Characterization of the porcine constitutive androstane receptor (CAR) and its splice variants

Nuclear Receptor Pathways Mediating the Development of Boar Taint

The nuclear receptors PXR, CAR, and FXR are activated by various ligands and function as transcription factors to control the expression of genes that regulate the synthesis and metabolism of androstenone and skatole. These compounds are produced in entire male pigs and accumulate in the fat to cause the development of a meat quality issue known as boar taint. The extent of this accumulation is influenced by the synthesis and hepatic clearance of androstenone and skatole. For this reason, PXR, CAR, and FXR-mediated signaling pathways have garnered interest as potential targets for specialized treatments designed to reduce the development of boar taint. Recent research has also identified several metabolites produced by gut microbes that act as ligands for these nuclear receptors (e.g., tryptophan metabolites, short-chain fatty acids, bile acids); however, the connection between the gut microbiome and boar taint development is not clear. In this review, we describe the nuclear receptor signaling pathways that regulate the synthesis and metabolism of boar taint compounds and outline the genes involved. We also discuss several microbial-derived metabolites and dietary additives that are known or suspected nuclear receptor ligands and suggest how these compounds could be used to develop novel treatments for boar taint.

Primary hepatocytes isolated from human and porcine donors display similar patterns of cytochrome p450 expression following exposure to prototypical activators of AhR, CAR and PXR

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CYP mRNA induction were compared between human and porcine primary hepatocytes.

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Both human and porcine primary hepatocytes responded to prototypical CYP inducers.

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CYP mRNA induction displayed similar patterns in human and porcine primary hepatocytes.

The hepatic cytochrome p450’s (CYP) are of major importance for the metabolism of xenobiotics and knowledge about their regulation is crucial. This knowledge often originates from cell models; primary human hepatocytes (PHH) being the gold standard. However, due to limited availability of high-quality human donor organs, basic knowledge on alternative models are needed. Primary porcine hepatocytes (PPH) have been suggested as an alternative to PHH. Unfortunately, data comparing the response in gene-transcription to standard CYP inducers between PHH and PPH are missing. In the present study we, cultured PHH and PPH under the same conditions, treated them with standard inducers of the CYP1-3 and determined the response in gene and protein expression. The results demonstrated that in both species TCDD and omeprazole caused an increase in CYP1A/B expression. In PPH, CITCO increased the content of CYP1A/B. For the CYP2B/C/D’s, phenobarbital and rifampicin caused increases in expression. For the CYP2D’s, TCDD and omeprazole caused increased gene expression in PPH, which were not the case for PHH. Both phenobarbital, rifampicin and omeprazole increased CYP3A expression in PHH and PPH. Moreover, TCDD increased the gene expression of CYP3A in PPH; this was not the case for PHH. Multivariate data analysis found no difference in gene expression between PHH and PPH for phenobarbital, rifampicin and CITCO. However, differential clustering was observed for TCDD and omeprazole. In conclusion, despite model specificity, there are a high number of similar responses, and experiments investigating mRNA regulation made in PPH permits for a reliable translation into human setting.

Regulation of CAR and PXR Expression in Health and Disease

Pregnane X receptor (PXR, NR1I2) and constitutive androstane receptor (CAR, NR1I3) are members of the nuclear receptor superfamily that mainly act as ligand-activated transcription factors. Their functions have long been associated with the regulation of drug metabolism and disposition, and it is now well established that they are implicated in physiological and pathological conditions. Considerable efforts have been made to understand the regulation of their activity by their cognate ligand; however, additional regulatory mechanisms, among which the regulation of their expression, modulate their pleiotropic effects. This review summarizes the current knowledge on CAR and PXR expression during development and adult life; tissue distribution; spatial, temporal, and metabolic regulations; as well as in pathological situations, including chronic diseases and cancers. The expression of CAR and PXR is modulated by complex regulatory mechanisms that involve the interplay of transcription factors and also post-transcriptional and epigenetic modifications. Moreover, many environmental stimuli affect CAR and PXR expression through mechanisms that have not been elucidated.

