Decreased Bile-Acid Synthesis in Livers of Hepatocyte-Conditional NADPH–Cytochrome P450 Reductase–Null Mice Results in Increased Bile Acids in Serum

Abstract

NADPH-cytochrome P450 reductase (Cpr) is essential for the function of microsomal cytochrome P450 monooxygenases (P450), including those P450s involved in bile acid (BA) synthesis. Mice with hepatocyte-specific deletion of NADPH-cytochrome P450 reductase (H-Cpr-null) have been engineered to understand the in vivo function of hepatic P450s in the metabolism of xenobiotics and endogenous compounds. However, the impact of hepatic Cpr on BA homeostasis is not clear. The present study revealed that H-Cpr-null mice had a 60% decrease in total BA concentration in liver, whereas the total BA concentration in serum was almost doubled. The decreased level of cholic acid (CA) in both serum and livers of H-Cpr-null mice is likely due to diminished enzyme activity of Cyp8b1 that is essential for CA biosynthesis. Feedback mechanisms responsible for the reduced liver BA concentrations and/or increased serum BA concentrations in H-Cpr-null mice included the following: 1) enhanced alternative BA synthesis pathway, as evidenced by the fact that classic BA synthesis is diminished but chenodeoxycholic acid still increases in both serum and livers of H-Cpr-null mice; 2) inhibition of farnesoid X receptor activation, which increased the mRNA of Cyp7a1 and 8b1; 3) induction of intestinal BA transporters to facilitate BA absorption from the intestine to the circulation; 4) induction of hepatic multidrug resistance-associated protein transporters to increase BA efflux from the liver to blood; and 5) increased generation of secondary BAs. In summary, the present study reveals an important contribution of the alternative BA synthesis pathway and BA transporters in regulating BA concentrations in H-Cpr-null mice.

Effect of Gender and Various Diets on Bile Acid Profile and Related Genes in Mice

“…For example, in the same C57BL/6J mice, the serum concentrations of CA, CDCA, αMCA, and βMCA were increased in mice fed 30 mg/kg berberine compared to the control mice, while the liver concentrations of all of these BAs were not altered by berberine (Guo et al, 2016). While the same phenomenon was found in different mice models (germ-free and hepatocyte-specific deletion of NADPH cytochrome P450 reductase null mice), or in varied pathological situations (mice after bile duct ligation) (Zhang et al, 2012b;Cheng et al, 2014;Selwyn et al, 2015). Thus, it is apparent from these numerous studies that changes in concentrations of BAs in the serum do not predict what happens to BA concentrations in the liver.…”

Effects of Diminished NADPH:cytochrome P450 Reductase in Human Hepatocytes on Lipid and Bile Acid Homeostasis

“…Slightly increased CDCA conjugates in HepaRG −POR cells may indicate increased bile acid synthesis via the alternative acidic pathway with the POR-independent, mitochondrial CYP27A1 (Monte et al, 2009) predominantly leading to CDCA synthesis (Alnouti, 2009;Pikuleva and Waterman, 2013). Enhanced alternative bile acid synthesis was also reported in Por knockout mice, where an increase in CDCA and TCDCA in the liver was observed (Cheng et al, 2014b). The observed downregulation of nuclear receptor expression in our cell model could be in response to the altered bile acid composition with decreased levels of TCA and GCA and unchanged TCDCA levels while levels of GCDCA were increased.…”

“…Genomic deletion of Por in mice leads to embryonal death around day 13 due to severe disturbances in retinoid and cholesterol homeostasis ( Shen et al, 2002 ; Otto et al, 2003 ; Schmidt et al, 2009 ). So far effects of Por knockout in liver on drug as well as endogenous metabolism were extensively studied in mouse models ( Gu et al, 2003 ; Henderson et al, 2003 ; Wang et al, 2005 ; Weng et al, 2005 ; Finn et al, 2009 ; Cheng et al, 2014b ). In addition to diminished CYP activity levels ( Gu et al, 2003 ; Henderson et al, 2003 ; Finn et al, 2007 ), these mice developed severe hepatic lipidosis as well as reduced circulating cholesterol, triglyceride levels and gallbladder bile volume ( Gu et al, 2003 ; Henderson et al, 2003 ; Wu et al, 2003 ).…”

“…Moreover, in-depth studies in liver specific Por knockout mice revealed diverse effects on bile acid synthesis. While total amounts of bile acids in mouse liver were only slightly decreased, concentrations of chenodeoxycholic acid (CDCA) and its gut microbial product lithocholic acid (LCA) were increased, probably due to induction of the alternative bile acid synthesis pathway ( Cheng et al, 2014b ; Gethings et al, 2021 ). In humans the multistep bile acid biosynthesis is mainly mediated by CYPs 7A1, 7B1, 8B1 and the mitochondrial, POR-independent CYP27A1, resulting in a substantially different composition of the bile acid pool.…”

Adaptive Hepatic and Intestinal Alterations in Mice after Deletion of NADPH-Cytochrome P450 Oxidoreductase (Cpr) in Hepatocytes

“…These protein data further demonstrate that CAR and Nrf2 are activated in livers of H-Cpr-null mice. In addition, we have demonstrated that depletion of Cpr in mouse hepatocytes leads to increased level of lithocholic acid (LCA) in mouse serum and liver (Cheng et al, 2014). Lithocholic acid is an activator of PXR nuclear receptor (Staudinger et al, 2001).…”