DPP4 and pancreatic insulinoma: In the context of DPP-IV inhibitor-induced insulin secretion from pancreatic β cells, previous studies reported that GLP-1 stimulated proinsulin gene expression and proinsulin biosynthesis in insulinoma βTC-1 cells 30, the cAMP analogue dibutyryl cAMP stimulated insulin biosynthesis in cultured insulinoma cells (RINm5F) 31, and forskolin-stimulated insulin mRNA levels in normal human islets 32.