Human and rat bile acid-CoA:amino acid N-acyltransferase are liver-specific peroxisomal enzymes: implications for intracellular bile salt transport

Structural insights into human ABCD3-mediated peroxisomal acyl-CoA translocation

Human ABC transporters ABCD1-3 are all localized on the peroxisomal membrane and participate in the β-oxidation of fatty acyl-CoAs, but they differ from each other in substrate specificity. The transport of branched-chain fatty acids from cytosol to peroxisome is specifically driven by ABCD3, dysfunction of which causes severe liver diseases such as hepatosplenomegaly. Here we report two cryogenic electron microscopy (cryo-EM) structures of ABCD3 bound to phytanoyl-CoA and ATP at resolutions of 2.9 Å and 3.2 Å, respectively. A pair of phytanoyl-CoA molecules were observed in ABCD3, each binding to one transmembrane domain (TMD), which is distinct from our previously reported structure of ABCD1, where each fatty acyl-CoA molecule strongly crosslinks two TMDs. Upon ATP binding, ABCD3 exhibits a conformation that is open towards the peroxisomal matrix, leaving two extra densities corresponding to two CoA molecules deeply embedded in the translocation cavity. Structural analysis combined with substrate-stimulated ATPase activity assays indicated that the present structures might represent two states of ABCD3 in the transport cycle. These findings advance our understanding of fatty acid oxidation and the molecular pathology of related diseases.

Sex-specific effects of gut microbiome on shaping bile acid metabolism

Gut microbiome is a group of microorganisms that plays important roles in contributing to health and diseases. These bacterial compositions have been demonstrated to impact bile acids (BAs) profiles, either by directly metabolizing primary BAs to secondary BAs or indirect ways through host metabolism by influencing BAs synthesis, transportation and conjugation in liver. It has been observed sexually dimorphic gut microbiome and bile acids composition, with variations in expression levels of bile acid metabolizing genes in the liver. However, associations between sex-specific differences in gut microbiome and BAs profiles are not well understood. This study aimed to investigate whether gut microbiome could influence BAs profiles in host in a sexspecific manner. We transplanted cecum feces of male and female C57BL/6 mice to male mice and measured BAs concentrations in feces, serum and liver samples 7 days after fecal transplantation. We found different BAs profiles between mice with male and female gut microbiome, including altering levels and proportions of secondary BAs. We also observed varied expression levels of genes related to bile acid metabolism in the liver and distal ileum. Our results highlight sex-specific effects of gut microbiome on shaping bile acid metabolism through gut bacteria and regulation of host genes.

The underappreciated diversity of bile acid modifications

The repertoire of modifications to bile acids and related steroidal lipids by host and microbial metabolism remains incompletely characterized. To address this knowledge gap, we created a reusable resource of tandem mass spectrometry (MS/MS) spectra by filtering 1.2 billion publicly available MS/MS spectra for bile-acid-selective ion patterns. Thousands of modifications are distributed throughout animal and human bodies as well as microbial cultures. We employed this MS/MS library to identify polyamine bile amidates, prevalent in carnivores. They are present in humans, and their levels alter with a diet change from a Mediterranean to a typical American diet. This work highlights the existence of many more bile acid modifications than previously recognized and the value of leveraging public large-scale untargeted metabolomics data to discover metabolites. The availability of a modification-centric bile acid MS/MS library will inform future studies investigating bile acid roles in health and disease.

Microbially catalyzed conjugation of GABA and tyramine to bile acids

Bile acids (BAs) are cholesterol-derived molecules that aid in digestion and nutrient absorption, regulate host metabolic processes, and influence physiology of the gut microbiota. Both the host and its microbiome contribute to enzymatic modifications that shape the chemical diversity of BAs in the gut. Several bacterial species have been reported to conjugate standard amino acids to BAs, but it was not known if bacteria conjugate BAs to other amine classes. Here, we show that Bacteroides fragilis strain P207, isolated from a bacterial bloom in the J-pouch of a patient with ulcerative colitis pouchitis, conjugates standard amino acids and the neuroactive amines γ-aminobutyric acid (GABA) and tyramine to deoxycholic acid. We extended this analysis to other human gut isolates and identified species that are competent to conjugate GABA and tyramine to primary and secondary BAs, and further identified diverse BA-GABA and BA-tyramine amides in human stool. A longitudinal metabolomic analysis of J-pouch contents of the patient from whom B. fragilis P207 was isolated revealed highly reduced levels of secondary bile acids and a shifting BA amide profile before, during, and after onset of pouchitis, including temporal changes in several BA-GABA amides. Treatment of pouchitis with ciprofloxacin was associated with a marked reduction of nearly all BA amides in the J-pouch. Our study expands the known repertoire of conjugated bile acids produced by bacteria to include BA conjugates to GABA and tyramine and demonstrates that these molecules are present in the human gut. IMPORTANCE BAs are modified in multiple ways by host enzymes and the microbiota to produce a chemically diverse set of molecules that assist in the digestive process and impact many physiological functions. This study reports the discovery of bacterial species that conjugate the neuroactive amines, GABA and tyramine, to primary and secondary BAs. We further present evidence that BA-GABA and BA-tyramine conjugates are present in the human gut, and document a shifting BA-GABA profile in a human pouchitis patient before, during, and after inflammation and antibiotic treatment. GABA and tyramine are common metabolic products of the gut microbiota and potent neuroactive molecules. GABA- and tyramine-conjugated BAs may influence receptor-mediated regulatory mechanisms of humans and their gut microbes, and absorption of these molecules and their entry into enterohepatic circulation may impact host physiology at distal tissue sites. This study defines new conjugated bile acids in the human gut.

Microbially-catalyzed conjugation of GABA and tyramine to bile acids

Bile acids (BAs) are cholesterol-derived molecules that aid in digestion and nutrient absorption, regulate host metabolic processes, and influence physiology of the gut microbiota. Both the host and its microbiome contribute to enzymatic modifications that shape the chemical diversity of BAs in the gut. Several bacterial species have been reported to conjugate standard amino acids to BAs, but it was not known if bacteria conjugate BAs to other amine classes. Here, we show that Bacteroides fragilis strain P207, isolated from a bacterial bloom in the J-pouch of a patient with ulcerative colitis (UC) pouchitis, conjugates standard amino acids and the neuroactive amines γ-aminobutyric acid (GABA) and tyramine to deoxycholic acid. We extended this analysis to other human gut isolates and identified species that are competent to conjugate GABA and tyramine to primary and secondary BAs, and further identified diverse BA-GABA and BA-tyramine amides in human stool. A longitudinal metabolomic analysis of J-pouch contents of the patient from whom B. fragilis P207 was isolated revealed highly reduced levels of secondary bile acids and a shifting BA amide profile before, during, and after onset of pouchitis, including temporal changes in several BA-GABA amides. Treatment of pouchitis with ciprofloxacin was associated with a marked reduction of nearly all BA amides in the J-pouch. Our study expands the known repertoire of conjugated bile acids produced by bacteria to include BA conjugates to GABA and tyramine and demonstrates that these molecules are present in the human gut.

Identification of bile acid-CoA:amino acid N-acyltransferase as the hepatic N-acyl taurine synthase for polyunsaturated fatty acids

N-acyl taurines (NATs) are bioactive lipids with emerging roles in glucose homeostasis and lipid metabolism. The acyl chains of hepatic and biliary NATs are enriched in polyunsaturated fatty acids (PUFAs). Dietary supplementation with a class of PUFAs, the omega-3 fatty acids, increases their cognate NATs in mice and humans. However, the synthesis pathway of the PUFA-containing NATs remains undiscovered. Here, we report that human livers synthesize NATs and that the acyl-chain preference is similar in murine liver homogenates. In the mouse, we found that hepatic NAT synthase activity localizes to the peroxisome and depends upon an active-site cysteine. Using unbiased metabolomics and proteomics, we identified bile acid-CoA:amino acid N-acyltransferase (BAAT) as the likely hepatic NAT synthase in vitro. Subsequently, we confirmed that BAAT knockout livers lack up to 90% of NAT synthase activity and that biliary PUFA-containing NATs are significantly reduced compared with wildtype. In conclusion, we identified the in vivo PUFA-NAT synthase in the murine liver and expanded the known substrates of the bile acid-conjugating enzyme, BAAT, beyond classic bile acids to the synthesis of a novel class of bioactive lipids.

