Zearalenone activates pregnane X receptor, constitutive androstane receptor and aryl hydrocarbon receptor and corresponding phase I target genes mRNA in primary cultures of human hepatocytes

The mycotoxin zearalenone (ZEN) is found worldwide as a contaminant in cereals and grains. ZEN subchronic and chronic toxicities are dominated by reproductive disorders in different mammalian species which have made ZEN established mammalian endocrine disrupter. Over the last 30 years of ZEN biotransformation study, the toxin was thought to undergo reductive metabolism only, with the generation in several species of α- and β-isomers of zearalenol. However, recent investigations have noticed that the mycoestrogen is prone to oxidative metabolism leading to hydroxylation of ZEN though the involvement of different cytochromes P450 (CYPs) isoforms. The aim of the present study was to further explore the effect of ZEN on regulation of some CYPs using primary cultures of human hepatocytes. For this aim, using real time RT-PCR, we monitored in a first time, the effect of ZEN on mRNA levels of pregnane X receptor (PXR), constitutive androstane receptor (CAR) and aryl hydrocarbon receptor (AhR), nuclear receptors known to be involved in the regulation of some CYPs. In a second time, we looked for ZEN effect on expression of PXR, CAR and AhR corresponding phase I target genes (CYP3A4, CYP3A5, CYP2B6, CYP2C9, CYP1A1 and CYP1A2). Finally, we realised the luciferase assay in HepG2 treated with the toxin and transiently transfected with p-CYP3A4-Luc in the presence of a hPXR vector or transfected with p-CYPA1-Luc.Our results clearly showed that ZEN activated human PXR, CAR and AhR mRNA levels in addition to some of their phase I target genes mainly CYP3A4, CYP2B6 and CYP1A1 and at lesser extent CYP3A5 and CYP2C9 at ZEN concentrations as low as 0.1 μM.

Pregnane X receptor as a therapeutic target to inhibit androgen activity

The androgen-androgen receptor signaling pathway plays an important role in the pathogenesis of prostate cancer. Accordingly, androgen deprivation has been the most effective endocrine therapy for hormone-dependent prostate cancer. Here, we report a novel pregnane X receptor (PXR)-mediated and metabolism-based mechanism to reduce androgenic tone. PXR is a nuclear receptor previously known as a xenobiotic receptor regulating the expression of drug metabolizing enzymes and transporters. We showed that genetic (using a PXR transgene) or pharmacological (using a PXR agonist) activation of PXR lowered androgenic activity and inhibited androgen-dependent prostate regeneration in castrated male mice that received daily injections of testosterone propionate by inducing the expression of cytochrome P450 (CYP)3As and hydroxysteroid sulfotransferase (SULT)2A1, which are enzymes important for the metabolic deactivation of androgens. In human prostate cancer cells, treatment with the PXR agonist rifampicin (RIF) inhibited androgen-dependent proliferation of LAPC-4 cells but had little effect on the growth of the androgen-independent isogenic LA99 cells. Down-regulation of PXR or SULT2A1 in LAPC-4 cells by short hairpin RNA or small interfering RNA abolished the RIF effect, indicating that the inhibitory effect of RIF on androgens was PXR and SULT2A1 dependent. In summary, we have uncovered a novel function of PXR in androgen homeostasis. PXR may represent a novel therapeutic target to lower androgen activity and may aid in the treatment and prevention of hormone-dependent prostate cancer.

Evaluation of computational docking to identify pregnane X receptor agonists in the ToxCast database

BACKGROUND

The pregnane X receptor (PXR) is a key transcriptional regulator of many genes [e.g., cytochrome P450s (CYP2C9, CYP3A4, CYP2B6), MDR1] involved in xenobiotic metabolism and excretion.

OBJECTIVES

As part of an evaluation of different approaches to predict compound affinity for nuclear hormone receptors, we used the molecular docking program GOLD and a hybrid scoring scheme based on similarity weighted GoldScores to predict potential PXR agonists in the ToxCast database of pesticides and other industrial chemicals. We present some of the limitations of different in vitro systems, as well as docking and ligand-based computational models.

