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Molecular and Cellular Biology, May 2007, p. 3569-3577, Vol. 27, No. 10
0270-7306/07/$08.00+0     doi:10.1128/MCB.01447-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Differential Activation of Insulin Receptor Substrates 1 and 2 by Insulin-Like Growth Factor-Activated Insulin Receptors

Adam Denley ,^1,2,, Julie M. Carroll,^1, Gemma V. Brierley,³ Leah Cosgrove,³ John Wallace,² Briony Forbes,² and Charles T. Roberts Jr.^1*

Department of Pediatrics, Oregon Health and Science University, Portland, Oregon 97239,¹ School of Molecular and Biomedical Science, University of Adelaide, Adelaide 5005, Australia,² CSIRO Division of Molecular and Health Technologies, Adelaide 5000, Australia³

Received 4 August 2006/ Returned for modification 13 October 2006/ Accepted 13 February 2007

The insulin-like growth factors (insulin-like growth factor I [IGF-I] and IGF-II) exert important effects on growth, development, and differentiation through the IGF-I receptor (IGF-IR) transmembrane tyrosine kinase. The insulin receptor (IR) is structurally related to the IGF-IR, and at high concentrations, the IGFs can also activate the IR, in spite of their generally low affinity for the latter. Two mechanisms that facilitate cross talk between the IGF ligands and the IR at physiological concentrations have been described. The first of these is the existence of an alternatively spliced IR variant that exhibits high affinity for IGF-II as well as for insulin. A second phenomenon is the ability of hybrid receptors comprised of IGF-IR and IR hemireceptors to bind IGFs, but not insulin. To date, however, direct activation of an IR holoreceptor by IGF-I at physiological levels has not been demonstrated. We have now found that IGF-I can function through both splice variants of the IR, in spite of low affinity, to specifically activate IRS-2 to levels similar to those seen with equivalent concentrations of insulin or IGF-II. The specific activation of IRS-2 by IGF-I through the IR does not result in activation of the extracellular signal-regulated kinase pathway but does induce delayed low-level activation of the phosphatidylinositol 3-kinase pathway and biological effects such as enhanced cell viability and protection from apoptosis. These findings suggest that IGF-I can function directly through the IR and that the observed effects of IGF-I on insulin sensitivity may be the result of direct facilitation of insulin action by IGF-I costimulation of the IR in insulin target tissues.

Published ahead of print on 26 February 2007.

Present address: Department of Molecular and Experimental Medicine, The Scripps Research Institute, La Jolla, CA 92037.

These authors contributed equally to this work.

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