The ␤-cell is a highly specialized cell with a unique differentiation that optimizes glucose-induced insulin secretion (GIIS). Here, we evaluated changes in gene expression that accompany ␤-cell dysfunction in the db/db mouse model of type 2 diabetes. In db/db islets, mRNA levels of many genes implicated in ␤-cell glucose sensing were progressively reduced with time, as were several transcription factors important for the maintenance of ␤-cell differentiation. Conversely, genes normally suppressed in ␤-cells, such as a variety of stress response mediators and inhibitor of differentiation/ DNA binding 1, a gene capable of inhibiting differentiation, were markedly increased. We assessed whether this global alteration in the pattern of ␤-cell gene expression was related more to chronic hyperglycemia or hyperlipidemia; db/db mice were treated with phlorizin, which selectively lowered plasma glucose, or bezafibrate, which selectively lowered plasma lipids. GIIS as well as the majority of the changes in gene expression were completely normalized by lowering glucose but were unaffected by lowering lipids. However, the restoration of GIIS was not accompanied by normalized uncoupling protein 2 or peroxisome proliferator-activated receptor ␥ mRNA levels, which were upregulated in db/db islets. These studies demonstrate that hyperglycemia, independent of plasma lipid levels, is sufficient for the loss of ␤-cell differentiation and secretory function in db/db mice. Diabetes 54:2755-2763, 2005 T he ␤-cell possesses a unique metabolic profile that allows it to respond to nutrient secretagogues over their physiological concentration range (1). During the progression to type 2 diabetes, an early observed functional defect is the selective loss of acute glucose-induced insulin secretion (GIIS), whereas secretion in response to other secretogogues is preserved (2,3). The mechanism(s) responsible for the selective loss of GIIS in type 2 diabetes remains unknown. It has been proposed that chronically elevated blood glucose and fatty acid levels, resulting from obesity and insulin resistance, lead to a loss of the unique expression pattern of genes necessary for appropriate GIIS (4,5). This exacerbates hyperglycemia and hyperlipidemia, which causes further ␤-cell dedifferentiation. However, the relative importance of hyperglycemia and hyperlipidemia as potential causes of ␤-cell failure in diabetes remains an unresolved issue.In the present study, we have used the C57BL/KsJ db/db mouse model to evaluate possible molecular mechanisms that underlie ␤-cell dysfunction in diabetes. Diabetes arises in db/db mice because of insufficient ␤-cell compensation for time-dependent increases in obesity and insulin resistance (6,7). The mice display marked hyperglycemia and hyperlipidemia in association with insulin secretory defects that resemble those found in human diabetes. The deterioration in islet structure and insulin secretory capacity in db/db mice can be delayed with dietary restrictions (8) and improved with the use of insulin-sensiti...