A systematic analysis of predicted phosphorylation sites within the human pregnane X receptor protein

Oleanolic Acid Up-Regulated UGT1A1 and Antagonized Inflammation by Affecting the Binding of PXR and PKCα to HSP90α and SRC1

Our previous studies have demonstrated that oleanolic acid (OA) can induce UGT1A1 expression in HepG2 cells by activating PXR and alleviate inflammatory damage caused by alpha-naphthyl isothiocyanate (ANIT). Activation of PKCα by phorbol-12-myristate-13-acetate (PMA) can significantly down-regulate the expression of UGT1A1 and counteract the inductive effect of OA on UGT1A1. This study aimed to explore the molecular mechanism of OA in up-regulating UGT1A1 and antagonizing inflammation based on the interaction of PXR and PKCα with HSP90α and SRC1. The expressions of PKCα, PXR, and UGT1A1, and the binding of PKCα and PXR to HSP90α and SRC1 were detected in HepG2 cells and in rats. The activation of PKCα induced by PMA or ANIT led to hyperinflammatory response and increased transfer of PKCα to the membrane, accompanied by decreased binding of PKCα to HSP90α and increased binding of PXR to HSP90α in the cytoplasm, which decreased the nuclear translocation of PXR and its binding to SRC1, and finally down-regulated the expression of UGT1A1. OA significantly inhibited the transfer of PKCα to cell membrane induced by PMA or ANIT, resulting in increased binding of PKCα to HSP90α and decreased binding of HSP90α to PXR in the cytoplasm. This facilitates PXR to enter the nucleus and increases its binding with SRC1, up-regulates UGT1A1 expression and inhibits inflammatory response. OA can affect the binding of PXR and PKCα with HSP90α and SRC1 to up-regulate the expression of UGT1A1 and finally antagonize inflammatory injury.

Insights of direct and indirect regulation of PXR through phosphorylation in fatty liver disease

The pregnane X receptor (PXR), a ligand-activated nuclear receptor, regulates the transcription of several genes that encode many enzymes and transporters related to drug metabolism. PXR also performs an important role as a physiological sensor in the modulation of endobiotic metabolism for hormones, bile acids, cholesterol, fatty acids, and glucose. Dysregulation of these PXR-mediated pathways is implicated in the progression of metabolic dysfunction-associated steatohepatitis (MASH), contributing to the complex interplay of factors involved in chronic liver disease development and exacerbation affecting millions worldwide. This review highlights the current knowledge of PXR expression and its role in endobiotic metabolism related to MASH development, which is associated with diverse causes and dire outcomes. This review focuses on elucidating the molecular pathways associated with PXR activation directly or indirectly and PXR interaction with other regulatory factors. Although there is still much to comprehend about the intricate details of these pathways, the conclusion is drawn that PXR exerts a crucial role in the pathological and physiological pathways of hepatic cellular processes, which holds promise as a potential pharmacological target for exploring novel therapeutic approaches for MASH treatment and/or prevention. SIGNIFICANCE STATEMENT: The pregnane X receptor (PXR) plays a fundamental role in regulating gene expression involved in xenobiotic and endobiotic metabolism. Dysregulation of PXR-mediated pathways is related to the development of metabolic dysfunction-associated steatohepatitis. The ligand-independent pathways regulating PXR hepatic functions through phosphorylation shed light on possible indirect molecular mechanisms and pathways that regulate PXR activity and function. Understanding these pathways may provide insight into new pharmaceutical interventions for metabolic dysfunction-associated steatohepatitis development.

Phosphorylation of nuclear receptors: Novelty and therapeutic implications

Nuclear receptors (NR) collectively regulate several biological functions in various organs. While NRs can be characterized by activation of the transcription of their signature genes, they also have other diverse roles. Although most NRs are directly activated by ligand binding, which induces cascades of events leading to gene transcription, some NRs are also phosphorylated. Despite extensive investigations, primarily focusing on unique phosphorylation of amino acid residues in different NRs, the role of phosphorylation in the biological activity of NRs in vivo has not been firmly established. Recent studies on the phosphorylation of conserved phosphorylation motifs within the DNA- and ligand-binding domains confirmed has indicated the physiologically relevance of NR phosphorylation. This review focuses on estrogen and androgen receptors, and highlights the concept of phosphorylation as a drug target.

Nuclear Receptor PXR Confers Irradiation Resistance by Promoting DNA Damage Response Through Stabilization of ATF3

Low response rate to radiotherapy remains a problem for liver and colorectal cancer patients due to inappropriate DNA damage response in tumors. Here, we report that pregnane X receptor (PXR) contributes to irradiation (IR) resistance by promoting activating transcription factor 3 (ATF3)-mediated ataxia-telangiectasia-mutated protein (ATM) activation. PXR stabilized ATF3 protein by blocking its ubiquitination. PXR–ATF3 interaction is required for regulating ATF3, as one mutant of lysine (K) 42R of ATF3 lost binding with PXR and abolished PXR-reduced ubiquitination of ATF3. On the other hand, threonine (T) 432A of PXR lost binding with ATF3 and further compromised ATM activation. Moreover, the PXR–ATF3 interaction increases ATF3 stabilization through disrupting ATF3–murine double minute 2 (MDM2) interaction and negatively regulating MDM2 protein expression. PXR enhanced MDM2 auto-ubiquitination and shortened its half-life, therefore compromising the MDM2-mediated degradation of ATF3 protein. Structurally, both ATF3 and PXR bind to the RING domain of MDM2, and on the other hand, MDM2 binds with PXR on the DNA-binding domain (DBD), which contains zinc finger sequence. Zinc finger sequence is well known for nuclear receptor peroxisome proliferator-activated receptor-γ (PPARγ) playing E3 ligase activity to degrade nuclear factor κB (NFκB)/p65. However, whether zinc-RING sequence grants E3 ligase activity to PXR remains elusive. Taken together, these results provide a novel mechanism that PXR contributes to IR resistance by promoting ATF3-mediated ATM activation through stabilization of ATF3. Our result suggests that targeting PXR may sensitize liver and colon cancer cells to IR therapy.

Insights into the critical role of the PXR in preventing carcinogenesis and chemotherapeutic drug resistance

Pregnane x receptor (PXR) as a nuclear receptor is well-established in drug metabolism, however, it has pleiotropic functions in regulating inflammatory responses, glucose metabolism, and protects normal cells against carcinogenesis. Most studies focus on its transcriptional regulation, however, PXR can regulate gene expression at the translational level. Emerging evidences have shown that PXR has a broad protein-protein interaction network, by which is implicated in the cross signaling pathways. Furthermore, the interactions between PXR and some critical proteins (e.g., p53, Tip60, p300/CBP-associated factor) in DNA damage pathway highlight its potential roles in this field. A thorough understanding of how PXR maintains genome stability and prevents carcinogenesis will help clinical diagnosis and finally benefit patients. Meanwhile, due to the regulation of CYP450 enzymes CYP3A4 and multidrug resistance protein 1 (MDR1), PXR contributes to chemotherapeutic drug resistance. It is worthy of note that the co-factor of PXR such as RXRα, also has contributions to this process, which makes the PXR-mediated drug resistance more complicated. Although single nucleotide polymorphisms (SNPs) vary between individuals, the amino acid substitution on exon of PXR finally affects PXR transcriptional activity. In this review, we have summarized the updated mechanisms that PXR protects the human body against carcinogenesis, and major contributions of PXR with its co-factors have made on multidrug resistance. Furthermore, we have also reviewed the current promising antagonist and their clinic applications in reversing chemoresistance. We believe our review will bring insight into PXR-targeted cancer therapy, enlighten the future study direction, and provide substantial evidence for the clinic in future.

