Insulin receptor substrate (IRS) proteins IRS-1 and IRS-2 differential signaling in the insulin/insulin-like growth factor-I pathways in fetal brown adipocytes

Mol Endocrinol. 1998 May;12(5):688-97. doi: 10.1210/mend.12.5.0106.


In the present study we have investigated the contribution of the insulin receptor substrate proteins (IRS-1 and IRS-2) to the insulin/insulin like growth factor I (IGF-I)-signaling pathways in fetal rat brown adipocytes, a model that expresses both insulin and IGF-I receptors. Insulin/IGF-I rapidly stimulated IRS-1 and IRS-2 tyrosine phosphorylation, their association with p85alpha, and IRS-1- and IRS-2-associated phosphatidylinositol (PI) 3-kinase activation to the same extent, the effect of insulin being stronger than the effect of IGF-I at the same physiological dose (10 nM). Furthermore, insulin/IGF-I stimulated IRS-1-associated Grb-2 phosphorylation. However, IRS-2-associated Grb-2 phosphorylation was barely detected. Pull-down experiments with glutathione-S-transferase-fusion proteins containing SH2-domains of p85alpha revealed a strong association between IRS-1 and IRS-2 with p85alpha in response to insulin/IGF-I, the insulin effect being stronger than IGF-I. However, the Grb-2-SH2 domain showed functional differences. While a strong association between IRS-1/Grb-2 was found, IRS-2/Grb-2 association was virtually absent in response to insulin/IGF-I, as also demonstrated in competition studies with a phosphopeptide containing the phosphotyrosine 895 residue within the putative Grb-2-binding domain. Finally, insulin/IGF-I stimulated tyrosine phosphorylation of the three SHC proteins (46, 52, and 66 kDa). Moreover, insulin/IGF-I markedly increased the amount of Grb-2-associated SHC proteins by the same extent. Our results suggest that both IRS-1 and IRS-2 are required for phosphatidylinositol 3-kinase activation that leads to adipogenic and thermogenic differentiation of fetal brown adipose tissue; meanwhile, IRS-1 and SHC, but not IRS-2, associate with Grb-2 leading to the ras-mitogen-activated protein kinase-signaling pathway required for fetal brown adipocyte proliferation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adaptor Proteins, Signal Transducing*
  • Adipocytes / metabolism
  • Adipocytes / physiology*
  • Adipose Tissue, Brown / metabolism
  • Adipose Tissue, Brown / physiology*
  • Amino Acid Sequence
  • Animals
  • Enzyme Activation / drug effects
  • Fetus
  • GRB2 Adaptor Protein
  • Insulin / metabolism
  • Insulin / physiology*
  • Insulin Receptor Substrate Proteins
  • Insulin-Like Growth Factor I / metabolism
  • Insulin-Like Growth Factor I / physiology*
  • Intracellular Signaling Peptides and Proteins
  • Molecular Sequence Data
  • Phosphatidylinositol 3-Kinases / metabolism
  • Phosphoproteins / metabolism
  • Phosphoproteins / physiology*
  • Phosphorylation
  • Protein Binding
  • Proteins / metabolism
  • Rats
  • Rats, Wistar
  • Receptor, Insulin / physiology*
  • Signal Transduction / physiology*
  • Tyrosine / metabolism
  • src Homology Domains / physiology


  • Adaptor Proteins, Signal Transducing
  • GRB2 Adaptor Protein
  • Grb2 protein, rat
  • Insulin
  • Insulin Receptor Substrate Proteins
  • Intracellular Signaling Peptides and Proteins
  • Irs1 protein, rat
  • Irs2 protein, rat
  • Phosphoproteins
  • Proteins
  • Tyrosine
  • Insulin-Like Growth Factor I
  • Phosphatidylinositol 3-Kinases
  • Receptor, Insulin