Recent studies ascribe a major role to pancreatic ␤-cell loss in type 2 diabetes. We investigated the dynamics of ␤-cell mass during diabetes evolution in Psammomys obesus, a model for nutrition-dependent type 2 diabetes, focusing on the very early and the advanced stages of the disease. P. obesus fed a high-calorie diet for 26 days developed severe hyperglycemia, ␤-cell degranulation, and markedly reduced pancreatic insulin content. Reducing calories for 7 days induced normoglycemia in 90% of the animals, restoring ␤-cell granulation and insulin content. To dissociate effects of diet from blood glucose reduction, diabetic animals received phlorizin for 2 days, which normalized glycemia and increased the pancreatic insulin reserve to 50% of control, despite a calorie-rich diet. During diabetes progression, ␤-cell mass decreased initially but recovered spontaneously to control levels, despite persistent hyperglycemia. Strikingly, however, ␤-cell mass did not correlate with degree of hyperglycemia or pancreatic insulin content. We conclude that reduced insulin reserve is the main cause of diabetes progression, whereas irreversible ␤-cell mass reduction is a late event in P. obesus. The rapid recovery of the pancreas by phlorizin-induced normoglycemia implies a causal relationship between hyperglycemia and islet dysfunction. Similar mechanisms could be operative during the evolution of type 2 diabetes in humans. Diabetes 54:138 -145, 2005 P ancreatic insulin reserve is an important parameter of islet function, with tight coupling between insulin secretion and production being necessary for the adequate functioning of the ␤-cell (rev. in 1). Insulin store size is determined by the balance between the rates of insulin biosynthesis and secretion as well as by the number and volume of the ␤-cells, i.e., the ␤-cell mass. In the adult, ␤-cell mass varies to adapt insulin secretion to long-term changes in insulin demand. This has been demonstrated repeatedly under physiological as well as pathological conditions of insulin resistance (rev. in 2). Although mainly demonstrated in rodents, this also seems to be the case in obese humans (3,4). The deterioration of the metabolic state in type 2 diabetes results mainly from progressive ␤-cell failure (5,6), hence the importance of determining whether functional ␤-cell mass is reduced in type 2 diabetes. This question remains controversial. Whereas some authors observed no change in ␤-cell mass (7-9), recent studies, using a large series of pancreata from patients with type 2 diabetes matched by appropriate control subjects, describe reduced relative ␤-cell volume or ␤-cell mass in humans with both impaired fasting glucose and established type 2 diabetes (4,10,11). The finding of reduced ␤-cell mass already in individuals with impaired fasting glucose has led to emphasizing its pathophysiological importance (9). In their recent evaluation of islets isolated from the pancreas of normal and type 2 diabetic cadaveric donors, Deng et al. (12) observed that in addition to a marked r...

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 ␤-cell proliferative activity, as opposed to a prolonged increase in the rate of ␤-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 ␤-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 ␤-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 ␤-cell mass in hyperglycemic P. obesus as a result of progressive ␤-cell death without an adequate increase in the rate of ␤-cell proliferation. Diabetes 50 (Suppl. 1):S113-S117, 2001 THE PSAMMOMYS OBESUS MODEL OF TYPE 2 DIABETESThe gerbil Psammomys obesus, nicknamed the sand rat, is a diurnal rodent that serves as a model for type 2 diabetes. In its native arid habitat in the North African and Eastern Mediterranean deserts, the P. obesus feeds mainly on the low-calorie salt bush (Atriplex halimus) and exhibits neither obesity nor diabetes. However, when transferred to laboratory conditions of nutritional abundance, it shows a propensity to develop diabetes associated with moderate obesity and insulin resistance (1,2). The genetic background for nutrition-evoked diabetes was demonstrated in the Jerusalem colony of P. obesus by selection of two outbred lines of animals: diabetes-prone (DP) and partially diabetes-resistant (DR) P. obesus (3,4). Most animals of the DP line under the age of 4 months develop diabetes when switched from ...

Decreased cellular GSH content is a common finding in experimental and human diabetes, in which increased oxidative stress appears to occur. Oxidative stress has been suggested to play a causative role in the development of impaired insulin action on adipose tissue and skeletal muscle. In this study we undertook to investigate the potential of GSH depletion to induce insulin resistance, by utilizing the GSH synthesis inhibitor, -buthionine-[S,R]-sulfoximine (BSO). GSH depletion (20-80 % in various tissues), was achieved in i o by treating rats for 20 days with BSO, and in itro (80 %) by treating 3T3-L1 adipocytes with BSO for 18 h. No demonstrable change in the GSH\GSSG ratio was observed following BSO treatment. GSH depletion was progressively associated with abnormal glucose tolerance test, which could not be attributed to impaired insulin secretion. Skeletal muscle insulin responsiveness was unaffected by GSH depletion, based on normal glucose response to exogenous insulin, 2-deoxyglucose uptake measurements in isolated soleus muscle, and on normal skeletal muscle expression of GLUT4

T ype 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. RESULTSDefective 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 was the apparent lack of pancreas duodenum homeobox gene-1 (PDX-1), the main mediator of glucose-regulated insulin gene expression, which also plays a key role in pancreatic development (5,6). Defective PDX-1 expression in pancreatic islets of normoglycemic and...