Toll-like receptor 4 deficiency attenuates insulin resistance and endothelial dysfunction associated with obesity and diabetes in mice

Abstract

BACKGROUND: Aging and obesity are major risk factors for endothelial dysfunction and metabolic syndrome. Endothelial dysfunction is characterized by an impaired release of endothelium-derived relaxing (EDRF) and hyperpolarizing (EDHF) factors and enhanced production of contracting factors (EDCF). The Toll-like receptor 4 (TLR4) is a major target for lipopolysaccharide and saturated fatty acids, both of which are potent inducers of inflammation and insulin resistance. The present study was designed to evaluate the role of TLR4 in modulating metabolism and endothelial function in mice with loss-of-function mutation of TLR4. METHODS: TLR4(-/-) (C3H/HeJ) and wild type (C3H/HeOuJ) mice were subjected to standard or high fat diet. A type-2 diabetes animal model, double knockout in leptin receptor (Lepr) and TLR4 (DKO), was obtained by crossing Lepr(db/+) and TLR4(-/-) mice. Glucose and insulin tolerance tests (GTT and ITT) were carried out. Systolic blood pressure was measured by tail-cuff method. The animals were sacrificed at the age of 12 weeks. Rings of aorta, carotid artery and mesenteric artery (with or without endothelium) were suspended in a wire-myograph for measuring changes in isometric tension. Endothelial function was assessed by recording the responses to endothelium-dependent vasodilator and vasoconstrictor agonists. RESULTS: TLR4(-/-) mice under high fat diet feeding showed a better insulin sensitivity and lower blood pressure than wild type mice. DKO mice also demonstrated a significantly lower fasting blood glucose, serum cholesterol, lower blood pressure and better insulin sensitivity than Lepr(db/db) control mice. Acetylcholine-induced EDCF-mediated responses in carotid arteries were enhanced by aging, high fat diet, and genetic obesity, but were significantly attenuated by TLR4 deficiency. The contractions were inhibited by indomethacin, SC560, and S18886, but not NS398, suggesting the involvement of COX-1. Apocynin, MnTMPyP, catalase but not deferoxamine also inhibited the contractions, suggesting that reactive oxygen species may play a role. Acetylcholine-evoked hyperpolarizations were estimated in mesenteric arteries as endothelium-dependent relaxations blocked by Tram- 34 and UCL-1684. The acetylcholine-induced EDHF responses were potentiated in TLR4(-/-) mice fed with control and high fat diet. The acetylcholine and sodium nitroprusside-induced relaxations in the aorta were not different in wild type and TLR4(-/-) mice. CONCLUSION: Toll-like receptor 4 deficiency can prevent aging and obesity-induced insulin resistance and endothelial dysfunction possibly by decreasing oxidative stress.