A Dual Coverage Monitoring of the Bile Acids Profile in the Liver-Gut Axis throughout the Whole Inflammation-Cancer Transformation Progressive: Reveal Hepatocellular Carcinoma Pathogenesis

Hepatocellular carcinoma (HCC) is the terminal phase of multiple chronic liver diseases, and evidence supports chronic uncontrollable inflammation being one of the potential mechanisms leading to HCC formation. The dysregulation of bile acid homeostasis in the enterohepatic circulation has become a hot research issue concerning revealing the pathogenesis of the inflammatory-cancerous transformation process. We reproduced the development of HCC through an N-nitrosodiethylamine (DEN)-induced rat model in 20 weeks. We achieved the monitoring of the bile acid profile in the plasma, liver, and intestine during the evolution of "hepatitis-cirrhosis-HCC" by using an ultra-performance liquid chromatography-tandem mass spectrometer for absolute quantification of bile acids. We observed differences in the level of primary and secondary bile acids both in plasma, liver, and intestine when compared to controls, particularly a sustained reduction of intestine taurine-conjugated bile acid level. Moreover, we identified chenodeoxycholic acid, lithocholic acid, ursodeoxycholic acid, and glycolithocholic acid in plasma as biomarkers for early diagnosis of HCC. We also identified bile acid-CoA:amino acid N-acyltransferase (BAAT) by gene set enrichment analysis, which dominates the final step in the synthesis of conjugated bile acids associated with the inflammatory-cancer transformation process. In conclusion, our study provided comprehensive bile acid metabolic fingerprinting in the liver-gut axis during the inflammation-cancer transformation process, laying the foundation for providing a new perspective for the diagnosis, prevention, and treatment of HCC.

Investigation of dichlorodiphenyltrichloroethane (DDT) on xenobiotic enzyme disruption and metabolomic bile acid biosynthesis in DDT-sprayed areas using wild rats

Dichlorodiphenyltrichloroethane (DDT) is an organochlorine insecticide used worldwide. Several studies have reported the toxic effects of DDT and its metabolites on steroid hormone biosynthesis; however, its environmental effects are not well understood. This study examined wild rats collected in DDT-sprayed areas of South Africa and quantified plasma metabolites using liquid chromatography quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS). Fold change analysis of the metabolome revealed the effect of DDT on bile acid biosynthesis. Gene expression of the related enzyme in rat liver samples was also quantified. Significant association was found between DDT and gene expression levels related to constitutive androstane receptor mediated enzymes, such as Cyp2b1 in rat livers. However, our results could not fully demonstrate that enzymes related to bile acid biosynthesis were strongly affected by DDT. The correlation between DDT concentration and gene expression involved in steroid hormone synthesis in testis was also evaluated; however, no significant correlation was found. The disturbance of metabolic enzymes occurred in rat liver in the target area. Our results suggest that DDT exposure affects gene expression in wild rats living in DDT-sprayed areas. Therefore, there is a need for DDT toxicity evaluation in mammals living in DDT-sprayed areas. We could not find an effective biomarker that could reflect the mechanism of DDT exposure; however, this approach can provide new insights for future research to evaluate DDT effects in sprayed areas.

The physiological functions of human peroxisomes

Peroxisomes are subcellular organelles that play a central role in human physiology by catalyzing a range of unique metabolic functions. The importance of peroxisomes for human health is exemplified by the existence of a group of usually severe diseases caused by an impairment in one or more peroxisomal functions. Among others these include the Zellweger spectrum disorders, X-linked adrenoleukodystrophy, and Refsum disease. To fulfill their role in metabolism, peroxisomes require continued interaction with other subcellular organelles including lipid droplets, lysosomes, the endoplasmic reticulum, and mitochondria. In recent years it has become clear that the metabolic alliance between peroxisomes and other organelles requires the active participation of tethering proteins to bring the organelles physically closer together, thereby achieving efficient transfer of metabolites. This review intends to describe the current state of knowledge about the metabolic role of peroxisomes in humans, with particular emphasis on the metabolic partnership between peroxisomes and other organelles and the consequences of genetic defects in these processes. We also describe the biogenesis of peroxisomes and the consequences of the multiple genetic defects therein. In addition, we discuss the functional role of peroxisomes in different organs and tissues and include relevant information derived from model systems, notably peroxisomal mouse models. Finally, we pay particular attention to a hitherto underrated role of peroxisomes in viral infections.

Dysfunctional peroxisomal lipid metabolisms and their ocular manifestations

Retina is rich in lipids and dyslipidemia causes retinal dysfunction and eye diseases. In retina, lipids are not only important membrane component in cells and organelles but also fuel substrates for energy production. However, our current knowledge of lipid processing in the retina are very limited. Peroxisomes play a critical role in lipid homeostasis and genetic disorders with peroxisomal dysfunction have different types of ocular complications. In this review, we focus on the role of peroxisomes in lipid metabolism, including degradation and detoxification of very-long-chain fatty acids, branched-chain fatty acids, dicarboxylic acids, reactive oxygen/nitrogen species, glyoxylate, and amino acids, as well as biosynthesis of docosahexaenoic acid, plasmalogen and bile acids. We also discuss the potential contributions of peroxisomal pathways to eye health and summarize the reported cases of ocular symptoms in patients with peroxisomal disorders, corresponding to each disrupted peroxisomal pathway. We also review the cross-talk between peroxisomes and other organelles such as lysosomes, endoplasmic reticulum and mitochondria.

Bile acid metabolism dysregulation associates with cancer cachexia: roles of liver and gut microbiome

BACKGROUND

Cancer cachexia is a multifactorial metabolic syndrome in which bile acid (BA) metabolism might be involved. The aim of the present study was to clarify the contribution of liver and gut microbiota to BA metabolism disturbance in cancer cachexia and to check the possibility of targeting BA metabolism using agents such as tauroursodeoxycholic acid (TUDCA) for cancer cachexia therapy.

METHODS

The BA profiles in liver, intestine, and serum of mice with cancer cachexia induced by inoculation of colon C26 tumour cells were analysed using metabolomics methods and compared with that of control mice. Proteomic analysis of liver protein expression profile and 16S rRNA gene sequencing analysis of gut microbiota composition in cancer cachexia mice were conducted. Expression levels of genes related to farnesoid X receptor (FXR) signalling pathway in the intestine and liver tissues were analysed using RT-PCR analysis. The BA profiles in serum of clinical colon cancer patients with or without cachexia were also analysed and compared with that of healthy volunteers. The effects of TUDCA in treating cancer cachexia mice were observed.

RESULTS

In the liver of cancer cachexia mice, expression of BA synthesis enzymes was inhibited while the amount of total BAs increased (P < 0.05). The ratios of conjugated BAs/un-conjugated BAs significantly increased in cancer cachexia mice liver (P < 0.01). Gut microbiota dysbiosis such as decrease in Lachnospiraceae and increase in Enterobacteriaceae was observed in the intestine of cancer cachexia mice, and microbial metabolism of BAs was reduced. Increase in expression of FGF15 in intestine (P < 0.01) suggested the activation of FXR signalling pathway which might contribute to the regulation of BA synthesis enzymes, transporters, and metabolic enzymes. Increase in the BA conjugation was observed in the serum of cancer cachexia mice. Results of clinical patients showed changes in BA metabolism, especially the increase in BA conjugation, and also suggested compensatory mechanism in BA metabolism regulation. Oral administration of 50 mg/kg TUDCA could significantly ameliorate the decrease in body weight (P < 0.001), muscle loss (P < 0.001), and atrophy of heart and liver (P < 0.05) in cancer cachexia mice without influence on tumour growth.

CONCLUSIONS

Bile acid metabolism dysregulation such as decrease in BA synthesis, increase in BA conjugation, and decrease in BA microbial metabolism was involved in development of cancer cachexia in mice. Targeting BA metabolism using agents such as TUDCA might be helpful for cancer cachexia therapy.

CYP3A deficiency alters bile acid homeostasis and leads to changes in hepatic susceptibility in rats

Cytochrome P450 3A (CYP3A) as an important enzyme metabolizes many drugs and a variety of endogenous substances. Bile acids (BA) regulate physiological function by activating BA receptors. In this study, CYP3A1/2 gene knockout (KO) and wild-type (WT) rats were used to investigate the regulatory effects of CYP3A on BA homeostasis and liver function. Compared with WT rats, BA concentrations in serum, liver and small intestine of CYP3A1/2 KO rats increased significantly, which was due to the decrease of catabolism and the increase of synthesis. In particular, the composition of serum BA (overall hydrophobicity) presented an age- and CYP3A-dependent manner. With the aging of WT rats, the serum BA became more hydrophobic, while this trend was delayed in CYP3A1/2 KO rats. Moreover, the level of serum total cholesterol, the precursor of BA synthesis, decreased by about 20% in CYP3A1/2 KO rats, which is due to the low synthesis but high biotransformation rate. The increase of BA pool further led to the change of transcription level of BA receptor in liver (pregnane X receptor) and small intestine (Takeda G-protein receptor 5), and affected the function and morphology of CYP3A1/2 KO rat liver. In conclusion, CYP3A is a key regulator of BA homeostasis in rats, especially in regulating BA pool size, composition and balance of anabolism, and prevents susceptibility to hepatotoxicity under BA overload.

