Novel Transgenic Mice for Inducible Gene Overexpression in Pancreatic Cells Define Glucocorticoid Receptor-Mediated Regulations of Beta Cells

Abstract

Conditional gene deletion in specific cell populations has helped the understanding of pancreas development. Using this approach, we have shown that deleting the glucocorticoid receptor (GR) gene in pancreatic precursor cells leads to a doubled beta-cell mass. Here, we provide genetic tools that permit a temporally and spatially controlled expression of target genes in pancreatic cells using the Tetracycline inducible system. To efficiently target the Tetracycline transactivator (tTA) in specific cell populations, we generated Bacterial Artificial Chromosomes (BAC) transgenic mice expressing the improved Tetracycline transactivator (itTA) either in pancreatic progenitor cells expressing the transcription factor Pdx1 (BAC-Pdx1-itTA), or in beta cells expressing the insulin1 gene (BAC-Ins1-itTA). In the two transgenic models, itTA-mediated activation of reporter genes was efficient and subject to regulation by Doxycycline (Dox). The analysis of a tetracycline-regulated LacZ reporter gene shows that in BAC-Pdx1-itTA mice, itTA is expressed from embryonic (E) day 11.5 in all pancreatic precursor cells. In the adult pancreas, itTA is active in mature beta, delta cells and in few acinar cells. In BAC-Ins1-itTA mice tTA is active from E13.5 and is restricted to beta cells in fetal and adult pancreas. In both lines, tTA activity was suppressed by Dox treatment and re-induced after Dox removal. Using these transgenic lines, we overexpressed the GR in selective pancreatic cell populations and found that overexpression in precursor cells altered adult beta-cell fraction but not glucose tolerance. In contrast, GR overexpression in mature beta cells did not alter beta-cell fraction but impaired glucose tolerance with insufficient insulin secretion. In conclusion, these new itTA mouse models will allow fine-tuning of gene expression to investigate gene function in pancreatic biology and help us understand how glucocorticoid signaling affects on the long-term distinct aspects of beta-cell biology.

Glucocorticoid hormones and energy homeostasis

“…Indeed, deletion of the GR specifically in pancreatic precursor cells at the fetal stage counteracts the deleterious effects of under-nutrition on β-cell mass and dysfunction in later life [81]. In the adult, time-restricted over-expression of the GR specifically in mature β-cells did not affect β-cell numbers but led to impaired glucose tolerance in mice, correlating with impaired insulin secretion [82]. In aged mice over-expressing the GR in mature pancreatic β-cells, this genetic modification leads to overt hyperglycaemia, indicating that continuous over-activation of the GC/GR axis during aging significantly contributes to the progressive cascade from the glucose intolerant to the full-blown diabetic state [83,84].…”

Thiobenzothiazole-modified Hydrocortisones Display Anti-inflammatory Activity with Reduced Impact on Islet β-Cell Function

“…The distribution of the GR in virtually all tissues provides numerous targets for GC action, including effects in pancreatic ␤-cells. Overexpression of the GR targeted to pancreatic ␤-cells impairs insulin secretion (4), whereas deletion of the GR in pancreatic progenitor cells promotes ␤-cell mass expansion (5,6). Thus, current GCs regulate ␤-cell development and suppress IL-1␤-induced inflammatory signaling responses in pancreatic ␤-cells (7,8) but also markedly impair adult ␤-cell function and mass (see Ref.…”

Association between Higher Serum Cortisol Levels and Decreased Insulin Secretion in a General Population

“…In vitro experiments with cultured ß-cells showed that GC suppress insulin secretion [16]. Furthermore, in vivo experiments with transgenic mice that over express GC receptor specifically in ß-cells showed decreased insulin secretion during a glucose load [17, 18]. GCs thus appear to suppress ß-cell function directly, suggesting that higher GC levels increase the risk of impaired glucose metabolism independent of their induction of insulin resistance.…”