Overcoming drug resistance by regulating nuclear receptors

Chemical manipulation of an activation/inhibition switch in the nuclear receptor PXR

Nuclear receptors are ligand-activated transcription factors that can often be useful drug targets. Unfortunately, ligand promiscuity leads to two-thirds of receptors remaining clinically untargeted. PXR is a nuclear receptor that can be activated by diverse compounds to elevate metabolism, negatively impacting drug efficacy and safety. This presents a barrier to drug development because compounds designed to target other proteins must avoid PXR activation while retaining potency for the desired target. This problem could be avoided by using PXR antagonists, but these compounds are rare, and their molecular mechanisms remain unknown. Here, we report structurally related PXR-selective agonists and antagonists and their corresponding co-crystal structures to describe mechanisms of antagonism and selectivity. Structural and computational approaches show that antagonists induce PXR conformational changes incompatible with transcriptional coactivator recruitment. These results guide the design of compounds with predictable agonist/antagonist activities and bolster efforts to generate antagonists to prevent PXR activation interfering with other drugs.

Induction effect of antiretroviral bictegravir on the expression of Abcb1, Abcg2 and Abcc1 genes associated with P-gp, BCRP and MRP1 transporters present in rat peripheral blood mononuclear cells

BACKGROUND

Antiretrovirals have the potential to cause drug interactions leading to inefficacy or toxicity via induction of efflux transporters through nuclear receptors, altering drug concentrations at their target sites.

RESEARCH DESIGN AND METHODS

This study used molecular dynamic simulations and qRT-PCR to investigate bictegravir's interactions with nuclear receptors PXR and CAR, and its effects on efflux transporters (P-gp, BCRP, MRP1) in rat PBMCs. PBMC/plasma drug concentrations were measured using LC-MS/MS to assess the functional impact of transporter expression.

RESULTS

Bictegravir significantly increased the expression of ABC transporters, with Car identified as a key mediator. This suggests that bictegravir's influence on nuclear receptors could affect drug transport and efficacy at the cellular level.

CONCLUSIONS

Bictegravir activates nuclear receptors enhancing efflux transporter expression. Understanding these interactions is crucial for preventing drug-drug interactions and reducing toxicity in clinical use. Combining CAR antagonists with bictegravir may prevent drug resistance and toxicity. However, these findings are based on preclinical data and necessitate further clinical trials to confirm their applicability in clinical settings.

Zinc finger proteins and ATP-binding cassette transporter-dependent multidrug resistance

BACKGROUND

Multidrug resistance (MDR) remains a significant challenge in cancer treatment, leading to poor clinical outcomes. Dysregulation of ATP-binding cassette (ABC) transporters has been identified as a key contributor to MDR. Zinc finger proteins (ZNPs) are key regulators of transcription and have emerged as potential contributors to cancer drug resistance. Bridging the knowledge gap between ZNPs and MDR is essential to understand a source of heterogeneity in cancer treatment. This review sought to elucidate how different ZNPs modulate the transcriptional regulation of ABC genes, contributing to resistance to cancer therapies.

METHODS

The search was conducted using PubMed, Google Scholar, EMBASE and Web of Science.

RESULTS

In addition to ABC-blockers, the transcriptional features regulated by ZNP are expected to play a role in reversing ABC-mediated MDR and predicting the efficacy of anticancer treatments. Among the ZNP-induced epithelial to mesenchymal transition, SNAIL, SLUG and Zebs have been identified as important factors in promoting MDR through activation of ATM, NFκB and PI3K/Akt pathways, exposing the metabolism to potential ZNP-MDR interactions. Additionally, nuclear receptors, such as VDR, ER and PXR have been found to modulate certain ABC regulations. Other C2H2-type zinc fingers, including Kruppel-like factors, Gli and Sp also have the potential to contribute to MDR.

CONCLUSION

Besides reviewing evidence on the effects of ZNP dysregulation on ABC-related chemoresistance in malignancies, significant markers of ZNP functions are discussed to highlight the clinical implications of gene-to-gene and microenvironment-to-gene interactions on MDR prospects. Future research on ZNP-derived biomarkers is crucial for addressing heterogeneity in cancer therapy.

Natural compounds targeting nuclear receptors for effective cancer therapy

Human nuclear receptors (NRs) are a family of forty-eight transcription factors that modulate gene expression both spatially and temporally. Numerous biochemical, physiological, and pathological processes including cell survival, proliferation, differentiation, metabolism, immune modulation, development, reproduction, and aging are extensively orchestrated by different NRs. The involvement of dysregulated NRs and NR-mediated signaling pathways in driving cancer cell hallmarks has been thoroughly investigated. Targeting NRs has been one of the major focuses of drug development strategies for cancer interventions. Interestingly, rapid progress in molecular biology and drug screening reveals that the naturally occurring compounds are promising modern oncology drugs which are free of potentially inevitable repercussions that are associated with synthetic compounds. Therefore, the purpose of this review is to draw our attention to the potential therapeutic effects of various classes of natural compounds that target NRs such as phytochemicals, dietary components, venom constituents, royal jelly-derived compounds, and microbial derivatives in the establishment of novel and safe medications for cancer treatment. This review also emphasizes molecular mechanisms and signaling pathways that are leveraged to promote the anti-cancer effects of these natural compounds. We have also critically reviewed and assessed the advantages and limitations of current preclinical and clinical studies on this subject for cancer prophylaxis. This might subsequently pave the way for new paradigms in the discovery of drugs that target specific cancer types.

