Estrogen deficiency aggravates insulin resistance and induces ß-cell loss and diabetes in female New Zealand obese mice.

In several rodent strains such as the New Zealand Obese (NZO) mouse, the incidence of obesity-associated diabetes mellitus is much higher in males than in females. In the present study, we investigated the effects of ovariectomy on glucose homeostasis in female NZO mice in order to elucidate the mechanism of their diabetes resistance. NZO females were ovariectomized at the age of 4 weeks, received a high-fat diet and body weight, body fat, glucose and insulin tolerance were investigated in comparison to sham-operated mice. In a second experiment, operated mice were fed a carbohydrate-free diet up to the age of 19 weeks before they received the high-fat diet. In comparison with a sham-operated control group, ovariectomized female NZO mice exhibited similar body weights, a reduced glucose tolerance, developed significantly higher blood glucose levels, lost insulin producing ß-cells, which finally resulted in a diabetes prevalence of 73% at the age of 16 weeks vs. 25% in controls. Similar to male NZO mice, ovariectomized females presented a more severe insulin resistance in the insulin tolerance test than sham-operated controls. Furthermore, the more severe insulin resistance in ovariectomized mice preceded the development of diabetes and pancreatic insulin depletion that was caused by a dietary regimen of carbohydrate restriction and subsequent re-exposure. In summary our data demonstrate that estrogen protects NZO females from ß-cell loss and obesity-associated diabetes mellitus. This effect is due to a reduced insulin resistance and possibly also to a reduced sensitivity of ß-cells to glucolipotoxic conditions.

Diet dependence of diabetes in the New Zealand Obese (NZO) mouse: total fat, but not fat quality or sucrose accelerates and aggravates diabetes.

BACKGROUND: Obesity and diabetes in mice can be modified by dietary variables. Here we systematically analysed the effect of the sucrose and fat content and of the fat quality in New Zealand Obese mice, a mouse model of the metabolic syndrome. RESULTS: Male NZO mice fed a semi-purified diet with sucrose exhibited an identical weight gain and diabetes incidence as controls without sucrose. In contrast, mice on a chow diet gained weight more slowly and developed diabetes approximately 10 weeks later than those on the semi-purified diet (energy density 3.05 vs. 3.85 kcal/g; fibre content 12.9 vs. 4.7%). In a second experimental series, neither the fat content (10 vs. 40% of the total energy) nor the quality of the fat (lard, safflower oil, or fish oil) of semi-purified diets modified weight gain. However, diabetes started approximately 2 weeks earlier and appeared more severe (blood glucose 30 vs. 20 mmol/l at week 13) in the high-fat diet group (energy density 4.58 kcal/g; fibre content 5.7%). CONCLUSIONS: Obesity in NZO mice develops independent of the dietary sucrose or fat content, and of the fat quality. However, the dietary fat content accelerates the onset of diabetes without enhancing adiposity. In contrast, chow diet exerts an anti-adipogenic/anti-diabetogenic effect that appears to be due to its lower caloric density and/or its higher fibre content.

Dissociation of lipotoxicity and glucotoxicity in a mouse model of obesity associated diabetes: role of forkhead box O1 (FOXO1) in glucose-induced beta cell failure.

AIMS/HYPOTHESIS: Carbohydrate-free diet prevents hyperglycaemia and beta cell destruction in the New Zealand Obese (NZO) mouse model. Here we have used a sequential dietary regimen to dissociate the effects of obesity and hyperglycaemia on beta cell function and integrity, and to study glucose-induced alterations of key transcription factors over 16 days. METHODS: Mice were rendered obese by feeding a carbohydrate-free diet for 18 weeks. Thereafter, a carbohydrate-containing diet was given. Plasma glucose, plasma insulin and total pancreatic insulin were determined, and forkhead box O1 protein (FOXO1) phosphorylation and the transcription factors pancreatic and duodenal homeobox 1 (PDX1), NK6 homeobox 1 protein (NKX6.1) and v-maf musculoaponeurotic fibrosarcoma oncogene family, protein A (avian) (MAFA) were monitored by immunohistochemistry for 16 days. RESULTS: Dietary carbohydrates produced a rapid and continuous increase in plasma glucose in NZO mice between day 2 and 16 after the dietary challenge. Hyperglycaemia caused a dramatic dephosphorylation of FOXO1 at day 2, followed by a progressive depletion of insulin stores. The loss of beta cells was triggered by apoptosis (detectable at day 8), associated with reduction of crucial transcription factors (PDX1, NKX6.1 and MAFA). Incubation of isolated islets from carbohydrate-restricted NZO mice or MIN6 cells with palmitate and glucose for 48 h resulted in a dephosphorylation of FOXO1 and thymoma viral proto-oncogene 1 (AKT) without changing the protein levels of both proteins. CONCLUSIONS/INTERPRETATION: The dietary regimen dissociates the effects of obesity (lipotoxicity) from those of hyperglycaemia (glucotoxicity) in NZO mice. Obese NZO mice are unable to compensate for the carbohydrate challenge by increasing insulin secretion or synthesising adequate amounts of insulin. In response to the hyperglycaemia, FOXO1 is dephosphorylated, leading to reduced levels of beta cell-specific transcription factors and to apoptosis of the cells.