Hepatic bile acid metabolism and expression of cytochrome P450 and related enzymes are altered in Bsep −/− mice

Abstract

The bile salt export pump (BSEP/Bsep; gene symbol ABCB11/Abcb11) translocates bile salts across the hepatocyte canalicular membrane into bile in humans and mice. In humans, mutations in the ABCB11 gene cause a severe childhood liver disease known as progressive familial intrahepatic cholestasis type 2. Targeted inactivation of mouse Bsep produces milder persistent cholestasis due to detoxification of bile acids through hydroxylation and alternative transport pathways. The purpose of the present study was to determine whether functional expression of hepatic cytochrome P450 (CYP) and microsomal epoxide hydrolase (mEH) is altered by Bsep inactivation in mice and whether bile acids regulate CYP and mEH expression in Bsep (-/-) mice. CYP expression was determined by measuring protein levels of Cyp2b, Cyp2c and Cyp3a enzymes and CYP-mediated activities including lithocholic acid hydroxylation, testosterone hydroxylation and alkoxyresorufin O-dealkylation in hepatic microsomes prepared from female and male Bsep (-/-) mice fed a normal or cholic acid (CA)-enriched diet. The results indicated that hepatic lithocholic acid hydroxylation was catalyzed by Cyp3a/Cyp3a11 enzymes in Bsep (-/-) mice and that 3-ketocholanoic acid and murideoxycholic acid were major metabolites. CA feeding of Bsep (-/-) mice increased hepatic Cyp3a11 protein levels and Cyp3a11-mediated testosterone 2β-, 6β-, and 15β-hydroxylation activities, increased Cyp2b10 protein levels and Cyp2b10-mediated benzyloxyresorufin O-debenzylation activity, and elevated Cyp2c29 and mEH protein levels. We propose that bile acids upregulate expression of hepatic Cyp3a11, Cyp2b10, Cyp2c29 and mEH in Bsep (-/-) mice and that Cyp3a11 and multidrug resistance-1 P-glycoproteins (Mdr1a/1b) are vital components of two distinct pathways utilized by mouse hepatocytes to expel bile acids.

Prognostic roles of tetrahydroxy bile acids in infantile intrahepatic cholestasis

“…The most abundant CYP enzyme family in the human liver is the CYP3A subfamily, which includes three isoforms: CYP3A4, CYP3A5, and CYP3A7 (26). Cyp3a11 (human homolog of CYP3A5) has been proposed to be involved in THBA formation in mice (27). However, no studies have examined the relationship between CYP3A4 or CYP3A5 and THBAs in humans.…”

Comprehensive bile acid profiling in hereditary intrahepatic cholestasis: Genetic and clinical correlations

“…In animal model of PFIC2, the Abcb11 knockout ( Abcb11 −/− ) mouse, only mild but persistent cholestasis is observed and lifespan is normal. Bile acid analyses suggest that these mice alleviate cholestatic stress in at least two ways . First, they substantially increase synthesis of hydrophilic tetrahydroxylated bile acids (THBAs), thereby reducing hepatocellular bile acid toxicity.…”

Hydrophilic bile acids prevent liver damage caused by lack of biliary phospholipid in Mdr2 mice

“…On the other hand, the mouse model of PFIC2, the Bsep / mouse, displays only a mild phenotype, with no progressive liver damage. A key feature of the hepatoprotective mechanism in the Bsep / mice appears to be the ability of their hepatocytes to shift the composition of their bile acid to a more hydrophilic, and less toxic, one by upregulating a series of P450 enzymes, favoring production of the less hydrophobic primary bile acids (-MCA over CA) and conversion of trihydroxy bile acids into THBAs (45). The reduced hepatoxicity of these bile acids is significant because these mice do have 5-fold higher levels of bile acids in the liver, but they do not suffer the pathology expected of a cholestatic phenotype.…”

“…This hepatoprotective response appears to coincide with a change in expression of the Cyp2c family genes, downstream of Cyp27a1 in the production of MCA (31), particularly Cyp2c55. Cyp2c70 and nine other members of the Cyp2c family, plus the bile acid 6-hydroxylase Cyp3a11 (32,45), were expressed at higher levels in the Bsep / and/or DKO livers than in the Mdr2 / mouse, making these likely candidates for the enzymes responsible for THBA production from trihydroxyl bile acids in Bsep / and DKO mice. Taken together, the dramatic hydrophilic shift of bile acid composition, coupled with the presence of an alternative canalicular transporter with the ability to transport hydrophilic bile acids, to promote bile flow in the DKO mice, protect them from the toxicity of nonmicelle-bound bile acids in the bile duct.…”