Control of pancreatic β cell regeneration by glucose metabolism

Recent studies revealed a surprising regenerative capacity of insulin-producing β cells in mice, suggesting that regenerative therapy for human diabetes could in principle be achieved. Physiologic β cell regeneration under stressed conditions relies on accelerated proliferation of surviving β cells, but the factors that trigger and control this response remain unclear. Using islet transplantation experiments, we show that β cell mass is controlled systemically rather than by local factors such as tissue damage. Chronic changes in β cell glucose metabolism, rather than blood glucose levels per se, are the main positive regulator of basal and compensatory β cell proliferation in vivo. Intracellularly, genetic and pharmacologic manipulations reveal that glucose induces β cell replication via metabolism by glucokinase, the first step of glycolysis, followed by closure of K(ATP) channels and membrane depolarization. Our data provide a molecular mechanism for homeostatic control of β cell mass by metabolic demand.

Discovery of a novel site regulating glucokinase activity following characterization of a new mutation causing hyperinsulinemic hypoglycemia in humans

Type 2 diabetes is a global problem, and current ineffective therapeutic strategies pave the way for novel treatments like small molecular activators targeting glucokinase (GCK). GCK activity is fundamental to beta cell and hepatocyte glucose metabolism, and heterozygous activating and inactivating GCK mutations cause hyperinsulinemic hypoglycemia (HH) and maturity onset diabetes of the young (MODY) respectively. Over 600 naturally occurring inactivating mutations have been reported, whereas only 13 activating mutations are documented to date. We report two novel GCK HH mutations (V389L and T103S) at residues where MODY mutations also occur (V389D and T103I). Using recombinant proteins with in vitro assays, we demonstrated that both HH mutants had a greater relative activity index than wild type (6.0 for V389L, 8.4 for T103S, and 1.0 for wild type). This was driven by an increased affinity for glucose (S(0.5), 3.3 ± 0.1 and 3.5 ± 0.1 mm, respectively) versus wild type (7.5 ± 0.1 mm). Correspondingly, the V389D and T103I MODY mutants had markedly reduced relative activity indexes (<0.1). T103I had an altered affinity for glucose (S(0.5), 24.9 ± 0.6 mm), whereas V389D also exhibited a reduced affinity for ATP and decreased catalysis rate (S(0.5), 78.6 ± 4.5 mm; ATP(K(m)), 1.5 ± 0.1 mm; K(cat), 10.3 ± 1.1s(-1)) compared with wild type (ATP(K(m)), 0.4 ± <0.1; K(cat), 62.9 ± 1.2). Both Thr-103 mutants showed reduced inhibition by the endogenous hepatic inhibitor glucokinase regulatory protein. Molecular modeling demonstrated that Thr-103 maps to the allosteric activator site, whereas Val-389 is located remotely to this position and all other previously reported activating mutations, highlighting α-helix 11 as a novel region regulating GCK activity. Our data suggest that pharmacological manipulation of GCK activity at locations distal from the allosteric activator site is possible.

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.

Glucokinase activation induces potent hypoglycemia without recruiting insulin and inhibits food intake in chicken

Glucose homeostasis exhibits several peculiarities in chickens (in short, presence of high glycemia and resistance to high doses of exogenous insulin). Though the full chicken glucokinase gene sequence is still lacking, several results suggest its existence. The functionality of chicken glucokinase (GK) has been further investigated using an activator of mammalian GK (GKA). In vitro, GKA decreased GK's S0.5(a) in a glucose-dependent manner in liver homogenates from either fasted or fed chickens; it also increased GK Vmax(a) in homogenates from fed chickens. In vivo, acute oral GKA administration (10-100 mg/kg) induced a potent and dose dependent hypoglycemic effect in fed chickens (starting between 15 and 45 min with a maximum effect at 40 mg/kg, P<0.0001). At this dose, plasma insulin levels showed erratic and minor changes in the early times (an increase at 5 min and a decrease at 10 min, P<0.05). At 90 min, when hypoglycemia had developed plasma insulin levels decreased under controls and plasma pancreatic glucagon levels increased over controls. Also at 40 mg/kg, GKA transiently inhibited food intake at about 3h (P<0.0001). In conclusion, GKA is a potent activator of chicken GK evidencing that the structure and the activity of chicken GK are similar to those of mammalian GK. At variance with results obtained in mammals, the potent GKA hypoglycemic action appears to rely mostly on an effect on liver GK in chicken. This fits with previous results and further support the hypothesis of a "deficient coupling" between Β-cell metabolism and insulin release in this species.

