GLP-1 Metabolite Does Not Signal through the IGF-I Receptor in Mouse Embryo Fibroblasts
Presentation Number: LBF-077
Date of Presentation: March 6th, 2015
Julie M Carroll, Ashley White and Charles T Roberts Jr.*
Oregon National Primate Research Center, Beaverton, OR
The incretin hormone GLP-1 is secreted by intestinal L cells and exhibits insulinotropic effects on the pancreatic beta cell through the adenylyl cylase-linked, G protein-coupled GLP-1 receptor to facilitate glucose disposal. Full-length (7-37)GLP-1 is rapidly converted to the more biologically active 7-36amide form, which is subsequently processed to the major (9-36amide) metabolite by dipeptidyl peptidase (DPP)-4, particularly in the hepato-portal bed (1). Injectable GLP-1 analogs resistant to DPP-4 and orally available small-molecule DPP-4 inhibitors are important classes of current type-2 diabetes therapies. The DPP-4-generated metabolite, originally considered inactive, has recently been shown to exert insulin-mimetic and cardioprotective effects that are not clearly ascribable to the classical GLP-1 receptor (2,3). The nature of the metabolite’s receptor and related signaling pathways represents a significant gap in our understanding of overall GLP-1 biology. It has been recently reported that nM concentrations of the GLP-1 metabolite bind to and to activate the IGF-I receptor, but not the insulin receptor, in cardiac fibroblasts (4). If firmly established, this would have important ramifications for both GLP-1 and IGF biology, in that some biological effects previously attributed to IGF-I could conceivably be the result of GLP-1 metabolite action in vivo. However, maximal postprandial concentrations of the GLP-1 metabolite in human circulation are in the low pM range (1,5), and the sequence homology between GLP-1 and IGF-I proposed in support of IGF-I receptor cross-activation primarily involved the signal peptide of IGF-I, which is not present in the mature hormone, with the remaining homology between the N-terminus of mature IGF-I and GLP-1 being shared with insulin. We therefore evaluated the ability of intact GLP-1 and its metabolite to activate the IGF-I and insulin receptors in engineered mouse embryo fibroblast cell lines expressing high levels of one receptor or the other, a well-established model of IGF/insulin signaling. Receptor autophosphorylation as well as phosphorylation of the downstream targets Erk and Akt were determined using sensitive and specific ELISAs. While IGF-I and insulin exhibited robust, dose-dependent activation of their cognate receptors and signaling pathways over a 0.1 to 10-nM dose range, neither GLP-1 nor its metabolite exhibited activation of any parameter at the concentrations tested. These data suggest that direct activation of the IGF-I receptor by GLP-1 or its metabolite is unlikely to occur in vivo and that any apparent crosstalk seen at pharmacological doses may be cell type-specific or reflect indirect post-receptor signaling pathway interactions.
Nothing to Disclose: JMC, AW, CTR Jr.