Essentiality of intron control in the induction of c-fos by glucose and glucoincretin peptides in INS-1 beta-cells

Glucose controls long-term processes in the pancreatic beta-cell such as me tabolic enzymes gene expression, cell growth, and apoptosis. Such control i s likely mediated via the expression of immediate-early response genes sinc e several of these genes including c-fos are strongly induced by glucose in the beta-cell line INS-1, provided costimulation with cAMP-raising glucoin cretin hormones. this study addresses the mechanism of c-fos gene activatio n by glucose. Glucose in the presence of chlorophenylthio-cAMP generated a low threefold induction of the c-fos/basic luciferase reporter gene, which includes only the c-fos promoter. In contrast, the c-fos/intron construct c ontaining the first intron in addition to promoter elements showed a pronou nced 16-fold induction, comparable to the increased c-fos mRNA accumulation . Similar observations were made with glucose in combination with the gluco incretins glucagon-like peptide 1, glucose-dependent insulinotropic polypep tide, and pituitary adenylyl cyclase-activating peptide 38. Deletion of a 1 19 bp region in intron 1 that includes a transcriptional arrest site did no t affect the inductive process. In contrast, a 534 bp deletion comprising a major part of the intron reduced the induction by 75%. At the promoter lev el, mutating the cAMP response element reduced by more than 60% the transcr iptional activation whereas mutating the serum response element had no effe ct . Inhibitors of protein kinase A and Ca2+/calmodulin-dependent protein k inase each reduced by 50% the reporter gene activation and together fully p revented the glucose-glucoincretin effect. In conclusion, the strong induct ion of c-fos by glucose and glucoincretins results from Ca2+ and cAMP signa ling pathways addressing both the CRE in the promoter and essential respons e element(s) in the first intron that are unrelated to the transcription ar rest site.