Glucose and aging control the quiescence period that follows pancreatic beta cell replication

Pancreatic beta cell proliferation has emerged as the principal mechanism for homeostatic maintenance of beta cell mass during adult life. This underscores the importance of understanding the mechanisms of beta cell replication and suggests novel approaches for regenerative therapy to treat diabetes. Here we use an in vivo pulse-chase labeling assay to investigate the replication dynamics of adult mouse beta cells. We find that replicated beta cells are able to re-enter the cell division cycle shortly after mitosis and regain their normal proliferative potential after a short quiescence period of several days. This quiescence period is lengthened with advanced age, but shortened during injury-driven beta cell regeneration and following treatment with a pharmacological activator of glucokinase, providing strong evidence that metabolic demand is a key determinant of cell cycle re-entry. Lastly, we show that cyclin D2, a crucial factor in beta cell replication, is downregulated during cell division, and is slowly upregulated post-mitosis by a glucose-sensitive mechanism. These results demonstrate that beta cells quickly regain their capacity to re-enter the cell cycle post-mitosis and implicate glucose control of cyclin D2 expression in the regulation of this process.

Discovery, structure-activity relationships, pharmacokinetics, and efficacy of glucokinase activator (2R)-3-cyclopentyl-2-(4-methanesulfonylphenyl)-N-thiazol-2-yl-propionamide (RO0281675)

Glucokinase (GK) is a glucose sensor that couples glucose metabolism to insulin release. The important role of GK in maintaining glucose homeostasis is illustrated in patients with GK mutations. In this publication, identification of the hit molecule 1 and its SAR development, which led to the discovery of potent allosteric GK activators 9a and 21a, is described. Compound 21a (RO0281675) was used to validate the clinical relevance of targeting GK to treat type 2 diabetes.

4-Substituted-7-N-alkyl-N-acetyl 2-aminobenzothiazole amides: drug-like and non-xanthine based A2B adenosine receptor antagonists

7-N-Acetamide-4-methoxy-2-aminobenzothiazole 4-fluorobenzamide (compound 1) was chosen as a drug-like and non-xanthine based starting point for the discovery of A(2B) receptor antagonists because of its slight selectivity against A(1) and A(2A) receptors and modest A(2B) potency. SAR exploration of compound 1 described herein included modifications to the 7-N-acetamide group, substitution of the 4-methoxy group by halogens as well as replacement of the p-flouro-benzamide side chain. This work culminated in the identification of compound 37 with excellent A(2B) potency, modest selectivity versus A(2A) and A(1) receptors, and good rodent PK properties.

Clinical heterogeneity in monogenic diabetes caused by mutations in the glucokinase gene (GCK-MODY)

OBJECTIVE

To evaluate the heterogeneity in the clinical expression in a family with glucokinase mature-onset diabetes of the young (GCK-MODY).

RESEARCH DESIGN AND METHODS

Members (three generations) of the same family presented either with overt neonatal hyperglycemia, marked postprandial hyperglycemia, or glucosuria. Homeostasis model assessment of insulin resistance (HOMA(IR)) and insulinogenic and disposition indexes were calculated. Oral glucose tolerance test (OGTT) results in the GCK mutation carriers from this family were compared with those from other subjects with GCK mutations in the same codon (GCK(261)), with other missense and other types of GCK mutations in different codons from the European MODY Consortium database (GCK(m)).

RESULTS

Mutation G261R was found in the GCK gene. During the OGTT, glucose (P = 0.02) and insulin (P = 0.009) response at 2 h as well as at the 2-h glucose increment (GCK(261) versus other missense GCK mutations, P = 0.003) were significantly higher in GCK(261) than in GCK(m) carriers.

CONCLUSIONS

Differing from other GCK(m) carriers, the glucose and insulin response to oral glucose was significantly higher in GCK(261) carriers, indicating clinical heterogeneity in GCK-MODY.

Extremes of clinical and enzymatic phenotypes in children with hyperinsulinism caused by glucokinase activating mutations

OBJECTIVE

Heterozygous activating mutations of glucokinase have been reported to cause hypoglycemia attributable to hyperinsulinism in a limited number of families. We report three children with de novo glucokinase hyperinsulinism mutations who displayed a spectrum of clinical phenotypes corresponding to marked differences in enzyme kinetics.

RESEARCH DESIGN AND METHODS

Mutations were directly sequenced, and mutants were expressed as glutathionyl S-transferase-glucokinase fusion proteins. Kinetic analysis of the enzymes included determinations of stability, activity index, the response to glucokinase activator drug, and the effect of glucokinase regulatory protein.

