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Interleukin-6 Depletion Selectively Improves Hepatic Insulin Action in Obesity

Department of Pathology and Laboratory Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642

Address all correspondence and requests for reprints to: Dr. Robert A. Mooney, Department of Pathology and Laboratory Medicine, University of Rochester School of Medicine and Dentistry, 601 Elmwood Avenue, Rochester, New York 14642. E-mail: robert_mooney{at}urmc.rochester.edu.

Obesity and insulin resistance are considered chronic inflammatory states, in part because circulating IL-6 is elevated. Exogenous IL-6 can induce hepatic insulin resistance in vitro and in vivo. The importance of endogenous IL-6, however, to insulin resistance of obesity is unresolved. To test the hypothesis that IL-6 contributes to the inflammation and insulin resistance of obesity, IL-6 was depleted in Lep^ob mice by injection of IL-6-neutralizing antibody. In untreated Lep^ob mice, signal transducer and activator of transcription-3 (STAT3) activation was increased compared with that in lean controls, consistent with an inflammatory state. With IL-6 depletion, activation of STAT3 in liver and adipose tissue and expression of haptoglobin were reduced. Expression of the IL-6-dependent, hepatic acute phase protein fibrinogen was also decreased. Using the hyperinsulinemic-euglycemic clamp technique, insulin-dependent suppression of endogenous glucose production was 89% in IL-6-depleted Lep^ob mice, in contrast to only 32% in Lep^ob controls, indicating a marked increase in hepatic insulin sensitivity. A significant change in glucose uptake in skeletal muscle after IL-6 neutralization was not observed. In a direct comparison of hepatic insulin signaling in Lep^ob mice treated with anti-IL-6 vs. IgG-treated controls, insulin-dependent insulin receptor autophosphorylation and activation of Akt (pSer⁴⁷³) were increased by nearly 50% with IL-6 depletion. In adipose tissue, insulin receptor signaling showed no significant change despite major reductions in STAT3 phosphorylation and haptoglobin expression. In diet-induced obese mice, depletion of IL-6 improved insulin responsiveness in 2-h insulin tolerance tests. In conclusion, these results indicate that IL-6 plays an important and selective role in hepatic insulin resistance of obesity.

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