title: Continually high insulin levels impair Akt phosphorylation and glucose transport in human myoblasts.
abstract: Chronic hyperinsulinemia is both a marker and a cause for insulin resistance. This study analyzes the effect of long-term exposure to high insulin levels on the insulin-signaling pathway and glucose transport in cultured human myoblasts. Human myoblasts were grown in the presence of low (107 pmol/L, SkMC-L) or high (1430 pmol/L, SkMC-H) insulin concentrations for 3 weeks. Glucose transport, insulin receptor (IR), and IR substrate 1 (IRS1) phosphorylation, phosphatidylinositol 3'-kinase (PI3K) activity, as well as Akt-Ser473 phosphorylation have been investigated at the end of the incubation period and after a further short-term insulin stimulation. At the end of the incubation period, IR, IRS1, p85/PI3K, Akt, and GLUT4 protein expression levels were similar in both culture conditions. Basal glucose transport was similar in SkMC-L and SkMC-H, but after short-term insulin stimulation significantly increased (P < .01) only in SkMC-L. IR binding was down-regulated in SkMC-H (P < .01), but IR and IRS1 tyrosine phosphorylation and PI3K activity were significantly higher (P < .01) in SkMC-H than SkMC-L. Despite increased PI3K activation, Akt-Ser473 phosphorylation was similar in SkMC-L and SkMC-H. After a short-term insulin stimulation (10 nmol/L insulin for 10 minutes), IR and IRS1 tyrosine phosphorylation, PI3K activation, and Akt-Ser473 phosphorylation significantly increased (P < .01 and P < .05 for Akt) in SkMC-L but not in SkMC-H. Serine phosphorylation of IRS1 was similar in SkMC-L and SkMC-H. Moreover, in the SkMC-H, insulin stimulation was associated with the inhibition of IRS1 tyrosine dephosphorylation (P < .05). In summary, continuous exposure of cultured myoblasts to high insulin levels induces a persistent up-regulation of IR, IRS1, and PI3K activity associated with the demodulation of insulin signaling. Moreover, the impairment of the insulin-signaling steps between PI3K and Akt is concomitant with the desensitization of glucose transport. These alterations may contribute to the derangement insulin-signaling pathway states of hyperinsulinemia such as obesity and type 2 diabetes.
authors.affiliation: Section of Diabetes, Department of Endocrinology and Metabolism, School of Medicine, Ospedale Cisanello, University of Pisa, 56124 Pisa, Italy.
authors.forename: Anna
authors.initials: A
authors.lastname: Bertacca
authors.forename: Annamaria
authors.initials: A
authors.lastname: Ciccarone
authors.forename: Paolo
authors.initials: P
authors.lastname: Cecchetti
authors.forename: Barbara
authors.initials: B
authors.lastname: Vianello
authors.forename: Incoronata
authors.initials: I
authors.lastname: Laurenza
authors.forename: Margherita
authors.initials: M
authors.lastname: Maffei
authors.forename: Chiara
authors.initials: C
authors.lastname: Chiellini
authors.forename: Stefano
authors.initials: S
authors.lastname: Del Prato
authors.forename: Luca
authors.initials: L
authors.lastname: Benzi
mesh_terms: D001692:Biological Transport; D002478:Cells, Cultured; D005947:Glucose / Q000378:metabolism*; D006801:Humans; D006946:Hyperinsulinism / Q000378:metabolism*; D055504:Insulin Receptor Substrate Proteins; D032446:Myoblasts / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D011972:Receptor, Insulin / Q000378:metabolism; D015398:Signal Transduction
chemical_list: C526292:IRS1 protein, human; D055504:Insulin Receptor Substrate Proteins; D010750:Phosphoproteins; D019869:Phosphatidylinositol 3-Kinases; D011972:Receptor, Insulin; D051057:Proto-Oncogene Proteins c-akt; D005947:Glucose