Phosphorylation of CCAAT/enhancer-binding protein α regulates GLUT4 expression and glucose transport in adipocytes

Abstract

The transcription factor CCAAT/enhancer-binding protein α (C/EBPα) is required during adipogenesis for development of insulin-stimulated glucose uptake. Modes for regulating this function of C/EBPα have yet to be determined. Phosphorylation of C/EBPα 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/EBPα 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/EBPα, enforced expression of S21A-C/EBPα resulted in normal lipid accumulation and expression of many adipogenic markers. However, S21A-C/EBPα had impaired ability to activate the Glut4 promoter specifically, and S21A-C/EBPα 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/EBPα expression, and when exogenous GLUT4 expression was enforced to normalize expression in S21A-C/EBPα cells, insulin-responsive glucose transport was reconstituted, suggesting that the primary defect was a deficit in GLUT4 levels. Mice in which endogenous C/EBPα was replaced with S21A-C/EBPα 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/EBPα on Ser-21 may regulate adipocyte gene expression and whole body glucose homeostasis. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.