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To define the specific role of IGF-I receptor (IGF-IR) in adipogenic and thermogenic differentiation of brown adipocytes during late fetal life, we have established immortalized brown adipocyte cell lines from fetuses of IGF-IR-deficient mice (IGF-IR(-/-)) as well as from wild-type mice (IGF-IR(+/+)). IGF-IR(-/-) cells showed an increased insulin sensitivity regarding insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation despite a substantial reduction in IRS-1 protein content. Furthermore, insulin-induced total and IRS-1-associated phosphatidylinositol 3-kinase activities were augmented in IGF-IR-deficient cells compared with wild-type cells. Downstream phosphatidylinositol 3-kinase activation of Akt, but not p70s6 kinase, were elicited at lower doses of insulin in IGF-IR(-/-) brown adipocytes. Activation of protein kinase Czeta by insulin was similar in both cell types as was insulin-induced glucose uptake. Treatment of wild-type brown adipocytes with insulin for 12 h up-regulated fatty acid synthase (FAS) and adipocyte determination and differentiation (ADD1/SREBP) mRNAs; this effect was impaired in the absence of IGF-IR. At the protein level, insulin increased FAS content and the amount of the mature form of adipocyte determination and differentiation (ADD1/SREBP) in the nucleus in wild-type cells, but not in IGF-IR(-/-) cells. Furthermore, 24 h of insulin stimulation induced the expression of both uncoupling protein-1 and CCAAT/enhancer-binding protein alpha (C/EBPalpha) in wild-type brown adipocytes; these effects were abolished in IGF-I-R(-/-) cells. Retrovirus-mediated reexpression of peroxisomal proliferator-activated receptor gamma (PPARgamma) in IGF-IR(-/-) brown adipocytes could overcome FAS mRNA impairment, bypassing insulin signaling. However, insulin further increased FAS mRNA expression in C/EBPalpha-IGF-IR(-/-) cells, but not in PPARgamma-IGF-IR(-/-) cells. In addition, fetal brown adipocytes lacking IGF-IR up-regulated uncoupling protein-1 expression in the absence of insulin when PPARgamma, but not C/EBPalpha, was overexpressed. These data provide strong evidence for a critical role of IGF-IR in the differentiation of the brown adipocyte phenotype in fetal life; this effect is mimicked by PPARgamma in an insulin-independent manner.

Insulin receptor substrate-2-de¢cient (IRS-23/3 ) mice develop type 2 diabetes. We have investigated the molecular mechanisms by which IRS-2 3/3 immortalized brown adipocytes showed an impaired response to insulin in inducing GLUT4 translocation and glucose uptake. IRS-2-associated phosphatidylinositol 3-kinase (PI 3-kinase) activity was blunted in IRS-2 3/3 cells, total PI 3-kinase activity being reduced by 30%. Downstream, activation of protein kinase C (PKC) j j was abolished in IRS-2 3/3 cells. Reconstitution with retroviral IRS-2 restores IRS-2/PI 3-kinase/PKCj j signalling, as well as glucose uptake. Wild-type cells expressing a kinase-inactive mutant of PKCj j lack GLUT4 translocation and glucose uptake. Our results support the essential role played by PKCj j in the insulin resistance and impaired glucose uptake observed in IRS-2-de¢-cient brown adipocytes. ß