The constitutive androstane receptor and pregnane X receptor in the brain

Since their discovery, the orphan nuclear receptors constitutive androstane receptor (CAR;NR1I3) and pregnane X receptor (PXR;NR1I2) have been regarded as master regulators of drug disposition and detoxification mechanisms. They regulate the metabolism and transport of endogenous mediators and xenobiotics in organs including the liver, intestine and brain. However, with proposals of new physiological functions for NR1I3 and NR1I2, there is increasing interest in the role of these receptors in influencing brain function. This review will summarise key findings regarding the expression and function of NR1I3 and NR1I2 in the brain, hereby highlighting the need for further research in this field.

DNA elements for constitutive androstane receptor- and pregnane X receptor-mediated regulation of bovine CYP3A28 gene

The regulation of cytochrome P450 3A (CYP3A) enzymes is established in humans, but molecular mechanisms of its basal and xenobiotic-mediated regulation in cattle are still unknown. Here, ~10 kbp of the bovine CYP3A28 gene promoter were cloned and sequenced, and putative transcription factor binding sites were predicted. The CYP3A28 proximal promoter (PP; -284/+71 bp) contained DNA elements conserved among species. Co-transfection of bovine nuclear receptors (NRs) pregnane X and constitutive androstane receptor (bPXR and bCAR) with various CYP3A28 promoter constructs into hepatoma cell lines identified two main regions, the PP and the distal fragment F3 (-6899/-4937 bp), that were responsive to bPXR (both) and bCAR (F3 fragment only). Site-directed mutagenesis and deletion of NR motif ER6, hepatocyte nuclear factor 1 (HNF-1) and HNF-4 binding sites in the PP suggested either the involvement of ER6 element in bPXR-mediated activation or the cooperation between bPXR and liver-enriched transcription factors (LETFs) in PP transactivation. A putative DR5 element within the F3 fragment was involved in bCAR-mediated PP+F3 transactivation. Although DNA enrichment by anti-human NR antibodies was quite low, ChIP investigations in control and RU486-treated BFH12 cells, suggested that retinoid X receptor α (RXRα) bound to ER6 and DR5 motifs and its recruitment was enhanced by RU486 treatment. The DR5 element seemed to be recognized mainly by bCAR, while no clear-cut results were obtained for bPXR. Present results point to species-differences in CYP3A regulation and the complexity of bovine CYP3A28 regulatory elements, but further confirmatory studies are needed.

Small-molecule modulators of PXR and CAR

Two nuclear receptors, the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), participate in the xenobiotic detoxification system by regulating the expression of drug-metabolizing enzymes and transporters in order to degrade and excrete foreign chemicals or endogenous metabolites. This review aims to expand the perceived relevance of PXR and CAR beyond their established role as master xenosensors to disease-oriented areas, emphasizing their modulation by small molecules. Structural studies of these receptors have provided much-needed insight into the nature of their binding promiscuity and the important elements that lead to ligand binding. Reports of species- and isoform-selective activation highlight the need for further scrutiny when extrapolating from animal data to humans, as animal models are at the forefront of early drug discovery. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

Investigation of the dominant positive effect of porcine farnesoid X receptor (FXR) splice variant 1

