The induction of human UDP-glucuronosyltransferase 1A1 mediated through a distal enhancer module by flavonoids and xenobiotics

Identification of Bromophenols' glucuronidation and its induction on UDP- glucuronosyltransferases isoforms

Bromophenols (BPs) are prominent environmental pollutants extensively utilized in aquaculture, pharmaceuticals, and chemical manufacturing. This study aims to identify UDP- glucuronosyltransferases (UGTs) isoforms involved in the metabolic elimination of BPs. Mono-glucuronides of BPs were detected in human liver microsomes (HLMs) incubated with the co-factor uridine-diphosphate glucuronic acid (UDPGA). The glucuronidation metabolism reactions catalyzed by HLMs followed Michaelis-Menten or substrate inhibition kinetics. Recombinant enzymes and inhibition experiments with chemical reagents were employed to phenotype the principal UGT isoforms participating in BP glucuronidation. UGT1A6 emerged as the major enzyme in the glucuronidation of 4-Bromophenol (4-BP), while UGT1A1, UGT1A6, and UGT1A8 were identified as the most essential isoforms for metabolizing 2,4-dibromophenol (2,4-DBP). UGT1A1, UGT1A8, and UGT2B4 were deemed the most critical isoforms in the catalysis of 2,4,6-tribromophenol (2,4,6-TBP) glucuronidation. Species differences were investigated using the liver microsomes of pig (PLM), rat (RLM), monkey (MyLM), and dog (DLM). Additionally, 2,4,6-TBP effects on the expression of UGT1A1 and UGT2B7 in HepG2 cells were evaluated. The results demonstrated potential induction of UGT1A1 and UGT2B7 upon exposure to 2,4,6-TBP at a concentration of 50 μM. Collectively, these findings contribute to elucidating the metabolic elimination and toxicity of BPs.

The Role of Endocrine Disruption Chemical-Regulated Aryl Hydrocarbon Receptor Activity in the Pathogenesis of Pancreatic Diseases and Cancer

The aryl hydrocarbon receptor (AHR) serves as a ligand-activated transcription factor crucial for regulating fundamental cellular and molecular processes, such as xenobiotic metabolism, immune responses, and cancer development. Notably, a spectrum of endocrine-disrupting chemicals (EDCs) act as agonists or antagonists of AHR, leading to the dysregulation of pivotal cellular and molecular processes and endocrine system disruption. Accumulating evidence suggests a correlation between EDC exposure and the onset of diverse pancreatic diseases, including diabetes, pancreatitis, and pancreatic cancer. Despite this association, the mechanistic role of AHR as a linchpin molecule in EDC exposure-related pathogenesis of pancreatic diseases and cancer remains unexplored. This review comprehensively examines the involvement of AHR in EDC exposure-mediated regulation of pancreatic pathogenesis, emphasizing AHR as a potential therapeutic target for the pathogenesis of pancreatic diseases and cancer.

Drug-drug interactions that alter the exposure of glucuronidated drugs: Scope, UDP-glucuronosyltransferase (UGT) enzyme selectivity, mechanisms (inhibition and induction), and clinical significance

Drug-drug interactions (DDIs) arising from the perturbation of drug metabolising enzyme activities represent both a clinical problem and a potential economic loss for the pharmaceutical industry. DDIs involving glucuronidated drugs have historically attracted little attention and there is a perception that interactions are of minor clinical relevance. This review critically examines the scope and aetiology of DDIs that result in altered exposure of glucuronidated drugs. Interaction mechanisms, namely inhibition and induction of UDP-glucuronosyltransferase (UGT) enzymes and the potential interplay with drug transporters, are reviewed in detail, as is the clinical significance of known DDIs. Altered victim drug exposure arising from modulation of UGT enzyme activities is relatively common and, notably, the incidence and importance of UGT induction as a DDI mechanism is greater than generally believed. Numerous DDIs are clinically relevant, resulting in either loss of efficacy or an increased risk of adverse effects, necessitating dose individualisation. Several generalisations relating to the likelihood of DDIs can be drawn from the known substrate and inhibitor selectivities of UGT enzymes, highlighting the importance of comprehensive reaction phenotyping studies at an early stage of drug development. Further, rigorous assessment of the DDI liability of new chemical entities that undergo glucuronidation to a significant extent has been recommended recently by regulatory guidance. Although evidence-based approaches exist for the in vitro characterisation of UGT enzyme inhibition and induction, the availability of drugs considered appropriate for use as 'probe' substrates in clinical DDI studies is limited and this should be research priority.

Regulation of Nuclear Receptors PXR and CAR by Small Molecules and Signal Crosstalk: Roles in Drug Metabolism and Beyond

Pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are ligand-activated transcription factors that regulate the expression of drug metabolizing enzymes and drug transporters. Since their discoveries, they have been studied as important factors for regulating processes related to drug efficacy, drug toxicity, and drug-drug interactions. However, their vast ligand-binding profiles extend into additional spaces, such as endogenously produced chemicals, microbiome metabolites, dietary compounds, and environmental pollutants. Therefore, PXR and CAR can respond to an enormous abundance of stimuli, resulting in significant shifts in metabolic programs and physiologic homeostasis. Naturally, PXR and CAR have been implicated in various diseases related to homeostatic perturbations, such as inflammatory bowel disorders, diabetes, and certain cancers. Recent findings have injected the field with new signaling mechanisms and tools to dissect the complex PXR and CAR biology and have strengthened the potential for future PXR and CAR modulators in the clinic. Here, we describe the historical and ongoing importance of PXR and CAR in drug metabolism pathways and how this history has evolved into new mechanisms that regulate and are regulated by these xenobiotic receptors, with a specific focus on small molecule ligands. To effectively convey the impact of newly emerging research, we have arranged five diverse and representative key recent advances, four specific challenges, and four perspectives on future directions. SIGNIFICANCE STATEMENT: PXR and CAR are key transcription factors that regulate homeostatic detoxification of the liver and intestines. Diverse chemicals bind to these nuclear receptors, triggering their transcriptional tuning of the cellular metabolic response. This minireview revisits the importance of PXR and CAR in pharmaceutical drug responses and highlights recent results with implications beyond drug metabolism.

