Heme Biosynthesis Regulates BCAA Catabolism and Thermogenesis in Brown Adipose Tissue
The distinctive color of brown adipose tissue (BAT) is attributed to its high content of heme-rich mitochondria. Despite this, the mechanisms by which BAT regulates intracellular heme levels remain largely unexplored. Here, we demonstrate that heme biosynthesis is the primary source of heme in brown adipocytes. Inhibiting heme biosynthesis results in an accumulation of the branched-chain amino acids (BCAAs) valine and isoleucine, due to a heme-associated metabolon that channels BCAA-derived carbons into heme biosynthesis. Heme synthesis-deficient brown adipocytes display reduced mitochondrial respiration and lower UCP1 levels compared to wild-type cells. While exogenous heme supplementation can restore intracellular heme 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 heme synthesis. Finally, disruption of heme 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 heme biosynthesis as a key regulator of thermogenesis and sex-dependent BCAA homeostasis. Overall design: Upload contains multiple RNAseq datasets derived from cultured adipocytes or harvested BAT samples. Experiment 1-Differentiation time course of immortalized brown (B9), primary brown (pBAT), and primary white (pWAT) adipocytes harvested at day 0, 2, 4, or 6 of differentiation (associated with Ext. Fig 1).  Experiment 2- pBAT adipocytes differentiated in the presence of heme synthesis inhibitor succinylacetone (SA), heme-arginate (HA), or both (associated with Ext. Fig. 2 and 4), Experiment 3-BAT samples collected from WT and brown adipocyte-specific Alas1 KO (BAKO) mice (associated with Fig. 6, Ext. Fig. 9).