title: High glucose induces cardiac insulin-like growth factor I resistance in ventricular myocytes: role of Akt and ERK activation.
abstract: OBJECTIVE
  Cardiac resistance to IGF-1 occurs in diabetes and is attributed to cardiac dysfunction in diabetes. However, the mechanism of action responsible for cardiac IGF-1 resistance is still unknown. This study was designed to examine the impact of high glucose on IGF-1-induced contractile response and activation of serine-threonine kinase Akt as well as extracellular signal-regulated kinase (ERK1/2) in cardiac myocytes.
  
  
  METHODS
  Isolated adult rat ventricular myocytes were cultured for 12-18 h in a serum-free medium containing either normal (NG, 5.5 mM) or high (HG, 25.5 mM) glucose. Mechanical properties were evaluated using an IonOptix MyoCam system. Myocytes were electrically stimulated at 0.5 Hz and contractile properties analyzed included peak shortening (PS), time-to-PS (TPS) and time-to-90% relengthening (TR(90)). Intracellular Ca(2+)-induced Ca(2+) release was measured as fura-2 fluorescence intensity change (DeltaFFI). Protein levels of total and phosphorylated Akt and ERK1/2, indicators of Akt and ERK1/2 activation, IGF-1 receptors (pro-IGF-1R and IGF-1Ralpha) as well as the glucose transporter GLUT4 were assessed by Western blot.
  
  
  RESULTS
  IGF-1 (10(-10)-10(-6) M) elicited a dose-dependent increase in PS and DeltaFFI in myocytes maintained in NG medium. However, IGF-1 induced a negative response on PS and DeltaFFI in HG myocytes. The IGF-1-induced responses in NG or HG myocytes were blunted by the IGF-1 receptor antagonist H-1356. Western blot analysis revealed that IGF-1Ralpha but not pro-IGF-1R was reduced in HG myocytes. While IGF-1 (10(-6) M) upregulated total Akt protein levels in both NG and HG myocytes, it only induced a significant activation of Akt in NG but not HG myocytes. IGF-1 elicited comparable ERK1/2 activation in both NG and HG myocytes.
  
  
  CONCLUSIONS
  These results suggest that the cardiac IGF-1 resistance in diabetes is likely attributed, at least in part, to reduced IGF-1R and attenuated IGF-1-induced Akt phosphorylation under elevated extracellular glucose.
authors.affiliation: University of Wyoming College of Health Sciences, Laramie, WY 82071, USA. jren@uwyo.edu
authors.forename: Jun
authors.initials: J
authors.lastname: Ren
authors.forename: Jinhong
authors.initials: J
authors.lastname: Duan
authors.forename: Kadon K
authors.initials: KK
authors.lastname: Hintz
authors.forename: Bonnie H
authors.initials: BH
authors.lastname: Ren
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism; D048429:Cell Size / Q000187:drug effects; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004351:Drug Resistance; D005947:Glucose / Q000494:pharmacology*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D008297:Male; D008353:Mannitol / Q000494:pharmacology; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000378:metabolism; D009842:Oligopeptides / Q000494:pharmacology; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats
chemical_list: D009842:Oligopeptides; D011518:Proto-Oncogene Proteins; C411572:H 1356; D008353:Mannitol; D007334:Insulin-Like Growth Factor I; C494920:Akt1 protein, rat; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D005947:Glucose; D002118:Calcium