Organic anion-transporting polypeptide 1a4 (Oatp1a4) is important for secondary bile acid metabolism

Quantitative bile acid profiling in healthy adult dogs and pups from serum, plasma, urine, and feces using LC-MS/MS

Synthesis and secretion of bile acids (BA) is a key physiological function of the liver. In pathological conditions like portosystemic shunt, hepatic insufficiency, hepatitis, or cirrhosis BA metabolism and secretion are disturbed. Quantification of total serum BA is an established diagnostic method to assess the general liver function and allows early detection of abnormalities, liver disease progression and guidance of treatment decisions. To date, data on comparative BA profiles in dogs are limited. However, BA profiles might be even better diagnostic parameters than total BA concentrations. On this background, the present study analyzed and compared individual BA profiles in serum, plasma, urine, and feces of 10 healthy pups and 40 adult healthy dogs using ultra-high performance liquid chromatography coupled to electrospray ionization mass spectrometry. Sample preparation was performed by solid-phase extraction for serum, plasma, and urine samples or by protein precipitation with methanol for the feces samples. For each dog, 22 different BA, including unconjugated BA and their glycine and taurine conjugates, were analyzed. In general, there was a great interindividual variation for the concentrations of single BA, mostly exemplified by the fact that cholic acid (CA) was by far the most prominent BA in blood and urine samples of some of the dogs (adults and pups), while in others, CA was under the detection limit. There were no significant age-related differences in the BA profiles, but pups showed generally lower absolute BA concentrations in serum, plasma, and urine. Taurine-conjugated BA were predominant in the serum and plasma of both pups (68%) and adults (74-75%), while unconjugated BA were predominant in the urine and feces of pups (64 and 95%, respectively) and adults (68 and 99%, respectively). The primary BA chenodeoxycholic acid and taurocholic acid and the secondary BA deoxycholic acid and lithocholic acid were the most robust analytes for potential diagnostic purpose. In conclusion, this study reports simultaneous BA profiling in dog serum, plasma, urine, and feces and provides valuable diagnostic data for subsequent clinical studies in dogs with different kinds of liver diseases.

Generation of Slco1a4-CreERT2-tdTomato Knock-in Mice for Specific Cerebrovascular Endothelial Cell Targeting

The cerebrovascular endothelial cells with distinct characteristics line cerebrovascular blood vessels and are the fundamental structure of the blood-brain barrier, which is important for the development and homeostatic maintenance of the central nervous system. Cre-LoxP system-based spatial gene manipulation in mice is critical for investigating the physiological functions of key factors or signaling pathways in cerebrovascular endothelial cells. However, there is a lack of Cre recombinase mouse lines that specifically target cerebrovascular endothelial cells. Here, using a publicly available single-cell RNAseq database, we screened the solute carrier organic anion transporter family member 1a4 (Slco1a4) as a candidate marker of cerebrovascular endothelial cells. Then, we generated an inducible Cre mouse line in which a CreERT2-T2A-tdTomato cassette was placed after the initiation codon ATG of the Slco1a4 locus. We found that tdTomato, which can indicate the endogenous Slco1a4 expression, was expressed in almost all cerebrovascular endothelial cells but not in any other non-endothelial cell types in the brain, including neurons, astrocytes, oligodendrocytes, pericytes, smooth muscle cells, and microglial cells, as well as in other organs. Consistently, when crossing the ROSA26LSL-EYFP Cre reporter mouse, EYFP also specifically labeled almost all cerebrovascular endothelial cells upon tamoxifen induction. Overall, we generated a new inducible Cre line that specifically targets cerebrovascular endothelial cells.

