Tissue-specific regulation of pregnane X receptor in cancer development and therapy

Pregnane X receptor inhibits the transdifferentiation of hepatic stellate cells by down-regulating periostin expression

Pregnane X receptor (PXR) is a xenobiotic-sensing nuclear receptor that plays a key role in drug metabolism. Recently, PXR was found to attenuate the development of liver cancer by suppressing epithelial-mesenchymal transition (EMT) in liver cancer cells in a mouse model of two-stage chemical carcinogenesis. To elucidate the role of PXR in the EMT of liver cancer cells, we focused on its role in hepatic stellate cells (HSCs), which are components of the tumor microenvironment in hepatocellular carcinoma (HCC). Human HSC-derived LX-2 cells stably expressed destabilization domain (DD)-fused human PXR (hPXR-LX2 cells). Human HCC-derived HepG2 cells were transfected with the EMT marker VIM promoter-regulated reporter plasmid and co-cultured with hPXR-LX2 cells or treated with hPXR-LX2-derived conditioned medium (CM). Co-culture or CM treatment increased reporter activity in HepG2 cells. This induction was attenuated upon PXR activation in hPXR-LX2 cells by treatment with the DD-stabilizing chemical Shield-1 and the human PXR ligand rifampicin. PXR activation in hPXR-LX2 cells exhibited inhibition of TGF-β1-induced transdifferentiation, supported by observations of morphological changes and protein or mRNA levels of the transdifferentiation markers COL1A1 and FN1. PXR activation in hPXR-LX2 cells also attenuated the mRNA levels of the key transdifferentiation factor, POSTN. Treatment of hPXR-LX2 cells with recombinant POSTN restored the PXR-mediated suppression of transdifferentiation. Reporter assays with the POSTN promoter showed that PXR inhibited the NF-κB-mediated transcription of POSTN. Consequently, PXR activation in HSCs is expected to inhibit transdifferentiation by down-regulating POSTN expression, thereby suppressing EMT of liver cancer cells.

Regulation of PXR in drug metabolism: chemical and structural perspectives

INTRODUCTION

Pregnane X receptor (PXR) is a master xenobiotic sensor that transcriptionally controls drug metabolism and disposition pathways. PXR activation by pharmaceutical drugs, natural products, environmental toxins, etc. may decrease drug efficacy and increase drug-drug interactions and drug toxicity, indicating a therapeutic value for PXR antagonists. However, PXR's functions in physiological events, such as intestinal inflammation, indicate that PXR activators may be useful in certain disease contexts.

AREAS COVERED

We review the reported roles of PXR in various physiological and pathological processes including drug metabolism, cancer, inflammation, energy metabolism, and endobiotic homeostasis. We then highlight specific cellular and chemical routes that modulate PXR activity and discuss the functional consequences. Databases searched and inclusive dates: PubMed, 1 January 1980 to 10 January 2024.

EXPERT OPINION

Knowledge of PXR's drug metabolism function has helped drug developers produce small molecules without PXR-mediated metabolic liabilities, and further understanding of PXR's cellular functions may offer drug development opportunities in multiple disease settings.

The F-box-only protein 44 regulates pregnane X receptor protein level by ubiquitination and degradation

Pregnane X receptor (PXR) is a ligand-activated nuclear receptor that transcriptionally upregulates drug-metabolizing enzymes [e.g., cytochrome P450 3A4 (CYP3A4)] and transporters. Although the regulation of PXR target genes is well-characterized, less is known about the regulation of PXR protein level. By screening an RNAi library, we identified the F-box-only protein 44 (FBXO44) as a novel E3 ligase for PXR. PXR abundance increases upon knockdown of FBXO44, and, inversely, decreases upon overexpression of FBXO44. Further analysis revealed that FBXO44 interacts with PXR, leading to its ubiquitination and proteasomal degradation, and we determined that the F-box associated domain of FBXO44 and the ligand binding domain of PXR are required for the functional interaction. In summary, FBXO44 regulates PXR protein abundance, which has downstream consequences for CYP3A4 levels and drug-drug interactions. The results of this study provide new insight into the molecular mechanisms that regulate PXR protein level and activity and suggest the importance of considering how modulating E3 ubiquitin ligase activities will affect PXR-mediated drug metabolism.

