pancreatic isoform, facilitating its nuclear distribution in β-cells. Despite the difference in subcellular distribution, overexpression of GCK isoforms similarly enhanced glucose uptake and β-cell proliferation in vitro. Overexpression of hepatic or pancreatic GCK also similarly enhanced β-cell proliferation in normal diet mice without affecting fasting glucose and intraperitoneal glucose tolerance tests (IPGTT). Our further study on human GCK sequences identified disproportional GCK amino acid variants in exon 1, while mutations linked to maturity onset diabetes of the young type 2 (MODY2) were disproportionally found in exons 2 through 10. Our results therefore indicate functional conservation between the two major GCK isoforms despite their distinct subcellular distribution.

Carbohydrate Kinases: A Conserved Mechanism Across Differing Folds

Carbohydrate kinases activate a wide variety of monosaccharides by adding a phosphate group, usually from ATP. This modification is fundamental to saccharide utilization, and it is likely a very ancient reaction. Modern organisms contain carbohydrate kinases from at least five main protein families. These range from the highly specialized inositol kinases, to the ribokinases and galactokinases, which belong to families that phosphorylate a wide range of substrates. The carbohydrate kinases utilize a common strategy to drive the reaction between the sugar hydroxyl and the donor phosphate. Each sugar is held in position by a network of hydrogen bonds to the non-reactive hydroxyls (and other functional groups). The reactive hydroxyl is deprotonated, usually by an aspartic acid side chain acting as a catalytic base. The deprotonated hydroxyl then attacks the donor phosphate. The resulting pentacoordinate transition state is stabilized by an adjacent divalent cation, and sometimes by a positively charged protein side chain or the presence of an anion hole. Many carbohydrate kinases are allosterically regulated using a wide variety of strategies, due to their roles at critical control points in carbohydrate metabolism. The evolution of a similar mechanism in several folds highlights the elegance and simplicity of the catalytic scheme.

Genotype effects of glucokinase regulator on lipid profiles and glycemic status are modified by circulating calcium levels: results from the Korean Genome and Epidemiology Study

Single nucleotide polymorphisms (SNPs) in the glucokinase regulator (GCKR) are associated with major cardiovascular risk factors (ie, lipid profile and glycemic status). Recently, GCKR was shown to be related to circulating calcium levels involved in lipid and glycemic controls. Therefore, we hypothesized that GCKR SNPs are associated with major cardiovascular risk factors in the Korean population, and the association is modified by circulating calcium levels. Epidemiological data and GCKR SNPs (rs780093T>C, rs780094 T>C, and rs1260326 T>C) were collected from a subset of Ansung-Ansan cohort in the Korean Genome and Epidemiology Study (n = 7815). Consistent with the results of previous studies, GCKR SNPs were significantly associated with decreased total cholesterol and triglyceride levels and increased glucose levels and insulin resistance. Minor C allele carriers, particularly CC homozygotes, had lower serum calcium levels than TT homozygotes for all 3 SNPs. Particularly, the effect of GCKR SNPs on total cholesterol, triglyceride, fasting glucose, and insulin resistance was apparent when serum calcium levels were in normal range (8.8-10.1 mg/dL). When serum calcium levels were high (≥10.2 mg/dL), CC homozygotes also had significantly lower triglyceride and higher fasting glucose than TT homozygotes. However, the associations were not observed when serum calcium levels were low (<8.8 mg/dL). In conclusion, GCKR SNPs are associated with lipid profiles and glycemic status in the Korean population, and the genetic effect is modified by basal circulating calcium levels, particularly in normal or high ranges. It provides important information for individualized prevention and management of cardiovascular risk associated with GCKR SNPs.

The Effect of Small Doses of Fructose and Its Epimers on Glycemic Control: A Systematic Review and Meta-Analysis of Controlled Feeding Trials

Objective: Contrary to the concerns that fructose may have adverse metabolic effects, an emerging literature has shown that small doses (≤10 g/meal) of fructose and its low-caloric epimers (allulose, tagatose, and sorbose) decrease the glycemic response to high glycemic index meals. Whether these acute reductions manifest as sustainable improvements in glycemic control is unclear. Our objective was to synthesize the evidence from controlled feeding trials that assessed the effect of small doses of fructose and its low-caloric epimers on glycemic control. Methods: We searched MEDLINE, EMBASE, and the Cochrane Library through April 18, 2018. We included controlled feeding trials of ≥1 week that investigated the effect of small doses (≤50 g/day or ≤10% of total energy intake/day) of fructose and its low-caloric epimers on HbA_1c, fasting glucose, and fasting insulin. Two independent reviewers extracted data and assessed risk of bias. Data were pooled using the generic inverse variance method and expressed as mean differences (MDs) with 95% confidence intervals (CIs). Heterogeneity was assessed using the Cochran Q statistic and quantified using the I² statistic. Grading of Recommendations Assessment, Development and Evaluation (GRADE) assessed the certainty of the evidence. Results: We identified 14 trial comparisons (N = 337) of the effect of fructose in individuals with and without diabetes, 3 trial comparisons (N = 138) of the effect of allulose in individuals without diabetes, 3 trial comparisons (N = 376) of the effect of tagatose mainly in individuals with type 2 diabetes, and 0 trial comparisons of the effect of sorbose. Small doses of fructose and tagatose significantly reduced HbA_1c (MD = -0.38% (95% CI: -0.64%, -0.13%); MD = -0.20% (95% CI: -0.34%, -0.06%)) and fasting glucose (MD = -0.13 mmol/L (95% CI: -0.24 mmol/L, -0.03 mmol/L)); MD = -0.30 mmol/L (95% CI: -0.57 mmol/L, -0.04 mmol/L)) without affecting fasting insulin (p > 0.05). Small doses of allulose did not have a significant effect on HbA_1c and fasting insulin (p > 0.05), while the reduction in fasting glucose was of borderline significance (p = 0.05). The certainty of the evidence of the effect of small doses of fructose and allulose on HbA_1c, fasting glucose, and fasting insulin was graded as low. The certainty of the evidence of the effect of tagatose on HbA_1c, fasting glucose, and fasting insulin was graded as moderate. Conclusions: Our results indicate that small doses of fructose and tagatose may improve glycemic control over the long term. There is a need for long-term randomized controlled trials for all four sugars to improve our certainty in the estimates.