Increased Perfluorooctanesulfonate (PFOS) Toxicity and Accumulation Is Associated with Perturbed Prostaglandin Metabolism and Increased Organic Anion Transport Protein (OATP) Expression

Perfluorooctanesulfonate (PFOS) is a widespread environmental pollutant with a long half-life and clearly negative outcomes on metabolic diseases such as fatty liver disease and diabetes. Male and female Cyp2b-null and humanized CYP2B6-transgenic (hCYP2B6-Tg) mice were treated with 0, 1, or 10 mg/kg/day PFOS for 21 days, and surprisingly it was found that PFOS was retained at greater concentrations in the serum and liver of hCYP2B6-Tg mice than those of Cyp2b-null mice, with greater differences in the females. Thus, Cyp2b-null and hCYP2B6-Tg mice provide new models for investigating individual mechanisms for PFOS bioaccumulation and toxicity. Overt toxicity was greater in hCYP2B6-Tg mice (especially females) as measured by mortality; however, steatosis occurred more readily in Cyp2b-null mice despite the lower PFOS liver concentrations. Targeted lipidomics and transcriptomics from PFOS-treated Cyp2b-null and hCYP2B6-Tg mouse livers were performed and compared to PFOS retention and serum markers of toxicity using PCA. Several oxylipins, including prostaglandins, thromboxanes, and docosahexaenoic acid metabolites, are associated or inversely associated with PFOS toxicity. Both lipidomics and transcriptomics indicate PFOS toxicity is associated with PPAR activity in all models. GO terms associated with reduced steatosis were sexually dimorphic with lipid metabolism and transport increased in females and circadian rhythm associated genes increased in males. However, we cannot rule out that steatosis was initially protective from PFOS toxicity. Moreover, several transporters are associated with increased retention, probably due to increased uptake. The strongest associations are the organic anion transport proteins (Oatp1a4-6) genes and a long-chain fatty acid transport protein (fatp1), enriched in female hCYP2B6-Tg mice. PFOS uptake was also reduced in cultured murine hepatocytes by OATP inhibitors. The role of OATP1A6 and FATP1 in PFOS transport has not been tested. In summary, Cyp2b-null and hCYP2B6-Tg mice provided unique models for estimating the importance of novel mechanisms in PFOS retention and toxicity.

Sex Differences in Blood–Brain Barrier Transport of Psychotropic Drugs

Treatment of neuropsychiatric disorders relies on the effective delivery of therapeutic molecules to the target organ, the brain. The blood–brain barrier (BBB) hinders such delivery and proteins acting as transporters actively regulate the influx and importantly the efflux of both endo- and xeno-biotics (including medicines). Neuropsychiatric disorders are also characterized by important sex differences, and accumulating evidence supports sex differences in the pharmacokinetics and pharmacodynamics of many drugs that act on the brain. In this minireview we gather preclinical and clinical findings on how sex and sex hormones can influence the activity of those BBB transporter systems and affect the brain pharmacokinetics of psychotropic medicines. It emerges that it is not well understood which psychotropics are substrates for each of the many and not well-studied brain transporters. Indeed, most evidence originates from studies performed in peripheral tissues, such as the liver and the kidneys. None withstanding, accumulated evidence supports the existence of several sex differences in expression and activity of transport proteins, and a further modulating role of gonadal hormones. It is proposed that a closer study of sex differences in the active influx and efflux of psychotropics from the brain may provide a better understanding of sex-dependent brain pharmacokinetics and pharmacodynamics of psychotropic medicines.

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

Diet is an important factor for many diseases. Previous studies have demonstrated that several diets had remarkable effects on bile acid (BA) homeostasis, but no comprehensive information for both genders has been reported. Therefore, the current study characterized the nine most used laboratory animal diets fed to both genders of mice for a comparable evaluation of the topic. The results revealed that marked gender difference of BA homeostasis is ubiquitous in mice fed the various diets, and of the nine diets fed to mice, the atherogenic and calorie-restricted diets had the most marked effects on BA homeostasis, followed by the laboratory chow and essential fatty acid-deficient diets. More specifically, females had higher concentrations of total BAs in serum when fed six of the nine diets compared with male mice, and 26 of the 35 BA-related genes had marked gender difference in mice fed at least one diet. Although mice fed the calorie-restricted and atherogenic diets had increased BA, which was more pronounced in serum than liver, the intestinal farnesoid X nuclear receptor-fibroblast growth factor 15 axis changed in the opposite direction and resulted in different hepatic expression patterns of Cyp7a1 Compared with AIN-93M purified diet, higher hepatic expression of multidrug resistance-associated protein 3 was the only alteration in mice fed the laboratory chow diet. The other diets had little or no effect on BA concentrations in the liver and plasma or in the expression of BA-related genes. This study indicates that gender, the atherogenic diet, and the calorie-restricted diet have the most marked effects on BA homeostasis. SIGNIFICANCE STATEMENT: Previous evidence suggested that various diets have effect on bile acid (BA) homeostasis; however, it is not possible to directly compare these findings, as they are all from different studies. The current study was the first to systematically investigate the influence of the nine most used experimental mouse diets on BA homeostasis and potential mechanism in both genders of mice and indicates that gender, the atherogenic diet, and the calorie-restricted diet have the most marked effects on BA homeostasis, which will aid future investigations.