Oridonin alleviates hyperbilirubinemia through activating LXRα-UGT1A1 axis

Hyperbilirubinemia is a serious hazard to human health due to its neurotoxicity and lethality. So far, successful therapy for hyperbilirubinemia with fewer side effects is still lacking. In this study, we aimed to clarify the effects of oridonin (Ori), an active diterpenoid extracted from Rabdosia rubescens, on hyperbilirubinemia and revealed the underlying molecular mechanism in vivo and in vitro. Here, we showed that liver X receptor alpha (LXRα) deletion eliminated the protective effect of Ori on phenylhydrazine hydrochloride-induced hyperbilirubinemia mice, indicating that LXRα acted as a key target for Ori treatment of hyperbilirubinemia. Ori significantly increased the expression of LXRα and UDP-glucuronosyltransferase 1A1 (UGT1A1) in the liver of wild-type (WT) mice, which were lost in LXRα^-/- mice. Ori or LXR agonist GW3965 also reduced lipopolysaccharide/D-galactosamine-induced hyperbilirubinemia via activating LXRα/UGT1A1 in WT mice. Liver UGT1A1 enzyme activity was elevated by Ori or GW3965 in WT mice. Further, Ori up-regulated LXRα gene expression, increased its nuclear translocation and stimulated UGT1A1 promoter activity in HepG2 cells. After silencing LXRα by siRNA, Ori-induced UGT1A1 expression was markedly reduced in HepG2 cells and primary mouse hepatocytes. Taken together, Ori stimulated the transcriptional activity of LXRα, resulting in the up-regulation of UGT1A1. Therefore, Ori or its analogs might have the potential to treat hyperbilirubinemia-related diseases through modulating LXRα-UGT1A1 signaling.

Neobavaisoflavone Induces Bilirubin Metabolizing Enzyme UGT1A1 via PPARα and PPARγ

UDP-glucuronosyltransferase 1A1 (UGT1A1) is an essential enzyme in mammals that is responsible for detoxification and metabolic clearance of the endogenous toxin bilirubin and a variety of xenobiotics, including some crucial therapeutic drugs. Discovery of potent and safe UGT1A1 inducers will provide an alternative therapy for ameliorating hyperbilirubinaemia and drug-induced hepatoxicity. This study aims to find efficacious UGT1A1 inducer(s) from natural flavonoids, and to reveal the mechanism involved in up-regulating of this key conjugative enzyme by the flavonoid(s) with strong UGT1A1 induction activity. Among all the tested flavonoids, neobavaisoflavone (NBIF) displayed the most potent UGT1A1 induction activity, while its inductive effects were confirmed by both western blot and glucuronidation activity assays. A panel of nuclear receptor reporter assays demonstrated that NBIF activated PPARα and PPARγ in a dose-dependent manner. Meanwhile, we also found that NBIF could up-regulate the expression of PPARα and PPARγ in hepatic cells, suggesting that the induction of UGT1A1 by NBIF was mainly mediated by PPARs. In silico simulations showed that NBIF could stably bind on pocket II of PPARα and PPARγ. Collectively, our results demonstrated that NBIF is a natural inducer of UGT1A1, while this agent induced UGT1A1 mainly via activating and up-regulating PPARα and PPARγ. These findings suggested that NBIF can be used as a promising lead compound for the development of more efficacious UGT1A1 inducers to treat hyperbilirubinaemia and UGT1A1-associated drug toxicities.

Oleanolic Acid and Ursolic Acid Induce UGT1A1 Expression in HepG2 Cells by Activating PXR Rather Than CAR

Background: Oleanolic acid (OA) and its isomer ursolic acid (UA) have recently emerged as research foci based on their biologic activities. We previously demonstrated that UA can inhibit the activities of UGT1A3 and UGT1A4, and OA inhibits UGT1A3 activity in liver microsomes. However, whether OA and UA affect the expression of UGT1As in HepG2 cells and the underlying regulatory mechanism remain unclear.

Purpose: The present study aimed to explore the effect of OA and UA on the expression of UGT1As in HepG2 cells and the regulatory mechanisms on UGT1A1 based on the pregnane X receptor (PXR) and constitutive androstane receptor (CAR) signaling pathways.

Methods: We analyzed the effect of OA and UA on UGT1A expression and on the PXR/CAR regulatory pathway in HepG2 cells, hPXR-silenced HepG2 cells, and hCAR-silenced HepG2 cells by Q-PCR, Western blotting, and dual-luciferase reporter gene assays.

Results: In HepG2 cells, OA and UA both significantly induced the expression of UGT1A1, UGT1A3, UGT1A4, and UGT1A9 and upregulated the expression of PXR. However, OA and UA did not affect CAR expression. A dual-luciferase reporter assay showed that OA and UA could markedly promote PXR-mediated UGT1A1 luciferase activity, whereas OA and UA did not affect CAR-mediated UGT1A1 luciferase activity. In hPXR-silenced HepG2 cells, OA and UA did not elevate UGT1A1 activity compared to the control group. However, the expression of UGT1A1 in hCAR-silenced HepG2 cells was markedly elevated compared to the control group or with non-silenced HepG2 cells treated with OA (10, 20, and 40 μM) or UA (10, 20, and 40 μM).

Conclusions: OA and UA significantly induce the expression of UGT1A1, UGT1A3, UGT1A4, and UGT1A9 in HepG2 cells, and their induction on UGT1A1 is mediated by PXR activation, not CAR.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

Use of quercetin in animal feed: effects on the P-gp expression and pharmacokinetics of orally administrated enrofloxacin in chicken

Modulation of P-glycoprotein (P-gp, encoded by Mdr1) by xenobiotics plays central role in pharmacokinetics of various drugs. Quercetin has a potential to modulate P-gp in rodents, however, its effects on P-gp modulation in chicken are still unclear. Herein, study reports role of quercetin in modulation of P-gp expression and subsequent effects on the pharmacokinetics of enrofloxacin in broilers. Results show that P-gp expression was increased in a dose-dependent manner following exposure to quercetin in Caco-2 cells and tissues of chicken. Absorption rate constant and apparent permeability coefficient of rhodamine 123 were decreased, reflecting efflux function of P-gp in chicken intestine increased by quercetin. Quercetin altered pharmacokinetic of enrofloxacin by decreasing area under curve, peak concentration, and time to reach peak concentration and by increasing clearance rate. Molecular docking shows quercetin can form favorable interactions with binding pocket of chicken xenobiotic receptor (CXR). Results provide convincing evidence that quercetin induced P-gp expression in tissues by possible interaction with CXR, and consequently reducing bioavailability of orally administered enrofloxacin through restricting its intestinal absorption and liver/kidney clearance in broilers. The results can be further extended to guide reasonable use of quercetin to avoid drug-feed interaction occurred with co-administered enrofloxacin or other similar antimicrobials.

