Browsing by Author "Baun, D."

Background and purpose: The transcription factor peroxisome proliferator-activated receptor gamma (PPAR gamma) is essential for glucose homeostasis. PPAR gamma ligands reducing insulin levels in vivo are used as drugs to treat type 2 diabetes mellitus. Genes regulated by PPAR gamma have been found in several tissues including insulin-producing pancreatic islet beta-cells. However, the role of PPAR gamma at the insulin gene was unknown. Therefore, the effect of PPAR gamma and PPAR gamma 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 PPAR gamma-responsive promoter regions. Key results: Rosiglitazone caused a PPAR gamma-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 PPAR gamma, 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 PPAR gamma/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 PPAR gamma target that may contribute to the action of thiazolidinediones in type 2 diabetes mellitus.

Cyclosporin A and tacrolimus are clinically important immunosuppressive drugs. They share a diabetogenic action as one of their most serious adverse effects. The underlying mechanism is unknown. Previous studies have shown that tacrolimus can inhibit insulin gene transcription at high concentrations in tumor cell lines. To study insulin gene transcription in normal, mature pancreatic islet cells, we used a novel approach in the present study. Transgenic mice that carry a human insulin promoter-reporter gene were generated. The human insulin promoter directed transcription in pancreatic islets and conferred a normal, physiological glucose response to reporter gene expression in isolated islets. After stimulation with glucose, human insulin promoter-mediated gene expression was inhibited in normal, mature islet cells by both tacrolimus and cyclosporin A to a large extent ( approximately 70%) and with high potency at concentrations that are known to inhibit calcineurin phosphatase activity (IC50 values of 1 and 35 nM, respectively). Furthermore, glucose stimulated calcineurin phosphatase activity in mouse pancreatic islets, further supporting the view that calcineurin phosphatase activity is an essential part of glucose signaling to the human insulin gene. The high potency of cyclosporin A and tacrolimus in normal islets suggests that inhibition of insulin gene transcription by cyclosporin A and tacrolimus is clinically important and is one mechanism of the diabetogenic effect of these immunosuppressive drugs.