Pigs are well recognized as a model for humans in research studies due to similarities in metabolism and physiology between the two species. The potential for pigs to model humans in studying metabolic diseases is highly dependent on similarities in hepatic metabolism between the two species, including similarities in the farnesoid X receptor (FXR; NR1H4) which regulate bile acid homeostasis. During initial cloning of porcine FXR (pFXR), an alternative splice variant (pFXR-SV1) was isolated which contained a four amino acid (MYTG) insert that exerted a dominant positive effect on the wild type receptor (pFXR-WT). The current study investigated the role of this insert in the dominant positive effect. Individual point mutations were made to the first three amino acids of the MYTG insert. Mutations of the methionine (M) or threonine (T) to alanine (A) reduced the dominant positive effect, while mutation of the tyrosine (Y) to either A or phenylalanine (F) completely abolished the dominant positive effect. Treatment with the tyrosine phosphatase inhibitor sodium orthovanadate (Na3VO4) increased the dominant positive effect of pFXR-SV1 by about 30%. These results suggest that the dominant positive effect may be dependent on the phosphorylation status of the tyrosine in the MYTG insert. The human variant hFXRα+ has the same MYTG insert as pFXR-SV1, but did not cause a dominant positive effect on hFXR-WT and significantly reduced the activity of hFXR-WT. Thus, although the MYTG insert is conserved in both human and pig, the effects of this insert are different in the two species.

Metabolism of androstenone, 17β-estradiol and dihydrotestosterone in primary cultured pig hepatocytes and the role of 3β-hydroxysteroid dehydrogenase in this process

Steroids metabolism plays an important role in mammals and contributes to quality of pig meat. The main objective of this study was to identify metabolites of androstenone, 17β-estradiol and dihydrotestosterone using primary cultured pig hepatocytes as a model system. The role of 3β-hydroxysteroid dehydrogenase (3βHSD) in regulation of steroid metabolism was also validated using trilostane, a specific 3βHSD inhibitor. Steroid glucuronide conjugated metabolites were detected by liquid chromatography time of flight mass spectrometry (LC-TOF-MS). 3βHSD enzyme was essential for metabolism of androstenone to 5α-androst-16-en-3β-ol, which then formed androstenone glucuronide conjugate. Metabolism of 17β-estradiol was accompanied by formation of glucuronide-conjugated estrone and glucuronide-conjugated estradiol. The ratio of the two metabolites was around 5∶1. 3βHSD enzyme was not involved in 17β-estradiol metabolism. 5α-Dihydrotestosterone-17β-glucuronide was identified as a dihydrotestosterone metabolite, and this metabolism was related to 3βHSD enzyme activity as demonstrated by inhibition study.

Regulation of porcine hepatic cytochrome p450 - implication for boar taint

Cytochrome P450 (CYP450) is the major family of enzymes involved in the metabolism of several xenobiotic and endogenous compounds. Among substrates for CYP450 is the tryptophan metabolite skatole (3-methylindole), one of the major contributors to the off-odour associated with boar-tainted meat. The accumulation of skatole in pigs is highly dependent on the hepatic clearance by CYP450s. In recent years, the porcine CYP450 has attracted attention both in relation to meat quality and as a potential model for human CYP450. The molecular regulation of CYP450 mRNA expression is controlled by several nuclear receptors and transcription factors that are targets for numerous endogenously and exogenously produced agonists and antagonists. Moreover, CYP450 expression and activity are affected by factors such as age, gender and feeding. The regulation of porcine CYP450 has been suggested to have more similarities with human CYP450 than other animal models, including rodents. This article reviews the available data on porcine hepatic CYP450s and its implications for boar taint.

Function of alternative splicing

Almost all polymerase II transcripts undergo alternative pre-mRNA splicing. Here, we review the functions of alternative splicing events that have been experimentally determined. The overall function of alternative splicing is to increase the diversity of mRNAs expressed from the genome. Alternative splicing changes proteins encoded by mRNAs, which has profound functional effects. Experimental analysis of these protein isoforms showed that alternative splicing regulates binding between proteins, between proteins and nucleic acids as well as between proteins and membranes. Alternative splicing regulates the localization of proteins, their enzymatic properties and their interaction with ligands. In most cases, changes caused by individual splicing isoforms are small. However, cells typically coordinate numerous changes in 'splicing programs', which can have strong effects on cell proliferation, cell survival and properties of the nervous system. Due to its widespread usage and molecular versatility, alternative splicing emerges as a central element in gene regulation that interferes with almost every biological function analyzed.