The Interface between Cell Signaling Pathways and Pregnane X Receptor

Highly expressed in the enterohepatic system, pregnane X receptor (PXR, NR1I2) is a well-characterized nuclear receptor (NR) that regulates the expression of genes in the liver and intestines that encode key drug metabolizing enzymes and drug transporter proteins in mammals. The net effect of PXR activation is to increase metabolism and clear drugs and xenobiotics from the body, producing a protective effect and mediating clinically significant drug interaction in patients on combination therapy. The complete understanding of PXR biology is thus important for the development of safe and effective therapeutic strategies. Furthermore, PXR activation is now known to specifically transrepress the inflammatory- and nutrient-signaling pathways of gene expression, thereby providing a mechanism for linking these signaling pathways together with enzymatic drug biotransformation pathways in the liver and intestines. Recent research efforts highlight numerous post-translational modifications (PTMs) which significantly influence the biological function of PXR. However, this thrust of research is still in its infancy. In the context of gene-environment interactions, we present a review of the recent literature that implicates PXR PTMs in regulating its clinically relevant biology. We also provide a discussion of how these PTMs likely interface with each other to respond to extracellular cues to appropriately modify PXR activity.

PXR phosphorylated at Ser350 transduces a glucose signal to repress the estrogen sulfotransferase gene in human liver cells and fasting signal in mouse livers

Hepatic estrogen sulfotransferase (SULT1E1), the enzyme that inactivates estrogen, regulates metabolic estrogen homeostasis. Here, we have demonstrated how nuclear receptor PXR regulated the SULT1E1 gene in response to glucose in human hepatoma-derived cells and in response to fasting in mouse livers. The SULT1E1 gene was activated by a nuclear receptor HNF4α-RORα complex binding on an upstream enhancer of the SULT1E1 promoter in cells cultured in high glucose medium (Hu and Negishi, 2020). The SULT1E1 gene was repressed in cells cultured in low glucose medium, in which PXR was phosphorylated at Ser350 by vaccinia virus-related kinase 1. Phosphorylated PXR interacted with this complex, retaining HNF4α on and dissociating RORα from the enhancer as a phosphorylated PXR complex. Therefore, in response to low glucose, phosphorylated PXR transduced a low glucose signal to repress the SULT1E1 gene in cells. Hepatic Sult1e1 mRNA was induced in PXR wild type (WT) male mice in response to fasting, whereas this induction was synergistically increased in phosphorylation-blocking PXR Ser347Ala (Ser350 in human) KI males over that observed in PXR WT males. As phosphorylated PXR repressed the Sult1e1 gene, it increased its binding to the Sult1e1 promoter in WT males. The absence of phosphorylated PXR resulted in the synergistic activation of the Sult1e1 gene in PXR KI males. Apparently, phosphorylated PXR functioned as a transcriptional repressor to the SULT1E1/Sult1e1 gene in human liver cells and mouse livers.

Nuclear receptor phosphorylation in xenobiotic signal transduction

Nuclear pregnane X receptor (PXR, NR1I2) and constitutive active/androstane receptor (CAR, NR1I3) are nuclear receptors characterized in 1998 by their capability to respond to xenobiotics and activate cytochrome P450 (CYP) genes. An anti-epileptic drug, phenobarbital (PB), activates CAR and its target CYP2B genes, whereas PXR is activated by drugs such as rifampicin and statins for the CYP3A genes. Inevitably, both nuclear receptors have been investigated as ligand-activated nuclear receptors by identifying and characterizing xenobiotics and therapeutics that directly bind CAR and/or PXR to activate them. However, PB, which does not bind CAR directly, presented an alternative research avenue for an indirect ligand-mediated nuclear receptor activation mechanism: phosphorylation-mediated signal regulation. This review summarizes phosphorylation-based mechanisms utilized by xenobiotics to elicit cell signaling. First, the review presents how PB activates CAR (and other nuclear receptors) through a conserved phosphorylation motif located between two zinc fingers within its DNA-binding domain. PB-regulated phosphorylation at this motif enables nuclear receptors to form communication networks, integrating their functions. Next, the review discusses xenobiotic-induced PXR activation in the absence of the conserved DNA-binding domain phosphorylation motif. In this case, phosphorylation occurs at a motif located within the ligand-binding domain to transduce cell signaling that regulates hepatic energy metabolism. Finally, the review delves into the implications of xenobiotic-induced signaling through phosphorylation in disease development and progression.

Phosphorylation Modulates the Coregulatory Protein Exchange of the Nuclear Receptor Pregnane X Receptor

The pregnane X receptor (PXR), or nuclear receptor (NR) 1I2, is a ligand-activated NR superfamily member that is enriched in liver and intestine in mammals. Activation of PXR regulates the expression of genes encoding key proteins involved in drug metabolism, drug efflux, and drug transport. Recent mechanistic investigations reveal that post-translational modifications (PTMs), such as phosphorylation, play a critical role in modulating the bimodal function of PXR-mediated transrepression and transactivation of target gene transcription. Upon ligand binding, PXR undergoes a conformational change that promotes dissociation of histone deacetylase-containing multiprotein corepressor protein complexes while simultaneously favoring recruitment histone acetyl transferase-containing complexes. Here we describe a novel adenoviral vector used to deliver and recover recombinant human PXR protein from primary cultures of hepatocytes. Using liquid chromatography and tandem mass spectrometry we report here that PXR is phosphorylated at amino acid residues threonine 135 (T135) and serine 221 (S221). Biochemical analysis reveals that these two residues play an important regulatory role in the cycling of corepressor and coactivator multiprotein complexes. These data further our foundational knowledge regarding the specific role of PTMs, namely phosphorylation, in regulating the biology of PXR. Future efforts are focused on using the novel tools described here to identify additional PTMs and protein partners of PXR in primary cultures of hepatocytes, an important experimental model system. SIGNIFICANCE STATEMENT: Pregnane X receptor (PXR), or nuclear receptor 1I2, is a key master regulator of drug-inducible CYP gene expression in liver and intestine in mammals. The novel biochemical tools described in this study demonstrate for the first time that in cultures of primary hepatocytes, human PXR is phosphorylated at amino acid residues threonine 135 (T135) and serine 221 (S221). Moreover, phosphorylation of PXR promotes the transrepression of its prototypical target gene CYP3A4 through modulating its interactions with coregulatory proteins.

Reviewing the mechanisms of natural product-drug interactions involving efflux transporters and metabolic enzymes

The World Health Organization (WHO) and other worldwide health agencies have recently taken initiatives to encourage the use of traditional medicine and/or complementary/alternative medicine in order to promote well-being and public health. In this way, one of the WHO's concerns is the safe use of these therapies. Phytotherapy is a strategy consisting of the use of medicinal plants (MP) and/or herbal medicinal products (HMP) for medicinal purposes. The use of phytotherapy concomitantly with drugs may cause interactions compromising the expected pharmacological action or generating toxic effects. These interactions are complex processes that may occur with multiple medications targeting different metabolic pathways, and involving different compounds present in MP and HMP. Thus, the aim of this review was to summarize the main MP- and HMP-drug interactions that involve specific transporters (P-glycoprotein and BCRP) and CYP450 enzymes (CYP3A4 and CYP2D6), which play relevant roles in the mechanisms of interactions. Firstly, multiple databases were used to search studies describing in vitro or in vivo MP and HMP-drug interactions and, after that, a systematic note-taking and appraisal of the literature was conducted. It was observed that several MP and HMP, metabolic pathways and transcription factors are involved in the transporters and enzymes expression or in the modulation of their activity having the potential to provide such interactions. Thus, the knowledge of MP- and HMP-drug interaction mechanisms could contribute to prevent harmful interactions and can ensure the safe use of these products to help the establishment of the therapeutic planning in order to certify the best treatment strategy to be used.