Effects of aqueous extract of Ruta graveolens and its ingredients on cytochrome P450, uridine diphosphate (UDP)-glucuronosyltransferase, and reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H)-quinone oxidoreductase in mice

Ruta graveolens (the common rue) has been used for various therapeutic purposes, including relief of rheumatism and treatment of circulatory disorder. To elucidate the effects of rue on main drug-metabolizing enzymes, effects of an aqueous extract of the aerial part of rue and its ingredients on cytochrome P450 (P450/CYP), uridine diphosphate (UDP)-glucuronosyltransferase, and reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H):quinone oxidoreductase were studied in C57BL/6JNarl mice. Oral administration of rue extract to males increased hepatic Cyp1a and Cyp2b activities in a dose-dependent manner. Under a 7-day treatment regimen, rue extract (0.5 g/kg) induced hepatic Cyp1a and Cyp2b activities and protein levels in males and females. This treatment increased hepatic UDP-glucuronosyltransferase activity only in males. However, NAD(P)H:quinone oxidoreductase activity remained unchanged. Based on the contents of rutin and furanocoumarins of mouse dose of rue extract, rutin increased hepatic Cyp1a activity and the mixture of furanocoumarins (F_mix) increased Cyp2b activities in males. The mixture of rutin and F_mix increased Cyp1a and Cyp2b activities. These results revealed that rutin and F_mix contributed at least in part to the P450 induction by rue.

Small-molecule modulators of the constitutive androstane receptor

INTRODUCTION

The constitutive androstane receptor (CAR) induces drug-metabolizing enzymes for xenobiotic metabolism.

AREAS COVERED

This review covers recent advances in elucidating the biological functions of CAR and its modulation by a growing number of agonists and inhibitors.

EXPERT OPINION

Extrapolation of animal CAR function to that of humans should be carefully scrutinized, particularly when rodents are used in evaluating the metabolic profile and carcinogenic properties of clinical drugs and environmental chemicals. Continuous efforts are needed to discover novel CAR inhibitors, with extensive understanding of their inhibitory mechanism, species selectivity, and discriminating power against other xenobiotic sensors.

Transcriptional regulation of human UDP-glucuronosyltransferase genes

Glucuronidation is an important metabolic pathway for many small endogenous and exogenous lipophilic compounds, including bilirubin, steroid hormones, bile acids, carcinogens and therapeutic drugs. Glucuronidation is primarily catalyzed by the UDP-glucuronosyltransferase (UGT) 1A and two subfamilies, including nine functional UGT1A enzymes (1A1, 1A3-1A10) and 10 functional UGT2 enzymes (2A1, 2A2, 2A3, 2B4, 2B7, 2B10, 2B11, 2B15, 2B17 and 2B28). Most UGTs are expressed in the liver and this expression relates to the major role of hepatic glucuronidation in systemic clearance of toxic lipophilic compounds. Hepatic glucuronidation activity protects the body from chemical insults and governs the therapeutic efficacy of drugs that are inactivated by UGTs. UGT mRNAs have also been detected in over 20 extrahepatic tissues with a unique complement of UGT mRNAs seen in almost every tissue. This extrahepatic glucuronidation activity helps to maintain homeostasis and hence regulates biological activity of endogenous molecules that are primarily inactivated by UGTs. Deciphering the molecular mechanisms underlying tissue-specific UGT expression has been the subject of a large number of studies over the last two decades. These studies have shown that the constitutive and inducible expression of UGTs is primarily regulated by tissue-specific and ligand-activated transcription factors (TFs) via their binding to cis-regulatory elements (CREs) in UGT promoters and enhancers. This review first briefly summarizes published UGT gene transcriptional studies and the experimental models and tools utilized in these studies, and then describes in detail the TFs and their respective CREs that have been identified in the promoters and/or enhancers of individual UGT genes.

Effects of co-treatment with sulforaphane and autophagy modulators on uridine 5'-diphospho-glucuronosyltransferase 1A isoforms and cytochrome P450 3A4 expression in Caco-2 human colon cancer cells

Sulforaphane (SFN), which is highly enriched in cruciferous vegetables, has been investigated for its cancer chemopreventive properties and ability to induce autophagy. Uridine 5'-diphospho (UDP)-glucuronosyltransferase (UGT)1A induction is one of the mechanisms that is responsible for the cancer chemopreventive activity of SFN. The current study demonstrates that rapamycin may enhance the chemopreventive effects of SFN on Caco-2 cells; this may be partially attributed to nuclear translocation of nuclear factor erythroid 2-related factor 2 (Nrf2)- and human pregnane X receptor (hPXR)-mediated UGT1A1, UGT1A8 and UGT1A10 induction. These results indicate that targeting autophagy modulation may be a promising strategy for increasing the chemopreventive effects of SFN in cases of colon cancer.

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

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

Age-related changes in mRNA levels of hepatic transporters, cytochrome P450 and UDP-glucuronosyltransferase in female rats

Hepatic transporters and metabolic enzymes affect drug pharmacokinetics. Limited information exists on the alteration in mRNA levels of hepatic transporters and metabolic enzymes with aging. We examined the effects of aging on the mRNA levels of representative hepatic drug transporters and metabolic enzymes by analyzing their levels in 10-, 30- and 50-week-old male and female rats. Levels of mRNA of drug transporters including multidrug resistance protein (Mdr)1a, multidrug resistance-associated protein (Mrp)2, breast cancer resistance protein (Bcrp) and organic anion-transporting polypeptide (Oatp)1a1, and the metabolic enzymes cytochrome P450 (CYP)3A1, CYP3A2 and UDP-glucuronosyltransferase (UGT)1A1 were analyzed using real-time reverse transcriptase polymerase chain reaction. The mRNA levels of transporters in male rats did not decrease with age, while the mRNA levels of Bcrp and Oatp1a1 in female rats decreased with age. The mRNA levels of CYP3A1 and CYP3A2 in male rats were higher than those in female rats. The mRNA levels of metabolic enzymes decreased with age in female but not male rats. In particular, the mRNA levels of UGT1A1 in 10-week-old female rats were higher than those in male rats. mRNA expression of hepatic transporters and metabolic enzymes are more susceptible to aging in female than male rats. The age-related decreases in the mRNA levels of Bcrp, Oatp1a1, CYP3A1 and CYP3A2 in female rats may affect the metabolism and transport of substrates. This study showed that aging affected the mRNA expression of hepatic transporters and metabolic enzymes in rats.

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.

