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J. Biol. Chem., Vol. 283, Issue 6, 3465-3475, February 8, 2008

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Rapid Turnover of the mTOR Complex 1 (mTORC1) Repressor REDD1 and Activation of mTORC1 Signaling following Inhibition of Protein Synthesis^*

Scot R. Kimball¹², A. N. Dang Do¹, Lydia Kutzler, Douglas R. Cavener, and Leonard S. Jefferson

From the Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033 and the Department of Biology, The Pennsylvania State University, University Park, Pennsylvania 16802

mTORC1 is a complex of proteins that includes the mammalian target of rapamycin (mTOR) and several regulatory proteins. It is activated by a variety of hormones (e.g. insulin) and nutrients (e.g. amino acids) that act to stimulate cell growth and proliferation and repressed by hormones (e.g. glucocorticoids) that act to reduce cell growth. Curiously, mTORC1 signaling is reported to be rapidly (e.g. within 1-2 h) activated by inhibitors of protein synthesis that act on either mRNA translation elongation or gene transcription. However, the basis for the mTORC1 activation has not been satisfactorily delineated. In the present study, mTORC1 signaling was found to be activated in response to inhibition of either the initiation or elongation phases of mRNA translation. Changes in mTORC1 signaling were inversely proportional to alterations in the expression of the mTORC1 repressor, REDD1, but not the expression of TRB3 or TSC2. Moreover the cycloheximide-induced increase in mTORC1 signaling was significantly attenuated in cells lacking REDD1, showing that REDD1 plays an integral role in the response. Finally, the half-life of REDD1 was estimated to be 5 min or less. Overall, the results are consistent with a model in which inhibition of protein synthesis leads to a loss of REDD1 protein because of its rapid degradation, and in part reduced REDD1 expression subsequently leads to de-repression of mTORC1 activity.

Received for publication, August 9, 2007 , and in revised form, November 14, 2007.

^* This study was supported in part by Grant DK-13499 from the National Institutes of Health and a grant from the Pennsylvania Department of Health using Tobacco Settlement Funds. 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 Both authors contributed equally to this work.

² To whom correspondence should be addressed: Dept. of Cellular and Molecular Physiology, H166, The Pennsylvania State University College of Medicine, 500 University Drive, Hershey, PA 17033. Tel.: 717-531-8970; Fax: 717-531-7667; E-mail: skimball{at}psu.edu.

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