Sustained endotoxemia leads to marked down-regulation of early steps in the insulin-signaling cascade.
OBJECTIVE
To determine the effects of sustained, 3-day endotoxin infusion on early steps of the insulin-signaling pathway in rat liver and skeletal muscle in vivo; to examine insulin signaling in well-established acute endotoxin models of insulin resistance.


METHODS
Prospective, controlled animal study.


METHODS
University research laboratory.


METHODS
Male Sprague-Dawley rats: 24 in the 3-day endotoxin study, 22 in each acute endotoxin study.


METHODS
In prolonged endotoxemia studies, endotoxin (1 mg.kg-1.24 hrs-1) was administered via jugular venous catheter for 74 hrs. Insulin was then injected, and liver and skeletal muscle were removed after 5 mins. In acute endotoxemia studies, an endotoxin bolus (1 mg/kg) was administered, and insulin-signaling responses were studied after 4 hrs.


RESULTS
In liver of rats with sustained endotoxemia, there were significant decreases in insulin-stimulated tyrosine phosphorylation of insulin receptors (74%), insulin receptor substrate (IRS)-1 (74%), and IRS2 (53%); binding of the p85 subunit of phosphatidylinositide 3-kinase to IRS1 (80%); and IRS1-precipitable phosphatidylinositide 3-kinase activity (>90%). These findings were associated with significant reductions in abundance of insulin receptors (37%), IRS1 (60%), and IRS2 (23%). Signaling in skeletal muscle was similarly affected, with reduced IRS1 phosphorylation (49%), IRS1 abundance (50%), and binding of p85 to IRS1 (57%). Insulin signaling 4 hrs after endotoxin administration was not different from controls.


CONCLUSIONS
Prolonged endotoxemia is associated with marked deficits in early steps of the insulin-signaling pathway, which are at least partly explained by reduced abundance of the insulin receptor and IRS proteins. Signaling defects were not evident 4 hrs after endotoxin administration under conditions of adequate nutrition, indicating that insulin resistance develops gradually, may require concomitant malnutrition, and is not reversed by the development of endotoxin tolerance.
McCowen
K C
KC
Research Division of the Joslin Diabetes Center, Beth Israel Deaconess Medical Center, Boston, MA, USA.
Ling
P R
PR
Ciccarone
A
A
Mao
Y
Y
Chow
J C
JC
Bistrian
B R
BR
Smith
R J
RJ
D000704:Analysis of Variance; D000818:Animals; D015536:Down-Regulation; D004435:Eating; D019446:Endotoxemia / Q000378:metabolism*; D004926:Escherichia coli; D007328:Insulin / Q000378:metabolism*; D055504:Insulin Receptor Substrate Proteins; D008070:Lipopolysaccharides; D008099:Liver / Q000378:metabolism*; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction
D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D008070:Lipopolysaccharides; D019869:Phosphatidylinositol 3-Kinases; D010750:Phosphoproteins; C526296:Irs1 protein, rat