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J. Biol. Chem., Vol. 283, Issue 1, 311-323, January 4, 2008

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Insulin Releases Glut4 from Static Storage Compartments into Cycling Endosomes and Increases the Rate Constant for Glut4 Exocytosis^*

From the Department of Biochemistry and Molecular Biology, University of Nevada, Reno, Nevada 89557

In adipocytes, insulin triggers the redistribution of Glut4 from intracellular compartments to the plasma membrane. Two models have been proposed to explain the effect of insulin on Glut4 localization. In the first, termed dynamic exchange, Glut4 continually cycles between the plasma membrane and intracellular compartments in basal cells, and the major effect of insulin is through changes in the exocytic and endocytic rate constants, k_ex and k_en. In the second model, termed static retention, Glut4 is packaged in specialized storage vesicles (GSVs) in basal cells and does not traffic through the plasma membrane or endosomes. Insulin triggers GSV exocytosis, increasing the amount of Glut4 in the actively cycling pool. Using a flow cytometry-based assay, we found that Glut4 is regulated by both static and dynamic retention mechanisms. In basal cells, 75-80% of the Glut4 is packaged in noncycling GSVs. Insulin increased the amount of Glut4 in the actively cycling pool 4-5-fold. Insulin also increased k_ex in the cycling pool 3-fold. After insulin withdrawal, Glut4 is rapidly cleared from the plasma membrane (t of 20 min) by rapid adjustments in k_ex and k_en and recycled into static compartments. Complete recovery of the static pool required more than 3 h, however. We conclude that in fully differentiated confluent adipocytes, both the dynamic and static retention mechanisms are important for the regulation of plasma membrane Glut4 content. However, cell culture conditions affect Glut4 trafficking. For example, replating after differentiation inhibited the static retention of Glut4, which may explain differences in previous reports.

Received for publication, July 13, 2007 , and in revised form, October 23, 2007.

^* This work was supported by National Institutes of Health Grant DK56197. 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1-S3.

¹ To whom correspondence should be addressed: Dept. of Biochemistry and Molecular Biology, Mailstop 330, University of Nevada, Reno, NV 89557. Tel.: 775-784-1155; Fax: 775-784-1419; E-mail: cmastick{at}unr.edu.

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