Plasma Retinol-Binding Protein-4 Concentrations Are Elevated in Human Subjects With Impaired Glucose Tolerance and Type 2 Diabetes

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Abstract
OBJECTIVE—The dysregulation of adipokines is closely associated with the pathogenesis of insulin resistance and type 2 diabetes. Retinol-binding protein-4 (RBP4), a new adipokine, was recently reported to provide a link between obesity and insulin resistance. Here, we examined the relation between plasma RBP4 concentrations and various metabolic parameters in humans. RESEARCH DESIGN AND METHODS—An enzyme-linked immunosorbent assay was developed to measure human RBP4 plasma concentrations, which were then compared with various parameters related to insulin resistance in subjects with normal glucose tolerance (NGT; n = 57), impaired glucose tolerance (IGT; n = 48), and type 2 diabetes (n = 49). RESULTS—Plasma RBP4 concentrations were higher in the IGT and type 2 diabetic groups than in the NGT group (median 18.9 [range 11.2–45.8], 20.9 [9.9–48.5], and 18.1 μg/ml [9.3–30.5], respectively). However, no difference was found between plasma RBP4 concentrations in the IGT and type 2 diabetic groups. Plasma RBP4 concentrations were found to be associated with sex, waist circumference, fasting plasma glucose, and insulin resistance. Of these, sex and fasting plasma glucose levels were found to be independent determinants of plasma RBP4 concentration. CONCLUSIONS—Plasma RBP4 concentrations were found to be elevated in subjects with IGT or type 2 diabetes and to be related to various clinical parameters known to be associated with insulin resistance.