Abstract
Objective The canonical target of the glucagon-like peptide-1 receptor (GLP-1R), Protein Kinase A (PKA), has been shown to stimulate mechanistic Target of Rapamycin Complex 1 (mTORC1) by phosphorylating the mTOR-regulating protein Raptor at Ser791 following β-adrenergic stimulation. The objective of these studies is to test whether GLP-1R agonists similarly stimulate mTORC1 via PKA phosphorylation of Raptor at Ser791 and whether this contributes to the weight loss effect of the therapeutic GLP-1R agonist liraglutide.
Methods We measured phosphorylation of the mTORC1 signaling target ribosomal protein S6 in Chinese Hamster Ovary cells expressing GLP-1R (CHO-Glp1r) treated with liraglutide in combination with PKA inhibitors. We also assessed liraglutide-mediated phosphorylation of the PKA substrate RRXS*/T* motif in CHO-Glp1r cells expressing Myc-tagged wild-type (WT) Raptor or a PKA-resistant (Ser791Ala) Raptor mutant. Finally, we measured the body weight response to liraglutide in WT mice and mice with a targeted knock-in of PKA-resistant Ser791Ala Raptor.
Results Liraglutide increased phosphorylation of S6 and the PKA motif in WT Raptor in a PKA-dependent manner. Liraglutide failed to stimulate phosphorylation of the PKA motif in Ser791Ala Raptor. Ser791Ala Raptor knock-in mice were resistant to liraglutide-induced weight loss.
Conclusion GLP-1R agonists promote PKA-mediated phosphorylation of Raptor at Ser791, and this facilitates liraglutide-induced weight loss.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Details about generation of Ser791Ala Raptor mutant mice provided (Methods, Supp Fig. 1); additional experiment showing effects of liraglutide in Ser791Ala Raptor mutant mice on food intake and energy expenditure (Figure 3K-3N). Insulin measurements added (Supp. Fig. 3C); absolute data values provided (Fig. 3A, C, E, G, I, K, M and Supp. Fig. 2).
