Protein kinase C Theta inhibits insulin signaling by phosphorylating IRS1 at Ser(1101).
ABSTRACT Obesity and stress inhibit insulin action by activating protein kinases that enhance serine phosphorylation of IRS1 and have been thus associated to insulin resistance and the development of type II diabetes. The protein kinase C (PKC) is activated by free-fatty acids, and its activity is higher in muscle from obese diabetic patients. However, a molecular link between PKC and insulin resistance has not been defined yet. Here we show that PKC phosphorylates IRS1 at serine 1101 blocking IRS1 tyrosine phosphorylation and downstream activation of the Akt pathway. Mutation of Ser(1101) to alanine makes IRS1 insensitive to the effect of PKC and restores insulin signaling in culture cells. These results provide a novel mechanism linking the activation of PKC to the inhibition of insulin signaling.
SourceAvailable from: Jonas T. Treebak[Show abstract] [Hide abstract]
ABSTRACT: Skeletal muscle constitutes 40% of individual body mass and plays vital roles in locomotion and whole-body metabolism. Proteomics of skeletal muscle is challenging due to highly abundant contractile proteins that interfere with detection of regulatory proteins. Using a state-of-the art mass spectrometric (MS) workflow and a strategy to map identifications from the C2C12 cell line model to tissues, we identified a total of 10,218 proteins, including skeletal muscle specific transcription factors like myod1 and myogenin and circadian clock proteins. We obtain absolute abundances for proteins expressed in a muscle cell line and skeletal muscle, which should serve as a valuable resource. Quantitation of protein isoforms of glucose uptake signaling pathways and in glucose and lipid metabolic pathways provides a detailed metabolic map of the cell line compare to tissue. This revealed unexpectedly complex regulation of AMP-activated protein kinase and insulin signaling in muscle tissue at the level of enzyme isoforms. Copyright © 2015, The American Society for Biochemistry and Molecular Biology.Molecular & Cellular Proteomics 01/2015; DOI:10.1074/mcp.M114.044222 · 7.25 Impact Factor
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ABSTRACT: Although an elevated triacylglycerol (TAG) content in non-adipose tissues is often associated with insulin resistance, the mechanistic relationship remains unclear. Data support roles for intermediates in the glycerol-3-phosphate pathway of TAG synthesis: diacylglycerol (DAG), which may cause insulin resistance in liver by activating PKCϵ,and phosphatidic acid (PA), which inhibits insulin action in hepatocytes by disrupting the assembly of mTOR and rictor. To determine whether increases in DAG and PA impair insulin signaling when produced by pathways other than that of de novo synthesis, we examined primary mouse hepatocytes after enzymatically manipulating the cellular content of DAG or PA. Over-expressing phospholipase D-1 or -2 inhibited insulin signaling and was accompanied by an elevated cellular content of total PA, without a change in total DAG. Over-expression of diacylglycerol kinase-θ inhibited insulin signaling, and was accompanied by an elevated cellular content of total PA and a decreased cellular content of total DAG. Over-expressing glycerol-3-phosphate acyltransferase-1 or -4 inhibited insulin signaling and increased the cellular content of both PA and DAG. Insulin signaling impairment caused by over-expression of phospholipase D-1/2 or diacylglycerol kinase-θ was always accompanied by disassociation of mTOR/rictor and reduction of mTORC2 kinase activity. However, although the protein ratio of membrane to cytosolic PKCϵ increased, PKC activity itself was unaltered. These data suggest that PA, but not DAG, is associated with impaired insulin action in mouse hepatocytes. Copyright © 2014, The American Society for Biochemistry and Molecular Biology.Journal of Biological Chemistry 12/2014; DOI:10.1074/jbc.M114.602789 · 4.60 Impact Factor
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ABSTRACT: Insulin resistance in metabolic syndrome subjects is profound in spite of muscle insulin receptor and insulin-responsive glucose transporter (GLUT4) expression being nearly normal. Insulin receptor tyrosine kinase phosphorylation of insulin receptor substrate-1 (IRS-1) at Tyr896 is a necessary step in insulin stimulation of translocation of GLUT4 to the cell surface. Serine phosphorylation of IRS-1 by some kinases diminishes insulin action in mice. We evaluated the phosphorylation status of muscle IRS-1 in 33 subjects with the metabolic syndrome and seventeen lean controls. Each underwent euglycemic insulin clamps and a thigh muscle biopsy before and after 8 weeks of either strength or endurance training. Muscle IRS-1 phosphorylation at six sites was quantified by immunoblots. Metabolic syndrome muscle IRS-1 had excess phosphorylation at Ser337 and Ser636 but not at Ser307, Ser789, or Ser1101. Ser337 is a target for phosphorylation by glycogen synthase kinase 3 (GSK3) and Ser636 is phosphorylated by c-Jun N-terminal kinase 1 (JNK1). Exercise training without weight loss did not change the IRS-1 serine phosphorylation. These data suggest that baseline hyperphosphorylation of at least two key serines within muscle IRS-1 diminishes the transmission of the insulin signal and thereby decreases the insulin-stimulated translocation of GLUT4. Excess fasting phosphorylation of muscle IRS-1 at Ser636 may be a major cause of the insulin resistance seen in obesity and might prevent improvement in insulin responsiveness when exercise training is not accompanied by weight loss. © 2014 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society.12/2014; 2(12). DOI:10.14814/phy2.12236