title: Forskolin suppresses insulin gene transcription in islet beta-cells through a protein kinase A-independent pathway.
abstract: This work was designed to evaluate the effect of cAMP on insulin gene regulation. We studied the effects of forskolin on insulin gene transcription in the INS-1 beta-cell line, confirming key results in primary cultures of human islet cells. Forskolin increased intracellular cAMP and cAMP-responsive element-binding activity. Insulin gene transcription was studied using a reporter construct in which the human insulin promoter was fused to luciferase. When cells were treated with forskolin for 12 h, insulin promoter activity was decreased 2- to 3-fold, whereas islet amyloid polypeptide promoter activity was significantly increased. This effect of forskolin on the insulin gene was time- and concentration-dependent, and was mimicked by 8-bromo-cAMP. Mutagenesis of the CRE-like elements in the insulin promoter had no effect on the forskolin-induced suppression, but dramatically decreased basal insulin promoter activity. Inhibition of PKA with H-89 also did not reverse the forskolin-induced suppression of insulin transcription. However, this effect was completely reversed by inhibition of cellular MAP kinase activity with PD98059 or U0126. These results demonstrate that forskolin suppresses insulin transcription in INS-1 cells through a PKA-independent mechanism that probably involves MAP kinase signalling.
authors.affiliation: Department of Medicine, Center for Basic Research in Digestive Diseases, Mayo Clinic and Foundation, Guggenheim 17, Rochester, MN 55905, USA.
authors.forename: Wei-Qun
authors.initials: WQ
authors.lastname: Ding
authors.forename: Maoqing
authors.initials: M
authors.lastname: Dong
authors.forename: Dora
authors.initials: D
authors.lastname: Ninova
authors.forename: Eileen L
authors.initials: EL
authors.lastname: Holicky
authors.forename: Mark D
authors.initials: MD
authors.lastname: Stegall
authors.forename: Laurence J
authors.initials: LJ
authors.lastname: Miller
mesh_terms: D001483:Base Sequence; D002460:Cell Line; D005576:Colforsin / Q000494:pharmacology*; D000242:Cyclic AMP / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology*; D015536:Down-Regulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005419:Flavonoids / Q000494:pharmacology; D005786:Gene Expression Regulation; D006801:Humans; D007328:Insulin / Q000096:biosynthesis / Q000235:genetics*; D007515:Islets of Langerhans / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D020935:MAP Kinase Signaling System; D020928:Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors; D011401:Promoter Regions, Genetic; D014158:Transcription, Genetic
chemical_list: D004791:Enzyme Inhibitors; D005419:Flavonoids; D007328:Insulin; D005576:Colforsin; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D020928:Mitogen-Activated Protein Kinases; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one