Protocatechuic acid inhibits LPS-induced mastitis in mice through activating the pregnane X receptor

Mastitis refers to the inflammation in the mammary gland caused by various reasons. Protocatechuic acid (PCA) exerts anti-inflammatory effect. However, no studies have shown the protective role of PCA on mastitis. We investigated the protective effect of PCA on LPS-induced mastitis in mice and elucidated its possible mechanism. LPS-induced mastitis model was established by injection of LPS into the mammary gland. The pathology of mammary gland, MPO activity and inflammatory cytokine production were detected to evaluate the effects of PCA on mastitis. In vivo, PCA significantly attenuated LPS-induced mammary pathological changes, MPO activity, TNF-α and IL-1β production. In vitro, the production of inflammatory cytokines TNF-α and IL-1β was significantly reduced by PCA. Furthermore, LPS-induced NF-κB activation was also inhibited by PCA. In addition, PCA was found to activate pregnane X receptor (PXR) transactivation and PCA dose-dependently increased the expression of PXR downstream molecule CYP3A4. In addition, the inhibitory effect of PCA on inflammatory cytokine production was also reversed when PXR was knocked down. In conclusion, the protective effects of PCA on LPS-induced mastitis in mice through regulating PXR.

Gefitinib Inhibits Rifampicin-Induced CYP3A4 Gene Expression in Human Hepatocytes

During multidrug combination chemotherapy, activation of the nuclear receptor and the transcription factor human pregnane xenobiotic receptor (hPXR) has been shown to play a role in the development of chemoresistance. Mechanistically, this could occur due to the cancer drug activation of hPXR and the subsequent upregulation of hPXR target genes such as the drug metabolism enzyme, cytochrome P450 3A4 (CYP3A4). In the context of hPXR-mediated drug resistance, hPXR antagonists would be useful adjuncts to PXR-activating chemotherapy. However, there are currently no clinically approved hPXR antagonists in the market. Gefitinib (GEF), a tyrosine kinase inhibitor used for the treatment of advanced non-small-cell lung cancer and effectively used in combinational chemotherapy treatments, is a promising candidate owing to its hPXR ligand-like features. We, therefore, investigated whether GEF would act as an hPXR antagonist when combined with a known hPXR agonist, rifampicin (RIF). At therapeutically relevant concentrations, GEF successfully inhibited the RIF-induced upregulation of endogenous CYP3A4 gene expression in human primary hepatocytes and human hepatocells. Additionally, GEF inhibited the RIF induction of hPXR-mediated CYP3A4 promoter activity in HepG2 human liver carcinoma cells. The computational modeling of molecular docking predicted that GEF could bind to multiple sites on hPXR including the ligand-binding pocket, allowing for potential as a direct antagonist as well as an allosteric inhibitor. Indeed, GEF bound to the ligand-binding domain of the hPXR in cell-free assays, suggesting that GEF directly interacts with the hPXR. Taken together, our results suggest that GEF, at its clinically relevant therapeutic concentration, can antagonize the hPXR agonist-induced CYP3A4 gene expression in human hepatocytes. Thus, GEF could be a potential candidate for use in combinational chemotherapies to combat hPXR agonist-induced chemoresistance. Further studies are warranted to determine whether GEF has sufficient hPXR inhibitor abilities to overcome the hPXR agonist-induced chemoresistance.