METHODS

Each ToxCast compound was docked into the five published crystallographic structures of human PXR (hPXR), and 15 compounds were selected based on their consensus docking scores for testing. In addition, we used a Bayesian model to classify the ToxCast compounds into PXR agonists and nonagonists. hPXR activation was determined by luciferase-based reporter assays in the HepG2 and DPX-2 human liver cell lines.

RESULTS

We tested 11 compounds, of which 6 were strong agonists and 2 had weak agonist activity. Docking results of additional compounds were compared with data reported in the literature. The prediction sensitivity of PXR agonists in our sample ToxCast data set (n = 28) using docking and the GoldScore was higher than with the hybrid score at 66.7%. The prediction sensitivity for PXR agonists using GoldScore for the entire ToxCast data set (n = 308) compared with data from the NIH (National Institutes of Health) Chemical Genomics Center data was 73.8%.

CONCLUSIONS

Docking and the GoldScore may be useful for prioritizing large data sets prior to in vitro testing with good sensitivity across the sample and entire ToxCast data set for hPXR agonists.

Estrogen and progesterone receptors: from molecular structures to clinical targets

Research involving estrogen and progesterone receptors (ER and PR) have greatly contributed to our understanding of cell signaling and transcriptional regulation. In addition to the classical ER and PR nuclear actions, new signaling pathways have recently been identified due to ER and PR association with cell membranes and signal transduction proteins. Bio-informatics has unveiled how ER and PR recognize their ligands, selective modulators and co-factors, which has helped to implement them as key targets in the treatment of benign and malignant tumors. Knowledge regarding ER and PR is vast and complex; therefore, this review will focus on their isoforms, signaling pathways, co-activators and co-repressors, which lead to target gene regulation. Moreover it will highlight ER and PR involvement in benign and malignant diseases as well as pharmacological substances influencing cell signaling and provide established and new structural insights into the mechanism of activation and inhibition of these receptors.

Medical treatment of primary sclerosing cholangitis: a role for novel bile acids and other (post-)transcriptional modulators?

Primary sclerosing cholangitis (PSC) is a rare chronic cholestatic disease of the liver and bile ducts that is associated with inflammatory bowel disease, generally leads to end-stage liver disease, and is complicated by malignancies of the biliary tree and the large intestine. The pathogenesis of PSC remains enigmatic, making the development of targeted therapeutic strategies difficult. Immunosuppressive and antifibrotic therapeutic agents were ineffective or accompanied by major side effects. Ursodeoxycholic acid (UDCA) has consistently been shown to improve serum liver tests and might lower the risk of colon carcinoma and cholangiocarcinoma by yet unknown mechanisms. Whether "high dose" UDCA improves the long-term prognosis in PSC as suggested by small pilot trials remains to be demonstrated. The present overview discusses potential therapeutic options aside of targeted immunological therapies and UDCA. The C23 bile acid norUDCA has been shown to markedly improve biochemical and histological features in a mouse model of sclerosing cholangitis without any toxic effects. Studies in humans are eagerly being awaited. Nuclear receptors like the farnesoid-X receptor (FXR), pregnane-X receptor (PXR), vitamin D receptor (VDR), and peroxisome-proliferator-activator receptors (PPARs) have been shown to induce expression of diverse carriers and biotransformation enzymes of the intestinal and hepatic detoxification machinery and/or to modulate fibrogenesis. Pros and cons of respective receptor agonists for the future treatment of PSC are discussed in detail. In our view, the novel bile acid norUDCA and agonists of PPARs, VDR, and PXR appear particularly attractive for further studies in PSC.

Structural basis of human pregnane X receptor activation by the hops constituent colupulone

Hops extracts are used to alleviate menopausal symptoms and as an alternative to hormone replacement therapy, but they can produce potentially harmful drug-drug interactions. The nuclear xenobiotic receptor pregnane X receptor (PXR) is promiscuously activated by a range of structurally distinct chemicals. It has a key role in the transcriptional regulation of genes that encode xenobiotic metabolism enzymes. In this study, hops extracts are shown to induce the expression of numerous drug metabolism and excretion proteins. The beta-bitter acid colupulone is demonstrated to be a bioactive component and direct activator of human PXR. The 2.8-A resolution crystal structure of the ligand binding domain of human PXR in complex with colupulone was elucidated, and colupulone was observed to bind in a single orientation stabilized by both van der Waals and hydrogen bonding contacts. The crystal structure also indicates that related alpha- and beta-bitter acids have the capacity to serve as PXR agonists as well. Taken together, these results reveal the structural basis for drug-drug interactions mediated by colupulone and related constituents of hops extracts.

