Down regulation of the liver lipid metabolism induced by hypothyroidism in mice: metabolic flexibility favors compensatory mechanisms in white adipose tissue

In mammals, the maintenance of energy homeostasis relies on complex mechanisms requiring tight synchronization between peripheral organs and the brain. Thyroid hormones (TH), among their pleiotropic actions, play a central role in these regulations. Hypothyroidism, which is characterized by low circulating TH levels, slows down the metabolism, leading to a reduction in energy expenditure, as well as in lipid and glucose metabolism, and to insulin resistance. Our objective was to evaluate whether metabolic deregulations induced by hypothyroidism could be avoid by regulatory mechanisms involved in metabolic flexibility. To this aim, we compared the response to hypothyroidism in two mouse strains, the wild-derived WSB/EiJ mouse strain characterized by a diet-induced obesity (DIO) resistance due to its high metabolic flexibility phenotype and the C57BL/6J mice, prone to DIO. Adult mice were fed with a low-iodine diet supplemented with 6-n-propyl-2-thiouracyl (PTU) for 7 weeks to induce hypothyroidism. Our results show that hypothyroidism, characterized by a decrease in serum T4 levels, led to metabolic deregulations, as an alteration of lipid metabolism in the liver of both strains. However, the decrease in hepatic lipid synthesis was compensated in WSB/EiJ mice by a mobilization of lipid reserves from white adipose tissue, but not in the C57BL/6J mice. No peripheral or hypothalamic inflammatory response to hypothyroidism was observed in both strains. Moreover, gene expression analysis showed that hypothyroidism stimulates the hypothalamic orexigenic circuit in both strains, but unchanged Mc4r and LepR expression in hypothyroid WSB/EiJ mice strain, which reflect their adaptability to maintain their body weight, contrary to C57BL/6J mice. Our results show that WSB/EiJ mice displayed a phenotype of resistance to metabolic dysregulations induced by hypothyroidism, by compensatory mechanisms. This response as well as their resistance to HFD-induced obesity highlights their adaptive capacities to maintain metabolic homeostasis, namely, their high metabolic flexibility, despite serum hypothyroidism. This model sheds light on the importance of local thyroid homeostasis to maintain lipid metabolism and metabolic homeostasis. ### Competing Interest Statement The authors have declared no competing interest.