Sex-, Age-, and Race/Ethnicity-Dependent Variations in Drug-Processing and NRF2-Regulated Genes in Human Livers

Individual variations in xenobiotic metabolism affect the sensitivity to diseases. In this study, the impacts of sex, age, and race/ethnicity on drug-processing genes and nuclear factor erythroid 2-related factor 2 (NRF2) genes in human livers were examined via QuantiGene multiplex suspension array (226 samples) and quantitative polymerase chain reaction (qPCR) (247 samples) to profile the expression of nuclear receptors, cytochrome P450s, conjugation enzymes, transporters, bile acid metabolism, and NRF2-regulated genes. Sex differences were found in expression of about half of the genes, but in general the differences were not large. For example, females had higher transcript levels of catalase, glutamate-cysteine ligase catalytic subunit (GCLC), heme oxygenase 1 (HO-1), Kelch-like ECH-associated protein 1 (KEAP1), superoxide dismutase 1, and thioredoxin reductase-1 compared with males via qPCR. There were no apparent differences due to age, except children had higher glutamate-cysteine ligase modifier subunit (GCLM) and elderly had higher multidrug resistance protein 3. African Americans had lower expression of farnesoid X receptor (FXR) but higher expression of HO-1, Caucasians had higher expression of organic anion transporter 2, and Hispanics had higher expression of FXR, SULT2A1, small heterodimer partner, and bile salt export pump. An examination of 34 diseased and control human liver samples showed that compared with disease-free livers, fibrotic livers had higher NAD(P)H-quinone oxidoreductase 1 (NQO1), GCLC, GCLM, and NRF2; hepatocellular carcinoma had higher transcript levels of NQO1 and KEAP1; and steatotic livers had lower GCLC, GCLM, and HO-1 expression. In summary, in drug-processing gene and NRF2 genes, sex differences were the major findings, and there were no apparent age differences, and race/ethnicity differences occurred for a few genes. These descriptive findings could add to our understanding of the sex-, age-, and race/ethnicity-dependent differences in drug-processing genes as well as NRF2 genes in normal and diseased human livers. SIGNIFICANCE STATEMENT: In human liver drug-processing and nuclear factor erythroid 2-related factor 2 genes, sex differences were the main finding. There were no apparent differences due to age, except children had higher glutamate-cysteine ligase modifier subunit, and elderly had higher multidrug resistance protein 3. African Americans had lower expression of farnesoid X receptor (FXR) but higher expression of heme oxygenase 1, Caucasians had higher expression of organic anion transporter 2, and Hispanics had higher expression of FXR, small heterodimer partner, SULT2A1, and bile salt export pump.

Effects of Absence of Constitutive Androstane Receptor (CAR) on Bile Acid Homeostasis in Male and Female Mice

