Fxr-/- mice adapt to biliary obstruction by enhanced phase I detoxification and renal elimination of bile acids

Farnesoid X receptor knockout (Fxr(-/-)) mice cannot upregulate the bile salt export pump in bile acid loading or cholestatic conditions. To investigate whether Fxr(-/-) mice differ in bile acid detoxification compared with wild-type mice, we performed a comprehensive analysis of bile acids extracted from liver, bile, serum, and urine of naive and common bile duct-ligated wild-type and Fxr(-/-) mice using electrospray and gas chromatography mass spectrometry. In addition, hepatic and renal gene expression levels of Cyp2b10 and Cyp3a11, and protein expression levels of putative renal bile acid-transporting proteins, were investigated. We found significantly enhanced hepatic bile acid hydroxylation in Fxr(-/-) mice, in particular hydroxylations of cholic acid in the 1 beta, 2 beta, 4 beta, 6 alpha, 6 beta, 22, or 23 position and a significantly enhanced excretion of these metabolites in urine. The gene expression level of Cyp3a11 was increased in the liver of Fxr(-/-) mice, whereas the protein expression levels of multidrug resistance-related protein 4 (Mrp4) were increased in kidneys of both genotypes during common bile duct ligation. In conclusion, Fxr(-/-) mice detoxify accumulating bile acids in the liver by enhanced hydroxylation reactions probably catalyzed by Cyp3a11. The metabolites formed were excreted into urine, most likely with the participation of Mrp4.