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.

Prolactin potentiates insulin-stimulated leptin expression and release from differentiated brown adipocytes

“…adipogenic and thermogenic gene expression upon insulin or insulin-like growth factor stimulation, already demonstrated in primary brown adipocytes (Valverde et al 1997, Teruel et al 1998, Mur et al 2003. The role of IRS-1, -2 and -3 in PRL signaling is to be phosphorylated by JAK2, thereby providing docking sites for p85 PI 3-kinase and activating PI 3-kinase (Yamauchi et al 1998).…”

Prevention of Obesity and Insulin Resistance in Mice Lacking Plasminogen Activator Inhibitor 1

“…The UCPs are mitochondrial inner membrane proteins that play important roles in whole-body energy expenditure (46,47,50,51). UCP-1 is exclusively expressed in brown adipose tissue, whereas UCP-2 is widely expressed in many tissues and UCP-3 is primarily expressed in skeletal muscle.…”

“…Glucose uptake. 2-Deoxyglucose uptake was measured (46). Primary mouse WT and PAI-1 Ϫ/Ϫ adipocytes (day 10 after differentiation) in six-well plates were cultured overnight in 10% fetal serum-Dulbecco's modified Eagle's medium with low glucose (1 g/l).…”

Susceptibility to Apoptosis in Insulin-like Growth Factor-I Receptor-deficient Brown Adipocytes

“…However, IGF-IRdeficient brown adipocytes did not respond to insulin by inducing brown adipocyte differentiation-related genes such as the uncoupling protein-1 (UCP-1) and fatty acid synthase (FAS). This was due to a lack of expression of transcription factors that up-regulate these genes in response to insulin (Mur et al, 2003). In the present article, we demonstrate that IGF-IR deficiency in brown adipocytes confers increased susceptibility to programmed cell death after serum withdrawal, compared with the wild type.…”

“…Ϫ/Ϫ -immortalized brown adipocytes displayed an increased insulin sensitivity regarding IRS-1/Grb-2/MAPK and IRS-1/PI 3-kinase/Akt signaling pathways compared with wildtype cells (Mur et al, 2002(Mur et al, , 2003. Accordingly, we cultured IGF-IR Ϫ/Ϫ brown adipocytes for 16 h in serum-free medium without or with insulin (100 nM) either in the absence or in the presence of 10 M LY294002 or 20 M PD98059.…”