Collagen release by human hepatic stellate cells requires vitamin C and is efficiently blocked by hydroxylase inhibition

Liver fibrosis is characterized by the accumulation of extracellular matrix proteins, mainly composed of collagen. Hepatic stellate cells (HSCs) mediate liver fibrosis by secreting collagen. Vitamin C (ascorbic acid) is a cofactor of prolyl-hydroxylases that modify newly synthesized collagen on the route for secretion. Unlike most animals, humans cannot synthesize ascorbic acid and its role in liver fibrosis remains unclear. Here, we determined the effect of ascorbic acid and prolyl-hydroxylase inhibition on collagen production and secretion by human HSCs. Primary human HSCs (p-hHSCs) and the human HSCscell line LX-2 were treated with ascorbic acid, transforming growth factor-beta (TGFβ) and/or the pan-hydroxylase inhibitor dimethyloxalylglycine (DMOG). Expression of collagen-I was analyzed by RT-qPCR (COL1A1), Western blotting, and immunofluorescence microscopy. Collagen secretion was determined in the medium by Western blotting for collagen-I and by HPLC for hydroxyproline concentrations. Expression of solute carrier family 23 members 1 and 2 (SLC23A1/SLC23A2), encoding sodium-dependent vitamin C transporters 1 and 2 (SVCT1/SVCT2) was quantified in healthy and cirrhotic human tissue. In the absence of ascorbic acid, collagen-I accumulated intracellularly in p-hHSCs and LX-2 cells, which was potentiated by TGFβ. Ascorbic acid co-treatment strongly promoted collagen-I excretion and enhanced extracellular hydroxyproline concentrations, without affecting collagen-I (COL1A1) mRNA levels. DMOG inhibited collagen-I release even in the presence of ascorbic acid and suppressed COL1A1 and alpha-smooth muscle actin (αSMA/ACTA2) mRNA levels, also under hypoxic conditions. Hepatocytes express both ascorbic acid transporters, while p-hHSCs and LX-2 express the only SVCT2, which is selectively enhanced in cirrhotic livers. Human HSCs rely on ascorbic acid for the efficient secretion of collagen-I, which can be effectively blocked by hydroxylase antagonists, revealing new therapeutic targets to treat liver fibrosis.

Simultaneous Induction of Glycolysis and Oxidative Phosphorylation during Activation of Hepatic Stellate Cells Reveals Novel Mitochondrial Targets to Treat Liver Fibrosis

Upon liver injury, hepatic stellate cells (HSCs) transdifferentiate to migratory, proliferative and extracellular matrix-producing myofibroblasts (e.g., activated HSCs; aHSCs) causing liver fibrosis. HSC activation is associated with increased glycolysis and glutaminolysis. Here, we compared the contribution of glycolysis, glutaminolysis and mitochondrial oxidative phosphorylation (OXPHOS) in rat and human HSC activation. Basal levels of glycolysis (extracellular acidification rate ~3-fold higher) and particularly mitochondrial respiration (oxygen consumption rate ~5-fold higher) were significantly increased in rat aHSCs, when compared to quiescent rat HSC. This was accompanied by extensive mitochondrial fusion in rat and human aHSCs, which occurred without increasing mitochondrial DNA content and electron transport chain (ETC) components. Inhibition of glycolysis (by 2-deoxy-D-glucose) and glutaminolysis (by CB-839) did not inhibit rat aHSC proliferation, but did reduce Acta2 (encoding α-SMA) expression slightly. In contrast, inhibiting mitochondrial OXPHOS (by rotenone) significantly suppressed rat aHSC proliferation, as well as Col1a1 and Acta2 expression. Other than that observed for rat aHSCs, human aHSC proliferation and expression of fibrosis markers were significantly suppressed by inhibiting either glycolysis, glutaminolysis or mitochondrial OXPHOS (by metformin). Activation of HSCs is marked by simultaneous induction of glycolysis and mitochondrial metabolism, extending the possibilities to suppress hepatic fibrogenesis by interfering with HSC metabolism.

Experimental Evidence of Liver Injury by BSEP-Inhibiting Drugs With a Bile Salt Supplementation in Rats

The bile salt export pump (BSEP, ABCB11) mediates bile acid efflux from hepatocytes into bile. Although the inhibition of BSEP has been implicated as an important mechanism of drug-induced liver injury (DILI), liver injury caused by BSEP-inhibiting drugs is rarely reproduced in experimental animals, probably due to species differences in bile acid composition between humans and rodents. In this study, we tested whether supplementation with chenodeoxycholic acid (CDCA) sodium, a hydrophobic bile salt, could sensitize rats to liver injury caused by a BSEP-inhibiting drug. A potent BSEP inhibitor, ketoconazole (KTZ), which is associated with clinical DILI, was intragastrically administered simultaneously with CDCA at a nontoxic dose once a day for 3 days. Plasma transaminase levels significantly increased in rats receiving CDCA+KTZ, whereas neither treatment with CDCA alone, KTZ alone nor a combination of CDCA and miconazole, a safe analog to KTZ, induced liver injury. In CDCA+KTZ-treated rats, most bile acid species in the liver significantly increased compared with treatment with vehicle or CDCA alone, suggesting that KTZ administration inhibited bile acid excretion. Furthermore, hepatic mRNA expression levels of a bile acid synthesis enzyme, Cyp7a1, and a basolateral bile salt influx transporter, Ntcp, decreased, whereas a canalicular phosphatidylcholine flippase, Mdr2, increased in the CDCA+KTZ group to compensate for hepatic bile acid accumulation. In conclusion, we found that oral CDCA supplementation predisposed rats to KTZ-induced liver injury due to the hepatic accumulation of bile acids. This method may be useful for assessing the potential of BSEP-inhibiting drugs inducing liver injury in vivo.

Impaired Hepatic Vitamin A Metabolism in NAFLD Mice Leading to Vitamin A Accumulation in Hepatocytes

BACKGROUND & AIMS

Systemic retinol (vitamin A) homeostasis is controlled by the liver, involving close collaboration between hepatocytes and hepatic stellate cells (HSCs). Genetic variants in retinol metabolism (PNPLA3 and HSD17B13) are associated with non-alcoholic fatty liver disease (NAFLD) and disease progression. Still, little mechanistic details are known about hepatic vitamin A metabolism in NAFLD, which may affect carbohydrate and lipid metabolism, inflammation, oxidative stress and the development of fibrosis and cancer, e.g. all risk factors of NAFLD.

METHODS

Here, we analyzed vitamin A metabolism in 2 mouse models of NAFLD; mice fed a high-fat, high-cholesterol (HFC) diet and Leptinob mutant (ob/ob) mice.

RESULTS

Hepatic retinol and retinol binding protein 4 (RBP4) levels were significantly reduced in both mouse models of NAFLD. In contrast, hepatic retinyl palmitate levels (the vitamin A storage form) were significantly elevated in these mice. Transcriptome analysis revealed a hyperdynamic state of hepatic vitamin A metabolism, with enhanced retinol storage and metabolism (upregulated Lrat, Dgat1, Pnpla3, Raldh's and RAR/RXR-target genes) in fatty livers, in conjunction with induced hepatic inflammation (upregulated Cd68, Tnfα, Nos2, Il1β, Il-6) and fibrosis (upregulated Col1a1, Acta2, Tgfβ, Timp1). Autofluorescence analyses revealed prominent vitamin A accumulation in hepatocytes rather than HSC in HFC-fed mice. Palmitic acid exposure increased Lrat mRNA levels in primary rat hepatocytes and promoted retinyl palmitate accumulation when co-treated with retinol, which was not detected for similarly-treated primary rat HSCs.

CONCLUSION

NAFLD leads to cell type-specific rearrangements in retinol metabolism leading to vitamin A accumulation in hepatocytes. This may promote disease progression and/or affect therapeutic approaches targeting nuclear receptors.

