Peroxisomal bile acid-CoA:amino-acid N-acyltransferase in rat liver

Bile acid-CoA:amino-acid N-acyltransferase activity was measured in subcellular fractions of rat liver homogenate. The conversion of [14C]choloyl-CoA and [14C]chenodeoxycholoyl-CoA into the corresponding [14C]tauro- and glyco-bile acids was calculated after isolation of the product by high performance liquid chromatography. There was an enrichment of bile acid-CoA:amino-acid N-acyltransferase activity in the light mitochondrial (L) fraction and to a lesser extent in the microsomal fraction. Surprisingly, no enrichment was found in the cytosolic fraction. Subfractionation of the L-fraction by Nycodenz gradient centrifugation, showed that the activity of the N-acyltransferase had a bimodal distribution and co-sedimented with peroxisomes (particulate catalase) and microsomes (esterase). The highest specific amidation activity of both choloyl-CoA and chenodeoxycholoyl-CoA was always found in the most peroxisome-rich fractions. [14C]Taurocholate formation in the peroxisomal fraction was 2.2 mumol/mg of protein/min. Striking differences were observed in the Km values and the saturation concentrations for glycine and taurine. The peroxisomal amidation of [14C]choloyl-CoA had a Km for taurine of 0.9 x 10(-3) M and for glycine of 17 x 10(-3) M. The results are consistent with the possibility that most of de novo synthesized bile acids conjugate to taurine by a peroxisomal bile acid-taurine N-acyltransferase in rat liver. The bile acids deconjugated in the gut and recirculating to the liver may be activated and amidated by the microsomal enzyme system prior to biliary secretion.     

Characterization of an acyl-coA thioesterase that functions as a major regulator of peroxisomal lipid metabolism.

Peroxisomes function in beta-oxidation of very long and long-chain fatty acids, dicarboxylic fatty acids, bile acid intermediates, prostaglandins, leukotrienes, thromboxanes, pristanic acid, and xenobiotic carboxylic acids. These lipids are mainly chain-shortened for excretion as the carboxylic acids or transported to mitochondria for further metabolism. Several of these carboxylic acids are slowly oxidized and may therefore sequester coenzyme A (CoASH). To prevent CoASH sequestration and to facilitate excretion of chain-shortened carboxylic acids, acyl-CoA thioesterases, which catalyze the hydrolysis of acyl-CoAs to the free acid and CoASH, may play important roles. Here we have cloned and characterized a peroxisomal acyl-CoA thioesterase from mouse, named PTE-2 (peroxisomal acyl-CoA thioesterase 2). PTE-2 is ubiquitously expressed and induced at mRNA level by treatment with the peroxisome proliferator WY-14,643 and fasting. Induction seen by these treatments was dependent on the peroxisome proliferator-activated receptor alpha. Recombinant PTE-2 showed a broad chain length specificity with acyl-CoAs from short- and medium-, to long-chain acyl-CoAs, and other substrates including trihydroxycoprostanoyl-CoA, hydroxymethylglutaryl-CoA, and branched chain acyl-CoAs, all of which are present in peroxisomes. Highest activities were found with the CoA esters of primary bile acids choloyl-CoA and chenodeoxycholoyl-CoA as substrates. PTE-2 activity is inhibited by free CoASH, suggesting that intraperoxisomal free CoASH levels regulate the activity of this enzyme. The acyl-CoA specificity of recombinant PTE-2 closely resembles that of purified mouse liver peroxisomes, suggesting that PTE-2 is the major acyl-CoA thioesterase in peroxisomes. Addition of recombinant PTE-2 to incubations containing isolated mouse liver peroxisomes strongly inhibited bile acid-CoA:amino acid N-acyltransferase activity, suggesting that this thioesterase can interfere with CoASH-dependent pathways. We propose that PTE-2 functions as a key regulator of peroxisomal lipid metabolism.     

Measurement and subcellular distribution of choloyl-CoA synthetase and bile acid-CoA:amino acid N-acyltransferase activities in rat liver

An improved method for assaying choloyl-CoA synthetase activity (E.C. 6.2.1.7) and two methods for specific measurement of bile acid-CoA:amino acid N-acyltransferase activity (E.C. 2.3.1) are described. The methods are shown to be reproducible, linear with respect to time and enzyme protein, and result in estimates of enzymic activity that conform to the theoretical stoichiometry of the individual reactions. Utilizing these methods, the subcellular distribution of the rat liver enzymic activity catalyzing the formation of glycine and taurine conjugates of bile acids is shown. Choloyl-CoA synthetase is associated with the microsomal membranes and bile acid-CoA:amino acid N-acyltransferase activity with the postmicrosomal supernatant. No significant amino acid N-acyltransferase activity is present in the lysosome fraction. These studies provide methods that will permit further study of the individual enzymic reactions involved in the intrahepatic conjugation of bile acids with amino acids.     