Direct inhibitory effect of glucocorticoids on corticotrophin-releasing hormone gene expression in neurones of the paraventricular nucleus in rat hypothalamic organotypic cultures

Corticotrophin-releasing hormone (CRH) in the parvocellular neurosecretory neurones of hypothalamic paraventricular nucleus governs neuroendocrine stress cascade and is the major target of the negative feedback effect of corticosteroids. To assess whether glucocorticoids exert their inhibitory effect on CRH expression directly on parvocellular neurones or indirectly through a complex neuronal circuit, we examined the effect of corticosterone (CORT) and dexamethasone (DEX) on CRH mRNA levels in slice explant cultures of the rat hypothalamus. Organotypic slice cultures were prepared from 6 days old rat pups and maintained in vitro for 14 days. CRH mRNA expression was measured by in situ hybridisation histochemistry. Under basal conditions, CRH mRNA expressing cells were exclusively revealed in the paraventricular region along the third ventricle. Inhibition of action potential spike activity by tetrodotoxin (TTX, 1 microm) reduced CRH mRNA signal in the organotypic cultures. CORT (500 nm) or DEX (50 nm) treatment for 24 h significantly inhibited CRH expression in the parvocellular neurones and this effect of corticosteroids was not affected following blockade of voltage dependent sodium channels by TTX. Forskolin-stimulated CRH mRNA levels in the paraventricular nucleus were also inhibited by CORT or DEX in the presence and in the absence of TTX. These studies identify paraventricular CRH neurones as direct target of corticosteroid feedback. Type II corticosteroid receptor agonists act directly on paraventricular neurones to inhibit basal and forskolin-induced CRH mRNA expression in explant cultures of the rat hypothalamus.

Chlordecone, a mixed pregnane X receptor (PXR) and estrogen receptor alpha (ERalpha) agonist, alters cholesterol homeostasis and lipoprotein metabolism in C57BL/6 mice

Chlordecone (CD) is one of many banned organochlorine (OC) insecticides that are widespread persistent organic pollutants. OC insecticides alter lipid homeostasis in rodents at doses that are not neurotoxic or carcinogenic. Pretreatment of mice or rats with CD altered tissue distribution of a subsequent dose of [(14)C]CD or [(14)C]cholesterol (CH). Nuclear receptors regulate expression of genes important in the homeostasis of CH and other lipids. In this study, we report that CD suppresses in vitro reporter systems for human liver X receptors (LXRs) and activates those for human farnesoid X receptor (FXR), pregnane X receptor (PXR) and estrogen receptor alpha (ERalpha) in a concentration-dependent manner (0-50 muM). Consistent with human PXR activation in vitro, three days after a single dose of CD (15 mg/kg) hepatic microsomal CYP3A11 protein increases in C57BL/6 mice. CD decreases hepatic CH ester content without altering total CH concentration. Apolipoprotein A-I (apoA-I) contents of hepatic lipoprotein-rich and microsomal fractions of CD-treated mice are higher than controls. There is a significant reduction in non-high density lipoprotein CH but not apolipoprotein B-48/100 (apoB-48/100) in plasma from CD-treated mice after a 4 h fast. At 14 days after 15 mg CD/kg apoA-I and apoB-100 proteins but not CYP3A11 protein in hepatic microsomes are similar to controls. This work indicates that altered CH homeostasis is a mode of OC insecticide action of relevance after a single dose. This at least partially explains altered CH tissue distribution in CD-pretreated mice.