Effects of nuclear receptor transactivation on boar taint metabolism and gene expression in porcine hepatocytes

The accumulation of the testicular steroid androstenone (AND) and tryptophan degradation product skatole (3MI) in fat results in boar taint, an off odor and flavor in boar meat. Increasing boar taint metabolism in the liver may help limit the deposition of AND and 3MI in fat, thereby improving meat quality. The effects of transactivation of the nuclear receptors constitutive androstane receptor (CAR), pregnane X receptor (PXR), and farnesoid X receptor (FXR) on the expression levels of several transcripts of interest and the metabolism of AND and 3MI in primary porcine hepatocytes were tested. Primary cells were isolated from mature boars, and transcript expression levels were assayed using real-time PCR. The transcripts of interest included porcine orthologs of common phase I and phase II metabolic enzymes and transcripts previously shown to be differentially expressed in boars with high boar taint levels. Transactivation of CAR, PXR, or FXR resulted in altered expression of several transcripts, including increased expression of cytochrome P450 (CYP) 2B22 by CAR, of CYP2A19, CYP2B22, CYP2C33, and CYP2C49 by PXR, of CYP2C33 and CYP2E1 by FXR, and of CYP19A2 by all three receptors. Only transactivation of PXR had a significant effect on AND metabolism, resulting in 7.5±1.5% of the initial level of AND remaining compared to 21.4±3.1% remaining with control dimethyl sulfoxide (DMSO) treatment. FXR had the greatest effect on 3MI metabolism, increasing the expression of CYP2E1 by 1.29-fold and increasing the production of the key metabolite 6-hydroxy-3-methylindole (6-OH-3MI), while decreasing 5-hydroxy-3-methylindole (5-OH-3MI) production. 3-Hydroxy-3-methyloxindole (HMOI) production was increased by CAR transactivation, while indol-3-carbinol (I3C) production was increased by PXR and FXR transactivation, and by treatment with 5β-dihydrotestosterone (5β-DHT). From this, it can be concluded that selective transactivation of PXR and FXR may be a viable means of decreasing boar taint by increasing the hepatic metabolism of AND and 3MI.

Effects of nuclear receptor transactivation on steroid hormone synthesis and gene expression in porcine Leydig cells

Male pigs are routinely castrated at a young age to prevent the formation of androstenone, a 16-androstene testicular steroid that is a major component of boar taint. The practice of castration has been increasingly viewed as unfavorable, due to both economic considerations and animal welfare concerns. Other means of controlling boar taint, including reducing the synthesis of androstenone in the testes, would eliminate the need for castration. In this study, we determined the effects of transactivation of three nuclear receptors, the constitutive androstane receptor (CAR), pregnane X receptor (PXR), and farnesoid X receptor (FXR), on gene expression and steroid hormone metabolism in primary porcine Leydig cells. Primary cells were isolated from mature boars, and transcript expression levels were assayed using real-time PCR. The transcripts of interest included porcine orthologs of common phase I and phase II metabolic enzymes, enzymes involved in steroidogenesis, and transcripts previously shown to be differentially expressed in boars with high androstenone and boar taint levels. Transactivation of CAR, PXR, or FXR increased the expression of several genes involved in steroidogenesis, including cytochrome B5A (CYB5A) and cytochrome B5 reductase 1 (CYB5R1), as well as hydroxysteroid (17-beta) dehydrogenase 4 (HSD17B4) and retinol dehydrogenase 12 (RDH12). Treatment with (6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime (CITCO), a CAR agonist, or rifampicin (RIF), a PXR agonist, resulted in significantly (p<0.05) decreased sex steroid production and significantly (p<0.05) increased production of 16-androstene steroids. Treatment with the FXR agonist chenodeoxycholic acid (CDCA) resulted in significantly (p<0.05) decreased sex steroid production. These results indicate that transactivation of these nuclear receptors may lead to increased levels of 16-androstene steroids, likely by altering the activity of CYP17A1 through CYB5A and CYB5R1 to the andien-β synthase reaction and away from the 17α-hydroxylase and C17, 20 lyase reactions.