Regulation of CAR and PXR Expression in Health and Disease

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

Microbiota-Derived Metabolites in Tumor Progression and Metastasis

Microbial communities and human cells, through a dynamic crosstalk, maintain a mutualistic relationship that contributes to the maintenance of cellular metabolism and of the immune and neuronal systems. This dialogue normally occurs through the production and regulation of hormonal intermediates, metabolites, secondary metabolites, proteins, and toxins. When the balance between host and microbiota is compromised, the dynamics of this relationship change, creating favorable conditions for the development of diseases, including cancers. Microbiome metabolites can be important modulators of the tumor microenvironment contributing to regulate inflammation, proliferation, and cell death, in either a positive or negative way. Recent studies also highlight the involvement of microbiota metabolites in inducing epithelial-mesenchymal transition, thus favoring the setup of the metastatic niche. An investigation of microbe-derived metabolites in "liquid" human samples, such as plasma, serum, and urine, provide further information to clarify the relationship between host and microbiota.

CYP3A4 genetic variants are associated with susceptibility of non-small cell lung cancer in a Shaanxi Han population

PURPOSE

Lung cancer (LC) is one of the fastest-growing malignant tumors in the world in terms of morbidity and mortality. CYP3A4 plays a crucial role in the occurrence of LC. Little is known about the contribution of CYP3A4 polymorphisms for non-small cell lung cancer (NSCLC) risk. This study aimed to explore the correlation of CYP3A4 genetic variants (rs3735451, rs4646440, rs35564277, and rs4646437) with NSCLC risk.

METHODS

Four single nucleotide polymorphisms (SNPs) were genotyped by Agena MassARRAY in this case-control study (507 NSCLC patients and 505 controls) among a Shaanxi Han population. Hardy-Weinberg equilibrium (HWE) of each SNP in controls was evaluated by exact test. The association of CYP3A4 polymorphisms with NSCLC risk was explored by calculating odds ratios (OR) and 95% confidence intervals (CI) using logistic regression analysis with adjustment for age and gender.

RESULTS

Our research revealed that rs4646440 was significantly associated with an increased risk of NSCLC (OR 2.64, p = .005), while rs4646437 played a protective role in NSCLC risk (OR 0.48, p = 4.00 × 10^-7). Stratified analyses indicated that rs4646440 significantly enhanced the susceptibility of NSCLC in BMI > 24 kg/m², non-smokers and non-drinkers (OR 14.29, p = .012; OR 1.56, p = .023; OR 1.67, p = .031, respectively). Besides, we observed that rs3735451 exhibited an increased risk of NSCLC in BMI > 24 kg/m² (OR 2.47, p = .030), whereas rs4646437 had a reduced risk of NSCLC in BMI ≤ 24 kg/m² (OR 0.47, p = 5.17 × 10^-5). We also found that rs35564277 was considered as a protective factor of NSCLC in non-smokers (OR 0.50, p = .032).

CONCLUSION

Our study indicated that CYP3A4 genetic variants were associated with NSCLC susceptibility in a Shaanxi Han population.

[Pregnane X receptor promotes programmed cell death protein 4 expression in HepG2 cells]

OBJECTIVE

To investigate the role of pregnane X receptor (PXR) in the regulation of programmed cell death proteins (PDCDs) in HepG2 cells and explore the underlying molecular mechanism.

METHODS

HepG2 cells were treated with PXR agonist rifampicin (10 μmol/L) or SR12813 (1 μmol/L) for 24 h, using DMSO as the negative control. HepG2 cells were infected with constitutively activated PXR adenovirus (VP-PXR) for 36 h, with the cells infected with Mock as the negative control. The mRNA levels of PDCD2, PDCD4, PDCD5, and PDCD6 and the expression of miRNA21 were detected using qRT-PCR, and the protein level of PDCD4 was detected with Western blotting. Bioinformatic analysis was performed to predict the potential PXRresponsive elements (PXREs) motifs in the promotor region of human PDCD4.