Differential regulation of cytosolic and peroxisomal bile acid amidation by PPAR alpha activation favors the formation of unconjugated bile acids

In human liver, unconjugated bile acids can be formed by the action of bile acid-CoA thioesterases (BACTEs), whereas bile acid conjugation with taurine or glycine (amidation) is catalyzed by bile acid-CoA:amino acid N-acyltransferases (BACATs). Both pathways exist in peroxisomes and cytosol. Bile acid amidation facilitates biliary excretion, whereas the accumulation of unconjugated bile acids may become hepatotoxic. We hypothesized that the formation of unconjugated and conjugated bile acids from their common substrate bile acid-CoA thioesters by BACTE and BACAT is regulated via the peroxisome proliferator-activated receptor alpha (PPARalpha). Livers from wild-type and PPARalpha-null mice either untreated or treated with the PPARalpha activator WY-14,643 were analyzed for BACTE and BACAT expression. The total liver capacity of taurochenodeoxycholate and taurocholate formation was decreased in WY-14,643-treated wild-type mice by 60% and 40%, respectively, but not in PPARalpha-null mice. Suppression of the peroxisomal BACAT activity was responsible for the decrease in liver capacity, whereas cytosolic BACAT activity was essentially unchanged by the treatment. In both cytosol and peroxisomes, the BACTE activities and protein levels were upregulated 5- to 10-fold by the treatment. These effects caused by WY-14,643 treatment were abolished in PPARalpha-null mice. The results from this study suggest that an increased formation of unconjugated bile acids occurs during PPARalpha activation.     

Hepatocyte nuclear factor 4alpha is a central regulator of bile acid conjugation

Hepatocyte nuclear factor 4alpha (HNF4alpha) has an important role in regulating the expression of liver-specific genes. Because bile acids are produced from cholesterol in liver and many enzymes involved in their biosynthesis are preferentially expressed in liver, the role of HNF4alpha in the regulation of bile acid production was examined. In mice, unconjugated bile acids are conjugated with taurine by the liver-specific enzymes, bile acid-CoA ligase and bile acid-CoA:amino acid N-acyltransferase (BAT). Mice lacking hepatic HNF4alpha expression exhibited markedly decreased expression of the very long chain acyl-CoA synthase-related gene (VLACSR), a mouse candidate for bile acid-CoA ligase, and BAT. This was associated with markedly elevated levels of unconjugated and glycine-conjugated bile acids in gallbladder. HNF4alpha was found to bind directly to the mouse VLACSR and BAT gene promoters, and the promoter activities were dependent on HNF4alpha-binding sites and HNF4alpha expression. In conclusion, HNF4alpha plays a central role in bile acid conjugation by direct regulation of VLACSR and BAT in vivo.     

Purification and characterization of bile acid-CoA:amino acid N-acyltransferase from rat liver

An in vitro study of bile acid-CoA:amino acid N-acyltransferase activity of rat liver was undertaken in order to determine whether separate amino acid-specific enzymes catalyzed the formation of glycine and taurine conjugates of bile acids as postulated by others. Polyacrylamide gel electrophoresis of 200-fold purified enzyme localized the glycine- and taurine-dependent activities to a single band. Both activities were optimal at pH 7.8 and showed similar loss of activity at pH 6.0, pH 9.0, in the presence of 5,5'-dithiobis(2-nitrobenzoic acid), and at temperatures exceeding 50 degrees. With the purified fraction, Km for glycine was 31 mM and Km for taurine was 0.8 mM. Km for several bile acid-CoA substrates was approximately 20 micron and independent of the amino acid acceptor. Only amino acids with terminal alpha- or beta-amino groups were active as acyl acceptors. Acyl donors were limited to bile acid-CoA derivatives. The data support the conclusion that the rat has a single bile acid-CoA:amino acid N-acyltransferase. The substrate kinetics are consistent with previous observations that taurine conjugates predominate in rat bile at normal hepatocellular concentrations of glycine and taurine.     

