Synergistic endocrine induction by GLP-1 and TGF-beta in the developing pancreas

Glucagon-like peptide-1 (GLP-1) is known to stimulate glucose-dependent insulin production and secretion by pancreatic beta-cells. Preliminary evidence suggests that GLP-1 may also influence endocrine differentiation from pancreatic progenitor cells. Additionally, TGF-beta signaling can also control endocrine differentiation by both inhibiting proliferation and enhancing differentiation of endocrine progenitor cells to become mature beta-cells. Here we document synergy of these two signaling pathways in the differentiation of endocrine cells in the developing pancreas. Embryonic pancreas was harvested from mice at day 11.5 and cultured for six days with GLP-1 agonist, exendin-4, and/or TGF-beta1 ligand. Also, a pan-neutralizing TGF-beta isoform antibody was used alone or with exendin-4 to study TGF-beta inhibition in this system. Pancreatic cultures were processed for immunohistochemistry. Exogenous TGF-beta1 and exendin-4 each individually enhanced both insulin and glucagon differentiation dose-dependently. However, when combined there was an additive effect to a 4.5-fold increase in insulin-positive differentiation. We also saw suppression of amylase-positive differentiation. Surprisingly, TGF-beta pan-neutralizing antibody also gave an augmentation of endocrine differentiation by 1.5 to 2-fold, but no synergistic effect was seen with exendin-4. We conclude that TGF-beta isoforms have a specific synergistic role with GLP-1 pathway signaling in early pancreatic development, toward endocrine differentiation and away from acinar differentiation.