Quantitative relationship between rifampicin exposure and induction of Cyp3a11 in SXR humanized mice: extrapolation to human CYP3A4 induction potential

The SXR humanized mouse model was used to quantitatively assess an in vivo induction response of the human PXR agonist, rifampicin. Three days of rifampicin treatment increased RNA expression and microsomal enzyme activity of CYP3A11, as well as significantly reduced triazolam plasma exposure. These results indicate that the humanized SXR mouse can be used as a model to predict human CYP3A4 induction and the resulting pharmacokinetic changes of CYP3A4 substrates in humans.

Nuclear receptor drug discovery

Nuclear receptors (NR) are ligand-activated transcription factors that regulate the activation of a variety of important target genes. There are 48 genes that encode NRs in the human genome, and these receptors now represent one of the most important targets for therapeutic drug development. Successful identification of selective NR modulators has transformed the NR drug discovery strategy from the designing of synthetic compounds that mimic the full function of cognate ligands to developing compounds that selectively modulate the functional activity of an NR in a manner that is distinct from the cognate ligands. Current efforts regarding NR drug development continue to focus on improving the function and tissue selectivity of drug candidates to reduce undesirable side effects. This review focuses on modulators of the glucocorticoid receptor (GR), androgen receptor (AR), and pregnane X receptor (PXR).

Regulation of CYP3A4 and CYP2B6 expression by liver X receptor agonists

The liver X receptor (LXR) agonists, 24(S),25-epoxycholesterol and T0901317, were previously shown to be capable of inducing CYP3A expression in primary cultured rodent hepatocytes through activation of the pregnane X receptor (PXR). In this study, the abilities of these two LXR agonists to regulate CYP3A4 and CYP2B6 mRNA expression in primary cultures of human hepatocytes were evaluated. Treatment with 10 or 30 microM of the endogenous oxysterol, 24(S),25-epoxycholesterol, had no effect on CYP3A4 mRNA content in five preparations of primary cultured human hepatocytes, while 30 microM 24(S),25-epoxycholesterol treatment increased CYP2B6 mRNA content by approximately two-fold. By comparison, treatment with the synthetic LXR agonist, T0901317, potently increased CYP3A4 and CYP2B6 mRNA levels in the human hepatocyte cultures, producing multi-fold increases at 10nM. Using a HepG2-based transactivation assay, T0901317 activated human PXR with an EC(50) approximately 20nM, which was more than 10-fold lower than that of the potent PXR ligand, SR-12813, while treatment with 24(S),25-epoxycholesterol failed to induce reporter expression in this assay. Therefore, while 24(S),25-epoxycholesterol-mediated PXR activation and CYP3A induction does not appear to be conserved from rodent to human, T0901317 is among the most potent known activators of human PXR.

In vitro studies of steroid hormones in neurofibromatosis 1 tumors and Schwann cells

The most common NF1 feature is the benign neurofibroma, which consists predominantly of Schwann cells. Dermal neurofibromas usually arise during puberty and increase in number throughout adulthood. Plexiform neurofibromas, associated with larger nerves, are often congenital and can be life threatening. Malignant peripheral nerve sheath tumors (MPNST) in NF1 are believed to arise from plexiforms in 5%-10% of patients. There are reports of increased potential for malignant transformation of plexiform tumors and increase in dermal neurofibromas, during pregnancy. These observations suggest that steroid hormones influence neurofibroma growth, and our work is the first to examine steroid hormone receptor expression and ligand-mediated cell growth and survival in normal human Schwann cells and neurofibroma-derived Schwann cell cultures. Immunohistochemistry and real-time PCR showed that estrogen receptors (ERs), progesterone receptor (PR), and androgen receptor are differentially expressed in primary neurofibromas and in NF1 tumor-derived Schwann cell cultures compared to normal Schwann cells. However, there is substantial heterogeneity, with no clear divisions based on tumor type or gender. The in vitro effects of steroid hormone receptor ligands on proliferation and apoptosis of early passage NF1 tumor-derived Schwann cell cultures were compared to normal Schwann cell cultures. Some statistically significant changes in proliferation and apoptosis were found, also showing heterogeneity across groups and ligands. Overall, the changes are consistent with increased cell accumulation. Our data suggest that steroid hormones can directly influence neurofibroma initiation or progression by acting through their cognate receptor, but that these effects may only apply to a subset of tumors, in either gender.

