Human RBP4 adipose tissue expression is gender specific and influenced by leptin

The role of retinol-binding protein-4 (RBP4) in human insulin resistance remains controversial, which may in part be explained by a gender-specific secretion of RBP4 in adipose tissue (AT). The aim of the study was to determine gender-specific depot expression of RBP4 and to identify metabolic parameters and cytokines/adipokines associated with RBP4.

The study is an ex-vivo prospective analysis of paired AT-samples from 22 men and 26 women of similar age [men: 43·4 ± 13 (mean ± SD)years, women: 44·1 ± 12 years], BMI (men: 41·9 ± 18kg/m(2) , women: 38·4 ± 11kg/m(2) ) and homeostasis model assessment of insulin resistance taken during elective surgery and ex-vivo culture using visceral-AT (VAT)-explants (n = 10). Plasma RBP4 and cytokines were measured by ELISA and mRNA expression in AT by real-time PCR. VAT-explants were cultured with recombinant leptin and insulin and RBP4 determined by western blot analyses.

Overall subcutaneous AT (SCAT)-RBP4 mRNA expression was higher than VAT-expression [3·1 ± 0·26 signal units (SU; mean ± SE) vs 1·79 ± 0·18SU, n = 48, P < 0·0001], but neither correlated with circulating RBP4. SCAT-RBP4 expression was higher in women and correlated with BMI (r =-0·5, P = 0·009) and fat mass (r= -0·5, P = 0·002). VAT-RBP4 correlated positively with GLUT-4 expression and adiponectin in men only (r= 0·54, P = 0·03 and r = 0·64, P < 0·002, respectively) when correcting for age and fat mass. Multiple regression determined leptin AT-expression as a positive predictor of AT-RBP4 in women (SCAT: β = 0·50, P = 0·002; VAT: β = 0·58, P = 0·003) and adiponectin for VAT-RBP4 in men (β = 0·69; P=0·001). AT-RBP4 mRNA expression showed no relation with insulin resistance. Leptin stimulated RBP-4 secretion ex-vivo, whilst insulin did not affect RBP4.

AT-derived RBP4-mRNA expression is gender specific and regulated by leptin. Circulating RBP4 levels appear to be independent of AT-RBP4 secretion.