Endocrine Abstracts

Objectives: We have recently identified an islet renin-angiotensin system in the pancreas which was subjected to upregulation in an obesity-induced mouse model of Type 2 diabetes mellitus (T2DM). Blockade of the AT₁ receptor (AT1R) activation in this diabetic model improves islet function. It is well known that hyperglycemia plays a pivotal role in beta-cell dysfunction and T2DM. Nevertheless, glucotoxicity-induced AT1R activation and its consequence in oxidative stress-mediated beta-cell dysfunction are largely undefined. Accordingly, the present study was designed to investigate the in-vitro effects of chronic hyperglycemia on the expression changes in AT1R and insulin release using an INS-1E beta-cell line.

Methods: An INS-1E beta-cell line was cultured and incubated in different concentrations of glucose with varying time course. Immunocytochemistry was employed for precise localization of AT1R in INS-1E cells. The effects of hyperglycemia-induced AT1R expression changes in gene and protein levels were examined by real-time PCR and Western blot analysis, respectively. Glucotoxicity-induced AT1R activation-mediated secretory dysfunction was assessed by insulin release from INS-1E cells. AT1R activation mediated oxidative stress was also assessed by changes in NADPH oxidase expression.

Results: Immunoreactivity for AT1R was specifically localized to the cell membrane of INS-1E cells. AT1R expression at the gene and protein levels was dose dependently upregulated by chronic exposure to hyperglycemia, as demonstrated by real-time PCR and Western blot analyses, respectively. Chronic exposure of INS-1E cells to hyperglycemia impairs glucose-stimulated insulin release, which was specifically mediated by AT1R activation. AT1R-mediated NADHP oxidase activity/expression was also upregulated by hyperglycemia.

Conclusions: These data indicate that chronic hyperglycemia upregulates AT1R located on beta cells, thus impairing insulin secretion. The regulatory pathway may be mediated by an AT1R-mediated NADPH oxidase-dependent generation of reactive oxygen species.