Pancreatic glucagon-like peptide-1 receptor couples to multiple G proteinsand activates mitogen-activated protein kinase pathways in Chinese hamsterovary cells

Chinese hamster ovary (CHO) cells stably expressing the human insulin recep tor and the rat glucagon-like peptide-1 (GLP-1) receptor (CHO/GLPR) were us ed to study the functional coupling of the GLP-1 receptor with G proteins a nd to examine the regulation of the mitogen-activated protein (MAP) kinase signaling pathway by GLP-1. We showed that ligand activation of GLP-1 recep tor led to increased incorporation of GTP-azidoanilide into G(s)alpha, G(q/ 11)alpha, and G(i1.2)alpha, but not G(i3)alpha. GLP-1 increased p38 MAP kin ase activity 2.5- and 2.0-fold over the basal level in both CHO/GLPR cells and rat insulinoma cells (RIN 1046-38), respectively. Moreover, GLP-1 induc ed phosphorylation of the immediate upstream kinases of p38, MKK3/MKK6, in CHO/GLPR and RIN 1046-38 cells. Ligand-stimulated GLP-1 receptor produced 1 .45- and 2.7-fold increases in tyrosine phosphorylation of 42-kDa extracell ular signal-regulated kinase (ERK) in CHO/GLPR and RIN 1046-38 cells, respe ctively. In CHO/GLPR cells, these effects of GLP-1 on the ERK and p38 MAP k inase pathways were inhibited by pretreatment with cholera toxin (CTX), but not with pertussis toxin. The combination of insulin and GLP-1 resulted in an additive response (1.6-fold over insulin alone) that was attenuated by CTX. In contrast, the ability of insulin alone to activate these pathways w as insensitive to either toxin. Our study indicates a direct coupling betwe en the GLP-1 receptor and several G proteins, and that CTX-sensitive protei ns are required for GLP-1-mediated activation of MAP kinases.