Bile acid-based therapies for non-alcoholic steatohepatitis and alcoholic liver disease

Bile acids are synthesized from cholesterol only in hepatocytes. Bile acids circulating in the enterohepatic system act as physiological detergent molecules to help solubilize biliary cholesterol and emulsify dietary lipids and fat-soluble vitamins in small intestine. Bile acids are signaling molecules that activate nuclear receptor farnesoid X receptor (FXR) and cell surface G protein-coupled receptor TGR5. FXR critically regulates bile acid homeostasis by mediating bile acid feedback inhibition of hepatic bile acid synthesis. In addition, bile acid-activated cellular signaling pathways regulate metabolic homeostasis, immunity, and cell proliferation in various metabolically active organs. In the small and large intestine, gut bacterial enzymes modify primary bile acids to generate secondary bile acids to help shape the bile acid pool composition and subsequent biological effects. In turn, bile acids exhibit anti-microbial properties and modulate gut microbiota to influence host metabolism and immunity. Currently, bile acid-based therapies including systemic and intestine-restricted FXR agonists, TGR5 agonists, fibroblast growth factor 19 analogue, intestine FXR antagonists, and intestine apical sodium-bile acid transporter (ASBT) inhibitors have been developed as promising treatments for non-alcoholic steatohepatitis (NASH). These pharmacological agents improved metabolic and inflammatory disorders via distinct mechanisms of action that are subjects of extensive research interest. More recently, human and experimental alcoholic liver disease (ALD) has been associated with disrupted bile acid homeostasis. In additional, new findings showed that targeting bile acid metabolism and signaling may be promising therapeutic approaches for treating ALD.

Evolutionary Analysis of Bile Acid-Conjugating Enzymes Reveals a Complex Duplication and Reciprocal Loss History

To fulfill their physiological functions, bile acids are conjugated with amino acids. In humans, conjugation is catalyzed by bile acid coenzyme A: amino acid N-acyltransferase (BAAT), an enzyme with a highly conserved catalytic triad in its active site. Interestingly, the conjugated amino acids are highly variable among mammals, with some species conjugating bile acids with both glycine and taurine, whereas others conjugate only taurine. The genetic origin of these bile acid conjugation differences is unknown. Here, we tested whether mutations in BAAT’s catalytic triad could explain bile acid conjugation differences. Our comparative analysis of 118 mammals first revealed that the ancestor of placental mammals and marsupials possessed two genes, BAAT and BAATP1, that arose by a tandem duplication. This duplication was followed by numerous gene losses, including BAATP1 in humans. Losses of either BAAT or BAATP1 largely happened in a reciprocal fashion, suggesting that a single conjugating enzyme is generally sufficient for mammals. In intact BAAT and BAATP1 genes, we observed multiple changes in the catalytic triad between Cys and Ser residues. Surprisingly, although mutagenesis experiments with the human enzyme have shown that replacing Cys for Ser greatly diminishes the glycine-conjugating ability, across mammals we found that this residue provides little power in predicting the experimentally measured amino acids that are conjugated with bile acids. This suggests that the mechanism of BAAT’s enzymatic function is incompletely understood, despite relying on a classic catalytic triad. More generally, our evolutionary analysis indicates that results of mutagenesis experiments may not easily be extrapolatable to other species.

Glycine Metabolism and Its Alterations in Obesity and Metabolic Diseases

Glycine is the proteinogenic amino-acid of lowest molecular weight, harboring a hydrogen atom as a side-chain. In addition to being a building-block for proteins, glycine is also required for multiple metabolic pathways, such as glutathione synthesis and regulation of one-carbon metabolism. Although generally viewed as a non-essential amino-acid, because it can be endogenously synthesized to a certain extent, glycine has also been suggested as a conditionally essential amino acid. In metabolic disorders associated with obesity, type 2 diabetes (T2DM), and non-alcoholic fatty liver disease (NAFLDs), lower circulating glycine levels have been consistently observed, and clinical studies suggest the existence of beneficial effects induced by glycine supplementation. The present review aims at synthesizing the recent advances in glycine metabolism, pinpointing its main metabolic pathways, identifying the causes leading to glycine deficiency-especially in obesity and associated metabolic disorders-and evaluating the potential benefits of increasing glycine availability to curb the progression of obesity and obesity-related metabolic disturbances. This study focuses on the importance of diet, gut microbiota, and liver metabolism in determining glycine availability in obesity and associated metabolic disorders.

Hormone-sensitive lipase is a retinyl ester hydrolase in human and rat quiescent hepatic stellate cells

Hepatic stellate cells (HSC) store vitamin A as retinyl esters and control circulating retinol levels. Upon liver injury, quiescent (q)HSC lose their vitamin A and transdifferentiate to myofibroblasts, e.g. activated (a)HSC, which promote fibrosis by producing excessive extracellular matrix. Adipose triglyceride lipase/patatin-like phospholipase domain-containing protein 2 (ATGL/PNPLA2) and adiponutrin (ADPN/PNPLA3) have so far been shown to mobilize retinol from retinyl esters in HSC. Here, we studied the putative role of hormone-sensitive lipase (HSL/LIPE) in HSC, as it is the major retinyl ester hydrolase (REH) in adipose tissue. Lipe/HSL expression was analyzed in rat liver and primary human and rat qHSC and culture-activated aHSC. Retinyl hydrolysis was analyzed after Isoproterenol-mediated phosphorylation/activation of HSL. Primary human HSC contain 2.5-fold higher LIPE mRNA levels compared to hepatocytes. Healthy rat liver contains significant mRNA and protein levels of HSL/Lipe, which predominates in qHSC and cells of the portal tree. Q-PCR comparison indicates that Lipe mRNA levels in qHSC are dominant over Pnpla2 and Pnpla3. HSL is mostly phosphorylated/activated in qHSC and partly colocalizes with vitamin A-containing lipid droplets. Lipe/HSL and Pnpla3 expression is rapidly lost during HSC culture-activation, while Pnpla2 expression is maintained. HSL super-activation by isoproterenol accelerates loss of lipid droplets and retinyl palmitate from HSC, which coincided with a small, but significant reduction in HSC proliferation and suppression of Collagen1A1 mRNA and protein levels. In conclusion, HSL participates in vitamin A metabolism in qHSC. Equivalent activities of ATGL and ADPN provide the healthy liver with multiple routes to control circulating retinol levels.

Overexpression of Nudt7 decreases bile acid levels and peroxisomal fatty acid oxidation in the liver

Lipid metabolism requires CoA, an essential cofactor found in multiple subcellular compartments, including the peroxisomes. In the liver, CoA levels are dynamically adjusted between the fed and fasted states. Elevated CoA levels in the fasted state are driven by increased synthesis; however, this also correlates with decreased expression of Nudix hydrolase (Nudt)7, the major CoA-degrading enzyme in the liver. Nudt7 resides in the peroxisomes, and we overexpressed this enzyme in mouse livers to determine its effect on the size and composition of the hepatic CoA pool in the fed and fasted states. Nudt7 overexpression did not change total CoA levels, but decreased the concentration of short-chain acyl-CoAs and choloyl-CoA in fasted livers, when endogenous Nudt7 activity was lowest. The effect on these acyl-CoAs correlated with a significant decrease in the hepatic bile acid content and in the rate of peroxisomal fatty acid oxidation, as estimated by targeted and untargeted metabolomics, combined with the measurement of fatty acid oxidation in intact hepatocytes. Identification of the CoA species and metabolic pathways affected by the overexpression on Nudt7 in vivo supports the conclusion that the nutritionally driven modulation of Nudt7 activity could contribute to the regulation of the peroxisomal CoA pool and peroxisomal lipid metabolism.

Duodenal-jejunal lining increases postprandial unconjugated bile acid responses and disrupts the bile acid-FXR-FGF19 axis in humans

BACKGROUND AND AIMS

Placement of the duodenal-jejunal bypass liner (DJBL) leads to rapid weight loss and restoration of insulin sensitivity in a similar fashion to bariatric surgery. Increased systemic bile acid levels are candidate effectors for these effects through postprandial activation of their receptors TGR5 and FXR. We aimed to quantify postprandial bile acid, GLP-1 and FGF19 responses and assess their temporal relation to the weight loss and metabolic and hormonal changes seen after DJBL placement.

METHODS

We performed mixed meal testing in 17 obese patients with type 2 diabetes mellitus (DM2) directly before, one week after and 6 months after DJBL placement.

