Cyp7a1 is continuously increased with disrupted Fxr-mediated feedback inhibition in hypercholesterolemic TALLYHO/Jng mice

TALLYHO/Jng (TH) mice reveal hypercholesterolemia at an early age before their plasma glucose levels have increased. The increased plasma cholesterol should be related to bile acids (BAs) metabolism, because cholesterol is the precursor of BAs and BAs change cholesterol metabolism in a feedback manner. We analyzed the BAs pool size, BAs composition, and expression levels of several proteins that have key roles in BAs synthesis, excretion, and reabsorption and compared them to those of C57BL/6 (B6) mice to study BAs metabolism in TH mice. TH mice exhibited an increased total BAs pool size, increased BAs content in the cecum feces, and an increased ratio of muricholic acid (MCA)/cholic acid (CA). The mRNA and protein levels of cholesterol 7 alpha-hydroxylase (Cyp7a1) and the ATP-binding cassette sub-family G member 5 (Abcg5) were elevated in the liver but not in the apical sodium bile acid transporter (Asbt) and organic solute transporters (Osts) in the ileum. These results indicate that synthesis and the excretion of BAs from the liver to the gallbladder might be elevated, but the reabsorption rate of BAs in the ileum might be reduced. The declined expression of fibroblast growth factor 15 (Fgf15) and fibroblast growth factor receptor 4 (Fgfr4) was respectively identified in the ileum and the liver, indicating the disrupted feedback inhibition of Cyp7a1. Consequently, hypercholesterolemia in TH mice might increase the BAs amounts via the interrupted Fxr/Fgf15/Fgfr4-mediated feedback regulation of Cyp7a1.