title: Essential role of insulin-like growth factor I receptor in insulin-induced fetal brown adipocyte differentiation.
abstract: 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.
authors.affiliation: Instituto de Bioquímica/Departamento de Bioquímica y Biología Molecular, Centro Mixto, Facultad de Farmacia, Ciudad Universitaria, 28040 Madrid, Spain.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Mur
authors.forename: Mónica
authors.initials: M
authors.lastname: Arribas
authors.forename: Manuel
authors.initials: M
authors.lastname: Benito
authors.forename: Angela M
authors.initials: AM
authors.lastname: Valverde
mesh_terms: D017667:Adipocytes / Q000166:cytology / Q000378:metabolism; D002001:Adipose Tissue, Brown / Q000166:cytology* / Q000196:embryology / Q000378:metabolism; D000818:Animals; D001833:Body Temperature Regulation / Q000235:genetics; D022763:CCAAT-Enhancer-Binding Protein-alpha / Q000235:genetics; D022762:CCAAT-Enhancer-Binding Proteins / Q000235:genetics; D002454:Cell Differentiation / Q000187:drug effects; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000235:genetics; D064429:Fatty Acid Synthases / Q000235:genetics; D005260:Female; D015870:Gene Expression / Q000502:physiology; D005947:Glucose / Q000493:pharmacokinetics; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D007328:Insulin / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008817:Mice, Mutant Strains; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010750:Phosphoproteins / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation; D011247:Pregnancy; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D017526:Receptor, IGF Type 1 / Q000235:genetics* / Q000378:metabolism*; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D051780:Sterol Regulatory Element Binding Protein 1; D014157:Transcription Factors / Q000235:genetics
chemical_list: D022763:CCAAT-Enhancer-Binding Protein-alpha; D022762:CCAAT-Enhancer-Binding Proteins; D004268:DNA-Binding Proteins; D064429:Fatty Acid Synthases; D005947:Glucose; D007004:Hypoglycemic Agents; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D019869:Phosphatidylinositol 3-Kinases; D010750:Phosphoproteins; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D017526:Receptor, IGF Type 1; D018160:Receptors, Cytoplasmic and Nuclear; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D051780:Sterol Regulatory Element Binding Protein 1; D014157:Transcription Factors; C526295:Irs1 protein, mouse; C497388:Srebf1 protein, mouse