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J. Biol. Chem., Vol. 279, Issue 40, 41468-41476, October 1, 2004

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The Transcription Factor CCAAT/Enhancer-binding Protein Is Required for the Intracellular Retention of GLUT4^*

Nadine Wertheim, Zhenjian Cai, and Timothy E. McGraw

From the Department of Biochemistry, Weill Medical College of Cornell University, New York, New York 10021

Insulin modulates glucose uptake into adipocytes by regulating the trafficking of the GLUT4 glucose transporter. GLUT4 is mostly excluded from the surface of unstimulated cells because it is much more slowly exocytosed than it is endocytosed. GLUT4 traffics through an adipocyte-specific, specialized endosomal recycling pathway that only partially overlaps with compartments of the general endosomal recycling pathway. Insulin stimulates GLUT4 exocytosis and partially inhibits its endocytosis, resulting in GLUT4 redistribution to the cell surface. Insulin does not stimulate glucose uptake into adipocytes lacking the CCAAT/enhancer-binding protein (C/EBP) transcription factor. Here we show that these adipocytes do not properly traffic GLUT4. In these adipocytes, GLUT4 was rapidly exocytosed in basal conditions, resulting in an accumulation of GLUT4 on the plasma membrane. Although the kinetics of GLUT4 trafficking were altered, GLUT4 was still targeted to specialized intracellular compartments in adipocytes lacking C/EBP, demonstrating an uncoupling of the targeting of GLUT4 to a specialized, adipocyte-specific insulin-regulated pathway from the regulation of the movement of GLUT4 through this pathway. Re-expression of C/EBP in adipocytes lacking C/EBP restored normal GLUT4 trafficking. We propose that C/EBP controls the expression of the proteins that determine the basal, slow exocytosis of GLUT4, but not the proteins required to make the adipocyte-specific compartments through which GLUT4 traffics. Furthermore, these data support a model in which insulin stimulates GLUT4 exocytosis by releasing an inhibitor of GLUT4 movement to the cell surface, and it is this clamp on basal exocytosis that is missing in adipocytes lacking C/EBP.

Received for publication, May 7, 2004 , and in revised form, July 22, 2004.

^* This work was supported by National Institutes of Health Grants DK52852 and DK57689 (to T. E. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed: 1300 York Ave., New York, NY 10021. Tel.: 212-746-4982; E-mail: temcgraw{at}med.cornell.edu.

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