Functional characterization of MODY2 mutations in the nuclear export signal of glucokinase

Glucokinase (GCK) plays a key role in glucose homeostasis. Heterozygous inactivating mutations in the GCK gene cause the familial, mild fasting hyperglycaemia named MODY2. Besides its particular kinetic characteristics, glucokinase is regulated by subcellular compartmentation in hepatocytes. Glucokinase regulatory protein (GKRP) binds to GCK, leading to enzyme inhibition and import into the nucleus at fasting. When glucose concentration increases, GCK-GKRP dissociates and GCK is exported to the cytosol due to a nuclear export signal (NES). With the aim to characterize the GCK-NES, we have functionally analysed nine MODY2 mutations located within the NES sequence. Recombinant GCK mutants showed reduced catalytic activity and, in most cases, protein instability. Most of the mutants interact normally with GKRP, although mutations L306R and L309P impair GCK nuclear import in cotransfected cells. We demonstrated that GCK-NES function depends on exportin 1. We further showed that none of the mutations fully inactivate the NES, with the exception of mutation L304P, which likely destabilizes its α-helicoidal structure. Finally, we found that residue Glu300 negatively modulates the NES activity, whereas other residues have the opposite effect, thus suggesting that some of the NES spacer residues contribute to the low affinity of the NES for exportin 1, which is required for its proper functioning. In conclusion, our results have provided functional and structural insights regarding the GCK-NES and contributed to a better knowledge of the molecular mechanisms involved in the nucleo-cytoplasmic shuttling of glucokinase. Impairment of this regulatory mechanism by some MODY2 mutations might contribute to the hyperglycaemia in the patients.

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.

Early gene-diet interaction between glucokinase regulatory protein (GCKR) polymorphism, vegetable and fish intakes in modulating triglyceride levels in healthy adolescents

BACKGROUND AND AIMS

The benefits of dietary vegetable and fish consumptions on improving glucose and lipid metabolism have been well established. Recently, the T-allele of a common genetic variant rs780094 at glucokinase regulatory protein (GCKR) was reported to be associated with elevated triglyceride (TG) levels but reduced fasting plasma glucose (FPG) and type 2 diabetes risk. However, the dietary modulation on genetic risk is not clearly understood.

METHODS AND RESULTS

A cohort of 2095 Chinese adolescents (mean age 15.6 ± 2.0 years, 45.3% male) recruited from a population-based school survey for cardiovascular risk factor assessment, with dietary data including weekly vegetable and fish consumptions as well as clinical data were genotyped for the GCKR rs780094 polymorphism. In the linear regression analysis with adjustment for sex, age, body mass index, and socioeconomic status (school banding, paternal and maternal education levels), the frequency of vegetable intake per week was inversely associated with FPG (P = 0.044). Individuals with low fish intake generally had elevated TG levels but reduced TC, HDL-C and LDL-C (0.006 < P < 0.029). We also observed significant associations of the minor T-allele of GCKR rs780094 with decreased FPG (P = 0.013) and increased TG levels (P = 2.7 × 10(-8)). There were significant gene-diet interactions between rs780094 and vegetable consumption (P(interaction) = 0.009), and between rs780094 and fish consumption (P(interaction) = 0.031) in modulating TG levels. The T-allele of GCKR locus was associated with higher TG levels amongst individuals with ≥7 vegetable meals per week (P = 6.4 × 10(-9)), and among individuals with <7 fish meals per week (P = 0.020 and 7.0 × 10(-7) for 4-6 and ≤3 meals per week, respectively). High intake of vegetable exerted a reduction in TG levels only among CC genotype carriers (Ptrend = 0.020), while high intake of fish was associated with reduced TG levels only among TT genotype carriers (Ptrend = 0.026).

CONCLUSIONS

In summary, our data indicated that the favorable associations of higher vegetable and fish intakes on TG levels are dependent on the genetic background of an individual. In particular, at-risk TT- genotype carriers of the GCKR variant may derive more benefits from a high fish intake, while the CC-genotype carriers may find further benefits from a high consumption of vegetable.

Glucokinase regulatory protein: complexity at the crossroads of triglyceride and glucose metabolism

PURPOSE OF REVIEW

Glucokinase regulator (GCKR) encodes glucokinase regulatory protein (GKRP), a hepatocyte-specific inhibitor of the glucose-metabolizing enzyme glucokinase (GCK). Genome-wide association studies have identified a common coding variant within GCKR associated with multiple metabolic traits. This review focuses on recent insights into the critical role of GKRP in hepatic glucose metabolism that have stemmed from the study of human genetics. This knowledge has improved our understanding of glucose and lipid physiology and informed the development of targeted molecular therapeutics for diabetes.

RECENT FINDINGS

Rare GCKR variants have effects on GKRP expression, localization, and activity. These variants are collectively associated with hypertriglyceridaemia but are not causal. Crystal structures of GKRP and the GCK-GKRP complex have been solved, providing greater insight into the molecular interactions between these proteins. Finally, small molecules have been identified that directly bind GKRP and reduce blood glucose levels in rodent models of diabetes.

SUMMARY

GCKR variants across the allelic spectrum have effects on glucose and lipid homeostasis. Functional analysis has highlighted numerous molecular mechanisms for GKRP dysfunction. Hepatocyte-specific GCK activation via small molecule GKRP inhibition may be a new avenue for type 2 diabetes treatment, particularly considering evidence indicating GKRP loss-of-function alone does not cause hypertriglyceridaemia.

