ApoE is a major determinant of hepatic bile acid homeostasis in mice

Apolipoprotein E (ApoE) plays a central role in lipid transport and cholesterol metabolism, with surplus cholesterol being removed from the liver through bile acid (BA) synthesis. Furthermore, BAs are of critical importance in fat absorption by forming intestinal lipid-bile salt mixed micelles. To define ApoE's role in BA homeostasis, the metabolism of cholesterol and BA was investigated in liver tissue and gallbladder bile of ApoE-deficient mice given a chow or high-cholesterol/high-fat diet (HCHF) diet for 6 months. When compared to wild-type mice, muricholic acid (MCA) and chenodeoxycholic acid (CDCA) increased approximately 15-, 82-, 22- and 38-fold, respectively, in hepatic tissue of ApoE-deficient mice given a chow or HCHF diet. Moreover, ApoE-deficient mice on an HCHF diet increased the amounts of hepatic free cholesterol, MCA and CDCA by 61%, 61% and 50% (P<.05). Conversely, total cholesterol and cholesterol esters were unchanged, and the bile acids taurohyodeoxycholic acid, taurodeoxycholic acid and hyodeoxycholic acid decreased to one third as compared to the chow diet (P<.05). Additionally, quantitative reverse-transcription polymerase chain reaction assays revealed induced expression of the bile acid receptor (Fxr) and associated transcription factors, i.e.Fxr, Lrh, Lxra and Srebp1c. Transcript expression of Cyp2a12, Cyp1b1, Cyp2e1, Cyp3a16 and Cyp4a10 was also induced. Note that Cyp4a10 catalyzes ω-hydroxylation of arachidonic acid to epoxy- and hydroxyeicosatrienoic acids to control vascular tone. Altogether, MCA and CDCA synthesis is selectively induced in ApoE-deficient mice. These hydrophilic BAs alter micellar size and structure to lower intestinal cholesterol solubilization. Furthermore, CDCA and MCA are potent FXR agonist and antagonist, respectively, and function in a regulatory loop to mitigate impaired ApoE function.