Brown and beige adipocytes express uncoupling protein 1 (UCP1), a mitochondrial protein that dissociates respiration from ATP synthesis and promotes heat production and energy expenditure. However, UCP1−/− mice are not obese1–5, consistent with the existence of alternative mechanisms of thermogenesis6–8. Here we describe a UCP1-independent mechanism of thermogenesis involving ATP-consuming metabolism of monomethyl branched-chain fatty acids (mmBCFA) in peroxisomes. These fatty acids are synthesized by fatty acid synthase using precursors derived from catabolism of branched-chain amino acids9 and our results indicate that β-oxidation of mmBCFAs is mediated by the peroxisomal protein acyl-CoA oxidase 2 (ACOX2). Notably, cold exposure upregulated proteins involved in both biosynthesis and β-oxidation of mmBCFA in thermogenic fat. Acute thermogenic stimuli promoted translocation of fatty acid synthase to peroxisomes. Brown-adipose-tissue-specific fatty acid synthase knockout decreased cold tolerance. Adipose-specific ACOX2 knockout also impaired cold tolerance and promoted diet-induced obesity and insulin resistance. Conversely, ACOX2 overexpression in adipose tissue enhanced thermogenesis independently of UCP1 and improved metabolic homeostasis. Using a peroxisome-localized temperature sensor named Pexo-TEMP, we found that ACOX2-mediated fatty acid β-oxidation raised intracellular temperature in brown adipocytes. These results identify a previously unrecognized role for peroxisomes in adipose tissue thermogenesis characterized by an mmBCFA synthesis and catabolism cycle.
