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

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

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

Proanthocyanidin B2 derived metabolites may be ligands for bile acid receptors S1PR2, PXR and CAR: an in silico approach

Bile acids (BAs) act as signaling molecules via their interactions with various nuclear (FXR, VDR, PXR and CAR) and G-protein coupled (TGR5, M3R, S1PR2) BA receptors. Stimulation of these BA receptors influences several processes, including inflammatory responses and glucose and xenobiotic metabolism. BA profiles and BA receptor activity are deregulated in cardiometabolic diseases; however, dietary polyphenols were shown to alter BA profile and signaling in association with improved metabolic phenotypes. We previously reported that supplementing mice with a proanthocyanidin (PAC)-rich grape polyphenol (GP) extract attenuated symptoms of glucose intolerance in association with changes to BA profiles, BA receptor gene expression, and/or downstream markers of BA receptor activity. Exact mechanisms by which polyphenols modulate BA signaling are not known, but some hypotheses include modulation of the BA profile via changes to gut bacteria, or alteration of ligand-availability via BA sequestration. Herein, we used an in silico approach to investigate putative binding affinities of proanthocyanidin B2 (PACB2) and PACB2 metabolites to nuclear and G-protein coupled BA receptors. Molecular docking and dynamics simulations revealed that certain PACB2 metabolites had stable binding affinities to S1PR2, PXR and CAR, comparable to that of known natural and synthetic BA ligands. These findings suggest PACB2 metabolites may be novel ligands of S1PR2, CAR, and PXR receptors.Communicated by Ramaswamy H. Sarma.

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.

Why Do Dietary Flavonoids Have a Promising Effect as Enhancers of Anthracyclines? Hydroxyl Substituents, Bioavailability and Biological Activity

Anthracyclines currently play a key role in the treatment of many cancers, but the limiting factor of their use is the widespread phenomenon of drug resistance and untargeted toxicity. Flavonoids have pleiotropic, beneficial effects on human health that, apart from antioxidant activity, are currently considered small molecules-starting structures for drug development and enhancers of conventional therapeutics. This paper is a review of the current and most important data on the participation of a selected series of flavonoids: chrysin, apigenin, kaempferol, quercetin and myricetin, which differ in the presence of an additional hydroxyl group, in the formation of a synergistic effect with anthracycline antibiotics. The review includes a characterization of the mechanism of action of flavonoids, as well as insight into the physicochemical parameters determining their bioavailability in vitro. The crosstalk between flavonoids and the molecular activity of anthracyclines discussed in the article covers the most important common areas of action, such as (1) disruption of DNA integrity (genotoxic effect), (2) modulation of antioxidant response pathways, and (3) inhibition of the activity of membrane proteins responsible for the active transport of drugs and xenobiotics. The increase in knowledge about the relationship between the molecular structure of flavonoids and their biological effect makes it possible to more effectively search for derivatives with a synergistic effect with anthracyclines and to develop better therapeutic strategies in the treatment of cancer.

Phosphorylation-Induced Ubiquitination and Degradation of PXR through CDK2-TRIM21 Axis

Pregnane X receptor (PXR) is a member of the nuclear receptor superfamily that is activated by a variety of endogenous metabolites or xenobiotics. Its downstream target genes are involved in metabolism, inflammation and processes closely related to cancer. However, the stability regulation of PXR protein resulting from post-translational modification is still largely undefined. In the present study, primary mouse hepatocytes, hepatoma HepG2 cells and HEK 293T cells were used to investigate gene expression and protein interactions. The role of kinases was evaluated by RNA interference and overexpression constructs with or without PXR phosphorylation site mutations. The activity of CYP3A4 and P-gp was determined by enzymatic and substrate accumulation assays. It was found that E3 ubiquitin ligase TRIM21 mediates the ubiquitination and degradation of PXR and plays an important role in regulating the activity of PXR. On this basis, PXR phosphorylation-associated kinases were evaluated regarding regulation of the stability of PXR. We found cyclin dependent kinase 2 (CDK2) exclusively phosphorylates PXR at Ser350, promotes its disassociation with Hsp90/DNAJC7, and leads to subsequent TRIM21-mediated PXR ubiquitination and degradation. As well-known CDK inhibitors, dinaciclib and kenpaullone stabilize PXR and result in elevated expression and activity of PXR-targeted DMETs, including carboxylesterases, CYP3A4 and P-gp. The suppressed degradation of PXR by CDK2 inhibitors denotes dinaciclib-induced promotion of PXR-targeted genes. The findings of CDK2-mediated PXR degradation indicate a wide range of potential drug–drug interactions during clinical cancer therapy using CDK inhibitors and imply an alternative direction for the development of novel PXR antagonists.

The Interface between Cell Signaling Pathways and Pregnane X Receptor

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

Variation in Expression of Cytochrome P450 3A Isoforms and Toxicological Effects: Endo- and Exogenous Substances as Regulatory Factors and Substrates

The CYP3A subfamily, which includes isoforms CYP3A4, CYP3A5, and CYP3A7 in humans, plays important roles in the metabolism of various endogenous and exogenous substances. Gene and protein expression of CYP3A4, CYP3A5, and CYP3A7 show large inter-individual differences, which are caused by many endogenous and exogenous factors. Inter-individual differences can cause negative outcomes, such as adverse drug events and disease development. Therefore, it is important to understand the variations in CYP3A expression caused by endo- and exogenous factors, as well as the variation in the metabolism and kinetics of endo- and exogenous substrates. In this review, we summarize the factors regulating CYP3A expression, such as bile acids, hormones, microRNA, inflammatory cytokines, drugs, environmental chemicals, and dietary factors. In addition, variations in CYP3A expression under pathological conditions, such as coronavirus disease 2019 and liver diseases, are described as examples of the physiological effects of endogenous factors. We also summarize endogenous and exogenous substrates metabolized by CYP3A isoforms, such as cholesterol, bile acids, hormones, arachidonic acid, vitamin D, and drugs. The relationship between the changes in the kinetics of these substrates and the toxicological effects in our bodies are discussed. The usefulness of these substrates and metabolites as endogenous biomarkers for CYP3A activity is also discussed. Notably, we focused on discrimination between CYP3A4, CYP3A5, and CYP3A7 to understand inter-individual differences in CYP3A expression and function.

