Disease relevance of Gck

• This study has confirmed the results of a previous report that human subjects carrying mutations in Gck had reduced birth weights and has provided direct evidence for a link between insulin and fetal growth [1].
• GLK-deficient mice die perinatally with severe hyperglycemia [2].
• Patients with maturity-onset diabetes of the young (MODY) have heterozygous point mutations in the glucokinase gene that result in reduced enzymatic activity and decreased insulin secretion [3].
• Glucokinase (GK) gene mutations cause diabetes mellitus in both humans and mouse models, but the pathophysiological basis is only partially defined [4].
• These defects were not due to a decrease in glucokinase or insulin gene transcription. beta cell mass adjusted for body weight was not reduced in the (-/-) animals, although pancreatic insulin content adjusted for pancreas weight was slightly lower (0.06+/-0.01 vs. 0.10+/-0.01 microg/mg, P < 0.01) than in the (+/+) animals [5].

High impact information on Gck

• In mice with only one GLK allele, blood glucose levels are elevated and insulin secretion is reduced [2].
• As phosphorylation by glucokinase (GLK) appears to be the rate-limiting step for glucose catabolism in beta cells, this enzyme may be the glucose sensor [2].
• Transgenic knockouts reveal a critical requirement for pancreatic beta cell glucokinase in maintaining glucose homeostasis [2].
• We detected in the liver insulin and other islet-specific transcripts, including proinsulin-processing enzymes, beta-cell-specific glucokinase and sulfonylurea receptor [6].
• Mice haploinsufficient for beta cell glucokinase (Gck) were unable to increase their beta cell mass in response to insulin resistance produced by high-fat feeding [7].

Chemical compound and disease context of Gck

• These findings demonstrate a key role for glucokinase in glucose homeostasis and implicate both islets and liver in the MODY disease [3].
• However, these mice exhibited impaired glucose tolerance due in part to reduced expression of hepatic glucokinase, and hyperammonemia from reduced expression of hepatic carbamoyl phosphate synthase-I [8].
• Systemic treatment with sympatholytic dopamine agonists improves aberrant beta-cell hyperplasia and GLUT2, glucokinase, and insulin immunoreactive levels in ob/ob mice [9].

Biological context of Gck

• To elucidate the interplay between insulin resistance and insulin secretory defect for the development of NIDDM, we generated double knockout mice with disruption of IRS-1 and beta cell GK genes by crossing the mice with each of the single gene knockout [10].
• Hepatic glucokinase (GK) catalyzes the phosphorylation of glucose to glucose 6-phosphate (G6P), a step which is essential for glucose metabolism in liver as well as for the induction of glycolytic and lipogenic genes [11].
• Liver glucokinase gene expression is controlled by the onecut transcription factor hepatocyte nuclear factor-6 [12].
• AIMS/HYPOTHESIS: Glucokinase plays a key role in glucose homeostasis and the expression of its gene is differentially regulated in pancreatic beta cells and in the liver through distinct promoters [12].
• METHODS: We tested the binding of recombinant HNF-6 to DNA sequences from the mouse hepatic glucokinase promoter in vitro and the effect of HNF-6 on promoter activity in transfected cells [12].

Anatomical context of Gck

• To determine whether the effect of SREBP-1c requires GK expression and subsequent glucose metabolism, a transcriptionally active form of SREBP-1c was overexpressed both in vivo and in primary cultures of control and hGK-KO hepatocytes [11].
• Insulin regulates GK activity by modulating its association with secretory granules, although little is known about the mechanisms involved in regulating this association [13].
• We used MIN6 insulin-secretory cells and organelle fractionation to determine the effects of glucose on the subcellular distribution of glucokinase [14].
• A construct containing 1,000 bp of 5'-flanking DNA was efficiently expressed in HIT M2.2.2 cells, a beta-cell-derived line that makes both insulin and glucokinase, but not in NIH 3T3 cells, a heterologous cell line [15].
• The present study was undertaken to test the hypothesis that exposure to high glucose concentrations enhances insulin secretion in pancreatic islets from glucokinase-deficient mice [16].

Associations of Gck with chemical compounds

• Heterozygous mice with beta cell glucokinase (GK) gene knockout showed impaired glucose tolerance due to decreased insulin secretion to glucose [10].
• Mutation of cysteine 371 to serine blocks S-nitrosylation of GK and causes GK to remain tightly bound to secretory granules [13].
• We show here that overexpressing SREBP-1c specifically in the liver of diabetic mice induces GK and lipogenic enzyme gene expression and represses the expression of phosphoenolpyruvate carboxykinase, a key enzyme of the gluconeogenic pathway [17].
• Positional candidate gene analyses revealed an A to T transversion mutation in exon 9 of the glucokinase gene, resulting in an isoleucine to phenylalanine change at amino acid 366 (I366F) [18].
• GK-deficient islets isolated from homozygotes showed defective insulin secretion in response to glucose, while they responded to other secretagogues: almost normally to arginine and to some extent to sulfonylureas [19].

Physical interactions of Gck

• RESULTS: HNF-6 bound to the hepatic promoter of the glucokinase gene and stimulated its activity [12].
• GK was also found to interact stably with neuronal NOS as detected by coimmunoprecipitation and fluorescence resonance energy transfer [13].

Co-localisations of Gck

• Subfractionation of the insulin granule components by hypotonic lysis followed by sucrose gradient centrifugation showed that glucokinase colocalized with the granule membrane marker phogrin and not with insulin [14].

Regulatory relationships of Gck

• We hypothesized that HNF-6 controls the activity of the hepatic glucokinase promoter [12].
• This suggested that IL-1beta and TNF-alpha downregulate gene expression of GLUT2 and glucokinase in pancreatic beta-cells [20].
• The glucokinase regulatory protein (GKRP) inhibits glucokinase competitively with respect to glucose by forming a protein-protein complex with this enzyme [21].
• Moreover, high fructose 2,6-bisphosphate levels downregulated glucose-6-phosphatase and upregulated glucokinase gene expression, thereby reversing the insulin-resistant pattern of hepatic gene expression of these two key glucose-metabolic enzymes [22].

Other interactions of Gck

• Inactivation of the hnf6 gene did not modify the pattern of deoxyribonuclease I hypersensitive sites but was associated with a decrease of liver glucokinase mRNA to half the control value [12].
• BAD is required to assemble the complex in that Bad-deficient hepatocytes lack this complex, resulting in diminished mitochondria-based glucokinase activity and blunted mitochondrial respiration in response to glucose [23].
• Pancreatic beta-cell-specific targeted disruption of glucokinase gene. Diabetes mellitus due to defective insulin secretion to glucose [19].
• Reduced islet glucokinase activity causes mildly elevated fasting blood glucose levels [3].
• 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 [24].

Analytical, diagnostic and therapeutic context of Gck

• Animal model for maturity-onset diabetes of the young generated by disruption of the mouse glucokinase gene [3].
• A complementary DNA for glucokinase (GK) was cloned from mouse liver total RNA by a combination of the polymerase chain reaction (PCR) and mouse liver cDNA library screening [25].
• METHODS: We performed hepatocyte-specific gene knockout of glucokinase in mice using Cre-loxP gene targeting strategy [26].
• 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 [27].
• These findings suggested that insulin and glucokinase might be expressed in skeletal muscle, using adeno-associated viral 1 (AAV1) vectors as a new gene therapy approach for diabetes [28].

References