Glucose Toxicity in β-Cells: Type 2 Diabetes, Good Radicals Gone Bad, and the Glutathione Connection

Abstract

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 β-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 β-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 β-cell function.