Molecular markers of chemotherapeutic response and toxicity in colorectal cancer

Outcomes in colorectal cancer have improved over the last 15 years; this is in part due to the optimization of 5-fluorouracil schedules and the introduction of new and effective chemotherapeutic agents, such as irinotecan and oxaliplatin. However, not all patients respond to these agents and a proportion may suffer severe side effects from particular chemotherapy drugs. These observations have resulted in a concerted research effort to identify markers of chemotherapy efficacy and toxicity. Here we review the evidence for using molecular markers to individualize chemotherapy treatment in colorectal cancer.

Anti-cancer potential of flavonoids: recent trends and future perspectives

Cancer is a major public health concern in both developed and developing countries. Several plant-derived anti-cancer agents including taxol, vinblastine, vincristine, the campothecin derivatives, topotecan, irinotecan and etoposide are in clinical use all over the world. Other promising anti-cancer agents include flavopiridol, roscovitine, combretastatin A-4, betulinic acid and silvestrol. From this list one can well imagine the predominance of polyphenols, flavonoids and their synthetic analogs in the treatment of ovarian, breast, cervical, pancreatic and prostate cancer. Flavonoids present in human diet comprise many polyphenolic secondary metabolites with broad-spectrum pharmacological activities including their potential role as anti-cancer agents. A positive correlation between flavonoids-rich diet (from vegetables and fruits) and lower risk of colon, prostate and breast cancers lead to a question that whether flavonoids mediate the protective effects as chemopreventive agents or can interact with different genes and proteins to play role in chemotherapy. The current review emphasizes onto the therapeutic potential of flavonoids and their synthetic analogs as anti-cancer agents by providing new insights into the factors, regulation and molecular mechanisms along with their significant protein interactions.

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.

Nuclear receptors and endobiotics glucuronidation: the good, the bad, and the UGT

The recent progresses in molecular biology and pharmacology approaches allowed the characterization of a series of nuclear receptors (NRs) as efficient regulators of uridine diphosphate glucuronosyltransferase (UGT) genes activity. These regulatory processes ensure an optimized UGT expression in response to specific endo- and/or exogenous stimuli. Many of these NRs are activated by endobiotics that also are substrates for UGTs. Thus, by activating their receptors, these endogenous substances control their own conjugation, leading to the concept that glucuronidation is an important part of feed-forward/feedback mechanisms by which bioactive molecules control their own concentrations. On the other hand, numerous studies have established the pharmacological relevance of NR-UGT regulatory pathways in the response to therapeutic ligands. The present review article aims at providing a comprehensive view of the physiological and pharmacological importance of the NR regulation of the expression and activity of endobiotics-conjugating UGT enzymes. Selected examples will illustrate how the organism profits from the feed-forward/feedback mechanisms involving NR-UGT pathways, but also how such regulatory processes are involved in the initiation and/or progression of several pathological situations. Finally, we will discuss how the present pharmacopeia involves NR-dependent regulation of endobiotics glucuronidation, and whether the unexploited NR-UGT axes could serve as pharmacological targets for novel therapeutics to restore endobiotics homeostasis.

Chrysin ameliorates chemically induced colitis in the mouse through modulation of a PXR/NF-κB signaling pathway

Targeted activation of pregnane X receptor (PXR) in recent years has become a therapeutic strategy for inflammatory bowel disease. Chrysin is a naturally occurring flavonoid with anti-inflammation activity. The current study investigated the role of chrysin as a putative mouse PXR agonist in preventing experimental colitis. Pre-administration of chrysin ameliorated inflammatory symptoms in mouse models of colitis (dextran sodium sulfate- and 2,4,6-trinitrobenzene sulfonic acid-induced) and resulted in down-regulation of nuclear transcription factor κB (NF-κB) target genes (inducible NO synthase, intercellular adhesion molecule-1, monocyte chemotactic protein-1, cyclooxygenase 2, tumor necrosis factor-α, and interleukin 6) in the colon mucosa. Chrysin inhibited the phosphorylation/degradation of inhibitor κBα (IκBα), which correlated with the decrease in the activity of myeloperoxidase and the levels of tumor necrosis factor-α and interleukin 6 in the colon. Consistent with the in vivo results, chrysin blocked lipopolysaccharide -stimulated nuclear translocation of NF-κB p65 in mouse macrophage RAW264.7. Furthermore, chrysin dose-dependently activated human/mouse PXR in reporter gene assays and up-regulated xenobiotic detoxification genes in the colon mucosa, but not in the liver. Silencing of PXR by RNA interference demonstrated necessity of PXR in mediating chrysin's ability to induce xenobiotic detoxification genes and NF-κB inactivation. The repression of NF-κB transcription activity by chrysin was confirmed by in vitro PXR transduction. These findings suggest that the effect of chrysin in preventing chemically induced colitis is mediated in large part by a PXR/NF-κB pathway. The data also suggest that chrysin or chrysin-like flavonoids could be further developed as intestine-specific PXR activators.

Diet-relevant phytochemical intake affects the cardiac AhR and nrf2 transcriptome and reduces heart failure in hypertensive rats

Intake of phytochemical-rich diets is inversely related to hypertension. Phytochemicals alter in vitro aryl hydrocarbon receptor (AhR) and NF-E2 related factor (nrf2) transcription factor activity and related genes pertinent to antioxidant defense. However, it is unknown if these molecular effects occur in the heart with dietary intake of physiologically relevant phytochemicals and if this correlates with reduced hypertension-associated heart failure. This extended feeding study used whole grapes as a model of a phytochemical-rich food and hypertensive heart failure-prone rats to assess mechanisms of effect. Grape intake reduced cardiac hypertrophy and fibrosis and improved diastolic function. At the development of diastolic dysfunction, hypertensive rats show reduced AhR activity, reduced expression of AhR-regulated genes, reduced glutathione and reduced activity of glutathione-regulating proteins. However, grape intake significantly increased cardiac AhR and nrf2 activity, Phase I/II gene transcripts and protein activity related to antioxidant defense. Heart failure is the leading cause of morbidity and mortality in the aged and the intake of phytochemicals from fruits and vegetables decreases with age. Concentrated antioxidant nutrient trials have failed to affect heart failure. However, this study demonstrates that diet-relevant intake of non-nutrient phytochemicals significantly reduces heart failure progression. Therefore, this study suggests that higher intake of phytochemical-containing foods may achieve cardiac benefits that isolated antioxidant supplements may not. In summary, intake of diet-relevant phytochemicals altered the cardiac antioxidant transcriptome, antioxidant defense, oxidative damage and fibrosis. Regular phytochemical intake may therefore increase cardiac resistance to cardiac pathology instigated by prolonged hypertension.

