Abstract
Insulin receptor substrate (IRS) proteins are tyrosine phosphorylated and mediate multiple signals during activation of the receptors for insulin, insulin-like growth factor 1 (IGF-1), and various cytokines. In order to distinguish common and unique functions of IRS-1, IRS-2, and IRS-4, we expressed them individually in 32D myeloid progenitor cells containing the human insulin receptor (32DIR). Insulin promoted the association of Grb-2 with IRS-1 and IRS-4, whereas IRS-2 weakly bound Grb-2; consequently, IRS-1 and IRS-4 enhanced insulin-stimulated mitogen-activated protein kinase activity. During insulin stimulation, IRS-1 and IRS-2 strongly bound p85α/β, which activated phosphatidylinositol (PI) 3-kinase, protein kinase B (PKB)/Akt, and p70s6k, and promoted the phosphorylation of BAD. IRS-4 also promoted the activation of PKB/Akt and BAD phosphorylation during insulin stimulation; however, it weakly bound or activated p85-associated PI 3-kinase and failed to mediate the activation of p70s6k. Insulin strongly inhibited apoptosis of interleukin-3 (IL-3)-deprived 32DIR cells expressing IRS-1 or IRS-2 but failed to inhibit apoptosis of cells expressing IRS-4. Consequently, 32DIR cells expressing IRS-4 proliferated slowly during insulin stimulation. Thus, the activation of PKB/Akt and BAD phosphorylation might not be sufficient to inhibit the apoptosis of IL-3-deprived 32DIR cells unless p85-associated PI 3-kinase or p70s6k are strongly activated.
ACKNOWLEDGMENTS
This work was supported by DK38712 (M.F.W.) and Juvenile Diabetes Foundation Research grant 197043 (M.G.M.). T.U. was supported by an American Diabetes Association Mentor-Based Fellowship and the Nakatomi Foundation Research Grant from Japan.