In vivo and in vitro evidence that chronic activation of the hexosamine biosynthetic pathway interferes with leptin-dependent STAT3 phosphorylation

We previously reported that a 2-day peripheral infusion of glucosamine caused leptin resistance in rats, suggesting a role for the hexosamine biosynthetic pathway (HBP) in the development of leptin resistance. Here we tested leptin responsiveness in mice in which HBP activity was stimulated by offering 30% sucrose solution in addition to chow and water or by infusing glucosamine. Mice were leptin resistant after 33 days of access to sucrose. Resistance was associated with increased activity of the HBP and with phosphorylation of transcription factor signal transducer and activator of transcription-3 Tyr705 [pSTAT3(Y705)] but inhibition of suppressor of cytokine signaling 3 in the liver and hypothalamus. Intravenous infusion of glucosamine for 3 h stimulated pSTAT3(Y705) but prevented leptin-induced phosphorylation of STAT3(S727). In an in vitro system, glucose, glucosamine, and leptin each dose dependently increased O-linked β-N-acetylglucosamine (O-GlcNAc) protein and pSTAT3(Y705) in HepG2 cells. To test the effect of glucose on leptin responsiveness cells were incubated in 5.5 mM (LG) or 20 mM (HG) glucose for 18 h and were treated with 0 or 50 ng/ml leptin for 15 min. HG alone and LG + leptin produced similar increases in O-GlcNAc protein, glutamine fructose-6-phosphate amidotransferase (GFAT), and pSTAT3(Y705) compared with LG media. Leptin did not stimulate these proteins in HG cells, suggesting leptin resistance. Leptin-induced pSTAT3(S727) was prevented by HG media. Inhibition of GFAT with azaserine prevented LG + leptin and HG stimulation of pSTAT3. These data demonstrate development of leptin resistance in sucrose-drinking mice and provide new evidence of leptin-induced stimulation of the HBP.

Discovery of Small-Molecule Glucokinase Regulatory Protein Modulators That Restore Glucokinase Activity

In the nuclei of hepatocytes, glucokinase regulatory protein (GKRP) modulates the activity of glucokinase (GK), a key regulator of glucose homeostasis. Currently, direct activators of GK (GKAs) are in development for the treatment of type 2 diabetes. However, this approach is generally associated with a risk of hypoglycemia. To mitigate such risk, we target the GKRP regulation, which indirectly restores GK activity. Here we describe a screening strategy to look specifically for GKRP modulators, in addition to traditional GKAs. Two high-throughput screening campaigns were performed with our compound libraries using a luminescence assay format, one with GK alone and the other with a GK/GKRP complex in the presence of sorbitol-6-phosphate (S6P). By a subtraction method in the hit triage process of these campaigns, we discovered two close analogs that bind GKRP specifically with sub-µM potency to a site distinct from where fructose-1-phosphate binds. These small molecules are first-in-class allosteric modulators of the GK/GKRP interaction and are fully active even in the presence of S6P. Activation of GK by this particular mechanism, without altering the enzymatic profile, represents a novel pharmacologic modality of intervention in the GK/GKRP pathway.

A panel of diverse assays to interrogate the interaction between glucokinase and glucokinase regulatory protein, two vital proteins in human disease

Recent genetic and clinical evidence has implicated glucokinase regulatory protein (GKRP) in the pathogenesis of type 2 diabetes and related traits. The primary role of GKRP is to bind and inhibit hepatic glucokinase (GCK), a critically important protein in human health and disease that exerts a significant degree of control over glucose metabolism. As activation of GCK has been associated with improved glucose tolerance, perturbation of the GCK-GKRP interaction represents a potential therapeutic target for pharmacological modulation. Recent structural and kinetic advances are beginning to provide insight into the interaction of these two proteins. However, tools to comprehensively assess the GCK-GKRP interaction, particularly in the context of small molecules, would be a valuable resource. We therefore developed three robust and miniaturized assays for assessing the interaction between recombinant human GCK and GKRP: an HTRF assay, a diaphorase-coupled assay, and a luciferase-coupled assay. The assays are complementary, featuring distinct mechanisms of detection (luminescence, fluorescence, FRET). Two assays rely on GCK enzyme activity modulation by GKRP while the FRET-based assay measures the GCK-GKRP protein-protein interaction independent of GCK enzymatic substrates and activity. All three assays are scalable to low volumes in 1536-well plate format, with robust Z’ factors (>0.7). Finally, as GKRP sequesters GCK in the hepatocyte nucleus at low glucose concentrations, we explored cellular models of GCK localization and translocation. Previous findings from freshly isolated rat hepatocytes were confirmed in cryopreserved rat hepatocytes, and we further extended this study to cryopreserved human hepatocytes. Consistent with previous reports, there were several key differences between the rat and human systems, with our results suggesting that human hepatocytes can be used to interrogate GCK translocation in response to small molecules. The assay panel developed here should help direct future investigation of the GCK-GKRP interaction in these or other physiologically relevant human systems.

Antidiabetic effects of glucokinase regulatory protein small-molecule disruptors

Glucose homeostasis is a vital and complex process, and its disruption can cause hyperglycaemia and type II diabetes mellitus. Glucokinase (GK), a key enzyme that regulates glucose homeostasis, converts glucose to glucose-6-phosphate in pancreatic β-cells, liver hepatocytes, specific hypothalamic neurons, and gut enterocytes. In hepatocytes, GK regulates glucose uptake and glycogen synthesis, suppresses glucose production, and is subject to the endogenous inhibitor GK regulatory protein (GKRP). During fasting, GKRP binds, inactivates and sequesters GK in the nucleus, which removes GK from the gluconeogenic process and prevents a futile cycle of glucose phosphorylation. Compounds that directly hyperactivate GK (GK activators) lower blood glucose levels and are being evaluated clinically as potential therapeutics for the treatment of type II diabetes mellitus. However, initial reports indicate that an increased risk of hypoglycaemia is associated with some GK activators. To mitigate the risk of hypoglycaemia, we sought to increase GK activity by blocking GKRP. Here we describe the identification of two potent small-molecule GK-GKRP disruptors (AMG-1694 and AMG-3969) that normalized blood glucose levels in several rodent models of diabetes. These compounds potently reversed the inhibitory effect of GKRP on GK activity and promoted GK translocation both in vitro (isolated hepatocytes) and in vivo (liver). A co-crystal structure of full-length human GKRP in complex with AMG-1694 revealed a previously unknown binding pocket in GKRP distinct from that of the phosphofructose-binding site. Furthermore, with AMG-1694 and AMG-3969 (but not GK activators), blood glucose lowering was restricted to diabetic and not normoglycaemic animals. These findings exploit a new cellular mechanism for lowering blood glucose levels with reduced potential for hypoglycaemic risk in patients with type II diabetes mellitus.