The human bile acid-CoA:amino acid N-acyltransferase functions in the conjugation of fatty acids to glycine

Bile acid-CoA:amino acid N-acyltransferase (BACAT) catalyzes the conjugation of bile acids to glycine and taurine for excretion into bile. By use of site-directed mutagenesis and sequence comparisons, we have identified Cys-235, Asp-328, and His-362 as constituting a catalytic triad in human BACAT (hBACAT) and identifying BACAT as a member of the type I acyl-CoA thioesterase gene family. We therefore hypothesized that hBACAT may also hydrolyze fatty acyl-CoAs and/or conjugate fatty acids to glycine. We show here that recombinant hBACAT also can hydrolyze long- and very long-chain saturated acyl-CoAs (mainly C16:0-C26:0) and by mass spectrometry verified that hBACAT also conjugates fatty acids to glycine. Tissue expression studies showed strong expression of BACAT in liver, gallbladder, and the proximal and distal intestine. However, BACAT is also expressed in a variety of tissues unrelated to bile acid formation and transport, suggesting important functions also in the regulation of intracellular levels of very long-chain fatty acids. Green fluorescent protein localization experiments in human skin fibroblasts showed that the hBACAT enzyme is mainly cytosolic. Therefore, the cytosolic BACAT enzyme may play important roles in protection against toxicity by accumulation of unconjugated bile acids and non-esterified very long-chain fatty acids.     

Conserved residues in the putative catalytic triad of human bile acid Coenzyme A:amino acid N-acyltransferase

Human bile acid-CoA:amino acid N-acyltransferase (hBAT), an enzyme catalyzing the conjugation of bile acids with the amino acids glycine or taurine has significant sequence homology with dienelactone hydrolases and other alpha/beta hydrolases. These enzymes have a conserved catalytic triad that maps onto the mammalian BATs at residues Cys-235, Asp-328, and His-362 of the human sequence, albeit that the hydrolases contain a serine instead of a cysteine. In the present study, the function of the putative catalytic triad of hBAT was examined by chemical modification with the cysteine alkylating reagent N-ethylmaleimide (NEM) and by site-directed mutagenesis of the triad residues followed by enzymology studies of mutant and wild-type hBATs. Treatment with NEM caused inactivation of wild-type hBAT. However, preincubation of wild-type hBAT with the substrate cholyl-CoA before NEM treatment prevented loss of N-acyltransferase activity. Substitution of His-362 or Asp-328 with alanine results in inactivation of hBAT. Although substitution of Cys-235 with serine generated an hBAT mutant with lower N-acyltransferase activity, it substantially increased the bile acid-CoA thioesterase activity compared with wild type. In summary, data from this study support the existence of an essential catalytic triad within hBAT consisting of Cys-235, His-362, and Asp-328 with Cys-235 serving as the probable nucleophile and thus the site of covalent attachment of the bile acid molecule.     

Quantification and regulation of the subcellular distribution of bile acid coenzyme A:amino acid N-acyltransferase activity in rat liver

Bile acid coenzyme A:amino acid N-acyltransferase (BAT) is responsible for the amidation of bile acids with the amino acids glycine and taurine. To quantify total BAT activity in liver subcellular organelles, livers from young adult male and female Sprague-Dawley rats were fractionated into multiple subcellular compartments. In male and female rats, 65-75% of total liver BAT activity was found in the cytosol, 15-17% was found in the peroxisomes, and 5-10% was found in the heavy mitochondrial fraction. After clofibrate treatment, male rats displayed an increase in peroxisomal BAT specific activity and a decrease in cytosolic BAT specific activity, whereas females showed an opposite response. However, there was no overall change in BAT specific activity in whole liver homogenate. Treatment with rosiglitazone or cholestyramine had no effect on BAT activity in any subcellular compartment. These experiments indicate that the majority of BAT activity in the rat liver resides in the cytosol. Approximately 15% of BAT activity is present in the peroxisomal matrix. These data support the novel finding that clofibrate treatment does not directly regulate BAT activity but does alter the subcellular localization of BAT.     

A coupled assay for bile acid:CoA ligase

A new assay for the enzyme bile acid:CoA ligase is presented. The new assay is designed to supplant the existing radiometric assays which require radiolabeled bile acids. The new assay couples the formation of bile acid-CoA to its glycination in a reaction catalyzed by bile acid-CoA:glycine N-acyltransferase. The coupling reaction utilizes [14C]glycine and the bile acid-CoA is quantitatively converted to [14C]glycobile acid. The [14C]glycobile acid is isolated by solvent extraction and quantitated by liquid scintillation counting. The method is shown to be accurate, highly sensitive, and applicable to a wide variety of bile acids.     

