Circulating RBP4 is associated with obesity related glucose and lipid metabolism alterations in children and adolescents

Retinol binding protein 4 (RBP4) was proposed as novel adipokine that potentially links obesity, insulin resistance and cardiovascular complications. In this study, we applied experimental, clinical and genetic approaches to investigate associations of RBP4 with obesity, insulin resistance and vascular alterations in children and adolescents. We recruited 62 obese (age 11.9±0.3 years, BMI-SDS 2.26±0.06) and 69 healthy lean children and adolescents (age 12.3±0.4 years, BMI-SDS -0.28±0.09). To identify a potential relation of RBP4 with normal, physical development, we analysed circulating RBP4 levels in dependence of pubertal development. RBP4 concentrations were significantly increased in lean and obese adolescents compared to pre-pubertal and pubertal children (p<0.01, p<0.05) whereas Matsuda insulin sensitivity index declined during puberty. In our pediatric cohort, circulating RBP4 levels were positively correlated to BMI-SDS (r=0.239, p<0.01). To assess whether the association of RBP4 with obesity may be attributable to adipogenesis, we evaluated RBP4 mRNA and protein secretion during adipocyte differentiation using the human SGBS cell line. In preadipocytes, RBP4 mRNA expression and protein secretion was nearly undetectable but it was strongly increased in in vitro differentiating adipocytes. In a next step, we investigated associations of circulating RBP4 levels and obesity-related metabolic and vascular alterations. Our results revealed significant correlations between circulating RBP4 levels and Matsuda insulin sensitivity index (r=-0.345, p<0.0001), serum levels of triglycerides (r=0.213, p<0.05), LDL-cholesterol (r=0.245, p<0.01), total cholesterol (r=0.225, p<0.05) and mean systolic 24h blood pressure (r=0.225, p<0.05). We did not find an association between RBP4 levels and the cardiovascular parameters reactive hyperemia index and intima media thickness. Also, number and functional capacity of circulating endothelial progenitor cells (CPCs), as determined as CD34+/KDR+ cells in peripheral blood using flow cytometry and by migration assay, was not correlated to circulating RBP4 levels. Finally, we genotyped the recently identified polymorphism -803G>A in the RBP4 gene that was previously shown to be associated with increased risk for type 2 diabetes. In our cohort, the -803G>A polymorphism had no impact on circulating RBP4 levels and metabolic parameters. In conclusion, circulating RBP4 levels are related to obesity and ensuing metabolic complications but not to the regenerative capacity and function of the vascular endothelium suggesting that RBP4 is not a vasoactive protein as described for other adipokines.