Time-course changes in immunoreactivities of glucokinase and glucokinase regulatory protein in the gerbil hippocampus following transient cerebral ischemia

Glucose is a main energy source for normal brain functions. Glucokinase (GK) plays an important role in glucose metabolism as a glucose sensor, and GK activity is modulated by glucokinase regulatory protein (GKRP). In this study, we examined the changes of GK and GKRP immunoreactivities in the gerbil hippocampus after 5 min of transient global cerebral ischemia. In the sham-operated-group, GK and GKRP immunoreactivities were easily detected in the pyramidal neurons of the stratum pyramidale of the hippocampus. GK and GKRP immunoreactivities in the pyramidal neurons were distinctively decreased in the hippocampal CA1 region (CA), not CA2/3, 3 days after ischemia-reperfusion (I-R). Five days after I-R, GK and GKRP immunoreactivities were hardly detected in the CA1, not CA2/3, pyramidal neurons; however, at this point in time, GK and GKRP immunoreactivities were newly expressed in astrocytes, not microglia, in the ischemic CA1. In brief, GK and GKRP immunoreactivities are changed in pyramidal neurons and newly expressed in astrocytes in the ischemic CA1 after transient cerebral ischemia. These indicate that changes of GK and GKRP expression may be related to the ischemia-induced neuronal damage/death.

Structural basis for regulation of human glucokinase by glucokinase regulatory protein

Glucokinase (GCK) is responsible for maintaining glucose homeostasis in the human body. Dysfunction or misregulation of GCK causes hyperinsulinemia, hypertriglyceridemia, and type 2 diabetes. In the liver, GCK is regulated by interaction with the glucokinase regulatory protein (GKRP), a 68 kDa polypeptide that functions as a competitive inhibitor of glucose binding to GCK. Formation of the mammalian GCK-GKRP complex is stimulated by fructose 6-phosphate and antagonized by fructose 1-phosphate. Here we report the crystal structure of the mammalian GCK-GKRP complex in the presence of fructose 6-phosphate at a resolution of 3.50 Å. The interaction interface, which totals 2060 Å(2) of buried surface area, is characterized by a small number of polar contacts and substantial hydrophobic interactions. The structure of the complex reveals the molecular basis of disease states associated with impaired regulation of GCK by GKRP. It also offers insight into the modulation of complex stability by sugar phosphates. The atomic description of the mammalian GCK-GKRP complex provides a framework for the development of novel diabetes therapeutic agents that disrupt this critical macromolecular regulatory unit.

Molecular basis for the role of glucokinase regulatory protein as the allosteric switch for glucokinase

Glucokinase (GK) is a monomeric allosteric enzyme and plays a pivotal role in blood glucose homeostasis. GK is regulated by GK regulatory protein (GKRP), and indirectly by allosteric effectors of GKRP. Despite the critical roles of GK and GKRP, the molecular basis for the allosteric regulation mechanism of GK by GKRP remains unclear. We determined the crystal structure of Xenopus GK and GKRP complex in the presence of fructose-6-phosphate at 2.9 Å. GKRP binds to a super-open conformation of GK mainly through hydrophobic interaction, inhibiting the GK activity by locking a small domain of GK. We demonstrate the molecular mechanism for the modulation of GK activity by allosteric effectors of GKRP. Importantly, GKRP releases GK in a sigmoidal manner in response to glucose concentration by restricting a structural rearrangement of the GK small domain via a single ion pair. We find that GKRP acts as an allosteric switch for GK in blood glucose control by the liver.

Evolution of hepatic glucose metabolism: liver-specific glucokinase deficiency explained by parallel loss of the gene for glucokinase regulatory protein (GCKR)

BACKGROUND

Glucokinase (GCK) plays an important role in the regulation of carbohydrate metabolism. In the liver, phosphorylation of glucose to glucose-6-phosphate by GCK is the first step for both glycolysis and glycogen synthesis. However, some vertebrate species are deficient in GCK activity in the liver, despite containing GCK genes that appear to be compatible with function in their genomes. Glucokinase regulatory protein (GCKR) is the most important post-transcriptional regulator of GCK in the liver; it participates in the modulation of GCK activity and location depending upon changes in glucose levels. In experimental models, loss of GCKR has been shown to associate with reduced hepatic GCK protein levels and activity.

METHODOLOGY/PRINCIPAL FINDINGS

GCKR genes and GCKR-like sequences were identified in the genomes of all vertebrate species with available genome sequences. The coding sequences of GCKR and GCKR-like genes were identified and aligned; base changes likely to disrupt coding potential or splicing were also identified.

CONCLUSIONS/SIGNIFICANCE

GCKR genes could not be found in the genomes of 9 vertebrate species, including all birds. In addition, in multiple mammalian genomes, whereas GCKR-like gene sequences could be identified, these genes could not predict a functional protein. Vertebrate species that were previously reported to be deficient in hepatic GCK activity were found to have deleted (birds and lizard) or mutated (mammals) GCKR genes. Our results suggest that mutation of the GCKR gene leads to hepatic GCK deficiency due to the loss of the stabilizing effect of GCKR.