Combination of Paclitaxel and PXR Antagonist SPA70 Reverses Paclitaxel-Resistant Non-Small Cell Lung Cancer

Paclitaxel (PTX) is one of the most efficient drugs for late-stage non-small cell lung cancer (NSCLC) patients. However, most patients gradually develop resistance to PTX with long-term treatments. The identification of new strategies to reverse PTX resistance in NSCLC is crucially important for the treatment. PTX is an agonist for the pregnane X receptor (PXR) which regulates PTX metabolism. Antagonizing PXR, therefore, may render the NSCLC more sensitive to the PTX treatment. In this study, we investigated the PXR antagonist SPA70 and its role in PTX treatment of NSCLC. In vitro, SPA70 and PTX synergistically inhibited cell growth, migration and invasion in both paclitaxel-sensitive and paclitaxel-resistant A549 and H460 lung cancer cells. Mechanistically, we found PTX and SPA70 cotreatment disassociated PXR from ABCB1 (MDR1, P-gp) promoter, thus inhibiting P-gp expression. Furthermore, the combination regimen synergistically enhanced the interaction between PXR and Tip60, which abrogated Tip60-mediated α-tubulin acetylation, leading to mitosis defect, S-phase arrest and necroptosis/apoptosis. Combination of PXT and SPA70 dramatically inhibited tumor growth in a paclitaxel-resistant A549/TR xenograft tumor model. Taken together, we showed that SPA70 reduced the paclitaxel resistance of NSCLC. The combination regimen of PTX and SPA70 could be potential novel candidates for the treatment of taxane-resistant lung cancer.

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.

Pregnane X Receptor and the Gut-Liver Axis: A Recent Update

It is well-known that the pregnane X receptor (PXR)/Nr1i2 is a critical xenobiotic-sensing nuclear receptor enriched in liver and intestine and is responsible for drug-drug interactions, due to its versatile ligand binding domain (LBD) and target genes involved in xenobiotic biotransformation. PXR can be modulated by various xenobiotics including pharmaceuticals, nutraceuticals, dietary factors, and environmental chemicals. Microbial metabolites such as certain secondary bile acids (BAs) and the tryptophan metabolite indole-3-propionic acid (IPA) are endogenous PXR activators. Gut microbiome is increasingly recognized as an important regulator for host xenobiotic biotransformation and intermediary metabolism. PXR regulates and is regulated by the gut-liver axis. This review summarizes recent research advancements leveraging pharmaco- and toxico-metagenomic approaches that have redefined the previous understanding of PXR. Key topics covered in this review include: (1) genome-wide investigations on novel PXR-target genes, novel PXR-DNA interaction patterns, and novel PXR-targeted intestinal bacteria; (2) key PXR-modulating activators and suppressors of exogenous and endogenous sources; (3) novel bidirectional interactions between PXR and gut microbiome under physiologic, pathophysiological, pharmacological, and toxicological conditions; and (4) modifying factors of PXR-signaling including species and sex differences and time (age, critical windows of exposure, and circadian rhythm). The review also discusses critical knowledge gaps and important future research topics centering around PXR. SIGNIFICANCE STATEMENT: This review summarizes recent research advancements leveraging O'mics approaches that have redefined the previous understanding of the xenobiotic-sensing nuclear receptor pregnane X receptor (PXR). Key topics include: (1) genome-wide investigations on novel PXR-targeted host genes and intestinal bacteria as well as novel PXR-DNA interaction patterns; (2) key PXR modulators including microbial metabolites under physiological, pathophysiological, pharmacological, and toxicological conditions; and (3) modifying factors including species, sex, and time.

High Pregnane X Receptor (PXR) Expression Is Correlated with Poor Prognosis in Invasive Breast Carcinoma

Pregnane X Receptor (PXR) is involved in human cancer, either by directly affecting carcinogenesis or by inducing drug-drug interactions and chemotherapy resistance. The clinical significance of PXR expression in invasive breast carcinoma was evaluated in the present study. PXR protein expression was assessed immunohistochemically on formalin fixed paraffin-embedded breast invasive carcinoma tissue sections, obtained from 148 patients, and was correlated with clinicopathological parameters, molecular phenotypes, tumor cells' proliferative capacity, and overall disease-free patients' survival. Additionally, the expression of PXR was examined on human breast carcinoma cell lines of different histological grade, hormonal status, and metastatic potential. PXR positivity was noted in 79 (53.4%) and high PXR expression in 48 (32.4%), out of 148 breast carcinoma cases. High PXR expression was positively associated with nuclear grade (p = 0.0112) and histological grade of differentiation (p = 0.0305), as well as with tumor cells' proliferative capacity (p = 0.0051), and negatively with luminal A subtype (p = 0.0295). Associations between high PXR expression, estrogen, and progesterone receptor negative status were also recorded (p = 0.0314 and p = 0.0208, respectively). High PXR expression was associated with shorter overall patients' survival times (log-rank test, p = 0.0009). In multivariate analysis, high PXR expression was identified as an independent prognostic factor of overall patients' survival (Cox-regression analysis, p = 0.0082). PXR expression alterations were also noted in breast cancer cell lines of different hormonal status. The present data supported evidence that PXR was related to a more aggressive invasive breast carcinoma phenotype, being a strong and independent poor prognosticator.

Building a Chemical Toolbox for Human Pregnane X Receptor Research: Discovery of Agonists, Inverse Agonists, and Antagonists Among Analogs Based on the Unique Chemical Scaffold of SPA70

Pregnane X receptor (PXR) plays roles in detoxification and other physiological processes. PXR activation may enhance drug metabolism (leading to adverse drug reactions) or inhibit inflammation. Therefore, PXR agonists, antagonists, and inverse agonists may serve as research tools and drug candidates. However, a specific PXR modulator with an associated structure-activity relationship is lacking. Based on the scaffold of specific human PXR (hPXR) antagonist SPA70 (10), we developed 81 SPA70 analogs and evaluated their receptor-binding and cellular activities. Interestingly, analogs with subtle structural differences displayed divergent cellular activities, including agonistic, dual inverse agonistic and antagonistic, antagonistic, and partial agonistic/partial antagonistic activities (as in compounds 111, 10, 97, and 42, respectively). We generated a pharmacophore model that represents 81 SPA70 analogs, and docking models that correlate strong interactions between the compounds and residues in the AF-2 helix with agonistic activity. These compounds are novel chemical tools for studying hPXR.