RESULTS

Both fasting and postprandial bile acid levels were unchanged at 1 week after implantation, and greatly increased 6 months after implantation. The increase consisted of unconjugated bile acid species. 3 hour-postprandial GLP-1 levels increased after 1 week and were sustained, whereas FGF19 levels and postprandial plasma courses were unaffected.

CONCLUSIONS

DJBL placement leads to profound increases in unconjugated bile acid levels after 6 months, similar to the effects of bariatric surgery. The temporal dissociation between the changes in bile acids, GLP-1 and FGF19 and other gut hormone responses warrant caution about the beneficial role of bile acids after DJBL placement. This observational uncontrolled study emphasizes the need for future controlled studies.

Bile Acids as Potential Biomarkers to Assess Liver Impairment in Polycystic Kidney Disease

Polycystic kidney disease is characterized by the progressive development of kidney cysts and declining renal function with frequent development of cysts in other organs including the liver. The polycystic kidney (PCK) rat is a rodent model of polycystic liver disease that has been used to study hepatorenal disease progression and evaluate pharmacotherapeutic interventions. Biomarkers that describe the cyst progression, liver impairment, and/or hepatic cyst burden could provide clinical utility for this disease. In the present study, hepatic cyst volume was measured by magnetic resonance imaging in PCK rats at 12, 16, and 20 weeks. After 20 weeks, Sprague Dawley (n = 4) and PCK (n = 4) rats were sacrificed and 42 bile acids were analyzed in the liver, bile, serum, and urine by liquid chromatography coupled to tandem mass spectrometry. Bile acid profiling revealed significant increases in total bile acids (molar sum of all measured bile acids) in the liver (13-fold), serum (6-fold), and urine (3-fold) in PCK rats, including those speciated bile acids usually associated with hepatotoxicity. Total serum bile acids correlated with markers of liver impairment (liver weight, total liver bile acids, total hepatotoxic liver bile acids, and cyst volume [ r > 0.75; P < 0.05]). Based on these data, serum bile acids may be useful biomarkers of liver impairment in polycystic hepatorenal disease.

Liver-specific knockout of histone methyltransferase G9a impairs liver maturation and dysregulates inflammatory, cytoprotective, and drug-processing genes

Methyltransferase G9a is essential for a key gene silencing mark, histone H3 dimethylation at lysine-9 (H3K9me2). Hepatic G9a expression is down-regulated by xenobiotics and diabetes. However, little is known about the role of G9a in liver. Thus, we generated mice with liver-specific knockout (Liv-KO) of G9a. Adult G9a Liv-KO mice had marked loss of H3K9me2 proteins in liver, without overt liver injury or infiltration of inflammatory cells. However, G9a-null livers had ectopic induction of certain genes normally expressed in neural and immune systems. Additionally, G9a-null livers had moderate down-regulation of cytoprotective genes, markedly altered expression of certain important drug-processing genes, elevated endogenous reactive oxygen species, induction of ER stress marker Chop, but decreased glutathione and nuclear Nrf2. microRNA-383, a negative regulator of the PI3K/Akt pathway, was strongly induced in G9a Liv-KO mice. After LPS treatment, G9a Liv-KO mice had aggravated lipid peroxidation and proinflammatory response. Taken together, the present study demonstrates that G9a regulates liver maturation by silencing neural and proinflammatory genes but maintaining/activating cytoprotective and drug-processing genes, in which the G9a/miR-383/PI3K/Akt/Nrf2 (Chop) pathways may play important roles. G9a deficiency due to genetic polymorphism and/or environmental exposure may alter xenobiotic metabolism and aggravate inflammation and liver dysfunction.

Species differences in bile acids II. Bile acid metabolism

One of the mechanisms of drug-induced liver injury (DILI) involves alterations in bile acid (BA) homeostasis and elimination, which encompass several metabolic pathways including hydroxylation, amidation, sulfation, glucuronidation and glutathione conjugation. Species differences in BA metabolism may play a major role in the failure of currently used in vitro and in vivo models to predict reliably the DILI during the early stages of drug discovery and development. We developed an in vitro cofactor-fortified liver S9 fraction model to compare the metabolic profiles of the four major BAs (cholic acid, chenodeoxycholic acid, lithocholic acid and ursodeoxycholic acid) between humans and several animal species. High- and low-resolution liquid chromatography-tandem mass spectrometry and nuclear magnetic resonance imaging were used for the qualitative and quantitative analysis of BAs and their metabolites. Major species differences were found in the metabolism of BAs. Sulfation into 3-O-sulfates was a major pathway in human and chimpanzee (4.8%-52%) and it was a minor pathway in all other species (0.02%-14%). Amidation was primarily with glycine (62%-95%) in minipig and rabbit and it was primarily with taurine (43%-81%) in human, chimpanzee, dog, hamster, rat and mice. Hydroxylation was highest (13%-80%) in rat and mice followed by hamster, while it was lowest (1.6%-22%) in human, chimpanzee and minipig. C6-β hydroxylation was predominant (65%-95%) in rat and mice, while it was at C6-α position in minipig (36%-97%). Glucuronidation was highest in dog (10%-56%), while it was a minor pathway in all other species (<12%). The relative contribution of the various pathways involved in BA metabolism in vitro were in agreement with the observed plasma and urinary BA profiles in vivo and were able to predict and quantify the species differences in BA metabolism. In general, overall, BA metabolism in chimpanzee is most similar to human, while BA metabolism in rats and mice is most dissimilar from human.

Peroxisomes and Their Central Role in Metabolic Interaction Networks in Humans

Peroxisomes catalyze a number of essential metabolic functions and impairments in any of these are usually associated with major clinical signs and symptoms. In contrast to mitochondria which are autonomous organelles that can catalyze the degradation of fatty acids, certain amino acids and other compounds all by themselves, peroxisomes are non-autonomous organelles which are highly dependent on the interaction with other organelles and compartments to fulfill their role in metabolism. This includes mitochondria, the endoplasmic reticulum, lysosomes, and the cytosol. In this paper we will discuss the central role of peroxisomes in different metabolic interaction networks in humans, including fatty acid oxidation, ether phospholipid biosynthesis, bile acid synthesis, fatty acid alpha-oxidation and glyoxylate metabolism.

Activin A is a prominent autocrine regulator of hepatocyte growth arrest

Activin A, a multifunctional cytokine, plays an important role in hepatocyte growth suppression and is involved in liver size control. The present study was aimed to determine the cell location of activin A in the normal rat liver microenvironment and the contribution of activin A signaling to the hepatocyte phenotype to obtain insight into molecular mechanisms. Immunohistochemical and in situ hybridization analyses identified hepatocytes as the major activin A‐positive cell population in normal liver and identified mast cells as an additional activin A source. To investigate paracrine and autocrine activin A‐stimulated effects, hepatocytes were cocultured with engineered activin A‐secreting cell lines (RF1, TL8) or transduced with an adeno‐associated virus vector encoding activin βA, which led to strikingly altered expression of cell cycle‐related genes (Ki‐67, E2F transcription factor 1 [E2F1], minichromosome maintenance complex component 2 [Mcm2], forkhead box M1 [FoxM1]) and senescence‐related genes (cyclin‐dependent kinase inhibitor 2B [p15^INK4b/CDKN2B], differentiated embryo‐chondrocyte expressed gene 1 [DEC1]) and reduced proliferation and induction of senescence. Microarray analyses identified 453 differentially expressed genes, many of which were not yet recognized as activin A downstream targets (e.g., ADAM metallopeptidase domain 12 [Adam12], semaphorin 7A [Sema7a], LIM and cysteine‐rich domains‐1 [Lmcd1], DAB2, clathrin adaptor protein [Dab2]). Among the main activin A‐mediated molecular/cellular functions are cellular growth/proliferation and movement, molecular transport, and metabolic processes containing highly down‐regulated genes, such as cytochrome P450, subfamily 2, polypeptide 11 (Cyp2C11), sulfotransferase family 1A, member 1 (Sult1a1), glycine‐N‐acyltransferase (Glyat), and bile acid‐CoA:amino acid N‐acyltransferase (Baat). Moreover, Ingenuity Pathway Analyses identified particular gene networks regulated by hepatocyte nuclear factor (HNF)‐4α and peroxisome proliferator‐activated receptor gamma (PPARγ) as key targets of activin A signaling. Conclusion: Our in vitro models demonstrated that activin A‐stimulated growth inhibition and cellular senescence is mediated through p15^INK4b/CDKN2B and is associated with up‐ and down‐regulation of numerous target genes involved in multiple biological processes performed by hepatocytes, suggesting that activin A fulfills a critical role in normal liver function. (Hepatology Communications 2017;1:852‐870)

