Turnover of the active fraction of IRS1 involves raptor-mTOR- and S6K1-dependent serine phosphorylation in cell culture models of tuberous sclerosis

Mol Cell Biol. 2006 Sep;26(17):6425-34. doi: 10.1128/MCB.01254-05.

Abstract

The TSC1-TSC2/Rheb/Raptor-mTOR/S6K1 cell growth cassette has recently been shown to regulate cell autonomous insulin and insulin-like growth factor I (IGF-I) sensitivity by transducing a negative feedback signal that targets insulin receptor substrates 1 and 2 (IRS1 and -2). Using two cell culture models of the familial hamartoma syndrome, tuberous sclerosis, we show here that Raptor-mTOR and S6K1 are required for phosphorylation of IRS1 at a subset of serine residues frequently associated with insulin resistance, including S307, S312, S527, S616, and S636 (of human IRS1). Using loss- and gain-of-function S6K1 constructs, we demonstrate a requirement for the catalytic activity of S6K1 in both direct and indirect regulation of IRS1 serine phosphorylation. S6K1 phosphorylates IRS1 in vitro on multiple residues showing strong preference for RXRXXS/T over S/T,P sites. IRS1 is preferentially depleted from the high-speed pellet fraction in TSC1/2-deficient mouse embryo fibroblasts or in HEK293/293T cells overexpressing Rheb. These studies suggest that, through serine phosphorylation, Raptor-mTOR and S6K1 cell autonomously promote the depletion of IRS1 from specific intracellular pools in pathological states of insulin and IGF-I resistance and thus potentially in lesions associated with tuberous sclerosis.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adaptor Proteins, Signal Transducing
  • Animals
  • Cell Culture Techniques
  • Cells, Cultured
  • Humans
  • Insulin Receptor Substrate Proteins
  • Mice
  • Monomeric GTP-Binding Proteins / metabolism
  • Neuropeptides / metabolism
  • Phosphoproteins / deficiency
  • Phosphoproteins / metabolism*
  • Phosphoserine / metabolism*
  • Protein Kinases / metabolism*
  • Proteins / metabolism*
  • Ras Homolog Enriched in Brain Protein
  • Regulatory-Associated Protein of mTOR
  • Ribosomal Protein S6 Kinases / metabolism*
  • Subcellular Fractions
  • TOR Serine-Threonine Kinases
  • Tuberous Sclerosis / pathology*
  • Tuberous Sclerosis Complex 1 Protein
  • Tuberous Sclerosis Complex 2 Protein
  • Tumor Suppressor Proteins / deficiency

Substances

  • Adaptor Proteins, Signal Transducing
  • IRS1 protein, human
  • Insulin Receptor Substrate Proteins
  • Irs1 protein, mouse
  • Neuropeptides
  • Phosphoproteins
  • Proteins
  • RHEB protein, human
  • RPTOR protein, human
  • Ras Homolog Enriched in Brain Protein
  • Regulatory-Associated Protein of mTOR
  • TSC1 protein, human
  • TSC2 protein, human
  • Tsc1 protein, mouse
  • Tsc2 protein, mouse
  • Tuberous Sclerosis Complex 1 Protein
  • Tuberous Sclerosis Complex 2 Protein
  • Tumor Suppressor Proteins
  • Phosphoserine
  • Protein Kinases
  • MTOR protein, human
  • TOR Serine-Threonine Kinases
  • mTOR protein, mouse
  • Ribosomal Protein S6 Kinases
  • Monomeric GTP-Binding Proteins