HMOX1 Is Involved in the Upregulation of Mitochondrial Fatty Acid β‐Oxidation Through Heme Catalysis

Intracellular energy metabolism is strictly regulated by hormones and intracellular metabolites for the appropriate use of energy resources such as carbohydrates and fatty acids. Mitochondria are essential organelles in eukaryotic cells since they contain several metabolic machineries required for oxidative phosphorylation, such as the tricarboxylic cycle, electron transport chain, and fatty acid β‐oxidation (FAO). Previously, we reported that FAO activity is increased in CLPX‐knockout HepG2 cells (CLPX‐KO cells). CLPX is the substrate recognition subunit of the mitochondrial protease ClpXP that regulates intracellular heme levels through the degradation of ALAS1 in the mitochondrial matrix. ALAS1 is the rate‐limiting enzyme for heme biosynthesis; thus, we examined whether heme regulates FAO activity. We found that the inhibition of heme synthesis in CLPX‐KO cells decreased FAO activity; in contrast, hemin treatment increased FAO activity in wild‐type HepG2 cells (CLPX‐WT cells). Furthermore, pretreatment of CLPX‐WT cells with a chemical inhibitor or specific siRNA targeting the heme degradation enzyme HMOX1 blocked FAO activation induced by hemin, indicating that heme catabolites rather than heme itself are responsible for FAO activation by heme. Among the heme catabolites, we identified carbon monoxide (CO) as an essential metabolite for FAO activation through the activation of the LKB1/AMPK signaling pathway. In this manuscript, we revealed the previously unknown role of CO as an intrinsic inducer of mitochondrial FAO activity and revealed that an excess amount of heme increases FAO activity via CO production by HMOX1.