β-cell glucotoxicity in the Psammomys obesus model of type 2 diabetes

Deficient insulin secretion and relative hyperproinsulinemia are characteristic features of type 2 diabetes. The gerbil Psammomys obesus appears to be an ideal natural model of the human disease because it shows increased tendency to develop diet-induced diabetes, which is associated with moderate obesity. The disease is characterized by initial hyperinsulinemia, progressing to hypoinsulinemia associated with depleted pancreatic insulin stores and an increased proportion of insulin precursor molecules in the blood and islets. Although the proinsulin translational efficacy was found to be increased in hyperglycemic animals, insulin mRNA levels were not augmented and exhibited a gradual decrease with disease progression. The development of hyperglycemia was associated with a transient increase in β-cell proliferative activity, as opposed to a prolonged increase in the rate of β-cell death, culminating in disruption of islet architecture. The hypothesis that glucotoxicity is responsible in part for these in vivo changes was investigated in vitro in primary islet cultures. Islets from diabetes-prone P. obesus cultured at high glucose concentrations displayed changes in β-cell function that mimic those observed in diabetic animals. These changes include deficient insulin secretion, depleted insulin content, an increased proportion of insulin precursor molecules, a progressive increase of DNA fragmentation, and a transient proliferative response. Furthermore, insulin mRNA was not increased by short-term exposure of P. obesus islets to elevated glucose in vitro. It is proposed that β-cell glucotoxicity in P. obesus results from the inability of proinsulin biosynthesis to keep pace with chronic insulin hypersecretion. The resulting depletion of the insulin stores may be related to deficient glucose-regulated insulin gene transcription, possibly due to defective PDX-1 (pancreatic duodenal homeobox factor-1) expression in the adult P. obesus. An additional glucotoxic effect involves the loss of β-cell mass in hyperglycemic P. obesus as a result of progressive β-cell death without an adequate increase in the rate of β-cell proliferation.