Differential Mitogenic Signaling in Insulin Receptor-Deficient Fetal Pancreatic β-Cells

Abstract

Insulin receptor (IR) may play an essential role in the development of β-cell mass in the mouse pancreas. To further define the function of this signaling system in β-cell development, we generated IR-deficient β-cell lines. Fetal pancreata were dissected from mice harboring a floxed allele of the insulin receptor (IRLoxP) and used to isolate islets. These islets were infected with a retrovirus to express simian virus 40 large T antigen, a strategy for establishing β-cell lines (β-IRLoxP). Subsequently, these cells were infected with adenovirus encoding cre recombinase to delete insulin receptor (β-IR−/−). β-Cells expressed insulin and Pdx-1 mRNA in response to glucose. In β-IRLoxP β-cells, p44/p42 MAPK and phosphatidylinositol 3 kinase pathways, mammalian target of rapamycin (mTOR), and p70S6K phosphorylation and β-cell proliferation were stimulated in response to insulin. Wortmannin or PD98059 had no effect on insulin-mediated mTOR/p70S6K signaling and the corresponding mitogenic response. However, the presence of both inhibitors totally impaired these signaling pathways and mitogenesis in response to insulin. Rapamycin completely blocked insulin-activated mTOR/p70S6K signaling and mitogenesis. Interestingly, in β-IR−/− β-cells, glucose failed to stimulate phosphatidylinositol 3 kinase activity but induced p44/p42 MAPKs and mTOR/p70S6K phosphorylation and β-cell mitogenesis. PD98059, but not wortmannin, inhibited glucose-induced mTOR/p70S6K signaling and mitogenesis in those cells. Finally, rapamycin blocked glucose-mediated mitogenesis of β-IR−/− cells. In conclusion, independently of glucose, insulin can mediate mitogenesis in fetal pancreatic β-cell lines. However, in the absence of the insulin receptor, glucose induces β-cell mitogenesis.