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.

Correlation of rare coding variants in the gene encoding human glucokinase regulatory protein with phenotypic, cellular, and kinetic outcomes

Defining the genetic contribution of rare variants to common diseases is a major basic and clinical science challenge that could offer new insights into disease etiology and provide potential for directed gene- and pathway-based prevention and treatment. Common and rare nonsynonymous variants in the GCKR gene are associated with alterations in metabolic traits, most notably serum triglyceride levels. GCKR encodes glucokinase regulatory protein (GKRP), a predominantly nuclear protein that inhibits hepatic glucokinase (GCK) and plays a critical role in glucose homeostasis. The mode of action of rare GCKR variants remains unexplored. We identified 19 nonsynonymous GCKR variants among 800 individuals from the ClinSeq medical sequencing project. Excluding the previously described common missense variant p.Pro446Leu, all variants were rare in the cohort. Accordingly, we functionally characterized all variants to evaluate their potential phenotypic effects. Defects were observed for the majority of the rare variants after assessment of cellular localization, ability to interact with GCK, and kinetic activity of the encoded proteins. Comparing the individuals with functional rare variants to those without such variants showed associations with lipid phenotypes. Our findings suggest that, while nonsynonymous GCKR variants, excluding p.Pro446Leu, are rare in individuals of mixed European descent, the majority do affect protein function. In sum, this study utilizes computational, cell biological, and biochemical methods to present a model for interpreting the clinical significance of rare genetic variants in common disease.

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.

From genetic association to molecular mechanism

Over the past 3 years, there has been a dramatic increase in the number of confirmed type 2 diabetes (T2D) susceptibility loci, most arising through the implementation of genome-wide association studies (GWAS). However, progress toward the understanding of disease mechanisms has been slowed by modest effect sizes and the fact that most GWAS signals map away from coding sequence: the presumption is that their effects are mediated through regulation of nearby transcripts, but the identities of the genes concerned are often far from clear. In this review we describe the progress that has been made to date in translating association signals into molecular mechanisms with a focus on the most tractable signals (eg, KCNJ11/ABCC8, SLC30A8, GCKR) and those in which human, animal, and cellular models (FTO, TCF7L2, G6PC2) have provided insights into the role in T2D pathogenesis. Finally, the challenges for the field with the advent of genome-scale next-generation resequencing efforts are discussed.

Glucokinase activators (GKAs) promise a new pharmacotherapy for diabetics

The glucose-phosphorylating enzyme glucokinase, a promising target for developing new antidiabetic agents, was identified through the combined efforts of basic research and human biochemical genetics. Allosteric glucokinase activators (GKAs) were discovered by high-throughput screening of a large compound library and first reported in 2003. GKAs stimulate insulin release and glucose metabolism in the liver thereby lowering blood sugar, and promising trials in humans demonstrate that they are highly effective in patients with type 2 diabetes mellitus. Many companies are now attempting to develop effective and safe GKAs for treating diabetics.

Dynamics and control of the central carbon metabolism in hepatoma cells

BACKGROUND

The liver plays a major role in metabolism and performs a number of vital functions in the body. Therefore, the determination of hepatic metabolite dynamics and the analysis of the control of the respective biochemical pathways are of great pharmacological and medical importance. Extra- and intracellular time-series data from stimulus-response experiments are gaining in importance in the identification of in vivo metabolite dynamics, while dynamic network models are excellent tools for analyzing complex metabolic control patterns. This is the first study that has been undertaken on the data-driven identification of a dynamic liver central carbon metabolism model and its application in the analysis of the distribution of metabolic control in hepatoma cells.

RESULTS

Dynamic metabolite data were collected from HepG2 cells after they had been deprived of extracellular glucose. The concentration of 25 extra- and intracellular intermediates was quantified using HPLC, LC-MS-MS, and GC-MS. The in silico metabolite dynamics were in accordance with the experimental data. The central carbon metabolism of hepatomas was further analyzed with a particular focus on the control of metabolite concentrations and metabolic fluxes. It was observed that the enzyme glucose-6-phosphate dehydrogenase exerted substantial negative control over the glycolytic flux, whereas oxidative phosphorylation had a significant positive control. The control over the rate of NADPH consumption was found to be shared between the NADPH-demand itself (0.65) and the NADPH supply (0.38).

CONCLUSIONS

Based on time-series data, a dynamic central carbon metabolism model was developed for the investigation of new and complex metabolic control patterns in hepatoma cells. The control patterns found support the hypotheses that the glucose-6-phosphate dehydrogenase and the Warburg effect are promising targets for tumor treatment. The systems-oriented identification of metabolite dynamics is a first step towards the genome-based assessment of potential risks posed by nutrients and drugs.

The biochemical basis of hereditary fructose intolerance

Hereditary fructose intolerance is a rare, but potentially lethal, inherited disorder of fructose metabolism, caused by mutation of the aldolase B gene. Treatment currently relies solely on dietary restriction of problematic sugars. Biochemical study of defective aldolase B enzymes is key to revealing the molecular basis of the disease and providing a stronger basis for improved treatment and diagnosis. Such studies have revealed changes in enzyme activity, stability and oligomerisation. However, linking these changes to disease phenotypes has not always been straightforward. This review gives a general overview of the features of hereditary fructose intolerance, then concentrates on the biochemistry of the AP variant (Ala149Pro variant of aldolase B) and molecular pathological consequences of mutation of the aldolase B gene.

