The lipocalins retinol-binding protein-4, lipocalin-2 and lipocalin-type prostaglandin D2-synthase correlate with markers of inflammatory activity, alcohol intake and blood lipids, but not with insulin sensitivity in metabolically healthy 58-year-old Swedish men.
The lipocalins retinol-binding protein (RBP)-4, lipocalin-2 and lipocalin-type prostaglandin D-synthase (L-PGDS) have been suggested to mediate obesity-associated insulin resistance and other metabolic co-morbidities. The role of lipocalins is however controversial and it is unclear whether they have a physiological role in regulation of insulin sensitivity and metabolic function in clinically healthy humans. Therefore, we examined the correlations between serum levels of RBP-4, L-PGDS and lipocalin-2 and insulin sensitivity and other metabolic parameters in non-diabetic subjects selected to display variations in insulin sensitivity. 100 clinically healthy 58-year-old Swedish men were selected by stratified sampling among 818 screened subjects to represent quintiles of varying degrees of insulin sensitivity. Insulin sensitivity was measured by the euglycaemic hyperinsulinaemic clamp method. Serum levels of lipocalins and cytokines were determined using antibody-based techniques. Serum lipids were measured by standardized laboratory methods. None of the measured lipocalins showed any correlations with insulin sensitivity. However, we found that lipocalin-2 and L-PGDS were correlated with each other, but not with RBP-4. Lipocalin-2 and L-PGDS were positively correlated with soluble TNF- receptors 1 and 2 and negatively with alcohol consumption and serum HDL. Further, lipocalin-2 was correlated with interleukin-6 whereas RBP-4 was negatively correlated with TNF-a. ?These results suggest that RBP-4, lipocalin-2 and L-PGDS do not regulate insulin sensitivity in healthy men. Rather the expression levels of lipocalin-2 and L-PGDS, but not RBP-4, seemed to reflect inflammatory activity and were inversely correlated with alcohol intake and serum HDL levels.