Abcb11 deficiency induces cholestasis coupled to impaired β-fatty acid oxidation in mice

Sirtuin 1 activation alleviates cholestatic liver injury in a cholic acid-fed mouse model of cholestasis

Sirtuin1 (Sirt1; mammalian homolog of Saccharomyces cerevisiae enzyme Sir2) is a transcriptional and transactivational regulator of murine farnesoid X receptor (Fxr), which is the primary bile acid (BA) sensor, and critical regulator of BA metabolism in physiological and pathophysiological conditions. Previous studies have suggested compromised Sirt1 expression in rodent models of cholestatic liver injury. We hypothesized that Sirt1 could be potentially targeted to alleviate cholestatic liver injury. In cultured primary human hepatocytes, SIRT1 messenger RNA was down-regulated after GCA treatment, potentially through induction of microRNA (miR)-34a, whereas tauroursodeoxycholic acid induced SIRT1 expression without affecting miR-34a expression. Sirt1 expression was also significantly down-regulated in three mouse models of liver injury (bile duct ligation, 1% cholic acid [CA] fed, and the Mdr2^-/- mouse). Mice fed CA diet also demonstrated hepatic FXR hyperacetylation and induction of the Janus kinase/p53 pathway. Mice fed a CA diet and concurrently administered the Sirt1 activator, SRT1720 (50 mg/kg/day, orally), demonstrated 40% and 45% decrease in plasma alanine aminotransferase and BA levels, respectively. SRT1720 increased hepatic BA hydrophilicity by increasing tri- and tetrahydroxylated and decreasing the dihydroxylated BA fraction. SRT1720 administration also inhibited hepatic BA synthesis, potentially through ileal fibroblast growth factor 15- and Fxr-mediated inhibition of cytochrome p450 (Cyp) 7a1 and Cyp27a1, along with increased hepatic BA hydroxylation in association with Cyp2b10 induction. SRT1720 administration significantly induced renal multidrug resistance-associated protein 2 and 4, peroxisome proliferator-activated receptor gamma coactivator 1-α, and constitutive androstance receptor expression along with ∼2-fold increase in urinary BA concentrations.

CONCLUSION

SRT1720 administration alleviates cholestatic liver injury in mice by increasing hydrophilicity of hepatic BA composition and decreasing plasma BA concentration through increased BA excretion into urine. Thus, use of small-molecule activators of Sirt1 presents a novel therapeutic target for cholestatic liver injury. (Hepatology 2016;64:2151-2164).

Maternal bile acid transporter deficiency promotes neonatal demise

Intrahepatic cholestasis of pregnancy (ICP) is associated with adverse neonatal survival and is estimated to impact between 0.4 and 5% of pregnancies worldwide. Here we show that maternal cholestasis (due to Abcb11 deficiency) produces neonatal death among all offspring within 24 h of birth due to atelectasis-producing pulmonary hypoxia, which recapitulates the neonatal respiratory distress of human ICP. Neonates of Abcb11-deficient mothers have elevated pulmonary bile acids and altered pulmonary surfactant structure. Maternal absence of Nr1i2 superimposed on Abcb11 deficiency strongly reduces maternal serum bile acid concentrations and increases neonatal survival. We identify pulmonary bile acids as a key factor in the disruption of the structure of pulmonary surfactant in neonates of ICP. These findings have important implications for neonatal respiratory failure, especially when maternal bile acids are elevated during pregnancy, and highlight potential pathways and targets amenable to therapeutic intervention to ameliorate this condition.

