Abstract
BACKGROUND AND PURPOSE: The transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) is essential for glucose homeostasis. PPARgamma ligands reducing insulin levels in vivo are used as drugs to treat type 2 diabetes mellitus. Genes regulated by PPARgamma have been found in several tissues including insulin-producing pancreatic islet beta-cells. However, the role of PPARgamma at the insulin gene was unknown. Therefore, the effect of PPARgamma and PPARgamma ligands like rosiglitazone on insulin gene transcription was investigated. EXPERIMENTAL APPROACH: Reporter gene assays were used in the beta-cell line HIT and in primary mature pancreatic islets of transgenic mice. Mapping studies and internal mutations were carried out to locate PPARgamma-responsive promoter regions. KEY RESULTS: Rosiglitazone caused a PPARgamma-dependent inhibition of insulin gene transcription in a beta-cell line. This inhibition was concentration-dependent and had an EC(50) similar to that for the activation of a reporter gene under the control of multimerized PPAR binding sites. Also in normal primary pancreatic islets of transgenic mice, known to express high levels of PPARgamma, rosiglitazone inhibited glucose-stimulated insulin gene transcription. Transactivation and mapping experiments suggest that, in contrast to the rat glucagon gene, the inhibition of the human insulin gene promoter by PPARgamma/rosiglitazone does not depend on promoter-bound Pax6 and is attributable to the proximal insulin gene promoter region around the transcription start site from -56 to +18. CONCLUSIONS AND IMPLICATIONS: The human insulin gene represents a novel PPARgamma target that may contribute to the action of thiazolidinediones in type 2 diabetes mellitus.