DIFFERENTIAL SIGNALING BY INSULIN-RECEPTOR SUBSTRATE-1 (IRS-1) AND IRS-2 IN IRS-1-DEFICIENT CELLS

Mice made insulin receptor substrate 1 (IRS-I) deficient by targeted g ene knockout exhibit growth retardation and abnormal glucose metabolis m due to resistance to the actions of insulin-like growth factor 1 (IG F-1) and insulin (E. Araki et al., Nature 372:186-190, 1991; H. Tamemo to et al., Nature 372:182-186, 1993), Embryonic fibroblasts and 3T3 ce ll lines derived from IRS-1-deficient embryos exhibit no IGF-1-stimula ted IRS-1 phosphorylation or IRS-l-associated phosphatidylinositol 3-k inase (PI 3-kinase) activity but exhibit normal phosphorylation of IRS -2 and She and normal IRS-2-associated PI 3-kinase activity, IRS-1 def iciency results in a 70 to 80% reduction in IGF-l-stimulated cell grow th and parallel decreases in IGF-l-stimulated S-phase entry, PI 3-kina se activity, and induction of the immediate-early genes c-fos and egr- 1 but unaltered activation of the mitogen-activated protein kinases ER K 1 and ERK 2, Expression of IRS-1 in IRS-l-deficient cells by retrovi ral gene transduction restores IGF-l-stimulated mitogenesis, PI 3-kina se activation, and c-fos and egr-1 induction in proportion to the leve l of reconstitution, Increasing the level of IRS-2 in these cells by u sing a retrovirus reconstitutes IGF-1 activation of PI 3-kinase and im mediate-early gene expression to the same degree as expression of IRS- 1; however, IRS-2 overexpression has only a minor effect on IGF-1 stim ulation of cell cycle progression, These results indicate that IRS-1 i s not necessary for activation of ERK 1 and ERK 2 and that activation of ERK 1 and ERK 2 is not sufficient for IGF-l-stimulated activation o f c-fos and egr-1, These data also provide evidence that IRS-1 and IRS -2 are not functionally interchangeable signaling intermediates for st imulation of mitogenesis despite their highly conserved structure and many common functions such as activating PI 3-kinase and early gene ex pression.