Background: Increase in glucagon-like peptide-1 (GLP-1) activity has recently emerged as a useful therapeutic tool for the treatment of type 2 diabetes by enhancing the glycemic control and also helping in maintaining or even decreasing body weight of most patients. Lipotoxicity through increased free fatty acids (FFAs) levels is an important factor in the pathogenesis of type 2 diabetes resulting in defective β-cell proliferation and increased β cell apoptosis. The action of GLP-1 and its mimetics on pancreatic β-cells as well as on nervous and digestive systems are relatively well established. The effect of this peptide and its analogues in other tissues including adipose, muscles and liver, however, are still poorly defined. 
Aim: We investigated the potential beneficial effects of GLP-1 mimetics on those peripheral tissues using established cell lines under stressing levels of palmitic acid and glucose. 
Methods:  Using cell lines from liver (HepG2), adipose tissue (3T3-L1), skeletal muscle (L6) and pancreatic cells (BTC-6), we analyzed the effect of Exendin-4 on global protein expression pattern using LC-MS/MS Orbitrap system and label-free quantification with a focus on the MAPKs in the presence of stressing amounts of palmitic acid (400uM) and glucose (30mM). Dysregulated pathways were analyzed using Ingenuity Pathway Analysis Package. Results were validated using Western blotting and RT-PCR approaches for various incubation periods (1-24h). Cell viability assays were also performed. 
Results and Discussion: Under lipotoxic conditions, a set of proteins related to lipid homeostasis were modulated by Exendin-4 aside with key canonical signaling pathways such as EIF2, ILK, PKA and Rho. Interestingly, the phosphorylation levels of MAPKs (JNK, ERK1/2 and p38) were clearly increased in the presence of Exendin-4 alone. In the presence of Palmitic Acid or glucose, Exendin-4 displayed limited effect for short incubation periods but significantly attenuated MAPKs phosphorylation after 24h incubation. By contrast, the heat shock response, known to relieve cellular stress, was not significantly affected by Exendin-4 regardless the used conditions. These observations were confirmed using both Western blotting and RT-PCR. Furthermore, cell viability assays have shown that exendin-4 alleviated the palmitic acid-induced cell death. In conclusion, identifying new pathways modulated by GLP-1 analogues provides insight into its beneficial effects beyond glycaemic control, such as MAPKs activity, energy homeostasis in glucose up-taking organs and body weight decrease.

Protective effects of GLP-1 analogues against cellular stress in glucose uptaking tissues under lipotoxic conditions