In obesity and type 2 diabetes, expression of the GLUT4 glucose transporter is decreased selectively in adipocytes. Adipose-specific Glut4 (also known as Slc2a4) knockout (adipose-Glut4(-/-)) mice show insulin resistance secondarily in muscle and liver. Here we show, using DNA arrays, that expression of retinol binding protein-4 (RBP4) is elevated in adipose tissue of adipose-Glut4(-/-) mice. We show that serum RBP4 levels are elevated in insulin-resistant mice and humans with obesity and type 2 diabetes. RBP4 levels are normalized by rosiglitazone, an insulin-sensitizing drug. Transgenic overexpression of human RBP4 or injection of recombinant RBP4 in normal mice causes insulin resistance. Conversely, genetic deletion of Rbp4 enhances insulin sensitivity. Fenretinide, a synthetic retinoid that increases urinary excretion of RBP4, normalizes serum RBP4 levels and improves insulin resistance and glucose intolerance in mice with obesity induced by a high-fat diet. Increasing serum RBP4 induces hepatic expression of the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) and impairs insulin signalling in muscle. Thus, RBP4 is an adipocyte-derived 'signal' that may contribute to the pathogenesis of type 2 diabetes. Lowering RBP4 could be a new strategy for treating type 2 diabetes.

RBP4 is an adipocyte-secreted molecule that is elevated in the serum before the development of frank diabetes and appears to identify insulin resistance and associated cardiovascular risk factors in subjects with varied clinical presentations. These findings provide a rationale for antidiabetic therapies aimed at lowering serum RBP4 levels.

OBJECTIVE-We identified lipocalin 2 (Lcn2) as a gene induced by dexamethasone and tumor necrosis factor-␣ in cultured adipocytes. The purpose of this study was to determine how expression of Lcn2 is regulated in fat cells and to ascertain whether Lcn2 could be involved in metabolic dysregulation associated with obesity. RESEARCH DESIGN AND METHODS-We examined Lcn2expression in murine tissues and in 3T3-L1 adipocytes in the presence and absence of various stimuli. We used quantitative Western blotting to observe Lcn2 serum levels in lean and obese mouse models. To assess effects on insulin action, we used retroviral delivery of short hairpin RNA to reduce Lcn2 levels in 3T3-L1 adipocytes.RESULTS-Lcn2 is highly expressed by fat cells in vivo and in vitro. Expression of Lcn2 is elevated by agents that promote insulin resistance and is reduced by thiazolidinediones. The expression of Lcn2 is induced during 3T3-L1 adipogenesis in a CCAAT/enhancer-binding protein-dependent manner. Lcn2 serum levels are elevated in multiple rodent models of obesity, and forced reduction of Lcn2 in 3T3-L1 adipocytes improves insulin action. Exogenous Lcn2 promotes insulin resistance in cultured hepatocytes. T he worldwide epidemic of obesity and type 2 diabetes has focused attention on adipocyte biology and the role of adipose tissue in the integration of systemic metabolism (1). The discovery of leptin more than a decade ago established a paradigm in which secreted proteins from adipocytes coordinate energy balance and glucose homeostasis (2,3). Since that initial discovery, the number of adipocytederived signaling molecules has grown ever larger, and the term adipokine was coined to reflect that many of these molecules exert positive or negative actions on inflammation. Several adipokines promote insulin sensitivity, including leptin (2), adiponectin (4), and visfatin (5), while others induce insulin resistance, such as resistin (6) and retinol binding protein (RBP)4 (7). CONCLUSIONS-Lcn2Lipocalin 2 (Lcn2)-also known as neutrophil gelatinase-associated lipocalin, siderocalin, and 24p3-is a member of a large superfamily of proteins that includes RBP4. Lipocalins are small generally secreted proteins with a hydrophobic ligand binding pocket (8). Known ligands for lipocalins include retinol, steroids, odorants, pheromones, and, in the case of Lcn2, siderophores (9). Siderophores are small molecules used by bacteria to poach iron from their hosts, a necessary cofactor for the growth of some pathogens. Lcn2 is used by the mammalian-innate immune system to sequester siderophore and thus deprive the bacteria of iron. Mice lacking Lcn2 appear normal but die when exposed to siderophorerequiring strains of bacteria in quantities that are cleared easily by wild-type mice (10,11). Lcn2 can thus be considered an iron transport protein, and it has been implicated in the apoptotic induction of pro-B-cells (12) and in the biology of the genitourinary system, both as a developmental factor and as a protective mechanism in renal ischemia (13).In this s...

Intra-abdominal fat is associated with insulin resistance and cardiovascular risk. Levels of serum retinol-binding protein (RBP4), secreted by fat and liver cells, are increased in obesity and type 2 diabetes (T2D). Here we report that, in 196 subjects, RBP4 is preferentially expressed in visceral (Vis) versus subcutaneous (SC) fat. Vis fat RBP4 mRNA was increased approximately 60-fold and 12-fold in Vis and SC obese subjects respectively versus lean subjects, and approximately 2-fold with impaired glucose tolerance/T2D subjects versus normoglycemic subjects. In obese subjects, serum RBP4 was increased 2- to 3-fold, and serum transthyretin, which stabilizes RBP4 in the circulation, was increased 35%. Serum RBP4 correlated positively with adipose RBP4 mRNA and intra-abdominal fat mass and inversely with insulin sensitivity, independently of age, gender, and body mass index. RBP4 mRNA correlated inversely with GLUT4 mRNA in Vis fat and positively with adipocyte size in both depots. RBP4 levels are therefore linked to Vis adiposity, and Vis fat may be a major source of RBP4 in insulin-resistant states.