Functional evolution of the pregnane X receptor

The pregnane X receptor (PXR; NR1I2) is a nuclear hormone receptor (NR) that transcriptionally regulates genes encoding transporters and drug-metabolising enzymes in the liver and intestine. PXR activation leads to enhanced metabolism and elimination of xenobiotics and endogenous compounds such as hormones and bile salts. Relative to other vertebrate NRs, PXR has the broadest specificity for ligand activators by virtue of a large, flexible ligand-binding cavity. In addition, PXR has the most extensive sequence diversity across vertebrate species in the ligand-binding domain of any NR, with significant pharmacological differences between human and rodent PXRs, and especially marked divergence between mammalian and nonmammalian PXRs. The unusual properties of PXR complicate the use of in silico and animal models to predict in vivo human PXR pharmacology. Research into the evolutionary history of the PXR gene has also provided insight into the function of PXR in humans and other animals.

New opportunities for pregnane X receptor (PXR) targeting in drug development. lessons from Enantio- and species-specific PXR ligands identified from a discovery library of amino acid analogues

Nuclear hormone receptors (NHRs) are transcription factors that bind to lipophilic signaling molecules (ligands), and subsequently regulate the expression of target genes. Since NHR ligands have potential as therapeutic agents, one of the most active research areas in the NHR field is the synthesis and identification of small molecule ligands for NHRs. Wipf et al. have recently reported the creation of a discovery library using a new method for the synthesis of homoallylic amides, allylic amides and C-cyclopropylalkylamides. This article is intended to review the use of this discovery library to screen for activators for the pregnane X receptor (PXR), a master xenobiotic receptor that regulates the expression of Phase I and Phase II enzymes as well as drug transporters. Our screening of the discovery library identified potent PXR activators whose carbon scaffolds are distinct from the chemical structures of known PXR agonists. Moreover, we found that enantiomers of the same compound show a species-specific activation of PXR. The development of the discovery library and the implications of enantiospecificity of PXR ligand design represent the primary focus of this account.

Human pregnane X receptor antagonists and agonists define molecular requirements for different binding sites

The pregnane X receptor (PXR) is an important transcriptional regulator of the expression of xenobiotic metabolism and transporter genes. The receptor is promiscuous, binding many structural classes of molecules that act as agonists at the ligand-binding domain, triggering up-regulation of genes, increasing the metabolism and excretion of therapeutic agents, and causing drug-drug interactions. It has been suggested that human PXR antagonists represent a means to counteract such interactions. Several azoles have been hypothesized to bind the activation function-2 (AF-2) surface on the exterior of PXR when agonists are concurrently bound in the ligand-binding domain. In the present study, we have derived novel computational models for PXR agonists using different series of imidazoles, steroids, and a set of diverse molecules with experimental PXR agonist binding data. We have additionally defined a novel pharmacophore for the steroidal agonist site. All agonist pharmacophores showed that hydrophobic features are predominant. In contrast, a qualitative comparison with the corresponding PXR antagonist pharmacophore models using azoles and biphenyls showed that they are smaller and hydrophobic with increased emphasis on hydrogen bonding features. Azole antagonists were docked into a proposed hydrophobic binding pocket on the outer surface at the AF-2 site and fitted comfortably, making interactions with key amino acids involved in charge clamping. Combining computational and experimental data for different classes of molecules provided strong evidence for agonists and antagonists binding distinct regions on PXR. These observations bear significant implications for future discovery of molecules that are more selective and potent antagonists.