RESULTS

Child 1 had an ins454A mutation, child 2 a W99L mutation, and child 3 an M197I mutation. Diazoxide treatment was effective in child 3 but ineffective in child 1 and only partially effective in child 2. Expression of the mutant glucokinase ins454A, W99L, and M197I enzymes revealed a continuum of high relative activity indexes in the three children (26, 8.9, and 3.1, respectively; wild type = 1.0). Allosteric responses to inhibition by glucokinase regulatory protein and activation by the drug RO0281675 were impaired by the ins454A but unaffected by the M197I mutation. Estimated thresholds for glucose-stimulated insulin release were more severely reduced by the ins454A than the M197I mutation and intermediate in the W99L mutation (1.1, 3.5, and 2.2 mmol/l, respectively; wild type = 5.0 mmol/l).

CONCLUSIONS

These results confirm the potency of glucokinase as the pancreatic beta-cell glucose sensor, and they demonstrate that responsiveness to diazoxide varies with genotype in glucokinase hyperinsulinism resulting in hypoglycemia, which can be more difficult to control than previously believed.

A glucose sensor role for glucokinase in anterior pituitary cells

Enzymatic activity of glucokinase was demonstrated, quantitated, and characterized kinetically in rat and mouse pituitary extracts using a highly specific and sensitive spectrometric assay. A previously proposed hypothesis that the glucokinase gene might be expressed in the pituitary corticotrophic cells was therefore reexamined using mRNA in situ hybridization and immunohistochemical techniques. No evidence was found that corticotrophs are glucokinase positive, and the identity of glucokinase-expressing cells remains to be determined. The findings do, however, suggest a novel hypothesis that a critical subgroup of anterior pituitary cells might function as glucose sensor cells and that direct fuel regulation of such cells may modify the classical indirect neuroendocrine pathways that are known to control hormone secretion from anterior pituitary cells.

From clinicogenetic studies of maturity-onset diabetes of the young to unraveling complex mechanisms of glucokinase regulation

Glucokinase functions as a glucose sensor in pancreatic beta-cells and regulates hepatic glucose metabolism. A total of 83 probands were referred for a diagnostic screening of mutations in the glucokinase (GCK) gene. We found 11 different mutations (V62A, G72R, L146R, A208T, M210K, Y215X, S263P, E339G, R377C, S453L, and IVS5 + 1G>C) in 14 probands. Functional characterization of recombinant glutathionyl S-transferase-G72R glucokinase showed slightly increased activity, whereas S263P and G264S had near-normal activity. The other point mutations were inactivating. S263P showed marked thermal instability, whereas the stability of G72R and G264S differed only slightly from that of wild type. G72R and M210K did not respond to an allosteric glucokinase activator (GKA) or the hepatic glucokinase regulatory protein (GKRP). Mutation analysis of the role of glycine at position 72 by substituting E, F, K, M, S, or Q showed that G is unique since all these mutants had very low or no activity and were refractory to GKRP and GKA. Structural analysis provided plausible explanations for the drug resistance of G72R and M210K. Our study provides further evidence that protein instability in combination with loss of control by a putative endogenous activator and GKRP could be involved in the development of hyperglycemia in maturity-onset diabetes of the young, type 2. Furthermore, based on data obtained on G264S, we propose that other and still unknown mechanisms participate in the regulation of glucokinase.

Small Molecule Glucokinase Activators as Glucose Lowering Agents: A New Paradigm for Diabetes Therapy

Glucokinase (GK) is a molecular sensor that regulates glucose induced insulin secretion in pancreatic beta-cells and glucose homeostasis in the liver via catalysis of glucose to glucose-6-phosphate. The recent discovery and development of small molecule glucokinase activators represents a potentially important development for the management of type 2 diabetes. Since the discovery of the first orally active small molecule GK activator RO0281675, a number of research groups have reported the identification of potent activators. In this review, the biological significance of GK in whole body glucose homeostasis is briefly described coupled with the recent progress regarding the identification of novel small molecule GK activators.

