Insulin-induced Desensitization of Extracellular Signal-regulated Kinase Activation Results from an Inhibition of Raf Activity Independent of Ras Activation and Dissociation of the Grb2-SOS Complex

Raymond V Fucini,Shuichi Okada,Jeffrey E Pessin

doi:10.1074/jbc.274.26.18651

Abstract

Previous studies have suggested that the interaction between the small adaptor protein Grb2 with the Ras guanyl nucleotide exchange factor SOS is functionally important in the regulation of the Ras activation/inactivation cycle. To examine the relationship between the Grb2-SOS complex and Ras activation, we observed that insulin stimulation results in a rapid but transient activation of Ras and the extracellular-signal regulated kinase (ERK) followed by dissociation of the Grb2-SOS complex. Although treatment with the phorbol myristate acetate resulted in ERK activation and complete dissociation of the Grb2-SOS complex, there was no effect on subsequent insulin-stimulated Ras activation. Similarly, insulin stimulation followed by insulin removal resulted in a time-dependent restoration of the Grb2-SOS complex but which was significantly slower than the recovery of insulin-stimulated Ras activation. In addition, although insulin was able to activate Ras under these conditions, there was a complete desensitization of Raf and ERK activation. This apparent homologous desensitization of insulin action was specific for Raf and ERK as the insulin re-stimulation of insulin receptor autophosphorylation and protein kinase B activation were unaffected. Together, these data demonstrate the presence of a pathway independent of the Grb2-SOS complex that can lead to Ras activation but that the desensitization of Raf accounts for the homologous desensitization of ERK.

Highlights

Shc and IRS families of intracellular substrates [3, 4]
Consistent with previous findings [27, 28, 31, 37], 5 min of insulin stimulation resulted in a characteristic mobility shift of SOS that is indicative of its serine/threonine phosphorylation (Fig. 1A, lanes 1 and 2)
Following 5 min of insulin stimulation there was a marked reduction in the amount of SOS that was co-immunoprecipitated with Grb2 (Fig. 1B, lanes 1 and 2)

Summary

Introduction

Shc and IRS families of intracellular substrates [3, 4]. The phosphorylation of these proteins generates specific recognition sites for the src homology 2 (SH2) domain of the 25-kDa adaptor protein Grb2 [5,6,7]. Stimulation of the cells by both PMA and osmotic shock resulted in the phosphorylation of SOS and activation of ERK (Fig. 3A, lanes 3– 6). Stimulation of the cells with all three stimuli (insulin, PMA, and osmotic shock) induced a dissociation of Grb2 from SOS (Fig. 3B, lanes 1– 6).

Results

Conclusion