Substrate selectivity of human intestinal UDP-glucuronosyltransferases (UGTs): in silico and in vitro insights

The current drug development process aims to produce safe, effective drugs within a reasonable time and at a reasonable cost. Phase II metabolism (glucuronidation) can affect drug action and pharmacokinetics to a considerable extent and so its studies and prediction at initial stages of drug development are very imperative. Extensive glucuronidation is an obstacle to oral bioavailability because the first-pass glucuronidation [or premature clearance by UDP-glucuronosyltransferases (UGTs)] of orally administered agents frequently results in poor oral bioavailability and lack of efficacy. Modeling of new chemical entities/drugs for UGTs and their kinetic data can be useful in understanding the binding patterns to be used in the design of better molecules. This review concentrates on first-pass glucuronidation by intestinal UGTs, including their topology, expression profile, and pharmacogenomics. In addition, recent advances are discussed with respect to substrate selectivity at the binding pocket, structural requirements, and mechanism of enzyme actions.

Nuclear factor κB down-regulates human UDP-glucuronosyltransferase 1A1: a novel mechanism involved in inflammation-associated hyperbilirubinaemia

Jaundice or hyperbilirubinaemia is a common complication of sepsis. UGT1A1 (UDP-glucuronosyltransferase 1A1) is a critical gene for bilirubin metabolism and irinotecan detoxification. However, the molecular pathogenesis of hyperbilirubinaemia during inflammation needs to be further clarified. Human hepatic UGT1A1 expression was analysed by RT (reverse transcription)–PCR, qRT-PCR (quantitative real-time PCR) and Western blotting in response to LPS (lipopolysaccharide) stimulation. Transcription regulatory elements in the upstream promoter region of the human UGT1A1 gene were determined using EMSA (electrophoretic mobility-shift assay) and ChIP (chromatin immunoprecipitation). The important role of the transcription regulatory element was examined using a luciferase assay, and was determined by qRT-PCR using a transcription factor activation inhibitor. LPS down-regulated the UGT1A1 mRNA expression in human hepatoma cell lines. A newly identified NF-κB (nuclear factor κB)-binding site was located on the upstream promoter region (−725/−716) of the human UGT1A1 gene. LPS-induced NF-κB activation and specific binding to the NF-κB-binding site can suppress human UGT1A1 promoter activity and human UGT1A1 expression. We demonstrated that LPS mediates the suppression of human UGT1A1 expression through specific binding of NF-κB to this newly identified NF-κB-binding site in the upstream promoter of the human UGT1A1 gene. The present study may partly explain the molecular pathogenesis of inflammation-associated hyperbilirubinaemia.

Effect of aflatoxin B1 on UDP-glucuronosyltransferase mRNA expression in HepG2 cells

Aflatoxin B1 (AFB1) is a potent mycotoxin that induces hepatocellular carcinoma in many animal species, including humans. In this study, we examined the effects of AFB1 on UDP-glucuronosyltransferase (UGT) mRNA expression in HepG2 cells (human hepatocellular carcinoma cell line). The cells were treated with AFB1 for 48 h at a concentration of 10 μM, and their viability (87%) was not significantly different from that of control cells. Reverse transcription polymerase chain reaction (RT-PCR) analysis demonstrated that the mRNAs of four UGT1As (UGT1A1, UGT1A3, UGT1A4 and UGT1A9) and seven UGT2Bs (UGT2B4, UGT2B7, UGT2B10, UGT2B11, UGT2B15, UGT2B17 and UGT2B28) are expressed in HepG2 cells. The mRNAs of aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR), retinoid X receptor (RXR) and glucocorticoid receptor (GR) as transcriptional regulators were also detected. AFB1 significantly increased mRNA levels of UGT1A3, UGT2B10, UGT2B15 and UGT2B17 in HepG2 cells to 2.5-, 2.0-, 1.9- and 1.5-fold, respectively, whereas the mRNA levels of transcriptional regulators were hardly affected by AFB1. These findings suggest that AFB1 induces UGT2B isoforms rather than UGT1A isoforms in HepG2 cells, and that the change may closely contribute to the toxicity of AFB1.

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

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

Selective action of human sera differing in fatty acids and cholesterol content on in vitro gene expression

Serum constituents might directly affect metabolic diseases pathogenesis and are commonly used as diagnostic tool. The aim of this study was to investigate the human serum effect on in vitro gene expression, related to nutrients action and involved in lipid metabolism. In detail, 40 human sera were firstly analyzed in fatty acids profile by gas-chromatography. Then samples were tested through direct addition within culture medium on Hep G2 human hepatoma cells, comparing samples from hypercholesterolemic (average 273 mg/dl) versus normocholesterolemic male subjects (average 155 mg/dl), since this condition is a relevant disease risk factor and is typically consequent to nutritional style. Hypercholesterolemic sera produced a 0.4-fold reduction of sterol regulatory element binding protein 1c (SREBP-1c) mRNA (P < 0.05) and a 1.5-fold increase of UDP-glucuronosyltransferase 1A1 (UGT1A1) mRNA (P < 0.01). Samples with higher concentrations of n-6 fatty acids produced a higher expression of UGT1A1 mRNA. Total fatty acids [docosahexaenoic, eicosopentanoic, arachidonic, linolenic, and linoleic acid (DHA, EPA, AA, LNA, and LA, respectively)] in each serum resulted roughly inverse with trend of SREBP-1c mRNA expression. Serum AA, LA, and trans fatty acids were more abundant in hypercholesterolemic subjects (P < 0.01) while DHA as quota of detected fatty acids was significantly higher in normocholesterolemic subjects (P < 0.05). While it is not possible to indicate which component was responsible for the observed gene modulations, our data indicate that sera differing in lipid profiles, mainly associated with dietary behavior, differentially affect gene expression known to be involved in metabolic and nutritional related conditions.

Pharmacogene regulatory elements: from discovery to applications

Regulatory elements play an important role in the variability of individual responses to drug treatment. This has been established through studies on three classes of elements that regulate RNA and protein abundance: promoters, enhancers and microRNAs. Each of these elements, and genetic variants within them, are being characterized at an exponential pace by next-generation sequencing (NGS) technologies. In this review, we outline examples of how each class of element affects drug response via regulation of drug targets, transporters and enzymes. We also discuss the impact of NGS technologies such as chromatin immunoprecipitation sequencing (ChIP-Seq) and RNA sequencing (RNA-Seq), and the ramifications of new techniques such as high-throughput chromosome capture (Hi-C), chromatin interaction analysis by paired-end tag sequencing (ChIA-PET) and massively parallel reporter assays (MPRA). NGS approaches are generating data faster than they can be analyzed, and new methods will be required to prioritize laboratory results before they are ready for the clinic. However, there is no doubt that these approaches will bring about a systems-level understanding of the interplay between genetic variants and drug response. An understanding of the importance of regulatory variants in pharmacogenomics will facilitate the identification of responders versus non-responders, the prevention of adverse effects and the optimization of therapies for individual patients.

