Coronary artery disease is associated with higher epicardial Retinol‐binding protein 4 (RBP4) and lower glucose transporter (GLUT) 4 levels in epicardial and subcutaneous adipose tissue

Abstract

Retinol-binding protein 4 (RBP4), produced by adipocytes and hepatocytes, contributes to an unfavourable lipid profile and insulin resistance, which can contribute to the development of coronary artery disease (CAD). Recently, several studies have shown that epicardial adipose tissue (EAT) differs from subcutaneous adipose tissue (SAT) and plays a role on the physiopathology of CAD because of its proximity to the coronary arteries. We aimed to study the expression and secretion levels of RBP4 in both fat tissues and explore its possible association with CAD. Fifty-eight patients undergoing heart surgery were included in the study. We analysed RBP4 mRNA expression by real-time PCR, protein expression by Western blot and immunohistochemistry, and secretion of EAT and SAT explants from CAD and non-CAD patients by Enzyme Immunoassay. Retinol-binding protein 4 is expressed at similar levels in EAT and SAT, mainly from adipocytes. Protein levels were higher in EAT from CAD than non-CAD patients (0·63 ± 0·09 arbitrary units (a.u).; n = 10) vs (0·41 ± 0·04 a.u.; n = 13, P = 0·039). In contrast, GLUT4 mRNA levels were lower in EAT from CAD than non-CAD patients (6·55 ± 0·16 a.u.; n = 13) vs (7·21 ± 0·18 a.u.; n = 14, P = 0·012). We also found differential expression in SAT between samples from CAD and non-CAD patients [(6·63 ± 0·16 a.u.; n = 14) vs (7·21 ± 0·14 a.u.; n = 14, P = 0·009)]. Besides, EAT releases higher RBP4 levels than SAT after 3, 6, 24 and 48 h of culture. These levels were independent of CAD but significantly higher in diabetic than nondiabetic patients. Retinol-binding protein 4 levels behave differently in EAT and SAT with respect to CAD. However, both adipose tissues have lower GLUT4 levels in patients with CAD. These findings suggest a differential regulation of RBP4 production in EAT and SAT that may be influenced by local factors.