Glucagon-Like Peptide 1 Induces Pancreatic β-Cell Proliferation Via Transactivation of the Epidermal Growth Factor Receptor
- From the Molecular Nutrition Unit, Departments of Nutrition and Biochemistry, University of Montreal, the Centre de Recherche du CHUM and Institut du Cancer, Montreal, Quebec, Canada
We previously provided evidence that glucagon-like peptide 1 (GLP-1) induces pancreatic β-cell growth nonadditively with glucose in a phosphatidylinositol (PI) 3-kinase- and protein kinase C ζ-dependent manner. However, the exact mechanism by which the GLP-1 receptor (GLP-1R), a member of the G protein-coupled receptor (GPCR) superfamily, activates the PI 3-kinase signaling pathway to promote β-cell growth remains unknown. We hypothesized that the GLP-1R could activate PI 3-kinase and promote β-cell proliferation through transactivation of the epidermal growth factor (EGF) receptor (EGFR), an event possibly linked to GPCRs via activation of c-Src and the production of putative endogenous EGF-like ligands. Both the c-Src inhibitor PP1 and the EGFR-specific inhibitor AG1478 blocked GLP-1-induced [3H]thymidine incorporation in INS(832/13) cells as well as in isolated rat islets, while only AG1478 inhibited the proliferative action of betacellulin (BTC), an EGFR agonist. Both compounds also suppressed GLP-1-induced PI 3-kinase activation. A time-dependent increase in tyrosine phosphorylation of the EGFR in response to GLP-1 was observed in INS(832/13) cells. This transactivation of the EGFR was sensitive to both the pharmacological agents PP1 and AG1478. The action of GLP-1 and BTC on INS cell proliferation was found to be not additive. Overexpression of a dominant-negative EGFR in INS cells with a retroviral expression vector curtailed GLP-1-induced β-cell proliferation. GLP-1 treatment of INS cells caused a decrease in cell surface-associated BTC, as shown by FACS analysis. Also, the metalloproteinase inhibitor GM6001 and an anti-BTC neutralizing antibody suppressed the GLP-1 proliferative effect. Finally, coculturing the prostatic cancer cell line LNCaP that lacks GLP-1 responsiveness with INS cells increased LNCaP cell proliferation in the presence of GLP-1, thus revealing that INS cells secrete a growth factor in response to GLP-1. GM6001 and an anti-BTC neutralizing antibody suppressed increased LNCaP cell proliferation in the presence of GLP-1 in the coculture experiments. The results are consistent with a model in which GLP-1 increases PI 3-kinase activity and enhances β-cell proliferation via transactivation of the EGFR that would require the proteolytic processing of membrane-anchored BTC or other EGF-like ligands.
Address correspondence and reprint requests to Dr. Marc Prentki, CR-CHUM, Pavillon de Sève, Y-4603, 1560 Sherbrooke East, Montreal, PQ H2L 4M1, Canada. E-mail:.
Received for publication 20 December 2001 and accepted in revised form 14 October 2002.
BTC, betacellulin; EGF, epidermal growth factor; EGFR, EGF receptor; GLP-1, glucagon-like peptide 1; GLP-1R, GLP-1 receptor; GPCR, G protein-coupled receptor; KRBH, Krebs-Ringer bicarbonate HEPES; PI, phosphatidylinositol.