Mice with humanized FXR ligand-binding domain display distinct metabolic responses upon pharmacological FXR...

Farnesoid-X-receptor (FXR), a bile acid (BA)-activated nuclear receptor, is a therapeutic target for cholestatic and metabolic liver diseases. However, species differences in BA metabolism and FXR signaling hamper translation from mice to humans. The human FXR ligand-binding domain (LBD) structurally differs from the murine LBD, potentially impacting pharmacological responses. Therefore, we generated mice with "humanized" FXR by replacing the murine LBD by the human LBD (FXR-hLBD) and assessed its impact on BA and cholesterol metabolism. Male and female FXR-hLBD mice on wild-type (WT) or Cyp2c70^-/- backgrounds were compared with FXR-mLBD controls under non-stimulated conditions. Additionally, WT mice expressing FXR-mLBD or FXR-hLBD received either vehicle or obeticholic acid (OCA; 40 mg/kg/day, p.o.) for 7 days. FXR humanization did not alter hepatic or intestinal FXR expression levels. Under basal conditions, physiological parameters, liver pathology markers, and hepatic transcriptomes were similar between FXR-hLBD and FXR-mLBD mice on a WT C57BL/6J background and in mice with a human-like BA composition (Cyp2c70^-/-). OCA did, however, elicit markedly stronger transcriptional responses in FXR-hLBD mice, including more pronounced suppression of hepatic BA synthesis genes and differential regulation of BA transporters. Intriguingly, pathways involved in cell proliferation and fibrogenesis were induced in FXR-hLBD mice. Furthermore, OCA lowered plasma cholesterol to a greater extent in FXR-hLBD than FXR-mLBD mice, primarily due to a reduction in HDL-cholesterol. FXR-hLBD mice resemble FXR-mLBD controls under basal conditions but exhibit enhanced responses to FXR agonism by OCA. This model may improve preclinical evaluation of FXR-targeting drugs in a translation-relevant context.