The Role of Xenobiotic Receptors on Hepatic Glycolipid Metabolism

Background:

PXR (Pregnane X Receptor) and CAR (Constitutive Androstane Receptor) are termed as xenobiotic receptors, which are known as core factors in regulation of the transcription of metabolic enzymes and drug transporters. However, accumulating evidence has shown that PXR and CAR exert their effects on energy metabolism through the regulation of gluconeogenesis, lipogenesis and β-oxidation. Therefore, in this review, we are trying to summary recent advances to show how xenobiotic receptors regulate energy metabolism.

Methods:

A structured search of databases has been performed by using focused review topics. According to conceptual framework, the main idea of research literature was summarized and presented.

Results:

For introduction of each receptor, the general introduction and the critical functions in hepatic glucose and lipid metabolism have been included. Recent important studies have shown that CAR acts as a negative regulator of lipogenesis, gluconeogenesis and β -oxidation. PXR activation induces lipogenesis, inhibits gluconeogenesis and inhabits β-oxidation.

Conclusion:

In this review, the importance of xenobiotic receptors in hepatic glucose and lipid metabolism has been confirmed. Therefore, PXR and CAR may become new therapeutic targets for metabolic syndrome, including obesity and diabetes. However, further research is required to promote the clinical application of this new energy metabolism function of xenobiotic receptors.

Indirect activation of pregnane X receptor in the induction of hepatic CYP3A11 by high-dose rifampicin in mice

Rifampicin (RIF), a typical ligand of human pregnane X receptor (PXR), powerfully induces the expression of cytochrome P450 3A4 (CYP3A4) in humans. Although it is thought that RIF is not a ligand of rodent PXR, treatment with high-dose RIF (e.g. more than 20 mg/kg) increases the expression of CYP3A in the mouse liver. In this study, we investigated whether the induction of CYP3A by high-dose RIF in the mouse liver is mediated via indirect activation of mouse PXR (mPXR). The results showed that high-dose RIF increased the expression of CYP3A11 and other PXR-target genes in the liver of wild-type mice but not PXR-knockout mice. However, the results of reporter gene and ligand-dependent assembly assays showed that RIF does not activate mPXR in a ligand-dependent manner. In addition, high-dose RIF stimulated nuclear accumulation of mPXR in the mouse liver, and geldanamycin and okadaic acid attenuated the induction of Cyp3a11 and other PXR-target genes in primary hepatocytes, suggesting that high-dose RIF triggers nuclear translocation of mPXR. In conclusion, the present study suggests that high-dose RIF stimulates nuclear translocation of mPXR in the liver of mice by indirect activation, resulting in the transactivation of Cyp3a11 and other PXR-target genes.

Role of c-Jun-N-Terminal Kinase in Pregnane X Receptor-Mediated Induction of Human Cytochrome P4503A4 In Vitro

Cytochrome P450 CYP3A4 is the most abundant drug-metabolizing enzyme and is responsible for the metabolism of ∼50% of clinically available drugs. Induction of CYP3A4 impacts the disposition of its substrates and leads to harmful clinical consequences, such as failure of therapy. To prevent such undesirable consequences, the molecular mechanisms of regulation of CYP3A4 need to be fully understood. CYP3A4 induction is regulated primarily by the xenobiotic nuclear receptor pregnane-X receptor (PXR). After ligand binding, PXR is translocated to the nucleus, where it binds to the CYP3A4 promoter and induces its gene expression. PXR function is modulated by phosphorylation(s) by multiple kinases. In this study, we determined the role of the c-Jun N-terminal kinase (JNK) in PXR-mediated induction of CYP3A4 enzyme in vitro. Human liver carcinoma cells (HepG2) were transfected with CYP3A4 luciferase and PXR plasmids, followed by treatment with JNK inhibitor (SP600125; SP) and PXR activators rifampicin (RIF) or hyperforin. Our results indicate that SP treatment significantly attenuated PXR-mediated induction of CYP3A4 reporter activity, as well as gene expression and enzyme activity. JNK knockdown by siRNA (targeting both JNK 1 and 2) also attenuated CYP3A4 induction by RIF. Interestingly, SP treatment attenuated JNK activation by RIF. Furthermore, treatment with RIF increased PXR nuclear levels and binding to the CYP3A4 promoter; SP attenuated these effects. This study shows that JNK is a novel mechanistic regulator of CYP3A4 induction by PXR.

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.

Regulation of PXR and CAR by protein-protein interaction and signaling crosstalk

INTRODUCTION

Protein-protein interaction and signaling crosstalk contribute to the regulation of pregnane X receptor (PXR) and constitutive androstane receptor (CAR) and broaden their cellular function.

AREA COVERED

This review covers key historic discoveries and recent advances in our understanding of the broad function of PXR and CAR and their regulation by protein-protein interaction and signaling crosstalk.

EXPERT OPINION

PXR and CAR were first discovered as xenobiotic receptors; however, it is clear that PXR and CAR perform a much broader range of cellular functions through protein-protein interaction and signaling crosstalk, which typically mutually affect the function of all the partners involved. Future research on PXR and CAR should, therefore, look beyond their xenobiotic function.

Tryptophan 299 is a conserved residue of human pregnane X receptor critical for the functional consequence of ligand binding

PXR is a xenobiotic receptor that regulates drug metabolism by regulating the expression of drug-metabolizing enzymes including CYP3A4. It can be modulated by chemicals with different structures, functional groups and sizes. X-ray crystal structures of the ligand binding domain of human PXR (hPXR) alone or bound with agonists reveal a highly hydrophobic ligand binding pocket where the aromatic amino acid residue W299 appears to play a critical role in ligand binding. Here, we have investigated the role of W299 on the functional consequence of hPXR ligand binding. We first found that substitution of W299 with a hydrophobic residue retained its response to rifampicin, but substitution with a charged residue altered such agonist response in activating the transcription of CYP3A4. The activity of hPXR mutants on CYP3A4 expression correlates with the ability of hPXR mutants to interact with co-activator SRC-1. We further demonstrated that the effect of replacing W299 by residues with different side chains on hPXR's function varied depending on the specific agonist used. Finally we interpreted the cellular activity of the hPXR mutants by analyzing reported crystallographic data and proposing a model. Our findings reveal the essential role of W299 in the transactivation of hPXR in response to agonist binding, and provide useful information for designing modulators of hPXR.