Discovery of a Highly Active Ligand of Human Pregnane X Receptor: A Case Study from Pharmacophore Modeling and Virtual Screening to “In Vivo” Biological Activity

The human pregnane X receptor (hPXR) is a nuclear receptor that regulates the expression of phase I and II drug-metabolizing enzymes as well as that of drug transporters. In addition, this receptor plays a critical role in cholesterol homeostasis and in protecting tissues from potentially toxic endobiotics. hPXR is activated by a broad spectrum of low-affinity compounds including xenobiotics and endobiotics such as bile acids and their precursors. Crystallographic studies revealed a ligand binding domain (LBD) with a large and conformable binding pocket that is likely to contribute to the ability of hPXR to respond to compounds of varying size and shape. Here, we describe an in silico method that allowed the identification of nine novel hPXR agonists. We further characterize the compound 1-(2-chlorophenyl)-N-[1-(1-phenylethyl)-1H-benzimidazol-5-yl]methanesulfonamide (C2BA-4), a methanesulfonamide that activates PXR specifically and more potently than does the reference compound 4-[2,2-bis(diethoxyphosphoryl)ethenyl]-2,6-ditert-butyl-phenol (SR12813) in our stable cell line expressing a Gal4-PXR and a GAL4 driven luciferase reporter gene. Furthermore treatment of primary human hepatocytes with C2BA-4 results in a marked induction of the mRNA expression of hPXR target genes, such as cytochromes P450 3A4 and 2B6. Finally, C2BA-4 is also able to induce hPXR-mediated in vivo luciferase expression in HGPXR stable bioluminescent cells implanted in mice. The study suggests new directions for the rational design of selective hPXR agonists and antagonists.

Cytochrome P450 enzyme levels in HepG2 cells and cryopreserved primary human hepatocytes and their induction in HepG2 cells

Early in vitro toxicity screening might improve the success rate of new chemical entities in pharmaceutical development. In previous studies, the advantage of cytotoxicity screening with the HepG2 cell line was shown. Cytotoxicity could be identified for 70% of the compounds in these assays as compared with known toxicity in either in vitro assays in primary hepatocytes, in in vivo assays in rats, or in (pre-)clinical development in humans. The low Phase I and II enzyme levels in HepG2 cells might have been responsible for the fact that 30% of the compounds scored negative. Therefore, we performed two follow-up studies in which Cytochrome P450 (CYP) enzymes and Phase II metabolism were examined. In the present study, the transcript levels of CYP1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, and 3A4 were measured with quantitative PCR. Results showed that transcripts of all CYPs were present in HepG2 cells, however, mRNA levels of most CYPs were dramatically lower than in primary human hepatocytes. These results were confirmed with luminometric assays which were used to measure the enzyme activities of CYP1A1, 1A2, 2C9, and 3A4. Regulation of CYP1A1, 1A2, 2B6, 2C8, 2D6, 2E1, and 3A4 by the aryl hydrocarbon receptor, pregnane X receptor and constitutive androstane receptor was studied in HepG2 cells at the mRNA and/or enzyme level. Regulation of CYP1A1, 1A2, 2B6, and 3A4 mRNA levels was similar to the regulation in primary human hepatocytes. In contrast, CYP2C8 mRNA levels are inducible in primary human hepatocytes, but not in HepG2 cells, after treatment with PXR/CAR activators. Consistent with other studies, CYP2D6 and 2E1 transcript levels were not changed after treatment with AhR, PXR, and CAR activators. Moreover, CYP1A1 and 1A2 enzyme levels could be induced by AhR agonists and CYP3A4 by PXR agonists. As a consequence of the low levels of CYPs in HepG2 cells, cytotoxicity of several compounds might have been missed or underestimated as compared with cytotoxicity in primary human hepatocytes. Inducing HepG2 cells with particular receptor stimulators might lead to higher toxicity for several of the tested compounds. Compared to primary human hepatocytes, HepG2 cells are a relatively easy-to-handle tool to study the up-regulation of CYP1A1, 1A2, 2B6, and 3A4.

