Differential role of insulin receptor substrate (IRS)-1 and IRS-2 in L6 skeletal muscle cells expressing the Arg(1152)-> Gln insulin receptor

In L6 muscle cells expressing the Arg(1152) --> Gln insulin receptor (Mut), basal tyrosine phosphorylation of insulin receptor substrate (IRS)-1 was i ncreased by 35% compared with wild-type cells (WT), Upon exposure to insuli n, IRS-1 phosphorylation increased by 12-fold in both the Mut and WT cells. IRS-2 was constitutively phosphorylated in Mut cells and not further phosp horylated by insulin. The maximal phosphorylation of IRS-S in basal Mut cel ls was paralleled by a 4-fold increased binding of the kinase regulatory lo op binding domain of IRS-S to the Arg(1152) --> Gln receptor. Grb2 and phos phatidylinositol 3-kinase association to IRS-1 and IRS-2 reflected the phos phorylation levels of the two IRSs, Mitogen-activated protein kinase activa tion and [H-3]thymidine incorporation closely correlated with IRS-1 phospho rylation in Mut and WT cells, while glycogen synthesis and synthase activit y correlated with IRS-S phosphorylation, The Arg(1152) Gln mutant did not s ignal Shc phosphorylation or Shc-Grb2 association in intact L6 cells, while binding Shc in a yeast two-hybrid system and phosphorylating Shc in vitro. Thus, IRS-2 appears to mediate insulin regulation of glucose storage in Mu t cells, while insulin-stimulated mitogenesis correlates with the activatio n of the IRS-1/mitogen-activated protein kinase pathway in these cells. IRS -1 and She-mediated mitogenesis may be redundant in muscle cells.