AbstractExendin‐4 (EX‐4), a long acting agonist of GLP‐1, induces an endocrine phenotype in Capan‐1 cells. Under culture conditions which include serum, ∼10% of the cells contain insulin and glucagon. When exposed to EX‐4 (0.1 nM, up to 5 days), the number of cells containing insulin and glucagon increased to ∼40%. Western blot analysis detected a progressive increase in protein levels of glucokinase and GLUT2 over 3 days of EX‐4 treatment. We explored the sequence of activation of certain transcription factors known to be essential for the beta cell phenotype: PDX‐1, Beta2/NeuroD, and hepatocyte nuclear factor 3β (HNF3β). Double immunostaining showed that PDX‐1 coexisted with insulin and glucagon in EX‐4‐treated cells. Treatment caused an increase in PDX‐1 protein levels by 24 h and induced its nuclear translocation. Beta2/NeuroD protein levels also increased progressively over 24 h. HNF3β protein level increased twofold as early as 6 h after EX‐4 treatment. EMSA results indicated that EX‐4 caused a 12‐fold increase in HNF3β binding to PDX‐1 promoter area II. Beta2/NeuroD protein levels progressively increased after 24 h treatment. Differentiation to insulin‐producing cells was also seen when Capan‐1 cells were transfected with pdx‐1, with 80% of these cells expressing insulin 3 days after transfection. PDX‐1 antisense totally inhibited such conversion. During the differentiation of duct cells to endocrine cells, cAMP levels (EX‐4 is a ligand for the GLP‐1, G‐protein coupled receptor) and MAP kinase activity increased. Our results indicate that EX‐4 activates adenylyl cyclase and MAP kinase which, in turn, may lead to activation of transcription factors necessary for an endocrine phenotype. Published 2002 Wiley‐Liss, Inc.