Significance of aldo-keto reductase 1C3 and ATP-binding cassette transporter B1 in gain of irinotecan resistance in colon cancer cells

Irinotecan (CPT11) is widely prescribed for treatment of various intractable cancers such as advanced and metastatic colon cancer cells, but its continuous treatment promotes the resistance development. In this study, we established CPT11-resistant variants of three human colon cancer (DLD1, RKO and LoVo) cell lines, and found that gain of the resistance elicited an up-regulation of aldo-keto reductase (AKR) 1C3 in the cells. Additionally, the sensitivity to CPT11 toxicity was decreased and increased by overexpression and knockdown, respectively, of the enzyme. Moreover, the resistant cells suppressed formation of reactive 4-hydroxy-2-nonenal by CPT11 treatment, and the suppressive effect was almost completely abolished by addition of an AKR1C3 inhibitor. These results suggest that up-regulated AKR1C3 contributes to promotion of the chemoresistance by detoxifying the reactive aldehyde. Western blot and real-time polymerase-chain reaction analyses and ATP-binding cassette (ABC) B1-functional assay revealed that, among three ABC transporters, ABCB1 was the most highly up-regulated by development of the CPT11 resistance, inferring a significant contribution of pregnane-X receptor-dependent signaling to the ABCB1 up-regulation. The combined treatment with inhibitors of AKR1C3 and ABCB1 potently sensitized the resistant cells to CPT11 and its active metabolite SN38. Taken together, our results suggest that combination of AKR1C3 and ABCB1 inhibitors is effective as adjuvant therapy to enhance CPT11 sensitivity of intractable colon cancer cells.

Associations between Pregnane X Receptor and Breast Cancer Growth and Progression

Pregnane X receptor (PXR, NR1I2) is a member of the ligand-activated nuclear receptor superfamily. This receptor is promiscuous in its activation profile and is responsive to a broad array of both endobiotic and xenobiotic ligands. PXR is involved in pivotal cellular detoxification processes to include the regulation of genes that encode key drug-metabolizing cytochrome-P450 enzymes, oxidative stress response, as well as enzymes that drive steroid and bile acid metabolism. While PXR clearly has important regulatory roles in the liver and gastrointestinal tract, this nuclear receptor also has biological functions in breast tissue. In this review, we highlight current knowledge of PXR’s role in mammary tumor carcinogenesis. The elevated level of PXR expression in cancerous breast tissue suggests a likely interface between aberrant cell division and xeno-protection in cancer cells. Moreover, PXR itself exerts positive effect on the cell cycle, thereby predisposing tumor cells to unchecked proliferation. Activation of PXR also plays a key role in regulating apoptosis, as well as in acquired resistance to chemotherapeutic agents. The repressive role of PXR in regulating inflammatory mediators along with the existence of genetic polymorphisms within the sequence of the PXR gene may predispose individuals to developing breast cancer. Further investigations into the role that PXR plays in driving tumorigenesis are needed.

Strategies for developing pregnane X receptor antagonists: Implications from metabolism to cancer

Pregnane X receptor (PXR) is a ligand-activated nuclear receptor (NR) that was originally identified as a master regulator of xenobiotic detoxification. It regulates the expression of drug-metabolizing enzymes and transporters to control the degradation and excretion of endobiotics and xenobiotics, including therapeutic agents. The metabolism and disposition of drugs might compromise their efficacy and possibly cause drug toxicity and/or drug resistance. Because many drugs can promiscuously bind and activate PXR, PXR antagonists might have therapeutic value in preventing and overcoming drug-induced PXR-mediated drug toxicity and drug resistance. Furthermore, PXR is now known to have broader cellular functions, including the regulation of cell proliferation, and glucose and lipid metabolism. Thus, PXR might be involved in human diseases such as cancer and metabolic diseases. The importance of PXR antagonists is discussed in the context of the role of PXR in xenobiotic sensing and other disease-related pathways. This review focuses on the development of PXR antagonists, which has been hampered by the promiscuity of PXR ligand binding. However, substantial progress has been made in recent years, suggesting that it is feasible to develop selective PXR antagonists. We discuss the current status, challenges, and strategies in developing selective PXR antagonists. The strategies are based on the molecular mechanisms of antagonism in related NRs that can be applied to the design of PXR antagonists, primarily driven by structural information.

Characteristics of Nur77 and its ligands as potential anticancer compounds (Review)

Nuclear receptor subfamily 4 group A member 1 (NR4A1; also termed Nur77/TR3/NGFIB), a member of the nuclear receptor superfamily, is expressed as an early response gene to regulate the expression of multiple target genes. Nur77 has the typical structure of a nuclear receptor, including an N‑terminal domain, a DNA binding domain, and a ligand‑binding domain. The expression and localization of Nur77 are closely associated with its roles in cell proliferation and apoptosis. Nur77 was first identified as an orphan receptor, the endogenous ligand of which has not yet been identified; however, an increasing number of compounds targeting Nur77 have been reported to have beneficial effects in the treatment of cancer and other diseases. This review provides a brief overview of the identification, structure, expression and localization, transcriptional role and non‑genomic function of Nur77, and summarizes the ligands that have been shown to interact with Nur77, including cytosporone B, cisplatin, TMPA, PDNPA, CCE9, THPN, Z‑ligustilide, celastrol and bisindole methane compounds, which may potentially be used to treat cancer in humans.

Activating the pregnane X receptor by imperatorin attenuates dextran sulphate sodium-induced colitis in mice

BACKGROUND AND PURPOSE

Activation of the human pregnane X receptor (PXR; NR1I2) has potential therapeutic uses for inflammatory bowel disease (IBD). Imperatorin (IMP), a naturally occurring coumarin, is the main bioactive ingredient of Angelica dahurica Radix, which is regularly used to treat the common cold and intestinal disorders. However, there are no data on the protective effects of IMP against IBD.

EXPERIMENTAL APPROACH

The effects of IMP on PXR-modulated cytochrome P450 3A4 (CYP3A4) expression were assessed using a PXR transactivation assay, a mammalian two-hybrid assay, a competitive ligand-binding assay, analysis of CYP3A4 mRNA and protein expression levels and measurement of CYP3A4 activity using a cell-based reporter gene assay and in vitro model. The inhibitory effects of IMP on NF-κB activity were evaluated by a reporter assay and NF-κB p65 nuclear translocation. The anti-IBD effects of IMP were investigated in a dextran sulphate sodium (DSS)-induced colitis mouse model. Colon inflammatory cytokines were assessed by elisa.