RESULTS

The expressions of PDCD5 and PDCD6 mRNA did not differ significantly between rifampicin-treated and the control cells, while PDCD4 mRNA expression increased (t=4.209, P=0.008) and PDCD2 mRNA decreased significantly (t=-2.875, P=0.017) in rifampicin-treated cells. The mRNA expressions of PDCD2, PDCD5 and PDCD6 showed no significant difference between SR12813-treated cells and the control cells, while PDCD4 mRNA expression increased obviously in SR12813-treated cells (t=4.574, P=0.006). The PXR target gene MDR1 also increased significantly in the rifampicin- and SR12813-treated cells compared with the control cells (P=0.020 and 0.01, respectively). Infection of the cells with VP-PXR adenovirus resulted in significantly increased expression of PDCD4 and MDR1 mRNA as compared with Mock group (t=3.343, P=0.000; t=3.343, P=0.024, respectively) without causing obvious changes in PDCD2 and PDCD6 mRNA expressions. The protein level of PDCD4 increased significantly in both rifampicin (t= 2.779, P=0.025) group and VP- PXR group (t=3.066, P=0.012). The expression of miRNA21, the negative regulatory factor of PDCD4, did not differ significantly between PXR agonist group and the control group. Informatic analysis revealed the presence of putative PXREs in the 5'-flanking region of PDCD4 gene.

CONCLUSIONS

Our findings demonstrate that PXR agonism in HepG2 cells increases the expression of PDCD4, which is independent of miRNA21 pathway, and PDCD4 may be a target gene of PXR in HepG2 cells.

Germline Polymorphisms in the Nuclear Receptors PXR and VDR as Novel Prognostic Markers in Metastatic Colorectal Cancer Patients Treated With FOLFIRI

Nuclear receptors act as mediators of cancer-related inflammation and gene expression. They have a regulatory effect on genes encoding proteins related to drug adsorption, distribution, metabolism, and excretion. The aim of the present study was to highlight novel prognostic markers among polymorphisms in genes encoding for nuclear receptor proteins and inflammation-related cytokines in patients treated with a FOLFIRI regimen. This study included two independent cohorts comprising a total of 337 mCRC patients homogeneously treated with first-line FOLFIRI. Genotyping of 246 haplotype-tagging polymorphisms in 22 genes was performed using bead array technology. The NR1I2 (PXR)-rs1054190 and VDR-rs7299460 polymorphisms were significantly associated with patient overall survival (OS). A detrimental effect of the NR1I2 rs1054190-TT genotype on OS was observed in both the discovery and replication cohorts (HR = 6.84, P = 0.0021, q-value = 0.1278 and HR = 3.56, P = 0.0414, respectively). Patients harboring the NR1I2 rs1054190-TT genotype had a median OS of 9 months vs. 21 months in patients with C-allele (P < 0.0001 log-rank test). VDR rs7299460-T was consistently associated with a longer OS in both cohorts (discovery: HR = 0.61, P = 0.0075, q-value = 0.1535; replication: HR = 0.57, P = 0.0477). Patients with the VDR rs7299460-T allele had a median OS of 23 months compared to 18 months in those with the CC genotype (P = 0.0489, log-rank test). The NR1I2-rs1054190 polymorphism also had an effect on the duration of progression-free survival, consistent with the effect observed on OS. Two novel prognostic markers for mCRC treated with FOLFIRI were described and, if validated by prospective trials, have a potential application in the management of these patients.

Clinical applications of small molecule inhibitors of Pregnane X receptor

The canonical effect of Pregnane X Receptor (PXR, NR1I2) agonism includes enhanced hepatic uptake and a concomitant increase in the first-pass metabolism and efflux of drugs in mammalian liver and intestine. In patients undergoing combination therapy, PXR-mediated gene regulation represents the molecular basis of numerous food-drug, herb-drug, and drug-drug interactions. Moreover, PXR activation promotes chemotherapeutic resistance in certain malignancies. Additional research efforts suggest that sustained PXR activation exacerbates the development of fatty liver disease. Additional metabolic effects of PXR activation in liver are the inhibition of fatty acid oxidation and gluconeogenesis. The identification of non-toxic and selective PXR antagonists is therefore of current research interest. Inhibition of PXR should decrease adverse effects, improve therapeutic effectiveness, and advance clinical outcomes in patients with cancer, fatty liver, and diabetes. This review identifies small molecule PXR antagonists described to date, discusses possible molecular mechanisms of inhibition, and seeks to describe the likely biomedical consequences of the inhibition of this nuclear receptor superfamily member.