The network of glucokinase-expressing cells in glucose homeostasis and the potential of glucokinase activators for diabetes therapy

The glucose-phosphorylating enzyme glucokinase has structural, kinetic, and molecular genetic features that are ideal for its primary role as glucose sensor in a network of neuro/endocrine sentinel cells that maintain glucose homeostasis in many vertebrates including humans. The glucokinase-containing, insulin-producing beta-cells of the pancreas take the prominent lead in this network, functioning in the aggregate as the master gland. The beta-cells are also conceptualized as the prototype for all other glucose sensor cells, which determines our current understanding of many extrapancreatic glucose sensors. About 99% of the enzyme resides, however, in the hepato-parenchymal cells and serves its second role in a high-capacity process of blood glucose clearance. Two examples strikingly illustrate how pivotal a position glucokinase has in the regulation of glucose metabolism: 1) activating and inactivating mutations of the enzyme cause hypo- and hyperglycemia syndromes in humans described collectively as "glucokinase disease" and fully explained by the glucose sensor paradigm, and 2) glucokinase activator drugs (GKAs) have been discovered that bind to an allosteric site and increase the kcat and lower the glucose S(0.5) of the enzyme. GKAs enhance glucose-stimulated insulin release from pancreatic islets and glucose disposition by the liver. They are now intensively explored to develop a novel treatment for diabetes. Future biophysical, molecular, genetic, and pharmacological studies hold much promise to unravel the evolving complexity of the glucokinase glucose sensor system.

Targeting glucokinase activation for the treatment of type 2 diabetes--a status review

Glucokinase (GK) plays a key role in glucose homeostasis. Developments over the past decade, such as the determination of the function of GK regulatory protein, the discovery of GK mutations related to maturity onset of diabetes of the young, permanent neonatal diabetes mellitus and persistent hyperinsulinemia hypoglycemia of infancy, and the discovery of novel GK activators (GKAs) and their X-ray co-crystal structures with GK, have significantly enhanced our understanding of GK structure and function. This review discusses key publications on GKAs that report full characterization, key compound disclosures from patents, and a current hypothesis on the mechanism of GK activation based on the co-crystal structures of GK-GKA complexes.

Glucokinase and glucose homeostasis: proven concepts and new ideas

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

Insights into the structure and regulation of glucokinase from a novel mutation (V62M), which causes maturity-onset diabetes of the young

Glucokinase (GCK) serves as the pancreatic glucose sensor. Heterozygous inactivating GCK mutations cause hyperglycemia, whereas activating mutations cause hypoglycemia. We studied the GCK V62M mutation identified in two families and co-segregating with hyperglycemia to understand how this mutation resulted in reduced function. Structural modeling locates the mutation close to five naturally occurring activating mutations in the allosteric activator site of the enzyme. Recombinant glutathionyl S-transferase-V62M GCK is paradoxically activated rather than inactivated due to a decreased S0.5 for glucose compared with wild type (4.88 versus 7.55 mM). The recently described pharmacological activator (RO0281675) interacts with GCK at this site. V62M GCK does not respond to RO0281675, nor does it respond to the hepatic glucokinase regulatory protein (GKRP). The enzyme is also thermally unstable, but this lability is apparently less pronounced than in the proven instability mutant E300K. Functional and structural analysis of seven amino acid substitutions at residue Val62 has identified a non-linear relationship between activation by the pharmacological activator and the van der Waals interactions energies. Smaller energies allow a hydrophobic interaction between the activator and glucokinase, whereas larger energies prohibit the ligand from fitting into the binding pocket. We conclude that V62M may cause hyperglycemia by a complex defect of GCK regulation involving instability in combination with loss of control by a putative endogenous activator and/or GKRP. This study illustrates that mutations that cause hyperglycemia are not necessarily kinetically inactivating but may exert their effects by other complex mechanisms. Elucidating such mechanisms leads to a deeper understanding of the GCK glucose sensor and the biochemistry of beta-cells and hepatocytes.

Rapid translocation of hepatic glucokinase in response to intraduodenal glucose infusion and changes in plasma glucose and insulin in conscious rats