Accumulation of BAs in hepatocytes has a role in liver disease and also in drug-induced liver injury. The Constitutive Androstane Receptor (CAR) has been shown to protect against BA-induced liver injury. The polymorphism of CAR has recently been shown to modify the pharmacokinetics and pharmacodynamics of various drugs. Thus it was hypothesized that polymorphism of CAR may also influence BA homeostasis. Using CAR-null and WT mice, this study modeled the potential consequences of CAR polymorphism on BA homeostasis. Our previous study showed that chemical activation of CAR decreases the total BA concentrations in livers of mice. Surprisingly the absence of CAR also decreased the BA concentrations in livers of mice, but to a lesser extent than in CAR-activated mice. Neither CAR activation nor elimination of CAR altered the biliary excretion of total BAs, but CAR activation increased the proportion of 6-OH BAs (TMCA), whereas the lack of CAR increased the excretion of TCA, TCDCA and TDCA. Serum BA concentrations did not parallel the decrease in BA concentrations in the liver in either the mice after CAR activation or mice lacking CAR. Gene expression of BA synthesis, transporter and regulator genes were mainly similar in livers of CAR-null and WT mice. In summary, CAR activation decreases primarily the 12-OH BA concentrations in liver, whereas lack of CAR decreases the concentrations of 6-OH BAs in liver. In bile, CAR activation increases the biliary excretion of 6-OH BAs, whereas absence of CAR increases the biliary excretion of 12-OH BAs and TCDCA.

Pharmacological Activation of PXR and CAR Downregulates Distinct Bile Acid-Metabolizing Intestinal Bacteria and Alters Bile Acid Homeostasis

The gut microbiome regulates important host metabolic pathways including xenobiotic metabolism and intermediary metabolism, such as the conversion of primary bile acids (BAs) into secondary BAs. The nuclear receptors pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are well-known regulators for xenobiotic biotransformation in liver. However, little is known regarding the potential effects of PXR and CAR on the composition and function of the gut microbiome. To test our hypothesis that activation of PXR and CAR regulates gut microbiota and secondary BA synthesis, 9-week-old male conventional and germ-free mice were orally gavaged with corn oil, PXR agonist PCN (75 mg/kg), or CAR agonist TCPOBOP (3 mg/kg) once daily for 4 days. PCN and TCPOBOP decreased two taxa in the Bifidobacterium genus, which corresponded with decreased gene abundance of the BA-deconjugating enzyme bile salt hydrolase. In liver and small intestinal content of germ-free mice, there was a TCPOBOP-mediated increase in total, primary, and conjugated BAs corresponding with increased Cyp7a1 mRNA. Bifidobacterium, Dorea, Peptociccaceae, Anaeroplasma, and Ruminococcus positively correlated with T-UDCA in LIC, but negatively correlated with T-CDCA in serum. In conclusion, PXR and CAR activation downregulates BA-metabolizing bacteria in the intestine and modulates BA homeostasis in a gut microbiota-dependent manner.

β-Klotho deficiency shifts the gut-liver bile acid axis and induces hepatic alterations in mice

β-Klotho (encoded by Klb) is an obligate coreceptor, mediating both fibroblast growth factor (FGF)15 and FGF21 signaling. Klb^-/- mice are refractory to metabolic FGF15 and FGF21 action and exhibit derepressed (increased) bile acid (BA) synthesis. Here, we deeply phenotyped male Klb^-/- mice on a pure C57BL/6J genetic background, fed a chow diet focusing on metabolic aspects. This aims to better understand the physiological consequences of concomitant FGF15 and FGF21 signaling deficiency, in particular on the gut-liver axis. Klb^-/- mice present permanent growth restriction independent of adiposity and energy balance. Klb^-/- mice also exhibit few changes in carbohydrate metabolism, combining normal gluco-tolerance, insulin sensitivity, and fasting response with increased gluconeogenic capacity and decreased glycogen mobilization. Livers of Klb^-/- mice reveal pathologic features, including a proinflammatory status and initiation of fibrosis. These defects are associated to a massive shift in BA composition in the enterohepatic system and blood circulation featured by a large excess of microbiota-derived deoxycholic acid, classically known for its genotoxicity in the gastrointestinal tract. In conclusion, β-Klotho is a gatekeeper of hepatic integrity through direct action (mediating FGF21 anti-inflammatory signaling) and indirect mechanisms (mediating FGF15 signaling that maintains BA level and composition).

