Heat production from adipose-tissue macrophages regulates lipolysis and diet-induced obesity

Recently white adipose tissues (WAT) have been shown to be able to generate heat upon cold exposure through the expression of uncoupling protein-1 (UCP1), a molecule regulating mitochondrial proton gradient. Although the role of UCP1 has been extensively investigated in adipocytes, it is currently unknown whether UCP1 in the microenvironment of WAT such as adipose-tissue macrophages (ATM) could play a role in thermogenesis. Given the critical role of hypoxia in regulating obesity and insulin resistance, we challenged our myeloid-specific hypoxia-inducible factor-1a (Hif-1a) knockout (KO) mice to high fat diet and observed that these mice were not only protected from diet-induced obesity but also resistant to the body temperature changes upon cold exposure, the latter effect being completely lost upon surgical removal of WAT. Mechanistically, ATM deficient for HIF-1a generated heat production through increased expression of UCP1 and increased mitochondrial functions, all of which lead to an enhanced lipolysis in neighboring adipocytes. Lastly, we observed a number of co-localized areas between UCP-1 and CD68 in WAT of lean mice as well as individuals whereas such areas were not observed in an obese mice or human subjects. In conclusion, we demonstrate a novel mechanism by which ATM regulate obesity through UCP1-mediated heat generation. Overall design: The investigation of gene expression change in FACS-sorted adipose tissue macrophages (ATM) from hMRP8cre+;Hif-1afl/fl;ApoE-/- mice or hMRP8cre-;Hif-1afl/fl;ApoE-/- mice.