Current Concepts in Drug-Induced Bile Salt Export Pump (BSEP) Interference

Numerous drugs have been shown to inhibit the activity of the Bile Salt Export Pump (BSEP in humans, Bsep in animals), and this is now considered to be one of several mechanisms by which idiosyncratic drug-induced liver injury (DILI) may be initiated in susceptible patients. The potential importance of BSEP inhibition by drugs has been recognized by the European Medicines Agency and the International Transporter Consortium, who have recommended that it should be evaluated during drug development when evidence of cholestatic liver injury has been observed in nonclinical safety studies or in human clinical trials. In addition, some pharmaceutical companies have proposed evaluation and minimization of BSEP inhibition during drug discovery, when there is a chemical choice, to help reduce DILI risk. The methods that can be used to assess and quantify BSEP inhibition, and key gaps in our current understanding of the relationship between this process and DILI, are discussed.

Dietary nicotinic acid supplementation ameliorates chronic alcohol-induced fatty liver in rats

BACKGROUND

Alcohol abuse frequently causes niacin deficiency in association with the development of alcoholic liver disease. The objective of the present study was to determine whether dietary nicotinic acid (NA) deficiency exaggerates and whether dietary NA supplementation alleviates alcohol-induced fatty liver.

METHODS

Male Sprague-Dawley rats were pair-fed with 4 isocaloric liquid diets: control, ethanol (EtOH), EtOH with dietary NA deficiency, and EtOH with dietary NA supplementation, respectively, for 8 weeks. The control and EtOH diets contained normal levels of NA (7.5 mg/l). Dietary NA deficiency (0 mg NA/l) was achieved by removing NA from the vitamin mix, while NA was added to the liquid diet at 750 mg/l for dietary NA supplementation.

RESULTS

Chronic EtOH feeding induced significant lipid accumulation in the liver, which was not worsened by dietary NA deficiency, but was ameliorated by dietary NA supplementation. Liver total NAD, NAD(+) , and NADH levels were remarkably higher in the NA supplemented group than the NA deficient or EtOH alone groups. Dietary NA supplementation to EtOH-fed rats increased the protein levels of hepatic cytochrome P450 4A1 (CYP4A1) and acyl-coenzyme A oxidase 1 without affecting their mRNA levels. Interestingly, we found dietary NA supplementation reduced the ubiquitination level of CYP4A1. In addition, hepatic fatty acid synthase expression was reduced, while the serum β-hydroxybutyrate and adiponectin concentrations were significantly elevated by dietary NA supplementation. Moreover, dietary NA supplementation modulated EtOH-perturbed liver and serum metabolite profiles.

CONCLUSIONS

These results demonstrate that alcoholic fatty liver was not exaggerated by dietary NA deficiency, but was ameliorated by dietary NA supplementation. Increased hepatic fatty acid oxidation and decreased hepatic de novo lipogenesis contribute to the effects of dietary NA supplementation.

Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Microbiome remodelling leads to inhibition of intestinal farnesoid X receptor signalling and decreased obesity

The antioxidant tempol reduces obesity in mice. Here we show that tempol alters the gut microbiome by preferentially reducing the genus Lactobacillus and its bile salt hydrolase (BSH) activity leading to the accumulation of intestinal tauro-β-muricholic acid (T-β-MCA). T-β-MCA is an farnesoid X receptor (FXR) nuclear receptor antagonist, which is involved in the regulation of bile acid, lipid and glucose metabolism. Its increased levels during tempol treatment inhibit FXR signalling in the intestine. High-fat diet-fed intestine-specific Fxr-null (Fxr(ΔIE)) mice show lower diet-induced obesity, similar to tempol-treated wild-type mice. Further, tempol treatment does not decrease weight gain in Fxr(ΔIE) mice, suggesting that the intestinal FXR mediates the anti-obesity effects of tempol. These studies demonstrate a biochemical link between the microbiome, nuclear receptor signalling and metabolic disorders, and suggest that inhibition of FXR in the intestine could be a target for anti-obesity drugs.

