title: The phosphorylation of Ser318 of insulin receptor substrate 1 is not per se inhibitory in skeletal muscle cells but is necessary to trigger the attenuation of the insulin-stimulated signal.
abstract: The Ser/Thr phosphorylation of insulin receptor substrate 1 (IRS) is one key mechanism to stimulate and/or attenuate insulin signal transduction. Using a phospho-specific polyclonal antibody directed against phosphorylated Ser(318) of IRS-1, we found a rapid and transient insulin-stimulated phosphorylation of Ser(318) in human and rodent skeletal muscle cell models and in muscle tissue of insulin-treated mice. None of the investigated insulin resistance-associated factors (e.g. high glucose, tumor necrosis factor-alpha, adrenaline) stimulated the phosphorylation of Ser(318). Studying the function of this phosphorylation, we found that replacing Ser(318) by alanine completely prevented both the attenuation of insulin-stimulated Akt/protein kinase B Ser(473) phosphorylation and glucose uptake after 60 min of insulin stimulation. Unexpectedly, after acute insulin stimulation, we observed that phosphorylation of Ser(318) is not inhibitory but rather enhances insulin signal transduction because introduction of Ala(318) led to a reduction of the insulin-stimulated Akt/protein kinase B phosphorylation. Furthermore, replacing Ser(318) by glutamate, i.e. mimicking phosphorylation, improved glucose uptake after acute insulin stimulation. These data suggest that phosphorylation of Ser(318) is not per se inhibitory but is necessary to trigger the attenuation of the insulin-stimulated signal in skeletal muscle cells. Investigating the molecular mechanism of insulin-stimulated Ser(318) phosphorylation, we found that phosphatidylinositol 3-kinase-mediated activation of atypical protein kinase C-zeta and recruitment of protein kinase C-zeta to IRS-1 was responsible for this phosphorylation. We conclude that Ser(318) phosphorylation of IRS-1 is an early physiological event in insulin-stimulated signal transduction, which attenuates the continuing action of insulin.
authors.affiliation: Department of Internal Medicine IV, University Hospital Tuebingen, Germany.
authors.forename: Cora
authors.initials: C
authors.lastname: Weigert
authors.forename: Anita M
authors.initials: AM
authors.lastname: Hennige
authors.forename: Tasja
authors.initials: T
authors.lastname: Brischmann
authors.forename: Alexander
authors.initials: A
authors.lastname: Beck
authors.forename: Klaus
authors.initials: K
authors.lastname: Moeschel
authors.forename: Myriam
authors.initials: M
authors.lastname: Schaüble
authors.forename: Katrin
authors.initials: K
authors.lastname: Brodbeck
authors.forename: Hans-Ulrich
authors.initials: HU
authors.lastname: Häring
authors.forename: Erwin D
authors.initials: ED
authors.lastname: Schleicher
authors.forename: Rainer
authors.initials: R
authors.lastname: Lehmann
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002478:Cells, Cultured; D003847:Deoxyglucose / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D006801:Humans; D007328:Insulin / Q000494:pharmacology* / Q000502:physiology; D055504:Insulin Receptor Substrate Proteins; D008297:Male; D051379:Mice; D018485:Muscle Fibers, Skeletal / Q000187:drug effects* / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010750:Phosphoproteins / Q000737:chemistry / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D010768:Phosphoserine / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D000099221:Protein Kinase C zeta
chemical_list: D003847:Deoxyglucose; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010750:Phosphoproteins; D010768:Phosphoserine; D011493:Protein Kinase C; D000099221:Protein Kinase C zeta; C526292:IRS1 protein, human; C526295:Irs1 protein, mouse