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J. Biol. Chem., Vol. 282, Issue 31, 22513-22524, August 3, 2007

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Insulin-like Growth Factor Type-I Receptor Internalization and Recycling Mediate the Sustained Phosphorylation of Akt^*

Robert J. Romanelli, Andrew P. LeBeau^¶1, Clifton G. Fulmer, Deborah A. Lazzarino, Alan Hochberg^¶, and Teresa L. Wood²

From the Department of Neurology and Neurosciences, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, Newark, New Jersey 07103, Department of Neural and Behavioral Sciences, Penn State College of Medicine, Hershey, Pennsylvania 17033, and ^¶ProSanos Corporation, La Jolla, California 92037

Previously we demonstrated that insulin-like growth factor-I mediates the sustained phosphorylation of Akt, which is essential for long term survival and protection of glial progenitors from glutamate toxicity. These prosurvival effects correlated with prolonged activation and stability of the insulin-like growth factor type-I receptor. In the present study, we investigated the mechanisms whereby insulin-like growth factor-I signaling, through the insulin-like growth factor type-I receptor, mediates the sustained phosphorylation of Akt. We showed that insulin-like growth factor-I stimulation induced loss of receptors from the cell surface but that surface receptors recovered over time. Blocking receptor internalization inhibited Akt phosphorylation, whereas inhibition of receptor trafficking blocked receptor recovery at the cell surface and the sustained phosphorylation of Akt. Moreover the insulin-like growth factor type-I receptor localized with the transferrin receptor and Rab11-positive endosomes in a ligand-dependent manner, further supporting the conclusion that this receptor follows a recycling pathway. Our results provide evidence that ligand stimulation leads to internalization of the insulin-like growth factor type-I receptor, which mediates Akt phosphorylation, and that receptor recycling sustains Akt phosphorylation in glial progenitors. Mathematical modeling of receptor trafficking further supports these results and predicts an additional kinetic state of the receptor consistent with sustained Akt phosphorylation.

Received for publication, May 24, 2007

^* This work was supported by NINDS, National Institutes of Health Grants NS37560 and NS050742 (to T. L. W.). 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 Present address: Accelrys, Inc., San Diego, CA 92121.

² To whom correspondence should be addressed: Dept. of Neurology and Neurosciences H506, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, 185 S. Orange Ave., Newark, NJ 07103. Tel.: 973-972-6529; E-mail: woodte{at}umdnj.edu.

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