Purpose of the investigation: Insulin receptor (IR) may play an essential role in the development of cell mass in the mouse pancreas. To further define the function of this signaling system in cell development, we have generated IR-deficient cell lines.

Methods: 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 SV40 large T antigen, a strategy for establishing cell lines (-IRLoxP). Subsequently, these cells were infected with adenovirus encoding cre recombinase to delete insulin receptor (-IR-).

Results: cells expressed insulin and Pdx-1 mRNA in response to glucose. In -IRLoxP cells, p44/p42 MAPK and PI 3 kinase pathways, mTOR and p70S₆K phosphorylation and -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 -IR- cells, glucose failed to stimulate PI 3 kinase activity, but induced p44/p42 MAPKs and mTOR/p70S₆K phosphorylation and -cell mitogenesis. PD98059, but not wortmannin, inhibited glucose-induced mTOR/p70S₆K signaling and mitogenesis in those cells. Finally, rapamycin blocked glucose-mediated mitogenesis of -IR- cells.

Conclusions: Independently of glucose, insulin can mediate mitogenesis in fetal pancreatic -cell lines. However, in the absence of the insulin receptor, glucose induces -cell mitogenesis.