GLP-1 Receptor Agonist Exendin-4 Improves Glycemic Control Through Beta Cell and Non-beta Cell Mechanism

Type 2 diabates is characterized by insulin resistance and progressive beta cell loss. Glucagon like peptide-1 (GLP-1) based therapy has been applied for clinical use for the treatment of type 2 diabetes. Both GLP-1 analogues and DPPIV inhibitors normalized blood glucose control both in rodent models and human beings. However, the detailed underlying mechanism for the beneficial effects is still unclear and under hot discussion. In this dissertation, we used both in vitro INS-1E cell line and human Islet Amyloid Polypeptide (hIAPP) transgenic or normal non-diabetic rodents to evaluate the intra- and extra- pancreas effects of GLP-1 analogue exendin-4. We explored its anti-apoptotic effects in hIAPP treated INS-1E cells and found that co-incubation with exendin-4 significantly abrogated hIAPP induced INS-1E cell apoptosis. AKT was inhibited by hIAPP treatment in the INS-IE cells but re-activation by transient over-expressing constitutively activated AKT1 significantly attenuated hIAPP induced INS-1E cell apoptosis, suggesting that inhibited AKT was at least involved in the apoptotic effects of hIAPP in the in vitro system. Treatment of exendin-4 re-activated hIAPP inhibited AKT phosphorylation in the INS-1E cells and promoted cytosolic translocation of forkhead box protein O1 (FOXO1) and the nucleus translocation of Pancreatic and Duodenal Homeobox-1 (Pdx-1). Interestingly, we also observed inhibited mitochondrial biogenesis after hIAPP treatment which could be partially restored by exendin-4 treatment. Based on these in vitro data, we concluded that exendin-4 protected hIAPP induced INS-IE cell death and potentially protected the beta cells from amyloid toxicity in vivo. Indeed, we found that daily injection with 24nM/kg exendin-4 for 4 weeks significantly lowered random blood glucose in hIAPP homozygous transgenic mice due to improvement in both beta cell morphology and insulin secretion, we will confirm these preliminary data in the following experiment. We also proved that hIAPP induced ER stress in vitro, but inhibition of endoplasmic reticulum (ER) stress did not significantly inhibited hIAPP evoked INS-1E cell death. The reactive oxygen species (ROS) level and JNK phosphorylation, which was potentially correlated with hIAPP induced INS-1E cell apoptosis was not affected by neither chemical chaperone treatment nor molecular chaperone over-expression. In consistence, exendin-4 did not alter ER stress level in the hIAPP treated cells in the in vitro system, suggesting that ER stress inhibition might not be an appropriate target for antagonizing beta cell amyloid toxicity.;We further used normal non-diabetic mice to evaluate the role of aging on the therapeutic effects of exendin-4. We optimized the dosage and treating period of exendin-4 and used a relatively lower dose and shorter treatment time. We observed that IOnM/kg treatment of exendin-4 did not significantly change beta cell proliferation or insulin secretion in both young and aging mice, however, we observed a significantly improved glucose response in the oral glucose tolerance test (OGTT) in both groups of mice after exendin-4 treatment. We further tested the insulin sensitivity and found that exendin-4 improved insulin sensitivity in aging mice but not in young mice. On the contrary, it was intriguing to see that the gluco-neogenesis correlated gene expression was significantly inhibited by exendin-4 treatment in young but not in aging mice. These data suggested that the extra-pancreas effects of exendin-4 was not attenuated by aging and that the glucose lowering effects of exendin-4 in young and aging mice might be slightly different.;The above data was first proposed by our group and was important for the clinical application of GLP-1 based therapy.