Inhibition of Phosphatidylinositol-3-kinase Enhances Insulin Stimulation of Insulin Receptor Substrate 1 Tyrosine Phosphorylation and Extracellular Signal-Regulated Kinases in Mouse R− Fibroblasts

Insulin stimulates phosphatidylinositol-3-kinase (PI3K) and extracellular signal-regulated kinases (ERK) in various mammalian cells. To study the role of PI3K in insulin stimulation of ERK, we employed PI3K inhibitor LY294002 and mouse embryonic R− fibroblasts lacking IGF-1 receptors. In these R− cells, PI3K inhibition by LY294002 enhanced insulin stimulation of ERK phosphorylation whereas LY294002 inhibited insulin stimulation of Akt phosphorylation. The enhanced insulin stimulation of ERK phosphorylation was accompanied by increased IRS-1 tyrosine phosphorylation. Insulin stimulation of insulin receptor tyrosine phosphorylation was not altered. PI3K inhibition increased IRS-1–Grb2 complex formation and ras activity following insulin treatment of cells. Increased insulin stimulation of ERK by PI3K inhibition was mediated by the MEK/ERK pathway, but did not involve inhibitory Ser259 phosphorylation of raf that was reported to be mediated by Akt. In summary, PI3K inhibition in R− cells enhanced insulin sti...