Human mediastinal adipose tissue displays certain characteristics of brown fat

Abstract

Background:The amount of intra-thoracic fat, of which mediastinal adipose tissue comprises the major depot, is related to various cardiometabolic risk factors. Autopsy and imaging studies indicate that the mediastinal depot in adult humans could contain brown adipose tissue (BAT). To gain a better understanding of this intra-thoracic fat depot, we examined possible BAT characteristics of human mediastinal in comparison with subcutaneous adipose tissue.Materials and methods:Adipose tissue biopsies from thoracic subcutaneous and mediastinal depots were obtained during open-heart surgery from 33 subjects (26 male, 63.7±13.8 years, body mass index 29.3±5.1 kg m−2). Microarray analysis was performed on 10 patients and genes of interest confirmed by quantitative PCR (qPCR) in samples from another group of 23 patients. Adipocyte size was determined and uncoupling protein 1 (UCP1) protein expression investigated with immunohistochemistry.Results:The microarray data showed that a number of BAT-specific genes had significantly higher expression in the mediastinal depot than in the subcutaneous depot. Higher expression of UCP1 (24-fold, P<0.001) and PPARGC1A (1.7-fold, P=0.0047), and lower expression of SHOX2 (0.12-fold, P<0.001) and HOXC8 (0.14-fold, P<0.001) in the mediastinal depot was confirmed by qPCR. Gene set enrichment analysis identified two gene sets related to mitochondria, which were significantly more highly expressed in the mediastinal than in the subcutaneous depot (P<0.01). No significant changes in UCP1 gene expression were observed in the subcutaneous or mediastinal depots following lowering of body temperature during surgery. UCP1 messenger RNA levels in the mediastinal depot were lower than those in murine BAT and white adipose tissue. In some mediastinal adipose tissue biopsies, a small number of multilocular adipocytes that stained positively for UCP1 were observed. Adipocytes were significantly smaller in the mediastinal than the subcutaneous depot (cross-sectional area 2400±810 versus 3260±980 μm2, P<0.001).Conclusions:Human mediastinal adipose tissue displays some characteristics of BAT when compared with the subcutaneous depot at microscopic and molecular levels.

Thermogenic potential and physiological relevance of human epicardial adipose tissue

“…It is important to note that Cheung et al 74 did not observe any associations between brown-like features of human mediastinal fat and any of the physiological parameters, similar to our observations where associations among circulating lipids and brown fat gene expression were exclusive to epicardial fat. 6 Future studies would be required to either completely dismiss or acknowledge a role for mediastinal fat and its thermogenic properties in terms of physiological relevance in humans.…”

“…However, neither UCP1 protein (at the tissue level) nor any of the beige markers were detectable in this depot. 74 In contrast, Sacks et al 70 noticed higher expression of beige marker CD137 in paracardial fat (even higher than epicardial fat); however, as they also could not locate UCP1 protein in this depot, they attributed the beige phenotype only to epicardial adipose tissue. We have observed gene expression of various beige fat markers in the mediastinal fat depot similar to the epicardial fat depot, however at lower expression levels (unpublished observations), leading to the suggestion that the mediastinal fat depot is likely to be a beige fat depot that possesses higher mitochondrial activity.…”

“…In a recent study, Cheung et al 74 compared the gene expression data of the human mediastinal fat depot with murine brown and white fat depots using microarrays, and found gene set enrichments that pointed toward a higher mitochondrial activity for this depot relative to subcutaneous fat depot. They further reported that the human mediastinal depot expresses UCP1 mRNA and contains multilocular UCP1-immunopositive adipocytes, thereby exhibiting brown fat-like features.…”

A stringent validation of mouse adipose tissue identity markers

“…Despite the fact that a post hoc interpretation would eliminate all until now suggested markers, we believe that with the information obtained here about marker genes it is now possible and important to reevaluate the conclusions from human studies that have been based on the expression of marker genes (5,8,13,16,17,30,34,40,48). As most of the marker genes used proved to be uninformative in our study, the revised results of the human studies now comes to rely on the observations of very few marker genes.…”

Evaluation of reference genes for gene expression studies in human brown adipose tissue

“…Some examples of reference genes that has been used is TATAbinding protein (TBP), 14,18 cyclophilin A (PPIA), 13,19 b actin (ACTB), 15,20 glyceraldehyde-3-phosphate dehydrogenase (GAPDH), 13 large ribosomal protein P0 (RPLP0) 16,17 and 18S rRNA. 21 In our study, all these reference genes, with the exception of 18S, seemed reasonable robust.…”