Compartment-specific gene regulation of the CAR inducer efavirenz in vivo

Nuclear receptors such as the constitutive androstane receptor (CAR) are central factors that link drug exposure to the activities of drug metabolism and elimination. In order to determine the in vivo effects of efavirenz, a CAR activator, the expression of target genes was determined in duodenal biopsies obtained from 12 healthy volunteers before treatment and after 10 days of treatment with efavirenz; concomitant administration of the cholesterol inhibitor ezetimibe produced no significant difference. However, in in vitro studies, efavirenz significantly increased CYP2B6 expression in several cell types, suggesting that the drug transactivates CAR. This hypothesis is supported by our findings that there is significant induction of CAR target genes in in vivo peripheral blood mononuclear cells (PBMCs) isolated from healthy volunteers treated with multiple doses of efavirenz. The impact of efavirenz on hepatic metabolism in vivo was confirmed by significant changes in plasma 4β-hydroxycholesterol and bilirubin levels and the area under the curve (AUC) of efavirenz. Induction of CYP2B6 mRNA expression correlated with the decrease in the AUC of efavirenz (r = 0.61; P = 0.036). Taken together, our results provide evidence that efavirenz exerts compartment-specific inductive capacity in vivo.

Enzymatic synthesis of substituted epicatechins for bioactivity studies in neurological disorders

Glucuronidated and/or methylated metabolites of the proanthocyanidin (PA) monomer (-)-epicatechin are detected in both blood and brain following feeding of rodents with a monomeric grape seed PA extract shown to reduce symptoms in a mouse model of Alzheimer's disease. To generate metabolites for future mechanistic studies, we investigated the ability of recombinant human glucuronosyl transferases of the UGT1A and UGT2B families to glucuronidate epicatechin or 3'-O-methyl epicatechin in vitro. Of twelve enzymes tested, UGT1A9 was the most efficient, producing epicatechin 3'-O-glucuronide as the major product. Incubation of UGT1A9 with 3'-O-methyl-epicatechin resulted in two major products, one of which was identified as 3'-O-methyl-epicatechin 5-O-glucuronide, a major metabolite found in blood plasma and brain tissue of the rodents following feeding with a grape seed extract. We also investigated in vitro methylation of epicatechin and epicatechin glucuronides by human catechol O-methyltransferase. Enzymatic production of 3'-O-methyl-epicatechin 5-O-glucuronide was optimized to 50% overall yield. These studies form a basis for generation of mg quantities of pure epicatechin (methyl) glucuronides of biological significance, and provide clarification of structure of previously identified epicatechin metabolites.

Molecular mechanisms of drug transporter regulation

Interindividual differences in drug transporter expression can result in variability in drug response. This variation in gene expression is determined, in part, by the actions of nuclear hormone receptors that act as xenobiotic- and endobiotic-sensing transcription factors. Among the ligand-activated nuclear receptors, signaling through the pregnane X receptor (PXR), constitutive androstane receptor (CAR), farnesoid X receptor (FXR), and vitamin D receptor (VDR) constitute major pathways regulating drug transporter expression in tissues. Hence, these endobiotic- and xenobiotic-sensing nuclear receptors are intrinsically involved in environmental influences of drug response. Moreover, because nuclear receptor genes are polymorphic, these transcription factors are also thought to contribute to heritability of variable drug action. In this chapter, the molecular aspects of drug transporter gene regulation by ligand-activated nuclear receptors will be reviewed including their clinical relevance.

Which sources of flavonoids: complex diets or dietary supplements?

There is increasing interest in the potential health benefits of dietary flavonoids. Fruits and vegetables, tea, and cocoa are rich natural sources of flavonoids. Epidemiological studies have indicated that consumption of these foods is likely to be associated with a reduced risk of cardiovascular disease, but the etiology of this benefit is not yet clearly defined. Furthermore, in some acute interventions, a positive effect of tea and cocoa on vascular function has been reported. An alternative source of flavonoids is dietary supplements, which have become increasingly popular in the recent past. In this context, it needs to be critically evaluated whether vascular health-promoting and other positive properties of flavonoid-rich diets can be replaced by purified flavonoids as dietary supplements. Plant sources of flavonoids contain a complex mixture of secondary plant metabolites and not only flavonoids per se. This complex mixture of secondary plant metabolites cannot be simply exchanged by single purified compounds as dietary supplements. If flavonoids are given as dietary supplements, toxicity issues as well as nutrient drug interactions need to be taken into account. Purified flavonoids given in high doses as dietary supplements may affect trace element, folate, and vitamin C status. Furthermore, they may exhibit antithyroid and goitrogenic activities. In this review article, the available literature on the safety issues surrounding high dose supplemental flavonoid consumption has been summarized.

Synergistically enhanced CYP2B6 inducibility between a polymorphic mutation in CYP2B6 promoter and pregnane X receptor activation

CYP2B6 is a highly inducible and polymorphic enzyme involved in the metabolism of an increasing number of clinically important drugs. Significant interindividual variability in CYP2B6 expression has been attributed to either genetic polymorphisms or chemical-mediated induction through the activation of constitutive androstane receptor and/or pregnane X receptor (PXR). It was reported that the -82T→C substitution within the CYP2B6*22 allele creates a functional CCAAT/enhancer-binding protein (C/EBP) binding site and enhances the basal expression of the CYP2B6 gene. Here, we explored whether this polymorphic mutation could affect drug-mediated induction of CYP2B6. Cell-based promoter reporter assays demonstrated that CYP2B6 luciferase activity was synergistically enhanced in the presence of both -82T→C mutation and rifampicin (RIF)-activated PXR. On the other hand, this synergism was attenuated by disrupting the C/EBP binding site or knocking down C/EBPα expression. Mechanistic studies revealed that C/EBPα plays an important role in such synergism by directly interacting with PXR; enhancing RIF-mediated recruitment of PXR to the -82T→C harboring CYP2B6 promoter; and looping the PXR-bound distal phenobarbital-responsive enhancer module toward the proximal C/EBP binding site. Furthermore, the genotype-phenotype association was evaluated in cultured human primary hepatocytes from 44 donors. Interestingly, RIF-mediated induction of CYP2B6 in four -82T/C carriers was higher compared with that in the reference -82T/T homozygotes. Together, our results demonstrate, for the first time, a synergistic interplay between a CYP2B6 polymorphism and PXR-mediated induction, which may contribute to the large individual variations and inducibility of CYP2B6 in humans.