LC-MS-based metabolomics: an update

Liquid chromatography-mass spectrometry (LC-MS)-based metabolomics can have a major impact in multiple research fields, especially when combined with other technologies, such as stable isotope tracers and genetically modified mice. This review highlights recent applications of metabolomic technology in the study of xenobiotic metabolism and toxicity, and the understanding of disease pathogenesis and therapeutics. Metabolomics has been employed to study metabolism of noscapine, an aryl hydrocarbon receptor antagonist, and to determine the mechanisms of liver toxicities of rifampicin and isoniazid, trichloroethylene, and gemfibrozil. Metabolomics-based insights into the pathogenesis of inflammatory bowel disease, alcohol-induced liver diseases, non-alcoholic steatohepatitis, and farnesoid X receptor signaling pathway-based therapeutic target discovery will also be discussed. Limitations in metabolomics technology such as sample preparation and lack of LC-MS databases and metabolite standards, need to be resolved in order to improve and broaden the application of metabolomic studies.

Defective canalicular transport and toxicity of dietary ursodeoxycholic acid in the abcb11-/- mouse: transport and gene expression studies

The bile salt export pump (BSEP), encoded by the abcb11 gene, is the major canalicular transporter of bile acids from the hepatocyte. BSEP malfunction in humans causes bile acid retention and progressive liver injury, ultimately leading to end-stage liver failure. The natural, hydrophilic, bile acid ursodeoxycholic acid (UDCA) is efficacious in the treatment of cholestatic conditions, such as primary biliary cirrhosis and cholestasis of pregnancy. The beneficial effects of UDCA include promoting bile flow, reducing hepatic inflammation, preventing apoptosis, and maintaining mitochondrial integrity in hepatocytes. However, the role of BSEP in mediating UDCA efficacy is not known. Here, we used abcb11 knockout mice (abcb11-/-) to test the effects of acute and chronic UDCA administration on biliary secretion, bile acid composition, liver histology, and liver gene expression. Acutely infused UDCA, or its taurine conjugate (TUDC), was taken up by the liver but retained, with negligible biliary output, in abcb11-/- mice. Feeding UDCA to abcb11-/- mice led to weight loss, retention of bile acids, elevated liver enzymes, and histological damage to the liver. Semiquantitative RT-PCR showed that genes encoding Mdr1a and Mdr1b (canalicular) as well as Mrp4 (basolateral) transporters were upregulated in abcb11-/- mice. We concluded that infusion of UDCA and TUDC failed to induce bile flow in abcb11-/- mice. UDCA fed to abcb11-/- mice caused liver damage and the appearance of biliary tetra- and penta-hydroxy bile acids. Supplementation with UDCA in the absence of Bsep caused adverse effects in abcb11-/- mice.

Metabolomics reveals that tumor xenografts induce liver dysfunction

Metabolomics, based on ultraperformance liquid chromatography coupled with electrospray ionization quadrupole mass spectrometry, was used to explore metabolic signatures of tumor growth in mice. Urine samples were collected from control mice and mice injected with squamous cell carcinoma (SCCVII) tumor cells. When tumors reached ∼2 cm, all mice were killed and blood and liver samples collected. The urine metabolites hexanoylglycine, nicotinamide 1-oxide, and 11β,20α-dihydroxy-3-oxopregn-4-en-21-oic acid were elevated in tumor-bearing mice, as was asymmetric dimethylarginine, a biomarker for oxidative stress. Interestingly, SCCVII tumor growth resulted in hepatomegaly, reduced albumin/globulin ratios, and elevated serum triglycerides, suggesting liver dysfunction. Alterations in liver metabolites between SCCVII-tumor-bearing and control mice confirmed the presence of liver injury. Hepatic mRNA analysis indicated that inflammatory cytokines, tumor necrosis factor α, and transforming growth factor β were enhanced in SCCVII-tumor-bearing mice, and the expression of cytochromes P450 was decreased in tumor-bearing mice. Further, genes involved in fatty acid oxidation were decreased, suggesting impaired fatty acid oxidation in SCCVII-tumor-bearing mice. Additionally, activated phospholipid metabolism and a disrupted tricarboxylic acid cycle were observed in SCCVII-tumor-bearing mice. These data suggest that tumor growth imposes a global inflammatory response that results in liver dysfunction and underscore the use of metabolomics to temporally examine these changes and potentially use metabolite changes to monitor tumor treatment response.