Belinostat, at Its Clinically Relevant Concentrations, Inhibits Rifampicin-Induced CYP3A4 and MDR1 Gene Expression

Activation of human pregnane X receptor (hPXR) has been associated with induction of chemoresistance. It has been proposed that such chemoresistance via cytochrome P450/drug transporters can be reversed with the use of antagonists that specifically abrogate agonist-mediated hPXR activation. Unfortunately, proposed antagonists lack the specificity and appropriate pharmacological characteristics that allow these features to be active in the clinic. We propose that, ideally, an hPXR antagonist would be a cancer drug itself that is part of a "cancer drug cocktail" and effective as an hPXR antagonist at therapeutic concentrations. Belinostat (BEL), a histone deacetylase inhibitor approved for the treatment of relapsed/refractory peripheral T-cell lymphoma, and often used in combination with chemotherapy, is an attractive candidate based on its hPXR ligand-like features. We sought to determine whether these features of BEL might allow it to behave as an antagonist in combination chemotherapy regimens that include hPXR activators. BEL represses agonist-activated hPXR target gene expression at its therapeutic concentrations in human primary hepatocytes and LS174T human colon cancer cells. BEL repressed rifampicin-induced gene expression of CYP3A4 and multidrug resistance protein 1, as well as their respective protein activities. BEL decreased rifampicin-induced resistance to SN-38, the active metabolite of irinotecan, in LS174T cells. This finding indicates that BEL could suppress hPXR agonist-induced chemoresistance. BEL attenuated the agonist-induced steroid receptor coactivator-1 interaction with hPXR, and, together with molecular docking studies, the study suggests that BEL directly interacts with multiple sites on hPXR. Taken together, our results suggest that BEL, at its clinically relevant therapeutic concentration, can antagonize hPXR agonist-induced gene expression and chemoresistance.

A Xenobiotic Detoxification Pathway through Transcriptional Regulation in Filamentous Fungi

Fungi are known to utilize transcriptional regulation of genes that encode efflux transporters to detoxify xenobiotics; however, to date it is unknown how fungi transcriptionally regulate and coordinate different phases of detoxification system (phase I, modification; phase II, conjugation; and phase III, secretion). Here we present evidence of an evolutionary convergence between the fungal and mammalian lineages, whereby xenobiotic detoxification genes (phase I coding for cytochrome P450 monooxygenases [CYP450s] and phase III coding for ATP-binding cassette [ABC] efflux transporters) are transcriptionally regulated by structurally unrelated proteins. Following next-generation RNA sequencing (RNA-seq) analyses of a filamentous fungus, Sclerotinia homoeocarpa, the causal agent of dollar spot on turfgrasses, a multidrug resistant (MDR) field strain was found to overexpress phase I and III genes, coding for CYP450s and ABC transporters for xenobiotic detoxification. Furthermore, there was confirmation of a gain-of-function mutation of the fungus-specific transcription factor S. homoeocarpa XDR1 (ShXDR1), which is responsible for constitutive and induced overexpression of the phase I and III genes, resulting in resistance to multiple classes of fungicidal chemicals. This fungal pathogen detoxifies xenobiotics through coordinated transcriptional control of CYP450s, biotransforming xenobiotics with different substrate specificities and ABC transporters, excreting a broad spectrum of xenobiotics or biotransformed metabolites. A Botrytis cinerea strain harboring the mutated ShXDR1 showed increased expression of phase I (BcCYP65) and III (BcatrD) genes, resulting in resistance to fungicides. This indicates the regulatory system is conserved in filamentous fungi. This molecular genetic mechanism for xenobiotic detoxification in fungi holds potential for facilitating discovery of new antifungal drugs and further studies of convergent and divergent evolution of xenobiotic detoxification in eukaryote lineages.IMPORTANCE Emerging multidrug resistance (MDR) in pathogenic filamentous fungi is a significant threat to human health and agricultural production. Understanding mechanisms of MDR is essential to combating fungal pathogens; however, there is still limited information on MDR mechanisms conferred by xenobiotic detoxification. Here, we report for the first time that overexpression of phase I drug-metabolizing monooxygenases (cytochrome P450s) and phase III ATP-binding cassette efflux transporters is regulated by a gain-of-function mutation in the fungus-specific transcription factor in the MDR strains of the filamentous plant-pathogenic fungus Sclerotinia homoeocarpa This study establishes a novel molecular mechanism of MDR through the xenobiotic detoxification pathway in filamentous fungi, which may facilitate the discovery of new antifungal drugs to control pathogenic fungi.

