Divergent Mechanisms for Homologous Desensitization of p21 by Insulin and Growth Factors

doi:10.1074/jbc.270.40.23421

Divergent Mechanisms for Homologous Desensitization of p21 by Insulin and Growth Factors (*)

From the Program in Molecular Medicine, and Department of Biochemistry and Molecular Biology, University of Massachusetts Medical Center, Worcester, Massachusetts 01605

Abstract

Previous work suggested that desensitization of p21 in response to growth factors such as epidermal growth factor (EGF) results from receptor down-regulation. Here we show that p21 is desensitized by insulin in 3T3-L1 adipocytes in the continued presence of activated insulin receptors, while loss of epidermal growth factor and platelet-derived growth factor (PDGF) receptors in response to their ligands correlates with p21desensitization. Furthermore, elevated amounts of Grb2/Shc complexes persisted throughout p21 desensitization by insulin. However, immunoblotting of anti-Son-of-sevenless (Sos) 1 and 2 immunoprecipitates with anti-Grb2 antisera revealed that p21desensitization in response to insulin and PDGF, but not EGF, is associated with a marked decrease in cellular complexes containing Sos and Grb2 proteins. Nonetheless, the desensitization of p21in response to these stimuli was homologous, in that each peptide could reactivate [P]GTP loading of p21 after desensitization by any of the others. Taken together, these data indicate that insulin, EGF, and PDGF all cause disassembly of Sos proteins from signaling complexes during p21desensitization, but at least two mechanisms are involved. Insulin elicits dissociation of Sos from Grb2 SH3 domains, whereas EGF signaling is reversed by receptor down-regulation and Shc dephosphorylation, releasing Grb2 SH2 domains. PDGF action triggers both mechanisms of Grb2 disassembly, which probably operate in concert with GAP to attenuate p21 signaling.

Footnotes

↵* This work was supported by Grant DK30648 from the National Institutes of Health (to M. P. C.) and a postdoctoral fellowship from the Juvenile Diabetes Foundation International (to A. D. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
↵¹ The abbreviations used are:

Sos

son-of-sevenless

EGF

epidermal growth factor

PDGF

platelet-derived growth factor

PAGE

polyacrylamide gel electrophoresis

PMA

phorbol 12-myristate 13-acetate.
- Received July 14, 1995.
- Revision received August 2, 1995.