KEY RESULTS

IMP activated CYP3A4 promoter activity, recruited steroid receptor coactivator 1 to the ligand-binding domain of PXR and increased the expression and activity of CYP3A4. PXR knockdown substantially reduced IMP-induced increase in CYP3A4 expression. Furthermore, IMP-mediated PXR activation suppressed the nuclear translocation of NF-κB and down-regulated LPS-induced expression of pro-inflammatory genes. Nevertheless, PXR knockdown partially reduced the IMP-mediated inhibition of NF-κB. IMP ameliorated DSS-induced colitis by PXR/NF-κB signalling.

CONCLUSIONS AND IMPLICATIONS

IMP acts as a PXR agonist to attenuate DSS-induced colitis by suppression of the NF-κB-mediated pro-inflammatory response in a PXR/NF-κB-dependent manner.

Epigenetics and Epi-miRNAs: Potential markers/therapeutics in leukemia

Epigenetic variations can play remarkable roles in different normal and abnormal situations. Such variations have been shown to have a direct role in the pathogenesis of various diseases either through inhibition of tumor suppressor genes or increasing the expression of oncogenes. Enzymes involving in epigenetic machinery are the main actors in tuning the epigenetic-based controls on gene expressions. Aberrant expression of these enzymes can trigger a big chaos in the cellular gene expression networks and finally lead to cancer progression. This situation has been shown in different types of leukemia, where high or low levels of an epigenetic enzyme are partly or highly responsible for involvement or progression of a disease. DNA hypermethylation, different histone modifications, and aberrant miRNA expressions are three main epigenetic variations, which have been shown to play a role in leukemia progression. Epigenetic based treatments now are considered as novel and effective therapies in order to decrease the abnormal epigenetic modifications in patient cells. Different epigenetic-based approaches have been developed and tested to inhibit or reverse the unusual expression of epigenetic agents in leukemia. The reciprocal behavior of miRNAs in the regulation of epigenetic modifiers, while being regulated by them, unlocks a new opportunity in order to design some epigenetic-based miRNAs able to silence or sensitize these effectors in leukemia.

GALNT14 Involves the Regulation of Multidrug Resistance in Breast Cancer Cells

GALNT14 is a member of N-acetylgalactosaminyltransferase enzyme family and mediates breast cancer cell development. Here, we find that GALNT14 regulates multidrug resistance (MDR) in breast cancer. The expression of GALNT14 is associated with MDR in breast cancer. Higher level of GALNT14 facilitates MCF-7 cells to resist Adriamycin, whereas knockdown of GALNT14 sensitizes cells to Adriamycin. Moreover, the expression of GALNT14 associates with the expression of P-gp, the efflux pump localized on the cell membrane, which could be the underlying mechanism of how GALNT14 induces MDR. In-depth analysis shows that GALNT14 regulates the stability of P-gp. Finally, GALNT14 associates with higher level of P-gp in chemotherapy-resistant human breast cancer tissues. Taken together, our studies reveal a molecular mechanism in breast cancer MDR.

High-Throughput Screening Identifies 1,4,5-Substituted 1,2,3-Triazole Analogs as Potent and Specific Antagonists of Pregnane X Receptor

Human pregnane X receptor (hPXR) is a nuclear receptor that regulates the expression of phase I and phase II drug-metabolism enzymes, as well as that of drug transporters. hPXR is a "xenobiotics sensor" and can be activated by structurally diverse compounds. The activation of hPXR by its agonists increases the clearance of xenobiotics by increasing the expression of drug-metabolism enzymes and drug transporters, possibly leading to drug toxicity, drug resistance, and other adverse drug reactions. Therefore, hPXR antagonists might attenuate agonist-mediated activation of hPXR and reduce the risk of adverse drug reactions. Several hPXR antagonists have been reported, but none of them is specific for hPXR. In this study, we present the first large-scale, unbiased, cell-based high-throughput screen to identify specific hPXR antagonists. Among the 132,975 compounds screened, we identified the 1,4,5-substituted 1,2,3-triazole analogs as potent and specific hPXR antagonists by sequentially performing primary screening, retesting, and dose-response analysis using cell-based hPXR gene reporter and receptor binding assays, as well as receptor and promoter specificity assays. The compound SJ000076745-1 is the most potent and specific hPXR antagonist in the 1,4,5-substituted 1,2,3-triazole chemical class, having a cell-based hPXR antagonist 50% inhibitory concentration (IC₅₀) value of 377 ± 16 nM and an hPXR binding inhibitory IC₅₀ value of 563 ± 40 nM.

SPA70 is a potent antagonist of human pregnane X receptor

Many drugs bind to and activate human pregnane X receptor (hPXR) to upregulate drug-metabolizing enzymes, resulting in decreased drug efficacy and increased resistance. This suggests that hPXR antagonists have therapeutic value. Here we report that SPA70 is a potent and selective hPXR antagonist. SPA70 inhibits hPXR in human hepatocytes and humanized mouse models and enhances the chemosensitivity of cancer cells, consistent with the role of hPXR in drug resistance. Unexpectedly, SJB7, a close analog of SPA70, is an hPXR agonist. X-ray crystallography reveals that SJB7 resides in the ligand-binding domain (LBD) of hPXR, interacting with the AF-2 helix to stabilize the LBD for coactivator binding. Differential hydrogen/deuterium exchange analysis demonstrates that SPA70 and SJB7 interact with the hPXR LBD. Docking studies suggest that the lack of the para-methoxy group in SPA70 compromises its interaction with the AF-2, thus explaining its antagonism. SPA70 is an hPXR antagonist and promising therapeutic tool.The xenobiotic-activated human pregnane X receptor (hPXR) regulates drug metabolism. Here the authors develop hPXR modulators, which are of potential therapeutic interest and functionally and structurally characterize the antagonist SPA70 and the structurally related agonist SJB7.

Pregnane X receptor is associated with unfavorable survival and induces chemotherapeutic resistance by transcriptional activating multidrug resistance-related protein 3 in colorectal cancer

Background

Although chemotherapy represents a predominant anti-cancer therapeutic modality, drug treatment efficacy is often limited due to the development of resistant tumor cells. The pregnane X receptor (PXR) affects chemotherapeutic effects by regulating targets involved in drug metabolism and transportation, but the regulatory mechanism is poorly understood.