Rat liver bile acid CoA:amino acid N-acyltransferase: expression,characterization, and peroxisomal localization

Bile acid CoA:amino acid N-acyltransferase (BAT) is responsible for the amidation of bile acids with the amino acids taurine and glycine. Rat liver BAT (rBAT) cDNA was isolated from a rat liver lambdaZAP cDNA library and expressed in Sf9 insect cells using a baculoviral vector. rBAT displayed 65% amino acid sequence homology with human BAT (hBAT) and 85% homology with mouse BAT (mBAT). Similar to hBAT, expressed rBAT was capable of forming both taurine and glycine conjugates with cholyl-CoA. mBAT, which is highly homologous to rBAT, forms only taurine conjugated bile acids (Falany, C. N., H. Fortinberry, E. H. Leiter, and S. Barnes. 1997. Cloning and expression of mouse liver bile acid CoA: Amino acid N-acyltransferase. J. Lipid Res. 38: 86-95). Immunoblot analysis of rat tissues detected rBAT only in rat liver cytosol following homogenization and ultracentrifugation. Subcellular localization of rBAT detected activity and immunoreactive protein in both cytosol and isolated peroxisomes. Rat bile acid CoA ligase (rBAL), the enzyme responsible for the formation of bile acid CoA esters, was detected only in rat liver microsomes. Treatment of rats with clofibrate, a known peroxisomal proliferator, significantly induced rBAT activity, message, and immunoreactive protein in rat liver. Peroxisomal membrane protein-70, a marker for peroxisomes, was also induced by clofibrate, whereas rBAL activity and protein amount were not affected. In summary, rBAT is capable of forming both taurine and glycine bile acid conjugates and the enzyme is localized primarily in peroxisomes in rat liver.     

Partitioning of bile acids into subcellular organelles and the in vivo distribution of bile acids in rat liver.

1. The subcellular distribution of conjugates of cholic acid and chenodeoxycholic acid between cytosol, nuclei, mitochondria and microsomes in rat liver has been determined. 2. The partition coefficients for the distribution of these bile acids between subcellular fractions and buffer have been measured and used to construct a compartmental model of the amounts of conjugated bile acids present in the different subcellular organelles in vivo. 3. This model indicates that a large percentage of the bile acid in the rat liver is found in the nuclear fraction; 42% of the cholic acid conjugates and 27% of the chenodeoxycholic acid conjugates. Substantial amounts of bile acid are also present in microsomes and mitochondria suggesting that published estimates of the amounts of bile acids in these fractions are underestimates. 4. The model also allows the amount of bile acid which is in free solution in cytosol to be determined; 10.9% of the cholic acid conjugates and 4.1% of the chenodeoxycholic acid conjugates in rat liver were present in this fraction. Knowlege of the amount of free bile acid allows possible roles of the cytosolic bile binding proteins to be assessed.     

Purification and characterization of bile acid-CoA:amino acid N-acyltransferase from human liver

The bile acid-conjugating enzyme, bile acid-CoA: amino acid N-acyltransferase, was purified 480-fold from the soluble fraction of homogenized frozen human liver. Purification was accomplished by a combination of anion exchange chromatography, chromatofocusing, glycocholate-AH-Sepharose affinity chromatography, and high performance liquid chromatography (HPLC) gel filtration. Following purification, the reduced, denatured enzyme migrated as a single 50-kDa protein band by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A similar molecular mass was obtained for the native enzyme by HPLC gel filtration. Elution from the chromatofocusing column suggested an apparent isoelectric point of 6.0 (+/- 0.2). Using a rabbit polyclonal antibody raised against the purified enzyme, Western blot analysis using 100,000 x g human liver supernatant confirmed that the affinity-purified polyclonal antibody was specific for human liver bile acid-CoA:amino acid N-acyltransferase. The purified enzyme utilized glycine, taurine, and 2-fluoro-beta-alanine (a 5-fluorouracil catabolite), but not beta-alanine, as substrates. Kinetic studies revealed apparent Km values for taurine, 2-fluoro-beta-alanine, and glycine of 1.1, 2.2, and 5.8 mM, respectively, with corresponding Vmax values of 0.33, 0.19, and 0.77 mumol/min/mg protein. These data demonstrate that a single monomeric enzyme is responsible for the conjugation of bile acids with glycine or taurine in human liver.     