Interpretation of scoring functions using 3D molecular fields. Mapping the diacyl-hydrazine-binding pocket of an insect ecdysone receptor

A method is presented for the interpretation of receptor docking score values (rough measures of binding affinities) of ligands in terms of 3D molecular field interaction contributions. The FlexX and FlexX-Pharm methods were used to dock the structures of designed sets of ligands into the ligand-binding pocket of a selected receptor. In the next step the relationship was investigated between the FlexX and CScore scores and 3D molecular fields obtained for the docked conformations of the ligands, using the CoMFA (Comparative Molecular Field Analysis) and CoMSIA (Comparative Molecular Similarity Indices Analysis) methods. The approach yielded highly significant CoMFA and CoMSIA models demonstrating that a high portion of the variance in the docking score values of the ligands can be explained by steric, electrostatic, hydrophobic, and hydrogen bond donor and acceptor molecular field interaction contributions. The approach was exemplified by using the crystal structure of the ligand-binding domain of the ecdysone receptor (EcR) of the moth Heliotis virescens as well as virtual molecule libraries of analogues of known diacyl-hydrazine (DAH) type ecdysteroid agonists. By docking appropriately designed virtual compound libraries into the DAH binding pocket of EcR followed by CoMFA and CoMSIA of the docked conformations, hitherto unexplored regions of the receptor cavity could be mapped. By mapping the significant molecular field interaction contributions onto the model of the receptor-ligand complex, important receptor-ligand interactions could be highlighted that may help the design of novel highly scored receptor ligands. An advantage of the method is that no experimental biological activity data are required to exhaustively map the receptor-binding site.

Non-monotonic dose-response relationship in steroid hormone receptor-mediated gene expression

Steroid hormone receptors are the targets of many environmental endocrine active chemicals (EACs) and synthetic drugs used in hormone therapy. While most of these chemical compounds have a unidirectional and monotonic effect, certain EACs can display non-monotonic dose-response behaviors and some synthetic drugs are selective endocrine modulators. Mechanisms underlying these complex endocrine behaviors have not been fully understood. By formulating an ordinary differential equation-based computational model, we investigated in this study the steady-state dose-response behavior of exogenous steroid ligands in an endogenous hormonal background under various parameter conditions. Our simulation revealed that non-monotonic dose-responses in gene expression can arise within the classical genomic framework of steroid signaling. Specifically, when the exogenous ligand is an agonist, a U-shaped dose-response appears as a result of the inherently nonlinear process of receptor homodimerization. This U-shaped dose-response curve can be further modulated by mixed-ligand heterodimers formed between endogenous ligand-bound and exogenous ligand-bound receptor monomers. When the heterodimer is transcriptionally inactive or repressive, the magnitude of U-shape increases; conversely, when the heterodimer is transcriptionally active, the magnitude of U-shape decreases. Additionally, we found that an inverted U-shaped dose-response can arise when the heterodimer is a strong transcription activator regardless of whether the exogenous ligand is an agonist or antagonist. Our work provides a novel mechanism for non-monotonic, particularly U-shaped, dose-response behaviors observed with certain steroid mimics, and may help not only understand how selective steroid receptor modulators work but also improve risk assessment for EACs.

The cholesterol-raising factor from coffee beans, cafestol, as an agonist ligand for the farnesoid and pregnane X receptors

Cafestol, a diterpene present in unfiltered coffee brews such as Scandinavian boiled, Turkish, and cafetière coffee, is the most potent cholesterol-elevating compound known in the human diet. Several genes involved in cholesterol homeostasis have previously been shown to be targets of cafestol, including cholesterol 7alpha-hydroxylase (CYP7A1), the rate-limiting enzyme in bile acid biosynthesis. We have examined the mechanism by which cafestol elevates serum lipid levels. Changes in several lipid parameters were observed in cafestol-treated APOE3Leiden mice, including a significant increase in serum triglyceride levels. Microarray analysis of these mice identified alterations in hepatic expression of genes involved in lipid metabolism and detoxification, many of which are regulated by the nuclear hormone receptors farnesoid X receptor (FXR) and pregnane X receptor (PXR). Further studies demonstrate that cafestol is an agonist ligand for FXR and PXR, and that cafestol down-regulates expression of the bile acid homeostatic genes CYP7A1, sterol 12alpha-hydroxylase, and Na(+)-taurocholate cotransporting polypeptide in the liver of wild-type but not FXR null mice. Cafestol did not affect genes known to be up-regulated by FXR in the liver of wild-type mice, but did increase expression of the positive FXR-target genes intestinal bile acid-binding protein and fibroblast growth factor 15 (FGF15) in the intestine. Because FGF15 has recently been shown to function in an enterohepatic regulatory pathway to repress liver expression of bile acid homeostatic genes, its direct induction in the gut may account for indirect effects of cafestol on liver gene expression. PXR-dependent gene regulation of cytochrome P450 3A11 and other targets by cafestol was also only seen in the intestine. Using a double FXR/PXR knockout mouse model, we found that both receptors contribute to the cafestol-dependent induction of intestinal FGF15 gene expression. In conclusion, cafestol acts as an agonist ligand for both FXR and PXR, and this may contribute to its impact on cholesterol homeostasis.