Methods

Oxaliplatin (L-OHP) content in tumor cells was analyzed by mass cytometry. The roles of PXR on cancer cell proliferation, apoptosis and tumor growth with L-OHP-treated were investigated by MTS, colony formation, flow cytometry and xenograft tumor assays. Luciferase reporter, Chromatin-immunoprecipitation and Site-directed mutagenesis were evaluated the mechanisms. The PXR and multidrug resistance-related protein 3 (MRP3) expressions were examined by western blot, RT-PCR or immunohistochemistry of TMA. Kaplan-Meier and Cox regression were adopted to analyze the prognostic value of PXR in colorectal cancer (CRC).

Results

PXR over-expression significantly increased oxaliplatin (L-OHP) transport capacity with a reduction of its content and repressed the effects of L-OHP on tumour cell proliferation and apoptosis. Conversely, PXR knockdown augments L-OHP-mediated cellular proliferation and apoptosis. Moreover, PXR significantly reduced the therapeutic effects of L-OHP on tumor growth in nude mice. Further studies indicated a positive correlation between PXR and MRP3 expression and this finding was confirmed in two independent cohorts. Significantly increased MRP3 expression was also found in PXR over-expressing cell lines. Mechanistically, PXR could directly bind to the MRP3 promoter, activating its transcription. The PXR binding sites were determined to be at -796 to -782bp (CTGAAGCAGAGGGAA) and the key binding sites were the “AGGGA” (-787 to -783bp) on the MRP3 promoter. Accordingly, blockade of MRP3 diminishes the effects on drug resistance of PXR. In addition, PXR expression is significantly associated with poor overall survival and represents an unfavorable and independent factor for male or stage I + II CRC patient prognosis.

Conclusions

PXR is a potential biomarker for predicting outcome and activates MRP3 transcription by directly binding to its promoter resulting in an increased L-OHP efflux capacity, and resistance to L-OHP or platinum drugs in CRC. Our work reveals a novel and unique mechanism of drug resistance in CRC.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-017-0641-8) contains supplementary material, which is available to authorized users.

Microdose-Induced Drug-DNA Adducts as Biomarkers of Chemotherapy Resistance in Humans and Mice

We report progress on predicting tumor response to platinum-based chemotherapy with a novel mass spectrometry approach. Fourteen bladder cancer patients were administered one diagnostic microdose each of [¹⁴C]carboplatin (1% of the therapeutic dose). Carboplatin-DNA adducts were quantified by accelerator mass spectrometry in blood and tumor samples collected within 24 hours, and compared with subsequent chemotherapy response. Patients with the highest adduct levels were responders, but not all responders had high adduct levels. Four patient-derived bladder cancer xenograft mouse models were used to test the possibility that another drug in the regimen could cause a response. The mice were dosed with [¹⁴C]carboplatin or [¹⁴C]gemcitabine and the resulting drug-DNA adduct levels were compared with tumor response to chemotherapy. At least one of the drugs had to induce high drug-DNA adduct levels or create a synergistic increase in overall adducts to prompt a corresponding therapeutic response, demonstrating proof-of-principle for drug-DNA adducts as predictive biomarkers. Mol Cancer Ther; 16(2); 376-87. ©2016 AACR.

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.

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.

Pregnane-10,2-carbolactones from a Hawaiian Marine Sponge in the Genus Myrmekioderma

Four new pregnanes, 3β,4β-dihydroxy-17-methyl-17α-pregna-5,13-diene-10,2-carbolactone (1), 6β-chloro-3β-hydroxy-17-methyl-17α-pregna-4,13-diene-10,2-carbolactone (2), 3β-hydroxy-6β-methoxy-17-methyl-17α-pregna-4,13-diene-10,2-carbolactone (3), and 3β,6β-dihydroxy-17-methyl-17α-pregna-4,13-diene-10,2-carbolactone (4), were isolated from an undescribed species of Myrmekioderma Ehlers along with the known pregnane carbolactone (5). The structures of the new compounds were determined by spectroscopic methods and comparison with previously described compounds. Compound 5 showed almost 4-fold activation of pregnane X receptor, while 2 inhibited BACE1 with an IC50 value of 82 μM.

Drug Transporters and Na+/H+ Exchange Regulatory Factor PSD-95/Drosophila Discs Large/ZO-1 Proteins

Drug transporters govern the absorption, distribution, and elimination of pharmacologically active compounds. Members of the solute carrier and ATP binding-cassette drug transporter family mediate cellular drug uptake and efflux processes, thereby coordinating the vectorial movement of drugs across epithelial barriers. To exert their physiologic and pharmacological function in polarized epithelia, drug transporters must be targeted and stabilized to appropriate regions of the cell membrane (i.e., apical versus basolateral). Despite the critical importance of drug transporter membrane targeting, the mechanisms that underlie these processes are largely unknown. Several clinically significant drug transporters possess a recognition sequence that binds to PSD-95/Drosophila discs large/ZO-1 (PDZ) proteins. PDZ proteins, such as the Na(+)/H(+) exchanger regulatory factor (NHERF) family, act to stabilize and organize membrane targeting of multiple transmembrane proteins, including many clinically relevant drug transporters. These PDZ proteins are normally abundant at apical membranes, where they tether membrane-delimited transporters. NHERF expression is particularly high at the apical membrane in polarized tissue such as intestinal, hepatic, and renal epithelia, tissues important to drug disposition. Several recent studies have highlighted NHERF proteins as determinants of drug transporter function secondary to their role in controlling membrane abundance and localization. Mounting evidence strongly suggests that NHERF proteins may have clinically significant roles in pharmacokinetics and pharmacodynamics of several pharmacologically active compounds and may affect drug action in cancer and chronic kidney disease. For these reasons, NHERF proteins represent a novel class of post-translational mediators of drug transport and novel targets for new drug development.

Pregnane X Receptor Expression in Human Pancreatic Adenocarcinoma: Associations With Clinicopathologic Parameters, Tumor Proliferative Capacity, Patients' Survival, and Retinoid X Receptor Expression

OBJECTIVES

Pregnane X receptor (PXR) has been involved in human malignancy, either by directly affecting carcinogenesis or by inducing drug-drug interactions and chemotherapy resistance. The present study aimed to assess the clinical significance of PXR in pancreatic adenocarcinoma.