A SUMO-acetyl switch in PXR biology

Post-translational modification (PTM) of nuclear receptor superfamily members regulates various aspects of their biology to include sub-cellular localization, the repertoire of protein-binding partners, as well as their stability and mode of degradation. The nuclear receptor pregnane X receptor (PXR, NR1I2) is a master-regulator of the drug-inducible gene expression in liver and intestine. The PXR-mediated gene activation program is primarily recognized to increase drug metabolism, drug transport, and drug efflux pathways in these tissues. The activation of PXR also has important implications in significant human diseases including inflammatory bowel disease and cancer. Our recent investigations reveal that PXR is modified by multiple PTMs to include phosphorylation, SUMOylation, and ubiquitination. Using both primary cultures of hepatocytes and cell-based assays, we show here that PXR is modified through acetylation on lysine residues. Further, we show that increased acetylation of PXR stimulates its increased SUMO-modification to support active transcriptional suppression. Pharmacologic inhibition of lysine de-acetylation using trichostatin A (TSA) alters the sub-cellular localization of PXR in cultured hepatocytes, and also has a profound impact upon PXR transactivation capacity. Both the acetylation and SUMOylation status of the PXR protein is affected by its ability to associate with the lysine de-acetylating enzyme histone de-acetylase (HDAC)3 in a complex with silencing mediator of retinoic acid and thyroid hormone receptor (SMRT). Taken together, our data support a model in which a SUMO-acetyl 'switch' occurs such that acetylation of PXR likely stimulates SUMO-modification of PXR to promote the active repression of PXR-target gene expression. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

Mechanisms of xenobiotic receptor activation: Direct vs. indirect

The so-called xenobiotic receptors (XRs) have functionally evolved into cellular sensors for both endogenous and exogenous stimuli by regulating the transcription of genes encoding drug-metabolizing enzymes and transporters, as well as those involving energy homeostasis, cell proliferation, and/or immune responses. Unlike prototypical steroid hormone receptors, XRs are activated through both direct ligand-binding and ligand-independent (indirect) mechanisms by a plethora of structurally unrelated chemicals. This review covers research literature that discusses direct vs. indirect activation of XRs. A particular focus is centered on the signaling control of the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), and the aryl hydrocarbon receptor (AhR). We expect that this review will shed light on both the common and distinct mechanisms associated with activation of these three XRs. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

Transcription regulation of nuclear receptor PXR: Role of SUMO-1 modification and NDSM in receptor function

Pregnane & Xenobiotic Receptor (PXR) is one of the 48 members of the nuclear receptor superfamily of ligand-modulated transcription factors. PXR plays an important role in metabolism and elimination of diverse noxious endobiotics and xenobiotics. Like in case of some nuclear receptors its function may also be differentially altered, positively or negatively, by various post-translational modifications. In this context, regulation of PXR function by SUMOylation is the subject of present investigation. Here, we report that human PXR is modified by SUMO-1 resulting in its enhanced transcriptional activity. RT-PCR analysis showed that PXR SUMOylation in presence of rifampicin also enhances the endogenous expression levels of key PXR-regulated genes like CYP3A4, CYP2C9, MDR1 and UGT1A1. In addition, mammalian two-hybrid assay exhibited enhanced interaction between PXR and co-activator SRC-1. EMSA results revealed that SUMOylation has no influence on the DNA binding ability of PXR. In silico analysis suggested that PXR protein contains four putative SUMOylation sites, centered at K108, K129, K160 and K170. In addition to this, we identified the presence of NDSM (Negative charge amino acid Dependent SUMOylation Motif) in PXR. Substitution of all its four putative lysine residues along with NDSM abolished the effect of SUMO-1-mediated transactivation function of PXR. Furthermore, we show that interaction between PXR and E2-conjugation enzyme UBCh9, an important step for implementation of SUMOylation event, was reduced in case of NDSM mutant PXRD115A. Overall, our results suggest that SUMOylation at specific sites on PXR protein are involved in enhancement of transcription function of this receptor.

Threonine-408 Regulates the Stability of Human Pregnane X Receptor through Its Phosphorylation and the CHIP/Chaperone-Autophagy Pathway

The human pregnane X receptor (hPXR) is a xenobiotic-sensing nuclear receptor that transcriptionally regulates drug metabolism-related genes. The aim of the present study was to elucidate the mechanism by which hPXR is regulated through threonine-408. A phosphomimetic mutation at threonine-408 (T408D) reduced the transcriptional activity of hPXR and its protein stability in HepG2 and SW480 cells in vitro and mouse livers in vivo. Proteasome inhibitors (calpain inhibitor I and MG132) and Hsp90 inhibitor geldanamycin, but not Hsp70 inhibitor pifithrin-μ, increased wild-type (WT) hPXR in the nucleus. The translocation of the T408D mutant to the nucleus was significantly reduced even in the presence of proteasome inhibitors, whereas the complex of yellow fluorescent protein (YFP)-hPXR T408D mutant with heat shock cognate protein 70/heat shock protein 70 and carboxy terminus Hsp70-interacting protein (CHIP; E3 ligase) was similar to that of the WT in the cytoplasm. Treatment with pifithrin-μ and transfection with anti-CHIP small-interfering RNA reduced the levels of CYP3A4 mRNA induced by rifampicin, suggesting the contribution of Hsp70 and CHIP to the transactivation of hPXR. Autophagy inhibitor 3-methyladenine accumulated YFP-hPXR T408D mutant more efficiently than the WT in the presence of proteasome inhibitor lactacystin, and the T408D mutant colocalized with the autophagy markers, microtubule-associated protein 1 light chain 3 and p62, which were contained in the autophagic cargo. Lysosomal inhibitor chloroquine caused the marked accumulation of the T408D mutant in the cytoplasm. Protein kinase C (PKC) directly phosphorylated the threonine-408 of hPXR. These results suggest that hPXR is regulated through its phosphorylation at threonine-408 by PKC, CHIP/chaperone-dependent stability check, and autophagic degradation pathway.

SUMOylation and Ubiquitylation Circuitry Controls Pregnane X Receptor Biology in Hepatocytes

Several nuclear receptor (NR) superfamily members are known to be the molecular target of either the small ubiquitin-related modifier (SUMO) or ubiquitin-signaling pathways. However, little is currently known regarding how these two post-translational modifications interact to control NR biology. We show that SUMO and ubiquitin circuitry coordinately modifies the pregnane X receptor (PXR, NR1I2) to play a key role in regulating PXR protein stability, transactivation capacity, and transcriptional repression. The SUMOylation and ubiquitylation of PXR is increased in a ligand- and tumor necrosis factor alpha -: dependent manner in hepatocytes. The SUMO-E3 ligase enzymes protein inhibitor of activated signal transducer and activator of transcription-1 (STAT1) STAT-1 (PIAS1) and protein inhibitor of activated STAT Y (PIASy) drive high levels of PXR SUMOylation. Expression of protein inhibitor of activated stat 1 selectively increases SUMO(3)ylation as well as PXR-mediated induction of cytochrome P450, family 3, subfamily A and the xenobiotic response. The PIASy-mediated SUMO(1)ylation imparts a transcriptionally repressive function by ameliorating interaction of PXR with coactivator protein peroxisome proliferator-activated receptor gamma coactivator-1-alpha. The SUMO modification of PXR is effectively antagonized by the SUMO protease sentrin protease (SENP) 2, whereas SENP3 and SENP6 proteases are highly active in the removal of SUMO2/3 chains. The PIASy-mediated SUMO(1)ylation of PXR inhibits ubiquitin-mediated degradation of this important liver-enriched NR by the 26S proteasome. Our data reveal a working model that delineates the interactive role that these two post-translational modifications play in reconciling PXR-mediated gene activation of the xenobiotic response versus transcriptional repression of the proinflammatory response in hepatocytes. Taken together, our data reveal that the SUMOylation and ubiquitylation of the PXR interface in a fundamental manner directs its biologic function in the liver in response to xenobiotic or inflammatory stress.