Evolution of vertebrate glucokinase regulatory protein from a bacterial N-acetylmuramate 6-phosphate etherase

Mammalian GKRP [GK (glucokinase) regulatory protein], a fructose 6-phosphate and fructose 1-phosphate sensitive inhibitor of GK, appears to have resulted from the duplication of a gene similar to bacterial N-acetylmuramate 6-phosphate etherase MurQ. In the present study, we show that several genomes of primitive eukaryotes encode a GKRP-like protein with two MurQ repeats. Recombinant Haemophilus influenzae MurQ and the GKRP homologue of the amoeboflagellate Naegleria gruberi both behaved as excellent N-acetylmuramate 6-phosphate etherases, with Kcat values (83 and 20 s(-1)) at least as high as that reported for Escherichia coli MurQ. In contrast, rat and Xenopus GKRP displayed much lower etherase activities (Kcat=0.08 and 0.05 s(-1) respectively). The etherase activity of rat GKRP was inhibited by ligands (fructose 6-phosphate, fructose 1-phosphate and sorbitol 6-phosphate) known to regulate its interaction with GK and by mutations affecting the binding of these phosphate esters. This indicated that these phosphate esters all bind to a single regulatory site, which evolved from the original catalytic site. Sorbitol 6-phosphate and other phosphate esters also inhibited the etherase activity of Xenopus GKRP, but did not affect its ability to inhibit GK. Thus, unlike what was previously thought, Xenopus GKRP has a binding site for phosphate esters, but this site is uncoupled from the GK-binding site. Taken together, these data indicate that duplication of the murQ gene led to a eukaryotic-type etherase, which subsequently evolved to GKRP by acquiring a new binding specificity while losing most of its etherase activity.

The P446L variant in GCKR associated with fasting plasma glucose and triglyceride levels exerts its effect through increased glucokinase activity in liver

Genome-wide association studies have identified a number of signals for both Type 2 Diabetes and related quantitative traits. For the majority of loci, the transition from association signal to mutational mechanism has been difficult to establish. Glucokinase (GCK) regulates glucose storage and disposal in the liver where its activity is regulated by glucokinase regulatory protein (GKRP; gene name GCKR). Fructose-6 and fructose-1 phosphate (F6P and F1P) enhance or reduce GKRP-mediated inhibition, respectively. A common GCKR variant (P446L) is reproducibly associated with triglyceride and fasting plasma glucose levels in the general population. The aim of this study was to determine the mutational mechanism responsible for this genetic association. Recombinant human GCK and both human wild-type (WT) and P446L-GKRP proteins were generated. GCK kinetic activity was observed spectrophotometrically using an NADP⁺-coupled assay. WT and P446L-GKRP-mediated inhibition of GCK activity and subsequent regulation by phosphate esters were determined. Assays matched for GKRP activity demonstrated no difference in dose-dependent inhibition of GCK activity or F1P-mediated regulation. However, the response to physiologically relevant F6P levels was significantly attenuated with P446L-GKRP (n = 18; P ≤ 0.03). Experiments using equimolar concentrations of both regulatory proteins confirmed these findings (n = 9; P < 0.001). In conclusion, P446L-GKRP has reduced regulation by physiological concentrations of F6P, resulting indirectly in increased GCK activity. Altered GCK regulation in liver is predicted to enhance glycolytic flux, promoting hepatic glucose metabolism and elevating concentrations of malonyl-CoA, a substrate for de novo lipogenesis, providing a mutational mechanism for the reported association of this variant with raised triglycerides and lower glucose levels.

Assessing the potential of glucokinase activators in diabetes therapy

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.

Human pancreatic beta-cell glucokinase: subcellular localization and glucose repression signalling function in the yeast cell

Human GK(beta) (pancreatic beta-cell glucokinase) is the main glucose-phosphorylating enzyme in pancreatic beta-cells. It shares several structural, catalytic and regulatory properties with Hxk2 (hexokinase 2) from Saccharomyces cerevisiae. In fact, it has been previously described that expression of GK(beta) in yeast could replace Hxk2 in the glucose signalling pathway of S. cerevisiae. In the present study we report that GK(beta) exerts its regulatory role by association with the yeast transcriptional repressor Mig1 (multicopy inhibitor of GAL gene expression 1); the presence of Mig1 allows GK(beta) to bind to the SUC2 (sucrose fermentation 2) promoter, helping in this way in the maintenance of the repression of the SUC2 gene under high-glucose conditions. Since a similar mechanism has been described for the yeast Hxk2, the findings of the present study suggest that the function of the regulatory domain present in these two proteins has been conserved throughout evolution. In addition, we report that GK(beta) is enriched in the yeast nucleus of high-glucose growing cells, whereas it shows a mitochondrial localization upon removal of the sugar. However, GK(beta) does not exit the nucleus in the absence of Mig1, suggesting that Mig1 regulates the nuclear exit of GK(beta) under low-glucose conditions. We also report that binding of GK(beta) to Mig1 allows the latter protein to be located at the mitochondrial network under low-glucose conditions.

Biophysical characterization of the interaction between hepatic glucokinase and its regulatory protein: impact of physiological and pharmacological effectors

Glucokinase (GK) is a key enzyme of glucose metabolism in liver and pancreatic beta-cells, and small molecule activators of GK (GKAs) are under evaluation for the treatment of type 2 diabetes. In liver, GK activity is controlled by the GK regulatory protein (GKRP), which forms an inhibitory complex with the enzyme. Here, we performed isothermal titration calorimetry and surface plasmon resonance experiments to characterize GK-GKRP binding and to study the influence that physiological and pharmacological effectors of GK have on the protein-protein interaction. In the presence of fructose-6-phosphate, GK-GKRP complex formation displayed a strong entropic driving force opposed by a large positive enthalpy; a negative change in heat capacity was observed (Kd = 45 nm, DeltaH = 15.6 kcal/mol, TDeltaS = 25.7 kcal/mol, DeltaCp = -354 cal mol(-1) K(-1)). With k(off) = 1.3 x 10(-2) s(-1), the complex dissociated quickly. The thermodynamic profile suggested a largely hydrophobic interaction. In addition, effects of pH and buffer demonstrated the coupled uptake of one proton and indicated an ionic contribution to binding. Glucose decreased the binding affinity between GK and GKRP. This decrease was potentiated by an ATP analogue. Prototypical GKAs of the amino-heteroaryl-amide type bound to GK in a glucose-dependent manner and impaired the association of GK with GKRP. This mechanism might contribute to the antidiabetic effects of GKAs.