Stable isotope- and mass spectrometry-based metabolomics as tools in drug metabolism: a study expanding tempol pharmacology

The application of mass spectrometry-based metabolomics in the field of drug metabolism has yielded important insights not only into the metabolic routes of drugs but has provided unbiased, global perspectives of the endogenous metabolome that can be useful for identifying biomarkers associated with mechanism of action, efficacy, and toxicity. In this report, a stable isotope- and mass spectrometry-based metabolomics approach that captures both drug metabolism and changes in the endogenous metabolome in a single experiment is described. Here the antioxidant drug tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) was chosen because its mechanism of action is not completely understood and its metabolic fate has not been studied extensively. Furthermore, its small size (MW = 172.2) and chemical composition (C(9)H(18)NO(2)) make it challenging to distinguish from endogenous metabolites. In this study, mice were dosed with tempol or deuterated tempol (C(9)D(17)HNO(2)) and their urine was profiled using ultraperformance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry. Principal component analysis of the urinary metabolomics data generated a Y-shaped scatter plot containing drug metabolites (protonated and deuterated) that were clearly distinct from the endogenous metabolites. Ten tempol drug metabolites, including eight novel metabolites, were identified. Phase II metabolism was the major metabolic pathway of tempol in vivo, including glucuronidation and glucosidation. Urinary endogenous metabolites significantly elevated by tempol treatment included 2,8-dihydroxyquinoline (8.0-fold, P < 0.05) and 2,8-dihydroxyquinoline-β-d-glucuronide (6.8-fold, P < 0.05). Urinary endogenous metabolites significantly attenuated by tempol treatment including pantothenic acid (1.3-fold, P < 0.05) and isobutrylcarnitine (5.3-fold, P < 0.01). This study underscores the power of a stable isotope- and mass spectrometry-based metabolomics in expanding the view of drug pharmacology.

Repression of hepatobiliary transporters and differential regulation of classic and alternative bile acid pathways in mice during pregnancy

During pregnancy, proper hepatobiliary transport and bile acid synthesis protect the liver from cholestatic injury and regulate the maternal and fetal exposure to bile acids, drugs, and environmental chemicals. The objective of this study was to determine the temporal messenger RNA (mRNA) and protein profiles of uptake and efflux transporters as well as bile acid synthetic and conjugating enzymes in livers from virgin and pregnant mice on gestational days (GD) 7, 11, 14, and 17 and postnatal days (PND) 1, 15, and 30. Compared with virgins, the mRNAs of most transporters were reduced approximately 50% in pregnant dams between GD11 and 17. Western blot and immunofluorescence staining confirmed the downregulation of Mrp3, 6, Bsep, and Ntcp proteins. One day after parturition, the mRNAs of many uptake and efflux hepatobiliary transporters remained low in pregnant mice. By PND30, the mRNAs of all transporters returned to virgin levels. mRNAs of the bile acid synthetic enzymes in the classic pathway, Cyp7a1 and 8b1, increased in pregnant mice, whereas mRNA and protein expression of enzymes in the alternative pathway of bile acid synthesis (Cyp27a1 and 39a1) and conjugating enzymes (Bal and Baat) decreased. Profiles of transporter and bile acid metabolism genes likely result from coordinated downregulation of transcription factor mRNA (CAR, LXR, PXR, PPARα, FXR) in pregnant mice on GD14 and 17. In conclusion, pregnancy caused a global downregulation of most hepatic transporters, which began as early as GD7 for some genes and was maximal by GD14 and 17, and was inversely related to increasing concentrations of circulating 17β-estradiol and progesterone as pregnancy progressed.