Serine 350 of human pregnane X receptor is crucial for its heterodimerization with retinoid X receptor alpha and transactivation of target genes in vitro and in vivo

The human pregnane X receptor (hPXR), a member of the nuclear receptor superfamily, senses xenobiotics and controls the transcription of genes encoding drug-metabolizing enzymes and transporters. The regulation of hPXR's transcriptional activation of its target genes is important for xenobiotic detoxification and endobiotic metabolism, and hPXR dysregulation can cause various adverse drug effects. Studies have implicated the putative phosphorylation site serine 350 (Ser(350)) in regulating hPXR transcriptional activity, but the mechanism of regulation remains elusive. Here we investigated the transactivation of hPXR target genes in vitro and in vivo by hPXR with a phosphomimetic mutation at Ser(350) (hPXR(S350D)). The S350D phosphomimetic mutation reduced the endogenous expression of cytochrome P450 3A4 (an hPXR target gene) in HepG2 and LS180 cells. Biochemical assays and structural modeling revealed that Ser(350) of hPXR is crucial for formation of the hPXR-retinoid X receptor alpha (RXRα) heterodimer. The S350D mutation abrogated heterodimerization in a ligand-independent manner, impairing hPXR-mediated transactivation. Further, in a novel humanized transgenic mouse model expressing the hPXR(S350D) transgene, we demonstrated that the S350D mutation alone is sufficient to impair hPXR transcriptional activity in mouse liver. This transgenic mouse model provides a unique tool to investigate the regulation and function of hPXR, including its non-genomic function, in vivo. Our finding that phosphorylation regulates hPXR activity has implications for development of novel hPXR antagonists and for safety evaluation during drug development.

Effect of erythropoietin on hepatic cytochrome P450 expression and function in an adenine-fed rat model of chronic kidney disease

BACKGROUND AND PURPOSE

Erythropoietin (EPO) is used to treat anaemia associated with chronic kidney disease (CKD). Hypoxia is associated with anaemia and is known to cause a decrease in cytochrome P450 (P450) expression. As EPO production is regulated by hypoxia, we investigated the role of EPO on P450 expression and function.

EXPERIMENTAL APPROACH

Male Wistar rats were subjected to a 0.7% adenine diet for 4 weeks to induce CKD. The diet continued for an additional 2 weeks while rats received EPO by i.p. injection every other day. Following euthanasia, hepatic P450 mRNA and protein expression were determined. Hepatic enzyme activity of selected P450s was determined and chromatin immunoprecipitation was used to characterize binding of nuclear receptors involved in the transcriptional regulation of CYP2C and CYP3A.

KEY RESULTS

EPO administration decreased hepatic mRNA and protein expression of CYP3A2 (P < 0.05), but not CYP2C11. Similarly, EPO administration decreased CYP3A2 protein expression by 81% (P < 0.001). A 32% decrease (P < 0.05) in hepatic CYP3A enzymatic activity (Vmax ) was observed for the formation of 6βOH-testosterone in the EPO-treated group. Decreases in RNA pol II recruitment (P < 0.01), hepatocyte nuclear factor 4α binding (P < 0.05) and pregnane X receptor binding (P < 0.01) to the promoter region of CYP3A were also observed in EPO-treated rats.

CONCLUSIONS AND IMPLICATIONS

Our data show that EPO decreases the expression and function of CYP3A, but not CYP2C in rat liver.

Threonine-290 regulates nuclear translocation of the human pregnane X receptor through its phosphorylation/dephosphorylation by Ca2+/calmodulin-dependent protein kinase II and protein phosphatase 1

The human pregnane X receptor (hPXR) is recognized as a xenobiotic-sensing nuclear receptor that transcriptionally regulates the gene expression of drug-metabolizing enzymes and transporters. Our study elucidates the mechanism by which the localization of hPXR is regulated through threonine-290. A phosphomimetic mutation at threonine-290 (T290D) retained hPXR in the cytoplasm of HepG2, HuH6, and SW480 cells in vitro and the mouse liver in vivo even after treatment with rifampicin, and a phosphodeficient mutation (T290A) translocated from the cytoplasm to the nucleus as the wild-type hPXR. The amount of the unphosphorylated wild-type yellow fluorescent protein-hPXR fusion protein but not the T290A mutant increased on Phos-tag gels in response to stimulations with rifampicin and cyclin-dependent kinase 2 inhibitor roscovitine, and a marked increase was observed in the unphosphorylated levels of the T290A mutant in nontreated cells. The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN93 [2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine)] and transfection with anti-CaMKII small-interfering RNA (siRNA) enhanced the unphosphorylated levels of the wild-type protein. CaMKII directly phosphorylated the threonine-290 of hPXR, and the T290A mutant conferred resistance to CaMKII. The protein phosphatase (PP) inhibitor okadaic acid (100 nM) and transfection with anti-PP1 siRNA but not anti-PP2A siRNA led to reduced expression of CYP3A4 mRNA. After the rifampicin and roscovitine stimulations, PP1 was recruited to the wild-type hPXR but not the T290A mutant. These results suggest that phosphorylation at threonine-290 by CaMKII may impair the function of hPXR by repressing its translocation to the nucleus, and dephosphorylation by PP1 is necessary for the xenobiotic-dependent nuclear translocation of hPXR.

Pregnane X receptor and drug-induced liver injury

INTRODUCTION

The liver plays a central role in transforming and clearing foreign substances. The continuous exposure of the liver to xenobiotics sometimes leads to impaired liver function, referred to as drug-induced liver injury (DILI). The pregnane X receptor (PXR) tightly regulates the expression of genes in the hepatic drug-clearance system and its undesired activation plays a role in DILI.

AREAS COVERED

This review focuses on the recent progress in understanding PXR-mediated DILI and highlights the efforts made to assess and manage PXR-mediated DILI during drug development.

EXPERT OPINION

Future efforts are needed to further elucidate the mechanisms of PXR-mediated liver injury, including the epigenetic regulation and polymorphisms of PXR. Novel in vitro models containing functional PXR could improve our ability to predict and assess DILI during drug development. PXR inhibitors may provide chemical tools to validate the potential of PXR as a therapeutic target and to develop drugs to be used in the clinic to manage PXR-mediated DILI.

Roles of xenobiotic receptors in vascular pathophysiology

The pregnane X receptor (PXR) and constitutive androstane receptor (CAR), 2 closely related and liver-enriched members of the nuclear receptor superfamily, and aryl hydrocarbon receptor (AhR), a nonnuclear receptor transcription factor (TF), are major receptors/TFs regulating the expression of genes for the clearance and detoxification of xenobiotics. They are hence defined as "xenobiotic receptors". Recent studies have demonstrated that PXR, CAR and AhR also regulate the expression of key proteins involved in endobiotic responses such as the metabolic homeostasis of lipids, glucose, and bile acid, and inflammatory processes. It is suggested that the functions of PXR, CAR and AhR may be closely implicated in the pathogeneses of metabolic vascular diseases, such as hyperlipidemia, atherogenesis, and hypertension. Therefore, manipulation of the activities of these receptors may provide novel strategies for the treatment of vascular diseases. Here, we review the pathophysiological roles of PXR, CAR and AhR in the vascular system.