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.

Glucokinase activator PSN-GK1 displays enhanced antihyperglycaemic and insulinotropic actions

AIMS/HYPOTHESIS

We evaluated the insulinotropic and antihyperglycaemic actions of glucokinase activators (GKAs), especially through acute and subchronic studies in rodent diabetes models with (2R)-2-(4-cyclopropanesulphonylphenyl)-N-(5-fluorothiazol-2-yl)-3-(tetrahydropyran-4-yl)propionamide (PSN-GK1), a novel and potent GKA.

MATERIALS AND METHODS

The action of PSN-GK1 on or in the following were investigated: (1) on human liver glucokinase, insulin secretion from MIN6 cells and 2-deoxy-D: -[(3)H]glucose (2-DG) uptake into rat hepatocytes; and (2) in Zucker diabetic fatty rats and in non-diabetic C57Bl/6, diabetic db/db and ob/ob mice.

RESULTS

At 5 mmol/l glucose, PSN-GK1 activated glucokinase (4.3-fold, median effective concentration [EC(50)] 130 nmol/l), increased MIN6 insulin secretion (26-fold, EC(50) 267 nmol/l) and 2-DG hepatocytic uptake (threefold, EC(50) 1 micromol/l); at higher glucose concentrations, EC(50)s and fold-effectiveness were both lower. In C57Bl/6 mice, PSN-GK1 reduced blood glucose at 1 and 10 mg/kg (by mouth), but insulin was increased significantly at only the higher dose. In hyperinsulinaemic 10-mmol/l glucose clamps, PSN-GK1 increased 2-DG incorporation into liver glycogen sixfold, directly demonstrating liver effects. PSN-GK1 improved glycaemic profiles in db/db mice and Zucker diabetic fatty rats, diabetic animal models in which GKA efficacy has not previously been described, without causing hypoglycaemia. In ob/ob mice, it dose-dependently reduced excursions in OGTTs. Moreover, after subchronic administration, no tachyphylaxis was evident and glycaemia was improved without alterations to lipid levels, liver weight, glycogen content or body weight.

CONCLUSIONS/INTERPRETATION

PSN-GK1 was potently antihyperglycaemic through its effects on insulin release and hepatic glucose metabolism. It is one of the most potent GKAs described in the literature and is active in diabetic animal models where GKAs have not been reported to show efficacy to date. Ongoing human trials are investigating the potential of this novel therapeutic approach.

A defect in glucose-induced dissociation of glucokinase from the regulatory protein in Zucker diabetic fatty rats in the early stage of diabetes

Effect of stimulation of glucokinase (GK) export from the nucleus by small amounts of sorbitol on hepatic glucose flux in response to elevated plasma glucose was examined in 6-h fasted Zucker diabetic fatty rats at 10 wk of age. Under basal conditions, plasma glucose, insulin, and glucagon were ∼8 mM, 2,000 pmol/l, and 60 ng/l, respectively. Endogenous glucose production (EGP) was 44 ± 4 μmol·kg−1·min−1. When plasma glucose was raised to ∼17 mM, GK was still predominantly localized with its inhibitory protein in the nucleus. EGP was not suppressed. When sorbitol was infused at 5.6 and 16.7 μmol·kg−1·min−1, along with the increase in plasma glucose, GK was exported to the cytoplasm. EGP (23 ± 19 and 12 ± 5 μmol·kg−1·min−1) was suppressed without a decrease in glucose 6-phosphatase flux (145 ± 23 and 126 ± 16 vs. 122 ± 10 μmol·kg−1·min−1without sorbitol) but increased in glucose phosphorylation as indicated by increases in glucose recycling (122 ± 17 and 114 ± 19 vs. 71 ± 11 μmol·kg−1·min−1), glucose-6-phosphate content (254 ± 32 and 260 ± 35 vs. 188 ± 20 nmol/g liver), fractional contribution of plasma glucose to uridine 5′-diphosphate-glucose flux (43 ± 8 and 42 ± 8 vs. 27 ± 6%), and glycogen synthesis from plasma glucose (20 ± 4 and 22 ± 5 vs. 9 ± 4 μmol glucose/g liver). The decreased glucose effectiveness to suppress EGP and stimulate hepatic glucose uptake may result from failure of the sugar to activate GK by stimulating the translocation of the enzyme.

Effect of berberine on hepatic glucokinase and its regulatory protein in insulin-resistant rats

AIM: To investigate the effects of berberine on hepatic glucokinase (GK) activity, protein expression and GK's regulatory protein (GKRP) in insulin-resistant rats.

METHODS: Male Wistar rats were fed with high-fat diet for 8 wk to induce the model of insulin resistance. Then the insulin-resistant rats were randomly divided into 3 groups: model, berberine, and metformin groups which were treated with the corresponding preparations. Meanwhile, a normal control group was designed. The differences in insulin sensitivity, hepatic glycogen, hepatic GK activity, and protein expression (using Western blot) of hepatic GK and GKRP were compared among the groups.

RESULTS: As compared with those in model group, the insulin sensitivity (-4.93 ± 0.30 vs -5.35 ± 0.40, P < 0.05), hepatic glycogen level (136.58 ± 52.57 µg/g vs 65.88 ± 27.80 µg/g, P < 0.05), GK activity (226.55 ± 10.62 µkat/g vs 92.69 ± 6.43 µkat/g, P < 0.05), and hepatic GK protein expression (1.71±0.49 vs 1.24±0.22, P < 0.05) were significantly elevated in berberine group, while the expression of GKRP protein weaker (1.19 ± 0.20 vs 1.94 ± 0.56, P < 0.01). Meanwhile, the hepatic glycogen level in berberine group was markedly higher than that in metformin group (136.584 ± 52.574 µg/g vs 89.427 ± 31.971 µg/g, P < 0.05), but the other indexes were not significantly different between them (all P > 0.05).

