Sustained endotoxemia leads to marked down-regulation of early steps in the insulin-signaling cascade

Objectives: 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 endo toxin models of insulin resistance. Design: Prospective, controlled animal study. Setting: University research laboratory. Subjects: Male Sprague-Dawley rats: 24 in the 3-day endotoxin study, 22 in each acute endotoxin study, Interventions: In prolonged endotoxemia studies, endotoxin (1 mg.kg(-1).24 hrs(-1)) was administered via jugular venous catheter for 74 hrs, Insulin w as 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, Measurements and Main Results:ln liver of rats with sustained endotoxemia, there were significant decreases in insulin-stimulated tyrosine phosphoryla tion of insulin receptors (74%), insulin receptor substrate (IRS)-1 (74%), and IRS2 (53%); binding of the p85 subunit of phosphatidylinositide 3-kinas e to IRS1 (80%); and IRS1-precipitable phosphatidylinositide 3-kinase activ ity (>90%). These findings were associated with significant reductions in a bundance of insulin receptors (37%), IRS1 (60%), and IRS2 (23%). Signaling in skeletal muscle was similarly affected, with reduced IRS1 phosphorylatio n (49%), IRS1 abundance (50%), and binding of p85 to IRS1 (57%). Insulin si gnaling 4 hrs after endotoxin administration was not different from control s. Conclusions: Prolonged endotoxemia is associated with marked deficits in ea rly steps of the insulin-signaling pathway, which are at least partly expla ined by reduced abundance of the insulin receptor and IRS proteins. Signali ng defects were not evident 4 hrs after endotoxin administration under cond itions of adequate nutrition, indicating that insulin resistance develops g radually, may require concomitant malnutrition, and is not reversed by the development of endotoxin tolerance.