METHODS

Pregnane X receptor and its heterodimers' PXR/retinoid X receptor α (RXR-α), RXR-β, and RXR-γ expression were assessed immunohistochemically on tumoral samples from 55 pancreatic adenocarcinoma patients and were associated with clinicopathologic parameters, tumor proliferative capacity, and patients' survival.

RESULTS

Enhanced PXR expression was noted in 24 (43.6%) of 55 pancreatic adenocarcinoma cases. Pancreatic adenocarcinoma patients presenting increased histological grade of tumor differentiation showed a significant increased incidence of elevated PXR expression (P = 0.023). Enhanced PXR/RXR-β expression was significantly associated with smaller tumor size and earlier clinical stage (P = 0.005 and P = 0.003, respectively). Elevated PXR/RXR-γ expression was significantly associated with smaller tumor size and earlier clinical stage (P = 0.012 and P = 0.014, respectively) and borderline with the absence of lymph node metastases (P = 0.056). In addition, pancreatic adenocarcinoma patients presenting enhanced PXR/RXR-γ expression showed marginally longer survival times compared with those with decreased expression (log-rank test, P = 0.053).

CONCLUSIONS

This study supported evidence that PXR and its copartners' overexpression may be associated with favorable clinicopathologic parameters and better outcome in pancreatic adenocarcinoma.

Mechanisms of Drug Resistance in Veterinary Oncology- A Review with an Emphasis on Canine Lymphoma

Drug resistance (DR) is the major limiting factor in the successful treatment of systemic neoplasia with cytotoxic chemotherapy. DR can be either intrinsic or acquired, and although the development and clinical implications are different, the underlying mechanisms are likely to be similar. Most causes for DR are pharmacodynamic in nature, result from adaptations within the tumor cell and include reduced drug uptake, increased drug efflux, changes in drug metabolism or drug target, increased capacity to repair drug-induced DNA damage or increased resistance to apoptosis. The role of active drug efflux transporters, and those of the ABC-transporter family in particular, have been studied extensively in human oncology and to a lesser extent in veterinary medicine. Methods reported to assess ABC-transporter status include detection of the actual protein (Western blot, immunohistochemistry), mRNA or ABC-transporter function. The three major ABC-transporters associated with DR in human oncology are ABCB1 or P-gp, ABCC1 or MRP1, and ABCG2 or BCRP, and have been demonstrated in canine cell lines, healthy dogs and dogs with cancer. Although this supports a causative role for these ABC-transporters in DR cytotoxic agents in the dog, the relative contribution to the clinical phenotype of DR in canine cancer remains an area of debate and requires further prospective studies.

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.

Targeting xenobiotic receptors PXR and CAR in human diseases

Nuclear receptors such as the pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are xenobiotic receptors regulating not only drug metabolism and disposition but also various human diseases such as cancer, diabetes, inflammatory disease, metabolic disease and liver diseases, suggesting that PXR and CAR are promising targets for drug discovery. Consequently, there is an urgent need to discover and develop small molecules that target these PXR- and/or CAR-mediated human-disease-related pathways for relevant therapeutic applications. This review proposes approaches to target PXR and CAR, either individually or simultaneously, in the context of various human diseases, taking into consideration the structural differences between PXR and CAR.

Biology of PXR: role in drug-hormone interactions

Hormonal homeostasis is essential for a variety of physiological and pathological processes. Elimination and detoxification of xenobiotics, such as drugs introduced into the human body, could disrupt the balance of hormones due to the induction of drug metabolizing enzymes (DMEs) and transporters. Pregnane X receptor (PXR, NR1I2) functions as a master xenobiotic receptor involved in drug metabolism and drug-drug interactions by its coordinated transcriptional regulation of phase I and phase II DMEs and transporters. Recently, increasing evidences indicate that PXR can also mediate the endocrine disruptor function and thus impact the integrity of the endocrine system. This review focuses primarily on the recent advances in our understanding of the function of PXR in glucocorticoid, mineralocorticoid, androgen and estrogen homeostasis. The elucidation of PXR-mediated drug-hormone interactions might have important therapeutic implications in dealing with hormone-dependent diseases and safety assessment of drugs.

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.

Bazedoxifene-scaffold-based mimetics of solomonsterols A and B as novel pregnane X receptor antagonists

Pregnane X receptor (PXR), a member of the NR1I nuclear receptor family, acts as a xenobiotic sensor and a paramount transcriptional regulator of drug-metabolizing enzymes and transporters. The overexpression of PXR in various cancer cells indicates the importance of PXR as a drug target for countering multidrug resistance in anticancer treatments. We describe the discovery of novel bazedoxifene-scaffold-based PXR antagonists inspired by the marine sulfated steroids solomonsterol A and B as natural leads. A luciferase reporter assay on a PXR-transfected HepG2 cell line identified compounds 19-24 as promising PXR antagonists. Further structure-activity relationship studies of the most active PXR antagonist from the series (compound 20, IC50 = 11 μM) revealed the importance of hydroxyl groups as hydrogen-bond donors for PXR antagonistic activity. PXR antagonists 20 and 24 (IC50 = 14 μM), in addition to the downregulation of PXR expression, exhibited inhibition of PXR-induced CYP3A4 expression, which illustrates their potential to suppress PXR-regulated phase-I drug metabolism.

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.

Marked differences in the effect of antiepileptic and cytostatic drugs on the functionality of P-glycoprotein in human and rat brain capillary endothelial cell lines

PURPOSE

The expression of P-glycoprotein (Pgp) is increased in brain capillary endothelial cells (BCECs) of patients with pharmacoresistant epilepsy. This may restrict the penetration of antiepileptic drugs (AEDs) into the brain. However, the mechanisms underlying increased Pgp expression in epilepsy patients are not known. One possibility is that AEDs induce the expression and functionality of Pgp in BCECs. Several older AEDs that induce human cytochrome P450 enzymes also induce Pgp in hepatocytes and enterocytes, but whether this extends to Pgp at the human BBB and to newer AEDs is not known.