U0126, a mitogen-activated protein kinase kinase 1 and 2 (MEK1 and 2) inhibitor, selectively up-regulates main isoforms of CYP3A subfamily via a pregnane X receptor (PXR) in HepG2 cells

Hepatocyte tumor cell lines lack the expression or induction properties of major cytochrome P450 (CYP) enzymes compared to primary human hepatocytes. The Ras/Raf/MEK/ERK signaling cascade contributes to hepatocarcinogenesis, dedifferentiation and loss of hepatocyte drug metabolism in hepatocyte tumors. In the present study, we examined whether MEK1/2 inhibitors can restore the expression of CYP genes in hepatocarcinoma HepG2 cells. We found that U0126, a prototype dual MEK1/2 inhibitor, is a potent inducer of CYP3A4, CYP3A5 and CYP3A7 mRNA expression (>100-fold) in HepG2 cells and CYP3A4 mRNA expression in primary human hepatocytes. This U0126-mediated induction is sensitive to the transcriptional inhibitor actinomycin D and was not detected for CYP2B6 or MDR1 mRNA expression. In gene reporter assays, U0126 activates a CYP3A4 promoter luciferase reporter construct containing PXR response elements (PXREs), but not a construct containing mutated PXREs. Based on a ligand binding assay and the examination of a PXR mutant expressing an obstructed ligand binding pocket, we found that U0126 is a ligand of PXR. We also found that U0126 up-regulates the mRNA expression of the nuclear receptors HNF4α, CAR, VDR and PXR but abolishes small heterodimer partner (SHP) corepressor expression in HepG2 cells. The MEK1/2 inhibitors PD0325901 and PD184352, as well as dominant-negative MEK1 expression, also down-regulate SHP mRNA expression. In contrast, dominant-negative MEK1 expression does not significantly induce CYP3A4 gene in HepG2 cells. In conclusion, we found that U0126 is an atypical PXR ligand that via direct (binding and activation of PXR) and indirect (SHP dowregulation) mechanisms selectively restores CYP3A genes in HepG2 cells.

Stability of the human pregnane X receptor is regulated by E3 ligase UBR5 and serine/threonine kinase DYRK2

The hPXR (human pregnane X receptor), a major chemical toxin sensor, is a ligand-induced transcription factor activated by various xenobiotics and toxins, resulting in the transcriptional up-regulation of detoxifying enzymes. To date, little is known about the upstream regulation of hPXR. Using MS analysis and a kinome-wide siRNA screen, we report that the E3 ligase UBR5 (ubiquitin protein ligase E3 component n-recognin 5) and DYRK2 (dual-specificity tyrosine-phosphorylation-regulated kinase 2) regulate hPXR stability. UBR5 knockdown resulted in accumulation of cellular hPXR and a concomitant increase in hPXR activity, whereas the rescue of UBR5 knockdown decreased the cellular hPXR level and activity. Importantly, UBR5 exerted its effect in concert with the serine/threonine kinase DYRK2, as the knockdown of DYRK2 phenocopied UBR5 knockdown. hPXR was shown to be a substrate for DYRK2, and DYRK2-dependent phosphorylation of hPXR facilitated its subsequent ubiquitination by UBR5. This is the first report of the post-translational regulation of hPXR via phosphorylation-facilitated ubiquitination by DYRK2 and UBR5. The results of the present study reveal the role of the ubiquitin-proteasomal pathway in modulating hPXR activity and indicate that pharmacological inhibitors of the ubiquitin-proteasomal pathway that regulate hPXR stability may negatively affect treatment outcome from unintended hPXR-mediated drug-drug interactions.

Identification and characterization of phosphorylation sites within the pregnane X receptor protein

Pregnane X receptor (PXR) is a xenobiotic sensor regulating the expression of genes involved in xenobiotic detoxification and elimination. Phosphorylation plays an important role in modulating PXR activity and several phosphorylation sites have been predicted and characterized in in vitro experiments. Although PXR has been shown to be a phosphoprotein in vivo, the exact residues that are phosphorylated remain elusive. Using mass spectrometry, we identified for the first time S114, T133/135, S167, and S200 residues that are phosphorylated in PXR following an in vitro kinase assay using cyclin-dependent kinase 2. We further found that the phosphorylation at S114, T133, and T135 occurred in PXR isolated from cells. We tested the phosphodeficient and phosphomimetic mutants corresponding to all the sites identified and determined that phosphorylation at S114 attenuates the transcriptional activity of PXR, consistent with the observation that the S114D mutant displayed reduced association with the PXR-targeted DNA response element. Phosphomimetic mutations at either T133 or T135 did not show a significant change in transcriptional activity however, the dual phosphomimetic mutant T133D/T135D displayed reduced transcriptional activity. Subcellular localization studies showed a varied distribution of the mutants suggesting that the regulation of PXR is much more complex than what we can observe by just overexpressing the mutants. Thus, our results provide the first direct evidence that PXR is phosphorylated at specific residues and suggest that further investigation is warranted to fully understand the regulation of PXR by phosphorylation.

Sulfotransferase genes: regulation by nuclear receptors in response to xeno/endo-biotics

Pregnane X receptor (PXR) and constitutive active/androstane receptor (CAR), members of the nuclear receptor superfamily, are two major xeno-sensing transcription factors. They can be activated by a broad range of lipophilic xenobiotics including therapeutics drugs. In addition to xenobiotics, endogenous compounds such as steroid hormones and bile acids can also activate PXR and/or CAR. These nuclear receptors regulate genes that encode enzymes and transporters that metabolize and excrete both xenobiotics and endobiotics. Sulfotransferases (SULTs) are a group of these enzymes and sulfate xenobiotics for detoxification. In general, inactivation by sulfation constitutes the mechanism to maintain homeostasis of endobiotics. Thus, deciphering the molecular mechanism by which PXR and CAR regulate SULT genes is critical for understanding the roles of SULTs in the alterations of physiological and pathophysiological processes caused by drug treatment or environmental exposures.

Induced CYP3A4 expression in confluent Huh7 hepatoma cells as a result of decreased cell proliferation and subsequent pregnane X receptor activation

We have previously shown that confluent growth of the human hepatoma cell line Huh7 substantially induces the CYP3A4 mRNA, protein, and activity levels. Here, the mechanisms behind were investigated, and a transcriptome analysis revealed significant up-regulation of liver-specific functions, whereas pathways related to proliferation and cell cycle were down-regulated in the confluent cells. Reporter analysis revealed that the CYP3A4 gene was transcriptionally activated during confluence in a process involving pregnane X receptor (PXR). PXR expression was increased, and PXR protein accumulated in the nuclei during confluent growth. The PXR ligand rifampicin further increased the expression of CYP3A4, and siRNA-mediated knock-down of PXR in confluent cells resulted in decreased CYP3A4 expression. Cyclin-dependent kinase 2 (CDK2), a known modulator of the cell cycle and a negative regulator of PXR, was more highly expressed in proliferating control cells. Trypsinization of the confluent cells and replating them subconfluent resulted in a decrease in CYP3A4 and PXR expression back to levels observed in subconfluent control cells, whereas the CDK2 levels increased. Knock-down of CDK2 in proliferating control cells increased the CYP3A4 and PXR protein levels. Moreover, the CDK inhibitor roscovitine stimulated the expression of CYP3A4. A phosphorylation-deficient mutation (S350A) in the PXR protein significantly induced the CYP3A4 transcription. In conclusion, the data strongly suggest that the increased CYP3A4 expression in confluent Huh7 cells is caused by the endogenous induction of PXR as a result of cell-cell contact inhibited proliferation and subsequent decreased CDK2 activities, indicating an endogenous, non-ligand-dependent regulation of PXR and CYP3A4, possibly of physiologic and pharmacological significance.