Pregnane X receptor activation ameliorates DSS-induced inflammatory bowel disease via inhibition of NF-kappaB target gene expression

Pregnane X receptor (PXR) expression was shown to be protective in inflammatory bowel disease (IBD). However, the mechanism by which PXR provides protection remains unclear. Wild-type and Pxr-null mice were treated with the PXR agonist pregnenolone-16alpha-carbonitrile or vehicle and administered 2.5% dextran sulfate sodium (DSS) in drinking water to induce IBD. Typical clinical symptoms were evaluated on a daily basis. In vivo intestinal permeability assays and proinflammatory cytokine analysis were performed. PXR agonist-treated mice were protected from DSS-induced colitis compared with vehicle-treated mice, as defined by body weight loss, diarrhea, rectal bleeding, colon length, and histology. Pregnenolone-16alpha-carbonitrile did not decrease the severity of IBD in Pxr-null mice. PXR agonist treatment did not increase epithelial barrier function but did decrease mRNA expression of several NF-kappaB target genes in a PXR-dependent manner. The present study clearly demonstrates a protective role for PXR agonist in DSS-induced IBD. The data suggest that PXR-mediated repression of NF-kappaB target genes in the colon is a critical mechanism by which PXR activation decreases the susceptibility of mice to DSS-induced IBD.

Pregnane X receptor and natural products: beyond drug-drug interactions

The pregnane X receptor (PXR, NR1I2) is a member of the nuclear receptor superfamily that is activated by a myriad of compounds and natural products in clinical use. Activation of PXR represents the basis for several clinically important drug-drug interactions. Although PXR activation has undesirable effects in patients on combination therapy, it also mediates the hepatoprotective effects exhibited by some herbal remedies. This review focuses on PXR activation by natural products and the potential therapeutic opportunities presented. In particular, the biological effects of St. John's Wort, gugulipid, kava kava, Coleus forskolii, Hypoxis, Sutherlandia, qing hao, wu wei zi, gan cao and other natural products are discussed. The impact of these natural products on drug metabolism and hepatoprotection is highlighted in the context of activation and antagonism of PXR.

The flame retardants, polybrominated diphenyl ethers, are pregnane X receptor activators

Polybrominated diphenyl ethers (PBDEs) are used as flame retardants and are universally present in the environment. An exponential increase in PBDE concentrations in the U.S. population have been reported over the last 3 decades. PBDEs 47 (tetraBDE) and 99 (pentaBDE) are the most commonly detected PBDE congeners in the environment and in human samples. PBDE209 (decaBDE) is the only remaining PBDE flame retardant commercially manufactured in the United States. Several PBDEs are known to induce cyp3a in rats, but the mechanism of induction remains unclear. The goal of this study was to clarify the mechanism by which PBDE congeners induce cyp3a. Treatment of C57BL6 mice with PBDEs 47, 99, and 209 induced gene expressions of cyp3a11 and 2b10, but not cyp1a1/2. Because the first two genes are known target genes of pregnane X receptor (PXR), a ligand-activated transcription factor in the nuclear hormone receptor superfamily, we hypothesized that PBDE congeners are PXR activators. Using reporter gene luciferase assays, the present data show that PBDEs 47, 99, and 209 activated PXR and its human counterpart, steroid X receptor, but not aryl hydrocarbon receptor. Furthermore, induction of cyp3a11 and 2b10 by PBDEs 47, 99, and 209 was markedly suppressed in PXR-knockout mice, indicating that PBDE congeners activate PXR in vivo. In summary, our study provides the first evidence that PBDEs are activators for xenobiotic nuclear receptor.