Gender-divergent expression of lipid and bile acid metabolism related genes in adult mice offspring of dams fed a high-fat diet

Maternal high-fat diet (HFD) consumption during pregnancy and lactation affects metabolic outcomes and lipid metabolism of offspring in later life in a gender-specific manner. However, it is not known whether maternal HFD alters bile acid metabolism in adult mice offspring. The purpose of this study was to elucidate the relationship between maternal HFDinduced metabolic diseases and bile acid metabolism in male and female adult mice offspring. Female mice were fed either standard chow (C) or HFD (H) for 10 weeks pre-pregnancy until lactation. After weaning, offspring were fed a chow diet until 11 weeks of age, then challenged with either C or H diet for 4 weeks, and divided into eight groups in accordance with mother's and offspring's diets: male(M) CC, MHC, MCH, MHH, female(F) CC, FHC, FCH, and FHH. MHH showed greater weight gain compared to FHH. Liver weight was higher in MHH than in FHH. Serum total cholesterol levels were higher in MHH than in MHC, and tended to be higher in MHH than in FHH. Serum glucose levels were higher in MHH than in MHC. Hepatic triglyceride levels were higher in MHH than in MHC. Hepatic mRNA expression of bile acid uptake transporters Oatp1a1 and Oatp1b2 was increased in MHH, compared to MCH. Hepatic mRNA expression of HMGCoAR, Cyp7a1, Sult2a1, and Oatp1a4 was increased in FHH, compared to FCH. In conclusion, maternal HFD consumption may promote bile acid synthesis, sulfation and excretion in female offspring fed a HFD, which may confer resistance to HFDinduced metabolic phenotypes.

Bile Acid Uptake Transporters as Targets for Therapy

Bile acids are potent signaling molecules that regulate glucose, lipid and energy homeostasis predominantly via the bile acid receptors farnesoid X receptor (FXR) and transmembrane G protein-coupled receptor 5 (TGR5). The sodium taurocholate cotransporting polypeptide (NTCP) and the apical sodium dependent bile acid transporter (ASBT) ensure an effective circulation of (conjugated) bile acids. The modulation of these transport proteins affects bile acid localization, dynamics and signaling. The NTCP-specific pharmacological inhibitor myrcludex B inhibits hepatic uptake of conjugated bile acids. Multiple ASBT-inhibitors are already in clinical trials to inhibit intestinal bile acid uptake. Here, we discuss current insights into the consequences of targeting bile acid uptake transporters on systemic and intestinal bile acid dynamics and discuss the possible therapeutic applications that evolve as a result.

ATP11C targets basolateral bile salt transporter proteins in mouse central hepatocytes

ATP11C is a homolog of ATP8B1, both of which catalyze the transport of phospholipids in biological membranes. Mutations in ATP8B1 cause progressive familial intrahepatic cholestasis type1 in humans, which is characterized by a canalicular cholestasis. Mice deficient in ATP11C are characterized by a conjugated hyperbilirubinemia and an unconjugated hypercholanemia. Here, we have studied the hypothesis that ATP11C deficiency interferes with basolateral uptake of unconjugated bile salts, a process mediated by organic anion-transporting polypeptide (OATP) 1B2. ATP11C localized to the basolateral membrane of central hepatocytes in the liver lobule of control mice. In ATP11C-deficient mice, plasma total bilirubin levels were 6-fold increased, compared to control, of which ∼65% was conjugated and ∼35% unconjugated. Plasma total bile salts were 10-fold increased and were mostly present as unconjugated species. Functional studies in ATP11C-deficient mice indicated that hepatic uptake of unconjugated bile salts was strongly impaired whereas uptake of conjugated bile salts was unaffected. Western blotting and immunofluorescence analysis demonstrated near absence of basolateral bile salt uptake transporters OATP1B2, OATP1A1, OATP1A4, and Na(+) -taurocholate-cotransporting polypeptide only in central hepatocytes of ATP11C-deficient liver. In vivo application of the proteasome inhibitor, bortezomib, partially restored expression of these proteins, but not their localization. Furthermore, we observed post-translational down-regulation of ATP11C protein in livers from cholestatic mice, which coincided with reduced OATP1B2 levels.

