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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

1998; Oxford University Press; Volume: 12; Issue: 5 Linguagem: Inglês

10.1210/mend.12.5.0106

1944-9917

Ángela M. Valverde, Margarita Lorenzo, Sebastián Pons, Morris F. White, Manuel Benito,

Pancreatic function and diabetes

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.