Background: Secretion of catecholamines by adipose tissue M2 macrophages was recently proposed as a molecular mechanism leading to activation of brown adipose tissue and heat generation in mice. However, published data are conflicting and it is not clear whether this pathway might play a role in humans. To shed light on this, we studied macrophage polarization in human white and brown adipose tissue and related it to thermogenic gene expression.
Methods: Paired samples of subcutaneous (s.c.), white and deep neck (dn) adipose tissue (AT) samples were collected from n=12 patients undergoing neck surgery for malignancies or nodular goiter. RNA expression of macrophage markers and thermogenic genes was analyzed by qRT-PCR.
Results: The expression of UCP1, PRDM16 and other thermogenic genes was significantly enriched in dn AT, identifying the depot as brown adipose tissue. Expression of the common macrophage marker CD68 as well as the M2 marker genes CD163, CD206/MRC1 and CD301/MGL1 were significantly increased in dn compared to sc adipose tissue (CD68 1.8-fold, CD163 2.1-fold, CD206 4-fold, CD301 2.2-fold). In contrast, CD11c, a marker for M1 macrophage polarization, was not differentially expressed between both depots. There was no correlation between any studied macrophage marker and the expression of UCP1 in both sc and dn adipose tissue.
Conclusion: Our data clearly show an enrichment of alternatively activated M2 macrophages in human deep neck AT. However we could not detect any relationship between the presence of M2 macrophages and UCP1 expression. This suggests that in humans macrophages play no major role for brown AT activation or white AT browning.
27 - 29 Sep 2018
European Society for Paediatric Endocrinology