The regulation of UDP-glucuronosyltransferase genes by tissue-specific and ligand-activated transcription factors

Elucidation of the mechanisms regulating UGT genes is of prime importance if the adverse effects of interactions between drugs primarily eliminated by glucuronidation are to be minimized, and if UGT expression is to be manipulated for therapeutic effect. The factors controlling UGT gene expression in the liver include the liver-enriched transcription factors, HNF-1alpha and HNF-4alpha, several members of the nuclear-receptor family (CAR, PXR, FXR, LXR, and PPAR), the arylhydrocarbon receptor, and transcription factors involved in stress responses (Nrf2, Maf). HNF-1alpha, in concert with the intestine-specific transcription factor, Cdx2, and Sp1 regulate UGT gene expression in the gastrointestinal tract, whereas the genes for the major androgen-glucuronidating enzymes, UGT2B15 and UGT2B17, are upregulated by estrogens in breast cell lines and downregulated by androgens in prostate-derived cells. Despite this knowledge, the complex interactions between these transcription factors and their coregulators has not been determined, and the mechanisms regulating UGT gene expression in organs and tissues, other than the liver, gastrointestinal tract, breast, and prostate, remain to be elucidated.

Induction of UGT1A1 and CYP2B6 by an antimitogenic factor in HepG2 cells is mediated through suppression of cyclin-dependent kinase 2 activity: cell cycle-dependent expression

Hepatocyte growth factor (HGF), an antimitogenic factor for HepG2 cells, increased mRNA and protein levels of UGT1A1 and CYP2B6, as well as the endogenous cyclin-dependent kinase (CDK) inhibitors p16, p21, and p27 in HepG2 cells but not in HuH6, Caco2, or MCF7 cells. Treatment with 1,4-diamino-2,3-dicyano-1,4-bis(methylthio)butadiene (U0126) (an extracellular signal-regulated kinase inhibitor) suppressed the HGF-induced expression of UGT1A1 and CYP2B6, as well as p16, p21, and p27 in HepG2 cells. The CDK inhibitor roscovitine also enhanced the expression of UGT1A1, CYP2B6, and CYP3A4. Transfection of anti-CDK2 siRNA led to elevated levels of UGT1A1, CYP2B6, and CYP3A4 in HepG2 and SW480 cells, whereas anti-CDK4 small interfering RNA (siRNA) did not significantly enhance the expression of these enzymes. In fact, CDK2 activity was decreased in HGF-treated HepG2 cells. In cells arrested in S phase by a thymidine block and then released into a synchronous cell cycle, there was a clear dissociation among the activation of CDK2 and the expression of UGT1A1, CYP2B6, and CYP3A4. Furthermore, the induction of CYP3A4 but not UGT1A1 or CYP2B6 mRNA expression by roscovitine was repressed in pregnane X receptor (PXR) siRNA-transfected HepG2 cells. Transfection with constitutive androstane receptor siRNA or PXR siRNA in HepG2 cells did not repress the HGF-stimulated expression of UGT1A1 mRNA. Taken together, our results show that the expression of UGT1A1 and CYP2B6 is negatively regulated through a CDK2 signaling pathway linked to cell cycle progression in HepG2 and SW480 cells, the mechanism of which may differ from that of CYP3A4 expression through PXR phosphorylated by CDK2.

Modulation of the activity of ABC transporters (P-glycoprotein, MRP2, BCRP) by flavonoids and drug response

The present article aims to review the up-to-date information on the most recent studies of the interaction of flavonoids with ABC transporters, in particular the drug pharmacokinetic consequences of such a relationship. In addition, the modulation of the expression of the ABC transporters by flavonoids is also illustrated. Flavonoids are a large group of plant polyphenols present extensively in our daily diets and herbal products. High intake of isoflavones has been associated with a variety of beneficial effects on several common diseases. These polyphenols interact with ABC drug transporters involved in drug resistance and drug absorption, distribution and excretion. A number of studies have demonstrated inhibition of drug transporters by flavonoids. This flavonoid-ABC-transporter interaction could be beneficial for poorly absorbed drugs but could also result in severe drug intoxication, especially drugs with a narrow therapeutic window. On the other hand, flavonoids are themselves substrates of ABC transporters. These proteins can affect the oral availability and tissue distribution of these compounds, modifying their beneficial effects. The challenge is to find a suitable way to predict harmful drug-flavonoid interactions mediated by these transporters.

Hepatic OATP1B transporters and nuclear receptors PXR and CAR: interplay, regulation of drug disposition genes, and single nucleotide polymorphisms

Drug uptake transporters are now increasingly recognized as clinically relevant determinants of variable drug responsiveness and unexpected drug-drug interactions. Emerging evidence strongly suggests members of the organic anion transporting polypeptide (OATP) family appear to be particularly important to the disposition of many drugs in clinical use today. Specifically, the liver-enriched OATP1B subfamily members OATP1B1 and OATP1B3 exhibit broad substrate specificity and the ability to transport drugs which are ligands for xenobiotic sensing nuclear receptors such as the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Accordingly, OATP1B transporters may indirectly regulate expression of drug metabolism genes via modulation of the intracellular concentration of PXR and CAR ligands. Moreover, a number of functionally important single nucleotide polymorphisms (SNPs) in OATP1B transporters have been described. In this review, a brief summary of known SNPs in PXR and CAR will be followed by an in-depth outline of OATP1B1 and OATP1B3 transporters particularly in relation to the known SNPs in these OATPs and the interplay between OATP1B transporters with PXR and CAR, both in vitro and in vivo.