Rifaximin, a pregnane X receptor (PXR) activator regulates apoptosis in a murine model of breast cancer

The present study was proposed to investigate the effect of rifaximin (RFX) on methyl nitrosourea (MNU) induced mammary gland carcinoma in albino wistar rats.

Nutrient-Gene Interaction in Colon Cancer, from the Membrane to Cellular Physiology

The International Agency for Research on Cancer recently released an assessment classifying red and processed meat as "carcinogenic to humans" on the basis of the positive association between increased consumption and risk for colorectal cancer. Diet, however, can also decrease the risk for colorectal cancer and be used as a chemopreventive strategy. Bioactive dietary molecules, such as n-3 polyunsaturated fatty acids, curcumin, and fermentable fiber, have been proposed to exert chemoprotective effects, and their molecular mechanisms have been the focus of research in the dietary/chemoprevention field. Using these bioactives as examples, this review surveys the proposed mechanisms by which they exert their effects, from the nucleus to the cellular membrane. In addition, we discuss emerging technologies involving the culturing of colonic organoids to study the physiological effects of dietary bioactives. Finally, we address future challenges to the field regarding the identification of additional molecular mechanisms and other bioactive dietary molecules that can be utilized in our fight to reduce the incidence of colorectal cancer.

Functions of pregnane X receptor in self-detoxification

Pregnane X receptor (PXR, NR1I2), a member of the nuclear receptor superfamily, is a crucial regulator of nutrient metabolism and metabolic detoxification such as metabolic syndrome, xenobiotic metabolism, inflammatory responses, glucose, cholesterol and lipid metabolism, and endocrine homeostasis. Notably, much experimental and clinical evidence show that PXR senses xenobiotics and triggers the detoxification response to prevent diseases such as diabetes, obesity, intestinal inflammatory diseases and liver fibrosis. In this review we summarize recent advances on remarkable metabolic and regulatory versatility of PXR, and we emphasizes its role and potential implication as an effective modulator of self-detoxification in animals and humans.

The expression levels of CYP3A4 and CYP3A5 serve as potential prognostic biomarkers in lung adenocarcinoma

Lung adenocarcinoma remains to be a high-mortality disease with few effective prognostic biomarkers. Novel biomarkers are urgently demanded to supplement the current prognostic biomarkers. Here, we explored the prognostic value of CYP3A4 and CYP3A5 in lung adenocarcinoma. The tissue microarray was made up of lung adenocarcinoma samples and corresponding normal lung tissues from Nanjing Medical University affiliated Cancer Hospital Tissue Bank. The expression of CYP3A4, together with CYP3A5, was detected by the chip data from Gene Expression Omnibus datasets and immunohistochemistry staining of the tissue microarray. Then, we assessed the relationships between CYP3A4 or CYP3A5 expression level and clinicopathological factors to estimate the clinical significance. Kaplan-Meier curves were applied to analyze the prognosis. Univariate and multivariate Cox analyses were subsequently applied to identify the independent prognostic factors. Immunohistochemistry staining results showed that by comparison with matched normal tissues, CYP3A4 was frequently hyper-expressed in lung adenocarcinoma tissues while CYP3A5 was hypo-expressed, which was consistent with the Gene Expression Omnibus analysis. Kaplan-Meier analysis indicated that high-CYP3A4 or low-CYP3A5 expression level predicted poor survival in lung adenocarcinoma patients. Multivariate Cox analysis found that hypo-expression of CYP3A5 was an independent prognostic factor. Further study revealed that combination of these two markers exhibited a more powerful predictor of poor prognosis, which could target to more accurate survival of lung adenocarcinoma. Our findings indicate that combination of CYP3A4 and CYP3A5 may serve as a novel prognostic biomarker in lung adenocarcinoma, which contribute to the precision of predicting the survival in lung adenocarcinoma.