Complex interaction between circadian rhythm and diet on bile acid homeostasis in male rats

Desynchronization between the master clock in the brain, which is entrained by (day) light, and peripheral organ clocks, which are mainly entrained by food intake, may have negative effects on energy metabolism. Bile acid metabolism follows a clear day/night rhythm. We investigated whether in rats on a normal chow diet the daily rhythm of plasma bile acids and hepatic expression of bile acid metabolic genes is controlled by the light/dark cycle or the feeding/fasting rhythm. In addition, we investigated the effects of high caloric diets and time-restricted feeding on daily rhythms of plasma bile acids and hepatic genes involved in bile acid synthesis. In experiment 1 male Wistar rats were fed according to three different feeding paradigms: food was available ad libitum for 24 h (ad lib) or time-restricted for 10 h during the dark period (dark fed) or 10 h during the light period (light fed). To allow further metabolic phenotyping, we manipulated dietary macronutrient intake by providing rats with a chow diet, a free choice high-fat-high-sugar diet or a free choice high-fat (HF) diet. In experiment 2 rats were fed a normal chow diet, but food was either available in a 6-meals-a-day (6M) scheme or ad lib. During both experiments, we measured plasma bile acid levels and hepatic mRNA expression of genes involved in bile acid metabolism at eight different time points during 24 h. Time-restricted feeding enhanced the daily rhythm in plasma bile acid concentrations. Plasma bile acid concentrations are highest during fasting and dropped during the period of food intake with all diets. An HF-containing diet changed bile acid pool composition, but not the daily rhythmicity of plasma bile acid levels. Daily rhythms of hepatic Cyp7a1 and Cyp8b1 mRNA expression followed the hepatic molecular clock, whereas for Shp expression food intake was leading. Combining an HF diet with feeding in the light/inactive period annulled CYp7a1 and Cyp8b1 gene expression rhythms, whilst keeping that of Shp intact. In conclusion, plasma bile acids and key genes in bile acid biosynthesis are entrained by food intake as well as the hepatic molecular clock. Eating during the inactivity period induced changes in the plasma bile acid pool composition similar to those induced by HF feeding.

Changes in hepatic gene expression and serum metabolites after oral administration of overdosed vitamin-E-loaded nanoemulsion in rats

Vitamin-E-loaded nanoemulsion (Vit E-NE) was produced, and the effects of repeated oral administration of Vit E-NE (2 g/kg/day) for five days on hepatic gene expression and serum metabolites were investigated in rats. The mean particle diameter and zeta potential of Vit E-NE was 112 nm and 56 mV, respectively. Vit E-NE administered rats showed significantly higher triglyceride content than of standard diet (control) or Vit E control emulsion (Vit E-CE) group but no toxicity symptoms were found in blood biochemical analysis. Next generation sequencing analysis of rat liver revealed that several genes related to energy and xenobiotic metabolism (CYP1A1 and glutathione S-transferase) were significantly altered. Serum metabolites (B-hydroxybutyrate and palmitoleic acid) indicating ketone body production and activation of stearoyl-CoAdesaturase were significantly increased by administration of Vit E-NE. The results of this study suggest that excessive consumption of edible nano-sized food ingredients can possibly cause adverse effects.

The interrelationship between bile acid and vitamin A homeostasis

Vitamin A is a fat-soluble vitamin important for vision, reproduction, embryonic development, cell differentiation, epithelial barrier function and adequate immune responses. Efficient absorption of dietary vitamin A depends on the fat-solubilizing properties of bile acids. Bile acids are synthesized in the liver and maintained in an enterohepatic circulation. The liver is also the main storage site for vitamin A in the mammalian body, where an intimate collaboration between hepatocytes and hepatic stellate cells leads to the accumulation of retinyl esters in large cytoplasmic lipid droplet hepatic stellate cells. Chronic liver diseases are often characterized by disturbed bile acid and vitamin A homeostasis, where bile production is impaired and hepatic stellate cells lose their vitamin A in a transdifferentiation process to myofibroblasts, cells that produce excessive extracellular matrix proteins leading to fibrosis. Chronic liver diseases thus may lead to vitamin A deficiency. Recent data reveal an intricate crosstalk between vitamin A metabolites and bile acids, in part via the Retinoic Acid Receptor (RAR), Retinoid X Receptor (RXR) and the Farnesoid X Receptor (FXR), in maintaining vitamin A and bile acid homeostasis. Here, we provide an overview of the various levels of "communication" between vitamin A metabolites and bile acids and its relevance for the treatment of chronic liver diseases.

Metabolic Interplay between Peroxisomes and Other Subcellular Organelles Including Mitochondria and the Endoplasmic Reticulum

Peroxisomes are unique subcellular organelles which play an indispensable role in several key metabolic pathways which include: (1.) etherphospholipid biosynthesis; (2.) fatty acid beta-oxidation; (3.) bile acid synthesis; (4.) docosahexaenoic acid (DHA) synthesis; (5.) fatty acid alpha-oxidation; (6.) glyoxylate metabolism; (7.) amino acid degradation, and (8.) ROS/RNS metabolism. The importance of peroxisomes for human health and development is exemplified by the existence of a large number of inborn errors of peroxisome metabolism in which there is an impairment in one or more of the metabolic functions of peroxisomes. Although the clinical signs and symptoms of affected patients differ depending upon the enzyme which is deficient and the extent of the deficiency, the disorders involved are usually (very) severe diseases with neurological dysfunction and early death in many of them. With respect to the role of peroxisomes in metabolism it is clear that peroxisomes are dependent on the functional interplay with other subcellular organelles to sustain their role in metabolism. Indeed, whereas mitochondria can oxidize fatty acids all the way to CO₂ and H₂O, peroxisomes are only able to chain-shorten fatty acids and the end products of peroxisomal beta-oxidation need to be shuttled to mitochondria for full oxidation to CO₂ and H₂O. Furthermore, NADH is generated during beta-oxidation in peroxisomes and beta-oxidation can only continue if peroxisomes are equipped with a mechanism to reoxidize NADH back to NAD⁺, which is now known to be mediated by specific NAD(H)-redox shuttles. In this paper we describe the current state of knowledge about the functional interplay between peroxisomes and other subcellular compartments notably the mitochondria and endoplasmic reticulum for each of the metabolic pathways in which peroxisomes are involved.

Hepatic dysfunction in peroxisomal disorders

The peroxisomal compartment in hepatocytes hosts several essential metabolic conversions. These are defective in peroxisomal disorders that are either caused by failure to import the enzymes in the organelle or by mutations in the enzymes or in transporters needed to transfer the substrates across the peroxisomal membrane. Hepatic pathology is one of the cardinal features in disorders of peroxisome biogenesis and peroxisomal β-oxidation although it only rarely determines the clinical fate. In mouse models of these diseases liver pathologies also occur, although these are not always concordant with the human phenotype which might be due to differences in diet, expression of enzymes and backup mechanisms. Besides the morphological changes, we overview the impact of peroxisome malfunction on other cellular compartments including mitochondria and the ER. We further focus on the metabolic pathways that are affected such as bile acid formation, and dicarboxylic acid and branched chain fatty acid degradation. It appears that the association between deregulated metabolites and pathological events remains unclear.

Clinical significance of intragraft miR-122 and -155 expression after liver transplantation

AIM

Recurrent hepatitis C (RHC) and acute cellular rejection (AR) remain critical problems following liver transplantation (LT) in hepatitis C virus (HCV) positive recipients because of the similar clinical features. Discrimination between these conditions can be problematic, and adjunctive biomarkers would be useful to discriminate these processes. The aim of our study was to investigate the possibility of the intragraft miR-122 and -155 expression as new biomarkers after LT.

METHODS

A total of 29 HCV positive recipients were enrolled in this study. Intragraft expressions of miR-122 and -155 were studied between RHC predominant (n = 17) and AR predominant cases (n = 12) using quantitative reverse transcription polymerase chain reaction. Furthermore, we investigated the correlations between these expression levels and clinical serum parameters.

RESULTS

Intragraft miR-122 expression had a good correlation with serum alkaline phosphatase (P = 0.02), but it was not correlated with the serum HCV viral load. The expression levels of miR-122 in the AR group were significantly higher than those in the RHC group (P = 0.0006) and, inversely, the expression levels of miR-155 in the AR group were significantly lower than those in the RHC group (P = 0.01).

