Mechanism of induction of muscle protein loss by hyperglycaemia.
Treatment of murine myotubes with high glucose concentrations (10 and 25 mM) stimulated protein degradation through the ubiquitin-proteasome pathway, and also caused activation (autophosphorylation) of PKR (double-stranded-RNA-dependent protein kinase) and eIF2alpha (eukaryotic initiation factor 2alpha). Phosphorylation of PKR and eIF2alpha was also seen in the gastrocnemius muscle of diabetic ob/ob mice. High glucose levels also inhibited protein synthesis. The effect of glucose on protein synthesis and degradation was not seen in myotubes transfected with a catalytically inactive variant (PKRDelta6). High glucose also induced an increased activity of both caspase-3 and -8, which led to activation of PKR, since this was completely attenuated by the specific caspase inhibitors. Activation of PKR also led to activation of p38MAPK (mitogen activated protein kinase), leading to ROS (reactive oxygen species) formation, since this was attenuated by the specific p38MAPK inhibitor SB203580. ROS formation was important in protein degradation, since it was completely attenuated by the antioxidant butylated hydroxytoluene. These results suggest that high glucose induces muscle atrophy through the caspase-3/-8 induced activation of PKR, leading to phosphorylation of eIF2alpha and depression of protein synthesis, together with PKR-mediated ROS production, through p38MAPK and increased protein degradation.
Russell
Steven T
ST
Nutritional Biomedicine, School of Life and Health Sciences, Aston University, Birmingham B47ET, UK.
Rajani
Seema
S
Dhadda
Raj S
RS
Tisdale
Michael J
MJ
D000818:Animals; D001284:Atrophy; D053148:Caspase 3 / Q000378:metabolism; D053181:Caspase 8 / Q000378:metabolism; D002460:Cell Line; D003920:Diabetes Mellitus / Q000201:enzymology; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism; D005947:Glucose / Q000494:pharmacology; D006943:Hyperglycemia / Q000378:metabolism*; D008297:Male; D051379:Mice; D008954:Models, Biological; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000201:enzymology / Q000473:pathology; D009124:Muscle Proteins / Q000172:deficiency* / Q000378:metabolism; D009218:Myosins / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D014176:Protein Biosynthesis / Q000187:drug effects; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D017382:Reactive Oxygen Species / Q000378:metabolism; D025801:Ubiquitin / Q000378:metabolism; D019892:eIF-2 Kinase / Q000378:metabolism
D015852:Eukaryotic Initiation Factor-2; D009124:Muscle Proteins; D017382:Reactive Oxygen Species; D025801:Ubiquitin; D019892:eIF-2 Kinase; D053148:Caspase 3; D053181:Caspase 8; D046988:Proteasome Endopeptidase Complex; D009218:Myosins; D005947:Glucose