Pregnane X receptor, constitutive androstane receptor and hepatocyte nuclear factors as emerging players in cancer precision medicine

Great research effort has been focused on elucidating the contribution of host genetic variability on pharmacological outcomes in cancer. Nuclear receptors have emerged as mediators between environmental stimuli and drug pharmacokinetics and pharmacodynamics. The pregnane X receptor, constitutive androstane receptor and hepatocyte nuclear factors have been reported to regulate transcription of genes that encode drug metabolizing enzymes and transporters. Altered nuclear receptor expression has been shown to affect the metabolism and pharmacological profile of traditional chemotherapeutics and targeted agents. Accordingly, polymorphic variants in these genes have been studied as pharmacogenetic markers of outcome variability. This review summarizes the state of knowledge about the roles played by pregnane X receptor, constitutive androstane receptor and hepatocyte nuclear factor expression and genetics as predictive markers of anticancer drug toxicity and efficacy, which can improve cancer precision medicine.

Pregnane X Receptor and Cancer: Context-Specificity is Key

Pregnane X receptor (PXR) is an adopted orphan nuclear receptor that is activated by a wide-range of endobiotics and xenobiotics, including chemotherapy drugs. PXR plays a major role in the metabolism and clearance of xenobiotics and endobiotics in liver and intestine via induction of drug-metabolizing enzymes and drug-transporting proteins. However, PXR is expressed in several cancer tissues and the accumulating evidence strongly points to the differential role of PXR in cancer growth and progression as well as in chemotherapy outcome. In cancer cells, besides regulating the gene expression of enzymes and proteins involved in drug metabolism and transport, PXR also regulates other genes involved in proliferation, metastasis, apoptosis, anti-apoptosis, inflammation, and oxidative stress. In this review, we focus on the differential role of PXR in a variety of cancers, including prostate, breast, ovarian, endometrial, and colon. We also discuss the future directions to further understand the differential role of PXR in cancer, and conclude with the need to identify novel selective PXR modulators to target PXR in PXR-expressing cancers.

Drug metabolism and clearance system in tumor cells of patients with multiple myeloma

Resistance to chemotherapy is a major limitation of cancer treatments with several molecular mechanisms involved, in particular altered local drug metabolism and detoxification process. The role of drug metabolism and clearance system has not been satisfactorily investigated in Multiple Myeloma (MM), a malignant plasma cell cancer for which a majority of patients escapes treatment. The expression of 350 genes encoding for uptake carriers, xenobiotic receptors, phase I and II Drug Metabolizing Enzymes (DMEs) and efflux transporters was interrogated in MM cells (MMCs) of newly-diagnosed patients in relation to their event free survival. MMCs of patients with a favourable outcome have an increased expression of genes coding for xenobiotic receptors (RXRα, LXR, CAR and FXR) and accordingly of their gene targets, influx transporters and phase I/II DMEs. On the contrary, MMCs of patients with unfavourable outcome displayed a global down regulation of genes coding for xenobiotic receptors and the downstream detoxification genes but had a high expression of genes coding for ARNT and Nrf2 pathways and ABC transporters. Altogether, these data suggests ARNT and Nrf2 pathways could be involved in MM primary resistance and that targeting RXRα, PXR, LXR and FXR through agonists could open new perspectives to alleviate or reverse MM drug resistance.

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.