Dietary phytochemicals as modulators of human pregnane X receptor

As a promiscuous xenobiotic sensor, pregnane X receptor (PXR) plays a crucial role in drug metabolism. Since dietary phytochemicals exhibit the potential to modulate human PXR, this review aims to summarize the plant-derived PXR modulators, including agonists, partial agonists, and antagonists. The crystal structures of the apo and ligand-bound forms of PXR especially that of PXR complexed with binary mixtures are summarized, in order to provide the structural basis for PXR binding promiscuity and synergistic activation of PXR by composite ligands. Furthermore, this review summarizes the characterized agonists, partial agonists, and antagonists of human PXR from botanical source. Contrary to PXR agonists, there are only a few antagonists obtained from botanical source due to the promiscuity of PXR. It is worth noting that trans-resveratrol and a series of methylindoles have been identified as partial agonists of PXR, both in activating PXR function, but also inhibiting the effect of other PXR agonists. Since antagonizing PXR function plays a crucial role in the prevention of drug-drug interactions and improvement of therapeutic efficacy, further research is necessary to screen more plant-derived PXR antagonists in the future. In summary, this review may contribute to understanding the roles of phytochemicals in food-drug and herb-drug interactions.

Flavonoid and cannabinoid impact on the ocular surface

PURPOSE OF REVIEW

To evaluate the impact of flavonoids and cannabinoids as anti-inflammatory and antiallergic treatments on the anterior surface of the eye.

RECENT FINDINGS

Allergic conjunctivitis and dry eye syndrome are common ocular surface diseases that have been treated with traditional pharmacological measures, e.g. corticosteroids, antihistamines. Given the side-effect profiles of these medications and the growing interest in complementary treatment modalities as part of integrative medical interventions, well known flavonoids, such as quercetin and catechin, are under investigation for topical and systemic application methods for relief. As flavonoid derivatives, pycnogenol and epigallocatechin gallate have alleviated dry eye symptoms, including lacrimal gland inflammation, tear secretion, and the stability of the tear film. Research on ocular cannabinoid receptors and response to synthetic cannabinoids are also being considered for therapy of anterior ocular disorders. The expansion of herbal formulations provides a framework for future treatment regimens for ocular surface disorders.

SUMMARY

Flavonoids and cannabinoids show promise as potential complementary treatment for allergic diseases because of their anti-inflammatory and antiallergic properties. Several studies implementing ocular and systemic application of these compounds show potential in becoming adjuvant treatment strategies for improving quality of life while also managing ocular surface disease processes.

Comparative Analysis on Single- and Multiherb Strategies in Coronary Artery Atherosclerosis Therapy

Herbal medicine unswervingly serves human health by modernizing preparation and administration. Coronary artery atherosclerosis is a serious threat to human health and survival all over the world. Following experimental and clinical evidence, we collected four herbal treatments containing herbal strategy I (San Qi), II (Injectio Salvia Miltiorrhizae), III (Danhong injection), and IV (Taoren Honghua Jian granule) against coronary artery disease. In order to analyze their similarities and differences in controlling coronary artery atherosclerosis, we investigated each herb of four strategies and revealed that the number of active components and molecule targets is increasing with the herb category of herbal strategy. Nitric oxide-associated carbonate dehydratase activity and nitrogen metabolism are tacitly enriched by target corresponding genes with statistical significance in four strategies. The herbal strategy with multiherb not merely possesses more amounts and interactions of target proteins than the strategy with single-herb but also enlarges interaction partners of target proteins like PTPN11 and STAT3 in strategy II, III, and IV. Whereas single-herb also involves regulating network core proteins in consistent with compatibility, such as SRC and PIK3R1 that are mostly targeted by strategy I, III, and IV. Comparing the targets of the herbal strategies and three existing drugs (atenolol, pravastatin and propranolol) and the symbols of coronary artery atherosclerosis, we discovered that MAOA, HTR1A, and ABCG2 are overlapping in the three groups. Hence, our work enables people to better understand the connections and distinctions of single- and multiherb on the healing of coronary artery atherosclerosis.

Nelfinavir and Its Active Metabolite M8 Are Partial Agonists and Competitive Antagonists of the Human Pregnane X Receptor

The HIV protease inhibitor nelfinavir is currently being analyzed for repurposing as an anticancer drug for many different cancers because it exerts manifold off-target protein interactions, finally resulting in cancer cell death. Xenosensing pregnane X receptor (PXR), which also participates in the control of cancer cell proliferation and apoptosis, was previously shown to be activated by nelfinavir; however, the exact molecular mechanism is still unknown. The present study addresses the effects of nelfinavir and its major and pharmacologically active metabolite nelfinavir hydroxy-tert-butylamide (M8) on PXR to elucidate the underlying molecular mechanism. Molecular docking suggested direct binding to the PXR ligand-binding domain, which was confirmed experimentally by limited proteolytic digestion and competitive ligand-binding assays. Concentration-response analyses using cellular transactivation assays identified nelfinavir and M8 as partial agonists with EC50 values of 0.9 and 7.3 µM and competitive antagonists of rifampin-dependent induction with IC50 values of 7.5 and 25.3 µM, respectively. Antagonism exclusively resulted from binding into the PXR ligand-binding pocket. Impaired coactivator recruitment by nelfinavir as compared with the full agonist rifampin proved to be the underlying mechanism of both effects on PXR. Physiologic relevance of nelfinavir-dependent modulation of PXR activity was investigated in respectively treated primary human hepatocytes, which showed differential induction of PXR target genes and antagonism of rifampin-induced ABCB1 and CYP3A4 gene expression. In conclusion, we elucidate here the molecular mechanism of nelfinavir interaction with PXR. It is hypothesized that modulation of PXR activity may impact the anticancer effects of nelfinavir. SIGNIFICANCE STATEMENT: Nelfinavir, which is being investigated for repurposing as an anticancer medication, is shown here to directly bind to human pregnane X receptor (PXR) and thereby act as a partial agonist and competitive antagonist. Its major metabolite nelfinavir hydroxy-tert-butylamide exerts the same effects, which are based on impaired coactivator recruitment. Nelfinavir anticancer activity may involve modulation of PXR, which itself is discussed as a therapeutic target in cancer therapy and for the reversal of chemoresistance.