Genetic polymorphisms in the TATA box and upstream phenobarbital-responsive enhancer module of the UGT1A1 promoter have combined effects on UDP-glucuronosyltransferase 1A1 transcription mediated by constitutive androstane receptor, pregnane X receptor, or glucocorticoid receptor in human liver

Transcription of UDP-glucuronosyltransferase (UGT) 1A1 is regulated by the transcription factors, constitutive androstane receptor (CAR), pregnane X receptor (PXR), glucocorticoid receptor (GR), hepatocyte nuclear factor (HNF) 1 alpha, and HNF4 alpha. The purpose of this study was to determine whether the genetic polymorphisms in the RNA polymerase II core promoter and the upstream phenobarbital-responsive element module (PBREM) of the UGT1A1 promoter have combined effects on UGT1A1 transcription mediated by the transcription factors. A polymorphism of A(TA)(5-8)TAA in the UGT1A1 TATA box and a single nucleotide polymorphism of -3279T>G in PBREM were genotyped in 98 human liver samples. Relative mRNA levels of CAR, PXR, GR, HNF1 alpha, HNF4 alpha, and UGT1A1 were quantified by a multiplex branched DNA technique. Correlations of mRNA levels between UGT1A1 and the transcription factors were established in liver samples with different combined genetic polymorphisms. Correlation of mRNA levels between UGT1A1 and CAR, PXR, or GR, but not HNF1 alpha or HNF4 alpha, was abolished in the samples with the combined genotype of TA7/7 plus -3279G/G, which was also associated with significantly lower UGT1A1 mRNA levels compared with other combined genotypes. Correlations of mRNA levels between UGT1A1 and CAR or PXR were reduced but not abolished in the samples with the combined genotype of TA6/7 plus -3279 G/G, which showed significantly lower UGT1A1 mRNA levels compared with the combined genotype of TA6/7 plus -3279T/G and other genotypes containing TA6/6. In conclusion, the combined genotypes containing A(TA)(7)TAA and -3279G decrease UGT1A transcription mediated by CAR, PXR, or GR but not by HNF1 alpha or HNF4 alpha.

Cruciferous vegetable feeding alters UGT1A1 activity: diet- and genotype-dependent changes in serum bilirubin in a controlled feeding trial

Chemoprevention by isothiocyanates from cruciferous vegetables occurs partly through up-regulation of phase II conjugating enzymes, such as UDP-glucuronosyltransferases (UGT). UGT1A1 glucuronidates bilirubin, estrogens, and several dietary carcinogens. The UGT1A1*28 polymorphism reduces transcription compared with the wild-type, resulting in decreased enzyme activity. Isothiocyanates are metabolized by glutathione S-transferases (GST); variants may alter isothiocyanate clearance such that response to crucifers may vary by genotype. We evaluated, in a randomized, controlled, crossover feeding trial in humans (n = 70), three test diets (single- and double-"dose" cruciferous and cruciferous plus apiaceous) compared with a fruit and vegetable-free basal diet. We measured serum bilirubin concentrations on days 0, 7, 11, and 14 of each 2-week feeding period to monitor UGT1A1 activity and determined effects of UGT1A1*28 and GSTM1/GSTT1-null variants on response. Aggregate bilirubin response to all vegetable-containing diets was statistically significantly lower compared with the basal diet (P < 0.03 for all). Within each UGT1A1 genotype, lower bilirubin concentrations were seen in *1/*1 in both single- and double-dose cruciferous diets compared with basal (P < 0.03 for both); *1/*28 in double-dose cruciferous and cruciferous plus apiaceous compared with basal, and cruciferous plus apiaceous compared with single-dose cruciferous (P < 0.02 for all); and *28/*28 in all vegetable-containing diets compared with basal (P < 0.02 for all). Evaluation of the effects of diet stratified by GST genotype revealed some statistically significant genotypic differences; however, the magnitude was similar and not statistically significant between genotypes. These results may have implications for altering carcinogen metabolism through dietary intervention, particularly among UGT1A1*28/*28 individuals.

Citrus fruit intake is associated with lower serum bilirubin concentration among women with the UGT1A1*28 polymorphism

UDP-glucuronosyltransferase (UGT) 1A1 glucuronidates bilirubin, estrogens, and xenobiotic compounds. The UGT1A1*28 polymorphism results in lower promoter activity due to 7 thymine-adenine (TA) repeats rather than the more common 6 TA repeats. Previously, we showed that serum bilirubin, a marker of UGT1A1 activity, was lower among individuals homozygous for the UGT1A1*28 polymorphism (7/7) when randomized to a high fruit and vegetable (F&V) diet, whereas there was no effect in individuals with the wild-type (6/6) and heterozygous (6/7) genotypes. Our objective here was to determine if we could detect genotype x diet interactions on bilirubin concentrations in an observational study. Healthy nonsmoking men (n = 146) and women (n = 147), recruited from the Seattle area, provided blood samples for genotyping and bilirubin measurements. We used multiple linear regression to assess the relationships among UGT1A1 genotype, bilirubin concentrations, and consumption of specific F&V [cruciferous vegetables, citrus fruits, and soy foods (n = 268)] based on FFQ and F&V from 6 botanical families [Cruciferae, Rosaceae, Rutaceae, Umbelliferae, Solanaceae, and Leguminosae (n = 261)] based on 3-d food records. We observed a significant interaction of UGT1A1 genotype and citrus consumption among women. Women with the 7/7 genotype who consumed > or = 0.5 daily servings of citrus fruit or foods from the Rutaceae botanical family had approximately 30% lower serum bilirubin than those with the same genotype who consumed less, whereas 6/6 and 6/7 genotypes did not differ by consumption (P for interaction = 0.006 and 0.03, respectively). These results suggest that citrus consumption may increase UGT1A1 activity among women with the 7/7 genotype.

Effects of endocrine disruptors on genes associated with 17beta-estradiol metabolism and excretion

In order to provide a global analysis of the effects of endocrine disruptors on the hormone cellular bioavailability, we combined 17beta-estradiol (E2) cellular flow studies with real-time PCR and Western blot expression measurements of genes involved in the hormone metabolism and excretion. Three endocrine disruptors commonly found in food were chosen for this study, which was conducted in the estrogen receptor (ER) negative hepatoblastoma HepG2 cell line: bisphenol A (BPA), genistein (GEN) and resveratrol (RES). We showed that 24 h after a single dose treatment with genistein, resveratrol or bisphenol A, the expression of ATP-binding cassette transporters (the multidrug resistance or MDR, and the multidrug resistance associated proteins or MRP) uridine diphosphate-glucuronosyltransferases (UGT) and/or sulfotransferases (ST) involved in 17beta-estradiol elimination process were significantly modulated and that 17beta-estradiol cellular flow was modified. Resveratrol induced MDR1 and MRP3 expressions, bisphenol A induced MRP2 and MRP3 expressions, and both enhanced 17beta-estradiol efflux. Genistein, on the other hand, inhibited ST1E1 and UGT1A1 expressions, and led to 17beta-estradiol cellular retention. Thus, we demonstrate that bisphenol A, genistein and resveratrol modulate 17beta-estradiol cellular bioavailability in HepG2 and that these modulations most probably involve regulations of 17beta-estradiol phase II and III metabolism proteins. Up to now, the estrogenicity of environmental estrogenic pollutants has been based on the property of these compounds to bind to ERs. Our results obtained with ER negative cells provide strong evidence for the existence of ER-independent pathways leading to endocrine disruption.