Defective Glucose-Regulated Insulin Gene Expression Associated With PDX-1 Deficiency in the Psammomys obesus Model of Type 2 Diabetes

Type 2 diabetes is associated with obesity, impaired insulin action, and defective insulin secretion (1). Although the relative contributions of insulin resistance and insulin deficiency to the pathogenesis of type 2 diabetes are debated, considerable evidence supports a dominating role for deficient � -cell function in all stages of the disease (2). The objective of this study was to define the molecular basis for � -cell dysfunction in nutritiondependent diabetes. Our experimental model was Psammomys obesus, a rodent model of type 2 diabetes that exhibits genetic predisposition for nutrition-induced hyperglycemia. The progression from the normoglycemic-normoinsulinemic phase to overt diabetes parallels the stages observed in obese type 2 diabetic patients. Thus, we have shown in analogy to the human disease that hyperglycemic P. obesus exhibit reduced insulin response to glucose stimulation; increased relative levels of insulin precursor molecules in the plasma and in the pancreas, associated with depletion of pancreatic insulin stores; and reduced pancreatic � -cell mass (3,4). We hypothesized that the increased vulnerability of diabetes-prone animals to dietary overload was related to inappropriate insulin production due to a defective regulation of insulin gene expression. RESULTS Defective glucose-regulated insulin gene expression in P. obesus. Feeding high-energy diet to diabetes-prone P. obesus for 4 days resulted in hyperglycemia associated with an 80‐90% depletion of islet insulin content. Insulin mRNA levels analyzed by relative quantitative reverse transcriptase‐polymerase chain reaction (RT-PCR) remained unchanged during the first 7 days of high-energy diet, despite hyperglycemia. Thereafter, insulin mRNA gradually decreased and by day 21 reached 15% of basal values. The failure of P. obesus � -cells to increase insulin gene expression in response to hyperglycemia may result from unresponsiveness of the insulin promoter to acute glucose stimulation or from toxic effects of chronic hyperglycemia. To identify the correct alternative, we studied the insulin gene response in isolated P. obesus and rat islets after short-term exposure to high glucose. Despite normal basal insulin gene expression, no increase in insulin mRNA was observed in prediabetic P. obesus islets incubated for 20‐22 h with 22.2 mmol/l glucose. In contrast, a threefold increase in insulin mRNA was observed in similarly treated rat islets. Thus, insulin gene expression in P. obesus � -cells does not respond to short-term glucose stimulation. Because deficient insulin gene transcription may, in part, account for these findings, we initiated studies on the effect of nutritional overload on various insulin transcription factors in the diabetes-prone P. obesus. An unexpected observation