Improved insulin sensitivity by calorie restriction is associated with reduction of ERK and p70S6K activities in the liver of obese Zucker rats.
Calorie restriction (CR) improves obesity-related insulin resistance through undefined molecular mechanisms. Insulin receptor substrate (IRS)-1 serine/threonine kinases have been proposed to modulate insulin sensitivity through phosphorylation of IRS proteins. The aim of this study is to test the hypothesis that changes in the activity of IRS1 serine/threonine kinases may underlie the molecular mechanism of CR in improving insulin sensitivity. Obese and lean Zucker rats were subjected to 40% CR or allowed to feed ad libitum (AL) for 20 weeks; body weight and insulin sensitivity were monitored throughout this period. The activity of IRS1 serine/threonine kinases - including JNK, ERK, MTOR/p70(S6K) (RPS6KB1 as listed in the MGI Database), glycogen synthase kinase 3beta (GSK3B), AMPK (PRKAA1 as listed in the MGI Database), and protein kinase C (PRKCQ) in liver tissue extracts was measured by an in vitro kinase assay using various glutathione-S-transferase (GST)-IRS1 fragments as substrates, while phosphorylation of IRS1 and serine kinases was determined by western blotting using phosphospecific antibodies. CR in obese rats significantly reduced body weight and increased insulin sensitivity compared to AL controls. Serine kinase activity toward IRS1(S612) (corresponding to S616 in human IRS1) and IRS1(S632/635) (corresponding to S636/639 in human IRS1) was increased in obese rats compared to lean littermates, and was markedly decreased following CR. Concomitantly, obesity increased and CR decreased the activity of hepatic ERK and p70(S6K) against IRS1. The close association between the activity of hepatic ERK and p70(S6K) with insulin resistance suggests an important role for ERK and p70(S6K) in the development of insulin resistance, presumably via phosphorylation of IRS proteins.
Section of Endocrinology, The University of Chicago, 900 E 57th Street, KCBD Room 8122, Chicago, Illinois 60637, USA.
Yanbin
Y
Zheng
Wenshuo
W
Zhang
Elisha
E
Pendleton
Sanhua
S
Leng
Jiong
J
Wu
Ridong
R
Chen
Xiao Jian
XJ
Sun
D000818:Animals; D031204:Caloric Restriction; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D055504:Insulin Receptor Substrate Proteins / Q000378:metabolism*; D007333:Insulin Resistance; D008099:Liver / Q000201:enzymology; D008297:Male; D009765:Obesity / Q000201:enzymology* / Q000628:therapy; D010766:Phosphorylation; D051381:Rats; D011924:Rats, Zucker; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism*
D055504:Insulin Receptor Substrate Proteins; C526296:Irs1 protein, rat; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D048049:Extracellular Signal-Regulated MAP Kinases