Identification of Intracellular and Physiological Targets Transducing the Weight-Lowering Actions of Glucagon-Like Peptide 1 Receptor Agonists

Abstract

This thesis identifies the molecular and physiological targets downstream of glucagon-like peptide-1 receptor (GLP-1R) activation that promote the weight-lowering effect of GLP-1R agonists. The first project builds on studies demonstrating the importance of mTORC1-mediated intracellular signaling in the ventromedial hypothalamus in mediating feeding suppression by GLP-1R agonists. As the canonical target of the GLP-1R, Protein Kinase A (PKA), stimulates mechanistic Target of Rapamycin Complex 1 (mTORC1) by phosphorylating the mTOR-regulating protein Raptor at Ser791 following β-adrenergic stimulation, we hypothesize that GLP-1R agonists similarly stimulate mTORC1 via PKA phosphorylation of Raptor at Ser791, which contributes to the weight loss effect of the therapeutic GLP-1R agonist liraglutide. In cells expressing GLP-1R, we showed that liraglutide increased phosphorylation of S6 and the PKA motif in wildtype Raptor in a PKA-dependent manner and failed to stimulate phosphorylation of the PKA motif in Ser791Ala Raptor. Compared to control mice, mice with a targeted knock-in of Ser791Ala Raptor were resistant to liraglutide-induced weight loss. These findings suggest that liraglutide promotes PKA-mediated phosphorylation of Raptor at Ser791, which in turn facilitates liraglutide-induced weight loss. The second project investigated the mechanisms engaged by liraglutide to increase FGF21 and the metabolic relevance of liraglutide-induced FGF21. We showed that liraglutide increases FGF21 levels via neuronal GLP-1R activation. We also demonstrated that lack of liver Fgf21 confers partial resistance to liraglutide-induced suppression of food intake and body weight. Since FGF21 reduces carbohydrate intake, we tested whether the contribution of FGF21 to liraglutide-induced weight loss is dependent on dietary carbohydrate content. In control and liver Fgf21 knockout (LivFgf21-/-) mice fed calorically matched diets with low- (LC) or high-carbohydrate (HC) content, we found that only HC-fed LivFgf21-/- mice were resistant to liraglutide-induced weight loss. Similarly, liraglutide-induced weight loss was partially impaired in LivFgf21-/- mice fed a high-fat, high-sugar (HFHS) diet. Lastly, we showed that loss of neuronal β-Klotho diminishes liraglutide-induced weight loss in mice fed a HC or HFHS diet, indicating that FGF21 mediates liraglutide-induced weight loss via neuronal FGF21 action. These findings support a novel role for a GLP-1R-FGF21 axis in weight regulation in the presence of high dietary carbohydrate content.