The Gut Microbial Endocrine Organ in Type 2 Diabetes

Historically, the focus of type II diabetes mellitus (T2DM) research has been on host metabolism and hormone action. However, emerging evidence suggests that the gut microbiome, commensal microbes that colonize the gastrointestinal tract, also play a significant role in T2DM pathogenesis. Specifically, gut microbes metabolize what is available to them through the host diet to produce small molecule metabolites that can have endocrine-like effects on human cells. In fact, the meta-organismal crosstalk between gut microbe-generated metabolites and host receptor systems may represent an untapped therapeutic target for those at risk for or suffering from T2DM. Recent evidence suggests that gut microbe-derived metabolites can impact host adiposity, insulin resistance, and hormone secretion to collectively impact T2DM progression. Here we review the current evidence that structurally diverse gut microbe-derived metabolites, including short chain fatty acids, secondary bile acids, aromatic metabolites, trimethylamine-N-oxide, polyamines, and N-acyl amides, that can engage with host receptors in an endocrine-like manner to promote host metabolic disturbance associated with T2DM. Although these microbe-host signaling circuits are not as well understood as host hormonal signaling, they hold untapped potential as new druggable targets to improve T2DM complications. Whether drugs that selectively target meta-organismal endocrinology will be safe and efficacious in treating T2DM is a key new question in the field of endocrinology. Here we discuss the opportunities and challenges in targeting the gut microbial endocrine organ for the treatment of diabetes and potentially many other diseases where diet-microbe-host interactions play a contributory role.

Characterization of human pregnane X receptor activators identified from a screening of the Tox21 compound library

The pregnane X receptor (PXR; NR1I2) is an important nuclear receptor whose main function is to regulate enzymes within drug metabolism. The main drug metabolizing enzyme regulated by PXR, cytochrome P450 (CYP) 3A4, accounts for the metabolism of nearly 50% of all marketed drugs. Recently, PXR has also been identified as playing a role in energy homeostasis, immune response, and cancer. Due to its interaction with these important roles, alongside its drug-drug interaction function, it is imperative to identify compounds which can modulate PXR. In this study, we screened the Tox21 10,000 compound collection to identify hPXR agonists using a stable hPXR-Luc HepG2 cell line. A pharmacological study in the presence of a PXR antagonist was performed to confirm the activity of the chosen potential hPXR agonists in the same cells. Finally, metabolically competent cell lines - HepaRG and HepaRG-PXR-Knockout (KO) - were used to further confirm the potential PXR activators. We identified a group of structural clusters and singleton compounds which included potentially novel hPXR agonists. Of the 21 selected compounds, 11 potential PXR activators significantly induced CYP3A4 mRNA expression in HepaRG cells. All of these compounds lost their induction when treating HepaRG-PXR-KO cells, confirming their PXR activation. Etomidoline presented as a potentially selective agonist of PXR. In conclusion, the current study has identified 11 compounds as potentially novel or not well-characterized PXR activators. These compounds should further be studied for their potential effects on drug metabolism and drug-drug interactions due to the immense implications of being a PXR agonist.

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

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

The constitutive androstane receptor and pregnane X receptor in the brain

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

Dual Effects of Chinese Herbal Medicines on Angiogenesis in Cancer and Ischemic Stroke Treatments: Role of HIF-1 Network

Hypoxia-inducible factor-1 (HIF-1)-induced angiogenesis has been involved in numerous pathological conditions, and it may be harmful or beneficial depending on the types of diseases. Exploration on angiogenesis has sparked hopes in providing novel therapeutic approaches on multiple diseases with high mortality rates, such as cancer and ischemic stroke. The HIF-1 pathway is considered to be a major regulator of angiogenesis. HIF-1 seems to be involved in the vascular formation process by synergistic correlations with other proangiogenic factors in cancer and cerebrovascular disease. The regulation of HIF-1-dependent angiogenesis is related to the modulation of HIF-1 bioactivity by regulating HIF-1α transcription or protein translation, HIF-1α DNA binding, HIF-1α and HIF-1α dimerization, and HIF-1 degradation. Traditional Chinese herbal medicines have a long history of clinical use in both cancer and stroke treatments in Asia. Growing evidence has demonstrated potential proangiogenic benefits of Chinese herbal medicines in ischemic stroke, whereas tumor angiogenesis could be inhibited by the active components in Chinese herbal medicines. The objective of this review is to provide comprehensive insight on the effects of Chinese herbal medicines on angiogenesis by regulating HIF-1 pathways in both cancer and ischemic stroke.

Drug discovery technologies to identify and characterize modulators of the pregnane X receptor and the constitutive androstane receptor

The pregnane X receptor (PXR) and the constitutive androstane receptor (CAR) are ligand-activated nuclear receptors (NRs) that are notorious for their role in drug metabolism, causing unintended drug-drug interactions and decreasing drug efficacy. They control the xenobiotic detoxification system by regulating the expression of an array of drug-metabolizing enzymes and transporters that excrete exogenous chemicals and maintain homeostasis of endogenous metabolites. Much effort has been invested in recognizing potential drugs for clinical use that can activate PXR and CAR to enhance the expression of their target genes, and in identifying PXR and CAR inhibitors that can be used as co-therapeutics to prevent adverse effects. Here, we present current technologies and assays used in the quest to characterize PXR and CAR modulators, which range from biochemical to cell-based and animal models.