The rate of liver glucokinase (GK) translocation from the nucleus to the cytoplasm in response to intraduodenal glucose infusion and the effect of physiological rises of plasma glucose and/or insulin on GK translocation were examined in 6-h-fasted conscious rats. Intraduodenal glucose infusion (28 mg.kg(-1).min(-1) after a priming dose at 500 mg/kg) elevated blood glucose levels (mg/dl) in the artery and portal vein from 90 +/- 3 and 87 +/- 3 to 154 +/- 4 and 185 +/- 4, respectively, at 10 min. At 120 min, the levels had decreased to 133 +/- 6 and 156 +/- 5, respectively. Plasma insulin levels (ng/ml) in the artery and the portal vein rose from 0.7 +/- 0.1 and 1.8 +/- 0.3 to 11.8 +/- 1.5 and 20.2 +/- 2.0 at 10 min, respectively, and 12.4 +/- 3.1 and 18.0 +/- 4.8 at 30 min, respectively. GK was rapidly exported from the nucleus as determined by measuring the ratio of the nuclear to the cytoplasmic immunofluorescence (N/C) of GK (2.9 +/- 0.3 at 0 min to 1.7 +/- 0.2 at 10 min, 1.5 +/- 0.1 at 20 min, 1.3 +/- 0.1 at 30 min, and 1.3 +/- 0.1 at 120 min). When plasma glucose (arterial; mg/dl) and insulin (arterial; ng/ml) levels were clamped for 30 min at 93 +/- 7 and 0.7 +/- 0.1, 81 +/- 5 and 8.9 +/- 1.3, 175 +/- 5 and 0.7 +/- 0.1, or 162 +/- 5 and 9.2 +/- 1.5, the N/C of GK was 3.0 +/- 0.5, 1.8 +/- 0.1, 1.5 +/- 0.1, and 1.2 +/- 0.1, respectively. The N/C of GK regulatory protein (GKRP) did not change in response to the intraduodenal glucose infusion or the rise in plasma glucose and/or insulin levels. The results suggest that GK but not GKRP translocates rapidly in a manner that corresponds with changes in the hepatic glucose balance in response to glucose ingestion in vivo. Additionally, the translocation of GK is induced by the postprandial rise in plasma glucose and insulin.

Allosteric activators of glucokinase: potential role in diabetes therapy

Glucokinase (GK) plays a key role in whole-body glucose homeostasis by catalyzing the phosphorylation of glucose in cells that express this enzyme, such as pancreatic beta cells and hepatocytes. We describe a class of antidiabetic agents that act as nonessential, mixed-type GK activators (GKAs) that increase the glucose affinity and maximum velocity (Vmax) of GK. GKAs augment both hepatic glucose metabolism and glucose-induced insulin secretion from isolated rodent pancreatic islets, consistent with the expression and function of GK in both cell types. In several rodent models of type 2 diabetes mellitus, GKAs lowered blood glucose levels, improved the results of glucose tolerance tests, and increased hepatic glucose uptake. These findings may lead to the development of new drug therapies for diabetes.

Glucokinase gene locus transgenic mice are resistant to the development of obesity-induced type 2 diabetes

Transgenic mice that overexpress the entire glucokinase (GK) gene locus have been previously shown to be mildly hypoglycemic and to have improved tolerance to glucose. To determine whether increased GK might also prevent or diminish diabetes in diet-induced obese animals, we examined the effect of feeding these mice a high-fat high-simple carbohydrate low-fiber diet (HF diet) for 30 weeks. In response to this diet, both normal and transgenic mice became obese and had similar BMIs (5.3 +/- 0.1 and 5.0 +/- 0.1 kg/m2 in transgenic and non-transgenic mice, respectively). The blood glucose concentration of the control mice increased linearly with time and reached 17.0 +/- 1.3 mmol/l at the 30th week. In contrast, the blood glucose of GK transgenic mice rose to only 9.7 +/- 1.2 mmol/l at the 15th week, after which it returned to 7.6 +/- 1.0 mmol/l by the 30th week. The plasma insulin concentration was also lower in the GK transgenic animals (232 +/- 79 pmol/l) than in the controls (595 +/- 77 pmol/l), but there was no difference in plasma glucagon concentrations. Together, these data indicate that increased GK levels dramatically lessen the development of both hyperglycemia and hyperinsulinemia associated with the feeding of an HF diet.

Characterization of glucokinase regulatory protein-deficient mice

The glucokinase regulatory protein (GKRP) inhibits glucokinase competitively with respect to glucose by forming a protein-protein complex with this enzyme. The physiological role of GKRP in controlling hepatic glucokinase activity was addressed using gene targeting to disrupt GKRP gene expression. Heterozygote and homozygote knockout mice have a substantial decrease in hepatic glucokinase expression and enzymatic activity as measured at saturating glucose concentrations when compared with wild-type mice, with no change in basal blood glucose levels. Interestingly, when assayed under conditions to promote the association between glucokinase and GKRP, liver glucokinase activity in wild-type and null mice displayed comparable glucose phosphorylation capacities at physiological glucose concentrations (5 mM). Thus, despite reduced hepatic glucokinase expression levels in the null mice, glucokinase activity in the liver homogenates was maintained at nearly normal levels due to the absence of the inhibitory effects of GKRP. However, following a glucose tolerance test, the homozygote knockout mice show impaired glucose clearance, indicating that they cannot recruit sufficient glucokinase due to the absence of a nuclear reserve. These data suggest both a regulatory and a stabilizing role for GKRP in maintaining adequate glucokinase in the liver. Furthermore, this study provides evidence for the important role GKRP plays in acutely regulating of hepatic glucose metabolism.

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

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