title: Phosphorylation of CCAAT/enhancer-binding protein alpha regulates GLUT4 expression and glucose transport in adipocytes.
abstract: The transcription factor CCAAT/enhancer-binding protein alpha (C/EBPalpha) is required during adipogenesis for development of insulin-stimulated glucose uptake. Modes for regulating this function of C/EBPalpha have yet to be determined. Phosphorylation of C/EBPalpha on Ser-21 has been implicated in the regulation of granulopoiesis and hepatic gene expression. To explore the role of Ser-21 phosphorylation on C/EBPalpha function during adipogenesis, we developed constructs in which Ser-21 was mutated to alanine (S21A) to model dephosphorylation. In two cell culture models deficient in endogenous C/EBPalpha, enforced expression of S21A-C/EBPalpha resulted in normal lipid accumulation and expression of many adipogenic markers. However, S21A-C/EBPalpha had impaired ability to activate the Glut4 promoter specifically, and S21A-C/EBPalpha expression resulted in diminished GLUT4 and adiponectin expression, as well as reduced insulin-stimulated glucose uptake. No defects in insulin signaling or GLUT4 vesicle trafficking were identified with S21A-C/EBPalpha expression, and when exogenous GLUT4 expression was enforced to normalize expression in S21A-C/EBPalpha cells, insulin-responsive glucose transport was reconstituted, suggesting that the primary defect was a deficit in GLUT4 levels. Mice in which endogenous C/EBPalpha was replaced with S21A-C/EBPalpha displayed reduced GLUT4 and adiponectin protein expression in epididymal adipose tissue and increased blood glucose compared with wild-type littermates. These results suggest that phosphorylation of C/EBPalpha on Ser-21 may regulate adipocyte gene expression and whole body glucose homeostasis.
authors.affiliation: Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI 48109-0622, USA.
authors.forename: Hyuk C
authors.initials: HC
authors.lastname: Cha
authors.forename: Nikhil R
authors.initials: NR
authors.lastname: Oak
authors.forename: Sona
authors.initials: S
authors.lastname: Kang
authors.forename: Tuan-Ahn
authors.initials: TA
authors.lastname: Tran
authors.forename: Susumu
authors.initials: S
authors.lastname: Kobayashi
authors.forename: Shian-Huey
authors.initials: SH
authors.lastname: Chiang
authors.forename: Daniel G
authors.initials: DG
authors.lastname: Tenen
authors.forename: Ormond A
authors.initials: OA
authors.lastname: MacDougald
mesh_terms: D041721:3T3-L1 Cells; D017667:Adipocytes / Q000166:cytology*; D000818:Animals; D001692:Biological Transport; D022763:CCAAT-Enhancer-Binding Protein-alpha / Q000378:metabolism*; D005786:Gene Expression Regulation; D005947:Glucose / Q000378:metabolism; D051275:Glucose Transporter Type 4 / Q000378:metabolism / Q000502:physiology*; D006706:Homeostasis; D051379:Mice; D010766:Phosphorylation; D010957:Plasmids / Q000378:metabolism; D012190:Retroviridae / Q000378:metabolism; D012694:Serine / Q000737:chemistry; D015533:Transcriptional Activation
chemical_list: D022763:CCAAT-Enhancer-Binding Protein-alpha; D051275:Glucose Transporter Type 4; C495693:Slc2a4 protein, mouse; D012694:Serine; D005947:Glucose