Effect of panaxytriol on cytochrome P450 3A4 via the pregnane X receptor regulatory pathway

Panaxytriol (PXT) is one of the major effective components of red ginseng and Shenmai injection. The present study aimed to explore the effect of PXT on cytochrome P450 3A4 (CYP3A4) based on the pregnane X receptor (PXR)-CYP3A4 regulatory pathway in HepG2 cells and hPXR-overexpressing HepG2 cells treated with PXT for different time periods using quantitative polymerase chain reaction, Western blot, and dual-luciferase reporter gene assays. PXT could upregulate the levels of PXR and CYP3A4 mRNA in HepG2 cells treated with PXT for 1 hr, with no impact on the expression of their protein levels. The expression levels of both PXR and CYP3A4 mRNA and protein in HepG2 cells treated with PXT for 24 hr increased in a concentration-dependent manner. The effects of PXT on the expression of PXR and CYP3A4 mRNA and protein in hPXR-overexpressing HepG2 cells were similar to those in HepG2 cells. Moreover, the influence trend of PXT on CYP3A4 was consistent with that of PXR in HepG2 cells and hPXR-overexpressing HepG2 cells. The dual-luciferase reporter gene assay in HepG2 cells further demonstrated that PXT treatment for specific time periods could significantly induce the expression of CYP3A4 mediated by the PXR regulatory pathway.

Quercetin‑3‑O‑β‑D‑glucoside decreases the bioavailability of cyclosporin A through regulation of drug metabolizing enzymes, transporters and nuclear receptors in rats

Quercetin is a flavonoid compound that is widely present in food and drink. Quercetin-3-O-β-D-glucoside (Q3GA) is a major metabolite of quercetin. The aim of the present study was to investigate the effect of Q3GA on the pharmacokinetics of orally and intravenously administered cyclosporin A (CsA) in rats, and to assess the effect of Q3GA on drug-metabolizing enzymes (DMEs), drug transporters (DTs) and nuclear receptors (NRs). The pharmacokinetic parameters of CsA were measured following oral (10 mg/kg) and intravenous (2.5 mg/kg) administration of CsA in the presence or absence of Q3GA. The mRNA and protein expression levels of DMEs, DTs and NRs in the liver and small intestine were detected by quantitative polymerase chain reaction and western blot analysis. The results indicated that the intravenous administration of Q3GA (2.5, 5 or 10 mg/kg) for 7 consecutive days reduced the bioavailability of oral CsA. By contrast, the pharmacokinetics of the intravenous administration of CsA were not affected by Q3GA. However, the mRNA and protein expression levels of DMEs and DTs were inhibited by Q3GA. The activation of DMEs and DTs by NRs, and the interplay between DMEs and DTs, may explain these results. The present study identified a novel flavonoid-drug interaction, which may have implications for patients taking CsA and quercetin supplements or on a quercetin-containing diet.

Structure-Dependent Modulation of Aryl Hydrocarbon Receptor-Mediated Activities by Flavonoids

Dietary flavonoids are used in treatment of multiple diseases, and their antiinflammatory effects in the intestine are due, in part, to interactions with gut microflora and possibly due to modulation of aryl hydrocarbon receptor (AhR) signaling. In this study, we investigated the structure-dependent AhR activity of 14 flavonoids in Caco2 colon cancer cells using induction of CYP1A1 and UGT1A1 gene expression as endpoints. A major structural determinant for AhR activation was the number of hydroxyl groups where pentahydroxyflavonoids (with the exception of morin) > hexahydroxyflavonoids > tetra-/trihydroxyflavonoids, and some of the latter compounds such as apigenin exhibited AhR antagonist activity for induction of CYP1A1. Simulations suggest that while quercetin and apigenin interact primarily with the same residues, the strength of interactions between specific AhR residues with CYP1A1 agonist, quercetin, in comparison with CYP1A1 antagonist, apigenin, is different; thus, such interactions are presumably indicative of potential switches for modulating CYP1A1 activity. The structure-dependent effects of the hydroxyl flavonoids on induction of UGT1A1 were similar to that observed for induction of CYP1A1 except that luteolin and apigenin induced UGT1A1 levels similar to that observed for TCDD, whereas both compounds were AhR antagonists for CYP1A1. Thus, the effects of the flavonoids in Caco2 cells on Ah-responsiveness and interactions with butyrate were both ligand structure- and response-dependent and these activities are consistent with hydroxyflavonoids being selective AhR modulators.

The Potential and Action Mechanism of Polyphenols in the Treatment of Liver Diseases

Liver disease, involving a wide range of liver pathologies from fatty liver, hepatitis, and fibrosis to cirrhosis and hepatocellular carcinoma, is a serious health problem worldwide. In recent years, many natural foods and herbs with abundant phytochemicals have been proposed as health supplementation for patients with hepatic disorders. As an important category of phytochemicals, natural polyphenols have attracted increasing attention as potential agents for the prevention and treatment of liver diseases. The striking capacities in remitting oxidative stress, lipid metabolism, insulin resistance, and inflammation put polyphenols in the spotlight for the therapies of liver diseases. It has been reported that many polyphenols from a wide range of foods and herbs exert therapeutic effects on liver injuries via complicated mechanisms. Therefore, it is necessary to have a systematical review to sort out current researches to help better understand the potentials of polyphenols in liver diseases. In this review, we aim to summarize and update the existing evidence of natural polyphenols in the treatment of various liver diseases by in vitro, in vivo, and clinical studies, while special attention is paid to the action mechanisms.

Cancer chemoprevention through dietary flavonoids: what's limiting?

Flavonoids are polyphenols that are found in numerous edible plant species. Data obtained from preclinical and clinical studies suggest that specific flavonoids are chemo-preventive and cytotoxic against various cancers via a multitude of mechanisms. However, the clinical use of flavonoids is limited due to challenges associated with their effective use, including (1) the isolation and purification of flavonoids from their natural resources; (2) demonstration of the effects of flavonoids in reducing the risk of certain cancer, in tandem with the cost and time needed for epidemiological studies, and (3) numerous pharmacokinetic challenges (e.g., bioavailability, drug–drug interactions, and metabolic instability). Currently, numerous approaches are being used to surmount some of these challenges, thereby increasing the likelihood of flavonoids being used as chemo-preventive drugs in the clinic. In this review, we summarize the most important challenges and efforts that are being made to surmount these challenges.

