Evidence for glucose and sorbitol‐induced nuclear export of glucokinase regulatory protein in hepatocytes

Mukhtar, Mohammed Hasan; Stubbs, Mark; Agius, Loranne

doi:10.1016/s0014-5793(99)01580-x

Cited by 44 publications

(58 citation statements)

References 23 publications

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“…Based on this observation and the structural information, it is possible that the glucose and compound A have additive effects in changing GK structure to a more active form that precludes its association with GKRP. It has been reported that inactive GK is localized in the nucleus as a complex with GKRP in hepatocytes and that GK is released from the complex to the cytoplasm when the glucose concentration is elevated (19). We observed that treatment with compound A at a low glucose concentration enhanced the translocation of GK from the nucleus to the cytoplasm in both primary cultured rat hepatocytes and in rat liver tissue.…”

Section: Discussionmentioning

confidence: 47%

An Allosteric Activator of Glucokinase Impairs the Interaction of Glucokinase and Glucokinase Regulatory Protein and Regulates Glucose Metabolism

Futamura

Hosaka

Kadotani

et al. 2006

Journal of Biological Chemistry

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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 GKRPassociated 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.There are three key aspects of type 2 diabetes pathogenesis, which are the focus of current and future therapies: insulin resistance, defective insulin secretion, and increased hepatic glucose production. Glucokinase (GK) 2 is the predominant glucose phosphorylation enzyme in pancreatic ␤-cells and hepatocytes. GK plays an important role as a glucose sensor for controlling plasma glucose homeostasis by enhancing insulin secretion from pancreatic ␤-cells and glucose metabolism in the liver (1, 2), which provides rational expectations that enhancement of GK activity would be a novel therapeutic strategy for type 2 diabetes. Consistent with this rationale, recently discovered small molecule allosteric activators of GK have been demonstrated to have antidiabetic efficacy in rodents (3, 4).To investigate the mechanism of action of GK activators, the interaction between GK and glucokinase regulatory protein (GKRP) is a key aspect. It is well known that hepatic GK activity is modulated by the endogenous inhibitor, glucokinase regulatory protein (GKRP) (5-8). GK is localized in the nucleus as an inactive complex with GKRP at low glucose concentrations and is dissociated from the GK⅐GKRP complex and translocated to the cytoplasm at high glucose concentrations, which triggers glucose disposal (9). Modulators of the GK-GKRP interaction have been shown to enhance hepatic glucose disposal (7). We recently solved the co-crystal structure of hepatic GK complex with GK activator, in which GK undergoes a large conformational change between the active and inactive forms at diffe...

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Section: Discussionmentioning

confidence: 47%

An Allosteric Activator of Glucokinase Impairs the Interaction of Glucokinase and Glucokinase Regulatory Protein and Regulates Glucose Metabolism

Futamura

Hosaka

Kadotani

et al. 2006

Journal of Biological Chemistry

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“…The finding that the GK activators caused dissociation of bound GK and translocation to the cytoplasm appeared incongruous with the size exclusion data showing that GKA1 does not dissociate GK from GKRP. High glucose concentration and glucose analogs that are not metabolized by GK to a significant extent (5-thioglucose and mannoheptulose) also cause translocation of GK from the nucleus (18,27,36). It has been generally assumed that the translocation induced by glucose is due to dissociation of GK from GKRP.…”

Section: Discussionmentioning

confidence: 99%

“…We propose the following hypothesis to account for these observations. Translocation of GK from the nucleus to the cytoplasm may require two complementary mechanisms: release of GK from GKRP, which is localized predominantly but not exclusively in the nucleus (18,27), and transfer to another GK receptor that is localized predominantly in the cytoplasm (3). Various GK binding proteins have been identified by the yeast two-hybrid system (40 -42) or by random peptide library screening (41), which are potential candidates for the cytoplasmic receptor.…”

