Human Epicardial Adipose Tissue Expresses Glucose-dependent Insulinotropic Polypeptide, Glucagon and Glucagon-Like Peptide 1 Receptors as Potential Targets of Pleiotropic Therapies

Background: Human Epicardial Adipose Tissue (EAT) plays a crucial role in the development and progression of coronary artery disease, atrial fibrillation and heart failure. Microscopically, EAT is composed of adipocytes, nerve tissues, inflammatory, stromovascular and immune cells. EAT is a white adipose tissue, albeit it also has brown-fat like or beige fat features. No muscle fascia divides EAT and myocardium; this allows a direct interaction and cross talk between the epicardial fat and the myocardium. Thus, it might be a therapeutic target for pharmaceutical compounds acting on G-Protein-Coupled Receptors, such as those for Glucose-dependent Insulinotropic Polypeptide (GIP), Glucagon (GCG) and Glucagon-Like Peptide-1 (GLP-1), whose selective stimulation with innovative drugs has demonstrated beneficial cardiovascular effects. The precise mechanism of these novel drugs and their tissue and cellular target(s) need to be better understood. Aim: We evaluate whether human EAT expresses GIP, GCG and GLP-1 receptors and whether their presence is related to EAT transcriptome. We also investigated protein expression and cell type localization specifically for GIPR and GCGR. Methods: EAT samples were collected from 33 patients affected by cardiovascular diseases undergoing open heart surgery (90.9% males, age 67.2±10.5 years mean ± SD). Microarray and immunohistochemistry analysis were performed. Results: Microarray analysis showed that GIPR and GCGR messenger Ribonucleic Acids (mRNAs) are expressed in EAT, beyond confirming the previously found GLP-1(3776±1377 arbitrary unit (A.U.), 17.77±14.91 A.U., and 3.41±2.27 A.U., respectively). The immunohistochemical analysis consistently indicates that GIPR and GCGR are expressed in EAT, mainly in macrophages, isolated and in crown-like structures. In contrast, only some mature adipocytes of different sizes showed cytoplasmic immunostaining, similar to endothelial cells and pericytes in the capillaries and pre-capillary vascular structures. Notably, EAT GIPR is statistically associated with the low expression of genes involved in Free Fatty Acid (FFA) oxidation and transport and those promoting FFA biosynthesis and adipogenesis (p<0.01). EAT GCGR, in turn, is related to genes involved in FFA transport, mitochondrial fatty acid oxidation, and white-to-brown adipocyte differentiation, in addition to genes involved in the reduction of fatty acid biosynthesis and adipogenesis (p<0.01). Conclusions: Having reported the expression of the GLP-1 receptor previously, here, we showed that GIPR and GCGR similarly present at mRNA and protein levels in human EAT, particularly in macrophages and partially adipocytes, suggesting these G-protein-coupled receptors as pharmacological targets on the ongoing innovative drugs, which seem cardiometabolically healthy well beyond their effects on glucose and body weight..