Radioassay of bile acid coenzyme A:glycine/taurine: N-acyltransferase using an n-butanol solvent extraction procedure

A rapid, specific, and sensitive radioassay for measuring bile acid CoA:glycine/taurine: N-acyltransferase (EC 2.3.1) has been developed. In this assay, 3H-labeled amino acids (glycine or taurine) are conjugated with unlabeled bile acid CoA derivatives to form 3H-labeled bile acid amidates. Following incubation, the 3H-labeled bile acid amidate is separated from the unreacted amino acid by an n-butanol extraction method. The extraction procedure was developed by evaluating the effects of buffer concentration and pH on the recovery of radiolabeled bile acid amidate standards in the presence of human hepatic cytosol. Highest recovery (greater than 90%) of bile acid amidate standards occurred under acidic conditions (pH 2) in the presence of 1% (w/v) SDS. When the radioassay and accompanying n-butanol extraction procedure were utilized to study the amidation of glycine or taurine with cholic acid in human hepatic cytosol, a single peak of radioactivity corresponding with either authentic glycocholate or taurocholate was detected in the n-butanol phase by high-performance liquid chromatography. This assay for bile acid CoA:glycine/taurine: N-acyltransferase activity was linear with incubation time and protein concentration. This assay should be useful in the biochemical studies of this enzyme, as well as in the examination of bile acid amidation in clinical liver specimens.     

Purification and characterization of the enzymes of bile acid conjugation from fish liver

1. The two steps in bile acid conjugation have been studied in subcellular fractions of liver from three species of fish; vermillion rockfish, canary rockfish and ling codfish. 2. The bile acid: coenzyme A (CoA) ligase activity in homogenates and isolated microsomes is undetectable due to indeterminate factors. 3. A purification scheme is presented which eliminates the interfering factors. The purified ligase was found to have a lower affinity for bile acids as compared to the mammalian form and to be present in much lower titer. 4. Since it appears to be the rate controlling enzyme in all species, it is expected that the rate of bile acid conjugation is much slower in non-mammalian liver as compared to mammalian liver. 5. The bile acid-CoA:taurine N-acyltransferase was found to exist as a dimer of molecular weight 100,000, in contrast to the monomeric mammalian forms. 6. The only major kinetic difference is that the fish liver forms have rates of glycine conjugation which are only 1-2% of the rate with taurine, in part due to a very high Km for glycine.     

The identification of a succinyl-CoA thioesterase suggests a novel pathway for succinate production in peroxisomes

Dicarboxylic acids are formed by omega-oxidation of fatty acids in the endoplasmic reticulum and degraded as the CoA ester via beta-oxidation in peroxisomes. Both synthesis and degradation of dicarboxylic acids occur mainly in kidney and liver, and the chain-shortened dicarboxylic acids are excreted in the urine as the free acids, implying that acyl-CoA thioesterases (ACOTs), which hydrolyze CoA esters to the free acid and CoASH, are needed for the release of the free acids. Recent studies show that peroxisomes contain several acyl-CoA thioesterases with different functions. We have now expressed a peroxisomal acyl-CoA thioesterase with a previously unknown function, ACOT4, which we show is active on dicarboxylyl-CoA esters. We also expressed ACOT8, another peroxisomal acyl-CoA thioesterase that was previously shown to hydrolyze a large variety of CoA esters. Acot4 and Acot8 are both strongly expressed in kidney and liver and are also target genes for the peroxisome proliferator-activated receptor alpha. Enzyme activity measurements with expressed ACOT4 and ACOT8 show that both enzymes hydrolyze CoA esters of dicarboxylic acids with high activity but with strikingly different specificities. Whereas ACOT4 mainly hydrolyzes succinyl-CoA, ACOT8 preferentially hydrolyzes longer dicarboxylyl-CoA esters (glutaryl-CoA, adipyl-CoA, suberyl-CoA, sebacyl-CoA, and dodecanedioyl-CoA). The identification of a highly specific succinyl-CoA thioesterase in peroxisomes strongly suggests that peroxisomal beta-oxidation of dicarboxylic acids leads to formation of succinate, at least under certain conditions, and that ACOT4 and ACOT8 are responsible for the termination of beta-oxidation of dicarboxylic acids of medium-chain length with the concomitant release of the corresponding free acids.     

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. This article is part of a Special Issue entitled: Metabolic Functions and Biogenesis of Peroxisomes in Health and Disease.     

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.     

Peroxisomes and bile acid biosynthesis

Peroxisomes play an important role in the biosynthesis of bile acids because a peroxisomal beta-oxidation step is required for the formation of the mature C24-bile acids from C27-bile acid intermediates. In addition, de novo synthesized bile acids are conjugated within the peroxisome. In this review, we describe the current state of knowledge about all aspects of peroxisomal function in bile acid biosynthesis in health and disease. The peroxisomal enzymes involved in the synthesis of bile acids have been identified, and the metabolic and pathologic consequences of a deficiency of one of these enzymes are discussed, including the potential role of nuclear receptors therein.