The relentless decline in ␤-cell function frequently observed in type 2 diabetic patients, despite optimal drug management, has variously been attributed to glucose toxicity and lipotoxicity. The former theory posits hyperglycemia, an outcome of the disease, as a secondary force that further damages ␤-cells. The latter theory suggests that the often-associated defect of hyperlipidemia is a primary cause of ␤-cell dysfunction. We review evidence that patients with type 2 diabetes continually undergo oxidative stress, that elevated glucose concentrations increase levels of reactive oxygen species in ␤-cells, that islets have intrinsically low antioxidant enzyme defenses, that antioxidant drugs and overexpression of antioxidant enzymes protect ␤-cells from glucose toxicity, and that lipotoxicity, to the extent it can be attributable to hyperlipidemia, occurs only in the context of preexisting hyperglycemia, whereas glucose toxicity can occur in the absence of hyperlipidemia. Diabetes 53 (Suppl. 1):S119 -S124, 2004

Chronic exposure to hyperglycemia can lead to cellular dysfunction that may become irreversible over time, a process that is termed glucose toxicity. Our perspective about glucose toxicity as it pertains to the pancreatic ␤-cell is that the characteristic decreases in insulin synthesis and secretion are caused by decreased insulin gene expression. The responsible metabolic lesion appears to involve a posttranscriptional defect in pancreas duodenum homeobox-1 (PDX-1) mRNA maturation. PDX-1 is a critically important transcription factor for the insulin promoter, is absent in glucotoxic islets, and, when transfected into glucotoxic ␤-cells, improves insulin promoter activity. Because reactive oxygen species are produced via oxidative phosphorylation during anaerobic glycolysis, via the Schiff reaction during glycation, via glucose autoxidation, and via hexosamine metabolism under supraphysiological glucose concentrations, we hypothesize that chronic oxidative stress is an important mechanism for glucose toxicity. Support for this hypothesis is found in the observations that high glucose concentrations increase intraislet peroxide levels, that islets contain very low levels of antioxidant enzyme activities, and that adenoviral overexpression of antioxidant enzymes in vitro in islets, as well as exogenous treatment with antioxidants in vivo in animals, protect the islet from the toxic effects of excessive glucose levels. Clinically, consideration of antioxidants as adjunct therapy in type 2 diabetes is warranted because of the many reports of elevated markers of oxidative stress in patients with this disease, which is characterized by imperfect management of glycemia, consequent chronic hyperglycemia, and relentless deterioration of ␤-cell function. Diabetes 52: 581-587, 2003

Chronic exposure of pancreatic islets to supraphysiologic concentrations of glucose causes adverse alterations in ␤ cell function, a phenomenon termed glucose toxicity and one that may play a secondary pathogenic role in type 2 diabetes. However, no mechanism of action has been definitively identified for glucose toxicity in ␤ cells. To ascertain whether chronic oxidative stress might play a role, we chronically cultured the ␤ cell line, HIT-T15, in medium containing 11.1 mM glucose with and without the antioxidants, N-acetyl-L-cysteine (NAC) or aminoguanidine (AG). Addition of NAC or AG to the culture medium at least partially prevented decreases in insulin mRNA, insulin gene promoter activity, DNA binding of two important insulin promoter transcription factors (PDX-1͞STF-1 and RIPE-3b1 activator), insulin content, and glucose-induced insulin secretion. These findings suggested that one mechanism of glucose toxicity in the ␤ cell may be chronic exposure to reactive oxygen species, i.e., chronic oxidative stress. To ascertain the effects of these drugs on diabetes, NAC or AG was given to Zucker diabetic fatty rats, a laboratory model of type 2 diabetes, from 6 through 12 weeks of age. Both drugs prevented a rise in blood oxidative stress markers (8-hydroxy-2-deoxyguanosine and malondialdehyde ؉ 4-hydroxy-2-nonenal), and partially prevented hyperglycemia, glucose intolerance, defective insulin secretion as well as decrements in ␤ cell insulin content, insulin gene expression, and PDX-1 (STF-1) binding to the insulin gene promoter. We conclude that chronic oxidative stress may play a role in glucose toxicity, which in turn may worsen the severity of type 2 diabetes.

Antioxidant drugs have been reported to protect pancreatic islets from the adverse effects of chronic exposure to supraphysiological glucose concentrations. However, glucose has not been shown to increase intracellular oxidant load in islets, nor have the effects of increasing or inhibiting glutathione peroxidase (GPx) activity on islet resistance to sugar-induced oxidant stress been studied. We observed that high glucose concentrations increased intracellular peroxide levels in human islets and the pancreatic ␤ cell line, HIT-T15. Inhibition of ␥-glutamylcysteine synthetase (␥GCS) by buthionine sulfoximine augmented the increase in islet peroxide and decrease in insulin mRNA levels, content, and secretion in islets and HIT-T15 cells induced by ribose. Adenoviral overexpression of GPx increased GPx activity and protected islets against adverse effects of ribose. These results demonstrate that glucose and ribose increase islet peroxide accumulation and that the adverse consequences of ribose-induced oxidative stress on insulin mRNA, content, and secretion can be augmented by a glutathione synthesis inhibitor and prevented by increasing islet GPx activity. These observations support the hypothesis that oxidative stress is one mechanism for glucose toxicity in pancreatic islets. T ype 2 diabetes mellitus is polygenic in origin and usually begins in adulthood, although specific genes for subtypes of this disease that occur earlier in life, referred to collectively as maturity-onset diabetes of the young (MODY), have been identified (1). The onset of type 2 diabetes is insidious, thus hyperglycemia develops gradually and often goes untreated for years until symptoms become clinically obvious. Consequent chronic exposure of tissues to supraphysiologic levels of blood glucose can lead to adverse intracellular outcomes, a process known as glucose toxicity (2-5). Possible mechanisms of action for glucose toxicity include the formation of advanced glycosylation end products and glucosamine, increased protein kinase C activity with c-myc induction, autooxidation of glucose, and increased levels of reactive glycolytic intermediates such as glyceraldehyde-3-phosphate or dihydroxyacetone phosphate (6 -13). All these processes usually are accompanied by the formation of reactive oxygen species (ROS), setting up the potential for oxidative stress. Many weeks of exposure to high concentrations of glucose are necessary before glucotoxic effects are expressed by islet ␤ cells in vitro and in vivo (14 -20). Because isolated islets do not reliably survive in culture greater than 2 weeks, the use of short-term exposure to ribose, a sugar that generates ROS more potently than glucose, has become an accepted model for studying islet glucose toxicity (21,22). The use of ribose as a prooxidant sugar is especially valuable in adenoviral overexpression systems, because the overexpression effect is short-lived, lasting only days.In the pancreatic ␤ cell, glucose and ribose toxicity cause decreased insulin mRNA levels (14-22), one mechanism ...