CONCLUSION

Our study emphasizes a useful pattern of miR-122 and -155 as ancillary markers to discriminate AR predominant cases from RHC in HCV positive patients after LT.

Molecular mechanisms underlying the enhanced functions of three-dimensional hepatocyte aggregates

Three-dimensional (3D) culture of hepatocytes leads to improved and prolonged synthetic and metabolic functions, but the underlying molecular mechanisms are unknown. In order to investigate the role of 3D cell-cell interactions in maintaining hepatocyte differentiated functions ex vivo, primary mouse hepatocytes were cultured either as monolayers on tissue culture dishes (TCD) or as 3D aggregates in rotating wall vessel (RWV) bioreactors. Global gene expression analyses revealed that genes upregulated in 3D culture were distinct from those upregulated during liver development and liver regeneration. Instead, they represented a diverse array of hepatocyte-specific functional genes with significant over-representation of hepatocyte nuclear factor 4α (Hnf4a) binding sites in their promoters. Expression of Hnf4a and many of its downstream target genes were significantly increased in RWV cultures as compared to TCD. Conversely, there was concomitant suppression of mesenchymal and cytoskeletal genes in RWV cultures that were induced in TCDs. These findings illustrate the importance of 3D cell-cell interactions in maintaining fundamental molecular pathways of hepatocyte function and serve as a basis for rational design of biomaterials that aim to optimize hepatocyte functions ex vivo for biomedical applications.

The profile of bile acids and their sulfate metabolites in human urine and serum

The role of sulfation in ameliorating the hepatotoxicity of bile acids (BAs) in humans remains unknown due to the lack of proper analytical methods to quantify individual BAs and their sulfate metabolites in biological tissues and fluids. To this end, a simple and sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated to characterize the detailed BA profile in human urine and serum. The limit of quantification was 1ng/mL and baseline separation of all analytes was achieved within in a run time of 32min. The method was validated over the dynamic range of 1-1000ng/mL. The LC-MS/MS method was more accurate, precise, and selective than the commercially available kits for the quantification of sulfated and unsulfated BAs, and the indirect quantification of individual sulfated BAs after solvolysis. The LC-MS/MS method was applied to characterize the BA profile in urine and serum of healthy subjects. Thirty three percent of serum BAs were sulfated, whereas 89% of urinary BAs existed in the sulfate form, indicating the role of sulfation in enhancing the urinary excretion of BAs. The percentage of sulfation of individual BAs increased with the decrease in the number of hydroxyl groups indicating the role of sulfation in the detoxification of the more hydrophobic and toxic BA species. Eighty percent of urinary BAs and 55% of serum BAs were present in the glycine-amidated form, whereas 8% of urinary BAs and 13% of serum BAs existed in the taurine-amidated form.

Nuclear targeting of dystroglycan promotes the expression of androgen regulated transcription factors in prostate cancer

Dystroglycan is frequently lost in adenocarcinoma, but the mechanisms and consequences are poorly understood. We report an analysis of β-dystroglycan in prostate cancer in human tissue samples and in LNCaP cells in vitro. There is progressive loss of β-dystroglycan immunoreactivity from basal and lateral surfaces of prostate epithelia which correlates significantly with increasing Gleason grade. In about half of matched bone metastases there is significant dystroglycan re-expression. In tumour tissue and in LNCaP cells there is also a tyrosine phosphorylation-dependent translocation of β-dystroglycan to the nucleus. Analysis of gene expression data by microarray, reveals that nuclear targeting of β-dystroglycan in LNCaP cells alters the transcription of relatively few genes, the most unregulated being the transcription factor ETV1. These data suggest that proteolysis, tyrosine phosphorylation and translocation of dystroglycan to the nucleus resulting in altered gene transcription could be important mechanisms in the progression of prostate cancer.

Glutathione and antioxidant enzymes serve complementary roles in protecting activated hepatic stellate cells against hydrogen peroxide-induced cell death

BACKGROUND

In chronic liver disease, hepatic stellate cells (HSCs) are activated, highly proliferative and produce excessive amounts of extracellular matrix, leading to liver fibrosis. Elevated levels of toxic reactive oxygen species (ROS) produced during chronic liver injury have been implicated in this activation process. Therefore, activated hepatic stellate cells need to harbor highly effective anti-oxidants to protect against the toxic effects of ROS.

AIM

To investigate the protective mechanisms of activated HSCs against ROS-induced toxicity.

METHODS

Culture-activated rat HSCs were exposed to hydrogen peroxide. Necrosis and apoptosis were determined by Sytox Green or acridine orange staining, respectively. The hydrogen peroxide detoxifying enzymes catalase and glutathione-peroxidase (GPx) were inhibited using 3-amino-1,2,4-triazole and mercaptosuccinic acid, respectively. The anti-oxidant glutathione was depleted by L-buthionine-sulfoximine and repleted with the GSH-analogue GSH-monoethylester (GSH-MEE).

RESULTS

Upon activation, HSCs increase their cellular glutathione content and GPx expression, while MnSOD (both at mRNA and protein level) and catalase (at the protein level, but not at the mRNA level) decreased. Hydrogen peroxide did not induce cell death in activated HSCs. Glutathione depletion increased the sensitivity of HSCs to hydrogen peroxide, resulting in 35% and 75% necrotic cells at 0.2 and 1mmol/L hydrogen peroxide, respectively. The sensitizing effect was abolished by GSH-MEE. Inhibition of catalase or GPx significantly increased hydrogen peroxide-induced apoptosis, which was not reversed by GSH-MEE.

CONCLUSION

Activated HSCs have increased ROS-detoxifying capacity compared to quiescent HSCs. Glutathione levels increase during HSC activation and protect against ROS-induced necrosis, whereas hydrogen peroxide-detoxifying enzymes protect against apoptotic cell death.

Peroxisomes in human health and disease: metabolic pathways, metabolite transport, interplay with other organelles and signal transduction

Peroxisomes play a key role in human physiology as exemplified by the devastating consequences of a defect in peroxisome biogenesis as observed in patients affected by Zellweger syndrome. The main metabolic functions of peroxisomes in humans include: (1) fatty acid beta-oxidation; (2) etherphospholipid synthesis; (3) bile acid synthesis; (4) fatty acid alpha-oxidation, and (5) glyoxylate detoxification. Since peroxisomes lack a citric acid cycle and respiratory chain like mitochondria do, metabolism in peroxisomes requires continued cross-talk with other organelles, notably mitochondria and the endoplasmic reticulum in order to allow continued metabolism of the products generated by peroxisomes. Many of the metabolites which require peroxisomes for homeostasis, are involved in signal transduction pathways. These include the primary bile acids; platelet activating factor; plasmalogens, N-acylglycines and N-acyltaurines; docosahexaenoic acid as well as multiple prostanoids. The current state of knowledge in this area will be discussed in this review.

The glycine deportation system and its pharmacological consequences

The glycine deportation system is an essential component of glycine catabolism in man whereby 400 to 800mg glycine per day are deported into urine as hippuric acid. The molecular escort for this deportation is benzoic acid, which derives from the diet and from gut microbiota metabolism of dietary precursors. Three components of this system, involving hepatic and renal metabolism, and renal active tubular secretion help regulate systemic and central nervous system levels of glycine. When glycine levels are pathologically high, as in congenital nonketotic hyperglycinemia, the glycine deportation system can be upregulated with pharmacological doses of benzoic acid to assist in normalization of glycine homeostasis. In congenital urea cycle enzymopathies, similar activation of the glycine deportation system with benzoic acid is useful for the excretion of excess nitrogen in the form of glycine. Drugs which can substitute for benzoic acid as substrates for the glycine deportation system have adverse reactions that may involve perturbations of glycine homeostasis. The cancer chemotherapeutic agent ifosfamide has an unacceptably high incidence of encephalopathy. This would appear to arise as a result of the production of toxic aldehyde metabolites which deplete ATP production and sequester NADH in the mitochondrial matrix, thereby inhibiting the glycine deportation system and causing de novo glycine synthesis by the glycine cleavage system. We hypothesize that this would result in hyperglycinemia and encephalopathy. This understanding may lead to novel prophylactic strategies for ifosfamide encephalopathy. Thus, the glycine deportation system plays multiple key roles in physiological and neurotoxicological processes involving glycine.