Potential of rooibos, its major C-glucosyl flavonoids, and Z-2-(β-D-glucopyranosyloxy)-3-phenylpropenoic acid in prevention of metabolic syndrome

Risk factors of type 2 diabetes mellitus (T2D) and cardiovascular disease (CVD) cluster together and are termed the metabolic syndrome. Key factors driving the metabolic syndrome are inflammation, oxidative stress, insulin resistance (IR), and obesity. IR is defined as the impairment of insulin to achieve its physiological effects, resulting in glucose and lipid metabolic dysfunction in tissues such as muscle, fat, kidney, liver, and pancreatic β-cells. The potential of rooibos extract and its major C-glucosyl flavonoids, in particular aspalathin, a C-glucoside dihydrochalcone, as well as the phenolic precursor, Z-2-(β-D-glucopyranosyloxy)-3-phenylpropenoic acid, to prevent the metabolic syndrome, will be highlighted. The mechanisms whereby these phenolic compounds elicit positive effects on inflammation, cellular oxidative stress and transcription factors that regulate the expression of genes involved in glucose and lipid metabolism will be discussed in terms of their potential in ameliorating features of the metabolic syndrome and the development of serious metabolic disease. An overview of the phenolic composition of rooibos and the changes during processing will provide relevant background on this herbal tea, while a discussion of the bioavailability of the major rooibos C-glucosyl flavonoids will give insight into a key aspect of the bioefficacy of rooibos.

Intraperitoneal administration of apigenin in liver ischemia/reperfusion injury protective effects

BACKGROUND/AIMS

Hepatic injury caused by ischemia/reperfusion (I/R) is a clinical problem associated with major liver surgery. Among other flavonoids, apigenin has shown a promising effect on I/R cases. In this study, we have investigated the effects of apigenin after liver I/R injury in rats.

MATERIALS AND METHODS

Forty eight rats were randomized into the following eight groups: (1) Control-sham group: rats subjected to the surgical procedure, except for liver I/R; (2) DMSO group: rats subjected to surgery, except for liver I/R given the apigenin solvent dimethyl-sulfoxide intraperitoneally; (3) C60 group; (4) C120 group; (5) C240 group: rats underwent liver ischemia for 45 min followed by reperfusion for 60 min, 120 min, and 240 min; (6) AP60 group; (7) AP120 group; (8) AP240 group: rats underwent liver ischemia for 45 min, and then given apigenin (5 mg) intraperitoneally followed by reperfusion for 60 min, 120 min, and 240 min. Reverse transcription polymerase chain reaction was performed on liver tissues to measure BCL-2/BAX expression, enzyme-linked immunosorbent assay to measure M30/M65 and ICAM-1. Immunohistochemistry was used to identify M30 biomarker in liver tissues.

STATISTICAL ANALYSIS

Quantitative variables were tested by Kolmogorov-Smirnov test, repeated measures analysis of variance/Friedman test. Gene levels were assessed by Student's t-test/Mann-Whitney U-test.

RESULTS

BCL-2 levels were significantly higher in I/R apigenin groups than in I/R control groups. BAX levels were lower in the AP240 group than in C240 group. Prolongation of reperfusion resulted in increased activation of M30. ICAM-1 levels were lower in the AP240 group than in C240 group.

CONCLUSIONS

Apigenin seems to inhibit the process of apoptosis and ameliorate the hepatic I/R injury.

In vitro Effects of Four Native Brazilian Medicinal Plants in CYP3A4 mRNA Gene Expression, Glutathione Levels, and P-Glycoprotein Activity

Erythrina mulungu Benth. (Fabaceae), Cordia verbenacea A. DC. (Boraginaceae), Solanum paniculatum L. (Solanaceae) and Lippia sidoides Cham. (Verbenaceae) are medicinal plant species native to Brazil shortlisted by the Brazilian National Health System for future clinical use. However, nothing is known about their effects in metabolic and transporter proteins, which could potentially lead to herb-drug interactions (HDI). In this work, we assess non-toxic concentrations (100 μg/mL) of the plant infusions for their in vitro ability to modulate CYP3A4 mRNA gene expression and intracellular glutathione levels in HepG2 cells, as well as P-glycoprotein (P-gp) activity in vincristine-resistant Caco-2 cells (Caco-2 VCR). Their mechanisms of action were further studied by measuring the activation of human pregnane X receptor (hPXR) in transiently co-transfected HeLa cells and the inhibition of γ-glutamyl transferase (GGT) in HepG2 cells. Our results show that P-gp activity was not affected in any case and that only Solanum paniculatum was able to significantly change CYP3A4 mRNA gene expression (twofold decrease, p < 0.05), this being correlated with an antagonist effect upon hPXR (EC50 = 0.38 mg/mL). Total intracellular glutathione levels were significantly depleted by exposure to Solanum paniculatum (-44%, p < 0.001), Lippia sidoides (-12%, p < 0.05) and Cordia verbenacea (-47%, p < 0.001). The latter plant extract was able to decrease GGT activity (-48%, p < 0.01). In conclusion, this preclinical study shows that the administration of some of these herbal medicines may be able to cause disturbances to metabolic mechanisms in vitro. Although Erythrina mulungu appears safe in our tests, active pharmacovigilance is recommended for the other three species, especially in the case of Solanum paniculatum.

The regulation of human hepatic drug transporter expression by activation of xenobiotic-sensing nuclear receptors

INTRODUCTION

If a drug is found to be an inducer of hepatic drug metabolizing enzymes via activation of nuclear receptors such as pregnane X receptor (PXR) or constitutive androstane receptor (CAR), it is likely that drug transporters regulated through these same receptors will be induced as well. This review highlights what is currently known about the molecular mechanisms that regulate transporter expression and where the research is directed. Areas covered: This review is focused on publications that describe the role of activated hepatic nuclear receptors in the subsequent regulation of drug uptake and/or efflux transporters following exposure to xenobiotics. Expert opinion: Many of the published studies on the role of nuclear receptors in the regulation of drug transporters involve non-human test animals. But due to species response differences, these associations are not always applicable to humans. For this reason, some relevant human in vitro models have been developed, such as primary or cryopreserved human hepatocytes, human liver slices, or HepG2 or HuH7 cell lines transiently or stably transfected with PXR expression and reporter constructs as well as in vivo models such as PXR-humanized mice. These human-relevant test systems will continue to be developed and applied for the testing of investigational drugs.

A Molecular Aspect in the Regulation of Drug Metabolism: Does PXR-Induced Enzyme Expression Always Lead to Functional Changes in Drug Metabolism?

