Raptor Binds the SAIN (Shc and IRS-1 NPXY Binding) Domain of Insulin Receptor Substrate-1 (IRS-1) and Regulates the Phosphorylation of IRS-1 at Ser-636/639 by mTOR

Alexandros Tzatsos

doi:10.1074/jbc.m109.027748

Abstract

In normal physiological states mTOR phosphorylates and activates Akt. However, under diabetic-mimicking conditions mTOR inhibits phosphatidylinositol (PI) 3-kinase/Akt signaling by phosphorylating insulin receptor substrate-1 (IRS-1) at Ser-636/639. The molecular basis for the differential effect of mTOR signaling on Akt is poorly understood. Here, it has been shown that knockdown of mTOR, Raptor, and mLST8, but not Rictor and mSin1, suppresses insulin-stimulated phosphorylation of IRS-1 at Ser-636/639 and stabilizes IRS-1 after long term insulin stimulation. This phosphorylation depends on the PI 3-kinase/PDK1 axis but is Akt-independent. At the molecular level, Raptor binds the SAIN (Shc and IRS-1 NPXY binding) domain of IRS-1 and regulates the phosphorylation of IRS-1 at Ser-636/639 by mTOR. IRS-1 lacking the SAIN domain does not interact with Raptor, is not phosphorylated at Ser-636/639, and favorably interacts with PI 3-kinase. Overall, these data provide new insights in the molecular mechanisms by which mTORC1 inhibits PI 3-kinase/Akt signaling at the level of IRS-1 and suggest that mTOR signaling toward Akt is scaffold-dependent.

Highlights

MTORC1 [7, 8] and mSin1 as a Rictor-interacting protein that is required by mTORC2 to phosphorylate Akt at Ser-473 [5, 6]
The physiological relevance of these findings is supported by recent studies that showed (a) the phosphorylation of insulin receptor substrate-1 (IRS-1) at Ser-636/639 is increased in noninsulin-dependent diabetes mellitus subjects with a concomitant reduction in Akt activity [16], (b) rapamycin-mediated mTORC1 inhibition reduces in vivo the phosphorylation of IRS-1 at Ser-636/639 and stimulates insulin-mediated glucose uptake in skeletal muscle of human subjects [16, 17], and (c) adipose-specific knock-out of raptor results in lean mice that are resistant to diet-induced obesity and exhibit insulin hypersensitivity with improved glucose tolerance [18]
In this report I systematically knocked down all the known components of mTORC1 and mTORC2, and I demonstrate that the selective ability of mTORC1, but not mTORC2, to suppress the IRS-1-associated PI 3-kinase/Akt signaling depends on Raptor

Summary

Introduction

MTORC1 [7, 8] and mSin1 as a Rictor-interacting protein that is required by mTORC2 to phosphorylate Akt at Ser-473 [5, 6]. IRS-1 lacking the SAIN domain is not phosphorylated at Ser636/639, and it is resistant to mTORC1-mediated inhibition of PI 3-kinase/Akt signaling.

Results

Conclusion