CONCLUSIONS

ATP11C is essential for basolateral membrane localization of multiple bile salt transport proteins in central hepatocytes and may act as a gatekeeper to prevent hepatic bile salt overload. Conjugated hyperbilirubinemia and unconjugated hypercholanemia and loss of OATP expression in ATP11C-deficient liver strongly resemble the characteristics of Rotor syndrome, suggesting that mutations in ATP11C can predispose to Rotor syndrome. (Hepatology 2016;64:161-174).

Effect of nine diets on xenobiotic transporters in livers of mice

1. Lifestyle diseases are often caused by inappropriate nutrition habits and attempted to be treated by polypharmacotherapy. Therefore, it is important to determine whether differences in diet affect the disposition of drugs. Xenobiotic transporters in the liver are essential in drug disposition. 2. In the current study, mice were fed one of nine diets for 3 weeks. The mRNAs of 23 known xenobiotic transporters in livers of mice were quantified by microarray analysis, and validated by branched DNA assay. The mRNAs of 15 transporters were altered by at least one diet. Diet-restriction (10) and the atherogenic diet (10) altered the expression of the most number of transporters, followed by western diet (8), high-fat diet (4), lab chow (2), high-fructose diet (2) and EFA-deficient diet (2), whereas the low n-3 FA diet had no effect on these transporters. Seven of the 11 xenobiotic transporters in the Slc family, three of four in the Abcb family, two of four in the Abcc family and all three in the Abcg family were changed significantly. 3. This first comprehensive study indicates that xenobiotic transporters are altered by diet, and suggests there are likely diet-drug interactions due to changes in the expression of drug transporters.

Intestinal transport and metabolism of bile acids

In addition to their classical roles as detergents to aid in the process of digestion, bile acids have been identified as important signaling molecules that function through various nuclear and G protein-coupled receptors to regulate a myriad of cellular and molecular functions across both metabolic and nonmetabolic pathways. Signaling via these pathways will vary depending on the tissue and the concentration and chemical structure of the bile acid species. Important determinants of the size and composition of the bile acid pool are their efficient enterohepatic recirculation, their host and microbial metabolism, and the homeostatic feedback mechanisms connecting hepatocytes, enterocytes, and the luminal microbiota. This review focuses on the mammalian intestine, discussing the physiology of bile acid transport, the metabolism of bile acids in the gut, and new developments in our understanding of how intestinal metabolism, particularly by the gut microbiota, affects bile acid signaling.

Age- and sex-related differences of organic anion-transporting polypeptide gene expression in livers of rats

Organic anion-transporting polypeptides (Oatps) play important roles in transporting endogenous substances and xenobiotics into the liver and are implicated in drug-drug interactions. Many factors could influence their expression and result in alterations in drug disposition, efficacy and toxicity. This study was aimed to examine the development-, aging-, and sex-dependent Oatps expression in livers of rats. The livers from SD rats during development (-2, 1, 7, 14, 21, 28, 35, and 60 d) and aging (60, 180, 540 and/or 800 d) were collected and total RNAs were extracted, purified, and subjected to real-time PCR analysis. Total proteins were extracted for western-blot analysis. Results showed that Oatp1a1, Oatp1a4, Oatp1a5 and Oatp1b2 were all hardly detectable in fetal rat livers, low at birth, rapidly increased after weaning (21 d), and reached the peak at 60 d. The Oatps remained stable during the age between 60-180 d, and decreased at elderly (540 and/or 800 d). After birth, Oatp1a1, Oatp1a4, and Oatp1b2 were all highly expressed in liver, in contrast, Oatp1a5 expression was low. Oatp expressions are male-predominant in rat livers. In the livers of aged rats, the Oatp expression decreased and shared a consistent ontogeny pattern at the mRNA and protein level. In conclusion, this study showed that in rat liver, Oatp1a1, Oatp1a4, Oatp1a5 and Oatp1b2 gene expressions are influenced by age and gender, which could provide a basis of individual variation in drug transport, metabolism and toxicity in children, elderly and women.

Decreased bile-acid synthesis in livers of hepatocyte-conditional NADPH-cytochrome P450 reductase-null mice results in increased bile acids in serum

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.