Glucokinase (Gck) functions as a glucose sensor for insulin secretion, and in mice fed standard chow, haploinsufficiency of β cell-specific Gck (Gck +/-) causes impaired insulin secretion to glucose, although the animals have a normal β cell mass. When fed a high-fat (HF) diet, wild-type mice showed marked β cell hyperplasia, whereas Gck +/-mice demonstrated decreased β cell replication and insufficient β cell hyperplasia despite showing a similar degree of insulin resistance. DNA chip analysis revealed decreased insulin receptor substrate 2 (Irs2) expression in HF diet-fed Gck +/-mouse islets compared with wild-type islets. Western blot analyses confirmed upregulated Irs2 expression in the islets of HF diet-fed wild-type mice compared with those fed standard chow and reduced expression in HF diet-fed Gck +/-mice compared with those of HF diet-fed wild-type mice. HF diet-fed Irs2 +/-mice failed to show a sufficient increase in β cell mass, and overexpression of Irs2 in β cells of HF diet-fed Gck +/-mice partially prevented diabetes by increasing β cell mass. These results suggest that Gck and Irs2 are critical requirements for β cell hyperplasia to occur in response to HF diet-induced insulin resistance.

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Mice carrying a null mutation in the glucokinase (GK) gene in pancreatic ␤-cells, but not in the liver, were generated by disrupting the ␤-cell-specific exon. Heterozygous mutant mice showed early-onset mild diabetes due to impaired insulin-secretory response to glucose. Homozygotes showed severe diabetes shortly after birth and died within a week. 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. These data provide the first direct proof that GK serves as a glucose sensor molecule for insulin secretion and plays a pivotal role in glucose homeostasis. GK-deficient mice serve as an animal model of the insulin-secretory defect in human noninsulin-dependent diabetes mellitus.Glucokinase (GK), 1 mainly expressed in pancreatic ␤-cells and the liver, is thought to constitute a rate-limiting step in glucose metabolism in these tissues (1-4). Since insulin secretion parallels glucose metabolism and the high K m of GK (5-8 mM) ensures that it can change its enzymatic activity within the physiological range of glucose concentrations, GK has been proposed to act as a glucose sensor in the pancreatic ␤-cell (1, 5). Recently, mutations of the GK gene have been identified in patients with maturity-onset diabetes of the young, a subtype of early-onset non-insulin-dependent diabetes mellitus (NIDDM) (6 -8). However, since all the mutations in humans so far occur in the region of the gene that is common to pancreatic ␤-cells and hepatocytes (9), and are heterozygous, it may not have been possible to fully reveal physiological roles of pancreatic ␤-cell GK either in vivo or in vitro. To this end, mice carrying a null mutation in the GK gene in pancreatic ␤-cells, but not in the liver, were generated by homologous recombination. Heterozygous mutant mice showed early-onset mild diabetes resembling the phenotype for human maturity-onset diabetes of the young. Homozygotes showed severe diabetes shortly after birth and died within a week. GK-deficient islets showed defective insulin secretion in response to glucose, while they responded to other secretagogues: almost normally to arginine and to some extent to sulfonylureas. These data provide the first direct proof that GK serves as a glucose sensor molecule for insulin secretion and plays a pivotal role in glucose homeostasis. EXPERIMENTAL PROCEDURESCloning of the Mouse GK Gene, Construction of a Targeting Vector, and Homologous Recombinant Experiments-A DNA fragment including the pancreatic ␤-cell-specific exon 1␤ of the GK gene was cloned from a BALB/c mouse genomic library (Clontech). A BamHI site was introduced 30 base pairs 3Ј to the translation initiation codon of GK by the Kunkel method (10). A neomycin resistance gene (neo r ) with a pgk-1 promoter but without a poly(A) ϩ addition signal was substituted for the XbaI-BamHI fragment in the exon 1␤. A diphtheria toxin A fragment gene (DTA) with a MC1 promoter was lig...

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