Section: Discussionmentioning

confidence: 99%

“…Immunostaining and imaging. Hepatocyte monolayers were washed in PBS and fixed in 4% paraformaldehyde in PBS (27). They were stained for GK with a rabbit IgG against human GK: 318-405 (H88, sc-7908; Santa Cruz) and FITC-labeled anti-rabbit IgG (27).…”

Section: Methodsmentioning

confidence: 99%

“…Hepatocyte monolayers were washed in PBS and fixed in 4% paraformaldehyde in PBS (27). They were stained for GK with a rabbit IgG against human GK: 318-405 (H88, sc-7908; Santa Cruz) and FITC-labeled anti-rabbit IgG (27). Imaging for fluorescein isothiocyanate (FITC) fluorescence was performed using a Nikon Eclipse E400 epifluorescence microscope with a narrow-band filter (B-2EC).…”

Section: Methodsmentioning

confidence: 99%

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Stimulation of Hepatocyte Glucose Metabolism by Novel Small Molecule Glucokinase Activators

et al. 2004

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Glucokinase (GK) has a major role in the control of blood glucose homeostasis and is a strong potential target for the pharmacological treatment of type 2 diabetes. We report here the mechanism of action of two novel and potent direct activators of GK: 6-[(3-isobutoxy-5-isopropoxybenzoyl)amino]nicotinic acid (GKA1) and 5-({3-isopropoxy-5-[2-(3-thienyl)ethoxy] benzoyl}amino)-1,3,4-thiadiazole-2-carboxylic acid (GKA2), which increase the affinity of GK for glucose by 4-and 11-fold, respectively. GKA1 increased the affinity of GK for the competitive inhibitor mannoheptulose but did not affect the affinity for the inhibitors palmitoylCoA and the endogenous 68-kDa regulator (GK regulatory protein [GKRP]), which bind to allosteric sites or to N-acetylglucosamine, which binds to the catalytic site. In hepatocytes, GKA1 and GKA2 stimulated glucose phosphorylation, glycolysis, and glycogen synthesis to a similar extent as sorbitol, a precursor of fructose 1-phosphate, which indirectly activates GK through promoting its dissociation from GKRP. Consistent with their effects on isolated GK, these compounds also increased the affinity of hepatocyte metabolism for glucose. GKA1 and GKA2 caused translocation of GK from the nucleus to the cytoplasm. This effect was additive with the effect of sorbitol and is best explained by a "glucose-like" effect of the GK activators in translocating GK to the cytoplasm. In conclusion, GK activators are potential antihyperglycemic agents for the treatment of type 2 diabetes through the stimulation of hepatic glucose metabolism by a mechanism independent of GKRP. Diabetes 53:535-541, 2004

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Liver glucokinase: An overview on the regulatorymechanisms of its activity

2011

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SummaryBlood glucose is the primary cellular substrate and in vivo must be tightly maintained. The liver plays a key role in glucose homeostasis increasing or decreasing glucose output and uptake during fasting and feeding. Glucokinase (GCK) is central to this process. Its activity is modulated in a coordinated manner via a complex set of mechanisms: in the postprandial period, the simultaneous rise in glucose and insulin increases GCK activity by enhanced gene expression, changes in cellular location, and interaction with regulatory proteins. Conversely, in the fasting state, the combined decrease in glucose and insulin concentrations and increase in glucagon concentrations, halt GCK activity. Herein we summarize the current knowledge regarding the regulation of hepatic GCK activity.

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Evidence for glucose and sorbitol‐induced nuclear export of glucokinase regulatory protein in hepatocytes

Cited by 44 publications

References 23 publications

An Allosteric Activator of Glucokinase Impairs the Interaction of Glucokinase and Glucokinase Regulatory Protein and Regulates Glucose Metabolism

An Allosteric Activator of Glucokinase Impairs the Interaction of Glucokinase and Glucokinase Regulatory Protein and Regulates Glucose Metabolism

Stimulation of Hepatocyte Glucose Metabolism by Novel Small Molecule Glucokinase Activators

Liver glucokinase: An overview on the regulatorymechanisms of its activity

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