GLP-1 Can Protect Proinflammatory Cytokines Induced Beta Cell Apoptosis through the Ubiquitination

ABSTRACT

BACKGROUND: Proinflammatory cytokines are one of the causes of diabetes mellitus. However, the exact molecular mechanism by which proinflammatory cytokines induce beta-cell death remains to be clearly elucidated. Glucagon-like peptide-1 (GLP-1) affects the stimulation of insulin secretion and the preservation of beta-cells. Additionally, it may exert an antiapoptotic effect on beta cells; however, the mechanism underlying this effect has yet to be demonstrated. Therefore, we investigated the protective effects of GLP-1 in endoplasmic reticulum (ER)-mediated beta-cell apoptosis using proinflammatory cytokines. METHODS: To induce ER stress, hamster insulin-secreting tumor (HIT)-T15 cells were treated using a mixture of cytokines. Apoptosis was evaluated via MTT assay, Hoechst 33342 staining, and annexin/propidium iodide (PI) flow cytometry. The mRNA and protein expression levels of ER stress-related molecules were determined via PCR and Western blotting, respectively. Nitric oxide was measured with Griess reagent. The levels of inducible nitric oxide synthase (iNOS) mRNA and protein were analyzed via real-time PCR and Western blot, respectively. iNOS protein degradation was evaluated via immunoprecipitation. We pretreated HIT-T15 cells with exendin (Ex)-4 for 1 hour prior to the induction of stress. RESULTS: We determined that Ex-4 exerted a protective effect through nitric oxide and the modulation of ER stress-related molecules (glucose-regulated protein [GRP]78, GRP94, and CCAAT/enhancer-binding protein homologous protein [CHOP]) and that Ex-4 stimulates iNOS protein degradation via the ubiquitination pathway. Additionally, Ex-4 also induced the recovery of insulin2 mRNA expression in beta cells. CONCLUSION: The results of this study indicate that GLP-1 may protect beta cells against apoptosis through the ubiquitination pathway.