METHODS

This prompted us to study the effects of various old and new AEDs on Pgp functionality in the human BCEC line, hCMEC/D3, using the rhodamine 123 (Rho123) efflux assay. For comparison, experiments were performed in two rat BCEC lines, RBE4 and GPNT, and primary cultures of rat and pig BCECs. Furthermore, known Pgp inducers, such as dexamethasone and several cytostatic drugs, were included in our experiments.

RESULTS

Under control conditions, GPNT cells exhibited the highest and RBE4 the lowest Pgp expression and Rho123 efflux, while intermediate values were determined in hCMEC/D3. Known Pgp inducers increased Rho123 efflux in all cell lines, but marked inter-cell line differences in effect size were observed. Of the various AEDs examined, only carbamazepine (100 μM) moderately increased Pgp functionality in hCMEC/D3, while valproate (300 μM) inhibited Pgp.

CONCLUSIONS

These data do not indicate that treatment with AEDs causes a clinically relevant induction in Pgp functionality in BCECs that form the BBB.

Effect of pregnane X receptor expression on drug resistance in breast cancer

This study was conducted to investigate the effect of increased expression of the nuclear transcription factor receptor pregnane X receptor (PXR) on drug resistance of breast cancer cells. Western blotting was used to detect the expression of PXR in breast carcinoma cells. The PXR agonist SR12813 was used to upregulate the expression of PXR. Semi-quantitative polymerase chain reaction was used to detect PXR gene expression in normal and cancerous breast tissues, as well as the expression levels of the drug-resistant genes multidrug resistance protein 1 (MDR1) and breast cancer resistance protein (BCRP) in breast cancer cells. A Cell Counting Kit-8 assay was used to observe the sensitivity of the breast cancer cells to chemotherapeutic agents. Flow cytometry was used to investigate cell apoptosis. PXR expression was detected in normal and cancerous human breast tissues and in breast cancer cell lines. SR12813 treatment led to an increased expression of PXR protein and an increased expression of drug-resistant genes, MDR1 and BCRP, in MCF-7 and MDA-MB-231 cells. SR12813 pretreatment significantly increased the resistance of MDA-MB-231 cells to docetaxel. A marked increase in resistance to 4-hydroxytamoxifen was also observed in MCF-7 with SR12813 pretreatment. Additionally, we also found that pretreatment with SR12813 led to reduced apoptosis of the two cell strains induced by chemotherapeutic agents. In conclusion, PXR expression has an important effect on the sensitivity to chemotherapy of PXR-positive breast carcinoma. The inhibitory effect of PXR on cell apoptosis may contribute to the drug resistance of breast carcinoma.

Reversal of ATP-binding cassette drug transporter activity to modulate chemoresistance: why has it failed to provide clinical benefit?

Enhanced drug extrusion from cells due to the overexpression of the ATP-binding cassette (ABC) drug transporters inhibits the cytotoxic effects of structurally diverse and mechanistically unrelated anticancer agents and is a major cause of multidrug resistance (MDR) of human malignancies. Multiple compounds can suppress the activity of these efflux transporters and sensitize resistant tumor cells, but despite promising preclinical and early clinical data, they have yet to find a role in oncologic practice. Based on the knowledge of the structure, function, and distribution of MDR-related ABC transporters and the results of their preclinical and clinical evaluation, we discuss probable reasons why these inhibitors have not improved the outcome of therapy for cancer patients. We also outline new MDR-reversing strategies that directly target ABC transporters or circumvent relevant signaling pathways.

Yeast ABC proteins involved in multidrug resistance

Pleiotropic drug resistance is a complex phenomenon that involves many proteins that together create a network. One of the common mechanisms of multidrug resistance in eukaryotic cells is the active efflux of a broad range of xenobiotics through ATP-binding cassette (ABC) transporters. Saccharomyces cerevisiae is often used as a model to study such activity because of the functional and structural similarities of its ABC transporters to mammalian ones. Numerous ABC transporters are found in humans and some are associated with the resistance of tumors to chemotherapeutics. Efflux pump modulators that change the activity of ABC proteins are the most promising candidate drugs to overcome such resistance. These modulators can be chemically synthesized or isolated from natural sources (e.g., plant alkaloids) and might also be used in the treatment of fungal infections. There are several generations of synthetic modulators that differ in specificity, toxicity and effectiveness, and are often used for other clinical effects.

Differential regulation of CYP3A4 promoter activity by a new class of natural product derivatives binding to pregnane X receptor

The pregnane X receptor (PXR) regulates drug metabolism by regulating the expression of drug-metabolizing enzymes such as cytochrome P450 3A4 (CYP3A4), which is involved in the metabolism of >50% of clinically prescribed drugs. The activity of PXR can be controlled by the binding of small molecule agonists or antagonists. Because of its unique ligand binding pocket, PXR binds promiscuously to structurally diverse chemicals. To study the structure-activity relationship, novel modulators for PXR are needed. Here we report the virtual screening of ∼25,000 natural product derivatives from the ZINC database using the Molecular Operating Environment docking software tool against the PXR-rifampicin complex X-ray crystal structure. Our screening resulted in identification of compounds based on the lowest S score, which measures Gibbs free energy. Interestingly, we found that the compounds that bind directly to PXR, as revealed in an intrinsic tryptophan fluorescence assay, modulate CYP3A4 promoter activity differentially in HepG2 cells. Mutational analysis and docking studies showed that these compounds bind broadly in the ligand binding pocket but interact with different amino acid residues. We further investigated the mechanism of binding by analyzing the functional groups that are important for distinguishing agonists from antagonists. The approach we used to identify novel modulators that bind to PXR can be useful for finding novel modulators of PXR.

