Experimental endotoxemia induces adipose inflammation and insulin resistance in humans

Abstract

Objective: An emerging model of metabolic syndrome and type 2 diabetes is of adipose dysfunction with leukocyte recruitment into adipose leading to chronic inflammation and insulin resistance (IR). This study sought to explore potential mechanisms of inflammatory-induced IR in humans with a focus on adipose tissue.

Research design and methods: We performed a 60-h endotoxemia protocol (3 ng/kg intravenous bolus) in healthy adults (n = 20, 50% male, 80% Caucasian, aged 27.3 +/- 4.8 years). Before and after endotoxin, whole-blood sampling, subcutaneous adipose biopsies, and frequently sampled intravenous glucose tolerance (FSIGT) testing were performed. The primary outcome was the FSIGT insulin sensitivity index (S(i)). Secondary measures included inflammatory and metabolic markers and whole-blood and adipose mRNA and protein expression.

Results: Endotoxemia induced systemic IR as demonstrated by a 35% decrease in S(i) (3.17 +/- 1.66 to 2.06 +/- 0.73 x 10(-4) [microU * ml(-1) * min(-1)], P < 0.005), while there was no effect on pancreatic beta-cell function. In adipose, endotoxemia suppressed insulin receptor substrate-1 and markedly induced suppressor of cytokine signaling proteins (1 and 3) coincident with local activation of innate (interleukin-6, tumor necrosis factor) and adaptive (monocyte chemoattractant protein-1 and CXCL10 chemokines) inflammation. These changes are known to attenuate insulin receptor signaling in model systems.

Conclusions: We demonstrate, for the first time in humans, that acute inflammation induces systemic IR following modulation of specific adipose inflammatory and insulin signaling pathways. It also provides a rationale for focused mechanistic studies and a model for human proof-of-concept trials of novel therapeutics targeting adipose inflammation in IR and related consequences in humans.

FIG. 1.

Plasma levels of inflammatory and hormonal markers. A: Endotoxemia increased plasma TNF-α, MCP-1, and hsCRP. B: Plasma cortisol and free fatty acids increased significantly with a trend for increase in GH (P = 0.08). Inset graphs show the difference for 24 h after versus 24 h before LPS. *P < 0.0001 and †P < 0.005 in mixed-effects model.

FIG. 2.

Endotoxemia induced IR without change in pancreatic β-cell function. Endotoxemia suppressed the insulin sensitivity index (S_i) at FSIGT testing (A), whereas there was no change in the FSIGT test-derived acute insulin response to glucose (AIRG) (B), a measure of pancreatic β-cell function. Consistent with this, the HOMA of IR fell following LPS (C), while the HOMA-B estimate of β-cell function was unchanged (D). *ANOVA P < 0.001.

FIG. 3.

Endotoxemia induced adipose inflammation in humans. A: Endotoxemia increased adipose IL-6, TNF-α, and TNF-induced protein 3 (A20) mRNA levels. B: In parallel, mRNA levels of MCP-1 and CXCL10, monocyte, and T-cell chemokines were induced. C: Western blotting confirmed increases in adipose A20 and MCP-1 proteins (representative blot shown; densitometry n = 3; *ANOVA P < 0.05).

FIG. 4.

Differential modulation of adipose and whole-blood SOCS proteins. Endotoxemia induced (A) adipose and (B) whole-blood mRNA levels of SOCS-1 and SOCS-3, but not SOCS-2 and SOCS-6. *P < 0.0001 in ANOVA (adipose) or mixed-effects model (blood).

FIG. 5.

Western blot of insulin signaling proteins in adipose tissue. Relative to β-actin, endotoxin reduced protein levels of IRS-1 and GLUT4, increased phosphorylated-AKT (p-AKT), and had no effect on insulin receptor expression (representative blot shown; densitometry for IRS-1 and p-AKT n = 3; *ANOVA P < 0.01).