Involvement of γ-aminobutyric acid transporter 2 in the hepatic uptake of taurine in rats

Taurine is essential for the hepatic synthesis of bile salts and, although taurine is synthesized mainly in pericentral hepatocytes, taurine and taurine-conjugated bile acids are abundant in periportal hepatocytes. One possible explanation for this discrepancy is that the active supply of taurine to hepatocytes from the blood stream is a key regulatory factor. The purpose of the present study is to investigate and identify the transporter responsible for taurine uptake by periportal hepatocytes. An in vivo bolus injection of [(3)H]taurine into the rat portal vein demonstrated that 25% of the injected [(3)H]taurine was taken up by the liver on a single pass. The in vivo uptake was significantly inhibited by GABA, taurine, β-alanine, and nipecotic acid, a GABA transporter (GAT) inhibitor, each at a concentration of 10 mM. The characteristics of Na(+)- and Cl(-)-dependent [(3)H]taurine uptake by freshly isolated rat hepatocytes were consistent with those of GAT2 (solute carrier SLC6A13). Indeed, the K(m) value of the saturable uptake (594 μM) was close to that of mouse SLC6A13-mediated taurine transport. Although GABA, taurine, and β-alanine inhibited the [(3)H]taurine uptake by > 50%, each at a concentration of 10 mM, GABA caused a marked inhibition with an IC(50) value of 95 μM. The [(3)H]taurine uptake exhibited a significant reduction when the GAT2 gene was silenced. Immunohistochemical analysis showed that GAT2 was localized on the sinusoidal membrane of the hepatocytes predominantly in the periportal region. These results suggest that GAT2 is responsible for taurine transport from the circulating blood to hepatocytes predominantly in the periportal region.

The emerging role of acyl-CoA thioesterases and acyltransferases in regulating peroxisomal lipid metabolism

The importance of peroxisomes in lipid metabolism is now well established and peroxisomes contain approximately 60 enzymes involved in these lipid metabolic pathways. Several acyl-CoA thioesterase enzymes (ACOTs) have been identified in peroxisomes that catalyze the hydrolysis of acyl-CoAs (short-, medium-, long- and very long-chain), bile acid-CoAs, and methyl branched-CoAs, to the free fatty acid and coenzyme A. A number of acyltransferase enzymes, which are structurally and functionally related to ACOTs, have also been identified in peroxisomes, which conjugate (or amidate) bile acid-CoAs and acyl-CoAs to amino acids, resulting in the production of amidated bile acids and fatty acids. The function of ACOTs is to act as auxiliary enzymes in the α- and β-oxidation of various lipids in peroxisomes. Human peroxisomes contain at least two ACOTs (ACOT4 and ACOT8) whereas mouse peroxisomes contain six ACOTs (ACOT3, 4, 5, 6, 8 and 12). Similarly, human peroxisomes contain one bile acid-CoA:amino acid N-acyltransferase (BAAT), whereas mouse peroxisomes contain three acyltransferases (BAAT and acyl-CoA:amino acid N-acyltransferases 1 and 2: ACNAT1 and ACNAT2). This review will focus on the human and mouse peroxisomal ACOT and acyltransferase enzymes identified to date and discuss their cellular localizations, emerging structural information and functions as auxiliary enzymes in peroxisomal metabolic pathways.

Peroxisomal acyl-CoA synthetases

Peroxisomes carry out many essential lipid metabolic functions. Nearly all of these functions require that an acyl group-either a fatty acid or the acyl side chain of a steroid derivative-be thioesterified to coenzyme A (CoA) for subsequent reactions to proceed. This thioesterification, or "activation", reaction, catalyzed by enzymes belonging to the acyl-CoA synthetase family, is thus central to cellular lipid metabolism. However, despite our rather thorough understanding of peroxisomal metabolic pathways, surprisingly little is known about the specific peroxisomal acyl-CoA synthetases that participate in these pathways. Of the 26 acyl-CoA synthetases encoded by the human and mouse genomes, only a few have been reported to be peroxisomal, including ACSL4, SLC27A2, and SLC27A4. In this review, we briefly describe the primary peroxisomal lipid metabolic pathways in which fatty acyl-CoAs participate. Then, we examine the evidence for presence and functions of acyl-CoA synthetases in peroxisomes, much of which was obtained before the existence of multiple acyl-CoA synthetase isoenzymes was known. Finally, we discuss the role(s) of peroxisome-specific acyl-CoA synthetase isoforms in lipid metabolism.

Endogenous bile acid disposition in rat and human sandwich-cultured hepatocytes

Sandwich-cultured hepatocytes (SCH) are used commonly to investigate hepatic transport protein-mediated uptake and biliary excretion of substrates. However, little is known about the disposition of endogenous bile acids (BAs) in SCH. In this study, four endogenous conjugated BAs common to rats and humans [taurocholic acid (TCA), glycocholic acid (GCA), taurochenodeoxycholic acid (TCDCA), and glycochenodeoxycholic acid (GCDCA)], as well as two BA species specific to rodents (α- and β-tauromuricholic acid; α/β TMCA), were profiled in primary rat and human SCH. Using B-CLEAR® technology, BAs were measured in cells+bile canaliculi, cells, and medium of SCH by LC-MS/MS. Results indicated that, just as in vivo, taurine-conjugated BA species were predominant in rat SCH, while glycine-conjugated BAs were predominant in human SCH. Total intracellular BAs remained relatively constant over days in culture in rat SCH. Total BAs in control (CTL) cells+bile, cells, and medium were approximately 3.4, 2.9, and 8.3-fold greater in human than in rat. The estimated intracellular concentrations of the measured total BAs were 64.3±5.9 μM in CTL rat and 183±56 μM in CTL human SCH, while medium concentrations of the total BAs measured were 1.16±0.21 μM in CTL rat SCH and 9.61±6.36 μM in CTL human SCH. Treatment of cells for 24h with 10 μM troglitazone (TRO), an inhibitor of the bile salt export pump (BSEP) and the Na⁺-taurocholate cotransporting polypeptide (NTCP), had no significant effect on endogenous BAs measured at the end of the 24-h culture period, potentially due to compensatory mechanisms that maintain BA homeostasis. These data demonstrate that BAs in SCH are similar to in vivo, and that SCH may be a useful in vitro model to study alterations in BA disposition if species differences are taken into account.

Transfer of metabolites across the peroxisomal membrane

Peroxisomes perform a large variety of metabolic functions that require a constant flow of metabolites across the membranes of these organelles. Over the last few years it has become clear that the transport machinery of the peroxisomal membrane is a unique biological entity since it includes nonselective channels conducting small solutes side by side with transporters for 'bulky' solutes such as ATP. Electrophysiological experiments revealed several channel-forming activities in preparations of plant, mammalian, and yeast peroxisomes and in glycosomes of Trypanosoma brucei. The properties of the first discovered peroxisomal membrane channel - mammalian Pxmp2 protein - have also been characterized. The channels are apparently involved in the formation of peroxisomal shuttle systems and in the transmembrane transfer of various water-soluble metabolites including products of peroxisomal β-oxidation. These products are processed by a large set of peroxisomal enzymes including carnitine acyltransferases, enzymes involved in the synthesis of ketone bodies, thioesterases, and others. This review discusses recent data pertaining to solute permeability and metabolite transport systems in peroxisomal membranes and also addresses mechanisms responsible for the transfer of ATP and cofactors such as an ATP transporter and nudix hydrolases.

Hepatosteatosis in peroxisome deficient liver despite increased β-oxidation capacity and impaired lipogenesis

Peroxisome deficiency in liver causes hepatosteatosis both in patients and in mice. Here, we studied the mechanisms that contribute to this lipid accumulation and to activation of peroxisome proliferator activated receptor α (PPARα) by using liver-specific Pex5(-/-) mice (L-Pex5(-/-) mice). Surprisingly, steatosis was accompanied both by increased mitochondrial β-oxidation capacity, confirming previous observations, and by impaired de novo lipid synthesis mediated by reduced expression of sterol regulatory element binding protein 1c and its targets. As a consequence, when challenged with a high fat diet, L-Pex5(-/-) mice were protected from adiposity. Hepatic fatty acid uptake was strongly increased whereas the expression of apolipoproteins and the lipoprotein assembly factor microsomal triglyceride transfer protein were markedly reduced resulting in reduced secretion of very low density lipoproteins. Most of these changes seemed to be orchestrated by the endogenous activation of PPARα, challenging the assumption that PPARα activation in hepatocytes requires fatty acid synthase dependent de novo fatty acid synthesis. Expression of cholesterol synthesizing enzymes and cholesterol levels were not affected in peroxisome deficient liver. In conclusion, increased fatty acid uptake driven by endogenous PPARα activation and reduced fatty acid secretion cause hepatosteatosis in peroxisome deficient livers.