Hepatic thyroid hormone signalling modulates glucose homeostasis through the regulation of GLP-1 production via bile acid-mediated FXR antagonism

A greater understanding of the glucose homeostasis mediated by glucagon-like peptide-1 will facilitate the development of novel glucose-lowering treatments. Here we show that improved glucose metabolism in hypothyroid mice after treatment of T3, the active form of thyroid hormone, is accompanied with increased GLP-1 production and insulin secretion. Treatment of a GLP-1 receptor antagonist is able to attenuate the observed T3 effect on insulin and glucose levels, suggesting that GLP-1 is critically involved in the regulation of glucose homeostasis by T3. By using a mouse model lacking hepatic TH receptor beta and a liver-specific TR beta-selective agonist, we demonstrate that TR beta-mediated hepatic TH signalling is not only required for the regulation of GLP-1 production by T3 but also the insulinotropic and glucose-lowering effects of T3. Accordingly, administration of the liver-targeted TR beta-selective agonist is capable of increasing GLP-1 and insulin levels and alleviating hyperglycemia in diet-induced obesity. Mechanistically, through suppressing CYP8B1 expression, T3 shapes the bile acid composition and increases the levels of Farnesoid X receptor-antagonistic BAs, thereby potentiating the GLP-1 production and insulin secretion by repressing intestinal FXR signalling. Consistently, correlations between the T3 levels and either GLP-1 or FXR-antagonistic BA levels can be observed in euthyroid human subjects. Thus, our study reveals a previously undescribed role of hepatic TH signalling in glucose homeostasis through the regulation of GLP-1 production via BA-mediated FXR antagonism, which will underpin the development of novel therapeutics.