Oxazaphosphorine bioactivation and detoxification The role of xenobiotic receptors

Oxazaphosphorines, with the most representative members including cyclophosphamide, ifosfamide, and trofosfamide, constitute a class of alkylating agents that have a broad spectrum of anticancer activity against many malignant ailments including both solid tumors such as breast cancer and hematological malignancies such as leukemia and lymphoma. Most oxazaphosphorines are prodrugs that require hepatic cytochrome P450 enzymes to generate active alkylating moieties before manifesting their chemotherapeutic effects. Meanwhile, oxazaphosphorines can also be transformed into non-therapeutic byproducts by various drug-metabolizing enzymes. Clinically, oxazaphosphorines are often administered in combination with other chemotherapeutics in adjuvant treatments. As such, the therapeutic efficacy, off-target toxicity, and unintentional drug-drug interactions of oxazaphosphorines have been long-lasting clinical concerns and heightened focuses of scientific literatures. Recent evidence suggests that xenobiotic receptors may play important roles in regulating the metabolism and clearance of oxazaphosphorines. Drugs as modulators of xenobiotic receptors can affect the therapeutic efficacy, cytotoxicity, and pharmacokinetics of coadministered oxazaphosphorines, providing a new molecular mechanism of drug-drug interactions. Here, we review current advances regarding the influence of xenobiotic receptors, particularly, the constitutive androstane receptor, the pregnane X receptor and the aryl hydrocarbon receptor, on the bioactivation and detoxification of oxazaphosphorines, with a focus on cyclophosphamide and ifosfamide.

Characterization of porcine pregnane X receptor, farnesoid X receptor and their splice variants

The pregnane X receptor (PXR; NR1I2) and the farnesoid X receptor (FXR; NR1H4) regulate the expression of many major metabolic enzymes. With the pig being used as a model for humans in metabolic and toxicological studies and also an important food animal, we characterized the transactivation profile of the porcine orthologs of these receptors, pgPXR and pgFXR. We compared the transactivation profiles of these receptors and their splice variants to their human orthologs using mostly endogenous ligands. Five alternatively spliced variants were identified for pgFXR as part of this study, while five alternatively spliced variants of pgPXR had been previously described. Insertions and deletions within these splice variants generated truncated proteins or proteins with altered tertiary structures, resulting in altered transactivation. Realtime polymerase chain reaction analyses showed that the pgPXR variants were present in liver cDNA samples from 3.33% to 7.92% of the total pgPXR, while the pgFXR variants were present from 1.92% to 9.26% of the total pgFXR. pgFXR was fairly evenly expressed in seven different tissues. In a luciferase reporter assay, wild-type pgPXR (pgPXR-WT) and human PXR (hPXR) responded to 12 common ligands, with similar levels of activation occurring for six of these. Wild-type pgFXR (pgFXR-WT) significantly responded to three ligands, two of which also activated hFXR. 3-Methylindole (skatole) was identified as a novel inverse agonist for pgPXR-WT and pgFXR-WT as well as porcine constitutive androstane receptor. None of the pgPXR splice variants (SVs) were active in the luciferase reporter assay on their own; pgFXR-SV1 was activated by chenodeoxycholic acid to a similar degree as pgFXR-WT. When co-transfected with their corresponding wild-type proteins, pgPXR-SV1 and pgFXR-SV1 significantly increased receptor transactivation. In conclusion, pgPXR-WT and pgFXR-WT both responded to ligands that activated their human orthologs, and some of the alternatively spliced variants significantly altered pgPXR and pgFXR transactivation at in vivo expression levels.