Regulation of pregnane X receptor (PXR) function and UGT1A1 gene expression by posttranslational modification of PXR protein

Human UDP-glucuronosyltransferase (UGT) 1A1 is a critical enzyme responsible for detoxification and metabolism of endogenous and exogenous lipophilic compounds such as bilirubin. The present study shows how cyclin-dependent kinase (CDK) inhibitor roscovitine stimulated the expression of UGT1A1 in HepG2 cells. Pregnane X receptor (PXR)-mediated transactivation of UGT1A1 reporter gene was more prominently enhanced by roscovitine, compared with the basal-, constitutive androstane receptor (CAR)-, and aryl hydrocarbon receptor-mediated activities. We determined the regulatory mechanism of UGT1A1 expression through PXR's stimulation by roscovitine. Although phosphomimetic mutations at Thr290 and Thr408 retained the PXR protein in cytoplasm and attenuated the induction of UGT1A1 expression by both roscovitine and rifampicin, a mutation at Ser350 specifically reduced the activity of PXR induced by roscovitine. Immunoprecipitation analysis revealed that the T290D but not T408D mutant protein remained in cytoplasm by forming a complex with heat shock protein 90 and cytoplasmic CAR retention protein, whereas treatment with proteasome inhibitor MG-132 accumulated the T408D mutant protein in cytoplasm. Transfection with anti-CDK2 small interfering RNA (siRNA) but not anti-CDK1 or CDK5 siRNA led to enhanced expression of UGT1A1. S350D yellow fluorescent protein-PXR fusion protein could translocate from cytoplasm to nucleus similar to the wild-type protein but was detected as an acetylated protein, whose binding with retinoid X receptor (RXR) and histone deacetylase was impaired. Cotransfection with coactivator steroid receptor coactivator (SRC) 2 but not SRC-1 partly recovered its PXR activity. These results indicate that roscovitine stimulated the expression of UGT1A1 by inhibiting CDK2, which phosphorylated PXR at Ser350 to suppress binding with RXR and coactivator and maintain the acetylation of PXR protein.

The role of residues T248, Y249 and T422 in the function of human pregnane X receptor

The pregnane X receptor (PXR) is a key xenobiotic receptor that regulates the expression of numerous drug-metabolizing enzymes. Some posttranslational mechanisms modulate its transcriptional activity. Although several kinases have been shown to directly phosphorylate this receptor, little is known about phosphorylation sites of PXR. In the present work, we examined T248, Y249 and T422 putative phosphorylation sites determined based on in silico consensus kinase site prediction analysis. T248 and T422 residues are critical for the interaction of the PXR ligand-binding domain and the activation function-2 (AF2) domain. Site-directed mutagenesis analysis was performed to generate phospho-deficient and phospho-mimetic mutants. We examined transactivation activity of the PXR mutants in gene reporter assays, formation of PXRmutant/RXRα heterodimer, binding of PXR mutants to the CYP3A4 gene response element DR3 and CYP3A4 expression in HepG2 cells after expression of the mutants. We found that T248D mutant activated CYP3A4 transactivation constitutively regardless of the presence or absence of a ligand. Contrary, T248V mutant exhibited low basal and ligand-inducible transactivation capacity as compared to wild-type PXR. Dose-response analysis revealed reduced ligand-dependent transactivation potency of PXR Y249D mutant. Transactivation of the CYP3A4 promoter was abolished with T422A/D mutants. All PXR mutants formed heterodimer with RXRα at a similar level to that observed with wild-type PXR. The ability to bind to DNA in vitro was substantially decreased in case of T248D, T422D and T248V mutants. Our data thus indicate that phosphorylation of T248, Y249 and T422 residues may be critical for the both basal and ligand-activated function of PXR.

Pregnane X receptor as a target for treatment of inflammatory bowel disorders

Pregnane X receptor (PXR; NR1I2), a member of the nuclear receptor superfamily, has a major role in the induction of genes involved in drug transport and metabolism. Recent studies in mice have provided insight into a novel function for PXR in inflammatory bowel disease (IBD). The mechanism of the protective effect of PXR activation on IBD is not fully established, but is due in part to the attenuation of nuclear factor (NF)-κB signaling that results in lower expression of proinflammatory cytokines. Recent clinical trials with the antibiotic rifaximin, a PXR agonist in the gastrointestinal system, have revealed its potential therapeutic value in the treatment of intestinal inflammation in humans. Thus, PXR may be a novel target for IBD therapy.

Role of CAR and PXR in xenobiotic sensing and metabolism

INTRODUCTION

The xenobiotic detoxification system, which protects the human body from external chemicals, comprises drug-metabolizing enzymes and transporters whose expressions are regulated by pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). The progress made in a large number of recent studies calls for a timely review to summarize and highlight these key discoveries.

AREAS COVERED

This review summarizes recent advances in elucidating the roles of PXR and CAR in the xenobiotic detoxification system. It also highlights the progress in understanding the regulation of PXR and CAR activity at the post-translational levels, as well as the structural basis for the regulation of these two xenobiotic sensors.

EXPERT OPINION

Future efforts are needed to discover novel agonists and antagonists with species and isoform selectivity, to systematically understand the regulation of PXR and CAR at multiple levels (transcriptional, post-transcriptional and post-translational levels) in response to xenobiotics exposure, and to solve the structures of the full-length receptors, which will be enabled by improved protein expression and purification techniques and approaches. In addition, more efforts will be needed to validate PXR and CAR as disease-related therapeutic targets and thus expand their roles as master xenobiotic sensors.

Site-directed mutagenesis of human cytosolic sulfotransferase (SULT) 2B1b to phospho-mimetic Ser348Asp results in an isoform with increased catalytic activity

Human SULT2B1b is distinct from other SULT isoforms due to the presence of unique amino (N)- and carboxy (C)-terminal peptides. Using site-directed mutagenesis, it was determined that phosphorylation of Ser348 was associated with nuclear localization. To investigate the effects of this phosphorylation of Ser348 on activity and cellular localization, an in silico molecular mimic was generated by mutating Ser348 to an Asp. The Asp residue mimics the shape and charge of a phospho-Ser and homology models of SULT2B1b-phospho-S348 and SULT2B1b-S348D suggest a similar significant structural rearrangement in the C-terminal peptide. To evaluate the functional consequences of this post-translational modification and predicted rearrangement, 6His-SULT2B1b-S348D was synthesized, expressed, purified and characterized. The 6His-SULT2B1b-S348D has a specific activity for DHEA sulfation ten-fold higher than recombinant 6His-SULT2B1b (209.6 and 21.8pmolmin(-1)mg(-1), respectively). Similar to native SULT2B1b, gel filtration chromatography showed SULT2B1b-S348D was enzymatically active as a homodimer. Stability assays comparing SULT2B1b and SUL2B1b-S348 demonstrated that SULT2B1b is 60% less thermostable than SULT2B1b-348D. The increased stability and sulfation activity allowed for better characterization of the sulfation kinetics for putative substrates as well as the determination of dissociation constants that were difficult to obtain with wild-type (WT) 6His-SULT2B1b. The K(D)s for DHEA and PAPS binding to 6His-SULT2B1b-S348D were 650±7nM and 265±4nM, respectively, whereas K(D)s for binding of substrates to the WT enzyme could not be determined. Characterization of the molecular mimic SULT2B1b-S348D provides a better understanding for the role of the unique structure of SULT2B1b and its effect on sulfation activity, and has allowed for improved kinetic characterization of the SULT2B1b enzyme.

