Ei24 deficiency in brown adipocytes induces severe hypothermia under cold stress independent of UCP1 activity

Brown adipocytes facilitate non-shivering thermogenesis, which is critical for maintaining energy balance and heat production in response to environmental stimuli. Here, we delineate the physiological and biochemical role of etoposide-induced 2.4 ( Ei24 ) in adenosine triphosphate (ATP) production and thermogenesis in brown adipocytes. We generated Ei24 adipocyte-specific knockout (EiaKO) mice that exhibited brown adipose tissue hypertrophy, lipid accumulation, and various mitochondrial abnormalities. Despite mitochondrial defects, uncoupling protein 1 (UCP1) expression and activity remained unchanged. However, those impairments caused lethal hypothermia in mice subjected to cold challenge, underscoring the key role of Ei24 in mitochondrial functions. The morphological and functional defects of EiaKO brown adipocyte mitochondria were due to compromised proton-motive force, ATP synthesis, and fatty acid oxidation. Our findings highlight the critical role of Ei24 in sustaining mitochondrial function independently of UCP1 expression and activity, emphasizing its connection between mitochondrial respiration and thermogenesis in brown adipocytes.