Haem biosynthesis regulates BCAA catabolism and thermogenesis in brown adipose tissue.

The distinctive colour of brown adipose tissue (BAT) is attributed to its high content of haem-rich mitochondria. However, the mechanisms by which BAT regulates intracellular haem levels remain largely unexplored. Here we demonstrate that haem biosynthesis is the primary source of haem in brown adipocytes. Inhibiting haem biosynthesis results in an accumulation of the branched-chain amino acids (BCAAs) valine and isoleucine, owing to a haem-associated metabolon that channels BCAA-derived carbons into haem biosynthesis. Haem synthesis-deficient brown adipocytes display reduced mitochondrial respiration and lower UCP1 levels than wild-type cells. Although exogenous haem supplementation can restore intracellular haem levels and mitochondrial function, UCP1 downregulation persists. This sustained UCP1 suppression is linked to epigenetic regulation induced by the accumulation of propionyl-CoA, a byproduct of disrupted haem synthesis. Finally, disruption of haem biosynthesis in BAT impairs thermogenic response and, in female but not male mice, hinders the cold-induced clearance of circulating BCAAs in a sex-hormone-dependent manner. These findings establish adipose haem biosynthesis as a key regulator of thermogenesis and sex-dependent BCAA homeostasis.