PXR antagonists and implication in drug metabolism

Adopted orphan nuclear receptor (NR), pregnane X receptor (PXR), plays a central role in the regulation of xeno- and endobiotic metabolism. Since the discovery of the functional role of PXR in 1998, there is evolving evidence for the role of PXR agonists in abrogating metabolic pathophysiology (e.g., cholestasis, hypercholesterolemia, and inflammation). However, more recently, it is clear that PXR is also an important mediator of adverse xeno- (e.g., enhances acetaminophen toxicity) and endobiotic (e.g., hepatic steatosis) metabolic phenotypes. Moreover, in cancer therapeutics, PXR activation can induce drug resistance, and there is growing evidence for tissue-specific enhancement of the malignant phenotype. Thus, in these instances, there may be a role for PXR antagonists. However, as opposed to the discovery efforts for PXR agonists, there are only a few antagonists described. The mode of action of these antagonists (e.g., sulforaphane) remains less clear. Our laboratory efforts have focused on this question. Since the original discovery of azoles analogs as PXR antagonists, we have preliminarily defined an important PXR antagonist pharmacophore and developed less-toxic PXR antagonists. In this review, we describe our published and unpublished findings on recent structure-function studies involving the azole chemical scaffold. Further work in the future is needed to fully define potent, more-selective PXR antagonists that may be useful in clinical application.

Molecular approaches towards development of purified natural products and their structurally known derivatives as efficient anti-cancer drugs: current trends

Several natural products and their derivatives, either in purified or structurally identified form, exhibit immense pharmacological and biological properties, some of them showing considerable anticancer potential. Although the molecular mechanisms of action of some of these products are yet to be elucidated, extensive research in this area continues to generate new data that are clinically exploitable. Recent advancement in molecular biology, high throughput screening, biomarker identifications, target selection and genomic approaches have enabled us to understand salient interactions of natural products and their derivatives with cancer cells vis-à-vis normal cells. In this review we highlight the recent approaches and application of innovative technologies made to improve quality as well as efficiency of structurally identified natural products and their derivatives, particularly in small molecular forms capable of being used in "targeted therapies" in oncology. These products preferentially involve multiple mechanistic pathways and overcome chemo-resistance in tumor types with cumulative action. We also mention briefly a few physico-chemical features that compare natural products with drugs in recent natural product discovery approaches. We further report here a few purified natural products as examples that provide molecular interventions in cancer therapeutics to give the reader a glimpse of the current trends of approach for discovering useful anticancer drugs.

Modulation of xenobiotic receptors by steroids

Nuclear receptors (NRs) are ligand-activated transcription factors that regulate the expression of their target genes. NRs play important roles in many human diseases, including metabolic diseases and cancer, and are therefore a key class of therapeutic targets. Steroids play important roles in regulating nuclear receptors; in addition to being ligands of steroid receptors, steroids (and their metabolites) also regulate other NRs, such as the pregnane X receptor and constitutive androstane receptor (termed xenobiotic receptors), which participate in steroid metabolism. Xenobiotic receptors have promiscuous ligand-binding properties, and their structurally diverse ligands include steroids and their metabolites. Therefore, steroids, their metabolism and metabolites, xenobiotic receptors, steroid receptors, and the respective signaling pathways they regulate have functional interactions. This review discusses these functional interactions and their implications for activities mediated by steroid receptors and xenobiotic receptors, focusing on steroids that modulate pathways involving the pregnane X receptor and constitutive androstane receptor. The emphasis of the review is on structure-function studies of xenobiotic receptors bound to steroid ligands.

Piperine activates human pregnane X receptor to induce the expression of cytochrome P450 3A4 and multidrug resistance protein 1

Activation of the pregnane X receptor (PXR) and subsequently its target genes, including those encoding drug transporters and metabolizing enzymes, while playing substantial roles in xenobiotic detoxification, might cause undesired drug-drug interactions. Recently, an increased awareness has been given to dietary components for potential induction of diet-drug interactions through activation of PXR. Here, we studied, whether piperine (PIP), a major component extracted from the widely-used daily spice black pepper, could induce PXR-mediated expression of cytochrome P450 3A4 (CYP3A4) and multidrug resistance protein 1 (MDR1). Our results showed that PIP activated human PXR (hPXR)-mediated CYP3A4 and MDR1 expression in human hepatocytes, intestine cells, and a mouse model; PIP activated hPXR by recruiting its coactivator SRC-1 in both cellular and cell-free systems; PIP bound to the hPXR ligand binding domain in a competitive ligand binding assay in vitro. The dichotomous effects of PIP on induction of CYP3A4 and MDR1 expression observed here and inhibition of their activity reported elsewhere challenges the potential use of PIP as a bioavailability enhancer and suggests that caution should be taken in PIP consumption during drug treatment in patients, particularly those who favor daily pepper spice or rely on certain pepper remedies.

Pregnane X receptor dependent up-regulation of CYP2C9 and CYP3A4 in tumor cells by antitumor acridine agents, C-1748 and C-1305, selectively diminished under hypoxia

Induction of proteins involved in drug metabolism and in drug delivery has a significant impact on drug-drug interactions and on the final therapeutic effects. Two antitumor acridine derivatives selected for present studies, C-1748 (9-(2'-hydroxyethylamino)-4-methyl-1-nitroacridine) and C-1305 (5-dimethylaminopropylamino-8-hydroxy-triazoloacridinone), expressed high and low susceptibility to metabolic transformations with liver microsomes, respectively. In the current study, we examined the influence of these compounds on cytochrome P450 3A4 (CYP3A4) and 2C9 (CYP2C9) enzymatic activity and gene expression in HepG2 tumor cells. Luminescence and HPLC examination, real-time RT-PCR and western blot analyses along with transfection of pregnane X receptor (PXR) siRNA and CYP3A4 reporter gene assays were applied. We found that both compounds strongly induced CYP3A4 and CYP2C9 activity and expression as well as expression of UGT1A1 and MDR1 in a concentration- and time-dependent manner. C-1748-mediated CYP3A4 and CYP2C9 mRNA induction equal to rifampicin occurred at extremely low concentrations (0.001 and 0.01μM), whereas 10μM C-1305 induced three-times higher CYP3A4 and CYP2C9 mRNA levels than rifampicin did. CYP3A4 and CYP2C9 expressions were shown to be PXR-dependent; however, neither compound influenced PXR expression. Thus, the observed drug-mediated induction of isoenzymes occurs on a PXR-mediated regulatory level. Furthermore, C-1748 and C-1305 were demonstrated to be selective PXR agonists. These effects are hypoxia-inhibited only in the case of C-1748, which is sensitive to P450 metabolism. In summary, PXR was found to be a new target of the studied compounds. Thus, possible combinations of these compounds with other therapeutics might lead to the PXR-dependent enzyme-mediated drug-drug interactions.