Activation of thyroid hormone receptor‐β improved disease activity and metabolism independent of body weight in a mouse model of non‐alcoholic steatohepatitis and fibrosis

Background and Purpose Activation of hepatic thyroid hormone receptor β (THR‐β) is associated with systemic lipid lowering, increased bile acid synthesis, and fat oxidation. In patients with non‐alcoholic steatohepatitis (NASH), treatment with THR‐β agonists decreased hepatic steatosis and circulating lipids, and induced resolution of NASH. We chose resmetirom (MGL‐3196), a liver‐directed, selective THR‐β agonist, as a prototype to investigate the effects of THR‐β activation in mice with diet‐induced obesity (DIO) and biopsy‐confirmed advanced NASH with fibrosis. Experimental Approach C57Bl/6J mice were fed a diet high in fat, fructose, and cholesterol for 34 weeks, and only biopsy‐confirmed DIO‐NASH mice with fibrosis were included. Resmetirom was administered at a daily dose of 3 mg·kg −1 p.o., for 8 weeks. Systemic and hepatic metabolic parameters, histological non‐alcoholic fatty liver disease (NAFLD) activity and fibrosis scores, and liver RNA expression profiles were determined to assess the effect of THR‐β activation. Key Results Treatment with resmetirom did not influence body weight but led to significant reduction in liver weight, hepatic steatosis, plasma alanine aminotransferase activity, liver and plasma cholesterol, and blood glucose. These metabolic effects translated into significant improvement in NAFLD activity score. Moreover, a lower content of α‐smooth muscle actin and down‐regulation of genes involved in fibrogenesis indicated a decrease in hepatic fibrosis. Conclusion and Implications Our model robustly reflected clinical observations of body weight‐independent improvements in systemic and hepatic metabolism including anti‐steatotic activity.