Expression of cAMP-regulated guanine nucleotide exchange factors in pancreatic beta-cells

The insulinotropic hormone glucagon-like peptide-1 (GLP-1) binds to a Gs-coupled receptor on pancreatic beta-cells and potentiates glucose-induced insulin secretion, insulin gene transcription, and beta-cell growth. These stimulatory effects have been attributed to the elevation of intracellular cAMP levels, though it is now apparent that some stimulatory effects of GLP-1 occur independently of the cAMP-mediated activation of protein kinase A (PKA). The nature of this alternative, PKA-independent signaling pathway remains unknown. Here we present evidence for the expression of type 1 and type 2 cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs) in beta-cells. GEFs are activated by their binding of cAMP. Because cAMP-GEFs activate Ras/MAPK proliferation signaling pathways, they may play an important role in PKA-independent, GLP-1-mediated, signaling pathways in the regulation of beta-cell growth and differentiation.