Abstract
Liraglutide, a human long-lasting GLP-1 analogue, is currently regarded as a powerful treatment option for type 2 diabetes. Apart from glucoregulatory and insulinotropic actions, liraglutide increases β-cell mass through stimulation of β-cell proliferation and islet neogenesis, as well as inhibition of β-cell apoptosis. However, the underline molecular mechanisms have not been fully characterized. In this study, we investigated the mechanism by which liraglutide preserves islet β-cells in an animal model of overt diabetes, the obese db/db mice, and protects a mouse pancreatic β-cell line (βTC-6 cells) against apoptosis. Treatment of 12-week-old diabetic mice with liraglutide for 2 weeks had no appreciable effects on blood non-fasting glucose concentration, islet insulin content and body weight. However, morphological and biochemical examination of diabetic mouse pancreatic islets demonstrated that liraglutide restores islet size, reduces islet β-cell apoptosis and improves nephrin expression, a protein involved in β-cell survival signalling. Our results indicated that liraglutide protects βTC-6 cells from serum withdrawal-induced apoptosis through inhibition of caspase-3 activation. The molecular mechanism of the anti-apoptotic action of liraglutide in βTC-6-cells comprises stimulation of PI3-kinase-dependent AKT phosphorylation leading to the phosphorylation, hence inactivation of the pro-apoptotic protein BAD and inhibition of FoxO1 transcription factor. In conclusion, we provided evidence that the GLP-1 analogue liraglutide exerts important beneficial effects on pancreatic islet architecture and β-cell survival by protecting cells against apoptosis. These findings extend our understanding of the actions of liraglutide and further support the use of GLP-1R agonists in the treatment of patients with type 2 diabetes.