Pregnane X receptor (PXR, NR112) is a xenobiotic receptor whose primary function is to regulate the expression of drug-metabolizing enzymes (DMEs) and drug transporters. Drug-induced PXR activation and subsequent enzyme and transporter induction has been proposed to be an important mechanism for the drug-drug interactions. In addition to activating PXR, many pharmaceutical chemicals can also function as reversible or irreversible inhibitors of DMEs, which may also impact the pharmacokinetics and pharmacodynamics (PK/PD) of drugs. Therefore, we cannot simply conclude that the PXR-induced alteration in enzyme expression always reflects functional changes. We should consider both PXR activation and DMEs inhibition to improve drug safety in the clinic.

Mechanisms of xenobiotic receptor activation: Direct vs. indirect

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

Bioactivation of luteolin by tyrosinase selectively inhibits glutathione S-transferase

Glutathione S-transferase (GST) plays a significant role in the metabolism and detoxification of drugs used in treatment of melanoma, resulting in a decrease in drug efficacy. Tyrosinase is an abundant enzyme found in melanoma. In this study, we used a tyrosinase targeted approach to selectively inhibit GST. In the presence of tyrosinase, luteolin (10 μM) showed 87% GST inhibition; whereas in the absence of tyrosinase, luteolin led to negligible GST inhibition. With respect to GSH, both luteolin-SG conjugate and luteolin-quinone inhibited ≥90% of GST activity via competitive reversible and irreversible mixed mechanisms with Ki of 0.74 μM and 0.02 μM, respectively. With respect to CDNB, the luteolin-SG conjugate inhibited GST activity via competitive reversible mechanism and competitively with Ki of 0.58 μM, whereas luteolin-quinone showed irreversible mixed inhibition of GST activity with Ki of 0.039 μM. Luteolin (100 μM) inhibited GST in mixed manner with Ki of 53 μM with respect to GSH and non-competitively with respect to CDNB with Ki of 38 μM. Luteolin, at a concentration range of 5-80 μM, exhibited 78-99% GST inhibition in human SK-MEL-28 cell homogenate. Among the 3 species of intact luteolin, luteolin-SG conjugate, and luteoline-quinone, only the latter two have potential as drugs with Ki < 1 μM, which is potentially achievable in-vivo as therapeutic agents. The order of GST inhibition was luteolin-quinone >> luteolin-SG conjugate >>> luteolin. In summary, our results suggest that luteolin was bioactivated by tyrosinase to form a luteolin-quinone and luteolin-glutathione conjugate, which inhibited GST. For the first time, in addition to intracellular GSH depletion, we demonstrate that luteolin acts as a selective inhibitor of GST in the presence of tyrosinase. Such strategy could potentially be used to selectively inhibit GST, a drug detoxifying enzyme, in melanoma cells.

3-Hydroxyflavone and structural analogues differentially activate pregnane X receptor: Implication for inflammatory bowel disease

Pregnane X receptor (PXR; NR1I2) is a member of the superfamily of nuclear receptors that regulates the expression of genes involved in various biological processes, including drug transport and biotransformation. In the present study, we investigated the effect of 3-hydroxyflavone and its structurally-related analogues on PXR activity. 3-Hydroxyflavone, galangin, kaempferol, querceetin, isorhamnetin, and tamarixetin, but not but not datiscetin, morin, myricetin, or syringetin, activated mouse PXR, as assessed in a cell-based reporter gene assay. By comparison, 3-hydroxyflavone activated rat PXR, whereas 3-hydroxyflavone, galangin, quercetin, isorhamnetin, and tamarixetin activated human PXR (hPXR). A time-resolved fluorescence resonance energy transfer competitive ligand-binding assay showed binding to the ligand-binding domain of hPXR by 3-hydroxyflavone, galangin, quercetin, isorhamnetin, and tamarixetin. 3-Hydroxyflavone and galangin, but not quercetin, isorhamnetin, or tamarixetin, recruited steroid receptor coactivator (SRC)-1, SRC-2, and SRC-3 to hPXR. In LS180 human colon adenocarcinoma cells, 3-hydroxyflavone, quercetin, and tamarixetin increased CYP3A4, CYP3A5, and ABCB1 mRNA expression, whereas galangin and isorhamnetin increased CYP3A4 and ABCB1 but not CYP3A5 mRNA expression. Datiscetin, kaempferol, morin, myricetin, and syringetin did not attenuate the extent of hPXR activation by rifampicin, suggesting they are not hPXR antagonists. Overall, flavonols activate PXR in an analogue-specific and species-dependent manner. Substitution at the C2' or C5' position of 3-hydroxyflavone with a hydroxyl or methoxy group rendered it incapable of activating hPXR. Understanding the structure-activity relationship of flavonols in hPXR activation may facilitate nutraceutical development efforts in the treatment of PXR-associated intestinal diseases, such as inflammatory bowel disease.

Effect of Natural Polyphenols on CYP Metabolism: Implications for Diseases

Cytochromes P450 (CYPs) are a large group of hemeproteins located on mitochondrial membranes or the endoplasmic reticulum. They play a crucial role in the metabolism of endogenous and exogenous molecules. The activity of CYP is associated with a number of factors including redox potential, protein conformation, the accessibility of the active site by substrates, and others. This activity may be potentially modulated by a variety of small molecules. Extensive experimental data collected over the past decade point at the active role of natural polyphenols in modulating the catalytic activity of CYP. Polyphenols are widespread micronutrients present in human diets of plant origin and in medicinal herbs. These compounds may alter the activity of CYP either via direct interactions with the enzymes or by affecting CYP gene expression. The polyphenol-CYP interactions may significantly alter the pharmacokinetics of drugs and thus influence the effectiveness of chemical therapies used in the treatment of different types of cancers, diabetes, obesity, and cardiovascular diseases (CVD). CYPs are involved in the oxidation and activation of external carcinogenic agents, in which case the inhibition of the CYP activity is beneficial for health. CYPs also support detoxification processes. In this case, it is the upregulation of CYP genes that would be favorable for the organism. A CYP enzyme aromatase catalyzes the formation of estrone and estradiol from their precursors. CYPs also catalyze multiple reactions leading to the oxidation of estrogen. Estrogen signaling and oxidative metabolism of estrogen are associated with the development of cancer. Thus, polyphenol-mediated modulation of the CYP's activity also plays a vital role in estrogen carcinogenesis. The aim of the present review is to summarize the data collected over the last five to six years on the following topics: (1) the mechanisms of the interactions of CYP with food constituents that occur via the direct binding of polyphenols to the enzymes and (2) the mechanisms of the regulation of CYP gene expression mediated by polyphenols. The structure-activity relationship relevant to the ability of polyphenols to affect the activity of CYP is analyzed. The application of polyphenol-CYP interactions to diseases is discussed.