Chemoprotective epigenetic mechanisms in a colorectal cancer model: Modulation by n-3 PUFA in combination with fermentable fiber

Colorectal cancer is the third major cause of cancer-related mortality in both men and women worldwide. The beneficial role of n-3 polyunsaturated fatty acids (PUFA) in preventing colon cancer is substantiated by experimental, epidemiological, and clinical data. From a mechanistic perspective, n-3 PUFA are pleiotropic and multifaceted with respect to their molecular mechanisms of action. For example, this class of dietary lipid uniquely modulates membrane and nuclear receptors, sensors/ion channels, and membrane structure/cytoskeletal function, thereby regulating signaling processes that influence patterns of gene expression and cell phenotype. In addition, n-3 PUFA can synergize with other potential chemoprotective agents known to reprogram the chromatin landscape, such as the fermentable fiber product, butyrate. Nutri-epigenomics is an emerging field of research that is focused on the interaction between nutrition and epigenetics. Epigenetics refers to a group of heterogeneous processes that regulate transcription without changing the DNA coding sequence, ranging from DNA methylation, to histone tail modifications and transcription factor activity. One implication of the nutri-epigenome is that it may be possible to reprogram epigenetic marks that are associated with increased disease risk by nutritional or lifestyle interventions. This review will focus on the nutri-epigenomic role of n-3 PUFA, particularly DHA, as well as the combinatorial effects of n-3 PUFA and fermentable fiber in relation to colon cancer.

A functional polymorphism in the 3'-UTR of PXR interacts with smoking to increase lung cancer risk in southern and eastern Chinese smoker

Pregnane X receptor (PXR) is an important member of the nuclear receptor superfamily that copes with various endobiotic and xenobiotic stimuli, such as carcinogens by regulating an array of environmental response genes. Low PXR expression has been shown to promote tumor initiation and metastasis. The aim of the current study was to investigate whether the single nucleotide polymorphisms (SNPs) of PXR could alter lung cancer susceptibility in Chinese by affecting the function or expression of PXR. We genotyped three putatively functional SNPs of PXR (i.e., rs3814055C>T, rs3732360C>T, and rs3814058C>T) and analyzed their associations with lung cancer risk in a two-stage case-control study with a total of 1559 lung cancer cases and 1679 controls in the southern and eastern Chinese population. We found that in comparison to the rs3814058CC common genotype, the rs3814058T variants (TC/TT) which is located in the 3'-untranslated region (3'-UTR) of PXR conferred a consistently increased risk of lung cancer in both the southern Chinese (odd ratios (OR) = 1.24, 95% confidence interval (CI) = 1.03−1.49) and the eastern Chinese (OR = 1.33, 95% CI = 1.02−1.75). The variants also significantly interacted with smoking on increasing cancer risk (p = 0.023). Moreover, lung cancer tissues with the rs3814058T variants showed significantly lower PXR expression than those with rs3814058CC genotype in the smokers (p = 0.041). These results suggested that the rs3814058C>T polymorphism of PXR interacts with smoking on increasing lung cancer risk in Chinese smokers, which might be a functional genetic biomarker for lung cancer.

Role of pregnane X receptor in chemotherapeutic treatment

Pregnane X receptor (PXR) is a member of the nuclear receptor superfamily that differently expresses not only in human normal tissues but also in numerous types of human cancers. PXR can be activated by many endogenous substances and exogenous chemicals, and thus affects chemotherapeutic effects and intervenes drug-drug interactions by regulating its target genes involving drug metabolism and transportation, cell proliferation and apoptosis, and modulating endobiotic homeostasis. Tissue and context-specific regulation of PXR contributes to diverse effects in the treatment for numerous cancers. Genetic variants of PXR lead to intra- and inter-individual differences in the expression and inducibility of PXR, resulting in different responses to chemotherapy in PXR-positive cancers. The purpose of this review is to summarize and discuss the role of PXR in the metabolism and clearance of anticancer drugs. It is also expected that this review will provide insights into PXR-mediated enhancement for chemotherapeutic treatment, prediction of drug-drug interactions and personalized medicine.