ORIGINAL ARTICLE Circulating lipopolysaccharide-binding protein (LBP) as a marker of obesity-related insulin resistance JM Moreno-Navarrete 1 , F Ortega 1 , M Serino 2 , E Luche 2 , A Waget 2 , G Pardo 1 , J Salvador 3 , W Ricart 1 , G Fru ¨ hbeck 3 , R Burcelin 2 and JM Ferna ´ ndez-Real 1 OBJECTIVE: Lipopolysaccharide-binding protein (LBP) is a 65-kDa acute-phase protein present in blood at high concentrations, known to be derived from the liver. We aimed to gain insights into the association of circulating LBP with insulin resistance in humans and mice. METHODS, DESIGN AND MEASUREMENTS: We studied the cross-sectional (n ¼ 222) and weight loss-induced (n ¼ 34) associations of LBP (enzyme-linked immunosorbent assay) with inflammatory and metabolic parameters (including minimal model-measured insulin sensitivity), and the effects of high-fat diet (HFD), metformin and genetic insulin sensitization (glucagon-like peptide 1 receptor knockout model) in mice. RESULTS: Circulating LBP concentration was significantly increased in subjects with type 2 diabetes and dramatically increased in subjects with morbid obesity. LBP was significantly associated with insulin sensitivity and different inflammatory markers and decreased after weight loss (22.2 ± 5.8 vs 16.2 ± 9.3 mg ml À1 , Po0.0001) in association with changes in body mass index and insulin sensitivity. Circulating LBP concentration was increased in HFD mice, whereas decreased in glucagon-like peptide 1 receptor knockout mice (significantly more insulin sensitive than wild-type mice) and after metformin administration. CONCLUSION: LBP is an inflammatory marker associated with obesity-related insulin resistance. International Journal of Obesity (2012) 36, 1442 – 1449; doi:10.1038/ijo.2011.256; published online 20 December 2011 Keywords: LBP; LPS; insulin resistance INTRODUCTION In recent years, it has become evident that alterations in the function of the innate immune system are intrinsically linked to metabolic pathways. 1–5 Immune system homeostasis is chal- lenged by continuous external insults, like saturated fatty acid-rich diets, 6 pathogen-associated molecular patterns like lipopoly- saccharide (LPS), 7 burden of infection 8 and oxidative stress. 9 These continuous insults could result in a chronic low-level inflammation associated with insulin resistance. Lipopolysaccharide-binding protein (LBP) is a 65-kDa protein, an acute-phase reactant predominantly derived from the liver, present in blood at high concentrations (approximately 2–20 mg ml À1 ). 10 LBP plasma levels increase dramatically after inflammatory challenges, including bacterial sepsis. 10 Although the molecular structure of LBP is not entirely known, LBP clearly binds LPS (and LPS substructures, such as lipid IVa) through recognition of lipid A. 11 The plasma protein LBP dramatically accelerates binding of LPS monomers from aggregates to CD14, 12 thereby enhancing the sensitivity of cells to LPS. Furthermore, LBP acts as a lipid transfer protein, a function in keeping with its sequence homology to lipid transferases (phospholipid transfer protein and cholesterol ester transfer protein). LBP co-purifies with high-density lipoprotein (HDL) particles and additional studies have shown that LBP can transfer LPS to lipoproteins, neutralizing LPS effects. 13 Serum LBP concentration was described as a marker of coronary artery disease 14,15 and has been recently found to be associated with obesity and related disorders in apparently healthy Chinese subjects. 16 We here confirm the previously reported association of circulating LBP with insulin resistance and obesity in a Caucasian population. 14 To gain insights into these associations, we investigated the effects of diet weight loss in humans and the effects of high-fat diet (HFD), metformin and genetic insulin sensitization (glucagon-like peptide 1 receptor knockout (GLPIR KO) model) in mice. MATERIALS AND METHODS Cross-sectional study Participants’ recruitment. A total of 222 Caucasian men were recruited and studied. In all, 142 of them were recruited in an ongoing study dealing with non-classical cardiovascular risk factors in Northern Spain. Subjects were randomly localized from a census and they were invited to participate. The participation rate was 71%. A 75-g oral glucose tolerance test according to the American Diabetes Association criteria was performed in all subjects. All subjects with normal glucose tolerance (NGT) (n ¼ 87) had fasting plasma glucose o7.0 mM and 2-h post-load plasma glucose o7.8 mM after a 75-g oral glucose tolerance test. Glucose intolerance (GIT) was diagnosed in 40 subjects according to the American Diabetes Association criteria (post-load glucose between 7.8 and Received 4 March 2011; revised 7 November 2011; accepted 9 November 2011; published online 20 December 2011 1 Department of Diabetes, Endocrinology and Nutrition, Institut d’Investigacio ´ Biome ` dica de Girona (IdIBGi), CIBEROBN (CB06/03/010) and Instituto de Salud Carlos III (ISCIII), Girona, Spain; 2 Unite ´ Mixte de Recherche (UMR), Institut de Medecine Moleculaire de Rangueil, Institut National de la Sante ´ et de la Recherche Me ´ dicale U858, IFR31, Centre Hospitalier Universitaire Rangueil, Toulouse, France and 3 Department of Endocrinology and Metabolic Research Laboratory, Clı ´nica Universitaria de Navarra, CIBEROBN (CB06/03/1014) and Instituto de Salud Carlos III (ISCIII), Pamplona, Spain. Correspondence: Dr JM Ferna ´ndez-Real, Department of Diabetes, Endocrinology and Nutrition, Hospital of Girona ‘Dr Josep Trueta’, Carretera de Franc ¸a s/n, 17007, Girona, Spain. E-mail: jmfernandezreal.girona.ics@gencat.cat International Journal of Obesity (2012) 36, 1442 – 1449 & 2012 Macmillan Publishers Limited All rights reserved 0307-0565/12 www.nature.com/ijo