A PXR-mediated negative feedback loop attenuates the expression of CYP3A in response to the PXR agonist pregnenalone-16α-carbonitrile

The nuclear receptor superfamily of ligand-activated transcription factors plays a central role in the regulation of cellular responses to chemical challenge. Nuclear receptors are activated by a wide range of both endogenous and exogenous chemicals, and their target genes include those involved in the metabolism and transport of the activating chemical. Such target gene activation, thus, acts to remove the stimulating xenobiotic or to maintain homeostatic levels of endogenous chemicals. Given the dual nature of this system it is important to understand how these two roles are balanced, such that xenobiotics are efficiently removed while not impacting negatively on homeostasis of endogenous chemicals. Using DNA microarray technology we have examined the transcriptome response of primary rat hepatocytes to two nuclear receptor ligands: Pregnenalone-16α-carbonitrile (PCN), a xenobiotic PXR agonist, and lithocholic acid, an endogenous mixed PXR/VDR/FXR agonist. We demonstrate that despite differences in the profile of activated nuclear receptors, transcriptome responses for these two ligands are broadly similar at lower concentrations, indicating a conserved general response. However, as concentrations of stimulating ligand rises, the transcriptome responses diverge, reflecting a need for specific responses to the two stimulating chemicals. Finally, we demonstrate a novel feed-back loop for PXR, whereby ligand-activation of PXR suppresses transcription of the PXR gene, acting to attenuate PXR protein expression levels at higher ligand concentrations. Through in silico simulation we demonstrate that this feed-back loop is an important factor to prevent hyperexpression of PXR target genes such as CYP3A and confirm these findings in vitro. This novel insight into the regulation of the PXR-mediated regulatory signal networks provides a potential mechanistic rationale for the robustness in steroid homeostasis within the cell.

Pregnane X receptor is SUMOylated to repress the inflammatory response

Long-term treatment of patients with the macrolide antibiotic and prototypical activator of pregnane X receptor (PXR) rifampicin (Rif) inhibits the inflammatory response in liver. We show here that activation of the inflammatory response in hepatocytes strongly modulates SUMOylation of ligand-bound PXR. We provide evidence that the SUMOylated PXR contains SUMO3 chains, and feedback represses the immune response in hepatocytes. This information represents the first step in developing novel pharmaceutical strategies to treat inflammatory liver disease and prevent adverse drug reactions in patients experiencing acute or systemic inflammation. These studies also provide a molecular rationale for constructing a novel paradigm that uniquely defines the molecular basis of the interface between PXR-mediated gene activation, drug metabolism, and inflammation.

Activation of xenobiotic receptors: driving into the nucleus

IMPORTANCE OF THE FIELD

Xenobiotic receptors (XRs) play pivotal roles in regulating the expression of genes that determine the clearance and detoxification of xenobiotics, such as drugs and environmental chemicals. Recently, it has become increasingly evident that most XRs shuttle between the cytoplasm and nucleus, and activation of such receptors is directly associated with xenobiotic-induced nuclear import.

AREAS COVERED IN THIS REVIEW

The scope of this review covers research literature that discusses nuclear translocation and activation of XRs, as well as unpublished data generated from this laboratory. Specific emphasis is given to the constitutive androstane receptor (CAR), the pregnane X receptor and the aryl hydrocarbon receptor.

WHAT THE READERS WILL GAIN

A number of molecular chaperons presumably associated with cellular localization of XRs have been identified. Primary hepatocyte cultures have been established as a unique model retaining inactive CAR in the cytoplasm. Moreover, several splicing variants of human CAR exhibit altered cellular localization and chemical activation.

TAKE HOME MESSAGE

Nuclear accumulation is an essential step in the activation of XRs. Although great strides have been made, much remains to be understood concerning the mechanisms underlying intracellular localization and trafficking of XRs, which involve both direct ligand-binding and indirect pathways.

Flavonoids activate pregnane x receptor-mediated CYP3A4 gene expression by inhibiting cyclin-dependent kinases in HepG2 liver carcinoma cells

Background

The expression of the drug-metabolizing enzyme cytochrome P450 3A4 (CYP3A4) is regulated by the pregnane × receptor (PXR), which is modulated by numerous signaling pathways, including the cyclin-dependent kinase (Cdk) pathway. Flavonoids, commonly consumed by humans as dietary constituents, have been shown to modulate various signaling pathways (e.g., inhibiting Cdks). Flavonoids have also been shown to induce CYPs expression, but the underlying mechanism of action is unknown. Here, we report the mechanism responsible for flavonoid-mediated PXR activation and CYP expression.

Results

In a cell-based screen designed to identify compounds that activate PXR-mediated CYP3A4 gene expression in HepG2 human carcinoma cells, we identified several flavonoids, such as luteolin and apigenin, as PXR activators. The flavonoids did not directly bind to PXR, suggesting that an alternative mechanism may be responsible for flavonoid-mediated PXR activation. Consistent with the Cdk5-inhibitory effect of flavonoids, Cdk5 and p35 (a non-cyclin regulatory subunit required to activate Cdk5) were expressed in HepG2. The activation of Cdk5 attenuated PXR-mediated CYP3A4 expression whereas its downregulation enhanced it. The Cdk5-mediated downregulation of CYP3A4 promoter activity was restored by flavonoids, suggesting that flavonoids activate PXR by inactivating Cdk5. In vitro kinase assays showed that Cdk5 directly phosphorylates PXR. The Cdk kinase profiling assay showed that apigenin inhibits multiple Cdks, suggesting that several Cdks may be involved in activation of PXR by flavonoids.

Conclusions

Our results for the first time link the stimulatory effect of flavonoids on CYP expression to their inhibitory effect on Cdks, through a PXR-mediated mechanism. These results may have important implications on the pharmacokinetics of drugs co-administered with herbal remedy and herbal-drug interactions.

Nuclear receptor-mediated regulation of carboxylesterase expression and activity

IMPORTANCE OF THE FIELD

Emerging evidence demonstrates that several nuclear receptor (NR) family members regulate drug-inducible expression and activity of several important carboxylesterase (CES) enzymes in mammalian liver and intestine. Numerous clinically prescribed anticancer prodrugs, carbamate and pyrethroid insecticides, environmental toxicants and procarcinogens are substrates for CES enzymes. Moreover, a key strategy used in rational drug design frequently utilizes an ester linkage methodology to selectively target a prodrug, or to improve the water solubility of a novel compound.

AREAS COVERED IN THIS REVIEW

This review summarizes the current state of knowledge regarding NR-mediated regulation of CES enzymes in mammals and highlights their importance in drug metabolism, drug-drug interactions and toxicology.

WHAT THE READER WILL GAIN

New knowledge regarding the transcriptional regulation of CES enzymes by NR proteins pregnane x receptor (NR1I2) and constitutive androstane receptor (NR1I3) has recently come to light through the use of knockout and transgenic mouse models. Novel insights regarding the species-specific cross-regulation of glucocorticoid receptor (NR3C1) and PPAR-alpha (NR1C1) signaling and CES gene expression are discussed.

TAKE HOME MESSAGE

Elucidation of the role of NR-mediated regulation of CES enzymes in liver and intestine will have a significant impact on rational drug design and the development of novel prodrugs, especially for patients on combination therapy.