Development of BODIPY FL vindoline as a novel and high-affinity pregnane X receptor fluorescent probe

The pregnane X receptor (PXR) regulates the metabolism and excretion of xenobiotics and endobiotics by regulating the expression of drug-metabolizing enzymes and transporters. The unique structure of PXR allows it to bind many drugs and drug leads, possibly causing undesired drug–drug interactions. Therefore, it is crucial to evaluate whether chemicals or drugs bind to PXR. Fluorescence-based assays are preferred because of their sensitivity and nonradioactive nature. On the basis of our previously characterized 4 (BODIPY FL vinblastine), a high-affinity PXR probe, we developed 20 (BODIPY FL vindoline) and showed that it is a novel and potent PXR fluorescent probe with K_d of 256 nM in a time-resolved fluorescence resonance energy transfer (TR-FRET) binding assay with PXR. By using 20 (BODIPY FL vindoline) in the PXR TR-FRET assay, we obtained a more than 7-fold signal-to-background ratio and high signal stability (signal was stable for at least 120 min, and Z′-factor > 0.85 from 30 to 240 min). The assay can tolerate DMSO up to 2%. This assay has been used to evaluate a panel of PXR ligands for their PXR-binding affinities. The performance of 20 (BODIPY FL vindoline) in the PXR TR-FRET assay makes it an ideal PXR fluorescent probe, and the newly developed PXR TR-FRET assay with 20 (BODIPY FL vindoline) as a fluorescent probe is suitable for high-throughput screening to identify PXR-binding ligands.

Is a flavonoid-rich diet with steamer cooking safe during calcineurin inhibitors therapy?

WHAT IS KNOWN AND OBJECTIVE

Dietary therapy is recommended for decreasing the symptoms of the metabolic syndrome and the risk of type 2 diabetes and cardiovascular diseases in subjects on calcineurin inhibitors. However, food-drug interactions may occur particularly with patients on such immunosuppressive therapy. This article comments on the benefit/risk assessment of a flavonoid-rich diet and steam-cooking of such food during calcineurin inhibitors therapy.

COMMENT

Patients are commonly advised against consuming citrus fruits and juices, grape juice and green tea. High vegetable intake may however increase the risk of food-diet interactions by inhibiting drug metabolic enzymes and transporters. Vegetable glucosinolates are potential interactants and may lead to adverse effects of drugs with narrow therapeutic indices and in the presence of genetic polymorphism. Examples of food components with potential drug interactants include all members of the Brassicaceae family.

WHAT IS NEW AND CONCLUSION

The potential additive and synergistic effects of flavonoids with other molecules in interfering with drug bioavailability need to be taken into account. The risk is highest with drugs with a narrow therapeutic index and in subjects with genetic polymorphisms of proteins involved in the disposition of those drugs.

Interference of flavonoids with fluorescent intracellular probes: methodological implications in the evaluation of the oxidative burst by flow cytometry

The evaluation of oxidative burst is particularly relevant in many pathological and subclinical conditions. Flow cytometry provides quick and accurate measures of the reactive oxygen species production by leukocytes in most situations. However, spurious results, related to probes' efflux may be observed in several instances. Many factors affect the evaluation of the oxidative burst with fluorescent probes that require intracellular deacetylation and could be substrate of the multidrug resistance proteins (MDR). After discussing the implications of the efflux of fluorophores in the normalization strategies in flow cytometry assays, we have pointed out the possible interference of flavonoids with fluorescet probes' staining and signal. We have also reviewed the results from human intervention studies regarding the evaluation of oxidative burst with these probes. In vitro, at concentrations close to post-ingestion circulating levels, some flavonoids and their metabolites could interfere with probes' staining and fluorescence signal through different mechanisms, such as the inhibition of esterases, the modulation of the MDR-mediate efflux of probe and the inhibition of the oxidation of probe. These effects may explain the contrasting results obtained by human intervention studies. Finally, also inflammatory state or the use of drugs substrate of MDR proteins could affect the evaluation of the oxidative burst with intracellular probes.

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

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

Analysis of flavonoids regulating the expression of UGT1A1 via xenobiotic receptors in intestinal epithelial cells

UDP-glucuronosyltransferase (UGT) 1A1 is one of the major metabolic enzymes for the detoxification of harmful xenobiotics in intestines, and its expression is regulated by transcription factors like the aryl hydrocarbon receptor (AhR) and the pregnane X receptor (PXR). A screening assay using real-time PCR showed that baicalein and 3-hydroxyflavone induced human UGT1A1 mRNA expression in LS180 cells. Experimental results confirmed that these flavonoids increased UGT1A protein expression as well as its enzymatic activity. The results indicated that baicalein and 3-hydroxyflavone increased the transcriptional activity of UGT1A1 via AhR and PXR, respectively. Observation via immunofluorescence microscopy suggested that baicalein and 3-hydroxyflavone further induced nuclear translocation of AhR and PXR, respectively. In addition, direct interaction between baicalein and AhR or 3-hydroxyflavone and PXR were observed using the quartz crystal microbalance method. These results elucidate the molecular mechanism of flavonoid-induced UGT1A1 gene expression via xenobiotic receptors in the intestines.

