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Bile acids (BAs) are essential for lipid absorption, bile formation, and regulation of intermediary metabolism. Primary BAs are synthesized in the liver and conjugated with glycine or taurine (T) before biliary excretion. Humans produce two primary BAs, cholic acid (CA) and chenodeoxycholic acid (CDCA). BA synthesis is regulated by the farnesoid X receptor (FXR), which represses Cyp7a1, the rate-limiting enzyme of BA synthesis. Although mice are commonly used models, mice efficiently hydroxylate CDCA by Cyp2c70 at the 6β-position; thus, their dominant BAs are the hydrophilic muricholic acids (MCAs), in contrast to the hydrophobic human BA-pool. These species differences complicate the translational research of BAs. For example, because of the hydrophilic BA composition, the PFIC-2 model, bile salt export pump (Bsep)-null mouse shows moderate changes in phenotype compared to human PFIC-2 patients. To better understand the role of human-like BA metabolism, we engineered Cyp2c70-null mice.

BA concentrations in liver, bile, and plasma, and hepatic gene expression, were quantified in male and female wild-type and Cyp2c70-null mice. Hepatic and plasma concentrations of total (Σ)BAs increased in male mice (liver: +73%, plasma: +5-fold), and were markedly higher in female null mice (liver: +1.79-fold, plasma: +14-fold) compared to WT mice. MCA formation was practically abolished, and CDCA and its metabolites increased markedly, creating a human-like, hydrophobic BA composition in Cyp2c70-null mice. Contrary to significant increase in concentrations of CDCA and LCA in livers of Cyp2c70-null mice, hepatic FXR-SHP and intestinal FXR-Fgf15 pathways were minimally activated. Biliary excretion of ΣBAs decreases in male (-18%) and more in female (-25%) Cyp2c70-null mice, contrary to increased biliary excretion of CDCA (~16-fold) and LCA (~32-fold). Decreased ΣBA excretion was due to reduced biliary excretion of TCA in male mice (-19%), more markedly in female mice (-46%). The ratio of TCDCA/TCA in WT liver is 0.12, whereas it is 1.6 in male and 2.9 in female Cyp2c70-null mice. Murine Bsep has a higher affinity for TCDCA than TCA; therefore, TCDCA can inhibit biliary transport of TCA, especially in female (-46%) Cyp2c70-null mice. Since (T)CA is the natural ligand for murine FXR, due to lower excretion of TCA, the intestinal FXR-Fgf15 pathway (responsible for Cyp7a1 suppression) is not activated. In contrast, the hepatic FXR-pathway is activated, causing 12-α-hydroxylase Cyp8b1 suppression, especially in female Cyp2c70-null mice. In summary, Cyp2c70-null mice have a human-like hydrophobic BA-pool and exhibit PFIC-like cholestatic features, which will help clarify the pathomechanisms of various pediatric liver diseases.

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Mice With Human-Like Bile Acid Composition Develop PFIC-2- Like Cholestasis