Inhibition of PI 3-kinase and RAS blocks IGF-I and insulin-induced uncoupling protein 1 gene expression in brown adipocytes

Abstract

Fetal brown adipocytes expressed uncoupling protein 1 (UCP1) mRNA, this expression being blunted throughout culture for 24 h in a serum-free medium. At physiological doses, either insulin-like growth factor I (IGF-I) or insulin turned out to be as potent as dibutyryl cAMP (dbcAMP) in increasing UCP1 gene transcription rate (1 h) and also UCP1 mRNA accumulation (3 h), their maximal effect (15-fold increase) reached upon treatment for 24 h. Upon treatment with either IGF-I or insulin for 48 h, a 7-fold increase in the UCP1 protein content relative to levels in the control cells was found, this induction being abolished in the presence of cycloheximide. Moreover, either IGF-I or insulin transactivates the UCP1-chloramphenicol acetyl transferase (CAT) fusion gene after transient transfection of primary brown adipocytes, these effects being tissue-specific. Transient transfection of dominant-negative form of phosphatidylinositol (PI) 3-kinase completely blocked the transactivation of the fusion gene UCP1-CAT induced by either IGF-I or insulin, although inhibition of p70S6kinase with rapamycin does not preclude transactivation of the UCP1 promoter by insulin. Furthermore, transient transfection of dominant-negative form of p21-ras or treatment of cells with a mitogen-activated protein kinase kinase (MEK-1) inhibitor (PD098059) completely abolished insulin-induced UCP1-CAT transactivation. Cotransfection with dominant-negative p85 or with dominant-negative Ras also produced down-regulation of the insulin or IGF-I-induced 12-O-tetradecanoylphorbol-13-acetate response element (TRE)-CAT (five AP-1, activating protein-1, binding sites arranged in tandem) transactivation. In addition, insulin induced AP-1 DNA binding activity, this effect being totally prevented in the presence of MEK-1 inhibitor. These results strongly suggest that either IGF-I or insulin induced thermogenic-differentiation through AP-1 activity in a PI 3-kinase and Ras/MAPK dependent manner in brown adipocytes.

Extracellular signal-regulated kinase activation differentially regulates platelet-derived growth factor's actions in hepatic stellate cells, and is induced byIn Vivo liver injury in the rat

“…Inhibition of ERK activity was associated with inhibition of AP-1 binding activity in HSC nuclear extracts, in agreement with recent data obtained in insulin-stimulated brown adipocytes. 48 An interesting finding of this study is the critical role played by ERK in the regulation of STAT1 activation by PDGF. PDGF has been shown to trigger activation of STAT1, STAT3, and STAT5, 49 but the mechanism of activation of these proteins is only partially understood.…”

The brown adipose cell: a model for understanding the molecular mechanisms of insulin resistance

“…Moreover, insulin and IGF-I transactivate the fusion gene containing the UCP-1 full promoter controlling the reporter gene chloramphenicol acetyl transferase (CAT), in a tissue-specific manner. Inhibition of PI3K with LY294002 completely blocked insulin/IGF-I-induced transactivation of the UCP-1 promoter and UCP-1 mRNA accumulation (Teruel et al 1998). However, p70S6K inhibition by rapamycin did not preclude the transactivation of the UCP-1-CAT construct induced by IGF-I but partly inhibited IGF-I- induced UCP-1 mRNA expression (Valverde et al 1997a).…”

Akt mediates insulin induction of glucose uptake and up‐regulation of GLUT4 gene expression in brown adipocytes

“…Brown adipose tissue is a target tissue for insulin action, specially during late foetal development when insulin promotes both adipogenic and thermogenic differentiation of primary cells, as well as displays survival effects [1–3]. Glucose transport in foetal brown adipocytes is maintained mainly by the activity of the insulin‐regulated glucose transporter GLUT4, although the ubiquitous GLUT1 glucose transporter is often expressed at appreciable levels together with GLUT4.…”