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

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

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

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

A vinblastine fluorescent probe for pregnane X receptor in a time-resolved fluorescence resonance energy transfer assay

The pregnane X receptor (PXR) regulates the metabolism and excretion of xenobiotics and endobiotics by regulating the expression of drug-metabolizing enzymes and transporters. The unique structure of PXR allows the binding of many drugs and drug leads to it, possibly causing undesired drug-drug interactions. Therefore, it is crucial to evaluate whether lead compounds bind to PXR. Fluorescence-based assays are preferred because of their sensitivity and nonradioactive nature. One fluorescent PXR probe is currently commercially available; however, because its chemical structure is not publicly disclosed, it is not optimal for studying ligand-PXR interactions. Here we report the characterization of BODIPY FL-vinblastine, generated by labeling vinblastine with the fluorophore 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene (BODIPY FL), as a high-affinity ligand for human PXR with a Kd value of 673 nM. We provide evidence that BODIPY FL-vinblastine is a unique chemical entity different from either vinblastine or the fluorophore BODIPY FL in its function as a high-affinity human PXR ligand. We describe a BODIPY FL-vinblastine-based human PXR time-resolved fluorescence resonance energy transfer assay, which was used to successfully test a panel of human PXR ligands. The BODIPY FL-vinblastine-based biochemical assay is suitable for high-throughput screening to evaluate whether lead compounds bind to PXR.

Rotenone-induced oxidative stress and apoptosis in human liver HepG2 cells

Rotenone, a commonly used pesticide, is well documented to induce selective degeneration in dopaminergic neurons and motor dysfunction. Such rotenone-induced neurodegenration has been primarily suggested through mitochondria-mediated apoptosis and reactive oxygen species (ROS) generation. But the status of rotenone induced changes in liver, the major metabolic site is poorly investigated. Thus, the present investigation was aimed to study the oxidative stress-induced cytotoxicity and apoptotic cell death in human liver cells-HepG2 receiving experimental exposure of rotenone (12.5-250 μM) for 24 h. Rotenone depicted a dose-dependent cytotoxic response in HepG2 cells. These cytotoxic responses were in concurrence with the markers associated with oxidative stress such as an increase in ROS generation and lipid peroxidation as well as a decrease in the glutathione, catalase, and superoxide dismutase levels. The decrease in mitochondrial membrane potential also confirms the impaired mitochondrial activity. The events of cytotoxicity and oxidative stress were found to be associated with up-regulation in the expressions (mRNA and protein) of pro-apoptotic markers viz., p53, Bax, and caspase-3, and down-regulation of anti-apoptotic marker Bcl-2. The data obtain in this study indicate that rotenone-induced cytotoxicity in HepG2 cells via ROS-induced oxidative stress and mitochondria-mediated apoptosis involving p53, Bax/Bcl-2, and caspase-3.

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.

Flavonoids as dietary regulators of nuclear receptor activity

Metabolic diseases such as obesity, type II diabetes, and dyslipidemia are a rising cause of mortality worldwide. The progression of many metabolic diseases is fundamentally regulated on the transcriptional level by a family of ligand-activated transcription factors, called nuclear receptors, which detect and respond to metabolic changes. Their role in maintaining metabolic homeostasis makes nuclear receptors an important pharmaceutical and dietary target. This review will present the growing evidence that flavonoids, natural secondary plant metabolites, are important regulators of nuclear receptor activity. Structural similarities between flavonoids and cholesterol derivatives combined with the promiscuous nature of most nuclear receptors provide a wealth of possibilities for pharmaceutical and dietary modulation of metabolism. While the challenges of bringing flavonoid-derived therapeutics to the market are significant, we consider this rapidly growing field to be an essential aspect of the functional food initiative and an important mine for pharmaceutical compounds.

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.

Novel yeast-based strategy unveils antagonist binding regions on the nuclear xenobiotic receptor PXR

BACKGROUND

Ketoconazole binds to and antagonizes pregnane X receptor (PXR) activation.

RESULTS

Yeast high throughput screens of PXR mutants define a unique region for ketoconazole binding.

CONCLUSION

Ketoconazole genetically interacts with specific PXR surface residues.

SIGNIFICANCE

A yeast-based genetic method to discover novel nuclear receptor interactions with ligands that associate with surface binding sites is suggested. The pregnane X receptor (PXR) is a master regulator of xenobiotic metabolism, and its activity is critical toward understanding the pathophysiology of several diseases, including inflammation, cancer, and steatosis. Previous studies have demonstrated that ketoconazole binds to ligand-activated PXR and antagonizes receptor control of gene expression. Structure-function as well as computational docking analysis suggested a putative binding region containing critical charge clamp residues Gln-272, and Phe-264 on the AF-2 surface of PXR. To define the antagonist binding surface(s) of PXR, we developed a novel assay to identify key amino acid residues on PXR based on a yeast two-hybrid screen that examined mutant forms of PXR. This screen identified multiple "gain-of-function" mutants that were "resistant" to the PXR antagonist effects of ketoconazole. We then compared our screen results identifying key PXR residues to those predicted by computational methods. Of 15 potential or putative binding residues based on docking, we identified three residues in the yeast screen that were then systematically verified to functionally interact with ketoconazole using mammalian assays. Among the residues confirmed by our study was Ser-208, which is on the opposite side of the protein from the AF-2 region critical for receptor regulation. The identification of new locations for antagonist binding on the surface or buried in PXR indicates novel aspects to the mechanism of receptor antagonism. These results significantly expand our understanding of antagonist binding sites on the surface of PXR and suggest new avenues to regulate this receptor for clinical applications.

Evaluation of the toxicity of Pradosia huberi extract during the preimplantation in Wistar rats

The treatment during the embryonic preimplantation phase of Wistar rats with the Pradosia huberi extract did not interfere with the water and feed consumption, as well as upon the body-weight gain. However, it has expressed a decrease of the uterine implant number, followed by the preimplantation losses at all applied doses (1.22, 6.1, and 30.5 mg/kg), and the number of embryonic resorptions in the two highest doses (6.1 and 30.5 mg/kg). After the organ weighing (hypophysis, ovaries, and uterus), only the relative weight of the hypophysis was raised at the different doses (1.22, 6.1, and 30.5 mg/kg). It was concluded that the hydroalcoholic extract of Pradosia huberi compromises the reproductive ability during the embryonic preimplantation phase, suggesting a possible toxic effect upon the reproductive system of Wistar rats.