Epicardial adipose tissue has a unique transcriptome modified in severe coronary artery disease

Abstract

Objective: To explore the transcriptome of epicardial adipose tissue (EAT) as compared to subcutaneous adipose tissue (SAT) and its modifications in a small number of patients with coronary artery disease (CAD) versus valvulopathy.

Methods: SAT and EAT samples were obtained during elective cardiothoracic surgeries. The transcriptome of EAT was evaluated, as compared to SAT, using an unbiased, whole-genome approach in subjects with CAD (n = 6) and without CAD (n = 5), where the patients without CAD had cardiac valvulopathy.

Results: Relative to SAT, EAT is a highly inflammatory tissue enriched with genes involved in endothelial function, coagulation, immune signaling, potassium transport, and apoptosis. EAT is lacking in expression of genes involved in protein metabolism, tranforming growth factor-beta (TGF-beta) signaling, and oxidative stress. Although underpowered, in subjects with severe CAD, there is an expression trend suggesting widespread downregulation of EAT encompassing a diverse group of gene sets related to intracellular trafficking, proliferation/transcription regulation, protein catabolism, innate immunity/lectin pathway, and ER stress.

Conclusions: The EAT transcriptome is unique when compared to SAT. In the setting of CAD versus valvulopathy, there is possible alteration of the EAT transcriptome with gene suppression. This pilot study explores the transcriptome of EAT in CAD and valvulopathy, providing new insight into its physiologic and pathophysiologic roles.

Figure 1. Epicardial adipose tissue (EAT) is an anatomically distinct adipose depot

EAT lies on the myocardial surface without fascial separation such that epicardial adipocytes are in direct contact with the myocardium, even appearing to invaginate the myocardial surface on histology. In healthy adult subjects, EAT lies in the atrioventricular grooves and follows the branches of the major coronary arteries such that the coronary vessels are embedded within the EAT, although some patients may present with a massive epicardial fat pad as shown. Superficial to the epicardial fat are the visceral and parietal layers of the pericardial sac. Paracardial adipose tissue (PAT) is superficial to the pericardium and thus has no direct anatomical contact with the EAT or myocardium; it also has a separate microcirculation from EAT, as EAT shares the microcirculation of the heart. Paracardial fat is alternatively termed mediastinal fat. Although EAT and PAT are anatomically distinct, they are sometimes discussed under the umbrella term pericardial fat which refers to epicardial plus paracardial fat (6).

Figure 2. Microarray analysis protocol

Gene expression was evaluated using Genechip Human Gene 2.0 ST arrays (Affymetrix, Inc., Santa Clara, CA) which utilizes a whole-transcript design to assess >30,000 coding genes. The resulting data files were preprocessed with GenePattern (www.broadinstitute.org, Cambridge, MA). Files were then analyzed by two methods. Differential analysis identified individual genes with enrichment and GSEA identified enriched gene sets by gene ontology. Individual genes were considered statistically significant with an FDR <0.05 in the EAT versus SAT analysis, and in the EATCAD versus EATVal analysis at an FDR <0.15. Gene sets were considered enriched at statistical significance with nominal p-value <1%. Two comparisons were made: EAT versus SAT to define the transcriptome of EAT and EATCAD versus EATVal to clarify how the transcriptome of EAT is modified in CAD. GO: gene ontology; EAT: epicardial adipose tissue; SAT: subcutaneous adipose tissue; CAD: coronary artery disease; Val: valvulopathy.

Figure 3. Heat map of the 50 most enriched genes in EAT samples compared with SAT samples

Heat mapping demonstrates the unique features of EAT where expression values are represented as colors (red: increased expression; blue: decreased expression) with the degree of color saturation indicating the degree of expression. Heat map generated by Broad Institute’s Gene Set Enrichment Analysis. EAT: epicardial adipose tissue; SAT: subcutaneous adipose tissue.

Figure 4. Heat map of the 50 most enriched genes in EATCAD samples compared with EATVal samples

Heat mapping demonstrates the unique features of EAT where expression values are represented as colors (red: increased expression; blue: decreased expression) with the degree of color saturation indicating the degree of expression. Heat map generated by Broad Institute’s Gene Set Enrichment Analysis. EAT: epicardial adipose tissue; CAD: coronary artery disease; Val: valvulopathy.