CONCLUSION: Berberine may decrease insulin resistance by increasing of the hepatic GK activity.

An allosteric activator of glucokinase impairs the interaction of glucokinase and glucokinase regulatory protein and regulates glucose metabolism

Glucokinase (GK) plays a key role in the control of blood glucose homeostasis. We identified a small molecule GK activator, compound A, that increased the glucose affinity and maximal velocity (V(max)) of GK. Compound A augmented insulin secretion from isolated rat islets and enhanced glucose utilization in primary cultured rat hepatocytes. In rat oral glucose tolerance tests, orally administrated compound A lowered plasma glucose elevation with a concomitant increase in plasma insulin and hepatic glycogen. In liver, GK activity is acutely controlled by its association to the glucokinase regulatory protein (GKRP). In order to decipher the molecular aspects of how GK activator affects the shuttling of GK between nucleus and cytoplasm, the effect of compound A on GK-GKRP interaction was further investigated. Compound A increased the level of cytoplasmic GK in both isolated rat primary hepatocytes and the liver tissues from rats. Experiments in a cell-free system revealed that compound A interacted with glucose-bound free GK, thereby impairing the association of GK and GKRP. On the other hand, compound A did not bind to glucose-unbound GK or GKRP-associated GK. Furthermore, we found that glucose-dependent GK-GKRP interaction also required ATP. Given the combined prominent role of GK on insulin secretion and hepatic glucose metabolism where the GK-GKRP mechanism is involved, activation of GK has a new therapeutic potential in the treatment of type 2 diabetes.

Novel polymorphisms in the GCKR gene and their influence on glucose and insulin levels in a Danish twin population

AIM

The glucokinase regulatory protein gene is a candidate gene for Type 2 diabetes. This study reveals three new polymorphisms and examines the impact of one new and one known polymorphism on insulin secretion and parameters associated with the insulin resistance syndrome in Danish twins with different degrees of glucose tolerance.

METHODS

Single nucleotide polymorphism detection was performed in 20 healthy subjects and in 20 subjects with Type 2 diabetes. The effect of the polymorphisms on lipid, glucose and insulin measures was studied in 566 same-sex twins aged 55-74 years.

RESULTS

The new nucleotide (nt) 363 polymorphism was found only in subjects with impaired glucose tolerance and Type 2 diabetes. The nt 11216 polymorphism influenced insulin measured at 120 min during an oral glucose tolerance test (OGTT). Subjects with genotype C11216C/T11216C had 21% higher insulin values (P<0.05) than subjects with genotype T11216T. In twins discordant for this genotype, the C-allele was associated with significantly higher plasma insulin levels at all time points during the OGTT, higher beta-cell function and lower plasma glucose levels during the OGTT.

CONCLUSION

The C-allele of nt 11216 polymorphism was associated with increased insulin secretion, and may therefore exert a potentially protective effect against Type 2 diabetes. This remains to be shown in a larger study population.

Interaction of glucokinase with the liver regulatory protein is conferred by leucine-asparagine motifs of the enzyme

The glucokinase regulatory protein (GRP) plays a pivotal role in the regulation of metabolic flux in liver by the glucose-phosphorylating enzyme glucokinase. Random peptide phage display library screening for binding partners of GRP allowed the identification of an asparagine-leucine consensus motif. Asparagine-leucine motifs of glucokinase located in the hinge region, as well as in the large domain, were changed by site-directed mutagenesis. The L58R/N204Y and the L309R/N313Y glucokinase mutants showed a significantly reduced interaction with GRP. The L355R/N350Y mutant had a fivefold-higher binding affinity for GRP than wild-type glucokinase. Imaging of glucokinase and GRP fluorescence fusion proteins revealed that the L58R/N204Y glucokinase mutant lacked glucose-dependent translocation by GRP, whereas the L355R/N350Y glucokinase mutant was trapped in the nucleus due to high affinity for GRP. The results indicate that the L58/N204 motif in the hinge region confers binding to GRP, while the L355/N350 motif may modulate the binding affinity for GRP. This latter motif is part of the alpha10 helix of glucokinase and accessible to GRP in the free and complex conformation.

Hepatic adaptations to sucrose and fructose

The liver is an important site of postprandial glucose disposal, accounting for the removal of up to 30% of an oral glucose load. The liver is also centrally involved in dietary lipid and amino acid uptake, and the presence of either or both of these nutrients can influence hepatic glucose uptake. The composition of ingested carbohydrate also influences hepatic glucose metabolism. For example, fructose can increase hepatic glucose uptake. In addition, fructose extraction by the liver is exceedingly high, approaching 50% to 70% of fructose delivery. The selective hepatic metabolism of fructose, and the ability of fructose to increase hepatic glucose uptake can, under appropriate conditions (eg, diets enriched in sucrose or fructose, high fructose concentrations), provoke major adaptations in hepatic metabolism. Potential adaptations that can arise in response to these conditions and putative mechanisms driving these adaptations are the subject of this review.

Small molecule glucokinase activators as novel anti-diabetic agents

The monomeric enzyme GK (glucokinase) has a low affinity for glucose and, quantitatively, is largely expressed in the liver and pancreatic β-cells, playing a key ‘glucose sensing’ role to regulate hepatic glucose balance and insulin secretion. Mutations of GK in man can be inactivating, to cause a form of diabetes mellitus, or activating, to lower blood glucose levels. Recently, models of GK protein structure have helped to elucidate the role of inactivating and activating mutations, with the latter revealing an allosteric binding site, possibly for an unknown physiological activator. However, this discovery was pre-dated by Drug Discovery projects that have identified small organic molecules that activate pancreatic and liver GK enzyme activity. These compounds stimulate insulin secretion in islets and glucose metabolism in hepatocytes. The profile of these GK activators, both in vitro and in vivo and the potential role that GK activators play in lowering blood glucose levels in Type II diabetes mellitus will be discussed.