IF1 is a cold-regulated switch of ATP synthase to support thermogenesis in brown fat

Abstract

In thermogenic adipocytes, uncoupling protein-1 (UCP1) is a key mediator of non-shivering thermogenesis (NST) by uncoupling the electron transport chain from FoF1-ATP synthase-mediated ATP production. While regulatory mechanisms of UCP1 are important for NST, it is unknown whether also the activity of ATP synthase is modulated during NST. Here, we show a critical role of Inhibitory Factor 1 (IF1), an inhibitor of ATP synthase, for brown adipocyte energy metabolism. In mice, IF1 protein content is diminished in brown adipose tissue of mice after 5 days of cold exposure. Additionally, the capacity of ATP synthase to generate mitochondrial membrane potential (MMP) through ATP hydrolysis (the so-called reverse mode) was higher in mitochondria isolated from cold-adapted mice compared to mice housed at room temperature. While silencing of IF1 in cultured brown adipocytes did not affect MMP, IF1 overexpression resulted in an inability of mitochondria to sustain MMP upon adrenergic stimulation. The effects of IF1 overexpression on MMP were blunted when UCP1 was silenced or when a mutant IF1, incapable of binding to ATP synthase, was used. In brown adipocytes, IF1 ablation was sufficient to increase mitochondrial lipid oxidation and the cellular dependency on glycolysis to produce ATP. Conversely, IF1 overexpression blunted mitochondrial respiration without causing energetic stress, leading to a quiescent-like phenotype in brown adipocytes. In conclusion, our data show that the cold-induced downregulation of IF1 facilitates the reverse mode of ATP synthase and enables proper bioenergetic adaptation of brown adipose tissue to effectively support NST.