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J. Biol. Chem., Vol. 283, Issue 14, 9187-9195, April 4, 2008

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Inhibition of GLUT4 Translocation by Tbc1d1, a Rab GTPase-activating Protein Abundant in Skeletal Muscle, Is Partially Relieved by AMP-activated Protein Kinase Activation^*

Jose A. Chavez¹, William G. Roach¹, Susanna R. Keller, William S. Lane^¶, and Gustav E. Lienhard²

From the Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03755, the Department of Medicine-Division of Endocrinology, University of Virginia, Charlottesville, Virginia 22908, and the ^¶Mass Spectrometry and Proteomics Resource Laboratory, Center for Systems Biology, Harvard University, Cambridge, Massachusetts 02138

Insulin increases glucose transport by stimulating the trafficking of intracellular GLUT4 to the cell surface, a process known as GLUT4 translocation. A key protein in signaling this process is AS160, a Rab GTPase-activating protein (GAP) whose activity appears to be suppressed by Akt phosphorylation. Tbc1d1 is a Rab GAP with a sequence highly similar to that of AS160 and with the same Rab specificity as that of AS160. The role of Tbc1d1 in regulating GLUT4 trafficking has been unclear. Our previous study showed that overexpressed Tbc1d1 inhibited insulin-stimulated GLUT4 translocation in 3T3-L1 adipocytes, even though insulin caused phosphorylation on its single canonical Akt motif. In the present study, we show in 3T3-L1 adipocytes that Tbc1d1 is only 1/20 as abundant as AS160, that knockdown of Tbc1d1 has no effect on insulin-stimulated GLUT4 translocation, and that overexpressed Tbc1d1 also inhibits GLUT4 translocation elicited by activated Akt expression. These results indicate that endogenous Tbc1d1 does not participate in insulin-regulated GLUT4 translocation in adipocytes and suggest that the GAP activity of Tbc1d1 is not suppressed by Akt phosphorylation. In addition, we discovered that Tbc1d1 is much more highly expressed in skeletal muscle than fat and that the AMP-activated protein kinase (AMPK) activator 5'-aminoimidazole-4-carboxamide ribonucleoside partially reversed the inhibition of insulin-stimulated GLUT4 translocation by overexpressed Tbc1d1 in 3T3-L1 adipocytes. 5'-Aminoimidazole-4-carboxamide ribonucleoside activation of the kinase AMPK is known to cause GLUT4 translocation in muscle. The above findings strongly suggest that Tbc1d1 is a component in the signal transduction pathway leading to AMPK-stimulated GLUT4 translocation in muscle.

Received for publication, October 30, 2007 , and in revised form, January 14, 2008.

^* This work was supported by National Institutes of Health Grants DK25336 and DK42816. 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.

¹ These authors contributed equally to this work.

² To whom correspondence should be addressed. Tel.: 603-650-1627; Fax: 603-650-1128; E-mail: gustav.e.lienhard{at}dartmouth.edu.

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This article has been cited by other articles:

				K. Sakamoto and G. D. Holman Emerging role for AS160/TBC1D4 and TBC1D1 in the regulation of GLUT4 traffic Am J Physiol Endocrinol Metab, July 1, 2008; 295(1): E29 - E37. [Abstract] [Full Text] [PDF]