VMP1-related autophagy induced by a fructose-rich diet in ß-cells: its prevention by incretins.

The aim of the present study was to demonstrate the role of autophagy and incretins in the fructose-induced alteration of ß-cell mass and function. Normal Wistar rats were fed (3 weeks) with a commercial diet without (C) or with 10% fructose in drinking water (F) alone or plus sitagliptin (CS and FS) or exendin-4 (CE and FE). Serum levels of metabolic/endocrine parameters, ß-cell mass, morphology/ultrastructure and apoptosis, vacuole membrane protein 1 (VMP1) expression and glucose-stimulated insulin secretion (GSIS) were studied. Complementary to this, islets isolated from normal rats were cultured (3 days) without (C) or with F and F + exendin-4 or chloroquine. Expression of autophagy-related proteins [VMP1 and microtubule-associated protein light chain 3 (LC3)], apoptotic/antiapoptotic markers (caspase-3 and Bcl-2), GSIS and insulin mRNA levels were measured. F rats developed impaired glucose tolerance (IGT) and a significant increase in plasma triacylglycerols, thiobarbituric acid-reactive substances, insulin levels, homoeostasis model assessment (HOMA) for insulin resistance (HOMA-IR) and ß-cell function (HOMA-ß) indices. A significant reduction in ß-cell mass was associated with an increased apoptotic rate and morphological/ultrastructural changes indicative of autophagic activity. All these changes were prevented by either sitagliptin or exendin-4. In cultured islets, F significantly enhanced insulin mRNA and GSIS, decreased Bcl-2 mRNA levels and increased caspase-3 expression. Chloroquine reduced these changes, suggesting the participation of autophagy in this process. Indeed, F induced the increase of both VMP1 expression and LC3-II, suggesting that VMP1-related autophagy is activated in injured ß-cells. Exendin-4 prevented islet-cell damage and autophagy development. VMP1-related autophagy is a reactive process against F-induced islet dysfunction, being prevented by exendin-4 treatment. This knowledge could help in the use of autophagy as a potential target for preventing progression from IGT to type 2 diabetes mellitus.

Islet neogenesis-associated protein (INGAP)-positive cell mass, ß-cell mass, and insulin secretion: their relationship during the fetal and neonatal periods.

OBJECTIVES: To study the chronological appearance of pancreatic islet neogenesis-associated protein (INGAP)-positive cells and its correlation with the increase in ß-cell mass and function in fetal and neonatal rats. METHODS: Normal Wistar rat embryos (E) at gestational days 15, 17, and 19 (E15, E17, E19) and 7-day-old postnatal rats (P7) were humanely killed to determine body and pancreas weight; blood glucose; glucose and arginine-induced insulin secretion; real-time polymerase chain reaction of Pdx1 and Ngn3; quantitative immunomorphometric analysis of ß-cell replication and apoptosis rate, cytokeratin and INGAP cell mass, and Pdx-1- and Ngn3-positive cells. RESULTS: Body and pancreas weight increased with age (P7 > E19 > E17 > E15; P < 0.05). Neonates had higher blood glucose concentrations than embryos (P < 0.05). We recorded a simultaneous and significant age-dependent trend of increase in the number of ß- and Pdx-1-positive cells, ß- and cytokeratin-positive cell mass and ß-cell capacity to release insulin in response to glucose and arginine, and decreased ß-cell apoptotic rate. These changes closely paralleled the increase in INGAP-positive cell mass. CONCLUSIONS: These findings suggest that INGAP exerts a positive modulatory effect on ß-cell mass and its secretory function in fetal and neonatal rats, thus becoming a new component in the multifactorial regulation of such processes.